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Thermal Degradation
15 Thermal Degradation IAN C. McNEILL University of Glasgow, UK 452
15.1 THE STUDY OF THERMAL DEGRADATION
15.1.1 15.1.2 15.1.3 15.1.4 15.1.5
452 453 453 455 456
Introduction Literature Sources Methods of Study Pitfalls in Experimental Studies Kinetics of Degradation
15.2 PRIMARY DECOMPOSITION PROCESSES IN ADDITION POLYMERS
15.2.1 Depolymerization 15.2.2 Elimination 15.2.3 Cyclization 15.3 ACRYLATE AND METHACRYLATE POLYMERS
15.3.1 15.3.2 15.3.3 15.3.4 15.3.5
458 458 459 459 461 462
Poly(methyl methacrylate) Higher Polymethacrylates Polyacrylates Poly(methacrylic acid) and Polymethacrylamide Salts of Poly (methacrylic acid)
15.4 POLYSTYRENE AND RELATED POLYMERS
15.4.1 15.4.2 15.4.3 15.4.4 15.4.5 15.4.6
456 456 457 457
462
Polystyrene Chlorinated Polystyrenes Polystyrenes with 0- and Nscontaining Ring Substituents Poly (m-methylstyrene) Crosslinked Styrene Polymers Polyt a-Methylstyrene}
462 465 465 465 465 466 467
15.5 POLYALKENE AND DIENE POLYMERS
467 470
Polyethylene, Polypropylene and Polyisobutylene 15.5.2 Polyisoprene and Polybutadiene
15.5.1
15.6 NITRILE POLYMERS
471
15.6.1 a-Substituted Acrylonitrile Polymers 15.6.2 Polyacrylonitrile
471 472
15.7 CHLORINE AND BROMINE-CONTAINING POLYMERS
474
15.7.1 Poly (vinyl chloride) 15.7.1.1 Introduction 15.7.1.2 Observed characteristics 15.7.1.3 Initiation of dehydrochlorination 15.7.1.4 Mechanism of dehydrochlorinaion 15.7.1.5 Secondary reactions 15.7.2 Poly(vinyl bromide) 15.7.3 Chlorinated Rubber 15.7.4 Poly(vinylidene chloride) 15.7.5 Polych10 roprene
474 474 474 475 475 476 477 477 477 478
15.8 FLUORINE-CONTAINING POLYMERS
478
15.8.1 15.8.2 15.8.3 15.8.4 15.8.5
478 479 479 480 480
Introduction Perfluoroalkane Polymers Chlorofluoroalkane Polymers Hydrofluoroalkane Polymers Fluoromethacrylate Polymers
451
452
Thermal Degradation
15.9 POLY (VINYL ACETATE) AND RELATED POLYMERS
15.9.1 Poly(vinyl acetate) and Ethylene/Vinyl Acetate Copolymers 15.9.2 Poly (vinyl formate) and Poly(vinyl alcohol) 15.10 RANDOM COPOLYMERS
15.10.1 15.10.2 15.10.3 15.10.4
Methyl Methacrylate Copolymers Styrene Copolymers Vinyl Chloride Copolymers Ionomers
15.11 BINARY POLYBLENDS AND RELATED SYSTEMS
15.11.1 Binary Polyblends 15.11.1.1 Interactions due to small radical migration 15.11.1.2 Interactions due to small molecule migration 15.11.1.3 Other interactions 15.11.2 Block and Graft Copolymers 15.12 POLYMERS WITH HETEROATOMS IN THE BACKBONE 15.12.1 Aliphatic Polyethers and Polyesters 15.12.2 Poly (ethylene terephthalate) 15.12.3 Polycarbonates 15.12.4 Polysiloxanes 15.12.5 Other Polymers 15.13 DEGRADATION OF POLYMERS IN THE PRESENCE OF FIRE RETARDANTS
15.13.1 15.13.2 15.13.3 15.13.4
Poly(methyl methacrylate) with Ammonium Polyphosphate Polyurethane with Ammonium Polyphosphate Polypropylene with Chlorinated n-Alkanes Other Fire Retardant Additive Systems
15.14 REFERENCES
15.1 15.1.1
480 480 482 482 482 483 485 485 486 486 486 488 489 490 490 490 491 492 494 494 495 495 495 496 497 497
THE STUDY OF THERMAL DEGRADATION Introduction
Thermal degradation begins to be important when polymers are processed and fabricated for use. The threshold temperature for breakdown determines the upper limit of temperature in fabrication and the volatile products of degradation must be known in order to guarantee the safety of workers. These same properties are relevant to the potential use of a polymeric material and, in the extreme case, to the hazards presented by the polymer in a fire situation. However, there are also some useful aspects of thermal degradation. Examination of the breakdown products from polymers is increasingly used as a method of analysis. In a limited number of cases, controlled thermal degradation is important in recycling of plastics waste and in the special case of carbon fibre production, degradative reactions are an essential part of the manufacturing process. A detailed understanding of how polymers break down on heating is important in the design of materials with improved properties for particular applications. Over the past 40 years, a considerable body of knowledge has built up about the degradation of polymeric materials, extending from pure homopolymers of simple structure to complex materials such as random copolymers, block and graft copolymers, ionomers, polymer blends and polymer-plus-additive systems such as fire retardant composition. Patterns of behaviour have emerged and the approach adopted in this chapter is to try to identify these and relate stability and breakdown mechanism as far as possible to chemical structure. It might appear that it should be possible, to a large extent, to deduce the stability and mode of breakdown of a polymer by reference to bond energies and the pyrolysis behaviour of related structures in small molecules. In practice such reasoning often leads to erroneous conclusions because it fails to take account of the special long chain character of polymer molecules, which provides possibilities for unique types of reaction, proceeding along the chain or between adjacent monomer units in the chain, and because it does not recognize the importance of low concentrations of structural irregularities, which often determine the initiation of the degradation process.
Thermal Degradation
453
15.1.2 Literature Sources Early monographs on the degradation of polymers 1- 3 are still of interest, although now considerably out of date. Useful detailed treatments have been given within the past twenty yearst:" and briefer accounts appear in three modern texts." - 8 For current awareness, in addition to the main polymer journals, the reader is referred to the series Developments in Polymer Degradation, which contains review articles in particular subject areas and to the specialist journal Polymer Degradation and Stability.
15.1.3 Methods of Study When a polymer breaks down, there are four main product fractions. A complete understanding of the degradation process requires information about all four fractions. These are (a) permanent gases, such as hydrogen, methane and carbon monoxide, which cannot be trapped out in a simple trap at -196°C; (b) condensable gases and liquids, volatile at room temperature but not at -196 °C; (c) the tar and wax fraction of products, such as the dimer, trimer and other low volatility materials, which are volatile at degradation temperature but not at room temperature; and (d) the solid involatile residue consisting of partially degraded polymer or product depending on the temperature. The choice of method of study depends very much upon the objectives, which may be to establish one or more of the following: threshold temperature for breakdown, nature of the evolved volatile products, production of a convenient 'fingerprint' for identification or purity control, establishment of kinetics, or ultimate understanding of the mechanism of decomposition. For some purposes, heating in air may be satisfactory, but forfundamental mechanistic studies, it is customary to reduce the complexity of the problem by use of an inert atmosphere or vacuum. Probably the most widely used method is thermogravimetry (TG), in which the polymer sample is heated in a thermobalance and the weight is followed as a function of temperature or time. The information available consists of (a) the number of stages of breakdown, (b) a quantitative measure of the weight loss in any stage, and (c) the threshold temperatures and temperatures of maximum rate of weight loss for the processes occurring. The major weakness of TG as a tool for studying polymer breakdown is that it provides no information about the degradation products, which in most designs of thermobalance are also not accessible for study by other techniques. Less useful for 'degradation studies than TG are differential thermal analysis (DT A) and differential scanning calorimetry (DSC), both of which measure effects due to heat evolution or absorption by the polymer as its temperature is raised. DTA and DSC indicate the temperature regions of occurrence of decomposition processes, but do not distinguish these clearly from physical changes in the sample which also involve absorption or evolution of heat. Product analysis is not possible. Possibly the most sophisticated equipment available at the present time for degradation studies consists of the various instruments for pyrolysis of the polymer, often using flash conditions of a few seconds at a high temperature, followed by analysis of volatile products by GC, MS or, most recently, FTIR spectroscopy. These contrast with TG, DTA and DSC in providing no direct information about threshold temperatures for breakdown or number of stages involved, but give considerable data on the products. The latter may vary, however, depending on the choice of temperature. These pyrolysis methods are particularly useful in applications requiring 'fingerprinting' or the ability to detect small amounts of particular products, e.g. toxic compounds. They require use with caution in mechanistic studies, because not all of the product fractions may be accessible and the temperatures involved may fragment the polymer so extensively that the important initial degradation reactions may be obscured. An example of pyrolysis-MS data is given in Figure 1. In the methods considered so far, the products of degradation are not collected. A third type of experimental approach widely applied to polymer degradation studies, in which the products are normally collected, is evolved gas analysis (EGA). Some studies of this type depend on measurement of a single degradation product. Much of the work on poly(vinyl chloride) (PVC), for example, has been based on estimation of the HCl evolved on heating. A much more generally applicable approach of this type, used in very many investigations of degradation mechanism is thermal volatilization analysis (TVA).9-15 This technique depends on recording (as a function of temperature or time) the small pressure which develops in a continuously evacuated system when the polymer sample degrades to volatile products. The TVA curve obtained resembles a derivative TG
454
Thermal Degradation (0) 10
32 44
5
(b) ~
0
~ c:
'(ji
30
Q)
+.~
42
10
58
5
87
~
'5
233
Q) Q::
10
(e)
32 5
71
42 215 40
60
80
100
120
140
160
180
200
220
.240
260
m/z
Figure 1 Typical pyrolysis mass spectra for (a) poly(ethylene glycol), MW 1000; (b) poly(propylene glycol), MW 4000; (c) poly(tetramethylene glycol), MW 2100. Pyrolysis conditions: 10 s at 610 DC (reproduced by permission of the American Chemical Society from Macromolecules, 1985, 18, 496)
(0 )
! .g
(b)
~ ::J
0
·2
e
a:
(c)
150
400
Figure 2 TVA curves for degradation under vacuum at 10 "Crnin -1 of chain-chlorinated polystyrenes containing (a) 0.54, (b) 0.85 and (c) 1.35 CI atoms per styrene unit. Trap temperatures: - - ODC and -45 DC, ------ -75 DC, - - -100 DC. The shaded areas represent HCI production, the remainder being due mainly to styrene (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1987, 18, 213)
Thermal Degradation
455
curve, because the pressure is a measure of the relative rate of volatilization of the sample. By means of a system with various cold traps at temperatures from 0 °C to -196°C preceding Pirani pressure gauges a considerable amount of information about product volatility and changes in product composition during the heating programme may be collected in a single TVA experiment, in addition to data on number of stages of breakdown and their threshold and maximum rate temperatures. The power of the approach is illustrated in the data of Figure 2 for chain-chlorinated polystyrene samples. The TVA technique distinguishes clearly between the two main volatile products, HCl and styrene, and shows how the mode of breakdown varies with the extent of chlorination. A further advantage of the TVA approach is that it is nondestructive of the various product fractions, which are available for further study, e.g. by spectroscopic methods. Thus the collected volatile products may be separated by controlled warming from -196°C to the ambient temperature of the trap containing the products, using the pressure gauges of the TVA system to monitor the volatilization of each substance (subambient TVA).14 The tar/wax products which condense out on the cooled upper part of the TVA sample tube (and are therefore often referred to as the cold ring fraction) may also be removed for investigation. Finally, the involatile residue on the base of the TVA tube can often be examined by various spectroscopic methods and, if it is soluble, the molecular weight may also be measured. The application of more than one experimental approach to any degradation study is very desirable, not only to confirm results, but sometimes also to reveal interesting features. For example, TG measures the loss of both the volatile and cold ring fraction products, whereas TVA pressure gauges register only the volatile fraction. A degradation process showing extensive weight loss by TG, but little volatile product evolution by TVA, must therefore be one giving mainly cold ring fraction products.
15.1.4 Pitfalls in Experimental Studies Failure to recognize and control the variables in thermal degradation can lead to difficulties in interpretation of the results. Some of the main problems are noted below. The sample environment may influence behaviour in various ways. Degradation in air will usually, be more complex than in nitrogen due to additional oxidation reactions. An inert atmosphere is preferred to simplify interpretation of the basic processes induced by heat, but behaviour may differ greatly between a- static and a dynamic atmosphere, since if products are not quickly removed from the degrading sample, secondary reactions may occur. The most efficient removal is under continuous evacuation. Sample size, or in practice thickness, is sometimes very important and this possible variable is often overlooked. Its importance is illustrated in Figure 3 for two familiar polymers. Thick samples of PVC degrade more rapidly than thin samples even under vacuum because the H'Cl released (which is less rapidly removed) is a very effectivecatalyst for the dehydrochlorination reaction. In the (b)
(0 ) 1.0
~
e
~
e :J
0
·c
e
a::
.gc 0.8 0
~
.... s:
.~0.6 ~
"0 :J "0
.~ 0.4
0:::
0.2
Figure 3 Effect of sample thickness on degradation behaviour: (a) TVA curves (vacuum, 10°Cmin- 1 ) for 20mg PVC, - - as powder or thick film, ----- as thin film of 2.5 mg cm (b) TG curves (dynamic N 2 , 10°Cmin- 1 ) for PAN; (1) 1 mg, (2) 2 mg, -(3) 5 mg, and (4) 10 mg (reproduced from Eur. Polym. J., 1970, 6, 143, 1277) t
':
Thermal Degradation
456
case of polyacrylonitrile (PAN) the cyclization reaction of the nitrile groups which occurs between 250 and 300°C (10 °C min - 1 heating rate) is extremely exothermic and causes some fragmentation of the chain due to rapid heat build up in the sample, since the heat cannot be dissipated sufficiently rapidly. The extent of overheating and fragmentation increases with sample thickness, as the results for weight loss indicate. Many polymers break down in successive stages as the temperature is raised. For example, a side group elimination reaction will normally be followed at higher temperatures by fragmentation of the residual, modified polymer chain. The observed behaviour may then depend on whether the temperature is raised gradually or rapidly. Degradation reactions are often initiated or strongly influenced by impurities in the sample, especially if these are incorporated into the polymer chain. The importance of pure, well characterized samples in any mechanistic study cannot be overemphasized. No matter how sophisticated the degradation equipment used, the results are only as good as the purity of the sample or design of the experiment will allow.
15.1.5 Kinetics of Degradation Studies of reactions of small molecules have been greatly facilitated by careful kinetic measurements and the calculation of rate constants and activation energies. It is therefore not surprising that much effort has been devoted to trying to apply the same approach to polymer degradation. Many activation energies have been reported, but the wide divergence in results by different workers suggests that the approach is of limited value. There are several reasons for this. Firstly, the physical state of the sample is far from ideal (usually a viscous liquid and in some situations a solid) and may change as the reaction proceeds. Efficient removal of products is also difficult under some experimental conditions. Secondly, degradation reactions are often complex, involving several stages of reaction which may not be well separated in temperature range. Assumptions about reaction order and constancy of activation energy over a particular temperature range may therefore be questionable. Finally, many kinetic treatments of thermal degradation assume that the residual weight of the sample may be handled as if it were a concentration, which is of doubtful validity. The optimum experimental situation for kinetic measurements of polymer decomposition' involves a very thinly distributed sample, rapid temperature equilibration, mild isothermal conditions spanning a temperature range over which the product composition can be shown to be constant, efficient product removal and estimation of an evolved degradation product as a means of establishing the initial rate at each temperature. Even if these criteria are satisfied, there remains the possibility of misleading results if the same degradation product can be formed by more than one mechanism.
15.2 PRIMARY DECOMPOSITON PROCESS IN ADDITION POLYMERS Before consideration of the degradation behaviour of the various addition polymers in groups based upon their chemical structure, it is useful to indicate the three general types of primary thermal decomposition process which are observed, i.e. depolymerization, elimination and cyclization.
15.2.1
Depolymerization
The term depolymerization is used in this context to include all situations in which reduction of macromolecular size occurs without change of chemical composition or alteration of the monomer unit structure. In the absence of the reactions considered in Sections 15.2.2 and 15.2.3,homolysis of the polymer chain may occur as in Scheme 1. Subsequent behaviour then depends on the nature of the chain substituents X and Y. There are three situations: (a) X = Y = H, no monomer produced, extensive transfer by H abstraction [e.g. polyethylene (PE)]; (b) X = H, amount of monomer depends on nature of Y, extensive transfer by H abstraction [e.g. polypropylene (PP), polystyrene (PS), poly(methyl acrylate) (PMA)]; (c) X, Y # H, large amount of monomer (up to 100%) transfer present or absent depending on nature of X, Y, e.g. poly(methyl methacrylate) (PMMA), poly(ex-methyl styrene) (PAMS). Monomer results by depropagation from the polymer radical chain end produced on homolysis (Scheme 2).
457
Thermal Degradation
x
X
I I IWVCH 2CCH 2CM/\ ---I~. yI yI
X
MILCH). I
y
Scheme 1 Chain homolysis by random scission
Scheme 2
Monomer formation by depropagation
Transfer may be intramolecular ('backbiting') giving small fragments such as dimer, trimer, etc., or intermolecular, producing a macro radical in which the radical centre is not at the chain end; this undergoes scission, so that intermolecular transfer leads to a rapid drop in the molecular weight (MW) of the polymer. All the products of this type of degradation, whether monomer, dimer, etc., or lower MW polymer chains, have essentially the same chemical composition as the repeat structure. Homolysis of the C-C backbone, in the absence of structural abnormalities which constitute weak links, usually requires a temperature of above 250°C. The following processes, however, are commonly observed at somewhat (and in some cases considerably) lower temperatures.
15.2.2 Elimination If one of the bonds within a chain substituent group, or that attaching it to the backbone, is weaker than the C-C backbone, then an elimination reaction may occur at lower temperatures than depolymerization, or simultaneously, giving a product and a residual chain structure which differ in composition and structure from the repeat unit. For example, PVC loses HCI and poly(isopropyl acrylate) loses propylene, as shown in Scheme 3. If such a process occurs, then depolymerization is either prevented or impeded. Further decomposition of the residual, modified polymer takes place at higher temperatures, generally 400-500°C. vwCH 2 CHUlIfJo
b
----1~~
+ HCI
uwCH==CHMA
vwCH 2CHvw -----t~.
Scheme 3
I
..o-C
O~'OH
+
CH 2==CHMe
Elimination reactions
15.2.3 Cyclization A macromolecular chain presents a unique reaction situation in which, in certain cases, groups which are potential reactants at elevated temperature are located in close proximity. Thus intramolecular cyclization may occur, which mayor may not involve elimination of small molecules. For example, vinyl ketone polymers such as poly(methyl vinyl ketone) (PMVK), poly(methyl isopropenyl ketone) (PMIK) and poly(phenyl vinyl ketone) undergo random cyclization of adjacent monomer units with release of water. 16, 17 The reaction for PMVK is illustrated in Scheme 4.
XXX
H2
~
VWt.C
0
#Me(f
Scheme 4
---I~.
Me
Cyclization in PMVK
vw\'CH1XXX~
o
~
~ Me
+
Thermal Degradation
458
Such cyclization reactions give products of composition different from that of the polymer if a small molecule is released (as with PMVK), but in other cases (e.g. some nitrile and diene polymers), the change involves only structural rearrangement. Decomposition of the cyclized structures occurs subsequently, if the temperature is raised.
15.3 ACRYLATE AND METHACRYLATE POLYMERS 15.3.1 Poly(methyl methacrylate) The thermal degradation of PMMA is essentially a very simple reverse of its polymerization, leading to monomer in 100% yield. After homolysis of the chain, the macroradicals depropagate (Scheme 2) with a very long zip length, so that chains of sufficiently low degree of polymerization unzip completely." The degradation behaviour of PMMA, however, is influenced to a far greater extent than for most polymers by the mode of polymerization and the presence, even in very small amounts (e.g. 1%), of comonomer units in the chain. In PMMA produced by free radical polymerization, unsaturated chain ends are found to constitute weak sites at which degradation may be initiated (Scheme 5). Programmed heating experiments by TVA illustrate very clearly the contribution of end initiation in PMMA degradation (Figure 4a). In these curves, each peak is due to monomer production but the lower temperature peak is due to initiation at the unsaturated chain ends, whereas the second (and in most samples the main) peak is due to initiation by random scission of the backbone. The relative sizes of the two peaks are dependent on molecular weight. For a very low MW polymer, in which the zip length exceeds the chain length, the chains with unsaturated ends disappear completely in the first stage and since these chains comprise about 50% of the sample, the two peaks are of similar size. In samples of higher MW, however, initiation at unsaturated ends does not lead to complete unzipping of these chains, so that as the proportion of ends per unit weight of sample decreases (MW becoming larger), so does the size of the first TVA peak. In PMMA samples made by the anionic route, in which all the chain ends are saturated, degradation is initiated only by random scission and so the early TVA peak, with maximum at about 290-300°C (Figure 4a) is not observed (Figure 4b). The sensitivity of PMMA stability to various foreign units in the chain is due to several effects. Firstly, there may be blocking of the long unzipping reaction at the foreign unit, which reduces the Me
I
Me
I
Me
CH 2
II
AM.CH.2TCH2--C-CH21
-
Me
CH 2
-cH2~
+
oAMCH2{CH2-t. C0 2Me t0 2Me
C0 2Me t0 2Me C0 2Me
C0 2Me
L
depropagation
Scheme 5 End-initiated depolymerization of PMMA to monomer
(Q)
(b)
.r,'\."' \\ O V
I II o
. I I
.,I
))
,..,/'I.
-c-,
/
\
\
0\ \ 0
'
.'
\
,
\\ \,.\ \\
Figure 4 TVA curves for degradation under vacuum at 10°Cmin- 1 of (a) free radical PMMA samples, Mn : - - 20000, ------ 100000, .. , ... 480000, (b) anionic PMMA samples, Mn : - - - 60000, - ' - ' - 1500000 (reproduced from Eur. Polym. J., 1968,4,21)
Thermal Degradation
459
zip length of the monomer-producing reaction and so stabilizes the polymer. Some commercial PMMA samples are stabilized in this way by copolymerizing small amounts of ethyl acrylate or methacrylic acid. Secondly, there may be interunit reaction between the comonomer unit and the adjacent methyl methacrylate (MMA) chain unit. As well as leading to additional volatile products, this type of reaction may produce structures which block unzipping. Thirdly, certain foreign units in the PMMA chain (e.g. acrylonitrile) lead to backbone scission. Some small molecules exert a profound influence on the mode of decomposition of PMMA. Silver acetate has been found to greatly accelerate depolymerization; zinc bromide changes the entire character of the degradation reaction, with the formation of various volatile products, notably methyl bromide.l'': 19 The fire retardant additive ammoniumpolyphosphate acts by reaction with the polymer so as to interfere with the monomer-producing reaction.'?
15.3.2 Higher Polymethacrylates Although several other methacrylate ester polymers resemble PMMA in degrading thermally to give 100% monomer, this behaviour is by no means general for this class of polymers and some polymethacrylates, in contrast, give very little 'monomer. The four isomeric poly(butyl methacrylates) illustrate very well the complete spectrum of behaviour, comprising 100% depolymerization to monomer in the case of the isobutyl ester, very high monomer yield plus a little alkene for the n-butyl ester, high alkene yield plus some monomer for the s-butyl ester and very high alkene yield with very little monomer for the t-butyl ester.:" The explanation for these differences is that in some of the polymers, favourable conditions exist for a non-radical side group ester decomposition reaction to occur, in competition with free radical depolymerization to monomer, leading to alkene and carboxylic acid. The former process is favoured in cases where a six-membered ring transition state involving f3 hydrogen atoms in the ester alkyl group can be formed. This is illustrated in Scheme 6 for poly(s-butyl methacrylate). Elimination of the alkene leaves a methacrylic acid unit in the polymer chain. Me
Me
I V\I\ICH 2 Cvw I
VW
'<,
CH 2
Scheme 6
ItIW
2T
+
CH 2=CHEt
C
~~
t> .v--, d
CH
o/~
I
H
Et
0
H
Ester decomposition in poly(sec-butyl methacrylate)
The ease of occurrence of the ester decomposition depends on the number of f3 hydrogens available in the alkyl group, which in the above examples is 1 (iso), 2 (n), 5 (s) and 9 (t) respectively, so explaining the observed trend of behaviour. The behaviour of many other methacrylate ester polymers can be rationalized in the same way. . Some methacrylate polymers in which the ester group is substituted by halogens have recently been studied. Poly(2-bromoethyl methacrylate) behaves predominantly as would be expected from the above discussion, giving a high yield of 2-bromoethyl methacrylate (2-BEMA) monomer, although some ester decomposition also occurs.P Poly(2,3-dibromopropyl methacrylate) gives a much more complex pattern of degradation, in which reactions involving C-Br scission compete with the main reaction of depolymerization to monomer.P The behaviour of several fluorinecontaining polymethacrylates is considered in Section 15.8.5.
15.3.3 Polyacrylates Despite the close similarity in structure, the polyacrylates differ greatly in degradation behaviour from the polymethacrylates. Crosslinking, yellowing of the polymer, formation of a high proportion of short chain fragments and evolution of the alcohol and carbon dioxide are characteristics of the breakdown of many polyacrylate esters which are not found in the corresponding methacrylate polymers. Furthermore, although some of the latter give large yields of monomer, the monomer yields from the polyacrylates are extremely small, typically about 0.3% of the polymer weight.
Thermal Degradation
460
The polyacrylates have one feature in common with the polymethacrylates, however, in giving alkene formation from the ester side group if Phydrogens are available to form the transition state analogous to that illustrated in Scheme 6. This reaction is more important in polyacrylate degradation because of the absence of unzipping to monomer. PMA has been the subject of several studies. 2 4 - 2 6 During degradation, PMA undergoes both crosslinking and chain scission. Little monomer is produced, the main products consisting of chain fragments. There is some disagreement regarding the yields of the volatile products, methanol and CO 2 , which make up a small fraction of the degradation products. Poly(benzyl acrylate), which also has no alkyl Phydrogens, shows greater similarities in its degradation behaviour to PMA than to the higher alkyl polyacrylates.F' It degrades to give mainly chain fragments, but there is in this case a substantial yield of benzyl alcohol, as well as a small amount of CO 2 , Six polyacrylates with larger alkyl ester groups have been studied in some depth." Four behave in a broadly similar way: these are the ethyl, n-propyl, n-butyl and 2-ethylhexyl ester polymers. Degradation occurs in a single stage and gives increasing yields of chain fragments (15 to 580/0 by weight) with a corresponding decrease in volatile products, which consist (in-order of importance) of alcohol, CO 2 and alkene. Poly(isopropyl acrylate) (PIPA) and poly(t-butyl acrylate) (PTBA) show quite different behaviour. They are less stable, degrade in two stages and give nearly quantitative yields of alkene per acrylate unit, no alcohol, smaller amounts of CO 2 and some water. A mechanism can only be presented with confidence in the case of PIPA and PTBA, for which the nonradical ester decomposition route to alkene and carboxylic acid provides a satisfactory explanation (Scheme 7). There are six and nine Phydrogens respectively in these structures. The initially produced carboxylic acid groups dehydrate to give the six-membered cyclic anhydride structure. Carbon dioxide probably arises either by decarboxylation of some of the C0 2H groups or by decomposition involving the cyclic anhydride. V\J\,CH 2 C H - C H 2 - C H v w
I
I
~c 07" "OH
hC HO'7 ' 0
---.
+ CH 2==C-Me
I
H (Me)
Scheme 7
Ester decomposition in PIPA and PTBA
It is generally accepted that the other polyacrylates have mechanisms of breakdown in which random scission is important (Scheme 8). In contrast to the situation in thecases of PMMA, PS and PAMS, where one of the two macroradicals depolymerizes to monomer, the radicals produced by homolysis in PMA do not behave in this way. It may be assumed that the more reactive primary radical (B) stabilizes itself by intermolecular and intramolecular transfer of a tertiary H atom and radical (A)may also be involved in this way. The product in either case is a tertiary radical (C) which provides a site for various possible reactions, including scission and crosslinking. Scission leads to an unsaturated chain end and radical (A). -----t~...
\MI\CHzCH.
I
C0 2R (A)
+
·CH 2CHCH2CH\IWl
I
I
C0 2R C0 2R
1
(B)
intermolecular transfer of H from PMA molecule
vwCH 2 C\lW'
t0
2R
+ MeCHCH2CH~
t0 t0 2R
2R
(C)
Scheme 8
Random scission and intermolecular transfer in polyacrylate degradation
Thermal Degradation
461
It is particularly difficult to account convincingly for the production of large amounts of the alcohol from these polymers. The high yields from some polyacrylates suggest the possibility of a reaction which can proceed progressively along the chain following some initiation step. The radical (A) seems a likely precursor and several schemes have been proposed on this basis. Whatever the mechanism, there is evidence that several adjacent acrylate units are required for alcohol production. For example, although in poly(ethyl acrylate) (PEA) half the acrylate units degrade to ethanol, in styrene/ethyl acrylate copolymers/? with isolated ethyl acrylate (EA) units, no alcohol is formed. A possible mechanism for alcohol production in polyacrylates is shown in Scheme 9.
~(b) CYclizatio; C -c-, c-, RO/ '0\· r'~o 1"'0 '6R
OR
repeat of (b), (e)
---~~~
Scheme 9
RO·
further ROH
Alcohol production from polyacrylates
The formation of alkene and CO 2 from PEA, poly(propyl acrylate) (PPA), poly(butyl acrylate) (PBA) and poly(2-ethylhexyl acrylate) (P2EHA) can be explained by a side group ester decomposition of radical (C) as in Scheme 10. This mechanism also leads to unsaturation and provides further possibility for crosslinking.
!
(1) H elimination to give unsaturation
or (2) crosslinking or (3) chain scission Scheme 10
Formation of alkene and CO 2 from polyacrylates
15.3.4 Poly(methacrylic acid) and Polymethacrylamide The decomposition of poly(methacrylic acid) is a two-stage process, involving initial dehydration at around 200°C to give the cyclic anhydride (Scheme 11a). At higher temperatures, the polyanhydride breaks down to give various fragments, including as volatile products some CO 2 , CO and traces of alkenes, and leaving a small amount of carbonaceous residue. 30 - 33 The cyclic anhydride structure shown in Scheme 11a plays an important part in a number of degradation situations involving methacrylate polymers, including copolymers, polymer blends and polymer plus additive compositions (e.g. certain fire retardant mixtures). It has characteristic IR absorptions at 1805, 1760 and 1020 em -1 and begins to decompose at about 400°C under programmed heating at 10°C per minute, i.e. somewhat higher than the corresponding ester structure. Since the ring structure blocks the unzipping reaction in the PMMA chain, formation of even a few anhydride rings in the backbone has a marked effect. When subjected to a gradual increase of temperature, polymethacrylamide also breaks down in two stages, the first of which commences at about 200°C and gives mainly ammonia, with some water. The second stage decomposition, above 300°C, leads to extensive fragmentation to give a yellow cold ring fraction; about 200/0 of the original sample remains as a carbonaceous char at 500 °C, considerably more than in the case of the acid polymer. The degradation involves the formation of both imide and anhydride rings, the former being the predominant process (Scheme 11b).34
Thermal Degradation
462 Me
Me
~H ~-CH-~WV I 2 I
(a) ~
2
vwCH
O?"
Me
~-CH -~W\I
2~
2
I
I
+ H 2O
~C~ /C~
~C ~C O'? 'oH 0-7 'oH
Me
Me Me \l\l\J\CH 2'-... I/CH 2, I~ C C
(b) ~
I
~O
0
Me
Me
I C
I C
VWCH2'-...~/CH2'~~ +
NH 3
c? "-NH/ ~
~ '-... ~C, O'? NH O'/" NH 2 2
Scheme 11 Cyclization in poly(methacrylic acid) and polymethacrylamide
Poly(N-methyl methacrylamide) undergoes an analogous cyclization with eliminaton of methylamine.
15.3.5 Salts of Poly(methacrylic acid) Until recently, little was known about the decomposition of this group of ionic polymers. Detailed studies of the ammonium.I" alkali metal." alkaline earth metal " and zinc salts"? have now been carried out, using TVA and other techniques. These form the subject of a recent review.P" Ammonium polymethacrylate shows unique degradation behaviour in this group of materials. It undergoes a two-stage decomposition, in the first stage of which NH 3 and H 20 are eliminated in a cyclization. At higher temperatures cold ring fraction products, isocyanic acid, HCN, CO 2 , CO and CH 4 are formed. The alkali metal polymethacrylates degrade under programmed heating in a single process commencing at about 300 "C. They are therefore more stable than PMMA. The alkaline earth salts show still higher stability and a more complex breakdown. The principal degradation products of the alkali metal polymers appear as a cold ring fraction, but the alkaline earth metal salts give an increased proportion of other more volatile products. In all cases, CO 2 and a complex mixture of carbonyl-containing compounds appear as the volatile fraction. An unusual feature in the case of the alkaline earth polymethacrylates is the presence in the products of up to 60/0 by weight of dimethylketene. The complex degradation behaviour of these various salts can be related to a number of basic processes (Scheme .12). Differences are observed because of variations in the importance of competing reactions. Two key factors are the stability of the metal carbonate and the volatility of the monomer and isobutyrate (which determines whether the latter materials are able to escape from the hot zone without decomposition).
polymer
»>: ~
monomer
+ isobutyrate
carbonate
+ cyclic and acyclic ketones, etc.
oxide
+ CO 2
ca. 450 °C
C (ZnPMA only)
~
~
metal
dimethylketene
+ CO 2
Scheme 12 Degradation reactions in salts of poly(methacrylic acid)
15.4 POLYSTYRENE AND RELATED POLYMERS 15.4.1 Polystyrene Despite the considerable volume of research on polystyrene degradation over 40 years, there remain aspects of the degradation mechanism which are controversial. A number of recent contributions have provided additional information, without fully resolving the conflicting views.38 - 45
Thermal Degradation
463
The degradation behaviour of PS depends critically upon whether the temperature is below or above 300°C. Between 200 and 300 °C the molecular weight falls but no volatile products are evolved. The pattern of the fall in molecular weight as a function of time has led to the conclusion that it is due in anionically prepared PS samples to random scissions of the backbone. The reasoning is as follows. If the average number of scissions per molecule in time tis Sand Po and P, are the chain lengths of the polymer initially and at time t, then so that
r,
Po/(S
S
(PolPt )
+
(1)
1)
(2)
The fraction of bonds broken rx is given by a
=
=
SIP o
(lIP t )
(3)
(lIP o )
-
For random scission, every bond is equally likely to break and ex
=
(4)
kt
Thus a plot of rx VS. t should be linear through the origin. This has been found to be the case for PS made by the anionic route (Figure 5). Samples prepared by free radical polymerization, however, give linear plots which do not pass through the origin. This indicates a relationship (5)
a=p+kt
in which
p represents
a small fraction of weak links initially present in the sample.
10
10
5
5
v
Q )(
o
2
4
6
8
10
0
2
6
4
8
10
Time (h)
Figure 5 Graphs of fraction of bonds broken (a) as a function of time of heating at various indicated temperatures for (a) anionic PS, Mn = 229000, (b) free radical PS, Mn = 1490000 (reproduced from Eur. Polym. J., 1968, 4, 709)
Various suggestions have been made to explain weak link behaviour in PS. Opinion at present tends to favour peroxide linkages as the probable explanation. Random scissions beiow 300°C probably involve initial homolysis (Scheme 13). Since no volatile products result, the radicals A and B produced are believed to undergo a cage disproportionation; ~CH2CHCH2CHCH2CHV\M
I
Ph
I
Ph
I
Ph
(a)
----+
~H2CH·
I
Ph (A)
(b)
+ ·CH 2CHCH2CHv"M ---. MACH 2CH2 + CH 2=CCH2CH
I
I
Ph
Ph (B)
Scheme 13 Random scission and disproportionation (PS) PS 6-P
I
Ph (C)
I
Ph (D)
I
Ph
Thermal Degradation
464
depolymerization does not occur in this temperature region. It follows also that end structures (C) and (D) must be stable up to 300°C. Above 300 "C, PS degrades to give a mixture of products, the major component of which is monomer (yields reported vary between 40 and 60% by weight), with progressively smaller amounts of dimer, trimer, etc., plus small amounts of toluene (2%) and z-methylstyrene (0.50/0). Recent examinations of the cold ring fraction from PS degradation (dimer and larger fragments) indicate clearly the presence of benzylic ends of structure (C) and unsaturated ends of structure (D).42 Deuteration of the tertiary H position in PS gives a polymer which yields a much higher proportion of monomer than PS, whilst in polytc-methylstyrene) the presence of the z-methyl substituent results in 100% monomer production. These facts highlight the importance of transfer at the tertiary H atoms in the formation of less volatile products. Initiation of the production of volatiles from PS above 300 "C can result from homolysis at random or near (C) and (D) type chain ends. The first type has already been shown as reaction (a) in Scheme 13; the other two situations are shown in Scheme 14. IVVl.CH1CHCH 1CH1
I Ph
(a)
I Ph
~
NWCH 2CHCH 2
I Ph
MIl.CH1CHCH1CH==CH 1
I
Ph
.CH 2
I
Ph
(B)
(C)
I
+
(b)
-~~~
(E)
MA.CH1CH
I
Ph
+
·CH 2C=CH 2
I
Ph
(D)
Ph
(A)
(F)
Scheme 14 Scission at chain ends (PS)
Only the former of the macroradicals (A) and (B) is believed to lead directly to production of volatile products (Scheme 15). Monomer production involves depropagation of radical (A) by reaction (a) in Scheme 15, whilst backbiting intramolecular transfer (reactions b, c) account for the dimer, etc. Fragments such as the trimer, etc. can be formed in the same way by H abstraction at subsequent tertiary H sites along the chain. vwCH1CHCH1CHCH1CH
I
I
Ph
I
Ph
(a)
---I~.
vwCH 2CHCH 2 CH
I
Ph
Ph
\MICH2CH+CH 1CCH 2CH 1 I I
I Ph
I Ph
(c)
~ vwoCH1CH +
I
Ph (A)
+
CH 2=CH
I
Ph (A)
(A)
I Ph
I
Ph
monomer
CH 1=CCH1CH 2
I
I
Ph dimer
Ph
Scheme 15 Depropagation and intramolecular transfer (PS)
Radical (A) is thus the common precursor in the formation of all the major volatile products and the constant proportion of monomer to less volatile products which is found is determined by the competition between reactions (a) and (b) in Scheme 15. The zip length of these reactions of radical (A) has recently been estimated't' as about 50 units, which is much less than for the depropagation in PMMA or PAMS. The continued fall in molecular weight as PS degrades may result from further random scission or from intermolecular transfer. The latter process (Scheme 16) is considered to be the more important. Each of the radicals (A), (B), (E) and (F) could participate as R· in H abstraction (reaction a in Scheme 16). The tertiary macroradical (G) which is produced then undergoes scission (reaction b in Scheme 16) at one of the points indicated, to give the depropagating radical (A) and an unsaturated chain end (D). Reaction (a) in Scheme 16 is probably the only major reaction of radical (B), giving a methyl end, whereas if radical (A) participates in this way a benzylic end (C) is formed. The minor products, toluene and z-methylstyrene, result when radicals (E) and (F), produced in reactions (a) and (b) of Scheme 14, abstract a hydrogen atom in reaction (a) of Scheme 16.
Thermal Degradation
465 I
•
I
I
I
I
~H2CH+CH2CCH2+CHCH2CHrvvV
I
Ph
Ph
I
Ph
I
Ph
(G)
Scheme 16 Intermolecular transfer and chain scission (PS)
Many workers have argued that the unsaturated chain ends (D) provide the points at which depolymerization is initiated via reaction (b) of Scheme 14.This view has recently been challenged by others.t' who believe that the formation ofradical (A) by random scission and especially following intermolecular transfer provides the principal route to the formation of volatile products. The relative yields of toluene and «-methylstyrene, and in particular the very small amounts of the latter found in the products, tend to support this view and also to suggest that benzylic ends (C) might in fact be less stable than unsaturated ends (D). In 'PS with only benzylic ends, an increased proportion of toluene in the early stages of degradation has been attributed to additional initiation at these ends. 4 4 , 4 5
15.4.2 Chlorinated Polystyrenes The effect on PS stability and degradation mechanism of different extents and positions of chlorination has been the subject of recent studies.?" Ring-chlorinated (mainly para) PS behaves in most respects like PS, giving a large yield of monomer, but is somewhat less stable; this is due to a small amount of chain chlorination which unavoidably accompanies ring-chlorination. It is possible to chlorinate PS only in the backbone and such polymers lose HCI in the same temperature range as does PVC. Lightly chain-chlorinated PS also degrades to styrene, with lower stability than PS because of the backbone unsaturation resulting from H'Cl loss. More highly chain-chlorinated PS samples give HCI as almost the sole volatile product (see Figure 2), leaving a black poly(phenylacetylene) type material at 300°C.
15.4.3 Polystyrenes with 0 ... and N-containing Ring Stibstituents A series of substituted polystyrenes of this type has been studied.4 7 , 4 8 The p-methoxy-, p-hydroxyand p-amino-styrene polymers all show similarities in degradation behaviour to PS, but the HOand NH 2-containing materials also undergo crosslinking due to transfer at the substituent H atoms. In the Me 2N- and Et 2N-substituted polystyrenes, there are no H atoms available on the N atom for transfer, but the behaviour is complex because of alkyl group migrations and so less monomer is obtained. Very complex behaviour, including crosslinking, is observed in polystyrenes with acetoxy and acetamido substituents.
15.4.4 Poly(m-methylstyrene) The ring methyl substituent has little effect on the stability. This polymer gives comparable amounts of monomer and larger volatile fragments to PS. Xylene is a significant product of degradation. between 300 and 400°C. 4 9
15.4.5 Crosslinked Styrene Polymers The behaviour of copolymers of styrene with divinylbenzene (DVB) and trivinylbenzene, resulting in polystyrene chains linked at various points through the aromatic ring, has been examined. 50,51 At very low DVB contents the styrene yield i~ greater than that from PS and polymers with 25% DVB still give relatively high styrene yields, probably because the rigidity of the polymer structure inhibits transfer reactions. .
466
Thermal Degradation
15.4.6 Poly(a-methylstyrene) Replacement of the tertiary H atoms in PS leads to very different degradation behaviour. PAMS is much less thermally stable than PS, with volatile product evolution commencing at about 250°C and proceeding rapidly at 300°C. Monomer is produced in nearly 100% yield. The molecular weight falls during PAMS degradation, but to a much smaller extent than for PS. The initial rate of degradation has been found in solution studies to be proportional to the polymer MW up to about 6.x 105.52-59 The simplest picture of the degradation involves initiation by random scission, Scheme 17, reaction (a),followed by depropagation, reaction (b), with very long zip length and no transfer. Some treatments have assumed that both macroradicals (A) and (B) depolymerize to monomer. The radical (A)would certainly be expected to behave in this way, because it is the propagating radical in the polymerization, which has a very low ceiling temperature. Radical (B), however, like the corresponding radical in PS degradation, is much more likely to stabilize itself by H abstraction. Me
Me
Me
III ~ IWVCH2CCH 2CCH 2CJV\A IPh Ph I Ph I
Me
Me
Me
I I NVlICH2CCH 2C· I I Ph
I I
+ ·CH 2CNW
Ph
Ph
(A)
radical (A)
(B)
~ depolymerization to monomer
(b)
Me
radical (C)
(d)
-~~~
I~ MNCH 2CCH 2 + I
Me
I I
CH 2==CCH 2CJVlI',
I
Ph
Ph
(B)
Ph
(D)
Me unsaturated end (D)
(e) ----1~~
•
CH 2==C-CH 2
I
+
Ph (A)
Ph
H abstraction
Scheme 17
I
TJ\I\A
(C)
+
monomer
Degradation of polytc-methylstyrene]
The characteristics ofPAMS degradation have recently been reexamined'" in a study in which the experimentally observed variation of number and weight average degree of polymerization with conversion was compared with behaviour predicted in computer simulations. The latter were based on the criteria that when the radical undergoing depolymerization is shorter than the zip length, it volatilizes without leaving any fragment, and that either (a) both initially formed radicals depolymerize to the same extent, or (b) only one of the radicals depolymerizes, whereas the other is transformed into a stable molecule. Various zip lengths were also tested. The results, shown in Figure 6, illustrate very clearly that the mechanism involving depolymerization of both radicals ,gives a poor fit with the experimental data, whereas for a single radical depolymerization, a good fit over a wide range of polymer 'molecular weights can be achieved for a zip length of 2000. Various possibilities are considered" for the behaviour of radicals (A) and (B). Perhaps the most plausible course of events is as shown in Scheme 17, reactions (b)----(e), which accounts for both the formation of monomer as the only volatile product and for the observed fall in molecular weight of the polymer.
Thermal Degradation 9000
(0 )
9000
467
(b)
6000
3000
0.4
0.6
0.8
'A(c
1200 800 400
(0 )
(b)
1200
~--------------"'a....Q..o -a ......... "-
800 400
"-
Conversion
Figure 6 Comparison of experimental data for number average degree of polymerization vs. conversion for high and low MW PAMS samples with predicted curves based on (a) depolymerization of both radicals formed on backbone scission, (b) depolymerization of only one of the radicals. Zip lengths: 5000,- - 2000, ----- 1000,- - - 425, _._.324 monomer units respectively (reproduced by permission of Elsevier Applied Science Publishers Ltd. from Polym. Degradation Stab., 1985, 11, 167)
15.5 POLYALKENE AND DIENE POLYMERS 15.5.1 Polyethylene, Polypropylene and Polyisobutylene These polymers all degradeby random scissions of the chains, the products being dependent on the reactivity and possible reactions of the various radicals produced. The predominant productsappear as a cold ring fraction (CRF). When each of these polymers is degraded to beyond 800/0 weight loss, the percentage by weight of CRF for PE, PP and polyisobutylene (PIB) amounts to > 90%, 790/0 and 700/0 respectively. The main route to these products is intermolecular transfer. The more volatile products, on the other hand, arise from depolymerization or intramolecular transfer. These latter reactions are of minor importance in PE degradation, but lead to a series of alkenes and alkanes from C 2 size upwards.P'' The broad distribution of such products is clearly seen in the subambient TVA 14 curve for the condensable volatile product fraction from degradation of PE to 500 °C (Figure 7a). The main features of the degradation of PE are presented in Scheme 18. Although this picture represents the behaviour of pure PE, it has been found that the degradation is also sensitive to the presence of structural abnormalities such as branches''! and oxidized structures.V Even at temperatures much lower than the onset of volatilization (the latter occurs at about 370°C), PE shows a decrease in molecular weight due to scission at these structures, which provide 'weak . links'. PP is attacked by oxygen when heated in air. The oxidation mechanism and the mode of action of antioxidants are considered in Volume 6, Chapter 19. The threshold temperature for volatilization in PP degradation in vacuum is 100 ° lower than that for PE. The proportion of volatile products, although low, is greater than for PE and this fraction is complex in composition. Four major studies of PP degradation have come to very different conclusions regarding the nature and proportions of the volatile products. The explanation may be found in the difference in degradation conditions, Three of the investigations''?: 63, 64 were carried out in closed systems, initially evacuated to 10- 2-10- 4 mm Hg, whereas the most recent 65 used a dynamic system with helium carrier gas. Sample sizes also varied from several milligrams''?: 65 to one gram.P" The importance of low sample thickness and efficient product removal from the hot zone has been stressed above (Section 15.1.4). Unfortunately, in the PP degradation studies the most comprehensive and painstaking product investigation't" was based on the least satisfactory degradation conditions, under which secondary reactions in the molten polymer and gas phase appear very probable; this accounts for differences from the results of other workers. In a recent study of the degradation of atactic PP at 388°C for one hour,65 the major volatile products in order of importance were found to be 2,4-dimethyl-1-heptene, 2-pentene, propene, 2-methyl-1-pentene and
468
Thermal Degradation VVVlCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH:i1'\l\lV
depolymerization
5 4 3 2 1 2 /\/VVCH2CH2CH2CH2CH2CH2 ,,",,"
~OlecUIartransfer
//
¥~/; /VV\CH 2CH2CH2CH2 + CH 2==CH2
at C3)
v\AACH 2CH2CH2CHCH 2Me
!
inter-molecular transfer
VVV\CH2CH2CH2CH2CH2Me
(e.g.
(Similarly at C 2 -. propene at C4 -. pentene at C, -. hexene etc.)
+ VVV\CH2CH2CHCH2CH2CH2/VVV
!SCission
Scheme 18 Degradation of PE
(3)
(b)
(e) (A)
o -196OC
OOC
Figure 7 Subambient TVA curves for warm up from -196°C to 0 °C of condensable volatile products of degradation under vacuum to 500 °C at 10°C min -1 in a TVA system of (a) polyethylene, (b) polypropylene. Assignments: PE (1) ethylene, (2) propylene, (3) butenes, (4H7) mainly alkenes of increasing chain length; PP (A) propylene, (B) 2-methyl-1-pentene, (C) 2,4dimethyl-1-heptene, (D) larger fragments
Me
Me
/
I
Me
I
Me
Me
/
Me
Me
I
-/
Me
Me
/1
~H2CHCH2CHCH2CHCH2CHCH2CHCH2CHCH2CH'/V\A
Me
I
Me
Me
I
I
Me
!
Me
I
I
I
I,
f\/\I\,CH2CHCH2CHCH2CHCH2<;H + -cH 2CHCH2CHCH2CH'VVV (5)
(3)
(A)
(1) (B)
intramolecular transfer at C-5
~ ~ ~
Me
I
Me
I
I
Me I
I
Me
Me
Me
I
I
Me
I,
Me
I
Me
I
Me
I
I
MeCHCH 2CHrvv\ + ~H2CHCH29CH2CHCH2CHvv\" ~H2CHTCH2GCH2ICHCH2CH2 (a) (b) scission
~
at (b)
Me
Me
l
Me
Me
~H2~H
+ CH 2=CH
I
(A)
I
monomer
!
Me
I
Me
Me
-L / I vvvCH 2CH I CH29CH2CH2
Me
I
(A) + CH 2=CCH2CH 2 2-methyl-l-pentene
Me
I
Me
I
scission at (a)
Me
I
I
(A) + CH 2=CCH2CHCH2CH 2 2,4-dimethyl-l-heptene
Scheme 19
Me
Me
I
!
~
~ ~
~
~ ~
Me
I
(B) + -CHCH 2CH 2
~
o
c· ;::s ~
Me
/
~
I
Me
I
(A) + CH 2=CCH2CHCH2CH
!
SCission H abstraction
Me
/
Me
/
v'\/\I'CH2CHMe + CH==CHCH 2 2-pentene
Me
I
CH 2=CMe + (A) isobutene
Degradation of PP
~
0\ \0
470
Thermal Degradation
(in a much smaller amount) isobutene. This sequence was not greatly dependent on tacticity or temperature (388-438 °C). Subambient TVA data (Figure 7b) for the condensable volatile products of degradation of PP under TVA conditions confirm the presence of three of the above major volatile products. In Scheme 19, a mechanism of degradation of PP is presented in which the fall in MW during degradation is attributed to random scissions plus extensive intermolecular transfer involving tertiary H abstractions from the polymer by the primary radical (B), whilst the formation of the observed volatile products is explained mainly by depolymerization and intramolecular transfer reactions of the secondary radical (A). Although PIB begins to break down in the same temperature region as PP, the maximum rate of volatilization, under programmed heating conditions, occurs nearly 50°C lower. The thermal degradation has been the subject of several investigations. 6 o , 6 6 - 6 9 In common with PE and PP, when PIB is degraded the polymer MW falls rapidly and the largest product fraction is the CRF. In the case of PIB, however, monomer is also a major product, about 300/0 of the initial polymer weight, and there are only traces of other volatile materials. These differences result from the absence of tertiary H atoms and the greater tendency for the dimethyl-substituted radical chain end to depolymerize to monomer. The behaviour of PIB can be satisfactorily explained by a mechanism similar in most respects to that presented in Scheme 17 for PAMS, in which the transfer reactions involve abstraction of primary H atoms. In the case of PIB, however, intramolecular transfer reactions must also be involved.
15.5.2 Polyisoprene and Polybutadiene Work on the thermal degradation of these polymers prior to 1970 has been reviewed," revealing conflicting results. Structural rearrangements in the polymers prior to major decomposition form the subject of a further review."? These polymers have more recently been reexamined by TVA, TG and gravimetric measurements ofCRF products.V: 71, 72 If the temperature is gradually increased, polyisoprene (PI) breaks down in a single stage from about 300°C, whereas PB shows an initial weight loss of about 10% in the same temperature range, followed by decomposition of the remaining polymer at slightly higher temperatures than for PI (Figure 8). Both polymers degrade completely by 500°C. The TVA curves indicate a distribution of products.
1.0~-------~_
c 02
0.8
~
: 0.6 s: c-
°i ~
g 0.4
"°Vi CD
a::
G2
Figure 8 TG curves (dynamic N 2 , lO°Cmin- 1 ) for polyisoprene (1) and polybutadiene (2)
Gravimetric measurements based on isothermal degradation of PI at 340°C show the amounts of CRF and volatile products to be about 550/0 and 32 % respectively. The volatile fraction, when separated by subambient TVA (Figure 9) has been found to consist of two major components, isoprene and dipentene. IR spectroscopic examination of the CRF products indicates the presence of vinyl and cyclized structures. The polymer MW falls as degradation proceeds and some fall occurs even before volatilization is detected. These features are consistent with a mechanism based upon random scission of chains at the bonds linking the methylene groups, depolymerization (one
Thermal Degradation (0 )
471
(3)
20
30
40
50
60
70
35
40
80
(8)
o
5
10
r5
20
25 30 Time (min)
45
50
Figure 9 Subambient TVA curves for warm up from -196°C of condensable volatile products of degradation in vacuum under TVA conditions of (a) polybutadiene at 380°C, (b) polyisoprene at 340°C. Assignments: (1) ethane, ethene, (2) propane, (3) 1,3-butadiene, (4) C, hydrocarbons, (5) C 6 hydrocarbons, (6) 4-vinylcyc1ohexene, (7) higher MW fragments, (8) isoprene, (9) dipentene
unit giving isoprene, two units giving dipentene) and intramolecular and intermolecular transfer reactions. The degradation of polybutadiene (PB) is more complex. The first change to occur in the polymer is loss of the original unsaturation in an exothermic reaction in which polycondensed rings are formed. The small amount of weight loss which occurs at this stage gives both volatile and CRF products, the former in a slightly greater amount. Subambient TVA separation of the volatile fraction has indicated the presence of several products, the most important being 1,3-butadiene and 4-vinylcyclohexene (Figure 9). In the second and major stage of breakdown, a very complex product mixture is obtained, with hydrocarbons of all sizes from C 2 upwards; some aromatic products are also found. The CRF products are essentially PB oligomers, but some saturated aliphatic and alicyclic structures are also involved. The structure and stability of PB samples allowed to oxidize to various extents have been examined. Oxidation leads to a considerable lowering of the threshold temperature for the main decomposition process; in samples oxidized at 0-20°C an additional early stage of degradation commencing near 100°C is found, due to peroxide decomposition. Mechanisms have been presented. 7 3
15.6 NITRILE POLYMERS 15.6.1
a-Substituted Acrylonitrile Polymers
Four polymers of this structural type have been studied. These are polymethacrylonitrile (PMAN), polyphenylacrylonitrile (PPAN), poly(vinylidene cyanide) (PVCN) and poly-e-chloro acrylonitrile (PCAN). The structures, compared to polyacrylonitrile (PAN, 1), are shown below (2-5). Me
MACH 2CHJVVI
I CN
(1) PAN PS 6-P*
MNCH 2CJW\
(2) PMAN
(3) PPAN
CN
I
tN
Cl
CN
Ph
I I\M.CH2 C\I\M I
JlMCH 2
tMl\. I
CN (4) PVCN
IIMCH 2
tJIM I
CN (5) PCAN
472
Thermal Degradation
It is convenient to consider these polymers before discussing PAN because they exhibit much simpler patterns of degradation, provided they are in the pure state. In common with other polymers having the (-CH 2CXY-) repeat structure, such as PMMA and PAMS, PMAN, PPAN and PVCN, each depolymerizes to monomer in high yield. 74 -77 In the presence of certain impurities however, either as additives or as comonomer units, the behaviour may be modified to various extents and a very characteristic colouration reaction is observed. In the extreme situation the colouration reaction may completely suppress monomer production. Only PMAN has been studied in depth. 74,76,78,79 In the presence of the appropriate initiating structures, PMAN begins to discolour just above the softening point, from about 120°C, and the colour changes gradually from white through cream, yellow, orange and red to deep red. The same colour change can also be induced in solution, in the presence of alkalis. The IR spectrum of the polymer shows a progressive decrease in the nitrile absorption at 2210 cm - 1 and a corresponding increase in absorption in the 1693-1490 em - 1 region, consistent with the conversion of -C=:N side groups to (-C =N-)n conjugated sequences (Scheme 20).
C
C
C
III N
III
N
III N
C'
III
N
Scheme 20
Cyclization of nitrile groups in PMAN
The colouration reaction in PMAN may be initiated by various structures.?" notably carboxylic acids and phenols. Copolymerized methacrylic acid units are particularly effective.?" The gradual progression of conjugation in the reaction and the types of initiating structure involved have led to the view that the process is a concerted one with initial nucleophilic attack on the nitrile unit rather than one involving free radicals. The reversion of the solution colouration by strong acids tends to support this view. The suggested mechanism is shown in Scheme 21. At higher temperatures, the ladder structure splits out methane and undergoes fragmentation. Me
Me
Me
Me
C
C
C
C
N
N
Me
Me
IWI.CH2~CH2\1\N'
c1 6 H NIII Me
Me
III
IlNV'CH 2
v'VV'CH z
Me
Me
Me
Me
Me
Me
Me
Me
CHzv'\l\J\,
III
H z\l\l\l\.
a
VVWCH z
a Scheme 21
CHzVVV'v NH
Mechanism of colouration in PMAN
The fourth polymer of this group, PCAN, also shows simple degradation behaviour.t? but cyclization of the nitrile groups is not involved. The presence of the nitrile substituent on the same carbon as the chlorine so weakens the C~CI bond that this polymer is even less stable than PVC and resembles the latter in splitting out hydrogen chloride. The dehydrochlorination proceeds rapidly at 140°C.
15.6.2 Polyacrylonitrile As far as monomer production is concerned, there is a close parallel between PMMA and PMAN, which can give nearly 1000/0 monomer, and PMA and PAN, which give virtually none. However, because of the additional possibility of nitrile cyclization which occurs in PAN at temperatures lower than those needed to cause scission. PMA and PAN degrade in very different ways. The discolouration which occurs when PAN is heated is important both as an undesirable feature of acrylic textile fibres and as a synthetic route to carbon fibres, which are made by heating PAN to
473
Thermal Degradation
about 1000 °C under controlled conditions. The subject has recently been reviewed.P ' If PAN is maintained at 180-190°C for an extended period of time (up to 65 h) in the absence of air, the pale yellow colour which initially develops (1 h) progressively deepens to a tan colour. If the polymer is subjected to programmed heating in nitrogen or under vacuum, there is a rapid colour change at 260-270°C to deep brown and beyond 350°C the material becomes black. The change at 260-270 °C is associated with a very characteristic exotherm seen in the DTA or DSC trace. The size of the exotherm is influenced especially by sample size and can cause a surge in the polymer temperature of as much as 40°C above that of the immediate environment. In such a situation TG curves show a catastrophic weight loss of limited extent at the same temperature, but a comparison of TG and TVA data reveals that most of this weight loss is due to polymer fragments formed by chain scission resulting from the sudden overheating. The small amount of more volatile material simultaneously produced consists of NH 3 and HCN. The TVA curves also indicate the evolution above 350°C of a noncondensable gas, which proves to be hydrogen. Nitrogen is evolved at very high temperatures, around 850-900 °C. The various thermal analysis curves are compared in Figure 10. (b)
~
o
a:
(0 )
(c) IOO~----.oIl
220
240
260 Temperature (OC)
280
O~_---I._---'---""""'--"""""""---
200
400
600
800
Temperature (OC)
Figure 10 Thermal analysis curves for polyacrylonitrile: (a) DSC, dynamic N 2' at 10 DC min - 1 (right) and 5 DC min - 1 (left);(b) TVA, under vacuum, 10 DC min -1 with trap temperatures - - 0 DC, ------ -75 DC, - ' - ' - -196 DC; (c) TG, dynamic N 2 , 10°C min -1 (reproduced by permission from 'Developments in Polymer Degradation', ed. N, Grassie, Applied Science, London, 1977, vol. 1, p. 137)
The spectral changes which occur as PAN discolours show that nitrile groups are being converted into conjugated -C =N- sequences, but the reaction in this polymer essentially involves the rapid intensification of a single spectral colour rather than the gradual pale yellow to deep red transition which occurs in PMAN. Whereas the PMAN colouration is impurity initiated and avoidable, the reaction in PAN, as well as being susceptible to the same types of impurity, can be initiated at structures inherent in the PAN chain or normally present in the polymer as structural abnormalities. The ::CHCN structure may itself be sufficiently acidic,"? or, perhaps more probably, enamine or ketonitrile structural abnormalities may be responsible.P As a result of extensive studies of various features of the reaction.v" it was concluded that the colouration reaction in PAN, in the absence of additives, is a radical process. The exotherm is clearly associated with the nitrile cyclization reaction in which the length of the conjugated sequences is much shorter than in PMAN but the kinetic chain length is long, maintained by transfer as tertiary H sites. The reaction consequently leads to short ladder polymer segments linked by segments of unchanged PAN chain. Chain fragments result from scission at the latter segments, whilst elimination of side groups in the same structures leads to small amounts of HCN. Ammonia, on the other hand, is believed to result from terminal imine groups in the cyclic structures. Mechanisms have been suggested. 81 - 85 The exotherm which occurs in the degradation of pure PAN is an undesirable effect in carbon fibre production, since the rapid build up of heat fragments the chains. The exotherm is reduced if
Thermal Degradation
474
the reaction can be initiated by impurities or comonomers, or by controlled preheating in oxygen. The effect of oxygen on the reaction is a subject of controversy/" but it appears likely that the oxidized structures produced act as additional low temperature initiation sites and also stabilize the fibre structure through hydrogen bonding.
15.7 CHLORINE- AND BROMINE-CONTAINING POLYMERS 15.7.1
Poly(vinyl chloride)
15.7.1.1 Introduction Decomposition to HCl is a characteristic of the pyrolysis of chloroalkanes, but PVC liberates HCl at a much lower temperature than would be expected from studies of 1,3-chlorosubstituted model compounds. The reasons for the unexpectedly low stability of PVC and mechanism of breakdown of the polymer continue to be subjects of argument. Several useful general views of the subject have appeared recently. 5,8,85 - 90 PVC has become a high tonnage plastic because of the success of PVC stabilization technology. The mechanism of action of PVC stabilizers is also a controversial field. Stabilization aspects lie outwith the scope of this section and are dealt with elsewhere (see Volume 6, Chapter 19).
15.7.1.2 Observed characteristics When PVC is heated, the polymer begins to discolour and evolve HCl at about 150 "C, The degradation is acutely sensitive to sample form because the dehydrochlorination is autocatalytic: thick samples degrade more rapidly than thin films (Figure 3) since the HCl cannot escape as rapidly. The degradation is faster in oxygen than in an inert atmosphere or under vacuum and the polymer stability is also influenced by the method of preparation (bulk, emulsion, suspension). In isothermal degradation experiments, particularly in solution, induction periods are observed; the rate then increases and subsequently decreases, giving characteristic S-shaped curves of HCl evolution vs. time. Studies by TG (Figure 11)and TVA (Figure 3)indicate that for a gradual rise of temperature there are two stages of decomposition. The first of these corresponds to near quantitative loss of HC1, together with a little benzene and traces of ethylene. In the second stage, most of the products are coloured materials which collect as a cold ring fraction; the volatile fraction at this stage is complex and includes aromatic and aliphatic compounds. A black carbonaceous residue remains at 500 "C.
.~ 0.8
U
~
:c0' 0.6
'j) ~
C
::J
0.4
"0
'en
~ 0.2
------'-------'-----.&....------.1
0 ........ 100
200
300
400
500
Temperature (Oe)
Figure 11 TG curves (dynamic N 2 , 10°C min -1) for (1) poly(vinyl bromide), (2) poly(vinyl chloride) and (3) polychloroprene
As HCl is released from PVC the discolouration increases with a colour shift towards red. This is attributed to the build up of polyene sequences, which can be studied by UV spectroscopy (Figure 12). It appears that the average polyene length is fairly short, reaching only 6-10 double bonds in conjugation. During dehydrochlorination there is also an increase in MW due to crosslinking, and some cyclization occurs.
475
Thermal Degradation 115
1.0
0.8
Q,)
o
6 0.6
-eo
en
.0
0.4
0.2
Figure 12 Absorption spectra for suspension-polymerized PVC degraded at 150°C in solution in N 2 atmosphere for the times indicated (min) (reproduced from Eur. Polym. J., 1967,3,747)
15.7.1.3 Initiation of dehydrochlorination It is agreed that the low stability of PVC is the result of the initiation of dehydrochlorination at structural abnormalities such as branches, chioroallyl groups, oxygen-containing structures, end groups or head-to-head linkages. There is considerable disagreement as to which are responsible. The predominant view is that p-chloroallyl groups -CH=CHCHCI- are important. This would be expected from the stability sequence of various nonoxygenated structures in chlorinecontaining model compounds.l" shown in Scheme 22. It is also supported by the observed correlation between internal chain unsaturation and dehydrochlorination rate and by the considerable destabilizing effect of the presence of phenylacetylene groups copolymerized into the PVC chain.86,92 MeCH =CCH 2Me ~ MeCHCH 2CHMe > MeCHMe > CH 2 =CHCH 2CHCH 2Me >
I
I
CI
CI
I
CI
I
I
CI
CI
Me
Et
I
I
CH 2 =CHCHCH 2Me > MeCMe > MeCH 2CCH 2Me > MeCH =CHCH 2Me
I
CI
I
CI
I
CI
I
CI
Scheme 22 Stability of selected chlorine-containing model compounds
On the other hand, others argue in favour of carbonylallyl groups -COCH=CHCHCICH 2- as the structure responsible.i" A case can also be made for chain branching, however, on the basis of evidence such as model compound stabilities and the decrease of the stability of PVC with increase in polymerization temperature. Head-to-head structures are less likely as labile sites, however, since head-to-head PVC is not sufficiently different in stability from normal PVC. 93
15.7.1.4 Mechanism of dehydrochlorination It is now established that free radicals are involved in the decomposition. The most convincing evidence comes from (a) transfer in the degradation of PVC in the presence of labelled toluene.i" (b) initiation by PVC of the free radical depolymerization of PMMA in the temperature range of dehydrochlorination and well below the normal temperature for PMMA breakdown (see Section 15.11.1.1), (c) ESR evidence for the presence of radicals in degrading PVC,94 and (d) initiation of dehydrochlorination by azodiisobutyronitrile.f"
Thermal Degradation
476
Various radical chain processes have been suggested, based on initiation either at a labile centre or by random scission of a normal C-CI bond. The CI- radical produced abstracts a methylene hydrogen and propagation then occurs along the chain or occasionally by attack on a neighbouring chain, so ending a particular polyene sequence but continuing the reaction in the other chain. Termination is assumed to be by combination of pairs of chain carriers (Scheme 23). H(R)
H(R)
I
I
-c-
Initiation
I
~
-<;-
+ CI·
CI Propagation
CI·
+
\/\I\IVCH2CHCICH 2CHCICH2CHCIIVVV
~ HCI ~
CI·
~ HCI ~
CI·
+ +
+ +
/VVVCHCHCICH.2CHCICH2CHCI/VV\ VVV\CH::;:::=CHCH 2CHCICH2CHCI.lVV\
IVVVCH=CHCHCHCICH2CHC~ IV\ACH=CHCH=CHCH 2CHCI./VVl etc.
Transfer to another chain
CI·
+
~H2CHCICH2CHCICH2CHCIVVV'
~ HCI
+ +
/\/V\.CH2CHCICHCHCICH2CHCIIVVV
~
CI·
IVV\CH2CHCICH=CHCH2CHCIVVV\
~
propagation as above
~
products
Termination R·
+ CI.} + CI·
R·
+
CI·
CI·
Scheme 23 Free radical mechanism for PVC dehydrochlorination
This mechanism fails, however, to explain autocatalysis by HCI, the catalysis by acetic acid in PVC/PVAC blends (see Section 15.11.1.2) and the catastrophic degradation induced by Lewis acids such as ZnCI 2 • It is probable, therefore, that molecular elimination of HCI via a four-centre transition state and HCI-catalyzed elimination via a six-centre transition state (Scheme 24) occurs simultaneously with the free radical route, the relative importance of radical or molecular elimination being dependent to some extent upon the conditions. The catalysis route in Scheme 24 is readily modified to explain acetic acid catalysis.96
C!J
H
~H}H4H(.:HCl'VVV\-
HCl+
NVVCH 2CH=CHCHCIVVVl Scheme 24 Molecular and HCI-catalyzed dehydrochlorination of PVC
15.7.1.5 Secondary reactions The crosslinking which occurs during HCI loss can be reversed or prevented by heating the polymer with maleic anhydride, which suggests that Diels-Alder type crosslinking of polyene sequences normally occurs (Scheme 25).
477
Thermal Degradation /.CH--cH. CH-eH hCHIN'N' // ~ // ~ ~ r ~CH CH-cH'/ "CH--eH
#
~H
CH-eH
~
/
CH=CH
CH--CH
,
CHW\N
~
CH~H
'cH--eH/
"
CH=CH ~H=CH/
~H-CH/
\:H=CH~
CH=CH\MN
Scheme 25 Die1s-Alder crosslinking of polyene sequences in PVC degradation
Intramolecular cyclization reactions of polyene sequences account for the formation of benzene and other aromatic products and also provide a route to the carbonaceous residue formed at high temperatures.
15.7.2 Poly(vinyl bromide) Poly(vinyl bromide) (PVB) resembles PVC in breaking down in two stages (Figure 11). In the first of these, loss of hydrogen bromide occurs with the development of conjugation, and in the second, the polyene structure undergoes scission, cyclization and carbonization.?? The stability, however, is much lower than that of PVC. The yield ofHBr is close to the theoretically possible value; rather less benzene and more ethylene are formed than in PVC degradation, and small amounts of unidentified products of lower volatility than benzene are also present in the volatile fraction. The length of the conjugated sequences produced in PVB during degradation has been estimated'" from the UV spectra of partially degraded samples to be about 12-13 double bonds, which is longer than in the case of PVC. It has not been established unambiguously whether or not radicals are involved in PVB dehydrobromination. The ability of PVC to initiate low temperature depolymerization of PMMA in PVC/PMMA blends provides excellent evidence for small radical intermediates in PVC dehydrochlorination. Similar experiments with PVB/PMMA blends?" do not lead to destabilization of the PMMA, which might indicate a purely molecular elimination of HBr from PVB. Alternatively, however, the rate of reaction of Br- radicals with PMMA at 200°C may be too low.
15.7.3
Chlorinated Rubber
Commercial chlorinated natural rubber contains between 65% and 680/0 CI, introduced both by addition and substitution reactions. The structure is complex and includes some cyclized units in the chains. A detailed examination of the degradation has been carried out by TG and TVA. 9 9 The polymer is stable to 200°C when heated at 10°C per minute and loses about 67% of its weight in the first stage of breakdown, which reaches a maximum rate at about 300°C. Further but limited weight loss (30/0) occurs between 400 and 500 "C, leaving a carbonaceous residue. By far the greatest proportion (95%) of the volatile products at the first stage of decomposition consists of HCl; small amounts of CO and CO 2 and traces of CH 4 and C 2H4 are also formed. In the subsequent weight loss, volatile products detected are CH 4 , C zH 4 , HCl (traces) and Hz. The polymer discolours at even 1% dehydrochlorination, showing that conjugation develops along the chain as in PVC degradation. At the same extent, insolubility also results. The extent and pattern of HCl loss under isothermal conditions are inconsistent with the structure suggested in 1967 for chlorinated rubber;'?" an alternative polymer structure and degradation mechanism in accordance with the observed behaviour have been presented.??
15.7.4 Poly(vinylidene chloride) This polymer is somewhat less stable than PVC. It degrades at 120-220°C to give only HCl; at temperatures above 250°C other products are also present. Only one molecule of HCl per vinylidine chloride unit is produced. Poly(vinylidene chloride) (PVDC) discolours very rapidly, becoming yellow at 1% and dark brown at ·100/0 volatilization. It also becomes completely insoluble at 1% degradation. Although autocatalysis is' observed after 15% reaction, the initial loss of HCl appears to be zero order. A free radical dehydrochlorination mechanism, initiated at the chain ends and proceeding along the chains, has been suggested. 101
Thermal Degradation
478
15.7.5 Polychloroprene Polychloroprene (PC) is unstable in air. Unless stabilized by additives the polymer absorbs oxygen. Simultaneously, HCl is released at a rate closely related to that of oxygen uptake and the polymer discolours. In order to study the effect of heat on PC it is necessary to degrade the polymer in nitrogen or under vacuum.P: 102 TG experiments (Figure 11)indicate two-stage weight loss as for PVC, but in PC the temperature for maximum rate of weight loss is much higher and there is also far more carbonaceous residue (20% of the sample weight) at 500 "C. Comparison of weight loss and EGA data for HCl evolution from PC (Figure 13) illustrate clearly the presence of additional products. The primary weight loss is about 45%, whereas about 35% appears as HCl. This is less than 90% of the available HCl in terms of the chlorine content.
Figure 13 Comparison of total weight loss (2)and weight loss due to HCI (1)for polychloroprene. Heating rate:,10 °C min -1. The total weight loss was determined by TG in dynamic N 2 and HCI by titration of products condensed from degradation under vacuum by TV A (reproduced from Eur. Polym. J., 1971, 7, 569)
Product analysis following degradation in a TVA system has established that minor gaseous degradation products include ethylene and a trace of monomer; above 400 "C, methane and smaller amounts of hydrogen and propylene are also formed. In addition to the gases, there is a complex liquid fraction containing two dichloro-4-vinylcyclohexene isomers. 1 02 UV studies of PC samples at different extents of dehydrochlorination'{ have revealed that the predominant absorption are characteristic of trienes. This is to be expected if random loss of HCl occurs by the nonradical mechanism shown in Scheme 26. PC does not accelerate PMMA degradation in PC/PMMA blends.l''! which is also consistent with a nonradical dehydrochlorination.
Cl---H
H
Cl
H
Scheme 26
HCl+ H
H
Cl
H
Molecular elimination of HCl from polychloroprene
15.8 FLUORINE-CONTAINING POLYMERS 15.8.1
Introduction
The study of the thermal stability of some of these materials has revealed a greater dependence of behaviour on environment and experimental conditions than for most polymers. Particular problems are the effect of oxygen and the tendency of some of the degradation products to react with the
Thermal Degradation
479
apparatus, particularly if glass is involved. Several useful treatments of the degradation of this class of polymers have appeared" 103 104 which contain references to original sources. t
15.8.2 Perfluoroalkane Polymers Poly(tetrafluoroethylene) (PTFE) is one of the most stable polymers known. The stability has been attributed to the high strength of the C-F bond compared to that of the C-H bond (487 and 418 kJ mol- 1 in CF 4 and CH 4 respectively) and to the shielding effect on the backbone of the highly electronegative fluorine atoms. There is no significant volatilization in vacuum below 450°C and even two hours heating of the polymer at 500 °C leads to only 30% weight loss. The stability of PTFE can be compared to that of polyethylene and other F-containing polymers from the following stability sequence, for vacuum conditions: poly(tetrafluoroethylene) > poly(vinylidene fluoride) > poly(trifluoroethylene) > polyethylene> poly(vinyl fluoride) > poly(chlorotrifluoroethylene) >poly(hexafluoropropylene). Copolymerization of TFE with hexafluoropropylene destabilizes the polymer but does not alter the principal products. The homopolymer poly(hexafluoropropylene) (PHFP) is very much less stable and decomposes below 300°C. The destabilization inPHFP and the copolymer is attributed to the presence of tertiary fluorine. PTFE degrades in the absence of air to give 95% yield of monomer and a small amount of hexafluoropropylene. No hydrogen fluoride or chain fragments are produced except at very high temperatures (1200 DC). Although the site of initial scission (random, structural abnormality or chain end) has not been established, the proposed mechanism (Scheme 27) involves homolysis followed by depolymerization of the radicals to monomer, with a complete absence of transfer because of the absence of C-H bonds and the strength of the C-F bond. IVVVCF2CF2CF2CF2CF2CF2CF2CF2vvv'
-+
/\/VVCF2CF 2CF2CF2 -+ vvvCF2CF2
2 vV\A.CF 2CF2CF2CF2
+ CF 2=CF2 etc.
Scheme 27 Degradation of polytetrafluoroethylene
The stability of PTFE in air is comparable with that in an inert atmosphere, but the mode of breakdown is completely different. Monomer is almost entirely absent. The major product below 650°C is the highly toxic carbonyl fluoride (COF2);above this temperature the product decomposes to CO 2 and CF 4, 15.8.3 Chlorofluoroalkane Polymers Replacement of a fluorine atom by chlorine reduces the stability and alters the mechanism of decomposition, because the C-Cl bond is weaker than the C-C and C-F bonds. Thus when poly(chlorotrifluoroethylene) is degraded in vacuum, the volatile fraction of products drops to about 280;0 of the polymer weight. The volatile material is predominantly monomer, plus some C 3F sCI and C 3F4C12' The remaining products are polymer fragments of average MW900, which collect as a cold ring fraction. Since the weakest bond is the C-CI bond, a possible degradation mechanism involves homolysis of this bond as the first step, followed by chain scission and depolymerization. There is spectroscopic evidence for the CF2=CF- structures also formed in this mechanism (Scheme 28). Further macroradicals may result by attack of chlorine atoms.
1 VVV\CF 2CFC1
+
CF2=CFCI
etc.
Scheme 28 Degradation of polychlorotrifluoroethylene
480
Thermal Degradation
15.8.4 Hydrofluoroalkane Polymers The presence of C-H bonds in fluorine-containing polymers of this type leads to the appearance of HF as a degradation product. The three main polymers in this class, poly(vinyl fluoride) (-CH 2CHF-)n' poly(vinylidene fluoride) (-CH 2CF 2-)n and polytrifluoroethylene (-CF2CHF-)n degrade in vacuum to give approximate yields of HF of 25, 50 and 6% by weight respectively. The remaining products consist mainly of cold ring fraction. Discolouration of these polymers also occurs. The mechanisms of degradation have not been established. A possible explanation of the above features is that molecular elimination of HF proceeding systematically along the chain competes with backbone scission and disproportionation of the radicals formed. Various copolymers involving the above monomer structures have been made, but the degradation behaviour remains largely unexplored. CF2CH2/CF3CFCF2 copolymer degrades above 370°C in nitrogen and gives 950/0 volatilization at 500 °C, of which some 30-400/0consists of volatile products, mainly HF, the remainder being chain fragments. CF2CH2/CF2CFCI copolymer is considerably less stable and begins to lose hydrogen halides from 250°C. The relative proportions of HF and HCI are sensitive to conditions (N 2 or vacuum) and degradation temperature. 15.8.5 Fluoromethacrylate Polymers Understanding of the behaviour of F-containing polymers has very recently been broadened by studies of three methacrylate polymers in which the ester group contains fluorine'?" poly(2-fluoroethyl methacrylate) (PFEMA), poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) and poly(hexafluoroisopropyl methacrylate) (PHFPMA). The experimental approach used is flash pyrolysis of microgram quantities of polymer at 600°C in a helium atmosphere, followed by GC separation of volatile products and identification by MS. This produces a convenient picture of the number and relative importance of the volatile products. The characteristic features of the degradation of alkyl methacrylate polymers are considered elsewhere (see Section 15.3.2). In contrast to PEMA, PFEMA gives no monomer. In addition to the ester decomposition products vinyl fluoride and CO 2, acetaldehyde and fluoroacetaldehyde are produced. Explanations have been proposed for the latter products on the basis of methacrylic acid/FEMA and FEMA/radical end FEMA adjacent unit interactions. It is argued that the ester decomposition route is favoured by F atom activation of the f3 hydrogens, so suppressing depolymerization. PTFEMA gives predominantly monomer, as would be expected because of the absence of f3 hydrogens in the ester group. Much smaller amounts of CO 2, vinylidene fluoride, trifluoroacetaldehyde and trifluoroethanol are also observed and mechanisms have been suggested. Weakening of the alkyl-O bond by the CF3 group in the CF3CH2 substituent, for example, leads to CH 2=CF2 and CO 2. PHFPMA gives behaviour which is completely untypical of the thermal degradation of methacrylate polymers, since by far the major product is hexafluoroisopropyl formate. Also produced are CO 2, acetaldehyde, 1,1,1,3,3,3-hexafluoropropane and trifluoroacetaldehyde, all of which are plausible decomposition products of the formate. It is suggested 105 that the first step in the production of the formate is scission of the C-H bond.
15.9 POLY(VINYL ACETATE) AND RELATED POLYMERS 15.9.1 Poly(vinyl acetate) and Ethylene/Vinyl Acetate Copolymers In many respects, the degradation of poly(vinyl acetate) (PVAC) resembles that of PVC, although the thermal stability of PVAC is much higher. PVAC gives near quantitative yields of acetic acid on the basis of one molecule of acid per chain unit, with accompanying discolouration of the polymer, and there is a further stage of decomposition at higher temperatures in which the products are similar to those at the same stage of PVC breakdown. In an early investigation of the polymer it was concluded that molecular elimination of acetic acid occurs, initiated at the chain ends.'?" However, this conclusion is in conflict with the lack of MW dependence of the rate of weight loss found in a later study. 1 07 More recently, the TG and TVA techniques have been applied in a reinvestigation of PVA degradation'? and also in examination of the degradation behaviour of PVAC/PMMA 108 and
481
Thermal Degradation
PVAC/PS 1 0 9 blends, from which important evidence regarding the mechanism of PVAC breakdown has emerged. Under programmed heating conditions (5 or 10°C per minute) in nitrogen (TG) or under vacuum (TVA), the polymer begins to decompose near 250°C and the maximum rate at the first stage of degradation occurs at 320-330°C; the second-stage rate maximum is at about 440°C. The carbonaceous residue at 500 "C amounts to about 8% of the initial polymer weight. The products at the first stage consist of 950/0 acetic acid plus 5% made up of ketene, water, methane, carbon dioxide and carbon monoxide. The products at the second stage result from scissions of the conjugated polyene left after deacetylation. Only small amounts of volatiles are produced; the major products at this stage are a coloured tar and the carbonaceous residue. The development of conjugation has been followed both in PVAC and ethylene/vinyl acetate (E/VAC) copolymers'f using UV spectroscopy of partially degraded polymer films cast on silica TVA tubes, which could conveniently be inserted directly into the sample compartment of the spectrometer. Assignments of peaks (Figure 14) were based on comparisons with reported data on model polyenes and with the sequence distributions in the copolymers. The TVA behaviour of 12 and 33% VAC copolymers indicated that the VAC sections of the chains eliminate acetic acid before any degradation of the backbone structure occurs. The 12% VAC copolymer contains 18.6% of the VAC units in pairs, but only 3.7% in blocks of three or more. Absorption is therefore confined to the diene region. The 33% VAC copolymer contains 29.6% of the VAC groups in pairs, 14.70/0 in blocks of three and 10.90/0 in blocks of more than three. Absorption beyond the triene region is therefore almost completely absent. Triene peaks appear at 261, 272 and 283 nm in this copolymer (and also in partly degraded PC, discussed in Section 15.7.5, which also gives trienes), as in PVAC. It has been concluded from these studies that in degraded PVAC conjugation extends to about six double bonds, the average polyene length thus being shorter than for degraded PVC. PVAC becomes insoluble at very low extends of degradation.
(F)
3
2
250
300
350
400
225
250
275
Wavelength (nrn)
Figure 14 UV absorption spectra for PV AC (left) and ethylene/vinyl acetate copolymers (right) (as films), initially and after degradation to various temperatures at 5°C min - 1. PV AC: (A) undegraded, (B) 289°C, (C) 301 "C, (D) 313°C, (E) 332 "C, (F) 381°C; 33% VAC copolymer: (A) undegraded, (B) 323°C, (C) 350 -c, (D) 356°C, (E) 374 °C, (F) 425°C. The dotted curve is for a 12% VAC copolymer degraded to 356°C (reproduced by permission of John Wiley and Sons Ltd from J. Thermal Anal., 1969, 1, 389)
When PVAC and PMMA are degraded together in a blend, the production of MMA monomer is much increased in the .250-350 °C region, whereas acetic acid evolution is retarded compared to the behaviour of the pure polymers. This is taken as strong evidence that radical species are involved in the deacetylation of PVAC. In the molecular elimination of acetic acid originally proposed, the presence of the minor products of degradation (ketene, CO 2 , etc.) was explained by assuming a small amount of decomposition of acetic acid. There is evidence that this can occur at considerably elevated temperatures (in a unimolecular process), but similar behaviour at 300 °C is questionable. One of the further arguments in' favour of involvement of radicals in PVAC deacetylation is that it allows the presence of minor products to be explained by side reactions of the acetate radical proposed as intermediate and also provides a ready explanation for the rapid insolubilization. The proposed mechanism, analogous to that for PVC (Scheme 23) is shown in Scheme 29.
482
Thermal Degradation Acetic acid production VV'V'CH 2CHCH2CH/VV\
I
OAc
-+
OAc
+
AcOH
AcO·
-+
I
-vvvCHCHCH 2CHCH2CHV\IV)
• I
I
OAc
-+
+
AcOH
I
OAc
OAc
/V\IVCH=CH~HCHCH2CH\IV\.A
• I
I
OAc
-+
AcO.
+
OAc
·~CH=CHCH=CHCH2CH/V\I\.
I
etc.
OAc Other reactions of acetate radicals 2AcO.
-+
CH 2=C=0
+ AcOH
AcO·
-+
CH-2=C=0
+ ·OH
AcO·
-+
CO 2
Scheme 29
+ Me (
-+
(-+
H 20)
CH 4 )
Free radical mechanism for degradation of PVAC
The simultaneous occurrence of molecular elimination of acetic acid in PVA degradation (Scheme 30) is probable, however, because the decomposition is catalyzed by acetic acid. Such a mechanism also allows catalysis by HC1, in PVACjPVC blends.?" to be explained. After loss of the first acid molecule, progression along the chain would be dependent on allylic activation of the adjacent VAC unit. ~H2CH~HVVV'
? ~01 or O=T Me
-
Me
VV\/ICH 2CHCH=CHtVV\/'
I
or?
+
MeC0 2 H
Me
SCheme 30 Molecular elimination of acetic acid from PVAC
15.9.2 Poly(vinyl formate) and Poly(vinyl alcohol) Poly(vinyl formate) has been briefly studied.P The UV spectra of partly degraded polymer samples resemble those obtained with PVAC although poly(vinyl formate) is less thermally stable. Two-stage weight loss occurs and the reaction appears very sensitive to conditions. Formic acid is the main product at atmospheric pressure, but under vacuum noncondensable gases, probably CO and H 2 originating by reaction of formyl radicals, are also produced. Poly(vinyl alcohol) degradation appears to be quite complex, although the elimination reaction to produce water occurs and some conjugation develops.P Acetaldehyde, which could result from depolymerization, has also been reported as a product. The TVA data indicate at least four stages of breakdown when the temperature is gradually increased, commencing near 200°C.
Thermal Degradation
483
15.10 RANDOM COPOLYMERS 15.10.1
Methyl Methacrylate Copolymers
When the degradation behaviour is compared with that of PMMA, the effects of introducing a comonomer unit into the PMMA chain fall into four types: unrestricted depolymerization, blocking of depolymerization, chain scission at the comonomer unit and interunit cyclization. In most systems, even a small amount (e.g. 10/0) of the second monomer can have a significant effect on the stability. If the second monomer is a methacrylate, then provided side group ester decomposition to alkene and acid unit does not occur, unzipping may proceed through the foreign unit, producing the two monomers as products. This is the case with ethyl methacrylate as comonomer and other n-alkyl methacrylates would be expected to behave similarly. Phenyl methacrylatejMMA copolymers are unusual in that although both homopolymers degrade exclusively to monomer, the copolymers show higher thermal stability and give only a small amount of products other than the monomers. Cyclic anhydrides appear to be produced in small amounts in the backbone by interunit reaction and these block unzipping.l !" Further examples of anhydride formation are considered below. There are several systems in which a second comonomer unit, even in a small amount, will stabilize the PMMA structure by blocking unzipping. In these cases, the depropagation process, once initiated, proceeds as far as the first foreign unit and may then stop because of some alternative reaction (e.g. transfer) of the new type of radical chain end. This is the situation with acrylate comonomers such as methyllll or butyl acrylate.P? and also with styrene. 113 The stabilizing effect of methyl acrylate and styrene is clearly illustrated by the TVA curves in Figure 15. In particular, the end-initiated PMMA depolymerization is reduced or eliminated. (0 )
100
200
300
400
100
Temperature (Oe)
Figure 15 TVA curves for degradation under vacuum at 10°C min -1 of (a) copolymers of MMA with methyl acrylate: ------ 26/1, 8/1, compared with PMMA of comparable MW ( - - ); (b) copolymers of MMA with styrene: ------4/1, - - 1/1, compared with PMMA of comparable MW ( ) (reproduced from Eur. Polym. J., 1968,4, 21)
Several MMA copolymer systems have been studied in which the first effect observed as the polymer temperature is raised is scission at the comonomer unit. In the case of low concentrations of «-chloroacrylonitrile, this weak link scission leads to radicals which begin to depolymerize immediately, so that MMA monomer is produced at only 140°C, together with some HCI. 114 However, with acrylonitrile as comonomer, although the molecular weight falls sharply, it has been found that under isothermal conditions at 220°C there is an induction period before monomer production, suggesting that the initial weak link scission leads to molecules rather than radicals.U" Methyl methacrylate chain units participate readily in intramolecular cyclization with adjacent comonomer units of certain types. These reactions seem to be favoured in situations where five- and six-membered rings can be formed. The cases presented in Scheme 31 have been investigated in some detail. In the first three examples,33,97,116 the cyclization occurs at lower temperatures than depolymerization. If the comonomer unit concentration is very low, the cyclic structures formed act as blocking groups for unzipping; at higher concentrations, however, MMA production may be almost completely prevented because the MMA units are used up in cyclization: the MeOH etc. then becomes the major volatile product. In the VACjMMA system, 11 7 cyclization competes with MMA production and acetic acid evolution, since all three reactions occur in the same temperature range. The anhydride structure shown in Scheme 31, reaction (a), which has characteristic IR absorptions at 1805, 1760 and 1020 em -1, is also produced in interunit cyclization in copolymers of MMA with
Thermal Degradation
484 Me
Me
I I
I
VVVlCH2 C-CH 2-Cvvvv
I
#C
O~
C",
"oH 0-1'
~:ttHMe Me
(a)
2~
a
OH
Me (b)
ca. 100°C
•
~H2'Ct:
o
C~ CI MeO/ ~O
Me
0
+
MeCI
Me
I
(c)
I
ca. 100°C
V\/V1CH 2CHCH2C;vvv
I
0 Me
I I
~H2CHCH2C'VVV\
I
+ MeOH
.
• ~H2r:t:
+ MeBr
Br /C, MeO 0 Me
I
(d)
~H29HCH2TV\/\A
to
Me
ca. 250°C
•
~H2Lt:
+
MeC0 2Me
C~ Me6 '\0
/~
0
Scheme 31
Intramolecular cyclization reactions in MMA copolymers
methacrylate salts, and in a number of situations involving PMMA degradation in the presence of additives, including PVC, PVAC, polychloroprene, fire retardants and zinc bromide. 1 9 , 2 o , 10 2 , 10 8 , 1 18
15.10.2 Styrene Copolymers In styrene rich styrene/MMA (S/MMA) copolymers, the stability is intermediate between that of the two homopolymers, but the product distribution is strongly influenced by the presence of the ~1MA units. For example, in a 4/1 S/MMA copolymer, the yield of cold ring fraction products is about a third of that expected on the basis of the styrene content and the behaviour of PS, so that transfer reactions are considerably reduced.l P One of the major differences in behaviour between PMMA and PS is that PMMA radicals will depolymerize to monomer at temperatures as low as 140°C, whereas PS radicals do not begin to depolymerize until nearly 300°C. Thus although the initial effect of small amounts of «-chloroacrylonitrile ((X-CAN) is the same in both the PMMA and PS chains, scission leads to monomer production in the former case but not in the Iatter.I!" The initially formed radicals in S/(X-CAN copolymers are believed to stabilize themselves by disproportionation (Scheme 32). Styrene/ acrylonitrile (S/AN) copolymers also show accelerated chain scission. 119 In the temperature region of volatile product formation, AN monomer is found (in contrast to the behaviour of PAN) and the CI
I
/\/\/VCH 2 CH CH 2 CH CH 2 C CH 2 CHvv\/\
I
I
Ph
I
Ph
-+
I
CN
Ph
I
-+
./\/V\CH 2CHCH 2---+-CHCH=CCH 2CH,/\/\/\
I
Ph vvV\CH2CHCH 2
I
: I
I
Ph
+
CN
+
I
HCI
Ph
I
CN
I
Ph
CH 2CH=CCH 2CH\/\/V\
I
Ph Scheme 32
+
·CHCH=CCH 2CHvvv Ph
Ph
I
I
CN
I
Ph
Chain scission and disproportionation in styrene/a-chloroacrylonitrile copolymers
Thermal Degradation
485
proportion of CRF products increases with the AN content. The styrene units prevent nitrile cyclization and in consequence some of the AN units decompose to HCN. Copolymers of styrene with vinyl chloride are much less stable than PS. 1 2 0 It is not possible to make such copolymers rich in VC because of the very unfavourable reactivity ratios. The styrene rich copolymers behave in a very similar way to chain-chlorinated PS samples of comparable CI content (see Figure 2). HCI is evolved from about 200°C and the resulting double bonds destabilize the PS backbone with the result that decomposition to styrene, etc. occurs at slightly lower temperatures than in PS. Styrene/vinylidene chloride copolymers have been found to show styrene group activation of vinylidene chloride units towards HClloss. 1 2 1, 122 15.10.3
Vinyl Chloride Copolymers
The novel behaviour of VC/MMA copolymers has been referred to above. A similar cyclization reaction occurs with methyl acrylate as comonomer. In vinyl chloride/vinyl acetate copolymers, at both extremes of the composition range, incorporation of the comonomer unit results in destabilization. 123 Minimum stability was found to occur at approximately equimolar composition in bulk degradation but at 33 mol % VAC in solution studies. During the degradation of a 470/0 VAC copolymer, the HCI/acetic acid ratio remains the same during the degradation and conjugation develops along the chain as in the case of the homopolymers. It may be concluded 'that both the rate of acid evolution and the development of conjugation are increased by heterogeneity in the polymer chain. One or both chain units must therefore be destabilized by the immediate proximity of a unit of the opposite type. Two mechanisms may be contributing. Firstly, the inductive effect of the CI atoms may weaken the C-H bonds of methylene group and so facilitate elimination of acetic acid in an adjacent VAC unit. The next molecule to be eliminated will be HCI because of allylic activation. Secondly, there may be neighbouring group participation by the acetate groups in the elimination of HCI, following which there is allylic activation for the removal of an acetic acid molecule. 123 The variation of stability with composition for the VC/VAC copolymer system is illustrated in Figure 16.
...
::::J
Eo ::::J
o 2
·2
e a:
Figure 16 Comparison of rate of volatilization at 248°C, as measured under TVA conditions at 5°C min - 1, with copolymer composition, for vinyl chloride/vinyl acetate copolymers (reproduced from Eur. Polym. J., 1970, 6, 679)
15.10.4 Ionomers The degradation behaviour of certain copolymers containing methacrylate salt units has been examined within the last few years. The systems examined include MMA/alkali metal methacrylates (Li, Na and K), MMA/zinc methacrylate and the commercially important ionomers based on ethylene and methacrylate salts (Na and Zn ion). The behaviour of the MMA ionomers has been discussed in a recent review.P" A major feature of the degradation reaction is the evolution of methanol and the formation of six-membered anhydride rings in the polymer chain. The ethylene/methacrylate salt ionomers, as produced commercially, are in fact terpolymers, since some free methacrylic acid structures are also present. The salt unit concentrations of the ionomers which have been studied v " are somewhat less than 10 mol %. TG measurements have indicated stabilities for the Na and Zn ionomers which are fairly comparable with that of PEe The residues at
486
Thermal Degradation
500°C consist of Na 2C0 3 and ZnO respectively. All the product fractions have been examined in TVA experiments. The noncondensable gas fraction (absent in PE degradation) is mainly CO. The condensable volatile products contain the broad spectrum of products typical of PE breakdown, with an increased amount of propene, and CO 2 , dimethylketene and isobutene are also present. The cold ring fraction resembles that from PE; IR spectra have shown that the chain fragments contain some methacrylic acid units. Residual polymer from partial degradation at 440°C show no C0 2H groups but salt units are present. It has been concluded v" that the ethylene ionomers degrade by a mechanism in which the initial step is scission in the salt side group. Subsequent reactions of the radicals formed in this step account satisfactorily for all the observed features. Scheme 33 shows the behaviour as proposed in the case of the Na ionomer.
\ CO
I + VVV\CH2CH2CH2<;CH2CH2CH2VVV'
(B)
(A)
I
j volatile and cold ring fraction products by intramolecular and intermolecular transfer, as for PE (including original methacrylic acid groups)
+
(C)
(B)
(A)
Me
NaO·
Me
I 'VV'VC I
/vv\-
NadF~
----+
atm.
Na 2
°
COl]
Me
+
I
~vvv
I /C~
·0
--+
CO 2
+ radical (A)
~O
Na2C03 Scheme 33
Degradation mechanism for ethylene/sodium methacrylate ionomer
15.11 BINARY POLYBLENDS AND RELATED SYSTEMS 15.11.1
Binary Polyblends
When two polymers are mixed, the system which results is almost invariably heterogeneous, involving a continuous and a dispersed phase. The degree of dispersion depends on the method of mixing. Common procedures are grinding together powders, milling or casting a film from a common solution of the two component polymers, and the degree of dispersion increases in that sequence. This is very relevant to the degradation behaviour of binary polyblends. For example, a powder mixture of PVC and PMMA behaves very differently from a film cast from solution. I IS The degree of dispersion is important because of the types of reaction which can occur during degradation of these systems. Since there are two discrete phases, any new reactions involving interactions must involve either reaction at the phase boundary between the two polymers or migration of some chemical species from one phase into the other. The evidence from the variety of polyblends studied is that any observed interactions are due to migration effects rather than boundary reactions. Either small product molecules or reactive species such as radicals produced in one polymer phase migrate into the other phase. In some cases interaction leads to destabilization, in others to stabilization, so that such processes are of major relevance to practical applications. Some illustrative examples are considered; two reviews have been published.P?: 127
487
Thermal Degradation
;
e ~
o
·c ~
(b)
a:
200
300
400
Temperature (OC)
Figure 17 TVA curves for degradation of films with 1: 1 ratio by weight of PVC to PMMA under vacuum at 10°C min -1, (a) for simultaneous but separate degradation of the two polymers, and (b) as a blend in the same film. Trap temperatures: O°C and -45°C, ------ - 75°C, - - - -100 °C, -196 "C. The shaded area represents HCI production; below 400 °C the remainder is due to MMA evolution (reproduced from Eur. Polym. J; 1970, 6, 679) _0_0-
15.11.1.1 Interactions due to small radical migration When films of PVC and PMMA are degraded separately at 10°C per minute, dehydrochlorination of PVC is observed at 225°C, some 50°C below the onset of the PMMA depolymerization (Figure 17a). However, when the two polymers are present in the same film, cast from a common solution, dehydrochlorination and depolymerization both commence simultaneously at 225°C. Large amounts of MMA are produced at this stage and a second stage of MMA evolution occurs after dehydrochlorination, the maximum rate for this process being 30°C higher than that for PMMA degraded alone. Clearly there is extensive interaction in the blend, destabilizing the PMMA, but it is also apparent from Figure 17b that the PVC dehydrochlorination is slowed down. A detailed investigation of this system 118 led to the conclusion that the most important effects are due to migration of chlorine radicals present as intermediates in the PVC dehydrochlorination into the PMMA phase, where attack on the backbone CH 2 or side group Me structure'S leads to subsequent chain scission and depolymerization (Scheme 34). Me
I
Scheme 34
Depolymerization of PMMA resulting from CI- radical attack
Thermal Degradation
488
Blends of PVAC with PMMA show similarities in behaviour to the PVC/PMMA system, again due to migration of small radicals, in this case MeC0 2 - , from one polymer phase into the other.l'" Acetic acid, like HC1, does not interact with PMMA in such a way as to facilitate monomer production. It is interesting to compare the behaviour of PVC/PMMA blends with that of PVC/PS blends. Production of HCl is again retarded because of the loss from the PVC phase of some Cl- radicals, which are no longer available to take part in the dehydrochlorination chain reaction. These Clradicals are believed to abstract tertiary H atoms from th PS; the resulting radicals undergo scission, but depolymerization of the PS does not occur in this temperature region, Extraction of PS from partially degraded PVC/PS blends, however, shows that the MW of the PS falls more rapidly than when PS is degraded separately.P" The effects observed in the degradation of PS/PAMS and PS/poly(ethylene glycol) (PEG) blends l 2 9 , 1 3 o can be explained in a similar way. In the former case, high molecular weight PAMS has little effect but very low MW samples lead to increased styrene production under the isothermal conditions used. In the PEG blends, the PS shows a more rapid fall in MW. At the temperatures used, both PAMS and PEG produce radicals. For samples of low initial MW radicals can result which are sufficiently mobile to migrate into the PS phase and abstract tertiary H atoms, leading to subsequent chain scission.
15.11.1.2 Interactions due to small molecule migration Interesting examples have been found of interactions in degrading polymer blends resulting from migration of a small product molecule from one polymer phase into the other. The results include structural modification of the polymer, catalysis of the degradation process and inhibition. The first leads to changes in degradation products, whereas the others influence mainly the rate of temperature range of degradation. When PMMA is degraded in a blend along with polymers which degrade to produce HCI or acetic acid, some of the ester groups may be modified so that an MMA copolymer results. The effect of reaction with HCI is to produce anhydride rings in the PMMA chain (Scheme 35).From studies of MMA copolymers, it is known that such structures will block depolymerization (Section 15.10.1), so that the presence of quite low concentrations of these rings greatly reduces the zip length for depolymerization. This has two effects: MMA evolution is retarded and PMMA molecules remain in the degradation apparatus to higher temperatures than when PMMA is degraded alone. The latter effect results in small amounts of alternative decomposition of the MMA units which leads to MeOH and CO, not normally produced in PMMA breakdown. Me
Me
MIlCH).---CH2~"""" I
I
~C...........
o
0
I
Me
C-OMe
II
0
Scheme 3S Anhydride ring fraction in PMMA resulting from H'Cl attack
Blends of polychloroprene with PMMA do not show the chlorine radical migration effects observed in PVC/PMMA blends, nor the stabilization of the dehydrochlorination reaction, both of which are consistent with the nonradical mechanism proposed for HCI production in the former polymer, but there is a major effect of HCI in producing PMMA stabilization.V" Blends of PVC with PMMA also show the effect of HCI, but this is masked by the earlier destabilization due to CIradicals. Acetic acid from PYA also leads to anhydride rings in PMMA with similar effects.'?" Blends of PVC with PVAC 9 6 show mutual catalysis of the decomposition of each polymer by the primary decomposition product of the other. This is seen both in. TVA studies similar to those illustrated for the PVC/PMMA system and from direct measurements of acid production, as illustrated in Figure 18, which show clearly the destabilization of each polymer.
Thermal Degradation
489
1:'
1ii
(3)
': 60
:c 'iii en
8. () 40 ~ en
2
t 1:'
20
1:'
'0
(2) __ __----e~
100
200
Time (min)
Figure 18 EGA data for acid evolution from PVC (-0-) and PVAC (-e-) films degraded alone (curves 4 and 2) and as a 1: 1 by weight blend (curves 3 and 1),at 220°C under vacuum (reproduced by permission of John Wiley and Sons Ltd from J. Polym. Sci., Part A-I, 1974, 12, 387)
Fortunately, in two of the most commercially important binary polyblends, PSjpolybutadiene (PB) and PSjpolyisoprene (PI), 131,132 the interactive effects lead to stabilization: the composition of the degradation products is not significantly altered, but the PS component is considerably stabilized. In Figure 19, the blend shows a major change in TG behaviour in the temperature region of the first weight loss stage, associated mainly with PS. Product investigations by subambient TVA for isothermal degradations of PSjPB blends reveal that initially the volatile products are those associated with the first stage of PB breakdown and that when these cease to be evolved, PS decomposition begins. It was concluded that 4-vinylcyclohexene produced from the PB acts as a radical scavenger for PS radicals, by virtue of the large number of allylic H atoms; this hypothesis has received support in further studies.P:' In the PSjPI blends, stabilization is also observed and a similar effect, in this case due to dipentene, is believed to be responsible. A further example of a product reacting with a polymer radical is provided by the case of blends of PMMA with polylactide.P" in which acetaldehyde produced from the polylactide is believed to react with PMMA radicals.
15.11.1.3 Other interactions In several of the polyblends studied, there is reason to believe that conjugated structures produced in one component as a result of partial degradation can act as a 'radical sink' for radical intermediates involved in the degradation of the other. Stabilization effects of PVC, PVA and PAN on the production of styrene from PS may be examples of this type of interaction. Purely physical effects may also occur in some blends. In the case of PVCjPAN blends, for example, it has been suggested that the observed destabilization of the PVC component is due to the PAN making release of HCI from the mixture more difficult, so increasing autocatalysis. ....~
1.0~-------...
c:
0.8
o
~ 0.6 ~
.E 0' 'i) ~
C 0.4 ~
1:'
'ij
a=
0.2
Figure 19 TG curves (dynamic N 2, 10°C min -1) for polystyrene/polybutadiene systems: curve (1) predicted behaviour for 1: 1 mixture by weight of PS and PB in absence of interaction, (2) experimental behaviour of 1: 1 blend, and (3) experimental behaviour of 1: 1 block copolymer (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1985, 10, 319)
490
Thermal Degradation
In the rare situation of a homogeneous polymer blend, interactions involving dissimilar macroradical/macromolecule pairs are theoretically possible. Interactions in the homogeneous twocomponent system PMMA/bisphenol A polycarbonate have been explained on the basis of PMMA radical attack on the polycarbonate chain.l " 15.11.2 Block and Graft Copolymers Degradation of these systems, which physically can be regarded as blends with a very high dispersion of one phase in the other, has received little study. A detailed examination of the styrene/butadiene diblock copolymers.P" however, has revealed a further enhancement of the stabilization effects in PS/PB blends, as illustrated in Figure 19. For the two component polymers, the degradation reactions are well separated, in the blend they overlap, but in the block copolymer they occur simultaneously. The same stabilization mechanism has been proposed and the enhancement of stabilization is due partly to the fact that short chain fragments from the PB blocks are being produced in the temperature range of PS volatilization. These are believed to act as radical scavengers in the same way as the 4-vinylcyclohexene produced at lower temperatures. The degradation behaviour of a number of graft copolymers prepared by mastication of PVC in presence of another monomer has ·been studied. Graft copolymers of PVC were so produced using methyl, ethyl and heptyl methacrylate respectively and also with a styrene or styrene/MMA mixture. 1 2 0 , 1 3 7 Again the effect of the higher dispersion is to enhance the interaction effects (e.g. methacrylate monomer production, PS fall in MW) observed in the corresponding polyblends.
15.12 POLYMERS WITH HETEROATOMS IN THE BACKBONE 15.12.1
Aliphatic Polyethers and Polyesters
Six polymers of simple structure have been investigated in sufficient depth for mechanisms of decomposition to be presented on a firm basis of product separation and analysis. The structures are shown as (6}-(11) below. The four polyesters (8}-(11) are all derived from polymerization of cyclic molecules containing one (II) or two (8}-(IO) chain repeat units; these cyclic molecules are shown as structures (12}-(15).
o JVV\CH 20VVV'
'VVVCH2CH2{)1vv\-
II
Me 0
vVV\CH2COvvv
I
Me 0
I
1/
I
/VVVC -COVVVl
/VV\ICH-C~
I
Me (9)
(10)
o 0 " I V\/\I1CH 2CH 20C-CO/VVV (11)
(15)
Polyoxymethylene (6) as prepared has OH ends; the polymer is of low stability and degrades to formaldehyde in 1000/0 yield from about 100°C. The stability is improved by acetylating the chain ends, which indicates that end initiation of depolymerization plays an important part, but the product of degradation of the end-capped polymer is the same. The formation of formaldehyde has been explained 138 by a nonradical mechanism as shown in Scheme 36. A similar six-centre transition state may be envisaged, leading to the release of two molecules of HCHO at each step. \NV\OCH 20CH 20CH 2QTcH 2
k~
•
'VVV()CH20CH 20CH20
k
+
CH 20
etc.
?:c.i
H2
vvvOCH2 OCH 20CH2
MetJ-o
II
o Scheme 36 Degradation mechanism for normal and end-capped polyoxymethylene
Thermal Degradation
491
Poly(ethylene glycol) (7) polymers of low MW (1000 and 1500) have been studied.P? Water and ethylene glycol are believed to be produced in chain terminal reactions; the remaining products, consisting of chain fragments of lower MW acetaldehyde, formaldehyde, ethylene oxide and methoxy- and ethoxy-acetaldehyde, are all satisfactorily explained by a radical mechanism involving initial homolysis of C-C or C-O bonds. The polymer begins to give volatile products from about 300 aC. The degradation reactions of polyglycolide (8),140,141 polylactide (9)134,141, 142 and poly(tetramethylglycollide) (10)143,144 are each characterized by ester interchange reactions at lower temperatures leading to the cyclic dimer (12HI5) and also to oligomers in the cases of polyglycollide and polylactide. Cyclic oligomers and cyclic dimers can be produced by essentially similar intramolecular mechanisms. Carothers' picture of a ring ~ chain equilibrium involving the cyclic dimer as the breakdown product at elevated temperatures is only partially correct in these cases, since a higher proportion of cyclic oligomers than cyclic dimers is in fact observed. When the temperature is further raised, polymers (8) and (9) also undergo homolysis leading to additional products including CO 2 and ketenes. The homolysis also produces the cyclic dimer and chain fragments. In the case of poly(tetramethylglycollide) the cyclic dimer degrades quantitatively to methacrylic acid if it is not efficiently removed from the hot zone. Poly(ethylene oxalate) (11) differs from the above polymers by breaking down by homolysis at the oxalate linkage in the polymer chain, since in addition to 49% of ethylene oxalate (15) and 300/0 of oligomer by weight, a variety of other volatile products are produced from the lowest degradation temperatures and not only at higher temperatures as in the case of polyglycollide and polylactide. All the observed products are satisfactorily explained by the free radical mechanism shown in Scheme 37.1 4 5
° °
II " vvvCH 2CH20C-COCH2CH 2 vvv
---I".
?i
2IVV\CH 2CH20C'
00 00 00 0 II II II II II I II I I I I II .lVV\.CH2CH2o-C--r-c0(CH2CH20C-CO)"CH2CH20C-r-C---r-o-rCH2CH2To--t-c· ----. chain scission I
(f)
(e)
Scission at (e) (f)
~ _ _~.,
(a) --~~ (b) - - ~. (d) - - ~. (b) + (c) --~.,
(d)
(c)
(b)
(a)
monomer} oligomer major products
co
CO 2 ethylene carbonate ethylene
Scheme 37 Degradation mechanism for poly(ethylene oxalate)
15.12.2 Poly( ethylene terephthalate) The degradation of PET has recently been reviewed. 146 The behaviour is complex and sensitive to the presence of impurities. The thermal stability of the polymer is fairly high: it begins to break down between 250 and 300 "C, but evolution of volatile products only becomes rapid above 350 "C, The initial step in the degradation involves nonhomolytic scission at an ester linkage to produce acid and vinyl ester end groups (Scheme 38). Reaction of the latter with hydroxyl ester end groups in the PET by transesterification leads to the elimination of acetaldehyde (formally, vinyl alcohol is the product, but rearrangement occurs) which is the principal observed volatile product. These are only the initial
Scheme 38 Chain scission and acetaldehyde formation in PET degradation
Thermal Degradation
492
reactions in a complex pattern of breakdown which must also include homolytic reactions at higher temperatures since CO, CO 2 , CH 4 , C 2H 2 , C 2H4 and benzene have also been observed. The subambient TVA curve for the separation of the principal condensable products from a commercial PET sample degraded to 500°C under TVA conditions is illustrated in Figure 20. Terephthalic acid appears as a solid product of degradation by sublimation from the hot zone and the proportion of acid end groups in the degrading polymer increases as these are formed and the OH ends are used up. (2)
(3)
oae Figure 20 Subambient TVA curve for warm up from -196°C to O°C of condensable volatile products of degradation in vacuum to 500 °C at 10°C min -1 in a TVA system of poly(ethylene terephthalate). Assignments: (1) ethylene, (2) CO 2, (3) acetaldehyde, (4) unidentified minor product, (5) water, (6) unidentified high boiling liquid
15.12.3 Polycarbonates Aliphatic polycarbonates such as poly(trimethylene carbonate) (16) and poly(neopentylene carbonate) (17) degrade mainly by an intramolecular exchange reaction to give the cyclic monomer, chain fragments and CO 2 • A radical process involving random scission becomes more important as the temperature is increased. The introduction of a bulky phenyl substituent in poly(2-phenyltrimethylene carbonate) (18) hinders cyclic monomer formation and this polymer gives mainly short chain polymer fragments. Poly(p-xylene carbonate) (19) also degrades to give short chain fragments as the predominant product. In addition, CO 2 and p-tolualdehyde are formed.
° "
/\/\/'vCH 2CH2CH 20 C OIVVV
Me
I
./\/VI-CHf:CH 2OC O/VV'
I
Me (16)
°I
(17)
°II
.N\/\CH 2CHCH 20COvvv
I
Me (18)
(19)
The most extensively studied polycarbonate is the commercial polymer based on bisphenol A. This is more stable than the polycarbonates mentioned above and begins to lose weight at temperatures above 350°C, under programmed heating at 10°C per minute. The reported products of degradation include chain fragments, bisphenol A, phenol, diphenyl carbonate and CO 2 • The first processes to occur in the degrading polymer are believed to be hydrolysis and phenolysis at the carbonate group. Branching reactions occur due to thermal rearrangement of the carbonate group and eventual gelation takes place due to the removal of the volatile products formed during degradation. A mechanism has been proposed.I"? the main features of which are given in Scheme 39. There is some evidence from TVA studies of bisphenol A polycarbonate, alone and in polycarbonate/PMMA blends.l " that radical reactions also playa significant part in the degradation process.
2vvvO"
o
~O~~
+
"20 -
+
CO 2
~O" +Oo~oQ-{~ -~-o~oOi~+ 0" Me
0
Me
o
2
vvvQo~oO
-
o
2
H-
Me
Me
Me
Me
0
~O~004DO" Me
~0-'~'O~
CO 2 "
-
Me
~
~
s
"""t
f2.. ~
-. vvvQ0co~ "OiDO" ~
Me
.
+
0
II
~ ~C.H.OCOC.H.~~
Scheme 39
Me
AAA/OO~0
'--II
.
0
--...()
~-:,--C
0
~
~ """t ~ ~ ~
Me
c" ("Oot.
~
<-o
f~+~" +
CO 2
Principal reactons in the thermal degradation of bisphenol A polycarbonate
~
\0
W
Thermal Degradation
494
15.12.4 Polysiloxanes The stability of polydimethylsiloxanes has been found to increase with MW and to be highly sensitive to the presence of traces of KOH, used industrially as a polymerization catalyst. The stability is improved by replacing the normal hydroxyl end groups by trimethylsilyl structures, which indicates the importance of hydroxyl end initiation as a degradation route. These effects are illustrated in the TG data of Figure 21.
§ 0.8
U
.g
.E 0.6 C' '0) ~
'0 0.4 ::3
~
.iii
~ 0.2
0--""-----..........,;:---'""""'--------------' 400 500 200 300 100 Temperature
(Oe)
Figure 21 TG curves (dynamic N 2 , 10°C min -1) for poly(dimethyl siloxanes). Mn values of (1)94 500, (2) 111500, (3) 183000 and (4) 258000; (5) Mn 111 500 sample after end capping; and (6) Mn 111 500 sample plus 50/0 KOH (reproduced from Eur. Polym. J., 1978, 14,875)
The suggested mechanismsv'" to explain the formation of the same products (cyclic trimer and higher cyclic oligomers) from hydroxyl-ended, end-capped and KOH-containing polydimethylsiloxane are shown in Scheme 40. Me
Me Me
\/
\/
Me Me
Me
\/
vvvSi--
TJ r l , H----4)--Si-Q
/
Me
"-Me
Me
Me Me
Me
Me
/VVV~i--r-O-Si--{) ~:
/"
I
vvvo-----CSi--D--Si Me
/"
Me Me
Me
/ \
Me
Me
,,/Me
/
~i----r-O--Si---O
1
'+~
HO- VVV\()-,.-Si--o--Si ~/
Me
Me
1 Me Me Me 0 Me < > ,,~/ -vvvSi + Si Si I /1 I' OH Me 0 0 Me S(;
Me
'\../
,,/
\./
Me
Me
~Si/ I
!
-,
Me
/ \
Me
Me
+
08
+ IVVVSiMe20-
Scheme 40 Degradation mechanisms in OH-ended, end-capped and KOH-containing polydimethylsiloxanes as at present
The replacement of some of the methyl groups in polydimethylsiloxane by phenyl groups improves the thermal stability but reverses the dependence of stability on MW .!10ted above; in addition, benzene is produced in small amounts and insolubility develops, even at low temperatures. A process of OH-assisted cleavage of Si-phenyl bonds, leading to chain branching, has been proposed.v" which accompanies a similar process of cyclic oligomer formation to that given in Scheme 40 for polydimethylsiloxane.
15.12.5 Other Polymers Some aspects of the degradation of polyurethanes are considered subsequently in relation to the fire situation. A number of other polymers of complex composition involving 0 and N atoms in the chain structure, including some of the most thermally stable organic polymers" have been discussed
Thermal Degradation
495
in a recent monograph.l?" The degradation behaviour of urea formaldehyde resins has also been examined. 149
15.13 DEGRADATION OF POLYMERS IN THE PRESENCE OF FIRE RETARDANTS An essential requirement for the combustion of a polymer is that it produces a volatile fuel which can mix with oxygen. The process is started by heat and sustained by the heat of combustion of the fuel. In order to retard or prevent burning, this cycle must be interrupted. There are several ways of doing so: removal of heat, quenching of the flame, preventing access of oxygen or modifying the reaction which generates the volatile fuel. Fire retardant additives may therefore act in various ways and in anyone polymer/additive composition there may be more than one effect. Attention is limited here to effects (chemical or physical) on the polymer decomposition process and to examples of investigations in which the nature of the interaction as it affects the degradation mechanism of the polymer has been established. Some of the most detailed investigations have been made in the absence of oxygen; this is appropriate, however, because it reflects the situation in the condensed phase from which the fuel is generated, since the oxygen is removed from the polymer surface by the burning fuel. Useful recent reviews 1 5 0 , 1 5 1 and a monograpb 'V are available. 15.13.1
Poly(methyl methacrylate) with Ammonium Polyphosphate
Several detailed studies have been made in which the fire retardant additive is ammonium polyphosphate (APP). It is necessary first to understand the effect of heat on APP as the temperature is gradually increased. In the temperature region 10o-260°C, less than 50/0 weight loss (as ammonia and water) occurs. Some free acid groups are formed, which condense to form crosslinks. The physical state changes from powder to a glassy, hygroscopic solid, from which gas evolution is less easy. The P-O-P links produced are easily hydrolyzed to acidic groups. Between 260 and 350°C the rate of evolution of NH 3 and H 2 0 goes through a maximum and declines to zero after 20% weight loss. The product is polyphosphoric acid, a hygroscopic glass. In the final stage above 350°C (which may be too high a temperature region to influence some polymers), the polyphosphoric acid structure is fragmented with the formation of low volatility products. The effects of APP are therefore likely to be due to one or more of these: evolution of NH 3 and H 2 0 , production of polyphosphoric acid or acidic species derived from it, and the glassy state of the intermediate decomposition product. When PMMA is heated with APP, chemical changes in the PMMA occur only above 260°C in the heating programme, i.e. after NH 3 and H 2 0 evolution has ceased and polyphosphoric acid is present. The observed effects are believed to be due to mobile fragmentation products rather than the crosslinked polyphosphoric acid itself.20 The primary effect is to cause ester groups to be converted to anhydride rings, a small concentration of which is sufficient to interfere significantly with the depolymerization process. Thus the production of monomer (which is the volatile fuel in a fire situation) is slowed down. When the temperature is increased, the products include, in addition to monomer, methanol, CO 2 and CO. There is therefore a close parallel with the behaviour of PMMA in other acid-releasing environments, such as blends with polychloroprene (see Section 15.11.1.2). 15.13.2 Polyurethane with Ammonium Polyphosphate A comparison of the degradation behaviour of a polyurethane (PU) alone and in the presence of APP has revealed several interesting features.P? The PU studied had the structure shown in Scheme 41. Under programmed heating (10°C per minute), it begins to break down at 250 °C, the primary products being the reactants used in its preparation, butanediol (BD) and methylene bisphenyldiisocyanate (MBPI) (see Scheme 41). Since these are of low volatility, some decomposition occurs as they diffuse out of the polymer, leading to CO 2 , tetrahydrofuran, butadiene and water, as indicated in the subambient TVA data of Figure 22a. It is possible that the ease of ignition of PU foams is associated with the evolution of some butadiene when the polymer is heated. The THF and water derive from decomposition of BD, whereas CO 2 results from the condensation of pairs ofMBPI molecules to produce carbodiimides (CDI). Butadiene is believed to result from reaction of BD with CDI. PS6-Q
Thermal Degradation
496
butanediol Scheme 41
methylene-bis-phenyldiisocyanate
Primary decomposition in a polyurethane
(0 ) (I)
(3)
~
;o
(b )(1)
·c ~
a..
o
10
20 Time (min)
30
40
-196°C
Figure 22 Subambient TVA curves for warm up from -196°C to O°C of condensable volatile products of degradation under vacuum to 500°C at 10°C min -1 in a TVA system of (a) a polyurethane sample, (b) the same polyurethane sample in the presence of 10% of the fire retardant ammonium polyphosphate. Assignments: (1) CO 2 , (2) butadiene, (3) tetrahydrofuran, (4) dihydrofuran, (5) water, (6) formaldehyde, (7) aniline (reproduced by permission from 'Developments in Polymer Stabilization', ed. G. Scott, Applied Science, London, 1979, vol. 1, p. 197)
When APP is present with the PU, major effects are observed which can be attributed to the presence of polyphosphoric acid. The volatile products (Figure 22b) still include CO 2 , THF and water. Notable differences, however, are the complete absence of butadiene and the appearance of three undesirable toxic products: ammonia, formaldehyde and aniline. There are also changes in the less volatile products: no BD is formed, there is less CDI and MBPI, C =N and P-O-C structures are evident in the solid and there is a substantial residual char. The chemistry involved is complex, but in essence the changes are due to polyphosphoric acid acting as an acid catalyst and as a crosslinking agent. The burning process is affected in two ways: the amount of fuel (primary products and butadiene) is reduced and the formation of char impedes the transfer of heat to the polymer. Unfortunately, these benefits are offset by the formation of new and very toxic products. A further illustration of the effect of APP as a fire retardant is to be found in studies of the behaviour with poly(ethylene glycolj.P" In this case the APP transforms the predominantly radical process in the pure polymer into a hydrolytic reaction leading to different products. 15.13.3 Polypropylene with Chlorinated n-Alkanes The degradation of PP has been studied in the presence of Cereclor 70, a commercial chloroparaffin (CP, chloro-n-alkanes) fire retardant containing 700/0 chlorine.l ": 1S6 Unlike the situations above, where the temperature regions of decomposition of polymer and APP show overlap, PP and CP degrade under programmed heating conditions at very different temperatures, as may be seen from Figure 23.
Thermal Degradation
497
c:
-Bo .g
0.8
~ 0.6
'i) ~
-
e
0.4
~
"C
.c;;
~ 0.2
O------r..~--..........---"""'-------I------:l~ 200 300 400 500 Temperature (Oe)
Figure 23 TG curves (dynamic N 2, 5 °Cmin -1) for (1) polypropylene, (2) chloroparaffin fire retardant, (3) PP plus 30% CP by weight, and (4) calculated behaviour for the same mixture assuming no interaction (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1981, 3, 423)
One view of the effectivenessof fire retardants is the 'right place at the right time' concept, i.e. that both polymer and fire retardant should be decomposing simultaneously. On this basis, CP would not be predicted to be an effectivefire retardant for PP, since the decomposition of the fire retardant would be complete before the PP begins to decompose to produce the fuel for burning to occur. This proves not to be the case, however, as the TG curves reveal some form of interaction, since the behaviour of the mixture is not simply an addition of the weight losses of the components. The nature of the interaction has been established by EGA and MW measurements. It has been found that at 260-280°C the CP dehydrochlorination rate is reduced in the mixture and there is a substantial increase in chain scission in the PP. This type of behaviour is also observed in PVC/PS blends and the interaction in the PP/CP mixture has been explained in the same way. The physical state of the mixture is heterogeneous. The CP component produces Cl- radicals as chain carriers in the dehydrochlorination; some of these migrate into the PP phase where they abstract tertiary H atoms and so induce chain scission. This leads to an alteration in the product distribution from the PP in such a way as to increase the proportion of materials of higher limiting oxygen index, .so reducing flammability. The loss of Cl- chain carriers stabilizes the CP and the stability of the PP is reduced, so that in the mixture the PP produces volatile products in a temperature region where the CP flame-quenching effect (due to HCl) is effective. As in the other systems already considered, condensed phase interactions determine the effectiveness of the fire retardant.
15.13.4 Other Fire Retardant Additive Systems The dehydration reaction in aluminum oxide trihydrate is strongly exothermic (1170 J g- 1). Since alumina is inert towards polymers, it is probable that the effect of this very widely used fire retardant additive is simply to remove heat. Boron compounds used as fire retardants may also owe much of their effectiveness to such 'heat sink' effects due to dehydration or, for ammonium salts, loss of ammonia. A second mode of action may be connected with the formation of a glassy layer at the surface. Boron compounds, however, cannot be regarded as inert, since there is evidence of borate ester formation with cellulose, for example.P! The ultimate effect is increased char formation, which impedes heat transfer. Antimony compounds are generally used as fire retardants in synergistic combinations with halogen compounds. The mechanism of action is not well understood, but may be associated with effects due to the release of the hydrogen halide, acting possibly both in the condensed phase to modify the decomposition of the polymer and in the gas phase as a flame quencher.
15.14 REFERENCES 1. N. Grassie, 'Chemistry of High Polymer Degradation Processes', Butterworths, London, 1956. 2. H. H. G. Jellinek, 'Degradation of Vinyl Polymers', Academic Press, New York, 1956. 3. S. L. Madorsky, 'Thermal Degradation of Organic Polymers', Interscience, New York, 1964. 4. R. T. Conley, 'Thermal Stability of Polymers', Dekker, New York, 1970. 5. C. David, in 'Comprehensive Chemical Kinetics', ed. C. H. Bamford and C. F. H. Tipper, Elsevier, Amsterdam, 1975, vol. 14, p. 1.
498 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76.
Thermal Degradation N. Grassie and G. Scott, 'Polymer Degradation and Stabilization', Cambride University Press, Cambridge, 1985. W. Schnabel, 'Polymer Degradation: Principles and Practical Applications', Hanser, New York, 1981. T Kelen, 'Polymer Degradation', van Nostrand Reinhold, New York, 1983. I. C. McNeill, J. Polyrn. Sci., Part A-I, 1966, 4, 2479. I. C. McNeill, Eur. Polym. J., 1967, 3, 409. I. C. McNeill and D. Neil, in 'Thermal Analysis', eds. R. F. Schwenker and P. D. Garn, Academic Press, New York, 1969, p.353. D. L. Gardner and I. C. McNeill, J. Thermal Anal., 1969, 1, 389. I. C. McNeill, Eur. Polym. J., 1970, 6, 373. I. C. McNeill, L. Ackerman, S. N. Gupta, M. Zulfiqar and S. Zulfiqar, J. Polym. Sci., Polyrn. Chern. Ed., 1977,15,2381. I. C. McNeill, in 'Developments in Polymer Degradation', ed. N. Grassie, Applied Science, London, 1977, vol. 1, p. 43. N. Grassie and J. N. Hay, Makromol. Chem., 1963, 64, 82. I. C. McNeill and D. Neil, Eur. Polym. J., 1971, 7, 115. A. 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Grassie, Applied Science, London, 1982, vol. 4, p. 27. I. C. McNeill, L. Ackerman and S. N. Gupta, J. Polym. Sci., Polyrn. Chern. Ed., 1978, 16, 2169. I. C. McNeill and S. N. Gupta, Polym. Degradation Stab., 1980, 2, 95. I. C. McNeill and W. T. K. Stevenson, Polym. Degradation Stab., 1985,11, 123. N. Grassie and I. C. McNeill, J. Polym. Sci., 1958, 27, 207. N. Grassie and I. C. McNeill, J. Chem. Soc., 1956,3929. N. Grassie and I. C. McNeill, J. Polym. Sci., 1958, 30, 37.
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499
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Chem., 1966, 99, 59; 1967, 108, 241. 99. B. Dodson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1974, 12, 2305. 100. H. E. Parker, in 'Treatise on Coatings', eds. R. R. Myers and R. S. Long, Dekker, New York, 1967, p. 129. 101. G. M. Burnett and R. A. Haldon, Eur. Polym. J., 1967, 3, 449. 102. D. L. Gardner and I. C. McNeill, Eur. Polym. J., 1971, 7, 569, 593, 603. 103. L. A. Wall, 'Fluoropolymers', Wiley-Interscience, New York, 1972. 104. J. P. Critchley, G. J. Knight and W. W. Wright, 'Heat Resistant Polymers: Technologically Useful Materials', Plenum Press, New York, 1983. 105. P. K. Dhal, G. N. Babu and J. C. W. Chien, Polym. Degradation Stab., 1986,16,135. 106. N. Grassie, Trans. Faraday Soc., 1952, 48,379; 1953, 49, 835. 107. A. Servotte and V. Desreux, J. Polym. Sci., Part C, 1968, 22, 367. 108. A. Jamieson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1976, 14, 1839. 109. A. Jamieson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1976, 14, 603. 110. S. Zulfiqar, M. Zulfiqar, T. Kausar and I. C. McNeill, Polym. Degradation Stab., 1987, 17, 327. 111. N. Grassie and B. 1. D. Torrance, J. Polym. Sci., Part A-I, 1968,6, 3303, 3315. 112. N. Grassie and 1. D. Fortune, Makromol. Chem., 1972,162,93; 1973,168,1,13; 1973,169,117. 113. N. Grassie and E. Farish, Eur. Polym. J., 1967, 3, 305. 114. N. Grassie and E. M. Grant, Eur. Polym. L, 1966,2,255. 115. N. Grassie and H. W. Melville, Proc. R. Soc. London, Ser. A, 1949,199,39. 116. I. C. McNeill and T. Straiton, Eur. Polym. J., 1979, 15, 1043. 117. I. C. McNeill, A. Jamieson, D. J. Tosh and J. J. McClune, Eur. Polym. J., 1976, 12, 305. 118. I. C. McNeill and D. Neil, Eur. Polym. J., 1970,6, 143, 569. 119. N. Grassie and D. R. Bain, J. Polym. Sci., Part A-I, 1970, 8, 2653, 2665, 2679. 120. I. C. McNeill, D. Neil, A. Guyot, M. Bert and A. Michel, Eur. Polym. J., 1971, 7, 453. 121. G. M. Burnett and R. A. Haldon, Eur. Polym. J., 1968,4,83. 122. R. A. Haldon and 1. N. Hay, J. Polym. Sci., Part A-I, 1968, 6, 951. 123. N. Grassie, I. F. McLaren and I. C. McNeill, Eur. Polym. J., 1970, 6, 679, 865, 1437. 124. I. C. McNeill, in 'Developments in Polymer Degradation', ed. N. Grassie, Elsevier Applied Science, London, 1987, vol. 7, p. 1. 125. I. C. McNeill and M. Barbour, J. Anal. Appl. Pyrolysis, 1987,11, 163. 126. I. C. McNeill, in 'Developments in Polymer Degradation', ed. N. Grassie, Applied Science, London, vol. 1, p. 171. 127. I. C. McNeill, N. Grassie, 1. N. R. Samson, A. Jamieson and T. Straiton, J. Macromol. Sci., Chem., 1978, 12,503. 128. B. Dodson and I. C. McNeill, J. Polym. Sci., Part A-I, 1976, 14, 353. 129. D. H. Richards and D. A. Salter, Polymer, 1967, 8, 127. 130. L.-P. Blanchard, V. Hornof, Hong-ha Lam and S. Malhotra, Eur. Polym. J., 1974, 10, 1057. 131. I. C. McNeill, L. Ackerman and S. N. Gupta, J. Polym. Sci., Polym. Chern. Ed., 1978,16,2169. 132. I. C. McNeill and S. N. Gupta, Polym. Degradation Stab., 1980, 2, 95. 133. I. C. McNeill and W. T. K. Stevenson, Polym. Degradation Stab., 1985, 10, 211. 134. I. C. McNeill and H..A. Leiper, Polym. Degradation Stab., 1985, 11, 267, 309. 135. A. Rincon and I. C. McNeill, Polym. Degradation Stab., 1987, 18, 99. 136. I. C. McNeill and W. T. K. Stevenson, Polym. Degradation Stab., 1985, 10, 319. 137. A. Guyot, M. Bert, A. Michel and I. C. McNeill, Eur. Polym. J., 1971, 7, 471. 138. N. Grassie and R. S. Roche, Makromol. Chem., 1968,112, 16. 139. N. Grassie and G. A. Perdomo Mendoza, Polym. Degradation Stab., 1984, 9, 155. 140. I. C. McNeill and H. A. Leiper, Polym. Degradation Stab., 1985,12, 373. 141. I. Liiderwald, in 'Developments in Polymer Degradation', ed. N. Grassie, Applied Science, London, 1979, vol. 2, p. 77. 142. D. Garozzo, G. Montaudo and M. Giuffrida, Polym. Degradation Stab., 1986, 15, 143. 143. H. Diebig, J. Geiger and M. Sander, Makromol. Chem., 1971, 145, 133. 144. G. J. Sutton, B. J. Tighe and M. Roberts, J. Polym. Sci., Polym. Chern. Ed., 1973, 11, 1079.
500
Thermal Degradation
145. I. C. McNeill and M. L. Perreira Leao, Polym. Degradation Stab., 1985,15,345. 146. H. Zimmermann, in 'Developments in Polymer Degradation', ed. N. Grassie, Applied Science, London, 1984, vol. 5, p.79. 147. A. Davis and 1. H. Golden, J. Macromol. Sci., Rev. Macrornol. Chern., 1969,3,49. 148. N. Grassie and I. G. MacFarlane, Eur. Polym. J., 1978,14,875. 149. G. Camino, L. Costa and L. Trossarelli, Polym. Degradation Stab., 1983, 5, 161. 150. A. Tkac, in 'Developments in Polymer Stabilization', ed. G. Scott, Applied Science, London, 1982, vol. 5, p. 153. 151. M. M. Hirschler, in 'Developments in Polymer Stabilization', ed. G. Scott, Applied Science, London, 1982,vol. 5, p. 107. 152. C. F. Cullis and M. M. Hirschler, 'The Combustion of Organic Polymers', Clarendon Press, Oxford, 1981. 153. N. Grassie and M. Zulfiqar, in 'Developments in Polymer Stabilization', ed. G. Scott, Applied Science, London, 1979, vol. 1, p. 197. 154. N. Grassie and G. A. Perdomo Mendoza, Polym. Degradation Stab., 1985, 10, 43. 155. G. Camino and L. Costa, Polym. Degradation Stab., 1981, 3, 43; 1982,4, 167. 156. G. Camino, L. Costa and L. Trossarelli, Polym. Degradation Stab., 1982,4, 39, 133.
15.1 THE STUDY OF THERMAL DEGRADATION
15.1.1 15.1.2 15.1.3 15.1.4 15.1.5
452 453 453 455 456
Introduction Literature Sources Methods of Study Pitfalls in Experimental Studies Kinetics of Degradation
15.2 PRIMARY DECOMPOSITION PROCESSES IN ADDITION POLYMERS
15.2.1 Depolymerization 15.2.2 Elimination 15.2.3 Cyclization 15.3 ACRYLATE AND METHACRYLATE POLYMERS
15.3.1 15.3.2 15.3.3 15.3.4 15.3.5
458 458 459 459 461 462
Poly(methyl methacrylate) Higher Polymethacrylates Polyacrylates Poly(methacrylic acid) and Polymethacrylamide Salts of Poly (methacrylic acid)
15.4 POLYSTYRENE AND RELATED POLYMERS
15.4.1 15.4.2 15.4.3 15.4.4 15.4.5 15.4.6
456 456 457 457
462
Polystyrene Chlorinated Polystyrenes Polystyrenes with 0- and Nscontaining Ring Substituents Poly (m-methylstyrene) Crosslinked Styrene Polymers Polyt a-Methylstyrene}
462 465 465 465 465 466 467
15.5 POLYALKENE AND DIENE POLYMERS
467 470
Polyethylene, Polypropylene and Polyisobutylene 15.5.2 Polyisoprene and Polybutadiene
15.5.1
15.6 NITRILE POLYMERS
471
15.6.1 a-Substituted Acrylonitrile Polymers 15.6.2 Polyacrylonitrile
471 472
15.7 CHLORINE AND BROMINE-CONTAINING POLYMERS
474
15.7.1 Poly (vinyl chloride) 15.7.1.1 Introduction 15.7.1.2 Observed characteristics 15.7.1.3 Initiation of dehydrochlorination 15.7.1.4 Mechanism of dehydrochlorinaion 15.7.1.5 Secondary reactions 15.7.2 Poly(vinyl bromide) 15.7.3 Chlorinated Rubber 15.7.4 Poly(vinylidene chloride) 15.7.5 Polych10 roprene
474 474 474 475 475 476 477 477 477 478
15.8 FLUORINE-CONTAINING POLYMERS
478
15.8.1 15.8.2 15.8.3 15.8.4 15.8.5
478 479 479 480 480
Introduction Perfluoroalkane Polymers Chlorofluoroalkane Polymers Hydrofluoroalkane Polymers Fluoromethacrylate Polymers
451
452
Thermal Degradation
15.9 POLY (VINYL ACETATE) AND RELATED POLYMERS
15.9.1 Poly(vinyl acetate) and Ethylene/Vinyl Acetate Copolymers 15.9.2 Poly (vinyl formate) and Poly(vinyl alcohol) 15.10 RANDOM COPOLYMERS
15.10.1 15.10.2 15.10.3 15.10.4
Methyl Methacrylate Copolymers Styrene Copolymers Vinyl Chloride Copolymers Ionomers
15.11 BINARY POLYBLENDS AND RELATED SYSTEMS
15.11.1 Binary Polyblends 15.11.1.1 Interactions due to small radical migration 15.11.1.2 Interactions due to small molecule migration 15.11.1.3 Other interactions 15.11.2 Block and Graft Copolymers 15.12 POLYMERS WITH HETEROATOMS IN THE BACKBONE 15.12.1 Aliphatic Polyethers and Polyesters 15.12.2 Poly (ethylene terephthalate) 15.12.3 Polycarbonates 15.12.4 Polysiloxanes 15.12.5 Other Polymers 15.13 DEGRADATION OF POLYMERS IN THE PRESENCE OF FIRE RETARDANTS
15.13.1 15.13.2 15.13.3 15.13.4
Poly(methyl methacrylate) with Ammonium Polyphosphate Polyurethane with Ammonium Polyphosphate Polypropylene with Chlorinated n-Alkanes Other Fire Retardant Additive Systems
15.14 REFERENCES
15.1 15.1.1
480 480 482 482 482 483 485 485 486 486 486 488 489 490 490 490 491 492 494 494 495 495 495 496 497 497
THE STUDY OF THERMAL DEGRADATION Introduction
Thermal degradation begins to be important when polymers are processed and fabricated for use. The threshold temperature for breakdown determines the upper limit of temperature in fabrication and the volatile products of degradation must be known in order to guarantee the safety of workers. These same properties are relevant to the potential use of a polymeric material and, in the extreme case, to the hazards presented by the polymer in a fire situation. However, there are also some useful aspects of thermal degradation. Examination of the breakdown products from polymers is increasingly used as a method of analysis. In a limited number of cases, controlled thermal degradation is important in recycling of plastics waste and in the special case of carbon fibre production, degradative reactions are an essential part of the manufacturing process. A detailed understanding of how polymers break down on heating is important in the design of materials with improved properties for particular applications. Over the past 40 years, a considerable body of knowledge has built up about the degradation of polymeric materials, extending from pure homopolymers of simple structure to complex materials such as random copolymers, block and graft copolymers, ionomers, polymer blends and polymer-plus-additive systems such as fire retardant composition. Patterns of behaviour have emerged and the approach adopted in this chapter is to try to identify these and relate stability and breakdown mechanism as far as possible to chemical structure. It might appear that it should be possible, to a large extent, to deduce the stability and mode of breakdown of a polymer by reference to bond energies and the pyrolysis behaviour of related structures in small molecules. In practice such reasoning often leads to erroneous conclusions because it fails to take account of the special long chain character of polymer molecules, which provides possibilities for unique types of reaction, proceeding along the chain or between adjacent monomer units in the chain, and because it does not recognize the importance of low concentrations of structural irregularities, which often determine the initiation of the degradation process.
Thermal Degradation
453
15.1.2 Literature Sources Early monographs on the degradation of polymers 1- 3 are still of interest, although now considerably out of date. Useful detailed treatments have been given within the past twenty yearst:" and briefer accounts appear in three modern texts." - 8 For current awareness, in addition to the main polymer journals, the reader is referred to the series Developments in Polymer Degradation, which contains review articles in particular subject areas and to the specialist journal Polymer Degradation and Stability.
15.1.3 Methods of Study When a polymer breaks down, there are four main product fractions. A complete understanding of the degradation process requires information about all four fractions. These are (a) permanent gases, such as hydrogen, methane and carbon monoxide, which cannot be trapped out in a simple trap at -196°C; (b) condensable gases and liquids, volatile at room temperature but not at -196 °C; (c) the tar and wax fraction of products, such as the dimer, trimer and other low volatility materials, which are volatile at degradation temperature but not at room temperature; and (d) the solid involatile residue consisting of partially degraded polymer or product depending on the temperature. The choice of method of study depends very much upon the objectives, which may be to establish one or more of the following: threshold temperature for breakdown, nature of the evolved volatile products, production of a convenient 'fingerprint' for identification or purity control, establishment of kinetics, or ultimate understanding of the mechanism of decomposition. For some purposes, heating in air may be satisfactory, but forfundamental mechanistic studies, it is customary to reduce the complexity of the problem by use of an inert atmosphere or vacuum. Probably the most widely used method is thermogravimetry (TG), in which the polymer sample is heated in a thermobalance and the weight is followed as a function of temperature or time. The information available consists of (a) the number of stages of breakdown, (b) a quantitative measure of the weight loss in any stage, and (c) the threshold temperatures and temperatures of maximum rate of weight loss for the processes occurring. The major weakness of TG as a tool for studying polymer breakdown is that it provides no information about the degradation products, which in most designs of thermobalance are also not accessible for study by other techniques. Less useful for 'degradation studies than TG are differential thermal analysis (DT A) and differential scanning calorimetry (DSC), both of which measure effects due to heat evolution or absorption by the polymer as its temperature is raised. DTA and DSC indicate the temperature regions of occurrence of decomposition processes, but do not distinguish these clearly from physical changes in the sample which also involve absorption or evolution of heat. Product analysis is not possible. Possibly the most sophisticated equipment available at the present time for degradation studies consists of the various instruments for pyrolysis of the polymer, often using flash conditions of a few seconds at a high temperature, followed by analysis of volatile products by GC, MS or, most recently, FTIR spectroscopy. These contrast with TG, DTA and DSC in providing no direct information about threshold temperatures for breakdown or number of stages involved, but give considerable data on the products. The latter may vary, however, depending on the choice of temperature. These pyrolysis methods are particularly useful in applications requiring 'fingerprinting' or the ability to detect small amounts of particular products, e.g. toxic compounds. They require use with caution in mechanistic studies, because not all of the product fractions may be accessible and the temperatures involved may fragment the polymer so extensively that the important initial degradation reactions may be obscured. An example of pyrolysis-MS data is given in Figure 1. In the methods considered so far, the products of degradation are not collected. A third type of experimental approach widely applied to polymer degradation studies, in which the products are normally collected, is evolved gas analysis (EGA). Some studies of this type depend on measurement of a single degradation product. Much of the work on poly(vinyl chloride) (PVC), for example, has been based on estimation of the HCl evolved on heating. A much more generally applicable approach of this type, used in very many investigations of degradation mechanism is thermal volatilization analysis (TVA).9-15 This technique depends on recording (as a function of temperature or time) the small pressure which develops in a continuously evacuated system when the polymer sample degrades to volatile products. The TVA curve obtained resembles a derivative TG
454
Thermal Degradation (0) 10
32 44
5
(b) ~
0
~ c:
'(ji
30
Q)
+.~
42
10
58
5
87
~
'5
233
Q) Q::
10
(e)
32 5
71
42 215 40
60
80
100
120
140
160
180
200
220
.240
260
m/z
Figure 1 Typical pyrolysis mass spectra for (a) poly(ethylene glycol), MW 1000; (b) poly(propylene glycol), MW 4000; (c) poly(tetramethylene glycol), MW 2100. Pyrolysis conditions: 10 s at 610 DC (reproduced by permission of the American Chemical Society from Macromolecules, 1985, 18, 496)
(0 )
! .g
(b)
~ ::J
0
·2
e
a:
(c)
150
400
Figure 2 TVA curves for degradation under vacuum at 10 "Crnin -1 of chain-chlorinated polystyrenes containing (a) 0.54, (b) 0.85 and (c) 1.35 CI atoms per styrene unit. Trap temperatures: - - ODC and -45 DC, ------ -75 DC, - - -100 DC. The shaded areas represent HCI production, the remainder being due mainly to styrene (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1987, 18, 213)
Thermal Degradation
455
curve, because the pressure is a measure of the relative rate of volatilization of the sample. By means of a system with various cold traps at temperatures from 0 °C to -196°C preceding Pirani pressure gauges a considerable amount of information about product volatility and changes in product composition during the heating programme may be collected in a single TVA experiment, in addition to data on number of stages of breakdown and their threshold and maximum rate temperatures. The power of the approach is illustrated in the data of Figure 2 for chain-chlorinated polystyrene samples. The TVA technique distinguishes clearly between the two main volatile products, HCl and styrene, and shows how the mode of breakdown varies with the extent of chlorination. A further advantage of the TVA approach is that it is nondestructive of the various product fractions, which are available for further study, e.g. by spectroscopic methods. Thus the collected volatile products may be separated by controlled warming from -196°C to the ambient temperature of the trap containing the products, using the pressure gauges of the TVA system to monitor the volatilization of each substance (subambient TVA).14 The tar/wax products which condense out on the cooled upper part of the TVA sample tube (and are therefore often referred to as the cold ring fraction) may also be removed for investigation. Finally, the involatile residue on the base of the TVA tube can often be examined by various spectroscopic methods and, if it is soluble, the molecular weight may also be measured. The application of more than one experimental approach to any degradation study is very desirable, not only to confirm results, but sometimes also to reveal interesting features. For example, TG measures the loss of both the volatile and cold ring fraction products, whereas TVA pressure gauges register only the volatile fraction. A degradation process showing extensive weight loss by TG, but little volatile product evolution by TVA, must therefore be one giving mainly cold ring fraction products.
15.1.4 Pitfalls in Experimental Studies Failure to recognize and control the variables in thermal degradation can lead to difficulties in interpretation of the results. Some of the main problems are noted below. The sample environment may influence behaviour in various ways. Degradation in air will usually, be more complex than in nitrogen due to additional oxidation reactions. An inert atmosphere is preferred to simplify interpretation of the basic processes induced by heat, but behaviour may differ greatly between a- static and a dynamic atmosphere, since if products are not quickly removed from the degrading sample, secondary reactions may occur. The most efficient removal is under continuous evacuation. Sample size, or in practice thickness, is sometimes very important and this possible variable is often overlooked. Its importance is illustrated in Figure 3 for two familiar polymers. Thick samples of PVC degrade more rapidly than thin samples even under vacuum because the H'Cl released (which is less rapidly removed) is a very effectivecatalyst for the dehydrochlorination reaction. In the (b)
(0 ) 1.0
~
e
~
e :J
0
·c
e
a::
.gc 0.8 0
~
.... s:
.~0.6 ~
"0 :J "0
.~ 0.4
0:::
0.2
Figure 3 Effect of sample thickness on degradation behaviour: (a) TVA curves (vacuum, 10°Cmin- 1 ) for 20mg PVC, - - as powder or thick film, ----- as thin film of 2.5 mg cm (b) TG curves (dynamic N 2 , 10°Cmin- 1 ) for PAN; (1) 1 mg, (2) 2 mg, -(3) 5 mg, and (4) 10 mg (reproduced from Eur. Polym. J., 1970, 6, 143, 1277) t
':
Thermal Degradation
456
case of polyacrylonitrile (PAN) the cyclization reaction of the nitrile groups which occurs between 250 and 300°C (10 °C min - 1 heating rate) is extremely exothermic and causes some fragmentation of the chain due to rapid heat build up in the sample, since the heat cannot be dissipated sufficiently rapidly. The extent of overheating and fragmentation increases with sample thickness, as the results for weight loss indicate. Many polymers break down in successive stages as the temperature is raised. For example, a side group elimination reaction will normally be followed at higher temperatures by fragmentation of the residual, modified polymer chain. The observed behaviour may then depend on whether the temperature is raised gradually or rapidly. Degradation reactions are often initiated or strongly influenced by impurities in the sample, especially if these are incorporated into the polymer chain. The importance of pure, well characterized samples in any mechanistic study cannot be overemphasized. No matter how sophisticated the degradation equipment used, the results are only as good as the purity of the sample or design of the experiment will allow.
15.1.5 Kinetics of Degradation Studies of reactions of small molecules have been greatly facilitated by careful kinetic measurements and the calculation of rate constants and activation energies. It is therefore not surprising that much effort has been devoted to trying to apply the same approach to polymer degradation. Many activation energies have been reported, but the wide divergence in results by different workers suggests that the approach is of limited value. There are several reasons for this. Firstly, the physical state of the sample is far from ideal (usually a viscous liquid and in some situations a solid) and may change as the reaction proceeds. Efficient removal of products is also difficult under some experimental conditions. Secondly, degradation reactions are often complex, involving several stages of reaction which may not be well separated in temperature range. Assumptions about reaction order and constancy of activation energy over a particular temperature range may therefore be questionable. Finally, many kinetic treatments of thermal degradation assume that the residual weight of the sample may be handled as if it were a concentration, which is of doubtful validity. The optimum experimental situation for kinetic measurements of polymer decomposition' involves a very thinly distributed sample, rapid temperature equilibration, mild isothermal conditions spanning a temperature range over which the product composition can be shown to be constant, efficient product removal and estimation of an evolved degradation product as a means of establishing the initial rate at each temperature. Even if these criteria are satisfied, there remains the possibility of misleading results if the same degradation product can be formed by more than one mechanism.
15.2 PRIMARY DECOMPOSITON PROCESS IN ADDITION POLYMERS Before consideration of the degradation behaviour of the various addition polymers in groups based upon their chemical structure, it is useful to indicate the three general types of primary thermal decomposition process which are observed, i.e. depolymerization, elimination and cyclization.
15.2.1
Depolymerization
The term depolymerization is used in this context to include all situations in which reduction of macromolecular size occurs without change of chemical composition or alteration of the monomer unit structure. In the absence of the reactions considered in Sections 15.2.2 and 15.2.3,homolysis of the polymer chain may occur as in Scheme 1. Subsequent behaviour then depends on the nature of the chain substituents X and Y. There are three situations: (a) X = Y = H, no monomer produced, extensive transfer by H abstraction [e.g. polyethylene (PE)]; (b) X = H, amount of monomer depends on nature of Y, extensive transfer by H abstraction [e.g. polypropylene (PP), polystyrene (PS), poly(methyl acrylate) (PMA)]; (c) X, Y # H, large amount of monomer (up to 100%) transfer present or absent depending on nature of X, Y, e.g. poly(methyl methacrylate) (PMMA), poly(ex-methyl styrene) (PAMS). Monomer results by depropagation from the polymer radical chain end produced on homolysis (Scheme 2).
457
Thermal Degradation
x
X
I I IWVCH 2CCH 2CM/\ ---I~. yI yI
X
MILCH). I
y
Scheme 1 Chain homolysis by random scission
Scheme 2
Monomer formation by depropagation
Transfer may be intramolecular ('backbiting') giving small fragments such as dimer, trimer, etc., or intermolecular, producing a macro radical in which the radical centre is not at the chain end; this undergoes scission, so that intermolecular transfer leads to a rapid drop in the molecular weight (MW) of the polymer. All the products of this type of degradation, whether monomer, dimer, etc., or lower MW polymer chains, have essentially the same chemical composition as the repeat structure. Homolysis of the C-C backbone, in the absence of structural abnormalities which constitute weak links, usually requires a temperature of above 250°C. The following processes, however, are commonly observed at somewhat (and in some cases considerably) lower temperatures.
15.2.2 Elimination If one of the bonds within a chain substituent group, or that attaching it to the backbone, is weaker than the C-C backbone, then an elimination reaction may occur at lower temperatures than depolymerization, or simultaneously, giving a product and a residual chain structure which differ in composition and structure from the repeat unit. For example, PVC loses HCI and poly(isopropyl acrylate) loses propylene, as shown in Scheme 3. If such a process occurs, then depolymerization is either prevented or impeded. Further decomposition of the residual, modified polymer takes place at higher temperatures, generally 400-500°C. vwCH 2 CHUlIfJo
b
----1~~
+ HCI
uwCH==CHMA
vwCH 2CHvw -----t~.
Scheme 3
I
..o-C
O~'OH
+
CH 2==CHMe
Elimination reactions
15.2.3 Cyclization A macromolecular chain presents a unique reaction situation in which, in certain cases, groups which are potential reactants at elevated temperature are located in close proximity. Thus intramolecular cyclization may occur, which mayor may not involve elimination of small molecules. For example, vinyl ketone polymers such as poly(methyl vinyl ketone) (PMVK), poly(methyl isopropenyl ketone) (PMIK) and poly(phenyl vinyl ketone) undergo random cyclization of adjacent monomer units with release of water. 16, 17 The reaction for PMVK is illustrated in Scheme 4.
XXX
H2
~
VWt.C
0
#Me(f
Scheme 4
---I~.
Me
Cyclization in PMVK
vw\'CH1XXX~
o
~
~ Me
+
Thermal Degradation
458
Such cyclization reactions give products of composition different from that of the polymer if a small molecule is released (as with PMVK), but in other cases (e.g. some nitrile and diene polymers), the change involves only structural rearrangement. Decomposition of the cyclized structures occurs subsequently, if the temperature is raised.
15.3 ACRYLATE AND METHACRYLATE POLYMERS 15.3.1 Poly(methyl methacrylate) The thermal degradation of PMMA is essentially a very simple reverse of its polymerization, leading to monomer in 100% yield. After homolysis of the chain, the macroradicals depropagate (Scheme 2) with a very long zip length, so that chains of sufficiently low degree of polymerization unzip completely." The degradation behaviour of PMMA, however, is influenced to a far greater extent than for most polymers by the mode of polymerization and the presence, even in very small amounts (e.g. 1%), of comonomer units in the chain. In PMMA produced by free radical polymerization, unsaturated chain ends are found to constitute weak sites at which degradation may be initiated (Scheme 5). Programmed heating experiments by TVA illustrate very clearly the contribution of end initiation in PMMA degradation (Figure 4a). In these curves, each peak is due to monomer production but the lower temperature peak is due to initiation at the unsaturated chain ends, whereas the second (and in most samples the main) peak is due to initiation by random scission of the backbone. The relative sizes of the two peaks are dependent on molecular weight. For a very low MW polymer, in which the zip length exceeds the chain length, the chains with unsaturated ends disappear completely in the first stage and since these chains comprise about 50% of the sample, the two peaks are of similar size. In samples of higher MW, however, initiation at unsaturated ends does not lead to complete unzipping of these chains, so that as the proportion of ends per unit weight of sample decreases (MW becoming larger), so does the size of the first TVA peak. In PMMA samples made by the anionic route, in which all the chain ends are saturated, degradation is initiated only by random scission and so the early TVA peak, with maximum at about 290-300°C (Figure 4a) is not observed (Figure 4b). The sensitivity of PMMA stability to various foreign units in the chain is due to several effects. Firstly, there may be blocking of the long unzipping reaction at the foreign unit, which reduces the Me
I
Me
I
Me
CH 2
II
AM.CH.2TCH2--C-CH21
-
Me
CH 2
-cH2~
+
oAMCH2{CH2-t. C0 2Me t0 2Me
C0 2Me t0 2Me C0 2Me
C0 2Me
L
depropagation
Scheme 5 End-initiated depolymerization of PMMA to monomer
(Q)
(b)
.r,'\."' \\ O V
I II o
. I I
.,I
))
,..,/'I.
-c-,
/
\
\
0\ \ 0
'
.'
\
,
\\ \,.\ \\
Figure 4 TVA curves for degradation under vacuum at 10°Cmin- 1 of (a) free radical PMMA samples, Mn : - - 20000, ------ 100000, .. , ... 480000, (b) anionic PMMA samples, Mn : - - - 60000, - ' - ' - 1500000 (reproduced from Eur. Polym. J., 1968,4,21)
Thermal Degradation
459
zip length of the monomer-producing reaction and so stabilizes the polymer. Some commercial PMMA samples are stabilized in this way by copolymerizing small amounts of ethyl acrylate or methacrylic acid. Secondly, there may be interunit reaction between the comonomer unit and the adjacent methyl methacrylate (MMA) chain unit. As well as leading to additional volatile products, this type of reaction may produce structures which block unzipping. Thirdly, certain foreign units in the PMMA chain (e.g. acrylonitrile) lead to backbone scission. Some small molecules exert a profound influence on the mode of decomposition of PMMA. Silver acetate has been found to greatly accelerate depolymerization; zinc bromide changes the entire character of the degradation reaction, with the formation of various volatile products, notably methyl bromide.l'': 19 The fire retardant additive ammoniumpolyphosphate acts by reaction with the polymer so as to interfere with the monomer-producing reaction.'?
15.3.2 Higher Polymethacrylates Although several other methacrylate ester polymers resemble PMMA in degrading thermally to give 100% monomer, this behaviour is by no means general for this class of polymers and some polymethacrylates, in contrast, give very little 'monomer. The four isomeric poly(butyl methacrylates) illustrate very well the complete spectrum of behaviour, comprising 100% depolymerization to monomer in the case of the isobutyl ester, very high monomer yield plus a little alkene for the n-butyl ester, high alkene yield plus some monomer for the s-butyl ester and very high alkene yield with very little monomer for the t-butyl ester.:" The explanation for these differences is that in some of the polymers, favourable conditions exist for a non-radical side group ester decomposition reaction to occur, in competition with free radical depolymerization to monomer, leading to alkene and carboxylic acid. The former process is favoured in cases where a six-membered ring transition state involving f3 hydrogen atoms in the ester alkyl group can be formed. This is illustrated in Scheme 6 for poly(s-butyl methacrylate). Elimination of the alkene leaves a methacrylic acid unit in the polymer chain. Me
Me
I V\I\ICH 2 Cvw I
VW
'<,
CH 2
Scheme 6
ItIW
2T
+
CH 2=CHEt
C
~~
t> .v--, d
CH
o/~
I
H
Et
0
H
Ester decomposition in poly(sec-butyl methacrylate)
The ease of occurrence of the ester decomposition depends on the number of f3 hydrogens available in the alkyl group, which in the above examples is 1 (iso), 2 (n), 5 (s) and 9 (t) respectively, so explaining the observed trend of behaviour. The behaviour of many other methacrylate ester polymers can be rationalized in the same way. . Some methacrylate polymers in which the ester group is substituted by halogens have recently been studied. Poly(2-bromoethyl methacrylate) behaves predominantly as would be expected from the above discussion, giving a high yield of 2-bromoethyl methacrylate (2-BEMA) monomer, although some ester decomposition also occurs.P Poly(2,3-dibromopropyl methacrylate) gives a much more complex pattern of degradation, in which reactions involving C-Br scission compete with the main reaction of depolymerization to monomer.P The behaviour of several fluorinecontaining polymethacrylates is considered in Section 15.8.5.
15.3.3 Polyacrylates Despite the close similarity in structure, the polyacrylates differ greatly in degradation behaviour from the polymethacrylates. Crosslinking, yellowing of the polymer, formation of a high proportion of short chain fragments and evolution of the alcohol and carbon dioxide are characteristics of the breakdown of many polyacrylate esters which are not found in the corresponding methacrylate polymers. Furthermore, although some of the latter give large yields of monomer, the monomer yields from the polyacrylates are extremely small, typically about 0.3% of the polymer weight.
Thermal Degradation
460
The polyacrylates have one feature in common with the polymethacrylates, however, in giving alkene formation from the ester side group if Phydrogens are available to form the transition state analogous to that illustrated in Scheme 6. This reaction is more important in polyacrylate degradation because of the absence of unzipping to monomer. PMA has been the subject of several studies. 2 4 - 2 6 During degradation, PMA undergoes both crosslinking and chain scission. Little monomer is produced, the main products consisting of chain fragments. There is some disagreement regarding the yields of the volatile products, methanol and CO 2 , which make up a small fraction of the degradation products. Poly(benzyl acrylate), which also has no alkyl Phydrogens, shows greater similarities in its degradation behaviour to PMA than to the higher alkyl polyacrylates.F' It degrades to give mainly chain fragments, but there is in this case a substantial yield of benzyl alcohol, as well as a small amount of CO 2 , Six polyacrylates with larger alkyl ester groups have been studied in some depth." Four behave in a broadly similar way: these are the ethyl, n-propyl, n-butyl and 2-ethylhexyl ester polymers. Degradation occurs in a single stage and gives increasing yields of chain fragments (15 to 580/0 by weight) with a corresponding decrease in volatile products, which consist (in-order of importance) of alcohol, CO 2 and alkene. Poly(isopropyl acrylate) (PIPA) and poly(t-butyl acrylate) (PTBA) show quite different behaviour. They are less stable, degrade in two stages and give nearly quantitative yields of alkene per acrylate unit, no alcohol, smaller amounts of CO 2 and some water. A mechanism can only be presented with confidence in the case of PIPA and PTBA, for which the nonradical ester decomposition route to alkene and carboxylic acid provides a satisfactory explanation (Scheme 7). There are six and nine Phydrogens respectively in these structures. The initially produced carboxylic acid groups dehydrate to give the six-membered cyclic anhydride structure. Carbon dioxide probably arises either by decarboxylation of some of the C0 2H groups or by decomposition involving the cyclic anhydride. V\J\,CH 2 C H - C H 2 - C H v w
I
I
~c 07" "OH
hC HO'7 ' 0
---.
+ CH 2==C-Me
I
H (Me)
Scheme 7
Ester decomposition in PIPA and PTBA
It is generally accepted that the other polyacrylates have mechanisms of breakdown in which random scission is important (Scheme 8). In contrast to the situation in thecases of PMMA, PS and PAMS, where one of the two macroradicals depolymerizes to monomer, the radicals produced by homolysis in PMA do not behave in this way. It may be assumed that the more reactive primary radical (B) stabilizes itself by intermolecular and intramolecular transfer of a tertiary H atom and radical (A)may also be involved in this way. The product in either case is a tertiary radical (C) which provides a site for various possible reactions, including scission and crosslinking. Scission leads to an unsaturated chain end and radical (A). -----t~...
\MI\CHzCH.
I
C0 2R (A)
+
·CH 2CHCH2CH\IWl
I
I
C0 2R C0 2R
1
(B)
intermolecular transfer of H from PMA molecule
vwCH 2 C\lW'
t0
2R
+ MeCHCH2CH~
t0 t0 2R
2R
(C)
Scheme 8
Random scission and intermolecular transfer in polyacrylate degradation
Thermal Degradation
461
It is particularly difficult to account convincingly for the production of large amounts of the alcohol from these polymers. The high yields from some polyacrylates suggest the possibility of a reaction which can proceed progressively along the chain following some initiation step. The radical (A) seems a likely precursor and several schemes have been proposed on this basis. Whatever the mechanism, there is evidence that several adjacent acrylate units are required for alcohol production. For example, although in poly(ethyl acrylate) (PEA) half the acrylate units degrade to ethanol, in styrene/ethyl acrylate copolymers/? with isolated ethyl acrylate (EA) units, no alcohol is formed. A possible mechanism for alcohol production in polyacrylates is shown in Scheme 9.
~(b) CYclizatio; C -c-, c-, RO/ '0\· r'~o 1"'0 '6R
OR
repeat of (b), (e)
---~~~
Scheme 9
RO·
further ROH
Alcohol production from polyacrylates
The formation of alkene and CO 2 from PEA, poly(propyl acrylate) (PPA), poly(butyl acrylate) (PBA) and poly(2-ethylhexyl acrylate) (P2EHA) can be explained by a side group ester decomposition of radical (C) as in Scheme 10. This mechanism also leads to unsaturation and provides further possibility for crosslinking.
!
(1) H elimination to give unsaturation
or (2) crosslinking or (3) chain scission Scheme 10
Formation of alkene and CO 2 from polyacrylates
15.3.4 Poly(methacrylic acid) and Polymethacrylamide The decomposition of poly(methacrylic acid) is a two-stage process, involving initial dehydration at around 200°C to give the cyclic anhydride (Scheme 11a). At higher temperatures, the polyanhydride breaks down to give various fragments, including as volatile products some CO 2 , CO and traces of alkenes, and leaving a small amount of carbonaceous residue. 30 - 33 The cyclic anhydride structure shown in Scheme 11a plays an important part in a number of degradation situations involving methacrylate polymers, including copolymers, polymer blends and polymer plus additive compositions (e.g. certain fire retardant mixtures). It has characteristic IR absorptions at 1805, 1760 and 1020 em -1 and begins to decompose at about 400°C under programmed heating at 10°C per minute, i.e. somewhat higher than the corresponding ester structure. Since the ring structure blocks the unzipping reaction in the PMMA chain, formation of even a few anhydride rings in the backbone has a marked effect. When subjected to a gradual increase of temperature, polymethacrylamide also breaks down in two stages, the first of which commences at about 200°C and gives mainly ammonia, with some water. The second stage decomposition, above 300°C, leads to extensive fragmentation to give a yellow cold ring fraction; about 200/0 of the original sample remains as a carbonaceous char at 500 °C, considerably more than in the case of the acid polymer. The degradation involves the formation of both imide and anhydride rings, the former being the predominant process (Scheme 11b).34
Thermal Degradation
462 Me
Me
~H ~-CH-~WV I 2 I
(a) ~
2
vwCH
O?"
Me
~-CH -~W\I
2~
2
I
I
+ H 2O
~C~ /C~
~C ~C O'? 'oH 0-7 'oH
Me
Me Me \l\l\J\CH 2'-... I/CH 2, I~ C C
(b) ~
I
~O
0
Me
Me
I C
I C
VWCH2'-...~/CH2'~~ +
NH 3
c? "-NH/ ~
~ '-... ~C, O'? NH O'/" NH 2 2
Scheme 11 Cyclization in poly(methacrylic acid) and polymethacrylamide
Poly(N-methyl methacrylamide) undergoes an analogous cyclization with eliminaton of methylamine.
15.3.5 Salts of Poly(methacrylic acid) Until recently, little was known about the decomposition of this group of ionic polymers. Detailed studies of the ammonium.I" alkali metal." alkaline earth metal " and zinc salts"? have now been carried out, using TVA and other techniques. These form the subject of a recent review.P" Ammonium polymethacrylate shows unique degradation behaviour in this group of materials. It undergoes a two-stage decomposition, in the first stage of which NH 3 and H 20 are eliminated in a cyclization. At higher temperatures cold ring fraction products, isocyanic acid, HCN, CO 2 , CO and CH 4 are formed. The alkali metal polymethacrylates degrade under programmed heating in a single process commencing at about 300 "C. They are therefore more stable than PMMA. The alkaline earth salts show still higher stability and a more complex breakdown. The principal degradation products of the alkali metal polymers appear as a cold ring fraction, but the alkaline earth metal salts give an increased proportion of other more volatile products. In all cases, CO 2 and a complex mixture of carbonyl-containing compounds appear as the volatile fraction. An unusual feature in the case of the alkaline earth polymethacrylates is the presence in the products of up to 60/0 by weight of dimethylketene. The complex degradation behaviour of these various salts can be related to a number of basic processes (Scheme .12). Differences are observed because of variations in the importance of competing reactions. Two key factors are the stability of the metal carbonate and the volatility of the monomer and isobutyrate (which determines whether the latter materials are able to escape from the hot zone without decomposition).
polymer
»>: ~
monomer
+ isobutyrate
carbonate
+ cyclic and acyclic ketones, etc.
oxide
+ CO 2
ca. 450 °C
C (ZnPMA only)
~
~
metal
dimethylketene
+ CO 2
Scheme 12 Degradation reactions in salts of poly(methacrylic acid)
15.4 POLYSTYRENE AND RELATED POLYMERS 15.4.1 Polystyrene Despite the considerable volume of research on polystyrene degradation over 40 years, there remain aspects of the degradation mechanism which are controversial. A number of recent contributions have provided additional information, without fully resolving the conflicting views.38 - 45
Thermal Degradation
463
The degradation behaviour of PS depends critically upon whether the temperature is below or above 300°C. Between 200 and 300 °C the molecular weight falls but no volatile products are evolved. The pattern of the fall in molecular weight as a function of time has led to the conclusion that it is due in anionically prepared PS samples to random scissions of the backbone. The reasoning is as follows. If the average number of scissions per molecule in time tis Sand Po and P, are the chain lengths of the polymer initially and at time t, then so that
r,
Po/(S
S
(PolPt )
+
(1)
1)
(2)
The fraction of bonds broken rx is given by a
=
=
SIP o
(lIP t )
(3)
(lIP o )
-
For random scission, every bond is equally likely to break and ex
=
(4)
kt
Thus a plot of rx VS. t should be linear through the origin. This has been found to be the case for PS made by the anionic route (Figure 5). Samples prepared by free radical polymerization, however, give linear plots which do not pass through the origin. This indicates a relationship (5)
a=p+kt
in which
p represents
a small fraction of weak links initially present in the sample.
10
10
5
5
v
Q )(
o
2
4
6
8
10
0
2
6
4
8
10
Time (h)
Figure 5 Graphs of fraction of bonds broken (a) as a function of time of heating at various indicated temperatures for (a) anionic PS, Mn = 229000, (b) free radical PS, Mn = 1490000 (reproduced from Eur. Polym. J., 1968, 4, 709)
Various suggestions have been made to explain weak link behaviour in PS. Opinion at present tends to favour peroxide linkages as the probable explanation. Random scissions beiow 300°C probably involve initial homolysis (Scheme 13). Since no volatile products result, the radicals A and B produced are believed to undergo a cage disproportionation; ~CH2CHCH2CHCH2CHV\M
I
Ph
I
Ph
I
Ph
(a)
----+
~H2CH·
I
Ph (A)
(b)
+ ·CH 2CHCH2CHv"M ---. MACH 2CH2 + CH 2=CCH2CH
I
I
Ph
Ph (B)
Scheme 13 Random scission and disproportionation (PS) PS 6-P
I
Ph (C)
I
Ph (D)
I
Ph
Thermal Degradation
464
depolymerization does not occur in this temperature region. It follows also that end structures (C) and (D) must be stable up to 300°C. Above 300 "C, PS degrades to give a mixture of products, the major component of which is monomer (yields reported vary between 40 and 60% by weight), with progressively smaller amounts of dimer, trimer, etc., plus small amounts of toluene (2%) and z-methylstyrene (0.50/0). Recent examinations of the cold ring fraction from PS degradation (dimer and larger fragments) indicate clearly the presence of benzylic ends of structure (C) and unsaturated ends of structure (D).42 Deuteration of the tertiary H position in PS gives a polymer which yields a much higher proportion of monomer than PS, whilst in polytc-methylstyrene) the presence of the z-methyl substituent results in 100% monomer production. These facts highlight the importance of transfer at the tertiary H atoms in the formation of less volatile products. Initiation of the production of volatiles from PS above 300 "C can result from homolysis at random or near (C) and (D) type chain ends. The first type has already been shown as reaction (a) in Scheme 13; the other two situations are shown in Scheme 14. IVVl.CH1CHCH 1CH1
I Ph
(a)
I Ph
~
NWCH 2CHCH 2
I Ph
MIl.CH1CHCH1CH==CH 1
I
Ph
.CH 2
I
Ph
(B)
(C)
I
+
(b)
-~~~
(E)
MA.CH1CH
I
Ph
+
·CH 2C=CH 2
I
Ph
(D)
Ph
(A)
(F)
Scheme 14 Scission at chain ends (PS)
Only the former of the macroradicals (A) and (B) is believed to lead directly to production of volatile products (Scheme 15). Monomer production involves depropagation of radical (A) by reaction (a) in Scheme 15, whilst backbiting intramolecular transfer (reactions b, c) account for the dimer, etc. Fragments such as the trimer, etc. can be formed in the same way by H abstraction at subsequent tertiary H sites along the chain. vwCH1CHCH1CHCH1CH
I
I
Ph
I
Ph
(a)
---I~.
vwCH 2CHCH 2 CH
I
Ph
Ph
\MICH2CH+CH 1CCH 2CH 1 I I
I Ph
I Ph
(c)
~ vwoCH1CH +
I
Ph (A)
+
CH 2=CH
I
Ph (A)
(A)
I Ph
I
Ph
monomer
CH 1=CCH1CH 2
I
I
Ph dimer
Ph
Scheme 15 Depropagation and intramolecular transfer (PS)
Radical (A) is thus the common precursor in the formation of all the major volatile products and the constant proportion of monomer to less volatile products which is found is determined by the competition between reactions (a) and (b) in Scheme 15. The zip length of these reactions of radical (A) has recently been estimated't' as about 50 units, which is much less than for the depropagation in PMMA or PAMS. The continued fall in molecular weight as PS degrades may result from further random scission or from intermolecular transfer. The latter process (Scheme 16) is considered to be the more important. Each of the radicals (A), (B), (E) and (F) could participate as R· in H abstraction (reaction a in Scheme 16). The tertiary macroradical (G) which is produced then undergoes scission (reaction b in Scheme 16) at one of the points indicated, to give the depropagating radical (A) and an unsaturated chain end (D). Reaction (a) in Scheme 16 is probably the only major reaction of radical (B), giving a methyl end, whereas if radical (A) participates in this way a benzylic end (C) is formed. The minor products, toluene and z-methylstyrene, result when radicals (E) and (F), produced in reactions (a) and (b) of Scheme 14, abstract a hydrogen atom in reaction (a) of Scheme 16.
Thermal Degradation
465 I
•
I
I
I
I
~H2CH+CH2CCH2+CHCH2CHrvvV
I
Ph
Ph
I
Ph
I
Ph
(G)
Scheme 16 Intermolecular transfer and chain scission (PS)
Many workers have argued that the unsaturated chain ends (D) provide the points at which depolymerization is initiated via reaction (b) of Scheme 14.This view has recently been challenged by others.t' who believe that the formation ofradical (A) by random scission and especially following intermolecular transfer provides the principal route to the formation of volatile products. The relative yields of toluene and «-methylstyrene, and in particular the very small amounts of the latter found in the products, tend to support this view and also to suggest that benzylic ends (C) might in fact be less stable than unsaturated ends (D). In 'PS with only benzylic ends, an increased proportion of toluene in the early stages of degradation has been attributed to additional initiation at these ends. 4 4 , 4 5
15.4.2 Chlorinated Polystyrenes The effect on PS stability and degradation mechanism of different extents and positions of chlorination has been the subject of recent studies.?" Ring-chlorinated (mainly para) PS behaves in most respects like PS, giving a large yield of monomer, but is somewhat less stable; this is due to a small amount of chain chlorination which unavoidably accompanies ring-chlorination. It is possible to chlorinate PS only in the backbone and such polymers lose HCI in the same temperature range as does PVC. Lightly chain-chlorinated PS also degrades to styrene, with lower stability than PS because of the backbone unsaturation resulting from H'Cl loss. More highly chain-chlorinated PS samples give HCI as almost the sole volatile product (see Figure 2), leaving a black poly(phenylacetylene) type material at 300°C.
15.4.3 Polystyrenes with 0 ... and N-containing Ring Stibstituents A series of substituted polystyrenes of this type has been studied.4 7 , 4 8 The p-methoxy-, p-hydroxyand p-amino-styrene polymers all show similarities in degradation behaviour to PS, but the HOand NH 2-containing materials also undergo crosslinking due to transfer at the substituent H atoms. In the Me 2N- and Et 2N-substituted polystyrenes, there are no H atoms available on the N atom for transfer, but the behaviour is complex because of alkyl group migrations and so less monomer is obtained. Very complex behaviour, including crosslinking, is observed in polystyrenes with acetoxy and acetamido substituents.
15.4.4 Poly(m-methylstyrene) The ring methyl substituent has little effect on the stability. This polymer gives comparable amounts of monomer and larger volatile fragments to PS. Xylene is a significant product of degradation. between 300 and 400°C. 4 9
15.4.5 Crosslinked Styrene Polymers The behaviour of copolymers of styrene with divinylbenzene (DVB) and trivinylbenzene, resulting in polystyrene chains linked at various points through the aromatic ring, has been examined. 50,51 At very low DVB contents the styrene yield i~ greater than that from PS and polymers with 25% DVB still give relatively high styrene yields, probably because the rigidity of the polymer structure inhibits transfer reactions. .
466
Thermal Degradation
15.4.6 Poly(a-methylstyrene) Replacement of the tertiary H atoms in PS leads to very different degradation behaviour. PAMS is much less thermally stable than PS, with volatile product evolution commencing at about 250°C and proceeding rapidly at 300°C. Monomer is produced in nearly 100% yield. The molecular weight falls during PAMS degradation, but to a much smaller extent than for PS. The initial rate of degradation has been found in solution studies to be proportional to the polymer MW up to about 6.x 105.52-59 The simplest picture of the degradation involves initiation by random scission, Scheme 17, reaction (a),followed by depropagation, reaction (b), with very long zip length and no transfer. Some treatments have assumed that both macroradicals (A) and (B) depolymerize to monomer. The radical (A)would certainly be expected to behave in this way, because it is the propagating radical in the polymerization, which has a very low ceiling temperature. Radical (B), however, like the corresponding radical in PS degradation, is much more likely to stabilize itself by H abstraction. Me
Me
Me
III ~ IWVCH2CCH 2CCH 2CJV\A IPh Ph I Ph I
Me
Me
Me
I I NVlICH2CCH 2C· I I Ph
I I
+ ·CH 2CNW
Ph
Ph
(A)
radical (A)
(B)
~ depolymerization to monomer
(b)
Me
radical (C)
(d)
-~~~
I~ MNCH 2CCH 2 + I
Me
I I
CH 2==CCH 2CJVlI',
I
Ph
Ph
(B)
Ph
(D)
Me unsaturated end (D)
(e) ----1~~
•
CH 2==C-CH 2
I
+
Ph (A)
Ph
H abstraction
Scheme 17
I
TJ\I\A
(C)
+
monomer
Degradation of polytc-methylstyrene]
The characteristics ofPAMS degradation have recently been reexamined'" in a study in which the experimentally observed variation of number and weight average degree of polymerization with conversion was compared with behaviour predicted in computer simulations. The latter were based on the criteria that when the radical undergoing depolymerization is shorter than the zip length, it volatilizes without leaving any fragment, and that either (a) both initially formed radicals depolymerize to the same extent, or (b) only one of the radicals depolymerizes, whereas the other is transformed into a stable molecule. Various zip lengths were also tested. The results, shown in Figure 6, illustrate very clearly that the mechanism involving depolymerization of both radicals ,gives a poor fit with the experimental data, whereas for a single radical depolymerization, a good fit over a wide range of polymer 'molecular weights can be achieved for a zip length of 2000. Various possibilities are considered" for the behaviour of radicals (A) and (B). Perhaps the most plausible course of events is as shown in Scheme 17, reactions (b)----(e), which accounts for both the formation of monomer as the only volatile product and for the observed fall in molecular weight of the polymer.
Thermal Degradation 9000
(0 )
9000
467
(b)
6000
3000
0.4
0.6
0.8
'A(c
1200 800 400
(0 )
(b)
1200
~--------------"'a....Q..o -a ......... "-
800 400
"-
Conversion
Figure 6 Comparison of experimental data for number average degree of polymerization vs. conversion for high and low MW PAMS samples with predicted curves based on (a) depolymerization of both radicals formed on backbone scission, (b) depolymerization of only one of the radicals. Zip lengths: 5000,- - 2000, ----- 1000,- - - 425, _._.324 monomer units respectively (reproduced by permission of Elsevier Applied Science Publishers Ltd. from Polym. Degradation Stab., 1985, 11, 167)
15.5 POLYALKENE AND DIENE POLYMERS 15.5.1 Polyethylene, Polypropylene and Polyisobutylene These polymers all degradeby random scissions of the chains, the products being dependent on the reactivity and possible reactions of the various radicals produced. The predominant productsappear as a cold ring fraction (CRF). When each of these polymers is degraded to beyond 800/0 weight loss, the percentage by weight of CRF for PE, PP and polyisobutylene (PIB) amounts to > 90%, 790/0 and 700/0 respectively. The main route to these products is intermolecular transfer. The more volatile products, on the other hand, arise from depolymerization or intramolecular transfer. These latter reactions are of minor importance in PE degradation, but lead to a series of alkenes and alkanes from C 2 size upwards.P'' The broad distribution of such products is clearly seen in the subambient TVA 14 curve for the condensable volatile product fraction from degradation of PE to 500 °C (Figure 7a). The main features of the degradation of PE are presented in Scheme 18. Although this picture represents the behaviour of pure PE, it has been found that the degradation is also sensitive to the presence of structural abnormalities such as branches''! and oxidized structures.V Even at temperatures much lower than the onset of volatilization (the latter occurs at about 370°C), PE shows a decrease in molecular weight due to scission at these structures, which provide 'weak . links'. PP is attacked by oxygen when heated in air. The oxidation mechanism and the mode of action of antioxidants are considered in Volume 6, Chapter 19. The threshold temperature for volatilization in PP degradation in vacuum is 100 ° lower than that for PE. The proportion of volatile products, although low, is greater than for PE and this fraction is complex in composition. Four major studies of PP degradation have come to very different conclusions regarding the nature and proportions of the volatile products. The explanation may be found in the difference in degradation conditions, Three of the investigations''?: 63, 64 were carried out in closed systems, initially evacuated to 10- 2-10- 4 mm Hg, whereas the most recent 65 used a dynamic system with helium carrier gas. Sample sizes also varied from several milligrams''?: 65 to one gram.P" The importance of low sample thickness and efficient product removal from the hot zone has been stressed above (Section 15.1.4). Unfortunately, in the PP degradation studies the most comprehensive and painstaking product investigation't" was based on the least satisfactory degradation conditions, under which secondary reactions in the molten polymer and gas phase appear very probable; this accounts for differences from the results of other workers. In a recent study of the degradation of atactic PP at 388°C for one hour,65 the major volatile products in order of importance were found to be 2,4-dimethyl-1-heptene, 2-pentene, propene, 2-methyl-1-pentene and
468
Thermal Degradation VVVlCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH:i1'\l\lV
depolymerization
5 4 3 2 1 2 /\/VVCH2CH2CH2CH2CH2CH2 ,,",,"
~OlecUIartransfer
//
¥~/; /VV\CH 2CH2CH2CH2 + CH 2==CH2
at C3)
v\AACH 2CH2CH2CHCH 2Me
!
inter-molecular transfer
VVV\CH2CH2CH2CH2CH2Me
(e.g.
(Similarly at C 2 -. propene at C4 -. pentene at C, -. hexene etc.)
+ VVV\CH2CH2CHCH2CH2CH2/VVV
!SCission
Scheme 18 Degradation of PE
(3)
(b)
(e) (A)
o -196OC
OOC
Figure 7 Subambient TVA curves for warm up from -196°C to 0 °C of condensable volatile products of degradation under vacuum to 500 °C at 10°C min -1 in a TVA system of (a) polyethylene, (b) polypropylene. Assignments: PE (1) ethylene, (2) propylene, (3) butenes, (4H7) mainly alkenes of increasing chain length; PP (A) propylene, (B) 2-methyl-1-pentene, (C) 2,4dimethyl-1-heptene, (D) larger fragments
Me
Me
/
I
Me
I
Me
Me
/
Me
Me
I
-/
Me
Me
/1
~H2CHCH2CHCH2CHCH2CHCH2CHCH2CHCH2CH'/V\A
Me
I
Me
Me
I
I
Me
!
Me
I
I
I
I,
f\/\I\,CH2CHCH2CHCH2CHCH2<;H + -cH 2CHCH2CHCH2CH'VVV (5)
(3)
(A)
(1) (B)
intramolecular transfer at C-5
~ ~ ~
Me
I
Me
I
I
Me I
I
Me
Me
Me
I
I
Me
I,
Me
I
Me
I
Me
I
I
MeCHCH 2CHrvv\ + ~H2CHCH29CH2CHCH2CHvv\" ~H2CHTCH2GCH2ICHCH2CH2 (a) (b) scission
~
at (b)
Me
Me
l
Me
Me
~H2~H
+ CH 2=CH
I
(A)
I
monomer
!
Me
I
Me
Me
-L / I vvvCH 2CH I CH29CH2CH2
Me
I
(A) + CH 2=CCH2CH 2 2-methyl-l-pentene
Me
I
Me
I
scission at (a)
Me
I
I
(A) + CH 2=CCH2CHCH2CH 2 2,4-dimethyl-l-heptene
Scheme 19
Me
Me
I
!
~
~ ~
~
~ ~
Me
I
(B) + -CHCH 2CH 2
~
o
c· ;::s ~
Me
/
~
I
Me
I
(A) + CH 2=CCH2CHCH2CH
!
SCission H abstraction
Me
/
Me
/
v'\/\I'CH2CHMe + CH==CHCH 2 2-pentene
Me
I
CH 2=CMe + (A) isobutene
Degradation of PP
~
0\ \0
470
Thermal Degradation
(in a much smaller amount) isobutene. This sequence was not greatly dependent on tacticity or temperature (388-438 °C). Subambient TVA data (Figure 7b) for the condensable volatile products of degradation of PP under TVA conditions confirm the presence of three of the above major volatile products. In Scheme 19, a mechanism of degradation of PP is presented in which the fall in MW during degradation is attributed to random scissions plus extensive intermolecular transfer involving tertiary H abstractions from the polymer by the primary radical (B), whilst the formation of the observed volatile products is explained mainly by depolymerization and intramolecular transfer reactions of the secondary radical (A). Although PIB begins to break down in the same temperature region as PP, the maximum rate of volatilization, under programmed heating conditions, occurs nearly 50°C lower. The thermal degradation has been the subject of several investigations. 6 o , 6 6 - 6 9 In common with PE and PP, when PIB is degraded the polymer MW falls rapidly and the largest product fraction is the CRF. In the case of PIB, however, monomer is also a major product, about 300/0 of the initial polymer weight, and there are only traces of other volatile materials. These differences result from the absence of tertiary H atoms and the greater tendency for the dimethyl-substituted radical chain end to depolymerize to monomer. The behaviour of PIB can be satisfactorily explained by a mechanism similar in most respects to that presented in Scheme 17 for PAMS, in which the transfer reactions involve abstraction of primary H atoms. In the case of PIB, however, intramolecular transfer reactions must also be involved.
15.5.2 Polyisoprene and Polybutadiene Work on the thermal degradation of these polymers prior to 1970 has been reviewed," revealing conflicting results. Structural rearrangements in the polymers prior to major decomposition form the subject of a further review."? These polymers have more recently been reexamined by TVA, TG and gravimetric measurements ofCRF products.V: 71, 72 If the temperature is gradually increased, polyisoprene (PI) breaks down in a single stage from about 300°C, whereas PB shows an initial weight loss of about 10% in the same temperature range, followed by decomposition of the remaining polymer at slightly higher temperatures than for PI (Figure 8). Both polymers degrade completely by 500°C. The TVA curves indicate a distribution of products.
1.0~-------~_
c 02
0.8
~
: 0.6 s: c-
°i ~
g 0.4
"°Vi CD
a::
G2
Figure 8 TG curves (dynamic N 2 , lO°Cmin- 1 ) for polyisoprene (1) and polybutadiene (2)
Gravimetric measurements based on isothermal degradation of PI at 340°C show the amounts of CRF and volatile products to be about 550/0 and 32 % respectively. The volatile fraction, when separated by subambient TVA (Figure 9) has been found to consist of two major components, isoprene and dipentene. IR spectroscopic examination of the CRF products indicates the presence of vinyl and cyclized structures. The polymer MW falls as degradation proceeds and some fall occurs even before volatilization is detected. These features are consistent with a mechanism based upon random scission of chains at the bonds linking the methylene groups, depolymerization (one
Thermal Degradation (0 )
471
(3)
20
30
40
50
60
70
35
40
80
(8)
o
5
10
r5
20
25 30 Time (min)
45
50
Figure 9 Subambient TVA curves for warm up from -196°C of condensable volatile products of degradation in vacuum under TVA conditions of (a) polybutadiene at 380°C, (b) polyisoprene at 340°C. Assignments: (1) ethane, ethene, (2) propane, (3) 1,3-butadiene, (4) C, hydrocarbons, (5) C 6 hydrocarbons, (6) 4-vinylcyc1ohexene, (7) higher MW fragments, (8) isoprene, (9) dipentene
unit giving isoprene, two units giving dipentene) and intramolecular and intermolecular transfer reactions. The degradation of polybutadiene (PB) is more complex. The first change to occur in the polymer is loss of the original unsaturation in an exothermic reaction in which polycondensed rings are formed. The small amount of weight loss which occurs at this stage gives both volatile and CRF products, the former in a slightly greater amount. Subambient TVA separation of the volatile fraction has indicated the presence of several products, the most important being 1,3-butadiene and 4-vinylcyclohexene (Figure 9). In the second and major stage of breakdown, a very complex product mixture is obtained, with hydrocarbons of all sizes from C 2 upwards; some aromatic products are also found. The CRF products are essentially PB oligomers, but some saturated aliphatic and alicyclic structures are also involved. The structure and stability of PB samples allowed to oxidize to various extents have been examined. Oxidation leads to a considerable lowering of the threshold temperature for the main decomposition process; in samples oxidized at 0-20°C an additional early stage of degradation commencing near 100°C is found, due to peroxide decomposition. Mechanisms have been presented. 7 3
15.6 NITRILE POLYMERS 15.6.1
a-Substituted Acrylonitrile Polymers
Four polymers of this structural type have been studied. These are polymethacrylonitrile (PMAN), polyphenylacrylonitrile (PPAN), poly(vinylidene cyanide) (PVCN) and poly-e-chloro acrylonitrile (PCAN). The structures, compared to polyacrylonitrile (PAN, 1), are shown below (2-5). Me
MACH 2CHJVVI
I CN
(1) PAN PS 6-P*
MNCH 2CJW\
(2) PMAN
(3) PPAN
CN
I
tN
Cl
CN
Ph
I I\M.CH2 C\I\M I
JlMCH 2
tMl\. I
CN (4) PVCN
IIMCH 2
tJIM I
CN (5) PCAN
472
Thermal Degradation
It is convenient to consider these polymers before discussing PAN because they exhibit much simpler patterns of degradation, provided they are in the pure state. In common with other polymers having the (-CH 2CXY-) repeat structure, such as PMMA and PAMS, PMAN, PPAN and PVCN, each depolymerizes to monomer in high yield. 74 -77 In the presence of certain impurities however, either as additives or as comonomer units, the behaviour may be modified to various extents and a very characteristic colouration reaction is observed. In the extreme situation the colouration reaction may completely suppress monomer production. Only PMAN has been studied in depth. 74,76,78,79 In the presence of the appropriate initiating structures, PMAN begins to discolour just above the softening point, from about 120°C, and the colour changes gradually from white through cream, yellow, orange and red to deep red. The same colour change can also be induced in solution, in the presence of alkalis. The IR spectrum of the polymer shows a progressive decrease in the nitrile absorption at 2210 cm - 1 and a corresponding increase in absorption in the 1693-1490 em - 1 region, consistent with the conversion of -C=:N side groups to (-C =N-)n conjugated sequences (Scheme 20).
C
C
C
III N
III
N
III N
C'
III
N
Scheme 20
Cyclization of nitrile groups in PMAN
The colouration reaction in PMAN may be initiated by various structures.?" notably carboxylic acids and phenols. Copolymerized methacrylic acid units are particularly effective.?" The gradual progression of conjugation in the reaction and the types of initiating structure involved have led to the view that the process is a concerted one with initial nucleophilic attack on the nitrile unit rather than one involving free radicals. The reversion of the solution colouration by strong acids tends to support this view. The suggested mechanism is shown in Scheme 21. At higher temperatures, the ladder structure splits out methane and undergoes fragmentation. Me
Me
Me
Me
C
C
C
C
N
N
Me
Me
IWI.CH2~CH2\1\N'
c1 6 H NIII Me
Me
III
IlNV'CH 2
v'VV'CH z
Me
Me
Me
Me
Me
Me
Me
Me
CHzv'\l\J\,
III
H z\l\l\l\.
a
VVWCH z
a Scheme 21
CHzVVV'v NH
Mechanism of colouration in PMAN
The fourth polymer of this group, PCAN, also shows simple degradation behaviour.t? but cyclization of the nitrile groups is not involved. The presence of the nitrile substituent on the same carbon as the chlorine so weakens the C~CI bond that this polymer is even less stable than PVC and resembles the latter in splitting out hydrogen chloride. The dehydrochlorination proceeds rapidly at 140°C.
15.6.2 Polyacrylonitrile As far as monomer production is concerned, there is a close parallel between PMMA and PMAN, which can give nearly 1000/0 monomer, and PMA and PAN, which give virtually none. However, because of the additional possibility of nitrile cyclization which occurs in PAN at temperatures lower than those needed to cause scission. PMA and PAN degrade in very different ways. The discolouration which occurs when PAN is heated is important both as an undesirable feature of acrylic textile fibres and as a synthetic route to carbon fibres, which are made by heating PAN to
473
Thermal Degradation
about 1000 °C under controlled conditions. The subject has recently been reviewed.P ' If PAN is maintained at 180-190°C for an extended period of time (up to 65 h) in the absence of air, the pale yellow colour which initially develops (1 h) progressively deepens to a tan colour. If the polymer is subjected to programmed heating in nitrogen or under vacuum, there is a rapid colour change at 260-270°C to deep brown and beyond 350°C the material becomes black. The change at 260-270 °C is associated with a very characteristic exotherm seen in the DTA or DSC trace. The size of the exotherm is influenced especially by sample size and can cause a surge in the polymer temperature of as much as 40°C above that of the immediate environment. In such a situation TG curves show a catastrophic weight loss of limited extent at the same temperature, but a comparison of TG and TVA data reveals that most of this weight loss is due to polymer fragments formed by chain scission resulting from the sudden overheating. The small amount of more volatile material simultaneously produced consists of NH 3 and HCN. The TVA curves also indicate the evolution above 350°C of a noncondensable gas, which proves to be hydrogen. Nitrogen is evolved at very high temperatures, around 850-900 °C. The various thermal analysis curves are compared in Figure 10. (b)
~
o
a:
(0 )
(c) IOO~----.oIl
220
240
260 Temperature (OC)
280
O~_---I._---'---""""'--"""""""---
200
400
600
800
Temperature (OC)
Figure 10 Thermal analysis curves for polyacrylonitrile: (a) DSC, dynamic N 2' at 10 DC min - 1 (right) and 5 DC min - 1 (left);(b) TVA, under vacuum, 10 DC min -1 with trap temperatures - - 0 DC, ------ -75 DC, - ' - ' - -196 DC; (c) TG, dynamic N 2 , 10°C min -1 (reproduced by permission from 'Developments in Polymer Degradation', ed. N, Grassie, Applied Science, London, 1977, vol. 1, p. 137)
The spectral changes which occur as PAN discolours show that nitrile groups are being converted into conjugated -C =N- sequences, but the reaction in this polymer essentially involves the rapid intensification of a single spectral colour rather than the gradual pale yellow to deep red transition which occurs in PMAN. Whereas the PMAN colouration is impurity initiated and avoidable, the reaction in PAN, as well as being susceptible to the same types of impurity, can be initiated at structures inherent in the PAN chain or normally present in the polymer as structural abnormalities. The ::CHCN structure may itself be sufficiently acidic,"? or, perhaps more probably, enamine or ketonitrile structural abnormalities may be responsible.P As a result of extensive studies of various features of the reaction.v" it was concluded that the colouration reaction in PAN, in the absence of additives, is a radical process. The exotherm is clearly associated with the nitrile cyclization reaction in which the length of the conjugated sequences is much shorter than in PMAN but the kinetic chain length is long, maintained by transfer as tertiary H sites. The reaction consequently leads to short ladder polymer segments linked by segments of unchanged PAN chain. Chain fragments result from scission at the latter segments, whilst elimination of side groups in the same structures leads to small amounts of HCN. Ammonia, on the other hand, is believed to result from terminal imine groups in the cyclic structures. Mechanisms have been suggested. 81 - 85 The exotherm which occurs in the degradation of pure PAN is an undesirable effect in carbon fibre production, since the rapid build up of heat fragments the chains. The exotherm is reduced if
Thermal Degradation
474
the reaction can be initiated by impurities or comonomers, or by controlled preheating in oxygen. The effect of oxygen on the reaction is a subject of controversy/" but it appears likely that the oxidized structures produced act as additional low temperature initiation sites and also stabilize the fibre structure through hydrogen bonding.
15.7 CHLORINE- AND BROMINE-CONTAINING POLYMERS 15.7.1
Poly(vinyl chloride)
15.7.1.1 Introduction Decomposition to HCl is a characteristic of the pyrolysis of chloroalkanes, but PVC liberates HCl at a much lower temperature than would be expected from studies of 1,3-chlorosubstituted model compounds. The reasons for the unexpectedly low stability of PVC and mechanism of breakdown of the polymer continue to be subjects of argument. Several useful general views of the subject have appeared recently. 5,8,85 - 90 PVC has become a high tonnage plastic because of the success of PVC stabilization technology. The mechanism of action of PVC stabilizers is also a controversial field. Stabilization aspects lie outwith the scope of this section and are dealt with elsewhere (see Volume 6, Chapter 19).
15.7.1.2 Observed characteristics When PVC is heated, the polymer begins to discolour and evolve HCl at about 150 "C, The degradation is acutely sensitive to sample form because the dehydrochlorination is autocatalytic: thick samples degrade more rapidly than thin films (Figure 3) since the HCl cannot escape as rapidly. The degradation is faster in oxygen than in an inert atmosphere or under vacuum and the polymer stability is also influenced by the method of preparation (bulk, emulsion, suspension). In isothermal degradation experiments, particularly in solution, induction periods are observed; the rate then increases and subsequently decreases, giving characteristic S-shaped curves of HCl evolution vs. time. Studies by TG (Figure 11)and TVA (Figure 3)indicate that for a gradual rise of temperature there are two stages of decomposition. The first of these corresponds to near quantitative loss of HC1, together with a little benzene and traces of ethylene. In the second stage, most of the products are coloured materials which collect as a cold ring fraction; the volatile fraction at this stage is complex and includes aromatic and aliphatic compounds. A black carbonaceous residue remains at 500 "C.
.~ 0.8
U
~
:c0' 0.6
'j) ~
C
::J
0.4
"0
'en
~ 0.2
------'-------'-----.&....------.1
0 ........ 100
200
300
400
500
Temperature (Oe)
Figure 11 TG curves (dynamic N 2 , 10°C min -1) for (1) poly(vinyl bromide), (2) poly(vinyl chloride) and (3) polychloroprene
As HCl is released from PVC the discolouration increases with a colour shift towards red. This is attributed to the build up of polyene sequences, which can be studied by UV spectroscopy (Figure 12). It appears that the average polyene length is fairly short, reaching only 6-10 double bonds in conjugation. During dehydrochlorination there is also an increase in MW due to crosslinking, and some cyclization occurs.
475
Thermal Degradation 115
1.0
0.8
Q,)
o
6 0.6
-eo
en
.0
0.4
0.2
Figure 12 Absorption spectra for suspension-polymerized PVC degraded at 150°C in solution in N 2 atmosphere for the times indicated (min) (reproduced from Eur. Polym. J., 1967,3,747)
15.7.1.3 Initiation of dehydrochlorination It is agreed that the low stability of PVC is the result of the initiation of dehydrochlorination at structural abnormalities such as branches, chioroallyl groups, oxygen-containing structures, end groups or head-to-head linkages. There is considerable disagreement as to which are responsible. The predominant view is that p-chloroallyl groups -CH=CHCHCI- are important. This would be expected from the stability sequence of various nonoxygenated structures in chlorinecontaining model compounds.l" shown in Scheme 22. It is also supported by the observed correlation between internal chain unsaturation and dehydrochlorination rate and by the considerable destabilizing effect of the presence of phenylacetylene groups copolymerized into the PVC chain.86,92 MeCH =CCH 2Me ~ MeCHCH 2CHMe > MeCHMe > CH 2 =CHCH 2CHCH 2Me >
I
I
CI
CI
I
CI
I
I
CI
CI
Me
Et
I
I
CH 2 =CHCHCH 2Me > MeCMe > MeCH 2CCH 2Me > MeCH =CHCH 2Me
I
CI
I
CI
I
CI
I
CI
Scheme 22 Stability of selected chlorine-containing model compounds
On the other hand, others argue in favour of carbonylallyl groups -COCH=CHCHCICH 2- as the structure responsible.i" A case can also be made for chain branching, however, on the basis of evidence such as model compound stabilities and the decrease of the stability of PVC with increase in polymerization temperature. Head-to-head structures are less likely as labile sites, however, since head-to-head PVC is not sufficiently different in stability from normal PVC. 93
15.7.1.4 Mechanism of dehydrochlorination It is now established that free radicals are involved in the decomposition. The most convincing evidence comes from (a) transfer in the degradation of PVC in the presence of labelled toluene.i" (b) initiation by PVC of the free radical depolymerization of PMMA in the temperature range of dehydrochlorination and well below the normal temperature for PMMA breakdown (see Section 15.11.1.1), (c) ESR evidence for the presence of radicals in degrading PVC,94 and (d) initiation of dehydrochlorination by azodiisobutyronitrile.f"
Thermal Degradation
476
Various radical chain processes have been suggested, based on initiation either at a labile centre or by random scission of a normal C-CI bond. The CI- radical produced abstracts a methylene hydrogen and propagation then occurs along the chain or occasionally by attack on a neighbouring chain, so ending a particular polyene sequence but continuing the reaction in the other chain. Termination is assumed to be by combination of pairs of chain carriers (Scheme 23). H(R)
H(R)
I
I
-c-
Initiation
I
~
-<;-
+ CI·
CI Propagation
CI·
+
\/\I\IVCH2CHCICH 2CHCICH2CHCIIVVV
~ HCI ~
CI·
~ HCI ~
CI·
+ +
+ +
/VVVCHCHCICH.2CHCICH2CHCI/VV\ VVV\CH::;:::=CHCH 2CHCICH2CHCI.lVV\
IVVVCH=CHCHCHCICH2CHC~ IV\ACH=CHCH=CHCH 2CHCI./VVl etc.
Transfer to another chain
CI·
+
~H2CHCICH2CHCICH2CHCIVVV'
~ HCI
+ +
/\/V\.CH2CHCICHCHCICH2CHCIIVVV
~
CI·
IVV\CH2CHCICH=CHCH2CHCIVVV\
~
propagation as above
~
products
Termination R·
+ CI.} + CI·
R·
+
CI·
CI·
Scheme 23 Free radical mechanism for PVC dehydrochlorination
This mechanism fails, however, to explain autocatalysis by HCI, the catalysis by acetic acid in PVC/PVAC blends (see Section 15.11.1.2) and the catastrophic degradation induced by Lewis acids such as ZnCI 2 • It is probable, therefore, that molecular elimination of HCI via a four-centre transition state and HCI-catalyzed elimination via a six-centre transition state (Scheme 24) occurs simultaneously with the free radical route, the relative importance of radical or molecular elimination being dependent to some extent upon the conditions. The catalysis route in Scheme 24 is readily modified to explain acetic acid catalysis.96
C!J
H
~H}H4H(.:HCl'VVV\-
HCl+
NVVCH 2CH=CHCHCIVVVl Scheme 24 Molecular and HCI-catalyzed dehydrochlorination of PVC
15.7.1.5 Secondary reactions The crosslinking which occurs during HCI loss can be reversed or prevented by heating the polymer with maleic anhydride, which suggests that Diels-Alder type crosslinking of polyene sequences normally occurs (Scheme 25).
477
Thermal Degradation /.CH--cH. CH-eH hCHIN'N' // ~ // ~ ~ r ~CH CH-cH'/ "CH--eH
#
~H
CH-eH
~
/
CH=CH
CH--CH
,
CHW\N
~
CH~H
'cH--eH/
"
CH=CH ~H=CH/
~H-CH/
\:H=CH~
CH=CH\MN
Scheme 25 Die1s-Alder crosslinking of polyene sequences in PVC degradation
Intramolecular cyclization reactions of polyene sequences account for the formation of benzene and other aromatic products and also provide a route to the carbonaceous residue formed at high temperatures.
15.7.2 Poly(vinyl bromide) Poly(vinyl bromide) (PVB) resembles PVC in breaking down in two stages (Figure 11). In the first of these, loss of hydrogen bromide occurs with the development of conjugation, and in the second, the polyene structure undergoes scission, cyclization and carbonization.?? The stability, however, is much lower than that of PVC. The yield ofHBr is close to the theoretically possible value; rather less benzene and more ethylene are formed than in PVC degradation, and small amounts of unidentified products of lower volatility than benzene are also present in the volatile fraction. The length of the conjugated sequences produced in PVB during degradation has been estimated'" from the UV spectra of partially degraded samples to be about 12-13 double bonds, which is longer than in the case of PVC. It has not been established unambiguously whether or not radicals are involved in PVB dehydrobromination. The ability of PVC to initiate low temperature depolymerization of PMMA in PVC/PMMA blends provides excellent evidence for small radical intermediates in PVC dehydrochlorination. Similar experiments with PVB/PMMA blends?" do not lead to destabilization of the PMMA, which might indicate a purely molecular elimination of HBr from PVB. Alternatively, however, the rate of reaction of Br- radicals with PMMA at 200°C may be too low.
15.7.3
Chlorinated Rubber
Commercial chlorinated natural rubber contains between 65% and 680/0 CI, introduced both by addition and substitution reactions. The structure is complex and includes some cyclized units in the chains. A detailed examination of the degradation has been carried out by TG and TVA. 9 9 The polymer is stable to 200°C when heated at 10°C per minute and loses about 67% of its weight in the first stage of breakdown, which reaches a maximum rate at about 300°C. Further but limited weight loss (30/0) occurs between 400 and 500 "C, leaving a carbonaceous residue. By far the greatest proportion (95%) of the volatile products at the first stage of decomposition consists of HCl; small amounts of CO and CO 2 and traces of CH 4 and C 2H4 are also formed. In the subsequent weight loss, volatile products detected are CH 4 , C zH 4 , HCl (traces) and Hz. The polymer discolours at even 1% dehydrochlorination, showing that conjugation develops along the chain as in PVC degradation. At the same extent, insolubility also results. The extent and pattern of HCl loss under isothermal conditions are inconsistent with the structure suggested in 1967 for chlorinated rubber;'?" an alternative polymer structure and degradation mechanism in accordance with the observed behaviour have been presented.??
15.7.4 Poly(vinylidene chloride) This polymer is somewhat less stable than PVC. It degrades at 120-220°C to give only HCl; at temperatures above 250°C other products are also present. Only one molecule of HCl per vinylidine chloride unit is produced. Poly(vinylidene chloride) (PVDC) discolours very rapidly, becoming yellow at 1% and dark brown at ·100/0 volatilization. It also becomes completely insoluble at 1% degradation. Although autocatalysis is' observed after 15% reaction, the initial loss of HCl appears to be zero order. A free radical dehydrochlorination mechanism, initiated at the chain ends and proceeding along the chains, has been suggested. 101
Thermal Degradation
478
15.7.5 Polychloroprene Polychloroprene (PC) is unstable in air. Unless stabilized by additives the polymer absorbs oxygen. Simultaneously, HCl is released at a rate closely related to that of oxygen uptake and the polymer discolours. In order to study the effect of heat on PC it is necessary to degrade the polymer in nitrogen or under vacuum.P: 102 TG experiments (Figure 11)indicate two-stage weight loss as for PVC, but in PC the temperature for maximum rate of weight loss is much higher and there is also far more carbonaceous residue (20% of the sample weight) at 500 "C. Comparison of weight loss and EGA data for HCl evolution from PC (Figure 13) illustrate clearly the presence of additional products. The primary weight loss is about 45%, whereas about 35% appears as HCl. This is less than 90% of the available HCl in terms of the chlorine content.
Figure 13 Comparison of total weight loss (2)and weight loss due to HCI (1)for polychloroprene. Heating rate:,10 °C min -1. The total weight loss was determined by TG in dynamic N 2 and HCI by titration of products condensed from degradation under vacuum by TV A (reproduced from Eur. Polym. J., 1971, 7, 569)
Product analysis following degradation in a TVA system has established that minor gaseous degradation products include ethylene and a trace of monomer; above 400 "C, methane and smaller amounts of hydrogen and propylene are also formed. In addition to the gases, there is a complex liquid fraction containing two dichloro-4-vinylcyclohexene isomers. 1 02 UV studies of PC samples at different extents of dehydrochlorination'{ have revealed that the predominant absorption are characteristic of trienes. This is to be expected if random loss of HCl occurs by the nonradical mechanism shown in Scheme 26. PC does not accelerate PMMA degradation in PC/PMMA blends.l''! which is also consistent with a nonradical dehydrochlorination.
Cl---H
H
Cl
H
Scheme 26
HCl+ H
H
Cl
H
Molecular elimination of HCl from polychloroprene
15.8 FLUORINE-CONTAINING POLYMERS 15.8.1
Introduction
The study of the thermal stability of some of these materials has revealed a greater dependence of behaviour on environment and experimental conditions than for most polymers. Particular problems are the effect of oxygen and the tendency of some of the degradation products to react with the
Thermal Degradation
479
apparatus, particularly if glass is involved. Several useful treatments of the degradation of this class of polymers have appeared" 103 104 which contain references to original sources. t
15.8.2 Perfluoroalkane Polymers Poly(tetrafluoroethylene) (PTFE) is one of the most stable polymers known. The stability has been attributed to the high strength of the C-F bond compared to that of the C-H bond (487 and 418 kJ mol- 1 in CF 4 and CH 4 respectively) and to the shielding effect on the backbone of the highly electronegative fluorine atoms. There is no significant volatilization in vacuum below 450°C and even two hours heating of the polymer at 500 °C leads to only 30% weight loss. The stability of PTFE can be compared to that of polyethylene and other F-containing polymers from the following stability sequence, for vacuum conditions: poly(tetrafluoroethylene) > poly(vinylidene fluoride) > poly(trifluoroethylene) > polyethylene> poly(vinyl fluoride) > poly(chlorotrifluoroethylene) >poly(hexafluoropropylene). Copolymerization of TFE with hexafluoropropylene destabilizes the polymer but does not alter the principal products. The homopolymer poly(hexafluoropropylene) (PHFP) is very much less stable and decomposes below 300°C. The destabilization inPHFP and the copolymer is attributed to the presence of tertiary fluorine. PTFE degrades in the absence of air to give 95% yield of monomer and a small amount of hexafluoropropylene. No hydrogen fluoride or chain fragments are produced except at very high temperatures (1200 DC). Although the site of initial scission (random, structural abnormality or chain end) has not been established, the proposed mechanism (Scheme 27) involves homolysis followed by depolymerization of the radicals to monomer, with a complete absence of transfer because of the absence of C-H bonds and the strength of the C-F bond. IVVVCF2CF2CF2CF2CF2CF2CF2CF2vvv'
-+
/\/VVCF2CF 2CF2CF2 -+ vvvCF2CF2
2 vV\A.CF 2CF2CF2CF2
+ CF 2=CF2 etc.
Scheme 27 Degradation of polytetrafluoroethylene
The stability of PTFE in air is comparable with that in an inert atmosphere, but the mode of breakdown is completely different. Monomer is almost entirely absent. The major product below 650°C is the highly toxic carbonyl fluoride (COF2);above this temperature the product decomposes to CO 2 and CF 4, 15.8.3 Chlorofluoroalkane Polymers Replacement of a fluorine atom by chlorine reduces the stability and alters the mechanism of decomposition, because the C-Cl bond is weaker than the C-C and C-F bonds. Thus when poly(chlorotrifluoroethylene) is degraded in vacuum, the volatile fraction of products drops to about 280;0 of the polymer weight. The volatile material is predominantly monomer, plus some C 3F sCI and C 3F4C12' The remaining products are polymer fragments of average MW900, which collect as a cold ring fraction. Since the weakest bond is the C-CI bond, a possible degradation mechanism involves homolysis of this bond as the first step, followed by chain scission and depolymerization. There is spectroscopic evidence for the CF2=CF- structures also formed in this mechanism (Scheme 28). Further macroradicals may result by attack of chlorine atoms.
1 VVV\CF 2CFC1
+
CF2=CFCI
etc.
Scheme 28 Degradation of polychlorotrifluoroethylene
480
Thermal Degradation
15.8.4 Hydrofluoroalkane Polymers The presence of C-H bonds in fluorine-containing polymers of this type leads to the appearance of HF as a degradation product. The three main polymers in this class, poly(vinyl fluoride) (-CH 2CHF-)n' poly(vinylidene fluoride) (-CH 2CF 2-)n and polytrifluoroethylene (-CF2CHF-)n degrade in vacuum to give approximate yields of HF of 25, 50 and 6% by weight respectively. The remaining products consist mainly of cold ring fraction. Discolouration of these polymers also occurs. The mechanisms of degradation have not been established. A possible explanation of the above features is that molecular elimination of HF proceeding systematically along the chain competes with backbone scission and disproportionation of the radicals formed. Various copolymers involving the above monomer structures have been made, but the degradation behaviour remains largely unexplored. CF2CH2/CF3CFCF2 copolymer degrades above 370°C in nitrogen and gives 950/0 volatilization at 500 °C, of which some 30-400/0consists of volatile products, mainly HF, the remainder being chain fragments. CF2CH2/CF2CFCI copolymer is considerably less stable and begins to lose hydrogen halides from 250°C. The relative proportions of HF and HCI are sensitive to conditions (N 2 or vacuum) and degradation temperature. 15.8.5 Fluoromethacrylate Polymers Understanding of the behaviour of F-containing polymers has very recently been broadened by studies of three methacrylate polymers in which the ester group contains fluorine'?" poly(2-fluoroethyl methacrylate) (PFEMA), poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) and poly(hexafluoroisopropyl methacrylate) (PHFPMA). The experimental approach used is flash pyrolysis of microgram quantities of polymer at 600°C in a helium atmosphere, followed by GC separation of volatile products and identification by MS. This produces a convenient picture of the number and relative importance of the volatile products. The characteristic features of the degradation of alkyl methacrylate polymers are considered elsewhere (see Section 15.3.2). In contrast to PEMA, PFEMA gives no monomer. In addition to the ester decomposition products vinyl fluoride and CO 2, acetaldehyde and fluoroacetaldehyde are produced. Explanations have been proposed for the latter products on the basis of methacrylic acid/FEMA and FEMA/radical end FEMA adjacent unit interactions. It is argued that the ester decomposition route is favoured by F atom activation of the f3 hydrogens, so suppressing depolymerization. PTFEMA gives predominantly monomer, as would be expected because of the absence of f3 hydrogens in the ester group. Much smaller amounts of CO 2, vinylidene fluoride, trifluoroacetaldehyde and trifluoroethanol are also observed and mechanisms have been suggested. Weakening of the alkyl-O bond by the CF3 group in the CF3CH2 substituent, for example, leads to CH 2=CF2 and CO 2. PHFPMA gives behaviour which is completely untypical of the thermal degradation of methacrylate polymers, since by far the major product is hexafluoroisopropyl formate. Also produced are CO 2, acetaldehyde, 1,1,1,3,3,3-hexafluoropropane and trifluoroacetaldehyde, all of which are plausible decomposition products of the formate. It is suggested 105 that the first step in the production of the formate is scission of the C-H bond.
15.9 POLY(VINYL ACETATE) AND RELATED POLYMERS 15.9.1 Poly(vinyl acetate) and Ethylene/Vinyl Acetate Copolymers In many respects, the degradation of poly(vinyl acetate) (PVAC) resembles that of PVC, although the thermal stability of PVAC is much higher. PVAC gives near quantitative yields of acetic acid on the basis of one molecule of acid per chain unit, with accompanying discolouration of the polymer, and there is a further stage of decomposition at higher temperatures in which the products are similar to those at the same stage of PVC breakdown. In an early investigation of the polymer it was concluded that molecular elimination of acetic acid occurs, initiated at the chain ends.'?" However, this conclusion is in conflict with the lack of MW dependence of the rate of weight loss found in a later study. 1 07 More recently, the TG and TVA techniques have been applied in a reinvestigation of PVA degradation'? and also in examination of the degradation behaviour of PVAC/PMMA 108 and
481
Thermal Degradation
PVAC/PS 1 0 9 blends, from which important evidence regarding the mechanism of PVAC breakdown has emerged. Under programmed heating conditions (5 or 10°C per minute) in nitrogen (TG) or under vacuum (TVA), the polymer begins to decompose near 250°C and the maximum rate at the first stage of degradation occurs at 320-330°C; the second-stage rate maximum is at about 440°C. The carbonaceous residue at 500 "C amounts to about 8% of the initial polymer weight. The products at the first stage consist of 950/0 acetic acid plus 5% made up of ketene, water, methane, carbon dioxide and carbon monoxide. The products at the second stage result from scissions of the conjugated polyene left after deacetylation. Only small amounts of volatiles are produced; the major products at this stage are a coloured tar and the carbonaceous residue. The development of conjugation has been followed both in PVAC and ethylene/vinyl acetate (E/VAC) copolymers'f using UV spectroscopy of partially degraded polymer films cast on silica TVA tubes, which could conveniently be inserted directly into the sample compartment of the spectrometer. Assignments of peaks (Figure 14) were based on comparisons with reported data on model polyenes and with the sequence distributions in the copolymers. The TVA behaviour of 12 and 33% VAC copolymers indicated that the VAC sections of the chains eliminate acetic acid before any degradation of the backbone structure occurs. The 12% VAC copolymer contains 18.6% of the VAC units in pairs, but only 3.7% in blocks of three or more. Absorption is therefore confined to the diene region. The 33% VAC copolymer contains 29.6% of the VAC groups in pairs, 14.70/0 in blocks of three and 10.90/0 in blocks of more than three. Absorption beyond the triene region is therefore almost completely absent. Triene peaks appear at 261, 272 and 283 nm in this copolymer (and also in partly degraded PC, discussed in Section 15.7.5, which also gives trienes), as in PVAC. It has been concluded from these studies that in degraded PVAC conjugation extends to about six double bonds, the average polyene length thus being shorter than for degraded PVC. PVAC becomes insoluble at very low extends of degradation.
(F)
3
2
250
300
350
400
225
250
275
Wavelength (nrn)
Figure 14 UV absorption spectra for PV AC (left) and ethylene/vinyl acetate copolymers (right) (as films), initially and after degradation to various temperatures at 5°C min - 1. PV AC: (A) undegraded, (B) 289°C, (C) 301 "C, (D) 313°C, (E) 332 "C, (F) 381°C; 33% VAC copolymer: (A) undegraded, (B) 323°C, (C) 350 -c, (D) 356°C, (E) 374 °C, (F) 425°C. The dotted curve is for a 12% VAC copolymer degraded to 356°C (reproduced by permission of John Wiley and Sons Ltd from J. Thermal Anal., 1969, 1, 389)
When PVAC and PMMA are degraded together in a blend, the production of MMA monomer is much increased in the .250-350 °C region, whereas acetic acid evolution is retarded compared to the behaviour of the pure polymers. This is taken as strong evidence that radical species are involved in the deacetylation of PVAC. In the molecular elimination of acetic acid originally proposed, the presence of the minor products of degradation (ketene, CO 2 , etc.) was explained by assuming a small amount of decomposition of acetic acid. There is evidence that this can occur at considerably elevated temperatures (in a unimolecular process), but similar behaviour at 300 °C is questionable. One of the further arguments in' favour of involvement of radicals in PVAC deacetylation is that it allows the presence of minor products to be explained by side reactions of the acetate radical proposed as intermediate and also provides a ready explanation for the rapid insolubilization. The proposed mechanism, analogous to that for PVC (Scheme 23) is shown in Scheme 29.
482
Thermal Degradation Acetic acid production VV'V'CH 2CHCH2CH/VV\
I
OAc
-+
OAc
+
AcOH
AcO·
-+
I
-vvvCHCHCH 2CHCH2CHV\IV)
• I
I
OAc
-+
+
AcOH
I
OAc
OAc
/V\IVCH=CH~HCHCH2CH\IV\.A
• I
I
OAc
-+
AcO.
+
OAc
·~CH=CHCH=CHCH2CH/V\I\.
I
etc.
OAc Other reactions of acetate radicals 2AcO.
-+
CH 2=C=0
+ AcOH
AcO·
-+
CH-2=C=0
+ ·OH
AcO·
-+
CO 2
Scheme 29
+ Me (
-+
(-+
H 20)
CH 4 )
Free radical mechanism for degradation of PVAC
The simultaneous occurrence of molecular elimination of acetic acid in PVA degradation (Scheme 30) is probable, however, because the decomposition is catalyzed by acetic acid. Such a mechanism also allows catalysis by HC1, in PVACjPVC blends.?" to be explained. After loss of the first acid molecule, progression along the chain would be dependent on allylic activation of the adjacent VAC unit. ~H2CH~HVVV'
? ~01 or O=T Me
-
Me
VV\/ICH 2CHCH=CHtVV\/'
I
or?
+
MeC0 2 H
Me
SCheme 30 Molecular elimination of acetic acid from PVAC
15.9.2 Poly(vinyl formate) and Poly(vinyl alcohol) Poly(vinyl formate) has been briefly studied.P The UV spectra of partly degraded polymer samples resemble those obtained with PVAC although poly(vinyl formate) is less thermally stable. Two-stage weight loss occurs and the reaction appears very sensitive to conditions. Formic acid is the main product at atmospheric pressure, but under vacuum noncondensable gases, probably CO and H 2 originating by reaction of formyl radicals, are also produced. Poly(vinyl alcohol) degradation appears to be quite complex, although the elimination reaction to produce water occurs and some conjugation develops.P Acetaldehyde, which could result from depolymerization, has also been reported as a product. The TVA data indicate at least four stages of breakdown when the temperature is gradually increased, commencing near 200°C.
Thermal Degradation
483
15.10 RANDOM COPOLYMERS 15.10.1
Methyl Methacrylate Copolymers
When the degradation behaviour is compared with that of PMMA, the effects of introducing a comonomer unit into the PMMA chain fall into four types: unrestricted depolymerization, blocking of depolymerization, chain scission at the comonomer unit and interunit cyclization. In most systems, even a small amount (e.g. 10/0) of the second monomer can have a significant effect on the stability. If the second monomer is a methacrylate, then provided side group ester decomposition to alkene and acid unit does not occur, unzipping may proceed through the foreign unit, producing the two monomers as products. This is the case with ethyl methacrylate as comonomer and other n-alkyl methacrylates would be expected to behave similarly. Phenyl methacrylatejMMA copolymers are unusual in that although both homopolymers degrade exclusively to monomer, the copolymers show higher thermal stability and give only a small amount of products other than the monomers. Cyclic anhydrides appear to be produced in small amounts in the backbone by interunit reaction and these block unzipping.l !" Further examples of anhydride formation are considered below. There are several systems in which a second comonomer unit, even in a small amount, will stabilize the PMMA structure by blocking unzipping. In these cases, the depropagation process, once initiated, proceeds as far as the first foreign unit and may then stop because of some alternative reaction (e.g. transfer) of the new type of radical chain end. This is the situation with acrylate comonomers such as methyllll or butyl acrylate.P? and also with styrene. 113 The stabilizing effect of methyl acrylate and styrene is clearly illustrated by the TVA curves in Figure 15. In particular, the end-initiated PMMA depolymerization is reduced or eliminated. (0 )
100
200
300
400
100
Temperature (Oe)
Figure 15 TVA curves for degradation under vacuum at 10°C min -1 of (a) copolymers of MMA with methyl acrylate: ------ 26/1, 8/1, compared with PMMA of comparable MW ( - - ); (b) copolymers of MMA with styrene: ------4/1, - - 1/1, compared with PMMA of comparable MW ( ) (reproduced from Eur. Polym. J., 1968,4, 21)
Several MMA copolymer systems have been studied in which the first effect observed as the polymer temperature is raised is scission at the comonomer unit. In the case of low concentrations of «-chloroacrylonitrile, this weak link scission leads to radicals which begin to depolymerize immediately, so that MMA monomer is produced at only 140°C, together with some HCI. 114 However, with acrylonitrile as comonomer, although the molecular weight falls sharply, it has been found that under isothermal conditions at 220°C there is an induction period before monomer production, suggesting that the initial weak link scission leads to molecules rather than radicals.U" Methyl methacrylate chain units participate readily in intramolecular cyclization with adjacent comonomer units of certain types. These reactions seem to be favoured in situations where five- and six-membered rings can be formed. The cases presented in Scheme 31 have been investigated in some detail. In the first three examples,33,97,116 the cyclization occurs at lower temperatures than depolymerization. If the comonomer unit concentration is very low, the cyclic structures formed act as blocking groups for unzipping; at higher concentrations, however, MMA production may be almost completely prevented because the MMA units are used up in cyclization: the MeOH etc. then becomes the major volatile product. In the VACjMMA system, 11 7 cyclization competes with MMA production and acetic acid evolution, since all three reactions occur in the same temperature range. The anhydride structure shown in Scheme 31, reaction (a), which has characteristic IR absorptions at 1805, 1760 and 1020 em -1, is also produced in interunit cyclization in copolymers of MMA with
Thermal Degradation
484 Me
Me
I I
I
VVVlCH2 C-CH 2-Cvvvv
I
#C
O~
C",
"oH 0-1'
~:ttHMe Me
(a)
2~
a
OH
Me (b)
ca. 100°C
•
~H2'Ct:
o
C~ CI MeO/ ~O
Me
0
+
MeCI
Me
I
(c)
I
ca. 100°C
V\/V1CH 2CHCH2C;vvv
I
0 Me
I I
~H2CHCH2C'VVV\
I
+ MeOH
.
• ~H2r:t:
+ MeBr
Br /C, MeO 0 Me
I
(d)
~H29HCH2TV\/\A
to
Me
ca. 250°C
•
~H2Lt:
+
MeC0 2Me
C~ Me6 '\0
/~
0
Scheme 31
Intramolecular cyclization reactions in MMA copolymers
methacrylate salts, and in a number of situations involving PMMA degradation in the presence of additives, including PVC, PVAC, polychloroprene, fire retardants and zinc bromide. 1 9 , 2 o , 10 2 , 10 8 , 1 18
15.10.2 Styrene Copolymers In styrene rich styrene/MMA (S/MMA) copolymers, the stability is intermediate between that of the two homopolymers, but the product distribution is strongly influenced by the presence of the ~1MA units. For example, in a 4/1 S/MMA copolymer, the yield of cold ring fraction products is about a third of that expected on the basis of the styrene content and the behaviour of PS, so that transfer reactions are considerably reduced.l P One of the major differences in behaviour between PMMA and PS is that PMMA radicals will depolymerize to monomer at temperatures as low as 140°C, whereas PS radicals do not begin to depolymerize until nearly 300°C. Thus although the initial effect of small amounts of «-chloroacrylonitrile ((X-CAN) is the same in both the PMMA and PS chains, scission leads to monomer production in the former case but not in the Iatter.I!" The initially formed radicals in S/(X-CAN copolymers are believed to stabilize themselves by disproportionation (Scheme 32). Styrene/ acrylonitrile (S/AN) copolymers also show accelerated chain scission. 119 In the temperature region of volatile product formation, AN monomer is found (in contrast to the behaviour of PAN) and the CI
I
/\/\/VCH 2 CH CH 2 CH CH 2 C CH 2 CHvv\/\
I
I
Ph
I
Ph
-+
I
CN
Ph
I
-+
./\/V\CH 2CHCH 2---+-CHCH=CCH 2CH,/\/\/\
I
Ph vvV\CH2CHCH 2
I
: I
I
Ph
+
CN
+
I
HCI
Ph
I
CN
I
Ph
CH 2CH=CCH 2CH\/\/V\
I
Ph Scheme 32
+
·CHCH=CCH 2CHvvv Ph
Ph
I
I
CN
I
Ph
Chain scission and disproportionation in styrene/a-chloroacrylonitrile copolymers
Thermal Degradation
485
proportion of CRF products increases with the AN content. The styrene units prevent nitrile cyclization and in consequence some of the AN units decompose to HCN. Copolymers of styrene with vinyl chloride are much less stable than PS. 1 2 0 It is not possible to make such copolymers rich in VC because of the very unfavourable reactivity ratios. The styrene rich copolymers behave in a very similar way to chain-chlorinated PS samples of comparable CI content (see Figure 2). HCI is evolved from about 200°C and the resulting double bonds destabilize the PS backbone with the result that decomposition to styrene, etc. occurs at slightly lower temperatures than in PS. Styrene/vinylidene chloride copolymers have been found to show styrene group activation of vinylidene chloride units towards HClloss. 1 2 1, 122 15.10.3
Vinyl Chloride Copolymers
The novel behaviour of VC/MMA copolymers has been referred to above. A similar cyclization reaction occurs with methyl acrylate as comonomer. In vinyl chloride/vinyl acetate copolymers, at both extremes of the composition range, incorporation of the comonomer unit results in destabilization. 123 Minimum stability was found to occur at approximately equimolar composition in bulk degradation but at 33 mol % VAC in solution studies. During the degradation of a 470/0 VAC copolymer, the HCI/acetic acid ratio remains the same during the degradation and conjugation develops along the chain as in the case of the homopolymers. It may be concluded 'that both the rate of acid evolution and the development of conjugation are increased by heterogeneity in the polymer chain. One or both chain units must therefore be destabilized by the immediate proximity of a unit of the opposite type. Two mechanisms may be contributing. Firstly, the inductive effect of the CI atoms may weaken the C-H bonds of methylene group and so facilitate elimination of acetic acid in an adjacent VAC unit. The next molecule to be eliminated will be HCI because of allylic activation. Secondly, there may be neighbouring group participation by the acetate groups in the elimination of HCI, following which there is allylic activation for the removal of an acetic acid molecule. 123 The variation of stability with composition for the VC/VAC copolymer system is illustrated in Figure 16.
...
::::J
Eo ::::J
o 2
·2
e a:
Figure 16 Comparison of rate of volatilization at 248°C, as measured under TVA conditions at 5°C min - 1, with copolymer composition, for vinyl chloride/vinyl acetate copolymers (reproduced from Eur. Polym. J., 1970, 6, 679)
15.10.4 Ionomers The degradation behaviour of certain copolymers containing methacrylate salt units has been examined within the last few years. The systems examined include MMA/alkali metal methacrylates (Li, Na and K), MMA/zinc methacrylate and the commercially important ionomers based on ethylene and methacrylate salts (Na and Zn ion). The behaviour of the MMA ionomers has been discussed in a recent review.P" A major feature of the degradation reaction is the evolution of methanol and the formation of six-membered anhydride rings in the polymer chain. The ethylene/methacrylate salt ionomers, as produced commercially, are in fact terpolymers, since some free methacrylic acid structures are also present. The salt unit concentrations of the ionomers which have been studied v " are somewhat less than 10 mol %. TG measurements have indicated stabilities for the Na and Zn ionomers which are fairly comparable with that of PEe The residues at
486
Thermal Degradation
500°C consist of Na 2C0 3 and ZnO respectively. All the product fractions have been examined in TVA experiments. The noncondensable gas fraction (absent in PE degradation) is mainly CO. The condensable volatile products contain the broad spectrum of products typical of PE breakdown, with an increased amount of propene, and CO 2 , dimethylketene and isobutene are also present. The cold ring fraction resembles that from PE; IR spectra have shown that the chain fragments contain some methacrylic acid units. Residual polymer from partial degradation at 440°C show no C0 2H groups but salt units are present. It has been concluded v" that the ethylene ionomers degrade by a mechanism in which the initial step is scission in the salt side group. Subsequent reactions of the radicals formed in this step account satisfactorily for all the observed features. Scheme 33 shows the behaviour as proposed in the case of the Na ionomer.
\ CO
I + VVV\CH2CH2CH2<;CH2CH2CH2VVV'
(B)
(A)
I
j volatile and cold ring fraction products by intramolecular and intermolecular transfer, as for PE (including original methacrylic acid groups)
+
(C)
(B)
(A)
Me
NaO·
Me
I 'VV'VC I
/vv\-
NadF~
----+
atm.
Na 2
°
COl]
Me
+
I
~vvv
I /C~
·0
--+
CO 2
+ radical (A)
~O
Na2C03 Scheme 33
Degradation mechanism for ethylene/sodium methacrylate ionomer
15.11 BINARY POLYBLENDS AND RELATED SYSTEMS 15.11.1
Binary Polyblends
When two polymers are mixed, the system which results is almost invariably heterogeneous, involving a continuous and a dispersed phase. The degree of dispersion depends on the method of mixing. Common procedures are grinding together powders, milling or casting a film from a common solution of the two component polymers, and the degree of dispersion increases in that sequence. This is very relevant to the degradation behaviour of binary polyblends. For example, a powder mixture of PVC and PMMA behaves very differently from a film cast from solution. I IS The degree of dispersion is important because of the types of reaction which can occur during degradation of these systems. Since there are two discrete phases, any new reactions involving interactions must involve either reaction at the phase boundary between the two polymers or migration of some chemical species from one phase into the other. The evidence from the variety of polyblends studied is that any observed interactions are due to migration effects rather than boundary reactions. Either small product molecules or reactive species such as radicals produced in one polymer phase migrate into the other phase. In some cases interaction leads to destabilization, in others to stabilization, so that such processes are of major relevance to practical applications. Some illustrative examples are considered; two reviews have been published.P?: 127
487
Thermal Degradation
;
e ~
o
·c ~
(b)
a:
200
300
400
Temperature (OC)
Figure 17 TVA curves for degradation of films with 1: 1 ratio by weight of PVC to PMMA under vacuum at 10°C min -1, (a) for simultaneous but separate degradation of the two polymers, and (b) as a blend in the same film. Trap temperatures: O°C and -45°C, ------ - 75°C, - - - -100 °C, -196 "C. The shaded area represents HCI production; below 400 °C the remainder is due to MMA evolution (reproduced from Eur. Polym. J; 1970, 6, 679) _0_0-
15.11.1.1 Interactions due to small radical migration When films of PVC and PMMA are degraded separately at 10°C per minute, dehydrochlorination of PVC is observed at 225°C, some 50°C below the onset of the PMMA depolymerization (Figure 17a). However, when the two polymers are present in the same film, cast from a common solution, dehydrochlorination and depolymerization both commence simultaneously at 225°C. Large amounts of MMA are produced at this stage and a second stage of MMA evolution occurs after dehydrochlorination, the maximum rate for this process being 30°C higher than that for PMMA degraded alone. Clearly there is extensive interaction in the blend, destabilizing the PMMA, but it is also apparent from Figure 17b that the PVC dehydrochlorination is slowed down. A detailed investigation of this system 118 led to the conclusion that the most important effects are due to migration of chlorine radicals present as intermediates in the PVC dehydrochlorination into the PMMA phase, where attack on the backbone CH 2 or side group Me structure'S leads to subsequent chain scission and depolymerization (Scheme 34). Me
I
Scheme 34
Depolymerization of PMMA resulting from CI- radical attack
Thermal Degradation
488
Blends of PVAC with PMMA show similarities in behaviour to the PVC/PMMA system, again due to migration of small radicals, in this case MeC0 2 - , from one polymer phase into the other.l'" Acetic acid, like HC1, does not interact with PMMA in such a way as to facilitate monomer production. It is interesting to compare the behaviour of PVC/PMMA blends with that of PVC/PS blends. Production of HCl is again retarded because of the loss from the PVC phase of some Cl- radicals, which are no longer available to take part in the dehydrochlorination chain reaction. These Clradicals are believed to abstract tertiary H atoms from th PS; the resulting radicals undergo scission, but depolymerization of the PS does not occur in this temperature region, Extraction of PS from partially degraded PVC/PS blends, however, shows that the MW of the PS falls more rapidly than when PS is degraded separately.P" The effects observed in the degradation of PS/PAMS and PS/poly(ethylene glycol) (PEG) blends l 2 9 , 1 3 o can be explained in a similar way. In the former case, high molecular weight PAMS has little effect but very low MW samples lead to increased styrene production under the isothermal conditions used. In the PEG blends, the PS shows a more rapid fall in MW. At the temperatures used, both PAMS and PEG produce radicals. For samples of low initial MW radicals can result which are sufficiently mobile to migrate into the PS phase and abstract tertiary H atoms, leading to subsequent chain scission.
15.11.1.2 Interactions due to small molecule migration Interesting examples have been found of interactions in degrading polymer blends resulting from migration of a small product molecule from one polymer phase into the other. The results include structural modification of the polymer, catalysis of the degradation process and inhibition. The first leads to changes in degradation products, whereas the others influence mainly the rate of temperature range of degradation. When PMMA is degraded in a blend along with polymers which degrade to produce HCI or acetic acid, some of the ester groups may be modified so that an MMA copolymer results. The effect of reaction with HCI is to produce anhydride rings in the PMMA chain (Scheme 35).From studies of MMA copolymers, it is known that such structures will block depolymerization (Section 15.10.1), so that the presence of quite low concentrations of these rings greatly reduces the zip length for depolymerization. This has two effects: MMA evolution is retarded and PMMA molecules remain in the degradation apparatus to higher temperatures than when PMMA is degraded alone. The latter effect results in small amounts of alternative decomposition of the MMA units which leads to MeOH and CO, not normally produced in PMMA breakdown. Me
Me
MIlCH).---CH2~"""" I
I
~C...........
o
0
I
Me
C-OMe
II
0
Scheme 3S Anhydride ring fraction in PMMA resulting from H'Cl attack
Blends of polychloroprene with PMMA do not show the chlorine radical migration effects observed in PVC/PMMA blends, nor the stabilization of the dehydrochlorination reaction, both of which are consistent with the nonradical mechanism proposed for HCI production in the former polymer, but there is a major effect of HCI in producing PMMA stabilization.V" Blends of PVC with PMMA also show the effect of HCI, but this is masked by the earlier destabilization due to CIradicals. Acetic acid from PYA also leads to anhydride rings in PMMA with similar effects.'?" Blends of PVC with PVAC 9 6 show mutual catalysis of the decomposition of each polymer by the primary decomposition product of the other. This is seen both in. TVA studies similar to those illustrated for the PVC/PMMA system and from direct measurements of acid production, as illustrated in Figure 18, which show clearly the destabilization of each polymer.
Thermal Degradation
489
1:'
1ii
(3)
': 60
:c 'iii en
8. () 40 ~ en
2
t 1:'
20
1:'
'0
(2) __ __----e~
100
200
Time (min)
Figure 18 EGA data for acid evolution from PVC (-0-) and PVAC (-e-) films degraded alone (curves 4 and 2) and as a 1: 1 by weight blend (curves 3 and 1),at 220°C under vacuum (reproduced by permission of John Wiley and Sons Ltd from J. Polym. Sci., Part A-I, 1974, 12, 387)
Fortunately, in two of the most commercially important binary polyblends, PSjpolybutadiene (PB) and PSjpolyisoprene (PI), 131,132 the interactive effects lead to stabilization: the composition of the degradation products is not significantly altered, but the PS component is considerably stabilized. In Figure 19, the blend shows a major change in TG behaviour in the temperature region of the first weight loss stage, associated mainly with PS. Product investigations by subambient TVA for isothermal degradations of PSjPB blends reveal that initially the volatile products are those associated with the first stage of PB breakdown and that when these cease to be evolved, PS decomposition begins. It was concluded that 4-vinylcyclohexene produced from the PB acts as a radical scavenger for PS radicals, by virtue of the large number of allylic H atoms; this hypothesis has received support in further studies.P:' In the PSjPI blends, stabilization is also observed and a similar effect, in this case due to dipentene, is believed to be responsible. A further example of a product reacting with a polymer radical is provided by the case of blends of PMMA with polylactide.P" in which acetaldehyde produced from the polylactide is believed to react with PMMA radicals.
15.11.1.3 Other interactions In several of the polyblends studied, there is reason to believe that conjugated structures produced in one component as a result of partial degradation can act as a 'radical sink' for radical intermediates involved in the degradation of the other. Stabilization effects of PVC, PVA and PAN on the production of styrene from PS may be examples of this type of interaction. Purely physical effects may also occur in some blends. In the case of PVCjPAN blends, for example, it has been suggested that the observed destabilization of the PVC component is due to the PAN making release of HCI from the mixture more difficult, so increasing autocatalysis. ....~
1.0~-------...
c:
0.8
o
~ 0.6 ~
.E 0' 'i) ~
C 0.4 ~
1:'
'ij
a=
0.2
Figure 19 TG curves (dynamic N 2, 10°C min -1) for polystyrene/polybutadiene systems: curve (1) predicted behaviour for 1: 1 mixture by weight of PS and PB in absence of interaction, (2) experimental behaviour of 1: 1 blend, and (3) experimental behaviour of 1: 1 block copolymer (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1985, 10, 319)
490
Thermal Degradation
In the rare situation of a homogeneous polymer blend, interactions involving dissimilar macroradical/macromolecule pairs are theoretically possible. Interactions in the homogeneous twocomponent system PMMA/bisphenol A polycarbonate have been explained on the basis of PMMA radical attack on the polycarbonate chain.l " 15.11.2 Block and Graft Copolymers Degradation of these systems, which physically can be regarded as blends with a very high dispersion of one phase in the other, has received little study. A detailed examination of the styrene/butadiene diblock copolymers.P" however, has revealed a further enhancement of the stabilization effects in PS/PB blends, as illustrated in Figure 19. For the two component polymers, the degradation reactions are well separated, in the blend they overlap, but in the block copolymer they occur simultaneously. The same stabilization mechanism has been proposed and the enhancement of stabilization is due partly to the fact that short chain fragments from the PB blocks are being produced in the temperature range of PS volatilization. These are believed to act as radical scavengers in the same way as the 4-vinylcyclohexene produced at lower temperatures. The degradation behaviour of a number of graft copolymers prepared by mastication of PVC in presence of another monomer has ·been studied. Graft copolymers of PVC were so produced using methyl, ethyl and heptyl methacrylate respectively and also with a styrene or styrene/MMA mixture. 1 2 0 , 1 3 7 Again the effect of the higher dispersion is to enhance the interaction effects (e.g. methacrylate monomer production, PS fall in MW) observed in the corresponding polyblends.
15.12 POLYMERS WITH HETEROATOMS IN THE BACKBONE 15.12.1
Aliphatic Polyethers and Polyesters
Six polymers of simple structure have been investigated in sufficient depth for mechanisms of decomposition to be presented on a firm basis of product separation and analysis. The structures are shown as (6}-(11) below. The four polyesters (8}-(11) are all derived from polymerization of cyclic molecules containing one (II) or two (8}-(IO) chain repeat units; these cyclic molecules are shown as structures (12}-(15).
o JVV\CH 20VVV'
'VVVCH2CH2{)1vv\-
II
Me 0
vVV\CH2COvvv
I
Me 0
I
1/
I
/VVVC -COVVVl
/VV\ICH-C~
I
Me (9)
(10)
o 0 " I V\/\I1CH 2CH 20C-CO/VVV (11)
(15)
Polyoxymethylene (6) as prepared has OH ends; the polymer is of low stability and degrades to formaldehyde in 1000/0 yield from about 100°C. The stability is improved by acetylating the chain ends, which indicates that end initiation of depolymerization plays an important part, but the product of degradation of the end-capped polymer is the same. The formation of formaldehyde has been explained 138 by a nonradical mechanism as shown in Scheme 36. A similar six-centre transition state may be envisaged, leading to the release of two molecules of HCHO at each step. \NV\OCH 20CH 20CH 2QTcH 2
k~
•
'VVV()CH20CH 20CH20
k
+
CH 20
etc.
?:c.i
H2
vvvOCH2 OCH 20CH2
MetJ-o
II
o Scheme 36 Degradation mechanism for normal and end-capped polyoxymethylene
Thermal Degradation
491
Poly(ethylene glycol) (7) polymers of low MW (1000 and 1500) have been studied.P? Water and ethylene glycol are believed to be produced in chain terminal reactions; the remaining products, consisting of chain fragments of lower MW acetaldehyde, formaldehyde, ethylene oxide and methoxy- and ethoxy-acetaldehyde, are all satisfactorily explained by a radical mechanism involving initial homolysis of C-C or C-O bonds. The polymer begins to give volatile products from about 300 aC. The degradation reactions of polyglycolide (8),140,141 polylactide (9)134,141, 142 and poly(tetramethylglycollide) (10)143,144 are each characterized by ester interchange reactions at lower temperatures leading to the cyclic dimer (12HI5) and also to oligomers in the cases of polyglycollide and polylactide. Cyclic oligomers and cyclic dimers can be produced by essentially similar intramolecular mechanisms. Carothers' picture of a ring ~ chain equilibrium involving the cyclic dimer as the breakdown product at elevated temperatures is only partially correct in these cases, since a higher proportion of cyclic oligomers than cyclic dimers is in fact observed. When the temperature is further raised, polymers (8) and (9) also undergo homolysis leading to additional products including CO 2 and ketenes. The homolysis also produces the cyclic dimer and chain fragments. In the case of poly(tetramethylglycollide) the cyclic dimer degrades quantitatively to methacrylic acid if it is not efficiently removed from the hot zone. Poly(ethylene oxalate) (11) differs from the above polymers by breaking down by homolysis at the oxalate linkage in the polymer chain, since in addition to 49% of ethylene oxalate (15) and 300/0 of oligomer by weight, a variety of other volatile products are produced from the lowest degradation temperatures and not only at higher temperatures as in the case of polyglycollide and polylactide. All the observed products are satisfactorily explained by the free radical mechanism shown in Scheme 37.1 4 5
° °
II " vvvCH 2CH20C-COCH2CH 2 vvv
---I".
?i
2IVV\CH 2CH20C'
00 00 00 0 II II II II II I II I I I I II .lVV\.CH2CH2o-C--r-c0(CH2CH20C-CO)"CH2CH20C-r-C---r-o-rCH2CH2To--t-c· ----. chain scission I
(f)
(e)
Scission at (e) (f)
~ _ _~.,
(a) --~~ (b) - - ~. (d) - - ~. (b) + (c) --~.,
(d)
(c)
(b)
(a)
monomer} oligomer major products
co
CO 2 ethylene carbonate ethylene
Scheme 37 Degradation mechanism for poly(ethylene oxalate)
15.12.2 Poly( ethylene terephthalate) The degradation of PET has recently been reviewed. 146 The behaviour is complex and sensitive to the presence of impurities. The thermal stability of the polymer is fairly high: it begins to break down between 250 and 300 "C, but evolution of volatile products only becomes rapid above 350 "C, The initial step in the degradation involves nonhomolytic scission at an ester linkage to produce acid and vinyl ester end groups (Scheme 38). Reaction of the latter with hydroxyl ester end groups in the PET by transesterification leads to the elimination of acetaldehyde (formally, vinyl alcohol is the product, but rearrangement occurs) which is the principal observed volatile product. These are only the initial
Scheme 38 Chain scission and acetaldehyde formation in PET degradation
Thermal Degradation
492
reactions in a complex pattern of breakdown which must also include homolytic reactions at higher temperatures since CO, CO 2 , CH 4 , C 2H 2 , C 2H4 and benzene have also been observed. The subambient TVA curve for the separation of the principal condensable products from a commercial PET sample degraded to 500°C under TVA conditions is illustrated in Figure 20. Terephthalic acid appears as a solid product of degradation by sublimation from the hot zone and the proportion of acid end groups in the degrading polymer increases as these are formed and the OH ends are used up. (2)
(3)
oae Figure 20 Subambient TVA curve for warm up from -196°C to O°C of condensable volatile products of degradation in vacuum to 500 °C at 10°C min -1 in a TVA system of poly(ethylene terephthalate). Assignments: (1) ethylene, (2) CO 2, (3) acetaldehyde, (4) unidentified minor product, (5) water, (6) unidentified high boiling liquid
15.12.3 Polycarbonates Aliphatic polycarbonates such as poly(trimethylene carbonate) (16) and poly(neopentylene carbonate) (17) degrade mainly by an intramolecular exchange reaction to give the cyclic monomer, chain fragments and CO 2 • A radical process involving random scission becomes more important as the temperature is increased. The introduction of a bulky phenyl substituent in poly(2-phenyltrimethylene carbonate) (18) hinders cyclic monomer formation and this polymer gives mainly short chain polymer fragments. Poly(p-xylene carbonate) (19) also degrades to give short chain fragments as the predominant product. In addition, CO 2 and p-tolualdehyde are formed.
° "
/\/\/'vCH 2CH2CH 20 C OIVVV
Me
I
./\/VI-CHf:CH 2OC O/VV'
I
Me (16)
°I
(17)
°II
.N\/\CH 2CHCH 20COvvv
I
Me (18)
(19)
The most extensively studied polycarbonate is the commercial polymer based on bisphenol A. This is more stable than the polycarbonates mentioned above and begins to lose weight at temperatures above 350°C, under programmed heating at 10°C per minute. The reported products of degradation include chain fragments, bisphenol A, phenol, diphenyl carbonate and CO 2 • The first processes to occur in the degrading polymer are believed to be hydrolysis and phenolysis at the carbonate group. Branching reactions occur due to thermal rearrangement of the carbonate group and eventual gelation takes place due to the removal of the volatile products formed during degradation. A mechanism has been proposed.I"? the main features of which are given in Scheme 39. There is some evidence from TVA studies of bisphenol A polycarbonate, alone and in polycarbonate/PMMA blends.l " that radical reactions also playa significant part in the degradation process.
2vvvO"
o
~O~~
+
"20 -
+
CO 2
~O" +Oo~oQ-{~ -~-o~oOi~+ 0" Me
0
Me
o
2
vvvQo~oO
-
o
2
H-
Me
Me
Me
Me
0
~O~004DO" Me
~0-'~'O~
CO 2 "
-
Me
~
~
s
"""t
f2.. ~
-. vvvQ0co~ "OiDO" ~
Me
.
+
0
II
~ ~C.H.OCOC.H.~~
Scheme 39
Me
AAA/OO~0
'--II
.
0
--...()
~-:,--C
0
~
~ """t ~ ~ ~
Me
c" ("Oot.
~
<-o
f~+~" +
CO 2
Principal reactons in the thermal degradation of bisphenol A polycarbonate
~
\0
W
Thermal Degradation
494
15.12.4 Polysiloxanes The stability of polydimethylsiloxanes has been found to increase with MW and to be highly sensitive to the presence of traces of KOH, used industrially as a polymerization catalyst. The stability is improved by replacing the normal hydroxyl end groups by trimethylsilyl structures, which indicates the importance of hydroxyl end initiation as a degradation route. These effects are illustrated in the TG data of Figure 21.
§ 0.8
U
.g
.E 0.6 C' '0) ~
'0 0.4 ::3
~
.iii
~ 0.2
0--""-----..........,;:---'""""'--------------' 400 500 200 300 100 Temperature
(Oe)
Figure 21 TG curves (dynamic N 2 , 10°C min -1) for poly(dimethyl siloxanes). Mn values of (1)94 500, (2) 111500, (3) 183000 and (4) 258000; (5) Mn 111 500 sample after end capping; and (6) Mn 111 500 sample plus 50/0 KOH (reproduced from Eur. Polym. J., 1978, 14,875)
The suggested mechanismsv'" to explain the formation of the same products (cyclic trimer and higher cyclic oligomers) from hydroxyl-ended, end-capped and KOH-containing polydimethylsiloxane are shown in Scheme 40. Me
Me Me
\/
\/
Me Me
Me
\/
vvvSi--
TJ r l , H----4)--Si-Q
/
Me
"-Me
Me
Me Me
Me
Me
/VVV~i--r-O-Si--{) ~:
/"
I
vvvo-----CSi--D--Si Me
/"
Me Me
Me
/ \
Me
Me
,,/Me
/
~i----r-O--Si---O
1
'+~
HO- VVV\()-,.-Si--o--Si ~/
Me
Me
1 Me Me Me 0 Me < > ,,~/ -vvvSi + Si Si I /1 I' OH Me 0 0 Me S(;
Me
'\../
,,/
\./
Me
Me
~Si/ I
!
-,
Me
/ \
Me
Me
+
08
+ IVVVSiMe20-
Scheme 40 Degradation mechanisms in OH-ended, end-capped and KOH-containing polydimethylsiloxanes as at present
The replacement of some of the methyl groups in polydimethylsiloxane by phenyl groups improves the thermal stability but reverses the dependence of stability on MW .!10ted above; in addition, benzene is produced in small amounts and insolubility develops, even at low temperatures. A process of OH-assisted cleavage of Si-phenyl bonds, leading to chain branching, has been proposed.v" which accompanies a similar process of cyclic oligomer formation to that given in Scheme 40 for polydimethylsiloxane.
15.12.5 Other Polymers Some aspects of the degradation of polyurethanes are considered subsequently in relation to the fire situation. A number of other polymers of complex composition involving 0 and N atoms in the chain structure, including some of the most thermally stable organic polymers" have been discussed
Thermal Degradation
495
in a recent monograph.l?" The degradation behaviour of urea formaldehyde resins has also been examined. 149
15.13 DEGRADATION OF POLYMERS IN THE PRESENCE OF FIRE RETARDANTS An essential requirement for the combustion of a polymer is that it produces a volatile fuel which can mix with oxygen. The process is started by heat and sustained by the heat of combustion of the fuel. In order to retard or prevent burning, this cycle must be interrupted. There are several ways of doing so: removal of heat, quenching of the flame, preventing access of oxygen or modifying the reaction which generates the volatile fuel. Fire retardant additives may therefore act in various ways and in anyone polymer/additive composition there may be more than one effect. Attention is limited here to effects (chemical or physical) on the polymer decomposition process and to examples of investigations in which the nature of the interaction as it affects the degradation mechanism of the polymer has been established. Some of the most detailed investigations have been made in the absence of oxygen; this is appropriate, however, because it reflects the situation in the condensed phase from which the fuel is generated, since the oxygen is removed from the polymer surface by the burning fuel. Useful recent reviews 1 5 0 , 1 5 1 and a monograpb 'V are available. 15.13.1
Poly(methyl methacrylate) with Ammonium Polyphosphate
Several detailed studies have been made in which the fire retardant additive is ammonium polyphosphate (APP). It is necessary first to understand the effect of heat on APP as the temperature is gradually increased. In the temperature region 10o-260°C, less than 50/0 weight loss (as ammonia and water) occurs. Some free acid groups are formed, which condense to form crosslinks. The physical state changes from powder to a glassy, hygroscopic solid, from which gas evolution is less easy. The P-O-P links produced are easily hydrolyzed to acidic groups. Between 260 and 350°C the rate of evolution of NH 3 and H 2 0 goes through a maximum and declines to zero after 20% weight loss. The product is polyphosphoric acid, a hygroscopic glass. In the final stage above 350°C (which may be too high a temperature region to influence some polymers), the polyphosphoric acid structure is fragmented with the formation of low volatility products. The effects of APP are therefore likely to be due to one or more of these: evolution of NH 3 and H 2 0 , production of polyphosphoric acid or acidic species derived from it, and the glassy state of the intermediate decomposition product. When PMMA is heated with APP, chemical changes in the PMMA occur only above 260°C in the heating programme, i.e. after NH 3 and H 2 0 evolution has ceased and polyphosphoric acid is present. The observed effects are believed to be due to mobile fragmentation products rather than the crosslinked polyphosphoric acid itself.20 The primary effect is to cause ester groups to be converted to anhydride rings, a small concentration of which is sufficient to interfere significantly with the depolymerization process. Thus the production of monomer (which is the volatile fuel in a fire situation) is slowed down. When the temperature is increased, the products include, in addition to monomer, methanol, CO 2 and CO. There is therefore a close parallel with the behaviour of PMMA in other acid-releasing environments, such as blends with polychloroprene (see Section 15.11.1.2). 15.13.2 Polyurethane with Ammonium Polyphosphate A comparison of the degradation behaviour of a polyurethane (PU) alone and in the presence of APP has revealed several interesting features.P? The PU studied had the structure shown in Scheme 41. Under programmed heating (10°C per minute), it begins to break down at 250 °C, the primary products being the reactants used in its preparation, butanediol (BD) and methylene bisphenyldiisocyanate (MBPI) (see Scheme 41). Since these are of low volatility, some decomposition occurs as they diffuse out of the polymer, leading to CO 2 , tetrahydrofuran, butadiene and water, as indicated in the subambient TVA data of Figure 22a. It is possible that the ease of ignition of PU foams is associated with the evolution of some butadiene when the polymer is heated. The THF and water derive from decomposition of BD, whereas CO 2 results from the condensation of pairs ofMBPI molecules to produce carbodiimides (CDI). Butadiene is believed to result from reaction of BD with CDI. PS6-Q
Thermal Degradation
496
butanediol Scheme 41
methylene-bis-phenyldiisocyanate
Primary decomposition in a polyurethane
(0 ) (I)
(3)
~
;o
(b )(1)
·c ~
a..
o
10
20 Time (min)
30
40
-196°C
Figure 22 Subambient TVA curves for warm up from -196°C to O°C of condensable volatile products of degradation under vacuum to 500°C at 10°C min -1 in a TVA system of (a) a polyurethane sample, (b) the same polyurethane sample in the presence of 10% of the fire retardant ammonium polyphosphate. Assignments: (1) CO 2 , (2) butadiene, (3) tetrahydrofuran, (4) dihydrofuran, (5) water, (6) formaldehyde, (7) aniline (reproduced by permission from 'Developments in Polymer Stabilization', ed. G. Scott, Applied Science, London, 1979, vol. 1, p. 197)
When APP is present with the PU, major effects are observed which can be attributed to the presence of polyphosphoric acid. The volatile products (Figure 22b) still include CO 2 , THF and water. Notable differences, however, are the complete absence of butadiene and the appearance of three undesirable toxic products: ammonia, formaldehyde and aniline. There are also changes in the less volatile products: no BD is formed, there is less CDI and MBPI, C =N and P-O-C structures are evident in the solid and there is a substantial residual char. The chemistry involved is complex, but in essence the changes are due to polyphosphoric acid acting as an acid catalyst and as a crosslinking agent. The burning process is affected in two ways: the amount of fuel (primary products and butadiene) is reduced and the formation of char impedes the transfer of heat to the polymer. Unfortunately, these benefits are offset by the formation of new and very toxic products. A further illustration of the effect of APP as a fire retardant is to be found in studies of the behaviour with poly(ethylene glycolj.P" In this case the APP transforms the predominantly radical process in the pure polymer into a hydrolytic reaction leading to different products. 15.13.3 Polypropylene with Chlorinated n-Alkanes The degradation of PP has been studied in the presence of Cereclor 70, a commercial chloroparaffin (CP, chloro-n-alkanes) fire retardant containing 700/0 chlorine.l ": 1S6 Unlike the situations above, where the temperature regions of decomposition of polymer and APP show overlap, PP and CP degrade under programmed heating conditions at very different temperatures, as may be seen from Figure 23.
Thermal Degradation
497
c:
-Bo .g
0.8
~ 0.6
'i) ~
-
e
0.4
~
"C
.c;;
~ 0.2
O------r..~--..........---"""'-------I------:l~ 200 300 400 500 Temperature (Oe)
Figure 23 TG curves (dynamic N 2, 5 °Cmin -1) for (1) polypropylene, (2) chloroparaffin fire retardant, (3) PP plus 30% CP by weight, and (4) calculated behaviour for the same mixture assuming no interaction (reproduced by permission of Elsevier Applied Science Publishers Ltd from Polym. Degradation Stab., 1981, 3, 423)
One view of the effectivenessof fire retardants is the 'right place at the right time' concept, i.e. that both polymer and fire retardant should be decomposing simultaneously. On this basis, CP would not be predicted to be an effectivefire retardant for PP, since the decomposition of the fire retardant would be complete before the PP begins to decompose to produce the fuel for burning to occur. This proves not to be the case, however, as the TG curves reveal some form of interaction, since the behaviour of the mixture is not simply an addition of the weight losses of the components. The nature of the interaction has been established by EGA and MW measurements. It has been found that at 260-280°C the CP dehydrochlorination rate is reduced in the mixture and there is a substantial increase in chain scission in the PP. This type of behaviour is also observed in PVC/PS blends and the interaction in the PP/CP mixture has been explained in the same way. The physical state of the mixture is heterogeneous. The CP component produces Cl- radicals as chain carriers in the dehydrochlorination; some of these migrate into the PP phase where they abstract tertiary H atoms and so induce chain scission. This leads to an alteration in the product distribution from the PP in such a way as to increase the proportion of materials of higher limiting oxygen index, .so reducing flammability. The loss of Cl- chain carriers stabilizes the CP and the stability of the PP is reduced, so that in the mixture the PP produces volatile products in a temperature region where the CP flame-quenching effect (due to HCl) is effective. As in the other systems already considered, condensed phase interactions determine the effectiveness of the fire retardant.
15.13.4 Other Fire Retardant Additive Systems The dehydration reaction in aluminum oxide trihydrate is strongly exothermic (1170 J g- 1). Since alumina is inert towards polymers, it is probable that the effect of this very widely used fire retardant additive is simply to remove heat. Boron compounds used as fire retardants may also owe much of their effectiveness to such 'heat sink' effects due to dehydration or, for ammonium salts, loss of ammonia. A second mode of action may be connected with the formation of a glassy layer at the surface. Boron compounds, however, cannot be regarded as inert, since there is evidence of borate ester formation with cellulose, for example.P! The ultimate effect is increased char formation, which impedes heat transfer. Antimony compounds are generally used as fire retardants in synergistic combinations with halogen compounds. The mechanism of action is not well understood, but may be associated with effects due to the release of the hydrogen halide, acting possibly both in the condensed phase to modify the decomposition of the polymer and in the gas phase as a flame quencher.
15.14 REFERENCES 1. N. Grassie, 'Chemistry of High Polymer Degradation Processes', Butterworths, London, 1956. 2. H. H. G. Jellinek, 'Degradation of Vinyl Polymers', Academic Press, New York, 1956. 3. S. L. Madorsky, 'Thermal Degradation of Organic Polymers', Interscience, New York, 1964. 4. R. T. Conley, 'Thermal Stability of Polymers', Dekker, New York, 1970. 5. C. David, in 'Comprehensive Chemical Kinetics', ed. C. H. Bamford and C. F. H. Tipper, Elsevier, Amsterdam, 1975, vol. 14, p. 1.
498 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76.
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Thermal Degradation 77. 78. 79. 80. 81. 82. 83. 84.
499
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Chem., 1966, 99, 59; 1967, 108, 241. 99. B. Dodson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1974, 12, 2305. 100. H. E. Parker, in 'Treatise on Coatings', eds. R. R. Myers and R. S. Long, Dekker, New York, 1967, p. 129. 101. G. M. Burnett and R. A. Haldon, Eur. Polym. J., 1967, 3, 449. 102. D. L. Gardner and I. C. McNeill, Eur. Polym. J., 1971, 7, 569, 593, 603. 103. L. A. Wall, 'Fluoropolymers', Wiley-Interscience, New York, 1972. 104. J. P. Critchley, G. J. Knight and W. W. Wright, 'Heat Resistant Polymers: Technologically Useful Materials', Plenum Press, New York, 1983. 105. P. K. Dhal, G. N. Babu and J. C. W. Chien, Polym. Degradation Stab., 1986,16,135. 106. N. Grassie, Trans. Faraday Soc., 1952, 48,379; 1953, 49, 835. 107. A. Servotte and V. Desreux, J. Polym. Sci., Part C, 1968, 22, 367. 108. A. Jamieson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1976, 14, 1839. 109. A. Jamieson and I. C. McNeill, J. Polym. Sci., Polym. Chern. Ed., 1976, 14, 603. 110. S. 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