Chapter 1 Thermal Degradation of Polymers

1,2-Dichloride structures were studied by Erbe et al. [190] and are thought to be more stable than 2,4dichloropentane which represents the normal structure of the polymer. 9.5 WEAK LINKS IN PVC

The results given in the preceding section 9.4 on model compounds would seem to suggest that the normal structure of PVC is very stable. Anomalous weak links are thus probably responsible for the degradation of the polymer. The potential weak links in PVC are (i) 1,adichloride structures -CH2-CH-CH--CH2-

I

I

c1 c1 (ii) vinyl chlorine at chain ends

-cH=YH c1

(iii) primary chlorine at chain ends --CH2-CH2-C1 (iv) allylic chlorine in the chain --CH2-CH-CH=CH-CH2-

I c1

References p p . 165-1 7 3

90

(v) allylic chlorine at chain ends -CH2-CH-CH=CH2

I

c1 I

CH2 I (vi) tertiary chlorine -CH2-C-CH2-

I

c1 These structures are formed during preparation of PVC in the following ways. Structure (i) is formed by combination termination or tail-to-tail addition. Structures (ii) and (iii) are formed by disproportionation termination. Structure (iv) results from HC1 elimination. Structure (v) results from termination transfer to the monomer by C1abstraction and reinitiation by the radical formed. Structure (vi) results from termination transfer to the polymer by hydrogen abstraction and reinitiation at the radical site in the macroradical. From the considerations in the preceding section 9.4, it may be inferred that the order of increasing stability of these chain structures is (a) 0-chloro-unsaturated structures in the chain, (b) tertiary chlorine, (c) P-chloro-unsaturated structures (allylic chlorine) at chain ends; (d) tertiary hydrogen and normal head-to-tail structures; (e) vinyl chlorine at chain ends; (f) head-to-head structures. In order to verify these assumptions experimentally Gupta and St Pierre [191] prepared PVC containing the structures

Q

-CH,-CHCl-&CHCl-CH,-

I

and

CH3

(C) and (D) were obtained by copolymerizing vinyl chloride and, respectively, l-chloropropene and 2-chloropropene. The mole ratio of vinyl chloride t o the second monomers were, respectively, 4.33/1.08 and 7.40/1.06 in the copolymers.

91

200

240

200

320

Tern pemtu r e ("C)

Fig. 51. Relative thermal stability of polyvinylchloride samples by dynamic thermogravimetry. 0, PVC(A);0, PVC(E);A, PVC(C);V,PVC(D)[191].

The relative thermal stabilities obtained by dynamic thermogravimetry above 200°C and by isothermal methods at 220°C are given in Figs. 51 and 52. (A) is a slightly branched polymer sample produced using benzoyl peroxide. Polymer (B) is a linear syndiotactic sample prepared at low temperature. The figures show that copolymers (C) and (D) are both much less stable than (A) and (B). Tertiary hydrogen and tertiary chlorine are

Time (min)

Fig. 52. HCl evolution at 224OC.0, PVC(A);0, PVC(B); A, PVC(C);V,PVC(D) [191]. References p p . 165-1 73

92 thus weak spots for the initiation of dehydrochlorination. The distribution of polyene length is not given. The relative stabilities deduced from the study of model compounds are thus in agreement with these results obtained by Gupta and St. Pierre on polymers. PVC containing allylic chlorine has not been synthesized as yet. The study of its stability would give valuable information concerning the behaviour of the links that are supposed to be the weakest in PVC. It has, however, been shown in section 9.2.5 that if allylic chlorines are esterified by reaction with organometallic compounds, stabilization of the polymer occurs. 9.6 MECHANISM OF DEHYDROCHLORINATION OF THE POLYMER

Both free radical and molecular mechanisms have been proposed. 9.6.1 Free radical mechanisms

Free radical processes were first postulated by Arlman [ 1661, Winkler [192] and Stromberg et al. [165]. The rate of dehydrochlorination of PVC was reported t o be a 3/2 order reaction by Stromberg et al. [165] and the mechanism suggested was the following. Initiation

H H H H ( I l l

-c-c-c-cI l l

1

c 1 H c1 H

kl

H H H H I I I I

-c-c-q-c-++1 I I I C1 H

(95)

H

Propagation

H H H H I I I I

-C-C-C-C!-

I

I

I

I

+.C1

H C1 H C1 H H H H 1 1 1 1

-c-c-q--cI

I

H c1 Termination

I

C1

-

k3

k2

H H H I I I -C<-C-C1 1 . H C1

H H H H 1 1 1 1

-c-c-c=cI

I

H C1

H I 1 C1

+ c1*

+HCl

(97)

93 A 3/2 order reaction was also observed by Salovey and Bair [171]. The overall activation energy was reported t o be 27 kcal mole-' between 126 and 155OC and zero between 78O and 12OoC. A more complex chain mechanism was proposed. Initiation is identical to eqn. (95) but propagation proceeds according to

H H H H

I

l

l

1

4-C-C-C-

1

H

-

1

H H H H

I I I .

kP

1

I

-C-C-C=C-

--+

I

+ HCI

H

H C1

C1 H

Transfer occurs according t o H H H

I

I

*-(C=C),

I

H H H H

+ -c-c-c-c-

I

I

I

I

H C1 H C1

-

H H H I I 1

H H H H I I I I

c1

1 - 1 1 H H H

Taking into account their observation that the rate of dehydrochlorination in solution is first order and inversely proportional to the number average degree of polymerization, Bengough and Sharpe [ 1611 proposed the following end-initiated mechanism

I I

CHCl=CH-kH-CH2+HCl-CH2-

CHCl=CH-CH=CH-kH-CH,-

+ C1'

+ HC1 etc. References p p . 165-1 73

94

It may be that occasionally C1- atoms escape from their environment, attack a second polymer molecule and thus start a short chain. Between 1 7 8 and 212°C the rate expression proposed is

where [HCl] is expressed in mole 1-' and C, is the polymer concentration in polymole 1-' . Another route for initiation of PVC dehydrochlorination has been suggested by Russian workers [152] who maintain that a system of conjugated double bonds can be partly present in the triplet state even at room temperature if the sequence is long enough; the existence of triplet states of polyene chains at 170-200°C is certain. They propose that the polyene chain in the triplet state takes part in the reaction, for example,

t

t

-CH-CH=CH-CH=CH-CH=CH--CH-CH.-(CH=CH)4-CH-

1

c1

-

tl

+

c1 -(CH=CH),+H-

+ C1'

Conversion of the polyene into the ground singlet state yields energy that most probably excites the C-C1 bond into the triplet state. A chlorine atom is then formed and initiates dehydrochlorination by a radical chain mechanism. Another route for the formation of chlorine atoms is

t

--CH-(CH=CH),,-CH-

t

H

-kH-(CH=CH),-CH2-(a)

-(cH=cH),-~H-cH-

-

+ -( CH=CH),,,--CH%HI I

I

c1

-

C1

+ -(CH=CH),

-+H--CHI c1

(b) - ( C H = C H ) ~ + ~ - -+ c b

The triplet polyene interacts with another macromolecule containing hydrogen in the 0-position t o conjugated double bopds giving two

95 stabilized macro-radicals (a) and (b). The latter decomposes giving chlorine atoms which initiate polymer degradation. The polyene triplet state may also yield benzene

which is in fact observed in the degradation products. The participation of free radical intermediates has been proved by Bamford and Fenton [ 1931 by degrading PVC a t 180°C in labelled toluene. Since toluene becomes incorporated in the chain, free radicals must be intermediates. Investigation of the thermal degradation of mixtures of PVC and polymethylmethacrylate [194] shows that the elimination of HC1 is retarded in the presence of the methacrylate. Figure 53 shows the experimental results obtained by programmed or isothermal thermogravimetric analysis. The rate of gas evolution is measured with Pirani gauges situated behind cold traps, the temperatures of which are indicated on the figure. Methylmethacrylate is completely condensed at -lOO"C, whilst HC1 is not trapped at that temperature. The PVC is thus more stable in the mixture, but the polymethylmethacrylate is initially less stable and gives monomer at temperatures corresponding to PVC dehydrochlorination. This interaction is explained in terms of attack on polymethylmethacrylate by chlorine atoms from the degrading PVC. Many other polymer blends and block copolymers have been studied (section 14). A slight retardation of HC1 evolution is generally observed. The strongest evidence invoked for a radical elimination is the presence of an ESR absorption, which has been reported by many authors. Degraded solid PVC exhibits paramagnetic character, since a broad singlet line is observed with a polymer that has lost 10%of the total HC1 [195]. The signal intensity decreases with temperature but is stable over many weeks with the polymer in vacuo. This singlet has, however, been shown to be associated with the polyene system and not with a radical, since it is present in air and in solutiondegraded material and its intensity does not alter with time [ 1951. Stronger evidence for free radical generation in PVC at 220°C has been obtained more recently by Liebman et al. [196]. The radicals appear in the early stages of dehydrochlorination, and decay rapidly. Reference6 p p . 165-1 73

96

.-

C

e

-h-u

3-

c)

0

a

2-

--------___

1-

O r 4 200

-75.

300

400

E 0

Sample temperature (%I

Fig. 53a. Differential condensation TVA curves for 20 mg film samples of ( i ) P v c Breon 113, (ii) PMM FR 1. Heating rate 10 degC min-' [ 1941.

3-

-

> 1E. c

3

c a

a

.-

(i i)

C 0 L .-

-

a 3u c 0

a

200

300

E

Sample tempemture('C)

Fig. 53b. Differential condensation TVA curves for simultaneous degradation of PVC Breon 1 1 3 and PMM FR 1, 20 mg of each; ( i ) unmixed, (ii) mixed. Film samples, heating rate 1 0 degC min-' [ 1941.

97 Another type of argument to support a free radical mechanism was advanced by Palma and Carenza [ 1721. Thermal and y-initiated dehydrochlorination between 80 and 130°C were compared. In view of the resemblance shown by the kinetic data for polyene formation, the same mechanism was thought t o be operative in both cases. Since according to the authors, a free radical mechanism is clearly established for y-initiated processes, this is also operative in thermal degradation. Salovey and Bair [171] reported that the thermal degradation of PVC at 155°C is enhanced by irradiation with 1 MeV electrons. Since later stages of isothermal weight loss for thermal and radiolytic decomposition follow 3 / 2 order kinetics, a free radical mechanism is also postulated by these workers. 9.6.2 Molecular elimination

Many experimental results, however can not be explained by a free radical mechanism. Bengough and Sharpe [161] found that addition of radical inhibitors or radical precursors have no effect on the rate of degradation. But this is not a good argument, since little is known of the effect of such inhibitors at high temperatures. The effect of free radical sources like AIBN is also not conclusive since there is no agreement between the results in the literature. According to Bengough and Sharpe [161], they have no effect on the rate of dehydrochlorination in solution, while Geddes [ 1481 reports a strong acceleration effect of these additives in PVC films. Stronger evidence against the free radical mechanism arises from the catalytic effect of HC1. This is indeed difficult to explain if the dehydrochlorination involves free radicals. If, on the other hand, molecular elimination with a cyclic transition state is operative; namely, Cl---H * * *

IPCI

-CH2+H-CH-CHCl-

* * *

-HCl

-CH2+H=CH-CHCl-

the following modified process seems reasonable in the presence of HC1 [169].

p-3 $2 - - - -CH2
--+

.

-CH2-CH=CH-CHCl-+

2 HC1

*

It must also be noted that the products of chain termination H2 and C12, in the radical mechanism proposed by Stromberg et al. [165] (section 9.6.1) have not been detected. The following observations are in agreement with molecular elimination: (a) the model compounds in the gas phase d o not lose HC1 by a radical mechanism, and References p p . 165-1 73

98 (b) the dehydrochlorination of PVC proceeds violently in the presence of FeCl,, ZnClz, etc. Therefore, molecular, non-radical dehydrochlorination is suggested by many workers t o be the most probable mechanism for dehydrochlorination of PVC. A general kinetic treatment of polymer degradation by elimination has been presented by Tudos et al. [170, 1971. The detailed elementary reactions taking place in the course of degradation are not taken into account. The main features of the model are (i) The initiating step of the process is the random elimination reaction, which is unimolecular from the point of the polymer molecule. (ii) Chain propagation takes place in a series of activated unimolecular elimination reaction steps. The activation propagates in only one direction, determined by the head-to-tail linkage. (iii) The chain termination takes place either by the reaction chain reaching the end of a polymer sequence given a priori (it is determined by the length distribution of the polymer sequences), or by the occurrence of a reaction step interrupting the propagation of activation (that is, a reaction step which gives a product that has no activating effect on the neighbouring monomeric unit). The values calculated according to this model for the yield of HC1, and concentration and distribution of polyene sequences are in agreement with the experimental results that Tiidos et al. obtained for the degradation of polyvinylchloride. 9.7 CONCLUSION

It is quite clear that free radicals are formed during the thermal degradation of PVC. On the other hand, the existence of molecular elimination also seems probable. There are two ways of correlating the data. (a) Free radical and molecular elimination occur simultaneously and their relative importance depends on temperature, the former becoming more important at higher temperatures. This view is suppoked by the work of Rasuvaev et al. [ 1521 and seems very promising. (b) Dehydrochlorination occurs by a molecular elimination process, but free radicals are generated in the system by the breaking of weak bonds, such as peroxide or hydroperoxide, which become incorporated in the system during polymer synthesis, purification or processing.

10. Other vinyl polymers 10.1 POLYVINYLACETATE

When heated, polyvinylacetate liberates acetic acid, and conjugated double bonds are formed in the residual polymer. It was suggested, a.long

99 time ago [198],that the degradation proceeds by a non-radical chain mechanism initiated at chain ends and propagated from unit t o unit along the chain. The problem was more recently re-examined by Servotte and Desreux [ 1991.The loss of weight was measured a t constant temperatures between 230 and 300°C. Acetic acid proved t o be the most abundant volatile product formed (90-95%). The rate of weight loss was independent of the initial molecular weight, in disagreement with end initiated degradation. Insolubilization was found to develop in the residue even after a small weight loss. Infrared analysis of the heated polymer performed at different degrees of degradation shows that the absorption in the 1600 cm-' region (double bonds) is very weak, while progressive deacetylation of the polymer takes place. Some acetic ester groups, however, remained in the polymer when 70% weight had been lost. The residue was nevertheless observed to be deeply coloured in agreement with the existence of conjugated double bonds. Owing t o the important discrepancies between the results and mechanisms proposed by Grassie [ 1981 and Servotte and Desreux [ 1991,further work is needed. A thermal volatilization analysis of polyvinylacetate degradation has been reported by Gardner and McNeill [200] (Fig. 54).Two maxima are observed at 322 and 435°C. The Pirani gauge situated after the 0°C trap responds t o all volatile products, while the gauge situated after the -196°C trap responds only t o non-condensable gases. Infrared analysis has shown the presence of carbon monoxide and methane in this last fraction. The

t

0"

Temperature ( " C )

Fig. 54. Thermal volatilization analysis for polyvinylacetate. 100 mg film sample, 5 degC min-' 1200 1 . References p p . 165-1 7 3

100 differences in response between the -75OC and -196OC curves were assigned t o the formation of water, carbon dioxide and ketene. These last two products have also been identified in the products that are volatile at -75OC. The two peaks shown in Fig. 54 thus correspond to the same products: acetic acid, methane, water, ketene, carbon monoxide and dioxide. UV spectra (Fig. 55) of degraded polyvinylacetate films show the presence of new absorptions, which have been assigned by the authors to 2-4-6-octatriene chromophores (triad centred at 272 nm), and t o longer polyenes which extend the absorption up to 400 nm.

W a v e l e n g t h (nm)

Fig. 55. UV spectra for PVA as 2 0 mg films initially..(a) undegraded, (b) degraded to 289OC, (c) degraded to 301°C, ( d ) degraded t o 313OC, ( e ) degraded to 332OC, (f) degraded t o 3 8 l o C , at 5 degC min-' [ 2001. 10.2 POLY VINYLALCOHOL

The thermal degradation of polyvinylalcohol has been studied by several workers [201-2041. The main products of degradation have been shown to be water from an elimination reaction similar t o that observed with polyvinylacetate, and acetaldehyde from depolymerization. Small amounts of aldehydes and ketones of the general formula OCH-(CH=CH),-CH3 and CH3-CO-(CH=CH),-CH3 were also detected by Tsuchiya and Sumi [ 2041. Water is formed by a mechanism similar t o that for hydrogen chloride production from polyvinylchloride, and acetic acid from polyvinylacetate, leaving a conjugated polyene

101

-

structure; namely, -(CH*H2)n+H+H2--

I

OH

I

OH

+ nH20

-(CH=CH)n-CH-CH2-

I

OH Scission of some of the C-C bonds results in the formation of carbonyl ends according to -CH-CH,--( I OH

CH=CH)n+H-CH2-

I

OH

-

-CH-CH2-(CH=CH),-CH

I

OH

I1

+ CH3+H-

I

0

-CH-CH,-(CH=CH),-&CH2--CH1 I OH OH

OH

-

I OH

-CH--CH,--(CH=CH),--C=CH2 I I OH OH

+ CH-CH2-II

I

(99)

0

I

--CH-CH,-(

(98)

CH=CH),--C-CH3

II

OH 0 A thermogravimetric analysis by Gardner and McNeill [ 2001 has shown the existence of four degradation peaks (Fig. 56). The first corresponds to water elimination; absorbed water and chemically linked water may be involved. The second peak would correspond to the formation of water, acetaldehyde, formaldehyde and hydrogen. The two high temperature peaks have not been identified. The UV absorption spectrum is modified. Absorption appears between 250 and 400 nm. The new bands are similar to those observed with degraded polyvinylacetate but are much less resolved. The carbonization of polyvinylalcohol has been studied. After heating to constant weight at 25OoC, polyvinylalcohol was pyrolysed at 900°C. Hydrogen, carbon monoxide, carbon dioxide, methane, acetylene and ethane were identified in the volatile products [ 2051. When polyvinylalcohol is modified with phosphorous acid (monoesters are formed and also possibly some crosslinks), the dehydration is accelerated and occurs at lower temperatures [2061. Reference8 p p . 166-1 73

102

-

4-

> c

3

n

3-

c)

3

0

Tern perat u re ('C)

Fig. 56. Thermal volatilization analysis of polyvinyl alcohol. 100 mg film sample, 5 degC min-' [ 2001.

10.3 POLYCHLOROPRENE

Polychloroprene eliminates hydrogen chloride in high yield during its thermal degradation. Although its structure is analogous t o that of polyisoprene, its behaviour is quite similar t o that of polyvinylchloride. A typical thermogravimetric weight loss curve is shown in Fig. 57 [207]. In earlier work a sharp exotherm observed at 377OC for a heating rate of 10 degC/min [208] was found t o correspond t o the maximum rate of evolution of HC1. Thermogravimetric curves recorded at a heating rate of 200 degC/h [209] have proved that the reaction takes place in two stages: elimination of hydrogen chloride followed by decomposition of

200

I

1

1

300

400

500

600

700

700

Ternperat u r e ('C)

Fig. 57. Thermogravimetric curves for polychloroprene MC 30 (continuous line) and PC-A (dotted line) [207]. Dynamic nitrogen atmosphere, 10 degC min-', 10 mg samples.

103 the residue. It was also demonstrated that hydrogen chloride does not accelerate the dehydrochlorination as is observed with polyvinylchloride. Analysis of the volatile products has been performed [ 210, 2111. Heating for 20 min at 400°C removed 68% of the chlorine as hydrogen chloride and gave 2% monomer [210]. When the polymer is pyrolysed at 17OO0C on an electrically heated filament only monomer is formed, while polyvinylchloride heated in the same conditions yields hydrogen chloride and benzene [ 2111.

I

I

100

I

200

I

300

I

400

f

Temperature ( ' C )

Fig. 58. Differential thermal analysis of polychloroprenes MC 30 (upper curve) and PC-A (lower curve). Dynamic nitrogen atmosphere; 10 degC min-' , 20 mg samples; reference, glass beads [ 2071.

The thermal degradation of polychloroprene was reinvestigated more recently by Gardner and McNeill [ 2071 . The results of thermogravimetric, thermal volatilization and differential thermal analysis are given in Figs. 57-59 for comparison. HC1 evolution attains its maximum rate at about 35OoC as indicated by the weight loss curve, the first peak of the thermal volatilization curve and the sharp exotherm obtained by differential thermal analysis. Endothermic melting of the crystalline region of the polymer is responsible for the peak observed in the vicinity of 50°C in the DTA curve. Measurement of the quantity of HC1 evolved during heating was performed by titration [207]. Comparison of the data with the weight loss curve showed that HC1 does not entirely account for the weight References p p . 165-1 73

104

Temperature ("0

Fig. 59. TVA curves for polychloroprene MC 30 at heating rates of 10 degC min-' (upper curve) and 5 degC min-' (lower curve). 30 mg samples were used, as small pieces [ 2071.

loss below 40OoC. Some non-condensables are also evolved, in agreement with the results of thermal volatilization analysis. The kinetics of dehydrochlorination was also studied by isothermal methods [ 2071. Activation energies determined by different methods applied to isothermal and programmed techniques all extend from 33 to 42 kcal mole-' and the logarithm of the frequency factor lies between 10 and 12.4. Values of the activation energy for dehydrochlorination of polyvinylchloride extend from 22 to 33 kcal mole-' and are thus considerably lower. Detailed study of the products of degradation (gaseous, liquid and solid residue) was performed by the same workers [212]. Below 4OO0C, small amounts of ethylene and a trace of chloroprene were detected in addition t o hydrogen chloride. Above 4OO0C, methane became a significant product but smaller amounts of hydrogen, ethylene and propene were present. The liquid product found after degradation on the water-cooled part of the degradation tube is a complex mixture of dimers of chloroprene and less volatile components containing aromatic structures. The UV spectrum of the polymer residue showed similarities with that of polyvinylacetate; conjugated structures are formed but the polyene sequences are shorter than with polyvinylchloride. Triene structures predominate and there is little contribution from structures with more than ten double bonds in conjugation.

11. Polymers containing heteroatoms in the chain 11.1 POLYSILOXANES

The thermal stability of polydimethylsiloxane has been the subject of numerous studies [213-2241. When heated in an inert medium or in

105 vacuo [ 2131 it depolymerizes into cyclic low molecular weight products. It has also been demonstrated [214, 2151 that the thermal stability of polydimethylsiloxane essentially depends on the presence of impurities and traces of polymerization catalysts which are active centres for initiation of the degradation. Investigation of the thermal degradation of polysiloxanes with terminal hydroxy groups [219] indicates that, at low conversion to volatile products, increase in the molecular weight of the polymer occurs in the range 170--3OOOC. This was assigned t o a polycondensation involving the terminal hydroxy groups. Subsequent increase in temperature decomposes the polymer and decreases its molecular weight (Fig. 60). The volatile fraction consists mostly of cyclic trimer-hexamethylcyclo trisiloxane :

the mechanism of depolymerization proposed is CH3

I

CH3

I

CH3

I

CH3

I

--Si-O-Si-O-Si-O-Si-OH I I I I CH3 CH3 CH3 CH3

H3C H3C CH3 CH, Si<-si/o\&-CH3 I \I

-

-

I

H3C

. . :

H-6,

:

I

./ 0

-

s1

/ \

CH3 CH3

-

CH3

CH3 I -Si-OH

CH3

l o 1 CH3-Si \Si-CH3 + I I

The process is initiated at terminal hydroxy groups and favoured by the spiral-like structure of polysiloxanes. Replacement of the hydroxy groups by methyl, or blocking them by chelation t o copper, iron or zirconium acetylacetonates, considerably decreases the rate of decomposition of the polymer and increases its thermal stability (Table 9). However, pronounced crosslinking even at moderate temperatures was observed in the polymer stabilized by transition metal compounds. The effect of the metal additives during thermal ageing is associated with reactions leading References p p . 165-1 73

106

Temperature

("C)

Fig. 60. Molecular weight ( i ) of polysiloxane and its weight loss (ii) in thermal degradation during 4 h. TABLE 9 EFFECT OF TERMINAL GROUPS ON THERMOSTABILITY OF POLYDIMETHYLSILOXANE AT 400° FOR 4 h [219]

Terminal groups

Weight loss (%)

CH3

I I

84-8

-O--Si-OH CH3

FH3

32.3

-0-Si- CH3

I

CH3 11.7

FH3

4-Si-0-Fe

(AcAc)z

14.8

CH3

I I

+Si-O--Zr

(AcAc)

CH3

AcAc = acetylacetonate fragment.

8.2

107 to cleavage of the siloxane chain and insertion of metal hetero-atoms in the latter. Cyclic trimer-hexamethylcyclotrisiloxaneis also formed during thermal ageing of the stabilized polymer. Cleavage, however, would occur at random along the chain. The mechanism was confirmed by Thomas and Kendrick [220] who performed the thermogravimetric analysis of various samples of polydimethylsiloxane prepared with special emphasis on the exclusion of hydroxy groups. The rate of volatilization t o cyclic dimethylsiloxane was found to be independent of molecular weight. The activation energy is 42 k 3 kcal mole-'. The depolymerization is thus randomly initiated. Change in molecular weight distribution was also observed with hydroxy end-block polymer fractions. The chain lengthening process occurring in vacuo below the depolymerization temperature is assigned t o intermolecular condensation of terminal hydroxy groups in agreement with Rode et al. [219]. An apparent activation energy of 8.6 k 1kcal mole-' has been measured. 11.2 POLYOXYMETHYLENE

A study of the thermal stability of polyoxymethylene was made by Schweitzer et al. [ 2251. The rate of weight loss was measured at 222°C. Formaldehyde is evolved by a first-order reaction. The decomposition was assumed to take place mainly by unzipping from the chain end, since the rate of weight loss increases with molecular weight. This was confirmed later by Kern and Cherdron [226] who also showed that acetylation of end groups leads to improved thermal properties. The degradation was reinvestigated later by Grassie and Roche [227]. An inverse relationship between molecular weight and initial rate of volatilization was found. The change in molecular weight with percentage volatilization is given in Fig. 61. The position of this curve above the diagonal D indicates that the reaction is not a stepwise degradation in which a stable polymer molecule is produced, since each monomer molecule is liberated from the chain end. The results are in agreement with an end initiated chain process, the average length of which is less than the average degree of polymerization. There is also evidence that little or no transfer occurs. Acetylating the terminal OH groups of the polymer inhibits the chain end initiation for the chains terminated by OH groups. The degradation was also found to occur preferentially in the amorphous region of the polymer. Molecular, free radical and ionic mechanisms were considered in detail by Grassie and Roche [227]. The absence of any secondary volatile products, which would result from an end initiated radical reaction, are strong arguments against a free radical mechanism. Furthermore, inhibitors of radical reactions have no effect on the depolymerization. In the presence of basic catalysts the reaction is accelerated and undoubtedly References p p . 165-1 73

108

I

1

20

40

L

6(

%volatl I tzatlon

Fig. 6 1. Changes in the number average molecular weights of polyoxymethylene during degradation at 17OoC. (The molecular weights were obtained via viscosity measurements.) [227 1.

anionic. Anionic and molecular mechanisms can nevertheless not be distinguished for the uncatalysed reaction. The end initiated nature of the degradation of -OH terminated polyoxymethylene chains was also confirmed by Dudina et al. [228]. End-blocked polymers, on the other hand, degrade by random initiation. The kinetic chain length was shown t o be smaller than the macromolecular chain and formaldehyde participates in chain transfer. The reaction is first order for the end-blocked polymers. Deviation from the first-order law is observed for unstabilized polymers. This shows that the fragments formed by disappearance of the polymer active centres are more stable than the original polymer molecules. The activation energies of the degradation process are given in Table 10. The thermal properties of copolymers of trioxane with cyclic ethers and acetals, lactones and vinyl monomers have been reviewed by Jaacks [229]. In contrast to pure polyoxymethylene diols, they depolymerize only partially when heated to

200OC.

TABLE 10 TRUE ACTIVATION ENERGY ( E ) VALUES FOR FORMALDEHYDE POLYMERS [ 2281

0

II

End group

-OH

-C-CH3

-0-CH3

-C-C-OH

E (kcal mole-')

26

32

41

46

109 11.3 HIGHER POLYETHERS

A comparison of the thermal stability of polyethylene oxide, polypropylene oxide, polyethylene and polypropylene has been presented by Madorsky and Straus [230]. More recently, studies on the thermal degradation of polytetramethylene oxide (I), polyhexamethylene oxide (11) and polydioxolane (111)

have been reported by Blyumenfeld et al. [231]. Initiation was shown to occur at the carbon in a position t o the oxygen atom. Rupture of the C-H bond is followed by breaking of the main chain according t o

11.4 NYLON

The degradation of nylon-6,6.has been studied extensively [232-2351. A wide range of degradation products is reported. These include simple hydrocarbons, cyclopentanone, water, carbon monoxide, carbon dioxide and ammonia. Gelation also occurs in some cases and discolouration is observed [236]. A large variation (15-42 kcal mole-' ) rather than a single value has been reported for the activation energy. Residual water was also found to be very important. Owing to the variety of volatile products formed and the variation in the activation energy, a complex mechanism has t o be operative. Breaking of the -NH-CO- link, followed by decarbonylation and cyclization, were among the various steps proposed. F. Wiloth [ 2361 has recently re-examined the problem. Nylon-6,6 and nylon-6,lO were compared under different degradation conditions. Viscosity measurements, titration of acid and basic groups in the pyrolysed copolymer, and determinations of ammonia and carbon dioxide produced were performed. The considerably more pronounced instability of nylon-6,6 has been assigned t o the ring closure tendency of the adipic acid component. In the presence of water, the thermolysis is dominated by the reactions of end groups, and in dry, high-molecular weight systems by reactions involving chain-amide groups. The reactions proposed are presented in Schemes 11and 12. References p p . 165-1 73

110

6

u

60 f J

2

6

u ++-

x

I

z

Scheme 11. Thermolysis of low molecular weight nylon-6.6 in the presence of water [ 2361.

111

Other data were recently published by Peebles and Huffman [237]. The rate of gel and colour formation in nylon-6,6 is found t o be dependent on the rate of removal of the volatile products of degradation. CH2-CH2 I I CH2 CH2-CO-NH\

'CO-NH-

-

(1)+-NH2

See Scheme 11. Scheme 12. Thermolysis of high molecular weight product in absence of water.

If a sample of nylon is heated above its melting point in a sealed tube, the material remains soluble in organic solvents over extended periods of time; the intrinsic viscosity first passes through a maximum, then a minimum, followed by formation of insoluble material (Fig. 62). Discolouration is not observed. End-group concentration varies as shown in Fig. 63.

1lo

2

4

6

8 10 12 2 14 16 18 20 22 24 1 Time (h)

Fig. 62. Change in molecular weight as a function of time for nylon-66 at 282OC contained in a sealed tube such that the volatile materials were maintained over the melt [ 2371. References p p . 165-1 73

112

Tirne(h)

Fig. 63. End-group concentrations (in m.Eq./g) as a function of time [237]. Same conditions as in Fig. 62.

Comparison of these two figures indicates that the initial increase in molecular weight is due t o polymerization of the unreacted ends -NH2

+ NH2- + -NH-

+ NH3

Ammonolysis, namely,

and the usual hydrolysis of the amide group were also proposed to occur. Branches are formed at the site of the secondary amines:

0

II

-C--NH,

+ -NH-

-

-N-

I

+NH,

-c=o

Chain scission and crosslinking are thus occurring simultaneously. If the volatile materials formed are permitted to escape, rapid gelation and colour formation is observed even in the complete absence of oxygen. Ammonolysis and hydrolysis are, in fact, strongly reduced in these conditions. Careful analysis of the volatile products was also performed

113 TABLE 11 DEGRADATION PRODUCTS OF NYLON 66 AT 305OC [237]

Product Highly volatile materials

Less volatile materials Neutral fraction

Cyclic monomer Cyclopentanone Cyclopent ylidinec yclo pentanone Cy clopentylcyclopen tanone Hexylamine Hexamethyleneimine Hexamethy lenediamine [ b,e] -Pyridine Several unknown materials

Basic fraction

Polymeric residuum after hydrolysis (black) Neutral fraction

Acid fraction (straw-coloured)

Basic fraction (black oil)

a

3-Pentanonea Cyclopenty Ic yclopentanone, Cy clopenty Ic yclopentanol Cyclopenty lidinecyclopentanone Other unknowns, three in high concentration Succinic acid Glutaric acid Adipic acid Pimelic acida Suberic acida Many other unknowns, some in high concentration Hexylamine Hexamethy leneimine Hexameth ylenediamine 6,6’-Diaminodihexylamine Non-volatile black oil

Identification based on retention time only.

(Table 11).One major source of water is thermal cracking followed by dehydration

0 II -C-NH-CH2-

0 II + -C-NH2

L

+ CH,=CH-CN+H20 References pp. 165-1 73

114 Ammonia and carbon dioxide result from decarboxylation and deamination following hydrolysis of the amide group. Dimers of cyclopentanone and the cyclic monomer of nylon-6,6, namely,

0

r

II

0

II

C--(CH2)4*

LN--(cH,),-NJ I H

1

I H

are formed. 1 1 . 5 THERMOSETTING RESINS

Extensive discussion on the degradation mechanism of thermosetting resins is beyond the scope of this book. The subject was reviewed in 1970 by Conley [ 2381. Some of the recent results on two important classes of

a Temperature (OC)

Fig. 64. Thermogravimetric curves of phenolic condensation polymers [ 2391.Sample weight 100 mg, pressure 0.2 Tom; heating rate 9-10 degC min-'

.

115 thermosetting resins, epoxy- and phenol-formaldehyde polymers will be outlined briefly. Weight loss is initiated in the neighbourhood of 300°C for phenolformaldehyde polymers. A mechanism of degradation has been proposed by Conley [238] t o take into account the volatile products formed. It is summarized in Scheme 13. Oxidation is initiated at the site of the methylene group or at terminal - C H 2OH. Subsequent decomposition of the ketone and acid groups formed results in CO, C 0 2 and water production. Pure thermal degradation results in rupture in the a position to the methylene group. Char is formed under oxidizing conditions. A comprehensive study of the thermal degradation of epoxy resins has been reported by Lee [239]. Their stability was found to be lower than that of polycarbonate, polyphenylene sulphide and teflon (Fig. 64).

DER 331 resin

DEN 438 resin

Polycarbonate

Polyphenylene sulphide

MNA: methylbicyclo[ 2,2,1]hept-5-ene-2.3-dicarboxylicanhydride. MDA: methylene dianiline.

Many concurrent reactions are usually operative. Their relative importance depends on the structure of the resins and the type of curing agent. The degradation products are mostly phenolic compounds. Three mechanisms based on the principle of simple ether cleavage and on the experimental results obtained were proposed: (a) The residual epoxide isomerizes to an aldehyde which decomposes into propionaldehyde and/or ethane and carbon monoxide. This is summarized in Scheme 14, Route I and was proposed earlier by Anderson [ 2411. (b) The residual epoxide undergoes etherification to form 1,3-hydroxyl substituted diethers which dehydrate. Three routes are then possible References p p . 165-1 73

116

OH

OH

I>CH2

117

-0 \

CH3*

(111)

,CH2

\

CH, + ,CH*

OH

n + * O H

fJ--”o/

H2co

0

\

,CH2

I

COOH I

Scheme 13. Typical reactions proposed for resin decomposition at elevated temperatures. (Route I) oxidative degradation processes; (Route 11) fragmentation reactions; (Route 111) formation of benzenoid species [ 2381.

References p p . 165-1 73

118

+

Route 11-C

Route 11-B

I(

/( Etherification)

Etherification)

/(Dehydration)

~ O + H = C H - IC H z -

1 CHI=CH
I

I

/(Hydrogen abstraction)

CH, =CH+H,-O+€H

I

(Claisen rearrangement)

* I # I

OH

j(Cle=we)

CH,=CH, + C O

(Hydrogen abstraction)

('laben

?H OCH2+H=CH, \

CH,=CH+HO I(Decarbony1ation)

I

I(Hydrogen abstraction)

I

Polymer Scheme 14. Degradation of epoxide resins [239].

I

Polymer

0

+ CH,=C=CH,

120

/

-?-

gs Y + 8II Y

X

I

T-o++X

X

..yT -0I

T

p + + X

3

*I

I

8 - y=o 8

8+ t

57

8+

121 according to the mode of decomposition of the 1,%unsaturdted diethers formed: Routes 11-A, 11-B and 11-C. (c) Scission of 1,2,3-triethers, which are present in the cured epoxy resins, occurs according t o similar mechanisms, Route 111. In the presence of oxygen, hydroperoxides, that further decompose, are formed at allylic carbons, Route IV.

12. Fluorinated polymers Polytetrafluoroethylene (PTFE) has been known for about thirty years. It is the most stable addition polymer. Its exceptional thermal stability has been assigned to the high C-F bond strength and to the shielding effect of the highly electronegative fluorine atoms. Therefore much research has been devoted during the last decade t o the synthesis of a large variety of fluorine containing polymers. All of them, with the exception of copolymers of CF, =CF, and CF, -CF=CF, and polytetrafluoroethylene oxide, were found t o be less stable than PTFE. In this section the mechanism of PTFE degradation will be outlined, but since kinetic data on the other fluorinated polymers are very scarce, we shall only characterize their stability by thermogravimetric data (Table 1 2 and Figs. 65-67). Comprehensive reviews on the thermal behaviour of fluorinated polymers have been published by Wright [242, 2431 and Wall [ 2441. 12.1 POLYTETRAFLUOROETHYLENE

The thermal degradation of PTFE has been widely studied. The general kinetic pattern of the degradation between 360 and 600°C is as follows. (a) The rate of volatilization is independent of molecular weight [245, 2461. (b) The molecular weight decreases during pyrolysis [ 2461. (c) The rate of weight loss is reduced if reaction products are allowed to accumulate [245-2471. (d) The rate of weight loss in inert atmosphere or in vacuo is stated to be first order [245-2521, although Carroll and Manche [253] and Cox et al. [ 2541 disagree and report a zero-order reaction. (e) Activation energies and pre-exponential factors have been determined by various methods and are, respectively, of the order of 80 kcal mole-' and 10' s-I . (f) The volatile products formed between 500 and 12OO0C have been analysed by mass spectrometry [255]. The results are given in Table 13. (g) The nature of the reaction vessel has an effect on the rate of volatilization [ 2471. On the basis of these experimental results, an over-all mechanism of References p p . 165-1 73

122 TABLE 1 2 THERMAL STABILITY O F FLUORINE-CONTAINING POLYMERS IN VACUUM ~ 4 2 1 Polymer unit

Ethylenic homopolymers: +FHCH2+F2CH2-CFHCFCI--CFzCFH--CF2CFCI-

+F2CF(CF3 )--CF2CF(CSFllk * F ~ C F ( O C ~ H S)Diene polymer: 4-chloroperfluoroheptadiene-1,6 Ethylenic copolymers: CF2CH2/CF2CFCl (a) 19%CFzCHz (b) 53% CFzCHz (c) 67% CF2CH2

CF2CHz /CF2 CF(CF3 ) CF2CF2/CFZCF(CF3 ) CF,CFH/CFjNO CF~CF~/CFJNO

Styrene polymers: 0, 0-trifluorostyrene 2, 3, 4, 5, 6-pentafluorostyrene Condensation polymers: Perfluoroalkylene triazines (a) from perfluoroglutarodiamidine (b) from perfluoroglutarodiamidine and perfluorobutyroamidine (1:l)

Q,

Activation Arrhenius Rate o f energy factor w t. loss (heal mole-' ) (s-' at 35OoC ( % p e r min)

48 71 38 53 50 50 61 81 76 83 75 78 57 63

57 68 61 50 61 54 57 46 55 73 47 56 58

10'O

lozo

10' 10l2 10l2 10' 1019 1019 10'~ 1019

-

1019 10'

loz2 1015

lo2' loL5 1015 1015 10'6 10' 10' 10'2 10' 10' 'I

l o 2' lo2'

Temperature ( " C ) for 1 % w t . loss per min

0.1 0.02 4 20 0.005 410 330 2.6 0.02 415 0.2 370 0.3 370 0.3 365 2 ~ 1 0 - ~510 515 2.6 x 510 1.2 x -

2.2 x 77 "2600

0.1

-

-

512 290 240 300 390

0.18 370 0.06 390 0.12 385 0.06 390 395 0.06 0.04 395 0.04 410 2 ~ 1 0 - ~495 530 1.8 x 1000 255 1200 265 1200 270

64 65

1019 1017

4.8 0.03

340 395

39

lo6

6~

540

43

lo8

6 ~ l O - ~520

123 TABLE 12-continued

Polymer unit

Activation Arrhenius Rate of energy factor wt. loss (kcal mole-') ( s - l ) at 35OoC ( % p e r min)

(c) from perfluoroadipodiamidine and perfluorobutyroamidine ( 1:1.35) 31 Polyhexafluoropentylene 32 adipate Polyhexafluoropentylene adipatelisophthalate Tereph thal ylchloridel octafluorohexane diol Fluorosilicone

lo5 lo8

8x10-3

2.0 "1.7

58 44

1 0l6 1013

Temperature

("c)for I%

wt. loss per min

500 335

2.3 8~

0.01

420 440

random chain scission, followed by depolymerization and termination by disproportionation, has been proposed [ 246,24'11. The occurrence of chain transfer has been a subject for discussion. It was originally assumed to have a very low probability owing to the large C-F bond energy [ 2 5 6 ] , but it was later pointed out that the energetics of the transfer reaction are not unfavourable. Efforts have been made to improve the thermal stability of PTFE: initiation of polymerization with fluorocarbon catalysts, inclusion in the chain of structural units able to promote transfer of the depropagating chain and inclusion of additives for the same purpose [257-2591. Only the last method has had some success.

Fig. 65. Weight loss behaviour of polymers representing several degradation processes and a wide range of thermal stabilities. The curves show volatilization rates of polymers heated in nitrogen at atmospheric pressure and at a constant rate of temperature increase of 100 degC h-' [ 242 1. References p p . 165-1 7 3

124

Temperature

('C)

Fig. 66. Comparison of thermal stabilities of various fluorine-containing polymers in vacuo [ 2431; A CF3NO-CF2 CFH copolymer; B, CF3NO-CF2 CF2 copolymer; C, fluorosilicone A; D, methylmethacrylate octafluorocyclohexa-l,3-dienecopolymer; E, [ CHFCFCI], ; F, polyhexafluoropentylene adipate; G, [ CF2 CFCl], ; H, CF2 CFCI-CF2 CH2 copolymer (80/20); I, CF2 CFCl-CF2 CH2 copolymer (50/50);J, CF2 CFCI-CF2 CH2 copolymer (30/70); K, CF2 CH2-CF3CFCF2 copolymer; L, terephthalyl chloride-octafluorohexane-l,6-diol polymer; M, fluorosilicone B; N, [ CF2 CH2 I n ; 0, butadiene-octafluorohexa-l,3-diene copolymer; P, CF2 CF2 -CF3 CFCF2 copolymer; Q, [ CF2 CF2 ] n . A

200

240

280

320

360

400

440

480

5

Temperature ("C)

Fig. 67. Comparison of thermal stabilities of various fluorine-containing polymers in oxygen [ 243 1. A; CF3NO-CF2 CFH copolymer; B, CF3 N M F 2 CF2 copolymer; C, polyhexafluoropentylene adipate; D, fluorosilicone A; E, [ CHFCFCI],, ; F, CF2 CFCI-CF2 CH2 copolymer (50/50);G, CF2 CFCI-CF2 CH2 copolymer (30/70); H, CF2 CFCI-CF2 CH2 copolymer (80/20); I, [ CF2CFClI n ; J, [ CF2 CH2 ] n ; K, terephthalyl chloride-octafluorohexane-1,6-diol polymer; L, butadiene-octafluoroCF2C H 2 - C F 3CFCF2 copolymer; N, cyclohexa-1,3-diene copolymer; M, CF2 CF2-CF3CFCF2 copolymer; 0, [ CFz CF2 ] n .

125 TABLE 13 ANALYSIS OF VOLATILE PRODUCTS FROM PYROLYSIS OF POLYTETRAFLUOROETHYLENE AT ELEVATED TEMPERATURES IN A VACUUM [255]

At 5OO0C

Componenta

Expt.

HF CF4 CZ F4 C3F6

Vwr Total a

At 8OO0C

At 120OoC

1

2

3

4

5

6

0 1.5 94.8 3.7 0 ___ 100.0

0 1.2 95.1 3.7 0 100.0

0 1.6 92.5 5.9 0 100.0

0 1.8 89.9 6.5 1.8 -

0.2 1.7 81 .O 5.3 11.8 100.0

0.5 2.6 75.2 5.8 15.9 100.0

100.0

Components are in wt. % of total volatiles.

12.2 OTHER PERFLUORO AND HYDROFLUORO ADDITION POLYMERS

Polyhexafluoropropene and polyperfluorohept-1-ene have low thermal stability (Table 12). Perfluoroacenaphthylene polymer is not more stable than polytrichlorofluoroethylene [ 243, 2601. Perfluorobutyne-2 yields s highly branched and crosslinked material which is more stable than PTFE [261]. Perfluorallene also gives a highly crosslinked solid, the therma, behaviour of which has not been characterized [ 2621. The thermal stability of polytetrafluoroethylene oxide and PTFE have been compared under the same conditions by Donato et al. [263] between 450 and 600°C. The decomposition rate has a maximum at 628°C for the oxide and at 568°C for PTFE. The activation energy for the first-order degradations are 98 kcal mole-' between 8.5 and 85% for the oxide polymer and 85 kcal mole-' between 523 and 571°C for PTFE. The rate of weight loss is less than 1.2% per min for both polymers below T = 550°C for the oxide and T = 590°C for PTFE. The oxide, however, loses weight below 390°C whereas PTFE does not. The main components of the volatile material are trifluoroacetyl fluoride, carbonyl fluoride and tetrafluoroethylene. An end-initiated thermal degradation with small zip length is proposed. Madorsky et al. [ 2641 have studied polyvinylfluoride, polyvinylidene fluoride and polytrifluoroethylene degradation in the range 372-500°C. Their behaviour is shown in Table 12. Hydrogen fluoride is an important volatile product. Polyvinylfluoride was reinvestigated recently [2651. It is stable in vacuo up to 300°C. The degradation products of polyvinylidene fluoride were analysed by Pravednikov et al. [266] and a mechanism of degradation was proposed. References p p . 165-1 73

126 Various other polymers have been prepared [242-2441. stable than PTFE.

All are less

12.3 CHLOROFLUOROPOLYMERS

The thermal degradation of polychlorotrifluoroethylene has been thoroughly studied [ 2671 . Only 28% of degradation products (mainly monomer) is volatile at room temperature. The remaining 72% has an average molecular weight of 900 but is volatile at the temperature of pyrolysis. The degradation of this polymer commences 150 degC lower than for PTFE. This was attributed t o the lower C - C l bond strength (81 kcal mole-') and confirmed by infrared methods: CF=CH2 groups were detected, confirming the splitting off of chlorine [268]. The thermal behaviour of other chlorofluoropolymers is given in Fig. 66 and Table 12. 12.4 FLUOROSTYRENE POLYMERS

Poly-a,p,@trifluorostyrene has slightly lower stability than polystyrene (Table 12). The relatively large amount of monomer formed suggests that chain scission is followed by depolymerization. Poly-2,3,4,5,6 pentafluoropolystyrene is much more stable than polystyrene (Table 12). Wall et al. [269] attributed this increased stability to the loss of resonance interaction between the phenyl group and the chain because of the presence of the fluorine atoms. Perfluorostyrene polymerized by yirradiation has, however, been found recently t o have a stability close to that of polystyrene: a rate of weight loss of 0.4% per min was found at 335°C and complete decomposition of the polymer is observed at 432'C [ 2701. 12.5 COPOLYMERS

Various copolymers of the fluorinated monomers have been prepared [ 2431. Copolymers of tetrafluoroethylene and hexafluoropropene have under vacuo a stability close t o that of teflon (Fig. 66). Copolymers with trifluoronitrosomethane have a lower stability than PTFE (Figs. 66 and 67). The copolymer of vinylidenefluoride and hexafluoropropene is one of the most stable elastomers available at the present time (Fig. 66 and Table 12). For a typical copolymer containing 70% vinylidene fluoride, the rate of weight loss is 0.04% per min at 350°C and the activation energy is 57 t o 46 kcal mole-', according t o Wright [243]. A wide range of other addition copolymers have been synthetized and examined for thermal stability in vacuo and in oxygen. Typical weight loss data are given in Fig. 65.

127 12.6 FLUOROAROMATIC POLYMERS

Perfluoropolyphenylene shows about the same stability as the unfluorinated analogues [ 2711. Perfluoropolyphenylene ethers are less stable than the corresponding polyphenylene ethers [ 2721. Perfluoropoly-p-phenylenesulphide is less stable than p-polyphenylenesulphide in the range 360-440°C [273]. 12.7 OTHER CONDENSATION POLYMERS [ 2431

Linear aliphatic fluorinated polyurethanes have been tested with respect to the location of the fluorine atoms in the diol or diisocyanate component. Fluorine atoms introduced into the diisocyanate unit increase the stability of the urethane group, but the presence of these atoms in the diol unit favours dissociation of the urethane group to monomer. Comparison of PTFE and perfluoroamidine polymers shows that introduction of triazine rings in fluoromethylene ,chains enhances their stability. Various fluorinated polyesters have been synthetized. The thermal stabilities of some of them in oxygen and in vacuo are compared in Figs. 66 and 67. The terephthalyl chloride/octafluorohexane-l,6diolpolymer shows a good stability in vacuo. Polymers containing triazine in the chain

X

decompose by a complex mechanism in which hydrolysis probably plays an important part. Some of them can be used for a short period in air at 300-350°C. Polybenzimidazoles of the type

L

Jn References pp. 165-1 73

128 readily decomposes with elimination of HF. This is attributed to a reaction of the hydrogen atom from the imino group with the fluorine atoms. 12.8 EVOLUTION OF HF FROM HYDROFLUORO AND PERFLUORO POLYMERS

The evolution of hydrogen fluoride or other products giving rise to Fions in aqueous solution has been studied quite recently for a number of hydrofluoro and perfluoro polymers heated in nitrogen or in air [274]. Such data are important since they indicate the maximum temperature of use for such polymers and also the risks of corrosion in high-temperature applications. The results obtained for degradation in a nitrogen atmosphere are given in Tables 1 4 and 15. They can be compared with the data for degradation in air (Tables 1 6 and 17). The temperature for initial weight loss and initial F- yield are the same in many cases. For other polymers, however, F- is detected at temperatures much lower than those for weight loss. This has been observed for instance with polyvinylfluoride, polyvinylidene fluoride and some Viton rubbers which are important commercial products. TABLE 14 EVOLUTION OF HYDROGEN FLUORIDE FROM PERFLUOROPOLYMERS IN NITROGEN [ 2741 Polymer structure

Temperature ("C) for Initial weight loss

fCFzCFz-3 Platinum boat 350 Calcium fluoride boat f-CFzCF2 /CF,CFCF2 -3 400 f-P-c6F44 High molecular weight 330 Low molecular weight 350 150 P-C~FSC~F~C~F~C ~FS f-m-c6F44

High molecular weight Low molecular weight m-C6Fs C6F4C6 F4 c6 F5

f-c6F4

s+

Crosslinked Linear +CSNF34 +C6F4CFz o-+ +C6F3(CF3)CF20+

1% weight loss

Initial F yield

F yield

Final total yield of F (%)

432

289 190 293

510 518 530

11.6 11.2 6.6

271 337

504 512 > 700

22.9 12.8

457 376 393 175

-

1%

150 120 120

162 144 126

160 212

-

403 502 > 700

200 170 150 40 100

315 215 280 60 128

180 212 291 168 198

374 467 355 437 486

-

16.7 13.7

-

31.3 16.1 56.9 7.0 5.1

129 TABLE 15 EVOLUTION O F HYDROGEN FLUORIDE FROM HYDROFLUOROPOLYMERS IN NITROGEN [274] ~~

~~

Polymer structure

Temperature ("C) for 1%

Initial weight loss

I% weight loss

Initial

240 240 320 340 290 270 350 400 170 250 270 220 340 340 213 250 270

320 320 390 365 305 350 410 440 320 305 295 270' 375 365 255 280 350

177 337 284 396 475 329 413 492 312 136 457 430 141 271 442 361 188 292 >SO0 337 >SO0 254 494 192 366 244 368 205 260 329 244 499 437

F

yield

Final total yield of

F yield F(%)

70.8 70.5 8.5 0.9 4.8 12.9 13.2 13.5 28.5 1.1 1.7 4.8 35.3 41.0 87.6 33.7 32.9

TABLE 16 EVOLUTION OF HYDROGEN FLUORIDE FROM PERFLUOROPOLYMERS IN AIR [274] Polymer structure

fCF2CF2-3 Platinum boat Calcium fluoride boat ~CFZCF~/CF~CFCF~+ fp-c6F4+ High molecular weight Low molecular weight P-C6FSC6F4C6F4C6FS fm-C6F4+ High molecular weight Low molecular weight m-C6F5C6F4C6F4C6F5 fCbF4S+ Crosslinked Linear fCSNF3+ fc6F4CF20+ fC6F3(CF3)CF20+

Temperature ( " C ) for

F yield

Final total yield of F(%)

424

236 239 298

433 3 59 429

68.2 70.7 78.4

280 300 150

344 384 164

180 337 198

354 462 587

50.7 54.6 2.0

120 140 110

163 186 124

142 122 70

402 368 579

70.0 61.3 1.7

200 160 180 50 100

243 270 290 75 122

154 191 286 182 163

334 364 410 310 372

79.4 82.5 72.6 25.2 12.9

Initial weight loss

1% Initial weight loss F yield

300

392

400

1%

References p p . 165-1 73

130 TABLE 17 EVOLUTION OF HYDROGEN FLUORIDE FROM HYDROFLUOROPOLYMERS IN AIR [274] Polymer structure

Temperature ("C) for Initial weight loss

1% weight loss

Initial 1% F F yield yield

190 250 300 290 300 340 380 370 200 200 240 260 250 180 190 240 295

250 320 330 325 320 395 420 425 360 250 270 290 360 250 255 260 380

153 214 311 294 294 143 195 259 220 199 243 269 175 186 153 221 280

276 386 373 359 387 431 417 420 383 334 348 333 361 327 266 296 420

Final total yield of F(%)

93.7 75.4 36.0 63.4 6.9 54.2 54.7 46.5 56.2 31.7 25.9 36.3 54.0 59.7 91.2 51.7 61.3

The nature of the primary breakdown products that give rise t o the Fions is not yet determined for perfluoropolymers. Hydrogen fluoride is evolved directly from hydrofluoropolymers. 12.9 CONCLUSION

The preceding discussion shows that PTFE is the most stable addition polymer. Until now, from the large range of fluorinated polymers and copolymers, only two are found to have a thermal stability approaching that of PTFE. Other fluorinated polymers show no advantage with respect to degradation in vacuo over the unfluorinated analogues. In oxygen, however, fluorinated polymers are often more stable.

13. Thermostable organic polymers 13.1 INTRODUCTION

One of the greatest disadvantages of high polymers, when compared to metal and stone as everyday materials, is the limited range of temperature

131 within which they can be used. Therefore extensive research has been undertaken during the last twenty years to improve the thermal stability of existing polymers and to synthetize new thermally stable polymers. The number of papers published in this field increases exponentially each year and exceeds one thousand at the time of writing; they mainly report the synthesis of new polymers and the results of thermogravimetric analysis. Also, many reviews have been published [ 275-2841. We will discuss in this section the various ways that can be used to improve the thermal stability of polymers. The synthesis and thermal behaviour of some typical heat-resistant polymers (sometimes commercially available) will then be given. The volatilization of these materials has very seldom been thoroughly studied: orders of reaction, activation energies and pre-exponential factors have generally not been determined. Therefore the thermal stability of the polymers will be characterized in an arbitrary way for the purpose of comparison. It must be stressed, however, that the physical properties of a polymer are at least as important for use at high temperature as the volatilization characteristics; an infusible polymer is very difficult t o process, and a heat resistant polymer with a low softening temperature is often useless. The softening temperature corresponds to the loss of mechanical properties. It can be measured by the standard heat deflection test. A t the end of the section, some recent studies on the thermal degradation of polymers with aromatic rings will be reviewed, but extensive discussion of results described in the literature on thermostable polymers is not possible within the scope of this chapter. Some improvement in the physical properties of polymers with respect to temperature can be obtained by increasing the chain interactions. The useful temperature range can also be increased through greater crystallinity. Low density amorphous polyethylene has a heat deflection temperature at 66 lb/in2 (ASTM-D648) of 38-50°C, whereas it is 60-88°C for crystalline high density polyethylene. Since isotactic polymers are generally more crystalline, they usually have better mechanical properties than the corresponding atactic polymers. The thermal stability, however, is not affected by crystallinity if volatilization occurs above the melting point. End groups and weak links, which are responsible in many cases for thermal degradation, depend on the methods used for polymer synthesis. Therefore the rate and mechanism of volatilization are often very different for isotactic and atactic polymers. The useful temperature can be improved by the intermolecular attraction induced by polar groups between the chains. The heat deflection temperature for high density polyethylene (-CH, -CH,-), , polyoxymethylene - ( C H , +),and nylon-66 (-R-CO-NH-R'-)n are, respectively, 6O-8O0C, 170°C and 185°C. The nature of the side chains has a profound effect on the thermal behaviour of polymers as reflected by the melting point (Table 18). The data in this table also show that branching of the side chain in the References pp. 165-1 73

132 TABLE 18 MELTING POINTS O F CRYSTALLINE POLYMERS Melting point ("C)

Polymer 3 C H 2 -CH2 -)n -( CH2-O-), -( R-CO-NH-R'

In

-( CH-CHz-),

I

A -( CH-CH2 ),

I

R

(CHR 7H-CH2)n

3

I

High density polyethylene Polyoxymethylene Polyhexamethylene adipamide (Nylon 6-6) A = CH3 polypropylene A = C1 polyvinylchloride A = C6Hs polystyrene A = CN polyacrylonitrile R = CH3 polypropylene R = C2Hs poly(butene-1) R = C3H7 poly(pentene-1) R = C4H9 poly(hexene-1) R=R'=H poly( butene-l) R = CH3R' = H poly(3-methylbutene-1) R = C6H5R' = H poly(3-phenylbutene-1)

138 175 255-265 198-212 21 2 230 317 198-212 (116) 124-142 130 55 (116) 124-142 300 360

CH~R'

position to the double bond raises the melting point because it restricts the freedom of rotation. The most thoroughly investigated and most efficient route t o high temperature polymers involves increasing the useful temperature by chain stiffening. If an inflexible ring is built into the backbone, softening temperature can be raised significantly. Typical ring systems that have been introduced into polymer chains are given in Table 19. They can be of the aromatic hydrocarbon or heterocycle type. TABLE 1 9 INFLEXIBLE RINGS FOR CHAIN STIFFENING

Polyphenylene

Poly thiazole

Polydiazine

Polyoxadiazole

Polydiimide

Polytriazine

Polytriazole

Pol y imide

Poly imidazopy rrolone

133 13.2 POLYMERS CONTAINING AROMATIC HYDROCARBONS IN THE MAIN CHAIN

Some of the most important structures are summarized in Table 20. Many of them are commercial products. Their stability is often very good. Representative data are given in Fig. 68. TABLE 20 STRUCTURES OF SOME AROMATIC POLYMERS

poly -p-phenylene

polytolylene n

poly-p-xylylene

polyphenylene oxide

azopolymers

polysulphone

polysulphide

Linear p-polyphenylene is thermally stable and crystalline, but brittle, insoluble and infusible. Therefore there has been considerable effort to insert flexible links such as -0-, -CO-, -NH-, 0-CO--, -0-C0-0, N,, -S- or SO2 without loss of thermal stability. Poly-p-xylylene has a melting point of 4OO0C and its mechanical properties are good. It is, however, insoluble and cannot be thermoprocessed. If one or more References p p . 1 6 5 - 173

134 hydrogens of the aromatic ring are substituted by halogen, acetyl, alkyl or ester groups, more soluble polymers are obtained. Various copolymers have also been synthetized. Poly-p-phenylene oxide is insoluble. Substitution in the aromatic ring increases the solubility but decreases the softening temperature. Thermal stability is sometimes but not always lower. The chloro-derivatives are usually more stable than the bromoderivatives. The azopolymers are crystalline, cannot be thermoprocessed and are always coloured. 1001

90

-

$

00-

kn

70-

2

60-

c

E

c

0

1 N

50-

5

c 40-

0

c

-

30-

c

g 200

-200

n

1

1

I

250

300

350

400

Tern perature('

I

I

1

450

500

550

C)

Fig. 68. LOSSin weight of various polyphenylene-type polymers after heating in vacuo for 2 h at different temperatures [ 2761. A, poly-p-2,3,5,6-tetramethylphenylene methylene; B, poly-9,lO-anthrylene ethylene; C, poly-p-2,5-dimethoxyphenylene ethylene; D, poly-p-2,5-dimethylphenylenemethylene; E, poly-p-phenylene ethylene; F, poly-p-2,3,5,6-tetramethylphenyleneethylene; G , poly-2,6-naphthylene ethylene; H, poly-p,p'-diphenylenemethylene; I, poly-m-phenylene;J, poly-p-phenylene.

Many thermoplastics can be obtained by condensation of bisphenol A with various reactants. Typical examples are given in Table 21. These polymers are generally transparent plastics. Polycarbonate prepared from bisphenol A has high toughness, dimensional stability and self-extinguishing properties. Many other polycarbonates, not derived from bisphenol A, have been synthetized. Thermal stability is improved when the backbone does not contain aliphatic groups. The >C(CH,), groups, in fact, constitute the weakest link. This is true for all the polymers containing aromatics in the main chain. Weight loss starts at 375-458OC according to the chemical structure [ 2851. Polysulphones from bisphenol A have high heat deflection temperature and good oxidation resistance and mechanical properties. They can be used up t o 260°C. Polyphenylene sulphides are semitransparent and melt at 270-29OOC. They show little volatilization

135 below 400°C [286, 2871. Various polyaromatic esters and amides have also been prepared. Para-linked polymers are always more stable than the meta isomers. Ortho-linking always results in a decrease in thermal stability. TABLE 21 POLYMERS FROM BISPHENOL A

polysulphonate

13.3 HETEROCYCLIC POLYMERS

A large variety of polymers containing heterocyclic rings in the chain have been synthetized. Some of these structures are given in Table 22. The most stable compounds are those that do not contain aliphatic groups in the chain. A very high thermal stability is obtained from this class of polymers: many of them are commercially available. There have been various studies of the mechanism of degradation. Some data on the thermal stability of the most representative polymers are given below. References u p . 165-1 73

TABLE 22 THERMAL STABILITY (TEMPERATURE A T WHICH DECOMPOSITION BEGINS) OF SOME POLYMERS CONTAINING HETEROCYCLES IN THE CHAIN [280] Chain unit and polymer

Starting materials Dicarbox ylic acid or deriua tiu e

Second component

Diphenyl terephthalate

Benzene-l,2,4,5-tetraamine

Diphenyl isophthdate

Benzene-l,2,4,5-tetraamine

Diphenyl terephthalate

Biphenyl-3,3',4,4'-tetraamine

Temp. ("C,

5 00

5 00

550

Poly-2,2'-p-phenylene-5, 5'-bibenzimidazolyl

Diphenyl isophthalate

Biphenyl-3,3', 4,4'-tetraamine

550

w

w

Diphenyl isophthalate

490

Diphenylmethane-3,3', 4,4'. tetra-amine Poly-2,2' -rn-phenylene-5,5'-dibenzimidazolylmethane

Diphenyl isoph thalate

4 90

2,2'-DimythyIbiphenyl3,3 ,4,4 -tetra-amine

Poly-4,4'-dimethyl-2,2'-rn-phenylene-5, 5'-bibenzimidazolyl

Diphenyl isophthalate

Diphenyl isophthalate

2,2'-DimethyI,diphyylmethane-3,3 ,4,4 -tetraamine

4, 4'-Diaminobiphenyl3,3 -diol

400

Poly-4,4'-dimethyI-rn-phenylene-5, 5'-dibenzimidazolylmethane

-

c

<

:

~

:

>\ c

~

500

Poly-2,2'-rn-phenylene-6,6'bibenzooxazolyl

a

$ I'

Diphenyl isophthalate

3', 3"-Diamino;2, a-diphenylpropane4 , 4 -diol

II

Poly-2', 2"-rn-phenylene-2,2-dibenzoxazol-5', 5"-ylpropane

2

-

500

b

P

o,

7

b

c;

Pyromellitic anhydride

Phenoxy benzene-pp'-diamine

450 Poly -pp'-phenoxyphenylpyromellitimide

TABLE 22-continued Starting materials Dicarboxylic acid or derivative

Second component

Pyromellitic anhydride

p-Diphenoxybenzenep'p''diamine

Chain unit and polymer

Temp. ("C)

-N N c o C o \

'co

C0lN

~

-

-

- 0

-

Poly-p-diphenoxybenzene-p>" -pyromellitimide

Pyromellitic anhydride

450

Anilinephthalein

0 Polydiphenylphthalide-pp'-pyromellitimide

Pyromellitic anhydride

Biphenylylene-pp'-dihydrazine

400

Poly-N-p-benzidinopyromellitimide

Diphenyl isophthalate

550

4,4'-Diaminobiphenyl-3,3'dithiol Poly-2,2'-m-phenylene-6,6' -bibenzothiazolyl

~

Pyromellitic anhydride

Benzene-l,2,4,5-tetramine

p-Bisdiazoxylene

p-Diethynylbenzene

450

0 Polybenzimidazopyrrone

N-N

ok,$j-

550

H Poly-5-p-phenylenepyrazol-3-yl

N-N Terephthaloyl chloride

Terephthalodihydrazide

'0'

-

400

Poly-5-p-phenyleneoxadiazol-2-yI

N-N

g'

Isophthaloy 1 chloride

Isophthalodihy drazide

40,AfJ

400

Poly-5-rn-phenyleneoxadiazol-2-yl b b-

VI o,

I

k

y

menoxybenzenepp'-dicarbonyl chloride

Phenoxybenzene-ppldicarbohydrazide

430

Poly -5-ppr-phenoxyphenyleneoxadiazol-2-yl

TABLE 22-continued Starting materials Dicarbox ylic acid or derivative

Chain unit and polymer

Temp. ("C)

Second component

N-N

Polyisophthalohydrazide

Aniline

4 .I N , y ) -

45 0

C6H5 Poly-4-phenyl-5-rn-phenylenetriazol-3-yl p-Diglyoxyloylbenzene

Biphenyl-3,3', 4,4'tetra-amine

470

Benzidine-3,3'dicarboxylic acid

p-Phenylene di-isocyanate

540

Poly-1, l', 2, ?,3,31,4,4r-octahydro-2,2',4,4'-tetraoxo-3,3 -p-phenylene-6,6'-biquinazolinyl

141 Polyimide. A typical polyimide results from the condensation of The final insoluble pyromellitic dianhydride and bi~-4(aminophenyl)ether. polydiimide is obtained by dehydration of a soluble polyamic acid, namely, 0 II

0

0

pyromellitic dianhydride

bis-4(aminopheny1)ether

Soluble polyamic acid

Insoluble polydiimide This commercial polymer is stable for a year at 275°C and can be used for 200 h at 358°C and 10 min at 377°C. At 600°C,volatiles are formed. They include CO, CO,, H,O and H, [288]. The activation energy of decomposition in vacuo in the range 585-632"C was estimated [289]to be 74 kcal mole-'. The purified polymer decomposes primarily by breaking of the amide group. Various other polyimides have been synthetized.

M-Si'

Polyoxadiazole. A typical polyoxadiazole is [--AI-C,~,C--] but various others have been synthetized. Some polyoxadiazole films have measurable fibre properties even after prolonged heating to a temperature of 400°C in air or nitrogen [290].Aliphatic linked polymers are of lower stability. Polybenzimidazole. Polymers of the type

References p p . 165-1 73

142 are commercially available. They can be used for 10 min at 650°C and 300 h a t 320°C. If the phenyl group is substituted in the 1-4 position, the weight loss is 1%after 1 day in nitrogen at 400°C. If the sample is then heated for one hour at successively higher temperatures the cumulative weight losses are as follows: 2% at 45OoC, 2% at 5OO0C,1.7%at 550°C and 4.7% at 600°C [291]. The polymers also show remarkable resistance to oxidative degradation. Polybenzothiazole. The polymers of general formula

can be used for 10 min at 538°C and 200 h at 330-343°C [292]. Table 22 summarizes data on the thermal stability of various other heterocyclic polymers. Aromatic ladder polymers. Ladder polymers which withstand red heat are obtained by cyclization of polyacrylonitrile [ 293, 2941 :

The softening point determined by thermomechanical methods may be as high as 500°C. A true ladder polymer can also be obtained by condensation of an aromatic tetracarboxylic acid or a dianhydride with an aromatic tetramine. An example is

ladder structure

143 By varying the heating rate, cyclodehydration takes place, giving either a semi-ladder or ladder structure. The semi-ladder polybenzimidazolone is soluble in some solvents. The ladder polymer subsequently formed is insoluble. This type of polymer is stable to 450-650”C in nitrogen. 13.4 MECHANISM OF DEGRADATION OF SOME POLYMERS CONTAINING AROMATIC RINGS IN THE CHAIN

The thermal behaviour of five thermostable polymers has been compared by Davis [ 2951. These are Poly [ 2,2-propane-bis-(4-phenylcarbonate)]

PC

1, Poly [ 2,6-dimethylphenylene ether]

PPO

A polysulphone prepared from bisphenol A and 4,4’-dichlorodiphenylsulphone PS

A polyarylate prepared from terephthaloylchoride and bisphenol A

TD

References p p . 165-1 73

144 A polyarylate prepared from terephthaloylchloride and phenolphthalein TPP

Gel is formed with all the polymers. The 3'6 gel formation as a function of time is given in Fig. 69. Gel does not form in the polyesters and polycarbonates if the volatile products are not removed. The relative

Time (h)

Fig. 69. Gel content of polymer PC, PPO, TP, and TD as a function of time at 37OoC and of PS at 38OoC [ 2951.

Temperature ("C)

Fig. 70. Relative gas evolution of five aromatic polymers heated in nitrogen at 32 degC min-' [ 2951.

145 TABLE 23 RATE OF GAS EVOLUTION AND GAS COMPOSITION FOR THE POLYMERS AT 38OoC [295] Polymer

PS TD PPO PC TPP

Rate of gas evolution (em3 at NTP per g polymer)

co

coz

0.3 1 4 4 5

9 34.5 7.2 3 79

59 4 95 20.9

% Gas composition -___

I

CH4

CzH6 Hz

35 5 44 2 -

-

0.3 2 -

3.2 0.2 41.4 0.1

so2

42.2 -

-

C6H6

C7Hs

1 -

3.6 0.1 1.3 -

m ~ o u n tof volatiles formed is given in Fig. 70 and their composition in Table 23. If volatile and gel formation are considered, it appears that polysulphone is the most stable of these five polymers. In these highly aromatic polymers, it is the nonaromatic linkages which are the most labile. The large amount of CO, formed in polycarbonates indicates that the carbonate linkage is the weakest. Methane formation in polycarbonate, polyarylate and polysulphone would result from a loss of CH; followed by hydrogen abstraction from the isopropylidene moiety. The large yield of H, and CH, in the degradation of the polyphenylene ether is indicative of the lability of the CH, groups on the aromatic ring. A detailed study of the same aromatic polysulphone (PS) has been reported by Davis [296]. The gas composition does not change with the time of heating. Volatile liquid and solid products have been separated and identified by gas chromatography and mass spectrometry; the results are given in Table 24. After three hours heating, a gel is formed. The amount of gel is not affected by the presence of the volatile pyrolysis products. If the theory of Charlesby-Pinner is applied to the gel formation data, a straight line with a positive intercept of 0.35 is obtained. This means that chain scission also occurs. The rate of SOz evolution is in agreement with the results of Levy and Ambrose [297] on the pyrolysis TABLE 24 VPC/MASS SPECTROGRAPHIC ANALYSIS OF THE VOLATILE LIQUID AND SOLID PRODUCTS OF HEATED POLYSULPHONE [296] Product

Relat. Product amount

Phenol p-Cresol p-Ethyl phenol p-Isopropylphenyl phenol Bisphenol A

200 1 2 2 1

<

Relat. amount

Diphenyl ether p-Tolyl phenyl ether p-Ethyl diphenyl ether p-Isopropenyl diphenyl ether Two isomers of tolyl(ethy1-pheny1)ether

5 20 5 20 10

References p p . 165-1 73

146 of diphenylsulphone. This led Davis [296] to propose a similar mechanism for SOz production in the polymer; namely,

radical abstraction e.g. CH3

...

radical-radical recombination

4 eo -o I -

or

radical addition to other polymer chains

CH3

CH3 A e e H 3 I CH3 Subsequent breakdown of the isopropylidene linkage at the chain ends produced by (1)would account for the ether products; namely,

. .. -

(_>.e& CH2

and 2 isomers of H3C I CH3 The mechanism of formation of phenol is not clear. It may arise from pyrolysis and interaction of phenolic end groups. The detailed data on the thermal stability of the poly[2,2-propanebis(4-phenylcarbonate)], PC, have been reviewed by Davis and Golden [298]. If the degradation products are not removed, a decrease in molecular weight obeying first-order kinetics, with an activation energy of 39 kcal mole-', is observed. In an evacuated system gelation occurs with an activation energy of 27 heal mole-'. The difference between degradation in a closed or in an evacuated system is due t o ' the

/ \

147

0

6

4

2 Time ( h )

Fig. 71. Amount of volatile products evolved from polycarbonate at 36OoC with respect to time: A, COz ; B, bisphenol A; C, phenol; D, 2-(para-hydroxyphenyI)-2phenylpropane; E, CO; F, CH4 ; G , diphenyl carbonate [ 2981.

competition between condensation and hydrolysis reactions. The most important volatile liquid and solid products formed are phenol, diphenyl carbonate, and 2(4-hydroxyphenyl)-2-phenylpropane. The gaseous products evolved are given in Table 23 and their rate of evolution in Fig. 71. The mechanism of degradation of the polycarbonate was elucidated by the detailed study of a model compound diphenylcarbonate. The following degradation paths are reported to justify the formation of the major volatiles, C 0 2 and bisphenol A, crosslinking and chain scission. Chain scission in closed systems occurs by step (6); gel is formed by step (8);namely,

COOH

G

- 4 3 - 3 4 3 - ( ; COOH

G

o

H2O + o

o c

o

; 4

3

e

Z

o F

-

2

-

o -

0

+

H

2

0

0

H + CO,

(6) References PP. 165-1 73

148

(7)

COOH

/coo0\

/

0

0

+H

\o /

e + C02

The liberation of gaseous compounds during pyrolysis of polycarbonate has also been studied by Kammermaier [299]. Three polyphenylene oxides were investigated by Powell et al. [300]. These are

(a) Poly (1,3-phenylene oxide) L

CH3 (b) Poly( 2,6-dimethyl-l,4-phenyleneoxide) [ - O p ] n

(c) Poly (2,5-dimethoxy-l,4-phenylene oxide)

149

I

1

100

I

200

300

400

500

600

700

800

900

Temperature ("C)

Fig. 72. Thermogravimetric analyses of poly( 1,3-phenylene oxide) [ 3001. 1, Aminco thermobalance, vacuum 0.3 mm, 3 degC min-' ; 2, Chevenard thermobalance, nitrogen, 24 degC min-' ; 3, Chevenard thermobalance, air, 24 degC min-'

.

Polymer (a) has number average molecular weight of 7.200, and the thermogravimetric analysis is given in Fig. 72. The major weight loss occurs between 500 and 600°C. The analysis of volatile products at different temperatures has been performed (Table 25). Some products sublimed; and, according t o mass spectral analysis, they may correspond to the chain fragments

4

HO

OH

TABLE 25 ANALYSES OF VOLATILES FROM POLY(1,B-PHENYLENE OXIDE [300] Temp. mnge

("C) ~~

H2

CH4

H2O

CO

C02

C6H6

26.0 37.3 61.5

7.3 7.2 7.5

32.8 16.4 7.7

23.2 35.1 21.8

9.6 3.7 1.4

1.1 0.3 0.1

Weight volatiles (%)"

Total weight loss (%)a

~~~~

20-450 450-550 550-620 ~~

a

Volatile products (mole %)

~

~~

~

3.7 5.8 3.9

55.5 7.0 3.9

~

Of starting material. References p p . 165-1 73

150 The composition of the residue is C6H1,600,1. A mechanism including chain breaking at the ether linkage has been proposed. Subsequent reactions of the phenoxyradical give crosslinking and formation of C 0 2 , CO, CH4, H2 and H20. The proportion volati1e:sublimate:residue is about 13%: 66% : 2196, as determined from isothermal experiments at 450, 550 and 62OoC. The polyether (b) substituted by methyl groups degrades about 100 degC below the Qnsubstituted polymer. Much more methane is formed. Scission of the methyl group is thus an important process which adds to random chain scission. If the aromatic ring is substituted by methoxygroups (polymer (c)), considerable weight loss occurs below 30OoC. Methane and methanol are produced and indicate that CHj and .0CH3 radicals are formed by side chain scission. Three aromatic polyesters were studied by Powell et al. [301]; namely, (i) Poly-p-phenylene isophthalate-co-terephthalate

r o

1

(ii) A condensation polymer from 4,4'-dihydroxydiphenyldimethylmethane and isophthalic acid r

1

(iii) A condensation polymer from 4,4'dihydroxydiphenylether and 5-amyloxyisophthalic acid

The highest rate of degradation of polyester (i) is observed between 500 and 55OoC. Differential thermal analysis indicates that decomposition occurs around 465OC. CO and C 0 2 are the major products formed in this

151 TABLE 26 ANALYSES OF VOLATILES FROM POLY@-PHENYLENE ISOPHTHALATE-COTEREPHTHALATE) [ 3011 Temp. range ( OC)

Weight volatiles

Volatile products (mole %) HZ

CH4

H2O

co

(%)'.)a

~

20-4 50 450-550 550-620

2.0 10.9 22.7

0.4 1.8 3.6

0.4 0.7 1.8

61.7 57.9 50.3

34.8 27.2 21.0

(%la

C6H6

c02

~~

~~

0.7 1.5 0.6

Total weight loss

29 8 3

~~

52.3 8.0 3.2

Of starting material.

first step (Table 26); they result from breakdown of the ester group. Hydroquinone appears in the sublimate. A t higher temperatures, formation of hydrogen and methane'is indicative of some ring breakdown. After removal of the major part of the ester linkages, the principal reaction is believed to be the formation of free radicals that recombine t o form polyphenyl and polyphenylether structures. Polymers (ii) and (iii) degrade about 100 degC below polymer (i). The major solid product of polymer (ii) is bisphenol A; CO and COz are the most important gaseous products. The mechanisms of degradation of polymers (i) and (ii) thus present important similarities. With polymer (iii), the amyloxy group is almost completely removed below 350°C. Weakness of the alkoxy linkage on aromatic groups has previously been reported for polyethers. The thermal behaviour of sulphur containing polymers has been reported on [ 3021. The major breakdown in poly( 1,Cphenylene sulphide)

occurs around 500" C. Hydrogen sulphide is the predominant volatile product at the lower temperatures and hydrogen at higher temperatures. A colourless, liquid to waxy condensate also separates. The structures suggested by mass spectral analysis to be present are

References p p . 165-1 73

152 The following simplified scheme represents the most important reactions. The primary step is cleavage of carbonsulphur bonds. Above 45OoC, the evolution of hydrogen becomes very important and results in extensive crosslinking in the residue [ 3021 .

I I The polysulphonate prepared by condensation of diphenylether-4-4'disulphonylchloride and 4,4'-dihydroxydiphenyl; namely,

decomposes at relatively low temperatures. Loss of weight becomes appreciable at 300°C. The maximum rate of SO, evolution is observed in the range 250-350°C. A review and some new results on the thermal degradation of poly-p-xylylene have been presented by Jellinek and Lipovac [ 3031. Little volatile material is formed but appreciable amounts of dimer, trimer, tetramer and pentamer were isolated. Typical vacuum volatilization curves are given in Fig. 73. It has been proposed that the mechanism consists of random chain scission a t abnormal structures in the chain, followed by a depropagation reaction resulting in low molecular weight polymer but very little monomer.

153

Tirne(rnin)

Fig. 7 3 . Typical vacuum volatilization curves (mo = 2.0 X monomeric unit moles); ( i ) 408OC, (ii) 436OC, (iii) 424OC, (iv) 45OoC, (v) 515OC [303].

14. Copolymers and polymer blends 14.1 COPOLYMERS

The presence of a comonomer can deeply affect the thermal behaviour of polymers. A comonomer can in some cases confer stability but in others may render a homopolymer unstable. Some important systems will now be described and discussed; a review on the subject has been published by Grassie [ 3041. 14.1.1 Destabilized homopolymers

( a ) Polymethacrylonitrile and polyacrylonitrile containing acrylic acid It was shown many years ago that if some methacrylic acid is copolymerized with methacrylonitrile, the discolouration reaction References p p . 165-1 73

154 observed with pure polymethacrylonitrile is accelerated [ 305-3081. According to the authors, the effect of the acid is to initiate the condensation of neighbouring nitrile groups according to

The conjugated carbon-nitrogen sequences formed are responsible for the observed colour. Radicals do not seem to be involved in this reaction. Identical effects are observed when acrylonitrile is copolymerized with acrylic acid. ( b ) Polyacry lonitrile containing methylvinylketone

The thermal degradation of polymethylvinylketone is a random reaction in which water is liberated from pairs of adjacent units resulting in cyclization and conjugated sequences of limited length [309]. When methylvinylketone units are incorporated into polyacrylonitrile, the rate of thermal colouration is greater than with pure polyacrylonitrile [309]. The acceleration effect of methylvinylketone units has been ascribed to their behaving as initiators according t o one of the following

155

I

1

I

I

Propagation can apparently pass through methylvinylketone units, conjugation being preserved. ( c ) Copolymers of vinylchloride and vinylacetate [310] The rates of production of volatile material from polyvinylacetate, polyvinylchloride and vinylacetate-vinylchloride copolymers, covering the entire composition range, have been compared by thermal volatilization analysis. It has been found that, at both extremes of the composition range, incorporation of the comonomer unit induces destabilization. Minimum stability occurs for composition of approximately 40-50 mole 7% vinylacetate. The rate of volatilization as a function of the composition of the copolymers is given in Fig. 74. The results were confirmed by a study of the thermal degradation in tritolylphosphate solution. The stability of the copolymers is a minimum at 30-40 mole 7% vinylacetate. HC1 and acetic acid catalyse the degradation of the

I

,

20

,

40 60 Mole % V A

00

L

0

Fig. 74. Compaf;ison of rate of volatilization for copolymers of vinyl chloride and vinyl acetate at 248 C as measured by Pirani reading, with copolymer composition, for heating rate of 5 degC min-' . References p p . 165-1 73

156 copolymer. Many other organic acids were found to have no catalytic effect. The UV spectrum is quite similar t o that reported for pure PVC. The variation of absorbance at three wavelengths with mole % vinylacetate in the copolymer, for 5%degraded samples, is given in Fig. 75. It is apparent

0

I

I

20

40

60

a0

0

Mole % V A

Fig. 75. Comparison of absorbance by copolymers of vinyl chloride and vinyl ace ate at three maxima in the UV spectrum for 5% degraded samples with copolymer composition.

that for compositions in the range 20-30 mole % vinylacetate longer sequences are formed. Grassie et al. [ 3101 proposed that both the over-all rate and the development of conjugation are increased considerably by some heterogeneity of the chain units. One or both units are labilized by the immediate proximity of a unit of the opposite type. ( d ) Copolymers of styrene and acrylonitrile [311] When acrylonitrile units are copolymerized into polystyrene, the rate of volatilization measured by thermal volatilization analysis increases in direct proportion t o the acrylonitrile content. From the changes in molecular weight that occur during the reaction, it is clear that the primary effect of the acrylonitrile units is to cause an increased rate of chain scission but the unzipping process which follews chain scission is not greatly affected. Acrylonitrile monomer thus appears among, the volatile products. The rate coefficient of the chain scission process associated with acrylonitrile units is about thirty times that for “normal scission” in styrene segments of the polymer chain. The proportion of chain fragments (dimer, trimer, etc.) increases with acrylonitrile content; these fragments also incorporating acrylonitrile units. Yellow colouration develops in the residues from copolymers with high acrylonitrile content

157 at advanced stages of degradation. Infrared and ultraviolet analysis suggest that this is due t o conjugated unsaturation in the polymer chain which may be associated with liberation of hydrogen cyanide. ( e ) Copolymers, methylmethacrylatel-chloroacrylonitrile and styreneh-chloroacry lonitrile a-Chloroacrylonitrile induces instability into polystyrene and polymethylmethacrylate [312]. With both copolymers as with the homopolymer of a-chloroacrylonitrile, the threshold degradation temperature is around 14OoC, which suggests that the same initiation process is involved. Random chain scission occurs in both copolymers; methylmethacrylate, hydrogen chloride and some a-chloroacrylonitrile are liberated during the degradation of the copolymer of methylmethacrylate/a-chloroacrylonitrile, but styrene is not evolved from the second copolymer. Grassie and Grant [ 3121 proposed that initiation occurs at the C-Cl bond of the a-chloroacrylonitrile unit, and the mechanisms suggested were as follows. For the m$thylmethacrylate copolymers CH3 c1 I I -CH2+-CH2-C
I

COOCH,

I

CN

CH3

I

____+

I COOCH,

CH3 I -CH,-C-CH

I

CH3 I 2-&CH2-C-----

COOCH3

CH3

I -CH2 <-CH I

I

CN

COOCH3 CN

I

CH3 I --.CH,-C-CH=C-CH;

I

COOCH,

CH3

=C-CH,-C-

I

I

COOCH, CN

+ c1*

I

I

I I

+ HC1

COOCH3

CH3 .I + C-

7

I

COOCH3

1

monomer References pp. 165-1 73

158 For the styrene copolymers

I

CN Ph Ph I -CH2-CH-CH2-&CH2+&H-

+ C1-

I

CN

I

Ph

Ph

I

-CH2-CH-CH=C-CH2-CH-

I

I

+ HC1

CN The two C-C bonds in the /3 position t o the ethylene unsaturation and the aromatic ring are much reduced in strength compared with normal C - C bonds. Scission of one of these bonds occurs and stable molecules are formed by disproportionation of the radicals: Ph

I

-CH,--CH--GH;

Ph

I

+ *CH--CH=C-

I

CN Ph

I

-CH2--C=CH2

Ph

I

+ CHZ-CH=CI

CN

14.1.2 Stabilized homopolymers

( a ) Poly methylme thacry late containing acrylonitrile

Grassie and Melville [313] observed that the presence of acrylonitrile in polymethylmethacrylate causes an induction period (Fig. 76) in the production of methylmethacrylate monomer at 220°C. During this induction period the molecular weight decreases rapidly and tends to a limiting value close to that corresponding t o the average distance between acrylonitrile units. Later, the rate of monomer production builds up and passes through a maximum after 2.3 hours heating. This led Grassie and Melville to believe that the acrylonitrile units constitute “weak links” in the polymer structure.

159

B'

220°C

Time (min)

Fig. 76. Rate of evolution of monomer at 22OoC from a methyl methacrylateacrylonitrile copolymer containing 0.24 mole % acrylonitrile (mol. wt. = 617,000) [313].

This system was recently reexamined by Grassie and Farish [314] and the work extended t o copolymers containing higher proportions of acrylonitrile. The occurrence' of induction periods and rate maxima reported by Grassie and helville [313] at 220°C was confirmed. A comprehensive study of the degradation a t 280°C was performed. The changes in molecular weight with the extent of volatilization are given in Fig. 77. They are independent of the composition of the copolymers. The induction period observed at 22OoC for the volatilization has disappeared at 280OC.

%volatilization

Fig. 77. Changes in mol. wt. with volatilization at 28OoC of methyl methacrylateacrylonitrile copolymers. ( 0 , 410/1; 0, 40/1; 0 16/1; 0 , 8/1) [314]. References p p . 165-1 73

160 A unified picture of the reaction mechanism at 220 and 280°C was proposed by Grassie and Farish [314]. At 220°C the degradation is initiated at unsaturated chain ends but radical depolymerization cannot pass through acrylonitrile units. Degradation is thus stopped at the first acrylonitrile unit, thereby accounting for inhibition of monomer production at an early stage in the reaction. Chain scission, however, occurs at random at a very slow ra$e at 22OoC. This random scission produces radicals which disproportionate in a cage. Unsaturated chain ends are thus formed at this stage; namely,

The concentration of these unsaturated chain ends increases and, since they are unstable at 220"C, the rate of volatilization is accelerated. Since depolymerization cannot pass through acrylonitrile, the chain length of the residue is the average distance between adjacent acrylonitrile units. A t 28OoC there are two fundamental changes in the nature of the reaction. First, random chain scission to depropagating radicals occurs. Secondly, depropagation can pass through the acrylonitrile units which are then liberated as monomer. Thus, the stabilization effect of acrylonitrile observed at 220°C is lost at 280°C because acrylonitrile units are liberated in the depropagation process.

( b ) Polymethylmethacrylate containingphenylacetylene Comparison of the TVA thermograms of the copolymer and of pure polymethylmethacrylate [ 3151 shows that end initiation at polymethylmethacrylate units is inhibited in the copolymer. Degradation above 300°C is the result of scission at the unsaturated phenylethylene units. ( c ) Poly me thy lme thacry late containing meth y 1 and higher acry lates

By comparison with pure polymethylmethacrylate it was shown that volatilization in the neighbourhood of 300°C is almost completely eliminated [315, 3161 (Fig. 78). The acrylate units in the chain exert a blocking effect on the depolymerization process, as in the case of acrylonitrile copolymers. The high temperature peak, however, is not affected by the presence of acrylate since the depropagation process can pass through the second monomer and methylacrylate appears among the volatile products. The property of blocking the end-initiated monomer producing reaction is common to straight chain acrylate monomers.

161

Temperature ("C) of P M M A

Fig. 78. Effect of copolymerization with methyl acrylate on the thermal stability TVA thermograms for 26/1(dashed line), and 8/l(dotted line) copolymers, compared with that for polymethylmethacrylate of comparable molecular weight (full line) [316].

( d ) Copolymers of methylmethacrylate with styrene The degradation behaviour of these copolymers is interesting in view of the fact that both homopolymers undergo depolymerization t o yield monomer. Copolymers containing 50% and 20% styrene have been prepared [316]. The TVA thermogram of pure polystyrene is independent of molecular weight, and for a heating rate of 10 degC/min, a single peak with a maximum at 418OC is observed. When the copolymers are compared to polymethacrylate of comparable molecular weight, an important improvement in stability is observed in the copolymers (Fig. 79). The end-initiated reaction is eliminated. It is improbable that styrene exerts a blocking effect on the depropagation

Temperature ("C)

Fig. 79. Effect of copolymerization with styrene on the thermal stability. TVA thermograms for 4/1 (dashed line) and 1/1 (full line) copolymers, compared with that for polymethylmethacrylate of comparable mol. wt. (dotted line) [ 3161. References p p . 1 6 5 - - 1 7 3

162 reaction as observed for acrylonitrile, since polystyrene depolymerizes on heating, whereas polyacrylonitrile does not. Grassie and Farish [ 3171 have attributed this effect to the absence of terminal double bonds in the copolymers, which was previously demonstrated by Bevington et al. [ 3181. 14.2 POLYMER MIXTURES

The nature of the interactions between polymers in a blend depends strongly on the physical state of the system. It is well known that polymers are generally incompatible in the solid state; films cast from a solution containing a mixture of polymers are, in fact, usually opaque. It has been shown by microscope methods that incompatible pairs can, in certain cases, be included in a two-phase system [319]. One polymer tends to form a continuous phase while the other is dispersed in that continuous phase in the form of micelles of dimension (1-15) x cm. The polymer blends described next have been obtained by freezedrying a solution of the mixture (Richards and Salter [320]) or by casting films from solutions (McNeill et al. [321-325]), Their physical state is, however, not defined.

( a ) Polystyrene-poly-a-methylstyrenemixtures The degradation of polye-methylstyrene is unaffected by the presence of polystyrene, but depolymerization of the latter polymer can be brought about at temperatures below 3OO0C by heating in the presence of polye-methylstyrene [ 3201. The rate of polystyrene volatilization then varies as an inverse function of the molecular weight of poly-a-methylstyrene. The system is heterogeneous, consisting of micelles of poly-amethylstyrene embedded in a polystyrene matrix. It has been suggested that the poly-a-methylstyrene chain unzips completely to a monomer radical which diffuses into the polystyrene matrix and attacks a polystyrene molecule.

( b ) Polypmpylene-polystyrene and polypropylene-polymethylmethacrylate mixtures [326] Polymethylmethacrylate and polystyrene are accelerators of the degradation of polypropylene. Acetone ex traction of the degraded polypropylene reveals the formation of graft and block copolymers. The presence of these graft copolymers can be used in the modification of the ability of polypropylene to retain dye.

( c ) Polymethylmethacrylate-polystyrene mixtures No interaction is observed in this case [321].

163

(d) Polyvinylchloride-polymethylmethactylatemixtures The thermaholatilization curves for this mixture are given in Fig. 80. The polyvinylchloride is slightly more stable in the mixture, but polymethylmethacrylate is initially less stable and gives monomer at temperatures corresponding to polyvinylchloride dehydrochlorination [ 3221. Subsequent breakdown of the remaining polymethylmethacrylate, under programmed heating conditions, is shifted to higher temperatures.

Sample temperature ("C)

Fig. 80. TVA curves for simultaneous degradation of PVC Breon 113 and PMM anionic, 5 mg of each, ( i ) unmixed, (ii) mixed. Film samples, heating rate 5 degC

All the observed features of the interaction of polymethylmethacrylate and polyvinylchloride, when degraded as a blend, are explained by two simultaneous reactions [ 322, 3231. (i) A chlorine atom from the degrading polyvinylchloride attacks polymethylmethacrylate according to C1' + M,

Pi Pi

+

--*

+

HC1+ P i

Hydrogen abstraction

P,:

Chain scission

+ M,- j

P,:- 1

+ MI

Depropagation

This chlorine atom attack accounts for the decrease in the molecular weight of polymethylmethacrylate, monomer production at abnormally low temperature and the delay in the dehydrochlorination of polyvinylchloride. This delay in dehydrochlorination is general for blends of polyvinylchloride and any polymer containing hydrogen atoms that can be abstracted by chlorine atoms. References p p . 165-1 73

164 (ii) HC1 abstracted from polyvinylchloride reacts with the methacrylate ester group and accounts for the anhydride structure shown in the IR spectrum of degraded polymethylmethacrylate. Degradation of the anhydride group results in formation of COz , CO and methane. These ring structures in the chain act as blocking units in the depolymerization and drastically reduce the zip length. A displacement of the curve to higher temperature, corresponding to volatilization of polymethylmethacrylate, is the expected result. This was further studied by gas chromatography of the volatile products, by IR analysis of the residual polymethylmethacrylate and by pyrolysis of polymethylmethacrylate in the presence of HCl [324]. The behaviour of graft copolymers [323] prepared by mastication of polyvinylchloride in the presence of methacrylate monomer is similar to that of a mixture of both polymers. This is in agreement with the mechanism suggested. ( e ) Polyvinylchloride-polystyrenesystem [ 3231

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds t o the elimination of HCl and the second t o that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded; styrene evolution, in fact, does not occur below 35OoC. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 30OoC. According to McNeill et al. [ 3231, structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of C1- t o the aromatic ring, yielding a cyclohexadienyl-type radical which is unable t o induce depolymerization of the styrene chain. Delay in dehydrochlorination is also observed with graft copolymers, but some chain fragments are produced a t the same temperature as is HCl. The behaviour of random copolymers is quite different. The copolymers become progressively less stable as the vinylchloride content increases. Initiation of styrene production, and the temperature corresponding to the maximum rate of volatilization, are both displaced to lower temperature. ( f ) Polyvinylchloride-poly-a-methylstyrene system [ 3231

The delay in dehydrochlorination is particularly marked in this system and a-methylstyrene production is initiated earlier than in polyvinylchloride-polymethylmethacrylate mixtures.

165

Tern perature ("C)

Fig. 81. TVA behaviour of PVC + PS ( 1 : 4 ratio by weight) degraded simultaneously in twin-limbed tube, in unmixed and mixed condition. Heating rate 10 degC min-' [323].

( g ) Polychloroprene-polymethylmethacrylatemixtures

Evolution of hydrogen chloride from polychloroprene is unaffected by the presence of the second polymer. The system does not show any increased production of methacrylate monomer in the early stage of breakdown as was observed for mixtures of polyvinylchloride and Dolvmethvlmethacrvlate. REFERENCES

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