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Investigation of the chlorine balance in the degradation of unsaturated polyesters based on HET acid
Polymer Degradation and Stability 13 (1985) 147-160
Investigation of the Chlorine Balance in the Degradation of Unsaturated Polyesters Based on HET Acid J. K. Fink Institut fiir Chemische und Physikalische Technologie der Kunststoffe der Montanuniversit~it Leoben, A-8700 Leoben, Austria
(Received: 30 April, 1985)
ABSTRACT Unsaturated polyesters based on H E T acid, phthalic acid, maleic acid and 1,2-propanediol were cured with styrene. The cured and uncured products were degraded to 500°C both in nitrogen and in air. The amount of chlorine which remained in the residue and the chlorine recovered as hydrogen chloride were analyzed. The results were nearly independent of whether the pyrolysis was carried out in nitrogen or in air. Under the conditions oJ'the experiment only 30 % to 40'% of the chlorine volatilized was recovered as hydrogen chloride. The ease of chlorine abstraction from model compounds was also measured. Probably hexachlorocyclopentadiene, which is an intermediate in the degradation of H E T acid-containing materials, is the active substance from which the chlorine is abstracted in the course of degradation.
INTRODUCTION The use of HET acid anhydride (1,4,5,6,7,7-hexachloro-5-norbornene2,3-dicarboxylic acid anhydride) as a basic constituent in polyesters to achieve flame retardancy was suggested several years ago. 1 In a previous series of papers the main degradation paths in such systems have been established by pyrolysis-gas chromatography-mass spectrometry techniques. 2'3 It was found that the retro-Diels-Alder reaction of the HET 147 Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
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moiety to eject HEX ( = hexachlorocyclopentadiene) plays a central r61e among the primary reactions of thermal degradation in imparting flame retardancy to the resin. In the course of degradation of HET acidcontaining polyesters the formation of hydrogen chloride has also been observed by pyrolysis mass spectrometry. 4 However, no quantitative studies have been performed hitherto to estimate to what extent the chlorine, originally present in the system, is converted into hydrogen chloride during thermal degradation. Since this compound is believed to be most active compared with other chlorinecontaining compounds in flame poisoning, 5 the measurement of hydrogen chloride formation during thermal degradation is of particular interest in order to obtain information about the tendency of a chlorinecontaining flame retardant to act as a flame poisoning agent via hydrogen chloride. For this reason, pyrolysis experiments in air and in nitrogen were performed on HET acid-containing systems, the hydrogen chloride volatilized being trapped in an alkaline solution. The chlorine which remained in the residue was also determined. Further, the ease of chlorine abstraction from some selected model compounds, which are of interest in the present research, was determined. From these experiments some hints can be gained concerning the nature of the precursor molecule from which the chlorine is abstracted. EXPERI MENTAL Materials HET acid and 1,2-propanediol were donated by Vianova (Austria). HEX was supplied by EGA-Chemie and the other samples by Merck. Styrene was distilled in v a c u o before use, benzene was also distilled before use and the other chemicals were used without further purification. Preparation of the unsaturated polyesters The unsaturated polyesters were prepared by melt condensation. Nitrogen was passed through the melt, the condensation temperature being maintained at 180°C to prevent enhanced retro-Diels-Alder reaction of the HET moiety. Usually, an acid number below 50rag KOH/g polyester was attained within 5 h. Composition data are shown in Table 1. A hundred grams of the initial feed was placed in the reaction
Chlorine balance in the degradation of HET-acid-UP
149
TABLE 1 Compositionof the Monomer Mixturesused in the Preparation of Unsaturated Polyesters Sample codes
UPA UPB UPC
Relative amounts of the various compounds (mole ratios) (1)
(2)
(3)
(4)
0-5 0.4 0.3
0.0 0.1 0.2
0-5 0.5 0.5
1-1 1.1 1.1
Acid number (rag KOH/g)
35.7 46.2 36.1
Components: (1) HET acid; (2) phthalic anhydride; (3) maleic anhydride; (4) 1,2-propanediol. N. B. This Table should not be confused with a similar Table in a previous paper 3 where polyesters with different components were under investigation.
vessel. Because of the comparatively high volatility of the diol component (1,2 propanediol) a 10 ?/oexcess of the latter c o m p o u n d was used. A glassclear, brittle material was obtained in all cases. Curing of the unsaturated polyesters
0.7 parts by weight of unsaturated polyester were mixed with 0.3 parts by weight of styrene and homogenized by gentle warming. 1 ~o by weight of dibenzoylperoxide was dissolved in the mixture which was kept at 80 °C for 1 h and post-cured at 100°C overnight. To distinguish the styrene cured samples from the uncured samples the letter S is added to the respective sample code (e.g. UPAS). In a few cases other formulations were chosen, e.g. 40 % styrene and 60 ~o UP. These exceptions are indicated when they occur. Determination of the chlorine content
Solid samples were decomposed in the following way. 6 An approximately 0" 1-g sample was placed in a small crucible (height, 1 cm) filled completely with powdered calcium oxide. This crucible was placed in a bigger crucible so that the orifice of the smaller crucible faced the inner bottom of the bigger crucible which was also filled with calcium oxide. The quantity of calcium oxide was weighed. The crucible was heated for 15 min by means of a Bunsen burner. By this special arrangement the
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J. K. Fink
pyrolysis gases are forced to move downwards into the hotter zone before escaping into the environment. The calcium oxide was dissolved in glacial acetic acid and the chlorine determined potentiometrically with a silver-silver chloride electrode by titration with silver nitrate. The analysis was corrected for the chlorine content of the reagents, which was, in any case almost negligible. The hydrogen chloride in the pyrolysis gases, trapped in the alkaline absorption solution, was also determined by simple potentiometric titration after acidifying with glacial acetic acid. Since it is known that certain organic compounds may lose chlorine under certain conditions, 7 HEX, as a model for the chlorine containing pyrolysis vapors in the systems under investigation was treated under similar conditions. Only 5 mg of chlorine per gram of HEX were recovered by the chlorine analysis method described above. In the subsequent treatment the chlorine analysed in the absorption solution is described as hydrogen chloride. In general, the chlorine content given in the Tables is expressed as weight ~o.
Determination of chlorine abstraction from certain model compounds To estimate the relative ease of chlorine abstraction from certain model compounds, dibenzoyl peroxide was allowed to decompose almost completely in the presence of these compounds and the resulting amount of chlorobenzene was measured. The following substances were used as model compounds: HEX, HET acid-diethylester and carbon tetrachloride. HEX was analyzed among the products of pyrolysis of HET acid-containing polyesters, HET aciddiethylester serves as a model compound for a polyester chain and carbon tetrachloride was used as a reference substance, because the flame retardant action of this substance is well established and used for fire fighting. HET acid-diethylester was prepared from HET acid in an excess of ethanol in the presence of p-toluenesulfonic acid using a standard procedure. The substance was recrystallized from ethanol (MP: 60°C; ~o Clcalcd.47"80; ~ Clrouna, 47"37).
Pyrolysis equipment Typically, a 0-2-g sample was placed in a ceramic weighing boat; uncured polyesters were used in powder form, the cured samples as small pieces.
Chlorine balance in the degradation of HET-acid- UP
151
The boat was placed in the centre of a quartz tube, 400 m m long and with an inner diameter of 26 mm. The flow of nitrogen or synthetic air (20 Oz, 80~o N2)was arranged to be 200ml min -1 The issuing gas stream passed through a safety vessel and then bubbled through 50 ml of 0- IM aqueous N a O H which was vigorously stirred. The pyrolysis was started by placing over the quartz tube, a tube furnace which had been preheated to the selected temperature. The tube was kept in the oven for 30 min, after which it was removed from the oven and allowed to cool. The residue was reweighed and the chlorine content of both residue and absorption solution was determined as described above. The temperature profile inside the tube was measured in blank experiments. It varied along the tube but did not depend upon the gas flow. The measured values were 25-30 °C lower than the adjusted nominal temperature in the center of the furnace. The temperatures given in the Tables are those measured at the center of the tube where the weighing boat was situated. The temperatures are rounded to 5 °C. From blank experiments it was estimated that the sample would reach a temperature within 5 °C of the final temperature within 6 min after the start.
RESULTS A N D DISCUSSION
Undeeomposedsamples Because of the volatility of the monomeric compounds, certain amounts are lost by volatilisation, especially in the earlier stages of the condensation reaction. This was clear from the appearance of fine needles in the upper part of the condensation vessel. Thus, it is necessary to determine the chlorine content of the polyesters independently rather than from the monomer feed. The chlorine contents of the undecomposed samples are shown in Table 2. Apart from sample UPC, the chlorine content determined by analysis agrees well with the chlorine content calculated from the m o n o m e r feed. The chlorine values obtained from analysis were used in subsequent discussion and calculations.
Degradation experiments The results of the degradation of the cured and uncured samples in nitrogen and in air are shown in Table 3. In this Table the weight per cent
J. K. Fink
152
TABLE 2 Chlorine Contents of the Undecomposed Unsaturated Polyester Samples
Sample
%Cl calc. a
%Cl found
Sample
°/ooClfound for UPX × 0.70
%Cl found
UPA UPB UPC
34"4 29-6 24' 1
33'5 30"0 28.3
UPAS UPBS UPCS
23"4 21.0 19.8
22'7 21 "2 19.0
a Calculated for infinite chain length (end groups neglected) from the anhydrides and acid compositions given in Table 1, but neglecting the excess of diol. b 0-7 parts UPX cured with 0.3 parts styrene.
of residue, the chlorine content in the residue in weight per cent and the yield of chlorine trapped in the absorption solution are given. Further, from the initial chlorine content and the chlorine content in the residue the amount of chlorine volatilized in the course of pyrolysis has been calculated. From these data the ratio of chlorine recovered in the absorption solution and the total amount of chlorine volatilized was calculated. It is believed that the amount of volatile chlorine missing in this balance was either deposited in the tar fraction or was among the higher volatiles, but in any case bound in some organic molecule. For U P A the amount of residue, the chlorine content of the residue and the amount of volatile chlorine recovered are plotted in Fig. 1. Since the behaviours of the other samples are qualitatively similar, the graphs are not given here, but some interesting features will be discussed. It can be seen from Fig. 1 and Table 3 that, in general, the data are almost independent of the atmosphere in which the pyrolysis experiment was performed. This means that, apart from the initial steps of degradation, which may be initiated by dissolved oxygen, the 'catabolism' of chlorine under the conditions of the experiment is practically unaffected by the oxygen content of the environment. At lower pyrolysis temperatures (275 °C), for the uncured samples, the chlorine content of the residue increases slightly in comparison with the original samples. Obviously, this apparent increase in the chlorine content is due to preferential volatilization of non-chlorine containing substructures of the polyester. In the styrene cured samples, however, the trend is reversed. In all cases the most significant changes occur in the range 275-325 °C, corresponding to a maximum rate of degradation. In certain cases the
Chlorine balance in the degradation of HET-acid-UP
153
/
5O
-50
E
L.I
275
325
375 ToE
Fig. 1.
UPA: Per cent residue, per cent chlorine in the residue and volatile chlorine
recovered in the absorption solution as HCI, viz. temperature of pyrolysis in air and in nitrogen atmosphere. • Residue in air, © residue in nitrogen, • per cent chlorine in residue in air, A per cent chlorine in residue in nitrogen, • milligrams of volatile recoverablechlorine per gram sample in air, [] milligramsof volatile recoverablechlorine per gram sample in nitrogen. a m o u n t of recoverable volatile chlorine even passes through a m a x i m u m at a pyrolysis temperature of 325 °C. Further, the a m o u n t of residue at higher pyrolysis temperatures (375°C) is significantly enhanced in the case of the styrene cured samples compared with the uncured samples. There is still a small a m o u n t of chlorine in the residues from pyrolysis at 375 °C. Since the retro-Diels-Alder reaction should be complete under the conditions of pyrolysis, to yield (volatile) HEX this finding suggests that some transfer reactions may occur in the degrading polymer matrix. Inspection of the last column of Table 3 shows that only part of the chlorine volatilized is recovered in the absorption solution as hydrogen chloride. This is in striking contrast to PVC where practically all chlorine initially present in the polymer is converted to HC1, even at comparatively low pyrolysis temperatures. Table 3 shows that in the case of the uncured samples, the a m o u n t of chlorine recovered as HCI from the total volatile chlorine approaches a limiting value of c a . 30 ~o. Therefore, from HET acid, which contains six
Pyrolysis temperature (°C)
275 295 325 375 470 275 325 375 275 325 375 275 325 375 275 325
Sample
UPA UPA UPA U PA UPA UPAS U PAS U PAS UPB U PB UPB U PBS UPBS U PBS UPC UPC
N N N N N N N N N N N N N N N N
Atmosphere
85'3 30-2 20.7 15"2 12'8 97'3 36-6 25'4 69-6 19-1 15.5 87.7 40.3 22"4 80"0 17'4
Per cent residue
36" 1 21 '7 7-2 2' 6 1"8 21 '8 1-6 1-3 32"6 5"4 3"1 20-6 2.3 1-5 30"6 2"0
Per cent CI in residue
1-3 67'7 105'9 100' 6 107"4 0"5 96" 3 69"5 11'3 90"2 91.3 4'8 82'3 81.8 4"8 86-9
Milligrams of volatile Cl recovered per gram sample 2'7 26.9 32"0 33" 1 33-3 1'4 22" 1 22'4 7-3 29-0 29'5 3-1 20.2 20'9 3'8 28"0
Grams of volatile Cl per 100 g sample
TABLE 3 Experimental Data for Unsaturated Polyesters, Uncured and Cured with 0"3 Parts of Styrene
4'7 25" 1 33"2 30"4 32-3 3"3 43"6 31" 1 15"6 31"2 31-0 15"6 40"4 39"2 12"6 31-1
volatilised CI recovered
Per cenl o f
375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375
A, Air, N, Nitrogen.
UPC UPCS U PCS UPCS U PA UPA U PA UPAS U PAS UPAS UPB UPB UPB UPBS U PBS UPBS UPC U PC UPC U PCS UPCS UPCS N N N N A A A A A A A A A A A A A A A A A A 15"2 85"6 30"3 20"7 75"4 t7"8 12"8 68'2 28'5 21"7 70"5 18"4 13"7 86"9 35"5 22"3 68"4 16"9 13"0 83"8 16"1 21 "4
1'6 17"5 2"0 1'3 33'3 4"0 3'2 23'3 2"2 l"8 32"7 0"7 2'6 21 "5 2'0 2'I 30"9 2'8 1'7 19"3 0'6 2'3 81 '9 6"7 80"4 77"3 7"4 96'1 114"3 10"0 90'2 74"0 1"8 92'4 101 "5 2"7 84'8 84'3 10'7 83'0 86"9 2"8 73"0 82'9 28"1 4"0 18"2 18'8 8"4 32'8 33"1 6"8 22'0 22"3 6'9 29'9 29'6 2"5 20"4 20"7 7"2 27"9 28"1 2"9 18"9 18"6 29"2 16'5 44'1 41 "2 8"8 29"3 34~6 14"7 41"0 33~2 2"6 31'0 34"3 11"0 41 "5 40'8 15"0 29"8 30"9 9'8 38 "6 44"7
C.,
2.
J. K. Fink
156
TABLE 4 Experimental Data for Unsaturated Polyesters Cured in Air with Larger Proportions of Styrene and with Sb20 3
Sample
UPCS4" UPCS5 b UPCSb3/2 c
T (°C) Per cent residue
375 375 375
19.9 18.9 32.5
Per cent CI in residue
Milligrams of volatile Cl recovered per gram sample
0.7 0.5 0.9
68.3 62.5 12.3
Grams of Per cent o f volatile Cl volatile CI per I00 g recovered sample 16.8 14.1 16.1
40.7 44.3 7.6
a 0.4 parts styrene + 0.6 parts UPC. b 0"5 parts styrene + 0-5 parts UPC. c 0.3 parts styrene + 0.7 parts UPC + 0.2 parts Sb20 3.
chlorine atoms per molecule, two are converted into hydrogen chloride in the limiting case. In the case of the styrene cured samples the amount of chlorine recovered from the volatile chlorine is slightly enhanced (40-44 ~o) which corresponds to 2.4 to 2-6 chlorine atoms per HET acidunit being converted into HC1. Table 4 presents the results of degradation experiments of styrene cured unsaturated polyesters with larger amounts of styrene. The enhanced styrene content does not result in an enhanced release of hydrogen chloride. It is believed, therefore, that two chlorine atoms of the HET moiety can be abstracted very easily, resulting in the formation of hydrogen chloride, whereas the remaining chlorine atoms are attached comparatively more strongly to the substrate. Hydrogen chloride formation from an antimony trioxide containing styrene cured sample is also shown in Table 4. In this case the formation of hydrogen chloride is suppressed, thus reflecting the well known antimony chlorine synergism. To this point it is not clear whether the chlorine is removed from the HET acid moiety directly or from HEX which is known to be formed in the course of degradation. Neither is it clear whether the process is radical or ionic. In the gas phase ionic species are well known to occur in flames; thus, the electrical conductivity of flames is the principle on which flame ionization detection is based. Gaseous ions seem to be of minor importance in the mechanisms of degradation and fire retardation of polymers 9 and degradation reactions of carbon-carbon-backbonepolymers have been described in terms of radical processes. In the case of
Chlorine balance in the degradation of HET-acid-UP
157
cellulose, however, an ionic degradation mechanism has been suggested. l o Thus, there is no reason to discard, a priori, the possibility of ionic reaction mechanisms occurring in the degradation of polyesters or, more generally, in the condensed phase during degradation. Certain arguments support the view that the hydrogen chloride is formed from the HEX molecule rather than from the HET moiety. Ct 0
.Ct "~/C
C
CL CL CL
" Ct
Ct 0 HEX
0
-.
moteic onhydride
0 HET ocid onhydride
In HET acid there are two chlorine atoms aliphatically bound, two allylically, and two vinylically. One vinyl bond is associated with the two allylic bonded chlorines which are, in addition, bonded to bridge nodes. In the HEX molecule, however, two chlorine atoms bonded to the same carbon atom are in the allylic position with respect to two double bonds. It is to be expected, therefore, that the allylic chlorines in HEX are more reactive than the chlorines in the HET moiety. Abstraction of a single chlorine atom would result in an allylic radical, which is known to be comparatively less reactive. Abstraction of a second chlorine atom from the allyl position would lead to a carbene-like structure. It is not known how far the two residual electrons will interact with the double bonds as is the case in the cyclopentadienyl-anion. The experimental proof of these points is extremely difficult. For example, to establish whether the two chlorines are released from the same position, by isotope labelling of the allylic position and monitoring the hydrogen chloride released during pyrolysis in a mass spectrometer, is a rather formidable task. Chlorine abstraction In the course of this work an attempt was made to estimate the relative ease of chlorine abstraction from HEX and HET acid by means of model reactions in the liquid phase.
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J. K. Fink
The most straightforward method of estimating the ease o f abstraction of certain groups from a substance in polymer chemistry is to measure the transfer constants in vinyl polymerization. In the case of HEX this concept fails since HEX is known to react (e.g. with styrene) at appreciable rates with the double bond of vinyl monomers.11 Instead, HEX and HET acid ester were exposed to phenyl radicals generated from dibenzoylperoxide and the yield of chlorobenzene taken as a rough estimation o f the reactivity of the respective chlorines. In this system no complications with Diels-Alder reactions in which HEX could be involved are to be expected. In these experiments the concentration of dibenzoyl peroxide was chosen to be small compared with that of the chlorinecontaining c o m p o u n d so that the concentration of the latter may be regarded as constant during the course of the experiment. Benzene was added as diluent to establish a comparable reaction medium in all cases. The ease of chlorine abstraction was characterized by the quantity R as follows: R = 50. Cdpcl/CBPOo where C~,ci is the concentration of chlorobenzene after complete decomposition of dibenzoyl peroxide and Cavo0 is the initial concentration of dibenzoyl peroxide. Because at least two phenyl radicals can be generated from one molecule of dibenzoy peroxide, the m a x i m u m possible value of R is 100. The results are shown in Table 5. It turns out that chlorine in the HET moiety is so inactive that no chlorobenzene could be detected, whereas, in the case of HEX, 22 ~ of the maximum (theoretical) yield of chlorobenzene was obtained. On the TABLE 5
Yield of Chlorobenzene Obtained by Exposing HEX, HET Acid Diethylester and Carbon Tetrachloride to Phenyl Radicals Generated from Dibenzoyl Peroxide at 80 °C Substance
HEX HET acid diethylester CCI4
Concentration (mol/ litre )
Yield of chlorobenzene (mol/ litre)
Ra
2-26
1-82.10- 2
22
2.01 2.28
~0 6.40.10- 2
0 77
a Initial concentration of dibenzoyl peroxide, 4-13.10-2 mol/litre.
Chlorine balance in the degradation of HET-acid-UP
159
other hand, carbon tetrachloride is much more prone to release chlorine when attacked by a phenyl radical. Although there is no guarantee that the situation under the conditions of degradation is the same, these data at least do not contradict the view that HEX is to be assumed to be the starting point for chlorine release of HET acid-containing systems in the course of thermal degradation. According to Hilado's classification 12 the experimental conditions applied here correspond to the degradation and decomposition stage rather than to the oxidation stage which is accompanied by a visible flame. As pointed out above, in this stage only 30 % to 40 % of the total chlorine available is liberated as hydrogen chloride. Undoubtedly, in a real fire situation the volatiles in the hot flame zone will be decomposed to a greater extent so that a larger amount of chlorine radicals is available for flame poisoning. As pointed out in a previous paper, 3 besides the flame poisoning reaction, other mechanisms may contribute to flame retardancy in HET acid-containing systems, e.g. enhanced char formation, etc. At the moment it is not clear which type of flame retarding mechanism predominates. Thus, it is not possible to localize the (spatial) zone in which the flame retardant is most active. Also, the temperature profile typical of the burning of unsaturated polyesters is not known. These pieces of information could, however, give some hints as to what extent the differently bounded halogens in HET acid-containing systems are essential and used to impart flame retardancy in this particular system.
ACKNOWLEDGEMENT The author is indebted to Mr J. Glaser for performing the chlorine analysis. REFERENCES I. H. V. Boenig, Unsaturated polyesters: Structure and properties, Elsevier, Amsterdam, 173 (1964). 2. C. T. Vijayakumar, J. K. Fink and K. Lederer, Angew. Makromol. Chem. 113, 121 (1983).
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3. C. T. Vijayakumar and J. K. Fink, J. Appl. Polym. Sci. 27, 1629 (1982). 4. J. K. Fink, Thermochimica Acta, ~77,!377 (1983). 5. R. R. Hindersinn and G. Witschard, in, Flame retardancy of Polymeric Materials. (W. C. Kuryla and A. J. Papa (Eds)), Vol. IV, 19, Dekker, New York (1978). 6. DIN 53474, April 1961 Sect. 7.2 Testing Plastics, Determination of the Chlorine Content. 7. H. Roth, G. B~ihr, F. Hein, A. Hornig and F. Zinneke, in, Houben-Weyl, Methoden der Organischen Chemie (E. Miiller (Ed.)), Vol. II, 232 4. Ed., Thieme, Stuttgart (1953). 8. S.L. Madorsky, Thermaldegradation of organic polymers. 166, Wiley, New York (I 964). 9. C. F. Cullis and M. M. Hirschler, The combustion of organic polymers, Clarendon Press, Oxford, 185-92 (1981). 10. R. H. Barker and J. E. Hendrix, in, Flame retardancy of polymeric materials. (W. C. Kuryla and A. J. Papa (Eds)), Vol. V, Dekker, New York, 31 (1979). 11. H. Wollweber in, Houben-Weyl, Methoden de Organischen Chemie (E. MiJller Ed.) V/lc, Pt. 3. Ed. Thieme Stuttgart, 1040 (1970). 12. C. Hilado, Flammability handbook for plastics. (2nd edn). Technomic, Westport, Conn., 27 (1974).
Investigation of the Chlorine Balance in the Degradation of Unsaturated Polyesters Based on HET Acid J. K. Fink Institut fiir Chemische und Physikalische Technologie der Kunststoffe der Montanuniversit~it Leoben, A-8700 Leoben, Austria
(Received: 30 April, 1985)
ABSTRACT Unsaturated polyesters based on H E T acid, phthalic acid, maleic acid and 1,2-propanediol were cured with styrene. The cured and uncured products were degraded to 500°C both in nitrogen and in air. The amount of chlorine which remained in the residue and the chlorine recovered as hydrogen chloride were analyzed. The results were nearly independent of whether the pyrolysis was carried out in nitrogen or in air. Under the conditions oJ'the experiment only 30 % to 40'% of the chlorine volatilized was recovered as hydrogen chloride. The ease of chlorine abstraction from model compounds was also measured. Probably hexachlorocyclopentadiene, which is an intermediate in the degradation of H E T acid-containing materials, is the active substance from which the chlorine is abstracted in the course of degradation.
INTRODUCTION The use of HET acid anhydride (1,4,5,6,7,7-hexachloro-5-norbornene2,3-dicarboxylic acid anhydride) as a basic constituent in polyesters to achieve flame retardancy was suggested several years ago. 1 In a previous series of papers the main degradation paths in such systems have been established by pyrolysis-gas chromatography-mass spectrometry techniques. 2'3 It was found that the retro-Diels-Alder reaction of the HET 147 Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
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J. K. Fink
moiety to eject HEX ( = hexachlorocyclopentadiene) plays a central r61e among the primary reactions of thermal degradation in imparting flame retardancy to the resin. In the course of degradation of HET acidcontaining polyesters the formation of hydrogen chloride has also been observed by pyrolysis mass spectrometry. 4 However, no quantitative studies have been performed hitherto to estimate to what extent the chlorine, originally present in the system, is converted into hydrogen chloride during thermal degradation. Since this compound is believed to be most active compared with other chlorinecontaining compounds in flame poisoning, 5 the measurement of hydrogen chloride formation during thermal degradation is of particular interest in order to obtain information about the tendency of a chlorinecontaining flame retardant to act as a flame poisoning agent via hydrogen chloride. For this reason, pyrolysis experiments in air and in nitrogen were performed on HET acid-containing systems, the hydrogen chloride volatilized being trapped in an alkaline solution. The chlorine which remained in the residue was also determined. Further, the ease of chlorine abstraction from some selected model compounds, which are of interest in the present research, was determined. From these experiments some hints can be gained concerning the nature of the precursor molecule from which the chlorine is abstracted. EXPERI MENTAL Materials HET acid and 1,2-propanediol were donated by Vianova (Austria). HEX was supplied by EGA-Chemie and the other samples by Merck. Styrene was distilled in v a c u o before use, benzene was also distilled before use and the other chemicals were used without further purification. Preparation of the unsaturated polyesters The unsaturated polyesters were prepared by melt condensation. Nitrogen was passed through the melt, the condensation temperature being maintained at 180°C to prevent enhanced retro-Diels-Alder reaction of the HET moiety. Usually, an acid number below 50rag KOH/g polyester was attained within 5 h. Composition data are shown in Table 1. A hundred grams of the initial feed was placed in the reaction
Chlorine balance in the degradation of HET-acid-UP
149
TABLE 1 Compositionof the Monomer Mixturesused in the Preparation of Unsaturated Polyesters Sample codes
UPA UPB UPC
Relative amounts of the various compounds (mole ratios) (1)
(2)
(3)
(4)
0-5 0.4 0.3
0.0 0.1 0.2
0-5 0.5 0.5
1-1 1.1 1.1
Acid number (rag KOH/g)
35.7 46.2 36.1
Components: (1) HET acid; (2) phthalic anhydride; (3) maleic anhydride; (4) 1,2-propanediol. N. B. This Table should not be confused with a similar Table in a previous paper 3 where polyesters with different components were under investigation.
vessel. Because of the comparatively high volatility of the diol component (1,2 propanediol) a 10 ?/oexcess of the latter c o m p o u n d was used. A glassclear, brittle material was obtained in all cases. Curing of the unsaturated polyesters
0.7 parts by weight of unsaturated polyester were mixed with 0.3 parts by weight of styrene and homogenized by gentle warming. 1 ~o by weight of dibenzoylperoxide was dissolved in the mixture which was kept at 80 °C for 1 h and post-cured at 100°C overnight. To distinguish the styrene cured samples from the uncured samples the letter S is added to the respective sample code (e.g. UPAS). In a few cases other formulations were chosen, e.g. 40 % styrene and 60 ~o UP. These exceptions are indicated when they occur. Determination of the chlorine content
Solid samples were decomposed in the following way. 6 An approximately 0" 1-g sample was placed in a small crucible (height, 1 cm) filled completely with powdered calcium oxide. This crucible was placed in a bigger crucible so that the orifice of the smaller crucible faced the inner bottom of the bigger crucible which was also filled with calcium oxide. The quantity of calcium oxide was weighed. The crucible was heated for 15 min by means of a Bunsen burner. By this special arrangement the
150
J. K. Fink
pyrolysis gases are forced to move downwards into the hotter zone before escaping into the environment. The calcium oxide was dissolved in glacial acetic acid and the chlorine determined potentiometrically with a silver-silver chloride electrode by titration with silver nitrate. The analysis was corrected for the chlorine content of the reagents, which was, in any case almost negligible. The hydrogen chloride in the pyrolysis gases, trapped in the alkaline absorption solution, was also determined by simple potentiometric titration after acidifying with glacial acetic acid. Since it is known that certain organic compounds may lose chlorine under certain conditions, 7 HEX, as a model for the chlorine containing pyrolysis vapors in the systems under investigation was treated under similar conditions. Only 5 mg of chlorine per gram of HEX were recovered by the chlorine analysis method described above. In the subsequent treatment the chlorine analysed in the absorption solution is described as hydrogen chloride. In general, the chlorine content given in the Tables is expressed as weight ~o.
Determination of chlorine abstraction from certain model compounds To estimate the relative ease of chlorine abstraction from certain model compounds, dibenzoyl peroxide was allowed to decompose almost completely in the presence of these compounds and the resulting amount of chlorobenzene was measured. The following substances were used as model compounds: HEX, HET acid-diethylester and carbon tetrachloride. HEX was analyzed among the products of pyrolysis of HET acid-containing polyesters, HET aciddiethylester serves as a model compound for a polyester chain and carbon tetrachloride was used as a reference substance, because the flame retardant action of this substance is well established and used for fire fighting. HET acid-diethylester was prepared from HET acid in an excess of ethanol in the presence of p-toluenesulfonic acid using a standard procedure. The substance was recrystallized from ethanol (MP: 60°C; ~o Clcalcd.47"80; ~ Clrouna, 47"37).
Pyrolysis equipment Typically, a 0-2-g sample was placed in a ceramic weighing boat; uncured polyesters were used in powder form, the cured samples as small pieces.
Chlorine balance in the degradation of HET-acid- UP
151
The boat was placed in the centre of a quartz tube, 400 m m long and with an inner diameter of 26 mm. The flow of nitrogen or synthetic air (20 Oz, 80~o N2)was arranged to be 200ml min -1 The issuing gas stream passed through a safety vessel and then bubbled through 50 ml of 0- IM aqueous N a O H which was vigorously stirred. The pyrolysis was started by placing over the quartz tube, a tube furnace which had been preheated to the selected temperature. The tube was kept in the oven for 30 min, after which it was removed from the oven and allowed to cool. The residue was reweighed and the chlorine content of both residue and absorption solution was determined as described above. The temperature profile inside the tube was measured in blank experiments. It varied along the tube but did not depend upon the gas flow. The measured values were 25-30 °C lower than the adjusted nominal temperature in the center of the furnace. The temperatures given in the Tables are those measured at the center of the tube where the weighing boat was situated. The temperatures are rounded to 5 °C. From blank experiments it was estimated that the sample would reach a temperature within 5 °C of the final temperature within 6 min after the start.
RESULTS A N D DISCUSSION
Undeeomposedsamples Because of the volatility of the monomeric compounds, certain amounts are lost by volatilisation, especially in the earlier stages of the condensation reaction. This was clear from the appearance of fine needles in the upper part of the condensation vessel. Thus, it is necessary to determine the chlorine content of the polyesters independently rather than from the monomer feed. The chlorine contents of the undecomposed samples are shown in Table 2. Apart from sample UPC, the chlorine content determined by analysis agrees well with the chlorine content calculated from the m o n o m e r feed. The chlorine values obtained from analysis were used in subsequent discussion and calculations.
Degradation experiments The results of the degradation of the cured and uncured samples in nitrogen and in air are shown in Table 3. In this Table the weight per cent
J. K. Fink
152
TABLE 2 Chlorine Contents of the Undecomposed Unsaturated Polyester Samples
Sample
%Cl calc. a
%Cl found
Sample
°/ooClfound for UPX × 0.70
%Cl found
UPA UPB UPC
34"4 29-6 24' 1
33'5 30"0 28.3
UPAS UPBS UPCS
23"4 21.0 19.8
22'7 21 "2 19.0
a Calculated for infinite chain length (end groups neglected) from the anhydrides and acid compositions given in Table 1, but neglecting the excess of diol. b 0-7 parts UPX cured with 0.3 parts styrene.
of residue, the chlorine content in the residue in weight per cent and the yield of chlorine trapped in the absorption solution are given. Further, from the initial chlorine content and the chlorine content in the residue the amount of chlorine volatilized in the course of pyrolysis has been calculated. From these data the ratio of chlorine recovered in the absorption solution and the total amount of chlorine volatilized was calculated. It is believed that the amount of volatile chlorine missing in this balance was either deposited in the tar fraction or was among the higher volatiles, but in any case bound in some organic molecule. For U P A the amount of residue, the chlorine content of the residue and the amount of volatile chlorine recovered are plotted in Fig. 1. Since the behaviours of the other samples are qualitatively similar, the graphs are not given here, but some interesting features will be discussed. It can be seen from Fig. 1 and Table 3 that, in general, the data are almost independent of the atmosphere in which the pyrolysis experiment was performed. This means that, apart from the initial steps of degradation, which may be initiated by dissolved oxygen, the 'catabolism' of chlorine under the conditions of the experiment is practically unaffected by the oxygen content of the environment. At lower pyrolysis temperatures (275 °C), for the uncured samples, the chlorine content of the residue increases slightly in comparison with the original samples. Obviously, this apparent increase in the chlorine content is due to preferential volatilization of non-chlorine containing substructures of the polyester. In the styrene cured samples, however, the trend is reversed. In all cases the most significant changes occur in the range 275-325 °C, corresponding to a maximum rate of degradation. In certain cases the
Chlorine balance in the degradation of HET-acid-UP
153
/
5O
-50
E
L.I
275
325
375 ToE
Fig. 1.
UPA: Per cent residue, per cent chlorine in the residue and volatile chlorine
recovered in the absorption solution as HCI, viz. temperature of pyrolysis in air and in nitrogen atmosphere. • Residue in air, © residue in nitrogen, • per cent chlorine in residue in air, A per cent chlorine in residue in nitrogen, • milligrams of volatile recoverablechlorine per gram sample in air, [] milligramsof volatile recoverablechlorine per gram sample in nitrogen. a m o u n t of recoverable volatile chlorine even passes through a m a x i m u m at a pyrolysis temperature of 325 °C. Further, the a m o u n t of residue at higher pyrolysis temperatures (375°C) is significantly enhanced in the case of the styrene cured samples compared with the uncured samples. There is still a small a m o u n t of chlorine in the residues from pyrolysis at 375 °C. Since the retro-Diels-Alder reaction should be complete under the conditions of pyrolysis, to yield (volatile) HEX this finding suggests that some transfer reactions may occur in the degrading polymer matrix. Inspection of the last column of Table 3 shows that only part of the chlorine volatilized is recovered in the absorption solution as hydrogen chloride. This is in striking contrast to PVC where practically all chlorine initially present in the polymer is converted to HC1, even at comparatively low pyrolysis temperatures. Table 3 shows that in the case of the uncured samples, the a m o u n t of chlorine recovered as HCI from the total volatile chlorine approaches a limiting value of c a . 30 ~o. Therefore, from HET acid, which contains six
Pyrolysis temperature (°C)
275 295 325 375 470 275 325 375 275 325 375 275 325 375 275 325
Sample
UPA UPA UPA U PA UPA UPAS U PAS U PAS UPB U PB UPB U PBS UPBS U PBS UPC UPC
N N N N N N N N N N N N N N N N
Atmosphere
85'3 30-2 20.7 15"2 12'8 97'3 36-6 25'4 69-6 19-1 15.5 87.7 40.3 22"4 80"0 17'4
Per cent residue
36" 1 21 '7 7-2 2' 6 1"8 21 '8 1-6 1-3 32"6 5"4 3"1 20-6 2.3 1-5 30"6 2"0
Per cent CI in residue
1-3 67'7 105'9 100' 6 107"4 0"5 96" 3 69"5 11'3 90"2 91.3 4'8 82'3 81.8 4"8 86-9
Milligrams of volatile Cl recovered per gram sample 2'7 26.9 32"0 33" 1 33-3 1'4 22" 1 22'4 7-3 29-0 29'5 3-1 20.2 20'9 3'8 28"0
Grams of volatile Cl per 100 g sample
TABLE 3 Experimental Data for Unsaturated Polyesters, Uncured and Cured with 0"3 Parts of Styrene
4'7 25" 1 33"2 30"4 32-3 3"3 43"6 31" 1 15"6 31"2 31-0 15"6 40"4 39"2 12"6 31-1
volatilised CI recovered
Per cenl o f
375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375 275 325 375
A, Air, N, Nitrogen.
UPC UPCS U PCS UPCS U PA UPA U PA UPAS U PAS UPAS UPB UPB UPB UPBS U PBS UPBS UPC U PC UPC U PCS UPCS UPCS N N N N A A A A A A A A A A A A A A A A A A 15"2 85"6 30"3 20"7 75"4 t7"8 12"8 68'2 28'5 21"7 70"5 18"4 13"7 86"9 35"5 22"3 68"4 16"9 13"0 83"8 16"1 21 "4
1'6 17"5 2"0 1'3 33'3 4"0 3'2 23'3 2"2 l"8 32"7 0"7 2'6 21 "5 2'0 2'I 30"9 2'8 1'7 19"3 0'6 2'3 81 '9 6"7 80"4 77"3 7"4 96'1 114"3 10"0 90'2 74"0 1"8 92'4 101 "5 2"7 84'8 84'3 10'7 83'0 86"9 2"8 73"0 82'9 28"1 4"0 18"2 18'8 8"4 32'8 33"1 6"8 22'0 22"3 6'9 29'9 29'6 2"5 20"4 20"7 7"2 27"9 28"1 2"9 18"9 18"6 29"2 16'5 44'1 41 "2 8"8 29"3 34~6 14"7 41"0 33~2 2"6 31'0 34"3 11"0 41 "5 40'8 15"0 29"8 30"9 9'8 38 "6 44"7
C.,
2.
J. K. Fink
156
TABLE 4 Experimental Data for Unsaturated Polyesters Cured in Air with Larger Proportions of Styrene and with Sb20 3
Sample
UPCS4" UPCS5 b UPCSb3/2 c
T (°C) Per cent residue
375 375 375
19.9 18.9 32.5
Per cent CI in residue
Milligrams of volatile Cl recovered per gram sample
0.7 0.5 0.9
68.3 62.5 12.3
Grams of Per cent o f volatile Cl volatile CI per I00 g recovered sample 16.8 14.1 16.1
40.7 44.3 7.6
a 0.4 parts styrene + 0.6 parts UPC. b 0"5 parts styrene + 0-5 parts UPC. c 0.3 parts styrene + 0.7 parts UPC + 0.2 parts Sb20 3.
chlorine atoms per molecule, two are converted into hydrogen chloride in the limiting case. In the case of the styrene cured samples the amount of chlorine recovered from the volatile chlorine is slightly enhanced (40-44 ~o) which corresponds to 2.4 to 2-6 chlorine atoms per HET acidunit being converted into HC1. Table 4 presents the results of degradation experiments of styrene cured unsaturated polyesters with larger amounts of styrene. The enhanced styrene content does not result in an enhanced release of hydrogen chloride. It is believed, therefore, that two chlorine atoms of the HET moiety can be abstracted very easily, resulting in the formation of hydrogen chloride, whereas the remaining chlorine atoms are attached comparatively more strongly to the substrate. Hydrogen chloride formation from an antimony trioxide containing styrene cured sample is also shown in Table 4. In this case the formation of hydrogen chloride is suppressed, thus reflecting the well known antimony chlorine synergism. To this point it is not clear whether the chlorine is removed from the HET acid moiety directly or from HEX which is known to be formed in the course of degradation. Neither is it clear whether the process is radical or ionic. In the gas phase ionic species are well known to occur in flames; thus, the electrical conductivity of flames is the principle on which flame ionization detection is based. Gaseous ions seem to be of minor importance in the mechanisms of degradation and fire retardation of polymers 9 and degradation reactions of carbon-carbon-backbonepolymers have been described in terms of radical processes. In the case of
Chlorine balance in the degradation of HET-acid-UP
157
cellulose, however, an ionic degradation mechanism has been suggested. l o Thus, there is no reason to discard, a priori, the possibility of ionic reaction mechanisms occurring in the degradation of polyesters or, more generally, in the condensed phase during degradation. Certain arguments support the view that the hydrogen chloride is formed from the HEX molecule rather than from the HET moiety. Ct 0
.Ct "~/C
C
CL CL CL
" Ct
Ct 0 HEX
0
-.
moteic onhydride
0 HET ocid onhydride
In HET acid there are two chlorine atoms aliphatically bound, two allylically, and two vinylically. One vinyl bond is associated with the two allylic bonded chlorines which are, in addition, bonded to bridge nodes. In the HEX molecule, however, two chlorine atoms bonded to the same carbon atom are in the allylic position with respect to two double bonds. It is to be expected, therefore, that the allylic chlorines in HEX are more reactive than the chlorines in the HET moiety. Abstraction of a single chlorine atom would result in an allylic radical, which is known to be comparatively less reactive. Abstraction of a second chlorine atom from the allyl position would lead to a carbene-like structure. It is not known how far the two residual electrons will interact with the double bonds as is the case in the cyclopentadienyl-anion. The experimental proof of these points is extremely difficult. For example, to establish whether the two chlorines are released from the same position, by isotope labelling of the allylic position and monitoring the hydrogen chloride released during pyrolysis in a mass spectrometer, is a rather formidable task. Chlorine abstraction In the course of this work an attempt was made to estimate the relative ease of chlorine abstraction from HEX and HET acid by means of model reactions in the liquid phase.
158
J. K. Fink
The most straightforward method of estimating the ease o f abstraction of certain groups from a substance in polymer chemistry is to measure the transfer constants in vinyl polymerization. In the case of HEX this concept fails since HEX is known to react (e.g. with styrene) at appreciable rates with the double bond of vinyl monomers.11 Instead, HEX and HET acid ester were exposed to phenyl radicals generated from dibenzoylperoxide and the yield of chlorobenzene taken as a rough estimation o f the reactivity of the respective chlorines. In this system no complications with Diels-Alder reactions in which HEX could be involved are to be expected. In these experiments the concentration of dibenzoyl peroxide was chosen to be small compared with that of the chlorinecontaining c o m p o u n d so that the concentration of the latter may be regarded as constant during the course of the experiment. Benzene was added as diluent to establish a comparable reaction medium in all cases. The ease of chlorine abstraction was characterized by the quantity R as follows: R = 50. Cdpcl/CBPOo where C~,ci is the concentration of chlorobenzene after complete decomposition of dibenzoyl peroxide and Cavo0 is the initial concentration of dibenzoyl peroxide. Because at least two phenyl radicals can be generated from one molecule of dibenzoy peroxide, the m a x i m u m possible value of R is 100. The results are shown in Table 5. It turns out that chlorine in the HET moiety is so inactive that no chlorobenzene could be detected, whereas, in the case of HEX, 22 ~ of the maximum (theoretical) yield of chlorobenzene was obtained. On the TABLE 5
Yield of Chlorobenzene Obtained by Exposing HEX, HET Acid Diethylester and Carbon Tetrachloride to Phenyl Radicals Generated from Dibenzoyl Peroxide at 80 °C Substance
HEX HET acid diethylester CCI4
Concentration (mol/ litre )
Yield of chlorobenzene (mol/ litre)
Ra
2-26
1-82.10- 2
22
2.01 2.28
~0 6.40.10- 2
0 77
a Initial concentration of dibenzoyl peroxide, 4-13.10-2 mol/litre.
Chlorine balance in the degradation of HET-acid-UP
159
other hand, carbon tetrachloride is much more prone to release chlorine when attacked by a phenyl radical. Although there is no guarantee that the situation under the conditions of degradation is the same, these data at least do not contradict the view that HEX is to be assumed to be the starting point for chlorine release of HET acid-containing systems in the course of thermal degradation. According to Hilado's classification 12 the experimental conditions applied here correspond to the degradation and decomposition stage rather than to the oxidation stage which is accompanied by a visible flame. As pointed out above, in this stage only 30 % to 40 % of the total chlorine available is liberated as hydrogen chloride. Undoubtedly, in a real fire situation the volatiles in the hot flame zone will be decomposed to a greater extent so that a larger amount of chlorine radicals is available for flame poisoning. As pointed out in a previous paper, 3 besides the flame poisoning reaction, other mechanisms may contribute to flame retardancy in HET acid-containing systems, e.g. enhanced char formation, etc. At the moment it is not clear which type of flame retarding mechanism predominates. Thus, it is not possible to localize the (spatial) zone in which the flame retardant is most active. Also, the temperature profile typical of the burning of unsaturated polyesters is not known. These pieces of information could, however, give some hints as to what extent the differently bounded halogens in HET acid-containing systems are essential and used to impart flame retardancy in this particular system.
ACKNOWLEDGEMENT The author is indebted to Mr J. Glaser for performing the chlorine analysis. REFERENCES I. H. V. Boenig, Unsaturated polyesters: Structure and properties, Elsevier, Amsterdam, 173 (1964). 2. C. T. Vijayakumar, J. K. Fink and K. Lederer, Angew. Makromol. Chem. 113, 121 (1983).
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J. K. Fink
3. C. T. Vijayakumar and J. K. Fink, J. Appl. Polym. Sci. 27, 1629 (1982). 4. J. K. Fink, Thermochimica Acta, ~77,!377 (1983). 5. R. R. Hindersinn and G. Witschard, in, Flame retardancy of Polymeric Materials. (W. C. Kuryla and A. J. Papa (Eds)), Vol. IV, 19, Dekker, New York (1978). 6. DIN 53474, April 1961 Sect. 7.2 Testing Plastics, Determination of the Chlorine Content. 7. H. Roth, G. B~ihr, F. Hein, A. Hornig and F. Zinneke, in, Houben-Weyl, Methoden der Organischen Chemie (E. Miiller (Ed.)), Vol. II, 232 4. Ed., Thieme, Stuttgart (1953). 8. S.L. Madorsky, Thermaldegradation of organic polymers. 166, Wiley, New York (I 964). 9. C. F. Cullis and M. M. Hirschler, The combustion of organic polymers, Clarendon Press, Oxford, 185-92 (1981). 10. R. H. Barker and J. E. Hendrix, in, Flame retardancy of polymeric materials. (W. C. Kuryla and A. J. Papa (Eds)), Vol. V, Dekker, New York, 31 (1979). 11. H. Wollweber in, Houben-Weyl, Methoden de Organischen Chemie (E. MiJller Ed.) V/lc, Pt. 3. Ed. Thieme Stuttgart, 1040 (1970). 12. C. Hilado, Flammability handbook for plastics. (2nd edn). Technomic, Westport, Conn., 27 (1974).