DAVID P. BISHOP
DAVID A. SMITH
M
This review covers the literature up to July, 1966, The authors discuss critically the various degradation mechanisms put forward and conclude that the thermal degradation process, despite much work, is still imperfectly understood years there has been a demand for adhesives Ifornandrecent electrical insulators of good thermal stability use in many fields including the construction of rockets and high performance aircraft. Consequently there has been considerable interest in the behavior of thermosetting resins at high temperatures, particularly epoxide resins which are noted for their good mechanical, electrical, and adhesive properties. The purpose of this review is to collect the findings of the numerous workers in this field and to summarize the knowledge accumulated to date. Cross-linked (cured or hardened) epoxy resins are formed by reaction of a difunctional or polyfunctional cross-linking agent (hardener) with a difunctional or polyfunctional epoxide. Alternatively, Lewis acids such as boron trifluoride may be used to promote direct cross-linking through the epoxide groups. There are several different types of commercially available epoxides all of which may be cured with a variety of hardeners such as aliphatic and aromatic diamines, acid anhydrides, and dicarboxylic acids. It is therefore possible to formulate a very large number of resin structures with thermal stabilities dependent on the particular epoxide/ hardener combinations.
T H E R M A L DEGRADATION I N OX I D IZ I N G ATMOSPHERES Heot Aging Studies
Jacobi and Andre (74) have examined the heat aging characteristics of 66 different epoxide/hardener/filler combinations by baking disk shaped samples for 1000 32
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
hr. in a convection oven at 180" C. The samples were weighed and measured before and after baking to determine weight loss and size change. The nature of the epoxides used is not disclosed, and the only useful information which can be deduced from this work is that acid anhydrides tend to give cured resins of greater thermal stability than those hardened with diamines. Lemon (20) has carried out similar work on three epoxy resins (two epoxylated diphenylolpropane resins of different molecular weight and one novolak resin) cured with three different hardeners (aniline formaldehyde resins, hexahydrophthalic anhydride, and BF3 monoethylamine complex). Cured castings were heated in an air-circulated oven a t 200" C. for periods of up to 1000 hr. Samples were measured and weighed before and after heating. Considerable dimensional changes were observed for castings made from diphenylolpropane based resins cured with aniline formaldehyde and with BF3 monoethylamine complex. The corresponding novolak resins showed virtually no size change, and none of the resins cured with hexahydrophthalic anhydride showed significant dimensional changes. Weight loss was roughly proportional to dimensional changes; the novolak/hexahydrophthalic anhydride resin lost 2yo by weight after 1000 hr. a t 200" C. whereas the diphenylolpropane/BF3 monoethylamine resins lost up to 12.5% by weight under the same conditions. More work on these lines has been done by Ehlers ( 7 7) who suggests that epoxides such as I and I1 below should form more stable resins than epoxides based on diphenylolpropane or partially condensed phenol-formaldehyde systems.
0
0 I
°
O
m
o
250" C. The oxygen pressure in the system remained constant for a time, T , and then began to fall. T h e length of the induction period, T , was taken to represent the resistance of the resin to oxidation. The induction period decreased when either the initial oxygen pressure or the reaction temperature was increased, and a formula relating induction time to oxygen pressure and temperature was derived. The fall in oxygen pressure after the induction period was attributed to the formation of hydroperoxide groups in the resin. When oxidized resin samples were treated with acidified potassium iodide solution, iodine was liberated indicating the presence of peroxides. The iodine was titrated with sodium thiosulfate and it was found that the concentration of peroxides in the amine hardened resin was considerably higher than in the resin hardened with maleic anhydride. It was also shown that oxidation of the amine hardened resins began at about 150" C. whereas the acid anhydride resins were stable up to 200" C. Kovarskaya and Zhigunova ( 1 6 ) have considered the thermal and thermo-oxidative degradation of cured epoxide and epoxy/phenol resins. They did not attempt to elucidate the mechanisms of degradation reactions but showed that the epoxy/phenol resins were the more heat stable under both oxidizing and inert atmospheres. Some effects of oxidative attack on epoxy/phenol paint films have been recorded by Park and Blount (27). O n heat aging in air a t 150' to 200" C., the films not only lost weight but became increasingly hygroscopic. A quantitative study of the rate of oxygen absorption was made, and infrared analysis before and after oxidation revealed that -OH groups in the films were disappearing and at the same time C=O groups were being formed. Two possible mechanisms were postulated to explain these phenomena :
I1
Although work of this kind is valuable in assessing the usefulness of various epoxide/hardener combinations it gives no insight into the mechanism of thermal degradation. Consequently more attention has been paid to techniques which measure physical changes taking place in a resin during the degradation process or which enable degradation products to be collected and identified. Information obtained from this type of work leads to a more complete understanding of degradation reactions and provides a rational basis for improving the properties of the products under consideration.
A.
I I
0
H-C-OH
+
I I
I I
H-C-0-OH
+ CEO
B. -0-CH2-CH-CH2-0-
I
p0 -0-C
H=CH-CHz-O-
io.
-0-CH=CH-CH-0
I
Mechanistic Studies
Neiman et al. (26) have investigated the thermooxidative degradation of a low molecular weight epoxylated diphenylolpropane hardened with polyethylenepolyamine and maleic anhydride. The hardened resins were ground to a fine powder and samples were placed in a reaction vessel connected to a manometer. The vessel was evacuated, and pure dry oxygen was then introduced. The samples were heated in a thermostated oil bath to various temperatures in the range 160" to 34
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
+ HzO
0-OH
1
-HzO
0-CH=CH-(2-0-
I1
0 It was considered that A was the more likely mechanism because there was no evidence for the formation of -C=Cin the IR spectra of the films. These workers
also found that the initial molecular weight of the epoxide has a definite influence on the rate of degradation of the cured resin, and that unreacted epoxide groups in the resin are focal point8 for oxidation. Conley (8) has also followed the oxidative degradation of thin films of hardened epoxides by oixerving changes in their I R spectra. An epoxylated diphenylolpropane was fractionated to give a series of resins with molecular weights ranging from 340 to 10,000. Films were cast from solutions of resin and hardener in chloroform, or chloroform/benzene, onto rock salt disks, and curing was completed in an oven after the solvent had evaporated at room temperature. The hardeners used were mphenylenediamine, diethylenetriamine, and phthalic anhydride. The films on the rock salt disks were heated in a cell designed to permit recording of the I R spectra while maintained at the reaction temperature. Degradation was studied at temperatures between 175" and 250' C . depending on the resin fraction/hardener combination. The rate of degradation of the amine cured films could be followed via the development of an absorption band at 5.8 microns. The growth of thii band may be attributed to the formation of
\
C=O
/
groups in the resin. Possible mechanisms for the formation of these groups postulated by Park and Blount have already been mentioned, but Conley suggests a quite different mechanism involving the rupture of the "curelink" or amin-alcohol portion of the resin:
--qP4HoH*H+ CHOH-CH2-O-
-
H,-CHOH-CH,-+
'CH*-CHOH-CH~-O-
+
-CHOH-CH,-O
1 1
CHFCOH-CH~~ CHj-
F04 H z - O -
Conley concludes that the aromatic amine yields a cured resin with slightly greater resistance to oxidation TABLE 1.
than the aliphatic amine. The films hardened with phthalic anhydride were more resistant than any of the amine hardened resins but their mode of degradation was not discussed in any detail. Films formed from the high molecular weight fractions of the epoxide were shown to be more resistant to thermc-oxidative attack than those prepared from low molecular weight resin.
DEGRADATION I N VACUO Programmed Thermogmvim.tric Analysis
Apart from indicating the temperature range over which the thermal degradation of a resin takes place, thermcgravimetric techniques provide useful information on the kinetics of degradation reactions. H. C. Anderson (4, 3 studied the pyrolysis of the diglycidyl ether of Z,Z'-bis(p-phenylol)propane (5) hardened with maleic anhydride, and with m-phenylenediamine. Resin samples (10 mg.) were heated to 200" C. in the evacuated chamber of a thermogravimetric apparatus. Weight loss of the sample was then recorded continuously as a function of temperature, which was increased at a rate of 1" C. ( * 0 . l o C.) per min. up to 525" C. The data obtained in this way were used to evaluate the average activation energies, orders of reaction, and frequency factors for the degradation reactions using the following equations (72) : ln(dw/dt)
A l n Wr
In A = In(dw/dt) where dw/dt = rate
(E/R)A(I/ T ) A In Wr
=n-
- In Wr
+ E/RT
of loss in weight
Wr = weight loss at completion of reaction, minus total weight loss at time t
E = activation energy n = order of reaction A = frequency factor
R T
= gas constant
= absolute temperature
The values obtained are listed in Table I.
r
ACTIVATION ENERGIES FOR T H E DEGRADATION OF EPOXIDE RESINS
I
dorsky and Suaus (Zi
VOL 59
NO
8
AUGUST 1967
35
A straightforward way of studying thermal degradation is to heat the material in an evacuated system and to collect the volatile products in cooled traps
Lee (79) has carried out some programmed thermogravimetric work on diphenylolpropane and novolakbased epoxides cured with nadic methyl anhydride and p,p '-diaminodiphenylmethane. He did not calculate rate constants or activation energies, but confirmed that the epoxylated novolak resins are more stable than the epoxylated diphenylolpropane resins, and that anhydrides give more stable resins than amine hardeners. Isothermal Weight Loss Studies
Coleman (7) has determined rate constants for the isothermal degradation of two epoxides (I11 and I V below) hardened with rn-phenylenediamine : 0 II
IV
Samples were maintained at 250" C. in vacuo and weight loss was recorded continuously. The overall degradation reactions were shown to be first order, and the rate constants at 250" were found to be k = 7.8 X lo-* rnin.-l for the resin from epoxide I11 and k = 7.6 X 10-4 min.-' for the resin from epoxide IV. Coleman states that because these rate constants are very similar, lengthening the chain between the two epoxide groups in the monomer has no effect on the kinetics of pyrolysis of the cured resin. Although this appears to be true for these particular epoxides it is not so for other systems which have been studied (8, 27). Madorsky and Straus (27) have carried out isothermal weight loss studies on a variety of polyester, phenolic, and epoxy resins. Their results and the nature of the epoxy resin used are listed in Table I. 36
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Pyrolysis in Vacuo
The most straightforward method of studying the thermal degradation of any material is that of heating samples in an evacuated system and collecting the volatile degradation products in cooled traps. These products may then be separated and identified by a variety of analytical techniques. An early sample of the application of this approach to epoxide degradation is the work of Harms (73) who studied the I R spectra of the mixture of volatile pyrolysis products. He claimed that the spectra were characteristic for each resin examined and could be used for identification purposes. More recently Sugita and Ito (30) have carried out similar work on a much wider range of epoxides and hardeners. This work was intended to provide a simple means of identifying epoxide and hardener types present in samples of cured resin. The I R spectra of mixtures of degradation products are too complex to permit the positive identification of single components present, and consequently other methods of analysis have been favored by workers interested in the degradation process itself. For example Madorsky and Straus (27) and Lee (79) analyzed mixtures of products by mass spectrometry, and other workers (22-25, 29) have separated and identified individual products by gas chromatography and classical chemical methods. Madorsky and Straus (27) studied the degradation of an epoxylated novolak resin hardened with BF3 monoethylamine complex. Pyrolysis was carried out in vacuo at 360", 500°, goo", and 1200" C. on 10 to 50 mg. samples contained in a platinum tube. Any volatile products which did not condense in liquid nitrogencooled traps were collected as a gas fraction and analyzed by mass spectrometry. A second fraction, composed of products volatile at room temperature, was collected and analyzed, also by mass spectrometry. The remaining products, which were volatile at the pyrolysis temperature but not at room temperature, were dissolved in a solvent, and their average molecular weight was determined by a microcryoscopic technique. For
pyrolysis a t 360' C. the most abundant volatile product identified was carbon dioxide (16.2% by weight of volatile material). Appreciable quantities of carbon monoxide (4.7%), propylene (6.5%), and methyl chloride (5.1%) were detected, and small amounts of methane (1.0%), acetylene (0.3%), acetone (0.9%), and ethyl chloride (1.7%) were also found. At higher pyrolysis temperatures (1200" C.), carbon monoxide accounted for 25% of the volatile material; other identified products were benzene (8.1%), methane (4.3%), acetylene (2.5%), ethylene (3.0%), hydrogen (2.1%), carbon dioxide (1.8%), ethyl chloride (0.8%), and No attempt was made to cyclopentadienes (O.G%). characterize the nonvolatile residues and no degradation mechanisms were postulated. Neiman et al. (23, 24) have made a fairly extensive study of the thermal degradation of two epoxylated diphenylolpropanes having molecular weights of 500 (resin ED6) and 2000 (resin ED15) by pyrolysis in vacuo. Volatile degradation products of the epoxides and of the resins formed from them by cross-linking with polyethylenepolyamine and maleic anhydride were identified by a combination of gas chromatography and chemical methods. Methane, propylene, carbon monoxide, formaldehyde, acetaldehyde, and water were the principal products identified ; also, large quantities of carbon dioxide were evolved by the resin hardened with maleic anhydride. The following mechanisms were postulated for the breakdown of epoxide groups (24):
--R-o-cH2-cH-cH2
A
-
+d C H ,
--R-o-~H2
-R
I+
H,CO
CH8
+ CO
11
CH,J-C-H
-
--P -R
-I-CH,
CH,-C=O 1-44
-R
+ CH,CHO
Although this reaction scheme satisfactorily explains the formation of most of the volatile products found by Neiman, it gives little indication of the mechanism of the thermal breakdown of completely cured resins. The kinetics of the thermal degradation of the cured and uncured resins were evaluated on the basis of the increase in pressure in the system which was caused by gas evolution. Overall activation energies for the degradation reactions were calculated from a series of isothermal experiments; the results are listed in Table I. Moiseev, Neiman, et al. have obtained some electron paramagnetic resonance data which support the theory that degradation occurs through free radical mechanisms (22). I n our laboratories chemical evidence for the presence of stable free radicals in the tarlike residues of pyrolyzed resin samples has been found (75). I n further work (22) Moiseev, Neiman, et al. synthesized an epoxide based on diphenylolpropane in which the central carbon atom of the diphenylolpropane
grouping was Cl4-labeled. (This was achieved by preparing the diphenylolpropane from acetone which was CI4-1abeled a t the carbonyl group.) The thermal degradation of the labeled epoxide was studied at several temperatures, and the specific and total activities of the solid and volatile degradation products were determined. I t was found that practically all the introduced activity remained in the solid residue, which implies that gaseous breakdown products originate from the aliphatic part of the epoxide molecule rather than from the diphenylolpropane grouping. No work on the degradation of the cured epoxide is reported; in fact throughout Neiman's work there is no evidence of any attempt to prepare fully cured resins. This is surprising because it seems that degradation occurs readily at the epoxide groups and consequently if a cured resin is to have good thermal stability the epoxide groups should be completely reacted with the cross-linking agent. The most comprehensive study of epoxide degradation reported in the literature is that made by Lee (79) who used pyrolysis in vacuo in addition to TGA, DTA, and hot wire pyrolysis techniques. T h e resins investigated were epoxylated diphenylolpropanes and epoxylated novolaks hardened with nadic methyl anhydride and p,p '-diaminodiphenylmethane according to formulations recommended by the manufacturers of the epoxides. Polymer samples were pyrolyzed in vacuo a t 350', 450', and 475' C. The volatile degradation products were collected in cooled traps and analyzed by mass spectrometry. T h e relative quantities of the identified products from both hardened and nonhardened resins were listed and a number of possible degradation schemes were discussed. The reaction mechanisms postulated depend on the presence of free epoxide groups (or epoxide groups which have reacted together to form ether linkages) and account for the more volatile degradation products. The high boiling degradation products, which constituted the major part of the volatile material formed on pyrolysis, were found to be mostly phenols and cresols; possible mechanisms for their formation are discussed in a further paper ( 7 8 ) in which the thermal degradation of nonhardened epoxide resins is examined. I t is remarkable that no nitrogen containing compounds were found among the degradation products of the diamine cured resins. I t is also surprising to note that, although it has been established that acid anhydride hardeners yield more thermally stable cured resins than diamine hardeners, no attempt was made to elucidate the different degradation mechanisms which must operate. T h e mechanisms postulated by Lee and by Neiman et al. may all be applied equally well to noncured resins and to resins hardened with either diamines or diacid anhydrides.
D I F F E R E N T I A L T H E R M A L ANALYSIS (DTA) Anderson and Freeman (7) describe some exploratory work on the application of DTA to the study of epoxide reactions, but the subject is treated in more VOL. 5 9
NO. 8
AUGUST 1 9 6 7
37
detail by H. C. Anderson (2, 3, 6). The last has carried out differential thermal analysis, under nitrogen at atmospheric pressure, on six different epoxides both unreacted and reacted with various amine and anhydride hardeners. The DTA curves showed exothermic peaks for polymerization reactions and for isomerization of epoxy groups. Volatilization of the various hardeners gave rise to endothermic peaks, but it is surprising to note that for most resins no endothermic peaks were observed for degradation reactions. The explanation for this is probably that the exothermic peak due to isomerization of epoxy groups, which occurs between 300" and 400" C., completely masks the expected endothermic peak. H. C. Anderson observed the formation of volatile products in this temperature range which suggests that degradation and isomerization of epoxy groups occur simultaneously. In fact the isomerization of epoxy groups may be regarded as one of the degradation reactions occurring in incompletely cured resins. Further DTA work on epoxy resins carried out by Lee (79) confirms the occurrence of the exotherm between 300" and 400" C. and shows that the peak height is reduced when hardeners are present. This may be attributed to interfering endotherms associated with vaporization of hardeners and degradation reactions ; it would be interesting to know if the exothermic peak was absent for a completely cured resin. The DTA data concerning paper reinforced epoxy resins reported by Learmonth and Wilson (17)does not yield much further information; the presence of paper in the laminates complicates the possible degradation reactions. Although DTA pinpoints the temperatures at which exothermic and endothermic changes take place in a material, and gives a characteristic curve which may be used as a means of identification of a given material, it has not proved particularly useful in investigating degradation mechanisms in this field. More rewarding results have been obtained from methods which permit the isolation and identification of volatile degradation products.
HOT W I R E PYROLYSIS The thermal degradation of many polymers has been studied by pyrolyzing small samples on an electrically heated filament and sweeping the volatile products into a gas chromatograph in the carrier gas stream (usually nitrogen or helium). The consequent separation of products often enables them to be identified tentatively by their retention times, and if the separated products are collected, positive identification can be 'achieved by IR analysis and classical chemical methods. Very little work involving the collection of products has been reported, however. The technique has been applied to epoxides by Lee to separate and identify phenolic degradation products ( 7 9 ) , and by Stuart and Smith (29) (in our laboratories) to identify the principal degradation products of some aliphatic amine hardened epoxy resins. An attempt 38
INDUSTRIAL AND ENGINEERING CHEMISTRY
was made to ensure that the temperature of the heated filament was accurately known, and pyrolysis using an ascending stepped temperature sequence was investigated. The pattern of degradation did not change fundamentally over a temperature range of 350" to 750" C., except that larger quantities of volatile ploducts were produced at the higher temperatures. Various phenolic products were identified, but no compounds resulting from the degradation of the aliphatic amine hardeners were found. The authors are continuing to use this technique in addition to pyrolysis in vacuo in a systematic study of the thermal degradation of epoxy resins.
D ISCUSS I ON The degradation reactions formulated by Lee (78, / 9 ) depend on the etherification or polymerization of residual epoxide groups followed by dehydration and cleavage of the ether links-cg.,
0
0
- cH2- ~ - - C H ~ - O
I
-CH I I
OH
+
! I
1
;I
CH,=CH-CH,-O+
+
CH
CH,=CH-CFO CH,=CH,
+
CO
I t seems unlikely that the etherification reaction plays a major part in the degradation of cured epoxy resins because it applies equally to uncured and cured resins and does not account for the differing stabilities imparted to the network by different hardeners. Moreover I R work on the curing of epoxides indicates that after a proper curing sequence there are very few free epoxide groups remaining in the resin (10). Recent work in our laboratories has confirmed that dehydration is an early step in the degradation of amine cured resins. Water has been positively identified as a degradation product using the hot wire pyrolysis/gas chromatography technique, and IR work indicates that P. Bishop is a Research Fellow and David A . Smith is a Lecturer with the Department of Materials at the College of Aeronautics, Cranfield, Bedford, England. The authors acknowledge the fellows& from the Ministry of Aviation under which work in thisJeld is being carried out. AUTHORS David
Future work should concentrate on carefully characterized epoxides and hardeners, since impurities in commercial resins affect thermal stability
double bond structures are formed in the polymer during the early stages of degradation. Cleavage of ether links probably takes place to varying extents depending on the resin/hardener system. When bonds significantly weaker than C-0 bonds are present bonds-it seems likely that these will -e.g., N-C be broken preferentially. The suggestion that aryl alkyl ethers and aryl alkenyl ethers undergo Claisen type rearrangements is a useful one because it explains satisfactorily the presence of cresols and xylenols among the degradation products. The isomerization and subsequent breakdown of free epoxide groups has received considerable attention (79, 24). The DTA work of Anderson (2, 3) and Lee (79) suggests that this isomerization occurs between 300" and 400" C., and since some amine hardened resins begin to degrade at temperatures considerably lower than 300" C. it seems unlikely that the initial stages of degradation are due to the breakdown of epoxide groups. If this were the primary degradation mechanism, amine cured and anhydride cured resins would be expected to have similar thermal stabilities, and since this is not the case the authors consider that more attention should be paid to the breakdown of the cure link in hardened resins. It was noted previously that Lee found no nitrogen containing compounds among the degradation products of amine cured resins. It has been shown in these laboratories, however, that considerable quantities of N-substituted anilines are to be found among the degradation products of epoxylated diphenylolpropane cross-linked with @,@ '-diaminodiphenylmethane (28). This suggests that the cure link does play a part in the degradation reactions and it is hoped that further work will lead to a more complete understanding of the degradation process.
CONCLUSION O n reviewing the relevant literature, it becomes apparent that despite the considerable amount of work that has been done in this field there is still relatively little known about the mechanism of the thermal degradation of epoxy resins. Although it has been established that the degradation proceeds (partly at least) through a free
radical mechanism, it is impossible to state with any certainty the reactions which lead to the formation of particular breakdown products. In further studies on epoxide degradation the following points should be considered : -Sample geometry and physical state of the material may influence the course of degradation through secondary reactions, temperature gradients, and diffusion phenomena. -The results obtained in work of this kind may, to some extent, be a function of the methods of degradation and analysis. Too much reliance should not therefore be placed on data obtained from any one method. -Impurities and additives in commercially available epoxide resins may influence their thermal stability. I t is therefore desirable to work with carefully characterized epoxide and hardeners to establish the fundamental degradation reactions. REFE R ENCES (1) Anderson, D. A., Freeman, E. S., Anal. Chem. 31, 1697 (1958). (2) Anderson, H . C., ACS Symp., Div. of Paint, Plastics, and Printing Ink Chemistry, September 1959. (3) Anderson, H. C., Anal. Chem. 32, 1592 (1960). (4) Anderson, H. C., J . Apfil. Polymer Sci. 6 , 484 (1962). (5) Anderson, H. C., Kolloid-Zeitschrift und Zietschrift fur Polymere, Band 184, Heft 1, p. 26. (6) Anderson, H. C., Polymer 2, 451 (1961). ( 7 ) - Coleman, W. D., Naval Ordnance Laboratory Technical Report 61-147, September 1961. (8) Conky, R . T., SPE Baltimore-Washington Regional Tech.-Conf. (1964), Reprints, p. 118, 1964. (9) Dandoy, J., Ind. Chim. Belge 4, 535 (1962). (10) Dannenburg, H., Forbes, J. W., Jones, A. C., Anal. Chem. 32, 365 (1760). (11) Ehlers, G. F. L., Polymer 1, 304 (1960). (12) Freeman, E. S., Carroll, B. J., J . Phys. Chem. 62, 394 (1958). (13) Harms, D. L., Anal. Chem. 25, 1140 (1953). (14) Jacobi, C. H., Andre, L., Insulation (Libertyuille) 8 (7), 24 (1962). (15) Keenan, M . A., unpublished work, 1966. (16) Kovarskaya, B. M., Zhigunova, I. E., PlastichesskieMussy 1964, p. 17. (17) Learmonth, G. S . , Wilson, T., J . Afifil. PolymerSci. 8,2873 (1964). (18) Lee, L.-H., J . Apfil.PoiymerSci. 9, 1981 (1965). (19) Lee, L-H., J . PolymerSci., PartA 3 , 859 (1965). (20) Lemon, P. H. R. B., Brit. Plustics 36, 336 (1963). (21) Madorsky, S. L., Straus, S., Modern Plastics38, 134 (1961). (22) Moiseev, V. D., Neiman, M. B., Kovarskaya, B. M., Zenova, I. E., Guryanova, V. V., Soviet Plastics 1962, p. 12. (23) Neiman, M. B. Golubenkova L. I. Kovarska a, B. M., Strizhkova, A. S., Levantovskaya, L'I., Akutin, hi. S., Moiseev, $. D., Vysokomolekul. Soedin. 1, 1531 (1959). (24) Neiman M. B Kovarskaya B. M Golubenkova L. I Strizhkova, A. S., Levantovsiaya, I.?., Akutin, M.'S., J . 3olymerSci. 56, 3b3 (I
