Thermal degradation of compositions based on aromatic polyamides

Polymer Science U.S.S.R. Vol. 9-4, No. 8, pp. 1953-1959, 1982 Printed in Poland

0032-3950/82 $7.50+.0 0 © 1988 Pergamon Press Ltd.

THERMAL DEGRADATION OF COMPOSITIONS BASED ON AROMATIC POLYAMIDES* S.-S. A. PAVI,OVA, P. 1~. GRIBKOVA, T. N. BALYKOVA, T. V. P o I ~ A , L. G. KOMAROVA, N. I. BEKASOVAand V. V. KOl~S~rA~ A. N. lqesmeyanov Institute of Hotero-organic Compounds, U.S.S.R. Academy of Sciences

(Received 6 .May 1981) A study has been made of the thermal degradation (523-773 K) of blends made up of various relative amounts of an aromatic synthesized from terephthalic acid and aniline fluorene, and a polyamide with metacarborane fragments incorporated in the macrochain. The investigation (in the range 523-623 K) covered changes in molecular weight characteristics, and gelation of the systems, relative to the poly. amidocarborane concentration. I t is shown that a 0. 5% addition of polyamidoearborane has an inhibiting influence on aromatic polyamide degradation in the interval 523-573 K. At higher temperatures ( >t 598 K) polyamidocarborane causes crosslinking of the aromatic polyamide over the entire concentration range investigated (from 0. 5 t o 50 ~¢.%).

I x earlier communications [1, 2] we showed that the addition of polyamidocarborane (P/kC) to aromatic polyamides (PA) increases the thermal stability of the latter under vacuum and in an oxygen atmosphere. It was proposed that heating is accompanied by chemical interaction of these polymers. To verify this assumption and to determine the mechanism of the stabilizing action of PAC we investigated at high-temperature the behaviour of systems with differing contents of the polymeric components in some detail, along with changes in their molecular weight characteristics in the range of relatively low temperatures. The polyamides investigated were of the type of

_[_ o

()," "0

o

/•--NH--

In

PA

- - [ - - C--CB,oHloC--C--N H - - ~ - 0

0 PAC

* V y s o k o m o l . s o y e d . A 2 4 : N o . 8, 1 7 1 2 - 1 7 1 7 , 1982. 1953

~ - - ~ - - N H--~

,

S.-S. A. PAVLOVA et 0~.

1954

The Table contains data on the composition of composites containing these polyamides. The synthesis of the PA and PAC has been described in [3, 4]. Systems were prepared by dissolving the polymers in DMF, with subsequent precipitation into methanol. The resulting mixtures were extracted wlth metlmnol to remove DMF, and were vacuum dried to constant weight. Degradation was carried out under a 0' 133 Pa vacuum in the temperature range 323-773 K with heating for 1 hr at each temperature, followed chromatographic. analysis of the gaseous products of degradation along with a detailed investigation of the thermolyzed residues, using a number of methods. C H A N G E S I N M O L E C U L A R W E I G H T C H A R A C T E R I S T I C S OF P ~ k , Pz~kC AND

PA-PAC

COMPOSITIONS.

R E L A T I V E TO T H E D E G R A D A T I O N T E M P E R A T U R E

I

Make-up of the compositions, w t . %

[0], m3/kg

PA

PAC

initial composites I

100-0 99.5 97.0 90.0 75-0 50.0 0.0

0 0.5 3.0 10.0 25.0 50.0 100-0

0.104 0.114 0.100 0.096 0'091 0.087 0.086

!Mw x X 10 .3

132 134 132 120 70 46

I 7, [q], Mw × 7, [~], M w X r, [~], wt. % mS/kg ! X lO-3 wt. % m3/kg × 10-~ wt. % m3/k~ 548K 100 100 100 100 100 100 100

0-102 0.118 0.100 0.087 0.076 0.070 0.067

573K 246 132 134 103 55 30

100 100 100 100 56 51 48

0.142 0.114 0.124 Ol12 0.068 0.035 0.029

598K 426 125

263 52 20 2O

100 0.158 13 -2 5 0.076 28 0.060 33 0-025 30 i 0~020 30

1~017

Note. The initial polymers and the PA-PACcompositionsare completeiysoluble, y denotes the amount of sol frac" tion.

I t h a s p r e v i o u s l y boon s h o w n [5, 6], a n d s u b s t a n t i a t e d in t h e p r e s e n t investig a t i o n (Fig. 1), t h a t d e g r a d a t i o n o f P A a n d P A C differ m a r k e d l y f r o m one a n o t h e r . W h e r e a s a m i d e b o n d scission w i t h t h e f o r m a t i o n of c a r b o n m o n o x i d e s , a n d of low m o l e c u l a r weight d e g r a d a t i o n p r o d u c t s such as benzoic acid, benzonitrile a n d fluorene, is t h e m a i n d e g r a d a t i o n process o f P A , t h e d e g r a d a t i o n o f r A C is m a i n l y t h e result o f c a r b o r a n e ring o p e n i n g w i t h t h e evolution o f h y d r o g e n a n d m e t h a n e . A n insignificant a m o u n t of c a r b o n oxides are f o r m e d in t h e degrad a t i o n o f P A C a t t e m p e r a t m ' e s a b o v e 620 K ; t h e r e are no low m o l e c u l a r w e i g h t p r o d u c t s of P A C d e g r a d a t i o n . T o t a l w e i g h t losses for P A C a t 773 K a m o u n t to -~ 5 % , c o m p a r e d w i t h ~ 5 0 % for P A a t this t e m p e r a t u r e [5, 6]. On i n v e s t i g a t i n g t h e d e g r a d a t i o n of s y s t e m s w i t h different p o l y a m i d e ratios n n d o r v a c u u m it was f o u n d t h a t as t h e P A C c o n c e n t r a t i o n increases t h e a m o u n t of h y d r o g e n is increased, a n d t h e c o n t e n t of c a r b o n oxides reduced, while t h e d e g r a d a t i o n process o f s y s t e m s w i t h a P A G c o n c e n t r a t i o n f r o m 0.5 to 10% b y wt. a t t e m p e r a t u r e s u p to 673 K is identical to t h a t of t h e a r o m a t i c p o l y a m i d e . A t 673 K a n d a b o v e t h e a d d i t i o n of as little as 0 . 5 % of P A C leads to a r e d u c t i o n in t h e a m o u n t o f c a r b o n m o n o x i d e s b y ~-ono-half, c o m p a r e d w i t h t h e P A . A f u r t h e r increase in t h e PA(] c o n c e n t r a t i o n , a t 673 K a n d above, results in a m a r k e d r e d u c t i o n in t h e a m o u n t of carbon m o n o x i d e s (Fig. 1).

Thermal degrada*~ion of compositions based on aromatic PA

1955

This can be accounted for in terms of a strengthening of the stabilizing action of 1)A(~ as its concentration in the blend is increased, and also by the fact t h a t it is the concentration of the thermally more stable component (1)AC) that has increased. However, it appears from an analysis of the experimental results that the amount of carbon monoxide and hydrogen is not an additive value in the degradatioll of the compositions, i.e. it does not vary directly in proportion to a change in the ratio of the components. For instance when the 1)AC concentration is in the range from 25 to 50% by wt. decomposition of the })lends is practically identical to t h a t of pure I)AC. This experimental fact evidences the modifying action of 1)AC on the aromatic PA, i.e. it is to some extent proof that chemical interaction takes place between the two polymers.

b

t~

OL

o.8I

/ ¢~,-- 2

~o.4

~

÷

=C

0.53

,(/.e

2.0

10

25

5O 100

0.5 3 lO

2E

i I

50 100 f eAcJ , %

Fro. 1. Amo(mt of carbon oxides (a) and hydrogen (b) evolved in the degradation of compositions vs. their PAC concentration at different temperatures: •--573, 2--598, 3--623, 4--673, 5--723, 6--773 K.

To shed further light on the thermal behaviour of the blends we investigated structural transitions of PA, 1)AC and their blends in the range 473-598 K. Investigations of this type involving 1)A and 1)A0 were outlined in [7, 8] where it was shown that branching takes place in thermal degradation of polyamides of the type of 1)A in the temperature range 473-598 K, and, at 598 K and above, is followed by crosslinking [7], whereas the thermal degradation of PAC in the same temperature range is characterized by a reduction in viscosity and in MW [8], which fully agrees with experimental fmdings presented in the Table a.nd in Fig. 2. I n addition it is noteworthy t h a t even up to 598 K the solubility of P A in organic solvents is fully preserved, whereas already at 573 K formation of ~ 50% gel is observed for 1)A(J and for blends with a high 1)AC concentration (25 and 50% by wt.) {see Table}. The mode of change in molecular weight characteristies of the soluble fraction of the blend (Fig. 2} suggests t h a t processes of

1956

S.-S. A. PAV-~OVA e~ ~.

branching and subsequent crosslinking of the polymers take place along with scission of PAC chains. It appears from the data in Fig. 4 that at 598 K gelation in the blends conraining 0.5 and 3% by wt. PAC begins after 10 and 5 rain respectively. The blends with a high PAC concentration (25 and 50% by wt.) form a gel in the first minutes of heating without any induction period (Fig. 6).

ErlJ'"q'~Ik~qa

I

M,,,.~-"

.I// 5~a

57a #sa

54s

#Ta

5qa

5~

5~

r,K

FIG. 2

8za

#~

673

T,K Fro. 3

Fro. 2. Changes depending on the degradation temperature in the intrinsic viscosity (a) and MW (b) of compositions with PAC concentrations (wt. %) as follows: 1--0, 2--0.5, 3--3, 6--10, 5--25, 6--50, 7--100.

Fzg. 3. Temperature dependence of the amount of gel formed in the degradation of compositions with differing PAC concentrations (wt. %): 1--0, pure PA, 2--@5, 3--3, 4--10, 5--25, 6--50~o, 7--pure PAC. At first glance findings such as the inhibition of degradation processes up t o 573 K on adding inconsiderable amounts of PAO and the increased rate of gelation at 598 K would appear contradictory. Such apparent contradictions could in ,~ll probability be accounted for if one accepts the concept of a "critical" inhibitor concoxltration [12]. In the range 523-573 K there are marked differences in the extent to which IVIW and the onset of golation temperature v a r y with the PA0 concentration. At higher temperatures (above 573 K) those differences are practically unrelated to the PAC concentration. The inadequate course of golation in the blonds at 598 K is apparently duo to an intensification of various transitions occurring in earborano fragments of the PAO macroehains [SJ. I n addition it is noteworthy t h a t a specific feature duo to the nature of PAC as a polymeric stabilizer m a y appear in the present case: while inhibiting the degradation of P A at temperatures above 573 K PAO" m a y at the same time act as a multifunctional crosslinking agent.

Thermal degradation of compositions based on aromatic PA

1957

I t is interesting to find t h a t crosslinking processes take place more rapidly at temperatures above 598 K, even in a blend containing 0.5~/o by wt. PAC~ t h a n in the pure P A (see Fig. 3). The rapid erosslinking of P A accompanying the addition of small amounts of PAC is advantageous, since, judging by the data presented in the Table and in Fig. 1, it increases the thermal stability of the aromatic polyamide. Iu all probability this takes place at the expense of formation of more stable bonds of types B - - N , B--C, B - - O and B - - B . !

8o

I 2O I

0

20 qo Time, rain FIO.

4

60

0

10 ZO [PAC] , %

3O

FIG. 5

Fro. 4. Gelation kinetics at 598 K for compositions with differing PAC concentrations (hero and in Fig. 6): •--0.5, 2--3, d--50%, 5--pure PAC. FIG. 5. Gelation induction period vs. PAC concentration at 598 K. As in the case of the high temperature degradation processes, the composites containing a large amount of PAC behave like the initial PAC, even under low temperature degradation conditions. For instance, curves of gelation vs. temperature (Fig. 3) and heating time for mixtures containing from 25 to 50~o by wt. PAC are practically identical to those of PAC (Fig. 4). Moreover it is clear from these findings t h a t the compositions undergoing degradation do not behave like mechanical blends, but interact with one another, which leads to the formation of "copolymers" whose thermal behaviour resembles that of PAC already in the case of a 25~o P-~C concentration. I t should be said that although the amount of gel fraction formed at 598 K and above is only slightly dependent on the PAC concentration, it does nevertheless have a marked influence on the polymeric network density. It is clear from the kinetic data (Fig. 6) t h a t the densest network is formed in the pure PAC and in the composites that have high PAC concentrations. As regards swelling the composites containing 3 and 10% PAO are closer to the pure PA, though differing significantly from one another in regard to their gelation times.

1958

S.-S. A. PAV~OVAe t a l .

In the higher temperature region (~>598 K) it is with a PAC concentration of 0.5% by wt. that we obtain enhanced thermal properties of PA, probably through the formation of optimal network density. Thus in the light of our study of blends with differing ratios of the initial components it now appears that there are three ranges of PAC concentrations differing in their influence on the degradation of P A in the range of relatively low temperature (523-598 K). 14

7" w

5 I

20

I

I

40

60

T/me ~m/n

FIG. 6. Swelling of gel fraction vs. time of thermal degradation at 598 K for compositions with differing PAC concentrations. When the P A 0 concentration is 0.5% b y wt. the initial P A structure is preserved after heating even up to 573 K. In the concentration range from to 10% P A 0 the mode of variation in molecular weight characteristics is similar to that observed for the pure PA. Structural transitions similar to those occurring in the pure PAC occur, during degradation at 523-573 K, in the blends containing from 25 to 50% PA0. There are also three distinct temperature ranges in which the presence of ~)'5~/o PAC acts in a different way. In the first interval (473-873 K) the added P A 0 acts as an inhibitor of radical processes of P A decay, while in the second (>~598 K) it acts as a modifier, whose action leads to crosslinking of the PA. T h e other PAG concentrat':ons, in the range 598-623 K act as modifiers of the PA. When small amounts of PAC (0.5-3% b y wt.) are added the effect on degradation of the aromatic polyamide is similar to that observed for radical t y p e stabilizers.

Thermal degradation of compositions based on aromatic PA

1959

H i g h c o n c e n t r a t i o n s o f P A C (25-50~/o b y wt.) result in c h e m i c a l i n t e r a c t i o n b e t w e e n P A a n d P A C w i t h t h e f o r m a t i o n of a " c o p o l y m e r " w h o s e p r o p e r t i e s a p p r o x i m a t e to t h o s e o f t h e c a r b o r a n e - c o n t a i n i n g p o l y a m i d e . T h e m e c h a n i s m o f i n t e r a c t i o n o f t h e p o l y m e r s h a s y e t t o be a d e q u a t e l y e x p l a i n e d , a n d will be t h e s u b j e c t of f u r t h e r i n v e s t i g a t i o n s . Translated by R. J. A.. H~N[)Rr

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2.

3. 4. 5.

6.

7. 8.

9. 10. 11.

12.

VA, Yu. L. AVETISYAN, N. I. BEKASOVA, L. G. KOMAROVA, Ya. S. VYGODSKII and S. V. VINOGRADOVA, U.S.S.R. Pat. 53532: Byull. izob., 1~o. 42, 1976 V, V. KORSHA~, S.-S. A. PAVLOVA, P. N. GRIBKOVA, Yu. L. AVETISYAN and I. A. GRIBOVA, Vysokomol. soyed. 20: 772, 1978 (Translated in Polymer Sei. U.S.S.R. 20: 4, 869, 1978) V. V. KORSHAK, S. V. VINOGRADOVA and Ya. S. VYGODSKH~ U.S.S.R. Pat. 198644; Byull. izob., No. 14, 1967 V. V. KORSHAK, N. I. BEKAS OVA and L. G. KOMAROVA, U.S.S.R. Pat. 254074; Byull. izob., No. 31, 1969 V. V. KORSHAK, P. N. GRIBKOVA, T. N. BALY~OVA, L. G. KOMAROVA and N. I. BEKh, SOVA, Vysokomol. soyed. AI4: 1557, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 7, 1746, 1972) V. V. RODE, P. N. GRIBKOVA, Ya. S. VYGODSKII~ S. V. VINOGRADOVA and V. V. KORSHAK, Vysokomol. soyed. A1O: 2550, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 11, 2962, 1968) V. V. RODE, P. N. GRIBKOVA and V. V. KORSHAK, Vysokomol. soyed. Al1: 57, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 1, 60, 1969) S.-S. A. PAVLOVA, P. N. GRIBKOVA, T. N. BALYKOVA, Yu. L. AVETISYAN and V. V. KORSHAK, Vysokomol. soyed. A19: 161, 1977 (Translated in Polymer Sei. U.S.S.R. 19: l, 189, 1977) B. H. ZIMM and R. KILB, J. Polymer Sei. 37: 131, 19, 1959 Stareniye i stabilizatsiya polimerov (Ageing and Stabilization of Polymers) edited by M. B. Neiman, p. 103, Nauka, Moscow, 1964 I. FOIGT, Stabilizatsiya sintotieheskikh polimerov protiv deistviya sveta i tepla (Stabilization of Synthetic Polymers Against the Action of Light and Heat), p. 123, Khimiya, Moscow, 1972 Yu. A. SHLYAPNIKOV and V. B. MILLER, Zh. fiz. khimii 39: 16, 2418, 1965