CONCERNING

THE

THERMALLY

AGED

STRENGTH

AND FRACTURE

OF

POLYCARBONATE

A. N. Machyulis, P. G. Konovalov,

V. P. Korotkov, and M. I. Pugina

UDC 678:620.192.4.7

The change in the strength, fractography, and s u p e r m o l e c u l a r s t r u c t u r e of polycarbonate a f t e r t h e r m a l aging has been experimentally investigated. It is shown that the fall in the strength of the polycarbonate a f t e r aging (T -< 160°C) is chiefly r e l a t e d with the formation of dangerous defects as a r e s u l t of the destruction of the s u p e r m o l e c u l a r s t r u c t u r e at the s u r f a c e of the block. It is noted that under the same conditions t h e r e is some o r d e r i n g of the s u p e r m o l e c u l a r s t r u c t u r e of the polycarbonate at depths of 150 g o r m o r e , but this has p r a c t i c a l l y no effect on the strength c h a r a c t e r i s t i c s .

Polycarbonate (PC) is a promising s t r u c t u r a l m a t e r i a l that is being increasingly employed in machinebuilding and instrument-making. However, like many other m a s s - p r o d u c e d plastics, the physicomechanical p r o p e r t i e s of PC a r e significantly affected b y heat. Our object was to d e t e r m i n e the behavior of the strength of PC t h e r m a l l y aged at a t e m p e r a t u r e of 160°C and to study the f r a c t u r e fractography and the c h a r a c t e r i s t i c s of thermooxidative degradation in the p r e s e n c e of dynamic thermooxidation. In these e x p e r i m e n t s we employed Diflon 1-2 PC with a m o l e c u l a r weight of 32,500. The specimens were obtained .by injection molding at 100-105 MN/m 2, a molding t e m p e r a t u r e of 290-300°C, a mold t e m p e r a t u r e of 90-100°C, and a total molding cycle of ~20 sec. The gauge dimensions of the specimens used for determining the breaking s t r e s s w e r e 45 × 5 × 3 m m . D e r i v a t o g r a m s w e r e r e c o r d e d on a P a u l i k - P a u l i k - E r d e l y i derivatograph. The s t r u c t u r a l investigation was c a r r i e d out by m e a n s of an MBI-6 m i c r o s c o p e . It is known that in the case of PC t h e r m a l l y aged at t e m p e r a t u r e s not exceeding the glass transition point [1] the strength d e c r e a s e s only slightly, while in the event of extended storage (from two to four years) at r o o m t e m p e r a t u r e the strength even i n c r e a s e s , although the m a t e r i a l is also e m b r i t t l e d [2]. t ~ m ~. . . . . . . . . . . . . . . . .

so

25 - -

~4~

time, hours~ zs8 ..... 432

Fig. 1. T e n s i l e breaking s t r e s s of PC as a function of-the duration of t h e r m a l aging at 160°C.

In determining the tensile breaking s t r e s s (Orb) of the t h e r m a l l y aged PC (aging t e m p e r a t u r e 160 • 2°C, f r e e a c c e s s to air) we noted two phases in the fall of the breaking s t r e s s (Fig. 1). In the f i r s t phase the fall in strength is only slight, while in the second phase a f t e r 430 h aging e b is r e d u c e d by 50%. As the aging time increased, the PC b e c a m e m o r e brittle and the coefficient of variation of the strength i n c r e a s e d f r o m 6.5% for the starting PC to 18.5% a f t e r 430 h o u r s of aging. In o r d e r to d e t e r m i n e the causes of such a considerable loss of strength we investigated the s u p e r m o l e c u l a r s t r u c t u r e (SMS) and f r a c t u r e f r a c t o g r a p h y of the PC {Fig. 2).

Institute of Physicotechnical P r o b l e m s of Energetics, Academy of Sciences of the Lithuanian SSR, Kaunas. T r a n s l a t e d f r o m Mekhanika P o l i m e r o v , No. 5, pp. 951-952, September-October, 197I. Original a r t i c l e submitted March 25, 1971. © 1974 Consultants B~reau, a division of Plenum Publishing Corporation, 227 ~'est 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $I5.00.

848

Fig. 2. Micrograph of a PC block: a), b), c) Original; d), e), f) heated at 160°C for 430 h; a), d) rupture fractograph (magnification 10x); b), e), e), f) SMS (450×); b), e), surface l a y e r of samples; e), f) at a depth of 150/~ o

,! 2

tot

308

~00

500 208

300

600

500

Fig. 3. Dynamic t h e r m o g r a v i m e t r i c c u r v e s for PC: a) starting PC: b) a f t e r t h e r m a l aging at 160°C for 430 h. 1) 13.8 deg/min: 2) 8.0 deg/min: 3) 5.8 deg/min: 4) 2.6 deg/min.

It was noticed that both at the surface of the block (see Fig. 2b) and at a depth of 150 g (see Fig. 2c) or in even d e e p e r l a y e r s the SMS of the investigated PC consists of i m p e r f e c t spherulites f o r m e d from fibrillar s t r u c t u r e s . The formation of i m p e r f e c t s u p e r m o l e c u l a r s t r u c t u r e s is attributable to the weak tendency of PC to c r y s t a l l i z e , although it is known that when artificial s t r u c t u r e - f o r m e r s a r e introduced it is p o s sible to obtain a s t r u c t u r e with quite well-defined spherulites [3]. When the specimens investigated were t h e r m a l l y aged, the SMS on the surface of the block w e r e completely d e s t r o y e d with the formation of defects (see Fig. 2e), which a r e apparently important f r a c t u r e c e n t e r s that considerably r e d u c e ~b" In the p r e s e n c e of t h e r m a l aging the SMS at a depth of 150 g (see Fig. 2f} o r m o r e continue to grow and b e c o m e m o r e o r d e r e d and v e r y sharply defined. However, in b r i t t l e f r a c t u r e , when t h e r e are defects at the s u r f a c e of the specimen, their effect on the strength of the p o l y m e r block is not v e r y important.

An analysis of the f r a c t u r e f r a c t o g r a p h y showed that both for the starting PC (see Fig. 2a) and for the aged PC (see Fig. 2d) both specular and rough f r a c t u r e zones are observed. It should be noted that the f i r s t stage of f r a c t u r e (specular zone), which is definitely lmown to be governed by a fluctuation mechanism, always begins at the surface of the aged specimens (see Fig. 2d) but m a y also s t a r t at the c e n t e r of the untreated specimens (see Fig. 2a). This is evidently r e l a t e d with technological or s t r u c t u r a l factors. An investigation of the thermooxidative failure of PC in the p r e s e n c e of dynamic heating showed it to be highly r e s i s t a n t . Intense thermooxidative degradation begins only at 380-400°C, but with further r i s e in t e m p e r a t u r e p r o c e e d s v e r y rapidly, and at 500°C the entire specimen is completely d e s t r o y e d (Fig. 3). In o r d e r to calculate the activation e n e r g y of the o v e r - a l l t h e r m o o x i d a t i v e degradation p r o c e s s (E) from the e x p r e s s i o n E = - 4 . 3 5 d log ~/(d l / T ) k c a t / m o l e [4], we obtained the dynamic t h e r m o g r a v i m e t r i c c u r v e s at various heating r a t e s . It was established that both for the starting PC and for the aged PC E v a r i e s from 8 to 45 k c a l / m o l e in the c o u r s e of the degradation p r o c e s s . In this case no p a r t i c u l a r r e g u l a r i t i e s in the behavior of E on the t e m p e r a t u r e interval 370-500°C w e r e observed, except for the fact that the E for the starting PC v a r i e s l e s s and has h i g h e r c h a r a c t e r i s t i c s than that for the aged PC. 849

In the initial phase of PC thermooxidation the process is of the radical-chain type [51 and develops by way of the hydroperoxides, while subsequently carbonyl intermediate products, which undergo further changes and participate in the development of the reaction, are formed. The superposition of these two processes, which have different effects on the total r a t e of degradation, evidently also leads to different values of E. The fact that chemical degradation first becomes important only at 370°C confirms our previous assumption that the change in ~rb after thermooxidatton at 160°C is chiefly related with structural changes in the polymer leading to the formation of surface defects. LITERATURE 1. 2. 3. 4. 5.

850

CITED

H. Schnell, Chemistry and Physics of Polycarbonates, Interscience (1964). P . M . Ogibalov and V. I. Moroz, Mekhan. Polim., No. 6, 1130 (1970). M . S . Akutin and L. N. Magazinova, Mekhan. Polim., No. 3, 396 (1969). It. L. Friedmann, J. Polym. Sci., C6, 183 (1964). A . I . Sidneev, A. N. Pravedu~kov, and B. M. Kovarskaya, Vys. Molek. Soed., A10, 1178 (1968).