EFFECT THE

OF DURABILITY M. and

LIQUID

MEDIA OF

M. Yatsyuk, A. I. Soshko

AND

RADIATION

ON

POLYETHYLENE L~

V.

UDC 620.172:078. 674:677.521

Mitsai,

The s t a b i l i t y of t h e m e c h a n i c a l p r o p e r t i e s of p o l y m e r i c m a t e r i a l s c h a n g e s u n d e r c o n d i t i o n s in w h i c h the m a t e r i a l is s u b j e c t e d to t h e s i m u l t a n e o u s a c t i o n of e x t e r n a l and i n t e r n a l s t r e s s e s and of a l i q u i d m e d i u m [1-3]. T h i s o c c u r s n o t o n l y a s t h e r e s u l t of the s i m u l t a n e o u s a c t i o n of s t r e s s e s and a g g r e s s i v e m e d i a c a u s i n g c h e m i c a l ( i r r e v e r s i b l e ) c h a n g e s o r s w e l l i n g of t h e m a t e r i a l but a l s o a s a r e s u l t o f the a c t i o n of s u r f a c e - a c t i v e m e d i a [3!. Since h i g h - p o l y m e r s a r e u s e d a s c o r r o s i o n - r e s i s t a n t p r o b l e m s i n v o l v e d in the s t a b i l i t y of t h e i r p r o p e r t i e s u n d e r p a p e r r e p o r t s the r e s u l t s of s t u d i e s of the e f f e c t of v a r i o u s p o l y e t h y l e n e , a m a t e r i a l w i d e l y u s e d in i n d u s t r y and e a s i l y t i o n - i n d u c e d c h a n g e s on the r e l i a b i l i t y and d u r a b i l i t y of the

m a t e r i a l s m a i n l y in s u r f a c e - a c t i v e m e d i a the such conditions become very important. This liquid m e d i a on t h e m e c h a n i c a l p r o p e r t i e s of m a d e into s h a p e d a r t i c l e s . T h e e f f e c t of r a d i a p o l y m e r is a l s o e x a m i n e d .

As the a c t i v e m e d i a , l i q u i d s w e r e s e l e c t e d w h i c h d i f f e r e d both in t h e i r c h e m i c a l c o m p o s i t i o n and in t h e e x t e n t to w h i c h t h e y a f f e c t e d t h e s t r e n g t h c h a r a c t e r i s t i c s of the e x p e r i m e n t a l m a t e r i a l . T h e s e l i q u i d s i n c l u d e d s o l v e n t s , s u r f a c e - a c t i v e m e d i a , and d e t e r g e n t s . The s p e c i m e n s in the f o r m of d u m b b e l l s ( c r o s s s e c t i o n 5 x 0.15 to 0.10 m m , l e n g t h of w o r k i n g r e g i o n 12 m m ) w e r e cut f r o m a P 2 0 2 0 - T p o l y e t h y l e n e f i l m . The d u r a b i l i t y t e s t s w e r e c a r r i e d out w i t h a c o n s t a n t l o a d a f t e r t h e s p e c i m e n s had b e e n a t t a c k e d f o r a p r e d e t e r m i n e d t i m e by t h e l i q u i d m e d i u m . T e s t s w e r e c a r r i e d out in p a r a l l e l to s e p a r a t e the e f f e c t of the m e c h a n i c a l s t r e s s e s and t h e l i q u i d m e d i a on the p r o p e r t i e s of the p o l y e t h y l e n e . F o r t h i s p u r p o s e the u n s t r e s s e d p o l y e t h y l e n e s p e c i m e n s w e r e k e p t f o r 27 h in t h e e x p e r i m e n t a l m e d i a , a f t e r w h i c h t h e i r s t r e n g t h c h a r a c t e r i s t i c s w e r e d e t e r m i n e d . T h e y w e r e i r r a d i a t e d in an e l e c t r o n a c c e l e r a t o r ( e n e r g y 0.3 MeV, b e a m c u r r e n t 1600 pA) and the r a d i a t i o n d o s e w a s 12 MR. T h e r e s u l t s in T a b l e I i n d i c a t e t h a t a f t e r p r o l o n g e d e x p o s u r e in m o s t o f the m e d i a the m e c h a n i c a l c h a r a c t e r i s t i c s of the u n s t r e s s e d s p e c i m e n s a r e r e d u c e d . Thus, t h e d u r a b i l i t y of p o l y e t h y l e n e w a s r o u g h l y h a l v e d a f t e r a n 8 - h p e r i o d in t o l u e n e . An e v e n g r e a t e r e f f e c t of t h e s e m e d i a is o b s e r v e d with the s i m u l t a n e o u s TABLE

1 Medium

Tripolyphosphate Acetone Butyl alcohol ToIuene

Decrease* in the short- Decrease* ha the dataterm strength after pre- [bility in the media, % liminarv attack by the ',attar- - a- penoa ol - v the medium for 2~7h, q~ 27h sn 13 !9

26,5 29,5

14 23 35 52

12 20 31 50

Swelling perflay, o/o 100,3 99,1 99,5 98,8

*By c o m p a r i s o n with the u n t r e a t e d m a t e r i a l in a i r .

P h y s i c o m e c h a n i c a l I n s t i t u t e , A c a d e m y of S c i e n c e s of the U k r a i n i a n SSR, L ' v o v . T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a M e k h a n i k a M a t e r i a l o v , VoL 8, No. 1, pp. 78-80, J a n u a r y - F e b r u a r y , 1972. O r i g i n a l a r t i c l e s u b m i t t e d D e c e m b e r 26, 1970.

9 1974 Consultants Bureau, 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 $15.00.

73

~0

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~,0:

i

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1,2,3

I

"I I I II I a ~k. | | I ~" I I~| I ~Ik 9 9

7,a

r,~

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2,8

~t daN/mms Fig. 1. The durability of polyethylene in active media before (a) and after (b~ irradiation; 1, 2, 3 air, water, and soap; 4, 5) saturated s o lutions of soda and tripolyphosphate; 6) acetone; 7) butyl alcohol; 8) toluene. The broken line indicates the untreated polyethylene in air. action of a mechanical load (Fig. 1). Whereas w a t e r and soap at 18 to 20 ~ C have virtually no effect on the durability of polyethylene, saturated solutions of soda and sodium tripolyphosphate change it significantly. The durability of the polyethylene is d e c r e a s e d to the g r e a t e s t extent by the action of toluene. With s t r e s s e s cr < 0.8 daN/ram ~ the effect of toluene on the strength of the m a t e r i a l is g r e a t e r , a n d with an inc r e a s e in the s t r e s s the effect is reduced. This effect was previously observed in a study of the durability of an organic g l a s s in water and k e r o s e n e [1, 2]; it was explained by the change in the mechanism of b r e a k ing as a r e s u l t of the change in the relationship between the c r a c k - e x t e n s i o n rate in the plastic (at a given load) and the surface-diffusion rate of the medium. Thus, over a range of s t r e s s e s close to the breaking s t r e s s , the c r a c k - e x t e n s i o n rate is significantly g r e a t e r than the rate of diffusion of the medium and the effect of the liquid (adsorption lowering of strength) d e c r e a s e s . With fairly small loads the medium i n t e r acts with the m a t e r i a l at the apex of the c r a c k for a prolonged period and t h e r e f o r e exerts an effect on the strength of the material by adsorption and absorption. A significant change in the durability of polyethylene o c c u r s when it is deformed in alcohols and acetone and to a l e s s e r extent in soda and tripolyphosphate solutions. The c h a r a c t e r of the change in durability, however, is the same in all the experimental media. The d e c r e a s e in the strength of polyethylene under the simultaneous action of s t r e s s and a medium which does not markedly affect an u n s t r e s s e d p o l y m e r (soda, tripolyphosphate) is probably the result of surface physical p r o c e s s e s associated with the adsorption, making the deformation e a s i e r , and the d e s t r u c tion of the m a t e r i a l (Rebinder effect} at the apex of the crack. The reduction in the strength c h a r a c t e r istics of polyethylene under the action of solvents in which the polyethylene swells to a limited extent is also the r e s u l t of the appearance of the Rebinder effect. In this case, however, the breaking p r o c e s s is slightly changed by this swelling since the diffusion of the medium into the volume of the material leads to i r r e v e r s i b l e s t r u c t u r a l t r a n s f o r m a t i o n s [3, 4]. It is known that s t r u c t u r a l changes o c c u r when p o l y m e r s a r e irradiated. In polyethylene, for e x a m ple, hydrogen is liberated with a simultaneous i n c r e a s e in the degree of unsaturation of the molecules. The free radicals thus formed r e a c t with a molecule o r with other m a c r o r a d i c a l s producing branching [5] and this produces an increase in the strength of the polyethylene and in its r e s i s t a n c e to attack f r o m v a r i ous media [6]. Thus, the durability of irradiated polyethylene in all the experimental media (apart f r o m toluene) b e c o m e s even higher than in unirradiated specimens in air (Fig. 1, broken line). The effect of toluene on the durability of polyethylene which has been subiected to radiation is also d e c r e a s e d . The d u r a bility of the irradiated m a t e r i a l in toluene approximates to the durability of the untreated polyethylene in butyl alcohol. Thus, irradiation of polyethylene in the above r e g i m e i n c r e a s e s its mechanical strength and d u r a bility in liquid media and may be a means of essentially i n c r e a s i n g the durability of polyethylene goods. LITERATURE 1.

2.

74

CITED

A. N. Tynnyi and A. I. Soshke, Fiz.-Khim. Mekhan. Mat., No. ] (1967). A. I. Soshko and A. N. Tynnyi, Fiz.-Khim. Mekhan. Mat., No. 5 (J 965).

3,

4. 5. 6.

N. G. Kalinin, A. N. Kogut, A. I. Soshko, and A. N. Tynnyi, Fiz.-Khim. Mekhan. Mat., No. 1 (1969). t~. Ber, Design P r o p e r t i e s of Plastics [in Russian], Izd. Khimiya 0967). A. A. Tager, Physical Chemistry of P o l y m e r s [in Russian], Izd. Khimiya (~ 968). E. G. Yarmilko, L P. Lopatin, A. M. Kabachki, A. I. Soshko, and A. N. Tynnyi, Mekhanika Polimerov, No. 3 (1971).

75