CREEP IN

AND

AGGRESSIVE

LENGTH

OF

LIFE

OF

POLYCARBONATE

MEDIA

V. N. Kestel~man, and R. A. Denisova

M.

G.

Fal~kovskii,

UDC 678:539.376

Experimental data regarding creep of polycarbonate in air, water, and 10% citric acid are presented. It is shown that there is a correlation between the rate of creep and the length of life of polycarbonate, whieh enables its long term strength to be predicted.

The process o[ "creep" -deformation with time under the action of a constant stress -is of great theoretical and practical interest and has been the object of numerous and diverse investigations [1-4]. It must, however, be pointed out that in the majority of studies the phenomenon of creep and that of "static breakdown" (static fatigue) are considered separately. Besides, creep as a rule occurs under low stresses, and under such conditions the time to breakdown (length of life) cannot be determined experimentally. In consequence,, not the full creep curves, but only certain portions thereof are obtained. Studies on static fatigue, on the other hand, are mostly carried out at relatively high stresses, and without recording creep curves. From a scientific and practical point of view, it would seem to be important to carry out a combined examination of creep and "static fatigue" of solid polymers used as constructional materials. A number of investigations, [5-8] and others,have been devoted to this problem. The main result of these studies has been to establish a regular connection between the macroscopic characteristics of the breakdown processes and creep. It was shown that the rate of creep v a[ter the attainment of steady conditions is related to the length of life T by the empirical relationship

where the coefficients b and a in general depend on the temperature and the nature of the material. In [4] and [7] it was shown that for metals and caprone b ~ I. This confirms the close connectienbetween the breakdown processes and the steady-state creep of a given material. Besides, in the opinion of the authors, the controlling process is that of breakdown: every elementary act of breakdown is followed by an elementary act of deformation (creep) which, in turn, prepares the way for a subsequent act of breakdown.

meric

In [6] this direct relationship is disputed on the grounds that in studies carried out on different polymaterials, the values of b turned out to be greater or less than unity.

Our present investigation is evidently the first attempt at a fatigue and creep of polymers in an aggressive medium. The aim tween the length of life of polycarbonate in aggressive media and sibility of predicting the serviceable life of polymers under such

combined study of the processes of static was to test if there is a correlation beits creep rate; and to determine the posconditions.

Food Industry Technological Institute, Moscow. D. I. Medeleev Institute of Chemical Technology, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 725-727, July-Aug~tst, 1972. Original article submitted December 27, 1971.

] © 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 [¢/est 17th Street, New York, N. Y. 10011. 1 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.

629

po~ T~s~e]

i

"%

i

I

. . . . ... . . T.... V-" %h .

3

.....

/ .

600

L

,*~ t

[ ............... 1

L

0

I

"

~ _

6 kgf/cmZ

I .

.

.

.

-

--:

.

700

.

.

.

800

L ..... !

.

.

900

.

.

We have previously investigated the c r e e p of polyca rbonate in various a g r e s s i v e media. We established that polycarbonate shows little tendency to c r e e p with a c l e a r l y e x p r e s s e d portion of s t e a d y - s t a t e c r e e p [8-10].

_i

.......

. . . . . . .

~_[s ec'l]

Fig. 1. Graph of logarithm of length of life log r [sec] and rate of s t e a d y - s t a t e c r e e p log v [sec -1] of polycarbonate Dillon of M = 75000, against monoaxial s t r e t c h ing load ~, k g f / c m 2, in 10% citric acid at 80°C. '

'7

I S ~ ....

m_ t /g~$(5C

The investigation was c a r r i e d out in an apparatus that e n abled the samples to be tested in the appropriate medium while maintaining t e m p e r a t u r e and s t r e s s constant. Deformation of the samples with time was r e c o r d e d on a r e c o r d i n g cylinder capable of being rotated at v a r i o u s speeds. The specimens themselves were domestically produced, oriented Diflon K sheets (mol. wt. of p o l y m e r M = 33,000 and 75,000, respectively), tested in the form of dumbbells as p e r GOST 11262-65, stamped out parallel to the axis of orientation of the sheets. The thickness of the sheets c o m p r i s e d 0.035+0.048 mm. All e x p e r i m e n t s were c a r r i e d out no less than 5 times, the r e s u l t s being p r o c e s s e d mathematically.

"I

i

3

Fig. 2. Graph of log r = f [ l o g (l/v)] for monoaxially stretched p o l y c a r bonate Dillon at 80°C: 1) M =75000; 2) M = 33000. - ' - air; - - - citric acid; - - - water.

F r o m a plot of the experimentally determined creep, using as c o o r d i n a t e s the relative deformation e and time t, the s t e a d y state c r e e p d e / d t = v was determined. Since the length of life T was d e t e r m i n e d at the same time for the same samples, it was possible to examine the connection between ~-, v, and ~, as p e r the task that we had set o u r s e l v e s . In the experiments, the relationship between log T(O-) and log v (a) was studied for samples of p o l y carbonate Diflon of M 33,000 and 75,000 exposed to air, in water, and in 10% citric acid at 80 =~0.5°C. There is obviously a c l e a r connection between the p a r a m e t e r s studied. The experimental data p r e s e n t e d show that a change in the m o l e c u l a r weight of the polyca rbonate and in the external medium do not change the relationship between log r (~) and log v (cr) qualitatively. A quantitative c o m p a r i s o n of the experimental results showed that i n c r e a s i n g the m o l e c u l a r weight of the p o l y m e r inc r e a s e s its length of life both in a i r as also in the a g g r e s s i v e m e dia, while c o r r e s p o n d i n g l y diminishing the rate of s t e a d y - s t a t e creep. At the same time, for any given value of M, there is a tendency for a reduction in ~- and an i n c r e a s e in v in citric acid and water, as c o m p a r e d to air. Of p a r t i c u l a r importance is the fact that not only in air, but also in a g g r e s s i v e media the log ~-((r) c u r v e s c o r r e l a t e quite well with the c o r r e s p o n d i n g log v (cr) c u r v e s . This is c l e a r l y apparent f r o m Fig. 2, which depicts the relationships log T=f(log l/v). These are linear and a r e d e s c r i b e d by the one equation log T = a + b log (l/v), i.e., by Eq. 1 written in logarithmic form. C a l c u lations showed that coefficient b is close to unity.

Between the length of life of polycarbonate and its rate of s t e a d y - s t a t e c r e e p there is thus a c o r r e l a tion that can be e x p r e s s e d by the equation Tv =const. The c o r r e l a t i o n between the p a r a m e t e r s T and v is p r e s e r v e d for different m o l e c u l a r weights of polycarbonate, and in the p r e s e n c e of water o r 10 % c i t r i c acid. This link between the length of life and rate of s t e a d y - s t a t e c r e e p enables predictions to be made regarding the useful life of a m a t e r i a l under specific c o n ditions, based on the r e s u l t s of s h o r t - t e r m determinations of the rate of s t e a d y - s t a t e creep. LITERATURE 1. 2. 3.

630

CITED

I . A . Oding and V. S. Ivanova, Teplo~nergetika, 1, 23 (1955). I . A . Oding and V. S. Ivanova, in: Investigation of Heat R e s i s t a n c e of Metals fin Russian], 52, Moscow (1956), p. 174. V . S . Ivanova, Zavod. Lab., 2.~3, 601 (1957).

4. 5. 6. 7. 8. 9. 10.

S . N . Zhurkov and T. P. Sanfirova, Zh. Tekh. Fiz., 28, 1719 (1958). M . I . Bessonov and E. V. Kuvshinskii in: Physics of Solid Bodies [in Russian], 1. M o s c o w - L e n i n g r a d (1959), p. 265. M. L Bessonov and E. V. KuvshinsMi, VMS, 2, 397 (1960). V . R . Regel' and N. K. Chernyi, VMS, 4, 925 (1963). P . M . Ogibalov and V. I. Moroz-Shobolova, Mekhan. Polim., No. 1, 46 (1967). Straub and Oberbach (Shtraub and Oberbakh), Khim. i Tekhnol. P o l i m e r o v , No. 7, 113 (1966). L . N . Magazinova, V. N. Kestel'man, and R. A. Denisova, Tekhnologiya Mashinostroeniya, ~ 16 (1971).

631