Die Angewandte Makromolekulare Chemie 176/177 (1990) 87-92 (3056)

Institute of Plastics 170 Bulgaro-savetcka druzhba Blvd., Sofia, Bulgaria

INVESTIGATION OF THE THERMAL ACiEIN% OP POLYETHYLENE AND POLYSTYRENE WrTH THE PURPOSE OF PREDICTING THEIR SHELF-LIFE

Maria Trifonova, Sonya Evtimova, Lili Peeva

SUMMARY

The aim of the present work is to predict the warehouse storage li- fe of polyethylene low-density, polyethylene high-density and poly- styrene impact-resistent, using the method of temperature-time su- perposition. The properties, most representative for thermal age- ing, are the relative elongation at break for polyethylene and the impact resistance for polystyrene. The validity of the predicted shelf-life is verified by estimating the change of these properti- es under real conditions of warehouse storage.

1. INTRODUCTION When polymer materials are stored for a long time, due to ageing processes, their characteristics change irreversibly. The survey of scientific works, dealing with such processes, shows that diffe- rent approaches to predicting the shelf-life are employed, those, based oh temperature-time superposition pre~ailingl-~.

The method of temperature-time superposition is based on investiga- ting the kinetics of the change in the properties of the polymer materials during thermal ageing. The method demands the choice of

Paper presented at the lgth Colloquium of Danubian Countries on "Atmospheric Aging and Stabilization of Polymers" in Prague (Czechoslovakia), July 24 - 29, 1989

0 1990 HOthig & Wepf Vcdag, Basel mO3-3146/90/$03.00 87

p r o p e r t i e s t h a t c h a r a c t e r i z e t h e s e r v i c e a b i l i t y of t h e m a t e r i a l and meet t h e fol lowing r equ i r emen t s - sens i t i v i ty t o thermal age- i ng and monotonous change i n t i m e . The s h e l f - l i f e i s evaluated by monitoring t h e change i n t h e s e l e c t e d p r o p e r t i e s a t s e v e r a l temperatures , higher t h a t t h e one of warehouse s t o r a g e , and ex- t r a p o l a t i n g t o i t ’ ~ ~ .

Since t h e change i n polymer p r o p e r t i e s during ageing i s condi- t i oned by physicochemical processes , i t s mathematical desc r ip - t i o n r e q u i r e s r e l a t i o n s h i p s between the r e a c t i o n r a t e and tem- pe ra tu re . I f we assume t h a t t h e change i n a c e r t a i n parameter during thermal ageing obeys formally a f i r s t - o r d e r equa t ion , t h e r a t e cons t an t , c h a r a c t e r i z i n g t h i s change i s def ined a s the s lope of a s t r a i g h t l i n e . In order t o e x t r a p o l a t e t h e ra te con- s t a n t t o t h a t of the process a t normal temperature, Arrhenius equat ion i s used :

The s h e l f - l i f e T~~ i s c a l c u l a t e d by t h e equat ion

T~~ = n

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K=A. ( 1 )

( 2 ) ‘ n o - 1 nXcr i t

K2 0

Where n is a redundant c o e f f i c i e n t t h a t i s normally assumed t o be 0 . 5 , xo i s the i n i t i a l value of t he monitored parameter, xcrit i s the maximum allowable change i n t h e parameter, up t o which t h e polymer r e t a i n s i t s s e r v i c e a b i l i t y , and K z 0 i s the r a t e constant of t h e parameter change a t warehouse s to rade tem- pe ra tu re .

The p resen t work i s an attempt t o p r e d i c t t h e warehouse s to rage l i f e of t h r e e polymer m a t e r i a l s , using t h e method of temperatu- re-t ime approach.

2 . EXPERIMENTS An o b j e c t of i n v e s t i g a t i o n a r e polyethylene high-densi ty (PE- HD) w i t h a m e l t index 5 g/10 min, polyethylene low-density (PE- LD) w i th a melt index 0,2 g/10 min, and polystyrene impact-re- s i s t a n t (PS-I). The p r o p e r t i e s chosen as s e n s i t i v e t o thermal ageing a r e t h e r e l a t i v e e longa t ion a t break f o r polyethylene and impact s t r engka fo r impact-resist’ant polystyrene. The inves- t i g a t i o n s a r e c a r r i e d out a t t h e fol lowing temperatures: 8 0 ,

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100, and l l O ° C f o r PE-HD; 80, 90, and 100°C f o r PE-LD, and 60, 7 5 , and 8OoC f o r PS-I.

F ig . 1 . Change i n r e l a t i v e e longa t ion a t break of PE-HD dur ing thermal ageing.

3. RESULTS AND DISCUSSION Figures 1 , 2 and 3 show t h e experimental r e s u l t s f o r each of t h e i n v e s t i g a t e d polymer i n coord ina tes InX/Xo-T, where X and Xo a r e parameter va lues a t a given i n s t a n t o f thermal ageing and p r i o r t o i t , and T i s the du ra t ion of thermal ageing a t t h e cor - responding tempera ture . The va lues of t h e r a t e cons t an t s , c a l c u l a t e d a s a s lope of t h e corresponding stram$h l i n e a r e presented i n Tab. 1 .

The a n a l y t i c a l form o f t h e l i n e a r r e l a t i o n I n KT-1/T f o r each o f t h e i n v e s t i g a t e d polymers i s der ived by t h e l ea s t - squa re method on a computer. The c o r r e l a t i o n c o e f f i c i e n t s of t h e a n a l y t i c a l models of t he t h r e e polymers a r e over 0.9. The va lues of t he r a - t e cons t an t s a t t h e warehouse s to rage temperature (KZ0), ca lcu la - t ed by e x t r a p o l a t i o n , and t h e s h e l f - l i v e s , c a l c u l a t e d by E g . ( 2 ) f o r t h e t h r e e polymers a r e presented i n Tab. 2 .

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Fig.2. Change i n r e l a t i v e e l o n g a t i o n a t b reak o f PE-LD du r ing thermal ageing.

Fig. 3. Change i n t h e impact s t r e n g t h o f PS-I du r ing thermal ageing.

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Tab. 1 . Rate c o n s t a n t va lues (KT) of t h e m l a g e i n g of t h e i n v e s t i g a t e d polymers.

- Temperature of Rate c o n s t a n t , h-' thermal ageing,OC PE-HD PE-LD Ps-I -

6 0 3 . g o . 1 0-4 75 1 . 4 6 . 1 0 - 3 80 1 . 0 5 . 1 0 - 4 1 . 9 6 . 1 0 - 4 3 .60 .10 -3 90 3 . 8 3 . 1 ~ l - ~

1 0 0 2 .98 .1 o - ~ 7 .33 .1 o - ~ 1 1 0 7 .00 .1 f4

Tab.2. Rate cons t an t va lues a t warehouse s t o r a g e tempera ture ( K Z o ) and s h e l f - l i v e s o f t h e i n v e s t i g a t e d polymers.

-- She 1 f -1 i f e a t xcr it = O . 8x0, K2 0 Polymer

years

'PE-HD 0 .81 .1 0-6 PE- LD 1 .26 .1 0-6 PS-I 2 .36 .1 0-6

1 6 1 0

5

I n o rde r t o e s t i m a t e t h e c a l c u l a t e d s h e l f - l i v e s , t h e change i n the r e l a t i v e e longa t ion a t b reak f o r PE-HD and PE-LD and i n t h e impact s t r e n g t h fo r PS-I i s determined a f t e r a three-year s t o r a g e term. The r e s u l t s i n d i c a t e t h a t t he r e l a t i v e e longa- t i o n a t b reak of PE-HD and PE-LD i s not changed. The impact s t r e n g t h of PS-I i s 5 % sma l l e r t han the i n i t i a l va lue .

4. CONCLUSIONS Using t h e method o f temperature-time s u p e r p o s i t i o n , t h e warehou- s e s t o r a g e l i f e o f t h r e e polymer m a t e r i a l s - polye thylene high- d e n s i t y , po lye thylene low-density and po lys ty rene impac t - r e s i s - t a n t i S p r e d i c t e d . The change i n t h e c h a r a c t e r i s t i c s o f t he i n - v e s t i g a t e d polymers i s monitored under r e a l cond i t ions o f ware- house s t o r a g e . The r e s u l t s ob ta ined confirm t h e s h e l f - l i v e s p red ic t ed .

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