Strain-Enhanced Photodegradation of Polyethylene - ACS Publications

17 Strain-Enhanced Photodegradation of

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Polyethylene DJAFER BENACHOUR and C. E. ROGERS Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106 The deleterious effect of sunlight on polymeric materials has been ascribed to a complex set of reactions in which both the absorption of ultraviolet light and the presence of oxygen are participating events. The process has been termed oxidative photodegradation (in this paper, the term is used interchangeably with photooxidation), and it has been the object of many studies and review articles (1-8). These studies typically have focussed on the relationship between weathering and changes in a specific property, with mechanical and optical properties receiving considerable attention (9). However, despite intensive investigations separately on both photooxidation and deformation of polymers, very little work has been done to determine how deformation affects photodegradation. In the few studies in that area, it was found that mechanical stress accelerates polymer deterioration (10,11), but no attempt was made to establish correlations between structural changes induced by deformation and the enhanced degradation process. In the present paper, we describe how photodegradation of low density polyethylene films was enhanced by uniaxial elongation. An explanation of the enhancement process is given based on the photooxidation and deformation mechanisms, and the photodegradation products. Experiments done in the absence of an external stress showed that the effects of degradation crosslinking are significant at relatively short times of UV exposure, and confirmed that the photodegradation is essentially in the surface layers. The oxidized layer thickness appeared to remain more or less constant after a certain exposure. Experimental Material Commercial low d e n s i t y p o l y e t h y l e n e f i l m s were used (Mw 60,000; p = 0.920 g/ml; 55% c r y s t a l l i n i t y as measured by x-ray 0097-6156/81/0151-0263$05.00/0 © 1981 American Chemical Society Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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d i f f r a c t i o n ; T = 118°C; thickness = 2 m i l s ) . No attempt was made to remove a d d i t i v e s ( i n c l u d i n g any a n t i o x i d a n t s ) nor were s p e c i a l precautions taken to prevent a i r o x i d a t i o n of the samples before they were exposed to UV l i g h t . m

Photooxidation Procedure The photooxidation was c a r r i e d out i n a commercial weatherometer (Q-UV, Q-panel Co., Cleveland, Ohio). T h i s apparatus uses medium pressure mercury f l u o r e s c e n t UV lamps (Sunlamps F5-40, Westinghouse E l e c t r i c Corp.) which emit UV l i g h t i n the 273-378 nm range with a maximum i n t e n s i t y at 310 nm. F i l m s , f o r both mechanical and spectroscopy s t u d i e s , were a f f i x e d to the specimen panels of the weatherometer. Upon comp l e t i o n of the UV exposure, which occurred at 37°C ± 1°C i n the presence of a i r , the f i l m s were removed and kept a t room temperature i n the dark f o r at l e a s t 24 hours i n order to remove any v o l a t i l e o x i d a t i o n products. I n f r a r e d Spectroscopy The i n f r a r e d spectra of the d i f f e r e n t samples were taken with a F o u r i e r Transform i n f r a r e d spectrometer ( D i g i l a b FTS-14) using the double beam mode v s . a i r as r e f e r e n c e . 150 scans per sample and 100 scans per r e f e r e n c e , a t a r e s o l u t i o n of 4 cm , were taken f o r every sample. A l l s p e c t r a were stored on tape, and a d i g i t a l s u b s t r a c t i o n of the a f t e r - and- before UV exposure (or any other sample treatment) s p e c t r a was performed, whenever needed, by an o n - l i n e computer, thus p e r m i t t i n g a b e t t e r v i s u a l i z a t i o n of the s p e c t r a l changes i n the polymer by UV- photooxidation. In the case where the samples were kept elongated during UV exposure, a s p e c i a l f i l m s t r e t c h e r was used. T h i s s t r e t c h e r was made to f i t i n the FTIR sample h o l d e r , thus a l l o w i n g spectra to be taken w h i l e the f i l m s are kept s t r e t c h e d . _i

Mechanical

Experiments

A l l t e n s i l e and s t r e s s - r e l a x a t i o n measurements were done using an I n s t r o n T e n s i l e t e s t e r . The samples were cut i n t o the dumbbell shape corresponding to the ASTM D412 type C model ( t o t a l length: 4.5 i n . ; s t r a i g h t p a r t : 1.5 i n . ; width: 0.25 i n . ) . The samples were tested at a deformation r a t e of 1 in./min. f o r the simple t e n s i o n experiments. In the case of s t r e s s - r e l a x a t i o n measurements, the samples were p r e s t r a i n e d to 7% e l o n g a t i o n at e = 5 in./min. then allowed to s t r e s s r e l a x over a 20 minute p e r i o d . A l l mechanical t e s t i n g were c a r r i e d out at room temperature.

Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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B E N A C H O U R A N D ROGERS

Results and

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Discussion

E f f e c t s of Photooxidation

on Mechanical P r o p e r t i e s

I t i s w e l l known that mechanical p r o p e r t i e s of polymeric m a t e r i a l s are g r e a t l y d e t e r i o r a t e d by UV exposure (_2-j0 . The nature of t h i s d e t e r i o r a t i o n was determined using non-strained samples which were photooxidized at 37°C. Engineering s t r e s s s t r a i n curves as a f u n c t i o n of UV exposure are shown i n F i g u r e 1. The numbers next to each curve represent days of UV exposure. In terms of degradation, the p o i n t s of i n t e r e s t are: The l a r g e drop (fy 45%) i n the s t r e s s to break between the non-oxidized sample and the o x i d i z e d ones, and 2. Both the 5- and 10-day samples f a i l at the same s t r e s s l e v e l (a b ^ 90 x 10 grams/cm ), and d i f f e r mainly i n the u l t i m a t e e l o n g a t i o n . S i m i l a r r e s u l t s were observed i n the s t r e s s - r e l a x a t i o n experiments which are shown i n F i g u r e 2. The 5- and 10-day samples r e l a x to the same s t r e s s l e v e l . The major d i f f e r e n c e i n s t r e s s r e l a x a t i o n behavior among the d i f f e r e n t samples occurs during the very beginning of the r e l a x a t i o n process. For that reason, and i n order to b e t t e r i l l u s t r a t e the f i r s t minutes of r e l a x a t i o n , the time s c a l e i s l o g a r i t h m i c . We are presenting data f o r the 5- and 10-day samples only, but the same r e s u l t s were a l s o observed f o r samples exposed to UV l i g h t f o r 6, 7, 8 and 9 days, at 37°C. These r e s u l t s suggest that a f t e r about 5 days of UV exposure, the a p p l i e d load i s supported by the same c r o s s - s e c t i o n i n a l l these samples. T h i s means that the e f f e c t i v e f i l m thickness of non-degraded m a t e r i a l does not change a f t e r 120 hours of photooxidation. Based on these f i n d i n g s , we are proposing the schematic model p i c t u r e d i n F i g u r e 3: the o x i d a t i o n , s t a r t i n g at the s u r f a c e , penetrates i n to the m a t e r i a l bulk as the UV exposure i n c r e a s e s . A f t e r 5 days, the extent of bulk p e n e t r a t i o n l e v e l s o f f , and the thickness of the o x i d i z e d l a y e r remains e f f e c t i v e l y constant (such thickness was estimated, by taking the r a t i o of the load to break of the o x i d i z e d samples to that of the non-oxidized f i l m , to be a p p r o x i mately 45% of the o r i g i n a l t h i c k n e s s , i . e . , 0.45 x 2 = 0 . 9 m i l s ) . Further o x i d a t i o n w i l l f u r t h e r d e t e r i o r a t e the already o x i d i z e d l a y e r , r e s u l t i n g i n the formation of more i n c i p i e n t c r a c k s . T h i s crack d e n s i t y i n c r e a s e w i t h i n the o x i d i z e d l a y e r w i l l , i n t u r n , r a i s e the p r o b a b i l i t y of f a i l u r e of the sample, thus r e s u l t i n g i n a d r a s t i c decrease of the e l o n g a t i o n to break when the f i l m s are s t r e t c h e d . T h i s i s shown i n F i g u r e 4. I t can be seen that a t r e l a t i v e l y short UV exposures (2 to 4 days) there i s a very s l i g h t increase of the u l t i m a t e e l o n g a t i o n . T h i s r e s u l t , a l s o observed by other workers (11,12), i s a t t r i b u t e d to o x i d a t i v e c r o s s l i n k i n g which p r e v a i l s i n i t i a l l y . At longer UV exposures, c h a i n - s c i s s i o n becomes the dominant r e a c t i o n , thus r e s u l t i n g i n the sharp decrease of the u l t i m a t e e l o n g a t i o n a f t e r 4 days of photooxidation. 3

2

n


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DC

S

40 20

ol 0

Figure

7.

Figure 2.

1

100

i

200 STRAIN

.

300 ( % )

i

i

400

500

Engineering stress-strain curves as a function of time of UV (numbers next to each curve represent days of exposure at 37°C)

exposure

Stress-relaxation as a function of time of UV exposure (numbers curve represent days of exposure at 37°C)

on each


17.


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E f f e c t of S t r a i n on Photodegradation a) F o u r i e r Transform i n f r a r e d s p e c t r a : FTIR s p e c t r a of the f o l l o w i n g samples are shown i n F i g u r e 5: a) PE f i l m , non-prestrained, non-photooxidized, b) PE f i l m , exposed to UV l i g h t at 37°C f o r 120 hours, c) PE f i l m , p r e s t r a i n e d to 200% e l o n g a t i o n , then exposed to UV l i g h t at same temperature, f o r same p e r i o d while kept s t r e t c h e d . The c-spectrum shows more c a r b o x y l i c content (ketonic absorbance at 1716 cm~l), meaning that the p r e s t r a i n has enhanced the photodegradation of the polymer. I t must be pointed out that a c o r r e c t i o n i s needed to account f o r the change i n thickness upon s t r e t c h i n g of the f i l m s . For that reason a l l carbonyl contents have been r e f e r r e d to the t h i c k ness of the non-strained f i l m which has been photooxidized a t the same temperature f o r the same p e r i o d . The 2840 cm l band, which corresponds to the symmetric C-H s t r e t c h i n g of the methylene groups (14) was chosen as r e f e r e n c e f o r thickness c o r r e c t i o n (other bands can be used, but the choice was decided by the f a c t that the 2840 cm"* band changed i n i n t e n s i t y as the s t r a i n v a r i e d , but i t s l o c a t i o n and shape were not a f f e c t e d by the s t r a i n v a r i a t i o n . ) Thus, a l l the photooxidation data are given on a r e l a tive scale. b) Strain-Enhanced Photodegradation as a F u n c t i o n of UV Exposure: _

F i g u r e 6 shows the v a r i a t i o n of photodegradation as a funct i o n of time of UV exposure f o r samples p r e s t r a i n e d to d i f f e r e n t elongations as i n d i c a t e d by the numbers next to each curve. The degradation r a t e i n c r e a s e s as the s t r a i n i n c r e a s e s from 0 to ^ 300% and then decreases a t higher s t r a i n s . T h i s degradation r a t e dependence on s t r a i n i s b e t t e r i l l u s t r a t e d i f the degradation extent i s p l o t t e d as a f u n c t i o n of e l o n g a t i o n f o r a given UV exposure. c) Photodegradation as a F u n c t i o n of S t r a i n : F i g u r e 7 shows the r e l a t i v e o x i d a t i o n v a r i a t i o n as a f u n c t i o n of nominal s t r a i n f o r 5 days of UV exposure a t 37°C. From the data, i t appears that the enhanced photodegradation occurs i n three stages: Stage I; from 0 to ^ 120% s t r a i n . Only a s m a l l i n c r e a s e of degradation r a t e i s seen. Stage I I ; from 120% to 300% e l o n g a t i o n . A very pronounced i n c r e a s e of degradation r a t e i s observed. Stage I I I : 300% to 500% e l o n g a t i o n . A f t e r an a p p r e c i a b l e decrease, the degradation r a t e l e v e l s o f f . An e x p l a n a t i o n of the presence of these d i f f e r e n t stages i n



Figure 6.

Photodegradation

of

269

Polyethylene

Oxidation as a function of time of UV exposure for different (% prestrain indicated by numbers next to each curve)

prestrains


270

Figure

PHOTODEGRADATION

7.

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PHOTOSTABILIZATION

OF

Engineering stress (right ordinate) and oxidation (left ordinate) function of strain for a given UV exposure (5 days at 37°C)


COATINGS

as a

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the enhancement process should not only take i n t o account the photooxidation mechanism, but a l s o the morphological changes i n duced upon s t r e t c h i n g of the PE f i l m s . These morphological changes have been w e l l i n v e s t i g a t e d by P e t e r l i n who proposed a model of p l a s t i c deformation of PE (15). T h i s model, i n v o l v i n g a deformation process of three stages, can be summarized as follows: Stage I; Continuous deformation of the s p h e r u l i t i c s t r u c ture before the neck; Stage I I : Discontinuous transformation i n the neck, of the s p h e r u l i t i c i n t o the f i b r i l l a r s t r u c t u r e ; Stage I I I : P l a s t i c deformation of the f i b r i l l a r s t r u c t u r e a f t e r neck formation i s complete. The c o r r e l a t i o n between the stages of deformation and those of the enhanced photodegradation i s c l e a r l y shown i n F i g u r e 7 where both nominal s t r e s s - s t r a i n curve ( r i g h t ordinate) and photooxidation v s . e l o n g a t i o n curve ( l e f t ordinate) are p l o t t e d . The c l o s e correspondence between the d i f f e r e n t stages of photoo x i d a t i o n and deformation allows us to make the f o l l o w i n g suggestions: Stage I: 0 to £ 120% s t r a i n ; the s t r u c t u r e of the elongated polymer i s s t i l l very c l o s e to that of the o r i g i n a l m a t e r i a l , and, t h e r e f o r e , no l a r g e d i f f e r e n c e i s seen i n the photodegradation rate. Stage I I : 120% to 300% s t r a i n ; t h i s stage corresponds more or l e s s to the necking development r e g i o n . In t h i s stage d r a s t i c morphological changes occur v i a t i l t i n g , s l i p p a g e and t w i s t i n g of the l a m e l l a e , p u l l i n g of some chains out of the c r y s t a l s , formation of microcracks and microvoids, a l l of which r e s u l t i n a h i g h l y d i s r u p t e d s t r u c t u r e . T h i s d i s r u p t i o n should g r e a t l y favor o x i d a t i o n and t h i s i s seen i n the very pronounced increase of carbonyl content i n t h i s stage. Stage I I I : from 300% up to sample f a i l u r e (X 5.10). The neck i s more or l e s s f u l l y developed and a f i b r i l l a r s t r u c ture i s obtained. T h i s s t r u c t u r e i s l e s s s u s c e p t i b l e to degradat i o n because of i t s high degree of o r i e n t a t i o n and high c r y s t a l linity. T h i s e x p l a i n s the drop and then l e v e l l i n g o f f of the carbonyl content i n t h i s l a t t e r stage. In the previous experiment, the samples were kept s t r a i n e d during UV exposure and the t a k i n g of FTIR s p e c t r a . Thus, the enhancement of degradation may be a t t r i b u t e d to a combination of both s t r e s s and s t r a i n e f f e c t s . The s t r a i n e f f e c t s , i . e . , e f f e c t s due to morphological changes induced by the deformation, are c l e a r l y shown i n Stage I I . During the necking development r e g i o n , the a p p l i e d load remains more or l e s s constant w h i l e the e l o n g a t i o n i n c r e a s e s . Thus, the enhancement o c c u r r i n g i n Stage II can be a t t r i b u t e d mostly to s t r a i n e f f e c t s . The s t r e s s e f f e c t s , that i s e f f e c t s due to the a p p l i e d load per se (or stored energy), are manifested by the f a c t that d e s p i t e higher c r y s t a l l i n i t y - t h e r e f o r e l e s s o x i d a t i o n s u s c e p t i b i l i t y - t h e h i g h l y drawn


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Figure 8. Oxidation (5 days at 37°C) as a junction of strain for different samples: (a) prestrained with fixed-ends; (b) prestrained, then annealed with free-ends (for one day at 60°C); (c) prestrained, then relaxed at room temperature with free-ends


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polymers, A > 4, s t i l l show more degradation than the nons t r a i n e d polymer. T h i s can be a t t r i b u t e d to the higher r e a c t i v i t y toward o x i d a t i o n of the s t r e s s e d chemical bonds (16). I t was shown that under the a c t i o n of mechanical s t r e s s , a homolytic s c i s s i o n of macromolecular chains occurs and f r e e r a d i c a l s are formed. The "mechanical" r a d i c a l s w i l l enhance the o x i d a t i o n and w i l l i n i t i a t e l o c a l f r a c t u r e i n s t r e s s e d polymers (17). E f f e c t s of Annealing and R e l a x a t i o n with Free-Ends The next step was to see what happens to the enhanced photodegradation when the s t r e s s i s r e l i e v e d and the morphological changes are reduced (free-end samples). The data shown i n F i g u r e 8-b were obtained using the samples that were f i r s t elongated, then taken out of the s t r e t c h e r and annealed with free-ends a t 60°C f o r 24 hours before being photooxidized. The data i n F i g u r e 8-c were obtained using the samples which were f i r s t s t r a i n e d , then taken out of the s t r e t c h e r and allowed to r e l a x a t room temperature f o r one week before photooxidation. In both cases, the samples were exposed to UV l i g h t at 37°C for 5 days. As i t can be seen, the photodegradation s t i l l appears to have three stages corresponding to those observed i n the f i r s t case ( F i g . 8-a), but there i s a two-fold decrease i n the s c a l e of the degradation. The o v e r a l l e f f e c t s of annealing and r e l a x a t i o n are to allow the deformed s t r u c t u r e to reorganize i t s e l f and p a r t i a l l y r e t u r n to the o r i g i n a l r e l a x e d s t a t e v i a e l a s t i c and v i s c o e l a s t i c recovery. T h i s recovery i s not complete even a f t e r a p e r i o d of one week, and, on a molecular l e v e l , most l i k e l y c o n s i s t s of small c h a i n s h i f t s i n the c r y s t a l l i n e l a t t i c e and i n l a m e l l a e r o t a t i o n and s l i p . Thus, the e f f e c t s of anneal i n g and r e l a x a t i o n are to p a r t i a l l y reverse the e f f e c t s of drawing (18) and t h i s r e s u l t s i n the r e d u c t i o n of the enhancement of degradation. The f a c t that the necking development r e g i o n s t i l l shows a higher l e v e l of degradation suggests that i n t h i s stage, the s t r u c t u r e remains i n a s i g n i f i c a n t l y d i s r u p t e d s t a t e and, t h e r e f o r e , s t i l l shows more s u s c e p t i b i l i t y to degradation even a f t e r annealing and r e l a x a t i o n . Conclusions The photodegradation of low d e n s i t y polyethylene f i l m s i s g r e a t l y enhanced by u n i a x i a l e l o n g a t i o n , and the enhancement process i s c l o s e l y r e l a t e d to the morphological changes induced upon drawing of the polymer f i l m s . The necking development r e g i o n shows more degradation due to i t s h i g h l y d i s r u p t e d s t r u c t u r e . The enhancement can be reduced by annealing or r e l a x a t i o n , and s t r e s s ( a p p l i e d l o a d , or stored energy) appears to be the dominant f a c t o r of enhancement. The mechanical t e s t s confirmed that the o x i d a t i o n i s concentrated i n the s u r f a c e l a y e r s .


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Acknowledgement The Fellowship support of SONATRACH (National Oil and Gas Company of Algeria) is gratefully acknowledged. Literature Cited 1. Jellinek, H.H.G., "Degradation of Vinyl Polymers", Academic Press, New York, 1955. 2. Raff, R.A.V.; Doak, K. W., Eds. "Crystalline Olefin Poly mers", Part II, Interscience, New York, 1964, Chap. 8. 3. Pinner, S. H., Ed. "Weathering and Degradation of Plastics", Gordon and Breach, London, 1966. 4. Kamal, M. R., Ed. "Weatherability of Plastic Materials", Appl. Polym. Symp., 4, Interscience, New York, 1967. 5. Neiman, M. B., Ed. "Aging and Stabilization of Polymers", Consultant's Bureau, New York, 1965. 6. Scott, G., "Atmospheric Oxidation and Antioxidants", Elsevier, New York 1965. 7. Ershov, Yu. A.; Kuzina, S. I.; Neiman, M. B., Russ. Chem. Rev., 1969, 38, 147. 8. Hawkins, W. L., "Oxidative Degradation of High Polymers", in Oxidation and Combustion Reviews, Tipper, C.F.H., Ed., Vol. I, Elsevier, New York, 1965. 9. Howard, K. W., "The Effects of Weathering on the Engineering Behavior of Plastic Films", Ph.D. Thesis, University of California, Davis, 1976. 10. Kaufman, F. S., Jr., "A New Technique for Evaluating Outdoor Weathering Properties of High Density Polyethylene", in ref. 4. 11. Trozzolo, A. M., "Stabilization Against Oxidative Photode gradation", in "Polymer Stabilization", Hawkins, W. L., Ed., Wiley-Interscience, New York, 1972. 12. McKellar, J. F.; Allen, N. S., "Photochemistry of Man-Made Polymers", Applied Science Publishers, London, 1979. 13. Ranby, B.; Rabek, J. F., "Photodegradation, Photooxidation and Photostabilization of Polymers", John Wiley & Sons, New York, 1975. 14. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C., "Spectro metric Identification of Organic Compounds", 3rd Ed., John Wiley & Sons, New York, 1974. 15. Peterlin, A., J. Mat. Sci., 1971, 6, 490. 16. Zhurkov, S. N.; Kosukov, V. E., J. Polym. Sci., Polym. Phys. Ed., 1974, 12, 385. 17. Pratt, P. L., "Fracture", Ed., Chapman and Hall, London, 1969, p. 531. 18. Peterlin, A., Makromol. Chem., Suppl., 1979, 3, 215. RECEIVED

October 17, 1980.


Strain-Enhanced Photodegradation of Polyethylene - ACS Publications

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