Effects of Deformation on the Photodegradation of Low-Density

20

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Effects of Deformation on the Photodegradation of Low-Density Polyethylene Films DJAFER BENACHOUR and C. E. ROGERS Case Western Reserve University, Department of Macromolecular Science, Cleveland,OH44106

The effects of uniaxial or biaxial deformation on a polymeric material's resistance to ultraviolet radiation can be significant in many practical applications. In this study it was found that both uniaxial and biaxial elongation of low density polyethylene film enhances the photodegradation rate at 40°C. The enhancement process for uniaxial deformation has been shown to be closely related to the mechanism of deformation and the morphological changes induced upon elongation. The necking development region, where original material structure is most disrupted, showed the largest enhancement. Highly oriented material is less sensitive to photodegradation. The experimental evidence suggests that the increase in degradation rate may be attributed primarily to strain effects (morphological changes) with some contribution from stress per se (stored energy). Biaxial stretching was found to result in greater degradation, probably because of a larger decrease in film thickness and more constraint applied. A comparison of the nature of uniaxial and biaxial deformations gives some further insight into the drastic effects of photooxidative degradation on mechanical properties. Cyclic deformation (fatigue) involves a competition (dependent on deformation frequency, amplitude, and the number of cycles) between the formation of fatigue damages (microcracks, etc.) which promote degradation and orientation of structure which reduces the degradation process. Photooxidation and deformation of polyethylene (PE) have been intensively investigated, and mechanisms for each process have been suggested which are, more or less, well accepted (1-7). However, how photooxidation can be affected by deformation (type, extent ...) has not been given much attention until recently (811), In a previous paper, we reported data on the effects of 0097-6156/83/0220-0307$06.50/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

POLYMERS IN SOLAR ENERGY UTILIZATION

308


u n i a x i a l s t r e t c h i n g on the photodegradation behavior of low dens i t y p o l y e t h y l e n e (LDPE) f i l m s (11). The present paper i s a cont i n u a t i o n of the mentioned work, and deals w i t h the e f f e c t s of c y c l i c u n i a x i a l e l o n g a t i o n and b i a x i a l s t r e t c h i n g . The data are explained i n terms of the p h o t o o x i d a t i o n mechanism and the r e s u l t a n t degradation products and the type of deformation a p p l i e d and i t s e f f e c t s on the m a t e r i a l ( o r i e n t a t i o n , damages, morphol o g i c a l changes ...)· We a l s o give a b r i e f account o f the dependence of f a t i g u e l i f e (Np) on the p h o t o o x i d a t i o n extent of LDPE f ilms. Experimental Low d e n s i t y polyethylene f i l m s were used. The m a t e r i a l c h a r a c t e r i s t i c s are l i s t e d i n Table I . The samples were exposed Table I :

LDPE f i l m c h a r a c t e r i s t i c s

Thickness (mils)

1.25

Density (g/cc)

0.914

Molecular Weight M^

60.000

Melt Index (g/10 min) C r y s t a l l i n i t y (%) (estimated by IR and x-ray)

1.6

50

i n a Q-UV weatherometer (Q-Panel Co.) a t 40°C and ambient atmosphere. The o x i d a t i o n extent was f o l l o w e d by measuring the c a r bonyl absorbance a t 1716 cm~l u s i n g F o u r i e r Transform I n f r a r e d Spectroscopy ( D i g i l a b FTS-14). U n i a x i a l deformations were done u s i n g a s p e c i a l l y designed s t r e t c h e r (11) which was made t o f i t i n the FTIR sample h o l d e r , thus a l l o w i n g s p e c t r a to be taken w h i l e f i l m s are kept elongated. C y c l i c s t r e t c h i n g and f a t i g u e t e s t s were performed on an I n s t r o n T e n s i l e machine. The s t r a i n - c o n t r o l mode was used f o r f a t i q u e . A T. M. Long Company b i a x i a l f i l m s t r e t c h e r was used i n the constant r a t e of deformation mode f o r concurrent and s e q u e n t i a l b i a x i a l deformations. A l l s t r e t c h i n g s and mechanical t e s t i n g s were c a r r i e d out a t room temperature. R e s u l t s and D i s c u s s i o n E f f e c t s of Photodegradation on Fatigue L i f e : The p h o t o o x i d a t i o n behavior of the m a t e r i a l used i n t h i s study i s i l l u s t r a t e d i n F i g . 1 where the extent of o x i d a t i o n i s p l o t t e d as a f u n c t i o n of UV exposure time. N o t i c e the e x p o n e n t i a l shape of the curve. S i m i l a r behavior has been observed by other workers (1,3) f o r PE,

Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


20.

BENACHOUR AND ROGERS

Low-Density Polyethylene Films

6 UV

8

10

EXPOSURE

309

15 ( DAYS )

Figure 1. Photodegradation of LDPE at 40°C as a f u n c t i o n of exposure time i n the QUV apparatus.



310

as w e l l as polypropylene, f i l m s and i s due t o the a u t o c a t a l y t i c nature of the photooxidation process i n such m a t e r i a l s . Despite many s t u d i e s o f the e f f e c t s of photooxidation on mechanical p r o p e r t i e s (such as Young s modulus, t e n s i l e s t r e n g t h , u l t i m a t e e l o n g a t i o n , e t c . . . . ) , there i s very l i t t l e information about these e f f e c t s on f a t i g u e l i f e . For that reason, we s t u d i e d the f a t i g u e l i f e of LDPE f i l m s as a f u n c t i o n of UV expo sure. The r e s u l t s are shown i n F i g . 2 where logNp (Np being the number of c y c l e s sustained before f a i l u r e ) i s p l o t t e d vs. time of UV exposure. I n t h i s case the s t r a i n amplitude i s 8% and the frequency (ω) i s 10 cycles/min. The l i n e a r r e l a t i o n s h i p which i s observed can be described by an equation such as:


1

logNp = a + b t

(1)

where

a = f a t i g u e l i f e on non-oxidized f i l m under the considered f a t i g u e t e s t c o n d i t i o n s , a i s given by the i n t e r c e p t of the p l o t , b = constant, depending on f a t i g u e t e s t c o n d i t i o n s and m a t e r i a l c h a r a c t e r i s t i c s ; b i s given by the slope o f the curve, t = time of UV exposure. S i m i l a r behavior was observed f o r two other frequencies and the values of a and b, as a f u n c t i o n of ω, are l i s t e d i n Table I I . A l l frequencies used were lower than 2 Hz i n order to m i n i Table I I : Values of a and b as a f u n c t i o n of frequency (Equation (1)) Frequency: ω (cycles/min)

a

N-p a t t = 0

b

5

5.40

255 χ 1 0

10

5.00

100 χ 10

3

- 0.263

50

4.50

32 χ 1 0

3

- 0.253

3

- 0.283

mize any thermal e f f e c t s . The c o n s t a n t - s t r a i n mode was chosen to prevent sample f a i l u r e by creep. I t appears that a decreases as ω increases while b decreases a l s o but l e s s s t e e p l y (as a matter of f a c t , we can consider t h a t , w i t h i n the e r r o r margin of ± 10%, b remains constant.) The r e l a t i o n s h i p between f a t i g u e l i f e , o x i d a t i o n extent and UV exposure i s i l l u s t r a t e d i n F i g . 3 where both l o g N and l o g [carbonyl] are p l o t t e d vs. UV exposure ( f o r ω = 10 c y c l e s . ) I t can be seen that as the time of UV exposure i n c r e a s e s , i . e . , as [carbonyl] i n c r e a s e s , Np decreases sharply. F i g . 4 shows that logNp depends i n a l i n e a r f a s h i o n on l o g [ c a r b o n y l ] . Such a deF



20.



311

u.

ο

2

4 UV

Figure

6

8

EXPOSURE

10

15 (DAYS)

2. Dependence of f a t i g u e l i f e (Nf, the number of c y c l e s t o f a i l u r e ) on UV exposure time f o r LDPE. (Δε = 8%, ω = 10 cycles/min).



312


I 0

ι 2

ι 4 UV

Figure 3,

ι 6

1 8

EXPOSURE

1 10

1

15

1 20

(DAYS)

Fatigue l i f e and carbonyl content of LDPE as a f u n c t i o n of UV exposure (Δε = 8 % , ω = 10 cycles/min.)·





20.


313

314


N

F

= A[carbonyl]

B

(2)

where A and Β a r e c o n s t a n t s , depending on m a t e r i a l c h a r a c t e r i s t i c s and UV exposure and f a t i g u e t e s t c o n d i t i o n s . A and Β were c a l c u l a t e d (from the i n t e r c e p t s and s l o p e s , r e s p e c t i v e l y , of p l o t s s i m i l a r t o F i g . 4.) f o r a l l three frequencies used; the values are l i s t e d i n Table I I I . I t appears that w h i l e Β remains


Table I I I : Values of A and Β as a f u n c t i o n of frequency (Equation (2)) Frequency: ω (cycles/min)

A

Β

5

6.25

- 3.52

10

6.00

- 3.50

50

5.50

- 3.47

Note: A w i l l be the f a t i g u e l i f e of LDPE f i l m s c o n t a i n i n g 1% of carbonyl absorbance a t 1716 cm"! (most commercial f i l m s show such amounts of chromophores, probably r e s u l t i n g from o x i d a t i o n during processing.) more o r l e s s constant ( t h e r e f o r e , Β can be assumed not t o depend on frequency) A decreases as ω i n c r e a s e s . This i s a t t r i b u t e d mostly t o the higher deformation speed w i t h higher frequency ( s i m i l a r t o s t r e s s - s t r a i n experiments where the higher the de formation r a t e , the sooner the sample f a i l u r e . ) The r e l a t i o n s h i p between f a t i g u e l i f e and carbonyl content can be e x p l a i n e d as f o l l o w s : according t o the photooxidation mechanism of PE, carbonyl groups r e s u l t mainly from a N o r r i s h type I I r e a c t i o n , i . e . , f o r each carbonyl formation, there i s a s c i s s i o n of a segment of a molecule c h a i n . Such s c i s s i o n c r e ates a defect i n the s t r u c t u r e which can grow and propagate i n t o a microcrack under a p p l i c a t i o n o f a l o a d . Under c y c l i c l o a d i n g , i t i s understandable that the number of c y c l e s the sample can s u s t a i n w i l l be d i r e c t l y r e l a t e d t o the number of defects (such as microcracks, microvoids . . . ) , as i s c l e a r l y described by equa t i o n (2). More work i s needed t o see i f equation (2) holds f o r d i f f e r e n t s t r a i n amplitudes and d i f f e r e n t m a t e r i a l c h a r a c t e r i s t i c s ( d i f f e r e n t c r y s t a l l i n i t i e s , molecular weights . . . ) . E f f e c t s of U n i a x i a l E l o n g a t i o n : LDPE f i l m s were f i r s t s t r e t c h e d t o d i f f e r e n t elongations (deformation speed: 1 i n c h / min.), then photooxidized w h i l e being kept s t r e t c h e d . The extent of o x i d a t i o n , a t a given UV exposure (10 days a t 40°C), as a



20.



315

f u n c t i o n of draw r a t i o i s i l l u s t r a t e d i n F i g . 5. (The i n t e r p r e t a t i o n of the data has been r e p o r t e d , i n more d e t a i l , i n our pre vious paper (11), and only a b r i e f summary i s given here.) There i s enhancement of the degradation process due t o the deformation. The enhancement process i s r e l a t e d t o the deformation mechanism; the l a r g e r the d i s r u p t u r e of the f i l m s t r u c t u r e (necking r e g i o n development) the greater the enhancement. The o r i e n t a t i o n e f f e c t s , which takes place f o r λ > 4, tend t o reduce the enhance ment . The r e d u c t i o n of degradation enhancement due to o r i e n t a t i o n i s b e t t e r seen when samples are s t r e t c h e d and then the time to f a i l , under UV r a d i a t i o n , i s recorded. The r e s u l t s are shown i n F i g . 6 where one should n o t i c e the break i n s c a l e f o r the r e f e r ence (non-oxidized) sample. There i s a d r a s t i c decrease i n f a i l ure time (F.T.) f o r low draw r a t i o s 1 < λ < 1.7. This can be a t t r i b u t e d t o s t o r e d e l a s t i c energy which makes the chemical bonds more r e a c t i v e toward UV, even at low s t r e s s l e v e l s . As λ increases and the polymer s t r u c t u r e becomes more and more o r i ented, F.T. i n c r e a s e s s t e e p l y before reaching a p l a t e a u once the o r i e n t a t i o n process i s more or l e s s completed. I f we consider that p h o t o o x i d a t i o n i s oxygen d i f f u s i o n c o n t r o l l e d (1-5), the o r i e n t a t i o n e f f e c t i s to decrease such d i f f u s i o n by making the s t r u c t u r e much more compact so that the degradation w i l l be r e duced. In o r i e n t e d samples, o x i d a t i o n i s much more concentrated i n the surface l a y e r , thus decreasing the formation of microcracks w i t h i n the bulk of the sample which i n c r e a s e s i t s a b i l i t y t o r e s i s t f a i l u r e under UV r a d i a t i o n . E f f e c t s of C y c l i c U n i a x i a l S t r e t c h i n g : LDPE f i l m s were f a t i g u e d (Δε = 20%, ω = 10 c y c l e s / m i n . , f o r 10 c y c l e s ) before being put i n the weatherometer i n "the r e l a x e d s t a t e " , i . e . , w i t h f r e e ends. T h e i r photooxidation behavior i s compared to that of nonf a t i g u e d (reference) samples i n F i g . 7. The f a t i g u e d samples show more degradation which i s a t t r i b u t e d to damages r e s u l t i n g from c y c l i n g ( f a t i g u e damages: m i c r o c r a c k s , m i c r o v o i d s , microcrazes . . . ) . Such damages were c l e a r l y observed by o p t i c a l microscopy (12) and are known t o enhance the s u s c e p t i b i l i t y of LDPE f i l m s t o photodegradation. As Ν i n c r e a s e s , the f a t i g u e dam ages w i l l i n c r e a s e r e s u l t i n g i n more and more degradation as shown i n F i g . 8. An increase i n s t r a i n amplitude (Δε) a l s o i n c r e a s e s p l a s t i c deformation (non-recoverable elongation) as i l l u s t r a t e d i n F i g . 9. The number of c y c l e s i s given i n a l o g a r i t h m i c s c a l e , and the p l a s t i c deformation (P.D.) i s given i n percent: P.D. = (£ - l ) I £ , where £ and l are the sample lengths a f t e r and before f a t i gue, r e s p e c t i v e l y . Such p l a s t i c deformation i s accompanied by an o r i e n t a t i o n e f f e c t which, as we have seen, tends to lower degradation. F i g s . 10 and 11 show t h a t samples f a t i g u e d f o r 10^ c y c l e s degrade l e s s pendence can be described by the f o l l o w i n g equation: Q

Q

Q



316


CO of

Ζ Ο

ϋ Q Χ Ο

2

3 DRAW

4

5

RATIO

Figure 5. Dependence of o x i d a t i o n extent ( r e l a t i v e s c a l e ) of LDPE a f t e r UV exposure f o r 10 days a t 40°C on u n i a x i a l e l o n g a t i o n (draw r a t i o ) .



20.



317

"r 300

ο

φ

en Ο 1

ο

250

200

O

h

ο

ΗLU h-

O

o

o

150

100

-Ι 2

3 DRAW

4

5

RATIO

Figure 6. F a i l u r e time under UV exposure v s . u n i a x i a l draw r a t i o of LDPE.


318



60

τ*

FATIGUED

•

10

4

CYCLES

U = 20 V;

R E F E R E N C E

50

I

5 Ο

40

ΙΟ

m
Φ Φ

-

3 Φ

1.5

LU

ζο Q Χ

Ο

.5

DRAW

RATIO

Figure 14. E f f e c t s o f b i a x i a l s t r e t c h i n g ( s e q u e n t i a l d i r e c t i o n s ) on p h o t o o x i d a t i o n as a f u n c t i o n of the draw r a t i o .



20.

BENACHOUR AND

ROGERS

327


lamellae and t i l t i n g of chain a x i s i n lamellae occur i n order to " o r i e n t " chains p a r a l l e l to the f i l m surface. At such draw r a t i o s , an " o p t i c a l l y balanced s t a t e " i s reached (19). region 3: f o r λ > 3: mostly u n f o l d i n g of chain molecules from l a m e l l a e . A c l o s e look at F i g . 14 shows that there i s good correspon dence between deformation regions and o x i d a t i o n stages. First, there i s an increase i n o x i d a t i o n which can be a t t r i b u t e d to stored e l a s t i c energy. In the second stage, 1.7 < λ < 3, since there i s an " o p t i c a l l y balanced s t a t e " , i . e . , the s t r u c t u r e of the s t r e t c h e d f i l m i s very s i m i l a r to that of the non-deformed one, we expect the samples to show s i m i l a r o x i d a t i o n content. This i s seen by the decrease, then l e v e l l i n g o f f of the carbonyl content to a value close to that of the reference sample. At higher draw r a t i o s , the s t r e s s e f f e c t ( a p p l i e d c o n s t r a i n t ) be comes more and more s i g n i f i c a n t and the samples w i l l undergo more degradation as shown by the increase i n o x i d a t i o n f o r λ > 3. Conclusions A l l types of d e f o r m a t i o n - u n i a x i a l , b i a x i a l and c y c l i c u n i a x i a l stretching-enhance the photooxidation of LDPE f i l m s . A c l o s e r e l a t i o n s h i p e x i s t s between the enhancement process and the deformation mechanism: more d i s r u p t u r e of the s t r u c t u r e r e s u l t s i n a l a r g e r enhancement. Damages such as microcracks, microv o i d s , m i c r o c r a z e s - r e s u l t i n g from d i s r u p t u r e or f a t i g u e e f f e c t s increase the degradation r a t e while o r i e n t a t i o n decreases i t . The l a r g e r degradation extent e x h i b i t e d by b i a x i a l l y s t r e t c h e d samples (by comparison to u n i a x i a l l y elongated f i l m s ) i s a t t r i buted to the f u r t h e r decrease i n t h i c k n e s s and more c o n s t r a i n t applied. Ac knovle dgment s The Fellowship support of SONATRACH ( N a t i o n a l O i l and Company of A l g e r i a ) i s g r a t e f u l l y acknowledged.

Gas

Literature Cited 1.

2. 3. 4.

Ranby, B., Rabek, J . F . , "Photodegradation, Photooxidation and Photostabilization of Polymers", John Wiley & Sons, New York, 1975. Kamal, M. R., Ed. "Weatherability of Plastic Materials", Appl. Polym. Symp. 1967, 4, Interscience, New York. Hawkins, W. L., Ed. "Polymer Stabilization", Wiley-Interscience, New York, 1972. McKellar, J . F . , Allen, N. S., "Photochemistry of Man-Made Polymers", Applied Science Publishers, London, 1979.


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5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.


Hawkins, W. L., "Oxidative Degradation of High Polymers", in Oxidation and Combustion Reviews, Tipper, C.F.H., Ed., Vol. I., Elsevier, New York, 1965. Howard, K. W., "The Effects of Weathering on the Engineering Behavior of Plastic Films", Ph.D. Thesis, University of Cali fornia, Davis, 1976. Peterlin, A. J . Mat. Sci. 1971, 6, 490. Pabiot, J.; Verdu, J . Polym. Eng. Sci. 1981, 21(1), 32. Akay, G.; Tincer, T. Polym. Eng. Sci. 1981, 21(1), 8. Akay, G.; Tincer, T. Europ. Polym. J . 1980, 16, 597. Benachour, D.; Rogers, C. E., Photodegradation and Photo stabilization of Coatings, Winslow, F. H.; Pappas, S. P., Eds., ACS Symp. Ser. 1981, 151, 263. Benachour, D., "Effects of Deformation on the Photodegrada tion of Low Density Polyethylene Films", Ph.D. Thesis, Case Western Reserve University, Cleveland, 1982. Read, Β. Ε.; Stein, R. S. Macromolecules 1968, 1(2), 116. Okajima, S.; Kurihara, K.; Homma, K. J . Appl. Polym. Sci. 1967, 11, 1703. Okajima, S.; Homma, K. J . Appl. Polym. Sci. 1968, 12, 411. Okajima, S.; Homma, K.; Masuko, T.; Tanaka, H. J . Polym. Sci. 1969, A-1(7), 1997. Okajima, S.; Masuko, T.; Tanaka, H. J . Polym. Sci. 1970, A-2(8), 1565. Okajima, S.; Masuko, T.; Tanaka, H. J . Polym. Sci. 1969, A-1(7), 3351. Okajima, S.; Mori, K.; Morita, M.; Kurihara, K.; Tanaka, H. J . Polym. Sci. 1971, B-9, 729. Okajima, S.; Masuko. T.; Tanaka, H. J . Appl. Polym. Sci. 1972, 16, 441. Okajima, S.; Iwato, N.; Tanaka, H. J . Polym. Sci. 1971, B-9, 797. Okajima, S.; Masuko, T.; Tanaka, H. J . Appl. Polym. Sci. 1973, 17, 1715. Okajima, S.; Iwato, N.; Tanaka, H. J . Appl. Polym. Sci. 1973, 17, 2533. Okajima, S., Iwato, N.; Tanaka, H. J . Appl. Polym. Sci. 1975, 19, 303. Okajima, S.; Tanaka, H. J . Polym. Sci. 1977, B-15, 349.

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December 27,1982


Effects of Deformation on the Photodegradation of Low-Density

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