Comparison of Chemiluminescence with Impact Strength for

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26 Comparison of Chemiluminescence with Impact Strength for Monitoring Degradation of Irradiated Polypropylene 1

1

2

G. D. M E N D E N H A L L , H . K. A G A R W A L , J. M . C O O K E , and T. S. DZIEMIANOWICZ 1

2

2

Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, MI 49931 Plastics Technical Center, Himont U.S.A., Inc., Wilmington, D E 19808

The loss of impact strength of polypropylene was followed from sheets stored in air at 25°C and 60°C after irradiation with electron beams. A marked difference in efficacy of phenolic and thioetherbased stabilizers at the two temperatures was found, with the thioether active alone at 60°C but only synergistically at 25°C. This difference was also reflected qualitatively in differences in chemiluminescence emission from the samples. Irradiation of polymers with y- or electron beams is an attractive alternative to chemical sterilization because of i t s speed, ease of control, and the absence of residue. Radiation treatment of polypropylene, however, also initiates chemical changes which lead ultimately to embrittlement. These changes in physical properties may not become apparent until some time after the treatment. The ability of antioxidants to prevent radiation damage does not always follow the trends observed in thermal oxidation, which has stimulated efforts to develop new stabilizers or optimized combinations of existing ones. The testing of polymers stabilized against radiation damage raises the familiar questions (a) whether the use of elevated temperatures for accelerated aging is valid, and (b) whether new analytical techniques under actual use conditions can give the same information in comparable time. In this study we measured chemiluminescence of polypropylene stabilized with different combinations of antioxidants and irradiated to different extents, and made correlations with conventional impact strength measurements of the same materials. Chemiluminescence has been used by a number of workers to characterize the thermal oxidation of polypropylene (1_,2). This study allowed an opportunity to use fiber-optics for transmitting chemiluminescence from the heated sample to the detector, which promises to simplify greatly the apparatus required for the technique. 0097-6156/85/0280-0373$06.00/0 © 1985 American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

374

The l i g h t e m i s s i o n from a u t o x i d i z i n g o r g a n i c m a t e r i a l s can a r i s e from s e l f - r e a c t i o n o f p r i m a r y o r secondary a l k y l p e r o x y l o r alkoxyl radicals. In the p r e s e n t c a s e , the predominent s i t e o f p e r o x y l r a d i c a l a t t a c k w i t h h y d r o p e r o x i d e f o r m a t i o n i n PP i n v o l v e s the t e r t i a r y c e n t e r s , so t h a t the c h e m i l u m i n e s c e n c e p r o b a b l y a r i s e s (1) from p r i m a r y o r m e t h y l p e r o x y l r a d i c a l s formed from 3 - s c i s s i o n , eg. S i n c e the l i g h t e m i t t e d by the o x i d i z i n g polymer i s e x t r e m e l y f a i n t , i t i s however p o s s i b l e t h a t a minor r e a c t i o n pathway w i t h a r e l a t i v e l y h i g h quantum y i e l d c o u l d be the predominent s o u r c e o f excited states. The mechanism i n Scheme 1 makes p o l y p r o p y l e n e an a t t r a c t i v e s u b s t r a t e f o r s t u d y , because i t i m p l i e s a p r o p o r t i o n a l i t y between r a t e s o f l i g h t e m i s s i o n and s c i s s i o n o f polymer c h a i n s . Experimental

Himont p o l y p r o p y l e n e

( c o n t r o l l e d r h e o l o g y type w i t h a melt

f l o w r a t e o f 12 dg m c o n t a i n i n g 0.0100% o f a p h e n o l i c p r o c e s s i n g s t a b i l i z e r , was combined w i t h c a l c i u m s t e a r a t e , i n h i b i t o r s as r e q u i r e d , and e x t r u d e d t o g i v e 9" x 0.040" sheet w i t h the f o l l o w i n g compositions: 0.1%

Ca s t e a r a t e

Neat

x

Phenol Thio Comb

x x x

a. b.

0.030% G o o d r i t e 3114*

1.0%

DLTDP

b

x x

x x

1,3,5-Tris-(4-hydroxy-3,5-di-tert-butylbenzyl)cyanuric acid. Di-n-dodecyl 2,2 -thiodipropionate. 1

I r r a d i a t i o n procedure. The s h e e t s were i r r a d i a t e d w i t h a Van de G r a a f e l e c t r o n a c c e l e r a t o r (High V o l t a g e E n g i n e e r i n g , Model AK) w i t h an e l e c t r o n energy o f 2Mev and a dose r a t e o f about 0.3MR s A n y l o n m a t r i x r a d i o c h r o m a t i c f i l m ( F a r West T e c h n o l o g y ) was used f o r dosimetry. C a l c u l a t i o n s i n d i c a t e d t h a t the dosage a t the lower s u r f a c e o f t h e sheet was about 20% h i g h e r than a t the upper surface. Impact measurement. The impact t e s t s were conducted i n the u s u a l manner on s i n g l e s h e e t s w i t h a Gardner L a b o r a t o r y Impact T e s t e r (Model IG-1120) w i t h a 0.625" d i a m e t e r punch hammer. At l e a s t t e n drops were performed i n the c e n t e r 50% o f the s h e e t s and w i t h p o i n t s o f impact a t l e a s t 1" a p a r t . The f a i l u r e c r i t e r i o n was a b r i t t l e (not d u c t i l e o r t e a r ) b r e a k . The v a l u e s r e p o r t e d i n T a b l e I a r e 50% p r o b a b i l i t i e s (energy a t w h i c h 50% o f f a i l u r e s a r e brittle). The s t a n d a r d d e v i a t i o n o f the v a l u e s i s about 10%. Chemiluminescence. The PP s h e e t s were s t o r e d a t ambient temperature i n t h e d a r k . Chemiluminescence e m i s s i o n from the i n i t i a l b a t c h o f samples was measured 4, 19, 38, 52, 65, 80, and

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

94

26.

MENDENHALL ET AL.

Degradation of Irradiated Polypropylene

375

days a f t e r i r r a d i a t i o n from c i r c u l a r p i e c e s ( 3 / 8 " d i a m . , w t . 60.0 71.5 mg) o f PP f r e s h l y c u t from a sheet w i t h a c o r k b o r e r . The p i e c e s were weighed to ±0.1mg and p l a c e d i n a sample h o l d e r shown i n F i g u r e 1. The s t e e l - j a c k e t e d f i b e r - o p t i c c a b l e ( D o l a n - J e n n e r I n c . , h i g h - t e m p e r a t u r e v a r i a n t Model BXT424, 1/4" f i b e r b u n d l e ) was screwed i n t o the top o f the sample h o l d e r , w h i c h was then p l a c e d i n a 1 x 6" copper tube w h i c h had been immersed i n an e t h y l e n e g l y c o l b a t h a t 150.0 ± 0 . 1 ° C ( P o l y Temp Model 8 0 ) . A minor p r o b l e m w i t h the c a b l e was the c o a t i n g o f the ends o f the g l a s s f i b e r s w i t h d a r k m a t e r i a l w h i c h had to be sanded o f f a f t e r s e v e r a l months ( t h i s d e p o s i t p r o b a b l y a r o s e from d i f f e r e n t m a t e r i a l s c o n c u r r e n t l y under study). The o t h e r end o f the f i b e r - o p t i c c a b l e was connected t o an e n d - o n p h o t o m u l t i p l i e r system d e s c r i b e d elsewhere ( 3 ) . The d a t a were n o r m a l i z e d t o the average sample w e i g h t o f 70.0 mg. For p l o t t i n g , the t e x t f i l e s f o r i n d i v i d u a l e x p e r i m e n t s were l o a d e d i n t o a S p e r r y mainframe computer and p l o t t e d w i t h m o d i f i e d commercial s o f t w a r e (TELAGRAF; I s s c o G r a p h i c s , I n c . ) . Control e x p e r i m e n t s showed t h a t removal o f the s u r f a c e l a y e r w i t h a b l a d e j u s t b e f o r e e x a m i n a t i o n a t 150°C d i d not change the shape o f the chemiluminescence c u r v e . S e v e r a l samples were weighed b e f o r e and a f t e r the h i g h t e m p e r a t u r e e x a m i n a t i o n , and no s i g n i f i c a n t w e i g h t d i f f e r e n c e s were f o u n d . Chemiluminescence a t ambient t e m p e r a t u r e i n t h i s s t u d y was o b t a i n e d from c i r c u l a r samples 1.0" i n d i a m e t e r (average w t . 0 . 5 g ) , i r r a d i a t e d t o 5MR o n l y , w h i c h were p l a c e d i n the sample w e l l o f an a p p a r a t u s w i t h automated c o u n t i n g f u n c t i o n ( T u r n e r D e s i g n s , I n c . Model 20 L u m i n o m e t e r ) . The l i g h t e m i s s i o n was measured f o r s e v e r a l time p e r i o d s o f 120 seconds e a c h . Average v a l u e s and s t a n d a r d d e v i a t i o n s were t h e n o b t a i n e d w i t h p o c k e t c a l c u l a t o r s . Care was t a k e n i n the c h e m i l u m i n e s c e n c e work t o keep the samples c l e a n , unexposed t o room l i g h t s f o r more than a few m i n u t e s , and they were h a n d l e d g e n t l y w i t h g l o v e s o r t w e e z e r s . Oven a g i n g . P l a q u e s were p h y s i c a l l y s e p a r a t e d from each o t h e r d u r i n g a c c e l e r a t e d a g i n g on a r a c k i n an oven m a i n t a i n e d a t 60±1 C with forced a i r c i r c u l a t i o n . Results Chemiluminescence ( 1 5 0 ° C , 50 min) from i r r a d i a t e d samples a f t e r 4 and 65 days a r e p r e s e n t e d i n F i g u r e s 2 - 3 . The i r r a d i a t i o n d o s e , s t a b i l i z e r s , and ambient s t o r a g e t i m e s b e f o r e e x a m i n a t i o n a r e i n d i c a t e d on each f i g u r e . Smoothed, 3D p l o t s d e r i v e d from complete s e t s o f a g i n g d a t a f o r i n d i v i d u a l c o m p o s i t i o n s and dosages appear i n Figures 4-6. Note the c o n t o u r l i n e s on the xy p l a n e o f each f i g u r e , and t h a t the z a x i s has been expanded i n some f i g u r e s to r e v e a l d i f f e r e n c e s between the more h i g h l y s t a b i l i z e d s a m p l e s . In s p i t e o f l o s s e s o f l i g h t t h r o u g h the c a b l e and the absence o f any f o c u s s i n g l e n s e s , the chemiluminescence i n t e n s i t y was f u l l y sufficient for precise monitoring. The c u r v e s show a r i s e i n i n t e n s i t y from z e r o time whose magnitude r e f l e c t s the amount o f p e r o x i d i c i n i t i a t o r s p r e s e n t , and the a b i l i t y o f added s t a b i l i z e r s t o p r e v e n t the c h e m i l u m i n e s c e n t r e a c t i o n s . In g e n e r a l , the l i g h t e m i s s i o n at 150°C i n c r e a s e d w i t h i n c r e a s i n g number o f days from



Photomultiplier^. tube/ housing

Interface and power supplies >

(—Fiber-optic

cable

Countdisplay r

Copper tube

JLmJk

onstant ttemperature < bath

.

Apple II

Steel sample holder (thread 1/2" 20)

Sample

F i g u r e 1. Schematic o f a p p a r a t u s f o r measurement o f chemilum i n e s c e n c e a t 150°C


MENDENHALL ET AL.

ROH

+o»

+


Pv

*

ROO.

P v v

CH0

CH 002

i

LIGHT

Scheme 1 .

20

Mechanism.

40

60

80

100

80

100

80

100

0.5 Minutes

20

40

60

0.5 Minutes 5000 65 D a y s No MR

4000 3000 2000 1000 0 20

40

60

0.5 Minutes F i g u r e 2. R e p r e s e n t a t i v e p l o t s o f c h e m i l u m i n e s c e n c e , i n c o u n t s p e r 30-second i n t e r v a l s , v s . time a t 150°C.


378

POLYMER STABILIZATION AND DEGRADATION 25000-1

20

40

60

80

100

0.5 Minutes 25000-1

0.5 Minutes 5000-1 65 D a y s

4000 H-

'-

1

Comb

3000

2000

1000 o , 0

,

,

,

,

20

40

60

80

100

0.5 Minutes F i g u r e 3. R e p r e s e n t a t i v e p l o t s o f c h e m i l u m i n e s c e n c e , i n c o u n t s p e r 30-second i n t e r v a l s , v s . time a t 150°C.

F i g u r e 4. 3 D - P l o t s o f c h e m i l u m i n e s c e n c e v s . time a t 150°C, v s . a g i n g time a t 2 5 ° C


26. MENDENHALL ET AL.



379


380

the time o f i r r a d i a t i o n , and o t h e r w i s e e q u i v a l e n t samples showed more l i g h t w i t h h i g h e r r a d i a t i o n dose. The u n i r r a d i a t e d samples, as e x p e c t e d , showed l i t t l e change on s t a n d i n g . There was no pronounced d i f f e r e n c e whether o r not a sample was wrapped i n aluminum f o i l d u r i n g i r r a d i a t i o n , e x c e p t i n one c a s e , where the f o i l - w r a p p e d , sample dosed w i t h 5MR showed a chemiluminescence c u r v e e s s e n t i a l l y i d e n t i c a l t o the unwrapped sample w h i c h r e c e i v e d a dose o f 3MR. The s t a b i l i t y o r d e r deduced from the r e l a t i v e i n t e n s i t i e s i n the c h e m i l u m i n e s c e n c e c u r v e s a t 150°C ( F i g u r e s 2-6) i s comb >^ t h i o >> p h e n o l > n e a t , the former two showing l i t t l e d i f f e r e n c e between i r r a d i a t e d and unexposed samples. The o r d e r i n g i s i n good agreement w i t h the r a n k i n g d e r i v e d from the impact s t r e n g t h measurements ( T a b l e I and F i g u r e 7) o f the samples s t o r e d a t 60°C, Table I. Aging

Before

Gardner impact

strengths (in-lb) of

polypropylene

Time, days

Neat

Phenol

Thio

Comb

25

1 7 18 32

-22.5 11.4 4.6 2.0

22.0 23.0 14.0 3.5

22.6 25.4 7.8 2.0

22.2 25.0 21.8 19.8

60

3 7 18 32

3.0 2.0 2.0 2.0

10.3 5.0 2.0 3.7

23.8 20.2 21.0 20.6

21.5 24.2 19.4 19.4

26.2

27.8

31.0

26.6

T,°C

irradiation

but not a t 2 5 ° C When the l a t t e r d a t a became a v a i l a b l e , i t was h y p o t h e s i z e d t h a t b e t t e r agreement between c h e m i l u m i n e s c e n c e and impact d a t a might be o b t a i n e d i f the l i g h t e m i s s i o n a l s o c o u l d be measured a t ambient t e m p e r a t u r e . E x a m i n a t i o n o f the neat and i r r a d i a t e d (5MR) n e a t samples i n hand 110 days a f t e r i r r a d i a t i o n , w i t h t h e commercial a p p a r a t u s gave v a l u e s o f 0.4 ± 0.2 and 2.7 ± 0.3 c o u n t s / m i n u t e , r e s p e c t i v e l y , a t room temperature. A l t h o u g h the e m i s s i o n r a t e s were a t the t h r e s h h o l d o f d e t e c t a b i l i t y , the r e s u l t and h i g h p r e c i s i o n were s u f f i c i e n t l y e n c o u r a g i n g t h a t a second b a t c h o f p o l y p r o p y l e n e samples was i r r a d i a t e d , and the c h e m i l u m i n e s c e n c e was measured r e p e a t e d l y from the same samples d u r i n g ambient s t o r a g e f o r s e v e r a l weeks. The r e s u l t s a r e g i v e n i n T a b l e I I and F i g u r e 7, i n w h i c h the v a l u e s r e f l e c t the average o f ten 2-minute c o u n t i n g i n t e r v a l s . As we had hoped, t h i s a p p r o a c h r e s t o r e d a q u a l i t a t i v e agreement between the two methods. The r a n k i n g w i t h l i g h t e m i s s i o n a f t e r s i x days c o r r e s p o n d s t o the o r d e r from impact s t r e n g t h a f t e r about a month. The p l o t s o f chemilumi n e s c e n c e i n F i g u r e 7 i n c o r p o r a t e s i n g l e p o i n t s o b t a i n e d o v e r weeks of i n t e r m i t t a n t o b s e r v a t i o n and do not resemble t h e ones o b t a i n e d d u r i n g much s h o r t e r e x a m i n a t i o n times a t 150°C.


26. MENDENHALL ET AL.


381

Table I I . Chemiluminescence ( c o u n t s / 2 m i n . ) a t ambient t e m p e r a t u r e from p o l y p r o p y l e n e i r r a d i a t e d a t day 0 w i t h a dose o f 5MR. Day

Neat

6 7 11 12 14 16 21

11.7 10.8 5.8 4.0 2.8 2.0 2.2

± ± ± ± ± ± ±

Phenol 2.0 3.8 3.0 0.7 1.9 1.2 0.8

8.7 6.2 1.5 1.2 0.6 0.8 0.6

± ± ± ± ± ± ±

1.4 3.8 0.5 1.3 0.5 0.8 0.5

Comb

Thio 11.8 11.2 5.3 4.4 4.6 4.0 4.0

± ± ± ± ± ± ±

1.2 1.6 0.5 0.6 0.5 0.4 1.4

1.2 1.7 1.7 1.0 0.5 0.5 0.5

± ± ± ± ± ± ±

0.8 1.3 0.6 1.0 0.5 0.5 0.5

Discussion At f i r s t g l a n c e , the d a t a seem a l i t t l e i n c o n s i s t e n t because the c h e m i l u m i n e s c e n c e was measured (at 25°C o r 150°C) from samples w h i c h had s t o o d a t ambient t e m p e r a t u r e a f t e r i r r a d i a t i o n i n each case. A p p a r e n t l y the i r r a d i a t i o n and a m b i e n t - t e m p e r a t u r e s t o r a g e l e f t the t h i o e t h e r e s t e r e s s e n t i a l l y unchanged, so t h a t upon h e a t i n g to 150°C a f t e r v a r i o u s s t o r a g e p e r i o d s , n e a r l y the f u l l complement o f a n t i o x i d a n t became a v a i l a b l e to r e t a r d a u t o x i d a t i o n . The r e s u l t i n g chemiluminescence data c o r r e l a t e t h e r e f o r e w i t h aging r e s u l t s at 6 0 ° C , a t w h i c h t e m p e r a t u r e the t h i o e t h e r e s t e r i s an a c t i v e a n t i o x idant . The r e v e r s a l i n the o r d e r o f e f f e c t i v e n e s s o f the s t a b i l i z e r s toward impact s t r e n g t h on a g i n g at 60°C v s 25°C i s u n u s u a l because the temperature d i f f e r e n c e i s a r e l a t i v e l y modest o n e . The s t a b i l i z i n g e f f e c t s and s y n e r g i s m o f t h i o compounds w i t h p h e n o l s a t h i g h t e m p e r a t u r e s has been i n v e s t i g a t e d p r e v i o u s l y i n some d e t a i l ( 4 - 6 ) . The i n i t i a l , p r o g r e s s i v e d e c l i n e o f c h e m i l u m i n e s c e n c e a t ambient t e m p e r a t u r e ( T a b l e I I and F i g u r e 7) i s c o n s i s t e n t w i t h a d e c r e a s e i n i n i t i a t i n g s p e c i e s , such as t r a p p e d r a d i c a l s o r l a b i l e p e r o x i d e s , w h i c h a r e p r o d u c e d i n the i r r a d i a t i o n p r o c e s s i n amounts above the s t e a d y - s t a t e c o n c e n t r a t i o n s . The l i g h t e m i s s i o n from the "thio sample exceeded t h a t from the " n e a t " sample a f t e r two weeks, suggesting a s u l f i d e prooxidant e f f e c t (7). The s h o r t b u r s t o f l i g h t i n the t h i o e t h e r - s t a b i l i z e d runs a t 150°C ( F i g u r e 6) may s i m i l a r l y a r i s e from i n d u c e d d e c o m p o s i t i o n o f p e r o x i d i c s p e c i e s by the t h i o e t h e r to produce r a d i c a l s , even though r e a c t i o n s o f p e r o x i d i c compounds w i t h s u l f i d e s a r e g e n e r a l l y thought t o be predomi n e n t l y n o n - r a d i c a l i n nature (8). Any d e t a i l e d m e c h a n i s t i c i n t e r p r e t a t i o n o f the d a t a i s r a t h e r l i m i t e d by the p r e s e n c e o f p r o c e s s i n g s t a b i l i z e r i n a l l s a m p l e s , and by the unknown d e t a i l s o f the l i g h t - e m i t t i n g r e a c t i o n s . I n h i b i t o r s reduce o x i d a t i v e c h e m i l u m i n e s c e n c e by r e d u c i n g the r a t e o f l i g h t - p r o d u c i n g p e r o x y l s e l f - t e r m i n a t i o n s (Scheme 1 ) , by q u e n c h i n g e l e c t r o n i c a l l y e x c i t e d s t a t e s (9) o r s i m p l y by a b s o r p t i o n of emitted l i g h t . The second f a c t o r may be much l e s s i m p o r t a n t i n p o l y m e r s because o f lowered r a t e s d i f f u s i o n ( 1 0 ) . The r a t e o f d i f f u s i o n o f 2 , 4 - d i h y d r o x y b e n z o p h e n o n e i n p o l y p r o p y l e n e has been -11 2 -1 measured (11) as 5.5 x 10 cm s at 4 4 ° C . I f we assume t h a t 1 1


P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

0

10

20

30

40

Days After Irradiation

0

10

20

30

40

Days After Irradiation

5MR 25°C

X

\

\\

Thio

PhC e no omNb \ g

0

10

20

30

40

Days After Irradiation F i g u r e 7. Top and M i d d l e : L o s s o f impact s t r e n g t h o f p o l y p r o p y l e n e , i r r a d i a t e d t o 5MR, v s . s t o r a g e time a t 25 and 60°C. Bottom: Ambient chemiluminescence from i r r a d i a t e d (5MR) p o l y p r o p y l e n e samples v s . s t o r a g e time a t 25°C.


26.

383


MENDENHALL ET AL.

the c o n s t a n t f o r t h e s t a b i l i z e r s i n our systems i s o f s i m i l a r o r d e r of magnitude, we can c a l c u l a t e from the Smoluchowski e q u a t i o n (12,13) the c o r r e s p o n d i n g d i f f u s i o n - l i m i t e d r a t e c o n s t a n t f o r 4 -1 -1 s e l f - e n c o u n t e r , k,_,-- = 4 x 10 M s . For a second-order dirt quenching

p r o c e s s k^ = k ^ f f [P*][Q]» where [Q] i s the quencher,

can then v e r y r o u g h l y e s t i m a t e the pseudo f i r s t - o r d e r [Q] from the i n i t i a l and 0.02

c o n c e n t r a t i o n s o f our a d d i t i v e s . 4 4 1 * 3 x 10 = 6 s and

t h i o e t h e r these are 2 x 1 0 M =400 s

i f we

we

term k ^ f f For

phenol 4 *

2 x 10

c o r r e c t the above d i f f u s i o n - l i m i t e d

rate

c o n s t a n t f o r d i f f e r e n c e i n approximate m o l e c u l a r d i a m e t e r s . For m o l e c u l a r oxygen i n p o l y p r o p y l e n e we s i m i l a r l y c a l c u l a t e k 7 -1 -1 dirt (25°C) = 8 x 1 0 M s starting with a recently published e q u a t i o n ( 1 3 ) , and f o r a i r - s a t u r a t e d p o l y p r o p y l e n e , we e s t i m a t e f

=

s

T

n

e

s

f

e

^ d i f f ^2^ e s t i m a t e d , maximum r a t e s do n o t appear t o be c o m p e t i t i v e w i t h the e x c i t e d s t a t e l i f e t i m e s o f most s i m p l e a l i p h a t i c c a r b o n y l s , and the e s t i m a t e s a l s o suggest t h a t quenching by oxygen would predominate even i f t h e y were. Singlet s t a t e l i f e t i m e s a r e r a r e l y l o n g e r than a few nanoseconds, while,. a l i p h a t i c t r i p l e t c a r b o n y l decay c o n s t a n t s a r e u s u a l l y about 10 s ( 1 5 ) . The t r i p l e t l i f e t i m e o f e x c i t e d c a r b o n y l s i n s o l i d _g e t h y l e n e - c a r b o n monoxide copolymer has been e s t i m a t e d as o n l y 10 s ( 1 6 ) . R e c e n t l y a l o w e r i n g o f quenching r a t e s toward e x c i t e d s t a t e s i n d i s s o l v e d macromolecules has a l s o been demonstrated ( 1 7 ) . The r o l e o f p h y s i c a l quenching o f e x c i t e d s t a t e s i n polymer p h o t o o x i d a t i o n i s a c l o s e l y r e l a t e d q u e s t i o n . A l t h o u g h we acknowledge t h a t many d e t a i l s such as sample homogeneity a r e n o t known i n our system, W i l e s and C a r l s s o n c o n c l u d e d i n an e a r l i e r s t u d y t h a t p h y s i c a l quenching by p h o t o p r o t e c t i v e a g e n t s i n commercial ( s o l i d ) polymers was o f l e s s e r importance than o t h e r modes o f p r o t e c t i o n ( 1 8 ) . To the e x t e n t t h a t t h i s g e n e r a l i z a t i o n i s t r u e , the c h e m i l u m i n e s c e n c e i n t e n s i t i e s i n s o l i d s w i l l n o t i n g e n e r a l be reduced by p h y s i c a l q u e n c h i n g . The above c o n s i d e r a t i o n s b e a r on the ambient c h e m i l u m i n e s c e n c e from p o l y p r o p y l e n e , a l t h o u g h a t 150°C one would s t i l l e x p e c t t h a t the f l u o r e s c e n c e e m i s s i o n from s i n g l e t c a r b o n y l s would be r e l a t i v e l y u n a f f e c t e d by quenching from s t a b i l i z e r s i n our samples. One can a l s o i n f e r t h a t p h y s i c a l quenching a l o n e i s not s u f f i c i e n t t o e x p l a i n the d e c r e a s e i n l i g h t from the p o l y p r o p y l e n e sample w i t h b o t h p h e n o l and t h i o e t h e r ( T a b l e I I , comb), s i n c e t h e i n i t i a l r e d u c t i o n i n i n t e n s i t y i s more than can be a c c o u n t e d f o r by the combined e f f e c t s o f the a d d i t i v e s s e p a r a t e l y . The d i f f e r e n c e s i n c h e m i l u m i n e s c e n c e i n t e n s i t i e s o f the samples a t 25°C a r e d i s c e r n i b l e a t a much e a r l i e r p o i n t than the d i f f e r e n c e s i n impact s t r e n g t h s . T h i s o b s e r v a t i o n i s r e a s o n a b l e , s i n c e the former t e c h n i q u e measures a dynamic p r o c e s s a t the m o l e c u l a r l e v e l , w h i l e the l a t t e r responds t o the c u m u l a t i v e damage to the sample up t o the time o f t e s t i n g . The d a t a a r e not c o m p l e t e l y i n a c c o r d w i t h t h i s g e n e r a l i z a t i o n , s i n c e the l i g h t



384

emission from the neat and thio samples (Figure 7) i s identical up to about two weeks after irradiation, whereas the impact strengths (Figure 6) of these two samples are quite different after only one week. Although the comparison involved separate batches of irradiated polymers, the impact strengths showed good reproducibility in routine studies. Conclusions 1. Chemiluminescence at ambient temperature and at 150°C of several irradiated (electron-beam) polypropylene formulations can be qualitatively correlated with loss of impact strength. The correlation with chemiluminescence i s preserved at high vs. low temperatures, even though the ranking changes. 2. The thioether ester (DLTDP) shows a very strong synergistic effect with the phenolic stabilizer at room temperature, and i s active alone at 60°C for protection from loss of impact strength of the irradiated polypropylene. 3. The thioether alone at room temperature does not protect polypropylene from loss of physical properties after electron-beam treatment, although irradiated samples that are subsequently heated to 150°C are apparently protected from thermal oxidation. 4. Fiber-optics is a convenient technique to isolate a heated sample from cooled photon-detecting equipment. 5. The differences of the relative efficacy of the stabilizers at 25°C vs 60°C emphasizes the importance of monitoring changes in physical properties after irradiation under use conditions. 6. Irradiation of samples of polypropylene sealed in aluminum f o i l makes l i t t l e difference in their subsequent properties compared with unsealed samples under the same conditions. Although the number of samples was rather limited, and the results tend to raise rather than settle mechanistic questions, the study indicates the possible u t i l i t y of chemiluminescence for nondestructive evaluation of radiation-treated polyolefins. Acknowledgement s The authors are grateful to Himont, Inc. for a fellowship to H.K.A., support from 3M Co. for hardware and software development, and to Mr. George Turner of Turner Designs, Inc., for loan of a Luminometer. The assistance of Mr. Burt Blanch with the Gardner impact tests is gratefully acknowledged. Literature Cited 1. 2. 3.

George, G.A., Develop. Poly. Degrad., 1981, 3, 173 and references therein. Flaherty, K.F., M.S. Thesis, Michigan Technological University, Houghton, Michigan, 1982. Ogle, CA.; Martin, S.W.; Dziobak, M.P.; Urban, M.P.; Mendenhall, G.D., J. Org. Chem., 1983, 48, 3728.


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4.

5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.


De Paolo P.A.; Smith, N.P.; In "Stabilization of Polymers and Stabilizers Processes"; Gould, R.F., Ed.; ADVANCES IN CHEMINSTRY SERIES No. 85, American Chemical Society: Washington, D.C., 1968, p. 202. Ray, W.C.; Isenhart, K., Poly. Sci. and Eng., 1975, 15, 703 de Jonge, C.R.H.I.; Giezen, E.A.; van der Maeden, F.P.B.; Huysmans, W.G.B.; de Klein, W.J., Mijs, W.J. In "Stabilization and Degradation of Polymers"; Allara, D.L.; Hawkins, W.L., Eds.; ADVANCES IN CHEMISTRY SERIES No. 169, American Chemical Society: Washington, D.C., 1978; p. 399. Scott, G., Communication at the time this paper was presented Swern, D., "Organic Peroxides"; Wiley-Interscience: New York, 1971; Vol. II, p.73 and references therein. Vasil'ev, R.F., Prog. React. Kin., 1967, 4, 305. Pratte, J.F.; Webber, S.E., Macromolecules, 1983, 16, 1188. Westlake J.F.; Johnson, M., J. Appl. Poly. Sci., 1975, 19, 319. Smoluchowski, M.V., Z. Physik, 1916, 17, 557. Frost, A.A.; Pearson, R.G., "Kinetics and Mechanism"; John Wiley and Sons: New York, 1961; 2nd Ed., p. 271. Kiryushkin, S.G.; Gromov, B.A., Vysokomol. Soedin., Ser. A., 1972, 14, 1746. Wilson, T.; Halpern, A.M., J. Amer. Chem. Soc., 1980, 102, 7279. Heskins, M.; Guillet, J.E., Macromolecules, 1970, 31, 3224. Scaiano, J.C.; L i s s i , E.A.; Stewart, L.C.; J. Amer. Chem. Soc., 1984, 106, 1539. Wiles, D.M.; Carlsson, D.J., Poly. Degr. and Stab. 1980-81, 3, 61.

RECEIVED December 7, 1984


Comparison of Chemiluminescence with Impact Strength for

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