Investigation of Thermal Oxidation and ... - ACS Publications

17 Investigation of Thermal Oxidation and Stabilization of High-Density Polyethylene

Downloaded by NORTHWESTERN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch017

PAUL-LI H O R N G and P E T E R P. K L E M C H U K Additives Department, Plastics and Additives Division, CIBA-GEIGY Corporation, Ardsley, NY 10502

A commercial HDPE, unstabilized or stabilized with a hindered phenolic antioxidant, was subjected to thermal oxidation at 140°, 100° and 40°C by oxygen uptake. The oxidative induction times of the unstabilized samples were found to f i t into a linear apparent Arrhenius relationship. The calculated activation energy for thermooxidative degradation of the HDPE agrees with literature data. Ultimate elongation, carbonyl formation and molecular weight distribution were found to change little before the induction time was reached. The degree of chain breaking, calculated from molecular weight data, shows an average of about one scission per molecule caused the polymer to lose its elongation property totally. Stabilization provided by a phenolic antioxidant demonstrated a relatively long induction time; e.g., 4700 versus 35 hours at 100°C. Within the induction time, chain scissioning and elongation were nearly unaffected. After the induction time, chain scissioning became uninhibited and was manifested by loss of elongation. Mechanisms of chain scissioning and stabilization are discussed. Polyolefins are sensitive to heat- and light-induced oxidative degradation. Studies in the past on thermal oxidative stability of high density polyethylene (HDPE) have generated information on how HDPE is oxidized under thermal stress (1-4). Alkyl and peroxy radicals, hydroperoxides, beta-scission after hydroperoxide decomposition to carbonyl and an alkyl radical end group are recognized as the major elements in the general oxidation pathway. Stabilization through interruption of the degradation cycle has resulted in the development of effective stabilizer systems for the many uses of this polymer. Recently we have been studying both the molecular weight changes and the physical property changes in HDPE as a function of oxidation. Unstabilized and stabilized HDPE were evaluated by oxygen uptake at 0097-6156/85/0280-0235$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

236

140°, 100° and 40°C. T h i s paper p r e s e n t s t h e r e s u l t s o f our f i n d i n g s which p r o v i d e some i n s i g h t s i n t o t h e r e l a t i o n s h i p s among degree o f o x i d a t i o n , m o l e c u l a r weight changes, p h y s i c a l p r o p e r t y r e t e n t i o n , and s t a b i l i z a t i o n o f t h e polymer.


Experimental The polymer used was A l a t h o n 5496, DuPont HDPE. The s t a b i l i z e r used was tetrakis[methylene-(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane, r e f e r r e d t o as A01. Powdered samples o f HDPE, s t a b i l i z e d and u n s t a b i l i z e d , were p r e p a r e d and s u b j e c t e d t o o x i d a t i o n i n a c l o s e d system w i t h oxygen. Oxygen uptake was m o n i t o r e d p e r i o d i c a l l y at g i v e n t e m p e r a t u r e s . The i n d u c t i o n p e r i o d was p i c k e d from t h e curve where t h e onset o f a u t o c a t a l y t i c o x i d a t i o n took p l a c e . E l o n g a t i o n d a t a o f 5-mil f i l m s were g e n e r a t e d on an I n s t r o n T e n s i l e T e s t e r a c c o r d i n g t o ASTM D882 a t a p u l l i n g r a t e o f 50mm/min. M o l e c u l a r weights o f polymer samples were determined by h i g h temperat u r e g e l p e r m e a t i o n chromatography c a l i b r a t e d w i t h p o l y e t h y l e n e s t a n d a r d s , except t h e r e s u l t s from 40°C were done u s i n g t h e Q f a c t o r from polystyrene standards. C h a i n s c i s s i o n was c a l c u l a t e d as [Mn(unoxidized)/Mn(oxidized)]-l. C a r b o n y l absorbances were determined by I n f r a r e d S p e c t r o s c o p y a t 1710 cm" . 1

R e s u l t s and D i s c u s s i o n O x i d a t i o n Curves a t 140°C, 100°C and 40°C. Samples o f HDPE, u n s t a b i l i z e d o r s t a b i l i z e d w i t h 0.1% o f A01, were o x i d i z e d i n a c l o s e d system w i t h oxygen. The o x i d a t i o n curves a t 1 4 0 ° , 100° and 40°C a r e shown i n F i g u r e s 1-3. These d a t a i n d i c a t e t h e e f f e c t i v e n e s s o f A01 i n p r e v e n t i n g oxygen consumption a t b o t h h i g h and low t e m p e r a t u r e s . Once t h e i n d u c t i o n p e r i o d was passed a t 140°C and 100°C, t h e oxygen consumption r a t e s were v i r t u a l l y t h e same f o r the u n s t a b i l i z e d and s t a b i l i z e d samples. The u n s t a b i l i z e d HDPE consumed oxygen a t s i g n i f i c a n t r a t e s , even a t 40°C, w i t h the i n d u c t i o n p e r i o d l a s t i n g about two y e a r s . The need t o s t a b i l i z e polymers f o r use a t a l l temperat u r e s i s e v i d e n t from t h e s e d a t a . C o r r e l a t i o n o f I n d u c t i o n Times a t 1 4 0 ° , 100° and 40°C. The i n d u c t i o n times o b t a i n e d f o r t h e u n s t a b i l i z e d HDPE a t t h r e e temperatures a r e t a b u l a t e d i n T a b l e I and a l s o p l o t t e d a g a i n s t r e c i p r o c a l a b s o l u t e temperature i n an apparent A r r h e n i u s r e l a t i o n s h i p i n F i g u r e 4.

Table

I.

Stabilizer None 0.1% A01 *95

O x i d a t i o n o f HDPE by Oxygen Uptake. I n f l u e n c e o f Temperature on O x i d a t i v e I n d u c t i o n Time.

I n d u c t i o n Time a t Temperature, Hours 100°C 40°C 140°C 35 15,960* 1 4,700 39,480+** 120

weeks; **235+ weeks (no i n d i c a t i o n o f

degradation)

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

17.

E


High-Density Polyethylene

HORNG AND KLEMCHUK

0.1% A 0 1

1 0 - 1 Unstabilized H D P E

D

9 E

I 50

- I — 150

100

- 1 — 200

Hours

F i g u r e 1. Oxygen Uptake o f S t a b i l i z e d and U n s t a b i l i z e d 140°C and 1 Atmosphere

a>

8 -

HDPE a t

0.1% A 0 1

Unstabilized H D P E

Q.

E

CO co

o)

6—

=> 49 E 2-

1000

2000

3000 Hours

F i g u r e 2. Oxygen Uptake o f S t a b i l i z e d and U n s t a b i l i z e d 100°C and 1 Atmosphere

HDPE a t




238

10yrs -105.

1/T (°K), Shown as Degrees Centigrade

F i g u r e 4. A r r h e n i u s P l o t o f S t a b i l i z e d and U n s t a b i l i z e d HDPE D u r i n g O x i d a t i o n , Oxygen Uptake a t 1 Atmosphere and Three Temperatures


17.

HORNG AND KLEMCHUK

239



It i s i n t e r e s t i n g to note that they f a l l i n t o a l i n e a r r e l a t i o n s h i p w i t h a c a l c u l a t e d a c t i v a t i o n energy o f 23.7 K c a l / m o l e . Literature d a t a showed v a l u e s o f 21-26 K c a l / m o l e . (5,6) T h i s i s one o f t h e few i n s t a n c e s t h a t o x i d a t i v e i n d u c t i o n times from 140°C t o near ambient temperature have been a v a i l a b l e . N o t a b l y , t h e temperature range i n c l u d e s t h e m e l t i n g p o i n t o f t h i s HDPE polymer so t h e range from 140°C to 40°C i n c l u d e s a phase change. In t h i s i n s t a n c e , we d i d not f i n d a discrepancy i n p l o t t i n g above-melting experimental r e s u l t s along with those o b t a i n e d a t much lower t e m p e r a t u r e s . Our f i n d i n g on t h e l i n e a r i t y o f the A r r h e n i u s r e l a t i o n s h i p s u g g e s t s t h a t e x t r a p o l a t i o n o v e r a moderate temperature range i s warranted a t l e a s t f o r u n s t a b i l i z e d HDPE, p r o v i d e d s e v e r a l d a t a p o i n t s a r e a v a i l a b l e . P r o p e r t y C o r r e l a t i o n D u r i n g O x i d a t i o n a t 100°C and 40°C. A s e r i e s of u n s t a b i l i z e d and 0.1% A O l - s t a b i l i z e d HDPE samples were o x i d i z e d at 100°C and removed p e r i o d i c a l l y f o r e v a l u a t i o n . A monotonic change i n the r e t e n t i o n o f e l o n g a t i o n i n r e l a t i o n t o oxygen uptake was found as i n F i g u r e 5. C a r b o n y l f o r m a t i o n d u r i n g o x i d a t i o n was a l s o found t o c o r r e l a t e w i t h oxygen uptake i n a l i n e a r r e l a t i o n s h i p . 50% r e t e n t i o n o f e l o n g a t i o n was found t o c o r r e l a t e w i t h 0.25 c a r b o n y l absorbance o f 5-mil f i l m and z e r o e l o n g a t i o n was found t o c o r r e s p o n d t o 0.5 c a r b o n y l absorbance. O x i d a t i o n t o 6.5 ml-0 /g-HDPE o r 2.1 mmole Oj mmole HDPE d e s t r o y e d e l o n g a t i o n c o m p l e t e l y f o r b o t h u n s t a b i l i z e d and s t a b i l i z e d HDPE. However, t h e time i n v o l v e d f o r such a c a t a s t r o p h i c change was d r a m a t i c a l l y p r o l o n g e d from t h e u n s t a b i l i z e d t o A O l s t a b i l i z e d HDPE; 75 v s . 4700 hours ( F i g u r e 6 ) . At 40°C, o x i d a t i o n took p l a c e a t a r e l a t i v e l y low, but measurable rate. Samples s t a b i l i z e d w i t h 0.1% A01 showed no l o s s i n e l o n g a t i o n r e t e n t i o n i n o v e r f o u r y e a r s as compared t o t h e u n s t a b i l i z e d sample which showed a c a t a s t r o p h i c d e c r e a s e i n e l o n g a t i o n a f t e r t h e 95-week i n d u c t i o n p e r i o d ( T a b l e I I ) .

Table I I .

Change i n E l o n g a t i o n o f HDPE O x i d i z e d a t 40°C

Sample Unstabilized

Stabilized 0.1% A01

with

Time (wks) 0 105 115 120

Oxygen Uptake (ml/g) 0 0.25 0.53 1.0

0 105 150 235

0 0.10 0.12 0.1

Elongation (%) 750 200 165 55 720 690 800 650

The m o l e c u l a r weight d i s t r i b u t i o n change o f t h e u n s t a b i l i z e d HDPE was a l s o m o n i t o r e d and i s p l o t t e d i n F i g u r e 7. At 105 weeks, oxygen consumption o f t h e u n s t a b i l i z e d HDPE was about 0.25 ml/g and t h e sample showed a s l i g h t r e d u c t i o n i n t h e h i g h m o l e c u l a r weight f r a c tion. C o n t i n u i n g o x i d a t i o n showed e v i d e n c e o f more c h a i n s c i s s i o n i n g and l o w e r i n g o f average m o l e c u l a r w e i g h t s . Again, A O l - s t a b i l i z e d



POLYMER

STABILIZATION A N D

DEGRADATION

ml-0 /g-HDPE 2

F i g u r e 5. C o r r e l a t i o n o f P e r c e n t R e t e n t i o n o f E l o n g a t i o n and C a r b o n y l Absorbance t o t h e Degree o f O x i d a t i o n , Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 100°C and 1 Atmosphere

F i g u r e 6. Comparison o f E l o n g a t i o n R e t e n t i o n o f S t a b i l i z e d and U n s t a b i l i z e d HDPE, Oxygen Uptake a t 100°C and 1 Atmosphere


17.


HORNG AND KLEMCHUK

241

HDPE a f t e r f o u r y e a r s a t t h i s temperature e x h i b i t e d no change i n b o t h e l o n g a t i o n r e t e n t i o n and m o l e c u l a r weight r e t e n t i o n . C h a i n S c i s s i o n i n g and i t s E f f e c t on M o l e c u l a r Weight. Table I I I shows t h e m o l e c u l a r weight d a t a o f t h e u n s t a b i l i z e d HDPE when o x i d i z e d a t 100°C t o d i f f e r e n t oxygen uptake l e v e l s .

Table I I I .

M o l e c u l a r Weight Data and C h a i n S c i s s i o n o f U n s t a b i l i z e d HDPE O x i d i z e d a t 100°C

Oxygen Uptake (ml-0 / -HDPE)


2

Mw

g

0 0.12 2.3 3.8 9.2 11.1 19.1 25.7 S = Average

151,000 137,000 29,100 15,900 13,800 10,400 8,460 7,080

Mn 8,270 6,450 5,610 4,620 3,700 3,200 2,670 2,390

Mw/Mn 18.1 21.1 5.2 3.4 3.7 3.2 3.2 3.0

S 0 0.3 0.5 0.8 1.2 1.6 2.1 2.5

Scisssions p e r M o l e c u l e = [Mn(0)/Mn(t)] -1

Weight average m o l e c u l a r weight (Mw) was found t o be more p r o n o u n c e d l y a f f e c t e d a t t h e e a r l y s t a g e o f o x i d a t i o n , w h i l e number average m o l e c u l a r weight (Mn) was a f f e c t e d t o a l e s s e r degree. Based on t h e a v a i l a b l e d a t a , t h e g r e a t e s t change i n m o l e c u l a r weight took p l a c e between 0.12 and 2.3 ml 0 /g HDPE oxygen u p t a k e . In t h a t i n t e r v a l , Mw was reduced 80% bu? Mn o n l y about 15%. C l o s e r examinat i o n o f t h e m o l e c u l a r weight d i s t r i b u t i o n (MWD) c u r v e s ( F i g u r e 8) i n d i c a t e s t h i s was t h e i n t e r v a l where t h e l o s s o f t h e h i g h m o l e c u l a r weight f r a c t i o n was g r e a t e s t and so was t h e f o r m a t i o n o f lower m o l e c u l a r weight s p e c i e s . S t a t i s t i c a l l y speaking, longer chains have g r e a t e r p r o b a b i l i t y f o r o x i d a t i v e a t t a c k and c h a i n r u p t u r e t h a n do s h o r t e r c h a i n s . The m a t h e m a t i c a l moment o f Mw r e f l e c t s a h e a v i e r c o n t r i b u t i o n from h i g h e r m o l e c u l a r weight s p e c i e s t h a n Mn. Theref o r e , t h e s t a t i s t i c a l l y g r e a t e r s c i s s i o n i n g o f l o n g e r c h a i n s had a g r e a t e r impact on changes i n Mw t h a n on Mn. F i g u r e 8 and T a b l e I f u r t h e r i n d i c a t e when samples were o x i d i z e d from 0.12 ml 0^/g HDPE (35 h o u r s ) t o 2.3 ml 0 / HDPE oxygen consumption (40 h o u r s ) , t h e y had passed t h r o u g h t h e i n d u c t i o n p e r i o d and e n t e r e d i n t o t h e a u t o c a t a l y t i c region of oxidation. An e x a m i n a t i o n o f t h e s e q u e n t i a l change o f t h e MWD c u r v e s o f b o t h u n s t a b i l i z e d and s t a b i l i z e d samples ( F i g u r e s 8 and 9) shows o x i d a t i o n up t o about 0.1 ml-0 /g-HDPE had l i t t l e e f f e c t on average m o l e c u l a r weight as compared t o t h e o r i g i n a l . However, i t took about 4300 hours f o r t h e s t a b i l i z e d sample t o r e a c h t h a t p o i n t as opposed t o m e r e l y 35 hours f o r t h e u n s t a b i l i z e d sample. When o x i d a t i o n c o n t i n u e d p a s t t h e i n d u c t i o n p e r i o d , a s i g n i f i c a n t change i n m o l e c u l a r weight became e v i d e n t as a r e s u l t o f c h a i n s c i s s i o n i n g . O x i d a t i o n t h e r e a f t e r was a u t o c a t a l y t i c , and t h e m o l e c u l a r weight r e d u c t i o n was c a t a s t r o p h i c w i t h i n a s h o r t p e r i o d o f t i m e . g



Degree of Oxidation ml-Q

•

'

•


oooooo

2X10

6

10

10

6

10

5

2

g-HDPE

Wks

S

0

0

0

o.25

105

0.1

10

4

1 80

3

Molecular Weight

F i g u r e 7. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 40°C and 1 Atmosphere

Degree of Oxidation ml - Q

2x10

6

10

6

10

5

10

4

2

g-HDPE

Hrs

S

0 0.12 2.3 9.2 25.7

0 35 40 60 70

0 0.3 0.5 1.2 2.5

10

3

1 80

Molecular Weight

F i g u r e 8. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 100°C and 1 Atmosphere


17.

HORNG AND

KLEMCHUK

243



Average c h a i n s c i s s i o n s p e r m o l e c u l e as c a l c u l a t e d from Mn changes a r e p l o t t e d a g a i n s t oxygen uptake i n F i g u r e 10. T h i s g r a p h demonstrates by l i n e a r r e g r e s s i o n t h a t t h e c h a i n s c i s s i o n r a t e o f HDPE was s i m i l a r f o r b o t h t h e s t a b i l i z e d and u n s t a b i l i z e d samples when compared by degree o f o x i d a t i o n i n terms o f oxygen u p t a k e . A g a i n , t h e s t a b i l i z e d samples showed a l o n g i n d u c t i o n p e r i o d d u r i n g which c h a i n s c i s s i o n i n g was q u i t e i n s i g n i f i c a n t . A f t e r the induction p e r i o d , c h a i n s c i s s i o n i n g c o n t i n u e s w i t h i n c r e a s i n g oxygen consumption i n a linear fashion. The e m b r i t t l e m e n t p o i n t , z e r o e l o n g a t i o n , was found t o c o i n c i d e w i t h o n l y 1 and 1.2 average s c i s s i o n s p e r molec u l e f o r s t a b i l i z e d and u n s t a b i l i z e d HDPE, r e s p e c t i v e l y . C h a i n S c i s s i o n i n g and i t s E f f e c t on E l o n g a t i o n . Since a l i n e a r r e l a t i o n s h i p was found between oxygen uptake and c h a i n s c i s s i o n r a t e ( F i g u r e 10) and a l s o between oxygen uptake and r e t e n t i o n o f e l o n g a t i o n ( F i g u r e 5 ) , i t i s o b v i o u s t h a t the c h a i n s c i s s i o n r a t e and r e t e n t i o n o f e l o n g a t i o n can enter i n t o another f i r s t order r e l a t i o n ship. C h a i n s c i s s i o n i n g e f f e c t e d not o n l y m o l e c u l a r w e i g h t , but a l s o m e c h a n i c a l p r o p e r t i e s , such as e l o n g a t i o n : a f t e r the i n d u c t i o n peri o d , e l o n g a t i o n d e c r e a s e d r a p i d l y w i t h b o t h time and c h a i n s c i s s i o n ing (Figure 6 ) . O x i d a t i o n o f s e m i c r y s t a l l i n e polymers i s g e n e r a l l y c o n s i d e r e d t o o c c u r w i t h i n t h e amorphous r e g i o n which c a n be t r e a t e d as a boundary phase o f t h e n e i g h b o r i n g c r y s t a l l i n e r e g i o n s . P e t e r l i n ' s model (_7) o f t e n s i l e d e f o r m a t i o n e x p l a i n e d the c o n t r i b u t i o n o f t i e m o l e c u l e s i n the amorphous r e g i o n t o t h e n e c k i n g e l o n g a t i o n o f a s e m i - c r y s t a l l i n e polymer. S i n c e o x i d a t i o n t a k e s p l a c e m o s t l y i n amorphous r e g i o n s , t i e m o l e c u l e s which connect c r y s t a l l i t e s through amorphous r e g i o n s may be s c i s s i o n e d i n t h e o x i d a t i o n p r o c e s s r e s u l t i n g i n a d e c r e a s e o f e l o n g a t i o n and o t h e r p h y s i c a l p r o p e r t i e s . At l a t e r s t a g e s o f o x i d a t i o n when many c h a i n s i n t h e amorphous phase and a l s o a t t h e c r y s t a l l i n e boundary a r e r u p t u r e d , samples e x h i b i t b r i t t l e n e s s upon e x t e r n a l stress. Mechanism o f C h a i n S c i s s i o n i n g and S t a b i l i z a t i o n . A review o f the average number o f c h a i n s c i s s i o n s as a f u n c t i o n o f oxygen consumption shows t h e number o f oxygen m o l e c u l e s consumed p e r c h a i n s c i s s i o n i n c r e a s e d w i t h i n c r e a s i n g oxygen consumption. Beyond t h e i n d u c t i o n p e r i o d , t h e mmoles o f oxygen p e r c h a i n s c i s s i o n i n c r e a s e d r a p i d l y w i t h time; t h e d a t a a r e summarized i n T a b l e IV.

T a b l e IV.

Time (hours) 0 35 40 50 60 66 70

C a l c u l a t e d Oxygen M o l e c u l e s Consumed p e r C h a i n S c i s s i o n D u r i n g O x i d a t i o n o f U n s t a b i l i z e d HDPE a t 100°C Oxygen Uptake A B 0 0 0.12 0.04 2.3 0.7 3.8 1.2 9.2 3.1 19.1 6.3 25.7 8.5

S 0 0.3 0.5 0.8 1.2 2.1 2.5

Oxygen M o l e c u l e s Consumed per Chain S c i s s i o n 0.2 1.6 1.6 2.5 3.0 3.5

A = ml 0 /g HDPE; B = mmole 0 /mmole HDPE.


244


Degree of Oxidation ml-Q

2


g-HDPE

Hrs

S

Molecular Weight

F i g u r e 9. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f 0.1% A O l - S t a b i l i z e d HDPE a t 100°C and 1 Atmosphere

— stabilized with . 1 % A O 1 .--unstabilized

Oxygen Uptake, m l - 0 / g - H D P E 2

F i g u r e 10. Comparison o f C h a i n S c i s s i o n Rates i n S t a b i l i z e d and U n s t a b i l i z e d HDPE, O x i d a t i o n by Oxygen Uptake a t 100°C and 1 Atmosphere


17. HORNG AND KLEMCHUK


245


T h i s r e s u l t i s i n agreement w i t h those r e p o r t e d by M. I r i n g , e t a l (2) f o r p o l y p r o p y l e n e and p o l y e t h y l e n e . The oxygen consumption d a t a i n d i c a t e t h e o x i d a t i o n o f p o l y e t h y l e n e c o n s i s t s o f a complex group o f r e a c t i o n s beyond t h e i n d u c t i o n p e r i o d w i t h no s i n g l e , simple r e l a t i o n s h i p between t h e number o f oxygen m o l e c u l e s consumed and t h e number o f c h a i n s c i s s i o n s . Using t h e d a t a o f C h i e n (8), we c a l c u l a t e d t h e h a l f - l i f e o f p o l y e t h y l e n e h y d r o p e r o x i d e t o be 6.4 hours a t 100°C. Thus, s i n c e our s h o r t e s t i n d u c t i o n p e r i o d a t 100°C was over 35 h o u r s , i t i s r e a s o n a b l e t o p o s t u l a t e t h e s c i s s i o n i n g o f p o l y e t h y l e n e r e s u l t s from unimolecular homolytic d i s s o c i a t i o n of polyethylene hydroperoxide, a major o x i d a t i o n p r o d u c t : #

ROOH »

» R 0 + HO* > R'CHO + R"

R 0

#

Chain s c i s s i o n i n g o f p o l y e t h y l e n e v i a t h e a l k o x y r a d i c a l would y i e l d a t e r m i n a l aldehyde and s h o r t e r - c h a i n p o l y e t h y l e n e f r e e r a d i c a l . Both p r o d u c t s w i l l r e a c t w i t h oxygen. The aldehyde s h o u l d be o x i d i z e d r e a d i l y to the p e r a c i d v i a a c h a i n r e a c t i o n : 0 R'CHO + R 0 > R'CO*---—> R'CO -----> R'CO + R* 00* OOH #

2

The p o l y e t h y l e n e r a d i c a l would be expected t o r e a c t r e a d i l y w i t h oxygen and c o n t r i b u t e t o o x i d a t i v e p r o p a g a t i o n as another peroxy radical: RH

R"

#

+ 0

2

> R»0

# 2

> R"0 H 2

+

R

#

Since t h i s r e s u l t a n t hydroperoxide i s a terminal hydroperoxide, i t s decomposition w i l l not r e s u l t i n c h a i n s c i s s i o n i n g . Assuming each r e a c t i o n t o be 100% e f f i c i e n t , i . e . , h y d r o p e r o x i d e decomposition t o a l k o x y l r a d i c a l ; a l k o x y l r a d i c a l c h a i n s c i s s i o n t o aldehyde and s h o r t e r p o l y e t h y l e n e r a d i c a l ; aldehyde o x i d a t i o n t o p e r a c i d ; and oxygen r e a c t i o n w i t h t h e p r i m a r y p o l y e t h y l e n e r a d i c a l t o y i e l d a peroxy r a d i c a l ; t h e maximum number o f oxygen m o l e c u l e s a s s o c i a t e d w i t h each c h a i n s c i s s i o n would be t h r e e . Not s u p r i s i n g l y , the o x i d a t i o n p r o c e s s i s t o o complex t o have shown a s i n g l e r e l a t i o n s h i p between oxygen m o l e c u l e s consumed and t h e number o f c h a i n scissions. A t t h e time o f e m b r i t t l e m e n t , about 2 m o l e c u l e s o f oxygen had been consumed p e r c h a i n s c i s s i o n and i n c r e a s e d beyond t h i s p o i n t . The i n c r e a s e i n t h e number o f oxygen m o l e c u l e s p e r c h a i n s c i s s i o n beyond t h e i n d u c t i o n p e r i o d i s n o t s u r p r i s i n g s i n c e i n t h e u l t i m a t e o x i d a t i o n o f p o l y e t h y l e n e t o c a r b o n d i o x i d e and w a t e r , 1.5 m o l e c u l e s o f oxygen would be r e q u i r e d f o r each methylene group. T h i s means a polymer w i t h an i n i t i a l number average m o l e c u l a r weight o f 8,270 would r e q u i r e 886 m o l e c u l e s o f oxygen f o r complete c o n v e r s i o n t o c a r b o n d i o x i d e and water. The a d d i t i o n o f t h e p h e n o l i c a n t i o x i d a n t A01 t o t h e polymer demonstrated a l o n g i n d u c t i o n p e r i o d ( T a b l e I ) d u r i n g which c h a i n s c i s s i o n i n g and p h y s i c a l p r o p e r t y changes were n e g l i g i b l e . The e q u a l l y complex s t a b i l i z a t i o n c h e m i s t r y (9) f o r p h e n o l i c a n t i o x i d a n t s can be summarized by t h e t r a p p i n g o f p e r o x y and a l k o x y r a d i c a l s and a l s o by t h e d e c o m p o s i t i o n o f h y d r o p e r o x i d e w i t h phenols and i t s



246

transformation products. Figure 7 at 40°C oxidation and Figure 9 at 100°C oxidation showed the MWD was almost unchanged during the induction period of the stabilized polymer. The oxidative chain reaction leading to chain scissioning as discussed above, is interrupted at the expense of A01 which is nearly consumed at the end of the induction period. Beyond the induction period, oxidation continues uninhibitedly similar to the unstabilized undergoing oxidation at the beginning. Conclusions


1.

During the induction period, the stabilized and unstabilized samples underwent little chain scissioning and loss of elongation. When the induction period was passed, both stabilized and unstabilized samples exhibited considerable chain scissioning and loss of elongation during this uninhibited period of oxidation. 2. The time difference in the induction periods for AOl-stabilized and unstabilized HDPE was phenomenal; 120 versus ~1 hour at 140°C and 4700 versus 35 hours at 100°C, respectively. 3. At 40°C the oxidation rate was slow but measurable for unstabilized HDPE. The stabilization provided by AOl for more than four years was evident from the oxygen uptake, elongation and molecular weight data. 4. The Arrhenius plot of induction period vs. temperatures ranging from 140°C to 40°C for unstabilized HDPE suggests extrapolation is permissable provided several data points are available. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Chan, M., Proc. Int. Wire Cable Symp. 23rd, 1974, pp. 34-41. Iring, M.; Laszlo-Hedvig, S.; Barabas, K.; Kelen, T.; Tudos, F., Eur. Polym. J., 1978, 14, 439-442. Holmstrom A., in "Durability of Macromolecular Materials"; Eby, R. K., Ed.; ACS Symp. Ser. No. 95, ACS: Washington, D.C., 1979; Chap. 4. Holmstrom, A.; Sorvik, E. M., J. Polym. Sci. Polym. Chem. Ed., 1978, 16, 2555-86. Hawkins, W. L.; Matreyek, W.; Winslow, E. H., J. Polym Sci., 1959, 41, 1-11. Grieveson, B. M.; Howard, R. N.; Wright, B., in "Thermal Degradation of Polymers"; Society of Chemical Industry: London, 1961; pp. 413-20. Peterlin, A., Int. J. Polym. Mater., 1980, 8, 285-301. Chien, J. C. W., J. Polym. Sci., 1968, 6, 375-379. Pospisil, J. in "Development in Polymer Stabilization - I"; Scott, G. Ed.; Applied Science; London, 1979; Chap. 1.

RECEIVED October 26, 1984


Investigation of Thermal Oxidation and ... - ACS Publications

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