29 Molecular Weight Distribution Studies of Electrically
Durability of Macromolecular Materials Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.
Stressed Polyethylene JOHN A. CROWTHER, JULIAN F. JOHNSON, and JOHN TANAKA Institute of Materials Science and Department of Chemistry, University of Connecticut, Storrs, CT 06268
Polyethylene has become one of the most widely used materials for electrical insulation due to its excellent dielectric properties and low cost. Original estimates of its probable lifetime using Arrhenius type extrapolation led to lifetime predictions of the order of forty years or more and yet a substantial number of failures in transmission and distribution cables have been observed at times under ten years. For 1977 failures on HMW URD cable totalled 1697 a failure rate of 3,08 per 100 miles while XLP cables were 0.53 per 100 miles and increasing (5). This work was undertaken to find out the role played by the polymer molecule under separate thermal and electrical stress. Experimental All measurements of molecular weight distribution were made on a specially constructed high temperature size exclusion chromatograph. This instrument has been described i n detail elsewhere(1). Basically chromatographic conditions were: temperature 140°C, solvent o-dichlorobenzene chromatographic column 21 foot 3/8" O.D. i s packed with varying grades of controlled pore glass, detector infra-red operating at the C-H stretching frequency. The amount of eluent was recorded on an automatic recording balance. Samples w e r e p r e p a r e d f r o m a c o m m e r c i a l l y available DuPont Alathon high density polyethylene resin. T h i s resin was h o t pressed at l80°C b e t w e e n two s h e e t s of a l u m i n u m foil. The pressing operation was c o n d u c t e d t w i c e to insure uniform films. Thermal and E l e c t r i c a l S t r e s s i n g The t h e r m a l s t r e s s i n g was c a r r i e d o u t o n t h i n f i l m s , approximately 50um t h i c k , t o i n s u r e a l a r g e s u r f a c e area and adequate oxygen p e r m e a b i l i t y . S m a l l samples, a p p r o x i m a t e l y 2 5 mg i n w e i g h t , w e r e p l a c e d o n m i c r o s c o p e s l i d e s f o r s u p p o r t . These samples were t h e n t h e r m a l l y s t r e s s e d i n a s m a l l l a b o r a t o r y oven which m a i n t a i n e d t h e d e s i r e d t e m p e r a t u r e , t o +1°C. 0-8412-0485-3/79/47-095-421$05.00/0 ©
1979 A m e r i c a n C h e m i c a l Society
422
DURABILITY O F
MACROMOLECULAR
MATERIALS
F i v e t e m p e r a t u r e s were u s e d i n t h e t h e r m a l s t r e s s i n g e x periments. T h e s e t e m p e r a t u r e s were 60°C, 90°C, 130°C, 150°C, and 170°C. Samples were i n t r o d u c e d i n t o t h e o v e n as q u i c k l y as p o s s i b l e t o m i n i m i z e t h e t h e r m a l l o s s . The s a m p l e s w e r e t h e n w i t h d r a w n a t t i m e d i n t e r v a l s and a l l o w e d t o c o o l t o room temperature. E l e c t r i c a l s t r e s s i n g was a c c o m p l i s h e d w i t h an e l e c t r i c a l s t r e s s e r b u i l t f o r t h i s p u r p o s e . An A l l a n s o n l u m i n o u s t u b e t r a n s f o r m e r , N o r t h Tonawanda, N.Y., was u s e d t o o b t a i n t h e desired stress levels. This transformer i s capable of producing s t r e s s l e v e l s o f up t o 15,000 A.C. v o l t s a t 60 c y c l e s when s u p p l i e d t o i t s p r i m a r y w i t h 120 v o l t 60 c y c l e A . C . The s t r e s s l e v e l was e a s i l y c h a n g e d b y means o f a v a r i a b l e t r a n s f o r m e r p l a c e d i n t h e p r i m a r y c i r c u i t and t h e r e s u l t i n g s e c o n d a r y v o l t a g e was calibrated using a high voltage voltmeter. The sample c e l l was c o n s t r u c t e d o f P l e x i g l a s s w h i c h enc l o s e d t h r e e s i d e s o f a r e c t a n g u l a r box a r r a n g e m e n t . The s a m p l e c e l l a s s e m b l y was f i t t e d w i t h c o n t a c t s w h i c h c o u l d be s l i d on and o f f t h e s e c o n d a r y v o l t a g e t e r m i n a l s . The two s e c o n d a r y v o l t a g e c o n t a c t s w e r e c o n n e c t e d t o two s q u a r e aluminum e l e c t r o d e s w i t h c o p p e r w i r e . The b o t t o m e l e c t r o d e was 2" x 2" and s e c u r e d t o t h e b o t t o m o f t h e c e l l a s s e m b l y . The u p p e r e l e c t r o d e was 1" b y 1" and c o n t a c t was made w i t h a s p r i n g l o a d e d copper s t r i p . I n t h i s f a s h i o n a sample may be p l a c e d b e t w e e n t h e e l e c t r o d e s and c o n t a c t m a i n t a i n e d b y t h e s p r i n g , w i t h m i n i m a l sample d i s t o r t i o n . S a m p l e s f o r e l e c t r i c a l s t r e s s i n g w e r e c o m p r e s s i o n m o l d e d , as d i s c u s s e d p r e v i o u s l y , i n t o f i l m s a p p r o x i m a t e l y 100 ym t h i c k . The f i l m s w e r e v i s u a l l y i n s p e c t e d and s q u a r e s a p p r o x i m a t e l y 2" x 2" were c u t out o f the r e g i o n w i t h t h e l e a s t v i s i b l e f l a w s . The s q u a r e s w e r e t h e n p l a c e d b e t w e e n two p a r a l l e l p l a t e s a t t a c h e d t o a m i c r o m e t e r d i a l and t h e t h i c k n e s s was r e c o r d e d . These s a m p l e s o f known t h i c k n e s s w e r e t h e n p l a c e d b e t w e e n t h e e l e c t r o d e s and t h e v o l t a g e l e v e l r a i s e d t o r e s u l t i n a s t r e s s o f a p p r o x i m a t e l y 15 kv/mm. I t s h o u l d be n o t e d , a t t h i s p o i n t , t h a t t h e s t r e s s l e v e l s u s e d i n t h i s work a r e w e l l above t h a t o r d i n a r i l y found i n commercial cables. M o l e c u l a r Weight D i s t r i b u t i o n A n a l y s i s ( a ) E f f e c t s o f T h e r m a l S t r e s s i n g and O x i d a t i v e D e g r a d a t i o n A n a l y s i s o f t h e r m a l d e g r a d a t i o n has b e e n p e r f o r m e d on e l e c t r i c a l i n s u l a t i n g m a t e r i a l s e x t e n s i v e l y . These e x p e r i m e n t s h a v e , f o r t h e most p a r t , b e e n c o n d u c t e d t o e x t r a p o l a t e l i f e t i m e s a t s e r v i c e temperatures from experimental data obtained at e l e v a t e d t e m p e r a t u r e s . One o f t h e p r o b l e m s i s t h a t i t i s p o s s i b l e t h a t t h e d e g r a d a t i o n mechanism may change b e t w e e n t h e e l e v a t e d t e s t t e m p e r a t u r e s and t h e s e r v i c e t e m p e r a t u r e s e n c o u n t e r e d . P o l y e t h y l e n e m e l t s b e t w e e n 100°C and 135°C, and t h e r e f o r e t h e a c c e l e r a t e d t h e r m a l t e s t s have a l m o s t e x c l u s i v e l y b e e n on t h e
29.
CROWTHER E T A L .
Electrically
Stressed
Polyethylene
423
p o l y m e r i n t h e m e l t p h a s e . A change i n d e g r a d a t i o n mechanism i s a v e r y r e a l p o s s i b i l i t y i n t h i s c a s e b e c a u s e most p r o p e r t i e s o f a s y s t e m may n o t b e e x t r a p o l a t e d t h r o u g h a p h a s e t r a n s i t i o n i n a continuous, l i n e a r fashion. I n a n a t t e m p t t o d e t e c t s u b t l e changes i n 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 o f p o l y e t h y l e n e a t temperatures below t h e m e l t i n g p o i n t , t h e p r e c i s e h i g h t e m p e r a t u r e SEC p r e v i o u s l y d e s c r i b e d was u t i l i z e d . To o b t a i n a n e s t i m a t e o f t h e m a g n i t u d e o f t h e changes e x p e c t e d a t temperatures below t h e m e l t u s i n g an A r r h e n i u s e x t r a p o l a t i o n , t h e d a t a o f H o l m s t r o m a n d S o r v i k (2) was u s e d . I n t h i s w o r k a l o w d e n s i t y p o l y e t h y l e n e was s u b j e c t e d t o t h e r m a l s t r e s s e s f r o m 280°C t o 355°C i n a t m o s p h e r e s c o n t a i n i n g l e s s t h a n 1.2$ o x y g e n . The s a m p l e s w e r e t h e n a n a l y z e d u s i n g v a r i o u s t e c h n i q u e s i n c l u d i n g SEC measurements on m o l e c u l a r w e i g h t d i s t r i b u tions. The d a t a c o n t a i n e d i n t a b l e 1 i n t h e w o r k b y H o l m s t r o m and ' S o r v i k g i v e s t h e t e m p e r a t u r e , £xygen c o n t e n t , t i m e o f s t r e s s i n g , l i m i t i n g s o l u t i o n v i s c o s i t y , M , a n d M /M . The d a t a c h o s e n t o do t h e A r r h e n i u s e x t r a p o l a t i o n was pertaining t o an o x y g e n c o n t e n t o f l . l 6 o $ . A s t h e m o l e f r a c t i o n o f o x y g e n i n a i r u n d e r s t a n d a r d c o n d i t i o n s i s 0.2095* t h e d a t a t a k e n a t l.l60# s h o u l d g i v e a s l o w e r r a t e o f d e g r a d a t i o n t h a n t h a t o b t a i n ed under a t m o s p h e r i c c o n d i t i o n s . The m o d e l c h o s e n t o d e s c r i b e t h e d e g r a d a t i o n o f p o l y e t h y l e n e was random c h a i n s c i s s i o n . L e n z (3.) i n h i s s e c t i o n o n d e g r a d a t i o n r e a c t i o n s o f polymers c i t e s work w h i c h s u p p o r t s t h e c o n t e n t i o n t h a t p o l y e t h y l e n e does t h e r m a l l y d e g r a d e i n a random c h a i n s c i s s i o n manner a s o p p o s e d t o d e p o l y m e r i z a t i o n . F o r t h i s m o d e l a s t a t i s t i c a l treatment has been developed b y M o n t r o l l and S i m h a (k). The e x t e n t o f r e a c t i o n may b e r e l a t e d t o t h e number average molecular weight b y :
tEat
I
n
= M /(1-p) o
where = number a v e r a g e m o l e c u l a r M p
q
= molecular weight
weight
o f e a c h r e p e a t i n g segment
= extent o f reaction
I f the c o n d i t i o n p = p a t zero time i s imposed, the r e l a t i o n s h i p can be developed:
following
424
DURABILITY O F
MACROMOLECULAR
MATERIALS
Table I S p e c i f i c R a t e C o n s t a n t s ( m i n ^) Temp. (°C)
K
Source
Measured 130
5.94 ± 0.89 x 1 0 "
6
T h i s work
150
1.74 ± 0.19 x 1 0 "
5
"
170
4.23 * 0.89 x 1 0 "
5
"
284
7.02 ± 1.98 x 1 0 ~
6
333
2.17 * 0.65 x 1 0 "
5
355
2.00 ± 0.60 x 1 0 "
5
Ref. 2
"
Calculated 9
60
1.29 x 10""
Ref. 2
90
7.61 x 1 0 "
9
"
60
6.21 x 1 0 "
8
T h i s work
90
5.48 x 1 0 ~
7
T h i s work
29.
Electrically
CROWTHER E T A L .
Stressed
425
Polyethylene
P =
P
exp - k t
P =
extent o f r e a c t i o n a t time t
o
•where
extent o f r e a c t i o n time o k
rate
t
time
constant
W i t h t h i s e q u a t i o n t h e e x t e n t o f r e a c t i o n may b e c a l c u l a t e d a t any t i m e , t , a s l o n g a s t h e i n i t i a l m o l e c u l a r w e i g h t a n d s p e c i f i c r a t e c o n s t a n t a r e known. The d a t a f r o m H o l m s t r o m a n d S o r v i k f o r p o l y e t h y l e n e i n 1.160% o x y g e n a t m o s p h e r e s were t r e a t e d b y t h e u s e o f t h e s e e q u a t i o n s t o c a l c u l a t e r a t e c o n s t a n t s a s g i v e n i n t a b l e 1 a t 28H°C, 333°C and 355°C. From t h i s , u s i n g t h e A r r h e n i u s r e l a t i o n s h i p , c o n s t a n t s f o r t h e s y s t e m c a n b e c a l c u l a t e d w i t h A = 2.632 a n d t h e a c t i v a t i o n e n e r g y 14.13 + 1.20 K c a l / m o l . U s i n g t h e s e c o n s t a n t s r a t e e q u a t i o n s w e r e c a l c u l a t e d a t 60°C a n d 90°C. A d d i t i o n a l l y , using t h e technique described p r e v i o u s l y , we made measurements a t 60°C, 90°C, 130°C, 150°C, a n d 1T0°C. The d a t a f r o m t h e t h r e e h i g h e r t e m p e r a t u r e s were u s e d t o c a l c u l a t e r a t e constants and i n t u r n A r r h e n i u s e x t r a p o l a t i o n s f o r a i r ^ c o n t a i n i n g t h e n o r m a l amount o f oxygen y i e l d i n g A = 1.713 x 10 and a n a c t i v a t i o n e n e r g y o f 17.k3 ± 0.h9 K c a l / m o l . Table 2 g i v e s t h e c a l c u l a t e d weight average m o l e c u l a r weights f o r t h e s a m p l e s r u n a t 60°C b r a c k e t e d b e t w e e n s a m p l e s r u n w i t h o u t s t r e s s i n g . S i m i l a r d a t a a r e g i v e n a t 90°C i n t a b l e 3. T h e r e a p p e a r e d t o b e no d e t e c t a b l e d i f f e r e n c e b e t w e e n t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n s r u n a t 60°C a n d t h o s e r u n a t 90°C. S t a t i s t i c a l a n a l y s i s o f t h e u n s t r e s s e d runs y i e l d e d t h e f o l l o w i n g v a l u e s f o r m o l e c u l a r w e i g h t a n d 95$ c o n f i d e n c e l i m i t s . M = 2.057 + 0.39 x 10 * a n d M = 6.620 + 0.226 x 10 . The c a l c u l a t e d r a t e c o n s t a n t s from t h e two s e t s o f e x p e r i m e n t a l d a t a were u s e d t o p r e d i c t t h e number a v e r a g e m o l e c u l a r w e i g h t t h a t s h o u l d b e o b s e r v e d a f t e r v a r i o u s p e r i o d s o f s t r e s s i n g a t 60°C and 90°C. T a b l e k shows a summary o f t h e e n d p o i n t s . Changes a t 90°C o f a t l e a s t 80% s h o u l d h a v e b e e n o b s e r v e d compared w i t h t h e a c t u a l o b s e r v a t i o n o f no change d e t e c t e d w i t h i n t h e l i m i t s o f e x p e r i m e n t a l e r r o r . A l t h o u g h n o t as dramatic f o r t h e d a t a f r o m r e f e r e n c e 3 w i t h a much l o w e r oxygen c o n t e n t t h e changes w o u l d have s t i l l b e e n o b s e r v e d a s t h e y a r e f a r i n e x c e s s o f t h e u n c e r t a i n c i e s i n t h e method. The r e s u l t s j u s t d i s c u s s e d i n d i c a t e t h a t t h e d e g r a d a t i o n r a t e i s d e f i n i t e l y changing between t h e h i g h temperature s t r e s s i n g , >130°C, a n d t h e l o w t e m p e r a t u r e s t r e s s i n g , £90°C. k
1
n
w
426
DURABILITY O F M A C R O M O L E C U L A R M A T E R I A L S
Table I I Sample
Mn x 10
-4
Mw x 10
Unstressed
2.108
6.719
24 h r s . @ 60
1.980
6.431
Unstressed
2.043
6.771
Unstressed
1.996
6.267
5 1 h r s .
