U.S. Department of Energy

Abstract

A t t h e 1 3 0 ~ ~ tempera tu re t h e major a n t a g o n i s t i c r e a c t i o n i s t h a t w i t h m e t a l l i c copper . I n as l i t t l e as 25 h o u r s c o l o r a t i o n and f low may b e n o t i c e d i n t h e a r e a where t h e EVA comes i n c o n t a c t w i t h m e t a l l i c copper . The e f f e c t is more pronounced i n a i r (compare Tab les 31 and 32) t h a n i n n i t r o g e n . Th is i s c o n s i s t e n t w i t h t h e t h e o r y t h a t t h e m e t a l d i s c o l o r a t i o n r e a c t i o n involv ,es a hydroperox ide i n t e r m e d i a t e t h a t w i l l forms i n t h e p r e s e n c e o f oxygen. A l l t h e exper iments concern ing t h e the rmal a g i n g o f EVA w i l l b e c o n t i n u e d o u t , a t l e a s t t o t h e 3,000 hour mark,and a subsequen t r e p o r t w i l l g i v e a d d i t i o n a l d e t a i l s such a s t h e t o t a l i n t e g r a t e d t r a n s m i s s i o n , d i e l e c t r i c s t r e n g t h and t h e r e a c t i o n w i t h copper d u s t (moni tored by o p t i c a l t r a n s m i s s i o n ) . EMA, commercial f o r m u l a t i o n 13439, w a s t h e n e x t m a t e r i a l examined by t h e t h e r m a l a g i n g and is r e p o r t e d i n Tab les 33 through 38. Th is compound app e a r s t o b e s t a b l e th roughout t h e 80°C and 1 0 5 ~ ~ exposures w i t h r e s p e c t t o i t s mechanical p r o p e r t i e s , however a n i n c r e a s e i n s o l v e n t . s w e l l r a t i o and a d e c r e a s e i n g e l c o n t e n t s u g g e s t t h a t some s c i s s i o n o f t h e c r o s s l i n k s may g r a d u a l l y b e t a k i n g p l a c e . However, t h i s e f f e c t w i l l b e v e r i f i e d a s t h e exposure i s con t inued . A s w i t h t h e EVA,.the main e f f e c t n o t i c e d i n EMA i s t h a t o f copper m e t a l r e a c t i o n , a g a i n more n o t i c a b l e i n t h e p r e s e n c e o f a i r t h a n n i t r o g e n . A f t e r 400 h o u r s a t . 1 0 5 ~ ~ i n a i r , t h e specimen molded around a s t r i p o f copper had a s t r o n g ye l low c o l o r and showed s i g n s o f d e g r a d a t i o n ( b l i s t e r i n g , s s p a r a t i n n from t h e m e t a l , f l o w ) . Th is e f f e c t i s t y p i c a l f o r polymers a t h i g h t empera tu re and i s o l a t i o n from copper i s a requ i rement . No r e a c t i o n w a s n o t i c e d i n c o n t a c t w i t h aluminum o r 60/40 l e a d / t i n s o l d e r . I n f u t u r e exper iments n i c k e l , t i t a n i u m and s i l v e r w i l l a l s o b e i n c l u d e d i n t h e temperature-metal c o r r o s i o n tests. 0 The 130 C c o n d i t i o n a l s o r e s u l t e d i n t h e fo rmat ion o f c o l o r a t i o n i n t h e EMA specimens. Although no a c e t i c a c i d i s produced , a s i m i l a r r e a c t i o n i n v o l v i n g t h e c l e a v a g e o f t h e a c r y l i c e s t e r group i s l i k e l y , a s t h e C-0 bond is less s t a b l e t h a n t h e C-C bonds i n t h e backbone o f t h e r e s i n . T h i s i s a h i g h t e m p e r a t u r e f o r p o l y o l e f i n copolymer s e r v i c e and c o n s i d e r a b l y above t h e maximum module o p e r a t i n g t e m p e r a t u r e s expec ted i n f i e l d s e r v i c e . The a l i p h a t i c p o l y u r e t h a n e , 2-2591, was compara t ive ly t h e most t h e r m a l l y 0 s e n s i t i v e of t h e t h r e e c a n d i d a t e p o t t a n t s t e s t e d t o d a t e . Even a t 80 C t h e development o f a s m a l l amount of ye l low c o l o r ( a l t h o u g h f a i n t ) w a s m f i c e a b l e a t t h e 400 hour p o i n t . The 1 0 5 ' ~ c o n d i t i o n b r o u g h t more n o t i c e a b l e changes i n p r o p e r t y . The t e n s i l e s t r e n g t h and modulus were found t o s t e a d i l y d e c r e a s e t o a b o u t 40% of c o n t r o l , b u t o n l y i n t h e n i t r o g e n atmosp h e r e , t h e a i r exposed samples remaining uncllanyed. T h e nra j o ~ :.material change a t t h i s t e m p e r a t u r e was t h e fo rmat ion o f c o l o r a t i o n . A f t e r 400 h o u r s , i n b o t h a i r and n i t r o g e n , t h e t e s t specimens had developed a s t r o n g yellow-brown c o l o r and t h e s u r f a c e s exposed t o a i r had become s t l c k y , i n 0 d i c a t l n g c h a i n s c i s s i o n . The r e d c t i u n s w i t h copper m e t a l a t 105 C were i n t e n s e and a t t h e 400 hour mark t h e c o r r o s i o n specimens were ' found t o have degraded w i t h m e l t i n g and d a r k brown c o l o r a t i o n . The specimens had t o b e s c r a p p e d o f f t h e bot tom o f t h e t e s t c o n t a i n e r t o b e r e t r i e v e d . 0 The 130 C exposure was by f a r t h e most severe. . The p h y s i c a l i n t e g r i t y of t h e t es t specimens l a s t e d t o t h e 100 hour h a r k , however a t 400 h o u r s t h e , . t e n s i l e b a r s had a l l me l ted t o amorphous masses o f d a r k brown s t i c k y resiri. C o l o r a t i o n developed w i t h i n t h e f irst 25 h o u r s and became d a r k w i t h i n 100 h o u r s . The r e a c t i o n w i t h copper r e s u l t e d I n meicing and dey rdilclliulb~ul: Llle r e s i n w i t h t h e f i r s t 25 hour p e r i o d . No r e a c t i o n between the u r e t h a n e and o t h e r m e t a l s , aluminum o r s o l d e r , was d e t e c t a b l e a t aey o f t h e ' t e s t t i m e s o r I n c o n c l u s i o n , t h e p o l y o l e f i n b a s e d p o t t a n t s , EVA and EMA, a r e c o n s i d e r e d t o have good the rmal s t a b i l i t y o v e r t h e t i m e s and t e m p e r a t u r e s e x p l o r e d s o f a r . The p o l y u r e t h a n e i s more q u e s t i o n a b l e . The fo rmat ion o f c o l o r i n t h i s com0 pound a t t e m p e r a t u r e s a s low a s 80 C c o u l d p o s s i b l y p l a c e a l i m i t a t i o n on i t s u s e i n s o l a r module a p p l i c a t i o n s . A l l t h e p o t t a n t s remain t o b e e v a l u a t e d f o r performance o v e r l o n g p e r i o d s of t ime. One conspicuous r e s u l t o f t h e t h e r m a l ag ing work is t h e a d v e r s e r e a c t i o n w i t h copper . I t is f e l t t h a t i n no c i r c u m s t a n c e s s h o u l d t h e p o t t a n t compound c o n t a c t m e t a l l i c c o p p e r , as s i g n s o f " c o r r o s i o n " a r e n o t i c e d i n a lmos t a l l c a s e s . V I I I . SUPPORTING ACTIVITIES During t h e p a s t y e a r , Spr ingborn L a b o r a t o r i e s h a s engaged i n o t h e r a c t i v i t i e s i n s u p p o r t o f t h e FSA e f f o r t a d m i n i s t e r e d by JPL. Arrangements were made t o p r o v i d e two o t h e r c o n t r a c t o r s w i t h e x p e r i m e n t a l tes t modules f a b r i c a t e d w i t h i n m a t e r i a l s and t e c h n i q u e s d e s i g n e d t o e v a l u a t e a p a r t i c u l a r f a c e t o f PV module o p e r a t i o n . A. S c i e n c e A p p l i c a t i o n s , I n c . S c i e n c e A p p l i c a t i o n s , I n c . ' (SAI) h a s i n v e s t e d c o n s i d c r a b l c e f f o r t i n t h e development o f l i g h t t r a p p i n g technology and, a s a r e s u l t , h o l d s t h e u n i t e d S t a t e s P a t e n t No. 4 ,246,042 "Fixed S o l a r Energy C o n c e n t r a t o r " f o r l i g h t t r a p p i n g t e c h n i q u e s . Because o f t h e impor tance o f u s i n g l i g h t t r a p p i n g t e c h n i q u e s i n t h e d e s i g n o f s o l a r PV p a n e l s , JPL funded a s t u d y a t SAI t o i n v e s t i g a t e t h e c o s t b e n e f i t s o f l i g h t t r a p p i n g used i n PV modules under an agreement t h a t S A I ' s p a t e n t r i g h t s would n o t b e a f f e c t e d . Three p a i r s o f Spr ingborn L a b o r a t o r i e s ' 12 i n c h by 16 i n c h modules were p r e p a r e d f o r t h i s s t u d y . One module ( t h e c o n t r o l module) o f each p a i r used t h e " s t a n d a r d " d e s i g n . The o t h e r module o f each p a i r employed d e s i g n . . m o d i f i c a t i o n s , o n l y t o t h e d e g r e e needed, t o i n c o r p o r a t e d e s i r e d l i g h t t r a p p i n g f e a t u r e s . The two modules o f each p a i r were t h e n used s ide -by-s ide under n a t u r a l s u n l i g h t o r a t e s t s o u r c e t o demons t ra te t h e b e n e f i t s o f l i g h t t r a p p i n g i n terms of p o t e n t i a l module c o s t s and t h e amount o f s i l i c o n r e q u i r e d t o p r o v i d e a g i v e n p a n e l energy o u t p u t . Two c e l l c o n f i g u r a t i o n s were used , t h e S o l a r Power Corpora t ion 100 mrn d i a m e t e r s i l i c o n p h o t o v o l t a i c s o l a r c e l l and t h e 75 nun h a l f c e l l . P a i r s o f modules i n c l u d e : . One p a i r u s i n g 100 mm d i a m e t e r c e l l s ; t h e pack ing f r a c t i o n o f l i g h t t r a p p i n g module i s a n t i c i p a t e d t o b e 0.75. . One p a i r u s i n g 75 mm d i a m e t e r h a l f c e l l s , pack ing f r a c t i o n o f l i g h t t r a p p i n g module i s a n t i c i p a t e d t o b e '~0.50, . . One p a i r u s i n g 75 nun d i a m e t e r h a l f c e l l s , pack ing f r a c t i o n o f l i g h t t r a p p i n g module i s a n t i c i p a t e d t o b e Q 0.25. Design d e t a i l s f o r each p a i r o f modules inc lude : . Maxime and minump performance t o l e r a n c e s o f the s o l a ? c e l l s t o be used wi th bo th modules. . The r e l a t i v e l o c a t i o n and t h e p h y s i c a l and m a t e r i a l d e f i n i t i o n ' of t h e s e v e r a l l a y e r s (back cove r , s p a c e r s , p o t t a n t s , s u b s t r a t e and/or s u p e r s t r a t e , and t h e edge s e a l and gaske t ) t h a t make up t h e modules. . Type and a p p l i c a t i o n . o f d i f f u s e r e f l e c t i v e c o a t i n g a t t h e bottom of s ,u l j s t ra te and on c c l l ( top.zur faoc) metallization. . Ind iv idua l c e l l i