Protective Coatings and Sealants for Solar Applications - American

7 Protective Coatings and Sealants for Solar Applications Downloaded by UNIV OF SYDNEY on September 4, 2014 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch007

K. B. WISCHMANN Sandia National Laboratories, 7472, Albuquerque, NM 87185

An aging study has been completed which evaluated a number of polymeric materials for potential use as 1) protective coatings for back surfaces of mirrors and 2) solar heliostat edge seals. These investigations were conducted in an artificial weathering chamber that accelerated thermal cycling. We observed the primary mirror failure mode to be silver corrosion resulting from moisture exposure. To increase mirror longevity in current heliostat designs, intimate bonding at a l l the composite interfaces is essential to minimize moisture pathways to the silvered surface. With good adhesion, a KRATON rubber was found to exhibit superior back surface mirror protection, 12 months in environmental chamber with no corrosion. An ultraviolet stabilized butyl rubber appeared to be the best edge seal. A l l heliostats edge sealed with silicone materials showed silver corrosion which indicated either poor bonding or moisture permeation.

Many current solar heliostat and flat plate collector designs which have been field tested show a vulnerability to long-term outdoor weathering ( _1) · Therefore, the purpose of this investigation was to evaluate potential protective polymeric materials, i.e., coating, sealants, that could be used in various solar applications. Depending on the specific design, material weathering can occur in a variety of ways.

0097-6156/83/0220-0115$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF SYDNEY on September 4, 2014 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch007

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POLYMERS IN SOLAR ENERGY UTILIZATION

F o r example, t h e r e f l e c t i v i t y o f s i l v e r e d m i r r o r h e l i o s t a t s c a n d e t e r i o r a t e w i t h time i n t h e p r e s e n c e o f m o i s t u r e (.1>2_)· The mechanisms o f s i l v e r d e t e r i o r a t i o n a r e c u r r e n t l y an a r e a o f c o n jecture (3). In f l a t - p l a t e s o l a r c o l l e c t o r s h i g h t e m p e r a t u r e s , h u m i d i t y , ozone and u l t r a v i o l e t (UV) r a d i a t i o n a l l c o n t r i b u t e t o a g i n g p r o c e s s e s w h i c h l i m i t the l i f e t i m e o f s e a l a n t s , a d h e s i v e s and g a s k e t m a t e r i a l s ( 4 ) . T h i s work does not a d d r e s s t h e f u n d a m e n t a l q u e s t i o n o f m i r r o r corrosion mechanisms. R a t h e r , our s t u d y was limited to v i s u a l observations of weathering effects upon commercial products exposed t o extreme c o n d i t i o n s i n an e n v i r o n m e n t a l t e s t chamber. The principal parameters investigated were 1) the materials' ability to p r o t e c t s i l v e r e d m i r r o r s from m o i s t u r e and 2) the e f f e c t of mechanical s t r e s s ( c o e f f i c i e n t s of expansion mismatches) due t o t e m p e r a t u r e - h u m i d i t y c y c l i n g . Materials Back S u r f a c e - M a t e r i a l s e x a m i n e d f o r t h e p r o t e c t i o n o f t h e b a c k s u r f a c e o f m i r r o r s w e r e : 1) ESTANE 5714 ( a p o l y u r e t h a n e ) f r o m B. F. Goodrich, 2) P r o S e a l 890 (a p o l y s u l f i d e ) from Essex C h e m i c a l C o r p o r a t i o n , 3) SARAN ( p o l y v i n y l i d e n e c h l o r i d e ) f r o m Dow Chemical Company, and 4) KRATON (a styrenebutadience block c o p o l y m e r ) f r o m S h e l l C h e m i c a l Company. Edge S e a l s - The edge s e a l s employed i n t h i s s t u d y were DC-790 and DC-738 from Dow C o r n i n g , GE 1200 f r o m G e n e r a l E l e c t r i c ; a l l t h r e e a r e s i l i c o n e s . One b u t y l r u b b e r s e a l a n t , A d c o s e a l B-100 was s u p p l i e d by A d h e s i v e D e v e l o p m e n t and C h e m i c a l O p e r a t i o n s , I n c . Test specimens were p r e p a r e d by b o n d i n g a 4 χ 4 χ 1/8" Gardner m i r r o r t o an a p p r o p r i a t e s u b s t r a t e w i t h EC-3549 (amine c u r e d p o l y u r e t h a n e from 3M Co.) and t h e n s e a l e d a r o u n d t h e m i r r o r edge w i t h a b e a d o f the r e s p e c t i v e s e a l a n t s . S u b s t r a t e s i n c l u d e d i n t h i s t e s t were: 1) c e l l u l a r g l a s s ( S o l a r a m i c s C o . ) , 2) p o l y s t y r e n e foam (Dow C o r n i n g ) , 3) p o l y s t y r e n e foam w i t h added b u t y l r u b b e r pad, 4 and 5) p a p e r honeycomb s e a l w i t h e p o x y - f i b e r g l a s s and melamine r e s p e c t i v e l y , 6 and 7) s i n e - w a v e fiberglass sealed w i t h e p o x y - f i b e r - g l a s s and melamine r e s p e c t i v e (Items 4-7 were s u p p l i e d by P a r a b o l i t e , I n c . ) M

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Experimental Artificial weathering was conducted in a Conrad, Inc., E n v i r o n m e n t a l T e s t Chamber. The chamber was programmed t o c y c l e f r o m -29^0 t o 50°C t h r e e t i m e s d u r i n g 24 h o u r s , a c y c l e c o n s i s t s o f a two hour h o l d a t e a c h t e m p e r a t u r e extreme w i t h a two h o u r ramp i n between. The h u m i d i t y was m e a s u r e d a t 82% a t 50*C and 50% at 7 C , t h u s a d i s t i n c t f r e e z e - t h a w c y c l e o c c u r e d when a t e s t passed through 0 C ( 5 ) . T h i s chamber does n o t i n c o r p o r a t e UV r a d i a t i o n as an e n v i r o n m e n t a l s t r e s s . C

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

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Protective Coatings and Sealants

Protection

As m e n t i o n e d , one o f t h e p r i m a r y factors contributing to mirror d e t e r i o r a t i o n i s w a t e r i n i t s v a r i o u s forms ( 1.) · I n an e f f o r t to r e t a r d the d e t e r i o r a t i o n of s i l v e r e d m i r r o r s , s e v e r a l different commercial polymeric materials were e v a l u a t e d . Our s e l e c t i o n was g o v e r n e d by t h e m a t e r i a l s ' h y d r o p h o b i c n a t u r e o r known u s e i n w e a t h e r r e s i s t a n t a p p l i c a t i o n s . S p e c i f i c a l l y , these c h o i c e s were: 1) a p o l y u r e t h a n e , 2) a p o l y s u l f i d e , 3) p o l y v i n y l i d e n e c h l o r i d e and 4) a s t y r e n e - b u t a d i e n e c o p o l y m e r (hydrocarbon). One must r e a l i z e t h a t a n y p o l y m e r i c c o a t i n g w i l l p e r m e a t e w a t e r i n time, consequently, predicting "protected" mirror lifetimes becomes v e r y d i f f i c u l t . The c o a t i n g s were e v a l u a t e d on: 1) m i r r o r s which containd o n l y t h e s i l v e r and s t a n d a r d sacrificial copper l a y e r and 2) m i r r o r s h a v i n g the f a m i l i a r p r o t e c t i v e gray p a i n t . The f i r s t e x p e r i m e n t was d e s i g n e d to e l i m i n a t e the c o n t r i b u t i n g e f f e c t of gray p a i n t . The c o a t i n g s w i l l be d i s c u s s e d i n o r d e r o f t h e i r i n c r e a s i n g e f f e c t i v e n e s s . The p o l y s u l f i d e ( P r o S e a l 890) a l m o s t i m m e d i a t e l y a t t a c k e d t h e s i l v e r . T h i s i l l u s t r a t e s one o f t h e f i r s t p r o b l e m s w i t h m a t e r i a l s e l e c t i o n , m a t e r i a l s c o m p a t i b i l i t y . The s u l f u r i n t h i s c l a s s o f compounds was s i m p l y incompatible with s i l v e r . In f a c t , t h e p a i n t e d m i r r o r showed e v i d e n c e o f a t t a c k o n l y a few days a f t e r t h e u n p a i n t e d sample. The SARAN c o a t i n g began t o p e e l a f t e r 7 days i n d i c a t i n g v e r y p o o r a d h e s i o n . This peeling exemplified a n o t h e r c r i t i c a l problem a s s o c i a t e d w i t h p r o t e c t i n g any s u r f a c e f r o m m o i s t u r e , t h e n e c e s s i t y f o r i n t i m a t e a d h e s i o n between c o a t i n g and substrate. We b e l i e v e a k e y t o m i r r o r longevity i s the integrity o f t h e v a r i o u s bond i n t e r f a c e s . I f t h e r e i s i n t i m a t e bonding, only molecular water permeates t o t h e s i l v e r and n e g l i g i b l e damage o c c u r s ( 6 ) . E v e n t h o u g h SARAN h a s o u t s t a n d i n g moisture resistance, i f poor bonding e x i s t s then a void or delamination c a n o c c u r . As a r e s u l t , w a t e r w i l l c o n g r e g a t e and mirror degradation f o l l o w s by w h a t e v e r c o r r o s i o n mechanism. The polyurethane (ESTANE 5714) a p p e a r e d to o f f e r some d e g r e e o f p r o t e c t i o n ; however, w i t h t i m e i t a l s o began t o p e e l , f o l l o w e d by mirror c o r r o s i o n . The s t y r e n e b u t a d i e n e c o p o l y m e r (KRATON 1101) afforded the greatest p r o t e c t i o n , exceeding a l l other coated s a m p l e s i n d u r a b i l i t y . A s s u m i n g good b o n d i n g , t h i s r e s u l t was n o t unreasonable t o expect s i n c e t h e c o a t i n g was a h y d r o c a r b o n and w o u l d not be e x p e c t e d t o have an a f f i n i t y f o r w a t e r . From this limited study, hydrocarbon coatings with good bonding c h a r a c t e r i s t i c s w o u l d be t h e m a t e r i a l s o f c h o i c e when attempting to protect against moisture. These m a t e r i a l s a r e not i n t e n d e d as s u b s t i t u t e s f o r t h e t r a d i t i o n a l gray p a i n t p r o t e c t i v e c o a t i n g , b u t s i m p l y a s added m o i s t u r e barriers.



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Edge S e a l P r o t e c t i o n H e l i o s t a t s f o r l o n g term f i e l d use w i l l p r o b a b l y r e q u i r e edge seal protection. Materials for this application must have o u t s t a n d i n g w e a t h e r a b i l i t y . A r e v i e w o f e x i s t i n g c a n d i d a t e edge s e a l m a t e r i a l s i n d i c a t e s t h a t s i l i c o n e s p o s s e s s some o f the b e s t o v e r a l l w e a t h e r i n g p r o p e r t i e s . Inasmuch as t h e r e a r e no edge s e a l s specifically d e s i g n e d f o r s o l a r h a r d w a r e , the f o l l o w i n g commer c i a l l y a v a i l a b l e b u i l d i n g s e a l a n t s were s e l e c t e d f o r e v a l u a t i o n : silicones from Dow Corning (DC-790, 7 3 8 ) , G e n e r a l E l e c t r i c (GE 1200) and a b u t y l r u b b e r t e r m e d A d c o s e a l B-100. Some u n d e r s t a n d i n g of the chemistry of these compounds i s required to assure c o m p a t i b i l i t y w i t h t h e h e l i o s t a t d e s i g n . Most s i l i c o n e building s e a l a n t s a r e one component s y s t e m s t h a t r e l y on m o i s t u r e t o c u r e . As a r e s u l t , t h e y e l i m i n a t e b y - p r o d u c t s w h i c h c a n be p o t e n t i a l l y d e t r i m e n t a l t o t h e l o n g e v i t y o f aluminum o r s i l v e r m i r r o r s . The DC-790 l i b e r a t e s amine p r o d u c t s , DC-738 e m i t s a l c o h o l , GE 1200 y i e l d s a c e t i c a c i d and b u t y l r u b b e r o u t g a s s e s toluene (sealant s o l v e n t ) . By-products such as amines and a c e t i c a c i d w o u l d be considered incompatible with s i l v e r e d mirrors. Model h e l i o s t a t reflector segments along with controls were placed in the t e m p e r a t u r e - h u m i d i t y c y c l i n g chamber f o r e v a l u a t i o n s . D e g r a d a t i o n was f o u n d t o o c c u r as a v a r i e t y o f p i t t i n g o r s p o t s , s i l v e r d e l a m i n a t i o n s , d e n d r i t i c growths, c o l o r v a r i a t i o n s and many forms o f shadowing o r f o g g i n g . E a r l i e r b e l i e f s that d e t e r i o r a t i o n w o u l d b e g i n a t t h e edges and p r o p a g a t e inward are not n e c e s s a r i l y t r u e ; d e g r a d a t i o n appeared to s t a r t at v i r t u a l l y any l o c a t i o n w i t h no s y s t e m a t i c p a t t e r n . C e l l u l a r g l a s s was c o n s i d e r e d a p r o m i s i n g h e l i o s t a t s u b s t r a t e b e c a u s e the c o e f f i c i e n t s o f t h e r m a l e x p a n s i o n between the m i r r o r and s u b s t r a t e s were m a t c h e d . T e s t m i r r o r s were bonded and s e a l e d w i t h t h e above m e n t i o n e d s i l i c o n e s e a l a n t s and a g e d . W i t h i n t h r e e months a l l the t e s t e d s a m p l e s c r u m b l e d . The s u b s t r a t e s became f i l l e d w i t h w a t e r and c r u m b l e d d u r i n g t h e f r e e z e - t h a w c y c l i n g due t o c o e f f i c i e n t o f e x p a n s i o n m i s m a t c h e s between t h e w a t e r and g l a s s Ο). O b v i o u s l y t o use a c e l l u l a r g l a s s s u b s t r a t e , i t must be s e a l e d . A KRATON c o a t e d c e l l u l a r g l a s s a p p e a r s t o be a p r o m i s i n g c a n d i d a t e . T h i s sample showed no e v i d e n c e o f c r u m b l i n g a f t e r 12 months i n the e n v i r o n m e n t a l chamber (7_) · The use o f p o l y s t y r e n e foam as a h e l i o s t a t s u b s t r a t e has b e e n a p o p u l a r c h o i c e b e c a u s e o f low c o s t and e a s e o f f a b r i c a t i o n . However, v a r y i n g forms o f d e g r a d a t i o n o r c o r r o s i o n o c c u r r e d r e g a r d l e s s o f what edge s e a l was e m p l o y e d . The DC-790 and GE 1200 were anticipated to be inferior because of their outgassing p r o d u c t s ( a m i n e s , a c e t i c a c i d e ) , y e t t h e DC-738 w h i c h e l i m i n a t e s a r e l a t i v e l y benign by-product ( m e t h a n o l ) was j u s t as i n e f f e c t i v e . E i t h e r the s i l i c o n e edge s e a l s p e r m e a t e m o i s t u r e a t a r a t e h i g h e r t h a n a n t i c i p a t e d o r t h e r e i s p o o r b o n d i n g a t the v a r i o u s s e a l a n t i n t e r f a c e s . Once the m o i s t u r e has m i g r a t e d i n between the m i r r o r and s u b s t r a t e i t becomes t r a p p e d . Once t r a p p e d , t h e w a t e r c a n n o t


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e s c a p e s i n c e t h e module c a n n o t " b r e a t h e , " t h a t i s , m o i s t u r e c a n n o t freely condense and e v a p o r a t e during thermal cycling. Conseq u e n t l y , m o i s t u r e c a n f r e e z e and t h e r e s u l t i n g e x p a n s i o n c a n cause f u r t h e r d e l a m i n a t i o n s t h e r e b y a l l o w i n g more m o i s t u r e p e n e t r a t i o n . At h i g h e r temperatures the trapped water can lead to chemical d e g r a d a t i o n . Post-mortem e x a m i n a t i o n o f t h e module r e v e a l e d t h a t t h e a d h e s i v e (EC-3549) t o p o l y s t y r e n e foam i n t e r f a c e was h i g h l y pitted. The c o r r o s i o n observed virtually replicates t h e foams cellular structure, s e e F i g u r e 1. As a r e s u l t o f t h e c e l l u l a r s t r u c t u r e , t h e r e a r e l i t e r a l l y h u n d r e d s o f v o i d s where m o i s t u r e c a n p e r m e a t e , become t r a p p e d and c a u s e m i r r o r c o r r o s i o n . A g a i n , the presence of moisture p o i n t s out the importance o f bond

i n t e g r i t y throughout these composite s t r u c t u r e s which i n c l u d e s the f o l l o w i n g i n t e r f a c e s : s i l v e r t o g l a s s , gray p a i n t and adhesive to s u b s t r a t e . I f t h e r e i s n o t i n t i m a t e c o n t a c t a t each i n t e r f a c e a void or delamination r e s u l t s , creating an a r e a where harmful entities, i . e . , water, can d i f f u s e and i n i t i a t e degradation. Non-bonding a t these c r i t i c a l i n t e r f a c e s c a n o c c u r i n a v a r i e t y o f ways, f o r example, p o o r a d h e s i v e a p p l i c a t i o n s , u n c l e a n s u r f a c e s , or p o o r p r o c e s s c o n t r o l . One o f t h e b e t t e r h e l i o s t a t s e a l a n t s was f o u n d t o be a b u t y l r u b b e r , A d c o s e a l B-100. T h i s module i n c o r p o r a t e d a d e s i g n m o d i f i c a t i o n ; a 1/16" b u t y l r u b b e r p a d bonded between t h e m i r r o r a n d t h e p o l y s t y r e n e foam s u b s t r a t e . The module l o o k e d e x t r e m e l y good a f t e r 12 months a c c e l e r a t e d a g i n g . A s e a l a n t o f t h i s t y p e must be u s e d w i t h some d e g r e e o f c a u t i o n b e c a u s e b u t y l r u b b e r s a r e n o t n o r m a l l y c o n s i d e r e d a s t h e r m a l l y s t a b l e o r UV r e s i s t a n t a s s i l i c o n e s . The paper honeycomb substrates sealed with either epoxyf i b e r g l a s s o r melamine a l l showed some form o f m i r r o r d e g r a d a t i o n after 6 months and i n c r e a s e d deterioration a t 12 months. Post-mortem e x a m i n a t i o n o f some o f t h e s e modules d r a m a t i z e s t h e d e g r a d a t i o n e f f e c t s a t r r i b u t e d t o thermal expansion mismatches i n s a n d w i c h h e l i o s t a t s t r u c t u r e s . The s i n e wave r i b s were p e r p e n d i c u l a r t o t h e m i r r o r s u r f a c e a n d work a g a i n s t t h e m i r r o r d u r i n g thermal c y c l i n g c a u s i n g debonding i n t h e immediate c o n t a c t a r e a . The corrosion pattern replicates t h e s i n e wave s t r u c t u r e , s e e F i g u r e 2. T h u s , m i r r o r l o n g e v i t y i s j u s t a s much a f u n t i o n o f h e l i o s t a t d e s i g n as i s m a t e r i a l s e l e c t i o n . Two G a r d n e r m i r r o r c o n t r o l s a m p l e s ( g r a y p a i n b a c k i n g ) were included i n the aging study. The m i r r o r s w i t h o u t any added p r o t e c t i o n showed o n l y a s l i g h t amount o f p i t t i n g a f t e r 12 months. Survival o f t h e s e m i r r o r s w i t h a m i n i m a l amound o f damage was attributed to i t s a b i l i t y to "breathe." Another Gardner m i r r o r c o n t r o l was c o a t e d w i t h t h e a d h e s i v e , EC-3549; i t showed e v i d e n c e of d e t e r i o r a t i o n (spotting, shadows) a f t e r 6 months. T h i s c o r r o s i o n s u g g e s t s i n c o m p a t i b i l i t y between t h e a d h e s i v e and s i l v e r . The EC-3549 i s a polyurethane adhesive cured with an amine. A c c o r d i n g t o a l i t e r a t u r e s o u r c e ( 8 ) , t h e b a s i c i t y o f an amine i n the presence of moisture can r e s u l t i n silver deterioration. U n f o r t u n a t e l y , i t was v e r y d i f f i c u l t t o e v a l u a t e t h e e f f e c t i v e n e s s




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Figure 1. Polystyrene foam: adhesive, 3M EC-35^9; edge s e a l , GE 1200 c o n s t r u c t i o n s e a l a n t ; time aged, 12 mos.



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F i g u r e 2. F i b e r g l a s s f a c e , s i n e wave f i b e r g l a s s body: adhesive, 3M EC-35^9; edge s e a l , GE 1200 c o n s t r u c t i o n s e a l a n t ; time aged, 12 mos.


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of t h e above r e s p e c t i v e edge s e a l s b e c a u s e m a t e r i a l incompati b i l i t i e s , n e b u l o u s b o n d i n g a n d h e l i o s t a t d e s i g n s a p p e a r e d t o have a g r e a t e r b i a s on t h e r e s u l t s t h a n edge s e a l s e l e c t i o n .


Summary and

Conclusion

The f i n d i n g s i n t h i s aging study r e s u l t e d i n the f o l l o w i n g c o n c l u s i o n s w h i c h s h o u l d have a b e a r i n g on h e l i o s t a t d e s i g n s : 1. Edge S e a l c h o i c e s seemed o f m i n o r i m p o r t a n c e compared t o the o v e r a l l module d e s i g n and b o n d i n g i n t e g r i t y . A l t h o u g h t h e y age w e l l , s i l i c o n e edge s e a l s a p p e a r t o bond p o o r l y and/or permeate m o i s t u r e e a s i l y . 2. To m i n i m i z e m o i s t u r e d i f f u s i o n pathways e i t h e r a b e t t e r barrier i s needed, t h a t i s , an i m p r o v e d d e s i g n o r t o assure the i n t e g r i t y of the various bond i n t e r f a c e s , i . e . , edge s e a l s t o s u b s t r a t e s , s i l v e r t o g l a s s , e t c . 3. M i r r o r c o r r o s i o n was m a n i f e s t e d by a v a r i e t y o f forms such a s p i t t i n g , s p o t t i n g , d e l a m i n a t i o n s and d i s c o l o r a t i o n s w i t h no p a r t i c u l a r p a t t e r n . E f f o r t s were n o t made t o e l u c i d a t e c o r r o s i o n mechanisms, y e t we b e l i e v e t h e f i r s t step i n the degradation process i s a delamination or c r e a t i o n o f a v o i d a r e a a t some c r i t i c a l interface where h a r m f u l r e a c t a n t s , i . e . , water, c o l l e c t , thereby precipitating degradation. 4. Compatibility of various materials i n these h e l i o s t a t d e s i g n s i s an i m p o r t a n t c o n s i d e r a t i o n w h i c h i f p r o p e r l y a d d r e s s e d , c a n add l o n g e v i t y t o t h e s y s t e m . F o r example, an edge seal which outgasses a relatively benign by-product l i k e a l c o h o l (as opposed t o a c e t i c a c i d ) would be a l o g i c a l c h o i c e a s an edge s e a l a n t . I n g e n e r a l , b u t y l r u b b e r s e a l a n t s were s u p e r i o r t o s i l i c o n e s , providing

a UV s t a b i l i z e d

product

i s used.

Acknowledgment Work s u p p o r t e d by D e p a r t m e n t

o f E n e r g y #DE-AC04-76, DP00789

Literature Cited 1. 2. 3. 4.

Burolla, V. P., Roche, S. L . , "Silver Deterioration in Second Surface Mirrors," SAND79-8276, Jan. 1980. Lind, Μ. Α., Buckwalter, C. Q., Daniel, J. L . , Hartman, J . S., Thomas, M. T., Peterson, L. R., "Heliostat Mirror Survey and Analysis," Sept. 1979, Pacific Northwest Lab. 3194, UC-62. Second Solar Reflective Materials Workshop, Feb. 12-13, 1980, San Francisco, CA, SERI/TP-334-558. Mendelsohn, Μ. Α., Luch, R. M., Yoemon, F. Α., Navish, F. W., "Sealants of Solar Collectors," Westinghouse R&D Center, LASL Contract.


7.

WISCHMANN

5.

Allred, R. E . , Miller, D. W., Butler, B. C., "Environmental Testing of Solar Reflector Structures," Presentation at 1979 International Solar Energy Society Congress. Sharma, S. P., Thomas, J . H., Bader, F. E . , Electrochem, J., Soc., p. 2002, Dec. 1978. Allred, R. E . , Miller, D. W., Private Communication at Sandia Laboratory. Hamner, H. G., "Corrosion Data Survey, Metals Section", 5th Edition, 1974 National Association of Corrosion Engineers.

6. 7. 8.

22, 1982


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Protective Coatings and Sealants for Solar Applications - American

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