Reactivity of Polymers with Mirror Materials - ACS Symposium Series

8

Downloaded by STANFORD UNIV GREEN LIBR on May 6, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch008

Reactivity of Polymers with Mirror Materials S. K. BRAUMAN, D. B. MacBLANE, and F. R. M A Y O SRI International, Menlo Park, CA 94025

Of eight polymers screened, poly(methyl methacrylate) and possibly poly(vinylidene fluoride) and poly(ethylene terephthalate) show promise as protective coatings for solar mirrors. Polymer-coated mirrors were exposed in a Weather-Ometer and analyzed periodically for mirror and polymer degradation. Failures resulted from physical delamination and chemical reaction (a) at the polymer/mirror interface due to interaction with the degrading polymer or its additives and (b) at the mirror backside due to inadequate protection by the mirror backing or encapsulant. Organic polymers are being considered as protective coatings on silver or aluminum solar mirrors, either as backing substrates or transmitting superstrates. Although polymers offer the advantage of low initial cost, their long-term stability in the mirror applications is of concern. Environmentally induced polymer/mirror interface reactions could result in a costly loss in the mirror's reflectivity. Considerable information is available on the weatherability of isolated polymers (1), but not when they are in contact with a metal such as silver or aluminum. In an ongoing program we are studying the reactivity of candidate polymers with mirror materials. This report summarizes progress from initial studies designed to screen the interfacial stabilities of different polymer/metal combinations. The objective of this initial screening is the selection of promising polymer/mirror combinations for subsequent long-term, semiquantitative environmental evaluation. Experimental Section Polymers. The polymers used in this study included ethylene - (18 wt%) vinyl acetate, EVA (Du Pont Elvax 460); ethylene (65 0097-6156/83/0220-0125$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.

POLYMERS IN SOLAR ENERGY UTILIZATION

126

to 75%) - propylene copolymer, EP (Exxon Chemical V i s t a l o n MD 719); p o l y i s o b u t y l e n e , PIB (Exxon Chemical V i s t a n e x MM L-100); and poly(methyl m e t h a c r y l a t e ) , PMMA (Rohm and Haas P l e x i g l a s V811). Polymers obtained and used i n f i l m form i n c l u d e d Kynar, a p o l y ( v i n y l i d e n e f l u o r i d e ) , PVDF (Westlake P l a s t i c s ; 3 m i l ) ; b i a x i a l l y o r i e n t e d Kynar, PVDF (BlAX) (Pennwalt Corp.; 3.5 m i l ) ; Korad K l e a r , an a c r y l a t e / m e t h a c r y l a t e polymer made by m u l t i s t a g e emulsion p o l y m e r i z a t i o n , Korad^(Georgia P a c i f i c , G0212; 3 m i l ) (2); Llumar, poly(ethylene t e r e p h t h a l a t e ) ( I C I , Melinex 0 that i s aluminized and s o l d by M a r t i n Processing Inc., PET^/A1, 3 m i l ) ; and FEK-244, aluminized poly(methyl methacrylate) w i t h adhesive backing, PMMA^(3M)/Al/adhesive (3M; 4 m i l polymer). Some o f t h e bulk polymers were compounded w i t h 3 wt% carbon b l a c k (Cabot's Vulcan 3 ) , designated w i t h a s u b s c r i p t c, to reduce u l t r a v i o l e t absorption. f


f

Polymer/Mirror Assembly P r e p a r a t i o n . Only s i l v e r m i r r o r s were prepared i n our l a b o r a t o r i e s . For the bulk polymers EVA, PIB, EP, and PMMA, the polymer/mirror assemblies were prepared by s o l u t i o n ( t o l u e n e ) - c a s t i n g 2-4 m i l (dry) polymer f i l m s over a s i l v e r m i r r o r deposited on g l a s s ( a i r s u r f a c e o f 1/16-in. L i b b y Owens-Ford soda-lime f l o a t g l a s s ) . Some of the EVA-coated m i r r o r s were annealed f o r 5 minutes a t 90 to 100°C. A l l PMMAand PIB-coated m i r r o r s were annealed a t 80 to 85°C f o r 6 minutes. These combinations are designated polymer/Ag/glass. For t h e preformed commercial f i l m s K y n a r , Kynar(BIAX)^, and K o r a d , s i l v e r was deposited d i r e c t l y on the polymer; the m i r r o r backside was p r o t e c t e d w i t h a backing. These assemblies are designated polymer^/Ag/backing. For our purpose, no extensive attempt was made to optimize the r e f l e c t a n c e of our m i r r o r s by v a r y i n g the f a b r i c a t i o n procedures. f

f

A l l s i l v e r m i r r o r s were prepared by e l e c t r o l a s s d e p o s i t i o n using the commercial s o l u t i o n s and general procedures s u p p l i e d by London L a b o r a t o r i e s , L t d . (Woodbridge, Conn.). For Korad f i l m , however, the aqueous s o l u t i o n s were m o d i f i e d w i t h ethanol to improve adhesion o f the s i l v e r t o the polymer. An ethanol conc e n t r a t i o n o f 4.8 v o l % was used i n the s e n s i t i z i n g , s i l v e r i n g , and reducing s o l u t i o n s . The s i l v e r i n our m i r r o r s i s a p p r o x i mately 600 to 700 A t h i c k . I n i t i a l l y , the m i r r o r backs i n polymer^/Ag assemblies were protected by a gasketed, g l a s s cover, sandwich arrangement. Subsequently, we used two commercial m i r r o r backings: a t r a d i t i o n a l gray m i r r o r backing (Glidden Coatings and Resins) and an antique m i r r o r backing p a i n t (PPG 44425 d i l u t e d w i t h PPG GV 3003 reducer) E v a l u a t i o n . For screening s t a b i l i t i e s a t i n t e r f a c e s , polymer/mirror samples were exposed i n an A t l a s Weather-Ometer at 70°C and 50% r e l a t i v e humidity, using continuous r a d i a t i o n

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

8.

BRAUMAN E T AL.

Polymers and Mirror Materials

111


from a 6000-watt xenon a r c lamp w i t h b o r o s i l i c a t e f i l t e r s . Samples were p o s i t i o n e d w i t h the polymer f a c i n g the lamp. Exposed samples were removed at i n t e r v a l s f o r e v a l u a t i o n . F i r s t , the r e f l e c t a n c e of the i n t a c t polymer/mirror assembly was measured. Where p o s s i b l e , the polymer was then removed from the m i r r o r . The separated polymer f i l m was analyzed f o r changes i n t e n s i l e p r o p e r t i e s , molecular weight, and i n f r a r e d a b s o r p t i o n s . The m i r r o r i n t e r f a c e was analyzed by scanning e l e c t r o n microscopy. A B r i c e - P h o e n i x U n i v e r s a l L i g h t S c a t t e r i n g Photometer (1000 s e r i e s w i t h 3200-lumen h i g h pressure mercury l i g h t source) was used f o r measurement of the 90° (45° angle of i n c i d e n c e ) specular r e f l e c t a n c e of m i r r o r s . For a l l polymer/Ag/glass assemblies, the r e f l e c t a n c e was measured through the g l a s s . For a l l polymer^/ m i r r o r / b a c k i n g assemblies the measurements were made through the polymer f i l m . R e f l e c t a n c e was measured a t four to e i g h t d i f f e r e n t l o c a t i o n s on each i n t a c t m i r r o r sample before the polymer f i l m was removed; the r e s u l t s were averaged. The r e f l e c tance o f the m i r r o r assemblies i s referenced to that of a commercial s i l v e r m i r r o r that serves as a common standard. M i r r o r components were separated f o r a n a l y s i s e i t h e r by p e e l i n g the polymer from the glass o r by soaking the e n t i r e assembly i n water. T y p i c a l l y the metal remains on the polymer f i l m w i t h e i t h e r procedure. Normally we do not remove such r e s i d u a l metal o r even good m i r r o r s before t e n s i l e t e s t i n g o f the polymer, but the metal f i l m o r m i r r o r backing i s f i r s t broken up by. g e n t l y f l e x i n g the assembly o r m e t a l l i z e d polymer f i l m . E l o n g a t i o n and t e n s i l e s t r e n g t h a t break of the polymer f i l m were measured w i t h an I n s t r o n Model TM t e n s i l e t e s t e r (crosshead speed 2 i n . per min; 23°C). Polymer f i l m s were d i s s o l v e d i n a p p r o p r i a t e solvents f o r determination o f molecular weight by g e l permeation chromatography (GPC). GPC molecular weights are referenced to p o l y s t y r e n e standards, which are s u i t a b l e f o r determining r e l a t i v e changes i n molecular weights. Performance Ranking Using l o s s i n r e f l e c t a n c e as an i n d i c a t i o n o f m i r r o r f a i l u r e and change i n t e n s i l e p r o p e r t i e s as an i n d i c a t i o n of polymer f a i l u r e , we can rank the polymer/mirror assemblies s t u d i e d f o r o v e r a l l performance. I n Table I we rank a l l the assemblies i n terms of poor, i n t e r m e d i a t e , and good performance. FEK-244 o r PMMA(3M)/Al/adhesive i s the most durable of the polymer/mirror combinations s t u d i e d . Poor performers warrant no f u r t h e r study. Intermediate cases show some promise and could p o s s i b l y be improved w i t h m o d i f i c a t i o n o f the polymer/mirror assembly. Both p h y s i c a l and chemical f a i l u r e have been observed i n the polymer-




Table I* RANKING OF THE WEATHERING PERFORMANCE OF POLYMER/MIRROR ASSEMBLIES

Assembly

Category Poor

EVA/Ag/glass EP/Ag/glass EP /Ag/glass PIB/Ag/glass PIB /Ag/glass Korad /Ag/Antique f

Intermediate

PMMA/Ag/glass PVDF /Ag/s andwi ch PVDF /Ag/Glidden PVDF* (BIAX) /Ag/Glidden PET 7Al f

f

Good

PMMA (3M) / A l /adhesive/ g l a s s f


8.

BRAUMAN ET AL.


129

coated m i r r o r s . Our r e s u l t s i n d i c a t e , however, that both types of f a i l u r e can o f t e n be reduced by proper s e l e c t i o n o f the polymer/mirror assembly and the a d d i t i v e s i n the polymer.


Physical Failure:

Delamination

P h y s i c a l f a i l u r e was evident as delamination of polymer and m i r r o r during weathering of PMMA/Ag/glass. These m i r r o r s remained shiny and c l e a r throughout the exposure p e r i o d , but they showed some l o s s i n r e f l e c t a n c e a f t e r 9 days o f exposure, when some samples a l s o began t o delaminate (See F i g u r e l ) . A l l samples showed some delamination by 12 days when the t e s t was terminated. A n a l y s i s o f these delaminated samples by attenuated t o t a l r e f l e c t a n c e IR spectroscopy showed that weathering had no appreciable e f f e c t on the polymer surface that was i n contact w i t h s i l v e r o r g l a s s i n the samples. S p e c i f i c a l l y , there was no change i n the C=0 o r C-H peaks, and no OH o r C=C developed w i t h exposure. Furthermore, the d i s s o l v e d f i l m (completely s o l u b l e i n toluene) showed n e i t h e r l o s s i n peak molecular weight nor GPC peak broadening w i t h exposure. Although the polymer d i d not degrade, i t e x h i b i t e d a l o s s i n t e n s i l e s t r e n g t h a f t e r 9 days o f exposure corresponding to the l o s s i n r e f l e c t a n c e and onset of delamination (Figure 2; compare t h i s l o s s i n t e n s i l e s t r e n g t h to a s i m i l a r but l e s s extensive l o s s f o r PMMA^ i n Figure 1 ) . Because the polymer i n PMMA/Ag/glass showed no d e t e c t a b l e degradation during weathering, polymer degradation does not c o n t r i b u t e s i g n i f i c a n t l y t o delamination of PMMA. We b e l i e v e , however, that a p h y s i c a l change i n the polymer, p o s s i b l y i n d i c a t e d by the l o s s i n t e n s i l e s t r e n g t h , mechanically d i s r u p t s the s i l v e r , causing the l o s s i n r e f l e c t a n c e and eventual delamination. Stress r e l a x a t i o n and a d i f f e r e n c e i n c o e f f i c i e n t s o f thermal expansion between PMMA and g l a s s are f a c t o r s t h a t could c o n t r i b u t e to a p h y s i c a l change i n t h i s r i g i d polymer. Thus, t h i n n e r f i l m s i n PMMA/Ag/glass weather longer before delamination occurs. F u r t h e r more, delamination apparently can be avoided by e l i m i n a t i n g the g l a s s s u b s t r a t e from the m i r r o r assembly. P r e l i m i n a r y experiments i n progress i n d i c a t e that delamination does not occur under s i m i l a r exposure c o n d i t i o n s when the s i l v e r i s deposited d i r e c t l y onto a preformed polymer f i l m (3M, A c r y l a r X-2417) to form PMMA^/Ag/backing. Moreover, f o r the mixed a e r y l a t e Korad^ i n Koraa^/Ag/Antique, delamination was not apparent on weathering. These r e s u l t s i n d i c a t e that delamination can be reduced and that PMMA, t h e r e f o r e , shows p o t e n t i a l as a c o a t i n g m a t e r i a l f o r metal m i r r o r s . Chemical F a i l u r e :

Polymer/Mirror I n t e r f a c e

Reaction a t the polymer/mirror

i n t e r f a c e , observed o n l y f o r




130

100

10.0

I I-

'Ρ

C3 UJ

£

CC

χ

CO

Ο

CO

8.0

6.0

\

ζ 4.0 20

40

60

EXPOSURE TIME —

F i g u r e 2. Weathering o f PMMA

f

80

100

days

(3M)/Al/Adhesive.



BRAUMAN ET AL.


3

I PMMA/Ag/Glass

Ο 52 Κ · UJ

—

I

EXPOSURE TIME — days

F i g u r e 1. Weathering o f Poly(methyl methacrylate) Systi • PMMA/Ag/glass Ο PMMA/glass.



132


s i l v e r m i r r o r s coated w i t h EP, EVA, and PIB, was i n d i c a t e d by v a r y i n g degrees of l o s s i n r e f l e c t a n c e and degradation of the polymer (EP/Ag/glass >> EVA/Ag/glass > PIB/Ag/glass b e s t ) ; see Figures 3 to 5. The most n o t i c e a b l e change was y e l l o w i n g of the m i r r o r s ( i n d i c a t e d by arrows i n Figures 3 to 5) that preceded l o s s i n r e f l e c t a n c e . This d i s c o l o r a t i o n i s s e l e c t i v e , appearing only where the polymer was i n contact w i t h the s i l v e r . Furthermore, on prolonged exposure the l o s s i n molecular weight i s s l i g h t l y greater f o r the polymer over s i l v e r than f o r the polymer over g l a s s . Where the exposed polymer could be separated from the m i r r o r (EVA/Ag/glass), we found that most of the y e l l o w c o l o r remained w i t h the s i l v e r and not the polymer. This d i s c o l o r a t i o n probably i s not due to a r e a d i l y r e d u c i b l e form of s i l v e r (e.g., oxide, s u l f i d e ) because the c o l o r could not be removed by soaking the m i r r o r i n t e r f a c e i n a reducing s o l u t i o n of d i l u t e stannous c h l o r i d e . Yellowing of the m i r r o r s i s light-dependent. When EP/Ag/ g l a s s and PIB/Ag/glass samples were weathered i n the dark a t 70°C and 50% r e l a t i v e humidity, t h e i r m i r r o r s degraded and d i s c o l o r e d much more s l o w l y than those weathered i n the l i g h t (e.g., see Figure 4 ) . We conclude that we are observing r e a c t i o n at the polymers i l v e r i n t e r f a c e that r e s u l t s i n r e f l e c t a n c e l o s s . This i n t e r face r e a c t i o n could r e s u l t from degradation of the bulk polymer or i t s a d d i t i v e s , i n i t i a t i o n a t the polymer/mirror i n t e r f a c e , or r e a c t i o n w i t h the ambient atmosphere. I n t e r f a c e degradation apparently does not r e s u l t from any a p p r e c i a b l e r e a c t i o n of the coated m i r r o r w i t h c o r r o s i v e gases permeating i n from the atmosphere. Thus, when the degraded polymer was d i s s o l v e d o f f the m i r r o r of a weathered PIB/Ag/glass sample and the m i r r o r analyzed by X-ray/scanning e l e c t r o n microscopy, the i n t e r f a c e contained no s u l f u r (0.1% d e t e c t i o n l i m i t ) and no more c h l o r i n e than d i d an unweathered c o n t r o l . R a d i c a l s produced i n the degrading polymer matrix could r e a c t a t the polymer/mirror i n t e r f a c e e v e n t u a l l y causing m i r r o r f a i l u r e . Thus, f o r a l l three polymers, y e l l o w i n g and extensive degradation of the polymer (evidenced by l o s s i n molecular weight and where p o s s i b l e - i . e . , EVA - l o s s i n t e n s i l e p r o p e r t i e s ) occur simultaneously. Both processes precede any s i g n i f i c a n t l o s s i n r e f l e c t a n c e ; see Figures 3 to 5. Of the three polymers (PIB, EVA, EP) that e x h i b i t t h i s s e l e c t i v e y e l l o w i n g , only PIB/Ag/glass yellows and degrades without subsequent r a p i d , major l o s s i n r e f l e c t a n c e . Apparently, degradation o f the m i r r o r i s retarded when degradation of the polymer i s retarded by s h i e l d i n g the samples from the l i g h t .


BRAUMAN ET AL.


1.0 0.9

>

Reactivity of Polymers with Mirror Materials - ACS Symposium Series

Recommend Documents