The Reduction of Solar Light Transmittance in Thermal Solar

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Downloaded by UNIV OF LEEDS on September 15, 2016 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch005

The Reduction of Solar Light Transmittance in Thermal Solar Collectors as a Function of Polymer Outgassing R. M. LUCK and M. A. MENDELSOHN Westinghouse Research and Development Center, Pittsburgh,PA15235

The outgassing of polymeric materials used in the construction of thermal solar collectors is of great importance to solar collector manufacturers. The chemical products evolved through thermal degradation and outgassing usually condense on the glass or plastic glazing surfaces where they significantly reduce the transmittance of solar light and thereby reduce the efficiency of the solar collector This work was performed in an endeavor to determine the amount and type of condensable compounds given off by polymeric materials during thermal aging, and to relate these with the reduction in solar light transmittance and reduced solar collector efficiency. Sources of Outgassing Products Inside each solar collector two types of dew (condensation) points exist. One is a moisture dew point and the other a series of dew points resulting from organic compounds. The amount of a given vapor present in the air at any specific temperature/ pressure condition is essentially a function of its relative volatility and its thermodynamic activity. Air will hold more vapor, whether it be moisture or an organic compound, at a high temperature than it will at a low temperature. When the temperature of the collector glazing or frame drops below the moisture or chemical vapor dew point, condensation occurs. As the temperature of the collector rises above the dew point, the condensation on the glazing can evaporate. The evaporation of moisture is clean and complete, and no residues or deposits are left on the glazing. However, over long periods of time, moisture condensates slowly leach out sodium and other metal salts from the glass and a white deposit slowly forms. These salt deposits greatly reduce the transmittance of solar light through the glass(1). 0097-6156/83/0220-0081$06.00/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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Some condensates do not evaporate from the g l a z i n g i n a c l e a n manner and o f t e n leave a c o l o r e d v i s c o u s o i l y r e s i d u e , a white powdery d e p o s i t or a continuous s o l i d f i l m . These chemical d e p o s i t s are produced by three d i f f e r e n t processes o c c u r r i n g simultaneously w i t h i n the s o l a r c o l l e c t o r . F i r s t , condensable v o l a t i l e s are being produced by d i f f u s i v e l o s s of low molecular weight compounds o r i g i n a l l y present i n the p l a s t i c or e l a s t o m e r i c m a t e r i a l s . Second, the polymer i s being degraded and depolymeri z e d as a r e s u l t of exposure to elevated temperatures, water, u l t r a v i o l e t l i g h t , oxygen and ozone. T h i r d , the l i q u i d , t h i n f i l m chemical condensate that i s being formed on the g l a z i n g undergoes thermal, o x i d a t i v e , and u l t r a v i o l e t induced r e a c t i o n s to form a s o l i d polymeric d e p o s i t (2,J3,4). During the many condensation and evaporation c y c l e s that a s o l a r c o l l e c t o r experi e n c e s , these s o l i d d e p o s i t s s l o w l y b u i l d up reducing s o l a r l i g h t transmittance. The sources f o r outgassing i n c l u d e preformed s e a l s , s e a l c a u l k s , room temperature v u l c a n i z i n g polymers, thermal i n s u l a t i o n s , polymeric c o a t i n g s , and polymeric m a t e r i a l s used i n s o l a r collector structural applications. Experimental A t e s t was developed to study the outgassing and degradation of polymeric m a t e r i a l s . A sample of the polymeric m a t e r i a l to be evaluated i s placed i n the bottom of a g l a s s tube, and an i n f r a red sodium c h l o r i d e c r y s t a l i s mounted i n the open end of the tube. This assembly (Figure 1) i s then p o s i t i o n e d v e r t i c a l l y i n t o a c l o s e l y f i t t i n g hole i n the top of the oven so that only the lower two-thirds of the tube i s i n s i d e the oven. E s s e n t i a l l y , a l l of the condensable outgassing products condense on the bottom surface of the sodium c h l o r i d e c r y s t a l . Condensable v o l a t i l e s are observed as weight i n c r e a s e of the c r y s t a l and noncondensables by d i f f e r e n c e between t h i s and the t o t a l weight l o s s of the t e s t sample. The sodium c h l o r i d e c r y s t a l i s placed d i r e c t l y i n t o the i n f r a r e d spectrophotometer f o r determination of the chemical nature of the condensable product. The heat v u l c a n i z e d preformed s e a l s , g l a z i n g p l a s t i c s , absorber p l a t e polymers and i n s u l a t i o n m a t e r i a l s were evaluated as r e c e i v e d . The s e a l caulks and the room temperature v u l c a n i z i n g (RTV) m a t e r i a l s were cast i n approximately 1.5 mm t h i c k sheets on f l u o r o c a r b o n f i l m and room temperature v u l c a n i z e d f o r four to f i v e weeks p r i o r to t e s t i n g . In the standard outgassing t e s t , the oven was operated at 150°C f o r a p e r i o d of 225 hours. This i s not an u n r e a l i s t i c t e s t temperature s i n c e thermal s o l a r c o l l e c t o r s can reach 125-200°C. The sodium c h l o r i d e c r y s t a l , during the t e s t p e r i o d , reached an e q u i l i b r i u m temperature of 65+2°C. The compositions of the condensable products were i d e n t i f i e d by i n f r a r e d a n a l y s i s u s i n g a Perkin-Elmer I n f r a r e d Spectrophotometer. Relative light transmittance values were obtained on s e v e r a l condensable products

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


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Reduction of Light Transmittance

Glass Enclosure'

NaCI Crystal at 6 5 ± 2 ° C

Dry Air

Room Temperature~ 23°C Dimples in Glass Tube to Support Crystal

Oven Insulation

Oven @ 150° C

Seal Material

F i g u r e 1.

Outgassing measurement a p p a r a t u s .


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c o l l e c t e d on g l a s s d i s k s . These, as w e l l as s e v e r a l g l a s s g l a z i n g samples taken from operating s o l a r c o l l e c t o r s , were measured for t h e i r r e l a t i v e l i g h t t r a n s m i t t a n c e over the range of 400 to 900 nm using a Coleman Spectrophotometer w i t h a tungsten power supply. The composition and source of the polymeric m a t e r i a l s evaluated i n t h i s study are shown i n Table I .


R e s u l t s and D i s c u s s i o n Condensables are defined i n t h i s work as compounds which w i l l condense on a surface maintained a t 65+2°C. The noncondensables are those which w i l l not condense on that heated s u r f a c e . The noncondensable chemicals g e n e r a l l y do not adversely a f f e c t s o l a r l i g h t t r a n s m i t t a n c e i n a s o l a r c o l l e c t o r . They remain i n a gaseous s t a t e w h i l e i n s i d e the working c o l l e c t o r and u s u a l l y escape to the surrounding atmosphere. A l a r g e q u a n t i t y of noncondensable and condensable products i n d i c a t e s that changes w i t h i n the polymeric m a t e r i a l , which can adversely a f f e c t i t s p h y s i c a l and mechanical p r o p e r t i e s , have taken p l a c e . Outgassing of S i l i c o n e Polymers. The percent condensable versus time curves a t 150°C f o r the heat v u l c a n i z e d (HV) preformed s i l i c o n e s e a l s , and f o r the room temperature v u l c a n i z e d (RTV) s i l i c o n e s e a l a n t s are shown i n Figure 2. Samples of each c l a s s of s i l i c o n e were obtained from s e v e r a l sources f o r e v a l u a t i o n i n t h i s study. For ease of d i s c u s s i o n , however, only the h i g h and low extremes f o r each c l a s s of s i l i c o n e are p l o t t e d i n F i g u r e 2. A l l of the other s i l i c o n e s evaluated f e l l between these extremes. The RTV s i l i c o n e s , i n g e n e r a l , e x h i b i t c o n s i d e r a b l y greater outgassing of both condensable and noncondensable m a t e r i a l s than the HV s i l i c o n e s . Their condensable-time curves were u s u a l l y l i n e a r and only one condensable product was being evolved. The i n f r a r e d spectra of the condensable products from a l l of the s i l i c o n e s evaluated i n d i c a t e d these m a t e r i a l s to be predominately low molecular weight a l k y l l i n e a r and/or c y c l i c p o l y s i l o x a n e s (Table I I ) . S i l i c o n e A gave a n o n l i n e a r condensable-time curve and evolved two types of condensable products; an a l k y l l i n e a r and/or c y c l i c p o l y s i l o x a n e , and an aromatic e s t e r . The HV s i l i c o n e s evolved l e s s condensable and noncondensable compounds than t h e i r RTV c o u n t e r p a r t s . Their condensable-time curves were l i n e a r and e s s e n t i a l l y only one type of condensable compound was evolved. The condensable products from the HV s i l i cones were a l s o i d e n t i f i e d as a l k y l l i n e a r and/or c y c l i c p o l y siloxanes . None of the s i l i c o n e polymers s t u d i e d produced a maximum amount of condensables during the 225 hour t e s t . The outgassing products were being evolved and condensed at the same r a t e at the end of the t e s t period as were observed during the e a r l y stages of the t e s t . L i n e a r curves u s u a l l y i n d i c a t e a slow c o n t i n u a l


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TABLE I


Source and Composition of Polymers f o r Outgassing Studies

Sample A Β C D Ε F G H I J Κ L M Ν 0 Ρ Q R S Τ υ ν

Composition

Source/Identification

Dow Corning 93-067-2 S i l i c o n e (RTV) Dow Corning 732 S i l i c o n e (RTV) N.A. R e i s s 747 S i l i c o n e (HV) Pawling 96-B-24 S i l i c o n e (HV) P o l y s a r 100 B u t y l (HV) DuPont V i t o n Fluorocarbon (HV) Goodrich Hycar 4054 A c r y l i c (HV) DuPont Vamac E t h y l e n e - a c r y l i c (HV) A c r y l i c terpolymer (RTV) Tremco Mono Tremco B u t y l B u t y l (RTV) Gibson-Homans Hypalon Chlorosulfonated polyethylene (RTV) DuPont Nordel (EPDM) Ethylene-propylene terpolymer (HV) Ethylene-propylene Bio Energy System Sola R o l l (EPDM) terpolymer Polypropylene Comco (PP) Comco (PVC) Polyvinylchloride Rohm & Haas P l e x i g l a s (PMMA) Polymethylmethacrylate General E l e c t r i c Lexan (PC) Polycarbonate Eastman Chemical (CAB) C e l l u l o s e acetate butyrate F i l l o n (GRP-558) Glass r e i n f o r c e d polyester DuPont (FEP) Fluorocarbon Mobay Baytherm 851 Polyurethane Corning Glass Fiberglass

RTV = Room temperature v u l c a n i z e d HV = Heat v u l c a n i z e d



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Figure 2. Percent condensables and noncondensables o f s i l i c o n e s as a f u n c t i o n o f time a t 150 °C: (n,o) RTV s i l i cones; and preformed s i l i c o n e s .


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degradation (5,6>) of the polymer chain r a t h e r than a simple outgassing of low molecular weight m a t e r i a l s , such as p l a s t i c i z e r s and processing o i l s , which are incorporated i n t o the polymer but are not c h e m i c a l l y bonded. TABLE I I .

Major Bands ^ Wave Number cm

Interpretation

A

800, 1000«il00, 1260, 1490, 1570, 1590, 1720, 2800-3000

A l k y l l i n e a r or c y c l i c p o l y s i l o x a n e plus an aromatic ester

Β

800, 1000-1100, 1260, 2800-3000

A l k y l l i n e a r or c y c l i c polysiloxane

C

800, 1000-1100, 1260, 2800-3000


D

800, 1000-1100, 1260, 2800-3000


Sample


I n f r a r e d Spectrophotometric A n a l y s i s of The Condensable V o l a t i l e s From S i l i c o n e Polymers

The noncondensable v o l a t i l e s were u s u a l l y i d e n t i f i e d as moisture which had been adsorbed by the s i l i c o n e s . The moisture content of the s i l i c o n e s was l e s s than 0.5% by weight. Noncondensables v a l u e s greater than 0.5% by weight i n d i c a t e the presence of other low molecular weight, h i g h l y v o l a t i l e m a t e r i a l s such as a c e t i c a c i d and methanol. These m a t e r i a l s are generated and evolved d u r i n g the room temperature v u l c a n i z a t i o n of RTV s i l i c o n e s . Several RTV s i l i c o n e s evolved as much as 7% by weight of these noncondensable m a t e r i a l s . A p p a r e n t l y , many RTV s i l i c o n e s do not completely v u l c a n i z e even a f t e r four to f i v e weeks exposure to a i r p r i o r to t e s t i n g . Outgassing of Preformed Organic Polymers. The percent con densable-time curves f o r s e v e r a l heat v u l c a n i z e d (HV) preformed organic s e a l s are shown i n F i g u r e 3. The curves are g e n e r a l l y l i n e a r and only one condensable product was evolved. The f l u o r o carbon polymers u s u a l l y produced very l i t t l e i f any condensable products as i n d i c a t e d by sample F. The a c r y l i c polymers, such as sample G, produced a very s m a l l q u a n t i t y of u n i d e n t i f i e d products and gave l i n e a r , low slope condensable-time curves. The e t h y l e n e - a c r y l i c polymer (H) produced 4.2% by weight of t o t a l v o l a t i l e s . The condensables were i d e n t i f i e d as a c r y l i c fragments or o x i d i z e d ethylene fragments (Table I I I ) . The noncondensables were not i d e n t i f e d , but were considered to be mois ture i f t h e i r c o n c e n t r a t i o n was l e s s than 0.5% by weight.




F i g u r e 3. Percent condensables and noncondensables o f p r e formed s e a l s as a f u n c t i o n o f time a t 150 °C: (o) f l u o r o carbon; (•) a c r y l i c ; (o) ethylene a c r y l i c ; and (*) b u t y l .


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TABLE I I I .

Sample Ε

89

I n f r a r e d Spectrophotometric A n a l y s i s of The Condensable V o l a t i l e s From Preformed Organic Polymers Major Bands ^ Wave Number cm 1300, 1460, 1700, 2800-3000

Interpretation Stearic acid

F

I n s u f f i c i e n t sample f o r a n a l y s i s

G


H Downloaded by UNIV OF LEEDS on September 15, 2016 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch005


1380, 1420, 1710, 2800-3000

A c r y l i c or o x i d i z e d ethylene fragments

The b u t y l polymer (E) gave a nonlinear curve i n d i c a t i n g that p o s s i b l y two or more products were being condensed. Only one product, s t e a r i c a c i d , was i d e n t i f i e d by i n f r a r e d a n a l y s i s , how ever, i t i s p o s s i b l e that other o x i d i z e d hydrocarbons were present i n l e s s e r concentrations. S t e a r i c a c i d i s used i n many rubber formulations as an a c c e l e r a t o r a c t i v a t o r and as a l u b r i c a n t pro cessing a i d (_7) , and u s u a l l y e x i s t s i n the f r e e s t a t e i n the rub ber. I t i s t h e r e f o r e e a s i l y outgassed, as a r e the p l a s t i c i z e r s , and condenses on the cooler surfaces of the c o l l e c t o r . The s t e a r i c a c i d was s t i l l being evolved a f t e r 225 hours a t 150°C. Outgassing of Polymeric Caulking Compounds. The organic c a u l k i n g compounds evolved l a r g e q u a n t i t i e s of v o l a t i l e m a t e r i a l s even a f t e r four to f i v e weeks exposure to a i r p r i o r to t e s t i n g . The condensable-time curves f o r the polymeric c a u l k i n g m a t e r i a l s are shown i n Figure 4. The c h l o r o s u l f o n a t e d polyethylene (K) l i b e r a t e d 28.4% by weight of v o l a t i l e m a t e r i a l s . The condensable p o r t i o n , which accounted f o r almost h a l f of the v o l a t i l e products was i d e n t i f i e d as an a l k y l s u l f o n i c a c i d ester type m a t e r i a l . The noncondensables were not i d e n t i f i e d . The b u t y l compound (J) evolved about 15% by weight of v o l a t i l e m a t e r i a l s . The condensables were i d e n t i f i e d by i n f r a r e d a n a l y s i s to be e i t h e r o x i d i z e d p a r a f f i n i c o i l or o x i d i z e d b u t y l fragments or a combination of both (Table I V ) . The a c r y l i c terpolymer ( I ) produced 0.8% by weight of con densable m a t e r i a l which was i d e n t i f i e d as a c r y l a t e fragments and/ or o x i d i z e d p a r a f f i n i c o i l . The noncondensable v o l a t i l e s from t h i s group of organic c a u l k i n g compounds were not i d e n t i f i e d but a r e b e l i e v e d to c o n s i s t mostly of s o l v e n t s and monomeric d i l u e n t s .



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F i g u r e k. Percent condensables and noncondensables o f c a u l k i n g compounds as a f u n c t i o n o f time a t 150 °C: (•) a c r y l i c terpolymer; (o) b u t y l ; ( ) c h l o r o s u l f o n a t e d p o l y e t h y l e n e . Δ


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TABLE IV.

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I n f r a r e d Spectrophotometry A n a l y s i s of Condensable V o l a t i l e s From RTV Organic Polymers Major Bands ^ Wave Number cm

Sample



Interpretation

I

1160, 1220, 1380, 1460, 1720, 2800-3000

A c r y l a t e or o x i d i z e d o i l

J

1160, 1220, 1380, 1460, 1700, 2800-3000

Oxidized b u t y l fragments or o x i d i z e d o i l

Κ

1180, 1240, 1380, 1460, 1720, 2800-3000

A l k y l s u l f o n i c a c i d ester

Outgassing of Absorber P l a t e Polymers. Several polymers have been used as absorber p l a t e s f o r thermal s o l a r c o l l e c t o r s . The condensable-time curves f o r a few s e l e c t e d polymers a r e shown i n F i g u r e 5. The curves f o r these polymers a r e l i n e a r i n d i c a t i n g only one type of condensable v o l a t i l e product. The h i g h d e n s i t y polypropylene (N) showed l i t t l e o u t g a s s i n g , and produced only a small amount of condensable m a t e r i a l . The p o l y v i n y l c h l o r i d e polymer (0) produced 0.5% by weight of condensable product i d e n t i f i e d as o x i d i z e d hydrocarbons (Table V ) . The nonconden sables evolved from PVC were a c i d i c and c o n s i s t e d of a mixture of HC1 and low molecular weight hydrocarbons, such as ethylene and butylène. TABLE V.

Sample

I n f r a r e d Spectrophotometric A n a l y s i s of Condensable V o l a t i l e s From Absorber P l a t e Polymers Major Bands .. Wave Number cm""

Interpretation

L

1260, 1380, 1460, 1600, 2800-3000

Naphthenic o i l

M

1260, 1380, 1460, 1700, 2.800-3000

Oxidized p a r a f f i n i c processing o i l or o x i d i z e d EPDM f r a g ments

Ν 0

I n s u f f i c i e n t sample f o r a n a l y s i s 1260, 1460, 1700-1720, 2800-3000

Oxidized hydrocarbons




Figure 5. Percent condensables and noncondensables o f absorber p l a t e polymers as a f u n c t i o n o f time a t 150 °C: (o) EPDM source L; (•) EPDM source Μ; (Δ) PP; and (•) PVC.


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The EPDM polymers (L and M) produced d i f f e r e n t condensable m a t e r i a l s . Sample L evolved a s m a l l q u a n t i t y of naphthenic o i l , whereas sample M y i e l d e d a h i g h amount of o x i d i z e d hydrocarbons. Outgassing of Polymeric G l a z i n g . I n F i g u r e 6 are shown the condensable-time curves f o r s e v e r a l polymers that a r e being used as g l a z i n g (cover p l a t e s ) f o r thermal s o l a r c o l l e c t o r s . These polymers produced l i n e a r curves and only one condensable m a t e r i a l was evolved from each polymer. The polycarbonate (Q), the p o l y methylmethacrylate (Ρ), and the f l u o r i n a t e d ethylene-propylene (T) evolved e s s e n t i a l l y no condensable m a t e r i a l s . The g l a s s r e i n f o r c e d p o l y e s t e r (S) gave a l i n e a r curve i n d i c a t i n g a s i n g l e type of condensable m a t e r i a l i d e n t i f i e d as an aromatic e s t e r (Table V I ) . The c e l l u l o s e acetate b u t y r a t e (R) a l s o produced a l i n e a r condensable-time curve which c o n s i s t e d of CAB fragments. TABLE V I .

Sample

I n f r a r e d Spectrophotometric A n a l y s i s of Condensable V o l a t i l e s From G l a z i n g Polymers Major Bands ^ Wave Number cm

Interpretation

Ρ


Q


R

1080, 1180, 1730, 2800-3000

CAB

S

1070, 1130, 1280, 1570, 1590, 2800-3000

Aromatic e s t e r

Τ

fragments


Outgassing of Thermal I n s u l a t i o n . Very l i t t l e condensable products were evolved from e i t h e r the g l a s s wool or the p o l y urethane thermal i n s u l a t i o n s d u r i n g thermal aging. The p o l y urethane foam was s t o i c h i o m e t r i c a l l y balanced and u n i f o r m l y mixed to i n s u r e e s s e n t i a l l y complete and t o t a l r e a c t i o n of the i s o c y a ^ nate and the p o l y o l components. The polyurethane foam, however, d i d e x h i b i t moderatly h i g h noncondensables which were i d e n t i f i e d to be predominately the f l u o r o c a r b o n blowing agent which s l o w l y d i f f u s e s through the foam c e l l w a l l s (Figure 7 ) . The g l a s s wool e x h i b i t e d about 1% by weight of nonconden sables which c o n s i s t e d of adsorbed moisture and low molecular weight o i l fragments.



94


0

50

100 150 Time (hours)

200

225

Figure 6. Percent condensables and noncondensables o f glaze polymers as a f u n c t i o n o f time a t 150 °C: (ο) FMMA; (•) PC; Co) CAB; (Δ) GRP; and (•) FEP. 2.0

Γ

1.5h

ο ο c ο

c ο ο

1.0

0.5h

100 150 Time (hours)

225

Figure 7 · Percent condensables and noncondensables o f t h e r mal i n s u l a t i o n s as a f u n c t i o n o f time a t 150 °C: (o) p o l y urethane; and (•) f i b e r g l a s s .


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E f f e c t s of Condensable Deposits on R e l a t i v e L i g h t

Transmittance


The evolved compounds i n i t i a l l y condense on the g l a z i n g m a t e r i a l as l i q u i d s , and produce l i t t l e adverse e f f e c t upon the r e l a t i v e l i g h t transmittance. However, as these t h i n condensate f i l m s are exposed to heat and oxygen, over long periods of time, they slowly degrade and o x i d i z e , and are e v e n t u a l l y converted i n t o s o l i d m a t e r i a l s which g r e a t l y reduce l i g h t transmittance. The e f f e c t s of condensate d e p o s i t s on the r e l a t i v e l i g h t t r a n s mittance are shown i n Table V I I . Several of these values were obtained from working s o l a r c o l l e c t o r s w h i l e others were obtained from l a b o r a t o r y prepared samples. S a l t D e p o s i t s , The s a l t d e p o s i t s formed on a g l a s s g l a z i n g due to water l e a c h i n g , over a period of two to three years, produce a c o n s i d e r a b l e r e d u c t i o n i n l i g h t transmittance. A s a l t deposit having a d e n s i t y of 0.237g/square meter produces a 34% r e d u c t i o n i n r e l a t i v e l i g h t transmittance. This r e d u c t i o n i s caused by the r e f l e c t i o n and a b s o r p t i o n of the l i g h t by the s a l t c r y s t a l s (8). S i l i c o n e s Polymers, The outgassing products from s i l i c o n e polymers i n i t i a l l y condense on the g l a z i n g as l i q u i d a l k y l l i n e a r and/or c y c l i c p o l y s i l o x a n e s . These l i q u i d condensates have been observed to form on s o l a r c o l l e c t o r s a f t e r s e v e r a l days of t e s t i n g i n the A r i z o n a d e s e r t . The l i q u i d p o l y s i l o x a n e s do not a p p r e c i a b l y a f f e c t the r e l a t i v e l i g h t transmittance. However, as the t h i n f i l m i s exposed to the harsh environmental c o n d i t i o n s encountered during three months of desert t e s t i n g , i t i s s l o w l y converted i n t o a white c o l l o i d a l s i l i c a powder through the f o l l o w i n g type of o v e r a l l r e a c t i o n (9). 0—

Si(CH ) 3

S

(CH0 Si 9

X

0

0 — SifCH Ï

2

CH

I 0

Q

Si-0

Si—Ο Ι 0 Jn

A c o l l o i d a l s i l i c a deposit having a d e n s i t y of 0.129g/square meter can reduce the r e l a t i v e l i g h t transmittance by as much as 18%. B u t y l Polymers. The outgassing products from the b u t y l polymers a l s o i n i t i a l l y condense out as l i q u i d s and are slowly converted i n t o tan c o l o r e d s o l i d v a r n i s h - l i k e d e p o s i t s . These resinous d e p o s i t s are formed through a thermal o x i d a t i v e degradation process as f o l l o w s :



96

TABLE V I I


E f f e c t of Glaze Deposits on R e l a t i v e L i g h t Transmittance

Deposit Water Leaching of Glass Salts Salts S i l i c o n e Rubber Fragments Liquid Powder B u t y l Rubber Fragments Liquid Solid Varnish A c r y l i c Fragments Solid Film Stearic Acid Liquid Solid Processing O i l s Liquid Solid Varnish EPDM Fragments Liquid Solid Varnish PVC Fragments Liquid Varnish

Quantity (g/m ) 2

Relative Light Transmittance % Reduction

0.151 0.237

20 34

0.4 0.129

~1 18

0.2 0.08

~1 4

0.02

Λ,Ι

0.4 0.4

^1 5

0.4 0.2

^1 10

0.1 0.1

^1 4

0.05 0.1

^1 3


5.

LUCK AND MENDELSOHN


97

(2,3,10) R. •OH RR.


Molecular enlargement, c r o s s l i n k i n g , and polymer t e r m i n a t i o n occur during the l a s t step to form a r e s i n o u s m a t e r i a l , which produces a great r e d u c t i o n i n l i g h t transmittance. A c r y l i c Polymers. The a c r y l i c polymers u s u a l l y produce a c l e a r transparent and continuous f i l m on the g l a z i n g . These f i l m s produced r e l a t i v e l y l i t t l e e f f e c t on l i g h t t r a n s m i t t a n c e , and d i d not appear to change w i t h time. S t e a r i c A c i d . Many rubber formulations c o n t a i n s t e a r i c a c i d as a processing a i d . S t e a r i c a c i d i s e a s i l y outgassed, and condenses on the g l a z i n g i n e i t h e r the l i q u i d or s o l i d form depending on the temperature of the s o l a r c o l l e c t o r and the g l a z i n g . The l i q u i d s t e a r i c a c i d produces l i t t l e e f f e c t on r e l a t i v e l i g h t transmittance, whereas the c r y s t a l l i n e form produces a s i g n i f i c a n t reduction. Processing O i l s . The v a r i o u s o i l s used i n the manufacture of rubber products as processing a i d s and extenders are not chemic a l l y bonded to the b a s i c polymer. During thermal aging they slowly d i f f u s e out of the rubber, and condense on the g l a z i n g as l i q u i d s . They then undergo thermal o x i d a t i v e degradation, i n the same manner as the b u t y l condensates, to form r e s i n o u s , v a r n i s h l i k e deposits which g r e a t l y reduce the l i g h t transmittance. EPDM Polymers. The condensable m a t e r i a l s from the EPDM polymers, though d i f f e r e n t i n composition, s t i l l formed r e s i n o u s , v a r n i s h - l i k e products through a thermal o x i d a t i v e degradation process. The l i q u i d condensates d i d not adversely a f f e c t l i g h t transmittance. The s o l i d products produced a s i g n i f i c a n t reduct i o n s i m i l a r to the s o l i d product obtained from the b u t y l polymers and the processing o i l s . PVC Polymers. The condensable products from the PVC a l s o formed a s o l i d , v a r n i s h - l i k e m a t e r i a l . The percent l i g h t t r a n s mittance r e d u c t i o n was e s s e n t i a l l y equivalent to that obtained from the b u t y l polymers, the processing o i l s , and the EPDM p o l y mers . Conclusion Outgassing of polymeric m a t e r i a l s i s one of the major f a c t o r s reducing s o l a r l i g h t t r a n s m i t t a n c e i n s o l a r c o l l e c t o r s . V a r i o u s


98


m a t e r i a l s are evolved d u r i n g thermal aging and most of these adversely a f f e c t the e f f i c i e n c y of the s o l a r c o l l e c t o r . In order t o produce s o l a r c o l l e c t o r s , which w i l l d i s p l a y high e f f i c i e n c y f o r many years, i t i s extremely important t o u t i l i z e those polymeric m a t e r i a l s which are thermally and o x i d a t i v e l y s t a b l e , and do not produce m a t e r i a l s which have an adverse e f f e c t on the performance c h a r a c t e r i s t i c s of the s o l a r collector. Acknowledg ment


This work has been supported by the Solar Heating and C o o l i n g Research and Development Branch, O f f i c e of Conservation and Solar A p p l i c a t i o n , U.S. Department of Energy. Literature Cited 1.

Mendelsohn, Μ. Α . ; Luck, R. M . ; Yeoman, F. Α . ; and Navish, F. W., Ind. Eng. Chem. Prod. Res. Dev., 1981, 20, 508-514. 2. Gibroy, Η. M. "Durability of Macromolecular Materials", Eby, R. K . , E d . , American Chem. Soc., Washington, DC, 1979, pp 6374. 3. Shelton, J . R.; Pecsok, R. L.; and Koenig, J . L. i b i d , pp. 7596. 4. Grassie, N. "The Chemistry of High Polymer Degradation Processes", Interscience Publisher, Inc., New York, 1956, pp. 68-80. 5. Flynn, J . H . ; and Dickens, B. "Durability of Macromolecular Materials", Eby, R. Κ., E d . , American Chem. Soc., Washington, DC, 1979, pp. 108-115. 6. L i c a r i , J . J.; and Wegand, B. L . "Resins for Aerospace", May C. A. Ed. American Chem. Soc., Washington, DC, 1979, pp. 127137. 7. Winspear, G. G. "Rubber Handbook", R. T. Vanderbilt Co., Inc., New York, 1968, pp. 68 and 237-238. 8. Jaffe, H. H . ; and Orchin, M. "Theory and Application of Ultra violet Spectroscopy", John Wiley and Sons, Inc., New York, 1962, p. 4. 9. Rochow, E. G. "Chemistry of the Silicones", John Wiley and Sons, Inc., New York, 1951, p. 95. 10. Holmstrom, A. "Durability of Macromolecular Materials", Eby, R. Κ., E d . , American Chem. Soc., Washington, DC, 1979, pp. 45-62. RECEIVED November 22,

1982


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