SIR 82-2583

utdoor Exposure Tests of Solar

Absorptive Coatings

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

October 1 982

Prepared for U.S. Department of Energy

Office of Solar Heat Technolgies

-QC—

100

.1156

82-2583

1982

Active Solar Heating and Cooling Division

4iWashington, DC 20585

NBSIR 82-2583

OUTDOOR EXPOSURE TESTS OF SOLAR

ABSORPTIVE COATINGS

National Bureau of Standards

library, £-01 Admin. Bldg.

06f § I 1982

8ftoo“

• 'H

u

Larry W. Masters

James F. Seiler

Willard E. Roberts

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

October 1 982

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Solar Heating and Cooling Division

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This report Is a follow up to an earlier report (NBSIR 81-2232, January 1981)

In which data on the performance of selected absorptive coatings In both accelerated laboratory exposures and outdoor exposures at three sites were presented.

The research presented In this report focuses upon the results obtained by continuing the outdoor exposures of absorptive coatings using ASTM E781-81, Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to

Conditions Simulating Stagnation In Solar Collectors with Cover Plates.

Comparison of the results of the outdoor exposures with those obtained In

accelerated laboratory exposures Indicated that 1) the accelerated exposures, as described In ASTM E744-80, Standard Practice for Evaluating Solar Absorptive

Materials for Thermal Applications, provide more severe exposure conditions than

outdoor exposures, and 2) the degradation processes Induced by outdoor exposure

are adequately addressed by the accelerated laboratory exposures.

Key Words: absorptive coatings; accelerated laboratory exposures; degradation;

outdoor exposures; simulated stagnation exposure; solar energy.

ill
ACKNOWLEDGMENT

The authors gratefully acknowledge 1) the many suggestions provided by members

of ASTM E44.04 on Materials Performance In planning and carrying out this study,

2) the interest demonstrated by the Office of Solar Heat Technologies of the

Department of Energy in supporting the research, and 3) the cooperation of

manufacturers who provided absorptive coating test specimens for inclusion in

the study and commercial exposure test sites who performed outdoor exposures.

iv
TABLE OF CONTENTS

Page

ABSTRACT Ill

ACKNOWLEDGMENT iv

LIST OF TABLES vi

LIST OF FIGURES vl

1. INTRODUCTION 1

1.1 BACKGROUND 1 1.2 OBJECTIVE 1 1.3 SCOPE 1

2. MATERIALS AND TEST PROCEDURES 3

2.1 MATERIALS 3

2.2 TEST PROCEDURES 3

2.2.1 Property Measurement Tests 3

2.2.2 Outdoor Exposure Tests 3

3. RESULTS AND DISCUSSION 5

3.1 EXPOSURES AT GAITHERSBURG, MARYLAND 5 3.2 EXPOSURES AT PHOENIX, ARIZONA 6

3.3 EXPOSURES AT MIAMI, FLORIDA 6

3.4 COMPARISON OF OUTDOOR EXPOSURE RESULTS WITH ACCELERATED

LABORATORY RESULTS 7

3.4.1 Nickel Coated Foil 7 3.4.2 Anodized 6061 Aluminum 7 3.4.3 Anodized 1100 Aluminum 7 3.4.4 Copper Oxide 7 3.4.5 Black Chrome 8

3.4.6 Polyvinylidene Fluoride 8

3.4.7 Urethane 8

3.4.8 Alkyd 8

3.4.9 Epoxide 9

3.4.10 Silicone 9

3.4.11 Modified Polyester 9

3.4.12 Porcelain 9

4. CONCLUSIONS 10

5. BIBLIOGRAPHY 11

v
LIST OF TABLES

Page

1. Absorptive Coatings and Substrates Used in the Study 12

2. Results of Outdoor Exposure at Gaithersburg, Maryland 13

3. Results of Oudoor Exposure at Phoenix, Arizona 14

4. Results of Outdoor Exposure at Miami, Florida 15

LIST OF FIGURES

1 . Test Box Used for Outdoor Exposures 16

vi
1. INTRODUCTION

1.1 BACKGROUND

The purposes of absorptive coatings used in solar energy collectors for heating

and cooling applications are to absorb the sun's energy and convert it to thermal energy. The optical properties of absorptive coatings, i.e., solar absorptance (a) and infrared emittance (e), are important attributes and are useful

in assessing the performance and durability of coatings.

A 1977 report by Skoda and Masters [1].!/ identified a number of materials

related solar collector problems in operational systems. The report pointed

out the need for test methods to assess the effect of exposure conditions on

the performance of solar collector materials, such as absorptive coatings.

Subsequently, research was initiated to aid in developing the technical bases

for new test methods for absorptive coatings. Results of accelerated laboratory exposures and outdoor exposures under simulated stagnation conditions (up

to 12 months) were reported in 1981 [2]. These results were used in the development of ASTM E744-80, Standard Practice for Evaluating Solar Absorptive

Materials for Thermal Applications [3]. The outdoor exposure tests initiated

in the above study [2] x*ere continued to obtain additional data on the ability

of the coatings to withstand the simulated stagnation conditions and on the

comparison of simulated stagnation exposure results with those obtained from

accelerated laboratory exposures as included in ASTM E744-80 [3]. The additional data, for three outdoor exposure sites, are presented in this report.

The outdoor exposure data previously reported [2] are included along with the

new data for the convenience of the reader.

1.2 OBJECTIVE

The objective of the studies presented in this report was to obtain data on the

performance of both selective and non-selective absorptive coatings when

exposed outdoors to simulated stagnation conditions at three sites. Data on exposed coatings are needed to help assess the newly developed standard, ASTM

E781-81, Standard Practice for Evaluating Absorptive Solar Receiver Materials

when Exposed to Conditions Simulating Stagnation in Solar Collectors with Cover

Plates [4], which is one testing option provided in ASTM E744-80 [3]. Data are also needed to aid in comparing results obtained by the two testing methods in

E744-80: 1) accelerated laboratory exposure; and 2) simulated stagnation

exposure (E781-81).

1.3

SCOPE

Test specimens of four selective and eight non-selective absorptive coatings on various substrates were exposed outdoors at three sites in accordance with ASTM

E781-81 [4]. The solar absorptance and infrared emittance were measured before

exposure and at approximately four month exposure intervals. The three exposure sites were: 1) Gaithersburg, Maryland, 2) Phoenix, Arizona, and 3) Miami,

1/ Numbers in brackets refer to references in the Bibliography.

1
Florida. The exposure times were cumulative; that is, the specimens were retrieved at each measurement interval and reinstalled in the outdoor exposure

boxes after measurement of optical properties and visual Inspection.

The test data contained in this report are not recommended for design purposes.

The properties of absorptive coatings are likely to vary from manufacturer to

manufacturer and from one production run to another. Data to aid design of collectors must, therefore, be obtained for the specific material to be used. The

coatings used in this study were selected to cover a range of generic coatings

so that test methods developed based upon the data would be broadly applicable.

But the data on specific generic coatings do not necessarily represent the performance of all coatings within each generic grouping. In addition, the infrared emlttance data reported are total normal emittance rather than total hemispherical emittance.

2
2. MATERIALS AND TEST PROCEDURES

2.1 MATERIALS

Absorber materials included in the test program were nickel coated foil,

anodized aluminum, copper oxide, black chrome, polyvinylidene fluoride (two),

urethane, alkyd, silicone, modified polyester, epoxide, and porcelain enamel.

These were selected to provide a cross-section of generic types of absorber

materials which have been or could be used in solar collectors. Substrates

included steel, copper, and aluminum although not all absorbers were tested on

all substrates. Table 1 includes a list of the materials and the substrates on which they were applied.

2.2 TEST PROCEDURES

2.2.1 Property Measurement Tests

Absorptance was calculated from reflectance as measured using ASTM E424-71,

Method A [5], which utilizes an integrating sphere spectrophotometer, and total

normal infrared emittance was measured using ASTM E408-71 [6]. Absorptance is

reported as an integrated value from 0.35 to 2.1 ym for an Air Mass 2 solar

spectrum. In addition to optical measurements, the specimens were visually

inspected before and after aging.

2.2.2 Outdoor Exposure Tests

Triplicate specimens of each coating/substrate combination were exposed outdoors

in accordance with ASTM E781-81 [4]. The exposure boxes, which simulate a solar

collector, are shown in figure 1. The boxes included a single cover plate con- forming to Type 1 (tempered low iron glass) of E781-81, fiber glass insulation,

and a coated metallic specimen mounting plate. For exposure of selective coatings, the boxes contained a black chrome coated mounting plate while the boxes

for exposure of non-selective coatings contained a flat black coated mounting

plate. The Test Plan Option A of E781-81, in which specimens are returned to

the boxes for continued exposure after each nondestructive measurement of properties (each time increment), was used in the study. The orientation (tilt

angle) of the exposure boxes was adjusted four times each year to provide

increased solar flux as follows:

3
Exposure Schedule at Each Tilt Angle

Tilt Angle Gaithersburg^/ Phoenix^/ Miami

5° May-August April-August April-August

local latitude September September September

45° October-March October-February October-February

local latitude April March March

39° latitude

33° latitude

26° latitude

Exposures were performed at three sites having distinctly different climates:

Gaithersburg, Maryland; Phoenix, Arizona; and Miami, Florida. The Maryland

exposures were started in June 1978. Specimens were first retrieved for evaluation in October 1978. Subsequent exposure periods were: 1) from November

1978 to March 1979, 2) from April 1979 to August 1979, 3) from April 1980 to

August 1980, 4) from November 1980 to March 1981, and 5) from May 1981 to

September 1981.

The Arizona exposures were initiated in November 1978. The exposure periods

were: 1) from November 1978 to March 1979, 2) from April 1979 to August 1979,

3) from May 1980 to September 1980, 4) from November 1980 to March 1981, and

5) from May 1981 to September 1981.

The Florida exposure periods were: 1) from June 1978 to October 1978, 2) from

November 1978 to May 1979 (including a two month non-exposure period due to

storm damage at the test site), 3) from July 1979 to November 1979, 4) from May

1980 to September 1980), 5) from December 1980 to April 1981, and 6) from May

1981 to September 1981.

The intent, at the initiation of the study, was to have the same exposure

increments at all three sites throughout the test program. Weather and instrumentation problems caused some deviation from this intent as indicated in the

actual dates of specimen exposure.

The mounting plate temperature of the boxes exposed at the Maryland site was monitored periodically near solar noon. A thermocouple embedded in the back

side of the mounting plate was used for temperature measurements. Cumulative

readings of total direct irradiation were recorded at each site as a possible

basis for site comparison.

4
3. RESULTS AND DISCUSSION

Tables 2, 3, and 4 contain absorptance and Infrared emittance data obtained for

the three exposure sites. The initial absorptance and infrared emittance values (at time zero) are an average of ten specimens of each sample. With the

exception of the emittance values for absorber C5 (copper oxide), the optical

properties of each sample were reproducible within 0.01 unit from specimen to

specimen. The values reported in tables 2, 3, and 4 for time increments other

than 0 (zero) are the average of triplicate specimens.

During the study, a set of test specimens was stored as controls in the

laboratory at 20 ± 3°C and 50 ± 5 percent r.h. Measurement of the absorptance

and emittance after completion of all exposure tests demonstrated that no changes in the optical properties were induced in the control specimens.

The reader is referred to the previous report on this study [2] for photographs

of the coatings before and after many of the exposure tests; these photographs

illustrate the primary visual changes noted in the following discussion.

3.1 EXPOSURES AT GAITHERSBURG , MARYLAND

Table 2 contains the absorptance and infrared emittance data obtained for

specimens exposed for various time Increments at Gaithersburg, Md. The estimated total direct solar irradiation received during the 24 months of actual

exposure was 10.88 x 10^ j/m2.,l/ The maximum mounting plate temperature

observed with the selective coatings exposure box was 182 °C; that for the

nonselective boxes was 148 °C.

The data in table 2 show that little change was observed in the absorptance and

emittance of most absorber materials during the exposures. The indicated change

in absorptance and emittance of copper oxide must be interpreted with caution

because of the large variability noted in emittance for unexposed (time zero)

specimens. Black chrome on copper (D4) was observed to change in absorptance

from 0.96 to 0.91 over the exposure period, although the emittance remained

relatively unchanged. The emittance of the modified polyester specimens (Ml,

M4 and M8) was observed to increase by 0.05 to 0.10 units depending upon the

substrate

.

Visual inspection of the specimens at each exposure increment led to the

following observations:

1. The adhesive used to bond the nickel coated foil to the aluminum substrate

delaminated in several areas of the test specimens early in the course of

exposures. The delaminations appeared to increase in number and area with

increasing time of exposure. But the foil did not completely delaminate

from the substrate during the exposure test. Delamination of the foil was also reported during laboratory oven aging tests [2],

A/ This value of solar irradiation is estimated because of instrument failures

during approximately 10 percent of the exposure days.

5
2. The G9 ( polyvinyl idene fluoride on hot dipped galvanized steel) delaminated

slightly during the 24 month exposure time. Previous laboratory exposures

at 92°C and 97 percent r.h. were found to cause extensive delamination of

this coating [2 ]

.

3. The modified polyester (M) coatings were gradually removed from the

substrates with increasing time of exposure. The nature of the "removal"

is more aptly described as vaporization as opposed to delamination. For

the M8 specimens (modified polyester on aluminum), the loss of coating was nearly complete at the end of the exposure; for the Ml (steel) and M4

(copper) specimens, the loss of coating was less extensive than for the M8

specimens

.

3.2 EXPOSURES AT PHOENIX, ARIZONA

Table 3 contains the results of the outdoor exposures at Phoenix, Arizona. The

total direct solar irradiation received during the 20 months of actual exposure

was 12.66 x 109 J/m2. With the exception of C5 (copper oxide), the absorptance

of specimens did not change more than 0.02 units during the 20 months of exposure. As noted in section 3.1, the copper oxide changes must be interpreted

with caution. The emittance of the modified polyester specimens increased during the exposure, but not to the extent observed for the Maryland exposures.

The emittance of other samples did not change by more than 0.02 units.

Visual observations were as follows:

1. The adhesive used to bond the nickel coated foil to the aluminum substrate

delaminated in the same manner as noted in section 3.1.

2. The G9 (polyvinylidene fluoride on hot dipped galvanized steel) delaminated

during the fourth exposure interval. When the specimens were retrieved

after 16 months exposure, the coating on all three specimens was completely

delaminated from the substrate.

3.3 EXPOSURES AT MIAMI, FLORIDA

Table 4 contains the results of the outdoor exposures at Miami, Florida. The

total direct solar irradiation received during the 24 months of actual exposure was 13.62 x 109 j/m2.

The data in table 4 are similar to the data in table 2 (Maryland) regarding

1) a loss of absorptance of black chrome on copper (D4), and 2) an increase in

the infrared emittance of the modified polyester specimens (Ml, M4 and M8).

For most absorbers, the exposure resulted in little or no change in the optical

properties

.

Visual inspection of the test specimens provided the following observations:

1. The adhesive used to bond the nickel coated foil to the aluminum substrate

delaminated as noted for the exposures in sections 3.1 and 3.2.

6
2. The G9 (polyvinylidene fluoride on hot dipped galvanized steel) delaminated

slightly during the 24 month exposure. The extent of the delamination was comparable to that observed at the Gaithersburg, Maryland site.

3. The "removal" of modified polyester (M) reported for the Gaithersburg,

Maryland exposures (section 3.1) was also observed at the Miami site. The

extent of loss of coating was nearly complete for the M8 specimens (alumi- num substrate) and less extensive for the Ml (steel substrate) and M4

(copper substrate) specimens.

3.4 COMPARISON OF OUTDOOR EXPOSURE RESULTS WITH ACCELERATED LABORATORY RESULTS

As noted in the Introduction to this report, results of accelerated laboratory

exposures were previously reported [2] for the same coatings evaluated in this

study. Since ASTM E744-80 [3] provides the option of testing using either

accelerated laboratory exposure (Method A) or simulated stagnation exposure

(Method B) , it is important to compare the results obtained by outdoor exposure

(simulated stagnation) to those obtained by accelerated laboratory exposure.

The following discussion will highlight some of the major similarities and

differences in the results from the two types of exposure,.

3.4.1 Nickel Coated Foil

The optical properties of the nickel coated foil remained virtually unchanged

during both the accelerated laboratory exposures and the outdoor exposures.

The adhesive used to bond the foil to the substrate partially delaminated in

outdoor exposures at all three sites and in the accelerated laboratory exposures

involving oven aging.

3.4.2 Anodized 6061 Aluminum

The optical properties of B6 anodized aluminum specimens exhibited little if

any change during the outdoor exposures and in all the accelerated laboratory

exposures except the exposure to moisture using a condensation cycle. In the

latter laboratory exposure, the infrared emittance increased from 0.81 to 0.90 over a period of 12 weeks exposure time.

3.4.3 Anodized 1100 Aluminum

The optical properties of 37 anodized aluminum specimens exhibited little if

any change during the outdoor exposures and in all the accelerated laboratory

exposures except moisture exposure. Exposure at 92 °C and 97 percent r.h. led

to an increase of absorptance (from 0.90 to 0.93) while exposure using the

condensation cycle led to an increase in infrared emittance (from 0.82 to

0.91).

3.4.4 Copper Oxide

The outdoor exposures led to loss in absorptance of copper oxide specimens at

all three exposure sites; the absorptance after the final exposure increment had

changed from 0.95 to 0.90 (Maryland) , to 0.87 (Arizona) and to 0.86 (Florida).

7
Loss of absorptance was also observed in the laboratory tests involving 1) oven

aging, and 2) thermal cycling. The infrared emittance changes are of questionable significance because of the emittance variability noted previously for

the copper oxide specimens.

3.4.5 Black Chrome

Black chrome on steel specimens (D2) exhibited little, if any, change in optical

properties during the outdoor exposures and during the accelerated laboratory

exposures

.

Black chrome on copper specimens (D4) changed somewhat in absorptance during

the outdoor exposures; the Maryland exposures led to a change from 0.96 to

0.91, Arizona from 0.96 to 0.94 and Florida from 0.96 to 0.91. The absorptance

of D4 specimens decreased by 0.03 units or more in the following laboratory

exposures: oven aging, thermal cycling and moisture condensation cycle.

3.4.6 Polyvinylidene Fluoride

As mentioned in sections 3.1, 3.2 and 3.3, the G9 specimens (polyvinylidene

fluoride on hot dipped galvanized steel) exhibited coating delamination at all

three exposure sites, although the delamination was more extensive at the Arizona site than at the other sites. The laboratory exposures consisting of 1)

oven aging at 250°C and 2) aging at 92°C and 97 percent r.h. caused delaminations similar to those observed in the outdoor exposures.

The specimens of polyvinylidene fluoride on steel (HI) exhibited slight changes

in emittance during the outdoor exposures but little, if any, change in absorptance. Oven aging at 200 and 250°C led to changes in emittance; at 250°C, the

emittance decreased from 0.85 to 0.54 after 12 weeks exposure. Thermal cycling

in the laboratory also led to a slight decrease in emittance.

The H8 (polyvinylidene fluoride on aluminum) exhibited little, if any, change

in optical properties during outdoor exposure or during accelerated laboratory

exposure, except for 1) a decrease in emittance (from 0.88 to 0.52) in oven aging exposures at 250°C, and 2) a decrease in emittance (from 0.88 to 0.85)

in oven aging at 200°C and thermal cycling.

3.4.7 Urethane

Little or no changes in optical properties were obtained in the urethane

specimens during outdoor exposure and in laboratory oven aging at 150 and 200°C.

Oven aging at 250°C led to a removal of the urethane coating from both copper

and aluminum substrates but it is unlikely that urethane coatings would be

exposed to temperatures over 200°C in stagnating collectors. Thermal cycling

led to removal of the urethane on steel.

3.4.8 Alky

d

The outdoor exposures induced only slight changes in optical properties for the

alkyd specimens. Laboratory oven aging at 150 and 200°C also induced only

8
slight changes in optical properties. In oven aging exposures at 250°C and in

thermal cycling, the alkyd coating was removed from some substrates during

exposure. The 250°C oven aging exposure is probably unrealistic since it is

unlikely that alkyd absorptive coatings would be exposed to temperatures greater

than 200°C in stagnating collectors. Laboratory moisture exposure at 92 °C

and 97 percent r.h. led to a loss of absorptance (from 0.97 to 0.90) for alkyd on steel (Jl) specimens. As previously mentioned [2], laboratory moisture

tests led to corrosion of the surfaces of alkyd on aluminum specimens which

did not significantly affect the optical properties. Corrosion of specimens

was not observed following outdoor exposure but outdoor exposure in a well

sealed box may not be indicative of corrosion in collectors with poorly sealed

seams

.

3.4.9 Epoxide

Only slight changes in optical properties of the epoxide specimens were obtained

during the outdoor exposures. Laboratory oven aging at 200 and 250 °C led to

delamination of the coating from most substrates. Thermal cycling also led to

delarainatlon of the coating on steel and copper. Moisture exposures at 92 °C

and 97 percent r.h. led to a loss of absorptance (from 0.97 to 0.90) for both

steel and aluminum substrate specimens. Scanning Election Microscopy (SEM)

of laboratory exposed specimens showed degradation on the epoxide matrix during

moisture exposure [2]. The same type of degradation was observed in outdoor

exposure specimens from all three sites, but the extent of degradation was

less after outdoor exposure than after 21 weeks laboratory exposure at 92 °C

and 97 percent r.h.

3.4.10 Silicone

Specimens of silicone coating exhibited little or no change in optical

properties during the outdoor exposures. In the laboratory exposures, oven

aging at 250°C and thermal cycling led to removal of the coating on some substrates. Oven aging at 150 and 200°C led to little or no change in optical

properties

.

3.4.11 Modified Polyester

As noted in sections 3.1 and 3.3 of this report, the outdoor exposures in

Maryland and Florida led to "removal" of the modified polyester coating from

the substrates. The data in tables 2 and 4, however, indicate that the absorptance was not significantly changed although the eraittance increased. Oven

aging at 250°C led to removal of the coating as did thermal cycling. During

oven aging at 200 °C, changes in absorptance of up to 0.06 units and changes

in emittance of up to 0.04 units were noted.

3.4.12 Porcelain

The optical properties of poreclain specimens exhibited little or no change in

either the outdoor exposures or the accelerated laboratory exposures.

9
4 CONCLUSIONS

1.

The outdoor exposure data, developed using ASTM Standard Practice E781-81,

show that simulated stagnation conditions degrade some materials typically

used as absorptive coatings.

2. Degradation of absorptive coatings, such as change of optical properties

or appearance that was observed during outdoor exposures^ was also observed

in one or more of the accelerated laboratory exposures included in ASTM

E744-80. This indicates that the degradation processes induced by stagnation exposure are adequately addressed by the accelerated laboratory

exposures

3. Changes in properties were observed with accelerated laboratory exposures

but not with outdoor exposures for the following coatings: Anodized aluminum (moisture exposure), Urethane (oven aging and thermal cycling), Alkyd

(oven aging and moisture exposure), Epoxide (oven aging, thermal cycling

and moisture exposure) and Silicone (oven aging and thermal cycling). This

indicates that the accelerated laboratory exposures provide more severe exposure conditions than outdoor exposure. Frequently, 250°C oven aging of

non-selective coatings induced changes that were not observed in oven aging

at lower temperatures or in outdoor exposure tests. Since it is highly

unlikely that non-selective absorption coatings would even be exposed to

stagnation temperatures greater than 200°C, exposures at 250°C are probably

far too severe to simulate actual in-service conditions.

4.

Degradation of absorptive coatings, which can be observed visually or with

microscopy, does not always result in a change in optical properties. This

points out the need to visually inspect exposed coatings and to note

observable changes.

5.

The current exposure requirements in ASTM E781-81 stipulate a minimum

solar irradiation of 6.2 x 10^ J/m^, which is based upon the solar irradiation received in a 12 month period with an average daily value of 1.7 x 10^

J/m^(1500 Btu/f t^/day ) . The data obtained in this study show that, where

degradation was observed, it occurred within the time required to achieve

that total solar irradiation. Therefore, the current exposure requirements

of ASTM E781-81 appear to be adequate.

10
5. BIBLIOGRAPHY

1. Skoda, L.F. and Masters, L.W., "Solar Energy Systems - Survey of Materials

Performance," NBSIR 77-1314, National Bureau of Standards, Washington,

D.C., 20234, October 1977.

2. Masters, Larry W., Seiler, James F., Embree, Edward J., and

Roberts, Willard E., "Solar Energy Systems - Standards for Absorber Materials," NBSIR 81-2232, National Bureau of Standards, Washington, D.C.,

20234, January 1981.

3. ASTM E744-80, Standard Practice for Evaluating Solar Absorptive Materials

for Thermal Applications, Annual Book of Standards, Part 41, American

Society for Testing and Materials, 1916 Race Street, Philadelphia, PA,

19103.

4. ASTM E781-81, Standard Practice for Evaluating Absorptive Solar Receiver

Materials When Exposed to Conditions Simulating Stagnation in Solar

Collectors with Cover Plates, Annual Book of Standards, Part 41, American

Society for Testing and Materials, 1916 Race Street, Philadelphia, PA,

19103.

5. ASTM E424-71, Test for Solar Energy Transmittance and Reflectance

(Terrestrial) of Sheet Materials, Annual Book of Standards, Part 41,

American Society for Testing and Materials, 1916 Race Street, Philadelphia,

PA, 19103.

6. ASTM E408-71, Test for Total Normal Eraittance of Surface Using Inspection

Meter Techniques, Annual Book of Standards, Part 41, American Society for

Testing and Materials, 1916 Race Street, Philadelphia, PA, 19103.

11
Table 1. Absorptive Coatings and Substrates Used in the Study

Code Number^/ Absorptive Coating Substrate

A8 Nickel coated foil Aluminum, alodine

. B6 Anodized aluminum Aluminum, 6061, anodized

B7 Anodized aluminum Aluminum, 1100, anodized

C5 Copper oxide Copper

D2 Black chrome Steel, cold-rolled!/

D4 Black chrome Copper, DHP 1232/

G9 Polyvinylidene fluoride 20 gage, G90, hot dipped

galvanized steel

HI Polyvinylidene fluoride Steel, bonderized

HR Polyvinylidene fluoride Aluminum, alodine

11 Urethane Steel, bonderized

14 Urethane Copper, DHP 122

18 Urethane Aluminum, alodine

J1 Alkyd Steel, bonderized

J4 Alkyd Copper, DHP 122

J8 Alkyd Aluminum, alodine

Kl Epoxide Steel, bonderized

K4 Epoxide Copper, DHP 122

K8 Epoxide Aluminum, alodine

LI Silicone Steel, bonderized

L4 Silicone Copper, DHP 122

L8 Silicone Aluminum, alodine

Ml Modified polyester Steel, bonderized

M4 Modified polyester Copper, DHP 122

M8 Modified polyester Aluminum, alodine

N2 Porcelain enamel Steel, cold-rolled

N8 Porcelain enamel Aluminum, alodine

1 / 21

v

The letter refers to the absorptive coating and the number to the substrate.

Includes 0.5 mil Nickel flash.

Includes 0.1 - 0.2 mil Nickel flash.

12
Table

2.

Results

of

Outdoor

Exposure

at

Gaithersburg,

Maryland o00co

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^4 | c-J | tn |in | vo |r^T |
Cumulative

Time

of

Exposure

(Months)

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Figure 1. Test box used for outdoor exposures

16
NBS-1 14A (REV. 2-80) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 82-2583 October 1982

4. TITLE AND SUBTITLE

Outdoor Exposure Tests of Solar Absorptive Coatings

5. AUTHOR(S)

Larry W. Masters, James F. Seiler and Willard E. Roberts

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7 , Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE S. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

U.S. Department of Energy

Office of Solar Applications for Buildings

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report is a follow up to an earlier report (NBSIR 81-2232, January 1981) in which

data on the performance of selected absorptive coatings in both accelerated laboratory

exposures and outdoor exposures at three sites were presented. The research presented

in this report focuses upon the results obtained by continuing the outdoor exposures of

absorptive coatings using ASTM E781-81, Standard Practice for Evaluating Absorptive

Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar

Collectors with Cover Plates.

Comparison of the results of the outdoor exposures with those obtained in accelerated

laboratory exposures indicated that 1) the accelerated exposures, as described in ASTM

E744-80, Standard Practice for Evaluating Solar Absorptive Materials for Thermal

Applications, provide more severe exposure conditions than outdoor exposures, and 2)

the degradation processes induced by outdoor exposure are adequately addressed by the

accelerated laboratory exposures.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Absorptive coatings; accelerated laboratory exposures; degradation; outdoor exposures;

simulated stagnation exposure; solar energy

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[y~| Unlimited

22

(331 For Official Distribution. Do Not Release to NTIS

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

15. Price 20402.

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U S COMM* DC 6043-P80