NAT'L INST. OF STAND & TECH

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NBS TECHNICAL NOTE 1170

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Weathering Performance

of Cover Materials

for Flat Plate

Solar Collectors

QC

lUU

11/J

1661
NATIONAL BUREAU OF STANDARDS

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RATlOWAt BUMAO

OF STANCARDS

LIBRARY , . _, _

Weathering Performance of Cover Materials

for Flat Plate Solar Collectors

Elizabeth J. Clark and Willard E. Roberts

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for: U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued November 1982
National Bureau of Standards Technical Note 1170

Natl. Bur. Stand. (U.S.), Tech. Note 1170, 80 pages (Nov. 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

For Sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Price $5.00

(Add 25 percent for other than U.S. mailing.)

I
ABSTRACT

Weathering studies were performed to obtain data on the performance and

durability of cover plate materials for flat plate solar collectors used in

solar heating and cooling systems. Ten materials were evaluated to assess

their durability after natural weathering and artificial weathering with a xenon arc light. The materials were weathered for four years on small minicollectors in Arizona, Florida, and Maryland after which the solar energy

transmittance and the effect of dirt on the transmittance were measured. The

tensile properties of selected film materials were also assessed after weathering. The effects of the natural weathering are compared: (1) for materials

exposed as inner and outer cover plates for each weathering site; (2) for the

three weathering sites; and (3) with materials artificially weathered with a xenon arc light.

Key words: artificial weathering; cover plate materials; durability; natural

weathering; solar collectors; solar energy; solar energy

transmittance; tensile properties; weathering of cover plates.

iii
Table of Contents

Page

ABSTRACT iii

LIST OF TABLES v LIST OF FIGURES vii

1

.

INTRODUCTION 1

1 .

1

BACKGROUND 1

1 .2 RELATED DEVELOPMENTS 1

2. LABORATORY STUDIES AND FIELD EXPOSURES 3

2 .

1

MATERIALS 3

2 .2 WEATHERING PROCEDURES 3

2.2.1 Natural Weathering 3

2.2.2 Artificial Weathering with Xenon Arc Light 7 2.3 PROPERTY TESTS 7 2.3.1 Solar Energy Transmittance 7 2.3.1.1 Cleaning Procedure 9

2.3.2 Tensile Properties 10

3

.

RESULTS AND DISCUSSION 11

3.1 VISUAL OBSERVATIONS 11

3.2 SOLAR TRANSMITTANCE 13

3.2.1 Gaithersburg, Maryland Exposure 13

3.2.2 Miami, Florida Exposure 19

3.2.3 New River, Arizona Exposure 25

3.2.4 Comparison of Exposures 25

3.3 TENSILE PROPERTIES 35

4

.

SUMMARY AND CONCLUSIONS 44

4 .

1

WEATHERING PROCEDURES 44

4.2 PROPERTY TESTS 45

4 .

3

EFFECTS OF WEATHERING ON MATERIALS 46

5

.

FUTURE RESEARCH NEEDS 48

5 .

1

MATERIALS DEVELOPMENT 48

5.2 SERVICE LIFE PREDICTION 48

6

.

ACKNOWLEDGMENT 49

7

.

REFERENCES 50

APPENDIX - SOLAR ENERGY TRANSMITTANCE DATA OF COVER PLATE MATERIALS

AFTER NATURAL WEATHERING 52

IV
List of Tables

Page

Table 1. Solar Energy Transmittance of Unweathered Cover Plate

Materials 5

Table 2. Natural Weathering Exposure Data for Minicollectors 6

Table 3. Calculated Values for Energy Deposited on Cover Materials

During Artificial Weathering with Xenon Arc Light 9

Table 4. Tensile Strength and Yield Strength of Weathered

Fluorinated (ethylene propylene) Copolymer 36

Table 5. Breaking Factor and Elongation at Break of Weathered

Fluorinated (ethylene propylene) Copolymer 37

Table 6. Tensile Strength and Yield Strength of Weathered Poly(vinyl

fluoride) 38

Table 7. Breaking Factor and Elongation at Break of Weathered Poly-

( vinyl fluoride) 39

Table 8. Tensile Strength and Yield Strength of Weathered Poly-

(ethylene terephthalate) 41

Table 9. Breaking Factor and Elongation at Break of Weathered Poly-

(ethylene terephthalate) 42

Table Al . Solar Energy Transmittance of Cover Plate Materials After

Exposure as Single Covers to Natural Weathering in

Gaithersburg, Maryland 52

Table A2. Solar Energy Transmittance of Cover Plate Materials After

Exposure as Double Covers to Natural Weathering in

Gaithersburg, Maryland 53

Table A3. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Single Covers to Natural Weathering in

Gaithersburg, Maryland 54

Table A4. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Double Covers to Natural Weathering in

Gaithersburg , Maryland 55

Table A5. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Single Covers in

Gaithersburg , Maryland 56
List of Tables (Continued)

Page

Table A6. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Double Covers in

Gaithersburg y Maryland 57

Table A7. Solar Energy Transmittance of Cover Plate Materials After

Exposure as Single Covers to Natural Weathering in Miami,

Florida 58

Table A8. Solar Energy Transmittance of Cover Plate Materials After

Exposure as Double Covers to Natural Weathering in Miami,

Florida 59

Table A9. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Single Covers to Natural Weathering in

Miami, Florida 60

Table AlO. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Double Covers to Natural Weathering in

Miami, Florida 61

Table All. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Single Covers in

Miami , Florida 62

Table A12. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Double Covers in

Miami, Florida 63

Table A13. Solar Energy Transmittance of Cover Plate Materials After

Exposure as Single Covers to Natural Weathering in New

River , Arizona • 64

Table A14. Solar Energy Transmittance of Cover Plate Materials After

Exposure as Double Covers to Natural Weathering in New

River, Arizona 65

Table A15. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Single Covers to Natural Weathering in

New River , Arizona 66

Table A16. Solar Energy Transmittance Loss of Cover Plate Materials

After Exposure as Double Covers to Natural Weathering in

New River , Arizona 67

Table A17, Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Single Covers in

New River , Arizona 68

vi
List of Tables (Continued)

Page

Table A18. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Double Covers in

New River , Arizona 69

List of Figures

Page

Figure 1 . Minicollector for Natural Weathering Exposure 4

Figure 2. Spectral Distribution of Energy from the Sun and for a Xenon Arc Light 8

Figure 3a. Scanning Electron Micrographs of Poly(ethylene

terephthalate) Ater One Year Weathering in Miami, Florida ... 12

Figure 3b. Scanning Electron Micrographs of Polycarbonate After One

Year Weathering in Miami , Florida 12

Figure 4a. Solar Energy Transmittance of Glass (0.01% iron oxide)

After Weathering on Minicollectors in Gaithersburg,

Maryland 14

Figure 4b. Solar Energy Transmittance of Glass (0.10% iron oxide)

After Weathering on Minicollectors in Gaithersburg,

Maryland 14

Figure 4c. Solar Energy Transmittance of Fluorinated (ethylene

propylene) Copolymer After Weathering on Minicollectors

in Gaithersburg , Maryland 15

Figure 4d. Solar Energy Transmittance of Poly(vinyl fluoride) After

Weathering on Minicollectors in Gaithersburg, Maryland 15

Figure 4e. Solar Energy Transmittance of Poly(ethylene terephthalate)

After Weathering on Minicollectors in Gaithersburg,

Maryland 16

Figure 4f . Solar Energy Transmittance of Acrylic Film After

Weathering on Minicollectors in Gaithersburg, Maryland 16

Figure 4g. Solar Energy Transmittance of Poly(methyl methacrylate)

After Weathering on Minicollectors in Gaithersburg,

Maryland 17

Figure 4h. Solar Energy Transmittance of Polycarbonate After

Weathering On Minicollectors in Gaithersburg, Maryland 17

vii
List of Figures (Continued)

Page

Figure 4i. Solar Energy Transmittance of Fiber Reinforced Plastic

(1.0 mm) After Weathering on Minicollectors in

Gaithersburg, Maryland 18

Figure 4j . Solar Energy Transmittance of Fiber Reinforced Plastic

(1.5 mm) After Weathering on Minicollectors in

Gaithersburg, Maryland 18

Figure 5a. Solar Energy Transmittance of Glass (0.01% iron oxide)

After Weathering on Minicollectors in Miami, Florida 20

Figure 5b. Solar Energy Transmittance of Glass (0.10% iron oxide)

After Weathering on Minicollectors in Miami, Florida 20

Figure 5c. Solar Energy Transmittance of Fluorinated (ethylene

propylene) Copolymer After Weathering on Minicollectors in

Miami , Florida 21

Figure 5d. Solar Energy Transmittance of Poly (vinyl fluoride) After

Weathering on Minicollectors in Miami, Florida 21

Figure 5e. Solar Energy Transmittane of Poly (ethylene terephthalate)

After Weathering on Minicollectors in Miami, Florida 22

Figure 5f . Solar Energy Transmittance of Acrylic Film After Weathering

on Minicollectors in Miami, Florida 22

Figure 5g. Solar Energy Transmittance of Poly (methyl methacrylate)

After Weathering on Minicollectors in Miami, Florida 23

Figure 5h. Solar Energy Transmittance of Polycarbonate After

Weathering on Minicollectors in Miami, Florida 23

Figure 5i. Solar Energy Transmittance of Fiber Reinforced Plastic

(1.0 mm) After Weathering on Minicollectors in Miami,

Florida 24

Figure 5j . Solar Energy Transmittance of Fiber Reinforced Plastic

(1.5 mm) After Weathering on Minicollectors in Miami,

Florida 24

Figure 6a. Solar Energy Transmittance of Glass (0.01% iron oxide)

After Weathering on Minicollectors in New River, Arizona .... 26

Figure 6b. Solar Energy Transmittance of Glass (0.10% iron oxide)

After Weathering on Minicollectors in New River, Arizona .... 26

viii
List of Figures (Continued)

Page

Figure 6c. Solar Energy Transmit tance of Fluorinated (ethylene

propylene) Copolymer After Weathering on Minicollectors in

New River, Arizona 27

Figure 6d. Solar Energy Transmittance of Poly(vinyl fluoride) After

Weathering on Minicollectors in New River, Arizona 27

Figure 6e. Solar Energy Transmittane of Poly (ethylene terephthalate)

After Weathering on Minicollectors in New River, Arizona ... 28

Figure 6f . Solar Energy Transmittance of Acrylic Film After

Weathering on Minicollectors in New River, Arizona 28

Figure 6g. Solar Energy Transmittance of Poly(methyl methacrylate)

After Weathering on Minicollectors in New River, Arizona ... 29

Figure 6h. Solar Energy Transmittance of Polycarbonate After

Weathering on Minicollectors in New River, Arizona 29

Figure 6i. Solar Energy Transmittance of Fiber Reinforced Plastic

(1.0 mm) After Weathering on Minicollectors in New River,

Arizona 30

Figure 6j . Solar Energy Transmittance of Fiber Reinforced Plastic

(1.5 mm) After Weathering on Minicollectors in New River,

Arizona 30

Figure 7a. Comparison of Solar Transmittance of Fluorinated (ethylene

propylene) Copolymer After Natural Weathering as the Inner

Cover of a Minicollector and Artificial Weathering

with Xenon Arc Light 31

Figure 7b. Comparison of Solar Transmittance of Fluorinated (ethylene

propylene) Copolymer After Natural Weathering as the Outer

Cover of a Minicollector and Artificial Weathering with

Xenon Arc Light 31

Figure 8. Comparison of Solar Transmittance of Poly (methyl

methacrylate) After Weathering on a Minicollector as an Outer Cover and in an Artificial Weathering Device with

Xenon Arc Light 33

Figure 9. Comparison of Solar Transmittance of Polycarbonate After

Weathering on a Minicollector as an Outer Cover and in an Artificial Weathering Device with Xenon Arc Light 33

IX
List of Figures (Continued)

Page

Figure 10a. Solar Transmit tance of Fiber Reinforced Plastic (1.0 mm)

as a Function of Solar Radiation Exposure 34

Figure 10b. Solar Transmittance of Fiber Reinforced Plastic (1.0 mm)

as a Function of Months of Exposure 34

Figure 11. Tensile Strength of Poly(ethylene terephthalate) After

Natural Weathering In Arizona and Artificial Weathering

with Xenon Arc Light 43

Figure 12. Elongation at Break of Poly (ethylene terephthalate) After

Natural Weathering In Arizona and Artificial Weathering

with Xenon Arc Light 43
1 . INTRODUCTION

1.1 BACKGROUND

In 1976, due to a lack of durability data for solar collector cover materials,

the National Bureau of Standards (NBS) initiated a study, sponsored by the

Department of Energy (DoE), to obtain data as the technical basis for standards

for solar collector cover plate materials. A report on the study [1]* issued

in 1980 assessed problems with cover plate materials, listed performance

requirements, described cover plate properties, and outlined factors which

cause material degradation. It also contained data comparing cover materials

after heat aging, artificial weathering with a xenon arc light, and two years

natural outdoor weathering on minicollectors at three geographic locations.

The materials were evaluated by measurement of transmittance, linear dimensions,

and warpage. The report also contained two proposed standards for weathering

and evaluation of cover plate materials.

This is the final report on the study. The objective of the work described

in this report was: 1) to obtain additional natural weathering data for

comparison with data from accelerated laboratory tests, and 2) to assess changes in mechanical properties of a limited number of materials.

This report discusses the effects of four years of natural weathering on cover plate materials. Comparisons are made with materials weathered artificially

with xenon arc light. Solar transmittance was measured to assess the effect

of the weathering and the effect of dirt. Tensile properties of three film

materials were also measured after outdoor weathering, xenon arc artificial

weathering, and heat aging.

The earlier report [1] contained data on exposure of cover materials to heat

aging at 75, 100, 125, and 150°C, artificial aging with xenon arc light, and

natural aging for two years on minicollectors.

1.2 RELATED DEVELOPMENTS

Since 1976 progress has been made in identification and use of more durable

cover plate materials in solar collectors. In response to greater emphasis on material durability, increased efforts have been placed in development of

standards for evaluating cover plate materials for solar collectors, in generating data regarding durability of cover plate materials, and in writing criteria

or guidelines for cover plate materials in solar systems. Of critical significance has been the development of uniform methods to measure long-term performance of cover plate materials and to evaluate relative durabilities.

Progress in standards development has also been evidenced by activity in the

American Society for Testing and Materials (ASTM) . In 1978 ASTM formed

Committee E 44 on Solar Energy Conversion with an active task group on Cover

Plate Standards. Since then, the following new ASTM standards relating to

* Figures in brackets refer to references in Section 7.

1
cover plate materials have been approved: E 765, Practice for Evaluation of

Cover Materials for Flat Plate Solar Collectors [2]; E 782, Practice for Exposure of Cover Materials for Solar Collectors to Natural Weathering Under Conditions Simulating Operational Mode [3]; E 822, Practice for Determining

Resistance of Solar Collector Covers to Hail by Impact with Propelled Ice Balls

[4]; and E 881, Practice for Exposure of Solar Collector Cover Materials to

Natural Weathering Under Conditions Simulating Stagnation Mode [5]. Standards

E 765 and E 881 are based on work reported in [I] while initial work on Standard E 822 is described in [6] and [7]. Other standards are also being

developed in Committee E 44. These standards represent significant progress

in the development of uniform methods to measure long-term performance and to

evaluate relative durabilities of cover plate materials.

Other data on durability of cover plate materials have also been published

since 1976. Optical and mechanical property data of a number of cover plate

materials are listed in [8, 9]. The materials were weathered at ambient

temperatures , rather than the higher temperatures attained in solar collectors

.

Consequently, a user of these durability data should keep in mind that most

materials are less durable at high temperatures.

Several efforts have also been made to assist designers, manufacturers, or builders in selecting reliable components and materials [10, 11, 12, 13, 14].

Design guidelines or performance criteria for materials in solar systems are provided, and reference 14 contains a chapter discussing the properties of

glazing materials.
2. LABORATORY STUDIES AND FIELD EXPOSURES

2.1 MATERIALS

The cover plate materials utilized in this study were typical of those

commercially available in 1976. The properties of materials currently on the

market may differ from those described in this report. Properties of cover plate materials can be altered by changes in minor consitutents (i.e., iron

oxide content, stabilizers, plasticizers , antioxidants) or in processing

techniques. Many cover plate materials are marketed, and in order to improve

their performance, manufacturers sometimes modify the materials. The reader

is cautioned against direct application of the data in this report to materials

currently marketed.

The ten cover plate materials selected for testing were representative of the

materials and thicknesses used in solar collectors. Two of the materials were

glass sheet. They differ in iron oxide content which directly affects the

solar energy transmittance. Four of the materials were plastic films and four

materials were plastic sheet, two of which were fiber reinforced plastics (FRP),

These FRP materials were from separate manufacturers and contained different

resins. They are distinguished in this report by their thicknesses. The ten

materials are listed in table 1 along with their solar energy transmittance

values. Greater description of the materials along with graphs of the initial

spectral transmittance and the infrared transmittance are given in the earlier

report [1].

2.2 WEATHERING PROCEDURES

2.2.1 Natural Weathering

The cover plate materials were weathered on small insulated boxes (i.e.

minicollectors) (240 by 140 by 83 mm) which were designed to elevate the exposure temperature and thereby provide an exposure environment simulating that

in a collector. Both single and double cover assemblies were used. The

design of the minicollector is shown in figure 1.

The minicollectors were weathered at three sites: New River, Arizona; Miami,

Florida; and Gaithersburg, Maryland. The Arizona site had a hot, dry desert

climate while the Florida site was a hot, humid tropical climate with some industrial exposure. The Maryland site had a hot, humid subtropical climate.

A variable angle exposure was utilized to maximize solar radiation incident

upon the minicollectors. Details of the minicollector construction and

weathering exposure are contained in the earlier report [1].

A set of test specimens was removed for evaluation at 3, 6, 12, 18, 24, 36,

and 48 month intervals. In some cases, duplicate sets of test specimens were

removed. The exposure schedule and the solar radiation accumulated during

exposure are summarized in table 2. Comparing the solar radiation accumulated

at the three sites during simultaneous exposure, it is observed that the test

specimens received approximately 50 percent more solar radiation at New River

than at Gaithersburg, and about 25 percent more solar radiation at New River
OUTER DOUBLE

COVER

SPACER FOR DOUBLE

COVER

SINGLE COVER OR

INNER DOUBLE COVER

BOX

ABSORBER PLATE

INSUUTION

13mm(r')

MATERIALS:

BOX AND SPACER:

19mm,(H")W00D PAINTED

WHITE ON OUTSIDE

INSULATION:

50mm (2") FIBERGLASS BUNKET

ABSORBER:

BLACK PORCELAIN ENAMEL

ON STEEL, a=. 93, 6= .85

SEALANT:

BTV SUITABLE FDR OUTDOOR

EXPOSURE.

NOTE:

VENT HOLES WERE DRILLED

IN THE BOTTOM OF THE BOX

AND THE END OF THE SPACER

TO MINIMIZE MOISTURE

ACCUMULATION

140mm

mi

Figure 1. Minicollector for natural weathering exposure

4
Table 1. Solar Energy Transmit tance of Unweathered Cover Plate Materials

Material

Nominal

Thickness

mm

Solar Energy Transmittance

Control

Test Specimens

Average Standard

Deviation

Glass (0.01% iron oxide)

Glass (0.10% iron oxide)

Fluorinated (ethylene

propylene) copolymer

Poly (vinyl fluoride)

Poly(ethylene terephthalate)

Acrylic

Poly (methyl methacrylate)

Polycarbonate

Fiber reinforced plastic

Fiber reinforced plastic

3.2

3.2

0.025

0.10

0.13

0.076

1.5

1.02

1.02

1.5

90.5 90.5 90.9

90.8 91.0 90.7

87.4 87.4 87.5

95.9 96.1 96.1

92.7 92.4 92.4

86.4 86.4 86.3

90.6 90.8 90.7

91.0 91.0 91.1

88.2 88.0 88.2

86.5 86.3 86.2

81.7 80.0 74.6

77.2 77.8 79.5

90.7

87.4

96.0

92.5

86.3

90.7

91.0

88.1

86.3

78.5

0.2

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

2.3

Spectral transmittance was measured with an integrating sphere spectrophotometer,

The solar energy transmittance was calculated for air mass 2. Transmittance is

expressed in percentage.
Table 2. Natural Weathering Exposure Data for Minicollectors

Solar Radiation (GJ/m^)

Months

Exposure Dates

Gaithersburg,

Maryland

Miami

,

Florida

New River,

Arizona

3

9/1/77-12/1/77

12/20/77-3/20/78

1.176 1.455 2.012

1.422

6

9/1/77-3/1/78

3/24/78-9/24/78

2.251 2.864 3.487

5.123

12

9/1/77-9/1/78

3/1/78-3/1/79

4/1/78-4/1/79

7/29/78-7/29/79

5.477

5.429

6.344

6.523

8.006

8.259

18 9/1/77-3/1/79 7.680 9.333 11.642

24 9/1/77-9/1/79 10.720 13.125 16.370

36

9/1/77-9/1/80

4/1/78-4/1/81

4/17/78-4/17/81

16.402

16.693

19.398

19.859

24.414

48 9/1/77-9/1/81 22.076 26.333 33.038
than at Miami. Similarly, the test specimens at Miami received about 20

percent more solar radiation than those at Gaithersburg.

2.2.2 Artificial Weathering with Xenon Arc Light

Cover plate materials were exposed to xenon arc light in an artificial

weathering device using ASTM Method D 2565, Recommended Practice for Operating

Xenon Arc-Type (Water-Cooled) Light-Exposure Apparatus With and Without Water

for Exposure of Plastics [15]. A Type B apparatus was used with the irradiance

regulated through the use of a continuously controlling monitor that automatically maintained uniform intensity at preselected wavelengths of 340, 420, and

580 nm. A borosilicate glass filter with suitable transmittance was used to

filter the light and to provide radiation which simulated the solar spectrum at sea level. Test specimens were exposed to a continuous light cycle without

water spray. The ambient temperature within the chamber was approximately

40-50°C.

Test specimens were about 45 cm from the light source. Periodically, test

specimens were repositioned to essentially achieve uniform radiation. A set was removed for evaluation at time intervals ranging from 250 to 4000 hours.

The spectral energy distribution of a xenon arc light in an artificial

weathering device is shown in figure 2, In the ultraviolet and visible region

it closely simulates the spectral distribution of sunlight (expecially air mass 1) also shown in figure 2, although there are some differences in the near infrared region.

To facilitate comparison of the effects of natural weathering with xenon arc artificial weathering, the energy deposited on materials during xenon arc exposure was calculated. Although it is the ultraviolet radiation which causes damage to polymeric materials, these comparisons were done with total solar

energy because no measurements of only ultraviolet energy were available for

the outdoor weathering. The spectral curve of the 6500 Joule (6500 Watt)

xenon arc lamp [16] run at a controlled lamp output of 52 yj was used to calculate energy deposited. Integration under the curve and adjustment for distance (45 cm) of specimen from the lamp produced a value of 1140 J/m^ at the

test specimen. The hours of xenon arc exposure in the artificial weathering

device and the corresponding calculated energy values are shown in table 3.

2.3 PROPERTY TESTS

2.3.1 Solar Energy Transmittance

The solar energy transmittance of the materials was determined using Method A

of ASTM E 424-71, Standard Test Methods for Solar Energy Transmittance and

Reflectance (Terrestrial) of Sheet Materials [17]. The spectral transmittance
• E

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DS/S3inor-33NViaVIIUI 1VUi33dS

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Table 3. Calculated Values for Energy Deposited on Cover

Materials During Artificial Weathering with Xenon

Arc Light

Hours of

Exposure

Energy Deposited

(Calculated) GJ/m^

250 1.030

499 2.055

758 3.122

1000 4.118

2012 8.286

3000 12.355

4010 16.515

of the cover plate materials was measured utilizing a Cary 17D Spectrophotometer^ with a 76 mm diameter integrating sphere. Measurements of spectral

transmittance were made over the spectral range from 300 to 2150 nm. The solar

energy transmitted was obtained by integrating over the solar energy distribution, as reported by Parry Moon [18], for sea level and air mass 2 (AM 2).

The weighted ordinates calculation method from ASTM E 424 was used to integrate

the solar energy distribution at 50 nm intervals, normalized to 100. Further

details of the transmittance measurement are given in the earlier report [1].

2.3.1.1 Cleaning Procedure

The cover materials weathered outdoors were cleaned after the weathering to

determine the effect of dirt accumulation on transmittance. No cleaning was

done during weathering. The cleaning procedure consisted of immersing the

test specimens in a 0.1 percent solution of detergent in distilled water. A

soft brush was used to clean both sides of the test specimen which was then

rinsed with distilled water and air dried. Care was taken to avoid scratching

or stretching the plastic materials.

Certain trade names and company products are identified in order to adequately

specify the experimental procedure. In no case does such identification

imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the products are necessarily the best available for the

purpose.
2.3.2 Tensile Properties

The tensile properties of fluorinated (ethylene propylene) copolymer,

poly(vinyl fluoride), and poly (ethylene terephthalate) films were measured

using ASTM D 882, Methods for Tensile Properties of Thin Plastic Sheeting [19].

The tensile properties which were calculated were breaking factor, tensile

strength, elongation at break, yield strength, and elongation at yield. Tensile specimens of 13 by 100 mm were cut from the weathered materials. Where

sufficient material was available, five tensile specimens were taken from both

the longitudinal and the transverse directions. This was done to obtain data

regarding the anisotropic nature of the films. Tensile testing was performed

only on selected test specimens from the heat aging at 125°C, artificial

weathering with xenon arc, and outdoor weathering at Arizona and Florida

(poly(ethylene terephthalate) only).

10
3. RESULTS AND DISCUSSION

3.1 VISUAL OBSERVATIONS

The test specimens were visually examined during removal from the minicollector

boxes. Some of each type of glass test specimen had broken during the exposure.

This breakage was apparently due to the manner in which the boxes were held on

the exposure rack. It appeared not to be due to impact. Several test specimens

were cracked when they were removed from the test boxes. Both types of glass

were annealed. The test specimens had been cut from large sheets and the

edges had not been treated to reduce the stress levels. It is doubtful cracking would have occurred with tempered glass which is generally used in solar

collectors.

With the fluorinated (ethylene propylene) copolymer, there was little change in

appearance other than the accumulation of dirt. However, several of the double

cover minicollectors (some from Miami and some from Gaithersburg) developed

small holes 20-40 mm from the edge. The holes were reported to be caused by

birds pecking at the cover plates. The minicollectors in Arizona suffered damage due to a hail storm. Damage ranged from dents to holes of 10-30 mm diameter.

The poly(vinyl fluoride) became extremely taut across the boxes, apparently

caused by shrinkage of the material. The test specimens gradually yellowed as the exposure continued. The inner cover of the double cover minicollectors

exposed for 18 to 48 months in Arizona and for 24 to 48 months in Florida

became so brittle that they ruptured. The single and outer double covers accumulated dirt that could not be washed off with the cleaning procedure used

in this test program. However, it was noted that the dirt could be removed

with adhesive tape. (When the specimens were placed in the spectrophotometer

for transraittance measurements, adhesive tape was used to hold the material in

place. When the tape was removed, the dirt came off cleanly from under the

tape. It appeared that some of the outer degraded polymer that was holding the

dirt also came off the cover plate.)

The poly (ethylene terephthalate) outer covers exposed for one year and longer

in Miami developed a white, opaque appearance. Although the material was transparent prior to exposure, it became a milky white. Closer examination of the

test specimens found that the white material was present only on the exterior

surface. Apparently the environmental factors in Miami, e.g., more moisture

and an industrial atmosphere, caused changes in the outer surface exposed

directly to the weather. The white material was not present on the interior

side of the outer cover nor on the inner cover. Examination of outer surface

with a scanning electron microscope indicated minute cracking on the surface.

Figure 3a illustrates the small cracks seen after one year of weathering in

Miami, Florida.

The poly(ethylene terephthalate) test specimens from Arizona were damaged

apparently by a hailstorm. All of the outer double cover and single cover poly(ethylene terephthalate) test specimens on exposure at that time had holes

and cracks in them. The material had become extremely brittle. The inner

cover remained intact. All but one of the minicollectors with poly (ethylene

11
M^^

Figure 3a. Scanning electron micrographs of poly(ethylene terephthalate)

after one year weathering in Miami, Florida

Figure 3b. Scanning electron micrographs of polycarbonate

after one year weathering in Miami, Florida

12
terephthalate) from Arizona were terminated after 18 months due to damage. The

remaining minicollector suffered similar serious damage in a hail storm during

the 29th month of exposure.

The acrylic film was the first material to physically deteriorate during the

exposure. It cracked and broke into pieces at all three exposure sites. The

material failed most rapidly in Arizona, followed by Florida and Maryland.

The outer cover test specimen deteriorated more rapidly than the inner one. After 18 months exposure in Arizona and Florida, the outer covers had completely

disintegrated and blown away. The inner cover and single cover test specimens

exhibited somewhat less damage. All minicollectors with the acrylic film were

terminated at 18 or 24 months due to material failure.

The polycarbonate material visibly yellowed as it was exposed. In addition, the

test specimens exposed in Florida developed deposits on the outer surface. The

deposits were not washed off during the cleaning. Similar deposits developed

on the Maryland samples but not to the extent that they occurred on the Florida

specimens. Examination of the surface with a scanning electron microscope

showed small cracks after one year. These are seen in figure 3b. However, no measure of physical properties was made to determine if the material was weakened.

Both of the fiber reinforced materials yellowed during the outdoor exposure,

with the 1.5 mm FRP changing the most. The resin of this material also progressively developed a whitish appearance when weathered in Florida. This

change may be attributed to either moisture or the industrial atmosphere.

After 36 months exposure, the 1.5 mm fiber reinforced material exhibited a very significant amount of "fiber bloom" on the outer covers while the 1.0 mm

fiber reinforced material showed a smaller amount. "Fiber bloom" occurs when

fibers break away from the resin and protrude from the surface. The greatest

amount of "fiber bloom" occurred on the test specimens from Arizona. The

"fiber bloom" and yellowing continued to increase for both fiber reinforced

materials through the 48 months exposure.

3.2 SOLAR TRANSMITTANCE

3.2.1 Gaithersburg, Maryland Exposure

The solar energy transmittance of single and double covers after natural

weathering in Gaithersburg is compared for each cover material in figures 4a

to 4j . The lines represent a least squares, best fit equation drawn through the

data. The test specimens were washed prior to transmittance measurements. All

materials suffered some loss in transmittance although the amount of the loss

varied among the materials. In general, the inner cover transmittance changed

the least. For most materials there was almost no change in the inner cover;

however, both fiber reinforced plastics show a significant decline indicating

their temperature sensitivity. Usually the rate of decline for the single and

outer double covers was similar with the single cover transmittance, generally,

slightly worse than the outer cover. One notable exception to this was the

fluorinated (ethylene propylene) copolymer (fig. 4c) where the outer double

cover decreased more. The single and outer double covers were exposed to

13
100

95 -

90 -

85 -

80 .

75

§-

A—A SINGLE COUER

*---* DOUBLE COUER, OUTER

»---«> DOUBLE COUER, INNER

"T"

10

-1 1 \ r—

15 30 E5 30

MONTHS EXPOSURE

35 40

"~r" 45 50

Figure 4a.

S

LAR

EN

E

RG

Y

T

RA

N

S

PIIT

TA

N

CE

Solar energy transmittance of glass (0.01% iron oxide) after

weathering on mini collectors in Gaithersburg, Maryland

100

95 -

90

85 .

80

75

^—* SINGLE COUER

*•--* DOUBLE COUER,

0---0 DOUBLE COUER,

OUTER

INNER

10 15 20 as 30 35 40 45 50

MONTHS EXPOSURE

Figure 4b. Solar energy transmittance of glass (0.10% iron oxide) after

weathering on mini collectors in Gaithersburg, Maryland

14
s

LA

R

E

N

ER

G

V

T

R

1^N

Sn

ITT

N

CE

100

95 -

90

85 -

80 .

75

^—^SINGLE COUER

*---* DOUBLE COUER, OUTER

O---OD0UBLE COUER, INNER

10 15 30 E5 30

nONTHS EXPOSURE

35

—1—

40 45 50

Figure 4c.

S

LA

R

EN

ER

G

YTR

A

N

S

n

IT

T

A

N

E

100

Solar energy transmittance of fluorinated (ethylene propylene)

copolymer after weathering on minicollectors in Gaithersburg,

Maryland

95 -

90

85 .

80

75

''—* SINGLE COUER

*---* DOUBLE COUER, OUTER

o-'-e* DOUBLE COUER, INNER

10

1^ 15 20

"~r" 25 30—T

35 40 45 50

MONTHS EXPOSURE

Figure 4d. Solar energy transmittance of poly (vinyl fluoride) after

weathering on minicollectors in Gaithersburg, Maryland

15
s

LR

E

N

E

R

G

Y

T

R

A

N

Sn

IT

T

A

N

C

E

lee

95 .

90

85

80 .

75

SINGLE COUER

•—DOUBLE COUER, OUTER

o—o DOUBLE COUER, INNER

10 15

-T29

-J30 35

T

40 45 50

nONTHS EXPOSURE

Figure 4e. Solar energy transmittance of poly(ethylene terephthalate)

after weathering on minicollectors in Gaithersburg, Maryland

S

LA

R

E

N

E

R

G

V

TR

A

N

Sn

IT

TA

N

C

E

100

95 .

90

85

80 .

75

SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10

"T15 —

80 as —

r- 30

"T35 —

T" 40

—T

45 50

MONTHS EXPOSURE

Figure 4f . Solar energy transmittance of acrylic film after weathering

on minicollectors in Gaithersburg, Maryland

16
s

L

R

E

N

E

R

G

V

T

RA

N

Sn

ITT

N

C

E

S

LA

R

E

N

E

R

G

V

TR

N

S

n

IT

T

ftN

C

E

100

95 -

90 .

85 .

80 -

75

"—* SINGLE COUER

*---* DOUBLE COUER, OUTER

o---^* DOUBLE COUER, INNER

10

1^ 15

^1

20

I S5 30 —

T" 35

T

40 45 50

MONTHS EXPOSURE

Figure 4g. Solar energy transmittance of poly(methyl methacrylate)

after weathering on minicollectors in Gaithersburg, Maryland

100

95 .

90 .

85 .

80

75

^—A SINGLE COUER

*---* DOUBLE COUER, OUTER

o---« DOUBLE COUER, INNER

10

'TIS 20 25 30 35 40 45 50

Figure 4h,

nONTHS EXPOSURE

Solar energy transmittance of polycarbonate after

weathering on minicollectors in Gaithersburg, Maryland

17
s

LA

R

E

N

ER

G

V

T

R

N

Sn

ITT

A

N

C

E

100

95 -

90 -

85 -

80

75

70 -

65

60

^—* SINGLE COUER

*---* DOUBLE COUER, OUTER

<>---o DOUBLE COUER, INNER

10

T

15 20 as 30

MONTHS EXPOSURE

35

-~r- 40 45 50

Figure 41.

100

90

Figure 4j

Solar energy transralttance of fiber reinforced plastic

(1.00 mm) after weathering on mlnlcollectors in

Gaithersburg, Maryland

80 -

70 -

60

50

40

^SINGLE COUER

*---* DOUBLE COUER, OUTER

«---«> DOUBLE COUER, INNER

10 15 20

1^ 25 30 35 40 45 50

MONTHS EXPOSURE

Solar energy transmittance of fiber reinforced plastic

(1.5 mm) after weathering on mlnlcollectors in

Gaithersburg, Maryland

18
identical weathering with the exception of temperature. With the minicollector

exposure the single cover attains a higher temperature than the outer double

cover. Since the combination of elevated temperature and sunlight can have

synergistic effects in degrading materials, the somewhat higher temperature of

single covers is probably the cause of the greater losses of the single covers than the double outer covers.

Data for the solar transmittance values are given in Appendix A. The solar

energy transmittance after weathering in Gaithersburg is tabulated in table Al

for single covers and in table A2 for double covers. Transmittance losses due

to weathering are listed in table A3 for single covers and table A4 for double

covers. The solar energy transmittance losses due to the surface dirt on the

materials are listed in table A5 for single covers and table A6 for double

covers. For the single and outer double covers the dirt accumulation increased

for the first 12 months after which it tended to level off or decrease. For

the inner double covers the transmittance losses were generally less than one percent and were not cumulative with time, except for the fluorinated (ethylene

propylene) copolymer.

3.2.2 Miami, Florida Exposure

The solar energy transmittance of single and double covers after natural

weathering in Miami is compared for each cover material in figures 5a to 5j

The transmittance measurements were made after the test specimens were washed.

Again, in general, the inner cover lost the least transmittance. For the glass

materials there was little loss of transmittance. For the other materials, the

outer double and single covers showed greater transmittance losses, the rates

generally being similar. Contrary to most of the other materials, the outer

double polycarbonate and FRP covers had a greater rate of loss than the single

covers. The two fiber reinforced plastics suffered significant deterioration,

apparently due to the combination of moisture, heat, and sunlight. For both

FRP materials the transmittance of the inner cover decreased substantially

while the outer and single covers declined at faster rates. Figures 5i and 5j

illustrate the losses in the FRP materials.

The solar energy transmittance values after weathering in Miami are tabulated

in table A7 for single covers and table A8 for double covers. Transmittance

losses due to weathering are given in tables A9 and AlO for single and double

covers. The solar energy transmittance losses due to accumulation of dirt are given in table All for single covers and table A12 for double covers. Dirt caused decreases in transmittance for a number of materials. For any

given time period, all the materials seemed to be high or low, comparatively,

in the transmittance loss. This may have been related to the most recent

rainfall prior to completion of the weathering. Transmittance losses for the

inner covers seemed to increase as the weathering continued. The poor conditions of the minicollector boxes after many months of weathering may have

permitted dirt to enter and accumulate during the weathering.

19
s

L

A

R

E

N

E

R

G

Y

T

RA

N

S

n

IT

TA

N

CE

100

95 -

90

85

80

75

'V-~V---'^---

* I s

'

'^—^SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10 15 20 as 30

MONTHS EXPOSURE

35

"T"

40 —

T" 45 50

Figure 5a, Solar energy transmittance of glass (0.01% iron oxide) after

weathering on minicollectors in Miami, Florida

S

LA

R

E

N

E

RG

V

TR

A

N

Sn

ITT

A

N

C

E

100

95 .

90

85

80 .

75

_ _Q_ -* —

A—A SINGLE COUER

*--* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER —

T" 10 15 20 25 30

MONTHS EXPOSURE

—

35

—T

40 45 50

Figure 5b. Solar energy transmittance of glass (0.10% iron oxide) after

weathering on minicollectors in Miami, Florida

20
100

95 .

90

85 -

80

75

A—A SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

-i 1 1 \ 1 1 1 1 r~ 5 10 15 20 as 30 35 40 45

nONTHS EXPOSURE

50

igure 5c. Solar energy transmit tance of fluorinated (ethylene propylene)

after weathering on minicollectors in Miami, Florida

S

L

A

R

EN

E

R

G

V

TR

A

N

S

n

IT

TA

N

C

E

100

95 .

'^--r

90 .

85 .

80

75

A—* SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER —

10 15 —

r- 20 25 —

T" 30 —

35 40 45 50

MONTHS EXPOSURE

Figure 5d. Solar energy transmittance of poly(vinyl fluoride) after

weathering on minicollectors in Miami, Florida

21
s

L

1^R

E

N

ER

G

V

T

R

AN

Sn

ITTA

N

CE

100

95 .

"—* SINGLE COUER *"* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

90

85

80

75

.r-^-

10

1"^ 15 20

—r

25 30 35 40 —

45 50

nONTHS EXPOSURE

Figure 5e.

S

LA

R

EN

ER

G

Y

T

R

AN

S

11IT

TA

N

CE

Solar energy transmittance of poly(ethylene terephthalate)

weathering on minlcollectors in Miami, Florida

100

95

90 .

85

80 .

75

^—A SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10

"T"

15 20 25 30

nONTHS EXPOSURE

35 40

T

45 50

Figure 5f. Solar energy transmittance of acrylic film after

weathering on minlcollectors in Miami, Florida

22
100

95 -

90

85

80

75

o o

'^—* SINGLE COUER

*---* DOUBLE COUEP, OUTER

0---0 DOUBLE COUER, IMNER

-1 1 \

1 1

10 15 20 25 30

nONTHS EXPOSURE

35 40 45 50

Figure 5g.

S

LA

R

EN

ER

GV

T

R

A

N

S

n

IT

T

N

C

E

Solar energy transmittance of poly (methyl methacrylate)

after weathering on minicollectors in Miami, Florida

100

95 .

90 _

85 .

80

75

^—A SINGLE COUER

*---* DOUBLE COUER, OUTER

o---° DOUBLE COUER, INNER

10 15 20 25 30 35 40 45 50

nONTHS EXPOSURE

Figure 5h. Solar energy transmittance of polycarbonate after

weathering on minicollectors in Miami, Florida

23
s

LR

EN

ER

G

V

T

R

AN

S

IIITT

N

CE

100

95 -

90 .

70 .

65 .

60

A—* SINGLE COUER *•-* DOUBLE COUER, OUTER

<^--o DOUBLE COUER, INNER

10

1^ 15 30 25 30

MONTHS EXPOSURE

35

Figure 5i. Solar energy transmlttance of fiber reinforced plastic (1.0 mm)

after weathering on minicollectors in Miami, Florida

S

LA

R

E

N

ER

GY

TR

N

Sn

IT

TA

N

C

E

100

40

90 .

80 -

70 .

60

50 .

'^—A SINGLE COUER

*---* DOUBLE COUER, OUTER

<»--o DOUBLE COUER, INNER

10 15

"~r- 20 25 30 35

I 40 45 50

MONTHS EXPOSURE

Figure 5j . Solar energy transraittance of fiber reinforced plastic (1.5 mm)

after weathering on minicollectors in Miami, Florida

24
3.2.3 New River, Arizona Exposure

The solar energy transmittance of single and double covers after natural

weathering in New River is compared for each cover material in figures 6a to

63 . The materials were washed prior to these measurements. The corresponding

solar energy transmittance values are tabulated in tables A13 for single covers and table A14 for double covers. The values for the transmittance losses

caused by the weathering are listed in tables A15 and A16. With the exception

of fluorinated (ethylene propylene) copolymer, the film materials did not hold

up well in the weathering. Most of the film test specimens failed by the 18th

month of exposure. The glass materials and the fluorinated (ethylene propylene)

copolymer showed very little change in transmittance. For the other materials

the inner cover generally declined less than the covers exposed directly to

sunlight. The single and outer double covers usually decreased at at rates

much faster than the inner cover. The fiber reinforced materials had the

greatest transmittance losses, exceeding 12 percent for the single and outer

double covers after 48 months. Figure 6j illustrates that the outer cover of

the 1.5 mm fiber reinforced plastic declined less than the inner and single

covers. This indicates that the transmittance loss is caused by more than

sunlight. The early drop in the inner cover values point toward elevated

temperature as a factor.

Transmittance losses resulting from dirt accumulation are listed in table A17

for single covers and table A18 for double covers. Dirt accumulation fluctuated

somewhat for the various time periods. For the longest periods, the fiber

reinforced materials and the fluorinated (ethylene propylene) copolymer seemed

to collect the most dirt.

3.2.4 Comparison of Exposures

The materials followed the same general trends at all three locations. The

glass cover plate materials were not changed significantly by the weathering.

The fluorinated (ethylene propylene) transmittance shown in figure 7a remained

virtually unchanged after natural weathering as an inner cover and artificial

weathering with a xenon arc light. However, both the outer and single cover specimens declined about five percent at Florida and Gaithersburg. Figure 7b

illustrates this declined. Both figures 7a and 7b have the transmittance

plotted versus cumulative solar radiation at each site, rather than months

exposure. This was done to illustrate the effects of total sunlight exposure

rather than total time exposure.

The inner poly (vinyl fluoride) cover deteriorated most rapidly at New River,

followed by Miami. The deterioration caused the material to become brittle and

crumble. The transmittance of the outer and single poly (vinyl fluoride)

specimens decreased more at Gaithersburg and Miami than at New River.

The transmittance of the poly(ethylene terephthalate) declined more at Miami

and Gaithersburg than at New River; however, the material became brittle at New

River in about twelve months

.

25
s

L

AR

EN

ER

G

V

T

RA

N

Sn

IT

T

N

C

E

100

95 -

90 1

85 .

80 -

75

O

*

jt_-

^—A SINGLE COUER

*---* DOUBLE COUER, OUTER

o---«> DOUBLE COUER, INNER

T T

10 15 E0 25 30

MONTHS EXPOSURE

"T"

35 40 45 50

Figure 6a. Solar energy transmittance of glass (0.01% iron oxide) after

weathering on minicollectors in New River, Arizona

S

LAR

E

N

E

R

G

y

TR

N

Sn

IT

T

f\N

C

E

100

95

90

85 .

80

75

.^^^ — - — --A— _ — »_ _ -O 8-

^—* SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10

1^ 15 20 25 30 35 40 45 50

MONTHS EXPOSURE

Figure 6b. Solar energy transmittance of glass (0.10% iron oxide) after

weathering on minicollectors in New River, Arizona

26
s

LAR

EN

ER

G

V

TR

A

N

Sn

ITT

AN

C

E

100

S

L

AR

E

N

E

R

G

V

TR

A

N

9n

IT

T

AN

C

E

95 'i^r-r - —i^^=^:^.=.^^^—^:-^^ ^^*- _-^^ -__„

90 .

85 .

80

75

ft^

A—A SINGLE COUER

*--* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

T

10 15 30 as 30

MONTHS EXPOSURE

35 40 45 50

Figure 6c. Solar energy transmittance of fluorlnated (ethylene propylene)

copolymer after weathering on minicollectors in New River,

Arizona

100

95 -

90 -

85 -

80 .

75

^—^SINGLE COUER

*--* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10 15 20 £5

1^ 301^ 35 40 45 50

nONTHS EXPOSURE

Figure 6d. Solar energy transmittance of poly(vinyl fluoride) after

weathering on minicollectors in New River, Arizona

27
s

LAR

EN

E

R

G

Y

TR

A

N

Sn

ITTA

N

C

E.

100

ys .

90 -

S5

80 _

75

-^^"^1^^--'^= -_8

^—* SINGLE COUER

*---* DOUBLE COUER, OUTER

«>---o DOUBLE COUER, INNER

10

T

15

1^ 25

T

80 30

nONTHS EXPOSURE

35 40 45 50

Figure 6e. Solar energy transmittance of poly (ethylene terephthalate)

after weathering on minicollectors in New River, Arizona

S

LA

R

EN

ER

G

Y

T

RA

N

Sn

IT

T

A

N

C

E

100

95 .

90

85 -

80

75

"—* SINGLE COUER

*---* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

10 —

T"

15 20

"T"

25

"T"

30

"T"

35 40

1^ 45 50

nONTHS EXPOSURE

Figure 6f. Solar energy transmittance of acrylic film after weathering

on minicollectors in New River, Arizona

28
s

LAR

EN

E

RG

y

TR

A

N

Sn

IT

T

AN

C

E

100

95

90

85 -

80 .

75

^—^SINGLE COUER

*••* DOUBLE COUER, OUTER

0---0 DOUBLE COUER. INHER

~1 1 1 1 f—

10 15 20 35 30

MONTHS EXPOSURE

35 40 45 50

Figure 6g. Solar energy transmlttance of poly (methyl methacrylate)

after weathering on minicollectors in New River, Arizona

S

LAR

EN

ER

GY

TR

A

H

S

n

IT

TA

N

CE

100

95

90 -

85 .

80

75

^—^SINGLE COUER

*--* DOUBLE COUER, OUTER

0---0 DOUBLE COUER, INNER

n 1 1 1 1 1 1 r

5 10 15 20 25 30 35 40

MONTHS EXPOSURE

45 50

Figure 6h. Solar energy transmlttance of polycarbonate after

weathering on minicollectors in New River, Arizona

29
s

LAR

EN

ER

G

V

T

R

A

N

Sn

IT

T

N

C

E

100

95

90

85

80

75 -

70 -

65

£0

o —

SINGLE COUER *-* DOUBLE COUER, OUTER

°---o DOUBLE COUER, INNER

10 15 30 35 30

MONTHS EXPOSURE

-T 11 35 40 45 50

Figure 6i. Solar energy transmittance of fiber reinforced plastic

(1.0 mm) after weathering on minicollectors in New

River, Arizona

S

LAR

E

N

E

RG

V

T

RA

N

Sn

IT

TA

N

C

E

100

90

80

Figure 6j

70 .

60 -

50 .

40

SINGLE COUER *-* DOUBLE COUER, OUTER

o--« DOUBLE COUER, INNER

10 15 20 25 30 35 40 45 50

MONTHS EXPOSURE

Solar energy transmittance of fiber reinforced plastic

(1.5 mm) after weathering on minicollectors in New

River, Arizona

30
s

LA

R

e

N

E

RG

Y

T

R

N

Sn

IT

T

A

N

C

E

95 ''T^^'^^s'^'^^T^^^S^^^^eTi^^

90 -

85 _

80

75

- -D- .

^^ FLORIDA, INNER COUER

Q---° ARIZONA, INNER COUER

*---*nARVLAND, INNER COUER

0---0 ARTIFICIAL UEATHERING UITH XENON ARC LIGHT

e 10

1^ 15 30 25 —

T~ 30 35

SOLAR RADIATION GJ/H

Figure 7a. Comparison of solar transmittance of fluorinated (ethylene

propylene) copolymer after natural weathering as the inner cover of a minicollector and artificial weathering with

xenon arc light

S

LAR

E

N

ER

G

V

T

R

A

N

S

n

IT

TA

N

CE

100

95 -

90 _

85 -

80 -

75

'=—A FLORIDA, OUTER COUER

Q-'-i^ ARIZONA, OUTER COUER

*---*P1ARVLAND, OUTER COUER

0---0 ARTIFICIAL UEATHERING UITH XENON ARC LIGHT

10 15 20 25 30 35

SOLAR RADIATION GJ/fl'

Figure 7b. Comparison of solar transmittance of fluorinated (ethylene

propylene) copolymer after natural weathering as an outer

cover of a minicollector and artificial weathering with xenon

arc light

31
The acrylic film did not weather well at any of the sites. Failure occurred

when the material physically deteriorated by cracking or crumbling into pieces.

Generally, the material failed by the time it had been exposed to 12 GJ/m^ of

solar radiation. Failure occurred at New River first, followed by Miami. This

indicates that the solar energy was interacting with the chemical bonds in the

acrylic polymer resulting in degradation of the film.

The poly(methyl methacrylate) weathered comparatively well on the minicollectors.

As an inner cover the transmittance remained relatively unchanged (<1%). For

equal solar radiation exposure at the three sites, transmittance losses for the

outer and single covers were one to three percent higher at Miami and Gaithersburg than at New River. These changes compared well with the data from artificial weathering with a xenon arc light. Figure 8 compares the transmittance

of the outer covers and the specimens from xenon arc exposure. The rate of

change was essentially the same for the test specimens from New River and the

artificial weathering. Both represent exposures of high solar energy intensity

with minimal moisture. (Arizona has little rain and the artificial weathering

was done without water spray.) The decreases in transmittance from Miami and

Gaithersburg exposures were similar. These exposures involved moisture from

rain, dew, and humidity.

Polycarbonate also weathered comparatively well. The outer and single cover materials lost somewhat more transmittance at Miami and Gaithersburg than at

New River, for equal solar radiation exposure. The decrease in transmittance

at New River compared well with the xenon arc data. Again the transmittance

decreases at Miami and Gaithersburg were similar. The transmittance of the

outer covers and the test specimens from xenon arc exposure are compared in

figure 9.

The fiber reinforced plastics showed the greatest decreases in solar

transmittance. The transmittance of the inner covers of both materials

declined less than the single and outer covers. The 1.5 mm material changed

significantly more than the 1.0 mm material. For equal solar radiation exposure as single or outer double covers, the Florida and Maryland exposures

caused greater decreases in solar transmittance than did Arizona. This seems

to indicate that factors in additional to sunlight and heat were causing the

changes. Most likely, the presence of more moisture at these locations con- tributed to the deterioration. Figure 10a shows the solar transmittance as a function of solar radiation exposure for the 1.0 mm FRP material as an outer

cover, and figure 10b illustrates the same as a function of months of exposure.

Figures 10a illustrates that the transmittance loss due to artificial

weathering with xenon arc light exposure (sunlight with minimal heat and no moisture) was less than for exposures at any of the three outdoor weathering

sites. Clearly, the degradation is not due to sunlight alone. When comparing

the transmittance as a function of solar radiation (figure 10a), the rate of

decline was rapid at Miami, followed by Gaithersburg and New River. However

when comparing transmittance as a function of months exposure (figure 10b), the

rate of decline for New River is greater than Gaithersburg. Nevertheless, by

comparing figures 10a and 10b it can be seen that the same trends (rapid decline

in solar transmittance) occurred.

32
s

LA

R

EN

E

R

G

V

T

R

A

N

S

n

IT

T

A

N

C

E

100

95 -

90

85 -

80 .

75

SOLAR RADIATION GJ/fl

Figure 8. Comparison of solar transmittance of poly (methyl methacrylate)

after weathering on a minicollector as an outer cover and in an artificial weathering device with xenon arc light

100

95 -

90

85 .

80 -

75

*-

Q—o FLORIDA OUTER COUER

*--* ARIZONA OUTER COUER

^---onARYLAND OUTER COUER

'-"^ARTIFICIAL UEATHERING UITH XENON ARC LIGHT

"T"

10 15 20

-T~

25 30 35

SOLAR RADIATION GJ/M

Figure 9. Comparison of solar transmittance of polycarbonate after

weathering on a minicollector as an outer cover and in an artificial weathering device with xenon arc light

33
s

LA

R

EN

E

RG

V

T

R

A

N

Sn

IT

TA

N

C

E

Figure 10a.

SOLAR RADIATION GJ/PI

Solar transmlttance of fiber reinforced plastic (1.0 mm) as a function of solar radiation exposure

100

95 .

Figure 10b,

MONTHS EXPOSURE

Solar transmlttance of fiber reinforced plastic (1.0 mm) as a function of months of exposure

34
In comparing the effects of dirt for all samples, the general trend was for the

transmittance loss due to dirt to increase for about 12 to 18 months, then to

decrease and/or level off. A possible reason for this is that, when the sample

is new or only outdoors for a short period of time, the dirt comes off relatively easily with the washing procedure used. As the material ages, the dirt

is embedded deeper in the surface and cannot be washed off by the procedure

that was used. That could explain the leveling off or decrease in the apparent

amount of dirt on the surface. Also, the samples were exposed to various

amounts of rain. The specimens were brought indoors on a set schedule, if

there was any rain in the preceeding days or weeks, a decrease in the amount of

dirt on the surface may result.

The dirt seemed to accumulate most on the fluorinated (ethylene propylene) and on the fiber reinforced materials which became rough as their surfaces

weathered.

3.3 TENSILE PROPERTIES

Tensile properties of three plastic films (i.e. fluorinated (ethylene

propylene) copolymer, poly(vinyl fluoride) and poly (ethylene terephthalate)

)

were measured following exposure to heat aging at 125°C, artificial weathering

with xenon arc light, and natural weathering in New River, Arizona (for up to

36 months) and Miami, Florida for three months (poly (ethylene terephthalate)

only). Breaking factor, tensile strength elongation at break, yield strength,

and elongation at yield were calculated from the tensile data.

None of the tensile properties of the fluorinated (ethylene propylene)

copolymer changed dramatically. Tensile strength and yield strength data are presented in table 4. The tensile strength typically decreased about 25 percent in the longitudinal direction during each exposure, but with the sizable

standard deviations it is difficult to say how meaningful this was. The

fluorinated (ethylene propylene) was very thin, consequently, cutting it into

tensile strips without edge flaws was difficult. This and the thinness of the

material undoubtedly are the cause of the large standard deviations. The

breaking factor and the elongation at break data are given in table 5. The

elongation at break decreased about 35 percent in the xenon arc artificial

weathering but a corresponding decrease was not observed in the Arizona

weathering.

The tensile strength and yield strength of poly(vinyl fluoride) are presented in table 6. Heat aging at 125°C for 2000 hours caused the tensile

strength to decrease about 15 percent and xenon arc exposure for 4000 hours

resulted in about 10 percent loss. Outdoor weathering for 36 months caused

about 25 percent drop in tensile strength of the outer and single covers. The

inner cover tensile strength declined about 50 percent in the first year. The

inner covers exposed for longer periods of time become brittle and disintegrated during exposure. The breaking factor and elongation at break are given

in table 7. Both of these parameters showed declines similar to the tensile

strength.

35
Table 4. Tensile Strength and Yield Strength of Weathered

Fluorinated(ethylene propylene) Copolymer

Number

of Test

Specimens^'

Tensile Strength (MPa) Yield Strerigth (MPa)

Longitudinal^/ Transversek/ Longitudinal^/ Transverse^/

Exposure

L/T X a X X a X

None 5/5 25.96 6.20 19.25 5.17 9.95 0.27 10.66 0.59

Heat Aging - 125'C

100 hours 5/5 27.60 9.11 19.05 3.06 10.32 0.96 9.21 0.91

250 hours 5/5 23.97 5.16 19.96 1.68 4.61 0.28 4.90 0.25

1000 hours 5/6 23.34 2.02 16.68 3.18 5.02 0.21 4.66 0.35

2000 hours 11/11 19.80 5.76 16.83 4.49 5.67 0.39 .5.51 0.25

Xenon Arc Light

500 hours 6 23.71 5.28 8.70 1.02

1000 hours 5 25.80 0.77 8.23 0.57

3000 hours 6 19.54 5.27 6.33 1.16

4000 hours 10 21.94 5.01 5.60 0.32

Arizona - Outer Double Cover

3 months 5/4 26.14 1.98 19.36 0.62 5.22 0.62 5.02 0.26

6 months 6/5 27.70 4.06 25.15 1.74 5.13 0.53 5.46 0.36

12 months 5/5 24.84 2.04 21.24 5.25 5.32 0.47 4.42 0.44

18 months 6/6 28.11 3.72 24.36 2.78 5.65 0.53 5.28 0.15

24 months 4 23.76 3.41 8.73 0.73

36 months 5 19.73 3.31 8.98 0.20

Arizona - Inner Double Cover

6 months 6/6 23.21 6.54 19.71 3.22 5.02 0.49 4.42 0.16

12 months 6/6 26.66 1.91 22.78 4.08 5.00 1.00 5.38 0.25

18 months 5/6 24.65 4.01 19.34 4.42 4.30 0.25 4.48 0.33

24 months 5 20.05 4.18 8.50 0.31

36 months 5 18.34 4.99 7.88 0.56

Arizona - Single Cover

3 months 4/5 23.58 6.60 22.04 3.47 5.67 0.16 5.77 0.43

6 months 6/6 25.27 5.51 23.02 1.92 5.44 0.34 5.39 0.38

12 months 6/5 27.17 5.59 24.09 2.40 4.36 0.78 4.90 0.36

24 months 5 20.85 4.68 8.07 0.52

36 months 5 19.56 3.49 8.75 0.18

BJ Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

y X = average, a = standard deviation.

36
Table 5. Breaking Factor and Elongation at Break of Weathered

Fluorinated(ethylene propylene) Copolymer

Number

of Test

Specimens^/

Breaking Factor (N/m) Elongation at Break (%) | Longitudinal^ Transverse^/ Longitudinal^ Transversek/

Exposure

L/T X a X X X a

None 5/5 646 156 488 131 257 45 272 160

Heat Aging - 125°C

100 hours 5/5 632 205 402 75 221 75 246 77

250 hours 5/5 609 131 507 42 204 65 340 21

1000 hours 5/6 604 61 462 81 217 43 243 106

2000 hours U/U 555 86 460 84 211 65 276 79

Xenon Arc Light

500 hours 6 566 143 225 52

1000 hours 5 672 171 254 104

3000 hours 6 495 133 170 71

4000 hours 10 557 126 161 74

Arizona - Outer Double Cover

3 months 5/4 646 65 492 16 265 14 355 21

6 months 6/5 749 100 674 42 253 28 372 15

12 months 5/5 681 31 593 147 355 16 209 65

18 months 6/6 644 212 492 215 252 28 340 35

24 months 4 718 75 379 35

36 months 5 576 75 335 42

Arizona - Inner Double Cover

6 months 6/6 676 108 500 82 261 31 327 50

12 months 6/6 676 49 385 103 260 31 288 100

18 months 5/6 671 100 513 107 264 40 325 75

24 months 5 513 107 318 91

36 months 5 572 159 301 119

Arizona - Single Cover

3 months 4/5 542 129 560 89 185 34 332 45

6 months 6/6 642 140 584 49 238 82 311 21

12 months 6/5 737 126 672 72 266 45 343 14

24 months 5 630 140 339 99

36 months 5 497 89 316 45

£/ Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

b/ X = average, a = standard deviation.

37
Table 6. Tensile Strength and Yield Strength of Weathered Poly(vinyl fluoride)

Number

of Test

Specimens^/

Tensile Strength (MPa) Yield Strength (MPa) 1 Longitudinal^ Transverse^ LongitudinalW Transverse^/

Exposure

L/T X a X a X a X a

None 9/9 78.63 6.65 74.92 3.46 36.30 4.14 35.25 2.08

Heat Aging - 125°C

100 hours 4/5 77.64 1.72 73.81 5.69 35.26 0.99 36.67 3.72

250 hours 5/4 71.70 2.42 70.29 2.87 32.34 3.54 33.41 1.40

1000 hours 3/3 68.99 4.58 64.70 3.07 36.63 1.63 36.52 0.36

2000 hours 9/9 66.71 4.35 63.04 4.11 37.67 2.88 35.32 2.63

Xenon Arc Light

500 hours 4 70.72 2.73 31.98 0.63

1000 hours 3 69.10 0.63 33.35 0.72

3000 hours 3 68.08 3.74 33.98 0.36

4000 hours 9 70.04 4.29 35.37 2.28

Arizona - Outer Double Cover

3 months 5/5 70.39 2.61 67.25 2.99 35.08 1.52 34.31 0.69

12 months 4/5 70.69 4.02 68.47 3.09 32.77 2.09 32.42 0.53

24 months 4 69.75 1.57 31.88 1.98

36 months 5 61.05 5.09 34.50 2.67

Arizona - Inner Double Cover

3 months 4/5 69.95 3.58 66.25 1.23 38.15 1.08 35.59 1.92

12 months 2 42.90 1.12 37.05 0.45

Arizona - Single Cover

3 months£/ 4/5 69.74 3.78 67.22 6.35 34.83 0.89 33.69 1.92

3 months£/ 4/5 76.70 6.84 69.25 3.83 35.46 1.79 34.19 1.00

6 months 5/5 74.65 2.79 64.60 2.43 32.91 0.96 32.68 0.76

12 months 5/4 63.09 5.73 59.18 10.07 34.81 2.00 34.86 1.41

18 months 6/5 67.14 3.49 58.01 5.39 30.91 1.61 30.11 0.99

24 months 4 63.07 4.17 32.01 0.56

36 months 3 58.17 4.44 34.54 1.04

£/ Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

Rl -K = average, a = standard deviation.

£/ Two sets of minicollectors were exposed the same period of time but different dates.

38
Table 7. Breaking Factor and Elongation at Break of Weathered Poly (vinyl fluoride)

Number

of Test

Specimens£/

Breaking Factor (N/m) Elongation at Break (%)

Longitudinal^/ Transverse^ Longitudinal^/ Transversa^/

Exposure

L/T X X X a X a

None 9/9 7655 506 7258 221 205 13.0 191 11.1

Heat Aging - 125°C

100 hours 4/5 8357 185 7626 586 229 8.6 204 22.4

250 hours 5/4 7408 249 7263 303 221 6.6 198 8.9

1000 hours 3/3 7317 408 6685 320 187 29.6 219 16.9

2000 hours 9/9 6762 422 6483 455 195 11.5 175 14.1

Xenon Arc Light

500 hours 4 8182 284 216 3.9

1000 hours 3 7139 66 212 2.6

3000 hours 3 7033 385 204 22.7

4000 hours 9 6951 168 192 9.6

Arizona - Outer Double Cover

3 months 5/5 7272 270 6947 310 193 7.5 194 15.8

12 months 4/5 7303 415 7002 373 216 16.9 214 6.5

24 months 4 6923 82 183 7.4

36 months 5 6345 529 176 19.0

Arizona - Inner Double Cover

3 months 4/5 7226 369 6845 126 204 14.4 195 9.4

12 months 2 4433 116 101 47.8

Arizona - Single Cover

3 months^/ 4/5 7214 408 6844 657 201 14.4 202 21.1

3 months^/ 4/5 7925 706 7154 396 211 28.7 202 13.7

6 months 5/5 7713 287 6675 252 224 8.6 205 9.6

12 months 5/4 6519 592 6114 1040 175 32.8 181 54.9

18 months 6/5 7214 369 6303 545 213 19.8 220 58.8

24 months 4 6161 352 160 12.4

36 months 3 5592 429 149 25.5

£/ Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

^

X = average, a = standard deviation.

£/ Two sets of minicollectors were exposed the same period of time but different dates.

39
The tensile properties of the poly(ethylene terephthalate) changed more than

either of the other films. The tensile and yield strengths are presented in

table 8. Figure 11 displays the tensile strength (longitudinal) as a function

of solar radiation in Arizona and in artificial weathering. (The hours and

months of exposure in table 8 can be converted to solar radiation using tables

2 and 3.) As seen in this figure, the tensile strength shows a dramatic decline

after exposure in the xenon arc artificial weathering and in the outdoor

weathering. The tensile strength of the outer double and single covers dropped

about 50 percent after only 3 months exposure in Arizona and Florida and 500

hours in the xenon arc exposure. The inner covers, which were weathered

simultaneously, showed minor changes. The outer covers apparently absorbed most

of the ultraviolet radiation which was causing the material to degrade.

Consequently the inner cover degraded much more slowly. After 18 months the

single and outer double covers retained only about 20 percent of their original

strength, while the inner cover had dropped to about 60 percent. At that point,

the material was very brittle. A hail storm at the New River test site broke

holes in all of the remaining poly (ethylene terephthalate) minicollectors.

The breaking factor and elongation break are listed in table 9. Both factors

dropped rapidly in the specimens exposed directly to sunlight and more slowly

in the inner covers. In just 3 months the elongation at break dropped from 82

percent to 2.7 percent. The changes in the elongation at break are illustrated

in figure 12 for poly(ethylene terephthalate) exposed in Arizona and in an artificial weathering device with xenon arc. A scanning electron micrograph

of the surface of poly(ethylene terephthalate) after 12 months outdoor exposure

in Miami is shown in figure 3a. Many small cracks can be seen. These indicate

degradation of the material and are certainly related to the loss of strength.

From reviewing the data for the tensile properties, it is evident that all

three of the films were anisotropic (i.e., properties were not the same in

transverse and longitudinal directions). Of the five properties which were

calculated from the tensile measurements, it appears that the elongation at

break and tensile strength are the most sensitive factors in determining the

degradation in mechanical properties caused by weathering.

40
Table 8. Tensile Strength and Yield of Weathered Poly(ethylene terephthalate)

Number

of Test

Specimens^/

L/T

Tensile Strength (MPa) Yield Strength (MPa)

Longitudinal^ Transverse^/ Longitudinal^/ Transverse^/

Exposure

X a X a X a X a

None 10/12 213.28 10.75 159.37 6.71 85.17 2.77 90.15 4.79

Heat Aging - 125°C

250 hours

1000 hours

2000 hours

5/6

6/3

5/6

202.55

199.82

178.08

5.04

4.34

3.79

158.68

153.39

140.39

21.59

2.36

4.70

79.39

81.33

79.36

1.85

4.28

11.48

79.02

81.82

79.36

2.92

2.30

2.11

Xenon Arc Light

500 hours

1000 hours

3000 hours

4000 hours

4

4

4

7

112.03

103.69

56.98

35.27

1.24

3.25

6.87

5.21

84.29

70.87

2.10

6.83

Arizona - Outer Double Cover

3 months

12 months

18 months

6/6

6/6

4

96.47

72.01

49.86

10.40

3.41

9.98

49.31

45.23

1.79

2.36

Arizona - Inner Double Cover

3 months

12 months

18 months

6/4

6

6/6

195.14

167.49

130.13

3.14

16.82

10.04

141.02

106.71

3.84

37.84

104.90

96.65

85.53

3.19

2.63

9.31

109.24

82.85

4.82

1.56

Arizona - Single Cover

3 months£/

3 months^/

6 months

12 months

18 months

2/6

6/4

6/6

5

3

82.20

185.61

85.23

27.89

45.52

4.61

2.99

10.34

2.70

6.71

43.72

185.48

43.16

2.73

3.39

12.88

88.92

84.35

2.61

1.41

Florida -

3 months

Single Cover

Outer Double

Inner Double

5/6

6/4

6/4

103.23

115.94

207.50

2.47

3.03

3.72

54.37

62.18

155.90

2.55

5.24

5.92

105.68

101.13

1.85

3.81 103.89 2.08

fi/ Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

^

X = average, a =standard deviation.

£/ Two sets of minicollectors were exposed the same period of time but different dates.

41
Table 9. Breaking Factor and Elongation at Break of Weathered Poly (ethylene terephthalate)

Number

of Test

Specimens^/

L/T

Breaking Factor (N/m) Elongation at Break (%)

Longitudinal^ Transverse^/ Longitudinal^/ Transverse^

Exposure

X a X a X a X

None 10/12 26110 812 19889 742 82.0 6.6 123.1 10.9

Heat Aging - 125 °C

250 hours

1000 hours

2000 hours

5/6

6/3

5/6

25356

24885

23011

597

541

485

19812

19101

17529

2668

320

530

81.2

91.5

74.3

12.3

8.6

5.6

115.7

138.8

115.8

18.7

2.6

7.7

Xenon Arc Light

500 hours

1000 hours

3000 hours

4000 hours

4

4

4

7

14233

13307

7246

4386

147

499

868

616

10.4

6.7

2.1

1.3

3.6

1.8

0.4

0.3

Arizona - Outer Double Cover

3 months

12 months

18 months

6/6

6/6

5

12207

9089

6566

1334

353

1285

6168

5687

201

280

2.8

2.1

5.3

0.2

0.1

0.3

1.7

1.6

0.2

0.1

Arizona - Inner Double Cover

3 months

12 months

18 months

6/4

6

6/6

24253

21222

16949

380

2218

1490

17545

14533

462

3852

78.7

42.3

28.8

6.9

19.9

22.5

116.0

13.8

16.5

6.2

Arizona - Single Cover

3 months£/

3 months^/

6 months

12 months

18 months

2/6

6/4

6/6

5

3

10246

23654

10640

3487

5900

560

285

1251

371

867

5436

23552

5279

355

440

1534

2.7

57.9

2.4

1.0

3.8

5.0

0.4

0.1

0.7

1.4

56.8

1.4

0.1

6.0

0.1

Florida - 3 months

Single Cover

Outer Double

Inner Double

5/6

6/4

6/4

12636

14205

25809

348

400

464

6631

7721

13483

310

609

739

3.1

4.2

80.2

0.04

0.2

5.9

1.9

2.2

131.3

0.2

0.2

9.8

^

Number of specimens indicate number of strips obtained from weathered material.

L = Longitudinal direction, T = Transverse direction.

£/ X = average, a = standard deviation.

£/ Two sets of minicollectors were exposed the same period of time but different dates.

42
T

E

N

S

IL

E

S

T

R

E

N

G

TH

M

P

a

200 -

150

100

50

T

8 10 12

SOLPIR RflDIflTION (GJ/M^)

Figure 1

1

Tensile Strength of Poly (ethylene terephthalate) After

Natural Weathering in Arizona and Artificial Weathering

with Xenon Arc Light

80

60 -

40

20

6 8 10 12

SOLflR RflDWTION (GJ/M^)

14 16 18

Figure 12. Elongation at Break of Poly(ethylene terephthalate)

After Natural Weathering in Arizona and Artificial

Weathering with Xenon Arc Light

43
4. SUMMARY AND CONCLUSIONS

The key objective of this entire study described in this report and an earlier

report [1] was to obtain data needed as the technical basis for standards for

cover plate materials. An underlying assumption in the development of the

standards was that they should provide the user with a means to evaluate the

relative durabilities of potential cover plate materials. During the course of

this research, cover plate materials were aged using natural and artificial

weathering techniques, and an assessment of their performance was obtained by

measuring optical, physical and mechanical properties before and after

weathering. The natural and artificial weathering techniques were carefully

chosen to simulate one or more environmental conditions reached by the cover

material of a solar collector. The material properties which were measured

were selected because they were considered both critical to the performance of

a solar collector and likely to be affected by weathering. The cover plate

materials used were typical of those commercially available in 1976.

The large body of data which was assembled during the study now permits

reevaluation of the weathering procedures and property tests to determine their

adequacy. As a side benefit, while not being an objective of the study,

examination of the data also provides information on the durability of the

specific cover plate materials used in this study. However, the reader is

cautioned against direct extrapolation of these data to materials currently on

the market. The cover plate materials in this study are representative of

those on the market in 1976. No attempt was made to evaluate all types of

cover plate materials since this was not the purpose of the research. Many

cover plate materials are marketed and manufacturers sometimes modify their

materials in order to improve performance. Since the properties of cover plate

materials can be changed by alteration in minor constituents or processing

techniques, the properties of cover plate materials currently on the market may

differ from those described in this report.

The conclusions given below are drawn from the work in this report and they

relate to weathering procedures, property tests, and the effects of weathering

on cover materials. Additional conclusions reached in the earlier phase- of the

research are contained in the first report [1] on this project. Those conclusions address the measurement of transmittance, accelerated laboratory testing,

and outdoor weathering, and are not repeated here unless additional data supporting the conclusions are included in this report.

4.1 WEATHERING PROCEDURES

1 . Exposure of materials on minicollectors provided data useful in evaluating

candidate cover plate materials .

(a) Minicollectors provide a means of simultaneously exposing cover plate

materials to sunlight, elevated temperatures, moisture, and other natural

environmental conditions encountered by a cover plate on actual solar

collectors . Since material degradation generally occurs more rapidly at

elevated temperatures, a device which simultaneously exposes materials to

elevated temperatures and natural sunlight is an advance over the commonly

44
employed methods of natural weathering which involve exposure at ambient

temperature.

(b) Mlnlcollectors provide a means to generate data to evaluate materials in

both inner and outer cover plate applications . Due to their location in a solar collector, inner and outer covers encounter somewhat different

weathering. In some ways, the differences may be inconsequential; however,

other differences can be important. Outer cover plates experience higher

intensities of solar radiation but lower temperatures than inner cover

plates. In addition, outer cover plates generally have more exposure to

moisture and air pollutants. After minicollector exposure, differences

were observed between materials exposed as inner covers and those exposed

directly to the sun. Materials susceptible to damage caused by sunlight

and by heat were distinguished.

2. Artificial weathering with xenon arc light is helpful in distinguishing

materials sensitivity to natural sunlight . Polycarbonate and both fiber

reinforced plastics yellowed and had the largest solar transmittance losses

after both exposure in the xenon arc light and 48 months weathering in

Arizona. Similarly with the tensile property measurements, poly (ethylene

terephthalate) had a dramatic decrease in tensile strength and elongation

at break after just 500 hours of xenon arc exposure. Rapid declines in

tensile properties also occurred in the test specimens exposed to natural

sunlight. Susceptibility of materials to property changes due to damage

from sunlight alone could be determined in shorter periods of real time

using artificial weathering with xenon arc. However, periods of exposure

in an artificial weathering device must be sufficiently lengthy, with

property measurements being made at regular intervals, to establish a definite rate of degradation.

4.2 PROPERTY TESTS

1

.

Evaluation of materials for cover plates should include measurement of

both optical and mechanical properties . It must be emphasized that both

are critical to the performance of a cover plate material and a significant

decrease in either can cause the material to be unsuitable for use in a solar collector. Evaluation of weathered materials has shown that optical

properties may change little while mechanical properties decline rapidly

leading to failure of the material.

2. Tensile strength and elongation at break are useful parameters for

monitoring changes in mechanical properties of plastic film cover plate

materials caused by weathering . Mechanical property losses induced by

weathering on the minicollectors appear to be either: (1) a function of

accumulated exposure to sunlight (e.g., poly(ethylene terephthalate) and

acrylic film), or (2) a combination of elevated temperature and accumulated

exposure to sunlight (e.g., poly(vinyl fluoride) inner covers). Deterioration of mechanical properties occurred most rapidly in Arizona. Of the tensile properties studied, tensile strength and elongation at break were the

most sensitive in determining material degradation. Significant information

45
relating to strength as well as ductibillty (or brittleness) can be

extracted from the data from tensile measurements.

4.3 EFFECTS OF WEATHERING ON MATERIALS

1Significant solar energy transmittance losses result from retention of dirt

and dust on the cover plate surfaces . For most materials, the transmittance loss due to dirt increased for 12 to 18 months, then decreased or leveled off. These transmittance losses, which can be regained by washing

the material, were generally three to six percent, although, in a few

cases, they reached 17-20 percent.

2. The rates of material degradation vary at different geographic locations . Sun, heat, moisture, and air pollutants can cause material degradation

either independently or synergistically. The varied influence of these

multiple weather factors is illustrated by the different rates of degradation for materials weathered simultaneously at three separate sites. The

data clearly indicate that weather parameters in addition to sunlight are causing degradation. Although the materials weathered in Arizona were

exposed to the most sun and the highest temperatures, they did not usually

have the greatest transmittance losses. The materials from Florida and

Maryland suffered greater transmittance losses. This seems to indicate

that moisture contributed to the material degradation. The effect of pollutants is difficult to assess quantitatively since their concentrations were

not monitored.

3. Degradation rates vary among the cover plate materials . Comparison of all

materials at one site indicates that some cover plate materials degrade

more rapidly than others. Similarly, comparison of a material at one site

exposed as single cover, and as inner and outer covers shows that the type

of use in a solar collector can influence the rate of degradation.

4. Outdoor weathering caused no significant degradation of the glass cover plate materials . Solar transmittance measurements of the two glass

materials did not decline significantly.

5. The f luorinated(ethylene propylene) copolymer film was the only one of

the four films which did not suffer serious degradation in either the

optical or mechanical properties during either accelerated or natural

weathering . Although the test specimens exposed in Gaithersburg had a solar transmittance loss of seven percent after three and four years

exposure, examination of the material indicated the likely cause was permanent deposits of dirt or pollutants on the surface.

6. Poly(vinyl fluoride) was susceptible to deterioration resulting from the

combination of sunlight and heat . The inner covers became brittle and

disintegrated in some cases.

7. The mechanical properties of the acrylic film and the poly(ethylene

terephthalate) are seriously affected by exposure to solar radiation .

46
The acrylic film disintegrated on the minicollectors after 18-24 months

weathering. The poly(ethylene terephthalate) became brittle and lost

flexibility after only one to two years exposure in Arizona. This led to

failure of the material when a hailstorm occurred.

8. The solar transmittance losses of polycarbonate and poly(methyl

methacrylate) were generally about the same, with polycarbonate losses

being about one to two percent higher than those for poly (methyl methacrylate) . Polycarbonate had losses of five to seven percent at Florida and

Maryland and about two to four percent at Arizona. The materials from

Florida and Maryland exposures had some deposits on the surface which

contributed to the higher transmittance losses.

9. The fiber reinforced plastic materials suffered significant solar

transmittance losses after exposure on the minicollectors . Transmittance

losses appeared to be due to a combination of weathering factors, i.e.,

sunlight, heat, moisture, and other weathering. The outer and single

covers of the 1 mm FRP had transmittance losses of 10-25 percent while the

1.5 mm FRP transmittance losses were 15-30 percent. Inner cover losses

reached 10 percent and 20 percent, for the 1 mm and 1.5 mm FRP materials,

respectively.

47
5. FUTURE RESEARCH NEEDS

Additional research is needed 1) to develop improved polymeric glazing materials

and 2) to develop improved methods for reliably predicting the service life of

potential polymeric glazing materials.

5.1 MATERIALS DEVELOPMENT

As shown the data presented in this report, polymeric materials are frequently

susceptible to degradation stemming from the environmental factors accompanying

solar collector applications. Because of the degradation of many currently

available materials, research is needed to develop improved polymeric glazings.

In particular, glazing materials are needed which are more resistant to degradation processes, such as photodegradation, thermal degradation and oxidation,

than currently available materials.

5.2 SERVICE LIFE PREDICTION

Service life data are essential in the effective selection of materials for

specific solar collector applications. While methods have been developed to

aid in screening potential glazing materials, methods are not available to

reliably predict their service lives.

Research is needed to develop improved methods of predicting service life prior

to the use of materials in collector glazing applications.

Inherent in the research to develop improved predictive test methods are the

following specific needs:

1. Characterization of the mechanisms of degradation of polymeric glazing

materials. Knowledge of the degradation mechanisms is essential to ensure that accelerated tests induce the same degradation processes as those which occur in actual use.

2. Characterization of microstructural changes and identification of the

relationship between microstructural changes and engineering properties. This would permit the use of microstructural changes in predicting impending changes in engineering properties with the advantage

that microstructural changes can be detected at much earlier stages of

degradation than engineering properties.

3. Mathematical models to aid extrapolation of short-term test data to

long-term performance. In particular, models are needed to account

for the synergistic actions of multiple degradative factors, such as ultraviolet radiation, heat, moisture and air contaminants. The use

of probabilistic models offers promise for meeting these needs.

48
6 . ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions of Frank Davis, Edward

Embree, John Grimes, and John Michalak of NBS in performing the tensile testing

and related calculations described in this report. The authors also thank

Catherine Kelly of NBS for her assistance in making optical property measurements. The personnel at South Florida Test Services, Inc. and at DSET Laboratories are also acknowledged for their cooperation in maintaining the outdoor

exposure boxes. The authors also thank the Department of Energy for support of

this research.

49
7. REFERENCES

[1] Clark, E. J., Roberts, W. E., Grimes, J. W., and Embree, E. J., "Solar

Energy Sytems - Standards for Cover Plates for Flat Plate Solar

Collectors," National Bureau of Standards Technical Note 1132, (1980).

Available from Superintendent of Documents, U.S. Government Printing

Office, Washington, DC 20402.

[2] ASTM E 765, Practice for Evaluation of Cover Materials for Flat Plate

Solar Collectors, Annual Book of ASTM Standards, Part 41, American

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

19103.

[3] ASTM E 782, Practice for Exposure of Cover Materials for Solar Collectors

to Natural Weathering Under Conditions Simulating Operational Mode,

Annual Book of ASTM Standards, Part 41, American Society for Testing

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

[4] ASTM E 822, Practice for Determining Resistance of Solar Collector

Covers to Hail by Impact with Propelled Ice Balls, Annual Book of

Standards, Part 41, American Society for Testing and Materials,

1916 Race Street, Philadelphia, PA 19103.

[5] ASTM E 881, Practice for Exposure of Solar Collector Cover Materials

to Natural Weathering Under Conditions Simulating Stagnation Mode,

Annual Book of Standards, Part 41, American Society for Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103.

[6] Jenkins, D. and Mathey, R. G. , "Hail Impact Testing Procedure for Solar

Collector Covers," National Bureau of Standards, NBSIR 82-2487, (1982).

Available from National Technical Information Service, Springfield, VA

22161.

[7] Waksman, D., Streed, E. R. , Reichard, T. W. , and Cattaneo, L. E.,

"Provisional Flat Plate Solar Collector Testing Procedures: First

Revision," National Bureau of Standards, NBSIR 78-1305A, (1978).

Available from National Technical Information Service, Springfield, VA

22161, Order No. PB 283-721.

[8] Gilligan, J. E., Brzuskiewicz , J., Brzuskiewicz, J. E., and Mell, R.

,

Handbook of Materials for Solar Energy Utilization - Low Temperature

Applications, IIT Research Institute, Chicago, IL, prepared for the

U.S. Dept. of Energy, Report DE 81027310, (April 1980).

[9] Gilligan, J. E., Exposure Testing and Evaluation of Solar Utilization

Materials, IIT Research Institute, Chicago, IL, prepared for the U.S.

Dept. of Energy, Report DE 81027311, (April 1980).

50
[10] Holton, J, K. , et al., "Interim Performance Criteria for Solar Heating

and Cooling Systems in Residential Buildings, Second Edition," National

Bureau of Standards Report NBSIR 78-1562, (1978). Available from

National Technical Information Service, Springfield, VA 22161, Order

No. PB 289 767.

[11] "Interim Performance Criteria for Solar Heating and Cooling Systems in

Commercial Buildings," National Bureau of Standards Report NBSIR 76-1187,

(1976). Available from National Technical Information Service,

Springfield, VA 22161, Order No. PB 262 114.

[12] "Intermediate Minimum Property Standards Supplement - Solar Heating and

Domestic Hot Water Systems," U.S. Department of Housing and Urban

Development, (1977). Available from Superintendent of Documents, U.S.

Government Printing Office, Washington, DC 20402, Order No. SN 023-000-

90161-7.

[13] "Final Reliability and Materials Design Guidelines for Solar Domestic

Hot Water Systems," Argonne National Laboratory, Report ANL/SDP-11,

prepared for U.S. Department of Energy, (September 1981). Available

from National Technical Information Service, Springfield, VA 22161.

[14] "Solar Heating Materials Handbook - Environmental and Safety Considerations

for Selection," Sandia National Laboaratories, Report DOE/TIC-11374,

prepared for U.S. Department of Energy, (January 1981). Available from

National Technical Information Service, Springfield, VA 22161.

[15] ASTM D 2565, Recommended Practice for Operating Xenon Arc-Type (WaterCooled) Light- and Water-Exposure Apparatus for Exposure of Plastics,

Annual Book of ASTM Standards, Part 35, American Society for Testing

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

[16] "Recommendations for the Integrated Irradiance and the Spectral Distribution of Simulated Solar Radiation for Testing Purposes, (1972), International Commission on Illumination, Publication CIE No. 20 (TC-2.2)..,

Available from Secretary of U.S. National Committee of International

Commission on Illumination, National Bureau of Standards, Washington,

DC 20234.

[17] ASTM E 424, Standard Test Methods for Solar Energy Transmittance and

Reflectance (Terrestrial) of Sheet Materials, Annual Book of ASTM

Standards, Part 46, American Society for Testing and Materials, 1916 Race

Street, Philadelphia, PA 19103.

[18] Smithsonian Physical Tables, Table 1, Vol. 815, p. 273, (1954).

[19] ASTM D 882, Standard Test Methods for Tensile Properties of Thin Plastic

Sheeting, Annual Book of ASTM Standards, Part 35, American Society for

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

51
APPE^fDIX - SOLAR ENERGY TRANSMITTANCE DATA OF COVER PLATE MATERIALS AFTER NATURAL WEATHERING

Table Al. Solar Energy TransmltCance of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering in Gaithersburg , Maryland

^v Months Exposure 3 6 12a/ 12a/ 18 24 36S/ 36S/ 48

Materlal\solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) 90.7 89.1 90.3 91.3 90.2 91.1 88.5 90.0 81.2 90.2

Glass (0.10% iron oxide) 87.4 88.2 86.8 87.5 88.8 86.7 85.9 87.0 80.4 85.5

Fluorinated (ethylene

propylene) copolymer

96.0 96.3 95.2 95.6 96.0 93.4 93.6 94.0 95.5 93.4

Poly(vinyl fluoride) 92.5 90.7 89.5 87.3 91.9 89.2 87.6 83.4 82.0 80.6

Poly(ethylene terephthalate) 86.3 85.2 83.1 83.8 85.7 84.6 76.6 y 85.1 y

Acrylic 90.7 90.1 87.3 86.0 87.3 89.0 87.8 y y y

Poly (methyl methacrylate) 91.0 89.5 89.1 89.2 89.3 88.1 85.8 86.3 81.3 86.8

Polycarbonate 88.1 88.2 — 87.0 87.2 85.6 81.6 84.2 84.3 83.0

Fiber reinforced plastic (1.0 mm) 86.3 85.7 84.2 84.5 84.4 82.5 80.3 77.7 68.5 y

Fiber reinforced plastic (1.5 imn) 78.5 75.0 72.3 76.6 69.5 74.5 65.6 57.7 59.4 49.1

Transmittance was measured after the test specimens were washed. The solar energy transmittance was calculated

for air mass 2. Each transmittance value represents a separate test specimen. Transmittance is expressed in percentage.

y Two sets of minicollectors were exposed for the same length of time but different dates. y Material failed prior to this time.

52
Table A2. Solar Energy Transmlttance of Cover Plate Materials After Exposure

as Double Covers to Natural Weathering In Galthersburg, Maryland

Outer Cover

>^ Months Exposure 3 6 123/ 123/ 18 24 363/ 363/ 48 MaterlalN. Solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% Iron oxide) 90.7 89.3 87.8 91.0 91.8 90.3 90.1 90.3 89.4 89.8

Glass (0.10% iron oxide) 87.4 88.3 86.9 87.2 88.9 87.0 88.1 87.0 86.2 87.4

Fluorinated (ethylene

propylene) copolymer

96.0 95.9 95.6 95.8 96.0 95.9 96.5 95.4 95.8 95.2

Poly(vlnyl fluoride) 92.5 91.4 90.7 91.7 91.8 92.8 92.0 92.1 91.6 88.7

Poly( ethylene terephthalate) 86.3 86.0 84.9 85.6 85.9 85.7 86.0 86.0 84.2 V

Acrylic 90.7 90.3 90.0 89.5 90.3 89.9 b/ b/ b/ b/

Poly(methyl methacrylate) 91.0 90.9 89.8 91.0 90.7 90.8 - 90.4 91.3 90.4

Polycarbonate 88.1 87.9 87.1 88.3 87.9 88.3 87.9 84.1 88.3 87.9

Fiber reinforced plastic (1.0 mm) 86.3 84.6 85.2 81.6 83.1 84.0 82.7 77.9 80.0 79.3

Fiber reinforced plastic (1.5 mm) 78.3 74.7 67.6 67.1 68.1 65.8 61.7 - 72.7 68.1

I

Inner Cover

>s^ Months Exposure 3 6 123/ 12a/ 18 24 36a/ 36a/ 48 Materlal\^Solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) 90.7 89.3 87.8 91.0 91.8 90.3 90.1 90.3 89.4 89.8

Glass (0.10% iron oxide) 87.4 88.3 86.9 87.2 88.9 87.0 88.1 87.0 86.2 87.4

Fluorinated (ethylene

propylene) copolymer

96.0 95.9 95.6 95.8 96.0 95.9 96.5 95.4 95.8 95.2

Poly( vinyl fluoride) 92.5 91.4 90.7 91.7 91.8 92.8 92.0 92.1 91.6 88.7

Poly(ethylene terephthalate) 86.3 86.0 84.9 85.6 85.9 85.7 86.0 86.0 84.2 b/

Acrylic 90.7 90.3 90.0 89.5 90.3 89.9 b/ b/ b/ b/

Poly(methyl methacrylate) 91.0 90.9 89.8 91.0 90.7 90.8 - 90.4 91.3 90.4

Polycarbonate 88.1 87.9 87.1 88.3 87.9 88.3 87.9 84.1 88.3 87.9

Fiber reinforced plastic (1.0 mm) 86.3 84.6 85.2 81.6 83.1 84.0 82.7 77.9 80.0 79.3

Fiber reinforced plastic (1.5 mm) 78.3 74.7 67.6 67.1 68.1 65.8 61.7 - 72.7 68.1

Transmlttance was measured after the test specimens were washed. The solar energy transmlttance was calculated

for air mass 2. Each transmlttance value represents a separate test specimen. Transmlttance is expressed in percentage

5/ Two sets of mlnicollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

53
Table A3. Solar Energy Transmittance Loss of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering In Gaithersburg, Maryland

^S^ Months Exposure 3 6 12a/ 12a/ 18 24 36^/ 36^/ 48 Materlal\Solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) 1.6 0.4 -0.6 0.5 -0.4 2.2 0.7 9.5 0.5

Glass (0.10% iron oxide) -0.8 0.6 -0.1 -1.4 0.7 1.5 0.4 7.0 1.9

Fluorinated (ethylene

propylene) copolymer

-0.3 0.8 0.4 2.6 2.4 2.0 0.5 2.6

Poly(vinyl fluoride) 1.0 3.0 5.2 0.6 3.3 4.9 9.1 10.5 11.9

Poly(ethylene terephthalate) 1.1 3.2 3.5 0.6 1.7 9.7 b/ 1.2 b/

Acrylic 0.6 3.4 4.7 3.4 1.7 1.9 b/ b/ b/

Poly(methyl methacrylate) 1.5 1.9 1.8 1.7 2.9 5.2 4.7 9.7 4.2

Polycarbonate -0.1 - 1.1 0.9 2.5 6.5 3.9 3.8 5.1

Fiber reinforced plastic (1.0 mm) 0.6 2.1 1.8 1.9 3.8 6.0 8.6 17.8 -

Fiber reinforced plastic (1.5 mm) 3.5 6.2 1.9 9.0 4.0 12.9 20.8 19.1 29.4

The transmittance is expressed in percentage. This solar energy transmittance loss is the difference

between the average for the control test specimens and the values obtained after the test specimen was washed (table Al). Negative values represent a gain in comparison with the average for the controls.

This solar energy transmittance loss does not Include the loss due to dirt on the surface. Losses due

to dirt are tabulated in table A5.

£/ Two sets of minicollectors were exposed for the same length of time but different dates. ^ Material failed prior to this time.

54

i

1
Table A4. Solar Energy Transmlttance Loss of Cover Plate Materials After Exposure

as Double Covers to Natural Weathering In Galthersburg, Maryland

Outer Cover ^\ Months Exposure 3 6 125/ 123/ 18 24 36§/ 365/ 48

Materials. Solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) 1.1 0.6 0.0 -0.8 -0.4 1.5 1.7 6.8 0.1

Glass (0.10% iron oxide) -0.5 0.2 0.0 -1.6 -0.5 -0.5 3.6 4.7 1.2

Fluorlnated (ethylene

propylene) copolymer

0.2 0.7 0.2 0.5 2.9 4.3 6.9 6.8 6.9

Poly(vinyl fluoride) 2.0 3.2 4.8 -0.3 0.3 5.9 7.4 8.8 b/

Poly(ethylene terephthalate) 0.7 1.7 2.8 0.6 1.0 5.0 4.2 -0.8 b/

Acrylic 1.6 3.1 2.9 2.9 3.0 b/ b/ b/ b/

Poly(methyl methacrylate) 0.4 2.0 1.2 1.4 2.3 - 5.1 6.3 3.1

Polycarbonate -0.8 1.5 0.4 1.1 1.7 6.9 5.8 3.8 6.9

Fiber reinforced plastic (1.0 mm) 1.1 2.4 3.0 2.8 5.8 8.2 10.3 14.3 15.9

Fiber reinforced plastic (1.5 mm) 5.2 0.9 1.7 5.6 9.7 9.8 14.3 27.9 23.6

Inner Cover

>s^ Months Exposure 3 6 125/ 12§/ 18 24 36§/ 36a/ 48 MateriaTS Solar Radiation (GJ/m2) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) 1.4 2.9 -0.3 -1.1 0.4 0.6 0.4 1.3 0.9

Glass (0.10% iron oxide) -0.9 0.5 0.2 -1.5 0.4 -0.7 0.4 1.2 0.0

Fluorlnated (ethylene

propylene) copolymer

0.1 0.4 0.2 0.0 0.1 -0.5 0.6 0.2 0.8

Poly(vlnyl fluoride) 1.1 1.8 0.8 0.7 -0.3 0.5 0.4 0.9 3.8

Poly(ethylene terephthalate) 0.3 1.4 0.7 0.4 0.6 0.3 0.3 2.1 b/

Acrylic 0.4 0.7 1.2 0.4 0.8 b/ b/ b/ b/

PolyCmethyl methacrylate) 0.1 1.2 0.0 0.3 0.2 - 0.6 -0.3 0.6

Polycarbonate 0.2 1.0 -0.2 0.2 -0.2 0/2 4.0 -0.2 0.2

Fiber reinforced plastic (1.0 mm) 1.7 1.1 4.7 3.2 2.3 3.6 8.4 6.3 7.0

Fiber reinforced plastic (1.5 mm) 3.8 10.9 11.4 10.4 12.7 16.8 - 5.8 10.4

The transmlttance is expressed in percentage. This solar energy transmlttance loss is the difference between the average for the control test specimens and the values obtained after the test specimen was washed (table A2). Negative values represent a gain in comparison with the average for the controls. This solar energy transmlttance loss does not include the loss due to dirt on the surface. Losses due to dirt

are tabulated in table A6.

5/ Two sets of minlcollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

55
Table A5. Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials

After Exposure as Single Covers in Gaithersburg, Maryland

>y Months Exposure 3 6 12a/ 122/ 18 24 36§/ 36a/ 48 Materials^ Solar Radiation (GJ/m^) 1.176 2.251 5.429 7.477 7.680 10.720 16.402 16.693 22.076

Glass (0.01% iron oxide) -1.3 -0.2 3.6 4.7 4.9 4.1 1.8 1.6 1.1

Glass (0.10% iron oxide) 2.1 1.5 5.9 5.9 2.3 4.7 1.8 6.8 1.8

Fluorinated (ethylene

propylene) copolymer

4.3 1.4 12.3 1.2 9.4 7.6 15.8 10.0 11.0

Poly(vinyl fluoride) 1.2 2.6 6.5 5.8 1.9 1.8 5.7 10.3 5.0

Poly(ethylene terephthalate) 0.5 0.4 8.0 4.8 4.2 1.3 b/ 7.2 6.0

Acrylic 2.4 1.6 7.0 4.1 8.6 3.7 b/ b/ b/

Poly(methyl methacrylate) 1.4 2.6 8.8 3.9 3.2 3.9 2.2 4.9 6.2

Polycarbonate 0.9 - 8.7 4.2 5.3 3.1 1.3 4.8 5.3

Fiber reinforced plastic (1.0 mm) 0.6 1.9 9.8 4.4 2.5 2.9 0.3 3.7 -

Fiber reinforced plastic (1.5 mm) 0.0 2.4 10.4 4.6 3.9 4.3 3.3 6.3 1.3

The solar energy transmittance loss due to dirt is the difference between values obtained before and after washing the test specimen. Negative values represent a loss in transmittance after washing.

Transmittance is expressed in percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

56
Table A6. Solar Energy Transmit Cance Loss Due to Surface Dirt on Cover Plate Materials

After Exposure as Double Covers In Galthersburg, Maryland

Outer Cover

>v Months Exposure 3 6 125/ 125/ 18 24 36S/ 365/ 48

Materials. Solar Radiation (GJ/m^) 1.176 2.251 5.429 5.477 7.680 10.720 16.402 16.693 22.076

Glass (O.OIZ Iron oxide) 0.3 1.8 8.4 2.7 3.2 4.0 0.7 5.2 3.5

Glass (0.10% iron oxide) 1.2 1.8 8.2 4.8 1.8 4.8 2.2 3.5 7.0

Fluorlnated (ethylene

propylene) copolymer

1.3 2.9 3.5 8.8 6.2 7.0 4.8 8.0 6.9

Poly (vinyl fluoride) 0.9 6.0 5.5 6.8 3.5 3.0 1.6 10.6 -

Poly(ethylene terephthalate) 0.8 4.5 3.8 4.3 3.3 7.2 4.5 9.3 b/

Acrylic 1.0 3.4 4.9 3.3 4.2 b/ b/ b/ y

Poly(methyl methacrylate) 2.0 2.7 7.9 3.3 2.7 - 2.9 4.5 6.6

Polycarbonate 2.3 1.6 6.6 5.0 5.1 4.1 0.8 5.4 3.4

Fiber reinforced plastic (1.0 nn) 0.8 2.3 6.7 2.6 1.5 0.3 2.5 2.7 4.9

Fiber reinforced plastic (1.5 mm) 1.8 1.0 5.7 1.0 3.3 1.4 5.3 7.8 0.1

Inner Cover

Xy Months Exposure 3 6 125/ 125/ 18 24 365/ 365/ 48

MaterialN. Solar Radiation (GJ/m^) 1.176 2.251 5.429 7.477 7.680 10.720 16.402 16.693 22.076

Glass (O.OIX iron oxide) 0.0 - 1.7 0.0 0.2 0.3 1.9 0.8 0.4

Glass (0.10% iron oxide) 1.3 - 0.7 2.6 0.2 2.4 0.7 0.1 0.4

Fluorlnated (ethylene

propylene) copolymer

0.4 0.7 0.6 0.5 0.8 2.9 5.1 4.7 0.9

Poly (vinyl fluoride) 0.8 1.0 0.2 0.3 0.9 0.3 0.9 -0.2 y

Poly (ethylene terephthalate) 0.3 -0.6 1.0 0.6 0.6 0.5 0.0 1.2 b/

Acrylic 0.3 0.8 -0.1 0.4 0.3 b/ ^ y b/

Poly(methyl methacrylate) 0.8 0.2 0.9 0.9 -0.3 - 0.2 1.4 0.4

Polycarbonate -0.1 0.2 1.2 0.5 0.7 0.6 0.5 1.2 0.9

Fiber reinforced plastic (1.0 mm) -1.4 0.5 0.6 0.3 -0.8 -1.8 -0.4 1.2 1.5

Fiber reinforced plastic (1.5 mm) -0.3 -0.2 0.1 -2.3 0.3 -1.1 - 3.9 2.0

The solar energy transmittance loss due to dirt is the difference between values obtained before and after washing the test specimen. Negative values represent a loss in transmittance after washing. Transmittance is expressed in percentage.

5/ Two sets of minicollectors were exposed for the same length of time but different dates. V Material failed prior to this time.
Table A7. Solar Energy Transmit tance of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering in Miami, Florida

>>^^ Months Exposure 3 6 12S/ 125/ 18 24 365/ 36§/ 48 MaterialN^Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.398 19.398 19.859 26.333

Glass (0.01% iron oxide) 90.7 90.1 90.0 88.8 90.5 89.1 90.4 88.2 90.1 90.1

Glass (0.10% iron oxide) 87.4 88.0 87.0 88.6 87.0 85.9 87.3 86.9 85.9 87.0

Fluor inated (ethylene

propylene) copolymer

96.0 95.7 95.3 95.2 94.9 92.4 91.7 95.6 90.0 95.0

Poly(vinyl fluoride) 92.5 91.9 91.4 88.5 91.7 87.3 87.9 — 89.0 89.1

Poly(ethylene terephthalate) 86.3 86.3 86.1 81.4 78.6 81.5 81.5 85.5 — 79.6

Acrylic 90.7 88.8 89.7 87.5 88.7 87.3 W b/ b/ b/

Poly(methyl methacrylate) 91.0 90.3 89.1 88.8 87.7 86.9 86.1 87.8 86.5 86.5

Polycarbonate 88.1 88.4 88.0 85.6 84.6 83.5 80.2 84.2 — 84.5

Fiber reinforced plastic (1.0 mm) 86.3 84.9 85.0 82.2 84.4 77.2 75.5 65.0 67.2 64.2

Fiber reinforced plastic (1.5 mm) 78.5 72.9 71.5 70.0 70.6 58.9 63.0 62.2 61.6 51.0

Transmittance was measured after the test specimens were washed. The solar energy transmittance was calculated

for air mass 2. Each transmittance value represents a separate test specimen. Transmittance is expressed in percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

£/ Material failed prior to this time.

58
Table A8. Solar Energy Transmlttance of Cover Plate Materials After Exposure

as Double Covers to Natural Weathering in Miami, Florida

Outer Cover

X^ Months Exposure 3 6 12a/ 12a/ 18 24 36aj' 36a/ 48 MaterialS^^ Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 90.7 91.0 90.3 92.4 90.1 89.3 89.2 89.0 89.3 90.5

Glass (0.10% iron oxide) 87.4 87.7 87.4 86.6 86.8 86.5 86.6 86.9 86.4 86.3

Fluorinated (ethylene

propylene) copolymer

96.0 96.5 96.0 94.7 95.9 91.5 89.4 94.5 - 95.4

Poly(vinyl fluoride) 92.5 91.6 91.1 89.5 91.5 92.5 88.1 89.6 89.1 b/

Poly (ethylene terephthalate) 86.3 86.7 85.4 78.6 81.1 81.1 82.4 b/ 81.5 b/

Acrylic 90.7 87.5 88.1 88.6 88.0 b/ b/ b/ b/ b/

Poly(methyl methacrylate) 91.0 90.0 89.2 88.2 89.0 87.7 87.9 88.2 85.7 85.8

Polycarbonate 88.1 88.2 87.3 85.7 85.6 83.1 83.0 81.5 82.3 -

Fiber reinforced plastic (1.0 mm) 86.3 88.5 85.5 79.0 77.2 77.8 71.3 68.8 61.8 64.2

Fiber reinforced plastic (1.5 mm) 78.5 80.9 78.2 58.8 67.9 54.2 53.7 56.4 44.6 57.3

Inner Cover

^s^ Months Exposure 3 6 125/ 12§/ 18 24 362/ 365/ 48 MaterialN^Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 90.7 90.9 90.7 92.3 90.6 90.2 91.6 89.6 91.7 90.2

Glass (0.10% iron oxide) 87.4 87.7 87.2 88.0 86.9 87.3 88.4 87.2 87.9 87.4

Fluorinated (ethylene

propylene) copolymer

96.0 96.4 96.1 95.3 95.7 95.8 96.1 95.9 - 95.2

Poly(vinyl fluoride) 92.5 92.1 92.4 91.8 91.7 91.8 91.7 b/ 90.8 b/

Poly(ethylene terephthalate) 86.3 86.4 85.7 85.0 85.8 85.5 86.2 80.0 85.7 82.5

Acrylic 90.7 89.7 91.1 89.7 90.3 90.0 b/ b/ b/ b/

Poly(methyl methacrylate) 91.0 91.4 91.5 91.5 90.4 90.4 91.3 90.0 90.2 90.7

Polycarbonate 88.1 88.3 88.4 89.0 87.5 87.3 88.2 85.9 87.3 84.2

Fiber reinforced plastic (1.0 mm) 86.3 87.9 86.1 83.0 79.4 74.8 74.8 76.2 76.2 77.3

Fiber reinforced plastic (1.5 mm) 78.5 67.0 72.1 58.7 68.6 66.4 56.6 56.6 68.9 61.0

Transmlttance was measured after the test specimens were washed. The solar energy transmlttance was calculated for air mass 2. Each transmlttance value represents a separate test specimen. Transmlttance

Is expressed In percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates

V Material failed prior to this time.

59
Table A9. Solar Energy Transmlttance Loss of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering in Miami, Florida

^^ Months Exposure 3 6 12a/ 12a/ 18 24 36S/ 36a/ 48

MaterialS^^^Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 0.6 0.7 0.9 0.2 1.6 0.3 2.5 0.6 0.6

Glass (0.10% iron oxide) -0.6 0.4 -1.2 0.4 1.5 0.1 0.5 1.5 0.4

Fluor inated (ethylene

propylene) copolymer

0.3 0.7 0.8 1.1 3.6 4.3 0.4 6.0 1.0

Poly(vinyl fluoride) 0.6 1.1 4.0 0.8 5.2 4.6 - 3.5 3.4

Poly(ethylene terephthalate) 0.0 0.2 4.9 7.7 4.8 4.8 0.8 1/ 6.7

Acrylic 1.9 1.0 3.2 2.0 3.4 b/ b/ b/ b/

Poly(methyl methacrylate) 0.7 1.9 2.2 2.3 4.1 4.9 3.2 4,5 4.5

Polycarbonate -0.3 0.1 2.5 3.5 4.6 7.9 3.9 3.6

Fiber reinforced plastic (1.0 mm) 1.4 1.3 4.1 1.9 9.1 0.9 21.3 19.1 22.1

Fiber reinforced plastic (1.5 mm) 5.4 7.0 8.5 7.9 19.6 15.9 16.1 16.9 27.5

Transmlttance is expressed in percentage. This solar energy transmlttance loss is the difference between

the average for the control test specimens and the values obtained after the test specimen was washed

(table A7). Negative values represent a gain in comparison with the average for the controls. This

solar energy transmlttance loss does not indued loss due to dirt on the surface. Losses due to dirt are tabulated in table All.

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

V Material failed prior to this time.

60
Table AlO. Solar Energy Transmlttance Loss of Cover Plate Materials After Exposure

as Double Covers to Natural Weathering in Miami, Florida

Outer Cover

>v Months Exposure 3 6 125/ 122/ 18 24 36a/ 363/ 48 MaterialNvSolar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) -0.3 0.4 -1.7 0.6 1.4 1.5 1.7 1.4 0.2

Glass (0.10% iron oxide) 0.3 0.0 0.8 0.6 0.9 0.9 0.5 1.0 1.1

Fluorinated (ethylene

propylene) copolymer

-0.5 0.0 1.3 0.1 4.5 1.4 1.5 - 0.6

Poly(vinyl fluoride) 0.9 1.4 3.0 1.0 0.0 4.4 2.6 3.4 b/

Poly(ethylene terephthalate) -0.4 0.9 7.7 5.2 5.2 3.9 b/ 4.8 b/

Acrylic 3.2 2.6 2.1 2.7 b/ b/ b/ b/ b/

Poly(methyl methacrylate) 1.0 1.8 2.8 2.0 3.3 3.1 2.8 5.3 5.2

Polycarbonate -0.1 0.8 2.4 2.5 5.0 5.1 6.6 5.8 -

Fiber reinforced plastic (1.0 mm) -2.2 0.8 7.3 9.1 8.5 15.0 17.5 24.5 22.1

Fiber reinforced plastic (1.5 mm) -2.4 0.3 19.7 10.4 24.1 24.8 21.9 33.9 21.2

Inner Cover

x^ Months Exposure 3 6 12§/ 12a/ 18 24 36^/ 36a/ 48

Materials. Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) -0.2 0.0 -0.6 0.1 0.5 -0.9 1.1 -1.0 0.5

Glass (0.10% iron oxide) -0.3 0.2 -0.6 0.5 0.1 -1.0 0.2 -0.5 0.0

Fluorinated (ethylene

propylene) copolymer

-0.4 -0.1 0.7 0.3 0.2. -0.1 0.1 - 0.8

Poly(vlnyl fluoride) 0.4 0.1 0.7 0.8 0.7 0.8 b/ 1.7 b/

Poly (ethylene terephthalate) -0.1 0.6 1.3 0.5 0.8 0.1 6.3 0.6 3.8

Acrylic 1.0 -0.4 1.0 0.4 0.7 b/ b/ b/ b/

Poly (methyl methacrylate) -0.4 -0.5 0.0 0.6 0.6 -0.3 1.0 0.8 0.3

Polycarbonate -0.2 -0.3 0.9 0.6 0.8 -0.1 2.2 0.8 3.9

Fiber reinforced plastic (1.0 mm) -1.6 0.2 1.9 3.3 6.9 1.5 10.1 10.1 9.0

Fiber reinforced plastic (1.5 mm) 11.5 6.4 19.8 9.9 10.1 12.5 21.7 9.6 17.5

Transmlttance is expressed in percentage. This solar energy transmlttance loss is the difference between

the average for the control test specimens and the values obtained after the test specimen was washed

(table A8). Negative values represent a gain in comparison with the average for the controls. This solar energy transmlttance loss doe? not Indued loss due to dirt on the surface. Losses due to dirt

are tabulated in table A12. ^ Two sets of mlnicollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

61
Table All . Solar Energy Transmittance Loss Due to Surface Dirt on Cover Plate Materials

After Exposure as Single Covers in Miami, Florida

>v Months Exposure 3 6 12a/ 12a/ 18 24 362/ 365/ 48

Material^. Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 3.1 2.2 2.6 3.3 4.1 5.5 3.9 2.7 3.5

Glass (0.10% iron oxide) 1.6 1.9 3.5 3.8 4.0 4.4 6.7 4.9 0.8

Fluor inated (ethylene

propylene) copoljmier 2.5 7.1 6.2 8.5 5.9 7.7 12.4 2.9 7.4

Poly(vinyl fluoride) 2.

A

1.8 1.8 7.1 2.9 2.2 V 2.9 3.4

Poly(ethylene terephthalate) 2.5 3.6 3.4 7.1 10.2 5.7 8.1 b/ 3.2

Acrylic 1.6 4.8 3.0 3.9 1.9 y b/ b/ b/

Poly(inethyl methacrylate) 1.7 2.2 1.5 4.2 5.2 2.6 3.7 2.6 1.7

Polycarbonate 2.5 2.9 3.9 6.4 5.9 4.8 4.7 - 1.7

Fiber reinforced plastic (1.0 mm) 2.3 2.6 3.4 6.7 2.9 -1.1 4.5 2.9 8.8

Fiber reinforced plastic (1.5 mm) 3.7 0.5 6.2 4.3 5.9 6.5 6.7 6.6 17.0

The solar energy transmittance loss due to dirt is the difference between the value obtained before and after washing the test specimen. Negative values represent a loss in transmittance after washing.

Transmittance is expressed in percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

62
Table A12. Solar Energy Transmlttance Loss Due to Surface Dirt on Cover Plate Materials

After Exposure as Double Covers In Miami, Florida

Outer Cover

>y Months Exposure 3 6 12?/ 125/ 18 24 365/ 36a/ 48

Materials. Solar Radiation (GJ/m^) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 3.4 1.0 3.3 1.1 5.7 2.2 2.7 2.3 3.3

Glass (0.10% iron oxide) 1.7 2.1 1.8 5.7 3.9 2.1 4.7 3.1 16.1

Fluorlnated (ethylene

propylene) copolymer

6.2 5.8 6.2 7.4 2.9 0.9 11.4 - 10.6

Poly(vlnyl fluoride) 2.0 0.5 1.4 7.2 6.6 1.7 2.6 2.0 b/

Poly(ethylene terephthalate) 2.3 1.9 1.0 10.9 10.9 4.5 b/ 4.4 b/

Acrylic 0.1 0.7 2.5 2.7 b/ b/ b/ b/ b/

Poly(methyl methacrylate) -1.1 2.2 1.0 5.0 3.7 3.4 6.6 2.5 4.7

Polycarbonate -0.5 2.1 5.8 6.3 6.1 5.1 4.6 1.6 -

Fiber reinforced plastic (1.0 mm) 2.6 2.5 2.1 6.7 4.4 2.2 2.5 0.9 8.6

Fiber reinforced plastic (1.5 mm) 4.7 1.6 6.3 4.6 4.0 6.1 3.9 4.5 5.6

I

Inner Cover

^^ Months Exposure 3 6 125/ 125/ 18 24 36^/ 365/ 48

Materials. Solar Radiation (GJ/m2) 1.455 2.864 6.344 6.523 9.333 13.125 19.398 19.859 26.333

Glass (0.01% iron oxide) 3.0 1.6 3.2 2.4 8.1 3.1 2.8 0.8 11.1

Glass (0.10% iron oxide) 0.6 1.4 1.9 0.7 4.3 15.3 1.8 0.8 3.4

Fluorlnated (ethylene

propylene) copolymer

1.0 0.9 -0.6 0.3 0.9 2.7 0.7 - 10.4

Poly(vlnyl fluoride) 1.5 1.1 2.3 0.6 2.7 2.6 - 0.0 -

Poly(ethylene terephthalate) 0.8 0.2 -0.3 0.4 0.1 1.2 0.2 2.7 22.2

Acrylic -0.3 1.5 0.1 0.8 3.2 b/ b/ i/ b/

Poly (methyl methacrylate) -1.6 1.7 3.1 0.4 9.2 1.8 3.5 -0.3 11.3

Polycarbonate -1.6 1.0 7.2 0.3 5.6 6.2 2.7 -1.0 2.9

Fiber reinforced plastic (1.0 mm) 2.8 1.1 0.0 1.2 1.7 3.6 7.7 1.5 17.5

Fiber reinforced plastic (1.5 mm) 2.2 2.2 -0.1 1.3 3.9 2.3 5.2 -0.6 2.9

The solar energy transmlttance loss due to dirt is the difference between the value obtained before and after washing the test specimen. Negative values represent a loss in transmlttance after washing. Transmlttance is expressed in percentage.

SJ Two sets of minicollectors were exposed for the same length of time but different dates. V Material failed prior to this time.

63
Table A13. Solar Energy Transmlttance of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering in New River, Arizona

^^ \^ Months Exposure 32/ 33/ 62/ 6a/ 125/ 123/ 18 24 36 48 Material\^Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) 90.7 90.8 89.1 89.7 92.4 91.5 90.0 89.8 90.0 89.8 90.0

Glass (0.10% iron oxide) 87.4 86.9 87.4 87.1 89.3 87.2 87.1 88.0 87.2 89.4 86.6

Fluorinated (ethylene

propylene) copolymer

96.0 95.8 95.3 95.3 95.6 94.7 95.2 95.4 95.5 95.2 95.3

Poly(vinyl fluoride) 92.5 91.4 90.7 91.0 91.3 90.5 91.3 90.7 91.4 88.9 90.7

Poly(ethylene terephthalate) 86.3 84.9 85.3 85.9 84.9 86.8 83.1 79.7 b/ b/ V

Acrylic 90.7 89.1 89.8 90.4 90.5 89.7 86.8 b/ i/ y y

Poly(methyl methacrylate) 91.0 89.5 89.4 88.8 91.3 89.2 87.8 88.5 89.4 90.8 89.2

Polycarbonate 88.1 87.9 87.3 87.6 86.4 - 84.9 85.0 86.3 84.7 83.6

Fiber reinforced plastic (1.0 mm) 86.3 85.3 84.4 84.5 83.6 82.6 80.2 81.6 84.4 73.2 72.9

Fiber reinforced plastic (1.5 mm) 78.5 73.9 74.3 69.9 66.0 65.8 76.6 68.2 69.2 65.4 62.2

The solar energy transmlttance loss due to dirt is the difference between the value obtained after washing the test specimen. Negative values represent a loss in transmlttance after wa( Transmlttance is expressed in percentage. y Two sets of minicollectors were exposed for the same length of time but different dates. y Material failed prior to this time.

before

hing. and

64
Table A14. Solar Energy Transmlttance of Cover Plate Materials After Exposure

as Double Covers to Natural Weathering in New River, Arizona

Outer Cover

^V Months Exposure 3a/ 35/ 65/ 65/ 125/ 125/ 18 24 36 48 Material^v Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01? iron oxide) 90.7 90.6 89.8 91.1 92.1 91.0 90.2 90.2 91.2 91.9 90.1

Glass (0.10% iron oxide) 87.4 87.1 86.6 87.1 89.4 88.9 87.1 87.1 87.9 88.8 87.5

Fluorinated (ethylene

propylene) copolymer

96.0 95.6 95.3 95.7 95.6 95.9 95.8 94.9 95.5 95.8 95.3

Poly(vinyl fluoride) 92.5 91.8 90.4 89.7 91.7 90.7 91.3 b/ 89.5 90.4 V

Poly(ethylene terephthalate) 86.3 85.7 85.5 86.0 85.0 85.2 y 84.8 b/ b/ b/

Acrylic 90.7 89.8 90.0 90.8 90.7 89.2 w b/ b/ V b/

Poly(methyl methacrylate) 91.0 89.3 89.4 89.3 91.5 90.0 88.3 89.0 89.3 88.2 88.3

Polycarbonate 88.1 87.6 87.9 87.8 86.5 85.9 85.8 86.0 85.1 84.4 83.8

Fiber reinforced plastic (1.0 mm) 86.3 86.5 84.5 84.5 84.2 84.0 83.1 83.0 82.8 75.1 60.7

Fiber reinforced plastic (1.5 mm) 78.5 78.3 77.5 77.8 72.0 72.1 75.7 72.0 70.9 65.8 65.6

Inner Cover

>^ Months Exposure 35/ 35/ 65/ 65/ 125/ 125/ 18 24 36 48

Materials. Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) 90.7 90.6 88.3 90.1 92.3 90.3 90.4 90.1 89.7 90.0 90.0

Glass (0.10% iron oxide) 87.4 87.5 87.4 86.5 89.4 89.9 87.0 87.1 86.5 86.9 86.9

Fluorinated (ethylene

propylene) copolymer

96.0 95.7 95.7 95.8 95.6 96.3 96.0 95.8 95.9 95.3 95.4

Poly(vinyl fluoride) 92.5 91.8 91.1 91.7 91.4 92.1 91.8 \! b/ b/ b/

Poly(ethylene terephthalate) 86.3 85.7 85.5 84.5 84.9 85.7 b/ 85.1 b/ b/ b/

Acrylic 90.7 90.7 90.1 90.5 90.3 90.8 V 91.0 b/ b/ b/

Poly(methyl methacrylate) 91.0 89.9 90.7 90.6 91.3 90.7 90.6 90.4 90.8 90.1 90.1

Polycarbonate 88.1 87.4 87.8 87.8 88.1 87.6 87.6 87.5 87.2 - 87.4

Fiber reinforced plastic (1.0 mm) 86.3 84.2 83.0 84.5 82.7 80.7 82.4 77.8 80.1 78.8 76.3

Fiber reinforced plastic (1.5 mm) 78.5 70.7 67.9 69.5 64.0 70.7 65.8 59.1 68.4 64.7 -

Transmlttance was measured after the test specimens were washed. The solar energy transmlttance was calculated for air mass 2. Each transmlttance value represents a separate test specimen. Transmlttance

is expressed in percentage.

5/ Two sets of minlcollectors were exposed for the same length of time but different dates

b/ Material failed prior to this time.

65
Table A15. Solar Energy Transmlttance Loss of Cover Plate Materials After Exposure

as Single Covers to Natural Weathering in New River, Arizona

>s^ Months Exposure 3^/ 3^ 6^ 63/ 12^ 12^/ 18a/ 24 36 48 MaterialN. Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) -0.1 1.6 1.0 -1.7 -0.8 0.7 0.9 0.7 0.9 0.7

Glass (0.10% iron oxide) 0.5 0.0 0.3 -1.9 0.2 0.3 -0.6 0.2 -2.0 0.8

Fluorinated (ethylene

propylene) copolymer

0.2 0.7 0.7 0.4 1.3 0.8 0.6 0.5 0.8 0.7

Poly(vinyl fluoride) 1.1 1.8 1.5 1.2 2.0 1.2 1.8 1.1 3.4 1.8

Poly(ethylene terephthalate) 1.4 1.0 0.4 1.4 -0.5 3.2 6.6 y y y

Acrylic 1.6 0.9 0.3 0.2 1.0 3.9 ^ y y y

Poly(methyl methacrylate) 1.5 1.6 2.2 -0.3 1.8 3.2 2.5 1.6 0.2 1.8

Polycarbonate 0.2 0.8 0.5 1.7 - 3.2 3.1 1.8 3.4 4.5

Fiber reinforced plastic (1.0 nnn) 1.0 1.9 1.8 2.7 3.7 6.1 4.7 1.9 13.1 13.4

Fiber reinforced plastic (1.5 nnn) 4.6 4.2 8.6 12.5 12.5 1.9 10.3 13.1 16.3 16.3

Transmlttance is expressed in percentage. This solar energy transmlttance loss is the difference between

the average for the control test specimens and the value obtained after the test specimen was washed

(table A13). Negative values represent a gain in comparison with the average of controls. This solar energy

transmlttance loss does not include loss due to dirt on the surface. Losses due to dirt are tabulated in table A17. y Two sets of minicollectors were exposed for the same length of time but different dates. y Material failed prior to this time.

66
Table A16. Solar Energy Transmlttance Loss of Cover Plate Materials After Exposure as Double Covers to Natural Weathering in New River, Arizona

Outer Cover

>y Months Exposure 3a/ 35/ 61/ 62/ 125/ 12a/ 18 24 36 48

MateriarS. Solar Radiation (GJ/m2) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) 0.1 0.9 -0.4 -1.4 -0.3 0.5 0.5 -0.5 -1.2 0.6

Glass (0.10% iron oxide) 0.3 0.8 0.3 -2.0 -1.5 0.3 0.3 -0.5 -1.4 -0.1

Fluorinated (ethylene

propylene) copolymer

0.4 0.7 0.3 0.4 0.1 0.2 1.1 0.5 0.2 0.9

Poly(vinyl fluoride) 0.7 2.1 2.8 0.8 1.8 1.2 b/ 3.0 2.1 b/

Poly(ethylene terephthalate) 0.6 0.8 0.3 1.3 1.1 b/ 1.5 b/ b/ b/

Acrylic 0.9 0.7 -0.1 0.0 1.5 b/ i/ b/ b/ b/

Poly(methyl methacrylate) 1.7 1.6 1.7 -0.5 1.0 2.7 2.0 1.7 2.8 2.7

Polycarbonate 0.5 0.2 0.3 1.6 2.2 2.3 2.1 3.0 3.7 4.3

Fiber reinforced plastic (1.0 mm) -0.2 1.8 1.8 2.1 2.3 3.2 3.3 3.5 11.2 25.6

Fiber reinforced plastic (1.5 mm) 0.2 1.0 0.7 6.5 6.4 2.8 6.5 7.6 12.7 12.9

Inner Cover

>^ Months Exposure 3^ 35/ 65/ 65/ 125/ 125/ 18 24 36 48

MaterlajSv Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) 0.1 2.4 0.6 -1.6 0.4 0.3 0.6 1.0 - 0.7 0.7

Glass (0.10% iron oxide) -0.1 0.0 0.9 -2.0 -2.5 0.4 0.3 0.9 0.5 0.5

Fluorinated (ethylene

propylene) copolymer

0.3 0.3 0.2 0.4 -0.3 0.0 0.2 0.1 0.7 0.6

Poly(vlnyl fluoride) 0.7 1.4 0.8 1.1 0.4 0.7 b/ b/ b/ b/

Poly(ethylene terephthalate) 0.6 0.8 1.8 1.4 0.6 b/ 1.2 b/ b/ b/

Acrylic 0.0 0.6 0.2 0.4 -0.1 b/ -0.3 b/ b/ b/

Poly(methyl methacrylate) 1.1 0.3 0.4 -0.3 0.3 0.4 0.6 0.2 0.9 0.9

Polycarbonate 0.7 0.3 0.3 0.0 0.5 0.5 0.6 0.9 0.7

Fiber reinforced plastic (1.0 mm) 2.1 3.3 1.8 3.6 5.6 3.9 8.5 6.2 7.5 10.0

Fiber reinforced plastic (1.5 ram) 7.8 10.6 9.0 14.5 7.8 12.7 19.4 10.1 13.8 ^

Transmlttance is expressed in percentage. This solar energy transmlttance loss is the difference

between the average for the control test specimens and the value obtained after the test specimen

was washed (table A14). Negative values represent a gain in comparison with the average of controls

This solar energy transmlttance loss does not include loss due to dirt on the surface. Losses due

to dirt are tabulated in table A18. ^ Two sets of minicollectors were exposed for the same length of time but different dates. ^ Material failed prior to this time.

67
Table A17. Solar Energy Transmlttance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Single Covers In New River, Arizona

^v Months Exposure 3^7 32/ 61/ 6S/ 12S/ 12S/ 18 24 36 48

Mater iaiS^Solar Radiation (GJ/m2) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) 1.4 -0.4 1.2 4.9 1.9 4.5 2.3 3.2 -1.2 3.0

Glass (0.10% iron oxide) 0.4 -0.2 1.7 3.6 0.6 1.2 4.0 4.6 2.1 3.6

Fluor inated (ethylene

propylene) copolymer

2.5 1.8 2.4 1.1 3.1 4.3 3.8 5.4 5.8 6.3

Poly(vlnyl fluoride) 0.4 1.3 0.8 0.7 0.9 4.6 3.0 3.9 -1.4 4.9

Poly(ethylene terephthalate) 0.3 2.1 2.0 1.0 4.3 12.1 2.9 b/ y b/

Acrylic 0.0 1.9 1.6 1.7 1.6 7.8 y b/ y V

Poly(methyl methacrylate) 0.8 0.8 1.7 4.5 2.4 2.7 3.8 5.5 1.9 3.7

Polycarbonate 0.7 1.2 1.7 2.2 - 0.8 4.1 2.3 1.2 3.1

Fiber reinforced plastic (1.0 mm) 1.8 0.1 0.7 1.3 1.7 1.0 6.6 7.2 3.8 6.3

Fiber reinforced plastic (1.5 mm) 0.1 1.9 1.4 -0.7 0.2 13.6 3.5 8.4 4.5 2.7

The solar energy transmlttance loss due to dirt is the difference between the solar energy transmlttance measurements made before and after washing the test specimen. Negative values represent a loss in transmlttance after washing. Transmlttance is expressed in percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates. y Material failed prior to this time.

68
Table A18. Solar Energy Transmictance Loss Due to Surface Dirt on Cover Plate Materials After Exposure as Double Covers In New River, Arizona

Outer Cover

>«^ Months Exposure 3iy 35/ 65/ 65/ 12^/ 124/ 18 24 36 48 MaterlaKySolar Radiation (GJ/m^) 1.422 2.012 3. 487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% Iron oxide) 0.2 0.1 1.9 5.8 0.3 2.1 3.0 5.4 2.5 0.5

Glass (0.10% iron oxide) 0.2 0.5 1.0 4.6 1.8 3.5 2.8 4.1 1.5 3.9

Fluorinated (ethylene

propylene) copolymer

2.5 1.7 2.2 1.2 2.8 4.0 3.4 1.1 6.9 4.8

Poly(vinyl fluoride) 0.8 0.0 0.7 1.2 2.0 3.8 ^/ 2.3 3.9 b/

Poly(ethylene terephthalate) 0.1 0.9 1.9 1.2 1.4 b/ 7.6 b/ b/ b/

Acrylic 1.6 1.8 1.3 2.3 0.4 b/ b/ b/ t/ b/

Poly(methyl methacrylate) 0.0 2.0 1.4 4.1 3.5 2.0 2.9 4.8 1.2 3.2

Polycarbonate 1.1 1.7 1.4 2.0 2.0 1.0 3.4 2.6 0.0 0.9

Fiber reinforced plastic (1.0 mm) 0.8 1.1 0.5 3.7 0.7 5.5 3.5 4.8

1

1.9 6.2

Fiber reinforced plastic (1.5 mm) 0.8 0.8 3.8 2.9 2.0 3.0 3.8 9.1 8.2 7.7

Inner Cover

>v Months Exposure 33/ 35/ 65/ 65/ 125/ 125/ 18 24 36 48

Materiabv. Solar Radiation (GJ/m^) 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370 24.414 33.038

Glass (0.01% iron oxide) -0.5 -1.0 0.9 3.3 3.2 0.2 0.8 1.9 -0.3 4.3

Glass (0.10% iron oxide) -0.4 0.7 -0.1 2.5 1.7 0.5 0.1 0.6 0.1 3.7

Fluorinated (ethylene

propylene) copolymer

0.0 0.4 0.3 0.0 2.1 0.8 0.7 2.2 0.5 0.2

Poly(vlnyl fluoride) 0.3 1.0 0.6 0.0 2.3 0.4 b/ b/ b/ b/

Poly(ethylene terephthalate) 0.1 0.2 -0.5 -0.1 1.7 b/ 0.4 b/ b/ b/

Acrylic 0.9 1.2 0.8 0.2 1.9 b/ 1.8 b/ b/ b/

Poly(methyl methacrylate) -0.8 1.0 0.0 1.7 1.5 0.5 1.0 2.8 0.3 1.6

Polycarbonate 0.2 0.6 0.4 0.7 2.1 0.4 1.7 4.1 - 2.7

Fiber reinforced plastic (1.0 mm) 0.0 -1.2 0.3 0.0 0.8 0.0 3.7 5.9 2.7 2.8

Fiber reinforced plastic (1.5 mm) -0.7 -1.1 0.9 -0.2 6.4 9.4 2.2 4.3 0.2 -

The solar energy transmittance loss due to dirt is the difference between the solar energy transmittance measurements made before and after washing the test specimen. Negative values represent a loss in transmittance after washing. Transmittance is expressed in percentage.

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

b/ Material failed prior to this time.

69
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS IN 1170

2. Performing Organ. Report No 3. Publication Date November 1982

4. TITLE AND SUBTITLE

WEATHERING PERFORMANCE OF COVER MATERIALS FOR FLAT PLATE SOLAR COLLECTORS

5. AUTHOR(S)

Clark, Elizabeth J., Roberts. Willard E.

6. PERFORMING ORGANIZATION (/f joint or other thon NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

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

U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

10. SUPPLEMENTARY NOTES

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

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

bibliography or literature survey, mention it here)

Weathering studies were performed to obtain data on the performance and

durability of cover plate materials for flat plate solar collectors used in

solar heating and cooling systems. Ten materials were evaluated to assess their durability after natural weathering and artificial weathering with a xenon arc light. The materials were weathered for four years on small minicollectors in Arizona, Florida, and Maryland after which the solar energy

transmittance and the effect of dirt on the transmittance were measured. The

tensile properties of selected film materials were also assessed after weathering. The effects of the natural weathering are compared: (1) for materials

exposed as inner and outer cover plates for each weathering site; (2) for the

three weathering sites; and (3) with materials artificially weathered with a xenon arc light.

12. KEY WORDS (S/x to twelve entries; alphabetical order: caoitalize only prober names: and seoarate kev words by semicolons)

artificial weathering; cover plate materials; durability; natural

weathering; solar collectors; solar energy; solar energy

transmittance; tensile properties; weathering of cover plates.

13. AVAILABILITY

IX] Unlimited

I I For Official Distribution. Do Not Release to NTIS

[Xl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Q_] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

80

15. Price

$5.00

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