STUDIES OF ENCAPSULATION MATERIALS FOR TERRESTRIAL PHOTOVOLTAIC ARRAYS

ENERGY RESEARCH AND DEVROPMENT ADMlNlSTRATlON Division of Solar Energy

Fifth Quarterly Progress Report, September 16,1976-December 15,1976

Hkck Performed Under Contract No. NAS7-100-!&4328

Battelle Columbus Labratorim Columbus, Ohio

DISCLAIMER

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FIFTH QUARTERLY PROGRESS REPORT Covering the Period September 16, 1976, to December 15, 1976

STUDIES OF ENCAPSULATlON MATERIALS FOR TERWESTRIhAL PHOTOVOLTAIC ARRAYS, 7f +l+

JPL Contract No. 954328

Encapsulation Task - Low-Cost Silicon Solar Array Project ,

to

JET PROPULSION' LABORATORY .' CALIFORNIA INSTITUTE OF TECHNOLOGY i

NOTICE

the Unitad SDles nor the Un i td Stat- Energy RaePrch and Dcvelopmnt Administration, nor any of their employeu, nor any of their contractors. s u b ~ ~ n m c t o n . or their employees, m k u any

pmOLL( d k c l o ~ d . or represtnts lhat its UY would not infringe privately omed r u t s .

t l 5

This work was performed for the Jet Propulsion Laboratory, California Institute of Technology, under NASA Contract NAS7-100 for the U.S. Energy Research and Development Administration, Division of Solar Energy.

The JPL Low-Cost Silicon Solar Array Project is funded by ERDA and forms part of the ERDA Photovoltaic Conversion Program to initiate a maior effort toward the development of low-cost solar arrays.

. .

. , . . . December 27, 1976 . .

' -

. * . D. C. Carmictiael

Program Manager

BA'I I ELLE : Columbus Laboratories

505 King Avenue Columbus, Ohio 43201

TABLE'OF CONTENTS

Page

. . . . . . . . . . . . . . . . . . . . . ABSTRACT 1

STUDY 3 . EVALUATION O F TEST METHODS AND PROPERTIES O F CANDIDATE ENCAPSULATION MATERIALS . . . . . . . . . 3

INTRODUCTION . . . . . . . . . . . . . . . . . 3

TECHNICAL DISCUSSION OF RESULTS THIS QUARTER . . . . 4

Evaluations of Polymeric Mater ials a s Encapsulation Components . . . . . . . . . . . . . . . . 4

Bond-Strength Experiments . . . . . . . . a 4 Water -Vapor Transmission Experiments (WV TR) . . 8 ~hemilumine ' scence Experiments . . . 13 . . . . . Evaluations of Glass Encapsulant Components 15 Glass -to-Cell Adhesive Bonding . . . . . . . . 15 Organic Se.aling of Glass Encapsulation Systems . . 18

STUDY 4 . DEVELOPMENT OF ACCELERATED AND ABBREVIATED TESTING METHODS FOR PREDICTING PERFORMANCE O F ENCAPSULATION MATERIALS OVER 20-YEAR LIFETIME . 22

. . . . . . . . . . . . . . . . . . . . . INTRODUCTION 22

. . . . TECHNICAL DISCUSSION O F RESULTS THISQUARTER 23

. . . . . . . . . . . . . . Measures of Precis ion 24 . . . . Review of Published Information on Test Precis ion 25

. . . . . . . . . . . . . Mandei's Sensitivity Katio 27

. . . . . . . . . . . . Transformations of Scale 30 Inferences ConcerningtheSetlsitivityRatio . 31

. . . . . . . . . Sensitivity. Samplc Size. and Cost 31 . . . . . . . . . . . Example Sensitivity Analysis 32 Statist ical Experimental D c ~ i g n for Dctcrmining Sensitivity

Ratio for Two Test Methods Both of Which Measure . . . . . . . . . . . . . Proper ty Degradation 33

. . . . . . . . . . . . . . Concluding Remarks 34

PLANS .FOR NEXT QUARTER . . . . . . . . . . . . . . . . . . . . REFERENCES 35

. . . . . . . . . . . . . . . . . . . NEW TECHNOLOGY 36

LIST OF TABLES

Page

Table 1. Lap-Shear Strength of Specimens on "Teflon" FEP Subs t ra te Following UV Exposure . . . . . . . . . . . . . 6

Table 2. Lap-Shear Strength of Specimens on "Teflon" F E P Substra te Following Tempera ture Cycling ( -40 to t 9 0 C) . . . . . 7

Table 3. Lap-Shear Strength of Specimens on Weatherable Mylar Substra te Following UV Exposure. . . . . . . . . . 9

Table 4. Lap-Shear Strength of Specimens on Weatherable Myla-r Substra te Following Tempera ture Cycling (-40 t o t 9 0 C ) . . 10

Table 5. Water Vapor T ransmis s ion Rate of Adhesive-Fi lm . . . . . . . . . . . . . . . . . . Subsystems

Table 6. Chemiluminescence Measurements on UV- and Tempera tu re - . . . . . . . . . . . . . . . Cycle d Sample s

Table 7. Important P rope r t i e s of Candidate Adhesives f o r Bonding . . . . . . . . . Silicon Solar Cel ls to Cover Glass

Table 8. P rope r t i e s of Candidate Sealants fo r Glass Encapsulation . Table 9. Coefficient of Variation for Various ASTM T e s t s on

. . . . . . . . . . . . . . . Plas t ic Mater ia l s

Table 10. Exalnple of Cost Ratio a t Equal P rec i s ion a s a Function of Testing Cos ts and Sensitivity Ratio . . . . . . . . .

Tablc 1 1. Raw Data, Means and Standard Deviations fo r Calculation of Sensitivity for the Wire-Adhesion Example . . . . .

LIST O F FIGURES

. . . . . . . . Figure 1. Configurationof Lap-Shear Specimens 5

Figuse 2. F i l m Pouch (Pil low Pack) Used in Determining Water Vapor T ransmis s ion Through Fi lm/Adhesive Subsys tem . . . 11

F igure 3 . Configuration of Specimen Used in Chemiluminescence Exper iments . . . . . . . . . . . . . . . . . . 13

F igu re 4. Screening Tes t P r o g r a m on Adhesives f o r Glass Encapsulation Sys tems . . . . . . . . . . 19

LIST OF FIGURES (Continued)

Page

Figure 5. Illustration of Accuracy and Precision . . . . . . . . 24

Figure 6. Illustration of Relative Sensitivity of Two Tests, M and N . 27

Figure 7. ,Plot of Propert ies P1 and P2 a s Functions of Time . . . . . 2 9

Figure 8. Relative Sensitivity a s a Function of Test P2 for Hypothetical Examples . . . . . . . . . . . . . 2 9

FIFTH QUARTERLY PROGRESS REPORT

STUDIES OF ENCAPSULATION MATERIALS FOR BERRESTRlAL PHQTQVOLTABC ARRAYS

JPL Contract No. 954328

Encapsulation Task Low-Cost Silicon Solar Array Project

JET PROPULSION LABORATORY CALIFORNIA INSTITUTE O'F TECHNOLOGY

from

BATTELLE' Columbus Laboratories

December 27, 1976

ABSTRACT

This Fif th Qua r t e r iy P r o g r e s s Report on J P L Contract No. 954328 cove r s work conducted in the per iod f r o m September 16, 1976 through December .15, 1976. The r e s e a r c h i s pa r t of a five-task development project , the Low-Cost Silicon Solar A r r a y P ro j ec t (LSSA Pro jec t ) , being managed by the J e t Propuls ion Labora - t o ry f o r the Energy Resea rch and Development Administrat ion, Division of Solar Energy. The Encapsulation Task (Task 111) of th is project includes four s tudies ass igned to Bat te l le ' s Columbus Labora tor ies . Two of these studies (Studies 1 and 2) have been completed; the o the r s (Studies 3 and 4) a r e in p r o g r e s s and the effor ts

. th is q u a r t e r are discussed in th is repor t .

The final r epo r t on Study 1 was published and distr ibuted t h i s q u a r t e r and i s identified a s follows:

Report No. : ERDAI J,PL-954328-7614

Report Title: Review o f World Experience and Properties of Materials for Encapsulation of Terrestrial Photovoltaic Arrays

The f inal r epo r t on Study 2 i s in the final editing and reproduction phase and i s

identified a s follows:

1

Report No. : ERDAIJPL-954328-76/ 5

Report Title : Terrestrial Service Environments for Selected Geographic Locations

P r e s e n t effor ts i n Study 3 , entit led "Evaluation of Test Methods and Prop- e r t i e s of Candidate Encapsulation Materials", a r e concerned pr imar i ly with eval- uation of the proper t ies of candidate encapsulation mater ia l s and processes which have been identified based on the resu l t s of Study 1. Specifically, cover mater ia l s fo r modules , and adhesives fo r bonding ce l l s to cover mater ia l s and for bonding cover ma te r i a l s t o o ther encapsulation-system components, a r e being screened i n environments including UV radiation, tempera ture cycling, and high relative .humidity. Resul ts of the evaluations t& date a r e described in the report . These effor ts include investigations.of both polymeric and g lass encapsulation mater ials .

Effor ts this qua r t e r i n Study 4, entitled "Development of Accelerated and Abbreviated 'Testing Methods for Predicting Performance of Encapsulation Mate- r i a l s Over a 20-Year Lifetime", were concerned pr imar i ly with analyzing methods for discrimination among t e s t methods with r ega rd to precision, sensitivity, and cost. The background and procedures for such discrimination a r e discussed and exampies a r e given.

STUDY 3: EVALUATION O F TEST METHODS AND PROPERTIES O F CANDIDATE ENCAPSULATION MATERIALS

F. A. Sliemers , M. C. Brockway, A. R. Bunk, G. P. Name , G. B. ~Gaines, and D. C. Carmichael

INTRODUCTION

To date, the experience with encapsulation of photovoltaic so lar a r r a y s for t e r r e s t r i a l applications has been r a the r limited. Moreover , the encapsulant m a - te r ia l s used so f a r have been employed based on c r i t e r i a not ent i rely appropriate for the goals of the LSSA project, especially cost and ease of processing using automated methods. There i s a considerable need, therefore, for important in- formation on the propert ies and aging behavior of mater ia l s which a r e candidates for encapsulating low-cost photovoltaic a r r a y s and a r e prepared by methods suit- able for automated.processing. Also, bccause of the newness and some unique requirements of the t e r r e s t r i a l photovoltaic a r r a y s , standard methods of mea- suring the propert ies of candidate encapsulation may not be useable.

The objectives of this study, therefore, a r e to

(1) Review t e s t standards for appropriateness to the requirements of the candidate mater ia l s and processes , and to

(2) Measure propert ies of selected mater ia l s under conditions which will make feasible a choice among candidate mater ia l s and processes for use in future encapsulation systems.

As pa r t of the second a rea , mater ia l s and measurement techniques will a l so be experimentally evaluated to obtain infbrmation for use in development of aging t e s t s in Study 4.

Results of the analysis of the world kxperience with existing modules (Study 1) indicate that major problems with mater ia l s interactions and interfaces have overshadowed changes in bulk propert ies of mater ia l s . The experience shows that many module fai lures have occurred, for instance, a t the sea ls attaching the bottom cover to top cover and a t those interfaces forming the sea l of the e lec t r ica l leads to the protective covers. Fur ther encapsulation sys t em developments depend significantly on finding solutions to the sea l and other interface problems. Thus, a substantial portion of the cur rent Study 3 efforts i s being directed toward m a - t e r i a l propert ies involved in these problems. Environmental pa ramete r s of f i r s t concern a r e ultraviolet radiation, tempera ture (including tempera ture cycling), and relative humidity.

TECHNICAL DISCUSSION OF RESULTS THIS QUARTER

F o r purposes of report ing, the efforts of Study 3 a r e organized into studies of polymeric ma te r i a l s and studies of g lass mater ia l s . This division i s somewhat a r b i t r a r y because many of the encapsulation concepts employing glass ( a s the top cover ) employ polymeric ma te r i a l s a s well.

Evaluations of Polymer ic Materials a s Encapsulant Components

As noted in previous r epor t s , Study 3 efforts a r e emphasizing those mate- rials and ma te r i a l s interfaces which the world experience suggests a r e current ly m o s t troublesome. During this qua r t e r , principal efforts have been directed to- w a r d screening adhesives and sealants fo r use with candidate encapsulants. To date, effects on bond s t rength of exposures to UV radiation and tempera ture c y - cling have been determined using "Teflon" FEP and weatherable Mylar f i lms (which were the first ma te r i a l s received). These resu l t s a r e reported below. Measurements a l so have been made on water permeation r a t e s of adhesivelf i lm composites. Also repor ted a r e init ial resu l t s of chemiluminescence measure - ments on selected specimens. Evaluations of other mater ia l s have been initiated as they have been received.

Bond-Strength Experiments

Lap-shear specimens were prepared for bond-strength measurements using 2. 54 x 12.7-cm (1 x 5-in. ) s t r ip s of the selected f i lm mater ia l s . Two of these s t r i p s a r e used f o r each lap-joint. The bond a r e a i s f i r s t cleaned with a solvent wipe (differs with type of f i lm) , and then a thin f i lm of the selected adhesive i s applied with a sma l l b rush over a 6.45 cm2 (1 in. 2, a r ea . The two s t r ip s a r e joined using a 4-112 lb, hard-rubber-covered rol ler . * Specimens a r e allowed to se t o r c u r e for a t least 72 hours a t 22.8 C ( 7 3 F) and 50 percent relative humidity before being placed on test . Bond-strength measurements a r e made on lap-shear spec imens following exp0sur.e to a potentially degrading environment (UV radiation o r t empera tu re cycling). The specimen design i s shown in Figure 1.

Temperature-cycling exposure i s effected in a commerc ia l environmental chamber with the specimens attached to aluminum sheets. Temperature i s cycled in the range -40 to 90 C with a 4-hour cycle t ime. Relative humidity i s not con- trolled. Bond-strength measurements a r e performed a t 23 C on a "bench-type" Instron t e s t e r using a draw ra t e of 0.5 c m l m i n (0.2 in. Imin) .

Exposure to UV radiation i s effected by placing aluminum-backed specimens in a closed chamber in which a 6000-watt, xenon lamp is operating. Each .specimen is positioned 45. 7 c m (18 in. ) f r o m the lamp. The tempera ture i s not controlled;

"Rubber -covered roller supplied by Pressure Sensitive T a p e Council , 1201 Waukegan Road, Glenview, Illinois.

it can vary within the approximate range of 35 to 60 C. Radiation other than the UV component i s not f i l tered out. Bond-strength measurements a r e performed as described above.

Bonded A r e a 6.45 cm2 ( I in .2)

Area

( I in.) '

FIGURE 1. CONFIGURATION OF LAP-SHEAR SPECIMENS

Bond-strengths have been, determined thus f a r for specimens on two types! of f i lm substrates: "Teflon" F E P and'weatherable Mylar. Table 1 summarizes the resu l t s of the measurements for "Teflon" F E P specimens af te r U V exposure up to 1000 hours, a period which has been selected a rb i t r a r i ly a s a cut-off for these screening-type evaluations. F o r the lap-shear evaluations, the control was s tored in a constant-temperature room a t 23 C and 50 percent relative humidity. Note in Table 1 that in the majority of the cases f i lm elongation r a the r than bond fai lure occurred. Generally speaking, i t can be concluded that the adhesives a r e s t i l l s t ronger than the yield value of the f i lm substrate a f te r 1000 hours of U V exposure.

The same film-adhesive sys tems were examined for bond s trength af te r exposure to tempera ture cycling for the periods indicated in Table 2. Adhesive bond failure occurred in a l l specimens in which the moisture-cured silicone ad- hesive RTV 118 was used. The bond-strength values were not significantly lower than those measured af te r UV exposure and were on an average slightly higher than those of the controls. Therefore, even though the bonds failed adhesively, the bond-strength values were sti l l approximately the yield value of the f i lm sub- s t rate . The moisture-cured RTV 732 fa i lures were approximately half adhesive fa i lures and half yield o r film-substrate-elongation failures. There i s little dif- fe rence in s t r e s s values (in psi) between the specimens that failed and those in which the fi lm yielded. All specimens bonded with the other three adhesives ex-

hibited f i lm elongation. Of the specimens bonded with the five adhesives to "Teflon F E P and subjected to temperature cycling, none had a value lower than 6 . 6 psi nor more than 9.5 psi a f te r 1133 hours ' exposure before adhesive failure o r f i lm elongation occurred. The tes t was terminated a t this exposure t ime.

TABLE 1. LAP-SHEAR STRENGTH OF SPECIMENS ON "TEFLON" FEP SUBSTRATE FOLLOWING uv EXPOSURE(^)

I Elongation or Exposure Time, Adhesive Bond

Specimen ~dhes ive ( b, hr psi ~ e m a r k d ~ )

RTV 118 Ditto

RTV 732 Ditto

Silgrip SR-573 with 1- l/27o Benzoyl peroxide (cat. )

Ditto

Scotch Weld 2216 B/A (clear) Ditto

Cavalon 3100 S, Activator 3300 S Ditto

None (control) 122 485

1046

None (control) 25

122 485

None (control)

None (control) 134 784

None (control) 134 7 84

Adhesive failure Film elongation Film elongation Film elongation

Film elongation Film elongation Film elongation Film elongation

Film elongation

Film elongation Film elongation Film elongation

Film elongation Film elongation Film elollgation

Film elongation Film elongatiode) Film e . l ~ n ~ a t i o n ( ~ )

(a) Adhesives on "Teflon" FEP film 127 p m (0.005 in.) thick; adhesive area. 6.45 cm2 ( 1 in. 2). Controls stored in constant temperature room 22.8 C (73 F), 50 percent relative humidity.

(b) Silicone adhesives: RTV 118, RTV 732, and Silgrip SR-573 - supplier is G.E. ; Scotch-Weld 2216 B/A (clear) - supplier is 3M; Cavalon 3100 S with activator 3300 S - supplier is DuPont.

(c) Bond strength in psi based on area of adhesive.

(d) Adhesive failure indicates bond failure between "Teflon" FEP film and adhesive; cohesive failure indicates bond failure in the adhesive itself; "film elongationw indicates that film elongation occurred in portion of film not associated with bonded area.

( e ) Adhesive is brown ifter exposure. Starting color is yellow to amber.

TABLE 2. LAP-SHEAR STRENGTH OF SPECIMENS ON "TEFLON FEP SUBSTRATE FOLLOWING TEMPERATURE CYCLING (-40 to +90 C)

Elongation or Exposure Time, Adhesive Bond

Specimen ~dhes ive( b, hr ~trength(c), psi ~ e m a r k s ( ~ )

RTV 118 Ditto

RTV 732 Ditto

Silgrip SR- 573 with 1- l/2(lo Benzoyl peroxide (cat. )

Ditto

None (control) 598

1133

None (control) 598

1133

None (control)

Adhesive failure Adhesive failure Adhesive failure

Film elongation Adhesive failure Adhesive failure

Film elongation

Film elongation Film elongation

9- 1 Scotch Weld 2216 B/A (clear) None (control) 8.8 Film elongation 9- 13 Ditto 500 9.4 Film elongation 9-21 1034 9 . 1 Film elongation

10- 1 Cavalon 3100 S, Activator 3300 S None (control) 8 .8 Film elongation 10- 13 Ditto 500 9.4 Film elongatiode)

1034 10-21 9.2 Film elongation(e1

2 (a) Adhesives on "Teflon" FEP film, 127pm (0.005 in.) thick; adhesive area, 6.45 cm2 ( 1 in. ). Controls stored in constant temperature room 22.8 C (73 F), 50 percent relative humidity.

(b) Silicone adhesives: RTV 118, RTV 732, and Silgrip SR-573 - supplier is G.E. ; Scotch-Weld 2216 B/A (clear) - .supplier is 3M ; Cavalon 3100 S with activator 3300 S - supplier is DuPont.

(c) Bond strength in psi based on area of adhesive. (d) Adhesive failure indicates bond failure between FEP-Teflon film and adhesive; cohesive failure indicates bond

failure in the adhesive itself; "film elongation" indicates that film elongation occurred in portion of film not associated with bonded area.

(e) Adhesive is dark brown after exposure. Starting color is yellow to amber.

Evaluations of adhesives for bond s trength on weatherable Mylar fi lm, 0. 178-mm (0.007-in. ) thick have given values which a r e m o r e indicative of the s t rength of the adhesives themselves, because the yield value for Mylar for the thickness being used is higher than most bond strengths. Table 3 summarizes the r e s u l t s of the UV-exposed specimens af te r 329 hours. F o r the specimens ex- posed to UV radiations, two types of fai lure a r e noted. These fai lures a r e ei ther adhesive o r cohesive. (See note a t bottom of Table 3 for definitions. ) The speci- m e n s with s t ructural- type adhesives, namely the epoxy Scotch Weld 3520 and the ac ry l i c Cavalon 3 100 S , had "adhesive" failures. F o r the nonstructural-adhesive specimens, namely Scotch Gr ip 4693 (plast ic contact adhesive), moisture -cured sil icone e las tomer ic adhesive RTV 118, and silicone r e s in adhesive Silgrip SR-573 (with catalyst) , the fai lure was "cohesive". Specimens bonded with the s t ruc tura l adhesive Cavalon 3 100 S, a f t e r 239 hours ' exposure, had an average bond-strength value higher than any other adhesive l is ted in Table 3. However, the catalyst used to c u r e with acry l ic adhesive discolors and darkens sufficiently that it would not be practi.ca1 to use i t where sunlight has to pass through the adhesive layer.

Table 4 summar izes the data fo r bond-strength measurements on weather- ab le Mylar, 0, 178-mm (0.007-in.) thick af te r exposure to tempera ture cycling f o r 508 hours (127 cycles). In this table, it can be seen that some specimens had sufficient bond s trength to cause f i lm elongation to occur before the bond failed cohesively. There i s no significant difference in bond-strength values between the s t ruc tu ra l and nonstructural adhesive specimens af te r 508 hours ' exposure to t empera tu re cycling. Specimens bonded with Scotch Gr ip 4693 exhibited the high- e s t average bond-strength value (102 ps i ) of any of the adhesives studied. The un- known factor with this adhesive i s i t s UV stability. The 1000-hour, UV-exposed specimens will provide important data in determining the value of this mater ia l f o r proposed encapsulant applications.

Water-Vapor Transmission Experiments (WVTR)

Experience to date with encapsulation of photovoltaic modules with polymeric m a t e r i a l s indicates that water vapor. i s one of the important "contaminants" af - fecting the serv ice life of the enclosed so lar cells. Combinations of f i lms (and shee t s ) with var ious adhesives a r e being screened so a s to select a candidate l is t of p r e f e r r e d mater ia l s . The initial resu l t s have been obtained for Mylar and FEP f i lms bonded with seve ra l adhesives (sealants).

The r a t e of water vapor t ransmiss ion through f i lms/adhesives i s being studied through the use of a f i lm pouch (or "pillow pack1'). Figure 2 i s a sche- mat ic of the pouch. As indicated, two square f i lms, .8. 9 c m (3. 5 in. ) on a side, a r e bonded together along the outside edges of the fi lms. The bond a r e a i s 1.27 c m (0. 50 in. ) wide; the total bonded a r e a is thus 38.7 cm2 (6.0 in. 2). Water- permeat ion-rate calculations a r e based on the total f i lm a r e a containing the desiccant (0.08 m2).

Pouches were prepared for measurement of WVTR before and af te r tempera- tu re cycling (-40 C to 90 C ) and af te r UV exposure for 500 and 1000 hours.

TABLE 3 . LAP-SHEAR STRENGTH OF SPECIMENS ON WEATHERABLE MYLAR SUBSTRATE FOLLOWING uv EXPOSURE(^)

Specimen ~ d h e s i v e ( b,

Exposure Time, Adhesive Bond hr psi ~ e m a r k s ( ~ ) '

Scotch Grip 4693 Ditto

RTV 118 Ditto

Scotch-Weld 3520 B/A Ditto

Cavalon 3100 S, Activator 3303 S Ditto

None (control) 7 9 . 5 Cohe.sive fail 11re 239 88 Cohesive failure 239 7 6 Cohesive failure 239 7 1 Cohesive failure

None (control) 239 239 239

None (control) 239 239

None (control) 239 239 239

Adhesive failure Cohesive failure Cohesive failure Cohesive failure

Adhesive failure Adhesive failure Adhesive failure

Adhesive failure Adhesivc failure Adhesive failure Adhesive failure

17- 3 Silgrip SR-573 with 1- 1/2qo None (control) 66 Adhesive failure Benzoyl peroxide (cat . )

17-4 Ditto 239 65 Cohesive failure 11 . ' ,. 49' . ., ,

17-5 239 Cohesive failure

17-6 239 53 . Cohesive failure

2 ( a ) Pdh~-s ives on weatherable Mylar f i lm 1 7 8 p m (0 .007 in.) thick; adhesive area, 6 . 4 5 c m 2 ( 1 in. ). Controls stored

in constant temperature room 22:8 C ( 7 3 F). 50 percent relative humidity. (b) Scotch Grip 4693 and Scotch-Weld 3520 B/A supplied by 3M; RTV 118 and Silgrip SR-573 supplied by G. E. ;

Cavalon 3100 S, Activator 3303 S supplied by DuPont. (c) Bond strength in psi based on area of adhesive. (d) Adhesive failure indicates bond failure. hetween weatherable Mylar film and adhesive; cohesive failure indicates

bond failure in the adhesive itself.

TABLE 4. LAP-SHEAR STRENGTH OF SPECIMENS ON WEATHERABLE MYLAR SUBSTRATE FOLLOWING TEMPERATURE CYCLING (-40 to +90 c ) ( ~ )

.. . Adhesive Bond

Exposure Time, ,

Specimen ~ d h e s i v e d b, hr psi ~ e m a r k d

13- 1 Scotch Grip 4693 None (control) 68.5 Cohesive failure 13- 13 Ditto 508 97 Film elongation, then

cohesive failure 13- 14 Ditto 108 Ditto 13- 1 5 100 13- 16 105

RTV- 118 Ditto

None (control) 66. 5 Cohesive failure 508 88 Cohesive failure

Ditto 38 . Adhesive failure 47.5 Adhesive failure 94 Cohesive failure

. . .

1 5 - i Scotch Weld 3520 B/A None (control) 66 Adhesive failure 15- 13 Ditto 508 107 Film elongation.. then

cohesive failure 15- 14 Ditto \ 86 Film broke 15- 1 5 92 Film broke 15- 16 56 . Adhesive failure.

16- 1 Cavalon 3100 S, Activator 3303 S Nour (control) 98.5 Adhesive failure 16- 1 3 Ditto 508 88 Adhesive failurde) 16- 14 Ditto 98 Adhesive failure(e) 16- 1 5 53 Adhesive failure(e) 16- 16 99 Adhesivc failurde)

. .. 17- lC' . Silgrip SR- 573 plus 1- 1/270 None (control) 57 Adhesive failure, .

Benzoyl peroxide 17- 1 3 Ditto 508 109 Film elongation, then

coh~s ive failure 17- 14 . Ditto, J10 ,, . Film elongation, thcn

. . cohesive failufe 17-15 7 6 Cohesive failure 17-16 I, 95 Cohesive failure

(a) Adhesives on weatherable Mylar film 178pm (0. 007 inch) thick; adhesive area, 6 . 45,cm2 ( 1 in.2). Controls stored in constant temperature room 22 .8 C (73 F), 50 percent relative humidity.

(b) Scotch Grip 4693 and Scotch Weld 3520 B/A supplied by 3M Company; RTV 118 and Silgrip SR- 573 supplied by GE ; Cavalon 3100 S. Activator 3303 S supplied by DuPont.

(c) Bond strength in psi based on area of adhesive. (d) Adhesive failure indicates bond failure between weatherable Mylar film and adhesive; cohesive failure indicates

bond failure in the adhesive itself; "film elongation" indicates that film elongation occurred in portion of film not associated with bonded ,area.

(e) Adhesive is yellow t o brown (spotty) in color after exposure. Starting color is light yellow t o yellow.

\ Desiccant

FIGURE 2. FILM POUCH (PILLOW PACK) USED IN DETERMINING WATER VAPOR TRANSMISSION THROUGH FILM/ ADHESIVE SUBSYSTEM

(Drawing shows one-half of square pouch.)

Pouches a r e p repa red by bonding t h r e e s ides , inser t ing the desiccant (10 g r a m s of ~ r i e r i t e @ ) , and sealing the fourth side. Tempera ture c.ycling and UV exposures of the t e s t pouches were c a r r i e d out p r io r to adding the desiccant and sealing the . four th side. Following prepa.ration, the pouches a r e i n se r t ed in a c losed environ- men t i n which the re la t ive humidity i s control led a t 95 percen t and the t empera tu re i s 23 C. Per iodical ly , the pouches a r e removed f r o m the environment and weighed. The weight i nc rea se i s recorded; in al l . exper iments t o date, the weight i nc rea se has been a l inear function of exposure t ime. Thus, ' the slope of the weight-t ime re la t ionship i s proportional to the water-vapor t r ansmis s ion ra te .

Resu l t s to date a r e summar i zed in Table 5 for Mylar and F E P f i lms . Each r e su l t r ep re sen t s the average of two sepa ra t e measu remen t s ( two'pouches) . F o r 0. 05 1 - m m (2-mi l ) thick Mylar pouches, the WVTR i s approximately constant for a l l adhesives used. Values for the 0.05 1 -mm (2-mi l ) Mylar a r e approximately 2-112 t i m e s those of the 0. 178-mm (7-mi l ) ma te r i a l . The difference in the ex- t r e m e values i s approxirrlately 10 percent . WVTR values fo r the F E P pouches a r e lower. The difference in the ex t r eme values i s approximately 138 percent . A tentative conclusion i s that the data f o r the Mylar exper iments r ep re sen t the : permeat ion r a t e through the Mylar and a r e l i t t le affected by the selection of ad- hesive. The WVTR value for thc F E P f i lm i s <O. 146 g (24 h r ) - l m - 2 , judging f r o m the r e su l t s of the heat.- seale d pouch. cons iderab le differentiation, then, i s obtained among the s eve ra l adhesives . F r o m the data shown, Si lgr ip SR-574 gives the highest WVTR value; the other adhesives lead to values grouped in a much lower range. Of these , Cavalon 3 100 S gives the lowest value.

WVTR, g/24 hr/m2 at 23 C. 9570 R. H. (a) Weatherable "Teflon" FEP.

Nonweatherable 0.127 mm, 5 mil Mylar 0.051 mm, 0.178 mm, 228.5 hr

Adhesive T y p e . . . . . .,. , . . . . . 2 mil 7 mil . Unexposed UV Exposure

Silgrip SR-573 Heat -sealable silicone 5. 63 1.85 0.204 0.286

Silgrip SR-574 Pressure -sensitive silicone

Silicone RTV -118 Room temperature curing 5.41 silicone

Silastic 732 RTV Room temperature curing 5.18 silicone .

Scotchlweld 2216 B/A Room temperatlure. curing 5.30

epoxy

Acryloid B-7. Thermoplastic acrylic 5.72 2.25 0.273 0.218

Cavalon 3100 S Room temperature curing . 5.37 1.79 0.167 0.356 acrylic -accelerator 3300 S

None (Heat sealed) - - - 0.154 -

(a) Film pouches, 8.,9 cm x 8.9 c m (3. 5 inch hy 3.5 inch). overall, were preparcd with a 1.27 -cm (0. 50-inch) adhesive bond around the edges. Ten grams of ~ r i e r i t e @ were contained within each pouch. Values are the average of two samples.

(b) These values are preliminary. They are based on three data points. (c) Single determination. One .sample failed after 20 days (WVTR = 1.354 g/24 hr/m2).

After 228. 5 hours ' exposure to UV, the WVTR ra t e i nc reased very sl ightly for a l l the adhesives examined except f o r the Silgrip SR-574 and the Acryloid B-7. The pressure-sens i t ive silicone (Silgrip SR-574) may be "hardening" and the ac ry l i c (Acryloid B-7) could be cr.oss-linking act ions which could lower the WVTR. .The Cavalon 3 100 S, which exhibited the lowest WVTR before UV exposure (0. 167 g /24 h r /m2) , had the highest WVTR a f t e r exposu re (0.356 g/24 hr/rn2).

Chemilumine scence Experiments

Measurements of the photon emiss ion f r o m ma te r i a l s owing to oxidation. react ions (chemiluminescence) have been made to determine i f such measu remen t s can be used a s a diagnostic tool to evaluate degradation r a t e s . Only pre l iminary r e su l t s a r e available to date. Initial measu remen t s have been made-on "sandwich" specimens of the type shown in F igure 3. Mylar and F E P f i lms were used a s the sandwich ma te r i a l ; RTV-118 and Scotch Weld 2216 B/A were the adhesives. Ex- posure s have included UV radiation and t empera tu re cycling of the type descr ibed previously. Actual measurements (photon counting) were made in a n air enc losure at 26 C.

7.62 c m

- (3.0 in.) -1

Bonded Area

F i lm (Square)

/

4 6.35 cm /

/-(2.5 in.) - FIGURE 3. CONFIGURATION OF SPECIMEN USED IN

CHEMILUMINESCENCE EXPERIMENTS

The r e su l t s t o date a r e given in Table 6. Note that the emiss ion (net counts pe r second) i s a function of the volume of the f i lm and adhesive used. Differences in volume'of the film present no problems. The volumes of the adhesives a r e un- known and subject to sizable variability. Moreover , the 'bonded a r e a was not de- fined well due to smear ing along the edges of the bond. With th i s reserva t ion , i t can be noted that the emiss ion r a t e s for F E P specimens alone (no adhesive) a r e very s m a l l f o r a l l conditions; Mylar f i lms with no adhesives lead to slightly higher values. With some. exceptions, the emiss ion r a t e s f r o m the epoxy adhesives

TABLE G.T-'CHEMILUMI~~ESCENCE MEASUREMENTS ON

UV- AND TEMPERATURE-CYCLED SAMPLES

7

-. .

Emission .

Exposure, net counts/ Specimen Film Adhesive Test . hr s e J a )

. 2 - 1 - Mylar None Control None 53 - 2-3 Mylar- None UV 2 5 13

2 -5 My1,ar None Temp cycling 598 18 . '2-6 . Mylar ' . None ., Temp cycling . 598 . ,' 24

I

2-11 FEP 2 -13 FEP 2 -15 FEP 2 -16 FEP

None None None None

Control ~ o i e uv 2 5 Temp cycling 598 Temp cycling ' ,598

. 3-1 Mylar 2216 B/A Control None 744 I

, 3 r 3 , Mylar 2216 B/A . UV ' 25 1.006 .

3-5 Mylar ' 2216 B/A " Temp cycling 598 95 .

3 -6 Mylar 2216 B/A Temp cycling . . - 598 25 . . -

3 -1.1 FEP- 2216 B/A Control None . 585 3 -13 FEP 2216 B/A UV 2 5 555 3 -15 EEP 2216 B ~ A . Temp cycling 598 ,58 3-16 - FEP 2216 B/A Temp cycling 598 41 ',

4 -1 Mylar RTV-118 Control , . . None . 85 RTV-118, 4;3 . , Mylar w ' 2 5 10

4 -5 Mylar RTV-118, Temp cycling 598 , 13 4 -6 Mylar RTV-118 . ' Temp cycling 598 11 .

4-12 . FEP. ' RTV-118 Control None 1 4 -13 FEP . Rc1:~-118 U V . . ' 25 . 5 4 -15 FEP . RTV-118 - Temp cycling 598 22 4 -16 FEP RTV-118 , Temp cycling 5 9 8 .

I . o I \

(a) Samples were placed in a closed chamber in air at 26 C,for measurements of unfiltered chemiluminescence emission. Background counting rates were subtracted from observed

, "

emissions. - - . .

/ _ .

( 22 16 B/A) a r e higher than those f rom the' silicone (RTV-118). Unexposed speci- mens of Mylar12216 B/A gave a r a the r high count r a t e , and, under UV exposure, the count ra te was even higher. It was a l so noted that exposure to tempera ture cycling decreased the ra te substantially. Whether fur ther curing was the cause of this decrease i s not yet known. Temperature cycling gave the same t r end for F E P / 2216 B/A specimens. FEPIRTV-118 specimens gave low count r a t e s for a l l con- ditions. Fur the r analysis must await the accumulation of considerably m o r e data on these and other systems.

Evaluations of G l a s s Encapsulant Components

\

Study 1 identified three a r e a s of p r imary importance in long-life performance of encapsulation sys tems incorporating glass covers: (1) the. cover g l a s s / so la r - cel l organic bond, ( 2 ) the "hermetic" sealing of the a r r a y , ' and ( 3 ) the corrosion of e lec t r ica l conductors (metallization, interconnects, and leads) a t the interfaces with ei ther adhesives o r sealants. .Thus, the ma te r i a l propert ies of pr ime im- portance to long life a r e those bearing on the interactions and behavior a t these interfaces; and the methods evolved for life testing must reflect changes in prop- e r t i e s which a r e cr i t ical to interface - degradation problems.

Glass -to -Cell Adhesive Bondine

The organic adhesive layer between a cover g lass and a 'gil icon so lar cell, being investigated in this study, should serve severa l functions. F i r s t , i t should provide optical coupling with maximum t ransmiss ion over the wavelength range of 280 to 1100 nm. Second, i t should maintain i t s mechanical integrity plus the bond integri t ies a t both the glass-adhesive and cell-adhesive interfaces. Third, i t should avoid thermal s t r e s s levels which would damage the cel l , cel l metall iza- tion, o r interconnects. Finally, the adhesive should be noncorrosive.

Large thermal mismatches among organic adhesives, silicon cel ls , and soda-lime g1as.s (p re fe r red over borosil icate for i t s low cos t ) provide the potential for severe thermal s t r e s s over the specified tempera ture cycle between -40 and t 9 0 C. The most promising solution to this thermal s t r e s s problem appears to be the use of low-modulus adhesives.

The experimental studies being implemented include two subject a r e a s : (1 ) screening-type t e s t s of'cell-adhesive-glass interface samples and (2) adhesive- prnperty measurements.

Screening t e s t s a r e being performed on sandwich-type samples consisting of a single cell, adhesive, and glass. A portion of the samples contain leads to permi t p re - and posttest e lec t r ica l measurements on the cell. Sample sandwiches utilize ei ther soda-lime float glass o r 7740 borosil icate glass plates. Adhesives have been selected on the basis of Study 1 information and discussions with adhesive manufactures, and additional candidates a r e expected to be identified for screening evaluations.

15

\ TABLE 7. IMPORTANT PROPERTIES OF CANDIDATE . .

,- . Property -

Mechanical ~dhes ive ( a) Ypung's . Hardness Tensile . Lap Ultimate

Polymer . Commercial Product Modulus Shore A Strength, Peel Shear Elonga- Type Deslgnatlon Supplier (10-5 psi) Durometer ~ / m 2 Strength Strength tlon, 70

------------ Ideal adhesive------------- LOW Low - Hlgh Hlgh Hlgh Hlgh

. . - - Dimethyl RTV-602 GE 15 6. 9 x 1 0 ~ - - - - 200 -- silicone

5 .

Dimethyl RTV-615 SE . -- 35 6 . 4 ~ 1 0 ~ ~ - - - - 150 silicone . .

- - RTV- 121 - - - - - - - - - - - - - - 35 - - Dimethyl Sylgard-184 -DowCorning : -- 6.2x106 1 - -

~ .. 100

silicone

si l icone S i lg r ip 573 GE - - - - - - - - -- - - \

-adhesive- ( . , . Silicone RTV3140 Dow Corning - - 25 2.1x1d6 - - - - . . 450

' - rubber . , Silicone 282 Dow Corning . . - - - - -- . - - - - - - adhesive '

(a) Yasui, R. K . , and Goldsmith, J. V . , "Status of JPL Solar Powered Experiments for Terrestrial Applications",. Presented a t Ninth Intersocity Energy Conversion conference, .Sari Francisco, August 26-30, 1974.

, (b) ~ a s > i + .Rt K . , and Patterson.' R. E., "Utilization of Space Technology for Terrestrial ~ b l a r Power Applications".

. ' Conference Record of Tenth IEEE Photovoltaic Specialists Conference, November 13- 15, 1973, Palo Alto, California ( 1974).

. ' (c) ~et-Propulsion Laboratgry, ,California Snstitute of Technology, Private Communications (1976). '

(d) U:. S. coast Guard Station (Groton, ~onnec t i cu t ) ; Private communication (1976). - .

(e) ~ a l l e s , ' Y. , "Solar Arrays for Terrestrial Applications and Sounding ~al loons" , hesented at the Ninth IEEE ~hdtovoltaic ~ ~ e c i a l i s t s Conference, May 2-4, 1972, Silver Spring, Maryland.

a L

(.f) Salles, Y. ,. "Industrial ~ e v e l o ~ m e n t of Silicon solar Cells", Acta Electronica, 18 (4), 339-343 (1975). - RTC La Radiotechnique-Compelec (France), Private Communication (1976). , .

z .

ADHESIVES FOR BONDING SILICON SOLAR CELLS T O COVER GLASS

Characteristics ---.--

C 0- Optical Reason for Inclusion efficient Index Potentially in Tabular Comparison

of Brittle Percent of Stability Corrosive Reported in Considered for Expansion, Point, Transmission Refrac- t o t o Electrical Study 1 Study 3 screening

lo-s c C (280-2000 mm) tion UV Conductors References Evaluations

(0.3-0.9) <-40 100 -- . 20-year Noncorrosive - - - - stability

(-18 t o 177 C) <-59 - - - - - - - - (a , b, c ) Yes 29

(-18 t o 177 C) <-59 (350-800 mm) -- -- - - 27 81(b)

Yes

-- - - - - -- - - - - ( e , f, g) Yes

( -55 t o 150 C) -65 (350-800 m m ) 1 . 4 3 - - - - - - No 30 7 d b )

- - -60 - - -- - - Noncorrosive - - Yes

I. - .. - - -- - - - - s -- - - Yes

- ,

The idea l adhesive for bondingsil icon sola'r cel ls to glass coyers might be. specified qualitatively t o have the following property =haracter is t ics : 100 percent light t ransmiss ion over the 280 to 11 00-nm wavelength range, 20-year stability t o UV, a coefficient of the rma l expansion between the values for silicon, and the c,over g lass , br i t t le point (g l a s s transit ion) below -40 C, h igh ultimate elongation,

- high s t rength (lap' shee r , peel, and-tensile), low young's modulus, low harness , and to b e onc corrosive. Such a hypothetical "ideal" combination of propert ies ap- p e a r s to be most c losely approached by silicone polymers. However, as-yet- unidentified candidates of o ther polymer types may be included in screening tests . Table 7 indicates the proper t ies considered most important and l is ts candidate

/

m a t e r i a l s and available data.

The screening t e s t s being initiated subject sandwich samples to sequential t h e r m a l cycling and ,UV exposure for a l ternate periods of a week. After each. week o r mLltiple thereof, the samples will be removed f r o m te s t and visually/micro - scopically examine d for evidence of delamination o r other degradation. Samples with e l ec t r i ca l leads will be checked periodically for electrical- degradation of the cell. When relevant nondestructive diagnostic techniques a r e evolved, they can be used for init ial sample character izat ion and to monitor evidence of degradation. The t e s t program is shown schematically in F igure 4.

Selective property measurements on-adhesives will be made a s needed to supplemeit ava i lab le information and to a s s e s s affects 'of and/or process

' - variables . La ter the promising-mater ials and processes f rom the screening tes t s . will bk used to encapsulation samples for aging t e s t s and to 'prepare s im-

ple modules representat ive .of proposed .encapsulation-s ystem design concepts..

o r g a n i c Sealing of Glass Encapsulation Systems

At present it is a s sumed that any glass-encapsulated a r r a y must be sealed "hermetically" to achieve a 20-year performance goal. The scope of this par t of the study i s to investigate achieving adequate "hermeticity" with organic sealants.

F o r the simple glass-encapsulation concept of two g lass panes having cel ls between them, the sealing problem i s very s imi lar geometrically to that for in- sulated g lass units. Thus, the established sealing mater ia l s and procedures used f o r insulated-glass manufacture could se rve a s a s tar t ing point for mater ia l s selec- tion and processing for g lass encapsulated so lar a r r a y s . In fact, the sealing ma- t e r i a l s m a y have utility f o r so lar a r r a y s of geometr ical design differing greatly f r o m insulated-glass windows.

, To achieve m a x i m u m "hermeticity" and lifetime of insulated-glass products, a dual seal> i s employed. An inner sea l of butyl o r butyl-polyisobutylene i s em-. ployed a s the p r imary b a r r i e r to water vapor. A second backup o r edge sea l s e r v e s a s an. adhesive-mechanical seal. This edge sea l i s commonly a polysul- fide; h,oweve r, for higher temperature service .silicones a r e now being used. Insulated-glass units ,also incbrporate desiccants to prolong the lifetime for .

,khich low' internal- water-vapor levels may be maintained.

F I G

Periodic Electrical

3 : Measurements

- Fill Factor - Resistance Change - Iv

U R E 4: SCREENING T E S T P R O G R A M ON ADHESIVES F O R GLASS ENCAPSULATION'SY STEMS ,

It i s not intended to imply that only t h e ~ s e a l ma te r i a l s of the insulated- g l a s s indus t ry a r e viable s e a l candidates for sealing solar a r r a y s . The best s ea l m a t e r i a l m a y obviously va ry with the a r r a y design and other factors . Neverthe- l e s s , t h e sealants f r o m insulated-glass technology provide some candidate m a - t e r i a l s of demonstrated performance and reasonable cost.

\

'Table 8 identifies two representat ive mater ia l s for inner vapor sea ls and two examples of edge sea l s used for insulated glass. Selections f rom these m a - t e r i a l s and o ther candidates a r e being employed to sea l simple-geometry samples such as two-flat-glas s shee ts sea led around the per imeter . Their "hermeticity" can then be a s s e s s e d by subjecting the samples to high humidity and alternately to t h e r m a l cycling.

. PLANS FOR NEXT QUARTER

The lap-shear specimens on t e s t which were prepared of weatherable Mylar and of polycarbonate film (51-mm Lexan) will be evaluated a t 500 and 1000 hours ' exposure to UV and to t empera tu re cycling for lap-shear bond strengths. Ad- ditional specimens will be prepared of Korad A (acrylic* f i lm) and Tedlar (fluoro- carbon film): (The evaluations of lap-shear specimens evaluated af te r expo su re to UV and t empera tu re cycling a r e complete for "Teflon" FEP. )

A t h i rd t e s t will be init iated in which the lap-shear specimens will be sub- jected to 95 to 100 percent re1ati;e humidity a t 49 C (120 F), since high humidity can be ve ry detr imental t o a n adhesive bond with some types of adhesives. Te s t conditions selected a r e expected to acce lera te fai lures of s,uch bonds. .

Work will continue on cu r ren t expo su re studies involving "Teflon" F E P .and

. %

weatherable Mylar. Tes t specimens a lso will be prepared and put on t e s t for other f i lm m a t e r i a l s , e. g. , Tedlar and Korad A, and for cer ta in sheet mater ia l s , prin- cipally acry l ic , polycarbonate, and g lass -reinforced polyester.

To determine the feasibil i ty of si l icon-cell encapsulation by fi lm lamination, s e v e r a l types of film mate r i a l s in combination with var ious adhesives will be s c r e e n e d fo r this app l i~a t ibn . hon&k'&to-"$ilicon ce l l s , lead wi res , and to the film itself will be evaluated. In addition to bonding, conditions for assembly ( t empera tu re , presssure; and t ime in p r e s s ) will requi re study. Types of laminate- forming technique s ( p r e s s u r e lamination, vacuum forming) will a l so nee d

\ investigation.

An evaluation of existing information on methods of fluorinating f i lm ma-- t e r i a l s and a stbdy of selected commerc ia l coatings will a l so be conducted. The feasibil i ty of f l u ~ r i n a t i n g f i lm mater ia l s to provide specific propert ies (e. g. , im- proved WVTR proper t ies ) will receive f i r s t attention. Selected commerc ia l coat- ings a l s o will be screened for application on f i lm substrates . Selected coatings wkll be evaluated to determine i f improved UV res is tance and/or resis tance to water permeat ion resul t , and, i f so, to what extent.

TABLE 8. PROPERTIES OF CANDIDATE SEALANTS FOR GLASS ENCAPSULATION

Sealant Moirture Vapor Adhesion to Glass Tensile ~ r i t t l e Relative Service Commarcial Primary Trm srnission Peel Tensile Strength. Hardness. Temp. . UV Percent Temperature . .

Polymer Type Example Function Rate N/m N/mZ N/G2 Shore A C Resistance Elongation Range

Butyl- Tremco Polyisobut ylene JS- 78.3a)

Butyl Cross-link, Novagqd hot-melt ~ - 4 l d ~ )

K c.

T w o - p ~ r t Tremco Polysulfide J S - ~ O ' ~ ( ~ )

Silicone Rhodon- (one- ?art) Therm

542(

Moisture vapor barrier

Moisture vapor barrier

"edge" seal

"edge" seal

. 16 g./m2/24 hr (excellent adhesion (Honeywell tester) t o glass)

0.2 g./rn2/24 hr for (25-27) 1 . 3 ~ 1 0 ~ 20-mil film ASTM E96- 53T

3 g/m2/24 hr (cohesive failure) ~ 9 . 5 ~ 1 0 ~ 50-60 (Honeywell tester) (after

aging)

- - (cohesive failure) 2 . 9 ~ 1 0 ~ 26

-40 Excellent - - -46 t o 120 C

- - Excellent >3 0 0 -46 t o 116 C

- -. Good >250 - -

Excellent 6 00 -62 t o 204 C

(a) Product of the Tremco Manufacturing Co. , Cleveland, Ohio. (b) Pruduct of the Novagard Corporation, Trenton, N. J. (c) Product of Rhodia. IEC., Chemicals Division, Monmouth Junction, N. J.

STUDY 4. DEVELOPMENT OF ACCELERATED AND ABBREVIATED TESTING METHODS FOR PREDICTING PERFORMANCE OF ENCAPSULATION MATERIALS OVER 20- YEAR LIFETIME

G. Derringer, C. W. Kistler, J r . , R. E. Thomas, D, M. Bigg, R. D. Igou, G. B. Gaines, , .

and D. C. Carmichael

INTRODUCTION

To be economically feasible, photovoltaic solar a r rays should have a service life of a t least 20 years. Silicon solar cells, i f properly protected from the atmos- phere, a r e expected with reasonable confidence to have such a lifetime. The suc- ce s s of the Low-Cost Silicon Solar Array Project depends substantially on identify- ing a low-cost encapsulation material system that will protect the solar cell for that period of time with little loss in electrical output. The success of an encapsulation material, therefore, depends on i ts ability to withstand 20 years of outdoor service in various environments. Since 20 years of service testing i s obviously tmpractical, methods for short- t e rm (abbreviatedlaccelerated) aging tests must be developed from which service life can be predicted with acceptable confidence.

Although a specific accelerated/abbreviated test for the ulttmate encapsl.ilant sys tem cannot be verified until the system i s defined, the objecti.ves of this study a r e to accomplish the following significant steps in establishing such a test.

( 1) Develop a suitable methodology for designing the statistical and experimental techniques for such a test

( 2 ) Illustrate this methodology by.developing an experimental test design for an aging tes t on specific materials and material interfaces which a r e representative of candidate encapsula- tion systems.

Subsequent work would involve conducting such tests and analyzing the data in accordance with the tes t design ("test plan") to verify the methodology and tech- niques and to obtain high-confidence predictions of service life.

The methodology to be developed will describe the important factors that must be considered in the detailed test design and describe suitable statistical and experi- mental techniques for treating the tmportant factors including statistical data analysis and prediction methods.

The illustrative test design will describe the total elements of a test plan, including material selection, specimen design and number, environmental parame- t e r s and operating conditions, properties to be measured, instrumentation to be utilized, and data analysis methods. Inputs regarding the selection of materials and interfaces to be considered will come from Study 1. Study 2 will furnish environmental descriptions of various geographical sites of interest.

22

TECHNICAL DISCUSSION O F RESULTS THIS QUARTER

In previous r epo r t s on Study 4, s e v e r a l a spec t s re la t ive t o developing a . methodology fo r aging t e s t s have been t rea ted ; among these topics. a r e aging mech- aging mechanisms in po lymers .and in g l a s se s , sensi t iv i ty ana lyses r equ i r ed to give detectable p roper ty changes with s ta t i s t i ca l validity, and discr iminat ion a - = mong mathemat ica l models of aging. . . data during the acquisi t ion of such data. his qua r t e r , investigations wk're conducted 0.n the discr iminat ion among t e s t methods used in acquiring aging data on the b a s i s of p rec i s ion and cost ; t h i s r epo r t i s con- ce rned with these investigations.

- . In t he se inve s t ig i t ions , considerable at tention 2s being give*' to "abbre - . '

viated" test ing. In such test ing, s ample s a r e exposed under "normal" t e r r e s - t r i a l conditions for some per iod of t ime , ti. Using data collected in th i s time, in terval , . p roper t ies of the mate:?ial o r s y s t e m a r k forecas ted fo r a future t ime,

tf . Guideline values for ti and t f in Study 4 a r e 2 and 20 years , respect ively . . F o r success fu l predictibn over such a long period, pr0pertie.s m e a s u r e d in the ini t ia l 2 -year in te rva l have to be m e a s u r e d with hig.h precision. One, t .herefore, r equ i r e s a means of discriminating among var ious t e s t methods o n the bas i s of h rec i s ion a s well a s cost and other fac tors .

P r i o r to discussing precis ion, i t will be useful to differentiate between "precision" and "accuracy", since the two concepts a r e some t imes confused. In shor t , precis ion i s a m e a s u r e of 'c loseness of repea ted measu remen t s of a p roper ty whereas accu racy i s a m e a s u r e of how close a t e s t measu remen t i s to some known re fe rence value. These concepts a r e i l lus t ra ted in F igure 5. Here, the two bell-shaped curves r ep re sen t the s p r e a d of t e s t values fo r Testing Methods 1 and 2 with means and s tandard deviations p l , p2, al, and a2, respectively.

The t r u e re fe rence value i s indicated on the absc i s sa . In th i s example, Tes t Method 2 has a sma l l e r s tandard deviation than Test Method 1 and i s t he re - fo r e m o r e prec i se . On t,he other hand, the population mean K 2 fo r Method 2 i s . .

f a r the r f r o m t rue value than the medn of ' i e s t 1, p . Therefore , one sbys that Method 1 i s m o r e accura te than Method 2. Thus, a ccu racy and precis ion a r e two independent concepts. In th is Study, precis ion, not accuracy , i s of p r i m a r y concern s ince the in te res t i s in p roper ty changes. To i l lus t ra te th is , suppose that a t e s t i s inaccurate by a factor 6, i. e . , ins tead of getting the t rue value the measu r ing ins t rument gives q + 6 . Then for two q measu remen t s , a t t imes t l and t2, the r a t e of p roper ty change would be [ (q2 t 6) - (q l + 611 / ( tZ - t l ) = (q2 - q l ) / ( t2 - t The inaccurac ies cancel out in. measur ing the r a t e of change .

Reference or I I~ rue ' l Value '

FIGURE 5. ILLUSTRATION OF ACCURACY AND PRECISION

. .

Measures of Precision

Precision is a measure of the spread (gr dispersion) of data values around the i r mean value. An obvious measure of dispersion i s the mean deviation M, where

N is the number of data points,' Z i s the data mean, .and x i i s the ,value of an indi- vidual measurement. Although this measure is sometimes used, it i s difficult to manipulate mathematically and is , therefore, seldom used in statistical com- putations. A better measure of dispersion i s the standard deviation, S , where

This s ta t i s t i c i s the m o s t common m e a s u r e of dispersion. Inspection of Equation ( 2 ) revea ls that the s tandard deviation c a r r i e s the s a m e units a s the data. This i s usually a desirable proper ty for any m e a s u r e of dispersion. F o r comparing different testing methods, however, the s tandard deviation i s unsatisfactory. F o r example, suppose one i s using two t e s t s with modulus exp res sed i n ps i and elongation a t b reak expres sed i n percent to determine the degree of vulcanization of an e las tomer ic compound. The modulus shows a s tandard deviation of roughly 70 psi , and elongation, approximately 20 percent. Clear ly , knowledge of the s tandard deviations i s not enough information to favor one t e s t ove r the o ther because one cannot compare usefully psi and percentage units.

One logical a l ternat ive would be the use of the coefficient of variation, CV, where

Clearly, the coefficient of variation i s unit less, and a s such would be expected to allow comparison of different t e s t precis ions . This i s t r u e in some c a s e s but i s not t rue generally.

Review of Published Information on Tes t Prec is ion

Published information was reviewed to find information on precis ion, a c - curacy, repeatabil i ty, reproducibility, e t c . , a s applied to polymer testing. In this effort the Engineering Index, Chemical Abs t rac t s and NTIS f i les were inter - rogated. Out of a total of 249 hits only 21 documents appeared relevant t o the problem of t e s t comparison. Most of the 21 have been received and were found to he. of 1.ittle value with r e g a r d to t e s t discrimination.

Pe rhaps the l a rges t reposi tory of information on t e s t accu racy and precis ion i s the Amer ican Society for Testing and Mater ia ls published t e s t methods. Ac- cordingly, the 1975 Annual Book of ASTM Standards, P a r t 35 fo r General 'l 'est Methods for P l a s t i c s , was scrutinized for precis ion information. The methods f o r which the coefficient of variation was given o r was calculatable f r o m the given data a r e shown in Table 9 along with the i r respect ive coefficients of variation. This table r evea l s a wide range of precis ion, and underscores the need fo r a systematic methodology for t e s t discrimination. As noted previously, compar i son of coefficients of variation i s usually invalid for comparing different t e s t s , and such s ta t i s t i cs a r e of l imited value. The problem of t e s t discrimination, however, has not gone unattended. ~ a n d e l ( l , 2, recognized the problem and defined a r e l a - t ively sensit ive s ta t is t ic for t e s t discrimination.

TABLE 9. COEFFICIENT O F VARIATION FOR VARIOUS ASTM TESTS ON PLASTIC MATERIALS '

Coefficient of ~ a r i a t i o n ( a ) ,

Tes t percent

Density

Linear Dimenstional Change

Dilute Solution Viscosity

IR Absorbance of CH3 on Polyethylene Type I1

Coefficient of Linear Thermal Expansion

Water Vapor Transmiss ion

Barcol Hardness

IR Absorbance of CH3 on Polyethylene Type I11

High-Speed Tensile at Break

Tensile h p a c t Energy

IR Absorbance of CH?, on Polyethylene

Type I

High-Spee d Elongation a t Break

(a) Coefficient of variation is standard deviation expressed as percent of mean.

Mandel's Sensitivitv Ratio

Suppose that two t e s t methods, M and N, both measure the same property, say Q. If M i s a function of Q and N is a function of Q, it follows that M i s func- tionally related to N. Suppose that the s tandard deviation of t e s t M i s OM and that of t e s t N i s ON. Mandel has shown that the two t e s t s can be compared by a quan- t i ty called the relative sensitivity of t e s t M with respec t to t e s t N, RS(M/N), where

Note that the relative sensitivity is unitles s and favors t e s t M when RS(M/N)> 1 and t e s t N when RS(M/N)<l. This concept i s i l lustrated in Figure 6 where the different ials 'are shown a s AM and AN. Careful inspection of this figure will indicate that RS(M/N) is not necessar i ly constant, but will va ry f r o m point to point depending upon the shape of the sensitivity curve and whether o r not the s tandard deviations a r e constant. Thus, one cannot say that t e s t M i s "better" than t e s t N unless RS(M/N)>l for a l l values of M and N. In the special case where dM/dN, OM, and a~ a r e constant, the relative sensitivity will, of course , a l so be 'a constant.

Test N

FIGURE 6. ILLUSTRATION OF RELATIVE SENSITIVITY OF TWO TESTS, M AND N

As a n example, suppose that one has the propert ies , P.1 and P2, f r o m two different t e s t methods, each of which can follow the'degradation of, say, a poly- m e r f i lm as a function of exposure t ime, t. Fur the rmore , suppose that P1 and

P2 have equal s tandard deviations and decrease with exposure t ime, t , a c c o r d i n g to the following relat ions hips,

where P10 and PZ0 a r e init ial property levels for t e s t s P1 and P2, respectively, Pl( t ) and PZ(t) a r e property levels a t t ime t fo r t e s t s P1 and P2, and b l , b l l , and b2 a r e constants. Solving Equation ('6) for t and substituting in Equation (5) , one ge ts .

(7) P l ( t ) = P10 + b l

Then the relative sensit ivity of t e s t P1 with respect to t e s t P2, RS(P1/.P2), .is as follows:

where

and

As a specific example, suppose one has

P l ( t ) = 100 - 5t - 0.5t 2 O< t < 10 - - and

These relationships a r e shown in F igure 7.

Then yo = 1. 5 and y l = -0.01, and one has

This relationship is shown in F igure 8, and indicates that a t P2 = 50 (o r t = 5) the two t e s t s a r e equally sensitive. At P2 < 50 (t <5) , P1 i s m o r e sensit ive, and a t P2 > 50 (t >.5), PZ!i.s.more sensit ive. The r e su l t s a r e s imi lar if ;al 'f ozi in which c a s e yo and y l a r e both multiplied by the constant (jZ/al.

F I G U R E 7.

Time, t

P L O T CF P R O P E R T I E S PI A N D P2 A S F U N C T I O N S O F T I M E

F I G U R E 8. RELATIVE SENSITIVITY AS A F U N C T I O N O F T E S T P2 F O R H Y P O T H E T I C A L E X A M P L E S

Transformations of Scale

F o r any scheme fo r test-method discrimination to be completely valid, t ransformations of scale should not affect the discrimination results. F o r ex- ample, suppose one has determined that elongation a t break i s twice a s sensitive a measure of the degree of vulcanization of .rubber a s tensile strength. Lf one modifies the tensile t e s t to give resul ts on a logarithmic scale, is the sensitivity ra t io s t i l l 2 t o l ? Mandel has shown that the relative sensitivity i s unchanged by a t ransformation of scale for one o r both tes ts . This resul t i s important enough to repeat its deviation here.

F o r t e s t s M and N, the relative sensitivity i s given by (4). Now suppose that one t ransforms the scale of M as follows:

M* = f (M) (14)

where M" i s the t ransformed version of M. Then the relative sensitivity of M with respect to N is

Now since M i s a function of N, then so is Mi. Then by differentiating (14) with respect to N one obtains

Now by the law of propagation of e r r o r s , one can write

Then by substituting Equations ( 16) and (1 7) into Equation ( 15), one gets

Thus, changing the scale of M does not change the relative sensitivity.

Inferences Concerning the Sensitivity Ratio

The discussion of the sensitivity rat io thus f a r has dealt with population parameters , OM, aN, and slope, dM/dN. All of these pa ramete r s must be e s - t imated f rom actual data, and a r e , therefore, subject to random fluctuations. A means i s required to determine i f a calculated sensit ivity ratio i s significantly grea ter than unity. Unfortunately, a n exact solution i s not possible, and i t i s necessa ry to r e s o r t to an approximation. F i r s t , i t i s assumed that the slope dM/dN, est imated by, say, K, i s sufficiently prec ise that i t can be ignored a s a source of variation fo r the sensitivity ratio. This i s often a reasonable assump- tion. Since the sensitivity ratio would normally be est imated f rom data f r o m a well- de signe d experiment, the experimenter can usually minimize the variabili ty of K. The variation in the sensitivity rat io i s then assumed to be a resu l t only of the variation in es t imates of aN and OM, denoted SN and SM. It i s known that

2 2 the quantity ( S & I U ~ ) I ( S & I O ~ ) follows an F distribution, where i and j a r e i, j

the number of degrees of f reedom associated with S and SN, respectively. Now i t can be shown that the quantity [ I K I (S / S ) I 1% represents a lower con- fidence l imit for the sensitivity rat io I K I (oNyoMY If this lower l imit exceeds unity, it i s concluded that, a t the confidence. level chosen, M i s m o r e sensitive than N. If i t does not exceed unity, i t must be concluded that the data a r e con- sonant with the hypothesis of equal sensit ivit ies. In most instances, however, a choice has to be made between two tes t s , and i t would be sensible to select the one with the higher ca lcu la ted sensitivity, regard less of the lack of s ta t is t ical confidence a t the desired level.

Sensitivity, Sample Size,. and Cost

In discriminating among candidate t e s t methods, cost a s well a s precision must be considered. F o r example, i f a i i s the s tandard deviation for a single measurement f rom te s t i, the standard deviation for a mean of N observations i s a i / m . This i l lustrates that the precision of any t e s t method can be increased to any level desired simply by increasing the number of repl icates , N. It i s t he re - fore necessary to consider testing costs and sample s ize in any test-discr imina- tion methodology. These fac tors can be easi ly incorporated into the relative sensitivity concept.

Mandel has shown that i f RS(M/N) = k, a single measurement by method M has the same precision a s the mean of k2 measurement by method N. Fur the r - more , he has shown that i f CM i s the cost of a single measurement for method M, and CN i s the cost for method N, then the ratio of costs , C(M/N), to achieve equal precision i s

Some examples a r e given in Table 10. In example 1, ' f o r equal costs and a sen- s i t ivi ty ra t io of 1, the cos t ra t io is 1. I£ the cost of t e s t M i s doubled with no change in sensit ivity r a t io , , the cost ra t io i s a l so doubled. In example 5, i f t e s t N c o s t s twice a s much as t e s t M, but t e s t M is twice a s sensit ive a s t e s t N, then one would have to spend 8 t i m e s a s much for t e s t N to get the same precision a s i n t e s t M. One is therefore able to evaluate various test ing methods not only on the b a s i s of precis ion but a l s o on the basis of cost.

TABLE 10. EXAMPLE. OF COST RATIO A T EQUAL PRECISION AS A FUNCTION OF TESTING COSTS AND SENSITIVITY RATIO

Example s

E x a m ~ l e Sensitivitv Analvsis

As p a r t of a study of the adhesion of t i r e cord to t i r e - c a r c a s s compound, the wi re adhesion t e s t (ASTM D2229) was evaluated with two different types of testing. One was a jig with a hole through which the wi res were pulled out of the vulcanized rubber compound. In the other , the s tandard jig had only a slot, and it was f ea red that 'the slot was interfering with rubber 'chunks stuck on the wire a s the wire was pulled through. .Accordingly', a sensitivity study was run on the two methods. The raw data and data 'means and standard deviations a r e given in- ~ a b l k 11 for four compounds se led ted so that the full pract ical range of adhesion levels would be covered.

The f i r s t s t ep in the analysis was the regress ion of the data means for Tes t M onto those of Test N. The resulting equation was M = 8. 52 + 1.02 N. The derivative, dM/ dN; was then 1. 02. The pooled s tandard deviations were SM = 3.798 and SN = 3.379. .' he sensitivity of Test M with respec t to Test N, RS(M/N) was therefore 1. 02/(3.798/3.379) = 0.907. The quantity RS(N/M) was a l s o calculated and resul ted in a sensit ivity of 1. 04. Since the sensit ivit ies could not be judged s tahs t ica l ly different f r o m 1.0, it was concluded that the type of t e s t jig was not an import,ant variable.

TABLE 11. RAW DATA, MEANS AND STANDARD DEVIATIONS FOR CALCULATION O F SENSITIVITY FOR THE WIRE-ADHESION EXAMPLE

Wire Adhesion, lb/in. Compound A B C D

Hole J ig -- Tes t M

Raw 66.93 58. 73 45.47 89.33 68.33 55.33- 37.73

Data 92.0

72.8 55.73 50.73 91.6

Mean 69.35 56.60 44.64 90.98 Std. Dev. 3.066 1.858 6.539 1.440

Slot J i g -- Tes t N

Raw 49.67 46.67 39.47 79.27 52.6 53. 53 38.53 81.27

Data 56.6 51.93 33.40 85.47

Mean 52. 96 50. 71 37.13 82.00 Std. Dev. 3.479 4.996 3- 267 3. 164

Sta t is t ica l E x ~ e r i m e n t a l Design f o r Determining Sensit ivi tv Ratio fo r Two Tes t Methods

~ 0 t h - o f Which Measure P r o ~ e r t v Degradation

The goal of a n exper iment to de te rmine sensi t iv i ty r a t i o f o r two t e s t s i s the determinat ion of the r eg re s s ion equation of t e s t M r e s u l t s on those of t e s t N, a s wel l as the s tandard deviations of e a c h t e s t with a s much prec i s ion a s possible and a t a reasonable cost . Without p r i o r knowledge of the f o r m of the mathemat ica l re la t ionship between the two t e s t s , a t l e a s t four l eve l s of the p rope r ty being m e a - sured , should be evaluated, p re fe rab ly five ar six. These points should be s p r e a d ove r a s wide a pro'berty range a s poss ible s o that compar i sons wi l l be valid a t any t e s t l eve l ( s ) l ikely to be found in the future.

In Study 4 we want to d i sc r imina te among t e s t s which can follow r a t e of de- gradation. A reasonable design would be the following in which degradation of a plas t ic shee t i s used a s a n example , along with m e a s u r e m e n t s f r o m two nonde- s t ruc t ive t e s t s . Suppose one d e s i r e s s i x l eve l s a t which to evalute the two t e s t s . At each level a min imum of t h r e e rep l ica tes a r e r equ i r ed ; s o one r e q u i r e s a min imum of 3 x 6 = 18 sample s of the p las t i c sheet . P r e f e r ab ly , these should be taken from r andom areas of the i na t e r i a l t o be fully rcprcscn ta t ive . Three

, r

samples a r e randomly selected f rom the 18, and the remainder a r e put in equiva- lent positions in a Weatherometer. At the end of each of five weathering periods, t h r e e of the samples a r e randomly withdrawn f rom the chamber. The total weath- er ing period should be l ~ n g enough to allow nearly complete degradation of the plastic. If th is t ime is unknown, more samples will be required so that sample- withdrawal t imes can be determined in a sequential manner.

After all 15 ( o r m o r e ) samples have been withdrawn f rom the Weatherometer, they should be combined with the original three samples and tested in random o r - der by both methods. F o r each sample, a coin should be flipped to determine which of the two t e s t s t o run f i rs t . This allows for possible changes in the sample due t o testing.

The f i r s t s tep in analyzing the data is to calculate the means and standard deviations f o r each group of replicates for each test. Bartletts tes t (3) for .homo- geneity of variance i s then applied. If heterogeneity is not found to .be significant, the s tandard deviations fo r each tes t a r e pooled for a single estimate for each - -

tes t , i. e. , SM and SNi A regression analysis is then run for the means of tes t M onto those of t e s t N. First, a l inear relationship should be t r ied a s follows:

where bo and b l a r e fitted intercept and slope, respectively. If f i t i s not adequate a s judged by calculation of a n I? ratio for lack of f i t ( 4 ) , a quadratic equation should be fit ted a s follows:

and evaluated for lack of fit. The p r o c e . 5 ~ is.continued until adequate fit i s found. Once a n adequate equation i s obtained, the derivative of M with respect to N i s taken, and the relative sensitivity calculated f r o m Equation (4). If Equation ( 19) gave a n adequate fit, RS(M/N) will be a constant, otherwise it will be a function of N. The design and analysis a r e easi ly extended to cover any number of tes ts if the RS values a r e a l l constant. If they a r e not, l inear and.nonlinear program- ming methods a r e indi=ated.

Concluding Remarks

The relative sensitivity appears . to be a n .excellent cr i ter ion for discr imi - nating among various t e s t .methods. Mandel's resul ts indicate that such discrim- ination can be made not only on the basis of precision, but a lso with testing costs taken-into consideration. This methodology appears to be well suited to the goals of Study 4 and will be incorporated into the overal l methodology for designing ab- breviated and accelerated tests .

PLANS FOR NEXT QUARTER

Two principal tasks remain on Study 4: (1) finish the development of the methodology by treating sample selection, testing sequence, and selection of s t r e s s e s related to specific environments, and (2 ) relating the methodology to a specific example of the design of a n aging test . Following these tasks , the draft of the final report on Study 4 will be prepared.

REFERENCES

(1) Mandel, John, "The Statist ical Analysis of Experimental Data", Interscience, Chapter 14 (1964).

( 2 ) Mandel, J., and Strehler , R. D., "Precis ion Measurement and Calibration -- Statist ical Concepts and Procedures" , NBS Special Publication 300, Vol. 1, p 179.

(3) Dixon, N.. J . , and Massey, F. J . , "Introduction to Statist ical Analysis", Third Edition, McGraw Hill, p 308 (1967).

(4) Draper, N. R e , and Smith, H. , "Applied Regression Analysis", John Wiley and Sons, Inc., New York, New York, p 26 (1966).

N E W T E C H N O L O G Y

* .' No items of new technology have been reported thus far in Studies 3 and 4. . .