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Ethylene-Vinyl Acetate 

Potential Problems for 

Photovoltaic Packaging 

Preprint 

M.D. Kempe, G.J. Jorgensen, K.M. Terwilliger, 

T.J. McMahon, and C.E. Kennedy 

National Renewable Energy Laboratory 

T.T. Borek 

Sandia National Laboratories

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39915 

May 2006 
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ETHYLENE-VINYL ACETATE POTENTIAL PROBLEMS FOR PHOTOVOLTAIC 

PACKAGING* 

Michael D. Kempe1

, Gary J. Jorgensen1, Kent M. Terwilliger1, Tom J. McMahon1, Cheryl E. Kennedy1, 

and Theodore T. Borek2

 1

 National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401 2

 Sandia National Laboratories, 1515 Eubank Blvd. SE, Albuquerque, NM 87123 

ABSTRACT 

Photovoltaic (PV) devices are typically encapsulated 

using ethylene-vinyl acetate (EVA) to provide mechanical 

support, optical coupling, electrical isolation, and 

protection against environmental exposure. Under 

exposure to atmospheric water and/or ultraviolet radiation, 

EVA will decompose to produce acetic acid, lowering the 

pH and increasing the surface corrosion rates of 

embedded devices. Even though acetic acid is produced 

at a very slow rate, it may not take much to catalyze 

reactions that lead to rapid module deterioration. Another 

consideration is that the glass transition of EVA, as 

measured using dynamic mechanical analysis, begins at 

temperatures of about –15°C. Temperatures lower than 

this can be reached for extended periods of time in some 

climates. Because of increased moduli below the glass 

transition temperature, a module may be more vulnerable 

to damage if a mechanical load is applied by snow or wind 

at low temperatures. Modules using EVA should not be 

rated for use at such low temperatures without additional 

low-temperature mechanical testing beyond the scope of 

UL 1703. 

INTRODUCTION 

Polymeric encapsulant materials are used in PV 

modules to provide electrical insulation and protect them 

from mechanical damage and environmental corrosion. A 

well bonded polymer can protect a surface by physically 

preventing the accumulation of water at its bonding 

interface. Water can enhance corrosion by providing a 

means by which by-products can more easily diffuse away 

from the surface to allow further corrosion. The presence 

of counter ions (such as Cl-

, Br-, or acetate) similarly 

enhance corrosion by facilitating the diffusion of metal ions 

[1]. Furthermore, acidic materials can catalyze the 

oxidation of metals. 

In some of the early work performed at the Jet 

Propulsion Laboratory in the 1980s [2,3,4] a number of 

encapsulant materials were investigated, and of those with 

 

* This work has been authored by an employee or employees of the Midwest Research 

Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. 

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this work, or allow others to do so, for United States Government purposes.

adequate mechanical and optical properties, EVA was 

chosen because it was inexpensive. EVA continues as 

the dominant encapsulant in the PV industry even though 

it suffers from non-ideal mechanical and thermal 

properties, a high diffusivity for water, the need for vacuum 

lamination in a semi-batch manufacturing process, and the 

production of acetic acid. As next-generation crystalline 

silicon wafers are manufactured thinner, the mechanical 

properties of EVA may not be sufficient, especially at low 

service temperatures [5]. 

Because of the success of EVA with silicon-wafer 

based technologies, it has often been assumed that the 

generation of acetic acid is not a problem [2]. Typical 

modules with “breathable” packages should be less 

affected by acetic acid than those built with impermeable 

front- and back-sheets (e.g., glass), which trap 

decomposition products within the package. This type of 

“non-breathable” package is commonly used in thin-film 

devices. This problem is exacerbated by the thinness of 

the device structures, enabling small amounts of surface 

corrosion to produce significant deleterious effects. 

Further experimentation is necessary to evaluate the 

effect of different encapsulants on the stability of thin-film 

devices to determine if the higher costs of other 

encapsulants can be justified by an increase in durability. 

In this work, we demonstrate that the hydrolysis of 

vinyl-acetate monomers results in the production of acetic 

acid, which can accelerate corrosion. We further explain 

how the mechanical properties of EVA are non-ideal 

because of the presence of both a melting point and a 

glass transition temperature (Tg) within the operating limits 

of a module. 

EXPERIMENTAL 

The effused gases from the thermal decomposition of 

EVA were collected using a heating apparatus and an ion 

chromatograph (IC) vial that contained a weighed amount 

of 4.8-mM KOH. This collection solution was tested using 

IC analysis to determine the acetic acid formation rate. 

Dynamic mechanical analysis was performed on a TA 

Instruments Ares Rheometer (equipped with an IGC 

Polycold Systems Inc. cryogenic refrigeration unit model 

#PGC-100 and the Ares forced convection oven) using a 

rectangular torsional testing fixture. A TA Instruments 

DSC Q1000 was used for differential scanning calorimetry 

(DSC). Damp heat (85°C and 85% relative humidity [RH]) 

exposure was conducted in a Blue M environmental 

1
chamber. Aluminum mirrors were produced in the 

sputtering chamber of a Pernicka multichamber vacuum 

deposition system. The final Al thickness was 

approximately 800 ± 25 Å. 

RESULTS AND DISCUSSION 

Acetic acid formation 

The results for the measurement of acetic acid 

formation in EVA are summarized in Fig. 1. The slope of 

the curve indicates an activation energy of 90 kJ/mol and 

extrapolating to the typical average module temperature, 

27°C [6], predicts 0.00331 (ng/min/g). 

1

10

100

0.0024 0.00245 0.0025 0.00255 0.0026 0.00265 0.0027

1/Temperature (1/K)

Acid Formation Rate 

(ng/min/g)

Fig. 1. Nanograms acetate per minute per gram EVA. Acetate 

was also detected at 80°C (1/K=0.00283), but was not quantified. 

Acetic acid has a pKa of 4.76, so it will tend to buffer 

solutions to pH~4.76 corresponding to a hydronium ion 

concentration of 1.74⋅10-5 mol/L. As an order of 

magnitude estimate of the time necessary to reach 

pH~4.76, we assume 3.31⋅10-12 gAcid/gEVA/min and that the 

ratio of acetic acid to water in the polymer is the same as 

would be found for a solution in equilibrium with the 

polymer. This second assumption provides a relationship 

between the chemical potential of the acetic acid in the 

polymer as compared to an aqueous solution. At 27°C, 

EVA has at most 0.00223 gH2O/gEVA [6], and at pH~4.76, 

there will be equal amounts of acetic acid and acetate ions 

(or 2.136⋅10-6 gAcid/gH2O). Under these conditions, it will 

take about 1 day to approach a chemical potential for 

acetic acid roughly equivalent to pH=4.76. This estimation 

ignores the large acetic acid production during lamination 

at ~140°C and the affects of UV radiation. A module 

constructed using EVA will quickly equilibrate to a pH less 

than 4.76 if it has an impermeable back-sheet, and to a pH 

between 4.76 and 7.0 if it has a breathable back-sheet. 

Aluminized glass 

Because Al is often used for electrical interconnects in 

PV modules, the ability of EVA to protect it was 

investigated. In Fig. 2a, aluminized glass substrates were 

laminated with EVA using a soda-lime glass back-sheet 

and were exposed to 85°C and 85% RH for 1003-h. In the 

backlit photograph, some corrosion is visible primarily 

around the edges where the highest concentration of 

water was seen. In Fig. 2b, a similar substrate was 

exposed to 85°C and 100% RH for 1003-h and slightly 

greater corrosion was seen. This demonstrates that 

corrosion rates with impermeable back-sheets are 

somewhat limited by moisture ingress rates from the 

sides. 

(a) 

(b) 

(c) 

Fig. 2. Aluminum mirror laminated with EVA after 1003-h of 85°C 

(a) with a soda-lime glass back-sheet and 85% RH exposure, (b) 

with a soda-lime glass back-sheet and 100% RH exposure, and 

(c) without a back-sheet and 85% RH exposure. 

2
Aluminized glass substrates were also encapsulated 

without a back-sheet material. When exposed to damp 

heat (85°C, 85% RH) for 1003-h, these breathable 

packages do not show significant corrosion of the Al (Fig. 

2c). However, when this same breathable construction is 

exposed to 85°C and 100% humidity (not shown), the Al is 

completely oxidized in about 750-h. This indicates that 

saturation is far more corrosive than 85% RH. 

When comparing Fig. 2a (impermeable back-sheet) 

with Fig. 2c (EVA only), it is apparent that the escape of a 

volatile component (presumably acetic acid) is enabling 

the Al coating to withstand damp heat much better. If 

acetic acid is allowed to escape easily as it is formed, then 

the pH of the encapsulant will remain between 4.76 and 7; 

but if it is trapped, the pH will be a little below 4.76, leading 

to faster corrosion rates. 

Aluminized glass substrates were also encapsulated 

with an impermeable glass back-sheet with EVA located 

only at the center and the edges so that the effects of 

encapsulation could be directly compared to bare Al (Fig. 

3). After 1003-h of exposure, the Al was almost 

completely removed in areas in contact with the EVA with 

very little corrosion detectable on the bare Al. In this 

configuration, the areas covered by EVA have a reduced 

barrier to corrosion relative to unencapsulated Al. EVA 

catalyzes corrosion processes presumably because of the 

presence of acetic acid and a low pH. 

Fig. 3. Aluminum mirror laminated with EVA after 1003-h of 85°C 

and 85% RH exposure with a soda-lime glass back-sheet having 

EVA located only in the center and the outer ~25 mm. 

To help quantify the effective chemical potential of the 

acetic acid produced in EVA, Al mirrors were exposed to 

the vapors coming off of saturated salt solutions containing 

varying amounts of acetic acid (Fig. 4a). This was 

accomplished using an excess of KCl in an aqueous 

solution containing up to 20% (by volume relative to the 

water content) acetic acid. At 85°C, this produced a vapor 

at approximately 79% RH. 

The pH was estimated using BAKER-pHIX pH indicator 

strips. At 1%, 4%, and 7%, a pH of approximately 3, 2.5, 

and 2.5, respectively, was obtained. For higher 

concentrations, the pH was at least 2 or lower. The control 

solutions (no acetic acid) did not oxidize significantly and 

had an appearance similar to Fig. 2c after 1024-h. 

Solutions containing 10%, 15%, and 20% acetic acid 

experienced severe corrosion after about a 1-day exposure 

at 85°C (Fig. 4a, right side). At 4% and 7%, 200 to 300 h 

were necessary to produce severe corrosion. Finally, the 

1% solution experienced a small but significant amount of 

oxidation after 1024-h exposure at 85°C (Fig. 4b). The 

amount of corrosion seen at 1% is similar to that 

experienced at the perimeter of the samples with 

impermeable glass back-sheets (e.g., Fig. 2a). Therefore, 

as a first-order approximation, the 10.2-cm-square samples 

with glass back-sheets can trap enough acetic acid at 85°C 

to produce an acid environment with a chemical potential 

approximately equal to a solution at pH~3. However, the 

pH was measured at room temperature; the rate of acetic 

acid loss vs. its production rate changes with temperature, 

which may affect the final effective pH attained. 

(a) 

(b) 

EVA 

Al only 

(no EVA) 

Fig. 4. (a) Aluminum mirror laminated with EVA only. Sample on 

left placed on top of a jar (mirror/EVA side down) with a saturated 

salt solution. The sample jar on the right contained 20% acetic 

acid. (b) Backlit photo of an aluminum mirror after 1024-h at 

85°C exposure on top of a jar containing a saturated KCl with 1% 

acetic acid. 

These findings are consistent with the fact that Al 

oxides become highly soluble in water at pH~2.4 and only 

slightly soluble at pH~4 [1]. This analysis demonstrates the 

large potential for EVA to produce a corrosive environment 

in a sealed package where acetic acid cannot escape. 

Rheology 

Because PV encapsulant materials provide 

mechanical support to the cells [2], rheological 

3
measurements were made to determine where the phase 

transitions are and their effect on the dynamic mechanical 

moduli. In the temperature range of 80° to –40°C, the 

moduli increased by a factor of about 500 (Fig. 5). This 

large change in mechanical properties is due to the 

presence of both a melting point and a glass transition at 

or near temperatures that are commonly experienced by a 

module. For frequencies of 100-rad/s, the Tg was 

measured to be –15°C, which is much higher than the 

values of around –40°C frequently reported in the 

literature [2]. These other sources typically measured the 

Tg using DSC, therefore we performed DSC 

measurements (at a heating/cooling rate of 10°C/min) and 

found the center of the transition at –33°C and –37°C 

when cooling and heating, respectively (Fig. 6). This kind 

of a discrepancy is common for polymers, because the 

two methods are measuring very different phenomena 

associated with a second-order transition [7]. Because 

one of the primary purposes of using an encapsulant is to 

provide mechanical support, the Tg measured using 

dynamic mechanical analysis is more relevant. 

1 E+4

1 E+5

1 E+6

1 E+7

1 E+8

1 E+9

-60 -40 -20 0 20 40 60 80

Temperature (oC)

G' (Pa)

0

10

20

30

40

50

60

70

80

90

Phase Angle (deg)

109

G'

δ

100 rad/s

 10 rad/s

 1 rad/s

 0.1 rad/s

-15oC

-19o

C

-23o

C -27o

C

104

105

106

107

108

Fig. 5. Dynamic mechanical analysis of cured EVA while heating 

the sample. 

0

10

20

30

40

50

60

-90 -70 -50 -30 -10 10 30 50 70 90

Temperature (oC)

Heat Capacity (mJ / oC)

Tg Cooling

-33.1 o

C

Tg Heating -36.8 o

C

Fig. 6. Heat capacity of cured EVA measured by DSC at a 

heating/cooling rate of 10oC/min. 

For many environments, a temperature of –15°C is 

often reached, making cells in EVA-based modules 

significantly more susceptible to breakage from sudden 

impacts and rapid flexing. PV modules are typically rated 

for use in environments as low as –40°C, but this may be 

too extreme. This low temperature is often based on 

passing a qualification test (UL 1703 [8]) where the 

temperature of a module is cycled between 90°C and 

–40°C. If high winds were experienced at such low 

temperatures, the flexing of a module could break the thin 

cells or debond interconnects. Inclusion of some 

mechanical bending at low temperatures would be a good 

addition to UL 1703. Without further testing, a low 

temperature rating of –15°C may be more appropriate for 

modules constructed using EVA. 

CONCLUSIONS 

Photovoltaic PV devices are typically encapsulated 

using EVA to provide them with mechanical support, 

electrical isolation, and protection against environmental 

exposure. During exposure to water, heat, and/or UV 

radiation, EVA will decompose to produce acetic acid, 

thereby lowering the pH and generally increasing surface 

corrosion rates. EVA also goes through a Tg beginning at 

about –15°C, making its use at low temperatures a 

significant concern. Despite these shortcomings, EVA has 

proven to be adequate for most terrestrial environments 

using silicon-wafer-based devices. Thin-film PV 

technologies, however, are constructed using layers that 

are only a few microns thick, making them potentially 

much more sensitive to surface corrosion than are siliconwafer-based technologies. 

ACKNOWLEDGEMENTS 

This work was performed under DOE Contract No. 

DE-AC36-99GO10337. 

 REFERENCES 

[1] T.E. Graedel, “Corrosion Mechanisms for Aluminum Exposed 

to the Atmosphere,” J. Electrochem. Soc., 136(4) (1989) 204-212. 

[2] E.F. Cuddihy, C.D. Coulbert, R.H. Liang, A. Gupta, P. Willis, B. 

Baum, “Applications of Ethylene Vinyl Acetate as an 

Encapsulation Material for Terrestrial Photovoltaic Modules,” 

DOE/JPL/1012-87. 

[3] E. Cuddihy, C. Coulbert, A. Gupta, R. Liang, “Electricity from 

Photovoltaic Solar Cells, Flat-Plate Solar Array Project,” Final 

Report, Volume VII: Module Encapsulation, JPL Publication 86-31 

Jet Propulsion Laboratory, Pasadena, CA (1986). 

[4] J.L. Koenig, F.J. Boerio, E.P. Plueddemann, J. Miller, P.B. 

Willis, E. F. Cuddiy, “Chemical Bonding Technology: Direct 

Investigation of Interfacial Bonds,” DOE/JPL-1012-120 (1996). 

[5] C.P. Chen, “Fracture Strength of Silicon Solar Cells,” DOE/JPL 

Publication 1012-32, Jet Propulsion Laboratory, Pasadena, CA, 

(1979). 

[6] M.D. Kempe, “Modeling of rates of Moisture Ingress Into 

Photovoltaic Modules,” Solar Energy Materials and Solar Cells, 

Accepted, in press (2006). 

[7] K.P. Menard, Dynamic Mechanical Analysis: A Practical 

Introduction, CRC Press, New York, NY (1999). 

[8] UL 1703, Standard for Safety, Flat-Plate Photovoltaic Modules 

and Panels. 

4