G' (Pa) 

POTENTIAL PROBLEMS WITH ETHYLENE-VINYL ACETATE 

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 • Golden, CO 80401; 2 Sandia National Laboratories, Albuquerque, NM 87123 

Abstract Materials and Methods EVA Decomposes to Produce 

Photovoltaic (PV) devices are typically encapsulated using Acetic Acid (1) EVA was obtained from a commercial source and was well 

ethylene-vinyl acetate (EVA) to provide mechanical support, formulated for use in photovoltaic applications. 

1

(2) EVA acetic acid formation was evaluated by collecting the 

effused gases from the thermal decomposition of EVA 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.

Acid Formation Rate

(ng/min/g)

100

10

electrical isolation, optical coupling, 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 (3) Aluminum mirrors were produced in the sputtering chamber of 

using dynamic mechanical analysis, begins at temperatures of a Pernicka multichamber vacuum deposition system. The final 

about -15oC. Temperatures lower than this can be reached for Al thickness was approximately 800±25 Å. 

extended periods of time in some climates. Due to 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. 

EVA Produces Acidic Environments 

in Non-Breathable Packaging

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 extrapolate down to 27oC where EVA will produce 

3.31×10-12 gAcid/gEVA/min and assume 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 assumption provides a relationship 

between the chemical potential of the acetic acid in the polymer as 

compared to an aqueous solution. At 27oC EVA has at most 0.00223 

gH2O/gEVA [1], 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. 

A module constructed using EVA will quickly equilibrate to a pH less 

than 4.76 (probably around 2 to 3) if it has an impermeable back-sheet, 

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

[1] M. D. Kempe, Modeling of rates of Moisture Ingress Into Photovoltaic Modules, Solar Energy 

Materials and Solar Cells, Accepted 00 (2006) 000-000 

Alternative Encapsulants Reduce 

Rather than Accelerate Corrosion

Al only 

(A) 

Al / EVA 

Al / DC700 

(B) 

Al only 

Figure 4. Aluminum mirror laminated with (A) EVA (B) DC 700 acetic acid cure 

silicone after 1000 hours of 85°C and 85% RH exposure with a soda-lime glass 

back-sheet having polymer located only in the center and the outer ~25mm. 

Thermally Deduced Tg is Significantly 

Lower than the Mechanical Tg

(4) Dynamic mechanical analysis was performed on a TA 

Instruments Ares Rheometer using a rectangular torsional 

testing fixture. 

(5) A TA Instruments DSC Q1000 was used for differential 

scanning calorimetry (DSC). 

Aluminum Mirrors Oxidize When 

Acetic Acid is Trapped

(A) 

(B) 

Figure 2. Aluminum mirror laminated with EVA and a glass back-sheet after 

1003 hours of 85°C with (A) 85% RH exposure and (B) 100% RH exposure. 

Exposure to Acidic Solutions 

Quantifies Acetic Acid Exposure

(A) (B) 

Figure 5. (A) Aluminum mirror laminated using EVA without a back-sheet. 

The sample on the left was 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 hrs at 85oC 

exposure on top of a jar containing a saturated KCl with 1% acetic acid (pH ~ 

3). At 85oC this produced a vapor at approximately 79% RH. 

Low Temperature Mechanical Testing 

of Modules is Needed

1 E+9109 90 

0.0024 0.00245 0.0025 0.00255 0.0026 0.00265 0.0027 

1/Temperature (1/K) 

Figure 1. Nanograms Acetate per minute per gram EVA. Acetate was 

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

was performed using a humidified N2 carrier gas. 

Breathable Back-Sheet Allows Acetic 

Acid to Escape 

(A) 

(B) 

Figure 3. EVA on an Al mirror without a back-sheet, (A) 1003 h at 85oC/85% RH 

and (B) 357 h at 85oC/100% RH. Acetic acid escape is beneficial, but saturated 

water is very detrimental. 

A Low pH is Required to Oxidize the 

Aluminum Mirrors

The pH was estimated using BAKER-pHIX pH indicator strips. At 1%, 4%, 

and 7% acetic acid a pH of approximately 3, 2.5, and 2.5 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. 3a 

after 1024 hrs. Solutions containing 10%, 15% and 20% acetic acid experienced 

severe corrosion after about a 1 day exposure at 85oC (e.g. Fig. 5a Right Side). 

At 4% and 7%, 200 to 300 hours were necessary to produce severe corrosion. 

Finally, the 1% solution experienced a small but significant amount of oxidation 

after 1024 hrs exposure at 85oC (Fig. 5b). 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 

85oC to produce an acid environment with a chemical potential approximately 

equal to a solution at pH~3. 

These findings are consistent with the fact that Al oxides become highly 

soluble in water at pH~2.4 and are only slightly soluble at pH~4 [2]. This analysis 

demonstrates the large potential for EVA to produce a corrosive environment in a 

sealed package where acetic acid can not escape. More experimentation is 

necessary to determine the specific relevance to PV devices. 

[2] T. E. Graedel, Corrosion Mechanisms for Aluminum Exposed to the Atmosphere, J. 

Electrochem. Soc., 136(4) (1989) 204-212. 

Conclusions 

100 rad/s (1) During exposure to water and/or UV radiation EVA 

will decompose to produce acetic acid, thereby 

lowering the pH and generally increasing surface 

corrosion rates.

Heat Capacity (mJ / 

oC)

80 EVA DSC data 10 rad/s 1 E+8108 G' 

Phase Angle (deg)

70 1 rad/s

0.1 rad/s 60

1 E+7107 

50 Tg Cooling -23o

C -19oC 

-15o

C 40 (2) Thin film PV technologies are more likely to be -33.1 o

C -27o

1 E+6 106 C

sensitive to acetic acid induced corrosion than are 

crystalline wafer based technologies. 

30 

1 E+5105 δ 20

10

Tg Heating -36.8 o

C 

1 E+4104 (3) EVA goes through a Tg beginning at about -15°C, 0 

making its use at lower temperatures a significant -60 -40 -20 0 20 40 60 80

-90 -70 -50 -30 -10 10 30 50 70 90 Temperature (o

C) concern.

Temperature (oC) Figure 7. Dynamic mechanical analysis of cured EVA. The moduli increased by a 

Figure 6. Heat capacity of cured EVA measured by DSC at a heating/cooling rate factor of about 500 because of the presence of both a melting point and a glass (4) Without low temperature mechanical testing, a 

of 10oC/min. This is the usual method for determining the glass transition transition. For many environments, a temperature of –15°C is often reached, temperature rating of –15°C may be appropriate for

temperature (T ) even though mechanical methods are more relevant. making cells in EVA-based modules significantly more susceptible to breakage modules constructed using EVA. g

from sudden impacts and rapid flexing. 

The information contained in this poster is subject to a government license.

2006 IEEE 4th World Conference on Photovoltaic Energy Conversion • Waikoloa, Hawaii • May 7-12, 2006 • NREL/PO-520-39977

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