Curing Kinetics of EVA Using DSC, DMA and Model Free Kinetics

The use of solar panels is well-known for converting sunlight to electricity.

This so-called photovoltaic electrical power is expected to make an important contribution to providing a sustainable supply of energy in the future.

Introduction

A photovoltaic module consists of arrays of jointly connected solar cells. An important step in the manufacture of a photovoltaic module is encapsulation. In this production step, solar cells are encapsulated between a glass sheet and a Tedlar film as backing sheet.

Encapsulation is commonly performed using a 0.4-mm thick ethylene-vinyl acetate (EVA) film. It seals the module and protects it against environmental influences such as moisture, oxygen, and weathering. This is very important because a guaranteed lifetime of 25 years is nowadays usual. In this article, we show how DSC and DMA experiments followed by evaluation with model free kinetics were used to investigate the curing behavior of EVA during the lamination process. Studies like this allow the optimum lamination conditions to be determined; the results can also be used for quality control.

In modern photovoltaic modules, the solar cells are encapsulated between a glass sheet and a backing sheet, usually a Tedlar^® film [1, 2, 3], as illustrated in Figure 1. Sheets of ethylene-vinyl acetate (EVA) are placed between the solar cells and the backing sheet and the glass. In the production process, the sandwich is pressed into place and heated. The EVA cures and provides a permanent and tight seal. 

EVA has many excellent long-term properties such as its optical transmittance in the visible region, chemical resistance toward UV light, and electrical insulation. EVA is a block copolymer and in this application typically consists of 67% polyethylene and 33% vinyl acetate (see Figure 2). Uncured EVA is a thermoplastic material that on heating first exhibits a glass transition and then a melting process. In the curing process, the EVA chains underg§o crosslinking.

The curing reaction is initiated by a peroxide compound. This decomposes on heating and splits into two oxyradicals that promote the crosslinking of the EVA polymer. EVA only becomes mechanically and chemically resistant at the high temperatures that occur in photovoltaic modules (up to 80 °C) after this curing process.

It is important to determine the degree of cure of the EVA in order to optimize the lamination conditions and control the quality control of the finished photovoltaic modules. In the past, this was done using a method based on solvent extraction. Recently it was shown that DSC measurements can also be used [4].

In this article, we show how the curing reaction can be described by model free kinetics using data from DSC and DMA measurements. This allows the lamination process to be optimized with regard to temperature and lamination time. 

Conclusions

The kinetics of the curing reaction of EVA that occurs during the lamination of photovoltaic modules can be investigated using DSC and DMA measurements. This is done by performing measurements at different heating rates and evaluating the results using the model free kinetics method. The results show that both techniques can be used to describe the curing process of EVA.

The predictions made from model free kinetics about the isothermal curing behavior were confirmed experimentally. Both DSC and DMA measurements are suitable for the optimization of process parameters for the lamination of photovoltaic modules. DMA yields additional information with regard to the viscoelastic behavior of the EVA film in the finished photovoltaic modules.

The two techniques can be equally well used for the quality control of laminated photovoltaic modules. 

_{Curing Kinetics of EVA using DSC, DMA and Model Free Kinetics | Thermal Analysis Application No. UC 312 | Application published in METTLER TOLEDO Thermal Analysis UserCom 31}