EN IEC 62788-1-6 Measurement Procedures for Materials Used in Photovoltaic Modules - Part 1-6: Capsules - Test Methods for Determining the Degree of Cure in Ethylene-Vinyl Acetate

The standard “EN IEC 62788-1-6 Measurement procedures for materials used in photovoltaic modules - Part 1-6: Encapsulations - Test methods for determining the degree of cure in ethylene-vinyl acetate”, which is based on a European standard and later published by the International Electrotechnical Commission (IEC), describes the terminology, test equipment, test environment, sample preparation, test procedures and test report for measuring the degree of cure of the ethylene-vinyl acetate (EVA) encapsulation layer used in photovoltaic (PV) modules.

Differential scanning calorimetry (both residual enthalpy and melting/freezing protocols) and gel content methods are included in this standard. This procedure can be used by material or module manufacturers to verify that the crosslinking additive is present and active. The procedure can also be used to verify the module manufacturing (lamination) process for quality and process control purposes. The procedure can also be used to evaluate the uniformity of the EVA formulation within a roll and to compare the variation of the EVA formulation from roll to roll. This procedure can be applied to uncured or recently cured EVA sheet and to uncured or recently cured EVA from PV modules. This test procedure can also be applied to crosslinked ethylenic copolymers other than EVA.

The temperatures determined for calorimetry measurements in this procedure are optimized for EVA. Therefore, if the test procedure is applied to other encapsulation materials, the range of test temperatures can be adjusted according to the active temperature of the curing agent or the melting/freezing temperature of the base material.

For the purposes of this standard, in addition to the terms and definitions given in the IEC TS 61836 standard, the following definitions apply:

• The degree of cure is the unitless parameter characterizing the extent of crosslinking within EVA. Unlike crosslink density, which is a physical quantity defined by the theory of rubber elasticity, the degree of cure in a polymer can be assessed by any experimental method that separates partially cured samples against each other or against a reference material. The degree of cure can be related to the gel content, which is the mass percent of insoluble material (assumed to be crosslinked) in the sample.
• Heat flux is the thermal flux across a given area in the direction of the temperature gradient.
• Differential scanning calorimetry, The thermoanalytical technique described in the ISO 11357-1 standard refers to the measurement of the difference in the amount of heat flow required to change the temperature of a material sample and a reference as a function of temperature or time. Both the sample and the reference are kept at almost the same temperature during differential scanning calorimetry characterization. Differential scanning calorimetry can be applied to measure the amount of heat generated or absorbed during the processing (curing) of EVA. The effects of crosslinking resulting from changes in the molecular structure of EVA can also be studied using differential scanning calorimetry at phase transitions (glass transition, melting point and crystallization temperature). The determination of phase transition temperatures is described in ISO 11357-2 and ISO 11357-3 standards.
• A differential scanning calorimeter is an instrument used to measure the heat flow difference between a test crucible (containing the sample) and a reference (usually empty) crucible.
• Gel content is the percentage by mass of polymer that is insoluble in a specified solvent after extraction according to specified test conditions. A gel typically consists of insoluble cross-linked material.

In principle, the degree of cure of EVA can be rapidly determined using a secondary method such as differential scanning calorimetry as described in this standard. Established alternative secondary methods or special equipment as described in the annex to this standard can also be used for direct production and quality control purposes (Annex A).

If the results of the secondary method are to be compared between different institutions, they should be calibrated using a slower, more universal primary method, such as the gel content test described in this standard, similar to the procedures described in ISO 6427, ISO 10147 and ASTM D2765.

The primary method can also be applied for research and development if the results of the secondary method are to be compared between EVA formulations. A test procedure for the primary method is described in this standard. The results of the primary or secondary methods can be correlated with modulus qualification tests (IEC 61215 series of standards) or additional field durability data to determine the minimum degree of cure required.

Examples of correlations between secondary and primary methods are included in Annex A. Differential scanning calorimetry measurements for EVA can be interpreted with respect to the enthalpy of the crosslinking reaction or to the characteristics of the melting/freezing transition as described in this standard. Since the melting/freezing transition does not depend on the concentration of the residual peroxide, the differential scanning calorimetry melting/freezing method can be applied to samples obtained from modules deployed in the field. Limitations of the primary and secondary methods are included in Annex A.

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