What is EVA film for photovoltaic modules?

EVA (Ethylene-Vinyl Acetate Copolumer) film is a functional film processed by adding crosslinking agent, coupling agent and anti-ultraviolet agent to resin based on ethylene vinyl acetate copolymer (commonly known as thermoplastic resin). .

1. EVA characteristics
EVA film will produce cross-linking and curing reaction under certain temperature and pressure, so that the battery, glass and backplane will be bonded into a whole, which can not only provide strong mechanical protection, but also effectively protect the battery from the erosion of the external environment. So as to ensure the normal use of solar cells in the long-term outdoor sun and rain. In the process of component lamination, one end of the coupling agent in the melted EVA is combined with EVA, and the other end is combined with glass to increase the interaction between the two.

The performance of EVA mainly depends on the content of vinyl acetate (expressed as VA%) and the melt index (Melting ldex, referred to as MI). The softer the better. The melt index M1 refers to the weight value of the thermoplastic plastic passing through a standard capillary within 10 minutes under a certain temperature and pressure. The melt index is used to describe the melt fluidity in the component packaging process. The larger the Ml, the better the EVA fluidity and the better the tiling. However, due to the small molecular weight, the tensile strength and elongation at break of EVA itself also vary. If it decreases, it is easy to tear after bonding, and the peel strength decreases. Since the reactivity ratio of VA monomer during copolymerization is much smaller than the activity of vinyl monomer, the MI of EVA resin with high VA content will not be too high, such as EVA with VA content of 33%, the minimum MI is about 25 , At present, the EVA resin suitable for photovoltaic packaging in the industry, the VA content is generally 28%~33%, and the MI is 10~100.

In order to ensure the reliability of components, the cross-linking rate (also known as cross-linking degree) of EVA is generally controlled at 75% to 90%. If the cross-linking rate is too low, it means that the EVA has not fully reacted, and the cross-linking reaction may continue to occur during subsequent outdoor use, with risks such as bubbles and delamination; if the cross-linking rate is too high, the subsequent use process will be There will be cracks, resulting in the occurrence of battery cracks and so on. Generally, EVA manufacturers will recommend a range of lamination parameters (Table 1), and component manufacturers can optimize and adjust them according to the actual situation during the production process.

Lamination temperature/℃Vacuum time/minLamination time/min
Table 1 Range of common PV module lamination process parameters

In addition to VA, M1 and degree of crosslinking, the shrinkage rate, light transmittance, volume resistivity, etc. of EVA are also key factors to measure whether it can meet the requirements of component production and use. In addition, yellowing resistance, water absorption, breakdown Voltage, etc., also need to be confirmed. After the components are made, various reliability tests such as DH1000 and TC200 should be carried out in accordance with the relevant retest guidelines of the IEC61215 standard.

If the shrinkage rate of EVA is too large, it will lead to battery fragmentation and local lack of glue during the lamination process, so it needs to be strictly controlled. Usually, the test method for EVA shrinkage is: cut a sample with a length of 300mm × width of 100mm, of which the length of 300mm is taken along the longitudinal direction of the EVA, and the sample is placed on a piece of glass with a size of 300mm × 300mm, and then the glass is placed flat at 120 ° C. On the hot plate, after 3 minutes, see the value of the change in the length direction. During the test, it should be noted that the EVA must be kept flat, and the melting should extend from the middle of the sample to both sides, otherwise the shrinkage test result will be inaccurate.

The light transmittance of EVA will directly affect the output power of the module. In order to prevent yellowing in the early EVA, an anti-UV agent was added to its formula, so the light in the ultraviolet band was almost cut off. Now, in order to improve the output power of the module, the front layer of EVA (that is, between the battery and the glass) EVA that allows light in the ultraviolet band to pass through can be used, and the output power of the module can be increased by about 1W. The back layer of EVA (that is, between the battery and the backplane) still uses EVA with anti-UV yellowing, which will also affect the anti-UV of the backplane. Performance puts forward higher requirements.

The volume resistivity of EVA plays a vital role in the insulation performance of the module. It not only affects the wet leakage index and various long-term reliability indicators of the module, but also the black lines (also known as snail patterns) and One of the main influencing factors of PID phenomenon. With the needs of the application side and the improvement of technology, the volume resistivity of EVA has increased from 1013Ω·cm in the early days to 104Ω·cm now. In order to achieve better anti-PID effect, some manufacturers have now achieved 1015Ω·cm or above. .

2. Production and preservation of EVA
The production process of EVA film can be realized by casting method or calendering method. The calendering method mainly follows the Japanese Bridgestone process, and adjusts the film thickness by adjusting the gap between three or four calendering rolls. The advantage is that the thickness is uniform and suitable for products with high melting point and low resin viscosity; while casting The method is adopted by most other manufacturers, and its advantages are that the resin has a wide range of applications and the processing parameters are easy to adjust.

The shelf life of EVA is generally 6 months. It should be stored in a dark and ventilated place, and the ambient temperature should not exceed 30 °C and the relative humidity should not exceed 60%. It is necessary to avoid direct sunlight and flame, and avoid contact with water, oil, organic solvents, etc. After taking out the material, the EVA should not be exposed to the air for a long time, and at the same time, the EVA should not be subjected to heavy objects and heat sources to avoid deformation.

3. EVA crosslinking degree test
The degree of EVA crosslinking is a very important technical index in the encapsulation process of photovoltaic modules. At present, there are two test methods for EVA cross-linking degree. One is xylene extraction method, which uses the property of EVA after cross-linking to be insoluble in xylene solution to calculate and test the cross-linking degree of EVA; the other is differential scanning method. Calorimetry (DSC). The latter was first proposed and popularized by Trina Solar, and it was the first time to submit a new standard proposal to IEC on behalf of China, which was unanimously approved by the IEC/TC82 expert group and formally established. It was officially released in March, becoming the first IEC standard proposed and dominated by China’s photovoltaic industry.

The two test methods are described below.
1) Xylene extraction method
The required instruments and equipment are: a large-mouthed round-bottomed flask with a capacity of 500ml with a 24# ground mouth; a reflux condenser with a 24# ground mouth; an electric heating mantle equipped with a temperature controller; an electronic balance with an accuracy of 0.001g; a vacuum Oven and stainless steel wire mesh bag: Cut a 120-mesh stainless steel wire mesh with a size of 120mm × 60mm, fold it into 60mm × 60mm, fold the two sides inward by 5mm twice and fix it to make a top opening with a size of 60mm ×40mm mesh bag; the required chemical reagent is xylene (AR grade).

What is EVA film for photovoltaic modules?
Figure 1 Schematic diagram of xylene extraction method

Sample preparation: The weight of the laminated sample to be tested is greater than 1g, the sample must be full and without holes, and the EVA film is cut into small particles with a size of about 3mm × 3mm.

The test process is:
(1) Wash and dry the stainless steel wire mesh bag, and weigh it as W (accurate to 0.001g);
(2) Take a sample (0.5±0.01) g, put it into a stainless steel wire mesh bag, make a sample bag, and weigh it as W1.
(accurate to 0.001g);
(3) After sealing the sample bag with a thin iron wire, mark it, insert it from the side opening of the large-mouthed flask and seal the bottle mouth with a rubber stopper, add xylene solvent to the flask to 1/2 of the volume of the flask, and make the sample bag Fully immersed in solvent. It was heated to about 140°C, and the solvent was boiled and refluxed for 5 hours. The reflux speed is maintained at 20~40 drops/min;
(4) After refluxing, take out the sample pack and hang to remove solvent droplets. Then put the sample package into a vacuum oven, control the temperature at 140°C, dry for 3 hours, and completely remove the solvent;
(5) Take the sample package out of the oven, remove the iron wire, put it in a desiccator to cool for 20 minutes, take it out, and weigh it as W (accurate to 0.001g).
(6) Calculate the test results, the degree of crosslinking is
In the formula η- degree of crosslinking, %;
W1 – weight of stainless steel wire mesh empty bag, g;
W2 – total weight of sample package, g;
W3 – weight of sample package after solvent extraction and drying, g.

2) Differential Scanning Calorimetry
Differential Scanning Calorimetry (DSC) is a thermal analysis method. The required equipment is a differential scanning calorimeter, as shown in Figure 2. By measuring the heat flow difference between the sample and the reference during the heating process, the purpose of DSC analysis is achieved. During the test, the sample is placed in a certain atmosphere, and its temperature is changed or maintained at a certain temperature, and the heat flow change between the sample and the reference is measured. When the sample undergoes physical changes such as melting, evaporation, crystallization, phase change, or chemical changes, there will be endothermic or exothermic heat change information in the map, and then the characteristics of the sample can be inferred. DSC can be used to accurately measure phase transitions (Tg, Tm, Tc), thermal changes, curing reactions and other chemical changes. When the material is crystallized or cross-linked, the degree of disorder inside the material is reduced, and the free energy also drops to a relatively stable state. Therefore, when the material is cross-linked or crystallized, it must be accompanied by an exothermic reaction.

What is EVA film for photovoltaic modules?
Figure 2 Differential Scanning Calorimeter

Figure 3 shows the DSC heat flow patterns of uncrosslinked and crosslinked EVA samples.

Figure 3 DSC heat flow spectra of uncrosslinked and crosslinked EVA samples

Key points for sample preparation: The size of the laminated sample to be tested (clinker) should be greater than 10mm×10mm, and it should be noted that the sample should be full and without holes; the sample size of the unlaminated raw material (raw material) should be 100mm×100mm.

The test process is as follows:
(1) Take out an empty standard pan and top cover, put the pan and top cover together in an electronic balance for weighing, and record the overall weight;
(2) Cut the clinker sample off the back plate and part of the EVA, and only keep the EVA clinker sample with a width of about 2mm near the glass;
(3) Cut the EVA raw meal and clinker into 2mm×2mm samples respectively, put them into the balance and weigh them. The sample weight is required to be 7g±0.5mg, and the sample weight is recorded;
(4) Put the sample into the tray, the sample should touch the bottom of the tray as much as possible, and then press the upper cover tightly with a tablet press;
(5) Put the pressed samples into the automatic sampler of the equipment in turn, and input the corresponding tray weight and sample weight in turn;
(6) Set the test conditions: confirm that the reference plate is in the correct position, the test temperature range is 80~230°C, and the heating rate is 10°C/min. After the setting is completed, click “Apply” below to save the settings, and then click “Start Measurement”;
(7) Data analysis: Change the status at the bottom right to Complete, the DSC furnace will automatically cool down to the set temperature, and take out the test plate through the autosampler; find the peak position of about 150°C on the curve, click “Integrate Peak linear”, and select The two points on both sides of the wave crest and the tangent to the straight line are the limits of the range;
(8) Record the thermal melting values ​​H1 and H2 of the raw and cooked samples.
To calculate the test results, the degree of cross-linking is
In the formula, η——crosslinking degree, %;
H1——The enthalpy value of uncrosslinked EVA solid;
H2——The fixed enthalpy of EVA after cross-linking.

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