Determination of the gel content of cross-linked ethylene-vinyl acetate (EVA) based material

Transcription

1 Determination of the gel content of cross-linked ethylene-vinyl acetate (EVA) based material Method developed by Photovoltaik Institut Berlin AG (PI Berlin), based on soxhlet extraction 3 rd Ed, July 2011 Written by: Benjamin Lippke Romain Penidon Matthias Hanusch Photovoltaik-Institut Berlin AG Wrangelstraße 100 Germany D Berlin Photovoltaik-Institut Berlin AG Module technology Testing Consulting Development Research Wrangelstr Berlin Company site: Berlin Trade register: Amtsgericht Charlottenburg Nr. HRB B Managing board: Dr. Paul Grunow. Prof. Dr. Stefan Krauter. Dipl.-Ing. Sven Lehmann Head of Supervisory board: Prof. Dr. Rolf Hanitsch Fon: Fax: VAT No.: DE Swift-BIC: DRES DE BB IBAN: DE Bank account: Dresdner Bank BLZ Account: Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

2 Table of content: 1 EVA: encapsulant material of photovoltaic modules Introduction Recommendations for curing and curing-level by the Manufacturer Known problems with the EVA Gel content determination at PI Berlin Our offers Requirements Recommended testing procedures Advantages of our method Interpretation of the results Further recommendations Your EVA team at PI Berlin... 9 Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

3 1 EVA: encapsulant material of photovoltaic modules 1.1 Introduction In photovoltaic modules mostly an encapsulate material made of ethylene-vinyl acetate (EVA) is used. This encapsulate is produced as a film and is stored and delivered on rolls. The length and the width of the roles may vary. Most of the EVA producers recommended a storage below 30 C (optimum 22 C) and relative humidity below 50%, for a time not exceeding 6 months after the production, no direct sunlight and always wrapped tightly with the original packaging. A film cut in size should be stacked and used within 8 hours. The film is composed of a standard polymer EVA which is produced for example by DuPont. The polymer is a thermoplastic. The manufacturer of the film adds a curing agent and other chemicals. The curing agent is a peroxide which decomposes with the increase of the temperature and starts the reaction in the film. That should be the curing process in the laminator. When the curing process is finished the former thermoplastic becomes an elastomer which is not able to be melted anymore. The material is irreversibly cured. Fig. 1 Schematic curing process 1.2 Recommendations for curing and curing-level by the Manufacturer On the market there is a broad selection of different EVA-films form a wide selection of manufactures (Solutia, STR, Mitsui, DuPont, Bridgeston and many others). Each of the manufacturers offers different films with different curing properties and different recommended curing levels. These curing-level range from about 70% to 90% depending on the product. The manufacturer of the film states mechanical, chemical and optical properties in their datasheets which are for their recommended curing level. With different curing levels these properties might deviate from the data sheet. Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

4 1.3 Known problems with the EVA Evaporation of the curing agent before curing. o Due to wrong storage or mistakes on producer-side there is not enough curing agent left for the curing process. Too short curing time. o In order to achieve process optimization and increase the production, insufficient curing time can be selected. The result might be inhomogeneous curing across the modules, or homogeneous but only partial curing. Wrong curing parameters. o The temperature for curing is selected too high or too low. In combination or not with inappropriate curing time a partial curing might result or the material can be irreversibly damaged. Not uniformly cured. o Since the development ultra fast, fast, or similar curing sheets in combination with the increasing of module sizes the curing process might not be completed across the whole module. We found that the curing level could deviate up to 20% between the positions. The problem is the time difference between the curing temperature reaching the centre and the corner of the module. In addition thermal stress might result in bending of the module, which lifts parts of the module from the laminator and slows the heat flow. Error by the supplier o It could happen that the supplier does not put enough peroxide in the EVA, stores it too long or just mixes good EVA with bad EVA. o Not uniformly distributed curing agents and other chemicals in the foils cause local different properties. In highly optimized process it might leads to only localized curing. 2 Gel content determination at PI Berlin There is no standard about the determination of the gel content of cross-linked EVA based material and furthermore no pass criteria for this test. The used method was developed and became the standard procedure to determine the gel content at PI Berlin. The method and results are accepted from the leading credit institutions. 2.1 Our offers Gel-content determination through soxhlet-extraction. o This method proved to be the best way to eliminate the most uncertainties. The thermal history of the material after curing is not relevant. The method does not depend on a peroxide rest in the foils. PI Berlin developed a method which is suitable for all EVA foils so far. Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

5 o In addition it s also possible to analyze EVA samples with most of the other extraction procedures which are used by different manufacturer or suppliers. If you need the test in a special way or want an exact duplicate of the procedure of your manufacturer or supplier ask for an offer. Analysis by differential scanning calorimetry (DSC). o This method is not made at PI Berlin; we subcontract it anonymously to a third party and discuss the results with you. For this test the thermal history of the module has to be known and there has to be enough peroxide left to ensure a full curing. 2.2 Requirements Information o The maximum of information about the EVA and lamination process need to be provided, so that reliable results can be concluded and further recommendation for a better gel content can be provided. Due to different EVA qualities the extraction process can be different and former information would helpful to save time and money. Amount of the specimen o For the extraction an amount of approx 15gramms cured EVA is needed. Finished modules can be provided from which the samples of EVA will be prepared. This is a destructive process, since the module will be opened to prepare the EVA samples (two squares of 18cm²). The basic offer includes the test on two positions: center of the module and upper left corner. A complete mapping of the modules can be offered on up to nine positions. o For the DSC analysis just a few grams are required. It is the best for the process flow if the samples can be send in small sheets, were the test specimen can be collected from. The specimen itself will be just a few milligrams. Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

6 2.3 Recommended testing procedures Standard (minimum): Two positions, six to eight samples of each position. Full (special request): Up to nine positions are analyzed with six to eight samples of each position. This way it is possible to draw a gel-content map. It is possible to start the test with two positions and depending on the results to decide later to test more positions. If position 1 and position 2 have a curing grade of almost 90% then it is almost certain that the other positions will not deviate much from that result. J-box Fig. 2: Tested position on the PV module, possibility of gel content mapping up to 9 positions 3 Advantages of our method PI Berlin is working on this method now for more than three years and gathered information regarding its performance with customers and in test series. Providing enough samples and information about the tested material, our method gives results within an error of ±3% in a 95% confidence interval. Further it is possible to determine the curing level on different positions and therewith to produce a curing map of the module. The positions are tested usually 6 times: 2 times with a short extraction time (6 hours i.e. 45 cycles) and 4 times with long extraction time (11 hours i.e. 75 cycles). If the deviation between these extraction times is too high, we repeat the test using an extended extraction time (22h i.e. 150 cycles). It is relevant to repeat the test with 150 cycles: For samples with a gel content of min. 75% after 75 cycles and a maximum deviation to 45 cycles of 4%. Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

7 For samples with a gel content between 75% and 67% after 75 cycles, a maximum deviation to 45 cycles of 3%. Indeed with a longer extraction time the gel content could fall below our pass criteria (see Fig. 5: interpretation of the test results [page 8]). The method used is an extraction according to soxhlet using THF (tetrahydrofuran) as a solvent. This solvent has proven to give the most reliable results. With this extraction process it is possible to stay below any curing temperatures. That prevents further curing of the material during the extraction. Also this method does not need any inhibitors like BHT. Other solvents like xylene (dimethylbenzene) and toluene (methylbenzene) could also be used but they may cause a temperature problem. The boiling points are too high which increase the possibility to induce further or other unwanted curing reactions during the extraction. The filters which are used in the apparatus are having a mesh opening of 1.0mm² and a wire diameter of 0.3mm. During the sample preparation the problem of sticking dirt on the EVA appears. The dirt can be paste, rest of the cells and/or rests of the back sheet. In a test series it was found that the error produced by remaining paste is marginal and lies far within the deviation between the measured samples of one position. However rests of cells needed to be removed completely to avoid high failures during the analysis. 4 Interpretation of the results The curing of the EVA can be typically approximated as a reaction of the first order. A typical curing graph is shown in Figure 3. Gel-content [ ] 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 Gel-content over curing time Curing time [min] Gel-content Fig. 3: Gel content vs. curing time of standard cure EVA There is no standard about the determination of the gel content of cross-linked EVA based material and furthermore no pass criteria for this test. Nevertheless, since PI Berlin been working on this test, results from 20% to 90% obtained and nearly Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

8 65% of the 254 measured samples from several module producers have a gel content above 67% (see figure 4). Fig. 4: Overview of the distribution of 254 measured samples (positions), around 120 Modules from several producers Occasionally PI Berlin receive modules with a gel content of 0% - 10% on at least one position. Low gel-contents usually produce high deviations. However the difference between the positions does not depend on the gel-content level. We found position related differences up to 20%. Figure 5 shows types of EVA samples with different gel contents analyzed by PI Berlin. Gelcontent [%] Types of cured EVA Typ [ ] [X] >67% Good 50%< [X] <67% Critical 20%< [X] <50% partially cured [X] <20% not cured Fig. 5: Types and categories of cured EVA, this is based on PI Berlin analysis Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9

9 5 Further recommendations Combination with peel off test To get a good idea of how the encapsulant performs we recommend the combination with a peel off test. Comment: It is possible to have a curing level of 100% and still not sticking to the solar cell and/or the back sheet. This can be caused due to e.g. the wrong application of the aluminum paste on the back side of the module or some aerosols which are present in the factory atmosphere which have a separating characteristic. 6 Your EVA team at PI Berlin Romain Pénidon, M.Sc. phone: +49 (30) penidon@pi-berlin.com Matthias Hanusch, M.Sc. phone: +49(30) hanusch@pi-berlin.com Benjamin Lippke phone: +49 (30) lippke@pi-berlin.com Photovoltaik-Institut Berlin AG EVA Gel content determination/ 3 rd Ed, July /9