Curing treatment of ethylene vinyl acetate used for solar module

Abstract. Ethylene vinyl acetate (EVA) is commonly used as the encapsulant in the manufacture of solar modules. To withstand harsh environmental conditions, EVA must be cured in properly thermal treatment. This paper will present and discuss heat treatment of EVA and the extent to which EVA is cured (gel content) using heat treatment, and its correlation to solubility and differential scanning calorimeter (DSC) curves. Based on the correlation between the DSC and solubility techniques performed on a given EVA, the treatment needed to obtain sufficient curing can be deduced. EVA must be at least 80% cured to be acceptable for making solar modules, thermal treatment in the curing of EVA (JCC 105) must be done at 140 - 150 ◦C with a curing time of 10 - 20 minutes.

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JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2014, Vol. 59, No. 9, pp. 51-58 This paper is available online at CURING TREATMENT OF ETHYLENE VINYL ACETATE USED FOR SOLAR MODULE Nguyen Thuy Linh2, Nguyen Truong Minh3, Nguyen Trong Tung1, Nguyen Duc Thien1 and Duong Ngoc Huyen1 1School of Engineering Physics, Hanoi University of Science and Technology 2Faculty of Engineering and Technology, Pham Van Dong University, Quang Ngai 3Institute for Industrial Policies & Strategies, Hanoi Abstract. Ethylene vinyl acetate (EVA) is commonly used as the encapsulant in the manufacture of solar modules. To withstand harsh environmental conditions, EVA must be cured in properly thermal treatment. This paper will present and discuss heat treatment of EVA and the extent to which EVA is cured (gel content) using heat treatment, and its correlation to solubility and differential scanning calorimeter (DSC) curves. Based on the correlation between the DSC and solubility techniques performed on a given EVA, the treatment needed to obtain sufficient curing can be deduced. EVA must be at least 80% cured to be acceptable for making solar modules, thermal treatment in the curing of EVA (JCC 105) must be done at 140 - 150 ◦C with a curing time of 10 - 20 minutes. Keywords: EVA, curing, laminator, solar module. 1. Introduction Solar cells fabricated from inorganic semiconductors as Si or CIGS are normally in the form of fragile planar wafers or thin films which are easily broken by mechanical impact. In addition, solar cells are affected by variety of physical and chemical agents in the open environment [6]. These impacts can damage or deteriorate solar cells. In order to protect solar cells from environmental impacts over a long period of time, encapsulating and packing solar cells in solar modules with specific materials is essential. Besides protecting solar cells in severe weather conditions, the encapsulating materials must also allow passage of solar radiation in a large range of wavelengths, it must be impervious to moisture and be able to resist oxidation. Received December 7, 2014. Accepted December 24, 2014. Contact Duong Ngoc Huyen, e-mail address: huyen.duongngoc@hust.edu.vn 51 Nguyen Thuy Linh, Nguyen Truong Minh, Nguyen Trong Tung, Nguyen Duc Thien and Duong Ngoc Huyen Solar modules are diverse in terms of size, shape, power, composition, etc. In terms of structure and materials, solar modules can be divided into three types as shown in Figure 1. According to this classification, solar modules have a common intermediate buffer layer with connected components of different physical properties such as glass, solar plates, protective layer, etc. Ethylene vinyl acetate (EVA), a copolymer of ethylene (Et) and vinyl acetate (VA), has been found to be superior to other materials due to its high transmittance (> 90% in a wide range of solar radiation spectrum), high reliability and availability, and making EVA a common choice as encapsulant in the manufacture of solar modules [1, 3-5]. To increase strength, EVA is usually heated to effect chemical cross-linking and adhesion of polymer chains (part of the curing process). Determining conditions under which EVA should be heat treated to obtain an appropriate degree of cure is key to the making of high quality solar modules [2, 7]. Figure 1. Cross section of common solar modules: (a) crystalline Si modules, (b) thin-film modules, (c) reversed thin film modules To obtain a resultant poor solubility of cured EVA, the degree to which the EVA must be cured is determined based on the "dissolution" of EVA in a suitable solvent. Fresh EVA is completely soluble in toluene or xylene; otherwise, cured EVA will not dissolve because of the polymer chain cross-linking and sticking. The proportion of insoluble adhesive (gel) that is found in toluene after EVA is dissolved in it is the degree to which the EVA is the cured (percentage of EVA gel). Measuring the components of EVA dissolution in the solvent is a direct technique but it is time-consuming, inconvenient. Errors in measurement and calculation are common because the accuracy depends on both the measuring instruments used and the manipulation of the substances. In terms of thermodynamics, differential scanning calorimetry (DSC) curves in Figure 2 showed that from 60 ◦C to 70 ◦C and from 120 ◦C to 200 ◦C an endothermic (enthalpy change∆H 0) reaction occurs in EVA respectively. The first endothermic reaction occurs during the melting process and the exothermic reaction occurs later in the curing process. Depending on the thermal treatment, the DSC curve likely indicates the fact that the enthalpy change (∆H) during the curing process 52 Curing treatment of ethylene vinyl acetate used for solar module is inversely proportional to amount of EVA gel (cured EVA). By comparing the ∆H in the DSC curve, the degree to which EVA is cured can be indirectly determined. In this study, the degree to which EVA is cured is measured by the solubility, while thermal DSC techniques are presented, compared and discussed. Figure 2. DSC curve of EVA scanning in the temperature range from -100 ◦C to 250 ◦C [4] 2. Content 2.1. Experimental procedure and method Fresh EVA (JCC-105) in the form of 0.4 mm thick film, used to manufacture solar modules of the Tienyang Co., China, is used as the starting material in the experiments of this study. The EVA samples, in rectangular shape 2.5 cm  3.7 cm, are placed on a glass substrate and heated to different temperatures: 130 ◦C, 140 ◦C, 150 ◦C and 160 ◦C, for 2, 5, 10, 15, 20 and 30 minutes, respectively. After heat treatment, the degree to which the EVA samples are cured can be determined by their solubility and by thermal DSC techniques. EVA gel is obtained in the dissolved portion in toluene as follows: place an EVA in the amount of w1 ( 0.1 - 0.2 g) into 10 mL of toluene solvent and stir in an ultrasonic vibrator at 60 ◦C for 3 hours. The solution is then filtered through filter paper (w2 in weight) and dried at 60 ◦C for 3 hours. Due to the presence of an insoluble fraction of EVA gel on the surface, the weight of the dried filter paper is increased to w3. The degree to which EVA is cured is calculated by the formula: %gel = w3 w2 w1 (1) For comparison, a DSC curve of the EVA samples is also taken by a DSC instrument (1 Star System, Hanoi Pharmaceutical University). The degree to which the EVA is cured 53 Nguyen Thuy Linh, Nguyen Truong Minh, Nguyen Trong Tung, Nguyen Duc Thien and Duong Ngoc Huyen can be determined from the Enthalpy change ∆H using the formula: %gel = ∆H0 ∆H1 ∆H0 (2) where ∆H0 and ∆Ht are enthalpy of EVA and cured EVA, respectively, in the curing process. The results of the determination of the degree to which EVA is cured as calculated by formula (1) and (2) are then compared and evaluated. 2.2. Results and discussion * Degree of curing determined by the solubility method The degree in which the EVA samples were cured, as determined by the solubility method in toluene, are shown in Table 1 and presented in Figure 3. The 130, 140, 150 and 160 degree curves correspond to the samples treated at the temperatures of 130 ◦C, 140 ◦C, 150 ◦C and 160 ◦C. From the experiment results we can see that the degree to which EVA products are cured will increase with increased heat treatment temperature and time. Table 1. The degree to which EVA is cured as determined by the solubility method in toluene Treated time (min.) 2 5 10 15 20 30 Curing 130 ◦C 6 10 13 18 23 35 degree 140 ◦C 17 30 48 62 66 70 (%) 150 ◦C 22 31 54 73 75 76 160 ◦C 40 62 77 78 80 81 The degree to which EVA is cured when it is heated at 130 ◦C increases upwards with increased processing time, for example about 35% after being heated for 30 minutes. Completing the curing process takes a long time with heat treatment at 130 ◦C. At 140 ◦C, the cure is 70% completed after being heat treated for 30 minutes. From 0 to 30 minutes the degree to which the EVA is cured increases linearly, after which time it increases more slowly as it nears saturation. Similarly, the degree to which EVA is cured when heated at 150 ◦C, increases rapidly from 0 to 15 minutes and then slows until 76% saturation is reached at 30 minutes. The same degree of curing saturation is observed with samples that undergo treatment at 160 ◦C. Thus, the higher the temperature the greater the speed at which the EVA is cure and saturation is attained (the amount of EVA gel is at a maximum). However, in terms of methodology, a direct determination of EVA gel is done in several stages: weighing, stirring, filtration, and drying, and in the process measurement errors tend to accumulate. In addition, if the EVA gel on the solvent and equipment surface is not removed completely, the degree measured will be lower than that of the theoretical calculations. 54 Curing treatment of ethylene vinyl acetate used for solar module Figure 3. The degree to which EVA is cured as determined by the solubility method * The degree to which EVA is cured as determined by the DSC spectrum The DSC curve can be used to complement the solubility method as follows: Determine the DSC of the fresh EVA sample and EVA samples treated at 140 ◦C for 2, 5, 10, 15 and 20 minutes, respectively. Scan each of them at a temperature of 10 ◦C to 200 ◦C at a rate of 10 ◦C/min. The DSC of the starting material is recorded as shown in Figure 4. Figure 4. DSC spectra of fresh EVA scanning from 10 ◦C to 200 ◦C As can be seen in the DSC curve, the onset of an endothermic reaction (the melting process) is observed at 42.48 ◦C, reaching a maximum at 52.99 ◦C. The exothermic reaction (EVA curing process) starts at 142.22 ◦C and reaches a peak at 164.36 ◦C. The process involves the formation of free radical from additives as well as EVA molecules following by polymer chain cross-linking and chain clicking. The area under the DSC 55 Nguyen Thuy Linh, Nguyen Truong Minh, Nguyen Trong Tung, Nguyen Duc Thien and Duong Ngoc Huyen curve is proportional to the heat emitted from the curing process and proportional to the amount of cured EVA formed. Accordingly, Figures 5 and 6 show DSC curves for EVA samples treated at 140 ◦C for 2 minutes and for 20 minutes. From the curves we can see similar behavior in the endothermic and exothermic processes. However, the amount of enthalpy change is lower. For example, the ∆H for the melting process in fresh EVA is -31.94 J.g−1 while the∆H for a sample heated for 2 minutes and 20 minutes is -27.59 J.g−1 and -23.90 J.g−1, respectively. The ∆H in curing reaction (exothermic) decreases from 12.54 J.g−1 to 8.89 J.g−1 and 1.35 J.g−1, respectively. The change in DSC curves of EVA treated at 140 ◦C for different amounts of time is shown in Figure 7. The decreasing trend of ∆H in the curing region can be clearly observed from the curves. The∆H data due to DSC for EVA treated for different amounts of time is extracted and shown in Table 2. Figure 5. DSC curve for EVA heated at 140 ◦C for 2 minutes Figure 6. DSC curve for EVA heated at 140 ◦C for 20 minutes 56 Curing treatment of ethylene vinyl acetate used for solar module Figure 7. DSC curves for fresh EVA and EVA heated at 140 ◦C for different amounts of time Table 2. ∆H and degree to which EVA is cured when treated at 140 ◦C for different amounts of time Time 0 2 5 10 15 20 ∆H (J.g−1) 12.54 8.89 5.38 2.88 2.10 1.35 % gel (%) 0 29 52 77 83 89 Using the data for ∆H in Table 2, the degree to which EVA is cured can be determined using the following formula: %gel = ∆H0 ∆Ht ∆H0 (3) where ∆H0 is enthalpy of fresh EVA and ∆Ht is enthalpy of cured EVA treated for length of time t. The degree of cure that is calculated based on formula (3) is also showed in Table 2. Comparing this to the results obtained from the solubility method in Table 1, we can see a difference of about 20% in degree to which it is cured, with the higher value obtained from used of the DSC method. With respect to methodology, errors may arise depending on the experimental method used and instrument precision. The DSC method is considered to be more precise and its results could be used to calibrate the solubility method. Accordingly, the degree of cure in Table 1 should add 20% as calibration to measured value. Based on that consideration, an EVA sample treated at 160 ◦C for 15 minutes could attain a 100% cure. An 80% cure could be obtained after heat treatment at 140 ◦C, 150 ◦C and 160 ◦C for 20, 15 and 10 minutes of heating time, respectively. However, at higher temperatures, EVA viscosity is reduced causing the defect on the back 57 Nguyen Thuy Linh, Nguyen Truong Minh, Nguyen Trong Tung, Nguyen Duc Thien and Duong Ngoc Huyen protect layer. The best temperature for heat treatment is expected to be around 140 ◦C or 150 ◦C with a treatment time of around 20 minutes. 3. Conclusion The conditions under which EVA is heat treated have a strong effect on the degree to which EVA is cured and therefore the overall properties (mechanical, optical and chemical) of the EVA. Controlling the treatment temperature and heating time could enhance EVA quality, increase solar module efficiency and improve the solar module processing. For EVA (JCC 105) to be 80% cured, the ideal temperature was found to be 140 - 150 ◦C with a heating time of 10 - 20 minutes. Acknowledgments. This current work was financially supported by Project KC05-07/11-15. REFERENCES [1] K. Agroui, A. Belghachi, G. Collins, J. Farencd, 2007. Quality control of EVA encapsulant in photovoltaic module process and outdoor exposure. The Ninth Arab International Conference on Solar Energy (AICSE-9), Kingdom of Bahrain. [2] Claudio Messier, Roger Doudin, Markus Bucher, 2010. 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