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Vinyl Acetate Copolymer (EVA) by thermal analysis DSC and DMA
Polymer Testing 30 (2011) 236–242
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Characterisation
Investigation of Ethylene/Vinyl Acetate Copolymer (EVA) by thermal analysis DSC and DMA W. Stark*, M. Jaunich BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin, Federal Republic of Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Accepted 21 December 2010
Two types of commercially applied Ethylene/Vinyl Acetate Copolymers (EVA) for encapsulation of photovoltaic modules were investigated by the thermal analysis methods of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) in the temperature range from 150 C to 200 C. Glass transition, crystal melting and crosslinking were analyzed. The aims of the investigations were to gain more information for incoming goods control and to get information about the whole temperature dependent material properties in the investigated temperature range, starting at very low temperatures up to the crosslinking temperature region. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: EVA Thermal analysis DSC DMA Glass transition Crystal melting Cross-linking
1. Introduction Ethylene/Vinyl Acetate Copolymer (EVA) is used worldwide for encapsulation of photovoltaic modules. The following properties make EVA an outstanding material for encapsulation [1]: 1) High electrical resistivity which makes it a good electric insulator, 2) Low processing and cross-linking temperature, 3) Very low water absorption ratio, 4) Good optical transmission. The whole packaging of a photovoltaic module consists typically of a five-layer structure: glass front side/EVA for heat and environmental sealing/photovoltaic module/2nd EVA sealing film/back face protection. In this way the protection from environmental damage of the solar cells
* Corresponding author. Tel.: þ49 30 8104 1614; fax: þ49 30 8104 3328. E-mail address: [email protected] (W. Stark). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.12.003
circuit and electrical insulation are guaranteed. The steps in lamination are first melting of the EVA and bonding of the solar panel with the front side glass unit and the rear side. The back face protection is realized by either a glass or a multilayer sheet of Tedlar-Aluminium-Tedlar. The output box is also situated at the back side. After that step, the EVA is cross-linked by a thermally activated reaction with peroxides as crosslinking agent. A series of publications dealing with the properties of EVA in solar panels can be found [2–6]. The aim of our investigations was to demonstrate the competency of the thermal analysis methods DSC and DMA for incoming goods control and quality assurance in the technological process, as thermal analysis equipment, especially DSC, is often present in industry. From DSC measurements, crystal melting and crosslinking behaviour can be well investigated. The maximum endothermic heat release is correlated with the melting of the crystals of the most frequently appearing thickness [7–9]. The endothermic peak onset temperature and peak height give information about size and size distribution of crystals, the number of peaks gives an indication of different crystalline forms (often bimodal), and normalized enthalpy gives information about degree of crystallinity. Information
W. Stark, M. Jaunich / Polymer Testing 30 (2011) 236–242
concerning the cross-linking reaction can be gained from the exothermic process. The state of knowledge is summarised by the following information: 1.1. DSC-measurements starting at temperatures considerably below room temperature Vogel and Heinze [10] published DSC results from 23 C to 227 C. For non cross-linked material two peaks, one close to 50 C and one close to 70 C, and an exothermic double peak in the range from 140 C to 200 C are seen. For a cross-linked sample only a broad endothermic shoulder without a peak in the range 20 C–50 C followed by a distinct peak near 65 C appears. The exothermic peak is missing. Varghese et al. [11] showed a DSC curve from 80 C to 250 C. There is an endothermic step from about 40 C to 10 C passing into a first peak at about 50 C, which is followed by a second peak close to 85 C. Khonakdar et al. [12] published DSC curves for EVA copolymers and also for pure EVA in the range 75 C to 175 C. A peak near 85 C was found. For the blends, reduced crystallinity of the cross-linked material was reported. 1.2. DSC-measurements starting at or close to room temperature Shi et al. [13] have reported on DSC results in the range 0 C–140 C. A so called L-peak (lower temperature) and a H-peak were found which are attributed to the primary and secondary crystallization. Depending on the VAc content and the thermal treatment, the L-peak is in the range 40 C–60 C, the H-peak 80 C–90 C. Using WAXD it was found that many crystals became bigger and more perfect by thermal ageing. Marcilla et al. [14] published DSC results in a temperature range starting from 37 C to 157 C showing two endothermic peaks - a lower one near 47 C, a higher one near 70 C. Agroui et al. [15] investigated two EVA types designated slow and fast cure. DSC was measured from 40 C to 155 C. Two endothermic peaks were seen, a more pronounced one at 50 C and more a shoulder near 70 C. Reyes-Labarta et al. [16] measured mixtures of PE with EVA and also pure EVA. DSC results for EVA from 27 C to 227 C showed two endothermic peaks – one near 50 C and another about 70 C. An exothermic peak in the range 175–215 C appeared. After cross-linking, the lower peak had vanished. This was interpreted by the reduction in structural regularity by the created cross-links. Moly et al. [17] used DSC and WAXS. DSC performed in the range 40 C–160 C showed a broad shoulder from 40 C finished by a peak at 80 C. During cooling, a peak at 50 C followed by a wide shoulder was seen. For blends, a decrease in crystallinity by cross-linking was found which was explained by some hindrance to the ordered arrangement of the polymer chains. Dikobe and Luyt [18] investigated EVA-PP blends, and also both polymers separately, by DSC and DMA. In a DSC
237
experiment from 60 to 180 C, an endothermic peak close to 100 C was found. 1.3. DSC-measurements in the temperature range of crosslinking reaction For cross-linking of EVA, peroxide is added as curing agent. At temperatures above 160 C the peroxide decomposes. Free peroxide radicals abstract mainly the hydrogen located at the tertiary carbon of the acetate group of the vinyl acetate (VAc) co-monomers [10,16]. Often dicumyl peroxide (DCP) is used - Bianchi et al. [19]. The DSC heating rates were varied from 2.5 K/min to 40 K/min. For rates of 10 K/min and higher an analysable exothermic peak in the range 150 C–200 C was seen. Summarizing the DSC literature data, one can state that for EVA no results starting at very low and reaching up to crosslinking temperatures were found. Typical endothermic peaks caused by crystal melting are found whereby often two endothermic peaks appear: first peak around 40 C–60 C, second peak at 70–90 C. Sometimes, a broad endothermic shoulder starting from 40 C is observed. Cross-linking appears in the range 140 C–200 C. The measurement of cross-linked material in a second run shows changes in the endothermic peak shape. These changes are discussed as a consequence of the change in molecular structure by cross-linking. 1.4. DMA-measurements The DMA results of Dikobe and Luyt [18] measured at 1 Hz between 80 and þ80 C show a distinct decrease in storage modulus in the 20 C region. A second decrease appeared close to 40 C. The tand curves gave a first peak in the region 10 to 0 C, associated with the glass transition, and a second peak near 60 C. Varghese et al. [11] measured at 50 Hz. They found a decrease of storage modulus from about 30 to 0 C accompanied by a tand peak at about 10 C. From tand maxima, a glass transition temperature of 12 C for uncross-linked and 10.4 C for cross-linked material were given. Storage modulus, loss modulus and tand curves measured at 10 Hz were published by John et al. [20]. A sharp decrease in storage modulus at about 40 C and a second decrease at about 40 C were reported. The loss modulus had a first peak in the region of 130 C and a second close to 25 C. Tand showed maxima at about 130 C, 20 C and 50 C. The change in material properties in the temperature region around 40 to 0 C was brought about in connection with the glass transition. Tand maxima at 140 and 25 C and a distinct decrease in loss modulus from 40 C were found also by Khondakar et al. [21]. The increase of tand between 75 and 95 C was explained by Koshy et al. [22] as melting of crystalline segments. To summarize the DMA measurements, a first relaxation process appears in the 120 C range, followed with increasing temperature by a glass transition in the range 40 to 0 C. An additional decrease of storage modulus in
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the range from 40 C and a tand increase are explained by crystal melting. 2. Experimental 2.1. Material Two commercially available materials applied in photovoltaic module production were used: Material 1: SOLAR EVAÒ RC02B (curing type: fast) from Mitsui Chemicals Fabro [23]. Material 2: ETIMEXÒ VISTASOLARÒ 486.10 (fast cure) from ETIMEX Solar GmbH [24]. The materials were supplied as foils with a thickness of 0.64 mm (material 1) and 0.45 mm (material 2). For DSC measurements, small disks of about 8 mg were cut. For DMA, 6 mm wide cross-linked strips were prepared in a compression mould.
Fig. 2. DMA – loss modulus as function of temperature, material 1.
E00 ¼ jE jsind 00 E – storage modulus
2.2. Thermal analysis 2.2.1. Dynamic mechanical analysis In DMA, a sinusoidal mechanical excitation of the sample is executed whereby force, elongation and the phase shift between them are measured as a function of temperature. The complex modulus E* is calculated from the measured data and the geometry:
Edual cantilever ¼
l3 F 16 b h3 A
l – sample length, b – sample width, h – sample thickness, F – force, A – deflection. With the phase shift, the complex E* can be split into two components:
E0 ¼ jE jcosd 0 E – storage modulus
Fig. 1. DMA – storage modulus as function of temperature, material 1.
The loss factor tand is calculated from:
tand ¼
E00 E0
Characteristic changes of moduli E0 and E00 and tand caused by thermally activated molecular movements begin at definite temperatures. A DMA 242C, Netzsch, Germany, was used for the investigations with a dual cantilever holder and a sample length of 5 mm. The heating rate was 1 K/min and the amplitude 40 mm. During a single measurement run cycling between the 5 frequencies – 0.33; 1; 3.33; 10 and 33.3 Hz – was carried out automatically. 2.2.2. Differential scanning calorimetry DSC evaluates the change of temperature difference between a sample crucible and a reference crucible during non-isothermal or isothermal conditions. The difference is used to calculate the heat flow into or out of the sample. It can detect melting, evaporation, glass transition and also
Fig. 3. DMA – tand as function of temperature, material 1.
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239
Table 1 Material 1 - Loss modulus peak temperature of the glass transition for different frequencies. 00
Frequency, Hz
E peak temperature, C
0.33 1 3.33 10 33.33
32.1 30.4 28.4 27.2 25.0
occurrence of chemical reactions by the consumption or release of heat [9]. Its special advantage is that only a few mg of the material are required. A DSC 204 F1 Phoenix, Netzsch, Germany, was used with standard crucibles and manually perforated lids. All measurements were carried out in the non-isothermal mode with a heating and cooling rate of 10 K/min. 3. Results and discussion 3.1. Material 1
Fig. 5. DSC – heat flow in the 1st and 2nd run, material 1.
These values were used to determine an activation energy from the Arrhenius relation between frequency of the peak (f) and temperature [26]: Ea
3.1.1. DMA The DMA results are presented first because they will be embraced in the discussion of the DSC results. The DMA was measured for cross-linked samples only. The crosslinking conditions were: 30 min at 160 C using a heated compression mould. For better comparability, the results of the storage and loss modulus are shown in a normalized manner, where the values at 150 C were set to unity. The results for material 1 are given in Figs. 1–3. A small decrease in storage modulus together with a loss modulus and tan d peak in the range of 140 C can be explained by the secondary relaxation caused by side group or end group motion or backbone “crank-shift” type motions typical for many polymers [25]. The storage modulus curve shows a distinct decrease from 40 C. This change is accompanied by a peak in loss modulus and tand. There is a pronounced correlation between frequency and location of the peak. For the loss modulus, the values are shown in Table 1.
Fig. 4. DMA – Arrhenius plot of logarithm of measuring frequency versus 00 reciprocal E peak temperature, material 1.
f ¼ A e RT
Ea – apparent activation energy, R – gas constant. The correlation between ln(f) and 1/T is shown in Fig. 4. A linear relation was found. From the slope of the straight line, an apparent activation energy can be calculated. The activation energy found was 328 kJ/mol, a value in the range typical for a glass transition [27]. The temperature range of the glass transition is in accordance with published data in [11,18,20]. The next step in storage modulus between 40 and 60 C was not accompanied by a peak in loss modulus. This will be discussed together with the DSC data later. 3.1.2. DSC A basic raw material sample was measured in a 1st run from 150 to 200 C. The measurement was rerun with the now cured sample. The results for the 1st and 2nd runs are given in Fig. 5. At the beginning of the DSC measurement, the typical thermal imbalance between the crucibles must be adjusted
Fig. 6. DSC – temperature program.
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Fig. 7. DSC – heat flow results for the temperature program from Fig. 6, material 1.
[9]. Therefore, a settling region, in Fig. 5 from the start at 150 to 135 C, appears. From 115 to 95 C a double peak structure is seen. The reason for this is unknown. From its appearance it looks like the formation of an ordered structure (exothermic peak) followed directly by destruction of this structure (endothermic peak). An endothermic step from 40 to about 20 C is observed. Endothermic peaks appear in the 1st run at 47 and 72 C. Additionally, an exothermic peak at 158 C with an enthalpy of 14.9 J/g appears. A broad endothermic shoulder followed by double peaks in the range 50–100 C have also been reported in [10–17]. An endothermic peak is typically for crystal melting. Multiple endothermic peaks in DSC can be correlated to a lamellar thickness distribution – see for example [28]. The exothermic peak with a maximum at 158 C indicates the exothermic cross-linking reaction. Similar results were reported in [10,16,19]. The reaction enthalpy value is rather low in comparison to cross-linking of epoxy resins, where values in the range of some hundred J/g (400–600) are typical [29,30]. In the 2nd run the peak near 100 C is similar to the 1st run. The endothermic double peak between 40 and 80 C has
Fig. 8. DMA – storage modulus as function of temperature, material 2.
Fig. 9. DMA – loss modulus as function of temperature, material 2.
changed to a broad shoulder followed by a peak at 63 C. The higher the crystal thickness the higher is the melting temperature - Illers and Hendus [7]. Therefore, the change from a peak into a broad shoulder is an indication of a distribution of crystals with reduced thickness as a consequence of the thermal treatment in the first run. The exothermic reaction peak is missing in the 2nd run, indicating that the cross-linking process was finished. The step in the range 40 to about 20 C is in accordance with the glass transition discussed for DMA results. Similarity between 1st and 2nd runs show that the glass transition temperature was not noticeable changed by cross-linking. It is interesting that the DSC peak in the 2nd run at 63 C, indicating crystal melting, correlates well with the sharp decrease of the storage modulus of the cured sample in the area between 40 and 70 C. The change in the endothermic peak structure in the range from 30 to 80 C between the 1st and 2nd runs is indication that the crystal structure has changed. Some authors assume that this is a consequence of cross-linking [16,17]. To find out if the change in melting peaks in the area 30–80 C has its reason in cross-linking or thermal history, in an additional experiment, the 1st heating run
Fig. 10. DMA – tand as function of temperature, material 2.
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241
Table 2 Material 2 - Loss modulus peak temperature of the glass transition for different frequencies. 00
Frequency, Hz
E peak temperature, C
0.33 1 3.33 10 33.33
31.7 29.9 28.2 26.7 24.6
was stopped at 100 C, a cooling run and a 2nd heating run to 200 C were added followed by additional cooling and heating runs – the temperature program is illustrated in Fig. 6. All DSC curves are collected in Fig. 7. The clear change in the 30–80 C area from a double peak in the 1st heating to a broad shoulder followed by one peak in the 2nd heating comes from thermal treatment – heating to 100 C and cooling under definite conditions. This general change with the appearance of a broad shoulder instead of a peak is obviously not caused by crosslinking. However, in the 3rd run after cross-linking the remaining peak following the shoulder has shifted to lower temperatures. This effect comes from heating to 200 C including cross-linking and indicates an additional drop in crystal perfection. 3.2. Material 2 3.2.1. DMA The DMA results for material 2 are shown in Figs. 8–10. Generally, the curves are similar for both materials. Also, material 2 shows a significant decrease in storage modulus starting from about 40 C which, together with the peak in storage modulus, is typical for the glass transition. Just as for material 1, in the 50 C region a further drop in the storage modulus appears. This was discussed for material 1 to be caused by melting of crystalline regions. With material 2, the contact with the DMA clamp was lost behind 120 C and this also occurred when the measurements were repeated. Fig. 9 gives the course of the loss modulus. As with material 1, a peak in the area 130 to 150 C
Fig. 12. DSC – heat flow in the 1st, 2nd and 3rd heating run, material 2.
appears. The temperature values of the loss modulus peak are summarized in Table 2. From these data, an activation energy of 330 kJ/mol is calculated – a value nearly identical with that for material 1. The peak temperatures are very close to those of material 1 which means that the glass transition temperature is nearly identical for both materials. Fig. 10 illustrates the results for tand. The tand curves of material 2 resemble those of material 1 in shape but they show slightly higher values. 3.2.2. DSC In Fig. 11, DSC results for the 1st and 2nd runs are shown. The results of material 2 are similar to material 1. In the 1st run, a step followed by two endothermic peaks appears. The exothermic curing reaction is in the region of 130– 180 C, whereby there is indication of a double peak. The reaction enthalpy is found to be 16.0 J/g. For material 2 also, a separate thermal treatment with the temperature regime from Fig. 6 was applied. The results in Fig. 12 are also very similar to material 1 in Fig. 7. A difference in the 2nd run for material 2 is that an endothermic step at about 110 C appears for which we have no explanation. Also for material 2, the thermal treatment during 1st heating and 1st cooling causes the form of the endothermic peaks to change from two distinct peaks into a broad shoulder and only one peak, as for material 1. Also for material 2, the thermal treatment during 1st heating and 1st cooling caused a change in the DSC curve for the 2nd heating run. The two endothermic peaks in the 1st run have changed to a broad shoulder and only one remaining peak. This is similar to the results for material 1 and can be interpreted by a change in allocation of crystal size distribution as a consequence of both thermal treatment and of heating to 200 C including cross-linking. 4. Conclusions
Fig. 11. DSC – heat flow in the 1st and 2nd run, material 2.
Two commercially available types of EVA - SOLAR EVAÒ RC02B (material 1) from Mitsui Chemicals Fabro and ETIMEXÒ VISTASOLARÒ 486.10 (material 2) from ETIMEX Solar GmbH were investigated by DSC and DMA in the temperature range from 150–200 C. This wide temperature range, the synchronous application of DMA
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and DSC and the combination of the measurement of the original material with measurements of the material thermally treated by the measurement to 100 C and cured during the preceding measurement to 200 C go beyond previously published data. Both materials show very similar characteristic behaviour as a function of temperature which can be concluded as follows: – A glass transition appears in the range 40 to 20 C, – The glass transition is not shifted perceptibly by crosslinking, – For original, non thermally treated material two crystalline melting processes indicated by two endothermic peaks exist, – Crystal melting is accompanied by a dip in storage modulus, – For material heated to 100 C, cooled down and heated once more the endothermic double peak has changed into a broad endothermic shoulder followed by only one pronounced endothermic peak, – Cross-linking appears in the temperature region from 140 to 180 C, – After heating to 200 C including cross-linking the remaining endothermic peak shifted to lower temperature giving a hint that this process also has changed crystal perfection. Knowledge of the characteristic DMA and DSC curve progressions is of high interest for good incoming control of basic raw material. References [1] A. El Amrani, A. Mahrane, F.Y. Moussa, Y. Boukennous, Solar module fabrication, International Journal of Photoenergy (2007) Article Number: 27610. [2] T. Carlsson, P. Konttinen, U. Malm, P. Lund, Absorption and desorption of water in glass/ethylene-vinyl-acetate/glass laminates, Polymer Testing 25 (5) (2006) 615–622. [3] G. Oreski, G.M. Wallner, Evaluation of the aging behavior of ethylene copolymer films for solar applications under accelerated weathering conditions, Solar Energy 83 (7) (2009) 1040–1047. [4] S.I.A. Ayutthaya, J. Wootthikanokkhan, Investigation of the photodegradation behaviors of an ethylene/vinyl acetate copolymer solar cell encapsulant and effects of antioxidants on the photostability of the material, Journal of Applied Polymer Science 107 (6) (2008) 3853–3863. [5] K. Agroui, A. Belghachi, G. Collins, J. Farenc, Quality control of EVA encapsulant in photovoltaic module process and outdoor exposure, Desalination 209 (1–3) (2007) 1–9. [6] B. Lee, J.Z. Liu, B. Sun, C.Y. Shen, G.C. Dai, Thermally conductive and electrically insulating EVA composite encapsulants for solar photovoltaic (PV) cell, Express Polymer Letters 2 (5) (2008) 357–363. [7] K.H. Illers, H. Hendus, Schmelzpunkt und Kristallitgröße von aus Schmelze und Lösung kristallisiertem Polyäthylen, Die Makromolekulare Chemie 113 (1967) 1–22. [8] A. Frick, C. Stern, DSC-Prüfung in der Anwendung. Hanser Wirtschaft, 2006. [9] G.W. Ehrenstein, G. Riedel, P. Trawiel, Thermal Analysis of Plastics: Theory and Practice. Hanser Gardner Publications, 2004, ISBN:13: 9781569903629.
[10] J. Vogel, C. Heinze, Reactive processing of polyethylene and Poly (ethylene-co-vinyl acetate), Angewandte Makromolekulare Chemie 207 (1993) 157–171. [11] H. Varghese, T. Johnson, S.S. Bhagawan, S. Joseph, S. Thomas, G. Groeninckx, Dynamic mechanical behavior of acrylonitrile butadiene rubber/poly(ethylene-co-vinyl acetate) blends, Journal of Polymer Science Part B-Polymer Physics 40 (15) (2002) 1556–1570. [12] H.A. Khonakdar, S.H. Jafari, A. Haghighi-Asl, U. Wagenknecht, L. Haeussler, U. Reuter, Thermal and mechanical properties of uncrosslinked and chemically crosslinked polyethylene/ethylene vinyl acetate copolymer blends, Journal of Applied Polymer Science 103 (5) (2007) 3261–3270. [13] X.M. Shi, J. Zhang, D.R. Li, S.J. Chen, Effect of damp-heat aging on the structures and properties of ethylene-vinyl acetate copolymers with different vinyl acetate contents, Journal of Applied Polymer Science 12 (4) (2009) 2358–2365. [14] A. Marcilla, J.A. Reyes-Labarta, F.J. Sempere, DSC kinetic study of the transitions involved in the thermal treatment of polymers. Methodological considerations, Polymer 42 (12) (2001) 5343–5350. [15] K. Agroui, A. Maallemi, M. Boumaour, G. Collins, M. Salama, Thermal stability of slow and fast cure EVA encapsulant material for photovoltaic module manufacturing process, Solar Energy Materials and Solar Cells 90 (15) (2006) 2509–2514. [16] J.A. Reyes-Labarta, M.M. Olaya, A. Marcilla, DSC and TGA study of the transitions involved in the thermal treatment of binary mixtures of PE and EVA copolymer with a crosslinking agent, Polymer 47 (24) (2006) 8194–8202. [17] K.A. Moly, H.J. Radusch, R. Androsh, S.S. Bhagawan, S. Thomas, Nonisothermal crystallisation, melting behavior and wide angle X-ray scattering investigations on linear low density polyethylene (LLDPE)/ethylene vinyl acetate (EVA) blends: effects of compatibilisation and dynamic crosslinking, European Polymer Journal 41 (6) (2005) 1410. [18] D.G. Dikobe, A.S. Luyt, Morphology and properties of polypropylene/ ethylene vinyl acetate copolymer/wood powder blend composites, Express Polymer Letters 3 (3) (2009) 190–199. [19] O. Bianchi, R.V.B. Oliveira, R. Fiorio, J.D.N. Martins, A.J. Zattera, L.B. Canto, Assessment of Avrami, Ozawa and Avrami-Ozawa equations for determination of EVA crosslinking kinetics from DSC measurements, Polymer Testing 27 (6) (2008) 722–729 OK. [20] B. John, K.T. Varughese, Z. Oommen, P. Poetschke, S. Thomas, Dynamic mechanical behavior of high-density polyethylene/ ethylene vinyl acetate copolymer blends: the effects of the blend ratio, reactive compatibilization, and dynamic vulcanization, Journal of Applied Polymer Science 87 (13) (2003) 2083–2099. [21] H.A. Khonakdar, U. Wagenknecht, S.H. Jafari, R. Hassler, H. Eslami, Dynamic mechanical properties and morphology of polyethylene/ ethylene vinyl acetate copolymer blends, Advances in Polymer Technology 23 (4) (2004) 307–315. [22] A.T. Koshy, B. Kuriakose, S. Thomas, S. Varghese, Viscoelastic properties of silica-filled natural rubber and ethylene-vinyl acetate copolymer blend, Polymer-Plastics Technology and Engineering 33 (2) (1994) 149–159. [23] SOLAR EVAÒ RC02B: http://eu.mitsuichem.com/. [24] ETIMEXÒ VISTASOLARÒ: http://www.etimex-solar.com/. [25] Handbook of thermal analysis and calorimetry. in: S.Z.D. Cheng (Ed.), Applications to Polymers and Plastics, vol. 3. Elsevier, 2002. [26] J.D. Menczel, R. Bruce Prime, Thermal Analysis of Polymers, Fundamentals and Applications. John Wiley & Sons, 2009. [27] S. Vyazovkin, N. Sbirrazzuoli, I. Dranca1, Variation in activation energy of the glass transition for polymers of different dynamic fragility, Macromolecular Chemistry and Physics 207 (13) (2006) 1126–1130. [28] G.F. Shan, W. Yang, X.G. Tang, M.B. Yang, B.H. Xie, Q. Fu, Y.W. Mai, Multiple melting behaviour of annealed crystalline polymers, Polymer Testing 29 (2) (2010) 273–280. [29] H.Y. Cai, P. Li, G. Sui, Y.H. Yu, G. Li, X.P. Yang, S. Ryu, Curing kinetics study of epoxy resin/flexible amine toughness systems by dynamic and isothermal DSC, Thermochimica Acta 473 (1–2) (2008) 101–105. [30] G. Tripathi, D. Srivastava, Cure kinetics of ternary blends of epoxy resins studied by nonisothermal DSC data, Journal of Applied Polymer Science 112 (5) (2009) 3119–3126.
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Characterisation
Investigation of Ethylene/Vinyl Acetate Copolymer (EVA) by thermal analysis DSC and DMA W. Stark*, M. Jaunich BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin, Federal Republic of Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Accepted 21 December 2010
Two types of commercially applied Ethylene/Vinyl Acetate Copolymers (EVA) for encapsulation of photovoltaic modules were investigated by the thermal analysis methods of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) in the temperature range from 150 C to 200 C. Glass transition, crystal melting and crosslinking were analyzed. The aims of the investigations were to gain more information for incoming goods control and to get information about the whole temperature dependent material properties in the investigated temperature range, starting at very low temperatures up to the crosslinking temperature region. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: EVA Thermal analysis DSC DMA Glass transition Crystal melting Cross-linking
1. Introduction Ethylene/Vinyl Acetate Copolymer (EVA) is used worldwide for encapsulation of photovoltaic modules. The following properties make EVA an outstanding material for encapsulation [1]: 1) High electrical resistivity which makes it a good electric insulator, 2) Low processing and cross-linking temperature, 3) Very low water absorption ratio, 4) Good optical transmission. The whole packaging of a photovoltaic module consists typically of a five-layer structure: glass front side/EVA for heat and environmental sealing/photovoltaic module/2nd EVA sealing film/back face protection. In this way the protection from environmental damage of the solar cells
* Corresponding author. Tel.: þ49 30 8104 1614; fax: þ49 30 8104 3328. E-mail address: [email protected] (W. Stark). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.12.003
circuit and electrical insulation are guaranteed. The steps in lamination are first melting of the EVA and bonding of the solar panel with the front side glass unit and the rear side. The back face protection is realized by either a glass or a multilayer sheet of Tedlar-Aluminium-Tedlar. The output box is also situated at the back side. After that step, the EVA is cross-linked by a thermally activated reaction with peroxides as crosslinking agent. A series of publications dealing with the properties of EVA in solar panels can be found [2–6]. The aim of our investigations was to demonstrate the competency of the thermal analysis methods DSC and DMA for incoming goods control and quality assurance in the technological process, as thermal analysis equipment, especially DSC, is often present in industry. From DSC measurements, crystal melting and crosslinking behaviour can be well investigated. The maximum endothermic heat release is correlated with the melting of the crystals of the most frequently appearing thickness [7–9]. The endothermic peak onset temperature and peak height give information about size and size distribution of crystals, the number of peaks gives an indication of different crystalline forms (often bimodal), and normalized enthalpy gives information about degree of crystallinity. Information
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concerning the cross-linking reaction can be gained from the exothermic process. The state of knowledge is summarised by the following information: 1.1. DSC-measurements starting at temperatures considerably below room temperature Vogel and Heinze [10] published DSC results from 23 C to 227 C. For non cross-linked material two peaks, one close to 50 C and one close to 70 C, and an exothermic double peak in the range from 140 C to 200 C are seen. For a cross-linked sample only a broad endothermic shoulder without a peak in the range 20 C–50 C followed by a distinct peak near 65 C appears. The exothermic peak is missing. Varghese et al. [11] showed a DSC curve from 80 C to 250 C. There is an endothermic step from about 40 C to 10 C passing into a first peak at about 50 C, which is followed by a second peak close to 85 C. Khonakdar et al. [12] published DSC curves for EVA copolymers and also for pure EVA in the range 75 C to 175 C. A peak near 85 C was found. For the blends, reduced crystallinity of the cross-linked material was reported. 1.2. DSC-measurements starting at or close to room temperature Shi et al. [13] have reported on DSC results in the range 0 C–140 C. A so called L-peak (lower temperature) and a H-peak were found which are attributed to the primary and secondary crystallization. Depending on the VAc content and the thermal treatment, the L-peak is in the range 40 C–60 C, the H-peak 80 C–90 C. Using WAXD it was found that many crystals became bigger and more perfect by thermal ageing. Marcilla et al. [14] published DSC results in a temperature range starting from 37 C to 157 C showing two endothermic peaks - a lower one near 47 C, a higher one near 70 C. Agroui et al. [15] investigated two EVA types designated slow and fast cure. DSC was measured from 40 C to 155 C. Two endothermic peaks were seen, a more pronounced one at 50 C and more a shoulder near 70 C. Reyes-Labarta et al. [16] measured mixtures of PE with EVA and also pure EVA. DSC results for EVA from 27 C to 227 C showed two endothermic peaks – one near 50 C and another about 70 C. An exothermic peak in the range 175–215 C appeared. After cross-linking, the lower peak had vanished. This was interpreted by the reduction in structural regularity by the created cross-links. Moly et al. [17] used DSC and WAXS. DSC performed in the range 40 C–160 C showed a broad shoulder from 40 C finished by a peak at 80 C. During cooling, a peak at 50 C followed by a wide shoulder was seen. For blends, a decrease in crystallinity by cross-linking was found which was explained by some hindrance to the ordered arrangement of the polymer chains. Dikobe and Luyt [18] investigated EVA-PP blends, and also both polymers separately, by DSC and DMA. In a DSC
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experiment from 60 to 180 C, an endothermic peak close to 100 C was found. 1.3. DSC-measurements in the temperature range of crosslinking reaction For cross-linking of EVA, peroxide is added as curing agent. At temperatures above 160 C the peroxide decomposes. Free peroxide radicals abstract mainly the hydrogen located at the tertiary carbon of the acetate group of the vinyl acetate (VAc) co-monomers [10,16]. Often dicumyl peroxide (DCP) is used - Bianchi et al. [19]. The DSC heating rates were varied from 2.5 K/min to 40 K/min. For rates of 10 K/min and higher an analysable exothermic peak in the range 150 C–200 C was seen. Summarizing the DSC literature data, one can state that for EVA no results starting at very low and reaching up to crosslinking temperatures were found. Typical endothermic peaks caused by crystal melting are found whereby often two endothermic peaks appear: first peak around 40 C–60 C, second peak at 70–90 C. Sometimes, a broad endothermic shoulder starting from 40 C is observed. Cross-linking appears in the range 140 C–200 C. The measurement of cross-linked material in a second run shows changes in the endothermic peak shape. These changes are discussed as a consequence of the change in molecular structure by cross-linking. 1.4. DMA-measurements The DMA results of Dikobe and Luyt [18] measured at 1 Hz between 80 and þ80 C show a distinct decrease in storage modulus in the 20 C region. A second decrease appeared close to 40 C. The tand curves gave a first peak in the region 10 to 0 C, associated with the glass transition, and a second peak near 60 C. Varghese et al. [11] measured at 50 Hz. They found a decrease of storage modulus from about 30 to 0 C accompanied by a tand peak at about 10 C. From tand maxima, a glass transition temperature of 12 C for uncross-linked and 10.4 C for cross-linked material were given. Storage modulus, loss modulus and tand curves measured at 10 Hz were published by John et al. [20]. A sharp decrease in storage modulus at about 40 C and a second decrease at about 40 C were reported. The loss modulus had a first peak in the region of 130 C and a second close to 25 C. Tand showed maxima at about 130 C, 20 C and 50 C. The change in material properties in the temperature region around 40 to 0 C was brought about in connection with the glass transition. Tand maxima at 140 and 25 C and a distinct decrease in loss modulus from 40 C were found also by Khondakar et al. [21]. The increase of tand between 75 and 95 C was explained by Koshy et al. [22] as melting of crystalline segments. To summarize the DMA measurements, a first relaxation process appears in the 120 C range, followed with increasing temperature by a glass transition in the range 40 to 0 C. An additional decrease of storage modulus in
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the range from 40 C and a tand increase are explained by crystal melting. 2. Experimental 2.1. Material Two commercially available materials applied in photovoltaic module production were used: Material 1: SOLAR EVAÒ RC02B (curing type: fast) from Mitsui Chemicals Fabro [23]. Material 2: ETIMEXÒ VISTASOLARÒ 486.10 (fast cure) from ETIMEX Solar GmbH [24]. The materials were supplied as foils with a thickness of 0.64 mm (material 1) and 0.45 mm (material 2). For DSC measurements, small disks of about 8 mg were cut. For DMA, 6 mm wide cross-linked strips were prepared in a compression mould.
Fig. 2. DMA – loss modulus as function of temperature, material 1.
E00 ¼ jE jsind 00 E – storage modulus
2.2. Thermal analysis 2.2.1. Dynamic mechanical analysis In DMA, a sinusoidal mechanical excitation of the sample is executed whereby force, elongation and the phase shift between them are measured as a function of temperature. The complex modulus E* is calculated from the measured data and the geometry:
Edual cantilever ¼
l3 F 16 b h3 A
l – sample length, b – sample width, h – sample thickness, F – force, A – deflection. With the phase shift, the complex E* can be split into two components:
E0 ¼ jE jcosd 0 E – storage modulus
Fig. 1. DMA – storage modulus as function of temperature, material 1.
The loss factor tand is calculated from:
tand ¼
E00 E0
Characteristic changes of moduli E0 and E00 and tand caused by thermally activated molecular movements begin at definite temperatures. A DMA 242C, Netzsch, Germany, was used for the investigations with a dual cantilever holder and a sample length of 5 mm. The heating rate was 1 K/min and the amplitude 40 mm. During a single measurement run cycling between the 5 frequencies – 0.33; 1; 3.33; 10 and 33.3 Hz – was carried out automatically. 2.2.2. Differential scanning calorimetry DSC evaluates the change of temperature difference between a sample crucible and a reference crucible during non-isothermal or isothermal conditions. The difference is used to calculate the heat flow into or out of the sample. It can detect melting, evaporation, glass transition and also
Fig. 3. DMA – tand as function of temperature, material 1.
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Table 1 Material 1 - Loss modulus peak temperature of the glass transition for different frequencies. 00
Frequency, Hz
E peak temperature, C
0.33 1 3.33 10 33.33
32.1 30.4 28.4 27.2 25.0
occurrence of chemical reactions by the consumption or release of heat [9]. Its special advantage is that only a few mg of the material are required. A DSC 204 F1 Phoenix, Netzsch, Germany, was used with standard crucibles and manually perforated lids. All measurements were carried out in the non-isothermal mode with a heating and cooling rate of 10 K/min. 3. Results and discussion 3.1. Material 1
Fig. 5. DSC – heat flow in the 1st and 2nd run, material 1.
These values were used to determine an activation energy from the Arrhenius relation between frequency of the peak (f) and temperature [26]: Ea
3.1.1. DMA The DMA results are presented first because they will be embraced in the discussion of the DSC results. The DMA was measured for cross-linked samples only. The crosslinking conditions were: 30 min at 160 C using a heated compression mould. For better comparability, the results of the storage and loss modulus are shown in a normalized manner, where the values at 150 C were set to unity. The results for material 1 are given in Figs. 1–3. A small decrease in storage modulus together with a loss modulus and tan d peak in the range of 140 C can be explained by the secondary relaxation caused by side group or end group motion or backbone “crank-shift” type motions typical for many polymers [25]. The storage modulus curve shows a distinct decrease from 40 C. This change is accompanied by a peak in loss modulus and tand. There is a pronounced correlation between frequency and location of the peak. For the loss modulus, the values are shown in Table 1.
Fig. 4. DMA – Arrhenius plot of logarithm of measuring frequency versus 00 reciprocal E peak temperature, material 1.
f ¼ A e RT
Ea – apparent activation energy, R – gas constant. The correlation between ln(f) and 1/T is shown in Fig. 4. A linear relation was found. From the slope of the straight line, an apparent activation energy can be calculated. The activation energy found was 328 kJ/mol, a value in the range typical for a glass transition [27]. The temperature range of the glass transition is in accordance with published data in [11,18,20]. The next step in storage modulus between 40 and 60 C was not accompanied by a peak in loss modulus. This will be discussed together with the DSC data later. 3.1.2. DSC A basic raw material sample was measured in a 1st run from 150 to 200 C. The measurement was rerun with the now cured sample. The results for the 1st and 2nd runs are given in Fig. 5. At the beginning of the DSC measurement, the typical thermal imbalance between the crucibles must be adjusted
Fig. 6. DSC – temperature program.
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Fig. 7. DSC – heat flow results for the temperature program from Fig. 6, material 1.
[9]. Therefore, a settling region, in Fig. 5 from the start at 150 to 135 C, appears. From 115 to 95 C a double peak structure is seen. The reason for this is unknown. From its appearance it looks like the formation of an ordered structure (exothermic peak) followed directly by destruction of this structure (endothermic peak). An endothermic step from 40 to about 20 C is observed. Endothermic peaks appear in the 1st run at 47 and 72 C. Additionally, an exothermic peak at 158 C with an enthalpy of 14.9 J/g appears. A broad endothermic shoulder followed by double peaks in the range 50–100 C have also been reported in [10–17]. An endothermic peak is typically for crystal melting. Multiple endothermic peaks in DSC can be correlated to a lamellar thickness distribution – see for example [28]. The exothermic peak with a maximum at 158 C indicates the exothermic cross-linking reaction. Similar results were reported in [10,16,19]. The reaction enthalpy value is rather low in comparison to cross-linking of epoxy resins, where values in the range of some hundred J/g (400–600) are typical [29,30]. In the 2nd run the peak near 100 C is similar to the 1st run. The endothermic double peak between 40 and 80 C has
Fig. 8. DMA – storage modulus as function of temperature, material 2.
Fig. 9. DMA – loss modulus as function of temperature, material 2.
changed to a broad shoulder followed by a peak at 63 C. The higher the crystal thickness the higher is the melting temperature - Illers and Hendus [7]. Therefore, the change from a peak into a broad shoulder is an indication of a distribution of crystals with reduced thickness as a consequence of the thermal treatment in the first run. The exothermic reaction peak is missing in the 2nd run, indicating that the cross-linking process was finished. The step in the range 40 to about 20 C is in accordance with the glass transition discussed for DMA results. Similarity between 1st and 2nd runs show that the glass transition temperature was not noticeable changed by cross-linking. It is interesting that the DSC peak in the 2nd run at 63 C, indicating crystal melting, correlates well with the sharp decrease of the storage modulus of the cured sample in the area between 40 and 70 C. The change in the endothermic peak structure in the range from 30 to 80 C between the 1st and 2nd runs is indication that the crystal structure has changed. Some authors assume that this is a consequence of cross-linking [16,17]. To find out if the change in melting peaks in the area 30–80 C has its reason in cross-linking or thermal history, in an additional experiment, the 1st heating run
Fig. 10. DMA – tand as function of temperature, material 2.
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Table 2 Material 2 - Loss modulus peak temperature of the glass transition for different frequencies. 00
Frequency, Hz
E peak temperature, C
0.33 1 3.33 10 33.33
31.7 29.9 28.2 26.7 24.6
was stopped at 100 C, a cooling run and a 2nd heating run to 200 C were added followed by additional cooling and heating runs – the temperature program is illustrated in Fig. 6. All DSC curves are collected in Fig. 7. The clear change in the 30–80 C area from a double peak in the 1st heating to a broad shoulder followed by one peak in the 2nd heating comes from thermal treatment – heating to 100 C and cooling under definite conditions. This general change with the appearance of a broad shoulder instead of a peak is obviously not caused by crosslinking. However, in the 3rd run after cross-linking the remaining peak following the shoulder has shifted to lower temperatures. This effect comes from heating to 200 C including cross-linking and indicates an additional drop in crystal perfection. 3.2. Material 2 3.2.1. DMA The DMA results for material 2 are shown in Figs. 8–10. Generally, the curves are similar for both materials. Also, material 2 shows a significant decrease in storage modulus starting from about 40 C which, together with the peak in storage modulus, is typical for the glass transition. Just as for material 1, in the 50 C region a further drop in the storage modulus appears. This was discussed for material 1 to be caused by melting of crystalline regions. With material 2, the contact with the DMA clamp was lost behind 120 C and this also occurred when the measurements were repeated. Fig. 9 gives the course of the loss modulus. As with material 1, a peak in the area 130 to 150 C
Fig. 12. DSC – heat flow in the 1st, 2nd and 3rd heating run, material 2.
appears. The temperature values of the loss modulus peak are summarized in Table 2. From these data, an activation energy of 330 kJ/mol is calculated – a value nearly identical with that for material 1. The peak temperatures are very close to those of material 1 which means that the glass transition temperature is nearly identical for both materials. Fig. 10 illustrates the results for tand. The tand curves of material 2 resemble those of material 1 in shape but they show slightly higher values. 3.2.2. DSC In Fig. 11, DSC results for the 1st and 2nd runs are shown. The results of material 2 are similar to material 1. In the 1st run, a step followed by two endothermic peaks appears. The exothermic curing reaction is in the region of 130– 180 C, whereby there is indication of a double peak. The reaction enthalpy is found to be 16.0 J/g. For material 2 also, a separate thermal treatment with the temperature regime from Fig. 6 was applied. The results in Fig. 12 are also very similar to material 1 in Fig. 7. A difference in the 2nd run for material 2 is that an endothermic step at about 110 C appears for which we have no explanation. Also for material 2, the thermal treatment during 1st heating and 1st cooling causes the form of the endothermic peaks to change from two distinct peaks into a broad shoulder and only one peak, as for material 1. Also for material 2, the thermal treatment during 1st heating and 1st cooling caused a change in the DSC curve for the 2nd heating run. The two endothermic peaks in the 1st run have changed to a broad shoulder and only one remaining peak. This is similar to the results for material 1 and can be interpreted by a change in allocation of crystal size distribution as a consequence of both thermal treatment and of heating to 200 C including cross-linking. 4. Conclusions
Fig. 11. DSC – heat flow in the 1st and 2nd run, material 2.
Two commercially available types of EVA - SOLAR EVAÒ RC02B (material 1) from Mitsui Chemicals Fabro and ETIMEXÒ VISTASOLARÒ 486.10 (material 2) from ETIMEX Solar GmbH were investigated by DSC and DMA in the temperature range from 150–200 C. This wide temperature range, the synchronous application of DMA
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and DSC and the combination of the measurement of the original material with measurements of the material thermally treated by the measurement to 100 C and cured during the preceding measurement to 200 C go beyond previously published data. Both materials show very similar characteristic behaviour as a function of temperature which can be concluded as follows: – A glass transition appears in the range 40 to 20 C, – The glass transition is not shifted perceptibly by crosslinking, – For original, non thermally treated material two crystalline melting processes indicated by two endothermic peaks exist, – Crystal melting is accompanied by a dip in storage modulus, – For material heated to 100 C, cooled down and heated once more the endothermic double peak has changed into a broad endothermic shoulder followed by only one pronounced endothermic peak, – Cross-linking appears in the temperature region from 140 to 180 C, – After heating to 200 C including cross-linking the remaining endothermic peak shifted to lower temperature giving a hint that this process also has changed crystal perfection. Knowledge of the characteristic DMA and DSC curve progressions is of high interest for good incoming control of basic raw material. References [1] A. El Amrani, A. Mahrane, F.Y. Moussa, Y. Boukennous, Solar module fabrication, International Journal of Photoenergy (2007) Article Number: 27610. [2] T. Carlsson, P. Konttinen, U. Malm, P. Lund, Absorption and desorption of water in glass/ethylene-vinyl-acetate/glass laminates, Polymer Testing 25 (5) (2006) 615–622. [3] G. Oreski, G.M. Wallner, Evaluation of the aging behavior of ethylene copolymer films for solar applications under accelerated weathering conditions, Solar Energy 83 (7) (2009) 1040–1047. [4] S.I.A. Ayutthaya, J. Wootthikanokkhan, Investigation of the photodegradation behaviors of an ethylene/vinyl acetate copolymer solar cell encapsulant and effects of antioxidants on the photostability of the material, Journal of Applied Polymer Science 107 (6) (2008) 3853–3863. [5] K. Agroui, A. Belghachi, G. Collins, J. Farenc, Quality control of EVA encapsulant in photovoltaic module process and outdoor exposure, Desalination 209 (1–3) (2007) 1–9. [6] B. Lee, J.Z. Liu, B. Sun, C.Y. Shen, G.C. Dai, Thermally conductive and electrically insulating EVA composite encapsulants for solar photovoltaic (PV) cell, Express Polymer Letters 2 (5) (2008) 357–363. [7] K.H. Illers, H. Hendus, Schmelzpunkt und Kristallitgröße von aus Schmelze und Lösung kristallisiertem Polyäthylen, Die Makromolekulare Chemie 113 (1967) 1–22. [8] A. Frick, C. Stern, DSC-Prüfung in der Anwendung. Hanser Wirtschaft, 2006. [9] G.W. Ehrenstein, G. Riedel, P. Trawiel, Thermal Analysis of Plastics: Theory and Practice. Hanser Gardner Publications, 2004, ISBN:13: 9781569903629.
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