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Permeation of water vapour through polyethylene terephthalate (PET) films for back-sheets of photovoltaic modules
Polymer Testing 58 (2017) 153e158
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Analysis Method
Permeation of water vapour through polyethylene terephthalate (PET) films for back-sheets of photovoltaic modules P. Hülsmann*, G.M. Wallner Johannes Kepler University Linz, Institute of Polymeric Materials and Testing, Altenberger Straße 69, 4040 Linz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2016 Received in revised form 21 November 2016 Accepted 21 November 2016 Available online 23 December 2016
Different polyethylene terephthalate films were investigated as to the permeation and diffusity for water vapour and as to structure-property correlations. Water vapour transmission rate, permeability and diffusion coefficient were measured at temperatures 23, 38, 60 and 80 C using a special test device with a mass spectrometer detection unit. While the permeability was significantly dependent on temperature and specimen thickness, the effect of TiO2 white pigmentation was negligible. Based on the empirical data a model for the temperature and film thickness dependency of the water vapour permeability was established and validated. © 2016 Published by Elsevier Ltd.
Keywords: PET Permeation Diffusion Temperature dependency Thickness TiO2 Modelling
1. Introduction The renewable energy sector has grown immensely in recent years and has become an integral part of world energy production with a share of estimated 21% of the world's electricity production by the end of 2013 [1]. Renewables provide the opportunity of more independency from the market of fossil fuels, increasing security of supply, reducing fuel costs and the bonus of an emission free energy production [2]. Photovoltaic energy in particular is part of it with an increasing share worldwide. Solar cells can convert solar light directly to electricity. Usually several of those cells are connected in series building a photovoltaic module (PV-module). The long-term reliability of PV-modules has to be granted. 20 years and more are aspired lifetimes of PV-modules that are installed worldwide and, therefore, are exposed to different kinds of harsh climatic conditions. Especially the polymeric encapsulants and back-sheet materials are exposed to location specific environmental loading conditions. The degradation processes are dependent on temperature, UV-irradiation and permeated gases such as oxygen and water vapour [3e8]. A PV-module is composed of a glazing, two layers of an
* Corresponding author. E-mail address: [email protected] (P. Hülsmann). http://dx.doi.org/10.1016/j.polymertesting.2016.11.028 0142-9418/© 2016 Published by Elsevier Ltd.
encapsulant, e.g. ethylene-vinylacetate copolymer (EVA) and a back-sheet (see Fig. 1). Most back-sheets are multi-layer laminates providing excellent weatherability, good reflectance, flame resistance, a high electrical insulation and a good adhesion to the encapsulant. It consists of an outer layer highly stabilized (faced to the atmosphere), a thick core layer providing electrical safety and an adhesion layer for bonding to the encapsulant. The most common back-sheet material is polyethylene terephtalate (PET), which exhibits a good cost-performance ratio. PET is used for the outer layer as well for the core layer. The adhesion layer to the encapsulant of the PV-module is usually based on a thermoplastic, polar ethylene copolymers. The water ingress into PV-modules depends strongly from the ambient climate but also from specific material properties [9e11], which depend on the inner material structure. Material structure parameters but also film thickness play an important role. The permeability is reduced by the volume fraction of the impermeable crystalline phase [12] or the orientation of the amorphous phase [13]. A decreasing permeation can also be achieved by the incorporation of fillers or pigments into the polymer matrix [12,14,15]. So far, the effects of structural parameters, film thickness and temperature has not been investigated systematically for PET based back-sheet films. Hence, it is the main objective of this paper to measure the water vapour permeability of various PET back-sheet
154
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
Fig. 1. Structure of a PV-module existing of solar glass, encapsulation, solar cells and back-sheet.
films, to establish structure-property-correlations and to implement semi-empirical models for description of the permeability of PET films. 2. Experimental
different temperature levels of 23, 38, 60 and 80 C. The WVTR describes the mass transport of H2O at a certain temperature and a given partial pressure (p) of H2O. The permeability P is the WVTR value scaled to 1 mbar and is more useful when investigating temperature dependency of the permeation process (1).
WVTR ¼ P,DpH2O
2.1. Materials Seven commercially available PET-films were investigated. The films designated PET_1 to PET_7 are generally used for outside or core layers of multi-layer back-sheets. While the non-pigmented layers have a thickness roughly from 50 to 250 mm (PET_1 to PET_4), the outside layers are modified with TiO2 white pigments. Furthermore, the level of stabilisation is significantly higher for the outside layers. In Table 1 the film materials including specimen code, thickness, back-sheet layer type, visual appearance and qualitative degree of stabilization are listed. All general information were derived from data-sheet specifications provided by the manufacturer Mitsubishi Polyester Films (Wiesbaden, Germany). 2.2. Determination of permeation and diffusion properties The permeation and diffusion measurements of the investigated PET films were carried out with a special constructed test device. It consists of a large area sample holder placed in a climatic cabinet and a quadrupol mass spectrometer QMG 422 from Pfeiffer Vacuum to detect permeated gases. The climatic cabinet is used to control air temperature and humidity and gives the opportunity to characterize films over a wide temperature range and under realistic ambient climatic conditions [19]. Water vapour transmission rate (WVTR), permeability (P) and diffusion coefficient (D) were determined for each sample at
Table 1 Investigated PET films and data-sheet specifications. code
Thickness [mm]
back-sheet layer
visual appearance
stabilisation
PET_1 PET_2 PET_3 PET_4 PET_5 PET_6 PET_7
50 190 250 190 50 190 190
outside core core core outside core/outside core/outside
transparent transparent transparent translucent white white white
high low low low high middle middle
(1)
The diffusion coefficient D describes the speed of molecules through a defined area of the sample and is determined via the time-lag Q of the permeation measurement and the thickness d of the sample (2).
Q¼
d2 6,D
(2)
The activation energy EA is used to describe the temperature dependency based on an Arrhenius approach. EA can be separately calculated (3) for the permeation and the diffusion process by replacing the reaction rate VR by P respectively D. The higher EA the more significant is the acceleration of mass transport with increasing temperature [16].
d,lnðVR Þ EA ¼ RD , d,T 1
(3)
2.3. Structural characterisation of PET films The basic film characteristics were determined by Geretschl€ ager [17]. Besides the thickness, the thermal characteristics glass transition temperature (Tg), melting temperature (Tm) and crystallinity (a) were investigated using a PerkinElmer DSC 4000 differential scanning calorimetry. Two heating cycles from 0 to 300 to 0 C were done a rate of 10 C/min while flushing with Nitrogen. Thermal decomposition (Td) and the amount of non-volatile components (m800 C) were determined by a PerkinElmer STA 6000 simultaneous thermogravimetric and calorimetric analyser at a heating rate of 10 C/min. Nitrogen was applied to prevent premature oxidation.
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158 Table 2 Thickness, thermal characteristics (glass transition (Tg), melting temperature (Tm), decomposition temperature (Td), pyrolysis residues (m800 C) and degree of crystallinity (a) of the investigated samples [17]. Specimen
Thickness [mm]
Tg [ C]
Tm [ C]
Td [ C]
m800 C [%m]
a [%]
PET_1 Transparent PET_2 Transparent PET_3 Transparent PET_4 Translucent PET_5 White PET_6 White PET_7 White
52 ±1 193 ±1 252 ±1 188 ±0 48 ±1 190 ±2 193 ±1
75.5 ±0.0 78.1 ±0.9 78.0 ±0.0 77.9 ±1.5 e e 79.7 ±0.0 76.4 ±0.1
253.3 ±0.2 254.8 ±0.0 254.2 ±0.1 254.4 ±0.2 249.2 ±0.0 254.3 ±0.1 254.4 ±0.1
452.9 ±0.6 455.0 ±0.3 459.7 ±0.6 455.5 ±3.2 449.0 ±0.4 457.4 ±4.2 455.1 ±5.9
0.5
31.1
2.3
32.6
2.6
32.5
2.4
32.3
13.1
31.7
3.7
32.4
7.0
31.6
155
content. For the melting peak temperatures (Tm) an average value was of 254.2 ± 0.5 C was detected for the films PET_1 to PET_4 and PET_6 and _7. For the grade PET_5 a lower value of 249.2 C was detected. The decomposition peak temperature (Td) of 455.1 ± 2.5 C did not show significant differences. Also the degree of crystallinity was just slightly varying between 31 and 33%. Differences were found for the amount of non-organic fillers (m800 C) as organic decomposition residues. While the non-pigmented film PET_1 showed the lowest value with a content of 0.5% of nonvolatile components, which is most likely related to pyrolysis residues of PET, higher values were obtained for the thicker transparent films ranging from 2.3 to 2.6%. For the white pigmented films different values of 3.7% for PET_6, 7.0% for PET_7 and 13.1% for PET_5 were obtained. Hence, it can be concluded that the amount of pigmentation is different for the investigated white films. 3.2. Permeation and diffusion of water vapour
3. Results and discussion 3.1. Structural characteristics of the PET films The measured glass transition temperatures were all within the same range with a median Tg of 77.3 ± 1.4 C (see Table 2). For the white pigmented outer layer (PET_5) the transition was not pronounced in the DSC trace, which is related to the high pigmented
The results of the permeation measurements (P and WVTR) are depicted in Fig. 2. A clear dependency of the thickness can be stated. PET_1 and PET_5 with a thickness of around 50 mm showed the highest rates over the entire temperature range. PET_3 with a thickness of 250 mm provided the lowest values. Interestingly, no difference between filled and unfilled PET grades was observable, e.g. PET_1 and PET_5. Generally, P and WVTR should be decreased by the ratio of the non-permeable inorganic pigment. This effect is probably overlaid by an assumed error of ±20% of the measuring
Fig. 2. Diffusion coefficient D, permeability P and water vapour transmission rate WVTR of the analysed PET-film grades measured at 23, 38, 60 and 80 C.
156
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
Table 3 Activation energies (EA P and EA D) of the investigated PET films.
EA P [kJ/mol] EA D [kJ/mol]
PET_1
PET_2
PET_3
PET_4
PET_5
PET_6
PET_7
34.1 26.7
37.1 39.4
37.1 38.8
34.9 40.3
35.0 26.0
36.3 38.4
36.8 38.6
technique compared to a maximum filler content of 13.1% for PET_5. But also an insufficient bonding of the mineral filler to the polymer matrix featuring a bypass for water molecules within this interface could be plausible [14,15]. Also in Fig. 2 the evaluated diffusion coefficient D is plotted as function of temperature. For the 50 mm thick films, PET_1 and PET_5, the values were lower compared to the rest. Furthermore, the increase with temperature was less pronounced for the 50 mm samples (PET_1 and PET_5). In literature diffusion coefficients for PET films of 5,48E-13 m2/s measured at 27 C [9] and 9,1E-13 m2/s (25 C) [18] are reported. These values are within the order of magnitude. Differences may be related to different crystallinity values. By Summon [19] diffusion coefficients from 8,57E-13 to 0,52E-13 m2/s for a crystallinity range from 4 to 25% were stated. 3.3. Temperature dependency The temperature dependency of the mass transport processes (P and D) was characterized by the determination of the activation energy (EA P and EA D) via an Arrhenius fit (R2 ¼ 0,989 to 0,999) in combination with equation. (3). All investigated PET samples showed an EA P in the range of 34.1e37.1 kJ/mol (see Table 3). Based on an evaluated error of ±20% all PET grades were within the same range and comparable. Even by crossing Tg no significant deviation from the Arrhenius-law was observable (Fig. 2). This could be based on the fact that an increase of the molecular flux caused by a mobilisation of the amorphous phase (crossing Tg) is much lower than the acceleration by temperature of P. Jorgensen reported an EA
of permeability of water vapour of 53,07 kJ/mol (0,55 eV) for PET films [7] which is significantly higher than the values presented here. However, in this study no specific details to the material structure are given. The determined activation energies for the diffusion coefficient (EA D) were ranging from 38.4 to 40.4 kJ/mol for PET_2, _3, _4, _6 and _7. The 50 mm thick films PET_1 and _5 exhibited lower values of about 26 kJ/mol. Also the stabilization level of these films was higher. Interestingly, EA P of PET_1 and PET_5 was the same as of all other samples. This follows that the solubility for water vapour of the thinner PET films should change with increasing temperature. The permeability P is the product of diffusion coefficient D and the solubility S [12]. For the investigated films no significant effect of the white pigments on the water vapour diffusion and permeation coefficients was discerned. The evaluated activation energies of the diffusion process are slightly lower compared to the findings of Kempe. Here, a EA D of 44,38 kJ/mol (0,46 eV) for PET films was stated [9], again not giving any structural material characteristics. 3.4. Effect of the film thickness To investigate the effect of film thickness the permeation of the transparent films PET_1 to _3 with varying film thickness were evaluated. The permeability at the four temperature levels (23e80 C) is plotted over the thickness for these PET films in Fig. 3. A decrease of permeability with increasing thickness could be stated that followed a simple potential function. Such a potential function was fitted to each slope showing a similar gradient for all four temperature levels that ranges from x1,31 to x1,43. 4. Modelling of permeation To fit the temperature dependency of the measured permeation data an Arrhenius model was applied (see Fig. 2). This physical based approach is based on equation (3). It needs a measured value as input and can be used to extrapolate the permeability data over a wide temperature range. The temperature dependency gives the first part of the model. The second part of the model is given by the thickness dependency. Here, the measured permeability values of the samples
Fig. 3. Influence of thickness on permeability of transparent PET samples, measured at four different temperature levels. Fig. 4. Measured permeability of the transparent PET-films with Arrhenius fit.
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
PET_1, PET_2 and PET_3 were fitted with a potential equation in the form y ¼ a$x-b. A mean coefficient of determination of R2 ¼ 0,99 was reached for the three slopes (see Fig. 4). Finally both functions, the temperature and the thickness dependency, were merged to the following two dimensional model describing the permeability of PET films:
P¼
P0 db0
EA $ R
$e
157
samples (PET_1, PET_2, PET_3) varying in thickness but containing no fillers or pigments. To get an impression of the quality of the new model it has been compared to other PET qualities that have
1 1 T0 T
$db
ðb ¼ 1; 339Þ
(4)
Here, P is the permeability, P0 is a measured permeability for a temperature T0 and a thickness d0, b is a material specific constant, EA is the activation energy of the permeability in J/mol. T and d are the temperature ( C) and the thickness (mm) respectively. According to equation (1) the model can be extended by the partial pressure of water vapour pH2O resulting in the WVTR:
EA
$ R P WVTR ¼ 0b $e d0
1 1 T0 T
$db $pH2 O
(5)
Equations (4) and (5) can be used to calculate the permeability and the WVTR of PET films for different thickness, temperature and air humidity values. Fig. 5 shows the calculated permeability of PET films for a wide temperature range and various thicknesses based on equation (5). The development of equations (5) and (6) is based on three Fig. 6. Comparison of calculated and measured permeability of PET-films containing different amounts of pigments.
Fig. 5. Calculated permeability of PET films depending on thickness and temperature.
158
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
The water transport through PET-films was significantly influenced by the thickness of the films. The permeability decreased with the increase in thickness. Regarding the pigmentation no significant effect was found for TiO2 contents below 13 w%.
Acknowledgements The research work was carried out within the research project SolPol-3 “Solar-electrical Systems based on Polymeric Materials Novel Polymeric Encapsulation Materials for PV Modules” founded by the Austrian Climate and Energy Funds (). Permeation measurements have been done in cooperation with Fraunhofer ISE (BMU FKz 0329978).
References
Fig. 7. Calculated permeability of a 190 mm thick PET-film compared to measured values of PET_7 (190 mm). The results are in good agreement.
not been part of model building. Fig. 6 shows a comparison of modelled and measured permeability values plotted against film thickness. The measured PET grades (PET_5, PET_7 and PET_3) not used for model building having different amounts of fillers and are in a good agreement with the model. Similar can be stated for the calculation of the permeability with increasing temperature. Here calculated values are plotted versus measured value of PET_7 (see Fig. 7). Both graphs show a good correlation between measured and calculated data. A coefficient of determination of R2 ¼ 0,999 was reached for thickness depending part of the model and R2 ¼ 0,981 for the temperature dependency. 5. Summary and conclusion Seven different PET films were analysed regarding permeability, WVTR and diffusity of water vapour including analysis of the temperature dependency of each process. The investigations were carried out with special focus on polymer properties influencing the flux of water molecules as thickness and non-organic fillers. Based on the experimental data a semi-empirical model was deduced to calculate the permeability of PET films. Investigations of the permeability of the films showed a strong temperature dependency of the molecular flux of water vapour. All samples offered an EA of the permeability process (P) within the same range of (EA ~ 35 kJ/mol). Interestingly the activation energy (EA) of the diffusion process (D) was ~26 kJ/mol for the thinner stabilized samples (PET_1 and _5) and ~39 kJ/mol for the non- and low-stabilized PET film. This behaviour indicates a changing solubility of water vapour presumably related to the stabilization of the PET films. Also a crossing of Tg showed no influence of an Arrhenius approach.
[1] REN21, Renewables 2014 Global Status Report, 2015. [2] IEA, Technology Roadmap: Solar Photovoltaic Energy - 2014 Edition, 2015. [3] W. Herrmann, N. Bogdanski, F. Reil, et al., PV module degradation caused by thermomechanical stress: real impacts of outdoor weathering versus accelerated testing in the laboratory, in: Proceedings of the Special Patrol Insertion/ Extraction Optics and Photonics (SPIE ’10), 2010. San Diego, Calif, USA. [4] F.J. Pern, Ethylene-Vinyl Acetate (EVA) encapsulants for photovoltaic modules: degradation and discoloration mechanisms and formulation modifications for improved photostability, Angew. Makromol. Chem. 252 (1997) 195e216. [5] F.J. Pern, Factors that affect the EVA encapsulant discoloration rate upon accelerated, Sol. Energy Mater. Sol. Cells (1996) 587e615, 41/42. [6] G. Oreski, G.M. Wallner, Aging mechanisms of polymeric films for PV encapsulation, Sol. Energy 79 (2005) 612e617. [7] G.J. Jorgensen, K.M. Terwilliger, J.A. DelCueto, S.H. Glick, M.D. Kempe, J.W. Pankow, F.J. Pern, T.J. McMahon, Moisture transport, adhesion, and corrosion protection of PV module packaging materials, Sol. Energy Mater. Sol. Cells 90 (2006) 2739e2775. [8] M.D. Kempe, Acetic acid production and glass transition concerns with ethylene-vinyl acetate used in photovoltaic devices, Sol. Energy Mater. Sol. Cells 91 (2007) 315e329. [9] M.D. Kempe, Modeling of rates of moisture ingress into photovoltaic modules, Sol. Energy Mater. Sol. Cells 90 (2006) 2720e2738. € hl, Temperature-dependent water vapor and [10] P. Hülsmann, K.-W. Weiss, M. Ko oxygen permeation through different polymeric materials used in photovoltaic-modules, Prog. Photovoltaics Res. Appl. 2273 (2012). http://dx.doi.org/10. 1002/pip. €hl, Simulation of water vapor ingress into pv[11] P. Hülsmann, M. Heck, M. Ko modules under different climatic conditions, Hindawi Publishing Corporation, J. Mater. 2013 (2013). Article ID 102691, 7 pages, http://dx.doi.org/10. 1155/2013/102691. [12] J. Crank, G.S. Park, Diffusion in Polymers, Academic Press, Great Britain, 1968. [13] G.A.J. Orchard, P. Spiby, I.M. Ward, Oxygen and water vapor diffusion through biaxially Oriented poly(ethylene-terephthalate), J. Polym. Sci. Part B Polym. Phys. 28 (1990) 603e621. [14] H. Zweifel, S.E. Amos, R.D. Maier, M. Schiller, Plastics Additives Handbook, 2009, ISBN 3446408010, 9783446408012, Hanser. [15] G.K. van der Wel, O.C.G. Adan, Moisture in organic coatingsea review, Prog. Org. Coatings 37 (1999) 1e14. €hl, Measuring temperature dependent water [16] P. Hülsmann, D. Philipp, M. Ko vapor and gas permeation through high barrier films, Rev. Sci. Instrum. 80 (2009), 113901. €ger, Characterization of Pigmented and Reinforced Polymeric [17] K. Geretschla Materials for Solar Energy Technologies, Doctoral Thesis, 2015. Linz. [18] S. Marais, Q.T. Nguyen, C. Devallencourt, M. Metayer, T.U. Nguyen, P. Schaetzel, Permeation of water through polar and nonpolar polymers and copolymers: determination of the concentration-dependent diffusion coefficient, J. Polym. Sci. Part B Polym. Phys. 38 (2000) 1998e2008. [19] C. Summon, J. Yarwood, N. Everall, A FTIR-ATR study of liquid diffusion processes in PET films: comparison of water with simple alcohols, J. Polym. me 41 (2000) 2521e2534.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Analysis Method
Permeation of water vapour through polyethylene terephthalate (PET) films for back-sheets of photovoltaic modules P. Hülsmann*, G.M. Wallner Johannes Kepler University Linz, Institute of Polymeric Materials and Testing, Altenberger Straße 69, 4040 Linz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2016 Received in revised form 21 November 2016 Accepted 21 November 2016 Available online 23 December 2016
Different polyethylene terephthalate films were investigated as to the permeation and diffusity for water vapour and as to structure-property correlations. Water vapour transmission rate, permeability and diffusion coefficient were measured at temperatures 23, 38, 60 and 80 C using a special test device with a mass spectrometer detection unit. While the permeability was significantly dependent on temperature and specimen thickness, the effect of TiO2 white pigmentation was negligible. Based on the empirical data a model for the temperature and film thickness dependency of the water vapour permeability was established and validated. © 2016 Published by Elsevier Ltd.
Keywords: PET Permeation Diffusion Temperature dependency Thickness TiO2 Modelling
1. Introduction The renewable energy sector has grown immensely in recent years and has become an integral part of world energy production with a share of estimated 21% of the world's electricity production by the end of 2013 [1]. Renewables provide the opportunity of more independency from the market of fossil fuels, increasing security of supply, reducing fuel costs and the bonus of an emission free energy production [2]. Photovoltaic energy in particular is part of it with an increasing share worldwide. Solar cells can convert solar light directly to electricity. Usually several of those cells are connected in series building a photovoltaic module (PV-module). The long-term reliability of PV-modules has to be granted. 20 years and more are aspired lifetimes of PV-modules that are installed worldwide and, therefore, are exposed to different kinds of harsh climatic conditions. Especially the polymeric encapsulants and back-sheet materials are exposed to location specific environmental loading conditions. The degradation processes are dependent on temperature, UV-irradiation and permeated gases such as oxygen and water vapour [3e8]. A PV-module is composed of a glazing, two layers of an
* Corresponding author. E-mail address: [email protected] (P. Hülsmann). http://dx.doi.org/10.1016/j.polymertesting.2016.11.028 0142-9418/© 2016 Published by Elsevier Ltd.
encapsulant, e.g. ethylene-vinylacetate copolymer (EVA) and a back-sheet (see Fig. 1). Most back-sheets are multi-layer laminates providing excellent weatherability, good reflectance, flame resistance, a high electrical insulation and a good adhesion to the encapsulant. It consists of an outer layer highly stabilized (faced to the atmosphere), a thick core layer providing electrical safety and an adhesion layer for bonding to the encapsulant. The most common back-sheet material is polyethylene terephtalate (PET), which exhibits a good cost-performance ratio. PET is used for the outer layer as well for the core layer. The adhesion layer to the encapsulant of the PV-module is usually based on a thermoplastic, polar ethylene copolymers. The water ingress into PV-modules depends strongly from the ambient climate but also from specific material properties [9e11], which depend on the inner material structure. Material structure parameters but also film thickness play an important role. The permeability is reduced by the volume fraction of the impermeable crystalline phase [12] or the orientation of the amorphous phase [13]. A decreasing permeation can also be achieved by the incorporation of fillers or pigments into the polymer matrix [12,14,15]. So far, the effects of structural parameters, film thickness and temperature has not been investigated systematically for PET based back-sheet films. Hence, it is the main objective of this paper to measure the water vapour permeability of various PET back-sheet
154
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
Fig. 1. Structure of a PV-module existing of solar glass, encapsulation, solar cells and back-sheet.
films, to establish structure-property-correlations and to implement semi-empirical models for description of the permeability of PET films. 2. Experimental
different temperature levels of 23, 38, 60 and 80 C. The WVTR describes the mass transport of H2O at a certain temperature and a given partial pressure (p) of H2O. The permeability P is the WVTR value scaled to 1 mbar and is more useful when investigating temperature dependency of the permeation process (1).
WVTR ¼ P,DpH2O
2.1. Materials Seven commercially available PET-films were investigated. The films designated PET_1 to PET_7 are generally used for outside or core layers of multi-layer back-sheets. While the non-pigmented layers have a thickness roughly from 50 to 250 mm (PET_1 to PET_4), the outside layers are modified with TiO2 white pigments. Furthermore, the level of stabilisation is significantly higher for the outside layers. In Table 1 the film materials including specimen code, thickness, back-sheet layer type, visual appearance and qualitative degree of stabilization are listed. All general information were derived from data-sheet specifications provided by the manufacturer Mitsubishi Polyester Films (Wiesbaden, Germany). 2.2. Determination of permeation and diffusion properties The permeation and diffusion measurements of the investigated PET films were carried out with a special constructed test device. It consists of a large area sample holder placed in a climatic cabinet and a quadrupol mass spectrometer QMG 422 from Pfeiffer Vacuum to detect permeated gases. The climatic cabinet is used to control air temperature and humidity and gives the opportunity to characterize films over a wide temperature range and under realistic ambient climatic conditions [19]. Water vapour transmission rate (WVTR), permeability (P) and diffusion coefficient (D) were determined for each sample at
Table 1 Investigated PET films and data-sheet specifications. code
Thickness [mm]
back-sheet layer
visual appearance
stabilisation
PET_1 PET_2 PET_3 PET_4 PET_5 PET_6 PET_7
50 190 250 190 50 190 190
outside core core core outside core/outside core/outside
transparent transparent transparent translucent white white white
high low low low high middle middle
(1)
The diffusion coefficient D describes the speed of molecules through a defined area of the sample and is determined via the time-lag Q of the permeation measurement and the thickness d of the sample (2).
Q¼
d2 6,D
(2)
The activation energy EA is used to describe the temperature dependency based on an Arrhenius approach. EA can be separately calculated (3) for the permeation and the diffusion process by replacing the reaction rate VR by P respectively D. The higher EA the more significant is the acceleration of mass transport with increasing temperature [16].
d,lnðVR Þ EA ¼ RD , d,T 1
(3)
2.3. Structural characterisation of PET films The basic film characteristics were determined by Geretschl€ ager [17]. Besides the thickness, the thermal characteristics glass transition temperature (Tg), melting temperature (Tm) and crystallinity (a) were investigated using a PerkinElmer DSC 4000 differential scanning calorimetry. Two heating cycles from 0 to 300 to 0 C were done a rate of 10 C/min while flushing with Nitrogen. Thermal decomposition (Td) and the amount of non-volatile components (m800 C) were determined by a PerkinElmer STA 6000 simultaneous thermogravimetric and calorimetric analyser at a heating rate of 10 C/min. Nitrogen was applied to prevent premature oxidation.
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158 Table 2 Thickness, thermal characteristics (glass transition (Tg), melting temperature (Tm), decomposition temperature (Td), pyrolysis residues (m800 C) and degree of crystallinity (a) of the investigated samples [17]. Specimen
Thickness [mm]
Tg [ C]
Tm [ C]
Td [ C]
m800 C [%m]
a [%]
PET_1 Transparent PET_2 Transparent PET_3 Transparent PET_4 Translucent PET_5 White PET_6 White PET_7 White
52 ±1 193 ±1 252 ±1 188 ±0 48 ±1 190 ±2 193 ±1
75.5 ±0.0 78.1 ±0.9 78.0 ±0.0 77.9 ±1.5 e e 79.7 ±0.0 76.4 ±0.1
253.3 ±0.2 254.8 ±0.0 254.2 ±0.1 254.4 ±0.2 249.2 ±0.0 254.3 ±0.1 254.4 ±0.1
452.9 ±0.6 455.0 ±0.3 459.7 ±0.6 455.5 ±3.2 449.0 ±0.4 457.4 ±4.2 455.1 ±5.9
0.5
31.1
2.3
32.6
2.6
32.5
2.4
32.3
13.1
31.7
3.7
32.4
7.0
31.6
155
content. For the melting peak temperatures (Tm) an average value was of 254.2 ± 0.5 C was detected for the films PET_1 to PET_4 and PET_6 and _7. For the grade PET_5 a lower value of 249.2 C was detected. The decomposition peak temperature (Td) of 455.1 ± 2.5 C did not show significant differences. Also the degree of crystallinity was just slightly varying between 31 and 33%. Differences were found for the amount of non-organic fillers (m800 C) as organic decomposition residues. While the non-pigmented film PET_1 showed the lowest value with a content of 0.5% of nonvolatile components, which is most likely related to pyrolysis residues of PET, higher values were obtained for the thicker transparent films ranging from 2.3 to 2.6%. For the white pigmented films different values of 3.7% for PET_6, 7.0% for PET_7 and 13.1% for PET_5 were obtained. Hence, it can be concluded that the amount of pigmentation is different for the investigated white films. 3.2. Permeation and diffusion of water vapour
3. Results and discussion 3.1. Structural characteristics of the PET films The measured glass transition temperatures were all within the same range with a median Tg of 77.3 ± 1.4 C (see Table 2). For the white pigmented outer layer (PET_5) the transition was not pronounced in the DSC trace, which is related to the high pigmented
The results of the permeation measurements (P and WVTR) are depicted in Fig. 2. A clear dependency of the thickness can be stated. PET_1 and PET_5 with a thickness of around 50 mm showed the highest rates over the entire temperature range. PET_3 with a thickness of 250 mm provided the lowest values. Interestingly, no difference between filled and unfilled PET grades was observable, e.g. PET_1 and PET_5. Generally, P and WVTR should be decreased by the ratio of the non-permeable inorganic pigment. This effect is probably overlaid by an assumed error of ±20% of the measuring
Fig. 2. Diffusion coefficient D, permeability P and water vapour transmission rate WVTR of the analysed PET-film grades measured at 23, 38, 60 and 80 C.
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Table 3 Activation energies (EA P and EA D) of the investigated PET films.
EA P [kJ/mol] EA D [kJ/mol]
PET_1
PET_2
PET_3
PET_4
PET_5
PET_6
PET_7
34.1 26.7
37.1 39.4
37.1 38.8
34.9 40.3
35.0 26.0
36.3 38.4
36.8 38.6
technique compared to a maximum filler content of 13.1% for PET_5. But also an insufficient bonding of the mineral filler to the polymer matrix featuring a bypass for water molecules within this interface could be plausible [14,15]. Also in Fig. 2 the evaluated diffusion coefficient D is plotted as function of temperature. For the 50 mm thick films, PET_1 and PET_5, the values were lower compared to the rest. Furthermore, the increase with temperature was less pronounced for the 50 mm samples (PET_1 and PET_5). In literature diffusion coefficients for PET films of 5,48E-13 m2/s measured at 27 C [9] and 9,1E-13 m2/s (25 C) [18] are reported. These values are within the order of magnitude. Differences may be related to different crystallinity values. By Summon [19] diffusion coefficients from 8,57E-13 to 0,52E-13 m2/s for a crystallinity range from 4 to 25% were stated. 3.3. Temperature dependency The temperature dependency of the mass transport processes (P and D) was characterized by the determination of the activation energy (EA P and EA D) via an Arrhenius fit (R2 ¼ 0,989 to 0,999) in combination with equation. (3). All investigated PET samples showed an EA P in the range of 34.1e37.1 kJ/mol (see Table 3). Based on an evaluated error of ±20% all PET grades were within the same range and comparable. Even by crossing Tg no significant deviation from the Arrhenius-law was observable (Fig. 2). This could be based on the fact that an increase of the molecular flux caused by a mobilisation of the amorphous phase (crossing Tg) is much lower than the acceleration by temperature of P. Jorgensen reported an EA
of permeability of water vapour of 53,07 kJ/mol (0,55 eV) for PET films [7] which is significantly higher than the values presented here. However, in this study no specific details to the material structure are given. The determined activation energies for the diffusion coefficient (EA D) were ranging from 38.4 to 40.4 kJ/mol for PET_2, _3, _4, _6 and _7. The 50 mm thick films PET_1 and _5 exhibited lower values of about 26 kJ/mol. Also the stabilization level of these films was higher. Interestingly, EA P of PET_1 and PET_5 was the same as of all other samples. This follows that the solubility for water vapour of the thinner PET films should change with increasing temperature. The permeability P is the product of diffusion coefficient D and the solubility S [12]. For the investigated films no significant effect of the white pigments on the water vapour diffusion and permeation coefficients was discerned. The evaluated activation energies of the diffusion process are slightly lower compared to the findings of Kempe. Here, a EA D of 44,38 kJ/mol (0,46 eV) for PET films was stated [9], again not giving any structural material characteristics. 3.4. Effect of the film thickness To investigate the effect of film thickness the permeation of the transparent films PET_1 to _3 with varying film thickness were evaluated. The permeability at the four temperature levels (23e80 C) is plotted over the thickness for these PET films in Fig. 3. A decrease of permeability with increasing thickness could be stated that followed a simple potential function. Such a potential function was fitted to each slope showing a similar gradient for all four temperature levels that ranges from x1,31 to x1,43. 4. Modelling of permeation To fit the temperature dependency of the measured permeation data an Arrhenius model was applied (see Fig. 2). This physical based approach is based on equation (3). It needs a measured value as input and can be used to extrapolate the permeability data over a wide temperature range. The temperature dependency gives the first part of the model. The second part of the model is given by the thickness dependency. Here, the measured permeability values of the samples
Fig. 3. Influence of thickness on permeability of transparent PET samples, measured at four different temperature levels. Fig. 4. Measured permeability of the transparent PET-films with Arrhenius fit.
P. Hülsmann, G.M. Wallner / Polymer Testing 58 (2017) 153e158
PET_1, PET_2 and PET_3 were fitted with a potential equation in the form y ¼ a$x-b. A mean coefficient of determination of R2 ¼ 0,99 was reached for the three slopes (see Fig. 4). Finally both functions, the temperature and the thickness dependency, were merged to the following two dimensional model describing the permeability of PET films:
P¼
P0 db0
EA $ R
$e
157
samples (PET_1, PET_2, PET_3) varying in thickness but containing no fillers or pigments. To get an impression of the quality of the new model it has been compared to other PET qualities that have
1 1 T0 T
$db
ðb ¼ 1; 339Þ
(4)
Here, P is the permeability, P0 is a measured permeability for a temperature T0 and a thickness d0, b is a material specific constant, EA is the activation energy of the permeability in J/mol. T and d are the temperature ( C) and the thickness (mm) respectively. According to equation (1) the model can be extended by the partial pressure of water vapour pH2O resulting in the WVTR:
EA
$ R P WVTR ¼ 0b $e d0
1 1 T0 T
$db $pH2 O
(5)
Equations (4) and (5) can be used to calculate the permeability and the WVTR of PET films for different thickness, temperature and air humidity values. Fig. 5 shows the calculated permeability of PET films for a wide temperature range and various thicknesses based on equation (5). The development of equations (5) and (6) is based on three Fig. 6. Comparison of calculated and measured permeability of PET-films containing different amounts of pigments.
Fig. 5. Calculated permeability of PET films depending on thickness and temperature.
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The water transport through PET-films was significantly influenced by the thickness of the films. The permeability decreased with the increase in thickness. Regarding the pigmentation no significant effect was found for TiO2 contents below 13 w%.
Acknowledgements The research work was carried out within the research project SolPol-3 “Solar-electrical Systems based on Polymeric Materials Novel Polymeric Encapsulation Materials for PV Modules” founded by the Austrian Climate and Energy Funds (). Permeation measurements have been done in cooperation with Fraunhofer ISE (BMU FKz 0329978).
References
Fig. 7. Calculated permeability of a 190 mm thick PET-film compared to measured values of PET_7 (190 mm). The results are in good agreement.
not been part of model building. Fig. 6 shows a comparison of modelled and measured permeability values plotted against film thickness. The measured PET grades (PET_5, PET_7 and PET_3) not used for model building having different amounts of fillers and are in a good agreement with the model. Similar can be stated for the calculation of the permeability with increasing temperature. Here calculated values are plotted versus measured value of PET_7 (see Fig. 7). Both graphs show a good correlation between measured and calculated data. A coefficient of determination of R2 ¼ 0,999 was reached for thickness depending part of the model and R2 ¼ 0,981 for the temperature dependency. 5. Summary and conclusion Seven different PET films were analysed regarding permeability, WVTR and diffusity of water vapour including analysis of the temperature dependency of each process. The investigations were carried out with special focus on polymer properties influencing the flux of water molecules as thickness and non-organic fillers. Based on the experimental data a semi-empirical model was deduced to calculate the permeability of PET films. Investigations of the permeability of the films showed a strong temperature dependency of the molecular flux of water vapour. All samples offered an EA of the permeability process (P) within the same range of (EA ~ 35 kJ/mol). Interestingly the activation energy (EA) of the diffusion process (D) was ~26 kJ/mol for the thinner stabilized samples (PET_1 and _5) and ~39 kJ/mol for the non- and low-stabilized PET film. This behaviour indicates a changing solubility of water vapour presumably related to the stabilization of the PET films. Also a crossing of Tg showed no influence of an Arrhenius approach.
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