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Energy Procedia
Energy Procedia 00 (2011) Energy Procedia 15 (2012) 428 –000–000 435
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International Conference on Materials for Advanced Technologies 2011, Symposium O
Surface Treatment of Polyethylene Terephthalate (PET) Film for Lamination of Flexible Photovoltaic Devices C.S. Goh, S.C. Tan and S.L. Ngoh, J. Wei* Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
Abstract Roll-to-roll lamination of polyethylene terephthalate (PET) has been carried out using the thermal lamination technique. The laminated substrates show high optical transparency due to the absence of an interlayer. Surface treatment of the PET allows thermal lamination to be carried out at temperatures 20 C below the glass transition temperature of the PET substrates. X-ray photoelectron spectroscopy analysis has been conducted to show the presence of various chemical bonds on the coated PET. The possibility of bond formation after surface treatment has also been discussed to explain the phenomenon behind the low temperature thermal lamination. ©©2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energyof International Conference on Materials for Advanced Technologies. Research Institute of Singapore (SERIS) – National University of Singapore (NUS). Keywords: Lamination; Polyethylene terephthalate; silane
1. Introduction Flexible roll-to-roll photovoltaic devices require lamination and sealing for protection purposes. Polyethylene terephthalate (PET) is a low-cost flexible film that can be used as a substrate for photovoltaic devices. Lamination of large flexible PET films using adhesives poses the common problems of non-uniformity in adhesive thickness and high interfacial thickness. The optical transparency and the flexibility of the laminated PET substrates would be seriously affected if any of the abovementioned issues occurred. Till now, techniques developed for lamination of functional substrates are very limited [1-4], much less to say on large-area roll-to-roll lamination of functional substrates [5]. Extensive lamination studies have also been carried out for polyvinyl butyral lamination for solar cells.
* Corresponding author. Tel: +65 6793 8575; fax: +65 6791 6377 E-mail address:
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1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference on Materials for Advanced Technologies. doi:10.1016/j.egypro.2012.02.052
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1.4 m2 and 5.7 m2 bubble free sizes have been obtained with the autoclave technique [5]. Most of the reported research works to date on the lamination processes are for rigid substrates, little is mentioned on the lamination of large-area flexible substrates. Attempts will be made in this work to develop a suitable roll-to-roll thermal lamination technique for flexible functional PET substrates. Characterisation of the laminated PET substrates will be carried out to determine the nature of the chemical bonds formation during lamination and to determine the physical and mechanical properties of the laminated substrates. 2. Experimental methodology The substrate material chosen for lamination is pre-coated polyethylene terephthalate (PET) supplied by Innox Higga. The pre-coating process and material are proprietary to the supplier. The thickness of the PET substrate is 125 m. The schematic diagram for thermal lamination is shown in Fig. 1. In the thermal lamination process, the two substrates to be laminated were brought together using the heated nip rollers at a pressure of 0.41 MPa and at temperatures above 55 C. The current speed of lamination is set at 1 m/min and a web width of 500 mm is achievable with the Morton International Model 300 lamination equipment. Heated nip rollers PET Substrate 2
Laminated PET substrates
PET Substrate 1
Fig. 1. Schematic diagram showing the thermal lamination process.
Surface treatment is conducted on the pre-coated PET substrates to reduce the lamination temperature to below that of the glass transition temperature Tg of PET. Surface treatment is carried out using epoxybased silane coupling agent (termed as silane for future reference). The silane is first mixed with alcohol and the mixture is sprayed on the PET substrate. Residual amount of the mixture is then wiped off using alcohol. Thermal lamination can be carried out at temperatures as low as 55 ºC, which is 20 ºC below the Tg of PET. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical bonds on the surface of the as-received PET. The surface analysis conducted will determine the adequate type of chemicals for treating the PET substrates. Interfacial thickness of the laminated films was measured using the cross-sectioning technique. A representative portion of the laminated film is first cut and mounted using slow-cure epoxy resin. Grinding and polishing were carried out to achieve a scratch-free surface. The ScopeTek software linked to the optical microscope was used to measure the thickness of the adhesive interlayer. The adhesion strength of the thermal laminated PET was determined in accordance to ASTM D1876-08 T-peel tests using an Instron machine with a crosshead speed of 280 mm/min. A minimum of 10 peel test samples were used and the average strength after a plateau was reached during the test was recorded. Optical transmittance of the PET film was measured using the BYK Hazegard Plus. The transmittance, haze and
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clarity of the PET substrates and PET substrates with interlayer can be collected. For each result, eight sample points are taken from an area of 10 cm2. 3. Results and discussion The interfacial thickness of the thermal laminated sample is represented in Fig. 2. For lamination technique processed samples, no interlayer between the two PET substrates is present. The two substrates have melted at their interfaces under the pressure of the heated rollers.
Two PET substrates brought together with no interlayer
50m Fig. 2. Micrograph showing the absence of an interfacial layer between the two PET substrates.
The optical results of the PET samples are shown in Table 1. It can be seen that the highest transmittance occurs in the single-layer PET sample. The lowest transmittance is observed in the two PET samples due to the air gap between the unprocessed two-layer PET samples which tends to reflect light. The thermal laminated samples show good transmittance above 90%, and clarity near or equal to 100%. As a reference, the optical transmittance of glass is 91% and laminated glass with ZnS coating for architectural applications is 80% [6]. Table 1. Optical results of PET. Sample No sample 1 PET 2 PET 2 PET_Thermal Lamination
Transmittance 100 93.4 86.4 92.5
Haze 0 0.8 1.6 1.7
Clarity 100 100 99.6 99.8
XPS analysis was conducted to determine the coating on the as-received pre-coated PET substrate before any treatment was done. The coating was done at the supplier to enhance the adhesion of ink on the surface of the PET substrates. The results are benchmarked against blank PET with no surface treatment to determine the effect of the coating. The results from the XPS analysis are shown in Fig. 3. From the XPS results on the coated PET, it can be seen that a Si-O group is present, which is not detected in the blank PET without pre-coating.
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4
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(a)
Si-O
(b) Fig. 3. XPS analysis on (a) blank PET and (b) coated PET.
The results of the T peel tests on the thermal laminated pre-coated PET substrates with and without surface treatment are shown in Fig. 4. It can be seen from the graph that when silane treated pre-coated PET has been thermally laminated at 65 C, the peel strength is four times higher than non-treated precoated PET that is laminated at a higher temperature of 75 C. This result can be explained by the reactions that occur during silane surface treatment.
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5
600
Peel Strength/ N/m
500 400 300 200 100 0 Non-Treated_75 degrees C
Treated_65 degrees C
Lamination Technique
Fig. 4. T-peel test results of thermal laminated samples with and without silane surface treatment.
The presence of the Si-O group, which is most likely to be Si-OH, is receptive to the silane treatment applied. The likely chemical reactions can be shown as follows: OH
PET
Si
OH
O
+
Si
O
OH
OH
OH
> 55C
O
Si
O
O
(1)
PET
OH
OH
PET
Si
Si
O
H2 C
C H
OH O
Si
OH
(2)
OH
Equation (1) will proceed at a much higher rate than Eq. (2), due to the higher stability of the epoxy ring as compared to the hydroxyl group on the silane. Under temperature of more than 55 C and
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pressure, there is a possibility that the remaining hydroxyl groups on the silane (as highlighted in the equation) can react and form covalent bonds (Eq. (3)).
OH
PET
Si
O
Si
O O
OH
+ OH O
Si
O
PET
Si
OH
O
O
PET
O
Si
O
Si OH
O
O
O
Si OH
O
Si
PET
(3)
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OH
PET
Si
H2 C
O
7
OH
C H
O
Si
OH
OH
+ OH O
Si
O
Si
PET
Si
OH
O
H2 C
C H
OH
OH
OH
PET
O
X
Si
O
H2 C CH
OH
where X =
OH X
Si
O
(4)
PET
Si
OH
O
Equation (4) can also occur at a much slower rate (due to the small amount of hydroxyl-terminated end groups on the treated PET) when the two treated PET substrates are brought together at temperatures above 55 C. The hydroxyl end and the epoxy end of the silane treated PETs will react and form covalent bonds. This can also cause the two substrates to fuse together at temperatures below the T g of PET. Two hydroxyl end groups on the silane treated PET can also react at elevated temperatures. However, there is a lower tendency of this reaction occurring as there is a much lesser –OH terminated groups on the silane treated PET as compared to the epoxy terminated groups. At temperatures above the Tg of PET (75 C), the pre-coating on the PET can directly react and cause the PET substrates to fuse together (Eq. (5)).
2
PET
Si
OH
PET
Si
O
Si
PET
(5)
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To verify the exact reactions that have occurred on the PET, further XPS studies will be conducted to determine the reactions that have taken place after silane treatment of the coated PET substrates. The chemical reactions that can take place between the hydroxyl terminated groups on the silane and the epoxy and hydroxyl terminated groups on the silane treated PETs have rendered the PET surfaces to be more reactive. Covalent bonds formation can occur between the different terminated groups and hence lamination can take place even at lower temperatures. At higher temperatures, at or above the T g of PET, the Si-OH groups on the PET coating can react directly and form chemical bonds. However, the bonds are not as strong as those formed with silane treatment, as the peel strength of the laminated PETs with no silane treatment is still four times lower. One possible way of improving the peel strength of thermal laminated untreated PET is by using higher lamination temperatures above the T g of PET. However, in large-area flexible lamination this is not advisable, as warpage and stretching of the PET substrates can occur when the temperature becomes too high. 4. Conclusions The following conclusions can be drawn from this study: Thermal lamination technique has been developed for roll-to-roll lamination of flexible photovoltaic devices. High optical transmittance and zero interfacial thickness have been obtained for the thermally laminated samples. Chemical surface treatment on the pre-coated PET films has reduced the thermal lamination temperatures to as low as 20 ºC below the Tg of PET. T-peel test results show that treated PET samples have four times higher strength than non-treated PET samples. References [1] [2] [3] [4] [5] [6]
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