Table 1 and Fig. 3 summarize the results of the tensile tests. For PET elastic modulus values of about 4000MPa were obtained. In contrast, the fluoropolymers PVF and PVDF exhibit significant lower elastic modulus values, ranging from 1500 to about 2000MPa. The lowest elastic modulus of about 600MPa was determined for the EVA-PVF-PET-PVF laminate, the elastic modulus is dominated by the EVA elastomer. The plastic modulus values are ranging from 10 to 76MPa. PVDF showed a negative slope of the stress-strain curve after the yield point; the plastic modulus E2 was set zero.

Fig. 4 depicts the force versus displacement plots of the three thermoplastic laminates PVDF/PET, PVDF/SiOx-PET and PVF/PET-PVF. The curve of the PVDF/PET laminate without the silicon oxide barrier layer shows two levels of peel force. The high level is interpreted as a cohesive fracture in the peel arm and the low level can be attributed to adhesive fracture between the two peel arms (Moore and Williams, 2001). The peel force is independent of the configuration; the mean value is 9.1±0.3N. As to the peel angle ф of the PVDF/PET laminate significant differences between the two configurations were obtained. Configuration A (PVDF peel arm top) yielded an angle of 13.3±0.4°, whereas in configuration B (PET peel arm top) an angle of 11.9±0.6° was measured. This difference can be explained by gravity effects in combination with the low flexural stiffness of the laminate. Fig. 5 shows the curvature of the PVDF/PET laminate.

For the PVDF/SiOx-PET laminate with an 80 nm thick silicon oxide barrier layer no effect of the configuration on the peel angle was observed. For both configurations mean values of 12.7±0.4° were obtained. In contrast to the PVDF/PET laminate the peel force is significant higher (11.1±0.3N).

The PVF/PET-PVF laminate did not show stable peeling, the PVF peel arm broke before peeling could be established at a maximum force of 8.6±1.4N. For the PVF/PET-PVF laminate it was not possible to do the adhesive fracture toughness evaluation.

Due to the differences in the peel angle ф the adhesive fracture energy (GA) values of the PVDF/PET laminate without silicon oxide barrier layer scattered significantly. GA values of 192±23J/m2 were obtained. Doing the evaluation for each configuration A and B, Ga values of 210±12J/m2 and 175±17J/m2 were calculated for configuration A (PVDF peel arm top) and B (PET peel arm top), respectively. For the PVDF/SiOx-PET laminate (with the silicon oxide barrier layer) an enhanced GA value of 284±20J/m2 was determined. In comparison to common thermoplastic laminates which are used for packaging purposes the tested laminates showed equal or more than 30% higher GA values (Moore and Williams, 2001).

The investigations on the two PVDF/PET laminates with and without SiOx barrier layer indicated that the energy based fracture mechanics approach is much more

sensitive to describe the delamination behaviour compared to the usually applied load based analysis.

Fig. 6 indicates that the adhesion between PET and EVA is much higher than between PET and the other materials (PVF, PVDF). A peel force P of 142±6N, a peel angle ф of 77.3±0.8° and an adhesive fracture energy of 7895±454J/m2 were determined. The obtained GA value is significant higher than any in the literature reported value of GA for plastics. Hence, for the investigated laminates a delamination within the backsheet material may be more likely than delamination between backsheet material and EVA solar cells encapsulation material.

Acknowledgements

The research work of this paper was performed within project S.9 at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the Kplus — Program of the Austrian Ministry of Traffic, Innovation and Technology with the contributions by the University of Leoben, Graz University of Technology, Johannes Kepler University Linz, Joanneum Research ForschungsgmbH and Upper Austrian Research GmbH. The PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria.