G. Oreski1, G. M. Wallner2, A. Skringer3, P. Pert!3, A. Plessing3

1) Polymer Competence Center Leoben GmbH (PCCL), Austria

2>Institute of Materials Science and Testing of Plastics, University of Leoben, Austria

3) Isovolta AG, Werndorf/Graz, Austria

E-Mail: gernot. oreski@stud. unileoben. ac. at

Multi layer films, produced by ISOVOLTA AG (Werndorf/Graz, Austria), are used as backsheets for novel, flexible photovoltaic (PV) modules. Backsheet materials have to fulfil electrical insulation and moisture barrier properties. Multi layer polymer films allow a significant weight reduction and more flexibility of PV modules (Plessing, 2003). A specific failure mechanism frequently observed in multi layer films is the phenomenon of delamination. This paper focuses on the description and evaluation of the delamination behaviour of several tested multi layer films using two different methods.

To analyse the peel tests, on the one hand a conventional load based method with the result of the peel force per unit width is applied (according to ISO 11339). On the other hand, a more sophisticated energy based fracture mechanics approach has been developed (Kinloch et al., 1994). The energy based fracture mechanics method
yields the adhesive fracture energy GA [J/m2], which is supposed to be a material parameter.

The peel force indicates how difficult it is to peel one layer from another, while the adhesive fracture energy describes how well the two layers are stuck together. When the two peel arms are made of different materials with different stiffness, the peel angles will be ф < 90° and 0 > 90°.

To separate the two peel arms from each other energy in form of external work has to be provided. The adhesive fracture energy is calculated from this external work, which contains several deformation and failure processes.

Uext is the external work and Us the stored strain energy. Udt und Udb refer to the dissipated energy through irreversible tensile deformation and irreversible bending deformation of the peel arm. B is the specimen width and da the peel fracture length. In an ideal case (peel arms of infinite modulus, no irreversible bending deformation) Ga is related to the peel force P and the peel angle ф (index 1 and 2 refer to peel arm 1 and peel arm 2).

The energy dissipation Gdb is a complex function which is described by Kinloch et al. (1994). The adhesive fracture energy of the laminate is the sum of the values of the two peel arms.

The advantage of the energy based fracture mechanics method is that the results are totally independent of specimen geometry and testing parameters.

To calculate the adhesive fracture energy two experiments are required. First a T-Peel test has to be carried out to measure the peel force P and one of the peel angles. Second a tensile test of each peel arm has to be performed. The stress-strain curve is approximated to a bilinear model (s. Fig. 2), that describes the real material behaviour of many polymers quite well. The results of the bilinear model

are the elastic modulus Ei, the plastic modulus E2 and the yield strain ey, which is the intercept of the two lines.

Fig 2: Stress-strain curve of PET and bilinear model approximation

Experimental

Three multi layer film types (laminates) were characterized as to their delamination behaviour. The slash (/) indicates the investigated interface.

• poly vinylidene fluoride / polyethylene terephthalate (PVDF/PET)

• poly vinylidene fluoride / silicon oxide barrier layer — polyethylene terephthalate (PVDF/SiOx-PET)

• poly vinyl fluoride / polyethylene terephthalate — poly vinyl fluoride (PVF/PET — PVF)

• polyethylene terephthalate / ethylene vinyl acetate — poly vinyl fluoride — polyethylene terephthalate — poly vinyl fluoride (PET/EVA-PVF-PET-PET)

Whereas the investigation of the first three laminates focuses on the adhesion between various backsheet material layers, for the latter laminate type the adhesion between the EVA encapsulation and the PET backsheet material is characterized.

For the delamination tests rectangular specimens were used. The two parts of the laminate have already been adhered but there is a region of unadhered material. Both, the peel tests and the tensile tests were done using an Instron 4505 tensile testing machine. To eliminate the kinetic energy associated with a moving fracture the peel tests were conducted at a crosshead speed of i0mm/min. The peel force P and the peel angle ф were recorded. The peel tests were carried out using two different configurations. In configuration A (s. Fig. 1) the stiffer peel arm 2 is at the bottom. In configuration B the specimen was revolved through 180° (peel arm 2 at the top). The adhesive fracture energy was calculated using the T-Peel. exe program developed by the Adhesion, Adhesives and Composites Group at Imperial College London (http://www. me. ic. ac. uk/AACgroup/index. html).

For the tensile tests rectangular strips of a width of 10mm and a length of 100mm were used. The tensile tests were conducted at a testing speed of 10mm/min.