IMPROVEMENTS OF THE PEEL TEST FOR ADHESION EVALUATION OF THIN METALLIC-FILMS ON POLYMERIC SUBSTRATES

For evaluating the adhesion of thin metallic films with thicknesses in the range of 50-1000 nm on polymeric substrates, the following varian t of the peel test is often applied: a hot melt foil such as ethylene acrylic acid (EAA) is heat-laminated onto the metal side of the metal/ polymer composite and subsequently this flexible auxiliary foil is pee led off from the polymeric substrate, hopefully, together with the met al layer. In cases where the adhesion of the metal layer to the polyme ric substrate is very good (peel forces greater than or equal to 5 N/c m), instead of peeling off the metal layer, cohesive failure within th e laminated EAA foil occurs and the EAA starts to tear or even break d ue to overstretching. Two improvements of this standard peel test vari ant have been developed to overcome the problem of the limited EAA coh esive strength and thereby to extend the upper test limit. One is to g lue another thin and flexible but tearproof reinforcing foil [such as polyimide (PI) with a thickness of 7.5 mu m] with an epoxy adhesive on to the EAA foil, thus preventing the EAA from being excessively stretc hed. The other new method consists in using a semicrystalline auxiliar y polymeric foil with a very high tensile strength, such as poly(ethyl eneterephthalate) (PET) or poly(ethylenenaphthalate) (PEN), and pretre ating the surface of this auxiliary foil by UV-excimer laser irradiati on leading to amorphization of its near-surface zone. The amorphized s urface can then, just like the ordinary hot melt foil, be easily heat- laminated to the metal surface with excellent adhesion. As the polymer morphology is influenced by the excimer laser treatment to a maximum depth of only a few hundred nanometres (penetration depth of the UV ir radiation) with the bulk material remaining completely unchanged, the high tensile strength of the semicrystalline auxiliary foil is conserv ed, allowing high peel forces to be achieved without reaching the cohe sive failure limit of the foil.