Improvements of the peel test for adhesion evaluation of thin metallic films on polymeric substrates

Abstract

Abstraet—For evaluating the adhesion of thin metallic films with thicknesses in the range of 50-1000 nm on polymeric substrates, the following variant 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 peeled off from the polymeric substrate, hopefully, together with the metal layer. In cases where the adhesion of the metal layer to the polymeric substrate is very good (peel forces 5 N/cm), instead of peeling off the metal layer, cohesive failure within the laminated EAA foil occurs and the EAA starts to tear or even break due to overstretching. Two improvements of this standard peel test variant have been developed to overcome the problem of the limited EAA cohesive strength and thereby to extend the upper test limit. One is to glue another thin and flexible but tearproof reinforcing foil [such as polyimide (PI) with a thickness of 7.5 μm] with an epoxy adhesive onto the EAA foil, thus preventing the EAA from being excessively stretched. The other new method consists in using a semicrystalline auxiliary polymeric foil with a very high tensile strength, such as poly(ethyleneterephthalate) (PET) or poly(ethylenenaphthalate) (PEN), and pretreating the surface of this auxiliary foil by UV-excimer laser irradiation leading to amorphization of its near-surface zone. The amorphized surface 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 irradiation) with the bulk material remaining completely unchanged, the high tensile strength of the semicrystalline auxiliary foil is conserved, allowing high peel forces to be achieved without reaching the cohesive failure limit of the foil.