The 180° peel strength of four ethylene vinyl acetate copolymers with varying V.A. content was measured (Pol A: 28%, Pol B: 18%, Pol C: 12% and Pol D: 9.5% by weight). The polymers were applied as hot-melts with or without an inextensible fabric support on: (a) mild steel etched in hydrochloric acid; (b) chemically polished copper and; (c) oxidised copper with a fibrous topography. The peel loads for a given polymer decreased from oxidised copper to etched steel to polished copper. For a given substrate the peel loads generally decreased from polymer A to polymer D with a peak for polymer C. This trend was correlated to similar trends obtained in mechanical tests like the tearing energy, strain energy density and tensile strength to failure. The fractured parts of the bond were examined with a scanning electron microscope. There was microscopic evidence of an analogy between the peel load and the observed extent of polymer deformation in the fractured surfaces where the ductilty of the particular polymer and the topography of the substrate were clearly depicted. Contact angle measurement showed a cohesive type of failure in all cases. X-ray photo-electron spectroscopy and multiple internal reflection infra-red spectroscopy provided evidence of polymer oxidation by the substrate, mostly significant in the case of etched steel. The energy balance approach was employed in order to analyse the peel test results further. The polymers response to the imposed mechanical deformations during peeling was shown to control the measured peel load. For the unbacked samples the major energy loss mechanism is the stretching of the freed strip. The much higher peel loads of the backed samples were attributed to energy losses around the peel front. Finally, plastic bending of the polymer was found to account for a relatively smaller part of the total input energy for both backed and unbacked samples.
|Date of Award||1985|