Surface analysis and adhesion in fluoropolymer

Abstract

A semi-empirical way to elucidate adhesive bonding phenomena is based on the surface analysis method guided by qualitative fracture mechanics theory. The latter emphasizesinherent flaws, viscoelastic and plastic response of materials, crazing, crack propagation and interfacial separation; our method determines the extent of each mechanism. In untreated fluoropolymers, surface molecules transfer to the adhesive even though bond strength is negligible. Surface treatment increases the surface energy via a hydrocarbon layer, the thickness of which varies from ~20A ("Teflon" FEP, Type C or gold-crystallized) to >500A with sodium etching. Increased adhesive bond strength is equivalent and fracture occurs relatively deep in the fluoropolymer with an amount of plastic deformation related to strength. When the surface treatment is depleted by heat or light, bond strength varies with surface composition. Cohesive failure occurs in both "Teflon" FEP and "Teflon" PFA used as hot-melt adhesives, but FEP separates much closer to the substrate and with little deformation compared with PFA, which helps to explain the order of magnitude greater strength of the latter.