The Interphase in Adhesion

Abstract

The role of weak boundary layers (WBL) in determining the breaking stress of adhesive joints has been proposed to be that of a discrete surface layer of material with strength properties inferior to the bulk material from which it originated. The breaking strength behavior of various polyethylene-epoxy adhesive lap joints has been used as prima facie evidence for the presence or absence of WBL. So-called WBL in PE appear not to be removable by extraction, by abrasion or by fractionation but they seem to disappear when one uses PE as a hot-metal adhesive. We shall attempt to place the WBL matter in better perspective by discussing boundary layers in a general way; that is, evidence for their existence, what they might be where we can isolate and identify them, how they develop and how we can control their properties and, finally, how they influence joint behavior. We conclude that the WBL postulated to explain the behavior of unmodified PE is probably a fiction and that the mechanics of the composite system alone, independent of any material property change in the vicinity of the interface, can determine that the joint shall fail in a thin layer of PE (with properties no different from the bulk) near the interface. Modification of PE mechanical properties in a thin layer near the interface, as by crosslinking or the presence of a transcrystalline structure, can markedly change the mode and locus of failure. The basis for these changes may be as “simple” as a decrease in stress concentration at the joint edges associated with the rise in toughness of the PE produced by cross-linking or transcrystallinity in the surface region.