Adhesion at partially restructured polymer surfaces

The controlled modification of the surface structure of styrene-4-vinyl pyr idene (SVP) and of ethylene-acrylic acid (EAA) copolymers was achieved by t he timed exposure of the polymers to non-orienting (n-alkane) and orienting (water) fluids. The degree of surface modification was monitored by period ic measurements of non-dispersive surface energies from static contact angl e data. Assemblies of rigid PVC/SVP, of poly(methyl methacrylate) (PMMA)/EA A and of polypropylene (PP)/EAA were prepared with the SVP and EAA in Vario us stages of restructuring. Bond characteristics of these assemblies were e valuated from single lap-shear measurements. Bond strengths responded stron gly to the orientational states of the copolymer adhesives. In PVC/SVP and PMMA/EAA, significant increases in bond strength were observed when orienta tional changes raised the nondispersive contribution, gamma(nd), to the sur face energies of these copolymers. Surprisingly, full increments in bond st rengths were realized at only about 25-40% of the total increment in y(nd). Surface restructuring had little effect on adhesion with the non-polar PP substrate. Residual bond strengths, following controlled aging, varied dire ctly with the initial bond strength. The residual bond strengths may be use d to specify ideal restructuring events in the adhesives which would render the assemblies fully resistant to the chosen aging process. Acid/base inte raction concepts have been used to rationalize the observations. These sugg est that only relatively few polar elements need to migrate into the surfac e region of the adhesives to maximize initial bond strength, but larger num bers are needed to ensure adequate resistance to aging. (C) 1999 Elsevier S cience B.V. All rights reserved.