ADHESIVE STRENGTH OF DEFORMED EPOXY AND POLYESTER COATINGS

The adhesive strengths of a soft, ductile polyester paint and a stiff, brittle epoxy resin on metal substrates equibiaxially strained to dif ferent levels were investigated by pull-off testing. The stress state in the samples after straining was estimated from relaxation tests on unsupported films. It was found that the epoxy behaved elastically at low strains and plastically at higher strains, whereas the polyester r esponded plastically at all levels of imposed strain. The pull-off str ess of the polyester decreased strongly with increasing strain, indica ting the formation of defects at the interface. No influence of the el astic strain in the epoxy coating on the pull-off stress could be dete cted. In both the epoxy and the polyester the location of failure move d towards the interface with increasing strain. After the onset of int erfacial failure, the increase in area fraction interfacial failure wi th strain was found to proceed similarly in both materials.