ADHESION TO SKIN .1. PEEL TESTS WITH HARD AND SOFT MACHINES

A modified adhesive peel test is described in which a spring is inserted between the test machine cross-head and the peeling strip. In tests using two pressure sensitive surgical adhesives (uncrosslinked elastomers) this 'soft machine' gives results which differ significantly from those obtained with a conventional 'hard machine'. In particular, when peeling energy is plotted against peeling velocity, the soft machine reveals a regime of low energy peeling and a transition to the normal high energy peeling. The transition behaviour has been studied as a function of adhesive thickness, cross-head speed and spring stiffness. The phenomena revealed by soft machine testing are interpreted in terms of variations in crack-tip radius caused by flow of the uncrosslinked rubber. The practical implication is that far more information can be obtained from soft machine tests than from conventional hard machine tests. The problem of oscillating peel force is also eliminated.