Polymer to metal adhesion. The system polyvinyl acetate–steel

In the limit of small film thickness the effective tensile adhesion of polyvinyl acetate to steel is of the order of the tensile strength of the polymer. Furthermore, the dependence of the limiting tensile adhesion on temperature, rate of loading, molecular weight, molecular weight distribution, and plasticizer content is the same as that of the cohesional tensile strength suggesting that in thin adhesive films the mechanism of failure is in cohesion. At greater thicknesses, shear components derived from strains in the adhesive film initiate failure at progressively lower tensile loads, causing the effective tensile adhesion to be lowered. The above results are realized with clean, though at least partially oxidized metal surfaces and carefully purified polymer. Even minute traces of fatty acids as surface impurities are capable of reducing the adhesion to less than one‐half of the original value. In a particular example, 0.05 of a calculated monolayer of decanoic acid reduced the limiting effective tensile adhesion from 7000 to 3900 p.s.i. Inasmuch as the adsorbed impurity is soluble in the polymer, it can be desorbed on prolonged heating, and, provided the polymer film is thick enough, the original adhesive strength can be restored. In extremely thin films, some permanent loss in adhesion will result from the establishment of an adsorption equilibrium in which appreciable amounts of fatty acid are left on the surface. The effect of perfluorodecanoic acid on the adhesion of polyvinyl acetate to steel is almost exactly the same as that of decanoic acid. The adhesion of polyethylene to steel is likewise critically dependent on surface purity. The results of this research point to the need for extreme cleanliness in studies of adhesion, a fact which, though seemingly obvious, has all too often been overlooked in past investigations.