Irradiation grafting of bis(2-chloroethyl) vinyl phosphonate onto polypropylene is found to be a very effective way of improving its adhesionability toward metals (aluminum, steel, copper). At low grafting levels representing small radiation doses, a condition of greatly enhanced adhesion with small change in bulk mechanical properties of the polymer is achieved. This enables one to study the interfacial aspects of adhesion practically independent of gross changes in the mechanical properties of the bulk polymer. The results indicate that interfacial properties influence adhesive joint strength up to a critical “minimal value.” At and above this critical degree of surface attachment, the adhesive joint strength is optimized, and a condition is reached where the bulk mechanical (rheological) properties of the adhesive polymer (in both the bulk and boundary layer) become the sole determinant of the adhesive joint strength.
The effectiveness of other functional groups is shown by presenting polymermetal adhesion test results obtained by using polypropylene grafted with other monomers. Empirically, it is found that acids (electron accepting groups) are conducive to strong interfacial adhesion to metals while other functional groups exhibit specificity.