Platinum Metals Rev., 1976, 20, (1), 11
The Electroless Plating of Plastics
Significant advances in the plating of plastics with metals such as copper and nickel have been made since the introduction of the “one step” process in which the sensitiser and the activator are applied to the substrate by immersion in a solution containing palladium chloride and stannous chloride in hydrochloric acid. This mixed system was first developed in 1961 by C. R. Shipley, who maintained that the solution contained colloidal particles which, when adsorbed on the surface of the work, provided catalytic sites for the metal to be deposited. Later workers have claimed, however, that their catalyst systems are true solutions and that their activity is due to the presence of soluble tin-palladium complexes.
Controversy on this point has continued, but a paper by E. Matijević and P. Zuman of the Institute of Colloid and Surface Science at Clarkson College of Technology and A. L. Poskanzer of the Shipley Company, Newton, Mass., now shows not only that these solutions do contain colloidal particles but that, on separating these out in an ultracentrifuge, they are active while the supernatant solution is not (Plating, 1975, 62, (10), 958–965).
A total of 16 PdCl2/SnCl2 systems were investigated, all samples being put through the ultracentrifuge at 25,000 r.p.m., and the two phases were separated for further examination. Ten of the solutions, as prepared before separation, were studied by electron microscopy, and all showed the presence of colloidal particles. Light scattering, which has been used by other workers as a test for the presence of colloidal particles in such catalysts, was found not to be a satisfactory means of discrimination.
Assessment of activity was carried out with all the samples, using epoxy laminated boards and copper plating after standard cleaning and sensitising procedures. Each formulation was aged for one week, separated in the ultracentrifuge, and the residue and supernatant fractions tested separately. In all cases the supernatant liquor showed no activity, while all the residues were highly active.
As the authors conclude, systematic chemical analysis of the colloidal particles present in these proprietary catalysts would seem to be highly desirable if the precise mechanism of their activity is to be established.