Electroplating on crystalline polypropylene. I. Compression molding and adhesion

A direct relationship between polymer processing and metal adhesion is evident from studies of compression‐molded isotactic polypropylene (PP). Cooling rate, nature of the mold surface, and after‐plated annealing are shown to affect the peel adhesion of the plated components. This report described (1) the relation of compression molding variables to polymer surface morphology, (2) the oxidative cracking behavior of the surface due to pretreatment with chromic–sulfuric acid in terms of crystallite orientation and crystallinity, and (3) the effect of surface crack patterns on adhesion. The nature of the mold surface is the single most important variable for controlling the surface morphology of PP. Compression molding PP against oxidized aluminum or copper produces a spherulitic surface, whereas molding the polymer against Mylar or Teflon produces a transcrystalline surface. Surface etching of PP homopolymer produces sponge‐like crack patterns characteristic of the morphology. Radial patterns are observed on spherulitic surfaces and random patterns, on transcrystalline ones. The various surface patterns are developed in the oxidative process by swelling of amorphous material followed by oxidative stress cracking and dissolution. Metal‐to‐polymer adhesion, as measured by the peel test, may involve failure at the interface or within the polymer. Three factors are shown to be important: (a) the geometry of the interface, (b) the diminished strength of the polymer surface arising from attack by the oxidizing acid, and (c) the crystallinity of the fissured polymer surface. The highest peel values are associated with conditions that lead to deep and frequent fissuring of the polymer surface and minimum oxidative damage.