COATING ADHESION TO LOW SURFACE FREE-ENERGY SUBSTRATES

With the increased usage of plastics in the automotive industry, the science of polymer (paint) to polymer (plastic) adhesion has gained increased recognition. This paper attempts to delineate the basics to attaining adhesion to plastics and to describe the current means by which adhesion to low surface free energy plastics is attained. Adhesion to very low surface free energy substrates, such as thermoplastic olefins (TPOs; blends of poly(propylene) and rubber), is most often accomplished through the use of a surface pretreatment. The surface pretreatment, for example plasma discharge, flaming, chemical etching, or solvent degreasing, modifies the chemistry of the plastic surface mainly through oxidation and surface roughening. The change in polarity of the surface then affords greater adhesion of subsequent topcoats. Adhesion to low surface free energy plastics can also be attained through the use of adhesion promoters. The adhesion promoter most commonly consists of chlorinated poly(olefin) dissolved in a nonpolar solvent. When applied to the low surface free energy plastic, the solvents can swell and diffuse into the surface. Mechanical interlocking with rubber domains below the surface accounts for afforded adhesion. Modern techniques available to the coatings chemist are described in relation to adhesion achieved. New chemistries developed to replace chlorinated poly(olefins) are reviewed.