The use of various types of plastics as structural and decorative material has been growing for many years in a variety of applications. This includes end uses in the container and packaging industry, in the automotive industry in both the interior and exterior of cars, and also in consumer and industrial applications. The articles being produced require printing and/or coating, but the sensitivity of plastics to heat (e.g., all thermoplastics, by definition) or to solvent attack puts constraints on the type of coating or ink that can be used. Because radiation-curable coatings or inks are natural contenders for use on such substrates, they are growing in use and popularity. Nevertheless, the adhesion of radiation-curable coatings to certain plastics remains a challenge not only because of their low surface energy, but also due to wide variation in the surface properties for a given category of plastic. The low-surface-energy plastic substrates, e.g., thermoplastic olefins (TPOs), have a very high growth rate in a variety of applications, as a result of their superior flexibility, durability and lower cost.
After reviewing the main parameters or factors that can influence or affect adhesion of radiation-curing coatings to plastic substrates, this paper presents our proposal regarding possible solutions to obtain adhesion to difficult plastic substrates. To support this proposal, the performance of recently developed adhesion promoters is presented, as well as the methodology we recommend for the optimum selection of components for a radiation-curable formulation.