Types of plastic substrate

The choice of a particular coating for a plastic substrate is dependent upon the nature of the substrate itself. Some of the more common plastics materials that require coating are identified below. 

These plastics are stable up to relatively high temperatures, chemically inert and resistant to most common organic solvents. Hence as a class they present fewer problems to the coatings formulator. 

The most commonly used group of plastics in this class are polyurethanes which are resistant to most solvents, with the exception of n-methyl pyrollidone which swells the substrate but improves adhesion. RIM (reaction injection moulded) polyurethane finds use in the automotive sector for bumpers, spoilers and external trims. Polyurethanes are affected by heat because of their cellular structure, often craters or pimples are formed in the coating as a result of the substrate s porosity. Hence components are often manually prepared with primer filler, pore filler and several de-nib stages prior to the application of a topcoat. In recent years this high labour requirement has resulted in the replacement of polyurethane by alternative plastics. 

Also of considerable importance are the polyesters, sheet moulded compound (SMC), dough moulded compound (DMC) and glass reinforced polyester (GRP), these again are labour intensive substrates. 

The only other major group of thermosets are the phenol formaldehyde and urea formaldehyde plastics. These are often used for closures in the cosmetic industry and are either coated to colour co-ordinate the component or vacuum metallised. 

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