Scratch resistance behavior of automotive plastic coatings

Abstract

The sensitivity of automotive coatings, particularly coatings for plastics, to scratching has been a growing concern among automakers. Scratching may result from such predelivery events as polishing of minor defects embedded in the paint, or post-delivery events such as car wash bristles, dirt embedded under a cloth utilized in polishing the car, tree branches, and the like. Warranty cannot separate out which event is the more prevalent (e.g., predelivery or post-delivery to the customer) form of damage on plastics. Data available on coatings for metal, however, does suggest that isocyanate-based crosslinked systems perform more poorly than their melamine-based crosslinked counterparts when exposed to in-plant (predelivery) handling. This work attempts to correlate the scratch resistance behavior of coatings of plastics, both in their “green state” (right out of the oven, less than one week post-cure time) and in their infancy in the field (simulated 250 kJ Xenon arc Weather-ometer), to surface attributes such as toughness, hardness, and elasticity. Functional carbamate-melamine crosslinked one-component coatings and functionalized silane-melamine crosslinked one-component coatings appear to out-perform selected two-component coatings, which in turn outperform one-component hydroxyl functional acrylic or polyester melamine crosslinked coatings. Material attributes such as surface hardness, toughness (as measured through the method of essential work), and the ability to recover from an applied load are most important in the ability of the coating to resist damage.