The effects of weathering on the mechanical performance of automotive paint systems

The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in asse ssing the durability of coatings by measuring weathering-induced chemical c omposition changes have greatly increased our ability to discern superior f rom inferior coatings. However, different coatings will likely tolerate dif ferent amounts of weathering-induced chemical composition changes while sti ll maintaining their mechanical integrity. Thus, a means of linking chemica l composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy req uired to propagate a crack in a material, is a sensitive measure of the bri ttleness of a material and is relevant to a number of potential failure mec hanisms in automotive paint systems. The fracture energy of clearcoats can vary widely depending on the formulation of the clearcoat (initial chemical composition and additive package) and on the amount of weathering. Weather ing embrittles most coatings. Weathering-induced changes in the fracture en ergy are related to chemical composition changes occurring in the clearcoat . Because the brittlest materials will not crack without an applied stress, the stress distribution in complete paint systems as a function of weather ing must also be known to accurately anticipate mechanical failures. Measur ing thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weat hering. The presence of flaws in the clearcoat changes the stress distribut ion dramatically. Coupled with fracture energy measurements, the stress mea surements provide additional insight into paint system failure mechanisms. (C) 1999 Elsevier Science S.A. All rights reserved.