The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in assessing the durability of coatings by measuring weathering-induced chemical composition changes have greatly increased our ability to discern superior from inferior coatings. However, different coatings will likely tolerate different amounts of weathering-induced chemical composition changes while still maintaining their mechanical integrity. Thus, a means of linking chemical composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy required to propagate a crack in a material, is a sensitive measure of the brittleness of a material and is relevant to a number of potential failure mechanisms 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. Weathering embrittles most coatings. Weathering-induced changes in the fracture energy 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 weathering must also be known to accurately anticipate mechanical failures. Measuring thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weathering. The presence of flaws in the clearcoat changes the stress distribution dramatically. Coupled with fracture energy measurements, the stress measurements provide additional insight into paint system failure mechanisms.

The sources of stress in complete automative paint systems have been identified and measured as a function of weathering. In addition to the stresses developed during cure, the main sources of stresses developed during exposure are thermal expansion coefficient mismatch, humidity expansion mismatch, and densification of the clearcoat. Stresses generally increase during weathering due to a slow densification of the clearcoat and increasing water absorption and desorption stresses. Finite element analysis (FEA) was used to compute the stress distribution in full paint systems. Stresses are typically in-plane and highest in the primer and clearcoat. Stresses approaching those required to propagate cracks can be attained in weathered paint systems. The presence of flaws, either cracks or incipient delaminations, will lead to large stress concentrations that can give rise to peeling forces not present in coatings without cracks.

The fracture energy of five automotive clearcoats was measured using fracture mechanics methods originally developed for measuring the fracture energy of brittle inorganic thin films. The fracture energy of the clearcoats ranged from 12 J m−2 to 140 J m−2. A discrepancy between the actual fracture energy and the ‘apparent’ fracture energy was observed and attributed to differences in the tensile stress-strain behavior of the materials, in particular the yield strength. Correcting for differences in their tensile stress-strain behavior, the apparent fracture energy of the films ranged from 13 J m−2 to 330 J m−2. Contrary to expectations, the fracture energy of the clearcoats did not decrease with increasing cross-link density. This was ascribed to differences in chemical architecture and additives in these commercial materials. Measuring the toughness in this manner shows promise for evaluating the long-term cracking resistance of automotive clearcoats.