Rapid, reliable tests of clearcoat weatherability: a proposed protocol

Abstract

In this paper, a protocol is proposed for the rapid prediction of durability of automotive enamel clearcoats. The protocol is designed to minimize test time while maximizing test reliability. The protocol is based primarily on chemical measurements of degradation rates rather than observation of physical failure. There are two key aspects to the protocol. The first is the use of electron spin resonance spectroscopy (ESR) to measure the rate of formation of free radicals in the coating under near-ambient exposure conditions (photo-initiation rate). This measure is used to rapidly screen coatings under development and to monitor the weatherability of previously qualified materials. Extensive, conventional accelerated exposure tests are performed only on new materials with low photo-initiation rates. The second key aspect is the use of spectroscopic measurements of chemical degradation both to select an appropriate accelerated exposure and to determine the ratio of the rate of chemical change during accelerated exposure to that during natural exposure ( i.e. the acceleration factor). Knowing the acceleration factor and the time to failure in a valid accelerated exposure, it is possible to estimate the service life. The service life is then related to the photo-initiation rate and photo-initiation rate measurements are used in coating quality control. This report describes in detail the steps in the protocol, the kind of chemical measurements which may be required, and provides examples of its application.