The Effect of Non-radiation Factors on the Weathering of Silicone Hardcoats
In the course of many years of developing thermally cured hardcoat systems at Momentive Performance Materials Inc. (MPM), it has been observed that model predictions of time to failure for coatings subjected to cycled temperature, humidity, and radiation are not always accurate. Particularly in systems where cracking failures occur sooner than adhesion failures, these models seem to break down. This paper reviews the results from ongoing experiments at MPM that attempt to decouple the radiation factor from the typical weathering exposure conditions of a hardcoat system on a polycarbonate substrate. The goal is to identify a “best case” or “entitlement” lifetime for a coating that has not experienced any photodegradation. Additionally, these experiments provide direction as to which factors are most critical in driving cracking failure in hardcoat systems. Discovery of the driving forces for crack propagation in these systems should enable a new model to be derived that would be a better predictor of real-world failure.
KeywordsCracking Adhesion GMOD Polycarbonate Siloxane Topcoat Primer Automotive Glazing Coating Degradation Hardcoat Entitlement Fracture Mechanics Cycling Temperature Humidity Time-to-failure
The authors are grateful to helpful discussions with Mark Nichols of the Ford Motor Company and Professor Michael Thouless of the University of Michigan at Ann Arbor in the interpretation of the fracture energy equation. Additional thanks go to Thomas Ford and Robert Travis of Momentive Performance Materials Inc. for making most of the measurements reported in this paper.