Thermal yellowing and photobleaching of automotive topcoats

Thermal yellowing and photobleaching are common problems that are seen to some extent in all automotive topcoat systems today. These effects can cause difficulties in the matching of light colors in the assembly plant and then later upon delivery to the customer. Current literature offers no clear mechanism for the thermal yellowing and photobleaching. Based on observed formulation dependencies, a mechanism is proposed based on oxidation of amine compounds. We constructed a model in vitro system using a hydroperoxide to thermally oxidize amines in solution. In this system, a series of amines were tested and both yellowing and photobleaching were reproduced. Loss of basicity confirmed that amine oxidation occurred under these conditions. Both the location of the UV absorption peak and strong amine structure dependence indicate nitrone formation as the yellow chromophore.