International. Thermal yellowing and photobleaching are common problems that are seen to some extent in all top-layer systems for today's automobiles. These effects can cause difficulties in matching light colors at the assembly plant and then in delivery to the customer.
The current literature does not offer a clear mechanism for thermal yellowing and photobleaching. Based on the formulation dependencies observed, BASF automotive coating researchers proposed a mechanism based on the oxidation of amine compounds. The scientists built an in vitro system model 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. The loss of basicity confirmed that amine oxidation occurred under these conditions. Both the location of the PEAK OF UV absorption and the strong dependence of the amine structure indicate the formation of nitrone such as the yellow chromophore.
Source: Journal of Coatings Technology and Research.
