Mechanism and prevention of winter discoloration of light-colored PVC exterior trim

摘要

In recent years, trends towards monochromatic styling have increased the use of body side moldings matched to panel paint colors. These moldings can be painted to match, but are increasingly using pigmented flexible poly (vinyl chloride) to improve durability and reduce part cost. Even complicated "special effect" colors like metallics and pearlescents are now matched in pigmented vinyl. However, after several winters in severe climates like the Great Lakes region, some white moldings were observed to be noticeably yellowed. Since white is the most popular choice of car buyers, this is particularly troubling. The effect proved to be difficult to consistently reproduce in laboratory experiments. This paper will describe work done at The Geon Company to identify the likely chromophore via analyses of staining road "slop" and yellowed moldings. This paper will then describe the proposed mechanism whereby the chromophore appears in the part. The relative roles of transport and chemical phenomena in creating the observed effect will be proposed. Last, this paper will show how the yellowing can all but be eliminated by designing the plastic compound so as to retard and interrupt the proposed mechanism, while retaining other desirable physical and chemical properties.