Evaluation of factors affecting repaint adhesion of automotive coating systems

Abstract

As it is today, when for some reason a car body needs to be repainted in the factory, either because of defects, or because an additional colour is to be applied, the body must go through a step called sanding. Sanding is a process where the top layer of the car body coating is being grinded. This is required in order to prepare a surface on which a required adhesion can be achieved. However, with sanding comes consequences. The process of sanding takes time and additional place on the conveyor belt, which in turn costs a lot of money and reduces the productivity. An additional negative side effect is that the sanding causes pollutant particles in the air in the factory, which leads to other car body coatings to get defects, which in turn leads to additional car bodies requiring repainting and sanding. Thus, a vicious circle is created. It is therefore desired to skip sanding. It is suspected that different factors and processes from the painting process have different influence on the repaint adhesion. In this work, the goal was to evaluate some of these factors influence on the repaint adhesion. This was done by applying coatings to test panels in a procedure simulating the steps in the factory. The repaint adhesion was evaluated by exposing the panels to a high pressure car wash jet. Examples from the factors evaluated are, curing time and temperature. Also, different coating materials and properties of the repainted surface, such as surface energy and hardness were among the factors evaluated. This work resulted in the findings that, among the factors evaluated, the most significant was those concerning the curing of the clear coat, namely, curing time and temperature. Also, indications, that suggest that the amount of rest-isocyanate on the repainted surface could have an influence on the repaint adhesion, was observed.