Harry A Mitchell

Powder priming of SMC. Part II: Failure mechanism

Current sheet molding compound (SMC) panels, when coated with powder primer, show primer popping in the bake oven due to the degassing of the substrate moisture. In the absence of moisture, however, it was found that some SMCs showed severe popping, while other SMC materials did not. These results suggested that, contrary to general opinion, the moisture content in SMC might not be the only cause for popping. This report covers the results of a systematic study that was carried out to identify the factors that contribute to powder primer popping. Several potential variables that could affect popping, such as volatiles in the substrate, thermal conductivity of the substrate, static charges, and powder bake profile were studied. The experiments showed that the poly(vinyl acetate) in the low profile additive (LPA) is the main cause. More specifically, the micro void formation of this low profile additive that eliminates SMC shrinkage, and enables a smooth surface finish, also causes primer popping. Based on the experimental results, it was concluded that the air permeation into these micro voids is the reason behind the popping of the moisture free SMC substrates. This understanding of the failure mechanism, paves the way for the development of low moisture absorbing SMC materials that can be powder primed in our assembly plants, and could remove the barriers for the usage of SMC on GM vehicles.

Powder Priming of SMC. Part I: Assessment of the Current Technologies

A cosmetic defect called ‘popping’ on the surface of sheet molding compound (SMC) body panels that are coated with powder primers has become a major hurdle in expanding the usage of SMC in automotive applications. General Motors R&D, therefore, initiated a project to study the mechanism of popping and to explore different solutions to this cosmetic problem. The current report covers the benchmarking phase of this project, in which a variety of SMC materials and conductive primers/sealers were evaluated for their ability to produce a pop free surface. In this work, among other factors, the effects of molding pressure and the panel moisture content on the degree of popping have been studied. The experimental results showed that popping increased with the increase in moisture content and with the decrease in molding pressure. The extent of popping, however, varied with the type of SMC and the conductive primer. We found that none of the SMC materials or conductive primers were able to eliminate the pops completely at high moisture levels. More importantly, it was concluded that moisture is not the only cause of the popping and there are other factors that contribute. The results of the current work will be used in the next phase of the investigation, which is focused on identifying solutions to this cosmetic problem.