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.