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In order to reduce unwanted emissions, future GM vehicle assembly plants, as with many current plants, will be using powder primer technology.
(Published on August-September 2005 – JEC Magazine #19)
BY HAMID G. KIA, BHAVESH SHAH, TERRENCE J. WATHEN, HARRY A. MITCHELL, AND CHRISTINA R. BERGER, MATERIALS & PROCESSES LAB GM R&D CENTRE
In order to reduce unwanted emissions, future GM vehicle assembly plants, as with many current plants, will be using powder primer technology. However, powder priming of SMC has become a major problem due to the popping of the primer coating in the bake oven  (fig.1). Because of this cosmetic defect, a widespread usage of SMC in exterior applications would only happen if the panels could be coated without defects. GM R&D, therefore, initiated a project to investigate and identify the materials’ properties and process characteristics that lead to the failure of SMC panels during powder priming. In the first phase of this project, a variety of SMCs and conductive primers/sealers were evaluated for their performance in producing a pop-free surface when powder coated . It was concluded that popping increases with increase in moisture content of SMC. However, the results on dry panels varied from one substrate to another. It was observed that: - sheet metal did not show popping when coated with powder at 20°C, - SMCs such as SMC-1 and SMC-3  did not show popping when coated with powder in the dry state at 20°C, - SMCs such as SMC-2 and SMC-4  showed popping when coated with powder even in the dry state at 20°C.
SMC-4 is a commonly used SMC in the automotive industry and is used in a variety of cars. Therefore, it is important to find out why some SMC materials behave differently with respect to powder priming, and to that end, a close examination of the powder application process is needed in order to better understand the above anomalies. In this process, the powder is applied in a spray booth that is maintained at 20°C and 55% relative humidity. Then the coated parts are transferred to the bake oven to cure the powder. The powder primer melts in the bake oven and forms a very viscous liquid followed by polymerization and cross-linking to form a rigid film. The popping occurs when entrapped gasses force their way out of the coating while it is still in the liquid state and has not yet solidified. Therefore, the popping of the primer coating could occur through two mechanisms: 1) the expansion of any entrapped air in the powder, and 2) the degassing of SMC substrate. Each of these mechanisms could have multiple causes and should be analyzed individually.
The air entrapment in the powder could have three causes: a) chemical incompatibility at the interface of the powder and the conductive coating, b) the substrate static charges, and c) skin formation on the liquid film. The chemical incompatibility means that the powder finds it difficult to wet the surface, resulting in the entrapped air. The static charges of the substrate, on the other hand, could interfere with the normal release of the air that exists between the powder primer particles, or it could interfere with the way the charged particles are laying on the surface during the spray. The skin formation could become possible if there is a significant temperature gradient within the liquid film caused either by the heating process, or the thermal conductivity of the substrate (…) The full article is available at our web site www.jeccomposites.com, in the JEC Composites Magazine section.