The Rheology of Bulk Molding Compounds

In the last decades there has been considerable growth in research related to the development of new energy alternatives such as solar energy, wind energy, and fuel cells. Fuel cells, such as the direct-methanol and the hydrogen fuel cells, have emerged as one of the most promising alternatives. Bipolar plates play an important role in a fuel cell stack. Bipolar plates for hydrogen fuel cells are currently compression molded from bulk molding compounds that consist mainly of graphite powder and thermoset resins. Currently, the rheology of these compounds is not well understood and many fundamental questions remain to be answered. In this study, which is part of a larger effort to find a method for characterizing the flow properties of these compounds, we seek to determine whether or not the compounds slip at the surface of the mold in order to use this information in the search for a constitutive equation to describe their flow behavior. To determine the degree of slip at the surface we added pigments to cylindrical pucks made from the compounds before compressing them into disks between heated parallel plates. We photographed the pucks before and after compression and used the photographs to measure the location of the pigmented regions before and after compression. The results indicate that in some cases the material slips at the surface but that the pigment used as indicator tends to stick to the surface of the plates and affect the flow of the compounds between them. The roughness of the plates was also found to affect the flow of the compound between the plates. The next step in this investigation will be to analyze the data already on hand to screen out experimental artifacts and to use the results in the search for a rheological model that describes the flow behavior adequately.