One of the common defects in bulk molding compound (BMC) parts is mottling, which makes it difficult to mold a deep colored BMC parts. The detailed cause of mottling in BMC parts is still unknown. In this paper, the relationship between coloruniformity in BMC parts and dispersed particle size of the Low Profile Additive (LPA) was studied.Mottling of cast parts, which were composed of unsaturated polyester resin (UP), LPA, deep color pigment and catalyst, were observed with an electron microscope. It was observed that there were large LPA particles containing large voids in light colorparts, and small LPA particles with small voids in deep colored parts. It was assumed that the control of dispersed particle size of LPA and formed voids could allow BMC parts of deep uniform color. Addition of a lightly cross linked polystyrene (LCPSt),with a particle size of 5-30 micrometer (μm), reduced the dispersed particle size of LPA, and narrowed the distribution of LPA particle size. Consequently, color-uniformity improved.By controlling dispersed particle size of LPA, mottling in BMC parts could be reduced, and the applications and market of BMC and SMC expanded.
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