Investigation Of Bulk Molding Compound With Low Shrinkage

Bulk Molding Compound (BMC) plays a prominent role in the family of unsaturated polyester (UP) in the industry of compression molding. BMC have widely used in the industry of fiber reinforced plastics for its low price and good processing. However, a volume shrinkage rate of 7%～10% may occur after UP being solidified, which would cause a series of negative effects and heavenly hinder the industrialization of UP. In order to get access the problem, adding the low profile additives was considered the perfect resolution presently. Since the power fillers have the biggest mass ratio among BMC, Consequently, we investigated the volume shrinkage effects of different fillers to UP, including Al(OH)3, CaCO3. Furthermore, the effects of the different length of chopped glass fiber to the strength of BMC also been studied in this work.The addition of fillers has influence the filler/UP composite system. The volume shrinkage rate of adding mix filler with different ratio was lower than that of the single filler system according to the gradation theory. The mechanism of fillers affecting the crosslinking of UP can be concluded as follows:the fillers substitute the non-crosslinked part of resin, which decreases the volume shrinkage rate of the composite system. After the crosslinking of the composite system, the holes formed among the fillers and UP make the volume shrinkage rate decline. Styrene volatilize little at the effect of fillers, which increases the crosslinked density of UP, and resulting a higher volume shrinkage ratio. By studying the effects of different kinds of low profile additives on the volume shrinkage of UP and its mechanism, we found that the LPA could decrease the volume shrinkage rate through forming hole during the crosslinking of UP.The chopped glass fiber is the key factor to the strength of MBC. Hence, by studying the different lengths of chopped glass fiber (3mm,6mm,12mm,25mm) on the strength of BMC, the most appropriate length and fiber content were chose. Meanwhile, the reasonable mix ratio of powder fillers was determined with the gradation theory. On the basic of a fixed fiber content, the optimum mass ratio of different powders was determined. At that condition, it is able to avoid the fiber's formation of sphere, align in parallel or cluster, which affects the strength of BMC. At last, we provide three kinds of formulas suitable for different environment. When the glass content is 19%, filler content is 53%, the bending strength up to 95MPa, the products has both high strength and has a better appearance; When the glass content is 22%, filler content is 53%, the bending strength up to 98MPa,only for the internal components; When the glass content is 14%, filler content is 63%,the blending strength is only 87MPa,the requirements can be used for low blending produsts.