2015 Volume 48 Issue 11 Pages 922-926
Crystal morphology is important in the production of crystalline particles because the morphology can affect the downstream processes after crystallization, such as solid–liquid separation, and the product quality. In particular, needle-like or plate-like morphology is undesirable in terms of the production of crystalline particles with high purity. Methacrylic acid, which is important as a raw material of specialty acrylates for coating resins, has to be high purity, and purified by melt crystallization. Suspension melt crystallization is suitable for purification because of the large crystal surface area per unit volume. However, methacrylic acid crystals by melt crystallization are long rod-like or needle-like crystals in typical morphology. Therefore, it is desired to improve the morphology of methacrylic acid crystal in the melt crystallization process. The authors have proposed a concept of combining growth and dissolution processes in melt crystallization for a morphology improvement. The objective of this study was to obtain the basic data for a strategy for a morphology improvement of methacrylic acid crystals by controlling the melt crystallization condition. The growth rate and dissolution rate of a single crystal of methacrylic acid were investigated in the major and minor axis directions in terms of supersaturation/undersaturation and additives as impurities (maleic acid, acrylic acid, and water). Based on the ratio of the growth rate in the major axis direction to the growth rate in the minor axis direction opposite to supersaturation, methacrylic acid crystals will easily become needle-like under high supersaturation condition. Therefore, supersaturation should be kept at a low level in order to obtain methacrylic acid crystals with improved morphology. For dissolution, methacrylic acid crystals tended to dissolve in the major axis direction for all of the impurity conditions of mother liquor. From these results, it was concluded that the basic data to discuss a strategy for a morphology improvement of methacrylic acid crystals were obtained.