An Investigation of the Relationship between Glass Transition Temperatures and the Crystallinity of Spray-Dried Powders

The relationship between the glass transition temperature and the crystallinity of spray-dried powders has been investigated in this study, by spray drying solutions of lactose, sucrose, and ascorbic acid in a Buchi B-290 spray dryer (Buchi, Switzerland). The degree of crystallinity was assessed using water sorption tests and modulated differential scanning calorimetry (MDSC). The material with the lowest glass transition temperature, ascorbic acid, is almost completely crystalline under virtually all spray-drying conditions, whereas the material with the highest glass transition temperature, lactose, is partly amorphous and partly crystalline under different spray-drying conditions. With lactose, changing the inlet air temperature from 134 to 210 degrees C changes the degree of crystallinity by about 10%. For sucrose, a similar change in temperature changes the degree of crystallinity by a larger amount, from almost completely crystalline at high temperatures to very amorphous at lower temperatures. These results are consistent with the Williams-Landel-Ferry equation and with the theory that the rate of crystallization is greater when the difference between the material temperature and the glass transition temperature of the material is higher. Tests on sucrose and ascorbic acid using liquid chromatography and chemical assays have shown that the amount of degradation for these materials in the drying chamber is less than 10%.