Micronization of a Soft Material: Air-Jet and Micro-Ball Milling

The air-jet and ball-mill are frequently used in fine micronization of active pharmaceutical ingredients to the order of 1-5 mu m, which is important for increasing dissolution rates, and also for pulmonary delivery. In this study, we investigated the ability of air-jet and ball-mill to achieve adequate micronization on the lab scale using a model soft material, Pluronic (R) F-68. Material mechanical properties were characterized using the nanometer 600. Pluronic (R) F-68 was ball-milled in a micro-mill at different material weights and durations in liquid nitrogen vapor. In comparison, a lab scale air-jet mill was used at various milling parameters according to a full factorial design, where the response factors were particle yield and particle size distribution, which was analyzed using laser diffraction and scanning electron microscopy. The yield achieved with the micro-ball mill was 100% but was similar to 80% for the air-jet mill, which reduced the size of Pluronic (R) F-68 from 70 mu m to sizes ranging between 23-39 mu m median diameters. Ball milling produced particles less than 10 mu m after 15 min. Although air-jet milling proved capable of particle size reduction of the relatively soft material Pluronic (R) F-68, limitations to the lower size range achievable were observed. The feed rate of the material into the air jet mill was a significant factor and slower feed rates lead to smaller sizes by allowing more time for particle collisions and subsequent particle breakage to occur. Micro-ball milling under cold condition was more successful at achieving a lower range particle size reduction of soft materials.