Fundamental Studies of Comminution

Comminution is an important step in the production of many materials made from polymers for selective laser sintering, powder coatings and rotational molding compounds. A longstanding problem in grinding technology involves predicting the operating conditions such as temperature and minimum impact velocity for effective comminution. Although many materials break readily at low temperatures, some remain resistant to grinding even when immersed in liquid nitrogen. Here we outline a novel approach to characterize grinding based on 1-dimensional mechanics of particle-mill collisions. Collision of a slab particle with a rigid mill surface generates compression and tension waves that sweep across the particle at sonic velocity. If at any time, deformation exceeds the coherence limit, the particle ruptures. Large amplitude stress-strain measurements, conducted from ambient to cryogenic temperatures, provide the basis for calculation of minimum impact velocity for fracture. Further, a minimum particle size may be determined at which collisions, tempered by ambient resistance, are too slow to cause fracture. Cryogenic grinding tests on polyethylene and ethylene copolymer resins reveal differences in failure mode (ductile/brittle) consistent with model predictions.