Fine grinding of polymers in a vibrated bead mill

Three polymers, having various glass temperatures, have been ground in a shaker bead mill. One is a friable polymer (poly(vinyl acetate), PVA) and two are thermoplastic polymers (polystyrene (PS) and polyethylene (PE)). These materials have been ground for various operating conditions (bead and polymer loads, vibration frequency, temperature). It was found that, in the case of the friable polymer, the size reduction occurs mainly according to a cleavage mechanism. Nevertheless, in specific operating conditions, attrition may happen. In the case of the thermoplastic polymers, different steps appear during the size reduction: abrasion, creation of cracks and fragmentation. Then, the fragmentation mechanism depends on the polymer nature. It is shown that it is necessary to work with a temperature below the glass temperature of polymer. Generally, the rate of size reduction increases with a decrease of the bead load and of the powder load. In the case of PVA, the evolution of the size distributions has been predicted by two statistical laws (the normal law at the beginning of the experiments and a combination of the normal and the log-normal laws at the end), whereas for PS, only log-normal laws have permitted to predict the evolution of the size distributions. (C) 1999 Elsevier Science S.A. All rights reserved.