Liquid-bound granule impact deformation and coefficient of restitution

The impact behaviour of liquid-bound granules and pellets was studied using a novel but simple experimental technique. Cylindrical pellets (20 mm diameter and 25 mm long) were made from 10, 19, 31, 37 and 60 mu m glass ballotini with a range of binders (water, surfactant solutions and glycerol) and binder contents (0.40 to 0.55 m(3) binder/m(3) solid). These pellets were dropped from heights of 10 to 30 cm and the amount of impact deformation measured. Impacts were mostly plastic with a coefficient of restitution less than 1%. The energy conservation model of Hawkyard [J.B. Hawkyard, A theory for the mushrooming of flat-ended projectiles impinging bn a flat rigid anvil, using energy considerations, Int. J. Mech. Sci. 11 (1969) 313-333] for rigid-plastic materials was used to calculate the pellet's dynamic yield stress (Y) from the size of the deformed area. When water was used, Y increased exponentially with decreasing surface mean particle size. However, when glycerol was used, particle size had no measurable effect on Y in the range of conditions studied. Increasing binder viscosity and increasing binder surface tension both increased Y. The effect of binder content varied-increasing the amount of a viscous binder (glycerol) increased Y throughout the range of conditions studied, whereas when a low-viscosity binder (water) was used, Y passed through a maximum. Hence, there are (at least) three energy dissipation mechanisms that control impact deformation. These are due to interparticle friction, capillary and viscous forces. The effect of varying binder content in a particular system cannot be predicted a priori, unless the balance between these three mechanisms is known. Measurements of granule deformation must be made at high strain rates if dynamic effects are to be accounted for. The simple technique developed has the potential for being used to characterise different formulations in order to better predict their granulation behaviour. (C) 1998 Elsevier Science S.A. All nights reserved.