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 solution s and glycerol) and binder contents (0.40 to 0.55 m(3) binder/m(3) sol id). These pellets were dropped from heights of 10 to 30 cm and the am ount of impact deformation measured. Impacts were mostly plastic with a coefficient of restitution less than 1%. The energy conservation mod el of Hawkyard [J.B. Hawkyard, A theory for the mushrooming of flat-en ded projectiles impinging bn a flat rigid anvil, using energy consider ations, Int. J. Mech. Sci. 11 (1969) 313-333] for rigid-plastic materi als was used to calculate the pellet's dynamic yield stress (Y) from t he size of the deformed area. When water was used, Y increased exponen tially with decreasing surface mean particle size. However, when glyce rol was used, particle size had no measurable effect on Y in the range of conditions studied. Increasing binder viscosity and increasing bin der surface tension both increased Y. The effect of binder content var ied-increasing the amount of a viscous binder (glycerol) increased Y t hroughout the range of conditions studied, whereas when a low-viscosit y binder (water) was used, Y passed through a maximum. Hence, there ar e (at least) three energy dissipation mechanisms that control impact d eformation. These are due to interparticle friction, capillary and vis cous forces. The effect of varying binder content in a particular syst em cannot be predicted a priori, unless the balance between these thre e mechanisms is known. Measurements of granule deformation must be mad e at high strain rates if dynamic effects are to be accounted for. The simple technique developed has the potential for being used to charac terise different formulations in order to better predict their granula tion behaviour. (C) 1998 Elsevier Science S.A. All nights reserved.