Influence of Punch Velocity on the Compressibility of Granules

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Abstract
The influence of punch velocity over the range 1-30 mm/min on the compressibility of granules and particles was studied using a modified Kawakita equation. Granule strength (St) increased with increasing concentration and viscosity of binder solution as well as increasing mean molecular weight of binding agent. The compressibility of powders was evaluated by the modified Kawakita equation (K). For the granules, the relationship between pressure and reciprocal of porosity (I/ε) showed an inflection point, but for the particles, no such inflection point was found. The slope of low compression stress in the stage of densification by powder slippage and rearrangement was the constant K1; and slope of high compression stress in the stage of elastic, plastic deformation and plastic fracture of particles was the constant K2. The K1 values decreased with increasing punch velocity. An approximately linear relationship was observed between reciprocal of compressibility constant (I/K2) and granule strength (St). For crystalline lactose, K values decreased with increasing punch velocity, indicating that compressibility was lowered. Thus, compressibility was shown to be dependent on type of crystal. The quantity of stress relaxation increased with increasing punch velocity. Especially, constant a and b values in a cellulose system (HPC) were greater than those in a noncellulose system (PVP). As a good relationship was found between constants a, b and constant K2, materials which undergo plastic deformation and fragmentation have great stress relaxation. The radial tensile strength (σt) increased with increasing granule strength (St). We concluded from these findings that the σt value was affected by the contact area rather by than the number of contact points.