Impact breakage of acicular crystals

Acicular crystals are frequently encountered in pharmaceutical and fine chemical industries. They are prone to breakage, as a result of which their bulk behaviour changes drastically. It is therefore of great interest to be able to assess their breakage propensity quickly and preferably using a small quantity. An integrated experimental and modelling method is proposed to quantify the breakability of acicular crystals by aerodynamic dispersion using the disperser of Morphologi G3, in which a pulse of compressed gas induces particle impact. Needle-shaped crystals of β l-glutamic acid, benzothiazin and potassium phosphate are used as the model test materials. The extent of breakage as a function of dispersion pressure is obtained by quantifying the shift in particle size distribution provided by Morphologi G3. It increases as the dispersion pressure is increased for β l-glutamic acid and benzothiazin particles, while not noticeably for potassium phosphate crystals. The impact velocity of the particles at different pressures is estimated by computational fluid dynamics (CFD) calculations. Its effect on the extent of breakage is used to infer the ease with which the crystals break, expressed by a lumped parameter as the ratio of hardness over square of toughness. Benzothiazin breaks by fracture along both crystal length and width due to presence of cleavage planes, whilst β l-glutamic acid breaks only perpendicular to the length. Potassium phosphate undergoes minor chipping from the edges and corners.