EFFECT OF GLIDANT ADDITION ON FLOWABILITY OF SOLIDS 485 Table I The particle size of bulk solids below which impaired flow occurs Estimated critical Method of assessment Material particle size (gm) Source of flowability Silica sand 204 (25) Flow through orifice Quartz sand Sodium chloride Sodium carbonate Citric acid N-cyclohexyl 2- benzothiazole- sulphenamide Strontium nitrate Acetanilide Ballotini Lactose Light magnesia Heavy magnesia Quartz sand Glass beads Sand Griseofulvin Lactose Sodium borate Boric acid Calcium gluconate Coal Sulphathiazole 150 150 35O 150 175 4OO 35O 5O 120 250 158 250 300 300 200 250 150-300 100-250 (26) (27) (28) (29) (3O) (31) (32) 250 (33) lOO 4OO (34) (35) Slide down a roughened inclined plane Flow through orifice Flow through orifice Flow through funnel Flow through orifice Static angle of repose Angular characteristics Flow through orifices Angle of repose Angle of repose presence of 'fines' in a tablet granulation and in these cases the addition of a flow-aid such as a glidant should be considered. The addition of glidant material of similar chemical constitution to the bulk solid When fine particles of size less than the optimum for flowability are added to a bulk solid of similar chemical constitution there is often an improvement in the rate of flow through an orifice (7-9). The effect is demonstrated in Fig. 1 for systems of heavy grade magnesia. The improve- ment is dependent upon the size and concentration of the fine particles

486 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1400 1200 iooo 8OO 6OO 4oo I I I I I 0 20 40 60 80 I00 % w/w odded fine powder Figure 1 The effect of size and concentration of fine particles on the flow rate of magnesia (851 gm) through a circular hopper orifice 11.4 mm diameter. the smaller the particles the lower the concentration required to produce an increase in flow but not necessarily a greater flow rate. The effect has also been shown for lactose (Table II). Table II The effect of size and concentration of fine lactose particles on the flowability of lactose granules (1 242gm). Results interpreted from (7). Arithmetic mean size in micrometres of added fine particles 626 335 213 163 111 -74 Estimated percentage fine material required to produce optimum flow Indeterminate 75 50 40 25 15 Rate of flow at optimum Rate of flow of plain granule 1.15 1.32 1.38 1.44 1.38 1.19 The concentration of fine material that is required to produce a flow rate maximum is, however, strongly dependent upon the orifice diameter of the hopper the required concentration of glidant increases as the orifice