THE DISPERSION OF PIGMENTS INTO DRY POWDER BASE 471 could be cut to 15 min. The main difference in powder behaviour in the "mixing" bowls appears to be in the degree of fluidization achieved by reason of the design of the rotors used. In the Moritz mixer a single rotating disc fitted with angled radial blades is used at a speed of 160-320 rpm (depending on the powder). This gives a certain degree of fluidization, but this is appreciably less than that given by the second method (below). The layout is illustrated diagram- matically in Figs. 2a and 2b. Ves*el Wall - _ Figure 2a Plan view of inside of Moritz mixer Figure oeb Diagrammatic representation of powder movement in Moritz mixer Size reduction probably occurs both as a result of the powder particles coming into high speed collision with the rotor and vessel walls, and also by particle/particle collisions. The Papenmeier mixer, on the other hand, utilises a vertical combination of a propeller type rotor with two sets of angled paddles, which create a vortex with the aid of a variable baffle. Figs. 3a, $b and 3c illustrate the process. Variable Baffle •----Vessel Wall ]-J r opellet .... l/•••• Upp ..... gled Paddle -• Lox• e• .... anFled Paddle Figure $a Plan view of interior of Papenmeier mixer

472 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Variable Baffle -- ..• Upper angled Paddle •-------Lower angled Paddle Propeller , Fiõure $b Vertical section through Papenrn•i•r mixer Figure $c Diagrammatic representation of powder flow in Pap•nm•ier mixer Size reduction is again probably due to particle/equipment and particle/ particle collisions. The rotational speed is similar to that of the Moritz mixer (Figs. oea and oeb). One very significant difference between the two pieces of equipment is the size of motor needed, that for the Moritz being about 50 hp whilst the Papenmeier needs 175 hp for a similar load. Some of this difference is probably accounted for by the increased degree of fluidization in the Papenmeier. The No. 2 size Mikro pulveriser utilises a 10 hp motor. COLOUR ASSESSMENT In order to simplify the colour measurements which were made on the dry powders, the latter were combined with a small percentage of wax base and pressed into godets to produce smooth-surfaced cakes.