DIRECT MEASUREMENT OF POWDER COHESION USING A TORSIONAL DEVICE

A device for the direct measurement of cohesion, which is the shear st rength of a powder when no normal stress is applied to the plane of sh ear, is discussed. The apparatus, called the Warren Spring-Bradford co hesion tester, works according to the principle of the annular shear c ell. The central part is a hollow cylinder to which eight spokes are a ttached, connected by an outer ring. Two materials, lactose and soda a sh, have been tested extensively in a wide range of sizes. By plotting the cohesion versus the mean size, a clear distinction can be made be tween free-flowing powders, characterized by a size-independent cohesi on, and cohesive powders that exhibit a cohesion the value of which is a strong function of particle size. The size at which the transition between the two states occurred was approximately 55 mu m. Apart from dense copper powder the values of the cohesion could be correlated sat isfactorily with those derived from the Jenike shear cell. This cohesi on tester served its intended purpose very well, i.e. obtaining a qual itative comparison as regards the flowability of powders of different sizes.