The tensile strength and free volume of cohesive powders compressed by gasflow

We report measurements of tensile strength and average free volume for a se t of cohesive powders as a function of consolidation stress. Powders with a verage particle size between 7.8 and 19.2 mum were made from a styrene/buta diene copolymer, and were subsequently surface-treated with different conce ntrations of a submicron fumed silica. This silica acts as a flow control a dditive by controlling interparticle forces. The measurement technique cons ists of initialization of the sample by fluidization and subsequent consoli dation by compression under a given gas flow while continuously monitoring the sample volume. By reversing the gas flow, a tensile stress is applied t o the sample. For each consolidation state, we determine the tensile streng th and the average free volume of the powder. We find that the relation bet ween the free volume and the consolidation stress follows a logarithmic for m. The magnitude of the interparticle forces is estimated from bulk measure ments. At high consolidation stresses, the average tensile force per contac t increases proportionally to the square root of the consolidation force pe r contact. Physical implications of these results are discussed. (C) 2001 E lsevier Science B.V. All rights reserved.