Predicting the density and tensile strength of viscoelastic soy powder compacts

The key physical property of compact density is important for scale up and influences the mechanical properties such as tensile strength. Hence, the objective of this research was to develop semi-mechanistic models for density of Soy Flour (SF) and Soy Protein Concentrate (SPC) powder compacts as a function of viscoelastic properties and process variables. Each of the powders was compacted at a punch speed of 5 mm/min to final compaction pressures in the range of 26-230 MPa, in a 13 mm diameter cylindrical die. Pressure-time profile during compaction, compact height and force relaxation during the dwell time were also recorded. Compact density was measured under maximum pressure and 24 h post ejection. The force relaxation data was fit to linear Maxwell model for a viscoelastic solid (R-2 > 0.99) and the viscoelastic properties of the pure solid material were estimated by extrapolating to zero compact porosity. Although density of compacts from either powder was found to be comparable, SPC produced much stronger compacts compared to SF due to higher bonding. The estimated compact density under pressure was found to be much higher than the pycnometer measured true densities. The contact area between two particles was estimated using Lum and Duncan-Hewitt (1999) using median particle size, pressure-time during compaction and material viscoelastic properties. Power law and logarithmic models that express the compact density and tensile strength respectively as a function of total viscoelastic contact area in a compact were successfully developed (R-2 > 0.95-0.99). Thus, viscoelastic properties were found to influence compact density and tensile strength through their influence on the contact area. (C) 2012 Elsevier Ltd. All rights reserved.