Le Chatelier-Braun principle

Abstract

Le Chatelier-Braun principle was modified for calculating the rate of free energy production [RFEP] in the flow of poly vinyl chloride (PVC) plastisol. At low-to-intermediate shear rate, viscosity decreases with increasing shear rate and reaches a minimum. The behavior was explained by the dominance of the rheological effect at the lower shear rates and that of the thermodynamic effect at the higher shear rates. In the former, viscous resistance was reduced by stress-induced phase separation into an immobilized layer and a mobile phase. In the latter the RFEP increases, reaching a maximum at the minimum viscosity. In the pseudo-plastic flow region, the flow is stable and the RFEP is negative. At shear rates higher than those at minimum viscosity, the immobilized layer dilates and then fractures when the stress becomes higher than the strength of the layer. Both dilatation and fracture are manifestations of flow instability and RFEP is positive, indicating insufficient resistance against failure of the immobilized layer. At super high shear rates, a plug flow takes place with a very thin layer of plasticizer lubricating at the capillary wall. Pseudo-plastic behavior may be explained by two different mechanisms. One is that a very small amount of particles are present in the lubricating layer. The particles are expelled from the layer with the increase in shear rate. The other possibility is that even though the lubricating layer consists of pure plasticizer, the layer increases in thickness in order to avoid excessive secondary flow such as eddy formation.