(142bv) Coalescence Kinetics and Sintering in Polyester Particles

Coalescence involves elastic contact (neck formation), neck growth (entrapment of bubbles), and densification with the elimination of bubbles. Coalescence has been modeled by evaluating two isolated particles, which are represented by the idealized system of spherical drops. In Frenkel /Eshelby model, coalescence is driven by surface tension and opposed by Newtonian fluid viscosity. The Newtonian model over-predicted the coalescence rate and is not applicable to polymers which are likely to be non-Newtonian and visco-elastic. This model is extended and improved to include elastic effects. An extension of the Newtonian model to the viscoelastic case includes incorporating the Upper Convected Maxwell model (UCM). When steady state stresses are assumed, the rate of coalescence is under-predicted unless relaxation time is considered higher than the experimental data. When transient form of UCM was considered, visco-elasticity is found to be important at short times and at long times it converges to the Newtonian solution, In this work, Newtonian and UCM constitutive model parameters were obtained from the rheological data of linear and crosslinked polyesters. The steady and dynamic rheological data were used to identify an acceptable rate to measure the zero shear viscosity and the transient viscosity. The effects of viscosity, surface tension and relaxation time on the coalescence kinetics of polyesters are discussed.