Mass diffusion-controlled bubbling and optimum schedule of thermal degradation of polymeric binders in molded powders

Abstract

During the thermal removal of polymeric binders from molded powder specimens, the gas pressure of the species produced in the thermal degradation of the polymer may exceed the ambient pressure and then bubbling (a defect) occurs. Here the gas-pressure evolution in the early stages of thermal degradation is modeled using experimental degradation kinetics, thermodynamic models for the gas–liquid equilibrium, models for the diffusion coefficient, and the species conservation principle. The predicted results show a strong dependence of the instantaneous gas-pressure distribution within the specimen on the earlier temperature history and the initial concentration of the volatile product (pre-charge) formed during molding. Through simulations, the minimization of the processing time is addressed using successive constant (but different) heating-rate periods. It is shown that two distinct regimes exist, the pre-charge dominated and the generation-dominated regimes. A single-period heating-rate is dominated by the effect of the pre-charge which can cause bloating at low temperatures. A large number of periods allows for the progressive isolation of the effect of the pre-charge. It is shown that compared to the single-period, the processing time can be reduced by over 60% when a large number of varying heating-rate periods are used. Experiments are performed using poly[ethylene- co-(vinyl acetate)] (EVA) and silicon carbide particles, the results are compared with the predictions, and a good agreement is found.