Rotational moulding of PVC plastisol: modelling of initial stages of gelation

Abstract

The viscosity during the earlier stages of the gelation process of plastisols formulated with 15 different commercial plasticizers has been obtained. Plastisols prepared were rotomoulded and mouldings obtained were characterized according to their wall thickness distribution. This parameter strongly depends on the minimum viscosity and on the rate of viscosity change during gelation (slope of the curves during the viscosity increase), and also on the temperature where no more flow of the plastisol is observed (gelation temperature). These variables are conditioned by the characteristics of the plasticizer and PVC employed, among other variables. The effect of plate and arm velocity and rotation rate on the wall thickness distribution was also studied. A simple model has been proposed to correlate the initial stage of the gelation of PVC plastisols formulated with the phthalate plasticizers. The suggested model assumes that the plastisol is a suspension constituted by a continuous phase (initially pure plasticizer) and a discontinuous phase constituted by PVC particles. It has been considered that the viscosity of the plastisol is influenced by the average mass molecular weight of the continuous phase and the temperature. The quantity of PVC dissolved in the continuous phase has been calculated by using a generic n-order kinetic equation for PVC dissolution rate. Additionally, a swelling process of the PVC particle by the plasticizer has also been considered and its effect on the viscosity of the suspension has also been accounted for. The model proposed provides a very good fitting of the sharp viscosity increase observed and is useful to predict the evolution of the viscosity of the parts in a rotomoulding process.