Modeling plasticizer uptake in aqueous polymer dispersions

Abstract

The mass transport of water-insoluble plasticizers, emulsified in an aqueous, colloidal polymer dispersion (Aquacoat®) was investigated. A model was developed, which allows quantitative predictions and gives further insight into the transfer mechanism. Experiments proved the validity of the model under various conditions. There are two important mass transfer processes: (1) dissolution of the plasticizer droplets and (2) diffusion of the plasticizer within the polymer particles. Initially, after adding the plasticizer to the aqueous dispersion, dissolution governs the kinetics (dissolution rate < diffusion rate). But increasing amounts of plasticizer located within the polymer particles lead to decreasing concentration gradients and subsequent declining diffusion rates. A change of the governing mechanism is observed. During the final portion, diffusion controls the transfer kinetics (dissolution rate > diffusion rate). Dissolution and diffusion are taken into account simultaneously in the new model, which was used to determine the diffusion coefficients and dissolution rate constants of various plasticizers. Knowing these parameters, the minimum stirring time, necessary for a complete plasticizer uptake (to avoid inhomogeneous coatings) can be calculated.