THE DISTRIBUTION OF PLASTICIZERS BETWEEN AQUEOUS AND POLYMER PHASES IN AQUEOUS COLLOIDAL POLYMER DISPERSIONS

The distribution of water-soluble (triethyl citrate and triacetin) and water-insoluble (acetyltriethyl citrate, acetyltributyl citrate, dibutyl phthalate, dibutyl sebacate, diethyl phthalate and tributyl citrate) plasticizers between the aqueous and polymer phases in an aqueous colloidal ethylcellulose dispersion, Aquacoat(R), was determined. A separation scheme was developed, which allowed the determination of the amounts of plasticizers dissolved and, in the case of water-insoluble plasticizers, also emulsified in the aqueous phase, and dissolved in the colloidal polymer particles. The plasticized ethylcellulose dispersion was separated by centrifugation and/or ultracentrifugation to obtain the various plasticizer-containing fractions. The plasticizer concentration in each phase was determined by a HPLC assay. The extent of the plasticizer partitioning was investigated with respect to the type (water-soluble or water-insoluble) and concentration of the plasticizer, and the solids content of the polymer dispersion. Water-insoluble plasticizers mainly partitioned into the polymer particles due to the higher affinity towards the polymer phase, however, the amount of emulsified plasticizer droplets increased with increasing plasticizer concentration after a plasticization time of 24 h. An 'association' coefficient, which was obtained from the ratio of the plasticizer concentration in the polymer to the concentration in the aqueous phase, was used to characterize the plasticizer partitioning. The fraction of plasticizer taken up by the colloidal polymer particles increased with increasing solids content of the polymer dispersion. It is therefore recommended to add the plasticizer to the undiluted dispersions followed by dilution to the desired solids content just prior to coating, rather than first diluting the dispersion followed by addition of the plasticizer.