Improvement of the Low-Temperature Stability of an Aqueous Colloidal Ethylcellulose Dispersion, Aquacoat® ECD, and Preparation/Characterization of a Redispersible Aquacoat® ECD Powder

Abstract

Aquacoat® ECD, an aqueous ethylcellulose dispersion, is susceptible to low-temperature storage, resulting in irreversible coagulation as indicated by a strong increase in viscosity and particle size. This destabilization of the ethylcellulose dispersion is caused by the anionic surfactant sodium dodecyl sulfate, which precipitates at low temperatures because of its low Krafft point. This problem could be solved by using the more hydrophilic, ethoxylated, sodium dodecyl ether sulfate, which was an effective stabilizer at low temperatures. A redispersible ethylcellulose powder was prepared by freeze- or spray-drying of the original polymer dispersion (Aquacoat ECD). The pH of the dispersion medium had a strong influence on the redispersibility of the dried ethylcellulose particles because of the dissociation behavior. At a pH>7, polymer dispersions in the colloidal size range were obtained. At lower pH values, the dried ethylcellulose agglomerates could not be redispersed. Drug release studies from pellets coated with the redispersed and the original dispersion showed a comparable release pattern when using alkalized aqueous dispersion systems. Higher plasticizer concentration and curing of the coated pellets improved the film formation of the redispersed polymer particles.