Colloidal and thermal characteristics of concentrated dispersions of polymethacrylate-based latices for aqueous enteric coating.

摘要

We have used rheological and thermal methods to study the colloidal characteristics of a widely used technical latex. The dispersions of poly(methacrylic acid-ethyl acrylate) (Eudragit L100-55) were found to be stabilized by a combination of electrostatic and steric mechanisms termed as electrosteric stabilization. The electrosteric stabilization is considered to arise in part from dissolved polymer chains with charged carboxylic groups extending out into the continuous phase. The presence of dissolved polymer chains in the dispersion implies that coalescence and interpenetration will be facilitated during film formation, enabling a smooth continuous film to be formed. The extent of the stabilization layer and an effective hard-sphere volume was estimated to discuss the steady shear and viscoelastic properties in this context. The glass transition temperature (Tg) of the particles making up the dispersion has also been determined as a function of sorbed moisture and modeled by the Gordon-Taylor equation modified for specific interaction between water, surfactant, and polymer. This parameter at high moisture content can be used as a first approximation to the minimum film-forming temperature (MFT). Change in Tg (and thus MFT) with moisture content implies that the coating process must be controlled so as to produce a rate of drying slow enough to allow coalescence to occur.