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

Abstract

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.

Preparation and characterization of a customized cellulose acetate butyrate dispersion for controlled drug delivery

“…However, the addition of sodium chloride or potassium chloride did not cause any flocculation, suggesting absence of any electrostatic forces between the particles. These experiments explain the stability behind the polymeric particles of pseudolatex 13. The key issue in pseudolatex dispersion is long‐term stability.…”

“…The Dougherty‐Kreiger equation for a hard sphere system is given by η r = (1− Φ eff /Φ eff,m ) −[η] Φeff,m , where, Φ eff is the effective volume fraction, Φ eff,m is the maximum theoretical packing fraction corresponding to a hexagonally packed sphere structure (loose packing, coordination number 8)13 and [η] is the intrinsic viscosity. From all other values, Φ eff can be calculated and an estimate of Δ can be figured to fit the experimental points (Fig.…”

“…An Ostwald viscometer was used for the measurement of viscosity of pseudolatex as a function of solid content 13. Relative viscosity was calculated by estimating the viscosity of the dispersed medium (emulsion stabilizer solution), and by taking the ratio of viscosity of pseudolatex to dispersed medium.…”

Characterization of the interactions between polymethacrylate‐based aqueous polymeric dispersions and aluminum lakes

“…Steric stabilization (due to the presence of the nonionic surfactant polysorbate 80) may also play a role because of the high stability of Eudragit ® L 30 D‐55 to the dyes at pH 5.2. Paulsson and Singh12 have reported that an electrosteric stabilization mechanism from the dissolved polymer chains is also present in this latex. One reason why Eudragit ® L 30 D‐55 exhibits higher critical coagulation concentrations than Eudragit ® RS 30 D is because, like the dyes, it is negatively charged.…”

Properties of treated papers and plastic film influencing ethyl ester transfer

“…According to Schuman et al (2004b,c), latex films do not contain crystalline regions, i.e., they are totally amorphous. Depending on latex composition and RH of storage, T g can vary widely, typically from 30 to 130°C (Paulsson and Singh, 1999;Xu et al, 2005). For styrene-acrylate lattices, Nestorson et al (2007) determined a T g value in the range of 2-5°C.…”