Effect of Processing Shear Time on EW Formulations

Abstract

Concentrated aqueous emulsions, or EWs, are the dispersion of a water insoluble organic liquid into water. The formulation is achieved through the use of polymeric surfactants, which provide multiple anchoring points and steric stabilization to prevent coalescence. Formation of the EW is mechanically driven, requiring high-shear processing to reduce particle size and attain a stable formulation. In this study, the relationship between processing shear time, particle size, and stability was examined. Samples were prepared using a high hydrophile/lipophile balance (HLB) polymer (butyl block copolymer) paired with a low HLB polymer (nonionic block or random copolymer), varying polymer types and polymer ratios. Shear was applied for 10 min to 40 min. Emulsion dilution stability (ASTM E1116-98), particle size distribution (Malvern Mastersizer), and high temperature stability were used to characterize the samples. It was found that particle size decreased as shear input was increased, reaching an optimum after 25 min to 30 min of shear, depending on the polymer pair used. All samples performed similarly in dilution stability showing improved performance after 10 min of shear. After 1 month at 54°C, all samples showed no separation and demonstrated similar dilution performance, however, D90 values dropped significantly (>20 %) in samples with less than 20 min of shear, whereas samples receiving a minimum of 20 min to 30 min of shear showed decreases in D90 around 10 %. A likely explanation for the decreases in D90 observed after elevated temperature storage is that the addition of heat energy drove the particle surface area toward equilibrium according to the equation ΔA=W/γ, where ΔA is change in surface area, W is work, and γ is interfacial tension