Semibatch Seeded Emulsion Polymerization of Acrylic Monomers: Bimodal Particle Size Distribution

Abstract

Injecting sodium dodecyl sulfate (SDS) into the reaction medium induces a second crop of tiny primary particles (i.e., secondary nucleation) in semibatch seeded emulsion polymerization. Secondary nucleation may thus result in latex products with a bimodal particle size distribution (BPSD-Latices). The concentration of seed particles ([Seed]) is the most important variable for controlling the particle-size-distribution-related parameter (P), followed by the time when the surfactant is injected into the reactor (t s). The resultant latex particle size (d p) is not sensitive to changes in the concentration of injected SDS ([SDS]). Parameter P decreases with increasing [Seed] or t s. This implies retarded secondary nucleation during monomer addition when [Seed] or t s increases. An experiment with smaller values of [Seed] and t s favors formation of BPSD-Latices. The data clearly show a bimodal particle size distribution when P is greater than approximate 70%. Parameter P correlates well with the total seed particle surface area (A s) regardless of the seed particle size, and P (%) is proportional to A s (cm2) to the -0.319 power. Thus, A s is a very useful parameter for manipulation of the resultant latex particle size and particle size distribution.