Particle Nucleation in Semi-batch Surfactant-free Emulsion Polymerization of Acrylic Monomers

Based on the coagulative particle nucleation mechanism, a two-step model has been developed for the semi-batch surfactant-free emulsion polymerization of butyl acrylate, or butyl acrylate in the presence of a small amount of acrylic acid. During Stage 1, precursor particles are first formed by phase separation of the oligomeric radicals in the aqueous phase. These precursor particles are extremely unstable and tend to aggregate until a stable (primary) particle size is achieved. The size of the newly formed particles is controlled mainly by limited coagulation among the precursor particles in this stage. During the early part of Stage 2, the rate of aggregation of these precursor particles to form a primary particle may be slower than the rate of coagulation of the primary particles. This factor would lead to a decrease in the particle concentration with time. Later on, the particle concentration starts to level off and finally reaches a steady value due to the effect of limited coagulation. In this stage, the particle growth is achieved mainly by polymerization of the feed monomer inside the particles. This model has been assessed by a number of experiments. The reaction variables selected for this purpose are the concentrations of initiator, acrylic acid and NaHCO3, and the agitation speed. The proposed model predicts the experimental data of the particle concentration and particle size reasonably well.