Effect of C₈mimPF₆ on miniemulsion polymerization for application in new latex coating products

C₈mimPF₆, as a type of room temperature ionic liquid (RTIL) with non-volatility and a low melting point, may replace conventional coalescing agents in latex coatings, thus preventing volatile organic compound (VOC) emissions caused by coalescing agents. In this study, systematic investigations on the effect of various factors including initiator type, initiator concentration, temperature and C₈mimPF₆ concentration on the conversion of latex and droplet/particle size of a miniemulsion during polymerization have been conducted. The presence of C₈mimPF₆ has shown to have a marked effect on the reaction rate. Such an effect strongly depends on the type of initiator being used. For polymerization initiated by 2,2-azobis (isobutyronitrile) (AIBN), C₈mimPF₆ had a promoting effect on the reaction rate at low concentrations, but this effect might be reversed upon certain C₈mimPF₆ concentrations, e.g. 10 wt%. While initiated by H₂O₂/Vc, this promoting effect faded even at low C₈mimPF₆ concentrations. The different limiting factors, which determine the reaction rate with different types of initiator, may contribute to the results. For reactions initiated by hydrophobic AIBN, the reaction was dominated by kinetics. The presence of C₈mimPF₆ may cause an enhanced chain propagation rate and reduced chain termination rate, which may further contribute to the increase in reaction rate at lower concentrations of C₈mimPF₆. With hydrophilic H₂O₂/Vc, the resistance for the transfer of radicals into a droplet/particle might be increased significantly with increasing C₈mimPF₆ concentration due to a tighter interfacial structure at lower concentrations of C₈mimPF₆. Thus, such transfer of radicals may become a limiting step whilst the presence of C₈mimPF₆ increases the transfer resistance on radicals resulting in a decrease in reaction rate. The reaction temperature, which is related to the decomposition temperature of the initiator being used, was another factor affecting the conversion of latex and the size of latex particles. A higher temperature e.g. 50 °C promotes the coalescence of droplets/particles, and hence produces larger latex particles. In the presence of C₈mimPF₆, the reaction temperature could be significantly reduced to as low as 40 °C, which prevents phase separation. The final particle size depends on the nucleation mechanism as well as the coalescence of droplets/particles during polymerization.