Encapsulation of room temperature ionic liquids by miniemulsion polymerisation for application in low-emitting latex coating

Latex coating has gained increasing attention. It is prepared by mixing latex, a dispersion of polymer particles in water, with pigments, fillers, and additives. As water is used as solvent, emission of VOCs can be minimised. However, coalescing agent emission and excessive surfactant residual hinder its further applications. To solve these two problems, we tried to use room temperature ionic liquid (RTIL), a potential multi-functional additive with non-volatility, to replace coalescing agent, and employed silica nanoparticle as emulsifier to stabilise the latex. The preparation of latex containing 1-octyl-3-methylimidazolium hexafluorophosphate (C8mimPF6), the target RTIL in this study, could be achieved by encapsulating C8mimPF6 inside particles via miniemulsion polymerisation. Two kinds of emulsifiers, surfactant and silica nanoparticle were employed to stabilise miniemulsion/latex. To achieve this encapsulation, three steps were carried out in succession: preparation of miniemulsion, miniemulsion polymerisation, and characterisation of latex containing C8mimPF6. Factors, especially C8mimPF6, on initial droplet size and stability of miniemulsion, yield and stability of polymerisation process, and final functionality of latexes were systemically investigated. For miniemulsion stabilised by surfactant, sodium dodecyl sulfonate (SDSO), initial droplet size decreased apparently with the addition of C8mimPF6 up to 1 wt% due to the effect of C8mimPF6 on oil phase viscosity and interfacial tension. During storage, C8mimPF6 above 10 wt% led to the frequent coalescence of droplet as it decreased the absolute zeta potential. During polymerisation, when initiated by hydrophobic 2, 2-azobis (isobutyronitrile) (AIBN), C8mimPF6 had a promoting effect on the reaction rate at low concentrations, but this effect might reverse upon certain C8mimPF6 concentrations, e.g. 10 wt%; while initiated by hydrophilic hydrogen peroxide/ascorbic acid (H2O2/AAc), this promoting effect faded ev