Inverse microsuspension polymerisation of aqueous acrylic acid using redox initiators
In an inverse microsuspension polymerisation initiated by a redox pair, at least one component of the redox pair must be segregated from the monomer initially to restrict the extent of polymerisation before the dispersion is established. Normally, the monomer and the oxidant in aqueous solution are dispersed in an oil phase, and the aqueous reductant is separately added to start polymerisation. Thus, the reaction system initially consists of two types of aqueous drop and the distribution of reactants is heterogeneous in nature. In the literature, no report to date has been found concerning the mechanistic aspects of this polymerisation process. The present work concerns polymerisation of aqueous acrylic acid in drops that are dispersed in a paraffinic oil and stabilised with a non-ionic surfactant. A sodium metabisulphitel potassium bromate redox initiation system is used. Drop mixing in agitated dispersions has been investigated by examining the aqueous drop behaviour in simulation systems using various developed techniques. The evolution of aqueous drop size and size distribution has been characterised throughout the course of polymerisation using a freeze-fracture technique with electron microscope. The variations of polymerisation rate, limiting conversion, final particle size and size distribution have been investigated by altering several key parameters, such as agitation intensity, volume and composition of the aqueous reductant and ways of adding the aqueous reductant. Aqueous polymerisation of acrylic acid in a singlephase, initiated by the redox pair, has been also studied and a reaction scheme has been proposed. Kinetic relationships for polymerisations in both an aqueous homogeneous medium and a heterogeneous medium have been obtained and rationalised in terms of mechanism. A new hypothesis for the polymerisation process has been developed which differs from both conventional suspension and emulsion polymerisations. It is believed that the polymerisation takes place in the aqueous drops as a result of continuous simultaneous coalescence and break-up of the different types of aqueous drops. The surfactant has important effects on the course of polymerisation. A preliminary model has been developed for describing this specific polymerisation process. The publications arising from the present project are listed in Appendix VIII.