A New Approach to the Control of Polymer Composition in Emulsion Copolymerisation

Henry's law and its application to the precise control of the emulsion copolymerisation of certain chosen monomers was investigated at 50°C and 75°C.

The solubility, solubilisation and vapour pressure relationships of seven monomers namely vinyl chloride (VC), vinyl acetate (VAC), acrylonitrile (AN), vinylidene chloride (VDC), butadiene 1,3 (Buta ay; ethylene (E) and propylene (Pr), were investigated in water and Anne solutions of sodium dodecyl sulphate (sps) and aqueous sodium oleate (for propylene only). A combined pressure differential method (mercury manometer) and gas chromatographic technique was used in this study.

Co-solubilisation studies were also carried out on predetermined monomer pairs. Evidence was obtained to show that a solubilised monomer could act as a co-solvent for the other monomer in any comonomer pair chosen. This co-solvent effect was greater with some monomers e.g. acrylonitrile and vinyl acetate, than others e.g. vinyl chloride and vinylidene chloride. Co-solubilisation investigations were.
also carried out on copolymer latices of varying solids content. For the comonomer pairs studied, the cosolubilisation effect was marginally greater in the latices than in the corresponding soap solutions.
Over the surfactant concentration range examined, Henry's law was obeyed by all the monomers. For the more soluble monomers, namely acrylonitrile and vinyl acetate, deviations from Henry’s law occurred at relatively low pressures. For the less soluble monomers e.g. ethylene and vinylidene chloride, a linear relationship (between vapour pressure and amount of monomer solubilised) was obtained above atmospheric pressure.

Henry's constants (obtained experimentally) and monomer reactivity ratios (published values) were necessary prerequisites for the calculation of a particular initial comonomer charge ratio for a given pair of monomers to be copolymerised.
This was not a unique ratio but varied with other parameters chiefly the volume ratio of the head-space of the reactor to the liquid medium (see figure 5.2.3).

Using non-conventional copolymerisation conditions i.e. total absence of monomer globules in the aqueous reaction medium copolymers of a pre-determined homogeneous composition were obtained.: This was only possible by maintaining and monitoring the comonomer feed charge such that the amounts of comonomers at the reaction sites were precisely known and conditions were simulated such that Henry's law prevailed throughout the copolymerisation process.

Diffusional problems were overcome by maintaining a high rate of agitation, Despite the "starvation" conditions of copolymerisation, latices of a relatively high solids content were obtained (30% - 40%) over the period of polymerisation studied (10-14 ½ hours).

Copolymers of a homogeneous chemical composition were obtained thus confirming that Markevich's (164) copolymerisation theory for solution systems applies to emulsion copolymerisation.