Water dilutable secondary stabilisers in suspension polymerisation of vinyl chloride monomer

Abstract

One of the continuing trends in the development of superior quality suspension poly(vinyl chloride) (sPVC) resin has been the need to impart higher grain porosity, facilitating improved vinyl chloride monomer removal and faster, more uniform, plasticiser uptake. To achieve this, PVC producers have adopted the use of specific secondary suspending agents, traditionally in the form of low hydrolysis, low molecular weight poly(vinyl acetate) (PVAc).It is well known that low hydrolysis PVAc is insoluble in water, and as such must be supplied as alcohol or alcohol rich solutions, typically in methanol. The use of methanol presents safety and environmental hazards relating to its particular toxicity and flammability. These concerns led to the development, during the 1980s, of water based secondary suspending agents, with a low methanol content, typically at 55% hydrolysis. These partially hydrolysed PVAcs remained for a long time the optimum hydrolysis at which a stable, aqueous product could be supplied, while imparting good PVC grain porosity. Nevertheless, in certain sPVC formulations, aqueous based 55% hydrolysed secondary suspending agents were failing to achieve the same high porosity levels as the solvent based systems, which typically had hydrolysis levels below 50%.The recent development of a lower hydrolysis, water soluble, fully dilutable grade, named Alcotex WD30, has provided an ideal solution, giving all the advantages of both secondary systems. This has been achieved by the ionic modification of the polymer, imparting superior water solubility, and allowing the production of a stable, aqueous partially hydrolysed PVAc at less than 50% hydrolysis. It has been demonstrated, using experimental design software, that such a water based, modified secondary, of 45–50%hydrolysis, is anoptimum product for producing high porosity PVC, with an acceptable grain size, in conjunction with a 72·5% hydrolysed PVAc as the primary suspending agent. The actual molecular weight of the polymer, or the degree of ionic modification required to aid the solubility of the polymer in water, has a negligible effect on the PVC properties.