SynopsisThe formation of primary particles in agitated bulk polymerizations of vinyl chloride was found to proceed in two stages. The first stage occurred at the very beginning of the polymerization; the second stage started as the initially nucleated particles began to agglomerate, and continued up to at least 7% conversion. Upon formation, the primary particles were stable and did not agglomerate until reaching a limiting size, which was found to be lower at higher stirring speeds. The number of particles formed in the first stage was independent of agitation and other polymerization parameters. The rate of particle formation during the second stage was equal to the rate of particle agglomeration. Thus the total number of primary particles formed was determined almost exclusively by the rate of particle agglomeration. Addition of the surfactant Span 20 caused an increase in the total number of primary particles. Also, this addition increased the tendency of the particles to fuse together after agglomeration. These effects can be understood to be a consequence of particle destabilization by the surfactant. When a small amount of a high molecular weight PMMA was dissolved in the monomer, the polymerization behaved quite differently. In this case, the primary particles were prevented from agglomeration, reaching a limiting size independent of stirring speed.

The formation and agglomeration of PVC primary particles were studied in bulk polymerization experiments. In the absence of additives, the primary particles started to agglomerate at low conversions. The agglomeration conversion, as well as the size of the agglomerated particles, decreased when the agitation speed increased. At the highest speed tested, the agglomeration started already at 0.05 percent conversion. The primary particle size was about 0.16-0.18 pm, and seemed to be constant in the conversion interval studied (up to 5 percent). This indicated that the nucleation rate of primary particles was almost constant and that the growth rate of agglomerated particles was very low. The addition of sorbitan monolaurate produced a decrease in primary particle size. Polymeric additives such as PMMA, EVA, and PVAc stabilized primary particles against agglomeration but had no marked effect on the primary particle size. The monomer-soluble fraction of poly(viny1 alcohol-b-vinyl acetate) with high content acetate groups did not affect either the particle size or the agglomeration process.

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Abstract:The electrostatic contribution to the colloidal stability of PVC primary particles (R = 0.15 ~) dispersed in vinyl chloride, was calculated using models based on the Coulombic interactions and the DLVO theory. The calculations were based on: a) the particle charge as obtained from literature data on the electrophoretic mobility of PVC primary particles in VCM and b) on estimates of the Debye length as obtained from measurements of the electrical conductivity of VCM and of solutions of Bu4NBF 4 in VCM.The calculations showed that particle stability would decrease with particle size (experimentally-observed behaviour), only if the particle charge increased with size at a lower rate than in proportion to particle radius.The calculations also suggest that particle growth may be governed by a competitive growth mechanism of electrostatic origin. Particle growth is assumed to occur by absorption of many small, weakly charged basic particles from the monomer phase. According to the calculations, the electrostatic interaction between primary and basic particles may be such that the growth of the smaller primary particles is favoured over that of the larger ones.

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