Suspended emulsion polymerization of vinyl chloride was carried out in a 5 L autoclave. The influence of agitation, polymerization conversion, dispersant and surfactant on the average particle size (PS) and particle size distribution (PSD), particle morphology and porosity of polyvinyl chloride (PVC) resin was investigated. It showed that the agitator had great influence on the smooth operation of polymerization, PS and PSD. The PS increased and PSD became narrow as polymerization conversion became high. The porosity decreased with the increase of conversion. A convenient choice of additives, both dispersants and non-ionic surfactants, allows one to adjust PS and PSD. The PS decreased with the addition of polyvinyl alcohol or hydroxypropyl methylcellulose dispersants,and increased with the addition of Span surfactants. The addition of dispersants or surfactants also affected the morphology and porosity of resin, and PVC resin with looser agglomeration and homogeneous distribution of primary particles was prepared.
The kinetics of suspended emulsion polymerization of methyl methacrylate (MMA), in which water acted as the dispersed phase and the mixture of MMA and cyclohexane as the continuous phase, was investigated. It showed that the initial polymerization rate (Rp0) and steady-state polymerization rate (Rp) were proportional to the mass ratio between water and oil phase, and increased as the polymerization temperature, the potassium persulphate concentration and the Tween20 emulsifier concentration ([S]) increased. The relationships between the polymerization rate and and [S] were obtained as follows: The above exponents were close to those obtained from normal MMA emulsion polymerization. It also showed that the average molecular weight of the resulting poly(methyl methacrylate) decreased as the polymerization temperature, and [S] increased. Thus, MMA suspended emulsion polymerization could be considered as a combination of many miniature emulsion polymerizations proceeding in water drops and obeyed the classical kinetics of MMA emulsion polymerization.
Suspended emulsion polymerization was used to prepare poly(vinyl chloride) (PVC) resin. Fine PVC particles were formed at low polymerization conversions. The amount of fine particles decreases as conversion increases and disappears at conversions greater than 30%. Scanning electron micrographs show that PVC grains are composed of loosely coalesced primary particles, especially for PVC resins prepared in the presence of poly(vinyl alcohol) dispersant. The size of primary particles increases and porosity decreases with the increase of conversion. In view of the particle features of PVC resin, a particle formation mechanism including the formation of primary particles and grains is proposed. The formation process of primary particles includes the formation of particle nuclei, coalescence of particle nuclei to form primary particles,and growth of primary particles. PVC grains are formed by the coagulation of primary particles. The loose coalescence of primary particles is caused by the colloidal stability of primary particles and the low swelling degree of vinyl chloride in the primary particles.
