Vinyl Chloride Suspension Emulsion Polymerization Particle Formation Mechanism And Kinetics,

Suspended emulsion polymerization is a new polymerization process developed from emulsion polymerization and suspension polymerization to prepare porous polymer particles. This work investigates the mechanism of particle formation and polymerization kinetics of suspended emulsion polymerization of vinyl chloride (VC) based on the study of polymerization technology.The influences of stirring, initiator, suspending agent, emulsifier, water/VC ratio and polymerization temperate on particle size and size distribution were studied. It is shown that polyvinyl chloride (PVC) resin prepared by suspended emulsion polymerization containing particles with different degree of primary particles aggregation and the particle size distribution was relatively wider. The volume fraction of smaller aggregates decreased and the mean particle size increased as polymerization conversion increased. The mean particle size decreased by adding of poly(vinyl alcohol) (PVA) or cellulosic suspending agent. Upon decreasing the HLB of emulsifier, the average particle size increased and the particle size distribution became narrower. As the ratio of water/VC increased, the mean particle size decreased and the distribution became narrower too.The particle morphology of PVC resin was characterized by scanning electron micrography. It is shown that PVC grains were composed of primary particles which almost did not fuse together and had higher porosity and lower density in grain center than near grain skin. The diethylhexyl phthalate (DOP) plasticizer absorption of PVC grain became greater when using LW300 as suspending agent, Span60 as emulsifier or higher ratio of water/VC.The mechanism of particle formation can be concluded as following: the PVC oligoradicals rapidly precipitate in water droplets under very low conversion to form latex particles. The stability of latex particles is so poor that they tend to agglomerate to form primary particles. The primary particles grows through polymerization of the swollen monomer, which is diffusing from continue VC phase into water droplets during the polymerization. The primary particles near grain border have higher density and agglomeration because of the organic nature of PVC chains and repulsive force between internal particles.inThe polymerization kinetics of VC suspension emulsion polymerization was also studied based on experiments. For VC suspended emulsion polymerization initiated by potassium persulphate, the initial polymerization rate (R,,) is proportional to [I]133 and proportional to [RpJ0 72, where [I] and [RpJ are the concentration of initiator and water/VC ratio. The activation energy is 174KJ/mol.Applying the optimized polymerization technology, VC suspended emulsion polymerization was carried out to more than 70% conversion and the obtained PVC grains were still constructed by unfused primary particles and had DOP plasticizer absorption of 19.2g/gPVC and apparent density of 0.54g/cm3.