Quality Study And Optimization Of PVC Paste From Seeded Emulsion Polymerization

Seeded emulsion polymerization has been studied for a long time and it has been adopted in lots of actual production process. By general emulsion polymerization, the particle size is always lower than0.2μm and the viscosity of the paste prepared is very high. The average particle size should be controlled in the range of1μm to2μm so that the paste viscosity can be kept in lower level under the condition of lower plasticizer dosage. The seeded emulsion polymerization for preparing PVC is a kind of new technology in which the emulsion polymerization can be carried out in the presence of the latex particles (seed particles) and the feeding rate of emulsifying agent, initiator and monomer should be controlled strictly and precisely. In seeded emulsion polymerization, the post-added monomers will be polymerized in the existing particles and no new particles will be formed; so the volumes of the existing particles will be enlarged and the number of the particles in the emulsion polymerization system will not be changed. Therefore, the process condition, the polymerization additives and the instrument parameters will play an important role in the process of the seeded emulsion polymerization of VCM.In order to reduce the viscosity of the resin paste and to improve its processability so as to meet the needs for different end users, the seeded emulsion polymerization of VCM was carried out in a reactor of24m3in the present work. On the basis of the analysis for the effects on the quality of emulsion paste, the optimum process was obtained by adjusting the formulation and process parameters in the seeded emulsion polymerization. The effects of monomer conversion at the point of starting to post-add emulsifying agent, the feeding rate and total content of emulsifying agent, the content of initial emulsifying agent, seed emulsion, initiators, chain transfer agent and chain extending agent, feeding rate of monomer, inlet and outlet temperature on the average particle size and its distribution of the PVC paste were investigated. The results show that the paricles in PVC paste come from the primary particles with different diameters and the particle size distribution is broad and bi-modal; if the content of small particles is much higher, the paste viscosity will be high; on the other hand, if the content of large particles is very high and that of small particles is very low, then the insterspace ratio will be high so that the paste viscosity will be very high. Only when the ratio of content of small particles to that of large particles is in the range of15to25, the small particles will be effectively filled in interspace among the large particles and the particle size distribution will be reasonable to achieve the lowest paste viscosity. By adjusting the formulation and optimizing the process parameters, the paste viscosity is reduced to meet the users’needs, the polymerization time is shortened and the cost of the products is lowered.