Process Of Formation Of Particles In Bulk Polymerization Of Vinyl Chloride

Production of bulk(mass)poly(vinyl chloride)(PVC)resin has no centrifugation of the product and no drying treatment.Compared to suspension polymerization,the investment and operation cost of vinyl chloride(VC)bulk polymerization process is lower.M-PVC resins exhibit a narrow molecular weight distribution,uniform particle diameter,higher apparent density and porosity,covering a wide range of uses and having a promising expect.Production of M-PVC has several defects,such as high proportion of "B" level product(＞10%)and high content of residue monomer residue(＞5mg/kg), which affect the further development of M-PVC product.The earlier study on M-PVC technology were carried by foreign researchers and were involved in the quantitative description of primary particle and particle formation process,which can not be used to guide solving the resin quality issue.Domestic research of production of M-PVC is far behind foreign countries,particularly in respect of particle formation process.The present thesis focuses on the pre-polymerization of VC,to investigate the influences of polymerization temperature,the usage of initiator and agitation rate on the pre-polymerization conversion,distribution of molecular weight and particles of resin. The particle formation process of PVC was also studied,to give an instruction for lowing the proportion of "B" level PVC and monomer residue.The study on kinetics of bulk polymerization of vinyl chloride shows that,the influences of polymerization temperature,the usage of initiator and agitation rate on the pre-polymerization conversion,distribution of molecular weight and particles of resin have the same rule with the suspension polymerization.The initiator activity is in turn of Luperox 610＞Luperox 223≈Luperox 10.Replacing of Luperox 223 with Luperox 10 can improve the thermal properties of PVC resin.Luperox 610 and Luperox 223(1:1) composite may cause the polymerization process to be steady and enhance the production efficiency. The study of particle formation process of bulk polymerization of vinyl chloride shows that PVC particle size(D_V)grows with conversion.After a certain conversion,D_V increases slowly and the distribution also tends to definite value.This conversion is defined as critical conversion(f),and the corresponding particle size is called as critical particle size(D_Vf).As the agitation rate increased,f and D_Vfwere gradually decreased. D_V,f and D_Vfwas not influenced by polymerization temperature and usage of initiator.By using of optical microscope,scanning electron microscope(SEM)and mercury-pressure meter,the external morphology and the internal structure of PVC particles were revealed in three levels.Optical microscope photos show that PVC particle morphology is not regular when the conversion is low.As conversion increased,small particles gradually disappear and particle morphology is gradually become regular.SEM photos of resin particles from the external morphology show that the size of primary particles increase slightly as conversion increased.The surface of resin particles gradually become dense and the particles become roundness.Mercury-pressure measurements show that before the critical conversion rate,the porosity of PVC resin and pore volume increase with conversion rate.After the critical conversion,the porosity and pore volume decreased gradually.Based on the above results and Boisel's particle formation mechanism,the mechanism of the bulk polymerization of VC is proposed as follows:the system is homogeneous in the early period of pre-polymerization,the polymer chains is formed as VC initiated by free radicals.The polymer chains coagulate each other and precipitate from the monomer phase to form the basic particles.As conversion increased,basic particles agglomerate to form primary particles with a diameter of 0.1～0.3μm.As polymerization conversion increased,the number of basic particles within primary particles increases constantly,but the number of primary particles always invariable.Finally,primary particles agglomerate to form PVC grains with a diameter of 150～260μm.The thesis can be a reference of optimizing the critical conversion of pre-polymerization of VC and reducing the proportion of "B" level PVC and monomer residue.