Polymerization Methods Of Pvc

PVC is prepared by free radical addition polymerization. The polymerization methods mainly include suspension polymerization, lotion polymerization and bulk polymerization. Suspension polymerization is the main method, accounting for about 80% of the total output of PVC. Add pure water, liquefied VCM monomer and dispersant into the reactor, and then add initiator and other additives. When the temperature rises to a certain temperature, VCM monomer undergoes free radical polymerization to generate PVC particles. Continuous stirring makes the particle size uniform, and the generated particles are suspended in water. In addition, the micro suspension method is also used to produce PVC paste resin, which has good performance and paste forming property.
① Suspension polymerization method: the monomer is suspended and dispersed in the water phase in the form of micro drops, and the selected oil soluble initiator is dissolved in the monomer. The polymerization reaction is carried out in these micro drops. The heat of polymerization reaction is timely absorbed by water. In order to ensure that these micro drops are dispersed in the water as beads, suspension stabilizers, such as gelatin, polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, etc., need to be added. Organic peroxides and azo compounds are mostly used as initiators, such as diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, diethylhexyl peroxydicarbonate, azodiisobentronitrile, azodiisobutyronitrile, etc. Polymerization is carried out in a polymerizer with an agitator. After polymerization, the materials flow into the monomer recovery tank or the stripper for monomer recovery. Then it flows into the mixing kettle, washed with water, centrifuged for dehydration and dried to obtain finished resin. Vinyl chloride monomer shall be removed from the resin as much as possible. For PVC used for food packaging, the content of free monomer should be controlled below 1ppm. During polymerization, in order to ensure the resin with specified molecular weight and molecular weight distribution range is obtained and prevent explosion polymerization, the temperature and pressure during polymerization must be controlled. The particle size and particle size distribution of the resin are controlled by the stirring speed and the selection and amount of suspension stabilizer. The quality of resin is characterized by particle size and particle size distribution, molecular weight and molecular weight distribution, apparent density, porosity, fish eye, thermal stability, color, impurity content and free flow of powder. Polymerization reactor is the main equipment, which is made of steel reactor lined with stainless steel or enamel, equipped with agitator and heat transfer jacket for temperature control, or internal cooling drain pipe, reflux condenser, etc. In order to reduce the production cost, the volume of the reactor has gradually expanded from several cubic meters to more than ten cubic meters, and the maximum volume has reached 200 cubic meters (kettle reactor). The polymerizer shall be descaled after multiple use. PVC prepared with polyvinyl alcohol and cellulose ether as suspension stabilizer is generally loose, with many pores, large surface area, and easy to absorb plasticizer and plasticize.
② Lotion polymerization: the earliest method for industrial production of PVC. In lotion polymerization, in addition to water and vinyl chloride monomer, surfactants such as sodium alkyl sulfonate are added as emulsifiers to disperse the monomer in the water phase to form an lotion. Water soluble potassium persulfate or ammonium persulfate are used as initiators, and "oxidation reduction" initiator system can also be used. The polymerization process and suspension method are different. Polyvinyl alcohol is also added as emulsion stabilizer, dodecyl mercaptan as regulator and sodium bicarbonate as buffer. There are three polymerization methods: batch polymerization, semi continuous polymerization and continuous polymerization. The polymerization product is emulsion, and the particle size of lotion is 0.05-2 μ m. It can be directly applied or dried by spray to form powder resin. The lotion polymerization method has short polymerization cycle and is easy to control. The resin obtained has high molecular weight and uniform degree of polymerization. It is suitable for making PVC paste, artificial leather or impregnated products. The formula of lotion polymerization is complex, and the content of impurities in the product is high.
③ Bulk polymerization: the polymerization unit is special, mainly composed of vertical prepolymerization kettle and horizontal polymerization kettle with frame agitator. The polymerization is carried out in two stages. The monomer and initiator are prepolymerized for 1h in the prepolymer reactor to generate seed particles. At this time, the conversion rate reaches 8%~10%, and then flows into the second polymerization reactor, where the same amount of monomer as the prepolymer is added to continue polymerization. When the conversion rate reaches 85%~90%, the residual monomer is discharged, and then the finished product is obtained by crushing and sieving. The particle size and shape of the resin are controlled by the stirring speed, and the reaction heat is brought out by the reflux condensation of the monomer. The production process of this method is simple, the product quality is good, and the production cost is low.