Synthetic resins are high-molecular compounds, which are produced by combining low-molecular raw materials-monomers (such as ethylene, propylene, vinyl chloride, etc.) into large molecules through polymerization. There are four polymerization methods commonly used in the industry: bulk polymerization, suspension polymerization, emulsion polymerization and solution combination.
Bulk polymerization is the polymerization process of monomers under the action of initiator, heat, light and radiation without adding other medium. The characteristic is that the product is pure, without complicated separation and purification, the operation is simple, and the production equipment utilization rate is high. It can directly produce pipes, plates and other quality products, so it is also called block polymerization. The disadvantage is that the viscosity of the material continues to increase with the polymerization reaction, mixing and heat transfer is difficult, and the reactor temperature is not easy to control. Bulk polymerization is commonly used in the production of polymethyl methacrylate (commonly known as plexiglass), polystyrene, low density polyethylene, polypropylene, polyester, and polyamide resins.
Suspension polymerization refers to the polymerization process in which monomers are dispersed into droplets under the action of mechanical agitation or shaking and a dispersant, usually suspended in water, so it is also called bead polymerization. The characteristics are: there is a lot of water in the reactor, the viscosity of the material is low, and it is easy to transfer heat and control; after the polymerization, only simple separation, washing, drying and other processes are required to obtain the resin product, which can be directly used for molding processing; the product is purer , Evenly. The disadvantage is that the reactor production capacity and product purity are inferior to the bulk polymerization method, and the continuous method cannot be used for production. Suspension polymerization is widely used in industry. 75% of polyvinyl chloride resin is produced by suspension polymerization, and polystyrene is also mainly produced by suspension polymerization. The reactor is also gradually becoming larger.
Emulsion polymerization refers to the polymerization of monomers in water by mechanical stirring or shaking under the action of an emulsifier. The product of emulsion polymerization is latex, which can be used directly or destroyed, washed, dried, etc. Post-treatment process to obtain powder or needle polymer. Emulsion polymerization can obtain higher molecular weight polymers at a higher reaction rate. The viscosity of the material is low, it is easy to transfer heat and mix, production is easy to control, and residual monomers are easy to remove. The disadvantage of emulsion polymerization is that the emulsifier added during the polymerization affects the performance of the product. In order to obtain a solid polymer, the process of coagulation, separation, washing, etc. is used. The production capacity of the reactor is lower than the bulk polymerization method.
Solution polymerization is a polymerization reaction in which a monomer is dissolved in an appropriate solvent. The formed polymer is sometimes dissolved in a solvent, which is a typical solution polymerization, and the product can be used as a paint or adhesive. If the polymer is insoluble in the solvent, it is called precipitation polymerization or slurry polymerization. For example, the production of solid polymer requires precipitation, filtration, washing, and drying before it becomes a finished product. In solution polymerization, the production operation and reaction temperature are easy to control, but both require the recovery of solvent. Industrial solution polymerization can use continuous method and batch method, large-scale production often uses continuous method, such as polypropylene.