Synthetic resins are high-molecular compounds, which are produced by combining low-molecular raw materials-monomers (such as ethylene, propylene, vinyl chloride, etc.) into macromolecules through polymerization. Commonly used industrial polymerization methods include bulk polymerization, suspension polymerization, emulsion polymerization, solution polymerization, slurry polymerization, gas phase polymerization and so on. There are abundant sources of raw materials for the production of synthetic resins. In the early days, coal tar products and calcium carbide calcium carbide were the main products, but now oil and natural gas products are mainly used, such as ethylene, propylene, benzene, formaldehyde, and urea.
Bulk polymerization
Bulk polymerization is the polymerization process of monomers under the action of initiator or heat, light and radiation without adding other media. The characteristic is that the product is pure, does not require complicated separation and purification, the operation is relatively simple, and the utilization rate of production equipment is high. It can directly produce quality products such as pipes and plates, so it is also called block polymerization. The disadvantage is that the viscosity of the material increases continuously with the progress of the polymerization reaction, mixing and heat transfer are difficult, and the temperature of the reactor is not easy to control. Bulk polymerization is often used in the production of polyacrylic methyl acrylate (commonly known as plexiglass), polystyrene, low-density polyethylene, polypropylene, polyester and polyamide resins.
Suspension polymerization
Suspension polymerization refers to the polymerization process in which the monomers are dispersed into droplets under the action of mechanical stirring or shaking and dispersing agents, usually suspended in water, so it is also called bead polymerization. The characteristics are: there is a large amount of water in the reactor, the viscosity of the material is low, and it is easy to transfer and control heat; after polymerization, it only needs to go through simple separation, washing, drying and other processes to obtain a resin product, which can be directly used for molding and processing; the product is relatively pure , Uniform. The disadvantage is that the reactor production capacity and product purity are not as good as the bulk polymerization method, and the continuous method cannot be used for production. Suspension polymerization is widely used in industry.
Emulsion polymerization
Emulsion polymerization refers to polymerization in which monomers form an emulsion in water under mechanical stirring or shaking with the help of an emulsifier. The emulsion polymerization reaction product is latex, which can be used directly, or the latex can be destroyed, after washing, drying, and other post-treatment processes, to obtain powder or needle-like polymers. Emulsion polymerization can obtain polymers with higher molecular weights at higher reaction speeds. The viscosity of the materials is low, heat transfer and mixing are easy, production is easy to control, and residual monomers are easy to remove. The disadvantage of emulsion polymerization is that the emulsifier added in the polymerization process affects the performance of the product. In order to obtain the solid polymer, the consumption goes through the process of coagulation, separation, washing and so on. The production capacity of the reactor is lower than that of the bulk polymerization method.
Solution polymerization
Solution polymerization is carried out in the presence of a solvent, and the selected solvent must dissolve both the monomer and the polymer. During the polymerization process, the system is a uniform viscous solution, and the polymerization system is always in a homogeneous phase, with a long continuous operation period and easy operation. However, the viscosity of the system is relatively high. The advantage is that the homogeneous reaction is easier to control, and the molecular weight and its distribution can also be appropriately controlled. However, the solution polymerization system is viscous, causing difficulty and uneven heat and mass transfer.
Slurry polymerization
In slurry polymerization, a solvent or monomer itself is used as a dispersion medium. The resulting polymer is insoluble in the dispersion medium, but is dispersed in the form of particles in the form of a slurry. Some earlier documents have attributed it to heterogeneous solution polymerization. This kind of polymerization is characterized by low viscosity of the system, easy stirring, easy heat dissipation, and higher monomer concentration can be used to improve the productivity of unit equipment. At present, this method can be used for the production of high-density polyethylene and polypropylene.
Gas phase polymerization
In the gas phase polymerization, the gas phase monomer and the catalyst are introduced into the reactor according to the specified amount to synthesize in one step to obtain the dry polymer. The prerequisite for gas phase polymerization is that the catalyst selectivity and yield must be high enough, and the product obtained does not need to remove the remaining catalyst, which can greatly shorten the process. With the advent of Ziegler catalysts with high activity carriers, gas phase polymerization has so far become the mainstream in the manufacture of polyethylene or polypropylene. In addition, it can also be widely used for polymerization by free radical mechanism.
