MDPE is characterized by resistance to environmental stress cracking and long-term strength retention. The relative density of MDPE is 0.926-0.953, the crystallinity is 70% - 80%, the average molecular weight is 200000, the tensile strength is 8-24 MPa, the elongation at break is 50% - 60%, the melting temperature is 126-135 ℃, the melt flow rate is 0.1-35g/10 min, and the thermal deformation temperature (0.46 MPa) is 49-74 ℃. There are three methods: high pressure method, low pressure method and medium pressure method. High pressure method is used to produce low-density polyethylene. This method was developed early. The polyethylene produced by this method has accounted for about 2/3 of the total polyethylene production so far. However, with the development of production technology and catalyst, its growth rate has lagged behind that of low pressure method. Low pressure method includes slurry method, solution method and gas phase method. Slurry method is mainly used to produce high density polyethylene, while solution method and gas phase method can not only produce high density polyethylene, but also produce medium and low density polyethylene, also known as linear low density polyethylene, by adding comonomer. In recent years, various low-pressure methods have developed rapidly. Medium pressure method is still used by Philips, and high-density polyethylene is the main product. High pressure method uses oxygen or peroxide as initiator to polymerize ethylene into low-density polyethylene. Ethylene enters the reverberator (Figure 3) after secondary compression, polymerizes into polyethylene under the pressure of 100~300MPa, temperature of 200~300 ℃ and the effect of initiator. The reverberant is separated by decompression, so that the unregulated ethylene is recovered and recycled. Molten polyethylene is extruded and granulated after adding plastic additives. (See the color picture) There are two kinds of polymerization reverberators used: tube type reverberators (tube length can reach 2000m) and kettle type reverberators. The one-way conversion rate of tubular process is 20%~34%, and the annual production capacity of single line is 100kt. The single pass conversion rate of kettle process is 20%~25%, and the annual production capacity of single line is 180kt. The low-pressure method includes slurry method, solution method and gas phase method. Except for the solution method, the polymerization pressure is below 2MPa. The usual processes include catalyst production, ethylene polymerization, polymer separation and granulation. ① Polyethylene produced by slurry method is insoluble in solvent and looks like slurry. Slurry polymerization has mild conditions and is easy to operate. Alkyl aluminum is often used as activator, hydrogen is used as molecular weight regulator, and kettle type reverberator is often used. The polymer slurry from the polymerization kettle goes through the flash kettle, gas-liquid separator to the powder drying machine, and then goes to granulation (Figure 4). The production process also includes solvent recovery, solvent refining and other processes. If different polymerizers are used in series or in parallel, products with different molecular weight distribution can be obtained. ② Solution polymerization is carried out in solvent, but both ethylene and polyethylene are soluble in solvent, and the reaction system is homogeneous solution. The reaction temperature (≥ 140 ℃) and pressure (4-5MPa) are high. It is characterized by short polymerization time, high production intensity, and can also produce high, medium and low density polyethylene, which can better control the properties of commodities; However, the polymer obtained by solution method has low molecular weight, narrow molecular weight dispersion and low solid content. ③ Gas phase polymerization of ethylene in gaseous state usually uses fluidized bed reactor. There are two kinds of catalysts, chromium system and titanium system, which are quantitatively involved in the bed by the storage tank. High speed ethylene circulation is used to maintain the bed fluidization and eliminate polymerization reaction heat. The generated polyethylene is discharged from the bottom of the reverberator (Figure 5). The pressure of the reverberator is about 2MPa, and the temperature is 85~100 ℃. Gas phase method is the most important method to produce linear low density polyethylene. Gas phase method saves solvent recovery and polymer drying processes, and saves 15% of investment and 10% of operation cost compared with solution method. It is 30% of the traditional high-pressure method and 1/6 of the operation cost. Therefore, it has been carried out rapidly. However, the gas phase method needs to be further improved in terms of commodity quality and variety. The medium pressure method uses chromium catalyst loaded on silica gel to polymerize ethylene in the loop reactor under medium pressure to produce high-density polyethylene. Processing and use can be processed by blow molding, extrusion, injection molding and other methods, which are widely used in the production of films, hollow products, fibers and daily necessities. In practice, in order to improve the stability of polyethylene to ultraviolet radiation and oxidation effect, and improve processing and application performance, a small amount of plastic additives are required. The commonly used UV absorbers are o-hydroxybenzophenone or its alkoxy derivatives, etc. Carbon black is an excellent UV shielding agent. In addition, it also takes part in antioxidant, lubricant, colorant, etc., so as to expand the use range of polyethylene.