High pressure low density polyethylene (hp-ldpe, abbreviated as LDPE) has been industrialized for more than 70 years. Ethylene polymerizes into high molecular weight polyethylene by free radical mechanism under high temperature and high pressure, which has the following characteristics:

 (1) Ethylene polymerization is a strong exothermic process. The polymerization heat of ethylene is about 93.5kj/mol (or 3.3kj/g), and the specific heat capacity of ethylene is 2.51 ~ 2.85j / (g · ℃) at 235 MPa and 150 ~ 300 ℃.

(2) Under high pressure, ethylene has been compressed to a gas tight phase with a density of 0.5g/ml, which is similar to a liquid that can no longer be compressed. At this time, the distance between ethylene molecules is significantly shortened, which increases the collision probability between free radicals and ethylene molecules, so polymerization is easy to occur.

 (3) At high temperature, the free radical activity of the growing chain is large, and chain transfer reaction is easy to occur. The resulting polymer is a linear structure with more branched chains.

Linear low density polyethylene (LLDPE) is ethylene with a small amount α- Olefin copolymerization is formed on the main chain of linear ethylene with a very short molecular structure of comonomer branches. Linear low density polyethylene is non-toxic, tasteless and odorless milky white particles with a density of 0.918 ~ 0.935g/cm3. Compared with LDPE, it has higher softening temperature and melting temperature, high strength, good toughness, high rigidity, good heat resistance and cold resistance, good environmental stress cracking resistance, impact strength and tear strength, and can be resistant to acid, alkali and organic solvents. It is widely used in industry, agriculture, medicine, health and daily necessities.                                                                High density polyethylene (HDPE) is a white powder or granular product. Non toxic, tasteless, crystallinity 80% ~ 90%, softening point 125 ~ 135 ℃, service temperature up to 100 ℃; Hardness, tensile strength and creep are better than those of low density polyethylene; Good wear resistance, electrical insulation, toughness and cold resistance; Good chemical stability, insoluble in any organic solvent at room temperature, resistant to acid, alkali and various salts; The film has low permeability to water vapor and air and low water absorption; The aging resistance is poor, and the environmental stress cracking resistance is not as good as that of low-density polyethylene. In particular, thermal oxidation will degrade its performance. Therefore, antioxidants and ultraviolet absorbers must be added to the resin to improve this deficiency. The thermal deformation temperature of HDPE film under stress is low, which should be paid attention to in application.

Virgin LDPE granules