Polyethylene is divided into high-density polyethylene (HDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) according to the polymerization method, molecular weight and chain structure.
LDPE
Properties: tasteless, odorless, non-toxic, matte surface, milky white waxy particles, density of about 0.920 g/cm3, melting point 130 °C ~ 145 °C. Insoluble in water, slightly soluble in hydrocarbons, etc. It can resist most acid and alkali erosion, small water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.
Production process: there are mainly two kinds of high-pressure tube method and kettle method. In order to reduce the reaction temperature and pressure, the tubular process generally adopts low-temperature and high-activity primer initiation system, with high-purity ethylene as the main raw material, propylene, propane, etc. as the density modifier, and the high-activity initiator is used to carry out the polymerization reaction under the conditions of about 200 °C ~ 330 °C and 150 ~ 300MPa. The molten polymer that initiates polymerization in the reactor must be cooled and separated by high pressure, medium pressure and low pressure, and the high-pressure circulating gas is cooled and separated and sent to the ultra-high pressure (300 MPa) compressor inlet, the medium pressure circulating gas is cooled and separated and sent to the high-pressure (30 MPa) compressor inlet, and the low-pressure circulating gas is cooled and separated and sent to the low-pressure (0.5 MPa) compressor for recycling, while the molten polyethylene is sent to the granulator after high-pressure and low-pressure separation for water pelletizing during granulation , enterprises can add suitable additives according to different application fields, and the granules are packaged and shipped.
Uses: can use injection molding, extrusion, blow molding and other processing methods. Mainly used as agricultural film, industrial packaging film, pharmaceutical and food packaging film, mechanical parts, daily necessities, building materials, wire, cable insulation, coating and synthetic paper.
LLDPE
Properties: Due to the obvious difference in molecular structure of LLDPE and LDPE, the performance is also different. Compared with LDPE, LLDPE has excellent environmental stress cracking resistance, electrical insulation, high heat resistance, impact resistance and puncture resistance. Production process: LLDPE resin is mainly produced by full-density polyethylene equipment, and the representative production process is the Innovene process and UCC's Unipol process.
Uses: Through injection molding, extrusion, blow molding and other molding methods, the production of films, daily necessities, pipes, wires and cables, etc.
HDPE
Properties: natural color, cylindrical or flat round particles, smooth particles, the size of the particles should be 2 mm ~ 5 mm in any direction, no mechanical impurities, thermoplastic. The powder is a white powder, and the qualified product is allowed to have a yellowish color. It is insoluble in general solvents at room temperature, but it can swell when exposed to aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons for a long time, and slightly soluble in toluene and acetic acid at above 70 °C. Oxidation occurs when heated in air and under the influence of sunlight. Can resist most acids and alkalis. Low water absorption, can still maintain flexibility at low temperatures, high electrical insulation.
Production process: gas phase method and slurry method are adopted.
Uses: using injection molding, blow molding, extrusion, rotational molding and other molding methods, the production of film products, daily necessities and industrial hollow containers, pipes, rolling belts and ligature belts for packaging, ropes and cables, fishing nets and woven fibers, wires and cables, etc.
