Polyethylene can be 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, opalescent, waxy particles with a density of about 0.920 g/cm3 and a melting point of 130 °C~145 °C. Insoluble in water, slightly soluble in hydrocarbons, etc. It can resist the corrosion of most acids and alkalis, has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.
Production process: there are mainly two kinds of high-pressure pipe method and kettle method. In order to reduce the reaction temperature and pressure, the tubular process generally adopts a low temperature and high activity initiator initiated polymerization system. With high-purity ethylene as the main raw material, propylene, propane, etc. as the density regulator, the high activity initiator is used for polymerization at about 200 °C~330 °C and 150~300MPa. Molten polymer polymerized in the reactor must be cooled and separated by high pressure, medium pressure and low pressure. High pressure circulating gas is cooled and separated before being sent to the inlet of ultra-high pressure (300 MPa) compressor. Medium pressure circulating gas is cooled and separated before being sent to the inlet of high-pressure (30 MPa) compressor. Low pressure circulating gas is cooled and separated before being sent to the low-pressure (0.5 MPa) compressor for recycling. Molten polyethylene is cooled and separated before being sent to the inlet of low-pressure (0.5 MPa) compressor for recycling After low-pressure separation, it is sent to the granulator for water granulation. During granulation, enterprises can add appropriate additives according to different application fields, and the granules are packaged and delivered.
Application: It can be processed by injection molding, extrusion molding, blow molding, etc. It is 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: LLDPE and LDPE have different molecular structures and properties. 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 plant, and the representative production processes are Innovene process and UCC Unipol process.
Application: To produce films, daily necessities, pipes, wires and cables by injection molding, extrusion, blow molding and other molding methods.
HDPE
Property: natural color, cylindrical or oblate particles, smooth and clean, particle size in any direction should be 2 mm~5 mm, no mechanical impurities, thermoplastic. The powder is this white powder, and qualified products are allowed to have yellowish color. It is insoluble in general solvents at room temperature, but it can swell when contacting with aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons for a long time, and slightly soluble in toluene and acetic acid when above 70 °C. It is heated in the air and oxidized under the influence of sunlight. It can resist the corrosion of most acids and alkalis. It has low water absorption, can still maintain flexibility at low temperatures, and has high electrical insulation.
Production process: gas phase method and slurry method are adopted.
Application: The molding methods of injection molding, blow molding, extrusion molding, roll molding, etc. are used to produce film products, daily necessities, and hollow containers of various sizes for industrial use, pipe materials, calendering tapes and binding tapes for packaging, ropes, fishing nets, fibers for weaving, wires and cables, etc.