Polyethylene (PE) is a thermoplastic resin obtained by polymerizing ethylene. In industry, copolymers of ethylene with small amounts of alpha-olefins are also included. Polyethylene is odorless, non-toxic, feels like wax, has excellent low temperature resistance (minimum operating temperature can reach -100~-70°C), good chemical stability, and can resist most acid and alkali erosion (not resistant to oxidation nature acid). It is insoluble in common solvents at room temperature, with low water absorption and excellent electrical insulation.
In terms of molecular structure, low-density polyethylene molecules contain a variety of branched chains, and in 1000 carbon atoms, there are on average about 20-30 ethylene, butyl or longer branched chains. However, high-density polyethylene has very few branched chains, with only a few branched chains in an average of 1,000 carbon atoms, so its temperature resistance, corrosion resistance, hardness and mechanical properties are better than those of low-density polyethylene. However, solvent is used in production, the cost is high, the expansion of the finished product is not good, it is not suitable for thin film, and the electrical properties are relatively poor. The performance of medium density polyethylene is in between.
Polyethylene is a General plastics material that is resistant to various organic solvents, weak acids, and weak bases, but polyethylene is not resistant to strong acids, especially nitric acid, and can burn. Polyethylene has a wide range of raw material sources, large output, low cost, small specific gravity, and easy processing. In addition to being widely used in civil use, it can be used in industry as water pipelines, wire sheathing, ropes, containers, etc.
Polyethylene can be strengthened with glass fiber, can be copolymerized with other vertical types, and can also be cross-linked by radiant energy or chemical methods during modification, making it a thermosetting material with a network structure, thereby greatly improving its resistance to fire. Temperature performance (140°C) and mechanical strength. In addition, there is expanded polyethylene, which can be made into various foam products.
Polyethylene is very sensitive to environmental stress (chemical and mechanical action) and is less resistant to thermal aging than polymers in terms of chemical structure and processing. Polyethylene can be processed by conventional thermoplastic molding methods. It has a wide range of uses, mainly used to manufacture films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulating materials for TVs, radars, etc.
(Raw material of polyethylene)
3. Classification and use of polyethylene particles
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.
Properties: LDPE tasteless, odorless, non-toxic, dull surface, milky white waxy particles, density about 0.920 g/cm3, melting point 130℃～145℃. Insoluble in water, slightly soluble in hydrocarbons, etc. It can withstand the erosion 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 tube method and kettle method. In order to reduce the reaction temperature and pressure, the tubular process generally adopts a low-temperature high-activity initiator to initiate the polymerization system, high-purity ethylene is used as the main raw material, propylene, propane, etc. are used as density adjusters. The polymerization was carried out under the conditions of 330°C and 150-300MPa. The molten polymer that initiates polymerization in the reactor must be cooled and separated at high pressure, medium pressure and low pressure. After separation, it is sent to the inlet of the high-pressure (30 MPa) compressor, while 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. Perform water granulation while pelleting.
Uses: Injection molding, extrusion molding, blow molding and other processing methods can be used. 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.
(Low-density polyethylene film grade - LD165)
Properties: Because the molecular structures of LLDPE and LDPE are obviously different, the properties are also different. Compared with LDPE, LLDPE has excellent environmental stress crack resistance and electrical insulation, higher 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 Innovene process and UCC's Unipol process.
Uses: Produce films, daily necessities, pipes, wires and cables, etc. by means of injection molding, extrusion, blow molding and other molding methods.
(Linear low-density polyethylene rotomolding grade - 7149U)
Properties: HDPE natural, cylindrical or oblate particles, smooth particles, particle size should be 2 mm ~ 5 mm in any direction, no mechanical impurities, thermoplastic. The powder is white powder, and the qualified product is allowed to have a slight yellow color. It is insoluble in common solvents at room temperature, but can swell in aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons when contacted for a long time, and is slightly soluble in toluene and acetic acid at temperatures above 70 °C. Oxidation occurs when heated in air and under the influence of sunlight. Resistant to most acid and alkali erosion. It has low water absorption, can still maintain softness at low temperature, and has high electrical insulation.
Production process: two production processes of gas phase method and slurry method are adopted.
Uses: Use injection molding, blow molding, extrusion molding, rotomolding and other molding methods to produce film products, daily necessities and industrial hollow containers of various sizes, pipes, calendering tapes and ligatures for packaging, ropes, fishing nets and weaving. Fiber, wire and cable, etc.
(High Density Polypropylene HDPE/HD5502XA)
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