What is PE?
PE: Polyethylene (PE for short) is a thermoplastic resin obtained by polymerization of ethylene. In industry, it also includes copolymers of ethylene and a small amount of α-olefins. Polyethylene is odorless, non-toxic, feels like wax, has excellent low temperature resistance (the lowest operating temperature can reach -100~-70°C), good chemical stability, and can withstand the corrosion of most acids and alkalis (not resistant to acids with oxidizing properties). It is insoluble in general solvents at room temperature, has low water absorption, and excellent electrical insulation. Polyethylene is a polymer formed by the polymerization of ethylene monomers.
Polyethylene was synthesized by ICI in the UK in 1922, and industrial production began in 1939. It was officially produced in the United States. It was an important radar insulation material and military supplies during the war. After the war, Japan's Mitsui Petrochemical and Sumitomo Chemical (1958) began formal production. In 1975, the annual output of the factory was 1.407 million tons, second only to the United States. In 1933, the British Brunner Mond Chemical Industries discovered that ethylene can be polymerized to form polyethylene under high pressure. This method was industrialized in 1939 and is commonly known as the high-pressure method.
In 1953, K. Ziegler of the Federal Republic of Germany discovered that ethylene can also be polymerized at relatively low pressure using TiCl4-Al(C2H5)3 as a catalyst. This method was put into industrial production by the Federal Republic of Germany's Hoechst Company in 1955 and is commonly known as low-pressure polyethylene. In the early 1950s, the American Phillips Petroleum Company discovered that ethylene can be polymerized to form high-density polyethylene under medium pressure using chromium oxide-silica aluminum gel as a catalyst, and industrial production was achieved in 1957. In the 1960s, DuPont of Canada began to use ethylene and α-olefins to produce low-density polyethylene using a solution method. In 1977, Union Carbide and Dow Chemical Company of the United States successively used the low-pressure method to produce low-density polyethylene, called linear low-density polyethylene, among which Union Carbide's gas phase method was the most important. The performance of linear low-density polyethylene is similar to that of low-density polyethylene, but it also has some characteristics of high-density polyethylene. In addition, the energy consumption in production is low, so it has developed very rapidly and has become one of the most eye-catching new synthetic resins.
The core technology of the low-pressure method lies in the catalyst. The TiCl4-Al(C2H5)3 system invented by Ziegler of Germany is the first generation catalyst for polyolefins. The catalytic efficiency is low, and about several kilograms of polyethylene are obtained per gram of titanium. In 1963, the Belgian Solvay Company pioneered the second generation catalyst with magnesium compounds as carriers, and the catalytic efficiency reached tens of thousands to hundreds of thousands of grams of polyethylene per gram of titanium. The use of the second generation catalyst can also save the post-treatment process of removing catalyst residues. Later, a gas phase high-efficiency catalyst was developed. In 1975, the Italian Monte Edison Group developed a catalyst that can directly produce spherical polyethylene without granulation. It is called the third generation catalyst and is another revolution in the production of high-density polyethylene. Polyethylene is a crystalline thermoplastic resin. Their chemical structure, molecular weight, degree of polymerization and other properties depend to a large extent on the polymerization method used. The polymerization method determines the type and degree of branching. Crystallinity depends on the regularity of the molecular chain and the thermal history it has experienced. Polyethylene is very sensitive to environmental stress (chemical and mechanical effects), and its heat aging resistance is inferior to the chemical structure and processing of the polymer.
Polyethylene can be processed by the molding method of general thermoplastics (see plastic processing). It has a wide range of uses, mainly used to make films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulation materials for televisions, radars, etc. With the development of petrochemicals, polyethylene production has developed rapidly, and its output accounts for about 1/4 of the total plastic production. In 1983, the world's total polyethylene production capacity was 24.65Mt, and the capacity of the equipment under construction was 3.16Mt. According to the latest statistics in 2011, the global production capacity reached 96Mt. The development trend of polyethylene production shows that production and consumption are gradually shifting to Asia, and China is becoming the most important consumer market.
In nuclear physics, astrophysics, and reactor operation, polyethylene is used as a diffuser to measure neutrons. Made its own contribution to the research of nuclear physics.
Polyethylene (PE) is a kind of plastic. The convenient bags we often mention are polyethylene (PE). Polyethylene is the simplest polymer and the most widely used polymer material. It is made up of repeated –CH2– units. Polyethylene is formed by the addition polymerization of ethylene (CH2=CH2).
The performance of polyethylene depends on its polymerization method. High-density polyethylene (HDPE) is formed by Ziegler-Natta polymerization under medium pressure (15-30 atmospheres) and organic compound catalysis. Under these conditions, the polymerized polyethylene molecules are linear, and the molecular chains are very long, with a molecular weight of up to hundreds of thousands. If it is free radical polymerization under high pressure (100-300MPa), high temperature (190-210C), and peroxide catalysis, low-density polyethylene (LDPE) is produced, which is a branched compound structure.
General characteristics Polyethylene resin is a non-toxic, odorless white powder or granule with a milky white appearance and a waxy feel. It has a low water absorption rate of less than 0.01%. Polyethylene film is transparent and decreases with the increase of crystallinity. Polyethylene film has low water permeability but high air permeability. It is not suitable for fresh-keeping packaging but suitable for moisture-proof packaging. The flammability and oxygen index are 17.4. There is low smoke when burning, a small amount of molten droplets, yellow upper and blue lower flames, and a paraffin smell. Polyethylene has good water resistance. The surface of the product is non-polar, difficult to bond and print, and it is improved after surface treatment. It has many side chains and poor resistance to light degradation and oxidation.
Mechanical properties Polyethylene has general mechanical properties, low tensile strength, poor creep resistance, and good impact resistance. Impact strength LDPE>LLDPE>HDPE, other mechanical properties LDPE<LLDPE<HDPE. It is mainly affected by density, crystallinity and relative molecular weight. As these indicators increase, its mechanical properties increase. Environmental resistance Stress cracking is not good, but it improves when the relative molecular mass increases. Puncture resistance is good, among which LLDPE is the best.
Thermal properties
Polyethylene has low heat resistance, which improves with the increase of relative molecular mass and crystallinity. Low temperature resistance Good performance, the brittle temperature can generally reach below -50℃; and with the increase of relative molecular mass, the lowest can reach -140℃. Polyethylene has a large linear expansion coefficient, up to (20~24)×10-5/K. Thermal conductivity is relatively high.
Electrical properties
Because polyethylene is non-polar, it has excellent electrical properties with low dielectric loss and high dielectric strength. It can be used as frequency modulation insulation material, corona-resistant plastic, and high-voltage insulation material.
Environmental properties
Polyethylene is an alkane inert polymer with good chemical stability. It is resistant to corrosion by acid, alkali, and salt aqueous solutions at room temperature, but not resistant to strong oxidants such as fuming sulfuric acid, concentrated nitric acid, and chromic acid. Polyethylene is insoluble in general solvents below 60°C, but it will swell or crack if in long-term contact with aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, etc. When the temperature exceeds 60°C, it can be dissolved in a small amount in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil and paraffin; when the temperature is higher than 100°C, it can be dissolved in tetralin.
Because polyethylene molecules contain a small amount of double bonds and ether bonds, its weather resistance is poor, and sun exposure and rain will cause aging, and antioxidants and light stabilizers need to be added to improve it.
Molding characteristics
- Crystalline material, low moisture absorption, no need to fully dry, excellent fluidity, fluidity is sensitive to pressure, high-pressure injection is suitable for molding, uniform material temperature, fast filling speed, and sufficient pressure holding. It is not suitable to use direct gates to prevent uneven shrinkage and increased internal stress. Pay attention to the selection of gate positions to prevent shrinkage holes and deformation.
- Large shrinkage range and shrinkage value, obvious directionality, easy to deform and warp. The cooling speed should be slow, the mold should be equipped with a cold material hole and a cooling system.
- The heating time should not be too long, otherwise decomposition will occur.
- When the soft plastic part has a shallow side groove, it can be demolded by force.
- Melt rupture may occur, and it is not suitable to contact with organic solvents to prevent cracking.
Excellent performance of modified ultra-high molecular weight polyethylene pipe:
1. High impact resistance
Ultra-high molecular weight polyethylene's impact resistance and impact absorption energy are the best among plastics. It is difficult to crack whether it is a strong external impact or an internal pressure fluctuation. Its impact strength is 10 times that of nylon 66, 20 times that of polyvinyl chloride, and 4 times that of polyethylene. Especially in low temperature environments, its impact strength reaches a higher value. This flexibility of ultra-high molecular weight polyethylene provides a safe and reliable guarantee for the conveying system.
2. High wear resistance
Among many pipeline materials, ultra-high molecular weight polyethylene has the smallest friction coefficient. In addition, the ultra-high molecular weight chain is particularly long, which makes the wear resistance of modified ultra-high molecular weight polyethylene pipes 4-7 times higher than that of steel pipes and stainless steel pipes when transporting various slurries, and about 10 times higher than that of polyvinyl chloride pipes and polyethylene pipes, greatly improving the service life of the pipeline.
3. Corrosion resistance
Based on the fact that ultra-high molecular weight polyethylene is a saturated molecular group structure, its chemical stability is extremely high. It can resist the erosion of various highly corrosive media (acids, alkalis, salts) and organic solvents within a certain temperature and concentration range.
4. No scaling
Ultra-high molecular weight polyethylene has self-lubricity and non-adhesion, and the friction coefficient is the smallest. The inner wall of the pipeline produced by special technology is corrosion-resistant, wear-resistant, and scale-free, so the flow resistance is very small, and the flow velocity and flow rate can be maintained for a long time. Its inner diameter design can be reduced by 15.4% compared with steel pipes.
5. Weather resistance and aging resistance
On the one hand, because there are few unsaturated genes in the molecular chain of ultra-high pipes and the molecular weight is large; on the other hand, the addition of high-quality modifiers makes the service life of ultra-high pipes much longer than that of ordinary polyethylene pipes. After about 60 years of use, the ultra-high pipes can still maintain more than 70% of their mechanical properties.
6. Wide temperature adaptability
Can work at temperatures from -269℃ to 80℃ for a long time.
7. Flame retardancy
Special industries use natural fireproof environments, add flame retardants for modification, and use special processing technology to overcome the flammability of ultra-high molecular weight polyethylene itself and have flame retardant properties.
8. Light weight and easy installation
The product is flexible, can adapt to various geological conditions, and can be directly laid by bending; it adopts flange connection without gasket, which is quick and easy to connect.