Polyethylene is odorless and nontoxic, feels like wax, and has excellent low temperature resistance (the lowest service temperature can reach -70 ~- 100℃), good chemical stability, resistance to most acid and alkali corrosion (oxidation acid resistance), insolubility in general solvents at room temperature, low water absorption and excellent electrical insulation performance. However, polyethylene is very sensitive to environmental stress (chemical and mechanical effects) and has poor thermal aging resistance.
The properties of polyethylene vary from variety to variety, mainly depending on the molecular structure and density.
Production method and technology of polyethylene resin
Production methods of polyethylene: polyethylene can be divided into high pressure method, medium pressure method and low pressure method according to polymerization pressure; According to media, it can be divided into slurry method, solution method and gas phase method.
Main production technologies: At present, there are many companies with polyethylene technology in the world, including 7 companies with LDPE technology, 0 10 companies with LLDPE and full density technology, and 0/2 companies with HDPE technology. From the perspective of technical development, the production of LDPE by high pressure method is the most mature method in PE resin production, and both kettle method and tube method have matured. At present, these two production technologies coexist. Foreign companies generally use low-temperature and high-activity catalysts to initiate polymerization systems, which can reduce the reaction temperature and pressure.
The production of LDPE by high pressure method will develop towards large-scale and tubular production. However, the low-pressure production of HDPE and LLDPE mainly uses titanium-based catalysts and composite catalysts, most of which are used in Europe and Japan, and most of which are used in the United States.
At present, there are 1 1 kinds of polyethylene production technologies mainly used in the world, and there are 8 kinds of PE production technologies in China.
(1) high-pressure tubular and kettle reaction processes
(2) CX method of mitsui chemical low-pressure sludge process
(3)BP gas-phase innovative production process
(4) LPE process of Chevron-Phillips double loop reactor
(5) basta double-peak process of Nordic chemical industry.
(6) Unipol process adopts low pressure gas phase method.
(7) Hostalen process of Basel Polyolefin Company.
(8) Production process of 8)Sclartech solution
Catalyst technology: catalyst is the key part of polyethylene process and the focus of its technical development. Especially in 199 1 year, metallocene catalyst was industrialized in the United States, which made PE production technology enter a new development stage.
At present, most of the major PE manufacturers in the world are involved in the production of metallocene PE(mPE), such as Dow Chemical, Eastman, Asahi Kasei, Atofina, Chevron-Phillips and other companies.
Asahi Kasei Chemical Co., Ltd. of Japan purchased the patent Insite of Dow metallocene catalyst, and produced metallocene high density polyethylene (mHDPE) with Creolex brand by slurry method. Due to its superior performance, mPE 1995 has been commercialized, and the global consumption of mPE resin is doubling every year. It is estimated that by 20 10, the global mPE production capacity will reach170,000 tons, including 7 million tons of mlldpe and 6 million tons of mhdpe.
At present, PE catalyst has developed to the third generation, and Mitsui Chemical and Dow Chemical of Japan have developed a new generation of post-metallocene catalysts. Different from the traditional metallocene and Z-N catalysts, this catalyst can copolymerize polar monomers such as methyl methacrylate and vinyl acetate with olefins, so it can be used to develop new polyolefin resins with cohesiveness, oil resistance and gas barrier.
China attaches great importance to PE production technology, and PE production technology innovation has been included in the national technology innovation plan. In view of the problems that domestic PE production is mainly based on gas phase process, it is difficult to switch product grades and there are many transition materials, in recent years, domestic PE production enterprises have developed gas phase polyethylene polycondensation, super polycondensation and slurry polyethylene external circulation process, relying on the technical transformation of existing polyethylene production, and achieved practical results.
At present, most domestic Uuipol process plants are rebuilt and expanded by domestic condensation technology, and the output has exceeded the original design capacity of the plant 120% ~ 200%.
More than half of the total output of film low density polyethylene is made into films by blow molding. This kind of film has good transparency and certain tensile strength, and is widely used as packaging materials for various foods, clothing, medicines, fertilizers, industrial products and agricultural films (see color map). It can also be processed into a composite film by extrusion for packaging heavy objects. Since 1975, high-density polyethylene film has been developed, which has high strength, low temperature resistance, moisture resistance, good printability and processability. The biggest use of linear low density polyethylene is to make films. Its strength and toughness are better than that of low density polyethylene, and its puncture resistance and rigidity are also better. Although its transparency is poor, it is still slightly better than that of high density polyethylene. In addition, the polyethylene coating can be extruded on paper, aluminum foil or other plastic films to make polymer composites.
High density polyethylene (HDPE) is suitable for hollow products because of its high strength. Bottles, barrels, cans, cans and other containers can be blow molded, and large containers such as tankers, cans and storage tanks can be cast.
Polyethylene pipes can be produced by tubesheet extrusion, and high-density polyethylene pipes have high strength and are suitable for underground laying. The extrusion plate can be processed twice. High-density polyethylene can also be made into low-foaming plastics by foaming extrusion and foaming injection, which can be used as bed boards and building materials (see polymer materials for building).
Fiber China is called ethylene fiber, which is generally spun into synthetic fiber with low-pressure polyethylene. Polyethylene is mainly used to make fishing nets and ropes, or spun into short fibers for flocculation, and can also be used for industrial acid and alkali resistant fabrics. At present, ultra-high strength polyethylene fiber (strength can reach 3 ~ 4 GPA) has been developed, which can be used as a composite material for bulletproof vests, automobiles and offshore operations.
Miscellaneous products produced by injection molding include daily necessities, artificial flowers, turnover boxes (see color map), small containers, bicycle and tractor parts, etc. When manufacturing structural components, high-density polyethylene should be used.
The modified varieties of polyethylene modified by polyethylene mainly include chlorinated polyethylene, chlorosulfonated polyethylene, crosslinked polyethylene and blended modified varieties.
Chlorinated polyethylene is a random chloride obtained by partially replacing hydrogen atoms in polyethylene with chlorine. Chlorination is initiated by light or peroxide, and is mainly produced by water suspension method in industry. Due to the differences in molecular weight and its distribution, branching degree, chlorination degree after chlorination, chlorine atom distribution and residual crystallinity, chlorinated polyethylene can be obtained from rubber to hard plastics. It is mainly used as a modifier for PVC to improve its impact resistance. Chlorinated polyethylene itself can also be used as electrical insulation material and floor material.
Chlorosulfonated polyethylene When polyethylene reacts with chlorine containing sulfur dioxide, some hydrogen atoms in the molecule are replaced by chlorine and a small amount of sulfonyl chloride (-SO2Cl) groups to obtain chlorosulfonated polyethylene. The main industrial method is suspension method. Chlorosulfonated polyethylene has good ozone resistance, chemical corrosion resistance, oil resistance, heat resistance, light resistance, wear resistance and tensile strength. It is an elastomer with good comprehensive performance and can be used to make equipment parts that come into contact with food.
Cross-linked polyethylene adopts radiation method (X-ray, electron ray or ultraviolet radiation, etc.). ) or chemical method (peroxide or silicone crosslinking) to make linear polyethylene into network or bulk crosslinked polyethylene. Among them, silicone crosslinking method has simple process and low running cost, and molding crosslinking can be carried out step by step, so blow molding and injection molding are suitable. The heat resistance, environmental stress cracking resistance and mechanical properties of crosslinked polyethylene are greatly improved compared with polyethylene, and it is suitable for large pipelines, cables and wires, rotational plastic products and so on.
The blending modification of polyethylene, linear low density polyethylene and low density polyethylene can be used to process films and other products, and the performance of the products is better than that of low density polyethylene. A variety of thermoplastic elastomers can be prepared by blending polyethylene and ethylene propylene rubber.
Metallocene polyethylene
Metallocene polyethylene is a new type of thermoplastic, which is the most important technological progress in polyolefin industry in the 1990s and a major innovation after LLDPE production technology. Because it is polyethylene produced with metallocene (MAO) as polymerization catalyst, its performance is significantly different from that of PE polymerized with traditional Ziegler-Natta catalyst. The unique excellent performance of metallocene catalyst and its application in the synthesis of metallocene polyethylene have attracted wide attention in the market. Many world-famous large petrochemical companies have invested huge manpower and material resources in development and research, which has become a hot topic in polyolefin industry and even the whole plastic industry.
Early metallocene catalysts used in ethylene polymerization can only get wax with molecular weight of 20 ~ 30 thousand, and their catalytic activity is not high, so they have no practical significance, so they have not attracted attention and promotion. Until 1980, Professor Kaminsky of the University of Hamburg, Germany, found that ethylene polymerization was carried out with dicyclopentadienyl zirconium chloride (CP2ZrCl2) and methylaluminoxane (MAO) as cocatalyst in toluene solution. The activity of the catalyst is as high as 106g-PE/g-Zr, and the reaction rate is equivalent to that of the enzyme. MAO is a high oligomerization methylaluminoxane synthesized by dimethylaluminum and water under the condition of non-polymerization system. Professor kaminsky's discovery has injected vitality into the research of metallocene catalysts, attracted many companies to participate in the research and development, and made great progress. 199 1 year, American Exxon Company realized the application of metallocene catalyst in polyolefin industry for the first time, and produced the first batch of metallocene polyethylene (mPE) with the trade name "Exact".
Among metallocene polyolefins, mPE has the fastest development and is relatively mature. The main varieties are linear low density polyethylene (LLDPE) and very low density polyethylene (VLDPE). There are two series of mPE, one is film product grade mainly aimed at packaging field, and the other is plastomer with octene-1 as comonomer, which is called POP (polyolefin plastomer). MPE film grade has lower melting point and obvious melting zone, which is obviously superior to traditional polyethylene in toughness, transparency, thermal viscosity, heat sealing temperature and low odor. It can be used to produce heavy packaging bags, metal trash can liners, food packaging, stretched films and so on.
At present, the consumption of metallocene linear low density polyethylene accounts for about 15% of the total consumption of linear low density polyethylene, and it is estimated that this proportion will reach 22% by 20 10. According to statistics, at present, the annual output of metallocene polyethylene in the world is about150,000 tons, of which products used for food packaging account for 36% of the total consumption, non-food packaging accounts for 47%, and other aspects (medicine, automobiles, construction, etc. ) accounts for 17%.
Polyethylene is the most productive, fastest developing and most active variety in synthetic resin. Whether polyethylene can achieve high performance depends largely on the performance of the catalyst. Metallocene catalyst has excellent catalytic copolymerization ability, which can copolymerize most of the copolymers with ethylene and catalyze the polymerization of polar monomers, which is difficult for traditional catalysts to achieve. In terms of cycloolefin polymerization, traditional catalysts can only be ring-opening polymerization, while metallocene catalysts can be used for double bond addition polymerization.
Since many developed countries have adopted metallocene linear low density polyethylene instead of conventional linear low density polyethylene, the average annual consumption growth rate of metallocene linear low density polyethylene will be higher than that of linear low density polyethylene, reaching 15%. In the future, nearly half of the linear low density polyethylene production growth in developed countries will come from metallocene linear low density polyethylene. It is estimated that the demand for metallocene linear low density polyethylene will increase to 654.38+340,000 tons in the US market in 2009.
Development of polyethylene industry
Polyethylene (PE) is the most widely used synthetic resin in China. Mainly used for making films, containers, pipes, monofilaments, wires and cables, daily necessities, etc. It can also be used as high-frequency insulating material for TV and radar. With the development of petrochemical industry, polyethylene production has developed rapidly, accounting for about 1/4 of the total plastic production. The sustained and rapid development of China's national economy has created a good development atmosphere for the synthetic resin industry, and the polyethylene (PE) industry is growing at a faster speed.
From June to June 2008, the cumulative output of polyethylene resin in China was 3,520,250.09 tons, up 2.36% year-on-year. From June, 2008 to June, 2008, China imported 2537799893.00 kg of primary ethylene polymer, and remitted 4085020 175.00 USD. 97,449,745.00 kg of primary ethylene polymer was exported, and the foreign exchange earned by export was 65,438+052,849,306.00 USD.
During 2008-20 1 1 year, the new polyethylene projects in the Asia-Pacific region are mainly located in China, India and South Korea, and China will continue to be the power source. China is becoming the world's largest exporter of PE films and packaging bags, supplying a large number of products to North America, Western Europe and Japan. In addition, the strong demand for films, woven bags, pipes, cable materials, hollow containers and turnover boxes will drive the growth of polyethylene consumption. Therefore, the polyethylene production capacity in China will continue to grow rapidly. It is predicted that by 20 10, China's polyethylene output will reach about170,000 tons, and the demand will reach14140,000 tons. The market development prospects are promising.