Because polyethylene molecules contain a small amount of double bonds and ether bonds, its weather resistance is not good, and it will cause aging due to sun and rain, so antioxidants and light stabilizers need to be added to improve it. LDPE and HDPE are plastics with good processability because of their good fluidity, low processing temperature, moderate viscosity and low decomposition temperature, and they do not decompose at 300℃ in inert gas. However, the viscosity of LLDPE is slightly higher, and the motor power needs to be increased by 20% ~ 30%; Melt fracture is easy to occur, so it is necessary to increase the gap between dies and add processing AIDS; The processing temperature is slightly higher, reaching 200 ~ 2 15℃. Polyethylene has low water absorption and does not need to be dried before processing.
Polyethylene melt belongs to non-Newtonian fluid, and its viscosity fluctuates little with the change of temperature, while the increase of shear rate decreases rapidly and linearly, and LLDPE decreases the slowest.
Polyethylene products are easy to crystallize during cooling, so attention should be paid to the mold temperature during processing. In order to control the crystallinity of products and make them have different properties. The molding shrinkage of polyethylene is relatively large, which must be considered when designing dies.
manufacturing method
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.
High pressure process is used to produce low density polyethylene. This method was developed earlier, so far the polyethylene produced by this method accounts for about 2/3 of the total polyethylene production. However, with the development of production technology and catalyst, its growth rate has lagged far behind that of low-pressure method. As far as the implementation methods of low pressure method are concerned, there are slurry method, solution method and gas phase method. Slurry method is mainly used to produce high density polyethylene, while solution method and gas phase method can not only produce high density polyethylene, but also produce medium and low density polyethylene by adding monomers, also known as linear low density polyethylene. Various low-pressure processes have developed rapidly. The medium-pressure method is still only used by Philip Company, and its main product is high-density polyethylene. A method for polymerizing ethylene into low density polyethylene with oxygen or peroxide as initiator. After secondary compression, ethylene enters the reactor and is polymerized into polyethylene at the pressure of 100 ~ 300 MPa and the temperature of 200 ~ 300℃ under the action of initiator. The reactants were separated under reduced pressure to recover unreacted ethylene, and the molten polyethylene was extruded and granulated after adding plastic additives.
There are two kinds of polymerization reactors used: tubular reactor (up to 2000 meters in length) and kettle reactor. The one-way conversion rate of tubular process is 20% ~ 34%, and the annual production capacity of single line is 100kt. The single-pass conversion rate of kettle process is 20% ~ 25%, and the annual production capacity of single line is 180kt. There are three methods: slurry method, solution method and gas phase method. Except the solution method, the polymerization pressure is below 2MPa. The general steps are catalyst preparation, ethylene polymerization, polymer separation and granulation.
① The polyethylene produced by slurry method is insoluble in solvent and is in slurry form. Slurry polymerization is mild and easy to operate. Alkyl aluminum is often used as activator, hydrogen is often used as molecular weight regulator, and kettle reactor is often used. The polymer slurry from the polymerization kettle reaches the powder dryer through the flash kettle and the gas-liquid separator, and then is granulated (Figure 4). The production process also includes steps such as solvent recovery and solvent refining. By using different polymerization reactors in series or in parallel, products with different molecular weight distributions can be obtained.
② Solution polymerization is carried out in solvent, but both ethylene and polyethylene are dissolved in the solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥ 140℃) and pressure (4 ~ 5 MPa) are relatively high. It is characterized by short polymerization time and high production intensity, and can produce high, medium and low density polyethylene at the same time, which can better control the properties of the products; However, the polymer obtained by solution method has low molecular weight, narrow molecular weight distribution and low solid content.
(3) Gas-phase ethylene polymerization is carried out in a gaseous state, and generally a fluidized bed reactor is adopted. There are two kinds of catalysts, chromium-based and titanium-based, which are quantitatively added into the bed from the storage tank. High-speed ethylene circulation is adopted to maintain the bed laminar flow and eliminate the polymerization heat. The produced polyethylene is discharged from the bottom of the reactor (Figure 5). The pressure of the reactor is about 2MPa and the temperature is 85 ~ 100℃. Gas phase method is the most important method of production line low density polyethylene. The gas phase method saves solvent recovery and polymer drying, and saves investment 15% and operating cost 10% compared with the solution method. The investment is 30% of the traditional high-pressure method, and the operating cost is 1/6. So it is developing rapidly. However, the gas phase method needs to be further improved in product quality and variety. Using chromium catalyst supported on silica gel, ethylene was polymerized in a loop reactor at medium pressure to produce high density polyethylene.
Processing and application can be processed by blow molding, extrusion, injection molding, etc. , widely used in the manufacture of films, hollow products, fibers and daily necessities. In practical production, in order to improve the stability of polyethylene to ultraviolet rays and oxidation, and improve the processing and use performance, a small amount of plastic additives need to be added. The commonly used ultraviolet absorber is o-hydroxybenzophenone or its alkoxy derivatives, and carbon black is an excellent ultraviolet shielding agent. In addition, antioxidants, lubricants, colorants, etc. So as to expand the application range of polyethylene. There are many companies with polyethylene technology in the world, including 7 LDPE technologies, 7 LLDPE and full density technologies 10, and 2 HDPE technologies10. From the perspective of technical development, high-pressure production of LDPE is the most mature method in PE resin production, and kettle method and tube method are both mature, and these two production technologies coexist at the same time. 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.
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. Most major polyethylene manufacturers in the world participated in the production of metallocene polyethylene (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.
E catalyst has been developed to the third generation, and a new generation of post-metallocene catalyst has been developed by Mitsui Chemical and Dow Chemical in Japan. Different from the traditional metallocene and Z-N catalysts, this catalyst can polymerize 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 fact that domestic PE production is mainly based on gas phase method, it is difficult to switch product grades and there are many transition materials, domestic PE production enterprises rely on the technical transformation of existing polyethylene production to develop gas phase polyethylene polycondensation, super polycondensation process and slurry polyethylene external circulation process, and achieved practical results.
Most domestic production units of Uuipol process are rebuilt and expanded by domestic condensation technology, and the output has exceeded the original design capacity of the unit 120% ~ 200%.
Films are 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.
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).
Miscellaneous products produced by injection molding include daily necessities and can also be used for industrial acid and alkali resistant fabrics. 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. Artificial flowers, turnover boxes (see color map), small containers, bicycle and tractor parts, etc. When manufacturing structural components, high-density polyethylene should be used.
Polyethylene modification
The modified varieties of polyethylene mainly include chlorinated polyethylene, chlorosulfonated polyethylene, crosslinked polyethylene and * * * mixed 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.
* * * Polyethylene linear low-density polyethylene and low-density polyethylene mixed modification can be used to process films and other products, and the product performance is better than low-density polyethylene. Polyethylene and ethylene propylene rubber can be mixed to produce widely used thermoplastic elastomers.
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 in toluene solution, ethylene polymerization was carried out with a * * catalyst composed of dicyclopentadienyl zirconium chloride (CP2ZrCl2) and methylaluminoxane (MAO). The activity of the catalyst is as high as 106g-PE/g-Zr, and the reaction rate is equivalent to that of 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 for packaging field, and the other is plastic body with octene-1 as * * * monomer, which is called POP (polyolefin plastic). 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.
The consumption of metallocene linear low density polyethylene accounts for about 15% of the total consumption of linear low density polyethylene, and it is predicted that this proportion will reach 22% by 20 10. According to statistics, 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. ) accounted for 654.38+07%.
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 ability to catalyze * * * polymerization, which can make most * * * polymers polymerize with ethylene and polar monomers catalyze polymerization, 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 in the US market will increase to 654.38+340,000 tons in 2009.