HDPE is a thermoplastic polyolefin produced by ethylene polymerization. Although 1956 introduced HDPE, this kind of plastic has not reached the mature level. This universal material is still developing new uses and markets.
key property
HDPE is a nonpolar thermoplastic resin with high crystallinity. It turns out that the appearance of HDPE is milky white, and it has a certain degree of translucency in the thin section. PE has excellent tolerance to most household and industrial chemicals. Some kinds of chemicals can cause chemical corrosion, such as corrosive oxidant (concentrated nitric acid), aromatic hydrocarbon (xylene) and halogenated hydrocarbon (carbon tetrachloride). The polymer does not absorb moisture, has good water vapor resistance and can be used for packaging purposes. HDPE has good electrical properties, especially high dielectric strength, which makes it very suitable for wires and cables. Medium and high molecular weight grades have excellent impact resistance, even at room temperature and even at the low temperature of -40F. The unique feature of various grades of HDPE is the proper combination of four basic variables: density, molecular weight, molecular weight distribution and additives. Different catalysts are used to produce customized polymers with special properties. These variables are combined to produce HDPE grades for different purposes; Achieve the best balance of performance.
density
This is the main variable that determines the characteristics of HDPE, although the four variables mentioned do interact with each other. Ethylene is the main raw material of polyethylene, and a few other monomers, such as 1- butene, L- propylene or 1- octene, are often used to improve polymer properties. For HDPE, the content of the above monomers generally does not exceed 1%-2%. The addition of * * * monomer slightly reduced the crystallinity of the polymer. This change is generally measured by density, which has a linear relationship with crystallization rate. According to American general classification, the density of HDPE is 0.940g/ according to ASTM D 1248. C or above; The density of medium density polyethylene (MDPE) ranges from 0.926 to 0.940 g/cm3. Other classifications sometimes classify MDPE as high density polyethylene or LLDPE. Homopolymer has the highest density, the highest stiffness, good permeability resistance and the highest melting point, but it usually has poor environmental stress cracking resistance (ESCR). ESCR is the ability of polyethylene to resist cracking caused by mechanical or chemical stress. Higher density usually improves mechanical strength, such as tensile strength, stiffness and hardness; Thermal properties, such as softening point temperature and thermal deformation temperature; Impermeability, such as air permeability or water vapor permeability. The lower density improves its impact strength and e-SCR. The density of polymer is mainly affected by the amount of monomer added, but it is less affected by molecular weight. The high molecular weight percentage makes the density slightly lower. For example, homopolymer have different densities ov a wide range of molecular weights.
Production and catalyst
The most common production method of PE is slurry method or gas phase method, and a few are produced by solution method. All these processes are exothermic reactions, involving ethylene monomer, α -olefin monomer, catalyst system (possibly more than one compound) and various types of hydrocarbon diluents. Hydrogen and some catalysts are used to control the molecular weight. The slurry reactor is generally a stirred tank or a more commonly used large annular reactor, in which the slurry can be stirred circularly. When ethylene and monomers (as required) contact with the catalyst, polyethylene particles will be formed. After the diluent is removed, the polyethylene particles or powder particles are dried, and additives are added according to the dosage to produce pellets. Modern production lines of large reactors equipped with twin-screw extruders can produce more than 40,000 pounds of polyethylene per hour. The development of new catalyst is helpful to improve the performance of new brand HDPE. The two most commonly used catalysts are Philip's chromium oxide-based catalyst and titanium compound monoalkyl aluminum catalyst. The high density polyethylene produced by Philip catalyst has a medium molecular weight distribution; The molecular weight distribution produced by titanium monoalkyl aluminum catalyst is very narrow. The catalyst used to produce narrow MDW polymer in composite reactor can also be used to produce wide MDW grade. For example, two reactors in series that produce products with significantly different molecular weights can produce bimodal molecular weight polymers with full-width molecular weight distribution.
molecular weight
Higher molecular weight leads to higher polymer viscosity, but viscosity is also related to the temperature and shear rate used in the test. The molecular weight of materials is characterized by rheology or molecular weight measurement. Generally, the grade molecular weight of HDPE ranges from 40 000 to 300 000, and its weight average molecular weight roughly corresponds to the melting index range, that is, from 65 438+000 to 0.029/65 438+00 min. Generally, higher MW (lower melt index MI) improves melt strength, better toughness and ESCR, but higher MW makes processing easier.
This process is more difficult or requires higher pressure or temperature.
Molecular weight distribution (MWD): The WD of PE varies from narrow to wide depending on the catalyst used and the processing process.
The most commonly used MWD measurement index is the inhomogeneity index (HI), which is equal to the weight average molecular weight (MW) divided by the number average molecular weight (Mn). The index range of all HDPE grades is 4-30. Narrow MWD provides low warpage and high impact during molding. Medium to wide MWD provides machinability for most extrusion processes. Wide MWD can also improve melt strength and creep resistance.
additive
The addition of antioxidants can prevent the polymer from degradation during processing and prevent the finished product from oxidation in use. Antistatic additives are used in many packaging grades to reduce the adhesion of bottles or packages to dust and dirt. Specific applications require special additive formulations, such as copper inhibitors related to wire and cable applications. Excellent weather resistance and ultraviolet (or sunlight) resistance can be obtained by adding anti-ultraviolet additives. PE without UV protection or carbon black is recommended not to be used continuously outdoors. Advanced carbon black pigment provides excellent ultraviolet resistance and can be often used in outdoor applications, such as wires, cables, pools or pipes.
Job operation
Polyethylene can be made by many different processing methods. Using ethylene as the main raw material, propylene, 1- butene and hexene as * * * polymers, under the action of catalyst, the obtained polymers are subjected to flash evaporation, separation, drying, granulation and other processes to obtain a finished product with uniform particles. Include, for example, sheet extrusion, film extrusion, tube or profile extrusion, blow molding, injection molding and rotational molding.
▲ Extrusion: The melt index of brands generally used for extrusion production is less than 1, and the MWD is from medium width to wide width. In the process of processing, low MI can obtain suitable melt strength. Wider MWD grades are more suitable for extrusion because they have higher production speed, lower die pressure and lower melt fracture tendency.
Polyethylene has many extrusion applications, such as wires, cables, hoses, pipes and profiles. The applications of pipelines range from small cross-section yellow pipes of natural gas to thick-walled black pipes with a diameter of 48 inches for industrial and urban pipelines. As a substitute for rainwater drainage pipes and other concrete drainage pipes, large-diameter hollow wall pipes are developing rapidly.
Plates and thermoforming: The thermoformed lining of many large picnic refrigerators is made of PE, which has good toughness, light weight and durability. Other sheet and thermoformed products include fenders, tank linings, oil pans, transport boxes and tanks. A large number of rapidly growing sheet applications are plastic film or pond bottom village, which is based on the toughness, chemical resistance and impermeability of MDPE.
▲ Blow molding: HDPE 1/3 sold in the United States is used for blow molding. From bottles containing bleach, engine oil, detergent, milk and distilled water to large refrigerators, car fuel tanks and cans. The characteristics of blow molding grades, such as melt strength, ES-Cr and toughness, are similar to those used in sheet and thermoforming applications, so similar grades can be adopted.
Injection blow molding is usually used to make smaller containers (less than 16 ounces) for packaging medicines, shampoos and cosmetics. One advantage of this process is that the corners of the bottle are automatically removed, and there is no need for post-finishing steps like general blow molding. Although some narrow MWD grades are used to improve the surface finish, medium to wide MWD grades are generally used.
▲ Injection molding: There are countless applications of HDPE, ranging from reusable thin-walled beverage cups to 5 GSL cans, which consume 1/5 HDPE produced in China. Generally, the melt index of injection molding grade is 5 ~ 10, which has low fluidity of toughness and high fluidity of processability. Uses include thin-walled packaging of daily necessities and food; Tough and durable food and paint cans; High environmental stress cracking resistance applications, such as small engine fuel tanks and 90 gallon trash cans.
▲ rotational molding: the materials processed by this method are generally crushed into powder and melted and flowed in the thermal cycle. Rotational molding uses two types of polyethylene: universal and crosslinkable. General MDPE/HDPE usually has a density of 0.935-0.945g/cc and a narrow MWD, which makes the product have high impact and minimum warpage, and its melt index is generally 3-8. Higher MI grades are usually not suitable because they do not have the impact resistance and environmental stress cracking resistance required by rotational molding products.
The application of high-performance rotational molding takes advantage of its unique properties of chemical crosslinkable grade. These grades have good fluidity in the first stage of the molding cycle, and then cross-link to form their excellent environmental stress cracking resistance and toughness. Wear resistance and weather resistance. Crosslinkable PE is only suitable for large containers, from 500-gallon storage tanks for transporting various chemicals to 20,000-gallon agricultural storage tanks.
▲ film: PE film is generally processed by ordinary film blowing or flat extrusion. Most PE is used in films, and generally low density polyethylene (LDPE) or linear low density polyethylene (LLDPE) can be used. HDPE film grade is generally used in places where super tensile property and excellent impermeability are required. For example, HDPE film is often used in commodity bags, grocery bags and food packaging.
product performance
High density polyethylene (HDPE) is nontoxic, tasteless and odorless white particles, with a melting point of about 130℃ and a relative density of 0.94 1~0.960. It has good heat and cold resistance, good chemical stability, high rigidity and toughness and good mechanical strength. The dielectric properties and environmental stress cracking resistance are also good.
Packaging, storage and transportation
When storing, keep away from fire sources and keep warm. The warehouse should be kept dry and tidy. It is forbidden to mix any impurities, and it is forbidden to be exposed to the sun and rain. When transporting, it should be stored in a clean and dry covered carriage or cabin, and there should be no sharp objects such as nails. It is forbidden to mix with organic solvents such as flammable aromatic hydrocarbons and halogenated hydrocarbons.
Recycle
HDPE is the fastest growing part of the plastic recycling market. This is mainly because it is easy to reprocess, has minimal degradation characteristics, and is widely used in packaging applications. The main recycling is 25% recycled materials, such as post-consumer recycled products (PCR), which are reprocessed with pure HDPE and made into bottles that are not in contact with food. English name low density polyethylene
Plastic materials suitable for various molding processes of thermoplastic molding have good processability, such as injection molding, extrusion molding, blow molding, rotational moulding, coating, foaming process, thermoforming, hot air welding and hot welding.
Main applications
LDPE is a kind of film product, which is suitable for making films, repackaging films, cable insulation materials, blow molding and foaming products.
Such as agricultural film, plastic film, agricultural film, vegetable shed film, etc. Packaging film such as candy, vegetables, frozen food packaging; Blown film for liquid packaging (milk, soy sauce, juice. Tofu, soybean milk); Heavy packaging bag, shrink packaging film, elastic film, lining film; Building film, general industrial packaging film and food bag. LDPE is also used for injection molding products, such as small containers, lids, household goods, plastic flowers and injection-stretching-blow molding containers. Medical devices, pharmaceutical and food packaging materials, extruded pipes and plates, wire and cable cladding, special-shaped materials, thermoforming and other products; Blow-molding hollow products, such as food containers, dairy products and jams, medicines, cosmetics, chemical products containers, tanks and so on.
manufacturing method
Low density polyethylene can be divided into high pressure method and low pressure method according to polymerization methods. According to the type of reactor, it can be divided into kettle method and tube method. Taking ethylene as raw material, it is sent into a reactor and polymerized under high pressure compression under the action of initiator. After unreacted ethylene is removed from the material from the reactor by a separator, it is melted, extruded, granulated, dried, mixed and sent to packaging.
product performance
Low density polyethylene is milky white beads. Non-toxic, tasteless, odorless, dull surface. The density is 0.916 ~ 0.930g/cm3. Soft, with good ductility, electrical insulation, chemical stability, processability and low temperature resistance (-70℃), but poor mechanical strength, moisture resistance, gas resistance and solvent resistance. The molecular structure is irregular, the crystallinity (55%~65%) is low, and the crystallization melting point (108~ 126℃) is also low.
Packaging, storage and transportation
Products are packed in polyethylene heavy packaging film bags, and polypropylene woven bags can be added as outer packaging according to users' needs. Products should be stored in a clean and dry warehouse and can be transported by train, car and ship. Attention should be paid to fire prevention, waterproof, sun protection, dust prevention and pollution prevention during storage and transportation. The means of transport should be kept clean and dry, free from sharp objects such as nails, and need a shed or tarpaulin (LLDPE), which is made of ethylene and a small amount of higher α-olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene-1, etc.). ) Under the action of catalyst. However, according to ASTM D- 1248-84, the density range of 0.926 ~ 0.940g/cm3 belongs to medium density polyethylene (MDPE). The new generation LLDPE extends its density to plastic (0.890 ~ 0.9 1.5g/cm3) and elastomer (
The molecular structure of conventional LLDPE is characterized by its linear main chain, few or no long branches, but some short branches. The absence of long branches makes the polymer more crystalline.
Generally speaking, LLDPE resins are characterized by density and melt index. The density is determined by the concentration of monomers in the polymer chain. The concentration of * * * monomer determines the number of short branches in the polymer. The length of the short chain depends on the type of monomer. * * * The higher the monomer concentration, the lower the density of the resin. In addition, the melt index is a reflection of the average molecular weight of the resin, which is mainly determined by the reaction temperature (solution method) and the addition amount of chain transfer agent (gas phase method). The average molecular weight has nothing to do with the molecular weight distribution, but is mainly influenced by the type of catalyst.
LLDPE was industrially produced by Union Carbide Company of the United States in 1970s, which represented a major change in the catalyst and processing technology of polyethylene and greatly expanded the product range of polyethylene. With its excellent performance and low cost, LLDPE has replaced LDPE in many fields in a short time by replacing free radical initiator with coordination catalyst and high-pressure reactor with low-cost gas phase polymerization. At present, LLDPE has penetrated into almost all traditional polyethylene markets, including films, molds, pipes and wires and cables.
LLDPE products are non-toxic, tasteless and odorless, showing milky white particles. Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance, cold resistance, good environmental stress cracking resistance, tear resistance, acid resistance, alkali resistance and organic solvent resistance.
In 2005, the LLDPE output in China was 6.5438+0.88 million tons, accounting for about 35.5% of the total PE output. The consumption is 3.55 million tons, accounting for 33.8% of the total consumption of PE. It is predicted that LLDPE consumption will continue to grow at a rate of about 8% in the next 2-3 years. According to the current market price of 12000 yuan/ton, the market scale of LLDPE in China has exceeded 40 billion yuan.
Application field of LLDPE
The main application fields of LLDPE are agricultural films, packaging films, wires and cables, pipes and coated products.
Linear low density polyethylene (LLDPE) is mainly used for making films because of its high tensile strength, good puncture resistance and tear resistance. In 2005, the world LLDPE consumption was 65,438+0,665,438+0.7 million tons, up 6.4% year-on-year. In the consumption structure, film products still account for the largest proportion, with the consumption of 1 1.9 million tons, accounting for 73.6% of the total consumption, followed by injection molding, with the consumption of 1 1.48 million tons, accounting for about 7 1% of the total consumption of LLDPE.
In 2005, the total consumption of LLDPE and LDPE in China was 5.98 million tons, of which the consumption of LLDPE was 3.55 million tons, up 25.4% year-on-year, accounting for 59.4% of the total consumption of LLDPE/ LDPE. The consumption of low-density polyethylene was 2.43 million tons, up 0.7% year-on-year, accounting for 40.6% of the total consumption of LLDPE/ low-density polyethylene.
According to the consumption structure of LLDPE/LDPE, film is still the most consumed variety, with a consumption of 4.85 million tons, accounting for 77.5% of the total consumption of LLDPE/LDPE, of which 310.3 million tons of packaging film, accounting for 50% of the total consumption; 654.38+345,000 tons of agricultural film, accounting for 22.5% of the total consumption; 376,000 tons of special packaging films, accounting for 6% of the total consumption. Followed by injection molding products, the consumption is 557,000 tons, accounting for 8.9% of the total consumption. Followed by coated products, pipes and wires and cables, the consumption is 310.3 million tons,10.88 million tons and10.57 million tons respectively, accounting for 5%, 3% and 2.5% of the total consumption; Other consumption is 6.5438+0.88 million tons, accounting for 3% of the total consumption.
Judging from the consumption of LLDPE/LDPE from 2003 to 2005, the consumption ratio of film has been kept at around 77%, and the consumption ratio of the second largest variety of injection molding products has been hovering at around 9%. It is predicted that in the next 2 ~ 3 years, although the absolute consumption of various varieties will continue to increase, the consumption ratio will basically maintain the status quo; Due to the rapid growth of packaging film demand, the consumption proportion of agricultural film will drop to about 20%. Due to the continuous improvement of LLDPE performance, its application field is also expanding, and the market demand growth rate of LLDPE in the future will be much higher than that of LDPE and HDPE.
Classification of LLDPE
According to the monomer type of * * *, LLDPE is mainly divided into three kinds of * * * polymers: C4 (butene-1), C6 (hexene-1) and C8 (octene-1). Among them, butene polymer is the largest LLDPE resin in the world, and hexene polymer is the fastest growing LLDPE variety at present. In LLDPE resin, the typical dosage of * * monomer is 5% ~ 10% by weight, and the average dosage is about 7%. The average monomer content of metallocene-based LLDPE plastomer (mLLDPE) is more than 3 times that of traditional LLDPE. Chart 1 refers to the global output of three LLDPE monomers quoted from foreign journals 10.
Chart1:1global output of three LLDPE monomers from 997 to 2007.
(Chart description: Butene: Butene; Hexene: hexene; Octene: Octene)
At the end of 1984, the then United Carbon Company introduced the production of hexene * * * poly LLDPE, followed by Exxon, Mobil and other companies. Octene is almost always used as monomer in the low-pressure dissolution process of Dow Chemical Company, and octene is also used in the medium-pressure dissolution process of NOVA Chemical Company in Canada. The strength, tear resistance and processability of octene LLDPE resin are slightly better, but there is little difference between hexene * * * poly resin and octene * * * poly resin. At present, the main producers of hexene LLDPE resin are ExxonMobil Chemical Company, Eastman Chemical Company, Equistar Company and Chevron Phillips Chemical Company. In addition, Dow Chemical, Basell, Innovene and Samsung Total also produce hexene LLDPE.
Compared with common butene monomers, LLDPE produced with hexene and octene as monomers has better properties. The biggest use of LLDPE resin is to produce films. Films and products made of LLDPE resin with long-chain α -olefins (such as hexene and octene) as monomers are superior to LLDPE resin with butene as monomers in many aspects, such as tensile strength, impact strength, tear strength, puncture resistance and environmental stress cracking resistance. Since 1990s, foreign polyethylene manufacturers and users tend to replace butene with hexene and octene. It is reported that when octene is used as the monomer of * * *, the resin performance may not be further improved than that of hexene * * *, and the price is more expensive. Therefore, at present, the trend that major LLDPE producers abroad use hexene instead of butene is more obvious.