Polyethylene is a thermoplastic resin obtained by polymerizing ethylene. In industry, copolymers of ethylene with small amounts of alpha-olefins are also included. Polyethylene odorless, non-toxic, feel like wax, with excellent low temperature performance (minimum temperature up to -100 ~ -70 ° C), chemical stability, resistance to most acid-base erosion (intolerance with oxidation Nature of the acid). Insoluble in normal solvents at room temperature, water absorption, excellent electrical insulation.
|Short name||PE||Melting point||92 ℃|
|Full name||Polyethylene||Boiling point||270 ℃|
|Chemical formula||(C2H4)n||Water solubility||Poor|
High Density Polyethylene (HDPE )
"High Density Polyethylene", referred to as "HDPE." HDPE is a high degree of crystallinity, non-polar thermoplastic resin. The appearance of the original HDPE was milky white.
HDPE is a thermoplastic polyolefin produced by the copolymerization of ethylene. Although HDPE was introduced in 1956, the plastic has not reached the level of maturity. This versatile material continues to develop its new uses and markets.
The main features
HDPE is a high crystallinity, non-polar thermoplastic resin. The virgin HDPE has a milky white appearance and a somewhat translucent appearance on a thin section. PE has excellent resistance to most life and industrial chemicals. Certain types of chemicals can cause chemical corrosion, such as corrosive oxidants (concentrated nitric acid), aromatic hydrocarbons (xylene), and halogenated hydrocarbons (carbon tetrachloride). The polymer is non-hygroscopic and has good water vapor resistance for packaging applications.HDPE has good electrical properties, especially high dielectric strength, making it suitable for wire and cable. Medium to high molecular weight grades have excellent impact resistance, both at ambient and even low -40F. The unique properties of HDPE are the proper combination of four basic variables:. The different densities, molecular weights, molecular weight distributions and additives are used to produce tailor-made specialty polymers These variables combine to produce HDPE grades for different applications; the best balance of performance.
This is the main variable that determines the characteristics of HDPE, although the four variables mentioned do indeed affect each other. Ethylene is the main raw material for polyethylene. A few other comonomers, such as 1-butene, 1-hexene or 1-octene, are also frequently used to improve the properties of polymers. For HDPE, the content of these few monomers is generally not more than 1% -2%. The addition of comonomer slightly diminishes the crystallinity of the polymer. This change is generally measured by the density, the density and the crystallization rate was linear. The general classification of the United States according to ASTM D1248 requirements, HDPE density 0.940g /. C or more; medium density polyethylene (MDPE) density range 0.926 ~ 0.940g / CC. Other taxonomies sometimes classify MDPE as HDPE or LLDPE. Homopolymers have the highest density, maximum stiffness, good impermeability and highest melting point, but generally have poor ESCR. ESCR is the ability of PE to resist cracking caused by mechanical or chemical stress Higher densities generally improve mechanical strength such as tensile strength, stiffness and hardness; thermal properties such as softening point and heat distortion temperature; and impermeability, such as air permeability or water vapor permeability. Lower density improves its impact strength and E-SCR. The density of the polymer is mainly influenced by the addition of comonomer but to a lesser extent by the molecular weight. The high molecular weight percentage gives a slight decrease in density. For example, homopolymers have different densities over a wider range of molecular weights.
Production and catalyst
The most common method of PE production is by slurry or gas phase processing, and a few are produced by solution phase processing. All of these processes are exothermic reactions involving ethylene monomers, alpha-olefin monomers, catalyst systems (which may be more than one compound), and various types of hydrocarbon diluents. Hydrogen and some catalysts are used to control the molecular weight. Slurry reactors are typically stirred tanks or a more commonly used large loop reactor where the slurry is circulated and stirred. When ethylene and comonomer (as needed) come into contact with the catalyst, polyethylene particles are formed. After removal of the diluent, the polyethylene granules or granules are dried and dosed with additives to produce pellets. The modernization of a large reactor with a twin-screw extruder produces PE 400,000 pounds per hour or more. The development of new catalysts has contributed to improving the performance of new grades of HDPE. The two most commonly used catalyst types are Phillips chromium oxide based catalysts and titanium compound monoalkylaluminum catalysts. The HDPE produced by Philip-type catalysts has a medium-width molecular weight distribution; the mono-alkyl aluminum catalyst has a narrow molecular weight distribution. Catalysts for the production of narrow MDW polymers using composite reactors can also be used to produce wide MDW grades. For example, two tandem reactors producing significantly different molecular weight products can produce bimodal molecular weight polymers with a full-width molecular weight distribution.
Higher molecular weights result in higher polymer viscosities, although the viscosity is also related to the temperature and shear rate used for the test. The molecular weight of the material is characterized by rheological or molecular weight measurements.HDPE grades generally have a molecular weight range of 40 000 to 300 000. The weight average molecular weight roughly corresponds to the melt index range from 100 to 0. 029/10 minutes. Generally, higher MW (lower Melt Index MI) enhances melt strength, better toughness and ESCR, but higher MW enables .
The process is harder and requires higher pressure or temperature.
Molecular weight distribution (MWD): The WD of PE varies from narrow to wide depending on the catalyst and the process used.
The most commonly used MWD measurement index is the Uniformity Index (HI) which is equal to the weight average molecular weight (MW) divided by the number average molecular weight (Mn). This index range for all HDPE grades is 4-30. Narrow MWD provides low warpage and high impact during molding. Mid-to-wide MWD provides processability for most extrusion processes. Wide MWD also improves melt strength and creep resistance.
Antioxidants are added to prevent degradation of the polymer during processing and to prevent oxidation of the finished product during use. Antistatic additives are used in many packaging grades to reduce the adhesion of bottles or packages to dirt and dirt. Specific applications require special additive formulations, such as wire and cable uses related to copper inhibitors. Excellent weatherability and UV resistance (or daylight) can be added by adding anti-UV additives. No added anti-UV or carbon black PE, it is recommended not to continue to use outdoors. High-grade carbon black pigments provide excellent UV resistance and can be used outdoors often, such as wires, cables, troughs, or pipes.
PE can be manufactured using a wide range of different processing methods. Using ethylene as the main raw material, propylene, 1-butene and hexene as interpolymers, slurry polymerization or gas phase polymerization process is carried out under the action of catalyst, and the obtained polymer is flashed, separated, dried and granulated etc. Processes to obtain a uniform finished product. Including such as sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding and rotational molding.
PE Atomic structure diagram
▲ extrusion :. The grades used for extrusion typically have a melt index of less than 1 and a broad to broad MWD. Low MI achieves suitable melt strength during processing. The MWD grades are more suitable for extrusion because they have Higher production speeds, lower die pressure and reduced tendency to melt fracture.
PE has many extrusion applications such as wires, cables, hoses, tubes and profiles. Tubing applications range from small-section yellow tubes for natural gas to thick-walled black tubes for 48-inch diameter pipes for industrial and urban applications. Large-diameter hollow-walled pipes are used as substitutes for concrete drainage pipes and other sewer pipelines to grow rapidly.
Sheet and Thermoforming: Thermoformed linings for many large picnic reefers are made of PE for toughness, light weight and durability. Other sheet and thermoformed products include fenders, tank linings, pan baskets Transport boxes and cans. A large number of rapidly growing sheet applications are mulch or pond bottom which is based on MDPE for its toughness, chemical resistance and impermeability.
▲ blow molding: HDPE1 / 3 sold in the United States for blow molding purposes. These range from bottles filled with bleach, motor oil, detergents, milk and distilled water to large refrigerators, car fuel tanks and cartridges. Properties such as melt strength, ES-CR and toughness are similar to those used for sheet and thermoforming applications, so similar grades can be used.
Injection-blow molding is often used to make smaller containers (less than 16 ounces). An advantage of this process for packaging pharmaceuticals, shampoos and cosmetics is that the bottle is automatically deburred without the usual blow molding That post-dressing step. Although some narrow MWD grades are used to improve surface finish, the wide to broad MWD grades are commonly used.
▲ Injection: There are countless applications of HDPE, ranging from reusable thin-walled beverage cups to 5-GSL cans, which consume 1/5 of the domestic production of HDPE injection grade general melt index of 5 to 10, with toughness . Low flow grades and higher flow grade grades for workability include thin walled commodities for daily necessities and food packaging; tough, durable food and paint cans; high resistance to environmental stress cracking applications such as small engine fuel tanks and 90 - Gal's trash can.
▲ Rotational Molding: Materials that are machined using this method are generally crushed into a powder material that melts and flows in thermal cycling. Two types of PE are used. General Purpose and Crosslinkable General Purpose MDPE / HDPE grades typically range in density from 0.935 to 0.945 g / cc with narrow MWD to provide high impact and minimal warpage of products with a melt index typically in the 3-8 range. Higher MI grades are not generally suitable because they do not possess the desired impact and environmental stress cracking resistance of rotational molded articles.
High performance rotational molding applications utilize the unique properties of their chemically cross-linkable grades. These grades are fluid in the first stage of the molding cycle and then crosslink to form their excellent resistance to environmental stress cracking and toughness. Abrasion resistance and weatherability. Crosslinkable PE is uniquely suited for large containers ranging from 500 gallons of chemical storage tanks to 20,000 gallons of agricultural storage tanks.
▲ Film: PE film processing is generally used ordinary blown film processing or extrusion processing most of the PE for the film, general low-density PE (LDPE) or linear low-density PE (LLDPE) are available.HDPE film grade is generally used for superior The stretchability and excellent impermeability of the place. For example, HDPE film is commonly used in commodity bags, grocery bags and food packaging.
High-density polyethylene is non-toxic, tasteless, odorless white particles, melting point is about 130 ℃, the relative density of 0.941 ~ 0.960. It has good heat resistance and cold resistance, chemical stability, but also has high rigidity and toughness, good mechanical strength. Dielectric properties, environmental stress cracking resistance is also better.
Packaging and storage
Storage should be far away from the fire, heat insulation, warehouse should be kept dry, clean, non-mixing of any impurities, no sun, rain. Transportation should be stored in clean, dry, covered cabins or cabins with no sharp objects such as nails. Non-flammable aromatic hydrocarbons, halogenated hydrocarbons and other organic solvents mixed operation.
recycle and re-use
HDPE is the fastest growing part of the plastics recycling market. This is mainly because of its ease of reprocessing, minimal degradation and its extensive use in packaging applications. The main recycling is the use of 25% recycled material, such as post consumer consumables (PCR), with virgin HDPE for reprocessing to make non-food contact bottles.
Low density polyethylene(LDPE)
A plastic material that is suitable for a variety of molding processes in thermoforming. Forming processability, such as injection molding, extrusion, blow molding, rotary molding, coating, foam technology, thermoforming, hot air welding, thermal welding.
The main purpose
LDPE is for film products, suitable for film, packaging film, cable insulation materials, injection molding and foam products.
Such as agricultural film, film mulching film, plastic sheeting, vegetable greenhouse film; packing film such as candy, vegetables, frozen food packaging; liquid packaging blown film (milk, soy sauce, juice, tofu, soy milk) Bags, Shrink Packaging Film, Elastic Film, Lined Film; Architectural Film, General Industrial Packaging Film and Food Bags. LDPE is also used in injection molded products, such as small containers, lids, household products, plastic flowers, a stretch blow molding a plastic container. Medical equipment, pharmaceuticals and food packaging materials, extruded pipes, sheets, wire and cable coating, profile, thermoforming and other products; blow molding hollow molding products, such as food containers with dairy products and jam, medicine, cosmetics, chemicals Product containers, tanks and more.
Low-density polyethylene by polymerization method can be divided into high pressure and low pressure method. According to the reactor type can be divided into tank and tube method. Ethylene as raw material, into the reactor, under the action of the initiator to high pressure compression polymerization reaction, the material from the reactor, after removing unreacted ethylene by the separator, the melt extrusion granulation, drying, mixed Together, sent to the packaging.
Low-density polyethylene is milky white pearlescent particles. Non-toxic, tasteless, odorless, matte finish. Density of 0.916 ~ 0.930 g / cubic centimeter. Properties are softer, with good extensibility, electrical insulation, chemical stability, processability and low temperature resistance (resistant to -70 ℃), but the mechanical strength, moisture barrier, gas barrier and poor solvent resistance. Molecular structure is not regular, the degree of crystallinity (55% ~ 65%) is low, the crystal melting point (108 ~ 126 ℃) is also low.
Packaging and storage
The product is packed in polyethylene re-packaging film bag, which can be packed with polypropylene woven bag according to user's requirement. The product should be stored in a clean and dry warehouse, which can be transported by train, car and ship. Storage and transportation should pay attention to fire, water, sun, dust and pollution prevention. The means of transport should be kept clean and dry. There should be no sharp objects such as nails,
(LLDPE) is a high-pressure or low-pressure polymerization of ethylene with a small amount of higher alpha-olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene-1, etc.) under the action of a catalyst Density at 0.915 ~ 0.940 g / cubic centimeter. However, according to ASTM D-1248-84, 0.926 ~ 0.940 g / cm3 density range is medium density polyethylene (MDPE). The new generation of LLDPE expands its density to plastomers (0.890 to 0.915 g / cm3) and elastomers (<0.890 g / cm3). However, the SPI and APC only named LLDPE's range of Union Carbide and Dow Chemical Company for their earlier sales of plastomers and elastomers as Very Low Density Polyethylene (VLDPE) And ultra low density polyethylene (excluding elastomer) ULDPE) resins.
The molecular structure of conventional LLDPE is characterized by its linear backbone, with few or no long branches, but with some short branches. No long chain branching makes polymers more crystalline.
In general, LLDPE resins are characterized by density and melt index. Density is determined by the concentration of comonomer in the polymer chain. The concentration of comonomer determines the amount of short branches in the polymer. The length of the short chain branches depends on the type of comonomer. The higher the comonomer concentration, the lower the density of the resin. In addition, the melt index is a reflection of the average molecular weight of the resin and is mainly determined by the reaction temperature (solution method) and the addition of a chain transfer agent (gas phase method). The average molecular weight has nothing to do with the molecular weight distribution, which is mainly influenced by the type of catalyst.
The industrialization of LLDPE by Union Carbide in the 1970s represented a major revolution in polyethylene catalysts and process technology, resulting in a significant expansion of the polyethylene product range.LLDPE replaced the free-radical initiator with a coordination catalyst and was modified with a lower The cost of low-pressure gas-phase polymerization instead of high-pressure high-pressure reactor, in a relatively short period of time, with its excellent performance and lower cost, has replaced LDPE in many areas. At present, LLDPE penetrates almost all the traditional polyethylene markets, including film, molding, pipe and wire and cable.
LLDPE products non-toxic, tasteless, odorless, milky white particles. Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance and cold resistance. It also has good resistance to environmental stress cracking, tearing strength and other properties, and acid, alkali, organic solvents.