Classification And Properties Of Polyethylene

Classification and Characteristics of Polyethylene

There are two types of polyethylene according to density: hdpe with a density of 0.94 ~ 0.97 g/cm3 and LDPE with a density of 0.91 ~ 0.935 g/cm3. There are three types of polyethylene according to the traditional production methods, i.e., low-pressure polyethylene, medium-pressure polyethylene and high-pressure polyethylene.

High-density polyethylene (HDPE) is made by polymerizing ethylene at atmospheric pressure or tens of atmospheres and at a certain temperature under the action of a catalyst, so it is also called low-pressure polyethylene, but recently it can also be produced at high pressures (1,000 ~ 2,000 atmospheres). Since the density of this kind of polyethylene is generally between 0.94 ~ 0.97 g/cm3, it is more suitable to be called high-density polyethylene.

HDPE produced by Ziegler method, Phillips method and standard oil method has a density of about 0.94~0.97 g/cm3, high crystallinity, high strength, and is mainly used for injection molding and extrusion molding products, which is the first generation of HDPE. The disadvantages of these three methods are low catalyst efficiency, and the residual catalyst in the polyethylene product seriously affects the performance of the product.

The main properties of LDPE, such as tear strength, hardness, toughness, aging stability, etc. are related to MFR. Short branched chain and long branched chain is one of the main characteristics of LDPE. This is one of the main characteristics of LDPE differentiated from HDPE. As the degree of branched chain increases, the crystallinity and density decrease. Crystallinity and density are linearly related, and LDPE density has a greater impact on product performance. As density increases, tear strength, hardness increases, Vicat heat resistance increases; while impact strength decreases, turbidity decreases.The molecular weight distribution of LDPE has a close relationship with processing performance. The wider the molecular weight distribution, the easier the flow under high shear, the lower the die pressure of the extruder during the molding process, the better the appearance of the molded material, and the better the optical properties of the film. Molecular weight distribution also has an impact on the physical and mechanical properties of the product, the general molecular weight distribution is wide, the impact strength and resistance to stress cracking is reduced. the chemical structure of LDPE is similar to paraffin wax, does not contain polar groups, so it has good chemical stability, resistance to corrosion of acids, alkalis and salts in aqueous solutions; its electrical properties are extremely good, with a low electrical conductivity, low dielectric constant, low power factor, dielectric strength, high characteristics. High characteristics. However, LDPE poor heat resistance, but also not resistant to oxidative aging and light aging.

Classification and properties of polyethylene

1. Ultra high molecular weight polyethylene

Ultra High Molecular Weight Polyethylene (UHMWPE) is a high density polyethylene with a relative molecular mass of 500,000 to 5 million. Its outstanding advantages are excellent impact resistance, abrasion resistance, stress crack resistance and cold resistance. In addition, it also has very low water absorption, good chemical stability, high heat resistance and silent operation, oil-free lubrication and other properties. For example, China's production of a relative molecular mass of 700,000-1,200,000 ultra-high molecular weight polyethylene, its relative density of 0.955- 0.968, crystalline melting point of 192- 212'C, simply support the impact strength of the beam test, with a notched specimen also did not break. The coefficient of friction is 0.14-0.15 and the wear is 4.4-5.2mm when dry friction is applied to the abraded parts (45-gauge steel, surface hardness HRc50-55), and it is superior to many other engineering plastics in terms of impact and abrasion resistance.

Ultra-high molecular weight polyethylene is mainly used to manufacture special films, large containers, large conduits, plates, and a variety of requirements for impact resistance, friction-resistant mechanical parts, such as bearings, gears, guide bearings, sprockets, shells, gaskets, the textile industry in the bobbin casting, lifting prismatic box and the belly of the belt, the papermaking industry in the scraper and the molding board, the mining industry landing liner and the guide chain rail. It is especially suitable for low temperature devices, is a very promising engineering plastics.

UHMWPE can be produced by the typical Ziegler method. Due to the high melt viscosity of UHMWPE, the mobility is very low, the original can only be used for cold press sintering method or hot press method of molding, has been able to change to extrusion processing. When the manufacture of complex structure products, can be molded into simple parts, and then reprocessed with general mechanical processing methods. Nitrile rubber, chloroprene rubber or epoxy resin can also be used for bonding.

2. Heat-resistant polyethylene

The heat resistance of general-purpose polyethylene is as low as 100°C, and its application in engineering plastics is quite limited. Romania with sodium pentyl as catalyst produced a 200 ℃ high density polyethylene, its performance is close to PTFE, can be used as engineering plastics, used in the manufacture of mechanical parts.

3. Cross-linked polyethylene

Polyethylene (PE) crosslinking technology is one of the important means to improve its material properties. After crosslinking modified PE can make its performance has been greatly improved, not only significantly improve the mechanical properties of PE, environmental stress cracking resistance, chemical resistance, corrosion resistance, creep resistance and electrical properties and other comprehensive performance, but also very obviously improve the temperature resistance level, can make PE heat-resistant temperature from 70 ℃ to more than 100 ℃, which greatly broaden the application areas of PE.

Cross-linked polyethylene has the following advantages:

1.Heat-resistant performance: XLPE with reticulated three-dimensional structure has very excellent heat-resistant performance. It will not decompose and carbonize below 200℃, the long-term working temperature can reach 90℃, and the thermal life can reach 40 years.

2.Insulation performance: XLPE maintains the original good insulation properties of PE, and insulation resistance is further increased. Its dielectric loss angle tangent value is very small, and not much affected by temperature.

3.Mechanical properties: due to the establishment of a new chemical bond between the macromolecules, XLPE's hardness, stiffness, abrasion resistance and impact resistance have been improved, thus making up for the PE susceptible to environmental stress and cracking shortcomings.

4.Chemical resistance: XLPE has strong acid and alkali resistance and oil resistance, its combustion products are mainly water and carbon dioxide, less harmful to the environment, to meet the requirements of modern fire safety.

There are two kinds of cross-linking methods: chemical method and radiation method.

Chemical method (with organic peroxide as cross-linking agent) crosslinked polyethylene impact strength than the uncrosslinked 50 times, good processing fluidity, suitable for rotomolding, processing of large containers, such as gasoline tanks, automotive parts, agricultural composting tanks, chemical industry sewage tanks or drains and so on.

Radiation cross-linking method: polyethylene in the high-energy rays (such as γ rays, α rays, electron rays, etc.) or cross-linking agent under the action of its macromolecules to generate cross-linking, can improve its heat-resistant and other properties. Using cross-linked polyethylene as insulation cable, its long-term operating temperature can be increased to 90 ℃, can withstand the instantaneous short-circuit temperature of up to 170-250 ℃. Radiation method of crosslinking product insulation performance is particularly good, can be used for the manufacture of high temperature 125'C appliances and electric motors of the soft core wire insulation. 4.

4. Glass fiber reinforced high density polyethylene

The United States of America DuPont (DuPont) company has successfully developed a good adhesion with glass fibers of high-density polyethylene (brand Alathon G 0530). This polyethylene and glass fiber combination, high strength, good heat resistance, is a kind of engineering plastics. It can be processed by compression, injection molding, extrusion, blow molding, etc. It is used in the manufacture of pillars and general pillars for agriculture and fisheries, large pipes, automotive parts, mechanical parts (computer, projector covers), and household electrical parts, etc. 5.

5. Polyethylene wax

Low molecular weight polyethylene with a relative molecular mass of 1000~10000 is called polyethylene wax. In recent years, Japan's Mitsui Petrochemical Company has used Ziegler-type catalysts to produce high, medium and low density polyethylene wax. It is characterized by good chemical and thermal stability, softening point as high as 114~132'C, low viscosity, good compatibility with other waxes and resins, excellent electrical properties, white color, odorless and harmless. The high-density grade varieties are used as dye dispersant, rubber and plastic mixing agent, coating, printing and paper processing additives; the low-density grade varieties are mainly used as additives to improve the processing performance of plastics.

6. Chlorinated polyethylene

Chlorinated polyethylene (CPE) is a saturated polymer material, the appearance of white powder, non-toxic and tasteless, with excellent weather resistance, ozone resistance, chemical resistance and aging resistance, good oil resistance, flame retardant and coloring properties. Good toughness (still flexible at -30℃), good compatibility with other polymer materials, high decomposition temperature, decomposition of HCl, HCl can catalyze the dechlorination reaction of CPE.

Chlorinated polyethylene is a random chloride obtained by replacing some hydrogen atoms in polyethylene with chlorine, and its structure is equivalent to the terpolymer of ethylene, vinyl chloride, and dichloroethylene. The introduction of chlorine atoms into the polyethylene molecule reduces crystallinity, lowers the softening temperature and increases flexibility.

Depending on the molecular weight and distribution of the raw polyethylene, the degree of structural branching, the degree of chlorination and the degree of residual crystallinity, chlorinated polyethylene can be obtained from rubbery to hard plastic. Non-crystalline or slightly crystalline polyethylene is rubbery. If the crystallinity increases, it becomes an amorphous resin with increased rigidity and higher embrittlement temperature and softening point. In addition to the solvent method (chlorobenzene, carbon tetrachloride, etc. as solvents) for the highly chlorinated compounds used as coatings and adhesives, the aqueous phase suspension method is mainly used in the industry. According to the reaction temperature, it is divided into block chlorination (low temperature) and random chlorination (temperature above the melting point). Non-crystalline to slightly crystalline rubbery material is mainly produced by random chlorination.

Chlorinated polyethylene has the following characteristics: good resistance to low temperatures, good fluidity, good processability when used alone or in combination with other resins and rubbers, second only to chlorinated rubbers in terms of chemical resistance, good flammability (not easy to burn with a chlorine content of more than 25%), good weather resistance, good ozone resistance and good impact resistance.

Non-crystalline chlorinated polyethylene can be vulcanized into rubber products alone, with properties similar to those of chlorosulfonated polyethylene, and can also be used in combination with other rubbers. Chlorinated polyethylene with fillers, plasticizers and stabilizers (to prevent decomposition of hydrogen chloride by heat and depolymerization reaction) can be used as plastic. When mixed with PVC, modified PVC plastics can be produced to improve impact resistance, low temperature resistance and processing performance. It is also used as a permanent plasticizer, coating and adhesive. Chlorinated polyethylene is less expensive than chloroprene rubber and chlorosulfonated polyethylene.

7. Chlorosulfonated polyethylene

Chlorosulfonated polyethylene (CSM) was first industrialized by Dupont Company in 1952. Chlorosulfonated polyethylene is produced from low-density polyethylene or high-density polyethylene by chlorination and chlorosulfonation. It is a white or yellow elastomer, soluble in aromatic hydrocarbons and chlorinated hydrocarbons, insoluble in fats and alcohols, and insoluble in ketones and ethers.

CSM is a saturated elastomer with polyethylene as the main chain, average molecular weight 30,000~120,000. chlorosulfonated polyethylene is a white or milky white flaky or granular solid, relative density 1.07~1.28, Menni viscosity 30~90, brittleness temperature -56°C~40°C. CSM's chemical structure is completely saturated, with excellent ozone resistance, weatherability, heat resistance, flame resistance, water resistance, chemical drug resistance, and water resistance. CSM has excellent ozone resistance, weather resistance, heat resistance, flame retardancy, water resistance, chemical resistance, oil resistance, abrasion resistance, etc. The solubility parameter of CSM is δ=8.9, which is soluble in aromatic hydrocarbons and halogenated hydrocarbons, and is only soluble in ketone, ester, ether, and insoluble in aliphatic hydrocarbons and alcohols.

When polyethylene and chlorine containing sulfur dioxide, part of the hydrogen atom in the molecule is replaced by chlorine and a small amount of sulfonyl chloride (-SOiCl) group, the product is called chlorosulfonated polyethylene. It is rubbery, because it does not contain double bonds, so it is ozone-resistant, aging-resistant, chemical-resistant, and has good oil resistance and heat resistance (can be used for a long time below 120 ℃), good tensile strength, modulus and hardness is high, good abrasion resistance, and even do not use plasticizers in the 50 ℃ of the low temperature is not brittle, and resistance to corona discharge.

CSM due to the molecular structure contains chlorosulfonyl active groups, so it shows high activity, especially resistance to chemical media corrosion, resistance to ozone oxidation and resistance to oil erosion, flame retardant properties, but also weathering, heat resistance, resistance to ionic radiation, resistance to low temperatures, resistance to abrasion and electrical insulation and excellent mechanical properties. Earlier CSM was mostly developed for military engineering purposes. but its large permanent deformation also limits its use.

8. Copolymers of ethylene with other monomers

Ethylene can be copolymerized with other monomers in order to obtain ethylene polymers with a comprehensive range of properties. Important ethylene copolymers are; ethylene-propylene copolymer, ethylene-butylene-ethylene copolymer, ethylene-ethylene copolymer, ethylene-perchloroethylene copolymer, ethylene-triphenylene chloride copolymer, and so on. ethylene-ethylene copolymer, ethylene-perchloroethylene copolymer, ethylene-triethylene chloride copolymer, etc. Generally prepared by metallocene catalyst polymerization, more representative of polyolefin elastomers POE, polyolefin plastomers POP, etc., widely used in such as automotive exterior and interior, automotive belts and hoses, weathering, packaging adhesives, footwear, roofing membranes, flooring, fittings and so on. This section on ethylene copolymers will be dedicated to organizing relevant content to share in the future.