Polyethylene plastics

1. Polyethylene plastics
Polyethylene refers to a polymer formed by the polymerization of ethylene monomer radicals, and the English name is PE for short. PE is the raw material for synthesis from petroleum, and it has been the world's largest producer of plastic resins since 1965. At present, the main varieties of polyethylene are:
Low-density polyethylene (LDPE), high-density polyethylene (HDPE),
Linear Low Density Polyethylene (LLDPE), (Ultra-)High Molecular Weight Polyethylene (UHMWPE),
Metallocene polyethylene (m-PE)
There are also its modified varieties: ethylene-vinyl acetate (EVA)
Chlorinated polyethylene (CPE).
1. Structural properties of polyethylene plastics
PE is a linear polymer, which belongs to the high molecular weight long-chain aliphatic hydrocarbon, with molecular symmetry and non-polarity, small intermolecular force, low mechanical properties, good electrical insulation, low melting point and poor printability. PE has a regular structure and high linearity, so it is easy to crystallize. Crystallinity sorted from high to low: HDPE, LLDPE, LDPE. With the increase of crystallinity, the density, rigidity, hardness and strength of PE products increase, but the impact performance decreases.
(1) General performance PE resin is an odorless, non-toxic white powder or granule, with a milky white appearance and a wax-like feel, and a low water absorption rate, less than 0•01%. The PE film is transparent, and the transparency decreases with the increase of crystallinity. The water permeability of PE film is low but the air permeability is large, which is not suitable for fresh-keeping packaging but suitable for moisture-proof packaging. PE is flammable, the oxygen index is only 17•4, low smoke when burning, a small amount of molten dripping, the flame is yellow and blue, and there is a paraffin smell. PE has good water resistance. The surface of the product is non-polar, difficult to bond and print, and can only be improved by surface treatment.
(2) Mechanical properties The mechanical properties of PE are average, its tensile strength is low, creep resistance is not good, and impact resistance is good. The cracking resistance of PE to environmental stress is not good, but it improves with the increase of molecular weight. PE has good puncture resistance and LLDPE is the best.
(3) Thermal properties The heat resistance of PE is not high, and it improves with the increase of molecular weight and crystallinity. PE has good low temperature resistance, and the embrittlement temperature can generally reach below -50°C, and the lowest can reach -140°C with the increase of molecular weight. The linear expansion coefficient of PE is large, which is the largest in plastics. The thermal conductivity of PE is the highest among plastics.
(4) Electrical properties PE has no polarity, so the electrical properties are very good. Dielectric losses are low and vary very little with temperature and frequency. PE is one of the few plastic varieties with good corona resistance and high dielectric strength, so it can be used as a high-voltage insulating material.
(5) Environmental performance PE has good chemical stability. At room temperature, it can resist the corrosion of acid, alkali and salt aqueous solutions, including dilute sulfuric acid, dilute nitric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid and acetic acid, etc., but it is not resistant to strong oxidants such as oleum sulfuric acid, concentrated sulfuric acid and chromic acid. PE is insoluble in general solvents below 60°C, but it will swell or crack in long-term contact with aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, etc. After the temperature exceeds 60 °C, it can be dissolved in a small amount in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil and paraffin, and after the temperature exceeds 100 °C, it can be soluble in tetrahydronaphthalene.
PE has poor weather resistance, and it will cause aging in the sun and rain, so it is necessary to add antioxidants and light stabilizers to improve.
2. The application range of polyethylene plastics
(1) Film products
Film products are the most important use of PE. LDPE resin can account for more than 50% of film products, and can be used for food, daily necessities, vegetables, shrinkage, self-adhesive, garbage bags and other lightweight packaging films, agricultural mulch films, shed films, etc. HDPE resin can account for more than 10% of film products. Because of its high film strength, it is mainly used for heavy packaging films, tear films and T-shirt bags. The proportion of LLDPE resin used in film products is larger than that of LDPE, which can account for more than 70% of the resin. LLDPE film has the advantages of good elongation, high tensile strength, puncture resistance, environmental stress cracking resistance and low temperature impact resistance, and can be made into ultra-thin film, which is mainly used for packaging film, garbage bags, plastic wrap, self-adhesive film and ultra-thin mulch film.
(2) Injection molded products
PE is widely used in injection molded products because of its good processability, of which HDPE accounts for more than 30%, LDPE and LLDPE account for more than 10% each. Main production: daily necessities such as pots, barrels, boxes, warm bottle shells, cups, toys, etc., turnover boxes, corrugated boxes.
(3) Hollow products
HDPE resin is the main one, which can account for 20% of the resin consumption. Its products have the advantages of good stress crack resistance, good oil resistance, good low temperature impact resistance, etc., and can be used for the packaging of liquids such as food oil, wine, gasoline and chemical agents. In addition, there are hollow toys and so on.
(4) Pipe products
Mainly HDPE resin, mainly used for domestic water supply, gas transportation, agricultural drainage and irrigation, cable threading pipes, liquid straws and ballpoint pen refills. LDPE pipes can also be used for the packaging of cosmetics, medicines, toothpaste, shoe polish, etc.
(5) Silk products
HDPE is used as raw material for round wire, which is mainly used for weaving fishing nets, cables, industrial filters and civil screen nets. Flat wire is made of HDPE and LLDPE, which is mainly used for woven bags, woven cloth and tear film.
(6) Cable products
PE is widely used in the insulation and sheath materials of medium and high voltage cables, among which LDPE is the main one, and the maximum withstand voltage can reach 220kV.
(7) Other products
HDPE and LLDPE can be used for strapping belts. LLDPE can be used for profiles.
3. Other varieties of polyethylene
(1) Metallocene polyethylene (m-PE)
Metallocene polyethylene is a metallocene polymer with a catalyst used in the polymerization reaction, which is single-column because of its unique properties, and is referred to as m-PE in English.
The characteristics of metallocene polyethylene are: high molecular weight and narrow distribution, few and short branches, low density, high purity, high transparency, high tensile strength, high impact, low heat sealing temperature and wide range, puncture resistance, etc.
Films made of metallocene polyethylene have excellent strength, high transparency, and low sealing temperatures (better than LLDPE
20°C lower).
At present, due to the high price of metallocene polyethylene, it cannot replace traditional polyethylene for a while, but it can compete with EPR, EVA, etc. Its main applications are film products, such as LLDPE blended with metallocene HDPE, which can be used to produce grease inhibitory heavy packaging bags.
(2) Ultra-high molecular weight polyethylene (UHMWPE)
Ultra-high molecular weight polyethylene is a polyethylene polymer with a relative molecular weight of up to 1 million to 4 million, abbreviated as UHMWPE in English. Compared with ordinary PE, UHMWPE has the advantages of good wear resistance (15 times larger than POM), low friction (0.05~0.08 when the coefficient of friction is lubricated by oil), good impact resistance, good self-lubrication, good physiological compatibility and corrosion resistance, but its hardness, low strength, poor heat resistance, poor creep resistance and difficult molding processing.
UHMWPE is mainly used to produce pipes, profiles, hollow products and injection molded products, and the specific applications are as follows:
(1) Wear-resistant conveying pipeline. It can be used for coal, mineral powder, cement, lime, salt, grain transportation pipelines, and the wear resistance is 10~50 times higher than that of metal.
(2) Mechanical parts. It mainly uses its wear resistance, such as shuttle, shuttle rod, gear, connector, sweeping rod, buffer block and eccentric block in textile machinery, etc., used in papermaking machinery for water tank cover, wiper, compacted parts, joints and sealing shafts, etc., and used in machine tools for guide rails, slides and friction wheels.
(3) Medical materials. Taking advantage of the advantages of good physiological compatibility and corrosion resistance, it is used in orthopedic materials and artificial joints.
(4) Other products. Radiation resistance is used to manufacture covers for nuclear power plants, and body armor and skis are manufactured to make use of high impact.
(3) Ethylene-vinyl acetate copolymer (EVA)
The content of vinyl acetate (VA) in EVA varies greatly, and its uses vary greatly. The appearance of EVA is translucent or semi-milky white particles or powder, flammable, unextinguishable from fire, and has molten dripping, giving off a lactic acid smell. EVA is characterized by good flexibility, toughness, low temperature resistance, weather resistance, stress crack resistance, heat sealing, weldability, adhesion, transparency, high gloss, chemical resistance, ozone resistance, colorability, and compatibility with fillers.
EVA has good processability, processing characteristics and technology are the same as LDPE, but due to the lower melt viscosity and limited thermal stability, the processing temperature is 20-40°C lower than LDPE.
EVA is mainly used in the following aspects:
(1) Film is the main use of EVA, which can be used for packaging film and agricultural film, including heavy packaging film, frozen packaging film, food packaging film, composite film, heat shrinkable film, self-adhesive film and barrier cling film. The heat preservation, light transmittance, aging resistance, and non-dripping properties of EVA agricultural film are better than those of PE film.
(2) Injection molded products such as bicycle saddles, toys, sealed containers, artificial turf and automobile fenders, etc.
(3) Extruded products hoses, cable sheaths, suction pipes, etc.
(4) Foam products Sandals, slippers, soles, bicycle tires and toy wheels, etc.
(4) Chlorinated polyethylene (CPE)
The chlorine content of CPE varies greatly. It is a soft material when the chlorine content is 25%-40%, and a hard material when the chlorine content is greater than 40%. The commonly used CPE has a chlorine content of 30%-40%, and the two grades of 135A with a chlorine content of 35% and 140B with a chlorine content of 40% are the most commonly used.
CPE is a product of PE chlorination, the regularity of PE macromolecules is destroyed, the crystallization ability is reduced, and the flexibility of the product is improved, becoming an elastomer material. CPE has excellent impact resistance, flame retardancy (oxygen index of 27), heat resistance (can be used for a long time at a temperature of 120 °C), weather resistance, oil resistance, acid and alkali resistance, ozone aging resistance, high wear resistance, and good electrical insulation.
The processing of CPE can be divided into two types: direct processing and vulcanization processing.
Direct processing can be molded by injection molding, extrusion, calendering and other methods, and products such as calendering method to produce waterproof membranes, extrusion method to produce sealing strips for doors and windows, etc. Vulcanization can be used to manufacture imitation leather soles, etc.
The application of CPE can be divided into two categories: individual products and modifiers. Separate products are not as large as modifiers, and are mainly used to manufacture waterproof membranes, sealing strips, cable sheaths and conveyor belts. As an impact modifier, it can be used as a blend material of rigid PVC, PE and ABS, mainly to improve the impact performance and flame retardant performance of products. In addition, CPE can be used as a compatibilizer for blends.