What Is Polyethylene Resin
- LDPE, MDPE, HDPE, LLDPE
Polyethylene resin, referred to as PE resin, is a thermoplastic resin obtained by polymerization of ethylene. In industry, it also includes copolymers of ethylene and a small amount of α-olefins. Polyethylene is odorless, non-toxic waxy while granules, with excellent low temperature resistance, good chemical stability. It can withstand most acid and alkali erosion, low water absorption, and excellent electrical insulation. Polyethylene resin has a wide range of uses, mainly used to manufacture films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc. PE resin can also be sued as high-frequency insulation materials for TVs and radars.
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What Are Types of Polyethylene PE Resin
Polyethylene resin can be divided into high-density polyethylene HDPE resin, medium-density polyethylene MDPE resin, low-density polyethylene LDPE resin and linear low density polyethylene LLDPE resin according to the polymerization method, chain structure and molecular weight.
LDPE is a typical crystalline polymer with a crystallinity of 55%-65% and a melting point of 105~126°C.
LDPE is a non-polar material, which is easy to be charged with static electricity and has low surface energy. Therefore, corona treatment should be carried out before printing and compounding to improve the surface energy. During processing, anti-static should be paid attention to to avoid static electricity accumulation affecting product quality or electric spark discharge , causing a fire.
LDPE resin has excellent transparency and excellent heat sealing performance, and can be widely used in the production of transparent low temperature frozen packaging products.
LDPE has excellent moisture resistance and is a high-quality raw material for making dry food or packaging for items that require good moisture resistance. However, LDPE has a large gas barrier and is easy to pass through various gases.
Although LDPE has certain oil resistance, its oil resistance and organic solvent resistance are not as good as polypropylene. Therefore, when the thickness is small, it is not suitable for long-term storage of gasoline, alcohol, grease, etc. When using LDPE, the thickness should preferably exceed 50mm.
Medium-density polyethylene is a kind of polyethylene with a density of 0.926-0.94g/cm3 and the same performance as LDPE. Due to the increase in density, the crystallinity of MDPE is as high as 70% to 80%. The increase in density and crystallinity increases the melting temperature of MDPE and the hardness and strength of the product. MDPE is between LDPE and HDPE.
The melting point of HDPE high-density polyethylene is 126-136 ° C, and the crystallinity exceeds 90%. Its rigidity, toughness, mechanical strength, solvent resistance and stress crack resistance are better than LDPE. Due to the high strength of HDPE, it is one of the important uses of blown film grade HDPE to be used in garbage bags. HDPE is characterized by high strength and poor transparency.
In addition to some characteristics of LDPE, LLDPE also has the following characteristics.
LLDPE has excellent heat sealability. Although the melting temperature of LLDPE is about 5°C higher than that of LDPE, the heat sealability of LLDPE has good ion-like properties. Therefore, if LLDPE is used as the heat-sealing material, only a lower heat-sealing temperature can be used to achieve highly reliable heat-sealing strength, which is suitable for high-speed heat-sealing machines.
The melt viscosity of LLDPE is quite large, 10 times that of LDPE, and the melt viscosity of LLDPE is not sensitive to temperature, but very sensitive to processing stress. This means that LLDPE cannot reduce the viscosity by increasing the temperature, but can only increase the melt fluidity by increasing the processing speed.
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What Are Excellent Properties of Polyethylene Resin
In addition to high-fluidity PE such as plastic special materials, the tensile strength of PE resin is generally above 10MPa, and elongation at break can reach 500% or higher.
High density polyethylene has strong macromolecular structure, high crystallinity and high strength. Generally, the tensile strength and other properties of high-density polyethylene are significantly higher than those of low-density polyethylene. Among them, the tensile strength can reach more than 2 times that of low-density polyethylene. However, the impact strength of high-density polyethylene is lower than that of low-density polyethylene.
- PE is one of the plastic with the best chemical corrosion resistance, and its chemical corrosion resistance includes the following aspects.
- Polyethylene PE is resistant to most dilute acids, and is usually not corroded by various salts and their solutions. But it will be damaged by oxidizing concentrated acid, and it will also be corroded by oxidizing substances at high temperatures.
- PE is usually not corroded by alcohols, aldehydes, ketones and esters. However, at room temperature, it will cause a certain degree of swelling due to the action of aromatic hydrocarbons, aliphatic aromatic hydrocarbons, and halogenated hydrocarbons. The dissolution of LDPE above 60°C and HDPE above 80°C also increases. PE is less affected by halogen at room temperature, but the effect will be accelerated at high temperature.
- PE also absorbs oil, such as mineral oil and essential oil, which will escape through PE material.
- PE products may produce stress cracking under the action of stress cracking agents such as aliphatic and aromatic hydrocarbons, aldehydes, ketones, alcohols, concentrated sulfuric acid, detergents, soaps, oils and fats, and alkali metal hydroxides.
Under the action of ultraviolet rays and high-energy radiation, PE resin will degrade in the air, resulting in discoloration, surface cracking and embrittlement, loss of strength and loss of use value. Therefore, for outdoor applications or PE hollow containers that are exposed to direct sunlight, weather-resistant formulas should be used. That is, an appropriate amount of UV absorbers, sunscreens and other additives should be added to the PE main material to prevent the harm of ultraviolet light.
In most applications plastic containers for hollow goods, barrier properties are often very important. Among the barrier properties to various substances, the permeability to oxygen, carbon dioxide, nitrogen, organic solvents, and the permeability to water and water vapor are particularly important in practical applications. They can directly affect the protective effect of plastic containers on the packaged items.
Generally speaking, the barrier properties of Polyethylene resin improve with the increase of density. That is to say, HDPE resin has better barrier properties than LDPE resin. However, the barrier properties of different substances vary greatly. PE has excellent barrier properties to the penetration of water vapor. In particular, high-density polyethylene is one of the best plastics to block the penetration of water vapor.
What Are Excellent Properties of Polyethylene Resin
Polyethylene film. PE film is more breathable. And as the density increases, its air permeability decreases. PE film has moisture resistance and low moisture permeability. In particular, more than half of the total output of low-density polyethylene is blown into film. This kind of film has good transparency and certain tensile strength. Widely used as packaging materials for various food, medicine, chemical fertilizers, industrial products and agricultural films. PE resin can also be processed into a composite film by extrusion for packaging heavy objects.
Polyethylene hollow products. Polyethylene PE can be made into bottles, barrels, cans, tanks and other containers by blow molding, or can be made into large containers such as tanker tanks and storage tanks by casting. The selection of raw materials for polyethylene hollow products should pay attention to the influence of different densities and different melt flow rate resins on the properties of hollow products.
Polyethylene pipe sheet. High-density polyethylene pipes have high strength and are suitable for underground laying. The extruded sheet can be used for secondary processing, and high density PE can be made into low-foam plastics by foam extrusion and foam injection methods, which can be used as pallets and building materials.
Polyethylene fibers. Generally, low pressure PE is used as raw material and spun into synthetic fibers. It can also be used in industrial acid and alkali resistant fabrics, composite materials for automotive and offshore operations.
Polyethylene resin also has important applications in the electronics industry and is the main material for the manufacture of household appliances such as refrigerators and microwave ovens. Excellent thermal conductivity can effectively reduce the temperature of electronic products and improve the service life of electronic products.
The wear resistance and corrosion resistance of polyethylene resin make it widely used in the automotive industry. It can be used to manufacture body parts, wheels, frame and other parts. It has good structural strength and wear resistance, and can effectively reduce the wear and tear of vehicles.
In addition, polyethylene resin can also be used to manufacture special industry supplies such as printing machines, processing tools, and production tools. It has good corrosion resistance, wear resistance, and anti-skid properties, and can meet various needs of special industries.
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