Characteristics of Polyethylene

General characteristics

Polyethylene resin is non-toxic and tasteless white powder or particle, with milky white appearance, wax like feel, and low water absorption, less than 0.01%. The polyethylene film is transparent and decreases with the increase of crystallinity. The polyethylene film has low water permeability but high air permeability, so it is not suitable for fresh-keeping packaging but moisture-proof packaging. Flammability, oxygen index 17.4, low smoke during combustion, a small amount of molten droplets, yellow on the flame and blue on the flame, with paraffin smell. Polyethylene has good water resistance. The surface of the product is non-polar, difficult to bond and print, and has been improved after surface treatment. Many branched chains have poor resistance to light degradation and oxidation.

Its molecular weight ranges from 10000 to 100000. If the molecular weight exceeds 100,000, it is UHMWPE. The higher the molecular weight, the better the physical and mechanical properties, and the closer to the required level of engineering materials. However, the higher the molecular weight is, the more difficult it is to process. The melting point of polyethylene is 100~130  °C, and its low temperature resistance is excellent. Good mechanical properties can still be maintained at – 60  °C, but the service temperature is 80~110  °C.

It is insoluble in any known solvent at room temperature, and can be slightly dissolved in toluene, amyl acetate, trichloroethylene and other solvents above 70  °C.

Chemical properties

Polyethylene has good chemical stability and is resistant to dilute nitric acid, dilute sulfuric acid and any concentration of hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, acetic acid, ammonia, amines, hydrogen peroxide, sodium hydroxide, potassium hydroxide and other solutions at room temperature. But it is not resistant to strong oxidation corrosion, such as fuming sulfuric acid, concentrated nitric acid, chromic acid and sulfuric acid mixture. At room temperature, the above solvents will slowly erode polyethylene, while at 90~100  °C, concentrated sulfuric acid and concentrated nitric acid will quickly erode polyethylene to destroy or decompose it. Polyethylene is easy to be oxidized by light, heat and ozone. It is easy to be degraded under the action of ultraviolet light. Carbon black has excellent light shielding effect on polyethylene. After irradiation, crosslinking, chain breaking, formation of unsaturated groups and other reactions may occur.

Mechanical properties

Polyethylene has general mechanical properties, low tensile strength, poor creep resistance and good impact resistance. Impact strength LDPE>LLDPE>HDPE, the influence of crystallinity and relative molecular weight of other mechanical properties LDPE, with the improvement of these indicators, its mechanical properties increase. The environmental stress cracking resistance is not good, but it is improved when the relative molecular weight increases. Good puncture resistance, among which LLDPE is the best.

Thermal properties

The heat resistance of polyethylene is not high, which is improved with the increase of molecular weight and crystallinity. Low temperature resistance is good, and the brittleness temperature is generally below – 50  °C; The lowest temperature can reach – 140  °C with the increase of relative molecular weight. The linear expansion coefficient of polyethylene is large, up to (20~24) × 10-5/K。 High thermal conductivity.

Electrical properties

Because polyethylene is non-polar, it has excellent electrical properties with low dielectric loss and high dielectric strength. It can be used as frequency modulation insulation materials, corona resistant plastics, and high-voltage insulation materials.

Environmental characteristics

Polyethylene is an alkane inert polymer with good chemical stability. It is resistant to the corrosion of acid, alkali and salt aqueous solutions at room temperature, but not to strong oxidants such as oleum, concentrated nitric acid and chromic acid. Polyethylene is insoluble in general solvents below 60  °C, but 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 slightly dissolved in toluene, amyl acetate, trichloroethylene, turpentine, mineral oil and paraffin; The temperature is higher than 100  °C, and it is soluble in tetrahydronaphthalene.

Because polyethylene molecule contains a small amount of double bond and ether bond, the aging will be caused by sunlight and rain, so antioxidant and light stabilizer should be added to improve it.

Machining characteristics

LDPE and HDPE have good fluidity, low processing temperature, moderate viscosity, low decomposition temperature, and do not decompose at high temperature of 300  °C in inert gas, so they are plastics with good processing performance. However, the viscosity of LLDPE is slightly higher, and the motor power needs to be increased by 20%~30%; Melt fracture is easy to occur, so it is necessary to increase die clearance and add processing aids; The processing temperature is slightly higher, up to 200~215  °C. Polyethylene has low water absorption and does not need drying treatment before processing.

Polyethylene melt belongs to non Newtonian fluid, and its viscosity fluctuates less with temperature, while the shear rate decreases rapidly and linearly with the increase of shear rate, among which LLDPE decreases most slowly.

Polyethylene products are easy to crystallize in the cooling process, so pay attention to the mold temperature during processing. To control the crystallinity of the product, so that it has different properties. The molding shrinkage of polyethylene is large, so it must be considered when designing the mold.

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