Polyethylene is a high molecular compound formed by addition polymerization of ethylene. The actual molecular weight varies from 10000 to several million according to different polymerization conditions. The polyethylene first invented is low-density polyethylene obtained by high-pressure method, with a specific gravity of 0.9l0-0.925g/cm3. The specific gravity of polyethylene obtained by low pressure and medium pressure method is 0.94l-0.965g/cm3, which is called high-density polyethylene. Polyethylene is a white waxy translucent material, soft and tough, slightly elongate, non-toxic, flammable, melting and dripping when burning, giving off the smell of paraffin burning. The properties of polyethylene are related to its molecular weight and crystallinity.
Many mechanical properties of polyethylene are determined by the density and melt index of the material. From low density polyethylene to high density polyethylene, its density varies in the range of 0.90-0.96g/cm3. The melt flow index (MFI) of polyethylene varies from 0.3 to more than 25.0. Many important properties of polyethylene vary with density and melt index.
The glass transition temperature of polyethylene material is low, 125 ℃, but it can maintain its mechanical properties in a wide temperature range. The equilibrium melting point of linear high molecular weight polyethylene is 137 ℃, but it is generally difficult to reach the equilibrium point, and the melting point range during processing is usually 132-135 ℃. The ignition temperature of polyethylene is 340 ℃, the autoignition temperature is 349 ℃, and the ignition temperature of its dust is 450 ℃. The melt index of polyethylene is determined by its molecular weight. When polyethylene materials with different molecular weights are mixed, their melt index is also taken as a certain value according to a certain rule.
Polyethylene is water resistant, and its physical properties remain unchanged in high humidity or water. Concentrated sulfuric acid, concentrated nitric acid and other oxidants will slowly erode polyethylene. In aliphatic hydrocarbons, aromatic hydrocarbons and chlorinated hydrocarbons, polyethylene will swell, but the swelling agent will recover its original properties after volatilization. Under 60 ℃, polyethylene can withstand most solvents, but when the temperature is higher than 70 ℃, hydrocarbon solvents will quickly attack polyethylene. When the temperature continues to rise, polyethylene will dissolve in some solvents. Polyethylene separated from the solution is cooled to form a paste or gel according to the temperature.
Polyethylene is prone to photooxidation, thermal oxidation, ozone decomposition and halogenation. Due to chemical inertness and surface nonpolarity, polyethylene is difficult to bond and print. But after oxidant, flame and corona discharge treatment, polyethylene will have good adhesion and printing properties.
When polyethylene is exposed to radiation, cross-linking, chain breaking and formation of unsaturated groups will occur, but the main reaction is cross-linking. When polyethylene is irradiated in inert gas, hydrogen will overflow and lose weight; When polyethylene is irradiated in air, it will gain weight due to the addition of oxygen. After irradiation, unsaturated groups were added to polyethylene molecules, which led to the reduction of oxidation stability. The cross-linking reaction of polyethylene under irradiation is better than that of chain breaking and unsaturated group formation. The cross-linking reaction can improve the weather resistance of polyethylene, so the irradiated polyethylene products have better weather resistance than the non irradiated polyethylene products.
Polyethylene is slowly degraded in the air by the action of oxygen, and this process will be accelerated by the action of heat, ultraviolet ray and high-energy radiation. Degradation and aging are characterized by fading, brittleness and even damage of products. Carbon black has a significant light shielding effect on polyethylene. Adding 2% carbon black can effectively improve the service life of polyethylene products. In addition to carbon black, adding some ultraviolet absorbers into polyethylene can also play an anti-aging role.
Polyethylene plastic has poor thermal conductivity. In order to make the heat transfer into the whole volume of plastic powder particles quickly during the rotational molding process, the size of polyethylene powder particles used for rotational molding should meet certain requirements. The smaller the particle, the easier the heat is transferred, and the easier the temperature of the material will reach its melting point. However, when the particles are too small, the material is easy to absorb moisture and agglomerate, which is not conducive to rolling movement in the mold. Polyethylene plastics bought in the market are often granular materials, which need to be ground and screened to meet the requirements of rotational molding process.
Polyethylene is a kind of plastic with high toughness. When ordinary grinding machines are used for processing, its particles will be torn and pulled into a shape that is not conducive to re grinding. The pulverization of polyethylene granules requires special high-speed shredding equipment.
