Knowledge of rotational molding for PE (Polyethylene) and composite materials

Polyethylene is a high molecular compound formed by the addition polymerization of ethylene. The actual molecular weight varies from 10,000 to several million depending on the polymerization conditions. The polyethylene invented was low-density polyethylene obtained by high-pressure method, with a specific gravity of 0.910-0.925g/cm3. The polyethylene obtained by low-pressure and medium-pressure methods has a specific gravity of 0.941-0.965g/cm3, which is called high-density polyethylene. Polyethylene is a white waxy translucent material, soft and tough, slightly elongated, non-toxic, flammable, and melts and drips when burning, giving off the smell of burning paraffin. The properties of polyethylene are related to its molecular weight and its 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, the density varies in the range of 0.90-0.96g/cm3. The melt index (melt flow index) of polyethylene varies greatly, 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 relatively low, at 125°C, but it can maintain its mechanical properties in a wide temperature range. The equilibrium melting point of linear high molecular weight polyethylene is 137°C, but it is generally difficult to reach the equilibrium point. Usually, the melting point range during processing is 132-135°C. The ignition temperature of polyethylene is 340°C, the autoignition temperature is 349°C, and the ignition temperature of its dust is 450°C. The melt index of polyethylene is determined by its molecular weight. When polyethylene materials of different molecular weights are mixed, their melt index also takes 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 corrode polyethylene. In aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons, polyethylene will swell, but the original properties can be restored after the swelling agent evaporates. Below 60°C, polyethylene can resist solvents, but hydrocarbon solvents will quickly corrode polyethylene when the temperature is above 70°C. When the temperature continues to rise, polyethylene will dissolve in certain solvents. The polyethylene separated from the solution forms a paste or colloidal state after cooling, depending on the temperature.
Polyethylene is susceptible to photo-oxidation, thermal oxidation, ozone decomposition, and halogenation. Due to its chemical inertness and non-polar surface, polyethylene is difficult to bond and print. However, after being treated with oxidants, flames, and corona discharge, polyethylene has good adhesion and printing properties.
When polyethylene is irradiated, cross-linking, chain breaking, and unsaturated group formation reactions occur, but the main reaction is cross-linking. When polyethylene is irradiated in an inert gas, hydrogen overflow occurs and it loses weight; when polyethylene is irradiated in air, it gains weight due to the addition of oxygen. After irradiation, unsaturated groups are added to the polyethylene molecules, resulting in reduced oxidative stability. When irradiated, the cross-linking reaction of polyethylene is to the chain-breaking and unsaturated group formation reactions. The cross-linking reaction can improve the weather resistance of polyethylene, so irradiated polyethylene products have better weather resistance than non-irradiated polyethylene products.
Polyethylene degrades slowly under the action of oxygen in the air, and this process is accelerated by heat, ultraviolet rays, and high-energy radiation. The characteristics of degradation and aging are fading brittleness and even damage to the products. Carbon black has a significant light-shielding effect on polyethylene. Adding 2% carbon black can effectively increase the service life of polyethylene products. In addition to carbon black, adding certain ultraviolet absorbers to polyethylene can also play an anti-aging role.
Polyethylene plastic has poor thermal conductivity. In order to allow heat to be quickly transferred to the entire volume of plastic powder particles during rotational molding, the particle size of polyethylene powder used for rotational molding should meet certain requirements. The smaller the particles, the easier it is for heat to be transferred, and the easier it is for the temperature of the material to reach its melting point. However, if the particles are too small, the material is easy to absorb moisture and agglomerates, which is not conducive to tumbling movement in the mold. The polyethylene plastics purchased on the market are often granules, which need to be ground and sieved to meet the requirements of the rotational molding process.
Polyethylene is a plastic with high toughness. When processed by a conventional grinder, its granules will be torn into a shape that is not conducive to grinding again. The crushing of polyethylene granules requires special high-speed shredding equipment.