High density polyethylene (HDPE) is a white powder or granular product. It is nontoxic, tasteless, with a crystallinity of 80% - 90% and a softening point of 125-l 35 ℃, service temperature can reach 100 ℃; hardness, tensile strength and creep are better than LDPE; wear resistance, electrical insulation, toughness and cold resistance are better; chemical stability is good, insoluble in any organic solvent at room temperature, corrosion resistance of acid, alkali and various salts; permeability of film to water vapor and air is small, water absorption is low; aging resistance is poor, ring resistance is poor The environmental stress cracking property of LDPE is not as good as that of LDPE, especially the degradation of its performance due to thermal oxidation. Therefore, antioxidant and ultraviolet absorbent should be added to the resin to improve the deficiency. The heat deformation temperature of HDPE film is low under stress, so attention should be paid to the application.
The most common production method of PE is slurry or gas-phase processing, and a few of them are produced by solution phase processing. All these processes are exothermic reactions involving ethylene monomers, a-olefin monomers, catalyst systems (possibly more than one compound) and various types of hydrocarbon diluents. Hydrogen and some catalysts are used to control molecular weight. The slurry reactor is generally a stirred tank or a more commonly used large-scale ring reactor, in which the slurry can be circularly stirred. When ethylene and comonomer (as needed) come into contact with catalyst, polyethylene particles will be formed. After the diluent is removed, the polyethylene particles or powder particles are dried and additives are added according to the dosage to produce the particles. The modern production line of large-scale reactor with twin-screw extruder can produce more than 40000 pounds of PE per hour. The development of new catalysts contributes to the improvement of the performance of new grade HDPE. The two most commonly used catalysts are Phillip's chromium oxide based catalysts and titanium based alkylaluminum catalysts. The molecular weight distribution of HDPE produced by Phillip type catalyst is medium width, while that of titanium alkylaluminum catalyst is narrow. The catalyst used for the production of narrow MDW polymer in a duplex reactor can also be used to produce wide MDW grades. For example, two series reactors that produce significantly different molecular weight products can produce bimodal molecular weight polymers with a wide range of molecular weight distributions.