HDPE- High Density Polyethylene Characteristics & Applications
HDPE has good heat resistance and cold resistance, good chemical stability, but also has high rigidity and toughness, good mechanical strength. Dielectric properties, environmental stress and cracking resistance is also better. Hardness, tensile strength and creep are better than low density polyethylene, wear resistance, electrical insulation, toughness and cold resistance are better, but with low density insulation is slightly worse; chemical stability, at room temperature, insoluble in any organic solvents, acid, alkali and various salts of corrosion; thin film for water vapor and air permeability is small, water absorption is low , anti-aging performance, resistance to environmental cracking less than low-density polyethylene, especially the thermal oxidation will make its performance decline, so the resin needs to add antioxidants and ultraviolet absorbent to improve the deficiencies in this area. The thermal deformation temperature of HDPE film under stress is low, this point should be paid attention to when applied.
PE can be made with a wide variety of processing methods. With ethylene as the main raw material, propylene, 1-butene, cyclohexene as the copolymer, under the action of the catalyst, slurry polymerization or gas phase polymerization process, the polymer obtained by flash evaporation, separation, drying, granulation and other processes, to obtain a uniform particle of the finished product. such as sheet extrusion, film extrusion, tubing or profile extrusion blow molding, injection molding and rolling molding.
HDPE suitable for all kinds of molding process of thermoplastic molding, good molding processing, such as injection molding, extrusion, blow molding, rotary molding, coating, foaming process, thermoforming, heat seal welding, thermal welding and so on. Extrusion: The grade used for extrusion production generally has a melt index of less than 1 and a width to wide MWD. In the process of processing, the low MI can obtain the suitable melt strength.
Wider MWD grades are more suitable for extrusion because they have higher production speeds, lower die pressure and less melt fracture trends. PE has many plastic extrusion uses, such as wire, cable, hose, tubing and profile. Tubing applications range from thick-walled clarinet used in industrial and urban pipelines to a small section of natural gas yellow tube to 48in diameter.
Large-diameter hollow-wall tubes made of concrete rain drains and other sewer lines have grown rapidly. Sheet and thermoforming: The hot-formed lining of many large picnic freezers is made of PE, with toughness, light weight and durability. Other sheet and thermoforming products include fenders, tank linings, pan-basin shields, transport boxes and cans.
A large number of rapidly growing sheet applications are mulch or pond-bottom villages, which are based on MDPE with toughness, chemical resistance and impermeable.
Blow Moulding: More than HDPE1/3 sold in the United States for blow moulding purposes. These range from bottles of bleach, oil, detergents, milk and distilled water to large refrigerators, car fuel tanks and cans.
The characteristics of blow moulding, such as melt strength, ES-CR and toughness, are similar to those used for sheet and thermoforming applications, so similar grades can be used. Injection-blow molding is usually used to make smaller containers (less than 16oz) for packaging pharmaceuticals, shampoos and cosmetics. One of the advantages of this process is the production of bottles automatically to the corners, do not need to be like the general blow molding process, such as the late trimming steps.
Although some narrow MWD grades are used to improve surface finishes, generally use medium to wide MWD grades. Rotational moulding: The material used in this process is generally crushed into powder material, which melts and flows in the heat cycle. Two types of PE are used for rotational moulding: general and cross-linked. General-grade MDPE/HDPE the usual density range from 0.935 to 0.945g/cc, with narrow MWD, so that the product has a high impact and the smallest warpage, the melt index of the general range of 3-8.
Higher MI grades are usually not applicable because they do not have the desired impact and environmental stress cracking of the rolled products. Film: PE film processing general use of ordinary blown film processing or flat extrusion processing method. Most PE is used for thin films, universal low-density PE (LDPE) or linear low density PE (LLDPE) are available. HDPE film grade is generally used to require superior tensile and excellent anti-seepage of the place. For example, HDPE membranes are commonly used in commodity bags, grocery bags and food packaging.
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