1. Polyethylene (PE)
(1) Properties and Applications
Polyethylene (PE) is a typical thermoplastic material. It is an odorless, tasteless, non-toxic, and combustible white powder. Polyethylene resins used for molding are generally supplied in the form of wax-like pellets produced through extrusion and pelletizing, with a milky-white appearance.
The molecular weight of polyethylene typically ranges from 10,000 to 1,000,000. Polyethylene with a molecular weight exceeding 1,000,000 is classified as Ultra-High Molecular Weight Polyethylene (UHMWPE). In general, the higher the molecular weight, the better the physical and mechanical properties. However, processing becomes more difficult as molecular weight increases.
Therefore, the appropriate molecular weight and processing conditions should be selected according to the intended application. High-molecular-weight polyethylene is suitable for structural and load-bearing applications, while low-molecular-weight polyethylene is mainly used for coatings, polishes, lubricants, plasticizers, and similar applications.
The mechanical properties of polyethylene depend largely on factors such as molecular weight, branching degree, and crystallinity. High-Density Polyethylene (HDPE) typically exhibits a tensile strength of 20–25 MPa, whereas Low-Density Polyethylene (LDPE) generally has a tensile strength of only 10–12 MPa.
The elongation characteristics of polyethylene are mainly influenced by its density. As density and crystallinity increase, flexibility and extensibility tend to decrease.
Polyethylene also offers excellent electrical insulation properties. As a non-polar insulating material, its dielectric constant and dielectric loss are largely unaffected by temperature and frequency. Its outstanding high-frequency performance makes it particularly suitable for manufacturing insulation layers for various high-frequency cables and submarine cables.
(2) Types of Polyethylene
1. Low-Density Polyethylene (LDPE)
(1) Performance:
The density range of low-density polyethylene is 0.910–0.925 g/cm³. Its molecular structure is a branched-chain type with long and short branches attached to the main chain. For every 1000 carbon atoms on the main chain, there are about 50 or fewer ethyl, butyl, or longer branches. Compared with high-density and medium-density polyethylene, it has a lower crystallinity (55%–65%), a lower softening point (108°C–126°C), and a broader melt index range (0.2–80 g/10 min). Since the chemical structure of low-density polyethylene is similar to that of paraffin hydrocarbons and contains no polar groups, it exhibits good chemical stability and resistance to corrosion by acids, alkalis, and aqueous salt solutions. Its electrical properties are excellent, characterized by low conductivity, low dielectric constant, low dielectric loss, and high dielectric strength. However, low-density polyethylene has poor heat resistance and is not resistant to oxygen or light aging. Therefore, antioxidants and ultraviolet absorbers are commonly added to the resin to improve its aging resistance. Low-density polyethylene possesses good flexibility, extensibility, and transparency, but its mechanical strength is lower than that of high-density polyethylene and linear low-density polyethylene.
(2) Applications:
Low-density polyethylene is mainly used to produce film. Film products account for more than half of the total output of low-density polyethylene products, used in agricultural films and packaging for various foods, textiles, and industrial goods. Due to its excellent electrical insulation properties, low-density polyethylene is often used as a sheathing material for wires and cables. It is also used for injection-molded products such as various toys, lids, boxes, and containers. When blended with high-density polyethylene, it can be used in injection molding and blow molding to produce pipes and containers, etc.
2. High-Density Polyethylene (HDPE)
(1) Performance:
The density of high-density polyethylene is 0.941–0.965 g/cm³, and its molecular structure is linear with few branches, averaging only a few branches per 1000 carbon atoms. Compared with low-density polyethylene, high-density polyethylene has a crystallinity of 80%–90%, higher density, higher service temperature, greater hardness and mechanical strength, and good chemical resistance.
(2) Applications:
The applications of high-density polyethylene differ from those of low-density polyethylene. About 50%–70% of low-density polyethylene is used for film production, whereas high-density polyethylene is mainly used for manufacturing rigid hollow products, accounting for approximately 40%–65% of its total consumption. Specific applications include: blow molding to produce various bottles, cans, and industrial containers such as tanks and barrels; injection molding to manufacture everyday containers like basins, buckets, baskets, crates, and frames, as well as daily-use articles and furniture; extrusion molding to produce various pipes, strapping bands, fibers, and monofilaments. In addition, it can be used for manufacturing wire and cable sheathing materials and synthetic paper. When large amounts of inorganic calcium salts are added, it can also be used to produce calcium-plastic packaging boxes, furniture, doors, and windows. Recently, high-density polyethylene has been increasingly used to produce high-strength ultra-thin films for packaging food, agricultural products, and textiles.
