Polyethylene is a thermoplastic polymer with a variable crystalline structure and a vast range of applications depending on the particular type. It is one of the most widely produced plastics in the world, with tens of millions of tons produced worldwide each year.
Polyethylene (PE), light, versatile synthetic resin made from the polymerization of ethylene. Polyethylene is a thermoplastic polymer which is an important family member of polyolefin resins. It is one of the most widely produced plastics in the world, with tens of millions of tons produced worldwide each year. PE was first produced in 1933 in England by Imperial Chemical Industries Ltd (ICI) during studies of the effects of extremely high pressures on the polymerization of polyethylene. However, it was until 1953 that Karl Zieger and Guilio Natta successfully develop linear HDPE. In part for this innovation, Ziegler was awarded the Nobel Prize for Chemistry in 1963. Since that time, by using different catalysts and polymerization methods, scientists have produced polyethylene with various properties and structures. LLDPE, for example, was introduced by the Phillips Petroleum Company in 1968.
There are several types of polyethylene, and each one is best suited for a different set of applications. Generally speaking, High-Density Polyethylene (HDPE) is much more crystalline, and is often used in entirely different circumstances than Low-Density Polyethylene (LDPE). For example, LDPE is widely used in plastic packaging, such as for grocery bags or plastic wrap. HDPE, by contrast, has common applications in construction (for example, in its use in the fabrication of drain pipes). Ultrahigh Molecular Weight Polyethylene (UHMW) has high-performance applications in things such as medical devices and bulletproof vests. Being a thermoplastic material, Polyethylene has a melting point of 110oC to 130 oC. It can be heated to that temperature, cooled, and then reheated again without any significant degradation. The ability of polyethylene to liquefy at around 110oC makes it a good fit for injection molding.
LDPE is prepared from gaseous ethylene under very high pressures (up to about 350 megapascals, or 50,000 pounds per square inch) and high temperatures (up to about 350 °C [660 °F]) in the presence of oxide initiators. These processes yield a polymer structure with both long and short branches. Because the branches prevent the polyethylene molecules from packing closely together in hard, stiff, crystalline arrangements, LDPE is a very flexible material. Its melting point is approximately 110 °C (230 °F). Principal uses are in packaging film, trash and grocery bags, agricultural mulch, wire and cable insulation, squeeze bottles, toys, and housewares. The plastic recycling code of LDPE is #4.
LLDPE is structurally similar to LDPE. It is made by copolymerizing ethylene with 1-butene and smaller amounts of 1-hexene and 1-octene, using Ziegler-Natta or metallocene catalysts. The resultant structure has a linear backbone, but it has short, uniform branches that, like the longer branches of LDPE, prevent the polymer chains from packing closely together. Overall, LLDPE has similar properties to LDPE and competes for the same markets. The main advantages of LLDPE are that the polymerization conditions are less energy-intensive and that the polymer’s properties may be altered by varying the type and amount of its chemical ingredients. The plastic recycling code of LLDPE is #4.
HDPE is manufactured at low temperatures and pressures, using Ziegler-Natta and metallocene catalysts or activated chromium oxide (known as a Phillips catalyst). The lack of branches in its structure allows the polymer chains to pack closely together, resulting in a dense, highly crystalline material of high strength and moderate stiffness. With a melting point more than 20 °C (36 °F) higher than LDPE, it can withstand repeated exposure to 120 °C (250 °F) so that it can be sterilized. Products include blow-molded bottles for milk and household cleaners; blow-extruded grocery bags, construction film, and agricultural mulch; and injection-molded pails, caps, appliance housings, and toys. The plastic recycling code number of HDPE is #2.
Linear polyethylene can be produced in ultrahigh-molecular-weight versions, with molecular weights of 3,000,000 to 6,000,000 atomic units, as opposed to 500,000 atomic units for HDPE. These polymers can be spun into fibres and then drawn, or stretched, into a highly crystalline state, resulting in high stiffness and a tensile strength many times that of steel. Yarns made from these fibres are woven into bulletproof vests.
Packaging: High-Density Polyethylene (HDPE) is used to make crates, trays, and bottles for commonly available groceries. Bottle caps, cans, and drums are also made using HDPE. The high impact strength of HDPE makes it the material of choice for making such packaging items.
Low-Density Polyethylene (LDPE) is used to make films, plastic bags, trash bags, and other food packaging material. The good thing about LDPE is that it is low-cost and provides good physical flexibility.
Pipes and fittings: Polyethylene is used extensively to make a variety of pipes and fittings. HDPE is used in gas pipes, water pipes, sewer pipes, as well as coatings on steel pipes. HDPE provides excellent resistance to chemicals and hydrolysis which makes it the material of choice for those applications. LDPE, on the other hand, is used to make water pipes and hoses because of its low water absorption and plasticity.
Electrical: Polyethylene is a good insulator and is used in making insulation for coaxial cables and cable jacketing.
Medical: Certain specific types of polyethylene like ultra-high-molecular-weight polyethylene has high toughness and is resistant to cuts and wear. It is also chemical resistant. So, it is used to make artificial joints, knee replacements, and hip replacements. It is also used to make certain sections of implants.
Toys: One of the biggest applications of HDPE is toys. In fact, one-third of all toys are made using HDPE. High tensile strength is one of the useful properties of HDPE.
Consumer products: Garbage containers, kitchenware, housewares, ice boxes, bowls, buckets, ketchup bottles, etc. are all made using polyethylene.
Automotive: Fuel tanks in vehicles are made using polyethylene (HDPE).