Polyethylene

Polyethylene is a simple but versatile polymer. Polymers are large molecules composed of many identical smaller molecules. Many of the most common plastics used in everyday life are made from polyethylene. Because it can be used to manufacture so many plastic products, polyethylene is the most commonly used plastic component in the world.

Background

While people have used various substances to make plastic-like coatings for centuries, true plastics, as they are known today, were first developed from cellulose in the 1850s. Researchers continually looked for new ways to treat or modify cellulose to create lightweight, malleable materials for manufacturing. The start of World War II increased demand for consumer items produced without the metal, rubber, and other materials needed to equip the armed forces. This led to experiments with chemicals to create new lightweight, durable materials to replace raw materials that were in short supply. It was during this time that polyethylene was first developed.

Polyethylene quickly became important to the war effort as well and was initially used as a coating for wire cables that crossed the ocean to enable long-distance communication. Later, it was used as an insulator for radar cables. When the war ended in 1945, companies that manufactured polyethylene began developing consumer products using the new material, and its popularity grew. Polyethylene became the first plastic to sell more than one billion pounds in one year in the United States, and it remains the most popular form of plastic in the United States and around the world.

Overview

Polyethylene is made by polymerizing ethene, also known as ethylene. Polymerization is the process by which many identical small molecules are linked together to form larger molecules. Polymerization is accomplished through the application of heat, pressure, and/or a catalyst to induce the formation of long chains of molecules that make up a polymer.

The process of making polyethylene begins with either a natural gas, such as ethane or methane, or a combination of gases including propane. It may also be created from by-products of the process used to make gasoline from crude oil. The gas is polymerized using one of three main methods, depending on the type of product the polyethylene will be used to make. These three methods result in low density polyethylene (LDPE), high density polyethylene (HDPE), or linear low density polyethylene (LLDPE).

One of the earliest methods of polymerizing polyethylene was discovered accidentally in England in the 1930s. It involves treating ethylene gas with heat under high pressure. Under these conditions, the gas turns into a white powder that today is known as LDPE. This type of polyethylene is used most often for making blown film, a type of plastic often used for packaging. LDPE may be used to insulate electrical wires and components. The final products made from this type of polyethylene are weatherproof and resistant to chemicals. They do not allow water through easily, they may be translucent, and they are inexpensive to produce.

HDPE was discovered in 1952 after German and Italian researchers found that they could polymerize polyethylene without applying as much pressure if they added an aluminum catalyst. A catalyst is a chemical that helps other chemicals change. The resulting polyethylene was thicker and stiffer, so it could be used for different purposes than LDPE. For example, HDPE may be blown into molds to create detergent bottles and other chemical storage containers or products such as plastic piping and household goods. Products manufactured from this form of polyethylene are rigid enough to hold other substances; however, they are still flexible and resistant to the effects of other chemicals and can withstand cold temperatures.

Later in the 1950s, the DuPont Company developed a process for adding another substance, such as butene or hexene, to ethene before starting a low-pressure polymerizing process. This resulted in LLDPE. Like LDPE, LLDPE is most commonly used for packaging materials. Of the three types of polyethylene, HDPE is the most commonly produced, followed closely by LDPE. LLDPE is the least common type. Regardless of the process used to make it, polyethylene is most often formed into small pellets of various sizes and shapes, which are then shipped to manufacturers and used as raw materials in making a final consumer product.

Products made from polyethylene are found in nearly every aspect of life in the developed world. Manufacturers use polyethylene pellets in various ways. For example, they may inject melted polyethylene pellets into a mold to form an object or flatten it into sheets. Polyethylene may be combined with other materials and with other forms of polyethylene to create composite products, such as multilayer film or food storage containers with plastic linings, such as milk cartons.

Because of its versatility, polyethylene is used in everything from packing materials to children's toys and safety products to automobile components. A single consumer purchase may include multiple forms of polyethylene. For instance, a prescription medicine contained in a plasticized capsule may be packed in a bottle made of polyethylene components and then handed to a customer in a plastic bag made of polyethylene.

Although it is widely used and often part of disposable products, proponents say that polyethylene is environmentally friendly. It is recyclable, and programs to recycle polyethylene plastics are common and easily accessible. Because of its nature, polyethylene can be reused and made into new products nearly endlessly. When a polyethylene product cannot be recycled, it may be used as a fuel source for energy production.

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