Polyethylene, also known as polythene, was the first polymer to be discovered. Polyethylene is produced by reacting oxygen and ethene. In this reaction, small ethene molecules attach together to form long chain polymer molecules. This process is known as addition polymerisation.
Polyethylene has many uses in our world today, including mouldings such as plastic bottles, lids, caps, and containers; films such as Glad wrap and various plastic bags; and cable coverings such as pipes and insulating wire and cables. These attributes make polyethylene an extremely useful substance in both domestic and industrial applications, and it’s impressive to note that polyethylene has only been around since 1933.
Polyethylene is a thermoplastic material that is often described as wax-like. It is extremely tough, has excellent chemical resistance, is less dense than water, and is the simplest polymer. Ethene (C2H4), a simple hydrocarbon molecule consisting of two carbon atoms and four hydrogen atoms, is the main component in the production of polyethylene. Ethene is also one of the most widely used petrochemicals in the world. It is an unsaturated, colorless gas that can be ignited in the presence of oxygen.
Polyethylene is produced by allowing the free-roaming ethene gas molecules to bond together to form long chain polyethylene molecules. A catalyst must be used for this process to work. A catalyst is a substance that can alter the rate of a chemical reaction without undergoing any chemical change itself. During this process, thousands of ethene molecules bond to form each molecule of polyethylene. Polyethylene is simply a set of ethene molecules bonded together to form a chain. These chains can often stretch up to many times longer than the original ethene molecule.
Although normally ethene monomers have little attraction for one another, the polyethylene molecules have a strong attraction for one another. When polyethylene molecules are attracted and bond, high-density polyethylene is formed. Thus, polyethylene is either formed by low-density or high-density polymerisation. Ethene can undergo the process of polymerisation due to the fact that it is unsaturated and has a double bond between its two carbon atoms. Both high-density and low-density polyethylene have different uses. Low-density polyethylene is used in the production of products such as various bags, plastic bottles, cling wraps, and insulating cables. Low-pressure polymerisation with the use of certain catalysts has meant that the process of polymerization can be achieved at fairly low pressure (20 atmospheres or 2000 kPa) and at temperatures of approximately 100°C.
The reactor itself contains a polyethylene bed placed on a perforated plate. It works by allowing the recycled gas to enter near its base. The gas then passes through the plate and pushes up through the bed, causing it to bubble. Finally, the catalyst converts the ethene to polyethylene. Once the polyethylene is cooled, it forms a fine powder called “fluff,” which is then collected and transported to a storage bin. Any ethene gas that has not reacted passes through a compressor and then a cooler and is processed again.
High-density polyethylene is used to produce items such as lids, caps, baskets, bowls, and large containers such as garbage bins. These polyethylene products are created in what is known as high-pressure polymerization. This is a process that was originally used before low-pressure polymerization was discovered, and thus, it is a fairly simple process. Firstly, ethene gas is compressed and liquefied. From here, it is pumped into a large reactor at a pressure of up to 2660 atmospheres (266 MPa). Amongst this, oxygen and peroxides are pumped in to initiate the polymerization reaction. This process generates a huge amount of heat, so the most complex part of the system is the cooling facilities.
The many products of polyethylene, which are most commonly used, are generally manufactured using any of these five different techniques: Extrusion – film. This technique is used for the production of items such as plastic bags, such as garbage bags and cling wrap. These are the most common applications, while there are many others, these are the best examples of this technique. This final product is achieved by blowing air into a tube of molten plastic. This allows for extremely fine layers of the plastic to form.
Blow molding is used to make plastic bottles and some motor oils. This technique is much like the previous one, but rather than using such fine walls to create fine layers of the plastic, here, the blow mold allows for the molten plastic to be forced downwards into a mold. Air is then used to force the plastic against the walls. This way, this final product is thicker.
Injection molding is used to make anything from lids and caps to toys, baskets, and even garbage bins. Injection molding is the process where the polyethylene pellets are melted and shot into a mold where the plastic recools and hardens, leaving a rigid and firm final product.
Rotational molding is used in the production of things such as boats, playground equipment, and even canoes. In this process, a specific amount of solid polyethylene is placed inside a mold. This mold is then spun at a high speed while being heated. Once the mold is removed, a hard object is left much like that of injection molding.
Extrusion is used to insulate wires and cables, as well as to make pipes. To make pipes, the molten polyethylene is passed through a ring where the shape is decided, and then it enters a cooling chamber, thus creating a hard product. For wire, it is shot through a mold, gaining a coating, and then it enters a cooling agent, which hardens it. Thus, the wire is coated with a hard insulation cover. After investigating the uses and properties of polyethylene, I have found that it is useful due to the following factors: 1. Insulation properties, 2. Chemical resistance, 3. Strength, 4. Flexibility, 5. Non-toxicity, 6. Waterproofness, 7. Unreactivity, 8. Ability to be used in an extremely thin film, 9. Fairly low production cost, 10. Ease of production, 11. Ability to be formed in two separate ways, thus allowing for different products with different properties from the same polymer.
Polyethylene is used in millions of applications all around the world, due to the many different uses of this substance, it is hardly unexpected. Polyethylene is so widely used that for some of us, life would be almost impossible, for everyone reading this has probably come into contact with at least one polyethylene product today.
- (1993) Production of SBR, Kemcor Australia Resources Kit – Section 6, Kemcor Australia, Melbourne.
- Jones, Elvins. Miskin, Lukins. Sanders, Ross (1995) Chemistry One, Reed International Books, Melbourne.