What is Polyethlene ?
Polyethylene also known as polyethylene or polythene, was the first of the polymers to be discovered. Polyethene is a polymer produced by reacting oxygen and ethene, in this reaction the small ethene molecules attach together to form long chain polymer molecules. This process is known as addition polymerization.
Polyethene in our world today has many uses, some of these are: · moldings-plastic bottles, lids and caps, different types of containers. · films-glad wrap and various plastic bags. · cable coverings-various pipes and insulating wire and cables As you can see polyethene has a huge variety of both domestic and industrial uses, this is fairly impressive when you see that polyethene has only been around sense 1933.
Polyethene is a thermoplastic material which is often described as wax-like it is extremely tough and is has an excellent chemical resistance. It is also less dense than water and is the simplest polymer, these attributes lead to polythene being an extremely useful substance.
Ethene (C2H4), is a simple hydrocarbon molecule which consists of 2 carbon atoms and 4 hydrogen atoms. Ethane's main use is in the production of polythene yet it is one of the most widely used petrochemicals in the world. Ethene is an unsaturated colorless gas which can be ignited in the presence of oxygen. Below is a diagram of ethene: Polyethene is produced by allowing the free roaming ethene gas molecules to bond together to form long chain polyethene molecules. In order for this to work a catalyst must be used, 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 from each molecule of polyethene.
Polyethene is simply a set of ethene molecules bonded together to form a chain, these chains can often stretch up to many many times longer than the original ethene molecule. Below is a diagram of polyethene: Although normally ethene monomers have little attraction for one another, yet the polyethene molecules have a strong attraction for one another. When polyethene molecules are attracted and bond high-density polyethene is formed, thus polyethene is either formed by low-density or high-density polymerization. Ethene can undergo the process called polymerization due to the fact that it is unsaturated and because it has a double bond between its two carbon atoms. Both High-density and low-density polyethene have different uses.
Low-density polyethene is used in the production of products such as various bags, plastic bottles, cling wraps, and insulating cables. Low pressure polymerization 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 polyethene 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 polyethene, once the polyethene is cooled it forms a fine powder called 'fluff' which is then collected and transported to a storage bin. Any ethene gas which has not reacted passes through a compressor and then a cooler, and is processed again.
High-density polyethene is used to produce items such as lids, caps, baskets, bowls and large containers such as garbage bins. These polyethene products are created in what is know 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 polyethene 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 glad 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, this is used to make plastic bottles and some motor oils. This technique is much like the previous one but rather that using such fine walls to create fine layers of the plastic here the blow mould allows for the molten plastic to be forced downwards into a mould. Air is them used to force the plastic against the walls this way this final product is thicker. Injection molding, this is used to make anything from lids and caps to toys, baskets and even garbage bins. Injection molding, is the process where the polyethene pellets are melted and shot into a mould where the plastic re-cools and hardens leaving a rigid and firm final product.
Rotational molding, this technique is used in the production of things such as boats, playground equipment and even canoes. In this process a specific amount of solid polyethene is placed inside a mould, this mould is then spun at a high speed while being heated. Once the mould is removed a hard object is left much like that of injection molding. Extrusion, this is used to insulate wires and cables as well as making pipes. To make pipe, the molten polyethene is passed through through a ring where the shape is decided, it then enters a cooling chamber thus creating a hard product. For wire it is shot through a mould thus gaining a coating, it then enters a cooling agent which hardens it and thus the wire is coated with a hard insulation cover.
After investigating the uses and properties of polyethene I have found that it is useful due to the following factors:
1. Insulation properties
2. Chemical resistance
3. Strength
4. Flexibility
5. Non-toxic
6. Water proof
7. Unreactive
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.
Polyethene is used in millions of applications all around the world, due to the many different uses of this substance it is hardy unexpected. Polyethene it is so widely used that for some of us life would be almost impossible, for everyone of you reading this has probably come into contact with at least one polyethene product today.
Bibliography
(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
