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 polymerisation.
Polyethene in our world today has many uses, some of these are: ? mouldings-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. Ethene’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
colourless 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 polymerisation.
Ethene can undergo the process called polymerisation 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 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 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 polymerisation, this is a process that
was originally used before low-pressure polymerisation 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 polymerisation 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 moulding, 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 moulding, this is used to make anything from lids and caps to
toys, baskets and even garbage bins. Injection moulding, 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 moulding,
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 moulding. 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 chaimber 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. Flexability 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 seperate ways thus allowing for differnent 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 probabily 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.
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.
