Polyethylenealso 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