Need help with a specific question? Try posting them in our new Forum

  Student Engineer  

 Home

Polymerisation

Poly Vinyl Chloride

What is a polymer?

A process called polymerisation is used to join single molecules (monomers) together to form a chain of many, the above example shows Vinyl Chloride bonded into Poly Vinyl Chloride. The left portion of the image shows the vinyl chloride while on the right hand side these single molecules of vinyl chloride have bonded together to form poly vinyl chloride, "poly" meaning many. Polymers are often formed from crude oil however more recently polymers have been created from corn starch and vegetable fats to form Bioplastics.

What is polymerisation?

Polymerisation is the chemical process of monomers joining together to form polymers, often it takes many thousands of monomers to make a single polymer. 2 types of polymerisation reactions are listed below.

1. Addition polymerisation- where monomers addon to each other with the addition of a catalyst, these are usually alkenes such as ethene and propene. Alkenes can act as monomers because they have a double bond.

2.Condensation Polymerisation - This is when monomers join or polymerise with a byproduct such as water, carbon dioxide or ammonia. This usually requires two different types of monomers that join alternately.

Polymers can have different structures and these can affect their properties, some examples are shown below.

Linear Chain
Linear Chain Polymer
The chains when heated can flow easily this is a good example of the chain of a thermoplastic. Properties often associated with this type of chain are a weak material, ductile with a low density and melting point.

Branched Chain
Branched Chain Polymer
Branched chains will not flow as easily when heated they will have a higher melting point than linear chains and typically have the properties of a slightly less ductile material (stronger and stiffer).

Cross Linked
Cross Linked Polymer
The Cross linked bonding retains its shape when heated and is a good example of the bonding found in thermo set plastics. These also have a higher melting point than linear and branched bonds, they are usually a harder material but also more brittle.

The two main types of polymers are thermo sets and thermo plastics. Thermo plastics can be re-shaped after heating where as thermo sets cannot. Thermo plastics have the benefit of being recyclable and generally cheaper and easier to process than thermo set plastics.

Applications where polymers may be used.

Acrylic
Acrylic is a polymer called poly(methyl methacrylate) and is often used as an alternative to glass, examples of these include small fish tanks/aquariums visors in goggles and crash helmets covers for skylights and as it has good weatherproofing qualities it is commonly used for outdoor signage.

High Density Polythene (HDPE)
Babies baths, kitchen equipment, children’s toys, fabric filaments.
Polythene is cheap and acid resisting. A strong polymer which softens at aroiund 120 degrees celcius.

Low Density Polythene (LDPE)
Used in plastic bags, film and packaging. LDPE is a more flexible polymer than HDPE. This softens at around 85 degrees celcius.

PVC (Poly Vinyl Chloride)
Industrial and domestic piping, artificial leather (e.g. shoes), protective clothing.
PVC is reasonably hard and hardwearing it is also fairly cheap

PEEK (Poly Ether Ketone)
Used in aggressive environments such as nuclear plants, oil and geo thermal wells, high pressure steam valves, aircraft and car engine parts.
Peek has a high melting point, very good tensile strength, has a low coefficient of friction and is light weight with good chemical resistance. It is however very costly.

Polymers and the environment

Polymers are difficult to dispose of as they can take many years to degrade. Many carrier and refuse bags now include substances such as corn starch to help them break down a little sooner.

A majority of polymers can be recycled but the process of doing so can be very costly as the different polymers need to separated from each other.

EngineerStudent.co.uk © 2016