i

 

 

 

 

Using Hydrocarbons

 

Crude oil provides the raw materials (hydrocarbons) for making plastics.  When hydrocarbons are cracked alkenes are made (molecules with carbon to carbon double bond).  Ethene is an alkene, it is the starting material for many plastics.  

Plastics are very important to us, they were first made on large scale in the early 1930s.  If you look around your house you will find that most of the objects are now made from plastics.  Plastics are very large molecules.  Chemists call them POLYMERS.

Making Polymers

polymerisation animation

These are long chain molecules consisting thousands of carbon and hydrogen atoms (the word 'poly' means many e.g. a polygon - a shape having many sides).

Polymers are made by joining together thousands of smaller, reactive molecules called MONOMERS(the word 'mono' means one e.g. a monorail is a railway with one track only). 

polymer

The above reaction is called addition polymerisation.  One of the most common monomer is ethene.  When these react with each other, a long chain molecule is produced, commonly called polythene - POLY(ETHENE).

There are 2 types of reactions to make polymers.

Addition reaction

polymerisation

condensation reaction title

In this type of polymerisation a small molecule is always given off.
Nylon is an example of a polymer made in a condensation reaction..

The monomers have a reactive site at both ends of their molecules.
They join together, end to end, to make a long chain.

condensation reaction

Formation of Nylon


Properties and uses of polymers - different structure different properties different uses

Polymers have properties which make them very suitable for all sorts of objects.

Advantages:
  • They do not corrode.

  • They have low density, lighter than wood, stone or metal.

  • They can be produced in different colours, by adding pigments.

  • They have good elasticity and strength.

  • They are excellent thermal and electrical insulators.

  • They are easily moulded - this is the real meaning of plastic.

  • Can be made very strong.

  • They are uncreative, and do not corrode in air or water.  Many of them are not affected by acids or alkalis.

  •  

    Disadvantages:
  • They are difficult to dispose of.  Plastic bags and cartons do not rot when they are disposed of, so they pollute the environment.  However, biodegradable plastics rot away.
  • Some plastics catch fire very easily.
  • When they burn, they often produce harmful gases.  For example, PVC gives off fumes of hydrogen chloride when it burns.  If this is inhaled, it would form hydrochloric acid in your throat and eyes.
  • Aesthetically, they don't look as good as wood or stone.
  • Different polymers show these properties to different extent.   Some polymers can be remoulded as many times as desired after first softening them by heating.  These are called thermoplastic polymers.  They are sometimes called thermosoftening plastics.  

    Other polymers soften and can be moulded the first time they are heated, but can't be resoftened and remoulded.  These are called thermosetting polymers.  If you heat them strongly enough, they eventually break down and char.  They are hard and rigid.

    These different properties can be explained if you look at the arrangement of the polymer chains.

    The  diagrams below show the polymer chains in thermoplastic and thermoset polymers.

    Thermoplastic

    seperate polymer chains.


    In the thermoplastics the polymer chains are totally free to slide past each other, therefore it is easy to change the shape.

    packaging

    For example, polythene is very cheap and is easily moulded into strong, flexible containers.  However, it pollutes the environment as it does not rot.

    Thermoset


    Chains fixed together by strong bonds

    In thermosetting polymers the chains are cross-linked.  Instead of each chain being separate, adjacent chains are linked together.  This makes it difficult for polymer chains to move past each other, hence the polymer is hard and rigid.  Even when it is heated, the chains are still unable to move, so the polymer does not melt.

    13 amp plug

    This electrical plug is made from a thermosetting plastic which does not melt when it gets hot.

     

    tyre

    Rubber for car tires

    Rubber is natural polymer produced by rubber trees.  It is a runny, sticky liquid called latex.  In its raw form it far too soft and sticky to be of much use. But if sulphur is added to rubber, it makes cross-links between the polymer chains, and the rubber gets harder.  This is called vulcanising the rubber.  The higher the content of sulphur, the more cross links are formed and the harder it gets.  For example, eraser is soft rubber and has far less sulphur content in it then a car tire, which needs to be very hard.

    There are many different plastics with many different uses.

    Name of plastic: Polythene
    Monomer:

    Properties: Very cheap, strong and easily moulded
    Use: Carrier bags, moulded for containers, buckets, pop bottles, clingfilm etc.

    Example:

    packaging

    Name of plastic: Polypropene
    Monomer:

    Properties: Form strong fibres and has high elasticity, flexible, light and can be dyed
    Use: Car parts such as bumpers, battery cases, plastic chairs, ropes, fishing nets and carpets.

     

    Example:

    chair

    Name of plastic: Polystyrene
    Monomer:

    Properties: Cheap, easily moulded and can be expanded into foam
    Use: Plastic models, foam packaging, plastic cups,  radio outer cases and when expanded, as insulation.

     

    Example:

    radio

    Name of plastic: Polychloroethene (PVC: Polyvinyl chloride)
    Monomer:

    Properties: Cheap, strong, flexible yet strong, etc.
    Use: Pipes, gutters, window frames, electrical insulation of cables, floor tiles, rain coats, seat covers, records and wall paper.

     

    Example:

    window

    Name of plastic: Nylon - made from 2 different monomers
    Monomer:

    1,6 diaminohexane

    Hexanedioic acid
    Properties: Cheap, light, flexible, strong fibre, Can easily catch fire etc.
    Use: Ladies tights, ropes, bristle for brushes, carpets and clothing.

     

    Example:

    clothes

    Name of plastic: Phenolic resins-Bakelite - made from 2 different monomers
    Monomer:


    Phenol + Aldehyde
    Properties: Cheap, very strong but brittle, rigid, etc.
    Use: Electrical plugs, switches and saucepan handles.

     

    Example:

    13 amp plug

    Name of plastic: Perspex (acrylic)
    Monomer:

    Properties: Strong, clear and easily moulded
    Use: Car light reflectors, safety glasses, contact lenses, traffic signs and false teeth.

     

    Example:

    false teeth

    Name of plastic: PTFE
    Monomer:

    Properties: Very slippery, non-stick & hard
    Use: Non-stick surfaces, Teflon (pans, skis) and gaskets

     

    Example:

    pots and pans

    As fuels

    There are many different types of fuels within the fossil fuel family.  Different fuels are suitable for different purposes, because of the difference in their properties.  For this reason fuels combust in different ways.  Some of them burst into flames immediately and are dangerous therefore, must be handled with care, for example petrol.  Others are difficult to light, such as coal.  Hence a motor vehicle requires a fluid (liquid or a gas) fuel, where as heating a home can use either.

     

    So what makes a good fuel?

    When choosing a fuel one needs to consider the following!

    Availability
    Transport
    Storage
    Cost
    Combustibility
    Safety

    For what purpose
    Pollution

     

    Tags:Hydrocarbons, Polymers, Crude oil

     

     

     

    © 2012 science-resources.co.uk. All rights reserved | Design by W3layouts