Kareena’s Chemistry – Discovering and Making Composites

Ever heard of composites? Well listen up because these are really interesting materials!

They’re made of two or more materials (which you can tell  apart as they do not dissolve or blend into each other – although you may need a microscope to see these!!).

And when you mix these materials together, they create a mixture with different properties to those at the start.

Yeah, we’re a bit confused too! Let’s look at an example in…

Mud Bricks!

A really simple example are mud bricks. If you dry mud in a brick shape you’ve got a simple building material, but even though it’s strong if you try and squash them, they break easily if they bend. Which wouldn’t be good if say you’re a pig wanting to hide from a wolf with big breath!

Now, straw crumples easily but is actually really strong when it’s stretched.

So if you mix mud and straw you end up with a material that is resistant to both squeezing and tearing. It takes on the best bits from both parts and ends up making excellent building bricks.

In recent years there has been a huge increase in both the number of composites available and the number of applications using them. Many (but not all) of the modern composites are made from a plastic with something added to it. This might be glass fibres, carbon fibre, ceramic, metal or something else.

Good examples of sport equipment you may have are  tennis rackets and hockey sticks.  Older ones were made of wood and are noticeably heavier than the modern examples made of carbon fibre.  Chris Hoy’s track cycling bike is made mainly from carbon fibre – and that’s one of the reasons why he can go so fast!

Making a composite

So now we understand a bit about composites, why not make one ourselves!

Click here to download the handout for this experiment!

That way we can compare the properties of the composite we’ve made to the properties of the individual materials.

You’ll need:
  • Strips of fabric, about 5 cm x 3 cm (get a grown up to help cut the fabric into strips)
  • Flour – about two or three tablespoons per mould
  • Water
  • Oil or Vaseline
  • Mixing pot per child or group
  • Spoon
  • Plastic pot as mould for composite per child or pair (say a yoghurt pot)
  • Two other plastic tubs as moulds per child or pair
  • Cling film
  • Newspaper or old magazine per child or pair
What to do:
  • Put some flour into the mixing pot and add a little water. Mix the flour and water, adding more water slowly until the mixture looks like thick cream.
  • Grease the outside of the tub that will be used as the composite mould.
  • Dip a strip of fabric into the flour water mixture, get it well coated and then drape it over the composite mould. Continue adding strips of fabric until the mould is covered.
  • Add a second layer of flour-soaked fabric. Try to get the fabric strips lying in the opposite direction.
  • Add a third layer.
  • Place a piece of cling film on top of a newspaper or magazine and transfer the composite to the cling film.
  • Cover another tub with flour paste (don’t grease it). Add this tub to the cling film – not touching the composite covered tub.
  • Leave the tubs to dry in a warm place for at least two days. If the composites are turned after a day or two it will help them to dry faster.
  • When the composite has dried, carefully remove the plastic tub from the dried composite. This can be tricky and the tub may need to be pushed inwards to release it from the composite.

Now you can compare the properties of the fabric, the dried flour paste (which is very brittle) and the composite.  The composite should be much harder, stronger and rigid than the starting materials.

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