11.04.3 Ceramics, Polymers and Composites

Syllabus

- Most of the glass we use is soda-lime glass, made by heating a mixture of sand, Sodium Carbonate and limestone.
- Borosilicate glass, made from sand and Boron Trioxide, melts at higher temperatures than soda-lime glass.
- Clay ceramics, including pottery and bricks, are made by shaping wet clay and then heating in a furnace.
- The properties of polymers depend on what monomers they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced from Ethene.
- Thermosoftening polymers melt when they are heated. Thermosetting polymers do not melt when they are heated.
- Students should be able to:

1. explain how low density and high density poly(ethene) are both produced from Ethene
2. explain the difference between Thermosoftening and Thermosetting polymers in terms of their structures.
3. recall that most composites are made of two materials, a matrix or binder surrounding and binding together fibres or fragments of the other material, which is called the reinforcement.
4. recall some examples of composites.

compare quantitatively the physical properties of glass and clay ceramics, polymers, composites and metals given appropriate information:
explain how the properties of materials are related to their uses and select appropriate materials given appropriate information.

What does this mean?

Soda-lime glass and Borosilicate Glass

Every day glass is made mostly from sand (Silicon Oxide) mixed with Sodium Carbonate and Limestone (Calcium Carbonate).

The Carbonates break down to metal oxides.

Metal oxides are bases, Silicon Oxide is acidic and they neutralise each other forming a mixture of silicates we usually just call glass.

When hot the glass is soft and can be easily worked but will harden to be transparent.

But it has a low melting point so it would make a poor saucepan or beaker.

But if we change the ingredients a little so that they are mostly sand and Boron Trioxide we get a glass mostly made of Borosilicates.

These are still transparent but are much harder to melt.

Pyrex is an example.

Ceramics

We don't seem to need to know anything about the chemistry of ceramics.

We need to know that ceramics are commonly used in tiles, bricks and plates.

They are hard - difficult to scratch (tiles and plates)

Strong in compression - difficult to crush (bricks)

And have high melting points (fire-bricks and tiles for chimneys)

Ceramic brakes for sports cars also rely on their high melting points.

All ceramics are made from clay which is baked in a furnace

Specialty Polymers

All Polyethene is made from Ethene.

But there are different ways of forming the ethene that results in different properties.

At high pressure the process makes chains with very few branches the chains can pack together very tightly - High Density Polyethene (HDPE).

The intermolecular forces will be very strong.

It will be stronger and harder to melt.

At low pressure more branches are created and it is more difficult it is for the chains to pack together - Low Density Polyethene (LDPE).

This makes for weaker intermolecular forces, lower melting points and less strength.

Both LDPE and HDPE are thermosoftening polymers - meaning that the polymer softens as it is heated.

This is because individual chains are only held together by intermolecular forces.

Heat allows these to break and the chains move apart and start to flow.

Thermosetting polymers don't melt when heated.

Their chains are held together by covalent bonds which are much stronger than intermolecular forces.

Composites

Composites are made up of two (or more) materials and gets properties from both.

Reinforced concrete has the strength in compression of concrete as well as the strength in tension of the steel reinforcement.

Tall buildings are made of this.

The concrete is the Matrix, the bulk of the material that surrounds the steel

Lots of fibre glass has a matrix of plastic reinforced with glass fibres.

Even wood is a matrix of weak Lignin held together by strong cellulose fibres.

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