Choosing a material means matching its properties to the job
Definition
Physical property
A feature of a material that can be observed or measured without changing the material into a new substance.
- Ceramics, polymers, composites and metals each have their own physical properties.
- The right material for a job is the one whose properties best suit what it must do.
- You compare properties such as hardness, strength, density, flexibility and how brittle a material is.
Key Idea
- Different classes of material behave very differently, so each suits different uses.
- Selecting a material means weighing its physical properties against the demands of the job.
Glass ceramics: soda-lime and borosilicate
Definition
Brittle
A brittle material breaks or cracks rather than bending when a force is applied.
- Soda-lime glass is made by heating a mixture of sand, sodium carbonate and limestone until it melts.
- Borosilicate glass is made from sand and boron trioxide and melts at a higher temperature than soda-lime glass.
- Glass is hard, transparent and brittle, so it cracks rather than bends.
Example
- Borosilicate glass is used for cookware because its high melting point lets it survive strong heating.
- Soda-lime glass is cheaper and is used for windows and bottles.
Clay ceramics: shaped wet, then fired
- Clay ceramics include pottery and bricks.
- Wet clay is easily shaped into the object needed.
- The shaped clay is then heated in a furnace, which hardens it into a rigid ceramic.
- Clay ceramics are hard, brittle and good thermal and electrical insulators.
Note
- Firing the clay drives out water and bonds the particles, giving a hard, rigid material.
- Like glass, clay ceramics are strong under compression but brittle, so they shatter if dropped.
Polymers: properties depend on the monomers and the conditions
Definition
Polymer
A substance made from very large molecules in which many atoms are linked by strong covalent bonds.
Definition
Monomer
A small molecule that can join with many other monomer molecules to form a polymer.
Definition
Catalyst
A substance that increases reaction rate without being used up by providing an alternative pathway with lower activation energy.
- The properties of a polymer depend on the monomers used and the conditions under which it is made.
- Low-density poly(ethene) (LD) is made from ethene at a high temperature and pressure with a trace of oxygen.
- High-density poly(ethene) (HD) is made from ethene at a lower temperature and pressure using a catalyst.
- HD poly(ethene) is stronger and denser than LD poly(ethene), even though both are made from ethene.
Example
- LD poly(ethene) is flexible, so it is used for plastic bags and squeezy bottles.
- HD poly(ethene) is more rigid, so it is used for water pipes and crates.
Thermosoftening and thermosetting polymers behave differently on heating
- Thermosoftening polymers are made of separate, tangled polymer chains with only weak forces between them.
- They melt when heated and can be remoulded, because the weak forces are easily overcome.
- Thermosetting polymers have strong cross-links between the chains.
- They do not melt when heated, because the cross-links hold the chains firmly in place.
Common Mistake
- Do not say that all plastics melt when heated.
- Only thermosoftening polymers melt; thermosetting polymers keep their shape because of the cross-links.
Composites: a reinforcement held in a matrix
Definition
Density
Density is the mass of a substance per unit volume.
- A composite is made of two materials: a reinforcement of fibres or fragments held together by a matrix, or binder.
- The two parts combine their properties, often giving a material that is strong for its low density.
- Examples include fibreglass, carbon fibre, concrete and wood.
Exam technique
- When you compare materials, use the numbers you are given, such as density or strength, rather than vague words.
- Then link each property to the use, for example a low density but strong composite for a bike frame.
Matching the material to the job
- Metals are strong and can be bent and shaped, but many corrode and can be dense.
- Ceramics are hard and resist heat, but they are brittle.
- Polymers are light and flexible and do not corrode, but they can soften when heated.
- Composites can be strong for their weight, so the final choice depends on the properties the job needs.
Self review
- Name the raw materials heated together to make soda-lime glass.
- How is a clay ceramic such as a brick made?
- Why do LD and HD poly(ethene) have different properties if both are made from ethene?
- What is the difference between a thermosoftening and a thermosetting polymer?
- What are the two parts of a composite material?
