Ceramics, polymers, composites (AQA GCSE Chemistry): Revision Notes
Ceramics, polymers, composites
What are ceramics?
Ceramics are materials made by heating different substances at very high temperatures. There are three main types you need to know about:
The high-temperature heating process is what gives ceramics their unique properties of hardness and heat resistance.
Soda-lime glass is made by heating sand, sodium carbonate and limestone together. This is the most common type of glass we use every day.
Borosilicate glass is made by heating sand and boron trioxide. This type of glass is special because it has a higher melting point and can handle sudden temperature changes much better than ordinary glass. This makes it perfect for laboratory equipment and ovenware.
Clay ceramics are made by heating wet clay in a furnace. Examples include pottery, bricks and tiles.
Understanding composites
Composites are materials that combine two different substances to create something with better properties than either material on its own.
Example: Fibreglass Construction
Fibreglass combines:
- Glass fibres (the reinforcement) - provides strength
- Plastic matrix (the binder) - holds fibres together
Result: Strength of glass without brittleness
Modern composites often use carbon fibres or carbon nanotubes as reinforcement. These are more expensive but create very strong, lightweight materials. You'll find these in racing cars and aircraft.
The key thing to remember is that composites are often stronger than their individual parts. The reinforcement fibres give strength, while the matrix holds everything together.
Examples of composites:
- Wood: cellulose fibres held in a lignin matrix
- Concrete: small stones in a cement and sand binder
Thermosoftening polymers
These polymers soften and melt when you heat them up. Examples include poly(ethene) and poly(propene).
This happens because the polymer chains are only held together by weak intermolecular forces. When you warm the polymer, these weak forces get even weaker, letting the chains slide over each other easily. This makes the polymer soft and flexible.
Think of it like spaghetti - the individual strands can move around each other when they're warm and slippery. This analogy helps explain why thermosoftening polymers become flexible when heated.
Thermosetting polymers
These polymers are different because they have cross-links between the polymer chains. These cross-links are like bridges connecting the chains together.
Because of these strong cross-links, thermosetting polymers do not soften or melt when heated. Once they're set, they stay that way permanently.
This is the key difference from thermosoftening polymers!
Think of it like a rope net - even if you heat it up, the knots (cross-links) keep the structure rigid.
LDPE and HDPE - two important polymers
Both are made from ethene, but they have different properties due to different catalysts and reaction conditions:
LDPE (Low-density polyethene):
- Lower useful temperature (around 80°C)
- Low strength but high flexibility
- Good for plastic bags and food wrap
HDPE (High-density polyethene):
- Higher useful temperature (around 110°C)
- High strength but low flexibility
- Good for bottles and containers
The trade-off between strength and flexibility is a common theme in polymer design. HDPE's higher density gives it more strength but reduces flexibility, while LDPE's lower density provides more flexibility at the cost of strength.
Choosing the right material
When choosing materials, you need to think about what properties are most important for the specific application:
Material Selection Examples
- For hot drink cups: HDPE would be better than LDPE because it can handle higher temperatures without becoming too soft
- For aircraft parts: Carbon fibre composites give high strength with low weight
- For laboratory glassware: Borosilicate glass can handle temperature changes without cracking
Applications of carbon nanotube composites
Scientists are developing very thin carbon nanotube composite films that can be used as a layer on aircraft wings. When the wings get cold at high altitude, these films help prevent ice formation. This shows how new materials are constantly being developed for specific uses.
This application demonstrates how modern composite technology can solve very specific engineering problems, combining the unique properties of carbon nanotubes with practical aerospace needs.
Remember!
Key Points to Remember:
- Ceramics are made by heating materials at high temperatures - borosilicate glass is better for temperature changes than ordinary glass
- Composites combine reinforcement and matrix to be stronger than individual parts
- Thermosoftening polymers soften when heated due to weak forces between chains
- Thermosetting polymers don't soften when heated because of strong cross-links
- HDPE is stronger and handles higher temperatures than LDPE, but LDPE is more flexible