C2 Material Choices

Properties

• Strength (in tension or compression)
• Stiffness
• Melting point
• Hardness
• Density

Natural and Synthetic

• Natural- a material that occurs naturally but may need processing to make it useful ( cotton, silk and wool)
• Synthetic- a material made by a chemical process ( polythene, nylon and neoprene)

Long chain polymers

• A long chain molecule consisting of lots of atoms ( cotton, silk, wool, nylon)

Polymers, their uses and their properties

• Polythene- Used as food bags, need to have low stiffness and low density
• Polypropylene- Used to make synthetic limbs, needs to have high strength in compression and high hardness
• PVC- Used to make inflatables (bouncy castles), needs to have low stiffness and low density
• PET polyester- Used to make reheat-able food containers and drinks bottles, need to have a high melting point and high stiffness
• Polyester- Used to make hulls and sails, needs to have high strength in tension and compression
• Polycarbonate "glass"- Used as synthetic glass, needs to have high stiffness and high hardness
• Kevlar- Used to make helmets, needs to have high hardness, stiffness and density
• Neoprene- Used to make wetsuits, needs to have low density and stiffness
• Latex- Used to make gloves for surgery, needs to have low density and stiffness

Materials under the microscope

• The properties of a material depend on their makeup, from the large scale to the invisibly small
• The weave and the stitches of a woven shirt are visible to the naked eye, they are macroscopic
• Molecules and atoms in materials (descending order)
• Fabric
• Thread
• Fibre
• Molecule
• Atom

Models of molecules

No-one know what molecules look like so we use computer models to help to understand them.

Polymer discoveries

Most polymers were discovered by accident

• Polythene
• Neoprene
• Nylon

All were formed through the joining of small molecules into a long chain molecule (several small ethene molecules joined to become polythene)

Long and Short polymer chains

The properties of a polymer depends on the length of its molecules.The molecules in candle wax and polythene are similar; however candle wax is more brittle. Wax molecules are much shorter, only a few atoms, polythene contains thousands. Longer molecule- stronger material

Crystalline

Crystallinity makes polymers denser and stronger. Lining up polymers in neat, straight lines increases crystallinity. Crystalline polythene (or high density polythene HDPE) is used in milk bottles, normal polythene (low density polythene LDPE) is used in sandwich bags

Science can be used to change the properties of polymers

• Vulcanization- Sulfur is added to rubber when making tyres to make it more resistant to heat and wear exposed to when in use. This makes is more durable.The sulfur acts as a link between the long rubber molecules, it stops them from slipping over each other. The molecules are locked into an arrangement,making the rubber less flexible, stronger and harder
• Plasticizers- A plasticizer is usually a oily liquid added to polymers (usually PVC) to make them softer and more flexible. They force the polymer chains apart and weaken the forces between them. They can slide over each other more easily.

Sustainable- Using the Earth's resources in a way that can continue in the future, rather than destroying them

Sustainable Development- A plan for meeting people's present needs without spoiling the environment for the future

We are unsustainably using our metals, rocks, coal, oil and trees. These resources are an finite supply, they will not last forever.

But, we have started to use our trees sustainably. We plant new trees to replace the old trees we have used for making our products. But we have a long way to go.

The life of a product has 3 distinct phases

• They are made
• They are used
• They are thrown away

Each phase uses resources

• Cradle- Raw materials for making the product, the energy used to manufacture it
• Use- The energy needed to use it (petrol in car), the energy used to maintain it (cleaning, mending), the chemicals needed to maintain it
• Grave- The energy used to dispose it, the space needed to dispose it

Manufacturers are now assessing what happens to the materials in their products to reduce the resources used. This Life Cycle Assessment is part of a legislation to protect the environment

Life cycle of a polythene bottle

Cradle-

• Oil is extracted and taken to a refinery to be distilled
• Distilling separates the molecule according to size. The refinery breaks doen hydrocarbons with 20 or more C atoms and breaks them down into smaller molecules such as ethene.
• Ethene is piped to a petrochemical plant to be made in polythene
• The raw polythene is sent to a factory to be moulded

Use-

• The bottles are taken to a filling plant, are filled, and are sent to shops
• The drink is bought, consumed and thrown away

Grave- 4 options

• Recycling- Plastics are melted and remoulded BUT there are problems with sorting, cleaning and separating and money
• Recovering the chemicals- It is sometimes possible to convert the polymers in plastics into simple molecules, raw materials for the chemical industry
• Recovering the energy- Some polymers can be burnt. This reduces the dependance on crude oil for fuel.
• Landfill- Unfortunatly, most polymers end this way.