C2- Material choices

Solid materials can differ inthefollowing ways:

• melting point
• strength in tension (when pulled)
• Strength in compression (when pushed)
• stiffness
• hardness
• density

The sustainability of a product to a particular job depends on its properties.

How well a product does the job it was designed for depends on the effectiveness and durability of the materials used to make it.

• Rubber is used for car tyres because it is hard and elastic.
• Fibres are used to weave cloth into clothes.
• Plastics keep their shape when moulded into objects like washing up bowls.

Accuracy of a result depends on the quality of the measuring apparatus and the skill of the scientists taking the measurement.

For the data to be reliable there must be little variation between the values. However there is allways some variation between a set of measurements whatever is being measured.

• A single result may vary so repeats are needed.
• A result which is very different may be an outlier.
• Calculating the mean is a good way to estimate the true value.
• Many measurements need to be taken to find the true value.
• The range is the smallest to the largest result, excluding outliers.
• We can never be sure that a set of measurements gives the true value.
• Errors in measurement produce variations in data.
• Outliers can only be discarded if an error occured in the measurment.

All materials we use are chemicals or mixtures of chemicals:

• Metals are chemicals which are shiny, malleable and electric conductors.
• Ceramics include clay, glass and cement. They are hard and strong.
• Polymers are large molecules used to make rubbers, plastics and fibres.
• Concrete is a mixture of sand and cement.
• Bronze is a mixture of copper and tin.

Natural materials- From living things which need little processing are cotton and paper from plants and silk and wool from animals. Other natural raw materials which are extracted from the Earths crust are limestone, iron ore and crude oil.

Synthetic materials- manufactured by chemical reactions using raw materials. synthetic materials are alternatives to natural materials from living things. Synthetic materials have replaced natural materials because:

• Some natural materials are in short supply.
• They can be designed to give particular properties.
• They are often cheaper and can be made in the quantity needed.

Most hydrocarbons from crude oil are used as fuels.

When fuels burn in oxygen, carbon dioxide and water are made.

Burning a fuel like methane is a chemical reaction, so atoms are rearranged into new products.

The number of atoms of each element in the reactants must be the same as the products.

Crude oil consists mainly of a mixture of hydrocarbons, which are chain molecules of varying length up to 100 carbon atoms long.

As crude oil is a mixture its composition varies from place to place.

90% of crude oil is used as fuels.

• 3% of crude oils, mainly smaller hydrocarbon molecules are used to synthesise other chemicals. Examples include: ethanol and plastics.

Separated by fractional distillation:

• Oil is heated up which turns it all into gases.
• The distillation tower gets cooler as it gets higher.
• Gas molecules condense into liquids when they cool.
• Liquids with similar boiling points collect together. We call these fractions.
• Hydrocarbons in each fraction have boiling points within a range of tempertures.
• Molecule chains are similar sizes within each fraction.
• The smaller the molecule chain length the lower the boiling point.
• The smaller the molecule chain length, the smaller the forces between molecules.

• Attractive forces exist between molecules in crude oil, holding them together.
• As the hydrocarbon chain length increases, the force between these molecules increases.
• Large molecules need more energy to break them out of a liquidto form a gas, so have higher boiling points.

A polymer is a large chain molecule made by joining many smaller particles called monomers.

A polymer can have a chain of anything from hundreds to millions of carbon atoms.

A polymer is made by polymerisation.

Polymers with better properties mean some older materials have been replaced. Examples are plastic bottles and carbon fibre tennis rackets.

Ethane is a monomer used to make polyethene.

Different monomers produce different polymers.

PET is a polymer used to make drinks bottles. It is clear, strong, has a low density and does not shatter. This makes it a superior material to glass.

Polymer chains can be altered by replacing hydrogen stoms with other atomsor groups of atoms.

Each new polymer has its own set of properties and uses.

Materials such as kelvar have advantages over alternatives, but also have disadvantages.

Material choice will depend on comparing properties for different jobs, with cost being a factor.

• Small forces attract molecules to each other.
• The forces are strongest when the molecules are close together.
• The stronger the force:
  • The more energy is needed to separate the molecules.
  • The higher the melting point.

• Polymers can be made with properties that make them suitable for a range of different uses.
• The properties of polymers depend upon how their molecules are arranged and held together.

Low density polythene (LDPE)

• Has long molecules with branches.
• The branches keep molecule chains apart, so the forces between different molecules are weak.
• Items made with LDPE  e.g. plastic carrier bags are weak, flexible, soft and have low melting points.

High density polythene (HDPE)

• Long chains but no branches, so the molecules are aligned close to each other.
• HDPE is much stronger and is used to make long-lasting items which are hard and stiff, such as water pipes.
• HDPE has a high degree of crystallinity. This means there are lots of areas with regular patterns in the way the molecules line up.
• High crystalline polymers are strong with high melting points, but can be brittle.

• Making the molecule chain longer makes it stronger.
• Longer chains need more force to separate them.
• Longer chains have higher melting points than short chains.
• Plasticisers are used to make a polymer softer. They are small molecules inserted into polymer chains to keep them apart, weakening the forces between them.
• Plasticised PVC is still hardwearing and waterproof, but it is also flexible, making it a suitable material for rain coats.

• Thermoplastics soften when heated and can be moulded into shape.
• Thermosetting plastics do not soften when heated. They contain cross-links which lock the molecules together so they cannot melt.
• Crystallinity can be increased by removing branches on the main polymer chain and making the chains as flat as possible. This is so the molecule chains can line up neatly.
• Drawing polymers through a tiny hole when heated makes the molecule chains line up, increasing crystallinity and forming a higher tensile strength fibre.
• Materials which have been treated in this way include bullet-proof vests and sail material (kevlar).

• The width of a hair is about 0.1 millimetres.
• Microscopes are used to view small objects like human cells.
• Molecules and atoms are thousands of times smaller still.
• Nanoparticles are materials containing up to a thousand atoms.
• Nanoparticles:
  • Occur naturally, such as salt in seaspray
  • Occur by accident, such as solid particulates made when fuels burn.
  • Can be designed in laboratories.

• Nanotechnology is the use and control of very small structures. The size of these structures is measured in nanometres (nm). A nanometre is one millionth of a millimetre.
• An atom is about one-tenth of a nanometre in diameter.
• Nanoparticles can be built up from individual atoms. These structures are about the same size as some molecules.
• Carbon nanotubes are being designed in laboratories.
• Some nanoparticles are effective catalysts as they have a large surface area. Increasing surface area provides more sites for reactions to take place.
• Surface area increases when a lump of solid is cut up into bits.

• Nanotechnology builds structures from 10 atoms across (1nm) to a thousand atoms across (100nm).
• While the diameter of nanotubes is measured in nanometres, they can be millimetres long.
• Nanoparticles have very large surface areas. Becasue of this, they show different properties to larger particles of the same material.

100 millimetres = 1 metre

1000000 micrometres = 1 metre

1000000000 nanometres = 1 metre

• Silver nanoparticles are good at killing bacteria. They can be added to fibres and woven into socks put into wound dressings or put into plastic and made into food containers.
• Titanium oxide nanoparticles are put into sunscreen. They make the sunscreen transparent and absorb UV light.
• Nano particles can be mixed with other materials likemetals, ceramics and plastics. These combined materials are called composites.

• Composite materials are stronger and harder wearing. Adding nanoparticles:

• Plastic sports equipment makes it stronger.
• Tennis balls make them bouncy for longer.
• Rubber used in tyres make them hard wearing.

• Nanotechnology is the science of using and of making nanoparticles.
• Graphite forms in strong sheets that separate easily. Individual graphite sheets one-atom thick are known as graphene sheets.
• Graphene sheets can be rolled into carbon nanotubes. These are super-strength materials

• Silver nanoparticles can be washed out of clothes containing them and get into sewage works.
• Sewage works use bacteria to clean water. Silver nanoparticles could kill these useful bacteria.
• If silver nanoparticles are released into the environment they could kill lots of useful microorganisms.
• Nanoparticles are also used in cosmetics and sunscreens. The nanoparticles are added to materials that have allready been used and tested.
• Nanoparticles are small enough to pass through the skin into the blood and into body organs. The possible medical effects of this are not yet known.

• Little research has been carried out into the possible harmful effects of nanoparticles.
• One fear is that nanoparticles in the air may be breathed in and cause lung or brain damage.
• Some believe that because nanoparticles occur naturally such as in volcanic dust they do not pose danger.
• Others disagree because new nanoparticles with new properties have been developed.
• No one knows whether nanoparticles used in solids e.g. windows may be able to escape into the air.
• Some people want proof that nanoparticles do not pose a risk to the environment and health.
• Risk is defined as the change of an event occuring and the consequences if it did.