Comparing materials and measuring properties
Properties describe how a material behaves:
- Melting point is the temperature at which a solid turns to liquid
- Tensile strength is the force needed to break a material when it is being stretched
- Compressive strength is the force taken to crush a material when it is being squeezed
- Stiffness is the force needed to bend a material
- Hardness is how well the material stands up to wear. Hardness can be compared by scratching two materials together
- Density is the mass of a given volume of the material (mass per unit volume eg. g/cm^3)
Errors and variation in measurements:
Calculating the mean is a good way to estimate the true value. Many measurements need to be taken to find the true value.
Natural and synthetic materials
All the 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.
Natural materials are materials made from plant and animal products such as cotton, paper, silk and wool.
Synthetic materials are materials manufactured by chemical reactions using raw materials.
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
Crude oil and using hydrocarbons
Crude oil is a mixture of thousands of hydrocarbons. Hydrocarbons are chain molecules of varying length up to 100 carbon atoms long. Nearly 90% of crude oil is used is fuels. Around 3% of crude oil is used to synthesise other chemicals. It can also be used as a lubricant or as bitumen for roads and roofing.
Crude oil is separated by fractional distillation:
- The oil is heated up which turns it all into gases
- the distillation tower gets colder as it gets higher
- liquids with similar boiling points collect together. We call these fractions.
- gas molecules condense into liquids when they cool
Hydrocarbons in each fraction have boiling points within a range of temperatures. Molecule chain lengths are similar sizes within each fraction. The smaller the molecule chain length, the lower the boiling point and the smaller the forces between molecules.
A polymer is a large molecule made by joining many smaller molecules called monomers. A polymer can have a chain of anything from hundreds to millions of carbon atoms. A polymer is made by a process called polymeristion. Polymers with better properties mean some older materials have been replaced.
Polymer chains can be altered by replacing hydrogen atoms with other atoms or groups of atoms. Each new polymer has its own set of properties and uses. Material use 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 is:
- the more energy is needed to separate the molecules
- the higher the mellting point
Polymers can be made with properties that make them suitable for a range of different uses. The properties of the polymers depend on how their molecules are arranged and held together.
Polymers can be improved by:
- Increasing the molecule chain length which makes it stronger with a higher melting and boiling point.
- 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.
- Cross-links lock molecules together so they cannot melt.
- Increasing crystallinity is when branches on the main polymer chain are removed, making the chains as flat as possible so the molecules chains can line up neatly. High crystalline polymers are strong with high melting points, but can be brittle.
Thermoplastics soften when heated and can be moulded into shape
Thermosettting plastics do not soften when heated.
LDPE & HDPE
Low density polyethene (LDPE) has long molecules with branches. The branches keep molecule chains apart, so the forces between different molecules are weak. Items made from LDPE are weak, flexible, soft and have low melting points.
High density polyethene (HDPE) has long chains but NO branches, so the molecules are aligned close to each other. HDPE is much stronger and used to make long-lasting items which are hard and stiff, such as water pipes.
Nanoparticles are materials containing up to 1 thousand atoms. They can occur naturally (such as salt in seaspray); they can also occur by accident such as solid particulates that are made when fules burn; as well as this, they can be designed in laboratories.
Nanotechnology is the use and control of very small structures. The size of these strutures is measured in nanometres. One nanometre is one millionth of a millimetre.
An atom is about one tenth of a milimetre in diametrer.
Some nanoparticles are effective catalysts as they have a large surface area. Increasing surface area provides more sites for reactions to take place.
Nanotechnology builds structures from 10 atoms across (1 nm) up to a thousand atoms across (100 nm).
Nanoparticles have very large surface areas. Because of this, they show different properties to larger particles of the same material.
Silver nanoparticles are very good at killing bacteria. They can be: added to fibres and woven into socks; put into wound dressings; put into plastics and made into food containers.
Titanium oxide nanoparticles are put into sun cream. They make it transparent and absorb UV light.
Composite=a material consisting of a mixture of other materials. Composite materials are stronger and harder wearing.
Adding nanoparticles to:
- plastic sports equipment makes it stronger
- tennis balls makes them stay bouncy for longer
- rubber used in tyres makes them harder wearing
Graphite forms in strong sheets which separate easily. Individual graphite sheets are one atom thick and known as graphene sheets. They can be rolled into carbon nanotubes. These are super-strength materials.
Are nanoparticles safe?
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 sun creams. The nanoparticles are added to materials that have already been used and tested. Nanoparticles are small enough to pass through skin into blood and into body organs. The possible medical effects of this are not yet known.
While a lot of research is taking place into the use of nanoparticles, little has been carried out into their possible harmful effects. One fear is that nanoparticles in the air might be breathed in and cause lung or brain damage. Some people think that because nanoparticles occur naturally, such as in soot and volcanic dust, they pose no danger. Others disagree because new nanoparticles with new properties have been manufactured.
No one knows if nanoparticles used in solids, like windows and paintwork, can escape into the air. Some people want proof that new nanotechnologies will not create health and environmental risks. Risk is defined as the change of an event occuring, and the consequences if it did.