C2 - Material Choices

C2 - Material Choices

1. Properties of materials

   • Different materials behave in different ways. They have different properties. Solid materials Solid materials can differ in one or more of the following ways: melting point strength in tension (when pulled) strength in compression (when pushed) stiffness hardness density.
   • Plastics, rubbers and fibres
     • Many of the products that we use are made using plastics such as polythene and rubbers; and fibres such as cotton. The suitability of a material to a particular job depends on its properties.
       • a washing-up bowl may be made from the plastic poly(ethene), commonly called polythene, because this material is waterproof and can be melted and reformed into the desired shape
       • a bicycle tyre is made from rubber because it needs to be flexible and tough
       • a pair of tights may be made from nylon fibres because these are flexible and elastic

2. Choosing suitable materials

   • Suitable materials for making the uppers of shoes are cotton, leather, rubber and polythene. They are flexible, tough and light. Wood is too heavy and inflexible
     • Rubber and leather are both good materials for making the soles of shoes because they are flexible and durable ie hard-wearing. Cotton and polythene are flexible but not durable enough, and wood is durable but not flexible enough.

3. Effectiveness and durability

   • How well a product does the job it was designed for depends on the effectiveness and durability of the materials that were used to make it. Consider again the materials you would use to make a pair of shoes, but think about the materials that would be most durable and effective. A pair of shoes made with leather uppers and a rubber sole would be most effective and durable:
     • leather is waterproof and flexible; it can also be dyed in a range of colours, and can be given a shiny, polished surface
     • rubber is also waterproof and flexible; it is very hard-wearing, and will not slip easily when in contact with the ground
     • The properties of these materials make them suitable for making this product. Shoes made with leather uppers and rubber soles can carry out their job effectively and are durable, which means they last a long time before wearing out.

4. Accuracy and reliability of data

   • Many people may have ideas about why materials have different properties, but these opinions are not very useful if they are not supported by data. To justify an explanation, you need to have data to support it. This data may be obtained by taking measurements.
     • Accuracy
       • The accuracy of each measurement depends on the quality of the measuring apparatus and the skill of the scientists taking the measurement. If the apparatus is faulty, or the scientists make a mistake, the measurement may be inaccurate
     • Reliability
       • For the data to be reliable, the variation within the values must be small. There is always some variation in any set of measurements, whatever is being measured. There may be small differences in the composition of the rubber or the way the measuring apparatus is used.
         • In this set of data, each measurement is only slightly different from the others. The results are repeatable, meaning that each time a measurement is taken it has approximately the same value. We can say that this set of data is reliable.

5. Experimental design

   • A number of factors, also known as variables, may have an effect on the outcome of an experiment
     • e.g. The largest amount that the rubber stretched before breaking is a measure of its tensile strength.
       • In this experiment, the factor that is being investigated is the type of rubber. The outcome is the maximum increase in the length of the rubber before it breaks
         • Controlling other factors A positive feature of the design of this experiment is that other factors, which might also affect the amount by which the rubber stretches, are controlled. If these other factors were not controlled, this would be a design flaw in the experiment.
   • The less variation in the measurements, the more reliable they are. In other words, the smaller the range, the more reliable the data
     • This value for the mean gives a best estimate of the length that this rubber can be stretched before it breaks. This is a measure of the tensile strength of the rubber. By measuring this length for several samples and then calculating the mean, the scientists get a better estimate of the tensile strength of the rubber. They can also see how reliable the data is.
       • Perhaps the apparatus malfunctioned when this measurement was taken, or perhaps the person taking the measurement made a mistake.

6. Measuring tensile strength

   • To measure the tensile strength of samples of different types of rubber, first of all the length of each sample is measured. Then a mass is hung from the bottom of each sample, and the new length measured
     • The mass is increased in increments, and the new length of the rubber measured each time, until the rubber snaps. The largest amount the rubber stretches before breaking is a measure of its tensile strength
   • there are several factors, also called variables, which may affect the outcome. These include the
     • type of rubber width of rubber thickness of rubber length of rubber temperature.
       • The factor that is being assessed is the type of rubber. In a good experimental design, all of the other factors are controlled, ie kept constant, for all of the samples tested.
         • If the other factors are not controlled, they may have an effect on the outcome. This means that any measured effect on the outcome could be the result of one of these other factors, rather than the factor that is being assessed. The results of the investigation would then be unreliable.
   • Repeatable and reproducible data
     • Once factors have been correctly controlled and the test is carried out well, reliable data should be obtained. If someone was to carry out the test again using the same method and got the same result, the data is said to be repeatable
       • If a measurement was taken using a different method and got the same result, the result is referred to as reproducible
     • The fact that there is a real difference can be concluded from two observations: the mean for rubber type A data lies outside the range for rubber type B data the mean for rubber type B data lies outside the range for rubber type A data.
       • The range of the measurements from rubber type C overlap with the range of the measurements from rubber type D. Therefore, there is not a real difference between the tensile strengths of these two types of rubber

7. Crude oil and its uses

   • Today, synthetic materials offer us more durability at a lower cost than the natural materials we used to use.
     • The materials we use are chemicals or mixtures of chemicals. For example, sugar is a chemical, but chocolate is a mixture of chemicals, one of which is sugar.
     • Materials can be obtained or made from living things. Examples include cotton, paper, silk and wool. These are sometimes called natural materials.
     • Materials can also be made from chemicals. Examples of these include plastics such as polythene used in plastic shopping bags. These are called synthetic materials

8. Crude Oil

   • Crude oil is a raw material obtained from the Earth’s crust. It is a mixture of many different chemicals, most of which are hydrocarbons. These are chain molecules of varying length that are made from hydrogen and carbon atoms only.
     • Crude oil is not very useful until it has been processed at an oil refinery. The process of refining involves separating the hydrocarbons into fractions or batches using a technique called fractional distillation. Each fraction separates as they have different boiling points. The crude oil is heated in a furnace to around 400°C. This allows all of the hydrocarbons in the crude oil to move into the bottom of the fractionating tower. The tower is hottest at the bottom and coolest at the top
       • The smallest molecules contained in the crude oil have lower boiling points and so move to the top of the tower. This is because the forces between these molecules are weak, so little energy is required to vaporise them. Larger molecules will remain lower down the tower as they have higher boiling points. This is because forces between the molecules are stronger.
         • The fractions gathered still contain a mixture of hydrocarbons but they are now useful mixtures. The use depends on the properties of the fraction. The majority of fractions are used as different energy sources. Further refining can take place to isolate smaller molecules to be used in the chemical synthesis of other materials, such as polymers.
   • Size and properties of molecules
     • The size of a molecule has an effect on the properties that molecule shows.
       • For example, the boiling point of hydrocarbons increases as the number of carbon atoms in the molecule increases. This is because forces between molecules increase as the size of the molecule increases. Hydrocarbons with up to four carbon atoms are gases, five to 16 carbon atoms are liquids and hydrocarbons with more than 16 carbon atoms are solids.

9. Polymerisation

   • Some small molecules (monomers) can join together to make very long molecules called polymers. This process is called polymerisation.
     • Many polymers are made from chemicals that are obtained from crude oil. For example, molecules of ethene join together to make poly(ethene), commonly known as polythene.

10. Rearrangement of atoms

    • In a chemical reaction, the substances that react together are called the reactants, while the substances that are formed are called the products. reactants → products
      • The products have different properties from the reactants.
        • As a chemical reaction takes place, atoms in the reactants are rearranged to make the products. The number of atoms of each element in the reactants is the same in the products. No atoms are added or taken away. This is called the conservation of atoms.
          • When polythene is made from ethene, the polymer has the same number of carbon and hydrogen atoms as the ethene molecules it is made from

11. New synthetic materials

    • New synthetic materials are often used to make items that at one time were made from natural materials. The new material often has better properties, and may be cheaper to make.
      • Window frames used to be made from wood. This has to be painted regularly, and in time may rot. Many new window frames are made from uPVC, a synthetic polymer. The ‘u’ means it is unplasticised so is different from regular PVC. This can be made in a variety of colours, never needs to be painted, and does not rot.

12. Structure of Materials

    • The properties of a material depend on the arrangement of its particles. By modifying the arrangement of particles in a polymer, its properties can be changed.
      • Arrangement of particles
        • The properties of solid materials depend on how the particles in these materials are arranged, and how they are held together. A solid melts when the forces between its particles are broken. This requires energy.
          • The stronger the forces between the particles, the more energy is needed to break the particles out of the solid structure. The more energy that is needed, the higher the melting point of the solid.

13. Modifying polymers

    • Different polymers have different properties, depending on the small molecules they were made from. But the properties of a polymer can also be changed.
      • Polymer chains
        • Many polymers, such as poly(ethene), contain long molecules that lie side by side. These can uncoil and slide past each other, making the material flexible. Long polymer chains have stronger forces of attraction than shorter ones. By making the chains of a polymer longer, a stronger and less flexible material is produced.
      • Plasticisers
        • Plasticisers are small molecules that can be added to polymers during their manufacture. They push the polymer molecules slightly further apart, weakening the forces between them and making the material softer and more flexible.
          • Unplasticised PVC, usually called uPVC, is hard. It is used for pipes and window frames. Plasticised PVC is soft. It is used for clothing and flooring
      • Cross-links
        • Chemical bonds can be formed to link together the chains of some polymers. These cross-links make the material tougher and less flexible.

14. Different properties of polymers

    • Ethene can be polymerised in slightly different ways to make low-density polyethene (LDPE) and high-density polyethene (HDPE). These polymers have different properties because of the structure of their molecules
      • Low-density polyethene Some polymer chains have side branches. Low-density polyethene (LDPE) is like this. LDPE The side branches stop the polymer molecules from lining up regularly. Its structure is not crystalline. This means that: the material has a lower density than HDPE the forces of attraction between polymer molecules are weakened the material is less strong, and has a lower melting point, than HDPE.
      • High-density polyethene High-density polyethene (HDPE) does not have side branches. HDPE The polymer molecules line up regularly to give a crystalline structure. This means that: the material has a higher density than LDPE the forces of attraction between polymer molecules are strong the material is stronger, and has a higher melting point, than LDPE.
      • Various amounts of crystallinity By carefully controlling the amount of branching, it is possible to make polymers with various amounts of crystallinity. This means it is possible to make a polymer with the exact properties that are required for a particular purpose.

15. Nanotechnology

    • The use and control of tiny matter is called nanotechnology. The tiny matter is referred to as nanoparticles.
    • These particles are measured in nanometres (nm). A nanometre is one billionth of a metre (0.000 000 001m). Nanotechnology is concerned with the use and control of structures that are 1-100 nanometres in size
    • Some of these nanoparticles occur naturally, for example in volcanic ash. Some occur by accident, for example during the combustion of fuels. Many occur by design.
    • Properties of nanoparticles
      • Nanoparticles of a material show different properties compared to larger particles of the same material. Forces of attraction between surfaces can appear to be weak on a larger scale, but on a nanoscale they are strong.
        • One reason for this is the surface area to volume ratio. In nanoparticles this is very large. Atoms on the surface of a material are often more reactive than those in the centre, so a larger surface area means the material is more reactive
    • Uses of Nanoparticles
      • sports equipment: nanoparticles are added to materials to make them stronger whilst often being lighter. They have been used in tennis rackets, golf clubs and shoes
      • clothing: silver nanoparticles have been added to socks. This stops them from absorbing the smell of sweaty feet as the nanoparticles have antibacterial properties
      • healthcare: nanoparticles are used in sunscreens. They offer protection and can be rubbed in so there are no white marks
    • Harmful effects
      • There are some concerns that nanoparticles may be toxic to people. They may be able to enter the brain from the bloodstream and cause harm. Some people think more tests should take place before nanoparticles of a material are used on a wider scale.

Imágenes y archivos del mapa