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Plastics New

4.5.1 Describe a covalent bond.

In a covalent bond the outer electrons of some atoms come close enough to overlap and are shared between the nuclei, forming a covalent bond. Each pair of electrons is called a covalent bond. Mention of sigma (σ), pi (π), double or triple bonds is not required. Covalent bonds are strong bonds and examples of primary bonds (as are metallic and ionic bonds).

4.5.2 Describe secondary bonds as weak forces of attraction between molecules.

Kevlar atomic Structure

Hydrogen is positively charged and the Oxygen (one side) is negatively charged.

An interesting link on cross-linking.

4.5.3 Describe the structure and bonding of a thermoplastic.

Thermoplastics are linear chain molecules with weak secondary bonds between the chains.

4.5.4 Describe the effect of load on a thermoplastic with reference to orientation of the polymer chains.

Deformation occurs in two ways:

  • elastic, in which initially coiled chains are stretched and the material returns to its original size and shape when the load is removed
  • plastic, when at higher loads the secondary bonds between the chains weaken and allow the molecular chains to slide over each other, and the material does not return to its original size and shape when the load is removed. Creep and flow are important.
  • No quantitative details are required.

4.5.5 Explain the reversible effect of temperature on a thermoplastic, with reference to orientation of the polymer chains.

Increase in temperature causes plastic deformation.
  • The weak secondary bonds
  • That can easily broken by heat
  • Thus the long chains can slide over each other.
  • When plastically deformed and cooled at the same time new secondary bonds are made.

4.5.6 Explain how the reversible effect of temperature on a thermoplastic contributes to the ease of recycling of thermoplastics.

  • The Increase in temperature causes the weak secondary bonds to break (The heat is sufficient enough to break the secondary bonds but not the primary, covalent, bonds)
  • allows the long molecular chains to slide over each other, i.e. be reshaped into a new product.
  • when a plastically deformed and allowed to cool it will remain in the new shape
  • due to new secondary bonds being formed
  • recycling of plastic

Activity
Make a spinning wind twirler.

4.5.7 Draw and describe the structure and bonding of a thermoset.

Thermosets are linear chain molecules with strong primary bonds between adjacent polymer chains. This gives thermosets a rigid 3D structure.

4.5.8 Explain the non-reversible effect of temperature on a thermoset.

  • The heating process increases the number of permanent cross links
  • and so hardens the thermoset plastic
  • this cross linking is permanent

4.5.9 Discuss the properties and uses of polypropene and polyethene thermoplastic materials.

Polypropene

  • Properties
    • is resistant to fatigue
    • durable
    • higher melting point than other thermoplastics (160 Celsius)
    • high impact strength or toughness
    • high tensile strength
    • low density
    • corrosion resistant
    • retains stiffness at very high temperatures
    • can be easily coloured
  • Uses
    • in carpeting
    • ropes
    • dishwasher safe housewares
    • plastic containers that have hinges
    • bumpers on cars

Websites

Polyethene (Polyethylene)

  • Properties
    • high hardness
    • high stiffness
    • strong
    • corrosion resistant
    • high transparency and colourless
  • Uses
    • footwear
    • houseware
    • insulation
    • luggage

Websites

polypropylene Polyethene from science museum

4.5.10 Discuss the properties and uses of polyurethane and urea–formaldehyde (methanal) thermoset materials.

Polyurethane.

  • Properties
    • "Polyurethanes can be manufactured in an extremely wide range of grades, in densities from 6kg/m3 to 1220kg/m3 and polymer stiffnesses from very flexible elastomers to rigid, hard plastics." from Huntsman.com Look closely and you will see a wide range of applications according to the way it was manufactured
  • Uses
    • Note above

Websites

Urea–formaldehyde

4.5.11 Discuss the issues associated with the disposal of plastics, for example, polyvinyl chloride (PVC).

Although PVC disposal is problematic, PVC is still widely used as a structural material, for example, in windows and for guttering and drainpipes
PVC Products Greenpeace Campaign

Courtesy of Center for Health, Environment and Justice


References

Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.

Images are clickable links to its location.

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Page last modified on September 01, 2013, at 09:27 PM