3.2 Properties of Plastics (2017 Update)

National 5 Chemistry Unit 3: Chemistry in Society

• Topic 2: Plastics

• Key Area: Plastics

• Lourdes Secondary School

Making Plastics

• Plastics are also known as polymers (from "poly" meaning many and "mer" meaning unit).

• Polymers are made from monomers (single units), typically starting as unsaturated chemicals (e.g., ethene).

• Monomers are sourced from cracking fractions of crude oil.

• Naming polymers: Prefix "poly" to the monomer (e.g., ethene becomes poly(ethene)).

• Common polymers:

  • Ethene ➔ Polythene

  • Phenylethene ➔ Polystyrene

  • Poly(chloroethene)

  • Poly(propene) ➔ Polypropylene

  • Tetrafluoroethene ➔ PTFE

Addition Polymerisation

• Process where monomers join to form long chains: addition polymerisation.

• Involves breaking the carbon-carbon double bond, allowing chain formation.

• Example: Ethene molecules join after breaking double bonds.

• Diagrammatic representation can aid understanding (e.g., mice shaking hands).

Properties of Polymers

• Polymers can be large, containing 1000-10,000 carbon atoms.

• Properties depend on starting monomer; simple molecules with double bonds become more complex polymers.

• Identifying structure:

  • Monomer to Polymer: Draw three "H" shaped units linked by single bonds.

  • Finding repeating unit: Identify and revert back to carbon-carbon double bond in the chain.

Properties of Plastics

• Synthetic vs. Natural Materials:

  • Synthetic: Made from crude oil and processed chemically.

  • Natural: Sourced from plants and animals (e.g., wood, rubber).

• Advantages of Natural Materials:

  • Renewable, biodegradable, better environmental perception.

• Advantages of Synthetic Materials:

  • Alterable properties, lightweight, strong, flexible designs.

• Disadvantages of Natural Materials:

  • Fixed properties, difficult intricate designs.

• Disadvantages of Synthetic Materials:

  • Toxic fumes when burned, non-biodegradable.

The Problem with Plastics

• Plastics made from hydrocarbons tend to burn easily.

• General combustion equation for plastic:

  • plastic + oxygen ➔ carbon dioxide + water

• Byproducts include toxic gases from other elements in plastics (e.g., hydrogen chloride from PVC).

• Common Plastics:

  • Polyethene, Polypropene, Polystyrene, PVC, Acrylonitrile.

Properties and Uses of Plastics

• Types of Plastics:

  • Thermosoftening Plastics: Soft when heated; remouldable. (e.g., Polyethene, PVC)

  • Thermosetting Plastics: Stable at higher temperatures; decompose rather than melt. (e.g., Bakelite)

Natural Polymers

• Natural vs. Synthetic:

  • Natural polymers exist in nature; synthetic are man-made.

• Natural Polymers in Plants:

  • Rubber (from latex), cellulose (wood), starch.

• Natural Polymers in Animals:

  • Proteins (e.g., collagen, silk), enzymes act as biological catalysts.