Unit 4

Examples of Common Polymers

• Low-density polyethylene (LDPE) is used for squeeze bottles, toys, flexible pipes, and insulation covers for electric wires.
• High-density polyethylene (HDPE) is used for bottles, pipes, inner insulation of coax cable, plastic bags, and other applications.
• Polypropylene (PP) is used for auto parts, industrial fibers, food containers, bag liners, dishware, and textile/food wrapping.
• Polystyrene (PS) is used for petri dishes, CD cases, and plastic cutlery.
• Polytetrafluoroethylene (PTFE) is used for low friction bearings, non-stick pans, inner insulation of coax cables, and coatings against chemical attack.
• Polyvinyl chloride (PVC) is used for pipes, fencing, lawn chairs, handbags, curtains, non-food bottles, raincoats, toys, vinyl flooring, and electrical installation insulations.
• Styrene Butadiene Rubber (SBR) is used as an adhesive and bonding agent.

Classification of Polymers

• Based on Origin of Source:
  • Natural polymers: Found in nature, e.g., cellulose, rubber, proteins.
  • Semi-synthetic polymers: Modified natural polymers, e.g., rayon, cellophane.
  • Synthetic polymers: Man-made polymers, e.g., polyethylene, nylon.
• Based on Structure:
  • Linear polymers: Long, straight chains.
  • Branched chain polymers: Have side chains attached.
  • Cross-linked polymers: Chains connected by chemical bonds.
• Based on Molecular Forces:
  • Elastomers: Highly elastic, can stretch and return to original shape, e.g., rubber.
  • Fibers: Long, thin strands with high tensile strength, e.g., cotton, nylon.
  • Thermoplastics: Soften on heating and solidify on cooling, recyclable, e.g., polyethylene, nylon.
  • Thermosets: Harden irreversibly on heating, strong and durable, not recyclable, e.g., Bakelite, epoxy resins.
• Based on Mode of Polymerization:
  • Addition polymerization: Monomers add to form polymer without releasing small molecules, e.g., polyethylene, PVC.
  • Condensation polymerization: Monomers react, releasing small molecules like water, e.g., nylon, polyester.

Mechanism of Addition Polymerization

• In this process, monomers add together without losing small molecules.
• The polymer formed is an exact multiple of the monomer unit.
• Examples: Polyethylene, PVC, polypropylene.

Classification of Polymers Based on Mode of Polymerization

• Condensation Polymers:
  • Monomers react with loss of small molecules like water or HCl.
  • The polymer is not an exact multiple of the monomer unit.
  • Examples: Nylon, PET, Bakelite, Melamine-formaldehyde resin.

Classification of Polymers Based on Molecular Forces

• Elastomers:
  • High elasticity, tensile strength, flexibility, and resilience.
  • Return to original shape after deformation.
  • Examples: Natural rubber, polyurethanes, polybutadiene, silicone.

Classification of Polymers Based on Molecular Forces

• Fibers:
  • Long, thin strands with high tensile strength, flexibility.
  • Used in packaging, textiles, and non-woven fabrics.
  • Examples: Natural fibers - cotton, wool, silk. Synthetic fibers - polyester, nylon, acrylic.

Classification of Polymers Based on Molecular Forces

• Thermoplastics:
  • Soften on heating and solidify on cooling.
  • Held together by weak intermolecular forces.
  • Can be reshaped and recycled.
  • Examples: Polyethylene, nylon, polyvinyl chloride.

Classification of Polymers Based on Molecular Forces

• Thermosets:
  • Harden irreversibly on heating.
  • Molecules form a rigid, cross-linked network.
  • Strong, durable, and resistant to heat.
  • Not recyclable.
  • Examples: Vulcanized rubber, Bakelite, polyurethane, epoxy resin, vinyl ester resin.

Difference Between Thermoplastics & Thermosetting Plastics

• Thermoplastics:
  • Formed by addition polymerization.
  • Linear polymer chains.
  • Soften on heating, stiffen on cooling.
  • Can be remolded.
  • Usually soft, weak, less brittle.
  • Recyclable.
  • Soluble in some solvents.
• Thermosetting Plastics:
  • Formed by condensation polymerization.
  • Three-dimensional network structure with strong covalent bonds.
  • Do not soften on heating.
  • Cannot be remolded.
  • Typically hard, strong, more brittle.
  • Not recyclable.
  • Insoluble in most solvents.

Copolymers

• Homopolymers: Synthesized from a single type of monomer.
• Copolymers: Composed of two or more different monomer subunits linked together.

Plastic Additives

• Pigment: Adds color.
• Stabilizer: Prevents degradation (breakdown).
• Lubricant: Makes molding easier.
• Filler: Adds strength.
• Plasticizer: Increases flexibility.

Videos

• Learn about the history and discovery of vulcanized rubber, nylon, and Bakelite.

Petrochemicals

• Derived from natural gas liquids and oil refinery streams.
• Used to make plastics, fibers, and other synthetic materials.