2 - Commodity Thermoplastics

Overview of Commodity Thermoplastics

  • Commodity thermoplastics comprise approximately 90% by mass of thermoplastics produced.
  • They are categorized into six types with specific recycling codes (1-6).
    • Examples: Polyethylene, Polypropylene, PVC, Polystyrene, and Polyethylene Terephthalate (PET).

Key Types of Commodity Thermoplastics

1. Polyethylene (PE)

  • Dominates plastic production with excellent flexibility.
  • Characteristics:
    • Good chemical and solvent resistance.
    • Low mechanical properties: lower strength and low temperature limits.
    • Poor UV resistance; susceptible to embrittlement from sunlight.
  • Variants:
    • Low Density Polyethylene (LDPE):
    • Low density (~0.92 g/cm³), used in packaging, films, and coatings.
    • Linear Low Density Polyethylene (LLDPE):
    • Higher crystallinity and strength compared to LDPE.
    • Ultra Low Density Polyethylene (ULDPE):
    • Even lower density, greater flexibility, and crack resistance.

2. High Density Polyethylene (HDPE)

  • Characterized by highly linear chains with minimal branching, leading to high crystallinity (up to 90%).
  • Benefits:
    • High strength and load-bearing capability; good toughness.
    • Excellent chemical resistance and stress-cracking resistance.
  • Applications:
    • Common in injection and blow molded parts, piping, plastic lumber, and fuel tanks.

3. Polypropylene (PP)

  • Has a lower density than other common plastics (~0.9 g/cm³).
  • Characteristics:
    • Crystallinity depends on tacticity: isotactic (highly crystalline) vs. atactic (less crystalline).
    • Excellent fatigue resistance, ideal for living hinges.
    • Superior chemical and solvent resistance; resistant to solvents at room temperature.
  • Applications:
    • Used in furniture, films, bottles, and fibers.

4. Polyvinyl Chloride (PVC)

  • Amorphous plastic with high strength but prone to brittleness without plasticizers.
  • Features:
    • Highly compatible with various additives (especially plasticizers).
    • Applications range from rigid water piping to soft imitation leather.
    • Excellent electrical insulation properties; non-combustible.
  • Limitations:
    • Not generally resistant to solvents; prone to cracking when rigid.

5. Polystyrene (PS)

  • Amorphous with a large styrene group; known for its high optical clarity.
  • Characteristics:
    • Very low production cost; challenges in recycling due to cost.
  • Excellent for molds and foams.
  • Applications:
    • Used in consumer products and foams; forms the basis for acrylonitrile butadiene styrene (ABS) for impact resistance products (e.g., Lego bricks).

Conclusion

  • Understanding commodity thermoplastics is crucial for applications across a wide range of industries due to their properties, benefits, and limitations. Each type has distinct characteristics and uses.