4: Engineering Thermoplastics 2

Introduction to Engineering Thermoplastics

  • Discussion continues on various engineering thermoplastics, focusing on properties and applications.

Polymethylmethacrylate (PMMA)

  • Also known as acrylic or plexiglass.
  • Characteristics:
    • Amorphous, transparent plastic.
    • Low cost and mass-produced.
    • Easily injection moldable and impact resistant (though less so than polycarbonate).
  • Applications:
    • Used for unbreakable windows and glass replacements where impact resistance is needed.
    • Can be used as a clear lacquer or protective coating.

Cellulosics

  • Derived from plant fibers (unique among polymers).
  • Characteristics:
    • Excellent UV resistance, making them suitable for sunlight exposure.
  • Applications:
    • Common in goggles and sunglasses.
    • Types include:
    • Cellulose acetate: Used in films and fibers.
    • Cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP): Used for bulk polymer components with good molding characteristics.
    • Trade names: Cellophane (cellulose acetate), rayon (cellulosic fiber).

Polysulfone (PSU)

  • Common trade name: Udel.
  • Characteristics:
    • Strength similar to nylon but is transparent.
    • Highest use temperature for clear plastics around 80°C.
  • Comparison with polycarbonate shows similar tensile strength but higher operating temperature for PSU.

Polyphenylene Sulfide (PPS)

  • A crystalline polymer with high stiffness and strength.
  • Characteristics:
    • Room temperature strength similar to nylon.
    • High use temperature (~260°C).
    • Creep resistant and moisture resistant.
    • Good chemical and high-temperature solvent resistance.
  • Note: Not UV resistant.

Polyether Ketone (PEEK)

  • Higher strength and use temperature than PPS (up to 315°C).
  • Characteristics:
    • Good dimensional stability and inertness.
    • Low flammability, good resistance to hot water, steam, and chemicals.
    • Relatively good moldability compared to PPS.
  • Cost comparison:
    • PEEK is more expensive but necessary for extreme conditions; PPS is cheaper for moderate use.

Fluorocarbons

  • Example: Polytetrafluoroethylene (PTFE) or Teflon.
  • Characteristics:
    • High chemical inertness and excellent solvent resistance.
    • Expensive due to its non-moldable nature; produced through sintering.
    • Extremely low friction coefficients, good high-temperature resistance.
    • Limited strength and creep resistance.
  • Alternatives:
    • Fluorinated ethylene propylene (FEP): Similar to Teflon.
    • Perfluorinated alkoxy (PFA): Higher use temperature than Teflon with better creep resistance.
    • Vinylidene fluoride (PVDF): More moldable but lower inertness and use temperature.

Polyimides (PI)

  • Trade names include Vespel and Kapton.
  • Characteristics:
    • Very high-temperature resistance (260-315°C or more).
    • Good mechanical and electrical properties across a wide temperature range.
    • Excellent radiation resistance — suitable for spacecraft applications.
  • Processing challenges: Difficult to process due to high melt temperatures; thermosetting versions exist.
  • Costly compared to other thermoplastics.
  • Variants:
    • Polyamide imide (PAI) - Example: Torlon, has high strength and stiffness, injection moldable with post-curing.
    • Polyether imide (PEI): Most common trade name is Altem, amorphous structure, good dimensional stability, lower cost than PAI and PEEK.

Summary of Polyimide Variants

  • Polyamide imide:
    • High strength (23 ksi), high stiffness.
    • Injection moldable if pre-processed for lower molecular weight.
  • Polyether imide:
    • Similar strengths to PEEK, notch sensitive (cracks at sharp corners).
    • Excellent electrical properties, IR transparency, lower cost, and readily melt processed.

Conclusion

  • Engineering thermoplastics overview covers a variety of polymers with specific tasks and properties, aiding in material selection for engineering applications.