3: Engineering Thermoplastics

Introduction to Engineering Thermoplastics

  • Definition: Engineering thermoplastics are thermoplastics that exhibit superior strength, temperature resistance, chemical/solvent resistance, and wear characteristics compared to commodity thermoplastics.

  • Importance: Their enhanced properties justify their higher cost.

Major Engineering Thermoplastics

1. Polyamides (Nylons)

  • High strength polymers; one of the first engineering thermoplastics.

  • Yield strength comparison:

    • Example: Yield strength of 3,003 aluminum is about 6 KSI.

    • Neat nylon 66 has yield stress between 8-14 KSI based on moisture conditioning.

  • Temperature resistance: Continuous use temperatures can reach up to 20 °C.

  • Drawbacks:

    • High moisture absorption (up to 10% by weight) leading to volume changes and reduced stiffness/strength.

    • Common grades include nylon 6 and nylon 66:

    • Nylon 6 has better impact toughness but lower stiffness than nylon 66.

  • Applications include fibers and bulk components.

2. Thermoplastic Polyesters

  • Types:

    • Polyethylene Terephthalate (PET)

    • Polybutylene Terephthalate (PBT)

  • Properties:

    • High strength, good toughness, crystalline structure.

    • PET is used widely, also classifying it as a commodity thermoplastic.

    • Strengths similar to nylons.

  • Advantages over nylons:

    • Less moisture absorption, higher operating temperatures, and lower impact on mechanical properties.

  • Applications include fibers, films, blown beverage bottles, and injection molded mechanical parts (usually reinforced with 15-50% glass/mineral fillers).

3. Acetyls (Polyoxymethylene / POM)

  • Common brand: Delrin.

  • Properties:

    • Highly crystalline with exceptional hardness and toughness.

    • Lower moisture absorption than nylons.

    • Self-lubricating, but not highly abrasion-resistant.

    • High shrinkage during injection molding can occur.

  • Typical enhancements: Teflon or PTFE for lubrication and glass fillers for strength.

  • Cost: Slightly higher than nylons, comparable.

4. Polycarbonates

  • Common brand: Lexan.

  • Characteristics:

    • Amorphous, highly transparent.

    • Higher impact strength than PMMA (polymethylmethacrylate) – about 16 times.

  • Properties:

    • Tensile strength similar to nylons, but much higher impact strength.

    • Good UV resistance; molds well, used in safety applications and other mechanical components.

  • Cost: Roughly twice as expensive as nylons.

5. Ultra High Molecular Weight Polyethylene (UHMWPE)

  • Molecular weights significantly higher than regular polyethylene (3 to 5 million compared to 100 to 500,000).

  • Properties:

    • Lower density due to less crystallinity.

    • High alignment and crystallinity achieved through mechanical processing resulting in high strength fibers.

  • Applications:

    • Utilized in armor applications (bulletproof vests) under brands like Spectra and Dyneema.

    • Superior abrasion and wear resistance; coefficient of friction similar to Teflon.

Conclusion

  • Engineering thermoplastics represent a critical group of materials with enhanced mechanical properties for a variety of applications. Understanding their characteristics, strengths, and weaknesses aids in appropriate selection for engineering purposes.