10
  • Introduction

    This week we start our look at the properties and applications of some common engineering polymers


    Study Plan

    Study:
    1. Read over week 10 lecture presentation
    2. Read chapter 15.15 in Callister
    3. Work through the questions 1 to 3 in the Polymer engineering tutorial A below

    Activity:
    Look at the videos referenced in the study pack
  • Engineering Polymers

    AMORPHOUS THERMOPLASTICS

    Plastics that can be repeatedly solidified from the melt stage without significant crystallisation are described as amorphous or glassy amorphous thermoplastics. Basically, as the material cools, molecular mobility is sharply reduced to an extent that prevents molecular order - they can therefore regarded as super cooled liquids.

    Advantages of Amorphous Thermoplastics

    1.   Shrinkage between the melt and solid state (e.g. mould shrinkage) is less than that for crystalline thermoplastics. It is also more uniform and isotropic. Thus the dimensions of moulding, extrusions, etc. are more predictable. Warping, sinking and voiding are more easily avoided.

    2.   Usually transparent.

    3.   Attractive appearance, minimal sinks, good gloss and colour match.

    4.   Attractive appearance, minimal sinks, good gloss and colour match.

    5.   Stiffer and stronger than crystalline thermoplastics at their maximum continuous use temperature (because of sharp melting point).

    6.   Lower specific heat than crystalline thermoplastics. Hence more economical melt processing.

    7.   Generally more dimensionally stable than crystalline thermoplastics.

    8.   Lower elasticity at processing temperature than crystalline thermoplastics, hence appropriate for vacuum forming.

    9.   Good for structural foams.


    Disadvantages of Amorphous Thermoplastics

    1.   Poor fatigue resistance, and resistance to repeated impact.

    2.   Poor wear abrasion and general bearing qualities.

    3.   Poor solvent resistance.

    4.   Fibre reinforcement does not increase HDT significantly.


    Thus amorphous thermoplastics are preferred for applications which demand high dimensional accuracy and stability, good appearance, transparency or a combination of these qualities. They are preferred for enclosures. They should generally be avoided in applications involving mechanical abuse and/or contact with a wide range of chemicals.

    ABS: Acrylonitrile-Butadiene-Styrene

    Uses: Outer casing for modest duty domestic/electronic appliances, for its excellent appearance and impact toughness. Structural foam mouldings used for business machine housings.

    Pipe used in preference to PVC for low temperatures or for higher temperatures (50-70C). Vacuum formed ABS/PMMA(capped) sheet used for domestic baths, shower trays and caravan cladding.

    Electroplated grades are used for taps and shower fittings, etc.

    PMMA: Polymethylmethacrylate

    Uses: Aircraft glazing, lenses for optical equipment, diffusers, light rods, automobile lenses and trim, decorative encapsulations, capping for other thermoplastic sheet for outdoor applications, sanitary ware.

    PC: Polycarbonate

    Uses: Transparent-vandal proof glazing, riot shields, lenses, tail light lenses, sunglasses, safety glasses.

    Electrical-capacitors, heavy duty connectors, plugs, switch gear relays. Light machine casings, safety helmets, ice skates, metallised reflectors.

    uPVC: unplasticised Polyvinylchloride

    Uses: Rigid - pipe, guttering, glazing.

    Impact modified - pipe, window frames, bottles.

    Co-polymers - coatings, films, bottles, wire coatings.

    pPVC: plasticised Polyvinylchloride

    Uses: Fabric coatings, leather cloth (foamed), weather strip gaskets, shoe soles, thermoform film for bubble packs, sachets, flexible foam floor tiles, bottles and enclosures, heat shrinkable tubing and adhesive tape.

    Wire coverings for electrical insulation DOP up to 60C ester plasticised up to 105C.

    PS: Polystyrene

    Uses: Packaging: XPS (expanded PS); film vacuum formed containers (HIPS), bottle and jar caps Cold insulation: XPS

    Others: low cost toys.
  • POLYMER ENGINEERING TUTORIAL

    1. Give examples of suitable materials for use in FIVE of the following applications, giving reasons for your choices.

    i)   Automobile bumper.

    ii)   Plastic container for holding carbonated drinks

    iii)   Domestic bath,

    iv)   Body for a 13 amp plug,

    v   Contact lenses,

    vi   Vacuum cleaner body,

    vii   Brake fluid reservoir.


    2. Describe the major properties and applications of semi-crystalline thermoplastics and amorphous thermoplastics.

    3. Explain why thermosets find major application in the electronic industry. Explain the benefits of using PEEK or PES for these applications.