11
  • Introduction

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


    Study Plan

    Study:
    1. Read over week 11 lecture presentation
    2. Read chapter 15.15 in Callister
    3. Work through the questions 1 to 3 in the Polymer engineering tutorial B below

    Activity:
    1. Look at the videos referenced in the study pack
    2. Undertake the laboratory experiments on the mechanical properties of some common polymers and write an assessable lab report based on the work undertaken.
    3. View the VMSE on Tensile Testing of each of the polymers
  • Semi crystalline thermoplastics

    Plastics which solidify from the melt stage by the growth of crystals are described as crystalline or semi- crystalline thermoplastics. The degree of crystallisation depends on chemical structure (generic type, isotacticity), rate of cooling from the molten state and thermal history in the post moulded state. Very rapid cooling i.e. via chilling of thin sections can result in insignificant levels of crystallisation and this is used to effect in the production of transparent film (PE, PETP, PP).

    Increased crystallisation increases and improves hardness, modulus, strength, abrasion and wear resistance, creep resistance, barrier properties, shrinkage, density, opacity.

    Advantages of semi-crystalline Thermoplastics

    1.   Good solvent resistance compared with amorphous thermoplastics.

    2.   Excellent fatigue resistance.

    3.   Good wear and abrasion resistance.

    4.   Fibre reinforcement dramatically improves high temperature stiffness (HDT) and strength.

    5.   High melt strength improved blow mouldability.

    6.   Generally higher design strain than amorphous thermoplastics or thermosets hence better for snap fits.


    Dis-advantages of semi-crystalline Thermoplastics

    1.   High shrinkage, and because of variable degree of crystallinity shrinkage is variable and difficult to predict. Sinking, voiding and warping are all more likely than with amorphous thermoplastics. All but warping is improved by fibre reinforcement.

    2.   High elasticity at forming temperature makes vacuum forming difficult. Increased die swell makes accurate extrusion profiling difficult.

    3.   Generally difficult to adhesively bond.

    4.   Higher creep than amorphous thermoplastics.

    5.   Relatively poor dimensional stability.

    6.   When unreinforced, HDT is much lower than maximum continuous service temperature.

    Thus semi-crystalline thermoplastics are preferred for applications involving mechanical abuse or repeated/cyclic loading e.g. bearings, linkages, etc. They are less likely to be used unreinforced foe components requiring high dimensional accuracy or stability. When reinforced these aspects are improved (apart from warping) but this detracts from appearance. Hence not preferred for enclosures.

    POM: polyoxymethylene

    Uses: Small mechanisms - slides, gears, cams, bushes, bearings, bearing cages, castors, aerosol valves.

    Plumbing - taps, pipe fittings, cistern balls valves, shower heads, pump impellers and fans. Automotive - locks, clips and snap fit fasteners, instrument housing, carburettor bodies.

    Others - chain links.

    Fluorocarbons

    Uses: Non-stick applications e.g. PTFF low coeff of friction, diaphragms, seals, gasket, wire insulation, insulating tape, piston ring.

    PA(e.g. PA 6,6): polymide (e.g. Nylon 6,6)

    Uses: Gears, bearings, rollers, rotationally moulded petrol tanks, film for food packaging, nets and bolts, castors, trolley wheels, power tool casings, fan blades, ball bearing cages. Rocker box and timer chain covers.

    Polyethylene

    1. LDPE: Film for food packaging, moisture barriers in civil engineering, loose liners, blown bottles, moulded food containers, cable insulation.

    2. LLDPE: for stronger film.

    3. MDPE: Gas pipe.

    4. HDPE Water and sewerage pipes, bottle crates, heavy duty barrels, tanks.

    5. UHMWPE: Gears, slideways, cams, bearings, shute linings, mallet heads, artificial limb joints.

    6. XLPE: Rotationally moulded petrol tanks, wire and cable insulation, hot water pipe, shrink film and shrink tubing.

    PP: polypropylene

    Uses: Pipe for chemical plant, automotive cooling fans, radiator reservoirs, brake fluid reservoirs, washing machine tubs (glass fibre reinforced), tool handles, moulded chairs.
  • POLYMER ENGINEERING TUTORIAL

    1. Discuss the phenomenon of polymer embrittlement.

    2. Explain the mechanism of creep in polymers.

    3. Discuss the reasons for adding a plasticiser to PVC