12
  • Introduction

    This week we will complete the module by briefly looking at some of ways in which materials are processed and you will also have the final test assignment.


    Study Plan

    Study:
    1. Read over week 12 lecture presentation
    2. Read chapter 15.22 in Callister

    Activity:
    Watch this video: vacuum forming

    Resources:
    VMSE
  • Thermosets

    Thermosets in their stable cured state are very high molecular weight crosslinked polymers which on heating will not melt prior to thermal degradation. Prior to cure the resin is a low molecular with (hence low viscosity) quasi-stable liquid which will gel and cure under the influence of heat and/or the presence of a chemical initiator (catalyst). The rate of cure is determined by the reactivity of the resin and catalyst, their ratios, the temperature and the presence or otherwise of inhibitors or promoters (accelerators). The higher the rate of cure, and the thicker the part, the higher will be the peak exotherm (temperature generated due to cure). Post cure by soaking at elevated temperatures is commonly required to complete the crosslinking process.

    The equilibrium crosslink density is primarily determined by the functionality of the resin. Increased crosslink density confers:

    1.   increased hardness;
    2.   increased stiffness;
    3.   increased solvent resistance;
    4.   increased HDT;
    5.   decreased strain at break and toughness.

    Advantages

    1.   Low viscosity prior to fabrication hence can be cast, laminated, spray coated or compounded with high levels of filler or reinforcement. Low shear stresses during compounding allows long fibres to be incorporated without damage (unlike thermoplastics).
    2.   Strength, stiffness and toughness are less sensitive to temperature and thermoplastics.
    3.   Strength and stiffness is less sensitive to time under load hence lower creep and stress relaxation than thermoplastics.
    4.   Generally harder and more scratch resistant than thermoplastics.
    5.   Generally better solvent resistance than thermoplastics (particularly amorphous).
    6.   Generally better dimensional stability than thermoplastics.
    7.   Chemically active prior to and during cure hence excellent adhesion to substrates, fillers and fibres.
    8.   Generally better UV resistance than thermoplastics.

    Disadvantages

    1.   Significant shrinkage of unfilled resins during cure can lead to a high locked in strains.
    2.   Generally low strain at break and toughness.
    3.   Fillers and fibrous reinforcement reduce cure shrinkage. Fibrous reinforcement increases toughness. Hence unfilled unreinforced resins rarely used.
    4.   Slow production rates compared with hot melt thermoplastic processes.
    5.   Apart from gel coated laminates, thermosets are less attractive in appearance. Poorer surface finish, lower gloss, limited colours.
    6.   Thermosets cannot be recycled.
    7.   Resins and catalysts are often hazardous to health.
    8.   Mould release is difficult. Preparation of mould surfaces is labour intensive.

    Thus thermosets are used for adhesives, large laminated structures requiring stiffness, strength and corrosion resistance, large shell mouldings, chemical plant linings and coatings. They are only rarely used in consumer durables (where high production rates and good appearance is required).

    Epoxides

    Uses: Laminates - fabrics for aerospace, petrochemical + PCBs. Encapsulations - potting of electrical and electronic parts. Coatings - corrosion protection of steel pipe vessels.

    Adhesives.

    Castings - large electrical junction boxes.

    Mouldings - injection, high performance electrical appliances. Foams.

    Formaldehydes

    Uses: PF - handles for kitchenware, fuse boxes, distributor.

    MF - caps, light sockets. UF.

    Polyesters

    Uses: Valve seats, bearings, piston rings, thrust washers, arc cages, electrical insulators.