Fibre reinforcements

Orientation of fibres can be studied by RRIM by contact microradiography. In Fig. 6.12 the polymer matrix shows a typical random distribution. Each individual fibre can be readily seen to be surrounded by a coating of polymer, and exists in a straight needle-like form. Also some fibres are present as undispersed bundles observed by darker circles which represent vertically aligned fibres. 

It is necessary for the development of suitable methods of product fabrication to have some types of polyurethane elastomer which can be processed as solids using similar methods to those employed for conventional natural and synthetic rubbers, namely two-roll mill or internal mixing and compounding procedures, calendering, extrusion, moulding, fabrication of complex rubber-textile laminates and the like. 

Millable urethane elastomers are usually based on low molecular weight polymers, about 20000-30000 molecular weight, which are essentially linear in nature some branching is permissible but not to the extent that a three-dimensional structure is produced. In some grades a very small amount of branching is introduced for processing purposes. It is essential that the polymers used for millable urethanes contain no reactive isocyanate sites, otherwise storage instability results in the raw elastomer hence these materials are usually hydroxyl end-blocked for shelf stability purposes. 

Sulphur-curing grades are usually preferred by the rubber industry as they make use of established technology, but two other vulcanization techniques are also well established as they produce elastomers with certain superior properties these are crosslinking with peroxide or diisocyanate. Peroxide curing can be universally used as it has a free radical-type 

Reinforcing fillers can be classified by their aspect ratio (length v. diameter). Materials with low aspect ratio can confer increased stiffness (often by a factor of 2) but not increased strength. Fibres with large aspect ratios, e.g. asbestos, E-glass, carbon, cellulosics and aramid (Kevlar) increase not only stiffness but also tensile strength. 

The form of the fibres can be continuous strands, woven fibres and chopped (or discontinuous) fibres and they are used with both thermosets and thermoplastics. In the latter case, engineering thermoplastics are improved, but the tensile modulus of a commodity polymer, e.g. PP, can be increased 10-fold by the use of E-glass or chopped PET fibres, thus making it comparable in properties to an engineering polymer. 

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A. G. Causa, D. K. Kim, and R. S. Bhakuni, "Advances In MetaUic and Polymeric Fibre Reinforcement For Tyres," International Rubber... 

A. R. BunseU, ed.. Fibre Reinforcements for Composite Materials, Elsevier Science Publishing Co. Inc., New York, 1988. 

B. L. Riley, 2nd Int Conf. on Fibre Reinforced Composites, Proceedings, University of Liverpool, UK, 1986, p. 153. 

Tank material A mild-steel (MS) tank, with a wall thickness of 4-6 mm, and having an acid-proof lining of FRP (fibre-reinforced plastic), rubber or PVC will be suitable for this purpose. A healing arrangement, even if a cold process is adopted, will be ideal for... 

There is a simple way to estimate the modulus of a fibre-reinforced composite. Suppose we stress a composite, containing a volume fraction Vfo( fibres, parallel to the fibres (see Fig. 6.3(a)). Loaded in this direction, the strain, e , in the fibres and the matrix is the same. The stress carried by the composite is... 

Fig. 6.3. A fibre-reinforced composite loaded in the direction in which the modulus is (a) a maximum, (b) a minimum.

This gives us an upper estimate for the modulus of our fibre-reinforced composite. The modulus cannot be greater than this, since the strain in the stiff fibres can never be greater than that in the matrix. 

The two estimates, if plotted, look as shown in Fig. 6.4. This explains why fibre-reinforced composites like wood and GFRP are so stiff along the reinforced direction (the upper line of the figure) and yet so floppy at right angles to the direction of reinforcement (the lower line), that is, it explains their anisotropy. Anisotropy is sometimes what you want - as in the shaft of a squash racquet or a vaulting pole. Sometimes it is not, and then the layers of fibres can be laminated in a criss-cross way, as they are in the body shell of a Formula 1 racing car. 

As Fig. 20.7 shows, if DS eutectics ( DSEs ) prove successful, they will allow the metal temperature to be increased by =100°C above conventional DS nickel alloys, and the inlet temperature by =200°C (because of a temperature scaling effect caused by the blade cooling). Further improvements in alloy design are under way in which existing nickel alloys and DS eutectics are being blended to give a fibre-reinforced structure with precipitates in the matrix. 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

The generic thermosets are the epoxies and the polyesters (both widely used as matrix materials for fibre-reinforced polymers) and the formaldehyde-based plastics (widely used for moulding and hard surfacing). Other formaldehyde plastics, which now replace bakelite, are ureaformaldehyde (used for electrical fittings) and melamine-formaldehyde (used for tableware). 

Fig. 26.2. The microstructure of wood. Woods ore foams of relative densities between 0.07 and 0.5, with cell walls which ore fibre-reinforced. The properties ore very anisotropic, partly because of the cell shape and partly because the cell-wall fibres ore aligned near the axial direction.

Fig. 26.3. The molecular structure of a cell wall. It is a fibre-reinforced composite (cellulose fibres in o matrix of hemicellulose and lignin).

In the book of Exodus (Chapter 2) we read that the mother of Moses when she could no longer hide him, she took for him an ark of bullrushes and daubed it with slime and with pitch, and put the child therein and she laid it in the flags by the river s brink . Biblical commentaries indicate that slime is the same as bitumen but whether or not this is so we have here the precursor of our modem fibre-reinforced plastics boat. 

Plastics also find increasing use in vehicles for both water and air transport. Glass-fibre-reinforced plastic boats are widely used as a result of their economy in manufacture, ease of maintenance, lightness of weight and, for military purposes, antimagnetic characteristics. The non-corrosive nature of plastics also leads to their widespread use in boat fixtures and fittings. In aircraft, plastics are particularly useful on account of their low density. 

Some interesting differences are noted between amorphous and crystalline polymers when glass fibre reinforcement is incorporated into the polymer. In Figure 9.2 (ref. 10) it will be seen that incorporation of glass fibre has a minimal effect on the heat deflection temperature of amorphous polymers (polystyrene,... 

Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. 

As might be expected from a consideration of the factors discussed in Section 4.2, the imidisation process will stiffen the polymer chain and hence enhance Tg and thus softening points. Hence Vicat softening points (by Procedure B) may be as high as 175°C. The modulus of elasticity is also about 50% greater than that of PMMa at 4300 MPa, whilst with carbon fibre reinforcement this rises to 25 000 MPa. The polymer is clear (90% transparent) and colourless. 

Carbon-fibre-reinforced nylon 6 and nylon 6/12 mixtures have been offered commercially and found use in aerospace md tennis racket applications. 

The markets for polyetherimides arise to an extent from stricter regulations concerning flammability and smoke evolution coupled with such features as high strength, toughness and heat resistance. Application areas include car under-the-bonnet uses, microwave equipment, printed circuit boards and aerospace (including carbon-fibre-reinforced laminated materials). The polymer is also of interest in flim, fibre and wire insulation form. 

In addition to standard grades varying in the ABS/PC ratio, fire-retarded, glass-fibre-reinforced and glass-fibre-reinforced fire-retarded grades are available. Typical properties of three grades of ABS-PC alloys are given in Table 20.9. 

In 1997 it was reported that carbon-fibre reinforced PEEK had replaced aluminium in the fuel pump suction manifold of the Airbus. For this application the product has to withstand pressure thrusts of up to 30 bar and resist kerosene at operating temperatures in the range 40-200°C. The ventilation wheel for cooling the electric motor in the same application has also been converted from aluminium to PEEK. 

A large number of grades is available, one supplier alone offering about 40, including unreinforced, glass- and carbon-fibre reinforced, mineral filler reinforced, impact modified, elastomer modified, flame retardant and various combinations of the foregoing. 

Very low flammability with limiting oxygen indices in the range 35-50 and UL 94 V-0 ratings at as low a thickness as 0.4 mm in some instances, even without fibre reinforcement. 

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