Kevlar® fibre

Polymer-matrix composites for aerospace and transport are made by laying up glass, carbon or Kevlar fibres (Table 25.1) in an uncured mixture of resin and hardener. The resin cures, taking up the shape of the mould and bonding to the fibres. Many composites are based on epoxies, though there is now a trend to using the cheaper polyesters. 

A unidirectional fibre composite consists of 60% by volume of Kevlar fibres in a matrix of epoxy. Find the moduli and Comment on the accuracy of your value for E. Use the moduli given in Table 25.1, and use an average value where a range of moduli is given. 

The reinforcing filler usually takes the form of fibres but particles (for example glass spheres) are also used. A wide range of amorphous and crystalline materials can be used as reinforcing fibres. These include glass, carbon, boron, and silica. In recent years, fibres have been produced from synthetic polymers-for example, Kevlar fibres (from aromatic polyamides) and PET fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2. 

GFRP - Glass Fibre Reinforced Plastic KFRP - Kevlar Fibre Reinforced Plastic CFRP - Carbon Fibre Reinforced Plastic... 

Example 3.7 A thin unidirectional Kevlar fibre/epoxy composite has the properties listed below. If the fibres are aligned at 6 to the x-axis, show how... 

GFRP - Glass fibre reinforced plastic KFRP - Kevlar fibre reinforced plastic CRFP - Carbon fibre reinforced plastic... 

Advanced materials can be used in extreme conditions, e.g., high temperatures (> 200°C), severe chemical environments (e.g., polytetrafluoroethylene (PTFE) with concentrated H2SO4). They are often used as a critical component in a workpiece and are frequently reinforced with glass, carbon or aramid (e.g., Kevlar ) fibres. 

Although structurally Kevlar fibres are highly crystalline or ordered it is interesting to note that appreciable moisture is absorbed at equilibrium. For example in variant T950 the moisture uptake is about 5 % at 22 °C and 55 % relative humidity. As discussed in Sect. 4.1.4.1 it is likely that the water is retained in microvoids distributed close to the surface of the fibres. Certainly in the short term there appears to be little effect of moisture on the tensile properties. 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

Liquid crystalline aromatic polyamides were the first chemical class to be commercialised. The best known example is Kevlar fibre which is spun from liquid crystalline solution to obtain the benefit of the high orientation present in the nematic phase. Subsequently, melt-processable main-chain polyesters were developed and brought to the market (Amoco with Xydar, Hoechst-Celanese with Vectra). 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

Fig. 12.24 The repeat unit of poly(p-phenylene terephthalamide), PPTA, from which Kevlar fibres are produced.

Aramid (e.g. Kevlar 49 ) fibres do not melt, but they degrade on heating in vacuum above 375°C and will oxidize at 400°C. The tensile strength of commercial Kevlar fibres declines with temperature, to about 75% of the ambient value by the time that 1TC is reached. Unprotected yarns lose 25% of their tensile strength and 7% of their modulus, measured at room temperature, after heating for 96h in air at 205°C. Data on bare fibres can be misleading impregnation with a resin protects the fibres, and there is no observed adverse effect [3] when a normal cure cycle is carried out for Kevlar/epoxy at 180°C. 

Fig. 8.1 Using DuPont Thermo-Man to test Bristol Uniforms (UK) turnout gear made from Hainsworth TITAN fabric that incorporates DuPont Nomex and DuPont Kevlar fibres (note that the test is to EN 469 using a flame source of 84kWm" for 8 s) (a) before,...

Kevlar AS (Antistab) consists of selected fabrics made of DuPont Kevlar fibres that are... 

Fig. 10.2 Bristol Uniforms turnout gear made from Hainsworth TITAN fabric that incorporates DuPonUM Nomex and DuPonU Kevlar fibres.

A failure in 1983 when aramid fibres began to be used illustrates the problems that can arise. A ship. Ocean Builder, was to be used to erect an oil rig in the Gulf of Mexico. Five weeks before, 12 buoys were secured to anchors by Kevlar ropes. When the Ocean Builder arrived and the mooring lines were picked up, four ropes broke reportedly at 20% of break load. The up-and-down movement of the buoys had led to axial compression fatigue of the Kevlar fibres. 

In each case, find the mass of each constituent required per unit mass of composite. The densities (in kg m ) are glass fibre 2540 polypropylene 900 carbon fibre 1790 Kevlar fibre 1450 and epKno 1300. 

Carbon and Kevlar fibres are less widely used at present than glass fibres, on account of their much higher cost. Nevertheless, they are increasing rapidly in importance. This is due mainly to the quite exceptional specific modulus and specific strength which they offer (see Table 6.1) that is, they are not only stiff and strong but also of light weight. 

The behaviour of Kevlar fibre-epoxy laminates under static and fatigue loading—Part 2 modelling. Compos. Sci. Technol, 37 (4), 371-392. 

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