Short aramid fibres

Short aramid fibres lead to intermediate reinforcement between those obtained with short glass and carbon fibres. 

Table 6.9 Examples of enhancement ratios obtained with incorporation of short aramid fibres instead of short glass fibres in a thermoplastic composite...

Short glass fibre reinforcement the main thermoplastics are offered in such grades. Some short carbon or aramid fibre reinforced resins are also marketed. 

In all cases, carbon fibres lead to the highest mechanical performances compared to glass and aramid fibres. Nevertheless, their impact behaviour and price restrict their consumption. Glass fibres yield the cheapest composites but performances are more limited. Table 6.10 compares the properties of the main fibre types and shows some examples of properties for a nylon matrix reinforced with short fibres of the three types. 

Some basic property examples of short aramid, glass and carbon fibre reinforced polyamide are shown earlier in Table 6.30. 

The weak van der Waals bonds between the molecules in UHMWPE give it very poor heat resistance. The fibres melt at 150°C and their properties deteriorate as the temperature inaeases above room temperature. Under high stress, the fibres tend to creep extensively and can break after a short time under load. A secondary slow heating, under tension, when approaching the melting point, increases modulus and reduces CTeep. It is extremely resistant to chemical and biological attack and has better abrasion and fatigue resistance than aramid fibres. ... 

The measured increase due to the aramid adhesion treatment was different for each test 60 % in bundle pull-out, = 28 % in the 3-point, s 40 % in the 4-point short-beam and, = 60-80 % in transverse bending strength. Further improvement of the off-axis strength of aramid-epoxy composites is limited by the shear and transverse strength of the aramid fibres. In spite of the considerable adhesion improvement aramid-epoxy composites still have a lower shear and transverse strength than glass and carbon fibre composites. However, the corresponding strains are approximately equal because of the lower aramid-epoxy shear and transverse moduli. 

An interesting development for thermoplastics is a technique for pulping or fibrillation that greatly increases the surface area of short-length fibres of para-aramid, and renders them suitable for reinforcement of plastics and elastomers. While a typical staple fibre will have a surface area of about 0.1 m g K the new compounding process increases this to 7-9 g, so increasing the area... 

The shifts in the peak position of the 1610cm aramid Raman band, shown in Figure 8.4, can be used as calibration curves to monitor the ddbrmation of fibres in a composite under any state of stress or strain. Previous studies have shown [77-81] that it is possible to map out the distribution of stress or strain along a single short, discontinuous fibre in a low-modulus epoxy resin. This is described in detail next. 

Correa and co-workers [a.272] studied the thermal behaviour of short-fibre-reinforced PU composites by DSC and TG techniques and reported that the thermal resistance of aramid-fibre-reinforced composites was greater than that of carbon-fibre-reinforced composites or the pure matrix polymer. The DTG results are presented in Figure 31. 

Also, analysis of the kinetics and the glass transition temperature suggested greater interaction between aramid fibres and elastomer matrix. In particular, the degradation of PU or PU composites reinforced with aromatic PA or short carbon fibres followed first-order kinetics. 

Walton and Majumdar [122] studied the tensile creep of Kevlar fibres and the bending creep of cementitious composites made with these fibres (2.4% by volume of short random 2-dimensionally dispersed fibres). The creep coefficients of the aramid fibres themselves were found to be smaller than those of other synthetic fibres (polyethylene, polyvinylchloride and polycarbonate). The creep of the composite was of the same order of magnitude as that expected in a plain mortar matrix. 

For the global advanced composites market, the average cost of high-performance fibre reinforcements (carbon, aramid, high modulus polyethylene, boron, R/S/T-glass and some E-glass) is estimated from 5.5 to 6 per kg. This moderate price is due to the decrease in the carbon fibre price. Some grades could fall to less than 10/kg in the short or medium term. 

Short fibre reinforced polyamide E-glass Aramid Carbon... 

Although this author claims no special expertise in the toxicology of materials, it seems fair to say that the weight of opinion at present is reassuring about fibres in existing use. Glass, aramid and polyethylene fibres are all much safer than asbestos. In common with traditional materials such as wood and cotton, they must always be handled with care, especially if they are finely divided and therefore in respirable forms, i.e. small (<3 pm) diameter short fibres or fine dust. 

The word reinforcement will refer in this book exclusively to strong, stiff fibres. They can be made of glass, aramid (e.g. KevlaT (DuPont)) or high molecular weight polyethylene (e.g. Dyneema (DSM)), carbon/graphite, polyamide (nylon), jute, and so on. The fibres can be long, virtually continuous or short (e.g. 1mm). 

Short fibres of glass, rayon, aramid, asbestos and cellulose as reinforcing fillers, have been broadly used in rubber industries due to their high modulus, high strength and low creep. In recent years especially, natural fibres such as jute fibre, cellulose fibre, " coir fibre," " sisal fibre," " etc. have been also widely used in NR composites because they are enviromnental friendly, cheap, abundant and renewable. However, natural fibres also have some disadvantages such as moisture absorption, quality variations, low thermal stability and poor compatibility with the hydrophobic polymer matrix. 

Natural fibres such as flax, hemp, silk, jute, sisal, kenaf, cotton, etc are being used to reinforce matrices mainly thermoplastics and thermosets by many researchers. The principal synthetic fibres in commercial use are various types of glass, carbon, or aramid although other fibres, such as boron, silicon carbide, and aluminium oxide, are used in limited quantities. All these fibres can be incorporated into a matrix either in continuous lengths or in discontinuous (short) lengths. Both these fibres have some advantages and disadvantages. 

Fibre reinforcements Aramid, carbon, glass, natural fibres Mechanical strength used as short fibre, long fibre, spheres... 

A very tough 177-204 C versatile curing resin with a continuous service temperature range of -59 to 204 C, and short-term up to 232 C, and designed for RTM. Offers excellent compression properties after impact. Suitable for glass, carbon, aramid, silicon carbide and other fibre reinforcement. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

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