Fibres in composites

Kevlar has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectran is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlar is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectran ... 

R. M. Gill, Carbon Fibres in Composite Materials, Iliffe Books, London, 1972. 

Folydlacetylenes offer a unique opportunity of studying structure/property relationships in polymers. This paper is concerned with structural factors which control mechanical properties. The effect of the size of side-groups upon the Young s moduli of different polydiacetylenes is discussed briefly. The effect of internal and surface defects upon the strengths of individual fibres is also described. Examples are given of how Raman spectroscopy can be used to follow the deformation of fibres and it is shown how this can be extended to fibres in composites. The general mechanical properties of the composites are also described. 

This paper reviews recent work upon structure/mechanical-property relationships in polydiacetylenes. It is shown how this has led to the development of high strength polydiacetylene single crystal fibres and their performance as reinforcing fibres in composites is described. 

As already mentioned, specific mechanical properties are decisive when using natural fibres in composite materials. The fact that hemp, flax, and ramie natural fibres can compete with technical fibres is demonstrated in Fig 1. 

The final important form of carbon is the carbon fibre formed from polyacrylonitrile (PAN), cellulose or pitch and which is finding increasing use in fibre-reinforced composites. The corrosion of carbon fibres in composites designed for use under high temperature conditions is currently a severe limitation on their use. 

Starting from the cure reaction mechanism, a proper cure rate law, describing the evolution of the system from initial to final state, can be proposed. In the case of a mechanistic approach, in which the reaction model consists of a set of chemical reaction steps, a set of (stiff) coupled differential equations has to be solved to describe the evolution of the important reacting species of the system. In this case, effects of the composition of the fresh reaction mixture (such as a stoichiometric unbalance of resin and hardener, the concentration of accelerator, initiator or inhibitor) and the influence of additives (such as moisture and fibres in composites) can be studied. Because this set of equations may be rather complex and/or even partly unknown, various simplifications have to be made. 

Hepworth DG, Hobson RN, Bruce DM, Farrent JW (2000) The use of unretted hemp fibre in composite manufacture. Compos A 31 1279-1283 Idicula M, Boudenne A, Umadevi L, Ibos L, Candau Y, Thomas S (2006) Thermophysical properties of natural fibre reinforced polyester composites. Compos Sci Technol 66 2719-2725 Ioffe R, Andersons J, Wallstrom L (2003) Strength and adhesimt characteristics of elementary flax fibers with different surface treatments. Compos A 34 603-612 John MJ, Anandjiwala RD (2008) Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym Compos 29 187-207 John MJ, Anandjiwala RD, Thomas S (2009) Hybrid cranposites. In Thomas S, Pothan LA (eds) Natural fiber reinforced polymer composites macro to nanoscale. Old City, Philadelphia, pp 315-328... 

Chemical processes such as acetylation are in some cases used to increase the water repellency of fibres in composite wood products [85]. 

Mieck, K.P., Nechwatal, A., Knobelsdorf, C. (1994) Potential applications of natural fibres in composite materials, Mellianal Textilberichte 75, 892-98. 

The mechanical properties of asbestos fibre cements depend on the quality, amount and orientation of the fibres the data given in Table 3.1 should be considered as relating to orientation only. The asbestos fibre cements were considered as cheap and durable, with high corrosion and abrasion resistance, good performance in elevated temperature and good resistance against freeze/thaw cycles. Usually the volume fraction of fibres in composites was 6%, sometimes increased up to 20%. Asbestos fibres (mixed... 

Analysis of idealized distributions of short fibres in composite materials , Bulletin of the Academy of Polish Sciences, ser.sc.techn., 27(7) 601-9. 

It Is suggested that future studies should be directed towards the preparation of fibres with higher stiffness and strength and evaluation of the properties of such fibres In composites. 

Although the uses of ceramic fibres in composite structures lie mainly in ceramic-matrix and metal-matrix composites, where their outstanding chemical and thermal resistance are important, there are a few applications in organic polymers. Their relevant properties are low thermal expansion, low electrical conductivity, low dielectric constant, high stiffness, good compressive strength, and in most cases complete resistance to combustion. On the other hand they are very brittle, hard to process, and mostly considerably more expensive than carbon and para-aramid fibres. They have, for example, been used in hybrid structures with carbon and para-aramid and in electronic circuit boards. The fibres available or potentially available include alumina, combinations of alumina with... 

Kams, M., Ortmann, S., and Vogt, D. (2004) Use of Natural Fibres in Composites in the German Automotive Production 1996 till 2003, Nova-Institute, September 2004. 

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