Short fibre

Fillers. Some fillers, such as short fibres or flakes of inorganic materials, improve the mechanical properties of a plastic. Others, called extenders, permit a large volume of a plastic to be produced with relatively little actual resin. Calcium carbonate, silica and clay are frequently used extenders. 

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

The stress distribution in short fibres is often simplified to the form shown in Fig. 3.29(c)... 

It may be seen from Fig. 3.29 that due to the ineffective end portions of short fibres, the average stress in the fibre will be less than in a continuous fibre. The exact value of the average stress will depend on the length of the fibres. Using the stress distributions shown in Fig. 3.29(b) the fibre stresses may be analysed as follows. 

In practice it should be remembered that short fibres are more likely to be randomly oriented rather than aligned as illustrated in Fig. 2.35. The problem of analysing and predicting the performance of randomly oriented short fibres... 

Hull also proposed that the shear modulus and Poisson s Ratio for a random short fibre composite could be approximated by... 

Tsai, S.W. and Hahn, H.T. Introduction to Composite Materials, Technomic Westport, CT (1980). Folkes, M.J. Short Fibre Reinforced Thermoplastics, Research Studies E ress, Somerset (1982). Mathews, F.L. and Rawlings, R.D. Composite Materials Engineering and Science, Chapman and Hall, London (1993). 

An inherent problem with all of the above moulding methods is that they must, by their nature, use short fibres (typically 0.2-0.4 mm long). As a result the full potential of the reinforcing fibres is not realised (see Section 2.8.5). In recent years therefore, there have been a number of developments in reinforced... 

FIGURE 12.1 Length distribution of short Hbers after mixing. (From Ashida, M., Short Fibre-Polymer Composites, De, S.K. and White, J.R. (Eds.), Woodhead Publishing Ltd., Cambridge, U.K., 1996. With permission.)... 

Fillers and reinfordng agents (CB, mineral fillers, and short fibres)... 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

In the manufacture of man-made and synthetic textiles the base materials are extruded through tiny orifices producing continuous lengths of very fine fibres or filaments. The textiles are mostly used in this continuous filament form, but for some purposes the filaments may be cut up into short fibres and spun into a yam. 

Filament yams can exist in an almost twistless form, but this is not the case for staple fibre yams. The twisting of fibres together is still the most practical method of making short fibres into long continuous strands of yam suitable for weaving. See S-Twist and Z-Twist. 

A large variety of hose constructions are produced by the rubber industry for a wide range of applications. To contain transported liquids, usually under pressure, it is necessary to reinforce the hose construction with fabric, yam or short fibres to constrain expansion of the rubber. Although hand-built construction using fabric still represents a proportion of the production, it is also very common to use yams either braided or knitted directly onto the hose liner. For heavy duty purposes it is also necessary to use wire, in a braided form, to either reinforce or protect the hose from external damage. Use of short fibres as reinforcement represents a small proportion of hose production and requires specialised dies for extrusion to orientate the fibres circumferentially. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

In summary, therefore, processing, and in particular injection moulding, can introduce limited chemical degradation, local polymer orientation, orientation of short fibre reinforcements, internal stresses, warpage, shrinkage and defects such as weld lines and voids. 

Plastics, both thermoplastic and thermosetting, will deform under static load. This is known as creep. For this reason those materials whose prime function is mechanical are generally reinforced with mineral filler or short fibres, or else oriented by drawing. Many components have a limit on acceptable deformation, and the predicted creep strain at the end of life will be fed back to define either a maximum load, or mechanical dimensions large enough for the component to remain within the limitations on strain. Creep becomes more pronounced at higher temperatures. 

Most tests will be made on standard test pieces which may be pieces cut from a component or a sheet, or they may have been moulded separately from the same material. Where test pieces or sheet are produced for the trials it is important that they are produced in as near as possible the same way as the product and that the processing conditions are recorded. Different results can be expected from compression and injection moulding or from extrusion (where a choice is possible). Directional properties can result from the conditions of flowing and cooling in a mould. For example, in a study at ERA, the creep strain of unfilled HDPE, either individually moulded or cut from square plaques, varies by up to a factor of two depending on the orientation of flow [40]. This difference becomes even more marked with short fibre reinforcement. 

Fukuda, H. and Chou, T. W A probabilistic theory of the strength of short-fibre composites with variable fibre length and orientation,. /. Mater. Sci., 17, 1003 (1982). 

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