Fibres for polymer composites
When a load is applied to a fibre-reinforced composite consisting of a low-modulus matrix reinforced with high-strength, high-modulus fibres, the viscoelastic flow of the matrix under stress transfers the load to the fibre; this results in a high-strength, high-modulus material that determines the stiffness and strength of the composite and is in the form of particles of high aspect ratio (i.e. fibres), is well dispersed and bonded by a weak secondary phase (i.e. matrix).
Many amorphous and crystalline fibres can be used, including glass, and fibres produced from synthetic polymers, such as carbon fibre (made by elongating polyacrylonitrile and then placing its elongated form in various inert gases).
Making a fibre involves aligning the molecules of the material, and the high tensile strength is associated with improved intermolecular attraction resulting from this alignment. Polymeric fibres are made from those polymers whose chemical composition and geometry are basically crystalline and whose intermolecular forces are strong. As the extensibility of the material has already been utilised in the process of manufacture, such fibres have a low elongation.
The following sections discuss the manufacture, make-up and properties of fibres that can be used to upgrade polymers, cements, mortars and concretes. These include glass, carbon and Kevlar fibres that are used in conjunction with thermosetting polymers such as polyesters, vinylesters and epoxies to form civil engineering composites. Some typical properties of a number of these composites are then given.
Glass fibres are manufactured by drawing molten glass from an electric furnace through platinum bushings at high speed; Fig. 1 illustrates the procedure. The filaments cool from the liquid state at about 1200°C to room temperature in 10−5 seconds. On emerging from the bushings the filaments are treated with a lubricant or size and 200 filaments are bundled together to form a strand.
The main functions of the lubricant are to:
- facilitate the manufacturing of the strands and moulding of the composite
- reduce damage to the fibres during mechanical handling
- reduce the abrasive effect of the filaments against one another.
The following four types of glass fibre are the major ones used in construction:
- E-glass fibre of low alkali content is the commonest glass fibre on the market and is the major one used in composites in the construction industry. It was first used in 1942 with polyester resin to manufacture radomes (a structural weatherproof enclosure that protects radar equipment), and is now widely used with polyester, vinylester and epoxy resin.
- A-glass fibre of high alkali content was formerly used in the aircraft industry, and is now used for special manufactured articles in civil engineering.
- Z-glass (zirconia glass) was developed for reinforcing cements, mortars and concretes because of its high resistance to alkali attack.
- S2-glass fibre is used in extra-high-strength and high-modulus applications in aerospace and on occasions in civil engineering.
Strands of glass fibre are combined to form thicker parallel bundles called rovings which, when twisted, can form several different types of yarn; rovings or yarns can be used individually or in the form of woven fabric.
Glass strands and rovings for reinforcing thermosetting polymers may be used in a number of different forms:
- woven rovings, to provide high strength and stiffness characteristics in the direction of the fibres
- chopped fibres, to provide a randomly orientated dispersion of fibres in the composite
- chopped strand mat, to provide a quasi-isotropic reinforcement to the composite
- surface tissues, to provide a thin glass-fibre mat when a resin-rich surface of composite is required.
Carbon fibres are manufactured by controlled pyrolysis and cyclisation of certain organic fibre precursors (the raw material used to make carbon fibre). About 90% of carbon fibres are produced from polyacrylonitrile (PAN), the remaining 10% are made from rayon or petroleum pitch. The PAN precursor is manufactured by spinning to produce a round crosssection fibre; the yield is only 50% of the original precursor fibre. It can also be manufactured by a melt assisted extrusion as part of the spinning operation.
I-type and X-type rectangular cross-section carbon fibre composites are produced with a closer fibre packing in the composite compared to that of the circular fibre. The PITCH precursor fibres are derived from petroleum, asphalt, coal tar and PVC, the carbon yield is high but the uniformity of the fibre cross-sections is not constant from batch to batch. This non-uniformity is acceptable to the construction industry although it must be said that it is not acceptable to the aerospace industry.
The pitch fibre precursor is invariably used when ultra-high (European definition – see below) stiffness carbon fibres are used in construction. The conversion process for carbon fibre includes stabilisation at temperatures up to 400°C, carbonisation at temperatures from 800 to 1200°C and graphitisation in excess of 2000°C. Surface treatment of the fibres, which includes sizing and spooling, is then undertaken.
Figure 2 shows a diagrammatic representation of the process. Carbon fibres are organic polymers, characterised by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one manufacturing technique to another.
They are classified according to their mechanical properties as:
- high strength
- high modulus
- ultra-high modulus fibres (European definition).
The last two are alternatively defined in the USA and in some Asian countries as normal modulus and high-modulus fibres. Carbon filaments are typically between 5 and 8 mm in diameter and are combined into tows containing between 3000 and 12000 filaments.
A tow count is typically between 200 tex and 900 tex. The tows are twisted into yarns and woven into fabrics analogous to those described for glass. The techniques for the manufacture of the high modulus (HM) and the ultra-high modulus (UHM) (European definitions) are the same but the heat treatment temperature will be higher the higher the modulus of the fibres, thus, a more highly orientated fibre of crystallites will be formed for the UHM fibre.
Aramid (aromatic polyamide) fibres are produced by an extrusion and spinning process typically used to produce a thermoplastic acrylic fibre. A solution of the polymer in a suitable solvent at a temperature of between −50 and −80°C is extruded into a hot cylinder which is at a temperature of 200°C; this causes the solvent to evaporate and the resulting fibre is wound onto a bobbin.
To increase its strength and stiffness properties, the fibre undergoes a stretching and drawing process, thus aligning the molecular chains, which are then made rigid by means of aromatic rings linked by hydrogen bridges.
There are two grades of stiffness available; one has a modulus of elasticity of the order of 130 GPa – which is the one used in polymer composites for upgrading structural systems – and the other has a modulus of elasticity of 60 GPa and is used in bullet proof systems.
Aramid fibres are resistant to fatigue, both static and dynamic. They are elastic in tension but exhibit non-linear characteristics under compression and care must be taken when high strain compressive or flexural loads are involved. Aramid fibres exhibit good toughness and general damage tolerance characteristics.
Linear organic fibres
By orientating the molecular structure of simple thermoplastic polymers into one direction during their manufacture, a high-strength and high-modulus organic fibre can be produced. This fibre, in future, could be one of the major reinforcements for civil engineering structures.
With a relative density of 0.97, high-modulus polyethylene fibres, produced in the USA and The Netherlands, have mechanical properties of the same order as those of aramid fibres, with modulus of elasticity and tensile strength values of 117 GPa and 2.9 GPa, respectively. These values were determined at ambient temperature but will decrease rapidly with increasing temperature.
Furthermore, with non-cross linked thermoplastic polymer fibres, creep will be significant, but by crosslinking using radiation technology, creep problems can be overcome.
Synthetic fibres: The important fibres for upgrading cements and mortars or for use in reinforced earth situations are polypropylene, polyethylene, polyester and polyamide. The first two are utilised in the manufacture of cement/mortar composites; all are used in geosynthetics, especially to form geotextiles and geogrids.
Synthetic fibres are the only ones that can be engineered chemically, physically and mechanically to suit particular geotechnical engineering applications. The manufacture of synthetic fibres commences with the transformation of the raw polymer from solid to liquid either by dissolving or melting.
Synthetic polymers such as acrylic, modacrylic, aramid and vinyl polymers are dissolved into solution, whereas the polyolefin and polyester polymers are transformed into molten liquid; chlorofibre polymers can be transformed into a liquid by either means.
A spinneret consisting of many holes is used to extend the liquid polymer, which is then solidified into continuous filaments. The filaments undergo further extension in their longitudinal axes, thus further increasing the orientation of the molecular chain within the filament structure, with a consequent improvement in the stress–strain characteristics. Different types of synthetic fibre or yarn may be produced, including monofilament fibres, heterofilament fibres, multifilament yarns, staple fibres, staple yarns, split-film tapes and fibrillated yarns.
Natural fibres: In recent years there has been interest in the natural fibre as a substitute for glass fibre because of the potential advantages of weight saving, lower raw material price, and potential for recycling and renewing. Natural fibres are used to reinforce conventional thermoplastics, for example, injection moulding and press moulding interior parts for the automobile industry.
The fibres are generally short and randomly orientated. They are obtained from different parts of plants: jute, ramie, flax, kenaf and hemp are obtained from the stem whereas sisal, banana and pineapple from the leaf and cotton and kapok from seed.
All plant species are built up of cells and the components of natural fibres are cellulose, hemicellulose, lignin, pectin, waxes and water-soluble substances; the first three components govern the physical properties of the fibre; Pickering (2008) provides an overview of the types of natural fibre used in composites.
Currently, there is interest in converting the natural fibres into long, aligned reinforcement to exploit the inherent mechanical properties of plants in structural applications. Diversification of the market in geotextiles, which are required for temporary functions – for example, where biodegradation is desirable – temporary erosion control, building and construction materials is gradually taking place but there is little work being undertaken to use these fibres in civil engineering structural components.
This is owing to the following disadvantages compared with those fibre composites currently being used in civil engineering:
- lower strength properties, particularly impact strength
- variability in quality
- moisture absorption
- restricted maximum processing temperature
- lower durability
- poor fire resistance
- dimensional instability.
The advantage of fibre/polymer composite materials over the more conventional civil engineering materials is that they have high specific strength and high specific stiffness, achieved by the use of low-density fibres with high strength and modulus values Table 1.
Some strength and stiffness values of carbon, glass and Kevlar fibres have been mentioned in the previous sections. Table 1 gives a more comprehensive set of values. The degree of alignment of the small crystalline units in the carbon fibres varies considerably with the manufacturing technique, which thus affects the stiffness of the three types of fibre.
The arrangement of the layer planes in the cross-section of the fibre is also important, because it affects the transverse and shear properties. The strength and modulus of elasticity of glass fibres are determined by the three-dimensional structure of the constituent oxides, which can be of calcium, boron, sodium, iron or aluminium.
The structure of the network and the strength of the individual bonds can be varied by the addition of other metal oxides and so it is possible to produce glass fibres with different chemical and physical properties. The properties of carbon and glass fibres are anistropic and therefore the modulus of elasticity of both fibres along and transverse to the fibres will not be the same. The main factors that determine the ultimate strength of glass fibres are the processing conditions and the damage sustained during handling and processing.
The manufacturing processes for Kevlar fibres align the stiff polymer molecules parallel to the fibre axes, and the high modulus achieved indicates that a high degree of alignment is possible. When the fibres have been incorporated into a matrix material, composite action takes place and as discussed in the next chapter, a knowledge of the fibre alignment, fibre volume fraction and method of manufacture is necessary to obtain the optimum mechanical characteristic of the material.
Polymer composite properties
There have been several definitions of the meaning of advanced polymer composites. A clear definition is essential to their understanding, and in 1989 a study group of the Institution of Structural Engineers, the Advanced Polymer Composites Group, defined an advanced polymer composite for the construction industry as follows: ‘Composite materials consist normally of two discrete phases, a continuous matrix which is often a resin, surrounding a fibrous reinforcing structure. The reinforcement has high strength and stiffness whilst the matrix binds the fibres together, allowing stress to be transferred from one fibre to another producing a consolidated structure.
In advanced or high performance composites, high strength and stiffness fibres are used in relatively high volume fractions whilst the orientation of the fibres is controlled to enable high mechanical stresses to be carried safely. In the anisotropic nature of these materials lies their major advantage.
The reinforcement can be tailored and orientated to follow the stress patterns in the component leading to much greater design economy than can be achieved with traditional isotropic materials. The reinforcements are typically glass, carbon or aramid fibres in the form of continuous filament, tow or woven fabrics. The resins which confer distinctive properties such as heat, fibre or chemical resistance may be chosen from a wide spectrum of thermosetting or thermoplastic synthetic materials, and those commonly used are polyester, epoxy and phenolic resins.
More advanced heat resisting types such as vinylester and bismaleimides are gaining useages in high performance applications and advanced carbon fibre/thermoplastic composites are well into a market development phase.’
Structural polymer composites have a wide spectrum of mechanical properties. These properties will be dependent upon:
- the relative proportions of fibre and matrix materials (the fibre/matrix volume or weight ratio)
- the method of manufacture
- the mechanical properties of the component parts (a carbon fibre array will give greater stiffness to the composite than an identical glass fibre array)
- the fibre orientation within the polymer matrix (the fibre orientations can take the form of unidirectional, bi-directional, various off-axis directions and randomly orientated arrays).
The fibre arrangement within the matrix will influence the type and the mechanical properties of the composite material.
Table 2 gives typical mechanical properties of composites manufactured using long directionally aligned fibre reinforcement of glass, aramid and carbon with a fibre:matrix ratio by weight of 65:35. Table 3 shows typical mechanical properties of glass fibre composites manufactured by different techniques; it clearly illustrates the effect that the methods of fabrication have on the properties.
Table 4 shows the variation of the composite properties when the fibre:matrix ratio is changed, the method of manufacture and component parts of the composite remaining constant.