The most common types of fibres used in PMCs are glass, aramid, and carbon fibres [2- 4]
2.2.3.1 Glass Fibres
Glass fibres are extensively used in commercial PMC applications because of their good tensile strength, corrosion resistance, and good impact resistance incorporated with lower cost compared to aramid and carbon fibres [2, 4, 5, 52, 60]. In addition to commercial composite applications, glass fibres are also used primarily in the applications where good corrosion resistance is required such as piping in chemical industry and marine applications [2, 4]. However, glass fibres are not extensively used in high-performance applications where high strength and stiffness are required (such as the structural parts in aerospace sector) because of their inferior strength, modulus and fatigue resistance compared to carbon fibres [2, 4, 5].
The production of glass fibres [4, 52, 60, 61] starts by melting the mixture of silica (SiO2) with other oxides such as Al2O3 and B2O3, which are added to the silica mixture in
order to improve mechanical properties and workability. The melted mixture is then extruded through a platinum alloy bushing and then drawn into thin filaments. The thin filaments are quenched by air or water spray to achieve “rapid cooling rate”. The rapid cooling rate yields fibres with amorphous structure (glass) instead of semicrystalline
16 in isotropic properties of the fibres [4, 52, 60]. Finally, protective coatings such as polyester and epoxy are applied to protect the fibres during handling and improve fibre-to-matrix bonding [5, 61].
The most commonly used types of glass fibres are E-glass and S-glass. E-glass fibres are low cost, general purpose glass fibres with excellent electrical properties, while S-glass fibres are more expensive, high-performance glass fibres with superior strength, stiffness, and thermal stability than E-glass fibres [2, 4, 5, 60, 61]. The tensile strength and Young’s modulus for E-glass fibres are 3.5 GPa and 70 GPa respectively, and those for S2-glass fibres, which are the cheaper and lower performance version of S-glass fibres, are 4.5 GPa and 87 GPa respectively [5].
2.2.3.2 Aramid Fibres
Aramid fibres offer intermediate strength and modulus between glass and carbon fibres [4, 5]. With lowest density compared to other fibres (1.44 g/cm3), the specific tensile strength of aramid fibres can approach or exceed that of carbon fibres [2, 60, 62]. However, the specific modulus of aramid fibres is lower than that of carbon fibres [4, 5, 60]. Aramid fibres also offer higher toughness and damage tolerance compared to glass and carbon fibres, with good thermal stability [4, 60, 62]. With higher specific strength and modulus than glass fibres combined with high toughness and damage tolerance, aramid fibres are replacing glass fibres in high performance applications such as in interior structures of an aircraft, protective military armour, and sport equipment [2, 4, 5, 60]. Nevertheless, low compressive strength of aramid fibres (20% of tensile strength [2]) limits the use of aramid fibres in the applications with high-strain compressive or flexural loadings [5]. Toughness and high strain-to-failure of aramid fibres also makes it difficult to completely cut the fibres during machining [2, 4, 63].
Aramid fibres are produced by wet-spinning of aromatic polyamides (aramids) dissolved in sulphuric acid solution [4, 5, 52, 60, 62]. The microstructure of dissolved aramids are already arranged in crystalline structure with weak hydrogen bonds holding molecules in the transverse direction, directly resulting in anisotropic properties of aramid fibres [52, 60]. The dissolved solution of aramids is then extruded through the spinnerets, washed, dried, and finally wound up. Shearing during extrusion yields a higher degree of molecular orientation of aramid fibres along the fibre axis, resulting in higher degree of anisotropic properties [4, 52, 60, 62]. Heat treatment under tension at 150-550°C is carried out to increase molecular orientation in the fibre axis direction, improving modulus of the fibres in longitudinal direction [4, 5, 62].
17 The most prevalent aramid fibres are Kevlar 29, Kevlar 49, and Kevlar 149 [2, 5, 60]. Kevlar 29, which has highest toughness but lowest stiffness due to lowest degree of crystalline structure compared to the other two grades of Kevlar, is widely used in conventional PMC applications [2, 5, 60]. Kevlar 49 is also widely used in PMC applications such as military body armour and helmet [2, 5, 52]. In addition, Kevlar reinforced PMCs are also used in aerospace industry in applications such as the radome and fairing skins of the Airbus A-320 [52].
2.2.3.3 Carbon Fibres
Carbon fibres are defined as fibres containing at least 93% carbon content by weight [5]. The microstructure of carbon fibres is arranged in graphitic structure, which consists of parallel hexagonal basal planes of carbon atoms with covalent bonds holding carbon atoms in the basal planes and van der Waals bonds holding the basal planes in the transverse direction [1, 2], Figure 2.6. Due to the difference in strength of bonding in longitudinal and transverse directions, carbon fibres have higher tensile strength, modulus, electrical and thermal conductivity in the fibre axis direction than in the transverse direction [1]. This results in highly anisotropic properties of carbon fibres. In addition, the degree of anisotropy of carbon fibres increases with increasing graphitic structure [1, 2, 5].
Figure 2.6: Schematic diagram of graphitic structure in carbon fibres showing hexagonal basal plane of carbon atoms with covalent bonds between carbon atoms and van der Waals bonds between basal planes [1]
Among the reinforcing fibres for PMCs, carbon fibres are the most prevalent fibres used in high-performance PMC applications [2, 4]. Because of the high strength, high modulus, and light weight of high-performance carbon fibres, they are used for structural applications for aerospace sector where weight reduction combined with high strength and stiffness are critical [1, 2, 5, 64], refer to Figure 2.3. High thermal conductivity (900-
18 property for using carbon fibres in heat dissipation parts [5]. In addition, carbon fibres possess negative CTE, which enable the design of PMC parts with high dimensional stability [2, 4]. However, brittleness and low impact resistance are the main limitations to the application of carbon fibres in structural parts [1, 2, 5]. Strong bonding between carbon fibres and the matrix is therefore required to enable the matrix to support the fibres during compressive loads and interlamina shear [1]. From the machining point of view, abrasiveness and high electrical conductivity of carbon fibres causes problems during machining, e.g., high rate of abrasive wear on the tool, wear on the machine tool surfaces, and short-circuit of electrical equipment in machine tool [2, 4, 47].
In Table 2.1, specific tensile strength, specific modulus, and maximum usage temperature of carbon fibres are compared with glass and aramid fibres. High-strength carbon fibres offer higher specific strength than both E-glass and S-glass fibres. Although the specific strength of carbon fibres is lower than that Kevlar fibres, the maximum usage temperature and specific modulus of both high-strength and high-modulus carbon fibres are higher than Kevlar fibres. Considering specific modulus, carbon fibres (both high-strength and high-modulus) have the highest specific modulus when compared to glass and Kevlar fibres. However, the cost of carbon fibres is higher than those of glass fibres and Kevlar fibres, so they are limited to use in high-performance applications [2, 5].
Table 2.1: Comparison of specific tensile strength, specific modulus, and maximum usage temperature in an oxidizing atmosphere for glass, aramid (Kevlar), and carbon fibres, Note: adata from[65], bdata from[5], cdata from[66], ddata from [67]
Fibre material Density
a (g/cm3) Modulus of Elasticitya (GPa) Tensile Strengtha (GPa) Specific Tensile Strength (Pa/(kg/m3) Specific Modulus (Pa/(kg/m3) Maximum Usage Temperature in an oxidizing atmosphere (°C) E-glass 2.58 75 3.5 1.36 29.07 500b,c S-glass 2.46 90 4.5 1.83 36.59 Kevlar29 1.44 65 2.8 1.94 45.14 200b,d Kevlar49 1.44 125 3.5 2.43 86.81 T-300 carbon fibre (High- strength) 1.76 235 3.2 1.82 133.52 500b Amoco P-100 carbon fibre (High- modulus) 2.15 725 2.2 1.02 337.21
19 Carbon fibres for high-performance PMC applications are manufactured from either polyacrylonitrile (PAN) or mesophase pitch [1, 2, 4, 5].
PAN-based Carbon Fibres
The Polyacrylonitrile (PAN) precursor results from the polymerization of acrylonitrile monomer (CH2=CH-CN) [1, 5]. Since PAN tends to decompose before it melts, PAN
precursor fibres are produced by wet-spinning a solution produced by dissolving PAN in solvents such as sodium thiocyanate and dimethyl formamide [5, 64]. The PAN precursor solution is extruded through a spinnerette into a coagulation bath. In the coagulation bath, extruded PAN solution is precipitated into PAN fibres composed of a fibrillar, or ribbon like network structure with preferred orientation along the fibre axis direction due to the shearing force during extrusion [1, 5, 64]. Stretching is then applied to increase structural orientation along the fibre axis direction to yield carbon fibres with adequate strength and modulus in longitudinal direction after final heat treatment step [1, 5, 64]. After stretching, the as-spun PAN fibres have to be stabilized to be able to withstand high temperatures and maintain molecular and fibrillar orientation during carbonization step [5, 64]. Oxidative stabilization of PAN fibres is conducted in air at temperatures of 230-280°C under tension, which is applied to minimize the relaxation of PAN fibre structure [64]. This step will crosslink PAN fibres, forming ladder PAN polymers that can withstand high temperatures during the carbonization step [68]. The stabilized PAN fibres are then carbonized in an inert atmosphere, which prevents oxidization during the process [1], at temperatures of 980- 1595°C to convert them into carbon fibres [5]. During the carbonization step, non-carbon elements are eliminated in the form of gases such as water vapour, carbon dioxide (CO2),
ammonia (NH3), hydrogen cyanide (HCN), and methane (CH4) [5, 68]. Due to the fibrillar
or ribbon-like network structure of PAN fibres, PAN-based carbon fibres are composed of ribbons of hexagonal planes of carbon atoms in the form of turbostratic graphite layers in the structure [1, 5, 64, 68], Figure 2.7. If graphite fibres, which consist of 99% carbon content and have higher modulus than carbon fibres [5], are required, graphitization can be done in an inert atmosphere at the temperatures above 2700°C to increase degree of graphitic structures and, thus, modulus of the fibres [1, 5].
20
Figure 2.7: Schematic diagram representing microstrcuture of PAN-based carbon fibre by Johnson [69]
Due to this fibrillar nature and turbostratic structure, PAN-based carbon fibres are less sensitive to flaw-induced failure and, hence, have higher tensile strength than mesophase pitch-based carbon fibres [1, 5, 64, 69]. Johnson [69] was the first who explained this by using Reynolds and Sharp’s brittle-failure mechanism, Figure 2.8. Although fibrils in the structure of PAN-based carbon fibres are aligned and oriented parallel to the fibre axis direction, there are still some misalignments such as interlinks between turbostractic layers in the structure, Figure 2.8a. As tension is applied parallel to the direction of the fibre axis, the turbostractic layers are stretched and increasingly aligned until the movement is limited by interlinks between these layers. Consequently, tensile stress is building up within these interlinks. When the tensile stress reaches a sufficient level, these interlinks are broken, causing a crack in the direction normal to the fibre axis, Figure 2.8b. This crack within the structure will not cause failure unless its size is greater than critical flaw size of the fibre, Figure 8c. For the crack to be greater than critical flaw size of the fibre, the crack must occur within the interlinks having crystallite size greater than critical flaw size, or there must be continuity in graphitic structure surrounding the crack so that it can propagate [69]. In PAN- based carbon fibre, there is low degree of graphitization and spaces between turbostratic layers are larger than those of graphitic crystallite [5, 64]. This limits the size and continuity of graphitic crystallite, enabling a PAN-based carbon fibre to withstand higher tensile stress. Consequently, PAN-based carbon fibres can be produced to possess higher tensile strength (up to 7 GPa) than mesophase pitch-based carbon fibres (up to 4 GPa), which have higher degree of graphitic structure and lager crystallite size [1, 5, 64]. PAN-based carbon fibres are therefore the main precursors for producing high-strength carbon fibres [1, 2, 4, 5].
21
Figure 2.8: Diagrams representing Reynolds and Sharp mechanism of tensile failure in carbon fibres [69]
The less ordered, fibrillar structures with low degree of graphitization of PAN-based carbon fibres also results in lower elastic modulus of the carbon fibres that can be produced (up to 585 GPa) compared to mesophase pitch-based carbon fibres (up to 900 GPa) [1, 2, 4, 5, 64]. However, the elastic modulus of carbon fibres can be increased by increasing the final heat treatment temperature, which results in the increased molecular orientation of graphitic structure in the fibre axis direction [1, 2, 4, 5, 68]. The alignment of graphitic structure in carbon fibres increases with increasing heat treatment temperature, thus increasing elastic modulus of the carbon fibres [1, 2, 5, 64], Figure 2.9. In addition, tensile strength of carbon fibres also increases with increasing heat treatment temperature due to higher degree of molecular orientation along the fibre axis [1, 5, 64, 68]. Nevertheless, tensile strength of carbon fibres abruptly decreases when the heat treatment temperature exceeds 1600°C [70], beyond the red line in Figure 2.9. This is because the increased alignment of graphitic structure and the increase in crystallite size due to increasing heat treatment temperature provide greater continuity for the cracks during tensile loading to propagate, leading to flaw-induced failure as previously discussed [5, 70].
22
Figure 2.9: Effect of heat treatment temperature on tensile strength and modulus of carbon fibres [68]
Pitch-based Carbon Fibres
High performance pitch-based carbon fibres are manufactured from mesophase pitch, which is an anisotropic liquid crystalline with polyaromatic layers aligned parallel in longitudinal direction [1, 5, 64, 71]. Mesopahse pitch precursor can be produced by heating isotropic pitch, which is the precursor for low cost, low strength and low modulus pitch- based carbon fibres, to a temperature of 400-425ºC for up to 40 hours [1, 5]. Due to polyaromatics in the structure, mesophase pitch precursor has good thermal stability and it does not decompose before melting [5, 64]. As a consequence, mesophase pitch precursor fibres are manufactured by melt spinning process [1, 2, 4, 5, 64, 71].
The production process of mesophase pitch-based carbon fibres starts by melting the mesophase pitch precursor and extruding the melted precursor through a spinneret to form precursor fibres. The shearing force during extrusion enhances molecular orientation of mesophase pitch precursor fibres along the fibres direction, as in the case of PAN-based precursor fibres [5, 71]. The pitch precursor fibres are then stabilized in air at the temperatures of 230-280ºC in order to crosslink the thermoplastic pitch, converting into thermosetting pitch fibres that can withstand high temperatures and maintain molecular orientation during carbonization step [1, 64]. After being stabilized, mesopahse pitch fibres are carbonized in an inert atmosphere at the temperatures of 1000-2000ºC to convert the pitch fibres into carbon fibres [1, 5]. Similar to the production process of PAN-based carbon fibres, non-carbon elements are eliminated as gases such as water vapour, carbon dioxide (CO2), methane (CH4), and ammonia (NH3) [1, 5, 64, 71]. Then graphitization in an inert
23 As previously discussed, mesophase pitch-based carbon fibres have higher elastic modulus and lower tensile strength than PAN-based carbon fibres because of higher degree of graphitic crystallite and higher molecular orientation along the fibre axis direction [1, 2, 4, 5, 64, 71]. Similar to PAN-based carbon fibres, elastic modulus of the fibres continuously increases with increasing heat treatment temperature during carbonization process but tensile strength of the fibres increases until reaching a maximum at 1600ºC and abruptly decreases [2, 5, 64, 70], Figure 2.9. In addition, because of more alignment of graphitic structure along the fibre axis compared to PAN-based carbon fibres, mesophase pitch-based carbon fibres have higher thermal conductivity (900-1000 W/mK) in fibre axis direction than PAN-based carbon fibres (10-20 W/mK) [5, 71]. As a consequence, mesophase pitch-based carbon fibres are mainly used for the applications in which stiffness and good heat dissipation capability are more important than strength [5, 71].