Carbon materials are well- known for their structural applications in lightweight composites in aerospace and automotive applications. Recently, they have been increasingly investigated for electrical applications, including electromagnetic refl ection, heating and sensing. Carbon materials are typically built up in layers.
As they only conduct in the in- plane direction, their electrical properties depend greatly on the degree of preferred orientation of the carbon layers. This degree can be infl uenced during production by applying heat treatments and aligning carbon units, such as carbon fi bres in a composite structure. Hence, carbon can exhibit a range of electrical conductivity, which when compared to metals, is still rather low. Carbon is usually obtained by the pyrolysis of hydrocarbon gases and oils. Depending on their production parameters, different forms of carbon called allotropes exist. Next to amorphous carbon (carbon black), other allotropes of carbon are graphite and fullerene, such as carbon nanotubes.
2.8 Nickel- coated polyester fi bres through electroless deposition.
2.5.1 Carbon black (CB)
In amorphous carbon, the arrangement of carbon atoms is non- crystalline and irregular. It is the main constituent of carbon black (CB), a typical fi ller material for polymers and polymeric fi bres, as it provides electro- conductivity. There is a wide variety of CB products. Differences include particle size and particle size distribution, specifi c surface area, surface structure, amount of agglomeration, moisture content, and minor contaminants, such as metals, sulphur, oxygen and hydrogen (Peters, 1998). Compared to graphite and carbon nanotubes, CB is the least electro- conductive carbon material as it is amorphous.
In textiles, CB is either applied as fi ller material in polymers or as a layer. For the former, CB particles have been dispersed into different polymer solutions (Flandin et al. , 2001; Mallette et al. , 2001; Huang, 2002; Heiser et al. , 2004;
Hwang et al. , 2007b). Subsequent extrusion (Hitchcock et al. , 1999; Jimenez et al. , 2009) or electrospinning (Kim and Yang, 2003; Pedicini and Farris, 2004; Hwang et al. , 2007b; Tiwari et al. , 2008; Schiffman et al. , 2011) produces fi bres and fi bre webs with electrical properties depending on loading of CB in the polymer solution and the percolation threshold 2 , which is reported to be in the range of 2 to 12 vol.%
(Hwang et al. , 2007b). Normally, the electrical resistance ranges between 10 5 and 10 7 Ohm/cm. The same effect can be achieved by applying CB coatings or fi lms onto textile structures (Jin et al. , 2005; Koncar et al. , 2009; Xue et al. , 2011). In order to improve their electro- conductive properties, additional metallic particles or thin metallic layers are applied in or on CB containing fi brous and textile substrates (Chakravarthi et al. , 2011; Jalali et al. , 2011). Due to their high electrical resistance, CB-fi lled polymer composites are most often applied for antistatic, shielding and heating purposes (Koncar et al. , 2009). Their suitability as sensors and electrodes has also been explored (Chung, 2004; Xue et al. , 2011).
2.5.2 Graphite
Diamond, 3 graphite and fullerene are crystalline forms of carbon, in which each atom bonds with its neighbour forming a regular, repeating pattern. Graphite is the most common crystalline allotrope, with a layered, planar structure. In each layer, also referred to as graphene, each carbon atom forms three links with its neighbours, forming hexagonal rings. As carbon atoms have four electrons to share, each carbon atom can still bond to another carbon atom of another layer holding the material together (Sparrow, 1999).
The so- called fl exible graphite is a textile- compatible form of graphite, as it is a fl exible sheet made by compressing a collection of graphite fl akes without a binder. Due to its microstructure involving graphite layers that are preferentially parallel to the surface of the sheet, fl exible graphite has a good electrical and thermal conductivity in the plane of the sheet. It is particularly explored for shielding purposes and as a heating element (Chung, 2004).
2.5.3 Carbon nanotubes (CNT)
Another allotrope is fullerene, which has the form either of a hollow sphere, ellipsoid or cylinder. Fullerenes are similar in structure to graphite, but they may also contain pentagonal (or sometimes heptagonal) rings (Thostenson et al. , 2001). Typical fullerenes are carbon nanotubes, which are widely applied in composites. A carbon nanotube is produced by rolling a sheet at a specifi c angle.
The combination of the rolling angle and radius specifi es the nanotube properties, for example, whether the individual nanotube is a metal or semiconductor. Both, single- walled carbon nanotubes (SWCNT) and multi- walled carbon nanotubes (MWCNT) exist (Chou et al. , 2010).
Carbon nanotubes are applied in three different forms in textiles:
1. carbon nanotube fi bres;
2. fi ller material in polymeric fi bres and fi laments;
3. surface layers on textile structures.
Carbon nanotube fi bres
The development of continuous fi bres based on carbon nanotubes has recently gained a lot of interest. The most applied technologies include the spinning of carbon nanotubes from solutions (Bhattacharyya et al. , 2003) and gels (Li et al. , 2004; Koziol et al. , 2007; Chae et al. , 2007, 2009), as well as the dry spinning from multi- walled nanotubes grown on a substrate (Jiang et al. , 2002; Zhang et al. , 2007). These technologies are explained in more detail in Section 2.5. Carbon nanotubes are known for their high axial strength and stiffness. Fibres with a greater strength than para- aramid fi bres can be produced, depending on the production technologies and process parameters (Koziol et al. , 2007; Chou et al. , 2010).
Carbon nanotubes as fi ller material in polymeric fi bres and fi laments
Carbon nanotubes are used as fi llers in polymeric solutions to extrude or spin fi bres and fi laments. Researchers have explored, for example, the melt spinning of carbon nanotube loaded polypropylene (PP) (Kearns and Shambaugh, 2002;
Bhattacharyya et al. , 2003), poly(methyl methacrylate) (PMMA) (Haggenmueller et al. , 2000), polycarbonate (PC) (Sennett et al. , 2003; Potschke et al. , 2005;
Thomas et al. , 2006), polyimide (PI) (Siochi et al. , 2004), polyamide (PA) (Sandler et al. , 2004), poly(ethylene terephthalate) (PET) (Luo et al. , 2006), polyurethane (Chen et al. , 2006), and shape memory polyurethane (Hu and Meng, 2008). Furthermore, some attempts were made to produce carbon nanotube loaded fi bres through twin- screw extrusion with polyethylene and polyurethane (Chen et al. , 2006).
The electro- conductive properties of such fi bres depend on a range of factors, including the purity of the nanotubes, the homogeneity of the dispersion in the
polymeric host matrix, its loading and the aspect ratio between length and diameter for the connectivity between single nanotubes (Chung, 2004, Chou et al. , 2010).
However, the achievable electro- conductivity of these yarns is limited when comparing them to carbon nanotube fi bres.
Carbon nanotubes as surface layers on textile structures
Different technologies are employed to create carbon nanotube layers on textile materials. Chemical vapour deposition, for instance, was used to grow carbon nanotubes on woven fabrics (Baughman et al. , 2004; Veedu et al. , 2006; Zhang et al. , 2007). Another method that was applied in research is dye- printing of carbon nanotubes on polyester yarns (Zhang et al. , 2007; Fugetsu et al. , 2009). The achieved electrical resistance of the yarn by the latter method is 8 10 5 Ω /cm.