Preparation and Properties of Macro Plastic Fibres
The macro plastic fibres can be virgin and recycled polypropylene (PP), high-density polyethylene (HDPE) or polyethylene terephthalate (PET) fibres. PP fibres have been widely used in the concrete industry, due to its ease of production, high alkali resistance (Santos et al. 2005), and high tensile strength and Young’s modulus (Yin et al. 2013). However, their low density (around 0.9 g/cm3) may make the fibres ‘float up’ to the surface of concrete matrix (Auchey 1998). Low hydrophilic nature of PP fibres, which can be reflected by low wetting tension of about 35 mN/m, also significantly deteriorates workability of fresh concrete and bonding between the fibres and concrete (Ochi et al. 2007). HDPE fibres have slightly higher density (around 0.95 g/cm3) and are more hydrophilic than PP fibres. However, HDPE fibres have low tensile strength (ranging from 26 to 45 MPa), which significantly limits their applications (Auchey 1998). PET fibres have much higher density at 1.38 g/cm3 and better wetting tension of 40 mN/m than PP fibres, so they are easier to be mixed with concrete than either of PP or HDPE fibres. They also have high tensile strength and Young’s modulus (Ochi et al. 2007), which can effectively improve post-cracking performance of concrete. However, PET granules must be dried for at least 6 h before being processed into fibres. The PET granules are easily crystallised and stick on the inner wall of extruder. Hence, it is more difficult and costly to process PET than either of PP or HDPE. Moreover, alkali resistance of the PET fibres is questionable (EPC 2012; Silva et al. 2005). Therefore, the PP fibres have become the most common commercial concrete fibre, and PET fibres have attracted extensive research, but HDPE fibres are still rare in practice with very little research being reported in the literature. From the environmental and cost-saving perspective, researchers are now also investigating the use of recycled plastic fibres in concrete (Siddique et al. 2008). However, recycled plastics have uncertain processing and service history, impurities and varying degrees of degradation, leading to processing difficulties and unstable mechanical properties (Wang et al. 1994).
The physical and chemical characteristics of macro plastic fibres vary widely depending upon the manufacturing techniques. A popular technique involves melt spinning plastic granules into filaments and then hot drawing the monofilaments into fibres (Fraternali et al. 2011). In the study conducted by Ochi et al. (2007), PET granules were melted and extruded into monofilaments with a fineness of 60,000 dtex (dtex: grams per 10,000 m length). Then the monofilaments were hot drawn into 5000 dtex through a film orientation unit shown in Fig. 2.3 (Ochi et al. 2007). The resulting monofilaments were then indented and cut into fibres of 3040 mm long. This melt spinning and hot drawing process highly oriented the molecular chains of the PET fibres, inducing high crystallinity and thus
Fig. 2.3 Apparatus for PET fibre extrusion (Ochi et al. 2007) significantly improving tensile strength and Young’s modulus. Through this method, PET (Fraternali et al. 2011) and PP (Yin et al. 2015b) fibre of tensile strength above 450 MPa can be obtained.
Another popular processing technique is extruding PET, PP or HDPE granules through a rectangular die to form film sheets (0.2-0.5 mm thick). The resulting film sheets are then slit longitudinally into equal width tapes (1.0—1.3 mm wide) by a slitting machine. The tapes are then mechanically deformed using a patterned pin wheel, such as crimped and embossed. In some cases, the fibrillated tapes are also twisted before cutting to desired lengths (40-50 mm) (Kim et al. 2008). Kim et al.
(2010) used this technique to successfully prepare recycled PET fibre with 420 MPa tensile strength and 10 GPa Young’s modulus.
In order to reduce manufacturing costs, researchers have explored the potential of producing recycled plastic fibres just by mechanically cutting PET bottles, as reported by Fraternali et al. (2013), de Oliveira and Castro-Gomes (2011) and Foti
(2011) . Foti (2011) produced lamellar fibre and ‘O’-shaped annular fibre by this method. The special shape of the ‘O’-fibre can assist to bind the concrete on each side of a cracked section, thus improving ductility of the concrete. This technique though economical in smaller scale, cannot be used for a large-scale production. Firstly, the bottles should be washed before or after cutting which makes this process labour-intensive. Secondly, waste bottles have different history and degradation, which results in variable and poorer mechanical properties of the fibres. Moreover, the fibres produced through this technique only has a tensile strength of around 160 MPa and low Young’s modulus of about 3 GPa (Foti 2013), which are much lower than those of the fibres produced by the other two techniques.