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Flax fibre and its composites – a review

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In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an …

In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an
increasing requirement for developing sustainable materials. Flax fibres are cost-effective and offer specific
mechanical properties comparable to those of glass fibres. Composites made of flax fibres with thermoplastic,
thermoset, and biodegradable matrices have exhibited good mechanical properties. This
review presents a summary of recent developments of flax fibre and its composites. Firstly, the fibre
structure, mechanical properties, cost, the effect of various parameters (i.e. relative humidity, various
physical/chemical treatments, gauge length, fibre diameter, fibre location in a stem, oleaginous, mechanical
defects such as kink bands) on tensile properties of flax fibre have been reviewed. Secondly, the effect
of fibre configuration (i.e. in forms of fabric, mat, yarn, roving and monofilament), manufacturing processes,
fibre volume, and fibre/matrix interface parameters on the mechanical properties of flax fibre
reinforced composites have been reviewed. Next, the studies of life cycle assessment and durability
investigation of flax fibre reinforced composites have been reviewed.

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  • 1. Flax fibre and its composites – A review Libo Yan a,⇑ , Nawawi Chouw a , Krishnan Jayaraman b a Department of Civil and Environmental Engineering, The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand b Department of Mechanical Engineering, The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand a r t i c l e i n f o Article history: Received 11 October 2012 Received in revised form 1 February 2013 Accepted 12 August 2013 Available online 22 August 2013 Keywords: B. Natural fibre composites B. Mechanical properties a b s t r a c t In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an increasing requirement for developing sustainable materials. Flax fibres are cost-effective and offer spe- cific mechanical properties comparable to those of glass fibres. Composites made of flax fibres with ther- moplastic, thermoset, and biodegradable matrices have exhibited good mechanical properties. This review presents a summary of recent developments of flax fibre and its composites. Firstly, the fibre structure, mechanical properties, cost, the effect of various parameters (i.e. relative humidity, various physical/chemical treatments, gauge length, fibre diameter, fibre location in a stem, oleaginous, mechan- ical defects such as kink bands) on tensile properties of flax fibre have been reviewed. Secondly, the effect of fibre configuration (i.e. in forms of fabric, mat, yarn, roving and monofilament), manufacturing pro- cesses, fibre volume, and fibre/matrix interface parameters on the mechanical properties of flax fibre reinforced composites have been reviewed. Next, the studies of life cycle assessment and durability investigation of flax fibre reinforced composites have been reviewed. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, the use of bio-fibres to replace glass fibres as reinforcement in composites for engineering applications has gained popularity due to an increasing environmental concern and requirement for developing sustainable materials [1,4]. Approximately 43,000 tonnes of bio-fibres were utilised as rein- forcement in composites in the European Union (EU) in 2003 [2]. The amount increased to around 315,000 tonnes in 2010, which ac- counted for 13% of the total reinforcement materials (glass, carbon and natural fibres) in fibre reinforced composites. It is forecasted that about 830,000 tonnes of bio-fibres will be consumed by 2020 and the share will go up to 28% of the total reinforcement materials [3]. The United States (US) Department of Agriculture and the US Department of Energy had set goals of having at least 10% of all basic chemical building blocks be created from renew- able and plant-based sources in 2020, increasing to 50% by 2050 [4]. The explosive growth in bio-composites is indicative of their wide application in the future as the next generation structural materials. Bio-fibres are cost-effective with low density. These are biodegradable and non-abrasive. In addition, they are readily available and their specific mechanical properties are comparable to those of glass fibres used as reinforcement [5,6]. 2. Flax fibres Flax (Linum usitatissimum) is one of the most widely utilised bio-fibres. Flax is also one of the first to be extracted, spun and wo- ven into textiles. Flax in textile use was found in graves in Egypt dating back to 5000 BC [7]. Kvavadze et al. [8] have recently re- ported finding twisted wild flax fibres indicating that prehistoric hunter–gatherers were making cords for hafting stone tools, weav- ing baskets, or sewing garments around Dzudzuana Cave (Georgia) up to 30,000 years ago. Flax grown for fibre and linseed grown for seed oil are cultivars (varieties of the same plant species bred with an emphasis on the required product) [9]. Canada is the largest producer and exporter of flax in the world since 1994. In 2005/06, Canada produced about 1.035 million-tonnes and currently ships 60% of its flax exports to the EU, 30% to the US, and 4% to Japan [10]. Other leading produc- ers of flax are France, Belgium and the Netherlands, with nearly 130,000 acres under cultivation annually. In 2007, the EU produced 122,000 tonnes of flax fibres [11]. Climatic conditions in the re- gions are perfect for growing flax, and increasing worldwide de- mand for linen makes it an important cash crop. The growing cycle of flax is short, with only 100 days between sowing in March and harvesting in July in the Western European region [12]. Fine and regular long flax fibres are usually spun into yarns for linen textiles. Linen fabric maintains a strong traditional niche among high quality household textiles, such as bed linen, furnish- ing fabrics and interior decoration accessories. Shorter flax fibres produce heavier yarns suitable for kitchen towels, sails, tents and 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.08.014 ⇑ Corresponding author. Tel.: +64 9 373 7599x84521; fax: +64 9 373 7462. E-mail address: lyan118@aucklanduni.ac.nz (L. Yan). Composites: Part B 56 (2014) 296–317 Contents lists available at ScienceDirect Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
  • 2. canvas. Lower fibre grades as reinforcement and filler in compos- ites are used in automotive interior substrates and furniture [11]. 2.1. Structure Flax fibres are produced in the stems of flax bast plant. Like cot- ton, flax fibre is a cellulose polymer, but its structure is more crys- talline, making it stronger, crisper and stiffer to handle, and more easily wrinkled. A schematic view of the multi-scale structures of flax from stem to the cellulosic fibrils is given in Fig. 1 [13,15]. Flax plant ranges in length up to 90 cm which possesses strong fibres all along its stem, and average 12–16 lm in diameter [11]. At the macroscopic level, a flax stem is composed, from the outer towards the inner part, of bark, phloem, xylem and a central void. At the meso-scopic level, the cross-section of a bundle contains between 10 and 40 fibres which are linked together mainly by pectin [13]. The microstructure of a flax fibre is extremely complex due to the hierarchical organisation at different length scale and the dif- ferent materials present in variable proportions [14]. At the micro- scopic scale, each elementary fibre is itself made of concentric cell walls, which differ from each other in terms of thickness and arrangement of their constitutive components. At the centre of the elementary fibre, the concentric cylinders with a small open channel in the middle called the lumen, which contributes to water uptake as displayed in Fig. 1. The outer cell wall designed as the primary cell wall is only 0.2 lm thick [16]. On the outer side, the thin primary cell wall coats the thicker secondary cell wall which is responsible for the strength of the fibre and encloses the lumen. Each layer is composed of microfibrils of cellulose which run par- allel one to another and form a microfirilar angle with the fibre direction; this angle is minimum in the secondary cell wall [13]. The bulk of the fibre is essentially constituted by the layer S2 of the secondary cell wall (dominating the cross section), as shown in Fig. 2. This thickest cell wall (S2) contains numerous crystalline cellulose micro-fibrils and amorphous hemicellulose which are ori- ented at 10° (see Fig. 2) with the fibre axis and give fibre its high tensile strength [14,17]. At the nano-scale, a microfibril is consti- tuted of cellulose chains (crystalline zones) embedded in an amor- phous matrix mainly made of pectins and hemicelluloses [13]. The cellulose crystallites in the secondary cell wall are laid down in ori- ented, highly crystalline microfibrils which are glued together by the amorphous hemicellulose/pectic matrix [14]. These micro-fi- brils represent about 70% of the weight of a flax fibre and are likely to act as the reinforcement material within the fibre [18]. The angle between the axis and the fibrils of the fibre could affect the strength of the fibres. Generally, a fibre is more ductile if the mi- cro-fibrils have a spiral orientation or the fibre axis. 2.2. Chemical composition The chemical composition and location of constituents within the flax stem define the properties of flax fibre. Table 1 lists the compositions of flax fibres reported by different authors [20– 24,64]. The main constituents of a flax fibre consist of cellulose, hemicellulose, wax, lignin and pectin, in varying quantities. Cellu- lose, hemicellulose and lignin are basic components which deter- mine the physical properties of the fibres. Cellulose is the stiffest and the strongest organic constituent in the fibre. However, cellu- lose is a semicrystalline polysaccharidewith a large amount of hy- droxyl group, giving hydrophilic nature to natural fibre when used to reinforce hydrophobic matrices. The result is a very poor inter- face and poor resistance to moisture absorption [108]. In the com- posite materials, bio-fibres adhere poorly to hydrophobic matrices, often to the point that the composite is mechanically inferior to either the bio-fibres or the matrix material on their own. This calls for the fibre or matrix modification to improve the mechanical properties of the composite. Hemicellulose is strongly bound to cellulose fibrils presumably by hydrogen bonds. Hemicellulosic polymers are branched, fully amorphous and have a significantly lower molecular weight than cellulose. Because of its open struc- ture containing many hydroxyl and acetyl groups, hemicellulose is partly soluble in water and hygroscopic. Lignin and pectin act mainly as bonding agents [25]. Lignins are amorphous, highly com- plex, mainly aromatic, polymers of phenylpropane units but have the least water sorption of the natural fibre components [108]. The waxy substances of flax fibres affect the fibre wettability and adhesion characteristics. As shown in Table 1, flax fibre is rich in cellulose which accounts for about 70% of the total chemical com- position. This enables flax to be widely considered as reinforce- ment in composite. In Table 1, the variation of proportions of the Fig. 1. Flax structure from the stem to the cellulosic fibrils (reproduced with permission from [13,15]). Fig. 2. The micro-structure of a flax fibre cell (reproduced with permission from [14]). L. Yan et al. / Composites: Part B 56 (2014) 296–317 297
  • 3. constituents of flax fibres is due to the fact that the measured pro- portion is highly influenced by the species and the variety of the plant, agricultural variables such as soil quality, the weathering conditions, the level of plant maturity, and the quality of the ret- ting process and measurement conditions of that include or ex- clude moisture [14,33]. Consequently, these factors may have an impact on the physical and mechanical properties of flax fibres. 2.3. Tensile deformation Tensile properties of flax fibres are essential when considering as reinforcement in fibre reinforced polymer composites. The ten- sile deformation of a flax fibre is influenced by the specimens, even when these fibres are cultivated in the same location and the test parameters considered are identical. Charlet et al. [15] tested monofilament flax fibres using a universal MTS tensile testing ma- chine equipped with a 2N capacity load cell. The considered gauge length was 10 mm and the cross-head displacement rate was 1 mm/min. The tensile stress–strain curve of the flax fibre is given in Fig. 3. The response curve can be divided into three parts: (1) a first linear part (strain from 0% to 0.3%), this deformation associ- ates with a global loading of the fibre, through the deformation of each cell wall; (2) a second non-linear part (0.3–1.5%), the non-linear behaviour was interpreted as an elasto-visco-plastic deformation of the fibre, especially of the thickest cell wall (S2), since the alignment of the cellulosic micro-fibrils with the tensile axis led to the re-arrangement of the amorphous parts of the wall (mainly made of pectin and hemicelluloses); and (3) the final linear (1.5% to rupture). This linear part is thought to correspond to the elastic response of the aligned micro-fibrils to the applied tensile strain. A similar tensile response of a flax fibre was observed by Alix et al. [26], as shown in Fig. 6. Pillin et al. [27] evaluated the tensile deformation of different oleaginous flax fibres which were cultivated on the same geo- graphic area and lands in a temperate region (West of France). The varieties of oleaginous flax studied were Oliver, Hivernal, Alas- ka, Niagara and Everest. The test machine, gauge length and cross- head displacement rate used are identical to which considered by Charlet et al. [15]. The tensile deformations of these flax fibres are displayed in Fig. 4. It is observed that in the earlier stage of the loading the curve has a non-linear region with small deformations (0–0.5%). This behaviour can be explained by the reorganisation of the cellulose micro-fibrils in the direction of the fibre axis and shear during the tensile loading [14], since the micro-fibrils have a micro fibrillar angle of 10°, as shown in Fig. 2. For higher defor- mations (after 0.5%), a linear region of the stress–strain curve is ob- served which is characteristic of a Hookean behaviour. All the results reported by the authors [15,26,27] support the point that the angle between the axis and the fibril affects tensile properties of flax fibres remarkably. 2.4. Tensile failure mechanism Recently, some test methods and techniques have been devel- oped to evaluate and monitor the failure mechanisms of flax fibres in tension, e.g. in situ environmental scanning electron microscope (ESEM) study [16] and acoustic emission (AE) technique [28,29]. ESEM observation indicated that the fracture of flax initiates on tensile side of the fibre, then the cracks in primary cell wall widen, followed by the separation between primary and secondary cell wall. Next, extended plastic deformation of the fibrils appears in the secondary cell wall until to the completion of failure [16]. The primary cell wall (P zone in Fig. 2) breaks in a brittle manner, whereas in the secondary cell wall (S zone), due to its fibrillar nat- ure, a coarse crack grows, bridged by fibrils. The secondary cell wall is found to split relatively easily along the length direction, indicating that the lateral strength of the fibre is lower than its ten- sile strength, which also accounts for the lower compressive strength of the fibre compared to its tensile strength, i.e. the mea- sured tensile strength of elementary flax fibres was found to range between 1500 and 1800 MPa and the measured compressive strength was around 1200 MPa [19]. AE study has proved useful for its capability of real-time mon- itoring over the whole material volume and high sensitivity to any process generating stress waves. Studies by Romhány et al. [28,29] showed that there are three failure mechanisms of a tech- nical flax fibre: (1) longitudinal splitting of the pectin boundary layer among the elementary fibres (AE amplitude less than 35 dB, Table 1 Chemical composition of flax fibres as reported by different authors [20–24,64]. Cellulose (%) Hemi-cellulose (%) Pectin (%) Lignin (%) Wax (%) Moisture content (wt.%) Refs. 64.1 16.7 1.8 2.0 1.5 10.0 [20] 67 11 – 2.0 – – [21] 73.8 13.7 – 2.9 – 7.9 [22] 65 – – 2.5 – – [23] 62–72 18.6–20.6 2.3 2–5 1.5–1.7 8–12 [24] 71–75 18.6–20.6 2.2 2.2 1.7 10.0 [64] Fig. 3. Tensile stress–strain curve of an elementary flax fibre (reproduced with permission from [15]). Fig. 4. Tensile stress–strain curves for the different varieties of oleaginous flax fibres (reproduced with permission from [27]). 298 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 4. dB is a logarithmic measure of AE signal amplitude); (2) transverse cracking of the elementary fibre (35–60 dB); and (3) multiple frac- ture of elementary fibres and their micro-fibrils (over 60 dB), as shown in Fig. 5. 2.5. Factors affecting tensile properties Unlike synthetic fibres, natural fibres have significantly greater variability in their mechanical properties due to the conditions experienced in the field and the potential damage arising from the processes of production and measurement conditions. These factors which affect the mechanical properties of flax fibres are summarised in Table 2 [24,30,39]. In the process of production of flax fibres, there are several different stages: plant growth, harvest- ing, fibre extraction and supply. In each stage several factors can influence the quality of fibres. Except for the structure and prop- erty of the fibre itself, experimental conditions such as fibre gauge length, test speed, etc., all have effects on the properties of flax fi- bres. Additionally, various fibre surface treatments change the fibre properties considerably. In the following text, the parameters, with respect to environmental relative humidity (RH) [31], Duralin upgrading treatment [32], fibre length [19,32], different chemical treatments [26], fibre locations in the stems [13,33], microstruc- tural of the flax fibres [34], water treatment and drying cycle treat- ment [35], fibre mechanical defects such as kink bands [36], plants with different varieties [27,35,37], fibre diameter [27], and mea- surement gauge length [13,26,27,33,34] which affect the tensile properties of monofilament flax fibres are discussed. Since flax fibres are highly hydrophilic, their tensile moduli are strongly dependent on the environmental relative humidity (RH). Both static and dynamic moduli of flax fibres decreased remark- ably with an increase in RH (i.e. RH values of 30%, 45%, 60%, 75% and 90%, respectively) [31]. With an increase in RH from 30% to 90%, the reduction in static and dynamic moduli of flax fibres is 35.4% and 19.4%, respectively. High moisture absorption and poor dimensional stability (swelling) characteristics of natural fibres could degrade fibre ten- sile properties. Improving the poor environmental and dimen- sional stability of lignocellulosic materials is an effective way to modify the mechanical properties of these materials, e.g. flax fi- bres. Duralin treatment is a commercialised (CERES BV, Wagenin- gen, the Netherlands) upgrading process to improve the strength and reproducibility of the flax fibres, the so-called Duralin flax [32]. Duralin treatment consists of a steam or water-heating step of the rippled straw-flax at temperatures above 160o C during 30 min in an autoclave; followed by a drying step and a heating (curing) step above 150 °C for 2 h. Stamboulis et al. [32] investi- gated the effect of Duralin treatment on moisture absorption and tensile properties of flax fibres. Test results (Table 3) indicated that Duralin flax absorbed less moisture than the untreated green flax Fig. 5. Failure sequence in a technical flax fibre: (a) axial (longitudinal) debonding and fibrillation along the elementary fibres(AE amplitude <35 dB); (b) radial (transverse) cracking in the elementary fibres (amplified effect attributed to stress concentration) (AE amplitude 35–60 dB); (c) ‘‘tearing-type’’ fracture within and through the elementary fibres(AE amplitude 35–60 dB); (d) fracture completed by fracture of the elementary fibres and their constituting micro-fibrils(AE amplitude >60 dB) (reproduced with permission from [28]). Fig. 6. Tensile stress–strain curves for untreated and chemical treated flax fibres (reproduced with permission from [26]). L. Yan et al. / Composites: Part B 56 (2014) 296–317 299
  • 5. fibres. The Duralin fibres exhibited a higher and more uniform strength with less scatter. However, the average tensile strength changed with relative humidity as well as the tested fibre length. Generally, a higher tensile strength is observed for fibre with a shorter gauge length (3.5 mm vs. 8 mm) [32]. In other words, the tensile strength decreases with an increase in fibre length. The rea- sons lie on two-folds: one is that the longer the fibre, the higher its probability of containing a defect (e.g. kink bands) and thus of fail- ing prematurely compared to a shorter fibre. As the test length in- creases, the number of weak links or imperfections also increases, thus resulting in reduction in tensile strength. Secondly, the failure mechanism of technical fibres at shorter clamping length is differ- ent from that at longer clamping length. At large clamping length flax fibre failure takes place through the relatively weak pectin interphase that bonds the elementary fibres together. The pectin interphase is oriented predominantly in the length direction of the fibre, it breaks by shear failure. At clamping length below the elementary fibre length, failure can no longer take place through the pectin interphase, but the crack must now run through the stronger, cellulosic cell wall of the elementary fibres [19]. The main problem of natural fibre/polymer composites is the incompatibility between the hydrophilic natural fibres and the hydrophobic matrices. The hydrophilic characteristics of the natu- ral fibres (e.g. flax fibres) can lead to a poor fibre/matrix adhesion due to the presence of pendant hydroxyl and polar groups in the components. This nature leads to high moisture uptake which can seriously lower the tensile properties of the fibres themselves and thus lower the mechanical performance of bio-composites. To improve fibre/matrix interfacial bonding, chemical modifications have been considered for flax fibres. Alix et al. [26] performed five different chemical treatments, i.e. maleic anhydride (MA), acetic anhydride (Ac), silane (Si) and styrene (S), on flax fibres (cultivated in Hermes variety of the year 2004 in Normandy, France) to inves- tigate their effects on fibre tensile properties. It was found that the chemical treatments reduced the stiffness and the toughness of fi- bres, excepted for (Si) treatment (Table 4). It is believed that the significant enhancement in tensile properties with (Si) treatment is due to the possible grafting of silane (Si) with a long carbonyl chain between microfibrils. Besides, the removal of some sub- stances by chemical treatments leads to increase the ratio of cellu- lose in the material, the component which gives the mechanical properties of fibres. Fig. 6 gives the tensile stress–strain curves of the untreated and treated flax fibres. Obviously, the curve pattern of the untreated flax is similar to that given by Charlet et al. [15] in Fig. 3. The curves of these chemical treated flax fibres are also sim- ilar to the untreated one, indicating that the considered chemical treatment has insignificant effect on the curve pattern although fi- bre tensile strength and modulus are highly dependent on these chemical treatments. The tensile properties of flax fibres are not uniform along the length of a plant. Generally, in the stem the fibres are stronger and stiffer; at the mid-span and the tip the fibres have moderate properties. Flax fibres extracted from different locations in the stem also affect the tensile properties since fibres at different loca- tions have different chemical compositions and porosity, e.g. stud- ies by Charlet et al. [13,33]. Flax fibres in the study [13] are taken from long tows of the Hermes variety cultivated in 2002 and Aga- tha variety in 2003, respectively. These tows are issued from stems which have undergone retting, scotching and hackling operations. The tows, whose growth direction is known, are about 800 mm long. The top and the bottom zones are 150 mm long (Fig. 7). The fibres used for all the characterisations are taken in the middle of each zone of the tows (this corresponds to the same zone in the stems). Results indicated that the bottom fibres possess lower ten- sile properties than the others while the middle fibres exhibit the best ones. Biochemical analysis shows [33] that the variation in Table 2 Factors affecting the mechanical properties of flax fibres. Plant growth Specimens of plant, crop cultivation, crop geographical origin, fibre location in plant, local climate, e.g. rainfall and temperature during growth Harvesting stage Fibre ripeness, which effects: cell wall thickness, coarseness of fibres, adhesion between fibres and surrounding structure, size and shape of lumen, porosity, microfibril angle Fibre extraction stage Decortication process, type of retting method, separating conditions Supply stage Transportation conditions, storage conditions, age of fibres Measurement conditions Tensile speed, initial gauge length, moisture, temperature, different cross-section of fibres at different points Surface treatment Chemical treatment, upgrading treatment, water treatment, drying treatment, etc. Table 3 Average tensile properties of humidified flax fibres (reproduced with permission from [32]). Relative humidity (%) Flax fibre Fibre length (mm) Average tensile strength (MPa) Standard deviation (MPa) 30 Green 3.5 677 425 Duralin 809 134 66 Green 3.5 799 398 Duralin 1080 368 90 Green 3.5 818 318 Duralin 642 344 30 Green 8 619 461 Duralin 651 176 66 Green 8 760 390 Duralin 913 250 90 Green 8 761 369 Duralin 884 180 Table 4 Tensile properties of untreated and chemical treated flax fibres (reproduced with permission from [26]). Breaking strength (MPa) Breaking strain (%) Young’s modulus (GPa) Bundle diameter (lm) Number of fibres tested Untreated 300 ± 100 1.1 ± 0.4 30 ± 11 84 ± 20 23 MA 185 ± 60 1.2 ± 0.3 18 ± 5 88 ± 14 24 Ac 185 ± 85 0.8 ± 0.2 24 ± 10 77 ± 16 21 Si 555 ± 210 1.6 ± 0.6 40 ± 13 79 ± 13 22 S 245 ± 95 1.1 ± 0.4 28 ± 9 85 ± 17 21 The considered gauge length was 75 mm. 300 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 6. the tensile properties of the fibres from different locations is be- cause the middle fibres have the highest contents of cellulose; this supports the fact that cellulose of the natural fibre is the reinforce- ment material in the composite. One possible reason for lower ten- sile properties of fibre extracted from the bottom is that the bottom fibres should be more porous than the top fibres. The cells from the bottom part of the stem are likely stem not in the best environmental conditions to yield a dense core [33]. The tensile properties of flax fibres reported by Pillin et al. [27] are listed in Table 6. Compared to those in Tables 4–6, it is ob- served that the tensile properties of flax fibres are highly depen- dent on the gauge length considered for the measurement. The tensile strength of flax fibre decreases remarkably with an increase in gauge length, from 10 mm (Tables 5 and 6) to 75 mm (Table 4). Except for gauge length effect, the diameters of the fibres also im- pact the tensile properties. Young’s modulus and strength tend to decrease with the fibre diameter although a large scattering of the test results, as observed in [13,14,46]. Tables 4–6 also show that the tensile properties given by differ- ent authors scatter significantly. The dispersion of the fibre proper- ties is believed attributed to the variation in the cellulose content from an fibre to another and also as a result of the randomness of the location and size of defects along each fibre, rather than the scattering of the microstructural (with respect to fibre lumen diameter, porosity) of the flax fibres should be responsible for the scattering of the tensile properties [34]. In addition, the meth- ods of extraction of elementary fibres also lead to the scattering of the fibre properties. Physical treatments, such as stretching, thermo-treatment do not change the chemical composition of the fibres but change the fibre structure, surface properties and thereby influence the tensile properties of the fibres. le Duigou et al. [35] investigated the effect of water and drying cycle treatments on the tensile properties of flax fibres, respectively. For water treatment, flax fibres (cultivated in Hermès variety in France) were immersed in a distilled water bath at 23 °C for 72 h to clean the fibre surface, the fibres were then dried in air for 8 days in laboratory condition (23 °C and RH = 50%). Regarding to drying cycle treatment, flax fibres were dried at 105 °C for 14 h, then the fibres were dried in air for 8 days in lab- oratory condition (23 °C and RH = 50%). Results showed that the water treatment does not affect the fibre stiffness but results in a small drop in failure stress (À15%) and strain (À18%). However, the modulus of the drying cycle treated fibre drops by 20% (from 66.9 to 53.2 GPa), break stress by 45% (from 1057 to 601 MPa) and break strain by 33% (from 2.2% to 1.5%), compared to the un- treated one. The influence of drying in terms of fibre modification, reorganization of the microstructure and changes in pectin matrix/ cellulose microfibril interactions has been described in detail else- where [36,37], see Table 7. With regard to the moisture content or RH effect on the opti- mum properties of flax fibres for the use in composites, the mois- ture content at a given RH has a significant effect on the tensile performance of the composites made from flax fibres. Therefore, a drying process of flax fibres is an essential step, in despite of a reduction in tensile properties of flax fibres due to drying cycle has been observed. To have durable flax fibre reinforced composite with favourable tensile properties, suggestions are given on how to handle the fibres prior to or during manufacturing of the compos- ites: (1) during the separation of a single fibre cell from a bundle of fibre cells, damage of the fibre may occur, this may lead to a con- sequent decrease in the tensile properties of the fibres thus de- grades the mechanical properties of the composites [19], an effective fibre extraction processing should be considered, (2) fibre drying before processing is a significant step, and (3) an appropri- ate fibre treatment can be considered to reduce both the moisture content level very significantly. The use of one proper additive (coupling agents, lubricants, light stabilizers, colorants, flame retardants, foaming agents, odour reduction agents, and biocides) in very small quantities (0.5–5%) can significantly improve most of physical, chemical or mechancial properties of natural fibre rein- forced composite materials [84]. Cell wall defects in the fibre are also one of the most important parameters which determine the tensile properties of flax fibres. Fig. 7. Definition of the three locations of fibres in the oriented tows (reproduced with permission from [13]). Table 5 Tensile properties of flax fibres according to their location in the stem [adapted from 13,33,34]. Location Number of tested fibres Diameter (lm) Young’s modulus (GPa) Strength (MPa) Ultimate strain (%) Topa 36 19.0 ± 3.5 59.1 ± 17.5 1129 ± 390 1.9 ± 0.4 Middlea 37 19.6 ± 6.7 68.2 ± 35.8 1454 ± 835 2.3 ± 0.6 Bottoma 31 20.1 ± 4.1 46.9 ± 15.8 755 ± 384 1.6 ± 0.5 Topb 57 21.5 ± 5.3 51 ± 22 753 ± 353 1.8 ± 0.7 Middleb 45 21.3 ± 6.3 57 ± 29 865 ± 413 1.8 ± 0.7 Bottomb 59 21.3 ± 6.3 51 ± 26 783 ± 347 2.0 ± 0.9 –a 122 19.3 ± 5.5 63 ± 36 1250 ± 700 2.3 ± 1.1 The considered gauge length was10 mm. a Fibres from Hermes variety. b Fibres from Agatha variety. Table 6 Tensile properties of different oleaginous flax fibres [adapted from 27]. Number of fibres Diameter (lm) Tensile strength (MPa) Elastic modulus (GPa) Strain at failure (%) Hivernal 57 12.9 ± 3.3 1111 ± 544 71.7 ± 23.3 1.7 ± 0.6 Alaska 66 15.8 ± 4.1 733 ± 271 49.5 ± 3.2 1.7 ± 0.6 Niagara 71 15.6 ± 2.3 741 ± 400 45.6 ± 16.7 1.7 ± 0.6 Everest 76 21.2 ± 6.6 863 ± 447 48.0 ± 20.3 2.1 ± 0.8 Oliver 76 13.7 ± 3.7 899 ± 461 55.5 ± 20.9 1.7 ± 0.8 The considered gauge length was 10 mm. L. Yan et al. / Composites: Part B 56 (2014) 296–317 301
  • 7. Test results by Andersons et al. [38] supported well that tensile strength of flax fibre decreases with an increase in gauge length (Fig. 8). Furthermore, it was found that the fibre strength is to a large extent determined by cell wall defects as kink bands (Fig. 9), since the presence of kink bands limits the tensile strengths of flax fibres. Defect in fibre is also one reason for large dispersion in the tensile properties of flax fibres. The break of flax fibres under tension often occurs where the defect (i.e. kink band, as indicated by circle in Fig. 10) is situated, which has been ob- served by Bos and Donald [16] using the ESEM study. Tensile prop- erties of flax fibres reported by other different authors are also collected and displayed in Table 8. In all, when considering the environmental effects, higher RH values and high moisture uptake will degrade tensile properties of flax fibres significantly. Improving the poor environmental and dimensional stability of lignocellulosic materials is an effective way to modify the mechanical properties of these materials. With respect to measurement conditions, a longer gauge length de- creases the tensile strength of the flax fibres as a consequence of high risk of containing a defect (e.g. kink bands) where the occur- rence of fibre under tension is situated. A suitable chemical treat- ment such as silane (Si) can increase the tensile strength and strain of flax fibres. The tensile properties of flax fibre are not uniform along its length. Generally, in the stem the fibres are stronger and stiffer at the mid-span and the tip has moderate properties be- cause the fibres at middle and tip are rich in contents of cellulose. Also, the increase of fibre diameter tends to reduce the tensile strength and modulus of the fibres. All the test results indicate that the tensile properties reported by different authors scatter signifi- Fig. 8. Strength distribution of fibres A and B at 5 mm (a), 10 mm (b), and 20 mm (c) gauge length. Fibre A is produced by FinFlax Oy (Finland) and fibre B is produced by Ekotex (Poland) (reproduced with permission from [38]). Fig. 9. Kink bands in an elementary flax fibre as revealed by optical microscopy in transmitted polarised (a) and non-polarised (b) light in the same fibre fragment (fibre diameter is ca. 23 lm) (reproduced with permission from [38]). Fig. 10. Tensile fracture initiates at kink band of an elementary flax fibre. Scale bar of 50 lm (reproduced with permission from [16]). Table 7 Tensile properties of untreated, water and drying treated flax fibres [adapted from 35,37]. Fibre and reference Number of fibres tested Diameter (lm) Tensile strength (MPa) Elastic modulus (GPa) Strain at failure (%) Untreated [35] 98 25 1057 ± 462 66.9 ± 16.3 2.2 ± 0.8 Water treated [35] 98 25 913 ± 381 66.2 ± 15.0 1.8 ± 0.6 Dried [35] 98 25 601 ± 215 53.2 ± 7.0 1.5 ± 0.5 Untreated [37] 21 21.6 ± 1.0 1499 ± 346 64.1 ± 13.7 2.9 ± 0.7 Untreated [37] 23 23.9 ± 0.7 1317 ± 529 51.3 ± 12.0 3.3 ± 0.7 Dried [37] 23 20.9 ± 0.8 870 ± 266 59.2 ± 19.4 2.1 ± 0.3 Dried [37] 18 23.8 ± 0.7 711 ± 251 58.7 ± 15.9 1.7 ± 0.4 The considered gauge length was 10 mm. 302 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 8. cantly. The dispersion of the fibre properties is due to the variation in the cellulose content from an fibre to another and also due to the randomness of the location and size of defects along each fibre, rather than the scattering of the microstructural (with respect to fibre lumen diameter, porosity) of the flax fibres should be respon- sible for the scattering of the tensile properties. Additionally, the methods of extraction of elementary fibres also lead to the scatter- ing of the fibre properties. The selection of a suitable treatment such as Duralin treatment or drying cycle treatment offers a higher and more uniform strength of flax fibre with less scatter. 2.6. Comparison to glass and other bio-fibres The physical and tensile properties of various natural fibres and glass fibres are given in Table 9. Dittenber and GangaRao et al. [24] made a comparison between natural fibres with glass fibre in spe- cific Young’s modulus, cost per weight and cost per unit length to resist 100 kN load. The specific modulus was approximated using the average of the extreme values (the upper and lower values) of stiffness and the average of the extreme values of density found in the literature. It is observed that the specific Young’s modulus of Table 8 Physical and tensile properties of flax fibres by other authors. Diameter (lm) Relative density (g/cm3 ) Tensile strength (MPa) Elastic modulus (GPa) Strain at failure (%) Refs. 12–600 1.4–1.5 343–2000 27.6–103 1.2–3.3 [24] 10–60 1.52 840 100 1.8 [16] 10–60 1.52 1500 50 – [28] 76 ± 16 – 470 ± 165 37 ± 15 1.4 ± 0.5 [29] 17.8 ± 5.8 1.53 1339 ± 486 58 ± 15 3.27 ± 0.4 [14] – – 621 ± 295 51.7 ± 18.2 1.33 ± 0.56 [31] – – 600–2000 12–85 1–4 [40,41] – – 600–1500 50–80 1.4 [42] – 1.4 800–1500 60–80 1.2–1.6 [43] – 1.4–1.5 600–1100 45–100 1.5–2.4 [44,45] 12–34 – 1100 89 ± 35 – [46] 12.9 ± 3.3 – 1111 ± 544 71.7 ± 23.3 1.7 ± 0.6 [46] 15.8 ± 4.1 – 733 ± 271 49.5 ± 3.2 1.7 ± 0.6 [46] 15.6 ± 2.3 – 741 ± 400 45.6 ± 16.7 1.7 ± 0.6 [46] 21.2 ± 6.6 – 863 ± 447 48.0 ± 20.3 2.1 ± 0.8 [46] 13.7 ± 3.7 – 899 ± 461 55.5 ± 20.9 1.7 ± 0.6 [27] 15.8 ± 4.5 – 808 ± 442 51.1 ± 15.0 1.6 ± 0.4 [47] – – 365–1060 36.8–61.9 0.94–2.13 [48] 15 ± 0.6 1.53 1381 ± 419 71 ± 25 2.1 ± 0.8 [49] Table 9 Physical and tensile properties of natural fibres and glass fibres (reproduced with permission from [24]). Fibre type Diameter (lm) Relative density (g/cm3 ) Tensile strength (MPa) Elastic modulus (GPa) Specific modulus (GPa  cm3 /g) Elongation at failure (%) E-glass <17 2.5–2.6 2000–3500 70–76 29 1.8–4.8 Abaca – 1.5 400–980 6.2–20 9 1.0–10 Alfa – 0.89 35 22 25 5.8 Bagasse 10–34 1.25 222–290 17–27.1 18 1.1 Bamboo 25–40 0.6–1.1 140–800 11–32 25 2.5–3.7 Banana 12–30 1.35 500 12 9 1.5–9 Coir 10–460 1.15–1.46 95–230 2.8–6 4 15–51.4 Cotton 10–45 1.5–1.6 287–800 5.5–12.6 6 3–10 Curaua 7–10 1.4 87–1150 11.8–96 39 1.3–4.9 Flax 12–600 1.4–1.5 343–2000 27.6–103 45 1.2–3.3 Hemp 25–600 1.4–1.5 270–900 23.5–90 40 1–3.5 Henequen – 1.2 430–570 10.1–16.3 11 3.7–5.9 Isora – 1.2–1.3 500–600 – – 5–6 Jute 20–200 1.3–1.49 320–800 30 30 1–1.8 Kenaf – 1.4 223–930 14.5–53 24 1.5–2.7 Nettle – – 650 38 – 1.7 Oil palm – 0.7–1.55 150–500 80–248 0.5–3.2 17–25 Piassava – 1.4 134–143 1.07–4.59 2 7.8–21.9 PALF 20–80 0.8–1.6 180–1627 1.44–82.5 35 1.6–14.5 Ramie 20–80 1.0–1.55 400–1000 24.5–128 60 1.2–4.0 Sisal 8–200 1.33–1.5 363–700 9.0–38 17 2.0–7.0 Fig. 11. Comparison of potential specific modulus values and ranges between natural fibres and glass fibres (reproduced with permission from [24]). L. Yan et al. / Composites: Part B 56 (2014) 296–317 303
  • 9. flax is the second largest one followed by Ramie and the specific modulus of flax is greater than that of glass (Fig. 11). The compar- ison in cost per weight (Fig. 12) indicates that the unit price of flax fibre is also lower than that of glass fibres. Dittenber and GangaRao considered a better way to compare the costs of various fibres, as given in Fig. 13. In this figure, the range of values for cost per weight is multiplied by the range of values for the fibre density and an assumed 100 kN load and divided by the range of values for tensile strength. The resulting range of values indicates the po- tential cost per length of fibre material capable of resisting the 100 kN load. Based on the discussion, Ditterber and GangaRao [24] concluded that among various natural fibres, flax fibre offers the best potential combination of low cost, light weight, and high strength and stiffness for structural application. For structural application with bio-composites, the production yield of the fibre reinforcement should be sufficient. The estimated production volumes of several common used natural fibres which are commonly for composite fabrication are given in Table 10. It shows that cotton has the largest yield. However, cotton fibre in specific modulus and per unit cost is not desirable compared to flax, as shown in Figs. 11–13. Table 10 also shows that jute and flax also have the relatively high annual yield with favourable mechan- ical properties. Thus, when taking the cost, mechanical perfor- mance and yield into account, among various bio-fibres, flax, hemp and jute are the three most promising candidates that can be used to replace glass fibres in composite. 3. Polymer matrix In natural fibre/polymer composites, polymer matrix holds the fibres together to provide a shape and transfer the load to the fibres by adhesion and/or friction. Matrix also provides rigidity and shape to structural member, protects fibres from chemical and corrosion, influence the performance behaviours such as impact and ductility. The commonly used thermoplastic polymer matrix is polypropyl- ene (PP) and several synthetic thermoplastics such as polyethylene (PE), polystyrene (PS). The properties of the thermoplastics are listed in Table 11 [50]. The primary thermoset resins used are poly- ester, vinyl ester, and epoxy resins. A comparison of the typical thermoset properties is provided in Table 12 [50]. Thermoplastics have many advantages over thermoset polymers in bio-composites fabrication such as low processing, design flexibility, and ease of moulding complex parts. However, the development of thermo- plastic natural-fibre composites is restricted by the processing temperature. Generally, the temperature should be below 230 °C to avoid degradation of bio-fibres, e.g. PP and PE. Among the ther- moplastic polymers, PP is the most widely used in bio-composites due to its low density, good mechanical properties, relatively high temperature resistance, excellent processibility, and good impact resistance (e.g. studies in [53,55,56,58]). Although thermoplastic materials currently dominate as matrices for bio-fibres, nowadays more and more researchers are looking more toward to thermo- sets. This is because thermoset polymers outperform thermoplas- tics in some areas, including mechanical properties, chemical resistance, thermal stability, and overall durability. In addition, thermosets allow for more flexibility in structural fibre configura- tions and can be processed at room temperature or at tempera- tures comfortably within the safe range for natural fibres. Among thermosets, epoxy is the most common one (e.g. studies in [1,13,44,53,60,61,63]). Epoxy resins offer high mechanical perfor- mance (with respect to tensile strength and modulus, and com- pressive strength) and solvent resistance to environmental degradation. Vinyl ester is also widely used for its excellent chem- ical resistance, good thermal (better moisture resistance than epoxy when cured at room temperature) and impact properties (e.g. study in [57]). Most recently, the research of bio-fibres reinforced with biode- gradable polymers ‘‘green’’ composites has increased substantially. The biodegradable polymers can be classified based on the origin: naturally occurring or synthetic. Natural polymers are available in large quantities from renewable sources while synthetic polymers are produced from non-renewable petroleum-based resources [78]. Some of the biodegradable polymers are polyesteramide (PEA), polyhydroxybutyrate (PHB) [70,71], polyhydroxybutyrate- co-hydroxyvalerate (PHBV), polyactides (PLA) (e.g. study in [56]), and soy protein isolate resin (SPI) (e.g. study in [59]), as given in Table 13. One main limitation of these polymers is the high initial cost at this stage. Most biodegradable resins currently cost three to five times the commonly used resins such as PP, LDPE, and HDPE [65,66]. 4. Flax fibre reinforced composites Flax fibres as reinforcement material of composite are not only considered in the form of monofilament configuration [51,52]. Monofilament fibres are further processed into mats [e.g. studies in 44,53–55], rovings [e.g. studies in 56,57], yarns [e.g. studies in 58,58], and fabrics [e.g. studies in 1,60–63] in composites (Fig. 14). To date, a variety of manufacturing techniques have been developed to produce composites, such as film stacking [e.g. study in 53], vacuum infusion [e.g. studies in 1,60], hand lay-up [e.g. study in 61], compression moulding [e.g. studies in 36,51,52,54], filament winding [e.g. study 58], manual winding [e.g. study in 60], resin transfer moulding (RTM) [e.g. studies in 44,57], injection moulding [e.g. study in 51], and pultrusion [e.g. study in 56,73]. Fig. 12. Cost per weight comparison between glass and natural fibres (reproduced with permission from [24]). Fig. 13. Cost per unit length (capable of resisting 100 kN load) comparison between glass and natural fibres (reproduced with permission from [24]). 304 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 10. When selecting a manufacturing technique, the parameters includ- ing the targeted properties, size and shape of the composites, the properties of raw materials and manufacturing cost all should be taken into account [77]. The size of a composite is treated as a dominating factor for composite fabrication. For preliminary eval- uation of composites with small size, injection and compression mouldings are preferred as a consequence of their simplicity and fast processing period. For structure with large size, open moulding and autoclave processes (e.g. RTM and hand lay-up) are essential. Some manufacturing techniques are excluded for composites with specified shapes. Filament winding is the most suitable method for manufacturing composites pressure vessels and cylinders where the fibres normally are in the form of yarn [77]. Pultrusion is mainly used for producing long and uniform cross-section parts. In injection moulding, fibres are usually chopped into short accord- ing to the critical fibre length in which the stress should be fully transformed from the matrix to the fibre and the fibre can be loaded to its full capacity assuming a good interfacial bonding is achieved; the amount of the mixture can be pre-designed. Com- pression moulding technique is a combination of hot-press and autoclave processes. The fibres are usually in the forms of chopped fibres and mat. Hand lay-up is a labour-intensive process which is easy to deal with and cost effectively, it is widely used in civil infra- structure to retrofit and strengthen structure with carbon or glass fibre reinforced composites. Liquid composite moulding technique includes RTM, vacuum infusion, structural reaction injection moulding, and other subsets where the basic approach is to sepa- rately inject and liquid resin into a bed of stationary preforms [77]. The RTM and vacuum infusion enables the production of com- posites with high volume fraction and better strength-to-weight ratio [1]. The fibre preforms normally are fabric and mat. In partic- ular, theoretically, there is no limitation on the size of composites with RTM and vacuum infusion processes, which is critical for practical engineering application. It is well known that the fibre length or aspect ratio (length-to- diameter) has a great impact on the processing techniques. Gener- ally, long fibres have lower tensile properties than the short fibres. On the other hand, the manufacturing method also has a great influence on the remaining flax fibre length and length distribu- tion, which in turn influences greatly the tensile properties of the composites [51]. The critical length of a fibre in composite is deter- mined by fibre fracture, interfacial bonding strength, interfacial debonding, and interface friction and matrix plastic deformation. The higher the tensile strength of the fibre and the better the fi- bre/matrix adhesion, the shorter the minimum fibre length re- quired for effective transfer of the stress. Therefore, it is difficult to determine the optimal fibre length exactly [89]. For short flax fibres in composites considering injection mould- ing technique, the critical fibre length can be determined by using the Kelly and Tyson theory and the examples were discussed in [90]. Flax fibre reinforced with PP matrix is one widely used com- posite. It was found that flax/PP composites refer mainly on two Table 10 Estimated global production volume averages of different natural fibres (in million metric tons per year). Fibre type Production per year (million tonnes) Main producer countries Abaca 0.10 Philippines, Equator Cotton 25 China, USA, India, Pakistan Coir 0.45 India, Sri Lanka Flaxa 0.50–1.5 China, France, Belgium, Belarus, Ukraine Hempb 0.10 China Henequen 0.03 Mexico Jute 2.5 India, Bangladesh Kenaf 0.45 China, India, Thailand Ramie 0.15 China Silk 0.10 China, India Sisal 0.30 Brazil, China, Tanzania, Kenya a The real production of flax was underestimated because the production of flax in Canada is not considered for calculation. b China has announced plan to substantially increase the hemp production for textiles in the coming years to 1.5 million tonnes of fibre per year. Table 11 Properties of typical thermoplastic polymers used in natural fibre composite fabrication (reproduced with permission from [50]). Properties PP LDPE HDPE PS Density 0.899–0.920 0.910–0.925 0.94–0.96 1.04–10.6 Water absorption (24 h@20°C) 0.01–0.02 <0.015 0.01–0.2 0.03–0.10 Tg (°C) À10 to À230 À125 À133 to À1000 N/A Tm (°C) 160–176 105–116 120–140 110–1350 Heat deflection temp. (°C) 50–63 32–50 43–60 Max. 220 Coefficient of thermal expansion (mm/mm/°C Â 105 ) 6.8–13.5 10 12–13 6–8 Tensile strength (MPa) 26–41.4 40–78 14.5–38 25–69 Elastic modulus (GPa) 0.95–1.77 0.055–0.38 0.4–1.5 4–5 Elongation (%) 15–700 90–800 2.0–130 1–2.5 Izod impact strength (J/m) 21.4–267 >854 26.7–1068 1.1 PP = polypropylene, LDPE = low density polyethylene, HDPE = high-density polyethylene and PS = polystyrene. Table 12 Properties of typical thermoset polymers used in natural fibre composites (repro- duced with permission from [50]). Property Epoxy Polyester Vinyl ester Density (g/cm3 ) 1.1–1.4 1.2–1.5 1.2–1.4 Elastic modulus (GPa) 3–6 2–4.5 3.1–3.8 Tensile strength (MPa) 35–100 40–90 69–83 Compressive strength (MPa) 100–200 90–250 100 Elongation (%) 1–6 2 4–7 Cure shrinkage (%) 1–2 4–8 N/A Water absorption (24 h@20°C) 0.1–0.4 0.1–0.3 0.1 Izod impact strength (J/m) 0.3 0.15–3.2 2.5 L. Yan et al. / Composites: Part B 56 (2014) 296–317 305
  • 11. manufacturing routes, (1) mat technology, and (2) compound tech- nology [51]. Barkoula et al. [51] suggested a fibre length of approx- imately 25 mm for flax in the random mat using compression moulding technique and the short chopped fibre with a length of 10 mm for injection moulding technique. A similar fibre length (approximately 12 mm) of flax was suggested by Li and Sain [91] when manufacturing flax/PP composites considering the injection moulding technique. Peijs et al. [92] also recommended a fibre length of 25 mm for flax mat/PP matrix NMTs composites based on a film stacking method. With regard to the compounding pro- cess, it affects the shortening, fibrillation and the thermal deterio- ration of the fibres in early stages, the final properties of the product are already determined at the beginning of the production process [93]. Specht et al. [94] suggested the optimum lengths of Table 13 Properties of some biodegradable polymers used in bio-composites [56,67–71]. Property PEA PLA SPI PHB PHBV Density (g/cm3 ) 1.18 0.9–1.27 1.2–1.5 1.25 1.25 Elastic modulus (GPa) 0.42 1.5–2.7 0.1 0.93 2.38 Tensile strength (MPa) 16.4 60 6.0 21 25.9 Elongation (%) 85–119 8 170–236 5.2–8.4 1.4 Melting temperature (°C) 175 160–190 4–8 161 153 Glass transition temperature (°C) – 56–65 À10 À1 Fig. 14. Flax configuration in composite (a) mat, (b) roving, (c) fabric, (d) monofilament fibre and (e) yarn [adapted from 1,56,62]. 306 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 12. natural fibres for different compounding processes, i.e. for pelletiz- ing (with matrix), mixing (cascade mixing) and extruder com- pounding, the fibre length should be less than 3 mm. With respect to pultrusion or pull-drill-process of bast fibres, e.g. flax, the fibre length is good with the range between 10 and 30 mm. For hybrid fibre non-woven pre-consolidation and cut process, the suitable fibre length is less than 25 mm. 4.1. Flax mat reinforced polymer composites Oksman [44] studied the mechanical properties of traditionally retted unidirectional (UD) flax/epoxy composites and UD Arctic- Flax/epoxy using the RTM technique. Results showed that the (50/50) high quality ArcticFlax/epoxy composite has a stiffness of about 40 GPa and tensile strength of 280 MPa, as listed in Table 14. RTM showed to be a suitable processing technique for natural fibre composites when high quality laminates are preferred. Van de Weyenberg et al. [53] studied the effect of alkaline treat- ment on the flexural properties of UD flax mat reinforced epoxy composites using film stacking process (Fig. 15). Results indicated that alkalisation of flax fibres is a simple and effective method to enhance the fibre/epoxy matrix bonding thus improving the flex- ural properties of UD flax/epoxy composites (Table 15). Theoretically, high tensile strength of a natural fibre reinforced composite could be achieved by increasing the amount of the fibre used. Singleton et al. [54] investigated the effect of fibre volume fraction on the mechanical properties of flax mat/recycled HDPE composites by film stacking and compression moulding. It was ob- served that the tensile strength and modulus increased with an in- crease in fibre volume fraction (0%, 10%, 18%, 20% and 30%). However, the tensile strain at failure of the flax/HDPE composite decreased when the fibre volume increased. This is because the HDPE matrix had breaking strain more than 20% when failed. Char- py impact test indicated that the inclusion of flax mat increased the impact toughness of the composite significantly compared with pure HDPE while the largest impact energy occurred when the fibre volume fraction at 10%. The large toughness enhancement is believed attributable to a number of deformation and mecha- nisms acting in the notch tip process zone (termed Zone 1) and in the crack wake zone (termed Zone 2). The deformation in Zone 1 includes: (1) plastic deformation of the thermoplastic matrix; an example is by the nucleation, growth and coalescence of micro- voids indicated by ‘stress whitening’; (2) delamination cracking at or in front of the crack tip between plies (layers) of fibre and ma- trix and at polymer–polymer interfaces. The crack wake mecha- nisms in Zone 2 include: (1) crack bridging by the flax fibres; (2) crack bridging by highly ductile microscopic-sized ligaments of the polymer; (3) fibre slippage, fibre deformation, cracking, split- ting and fracture and fibre pull-out. Fibre surface condition is critical for the interfacial bond be- tween fibre and matrix. John and Anandjiwala [55] studied the ef- fect of Zein modification (2% solution) on the mechanical properties of flax mat/PP composites which fabricated using a compression moulding. It was found that the modification in- creased the tensile and flexural strength as a result of the improve- ment in interfacial bonding (Fig. 16). However, the modification decreased the impact strength of the composites. The decrease in impact strength may be interpreted by assuming that a better fi- bre/matrix adhesion results in shorter average pull-out lengths of the fibres. 4.2. Flax fabric reinforced polymer composites Yan et al. [1] studied the effect of alkali treatment on the mechanical properties of flax fabric reinforced epoxy composites fabricated using a vacuum bagging technique. SEM study indicated that the failure of fibre yarns along the load direction, debonding, fibre pull-out and brittle fracture of the matrix are the dominated failure mechanisms of flax fabric/epoxy composites (Fig. 17). Alkali treatment is beneficial to clean the fibre surface, modify the chem- istry on the surface, lower the moisture up take and increase the surface roughness. The treatment removes the impurities and waxy substances from the fibre surface and creates a rougher topography which facilitates the mechanical interlocking. Also, the purified fibre surface further enhances the chemical bonding between fibre and matrix. Alkali treatment improves the flax fi- bre/matrix adhesion thus increasing the tensile properties of flax fabric reinforced epoxy composites. However, the fracture tough- ness of flax fabric reinforced epoxy composite is dominated by the fibre volume fraction, rather than the reinforcement architec- ture. An improved in yarn and textile design leads to a superior balance of stiffness, strength and toughness of flax fabric rein- forced composites [60]. Assarar et al. [61] compared the tensile properties of flax- and glass-fabric reinforced epoxy composites which were fabricated by a hand lay-up process. It was found that the tensile strength of flax composites reached up to 380 MPa – making it close to that of glass-fabric reinforced epoxy composites. Liang et al. [63] made a comparative study of fatigue behaviour of flax fabric reinforced epoxy (FFRE) and glass fabric reinforced epoxy (GFRE) composites. Both composites are made of dry rollers of non-crimp fabrics with areal weights of flax of 235 g/m2 and glass of 434 g/m2 . Two stacking sequences of composites, i.e. [0/ 90]3S and (b) [±45]3S, were fabricated. The measured thickness and fibre volume fraction of FFRE and GFRE are 2.18 mm and 43.7%, and 2.33 mm and 42.5%, respectively. The tensile stress– strain curves of the composites are given in Fig. 20. The tension– tension fatigue loading test results indicated that the specific stress-number of cycles to failure curves, show that for the [0/ 90]3S specimens (Fig. 18), FFRE has lower fatigue endurance than GFRE, but the [±45]3S FFRE specimens offer better specific fatigue endurance than similar GFRE, in the studied life range (<2 Â 106 ), as shown in Fig. 19. 4.3. Flax roving reinforced polymer composites Nowadays there is a new interest in the area of developing fully biodegradable ‘‘Green’’ composites. It is generally believed that the ‘‘Green’’ composites are one of the key materials in all industries in coming centuries [77]. Oksman et al. [56] studied the flax roving as reinforcement in polylactic acid (PLA) polymer. Because of the brit- tle nature of PLA, triacetin was tested as plasticizer for PLA and PLA/flax composites to improve the impact properties. The mechanical properties of flax/PLA and flax/triacetin/PLA compos- ites were compared with flax/PP composites. All the composites were manufactured using a twin-screw extruder. The considered flax fibre content was 30 and 40 wt.%. It showed that the tensile strength of flax/PLA composite is about 50% better compared to similar flax/PP composites. Microscopy study showed a poor flax /PLA interfacial adhesion. Triacetin plasticizer did not improve the composite impact properties; it rather had a negative effect on tensile properties, as shown in Fig. 20. Andersons and Joffe [57] investigated the tensile strength of an UD flax roving/vinyl ester composite produced by the RTM tech- nique. Three different fibre rovings were considered. Roving N1 was made of fine processed long fibres used in textile industry. Roving N2 was produced from short flax fibres obtained as by- product from manufacturing of textile grade fibres. The twist indi- ces for N1 and N2 were about 100 turns/m. Roving N3 was over- twisted N2 (i.e. two N2 rovings loosely, about 50 turns/m, twisted together). Study indicated that the tensile strength of flax roving/ vinyl ester composite is a function of fibre volume fraction, the L. Yan et al. / Composites: Part B 56 (2014) 296–317 307
  • 13. average strength of long-fibre roving exceeds that of short-fibre roving by almost 40% at fibre volume of 17% (Fig. 21). SEM study showed the traces of fibre pull-out at the fracture surface, which corroborated the hypothesis that the mechanical interlocking and friction are the predominant mechanisms of apparent adhesion of plant fibres. 4.4. Flax monofilament fibre reinforced polymer composites Flax monofilament fibres as reinforcement have been consid- ered widely with various polymers. Barkoula et al. [51] studied the effect of fibre volume content (0%, 20%, 30% and 40%) and Table 14 Absolute and specific properties of composite and pure epoxy resin (reproduced with permission from [44]). Sample Fibre type Volume fraction (%) Density (g/cm3 ) Tensile strength (MPa) Specific strength (MPa/g cmÀ3 ) Elastic modulus (GPa) Specific modulus (GPa/g cmÀ3 ) Elongation at break (%) Epoxy – – 1.15 76 66 3.1–3.2 2.7 7.3 ArcticFlax/epoxy 1 ArcticFlax 21 1.22 193 ± 30 158 22 ± 4 18 0.9 ArcticFlax/epoxy 2 ArcticFlax 42 1.24 280 ± 15 221 35 ± 3 28 0.9 ArcticFlax/epoxy 3 ArcticFlax 47 1.32 279 ± 14 211 39 ± 6 29 0.8 UD-Flax/epoxy Flax 32 1.23 132 ± 4.5 107 15 ± 0.6 12 1.2 Fig. 15. Illustration of the film stacking method for the production of UD flax-epoxy composites (reproduced with permission from [53]). Table 15 Flexural properties of UD untreated and treated flax-epoxy composites (reproduced with permission from [53]). Treatment (wt.%) Longitudinal direction Transverse direction Tensile strength (MPa) Young’s modulus (GPa) Tensile strength (MPa) Young’s modulus (GPa) Untreated 218 ± 18 18 ± 3 8 ± 8 0.4 ± 0.2 1% NaOH 237 ± 12 23 ± 1 20 ± 4 2.3 ± 0.2 2% NaOH 261 ± 13 20 ± 2 15 ± 2 1.1 ± 0.1 3% NaOH 283 ± 20 22 ± 2 19 ± 4 1.2 ± 0.1 Fibre volume fraction is 40%. Alkali treated flax fibres for 20 min. Fig. 16. Effect of Zein modification on mechanical properties of composite (fibre vol. 30%) (reproduced with permission from [55]). Fig. 17. SEM micrograph of typical failure modes of untreated flax fabric reinforced composite in tension. (A) Failure of fibre; (B) fibre pull-out; (C) brittle fracture of epoxy matrix and (D) fibre debonding (reproduced with permission from [1]). 308 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 14. hydroxyvalerate weight content (0%, 8%, and 12%) on the mechan- ical properties of flax/polyhydroxybutyrate (PHB)/HV composites. In addition, the effect of manufacturing method (compression moulding of natural-fibre-mats (NMT) and injection moulding of short fibre compounds) on the mechanical performance of the flax/PHB composite was investigated. Results showed that the im- pact resistance and Young’s modulus of flax/PHB increased with an increase in fibre volume fraction, while the strength of flax/HPB/ Fig. 18. P–S–N (a) and specific S–N (b) behaviour of [0/90]3S FFRE and GFRE specimens (reproduced with permission from [63]). Fig. 19. S–N (a) and specific S–N (b) behaviour of [±45]3S FFRE and GFRE specimens (reproduced with permission from [63]). Fig. 20. Tensile properties: Flax/PLA vs. flax /PP tensile stress (a) and tensile modulus (b), tensile stress (c) and tensile modulus (d) of PLA with 5%, 10% and 15% triacetin and 40 wt.% flax fibres (reproduced with permission from [56]). L. Yan et al. / Composites: Part B 56 (2014) 296–317 309
  • 15. HV composite is almost constant with increasing fibre volume frac- tion (Fig. 22). There is no significant effect of processing methods on the stiffness, strength and elongation of the composite (Fig. 23). Modniks and Andersons [52] used a FEM model to predict the elastic properties of short flax/PP composites manufactured by compression moulding. The average length of flax fibre is 1.2 mm, the average apparent fibre diameter is 16 lm, and the lon- gitudinal stiffness of the fibres is 69 GPa for the shortest fibre length of 10 mm. Plain PP and PP modified by maleic anhydride grafted PP (MAPP) were used as matrices. The fibre volume frac- tions amounted to 0.13, 0.21, and 0.29. An elementary flax fibre is modelled as a cylindrical body, as shown in Fig. 24a. Its morphological layers are re-grouped for mechanical analysis, resulting in a three-layer cylinder with a lu- men as shown in Fig. 24b. The outermost layer of the model com- prises the primary cell wall and the outer layer, S3, of the secondary cell wall. The middle layer of the model corresponds to the thicker layer S2 of the secondary cell wall comprising heli- cally oriented cellulose fibrils, and the innermost model layer is the S1 layer of the secondary cell wall. The lumen radius was se- lected so that the lumen accounted for 1.5% of the fibre cross-sec- tion area [14]. The outer and inner model layer, L1 and L3, thickness was chosen at 1% of the fibre radius. The middle layer (S2) was treated as a unidirectional cellulose fibril composite, with reinforcement direction along a helix at a fixed angle to the fibre axis, as in [14]. A single fibre embedded in a block of matrix, Fig. 24c, was chosen as a unit cell (UC) of short-fibre-reinforced composite material. The matrix was assumed to cover the fibre so that the surfaces of the UC were at the same distance, c from fi- bre surface, see Fig. 24d. The method was applied to flax/PP composite with nearly uni- form fibre orientation distribution and a good agreement with experimentally determined stiffness was observed. Charlet et al. [13] studied the effect of volume fraction and fibre location on the tensile properties of UD flax/epoxy composites. The definition of bottom, middle and top location of the fibres was gi- ven in Fig. 7. The study of the tensile behaviour of UD composites as a function of the fibre content shows that the stiffness (Fig. 25a) and the strength (Fig. 25b) increase quasi linearly, whereas the ultimate strain remains nearly constant beyond Vf = 15% (Fig. 25c). The study of effect of fibre location on the tensile prop- erties of composites indicates that the composites reinforced with the bottom fibres exhibit the lowest properties in terms of stiffness and strength, in comparison with the other samples. These results are in accordance with those obtained from single fibre tensile tests (Table 5). The large scattering of the results observed in Figs. 25(d–f) can be ascribed to some particularities of flax fibres such as kink bands which are geometrical singularities that bring about stress concentrations in the fibres and in the matrix and con- Fig. 21. Effect of fibre volume fraction on the tensile strength of UD composite with different fibre rovings (reproduced with permission from [57]). Fig. 22. Effect of fibre and HV content on (a) Young’s modulus, (b) tensile strength, (c) elongation at break and (d) Izod impact resistance of flax/PHB/HV composites (reproduced with permission from [51]). 310 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 16. centric layers of cell walls in which cracks can easily develop as a consequence of a poor internal adhesion. In addition, the huge vari- ations of diameter along a single fibre are likely to contribute to the scattering of the mechanical properties not only for the single fi- bres but also for the composites. 4.5. Flax yarn reinforced polymer composites Rask et al. [58] applied synchrotron X-ray tomographic micros- copy (XTM) to observe in situ damage evolution in UD flax fibre yarn/PP composites loaded in uniaxial tension at stress levels be- tween 20% and 95% of the ultimate failure stress (about 110 MPa). Composites were manufactured using filament winding technique and followed by press consolidation. The geometry of the specimens is notched to have a non-uniform stress field (Fig. 26). Three dominating damage mechanisms have been identi- fied: (1) interface splitting cracks typically seen at the interfaces of bundles of un-separated fibres, (2) matrix shear cracks, and (3) fi- bre failures typically. It was observed that interface splitting cracks are found to initiate from the notches at 60 MPa. Matrix shear Fig. 23. (a) Young’s modulus, (b) tensile strength, (c) elongation at break and (d) Izod impact resistance of flax/PHB composites as a function of fibre volume fraction and manufacturing method (reproduced with permission from [51]). Fig. 24. FEM model of a flax fibre (a) and a fibre cross-section (b) showing model layers L1 (contains the primary cell wall and layer S3 of the secondary call wall), L2, and L3, that coincide with the morphological layers S2 and S1 of the secondary cell wall. Schematic of the unit cell comprising a fibre embedded in a block of matrix (c) and its orthogonal cross-sections (d) (reproduced with permission from [52]). L. Yan et al. / Composites: Part B 56 (2014) 296–317 311
  • 17. cracks initiate at 90 MPa. Fibre failures occur at 110 MPa. The given nominal stress values depend strongly on specimen geometry and fibre configuration, in addition to the properties of fibres, matrix, and interface. A study by Lodha and Netravali [59] concluded that the tensile properties of UD flax yarn/soy protein isolate (SPI) resin ‘‘green’’ composites were improved significantly by the treatment of a poly-carboxylic acid based modifier PhytagelÒ . Two PhytagelÒ modified SPI (PM-SPI) resins were used for making the composites: PM-SPI-4 resin, containing 40% PhytagelÒ and 12.5% glycerol and PM-SPI-2 resin, containing 20% PhytagelÒ and 12.5% glycerol. The composites were fabricated using manually winding. Lodha and Netravali also used the rule of mixture (ROM) to predict the theo- retically tensile properties of the composites. As shown in Table 16, ROM cannot predict the tensile properties of the composites be- cause the tested yarns lost some degree of alignment and acquired a wavy/non-straight orientation. In addition, as the resin shrank as the water evaporated during procuring and curing. Compared with the tensile properties of flax UD yarn/SPI composites [59] with flax UD yarn/PP composites given in [58], it is observed that the tensile strength of flax/SPI is significantly larger than that of flax/PP although the strengths of these two different matrices are Fig. 25. Influence of fibre volume fraction on the tensile properties of UD composites (fibre in middle location): (a) Young’s modulus; (b) strength and (c) ultimate strain, and influence of fibre location on the tensile properties of composites (fibre volume fraction of 20%): (d) Young’s modulus; (e) strength and (f) ultimate strain (reproduced with permission from [13]). Fig. 26. Schematic presentation of the characteristic damage mechanisms in flax yarn/PP composites (reproduced with permission from [58]). 312 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 18. insignificant, this may be attributable to the fact that fibre/SPI has a stronger interfacial adhesion because of highly polar groups on both cellulose of fibre and SPI polymer. 5. Environmental assessment of flax fibres With increasing environmental awareness, the application of natural fibres, e.g. flax, is growing rapidly due to market demands for ‘‘green’’ products. This calls for the investigation of the environ- mental performance of natural fibre reinforced composites. To this end, life cycle assessment (LCA) can be applied as a standardised method to quantify environmental impacts [95]. LCA analysis al- lows determining a detailed overview of all the environmental im- pacts related to products and/or processes, by a ‘‘cradle to grave’’ approach, thus determining all of the mass and energy flows deriv- ing from the manufacturing of a product, but also from the use dur- ing its life cycle. All these steps and flows are then linked to their direct and indirect environmental impacts, which must be deter- mined quantitatively. The entire process is regulated by ISO 14040 to 14043 standards [96]. The earlier LCA (ISO, 1997–1999 [97] and CML-2000 [98]) con- sists of four independent elements including: (1) the definition of goal and scope, (2) the life cycle inventory analysis, (3) the life cy- cle impact assessment, and (4) the life cycle interpretation. Later, a standard procedure (ISO-14044 [99]), a part of a global Eco-Design approach, was proposed to evaluate the environmental impacts throughout the life of a material or product based on the definition of a Functional Unit. In the last fifteen years, some studies based on LCA have been performed on flax fibres and/or its composites. Diener and Siehler [100] used LCA to deal with under-floor pan- els made from glass fibre mat reinforced PP and flax mat reinforced PP for Mercedes A class. The study showed that the flax reinforced panel scores better for all environmental impacts studied. For se- ven out of the ten impact categories, i.e. global warming (GWP), acidification (AP), eutrophication (EP), Ozone precursors, toxicity air, toxicity water, non-renewable energy, the environmental im- pact is reduced by close to 20%, in the remaining three cases (i.e., Ozone depletion, waste and resources) the reduction of impacts is higher (30–80%). These reductions of environmental impacts re- flect the fact that the manufacture of flax fibre mats required 80% less energy than fibreglass mats, the total energy savings for the entire component are smaller (14%) since the overall environmen- tal impact is dominated by PP input. Van der Werf and Turunen [101] quantified major environmen- tal impacts associated with the production of flax and hemp textile yarn by using LCA. It was found that the impacts of the hemp ref- erence scenario (traditional hemp warm water retting) and the flax scenario (dew retting of flax) were similar, except for the pesticide use (higher than flax) and water use during processing (higher for hemp). Later, in a study by González-García et al. [102], CML base- line 2000 methodology was selected to quantify the potential envi- ronmental impact associated with the production of flax and hemp fibres for speciality paper pulp. Specifically, GWP, AP, EP and pho- tochemical oxidant formation were evaluated. In addition, two flow indicators were considered: energy and pesticide use. System boundaries were covered from soil management up to straw pro- cessing and transportation of fibre bales to pulp mill. Production of all inputs for each system (fertilizers, pesticides, seeds, energy carriers) and their supply was also included, as well as machinery production, use and maintenance. It was found that production of hemp fibre reported higher values for all the impact categories analysed. On the contrary, flow indicators were more intensive in the flax scenario due to irrigation and pesticide consumption, as that observed by Van der Werf and Turunen [101]. Deng et al. [103] revealed that the overall weighted environ- mental scores of printed circuit boards (PCB) from flax/epoxy com- posites are significantly lower than the conventional glass/epoxy composites, especially in impact categories of climate change (60%), human toxicity (40%), fossil resources depletion (55%), pho- tochemical oxidant formation (45%) and freshwater eutrophication (58%), indicating bio-based materials as PCB substrate offer promising perspectives for final replacement of the conventional materials. Le Duigou et al. [86] noted that flax fibres consume little energy during their production (11.4 MJ/kg for Hackled flax fibres) compared to the same quantity of glass fibre (48.3 MJ/kg for glass fibre and 54.7 MJ/kg for glass mat [100]). Natural fibres, as flax fibres, have also the potential to store carbon dioxide (temporarily) during their growth resulting in CO2 emission reduc- tions [104]. A most recent study by Le Duigou et al. [88] evaluated the envi- ronmental impact of flax-based composites and flax-based sand- wich materials production using simplified LCA following the ISO 14044 standard. The flax mat reinforced PLLA composite and flax mat/PLLA/balsa bio-sandwich panels were compared with the ref- erence materials, glass mat reinforced unsatured polyester and glass mat/unsatured polyester/balsa sandwich. The study indicated that the bio-sandwich materials are very attractive in terms of environmental impacts. Further improvements in bio-composite and bio-sandwich mechanical strength are necessary if they are to be used in transport application compared to glass/polyester and glass/polyester/balsa sandwich. Dissanayake et al. [105] also confirmed that flax fibre produced by no till and warm water retting has an embodied energy of 59 GJ/ tonne of sliver (vs. 55 GJ/tonne for glass mat). The spinning process raised the embodied energy for flax yarn to 86 GJ/tonne (vs. 26 GJ/ tonne for continuous glass fibre). The validity of the ‘‘green’’ case for replacement of glass fibres by natural fibres is dependent on the chosen reinforcement form and associated processes. No-till method with water retting is identified as the most environmen- tally friendly for seven out of eight impact classification factors. To improve the case for flax fibres, the principal recommendation is for the use of organic fertiliser, biological control of pests and Table 16 Tensile and flexural properties of untreated and treated flax yarn/SPI composites (reproduced with permission from [59]). Samples Tensile strength (MPa) Elastic modulus (GPa) Predicted strength (MPa) Predicted modulus (GPa) Fracture strain (%) Flexural strength (MPa) Flexural modulus (MPa) Flexural strain (%) Yarna 360.2 13.0 – – 4.8 – – – SPIa 6.0 0.10 – – 206 – – – PM-SPI-2a 42.6 0.66 – – 28.9 – – – PM-SPI-4a 60.0 0.90 – – 19.5 – – – Flax yarn/SPI 197 ± 15 2.4 ± 0.3 160 5.7 11.2 ± 1.1 49 ± 9 2.8 ± 0.6 3.1 ± 1.0 Flax yarn/PM-SPI-2 220 ± 29 4.1 ± 0.2 190 6.4 7.5 ± 0.7 105 ± 9 7.8 ± 1.3 2.3 ± 0.6 Flax yarn/PM-SPI-4 174 ± 20 3.1 ± 0.4 200 6.5 8.8 ± 1.0 52 ± 8 4.5 ± 0.7 1.8 ± 0.2 a Average tensile property is listed, standard deviation is not considered. L. Yan et al. / Composites: Part B 56 (2014) 296–317 313
  • 19. conservation agriculture. The key consideration for reducing en- ergy consumption and impact potentials associated would be to produce aligned fibre reinforcement without the need for the en- ergy intensive spinning operation [105]. Based on the discussion above, to reduce the environmental im- pacts on the production of flax fibres as reinforcement for compos- ites, several agricultural operations can be considered by using no- till method in ground preparation, using organic fertiliser and bio- logical methods to control pets. Warm water retting can be adopted as an effective fibre extraction method. In addition, con- sidering the production of aligned fibre reinforcement without the need for energy intensive spinning operation is a key point to reduce energy consumption. The LCA concept sounds simple, but in fact, in reality the analysis is quite complex, primarily due to the difficulty in establishing the correct system boundaries, obtain- ing accurate data and interpreting the results correctly [106]. The future LCA on flax fibres should also consider some environmental impacts such as land and water use, impacts on biodiversity and soil fertility, soil erosion, noise and vibration [107]. The fully envi- ronmental superiority of bio-composites compared to synthetic fi- bre composites is still questionable because of their relatively excessive processing requirements, which in turn consume more energy. Therefore, careful life-cycle assessment of bio-composites is essential in order to retain the main advantage in the process of developing high performance bio-composites. Rarely publica- tions are available for flax fibre reinforced with biodegradable polymers such as for PLA, PHB, SPI, etc. Future work on LCA of flax fibre and its composites should focus on the biodegradable polymers. 6. Durability of flax fibre reinforced composites Although there are many promising achievements at laboratory or pilot scale, several challenges in producing bio-composites at the industrial scale are still exist. One major obstacle which needs to be overcome for successful commercialisation of bio-composites is the durability. Durability relates to resistance to deterioration resulting from external causes as well as internal causes. The lack of data related to the durability of natural fibre reinforced compos- ites is one major challenge that needed to be addressed prior to the widespread acceptance and implementation of bio-composite materials in different engineering areas. The life cycle of the bio- composites should be tailored to meet specific requirements. Compared to synthetic glass fibre reinforced composites, natu- ral flax fibre reinforced composites suffer from relatively poor moisture resistance because of the presence of hydroxyl and other polar groups in various constituents of the fibres. Flax fibres with a high moisture uptake will lead to a weak interfacial fibre/matrix bonding and thus compromise the mechanical properties of the composites. Flax fibre reinforced composites are very sensitive to influences from environmental agents such as hygrothermal aging and loading as well as prediction of lifetimes [79]. Therefore, it is necessary to enhance the hydrophobisity of the flax fibres by treat- ments with suitable coupling agents or by coating with appropriate resin in order to develop composites with better mechanical prop- erties and environmental performance. To date, only a few studies had dealt with the durability issues for flax fibre reinforced, although some authors (e.g. Ray and Rout [80]) mentioned that this will be a necessary area of research be- fore natural composites are accepted as primary structural compo- nents. Therefore, data on the effects of moisture on retention of mechanical properties of natural fibre reinforced composites dur- ing long-term service are crucial for them to be utilised in outdoor applications. Stamboulis et al. [81] confirmed that the develop- ment of fungus and bacteria in flax composites due to biodegrada- tion or moisture retention is a major concern in their development as structural materials. Different weathering conditions may also produce colour change, weight loss, surface roughening and strength reduction of the natural composites [82]. To have durable flax fibre reinforced composites, some signifi- cant studies have been conducted. Improving the poor environ- mental and dimensional stability of lignocellulosic materials is good to modify the tensile properties of flax fibres [32]. In the study by Stamboulis et al. [32], the environmental behaviour of flax mat reinforced composites is investigated by monitoring the mois- ture absorption and swelling, and measuring the residual mechan- ical properties of the composites at different moisture levels. It confirmed that the moisture absorption and swelling of the Duralin treated flax composites is approximately 30% lower than that of the composites based on untreated flax fibres. Improved understanding of interfacial properties is also essen- tial to optimise the mechanical properties and durability of bio- composites materials, but so far few data are available. Le Duigou et al. [109] investigated the interfacial bonding of flax fibre/ poly(l-lactide) composites by considering different thermal treat- ments, i.e. cooling rate and annealing. It concluded that when cool- ing rate is low, improved interfacial properties of the composites are observed. In addition, a proper modification, e.g. functionalizing, blending, on fibre surface (e.g. by acrylic acid (AA) and vinyl trimetoxy silane (VTMO) [110]) and polymer matrix is also benefit for the develop- ment of durable flax composites. Arbelaiz et al. [111] used several amounts of maleic anhydride-polypropylene copolymer (MAPP) as compatibilizer to treat flax fibres and PP matrix. Results showed that using MAPP as coupling agent, mechanical properties of com- posites improved, and water uptake rate clearly decreased. Similar results on flax fibre reinforced composites were observed by Gud- uri et al. [112] considering the Polypropylene-graft-Maleic anhy- dride (PP-g-MA, Grade: G-3015) and Polyethylene-graft-Maleic anhydride (PE-g-MA, Grade: G-2608) as compatibilizer. Oksman et al. [56] considered triacetin as plasticizer to improve the adhe- sion between fibre and matrix. To overcome the degradation of natural fibre reinforced com- posite, Thwe and Liao [113] considered hybridization of natural fi- bre with stronger and more corrosion-resistant synthetic fibre, for example, glass or carbon fibre, can improve the stiffness, strength and moisture resistance of the composite. Using a hybrid compos- ite that contains two or more types of different fibres, the advanta- ges of one type of fibre could complement what are lacking in the other. As a consequence, a balance in performance and cost could be achieved through proper material design. A similar study on bamboo-glass/PP hybrid composite indicated that the hybrid com- posite has better fatigue resistance than bamboo/PP composites et al. load levels tested. A similar study on silk/glass hybrid com- posites proved that the water uptakes of the hybrid composite were observed to be less than that of the silk fibre reinforced com- posites [114]. Cicala et al. [115] considered the hybridization of glass fibres with natural fibres (i.e. hemp, flax and kenaf) for applications in the piping industry. The pipe selected for the study was a curved fitting (90°) flanged at both ends designed to withstand an inter- nal pressure of 10 bar and in the presence of acid aqueous solu- tions. The hybrid composite laminates were tested after immersion in aqueous acid solutions for 40 days. The mechanical test showed that only small variations of the mechanical proper- ties after immersion were obtained. The use of hybrid lay-up led to a pipe which fulfilled the requirements of mechanical resis- tance for the intended use. A most recent study [116] on car- bon/flax hybrid composites suggested the application of this hybrid composite in structural applications (i.e. nautical and automotive). 314 L. Yan et al. / Composites: Part B 56 (2014) 296–317
  • 20. In order to have durable flax fibre reinforced composite, in near future, the hybridization of flax fibres with synthetic fibres can be considered. Regarding to the long-term consideration of replacement of synthetic fibres by natural fibres, works should be focused on the improvement of fibre/matrix adhesion, improvement of environmental and dimensional stability of fi- bres, and development of appropriate modification of fibre and/or polymer matrix. Works need to be done to explore the results of bio-composites exposed to durability concerns of moisture absorption and various weathering conditions in order to remove industry scepticism. 7. Application of flax fibre reinforced composites Recent work on flax composites reveals that the specific mechanical properties of flax composites are comparable to those of glass fibre reinforced composites. Bio-composites made of natu- ral fibres, i.e. in the forms of panels, tubes, sandwich plates, have been used to replace the wooden fittings, fixtures, furniture, and noise insulating panels in the last decade [4]. There is an increasing demand from automotive companies for materials with sound abatement capability as well as reduced weight for fuel efficiency. Natural fibres possess excellent sound absorbing efficiency and are more shatter resistant and have better energy management characteristics than glass fibre reinforced composites. In automotive parts, bio-composites not only reduce the mass of the component but also lower the energy needed for production by 80% [72]. Bio-composites can be designed for door panels, headrests, parcel shelves, roof upholstery to reduce the environmental impact, structural weight, and manufacturing costs. Other emerging markets are consumer applications such as tiles, flower pots, and marine piers [1]. Development of new com- posite products from the easily renewable natural materials has a strong potential to deliver novel biodegradable and/or readily recy- clable materials suitable for the packaging industry, thereby replacing not so easily renewable fossil fuel-based polymers/ plastics. One of the most important requirements for bio-composites is to be used as construction building materials. Bio-composites have the potential to eventually be lighter-weight and lower- cost than synthetic composites. Using materials like bio-com- posites that reduce construction waste and increase energy effi- ciency would provide a solution to immediate infrastructure needs while promoting the concept of sustainability [74]. To have a more sustainable construction industry, the EU recently established that in a medium term raw materials consumption must be reduced by 30% and that waste production in this sec- tor must be cut down by 40% [75]. Natural fibres are a renew- able resource and are available all most over the world. The use of natural fibres by the construction industry will help to achieve a more sustainable consumption pattern of building materials. Most recently, Yan and Chouw [76] investigated the feasibility of flax fabric reinforced epoxy composite tube as con- crete confinement. It was found that the flax/epoxy tube in- creased the axial compressive strength and structural ductility significantly, i.e. the 4-layer flax/epoxy tube confinement in- creased concrete compressive strength up to 54 MPa, compared with the unconfined concrete of 25 MPa. The pre-fabricated flax/ epoxy composite tube also acts as lightweight permanent form- work for fresh concrete to reduce the construction time and protects the encased concrete from a potentially harsh environ- ment, e.g. de-icing salts and other chemicals. Further studies of flax fibre reinforced composites as different structural elements are in progress, e.g. flax composite tube encased concrete as bridge pier [85,86] and PLLA/flax mat/balsa bio-sandwich in transport application [87,88]. 8. Future work A critical issue is that the properties of flax composites are dependent on the properties of the fibre and the adhesion between the fibre and the matrix. Chemical modifications of the matrix and fibre and use of adhesion promoters can be used in order to im- prove mechanical properties of natural composites. Modification relies on chemical and physical techniques, mainly focused on grafting chemical groups capable of improving the interfacial inter- actions between filler particles and polymer matrix. The main techniques have been summarised by La Mantia and Morreale [83] as follows: (1) Alkali treatment (mercerization), (2) Acetyla- tion, (3) Stearic acid treatment, (4) Benzylation, (5) Peroxide treat- ment, (6) Anhydride treatment, (7) Permanganate treatment, (8) Silane treatment, (9) Isocyanate treatment and (10) Plasma treat- ment. A proper selection of those techniques can improve the properties of natural composites. However, the high initial cost of some methods is a primary drawback when considered for industrial applications. Consequently, more attention on quicker, cheaper and environ- mentally friendly methods of modification, as well as understand- ing of the durability of the bio-composites are now required. 9. Conclusions Flax fibres are cost-effective materials have specific mechanical properties which have potential to replace glass fibres as reinforce- ment in composite. Their main disadvantage is the variability in their properties. Environmental effects (e.g. high relative humidity) will degrade the tensile properties of flax fibres. A suitable chemi- cal treatment (e.g. Silane) can increase the tensile strength and strain of the flax fibres. The tensile strength and modulus of flax fi- bres decrease with an increase in fibre length, fibre diameter and gauge length. Flax fibres at the mid-span and tip in the stem with high content of cellulose should be considered as the raw materi- als. Improving the poor environmental- and dimensional stability of lignocellulosic materials is an effective way to modify the mechanical properties of these materials. The tensile properties of flax fibres scatter significantly with the change in fibre diameter, gauge length. An appropriate treatment (e.g. Duralin treatment or drying cycle treatment) can be selected to achieve a higher and more uniform strength with less scatter. Flax fibre with thermoplastic, thermoset and biodegradable polymer matrices exhibit promising mechanical properties. A ma- jor limitation of using flax fibres as reinforcement in composites is the incompatibility which results in poor fibre/matrix interfacial bonding and thereby reduces the tensile properties. The selection of suitable manufacturing process and physical/chemical modifica- tion can improve the mechanical properties of flax composites. Flax composites have the potential to be the next generation materials for structural application for infrastructure, automotive industry and consumer applications. Future work on flax compos- ites should be focused on understanding the environmental assess- ment, durability, further improving the mechanical properties and moisture resistance. Additionally, novel manufacturing processes and surface modification methods should be further developed. Acknowledgements The authors are grateful to all the publishers (e.g. Elsevier Pub- lishers, SAGE, Springer, John Wiley and Sons) and authors who per- mitted to use figures and tables from their publications. This research was supported by the Engineering Faculty Research Development Fund (FRDF ID: 3702507) of the University of Auckland. The first author also wishes to thank the University of L. Yan et al. / Composites: Part B 56 (2014) 296–317 315
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