10 Natural fibre composites the macroscopic tests,the parameters of mechanical properties generally include such items as tensile strength and modulus,elongation,compressive strength and modulus,impact strength,and flexible strength and modulus. Researchers traditionally use the elongation,tensile strength and Young's modulus'.7 to evaluate the mechanical performance of wood fibres. As aforementioned,the mechanical performance of wood fibres is influenced by their structure;in addition,the mechanical performance is influenced by the growing parameters,e.g.area of growth,climate,and the age of the plant.72.7 Wood fibres generally display higher mechanical performance compared with other natural fibres (Table 1.4).However, due to wood source,growth conditions,and chemical and mechani- cal treatments,the strength of wood fibres varies considerably,which is one of the main drawbacks for all natural products.The range between minimum and maximum characteristic values of wood fibres is notice- ably wider than that of synthetic fibres although the wood fibres display a good Weibull modulus (Table 1.5)which describes the variability of the failure strength. The first report about the mechanical properties of wood fibres did not appear until the end of the 1950s.9 The development of spectroscopic(espe- cially X-ray diffraction(XRD)in 1912 and Raman spectroscopy in 1923)and microscopic (especially atomic force microscopy (AFM)in 1986)technol- ogies enabled the characterization of mechanical properties on micro-or nano-scales for heterogeneous polymer or composites.These developed technologies have enlarged understanding of the mechanical properties of wood fibre (wood fibre itself is a polymeric composite)and the structure- property relationship,and optimized the utilization of wood fibres as rein- forcements in composites.24 Nanoindentation and AFM techniques have been employed to investigate the micro-or nano-mechanical properties of Table 1.4 Mechanical properties of natural fibres Fibres Density Elongation Tensile Young's References (g/cm3) (%) strength modulus (MPa) (GPa) Flax 1.5 1.2-3.2 345-2000 15-80 74-76 Hemp 1.48 1.6 550-900 26-80 72,77,78 Sisal 1.5 3.0-7.0 468-700 9.4-22 79 Coir 1.2 17-47 175 4.0-6.0 1,71 Softwood 1.5 - 600-1020 18-40 80,81 Hardwood 1.2 37.9 82 E-glass 2.5 2.5 2000-3500 70 2 S-glass 2.5 2.8 4570 86 1 Aramid 1.4 3.3-3.7 3000-3150 63.0-67.0 Woodhead Publishing Limited,2014
10 Natural fi bre composites © Woodhead Publishing Limited, 2014 the macroscopic tests, the parameters of mechanical properties generally include such items as tensile strength and modulus, elongation, compressive strength and modulus, impact strength, and fl exible strength and modulus. Researchers traditionally use the elongation, tensile strength and Young’s modulus 1,71 to evaluate the mechanical performance of wood fi bres. As aforementioned, the mechanical performance of wood fi bres is infl uenced by their structure; in addition, the mechanical performance is infl uenced by the growing parameters, e.g. area of growth, climate, and the age of the plant. 72,73 Wood fi bres generally display higher mechanical performance compared with other natural fi bres (Table 1.4). However, due to wood source, growth conditions, and chemical and mechanical treatments, the strength of wood fi bres varies considerably, which is one of the main drawbacks for all natural products. The range between minimum and maximum characteristic values of wood fi bres is noticeably wider than that of synthetic fi bres although the wood fi bres display a good Weibull modulus (Table 1.5) which describes the variability of the failure strength. The fi rst report about the mechanical properties of wood fi bres did not appear until the end of the 1950s. 91 The development of spectroscopic (especially X-ray diffraction (XRD) in 1912 and Raman spectroscopy in 1923) and microscopic (especially atomic force microscopy (AFM) in 1986) technologies enabled the characterization of mechanical properties on micro- or nano-scales for heterogeneous polymer or composites. These developed technologies have enlarged understanding of the mechanical properties of wood fi bre (wood fi bre itself is a polymeric composite) and the structure– property relationship, 92 and optimized the utilization of wood fi bres as reinforcements in composites. 24 Nanoindentation and AFM techniques have been employed to investigate the micro- or nano-mechanical properties of Table 1.4 Mechanical properties of natural fi bres Fibres Density (g/cm 3) Elongation (%) Tensile strength (MPa) Young’s modulus (GPa) References Flax 1.5 1.2–3.2 345–2000 15–80 74–76 Hemp 1.48 1.6 550–900 26–80 72, 77, 78 Sisal 1.5 3.0–7.0 468–700 9.4–22 79 Coir 1.2 17–47 175 4.0–6.0 1,71 Softwood 1.5 – 600–1020 18–40 80, 81 Hardwood 1.2 – – 37.9 82 E-glass 2.5 2.5 2000–3500 70 1 S-glass 2.5 2.8 4570 86 1 Aramid 1.4 3.3–3.7 3000–3150 63.0–67.0 1
Wood fibres as reinforcements in natural fibre composites 11 Table 1.5 Weibull modulus of natural fibres Natural fibre Weibull Gauge References modulus length (mm) Flax 2.6 10 83 Hemp 2.86 10 84 Sisal 4.6 10 85 Coir 3.1 8 86 Wood 3-5 87 E-glass 6.61 10 88 S-glass 20.5 89 Aramid 10.4 5 90 Table 1.6 Effect of wood species on the mechanical properties of wood fibres Type of fibres MFA() Elastic modulus(GPa) References Softwood Spruce 13.49(CV 43.00%,earlywood) 93 Scots pine 21.00 (CV 16.00%,latewood) 93 (pulp) 12.2±1.6 94 Hardwood Oak 3±3 18.27 +1.74 (earlywood: Eucalyptus latewood =1:1) 94 (pulp) 9.10±1.60 wood fibres since the first report about the cell wall mechanics of softwood by using nanoindentation in 199793 By using these new techniques,researchers have revealed various novel properties of wood fibres.These findings support the selection of wood fibres as composites,and the understanding of the interaction mechanism between wood fibres and the matrices.For example:(i)the identification of different properties among wood species and across growing stages:at cell wall level,it seems that the elastic modulus of hardwood fibres is similar to that of softwood fibres;however,earlywood has a lower elastic modu- lus than latewood(Table 1.6);(ii)the anisotropic properties of wood fibres: Jager et al.36.97 employed the Vlassak model to evaluate the relationship between indentation modulus,indentation direction and elastic material constants of spruce cell wall material MPred(E G),and then using an error minimization procedure to analyse the values of the elastic mate- rial constants-the values for the longitudinal elastic modulus,transverse modulus and shear modulus are reported as 26.3,4.5 and 4.8 GPa,respec- tively;(iii)the interfacial compatibility between S,and S,layers:by using a nanoindentation-AFM technique the interfacial compatibility in the cell @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 11 © Woodhead Publishing Limited, 2014 wood fi bres since the fi rst report about the cell wall mechanics of softwood by using nanoindentation in 1997. 93 By using these new techniques, researchers have revealed various novel properties of wood fi bres. These fi ndings support the selection of wood fi bres as composites, and the understanding of the interaction mechanism between wood fi bres and the matrices. For example: (i) the identifi cation of different properties among wood species and across growing stages: at cell wall level, it seems that the elastic modulus of hardwood fi bres is similar to that of softwood fi bres; however, earlywood has a lower elastic modulus than latewood (Table 1.6); (ii) the anisotropic properties of wood fi bres: J ä ger et al . 96,97 employed the Vlassak model to evaluate the relationship between indentation modulus, indentation direction and elastic material constants of spruce cell wall material M pred ( Et , El , Gtl , υtt , υtl , δi ), and then using an error minimization procedure to analyse the values of the elastic material constants ‒ the values for the longitudinal elastic modulus, transverse modulus and shear modulus are reported as 26.3, 4.5 and 4.8 GPa, respectively; (iii) the interfacial compatibility between S 2 and S 3 layers: by using a nanoindentation-AFM technique the interfacial compatibility in the cell Table 1.5 Weibull modulus of natural fi bres Natural fi bre Weibull modulus Gauge length (mm) References Flax 2.6 10 83 Hemp 2.86 10 84 Sisal 4.6 10 85 Coir 3.1 8 86 Wood 3–5 – 87 E-glass 6.61 10 88 S-glass 20.5 – 89 Aramid 10.4 5 90 Table 1.6 Effect of wood species on the mechanical properties of wood fi bres Type of fi bres MFA (°) Elastic modulus (GPa) References Softwood Spruce Scots pine (pulp) – 5 – 13.49 (CV 43.00%, earlywood) 21.00 (CV 16.00%, latewood) 12.2 ± 1.6 93 93 94 Hardwood Oak Eucalyptus (pulp) 3 ± 3 – 18.27 ± 1.74 (earlywood: latewood = 1:1) 9.10 ± 1.60 94
12 Natural fibre composites wall of spruce was further investigated and it has been found that the S; layer has a less polar character than the S2 layer;hence polyurethane(PUR) was found to have a better adhesion to the S,layer and poorer adhesion to the S2 layer compared with urea formaldehyde(UF).It is proposed that dif- ferences in the polarity of the adhesives used and in the surface chemistry of the two cell wall surfaces examined account for the observed trends. In addition,nanoindentation and AFM have been widely used to reveal much more detail about wood fibres,e.g.cell wall lignification,9 melamine modification,100 stiffness and hardness of wood fibres,101 and conformability of wet wood fibres.102 1.2.3 Processing of wood fibres The separation of wood fibres includes two methods(Fig.1.4):a pulping process and a pulverizing process.Pulverizing is the process by which the wood is reduced into small particles (180-425 um).It is the main step for the production of wood flour,which is mainly used as filler in plastics.For dry mechanical processing,the final products typically have low aspect ratios (only 1-5).These low aspect ratios allow wood flour to be more easily metered and fed than individual wood fibres,which tend to bridge.However, the low aspect ratio limits the reinforcing ability.Pulping is the process by Applications of Classification of wood fibres wood fibres Processing of Chemical Paper wood fibres pulp Chemical Paperboard pulping Semi- Semi- Wood chemical chemical pulp Composite pulping Mechanical pulping Fibreboard Pulverizing Mechanical pulp Wood flour 1.4 Processing and applications of wood fibre. Woodhead Publishing Limited,2014
12 Natural fi bre composites © Woodhead Publishing Limited, 2014 wall of spruce was further investigated 98 and it has been found that the S 3 layer has a less polar character than the S 2 layer; hence polyurethane (PUR) was found to have a better adhesion to the S 3 layer and poorer adhesion to the S 2 layer compared with urea formaldehyde (UF). It is proposed that differences in the polarity of the adhesives used and in the surface chemistry of the two cell wall surfaces examined account for the observed trends. In addition, nanoindentation and AFM have been widely used to reveal much more detail about wood fi bres, e.g. cell wall lignifi cation, 99 melamine modifi cation, 100 stiffness and hardness of wood fi bres, 101 and conformability of wet wood fi bres. 102 1.2.3 Processing of wood fi bres The separation of wood fi bres includes two methods (Fig. 1.4): a pulping process and a pulverizing process. Pulverizing is the process by which the wood is reduced into small particles (180–425 μ m). It is the main step for the production of wood fl our, which is mainly used as fi ller in plastics. For dry mechanical processing, the fi nal products typically have low aspect ratios (only 1–5). These low aspect ratios allow wood fl our to be more easily metered and fed than individual wood fi bres, which tend to bridge. However, the low aspect ratio limits the reinforcing ability. Pulping is the process by Applications of wood fibres Paper Paperboard Composite Fibreboard Semichemical pulp Semichemical pulping Chemical pulping Chemical pulp Classification of wood fibres Processing of wood fibres Mechanical pulp Wood flour Mechanical pulping Pulverizing Wood 1.4 Processing and applications of wood fi bre
Wood fibres as reinforcements in natural fibre composites 13 which the macroscopic structure of raw wood is reduced to a fibrous mass.It is achieved by rupturing bonds within the wood structure.It can be accom- plished chemically,mechanically or by some combination of these treat- ments.These treatments are (i)chemical,03-10s(ii)mechanical and (iii) semi-chemical,which combines(i)and (ii)to separate wood fibres.0 The main commercial chemical treatment technique is the sulphate or kraft process;an acid sulfite process is also used.The chemical process involves the use of chemicals to degrade and dissolve lignin from the wood cell walls, releasing high cellulose content fibres.Chemical pulping processes yield pulps with higher strength compared with mechanical processes.However, these processes are low yield (40-55%)12(Table 1.7)and are very capital- intensive.1 Products from the chemical treatment process(chemical pulp) are always used for paper (e.g.tissue),paperboard,etc. Stone groundwood (SGW),pressure groundwood (PGW),refiner mechanical pulps(RMP)and thermomechanical pulps(TMP)are the main products of wet mechanical treatments.Wet mechanical treatment involves the use of mechanical force to separate the wood fibres.Mechanical defibra- tion of wood and chips results in only small material losses and the gross composition of the resulting pulps differ only slightly from that of the orig- inal.However,the fibre structure is somewhat damaged.Mechanical treat- ment under wet conditions can obtain higher yield (Table 1.7),but these processes are electrical energy-intensive and produce paper with lower strength,higher pitch content,and higher colour reversion rate compared with chemical processes.Mechanically produced pulp has a higher propor- tion of broken cell fragments (called 'fines')among the fibres.The mechan- ical pulps can be used for paper(printing paper),paperboard,composite and fibreboard. The semi-chemical techniques normally involve pretreatment of wood chips with a chemical method.There are several types of semi-chemical pulps in production,e.g.chemimechanical pulps (CMP),chemithermo- mechanical pulps(CTMP)and neutral sulfite semi-chemical (NSSC)pulps. NSSC is the most common product,made primarily from hardwood species and noted for its exceptional stiffness and high rigidity.The yield of semi- chemical pulping is 58.7-95%12(Table 1.7).Its primary use is for the pro- duction of paperboard as well as printing papers,greaseproof papers and bond papers.The semi-chemical pulps are still used for composite,but very much less for fibreboard. 1.2.4 Applications of wood fibres As a result of a growing awareness of the interconnectivity of global environ- mental factors,the principles of sustainability,industrial ecology and ecoeffi- ciency,and also green chemistry and engineering,are being integrated into the Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 13 © Woodhead Publishing Limited, 2014 which the macroscopic structure of raw wood is reduced to a fi brous mass. It is achieved by rupturing bonds within the wood structure. It can be accomplished chemically, mechanically or by some combination of these treatments. These treatments are (i) chemical, 103–105 (ii) mechanical 106,107 and (iii) semi-chemical, which combines (i) and (ii) to separate wood fi bres. 108–110 The main commercial chemical treatment technique is the sulphate or kraft process; an acid sulfi te process is also used. The chemical process involves the use of chemicals to degrade and dissolve lignin from the wood cell walls, releasing high cellulose content fi bres. Chemical pulping processes yield pulps with higher strength compared with mechanical processes. However, these processes are low yield (40–55%) 111,112 (Table 1.7) and are very capitalintensive. 111 Products from the chemical treatment process (chemical pulp) are always used for paper (e.g. tissue), paperboard, etc. Stone groundwood (SGW), pressure groundwood (PGW), refi ner mechanical pulps (RMP) and thermomechanical pulps (TMP) are the main products of wet mechanical treatments. Wet mechanical treatment involves the use of mechanical force to separate the wood fi bres. Mechanical defi bration of wood and chips results in only small material losses and the gross composition of the resulting pulps differ only slightly from that of the original. However, the fi bre structure is somewhat damaged. Mechanical treatment under wet conditions can obtain higher yield (Table 1.7), but these processes are electrical energy-intensive and produce paper with lower strength, higher pitch content, and higher colour reversion rate compared with chemical processes. Mechanically produced pulp has a higher proportion of broken cell fragments (called ʻ fi nes’) among the fi bres. The mechanical pulps can be used for paper (printing paper), paperboard, composite and fi breboard. The semi-chemical techniques normally involve pretreatment of wood chips with a chemical method. There are several types of semi-chemical pulps in production, e.g. chemimechanical pulps (CMP), chemithermomechanical pulps (CTMP) and neutral sulfi te semi-chemical (NSSC) pulps. NSSC is the most common product, made primarily from hardwood species and noted for its exceptional stiffness and high rigidity. The yield of semichemical pulping is 58.7–95% 129 (Table 1.7). Its primary use is for the production of paperboard as well as printing papers, greaseproof papers and bond papers. The semi-chemical pulps are still used for composite, but very much less for fi breboard. 1.2.4 Applications of wood fi bres As a result of a growing awareness of the interconnectivity of global environmental factors, the principles of sustainability, industrial ecology and ecoeffi - ciency, and also green chemistry and engineering, are being integrated into the
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© Woodhead Publishing Limited, 2014 Table 1.7 Pulp yield and relative strength achieved using various pulping methods Classifi cation Process Yield (%) Strength References Tensile index (Nm/g) Tear index (mN ·m 2/g) Burst index (kPa ·m 2/g) Chemical treatment Kraft pulping 40–50 92.0–98.5 8.6 6.8–7.3 113,114 Sulfi te pulping 45–55 85–132.7 7.4–12.2 4.43–6.42 115, 116 Soda pulping 45–55 69.9–83.6 3.2–9.2 4.2–7.34 117, 118 Mechanical treatment Stone groundwood (SGW) pulping 90–98.5 28.2 2.2 0.86 119,120 Pressure groundwood (PGW) pulping 95.5 18.4 2.8 0.90 119,120 RMP 90–97.5 28.1 2.8 0.66 119–121 TMP 91–95 27.6 3.2 1.10 120,122,123 Semi-chemical treatment Chemimechanical pulping (CMP) 80–90 49–63 5.45–5.5 3.11 115,120,124 Chemithermomechanical pulping (CTMP) 80–95 51.8 6.4 2.4 120,124,127 NSSC 58.7–80 30.90–35.57 3.73–4.08 1.38–1.60 126–128