Wood fibres as reinforcements in natural fibre composites 5 than 85%focus on wood fibres.In addition,the growing importance of wood fibres can be evidenced from the increasing number of publications in the past 10 years (Fig.1.2).It should be noted that these publications derive from the Google scholar database with the key word of wood fibre, and it is found that more than 75%of the publications are from jour- nals or conferences,and about 70%of these reports focus on wood fibre composites. The development of nanotechnology(NT)and biotechnology in the past 10 years has pushed the research and development of wood fibres a step fur- ther,and enlarged the role of wood fibres in many industrial sectors,such as pulp and paper,and material industries. 1.2 Wood fibres:nature and behaviour 1.2.1 Structure of wood fibres Wood fibres consist of both live and dead cells in the wood,depending on the location and the age of tree from which they are extracted.The hierarchical structure of wood fibres gives this fibrous material excellent performance properties,e.g.high strength to weight ratio.Wood fibres can be obtained from timber by chemical,mechanical,biological processes,and many com- bined processes. At the macroscopic level (normally 0.1-1 m),wood fibres mainly exist within the layer of xylem in the wood10(Fig.1.3(1)).The dark strip in the centre of the stem is the pith,which represents the tissues formed during the first year of growth.The inner part of the xylem layer consists of dark coloured heartwood.The lighter coloured outer part is sapwood,which con- ducts water from the roots to the foliage of the tree.Both inner and outer parts are organized with many concentric growth rings(annual increments), each of which is distinguished by earlywood,composed of large thin-walled cells produced during the spring when water is usually abundant,and the denser latewood,composed of small cells with thick walls (Fig.1.3(2c)11). In addition,the inner bark layer comprises the tissues outside the vascular cambium,including secondary phloem,which transports the nutrients from photosynthesis in the leaves to the rest of the tree,cork cambium (cork- producing cells),and cork cells.The outer bark,composed of dead tissue, protects the inner region from injury,disease and desiccation.At the meso- scopic level (normally 1-10 mm),wood fibres form a continuum of cellular material.12 At the microscopic level (normally 0.01-6 mm),two kinds of wood cells with different hierarchical structures,namely tracheids (in softwoods and hardwoods)and vessels (only in hardwoods),can be easily distinguished'3 (Fig.1.3(3)),and the dimensions of both wood fibres are shown in Table 1.1.14-18 Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 5 © Woodhead Publishing Limited, 2014 than 85% focus on wood fi bres. In addition, the growing importance of wood fi bres can be evidenced from the increasing number of publications in the past 10 years (Fig. 1.2). It should be noted that these publications derive from the Google scholar database with the key word of wood fi bre, and it is found that more than 75% of the publications are from journals or conferences, and about 70% of these reports focus on wood fi bre composites. The development of nanotechnology (NT) and biotechnology in the past 10 years has pushed the research and development of wood fi bres a step further, and enlarged the role of wood fi bres in many industrial sectors, such as pulp and paper, and material industries. 1.2 Wood fibres: nature and behaviour 1.2.1 Structure of wood fi bres Wood fi bres consist of both live and dead cells in the wood, depending on the location and the age of tree from which they are extracted. The hierarchical structure of wood fi bres gives this fi brous material excellent performance properties, e.g. high strength to weight ratio . Wood fi bres can be obtained from timber by chemical, mechanical, biological processes, and many combined processes. At the macroscopic level (normally 0.1–1 m), wood fi bres mainly exist within the layer of xylem in the wood 10 (Fig. 1.3(1)). The dark strip in the centre of the stem is the pith, which represents the tissues formed during the fi rst year of growth. The inner part of the xylem layer consists of dark coloured heartwood. The lighter coloured outer part is sapwood, which conducts water from the roots to the foliage of the tree. Both inner and outer parts are organized with many concentric growth rings (annual increments), each of which is distinguished by earlywood, composed of large thin-walled cells produced during the spring when water is usually abundant, and the denser latewood, composed of small cells with thick walls (Fig. 1.3(2c) 11 ). In addition, the inner bark layer comprises the tissues outside the vascular cambium, including secondary phloem, which transports the nutrients from photosynthesis in the leaves to the rest of the tree, cork cambium (corkproducing cells), and cork cells. The outer bark, composed of dead tissue, protects the inner region from injury, disease and desiccation. At the mesoscopic level (normally 1–10 mm), wood fi bres form a continuum of cellular material. 12 At the microscopic level (normally 0.01–6 mm), two kinds of wood cells with different hierarchical structures, namely tracheids (in softwoods and hardwoods) and vessels (only in hardwoods), can be easily distinguished 13 (Fig. 1.3(3) ), and the dimensions of both wood fi bres are shown in Table 1.1. 14–18
8 Woodhead Publishing Limited,2014
© Woodhead Publishing Limited, 2014 W Lumen S3 S2 S1 P M Earlywood Ray cells Thick walled tracheids Thin walled tracheids Earlywood Vessel Fibres Ray cells (2) Mesoscopic level Perforation plate Tracheids Vessel elements (3) Microscopic level (4) Ultrastructural level (5) Nanoscopic level (a) (b) (a) (b) (a) (b) (c) (d) (c) (d) M P S1 S S 2 3 (c) (d) 500 250 0 0 250 500 nm Pits 0 0 30 20 10 123 3 2 1 0 4 μm mμ 5 (6) Molecular level (1) Macroscopic level 1 nm Cellulose chains Disordered region 100 nm Cellulose nanocrystals Latewood Latewood Growing ring Pith Earlywood Latewood Heartwood Outer bark Inner bark Bark Sapwood 1.3 Wood fi bre from macroscopic to molecular levels: (1) macroscopic level; (2) mesoscopic level: 3D schematic of (a) softwood and (b) hardwood, scanning electron microscope (SEM) of (c) softwood and hardwood; (3) microscopic level; (4) ultrastructural level: (a) Raman image, (b) transmission electron microscope (TEM), (c) atomic force microscope (AFM) and (d) model of wood cell; (5) nanoscopic level: (a) schematic drawing of the cellulose aggregate structure in wood cell, (b) a schematic of cellulose fi brils laminated with hemicellulose and lignin, (c) AFM of a transverse section of the secondary wall of wood cell and (d) structure of cellulose in a wood cell wall; (6) molecular level
Wood fibres as reinforcements in natural fibre composites Table 1.1 Dimensions of typical softwood and hardwood fibres Types of fibres Length(mm) Width (um) Aspect ratio Softwood 2-6 20-40 50-200 Hardwood 1-2 10-50 28-86 Tracheids constitute over 90%of the volume of most softwood19 and 50% of the volume of hardwood.15 Their average length is usually between 2 and 6 mm,1416.s and their width is between 20 and 40 um,1417 with a length to width ratio (aspect ratio)often in excess of 50-200.In hardwood,the length of tracheids,which is only 1-2 mm.14.15 is considerably shorter than that of softwood tracheids,and the width is between 10 and 50 um'4.15,with a narrow aspect ratio of 28:86.14 In addition to tracheids,hardwoods have wider cells, namely vessel elements,which vary considerably in size and shape.20They are a series of broad and articulated cells(around 100 um),which are long(many centimetres)and their function is to channel sap in almost straight lines.In some wood species,they may account for up to 50%-60%of the volumetric composition,but usually less than 10%by weight.14The wide vessel elements of the early wood are found to be 13%-47%shorter than those of the late wood.21 At the ultrastructural level(normally 1-25 jm),the wood fibres are built up of four layers(Fig.1.3(4)).22-25These are middle lamella(M),primary wall (P),secondary wall(S),including the outer layer of the secondary wall(S), the middle layer of the secondary wall (S2),the inner layer of the secondary wall(S3),and the warty layer(W).2627 The middle lamella is located between the cells.This layer is highly rich in lignin;the concentration of lignin in this layer is about 70%80%,which is about twice that in secondary wall.The high concentration of lignin can cement the cells together very well,but in the processing of wood fibres,separation of the lignin remaining on the fibre surface can result in a decrease of inter-fibre bonding.The primary cell wall is a thin layer (0.1-0.2 um)which surrounds the protoplast during cell division and subsequent enlargement.44 It contains a randomly and loosely organized network of cellulose microfibrils.Due to the occurrence of pectin and protein, the properties of the primary wall layer differ from those of the secondary; in this layer strong interactions exist among the lignin,protein and pectin, as well as among the cellulose and hemicellulose.This obvious feature has a major influence on the separation of fibres.The secondary cell wall (1.2-5.4 um)contains much more ordered microfibrils than the primary cell wall.It comprises a series of layers,namely S1,S2 and S3.The warty layer(W)is com- parable in thickness(0.1-0.3 um)to the primary wall,and consists of four to six lamellae which spiral in opposite directions around the longitudinal axis @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 7 © Woodhead Publishing Limited, 2014 Tracheids constitute over 90% of the volume of most softwood 19 and 50% of the volume of hardwood. 15 Their average length is usually between 2 and 6 mm, 14,16,18 and their width is between 20 and 40 μ m, 14,17 with a length to width ratio (aspect ratio) often in excess of 50–200. In hardwood, the length of tracheids, which is only 1–2 mm, 14,15 is considerably shorter than that of softwood tracheids, and the width is between 10 and 50 μ m 14,15 , with a narrow aspect ratio of 28:86. 14 In addition to tracheids, hardwoods have wider cells, namely vessel elements, which vary considerably in size and shape. 20 They are a series of broad and articulated cells (around 100 μ m), which are long (many centimetres) and their function is to channel sap in almost straight lines. In some wood species, they may account for up to 50%–60% of the volumetric composition, but usually less than 10% by weight. 14 The wide vessel elements of the early wood are found to be 13%–47% shorter than those of the late wood. 21 At the ultrastructural level (normally 1–25 μ m), the wood fi bres are built up of four layers (Fig. 1.3(4)). 22–25 These are middle lamella (M), primary wall (P), secondary wall (S), including the outer layer of the secondary wall (S 1 ), the middle layer of the secondary wall (S 2 ), the inner layer of the secondary wall (S 3 ), and the warty layer (W). 26,27 The middle lamella is located between the cells. This layer is highly rich in lignin; the concentration of lignin in this layer is about 70%–80%, 28 which is about twice that in secondary wall. The high concentration of lignin can cement the cells together very well, but in the processing of wood fi bres, separation of the lignin remaining on the fi bre surface can result in a decrease of inter-fi bre bonding. The primary cell wall is a thin layer (0.1–0.2 μ m) which surrounds the protoplast during cell division and subsequent enlargement. 14 It contains a randomly and loosely organized network of cellulose microfi brils. Due to the occurrence of pectin and protein, the properties of the primary wall layer differ from those of the secondary; in this layer strong interactions exist among the lignin, protein and pectin, as well as among the cellulose and hemicellulose. This obvious feature has a major infl uence on the separation of fi bres. The secondary cell wall (1.2–5.4 μ m) contains much more ordered microfi brils than the primary cell wall. It comprises a series of layers, namely S 1 , S 2 and S 3 . The warty layer (W) is comparable in thickness (0.1–0.3 μ m) to the primary wall, and consists of four to six lamellae which spiral in opposite directions around the longitudinal axis Table 1.1 Dimensions of typical softwood and hardwood fi bres Types of fi bres Length (mm) Width ( μm) Aspect ratio Softwood 2–6 20–40 50–200 Hardwood 1–2 10–50 28–86
8 Natural fibre composites of the tracheid.4 The outer secondary cell wall (S)has a crossed fibrillar structure.Although the S layer has a large microfibril angle (MFA),about 50-70,2 this layer is considered to play an important role in determining the transverse mechanical properties and surface properties of fibres30-32 as well as pulp fibre properties.3 The main bulk of the secondary wall is contained in the middle secondary cell wall (S2,1-5 um).The microfibrils in this layer spi- ral steeply about the axial direction at an angle of around 5-3029.34 and have a pronounced influence on the properties of fibres.The S,layer is the thickest cell wall layer and controls the strength of the entire fibre.The inner second- ary wall(S3,0.1 um),sometimes also known as the tertiary wall,35 is at the lumen boundary and forms a barrier between the lumen and the rest of the cell wall.Compared with the other layers in the secondary wall,the S,layer contains the highest concentration of lignin,about 53%.36 In this thin layer the microfibrils form a flat helix.The microfibrils in the S,layer are oriented almost perpendicularly to the microfibrils in the S,layer with MFA between 50 and 9029 The innermost portion of the cell wall consists of the so-called warty layer,probably formed from protoplasmic debris.All softwoods have this segment in their cell wall;however,not all hardwoods do.29 At the nanoscopic level (Fig.1.3(5)),37-39 wood fibres have an important influence on the final performance of timbers.These influences include chemical reactions and physical effects.The wood fibres are built up by cel- lulose microfibrils(10-25 nm26.40),hemicelluloses and lignins due to the for- mation of lignin-carbohydrate complex(LCC)by covalent bonds.#Most of the microfibrils are not parallel to the cell axis and can form a particular angle,which is known as the MFA.The MFA was found to be a critical fac- tor in determining the physical(e.g.shrinkage4)and mechanical proper- ties (e.g.stiffness,44 and tensile strength45)of wood fibres. From the molecular point of view (Fig.1.3(6)46),the main chemical com- ponents of wood fibres are cellulose,hemicellulose and lignin.As shown in Table 1.2,4748 the dominant component in wood fibres is cellulose.The Table 1.2 Chemical compositions of hardwood and softwood fibres Types of wood fibres Cellulose Hemicellulose Lignin Extractives (%) (%) (%) (%) Original wood fibres Softwood 40-45 25-30 26-34 0-5 Hardwood 45-50 21-35 22-30 0-10 TMP wood fibres 37.07±0.6 29.2±0.1 13.8±0.7 0.8±0.6 Unbleached wood fibres Softwood 69.0±2.5 22.0±0.7 8.8±18 0.2±0.1 Hardwood 78±0.5 19.3±0.1 2.4±0.4 0.3±0.2 Bleached wood fibres Softwood 79.2±0.2 20.0±0.1 0.8±0.1 0±0 Hardwood 78±0.2 20.3±0.1 1.3±0.1 0.5±0.1 TMP thermomechanical pulps. Woodhead Publishing Limited,2014
8 Natural fi bre composites © Woodhead Publishing Limited, 2014 of the tracheid. 14 The outer secondary cell wall (S 1 ) has a crossed fi brillar structure. Although the S 1 layer has a large microfi bril angle (MFA), about 50–70°, 29 this layer is considered to play an important role in determining the transverse mechanical properties and surface properties of fi bres 30–32 as well as pulp fi bre properties. 33 The main bulk of the secondary wall is contained in the middle secondary cell wall (S 2 , 1–5 μ m). The microfi brils in this layer spiral steeply about the axial direction at an angle of around 5–30° 29,34 and have a pronounced infl uence on the properties of fi bres. The S 2 layer is the thickest cell wall layer and controls the strength of the entire fi bre. The inner secondary wall (S 3 , 0.1 μ m), sometimes also known as the tertiary wall, 35 is at the lumen boundary and forms a barrier between the lumen and the rest of the cell wall. Compared with the other layers in the secondary wall, the S 3 layer contains the highest concentration of lignin, about 53%. 36 In this thin layer the microfi brils form a fl at helix. The microfi brils in the S 3 layer are oriented almost perpendicularly to the microfi brils in the S 2 layer with MFA between 50° and 90°. 29 The innermost portion of the cell wall consists of the so-called warty layer, probably formed from protoplasmic debris. All softwoods have this segment in their cell wall; however, not all hardwoods do. 29 At the nanoscopic level (Fig. 1.3(5)), 37–39 wood fi bres have an important infl uence on the fi nal performance of timbers. These infl uences include chemical reactions and physical effects. The wood fi bres are built up by cellulose microfi brils (10–25 nm 26,40 ), hemicelluloses and lignins due to the formation of lignin–carbohydrate complex (LCC) by covalent bonds. 41 Most of the microfi brils are not parallel to the cell axis and can form a particular angle, which is known as the MFA. The MFA was found to be a critical factor in determining the physical (e.g. shrinkage 42,43 ) and mechanical properties (e.g. stiffness, 44 and tensile strength 45 ) of wood fi bres. From the molecular point of view (Fig. 1.3(6) 46 ), the main chemical components of wood fi bres are cellulose, hemicellulose and lignin. As shown in Table 1.2, 47,48 the dominant component in wood fi bres is cellulose. The Table 1.2 Chemical compositions of hardwood and softwood fi bres Types of wood fi bres Cellulose (%) Hemicellulose (%) Lignin (%) Extractives (%) Original wood fi bres Softwood Hardwood 40–45 45–50 25–30 21–35 26–34 22–30 0–5 0–10 TMP wood fi bres 37.07 ± 0.6 29.2 ± 0.1 13.8 ± 0.7 0.8 ± 0.6 Unbleached wood fi bres Softwood Hardwood 69.0 ± 2.5 78 ± 0.5 22.0 ± 0.7 19.3 ± 0.1 8.8 ± 1.8 2.4 ± 0.4 0.2 ± 0.1 0.3 ± 0.2 Bleached wood fi bres Softwood Hardwood 79.2 ± 0.2 78 ± 0.2 20.0 ± 0.1 20.3 ± 0.1 0.8 ± 0.1 1.3 ± 0.1 0 ± 0 0.5 ± 0.1 TMP, thermomechanical pulps
Wood fibres as reinforcements in natural fibre composites 9 cellulose of wood fibres is similar to other natural fibres,in that it con- sists of a linear chain of several hundred to over 10 000 B(1-4)linked D-glucose units49 laid down in microfibrils in which there is extensive hydrogen bonding between cellulose chains,producing a strong crystal- line structure in a crystalline region.505 Combined with the amorphous region,the cellulose microfibrils aggregate into larger microfibrils.31 The hydrogen bonds in the cellulose not only have a strong influence on the physical properties of the cellulose (e.g.solubility,hydroxyl reactivity) but also play an important role in its mechanical properties.so 1.2.2 Physical and mechanical properties of wood fibres The surface property is one of the key properties of wood fibres;it can affect the interfacial adhesion of resin on the surface of fibres and the mechanical properties of fibre-based composite.This property is influenced by fibre mor- phology,chemical composition,32 extractive chemicals and processing condi- tions.53 Table 1.3 shows the surface properties of wood fibres in comparison with other natural fibres.Due to the high polar character of the surface,the fibres are less compatible with non-polar resin.Therefore,the combination of the inherent polar and hydrophilic features of wood fibres and the non-polar characteristics of resins gives rise to difficulties in compounding these mate- rials,resulting in inefficient stress transfer of its composites under load.The use of different kinds of physical(i.e.corona discharge)and chemical surface treatment methods (i.e.coupling agents such as silanes)leads to changes in the surface structure of the fibres as well as to changes of surface properties. The mechanical properties of wood fibres are of great importance for their use in the paper76s and composite industries.The mechanical prop- erties of materials can be characterized from two methods,namely,macro- scopic tests (e.g.tensile test)and indentation tests.The macroscopic tests focus on measuring the mechanical performance of the whole sample,while the indentation tests focus on measuring a local area of the sample.70 In Table 1.3 Surface properties of natural fibres Fibres Surface y(mJ/m2) (Co-C/Co (mV) Splatesu (mV) References area (m2/g) Flax 0.31-0.79 23.85 0.88-0.95 -1.1--0.21 54-57 Hemp 0.75 31.6 0.91 -0.1 55,57,58 Sisal 1.63 32.90-48.35 0.76-0.88 -1.7--0.4 55-57,59 Coir 0.48 45.05 0.22 -4.6--3.8 56,59.60 Softwood 0.97 31 -18.6±5.0 61-63 Hardwood 1.34 32-47 -73 64-66 y:dispersive surface energy;Co:-potential initial value;C:C-potential final value; Cplateu:S-potential plateau value. Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 9 © Woodhead Publishing Limited, 2014 cellulose of wood fi bres is similar to other natural fi bres, in that it consists of a linear chain of several hundred to over 10 000 β (1 → 4) linked D-glucose units 49 laid down in microfi brils in which there is extensive hydrogen bonding between cellulose chains, producing a strong crystalline structure in a crystalline region. 50,51 Combined with the amorphous region, the cellulose microfi brils aggregate into larger microfi brils. 51 The hydrogen bonds in the cellulose not only have a strong infl uence on the physical properties of the cellulose (e.g. solubility, hydroxyl reactivity) but also play an important role in its mechanical properties. 50 1.2.2 Physical and mechanical properties of wood fi bres The surface property is one of the key properties of wood fi bres; it can affect the interfacial adhesion of resin on the surface of fi bres and the mechanical properties of fi bre-based composite. This property is infl uenced by fi bre morphology, chemical composition, 52 extractive chemicals and processing conditions. 53 Table 1.3 shows the surface properties of wood fi bres in comparison with other natural fi bres. Due to the high polar character of the surface, the fi bres are less compatible with non-polar resin. Therefore, the combination of the inherent polar and hydrophilic features of wood fi bres and the non-polar characteristics of resins gives rise to diffi culties in compounding these materials, resulting in ineffi cient stress transfer of its composites under load. The use of different kinds of physical (i.e. corona discharge) and chemical surface treatment methods (i.e. coupling agents such as silanes) leads to changes in the surface structure of the fi bres as well as to changes of surface properties. The mechanical properties of wood fi bres are of great importance for their use in the paper 67,68 and composite industries. 69 The mechanical properties of materials can be characterized from two methods, namely, macroscopic tests (e.g. tensile test) and indentation tests. The macroscopic tests focus on measuring the mechanical performance of the whole sample, while the indentation tests focus on measuring a local area of the sample. 70 In Table 1.3 Surface properties of natural fi bres Fibres Surface area (m 2/g) γd (mJ/m 2) ( ζ0− ζ∞)/ ζ0 (mV) ζplateau (mV) References Flax 0.31 ~ 0.79 23.85 0.88 ~ 0.95 −1.1 ~ −0.21 54–57 Hemp 0.75 31.6 0.91 −0.1 55, 57, 58 Sisal 1.63 32.90–48.35 0.76 ~ 0.88 −1.7 ~ −0.4 55–57, 59 Coir 0.48 45.05 0.22 −4.6 ~ −3.8 56, 59, 60 Softwood 0.97 31 – −18.6 ± 5.0 61–63 Hardwood 1.34 32–47 – −7.3 64–66 γd: dispersive surface energy; ζ0 : ζ-potential initial value; ζ∞ : ζ-potential fi nal value; ζplateau: ζ-potential plateau value