20 Natural fibre composites performance of the wood-PP composite was increased.170 Alberto et al.17 modified wood fibres with (i)cold water,(ii)hot water and(iii)hot water with sodium hydroxide (1%concentration),and found that the third treatment could increase the compatibility factor significantly for the fibres from Jambire and Wimbe with a compatibility factor of 84.77%and 83.77%respectively. The main mechanism of the reinforcement by alkaline treatment may be due to the degradation of hemicellulose and amorphous content,as the alkaline treatment products are more effective than the polar extractive treatment.172 Oxidation Oxidation modification can be achieved under mild conditions.In this case carboxyl groups,aldehyde groups and ketone groups can be introduced into the cellulose chains by the selective oxidation of primary or secondary hydroxyl groups in the chains.In 1938,Yackel et al.173 firstly employed NO,as oxidant to oxidate cellulose selectively.After that,various primary4-s and secondary-oxidative systems have been reported.Recently,due to the excellent selective oxidation,TEMPO-NaBr-NaClO and TEMPO-NaClO- NaCIO oxidative systems81.17-197 have received much attention around the world.Potthast et al.198 investigated the new functional groups on the surface of hemp fibres which were introduced by the TEMPO oxidation system. Results showed that the content of hydroxyl groups was influenced by the concentration of oxidant and the treatment time.Matsui et al.199.200 investi- gated the influence of ozone oxidation pretreatment on the graft copoly- merization of methyl methacrylate on the surface of hemp fibres and found that,with the increase of oxidation time,hydroperoxide (HPO)increased from 0 mol/cell.molecule to 160 mol/cell.molecule,and the CI of the fibres decreased from 69.7%to 68.3%,but the degree of grafting increased signif- icantly from 14%to 129%. Crosslinking Multifunctional compounds which have more than two functional groups have always been used as crosslink agents to crosslink the inter-chains of cellulose by reacting with the hydroxyl groups.The crosslink modifica- tion of cellulosic fibres has always been carried out by etherification201 and esterification.202 The crosslinking of cellulose has been found important for commercial application in textile finishing of cellulose-based fabrics with end-use properties,e.g.wrinkle resistance,permanent press and easy care properties.Wood fibres are the main contributor to the hygro-expansion (which is one of the drawbacks of wood fibres)of wood fibre-based com- posites.The crosslinking modification can reduce the transverse coefficient of hygro-expansion of the wood fibres203 and result in the improvement of environment and dimension stability of wood fibre-based composite.720 Woodhead Publishing Limited,2014
20 Natural fi bre composites © Woodhead Publishing Limited, 2014 performance of the wood-PP composite was increased. 170 Alberto et al . 171 modifi ed wood fi bres with (i) cold water, (ii) hot water and (iii) hot water with sodium hydroxide (1% concentration), and found that the third treatment could increase the compatibility factor signifi cantly for the fi bres from Jambire and Wimbe with a compatibility factor of 84.77% and 83.77% respectively. The main mechanism of the reinforcement by alkaline treatment may be due to the degradation of hemicellulose and amorphous content, as the alkaline treatment products are more effective than the polar extractive treatment. 172 Oxidation Oxidation modifi cation can be achieved under mild conditions. In this case carboxyl groups, aldehyde groups and ketone groups can be introduced into the cellulose chains by the selective oxidation of primary or secondary hydroxyl groups in the chains. In 1938, Yackel et al.173 fi rstly employed NO 2 as oxidant to oxidate cellulose selectively. After that, various primary 174–181 and secondary 185–186 oxidative systems have been reported. Recently, due to the excellent selective oxidation, TEMPO-NaBr-NaClO and TEMPO-NaClONaClO 2 oxidative systems 181,187–197 have received much attention around the world. Potthast et al.198 investigated the new functional groups on the surface of hemp fi bres which were introduced by the TEMPO oxidation system. Results showed that the content of hydroxyl groups was infl uenced by the concentration of oxidant and the treatment time. Matsui et al.199,200 investigated the infl uence of ozone oxidation pretreatment on the graft copolymerization of methyl methacrylate on the surface of hemp fi bres and found that, with the increase of oxidation time, hydroperoxide (HPO) increased from 0 mol/cell. molecule to 160 mol/ cell. molecule, and the CI of the fi bres decreased from 69.7% to 68.3%, but the degree of grafting increased significantly from 14% to 129%. Crosslinking Multifunctional compounds which have more than two functional groups have always been used as crosslink agents to crosslink the inter-chains of cellulose by reacting with the hydroxyl groups. The crosslink modifi cation of cellulosic fi bres has always been carried out by etherifi cation 201 and esterifi cation. 202 The crosslinking of cellulose has been found important for commercial application in textile fi nishing of cellulose-based fabrics with end-use properties, e.g. wrinkle resistance, permanent press and easy care properties. Wood fi bres are the main contributor to the hygro-expansion (which is one of the drawbacks of wood fi bres) of wood fi bre-based composites. The crosslinking modifi cation can reduce the transverse coeffi cient of hygro-expansion of the wood fi bres 203 and result in the improvement of environment and dimension stability of wood fi bre-based composite. 77,204
Wood fibres as reinforcements in natural fibre composites 21 Grafting Chemical modification through graft copolymerization is an effective method that improves the compatibility of wood and other natural fibres with hydrophobic matrices.The technique involves the grafting of various monomers onto the surface of cellulosic fibres.205.206 The reaction is usually initiated by free radicals of cellulose molecules.The cellulose is exposed to high-energy ionizing radiation.After treatment with the selected ions(tran- sition metal ions),oxidative reagents (as initiating agents),initiate free radi- cals on cellulose.207 The radical sites initiate the grafting of alkyl acrylates (such as methyl,ethyl,butyl and propyl),vinyl monomer (such as methyl methacrylate and acrylonitrile)to the cellulosic surface.Maleic anhydride (MA)grafting treatment has been reported to function efficiently for nat- ural fibre-based composite.Among the grafting treatments,MA grafting is the main method208-226 for the modification of wood fibres.The type and con- centration of MA can influence the mechanical performance of the compos- ite.It has been reported that MD411D displayed a better performance at 2%concentrations.210 Compared with the other natural fibres(e.g.cotton), wood fibre-based composites display better mechanical performance under low fibre loading (<10%).216 The length of fibres has a positive effect on the tensile modulus and modulus of elasticity (MOE).220 In addition,the other additives (e.g.compatibilizer210212)also affect the mechanical performance of composites.Other grafting methods,e.g.methyl methacrylate (MMA) grafting,227-230 styrene grafting,231-233 cetyl alcohols4 grafting and polymer grafting?35236 have also been reported. Coupling In wood fibre composite industries,the coupling modification is the most important method.Coupling agents can be defined as the substances that are used in small quantities to treat a surface so that bonding occurs between filler and matrix.Coupling agents can be subdivided into two broad catego- ries:bonding agents and surfactants (also known as surface active agents). At present,over 40 coupling agents have been used in the production and research of natural fibre composites.237 The most popular treatments include the use of silanes238-251 and isocyanates.236.248.252.253 1.3.3 Nanotechnology (NT)modification Nanotechnology (NT)is defined by the US National Nanotechnology Initiative as the understanding,manipulation and control of matter at dimensions around 1-100 nm.Currently,most major governments around the world are investing heavily in NT254 and many see it as the fuel for the next Industrial Revolution.With the large amount of fundamental research Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 21 © Woodhead Publishing Limited, 2014 Grafting Chemical modifi cation through graft copolymerization is an effective method that improves the compatibility of wood and other natural fi bres with hydrophobic matrices. The technique involves the grafting of various monomers onto the surface of cellulosic fi bres. 205,206 The reaction is usually initiated by free radicals of cellulose molecules. The cellulose is exposed to high-energy ionizing radiation. After treatment with the selected ions (transition metal ions), oxidative reagents (as initiating agents), initiate free radicals on cellulose. 207 The radical sites initiate the grafting of alkyl acrylates (such as methyl, ethyl, butyl and propyl), vinyl monomer (such as methyl methacrylate and acrylonitrile) to the cellulosic surface. Maleic anhydride (MA) grafting treatment has been reported to function effi ciently for natural fi bre-based composite. Among the grafting treatments, MA grafting is the main method 208–226 for the modifi cation of wood fi bres. The type and concentration of MA can infl uence the mechanical performance of the composite. It has been reported that MD411D displayed a better performance at 2% concentrations. 210 Compared with the other natural fi bres (e.g. cotton), wood fi bre-based composites display better mechanical performance under low fi bre loading (<10%). 216 The length of fi bres has a positive effect on the tensile modulus and modulus of elasticity (MOE). 220 In addition, the other additives (e.g. compatibilizer 210,212 ) also affect the mechanical performance of composites. Other grafting methods, e.g. methyl methacrylate (MMA) grafting, 227–230 styrene grafting, 231–233 cetyl alcohol 234 grafting and polymer grafting 235,236 have also been reported. Coupling In wood fi bre composite industries, the coupling modifi cation is the most important method. Coupling agents can be defi ned as the substances that are used in small quantities to treat a surface so that bonding occurs between fi ller and matrix. Coupling agents can be subdivided into two broad categories: bonding agents and surfactants (also known as surface active agents). At present, over 40 coupling agents have been used in the production and research of natural fi bre composites. 237 The most popular treatments include the use of silanes 238–251 and isocyanates. 236,248,252,253 1.3.3 Nanotechnology (NT) modifi cation Nanotechnology (NT) is defi ned by the US National Nanotechnology Initiative as the understanding, manipulation and control of matter at dimensions around 1–100 nm. Currently, most major governments around the world are investing heavily in NT 254 and many see it as the fuel for the next Industrial Revolution. With the large amount of fundamental research
22 Natural fibre composites under government funding today,NT has applications across nearly all economic sectors and allows the development of new critical enabling sci- ence with broad commercial potential,such as nano-structured materi- als,nanoscale-based manufacturing processes and nano-electronics.It has been demonstrated in recent years that NT can be used to modify natural fibres to introduce new functions onto the surface of fibres and enhance the performance of the final fibre-based products.This modification has been used in the textile255.256 and paper industries257 successfully.It is believed that the application of NT to the modification of wood fibres offers high economic potential for the development of fibre-based industry. Various approaches have been developed to immobilize nanoparticles on the surface of wood and other natural fibres,among which,layer-by-layer (LbL)deposition258-26 and the sol-gel process22-26 are the main approaches that have commonly been employed by researchers.Different kinds of nanoparticles (e.g.AgNPs,258.266-269 TiO,,269.270 Si,,27-273 ZnO274275)have been developed to impart multifunctional properties(e.g.anti-bacteria,UV resistant,anti-wrinkle finishing,water repellent)to fibres. A novel way of combining biological technology with NT was first reported by Juntaro et al.2%6 in 2007 This green technique firstly employed bacteria Gluconacetobacter xylinus strain BPR 2001 to treat natural fibres (hemp and sisal),then fabricated the bacterial cellulose on the surface of natural fibres.These modified natural fibres were then incorporated into the renewable polymers cellulose acetate butyrate (CAB)and poly-L-lactic acid(PLLA).It was found that the modified sisal PLLA composites showed an increase in parallel strength of 44%and an increase in off-axis composite strength of 68%. 1.4 Matrices (binders)of wood fibre composites Wood-based composites can be classified as follows,based on the form of their structural components: (i)Fibrous composites(composed of fibres in a matrix),such as fibreboard, MDF and wood plastic composite (WPC); (ii)Laminar composites (composed of layers of materials),such as ply- wood,woven and non-woven textile composite; (iii)Particulate composites(composed of particles in a matrix),such as par- ticle boards(PB). In wood fibre composites,wood fibres are the dispersed phase.The matrix may be inorganic compound,natural polymer or synthetic resin.The com- posites'shape,surface appearance,environmental tolerance and overall durability are dominated by the matrix,while the fibrous reinforcement Woodhead Publishing Limited,2014
22 Natural fi bre composites © Woodhead Publishing Limited, 2014 under government funding today, NT has applications across nearly all economic sectors and allows the development of new critical enabling science with broad commercial potential, such as nano-structured materials, nanoscale-based manufacturing processes and nano-electronics. It has been demonstrated in recent years that NT can be used to modify natural fi bres to introduce new functions onto the surface of fi bres and enhance the performance of the fi nal fi bre-based products. This modifi cation has been used in the textile 255,256 and paper industries 257 successfully. It is believed that the application of NT to the modifi cation of wood fi bres offers high economic potential for the development of fi bre-based industry. Various approaches have been developed to immobilize nanoparticles on the surface of wood and other natural fi bres, among which, layer-by-layer (LbL) deposition 258–261 and the sol-gel process 262–265 are the main approaches that have commonly been employed by researchers. Different kinds of nanoparticles (e.g. AgNPs, 258,266–269 TiO 2 , 269,270 SiO 2 , 271–273 ZnO 274,275 ) have been developed to impart multifunctional properties (e.g. anti-bacteria, UV resistant, anti-wrinkle fi nishing, water repellent) to fi bres. A novel way of combining biological technology with NT was fi rst reported by Juntaro et al.276 in 2007. This green technique fi rstly employed bacteria Gluconacetobacter xylinus strain BPR 2001 to treat natural fi bres (hemp and sisal), then fabricated the bacterial cellulose on the surface of natural fi bres. These modifi ed natural fi bres were then incorporated into the renewable polymers cellulose acetate butyrate (CAB) and poly-L-lactic acid (PLLA). It was found that the modifi ed sisal PLLA composites showed an increase in parallel strength of 44% and an increase in off-axis composite strength of 68%. 1.4 Matrices (binders) of wood fibre composites Wood-based composites can be classifi ed as follows, based on the form of their structural components: (i) Fibrous composites (composed of fi bres in a matrix), such as fi breboard, MDF and wood plastic composite (WPC); (ii) Laminar composites (composed of layers of materials), such as plywood, woven and non-woven textile composite; (iii) Particulate composites (composed of particles in a matrix), such as particle boards (PB). In wood fi bre composites, wood fi bres are the dispersed phase. The matrix may be inorganic compound, natural polymer or synthetic resin. The composites’ shape, surface appearance, environmental tolerance and overall durability are dominated by the matrix, while the fi brous reinforcement
Wood fibres as reinforcements in natural fibre composites 23 carries most of the structural load,thus providing macroscopic stiffness and strength.277 1.4.1 Inorganic compounds The inorganic compounds used for wood fibre composites are cement,clay and lime.In ancient Egypt and China,clay had first been used to make walls. In the early to mid-1940s,natural fibre-cement composite was first investi- gated in Australia.278 Wood fibre-inorganic compound composites are one of the most successful applications of wood fibres in the composite industry.They have been widely used as corrugated or flat roofing materials,cladding panels and water containers,in a large number of building and agricul- ture applications.One of the drawbacks associated with wood fibres in inorganic application is their dimensional instability when the compos- ites are subjected to changing relative humidity(RH)atmosphere.This instability is promoted by:(i)the water sensitivity of cellulose fibres, (ii)the effects of carbonation,high alkali content of the cement matrix and the generation of incompatible stresses.However,the addition of wood fibre can bring three benefits:(i)improving the toughness,tensile strength and the cracking deformation of the composite;(ii)increas- ing the solids retention and (iii)capturing CO2 and locking it up in buildings. 1.4.2 Natural polymers Natural polymer matrices include natural polymers and synthetic natural polymers.The natural polymers typically include starch and rubber.The synthetic natural polymers include PLA and poly(hydroxy butyrate)(PHB), etc.which are made from natural products(e.g.sugar,vegetable oil)by var- ious chemical polymerization methods. The use of starch to produce biodegradable plastics began in the 1970s. Starch-based products have prevailed all over the market,from packaging to the toy industry.2 Starch has been considered as one of the most promising materials for bio- degradable plastics because of its natural abundance and low cost.However, low water resistance and poor mechanical properties are the main drawbacks of starch-based plastics.Using wood fibre to reinforce thermoplastic starch did not gain attention until the end of the 1990s.In 2001,researchers280281 reported that (i)the addition of wood fibre could increase the modulus and the tensile strength of the composite by up to 156%and 120%,respectively, and reduce the water uptake of the composite,281 and (ii)the addition of wood Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 23 © Woodhead Publishing Limited, 2014 carries most of the structural load, thus providing macroscopic stiffness and strength. 277 1.4.1 Inorganic compounds The inorganic compounds used for wood fi bre composites are cement, clay and lime. In ancient Egypt and China, clay had fi rst been used to make walls. In the early to mid-1940s, natural fi bre‒cement composite was fi rst investigated in Australia. 278 Wood fibre–inorganic compound composites are one of the most successful applications of wood fibres in the composite industry. They have been widely used as corrugated or flat roofing materials, cladding panels and water containers, in a large number of building and agriculture applications. One of the drawbacks associated with wood fibres in inorganic application is their dimensional instability when the composites are subjected to changing relative humidity (RH) atmosphere. This instability is promoted by: (i) the water sensitivity of cellulose fibres, (ii) the effects of carbonation, high alkali content of the cement matrix and the generation of incompatible stresses. However, the addition of wood fibre can bring three benefits: (i) improving the toughness, tensile strength and the cracking deformation of the composite; (ii) increasing the solids retention and (iii) capturing CO2 and locking it up in buildings. 1.4.2 Natural polymers Natural polymer matrices include natural polymers and synthetic natural polymers. The natural polymers typically include starch and rubber. The synthetic natural polymers include PLA and poly(hydroxy butyrate) (PHB), etc. which are made from natural products (e.g. sugar, vegetable oil) by various chemical polymerization methods. The use of starch to produce biodegradable plastics began in the 1970s. Starch-based products have prevailed all over the market, from packaging to the toy industry. 279 Starch has been considered as one of the most promising materials for biodegradable plastics because of its natural abundance and low cost. However, low water resistance and poor mechanical properties are the main drawbacks of starch-based plastics. Using wood fi bre to reinforce thermoplastic starch did not gain attention until the end of the 1990s. In 2001, researchers 280,281 reported that (i) the addition of wood fi bre could increase the modulus and the tensile strength of the composite by up to 156% and 120%, respectively, and reduce the water uptake of the composite, 281 and (ii) the addition of wood
24 Natural fibre composites flour could influence the rheological behaviour of the composite and result in a slight decrease of mechanical performance. Wood flour has been used as filler for rubber matrix.The addition of wood flour could increase the modulus,hardness and tensile strength of rub- ber.Wood fibre-rubber composites can be processed by extrusion,injec- tion moulding,calendering and milling.They can be used for automotive tyres,sheeting,metal braids,belts,diaphragms and gaskets,roofing and ship fenders. PLA is a versatile biopolymer made from renewable agricultural raw materials,and is fully biodegradable.It can be processed similarly to poly- olefin.The final PLA-based products display good stiffness and strength. PLA-wood fibre composites can be processed by extrusion,injection mould- ing,film and sheet casting,and spinning,providing access to a wide range of materials.The main drawbacks of PLA-based products are low toughness and thermal stability.Due to their inferior interface,282 the addition of wood fibres without any treatment has negative effects on the mechanical per- formance.In addition,the low aspect ratio of wood fibre will reduce the strength of composites.22 Grafting23 and coupling284285 treatments of wood fibres,as aforementioned,have been reported to improve the strength of the PLA-wood fibre composites significantly.In addition,the nanoscale wood fibres without treatment have been found to have a positive reinforcement effect on the PLA matrix.286 1.4.3 Synthetic polymers The synthetic polymers that are used as the matrix for wood fibres include thermoplastics and thermosets.Extensive research has reported the incor- poration of wood fibre with various synthetic polymers.Wood-synthetic polymer composites,namely,WPCs,can be dated back to as early as 1900. Over the past 110 years,the matrices used for WPC have shifted from thermosets to thermoplastic.Currently,the raw materials for WPCs con- sist of a broad range of materials,from those manufactured from waste to those consisting of pulped wood and engineering resins.Formulating for the desired outcome is key for wood fibre-based composites outperforming conventional filled or reinforced products.The main applications of WPCs are decking,consumer goods,car interior parts,house-wares,construction, etc.WPCs have many inherent benefits,including consistent material qual- ity and attractive visual appearance. PP.250287-299 polyethylene (PE),28 polystyrene (PS)300-30 and poly(vinyl chloride)(PVC)304-307 are the most commonly used thermoplastic matri- ces.The use of thermoplastic composites is diverse,ranging from automo- tive applications where they serve as door panels and gearshift knobs,to Woodhead Publishing Limited,2014
24 Natural fi bre composites © Woodhead Publishing Limited, 2014 fl our could infl uence the rheological behaviour of the composite and result in a slight decrease of mechanical performance. Wood fl our has been used as fi ller for rubber matrix. The addition of wood fl our could increase the modulus, hardness and tensile strength of rubber. Wood fi bre‒rubber composites can be processed by extrusion, injection moulding, calendering and milling. They can be used for automotive tyres, sheeting, metal braids, belts, diaphragms and gaskets, roofi ng and ship fenders. PLA is a versatile biopolymer made from renewable agricultural raw materials, and is fully biodegradable. It can be processed similarly to polyolefi n. The fi nal PLA-based products display good stiffness and strength. PLA‒wood fi bre composites can be processed by extrusion, injection moulding, fi lm and sheet casting, and spinning, providing access to a wide range of materials. The main drawbacks of PLA-based products are low toughness and thermal stability. Due to their inferior interface, 282 the addition of wood fi bres without any treatment has negative effects on the mechanical performance. In addition, the low aspect ratio of wood fi bre will reduce the strength of composites. 282 Grafting 283 and coupling 284,285 treatments of wood fi bres, as aforementioned, have been reported to improve the strength of the PLA‒wood fi bre composites signifi cantly. In addition, the nanoscale wood fi bres without treatment have been found to have a positive reinforcement effect on the PLA matrix. 286 1.4.3 Synthetic polymers The synthetic polymers that are used as the matrix for wood fi bres include thermoplastics and thermosets. Extensive research has reported the incorporation of wood fi bre with various synthetic polymers. Wood‒synthetic polymer composites, namely, WPCs, can be dated back to as early as 1900. Over the past 110 years, the matrices used for WPC have shifted from thermosets to thermoplastic. Currently, the raw materials for WPCs consist of a broad range of materials, from those manufactured from waste to those consisting of pulped wood and engineering resins. Formulating for the desired outcome is key for wood fi bre‒based composites outperforming conventional fi lled or reinforced products. The main applications of WPCs are decking, consumer goods, car interior parts, house-wares, construction, etc. WPCs have many inherent benefi ts, including consistent material quality and attractive visual appearance. PP, 250,287–299 polyethylene (PE), 289 polystyrene (PS) 300–303 and poly(vinyl chloride) (PVC) 304–307 are the most commonly used thermoplastic matrices. The use of thermoplastic composites is diverse, ranging from automotive applications where they serve as door panels and gearshift knobs, to