Wood fibres as reinforcements in natural fibre composites 15 Table 1.8 Production of commercially important fibre sources during 2002-2011 Year Production of Production Production Production Production Production Fibre wood of cotton of flax (t) of hemp of jute (t) of sisal (t) share fibres(t) lint (t) @ of wood (%) 2002 167257547 18885577 781762 78110 2863037 284355 87.96 2003 170046697 19467665 786350 69824 2798567 305867 87.89 2004 174785477 24530832 1013105 72793 2562784 325679 85.98 2005 173985135 24476010 1008656 78664 2766596 330895 85.86 2006 175826129 24449953 661958 113270 2870460 366497 86.07 2007 180993713 25066925 541087 77079 2823671 370519 86.24 2008 177469786 22487098 527851 71336 2691315 370786 87.16 2009 160557039 20898428 382538 79721 3045089 393953 86.62 2010 170871722 23717040 315583 81048 2828533 363558 86.22 2011 173309240 26102935 315084 81964 2861996 411864 85.34 development of the next generation of materials,products and processes.130 In 2003,the UK government established highly ambitious long-term goals relat- ing to climate change,with the objective of moving towards a 'low carbon economy'and a target to cut carbon dioxide (CO2)emissions by 60%by mid- twenty-first century.The White Paper states that this should be achieved with- out detriment to the UK's competitiveness or security.Then,to effectively reduce CO,emissions while keeping economic growth,different countries have begun to search for new development paths,among which low-carbon development has become widely advocated.131 These actions have accelerated research in natural fibres for application in many industrial sectors. Wood fibres are the most important source among the natural fibres,e.g.as shown in Table 1.8(these data are arranged based on reference 132),show- ing a share of wood fibres of over 85%.Wood fibres are primarily used for the paper and paperboard industry (about 80.5%),representing over 55% of total paper and paperboard production.3 Some 1703%of wood fibres are used in composites,with wood-fibre-based composites making up over 80% of natural fibre reinforced composites.4 1.3 Modification of wood fibres for composites The use of wood fibre to make low cost and eco-friendly composite materi- als is a subject of great importance.However,certain drawbacks of natural fibres (e.g.higher polar and hydrophilic)cause natural fibres to be poorly compatible with polymers,which results in the loss of mechanical proper- ties upon atmospheric moisture adsorption.35 Compared with glass fibres, natural fibres show lower mechanical properties.In order to improve the mechanical properties and the interfacial property of natural fibres,various modifications of the natural fibres have been investigated.These modifica- tions are of three types:physical,chemical and NT. Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 15 © Woodhead Publishing Limited, 2014 development of the next generation of materials, products and processes. 130 In 2003, the UK government established highly ambitious long-term goals relating to climate change, with the objective of moving towards a ‘ low carbon economy’ and a target to cut carbon dioxide (CO 2 ) emissions by 60% by midtwenty-fi rst century. The White Paper states that this should be achieved without detriment to the UK’s competitiveness or security. Then, to effectively reduce CO 2 emissions while keeping economic growth, different countries have begun to search for new development paths, among which low-carbon development has become widely advocated. 131 These actions have accelerated research in natural fi bres for application in many industrial sectors. Wood fi bres are the most important source among the natural fi bres, e.g. as shown in Table 1.8 (these data are arranged based on reference 132), showing a share of wood fi bres of over 85%. Wood fi bres are primarily used for the paper and paperboard industry (about 80.5%), representing over 55% of total paper and paperboard production. 3 Some 17. 03% of wood fi bres are used in composites, with wood-fi bre-based composites making up over 80% of natural fi bre reinforced composites. 4 1.3 Modification of wood fibres for composites The use of wood fi bre to make low cost and eco-friendly composite materials is a subject of great importance. However, certain drawbacks of natural fi bres (e.g. higher polar and hydrophilic) cause natural fi bres to be poorly compatible with polymers, which results in the loss of mechanical properties upon atmospheric moisture adsorption. 65 Compared with glass fi bres, natural fi bres show lower mechanical properties. In order to improve the mechanical properties and the interfacial property of natural fi bres, various modifi cations of the natural fi bres have been investigated. These modifi cations are of three types: physical, chemical and NT. Table 1.8 Production of commercially important fi bre sources during 2002–2011 Year Production of wood fi bres (t) Production of cotton lint (t) Production of fl ax (t) Production of hemp (t) Production of jute (t) Production of sisal (t) Fibre share of wood (%) 2002 167 257 547 18 885 577 781 762 78 110 2 863 037 284 355 87.96 2003 170 046 697 19 467 665 786 350 69 824 2 798 567 305 867 87.89 2004 174 785 477 24 530 832 1 013 105 72 793 2 562 784 325 679 85.98 2005 173 985 135 24 476 010 1 008 656 78 664 2 766 596 330 895 85.86 2006 175 826 129 24 449 953 661 958 113 270 2 870 460 366 497 86.07 2007 180 993 713 25 066 925 541 087 77 079 2 823 671 370 519 86.24 2008 177 469 786 22 487 098 527 851 71 336 2 691 315 370 786 87.16 2009 160 557 039 20 898 428 382 538 79 721 3 045 089 393 953 86.62 2010 170 871 722 23 717 040 315 583 81 048 2 828 533 363 558 86.22 2011 173 309 240 26 102 935 315 084 81 964 2 861 996 411 864 85.34
16 Natural fibre composites 1.3.1 Physical modification Physical modification has always been carried out by using instruments to change the structural and surface properties of the fibres,with the aim of increasing the strength of fibres and the interfacial compatibility between wood fibre and matrices.Traditional methods involve thermotreat- ment,133.134 calendering35.136 and stretching of these,thermotreatment is the most useful to modify natural fibres.When the fibres are subjected to heat treatment above the glass transition temperature of lignin,it is pos- tulated that the lignin will be softened and migrate to the fibre surface. Kraft lignin has a glass transition temperature of 142C.134 Lignin begins to degrade at around 214C;hence,heating the fibres to 200C would be expected to cause some softening.138 Thermal treatments can increase the crystallinity,dimensional stability,hydrophobicity of lignocellulosic fibres. Thermal treatment is an efficient modification for forest products.The final properties of the products may significantly depend on the mod- ification of hemicelluloses.This treatment can improve the moisture resistance of wood-based panels.39.140 Hydrothermal treatment to mod- ify wood flour can increase the storage modulus of poly(lactic acid) (PLA)-wood flour composites by up to 55.65%without any other chem- ical reagents.4 Saturated steam under pressure at various temperatures above 100C results in a decrease in the thickness swelling of the panels while mechanical properties,flexural properties,internal bond strength and screw withdrawal resistance,decrease.142 Medium density fibreboard (MDF)panels made from thermally-treated wood fibres at 180C for 30 min appear to be a practical choice for achieving a low thickness swelling of MDF products. Surface modification by discharge treatment,14314 such as low-tempera- ture plasma,sputtering and corona discharge,is of great interest in relation to the improvement in functional properties of natural fibres.This technique was found to be effective for the improvement of the compatibility between hydrophilic fibres and a hydrophobic matrix.Scientists in some industrial- ized countries,such as France,Japan and the United States,have carried out surface treatment of different fibres with various plasma techniques since the 1960s.To date,scientists in most countries have studied this topic to develop their own industrial projects.Plasma technology has been widely used as an effective method for surface modification of natural fibres such as flax,145.146 sisal,147 and keratin.148 Plasma treatment (Fig.1.5)149-151 mainly causes chemical implantation,etching,polymerization,free radical forma- tion and crystallization,whereas the sputter etching brings about mainly physical changes,such as surface roughness,and this leads to increase in adhesion.145 Woodhead Publishing Limited,2014
16 Natural fi bre composites © Woodhead Publishing Limited, 2014 1.3.1 Physical modifi cation Physical modifi cation has always been carried out by using instruments to change the structural and surface properties of the fi bres, with the aim of increasing the strength of fi bres and the interfacial compatibility between wood fi bre and matrices. Traditional methods involve thermotreatment, 133,134 calendering 135,136 and stretching of these, thermotreatment is the most useful to modify natural fi bres. When the fi bres are subjected to heat treatment above the glass transition temperature of lignin, it is postulated that the lignin will be softened and migrate to the fi bre surface. Kraft lignin has a glass transition temperature of 142°C. 134 Lignin begins to degrade at around 214°C; hence, heating the fi bres to 200°C would be expected to cause some softening. 138 Thermal treatments can increase the crystallinity, dimensional stability, hydrophobicity of lignocellulosic fi bres. Thermal treatment is an effi cient modifi cation for forest products. The fi nal properties of the products may signifi cantly depend on the modifi cation of hemicelluloses. This treatment can improve the moisture resistance of wood-based panels. 139,140 Hydrothermal treatment to modify wood fl our can increase the storage modulus of poly(lactic acid) (PLA)–wood fl our composites by up to 55.65% without any other chemical reagents. 141 Saturated steam under pressure at various temperatures above 100°C results in a decrease in the thickness swelling of the panels while mechanical properties, fl exural properties, internal bond strength and screw withdrawal resistance, decrease. 142 Medium density fi breboard (MDF) panels made from thermally-treated wood fi bres at 180°C for 30 min appear to be a practical choice for achieving a low thickness swelling of MDF products. Surface modifi cation by discharge treatment, 143,144 such as low-temperature plasma, sputtering and corona discharge, is of great interest in relation to the improvement in functional properties of natural fi bres. This technique was found to be effective for the improvement of the compatibility between hydrophilic fi bres and a hydrophobic matrix. Scientists in some industrialized countries, such as France, Japan and the United States, have carried out surface treatment of different fi bres with various plasma techniques since the 1960s. To date, scientists in most countries have studied this topic to develop their own industrial projects. Plasma technology has been widely used as an effective method for surface modifi cation of natural fi bres such as fl ax, 145,146 sisal, 147 and keratin. 148 Plasma treatment (Fig. 1.5) 149–151 mainly causes chemical implantation, etching, polymerization, free radical formation and crystallization, whereas the sputter etching brings about mainly physical changes, such as surface roughness, and this leads to increase in adhesion. 145
Wood fibres as reinforcements in natural fibre composites 17 (a) (b) Upper electrode 888808g8888888 Glass plate Lower electrode (c) 1.5 Schematic of plasma treatment:(a)plasma lamp;(b)plasma system and(c)wood fibre after discharge treatment. The processing of wood fibres can result in various chemical compositions in the wood fibres,as shown in Table 1.2.The discharge treatment'52(diffuse coplanar surface barrier discharge (DCSBD)plasma)of wood fibres can result in polar carbonyl groups(C=O)and a considerable increase of free surface energy to reduce the water uptake of wood.The discharge treatment (corona discharge)of wood fibres obtained by mechanical processing was reported to produce 2.4 carboxyl and 10.9 carbonyl functions per hundred C9 units of lignin.153 The cold Ar plasma treatment can result in the genera- tion of higher phenoxy radical concentration in CTMP154 the concentration of which was four times that of TMP.By using heteronuclear single quantum coherence(2D-HSQC)spectroscopy and nuclear magnetic resonance spec- troscopy of carbon (1C-NMR),it was found that the generation of phenoxy radicals can lead to cross-linkages of lignin monomeric units and formation of new inter-monomeric C-C and C-O bonds.In both fibres,the chemical structure of lignin was heavily modified by plasma treatment and the CTMP forms much more radicals than chemical pulp.s5 In addition,a variety of surface modifications can be achieved depending on the type of discharge.Carlsson et al.156157 studied the effects of hydrogen @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 17 © Woodhead Publishing Limited, 2014 The processing of wood fi bres can result in various chemical compositions in the wood fi bres, as shown in Table 1.2. The discharge treatment 152 (diffuse coplanar surface barrier discharge (DCSBD) plasma) of wood fi bres can result in polar carbonyl groups (C=O) and a considerable increase of free surface energy to reduce the water uptake of wood. The discharge treatment (corona discharge) of wood fi bres obtained by mechanical processing was reported to produce 2.4 carboxyl and 10.9 carbonyl functions per hundred C9 units of lignin. 153 The cold Ar plasma treatment can result in the generation of higher phenoxy radical concentration in CTMP, 154 the concentration of which was four times that of TMP. By using heteronuclear single quantum coherence (2D-HSQC) spectroscopy and nuclear magnetic resonance spectroscopy of carbon ( 13 C-NMR), it was found that the generation of phenoxy radicals can lead to cross-linkages of lignin monomeric units and formation of new inter-monomeric C–C and C–O bonds. In both fi bres, the chemical structure of lignin was heavily modifi ed by plasma treatment and the CTMP forms much more radicals than chemical pulp. 155 In addition, a variety of surface modifi cations can be achieved depending on the type of discharge. Carlsson et al . 156,157 studied the effects of hydrogen Plasma zone Sample Upper electrode Plasma zone Sample Lower electrode 5.5 mm Glass plate (a) (c) (b) 1.5 Schematic of plasma treatment: (a) plasma lamp; (b) plasma system and (c) wood fi bre after discharge treatment
18 Natural fibre composites and oxygen plasma treatments on wood fibres and found that the hydrogen plasma treatment reduced the hydroxyl groups and the water absorption of the wood fibres.By contrast,the oxygen plasma treatment displayed an improvement of water wettability. 1.3.2 Chemical modification Chemical modifications utilize chemical agents to modify the surface of fibres or the whole fibre.They can modify the structure of wood fibres or introduce new hydrophobic functional groups into the surface of wood fibres to reduce the hydrophobicity of fibres.The modification can be classi- fied into five methods:mercerization,oxidation,crosslink,grafting and cou- pling agent treatment(Fig.1.6). Mercerization Mercerization is an old method of cellulose fibre modification,which is an alkaline treatment method for cellulose fibres.The process was devised in 1844 by John Mercer of Great Harwood,Lancashire,England,who treated cotton fibre with sodium hydroxide.'ss This treatment caused the fibres to swell;about 25%of hydrogen bonds are broken during the swelling process in the post-treatment(drying).159 These bonds will re-bond and the conse- quent effects of the re-bond have been reported in the literature:including (i)decreasing the spiral angle of the microfibrils and increasing the molec- ular direction;(ii)producing fibre fibrillation,i.e.axial splitting of the ele- mentary fibres (or microfibres that constitute the elementary fibre).160-1 This process leads to a decrease in fibre diameter,increasing the aspect ratio and the effective surface area available for wetting by a matrix in a compos- ite;there is also an increase in fibre density as a consequence of the collapse of its cellular structure;and (iii)changing the fine structure of the native cellulose I to cellulose II.163-166 These changes may result in an improvement in fibre strength and hence stronger composite materials.61.671 It was reported that after immersion in alkali for 48 h,the globular pul- trusion presented in the untreated fibre disappeared,leading to the forma- tion of a larger number of voids.Systematic investigations159 have already revealed three important phenomena of cellulose swelling in aqueous alkali,i.e.(i)the passing of the swelling value through a maximum,depend- ing on lye concentration;(ii)a qualitatively similar,but quantitatively dif- ferent,behaviour of all the alkali hydroxides in aqueous solution from LiOH to CsOH on interaction with cellulose in an aqueous medium;and (iii)a phase transition within the region of crystalline order above a lye (alkaline)concentration of 12-15%due to a so-called intracrystalline swelling caused by inclusion of NaOH and H2O into the crystallites. Woodhead Publishing Limited,2014
18 Natural fi bre composites © Woodhead Publishing Limited, 2014 and oxygen plasma treatments on wood fi bres and found that the hydrogen plasma treatment reduced the hydroxyl groups and the water absorption of the wood fi bres. By contrast, the oxygen plasma treatment displayed an improvement of water wettability. 1.3.2 Chemical modifi cation Chemical modifi cations utilize chemical agents to modify the surface of fi bres or the whole fi bre. They can modify the structure of wood fi bres or introduce new hydrophobic functional groups into the surface of wood fi bres to reduce the hydrophobicity of fi bres. The modifi cation can be classifi ed into fi ve methods: mercerization, oxidation, crosslink, grafting and coupling agent treatment (Fig. 1.6). Mercerization Mercerization is an old method of cellulose fi bre modifi cation, which is an alkaline treatment method for cellulose fi bres. The process was devised in 1844 by John Mercer of Great Harwood, Lancashire, England, who treated cotton fi bre with sodium hydroxide. 158 This treatment caused the fi bres to swell; about 25% of hydrogen bonds are broken during the swelling process in the post-treatment (drying). 159 These bonds will re-bond and the consequent effects of the re-bond have been reported in the literature: including (i) decreasing the spiral angle of the microfi brils and increasing the molecular direction; 1 (ii) producing fi bre fi brillation, i.e. axial splitting of the elementary fi bres (or microfi bres that constitute the elementary fi bre). 160–162 This process leads to a decrease in fi bre diameter, increasing the aspect ratio and the effective surface area available for wetting by a matrix in a composite; there is also an increase in fi bre density as a consequence of the collapse of its cellular structure; and (iii) changing the fi ne structure of the native cellulose I to cellulose II. 163–166 These changes may result in an improvement in fi bre strength and hence stronger composite materials. 161,167,168 It was reported that after immersion in alkali for 48 h, the globular pultrusion presented in the untreated fi bre disappeared, leading to the formation of a larger number of voids. Systematic investigations 159 have already revealed three important phenomena of cellulose swelling in aqueous alkali, i.e. (i) the passing of the swelling value through a maximum, depending on lye concentration; (ii) a qualitatively similar, but quantitatively different, behaviour of all the alkali hydroxides in aqueous solution from LiOH to CsOH on interaction with cellulose in an aqueous medium; and (iii) a phase transition within the region of crystalline order above a lye (alkaline) concentration of 12–15% due to a so-called intracrystalline swelling caused by inclusion of NaOH and H 2 O into the crystallites
Wood fibres as reinforcements in natural fibre composites 19 Cellulose fibre Coupling agent OH Si C1 C1 Monomer Grafting Cellulose fibre Coupling Crosslink HO、 OH O0OH NaOH Crosslinking agent Mercerization Oxidation Cell-ONa TEMPO 43长 1.6 Main chemical treatments and modification mechanism of natural fibre. The mercerization process increases the number of hydroxyl groups on the wood fibres surface,which in turn favours water absorption,169 therefore, wood fibres with mercerization should not be suitable for hydrophobic matri- ces.It was reported that the alkaline treated wood fibres incorporated in poly- propylene(PP)can induce the hexagonal phase of iPP and the mechanical @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 19 © Woodhead Publishing Limited, 2014 The mercerization process increases the number of hydroxyl groups on the wood fi bres surface, which in turn favours water absorption, 169 therefore, wood fi bres with mercerization should not be suitable for hydrophobic matrices. It was reported that the alkaline treated wood fi bres incorporated in polypropylene (PP) can induce the hexagonal phase of iPP and the mechanical O O O O O O OOO O O Br Br Monomer Cellulose fibre Coupling agent Si Si OH O Cellulose fibre Coupling OH O O O OH OH OH N-O• OH OH OH HO n O O O O O O O O O O O O O OH OH OH HO HO HO OH OH OH O OH O OH HO O Crosslinking agent Crosslink Oxidation Cell-ONa NaOH Mercerization TEMPO O HO HO HO HO O O O O O n O Grafting C1 C1 O O 1.6 Main chemical treatments and modifi cation mechanism of natural fi bre