Abstract
In the last two decades, laccases have received much attention from researchers because of their specific ability to oxidize lignin. This function of laccase is very useful for applications in several biotechnological processes, including delignification in the pulp and paper industry and the detoxification of industrial effluents from the textile and petrochemical industries. This review focuses on laccase-mediated fiberboard synthesis. Growing concerns regarding the emission of formaldehyde from wood composites has prompted industrialists to consider the fabrication of green composites. Laccase-mediated fiber treatments oxidize the lignin component without affecting the cellulose structure. As a result, free radicals are generated on the fiber surface, and these can act as potential reactive sites for further cross-linking reactions in board manufacturing. Binderless fiberboards prepared using such methods can be considered as green composites because the manufacturing process involves no additional resin.
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Laccase, an Emerging Tool to Fabricate Green Composites: A Review
Mohammed Nasir,a,* Rokiah Hashim,a,* Othman Sulaiman,a Noor Afeefah Nordin,a Junidah Lamaming,a and Mohd Asim b
In the last two decades, laccases have received much attention from researchers because of their specific ability to oxidize lignin. This function of laccase is very useful for applications in several biotechnological processes, including delignification in the pulp and paper industry and the detoxification of industrial effluents from the textile and petrochemical industries. This review focuses on laccase-mediated fiberboard synthesis. Growing concerns regarding the emission of formaldehyde from wood composites has prompted industrialists to consider the fabrication of green composites. Laccase-mediated fiber treatments oxidize the lignin component without affecting the cellulose structure. As a result, free radicals are generated on the fiber surface, and these can act as potential reactive sites for further cross-linking reactions in board manufacturing. Binderless fiberboards prepared using such methods can be considered as green composites because the manufacturing process involves no additional resin.
Keywords: Laccase structure; Radicals, Crystallinity index; Self-bonding
Contact information: a: Division of Bioresource, Paper and Coatings Technology, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia; b: Biocomposite Laboratory, INTROP, Universiti Putra Malaysia, Serdang, 43400, Selangor, Malaysia;
* Corresponding authors: mnknasir@gmail.com; hrokiah@usm.my
INTRODUCTION
Wood is the best example of a natural composite. The polysaccharides, cellulose and hemicellulose, of wood are embedded in a matrix of aromatic lignin (Moniruzzaman and Ono 2013). Likewise, commercial wood composites are made from wood-based materials bonded together with a synthetic adhesive using heat and pressure (Li et al. 2007). Therefore, in wood composites, the synthetic adhesive is the only foreign component that is not present in the natural wood. These synthetic adhesives are mostly formaldehyde-based, produced from non-renewable resources such as petroleum and natural gas, and they emit formaldehyde throughout their usable life (Li et al. 2009; Moubarik et al. 2009). The International Agency for Research on Cancer has classified formaldehyde gas as carcinogenic to humans (IARC 2004). Thus, to fabricate a completely natural wood composite, it should be free of synthetic adhesives. Many alternative methods are being studied, such as using natural adhesives, using lignin- or tannin-based adhesives, or modifying wood fiber to facilitate self-bonding. A number of research attempts have been conducted to reduce or replace the formaldehyde content in adhesives preparation, but none of them have been commercially applicable (Khan et al.2004; Khan and Ashraf 2006). The most successful attempt to utilize lignin as an adhesive for industrial applications was an investigation into substituting the phenol of phenol-formaldehyde (PF) resins with 50 wt% organosolv lignin obtained from groundnut shell lignin (GNSL) (Khan and Ashraf 2006; Nasir et al. 2013b).
Fiber modification is a new approach to initiate self-bonding between fibers in fiberboard. Several chemical, mechanical, and biological methods have been suggested to modify the physical, mechanical, or thermoplastic behaviors of cellulosic fibers (Gradwell 2004; Li et al. 2007; Nasir et al.2014b). Such methods include alkaline treatment, steam treatment, microwave treatment, enzyme treatment, or high-temperature treatment to modify the physico-chemical behavior of cellulosic fibers. Recently, there has been increasing interest in enzyme-assisted fiber modification. Several enzymes have been identified that specifically act on cellulose, hemicellulose, or lignin without affecting the other components (Chandra and Ragauskas 2002). Enzymatic treatments are often milder, causing less damage to the original structure, and more environmentally friendly compared to chemical treatments (Kunamneni et al. 2008a). Laccase is a well-established oxidoreductase enzyme that acts specifically on lignin in cellulose fiber hydrolysis. Research by Felby et al. (1997, 2002, 2004) and Kharazipour et al. (1997) revealed that wood fibers could be enzymatically activated in vitro by laccase enzymes; this treated fiber could be used to produce wood composites with enhanced self-bonding between fibers.
Laccase is a widely distributed enzyme in plants and fungi (Milstein et al. 1989). The majority of fungi that produce laccase belong to the class of white rot fungi involved in lignin degradation (Ohkuma et al. 2001; Kharazipour et al. 2008; Yu et al. 2009). Laccase action involves the oxidation of various phenolic polymers present in the lignin structure, with a concomitant reduction of oxygen to water (Witayakran and Ragauskas 2009). Recently, laccase has been utilized in the pulp and paper industry to improve the wet strength of fibers (Felby et al. 1997; Lund and Felby 2001; Mattinen et al. 2011). Laccase treatments usually involve the application of laccase enzymes to activate the lignin in fibers (a one-component system) or the addition of another component with laccase to act as a potential cross-linking agent (a two-component system) (Gochev and Krastanov 2007). Because laccase enzymes are too large to penetrate the cell wall (50 to 100 kDa), treatments are restricted to surface modification only (Kunamneni et al. 2007). Therefore, during enzyme hydrolysis, the free phenolic groups on the fiber surfaces act as potential reactive sites for laccase enzymes to create phenoxy radicals.
Regardless of the mechanisms by which laccase acts upon lignocellulose materials, its application can be very wide such as pulp bleaching, textile-dye bleaching, food improvement, bioremediation of soils and polymer synthesis (Kudanga et al. 2011; Widsten and Kandelbauer 2008). All the possible applications of laccase are under intensive investigation in order to replace the hazardous chemical treatments to environmental friendly enzymatic treatment. A successful example of laccase application is in pulp and paper industries, where it not only acts as a bio-bleaching agent but it also enhances the fiber-to-fiber bonding (Giardina et al. 2010). Based on the research progress of laccase in pulp and paper industries, it is believed that similar approach fiber to fiber bonding can be achieved in wood composite industries also. This review paper focuses on feasibility to prepare a binderless board, by understanding the laccase structure, lignin-laccase reaction mechanism, and fiber improvement.
BINDERLESS COMPOSITE BOARDS
The main problem with these thermosetting adhesives is the emission of volatile formaldehyde vapor, which is carcinogenic in nature (Que et al. 2007; González-García et al. 2011). Many developed countries have focused their research on developing wood composites free from formaldehyde-based adhesives. Table 1 summarizes the research and development into fiberboard prepared without formaldehyde-based adhesives.
Table 1. Important Developments in Fiberboard Preparation by Self-Bonding of Fiber
Thielemans et al. (2002) prepared a binderless board by heating and pressing cellulosic fibers at high temperature. Lignin, an amorphous component, starts plasticizing at a high temperature (above 200 °C) and behaves like a thermoplastic resin (Lora and Glasser 2002). Felby et al. (1997) and Kharazipour et al. (1997) suggested an enzyme-assisted composite fabrication without using any adhesive. Laccase, an oxidoreductase enzyme, was used to generate free radicals, which were expected to help in either the physical or chemical bonding of fibers by modifying the fiber as well as the lignin structure (Kharazipour et al. 1997; Yu et al. 2009). Hüttermann et al. (2001) prepared a binderless particle board using laccase treatment that exhibited improved tensile strengths but lower water resistance. Much research has been done to develop a completely natural fiberboard by treating fiber with laccase, but none of the methods have been commercialized yet (Milstein et al. 1994; Lund and Felby 2001; Felby et al. 2004; Nasir et al. 2013a; Nasir et al. 2014a; Nasir et al. 2014b). Felby et al.(2002) prepared a laccase-treated binderless board in a pilot-scale production. They found that the mechanical strength was good and comparable to the conventional urea formaldehyde-based resin boards but the dimensional stability was very poor. When the wax was applied in treated fiber to improve the dimensional stability, it inhibited the bonding effect of the enzyme (Felby et al. 2002). Thus, binderless boards cannot be regarded as commercially viable until they have been shown to achieve good dimensional stability along with mechanical strength.
LACCASE
Historical Development
Laccase was first discovered by Yoshida (1883) in latex produced from the Japanese lacquer tree (Rhus vernicifera) that hardened in the presence of air (Yoshida 1883; Giardina et al. 2010). It is widely distributed among various classes of angiosperm, gymnosperm, fungi, and bacteria. While laccase is involved in the synthesis and biopolymerization of lignin in higher plants (Raiskila 2008), it plays a major role in the biodegradation of lignin in wood-rotting fungi (Kunamneni et al. 2008b). Although laccase has low redox potential, it can oxidize the phenolic compounds of lignin. This oxidation property of laccase can be improved further by addition of natural or chemical mediators, promoting the oxidation of other the recalcitrant aromatic compounds (Moldes et al. 2010; Garcia-Ubasart et al.2011). Because of their wide reaction capability and broad substrate specificity, laccase enzymes possess great biotechnological potential (Kunamneni et al. 2008b; Garcia-Ubasart et al. 2012). The promising applications of laccase include textile-dye bleaching (Mendonça Maciel et al. 2010), pulp bleaching (Valls et al. 2010), food improvement (Gochev and Krastanov 2007; Mendonça Maciel et al.2010), bioremediation of soils and water (Murugesan 2003; Bustos-Ramírez et al. 2013), polymer synthesis (Wang et al. 2009), and the development of biosensors and biofuel cells (Kim et al. 2014; Fokina et al. 2015).
Laccase is a well-studied oxidoreductase enzyme that acts specifically on lignin and its constituent compounds (such as phenols, polyphenols, anilines, aryldiamines, methoxy-substituted phenols, hydroxyindols, and benzenethiols) (Kunamneni et al. 2007; Van de Pas et al. 2011). It is a compound containing multiple copper atoms that catalyzes the single-electron oxidation of phenolic compounds with a simultaneous reduction of oxygen to water (Zhou et al. 2009; Tian et al. 2012). Laccase treatment of lignocellulosic fibers causes many changes to the physical and chemical properties of the fibers (Garcia-Ubasart et al. 2012). The rate of laccase-catalyzed oxidation varies with the physical and chemical properties of the substrate.
Molecular Structure
Laccases (p-diphenol: dioxygen oxidoreductase, EC 1.10.3.2) are extracellular, monomeric glycoproteins, multinuclear enzymes with carbohydrate contents of 8% to 50% (Gochev and Krastanov 2007). The active site of laccase is comparable to that of ceruloplasmin, ascorbate oxidase, and bilirubin oxidase (Kunamneni et al. 2007). The molecular mass of laccase ranges from 50 to 100 kDa, depending on the source and the origin of the enzyme (Widsten and Kandelbauer 2008; Giardina et al. 2010). Like other enzymes, laccase has a tertiary structure, as revealed under X-ray crystallography (Piontek et al. 2002; Gochev and Krastanov 2007). The function of laccase is dependent on Cu atoms that are arranged in four sets (Enguita et al. 2003). These four Cu atoms, having different electron paramagnetic resonance (EPR) signals, exhibit three redox sites, namely Cu I, II, and III, which play a crucial role in the reaction mechanism (Claus 2004). Cu I is an oxidized form that is EPR detectible and emits a blue color at 600 nm (Bertrand et al. 2002; Enguita et al. 2003). Cu II and Cu III are closely related in structure, but the EPR signal of Cu III is not detectible (Enguita et al. 2003). However, laccase shows a very low redox potential (RP) ranges from 0.4 V to 0.8 V, the fungal laccases show the highest redox potential among all the sources of laccase (Gochev and Krastanov 2007).
Reaction Mechanism
Laccases is a low redox potential enzyme; hence it can only oxidize the phenolic compounds (lignin moieties) having lower redox potential than laccase. The addition of mediator can enhance the substrate range, allowing the oxidation of non-aromatic compounds having redox potentials higher than those of the laccases (Kunamneni et al. 2008a). Laccase utilizes oxygen as the electron acceptor and removes protons from the phenolic hydroxyl group (Kunamneni et al. 2008a). Thus, free radicals are formed on phenolic compounds that can spontaneously rearrange and lead to fission at the C-C or C-O bonds of the alkyl side chains or cause the cleavage of aromatic rings (Dashtban et al. 2010). The laccase catalysis mechanism involves three major reaction steps. Initially, Cu I is reduced by a reducing substrate and oxidizes itself (Claus 2004; Kunamneni et al. 2008a). The electron generated at Cu I is transferred internally to Cu II, then to Cu III; these three atoms are arranged in a triangular structure relative to each other (Bertrand et al. 2002; Claus 2004). In this process, oxygen is reduced to water at the tri-nuclear cluster of copper atoms. The O2 molecule binds to the tri-nuclear cluster of Cu atoms for asymmetric activation, and it is assumed that this O2 bond pocket restricts the entry of any other oxidizing agent except O2 (Dashtban et al. 2010; Giardina et al. 2010). Thus, a laccase enzyme can reduce one molecule of oxygen to two molecules of water through the single-electron oxidation of various aromatic compounds, such as phenols, polyphenols, anilines, aryl diamines, methoxy-substituted phenols, hydroxyindols, and benzenethiols (Widsten 2002; Zhou et al. 2009; Zakzeski et al. 2010).
A laccase enzyme can extend its substrate range from phenolic to non-phenolic organic substrates if the laccase is supplemented with a mediator (Gochev and Krastanov 2007; Lee et al. 2012). A mediator is generally a small-size compound, able to generate stable radicals during reaction (Cañas and Camarero 2010; Euring et al. 2011). These radicals react with various chemical compounds, including non-phenolics, that laccase alone cannot oxidize (Giardina et al. 2010). Some of the common mediators are 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), N-hydroxyphthalimide (NHPI), and 1-hydroxybenzotriazole (HOBT) (Gochev and Krastanov 2007).
LIGNIN
Lignin is a three-dimensional network of three types of phenyl-propanoid monomers with irregular repeating units. The most accepted theory of lignin’s biosynthesis is the radical coupling of three basic monolignols: p-coumaryl, coniferyl, and sinapyl alcohols (Gang et al. 1999; Raiskila 2008). The process of lignin polymerization is known as lignification, during which phenolic monomers produce radicals that couple together with other monomer radicals to form oligomers and, ultimately, a phenyl-propanoid polymer (Brunow 2005; Milstein et al. 1994). The lignin polymer exhibits various bonding such as β-O-4, β-5, 5-5, 5-3, or α-4 bonds. Among these possibilities for lignin polymerization, β-O-4 and 5-5 are the most common and are responsible for two-thirds of the total bonding (Sernek 2002; Brunow 2005).
Lignin Polymerization
Liu et al. (1994) studied lignin polymerization using chemicals and observed that phenolic monomers were polymerized by the catalytic action of phenol oxidases or peroxidases. A similar reaction was observed when lignin was oxidized by laccase enzymes (Boerjan et al. 2003). The enzymatic oxidation of lignin with the oxido-reductases laccase and peroxidase has been shown to increase the bonding strength of fibers in MDF (Felby et al. 2002; Nasir et al. 2013a). The idea of utilizing laccases as oxidizing enzymes for lignin bonding applications was based on the reactivity of phenoxy radicals in the plant cell wall (Barcelo 1997; Wang et al. 2013). In native plants, in-situ oxido-reductase catalysis activity initiates the polymerization through the cross-linking of phenoxy radicals, and the same technique can be applied in the in-vitro bonding of lignocellulosic materials (Cesarino et al. 2012; Koch and Schmitt 2013). The laccase activity during delignification can be improved by adding a redox mediator, which increases the range of substrates from phenolic to non-phenolic compounds (Gutiérrez et al. 2012; Rosado et al. 2012).
Lignin polymerization begins with the oxidation of the phenyl-propane hydroxyl groups (Kuzina et al.2011). The lignin precursors undergo dimerization through enzymatic dehydrogenation, which is initiated by electron transfer and yields resonance-stabilized phenoxy radicals (Chen et al. 2006; Vanholme et al. 2012; Zhang et al. 2012). Figure 1 displays the phenoxyl radical formed from laccase oxidation and the different forms of resonance.
Fig. 1. Generation of resonating phenoxyl radicals by enzymatic dehydrogenation of coniferyl alcohol (adapted from Freudenberg and Neish (1968))
In the resonance structures, the radical changes positions to stabilize the oxidized phenolic compound, but it forms various bonds with other radicals in any of the positions of the unpaired electron (Vanholme et al. 2012). Such monolignols, having free radicals, can undergo radical coupling reactions and produce a number of dimers, called dilignols (van Parijs et al. 2010; Vanholme et al.2012). The β-O-4 and β-5 bonding types result in a linear polymer structure. However, a branched polymer may form when nucleophilic compounds, such as alcohols, phenolic hydroxyl groups, or water, attack the benzyl carbon of the quinone methide intermediate (Cesarino et al. 2012; Rowell 2012).
In the single-electron transfer reaction, lignin molecules are converted into small precursors of lignin called lignols (Boerjan et al. 2003). These lignols react further with additional lignol radicals in a typical chain reaction to form bilignols (Boerjan et al. 2003). The bilignols then undergo further endwise polymerization instead of combining with one another (Boerjan et al. 2003; Mattinen et al.2008). This radical reaction led to a very complex lignin structure, which formed an infinitely random three-dimensional network in the middle lamella of woody plants.
LACCASE APPLICATION IN FIBERBOARD FABRICATION
Currently, laccase is considered one of the most inexpensive and widely available enzymes used in commercial applications (Brijwani et al. 2010; Cristóvão et al. 2011). Some of these applications are already in practice, such as pulp processing (Virk et al. 2012), detoxification of environmental pollutants (Harms et al. 2011), preventing wine browning (Osma et al. 2010), oxidation of dyes and their precursors (Kumar et al. 2011), and producing lignin from cellulosic material (Wang et al. 2014). Laccase has been used commercially as a potential delignification agent in pulp production since the last decade (Camarero et al. 2007; Rico et al. 2014; Wang et al. 2014). Relatively recent studies have started to apply laccase in wood composites to prepare completely natural boards (Huttermann et al.2001; Park et al. 2001; Felby et al. 2002, 2004; Widsten et al. 2004; Frihart and Service 2005). Table 2 presents the chronological development of laccase application in fiberboard by various methods.
Lignin polymerization is another approach, under intensive investigation for generating self-bonding between fibers (Mai et al. 2004; Savolainen et al. 2010). In the laccase-assisted lignin oxidation, various free radicals of phenols and polyphenols are formed. These free radicals are highly reactive and can participate in polymerization, de-polymerization, copolymerization, and grafting (Saastamoinen et al. 2012). Since the lignin structure is very similar to that of phenol–formaldehyde (PF) resins, similar polymerization can be achieved (Khan and Ashraf 2006; Laurichesse and Avérous 2014). To transform lignin into an insoluble adhesive, it must be additionally cross-linked; a lower number of free positions in the aromatic nuclei and a lower rate of reactivity limit the utility of lignin as an adhesive (Khan and Ashraf 2006). Furthermore, the methoxy or methoxy-equivalent groups present on the aromatic ring of lignin are considerably less reactive toward hydroxybenzyl alcohol groups than the hydroxyl groups found in phenol (Pizzi 2003; Khan and Ashraf 2006; Schorr et al.2014). Because of these reasons, lignin cannot be utilized as effectively as a potential adhesive as synthetic PF resins. Thus, potential cross-linking agents, such as polyisocyanates (Dunky 2003), epoxides (Zakzeski et al. 2010), polyols (Pizzi 2003), polyethyleneimine (Huang and Li 2007), maleic anhydride (Gu and Li 2010), proteins (Frihart 2010; Hamarneh et al. 2010), amines (Dunky 2003), or melamine (Amaral-Labat et al. 2012), are required to achieve the desired results. Due to the aforementioned concerns for safety and the environment, the use of formaldehyde as a possible agent of lignin polymerization was not included here. So far, all of these procedures, for different reasons, have not led to the development of any major practical applications. Laccase treatment to activate the lignin structure was a new technique developed in the early 1990s (Felby et al. 1997; Huttermann et al.2001; Wu et al. 2011).
Table 2. Chronological Development of Laccase Applications to Fiberboard Fabrication
Laccase is applied to wood composites with two goals: physical modification of fiber and chemical modification of fiber. The physical modifications may include changes in the crystallinity or the morphology of the fiber surface. Such changes may improve the mechanical strength and facilitate self-bonding of fibers by mechanical interlocking (Winandy and Rowell 2005). The chemical modifications include the activation of the lignin molecules of lignocellulose fibers to induce lignin polymerization reactions (Tamminen et al. 2010; Moilanen et al. 2011).
Crystallinity Index Improvement
Cellulose is a long, linear chain of D-glucose connected with ß-1,4-glycosidic bonds. The hydroxyl groups present in its basic structural unit link laterally by well-organized hydrogen bonding networks, giving rise to a crystalline structure (Janga et al. 2012). Though the first crystalline structure of cellulose was proposed by Carl von Nägeli in 1858 (Wilkie 1961), its structure is not yet fully understood because of its complexity (Quintana et al. 2015). The crystallinity Index (CrI) of cellulose is one of the most important parameters to study in determining the physical and mechanical behavior of cellulose fibers (Nasir et al. 2013a). There are various techniques to calculate the CrI of cellulosic fibers, but their values differ significantly depending on the method used (Bansal et al. 2010). Cellulose exists in four different crystalline forms (polymorphs): I, II, III, and IV. Cellulose I is native cellulose as it exists in its natural state, and the rest are all the result of some chemical modification (Ishikawa et al. 1997).
The crystallinity of cellulose plays an important role in the accessibility and longevity of cellulosic fiber (Awadel-Karim et al. 1999; Schenzel et al. 2005). Li and Pickering (2008) and Nasir et al.(2013a) studied the effect of laccase on cellulosic fiber and observed an up to 22% and 10% increase in the crystallinity index, respectively. Such an effect is consistent with removal of non-crystalline matter. Figure 2 shows the change in the CrI of rubber wood fiber treated with laccase in different time interval (Nasir et al. 2014b).
Fig. 2. Crystallinity index (Crl, %) of laccase-treated fiber at different time intervals (adapted from Nasir et al. (2014b))
With an increase in the crystalline to amorphous ratio, the rigidity of cellulose fibers increases but the flexibility decreases (Ishikawa et al. 1997). Thermogravimetric analysis (TGA) shows an interesting pattern of improved thermal stability when the change in crystallinity is considered (Li and Pickering 2008; Zeng et al. 2011). Laccase mediator-based hydrolysis removes the amorphous phenolic and non-phenolic components from the surface but does not affect the microfibril core, which remains crystalline (Quintana et al. 2015). Thus, the selective removal of amorphous components from the fiber increases the crystallinity of individual fibers.
Surface Modification
Enzymes can be used in cross-linking/self-bonding of wood fibers to prepare fiberboard without any external adhesive (Felby et al. 2002; Widsten and Kandelbauer 2008; Nasir et al. 2013a). Nasir et al.(2013a) treated rubber wood fiber with laccase and observed a smooth deposition of lignin onto the surface (Fig. 3). An enzymatically modified fiber can improve the inter-bonding strength of fiber in many ways, such as surface smoothness/roughness or adsorption/desorption behavior, that can lead to mechanical interlocking between fibers (Symington et al. 2009). Mechanical interlocking is a type of physical force in which two components of distinct interfaces are held together. This mechanism is similar to dovetail joints, where the surface of one component is embedded into another. In a laccase hydrolysis process, along with the breakdown of lignin the precipitation and adsorption of lignin also occurs simultaneously (Maximova 2004; Pribowo et al. 2012). A lignin-adsorbed surface can change the sorption characteristics, dimensional stability, and intermolecular adhesion of the fiber (Maximova 2004; Yu et al. 2009).
Fig. 3. SEM picture of (A) untreated and (B) treated fiber at 2000 × magnification (adapted from Nasir et al. (2013a))
Chemical Modification
Chemical modification involves the direct chemical reaction of components at the interface, either a free radical reaction (Zhou et al. 2009), ionic reaction (Shill et al. 2012), hydrogen bonding, or carbonyl bonding (Hill and Cetin 2000). Figure 4 shows the possible self-bonding reactions that may occur at the time of hot pressing. Laccase is a well-established approach to generate phenoxy radicals from lignin by the oxido-reduction process (Hüttermann et al. 2001; Mai et al. 2004). These free radicals undergo polymerization reactions and form a network of polymers by coupling (similar to thermoset adhesives) (Kunamneni et al. 2008a; Spulber et al. 2014). In recent studies, it has been well established that laccase enzymes obtained from fungi are best suited for the activation of native lignin and accelerating the oxido-reductase coupling of lignin (Ceylan et al. 2008; Liu et al. 2009; Witayakran and Ragauskas 2009; Bledzki et al. 2010; Singh and Singh 2014).
Another approach to improve the self-bonding of the fiber is to bring the copolymer matrix (lignin and hemicellulose) to the surface of the fiber so that it can take part in the auto-adhesion of the fiber when the fiber is pressed at high temperatures. Laccase is a specific enzyme that acts on lignin, but laccase supplemented with a mediator can act on a wide range of substrates (phenolic compounds) (Fillat and Roncero 2010; Lee et al. 2012). It can oxidize the variety of organic compounds present in plant cell walls, such as lignin, ortho- and para-diphenols, aminophenols, polyphenols, aryl diamines, polyamines, and some inorganic ions (Mattinen et al. 2011). These copolymers of the cell wall exhibit an amphiphilic nature and serve as both adsorbing surfaces and adsorbable amphiphiles (Tian et al.2012). It is evident that a lignocellulosic composite can be formed successfully if the wood surface is coated with a thermoplastic cell matrix such as lignin or hemicellulose (Gradwell 2004; Kumar et al.2009; Tian et al. 2012).
Fig. 4. The possible auto-adhesion reactions between two modified fibers (adapted from Widsten (2002))
Although a treated fiber exhibits improved physical and mechanical properties compared to an untreated fiber, its water resistance properties decrease (Li and Pickering 2008). The untreated fibers are bundled together and the surfaces are covered with non-cellulosic compounds, such as lignin, wax, and pectin, which restrict the water absorption. The removal of such compounds during laccase treatments separates the fiber bundles and exposes hydroxyl groups on the fiber surface. Much research has been proposed on producing wood composites from laccase-modified natural fiber, but fiber modification alone cannot achieve the minimum required strength (Lund and Felby 2001; Felby et al.2002). Thus, an enzyme-treated fiberboard can work excellently in combination with adhesive, or it can reduce the quantity of adhesive used.
CONCLUSIONS
Formaldehyde emission is a serious concern, and positive progress in laccase-based wood composites will lead to the manufacture of an eco-friendly, biodegradable composite. Laccase displays a versatile mode of action and has a tremendous scope for future work. It not only plays a role in the delignification of cellulosic fiber but has the capability to remove other phenolic as well as nonphenolic extractives. It is a potential tool to modify the physio-chemical properties of natural fiber by altering surface morphology, surface deposition, pulp grafting, and delignification. The optimum treated fiber exhibits enhanced crystallinity, improved thermal resistance, and higher mechanical strength that would be transferred to the product into which it is made. A binderless board can be formed either by liberating reactive radicals from lignin or by functionalizing lignocellulosic fibers, but further work is needed to improve the mechanical strength and water resistance. An integrated approach of fiber modification and lignin polymerization should be studied. Detailed study will be required to determine the radical reaction mechanism of laccase and the stability of oxidized lignin molecules through laccase reaction engineering.
ACKNOWLEDGEMENTS
The authors acknowledge the Universiti Sains Malaysia for providing a post-doctoral fellowship to Dr. Mohammad Nasir.
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Article submitted: April 28, 2015; Peer review completed: June 29, 2015; Revised version received and accepted: July 9, 2015; Published: July 22, 2015.
DOI: 10.15376/biores.10.3.Nasir