Abstract
Natural fibre or lignocellulosic fibres have aroused the attention of scientists in the polymer industry, given their eco-friendly, low density, and biodegradability. Nevertheless, the major obstacle to wide commercial applications is the incompatibility between polar natural plant fibres with non-polar polymer matrix due to different chemical constitutions. Therefore, surface treatment of natural fibres before they are implemented to prepare fibre-reinforced composites is considered. Chemical and physical treatments are not preferred as it involves hazardous chemicals and high energy consumption. This review article provides an overview of various environmentally friendly approaches, such as introducing bacterial nanocellulose, pre-treatment with bacterial cellulase, fungal treatment, and enzymatic treatments aimed to treat natural fibres. The implications of working mechanisms on the characteristics of fibre itself and polymer composites with reinforcement are reviewed. The application of treated fibres reinforced composites and comparison of biological treatment with other treatments are discussed in this article. It is evidenced that fibre that undergoes biological modifications facilitates better fibre-matrix interfacial adhesion, has stronger mechanical bonding with the matrix, along with the reduction of water uptake of the composites.
Introduction
The cutting-edge recognition of worldwide environmental issues, along with the exhaustion of petroleum resources, volatility of crude oil prices, and the preference to minimize waste disposal, have been put forth to sustain natural resources. Industries and the scientific community are giving more conscious consideration toward sustainability and energy costs, focusing on reducing environmental damage and pollution. Substantial studies attempt to utilise bio-based materials such as green composites as a substitute for non-renewable materials that jeopardize the environment.1–3 Natural fibres, namely bamboo, cotton, kenaf, flax, jute, hemp, rice husk, coffee husk, oil palm, so on, from plant origin, are introduced as lignocellulosic fibres (LCFs). These fibres have garnered interest in materials science because of their ecologically harmless, cheap, recyclability, and local availability.1,4–7 They also exhibit excellent specific mechanical properties in terms of strength, stiffness, as well as low acoustic and thermal conductivity.8,9 Being renewable, degradable, and neutral concerning CO2 emission, LCFs lighten the environmental impact and help in preventing global warming. 10
Natural or lignocellulosic fibres, in preference to glass fibres can be recognised as reinforcing agents in polymer composite materials. For Pommet et al., 8 a composite refers to combining at least two different materials with better performance than when the single component is used alone. In fact, incorporating LCFs into composite supports the production of lighter composite. The relatively lower density of LCFs (1.2–1.6 g/cm3) over a glass fibre (2.4 g/cm3) is a key benefit of creating lignocellulosic fibre composite where the ideal property is centrally weight reduction.11,12 Hence, the interest in commercializing natural fibre-based composites has risen in many sectors, i.e. infrastructure, automotive, and packaging.13–15 In light of this, natural fibre reinforced polymer composites (NFPC) substituting glass fibre reinforced composites (GFRCs) is a new evolution. Different LCFs (e.g. banana, hemp, sisal, flax, jute) have been extensively used and reported in the literature for lightweight composite production.8,14,16–20
Natural fibre reinforced polymer composites is prepared utilizing plant fibre as fillers and polymer matrices. Natural fibres are highly polar and hydrophilic in nature. Polymer matrices can be polar or non-polar, depending on the chain geometry. 21 The difference in chemical constitutions between matrices, specifically, non-polar and highly polar fibre, upsets the interfacial bonding between the two phases, which lowers the composite’s mechanical performance.14,19,22 Climate discrepancy, plant age, and extraction techniques also alter the structural and mechanical characteristics of the fibre. 13 The hydroxyl (-OH) group-rich cellulose structure furnishes the resulting natural fibres with hydrophilic properties. When natural fibres are exposed to a high humidity environment, these groups tend to bind with water molecules through hydrogen bonding (-H). 23 There is a need to reduce the number of cellulose hydroxyl groups available for moisture absorption, which affects the efficacy of natural fibre as composite reinforcement.
Superior fibre reinforcement composite properties require a well-bonded fibre-matrix. In composites, fibres carry the applied force, while the matrix shields the surface of fibres from mechanical damage and transfers stress between the fibres.24–26 Interfacial connection between the phases is correlated to mechanical interlocking, chemical, and physical bonding (van der Waals force and hydrogen bond). 22 The adhesive force between the two components should be maximized for excellent stress transferability from the matrix to the fibre to appear. Cellulose, lignin, hemicelluloses, pectin, and waxes are the components involved in the structure of lignocellulosic fibres. The outer layer of LCFs covered by lignin, waxes, and oils is incompatible with conventional resins and should be extracted by surface treatment. Unstable interfaces are generated when unmodified fibres are incorporated into a polymer matrix. This incompatibility interface is common between LCFs and thermoplastic matrices, such as polypropylene and polyethylene, due to their polarity. As the stress exerted is ineffectively transmitted from the matrix to the fibre, fibre’s desired reinforcement outcome is underutilised.14,25 In this regard, researchers attempted to overcome the aforementioned problems possessed by LCFs through surface modification before they are used with polymer matrices.
One of the proposed solutions to improve the LCF reinforced polymer interfacial adhesion is the fibre surface treatments, which have been found to be beneficial.27–31 These treatments include chemical, physical, and biological methods. Chemical treatment is the most widespread method involving alkali,32,33 silane, 34 peroxide, 15 steric acids. 5 Khoathane et al. 35 defined “chemical modification as a chemical reaction between some reactive constituents of the lignocellulosic fibre and a chemical reagent (with or without a catalyst) forming a covalent bond between the two”. The phase compatibility of polymer matrix and fibres is affected by the hydrophilic character of cellulose, which is bound to the matrix primarily through hydrogen bonding. 36 A chemical reaction occurs with the fibre components, modifying the fibre structure, reorientating the highly packed crystalline cellulose region into the amorphous region, and removing hydrogen bonds in the network structure. 35 The hydroxyl groups (OH) are substituted with less polar acetyl groups (CH3COO−) during acetylation, ionization of the OH group to the alkoxide during alkali treatment, formation of silanol (Si-OH) groups during silane treatment.5,11 These modifications help reduce the polar nature of fibre by substituting hydroxyl groups with hydrophobic functional groups or polymeric chains, removing impurities, and roughening the surface to introduce an interlocking system between LCF and polymer matrix regardless of the type of polymer matrix. Besides, fibre’s surface morphology, tensile strength, and polymer matrix’s wettability are improved, especially for the hydrophobic nature of the thermoplastic matrix.5,28,37 Typically, physical methods are employed to break down the fibre bundles into individual filaments and alter the properties of fibres without affecting the chemical composition extensively. 38 Plasma treatment, electric radiation, ultraviolet (UV), and corona treatment are examples of widely used physical surface modifications.1,39,40
Nevertheless, these two treatments are against the green image of the final composites. It involved large quantities of chemicals and solvents in chemical treatment, generating difficulty-to-dispose contaminants, creating extra expenses for the industry, and causing an environmental burden. Additionally, physical technique, for instance, plasma treatment, is a lengthy and high energy consumption process to obtain fibre, which is unfavourable for low-cost manufacturing.5,33 Alternatively, biologically treating natural fibres has received ample attention in recent years. The biological treatment utilizes naturally occurring microorganisms such as bacteria and fungi to eliminate undesirable components selectively and modify the fibre surface with lower energy input. This treatment also loosens up cellulose’s crystalline structure and facilitates the degradability to release fermentable sugar. 41 This ecological surface modification offers benefits such as cost-effectiveness, no chemical reagents involved, and encouraged the application towards natural fibre with minimal or no undesirable impact relative to the environment, as reported by many researchers.33,35,39,41 The catalysed reactions are specific and have a focused performance. 16 This paper provides an overview of different modifying agents and summarizes the green approaches for surface modification of natural fibres and their effects on fibre qualities. The employment of modified plant fibres as a reinforcement of composite is shown.
Surface modification of natural fibre/lignocellulosic material
Natural fibres exhibit a high tendency toward moisture absorption because of disordered and amorphous polymers, which lowers fibres stiffness and gives rise to internal stress because of swelling and micro-cracks. Consequently, their potential application is restricted as reinforcing polymers once the lack of proper fibre-matrix interfacial adhesion leads to non-uniform fibre dispersion within the matrix. Furthermore, inadequate adhesion between hydrophilic natural fibres and hydrophobic matrices weakens the interface bond strength and limits stress transfer efficiency along with the composites’ interface.22,27,42 Thus, as mentioned earlier, various approaches could solve natural fibre’s problem to qualify them for such application. Surface modifications are attracting considerable attention in developing composite materials as it ensures that natural fibre and hydrophobic matrix are compatible. 36 Generally, fibre surface modification methods are classified into three dominant groups, which are chemical, physical, or biological methods, intending to optimise the accessible area between the fibre surface and the polymer matrix.13,38 This section highlights the biological treatment of the natural fibres, addressing fibres with enzymes, fungi, and coating them with nanocellulose.
Biological treatment
Biological treatment as an eco-friendly and economically feasible alternative to chemicals method encompasses naturally occurring microorganisms, for instance, bacteria, fungi, and enzymes. 13 Pre-treatment of natural fibres via biological agents before composite production is a novel approach performed by several researchers.24,43,44 These modifications have an advantage relative to chemical and physical processes, including less energy consumption, being selective towards pectin and hemicellulose removal, and recycling of enzymes.16,39,40 When it comes to modifying the surface of natural fibres, green surface treatments should be considered.
Coating with bacterial nanocellulose
Cellulose, the main structural component of the plant biomass, is primarily derived from plants (kenaf, sisal, flax, hemp),3,12,45 wood,
37
bacteria,
8
and algae.46,47 It has a flat ribbon-like shape and is made up of long polymer chains of glucose monomer joined by B-1,4-glucan chain (Figure 1). Bacterial cellulose (BC), also termed microbial cellulose, is cellulose nanoparticles (with a diameter of 24–86 nm) that are highly crystalline and impurity-free.
48
When cellulose fibres disintegrated into micro dimensions, crystalline cellulose chains form microfibrils.4,29 Structure of cellulose.
Plant cellulose has a similar molecular formula as bacterial cellulose and differs in physical and chemical features.49,50 BC emerged as a promising biomaterial over plant cellulose because of its high tensile strength, excellent biological affinity, high crystallinity index, large water-holding capacity, and good biodegradability.40,51,52 Gram-positive bacteria species such as Sarcina ventriculi, and gram-negative bacteria species such as Acetobacter, Agrobacterium, Salmonella, and Escherichia, produce cellulose.34,51 Acetobacter species are the most effective cellulose makers among the previously mentioned bacteria. When cellulose-producing bacteria like Acetobacter xylinum are cultured, cellulose is selectively accumulated on the fibre’s surface, which the bacteria use as growth support during the fermentation process. Bacterial cellulose, a bacteria’s extracellular product, will develop and adhere to the fibre surfaces if adequate interactions exist between them. In this way, surfaces of natural fibre would undergo changes on a nanoscale.29,47
Fermentation of BC production is conducted in either static, agitated, or bioreactor cultures. The final morphology, structure, and characteristics of BC vary depending on the technique used. 46 Applying bacterial cellulose to natural fibres offers an alternative way of regulating the bonds between fibres and polymer matrices. This confirms previous findings by Pommet et al. 8 that BC coated natural fibres introduced nanocellulose at the fibre/matrix interface, which positively affects matrix stiffness around the natural fibre. The purpose of treating natural fibres with BC is to control the interaction of fibres with polymer matrices. In this way, green fibre-reinforced nanocomposites with improved characteristics and durability can be developed. 34
Enzymatic treatment
The application of enzymes is progressively popular in the natural fibre modification and textile industry.35,53 The excellent selectivity, specificity, and mild process conditions of enzymes bring about the separation of fibres from their non-fibrous components, which is one of the main reasons for adopting this technique. Enzymes are biocatalysts that act on a substrate with their specific enzyme to speed up biochemical reactions. Selecting suitable enzymes could present a long-term solution to environmental issues, such as chemical reagents in chemical treatment.19,39 Enzymatic hydrolysis is a catalytic process that causes a chemical reaction, degrading the hydrophilic and non-cellulosic surface impurities, enhancing the natural cellulosic fibres’ hydrophobicity.28,38,40 Hydrolases and oxidoreductases are the main enzymes involved in enzymatic hydrolysis. Cellulases, pectinases, proteases, and lipases are classified as hydrolases group, 53 whereas tyrosinase, laccase, or peroxidase are grouped under oxidoreductase and are studied for polymer modification. 35
Pectinases are a class of enzymes that hydrolyse pectins, release fibres from their non-fibre components. 39 Pectinolytic enzymes play a significant role in natural fibre retting and degumming by removing interlamellar pectin between the fibres.30,54 In the pectinases class, methyl esterase and depolymerase are the two dominant groups affecting different pectic polysaccharides. Methyl esterase removes the methyl group to form pectate lyase, whereas depolymerase (hydrolases and lyases) hydrolytically or trans-eliminatively cleaves the backbone of both pectin and pectate. As a result, fibre bundles are loosening from the woody stem, providing greater fibre separation and better performance.30,39
Laccase, a blue multi copper polyphenol oxidases enzyme with four copper atoms in its active site, is another enzyme that influences natural fibre extraction. Laccase is broadly distributed in insects, bacteria, and plants but is most dominant in fungi, especially white-rot fungi.53,55 This enzyme can only oxidise lignin phenolic compounds, which have a lower redox potential than laccase. The biodegradation of lignin by the laccase enzyme system is an oxidative process, which occurs by either radical hydrogen transfer, electron transfer, or ionic oxidation. Laccase utilises oxygen to remove protons from the phenolic hydroxyl group and give rise to free radicals on the phenolic compound. Laccase cleaves three types of lignin bonds: Cα- oxidation, Cα-Cβ cleavage, and aryl-alkyl cleavage. During delignification, laccase activity can be boosted by adding mediators, expanding the substrate range.24,56,57
Fungal laccase is preferred over bacterial or plant laccase in biotechnological applications because of its high redox potential (up to +800 mV). Thus, fungal laccase aids in lignin degradation and eliminates potentially hazardous phenols formed during the process.24,58 Laccases, along with other enzymes, have been found to alter cellulosic structures. The study performed by George et al. 16 successfully used enzyme blends to improve flax and hemp’s surface and thermal properties. In another study by the same author, using the same fibres, laccase treatments for both hemp and flax fibre composites are shown to decrease the moisture absorption relative to untreated fibre composites. 59 However, there is no change in the thermal properties of laccase-treated hemp and flax fibres, which might be due to these fibre types having low lignin content to be treated by laccase.
Xylanase breaks down xylan, a significant constituent of hemicelluloses. Endo-1,4- β-xylanase and β-xylosidase are the two essential xylan-degrading enzymes. Endo-1,4-β-xylanase (EC 3.2.1.8) hydrolyses the β-1,4-d xylosidic bonds in xylan and form xylo-oligosaccharides (XO). On the other hand, β-xylosidase (EC 3.2.1.37) releases xylose by eliminating the terminal xylose unit at the non-reducing end of the xylo-oligosaccharides.16,39,60,61
Pre-treatment with bacterial cellulase
Cellulases are gaining popularity for fibre modification and bioconversion, where the enzymes show their broader potential in the textile, pharmaceutical, food, and agriculture industries.18,20,41,59 Cellulase is an enzyme family that catalyses cellulolysis, or the hydrolysis of cellulose. They belong to an enzyme group called hydrolases. 31 Cellulase attacks the β-1,4-glucosidic linkage between glucose molecules in cellulose during the degradation process. They comprise three enzymes that work synergistically in the plant cell wall to hydrolyse cellulose microfibrils: endoglucanase (EG), exoglucanase, and β-glucosidase. EG targets the amorphous region of cellulose and breaks the β-1,4-linkages along the cellulose chain. Exoglucanase produces cellobiose units by attacking the cellulose chain’s end. 34 Finally, β-glucosidase breaks down the cellobiose into glucose. 62 The pore structure and surface of the cellulose fibres are supposed to change after enzymatic hydrolysis, resulting in wall stripping and forming fine fibrils.18,41 In the bio-processing of natural fibres, cellulase enzymes help to improve appearance, colour brightness, smoothen and soften the fibre surface. 20
Fungal treatment
Similarly, fungal treatment is an environmentally attractive option to bring about a desirable modification in improved chemistry and morphology of LCFs. Fungus is a member of the eukaryotic family that involves microorganisms such as yeast, moulds, and mushrooms. Approximately 1.5 million fungi species worldwide are classified into four groups: ascomycetes, basidiomycetes, zygomycetes, and deuteromycetes.34,63 Researchers utilised fungal methods to alter lignocellulosic fibres and noticed that this method imparts the same modifications as chemical approaches. White-rot basidiomycete fungi are versatile in treating natural fibres among the ligninolytic microorganisms.63–65 Extracellular oxidases are enzymes produced by white-rot fungus that react with lignin constituents, increasing hemicellulose solubility for metabolizing. It degrades lignin as well as other aromatic and non-aromatic compounds not linked to lignin.40,64 Ascomycetes fungi are subjected to the production of lipolytic enzymes (lipase) and used to degrade the pitch component (resinous hydrophobic components including resin acids, fatty acids, and natural oily materials) of the fibres. In addition, fungiform hyphae create tiny holes (pits) on the fibre surface, creating a rough appearance that allows the matrix to better interlock with the fibre.36,63
Effect of biological treatment on properties of natural fibres
Optimization of fibre surfaces using different biological treatments has a remarkable impact on fibre properties. Mechanical strength and surface features of jute fibres treated with various enzymes (pectinase, laccase, cellulase, and xylanase) were investigated by Karaduman et al. 19 Enzymatic treatment facilitates the extraction of pectin, hemicelluloses, and lignin from the fibre. Enzymes induced the fabrics to become more fibrillated, and the yarn twisted to open. Based on tensile test data, breaking force and Young’s modulus of jute fabrics reduced, except for xylanase-treated fabrics, which showed a higher Young’s modulus value. Vishnu Vardhini et al. 17 found that enzyme treatment improved the percentage of cellulose in the fibre by removing non-cellulosic components. In other words, making available more cellulosic material to interact with larger matrix surface area and thus contributes to enhancing stress transfer.
Detailed examinations of alfa fibres by Werchefani et al. 32 showed that the surface morphologies of unmodified fibre revealed cuticle, waxy materials, and non-crystalline parts. Alfa fibres were characterized by excellent separation into cellulose filaments for both enzymatic modifications, and the fibrils became visible and well exposed. In addition to this, the disentangled samples allowed a greater aspect ratio, thereby creating room for mechanical interlocking and stronger bond strength in composites. Flax and hemp fibres treated with a mixture of xylanase and cellulase have a rough surface appearance. Flax samples had more exposed fibre bundles, likely because of a higher proportion of hemicellulose and less cellulose. 16 This research work recognized the potential of the biological treatment to facilitate fibre bundles opening, roughen the surface, and create a much easier fibrillation process.
A study on hemp and flax fibres employing several enzymes (xylanase, pectin-methylesterase, polygalacturonase, laccase, and xylanase with cellulase background) revealed that xylanase and cellulase enzyme mixtures increased the cellulose content of the flax fibre, degraded pectin and hemicellulose. 16 Treatments with hemicellulase and pectinase enhanced the thermal properties and produced more homogenous fibre surfaces. One study has been set up to assess the activity of white-rot fungi, cellulase, and mixed enzyme (pectinase, cellulase, and xylanase) on the physical properties of jute fibres. 20 Although the increase in enzyme concentration lead to a drop in tensile strength of jute fibres, physical performance such as flexural rigidity, whiteness index, and elongation improved.
In their work, Liu et al. 55 used four enzymes with various concentrations, i.e. pectin lyase, xylanase, laccase, and cellulases, on bamboo fibres. To develop industrial-grade natural bamboo fibres, they were analysed based on their chemical composition, weight loss, toughness, and fineness. The authors outlined that the different enzymatic systems brought a pronounced increase in fineness, plausibly because of removing more polar hemicellulose fractions. Similar outcomes were shown on xylanase and pectinase-treated alfa fibres. In summary, enzymes decompose hydrophilic substances from the fibre bundles interface, which leads to reduced fibre diameter and length. 32
Fungi treatment (Ophiostoma ulmi) on hemp fibres was carried out by Gulati and Sain. 43 It has been observed that fungal-treated fibres are relatively free of impurities because of the action of fungus and the absence of water-soluble compounds on the fibres. After fungal treatment, an increase in acid-base characteristics of hemp fibre was shown, which positively influences their bonding with both acidic and basic resins. However, after treatment, a minor reduction in tensile strength has occurred because of the enzymes secreted by the fungus that induced chemical or structural changes. Pickering et al. 63 observed dull and striated fibre surfaces after fungal treatment. The production of pits on treated fibre surface observed with Phanerochaete sordida (D2B) fungi improved fibre morphology and higher crystallinity index, proving fungi’s ability to eliminate lignin and allowing better packing of cellulose chains. These results are in agreement with Jayapriya and Vigneswaran 65 findings which showed the removal of lignin from fungi-treated jute fibres, and leads to a rise in elongation percentage.
Surface coating with bacterial cellulose involves adding new material to the surface of fibres. 40 Several methods for coating natural fibres with BC have been explored, including culturing Acetobacter xylinum and coating hemp and sisal fibres with BC layers. 8 Based on scanning electron microscopy (SEM) studies, cellulose nanofibrils adhered to the fibre surface, which acted as an ideal substrate during the fermentation process. The modification system does not affect the mechanical behaviour of sisal fibres, in contrast to hemp fibres. When exposed to a bacteria-containing fermentation medium, hemp fibres lose considerable strength and Young’s modulus because of their non-cohesive structure. That could be related to the further separation of the hemp fibres into finer subfibres. Dai et al. 45 utilized oxidation/ultrasonication to yield nanocellulose from hemp fibres, which subsequently functioned as a “coupling agent” to treat fibres. Field emission gun-scanning electron microscopy micrographs illustrate that nanocellulose is effectively distributed along the stria and bonds the inter-fibril on the fibre surface. Modified fibres increased their crystallinity by approximately 20%, evidenced by the success of nanocellulose to penetrate and be compatible with microstructure layers of fibres. These findings further support nanocellulose modification as providing an extra benefit in improved tensile stress, tensile strain, and modulus.
For composite fabrication, Kalia and Vashistha 18 have subjected sisal fibres (Agave sisalana) to bacterial cellulase (Brevibacillus parabrevis) and microwave-assisted grafting. Compared to grafted sisal fibres, bacterial cellulase treatment improved the material’s thermal and crystallinity properties. It provided a smoothened, soft surface because of the removal of gum materials and surface-protruding fibrils. Degradation of cellulose by bacterial cellulase initiates wall stripping through hydrolysis of the β-1,4-glycosidic bond by peeling mechanism, leaving less hydrophilic fibre. Once the cell wall degrades, fibrillation increases, contributing to interfibre bonding. Hence, cellulase-modified fibre enhanced both the surface hydrophobicity and the mechanical properties of the fibres. Park et al. 62 modified cellulose fibres using cellulase from Trichoderma reesei and assessed the changes using a different methodology for high and low dosages of cellulase treatment. The authors noted that high-dose treatment resulted in a considerable drop in average fibre length and a rise in fines content of fibres, while minor changes in low-dosage treatments. In addition, the percentage of fibre degradation rose with hydrolysis time but was inversely related to the cellulase concentration.
Application of modified natural fibres as a reinforcement in composite
The composite material world is undergoing a revolution by pursuing environmentally friendly fibre and matrix materials. Natural fibres treated with biological techniques can be utilised as a reinforcing element to create eco-friendly bio-composites with a green image. When used as reinforcement for composite application, individual fibre strength cannot be ruled out as it dictates the overall strength of composite materials. 39 Further studies on the effect of biological treatments on composite performance are required to elucidate biologically treated fibres' behaviour in the final application.
Fibres and matrix are the two fundamental components of the fibre polymer composites determining the mechanical properties of the composite. The fibre-matrix interface served as a binder to transfer stress across particles; optimum reinforcement can be achieved through good interfacial bonding.26,35 Karaduman et al. 19 have prepared jute-fibre reinforced polyester composites through modification with various enzyme mixtures and treatment time. Enzyme treatment removed the amorphous and disordered polymers from the fibre surface. This removal creates more resin impregnation sites as fibre-matrix contact surface is increased. As a result, a rougher fibre surface is formed after removing impurities, pectin, hemicelluloses, and lignin from fibre, facilitating the fibre-matrix interlocking. In addition, polyester has a higher polarity as compared to the thermoplastic resin. A stronger fibre/matrix bond benefits the overall mechanical behaviour of the composite. The surface of hemp fibres modified with fungi created better bonding characteristics between plant fibres and composites. 63 Comparison of these fungi-treated polypropylene composites relative to the untreated fibre composites revealed a 22% improvement in tensile strength. Lignin removal (supported by colour change and XRD results) exposed more reactive hydroxyl sites and increased surface roughness, which are believed to increase hydrophilicity and retain composite strength despite decreasing fibre strength. These data obtained have implied that the overall mechanical strength of composite increases with the increment of interface roughness.19,63
To heighten composite performances, impact tests are conducted to assess the impact strength and toughness of structural materials, which are interconnected to the product implementation and durability. The impact strength of a composite is based on the reinforcement’s toughness properties, type of the interfacial region, and the frictional behaviour of the polymers. 17 Vishnu Vardhini et al. 17 compared alkali and enzymatic treatment effects on banana fibre-reinforced polypropylene composites at varied treatment concentrations. The impact strength of 20% laccase and xylanase treated fibre composites increased by 113.6% and 120.6%, respectively, compared to NaOH treatment, which increased by 98%. The authors pointed out that the fibre treated with 20% xylanase had the highest flexural and tensile strength readings. In addition, surface roughening of fibres and removal of non-cellulosic material allow for improved fibre dispersion in the polymer matrix and greater interface formation via mechanical interlocking. Therefore, they concluded enzymes could be a fit candidate for alkali treatment for banana fibres.
Regarding the flexural properties, O.ulmi treated hemp fibre-polyester composites obtained an improvement of 12% and 21% in flexural modulus and flexural strength, respectively. 43 The strengthened interfacial bonding was justified by the comparatively higher acid-base interactions between treated fibres and resin. Moreover, a reduction in water uptake behaviour by eliminating the voids were observed, which benefits the composites' durability when voids acted as reservoirs for water accumulation. The study by Karaduman et al. 19 also reported that the composites incorporated with laccase-treated jute fabrics (LC180) had greater tensile and flexural modulus than those made from unmodified jute fabrics. This outcome was due to the cleaning of amorphous lignin macromolecules, which increasingly exposed fibre to the matrix phase, consequently ameliorating composites’ mechanical performance.
The BC treated sisal fibres exhibited better interfacial shear strength (IFSS) than unmodified sisal. 8 In addition, bacterial cellulose also demonstrated its capacity to increase the IFSS of both modified hemp and sisal fibres to cellulose acetate butyrate (CAB) matrix. Higher IFSS can be attributed to a rougher surface caused by nanoscale cellulose on the surface and the coupling of bacterial cellulose fibrils with polymer molecules. Also, hydroxyl groups on the treated fibre surface and matrix appear to form hydrogen bonds with the carbonyl groups in PLLA, creating a solid bridging between modified fibre and CAB matrix.
The study by Li et al. 64 treated hemp fibres with white-rot fungi Phanerochaete sordida (D2B), Pycnoporus species (Pyc), and Schizophyllum commune (S.com) to study the strength of reinforced polypropylene composite affected by interfacial bonding variables. White-rot fungi have been found to degrade non-cellulosic compounds at a higher level and form microholes associated with exposure to more reactive hydroxyl sites, resulting in a strong degree of interfacial bonding and composite strength. Compared to untreated fibre composites, S.com treated fibre composites achieved the maximum tensile strength of 45 MPa with a 28% improvement. According to Karaduman et al., 19 polyester composites formulated with untreated fibres were observed with fibre pull-out and separated fibre bundles, clean fibre surfaces without resin adhesion, presenting the composites with poor mechanical properties. In contrast, enzyme-treated fibres (pectinase+ laccase+ cellulase) (PLC) showed a sharp-edged fracture region demonstrating better wettability and enhanced interfacial connection between the matrix and fibre.
Potential of the biological treatment and comparison with others treatment
Advantages and disadvantages of various pre-treatment methods.
Physical treatment aims to modify fibre’s structural and surface properties by reducing fibre hydrophilicity. Standard physical methods applied are related to plasma, ultrasounds, and UV light.1,40 This dry method reduces air, water, and land pollution compared to wet chemical methods. 14 Once the fibre is not immersed in any liquid medium; it has a higher resistance to water absorption than other types of surface treatments. 67 Despite this, physical treatment suffers from several pitfalls, such as only batch process applicable for plasma treatment, the complexity of corona treatment, costly and lengthy processing conditions.34,38,68
Alternatively, a chemical approach can also treat natural fibre, which enriches the interfacial adhesion by removing non-cellulosic compounds (hemicellulose, lignin, pectin, and oils). There is an introduction of a third material to improve the chemical interactions between the fibre and the polymer matrixes. 22 A significant drawback of chemical treatment is the possibility of fibre degradation when using excessive chemicals. 13 Along with that, the involvement of hazardous chemicals, inappropriate handling of chemical waste, and generating by-products that are hard to dispose. This issue appends extra cost to the final production cost, making this treatment less adopted in manufacturing inexpensive products36,69
Biological treatment is an environmental concern approach, selectively eliminating undesirable fibre components by specific enzymatic actions. 42 The major reason for embracing this treatment is the fact that the application of microorganisms such as bacteria, fungi, and enzymes, results in a less harsh mechanical post-treatment, implying minor fibre damage. 39 Beyond all these advantages, the downsides of this treatment are the enzymes cost and treatment time, which limits to only the pilot scale. 70 To make the process economically feasible, the cost efficiency of the process is the focal point. The possibility of recycling enzymes after each use and use of by-products may increase the attractiveness of the process. Furthermore, the incubation time that varies depending on the biomass composition can be minimized by using a suitable microbial consortium to promote adaptability and productivity.
To sum up, chemical treatment is still the most frequently used method, followed by physical treatment. The greener surface treatment, biological treatment, is making a turnover in the composite industry for surface modification of natural fibre. Further research on reducing the treatment cost and exploring the mechanisms behind the enzyme activity can intensify the sustainability of the processes. These issues may open up a new horizon for biologically treated fibre reinforced polymer composites and advanced applications.
Conclusion
With the increased environmental awareness, more efforts have been put into creating polymeric materials manufactured from renewable resources. Natural fibres can potentially compete with synthetic fibres with the utilisation as a reinforcing agent in polymer matrix composites since they are cost-effective, have ease of processing, and have potential mechanical properties. This review propounds the potential of biological treatments in modifying natural fibres and examines their feasibility as reinforcement for composites. After adapting biological surface treatments on the fibres, reducing the moisture uptake capacity, eliminating the non-crystalline parts of natural fibres, and improving fibre compatibility with hydrophobic matrices have occurred. Pre-treatments of natural fibres in fibre-reinforced composites showed changes in structure and surface morphology upon the action of different biological agents owing to the considerable improvements in the mechanical performance of composites. Current findings suggest that enzyme is a promising and sustainable modifier of the natural fibres used in composites. However, biological treatment is laborious and time-consuming, which is essential for further studies. Besides, the proper dosage of the response enzyme is necessary for a productive modification process. To further build up the market opportunities of natural fibre for composite applications, the production cost should be less than those of GFRC manufacturing and provide composite performance equivalent to GFRC. The presented challenges can be resolved to expand the application of superior natural fibres reinforced composites over synthetic fibres reinforced composites in various industrial applications.
Footnotes
Acknowledgements
The authors would like to acknowledge the financial support (1001/PTEKIND/8014123) provided by Universiti Sains Malaysia.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the (1001/PTEKIND/8014123) provided by Universiti Sains Malaysia.
