Plant-Based Natural Fibre Reinforced Composites: A Review on Fabrication, Properties and Applications

The increasing global environmental concerns and awareness of renewable green resources is continuously expanding the demand for eco-friendly, sustainable and biodegradable natural fibre reinforced composites (NFRCs). Natural fibres already occupy an important place in the composite industry due to their excellent physicochemical and mechanical properties. Natural fibres are biodegradable, biocompatible, eco-friendly and created from renewable resources. Therefore, they are extensively used in place of expensive and non-renewable synthetic fibres, such as glass fibre, carbon fibre and aramid fibre, in many applications. Additionally, the NFRCs are used in automobile, aerospace, personal protective clothing, sports and medical industries as alternatives to the petroleum-based materials. To that end, in the last few decades numerous studies have been carried out on the natural fibre reinforced composites to address the problems associated with the reinforcement fibres, polymer matrix materials and composite fabrication techniques in particular. There are still some drawbacks to the natural fibre reinforced composites (NFRCs)—for example, poor interfacial adhesion between the fibre and the polymer matrix, and poor mechanical properties of the NFRCs due to the hydrophilic nature of the natural fibres. An up-to-date holistic review facilitates a clear understanding of the behaviour of the composites along with the constituent materials. This article intends to review the research carried out on the natural fibre reinforced composites over the last few decades. Furthermore, up-to-date encyclopaedic information about the properties of the NFRCs, major challenges and potential measures to overcome those challenges along with their prospective applications have been exclusively illustrated in this review work. Natural fibres are created from plant, animal and mineral-based sources. The plant-based cellulosic natural fibres are more economical than those of the animal-based fibres. Besides, these pose no health issues, unlike mineral-based fibres. Hence, in this review, the NFRCs fabricated with the plant-based cellulosic fibres are the main focus.


Introduction
After energy production, which contributes greatly to carbon emissions, the textile and fashion industry is considered the second most polluting industry due to generating huge amounts of wastewater, and tonnes of textile waste that is directly buried in landfills or incinerated [1][2][3][4][5][6][7][8]. In the textile industry, production of fibres plays a major role, where fibres are classified as synthetic or man-made fibres and natural fibres. Synthetic fibres are produced from non-renewable and petroleum- The NFRCs are environmentally friendly, biodegradable, biocompatible, renewable and costeffective [5]. For example, these composites have the potential to replace steel in the automobile industry, which may reduce the total weight of a vehicle by around 25% and the consumption of nearly 250 million barrels of oil [5]. The NFRCs can be used in the production of bicycle frames, door and window frames, columns, ceilings and so on [28]. Besides, in recent years, along with the virgin natural fibres, the abundantly available short and non-spinnable waste natural fibres and recycled plastic materials have been used in fabricating the NFRCs. For example, waste oil plum fibres; wood flour and fibres; recyclable polymers (e.g., polyolefins); and short fibres of cotton, flax, sisal, jute, hemp and recycled plastics are used instead of virgin plastics to produce the bio-based green composites for applications wherein composites with strong mechanical properties are not required (e.g., panels, gardening products and packaging) [28][29][30][31][32].
Although some detailed reviews are available on the NFRCs [33][34][35][36], a single review covering the NFRCs' fabrication processes, the required properties of the natural fibres and the polymeric matrices, the detailed mechanical properties, the challenges associated with the fabrication of the NFRCs and their potential remedies with applications of the NFRCs is not present. Therefore, in this paper, we systematically review the fabrication procedures of natural fibre reinforced composites along with the physicochemical properties of the natural reinforcing materials and polymer matrices. The mechanical properties of the NFRCs, the factors affecting the mechanical properties of the NFRCs and their potential application areas are also summarised. As mentioned earlier, these composites go by many other names; we will mainly use natural fibre reinforced composites (NFRCs) or biocomposites in this review to avoid any misunderstanding of the readers. It is worth mentioning that among natural fibres, plant-based fibres are mostly used in the fabrication of NFRCs because of their price and safety. Therefore, the properties and applications of the natural fibre reinforced composites produced with the plant-based natural fibres are reviewed here.

Reinforcement and Polymer Matrix of Biocomposites
The prime objectives of fabricating the biocomposites include but not limited to the development of a new line of fibre composites to replace the use of plastics and other synthetic fibres that will not only be eco-friendly but sustainable throughout the manufacturing process and after the useful lifetime. Generally, the term "biocomposites" refers to fabricated composite materials where two or more constituents are joined together in which at least one element is obtained from natural or bio-based resources [17,37]. In a broad definition, it can be said that biocomposites can be formed in between wood and non-wood based natural fibres (e.g., hard and softwood, and cotton, jute and sisal fibres), in between biopolymers and natural fibres (e.g., PLA and sisal) or even in between biopolymers and natural fibres with synthetic or man-made fibres (PLA and glass fibres) [17,[37][38][39][40]. Figure 2 [ [41][42][43][44][45][46] shows the tree diagram of reinforcement and matrix materials associated with the fabrication of the NFRCs. NFRCs are fabricated with combinations of natural reinforcing agents and polymer matrices. The natural fibres originating from the plant, animal and mineral sources can be used directly, or in chopped fibrous strands, nonwoven mats or fabric forms. Composite constituent materials at a glance [41][42][43][44][45][46]. Natural fibres are broadly classified based on their origins, such as plant, animal and mineral sources ( Figure 2). The plant fibres are collected from seeds (e.g., kapok, oil palm, cotton and coir), bast (e.g., ramie, hemp, flax, jute and kenaf), straw (e.g., wheat, rice and corn), wood (e.g., soft and hardwood), grass (e.g., bamboo and bagasse) or leaves (e.g., pineapple, sisal and abaca). The animal fibres, also known as the protein fibres and mostly used in the textile industry, are the second most widely available natural fibres after the plant-based fibres, and are obtained from sheep, alpacas, cashmere, silk, chickens and ducks [11,[47][48][49]. Generally, the animal fibres are used in particles or chopped fibres while producing the biocomposites [9,11,24]. The protein fibres possess some excellent properties, such as inherent fire retardancy and thermal stability. However, the animal fibres are not widely used in the commercial production of the NFRCs due to the higher prices of the protein fibres (e.g., silk costs 2.6-40.0 US$/kg) compared to the plant fibres (e.g., hemp costs 1.0-2.1 US$/kg) [11,50,51]. Mineral fibres are nonmetallic and inorganic fibres produced from minerals. Asbestos and glass are the most used mineral sources for fabricating biocomposites. However, the application of asbestos fibres is currently banned due to their extensive carcinogenicity and other health issues [21,24]. As mentioned above, due to some issues associated with the animal and mineral fibres, such as cost and safety, natural plant-based fibres are mostly used in the fabrication of NFRCs, which is the focus of this study.

Plant-Based Natural Fibres
The most commonly used plant fibres are shown in Figure 3. The main components of the plant fibres or cellulosic fibres are cellulose, hemicellulose and lignin (Table 1) [52,53]. The amounts of cellulose, hemicellulose and lignin differ in the plants owing to their maturity, location of growth, environment and even species. Cellulose, as shown in Figure 4, is a natural polysaccharide wherein D-glucopyranose rings are connected with β (1→4) glycosidic linkages [10,53]. All plant fibres possess a crystalline structure with a nearly 65-70% cellulose that consists of C, H and O (having a common formula of (C 6 H 10 O 5 ) n [10,52]. Additionally, the final properties and characteristics of cellulosic fibres are modified with the presence of lignin and further non-cellulosic constituents. Due to the presence of a higher proportion of hydroxyl groups and hygroscopic nature, one of the important properties of the plant-based fibres is their higher moisture absorption capability, which is very often an essential requirement for the fabricated composites [22,52]. The chemical constituents and physical and mechanical properties of most widely used plant-based natural fibres are presented in Table 1.

Polymer Matrix
Biocomposites are fabricated with the addition of plant-based natural fibres and polymer matrices ( Figure 2). Therefore, it is very important to choose correct polymer matrices for the production of the biocomposites. Polymers are the macromolecules composed of single or many repeating units [61]. The polymeric chains in the matrix can be arranged in a unidirectional way or randomly result in a crystalline and amorphous structure, respectively. The polymers are generally divided into thermoplastic and thermosetting polymers ( Figure 2) [62]. Thermoplastic polymers (e.g., polypropylene, polycarbonate, polyimides, polyethylene terephthalate) bonded with weak van der Waals forces, are widely used in manufacturing composites where both the weaker and stronger mechanical properties are required [61,62]. These polymers show outstanding toughness and resistance to wear and tear [61]. On the other hand, the thermosetting polymers (e.g., epoxy, silicone, polyurethane) are moulded into 3D networks along with strong covalent bonds and cross-linking, which makes them tough. These polymers possess higher thermal stability, creep and chemical resistance, easy processability and good wetting properties [61,63]. In general, the synthetic polymers are neither biodegradable nor flame retardant (suppressed by the addition of the nature-based polymers). Some of the key properties of the polymer matrices are illustrated in Table 2.

Fabrication of Biocomposites
The fabrication process of the natural fibre reinforced composites (NFRCs) refers to the processes of preparing the preforms and then reinforcing the preforms with the polymer matrices [67]. The details of composite constituent (type, forms, etc.) are shown in Figure 2 [41][42][43][44][45][46]. There are a wide variety of processing techniques used to fabricate the composite materials. Application conditions for each of these techniques are quite different. Different types of composite fabrication methods can be classified according to the methods of a polymer matrix and reinforcement application to the mould, or according to the curing methods. However, all of these techniques can be broadly classified as open moulding or closed moulding techniques based on the application conditions of reinforcement and polymer matrix materials [68][69][70][71][72][73]. Open moulding is when the resin is exposed to the atmosphere during curing, and closed moulding is when the resin is not exposed to the atmosphere [74,75]. Each of these techniques has various application methods, as shown in Table 3 [42]. The composite fabrication technique is selected based on the constituent materials, the availability of the required tools and the properties required for the ultimate composite structures [76]. However, both the open and closed moulding techniques possess some benefits and drawbacks. Open moulding techniques are the most orthodox means of fabricating composites. They have been widely used because of their simplicity, low processing costs, unsophisticated techniques and variety of suitable reinforcements and laminating systems. Nevertheless, well-trained and highly skilled operators are essential to open moulding techniques to ensure the laminate quality, especially the void content and fibre volume fraction of the composite laminates [77]. On the other hand, the closed moulding techniques are preferred for making 3-dimensional composite parts of better quality, for less material waste and for perfect and aesthetic finishes of the parts [69, 78,79]. This is mostly an automated moulding technique with reduced material, labour and waste disposal costs, and greater productivity. Compression moulding [80], extrusion moulding [81,82] and injection moulding [83][84][85] are the most used open moulding techniques for the thermoplastic composites. Hand layup [86,87], resin transfer moulding (RTM) [88], vacuum-assisted resin transfer moulding (VaRTM) [89] and resin film infusion (RFI) [90] are used for the production of thermosetting composites.  Figure 5a shows the two most popular open moulding techniques named hand lay-up and spray-up techniques, which are most suitable for the large composite components such as boat hulls, turbine blades, large container tanks, swimming pools and various automotive parts [91]. In the hand lay-up technique, reinforcement materials and the polymer matrix are applied manually onto an open mould surface, one by one until the desired thickness of the component is obtained by successive layers [92,93]. On the other hand, in the case of the spray-up technique, the reinforcement fibres in the form of chopped strands and the polymer matrices are sprayed using spray gun onto an open mould surface until the desired thickness of the composite lamination is obtained [44]. In fact, the spray-up technique is the automated version of the traditional hand lay-up technique. Figure 5b represents the VaRTM composite fabrication process. Vacuum-assisted resin transfer moulding (VaRTM) is one of the most frequently used closed moulding processes in the fibre reinforced polymer composite industry. In this technique, the mould is filled with the reinforcement fibres and then closed, and simultaneously the air is pumped out and the polymer matrix is allowed to flow in to impregnate the fibres [94]. The reinforcement fibres are applied in a mould of the same geometry as the size and area of the required composite material [95]. The composite parts manufactured by this method possess fewer void parts with good surface finishes [96,97]. Currently, this fabrication technique is undergoing a wide variety of developments to produce more complex composite parts with the desired finishes at reasonable cost [95].
Asumani et al. [119] used alkali-treated, kenaf, short-fibre nonwoven mats to fabricate kenafpolypropylene composites by compression moulding technique and demonstrated improved mechanical properties of the composite material. The composites showed enhancements of 25%, 11% and 10% for tensile strength, modulus and flexural strength, respectively. Furthermore, alkali-silane combined treatment of kenaf fibre resulted in 75%, 94% and 50% increases in tensile strength, modulus and flexural strength of the composite, respectively. Similarly, Wambua et al. [137] applied the randomly oriented kenaf fibres and compared the composite properties to that of glass mat-polypropylene composites. Although the tensile modulus of kenaf-polypropylene composite was slightly higher (9.7%), the tensile strength, flexural strength, modulus and impact strength were lower (6.7%, 52%, 50% and 74%, respectively) than those of glass fibre mat-polypropylene, which might have been due to the lower interfacial bonding strength between the fibre and matrix. Cavalcanti et al. [138] investigated the cellulose-based composite that was manufactured using the hand layup technique, where jute, curaua and sisal fibres were the reinforcing agents and epoxy was the polymeric matrix. Pure jute fibre and jute blended with curaua and sisal fibres were chemically treated with (i) alkali and (ii) combination of alkali and silane for 1 h. Afterwards, it was found that the combined alkalisation and silanisation of jute fibre enhanced both the tensile strength (8.4%) and flexural strength (14%). Besides, the alkali treatment on the jute-sisal blended fibre reinforced composite demonstrated the maximal improvements. Singh et al. [139] investigated the influence of curing temperature on jute-epoxy composite properties. The jute woven preforms were used as the reinforcements and the hand layup fabrication method was applied, followed by the compression moulding technique. The authors concluded that 100 • C for flexural strength and 80 • C for impact strength are the optimum curing temperatures that yield the maximum output. Table 4 presents the literature available on natural fibre reinforced polymer composites; it summaries the fabric/matrix modifications and their effects on associated mechanical properties.

Factors Affecting the Mechanical Properties of Biocomposites
Many factors, such as type of polymer matrix and the types associated fibres; the fibre's origin, processing and forms; the fibre dispersion; the distribution in the matrix; the orientation; the fibrematrix interfacial interaction; and the techniques used in the composite's fabrication, directly affect the mechanical properties of the biocomposites [169][170][171][172][173][174]. Besides, as the strength of reinforcement fibre is higher than that of the matrix material, the strength of a biocomposite is more dependent on the fibre rather than the matrix [175][176][177][178]. Therefore, the strength of a biocomposite can be enhanced by increasing the amount of fibre in the composite structure to a certain extent [179,180]. On the other hand, when the amount of fibre exceeds the limit, the load transferred through the interface reduces, and the amount of porosity in the composite increases, which results in a greater reduction in the strength and stiffness of the composite material [179]. Moreover, with the increment of fibre content into the composite structure, the water uptake of the composite increases, which very often degrades the mechanical properties of the composite material [181][182][183]. Along with the amount of fibre content, the length of the reinforcement fibre plays an important role in the mechanical properties of the composite [24]. The higher the aspect ratio of the fibre, the better its load-bearing capacity [184]. However, it should be noted that a very long fibre can become entangled, resulting in non-uniform distribution of the fibre and the reinforcement [185]. The matrix is also an important constituent of the composite that protects the fibre's surface from externally applied forces. The polymer matrix transfers the forces externally applied to the composite to the reinforcement materials, and thus enhances the composite's longevity [186]. With the increased interfacial interaction between the fibres and the matrices, the biocomposites become stronger. As the applied load is transferred from the matrix to the reinforcement materials, the interfacial bonding is all important and is the true indicator of the composite properties [170,187]. To increase the interfacial bonding between the fibre and the matrix, it is necessary to improve the hydrophobicity of the fibres, the interfacial bonding between matrix and fibre and the roughness [188]. Bonding between fibre and matrix is usually enhanced by a few mechanisms, such as mechanical interlocking, chemical bonding, electrostatic bonding and inter-diffusion bonding [189][190][191][192][193][194][195]. Besides, the other factors such as temperature, pressure, time and resin viscosity seriously affect the properties of the biocomposites [139,[196][197][198].

Biodegradability of the Biocomposites
The natural fibre reinforced composites (NFRCs) show excellent biodegradability compared to the synthetic fibre-based materials and composites, which is one of the major aspects of fabricating NFRCs [199,200]. In general, the biodegradable materials are degraded and converted into CO 2 , H 2 O, hydrocarbons, methane and biomass under the aerobic or anaerobic conditions, due to the chemical or biological reactions [200][201][202]. In most of the cases, the material biodegradability is tested by the soil burial method (ASTM D5988/D5338 or ISO 14855) and the results are expressed by the weight loss (%) [199,[203][204][205][206]. Different factors such as the molecular weight, chemical structure, glass transition temperature (Tg), melting temperature (Tm), mechanical behaviour, crystallinity and crystal structure potentially impact the NFRC's biodegradability [199,203]. For example, a polymer with high crystallinity and a high melting temperature shows lower biodegradability-i.e., lower weight loss (%) [200,203]. A study over a 3-month period revealed that the biodegradability of the lyocell incorporated polyester composites is higher (75%) than that of the pure polyester, which might be due to the presence of natural lyocell fibre [207]. In another study, the biodegradability test of the kenaf/PLA composites using the garbage-processing machine showed the weight loss of 38% within one-month of composting [208]. Recently, Wu et al. found that the use of 40 wt.% palm fibre (PF) as a reinforcement with the polyhydroxyalkanoate (PHA) matrix increased the rate of biodegradation (~90%) compared to pure PHA (~20%) [209]. Similarly, the study of Mittal and Chaudhary revealed that the biodegradability of the epoxy resin increased while using the nature-based pineapple and coconut fibres as reinforcements with the epoxy resin. Pure epoxy exhibited only 10% weight loss, while the natural fibre reinforced composites showed 60% to 80% weight loss [210]. The authors concluded that upon burial of the composites into the soil, both the water molecules and microorganisms penetrated the polymeric backbone, which eventually degraded the higher cellulose contents of the natural pineapple and coir fibres, resulting in the higher weight losses (%) of the composite materials compared to the pure synthetic materials [210]. The biodegradability of some of the NFRCs reported in the literature is shown in Table 5. In 60 days, the PLA/Soy straw exhibited 90% degradation while the pure PLA showed 50%. [214]

Challenges Associated with the Biocomposites and Probable Fibre Modifications
The development, production and applications of the plant-based natural fibre reinforced composite materials (NFRCs) are increasing regularly due to their outstanding properties. However, worldwide researchers are facing a number of challenges relating to the development of 100% green biocomposites where both the polymer matrices and reinforcing materials come from natural and renewable resources. Additionally, poor interfacial bonding between the polymer matrix and the reinforcing material, due to the hydrophilic nature of plant-based natural fibres and hydrophobicity of the polymer matrices, is another point of consideration while fabricating the cellulose-based NFRCs [170,215]. Due to the low interfacial interaction, the NFRCs cannot ensure the desired mechanical properties, although they provide the most pursued biodegradability properties, and are significantly cheaper as compared to the high-performance synthetic fibres, such as carbon fibre [170,216]. On top of that, low thermal stability, high moisture absorbency and low wettability of cellulosic natural fibres are some other remarkable challenges in the production of the NFRCs [217]. Moreover, it is not always possible to ensure the similar properties among the natural fibres, as their properties vary based on the changes in weather, season, cultivation conditions and production processing [218,219]. Hence, necessary modifications of the reinforcement fibres must be taken into consideration before processing the improvement of interfacial bonding by enhancing the wettability of the fibres and controlling the moisture absorption of the fibres or matrix [220]. The fibre and polymer matrix interfacial bonding can be improved by physical and chemical fibre modification techniques. Interfacial bonding between the reinforcement fibres and the polymer matrix can be improved by applying some of the fibre modification techniques-both physical and chemical.
The physical modification of a cellulosic natural fibre refers to the changing of the surface properties such as surface energy, polarity, surface area, cleanliness and wettability. Corona treatment [160,[221][222][223], plasma treatment [224][225][226], ultraviolet (UV) treatment [227][228][229][230], fibre beating [231][232][233] and heat treatment [234] are some of the remarkable techniques for physical modifications of fibre without hampering the chemical structure of fibres [235][236][237]. In the case of the corona, for plasma and UV treatment, fibre surface energy is changed using a high voltage at low temperature and atmospheric pressure [160]. Consequently, wettability and fibre-matrix interfacial bonding is improved, which enhances the composite strength. Active surface area is enlarged by the process of fibre beating which allows a good interfacial interaction between fibre and matrix [162]. The heat treatment process removes the non-cellulosic components of plant fibres, such as pectin, lignin and hemicellulose, along with other dirt from the fibres, and allows the cleaned fibre to react with the matrix more actively [238]. The chemical treatment is mostly carried out to deteriorate the inherent hydrophilic nature of the natural fibres, which eventually assists in enhancing the interfacial bonding between the fibres and the matrix [239,240]. Over the past few decades, very intensive research work has continued to better understand the chemical treatment behaviour of cellulosic fibres. Presently, mercerisation [241][242][243][244][245], acetylation [246][247][248][249], peroxide treatment [250][251][252], permanganate treatment [253][254][255][256][257], silanization [258][259][260][261][262][263][264], acrylation [265][266][267][268][269], acrylonitrile grafting [270][271][272] and latex coating [273][274][275] are the chemical modification techniques most used to improve the reactivity of the natural fibres with the polymeric matrix materials. The hydrophilicity reduction of the cellulosic fibres by eliminating the proportions of hydroxyl groups (-OH) is the ultimate objective of the abovementioned chemical modification techniques [24]. While, due to the reduction in the number of -OH groups, the moisture absorbency of the fibres is reduced, the non-cellulosic components and dirt are removed as the by-products of the chemical reaction, which allows an improved load transfer capacity through the fibre-matrix interface. Figure 6 illustrates the often used chemical modification techniques associated with the cellulosic fibres [276].

Potential Application Areas of Biocomposites
Many countries across the world have imposed restrictions for industries in the use of oil-based raw materials while fabricating different products [277,278]. For example, in the year of 2005, the European Commission (EU) executed "European Guideline 2000/53/EG," which ensured the utilization of 85% recyclable materials while fabricating the automobile parts [21]. This amount was further augmented to 95% in the year of 2015 [22]. Light weight but greater mechanical strength; lower manufacturing costs; resistance to fatigue and corrosion; and greater availability, renewability and biodegradability than synthetic fibres, are some of the major reasons for this increased demand of NFRCs [279][280][281][282]. The potential application areas have been increasing dramatically over the last few decades, including but not limited to the automobile, packaging, military, sports, medical, building and constructions sectors [279][280][281]. Among these, the applications of NFRCs are the most prevalent in the automobile industry, due to the requirements for eco-friendly lightweight materials and associated costs [9,13,55]. Henry Ford fabricated the first NFRCs using hemp fibres in the year of 1940 [22]. Later, several car manufacturers reported the fabrication of car bodies and other associated parts from NFRCs in the 1950s and in between 1990 and 1996, which gradually increased and gained popularity [13,22], and in recent years from 2014 to 2019, an increase of 11% in production was reported [283].
The natural fibre-based composite market is expected to grow to $531.3 million US in the year 2019 from $289.3 million in 2010 with 28% of market shares occupied by natural fibres [283,284]. Table 6 shows the main producers and the annual production of some plant-based natural fibres commonly used in NFRCs. It has been reported that application of NFRCs would result in the reduction of nearly 20% of manufacturing costs and 30% of automobile weight which would also help in reducing the fuel consumption [9,55]. The European car manufacturers are making serious attempts at incorporating the NFRCs in various portions of cars, such as car seats, backrests, front and back door liners and in door-trim panels [285]. The German car manufacturers are currently focusing on the fabrication of exterior body parts for cars and trucks with bio-based composites [25,55,286]. It has been reported that in the year of 2004 the BMW group used 10 kilotonnes of renewable natural fibres while producing the NFRCs for their cars [287]. Table 7 shows the utilisation of different plant-based natural fibres in various parts of the automobiles [9,55,[288][289][290][291]. It has been reported that in the year of 2012, the European automobile industry has used 38% wood, 25% cotton, 19% flax, 8% kenaf, 5% hemp and 7% other natural fibres such as jute, coir, sisal and abaca to fabricate the natural fibre reinforced composites (NFRCs) to use in this particular industry [290]. Hence, it can be predicted that in near future the NFRCs would be used in a higher proportion in the automobile industry. In addition to the automobile industry, the biocomposites fabricated from the cellulosic natural fibres are extensively used in geotextiles, medicine, sports, construction and packaging (Table 8). After the automobile industry, the building and construction industries are the second-largest consumers of NFRCs owing to the demand for the eco-friendly green buildings, and load-bearing and non-load bearing capabilities of NFRCs [292,293]. Roofs, windows, panels and doors are built using the NFRCs [293]; for example, the composites fabricated from sisal fibres can be used in producing tanks, pipes and even roofing materials that could replace asbestos [294,295]. Table 8. Applications of cellulosic fibres in different industries [296][297][298][299].

Fibres
Industrial Applications

Conclusions
Plant-based natural fibre reinforced composites (NFRCs) have some excellent properties, such as biodegradability, biocompatibility and renewability. Hence, it is more appropriate to use the NFRCs instead of the petroleum-based synthetic materials, which are neither eco-friendly nor obtained from the renewable sources. This review thoroughly illustrated the required properties of the cellulosic fibres and the polymeric matrices for the fabrication of the NFRCs. Additionally, the mechanical properties, challenges and potential application areas of the NFRCs have been discussed. Although it is evident that the plant fibres are not free from some drawbacks, such as possessing higher moisture retention, these can be overcome by applying various physical and chemical modifications. Moreover, it is difficult to produce 100% green biocomposites wherein both the polymeric matrix and the reinforcing material are derived from the natural and renewable sources. Therefore, more focus is required to commercially develop pure green biocomposites to support sustainability. It is necessary to find new sources of natural biopolymers and fibres which can be used as both the polymer matrices and the reinforcing materials. Research also needs to find how can we utilize the abundantly available short and non-spinnable natural fibre wastes, either as reinforcing materials or as polymer matrices to fabricate fully green biocomposites.

Conflicts of Interest:
The authors declare no conflict of interest.