A state-of-the-art review on coir fiber-reinforced biocomposites

The coconut (Cocos nucifera) fruits are extensively grown in tropical countries. The use of coconut husk-derived coir fiber-reinforced biocomposites is on the rise nowadays due to the constantly increasing demand for sustainable, renewable, biodegradable, and recyclable materials. Generally, the coconut husk and shells are disposed of as waste materials; however, they can be utilized as prominent raw materials for environment-friendly biocomposite production. Coir fibers are strong and stiff, which are prerequisites for coir fiber-reinforced biocomposite materials. However, as a bio-based material, the produced biocomposites have various performance characteristics because of the inhomogeneous coir material characteristics. Coir materials are reinforced with different thermoplastic, thermosetting, and cement-based materials to produce biocomposites. Coir fiber-reinforced composites provide superior mechanical, thermal, and physical properties, which make them outstanding materials as compared to synthetic fiber-reinforced composites. However, the mechanical performances of coconut fiber-reinforced composites could be enhanced by pretreating the surfaces of coir fiber. This review provides an overview of coir fiber and the associated composites along with their feasible fabrication methods and surface treatments in terms of their morphological, thermal, mechanical, and physical properties. Furthermore, this study facilitates the industrial production of coir fiber-reinforced biocomposites through the efficient utilization of coir husk-generated fibers.


Introduction
Natural ber-reinforced composite materials have received continuous attention due to their industrial application potential. Natural bers are comparatively cheap, renewable, completely/partially recyclable, biodegradable, and ecofriendly, [1][2][3][4][5][6] and synthetic products 7-12 are continuously being replaced by natural products. [13][14][15][16] The lignocellulosic ber materials including ax, hemp, ramie, kenaf, jute, coir, hard and sowood materials, and rice husk are the biggest sources of biocomposite ller materials. 17,18 Their availability, costing, lower density, and overall convenient mechanical features have made them attractive ecological materials as compared to synthetic bers such as glass, carbon, nylon, and aramid. Natural bers have a long history of usage for various products ranging from housing to construction and clothing. [19][20][21][22] Natural ber-reinforced composites are used in diverse applications such as automobiles, aerospace, construction and building sector, consumer products, packaging, and biomedicine. However, nowadays, synthetic ber-reinforced products are still being used for producing composite materials because of the lack of adequate technology, research, and scientic innovations to utilize renewable natural bers as a prominent replacement for biocomposite production.
Natural bers are classied into different categories, such as animal, vegetable, and mineral bers, and are further classied as seed, bast, stalk, grass/reeds, wood (hard and so), and leaf bers. 23,24 Coir belongs to a popular seed ber group; besides, as a lignocellulosic material, coir remains neutral in terms of CO 2 emissions. 25,26 Lignocellulosic materials are in line with the Kyoto protocol in terms of minimizing greenhouse gas emissions. However, there are some plants such as the banana plant, which are cultivated primarily for fruits; although, their leover barks/leaves can be used as a potential biocomposite raw material. 27,28 This ber from banana is seldom used and is discarded just aer collecting fruits. Fibers from coconut fruits also have a similar phenomenon just aer collecting the fruits/ coconuts waterthey are discarded into the environment in general. Coconuts are grown in many parts of the world, especially in tropical and sub-tropical areas and play a signicant role in economic development. It was reported that around y billion coconuts are produced throughout the world accumulating a huge quantity of coir bers. 26,29 Coconut husks are used for culinary purposes aer extracting the copra and the interior liquid endosperm. The fruit shell of the coconut has a long decay time; hence, the transformation manufacturer and areas associated with high coconut consumption are facing challenges for disposing this waste through feasible and convenient disposal approaches. 30 Another challenging aspect of coconut is that the husk and coconut fruits can oat in ocean water without rotting for more than a month. Furthermore, durability is a major problem in natural ber-reinforced composites; however, since coir ber contains more lignin as compared to other natural bers, it is more durable. 31 Due to greater elongation at break properties, coir ber-reinforced composites are also stretchable up to their elastic limit without rupturing. 31 In this regard, bers obtained from coconut husk are currently attracting attention from researchers and industrialists to determine more convenient routes for utilization.
The manufacturing approaches to natural ber-reinforced composites are leaning toward novel and innovative routes for sustainable production. However, the biocomposite production from natural ber reinforcement depends on various factors like interfacial ber to matrix adhesions, length and contents of ber, treatments of bers, and the dispersions of polymers into the ber structure. In this regard, researchers are becoming more interested in biocomposite manufacturing research 4,32-37 and so coir ber-reinforced composites [38][39][40] are also getting signicant consideration. Different researchers have reported promising results on developed coir ber-reinforced biocomposites from different perspectives (thermal, mechanical, morphological, and so on). Rejeesh et al. 40 have suggested that coir berboards could function as an alternative ame retardant material to other plywoods. Olveira et al. 41 have proposed a design involving short coir ber reinforced with epoxy thermosets through applying uniaxial pressure, characterized in terms of exural properties, impact strength, and physical properties. The same study has further claimed that the perceived impact resistance and exural modulus were satisfactory when 35% ber volume with 375 g m À2 (ber grammage/density) was used, 41 although they found higher exural strengths at 300 g m À2 . Ayrilmis et al. 42 reported coir ber reinforcements with polypropylene (PP) in the presence of a coupling agent and found that the increased volume of the ber loading negatively inuenced the internal bonding strength and water resistance of the biocomposites. They also found an optimum ber loading of coir (60%), up to which the tensile and exural strengths of the composites increase. 42 Natural bers have very good compatibility with different thermoplastics, thermosetting polymers, or cementitious materials because of their lower density, better thermal insulation properties, mechanical properties, lower prices, unlimited availability, nontoxic-approaches, and problem-free disposals. Although the thermal, mechanical, and morphological properties of the natural bers have been studied by so many researchers, the studies on coir bers are still limited. Hence, this research reports various chemical, physical, morphological, and thermo-mechanical features of coir berreinforced biocomposites. The potential application and economical features of coconut ber-reinforced composites are further discussed and analyzed.

Coir fiber material
A coconut tree can produce 50 to 100 coconut fruits per year. 44 The photographs of the coconut palm tree, coconut fruits, coconut husk, and coir ber morphology are provided in Fig. 1. The extracted ber from the husks of the nut-shell is termed coir ber. The ber is extracted from the endocarp and external exocarp layers of coconut fruits. Generally, the extracted coir bers are a golden or brown-reddish color just aer removing and cleaning from coconut husks. The size of coir ber threads is normally within 0.01 to 0.04 inches in diameter. 45 Each coconut husk possesses 20 to 30% bers of various ber lengths. 46 The coconut palm tree can also be considered an integral ber-producing renewable resource due to the different parts of the palm like the petiole bark, leaf sheath, and leaf midrib. 47,48 The majority of palm coconuts are produced in Indonesia, Sri Lanka, Brazil, the Philippines, Vietnam, Thailand, Malaysia, Bangladesh, and India. 49-52 A study by Eldho et al. has mentioned that the coastal region of Asia produces 80% of the world's coconut bers. 53 The greater consumption of coconut fruits and water is generating green coconut trash, which is about 85% of the weight of the fruit. However, coir bers are used as ropes, yarns, cords, oor furnishing materials, mattresses, sacking, brushes, insulation materials, geotextiles, and rugs. Coir bers collected from coconut husks are thick and coarse, with some superior advantages like hard wearing capability, greater hardness quality (free from fragile characteristics like glass), better acoustic resistance, non-toxicity, moth-resistance, resistance to bacterial and fungal degradation, and they are not prone to exhibiting combustible properties. 42,54 Besides, coir bers have stronger resistance performances against moisture as compared to other plantbased natural bers along with the ability to withstand salty water from the sea and heat exposure. 42 The properties of mature coir bers are as follows: -100% naturally originated ber -Coir bers are strong and light -Coir bers easily withstand saline water -Coir bers easily withstand heat exposure -Plastic shrinkage is delayed in coir-based materials by controlling the cracks developed at the initial stage -The usage of coir in composite materials enhances thermal conductivity -Biodegradability and renewability -Higher water retention -Rot-resistant -Moth-resistant -Heat insulator -Have acoustic properties Coir bers can be of three types as shown in Fig. 2, namely, curled, bristol, and mat bers. 45 The curled bers are of inferior quality and are short staple bers. Bristol bers are coarse and thick, obtained from extractions of dry coconut husks, and are also termed as brown bers. Mat ber is the best coir ber type. It is obtained from retted coconut husks and has a longer and ner yarn. The mat ber is highly resistant against bacterial attack. 45

Retting of coir bers
Coir retting is performed in canals (a small area dug to store water), or rivers in riverine countries, or stored in watery areas; the coconut husks are submerged under the water by covering them with heavy soil. A mechanism regarding coir ber retting is depicted in Fig. 3. Compared to other natural bers like jute, coir bers require longer times by at least 4 to 12 months for biological retting processes. 55,56 The perfect retted coconut husks are separated from other poorly retted husks and washed with water to remove mud, sand, and slime from the surface. Aer that, the exocarp of the husk is easily peeled by hand. The coconut husks are then placed in a wooden box and beaten with wooden mallets or granite stones for further separation between the pith and coir bers. Another washing cycle is carried out to further remove the surface impurities and the bers are beaten  again to ensure further separation of the pith and coir. Finally, the retted coir materials are sun-dried by spreading them over a mat. The bers are then mechanically combed to process them for the next steps like spinning. The rotted husks could also be further mechanically processed for ber extractions. The machine also soens and removes the piths entirely from bers and provides parallel and clean bers. 45 The bers required spinning are rolled in a roller for sliver formations. It was also found that tidal force is better than stagnant water for retting the coconut husks. The progression of the retting process results in the decrease/deterioration of pectin, fat, pentosan, and tannin contents but there is no loss of lignin or cellulosic substances. 45,57,58 However, some of the researchers have also tried pollution-and hazard-free coir ber treatment by using closed anaerobic reactor-based technology. 59

Coir ber extractions
There are several de-husking procedures available for the separation of coconut husks from the surface of fruits. A skilled farmer could manually split and peel around 2000 coconuts in a single day (approximately), whereas the household could do 1 to 2 coconuts per day, and hotels 10 to 20 coconuts in a day. 46 An automatic de-husking machine could split and peel around 2000 coconuts every single hour. 46 The coconut husks are collected by the ber extraction industries from different sources that are not involved with direct de-husking operations (Fig. 3). The processes of ber extractions are dened depending on the usage and quality of the bers. Generally, the coconut husks in India are buried near the riverbanks in pits dug in a concrete tank lled with water. Sometimes, the coconut husks are also suspended through nets and weighted to ensure that they are submerged under the water in a river. Similar processes were described by Prashant et al. 46 for processing coconut husks to extract coir materials. A schematic ow process and ber extraction method is shown in Fig. 3 and 4.

Coir-based nanocellulose
Nanotechnology has become a hot topic nowadays, especially for nanocomposites developed through extracting  nanocellulose from different natural ber-based materials. [60][61][62][63][64] The cellulose brils can be easily cleaved when hydrolyzed with acidic solutions in small particles, which are termed microcrystalline cellulose, nanocellulose, cellulose nanowhiskers, and cellulose nanocrystals. 65 Nanocrystalline cellulose has certain benets as compared to other nano-structured materials. 65 The extraction of nanocellulose from coir husk could be another prominent raw material for nanocomposite production. Generally, coir ber-based manufacturing industries use the coir materials just aer the extraction without any additional processing. However, the nanotechnology-based functionalization or treatment of coir materials needs satisfactory and feasible extraction protocols. The separation of nanocellulose from coconut husk could open another new door for industrially advanced composite materials. There are several pretreatment methods used for isolating nanocellulose bers from coconut. Steam explosion is one of the most attractive and popular technologies in this regard. 53 Machado et al. 65 reported a plasticized nanocomposite developed from biodegradable cassava starch lm with glycerol and coir ber-derived nanocellulose (length/diameter value 38.9 AE 4.7 aer acidic hydrolysis, performed at 50 C for 10-15 min in the presence of 64% H 2 SO 4 ). They further found that the as-produced composites provided higher tensile modulus but there was a decline in the elongation modulus. 65

Coir ber compositions
The composition of ber depends on the types of extracted plants and agricultural conditions. 66,67 Generally, cellulose, lignin, and hemicelluloses are three chemical constituents of plant-based bers, whereas the cellulose and hemicelluloses are polysaccharides and lignin is a three-dimensional (3D) amorphous polyphenolic macromolecule, comprised of three different types of phenylypropane units. 68,69 The celluloses are crystalline, whereas lignin is amorphous. 70 However, the lignin is normally located at the ber surface, whereas the cellulose acts as the backbone of the natural bers. The coir bers are composed of cellulose, lignin, hemicellulose, pectin, ash, and other water-soluble elements as shown in Table 1. It was found that coir bers have approximately 40 to 50% lignin, 27 to 45% cellulose, 0.15 to 20% hemicellulose, 3.5% ash, and 9 to11% moisture content (Table 1). In contrast to other natural bers, coir bers contain more lignin but less cellulosic polymers. 71 However, the higher lignin contents of coir make it harder and naturally rigid. Besides, the resiliency, rot and damp-resistance properties and water absorption capability have made it exceptionally convenient for multifaceted applications. Coir also provides wonderful hard-wearing and endurance features along with weather resistance characteristics, which make it suitable for cords, brushes, and rope-based applications. The enriched lignin and cellulose contents of coir have made it an excellent candidate for biocomposite production as compared to other natural bers as a potential ller material due to its inherent properties like strength and modulus. 72 The higher lignin but relatively lower cellulose content of coir results in  elongation at break as well as the tensile strength of coir berreinforced composites.

Structural properties of coir ber
A typical FTIR analysis (spectra and associated peaks in tabulated form) of coir and other natural bers is shown in Fig. 5 and Tables 2 and 3. The peak at 3401 cm À1 is associated with O-H stretching vibrations, which is a typical characteristic of natural bers (Table 3). 2,79 The broad absorption peak is associated with the hydrophilic characteristics of the coconut materials, indicating the presence of the -OH group in aromatic and aliphatic alcohols. The peak at 2911 cm À1 is responsible for the symmetric and asymmetric stretching of C-H, which is related to the methylene and methyl groups. The aliphatic moieties of hemicellulose and cellulose are indicated by these two stretching peaks. 80,81 The absorption band at 1721 cm À1 is related to the stretching of C]O groups in the uronic ester and acetyl groups or carboxylic group of coumaric and ferulic acids of lignin. 81,82 The presence of amide I is reected by the peak at 1621 cm À1 . The vibration frequency depends on the hydrogen bonding nature of N-H and C]O groups and protein secondary structures. 80,81 The deformation of C-O is related to the peaks at 1030 and 1086 cm À1 . The overall FTIR study shows the signicant presence of the chemical constituents of coir materials. Some other relevant information on FTIR studies on coir materials is tabulated in Table 2.

Physical and mechanical properties of coir bers
The ultimate mechanical properties of the coir ber-reinforced biocomposites are also signicantly inuenced by the characteristics of the control coir materials. 71,88 In this regard, it is necessary to study the chemical and physical characteristics of coir materials before the fabrication of biocomposites. Some of the recently reported chemical and physical properties are tabulated in Tables 1 and 4 for coir materials and some other commonly used natural bers. The most signicant physical properties of the coir bers include density, strength, elastic modulus, and elongation at break, whereas the chemical characteristics are variable in terms of lignin, cellulose, and hemicelluloses. It could be concluded that coir bers have a density of around 1.15 to 1.45 g cm À3 , an elastic modulus of 4 to 7 GPa, 54 to 250 MPa strength, and 3 to 40% elongation at break (%), depending on the type, origin, nature, and processing of the ber ( Table 4). The different concentrations of lignin contained in coir also inuence the variable mechanical properties as shown in Table 5.

Treatment of coir bers
The interfacial adhesion characteristics between the natural ber and matrix is an extremely important parameter that signicantly affects the mechanical features of biocomposites through enabling stress transfer from the polymeric matrix to bers. 94 The chemical cross-linking or physical origination could impact the adhesion of the bers and polymers in the  biocomposites. Besides, the chemical bonding could also signicantly affect the biocomposite interface quality. As a polyphenolic element, lignin plays a major role in natural ber/matrix adhesions. Mir et al. 95 has reported that the treatment of coir ber in a single-stage by Cr 2 (SO 4 ) 3 $12H 2 O and double-stage by NaHCO 3 and CrSO 4 caused an increase in Young's modulus but a decrease in the tensile strength in terms of the increased span lengths of ber. However, the same study 95 further found that the treated coir bers provided higher tensile strengths as compared to untreated coir materials. Muensri et al. found an interesting effect on sodium chlorite treated coir bers, namely, a reduction in the lignin content from 42 to 21 wt% aer the treatment. 68 A proposed treatment process of coir is depicted in Fig. 6. The surface treatments of coir bers are bleaching, mercerization, dewaxing, acetylation, acrylation, cyanoethylation, benzoylation, silane treatment, stem explosion, isocyanate treatments, and so on. Some commonly implemented treatment processes are outlined in this section. 2.7.1 Mercerization or alkali treatment. This is the most commonly used and popular method for natural ber pretreatment to modify the surface. A disrupted hydrogen bond is created with the natural bers with enhanced surface roughness. 97 Different surface impurities like oil, wax, and fats   are removed from the cell membranes of the ber due to alkaline treatments. Alkaline reagents like NaOH aqueous solutions assist the natural bers to ionize -OH groups into the alkoxide. 98 The degree of polymerization, molecular orientation, and chemical composition are affected by the alkaline treatments, which impact the mechanical performances of the treated ber-based composites. A proposed reaction mechanism is shown in eqn (1).
The treatment of coir bers with silane reduces the -OH groups and enhances the surface interface. Silane coupling agents enhance the crosslinking in the interface area. 98 Silane functions perfectly to improve the interface between the natural bers and the associated matrix. Consequently, the mechanical features of the biocomposites are also improved. Javadi et al. 99 researched the silane treatment of coir bers, where a 2% concentration of silane (on the weight of coir) was used. They used a K-mixer instrument, where they operated the machine at 5000 rpm at 150 C. 99 The silane treatment could reduce the water absorption characteristics of natural ber-reinforced composites. 100 This mixer ensured the uniform dispersion of silane on coir bers. A silane treatment reaction mechanism 98 is shown in eqn (2) and (3).
2.7.3 Maleated coupling agents. The biocomposites are strengthened by using maleated coupling agents with natural bers and the associated matrix. Besides, the interfacial bonding of the ber and matrix is improved by using maleated coupling agents. Ayrilmis et al. 101 developed a composite panel for automotive applications (interior) by using maleic anhydride-graed polypropylene (PP) or MAPP with different loadings of coir and found an optimum recipe (3 wt% MAPP, 37 wt% PP, and 60 wt% coir ber).
2.7.4 Acetylation. The acetylation approach for treating the natural bers is also termed the esterication method to plasticize the cellulosic materials. 102 The natural ber acetylation is performed through graing acetyl groups with the cellulosic structures of bers. 102 A proposed reaction mechanism is shown in eqn (4).
Coir-OH + CH 3 CO-OH / coir-OCOCH 3 (4) 2.7.5 Benzoylation treatments. The hydrophilic nature of natural ber, as well as coir bers, creates adhesion problems with hydrophobic polymeric materials; the benzoylation treatment of natural bers could address this challenge to increase mechanical properties. The thermal stability of the coir ber could further be improved by using this method. 103,104 In this regard, alkaline treatment is initially carried out on the coir ber surface to ensure that -OH groups are exposed on the surface. Benzoyl chloride treatment is then conducted on the ber, which in turn replaces the -OH group and strongly attaches to the backbone of cellulose. The above-mentioned circumstances improve the hydrophobicity of bers, thus increasing the ber-to-polymer adhesions. 105 3. Polymers used for coir fiberreinforced composites Coir bers show tremendous potential for reinforcements with thermoplastic, 38,106-111 thermosetting, 112-119 and cementitious matrixes. [120][121][122][123][124][125] Thermoplastic polymers like polylactic acid (PLA), PP, polyethylene (PE) and high-density polyethylene (HDPE) are widely used for producing coir ber-reinforced biocomposites. The incorporation of thermoplastic polymers into coir enhances the thermomechanical properties of the biocomposite. The waxy layer of coir ber makes strong bonds with thermoplastic polymers, thus increasing the strength. 126 The use of thermosetting polymers like PES (polyester), MUF (melamine-urea-formaldehyde), epoxy resin, etc. is another promising area of research for coir ber-reinforced biocomposites. Biswas et al. 127 mentioned that the pretreatment of coir bers could provide better mechanical performances to the coir ber-reinforced thermosetting polymeric matrix. The pretreatment of coir ensures greater adhesion between the ber and polymeric matrix since normally (without treatment), hydrophilic bers restrict efficient adhesion with the polymers. 127 The biodegradability property of the composites made from coir/epoxy is enhanced aer the pretreatment, as reported by another study. 114 The cementitious matrix from coir and cement also shows great potential in developing composite panels for building and construction. Since the coir bers contain some outstanding features as an emerging natural ber, the manufacturing of light-weight cementitious matrix has gained popularity from coir ber-reinforced cement composites. The availability of raw materials and cheaper costs are some of the key features for the products of the construction and building sector, hence coir ber shows a new milestone in this perspective. Abraham et al. developed green building materials from optimized volumes of coir (10%), which provided satisfactory performance characteristics as roong tiles. 128 The mechanical and physical properties of different coir ber-reinforced composites are tabulated in Table 6. According to the results, it could be summarized that coir ber-reinforced composite materials are going to dominate the composite sectors in the near future.

Fabrication of coir fiber-reinforced composites
Fabrication is a very important aspect that requires focus for biocomposite manufacturing. Different manufacturing methods are used for coir ber-reinforced composites. The compression, extrusion, injection molding, RTM (resin transfer molding), and open molding methods are some of the popular fabrication techniques for coir ber-reinforced composites. However, some processing parameters (like ber volume, type of ber, temperature, pressure, moisture content, etc.) need to be considered during biocomposite manufacturing to produce successful products. Different fabrication methods are described in this section.

Compression molding
Compression molding is considered as the most suitable method for producing high-volume composite parts, both from thermoplastic or thermosetting polymers, or even cementitious materials. 2,3,129,130 Whether the ber length is long or short, both could be processed using the compression molding technique. It is nearly the same approach as the hand lay-up process, except that the matching dies used are closed during applying the pressure at a certain temperature for perfect curing. This method is more appropriate if the dimension of composite is smaller; however, open molding or hand layup is more feasible in the case of larger composite panels. Compression molding could be implemented in two different ways 131 as indicated below: Cold compression: operation is performed at room temperature without using any temperature on the mold.
Hot compression: the operation is carried out in terms of certain temperatures and pressures on the mold.
The high-quality composite panels could be manufactured by using this method through controlling and regulating some key parameters like temperature, pressure, and time. Besides, the physical dimensions of the composite panels like length, width, and thickness of the composites need to be selected carefully along with associated materials to be used for manufacturing the composites.

Extrusion molding
A screw extruder is used for this molding process at a specic speed and temperature. The composite materials need to cool down when the extrusion process is complete and could be molded further as per the desired specications. Extrusion molding is used for thermoplastic polymer reinforced composites with improved mechanical strength and stiffness. 132 Different studies have been conducted for coir ber reinforcements with the extrusion molding process. [133][134][135][136]

Injection molding
Injection molding facilitates diversied processing feasibility for polymeric composite manufacturing, especially for highvolume production. With shorter cycle time along with postpost-processing operation/functioning, the injection molding provides exceptional dimensional stability to the biocomposite materials. However, some limitations remain for using injection molding methods; e.g., it requires the lower molecular weight of polymers for maintaining adequate viscosity. Besides, the length of ber and processing temperatures also have less inuence on the produced biocomposite performance. [137][138][139] It has also been reported that plant ber reinforced with PP composites displayed higher performances in the case of injection molding as compared to the compression molding techniques. 139,140

RTM method
The RTM method provides high-quality nishing on composite surfaces with better dimensional accuracy. The thermoset polymeric resins are transferred to a closed mold at low temperature and pressure. Fibers of different forms could function as reinforcements by applying RTM methods. Although RTM is advantageous in terms of the ecological, economical, and technological perspectives, some factors also need consideration, such as ber concentrations, edge ow, and ber washing. 141 However, the most prominent advantage of using RTM methods for natural ber reinforcement is the positive contribution towards the strength and stiffness of the biocomposites. 142,143

Open molding
Thermoset polymer-reinforced composites with natural bers are manufactured by using this method. The biocomposites are cured at ambient temperature in an open mold where the natural mold (bers as reinforcement materials and thermoset as matrix materials) are placed. The investment in equipment is not high for producing high-volume thermoset polymeric composites by using this technology, although this method also has some critical drawbacks like longer curation time, manual labor, and higher waste generations with non-uniform products. 31 Through implementing spraying up/hand layup, the open molding process could be designed. In this regard, the open molding method is also considered the most economical method for biocomposite products.

Properties of coir fiber-reinforced composites
Tensile, exural, and impact properties are some of the significant mechanical properties of natural ber as well as coir berreinforced composites. The mechanical and physical properties of different coir and natural ber-based composites are tabulated (Table 6). It was found that coir bers provide signicant tensile, exural, impact, water absorption, and thickness swelling properties from developed biocomposites. However, different factors affect the mechanical performances of coir ber-reinforced composites as given below: -Types of coir ber -Geometry of coir ber -Processing of coir ber -Orientation of coir ber -Surface modication of coir ber, and -Fabrication of coir ber

Tensile properties
Tensile properties are mainly inuenced by the interfacial adhesion characteristics between the coir and matrix polymer.
Coir has greater proportions of lignin than other natural bers, which facilitates greater tensile strengths. 95 Siddika et al. 144 determined the tensile strength of coir ber-reinforced PP composites as per ASTM D 638-01 standard by using a universal testing machine with 4 mm min À1 crosshead movement. They conducted the test until the failure of the test samples. Romli et al. 145 researched the factorial design of coir-reinforced epoxy composites to investigate the effects of compression load, ber volume, and curation time and found that ber volume has the most signicant inuence on the produced composites (tested via ANOVA in terms of tensile strength).

Flexural properties
The exural strength of biocomposites indicates their resistance to bending deformations. The modulus of biocomposites and associated moments of inertia are two main dependent parameters of exural properties. 146 However, it is necessary to ensure an optimum loading of coir ber to achieve the required exural properties. Ferraz et al. 147 conducted a study on differently-treated coir ber-reinforced cementitious composites, where they found that hot water treatment provided an increase in the MOE (modulus of elasticity) but alkaline treatment caused a decline in the mechanical and physical properties of coir/cement composite panels. In another study by Prasad et al., 148 it was reported that exural strengths started to decline aer 20% coir ber loading, whereas it increased up to 20% ber loading (providing highest bending strength by 141.042 MPa). This test was conducted as per ASTM D 7264 on different coir ber loadings on polyester thermoset resins. 148 Siddika et al. 144 conducted a exural study according to the standard ASTM D 790-00 to assess the bending properties of biocomposites developed from coir. Coir ber reinforced with magnesium phosphate reinforced composites provided higher exural strengths with increased ber loading up to an optimum level then it declined again. 149

Impact strength
The Charpy impact strength testing equipment is used for impact strength measurements. The brittle and ductile transition of biocomposites could also be investigated by using this method. The level of bonding between the natural bers and matrix is responsible for the impact strengths of natural berreinforced composites. 146 The parameters such as the composition of natural bers like the toughness of polymers, surface treatments, and interfacial bonding between ber and matrix could enhance the biocomposites' tensile and exural performances but decline the impact strengths. 150 However, the serviceability of the natural ber-reinforced composite is dependent on the impact strength of natural bers. 146 Siddika et al. 144 performed the impact strength characterization by using a Charpy impact tester (MT3116) as per ASTM D 6110-97. The same study has further claimed that with the increased ber loading, more force is required for pulling-out the bers, hence the impact strength increases. 144 Padmaraj et al. 151 reported that alkali-treated coir ber-reinforced unsaturated polyester composites provided 22.2 kJ m À2 impact strength.

Coir ber-reinforced hybrid composites
Typically, hybrid composites are manufactured by reinforcing two or more different types of ber materials along with a common polymeric matrix. 168 Generally, hybrid composites reinforced with different natural bers demonstrate greater mechanical performances as compared to single-berreinforced composites, which are even competitive with synthetic ber-reinforced composites if the bers are carefully selected as per the requirements. 169 In the case of hybrid composites, the volume fraction of the associated bers strongly inuences the mechanical performances of the composites and stress transfer between the reinforcements (ber) and polymers in the matrix system. 170 Reinforcing synthetic bers with natural bers is also becoming a popular hybridization technology for developed hybrid composites. The natural bers show signicant potential in terms of replacing synthetic bers for developing hybrid composites having superior mechanical and functional properties through minimizing material and production costs. Tran et al. 89 reported that the reinforcement of bamboo with coir ber could positively inuence the failure at strain, hence the incorporated bamboo ber materials could enhance the stiffness of coir berreinforced polymeric composites (Table 7).

Morphological properties
The effects of adhesion properties on coir ber-reinforced composites were easily observed through the SEM (scanning electron microscopy) characterization of the biocomposites. 181 The poor interfacial adhesion between the coir ber and PBS matrix could create a gap and agglomeration during tensile strength testing for pulling out of the bers from the matrix. 91 However, the pretreatment of coir ber could overcome such problems and provide better compatibility between the ber and the matrix, thus providing better mechanical performance. If the bers are not treated, the interfacial region of the coir ber-based composites exhibits less compatibility, hence the composite can easily collapse. 91 Yan et al. 182 claimed that 5% alkaline treatment with NaOH for 30 min at 20 C provided a rough but cleaner surface as displayed through SEM analysis on coir ber-reinforced polymeric or cementitious composite panels. The failure surface of the coir ber/epoxy composite is shown in Fig. 7(a-d) before and aer the treatment across the direction of the applied load. However, treated fractured surfaces exhibited more pull-out of failed bers than the untreated ber composites Fig. 7(c and d). The alkali treatment of coir ber enhances the ber to matrix interfacial bonding, which leads to better tensile performances of biocomposites. The incorporation of more ber volume in biocomposites could minimize the strain fracture, as the increased llers lead to a decreased matrix quantity needed for elongation. 183

Physical properties
Water absorption and thickness swelling are two very important tests for assessing the dimensional stability of biocomposites. Natural bers absorb water from the surrounding environment or even in direct contact with the water and consequently, swelling occurs. 185 In this regard, it is important to investigate the water absorption properties of coir ber composites to ensure better serviceability during their usage. Water absorption has a positive relationship with the ber length; if the length is longer, then the water absorption is higher. 186 In general, the void content and composite density signicantly affect water absorption. The greater ber volume in the biocomposite is also responsible for greater water absorption. Biocomposites made with 20 wt% coir provided greater water absorption than 5 wt% coir ber. 186 The reason behind this may be that coir ber contains hydrophilic -OH groups, as seen in the FTIR study, hence the level of moisture absorption is also high. It could therefore be concluded that increased ber loading also increases the number of -OH groups in the composites, thus the water absorption is also increased. However, the pretreatment of coir ber could minimize the water absorption from associated composites as the treatment reduces the -OH groups from the bers as compared to the control. 133

Thermal properties
Thermogravimetric analysis (TGA) is a useful method for investigating the weight loss of biocomposite materials corresponding to different temperatures. The structural compositions of coir bers (lignin, cellulose, and hemicellulose) are responsible for thermal degradation due to the sensitivity to temperature. 105 The composition of biocomposites in terms of coir and matrix along with degradation behavior could be investigated by TGA analysis. Besides, the magnitude of peaks through derivative thermogravimetric (DTG) analysis could further provide the mutual effects of components in composite systems with respect to temperature. A typical mass loss curve for a coir ber-reinforced PP composite is illustrated in Fig. 8. The initial mass loss from room temperature (25 C) to 150 C is associated with water or moisture evaporations from the biocomposite panels. 187 The initial decomposition temperature for coir ber was observed at 190.18 C, whereas the coir ber/PP biocomposite exhibited decomposition at 211.2 C, which indicates that the incorporation of PP increased the thermal stability of the composite panels. The degradation of different polymers is indicated by the mass loss at certain temperatures: the degradation of hemicellulose occurred at 200-260 C, cellulose at 240-350 C, and lignin at 280-500 C. [187][188][189] However, the decomposition mass loss was 23.95 and 43.89% (Fig. 8) at  190.2-316.9 C and 316.9-475 C, exhibiting nearly the same behaviour. Some researchers also mentioned that the pretreatment of coir bers could also enhance the thermal stability of the biocomposites. 187 Singh et al. developed coir/carbon ber/ epoxy composites; the coir bers were 10, 20, and 30%, and the epoxy, hardener, and carbon materials were kept constant. 179 This study claimed that the incorporation of carbon ber and the treatment of coir bers increased the thermal stability in composite systems and weight loss became greater with the increased coir ber content Fig. 8(c).

Flame retardancy
Flammability characteristics are very important parameters for coir ber-reinforced biocomposites, and the manufacturing of panels with improved resistance/inhibition against re could enhance the market potential. The use of commercial re retardants could also enhance the ame retardancy of biocomposite materials. The commercially available re retardants are based on phosphate, nitrogen, halogen, and inorganic substances. 40 The main purpose of a re retardant is to inhibit the re from reinforced composites, and they function differently depending on the physical or chemical nature of the products in the solid, gas or liquid states. 191 It is reported that nitrogen and phosphorus-based re retardants generate very strong effects on lignocellulosic materials. The re retardants from boron-based compounds do not inuence the mechanical properties of biocomposite materials but resist decay. 40 Shukor et al. 192 conducted a study on ame retardancy in terms of measuring limiting oxygen index (LOI) as per the ASTM D 2863 standard and found satisfactory results ranging from 28.0 to 29.4. In another report, Jang et al. 193 assessed the ammability characteristics of coir ber-reinforced PLA composites and found that all the developed composite provided LOI values higher than 20. It was mentioned by previous researchers that LOI values higher than 20 are considered non-ammable materials. 194,195 However, Jang et al. 193 has further claimed that treating the coir bers could slightly enhance the LOI values of the composites.

Potential applications
Coir bers have a long-term tradition of usage in different application areas. For a long time, coir bers have been used as ropes, yarns, mats, oor furnishings, sackings, insulation panels, and geotextiles. 101 However, coir ber is showing new potential in terms of commercial prospects for manufacturing sustainable and green composite products. The light-weight, low-cost, and thermally conductive biocomposite panels are new and innovative additions of coir ber-reinforced composites. The coir-reinforced bers are widely used for composite panels, beams, and slabs. 196 Besides, coir-ber-based composites also show tremendous potential for seat cushioning in the automotive and construction sectors. 187,197 A funnel developed from coir-based materials provides good dimensional stability and mechanical strength. 198 The same study also reported ower pots having high water retention properties made of coir reinforced PP biocomposites. 198 Coir pith could also be used as lightweight and non-structural building materials through reinforcement with a cementitious matrix, providing thermal and acoustic performances with 3.97-4.35 MPa compressive strength and 0.99-1.26 g cm À3 bulk density. 199 Luz et al. developed a multilayered armor system by using 30% coir ber reinforced with epoxy resin to produce composite materials for ballistic performances. 200 They further claimed that the reported composite displayed similar performances to Kevlarbased materials. 200 The biocomposite materials made from coir/PP could be used for automotive parts. 126 Nadir et al. developed composite panels by reinforcing coir with PP for automotive interiors. 201 Coir-based materials could further be used as helmets, post-office boxes, and roong materials. 26 Different commercially available and ongoing research-based biocomposite products from coir bers are shown in Fig. 9.
Coir ber also shows tremendous application potential in the elds of furniture, aerospace (propellers, wings, and tails), boat hulls, sporting goods, cementitious particle boards, and packaging. 31,202 7. Economical aspects and environmental sustainability of coir fiber-reinforced composites The global composite market is booming with the continuously increasing demands of consumers. The world composite market was projected to be USD 74 billion by 2020, whereas this gure could be enhanced up to USD 112.8 billion by 2025 with an 8.8% compound annual growth rate (CAGR). 211 However, with the constantly increasing environmental awareness of the people, synthetic material-based composites are being replaced by biocomposite materials. The market volume of biocomposite in 2016 was USD 16.46 billion, whereas it was projected to be 36.76 billion by 2022 with a 14.44% CAGR from 2017. 212 The biocomposite market is still an untapped sector where there is the potential for a huge market with prominent demands. The biocomposite products are gaining tremendous attention from the aerospace, automotive, consumer and sporting goods, packaging, biomedical, and construction sectors. There are lots of efforts being made to explore more export-oriented coir ber and its associated markets, as most coir bers remain underutilized. In contrast to the potential competitiveness, the progress in the production of coir ber-reinforced biocomposites and associated employment generations is still low or constant. Coir-based industries are also facilitating huge employments from coconut cultivations to ber extractions, and associated biocomposite production. The total production of coir bers is 350 000 metric tonnes annually throughout the world. 213 Compared to other natural bers, coir ber and associated materials also contribute signicantly to biocomposite markets. Coir bers occupied around $369.7 million by 2019 which is expected to reach $525.7 million by 2027 with an 8.2% CAGR rate within this period. 214 However, due to constant demands for coir-based materials, there is an expansion in USA and Europe.
The processing associated with coir ber, like retting, is a major issue for generating pollutants. 215 Coconut husk retting in India is traditionally performed in water systems for 6 to 12 months long durations, which is an age-old process to extract coir bers. A large number of organic chemical substances like tannin, pectin, fat, phenolic compounds (toxic), and pentosans from coconut husks are liberated in the water systems. 215 Such retting processes of coconut husks also affect the living space of aquatic living agents like sh and also impact the tidal force of water sources. Besides, the air of the surrounding area of retting is affected by a bad smell that pollutes the surrounding atmosphere. The biological retting process for coconut husks is little bit different as compared to other natural bers like jute, as only pectin is decomposed from jute but beside the pectin, phenolic compounds are also decomposed and disintegrated from coir. The pectinolytic action of microorganisms like bacteria, yeast, and fungi degrades the ber-binding elements from husks and liberates them into the environment in large amounts, in terms of organic chemicals and materials. As the DO (dissolved oxygen) is decreased, the hydrogen sulphide, nitrate, and phosphate contents are increased as a consequence of retting-related waste generations in water systems. However, different studies are also trying to nd alternative routes for retting processes to eliminate such environmental challenges. 53 Recently, some of the manufacturers were also trying to treat coir bers with bleaching and scouring chemicals for ber-tomatrix adhesion improvements or coloration purposes to meet the demands of consumers; hence chemical-based waste is also polluting the water sources. 59 8. SWOT (strengths, weaknesses, opportunities, and threats) analysis of coir fibers and associated biocomposites 8.1 Strengths -Potential biodegradability feature -Awareness of sustainability throughout the world -Constantly increasing demands toward natural ber and associated byproduct-reinforced biocomposites -Lower density, higher stiffness, and higher strengths -Economical when produced on the industrial scale -Minimizes/eliminates hazardous effects from manufacturing operations -Renewability and recyclability -Requires less energy for processing

Weakness
-Differences in inherent characteristics -Weaker interfacial bonding -More feasible production technology is not yet invented

Opportunity
-Demands on eco-friendly sustainable products are increasing -Demands for a lightweight biocomposite material is high -Researchers and manufacturers are paying more attention to natural ber-reinforced biocomposites -Biocomposite manufacturing is also implementing state-ofthe-art technology with improved scientic inventions and knowledge -Manufacturers are trying to be more sustainable to cope with more customer demands

Threats
-Climate change is having a critical impact, affecting the availability of raw materials (plant-based) all over the world -Each specialized application needs specic highperformance bers -The cheaper price of synthetic materials -Non-homogeneous quality of the natural bers

Conclusion
This study has provided an overall discussion on coir ber as a potential ller material for producing biocomposite panels. The physical, chemical, morphological, thermal, and mechanical properties of coir ber materials, which affect the ultimate biocomposite features, have also been discussed in this review. The surface modications of natural bers like coir could also play a signicant role in the mechanical properties of biocomposite materials through improving the interfacial adhesion between the coir and matrix, which has been addressed. It was found that the mechanical properties of coir ber-based composites are dependent on the matrix used. Besides, the -OH groups of the treated coir materials decrease during the pretreatment processes; hence the untreated bers absorb more moisture in contrast to the treated bers. Another promising nding reported that the pretreatment could facilitate the reduction of the void content, which is a challenging problem in manufacturing biocomposites. However, the coir bers are suitable for particle boards, structural beams, thermal insulation, sound absorption panels, and so on. The coir berreinforced biocomposites also provide excellent mechanical performances and thermal stability. The hybrid composite materials developed from coir ber and other natural or synthetic bers could tune the improved thermo-mechanical performances of the composites. Furthermore, with the expansion of scientic innovations and technology, there are more areas of coir ber-reinforced composite applications, which also inuences the constantly increasing market for this emerging material. Further investigation is necessary to develop the coir ber-based composites from all the possible polymeric matrixes and dynamic characteristics like damping ratio and natural frequency.