Coconut fruit is made up of the coconut husk that bears the fibre (coir). Coconut husk contains about 20%–30% of coconut fibre [10], while the rest is the pith. Extraction of coir fibre can be carried out manually or by water retting [11]. Retting of coconut is a means of separating the leather-like exocarp from the fibrous mesocarp; this involves immersion of coconut husks in water for about 5 months [12] or 6–10 months [13].

In some research instances, coir fibres have been specified as water-retted, yet the retting details are often omitted. The Coir Board of India and The Coir Institute of Sri Lanka have established standardized retting procedures. Retting can be performed in various mediums, including deionized (DI) water, salt water (SW), or a chemical medium. Retted coir fibres exhibit differences in their crystalline index based on the medium used, and also in their tensile properties, with salt water-retted fibres demonstrating superior properties compared to those retted in deionized water [8]. During retting, coir fibre undergoes several stages, such as de-fiberizing and combing, where fibres from different sources may become intermixed. Combing is repeated as often as necessary without an automated gauge to indicate when the desired standard is achieved. The combing process can be quite severe and may cause significant surface damage to the fibres. This is expected to significantly affect the fibre diameter and, as a result, its mechanical properties. The de-fiberising process and combing should be better automated to achieve better yield and efficiency, which are likely to lead to improved properties [4]. Other natural fibres have been extracted either manually or through retting, as noted by [14–16]. Coir fibres typically range from 8 to 300 mm in length [17], with an average fineness of 27.94 Tex and an average diameter between 0.09 and 0.39 mm [18–20]. Coir obtained via retting or manual extraction can be utilized in various forms, including chopped fibres, unidirectional long lengths, or crushed particulates of different micron sizes for composite production.

Table 1 shows the chemical composition of coir fibre with other natural fibres. The overall characteristics of coir fibre are greatly affected by the concentration of its chemical constituents. The strength of natural fibres is notably determined by the amount of cellulose and lignin they contain [18]. Coir has one of the lowest densities among natural fibres considered for technical uses (refer to Table 1), which translates to considerable weight reduction and enhances specific properties. Lignin, an amorphous material [21], enhances the fibres’ elasticity and provides resistance to biodegradation. The lignin content in coir is over 60% higher than in other natural fibres, making it exceptionally suitable for outdoor applications due to its superior resistance to weathering and greater toughness. Lignin serves both as a binder for cellulose fibres and as an energy storage mechanism. Coir’s microfibrillar angle (MFA) is over 50% greater than that of other natural fibres, resulting in the lowest cellulose content. A higher microfibrillar angle correlates with reduced cellulose content and reduced strength [22]. With the lowest density (see Table 1 [2,10,15,18,23–29]), coir is ideal for lightweight applications.

Composites consist of reinforcements (fibre) and matrix; thus, the properties of the reinforcing fibres and the interfacial bond strength between the fibre and the matrix significantly determine the composites’ overall properties. Table 2 presents findings on the mechanical properties of coir fibres, highlighting the influence of origin and diameter. It indicates that fibres from Sri Lanka, Vietnam, and Brazil show increased tensile strength and stiffness with reduced diameter. However, this varies compared to fibres from other origins, underscoring the significance of fibre origin and diameter in mechanical properties [30–33]. As depicted in Fig. 4, tensile properties such as strength and elongation are affected by fibre diameter. Thicker fibres have more pores, leading to a larger [34] diameter [35], which correlates with a higher likelihood of failure due to the increased pore presence. The tensile strength of coir fibre is significantly influenced by the gauge length. Tensile properties are also influenced by the fibre extraction method [30], maturity [22], and processing conditions [10]. From Table 2, coir fibre diameter ranges from 0.006 mm (Vietnam) to 0.577 mm (Thailand), and tensile strength spans from 68.4 MPa (Tanzania) to 343 MPa (Vietnam). Notably, Vietnamese and Indian coir fibres exhibit the highest elongation at break, 63.8%, and Young’s modulus, 6 GPa, respectively. Vietnamese coir fibre, possessing a tensile strength of 343 MPa, can effectively compete with jute fibre and may be utilized in comparable applications with jute fibre. It is also suitable for applications where high elongation is crucial, for instance, in components that require energy absorption before plastic deformation, like foam padding. Conversely, coir fibres from Tanzania, Malaysia, and the Philippines, which exhibit lower tensile strength, can be appropriately used for products like brushes and foot mats.

Previous researches that have been carried out on coir as regards their tensile strength shows that the tensile strength of coir fibre linearly increases as the gauge length increases and vice-versa [46,47]. However, there exists a critical fibre length for optimal performance as discussed in Section 4.3.2.

Natural fibres are generally prone to thermal degradation due to temperature gradient. A fibre is thermally stable to the point it can withstand thermal degradation. Therefore, thermogravimetric analysis (TGA) of natural fibres is important. Thermal degradation of coir fibres mostly occurs in three stages [3,48]. The first stage is usually a result of loss in moisture or evaporation [3,18]; the second and third stages have been attributed to degradation of hemicellulose and degradation of cellulose, respectively. These stages with their corresponding loss in the weight of the fibres are displayed in Table 3; the loss in the weight of the fibre in the first stage (evaporation) is not more than 10%. In some cases, the weight loss of the fibre increases as the temperature increases. Some variations in the thermal stability of coir fibre have been attributed to the extraction processes. Thermal stresses have been noted to be induced by the high speed of the test, leading to the softening of the matrix, giving rise to mass loss or increased wear, and finally resulting in the formation of microcracks and debonding [49]. From Table 3, the least weight loss of about 2%–9% occurred as a result of thermal degradation, mostly in the first stage at temperatures between 25°C–150°C. Hence coir fibres can conveniently be used in composite applications where service temperature does not exceed 150°C, such as automobile dashboard/instrument panels, wind turbine blades and geotextiles. Moreover, specific treatments have been recognized for enhancing the thermal stability of natural fibres, such as noted by [50,51]. These treatments are discussed in further sections.

Coir fibres are typically multicellular, comprising xylem, parenchyma, phloem and xylem-parenchyma. The arrangement of these cells, their number and their shape, distinguish coir from other natural fibres by giving it its unique properties. The elementary fibres within the coir fibre bundle have a large number of microfibrils, and these elementary fibres are bonded together with the help of the middle lamella [39]. Scanning Electron Microscopy (SEM) is employed to examine the morphology of coir fibre. This morphology showcases numerous lumens and a central core, the lacuna, which contributes to the low density of coir. The surface of the fibre is rough with globular protrusions known as tyloses that aid in interlocking with the matrix. However, during resin impregnation, these pores might not be filled, resulting in less-than-optimal mechanical properties in the composites. The tyloses are longitudinally aligned, with occasional pits where protrusions are missing. The considerable inherent porosity of coir fibre is likely a factor in its reduced tensile strength. Tyloses enhance the interlocking of fibres and matrices within composites. The longitudinal cross-section shows a helical structure of the elementary fibres. Coir fibres are increasingly prone to intrinsic flaws as the stress application rate increases. Also, viscoelastic polymers exhibit increased apparent stiffness with rising stress application rates. The failure mechanisms in natural fibres, such as coir relate to the behaviour of the microfibrillar angle under tensile loading. The microfibril angle in the secondary wall layer significantly influences Young’s modulus, with this impact varying according to the radius of the fibre [39]. The multi-cellular microstructure of coir, composed of helically arranged elementary fibrils [22], leads to the uncoiling of these helices under tensile load at relatively low-stress levels. Additionally, the presence of lacunae, intrinsic porosity, and tyloses suggests that the tensile stress coir fibres can withstand can be significantly lower than that of a solid fibre with a comparable cross-section. The microfibril angle has been related to the Young’s modulus [22,40] using Eq. (4):(4) Ecf=E.cos2θ⌊k.(1−2cot2θ)2⌋E.cos2θ+k.(1−2.cot2θ)2 where θ is the microfibril angle, K is the bulk modulus, E calculated modulus and Ecf = is the observed modulus of fibre.

The properties of coir fibre are largely dependent on its microstructure. Fig. 5a,b shows the optical and scanning electron microscopy of the cross-section of the coir, while the coir surface is shown in Fig. 5c, where a rough surface can be observed. The rough surface has been reported to aid adhesion to the polymer matrix, thus providing mechanical interlocking of the fibre/polymer (see Section 4, for SEM of coir fibre in the polymer matrix). Fig. 5b shows elementary fibres. Fig. 5a shows the presence of smaller lumens and a larger one called lacuna [35]. These lumens have been noted to aid resin flow. The lacuna may not always be centrally located; the position of the lacuna depends on the sectioned part of the fibre (that is head, middle or tail) [4]. Lumen influences the mechanical properties of the fibre. The fibre porosity is in the range of 21.1% to 46.3 % [35,39]. The fibre surface displays a series of protrusions rather known as tyloses. Plants with many tyloses have been noted to be less prone to pathogen attack; hence, coir has a higher resistance to attack and much slower degradation than other natural fibers.

Coir possesses a good number of drawbacks which may limit its use as reinforcement in composites. The key drawbacks include:

Variability in the length of coir fibre, diameter and thus mechanical properties

Natural fibres are hydrophilic due to the presence of the hydroxyl (OH) group. Therefore, they are prone to moisture ingress. The degree of water absorption or moisture penetration in coir fibre is influenced by various factors, such as the temperature of the water and environmental conditions. Higher weight gain in the fibre has been observed at elevated temperatures compared to room temperature [50]. The rate of absorption increases with temperature, which is attributable to molecular processes (see Section 4.3). This brings about reduced interfacial bond strength between the fibre and the polymer matrix, hence, a significant reduction in their mechanical properties when compared with synthetic fibres such as glass and carbon. Other drawbacks include: limited length of the fibres, non-uniformity of the fibres, especially along their length because they possess complex multi-layered cell wall and lumen structures [15,19], reduced thermal stability, high porosity, harsh extraction processes, swelling of the fibres due to the plant constitution leading to debonding and seasonal planting conditions. Coir fibres possess much lower diameters (less than 0.5 mm); as a result, it is practically difficult to measure the fibre strain via an extensometer. They are prone to insect and microbial attacks. Some of these downsides of coir fibres are attributed to their chemical composition (see Table 1) as well as their structure (see Fig. 6) and inherent defects as a natural fibre. The chemical components of these fibres, such as cellulose (OH group), allow for high moisture ingress of the fibres. Besides, polymeric matrixes such as epoxy, which are often used with fibre reinforcement, have an entirely different structure, hence incompatible with the fibre and, as a result, poor stress transfer at the interface (poor interfacial bonding). Therefore, chemical treatment of coir before use in the composite is necessary to mitigate these drawbacks.

Coir fibres are washed and dried to remove surface debris before chemical treatments. Treatments that have been carried out on coir include alkali treatment [55–57], acetylation, use of silane and other coupling agents, oxidation, UV aging and grafting with acrylate monomer (EMA) using UV radiation. The main reason for surface treatments of natural fibres is to improve the interfacial bond strength between the fibre and the matrix. Others are to improve their resistance to thermal degradation [24,58–60], reduce water absorption [14,61,62], increase storage modulus [55], improve flame retardance [50], enhance damping factor [63], prevent microbial attack [64], and to resist the deterioration of their composites [65]. NaOH treatment has been the most widely reported and the most effective so far. Silane treatment has been reported as the most effective coupling agent for natural fibres. Therefore, these two treatments are subsequently reviewed in detail.

Table 5 shows that coir fibre composites have been manufactured with epoxy, polyester, polypropylene and polyurethane matrix. Epoxy and unsaturated polyester resins are thermoset resins that do not give off volatiles during cross-linking. Epoxy resin is a thermoset polymer that has been predominantly used for coir fibre matrices and even for high-performance composites. Epoxy possesses inherent advantages such as excellent wettability, and high thermal and dimensional stability; it is also less volatile and can successfully be used at room temperature. Unsaturated polyester cures at room temperature with methyl ethyl ketone peroxide used as an accelerator. Epoxy is predominantly used as a matrix to manufacture composites via HLU. In HLU, the reinforcing fibres are in most cases, manually impregnated by the resin, where the skill and expertise of the operator play an important role in the result of the composite [94]. In RTM, a mould or steel panel is used to form the panel surfaces. The reinforcing fibres are placed into the mould cavity; the mould is then closed before the introduction of the resin. RTM comes in different forms concerning how the resin is introduced into the mould. RTM includes resin infusion and vacuum-assisted resin transfer moulding (VARTM). Fibres are impregnated and preforms laminated using resin infusion. The vacuum pressure, resin inlet, resin trap and clamps are carefully monitored. Coir fibre composites with epoxy matrix have been manufactured using resin infusion as shown in Table 5.

Coir composites such as coir/PBS [73] have been manufactured using a hot press at a pressure of 10 MPa, temperature of 150°C and a duration of 10 min. Cooling was carried out by quenching in ice water. The parameters to watch are processing temperature, pressure, dwell time, sample dimensions, and the viscosity of the resin. Other important parameters include means of cooling such as quenching or air cooling. Coir/polybutylene succinate and coir/PLA have been manufactured using a hot press. A composite manufactured using HLU can be consolidated in a hot press platen.

The manufacturing of coir composites through injection moulding is not widely practised. However, Reference [1] successfully utilized injection moulding to produce coir fibre-reinforced polyvinyl chloride (PVC). Initially, the fibres were ground to various sieve sizes at different fibre volume fractions and then combined with the matrix material before being fed into the hopper. The rotating screw and the heater band within the barrel facilitated the formation of the composite. Key parameters in the manufacture of composites by injection moulding include injection temperature, pressure, rate, and holding pressure [1] manufactured a PVC/Coir composite using an injection pressure of 130 MPa, a temperature of 190°C, and a holding pressure of 190 MPa. In injection moulding, the mould clamping force is verified to ensure it is adequate to keep the mould closed. The required mould clamping force is calculated as follows:(6) F=Po(1− (rR)n)2πrdr

(7) F=πR2P0(nn+2)

(8) F=TOL(nn+1)

Eqs. (7) and (8) are for circular and rectangular plates, respectively. where F = mould clamping force, P0 = pressure at the gate; n = constant (pressure loss coefficient); R  = radius; T and P = width and length, respectively.

This expression is used to estimate the clamping force depending on the shape of the material.

Other natural fibres have been manufactured using injection moulding including ramie/PLA [95]. Compression moulding can be used to manufacture composites of thermosetting or thermoplastics matrix. The granular plastic can first be converted into sheets by placing the granules on a steel mould of the compression moulding machine and compressed at a certain pressure, temperature and duration. Afterward, the fibres are stacked onto the produced sheets according to the experimental design or stacking sequence adopted to form laminates [80,81,96,97]. For compression moulding, the compaction force ‘F’ is given by:(9) F=3ηV28πtS4

 S = platen separation at time, t

η = viscosity

v = volume of the preform

Other methods that have been used to manufacture natural fibre composites include vacuum infusion, filament winding and pultrusion [98].

Table 5 gives a summary of the mechanical properties, manufacturing techniques, fibre volume fractions and fibre orientation that have been used in coir fibre-reinforced composites. Table 5 shows that over 50% of the researchers used the HLU manufacturing method and epoxy matrix. The volume fraction used ranges between 10–65% with both random and unidirectional fibre orientations. Over 35% of the researchers manufactured coir composites with fibres unidirectionally oriented.

From the Table, fibre volume fractions of about 10% yielded poorer results than those of over 20% concerning mechanical properties. Randomly oriented fibres produce less desirable properties in composites than unidirectionally oriented fibres in terms of tensile and flexural strengths.

Fig. 8, deducted from Table 5, shows the tensile strength of the coir epoxy composite to range between 13.05 to 62.92 MPa (A–J). The highest tensile strength achieved was 62.92 MPa (J) with 49% fibre volume fraction unidirectionally oriented and stacked in three layers, manufactured using HLU. 50% of HLU-manufactured coir/epoxy composites with unidirectionally oriented coir fibres possess an average tensile strength of 41%. This is comparable to the tensile strengths of sisal, jute and Kenaf fibre-reinforced epoxy composites manufactured using HLU, as shown in Table 6. The impact strength of coir is over 90% higher than those of sisal, jute and Kenaf epoxy composites manufactured via HLU. However, the flexural properties of coir/epoxy composites were 15% lower than those of sisal, jute and Kenaf epoxy composites manufactured using HLU. Not much has been done on the creep behaviour of coir composites as has been recorded in jute-reinforced composites [99].

Fig. 9, deducted from Table 5, gives a clearer picture of the coir/polyester composite where the highest and the lowest tensile strengths were 29.1 and 4.13 MPa respectively with an average of 18.32 MPa. However, this cannot be compared with the tensile strength of other natural fibres using a polyester matrix, as shown in Table 6. The mechanical properties, as seen, have invariably been influenced by several factors such as the type of matrix, type of fibre [25,100], size of fibre [98], manufacturing method [101], fibre treatment [69,102–104], fibre orientation [105], stacking sequence [106] and fibre volume fraction [107–109]. Some of these factors will be reviewed in the subsequent section.

Fibre volume fraction, fibre treatment, fibre length and fibre orientation on the properties of the composites.

The thermal stability of composites is a subset of the thermal stability of both the resin and the fibre; treated fibres show better thermal stability in composites than untreated [36]. However, not all treatments lead to an improvement in thermal resistance. Enzymatic treatment improves thermal stability [138] as well as glycidyl methacrylate [58]. A reduction in void content and density of coir composites is observed with treated fibres, as shown in Fig. 11. Void is observed in the composite as a result of the inability of the matrix to displace all the air entrapped within the fibre as the fibre is being impregnated. The presence of void (see Eq. (10)), especially at a significant level, significantly reduces both the mechanical and physical properties of composites [139]. Voids can be attributed to partial wetting out of the fibres by the matrix and processing conditions [10]. The addition of a wetting agent brings about an improvement in interfacial compatibility and, hence, improved mechanical properties. Therefore, perfect impregnation and interfacial bond strength are imperative for improved composite structures.

High moisture ingress gives rise to poor wettability and weak interfacial bond strength between fibres and matrices, leading to the deterioration of composites. Water absorption of coir fibre can be determined using the following equation:(14) Wac=[MwtcMdc×100]%

Wac , Mwtc and Mdc represent water absorption of coir, the mass of the wet coir sample at time t and the constant mass of dry coir respectively. Moisture ingress is quickened by diffusion through the fibre/matrix interface. The diffusion behaviour of composites at room temperature is by Fick’s law given in Eq. (15):

(15) MoMx=1−8π2∑n=0∞⁡(1(2n+1)2exp−(2n+1)2πDth2) where Mo, Mx, Dt, h, n represent moisture uptake, maximum uptake of moisture, diffusivity, thickness of the composite and summation index, respectively.

Recent researches show that water absorption is higher in untreated coir fibre composites than in treated [36]. Water absorption significantly decreases on fibre treatment; this can be attributed to the reduction of the free hydroxyl (OH) group [10] and hence improved interfacial bonding between the fibres and the resin. The coating has been carried out on other natural fibres such as jute, sisal and bamboo to reduce the susceptibility of the composites to moisture ingress [140].