Scientific African 11 (2021) e00722 Contents lists available at ScienceDirect Scientific African journal homepage: www.elsevier.com/locate/sciaf The effect of natural fibre reinforcement on polyurethane composite foams – A review Charles Kuranchie, Abu Yaya, Yaw Delali Bensah∗ Department of Materials Science and Engineering, University of Ghana, Accra, Ghana a r t i c l e i n f o a b s t r a c t Article history: The use of natural fibre reinforced polyurethane composite foams is becoming increasingly Received 11 November 2020 popular in the cellular foam industry. This demand is due to the increase in the use of Revised 12 January 2021 ecofriendly, biodegradable, and sustainable materials to produce polyurethane composite Accepted 8 February 2021 materials for such applications. In this regard, natural fibres from agro-waste are being preferred to their synthetic counterparts due to readily availability, strength, lightweight, Keywords: biodegradability, and cost-effectiveness. In addition to their renewable nature, the use of Natural fibre natural fibre in polyurethane foams produces composite foams with better properties than Interfacial adhesion the neat polyurethane foams. This review explores the preparation, and properties of nat- Reinforcement ural fibre reinforced polyurethane composite foams and discusses the effect of chemical Polyurethane foams modification of these fibres on the interfacial adhesion of the fibre-polymer matrix sys- tem. It also assesses the trends and future potential of the global market of natural fibres and the polyurethane composite foams. © 2021 The Authors. Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Introduction Polyurethane foams (PUF) are especially useful group of cellular polymers with many applications in diverse industries. They are widely used in consumer bedding and cushioning products as upholstery, furniture, automotive seats, insulation and in applications where other materials might not be suitable due to the versatility of polyurethane chemistry [ 1 , 2 ]. The PUFs are normally produced via a polyaddition reaction between polyols and isocyanate with other additives to enhance the foaming process [3] . There are two main types of PUF, rigid polyurethane foams (RPUF) and flexible polyurethane foams (FPUF), which differ by the type of raw materials and additives used for their manufacture. The raw materials for their production are mainly from petrochemical sources which are now becoming unpopular due to environmental issues as- sociated with the synthesis of these materials [ 1 , 4 , 5 ]. The increasing demand for a clean environment has led to the use of natural resources to develop innovative green materials [6] . This has therefore prompted the polyurethane industry to develop composite foams from renewable materials especially polyols from both edible and non-edible plant oils [7] and isocyanates from amino acids or fatty acids [8] as well as using natural fibres from agro-waste as fillers [9] . The use of these eco-friendly sustainable materials effectively contributes to the reduction of the release of greenhouse gases, promotes envi- ∗ Corresponding author. E-mail address: ydbensah@ug.edu.gh (Y.D. Bensah). https://doi.org/10.1016/j.sciaf.2021.e00722 2468-2276/© 2021 The Authors. Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 ronmental sustainability and conservation and provides a cost-effective measure of producing composites [10] . Amongst the renewable materials that are being used in the polyurethane industry today, the use of natural fibres as fillers has gained considerable attention. Generally, fillers are known to improve the density, mechanical, optical, electrical, and thermal prop- erties of polymers. Additionally, they provide a cost-effective way of producing polymeric composites without compromising their inherent properties [11] . Several studies suggest that the addition of natural fibres to polyurethane composite foams improves the mechanical properties of such composite foams [12–16] . Apart from this, natural fibres make it possible for the modification of PUFs to exhibit properties such as thermal stability and conductivity, flame retardancy, acoustic, sound- absorbing, and insulation [17] . In recent times, studies have shown that the addition of nutmeg fillers, for instance, produces polyurethane composite foams with antibacterial and anti-ageing properties in addition to their expected physio-mechanical properties [5] . Over the last decade, several works have been published about the use of natural fibres as reinforcements in polymeric composites. For instance, Gowda et al. [18] published a comprehensive review of the use of natural fibres as a sustainable and renewable resource for developing eco-friendly composites. They discussed the various types of natural fibres that are being used extensively (e.g., silk, bamboo, kapok, coconut fibre, etc.), how the properties of these fibres can be enhanced through chemical treatment, the applications of their reinforced composites, etc. [18] . Their review talked generally about polymeric composite materials without emphasis on a particular polymer and their scope of fibre treatment was centred only on chemical treatment. Similarly, Abdul Karim et al. [19] published a review on natural fibres as promising environ- mentally friendly reinforcement for polymer composites. They discussed the various factors that affect the properties of natural fibres and how the interfacial adhesion with the polymer matrix can be improved to produce composites with im- proved properties for useful applications. Their emphasis was on jute, hemp, coir fibre reinforced polymer composites whose matrix was mainly polypropylene, polyethene, and polylactic acid [19] . Currently, there is little information on the review of natural fibre reinforced polyurethane composite foams. One of the interesting reviews published on polyurethane composite foams is that by Kauser [3] for high-performance applications with a focus on carbon-based nanofillers as reinforcements and applications of these composite foams in selected industries like the aerospace and automotive, sensors, fire-proof materials, and as biomedical materials. In this review, our emphasis is on the reinforcement of polyurethane composite foams using selected natural fibres with attention to their preparation, and properties. The effect of interfacial adhesion between the fibres and the polymer matrix and how this can be improved through fibre modification/treatment are briefly discussed. Lastly, it assesses the trends and future potential of the global market of natural fibres and the polyurethane composite foams as well as the numerous opportunities that can be harnessed by the research world. Natural fibres Generally, natural fibres are fibres obtained from vegetable or animal sources [20] and this includes all-natural cellulosic- fibres (cotton, jute, sisal, coir, etc.) and protein-based fibres such as wool and silk. Those from the vegetable/plant sources can be largely classified as either wood or non-wood fibres [ 21 , 22 ]. The wood fibres mainly consist of soft and hardwood, and recycled wood. Non-wood fibres come from various sources such as the leaf, straw, bast, grass, etc. [9] . Currently, non-wood fibres derived from agro-waste have gained considerable attention in the polyurethane industry especially for reinforcing applications [23] . The commonly used ones which are lignocellulosic include coconut husk, rice husk, wheat husk, palm kernel shell, jute, kenaf, flax, cotton husk, bast, banana fibre etc. [ 24 , 25 ]. They are normally a low- cost replacement to mineral fillers due to their ready availability, easy accessibility, low density, renewability, and their less abrasive effect on processing equipment [26] . In addition to this, natural fibres are abundantly available in various forms all over the world, they are non-toxic and can be chemically modified to improve their compatibility with the polymer matrix [27] . They exhibit high specific strength, stiffness and virtually no carbon dioxide emissions. These characteristics make natural/bio-fillers preferred to their mineral source counterparts [28] . Despite these interesting characteristics, the use of natural fibres in reinforcing thermoplastic polymers comes with its challenges [24] . Firstly, they are hydrophilic due to their lignocellulose nature and this makes blending with a hydrophobic polyurethane matrix less effective because of the poor interfacial adhesion between the fibres and the matrix. Furthermore, the polar nature of natural fibres promotes moisture absorption leading to swelling of these fibres which causes a reduction in their mechanical properties [ 9 , 10 , 29 ]. However, these drawbacks can be resolved by chemical modification of the fibres before their use [30] . The effect of this modification on the properties of fibre-reinforced polyurethane composites shall be discussed in a later section of this paper. Natural fibre reinforced polyurethane composite foams Several studies have shown that the addition of natural fibres to flexible polyurethane foams (FPUFs) results in enhanced mechanical properties as compared to the addition of mineral fillers such as CaCO 3. In one of such studies, Uzoma et al. [31] investigated the effect of using bambara nutshell and corn chaff on flexible polyurethane foam and compared it with that of a CaCO 3 filled foam. They discovered that the bambara nutshell filled foams had better resistance to tearing and shredding and hence was ideal for the making of upholstery products as compared to the CaCO 3 filled ones. On the other hand, the corn chaff filled foams showed higher compressive strength than the CaCO foams. Amongst the three fillers that 3 2 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 were used, the CaCO 3 showed the least compressive strength whereas the corn chaff gave the highest. Additionally, there was also an increase in density in the foams filled with the natural fillers as compared to the CaCO 3 filled ones [31] . Chris-Okafor et al. [16] investigated the effects of coconut husk and corn cob on the properties of FPUF. To demon- strate this, they varied the filler percentage (5%, 10%, 15%, 20%, 25%) and noted that the density and compression test in- creased with increasing filler concentration. However, the elongation at break and tensile strength decreased with increasing filler concentration. They asserted that this observation was attributable to the poor interaction between the fillers and the polyurethane matrix, and also the voids created by the fillers in the matrix phase. Their study also revealed that an increase in filler load also led to an increase in the indentation deflection force (IDF), a physical property that measures the firmness of polyurethane foams [16] . Further in the study, Chris-Okafor et al. [16] used a mixture of rice husk and corn cob as fillers to produce FPUF where the results of this, corroborated the finding in their previous study where they used coconut husk and corn hob. They observed that an increase in filler load led to a corresponding increase in density [16] . Apart from increasing the density and compressive strength, natural fillers have also been shown to improve the damping and thermal properties of foams as demonstrated by Shan et al. [32] using coir particles. They observed that coir particles had high decomposition temperatures and hence was suitable to be used as fillers. Furthermore, the addition of the fillers created a cellular structure with smaller cell size and higher cell density hence providing better damping for energy absorp- tion applications [32] . Also, Bryskiewicz et al. [12] discovered that the addition of walnut shells and hazelnut shells also resulted in improved thermal stability of PU foams. Furthermore, using about 13% of a combination of walnut and hazelnut shells resulted in an improvement in the flexibility of the foam as compared to the unfilled foams [12] . Another interesting aspect of natural fibres is that, it also helps to enhance other properties such as the insulation and sound absorption properties of rigid polyurethane foams. The work of Sair et al. [6] explored how the introduction of al- kalized alfa fibres could reinforce rigid polyurethane foam and improve their insulation properties. The normal sound ab- sorption of neat PU foams generally shoots steadily from 0.1 at 50 Hz to about 0.19 at 2 kHz. However, they observed that at low frequency (up to 350 Hz) both neat and reinforced foams recorded the same sound absorption coefficient. Beyond 350 Hz, the fibre reinforced composite foams had higher sound absorption coefficient because of the fillers. This observation was due to a reduction in transmission of vibrations because of the numerous small holes found on the hollow tubular structures of the alfa fibres [33] . In another study, Ekici et al. [34] sought how they could improve the sound absorption capacity of PU foams by adding natural tea leaf fibres. Their motivation was hinged on the fact that sound absorption is a relevant requirement for human comfort especially in automobiles and manufacturing environments where higher sound pressure is created. They reasoned that to produce economical insulation materials, the versatility of polyurethane could be explored by the introduction of tea leave fibres in the formulation of rigid PU foams. After preparing several formulations, they concluded that there was a steady increase in sound absorption as the filler content was increased with an ultimate value of sound absorption of 0.39 at 6.3 kHz. At 8% filler concentration, 80% increase in sound absorption was recorded [34] . Tao, Li and Cai [35] also studied the influence of both rice straw fibre and wheat straw fibre on the sound absorption properties of rigid polyurethane foams (RPUF). After preparing the foam composites, they measured their sound absorption at different frequencies. They observed that at 10% of fibre content (both rice and wheat straw fibre), the maximum sound absorption coefficient was recorded. However, the average sound absorption coefficient of the wheat fibre filled composite foams was higher than that of the rice fibre filled composite foam at 5% and 10% filler content. Using SEM images, they discovered that the addition of the straw fibres damaged the close cell structure of the polyurethane foam resulting in an open cell structure which might have accounted for the improvement in the absorption property [35] . Chen and Jiang [36] have also studied how the addition of bamboo leaf particles influences the acoustic property of polyurethane foams. In this study, they reported that the PU composite foam that contained 6% of the bamboo fibre gave the highest noise reduction coefficient and the maximum sound absorption coefficient at 6.3 kHz. Furthermore, they discovered that 8% of bamboo chips with a particle size range of 2–3 mm gave the highest sound insulation [36] . A look at Table 1 shows that the amount of the fibres that were added were largely dependant on the type of fibre, and the property that was needed to be enhanced since the fibres differed in their chemical composition and morphol- ogy. Despite the positive reinforcing effects of natural fibres, a few studies also suggest that their addition enhanced some properties of polyurethane foams and at the same time decreased other properties. Riberio da Silva and group [43] have produced polyurethane foams using tung oil with rice husk ash as a filler. They found that the addition of the rice husk ash resulted in noticeable changes in properties such as thermal conductivity, density, and foam morphology. However, the com- pression modulus, compressive strength, and storage modulus decreased with an increase in the filler content as compared to the unfilled foams [43] . Bryskiewicz et al. [12] also noted an observation like that of Riberio da Silva et al. [43] . Despite reporting an increase in foam flexibility at 13% filler concentration, they discovered that this property decreased when the concentration of the filler was increased to 18% [12] . A similar decline in density and mechanical properties was reported by Xue and group [44] . In their study, they used lignin-based polyol to make rigid polyurethane foams and reinforced with different ratios of pulp fibre (1%, 2% and 5% wt.). They discovered that the densities of the foams decreased as the content of the lignin increased. Through an SEM image of the resulting foam, they observed that the addition of the lignin and pulp fibre affected the cell shape of the foam resulting in irregular and large cells thereby affecting the density. They further observed that an increase in the lignin3 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 Table 1 Summary of selected fibres and property of their composite foams. Type of Natural filler % Filler added Property Enhanced Reference Bamboo leaf fibre 6% Sound absorption [36] Natural tea leaf fibre 20% Sound absorption and insulation [34] Hazelnut shell fibre 35% Flexural strength [12] Palm fibre 5% Compressive strength [37] Kenaf fibre 7.5% Compressive strength [38] Flax & jute fibre 38% Stiffness and strength [39] Sisal fibre 3% Flexural strength [40] Hemp fibre 15% Tensile strength [15] Alfa fibre 10% Acoustic property and tensile strength [6] Coconut shell powder 30% Tensile strength [14] Cloves 2% Compressive strength, impact strength and antibacterial property [5] Combination of wood, bamboo leaves & rice husk 35% Thermal insulation [41] 50–50 Coconut husk & rice husk 15% Flame retarding [16] Wood fibres 10% Compressive strength [42] content caused a decrease in the compressive strength of the foams. Surprisingly, the addition of the pulp fibres did not improve the compressive strength as the lignin content was increased [44] . Silva, Takahashi and Chaussy [45] had slightly different results out of their study as compared to the other authors cited above. They prepared composites of rigid PU foams using cellulose fibre residue and assessed the influence of the cellulose fibre on the structural, thermal, and mechanical properties of the composite foam. They reported that at 8–16% filler concentration, the density of the filled foam increased to about 10–28%. Furthermore, the addition of 1–8% resulted in a decrease in compressive strength as compared to the unfilled foams. However, when the filler concentration was increased to 16%, there was a slight increase in the compressive strength indicating that the filler did not have a significant change in the compressive strength of the filler foam. Despite these findings, they also discovered that the composite foams were susceptible to fungal attack. They concluded that at 16% filler concentration, the thermal conductivity decreased but the thermoxidative stability and mechanical properties remained unchanged. The composite foams will not put a burden on the environment after disposal since it can easily degrade through fungal attack [45] . From the review of the various studies mentioned in this paper, it is evident that the addition of natural fibres enhances the properties of polyurethane composite foams (PUCF). Even though some properties are enhanced, there is the likelihood that others may decrease with increasing fibre concentration. Therefore, it is imperative to know the kind of property one seeks to enhance/reinforce in a PUCF and the quantity of fibre that is required to bring out the reinforcement. Preparation of fibre reinforced polyurethane foams The processing of polyurethane foams requires the mixing of the reagents and additives in appropriate proportions to ensure control of the foaming process to produce foams with the desired properties and applications [2] . During the foaming process, a catalyst (e.g. stannous octoate) speeds up the reaction between a polyol and the isocyanate. Surfactant is added to control the interaction between the non-homogenous components of the reaction mixture. Also, cross-linking and chain extending agents are added to provide mechanical reinforcement to improve the physical properties via the modification of the polyurethane structure. Additionally, fillers are added to reinforce the polymer and reduce cost [2] . These fillers are normally introduced into the foam formulation via one of the two basic main components of polyurethane system (polyol/isocyanate) before they are mixed with the other raw materials [46] . Normally, these fillers are dispersed in the polyols to form a polyol premix. Catalysts, surfactants, and water are then added to improve the dispersion of the fillers in the polyol depending on the kind of filler used and the type of property that the formulation seeks to enhance. The resulting mixture is then added to a specified amount of isocyanate and mechanically mixed [46] . In instances where high-density foam is needed, the volume of water used in the formulation is reduced. In one study, Prociak et al. [47] used flax fibres to reinforce rigid PU foams where the flax fibres were fractionated and dried to constant weight before they were added to the polyol premix. They found that the number of flax fibres to be added was limited due to the increased viscosity of the polyol mixture. Others [48] have used a slightly different approach in their preparation of flexible polyurethane foam reinforced with rice husk with the objective of ascertaining the effect of the rice husk filler on the thermal and mechanical properties of the FPUF. They prepared a pre-polymer mix by adding the catalyst, surfactants, blowing agent, and other additives to the polyol. Then, the gradual addition of the filler via constant stirring to the polyol mixture before finally adding the isocyanate. It is worth noting that, it is not a rule of thumb to add the filler to the polyol, everything at a time. The addition of fillers is largely dependant on the type of foam (whether flexible or rigid) and the type of property and application that needs to be leveraged. In another instant, Gayathri and group [49] prepared flexible polyurethane foam modified with nano-silica, nano clay, and crumb rubber to improve their sound absorption, mechanical and thermal behaviour. In their preparation, they mixed the fillers (up to 2%) with the isocyanate instead of the polyol [49] as described in the method by Malewska4 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 and Prociak [46] and Prociak et al. [47] , who added the fillers to the polyol. Due to the different nature of foams that were produced by these groups, the reaction times during the foam formation differed significantly. Using a similar approach to that of Gayathri and group [49] , Jathin et al. [50] explored the effect of alumina filler on the mechanical properties of rigid polyurethane foams. They mixed the desired amount of the filler with diphenylmethane diisocyanate isocyanate (MDI) amidst stirring and this was added to the polyol premix. They reported that the compressive and flexural strength of the filled foams were better than the unfilled foams. From the review of the various methods of preparation in this section, it is apparent that the method of addition of the filler of the foam formulation does not have a significant effect on the foaming process. The only problem it poses is the increase in the reaction mixture viscosity, therefore, minimizing the amount of filler that can be added to the polyol premix which eventually affects the magnitude of improvement of a particular property [47] . Effects of fibre treatment One aspect of foam preparation that affects the properties of the composite foam is the modification of the fibre [21] . Naturally, an untreated fibre tends to absorb moisture which can cause delamination between the fibre and the polymer matrix and can severely affect the strength of the resulting composite material [51] . This is because natural fibres are hy- drophilic and therefore can minimize their reinforcing effect [52] . Secondly, the hydrophobic nature of the polyurethane matrix together with the hydrophilic fibres can lead to poor adhesion, interphase separation, and limited transfer of stress load in the composite foam. In addition to this, the presence of certain organic compounds such as waxes and pectin found on the surface of these fibres sometimes acts as barriers disrupting the effective interfacial adhesion between the filler and polymer matrix [ 13 , 53 , 54 ]. Therefore, to achieve a strong fibre-matrix interfacial bond, there is a need for fibre treatment/modification [55] . Through modification, all the organic compounds such as hemicellulose, waxes, pectin, on the filler surface are removed thereby changing the surface of the fillers and improving their adhesion to the reactive functional groups on the sites of the polymer matrix. There are mainly three methods for fibre treatment: chemical, physical, and biological [51] . Amongst these, chemical treatment is widely used. Generally, chemical treatment of fibres improves the stiffness, stability, and thermal conductivity of natural fibre-reinforced composites. Common treatment methods include alkalization, acetylation, and salinization [56] . Alkalization removes fats, hemicellulose, lignin as well as impurities that may be present on the surface of the fibres. Acety- lation results in the partial removal of the lignin and hemicellulose whereas salinization improves the interfacial adhesion between fibre and polymer matrix [56] . Several studies have shown that chemical modification of the fillers have a tremendous effect on their reinforcing ability. In one of such studies, Czlonka and group [13] investigated the effects of chemically treated eucalyptus fibre on the me- chanical, thermal, and insulating properties of rigid polyurethane composite foams. They employed a couple of treatments using maleic anhydride, alkali and silane and reported that the silane treated fibres showed remarkable enhancement of properties in their composite foams as compared to the others. Czlonka and group [13] observed that the unfilled foams had a high content of closed homogenous cells. The morphology of the foams modified with non-treated eucalyptus fibres showed more heterogeneous open cells with decreased porosity (91.5% to 89.2%). However, the chemically-treated eucalyptus filled foams had a more uniform, and numerous closed-cell structure like the foams that did not contain any fibres. Same can be said about their porosity ( > 90%) suggesting that chemical treatment increased the interfacial adhesion between the fillers and the polyurethane matrix. This accounted for an increase in their compressive strength which increased to about 20%. Similarly, the impact and flexural strength also increased by 48% and 6% respectively [13] . Due to the highly heterogeneous cells of the untreated fibres filled foams, their compressive, impact, and flexural strength decreased, even though the density increased. One factor that can be attributed to this low mechanical strength of the un- treated fibre foams was the high tendency of these fibres to agglomerate resulting in many voids which leads to interphase separation of the foam structure causing failure of the foam structure under compressive load. In another study, Chang, Sain and Kortschot [42] studied the influence of alkali-treatment of wood fibres on the com- pressive behaviour of their composite foams. They modified the surface of the wood fibres by immersing 10% of the fibres in a 2% NaOH solution for 8 h. Shown in Fig. 1 is the SEM micrograph of the untreated and treated fibre. From the SEM images in Fig. 1 , it is clear the alkali treatment influenced the surface morphology of the fibres. The untreated fibre had a rough surface as compared to the treated one which means a better bonding with the polymer matrix. This is because the hydrogen bonding on the surface of the fibres got disrupted as Na+ ions displaced + H on the fibre improving the interfacial bond with the polyurethane matrix. It is therefore not surprising that the treated fibre-filled foams had a 40% increase in strength and 64% in the flexural modulus [42] . Also, Sair et al. [6] prepared alfa fibre (AF) polyurethane composite and assessed its suitability as a thermal and acoustic insulating material for building applications. They treated the fibres at different concentrations of NaOH (0%, 5%, 7.5%, 10%, 12%) and the effect of this treatment is seen in the SEM images in Fig. 2 . The SEM images in Fig. 2 showed that as the concentration of the NaOH is increased, the amount of non-cellulosic material decreases exposing the cellulosic microfibrils of the fibre to be available for bonding with the polymer matrix. However, beyond 10% concentration of NaOH, the cellulosic microfibrils of the fibres are attacked and damaged affecting the topology of the fibre leading to a decrease in mechanical5 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 Fig. 1. SEM image fibre surface of the wood fibre (a) untreated wood fibre and (d) treated wood fibre [ 57 ] . Fig. 2. SEM images showing treated fibres at various NaOH concentrations (AF-5 = 5%; AF-7.5 = 7.5%; AF-10 = 10%; AF-12 = 12%) [ 6 ] . properties of the composite material. The untreated fibre (0%) has rough surfaces due to the presence of non-cellulosic compounds such as fats, waxes, polysaccharides, etc. Sair et al. [6] , found that 10% NaOH solution significantly improved the hydrophobic character, the mechanical and in- terfacial adhesion properties of the fibres. This was due to a 58% increase in the tensile strength of the treated alfa fibre composite as compared to the untreated fibre composite. Apart from this observation, they also found that the thermal conductivity increased with increase in the concentration of the alkali treating solution. 6 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 Fig. 3. SEM micrographs of untreated and treated hemp fibres. The image (a) is the untreated hemp fibre (control); (b) laccase treated; (c) xylanase- cellulase treated: (d) polygalacturonase treated. The areas indicated red from images (b) to (d) shows that enzyme-treated fibres were defibrillated and the fibre bundle exposed especially in the image (d) [ 62 ] . From the review of the various studies above, it is clear that the chemical treatment of the fibres enhances their in- terfacial adhesion and therefore results in the development of composite materials with better properties as compared to non-treated fibre composites. Despite the usefulness of chemical treatments in improving the compatibility of natural fibres with PU matrix, there appears to be a current drift towards using treatment methods which do not involve the use of chemicals in the bid to minimize environmental pollution and enhance ecological sustainability. This is due to the increased toxicity, hazards, and environmental challenges associated with the disposal of some solvents used for the chemical modification of natural fibres [ 53 , 58 ]. Physical treatment methods such as drying, stretching, calendaring, thermo-treatment, etc. can be looked at. These methods do not alter the chemical composition of the fibre but rather their surface and structural properties which thereby improves their bonding to polymer matrix [55] . In this regard, Cichosz and Masek [53] have demonstrated that thermal drying can be an effective way of improving the mechanical properties of a cellulose reinforced polymer composite. Another method of fibre treatment which is gaining attention lately is a biological treatment especially with the use of enzymes, fungus, and bacteria [ 59 , 60 ]. In the enzyme treatment option, specific enzymes are applied to remove the non- cellulosic components on the surface of the fibre [51] by inducing the release of pectic polymers from the middle lamella of the individual fibres. This weakens the bonding between the fibres resulting in a separation of individual fibres and small fibre bundle from larger fibre bundles [61] . Enzymes such as xylanase and laccase are known to improve the physical (hardness) and mechanical properties (flexural and tensile strength) of plant fibres by the increment of cellulose content and the subsequent removal of lignin and hemicellulose [59] . George et al. [62] investigated the effect of enzymatic treatment on the surface and thermal characterization of natural fibres. The work was carried out by subjecting hemp and flax fibres to five different enzyme systems (pectinase, oxidore- ductase, hemicellulose, and their combinations) and discovered that each treatment improved the surface topology of both fibres by the removal of contaminants from the surface of the fibres. This removal led to an increase in exposure of the individual bundles of the fibres as revealed by SEM micrographs in Fig. 3 . George and his team [62] concluded that the enzymatic treatment of natural fibres provides an inexpensive and envi- ronmentally friendly alternative in producing better polymer composites. Instead of enzymes, Noureddine [60] used bacteria (Acetobacter xylinum) to modify the surface of date palm fibres. The underlying principle of this is that, under the right conditions, cellulose producing bacteria will grow on plant fibres due to the hydrophilic nature of the fibres, and lead to the7 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 modification of the surface morphology of the fibres. After the process, the surface of the fibre which was initially smooth became rough due to the attachment of some bacteria cellulose. Compatibility of the bacteria cellulose and the fibre was improved by the removal of non-cellulosic materials on the surface of the fibre via Soxhlet extraction in acetone thereby increasing the hydrogen bonding between the fibre and the bacteria. This modification also resulted in an increase in Young modulus of the fibre compared to the fibre without the bacteria (from 13.5 GPa to 14.8 GPa). Thus, the deliberate introduction of nanosized bacteria cellulose on the surface of natural fibre can improve the adhesion of the fibre with a polymer matrix [60] . An advantage of enzymatic treatment over the chemical is that it is performed under milder conditions (relatively neutral pH and low temperature). Also, the use of enzymes is a better and environmentally safer option [61] , which, therefore, paves the way for other researchers to investigate how different fibre treatment using enzymes and bacteria can enhance the interfacial adhesion with the polyurethane matrix in other to produce PU composite foams with better properties. The only challenge that comes with the use of enzymes is the cost and it is also a time-consuming process [63] . Recent trends and opportunities for the future The use of natural fibres to reinforce polyurethane has become increasingly attractive due to the desire to produce com- posite materials that are renewable and biodegradable. Over the past decade, the global production of natural fibres has increased steadily to about 32 t as of 2018. Out of this, cotton accounted for 81%, followed by jute (7%), with wool, and coir having up to 3%. Interestingly, the farm value of natural fibres as of 2018 was $60 billion according to Bio-Based News [64] . According to researchandmarket.com, the current value of the foam industry is estimated to be $9.1 billion and this is expected to hit $15.75 billion by 2025. Presently, polyurethane foams have the largest share of the technical foam market. This is attributable to the increase in the demand for polyurethane foam-based materials that are used for diverse appli- cations in industries such as automotive, and transportation, construction, medical, etc. [65] . In 2014, the global market for bio-based polyurethane products was estimated to be around $29.9 million due to the increasing demand for eco-friendlier PU alternatives and this is expected to boost the market of these products [65] . This, therefore, provides a unique opportunity for researchers to exploit the unique attributes of natural fibres and in- creasing demand for bio-based polyurethane foams to develop composites that are biodegradable, lighter with better prop- erties than the conventional polyurethane foams. For instance, the building and construction industry can use natural fibre to reinforce polyurethane composite foam to produce panels that have better structural integrity and energy absorption [66] . Other industries can also harness the unique insulation properties of these composite foams for lightweight thermal applications [41] . Furthermore, there is also the opportunity for greener polyurethane composite foams reinforced with nanosized natural fibres with good flexibility, elasticity, tear strength, shock absorption, wound healing, antimicrobial, and insulation properties. This can serve very useful applications in the automotive, electronics, building and construction, and the biomedical industries [67] . The numerous sources from which fibres can be obtained provides a possibility for the discovery of novel natural fibres. Recently, new fibres such as dichrostacys cinerea fibres (DCF) has been discovered [68] . Analysis of this fibre showed that it had a high cellulose content (72.4%) than most known natural fibres such as abaca, coir, etc. and only comparable to cotton, kenaf, and pina [ 68 , 69 ]. Investigations by other researchers have also led to the identification of other new natural fibres such as sida rhombifolia fibre [70] , heteropogon contortus plant fibre [71] , cissus quadrangularis stem fibre [72] , Juncus effusus L. fibre [73] , etc. All of these new fibres have high cellulose content just like DCF [68] . In addition to discovering new fibres, there is also the need to research into ways of improving the quality of fibres by looking at the methods of fibre extraction and processing, fibre orientation, fibre treatment, fibre dispersion, fibre- polyurethane matrix adhesion, and the composite manufacturing process, etc. [ 24 , 74 ]. The versatility of polyurethanes coupled with the wide variety of sources from which natural fibres can be obtained makes it possible to develop PU composite materials tailored with specific properties and application. For instance, da Silva et al. [75] have developed polyurethane-based eco-composite foam using dog wool microparticles as fillers with potential to be used for insulation applications. There is also a possibility of developing PU composite foams reinforced with fibres that possess antibacterial and antiviral properties that can be used to disinfect contact surfaces. This can be an innovative contribution towards the current fight against the spread of the dreaded coronavirus. Ilangovan et al. [76] have taken giant strides in that regards by successfully extracting and characterizing cellulose fibres with antimicrobial properties from Kigelia africana , also known as the sausage plant. Conclusion Currently, the drive for a greener environment has triggered the need for new materials, processes and products that are sustainable and environmentally friendly. This has motivated researchers in the polyurethane industry to search and discover bio-based materials that are able to support the manufacture of composite products with superior properties and applica- tions. A review of several studies indicates that, plant-based natural fibres can be used as reinforcement in polyurethane composite foams. The advantages of these natural fibres over synthetic fibres have been their low cost, lightweight, high 8 C. Kuranchie, A. Yaya and Y.D. Bensah Scientific African 11 (2021) e00722 specific strength properties, extremely low carbon footprints, renewability, and biodegradability. As a result, the use of nat- ural fibres as reinforcement has gained a lot of attention leading to the development of polyurethane composite materials with improved mechanical properties, thermal stability and conductivity, flame retardation, insulation, and sound damping. Despite these improved properties, their hydrophilic nature still possesses a challenge due to poor dispersion and weak interfacial adhesion with the polyurethane matrix. To minimize this effect, fibres have been modified either chemically or biologically to improve their interfacial adhesion with the polymer matrix. The numerous on-going research to produce bio-based composite materials, therefore, presents a great opportunity for the exploration and expansion of the possibilities of using different types of natural fibres to produce polyurethane compos- ite materials with well-tailored properties and applications. Declaration of Competing Interest There is no conflict of interest or competing interest. Acknowledgments Charles Kuranchie acknowledges the support of BANGA-Africa project, at the University of Ghana, Accra, Ghana. Author Contributions Conceptualization and structure by Abu Yaya, Charles Kuranchie and Yaw Delali Bensah; original draft preparation by Charles Kuranchie and Abu Yaya; writing, review and editing by Charles Kuranchie, Abu Yaya and Yaw Delali Bensah; super- vision, resources and administration by Abu Yaya and Yaw Delali Bensah. All authors have read and agreed to this version of the manuscript for publication. References [1] N.V. Gama , A. Ferreira , A. Barros-Timmons , Polyurethane foams: past, present, and future, Materials (Basel) 11 (10) (2018) . [2] C. Oppon , P.M. Hackney , I. Shyha , M. Birkett , Effect of varying mixing ratios and pre-heat temp erature on the mechanical properties of polyurethane (PU) foam, Procedia Eng. 132 (2017) 701–708 . [3] A. Kausar , Polyurethane composite foams in high- performance applications : a review, Polym. Plast. Technol. 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