Key advances in development of straw fibre bio-composite boards: An overview

In recent years, considerable attention have been given to the development and utilization of biodegradable fibres for bio-composite boards. This is due to the increase in the environmental consciousness and the need for sustainable development which enable establishment of new materials majorly for packaging, aircraft, furniture, and automobile. Straw fibres (wheat, rice, and corn fibre) are the most available natural agricultural wastes products, which has been utilized for the production of these new materials. This paper hence reviews the enhancement in production methodology and properties of the straw fibres bio-composite boards to add further scientific knowledge to the potentiality of using agricultural fibres as value added products. The future replacement of conventional wood fibres for the production of bio-composite panels, especially with agricultural wastes, could be centered on straw fibres. The introduction of straw fibres in polymer matrices were presented based on various research outcomes. Biodegradable fibres could be regarded as a good fibrous composite material. Although, more efforts are still needed in developing facile straw fibre composite production methods and materials with robust industrial and domestic applications. Industrial adoption of these fibres would gear effort towards achieving a clean, and pollution free environment.


Introduction
The technologies development that facilitates ease of human existence have been intimately associated with the accessibility of suitable materials [1]. Research advancement in the understanding of a material type and production processes are usually the backbone to the stepwise progression of a technology [2]. Straw fibre is an agricultural natural polymer materials used to manufacture bio-composite boards (BCB) product. It is a generic name for various types of agricultural wastes like wheat, rice and corn [3] and one of the most available wastes resources. Straw fibre BCB is the combination of two or more biological/natural materials to form a new product with certain unique properties which are usually more superior in functionality and usage. It usually has features that are not exhibited by any of its components in isolation [4]. Example includes; particleboard, medium density fiberboard (MDF), and plastic bonded board etc.
In recent time, there has been continuous renew interest in the use of materials whose production components are made from natural or biodegradable sources. Unlike synthetic fibre based polymers composites, BCB can easily be disposed or decomposed after usage. As a result, many researchers are finding the needs to produce new materials using different experimental procedures. One of the major drivers of the renewed research interest in BCB is the concerns about sustainability and environmental issues that the world has faced in recent times. Green materials are also targeted or expected to create a friendly environment by preventing natural disasters [5]. The recent survey by nova-Institute stated that demand for BCB materials would increase in an upward progression till year 2020 [6]. It was also projected in another review that the production of composites from renewable sources will increase from 12% in 2010, 18% in 2020 and/to 25% in Manufacturing of BCB is also a concerned where high production volumes is required. The lack of suitable and generally acceptable manufacturing processes posed a great challenge for its mass production, although, numerous production procedures have been used since the idea of using straw fibre for BCB was conceptualized. A full adoption of the fibre in automobile industries becomes limited even though, a typical truck application might have a volume ranging between 5,000 and 20,000 parts per year, and car application between 80,000-500,000 parts per year [1].

Advances through fibre surface modifications
Straw fibre surface modification is a method of permanent restructuring of the fibre cell walls by grafting polymers on the fibre surface. The fibre cell wall can also be cross-linked with various chemicals. This process improves the dimensional stability, increases hydrophobicity, enhances fibre/matrix interfacial adhesion, and restricts the activities of the bio-deteriorating agent [18]. Fibre surface treatment mainly includes acetylation, silane and mercerization [19], hydrothermal extraction, rays or plasma [20], enzymatic [21,22], and steam explosion treatment [23]. Various types of chemicals have been used for the treatment of natural fibres. Alkali is one of the most commonly employed chemical treatment for modifying the cellulosic molecular structure of straw fibres. Its application enhanced the interlocking of fibres with its polymer matrix by removing the lignin, wax, and oil covering part of the fibres [24]. Also, it disrupts fibre clusters and forms amorphous at the expense of highly packed crystalline cellulose. Similarly, coupling agents are also adopted for blocking the hydroxyl groups on the fibre surface, thereby changing the hydrophilic nature of the fibre [25]. Numerous studies have been carried out to improve the adhesion between the straw fibre and matrix by modification of the fibre through physical, mechanical, and chemical methods.
Straw fibre was treated with alkali (NaOH) and subsequently hydrolyzed with H 2 SO 4 . The alkali treatment was found to be efficient in removing hemicelluloses and lignin, but the lignin was only fully removed after acid hydrolysis treatment [26]. It was found that the treatment induced a cellulose increase from 31 to 96% [26]. This indicated that apart from the main purpose of fibre treatment (Fibre/matrix adhesion), the treatment could also provide an opportunity of sustainable extraction of cellulose from the straw fibre. And another development has found that, 43% cellulose could be extracted by a dilute sulfuric acid/steam explosion of wheat fibre [27], due to slower hydrolysis of cellulose at low combined severity factors [28]. By using Alkaline solutions at three concentrations (2, 5 and 10%), to specifically treat both rice and wheat fibres with the aim of improving the fibre surface interface, it was reported that the alkaline treatment dissolved some fraction of hemicelluloses, waxes and lignin, such that the polarity of its surface increased. Furthermore, the treatments showed more influence on wheat straw than on rice straw fibres, as revealed by the Fourier Transform Infrared (FTIR) spectra and environmental scanning electron microscopy (ESEM) observations [29]. When the two fibres were further treated with silane, without or with alkaline treatment, it was observed that the silane treated fibres had a lower moisture sensitivity but higher surface energy of the fibres. The BCB reinforced with fibres treated by combined treatment of alkaline solution and silane had higher bending strength than those reinforced by unmodified fibres or by only silane treatment alone [29]. It has also been reported that the combination of hydrothermal and NaOH-delignification could remove up to 86% of lignin in wheat straw [30]. Therefore, the fibre surface treatment by alkali and subsequent modification with silane or in the presence of hydrothermal reaction should be encourage for more effective result.
Straw fibre can also be modified by acetic anhydride. Acetylation can be done by introducing an acetyl functional group (CH 3 COO-) into the organic compound. Esterification is the best method for acetylation of straw fibres due to its ability to plasticize cellulosic fibres. Fibre modification with acetic anhydride replaced the polymer hydroxyl groups of the cell wall with acetyl groups, thereby altering the properties of these polymers. Acetylation can reduce the hydrophilic nature of natural fibres [31]. It is a simple procedure for developing inexpensive and biodegradable thermoplastic BCB from straw. It was reported that when the acetylated rice straw was casted to form transparent films, all the cell surface of the fibre could be altered due to its irregular structure. The attachment of the acetyl group could be further confirmed by the x-ray diffraction (XRD) [32]. The compatibility of acetic acid with diethyl phthalate (plasticizer) strengthen the elongating capacity of the acetylated fibre [32] (Zhang, et al, 2013). In addition, the phosphotungstic acid (H 3 PW 12 O 40 ·6H 2 O) could be an effective catalyst for the acetylation of the straw based cellulose [33]. Similarly, Benzoylation of straw fibres could also take place by introducing benzoyl functional groups (C 6 H 5 C=O) at the fibre surface with the aid of benzoyl chloride. This method enhanced the hydrophobic nature of the fibres by increasing the hydrophobic linkages on the polymer matrix, thereby increasing the water resistance of the BCB [34,35]. Also, higher thermal stability and thermplasticity were observed [36]. It should be noted that prior to carrying out the reaction between the fibre and benzoyl groups, the fibre should be pretreated with NaOH aqueous solution in order to activate and expose the hydroxyl groups on the fibre surface [37]. Benzoyl functional groups are also required for the reaction between rice straw fibre and water glass (Na 2 (SiO 2 ) n O), which is another fibre surface treatment enhanced with Sodium silicofluoride, zinc polyphosphate and ammonium polyphosphate [38]. Poly(butyl acrylate) was also successfully coated on rice straw fibre by suspension polymerization of butyl acrylate (BA) monomer. It was reported that the procedures could increase the tensile strength by 7.98% (6 MPa) and lower its water absorption [39].
Linseed oil is known for its use as a chemical preservative for wood decking [40]. The linseed oil glyceride was synthesized by using a transesterification process with a glycerol/linseed oil molar ratio of 1.0, after which it was coated as a preservative on the wood surface [41]. Differently, the treatment was applied purposely for improving the fire performance of wheat straw fibre. The significant effect of the linseed oil treatment at the macroscopic level by flammability test and microscopic level by differential scanning calorimeter (DSC) was revealed, and it was found that, the linseed oil treatment retarded the flame spread and prevents BCB degradation [42]. Plasma treatment is another useful fibre surface modification technique for improving the polymeric materials by utilizing tools, such as electrons, high-energy photons, radicals and ions. The use of dielectric barrier-discharge plasma with water vapor as feeding gas led to the surface roughness of wheat straw fibre, and the generation of massive free radicals [43]. Also, the equilibrium contact angle and the surface total free energy were increased significantly after plasma modification [44]. Rice straw fibre treated with ultravioletozonolysis (UV/O 3 ) also showed rougher surface and improved adhesion with the polymer matrix. The tensile strength of UV/O 3 -treated BCB showed about 5% increase. In addition, it reduced processing time and raw material wastage during BCB manufacturing [45]. This could facilitate the commercial production of sustainable and environmentally friendly materials. Mechanical processing method can also be used for fibre surface treatment. Solid State Shear Milling process (SSSM) was adopted as fibre surface modification. The process gave wheat straw an ultra-fine particle size. Optical and electron microscopy showed that BCB produced by SSSM exhibited excellent dispersion of cellulose nanofibrils, hemicellulose, and lignin. The resultant polymer matrix and wheat straw BCB exhibited a fast crystallization rate (0.8 min crystallization half-time). This led to the enhancement in flexural modulus of the BCB [46].
Regardless of the fibre surface modification method adopted for BCB production, there are other factors that could influence the properties of the final products. Factors such as the fillers content, particle sizes, fibre processing methods, fibers characteristics, and the structure of the BCB. A detailed comparison of the surface modification techniques with regards to the aforementioned factors would enable accurate selection on the most suitable fibre treatment methods. Dilute sodium hydroxide (NaOH), dilute sulfuric acid (H 2 SO 4 ), hydrothermal (LHW), calcium hydroxide (CaOH) and other organic solvents such as methyl isobutyl ketone (MIBK) and aqueous ethanol (EtOH) surface treatment, were examined on properties of wheat straw fibre BCB by a number of researchers (table 1). The chemical composition was successfully altered as revealed by the different polar functional groups (O-H, C-H, CO, N-H, CH 2 , and C-OH) incorporated into the wheat fibre surfaces from the FTIR spectra of each treatment (figure 2). NaOH and H 2 SO 4 increased cellulose content by 29.6 and 35.1%, respectively. The surface cleaning of the treated fibres occurred in all the treatment but sufficient surface roughness was observed in the H 2 SO 4 , NaOH, and MIBK treated fibres. In addition, H 2 SO 4 . NaOH and H 2 SO 4 treated fibres achieved the most prominent surface modification [47]. Contrarily, it was found out that the steam cooking and steam explosion fibre treatment methods which were not considered in the previous work performed better than the other fibre treatments [48] 3. Development of straw fibre based bio-composite boards The synergy between polymer matrix and straw fibre depends largely on the fibre properties [19]. However, the production method employed in board formation could determine the final properties of the fabricated BCB.   [50][51][52] . The length of elongated fibre, linear density of the fibre bundles, tenacity of the fibre bundles and elongation of the fibre bundles were found ranging from 800-1300 μm, 37-45 dtex, 9-17 cN tex −1 and 1%-5% respectively [53]. The mechanical properties of wheat fibre bundles are inferior to the properties of conventional textile bast fibres. Due to the very low bending tenacity, the use of wheat fibre is only reasonable in form of whole stem for some technical applications [53], although, the fibre could be reconstituted to enhance the quality/properties of its product. In a comparative experiment conducted on straw fibres, the mechanical properties of wheat straw fibre composites showed better features compared to corncob and cornstalk fibre based composites, which may be due to a good compatibility between the more non-polar surface of wheat straw fibres and high density polyethylene (HDPE) [54]. In a similar research with different approach for investigating the potential inclusion of treated wheat fibre in a polymer matrix. The stems of wheat straw were reinforced with injection of the animal glue to improve the crushing force and strength of the straw fibre itself. This approach was found to improve its tenacity by about 165%, Young's Modulus of elasticity by 125% and breaking strain by 125%. The specific crushing force also reached 4.444 kN/m for the injected wheat straw, while it was 0.421 kN/ m for untreated straw [55].
Wheat fibre could be subjected to different mechanical processes, such as impact, cutting and ball milling (IM, CM and BM). The contact angle test shown that the interfacial adhesion was improved in the order of BM>IM>CM, whereas the fiber elongation (aspect ratio) decreased in the order of CM>IM>BM [56,57]. In addition, the energy required to fractionate wheat straw increased with decreasing final particle size, i.e. in the order of CM<IM<BM, the choice of an excessive milling, such as ball milling appeared not economically viable [57]. However, it was reported that high-energy ball milling process was an interesting way to produce the BCB and to improve their properties [58]. After successfully removing 50% of lignin from wheat fibres, it was confirmed that high lignin content in the lignocellulose fibres is not a necessary condition to obtain a good fiber/matrix adhesion, provided the lignin concentration is enough to cover fibre surface [59]. Although, further studies are still in progress to develop lignin extracting procedures that will allow reaching higher extraction level without degrading the inner structure of the fibres.
In an attempt to find more eco-friendly packaging materials that can be used in the food industries, the potential use of wheat fibres as filler in a poly-3-hydroxybutyrate-co-valerate (PHBV) biopolymer, was compared to brewing spent grains and olive mills. Due to 3.5-fold increase exhibited by the wheat straw, it was deduced that PHBV/wheat straw fibres BCB appeared as promising materials to reach the packaging requirements of respiring food products [60]. The comparison of the mechanical properties shown that the BCB exhibited higher Young modulus and lower values of stress and strain to bre ak [61] . Alkali treatment of wheat straw fibers further enhanced strain break and impact strength of PHBV composites by approximately 35% [62]. In another research, the impact of fibre moisture at the time of the manufacturing process was examined on wheat fibre/PHBV BCB. The relat ionships between the structure of PHBV/wheat fibre BCB and their final mechanical properties, with a special focus on the interphase role, was monitored [63]. The molecular weight of PHBV decreased significantly with increasing fibre moisture content, which may be due to hydrolysis reactions induced by residual water molecules. The BCB crystallinity increases along with initial fibre moisture content which was attributed to the observed decrease of molecular weight. Regardless of the structural differences, the tensile properties were similar for all the BCB [63]. Therefore, the initial fibre moisture content was not a predominant factor in controlling the mechanical properties of PHBV/wheat fibre BCB. It is worthy to note that PHBV-biomass (natural fibres) composite is considered as the most interesting material for many industrial sectors by virtue of its possibility of full biobased system (figure 3).
Hydrophobicity of wheat straw fibres could be modify using torrefaction treatment. This treatment improved fibre-matrix interfacial attraction of the BCB prepared by melt-extrusion. The molecular weight and crystallinity of matrix polymers remained intact when compared to untreated fibres. Furthermore, the fibre torrefaction treatment showed no relevant improvement on the mechanical properties of PHBV/wheat straw fibre BCB [64]. The lack of improvement in the BCB may be associated with a reduction in tensile failure strength and strain energy of oven dried and torrefied wheat straw fibres as shown by increasing torrefaction temperature from 200 to 300°C [65]. Higher storage modulus, hardness and tensile strength properties were achieved in BCB samples from polycaprolactone and wheat fibres [66]. Wheat fibres in a polymer matrix could be successfully applied as a low-cost substitute of lignocellulose wood composites. Although, the use of polyester (poly(butylene adipate-co-terephthalate)) and polyolefin (a copolymer of polyethylene and polypropylene) as polymer matrix should be avoided, since they exhibited low formation of intermolecular bonds and brittle nature respectively [67]. Recently, more attention was put on the use of wheat fibre gluten because the glassy gluten matrix has comparable properties as an epoxy resin [68]. Due to this, various methods were employed to process wheat gluten composites reinforced with other fibres. In one of the findings, wheat gluten BCB treated with alkaline and silane provided an 80% increase in strength and showed superior fiber-matrix interfacial adhesion. Furthermore, using extrusion of an aqueous gluten dough at 30-110°C and 70% ethanol at 110°C in a pressurized reactor, there was an improvement in the mechanical properties of the experimental samples [69]. In addition, the use of silica particles for coating the alumina particles with silane coupling agents in the wheat gluten matrix exhibited better physical and mechanical properties [70].
Various research has been carried out to characterize the behavior of the biomass when incorporated into polymer matrix. Rice straw/husk was observed to exhibit weak interaction with various polymer matrix, especially when compared with its contemporary products from wheat straw and wood composites [83] [84,85]. The weak bond-line interaction is as a result of the high content of silica, wax and low aspect ratio of fibres, although, it possessed some benefits over BCB produced from other fibre straw. The boards from rice husk is very tough, biodegradable, high resistance to weathering, low weight and economically affordable [86,87]. In addition, when compared with BCB from wheat straw, it has higher termite resistance with better dimensional stability. These properties make it suitable for various applications such as house and automotive constructions [88].
The possibility of producing rice husk composites with Acacia mimosa tannin-based resin as a polymer matrix has been investigated [89]. The high silica content in rice husk, often retards the surface interaction of the husk with polymer matrix. Although some improvements in the properties of the BCB have been reported to be related to the harmer-milling process of the rice husk fibre, further research is still required to ascertain the influence of different conversion methods on the properties of BCB filled with rice husk. Despite the issue of surface interaction with polymer matrix, some previous studies reported that rice husk BCB could be used in furniture and interior fitments manufacturing, although, the physical and mechanical properties of the boards were lower than those of the BCB made from wood particles [83][84][85]. Further research was carried out by using a fibre surface treatment with 4% wt of NaOH and 3% wt of Maleic anhydride polyethylene (MAPE) coupling agent [90]. Fourier transform infrared (FTIR) analysis was performed on the treated and untreated rice husk fibre. The implication of the surface modification and the effect of filler loading of 50 to 80% wt was examined. Substantial improvement in Young's modulus and tensile strength, as well as, the total reduction in hygroscopicity was noticed for the MAPE treated samples. The rice husk BCB treated with NaOH at 70% wt shown the highest tensile and modulus strength. Although the Scanning electron microscope showed a rough surface of the treated samples, but the fibre treatment proved effective [90].
Reduction of the silica in rice husk may result in better mechanical properties of the BCB. Battegazzore and his team [91] extracted silica powder from rice husk using a simple extraction approach for preparing polylactic BCB with different filler contents of 5, 10, 20, and 30 wt%. The examination of the morphological, thermal and mechanical properties of the BCB indicated that, the extraction of silica induced significant improvements in Young's modulus as well as a slight reduction of the oxygen permeability of polylactic. The collected values at 10 and 30% wt were compared with similar samples prepared using a commercial silica and they found that the mechanical properties turned out to be higher than those observed in homologous samples. The economic analysis of the materials and the whole process (BCB preparation and silica extraction) showed that the production with a 20 wt% silica can be considered economically viable, when the energy recovered from the rice husk burning is reused [72] [91]. In addition, the collected data by thermogravimetry and mechanical analysis showed that all the formulations investigated could be used to manufacture materials for some packaging applications such as boxes, containers and crates [91].
Contrary to the general findings that silica is the only chemical component that retards the surface interaction of BCB filled with rice husk fibre, Mohammadi-Rovshandeh and his group [92] investigated the possibility of using bleached rice husk (BRH) in powdery form and the effect of lignin removal on mechanical and thermal properties of husk polylactic and starch composites. It was revealed that the lignin removal resulted in an increase in the toughness and Young's modulus of the BCB. In addition, the filler improved the hardness and tensile strength of polylactic/starch blend. The result of the mechanical test showed that 5 to 10% of rice husk or BRH may be used in BCB. BRH samples displayed higher hardness, modulus and charpy impact strength when compared with BCB filled with rice husk. Regardless of the higher elongations values at the break of rice husk BCB, the mechanical properties shown a significant improvement from 25 to 50% due to the lignin removal. Also, the differential scanning calorimetry thermograms shown an increase in crystallization behavior and reduction of T g from 47 to 31°C in the BRH filled polymer composites. In relation to their observations, BRH could have the ability of wide use in the bio-plastics and food packaging industries [92]. Further chemical isolation may be needed to fully understand the behavior of rice husk with different matrix materials.
While the major findings based on the utilization of rice husk for BCB are rather extensive, the opportunities that may avail its stalk have not been fully tapped. This curiosity led Jayamani and his research group to study the performance of rice stalk filled BCB with maleic anhydride polypropylene (MAPP) serving as matrix materials [93]. Similar to what Chen et al did in 2015, the stalk was treated with 5% wt of NaOH at 30°C for 30 min, and then a combinations of 10 to 25% wt and 2% wt of MAPP was used. It was found that, the BCB treated with NaOH gave higher tensile strength, thermal stability, sound absorption and fibre-matrix adhesion than the untreated fibre BCB. The sound absorption measurement showed that the acoustic properties were closely related to higher fibre content. The changes in the peak of FTIR spectrum revealed the removal of lignin and hemicelluloses from the rice stem fibres [93]. Liu also assessed the flexural properties of BCB using rice stalk fibre as filler with bio-based adhesive (corn starch) serving as the matrix material. Other samples were produced using cassava and potato starch as polymer matrix. The rice stalk fibres were treated with NaOH and hot-water to evaluate the effect of the fibre pre-treatments on the BCB performance [94]. Due to the NaOH treatment, the rice stalk composite with corn-based adhesive showed higher flexural properties [94,95]. However, the BCB samples produced from hot-water treated rice stalk had a better interface and higher flexural properties. In addition, the strength and elastic modulus reached peak values at the starch content of 10% and a composite density of 0.7 kg m −3 . Finally, the microstructure under scanning electron microscope showed that corn-based adhesive could wrap the stalk surface more uniformly. The phenomenon was due to a better mobility of cornbased adhesive on the rice stalk surface, making it adhere better with the granules [94].

Corn fibre bio-composite boards
The properties of corn stalk fibres, such as the crystallinity, structures, morphology and defects, have great impacts if the fibres are reinforce by polymer matrix. Depending on the source and preparation methods, the chemical constituents of corn stalk lignocellulosic bio-fibres may vary. However, the main chemical composition of natural fibres generally includes cellulose, hemicellulose, lignin, and a small amount of wax and pectin [96]. The combined effect of these chemicals determines the overall mechanical and thermal properties of corn stalk fibres. The chemical composition of corn stalk fibre consists of 41.7% cellulose, 47% hemicelluloses, 7.4% lignin [97]. Corn stalk cellulose is a linear chain with an amorphous and crystalline region (figure 4), the former can easily be penetrated by water (hydrophilic). The mechanical properties of corn stalk fibres are influence by their moisture content and voids between fibres [19]. The need for corn fibre modification is necessary in order to form an interlocking network with the hydroxyl group on the cellulose chain. Also, fibre modification serves as a means of enhancing the fibre interfacial interaction with the polymer matrix. The degree of surface interaction between corn fibres and matrix material affects the physical and mechanical performance of bio-fibers reinforced board and the reinforcing efficiency of the fibers [98,99] . Therefore, surface modification treatment is an indispensable corn stalk fiber-matrix interfacial bonding treatment that could improve the bondability of the BCB mixture. Surface modification treatments used for corn fibre includes benzoylation, acetylation, alkali, and electric discharge treatment [100][101][102], although alkaline treatment is the most widely used among the fibre treatments.
In a research conducted in China, alkali treatment was used as fiber surface modification to improve the corn stalk fibre compatibility with the matrix material. The fibre rinds were ground to 3-4 mm in length before treatment. The fibres were dipped in 1% aqueous NaOH solution at 30°C for about 20 min and thoroughly washed in distilled water until it became neutral (pH 7). Excessive solvent and moisture was removed from the corn stalk fibers after treatment in a ZK350S vacuum drying oven at 90°C for 4hrs [103]. The surface morphology of the untreated fibres shown smooth surfaces with some impurity particles ( figure 5(a)), while after NaOH treatment, the fibre surfaces became cleaner with the presence of large number of node structures and micropores ( figure 5(b)) [104]. These indicated that corn stalk fibers were significantly modified [105,106]. These changes may be as a result of natural and artificial impurities removal (e.g. wax, lignin, pectin and oils) and an increase in the amount of exposed cellulose on the corn fiber surfaces [98,107]. The corn fibre treatment also increased the mechanical properties of the BCB board [108]. In a similar research, the flexural strength of corn fibre BCB showed a declining trend with an increase in the filler loading (10, 20, 30 and 40% wt) and the impact strength showed an increasing trend with the increase in the wire mesh size (50∼100, 100 and 300) [109] observed that the thermal conductivity and diffusivity of the corn husk composites decreased with fibre loading. Using periodic temperature ramp method, the incorporation of corn fibre induces a decrease in both thermal conductivity and thermal diffusivity [110].
In another very interesting research, the acoustic properties of NaOH treated corn fibres were experimentally investigated. In order to get an accurate results, five different types of boards were produced and the acoustical properties were studied using a two-microphone impedance tube test. The sound absorption and tensile properties of the treated fibres BCB were found to be better than the untreated ones; due to the higher airflow resistivity and low porosity. The scanning electron micrographs of the corn fibres surface also revealed that different sound absorption properties of these BCB may be due to the changes in roughness and lumen structures of the fibres [111].

Mechanical properties of straw fibre bio-composite boards
The strength of a material is one of the major consideration in the construction industries. The ability of a material to withstand stresses imposed by load is as a result of strength of such material [112]. Constructional failure in service is often caused by over-loading, which could be death or live load. Death/static load is a permanent loads which were fixed, while live/fluctuation load is a moveable loads exerted on the construction material [113]. Live load is a major reason for failure of materials in service because most users rarely obey restriction on structural failure factors. Straw fibres BCB is not an exception to this problem as Mohammed et al, extensively studied the energy features of rice husk/polypropylene BCB to ascertain fatigue behavior of straw fibres BCB under stresses. Based on the structural analysis, catastrophic failure in rice husk BCB was found when the interfacial fibres-matrix materials system turned weak due to fibre sliding and fibre pull-out during loading/ unloading process [114]. However, using different experimental approach, Kumar et al, observed improvement in the wear resistant of BCB when rice husk fibres were admixed with other type of fillers [115]. Improvement in wear resistant could be associated to the higher levels of cellulose, lignin, and crystallinity in straw fibres [116]. Generally, strength of a biopolymer material could be determined by various factors such as the density, stiffness, polymer matrix, material type, and processing conditions among other factors as presented in figure 6. Like every other materials, several studies on the mechanical properties of straw fibres BCB has been conducted and reported. The variation in the mechanical properties of straw fibres BCB collected from selected findings are shown in tables 2 and 3 in term of the fibre type, polymer matrix, fibre processing method, fibre treatment, board thickness, and fibre content. According to Maslowski et al, increasing filler content could decrease the rigidity (elastomer) of the BCB regardless of the type of cellulosic material but improved the mechanical properties (at limited fillers proportion) except for tensile properties due to the deterioration of fillers dispersion and formation of agglomerates [117]. However, this could be further affected by other factors as presented in tables 2 and 3.

Potential application of straw fibres bio-composite boards
The conventional uses of straw fibres have been in the pulp and paper industry, particleboard industry, building, packaging, animal feed and bedding [131]. However, there has been series of critical researches that established the potential functionality of the material for various applications. In recent years, continuous increase in the industrial development and urbanization brought about escalation in noise pollution. Finding an effective way to subdue the noise pollution has attracted the interest of many scientists. Meanwhile, it was previously established that a good noise insulator must possess internal resonant effect [132,133]. As a result, blended mastication-hot processing was used to fabricate discarded degummed corn husk fibre and thermoplastic polyurethane (TPU) particles into a micro-slit plate having the potential of an excellent absorption acoustic performance. The double layer micro-slit plate based on corn husk fibre and TPU was found having a good noise absorption band [134]. It was also reported in another findings that the fibro-granular BCB made from rice husk fibres are sustainable noise absorption material in high frequency as well as in low frequency regions. The influence of fibre diameter, thickness, grain size on noise absorption characteristics was also studied [135]. In another interesting research, sound absorption performance of corn husk fibres was assessed by using impedance tube method (ASTM 1050 standing test method). Just like rice husk, it was also found that corn husk  Table 2. Selected mechanical properties of straw fibre bio-composites (stress at break (σb), strain at break (ε b ) and young modulus (E)).

Fibre type
Polymer matrix Fibre processing method N.S: Not specified. had good sound absorption properties at high frequency and low frequency due to its thinner thickness and light weight [136]. Sari et al, also investigated the acoustic properties of corn husk fibres and polyester composites. The effect of volume fraction of corn husk fibres on the acoustic performance was studied. The samples of different volume fraction of fibres was tested in an impedance tube. Samples with 80% corn husk fibres volume revealed the highest acoustic performance compared to other samples [137]. Therefore, there is a big opportunity to use straw fibres BCB as a noise suppressant in buildings [138]. Development of materials with good thermal insulator in buildings is very paramount due to the high cost of cooling in summer and heating during winter. In respect of these needs, corn stalk particles (using cutting mill at 1000 rpm) were reinforced with epoxy resin. The effect of the epoxy/corn stalk particles ratio were assessed on the thermal properties of the BCB. Theoretically, when the thermal conductivity coefficients of the insulation materials are below 0.1 W m −1 K −1 , a material is classified as good insulator according to TS 805 EN 601 [139]. In relation to the standard value, the results showed that BCB with 60/30% corn stalk and epoxy resin had lowest thermal conductivity coefficients (0.075 W m −1 K −1 ). Interestingly, the mixtures (60/30%) also showed the lowest ultrasonic sound penetration velocity coefficient (0.15 km s −1 ) [139]. Luo et al, observed improvement in the thermal resistant of corn fibre reinforced with polylactide after fibres surface sizing treatment. It could be suggested that sizing treatment of corn fibre could be a promising method to produce high performance thermal insulator [140]. Similarly, surface treated wheat husk fibre BCB exhibited thermal conductivity as low as 0.0714 W m −1 K −1 [123]. Interestingly, the wheat fibre chaff showed excellent thermal conductivity coefficient under steady-state and dynamic conditions (0.307 W m −1 K −1 and 0.298 W m −1 K −1 , respectively) [141]. Several research also established the good thermal resistance of straw fibres BCB [127,[142][143][144][145][146]. Therefore, straw fibres BCB have good thermal insulation effect, which could be mainly used as filling materials in the refrigerator, chemical transportation pipeline insulation material, and thermal insulation layer of building wall surface etc just like in the case of rigid polyurethane foams manufactured from corn straw powder [147].
The use of low density and fire resistance materials in an uprising building is one of the major requirement in building construction. It is expected that the density of material should decrease as the building rises. Also, the use of low density materials has been one of the major considerations in aeronautic industries. Fabrication of low density materials with improved fire performance without jeopardizing its mechanical strength has been one of the yardsticks for selecting material for constructional purposes. According to Antunes, et al, rice straw fibre BCB is a suitable material for buildings refurbishment due to its bulk density, thermal conductivity, ultrasound velocity, flexural strength, abrasion and fire resistance [148]. Rice straw fibres reinforced with ammonium polyphosphate polyelectrolyte modified high-density polyethylene had good limited oxygen index (LOI) of 23.5%, and peak heat release rate of 488.4 kW m −2 [149]. It was also confirmed that the addition of ammonium polyphosphate increased the flame retardancy of the polypropylene and straw fibre BCB with a significant increase in LOI [125]. Furthermore, straw fibres BCB exhibited excellent flexural strength, modulus of elasticity, impact resistant, tensile strength, and young modulus when compared to the reference materials, despite having very low bulk density of 0.41 g cm −3 . As observed by Zou, et al, the low bulk density allows wheat straw fibres to be packed very tightly, leaving a few voids [50]. The inner voids in BCB were reduced at high concentrations of wheat fibre, thereby enabling production of lightweight materials with outstanding mechanical properties.
Stepha et al, also observed the lowest density in alkaline treated husks despite its higher thickness compared to the reference materials [123]. It is interesting to know that straw fibres also possessed good drilling abilities in addition to lightweight, making it easy to couple into certain shape for specific application. Jayaprakash et al, studied the drilling parameters of rice husk/epoxy resin BCB. Based on the optimization result from Taguchi design, it was reported that feed rate of 0.06 mm/rev, speed of 1250 rpm with the minimum thrust force of 217.23 N and torque of 0.83 Nm were selected for carbide tipped drill [150]. It could be seen that the addition of rice husk could reduce the cost associated with drilling time and the drill's life of the machine tools. Meanwhile, grafting of maleic anhydride on polymer matrices could further enhance the withdrawal strength of fasteners, thereby enable coupling of rigid structure at low cost. However, the quantity of the fillers should be less than 50% in order to obtain optimum result [151].

Life cycle of straw fibres bio-composite boards
The shelf-life of a material is an important factor to be considered before subjecting them to a particular use. The life cycle of straw fibre BCB has been assessed using various methods but the most advanced technology for this test is fluorescence lifetime. The Leica SP8 module with TCSPC (Time-Correlated Single Photon Counting) is an important set-up for this test. In a robust experiment recently conducted, the effect of different treatments (hydrothermal, plasma and alkaline treatments) on the life cycle of wheat husk fibre BCB were tested. Untreated and alkaline pre-treated wheat husks exhibited the highest and lowest fluorescence lifetime, respectively. In addition, both hydrothermal and plasma pre-treated wheat husk did not cause statistical significant differences in fluorescence lifetime when compared with reference materials. The higher lignin content in alkaline pretreated wheat husk (as reported) is responsible for the lowest fluorescence lifetime [123]. It was further confirmed by the highest peak of alkaline pre-treated husks in the normalized photon count of fluorescence decay curve. Nevertheless, degrade-ability of material after use is a positive benefits regardless of storage period of any products. Pradhan, et al, assessed the biodegradation of polylactic acid (PLA), and wheat straw BCB under simulated aerobic compost (ASTM D 5338). Close to 90% of the BCB materials were degraded within 70 days of exposure [152].Therefore, the BCB of wheat straw reinforced PLA have been clearly demonstrated to be compostable products despite the low fluorescence lifetime earlier recorded.

Conclusion and future prospect
Introducing wheat, rice and corn fibre into the polymer matrix could allow reduction in the manufacturing costs of BCB through the use of significant amount of inexpensive and renewable fillers as reported in this review. The straw fibre BCB has been proved suitable for many applications as backed up by various findings. The behavior of straw fibre reinforced polymer BCB depends on several factors, such as the interaction of fibre with the polymer matrix, fillers content, particle sizes, fibre processing methods, fibres chemical composition, physical, and mechanical properties etc In order to effectively expand the use of straw fibres for BCB and improve their performances, it is essential to understand their fibre characteristics. Wheat straw fibre bonded with wheat gluten seems to possess the most outstanding strength properties with stress at break of 40.90 MPa and also young modulus of 25.50 GPa. Based on the intended application, robust mechanical properties test would be needed to fully ascertain the differences in the various straw fibre BCB samples. It is positive to see that more research on straw fibre BCB are now focusing on the use of biodegradable polymer matrices and this should further be encouraged to increase the production of greener materials. The industrial production and applications of the fibres are still limited or undocumented even though most of the straw fibres type were experimentally converted to value-added products. The properties of straw fibres differ among cited works because different fibres and testing methods were employed. The straw fibre processing methods (e.g. extrusion, filament winding, compression, and injection moulding etc) and suitable processing conditions (e.g. board orientation, particle size, and moderate temperatures (below 200°C) had significant effect on the BCB parameters. Therefore, manufacturing engineers must identify the most suitable processing method to fabricate straw fibre reinforced BCB, also consider the production design, and the manufacturing cost. However, there is still an urgent needs for further research in order to experimentally establish the most appropriate production methods that would be fitted for industrial production of straw fibre BCB on large scale. The characteristics of the straw fibre BCB (e.g. board thickness and density) must also be consistently reported as this could greatly influence the properties of the final products and also would enable adequate comparison of the different research. All the fibre surface treatment considered were found suitable with some exceptional results. Although, few specific fibre comparison of the treatment methods were found, and for effective selection of the optimum and most suitable treatment, an holistic comparison of the treatments are needed for each fibre treatments discussed in this review. In addition, there are few development in the use of statistical design (e.g. Taguchi and response surface methodology) for optimization of the production variables. This would be needed for generating the optimum conditions and predictive model for the straw fibre BCB production. Understanding of the scientific rationale responsible for the interaction of material constituents form the major aspect of research breakthrough. However, the mechanism of formation for most of the reviewed works are still missing, this will be needed for future development of straw fibres BCB.