Renewable materials are a new class of materials with huge potential to replace traditional petroleum-based materials in some applications ^[1][2][3][1,2,3]. Natural fibres derived from natural renewable materials, i.e., animals and plants, are described as new ‘green’ reinforcing materials and can be used to improve the overall properties of various polymers [4]. This is a result of their attractive attributes, such as their light weight, availability, inexpensiveness, high specific mechanical properties, biocompatibility, biodegradability, and non-toxicity. It has been recognised that the inherited hydrophilicity of natural fibres limits their broad application, especially when considering long-term durability under cyclic loading [1]. Some reports have demonstrated that the presence of natural fibres in composite materials can adversely affect mechanical performance [1], emphasising that the significant moisture adsorption caused by the fibres’ presence leads to a decrease in tensile modulus and strength [1]. Given the limitations of natural fibres, research has been dedicated to mitigating these drawbacks at a relatively low cost.

Over the span of decades, increasing attention has been given to combining two fibres from different sources to overcome these hurdles, in order for these materials to be able to compete with synthetic-based fibres at the commercialisation stage [5]. With glass fibres dominating the reinforced composites, viz., 95% volume, the combination of glass fibres with natural fibres (coir [6], bamboo [7], jute ^[8][9][8,9], areca core-sheath [8], and flax [10]) has attracted attention from researchers and industry from both economic and ecological perspectives ^[6][7][11][6,7,11]. Glass fibre is utilised as a reinforcing agent due to the properties of the resulting materials, which include excellent mechanical properties, non-biodegradability, moisture insensitivity, corrosion resistance, and relatively low cost. Nonetheless, other synthetic fibres have been combined with natural fibres to develop new materials with fascinating properties for advanced applications when compared to individually reinforced polymers. Therefore, the hybridisation of fibres, e.g., synthetic, natural fibres, or other fibres, to serve as reinforcement for various polymers offers a new platform to explore with the aim of reaping the benefits of both. In the literature, bast fibres have received more attention due to their promising performance when compared to synthetic fibres, e.g., glass fibres, as well as other natural fibres [2]. The driving force has been their attractive attributes, which include renewability, lightweight, abundant availability, and excellent mechanical performance.

Bast fibres are one of the plant fibres commonly used in reinforced polymer composites. They are long and are obtained from the exterior part of the plant stalk. These fibres are often used in the automotive industry to replace traditional fibres in car components, such as door panels, seatbacks, etc. Bast fibres exhibit drawbacks similar to those of other natural fibres, including excessive moisture absorption, poor thermal stability, inherent variability, and high flammability. Hybridisation with other fibres has been the most promising solution for overcoming these limitations. Synthetic fibres (e.g., glass fibres, carbon fibres, and basalt fibres) and other natural fibres have been hybridised with bast fibres to develop hybrid composite materials that not only address the limitations of bast fibres but also with the aim of developing complete composite materials with balanced overall properties [12].

2. Chemical Treatment of Fibres

Similar to any other natural plant fibre, bast fibres are inherently hydrophilic because of their constituents, viz., cellulose, hemicellulose and lignin ^{[3][13][14][15][16][17][18][19][20]}[3,21,22,23,24,25,26,27,28]. These constituents have a large number of hydroxyl groups, which are responsible for the hydrophilic nature of these fibres, and hence their high moisture sensitivity. Despite the fact that high moisture sensitivity is the primary limiting factor for the industrial application of plant-based fibres, these surface functional groups are crucial for the chemical modification of the fibres. These modifications improve the polarity of the fibres, which significantly improves interfacial bonding with hydrophobic polymers ^[21][20]. Chemical treatment (e.g., alkali treatment, silane treatment) is the process most frequently used to improve the hydrophobicity of fibres. In most cases, alkali treatment is used prior to other treatments in order to remove residues on the surface of the fibres, thus promoting fibrillation and surface roughness ^[3][20][3,28].

2.1. Alkali Treatment

The most commonly used treatment for plant fibres is alkali treatment, which is also known as mercerisation ^[13][14][21][20,21,22]. This process involves the removal of impurities on the fibre surface, thus affording rough surfaces. A sodium hydroxide solution with concentrations ranging between 5 and 20% is usually used for treating plant-based fibres. The time often ranges between 1 and 5 h, depending on the temperature used. Elsewhere, ramie fibres have been treated with 5% NaOH solution for 4 h at room temperature ^[20][28]. The obtained treated fibres were washed and neutralised using deionised water containing acetic acid to ensure the complete removal of NaOH. The fibres were then dried in a vacuum oven at 98 °C for hybrid preparation. Fibrillation of the fibres was achieved through partial removal of the matrix that causes the fibrils to adhere together, i.e., gums and pectin. 

2.2. Coupling Agents

Silane treatment of ramie fibres was carried out in order to improve the interaction between the fibres and the host polymeric material ^[15][16][20][23,24,28]. Functionalised ramie fibres were found to be trapped within the host matrix, indicating that there was good wettability between the filler and the matrix ^[15][23]. Due to the strong interaction between the fibres and matrix, there was better stress transfer from the polymer to the fibres, resulting in enhanced mechanical properties. The weak interfacial adhesion between the untreated ramie fibres and the matrix resulted in fibre pullouts, leaving behind holes ^[15][23]. Elsewhere, NaOH treatment for kenaf and pineapple leaf fibre (PALF) was carried out prior to the silane treatment of the fibres ^[20][28]. Hybridised and unhybridised composites showed fibre–matrix debonding, fibre pullouts, and breakage, indicating that a large amount of load was being carried by the fibres. Moisture absorption and mechanical properties were found to be dependent on the type of fibre.

2.3. Microbiological Treatment

Biological treatments are recognised for their eco-friendly and energy-saving characteristics ^[22][29]. Angelini et al. ^[22][29] compared the defibrillation of the ramie fibres using enzymes and NaOH. They used two strains of Clostridium felsineum L (NCIMB 10690 (MIC 10690) and NCIMB 9539 (MIC 9539)) because of their high pectonolytic efficacy. In the case of alkali treatment, the contained fibres were subjected to 2% NaOH and immersed in boiling water for 2 h. The chemical treatment was found to be more efficient at removing hemicellulose and lignin when compared to the two enzymatic strains.

3. Bast Hybrid Composites

There are several factors, such as fibre loading, the effect of the treatment applied to the fibres, and the stacking of the fibres, that play an important role in the resultant properties of the bast hybrid composites ^[23][24][30,31]. Since these factors are essential for their intended applications, most studies based on bast hybrid composites have been dedicated to finding the balance between these factors ^{[25][26][27][28]}[32,33,34,35]. In addition, there have been some studies dedicated to finding a good combination between the fibres, since they display different properties ^[26][28][33,35]. In general, the combination of synthetic and bast fibre results in a composite product with enhanced mechanical performance and low moisture absorption. Synthetic fibres that are often combined with bast fibres include carbon, Kevlar, glass and basalt. The combination of bast as a reinforcing agent with other natural fibres with high impact resistance results in hybrids with inferior attributes compared to single-bast-fibre-reinforced composites ^[26][33]. In some studies, the presence of these fillers has been shown to result in a balance of the two fibres in terms of the resulting properties ^[28][35].

3.1. Thermoplastics

Melt compounding is commonly employed for thermoplastic polymers since they can be melted and moulded into the desired shape/structure ^{[28][29][30][31][32][33]}[35,36,37,38,39,40]. These techniques include melt compression, extrusion method, and injection moulding. Some of these techniques can be used alone or together in order to produce the desired hybrid composite products ^[28][35]. In addition, melt extrusion and injection moulding have been employed for short fibres, whereas melt compression can be applied for both short and long fibres, as well as woven or non-woven fibres ^[30][37]. In the case of compression moulding, higher contents (30–50 wt.%) of fibres can be used to reinforce thermoplastic polymers ^[31][38]. The use of more than 30% of fibres within hybrid composites for other melting processing techniques has been reported to reduce the overall performance of the hybrid composites ^[34][35][41,42]. Different thermoplastic polymers, such as polycaprolactone (PCL), polylactic acid (PLA), polypropylene, and polyethene, have been employed for the preparation of bast hybrid composites ^[33][34][35][40,41,42].

3.2. Thermosets

The thermoset resins most often used for manufacturing hybrid composites are epoxies ^{[36][37][38][39][40]}[45,46,47,48,49], melamines, phenolics [3], polyesters ^[23][24][30,31], and polyurethanes. Among the thermoset group, epoxy is the most commonly employed for the fabrication of hybrid composite materials ^{[36][37][38][39][40]}[45,46,47,48,49]. In this context, the resin undergoes crosslinking into a three-dimensional network through a curing process to afford a product that cannot be melted or reshaped again. Hybrid composites based on thermosets are often manufactured using vacuum fusion ^[39][40][48,49], compression moulding, and hand lay-up processes ^[36][37][38][45,46,47]. The latter is often used together with compression moulding in the desired mould ^[36][45]. In most cases, long fibres in either woven or non-woven form are preferentially utilised to prepare these hybrid composites because they can be used for structural applications, e.g., civil infrastructure applications. The arrangement of the layers or patterns affords different properties.

3.3. Summary

Other processes, such as the solvent evaporation method, have not been utilised to prepare hybrid composites. Regardless of whether long or short fibres are utilised as reinforcing agents in hybrid manufacturing, pretreatment is crucial to the resultant properties. The preparation methods are often chosen with the aim of achieving the desired properties at a relatively low cost. The sensitivity of the natural fibres to long exposure to high heat limits the number of polymers that can be used for the preparation of thermoplastic-based hybrid composites via melt compounding techniques. The limitations of thermosets include recyclability, due to the three-dimensional network formed due to the curing process.

4. Effects of Moisture Absorption

Moisture absorption by natural-fibre-based polymer composites (NFBPC) is one of the most common concerns or drawbacks affecting the mechanical and physical properties of the composite for high-end or outdoor applications. The hydrophilic nature of natural fibres causes weakness in the interfacial adhesion between the fibres and the polymer matrix in the composite. The adsorption of moisture by the composites relies on various factors, which include the humidity and temperature, the fibre type, the matrix, the reaction between the matrix and water, the volume fraction of the fibre, voids, and the difference in the water distribution within the composites ^{[41][42][43][44][45][46][47]}[50,51,52,53,54,55,56]. Moisture absorption by NFBPC is governed by three different mechanisms of moisture diffusion. The first is the diffusion of water molecules into the microscopic spaces between polymer chains, the second is the diffusion of water molecules via capillary transport into the gaps and flaws at the interface between the polymer and the fibre as a result of inadequate impregnation and wetting during the compounding process, and the third is the diffusion of water molecules via microscopic cracks in the polymer matrix as a result of fibre swelling ^{[45][46][47][48]}[54,55,56,57]. These mechanisms allow the moisture diffusion in polymeric composites to be further classified as either Fickian or non-Fickian, which determines how quickly the material can absorb water ^{[43][44][45][46][47][48][49][50]}[52,53,54,55,56,57,58,59]. Composite materials with high diffusion coefficients tend to absorb large quantities of water quickly, compared to those with low diffusion coefficients.

5. Properties of Bast-Fibre-Based Hybrid Composites

5.1. Mechanical Performance of Bast-Fibre-Based Hybrid

The incorporation of fibres into hybrid composites can positively and negatively influence the overall mechanical performance of the material or not influence it at all, especially when low loadings are incorporated. Prior to the fabrication of hybridised composite materials, the mechanical performance, density and morphological properties of each individual fibre and/or bulk fibre should be investigated in order to determine which fibres have greater levels of influence than the others. Natural fibres are isolated from different parts of the plants, i.e., leaf, stem, stalk, seeds, fruit, bast, etc., and offer different properties. This provides a platform from which to tailor the overall mechanical performance of the resulting hybridised materials.

5.1.1. The Effect of Fibre Treatment on the Properties of Bast-Fibre-Based Hybrids

As previously mentioned, bast fibres have been widely used in hybridisation processes to improve the overall performance of the resulting materials. Despite their usefulness in hybrid applications, there are some inherent drawbacks. These drawbacks include inherent polar and hydrophilic nature, low detectable defibrillation, which causes non-uniform dispersion within the polymer matrix, and a smooth surface, which leads to slippery fibre, resulting in fibre pullout and debonding, which can be attributed to the poor interfacial adhesion between fibres and polymers. This therefore leads to the resulting hybrid material possessing undesirable properties. In order to mitigate the aforementioned shortcomings of bast fibres, one of the major strategies is the chemical treatment of fibres in order to enhance their dispersion within the polymer and improve the interaction between them. The treatment of bast fibres is usually achieved by silane, alkaline, grafting, and acetylation. Alkali treatment using sodium hydroxide has been widely employed for treating bast fibres. The treated fibres were subsequently reinforced with hybrid composites with the aim of improving their interfacial adhesion and mechanical properties. Numerous researchers ^{[51][52][53][54][55][56]}[44,83,84,85,86,87] have studied the effect of the chemical modification of bast fibres on the mechanical performance of the resulting hybrid composite samples. The results have demonstrated that the chemical treatment of fibres leads to strong interaction between fibres and polymers, thus enhancing their performance.  This increased their surface area, thus allowing the polymer to penetrate between the individual fibres. Therefore, this resulted in a strong interaction between the fibres and the polymeric matrix, leading to an enhancement of the overall mechanical performance of the hybrid. The incorporation of the alkaline-treated ramie fibres into the polycaprolactone (PCL) hybrid composite containing borassus fibres resulted in a notable increase in both tensile strength and Young’s modulus, whereas ductility behaviour decreased. In fact, this is in agreement with the hypothesis that the tensile properties of composites improve when reinforced with fibres possessing a high aspect ratio. This increment is caused by good stress transfer from the polymeric matrix to the fibres, and the fibres are able to withstand stress. It has also been reported that the incorporation of treated ramie fibres does not influence the hardness of the ensuing composite samples. 

5.1.2. Effect of Loading of Bast Fibres on the Tensile Properties of Hybrid Composites

The incorporation of fibres has been reported to improve the tensile properties of the resulting hybrid materials ^{[26][51][57][58]}[33,44,89,90]. Even though the incorporation of bast fibres has been demonstrated to enhance the tensile properties, nevertheless, numerous researchers have reported that increasing fibre loading results in the enhancement of tensile properties ^{[26][51][57][58][59][60][61]}[33,44,89,90,91,92,93]. However, a significant improvement was noticeable when jute loading was increased from 20 to 80 wt.%. Similar findings have been reported in other studies ^{[26][51][58][59][60][61]}[33,44,90,91,92,93]. In these studies, it was reported that when the bast fibre loading was increased, the mechanical properties also increased, whereas tensile strain decreased. The improvement in tensile properties was due to the high tensile strength and modulus of the bast fibres. With increasing loading, bast fibres are able to withstand higher loads, while distributing lesser loads to other fibres in a hybrid material leads to enhanced tensile properties ^[26][33]. 

5.2. Viscoelastic Properties of Bast-Fibre-Based Hybrid

Dynamic Mechanical Analysis (DMA) is the most effective technique for determining the viscoelastic properties (stiffness and damping behaviour) of polymeric materials such as hybrid materials ^[62][94]. DMA monitors the manner in which viscoelastic behaviour varies with temperature, time, and frequency. The stiffness of the material is determined using storage modulus, whereas Tan δ is used to determine the elastic behaviour, viscosity, and impact properties of the polymeric material. The glass transition temperature (T_g), which is attained on the basis of damping behaviour, defines the temperature range in which the polymeric material transforms from a glassy to a rubbery state ^[61][93]. Comparably to mechanical properties, DMA depends on various factors, such as the fibre treatment, the orientation of the fibre, the fibre type, the fibre loading, etc. It is evident from previous studies that DMA provides valuable information about the changes in the viscoelastic behaviour of hybrid materials ^{[35][59][60][61][62][63][64][65]}[42,91,92,93,94,95,96,97]. Other researchers ^[35][59][64][42,91,96] have investigated the effect of fibre loading on the viscoelastic properties of hybrid samples. In general, because of the increase in fibre loading, the viscoelastic properties are improved. For instance, Shanmugam and Thiruchitrambalam ^[59][91] investigated the viscoelastic behaviour of an unsaturated polyester hybrid reinforced with palmyra palm and jute fibres with various fibre loadings (75/25, 50/50, 25/75). It was observed that the storage modulus of the hybrid composites gradually decreased when compared to the virgin resin.

6. Thermal Properties of Bast-Fibre-Based Hybrid

In-depth thermal properties analyses of bast-fibre-based hybrids have been performed previously ^{[35][66][67][68][69][70]}[42,98,99,100,101,102]. These studies focused on the effect of fibre treatment as well as varied fibre loading. Thermal decomposition and stability, as well as melting behaviour, were measured to determine the thermal properties of the hybrids. Thermogravimetric analysis (TGA) is used to study the thermal decomposition behaviour and stabilities of materials, whereas differential scanning calorimetry (DSC) is used to investigate their melting behaviour. It has been reported that the incorporation of fibres does not change the degradation pattern of the hybrid composites ^[35][66][67][42,98,99]. However, the introduction of bast fibres into hybrid materials improves their thermal stability due to the bonding of the fibres with the polymeric matrix and the high thermal stability of the bast fibres ^[65][97]. The thermal stability of hybrid materials is dependent on the properties of both fibres: the reinforcement and the polymeric matrix. The effect of fibre treatment on the thermal stability of hybrid composites has also been investigated ^[66][68][69][98,100,101]. It is well known that the chemical treatment of fibres with alkaline removes impurities and components that are thermally less stable. However, these researchers observed that the decomposition temperature of hybrid reinforced with treated fibres shifted to a higher temperature, indicating an improvement in thermal stability. Few studies have investigated the melting and crystallisation behaviour of hybrid composites ^{[35][67][69][70]}[42,99,101,102]. The melting temperature (T_m), melting enthalpy (Hm), crystallisation enthalpy (H_c), crystallisation temperature (T_c), and degree of crystallinity (X_c) were determined via analysis in order to fully understand the thermal properties. In these aforementioned studies, it was reported that the incorporation of bast fibres into hybrids influences their melting and crystallisation behaviour.

7. Flame Resistance

Most hybrid composite applications require them to meet certain fire regulatory criteria in order to ensure public safety ^{[5][71][72][73][74]}[5,103,104,105,106]. To examine the combustion performance of different hybrid products, a wide variety of techniques can be employed, depending on the desired application. Cone calorimetry is often used for simulation of real-life situations ^[4][74][75][4,106,107]. It provides useful data related to the combustion of materials, such as heat release rate (HRR), total smoke release (TSR), carbon dioxide production (CO₂P), total heat release (THR), and smoke production rate (SPR). For instance, pHRR provides essential data based on the intensity of the fire, viz., higher pHRR values indicate a major fire hazard. Owing to the high flammability of polymeric materials and natural fibres, different additional fillers can be incorporated to improve their flame-retardant properties ^{[4][5][71][72][73][74][75][76][77]}[4,5,103,104,105,106,107,108,109]. Silane treatment of bast fibres in flame-retardant hybrid systems has been reported in the literature ^[17][18][25,26]. A comparison between the treatment of silane-treated fibres with APP (AF) and introducing and mixing APP into PLA (APLA) before the introduction of silane-treated fibres was conducted by Li et al. ^[16][24]. It was reported that the incorporation of APP into PLA is an acceptable method for producing hybrids with acceptable flame-retardant properties. The composites exhibited LOI values of 28–37, with a UL94 V0 rating. Modification of the secondary filler rather than the bast fibres can also be adopted in order to improve the overall flame-retardant property of the resulting hybrid composites ^[78][79][110,111]. The treatment of the CNTs with a mixture of acids to introduce carboxyl groups afforded chlorination with thionyl chloride, resulting in functionalisation with 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) ^[78][110]. The resultant hybrid composite composed of ramie/CNT-DOPO/PLA had higher char residue than the untreated CNTs/ramie/PLA. This was a result of the ramie fibres acting as a carbon source for charring, in turn acting as a shield to protect the underlying materials. This layer delays the transportation of volatiles and heat transfer, thus promoting flame resistance performance. In situ synthesis of phosphorus- and nitrogen-containing FR on the surface of ramie fibres was carried out using a condensation reaction by Du et al. ^[71][103]. The hybrid was prepared using melt blending in the presence of PP-grafted maleic anhydride (PP-g-MAH) as a compatibiliser. pHHR decreased from 714 kWm⁻² (PP/RF) to 547 kWm⁻² (PP/RF/FR), and THR decreased from 96 MJm⁻² to 84 MJm⁻², respectively. It was concluded that the FR on the surface of the fibres degraded, creating a continuous char residue that protected the fibres from the heat source. Elsewhere, both melamine pyrophosphate (MPP) and aluminium hypophosphite (AP) were introduced into BF/PP composites at a mass ratio of 2:1 ^[76][108]. The addition of 30% FR resulted in pHRR and THR reduction by ~39% and ~21%. Metallic hydroxides are known for their flame-retardant properties, which result from a cooling effect due to endothermic reactions from their degradation ^[18][26]. The incorporation of metallic hydroxide into hybrid composites was also studied ^[18][26]. In their study, El-Sabbagh et al. ^[18][26] studied the influence of magnesium hydroxide (Mg(OH)₂) on the flame retardance of PP/flax composite materials with PP-g-MAH employed as a compatibilising agent. It was pointed out that despite the samples failing to satisfy the lowest UL rating, the samples exhibited a prolonged burning process without signs of dripping. This was ascribed to the low content of Mg(OH)₂ (i.e., 30%) since 50–60% is required to achieve the desired flame-retardant properties. In addition, a recyclability study on this composite indicated that an increase in the number of recycling cycles led to a reduction in flame resistance performance by almost 5%. The preparation method plays a critical role in the dispersion of the fillers, and thus the resultant properties. The incorporation of FR into bast-based composites can drastically improve the flame resistance performance of the resulting hybrid composite materials ^[80][112]. The use of these FRs as modifiers for the fibres serves as a suitable alternative for improving adhesion between the polymers and fibres while maintaining the desired flame resistance performance. The optimisation of the contents of the components and the processing method are critical for attaining acceptable flame-retardant properties.