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Mohammed, M. M., Rasidi, M., Mohammed, A. M., Rahman, R. B., Osman, A. F., Adam, T., Betar, B. O., and Dahham, O. S. (2022). "Interfacial bonding mechanisms of natural fibre-matrix composites: An overview," BioResources 17(4), Page numbers to be added.

Abstract

The development of natural fiber (NFr) composites for a variety of applications is on the rise. The optimization of the interfacial bonding (IFB) between the reinforcing NFr and polymer matrix is perhaps the single most critical aspect in the development of natural fibre polymer composites (NFPCs) with high mechanical performance. While the IFB is critical in determining the mechanical properties of the NFPCs, such as stress transfer, it is one of the least understood components. This article offers a summary of IFB mechanisms, different modification approaches targeted at lowering incompatibility and improving IFB, and evaluation of the impact of IFB. It has been found that 1) In general, interdiffusion, electrostatic adhesion, chemical reactions, and mechanical interlocking are accountable for the IFB; 2) the incompatibility of the fibre and matrix, which results in poor dispersion of the fiber, weak IFB, and ultimately worse composite quality, may be addressed through strategic modifications; and 3) Interfacial interactions between polymers and nanoparticles (NPs) are significantly improving their performance in areas like thermal, mechanical, robust IFB, and moisture absorption. As a result, this review study could be an important resource for scholars interested in coating and treating NFr to further enhance their surface characteristics.


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Interfacial Bonding Mechanisms of Natural Fibre-Matrix Composites: An Overview

Mohammed Mohammed,a,b,* MSM Rasidi,a,b,* Aeshah M. Mohammed,c Rozyanty Rahman,a,b Azlin F. Osman,a,b Tijjani Adam,d Bashir O. Betar,e and Omar S. Dahham f, g

The development of natural fiber (NFr) composites for a variety of applications is on the rise. The optimization of the interfacial bonding (IFB) between the reinforcing NFr and polymer matrix is perhaps the single most critical aspect in the development of natural fibre polymer composites (NFPCs) with high mechanical performance. While the IFB is critical in determining the mechanical properties of the NFPCs, such as stress transfer, it is one of the least understood components. This article offers a summary of IFB mechanisms, different modification approaches targeted at lowering incompatibility and improving IFB, and evaluation of the impact of IFB. It has been found that 1) In general, interdiffusion, electrostatic adhesion, chemical reactions, and mechanical interlocking are accountable for the IFB; 2) the incompatibility of the fibre and matrix, which results in poor dispersion of the fiber, weak IFB, and ultimately worse composite quality, may be addressed through strategic modifications; and 3) Interfacial interactions between polymers and nanoparticles (NPs) are significantly improving their performance in areas like thermal, mechanical, robust IFB, and moisture absorption. As a result, this review study could be an important resource for scholars interested in coating and treating NFr to further enhance their surface characteristics.

GRAPHICAL ABSTRACT

DOI: 10.15376/biores.17.4.Mohammed

Keywords: Natural fibre composite; Compatibility; Fibre modification; Interfacial bonding mechanisms

Contact information: a: Center of Excellence Geopolymer & Green Technology (CEGeoGTech), Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia; b: Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Arau 02600, Perlis, Malaysia; c: University of Bagdad College of Education for Pure Science Ibn-Alhaitham, Baghdad, Iraq; d: Faculty of Technology, Universiti Malaysia Perlis, Kampus Uniciti Alam Sg. Chuchuh, 02100 Padang Besar (U), Perlis, Malaysia; e: Research Center (NANOCAT), University of Malaya, Kuala Lumpur 50603, Malaysia; f: Department of Petroleum and Gas Refinery Engineering, Al-Farabi University College, Baghdad, Iraq; g: Department of Civil Engineering, College of Engineering, Cihan University-Erbil, Kurdistan Region, Iraq; *Corresponding authors: hmn7575@yahoo.com; Syahmie@unimap.edu.my

INTRODUCTION

The importance of composite materials in various NFPCs has grown as the demand for lighter density and high-performing materials has increased. In the aerospace and automotive industries, NFr reinforced composites have replaced cutting-edge metal alloys as structural components. The onus on current technology to deploy lightweight electric vehicles will undoubtedly enhance the importance of NFr composites among all reinforcing fibres composites in the next years. NFPCs have grown in prominence as a subset of composite materials during the previous decade. The advantages that natural fibres have over inorganic fillers and reinforcements, such as their availability, environmentally friendly nature, biodegradability, nontoxicity, cheap cost, manufacturing scalability, and high flexural and tensile modulus, are driving this rapid development (George et al. 2001; Mukhopadhyay et al. 2003). In the realm of NFPCs, readily available, renewable, and biodegradable NFr are replacing classic synthetic fibres such as glass and carbon.

NFPCs are the most exquisite and promising materials of this century. Their durability and integrity in diverse service settings can be affected by the response of their constituents, i.e., the fibre, polymer matrix, and the existing interface/interphase between the fibre and polymer matrix in that environment. The primary reasons for the usage of NFPCs in various applications have been reduced weight, reduced cost, increased performance, decreased fuel consumption, and decreased pollution (Balakrishnan et al. 2016). However, this study narrowly focuses on composites whose strength typically peaks at fibre contents of 40 to 55 wt% for matrix composites, with the reduction at higher fibre contents explained as a result of poor wetting leading to reduced stress transfer across the fibre–matrix interface and increasing porosity. Stiffness has been found to increase up to higher fibre contents of around 55 to 65 wt% with similar materials, possibly due to less dependency on interfacial strength than composite strength (Le and Pickering 2015; Madsen et al. 2009). In addition to being a problem for the short-term qualities of composites, high fibre volume fractions are also a concern due to the possibility of increased water absorption, resulting in the loss of longer-term characteristics. Significant progress has been achieved in this area, and there are a sufficient number of review papers available. Pickering et al. (2016), provided an overview of the performance characteristics of NFr reinforced polymer composites. Saba et al. (2016) compiled a list of dynamic mechanical characteristics of these composites. Vaisanen et al. (2017) conducted a critical review of the impacts of waste material. Faruk et al. (2012) reviewed all of the developments that occurred between 2000 and 2010. They discussed various NFr, their availability, treatment procedures, and the different polymer matrices employed. Sanjay et al. (2018), provide a complete analysis of the characterisation techniques and attributes of these composites. Sood and Dwivedi (2018) focused on the flexural characteristics of these NFPCs. Oqla et al. (2015), described NFPC-like conductive polymer composites. Thakur and Thakur (2014), described how natural cellulose fibre is processed and characterised in a thermoset matrix. Omrani et al. (2016) investigated the tribological characteristics and utilization of polymer matrix composites enhanced with these novel materials. Vaisanen et al. (2016), discussed the use of agricultural and forest industry waste and residues in these NFPCs. Regardless of these benchmarking properties, optimising the IFB between natural fibres and polymer matrix is one of the most important techniques for achieving the best NFPCs formulation (Thakur and Thakur 2014).

Surface treatment of fibres has been widely used to improve IFB by adding polar functional groups and improving acceptable surface roughness (Zheng et al. 2004; Li et al. 2017). Surface treatment is a meticulous and deliberate technique to change the chemical surface environment and topography of the fiber in order to trigger adhesion-promoting processes. The experiment and work done by Oushabi et al. (2018) indicated that the chemical treatments of date palm fibres, in particular the alkali treatment and the treatment with organosilanes, significantly enhanced the interfacial properties between the fibres and the organic matrices, such as polyurethane and epoxy, as determined by the pull-out tests. Similar research by Rizal et al. (2018), demonstrated that the alkali treatment of typha fiber enhanced the interfacial compatibility between the epoxy resin and typha fiber, resulting in enhanced mechanical capabilities and a hydrophobic composite. After 5% alkaline immersion, the tensile, flexural, and impact strengths of the typha fiber reinforced epoxy composite increased in comparison to the untreated typha fiber composite.

The IFB between NFr and matrix acts within an interaction or propagation region where two phases or elements interact mechanically, physically, and chemically, as shown in Fig. 1. The interfacial adhesion between the fibre and matrix is critical in determining the mechanical properties of the composite (Kabir et al. 2012). The IFB between the fibre and matrix is influenced by mechanical interlocking, chemical bonding, and attractive molecular forces. However, naturally hydrophilic NFr is inherently incompatible with hydrophobic polymers. In addition to the pectin and waxy components in NFr acting as an obstacle to interlocking with the nonpolar polymer matrix, the presence of a large number of hydroxyl groups inhibits its operative reaction with the matrix (Bledzki et al. 1998; Kazayawoko et al. 1999). Therefore, modifying the surface characteristics of NFr and hydrophobic polymer matrix is critical to develop a suitable composite with excellent IFB and efficient inherent stress convey across the interface region. Physical modifications (such as solvent extraction, heat treatment, corona, and plasma treatments), physic-chemical modification (such as laser, and UV bombardment) (John and Anandjiwala 2008), surface coating (Kabir et al. 2012), and chemical treatments including alkaline (Fiore et al. 2015), silane (Agrawal et al. 2000), acetylation (Tserki et al. 2005), benzoylation (Li et al. 2007), and coupling agents have all been tried to improve the compatibility and bonding between NFr and matrix (Saheb and Jog 1999; El-Abbassi et al. 2015). Chemical treatments and surface coatings, in particular, were shown to be useful methods for increasing the mechanical strength of composites by improving their wettability and interfacial characteristics. Kabir et al. (2012) highlighted the critical role of chemically treated NFr in enhancing the efficiency of their composites. Furthermore, Li et al. (2007), investigated various chemical treatment methods and their mechanism of action in improving the properties of composites. Mohanty et al. (2001), discussed how to modify the surface of NFr and how this affects their biocomposites. However, the majority of these surface treatments improved the surface of NFr, but the ultimate consequence was an improvement in the interfacial bonding (IFB) between the NFr and polymer matrix. Once the IFB is enhanced, all the mechanical characteristics of the NFPCs would be supported (Kabir et al. 2012).

These modifications significantly enhanced the IFB, wettability, roughness of NFPCs. However, such composites (with organic compounds) are not feasible in structural and high temperature applications. In general, the NFPCs are required to bear a high load in high structural applications (like tensile and compression). Improved compression strength, tensile strength, toughness, and stiffness are needed to withstand such a high load. Thus, to establish a durable interface region, an adequate scale of physicochemical interactions is required, which might be fostered by van der Waals, hydrogen, and covalent chemical bonding between NFr and matrix. Numerous surface treatments approaches have successfully improved IFB via chemical interactions and mechanical interlocking (George et al. 2001; Bogoeva-Gaceva et al. 2007; Li et al. 2007). However, a combination of modification approaches has also been used to achieve superior IFB (Karnani et al. 1997; Zhou et al. 2016). Recently, nano-reinforcement materials such as nano-CaCO3, zinc oxide NPs, and titanium dioxide have been employed to alter the polymer matrix and fibre in order to achieve the maximum possible IFB through synergy (Wang et al. 2012; Soltani et al. 2013; Li et al. 2017).

This paper focuses primarily on the IFB mechanisms of NFr composites and is organized into three main sections: an overview of the IFB mechanisms, different surface modification approaches targeted at improving IFB between NFr and polymer matrix, and the evaluation the impact of IFB on the mechanical performance of NFPCs. This article will serve as a foundation for future research and industrialisation of NFPCs. In the current situation, where traditional modification techniques are widely utilized to modify NFr for potential industrial applications, researchers and engineers cannot agree on which modification techniques provide the most significant benefits while maintaining the integrity of NFr. As a result, it is vital to highlight the advantages and disadvantages of present NFr surface modification approaches and to envision concrete paths for future improvement. In response to the rising number of publications, this review will discuss the most recent developments in NFr surface modification and the current state of modified NFPCs. Numerous modification strategies have been discussed in detail with suitable examples to demonstrate the effect of surface treatments on the interfacial characteristics. Additionally, the strengths and weaknesses of each modification strategy have been evaluated based on the existing literature.

Fig. 1. Schematic illustration of the interphase in the composite

NATURAL FIBRE POLYMER COMPOSITES

NFPCs are multi-phase materials composed of natural reinforcing fibres originating from plants or animals. The combination of fibre and polymer matrix produces synergistic qualities that cannot be obtained from either component alone. NFr provide several advantages over synthetic fibres, including superior rigidity and strength, low cost, environmentally benign nature, biodegradability, and the fact that they are renewable materials. This review concentrates on composites made from natural-derived fibres. NFr can be categorised generically into the stem, fruit, and leaf fibres, as illustrated in Fig. 2.

Fig. 2. Schematic representation natural fibres’ origin

CHEMICAL COMPOSITION OF NATURAL FIBRE

Natural fibres primarily consist of cellulose, lignin, hemicellulose, pectin, and waxy surface, etc. (Pickering et al. 2016). Hemicellulose and lignin are major components of the secondary cell wall. The crystalline and amorphous portions of cellulose vary from plant to plant and determine the stiffness and mechanical strength of the fibre (Suryanto et al. 2014). Figure 3 is a simplified model of composition of natural fibres that depicts the structure of the cellulose molecule, which is made of D-glucopyranose, which is interconnected with b-1,4- glycosidic linkage having a degree of polymerization of 10,000 (Kbir et al. 2013; Celino et al. 2014). Intramolecular hydrogen bonding is accomplished via the –OH group in cellulose. The microfibril angle is the alignment of this cellulose molecule with regard to the fibre axis, which, along with the degree of crystallinity of the molecule, defines the fibre strength. The larger the crystallinity and smaller the microfibril angle, the greater will be the strength of the fibre (Eicorn 2001; Reddy and Yang 2005). The presence of bulky pendant groups in the structure of hemicellulose and lignin causes them to be amorphous. Hemicellulose comprises a mixture of 5 and 6 carbon ring polysaccharides with 50 to 300 degrees of polymerization, whereas lignin contains an aliphatic and aromatic component composed of 4-hydroxy-3-methoxyphenyl propane. They are quite stable and insoluble in most solvents; however, they dissolve in alkali (John and Thomas 2008). NFr can be used to make sustainable composite materials because of their abundance and accessibility of availability.

Natural fibres generally contain large amounts of the hydroxyl group, which can form hydrogen bonds with polar molecules such as water. Water adsorption is primarily influenced by the relative humidity of the surrounding atmosphere, crystallinity, and amorphous content of cellulose. Cellulose with a high degree of crystalline structure can only accommodate water on its surface. In contrast, if the proportion of the amorphous phase is higher, then water molecules can permeate the interior of cellulose material (Gauthier et al. 1998). As a result, the intake of water molecules has unfavourable consequences. During composite manufacturing, the –OH content of the NFr is accountable for its inability to adapt to a hydrophobic matrix. Water is absorbed and transferred to the bulk of the fibre from the surface by diffusion or capillary action (Munoz and Garcia-Manrique 2015).

Fig. 3. Simplified model of the composition of natural fibres

Fig. 4. Schematic representation mechanism of water absorption

As shown in Fig. 4, water flows into the fibre-matrix interface region, which results in the leaching of water-soluble compounds from surface of the NFr and the debonding between NFr and matrix (Azwa et al. 2013). The strength of the composites produced is determined by the efficiency of the IFB between NFr and matrix. If the IFB between the NFr and the matrix is too robust, then the applied load will result in the formation of a fracture that does not distribute the stress from the matrix to the NFr, producing a brittle material that breaks down (Hidalgo-Salazar et al. 2013). Simultaneously, if the IFB is inadequate, the necessary stress transfer from matrix to NFr will not occur. As a result, the mechanical strength of the composites decreases with time. According to research conducted by many researchers, NFr have a higher water absorption capacity, which rises with increasing fibre loading and time spent in humid circumstances, resulting in poor transfer of stress from matrix to NFr, culminating in composite failure (Vijayakumar et al. 2014; Bujjibabu et al. 2018). Because of inherent flaws in NFr, surface treatment of the fibres can be accomplished through chemical treatments or additives during composite production.

SIGNIFICANCE OF FIBRE/MATRIX INTERFACE

When fibres are introduced to a matrix, the mechanical characteristics (e.g., tensile strength and bending stiffness) of the material can be increased. Fibres serve two functions: 1) carrying most of the compressive or tensile stress imparted to NFPCs, and 2) bridging matrix cracks and reducing crack propagation by dissipating energy around the fracture tip. The stress is transmitted to the NFr via the surrounding matrix when an NFPC is exposed to axial tensile stress, as shown in Fig. 5(a).

Fig. 5. (a) Simplified model of the load transferring at fibre/matrix interface and (b) fibre bridging

The process begins with matrix deformation due to the imposed load. The matrix deformation then causes shear stress at the IFB region between the NFr and matrix (Young 2015). To balance the shear stress, the tensile stress of the fibre is triggered. The fibre carries most of the tensile stress imparted to the NFPCs during this procedure. Figure 5(b) shows the mechanism of fibre bridging. Microcracks begin in the matrix because of its lower fracture strain than fibres. When the strain energy near the crack tip is more than the energy required to build a new surface, the crack will propagate. The stress carried by the matrix is transferred to the fibre via the interface when fibres bridge a crack in the matrix. Based on the interfacial shear strength of the fibre-matrix interface and the tensile strength of the fibre, interface failure (e.g., fibre pull-out and interface debonding) or fibre fracture will occur as the stress increases. The strain energy is dissipated around the crack tip by interface failure and fibre fracture, preventing crack propagation.

In NFPCs, the interface is the shared boundary between the fibre and the polymer matrix, through which load can be transferred from the matrix to the fibres based on strain compatibility. If the interface fails prematurely, then the deformation of the matrix (or strain) is incompatible with that of the fibre, preventing the load from being transferred from the weak component (matrix) to the robust component (fibre). In other words, it is impossible to accomplish the reinforcing impact of fibres on the polymer matrix. Consequently, strain compatibility at the interface is vital for the structural integrity of NFPC’s materials. Due to the different mechanical (e.g., elastic modulus) and physical (e.g., swelling ratio) characteristics of fibres and the matrix, it is difficult to establish strain compatibility under stressed and hostile environmental circumstances. Stress concentration is more likely to occur at the interfacial region than in the fibres or matrix, making it easier to develop microcracks at the IFB between fibre and matrix. The load-carrying capability of the NFPCs material declines as the microcracks propagate. It is critical to investigate the fibre-matrix interfacial characteristics to make the best use of NFPCs material.

Furthermore, the IFB between fibre and matrix is crucial for the long-term mechanical characteristics of NFPCs in hostile environments. For instance, when a NFPC composite is subjected to a high humidity environment (e.g., 95% RH), the mechanical characteristics of the complete NFPCs decline due to the degradation of the fibres, matrix, and IFB between fibre and matrix. The hydrolysis occurs in the matrix and NFr, causing the mechanical characteristics degradation of natural fibres and matrix. The transported moisture degrades the interfacial bonding by weakening the chemical bonds and mechanical interlocking at the interface of the fibre and matrix. In addition, the difference in swelling between the fibre and matrix causes microcracks at their interface (Bradley and Grant 1995; Heshmati 2017). When exposed to high humidity for an extended period of time, the microcracks generated by the swelling mismatch in the early stages of exposure create additional pathways for moisture diffusion, accelerating the propagation of the damage in the natural fibres, matrix, and the IFB between fibre and matrix. Compared to natural fibres derived from plants, synthetic fibres undergo comparatively little hydrolysis.

An investigation was carried out by Sanjeevi et al. (2021), to understand the effects of water absorption on the mechanical properties of hybrid phenol formaldehyde (PF) composite fabricated with Areca Fine Fibres (AFFs) and Calotropis Gigantea Fibre (CGF). The mechanical characteristics of the hybrid composites in a wet condition were inferior than those in a dry condition. The tensile strength of 45 wt% composite at wet condition was 40 MPa lower than the 35 wt% composite in a wet condition due to the penetration of water molecules into the fibre–matrix region. The penetration led to the change of dimensions of the composite specimens and caused weak IFB, thereby decreasing the tensile characteristics. In addition, Mohammed et al. (2017) studied the weather effects on mechanical properties of pure kenaf and the kenaf/glass fiber hybrid composites. The unexposed pure kenaf composites and the kenaf/GF hybrid composites showed an approximate tensile strength of 60.8 and 70.9 MPa, respectively. There was a decreasing trend in the tensile strengths of the pure kenaf composites and the kenaf/glass fibre hybrid composites. Based on the results, it can be observed that the pure kenaf composites showed a higher decrement as compared to the kenaf/GF hybrid composite. This was due to the inclusion of GF to the top and bottom layers of the pure kenaf composites, which worked as a barrier and impeded moisture transfer to the kenaf fibres. The ideal interface ought to fulfil the following objectives.

Bonding Layer

The appropriate interface should have the optimum bonding strength for the interface. When the matrix crack stretches to the IFB phase, the acceptable IFB between fibre and matrix can mitigate the stress concentration at the front end of the matrix crack, prevent fibre debonding, prevent crack deflection, prevent the crack from expanding into the fibre, and consume fracture energy (Hestiawan et al. 2018). To accurately capture the damage initiation and propagation at the constituent level, three-dimensional Representative Volume Element (RVE) models have been developed by modelling the fibre, the matrix, and the coating material separately. In addition, numerous geometrical parameters, including the random distribution of the fibres, the fiber-coating, and the matrix-coating interface decohesions, were considered (Daggumati et al. 2020). In addition, the extended finite element method (XFEM) technique was utilised to capture the coating and matrix material fracture behaviour (Daggumati et al. 2020).

Load Transfer

The IFB phase must be sufficiently strong to operate as a bridge between the matrix and the NFr, allowing stress to convey between them to occur (Merotte et al. 2018). As a result of the poor interface bonding, the interface tends to debond under low tension, making it difficult to convey the load properly. As a result, the strengthening effect of NFr is not fully realized, resulting in inferior mechanical characteristics in composites. If the strength of IFB between fibre and matrix is excessively high, then the interface will be unable to modify the stress distribution, resulting in brittle fracture when the composite is loaded. Only when the interface layer has adequate material, and bonding strength that enables the interface simultaneously convey load and modify stress, allowing composites to achieve superior mechanical qualities (Arbelaiz et al. 2005; Sain et al. 2005).

In a mechanically loaded composite, it is usually assumed that the fibres and the matrix are bonded together and that no different movement exists between them. The strain in each component of the composite can then be deemed same and equal to the composite’s overall strain (Yin et al. 2017). For an elastic response, the stress on a unidirectionally reinforced composite loaded parallel to the fibre axis σc, can be calculated by multiplying the elastic modulus in that direction Ec, by the measured strain εc

(1)

A weak interfacial bonding tends to dissipate more energy than that of a robust interfacial bonding. As it pertains to fiber-reinforced composites, debonding can improve the stick–slip friction under imposed cyclic loads, leading to enhanced damping qualities. Geethamma et al. (2005) examined the influence of chemical treatment of coir fiber on damping; the results indicated that the poorer the interfacial bonding, the better the damping capacity is. It has been found that the weak bonding between fibers and matrix could be attributed to the relative motion of fibers. To create composites with high damping, both energy-dissipating material and strong shear strain are required. Thus, improving energy dissipation behaviour through fibre surface treatment to strengthen interfacial damping effects is effective. It was presented that the interfacial damping mechanisms in fiber reinforced composite are closely related to interfacial debonding and friction. Recently, dampening behaviour has evolved to be a hot topic. Fiber reinforced composite with robust damping characteristic is cost-effective for increasing energy attenuation in engineering system applications (Tang and Yan 2020). The intrinsic viscoelastic damping of the matrix is the principal contributor to damping in fibre-reinforced composites. Under dynamic loading, it has been demonstrated that the matrix carries both extensional and shearing stresses. Particularly, damping behaviour related with stress distribution perpendicular to the reinforced fibre axis, commonly known as in-plane shear stress, is essentially entirely dictated by matrix damping (Treviso et al. 2015). It has been claimed that raising the matrix volume percentage at the expense of stiffness and strength can increase damping. The reason can be attributed to the higher stresses in the matrix due to closer spacing which resulted in higher energy dissipation capacity during the dynamic loading (Haddad and Feng 2003; Melo and Villena 2012).

Various types of fibres are currently employed for engineering applications, with carbon fibre dominating lightweight structural composites because of its high modulus and low density. Among the most popular natural fibrous materials, the damping behavior of short coir fiber reinforced natural rubber composites (Geethamma et al. 2005), banana fiber reinforced polyester (Pothan et al. 2003), flax- and hemp-fiber reinforced polypropylene (Wielage et al. 2003), ramie/glass hybrid fiber reinforced polyester composites (Romanzini et al. 2012) has been extensively studied in various applications. The results indicated that the storage modulus of the composites augmented with the increase of hemp fiber loading. The maximum damping ratio was achieved when the composites were reinforced with 30wt% noil hemp fiber (Etaati et al. 2014). In addition, Senthilvelan and Gnanamoorthy (2006) revealed that increasing the amount of short sisal fibre and short banana fibre in polyester composites might enhance their damping qualities. The importance of fibre in the damping behaviour of composites is growing. Moreover, damping composites comprised of high-performance fibre are utilized for a variety of purposes, such as sound absorption, energy storage, vibration control, etc.

Physical and Chemical Compatibility

Prestress will be formed at the interface if the thermal expansion coefficients of the NFr and the matrix are mismatched, particularly during the cooling process, affecting the performance of NFPCs (Lu and Oza 2013). The thermal expansion coefficients of the NFr and the matrix must be properly matched to avoid a thermal expansion mismatch between them (Krishnasamy et al. 2019). A coating can also be applied between the NFr and the matrix to act as a buffer zone, reducing the stress concentration induced by the thermal mismatch between the NFr and the matrix (Nurazzi et al. 2021). Chemical compatibility must be considered between the interface, NFr, and matrix; that is, the chemical composition of the NFr, interface, and matrix must be thermodynamically stable (Chawla 2008). TA Instruments Q400Ethermomechanical analysis equipment (TA Instruments, New Castle, DE, USA) was utilized to assess the matrix glass transition temperature, thermal expansion, and coefficient of natural fibre composites (Thomason et al. 2017).

Prevention or Inhibition of Oxidation

NFPCs are frequently subjected to oxidising conditions at elevated temperatures during their manufacturing or application. At high temperatures, interdiffusion between the matrix and the NFr and erosion in an oxidising environment significantly decrease the characteristics of the NFr (Matuana et al. 2011). Therefore, the IFB region should protect the NFr from damage caused by high temperatures. On the one hand, atomic diffusion and chemical reactivity between the NFr and matrix must be controlled, whereas erosion in an oxidising environment must be avoided (Fabiyi et al. 2008). In short, the resistance of NFr and even composites to high-temperature oxidation must be enhanced (Wang et al. 2005; Matuana et al. 2011). The low thermal stability increases the likelihood of cellulose degradation and the likelihood of volatile material emissions, both of which could negatively impact the composite’s characteristics. Therefore, processing temperatures are restricted to approximately 200 °C, but it is feasible to employ greater temperatures for brief durations. 60% of the thermal breakdown of most natural fibres occurred between 215 and 310 C with an apparent activation energy of 160 to 170 kJ/mol (Yao et al. 2008). By exposing samples to a specified radiant flux, cone calorimeters are commonly used to evaluate the flammability of polymer compounds. A cone calorimeter measures heat release rate (HRR), total heat released (THR), mass loss rate (MLR), time to ignition (TTI), smoke emission (SEA – specific extinction area), and average carbon monoxide (CO) and carbon dioxide (CO2). Lower HRR indicates smaller contribution to a fire (Stark et al. 2010; Zhang et al. 2012). Furthermore, thermogravimetric analysis (TGA) is utilized to study the thermal stability of NFr. A typical TGA curve for composite thermal degradability shows that a sample subjected to heat will slowly suffer weight drop, then the weight will drop sharply over a narrow range and finally turns back to zero slope as the reactant is exhausted. The shape of the TGA curve is determined by the kinetic parameters of the pyrolysis such as reaction order, frequency factor and activation energy while the values obtained depend upon atmosphere, sample mass, sample shape, flow rate, heating rate and the mathematical treatment used to evaluate the data (Azwa et al. 2013).

Appropriate Thickness of the Interface Layer

The IFB will be excessively strong if the interface layer is too thin, resulting in brittle fracture. If the interface layer is overly thick, then the IFB will be insufficiently robust, resulting in reducing the strength of NFPCs. It is possible to achieve a moderate bonding strength while preserving the strength and toughness of the composite by adjusting the thickness of the interface layer (El Jaouhari et al. 2018; Banerjee et al. 2019). Yu et al. (2011), fabricated SiC/SiC composites by coating SiC fibers with PyC layers with different thicknesses. When the thickness of the PyC interface layer is 0.1 μm, the flexural strength of the SiC/SiC composite reaches its maximum value, and the optimal thickness corresponding to the highest fracture toughness is 0.53 μm.

In general, the scanning electron microscope (SEM) is the mostly utilized technique for investigating fibre-matrix interactions at fracture surfaces and polymer distributions in plant fibre composites. It permits the monitoring of monomer-impregnated samples directly and after composites have cured, yielding information on the polymer-fibre interaction (George et al. 2001). Utilizing SEM, Migneault et al. (2015) examined the variation in wetting at the fibre-matrix interface of composites among the different fibres employed, including aspen wood, spruce bark, and spruce wood fibres. At the fibre-matrix interface, the SEM micrographs also revealed variations in interfacial adhesion and mechanical interlocking. The interpretation of the fibre-matrix adhesion is of special significance for the successful design and proper utilisation of NFPCs. There are several micromechanical testing methods for measuring the fibre-matrix interfacial adhesion. Examples include the single fibre fragmentation test (SFFT), the single fibre pull-out test (SFPT), and the microbond test (MT). The detailed summarisation of these tests can be found in the book of Kim and Mai (1998).

MECHANISMS OF INTERFACIAL BONDING

In NFPCs, the major components are the reinforcement NFr and the polymer matrix phases. Three key parameters influence the qualities and performance of composites: matrix, reinforcement, and interface. The interfacial region between the NFr and the matrix plays a significant role in determining the overall material behavior. In composites, the interface, which is frequently referred to as an intermediate area generated by the binding of the fibre and matrix, is a zone of compositional, structural, and characteristic gradients whose width typically ranges from a single atomic layer to micrometers (Zhou et al. 2016). At the interface, there is a strong link between the processes that take place at the atomic, microscopic, and macroscopic levels. Indeed, understanding the sequences of events that occur at these many levels is critical for comprehending the characteristics of IFB phenomena. The IFB between fibre and matrix is responsible for stress covey between the NFr and matrix and is primarily determined by the interfacial adhesion level. A sufficient interfacial strength guarantees that the maximum stress level is maintained without disturbance throughout the interface and from fibre to matrix. The molecular interaction at the interface, as well as the thickness and strength of the IFB region, impact the efficacy of load transmission (Drzal and Madhukar 1993; Awang 2013).

According to the interface definition, a fibre in NFPCs can be divided into two parts. One part does not come into contact with the matrix and retains the characteristics of the original fibres. The other part of the fibre is impacted by the matrix, resulting in characteristics that differ from the original fibre. This difference in the characteristics has two origins: first is the treatment of adsorbed materials on the fibre or matrix prior to composite formation. Functional groups can be added or removed from the initial surface of fibres using surface treatments. As a result, the surface of the fibre differs chemically and structurally from the original fibre. When the fibres or matrix are subjected to air prior being assembled, chemical species in the air may adsorb at the surface, altering/eliminating specific surface reactivity, resulting in a decrease in IFB (Drzal et al. 1983). The second reason is diffusion or chemical reactions between the fibres and matrix.

Physical Attraction Mechanism

The formation of an interface in composite materials includes physical attraction between electrically neutral bodies, molecular entanglement, inter-diffusion of elements, electrostatic adhesion attraction, chemical bonding reaction between groups on fibre and matrix surfaces, and mechanical interlocking (Drzal et al. 1983). In addition, there are forces with low energy, such as van der Waals forces and hydrogen bonding. Physical attractions (including electrostatic attraction and physical attraction between electrically neutral bodies) may occur before the NFr and matrix contact each other during the formation of IFB. Other mechanisms, such as molecular entanglement and interdiffusion, begin to operate after the NFr and matrix make contact. The final bond between the NFr and matrix is generated by all the mechanisms operating independently or, more likely, in concert (Kim and Mai 1998). Figure 6 depicts the individual bonding mechanisms.

Physical attraction between electrically neutral bodies leads in interfacial bonding on an atomic scale via electron interactions. This interaction is governed by van der Waal forces, which are the attraction between neutral molecules or atoms. It may also be determined by the acid-base interaction, i.e., the polar attraction of Lewis acids and bases, such as electron-deficient and electron-rich elements (Ebnesaiiad and Ebnesajjad 2014). When the distance between the atoms of the fibre and matrix is within a few atomic diameters, the interaction occurs (Kim and Mai 1998). This physical attraction is commonly described in terms of wettability (the ability of liquids to spread over a solid surface) (Kim and Mai 1998).

Fig. 6. Interfacial bonding formed by (a)molecular entanglement; (b) inter-diffusion of elements; (c) electrostatic attraction; (d) chemical reaction between groups on reinforcement and matrix surfaces; (e) mechanical interlocking

Interdiffusion Mechanism

Interdiffusion occurs as a result of intimate intermolecular interactions generated by Van der Waals forces or hydrogen bonding between the molecules of the NFr substrate and the matrix (Liu et al. 2012). Indeed, this adhesion mechanism consists of two stages: adsorption and diffusion. The first stage requires intimate interaction between two constituents, the NFr and matrix, guided by two actions: spreading and penetration. After adequate wetting, permanent adhesion is formed via molecular attractions such as covalent, electrostatic, and Van der Waals. On the other hand, enough wetness between the substrates results in the interdiffusion of NFr and matrix molecules. The extent and degree of diffusion are mostly determined by the chemical compatibility of the two ingredients and the penetrability of the substrate (Kim and Pal 2010). Despite the benefits, inter-diffusion can damage the NFr and lower the toughness of composites when exposed to a high-temperature oxidation environment. Under these conditions, various energy loss mechanisms such as fibre debonding, fibre bridging, fracture deflection, and fibre pull-out may impact the interface between the NFr and the matrix (Xu et al. 2021). The molecules that have diffused may become entangled with other molecules. The ambient temperature affects molecular entanglement (Kim and Mai 1998). As the temperature rises, the molecules in polymer chains gain sufficient energy to overcome local obstacles (such as molecule chains interpenetration) that obstruct molecular motion. When the temperature approaches the glass transition temperature, molecular motion becomes so active that even molecular entanglement is ineffective at preventing molecule chains from slipping (Sethi et al. 2015). As a result, the interfacial connection becomes weaker as the molecular entanglement is gradually unraveled.

Electrostatic Adhesion Mechanism

Electrostatic adhesion is a result of the formation of opposite charges (anionic and cationic) on the interacting surfaces of the NFr and matrix as a result of an electron or ion imbalance, as illustrated in Fig. 6(c); therefore, an interface region composed of two layers with opposite charges is produced, which accounts for the adhesion between the two components of the composite. The interfacial strength is reliant on the charge density (Kim and Mai 1998). When materials are separated, electrostatic attraction is efficient, and the active distance is in the centimeter range, which is greater than the range of other adhesion processes (Hays 1991). As previously stated, interactions based on van der Waal forces can only occur within a few atomic diameters. However, when substances are in close proximity, the electrostatic impact is relatively weaker compared to van der Waals forces (Petrie 2013).

Interestingly, electrostatic discharge treatments were applied to the surfaces of polymer and electrostatic fibres to produce electrostatic adhesion at the interface region, hence enhancing the performance of the composites (Rajak et al. 2019). In addition to this, the adsorption of particles on the surface of fibre through electrostatic interaction may also affect the surface adhesion between the fibres and matrix (Yamamoto et al. 2017; Yamamoto et al. 2016). Due to the aforementioned benefits, electrostatic adhesion was introduced into the fibre-reinforced composite structure (Heath et al. 2015, 2016) to increase the interfacial strength. As previously stated, the poor IFB between the fibre and matrix may limit the practical application of NFr. Thus, it is critical to optimise the matrix-fibre interaction mechanism to broaden the applications of fibre reinforced composites.

Chemical Bonding Mechanism

Chemical bonding occurs at the interface when compatible chemical groups are connected between the NFr and matrix. One common way of enhancing chemical bonding is by utilizing coupling agents, which are bifunctional molecules with one end of the molecule capable of reacting with the compatible chemical group on the fibres and the other end capable of reacting with the compatible chemical group in the matrix. Silane agents and maleated coupling agents are examples of traditional agents. For silane agents, one end of silanes can interact with hydroxyl groups (hydrophilic groups) of the NFr; while the other end interacts with the hydrophobic groups in the matrix (Li et al. 2007; Huda et al. 2008). The function of coupling agents is influenced by the environment when NFPCs with coupling agents are used in practical environments such as hygrothermal and UV temperatures. As a result, an appropriate coupling agent must be chosen based on the operating conditions of NFPCs (Yeh et al. 2021).

Mechanical Interlocking Mechanism

Mechanical interlocking happens between the matrix and rough surface of the natural fibre. The roughness of fibre surface may be defined by the regularity or irregularity valleys, and crevices of fibre surface (Liu et al. 2012). When the surface is rougher, the binding area between matrix and NFr augments, and thus the strength of bonding in interface region can be increased. Apart from the roughness of the NFr, the residual clamping stress induced by the differential in thermal expansion or shrinkage between the NFr and the matrix is also advantageous for the mechanical interlocking of the NFr and the matrix (Kim and Mai 1998). It occurs at the millimeter and micron length scales, while diffusion entanglement occurs at the nanoscale within the cell wall pores of natural fibre. By grafting NPs (e.g., nano clay and nanocarbon tube) to fibres, the surface roughness of the NFr can be increased, hence improving mechanical interlocking. Alkali treatment is a frequently used technique for increasing the mechanical interlocking of NFPCs. Natural fibres contain fat and wax substances that smooth the fibre surface and have a detrimental impact on interlocking. Alkali solution can be used to eliminate fat and wax, thereby roughening the surface of the fibre. As a result, mechanical interlocking can be enhanced. Following alkali treatment, grafting NPs to the NFr surface can augment the roughness and wettability of the fibre surface, hence improving IFB. Zinc oxide NPs have been increasingly common in NFPC research because they possess NPs characteristics and exhibit high chemical reactivity due to their intrinsic functional groups, water repellence, and resistance to ultraviolet radiation (Mohammed et al. 2019). Inorganic NPs were integrated into the cell wall and likely occupied vacuum spaces (micropores) that would otherwise be exposed to water molecules, lowering the hygroscopic qualities of the modified natural fibres (Donath et al. 2004; Hill 2007).

On the other hand, during hydrolysis and polycondensation, the hydroxyl groups of cell wall constituents that are primarily responsible for moisture absorption were probably blocked by forming hydrogen bonds with inorganic NPs, which results in a decrease in hygroscopicity, significantly improves IFB, and further increases other mechanical characteristics, such as tensile strength. Mohammed et al. (2019) reported that zinc oxide nanoparticle-treated kenaf fibre-reinforced unsaturated polyester composites exhibited increased tensile strength compared to untreated composites. The modulus, Tensile strength, break at elongation, flexural modulus, flexural strength, and impact strength of the composites with higher content kenaf/ZnO nanoparticle are 560 MPa, 58MPa, 1.8%, 1300 PMa, 68 MPa, and 31 MPa, respectively. As a result of the amalgamation of zinc oxide nanoparticles, the mechanical characteristics and IFB were improved, and the contact angle results revealed an improvement in wetting of the fibres with the addition of ZnO nanoparticles. Notably, the presence of water molecules at the interface showed a significant effect on the mechanical interlocking. Water in the interface, acting as a lubricant, would exacerbate the slip between the fibre and matrix. On the other hand, the shrinkage and swelling ratios of the fibre and matrix are different. By increasing the moisture content, the residual clamping stress on the interface induced by the shrinkage differential between the fibre and matrix can be reduced (Liu et al. 2015).

Permeability is another fibre-related aspect that affects matrix penetration. Permeability varies according to the surface properties and direction, for example, tangential, radial, and longitudinal. This mechanical interlocking process is commonly employed in polymer composites by etching the polymer surface to augment surface roughness, hence augmenting the surface contact with the fibre and mechanical interlocking (Kim and Pal 2010). In contrast, an augment in mechanical interlocking results in the improvement of other bonding systems.

In general, the fibre-matrix interfacial bonding mechanisms include inter-diffusion, electrostatic adhesion, chemical reactions, and mechanical interlocking. These mechanisms work together to cause adhesion, and one of them is usually dominant. These particular phenomena occur predominantly in all almost natural fibr composites, but they can be minimised or reduced to minimal with a very good process (Zhou et al. 2016).

Besides the aforementioned risk, an additional risk of utilizing NFr is the poor compatibility between fibres and polymer matrix, which results in a non-uniform dispersion formation of fibres within the matrix, and thus generates poor interfacial bonding characteristics. Most polymers, especially thermosetting, are non-polar (hydrophobic, water repelling) substances, which are not compatible with polar (hydrophilic, water attracted) wood fibres, and therefore cause poor adhesion between the fibre and matrix interaction surfaces. That will be discuss in the next section (Mohd Nurazzi et al. 2017).

COMPATIBILITY BETWEEN NATURAL FIBRE AND SYNTHETIC POLYMER

The cellulose structure of the fibres is differentiated by crystalline and amorphous areas. The crystallite area forms a large number density of intramolecular hydrogen bonds, which are strong in comparison to van der Waals interactions. This results in the formation of a cellulose block, making it harder for other chemicals to penetrate. However, the amorphous region readily absorbs dyes and resins. The hydrophilic hydroxyl groups in this area combine with water molecules from the surrounding environment. Water molecules are generally held by hemicellulose, lignin, pectin, and waxy compounds. This results in the NFr being hydrophilic and polar, which reduces its compatibility with non-polar/hydrophobic matrixes (Mwaikambo and Ansell 2002; Abdelmouleh et al. 2007). Polymeric matrices are the most commonly utilised in NFPCs today since they are light weight and can be produced at low temperatures. Both thermoplastic and thermoset polymers have been employed for matrices with NFr (Holbery and Houston 2006). Matrix selection is constrained by the temperature at which natural fibres degrade. Most of the NFr utilized for reinforcement in NFPCs are thermally unstable above 200 °C, although under some circumstances it is possible for them to be processed at higher temperature for a short period of time (Summerscales et al. 2010). Due to this constraint, only thermoplastics that soften below this temperature such as polyethylene (PE), polypropylene (PP), polyolefin, polyvinyl chloride, and polystyrene and thermosets (which can be cured below this temperature) are useable as a matrix material (dos Santos et al. 2008). Indeed, PP and PE are the two most commonly adopted thermoplastic matrices for NFPCs. The main thermosets used are unsaturated polyester (UP), epoxy resin, phenol formaldehyde, and vinyl ester (VE) resins. This hydrophilic property impedes the effectively react with the matrix.

Additionally, pectin and waxy materials coat the reactive functional groups of the fibre, acting as a hindrance to matrix interlock. To dilate the crystalline region, remove hydrophilic hydroxyl groups, remove surface impurities (waxy substances), and improve the effectiveness of interfacial bonding, the surface of the fibre must be treated using various chemical treatments, reactive additives, and coupling agents. Chemical and physical modification of natural fibres is required (Dash et al. 2000).

MODIFICATIONS OF NATURAL FIBRE COMPOSITE TO IMPROVE INTERFACIAL ADHESION

High Energy Treatments

Without utilising chemical agents, physical treatments of natural fibres change their structure and surface characteristics, consequently affecting their mechanical interaction with the polymer matrix. Radiation and discharge treatments such as gamma radiation, corona, and plasma treatments are the most frequently utilized physical treatments in NFr composites for improving the functional properties and changing in the surface structure and surface energy of the NFr (Gassan and Gutowski 2000; Khan et al. 2009a).

Gamma radiation treatment

Gamma radiation treatment of composite materials is not a novel concept. The majority of research examined the effects of gamma radiation on the various characteristics of composite materials. Gamma radiation might be an excellent alternative due to several advantages, including reduced time requirements, uninterrupted operation, environmental friendliness, and design flexibility (Shubhra and Alam 2001; Khan et al. 2009b). Gamma radiation is powerful ionising radiation that can alter the internal structure of a material and decrease its hydrophilicity, resulting in improved crosslinking between the NFr and matrix (Khan et al. 2009c). Numerous scientists have investigated the effect of gamma radiation on composite materials. Khan et al. (2009b) studied the mechanical properties of jute fabric-reinforced polypropylene composites following gamma radiation treatment and reported that irradiated composites outperformed all other non-irradiated composites (Khan et al. 2009c). Hoque et al. (2017) investigated the influence of gamma radiation on the mechanical qualities of raw and polyethylene glycol-modified bleached jute-reinforced polyester composites and discovered that gamma radiation increased mechanical properties up to a point, then began to degrade them. Gamma and electron beam radiation was also used on a PAN carbon fibre-based composite by Jafari et al. (2016). They revealed that, by increasing the gamma and electron doses, the thermal behavior of the composites indicated a higher decomposition degree as a function of the temperature. Besides these, Martínez-Barrera et al. (2020) studied the effects of gamma radiation on the physicochemical properties of polyester resin. According to their findings, commercially available catalysts are insufficient for the complete polymerization of polyester resin. In contrast, gamma radiation can complete such activity, resulting in a high degree of cross-linking and morphological change on the surface when used as a matrix on a composite material.

Corona discharge treatment

Corona discharge treatment is an effective method for surface modification of cellulosic fibres used to fabricate composite materials. It is a cost-effective and environmentally friendly treatment that improves the NFMIF in composites. Corona discharge is an alternative technology for modifying the surface property of fibres. It is used to activate the surfaces of synthetic polymers (for example, polyethylene before printing) and woods (e.g., graft polymerization of the vinyl monomer or ethylene imine to wood) (Uehara and Sakata 1990; Dong et al. 1993). Corona discharge is frequently utilized to enhance the wettability, adhesion, and hydrophilic properties of materials (Belgacem et al. 1994; Ooi et al. 2004) or to graft molecules on polymeric surfaces (Seto et al. 1999; Benhadi et al. 2011).

Compared with chemical activation, corona discharge activation of polymers (which includes lignocellulosic materials) has numerous advantages: exceptional efficiency and commercial feasibility (Mihailovic et al. 2011); environmental acceptability as a solvent-free, continuous process (Benhadi et al. 2011); capacity to modify surface properties without impacting bulk properties (Lei et al. 2000), can be conducted in a variety of atmospheres (Benhadi et al. 2011), and allows for the grafting of a wide variety of species (Bataille et al. 1994; Bataille et al. 1991). Treatment variables such as time and current determine the degree of improvement. Sakata et al. (1993), demonstrated that increasing the degree of corona treatment reduces the contact angle of a droplet of urea-formaldehyde resin on a wood surface. Improved wettability was said to contribute to increased binding strength in that situation.

The treatment induces oxidation on the surface of the specimen in the presence of oxidizing agents such as atomic oxides, oxygen-free radicals, and ozone. Due to oxidation, the surface of the specimen is cleansed while also increasing its mechanical characteristics (Tuominen et al. 2010). Corona treatment improves the bonding and compatibility of fibres and fillers, as Gholshan Tafti et al. (2018) concluded in their study. Tuominen et al. (2010) found that corona treatment improved the oxidation level and surface energy of the specimen. After the corona treatment, Gassan and Gutowski (2000) found fibre and matrix compatibility and specimen surface oxidation increased.

Plasma treatment

Plasma treatment is another effective physical approach for enhancing the surface characteristics of NFr and polymeric materials by harnessing high-energy photons, electrons, ions, radicals, and excited species. Plasma treatment introduces polar or excited groups onto the fibre surface or grafts a new polymer layer onto the fibre surface, allowing for strong covalent bonds between the NFr and the polymer matrix. Additionally, this process roughens the fibre surface, improving mechanical interaction between the fibres, polymer matrix, and IFB (Yuan et al. 2002). The alteration of NFr by treatment in cold oxygen plasma produced from a corona discharge under ideal operating circumstances transforms the fibre into a semi-active filler for the composite (Vladkova et al. 2004, 2006). As a result, the adhesion at the IFB region is augmented with the plasma treatment. The consequence is an enhancement in the mechanical characteristics of the composites (such as flexural strength and modulus, tensile strength, and modulus). Nevertheless, the fibre may degrade over a prolonged exposure time because of the continual effect of particles on the surface, which eventually reduces the IFB between NFr and the matrix (Morales et al. 2006; Oporto et al. 2009).

When subjected to electric field energy, free electrons in the plasma atmosphere begin colliding with neutral gas molecules and transfer energy. This results in the production of reactive species, which are predominantly made up of ions. By connecting these excited ions to the solid surface, the chemical and physical modification on the surface of the material is established. When treated with plasma, the NFr does not change its bulk properties; it only modifies its surface characteristics (Sun 2016).

Macedo et al. (2019) used cold plasma to treat kapok fibres and found changes in the interfacial adhesive characteristics of the polyethylene reinforced kapok fibre composite. After treatment, it was noticed that the mechanical strength of the specimen was augmented, while its thermal stability decreased. Gibeop et al. (2013), investigated the effects of plasma treatment on the mechanical parameters of a jute fibre composite reinforced with polylactic acid (PLA). It was revealed that following treatment, the strength of the specimen rose due to the plasma treatment’s “heating and etching action,” which also increased adhesion by roughening the surface of the fibre. Yuan et al. (2004), examined the change in mechanical characteristics of polypropylene composite reinforced by wood fibre. Increased interfacial adhesion and viscoelastic dissipation were seen due to the roughening effect on the fibre surface. Fazeli et al. (2019), studied the effect of atmospheric pressure plasma treatment on the interfacial adhesion of starch matrix and cellulosic fibre. It was observed that after plasma treatment at low pressures, the tensile strength of the specimen rose dramatically due to improved interfacial adhesion between matrix and fibre.

The primary impacts of plasma treatment on NFr are as follows: Cold plasma treatment can improve the mechanical properties of natural fibres. A considerable augment in tensile strength can be noticed when treated with low pressure air plasma, owing to the increased interfacial adhesion between the NFr and matrix. Surface modification by oxygen plasma treatment results in a significant increase in qualities such as wettability, toughness, compatibility, and IFB. Physical approaches do not alter the chemical composition of fibres, but they do change their surface and structural qualities. Table 1 summarises the significant impacts of different physical treatments on NFPCs.

Table 1. Impact of Physical Treatment on Natural Fiber Composites

Chemical Treatments

Natural fibres are hydrophilic due to their lignocellulosic composition, which contains highly polar hydroxyl groups. As a result, these fibres are fundamentally incompatible with hydrophobic thermoplastics such as polyolefins. The major limitations of employing these fibres as reinforcements in such matrices include poor interfacial adhesion between polar-hydrophilic fibres and non-polar hydrophobic matrix, and difficulties in mixing due to poor wetting of the fibres with the matrix. Therefore, it is imperative that NFr should be subjected to chemical modification to increase the compatibility and adhesion between NFr and matrix (John and Anandjiwala 2008). It would also be desirable that the chemical used for the modification of NFr preserves the biodegradable nature of NFr. Ideally, the chemicals utilised for modification should be derived from sustainable sources as well (Kabir et al. 2013).

Natural fibers have also been treated with various chemicals such as alkali, silane, water repelling agents, peroxides, permanganates, etc. It has been observed that some of these chemical treatments (for example, alkali treatment) can significantly enhance the mechanical characteristics of NFr by modifying their crystalline structure, as well as by removing weak components such as hemicelluloses and lignin from the fiber structure (Li et al. 2007). Also, moisture absorption and subsequent swelling of NFr can be reduced through selective chemical treatments (e.g., water repelling agents). Furthermore, chemical treatments (for example, using silane coupling agents) can increase the fibre-matrix interfacial interactions by forming strong chemical bonds, resulting in a significant improvement in composite mechanical performance (Xie et al. 2010).

Natural fibers are amenable to chemical surface treatment due to the presence of hydroxyl groups. The hydroxyl groups may participate in hydrogen bonding inside cellulose molecules, activating these groups, or they may introduce new moieties that form effective interlocks within the system. Chemical modification can increase surface properties such as wetting, adhesion, surface tension, and fibre porosity. The irregularities of the fiber surface play an important role in the mechanical interlocking at the interface. The interfacial qualities can be improved by modifying the components, which results in changes in physical and chemical interactions at the interface (John et al. 2008; John and Anandjiwala 2008).

Cao et al. (2006) examined the mechanical characteristics of bagasse fiber-reinforced polyester composites before and after alkali treatment. NaOH solutions of 1, 3, and 5% concentrations were employed. Superior properties were obtained for composites made from 1% NaOH-treated bagasse fibers. The authors observed a 13% improvement in tensile strength, 14% in flexural strength, and 30% in impact resistance, respectively, due to chemical modification. Researchers have reported the utilized of benzoic acid for chemical modification of bagasse fibers in polyvinyl chloride composites (Zheng et al. 2007). It was found that the tensile strength of modified composites rose by around 35 %, indicating improved reinforcing. The tensile strength of composites containing untreated fibres was 38 MPa, while that of composites with chemically modified fibres was 52 MPa.

Xue et al. (2007) investigated the influence of the maleic anhydride grafted polypropylene (MAPP) coupling agent on the mechanical characteristics of aspen fiber-reinforced polypropylene (PP) composites. The coupling agent enhanced the adhesion between fiber and matrix and augmented the tensile characteristics of the composites. The tensile strength of the composites containing compatibilizer increased by 15% and flexural strength registered an increase of 40%. The greater interfacial adhesion was also evident. In an interesting study, Teramoto et al. (2004) studied the biodegradation of aliphatic polyester composites reinforced by abaca fiber. Abaca fibers, chemically modified with acetic anhydride, showed greater resistance to biodegradation due to the existence of a protective covering on the fibers.

Alkaline treatment

When natural fibres reinforce thermoplastics and thermosets, alkaline treatment or mercerization is one of the most often utilized chemical treatments. The most significant modification caused by alkaline treatment is the disruption of hydrogen bonding in the network structure, hence increasing the roughness of the surface (Alnaid et al. 2018; Dahham et al. 2018). This treatment eliminates a certain amount of lignin, wax, and oils that cover the external surface of the fibre cell wall, depolymerizes cellulose and exposes the short length crystallites (Mohanty et al. 2001). The sensitive hydroxyl groups (OH) are degraded throughout the treatment and hence react with water (H-OH), leaving the ionized reactive molecules to produce alkoxide with NaOH (Eq. 2) (Li et al. 2007; Huda et al. 2008).

Fibre-cell-OH + NaOH = Fibre-cell-O-Na + H2O + Impurities (2)

As a result, the surface of the fibre becomes clean. As long as the matrix material distributes well over the cellulosic material, this is likely to expose more microvoids in the cellulosic material, which can be conducive to bonding. In other words, the removal of hydrophobic substances from the surface can result in a rougher surface, hence enhancing the stress transmission capacity between the ultimate cells. Furthermore, it reduces the diameter of the fibre and hence increases the aspect ratio (length/diameter). This increases the effective surface area of the fibre, which is necessary for optimal adherence to the matrix (Joseph et al. 2003; Alnaid et al. 2018; Dahham et al. 2018). The ionization of the hydroxyl group to the alkoxide is promoted by adding aqueous sodium hydroxide (NaOH) to NFr (Agrawal et al. 2000). Thus, alkaline processing directly affects the cellulose fibrils, the degree of polymerization, and the extraction of lignin and hemicellulosic compounds (Jahn et al. 2002). Alkaline treatment has two impacts on the fibre: (1) it enhances surface roughness, resulting in improved mechanical interlocking, and (2) it increases the amount of cellulose exposed on the fibre surface, hence increasing the number of potential reaction sites (Valadez-Gonzalez et al. 1999). This treatment dramatically improves the mechanical and thermal properties of the NFPCs. Excess delignification of the fibre can occur if the alkali concentration is higher than the recommended level, resulting in weakening or damage to the fibres (Li et al. 2007; Wang et al. 2007; Dahham et al. 2018). A schematic representation of the cellulose fibre structure before and after an alkali treatment is shown in Fig. 7. Treated fibres have lower lignin content, partial removal of wax and oil cover materials and distension of crystalline cellulose structure.

Cai et al. (2016) demonstrated that a 5% alkaline treatment enhanced the mechanical characteristics of abaca fibre reinforced epoxy composites. The 5 wt% alkali treatment increased the tensile strength and young’s modulus of composites by 8% and 35% respectively as compared to untreated abaca fibers. Also, Vilay et al(2008), carried out a study on the effect of chemical treatment of sodium hydroxide (NaOH) and fiber loading on mechanical properties of bagasse fiber reinforced polyester composites and and concluded that the higher tensile and flexural values were attained for treated fiber composites compared to those of untreated fiber based composites. The influence of alkali treatment on mechanical characteristics of bagasse fiber reinforced polyester composites was investigated by Cao et al(2006). From this study, the authors found that the alkali treatment improved the tensile strength by 13%, flexural strength by 14%, and impact energy by 30%, respectively.

Fig. 7. The schematic drawing of the hierarchical structure of natural fibres (a); the alkaline degradation process of natural fibres (b)

Silane treatment

The word silane, which literally has the chemical formula SiH4, also can be used to represent a family of reagents that can be used to modify cellulosic surfaces. Thus, silanes have been utilized as coupling agents to treat the surface of NFr. A typical silane coupling agent contains two reactive groups: one end with alkoxysilane groups reacts with hydroxyl-rich surfaces, such as wood or other NFr, while the other end interacts with the polymer matrix, allowing the NFr to adhere to the polymer matrix and thus stabilize the composite material (Zhou et al. 2015). Silane coupling agents may lower the number of cellulose hydroxyl groups in the IFB between NFr and matrix.

Trialkoxysilanes have received much attention as effective and practical agents for modification of lignocellulosic materials (Szlek et al. 2022). In the presence of moisture, the hydrolysable alkoxy group leads the production of silanols. The silanol then interacts with the hydroxyl group of the fibre, generating stable covalent bonds to the cell wall that are chemisorbed onto the surface of NFr. The fourth substituent attached to the silicon atom in a trialkoxysilane is often selected to be an alkyl chain (Szlek et al. 2022). As a result of the diffusion of hydrocarbon chains into the matrix, the hydrocarbon chains generated by the application of silane restrict the swelling of the fibres by establishing an entangled network (Dhakal et al. 2007). It was hypothesised that the hydrocarbon chains generated by the application of silane affected the wettability of the NFr, hence enhancing the chemical affinity of the polymer matrix.

Silane treatment boosts the tensile strength of the NFPCs, decreases the impact of moisture on the characteristics of the NFPCs, and augments adhesion, and thereby the strength of NFPCs (George et al. 2001; Bogoeva-Gaceva et al. 2007; Li et al. 2007). The surface of NFr has micro-pores, and silane coupling agents are surface coatings. This penetrates into the pores and develops mechanically interlocked coatings on fibre surfaces. Silane-treated fibre composites have been found to have better tensile strength characteristics than alkali-treated fibre composites (Valadez-Gonzalez et al. 1999). The most commonly used silanes are amino, methacryl, glycidoxy, vinyl, azide, and alkylsilanes. During the silane treatment of natural fiber as shown in the Fig. 8, the hydrolysis of alkoxy groups on silane takes place to form silanol (Si–OH) groups, which can then react with hydroxyl groups on the fiber surface (Pickering et al. 2016).

Fig. 8. Reaction of silane with NFr (R representing organic group, … representing hydrogen bonding) (Islam et al. 2021; Creative Commons Attribution (CC BY 4.0))

In a study by Seki (2009), the impact of oligomeric siloxane treatment of jute fabrics on mechanical characteristics of jute/epoxy and jute/polyester composites was studied. At first, the jute fabrics were treated with 5% (w/w) NaOH solution for 2 h, and then the alkali-treated jute fabrics were treated with 1% siloxane. Hand lay-up was used to make the jute/epoxy and jute/polyester composites. The mechanical characteristics such as tensile, flexure, and interlaminar shear strengths of the siloxane treated composites were increased by ~32%, ~22%, and ~109% for jute/epoxy composite and ~31%, ~37%, and ~103% for jute/polyester composite compared to untreated jute fiber composites. In the work of Wang et al. (2020), the surface of the bamboo fibers (BF) was treated with three kinds of silane coupling agents terminated with amino functional groups (KH550), epoxy functional groups (KH560), and methyl functional groups (KH570) to enhance IFB. The effects of silane treatment on the mechanical characteristics and thermal behavior of BF/polypropylene (PP) composites were examined. Mechanical test results showed that the order of modification effectiveness was KH570 > KH550 > KH560. KH570-treated fiber composite exhibited the best mechanical characteristics. The tensile strength and flexural strength of 5 wt% KH570 treatment reached to 36.1 and 54.7 MPa, which were 15.4% and 23.6% higher than those of UBF/PP composites.

Chemical coupling agents are often molecules with dual functionalities. The first function is to react with hydroxyl groups of cellulose and the second function is to react with functional groups of the matrix. Bledzki and Gassan (1999), outlined several mechanisms of coupling in materials, namely: (a) elimination of weak boundary layers; (b) creation of a strong and malleable layer; (c) The formation of a strongly crosslinked interphase area with a modulus intermediate to that of the substrate and that of the polymer; (d) improvement of the wetting between polymer and substrate; (e) formation of covalent bonds with both materials; and (f) alteration of acidity of substrate surface. Figure 9 shows the coupling agent (i.e., silane) mechanism between the hydrophilic fiber and hydrophobic polymer matrix. In this case, the compatibilizing agent is chemically bonded to the fiber and blended by wetting in polypropylene (PP) polymer chain (Arjmandi et al. 2017). As compatibilizer polyethylene-octene grafted maleic anhydride (POE-g-MAH) demonstrated faster mixing and better dispersion when blended with PP compared with conventional polyolefin elastomers (Lim et al. 2008; Bai et al. 2004). It is interesting to note that POE-g- MAH acts as compatibilizer by forming hydrogen bond between maleic anhydride group of POE and hydroxyl group of fibers.