Chemical,biological,and nanoclay treatments for natural plant fiber-reinforced polymer composites: A review

Abstract

Natural plant fiber-reinforced polymer composites have been in the limelight in the field of materials science for their mechanical properties, economy, and eco-friendliness. Properties of such novel composites depend on the adhesion and interaction between the fibers and the polymer matrix. Consequently, poor interaction can lead to declined mechanical properties, particularly strength. Surface modifications of fibers are carried out to enhance the bonding with the matrix by certain chemical treatments that remove hydroxyl groups in the amorphous cellulose region, making them hydrophobic and hence compatible with the matrix. Doing so also strengthens the composites, widening their scope of application. This review article provides comprehensive information about various surface modification techniques that include alkali, silane, acetylation, permanganate, peroxide, benzoylation, acrylation, acrylonitrile grafting, isocyanate, addition of maleated coupling agents, and fungal treatments. The working mechanisms and the effects of such treatments on mechanical strength are also elucidated. Furthermore, this review provides an overview of nanoclay inclusion in polymers, their addition techniques, and the augmentation of the mechanical properties of polymer matrix composites. The article concludes along with the field of applications, summary of pertinent challenges, and directions for future work.

Introduction

Environmental consequences of practices have led to a search for biodegradable materials for day today applications. Industries now give importance to sustainability and energy costs besides considering the strength and processability of materials for the application. Not until two decades back when the recyclability of materials was burgeoning, the renewable raw natural plant fibers saw the limelight. ¹ With the involvement of natural plant fiber-reinforced composites (NFRCs) in current research, extensive work has been carried out in implementing them in automotive and construction industries.^2-5

The salient features of the NFRCs include low density, low cost, good specific strength properties, ⁶ and high biodegradability. ⁷ These composites have better mechanical properties than their pure polymer counterparts.^8-11 In a comparative study with glass fibers, NFRCs have been verified as environmentally superior^12,13 consuming 80% less energy for production. ¹⁴ Their low abrasive nature makes them a better choice than glass fibers from manufacturing and safety aspects. The major limitations of NFRCs include water retention and poor thermal stability. ¹⁵ These properties depend on the structure which, in turn, is influenced by the conditions of growth, such as land, climate, and age of plant.

Though the idea of creating biofriendly materials is captivating for industrialists and environmentalists, the NFRCs are far below the glass fiber composites when mechanical properties are compared. The need for improving the mechanical properties of NFRCs has created a foray into a new set of research in composite science, which has highlighted the main cause for lower values of mechanical properties as the inadequate bonding between the natural plant fibers and the polymer matrix. ¹⁶ Having provided a direction to work on, a major research effort is involved in treatments of fiber surfaces that make them compatible with the matrix to provide greater strength for the composite.

In the quest for identifying procedures to increase the composite strength, surface treatments of fibers have proved to be of great help.^17-20 These treatments target the amorphous cellulose region to improve compatibility between the fiber and the polymer matrix. The amorphous portion of surface cellulose contains multiple hydroxyl groups, which render the fibers a polar nature and result in poor bonding with the predominantly nonpolar polymer resin. ²¹ Surface treatments help in enhancing the surface properties or reducing the polar nature by the removal of hydroxyl groups, thus improving the fiber–matrix adhesion and providing higher mechanical strength. ²² In recent years, the introduction of nanoclay dispersion in polymers has raised the attention due to their commendable property of strength enhancement through exfoliation. With 42% of the polymer global market focused on packaging, nanoclay/natural plant fiber hybrid composites are considered to be promising materials. ²³

Components of natural plant fibers

Plant fibers are lignocellulosic in nature, containing cellulose, hemicellulose, lignin, pectin, and waxy substances, as shown in Figure 1(a) and (b). Their relative proportions dictate the mechanical properties of the NFRCs along with aspect ratio, morphology, hydrophilic tendency, and the dimensional stability of the fibers used. The major component is the cellulose, as shown in Figure 2, which provides strength, stiffness, and structural stability to the fiber, having multiple hydroxyl groups. ¹ The cellulose in the fiber cell wall is present as crystalline regions and amorphous regions. The strong intramolecular hydrogen bonding in the crystalline region makes it impenetrable by other chemicals, including water. On the other hand, the amorphous regions easily absorb resins and dyes. The hydroxyl groups in this region easily combine with water molecules, making the fibers hydrophilic and polar in nature. ¹⁷

Figure 1.

(a) Structure of a plant fiber and (b) structural organization of the constituents in the fiber cell wall. ¹⁷

Figure 2.

Chemical structure of cellulose. ¹⁷

Hemicellulose is a group of polysaccharides (excluding pectin) that remains on removal of lignin, as shown in Figure 3. With low degree of polymerization, it exhibits high chain branching. Such branching strengthens the cell walls by interaction with cellulose and in some walls with lignin. Lignin has an aromatic structure and is amorphous in nature, as shown in Figure 4. While cellulose content increases from the primary to the secondary layers, lignin content decreases in this sequence. Lignin is less hydrophilic than the cellulose and hemicellulose. With an increase in lignin content, water absorption is reduced. Pectin comprises complex polysaccharides cross-linked with calcium ions and arabinose sugars. Lignin and pectin are coupled with the cellulose–hemicellulose network and provide adhesion between the molecules. Waxes, being minor elements of plant fibers, constitute different types of alcohols, which are insoluble in water and in several other acids. The chemical compositions of selected plants are presented in Table 1.

Figure 3.

Chemical structure of hemicellulose. ¹⁷

Figure 4.

Chemical structure of lignin. ¹⁷

Table 1.

Chemical composition of selected plant fibers.^17,24-26

Fibers	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Pectin (%)	Ash (%)
Abaca	56–63	15–17	7–9		3
Areca	57.35–58.21	13–15.42	23.17–24.16	—	—
Bagasse	55.2	16.8	25.3	—	—
Bamboo	26–43	15–26	21–31	—	2.3
Banana	60–65	6–19	5–10	—	9
Coconut	32–43.8	0.15–20	40–45	—	—
Cotton	82.7	5.7	0.7–1.6	—	—
Flax	71	18.6	2.2	20.6	2.3
Hemp	57–77	14–22.4	3.7–13	0.9	0.8
Henequen	77.6	4–8	13.1	—	—
Jute	41–71.5	13.6–22	12–26	0.2	0.5–2
Kenaf	31–57	21.5–23	15–19		2–5
Palm	60–65	—	21–29	—	—
Ramie	68.6–91	5–16.7	0.6–0.7	1.9	—
Sisal	47–78	10–24	7–11	10	0.6–1

Higher concentrations of hydroxyl rich cellulose, hemicellulose, and lignin attribute to poor bonding with matrix materials. ²⁷ The extent of interfacial interaction between the fiber and the matrix determines the stress transfer ability of the composite. ²⁸ In addition to the fiber composition, the interfacial bonding depends on the surface morphology of the fiber, polarity of the matrix, and the presence of reactive probes in the matrix. Hence, chemical treatments that use reagents to alter the structure and composition of fiber surface have a great tendency to lower moisture absorption and improve stress transfer capabilities.

The fiber failure occurs due to fibril slip under the application of tensile stress. As a result, the total stress is shared by only a few cells. Further increase in the stress leads to fiber failure by cell wall rupturing and decohesion of cells. Surface modifications remove the amorphous components in the fibrillar structure, thus changing the deformation behavior of the fibers. The brittleness of the fiber is substantially reduced upon treatments. Table 2 presents the mechanical and physical properties of plant fibers.

Table 2.

Mechanical and physical properties of selected plant fibers.^24,26,29,30

Fibers	Density (g cm−3)	Tensile strength (MPa)	Young’s modulus (GPa)	Moisture (wt%)	Elongation (%)
Abaca	1.5	400	12	15	3–10
Areca	0.7–0.8	147–322	1.12–3.15	7.32	10.23–13.15
Bagasse	1.25	290	17	8.8	1
Bamboo	1.5	575	27	—	—
Banana	1.3–1.35	345–520	17	—	1.7–2.6
Coconut	1.15–1.46	95–230	2.8–6	8	15–51.4
Cotton	1.5–1.6	287–597	5.5–12.6	8.5	7–8
Flax	1.4–1.5	345–1500	27.6–80	12	2.7–3.2
Hemp	1.48	550–900	70	12	1.6
Jute	1.3–1.5	200–450	20–55	17	1.5–1.8
Kenaf	1.22–1.4	295–1191	22–60	17	1.6
Palm	0.7–1.55	80–248	0.5–3.2	—	17–25
Ramie	1.5	220–938	44–128	8.5	2.5
Sisal	1.33–1.5	400–700	9–38	14	2–2.5

Surface modifications by chemical treatments

The two primary components of the fiber polymer composites, viz., fibers and matrix influence the mechanical properties of the composite. ³¹ Fibers carry the imposed load while matrix transfers the load between the fibers. Furthermore, the fiber–matrix interface should have enough strength for loads to traverse the interface without inducing cracking and delamination. Tensile strength of the fiber is influenced by the refinement of the fiber surface. The voids and the amorphous portions of the fiber are responsible for about 10% of moisture content. ³² With regard to water absorption, refinements by chemical treatments enhance the hydrophobicity of the fibers. However, the chemically treated fiber polymer composites show higher water absorption when compared to pure polymer (without reinforcements) due to decreased fiber–matrix adhesion. ³³ So, it is important for the user to define the required mechanical properties of the composite to find the “sweet spot” between the untreated NFRCs and pure polymers. These treatments are aimed at lowering the concentrations of voids and amorphous regions in the fibers.

Composites containing chemically treated fibers exhibit superior mechanical properties than untreated fiber-reinforced composites owing to better packaging of cellulose chains upon dissolution of lignin.^34,35 Priya and Tiwari ³⁶ showed that an improved interfacial bonding can be achieved through alkali, permanganate, benzoyl chloride, maleic anhydride (MA), and silane treatments, which improve the damping properties of the composite in comparison to those reinforced with untreated fibers. The commonly used surface modification treatments are described in the following sections.

Alkaline treatment

Alkaline treatment or mercerization involves the application of sodium hydroxide (NaOH) to the fibers. Alkali-sensitive hydroxyl groups (OH) present along the surface of fibers react with NaOH and release water molecules from the process. On reduction of the number of hydroxyl groups, the moisture absorption tendency is lowered. In addition, it also dissolves certain amounts of hemicellulose, lignin, pectin, and the waxy layer. The reaction mechanism between the fiber and alkali is shown in equation (1)

$\text{Fiber}-\text{OH}\hspace{0.17em}\text{+}\hspace{0.17em}\text{NaOH}\to \text{Fiber}-\text{O}-\text{Na}+{\text{H}}_2\text{O}$

1

Alkali treatment results in the improvement of fiber surface adhesion characteristics by producing a rough surface topography, which enhances the mechanical interlocking at the interface. ²⁸ This is achieved by removing natural and artificial impurities and disrupting hydrogen bonding in the network structure.^26,37 The fiber is also made more thermodynamically stable.^38,39 Alkali treatment eliminates microvoids, making the surface more even and improving the stress transfer capacity. A decrease in the fiber diameter that resulted an increase in aspect ratio can be noticed on NaOH treatment, which leads to good fiber–matrix interactions because of a larger effective surface area. ¹⁷ Being effective in removing the waxy layers and increasing the effective surface area, mercerization is often used as a pretreatment of fibers before other chemicals are used for strengthening.

Mechanical, thermal, and water retention behaviors are significantly improved with alkali/NaOH treatment. The concentration of NaOH solution and the soaking time required for obtaining the best mechanical properties should have optimum values. Increasing concentrations have been associated with the increase in mechanical properties. Further increase in concentration beyond the optimum value resulted in delignification of fibers and hence in damage.^26,40 An optimal 5% NaOH-treated Tridax procumbens fiber gives increased wettability, crystallinity, reduced amorphous region, and fiber diameter. ⁴¹ Alkaline treatment also improves the tensile modulus and the flexural strength of the NFRCs. ⁴² Table 3 provides an overview of the acceptable concentrations of NaOH and the soaking time, which give an optimum strength, before delignification, which leads to wear and tear of the fiber.

Table 3.

Overview of acceptable concentrations of NaOH and its soaking time.

Fiber–matrix composite	Applied treatment	Results	Reference
Oil palm-PF	Fibers were treated with 5% solution of NaOH for different time intervals, such as 72, 48, and 24 h at room temperature (≈24°C).	Highest flexural strength (24 h treated) as compared to other chemical treatments.	28
Jute-PLA	Fibers were soaked in 5, 10, and 15% NaOH solution for 6 h at 70°C and neutralized with 50% acetic acid. Composites were prepared using 0%, 5%, 10%, 15%, 20%, and 25% fibers.	Decrease in fiber diameter and density. Increase in tensile strength by 7.7% at 25% fiber loading and 10% NaOH compared to untreated fibers.	40
Jute-PP		Higher tensile strength was obtained at 20% fiber loading and 10% NaOH with the increase of 16% compared to untreated fiber.	43
Napier grass	Fibers were soaked in 10% NaOH for various time periods (3, 6, 12, 18, and 24 h).	Least fiber diameter and mass at 12 h. The 6 h soaking time yielded the highest strength. However, the fiber soaked for 24 h was observed with a higher degree of fiber surface damage. The fiber roughness was increased if the soaking time exceeded 18 h.	44
Henequen-HDPE	Fibers were immersed in 2% (w/v) NaOH solution at 25°C for 1 h.	It takes 100 h to attain adsorption equilibrium. However, silane treatment exhibited higher adsorption rate.	45
Sisal-PP	The fibers were treated in 6% NaOH at 21°C for 45 s, 15 min, and 45 min.	The fibers subjected to 45 min of treatment showed high levels of crystallinity with a more loosened cell wall structure. The average increase in tensile and shear strengths was 12.04% and 173%, respectively.	46
Sisal-LDPE	Fibers were immersed in 10% NaOH for 1 h.	Decrease in sorption capacity due to increase in crystallinity.	47
		Tensile properties increased with fiber loading and were more prominent for longitudinal orientation of fibers. Optimum fiber length was 5.8–9 mm.	48
Ladies finger fiber	Fibers were soaked in 2% NaOH for 2.5 h at 70°C.	Removal of hydrophilic hemicellulose improved surface roughness.	49
Kenaf-MAPP/MAPE	Fibers were soaked in 3%, 6%, and 9% NaOH for 24 h.	Optimum tensile strength and modulus of elasticity were observed for 6% NaOH samples.	50
Kenaf bast fibers	Fibers were soaked in 3%, 6%, and 9% of NaOH for 3 h at room temperature. Additionally, for the 6% NaOH concentration, the fibers were immersed in water bath at 95°C.	Cleanest fiber surface observed in 9% NaOH, although tensile strength decreased. However, 6% NaOH with higher temperature was effective in the cleaning process.	51
Banana-PLA	Fibers were soaked in 1 N NaOH for 1 h with 1, 3, and 5 wt% of fibers.	Slight increases in tensile modulus, impact and tensile strength at 3 wt% of fiber.	52
Banana-PP	Fibers were immersed in 2% and 10% NaOH for 1 h.	10% NaOH treatment showed a considerable increase in thermal conductivity.	53
Banana-PF	Fibers were treated with 4% NaOH solution for 2 h.	Slight increases in tensile strength and modulus. 117% increase in flexural strength was observed.	54
Banana/glass-PS	Chopped banana fibers were soaked in 1% NaOH for 1 h at room temperature (≈24°C).	The alkali treatment showed the best results for tensile and flexural properties when compared with benzoylation and PSMA treatments.	55
Banana fiber	Fibers were soaked in 5% (w/v) NaOH aqueous solution for 1 h at room temperature.Note: fiber-to-solution weight ratio = 1:15.	Decreased young’s modulus, tensile strength, and strain might be due to cellulose delignification and degradation during alkali treatment.	56
	Fibers were tested for finding optimal alkali conditions (concentration, soaking time, and temperature) for the best lignin removal.	The optimum conditions were found to be 11 g L−1 concentration, treatment time of 2.5 h, and 90°C temperature.	57
PALF-epoxy	PALFs were thoroughly washed with water and dried in an oven at 70°C for 24 h. Fibers were cut into 2–5 mm length. Later, the fibers were treated with NaOH solution of 3%, 5%, 7%, and 10% concentrations. Treated fibers were washed with deionized water repeatedly to remove the extra alkali chemical. Finally, the fibers were again dried in an oven and reinforced with epoxy to produce composite.	Increase in fiber density and decrease in fiber diameter for all concentration. Cellulose content, crystallinity, tensile strength, thermal resistance, and water retention properties were increased up to 7% concentration.	58
PALF-PLA	PALFs were dipped completely in NaOH (5% w/v) for 2 h at ambient condition (≈24°C) followed by washing with distilled water and dried for 2 days. Later, treated fibers were impregnated with PLA resin to produce composites.	Significant change in oxygen/carbon atomic ratio. Significant improvement in thermal stability, flexural strength and storage modulus. Impact strength of the composite is increased by 79%. Heat deflection temperature (171.3 C), which is close to the melting temperature of the neat polymer. Reduction in crystallization temperature by 14 C.	59
PALF-polycarbonate	Fibers were washed and immersed in water for 24 h at room temperature (≈24 C). Subsequently, the fibers were washed with acetone and dried in an oven at 120°C for 3 h. Finally, the fibers were immersed in NaOH solution (5 w/v) in water bath at 30 C for 5 h. The treated fibers were used as reinforcement in polycarbonate.	Young’s modulus of the treated fiber reinforced composite is increased by 30% than untreated.	60
WSF-epoxy	WSF was immersed in 1, 3, and 5 wt% NaOH solution separately for 2 h at ambient condition (≈24°C). The fibers were removed and washed with distilled water and dried in an oven for 72 h at 70°C. Later, these fibers were reinforced with epoxy.	Thermal stability of 3% WSF/epoxy composite was superior than other alkali treated and untreated composites. FTIR of all the treated composites proved partial removal of hemicellulose and lignin. All the treated reinforced composites exhibited shifting of the endothermic peaks toward higher temperature than untreated.	61

PF: phenol formaldehyde; PLA: polylactic acid; PP: polypropylene; LDPE: low-density polyethylene; HDPE: high-density polyethylene; WSF: wheat straw fiber; PALF: pineapple leaf fiber; MAPP: maleated polypropylene; MAPE: maleated polyethylene; FTIR: Fourier transform infrared; NaOH: sodium hydroxide; PSMA: polystyrene maleic anhydride.

Silane treatment

Silane (SiH₄) being a multifunctional molecule is added as a coupling agent to enhance the fiber–matrix interaction. It undergoes several stages of hydrolysis, condensation, and bond formation during the treatment of fibers. In the presence of moisture, silanol is formed. The reaction mechanism for the same is shown in equation (2)

${\text{CH}}_{\text{2}}\text{CHSi}{\left({\text{OC}}_{\text{2}}{\text{H}}_5\right)}_3+{\text{ H}}_{\text{2}}{\text{O CH}}_{\text{2}}\text{CHSi}{\left(\text{OH}\right)}_3+3{\text{C}}_2{\text{H}}_5\text{OH}$

2

During condensation, one end of silanol reacts with the cellulose hydroxyl group of the fiber and the other end reacts with the functional group of the matrix, thereby creating a chemical bond between the two, ¹⁷ as shown in equation (3)

${\text{CH}}_{\text{2}}\text{CHSi}{\left(\text{OH}\right)}_3+\text{ Fiber}-\text{OH }\to {\text{CH}}_{\text{2}}{\text{CHSi(OH)}}_2\text{O}-\text{Fiber}\hspace{0.17em}\text{+}\hspace{0.17em}{\text{H}}_2\text{O}$

3

Generally, silane treatment starts with a derivative of silane (typically amine) with its solution in alcohol/acetone. Upon soaking the fibers in the silane solution, the modified fibers interact better with the matrix than alkaline-treated fibers, leading to higher tensile strength, tensile modulus, flexural rigidity, and thermal stability.^26,42

Soaking of sisal fiber in 2% aminosilane containing 95% alcohol for 5 min at a pH value of 4.5–5.5 was done by Rong et al., ³⁷ which led to improved fiber stiffness by enhancing crystallinity of the hard cellulose. Silane solutions in a water ethanol mixture with concentration of 0.033% and 1% were also used by Valadez et al. ⁴⁵ and Agrawal et al. ⁶² to treat henequen fibers and oil palm fibers. Improved interfacial interactions and crystallinity percentage were observed. Zegaoui et al.^63,64 used treated natural hemp and basalt fibers reinforced in cyanate ester and benzoxazine, respectively. They reported the improvement in microhardness, flexural strength, and thermal stability. Atiqah et al. ⁶⁵ tested sugar palm fibers with silane, alkaline, and combination of silane and alkaline treatments. Doing so improved fiber roughness and thus, fiber–matrix bonding by removing the outer layers that contain impurities, less nodes, wax, and pectin. The works related to silane treatment on NFRC are summarized in Table 4.

Table 4.

The works related to silane treatment of fibers in NFRCs.

Fiber–matrix composites	Applied treatment	Results	References
Oil palm-PF	The alkali-treated fibers were dipped in alcohol water mixture (60:40) containing triethoxyvinylsilane coupling agent. The pH of the solution was maintained in between 3.5 and 4.	Poor fiber–matrix interfacial bonding and composite was found to be more hydrophobic.	28
Henequen-HDPE	Fibers were treated with 0.033% wt%/wt% aqueous silane solution (1% wt%/wt% silane + 0.5% wt%/wt% dicumyl peroxide). Acetic acid was added to maintain a pH of 3.5. Fibers were soaked for 1 h.	No increase in adsorption beyond 1.15 wt% of silane. Combination of silane and NaOH treatment increased tensile strength from 21 MPa to 27 MPa.	45
Sisal–epoxy	Fibers were treated with 2 wt% A-187 solution (pH = 3.5, room temperature) for 2 h.	Composite had higher impact strength compared to alkalized fibers.	66
Banana-PF	Alkali-treated fibers were dipped in alcohol–water mixture (60:40) containing 0.6% silane as coupling agent. The pH of the solution was maintained between 3.5 and 4.	Marginal increase in tensile and toughness properties. The flexural strength was increased by 160%.	52
Banana-PLA	Fibers were treated with APS in 40:60 water–ethanol. Acetic acid was added to maintain a pH of 4. Fibers were soaked for 3 h.	Tensile and impact strength were increased by 19.43% and 30.84%, respectively, and marginal increase in tensile modulus.	52
	Fibers were treated with bis-(3-triethoxysilylpropyl) tetrasulfane (Si69) in 40:60 water–ethanol. Acetic acid was added to maintain a pH of 4. Fibers were soaked for 3 h.	Increases in tensile and impact strength by 136% and 56.80%, respectively, and slight increase in tensile modulus.	67
Jute–epoxy	Fibers were treated with 1% oligomeric siloxane + 96% alcohol solution for 1 h.	Alkali treatment followed by silane treatment showed 12% and 7% increase in strength and modulus compared to alkali treatment alone.	68
Jute–polyester		Alkali treatment followed by silane treatment showed 20% and 8% increase in strength and modulus compared to alkali treatment alone.
	Fibers were treated with 0.1%, 0.3%, and 0.5% of γ-methacryloxy-propyl-trimethoxy-silane.	0.3% Silane-treated composites showed 40%, 30%, and 55% increases in tensile, flexural, and interlaminar shear strength, respectively.	69
PALF-PLA	5 wt% silane was hydrolyzed in a homogenous mixture of water and ethanol (40:60 wt%/wt%) and pineapple leaf fibers were immersed in silane solution for 3 h followed by drying of fibers in air for 3 days and dried in an oven at 80°C for 12 h. Later, the treated fibers were reinforced with PLA.	Change in oxygen/carbon atomic ratio was more than alkaline-treated fibers. Improvement in thermal stability. Impact strength was increased by 47%. However, it is less than alkaline-treated composites. 27% increase in flexural modulus. Increase in storage modulus than alkaline treated fibers. Reduction in heat deflection crystallization temperature than alkaline treatment.	59
PALF-polycarbonate	Fibers were cut into ≈2 mm length. 1% wt%/wt% silane was hydrolyzed by dissolving in methanol solution. pH value was maintained at 3.5. The fibers were soaked in the solution for 6 h followed by air drying for 3 days.	The silanized composites have less Young’s modulus than alkali-treated composites.	60
PALF-LDPE and PP	Silane 3 wt% was dissolved in a solution (400 mL) of ethanol and water (80:20). ≈15–20 g of PALF was mixed and stirred for 2 h. The fibers were dried in an oven at 70°C for 12 h. Later, the treated fibers were reinforced with LDPE and PP.	Hydrophilic tendency of the fibers reduced drastically. Tensile strength of the treated LDPE/PALF composites increased significantly. Increase in crystallinity size but % crystallinity decreases.	70

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; PP: polypropylene; PS: polystyrene; HDPE: high-density polyethylene; LDPE: low-density polyethylene; PLA: polylactic acid; PALF: pineapple leaf fiber; NaOH: sodium hydroxide; APS: 3-amino propyltriethoxysilane.

Acetylation treatment

Acetylation treatment involves the addition of reagents with an acetyl group (CH₃CO), such as acetic acid and acetic anhydride. The acetyl group reacts with the cellulose fibers, followed by the removal of hydroxyl groups, hence reducing the moisture absorption capacity. ⁷¹ While other chemical treatments leave more voids after the reaction, acetylation provides a rough surface topography with less number of voids, as a result of giving better interlocking capabilities. Acetylation could be performed with or without an acid catalyst, as shown in Figure 5(a) and (b), although the subsequent addition of acetic anhydride after soaking the fibers in acetic acid would respond faster. ¹⁷

Figure 5.

(a) Acetylation with acid catalyst and (b) acetylation without acid catalyst.

It is suggested that a limited amount of liquid acetic anhydride without a catalyst or cosolvent at 120–160°C can be used for acetylating the lignocellulosic mass. Acetic acid in small amounts could also be used to initiate the reaction by swelling the cell wall. ⁷²

In tensile testing, the fibrillation is found to occur due to removal of binding materials and appearance of some micropores in the treated fibers. Furthermore, for higher degrees of acetylation, the fibrillation increases leading to fiber damage and cracks. ³⁹ Moreover, improvement in dimensional stability, resistance to biological attack, and degradation due to ultraviolet energy can be observed in acetylated fiber composites. ⁷³ Acetylation reduces water absorption and hence improves dimensional stability.^72,74 Acetylation is also known to improve the tensile strength, stiffness, and the bond shear strength.^42,75 Table 5 summarizes pertinent works on acetylation treatment of NFRC.

Table 5.

The works related to acetylation treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Oil palm-PF	Fibers were treated with 2% NaOH for 30 min followed by acetylation with acetic anhydride in acetic acid medium with concentrated H2SO4 acting as a catalyst.	High strain value which results in enhanced elastic nature and high impact resistance.	28
Green flax-PP	The flax fibers were soaked in acetylating solution consisting of 250 mL toluene, 125 mL acetic anhydride, and a small amount of catalyst perchloric acid (60%) at 60°C for 1–3 h.	Moisture absorption was lowered by 50% at a relative humidity of 65%. Thermal stability increased due to increased degree of acetylation. About 25% improvement in strength properties was observed compared to untreated fiber composites.	39
Flax-PP		18% Acetylated flax fiber exhibited highest tensile and flexural properties. It was improved by 25% compared to untreated fiber composites. However, the addition of MAH resulted in improvement of mechanical properties by 20–35%.	39
Sisal-PP	Fibers were treated in acetic acid for 1 h at 21°C followed by(i) treatment with ethyl acetate at 70°C for 1 h.(ii) treatment with ethyl acetate containing H2SO4 as a catalyst for 10 min.(iii) no treatment.	The average increases in tensile strength and shear strength by 14.08% and 435%, respectively. The acetic acid treatment followed by ethyl acetate with H2SO4 resulted in high levels of cellulose swelling or loosened cell wall structure.	46
Sisal-LDPE	Alkali-treated fibers were soaked in glacial acetic acid for 1 h followed by soaking acetic anhydride containing two drops of concentrated H2SO4 for 2 min.	Decreased dielectric constant with increasing frequency. Low sorption of water compared to untreated.	47
Banana-PF	The pretreated fiber was soaked in a 50% solution of acetic acid for 5 min, followed by washing and air drying.	Fibrillated and rougher than untreated fibers. However, tensile properties are better than mercerization treatment.	54
(Rayon/cotton/wood)-PS	Fibers were dipped into acetic anhydride for 1 min and preheated to 120°C. Excess acetic anhydride and byproduct acetic acid were removed and the flakes were then dried in an oven at 105°C for 12 h.	Fibers had increased interfacial shear strength and surface free energy.	76
Test on following fibers:(1) Oil palm empty fruit bunch(2) Coconut fiber (coir)(3) Oil palm frond(4) Jute(5) Flax	Fibers were immersed in 100°C and 120°C acetic anhydride solutions for 10–180 min, followed by quenching in acetone at ambient temperature (≈24 C).	Tensile properties were slightly improved on acetylation, but not much improvement in impact strength. Treatment in 100°C acetic anhydride was better as it did not damage the cell wall. Treated fibers exhibited lower rate of water uptake and no substantial losses in properties, compared with untreated fibers.	77
WSF-PP	Two grams of straw fibers were immersed in a beaker containing acetic anhydride for 1 min followed by oven drying at 0.5, 1, 1.5, 2, 3, and 4 h.	More chemical and thermal stability yielding ester formation (1 h dried fiber). Decrease in crystallinity was observed.	78

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; PP: polypropylene; PS: polystyrene; WSF: wheat straw fiber; LDPE: low-density polyethylene; H₂SO₄: sulfuric acid; NaOH: sodium hydroxide; MAH: maleic anhydride.

Permanganate treatment

Potassium permanganate (KMnO₄) in acetone solution releases highly reactive Mn³⁺ ions. These ions react with the hydroxyl groups of the cellulose and form cellulose-manganate, as shown in equation (4). This initiates graft copolymerization, as shown in equation (5), providing high thermal stability to the fiber. In general, the experimentation is done using different concentrations of KMnO₄ solution in acetone with soaking (1–3 min) after alkaline pretreatment

$\text{Fiber}-{\text{OH + KMnO}}_{\text{4}}\to \text{Fiber}-\text{O}-\text{H}-\text{O}-\left({\text{MnO}}_2\right)-{\text{ O}}^{-}{\text{K}}^{+}$

4

$\begin{array}{l}\text{Cellulose}-\text{H}+\text{Mn}\left(\text{III}\right)\to \text{Cellulose}-\text{H}-\text{Mn}\left(\text{III}\right)\text{complex}\\ \text{Cellulose}-\text{H}-\text{Mn}\left(\text{III}\right)\to \text{Cellulose}+{\text{H}}^{+}+\text{Mn}\left(\text{III}\right)\end{array}$

5

Apart from the removal of cellulose hydroxyl groups, it also reacts with lignin and separates it from the cell wall. As a result, high hydrophobicity is observed. The hydrophilic tendency of fiber decreases as the KMnO₄ concentrations increase up to an optimum value. At a concentration of 1%, polar groups between fiber and matrix are formed leading to degradation of cellulosic fiber. ⁴⁷ The fiber surface is etched by the oxidization of permanganate and becomes physically rougher. In this case, the interfacial adhesion is improved by the induced mechanical interlocking, and the contact area between the fiber and the matrix is also increased. ⁴⁸ This mechanical bonding of the roughened fibers with the matrix due to oxidation can be contrasted with chemical bonding, as in the case of silane treatment. ⁷⁹ The ductility, impact strength, flexural strength, flexural modulus, and thermal stability of NFRCs are improved by permanganate treatment.^42,80-82 Using KMnO₄ improves fiber roughness ⁸³ and oxidizes the hydroxyl groups in cellulose to carboxyl and aldehyde groups. ⁸⁰ Table 6 presents pertinent works on permanganate treatment of NFRCs.

Table 6.

The works related to permanganate treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Oil palm-PF	Alkali pretreated fibers were immersed in permanganate solution (concentrations of 0.01%, 0.05%, and 0.1%) in acetone for about 2–3 min.	Highest tensile strength and modulus compared to other chemical treatments. Highly fibrillated structure and hence very good fiber matrix adhesion.	28
Sisal	Alkali-treated fibers were immersed in permanganate solutions (0.033%, 0.0625%, and 0.125% concentrations) in acetone for 1 min.	The hydrophilicity of fiber decreased with increasing concentration of KMnO4. However, highest cellulose degradation was observed at 1% KMnO4 concentration.	47
Sisal-LDPE	The alkali-treated fibers were soaked in KMnO4 solutions of different concentrations in acetone for 1 min.	At 1% concentration, higher degradation of the cellulose occurred, forming more polar groups than untreated composites.	47
		Optimum properties were obtained at 0.055% permanganate concentration. Tensile properties lie in between alkali and peroxide treatments.	48
Sisal-vinyl ester	Fibers were soaked in a 0.055% permanganate acetone solution for 2 min.	Interlaminar shear strength, tensile properties, and flexural properties were better than silane-treated fibers, but impact energy absorption was lower than untreated fibers.	79
Banana-PP	The alkali-treated fibers were soaked in 0.5% KMnO4-acetone solution for 30 min.	16% Increase in thermal diffusivity.	53
		Increases in tensile strength and modulus are 6.4% and 7.5%, respectively. Flexural strength and modulus were found to have increases of 5% and 10%, which is lower than alkali and silane treated.	84
Flax-LLDPE	Fibers were soaked in NaOH for 1 h. Then, soaked in KMnO4-acetone solution for 10 min.	Better tensile and water repellent properties as compared to alkali- and silane-treated fibers.	85

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; PP: polypropylene; LDPE: low-density polyethylene; LLDPE: linear low-density polyethylene; HDPE: high-density polyethylene; NaOH: sodium hydroxide; KMnO₄: potassium permanganate.

Peroxide treatment

Organic peroxides (RO–OR) are functional groups containing divalent O–O ion. These peroxides have a high tendency to decompose into free radicals (RO). These highly reactive free radicals react with the hydrogen in the hydroxyl groups, as shown in equation (6).

$$\begin{gathered} \stackrel{\text{•}}{\text{RO}}\hspace{0.17em}-\text{OR}\to 2\left[\stackrel{\text{•}}{\text{RO}}\right] \\ \stackrel{\text{•}}{\text{RO}}\hspace{0.17em}+\hspace{0.17em}\text{Fiber}-\text{H}\to \text{R}-\text{OH}\hspace{0.17em}\text{+}\hspace{0.17em}\text{Fiber} \end{gathered}$$

Additionally, when the free radical reacts with the matrix molecule, say polyethylene (PE-H), the reaction shown in equation (7) takes place to give polyethylene (PE) ⁴⁷

$\stackrel{\text{•}}{\text{RO}}\hspace{0.17em}+\hspace{0.17em}\stackrel{\text{•}}{\text{PE}}-\text{H}\to \text{ROH}+\stackrel{\text{•}}{\text{PE}}$

7

The PE radicals could react with each other leading to cross-linking or grafting onto cellulose fibers, as shown in equation (8)

$$\begin{gathered} \stackrel{\text{•}}{\text{PE}}\hspace{0.17em}+\stackrel{\text{•}}{\text{PE}}\to \stackrel{\text{•}}{\text{PE}\hspace{0.17em}}-\stackrel{\text{•}}{\text{PE}} \\ \stackrel{\text{•}}{\text{PE}}\hspace{0.17em}+\text{Fiber}\to \stackrel{\text{•}}{\text{PE}}\hspace{0.17em}-\text{Fiber} \end{gathered}$$

8

The commonly used organic peroxide treatments are benzoyl peroxide (BP, (C₆H₅CO)₂O₂) and dicumyl peroxide ((C₆H₃C(CH₃)₂O)₂). The reaction of peroxide decomposition occurs during the time of curing of composites at the interface. Higher temperature is favored for the decomposition of the peroxides. ²⁸ There exists a critical peroxide concentration, where the tensile strength of the composite reaches a maximum value for a given fiber content. At this stage, the fibers are covered with grafted polymer (matrix) molecules and further addition of peroxide contributes to some cross-linking of matrix molecules, which have negligible effect on the mechanical properties of the composite. ⁴⁸ Therefore, peroxide treatment reduces water absorption and improves tensile strength, modulus of elasticity, thermal stability, and interfacial adhesion between the fiber and matrix.^82,86-88 Table 7 summarizes pertinent works on some peroxide treatment of NFRCs.

Table 7.

The works related to peroxide treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	Reference
Oil palm-PF	Fibers were soaked in saturated solutions of peroxide in acetone.	Higher flexural modulus as compared to other chemical treatments.	34
Sisal-LDPE	The alkali-treated fibers were immersed in 1 L containing 6% solution of peroxide in acetone for 30 min.	Reduction in hydrophilicity was observed.	47
	The alkali-treated fibers were soaked in 6% DCP solution in acetone for 30 min. Similar procedure was adopted for BP.	Approximately 50% increase in tensile properties. DCP provided higher interfacial bonding than BP. The critical concentrations of DCP and BP were 4% and 6%, respectively.	48
Sisal	Alkali pretreated sisal fibers were treated with 5% BP in acetone for 30 and 45 min.	Increase in crystallinity index and higher decomposition of fiber at soaking time of 30 min.	89
Kenaf-PLA	Fibers were immersed in hydrogen peroxide (5% v/v) for 60 min at pH 11 and 80°C.	High tensile and flexural strength at 30% fiber loading, whereas, modulus was high at 40% fiber loading.	90
Jute-PLA	The alkali-treated fibers were soaked in 6% BP for 30 min.	Tensile and flexural properties were better than alkali and permanganate treated fibers but not as superior as silane treatments, whereas, the thermal stability was reduced.	91
PALP-HDPE	35 g of PALP was mixed in 500 cm3 beaker having 300 cm3 H2O2 and 1 g of NaOH at 85°C and constantly stirred for 1 h. Later, the powders were filtered, washed with distilled water, and dried in an oven at 50°C. The treated powders were reinforced with HDPE for composite fabrication.	Increase in tensile strength, tensile modulus, flexural strength, and abrasion resistance compared to untreated composites. Reduction in elongation at break.	92
Wheat straw-H2O2	Milled wheat straw (22.5 mm sieve size) were treated initially with 10 g NaOH followed by 25 g H2O2 per 100 g of straw.	Drastic exothermic reaction resulted (temperature is more than 100 C) with significant amount of steam generation. Chemically treated straw had higher ash content than untreated. Removal of lignin was 50%.	93

PALP: pineapple leaf powder; H₂O₂: hydrogen peroxide; NFRC: natural plant fiber-reinforced composite; PF: polypropylene; LDPE: low-density polyethylene; HDPE: high-density polyethylene; DCP: dicumyl peroxide; BP: benzoyl peroxide; NaOH: sodium hydroxide; PLA: polylactic acid.

Benzoylation treatment

Benzoylation is an effective method in enhancing the hydrophobicity and thermal stability of fibers. Benzoyl chloride, being the most used chemical in this treatment, includes the benzoyl (C₆H₅C=O) group, which is responsible for decreased hydrophilic nature of the treated fiber and improved interaction with the hydrophobic matrix.

During the process, the fibers are pretreated with alkali, as shown in Figure 6. On removal of the extractable materials like lignin, waxes, and oils covering the fiber, more reactive hydroxyl groups are exposed on the surface. These fibers are treated with BP. In this step, the benzoyl groups replace the hydroxyl groups on the fibers. This greatly enhances hydrophobicity and adhesion with the matrix.

Figure 6.

Benzoylation.

Benzoylation roughens the fiber surface leading to fibrillation and provides better mechanical interlocking with the polymer matrix. For a tensile test, fiber breakage was observed rather than fiber debonding due to better fiber/matrix adhesion. ⁷⁹ This technique improves the tensile strength and Young’s modulus, increasing the glass transition temperature and thermal stability while reducing moisture absorption.^94-96 It also reduces voids and improves the hardness. ⁹⁷ Table 8 lists pertinent works on benzoylation treatment in NFRCs.

Table 8.

The works related to benzoylation treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Jute–epoxy	Alkali pretreated fibers were soaked for 15 min in benzoyl chloride solution followed by soaking in ethanol for 1 h.	Improved storage modulus and thermal stability.	36
Flax-LDPE and HDPE	The fibers were immersed in 10% NaOH solution and agitated with benzoyl chloride for 15 min followed by soaking in ethanol for 1 h.	Highest tensile and impact strength were observed for LDPE and highest impact strength for HDPE. Resulted in less water absorption as compared to silane and peroxide treatments. Smooth fiber surface was observed.	37
Sisal-LDPE	The alkali-treated fibers were soaked in BP and acetone for 30 min	Tensile strength gets saturated after 6% of BP. Though not as good as DCP, values are still high as compared to other treatments.	49
Banana-PP	Chopped fibers (6 mm) were soaked in 2% NaOH solution for 30 min and well agitated with benzoyl chloride for another 30 min.	Significant increase in thermal conductivity.	54
		Increases in tensile strength and modulus were 13% and 5%, respectively. However, still lower than alkali and silane-treated fibers.	79
Sisal-PS	Alkali-treated fibers (6 mm) were suspended in 10% NaOH solution and agitated well with benzoyl chloride for 15 min, followed by soaking in ethanol for 1 h to remove the unreacted benzoyl chloride, and finally washed with water and dried.	Activation energy for glass transition temperature (Tg) was increased and maximum activation energy was observed.	90

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; LDPE: low-density polyethylene; HDPE: high-density polyethylene; PP: polypropylene; PS: polystyrene; BP: benzoyl peroxide; NaOH: sodium hydroxide.

Acrylation treatment

Acrylic acid (CH₂=CHCOOH) is used to enhance fiber–matrix (polypropylene) adhesion. The acrylic acid molecules react with the cellulose hydroxyl groups to provide cellulose radicals for the polymerization medium. The grafting of acrylic acid on the matrix is initiated by peroxide radicals. The O–O bond in peroxide removes hydrogen atoms from the cellulose hydroxyl groups. Hence, these acids form ester linkages with the hydroxyl groups, as shown in equation (9). This coupling mechanism between fiber and matrix enhances the stress transfer capacity at the interface and consequently improves composite properties. ⁹⁸ Acrylation reduces the fibers hydrophilicity.^99,100 Table 9 presents pertinent works on acrylation treatment in NFRCs.

$\text{Fiber}-\text{OH}+{\text{C}}_2{\text{H}}_3\text{COOH }\to \text{Fiber}-{\text{OOCC}}_{\text{2}}{\text{H}}_3+{\text{ H}}_{\text{2}}\text{O}$

9

Table 9.

The works related to acrylation treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Oil palm-PF	Fibers were immersed in 10% NaOH for 1 h at room temperature (≈24°C). The obtained wet product was further treated with different concentrations of acrylic acid. Furthermore, the reaction was carried out for 1 h at 50°C.	Decrease in tensile modulus. However, yielding high extensibility and increased impact resistance were observed	28
		At 40 wt% fiber with 50°C produced moderate level of moisture absorption capacity (≈50%) compared to other treatments.	33
Flax-LLDPE	Flax fibers were immersed in NaOH solution for 30 min followed by soaking in acrylic acid solution for 2 h at 50°C.	Highly smooth fiber surface was observed. Higher tensile strength and lower water absorption in comparison to silane, permanganate, and sodium chloride treatments.	87
Flax fibers	The permanganate-treated fibers were placed in a reaction bottle containing a solution consisting of known concentrations of citric acid and acrylic acid at different temperatures and durations.	Grafting increases with increasing concentration of acrylic acid. This is due to higher availability of monomer molecules in the proximity of cellulose macro radicals as well as in the polymerization medium at higher monomer concentration.	100
Jute-epoxy-phenolic resin	Alkali-treated fibers were reacted with aqueous 10 wt% acrylic acid at reflux temperature.	Increases in tensile and flexural strengths by 42.2% and 13.9%, respectively.	101

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; LLDPE: linear low-density polyethylene; NaOH: sodium hydroxide.

Acrylonitrile grafting

Acrylonitrile (AN) initiates free radicals, which react with cellulose molecules of the fiber by dehydrogenation and oxidation. The activated free-radical sites on the fiber surface interact with the monomer of the matrix. The generation of free radicals can be done in the reaction medium through a redox reaction and then transferred to the backbone or by direct oxidation of the backbone by transition metal ions (such as Ce⁴⁺, Cr⁶⁺, and V⁵⁺). ¹⁰² The overall reaction is shown in equation (10)

$\text{Fiber}-\text{OH}+{\text{CH}}_2=\text{CHCN}\to \text{ Fiber}-{\text{OCH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{CN}$

10

The lignin content in the fiber is very crucial in determining the extent of grafting. The presence of lignin generally retards the polymerization rate and acts as an inhibitor at higher lignin levels. ²⁸ Properties, such as the swelling behavior in different solvents, solubility, moisture absorption, chemical and thermal resistance, can be improved by grafting. ¹⁰² Table 10 provides some works on AN treatment available in the literature.

Table 10.

The works related to acrylonitrile treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Oil palm-PF	Alkali bleached (2% for 30 min) and oxidized (0.02 mL−1 KMnO4 for 10 min) fibers were washed with water and then mixed with 1% H2SO4 containing acrylonitrile in the ratio 30:1. Further, it was placed in a thermostatic water bath for 120 min at 50°C	Shows enhanced elastic nature due to higher strain values and slight improvement in the stiffness was observed	28
		At 40 wt% fiber with 50 C produced higher moisture absorption capacity (≈200%) compared to other treatments.	33
Agave Americana fibers	Wet fibers were added to nitric acid (0.277 mol L−1) containing ceric ammonium nitrate. Followed by the addition of monomer contents were stirred at an optimum temperature and time interval. The homopolymer was removed with the help of dimethylformamide and grafted fibers were thoroughly washed with distilled water and dried at 60°C.	Resistance to moisture, acid and base reduced with increasing percentage of graft. Graft copolymerization also resulted in an increase in the thermal stability of the fibers.	102
Sisal–polyester amide	[Cu2+–IO4] was used as an initiator in the graft copolymerization of AN into 5% alkali-treated sisal fibers at 40°C in an aqueous medium.	Higher tensile and flexural strengths as compared to other treatments. This study revealed the least degradability among all other treatments	103
PALF–redox pair	Known quantity of distilled water, concentrated HNO3, glacial acetic acid, butyl acetate, potassium permanganate, and 0.1 g of fibers were mixed in a vessel for 10 min. The fibers were removed and completely dipped in N-N-dimethylformamide before analyzing	Graft yield in modified fiber is lower than unmodified fibers.	104

NFRC: natural plant fiber-reinforced composite; PF: polypropylene; LDPE: low-density polyethylene; PALF: pineapple leaf fiber; PF: phenol formaldehyde; AN: acrylonitrile; KMnO₄: potassium permanganate; H₂SO₄: sulfuric acid; HNO₃: nitric acid.

Isocyanate treatment

Isocyanates are organic compounds that contain the isocyanate functional group R–N=C=O. These groups have high reactivity toward hydroxyl groups forming urethane linkages, as shown in equation (11). These are used to remove the hydroxyl groups on cellulose and lignin. In addition, isocyanate reacts with moisture on the fibers producing urea, which further reacts with hydroxyl groups in cellulose.

$\text{R}-\text{N}=\text{C}=\text{O}+\text{HO}-\text{Fiber }\to \text{R}-\text{NH}-\text{CO}-\text{O}-\text{Fiber}$

11

The major drawback of alkali treatment is that disrupts the hydrogen bonding in cellulosic hydroxyl groups, thereby making them more reactive than before. ⁴⁸ Isocyanate treatment is seen as a promising alternative to alkali treatment as it is more effective in removing the free hydroxyl groups on the fiber surface. Therefore, isocyanate treatment improves mechanical properties, such as tensile strength, and reduces water absorption.^105,106 Table 11 reports key works on isocyanate treatment.

Table 11.

The works related to isocyanate treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	References
Oil palm-PF	The alkali-treated fibers were soaked in chloroform containing dibutyltin dilaurate. Toluene diisocyanate was added dropwise, stirred for 2 h, and purified by refluxing with acetone.	At 40 wt% fiber with 50°C produced higher moisture absorption capacity (≈280%) compared to other treatments.	33
Sisal-LDPE	The required amount of CCl4 and a little dibutyltin dilaurate catalyst were added to the alkali-treated fibers. The urethane derivative was added dropwise to the flask with continuous stirring until the addition was completed. The reaction was allowed to continue for 1 h.	Decrease in dielectric constant after the treatment.	47
		Fibers showed superior tensile properties than alkaline and untreated fibers. The elongation at break also quadrupled as compared to alkaline treated fibers.	48
PALF-LDPE and PP	3 wt% isocyanate coupling agent was homogeneously mixed in toluene (400 mL). ≈16 g of PALF was constantly mixed and stirred in this solution for 30 min at 50°C. Subsequently, the fibers were dried in an oven for 12 h at 70°C.	Reduction in hydrophilic tendency than silane-treated composites. Decrease in % crystallinity. Increase in tensile strength than untreated composites.	70
Jute/hemp/flax-epoxy	Nonwoven fiber mat was allowed to swell in DMF for 30 min and was treated with isocyanate using dibutyltin dilaurate as a catalyst at a temperature of 125°C for 1 h. After the treatment, the mats were immersed in hot DMF and washed thoroughly before dried in an oven at 105°C.	Increased stiffness and reduction in impact strength by 17%.	107
Fibrous cellulose-PP	The predried fibers were treated with DIC. Composite contained 30 wt% fibers.	Increased tensile properties and elongation. SEM micrographs revealed improvement in interfacial adhesion.	108

DMF: dimethyl formamide; DIC: 1,6-diisocyanatohexane; NFRC: natural plant fiber-reinforced composite; PP: polypropylene; LDPE: low-density polyethylene; SEM: scanning electron microscope.

Maleated coupling agents

Addition of maleated coupling agents helps in increasing the interfacial bonding between the fiber and matrix. During grafting, MA reacts with the hydroxyl groups and removes them from the fiber cells. ¹⁰⁹ This results in a long chain polymer attached to the fiber surface through covalent carbon–carbon bonds, ¹⁷ as shown in Figure 7. Before the treatment, the copolymer is heated to 170 C, followed by the esterification reaction. After this treatment, the surface energy of cellulose fibers is increased to a level much closer to the surface energy of the matrix. This ensures higher interfacial adhesion due to better wettability of the fiber. ²⁶ The effectiveness of MAPP in enhancing the impact energy absorption, heat deflection temperature, and tensile and flexural strengths was reported by Daghigh et al. ¹¹⁰ for an optimal concentration of 2 wt% coupled with short Latania fiber. The effectiveness in enhancing the flexural strength and reducing the water absorption was given by Huang et al. ¹¹¹ for wood–polypropylene composites. Maleated polyolefins (MaPOs) possess an excellent balance of properties to bridge the interface between polar and nonpolar species. This along with their ease of production makes MaPOs very successful. ¹¹² Table 12 presents key works on maleated coupling agents.

Figure 7.

The reaction of cellulose fibers with hot MAPP copolymer.

Table 12.

The works related to maleated coupling agent treatment of fibers in NFRCs.

Fiber–matrix composite	Applied treatment	Results	Reference
WSF-PP	Extracted fibers were placed in hot xylene at 120°C for 3 h with 4 g predissolved MAPP.	Significant reduction in crystallinity and thermal stability is more than acetylation treated fibers.	78
Wood flour-HDPE	5% of MA-HDPE, MA-LLDPE, and MAPP were added to a 20 L blender along with HDPE and dried wood flour. The blended mixture was extracted from a screw extruder.	Increases in tensile strength and modulus properties became twice as compared with untreated fibers.	113
Sisal-PP	The fibers of different lengths (3, 6, and 10 mm) were immersed in hot MAPP solution in toluene at 100°C at different concentrations (0.3, 0.5, 1, and 2%), with different time periods of impregnation (3, 5, and 10 min).	Increases in tensile, flexural and impact strength are 50%, 30%, and 58%, respectively. Decrease in water absorption by 61%. High level of crystallinity was also observed.	114
Banana/hemp/sisal-Novolac resin	The fibers were esterified by using 2% MA in xylene, keeping the fiber-to-solvent ratio at 1:20 (w/v). The soaking of MA was allowed for 18 h at 65°C.	Water absorption decreased compared to untreated fibers. Above 50% fiber loading, it tends to increase with loading percentage. Tensile, flexural, and impact strengths peaked up at 50% loading and flexural modulus values were higher than untreated fibers.	115
Hildegardia-PP	10 wt% of PP-g-MAH (compatibilizer) was blended with 90 wt% of PP. The alkali treated fibers were added to it.	Tensile properties were higher with the addition of compatibilizer. The DRIFT spectra of the fibers suggested that a chemical bond existed between the fibers and matrix via compatibilizer.	116
PALF-MA-g-PP	Alkali pretreated fibers were treated with 2 N NaOH at 23°C for 60 h and washed with dilute HCl. The fibers were again washed with distilled water and dried in vacuum oven at 60°C for 24 h. Later, the treated fibers were reinforced with PP.	Treated fibers exhibited increased aspect ratio and matrix compatibility. Tensile strength, impact strength, and flexural modulus are increased by 9%, 30%, and 3%, respectively, than untreated fibers.	117

NFRC: natural plant fiber-reinforced composite; PP: polypropylene; HDPE: high-density polyethylene; PALF-MA-g-PP: pineapple leaf fiber maleic anhydride grafted polypropylene; PVOH: polyvinyl alcohol; NaOH: sodium hydroxide; MAPP: maleated polypropylene; LLDPE: linear low-density polyethylene; MA: maleic anhydride; PP-g-MAH: polypropylene-grafted maleic anhydride.

Biological treatments: Fungal and enzymes treatment

Since chemical treatments pollute the environment, increase the production cost and time consuming, hence, the recent studies are focused on biological treatments.^118-120 Being eco-friendly and a promising alternative for surface treatments, fungal treatment involves the production of enzymes. These enzymes act on the surface of the fibers to remove noncellulosic components, such as wax. White rot fungi produce extracellular oxidizing enzymes that react with lignin and assist in its removal, in addition to increasing hemicellulose solubility and reducing the hydrophobicity of the fiber. ¹²¹ Moreover, fungi produce hyphane that creates fine holes on the fiber surface and produces a rough surface for better interlocking with the matrix. Enzymes could directly be added to natural plant fibers without making use of fungi. Enzymes such as xylanase and laccase improve the hardness and tensile and flexural strengths by enhancing the cellulose content and removing lignin and hemicellulose.^122,123 Table 13 enumerates fungal treatment mentioned in the literature.

Table 13.

The works related to fungal and enzymes treatment of fibers in NFRCs.

Fiber–matrix composite [fungus used]	Applied treatment	Results	References
Hemp–polyester (Ophiostomaulmi)	Hemp fibers were soaked in 1.2 L of water containing 0.5% w/v glucose and 0.1% w/v yeast (Ophiostomaulmi). The material was sterilized for 20 min and cooled. 2 mL of spore suspension was added for every 50 mL of the medium. The material was then incubated on a rotary shaker (150 r min−1) for 4, 6, or 8 days. After treatment, fibers were removed from the flask and thoroughly washed with distilled water.	4 days treated hemp fibers were found to have improvements in flexural strength and modulus by 21% and 12%, respectively. Slight decrease in tensile strength, no significant changes in Young’s modulus, and improved resistance to moisture absorption.	118
Hemp-PP (Phanerochaetesordida (D2B), Pycnoporus species (Pyc), Schizophyllumcommune (S.com), Absidia (B101), Ophiostomnfloccosum (F13))	Fibers were sterilized in an autoclave for 15 min at 120°C. The fungi:hemp ratio was approximately 10 mg: 12 g and incubation was carried out at 27°C for 2 weeks. After treatment, the fibers were sterilized and washed in a fiber washer for 10 min before being dried in an oven at 80 C.	Less lignin was removed compared to alkali treatment. However, composite strengths were found to be better (22% increase) than the untreated fiber. This is due to the action of fungal hyphae, enabling good mechanical bonding of the matrix to the fiber.	124
Abaca–fungamix	A beaker containing 2.5 L of demineralized water was heated up to 50°C. Abaca fiber bundles were submerged in the water keeping the pH level as 7.5, 1.5 wt% of fungamix was mixed slowly in the beaker with homogenous stirring for 4 h. Later, abaca fibers were dried in an oven at 60°C. Subsequently, the enzyme was deactivated by adding NaOH and heating up to 95°C for 30 min. Further, these fibers were commingled with polypropylene to produce composites.	Fiber surface becomes smother due to removal of waxy materials and cuticles from the surface. Fiber average diameter was found out to be reduced drastically. TGA proved 8°C increase in decomposition temperature. The equilibrium moisture content of the treated fiber commingled composite found to be 45% lower than the untreated fiber composites. Tensile, flexural, and impact strength was increased by 45%, 10–35%, and 10–25%, respectively.	125
Date palm fibers–Acetobacter Xylinium	Date palm fibers were put in 250 mL Erlenmeyer flask, which contains 90 mL culture medium (50 g L−1 fructose, 5 g L−1 yeast extract, 5 g L−1 peptone, 2.7 g L−1 Na2HPO4, and 1.15 g L−1 citric acid). After autoclaving at 110°C for 50 min, the flask was inoculated with Acetobacter Xylinium. The content was agitated at 100 r min−1 under 30°C for 10 days.	Weight gained by the fibers due to bacterial adhesion is 4–6%. Tensile strength and elongation at break of the fibers were increased by 20% and 40%, respectively. The increase in interfacial shear strength was about 90%.	126
Flax/hemp-xylanase, xylanase (10% cellulase), polygalacturonase, laccase, and pectinmethylesterse	1.0 g of fiber was weighed in an Erlenmeyer flask and mixed with 2, 6, and 10 wt% enzymes stock at a specified optimized condition separately. Liquid-to-fiber ratio was maintained at 50:1. The mixture was agitated at 80 r min−1 for 90 min.	All the enzymes treated fibers showed reduction in lignin, hemicellulose and wax. However, xylanase (10% cellulase) treated fibers exhibited highest removal rate than others.	127
Hemp–chelator, pectinase, and laccase	10 g fibers were treated with EDTA of 200 mL and treated with pectinase (100 U) and laccase (100 U) separately.	EDTA treatment followed by pectinase-treated fibers exhibited highest reduction in noncellulosic compounds.	128
Wheat straw, PVOH, ligninolytic enzymes	Wheat straw fibers were mixed with 200 U mL−1 of ligninolytic enzymes. 15 g of the substrate was made up to 100 mL with 0.5 mM of a sodium malonate buffer having pH value of 4.5. Later, the treated fibers were reinforced with PVOH.	58.5% of delignification took place in the fibers surface. Treated fiber-reinforced composite showed maximum tensile strength and modulus of 46.12 and 1472 MPa, respectively. Decreased in elongation at break (11.32%) and water absorption behavior.	129

NFRC: natural plant fiber-reinforced composite; PP: polypropylene; PVOH: polyvinyl alcohol; EDTA: ethylenediaminetetraacetic acid; NaOH: sodium hydroxide; TGA: thermogravimetric analysis.

Nanoclay addition to polymer matrix: Nanocomposites

Nanoclays are inorganic nanoparticles (with size between 1 nm and 100 nm), which are used as reinforcements to enhance mechanical, thermal, or electrical properties. Nanoclay additives are widely used in industries, such as cable coatings, adhesives, inks, pharmaceutical, and automotives. ¹³⁰ Among the various inorganic nanoparticles, layered clay minerals have received more attention due to their commercial availability, low cost, relatively simple processability, and significant enhancements in properties. ¹³¹ Montmorillonite (MMT) is one of the most common nanoclay forms and is classified as magnesium aluminum silicate having sheets morphology. These sheets or platelets have permanent negative charge and are held together by cations, such as Na⁺ and Ca²⁺. With a particle thickness of 1 nm and 70–100 nm crosswise silica platelets, ¹³² the total surface area of MMT can be as large as 750 m² g⁻¹ and high aspect ratio of 70–150. ¹³³ Known for their availability and being inexpensive, minimal content (1–5 wt%) of nanoclay can improve the flexural modulus and lower the coefficient of linear thermal expansion of the polymer matrix. ¹³⁴ Moreover, the heat resistant property of the nanoclay can increase the heat distortion temperature of the composites ¹³⁵ by making them more dimensionally stable and flame retardant.^130,136 Nanoclay, by virtue of its high aspect ratio, provides a tortuous path that makes it hard for gases and vapors to pass through, thereby improving the barrier properties. Alamri and Low ¹³⁷ have summarized the advantages of polymer–clay nanocomposites as being excellent in physical (shrinkage, optical, dielectrics, and permeability), mechanical (toughness, strength, and modulus), and thermal (thermal expansion coefficient, flammability, decomposition, and thermal stability) properties.

Like natural plant fibers, the fiber–matrix interaction plays a major role in the properties of the composite. However, the properties of nanocomposites depend on the area of nanoclay exposed to the polymer matrix. Homogeneity in the nanoclay dispersion in polymer is linked with optimal properties. Sancaktar and Kuznicki ¹³⁸ proposed three stages of dispersing nanoparticles in polymeric matrices. The formed nanostructure of the mixture could be conventional, intercalated, or exfoliated, in which exfoliated being the best, as shown in Figure 8. The final mixture is typically a combination of these three morphologies. Due to their high aspect ratio, a small percentage of nanoclay particles properly dispersed in the matrix can generate a very large surface area for polymer/filler interactions.

Figure 8.

States of dispersion of nanoclay platelets. ¹³⁸

The final properties of nanostructure will mainly depend on the choice of the mixing technique and the resulting degree of exfoliation of the nanoclay platelets. ¹³⁹ The mixing techniques adopted are manual mixing, ¹³³ thinky mixing, magnetic stirring, sonication, ¹⁴⁰ three roll milling,^140,141 high shear mixing, ¹³⁹ high speed mixing via homogenizer, ¹¹⁴ direct melt compounding, and master batch dilution. ¹⁴² In the study of nanoclay–epoxy composites by Jumahat et al., ¹⁴³ the presence of clusters of intercalated nanomers and nanovoids reduced the compressive strength of the epoxy system. At high nanoclay loadings, mechanical properties deteriorated due to the existence of agglomerates and air bubble voids in the epoxy matrix. Abdollahi et al. ¹³¹ investigated MMT as a filler (1, 3, and 5 wt%) in alginate matrix for food packaging applications. The addition of nanoclay was done by mechanical stirring and sonication. Among them, 5 wt% showed reduced water absorption from 99.55% (neat alginate film) to 77.49%, but the contact angle was also reduced from 40.7° to 36.18°, making it more hydrophilic. On the other hand, the tensile strength was decreased beyond 1 wt% loading. This was a result of the interaction of MMT layers by forming stacked clay themselves through the polymer matrix, high interfacial area and energy, which was a result of higher MMT content.

Ho et al. ¹³³ added various amounts (0.5–8 wt%) of MMT in epoxy through mechanical stirring. The ultimate tensile strength was maximum at 5% with ductility dropping from 1%, and the higher weight concentrations were found to make it brittle. In Vickers hardness test, the 5% sample showed maximum value of 11.3 when compared to 9.8 for pure epoxy. As the wt% of nanoclay increased, “nanoribbon” structures of MMT interlocked with the epoxy network chains as a result of increasing the “grid lines” of the net. This would hinder the linking of epoxy chain networks making the composite brittle. Withers et al. ³¹ tested mechanical properties of unloaded, 2% and 4% loaded Cloisite B nanoclay, which is capable of bonding to epoxy resin compounds either noncovalently through hydrogen bonds or covalently by additional reaction. A loading of 2% showed a higher tensile strength with a 1000% higher fatigue life than the pristine composite when extrapolated to 10⁹ cycles or a simulated 10-year life cycle. The properties with 4% loading were found to deteriorate due to intercalation, which did not support uniform stress transfer like exfoliated as in the case of 2% loading. Alamri and Low ¹³⁷ investigated the effect of water absorption on the mechanical properties of nanoclay (modified MMT) recycled cellulose fiber-reinforced hybrid epoxy composites. They concluded that the presence of nanoclay platelet was found to slightly minimize the effect of moisture on the mechanical properties of the composites, as evidenced by improved flexural strength, modulus, and fracture toughness properties of the composites.

Shoumya and Kazi ¹⁴⁴ compared the properties of polyester reinforced with graded and nongraded nanoclays. MMT-graded clay was surface modified with 15–35 wt% octadecylamine and 5 wt% aminopropyltriethoxysilane, while the nongraded clay was collected from local areas and nonprocessed. It was experimentally found that the graded nanoclay improved tensile strength, modulus, and flexural strength, whereas nongraded clay degraded the same properties. Alfred et al. ¹⁴⁵ evaluated the properties of graded nanoclays (Nanomer I.28E and Cloisite 10A and 30B) and concluded that Cloisite 10A had the best dispersion due to highest intergallery spacing and viscosity.

Solyman et al. ¹⁴⁶ fabricated nanoclay composites by assembling polymeric thiol surfactants on gold nanoparticles followed by impregnation into the clay matrix. The results showed that the polymeric thiol surfactants assembled on gold nanoparticles were located in the interlayer space of the clay mineral and affected the clay structure. Moreover, the assembled polymeric thiol surfactants on gold nanoparticles exfoliated and increased the activity of nanoclay. Pankil et al. ¹⁴⁷ tested various surfactants (namely dodecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, tetraphenylphosphonium bromide, and zinc stearate) on unmodified MMT. The surfactants were chosen on the basis of long hydrophobic carbon chains attached to the central atom that results in increasing surface area. This will lead to better intercalation of organic cation between the layers of unmodified MMT clay. Basal spacing is found to be the highest with modification of MMT with tetraphenylphosphonium ion. The nanoclays are found to be more thermally stable with modification using surfactants.

Haq et al. ¹⁴⁸ observed that the fibers provided mainly the stiffness and strength to the composite while the nanoclay enhanced the hygrothermal and barrier properties. In their study on hemp-PP composites, the addition of Cloisite 30B nanoclay did not seem to alter the fiber–matrix adhesion. However, it increased the surface roughness and become tougher. The improvement in stiffness is attributed to the inorganic nanoclay fillers, which compensate for the low stiffness observed in organic fillers like the natural plant fibers in polymer matrix. ¹¹³ Behzad ¹⁴⁹ used modified MMT in rice husk high-density polyethylene (HDPE) composite and observed an increase in dynamic mechanical properties with the addition of nanoclay. He has concluded that 2 per hundred compounds (phc) of nanoclay intercalated exhibited good crystalline properties. Extending his study to wood flour/glass reinforced PP hybrid composites and concluded that an increase in the percentage of glass fibers resulted in increased tensile strength and decreased in impact strength and water absorption for an optimum concentration of nanoclay as 4 phc. ¹⁵⁰ The same author ¹⁵¹ studied the addition of nanoparticles in hemp-PP composites, which led to an increase in the tensile strength and modulus, decrease in the water absorption and thickness swelling, and a reduction in the impact strength.

Zhong et al. ¹⁵² observed that incorporation of organoclay in wood flour-HDPE composite reduced the thermal expansion and enhanced the mechanical properties while emphasizing that an ore compatibilizer is required in the presence of the nanoclay. A similar trend was observed by Najafi et al. ¹⁵³ with optimum nanoclay loading at 4 phc. Essabir et al. ¹⁵⁴ experimented with alfa, pine, bagasse, and coir fibers with nanoclay fillers and found that mechanical properties were optimal at 15 wt% of nanoclay loading. Although the addition of MMT in NFRCs increased the tensile strength and interfacial adhesion between the fibers and matrix, it also increased water absorption and decreased in biodegradability. ¹⁵⁵

Arulmurugan and Venkateshwaran ¹⁵⁶ exfoliated organically modified MMT in jute-PE composite using ultrasonication technique and concluded that best mechanical properties were observed at 5% clay loading. Ramakrishnan et al. ¹⁵⁷ added nanoclay to jute-PE composite using magnetic stirring method and observed the enhancement in tensile, flexural, impact strengths and modulus of elasticity compared to pure, fiber-reinforced polyester, and nanoclay-filled resin. Shahroze et al. ¹⁵⁸ added organomodified nanoclay to sugar palm fiber-reinforced unsaturated polyester resin composite. For an optimal value of 2%, the best performance for tensile properties was observed, while for 4%, the best performance of impact and flexural strengths was observed. Patel et al. ¹⁵⁹ tested the effect of nanoclay on bamboo fiber-reinforced polyester composites for loadings of 0–4 wt%. The optimal loading for the best tensile strength (66.9 MPa) and flexural strength (127 MPa) was obtained at 1%. Hasan et al. ¹⁶⁰ added MMT to jute–polyester resin composite in loadings of 1, 3, and 5% and tested by dynamic mechanical analysis and thermogravimetric analysis. The 5% and 1% sample showed better thermal stability and moisture resistance, respectively.

Applications of NFRCs

The previously discussed chemically treated NFRCs have applications in a wide array of industrial sectors, as shown in Figure 9. Strict rules on the recyclability of automobiles have raised the importance of NFRCs in the automotive industry in certain countries. In Europe, the EU “end of life vehicle” dictates that 95% (by weight) of the components must be recyclable. NFRCs the go-to material owing to their recyclability and light weight structural components used in automotive, which led to lower fuel consumption. A 25% reduction in vehicle weight is equivalent to a saving of 250 million barrels of crude oil and a reduction in carbon dioxide (CO₂) emissions of 220 billion pounds per year. ¹⁶¹

Figure 9.

Applications of natural plant fiber-reinforced composites.

NFRCs are used for various parts of an automobile, as shown in Figure 10. ¹⁶² This includes the Al-Qureshi designed MANACA—the first all-banana reinforced composite car panel ¹⁶³ and the efforts of Johnson Controls Interiors GmbH and Co. (Westendhof 8 Essen, Germany) in producing plant fiber-reinforced thermoplastics for the interior of cars. A sustainable vehicle “Eco Car” was developed by automotive engineers to run on biofuels, having body panels made of NFRCs in a biodegradable resin matrix. ¹⁶⁴ Table 14 presents the usage of NFRCs in automotive sector.^165-168

Figure 10.

Automobile components made of natural fiber-reinforced composites. ¹⁶²

Table 14.

Various applications of NFRCs in automotive sector. ^{165 –16 8}

Manufacturer	Model	Application
Mitsubishi	All models	Door, instrumental panels, and cargo floor area
Rover	2000 and some special models	Rear side storage shelf and panel, insulation
Renault	Twingo, Clio	Shelf in parcel area
Audi	Coupe, A2 to A4 (including Avant), Road Star A6, A8	Hatrack, dashboard area, windshield, pillar cover panel
BMW	All pilot type and 3, 5, and 7 series	Boot lining, special type of headline, and noise reduction panel
Volkswagen	Golf, A4, and Passat variant	Boot lid panel, door panel, and boot liner
Mercedes Benz	C, S, E, and A classes	Instrument panels support, door panels, molding rod/apertures, back rest panel, trunk panel, roof cover, interior insulation engine cover, and wheel box
Ford	Mondeo CD 162, some cars of focus	Boot liner, door inserts, floor trays, and door panels
Toyota	Celsior, Raum, Harrier, Brevis, and ES3	Tire cover, parts of door panel, floor mats spare, and some interior parts
Peugeot	Some cars of 406	Area of parcel shelf, all door panels
General motors	Blazer, Cadillac Deville	Floor mat and some parts of cargo area
Fiat	Alfa Romeo, 146, 156, 159 models, and some cars of Brava and Punto	Only door panels
Opel	Zafira, Vectra, and Astra	Door, pillar cover, instrumental and headliner panels
Citroen	C5	Only interior door paneling
Saab	9S	Door and window panels

NFRC: natural plant fiber-reinforced composite.

NFRCs have also led to two well-established products: Tufnol 6F/45 (cotton-epoxy composite), as shown in Figure 11(a), and Tespa (wood fiber-phenol) composites. ²⁴ Several establishments use chemically treated NFRCs, including Tipco Industries (Valsad, Gujarat, India) TIPWOOD®50EX in furnishing, NEC Corp. (Minato, Japan) and UNITIKA’s “TERRAMAC” (Osaka, Japan) in cell phones due to improved moisture resistance and the fall impact durability ¹⁶⁹ ; EastarBio (Eastman Chemical Company, Kingsport, Tennessee, USA), BioPET or Biomax (DuPont, Deleware, USA), polylactic acid (Dow-Cargill, Nebraska, USA), Ecoflex (BASF), and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (Metabolix, Woburn, Massachusetts, USA) in automobiles. Trend Hunter Eco Company (Toronto, Ontario, Canada) produces natural packaging material using coconut fiber-reinforced composites. DuPont Company produces tennis racquet with flax fibers-reinforced composites. Museeuw bikes have developed first racing bicycles (models MF1, MF3, and MF5) with hemp- and flax-reinforced epoxy composites.

Figure 11.

(a) Tufnol 6F/45 (cotton-epoxy composite) ²⁴ and (b) biodegradable cutlery. ⁵²

Jandas et al. ⁵² produced eco-friendly biodegradable cutlery using surface-treated banana fiber reinforced bio-nanocomposites, as shown in Figure 11(b). Similarly, Asim et al. ¹⁷⁰ reported that PALF fibers can be employed in diversified applications, such as decoration, sports, furniture, aerospace, and fabric industries, as shown in Figure 12. Also, window frame, fencing, and railings of NFRCs are manufactured by commercial companies for household applications, as shown in Figure 13. ¹⁷¹ Sapuan et al.^172,173 designed and fabricated a multipurpose table and telephone stand using banana fiber-reinforced composites, as shown in Figure 14(a) and (b). These biodegradable materials degrade in the natural condition leaving behind biomass and CO₂. ¹⁶¹ Filler composites made from municipal solid waste (MSW) are very promising while considering the annual MSW generation in populous countries, like India, could go up to 68.8 million tons.^174-176 Additionally, research on hybrid composites like MSW-banana and Bauhinia racemosa-glass fibers suggest good thermal and electrical insulation properties.^177,178

Figure 12.

Application of PALF fibers in various industries. ¹⁷⁰ (a) Decoration, (b) sports industries, (c) furniture industries, and (d) aerospace industries.

Figure 13.

Applications of NFRCs as construction materials. ¹⁷¹ (a) Railing, (b) window frame, and (c) fencing.

Figure 14.

(a) Multipurpose table ¹⁷² and (b) banana fibers-reinforced household telephone stand. ¹⁷³

In the future, applications of NFRCs are expected to be seen majorly in the field of eco-friendly materials. The search for the “green materials” which are environmentally compatible is a growing social concern, increasing the rate of depletion of petroleum resources and new environmental regulations. With cost-effective and easy processing technologies, the eco-composites have the potential to compete with commodity plastics with a promising double-digit growth in the future market. ¹⁶¹ Table 15 clearly reveals the modern day usage of NFRCs in various sectors.

Table 15.

Applications of NFRCs in various industries.^{170,171,179-183}

Fiber	Application
Pineapple	Decoration, sports, furniture, aerospace, and fabric industries
Bamboo and bagasse	Special type of window frame, fencing, and railing
Jute	Mainly used as packaging materials, door frames, chipboards, panels in buildings, roofing materials, door shutter, and in textile industry
Oil palm	As structural insulated panel, door frames, windows, and roofing material
Sisal	Panels, shutting plate window, doors, and in paper and pulp manufacturing unit
Hemp	Manufacturing of pipes, cordage, furniture, and electrical appliances
Kenaf	Insulations, bags, mobile cases (due to impact resistance), parts of boats and as packaging material
Flax	Railing system, door shutters. Laptop cases, snowboarding, bicycle frame, decking, and window frame
Cotton	Furniture, textile, and cordage
Coir	Storage tank, post boxes, helmets, mirror casing, building panels, roofing sheet, flush door shutters, voltage stabilizer covers, paper weights, projector cover, and seat upholstery
Wood	Panels, decking, window frames and fencing
Rice husk	Building panels and as dampers
Ramie	Packaging material, fish net, and used in making sewing thread

NFRC: natural plant fiber-reinforced composite.

Challenges and future work

The techniques discussed are not perfect panaceas for attaining the best performance out of NFRCs. Each of these techniques is faced by unique challenges. For chemical treatments, these include delignification of fibers (alkaline treatment), degradation by UV radiation (acetylation treatment), and adverse effects on the pristine internal configurations (permanganate and isocyanate treatment). The challenges with nanoclay composites which manifest as a reduction in the properties of the polymer include poor dispersion and agglomeration of clay particles causing premature failure of the system, weak filler–matrix interface bonding, reducing the stress transfer capability and increasing resin viscosity with the proportion of nanoparticles, thus limiting processability.

Further research should be carried out in the following areas but are not limited to these:

Employing multiple chemical treatments in succession to complement their individual effects, thereby bringing out the best properties of the NFRCs.

Employing chemical treatments prior to dispersing nanoclay to improve fiber–matrix interactions.

Testing new chemicals for their effectiveness in removing the hydroxyl group.

Testing other biological species to achieve similar or better performance than fungal treatment.

Employing alternative nanofillers to nanoclay that are hydrophobic in nature while providing appreciable mechanical properties.

Combining chemical treatments with heat treatments to achieve superior properties.

Employing bioresins to combine with chemically treated fibers to form superior and greener composites.

Advancement in the field of materials science will help to remove the mental block that prevents NFRCs from prevailing in the global market.

Conclusion

NFRCs, by virtue of their appreciable mechanical properties and environmental benefits, are promising materials for the future. Reducing the hydrophilicity of the plant fibers and improving their adhesion with the matrix, thereby achieving enhanced thermomechanical properties and commercializing the same is one of the aims of current research. This review propounds the removal of the hydroxyl groups as an effective way to achieve enhancement in these properties. This article delineated the common chemical treatments carried out to achieve superior properties along with their applications, effects, and working mechanisms. Another method to attain enhancement of properties is through the addition of nanoclay as reinforcement in the matrix. The addition may be selected to achieve conventional, intercalated, or exfoliated nanocomposites depending on the required properties and applications.

The presented challenges can be overcome to engender superior NFRCs, thus opening new vistas for greener products. Furthermore, the research at the intersection of nanocomposites and chemical treatment of plant fibers is in its early stages and is full of potential. We believe that this is the direction to head toward building a world with better materials.