Effect of fiber treatment on physical and mechanical properties of natural fiber-reinforced composites: A review
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S. Behnam Hosseini
,
Haitao Li
Abstract
Due to environmental and financial concerns, there is a growing demand for composite materials in a wide range of industries, including construction and automotive industries. In 2020, the market for wood plastic composites was estimated to be worth $5.4 billion. By 2030, it is expected to have grown to $12.6 billion, with a compound annual growth rate of 8.9% between 2021 and 2030. The fundamental disadvantage of reinforced composites by natural fibers is the different nature of the hydrophilic lignocellulosic and the hydrophobic thermoplastic polymers, although natural fibers would lower total costs. These composites typically fail mechanically as a result of fiber debonding, breaking, and pull-out. In a fiber-reinforced composite, the matrix’s function could be described as distributing the force to the added fibers using interfacial shear stresses. A strong connection between the polymeric matrix and the fibers is necessary for this procedure. Weak adhesion at the interface prevents the composite from being used to its maximum potential and leaves it open to attacks from the environment that could damage it and shorten its lifespan. Poor mechanical performance is caused by insufficient adhesion between hydrophobic polymers and hydrophilic fibers in natural fiber-reinforced polymer composites. Consequently, during the past 20 years, a variety of chemical, thermal, and physical methods have been employed to address these issues. These methods largely concentrated on the grafting of chemical groups that could enhance the interfacial contacts between the matrix and natural fibers. This review article aimed to give information on several types of fiber treatments and natural fiber-treated composites with a specific focus on their physical and mechanical properties.
1 Introduction
Wood plastic composite (WPC) is a synthetic composite made from a polymer matrix imbedded with wood fiber/wood flour. Thermosettings and thermoplastics are the two main categories of polymers. High-density polyethylene (HDPE), polypropylene (PP), and polyvinyl chloride (PVC) are the most often used thermoplastic polymers as a matrix for natural fibers (Table 1), whereas polyester, epoxy, phenolic, and resins are the most widely employed thermoset materials (Table 2) [1]. The fact that thermoplastic polymers can be cut, screwed, and nailed using tools previously used for wood construction and their processing temperatures, which are often lower than wood’s thermal degradation temperature (180–200°C), make them appealing for WPCs, where HDPE (83%), PP (9%), and PVC (7%) are the most common thermoplastic polymers used in WPCs [2].
Properties of typical thermoplastic polymers used in natural fiber composite fabrication (adapted from ref. [1])
| Property | PPa | LDPE | HDPE | PS | Nylon 6 | Nylon 6,6 |
|---|---|---|---|---|---|---|
| Density (g·cm−3) | 0.899–0.920 | 0.910–0.925 | 0.94–0.96 | 10.4–1.06 | 1.12–1.14 | 1.13–1.15 |
| WA-24 h (%) | 0.01–0.02 | <0.015 | 0.01–0.2 | 0.03–0.10 | 1.3–1.8 | 1.0–1.6 |
| T g (°C) | −10 to −23 | −125 | −133 to −100 | N/A | 48 | 80 |
| T m (°C) | 160–176 | 105–116 | 120–140 | 110–135 | 215 | 250–269 |
| Heat deflection Temp (°C) | 50–63 | 32–50 | 43–60 | Max. 220 | 56–80 | 75–90 |
| Coefficient of thermal expansion (mm−1·mm−1·°C × 105)·cm−1 | 6.8–13.5 | 10 | 12–13 | 6–8 | 8–8.86 | 7.2–9 |
| Tensile strength (MPa) | 26–41.4 | 40–78 | 14.5–38 | 25–69 | 43–79 | 12.4–94 |
| Elastic modulus (GPa) | 0.95–1.77 | 0.055–0.38 | 0.4–1.5 | 4–5 | 2.9 | 2.5–3.9 |
| Elongation (%) | 15–700 | 90–800 | 2.0–130 | 1–2.5 | 20–150 | 35–>300 |
| Izod impact strength (J·m−1) | 21.4–267 | >854 | 26.7–1,068 | 1.1 | 42.7–160 | 16–654 |
aPP = polypropylene, LDPE = low-density polyethylene, HDPE = high-density polyethylene and PS = polystyrene.
Properties of typical thermoset polymers used in natural fiber composites (adapted from ref. [1])
| Property | Polyester resin | Vinyl ester resin | Epoxy |
|---|---|---|---|
| Density (g·cm−3) | 1.2–1.5 | 1.2–1.4 | 1.1–1.4 |
| Elastic modulus (GPa) | 2–4.5 | 3.1–3.8 | 3–6 |
| Tensile strength (MPa) | 40–90 | 69–83 | 35–100 |
| Compressive strength (MPa) | 90–250 | 100 | 100–200 |
| Elongation (%) | 2 | 4–7 | 1–6 |
| Cure shrinkage (%) | 4–8 | N/A | 1–2 |
| WA (24 h@20°C) | 0.1–0.3 | 0.1 | 0.1–0.4 |
| Izod impact strength (J·m−1) | 0.15–3.2 | 2.5 | 0.3 |
In response to the rising demand for biodegradable, renewable, and sustainable materials, this class of composite material, also known as “green composites,” has emerged as a significant group of engineering materials. Due to the existence of wood as an organic material, WPCs have a sustainable nature that allows for environmental conservation and waste reduction. Such materials also offer substantial benefits that justifiably support their use [3]. Although the majority of polymers are non-biodegradable, wood fibers (as well as the other main natural-organic fillers including waster agriculture fibers) are usually added to the polymer matrix up to 40–70% by weight and reduce non-biodegradable proportion of WPC significantly [4]. Furthermore, it is possible to use waste or recycled polymer materials to produce WPCs with the minimum negative effect on the environment. Finally, the thermoplastic-based green composites are recyclable and more biodegradable due to their polymer features and natural fiber content, respectively.
The benefits of wood flour include its low cost, lightweight, and accessibility, making it an economically friendly material that can be used in composites [5–8]. The construction and automotive industries are where WPCs are most commonly used, but they are also employed in packaging, the creation of different home furnishings, office equipment, and other goods [5,6,9]. Salemane and Luyt [10] investigated how adding wood flour improved the mechanical properties of a composite made of wood and PP. Since fibers have substantially greater strength and stiffness amounts than matrices, adding fibers to a polymer matrix usually significantly improves the tensile properties of composites.
It is imperative to create alternative, affordable, and ecologically friendly natural fiber sources for plastic composites due to rising wood pricing and demand for traditional wood sectors. The composite’s mechanical, thermal, and other properties are quickly enhanced by the plant fiber utilized as a reinforcing material [11]. For their persuasive characteristics, including cheap prices, lightweight, low density, biodegradability, recyclability, high stiffness and strength, and renewable features, natural fibers are favored against synthetic fibers (Table 3) [12]. Plant-based natural fibers including flax [13,14], hemp [15,16], jute [17,18], sisal [19,20], kenaf [21,22], bagasse [11,23,24], banana [25,26] coir [27,28], plantain [29,30,31], and pineapple fibers [32,33] have been adopted by composite sectors as replacements for synthetic fibers in an attempt to substitute them due to their numerous benefits during the recent decades [34].
Physical and tensile properties of natural fibers and glass fibers (adapted from ref. [56])
| Fiber type | Diameter (µm) | Relative density (g·cm−3) | Tensile strength (MPa) | Elastic modulus (GPa) | Specific modulus (GPa × cm3·g−1) | Elongation at failure (%) |
|---|---|---|---|---|---|---|
| E-glass | <17 | 2.5–2.6 | 2,000–3,500 | 70–76 | 29 | 1.8–4.8 |
| Abaca | — | 1.5 | 400–980 | 6.2–20 | 9 | 1.0–10 |
| Alfa | — | 0.89 | 35 | 22 | 25 | 5.8 |
| Bagasse | 10–34 | 1.25 | 222–290 | 17–27.1 | 18 | 1.1 |
| Bamboo | 25–40 | 0.6–1.1 | 140–800 | 11–32 | 25 | 2.5–3.7 |
| Banana | 12–30 | 1.35 | 500 | 12 | 9 | 1.5–9 |
| Coir | 10–460 | 1.15–1.46 | 95–230 | 2.8–6 | 4 | 15–51.4 |
| Cotton | 10–45 | 1.5–1.6 | 287–800 | 5.5–12.6 | 6 | 3–10 |
| Curaua | 7–10 | 1.4 | 87–1,150 | 11.8–96 | 39 | 1.3–4.9 |
| Flax | 12–600 | 1.4–1.5 | 343–2,000 | 27.6–103 | 45 | 1.2–3.3 |
| Hemp | 25–600 | 1.4–1.5 | 270–900 | 23.5–90 | 40 | 1–3.5 |
| Henequen | — | 1.2 | 430–570 | 10.1–16.3 | 11 | 3.7–5.9 |
| Isora | — | 1.2–1.3 | 500–600 | — | — | 5–6 |
| Jute | 20–200 | 1.3–1.49 | 320–800 | 30 | 30 | 1–1.8 |
| Kenaf | — | 1.4 | 223–930 | 14.5–53 | 24 | 1.5–2.7 |
| Nettle | — | — | 650 | 38 | — | 1.7 |
| Oil palm | — | 0.7–1.55 | 150–500 | 80–248 | 0.5–3.2 | 17–25 |
| Piassava | — | 1.4 | 134–143 | 1.07–4.59 | 2 | 7.8–21.9 |
| PALF | 20–80 | 0.8–1.6 | 180–1,627 | 1.44–82.5 | 35 | 1.6–14.5 |
| Ramie | 20–80 | 1.0–1.55 | 400–1,000 | 24.5–128 | 60 | 1.2–4.0 |
| Sisal | 8–200 | 1.33–1.5 | 363–700 | 9.0–38 | 17 | 2.0–7.0 |
Natural fibers have many inherent drawbacks, including weak compatibility with polymer materials. The hydrophilic feature of the natural fiber that led to weak adhesion and moisture absorption causes weak compatibility in biocomposites. As a result, natural fibers must be pretreated in order to increase the biocompatibility between the fibers and matrix by the activation of fiber’s hydroxyl groups [35]. Pre-processing modifications of composite elements are performed to prevent the fibers’ hydrophilicity and to improve the interfacial characteristics of polymer and natural fibers. Chemical, mechanical, and thermal treatment methods represent the three basic types. Although chemical treatment is frequently employed, it has negative environmental effects. Hence, environmentally acceptable alternatives to chemical treatment including mechanical and thermal methods were recommended [36]. The fiber modification aims to increase the fibers’ hydrophobicity, interfacial connection between the fiber and matrix, roughness, and wettability as well as to reduce moisture absorption, which will improve the composites’ mechanical properties [37–41].
An overview of current developments in the field of natural fiber-treated composites is given in this study, and the effect of various treatments on the mechanical and physical characteristics of composites reinforced by natural fibers is highlighted.
2 Thermal treatment
Thermal treatment of wood, which significantly alters its physical and mechanical properties, is one of the most significant wood treatment techniques [42]. In recent years, the application of thermal modification has accelerated, and it is still expanding as an industrial technique [43]. Normal operating temperatures for thermal treatment are 180 to 260°C. The wood structure is not considerably affected by temperatures below 140°C, while unfavorable degradation occurs at temperatures above 260°C [44]. Wood’s chemical compounds are impacted by heat treatment. The temperature of the treatment has been discovered to have a greater impact on chemical modifications than duration [45].
The chemical modifications that take place in wood by thermal treatment have a number of positive effects on the physical characteristics of wood, including less swelling and shrinkage, improved durability to biological deterioration, alteration in color, and lower equilibrium moisture content. Thermally treated timber has a significant and substantial drawback because its mechanical features deteriorate, limiting its utilization for strength-bearing purposes, and its MOE decreases [46].
Ayrilmis et al. [47] studied how thermal treatment affected the mechanical futures and dimensional endurance of WPCs produced by the flat press method. The dry mixture of Eucalyptus camaldulensis and 50% PP powder was used to make the WPC samples in an autoclave for 20 and 40 min at three different temperatures (120, 150, and 180°C) by employing a standard flat press procedure in a laboratory environment. The study confirmed that depending on the temperature and time circumstances, the thermal treatment of the wood fibers resulted in decreases of 60 and 31% in the TS and water absorption (WA) after 28 days of water soaking, respectively (Table 4). This was mostly due to the thermal treatment’s hydrolysis of hemicelluloses. The TS and WA of the WPC panels were significantly affected by the thermal treatment of the wood fibers, especially over 150°C for 40 min. However, the treatment temperature had a greater impact on the TS and WA than the treatment duration (Table 4) [47]. Based on the duration and temperature circumstances, the modulus of elasticity, rupture, and the internal bond (IB) strength of the wood fibers treated by this method fell by 5 to 19%, 7 to 22%, and 9 to 28%, respectively, while the screw withdrawal resistance reduced from 3 to 11% (Table 5). The IB outcomes depicted that raising the intensity of the thermal treatment had a negative impact on the mechanical interlocking between the polymer and wood fibers [47].
The relationship between the WPC panels’ physical characteristics and the thermal treatment of the wood fibers (the ratio of wood fiber to PP: 50:50 by wt) (adapted from ref. [47])
| Thermal-treatment level (°C, min) | Physical properties | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Density (g·cm−3) | Thickness swelling (%) | WA (%) | |||||||
| 1-day | 7-days | 14-days | 28-days | 1-day | 7-days | 14-days | 28-days | ||
| Untreated reference | 0.82 | 3.98Aa | 5.06A | 7.82A | 8.15A | 6.45A | 16.53A | 24.12A | 25.94A |
| 120,20 | 0.80 | 2.94B | 3.82B | 4.97B | 5.20B | 4.82B | 14.72B | 21.59B | 23.18B |
| 120,40 | 0.82 | 2.64BC | 3.48BC | 4.59B | 4.84B | 4.39B | 14.09B | 20.96B | 21.46B |
| 150,20 | 0.81 | 2.28C | 3.15CD | 4.27B | 4.51C | 3.85C | 13.37BC | 19.77C | 21.12C |
| 150,40 | 0.82 | 2.05CD | 3.05D | 3.81C | 4.02C | 3.47CD | 12.96C | 19.25C | 20.58D |
| 180,20 | 0.83 | 1.84D | 2.28E | 3.17D | 3.39D | 3.15D | 10.88D | 17.92D | 18.64E |
| 180,40 | 0.81 | 1.72D | 2.05E | 3.06D | 3.25D | 2.91E | 9.79E | 16.95E | 17.84E |
a Letters show Duncan’s multiply range test (p < 0.01).
The relationship between the WPC panels’ mechanical characteristics and the thermal treatment of the wood fibers (the ratio of wood fiber to PP: 50:50 by wt) (adapted from ref. [47])
| Thermal-treatment level (°C, min) | Mechanical properties | |||
|---|---|---|---|---|
| Modulus of rupture (N·mm−2) | Modulus of elasticity (N·mm−2) | IB strength (N·mm−2) | Surface SWR (N) | |
| Untreated reference | 25.9Aa | 2682.4A | 0.95A | 1,010A |
| 120,20 | 24.7B | 2483.8B | 0.86B | 982AB |
| 120,40 | 24.2BC | 2434.5B | 0.83BC | 976AB |
| 150,20 | 23.7CD | 2358.3C | 0.77CD | 951BC |
| 150,40 | 23.0D | 2264.2D | 0.75D | 924CD |
| 180,20 | 21.7 F | 2225.8D | 0.71DE | 915CD |
| 180,40 | 21.0 F | 2102.5E | 0.68E | 894D |
aLetters show Duncan’s multiply range test (p < 0.01).
The creation of soluble acidic compounds, including acetic acid and formic acid, from the decomposition of hemicellulose may be responsible for the WPC panels’ loss of mechanical features [48]. The crystalline structure of cellulose (long chain) is broken down into shorter pieces by these acids, which speed up the cellulose depolymerization. Additionally, as the temperature and length of the treatment increase, C–C and C–O links cleave within the polymer itself. As a result, the copolymer system of lignin–hemicellulose separates, and hemicelluloses and amorphous cellulose are depolymerized [49]. The modulus of rupture (MOR) and modulus of elasticity (MOE) of the wood may be influenced by cellulose depolymerization and the decrease of its length chain [50]. Other studies also showed similar outcomes [51–54].
According to Hu et al. [55], the mechanical and physical characteristics of the bamboo plastic composite were examined in relation to the influence of bamboo fibers that had been modified by utilizing a vacuum-heated method. The outcomes showed that the bamboo fibers’ surface polarity and hemicellulose content could be decreased after the vacuum heat treatment, enhancing the interface compatibility between the bamboo fiber and polyethylene. The composite’s WA after 24 h was reduced by 73.01% when the temperature was 160°C in comparison with the control group, and the composite’s thickness swelling after 24 h was the lowest point at 180°C, which, compared to the control group, was 71.47% lower. The mechanical strength of the composites also exhibited a pattern of raising and reducing as a function of vacuum heat treatment temperature. The bending strength and modulus of the composites reached the highest amount at 180°C, increasing by 39.91 and 21.77%, respectively, compared to the control group. Thus, the findings showed that adding a compatibilizer and thermal treatment of the composites could greatly increase their bending strength and water resistance, but it may also somewhat diminish their impact strength [55].
2.1 NaOH
Alkali modification is advantageous for cleaning the surface of the fiber, altering the surface chemistry, reducing moisture absorption, and raising the roughness of the surface. The process helps clean away contaminants and waxy particles from the surface of the fiber and produces a smoother texture that makes mechanical interlocking easier. The chemical connection between the reinforcement and resin is also strengthened by the cleaned fiber surface [56].
Bharath and Basavarajappa [57] evaluated phenol-formaldehyde (PF) composites containing coconut tree leaf sheath (CLS) for their fire resistance capability. To create CLS samples, a conventional hot press was employed with 60 wt% untreated and treated CLS fibers with 5% NaOH. The results of the flammability test showed that the treated composites’ mass loss rate and flame propagation rate had been reduced, while their flame resistances had been increased. The limiting oxygen index test also showed that alkali-treated composites needed more oxygen to burn than untreated composite material [57].
Guo et al. [58] reported that following alkalization, some pectin, hemicellulose, lignin, and other low-molecule contaminants dissolve and are eliminated, along with a significant number of hydrogen bonds. The fiber’s surface gets rougher, which improves the fiber’s ability to bind to the resin at the interface. The remaining material is primarily composed of wood fiber. The hydroxyl of the crystalline wood fiber is exposed to the alkaline solution, which causes the wood fiber to become fluffy and better able to attach to the coupling agent. This reduces the hydrophilicity of the wood fiber and enhances the interface’s adhesion properties and wettability. The alkalization process reduces the fiber split, decreasing its diameter, while enhancing its aspect ratio, resulting in an expansion of the fiber’s interface with the matrix [58].
Li et al. [59] investigated the possibilities of adopting several kenaf core pre-treatment using HDPE. The study found that pre-treatment had little impact on fungus resistance and that the duration of pectinase or cellulase enzyme pre-treatment should be reduced to 30 or 60 min, respectively. The mechanical performance of a 60/40 kenaf/HDPE mixture was significantly enhanced by pre-treating the core particles with 1% NaOH or 1% HCl for 60 or 30 min, respectively. Pre-treating the kenaf fibers for 1 h with NaOH resulted in increases in MOE and MOR of approximately 50 and 82%, respectively [59].
Corn husk fiber (CHF) and recycled polystyrene foam (rPS) composites’ tensile, thermal, WA, and morphologies were studied by Chun et al. [60] in relation to their fiber loading and alkaline treatment. According to the tensile test results, rPS/CHF composites’ strength and modulus have increased by an average of 26 and 13%, respectively. A portion of the hemicellulose, lignin, and other compounds were eliminated from CHF after the alkaline treatment, and the fibers’ surface roughness was raised. The interfacial interlock between the treated fibers and the rPS polymer was enhanced. Because of this, there was an improvement in the stress transmission between the fiber and matrix, leading to composites with greater strength and modulus. Similar findings were observed in other works as well [61,62]. Compared to untreated rPS/CHF composites, the average WA is 29% lower in treated rPS/CHF composites. Additionally, Chun et al. [63,64] discovered that composite materials with treated natural filler will absorb less water.
Baffour-Awuah et al. [65] investigated the effects of treating wood flour with alkali and ultrasound methods on the mechanical characteristics of wood polypropylene composites. The findings demonstrated that lignin and hemicellulose were eliminated from the wood by the alkali treatment, and the amount of hydroxyl groups on the surface of the cellulose increased. This technique was made more effective by ultrasonic modification. When PP grafted with maleic acid was utilized as a compatibilizer, mechanical features of WPC specimens demonstrated that alkali treatment increased both composite strength and modulus. While stronger modulus results from the elimination of hemicellulose and lignin, which are less stiff than cellulose, the strength improvement is caused by enhanced adhesion between the matrix and the fiber. Composites having 1 and 3% NaOH-modified wood showed improvements in average tensile modulus of 13% (from 4.27 to 4.83 GPa) and 20% (from 4.27 to 5.13 GPa), respectively, as comparison to composites manufactured with untreated wood. It has been definitively proven that chemical treatment of wood combined with ultrasonic improves the mechanical properties of composites more effectively than chemical treatment [65]. The alkali solution penetrates the wood flour by utilizing ultrasound, which also gives each particle uniform vibrations. By combining with alkali treatment, ultrasound expedites the elimination of lignin from wood and boosts the amount of hydroxyl groups on the cellulose’s surface. The particle size of the wood is also decreased by the application of ultrasound. Thus, all of the mentioned processes have an impact on the stiffness and strength of WPCs produced by alkali-treated wood [65].
Cui et al. [66] investigated the role of weight fraction and wood fiber length, as well as three surface modifications, including silane, alkaline, and alkaline + silane methods, on the mechanical features of wood recycled plastic composites (WRPCs). The research showed that as the amount of wood fiber rose, flexural strengths increased and impact strengths decreased. Although the flexural modulus improved with longer fibers, the impact and flexural strengths reduced. The strongest mechanical features were observed in a composite composed of wood fibers treated by alkaline + silane methods along with MAPP. According to Figure 1, all three surface treatments enhanced the WRPC materials’ flexural strength in comparison with unreinforced samples. Although the findings for the silane treatment are relatively close to those for the combined treatment, the alkaline + silane-treated composites demonstrated the greatest flexural strength when compared to the other two types of composites. The greatest improvement was about 27% (50 wt%). Because of the complex microstructure of wood fiber, a surface treatment method may only be able to slightly alter the surface properties, which results in a limited increase in flexural strength [67]. The flexural modulus and surface treatment trends are quite close to flexural strengths, where 50 wt% treated wood fiber by alkaline + silane had the highest flexural modulus (Figure 2) [66].
![Figure 1
Flexural strengths of wood fiber composites as a function of wood fiber content with different surface treatment methods (adapted from ref. [66]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_001.jpg)
Flexural strengths of wood fiber composites as a function of wood fiber content with different surface treatment methods (adapted from ref. [66]).
![Figure 2
Flexural modulus of WRPC material as a function of wood fiber fraction with different surface treatment methods (adapted from ref. [66]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_002.jpg)
Flexural modulus of WRPC material as a function of wood fiber fraction with different surface treatment methods (adapted from ref. [66]).
Nam et al. [68] produced composite material using treated coconut shell fiber and polylactic acid after soaking it in a 5% NaOH solution for 72 h. In comparison to untreated coconut shell fiber, the research revealed that the shear strength and tensile strength had improved by 72.8%.
Rajeshkumar et al. [69] evaluated the effects of four levels of NaOH concentrations (5–20%) on the mechanical characteristics of epoxy composites reinforced by Phoenix sp. fibers. In comparison to untreated fiber composites, the results showed that the treated fibers had better interfacial adhesion with polymeric matrix, decreased mechanisms of failure (including fiber debonding and pull-outs), and had improved mechanical characteristics [69]. Figure 3 shows that reinforced specimens using 15% treated fiber have the highest tensile modulus (589.12 MPa), which is 18.25% higher than the untreated composite. According to the study’s findings, composites with 15% treated fiber have the best flexural characteristics (Figure 4), and their flexural strength (143.65 MPa) is much higher than that of the control samples (117.05 MPa). It was determined that the qualities of this composite material complied with the requirements of EN standards 312-2 and 312-3 and, therefore, could be utilized for industrial purposes such as car panels [69].
![Figure 3
The effect of NaOH treatment on tensile properties of reinforced composites (adapted from ref. [69]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_003.jpg)
The effect of NaOH treatment on tensile properties of reinforced composites (adapted from ref. [69]).
![Figure 4
The effect of NaOH treatment on flexural properties of reinforced composites (adapted from ref. [69]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_004.jpg)
The effect of NaOH treatment on flexural properties of reinforced composites (adapted from ref. [69]).
2.2 Polyvinyl alcohol (PVA)
A type of secure, eco-friendly, and water-soluble material that can be employed as fiber modification method is PVA. PVA units that have been somewhat alcoholic have both hydrophilic and hydrophobic OH groups and molecular chains, respectively, that are well compatible with the hydrophilic nature of cellulose. PVA’s hydroxyl groups can be bonded by borax, formaldehyde, and other chemicals. In order to increase the interfacial adhesion between natural fiber and polymer matrix, PVA can be employed to treat the fiber [70].
To assess the mechanical characteristics and water uptake of the composites, Hu et al. [70] coated the sisal fibers with an alkali + PVA using an ultrasonic process. The sisal-reinforced composites were manufactured using a twin-screw extruder. The study demonstrates that sisal fiber and HDPE can effectively be treated with an alkali/PVA coating compound to increase the interfacial adhesion, enhance the mechanical characteristics of the composite, and decrease WA. The interfacial adhesion can be clearly seen in the micromorphology of the composites’ cross sections. The interfacial adhesion between sisal fibers and the HDPE polymer has been considerably enhanced following surface modification, which is also supported by the scanning electron microscope (SEM) photos of the cross section and other scientists [71–75]. As depicted in Figure 5(a), weak mechanical property is often caused by the insufficient adhesion of sisal fibers to HDPE polymer. Figure 5(b) illustrates that the interfacial bonding enhances as the space between the alkali-treated fibers and polymer reduces. As shown in Figure 5(c), there is not much space between alkali + PVA-treated sisal fibers and HDPE. According to the observations, the sisal was only fractured at the cross section and not pulled out. Both tensile and flexural strengths increased initially and then declined when the sisal fiber amount was raised (Figures 6 and 7). The flexural and tensile strengths of the alkali/PVA-coated sisal fiber composites attained their highest amounts of 45.84 and 26.86 MPa, respectively, when the sisal fiber content reached 12%. The alkali/PVA coating treatment increased the tensile and flexural strength of composites by up to 18.85 and 18.90%, respectively, in comparison to untreated sisal fiber composite. However, once sisal fiber levels rose beyond 12%, fiber aggregation took place and led to weak distribution. The WA of sisal fiber/HDPE composites with a 12% filling content is shown in Figure 8. By sisal fiber’s surface treatment, the composite materials’ capacity to absorb water was drastically reduced. Untreated sisal composite absorbed 2.03% of water, but alkali-treated sisal composite absorbed 1.57% of water, 22.67% less than untreated sisal composite. The alkali/PVA coating treated sisal fiber composite absorbed 1.33% of water, 34.48% less than untreated sisal fiber composite [69].
![Figure 5
SEM images of sections of untreated sisal composites (a), alkali-treated sisal composites (b), and alkali/PVA coating compound-treated sisal composites (c) and (d). SEM: scanning electron microscope; PVA: polyvinyl alcohol (adapted from ref. [70]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_005.jpg)
SEM images of sections of untreated sisal composites (a), alkali-treated sisal composites (b), and alkali/PVA coating compound-treated sisal composites (c) and (d). SEM: scanning electron microscope; PVA: polyvinyl alcohol (adapted from ref. [70]).
![Figure 6
Tensile strength of composites (adapted from ref. [70]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_006.jpg)
Tensile strength of composites (adapted from ref. [70]).
![Figure 7
Flexural strength of composites (adapted from ref. [70]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_007.jpg)
Flexural strength of composites (adapted from ref. [70]).
![Figure 8
WA of composites (adapted from ref. [70]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_008.jpg)
WA of composites (adapted from ref. [70]).
2.3 Steam-exploded (SE) treatment
The impact of adding SE wood flour to a wood flour/plastic composite was studied by utilizing Cryptomeria japonica (Japanese cedar), Shorea negrosensis (red meranti), and Fagus crenata (beech) and three types of thermoplastic polymers including PVC, polymethylmethacrylate, and polystyrene [76]. The substitution of SE for wood flour often enhanced the wood plastic board’s modulus of elasticity and rupture, and resistance to water. However, depending on the plastic polymer and partially the wood species of SE, the best-performed composition of the board was varied. Despite plastic polymer, water durability was superior for all composites utilizing SE compared to untreated composites, and the SE wood species had minimal effects on the final board’s qualities [76].
2.4 Fungicide solutions
Ashori et al. [77] studied the impact of two chemical preservatives including “3-iodo-2-propynyl butylcarbamate (IPBC)” and “2 thiazol-4-yl-1H-benzoimidazole (TBZ)” on endurance against the Coriolus versicolor (white-rot fungus) as well as physical and mechanical characteristics of HDPE-reinforced composites by poplar wood flour (PF). The study revealed that treated composites had much lower strength losses and higher resistance against decay than untreated samples. Fungicide treatments enhanced TS and WA, but there was no obvious difference between the modified samples (Table 6). The weight loss of different treated composites varied from 1.1 to 4.5%, and the control had the greatest weight loss. Additionally, compared to the treated composites with TBZ, samples that had undergone IPBC treatment had slightly less weight loss. As a result, reinforced composites can be properly protected with IPBC and TBZ. Therefore, IPBC demonstrated better results than TBZ, and it is advised to protect WPCs [77].
Average mechanical and physical properties before and after fungal incubation (adapted from ref. 77)
| Treatment | Property | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| MOR (MPa) | MOE (MPa) | NI (kJ·m−2) | WA (%) | TS (%) | ||||||
| Before | After | Before | After | Before | After | Before | After | Before | After | |
| Control | 47.4 (1.2) | 31.7 (3.4) | 4,424 (76) | 2,466 (221) | 0.43 (0.08) | 0.34 (0.08) | 11.4 (1) | 14.9 (3.3) | 4.1 (1.2) | 5.0 (3.2) |
| A1 | 49.1 (0.9) | 34.0 (2.2) | 4,046 (115) | 2,730 (154) | 0.37 (0.09) | 0.31 (0.08) | 9.4 (1.1) | 10.8 (2.1) | 4.0 (0.9) | 4.5 (1.4) |
| A2 | 44.3 (0.8) | 35.9 (2.1) | 4,360 (97) | 2,877 (165) | 0.37 (0.09) | 0.35 (0.09) | 8.6 (0.5) | 10.0 (3.1) | 3.9 (1.4) | 4.4 (2.7) |
| A3 | 43.9 (1.4) | 39.6 (2.5) | 4,250 (136) | 3,192 (111) | 0.38 (0.09) | 0.37 (0.1) | 10.4 (2.2) | 11.3 (2.2) | 4.3 (1.3) | 4.7 (2.1) |
| B1 | 37.4 (1.9) | 26.8 (3.0) | 3,951 (117) | 2,339 (89) | 0.41 (0.07) | 0.31 (0.09) | 12.7 (0.9) | 14.7 (3.4) | 3.8 (0.5) | 4.1 (1) |
| B2 | 34.4 (2.4) | 27.8 (2.8) | 3,686 (112) | 2,395 (99) | 0.37 (0.09) | 0.34 (0.08) | 12.1 (1.3) | 14.1 (1.9) | 3.7 (3) | 4.2 (2.1) |
| B3 | 32.8 (1.9) | 27.2 (1.2) | 3,723 (80) | 2,627 (146) | 0.37 (0.08) | 0.35 (0.03) | 13.4 (1.4) | 14.5 (2.9) | 4.5 (1) | 4.9 (1.2) |
| A1B1 | 41.8 (2) | 37.8 (1.0) | 4,179 (116) | 3,080 (79) | 0.37 (0) | 0.36 (0.08) | 11.8 (1.3) | 13.8 (1.9) | 5.2 (4.1) | 5.8 (2.3) |
| A3B3 | 43.0 (1.7) | 40.8 (1.0) | 4,282 (89) | 3,437 (100) | 0.39 (0.02) | 0.37 (0.03) | 9.3 (1.5) | 10.2 (2.3) | 4.6 (3.2) | 5.0 (2.3) |
Note: Values listed in parentheses are the standard deviations based on four specimens.
The thermal decomposition, dimensional stability, and biological and mechanical properties of HDPE-reinforced composites by CCA-treated wood, which were collected after serving for 20 years, were examined by Tascioglu et al. [78]. With the exception of the Izod impact strength, the WPC’s dimensional stability and mechanical properties were enhanced by the recycled CCA-treated wood flour and coupling agent. The biological analysis confirmed that the treatment enhanced resistance to fungus and termites. Instead of treating wood with chemicals, this procedure could be utilized as a substitute recycling method.
Because the majority of WA occurs in the wood, increasing the wood flour percentage made the material more susceptible to decay and mass reductions. Several borate-based biocides, including zinc borate (ZB), disodium octaborate tetrahydrate, calcium/sodium borate, and boric acid, can stop the fungi and other biodeteriorates [79]. ZB, which has a high level of leaching resistance, broad efficacy against insects and fungi as well as very low mammalian toxicity and cost, is widely utilized commercially in a variety of wood composites, such as wood plastic and particleboard [80].
Hosseinihashemi and Badritala [79] studied the long-duration WA of wood flour prepared by ZB in a PP matrix as influenced by “Trametes versicolor” after 98 days of incubation. Since the WA and diffusion coefficients differed depending on the sample formulation, it was clear that the ZB pretreatment employed in the current investigation had a beneficial impact on the WA of WPCs. Figure 9 illustrates how the ZB-containing specimens had less WA than the untreated ones. In comparison to the untreated composites, the WA of the ZB-T-WPC and ZB-PT-WPC samples reduced by 4.6 and 27.9%, respectively. This is mostly due to ZB’s lower water solubility. Additionally, the average WA was higher in the ZB-T-WPC samples compared to the ZB-PT-WPC samples. It might be anticipated that ZB precipitation in cell lumens and cell walls will eventually decrease in WA [79].
![Figure 9
Typical WA curves for selected composite formulations. UT-WPC: untreated wood flour/plastic composite; ZB-T-WPC: zinc borate treated wood flour in manufacturing process/plastic composite; ZB-PT-WPC: zinc borate pretreated wood flour/plastic composite (adapted from ref. [79]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_009.jpg)
Typical WA curves for selected composite formulations. UT-WPC: untreated wood flour/plastic composite; ZB-T-WPC: zinc borate treated wood flour in manufacturing process/plastic composite; ZB-PT-WPC: zinc borate pretreated wood flour/plastic composite (adapted from ref. [79]).
Badritala et al. [80] investigated how ZB treatment affected the mechanical and morphological characteristics of wood flour in the PP matrix. Except for tensile strength, the ZB treatment had no effect on the composite’s mechanical features. In comparison to the untreated samples, ZB-containing samples had reduced flexural, tensile, and impact strengths. However, the hardness was only slightly enhanced by the ZB modification. According to SEM results, certain crystalline deposits of ZB were deposited on the outer surface of the wood fibers, increasing the surface area of the solids inside the WPC and decreasing the bond effectiveness of the polymer [80].
Gnatowski [81] investigated how zinc borate (ZnB) affected WPCs’ ability to absorb water. The findings demonstrated that two industrial WPCs with ZnB absorbed lower water in exposed situations. The moisture diffusion coefficient of WPCs made of HDPE and ZnB was studied by Jahadi et al. [82]. The findings indicate that the diffusion constant of the composites was reduced by the addition of ZnB (1%).
2.5 Chemical coupling agents
One of the major chemical techniques for enhancing interfacial adhesion is chemical coupling. In this technique, a substance is applied to the fiber surface to create a chemical link between the matrix and the fiber. The majority of researchers discovered that these medications were successful and improved interfacial adhesion [37]. In order to treat wood fibers, more than 40 different compatibilizers have been studied. The most important types include anhydrides, silanes, isocyanates, and anhydride-modified copolymers [83]. In general, the literature claims that using maleic anhydride (MA) grafted matrices as a coupling agent increases tensile strength and elongation at break [39]. The load of the coupling agent is a key factor in the treatment of fibers. Insufficient bonding between the fibers and matrix results from low values of coupling agents. In contrast, taking too much might cause substantial fiber’s agglomeration [84,85]. As a result, the best performance of the composite materials would need the appropriate amount of compatibilizers [86].
Cui et al. [87] treated the wood fibers using compatibilizers, including MA, methyl methacrylate (MMA), and KH550, in order to decrease the water uptake of composites. The outcomes showed that the three surface modifications could successfully reduce the TS and WA of WRPCs. The WA ratios of WRPCs with treated wood fibers by KH550, MMA, and MA were decreased by 10.0, 21.7, and 27.6%, respectively, compared to the untreated samples, and the impact toughness was improved by 18.4, 24.0, and 2.1%, respectively, after 4 weeks of soaking in hot water. When WRPC was treated with KH550 and MA, the tensile strength improved by 26.0 and 11.1%, respectively, but it reduced by 6.5% when treated with MMA. The effective approach for decreasing the WA of composites was treated wood fibers by coupling agents, and this type of composite had the best overall features [87].
In the study conducted by Khamtree et al. [88], alkaline, silane, and alkaline–silane treatments were employed to rubber wood flour (RWF) to create reinforced recycled polypropylene (rPP) composites. To assess the role of treatments in WA, morphological characterization, mechanical and thermal properties of WPCs, silane treatment was employed at different proportions and durations. In comparison to silane or alkaline alone, the results showed that RWF treated with an alkaline–silane mixture demonstrates superior properties. Additionally, treatment times mostly unchanged the mechanical strength, hardness, and WA, although silane amounts had a considerable impact on these properties. The highest water resistance, mechanical strength, and hardness of WPCs were achieved by an alkaline-silane treatment (5% silane for 2 h). This procedure also raised the crystallinity of the WPCs and enhanced the interfacial bonding of RWF and rPP [88].
According to Hou et al. [89], incorporating the two treatments for composites will improve bonding and might even have an unanticipated synergetic effect.
Ramlee et al. [90] studied the different characteristics of sugarcane bagasse (SCB) and oil palm empty fruit bunch (OPEFB) composites treated with 4% H2O2 and 2% silane. In accordance with the findings, a 24-h examination revealed that the silane-treated sample had the best mechanical characteristics and the least amount of thickness swelling and water uptake. Flexural strength (Figure 10) and flexural modulus (Figure 11) of reinforced composites were significantly improved with the addition of 2% silane-treated fibers compared to H2O2 and untreated composites. The research revealed that silane treatment enhanced the functionality of waste natural fibers, and green composites containing a mixture of different fibers meet both the physical and mechanical requirements for insulating board standards to create new classes of sustainable and eco-friendly construction materials such as thermal insulation [90].
![Figure 10
Flexural strength of treated reinforced composites (adapted from ref. [90]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_010.jpg)
Flexural strength of treated reinforced composites (adapted from ref. [90]).
![Figure 11
Flexural modulus of treated reinforced composites (adapted from ref. [90]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_011.jpg)
Flexural modulus of treated reinforced composites (adapted from ref. [90]).
Vinod et al. [91] assessed the role of NaOH, oxalic acid, and silane as treatments of soy stem fibers in the physical and mechanical features of reinforced composites. The study’s findings demonstrated that silane treatment of composites improved their mechanical performance when compared to NaOH, oxalic acid, and untreated fibers. The soy fibers’ thermal stability was enhanced after the silane treatment, and the silane-treated composite had a lower coefficient of thermal expansion [91]. The significant improvement in mechanical and physical properties of silane-treated composites was also confirmed by Ramesh et al. [92], where the tensile strength of silane-treated fiber composites varies in the range of 35–40 MPa, which is two times higher than that of untreated fiber composites.
2.6 Plasma treatment
By inserting functional groups like sulfonates and amine into wood fibers, plasma treatment is an environmentally friendly mechanical treatment that is applied to enhance the wood fibers’ surface features. It could be applied to remove contaminants from the fiber surface, increase fiber porosity, and the wood fiber defibrillation, which would then improve the mechanical interlocking of polymer on the surface of fibers. Along with the advantages that plasma treatment has over chemical-based treatments in terms of the environment, it also takes much less time (1–3 min) than traditional chemical processes, which need several days to treat fibers appropriately [36]. For the fibers of Spanish broom and beech veneer, plasma modification transforms the soft surface of the fibers into a cross-linked one (Figure 12) [93,94]. High plasma discharge power results in greater surface etching, which increases the degree of inhomogeneity of the fibers by creating deeper and wider cracks. On the fiber surface, which was peeled off, threads were produced [36].
![Figure 12
SEM pictures of: beech veneer’s surface (a) untreated, (b) 1 kW plasma treated fibers, (c) 2 kW plasma treated fibers; Spanish Broom fibers (d) untreated (e) 10 W plasma treated fibers (f) 70 W plasma treated fibers (adapted from ref. [36]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_012.jpg)
SEM pictures of: beech veneer’s surface (a) untreated, (b) 1 kW plasma treated fibers, (c) 2 kW plasma treated fibers; Spanish Broom fibers (d) untreated (e) 10 W plasma treated fibers (f) 70 W plasma treated fibers (adapted from ref. [36]).
It has been shown to be beneficial to treat WPC by low-pressure plasma to increase adhesion. Although treating WPCs with low-pressure plasmas is efficient, it is challenging to implement on an industrial scale since a vacuum is required, the treatment is not continuous, and it can only be applied to small pieces [95].
Hämäläinen and Kärki [96] used contact angle measurement with sessile drop technique and tensile strength testing of glued samples to examine the impact of atmospheric plasma treatment on PP and spruce (Picea abies) wood–plastic composite surfaces. WPC profiles that have been extruded are subjected to a plasma treatment. The findings demonstrate an enhancement in the tensile strength of glued specimens after plasma treatment as well as an improvement in the contact angle of WPC materials treated with plasma. According to the research findings, it is evident that atmospheric plasma processing enhances the bonding of WPCs, as proved by the results of sessile drop and tensile strength tests of glued specimens. An increase in polar groups in the Raman spectra could serve as a sign of the plasma treatment’s effectiveness [96].
2.7 Distillate treatment
Väisänen et al. [97] investigated how hardwood distillate (HWD) affected the characteristics of a commercial WPC made of thermally treated sawdust (Scots pine) in a polymeric matrix by applying several levels (1–8 wt%) of distillate to the mixture. The results of the mechanical experiments revealed that adding 1 wt% of HWD to WPCs improves their flexural and tensile properties but not their impact strength (Table 7). Incorporating up to 4 wt% HWD greatly reduced the WA of WPCs without decreasing the observed mechanical features, which is another major result from this study. Table 7 shows a substantial tensile modulus improvement occurred by incorporation of HWD (1 wt%). Additionally, improvements in other mechanical characteristics were seen by utilizing distillate modification for produced composites. The reason why composites are more rigid is that HWD occupies the cracks and spaces in the composites, which is also demonstrated by the fact that bending and strain were reduced on greater distillate amounts (above 4 wt%) [97].
Densities and mechanical properties of the studied materials (adapted from ref. [98])
| Material | Density (g·cm−3) | Tensile strength (MPa) | Tensile modulus (GPa) | Strain (mm) | Flexural strength (MPa) | Modulus of elasticity (GPa) | Bending (mm) | Charpy’s impact strength (kJ·m−2) |
|---|---|---|---|---|---|---|---|---|
| LG | 1.130 ± 0.002 | 22.41 ± 0.82 | 2.09 ± 0.20 | 1.86 ± 0.26 | 44.09 ± 2.00 | 3.05 ± 0.16 | 4.54 ± 0.28 | 11.57 ± 2.02 |
| LG + HWD1 | 1.131 ± 0.002 | 22.85 ± 0.31 | 2.33 ± 0.09a | 1.81 ± 0.12 | 45.27 ± 1.01 | 3.21 ± 0.08b | 4.42 ± 0.28 | 10.61 ± 1.27 |
| LG + HWD2 | 1.132 ± 0.002 | 22.26 ± 0.30 | 2.24 ± 0.06 | 1.83 ± 0.15 | 43.09 ± 0.83 | 3.11 ± 0.12 | 4.37 ± 0.30 | 11.17 ± 0.81 |
| LG + HWD4 | 1.138 ± 0.004 | 22.05 ± 0.44 | 2.16 ± 0.06 | 1.92 ± 0.17 | 42.86 ± 1.17 | 2.89 ± 0.09b | 4.72 ± 0.29 | 10.89 ± 1.13 |
| LG + HWD8 | 1.141 ± 0.003 | 19.67 ± 0.52a | 1.86 ± 0.11a | 1.85 ± 0.13 | 39.36 ± 0.63a | 2.73 ± 0.09a | 4.67 ± 0.27 | 9.42 ± 0.69b |
WPC: wood–plastic composites; LG: LunaGrain; HWD: hardwood distillate.
a p < 0.01 (highly significant difference compared with the unmodified WPC).
b p ≤ 0.05 (significant difference compared with the unmodified WPC).
Väisänen et al. [98] examined the effect of several treatments (alkali, enzymatic, steam, and wood distillate) on the properties of hemp fibers and their composites. Utilizing vacuum-assisted resin transfer molding, long (aligned) hemp fiber (Cannabis sativa L.) reinforced epoxy composites were manufactured. The findings demonstrate that the fibers’ tensile strength, Young’s modulus, and toughness all massively enhanced after steam treatment. Water uptake of the reinforced samples was dramatically reduced as a result of the treated hemp fibers (Figure 13). The fiber modification by the alkali method depicted the minimum WA levels among other processes.
![Figure 13
WA of different epoxy-hemp composites during 28 days of immersion (adapted from ref. [99]).](/document/doi/10.1515/rams-2023-0131/asset/graphic/j_rams-2023-0131_fig_013.jpg)
WA of different epoxy-hemp composites during 28 days of immersion (adapted from ref. [99]).
According to Baghaei et al. [99], an improvement in the interfacial adhesion between the polymer matrix and fibers may be utilized to explain alkali treatment led to a reduction in the WA. Another factor is that the elimination of hemicelluloses and other hydrophilic materials reduced fibers’ hydrophilicity. Another effective strategy to reduce the composite’s WA is enzyme treatment. Compared to other treatments, the treated fibers by wood distillate indicated low efficiency in water up taking properties. However, untreated fibers still differ significantly from fibers that have been treated using wood distillate [98].
3 Conclusion and future scope
Natural fibers have many benefits to utilize in composite materials including high elastic modulus, flexural strength, flexibility, low density, renewability, biodegradability, and recyclability. These qualities have made natural fiber-reinforced composites more desirable. The hydroxyl groups of hemicellulose, cellulose, and lignin, which compose natural fiber, form many hydrogen bonds in the wood’s structure. The hydroxyl groups may create new hydrogen bonds with water molecules, causing water uptake, fiber expansion, and the development of small cracks in the specimen, leading to the fiber debonding and the deterioration of the interface between polymer and fiber. Due to the hydrophobicity of the polymer and the hydrophilicity of the fiber, polymer matrix containing natural fibers typically exhibit weak interfacial adhesion and hence have a limited capability to transfer stress from the matrix to the reinforcement materials. The fiber surface may experience a variety of treatments, which are mainly categorized as chemical, physical, and thermal treatments, to enhance bonding properties, decrease WA, enhance weak wettability, and improve both structural and mechanical properties. The utilization of a wide range of fiber treatments on different properties of reinforced composites with natural fibers was discussed in this research. Generally, physical and thermal treatments compared to chemical methods are more environmentally friendly, but they require higher energy consumption, are relatively newer, and also have the appropriate potential to develop in the future. According to the results of the studies, the physical and mechanical properties of the fiber-treated composites were typically improved by using fiber treatments, and combined treatment methods may exhibit an unanticipated synergetic activity.
Future research should concentrate on creating fully bio-composites utilizing biodegradable or recycled polymers and a higher percentage of natural fibers, as well as focus on a better understanding of the natural fiber’s interaction with polymers through fiber modification to create environmentally friendly innovative products in all potential industries, including building construction, automotive, and outdoor and indoor applications. Natural fiber surfaces can be treated using traditional treatments (mainly chemical methods) and eco-friendly approaches to improve the interaction of fiber and polymer for a variety of purposes. Natural fiber modification using chemical methods, including benzoylation, peroxide, mercerization, acetylation, compatibilizers, and polymer grafting, requires the use of a large number of dangerous compounds. The manufacturing costs of natural fiber-reinforced composites may rise due to appropriate management of chemical waste. In order to treat the natural fibers, eco-friendly approaches such as bacteria, fungi, enzymes, plasma, cellulose, nanocellulose coating, and supercritical carbon dioxide should be employed. These methods are a great replacement for traditional techniques. Researchers should focus on two major areas of interest in future research projects based on the reviewed articles in the current study: firstly, moving toward environmentally friendly methods for fiber treatment, and secondly, the incorporation of two or more treatment methods simultaneously with the evaluation of different levels, ratios, durations, and other experimental situations to develop eco-friendly fiber modification techniques for the manufacturing of reinforced composites.
Acknowledgments
The authors would like to thank the Department of Furniture, Design and Habitat Brno, Mendel University in Brno for the close collaboration in this article.
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Funding information: The authors state no funding involved.
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Conflict of interest: David Hui, who is the co-author of this article, is a current Editorial Board member of Reviews on Advanced Materials Science. This fact did not affect the peer-review process. The authors declare no other conflict of interest.
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Data availability statement: Data sharing is not applicable to this article as no new data were created or analyzed in this study.
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- Exploring temperature-resilient recycled aggregate concrete with waste rubber: An experimental and multi-objective optimization analysis
- Study on aging mechanism of SBS/SBR compound-modified asphalt based on molecular dynamics
- Evolution of the pore structure of pumice aggregate concrete and the effect on compressive strength
- Effect of alkaline treatment time of fibers and microcrystalline cellulose addition on mechanical properties of unsaturated polyester composites reinforced by cantala fibers
- Optimization of eggshell particles to produce eco-friendly green fillers with bamboo reinforcement in organic friction materials
- An effective approach to improve microstructure and tribological properties of cold sprayed Al alloys
- Luminescence and temperature-sensing properties of Li+, Na+, or K+, Tm3+, and Yb3+ co-doped Bi2WO6 phosphors
- Effect of molybdenum tailings aggregate on mechanical properties of engineered cementitious composites and stirrup-confined ECC stub columns
- Experimental study on the seismic performance of short shear walls comprising cold-formed steel and high-strength reinforced concrete with concealed bracing
- Failure criteria and microstructure evolution mechanism of the alkali–silica reaction of concrete
- Mechanical, fracture-deformation, and tribology behavior of fillers-reinforced sisal fiber composites for lightweight automotive applications
- UV aging behavior evolution characterization of HALS-modified asphalt based on micro-morphological features
- Preparation of VO2/graphene/SiC film by water vapor oxidation
- A semi-empirical model for predicting carbonation depth of RAC under two-dimensional conditions
- Comparison of the physical properties of different polyimide nanocomposite films containing organoclays varying in alkyl chain lengths
- Effects of freeze–thaw cycles on micro and meso-structural characteristics and mechanical properties of porous asphalt mixtures
- Flexural performance of a new type of slightly curved arc HRB400 steel bars reinforced one-way concrete slabs
- Alkali-activated binder based on red mud with class F fly ash and ground granulated blast-furnace slag under ambient temperature
- Facile synthesis of g-C3N4 nanosheets for effective degradation of organic pollutants via ball milling
- DEM study on the loading rate effect of marble under different confining pressures
- Conductive and self-cleaning composite membranes from corn husk nanofiber embedded with inorganic fillers (TiO2, CaO, and eggshell) by sol–gel and casting processes for smart membrane applications
- Laser re-melting of modified multimodal Cr3C2–NiCr coatings by HVOF: Effect on the microstructure and anticorrosion properties
- Damage constitutive model of jointed rock mass considering structural features and load effect
- Thermosetting polymer composites: Manufacturing and properties study
- CSG compressive strength prediction based on LSTM and interpretable machine learning
- Axial compression behavior and stress–strain relationship of slurry-wrapping treatment recycled aggregate concrete-filled steel tube short columns
- Space-time evolution characteristics of loaded gas-bearing coal fractures based on industrial μCT
- Dual-biprism-based single-camera high-speed 3D-digital image correlation for deformation measurement on sandwich structures under low velocity impact
- Effects of cold deformation modes on microstructure uniformity and mechanical properties of large 2219 Al–Cu alloy rings
- Basalt fiber as natural reinforcement to improve the performance of ecological grouting slurry for the conservation of earthen sites
- Interaction of micro-fluid structure in a pressure-driven duct flow with a nearby placed current-carrying wire: A numerical investigation
- A simulation modeling methodology considering random multiple shots for shot peening process
- Optimization and characterization of composite modified asphalt with pyrolytic carbon black and chicken feather fiber
- Synthesis, characterization, and application of the novel nanomagnet adsorbent for the removal of Cr(vi) ions
- Multi-perspective structural integrity-based computational investigations on airframe of Gyrodyne-configured multi-rotor UAV through coupled CFD and FEA approaches for various lightweight sandwich composites and alloys
- Influence of PVA fibers on the durability of cementitious composites under the wet–heat–salt coupling environment
- Compressive behavior of BFRP-confined ceramsite concrete: An experimental study and stress–strain model
- Interval models for uncertainty analysis and degradation prediction of the mechanical properties of rubber
- Preparation of PVDF-HFP/CB/Ni nanocomposite films for piezoelectric energy harvesting
- Frost resistance and life prediction of recycled brick aggregate concrete with waste polypropylene fiber
- Synthetic leathers as a possible source of chemicals and odorous substances in indoor environment
- Mechanical properties of seawater volcanic scoria aggregate concrete-filled circular GFRP and stainless steel tubes under axial compression
- Effect of curved anchor impellers on power consumption and hydrodynamic parameters of yield stress fluids (Bingham–Papanastasiou model) in stirred tanks
- All-dielectric tunable zero-refractive index metamaterials based on phase change materials
- Influence of ultrasonication time on the various properties of alkaline-treated mango seed waste filler reinforced PVA biocomposite
- Research on key casting process of high-grade CNC machine tool bed nodular cast iron
- Latest research progress of SiCp/Al composite for electronic packaging
- Special Issue on 3D and 4D Printing of Advanced Functional Materials - Part I
- Molecular dynamics simulation on electrohydrodynamic atomization: Stable dripping mode by pre-load voltage
- Research progress of metal-based additive manufacturing in medical implants
Articles in the same Issue
- Review Articles
- Progress in preparation and ablation resistance of ultra-high-temperature ceramics modified C/C composites for extreme environment
- Solar lighting systems applied in photocatalysis to treat pollutants – A review
- Technological advances in three-dimensional skin tissue engineering
- Hybrid magnesium matrix composites: A review of reinforcement philosophies, mechanical and tribological characteristics
- Application prospect of calcium peroxide nanoparticles in biomedical field
- Research progress on basalt fiber-based functionalized composites
- Evaluation of the properties and applications of FRP bars and anchors: A review
- A critical review on mechanical, durability, and microstructural properties of industrial by-product-based geopolymer composites
- Multifunctional engineered cementitious composites modified with nanomaterials and their applications: An overview
- Role of bioglass derivatives in tissue regeneration and repair: A review
- Research progress on properties of cement-based composites incorporating graphene oxide
- Properties of ultra-high performance concrete and conventional concrete with coal bottom ash as aggregate replacement and nanoadditives: A review
- A scientometric review of the literature on the incorporation of steel fibers in ultra-high-performance concrete with research mapping knowledge
- Weldability of high nitrogen steels: A review
- Application of waste recycle tire steel fibers as a construction material in concrete
- Wear properties of graphene-reinforced aluminium metal matrix composite: A review
- Experimental investigations of electrodeposited Zn–Ni, Zn–Co, and Ni–Cr–Co–based novel coatings on AA7075 substrate to ameliorate the mechanical, abrasion, morphological, and corrosion properties for automotive applications
- Research evolution on self-healing asphalt: A scientometric review for knowledge mapping
- Recent developments in the mechanical properties of hybrid fiber metal laminates in the automotive industry: A review
- A review of microscopic characterization and related properties of fiber-incorporated cement-based materials
- Comparison and review of classical and machine learning-based constitutive models for polymers used in aeronautical thermoplastic composites
- Gold nanoparticle-based strategies against SARS-CoV-2: A review
- Poly-ferric sulphate as superior coagulant: A review on preparation methods and properties
- A review on ceramic waste-based concrete: A step toward sustainable concrete
- Modification of the structure and properties of oxide layers on aluminium alloys: A review
- A review of magnetically driven swimming microrobots: Material selection, structure design, control method, and applications
- Polyimide–nickel nanocomposites fabrication, properties, and applications: A review
- Design and analysis of timber-concrete-based civil structures and its applications: A brief review
- Effect of fiber treatment on physical and mechanical properties of natural fiber-reinforced composites: A review
- Blending and functionalisation modification of 3D printed polylactic acid for fused deposition modeling
- A critical review on functionally graded ceramic materials for cutting tools: Current trends and future prospects
- Heme iron as potential iron fortifier for food application – characterization by material techniques
- An overview of the research trends on fiber-reinforced shotcrete for construction applications
- High-entropy alloys: A review of their performance as promising materials for hydrogen and molten salt storage
- Effect of the axial compression ratio on the seismic behavior of resilient concrete walls with concealed column stirrups
- Research Articles
- Effect of fiber orientation and elevated temperature on the mechanical properties of unidirectional continuous kenaf reinforced PLA composites
- Optimizing the ECAP processing parameters of pure Cu through experimental, finite element, and response surface approaches
- Study on the solidification property and mechanism of soft soil based on the industrial waste residue
- Preparation and photocatalytic degradation of Sulfamethoxazole by g-C3N4 nano composite samples
- Impact of thermal modification on color and chemical changes of African padauk, merbau, mahogany, and iroko wood species
- The evaluation of the mechanical properties of glass, kenaf, and honeycomb fiber-reinforced composite
- Evaluation of a novel steel box-soft body combination for bridge protection against ship collision
- Study on the uniaxial compression constitutive relationship of modified yellow mud from minority dwelling in western Sichuan, China
- Ultrasonic longitudinal torsion-assisted biotic bone drilling: An experimental study
- Green synthesis, characterizations, and antibacterial activity of silver nanoparticles from Themeda quadrivalvis, in conjugation with macrolide antibiotics against respiratory pathogens
- Performance analysis of WEDM during the machining of Inconel 690 miniature gear using RSM and ANN modeling approaches
- Biosynthesis of Ag/bentonite, ZnO/bentonite, and Ag/ZnO/bentonite nanocomposites by aqueous leaf extract of Hagenia abyssinica for antibacterial activities
- Eco-friendly MoS2/waste coconut oil nanofluid for machining of magnesium implants
- Silica and kaolin reinforced aluminum matrix composite for heat storage
- Optimal design of glazed hollow bead thermal insulation mortar containing fly ash and slag based on response surface methodology
- Hemp seed oil nanoemulsion with Sapindus saponins as a potential carrier for iron supplement and vitamin D
- A numerical study on thin film flow and heat transfer enhancement for copper nanoparticles dispersed in ethylene glycol
- Research on complex multimodal vibration characteristics of offshore platform
- Applicability of fractal models for characterising pore structure of hybrid basalt–polypropylene fibre-reinforced concrete
- Influence of sodium silicate to precursor ratio on mechanical properties and durability of the metakaolin/fly ash alkali-activated sustainable mortar using manufactured sand
- An experimental study of bending resistance of multi-size PFRC beams
- Characterization, biocompatibility, and optimization of electrospun SF/PCL composite nanofiber films
- Morphological classification method and data-driven estimation of the joint roughness coefficient by consideration of two-order asperity
- Prediction and simulation of mechanical properties of borophene-reinforced epoxy nanocomposites using molecular dynamics and FEA
- Nanoemulsions of essential oils stabilized with saponins exhibiting antibacterial and antioxidative properties
- Fabrication and performance analysis of sustainable municipal solid waste incineration fly ash alkali-activated acoustic barriers
- Electrostatic-spinning construction of HCNTs@Ti3C2T x MXenes hybrid aerogel microspheres for tunable microwave absorption
- Investigation of the mechanical properties, surface quality, and energy efficiency of a fused filament fabrication for PA6
- Experimental study on mechanical properties of coal gangue base geopolymer recycled aggregate concrete reinforced by steel fiber and nano-Al2O3
- Hybrid bio-fiber/bio-ceramic composite materials: Mechanical performance, thermal stability, and morphological analysis
- Experimental study on recycled steel fiber-reinforced concrete under repeated impact
- Effect of rare earth Nd on the microstructural transformation and mechanical properties of 7xxx series aluminum alloys
- Color match evaluation using instrumental method for three single-shade resin composites before and after in-office bleaching
- Exploring temperature-resilient recycled aggregate concrete with waste rubber: An experimental and multi-objective optimization analysis
- Study on aging mechanism of SBS/SBR compound-modified asphalt based on molecular dynamics
- Evolution of the pore structure of pumice aggregate concrete and the effect on compressive strength
- Effect of alkaline treatment time of fibers and microcrystalline cellulose addition on mechanical properties of unsaturated polyester composites reinforced by cantala fibers
- Optimization of eggshell particles to produce eco-friendly green fillers with bamboo reinforcement in organic friction materials
- An effective approach to improve microstructure and tribological properties of cold sprayed Al alloys
- Luminescence and temperature-sensing properties of Li+, Na+, or K+, Tm3+, and Yb3+ co-doped Bi2WO6 phosphors
- Effect of molybdenum tailings aggregate on mechanical properties of engineered cementitious composites and stirrup-confined ECC stub columns
- Experimental study on the seismic performance of short shear walls comprising cold-formed steel and high-strength reinforced concrete with concealed bracing
- Failure criteria and microstructure evolution mechanism of the alkali–silica reaction of concrete
- Mechanical, fracture-deformation, and tribology behavior of fillers-reinforced sisal fiber composites for lightweight automotive applications
- UV aging behavior evolution characterization of HALS-modified asphalt based on micro-morphological features
- Preparation of VO2/graphene/SiC film by water vapor oxidation
- A semi-empirical model for predicting carbonation depth of RAC under two-dimensional conditions
- Comparison of the physical properties of different polyimide nanocomposite films containing organoclays varying in alkyl chain lengths
- Effects of freeze–thaw cycles on micro and meso-structural characteristics and mechanical properties of porous asphalt mixtures
- Flexural performance of a new type of slightly curved arc HRB400 steel bars reinforced one-way concrete slabs
- Alkali-activated binder based on red mud with class F fly ash and ground granulated blast-furnace slag under ambient temperature
- Facile synthesis of g-C3N4 nanosheets for effective degradation of organic pollutants via ball milling
- DEM study on the loading rate effect of marble under different confining pressures
- Conductive and self-cleaning composite membranes from corn husk nanofiber embedded with inorganic fillers (TiO2, CaO, and eggshell) by sol–gel and casting processes for smart membrane applications
- Laser re-melting of modified multimodal Cr3C2–NiCr coatings by HVOF: Effect on the microstructure and anticorrosion properties
- Damage constitutive model of jointed rock mass considering structural features and load effect
- Thermosetting polymer composites: Manufacturing and properties study
- CSG compressive strength prediction based on LSTM and interpretable machine learning
- Axial compression behavior and stress–strain relationship of slurry-wrapping treatment recycled aggregate concrete-filled steel tube short columns
- Space-time evolution characteristics of loaded gas-bearing coal fractures based on industrial μCT
- Dual-biprism-based single-camera high-speed 3D-digital image correlation for deformation measurement on sandwich structures under low velocity impact
- Effects of cold deformation modes on microstructure uniformity and mechanical properties of large 2219 Al–Cu alloy rings
- Basalt fiber as natural reinforcement to improve the performance of ecological grouting slurry for the conservation of earthen sites
- Interaction of micro-fluid structure in a pressure-driven duct flow with a nearby placed current-carrying wire: A numerical investigation
- A simulation modeling methodology considering random multiple shots for shot peening process
- Optimization and characterization of composite modified asphalt with pyrolytic carbon black and chicken feather fiber
- Synthesis, characterization, and application of the novel nanomagnet adsorbent for the removal of Cr(vi) ions
- Multi-perspective structural integrity-based computational investigations on airframe of Gyrodyne-configured multi-rotor UAV through coupled CFD and FEA approaches for various lightweight sandwich composites and alloys
- Influence of PVA fibers on the durability of cementitious composites under the wet–heat–salt coupling environment
- Compressive behavior of BFRP-confined ceramsite concrete: An experimental study and stress–strain model
- Interval models for uncertainty analysis and degradation prediction of the mechanical properties of rubber
- Preparation of PVDF-HFP/CB/Ni nanocomposite films for piezoelectric energy harvesting
- Frost resistance and life prediction of recycled brick aggregate concrete with waste polypropylene fiber
- Synthetic leathers as a possible source of chemicals and odorous substances in indoor environment
- Mechanical properties of seawater volcanic scoria aggregate concrete-filled circular GFRP and stainless steel tubes under axial compression
- Effect of curved anchor impellers on power consumption and hydrodynamic parameters of yield stress fluids (Bingham–Papanastasiou model) in stirred tanks
- All-dielectric tunable zero-refractive index metamaterials based on phase change materials
- Influence of ultrasonication time on the various properties of alkaline-treated mango seed waste filler reinforced PVA biocomposite
- Research on key casting process of high-grade CNC machine tool bed nodular cast iron
- Latest research progress of SiCp/Al composite for electronic packaging
- Special Issue on 3D and 4D Printing of Advanced Functional Materials - Part I
- Molecular dynamics simulation on electrohydrodynamic atomization: Stable dripping mode by pre-load voltage
- Research progress of metal-based additive manufacturing in medical implants
