A Comprehensive Review of Sustainability in Natural-Fiber-Reinforced Polymers

Abstract

Fiber-reinforced polymer composites (FRCs) from renewable and biodegradable fiber and sustainable polymer resins have gained substantial attention for their potential to mitigate environmental impacts. The limitations of these composites become evident when considered in the context of high-performance engineering applications, where synthetic fiber composites like glass or carbon FRCs typically dominate. A balance between the performance of the composite and biodegradability is imperative in the pursuit of what may be termed an environmentally conscious composite. This comprehensive review article provides some insight into the sustainability of FRCs, alongside detailing the sustainability considerations at various stages—materials, performance, applications, and end of life. The discussion also covers the different types of sustainable natural fibers and the types of polymer resins with some of the current achievements in the mechanical and functional properties of such composites, followed by a broad survey of their potential applications across diverse engineering applications.

1. Introduction

Material selection is now primarily focused on considering the sustainability aspects of materials. The huge demand and awareness of the environmental impact and proper balance between a material’s performance and recyclability are critical factors in the selection of materials for engineering product design. Taking into consideration sustainability, natural-fiber polymer composites are of high interest. In these composites, the reinforcement is selected from natural and renewable resources. Fibers such as flax, jute, coir, hemp, and kenaf are used as fiber reinforcement in woven mats or chopped fibers with proper polymer resins based on the application [1]. The confluence of environmental regulations, sustainability principles, escalating socioeconomic awareness, ecological consciousness, and the diminishing availability of fossil-fuel derivates had a substantial influence on the utilization of natural resources [2]. The use of natural fibers as reinforcement not only reduces waste-disposal issues but also reduces environmental pollution. Environmentally friendly materials (natural fibers) characterized by their favorable specific strength properties, low cost, and high stiffness in composites have good potential as a substitute for glass and carbon-fiber composites for nonstructural applications [3]. Even though natural-fiber-reinforced composites have good stiffness, it is generally lower than that of glass or carbon-fiber-reinforced composites. Glass and carbon fibers are known for their exceptional stiffness and strength, making them preferable for applications where rigidity is crucial. The choice depends on factors like cost, weight, and sustainability, with natural fibers often favored for their eco-friendly and cost-effective properties.

Synthetic polymers are widely employed as the polymer matrix in composite-material production, owing to their exceptional mechanical and adhesion performance, as well as notable characteristics such as chemical and moisture resistance, good fatigue resistance, temperature stability, and high electrical resistance. However, the environmental repercussions of these synthetic-fiber and polymer resins are extensive and substantial, being recognized for their nonbiodegradable nature, leading to a persistence in the environment that spans hundreds of years [4]. Furthermore, the manufacture of these fossil derivatives results in an extensive release of excess greenhouse gases, thereby fueling global warming. The concerns about fossil-fuel depletion, the awareness of environmental crises, and a wider responsibility to sustainability have obliged researchers and innovators to evolve toward the production of sustainable, biodegradable, and recyclable fiber-reinforced polymer composites [5,6,7]. With the rise of sustainability as a global demand across various industries, composite development is aligning more toward sustainable composites. The pursuit of fostering lightweight and high-performing materials at a lower cost has led to the selection of biodegradable and sustainable composite materials. Composites with either the reinforcement or matrix obtained from natural resources come under the category of partially biodegradable composites, while those fully made of natural resources are termed bio-based composites or fully biodegradable composites [8]. Natural fibers are regarded as appropriate materials for developing sustainable composites owing to their availability and renewable, lightweight, and cost-effective characteristics aligned with their good mechanical properties, nonabrasive nature, and biodegradability. The environmental footprints of these materials are much lower than synthetic fibers [9,10]. A sustainable methodology toward composite production comprising natural fibers is reinforcement, and a biodegradable polymer matrix enables the composting of these materials at their end of life with minimal energy. However, it is critical to note that these bio-based composites must provide sufficient mechanical performance to compete with synthetic composites. Significant research has been conducted on improving the mechanical properties of these natural-fiber-reinforced composites (NFRCs) by the addition of nanomaterials such as metal oxides, carbon nanotubes, and graphene to enhance the strength of the composites [3,11]. Obtaining a balance between the sustainability aspect and the performance of the composite is important when the applications of these composites are considered.

Previously published articles have explored natural-fiber composites and the ways to enhance the mechanical properties of these composites for engineering applications. There is a good gap for a comprehensive review addressing recent advances in sustainable fibers and polymer resins in environmentally friendly fiber-reinforced composites. This review article comprehensively elucidates the sustainability dimensions of natural-fiber reinforcement and polymer resins, encompassing both thermoplastic and thermoset resin. The mechanical properties of these sustainable resin-based composites are explored while shedding light on the various applications of these bio-based composites. Notably, this review stands out as distinctive in outlining the sustainability aspects of the entire composite-development spectrum, starting from the raw materials to end applications. It probes into both thermoset and thermoplastic resins, delivering a thorough understanding of their environmental impact. This review also details the mechanical performance of these sustainable composites and presents their practical applications.

2. Fiber Reinforcement and Its Sustainability

Generally, the composites for engineering applications are dominated by glass and carbon-fiber composites owing to their excellent mechanical performance, being lightweight and desirable for heavy-duty applications [12]. The most used synthetic polymers are epoxy, vinyl ester, and unsaturated polyester which serve as adhesives, coatings, and casting materials derived from petroleum-based sources. Natural-fiber composites compared to their synthetic-fiber competitors have relatively inferior mechanical, chemical, and thermal properties that limit them to specific applications. However, the environmental issues connected to the recyclability and biodegradability of these thermoset resins and synthetic reinforcements has caused a shift toward more environmentally friendly materials.

Natural fibers are a rising reinforcement used for automotive composite components to replace conventional glass-fiber composites [13]. The benefits encompass cost-effectiveness, good thermal and acoustical insulation properties, availability, enhanced CO₂ sequestration, energy recovery, reduced dermal and respiratory irritation, and reduced tool wear during machining [2,13]. The strength of these composites also depends on the fiber–matrix interaction, the compatibility and competitiveness of the reinforcement with the polymer resin, and the fiber–matrix interfacial adhesion. There is a lot of interesting research around building the interfacial adhesion of natural-fiber polymer composites and reducing the drawbacks of the water-absorption behavior of these composites. The majority of the research is centered on improving the mechanical properties, surface treatment through chemical modifications to improve the interfacial adhesion properties, better manufacturing processes, etc. [1,13,14,15]. Before selecting natural-fiber composites for engineering applications, it is mandatory to evaluate the properties of both the fiber and resin and the fiber–matrix interaction. Figure 1 schematically explains the levels of the criteria influencing the selection of natural-fiber composites for engineering applications. Here, the specific composite performance directly relates to how well the composite performs based on the mechanical properties, thermal conductivity, etc., whereas the natural-fiber properties focus on the intrinsic characteristics of the natural fibers used, which are the fiber type, length, orientation, etc. The polymer-base properties deal with the attributes of the polymer matrix, which include the type, composition, and compatibility. The composite characteristics encompass broader aspects like processing methods and the overall structure of the composite material. The general composite performance serves as a comprehensive evaluation level, ensuring that the selected natural-fiber composite not only meets the individual criteria but also excels in delivering desired outcomes across a range of performance indicators. The schematic diagram helps to illustrate how these levels of criteria collectively influence the decision-making process for selecting natural-fiber composites in engineering applications. For example, with the same polymer-resin system, different natural-fiber reinforcements can significantly change the properties and behavior of the final composite product [2,16]. The mechanical properties of various natural-fiber composites along with some synthetic fibers such as glass and carbon are indicated in

Table 1. Mechanical properties of various fiber reinforcements [19,20].

Fibres	Density (g cm−3)	Tensile Strength (MPa)	Young’s Modulus (GPa)	Elongation at Break (%)
Flax	1.4–1.5	345–1500	27–39	2.7–3.2
Hemp	1.47	550–900	38–70	1.6–4
Jute	1.3–1.49	393–800	13–26.5	1.16–1.5
Kenaf	1.5–1.6	350–930	40–53	1.6
Ramie	1.5–1.6	400–938	61.4–128	1.2–3.8
Sisal	1.45	468–700	9.4–22	3.7–4.3
Pineapple	1.52–1.56	170–1627	60–82	2.4
Banana	1.30–1.35	529–914	27–32	3
Coir	1.25–1.50	175	4–6.02	30
Basalt	2.65	1.8–2.5	80–93	3.1
E-glass	2.55	2000–3500	70–73	2.5–3.7
S-glass	2.5	4570	86	2.5
Carbon	1.4	4000	230–240	1.4–1.8

. Comparing these values gives an indication of the difference in the strength of these natural fibers with the synthetic reinforcement, helping us to choose the appropriate fiber type for applications. Nanocelluloses, derived from downsized plant-cellulose fibers produced industrially from renewable wood biomass, are gaining attention for their potential to replace petroleum-based materials and contribute to a more sustainable society, as reflected in the increasing scientific publications and patents. Cellulose nanonetworks (CNNeWs), cellulose nanofibrils (CNFs), and cellulose nanocrystals (CNCs) hold significant potential for contributing to a sustainable society [17]. A study by Thanga et al. [18] reported that silane treatment enhanced cellulose-nanofibers’ properties and their incorporation into epoxy nanocomposites showed promising physical, mechanical, and thermal characteristics, making them suitable for lightweight structural applications.

Currently various bio-based resins, both thermosets and thermoplastics, are studied for their recyclability using both natural- and synthetic-fiber reinforcements. The environmentally friendly resins formulated also exhibit improved mechanical performance through the incorporation of diverse natural fibers. The biodegradability of NFRCs is assigned to easiness in the breakdown of individual constituents within the composites. Biodegradability alongside recyclability, renewability, and sustainability hold benefits for present and future climatic deliberations [21]. There is an escalating focus on the demand for eco-friendly materials due to the continuous elevation of standards and regulations against harmful substances. Researchers are thus advocating the production of bio-based materials, particularly NFRCs, in this context. NFRCs, due to their eco-friendly nature and lower energy-consumption value of 9.5 MJ/kg compared to conventional synthetic-fiber composites that require 54.7 MJ/kg, have significantly contributed to the choice of selecting sustainable materials for engineering applications [22]. NFRCs are gaining market traction due to their lower environmental impacts compared to synthetic composites, driven by their reduced climate effects. Products with economic features such as biodegradability and renewability are experiencing a rise in market volume. Figure 2 presents the various characteristics related to sustainability. Moreover, the cultivation of natural fibers is an excellent way of revenue generation, and the waste residues during their production can again be used for landfills. The land used for farming can be utilized; moreover, crops such as hemp and flax yield seeds, substances, and oils for diverse applications, ranging from food and textiles to industrial and medicinal purposes [23]. Importantly, the mass produced by these materials is biodegradable at the end of their life cycle.

NFRCs have a volume fraction in the range of 60–70% with the rest of the polymer matrix. The sustainability aspects of these composites are derived from most of the constituents in these composites being obtained from living plant and animal sources. The market share of the United States stated that the composite market share was GBP 2.7 billion in 2006, rising to GBP 3.3 billion in 2012, with an expected annual increase of 3.3%. Between 1994 and 2004, there was a significant increase in market share, with a growth rate of 13%, equivalent to 275 million kilograms. [24]. The global annual market growth for NFRCs averaged 38% from 2003 to 2007, with Europe holding the highest annual growth rate of 48%. Industries based on composite materials have established their success worldwide, and currently, NFRCs are making effective contributions to these sectors. The life-cycle-assessment studies support the ecological benefits of composite materials over the aluminum-based structural components. The aircraft industries have supported the reduction in CO₂ contents by 15–20% with the inclusion of composite materials. The lignin, cellulose, and hemicellulose constituents in natural fibers make them environmentally friendly compared to traditional composites [25]. The abundance of cellulose helps in decomposing the material naturally without any requirement for additional energy. Some of the biodegradable natural-fiber composites with the polylactic-acid-based-resin system degrade easily [26]. The energy associated with the burning of China reed strands is estimated at 14 MJ/kg. The incineration process does not release CO₂ into the atmosphere. The burning of these natural fibers results in positive carbon credits and reduces risks to the environment. The gross energy requirement (GER) for natural fibers and residues utilized for energy generation at the end of their life-cycle assessment is illustrated in Figure 3.

3. Thermoset Matrices and Their Sustainability

The resin systems bind the fibers together, transfer the load within the reinforcements, and protect them from environmental factors such as moisture, chemicals, and abrasion along with providing chemical and environmental resistance. Generally, synthetic or petroleum-based polymers are used for the manufacturing of fiber-reinforced composite materials, and these polymers are produced from petroleum-based resources. Synthetic polymers are of two types: thermosets and thermoplastics. The thermoset polymers are irreversible and cannot be remolded by heating once they are cured during the composite fabrication. The thermoset polymers having unlinked molecules when added with curing agents with the support of heat initiate the chemical reaction for the curing of the polymer. During this process, the molecules cross-link and form long molecular chains with cross-linked structures transferring them to solid form. The thermoset matrix material has better modulus, creep, and resistance to thermal and chemical environments when compared to thermoplastic composites. These resin systems have the drawback of being brittle and possessing lower fracture toughness at room temperature [27,28]. Commonly used thermosets to produce fiber-reinforced composites are epoxy, vinyl ester, unsaturated polyester, and phenolics.

With greater interest in sustainable composites, thermoset resin faces great challenges in terms of reuse and recycling. The lack of recycling in turn causes environmental concerns after the end of the life cycle. The permanent irreversible chemical chains make them unfit to be remolded, reprocessed, and recycled. Some recent research works [29,30,31,32,33] have focused on the recycling of these thermoset composites and explain the ways of developing thermosets that are less inert and receptive with the inclusion of dynamic covalent bonds into the thermosetting material. A research work by Lorena et al. [34] performed the recycling of a flax-fiber bio-based thermosetting composite under mild recycling conditions extracting the fiber reinforcement and epoxy resin. The epoxy composite’s disposal approach involved employing a chemical-recycling process, enabling the retrieval of the original material with a substantial yield in recovery.

The global market for thermosets, specifically epoxy, dominates with a 70% share [35] due to their excellent mechanical properties, dimensional stability, and adhesive properties. Among these epoxy thermosets, 90% are sourced from the petroleum monomer diglycidyl ether of bisphenol A (DGEBA), which causes health hazards to humans and is not renewable [36]. Both bisphenol A (BPA) and epichlorohydrin pose hazards to living organisms [37,38]. The alternative means of developing these thermoset polymers from renewable sources and developing their mechanical properties are of greater importance in the current research. Sources such as plant oil, itaconic acid, rosin, lignin, furan, etc. are some of the examples [39]. He et al. [27] presented an eco-friendly method of producing thermoset composites with the inherent characteristics of polyimine-based covalent adaptable networks (CANs). The initial step involves the creation of polyimine thin films through the recycling of composite scraps, serving as the raw material for subsequent composite manufacturing. Another work by Monteserin et al. [40] presented the development of environmentally friendly epoxy vitrimers that are formulated using a Schiff base derived from the reaction between a petroleum-derived diamine (4,4′-diamine diphenyl methane, DDM) and two key components—a bio-based compound, vanillin, and an epoxy resin derived from linseed oil (LO). Notably, these vitrimers exhibit the ability to be reprocessed and recycled under mild conditions.

The recent research focus on vitrimer-based natural-fiber composites is because of the self-healing ability of the vitrimers. Mingen et al. [41] studied hemp-based, recyclable, bio-based matrix composites by developing a dual-network vitrimer matrix from hempseed oil and limonene derivatives. The fiber–matrix adhesion of the composites was further enhanced by the introduction of amino silane into the vitrimer matrix, which increased the cross-link density and the toughness of the matrix. The imine and hydroxy-ester dynamic bonds of the matrix allow the recycling of the composites with a mild and low-cost aminolysis process. A research study by Ali et al. [42] developed discontinuous flax-fiber, vitrimer-based composites that are sustainable and evaluated the repair performance. The interfacial shear strength (IFSS) properties of the composites were compared with a standard epoxy and achieved a high level of adhesion compared to the standard epoxy. The IFSS value of the epoxy-based composites was 11.8 MPa, whereas the vitrimer-based composites registered the IFSS value as 20.0 MPa. The low-temperature repair of the vitrimer-based composites was achieved while placing the sample between the heated plates in a 50 kN load cell UTM, while applying a pressure of 0.69 MPa for 5 min at 120 °C. The repair capabilities of the composites were observed using the end-to-end and single-patch repair methods. The research work by Li et al. [43] studied recyclable, high-performance, ramie-yarn-reinforced, polyimine vitrimer composites and observed self-healing, moldable, and good water-barrier properties. The experimental findings observed that the tensile strength values of the vitrimer-based composites were superior when compared to most of the natural-fiber polymer composites with the same fiber and volume fraction. Moreover, the vitrimer-based natural fibers were capable of undergoing physical recycling at least nine times without any degradation in performance. The tensile strength and modulus values of the recycled composites (even after the ninth time of recycling) were still comparable with the properties of the initial samples.

4. Thermoplastic Matrices and Their Sustainability

Numerous industrial polymer-composite products find applications across diverse fields, including aerospace, marine, sports industries, and beyond. The demand and use for polymer composites are increasing day by day [15,44]. Consequently, environmental issues have surged over the past years. The use of thermoplastic polymers as the matrix has been tremendously increasing in the area of natural- and synthetic-fiber-reinforced polymer composites due to their high performance and recyclability [45]. The management of waste and waste disposal plays a pivotal role in sustainability. Moreover, the need and awareness for sustainable materials are growing among composite industries, leading to the use of thermoplastic polymers instead of traditional materials.

Thermoplastic polymers are composed of linear molecular chains that soften and harden due to the application of heating and cooling. These polymer chains are linked to each other via intermolecular entanglement such as van der Waals forces or dipole–dipole interactions. As a result, when these polymers are reheated such weak entanglement breaks down to a molten stage and is easily reformed into a solid by cooling [46]. The nondegradation nature of thermoplastic carbon chains during heating makes this polymer more vulnerable to recycling. Figure 4 shows the molecular structure of thermoplastic and thermosetting polymers. Thermoplastic polymers are distinguished into crystalline, semi-crystalline, and amorphous polymers on the basis of their polymer arrangements. Crystalline polymers are polymer chains arranged in ordered, repeating, and three-dimensional patterns with a higher impact performance. High- and low-density polyethylene (HDPE and LDPE) and polypropylene (PP) come under this category because the glass transition state of these polymers is at negative degree Celsius. The random arrangement of molecular chains in polymers is referred to as amorphous thermoplastic polymers, for example, polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyphenyl sulphone (PPSU), polycarbonate (PC), and acrylonitrile butadiene styrene (ABS) [47,48]. Lastly, in the case of polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), these polymers contain both amorphous and crystalline structures and are known as semi-crystalline polymers. Figure 5 shows the different types of thermoplastic polymers classified based on their performance and structure [49]. These thermoplastic polymers are compatible with natural fibers as matrices for building structures and construction sectors due to their economical, durability, damage-tolerance, and flame- and chemical-resistance properties [50].

Thermoplastic polymers can be recycled by sustainable means such as mechanical, thermal, and chemical processes, making them prone to reuse [52]. The process of mechanical recycling involves the granulating, squashing, grinding, and eventual milling of clean polymers and their composites. Only thermoplastic materials like PE, PP, PET, and PVC may be recycled by this method. Mechanical recycling is a low-cost procedure, although it requires a larger initial machinery investment. The crucial drawback of mechanical recycling is the lowering of molecular weight due to the breakage of molecular chain links under the action of environmental moisture and acid. Moreover, the involvement of nonhomogeneous plastic wastes affects the performance of recycled composite parts [53]. When considering factors like time, carbon footprint, environmental effect, and cost management, this recycling procedure is the most efficient and sustainable one.

Chemical recycling is considered as supplementary to mechanical recycling. By the process of chemical recycling, thermoplastic polymers are chemically converted into monomers. Once again, these monomers are subjected to new polymerization to obtain similar polymers or compatible polymer components. The requirement of high expertise for handling the process and high investment make this less vulnerable for industries [54,55], although chemical recycling, on the other hand, resulted in savings of 3.5 billion barrels of oil for the processing of the global production submission warrant (PSW) with a savings of USD 176 billion US [56]. In the thermal-recycling process (thermochemical), thermoplastic polymers are decomposed into solid mixtures such as gas (CO, CH₄, CO₂), oils (toluene, phenols, benzene), and polyaromatic chars. These compounds have a higher calorific value and vary in molecular weight, making them more suitable as fuels and sources for different manufacturing processes [57,58]. This method is preferably suggested for those polymers that cannot be recycled using mechanical recycling methods like fiber-reinforced composites, multilayered packaging materials, and mixtures of PS/PE/PP. The major advantage of the pyrolysis process is the high temperature resulting in the breakage of molecular bonds that rely on random fragmentation, depolymerization, and polymer type.

Apart from synthetic thermoplastic polymers, there are bio-based thermoplastic polymers, in which the monomers are derived from renewable or organic matter including starch or plants. If a polymer composite has at least one bio-based or biodegradable component, it is considered a biocomposite. The sustainable way of reducing environmental impacts caused by synthetic or oil-derived polymers is to replace them with biopolymers. Apart from reducing carbon emissions, this also imparts several advantages, such as biodegradability, biocompatibility, reduction in global warming, and carbon dioxide sequestration [59]. Polylactic acid (PLA), polyhydroxyalkanoates (PHAs), chitosan, and starch are biopolymers obtained from renewable raw materials and are degradable, while nonbiodegradable biopolymers like PE, PP, PVC, and PET are also made from renewable sources. On the contrary, there are several fossil resources derived from biodegradable biopolymers, for example, poly (butylene adipate-co-terephthalate), polycaprolactone, and poly (butylene succinate) [60]. Figure 6 depicts (a) various types of biopolymers, their origin and degradability, and (b) the classification of biopolymers based on monomeric units [61]. The use of biopolymers has significantly reduced the environmental impacts of products during their entire life cycle. However, this effect can be further reduced by considering the recyclability of bio-based polymer systems. Recycled biopolymers are currently used for several applications in industries by considering the importance of sustainability [62,63]. Numerous researchers have examined the feasibility of repurposing biodegradable polymers to mitigate the ecological consequences associated with their life cycle [64,65,66]. A study conducted by Badia et al. [67] on the mechanical recycling of PLA found that mechanical recycling is a cost-effective and simple approach. Similar conclusions were also made in other studies [68,69].

5. Performance of Sustainable Composites

One of the main concerns today should be protecting the environment and ensuring its security to meet future needs. Compared with fully synthetic composites (both reinforcement and matrix are synthetic materials), sustainable composite materials (either one or both materials are sustainable) have less performance. Although, taking sustainability into account, the weak performance of sustainable composites can be neglected. Nevertheless, nowadays, sustainable composites exhibit a wide range of properties that make them compete with traditional materials in various areas such as packaging, nonstructural applications, biomedicals, etc. The performance of sustainable composites may depend on the properties of the reinforcement (orientation and stacking order, moisture absorption, volume fraction, hybridization, surface treatment) and matrix (fiber–polymer adhesion, molecular weight, curing, volume fraction, viscosity). Figure 7 shows the various factors affecting the performance of fiber-reinforced polymer composites [7]. Improving the mechanical properties of composites is most effective when the fibers are oriented in a manner parallel to the applied force. Yet, achieving this alignment is a formidable task with natural fibers because they tend to assume random orientations. When the angle at which the fibers are oriented relative to the test direction increases, the tensile strength and Young’s modulus of FRP (fiber-reinforced polymer) significantly decrease [70]. To address this challenge, natural fibers are frequently carded and assembled into sheets before being impregnated with the matrix material to attain a high degree of fiber alignment [71]. Mostly, it is observed that the presence of moisture in natural fibers reduces the mechanical performance of composites. Natural fibers typically exhibit hygroscopic properties, affecting the mechanical characteristics of their composites. Additionally, two key factors influencing the properties of composites reinforced with short natural fibers are fiber dispersion and volume fraction. These composites typically involve hydrophilic fibers and hydrophobic matrices. Longer fibers tend to cluster together. Effective fiber dispersion enhances the quality of interfacial bonding and minimizes the presence of voids by ensuring the fibers are well-encased by the matrix material [7]. Moreover, it is possible to subject the fibers to surface treatments or modifications to introduce specific functionalities and enhance their mechanical properties. Similarly, the polymer matrix also plays a crucial role in determining the performance of FRP composites. It protects the fibers from external mechanical damage, enables the transfer of loads through the fiber, and acts as a shield against severe environments. The fiber–matrix adhesion plays a pivotal role in stress transfer. Some of the performances of sustainable composites are discussed below:

Studies conducted by Alliyankal Vijayakumar et al. [15,44] have shown that hybridization and effective stacking of flax and bamboo fabrics in epoxy resin increased mechanical, thermal, and tribological performance and decreased the water-absorption properties of composites by 8.3% (tensile), 25% (flexural), 4.4% (impact), 14.6% (interlaminar shear), 65–94% (wear), and 34.75% (water diffusion) compared with pure epoxy. The authors proposed this hybrid composite as an alternative to plastic gears in toys, food-processing machines, robotics, and so on. Lowering plastic consumption and environmental impacts promotes the use of sustainable composite parts instead of conventional materials. Similarly, Crossley et al. [72] examined the mechanical, acidity, and fire-resistant properties of a renewable thermosetting resin (called poly furfuryl alcohol or furan resin) reinforced with flax and E-glass fibers. In addition, characteristics of the same were compared with composites of flax and E-glass fibers in unsaturated polyester, epoxy, and phenolic resins. It was found that furan-resin, E-glass laminates were discovered to exhibit mechanical properties like those of existing resin systems, along with exceptional fire resistance. Their fire resistance matches that of phenolic resin and surpasses the performance of polyester and epoxy. On the other hand, both phenolic and furan-flax laminates exhibited diminished mechanical performance when compared to polyester and epoxy. When reinforced with natural fiber (flax), they gained the full potential to develop a completely bio-derived, sustainable-based composite with structural–mechanical capabilities and fire-resistant properties.

In sandwich structures, the use of the same thermoplastic polymer as the skin and the core improves the ease of recycling in composites. Passaro et al. [45] developed a commingled E-glass/polypropylene-based sandwich panel with polypropylene foam as the core for the insulating walls of containers during food transportation. They found that the use of the same polymer for the skin and core increased the performance by improving the skin–core adhesion and reduced the hassle of recycling. Likewise, the mechanical properties, including tensile strength, tensile modulus, and impact strength, of unidirectional composites made from PP and coconut-fiber reinforcement were observed to improve as the fiber loading increased. The composites, reinforced with coconut fibers up to a weight percentage of 30%, exhibited an optimal combination of mechanical properties. The chemical treatment of the coir fiber resulted in enhanced mechanical characteristics and reduced water-absorption tendencies in the composite materials, attributed to the improved bonding between the coconut fiber and the PP matrix material [73,74].

Oksman et al. [75] noted a substantial enhancement, approximately 50%, in the tensile strength of a PLA/flax composite when compared to PP/flax composites of a similar nature. This increase in tensile strength was accompanied by an improvement in stiffness, with the PLA composite exhibiting a stiffness ranging from 3.4 to 8.4 GPa after the inclusion of 30 wt.% flax fibers. Furthermore, the thermal properties of PLA, which are typically seen as a drawback, were observed to improve upon the introduction of flax fibers. In recent years, there has been significant interest in utilizing cellulose nanofibers (CNFs) and nanowhiskers (CNWs) derived from renewable biomass as a viable alternative to micron-sized reinforcements in composite materials. This interest is particularly focused on improving the barrier properties of PLA (polylactic acid) for applications in the packaging sector. Jonoobi et al. [76] documented a notable enhancement in the tensile modulus and strength. Specifically, they observed an increase from 2.9 to 3.6 GPa in tensile modulus and from 58 to 71 MPa in tensile strength for nanocomposites containing 5 wt.% CNF extracted from kenaf pulp. Examining the mechanical characteristics of natural-fiber composites and comparing these biodegradable materials across different types is crucial for gaining insights into their responses to various mechanical tests. This knowledge, in turn, aids in the informed selection of these composites for engineering applications. Figure 8 represents the mechanical properties of biodegradable composites, illustrating the relationship between the volume fraction of the fibers utilized and the corresponding tensile strength and modulus attained.

6. Applications of Sustainable Composites

Natural-fiber-reinforced polymer composites and thermoplastic and thermoset composites have gained popularity across various industries, including automotive, aerospace, marine, roofing, interiors, biomedical, electrical components, packaging, and construction. This popularity stems from their favorable attributes, such as strong mechanical performance, cost-effective production, vibration-damping capacity, lightweight nature, biodegradability, and so on. Natural-fiber-reinforced polymer composites have a longstanding history of application in the automobile industry. Pioneers in this field, including Henry Ford and George Washington Carver, made early attempts to utilize natural fibers like hemp and flax as far back as 1941. Similarly, prominent automobile manufacturers, including Toyota, Mercedes-Benz, General Motors, Ford, Chrysler, Daimler, BMW, and Audi, have adopted the use of bio-based composites in a wide range of applications [81,82]. For instance, Ford incorporates bio-based materials such as soy foam for cushions and seats, along with hemp-fiber composites for front grills. Similarly, Mercedes-Benz utilizes jute-based biocomposites for interior panels, flax-reinforced composites for trunk covers and shelves, and sisal-fiber composites for rear-panel shelves [83]. Figure 9 shows some of the applications of natural-fiber-reinforced composites, including serving as a durable material for pressure vessels when combined with thermoset resin, eco-friendly biopackaging products, lightweight yet sturdy inner-door panels made from kenaf/PP, robust frames for sports cycles, and even as components in high-end vehicles like the Mercedes Benz E-Class, incorporating natural fibers such as hemp, sisal, flax, wool, and others, enhancing sustainability and reducing environmental impact.

Wood–plastic composites find application in the manufacturing of tables, decks, benches, floorings, and landscape timbers. Additionally, biocomposites are employed in the repair and rehabilitation of various structural components. Natural-fiber composites are used as insulating and soundproofing materials due to their excellent thermal and acoustic properties. In particular, hemp/lime/concrete composites have proven to be superior to other binders in terms of sound absorption [84,85]. Recycling PP through comprehensive recycling processes, including washing and pelletizing, has yielded improved results in impact strength and ultimate elongation values. However, it is worth noting that these results indicate a decrease in toughness due to contamination. Considering these properties, recycled PP can be effectively utilized for manufacturing external unit panels for room air-conditioners, parts of washing-machine cleaners, and base units.

From a broader standpoint, recycling waste PET plastics can have a substantial impact on both the economy and the environment. It plays a crucial role in transforming the physical and engineering characteristics of construction materials like asphalt and building concrete. Simultaneously, it reduces the reliance on natural resources, minimizes environmental pollution, and leads to cost and energy savings [86]. Laria et al. conducted a study on the processing and mechanical properties of recycled LDPE-HDPE and PET composites for use in building components. In tests involving tensile, flexural, and compressive strength, the mechanical resistance of these composites was found to be approximately 60% of that of virgin material [87]. Biocomposites, due to their relatively low flame retardancy, raise concerns about their suitability for use in aircraft applications. The external structure of an aircraft requires materials with high flame resistance. However, for the interior structure, including areas like the cabin, decks, seats, and floors, which are less susceptible to fire hazards, biocomposites can be considered. For instance, cabin interior panels have been manufactured using phenolic resin and woven flax, which may offer a good balance of properties for these specific applications [88]. Apart from engineering applications, green composite materials are also used for entertainment and sporting goods. Polymer composites reinforced with natural fibers are used for musical instruments, boat hulls, toys, canoes, tennis rackets, snowboards, bicycle frames, and surfboards. These materials are considered safe for health and provide vibration features.

Figure 9. Application of natural-fiber-reinforced composites as a (a) pressure vessel with thermoset resin, (b) biopackaging product, (c) kenaf/polypropylene inner-door panel, (d) sports-cycle frame, and (e) Mercedes Benz E-Class components with hemp, sisal, flax, wool, and other natural fibers [89,90,91].

Figure 9. Application of natural-fiber-reinforced composites as a (a) pressure vessel with thermoset resin, (b) biopackaging product, (c) kenaf/polypropylene inner-door panel, (d) sports-cycle frame, and (e) Mercedes Benz E-Class components with hemp, sisal, flax, wool, and other natural fibers [89,90,91].

7. Conclusions

To reduce the adverse environmental impact of advanced materials, achieving complete degradation is essential, yet it remains a considerable challenge. To address these issues, researchers must consistently seek materials that are entirely combustible or biodegradable. The present article outline and compares conventional synthetic-fiber-reinforced thermoset composites with bio-based composites on the aspects of sustainability, recyclability, and the prospect of acquiring the materials from renewable sources. The shift in the use and demands of conventional polymer materials derived from fossil fuels to more environmentally friendly alternatives obtained from renewable resources is expressed in this article. Bio- or natural-fiber-reinforced composites have the potential to serve as alternatives in smart and multifunctional applications. The use of nonbiodegradable materials has created and exacerbated environmental and ecological issues, necessitating a shift toward more environmentally friendly alternatives. Biocomposites or natural-fiber-reinforced composites offer several advantages, and many industrial sectors have started to recognize these benefits; the commercialization of manufactured biocomposite materials is still in its early stages. The advantages of biocomposites or natural-fiber-reinforced composites are renewability, environmental friendliness, weight reduction, biodegradability, cost-effectiveness, etc. [92]. There are certain drawbacks, such as the potential release of harmful chemicals from these materials. However, with growing interest from both academic and industrial sources, it is evident that the commercialization of biocomposites is on an upward trajectory. This is likely to happen as people become more environmentally conscious, manufacturing processes become more efficient, and new applications for biocomposites are discovered.