Natural Fiber Composite Filaments for Additive Manufacturing: A Comprehensive Review

Abstract

This research explores the potential and significance of 3D printing natural fiber composite (NFC) materials. The primary objective is to investigate the mechanical, thermal, and environmental properties of NFC filaments, mainly focusing on biodegradable, renewable fibers such as jute, hemp, flax, and kenaf. In addition to studying the properties of NFCs, our research delves into the challenges associated with processing, including moisture absorption and fiber-matrix interfacial bonding. The novelty of this work lies in the convergence of traditional composite materials with the versatility of 3D printing technology. NFC filaments offer unique advantages in terms of sustainability, and we examine their potential contributions to the circular economy. By using eco-friendly NFC materials in 3D printing, we aim to present a viable, environmentally responsible alternative to conventional synthetic composites. The importance of 3D printing NFCs stems from the ways their use can align with sustainability goals. These materials provide the advantages of renewability, reduced carbon impact, and in some cases, biodegradability. Their applications extend to various industries, such as automotive, construction, and packaging, where eco-friendly materials are increasingly sought. Such applications showcase the ways in which NFC-based 3D printing can contribute to a more environmentally responsible and sustainable future. This research explores the mechanical, thermal, and environmental properties of NFC materials, highlighting their unique advantages for 3D printing and the potential to have eco-friendly applications in diverse industries.

1. Introduction

A major paradigm shift in additive manufacturing occurred recently, leading to a focus on discovering and applying novel materials to expand three-dimensional (3D) printing techniques [1]. Natural fiber composite filaments have received considerable interest among these emerging materials due to their unique mechanical properties, eco-friendliness, and renewable nature [2]. One of the most fascinating developments is the use of natural fiber composite filaments in AM techniques. This novel approach combines the benefits of naturally occurring, sustainable fibers with the adaptability and precision of additive manufacturing techniques [3]. Fused deposition modeling (FDM) is becoming popular for developing intricate structures using biodegradable polymer composite materials in various industries, including medicine, automotive, sensors, and pharmaceuticals. This approach is known as fused filament fabrication (FFF) [4]. Various additive manufacturing (AM) techniques have also been used to create prototypes. Due to its low cost, user-friendliness, and streamlined control, which requires minimal human input and understanding of the process, FFF offers a compelling advantage. Polymers are the primary raw material for FFF, and the finished products are produced sequentially under the direction of G-code instructions derived from CAD data [5]. Famous thermoplastic polymers like acrylonitrile butadiene styrene (ABS), polylactic acid/polylactide (PLA), polypropylene (PP), nylon, polyamide, and polycarbonate (PC) are extensively used within this framework [6]. Materials are typically used in the automotive, electrical, medical, and architectural industries based on their inherent strength and processability [7].

While FFF first found applications in manufacturing prototypes with low stress requirements, its value is now seen in a more extensive range of applications, including in toys for the home [8]. The creation of composite filaments from natural fibers like hemp, flax, and bamboo has advanced significantly as a result of recent research efforts [9]. These filaments have improved mechanical qualities, lower environmental impact, and a more comprehensive range of applications [10]. Investigations of natural fiber composites as a viable alternative to typical polymer-based filaments have gained traction as the global emphasis on eco-friendly solutions and reducing the carbon footprint of manufacturing has grown [11]. The employment of natural fiber composite filaments in AM processes is one of the most exciting advances in this field [12]. This innovative method combines the advantages of renewable and sustainable natural fibers with the adaptability and accuracy of additive manufacturing processes [13]. The creation of composite filaments from natural fibers has advanced significantly as a result of recent research efforts [14]. These filaments have improved mechanical qualities, lower environmental impact, and a wider range of applications [9]. Investigations of natural fiber composites as a viable alternative to typical polymer-based filaments have gained traction. Combining insights from materials science, mechanical engineering, and additive manufacturing technologies, this research aims to comprehensively assess this developing field’s accomplishments, obstacles, and future possibilities. This review aims to shed light on the history of natural fiber composite filaments by examining the current literature and critically evaluating experimental findings, thereby adding to the more significant debate on sustainable additive manufacturing materials and processes. In addition, this work contributes to the evolving field of additive manufacturing by analyzing the synthesis, processing techniques, and wide range of applications of natural fiber composite filaments (Figure 1).

1.1. Natural Fiber-Reinforced Polymer Composites (NFRPC)

Researchers are exploring the use of NFRPCs to bridge the gap between sustainability and material performance. Fiber-reinforced polymer (FRP) was initially introduced by Owens Corning in 1935, with the first iteration using glass fiber [15]. Composites are becoming increasingly important, and their use is expanding in many areas of modern life. Eco-friendly, lightweight, robust, renewable, inexpensive, biodegradable, and sustainable composite materials can be made from natural fibers [15]. Compared to traditional synthetic fibers, natural fibers are preferable due to their superior mechanical characteristics. Thus, natural fibers have recently attracted the attention of many researchers and scientists for usage as an alternative reinforcement in polymer composites [16]. They are affordable, recyclable, renewable, use less energy, pose fewer health risks, are not abrasive to machinery, and do not irritate the skin. They have thermoplastic and thermosetting properties and can be employed as a reinforced material. Several thermosetting resins are commonly employed in composite materials, including epoxy, unsaturated polyester resins, polyester, polyurethane, and phenolic resins [17]. They have acceptable mechanical characteristics and are reasonably priced. Natural fibers are gaining increased consideration from academics and industrial users because they are extremely strong and lightweight as well as being much more environmentally friendly than regular composites. Natural composites are being used ever more frequently because they are biodegradable and non-carcinogenic [18].

Ensuring efficient bonding at the fiber-matrix interface is a critical difficulty with NFRPCs. Recent research has concentrated on fiber-surface treatments that encourage adhesion. Alkali treatments have been used to improve interfacial bonding by removing impurities and roughening the surface of fibers. Natural fibers can absorb moisture and cause composite materials to degrade because they are intrinsically hydrophilic [19]. Hydrophobic coatings and the incorporation of desiccants into the matrix are now being researched as potential solutions to this problem.

Natural fiber is a relatively economical material that is used in various industries and applications, including packaging, automotive, building and construction, covering the interiors of railway coaches, and warehouses. It is also used to replace expensive glass fiber in various industries. The poor mechanical properties of NFRCs are one of their drawbacks [20]. The hybridization process is one way to improve the mechanical performance of NFRCs and broaden their applications. The water-absorption rates for hemp/flax/epoxy, hemp/jute/flax/epoxy, and hemp/jute/epoxy blends are affected by the hybridization process, which alters the behavior of hybrid composites. These rates are 2.8 percent, 3 percent, and 4.5 percent, respectively, when hybridization is done by bi-directional weaving and hand layup compression [20]. The water-absorption rate of the flax-fiber composites was discovered to be 12 times higher than that of the glass-fiber composites via vacuum-assisted resin infusion. The increased moisture absorption of natural fibers is the main drawback of these materials. Therefore, chemical treatments must mitigate excessive moisture absorption [21]. Many parameters, including fiber length, fiber aspect ratio, fiber-matrix adhesion, etc., significantly affect the mechanical properties of natural fibers after chemical treatment [9]. Through chemical modification using alkaline treatment, silane treatment, acetylation treatment, benzoylation treatment, and peroxide treatment, the matrix’s adherence to NFRC is improved and its hydrophilicity is lowered [22].

1.2. Properties of Natural Fibers

Fiber diameter, fiber length, and cell-wall thickness are three crucial characteristics of natural fibers. Because of the wide variety in fiber diameters (5–76 μm), fiber bundle widths (10–1000 μm), and fiber lengths, the properties of polymer composites made with natural fibers can vary greatly (1.2–300 mm) [23]. Optimizing production processes wherein natural fibers are used as reinforcement is difficult due to the inherent variances in natural fibers. Thus, accurate assessment and management of feedstock material is critical to obtaining the desired results from NFRCs. Cellulose, a natural polymer composed of three repeating hydroxyl groups, is one of the NFRC’s components [24]. Plants typically store cellulose as a linear, ribbon-shaped polymer of glucose. Chemical and solution treatments can degrade cellulose, despite the material’s resistance to hydrolysis, strong alkalis, and oxidants. Hemicelluloses, which are low-molecular-weight polysaccharides, act as a matrix for the bonds between cellulose microfibrils, which are the basic constituents of NFRC cells. Hemicelluloses hydrolyze efficiently in the presence of weak acids because of their hydrophilicity. A complex hydrocarbon polymer called lignin gives NFRCs strength, facilitates water transport, and resists most microbial attacks. It is hydrophobic, hydrolyzes in concentrated hydrochloric acid, and is easily oxidized. Pectin is a heteropolysaccharide that give flexibility to plant cell walls and is used as thickener. NFRCs are naturally polar and hydrophilic due to their abundance of hydroxyl groups. However, most polymer matrixes are hydrophobic by nature [25].

The properties of the natural fibers that influence those of NFRCs are physical, chemical, geometrical, thermal, and mechanical, as shown in Figure 2. Mechanical properties of natural fibers are directly influenced by structural and dimensional changes, such as changes in fiber density (cell wall-lumen ratio) and microfibril angle (MFA). Synthetic fibers like E-glass fiber have a tensile strength of about 2000–3500 MPa. These natural fibers, however, have a wide range of tensile strengths and Young’s moduli, with a typical tensile strength between 100 and 1000 MPa [26].

The mechanical performance of a hand-layup kenaf/polyester composite was examined by Mohammed et al. to elucidate the effects of layering NFRCs [25]. The influence of the variable fiber-volume fraction (5–25%) on the mechanical specifications of kenaf/polyester composites was studied by Hussein et al. Their findings demonstrated that as the fiber volume reached 10%, tensile strength increased [27]. In contrast to composites without filler, Jenish et al. studied Cissus quadrangularis stem fiber /epoxy resin with various concentrations of coconut-shell ash powder. The tensile and flexural strength values for Cissus quadrangularis stem fiber stem fiber/epoxy composite with five wt. percent coconut-shell ash particulate-were inferred to be 110.31 and 136.11 MPa, respectively [28].

1.3. Advantages and Challenges of Using Natural Fibers in Composites

Material and product development relies heavily on considerations like biodegradability and recyclability. The advantages of natural fibers over synthetic fibers in terms of technology, economy, and ecology are displayed in Figure 3. Carbon and glass fibers, two common types of synthetic fibers used as reinforcement, have the central problem of not being biodegradable or recyclable when they end their useful lives. Their processing also carries risks of harm to human health [29]. The focus of scientific inquiry has recently shifted from single-component materials to composites reinforced with natural fibers. The fact that plant fibers do not increase greenhouse gas emissions is remarkable. It is widely believed that carbon dioxide (CO₂) emissions from burning materials made from petroleum-based products are the primary cause of the greenhouse effect and, consequently, of climate change [30].

More cellulose-rich fibers with higher tensile strength and values for Young’s modulus values include curauá, sisal, and ramie. However, many factors, including those environmental and processing circumstances already mentioned, can affect their behavior [31]. One such aspect is the large variability in fiber quality, which leads to high variation in material properties. Therefore, more research on these characteristics is required with respect to methods of fiber preservation. Another critical factor is that it is difficult to compare the results of different investigations due to the absence of specific information regarding experimental testing methodologies for single fibers. Tensile strength is calculated using the entire cross-section of the sample, and even a small measurement error can have a significant impact on the results [32].

1.4. Natural Fiber Types

Natural fibers are fibers that are free of dyes, artificial additives and synthetics. Plants and animals both produce natural fibers. Natural fibers come from both renewable and nonrenewable resources. Natural fibers are hair-like raw materials derived directly from vegetables, plants, animals, or minerals. These raw materials are converted into nonwoven fabrics and used as composite components. The classification of natural fibers into plant, animal, and mineral fibers is depicted in Figure 4. The primary difference between animal and plant fibers is that animal fibers are composed primarily of protein, whereas plant fibers are promarily cellulose.

Plant fibers: Plant fibers consist primarily of cellulose fibers. Plant fibers are divided into primary and secondary fibers, depending on their purpose. The fibers of primary plants are used to cultivate secondary plants. These fibers can be further categorized as seed fiber (fibers from the seed and seed case, e.g., cotton and kapok), leaf fiber (leaves, e.g., agave, abaca, henequen, pineapple fiber, and sisal), skin fiber (the skin or bast that surrounds the stem of a plant (e.g., flax, ramie, jute, banana, hemp, kenaf, rattan, vine fibers, soybean fiber, and banana fibers), and fruit fiber (coconut (coir) fiber) [33]. Fibers from stalks, cane, grass, and reeds are from original natural sources (e.g., bamboo, bagasse, sabai, communis, and phragmites).

Animal fibers: Animal fibers are derived from wool, silk, hair/fur, alpaca fiber, and avian fiber.

Mineral fibers: Mineral fibers can be found or modified minimally to become asbestos, ceramic fiber, or metal fiber.

The trajectory of NFRPC research is shifting from basic understanding to optimization and application. As methodologies to enhance interface adhesion and reduce moisture sensitivity mature, NFRPCs are poised to become a mainstay in sustainable materials.

2. Additive Technologies for Natural Fiber Composites

2.1. Polymer-Based Fused Deposition Modeling (FDM)

Fused deposition modeling is an additive manufacturing technique that utilizes compact desktop-sized machines to produce rapid prototyped 3D printed structures. This process uses polymer filaments heated to a molten state and extruded through a nozzle typically ranging in size from 0.2 mm to 1.0 mm and made of brass. First, a model is designed on a CAD software 24.2. Next, the 3D printer software slices the 3D model into 2D layers stacked on top of each other, making a layer-by-layer code (G-code), which supplies the instructions to the printer head to control all of the printing parameters. The slicer supports the 3D model if any structures are overhanging or unstable. A spool of polymer filaments typically made from polylactic acid (PLA) is loaded onto the 3D printer, and the filament is loaded between two gears that guide the filament from the spool to the extruder head. There, the filament is heated to a molten state and extruded from the nozzle. The extruder and heater are attached to an axis that moves in the x, y and z directions, facilitating material deposition in complex shapes and structures [34].

There are printing parameters to consider when designing a 3D printing model. The most critical parameters are print speed, print temperature, and layer height. Print speed is the speed at which the printer deposits material onto the build plate and is typically measured in mm/s. Print temperature is the temperature at which the filament is extruded, and layer height is the thickness of each layer of the part. The speed and temperature can be different for the first layer than for the rest of the printed part because good adhesion between the first layer and the build plate needs to be established for a good-quality printed part. The manufacturer gives a print temperature for the filaments, but this value can also be obtained from thermal-stability analysis. The optimal layer height depends on the nozzle diameter; a good layer thickness is about 50–60% of the nozzle diameter. This thickness facilitates proper adhesion between the layers to maintain the properties of the printed part. Optimal process conditions can be obtained by analyzing the properties of the filaments.

Other than the abovementioned parameters, other physical parameters of each layer affect the characteristics and properties of printed parts: air gap, perimeter, and raster. The air gap is the distance between a raft, a support that can be printed as the base for a part, and the bottom of the model. Raster width is the width of a single line of material deposited, and raster angle is the angle at which the infill is printed [35]. The number of perimeters and thickness of the perimeter also make a difference in the print quality of the end product. Gebisa et al. studied the effect of printing parameters on the tensile properties of high-performance ULTEM 9085 using complete factorial design of experiments and found that the raster or infill angle has the greatest effect on tensile strength and strain [8]. Sanatgar et al. investigated the effects of print temperature, print speed, and build-plate temperature on the adhesion properties of polymers (PLA, nylon, PA66) and nanocomposites (carbon nanotubes and PLA: CNT/PLA; carbon black and PLA: CB/PLA) in textiles [36]. Build-plate temperature had little effect on the adhesion properties when it was lower than the glass-transition temperature. Linear and quadratic correlations were found between print temperature and print speed, respectively, and adhesion properties. Wang et al. studied the process parameters of printing glass fiber (GF) and carbon fiber (CF) composite with polyetheretherketone (PEEK) [37]. Print temperature, build-plate temperature, print speed, and layer height were investigated and related to mechanical and microstructural properties of the GF/PEEK and CF/PEEK. The ideal layer height and print speed for composites were 1 mm and 5 mm/s, and all mechanical parameters of the composites decreased as print speed and layer height increased. Tensile and flexural strength increased at higher print and build-plate temperatures, but these two factors had no effect on impact strength.

Materials Compatibility and Natural Fiber Composites in FDM

The limitation of FDM is the need for a thermoplastic polymer to be used in the 3D printing process. The raw materials needed for FDM printers are filaments that must be molten. This can be a challenge for 3D printing of natural fiber composite because natural fiber decomposes at a lower temperature compared to thermoplastic polymers, making the composite challenging to manufacture. Examples of thermoplastic polymers typically used in the FDM technique are polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyamide (PA)/nylon, polyethylene terephthalate glycol (PETG), and elastomers. PLA is a biodegradable polymer typically made from fermented plant-based starch that breaks down into lactic acid over time, making it a safe and cost-effective choice for 3D printing filaments. However, PLA’s degradability and poor mechanical strength makes it a poor choice for 3D printing structures that need to withstand high temperatures or force. ABS, by contrast, exhibits good thermal stability, making it a good filament with which to print functional parts, but it produces styrene gas when subjected to high print temperatures. Similarly, PA is also typically used to print functional parts because of its excellent mechanical properties and thermal stability. PETG is one of the best materials for printing because of its superior chemical, mechanical and heat stability, but it is typically more expensive than other 3D-printing filaments. Thermoplastic polyurethane, copolyester and elastomers are elastic filaments used for printing parts that require high impact resistance, but the elastic nature of these filaments make them challenging to control and feed into the nozzle. Other than that, complex overhanging structures are not suitable for manufacture with elastic filaments and require support structures.

Findings in the literature have shown that natural fibers improve the properties of some of the abovementioned polymers. Kamarudin et al. investigated the properties of up to 5 wt.% alkali-pretreated kenaf fiber in a PLA matrix with the addition of epoxidized jatropha oil as a plasticizer [38]. There was a slight increase in flexural strength and up to a tenfold increase in impact strength for treated kenaf-fiber composites. Adding the plasticizer decreased water absorption and retention, and it was concluded that the fiber pretreatment improved fiber-polymer adhesion. Agaliotis et al. studied the tensile properties of 3D-printed PLA-henequen fiber-flour composites and showed similar mechanical properties with the addition of 1% fiber flour [39]. However, there is an increase in ultimate tensile strength and strain in specimens with maleic anhydride. Duigou et al. reported a study of using continuous flax fiber coated with PLA for 3D printing, which is a different approach compared to mixing short-natural-fiber composites using high torque and temperature [40]. These biocomposites contained 35 wt.% flax fiber and exhibited significantly higher tensile strength and modulus compared to short-fiber printed composites. Ahmad et al. developed ABS-oil palm-fiber composite filaments in which 5 wt.% of natural fiber was added to the composite mixture [41]. The values of Young’s modulus and tensile strength for the composite printed parts increased at the expense of flexural strength, which is acceptable because of the higher stiffness of fiber-reinforced composites. There is little literature on composites of nylon and natural fibers because of the high thermal stability of nylon, which makes it incompatible with natural fibers, which have much lower thermal stability. However, extensive research has been done to examine composites of carbon or glass fiber and nylon [42,43,44,45]. The mechanical properties increase significantly with the addition of carbon fiber to nylon. Balla et al. 3D printed up to 35 wt.% of soybean-hull fiber mixed with TPC to test the mechanical properties and water uptake of the resulting material [14,46]. They discovered that sulfuric-acid pretreatment increased the mechanical properties of soy-hull-reinforced 3D-printed composites and that water uptake is lower at lower percentages of natural fibers. Zhou et al. studied high loading of lignin in TPU for 3D printing composites and found that the mechanical properties peak at 50 wt.% lignin [47]. They further investigated the addition of carbon fiber and showed a 67% improvement in tensile strength, which is more than the tensile strength exhibited by a composite with 20 wt.% kenaf fiber.

2.2. Resin Based Stereolithography (SLA)

2.2.1. Photopolymerization Mechanisms

Stereolithography is a rapid prototyping method wherein an ultraviolet (UV) spectrum light source is used to cure photosensitive polymeric resin to produce 3D-printed structures. The resin contains photo initiators that undergo a chemical reaction when exposed to UV light and bond monomers and oligomers into a polymer. SLA printers are designed as upside-down printers, where the build plate moves vertically into a vat of photosensitive resin and the bottom of the vat is clear. There is a small distance between the build plate and the bottom of the resin-filled vat, and the material is cured by the UV light from under the vat. When a layer is finished, the build plate moves up by the height of a layer to allow liquid resin to go between the bottom of the vat and the top of the previous layer. After printing, the part is post-processed by further UV curing under visible light or under UV light for a faster curing time.

The printing parameters to consider that affect dimensional accuracy in SLA printing are layer height, exposure time, build angle, and support density. Badanova studied the effects of SLA-printing parameters on castable wax and found that build angle plays the most important role in dimensional accuracy. A zero-degree build angle and a 0.25 μm layer height were optimal for castable wax [48]. The effects of post-curing processes used with SLA, namely, treatment in a microwave, conventional oven, and UV chamber, on the mechanical properties of printed parts were studied. The parameters investigated were build angle, solidification factor, and curing time. The microwave and UV post-curing was found to lead to an increase in mechanical strength, while the heat from the conventional oven led to overcuring and a decrease in strength [49]. A study on the effects of UV SLA in increasing solids loading and the printability of ceramics was performed wherein combinations of dispersant, photo initiator, plasticizer, monomer, and alumina powder were prepared with varying loading of alumina powder. With this method, the highest flexural strength was found at 52 vol.% solids loading. Sintered part density increased while shrinkage decreased with increasing solids loading. Rheological results showed a significant increase in viscosity with increasing solids loading [50]. Hu investigated the 3D printing of Mn-Zn ferrite ceramics using an SLA technique wherein the layer height, print speed and laser power were varied. They concluded that the optimal parameters to print Mn-Zn ferrite ceramic parts were 2 μm layer height, 2500 mm/s print speed, and laser power of 240 mW. These paramets produced the best dimensional accuracy and the least shrinkage [51].

2.2.2. Formulation of Photopolymerizable Resin with Natural Fibers

Although SLA is the oldest form of 3D printing, research around natural fiber composites based on SLA is limited, possibly due to the poor compatibility and bonding between thermoset resins and natural fibers. Another reason could be that natural fiber may hinder or slow the photopolymerization process by blocking the UV light from reacting with the polymer matrix. SLA printing is normally used for printing parts with a smooth finish and rarely exhibits good chemical or mechanical properties for most applications. However, researchers have been trying to add other substances to the mixture of photopolymer and natural fibers to help with bonding and curing. Palaganas extracted cellulose nanocrystals (CNCs) from hydrolyzed abaca-pulp fibers and mixed up to 1.2 wt.% of CNCs with polyethylene glycol diacrylate (PEGDA) and a photo initiator to create a hydrogel precursor. There were no significant differences in elastic modulus at different CNC loading wt.%, but adding 0.3 wt.% CNC loading resulted in a 100% increase in tensile strength, 110% increase in elongation and 300% improvement in fracture energy. Adding 1.2 wt.% loading of CNC increased the tensile strength, elongation, and fracture energy by 33%, 40%, and 100%, respectively. This study provides a potential formula for a biocompatible 3D-printed CNC resin that can be used in tissue engineering. Using lignocellulosic natural fibers [52]. Romero-Ocana tested 5 wt.% loading of wheat-straw and rice-straw composites with photosensitive resin for SLA 3D printing. The fibers were milled and sifted to <45 µm, <125 µm and <250 µm to investigate the effects of particle size of the two cereal straws on mechanical properties. Both straw composites with <45 µm fibers showed higher elastic modulus compared to clean resin. Tensile strength and elongation decreased significantly for all composites with straw fiber added [53]. Feng studied the SLA printing of lignin-coated CNCs mixed with methacrylate resin and the effect of loading 0.1, 0.5 and 1 wt.% CNCs on the thermal and mechanical properties of the composite. Loading of lignin (1 wt.%) CNCs resulted in the worst performance in terms of mechanical and thermal properties. CNC-loaded (0.1 wt.%) post-cured composites showed a small increase in tensile strength and deformation and a drastic increase in tensile modulus, while a 0.5 wt.% addition resulted in a significant improvement in tensile strength and modulus, with a small decrease in deformation. Adding 0.1 and 0.5 wt.% CNCs improved the thermal stability by increasing the critical temperatures of the composites, making them more resistant to heat [54].

2.3. Powder-Based Sintering and Deposition Processes

2.3.1. Selective Laser Sintering (SLS)

Selective laser sintering converts powder into complex and functional structures by melting thermoplastic with a laser beam. The powder reservoir is filled with a thermoplastic powder such as nylon, then packed in a thin and uniform layer by a rotating roller. The layer of powder is heated to a point close to the melting temperature to facilitate laser sintering and prevent warping from a large temperature gradient. A high-powered laser beam is directed onto a mirror, where a galvo motor focuses the beam to selectively heat regions on the powder layer. This heating fuses the powder particles to form a solid part layer. The particles that make up the powder are round, smooth, uniform, and less than 100 μm in diameter. Once a layer is sintered, the next layer is filled with powder and the process repeats. Unheated powder surrounds printed layers and also acts as a support for the printed part. Wang et al. investigated the relationship between SLS process parameters and shrinkage using a neural network model [55]. Their study suggests that shrinkage ratio decreases with increasing layer thickness, interval time, and laser power, while it increases with increasing scanning speed and hatch spacing. The surrounding temperature increases shrinkage from 82–85 °C and results in a rapid decrease at temperatures higher than 85 °C.

2.3.2. Binder Jetting (BJ)

Binder jetting is a mechanism similar to SLS and 2D printing wherein a layer of powder is deposited onto a bed and a binder jetting head deposits a binding agent where the layer needs to be solidified. Unlike SLS, BJ does not use heat or a laser, which allows for any material to be the feedstock. Depending on the material and binding agent, some printed parts need to be post processed by heat or chemicals. The printed part can undergo a debinding process wherein excess binding agent left on the printed part is removed using heat or solvent. The final step is sintering the printed part at a high temperature to solidify and improve the properties.

2.3.3. Natural Fibers in SLS and BJ

Natural fibers are rarely used in composites created using SLS and BJ because of the high operating temperature, which degrades most biomaterials. However, there has been work on modifying natural fibers to incorporate them into composites made by these techniques. Idriss examined how the processing parameters of sisal fiber-polyether sulfone (SFPC) composites (weight percentage ratio of 1:5 for sisal fiber:polyethersulfone) affect the mechanical properties of printed parts. Tensile and flexural strength increased with increasing preheating temperature and laser power and decreasing scan speed. The results were compared to wood-fiber composites, and sisal-fiber composites showed an increase of over 100% in flexural strength and an increase of 50% in tensile strength [56]. In a technique similar to SLS, Coelho manufactured a composite of gypsum and sisal fiber using binder jetting technology. Samples were tested with and without fibers and with and without post-processing adhesive infiltration. Adding fibers increased porosity by 4% due to poor fiber-gypsum interfaces, which decreased the tensile strength of green parts with fibers. However, the strength of fiber-impregnated samples increased by 41% with post-processing, possibly due to the presence of curing resin in the available pores [57]. Evdokimov investigated the characteristics of five wood types, namely beech, oak, larch, alder, and pine wood for creating fibrous composites with binder jetting and concluded that pine-wood fibers were the most suitable because of their elasticity, length, lower energy requirements, and small particle size [58]. Adjary et al. mixed alkali lignin and polyamide 12 (PA12) at a ratio of 40:60 wt.% to prepare 3D-printed composites with the SLS technique. Addition of lignin decreased mass loss from 97% for pure PA12 to 77% for composite and increased porosity by 14% but also increased Young’s modulus by 16% [59]. Figure 5 shows a summary of 3D-printing techniques and the typical matrices used.

3. Mechanical, Thermal, and Chemical Properties of Natural Fiber Composites

3.1. Tensile Strength, Flexural Strength, and Impact Resistance

Natural fibers have been proven to improve mechanical properties and provide reinforcement to the filaments and printed parts. The mechanical properties are inherently influenced by the cellulose content of the fibers and the spiral angle formed by the microfibrils in relation to the fiber axis in their inner structure. Factors like the source of the fibers, their age, and other variables influence both properties and structure [60]. The attributes of natural fiber composites are influenced not only by the inherent qualities of the natural fibers, but also by various testing parameters, such as fiber length, test speed, temperature, etc. Introducing fibers into a polymer matrix is an important step because it significantly enhances the composite’s mechanical properties. Readily available natural fibers can provide reinforcement through fiber alignment, and fiber pretreatments help to increase contact with the polymer matrix, which improves the mechanical properties of the composite [14,61]. This phenomenon aligns with the Rule of Mixture theory, which has been employed to approximate composite moduli, enabling the adjustment of properties to various levels [62].

Nonetheless, it is crucial to emphasize that variances in mechanical properties are a common occurrence, and these deviations can be ascribed to a multitude of factors, including differences in process parameters, manufacturing techniques, fiber treatments, and the wide-ranging sources of both fibers and polymer matrices. An overview of the tensile strength, flexural strength, and impact strength of natural fiber composites is given in Table 1. Investigations into the effects of fiber volume fractions and pretreatments on hemp fiber with a polylactic acid (PLA) matrix provided valuable insights. Elastic moduli increased with the addition of hemp fibers, regardless of fiber weight percentage and pretreatments. However, increasing fiber volume decreased tensile strength and modulus. Pretreatment of hemp fibers and weight-percentage loading showed little to no effect on the flexural modulus, but alkali and silane pretreatment resulted in high flexural strength for both 30% and 50% hemp fiber-loaded composites [63].

In the pursuit of creating 100% bio-based, biodegradable and sustainable composites, various ratios of abaca fibers and bio-polyethylene (bioPE) have been employed, including 20/80, 30/70, 40/60, and 50/50 wt.%. The volume fraction of abaca fibers contributed almost linearly to the composites’ tensile strength, with maximum tensile properties achieved at 50 wt.% fiber loading. Moreover, it has been observed that the measures of mechanical properties increased in tandem with the weight fraction of abaca fibers and that the use of maleic acid as a coupling agent further improved these properties [64]. The development of bast-fiber composites by compression molding and chemically pretreating jute fibers with amino-silicone oil has been explored. The experimental design varied molding temperature, compression pressure, and pressure time. Box-Behnken design (BBD) response-surface methodology (RSM) was utilized to predict tensile, flexural, and impact strengths. as well as optimize compression molding conditions to provide the best composite mechanical properties. Table 1 shows some other work related to optimization of natural fibers with matrix material.

Table 1. Mechanical Properties of Natural fibers.

Natural Fiber	wt.% Loading	Matrix Material	Tensile Strength (MPa)	Flexural Strength (MPa)	Impact Strength	References
Kenaf	30–40	PLA	50–61	58–62	15–48 kJ/m2	[65]
	40	PP	90	50	-	[66]
	30	PLA	36.18	64.90	116.6 J/m	[38]
Jute	50	Epoxy	39.52	89.62	2.22 J	[67]
	26.9	Epoxy	70.4	84	-	[68]
	33	PP	27.49	43.33	25.54 kJ/m2	[69]
Flax	37.9	Epoxy	95.4	95	-	[68]
	20	PLA/PCL (70:30)	49–60	-	3.3–6 kJ/m2	[70]
	22	PLA	-	160–185	-	[71]
Hemp	50	Epoxy	22.43	57.11	1.25 J	[67]
	30–50	PLA	39–65	51–113	-	[63]
	30	Polybenzoxazine	52	122	4.23 kJ/m2	[72]
Abaca	10–30	PP	22–30	46–54	0.040–0.048 kJ/m2	[73]
	20–50	BioPE	26.64–47.73	-	-	[64]
	30	HDPE	33.13	-	-	[74]
Coir	10–30	PP	24–30	48–57	0.040–0.055 kJ/m2	[73]
	84–90	Melamine-Urea-Formaldehyde (MUF) Biopolymer	3.05–4.4	2.099–5.149	-	[75]
	5–30	Bakelite resin	-	53–61	-	[76]

Table 1. Mechanical Properties of Natural fibers.

Natural Fiber	wt.% Loading	Matrix Material	Tensile Strength (MPa)	Flexural Strength (MPa)	Impact Strength	References
Kenaf	30–40	PLA	50–61	58–62	15–48 kJ/m2	[65]
	40	PP	90	50	-	[66]
	30	PLA	36.18	64.90	116.6 J/m	[38]
Jute	50	Epoxy	39.52	89.62	2.22 J	[67]
	26.9	Epoxy	70.4	84	-	[68]
	33	PP	27.49	43.33	25.54 kJ/m2	[69]
Flax	37.9	Epoxy	95.4	95	-	[68]
	20	PLA/PCL (70:30)	49–60	-	3.3–6 kJ/m2	[70]
	22	PLA	-	160–185	-	[71]
Hemp	50	Epoxy	22.43	57.11	1.25 J	[67]
	30–50	PLA	39–65	51–113	-	[63]
	30	Polybenzoxazine	52	122	4.23 kJ/m2	[72]
Abaca	10–30	PP	22–30	46–54	0.040–0.048 kJ/m2	[73]
	20–50	BioPE	26.64–47.73	-	-	[64]
	30	HDPE	33.13	-	-	[74]
Coir	10–30	PP	24–30	48–57	0.040–0.055 kJ/m2	[73]
	84–90	Melamine-Urea-Formaldehyde (MUF) Biopolymer	3.05–4.4	2.099–5.149	-	[75]
	5–30	Bakelite resin	-	53–61	-	[76]

3.2. Thermal Stability and Thermal Conductivity

Natural fibers exhibit biodegrade relatively rapidly, which contributes to the flammability of composites containing them. The decomposition of these natural fibers occurs at an accelerated pace in a polymer matrix. It is essential to recognize that decomposition unfolds at varying temperatures: cellulose undergoes decomposition first, followed by hemicellulose, and eventually, lignin. Some factors that influence the thermal properties of natural fiber composites are the fiber type, surface pretreatment, type of polymer matrix, other addition of fillers, and fiber content and orientation [77]. The thermal stability of natural fiber composites can be quantified by thermogravimetric analysis (TGA), wherein the sample is subjected to various temperatures in a controlled environment [78,79]. The specific temperature and thermogravimetric method vary from one composite to another, but a starting guideline can be obtained by implementing ASTM E2250. The typical decomposition pattern for lignocellulosic biomass consists of three stages. Stage one involves the removal of water from the natural fiber (60–100 °C). Stage two entails the breakdown of the primary components of the fibers, including hemicellulose, cellulose, and lignin (200–500 °C). The final stage results in the formation of ash, which remains as a residual substance.

Several researchers have employed thermogravimetric analysis to assess the thermal stability of natural fiber composites. For example, a composite of bamboo fiber and epoxy resin exhibited first-stage weight loss at temperatures under 155 °C. Second-stage decomposition occurred from 199–399 °C and signified the degradation of cellulose and hemicellulose, while the final stage occurred from 364–499 °C, signifying the degradation of lignin [80]. In another study, sugar-palm fibers were mixed with phenolic resin at a 30 wt.% loading. TGA results revealed that the initial weight loss occurred from 30–200 °C, when moisture is lost. The second stage involves hemicellulose, cellulose, and lignin degradation and occurred between 300 °C and 400 °C. The final phase occurred at 300–400 °C, which corresponds to losing small groups and water bonds in the chains of the chemical structures. The chemical treatment negatively affected the thermal stability of the composite [81]. Bessa investigated the thermal characteristics of benzoxazine resin composites reinforced with Arundo donax L. (ADL) fibers. It was shown that the second and third stage of decomposition occurred at 200–300 °C and 350–500 °C, respectively [82]. Fique fibers are typically grown in the South American Andean region and have been used as structural reinforcement material for polymer composites. At 60–100 °C, moisture evaporation occurred; at 250–350 °C, hemicellulose decomposed; at 350–600 °C, cellulose degraded. Incorporating fique fiber into the LLDP matrix decreased the time to onset of degradation and the peak degradation temperature compared to pure LLDP composite. For the epoxy resin composite, fiber incorporation increased the time to the onset of degradation and the peak degradation temperature compared to the pure epoxy [83].

Epoxy is a resin commonly used in natural fiber composites due to its inherent thermal stability and pose good thermal compatibility to various natural fibers. Sisal and hybrid fibers have been investigated as reinforcements for epoxy. Investigations involving sisal and hybrid fibers as reinforcements for epoxy composites revealed that hybridizing sisal with curauá fiber and ramie fiber enhanced the composites’ thermal stability compared to pure sisal composites [84]. Similarly, hybridizing epoxy composites with jute fiber and oil-palm fiber increased the maximum degradation temperature compared to pure oil-palm fiber composites [85]. However, in the case of banana and jute fibers, hybridization had a lesser impact compared to the fiber content. Increasing the fiber content decreased in the thermal diffusivity and specific heat capacity of the jute/banana hybrid composite [86]. PLA is another common matrix used in natural fiber composites due to its biodegradability and compatibility with natural fibers, which arises from its low glass-transition temperature. Similarly to epoxy resin, PLA composites are influenced by hybridization with flax and basalt natural fiber. The hybrid composite exhibited an increased temperature of degradation onset compared to the flax + PLA composite, with the addition of basalt fiber contributing to improved thermal stability. The addition of basalt fiber also increased the thermal stability of the composite [87]. Jute and PLA are recognized as a promising combination for natural fiber composites. Initially, weight loss in jute-PLA composites can be attributed to water evaporation, which is followed by the thermal decomposition of jute fiber at approximately 250 °C. In contrast, the PLA matrix experiences degradation at around 340 °C, with the composite decomposing most rapidly at approximately 390 °C. During this stage, there is noticeable and significant weight loss due to the degradation of cellulose and the release of volatile compounds produced by PLA, such as carbon monoxide and dioxide [88].

Table 2. Material with temperature range for decomposition.

Material	Temperature (°C)	Weight Loss during Decomposition	Reference
Bamboo Fiber and Epoxy Resin	60–155	Moisture loss
	199–399	Cellulose and hemicellulose	[80]
	364–499	Lignin
Sugar Palm Fiber and Phenolic Resin	30–200	Moisture loss	[81]
	300–400	Hemicellulose, cellulose, lignin
	300–400	Small groups and water bonds
		in the chemical structures
Arundo donax L. (ADL) Fibers and	200–300	Second stage of decomposition	[82]
Benzoxazine Resin Composite	350–500	Third stage of decomposition
Fique Fiber and LLDP Matrix	60–100	Moisture evaporation	[83]
	250–350	Hemicellulose
	350–600	Cellulose
Epoxy Resin Composites with
Sisal and Hybrid Fibers	Varies	Improved thermal stability in	[84]
		hybrid composites
Epoxy Composites with Jute and	Varies	Increased maximum degradation	[85]
Oil Palm Fiber		temperature in hybrid composites
Jute and PLA Composite	Water evaporation at	Water evaporation, jute fiber	[86]
	lower temperatures	decomposition, PLA matrix

describes the range of weight loss associated with decomposition.

4. Tribological Properties

Tribological properties describe the behavior and characteristics of a material when it interacts with other materials, especially in terms of friction, wear, and lubrication. These properties are crucial to evaluating the compatibility of materials in a system with mechanical parts. Factors that can quantify tribological properties include frictional and wear resistance and lubrication effectiveness [89]. The wear resistance of composite materials varies depending on the orientation of the reinforcing fibers. In jute fiber-reinforced epoxy composites, there is a hierarchical trend in wear resistance, with the anti-parallel orientation surpassing the parallel and normal orientations in terms of the sliding direction and applied force. In the anti-parallel orientation, the mats were strategically arranged to align parallel to the direction of the applied force while remaining perpendicular to the sliding direction. Under experimental conditions characterized by a load of 30 N and a sliding velocity of 3 m/s, the anti-parallel-oriented mat exhibited a maximum volume loss of merely 10 mm³ when subjected to an extensive 11 km sliding distance against a stainless steel surface [90].

In a comparison of white and brown coconut fibers, the brown fiber-reinforced composite displayed better wear resistance due to the greater hardness and porosity and lower median pore diameter of the brown fibers. Interestingly, an NaOH treatment had no observable effect on wear resistance, but aligning the fibers perpendicularly to the direction of rubber disc had a small effect [91]. Another study explored hybridizing sisal fibers and coconut sheath with polyester as the polymer matrix. In single-fiber composites, both mass loss and specific wear rate (SWR) consistently increased with sliding distance. Conversely, hybrid composites exhibited a unique trend: mass loss and SWR initially increased up to 1800 mm of sliding distance and then declined. This shift can be attributed to the hybrid composites having lower porosity and void content compared to single-fiber composites. Regardless of the type of composite, the coefficient of friction (COF) consistently increased as sliding distance increased. This effect is due to increased contact between the polymer surface and abrasive grit as the sliding distance increases [92]. Liu investigated the influence of mixing silane-treated corn-stalk fiber with nitrile butadiene rubber-modified phenolic resin on the wear resistance of the composite. The different weight percentages (1–13 wt.%) of fibers loaded in the composite samples had minimal effect on the COF, which increased up to 150 °C. The COF decreased at temperatures higher than 150 °C, possibly due to composite degradation [93].

5. Applications of Natural Fiber-Reinforced Polymer Composites in Various Industries

Natural fiber composites are a viable option for several applications due to their low production cost [94]. Possible applications include car bodies and interiors, storage devices, walls and ceilings in commercial and industrial spaces, and even false ceilings. Figure 6 depicts many applications in the automobile, transportation and aerospace, construction and building, electronics, sports, residence construction, and material handling and storage industries. The ever-increasing size of aircraft wings and wind-turbine blades is continuously testing advanced materials, designs, and fabrication techniques. Composites will be made with processed fiber and resin as fillers [95]. Research efforts focus on using high-quality components and streamlined production procedures. When weight-bearing ability is a primary concern, NFPCs can be used instead of glass in several contexts. The superior mechanical characteristics and lower cost of fiber-reinforced composites make them superior to polymer resins for structural applications.

Commercial aircraft use composites in construction: 50% in the Boeing 787 Dreamliner, 53% in the Airbus A350 53%, and 25% in the Airbus 380. Fuel efficiency can be significantly enhanced by using lighter materials. Using composites reinforced with natural fibers allows for a 35% reduction in aircraft weight. The Mercedes Benz E-class was the first to employ NFRCs for the inner door panel, combining flax and sisal with epoxy polymer, achieving weight savings of 20% [96].

Numerous new applications for biofiber materials have emerged, including window frames and decking, dashboards and parcel shelves, door panels, seat cushions, and backrests, and cabin linings. More industries, such as the automotive, aerospace, marine, electrical and electronics, sports, recreation equipment, and machinery office-equipment sectors, are switching to NFCs from petroleum-based fibers, as are the automobile and construction industries, because of its low-cost processing, excellent comparative mechanical properties, ability to withstand corrosion and fatigue, and recyclability. Many companies use natural fibers. For instance Daimler Benz constructed dashboards, center-armrest consoles, and seat shells out of natural fibers. Volkswagen utilized natural fibers to create the boot-lid finish panel, seatback, boot lining, and door panel for the Passat, A4, Golf, and Bora. The use of cellulose fibers like flax and abaca increased NFC use in several vehicles by about 98% over older models [97].

The European Commission issued directives in 2000/53/EC mandating that 85% of the weight of products, particularly vehicles, be recyclable by 2005. By 2015, manufacturers were to achieve a recycling rate of 95%. Automakers must incorporate biofibers in composites to balance sustainability and cost [98].

Biofibers are used extensively in the production of door panels, dashboards, parcel shelves, seat cushions, backrests, and cabin linings. More industries, such as the automotive, aerospace, marine, electrical and electronics, sports, recreation equipment, and machinery office-equipment sectors, are switching to natural fibers from petroleum-based fibers [99]. The automotive manufacturing applications of natural fiber-reinforced polymer composites are depicted in Figure 7. Natural fibers were used by Opel’s other producers on the instrument panel, headliner panel, door panels, and pillar-cover panel of models such the Astra, Vectra, and Zafira. The Citroën C5 model included interior door-paneling components made from natural fibers. The Raum, Brevis, Harrier, and Celsius Toyota models included door panels, seat backs, floor mats, and spare tire covers made from natural fiber. Mitsubishi (Space Star, Colt Cargo) used natural fibers for flooring, instrument panels and door panels. Rear parcel shelves made from natural fibers used by Renault (Clio, Twingo), and Volkswagen (Golf A4, Passat, Variant, Bora) includes natural fiber in the door panel, seat back, boot-lid finish panel, and boot liner. The Lotus Eco Elise model includes body panels, spoilers, seats, and interior carpets made from natural fibers [100].

Hybrid polymer-based composites increasingly use synthetic fibers, including glass, carbon, and aramid, for their superior stiffness and strength. Biodegradability, high initial processing costs, non-recyclability, high energy consumption, machine abrasion, and potential health risks are some of the main disadvantages of using these fibers [101].

Some challenges of using natural fibers in composites are their relatively low degradation temperature and high flammability. Additionally, variability in fiber characteristics and properties contributes to large scatter in the properties of NFRCs. Certain fibers are not compatible with certain polymer matrices and may be susceptible to damage from insects and fungi. Product performance and reliability depend on whether the natural fiber causes higher moisture absorption (swelling), and variation in natural fiber characteristic features/quality depends on the conditions of cultivation and weather, a complex supply chain, and geographical availability. NFRCs exhibit low thermal conductivity compared to glass fiber-reinforced composites, but natural fibers exhibit poor wettability, resulting in weak fiber-matrix interfacial bonding and worse mechanical properties. They are also subject to fluctuations in price and supply due to weather and crop production, as weather and crop yield can significantly affect both prices and supplies. NFRCs have a service temperatures lower than 200 °C, so they can be used only for non-structural components [11].

The fundamental drawback of incorporating NFRCs into a polymer matrix is the absence of interfacial interaction between the two components, which results in a less ideal end product. Chemical modifications such as alkaline treatment can strengthen the bond between the NFRC and the matrix. Morphological examination demonstrates that when non-cellulose materials are removed from the surface of alkali-treated particles, the interfacial adhesion between the particles and matrix is improved [12].

The mechanical characteristics and dimensional stability of composites are reduced by humidity and aging. Matrix materials like polyester are quickly hydrolyzed by moisture, lowering the molecular weight of the polymer. Temperature and humidity affect how quickly materials decompose through hydrolysis [102]. Salem et al. examined the effects of fiber loading (10 to 40) wt percent) on the ability of a kenaf/polyester composite’s to absorb water at room temperature. Because of the high hydrophilicity of NFRCs, natural fiber composites typically absorb 2% of their weight in moisture within 24 h, 5% within a week, and up to 22% after many months [103]. Moisture-exposed composites have different surface morphologies than their dry counterparts, with increased permeability, swelling, gaps, adsorption in cracks, and fissuring around the NFRCs. Between 215 and 310 degrees Celsius, most natural fibers undergo around 60 percent thermal degradation has been found [104]. Solar ultraviolet (UV) radiation causes the breakdown of covalent bonds in organic polymers, leading to consequences such as yellowing, discoloration, weight loss, surface roughening, deterioration in mechanical properties, embrittlement, and a pronounced decline in the thermal stability of natural fiber composites [105].

Natural fibers often have their surfaces treated to improve their properties before they are used to make composite materials. Surface modification improves fiber-matrix interfacial bonding, roughness, and wettability, hence decreasing the fibers’ moisture absorption. The resulting composites have enhanced mechanical properties. Hybrid composites benefit from fiber treatment because of this enhanced performance [18].

Physical treatment: Adhesion among the fiber surface and the polymer matrix can be enhanced by applying physical treatments that affect the fiber surface and increase fiber strength. The interfacial bonding between the NFRC and the matrix is improved through physical treatment, which reinforces the interface without affecting the chemical properties of the fiber [106]. Plasma, corona, electron radiation, ultraviolet, heat treatments, and fiber beating are used to modify the surface of natural fibers. Gassan and Gutowski showed that Corona treatment modifies cellulose fibers’ surface energy, making them more amenable to incorporation into the NFRC. This technique employs high voltage at low temperatures to generate plasma at atmospheric pressure. In plasma therapy, the gas type, rate of gas flow, gas pressure, and gas concentration are controlled [107]. The NFRCs are heated to a temperature close to their degradation point during heat treatment. However, this treatment affects the NFRC’s physical, mechanical, and chemical characteristics, including their chemical composition, fiber strength, and cellulose crystallinity. These processes affect the fibers’ surface characteristics without changing their structural makeup, enhancing the bond between the fiber and the matrix. When comparing chemical and physical treatments, the primary difference is the cost of the equipment for physical treatments [108].

Chemical treatment: Enhancing adhesion to the fiber matrix through chemical bonding or mechanical interlocking at the interface, as is done in many chemical treatments, is a common way to reduce the fibers’ water absorption [109]. Compounds used for this purpose include acetic acid, benzoyl chloride, sodium hydroxide, silane, malleated coupling agents, peroxides, isocyanates, and stearic acid. Sumrith et al. used water-hyacinth fibers treated with NaOH and silane to form reinforced bio-epoxy-based composites by casting. The results show that hyacinth fibers can be used with bio-epoxy polymer as a reinforcement material to produce biobased, eco-friendly composites with lightweight structures [110].

Chemical methods result in better enhancement of properties than physical methods. Mechanical and thermal properties are enhanced by chemical concentration and are dependent on exposure time. Sometimes, the mechanical and thermal properties resulting from combined treatment with two chemicals were superior to those resulting from either treatment used alone [111].

Improvements in fiber-matrix interfacial adhesion, roughness, and wettability were observed in chemically and physically modified natural fibers. Most treatments also lessen natural fibers’ hydrophilicity and their propensity to absorb moisture. Depending on fiber, matrix, filler, and other factors, fabric polymer composites can exhibit improved mechanical and thermal properties. Fiber content, aspect ratio, and fiber orientation are important characteristics of composites. The manufacturing process and the techniques used to create the fiber (cylindrical, spherical, or rectangular cross-sectioned prisms or platelets) are significant as well [112].

The matrix contributes significantly to the composite’s shear properties, compression, and transverse modules (mechanical properties). Matrix properties like curing temperature, duration, viscosity, and fiber-bonding behavior influence the manufacturing strategy [113]. The matrix in composites binds the fibers and distributes loads between them. Aims include the optimum orientation and location of the fibers, shielding them from damaging environmental factors such as temperature, humidity, chemicals, and abrasion, and supporting interlaminar shear. The matrix enhances the composite’s transverse properties [114]. Limited shrinkage, dimensional consistency, chemical resistance, reduced coefficient of thermal expansion, decreased water absorption, resistance to damage from significant temperature increases, elasticity for load transfer to the fiber, excellent flow characteristics that allow the matrix to penetrate fiber bundles and remove voids during compacting, and the ability to be easily shaped into the final composite form are highly desirable properties of a composite matrix [115]. The fiber/matrix employed and the composite’s intended use will determine the best technique for fiber modification. To use NFRCs and hybrid composites, it is crucial to understand the interfacial properties and bonding mechanisms of the fiber-matrix interaction, which necessitates extensive research [116]. Inadequate adhesion between the fibers and the matrix will lead to inefficient load distribution, reducing the composite materials’ mechanical properties (such as tensile, flexural, and impact strength). Obtaining the correct mechanical and thermal properties in composites involves finding the right fiber/matrix combination [117].

The matrix protects the reinforcement fibers from wear and tear caused by mechanical and chemical abrasion, keeps the fibers in their proper positions, and transfers stress adequately between them.

Matrix types include ceramic, metallic, and polymeric matrices. There are two broad categories of polymer matrix: thermosets and thermoplastics. Polymeric (thermoset or thermoplastic) matrices are most typically utilized to make hybrid natural fiber composites because they can be produced at low temperatures and are lightweight. Thermoplastic polymeric matrices include nylon, cellulose acetate, polylactic acid, polycarbonate, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polylactic acid, and polyether-ether ketone, while thermoset polymeric matrices include epoxy, phenolic, polyester, polyamide, polyurethane, and ester vinyl chloride. Epoxy and unsaturated polyesters are the primary thermoset resins used in hybrid natural composites [118]. Epoxy resins are resistant to environmental deterioration and have high mechanical strength. Epoxy resins are known for their strong adhesive properties and are simple to use and cure. A crucial advantage of epoxy resins over phenolic, polyester, and vinyl ester resins is that no volatile substances are created during curing. Additionally, compared to polyesters, epoxy resins experience less shrinkage [119]. The relative brittleness of epoxy resins, which has serious negative effects on the interlaminar characteristics of the matrix and the fiber reinforcement, is a disadvantage. Epoxy resin undergoes a catalytic chemical reaction when it cures from a liquid to a solid, forming strong bonds that are difficult to undo or reform. This reaction results in this matrix material having superior performance compared other matrices, but recycling this material is still difficult. Most natural fibers become unstable at temperatures above 200 °C, and matrices that call for high processing temperatures are ineffective for producing NFRCs or hybrid natural fiber composites [120].

5.1. Automotive Industry

Natural fiber composites have made a positive impact in the automotive industry due to their light weight and desirable mechanical properties. Their light weight decreases the overall weight of the vehicle, hence decreasing fuel consumption, which makes the vehicles more sustainable. Natural fibers have been investigated for potential automotive applications by several authors in the literature. The structural design of a bonnet containing flax-fiber composite was investigated through impact-damage analysis. The structure was first manufactured using resin-transfer molding. Steel panel and flax-vinyl ester panels were compared in compression and impact tests, and it was concluded that the composite resulted in a 31.7% weight reduction with similar mechanical properties compared with the steel panels [121]. Another composite investigated for automotive applications, especially for the front cabinet, body sheet and lighting frames, is caryota fiber and polyester with 40% weight fiber loading. A 50 mm fiber length and 40% weight of fiber loading had the best mechanical properties [122]. Nachippan investigated the impact strength of automotive bumpers and bodies using glass fiber, natural fiber (hemp), and hybrid composites mixed with epoxy. They concluded that hybrid composites possess better impact strength and can protect the vehicle body and passengers better than glass-fiber composites [123]. Another study investigated the applications of composites of alkali-treated sugarcane bagasse and palm-sheath fibers in an epoxy matrix in automobile dashboards. A matrix: fiber ratio of 60:40 with a hybrid combination of treated 40% palm and 60% bagasse showed the highest tensile strength, Young’s modulus, elongation, flexural strength, impact strength and hardness, making it a superior composite for dashboard applications [124]. Another investigation subjected Desmostachya bipinnata fibers to silane treatment to fabricate brake-pad composites. The pretreatment increased hydrophobicity, hardness and shear strength, and the wear rate of treated fiber-composite brake pads was lower. This result shows that pretreated Desmostachya bipinnata fibers are suitable for use in brake pads because of their wear resistivity and high coefficient of friction [125].

Vehicle manufacturers have started using natural fibers, especially lignocellulosic biomass waste, in their vehicles. Mercedes Benz, one of the leading automobile manufacturers from Germany, included jute and flax fibers in an epoxy matrix in their A- and E-class models, mainly in their sunroof, gearbox and door panels. The sunroof frame was developed using 70% renewable materials, mostly sourced from natural fibers such as jute ad sisal. and offers up to 50% reduction in weight compared to a traditional metal sun roof frame [126,127]. Before Mercedes Benz, BMW had been one of the first car companies to use natural fibers in their cars, where flax and sisal were used in the car door lining and some of the vehicle panels, which increased the vehicles’ impact resistance. BMW also incorporated wood fiber and cotton for a high-quality sound system and as filler for car seats, and this method has been used in many car manufacturers after them [128]. Additionally, Audi used flax and sisal fibers in their door panels, with the polymer matrix being polyurethane (PU) [129]. In the 2003 Toyota RAUM, the spare-tire cover was made out of sustainable composites based on kenaf fibers reinforced with sugarcane- and sweet potato-based PLA [130].

5.2. Aerospace Industry

Even though research is being done into developing sustainable composite materials for the aerospace sector, the composites currently used in aircrafts are mainly made from carbon fiber, aluminum, and titanium due to their high strength-to-weight ratio. The widely available Boeing 787 Dreamline and Airbus 350 XWB include some composite parts, including but not limited to turbine housing, wing panels and stabilizers. This choice has shown to increase passenger comfort and noise cancellation, while reducing weight and fuel consumption [131]. Other common applications include interior parts such as seating, cabin dividers, restrooms panels, overhead bins, flooring, and wall coverings [132]. Within the aviation sector, critical objectives include reducing aircraft weight, enhancing fuel efficiency, increasing payload capacity, and improving aircraft maneuverability [133]. These objectives can be effectively achieved by employing NFRP composites, which not only contribute to heightened aircraft efficiency through reduced fuel consumption and emissions but also offer remarkable strength and stiffness, facilitating the fabrication of intricate aerodynamic shapes. NFRP composites exhibit excellent fatigue resistance and corrosion resilience [132,134].

More than 200 aircraft components are crafted from NFRP composites, encompassing elements such as cabin furnishings, sidewalls, and propulsion systems [133,134]. This significant shift towards NFRP composites in aviation has been supported by the European Union through initiatives like the Cayley project, a collaborative effort involving Boeing Research and Technology, Invent GmbH, Aimplas, and Lineo. The aim of this project is to develop environmentally friendly interior panels for aircraft by utilizing flax fiber and recycled thermoplastic sheets [135]. Another EU-funded project, known as Eco-Compass, is dedicated to the development of eco-friendly composites derived from bioresources and recycled materials for aviation applications. The primary focus of this project is to explore alternative ways to reduce the use of carbon- and glass-reinforced polymer composites in aircraft by incorporating bioresins, natural fibers, or recycled carbon fibers in secondary structural and interior applications. Flax and ramie fibers have demonstrated potential for incorporation into NFRP composites for aircraft, offering improvements in tensile strength and flame-retardant properties [136,137].

Similar to the Boeing 787 and the Airbus 350 XWB mentioned earlier, the Boeing 737 aircraft also used natural fiber composites, namely flax/epoxy sandwich composites in the sidewall panels, with the flax fabrics treated to be halogen-free and fire-retardant. These flax/epoxy panels are not only 35% lighter than carbon/epoxy composites, but also have a structure similar to that of glass/epoxy unidirectional composites [132]. Beyond epoxy, phenolics and engineering thermoplastics such as polyphenylene sulfide, polyether ether ketone, polyamide (PA), and polycarbonate (PC) have garnered significant attention in the aviation industry. This attention is primarily due to their excellent fire resistance, aligning with the stringent flame, smoke, and toxicity regulations applicable to aviation materials [138]. For instance, polyether ether ketone (PEEK) was reinforced with chopped glass and carbon fiber by Victrex Europa GmbH, who developed VICTREX^® PEEK for aircraft applications. This composite exhibits mechanical strength and dimensional stability comparable to those of metal alloys, withstanding high temperatures and displaying resistance to corrosion, chemicals, wear, and abrasion [131].

5.3. Medical Industry

In the biomedical industry, lignocellulosic fibers rarely are used, except for in recent novel regenerative studies, but the wearable technology sector has taken this opportunity to utilize biomass waste to develop a strain sensor for biomedical applications. The sensor was constructed from a conductive and flexible composite in which carbonized walnut shell powder was blended with polydimethylsiloxane (PDMS). The composite demonstrated exceptional sensitivity in detecting human body movements. The potential applications for this innovative sensor are boundless, with the tantalizing prospect of it evolving into a versatile and adaptable technology for wearable, flexible sensors that enhance our daily lives [139,140].

Animal-based fibers such as silk and chitosan and cellulose-based nanoparticles have been studied. with promising results. Silkworm- and spider-produced silk have been applied in medical components such as sutures, sponges, and films [141,142,143,144]. The allure of silk lies in its distinctive usefulness as a scaffold biomaterial, in particular because of its ability to degrade gradually. This unique feature aligns perfectly with the demands of biomedical applications, offering a material that seamlessly integrates with the body’s natural processes. One particularly striking advantage of employing animal-based fibers like silkworm silk and spider silk in medical implants and bone repairs is the elimination of the need for a secondary surgical procedure to remove metallic implants. This groundbreaking advancement not only enhances patient comfort, but also reduces the associated risks and costs, revolutionizing the field of medical interventions [143,145].

Steering our focus towards the world of advanced technology and biosensors, we encounter an exciting development in the form of nanocomposites. These nanocomposites, designed to encapsulate gold nanoparticles with the aid of camphorsulfonic acid (CSA) surfaces, have applications in electrical devices and biosensors. The nanocomposites showed high solubility in water and select organic solvents, rendering them exceptionally versatile for an array of applications [146]. Another noteworthy innovation is a self-cleaning and antimicrobial hybrid system. This ingenious system is based on a foundation of polybutylene succinate (PBS) and polybutylene adipate-co-terephthalate (PBAT), fortified with the inclusion of lignin and zinc nanoparticles. The result is a material with the remarkable capability to repel dirt and hinder the growth of harmful microbes. This development not only had immediate implications for maintaining cleanliness and hygiene, but also holds significant promise for various sectors that demand durable and hygienic surfaces [147].

5.4. Construction Industry

Geopolymer materials have attracted significant interest for their remarkable technical characteristics, which closely resemble those of conventional Portland cement-based materials, all while significantly mitigating carbon dioxide (CO₂) emissions and minimizing their overall environmental footprint. Furthermore, integrating natural fibers, including but not limited to bamboo, flax, hemp, and jute, within the geopolymer matrix is a compelling avenue of exploration. These natural fibers serve as a reinforcing component, enhancing the tensile and flexural strength while reducing material density, creating lightweight construction materials. Another advantage of using these natural fiber composites on the interior of buildings include improved thermal insulation, which leads to energy efficiency and better soundproofing.

Different aspects of natural fiber-reinforced geopolymers have been studied in the literature. One of the important factors in choosing materials for construction is flame-retardation properties, which allow the building material to act as a flame retardant in case of a fire. Silva studied the fire-resistance and head-absorption capabilities of a jute fiber-reinforced pozzolan-based geopolymer. The samples demonstrated an impressive ability to absorb nearly 65% of the heat generated by the flames, with minimal degradation [148]. Other common properties sought after by the construction industry are high mechanical strength and hydric properties. Bast-hemp fibers were investigated with and without sodium hydroxide pretreatment at 3% weight loading in diatomite geopolymer, and the composite showed an almost 100% increase in compressive strength and a 35% increase in flexural strength while maintaining thermal conductivity [149]. Flax linen, cotton and abaca fibers also showed an increase in compressive strength after being mixed with geopolymer. Alkali treatment of the abaca fiber facilitated fiber bonding with fly ash and increased the flexural strength of the composite [150]. When untreated cotton- and flax-based linens were investigated for use in fly-ash composite, the compressive and flexural strength of the composites increased, but the flexural strength of the linen-fly ash composite increased significantly, by 60% [45,151].

With a rising global emphasis on sustainable materials, recent breakthroughs in FFFs have broadened their usefulness across numerous domains. Material innovation has witnessed the development of novel hybrid resins for DLP 3D printing of biocompatible scaffolds using renewable resources [152], extrusion methods for high-performance thermoplastics derived from recycled materials [153], and laser-sintering techniques for silicon carbide-based ceramics utilizing post-industrial waste [154]. Process improvements include adaptive process control for laser powder bed fusion [155] AM to optimize material usage, in situ monitoring using computer vision to minimize material consumption, and multi-material AM systems with extrusion-based approaches that facilitate the effective integration of sustainable materials [156]. Personalized orthotics and prosthetics made of recyclable and biocompatible materials, aerospace parts like rocket engines and aircraft parts made of lightweight, sustainable materials, and medical devices like implants and surgical guides that are made with biocompatibility and sustainability in mind are just a few of the numerous applications of additive manufacturing (AM) [157,158]. These developments highlight AM’s revolutionary potential to promote a circular economy and advance environmentally friendly manufacturing techniques, opening the door for more ground-breaking discoveries in the years to come.

6. Challenges and Opportunities

Additive manufacturing and the incorporation of natural fibers in different additive manufacturing technologies may have great benefits for many industries by improving composite properties. The new materials have similar or better characteristics compared to synthetic fibers, with a smaller carbon footprint. However, there are challenges and aspects of this sector that need to be explored and better understood.

First, because natural fiber composites emerged only recently, there are limited data on the durability, safety and recyclability of NFCs. Although PLA is a commonly used polymer composite matrix and is commonly derived from bio-based sources, the recyclability of NFCs remains uncertain. Some composites may contain precursors and chemical agents based on formaldehyde, which can emit volatile compounds that may pose a safety concern. In outdoor applications of composites, the durability of the composites is relevant. In addition, some composite additives can seep out when subjected to moisture, high humidity or rain, which can potentially cause chemical leakage into the water system.

The development of NFCs is gaining momentum and holds promise as a sustainable source of material for various new applications. More work needs to be done to overcome the challenges associated with the development of NFCs at a large scale, such as cost and the constraint of technological adaptation. NFCs inherently exhibit relatively inferior mechanical properties due to the incompatibility between natural fibers and the polymer matrix and the inherently weaker nature of natural fibers compared to their synthetic counterparts. A potential solution to address these drawbacks involves the modification of natural fibers through physical, chemical or biological means. For instance, alkalization alters the fibers’ structures, substantially reducing the fibers’ capacity for moisture absorption and thereby enhancing the interfacial adhesion between the fibers and the polymer matrix. Despite these limitations, the use of NFCs is growing, especially in the automotive sector. There, long fibers like hemp, kenaf, and flax are increasingly being integrated into various automotive components. The adaptability of wood-plastic composites for construction highlights the diverse uses of NFCs. Additionally, these materials have been successfully used in electrical devices and sporting goods, highlighting their potential for a significant market presence.

Future directions for NFCs should be include development of new composite materials; improving fiber-matrix interfacial properties, fiber homogeneity and alignment and interlayer bonding; addressing porosity issues; and improving printability. One drawback of natural fibers is their vulnerability to fluctuations in supply, price and quality with different seasons, climates and temperatures, which can significantly affect the production and properties of NFCs. One potential solution to this problem can include hybridization, whether between natural fibers or natural fiber and synthetic fiber, which can optimize the properties of the resulting composites. Future research can also focus on ensuring the long-term stability of NFCs in outdoor applications where the composite parts are subjected to weather changes, UV radiation and humidity.

7. Conclusions

In conclusion, the development of additive manufacturing has ushered in a new phase in materials science that is marked by investigations into natural fiber-reinforced polymer composites. The creation of unique composite materials that can resist structural failure while being environmentally sustainable has been made possible by the collaborative fusion of traditional materials-science paradigms with state-of-the-art manufacturing techniques. This approach has produced groundbreaking results. The progress made in 3D printing highlights a broad area of study in material science and opens hitherto unexplored possibilities for developing sustainable solutions.

A vital component of this development is a dedication to environmental sustainability, as demonstrated by the purposeful inclusion of natural fiber-reinforced polymer composites in 3D printing, which is a significant paradigm shift. The combination of these compositional ingredients with additive manufacturing techniques has created materials with increased mechanical strength and improved biodegradability, substantially reducing the carbon footprint associated with traditional manufacturing methods. Therefore, the combination of natural fibers and polymers with additive manufacturing is a revolutionary partnership that paves the way for the development of materials that are resilient to harsh mechanical demands and consistently environmentally responsible. This revolutionary finding highlights the potential of additive manufacturing to become a leader in the production of sustainable materials, and it calls for more scholarly investigation and discussion in the context of modern materials science, with the following specific focus points.

Material Diversity: Natural fibers such as jute, flax, hemp, and kenaf have been successfully integrated into polymer matrices. These composites exhibit unique mechanical and thermal properties, often on par with those of their synthetic counterparts.

Enhanced Properties: Advancements in fiber treatments and matrix formulations have optimized fiber-matrix adhesion, leading to improved tensile strength, rigidity, and durability of the composites.

Hybrid Systems: Recent studies have showcased the promise of hybrid composites, which combine natural fibers with synthetic ones, resulting in materials that harness the benefits of both.

Processing Techniques: Enhanced FFF methodologies specific to natural fiber composites have been developed to address challenges like moisture absorption and ensure uniform fiber distribution.

Natural fiber-reinforced polymer composites present a two-fold advantage. Environmentally, they promote sustainability through biodegradability and reduced carbon footprints. Economically, they offer the potential for cost reductions, leveraging abundantly available natural resources. As industries globally grapple with mounting environmental concerns, the adoption of such sustainable materials is both viable and imperative.

A new age of materials research has begun with FFF, especially concerning the enormous potential of natural fiber-reinforced polymer composites. Innovative discoveries have been made possible by the fusion of conventional materials science with cutting-edge production techniques, leading to the development of composite materials that are both strong and ecologically friendly.

The research presented in this publication makes a substantial contribution to the fields of sustainable manufacturing and materials science. This paper provide helpful answers to environmental problems and improves our knowledge of natural fiber-reinforced polymer composites. Eco-friendly materials are essential to reducing the size of our carbon footprint and protecting the environment for coming generations, and this research is a vital first step toward a more sustainable and ecologically conscious industrial sector.