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The escalation of ecological awareness and sustainability has motivated many researchers to foster bio-composite studies and advancement in employing natural fibers and resin. During the last few decades, composites have emerged with various environmental impacts such as degradation, incineration, and toxicity. The credence is that bio-composite materials will downsize the need for environmentally and economically synthetic polymers. To enhance the degradability to the maximum extent, natural resin and natural fiber must be acquired from natural resources. A sequence of treatments must be followed throughout the resin synthesis process to obtain a usable and effective form of natural resin. Natural fibers are becoming more and more dominant over synthetic fibers because of their superior strength, stiffness, durability, and lack of toxicity. In comparison to synthetic fibers, biocomposites have the potential to diminish material expenses while improving mechanical properties. The processing techniques of compression molding, injection molding, and extrusion are frequently employed for biopolymer composite preparations. The ultimate properties of the composite are determined by the degree of adhesion between the matrix and fiber. To assess the mechanical and thermal properties of biocomposites, tests such as tensile, flexural, impact, thermogravimetric analysis, and dynamic mechanical analyzer are conducted. Many applications of bio-composites have created new opportunities for research and business ventures. Bio-composites are non-abrasive, degradable, and used for various purposes like packaging, medicine, agriculture, and the automotive industry. The undesirable factors like degradation, incineration, and recycling problem of non-biodegradable composite have induced the research and evolution of bio-composite. This decisive review would manifest a summary concerning the framework of natural resins, natural fibers, and bio-composites, the factors affecting the characteristics of bio-composites, and the future prospects for this field.
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The increasing ecological awareness and sustainability have motivated many researchers to synthesize green resin using derived natural products [1]. Natural reinforced composites which are lightweight and eco-friendly are increasingly being developed by researchers. During the last few decades, synthetic resins were mostly used as engineering plastics. Most of the materials like polymers and plastics are extracted from petroleum-based products. Most of the plastics which are being used cause serious environmental impacts like soil eruption, land pollution, and ocean pollution [2]. Disposal methods like incineration also cause some effects on the environment. The increasing and rapid depletion of landfill space available for discarded wastes encourages the use of biodegradable polymer materials [3]. Synthetic resin has a wide range of characteristics like easy processing, high mechanical properties, good adhesion, etc. Biodegradable polymers are polymers that are degradable to carbon dioxide and water [4]. Materials produced from renewable feedstocks are expected to increase from 5% in to 12% in to 18% in and 25% in [5].
Most naturally occurring green resins occur in the form of gum tissue and mucilaginous in a wide range of plants, animals, and microbe [6]. As compared to synthetic resins, green resins are non-toxic, cheaper, and have no impact on humans, they are mostly employed in the pharmaceutical and food industries, but little study has been conducted on the synthesis of green resin as an appropriate composite matrix [7]. The composite property of the green resin is greatly influenced by the process variables including pressure, temperature, and catalyst. The characteristics of the utilized reinforcement impact the performance of the green resin [8]. Nowadays landfills are decreasing which results in the availability of waste to eliminate waste. From a survey, Stevens ES found that the number of landfills fell from to between and in the US [9]. In automobiles, natural fiber-based composite materials caught the interest for greater fuel efficiency and cheaper and ecological sustainability. Natural fibers have gained interest in automotive industries [10].
The credence is that bio-composite materials will downsize the need for synthetic polymers both environmentally and economically. To intensify the property of degradability in bio-composites, both the reinforcement and matrix must be derived from natural resources [11]. The drive to use bio-composites in the place of synthetic polymers is to improve performance. Biocomposites with natural fiber reinforcement display notable mechanical qualities required for many applications. The chemical composition and physical characteristics of natural fiber and biopolymer, surface modification of fiber, composite processing techniques, processing environment, fiber loading concentration, fiber orientation in the matrix, copolymerization, and plasticization are just a few of the factors that affect the mechanical properties of biocomposites [12]. These bio-composites are used for various purposes like packaging, medicine, agriculture, the automotive industry, etc. Moreover, bio-composites may reduce material waste. These composites provide excellent thermal and acoustic insulating properties, high rigidity, and great fracture resistance [13]. Tensile tests are one of the most commonly used methods for figuring out the mechanical characteristics and understanding the structural layout of biocomposites. Biopolymers are reinforced to the matrix to increase the tensile properties of the composite because fibers are stronger and more rigid than biopolymers [14]. The flexural characterization test is the second most recommended mechanical test, and flexural stiffness is a factor in determining deformability. Young's modulus and moment of inertia, which are functions of the cross-sectional geometry of a material, are necessary for flexural properties [15]. FTIR spectroscopy, laser Raman spectroscopy, solid-state nuclear magnetic resonance (ssNMR) spectroscopy, ion scattering spectroscopy, Auger electron spectroscopy, x-ray photoelectron spectroscopy, wide-angle x-ray scattering (WAXS), and contact angle measurement are the most frequently used characterization techniques for identifying the interface of biopolymer composites.
γ-ray treatment promotes the mechanical strength of natural fibers by increasing the inter-crosslinking of cellulose molecules [16]. In the early days, natural fibers are generally used in rope manufacturing, fabrics, and carpets. Nowadays, these natural fibers are used in various sectors like automobiles, civil and paper industries [17]. Natural fibers are not only used in structural applications but also as fillers in polymer preparation to achieve advantageous properties [18]. The rising levels of societal concern and environmental awareness worldwide, the rapid depletion of petroleum supplies, the idea of sustainability, and new environmental legislation have all combined to spur the quest for new environmentally friendly techniques and products [19]. Since many of the resins and fibers used are synthetic and non-biodegradable they do not degrade and aggravate environmental problems, to overcome this problem many countries' governments have implemented laws to use recycled products. This article offers a thorough analysis of natural fiber and its applications, as well as its mechanical attributes, manufacturing method, morphological study qualities, and potential to outperform conventional fibers.
For a few decades, waste vegetable oil is being used in biodiesel production. Bio resins are used in various industries for various applications. By reaction with biobased hardener waste, vegetable oil can be converted to bio epoxy resin which is used in the printing ink, and control release system [20]. The classification of bio-resin is illustrated in figure 1. The green resins can be of marine, terrestrial, animal, or microbial origin depending on where they came from. Agar, alginic acid, and laminarin are a few of the marine resins that are made from algae. Arabica, ghatti, guar, starch, and pectin are a few of the resins with a plant origin. The green resins can either be straight or branching depending on their morphology [21].
Figure 1. Classification of bio-resins [21].
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Standard image High-resolution imageCitric acid is recognized as an environmentally friendly compound and resin made from this has demonstrated its ability to replace synthetic resin. When citric acid is combined with another compound it exhibits good resistance against biological attack, dimensional stability, and compression strength [22]. Citric acid can react with a crosslinker that has high adhesion and bonding properties since it has triple carboxyl groups. Companies in several nations are addressing issues connected to product life because of environmental concerns along with regulatory requirements [23]. A bio-sourced thermoset resin developed using furfuryl alcohol and tannins is used as a matrix in the automotive industry for brake pads. These brake pads exhibit excellent wear resistance and very good braking properties as comparable to phenolic resins [24]. Natural oils like algae and cottonseed oil are used to synthesize bio epoxy resins which can be used as a great alternative to synthetic resins and can be used in different sectors [25]. Similar to how rubber is extracted, grooves are made on the stem of the urushi tree to obtain the resin. Urushi resin is primarily used in the production of lacquer, which is mostly applied to dinnerware, wooden furniture, and military armor [26]. Resins made from cashew nut shell liquid can replace synthetic resins in many applications. They find use in frictional components like clutches and brakes. They are also an effective binding agent when particle boards are made [27].
To improvise the yield and modify oils and some triglycerides in beans for the synthesis of resin biotechnology is used. These naturally derived resins will be more affordable than other resins and can be used in various cutting-edge industrial fields. The process variables used during the synthesis of bioresin, such as pressure, temperature, catalyst, initiator, and so forth, have a significant impact on the composite property [28].
Over synthetic fibers, natural fibers are mostly preferred because of their wear and tear properties. Also, water bodies and landmass are less contaminated by utilizing natural fibers [29]. From a study, Jawaid M H observed that every year 3 million tonnes of natural fibers are generated for various applications like building materials, paper making, clothing, packaging, etc [30]. Lightweight, a high strength-to-weight ratio, resilience to fracturing while manufacturing, and recyclability are some advantages of plant fibers [31].
Two main categories can be used to describe natural fibers, namely plant fibers, and animal fibers as shown in figure 2.
Figure 2. Categorization of natural fibers [1, 32].
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Standard image High-resolution imageAnimal fiber primarily comprises proteins and natural fibers primarily comprise cellulose. When compared to plant fibers, animal fibers possess better flexibility, high surface roughness, and low hydrophilic nature [33]. Animal fibers are mostly used as reinforcements in the manufacturing of polymer composites. A crucial technique that controls the qualitative and quantitative characteristics of fiber is fiber extraction. The process of fiber extraction is determined by the type of fiber and the necessary application [29]. Depending on where the animal fibers come from, different procedures are used to extract them. For example, the wool is harvested by hand and then further cleansed to remove contaminants. The most popular animal fiber nowadays is wool, which comes from creatures like sheep, bison, rabbits, alpacas, rabbits, and others. Australia, China, and New Zealand are considered to be the major producers of wool [34]. Wool is mostly used because it possesses some significant characteristics like a low rate of heat release and low rate of flammability when compared to other animal fibers. Silk is also an animal fiber that is procured from approximately species of butterfly larvae and also from spiders [33].
Chitin, a highly organized protein, gives the fiber remarkable mechanical strength and chemical resistance. The toughest silk is recognized as dragline silk, which has a tensile strength of MPa.
There are numerous techniques for retting to extract fibers from plant sources, just like with animal fibers. As an alternative to glass fiber, plant-based fibers with good mechanical characteristics including jute, sisal, and hemp are utilized. Sisal and kenaf are frequently used when creating natural polymer composites [35]. Figure 3 lists many classifications of plant-based natural fibers and the functions of those fibers. Plant fibers are categorized into two, where one is primary fibers which include sisal, hemp, cotton, jute, etc and the second is secondary fibers which include oil palm, coir, banana fiber, etc [29].
Figure 3. Categorization of plant-based natural fibers and their applications [36&#;38].
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Standard image High-resolution imageThe primary components of plant fibers are cellulose, hemicelluloses, lignin, pectin, and wax, though the proportions of these components vary from one plant to the next and within the same plant [39]. Plant fibers are broadly classified according to their botanical source for commercial purposes as bark, leaf, seed, fruit, stalk, grass, and wood fibers. Bast fibers are those that are derived from the stem of a plant, which includes kenaf, hemp, flax, jute, and banana; leaf fibers, which include sisal, abaca, and PALF (pineapple leaf fibers); seed fibers, which include coir, cotton, and soya; and fruit fibers. Both softwood and hardwood are the main types of wood fibers [31]. Plant fibers exhibit good flexural and impact strength and high moisture resistance. The elemental composition of the constituents that make up plant fibers determines their thermal and mechanical capabilities. Plant fiber has a compound called pectin that clubs the cellulose molecules together, improving strength and water resistance [40].
Plant oils and natural fibers are utilized as raw materials for the manufacturing of biocomposite. It was estimated that 26 million tons of natural fiber are used for production approximately. Figure 4 illustrates how different natural fibers are utilized globally for the creation of composites [41].
Figure 4. Worldwide use of natural fibers for materials [41].
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Standard image High-resolution imageThe usage of natural fibers leads to various other problems like weak performance because of the poor interfacial bonding relied upon with the hydrophobicity of polymer [42]. Natural fibers which are modified by surface treatment to overcome the problem of adsorption of water and compatibility of resin help to facilitate good adhesion from the matrix to the fiber [43]. Natural fibers are hydrophilic and hence to resist water leaching, silane treatment is used for the stabilization of polymer composites. Several factors like diffusivity, temperature, permeability nature of the fiber, and fiber volume fraction are taken into account when composite materials absorb water [44].
Table 1 lists the mechanical properties of natural fibers and also some natural fibers that possess the same strength as E-glass fibers. Natural fibers with properties like porous, non-abrasive, and hygroscopic fibers can be used in many applications like home appliances, automobiles, and the architecture sector to make soundproofing and long-lasting fabrics [45]. Table 2 lists the application of natural fibers.
Table 1. Mechanical properties of natural fibers.
S.noFiberDensity (g cm&#;3)Elastic Modulus (GPa)Elongation (%)Tensile strength (MPa)References1Kenaf1.3&#;1.59&#;551.4&#;2.&#;700[46&#;48]2Flax1.3&#;1.58.5&#;401.9&#;&#;[32, 47, 49]3Cotton1.5&#;1.65.5&#;15.12.1&#;&#;800[32, 47, 50]4Hemp1.1&#;1.614.4&#;700.8&#;&#;[32, 46, 47, 49]5Coir1.15&#;1.63&#;714&#;&#;593[47, 48, 50]6Jute1.3&#;1.525&#;811.1&#;3.&#;850[32, 47, 50]7Banana0.5&#;1.54&#;32.72.4&#;3.&#;789[32, 46]8Bamboo1.2&#;1.527&#;401.9&#;3.&#;575[46, 48]9Bagasse1.1&#;1.65.1&#;6.26.3&#;7.&#;350[46, 48, 49]Table 2. Application of natural fibers.
Fiber typeApplicationReferencesWood fiberFittings for the Ford Freestar's sliding doors and seatbacks[1, 51]Coconut fiberthe headrests, rear cushions, and seat bases[23, 51]CottonSoundproofing[51, 52]Kenaf and flax mixtureEntrance panel inlays and shipping baskets[1, 51, 52]Abacaflooring bodywork[51]CornFord mounts Goodyear tires[1, 51]KenafLexus package shelves[1, 24, 51]Due to their lower microbiological resistance, natural fibers have poor impact strength, and their mechanical properties are influenced by their colour, temperature effects, and odour [52]. A study reveals good results in automotive applications when banana fibers are combined with leather [53]. Kenaf fiber has some significant properties like it absorbs nitrogen and phosphorous from soil and taking carbon dioxide from the air and these fibers can be easily recycled [54].
Natural fibers are a sustainable resource that is considered to be renewable ad biodegradable and they can be used without damaging the environment. Natural fibers such as hemp, jute, sisal, bamboo, and coir can be used as a replacement for synthetic fibers with improved mechanical properties. Structure, cell dimension, density, microfibrillar angle, chemical composition, mechanical qualities, and the interaction of a fiber with a matrix under particular environmental circumstances are significant aspects that affect a fiber's overall performance for a particular application [15].
The fiber depiction represents the morphological characteristics of natural fibers such as surface roughness and cell wall structure are analyzed and determined using a scanning electron microscope (SEM). This SEM uses a high electron beam to give a high-resolution image of the fiber using a backscattered electron signal. The crystallographic structure, crystal size, and crystallinity index of the fiber are examined using x-ray diffraction [55]. A technique called Fourier-transformed infrared spectroscopy (FTIR Spectroscopy) is used to analyze the molecular structure of fibers and also the chemical bonding qualitatively and quantitatively [56]. The band values which are around and &#;1 had been observed in a study of seagrass fiber for lignin and pectin [57]. The exact diameter of the fibers is revealed by TEM micrographs. TEM micrographs could be used to investigate even the tiniest aspects of the fibers. With the help of TEM micrographs, the transverse dimensions of the many sublayers that make up a cell wall may be examined. These techniques, however, demand a meticulous procedure for sample preparation [58]. Using a DSC machine, differential scanning calorimetry (DSC) analysis is carried out. Using DSC analysis, the melting peak and Tg are identified as standard [59].
Biodegradable polymers have been developed successfully and are used in various fields and biodegradation depends on environmental temperature, pH, and microbial activity [20]. Even though the contribution of biopolymers is less to the biopolymer market, it is analyzed that biopolymers help to replace petroleum-based products up to 30 to 90 percent. Regarding manufacture and use, the two most popular biopolymers are PLA and PHA [60]. Renewable raw materials replacing synthetic materials meet the green chemistry principle [61]. Several different kinds of polymers have been created from a variety of biofuels, including starch, which is employed to manufacture bioplastics, fermented sugars, which create polylactic acid (PLA) through the creation of lactic acid, and polysaccharides, which create polyhydroxyalkanoates [62].
To manufacture green composites, some oils are modified chemically to obtain resin. The first fiber-reinforced composite for car bodies was made using resin from soyabean oil by Henry Ford in the year as soybean contains 20% oil [63]. The so-called green composites in which non-biodegradable resin is reinforced with degradable fibers are returned to natural or industrial metabolism but not to a food stack. In terms of availability, affordability, and degradability, plant and vegetable oils are among the best sources of sustainable resources [64].
Figure 5 depicts the categorization of biopolymers in terms of renewable raw resources and crude oil. Although biopolymers have a wide range of uses, they have significant disadvantages, such as poor mechanical characteristics and a low durable degradation ratio. Biopolymers are hydrophilic as well [66]. Calori IR et al stated that natural biopolymers are obtained from microbes, proteins, and polysaccharides. Biopolymers that are obtained from polysaccharides are cellulose, starch, chitin, dextrin, and alginate [67].
Figure 5. Categorization of biopolymers [65].
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Standard image High-resolution imageBiological molecules formed by a large chain of residues from amino acids are referred to as proteins. Proteins are of two types, one is animal proteins which include silk, whey, collagen, and keratin and the next is plant proteins which include gluten, soy protein, and zein. In the production of bio-composites, gelatin is the mostly preferred biopolymer because of its cost-effectiveness, biodegradability, and biocompatibility [68]. Soy protein isolate which is extracted from soya bean oil has 90% protein and is used as a restoration for petroleum-based polymers. Cellulose is considered the most eco-friendly polysaccharide because of its low requirement of energy during manufacturing and it is easy to recycle by the process of combustion [69].
Polymeric substances called biopolymers are produced from biological sources. The significance is that biopolymers have been studied for use in sorption and other industrial applications due to their renewability, abundance, biodegradability, and other special qualities like high adsorption capacity and ease of functionalization. Because of PLA's exceptional mechanical and barrier qualities, it may be used to create a variety of biomaterials for use in the creation of textiles, packaging, biomedical products, and automobiles [12]. Because of its hydrophobicity, biocompatibility, biodegradability, and thermoplastic characteristics, PHA is a desirable biopolymer used in pharmaceuticals, tissue engineering, and traditional medical devices [70].
Bio-degradable composites refer to reinforcing natural fibers from plant and animal sources with natural biopolymers. They use adhesion methods for reinforcement such as mechanical bonding, electrostatic attraction, chemical bonding, interdiffusion, and adsorption. Bio-composites are attractive to manufacturers because of their flexibility during the processing of composites, low cost, and high specific strength [71].
Bio-composites can be categorized as bio-based composites, green composites, and sustainable bio-composites. Natural resources are used to create the components of bio-based composites. In green composites, the constituents include polymeric matrix which can be acquired from biomass or petroleum-based products, and bio-composite which is degradable. The lifecycle of green composite is shown in figure 6. One or more of the components of a sustainable bio-composite are from natural sources [72].
Figure 6. Lifecycle of green composites.
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Standard image High-resolution imageThe use of lignocellulosic fibers which are nonabrasive and lightweight is used to make the composite greener A survey implies that when the demand for green composites increases, the cost of the resin may decrease and come down [73]. Composites can be made by combining the fiber with the matrix through two different methods which are bulk composite and laminate composite. To produce a low-cost bio composite, new techniques, and technologies have been developed. Christian SJ mentioned that bio-composites exhibit a maximum tensile strength of 20 to 200 MPa and stiffness of about 1 to 4 GPa [60].
Bio-composites strongly contribute to switching from fossil-based materials. Bio-composites can be processed using three techniques like compression molding, injection molding, and extrusion.
The processing techniques of compression molding, injection molding, and extrusion are frequently employed for biocomposite preparations. For natural fiber-reinforced thermoplastics, thermoforming, compounding, and long fiber thermoplastic-direct (LFT-D) techniques are also employed. In compression molding, flat semi-finished goods or hybrid fleeces that are either precisely trimmed to the appropriate part size or are larger than the form are typically utilised [74]. By using the compression molding process and the in situ polymerization method, thermoplastic biodegradable composites based on a PLA/PCL matrix and silane coupling agents to improve interfacial adhesion was created. Tensile and impact strength were examined in relation to fiber length and content, and it was discovered that the same were at their highest levels when a 45-weight percent of fiber with 5&#;6 mm length was added [75].
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The composites which are based on plant fibers are made using waste wood products, wood scraps, etc. These are called plastic lumber and are considered a partial solution for waste disposal issues [76]. Fiber boards made from sugarcane extraction waste called bagasse are used in several manufacturing units. Starch and its blends are also used in the manufacturing of green composites [77].
Due to their increased eco-efficiency, recyclability, and non-toxicity, natural fiber-reinforced composites are predominantly employed in the application of automotive parts [78]. Many experimental analyses carried out on PLA resins when combined with kenaf and jute fiber are more suitable in the manufacturing of automobile interior parts [79]. The density of bio-composites is 20%&#;30% lighter than glass fibers and is shown in figure 7 [80].
Figure 7. Density of PLA and PP composites [32].
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Standard image High-resolution imageThe polymers that come from plants, animals, and bacteria are known as biopolymers. They are widely accessible renewable resources that are typically used to make environmentally acceptable bioplastics [56]. Composites are mixes of heterogeneous structural materials made by combining two or more constituent elements that have a wide range of characteristics. Biocomposites refer to biodegradable materials that are reinforced with a variety of natural fibers derived from plant and animal sources and natural or manufactured biopolymers [60].
The properties of the biocomposite as shown in figure 8 can be determined by mechanical testing which includes tensile test, flexural test, and impact test; thermal testing which includes Dynamic mechanical analysis (DMA), Differential scanning calorimetry (DSC), and Thermogravimetric analysis (TGA); and material characterization which includes Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and Energy-dispersive x-ray analysis (EDAX).
Figure 8. Experimental observation.
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Standard image High-resolution imageSaravanan et al carried out a mechanical test on two specimens, the first specimen is a natural fiber with resin and the second specimen is a natural fiber/resin with aluminum. The results including total deformation, equivalent elastic strain, equivalent stress, and shear stress are taken into account and the corresponding standard error plot between this specimen is shown in figure 9.
Figure 9. Error plot of two specimens during the mechanical test (a) total deformation (b) equivalent strain (c) equivalent stress (d) shear stress [80].
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Standard image High-resolution imageThe results show that sisal and flax are more mechanically superior to jute and leather. Better performance is aided by the deformation of flax and sisal, which is observed to be 1.026 mm, 0.877 mm, and 1.161 mm, and 0.898 mm, respectively. Leather is inappropriate for manufacturing due to its high deformation and generated stresses. When compared to other fibers, flax is appropriate for making automotive panels due to its deformation property. When compared to other natural fibers, sisal is found to be superior because it has the lowest equivalent and shear stress values [80].
Hemp fibers are taken to examine the property with and without alkali treatment. The alkali treatment involves the process of soaking the fiber in a sodium hydroxide solution. Flax/Hemp fiber biocomposites exhibit a reduction of 30% tensile strength and 27% flexural strength in comparison to epoxy flax/hemp fiber composites [81].
Examining the viscoelastic and dynamic characteristics of composites is accomplished using a technique called DMA (dynamic mechanical analysis). The investigation of dynamic properties by Pothan et al of the banana fiber-reinforced composite at various conditions like 0.1, 1, and 10Hz for various fiber volume fractions of 10, 20, 30, and 40%. When these composites are examined, it was observed that at higher-temperature regions the storage modulus of neat resin is lesser than the composites, and at low-temperature regions, the storage modulus of fiber-filled polyester is lower than the neat polyester [82].
The tensile characteristics of biocomposites are significantly influenced by the reinforced fiber content. By adding 0&#;30 weight percent of coconut fiber as reinforcement, PHB composites were created. The composite made with 10-weight percent coconut fiber produced the greatest results in the trial, improving tensile strength and elongation at the break by 35 and 25%, respectively, in comparison to pure PHB composite [83]. However, Petinakis et al found that the amount of wood flour in the composite wood flour fiber/PLA fiber did not significantly affect its tensile strength. With the addition of a coupling agent, methylene diphenyl-diisocyanate (MDI), they increased the tensile strength and tensile modulus of the biopolymer composites by 10 and 135%, respectively [84].
Sawpan et al evaluated the flexural characteristics of thermoplastic (PLA) and thermosetting (unsaturated polyester resin) composites reinforced with 30-weight percent hemp fiber. It was determined that when the fiber loading increased, Young's modulus rose but the flexural strength fell. It is noticed that increasing the fiber content causes more kinks, which lowers the flexural stress of the composites [85].
The heat deflection temperature (HDT) of biocomposites is determined through dynamic mechanical thermal analysis (DMTA). The DMTA test includes both thermal (DSC and TGA) and mechanical test methodologies [86]. By including triacetin as a plasticizer, Oksman et al created PLA composites reinforced with flax fibers (30 and 40 wt%) in a twin-screw extruder. The addition of plasticizer reduced the thermal characteristics, however, the DMTA analysis revealed that the storage modulus rose and the material's ability to soften from 50 to 60 °C was improved. After mixing kenaf fibers (0 to 40 weight percent) and polyethylene glycol (PEG) (as plasticizer) in a mixer, compression molding was used to create PLA composites [87].
In unidirectional flax fiber-reinforced composite laminate, it was observed that delamination of various sizes and locations had a substantial impact on the composite's dynamic mechanical properties during the dynamic mechanical analyzer test. The rise in loss factor shows the presence of delamination, and the delamination may be consistently detected using test parameters with low frequency and amplitude. It showed that the effect of the delamination's transverse position on its detectability was more significant than its size. Additionally, it is noticed that the frequency has a bigger impact on the energy dissipation of composites than the amplitude does [88].
The thermal properties of natural fiber composites obtained using DMA are determined by the physical or structural arrangement of phases, morphology, and the type of natural composite materials. The addition of fillers, fiber content and orientation, and fiber chemical treatment have been demonstrated in the literature to alter the dynamic mechanical properties of composite material [89]. The effect of the hybridization of bamboo and kenaf fibers on the thermal properties of various configurations in epoxy resin-based hybrid composites was examined by Chee et al using thermo mechanical analysis (TMA) and DMA analysis, the epoxy resin and 100% Kenaf had storage modulus of 449 and 775 MPa respectively, whereas the composite with 100% Bamboo had storage n = modulus of 979 MPa [90].
When hemp fibers are observed through SEM (Scanning Electron Microscope) analysis, the fiber without alkali treatment forms a layer of non-cellulose and the fiber treated by alkali treatment removes the non-cellulose layer and exhibits higher adhesion properties because the bonding of single fibers. The fractography of the composite was shown in figure 10. Figure 10(a) shows the SEM analysis result of fiber without alkali treatment and 10(b) shows the result of fiber with alkali treatment [91].
Figure 10. SEM analysis fractography of hemp fiber [91].
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Standard image High-resolution imageYear to year the demand for bio-composites has been increasing. The anticipated growth rate for the worldwide bio-composite market is 9.59%, with a target valuation of USD 41 billion by [46]. Nanofibers which are extracted from natural fibers possess an imperative role in structural applications. In global production capacity, bio-based plastics like PE and PET, a group of bio-degradable products, show the strongest growth [92].
Wood fibers are widely in the production of bio-composites for various applications in recent days. In the future, bast fibers (hemp, flax, and jute) emerges because of their improved property of tensile strength and stiffness. The utilization of bast fibers has been predicted through research, as seen in figure 11 below.
Figure 11. 10-year forecast of bast fibers [93].
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Standard image High-resolution imageThe construction of green buildings is a challenging application for future use. Green buildings majorly use bio-composites to develop a healthy and environmentally friendly place for people to live and work. The benefits of constructing green buildings depend on the factors like component weight, production, environment, etc [93].
Future research on biocomposites should focus on the following aspects listed below:
1.
To make cost-effective and eco-friendly composites, surface modification techniques must be improved. Also, research must be carried out to predict suitable techniques for suitable composite preparation.
2.
Durability is the major unpredictable factor. New techniques or methods should be developed to investigate and evaluate biopolymer composites' properties of durability and biodegradability.
3.
Biocomposites are used in various sectors like automobiles, aerospace, electronics, etc. More research has to be carried out in the field of medical scaffolds.
4.
The long performance of the manufactured composite has to be observed and increased.
5.
The study of nanotechnology and the use of nanotechnology possess an imperative role in improvising the characteristics of biopolymer composites [ 94 ].
The desire for more environmentally friendly materials in the modern world has led researchers to focus on natural cellulosic fibers, which have successfully replaced synthetic fibers in a variety of diverse uses. In recent times, several challenges have been engrossed with the exploitation of petroleum products such as high cost and degradation of the environment. Since then, scientists have worked to create bio-composites as a replacement for products manufactured by petroleum.
Bio-composites have replaced synthetic composites in various fields like research, commercialization, and development. The usage of synthetic composites was reduced due to decreased petroleum resources and increased environmental hazards. To explore more in the field of research and industries the use of bio-composites has opened a pathway. Bio resin, natural fibers, biopolymers, bio-composites, and the future of bio-composites have been discussed in detail. Several forms of natural fibers and their use in many sectors are also addressed, along with a detailed discussion of how to categorize natural fibers. It is clear from the graphical data that flax and sisal have superior mechanical qualities over jute and leather. Sisal and flax are regarded as acceptable materials for making panels and other parts for automobiles since they have the lowest equivalent stress values of 68.09 GPa and 62.064 GPa, respectively, and the lowest deformation values of 1.026 mm and 0. mm, respectively.
The extensive overview of numerous natural fiber qualities and their characterization using various methodologies described in this study offer insight into natural fiber composites and be especially useful to younger researchers in this subject. Furthermore, the behavior and material reinforcement qualities have a significant impact on the performance of fiber-reinforced polymer composites. It was found that chemical and surface treatments could improve the thermochemical and mechanical characteristics of natural fibers. The employment of bio-composites has the benefits of being eco-friendly, lightweight, renewable, and non-corrosive and also employing high stiffness and high specific strength. The investigation exposed that bio-composites are suitable for various industries including aerospace, biomedical, automotive, and construction.
All data that support the findings of this study are included within the article (and any supplementary files).
The authors declare that there is no conflict of interest.
There is no funding provided for this study.
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