Green composites prepared from soy protein, polylactic acid (PLA), starch, cellulose, chitin: a review

Abstract

Any new product that we develop for the world should undergo a life cycle analysis (LCA) from birth to death. Environmental impact can be enhanced by lowering reliance on the resources derived from naphtha. The challenges accompanying the generation of plastic-derived waste can be substantially reduced by employing biodegradable polymers. Green composites have gained significance over traditional petroleum-based products because they are derived from natural resources, environment friendly, recyclable, non-toxic, and biodegradable. In addition, the fabricated composites should exhibit optimized weight and required mechanical properties to meet the performance requirements. Green composites comprise two phases: a bio-based polymer matrix integrated with natural fiber reinforcements like jute, hemp, sisal, flax, banana fiber (abaca), etc. Owing to the inherent benefits of natural fiber reinforcements such as low density (synthetic fibers like carbon and E-glass fibers exhibit density values ranging from 1.8 to 2.1 g/cm³ and 2.54 g/cm³, respectively as opposed to natural fibers spanning from 1.25 to 1.6 g/cm³) and higher acoustic damping in composites, they can be widely used in interior automotive applications. Potential applications in the automotive sector include interior structures such as door panel inserts, armrests, seatback lining, seat bottoms, and under-floor body panels. Green composites formulated from soy protein, polylactic acid (PLA), starch, cellulose, and chitin and along with their processing techniques and applications have been discussed here.

Graphical abstract

1 Introduction

Monolithic materials often cannot fulfill the requirements of today’s sophisticated technologies, whereas composite materials that comprise two or more components (conglomerated at the macroscopic level) led to the enhancement of several properties compared to their parent materials. Such materials are composed of a continuous matrix phase and a discontinuous reinforcing phase. The former serves to bind the reinforcement (which can either be particles, fibers, or flakes) and protects them from the environment while distributing load to the reinforcement, whereas the latter functions to bear the load while being embedded in the matrix [1].

Figure 1 represents the categorization of composites in accordance with the type of matrix utilized.

Fig. 1

Categorization of composites

Among these different types of composites, PMCs derived from hydrocarbons are extensively used ascribed to several advantages, including greater specific modulus (ratio of modulus of elasticity to density) and specific strength (ratio of strength to density) values [2].

Figure 2 represents the classification of various synthetic polymer matrices along with reinforcements used [3].

Fig. 2

Classification of synthetic polymer matrices along with reinforcements used

Synthetic polymer matrix composites along with reinforcements posed some problems, which are stated as follows:

• i. Fabrication of such composites involved the consumption of large amounts of energy.

• ii. They are derived from exhaustible petroleum-based products.

• iii. Processing techniques involved might pose some health issues such as respiratory problems [4].

• iv. Due to the non-biodegradability of such composites, large amounts of plastic-derived waste were generated [4, 5].

Green composites are categorized as a specific group of composites comprising at least one component (reinforcement or matrix) acquired from environmental assets [6]. Green composites incorporated with natural fiber reinforcing agents have been availed by humankind since the ancient era of human civilization [7]. For example, in prehistoric Egypt, people used mudbricks made up of an admixture of mud, sand, loam, and water with straw or rice husk as the reinforcing elements. Straw as a reinforcement served to enhance the strength of mudbricks [8].

Green composites, being designed to be materials with a minimal ecological footprint [9], have attained significance over the years due to rising environmental pollution, exhaustible petroleum resources, and increasing environmental awareness. The polymer composites fabricated from natural reinforcements (fibers) and matrices were biodegradable, non-toxic, environment-friendly, non-abrasive, and lightweight compared to their synthetic counterparts.

In green composites also the matrix binds the reinforcing agent and transfers load while the reinforcement renders strength by bearing the load. They can be further categorized based on the matrix and reinforcing materials used as follows:

Figure 3 represents the categorization of green composites characterized by the matrix type and reinforcement used, which includes [10] the following:

• i. Completely renewable composites, composed of both naturally derived matrices and reinforcements.

• ii. Partially renewable composites comprising synthetic materials reinforcing a naturally derived matrix phase.

• iii. Partially renewable composites comprising naturally derived reinforcing materials (biopolymers) integrated within a synthetic matrix.

Fig. 3

Categorization of green composites

This review comprehends green composites prepared from entirely renewable sources associated with naturally derived matrices (biopolymers) as well as reinforcements. The aim was to analyze the potential for substituting synthetic fiber reinforcements with natural ones incorporated within the biobased polymer matrices. Green composites prepared from soy protein, polylactic acid (PLA), cellulose, chitin, and starch, along with their processing methodologies, and properties, have been discussed here. It also provides an insight into various applications encompassing EMI shielding materials, pH-sensitive films, hydrogels for effluent treatment, and aerogels for oil–water emulsion separation.

2 Reinforcing element

Natural fiber reinforcing agents used in green composites can be broadly categorized into two types: plant fibers and animal fibers, depending on the sources from which they are obtained.

Figure 4 represents the classification of natural fibers used as green composite reinforcements. Vegetable fibers are composed of polysaccharides, whereas animal fibers constitute proteins. These natural fibers reinforce bio-based polymer matrices after extraction and segregation from all the binding entities existing in the plant-based and animal-based fibers [11]. Constituents of natural fibers include cellulosic microfibers dispersed in hemicellulose and lignin matrix in an amorphous phase. Based on the type of natural fibers, the amount of cellulose and lignin varies from 60 to 80 weight percent and 5 to 20 weight percent, respectively, with moisture level up to 20 weight percent [3].

Fig. 4

Categorization of natural fibers used as green composite reinforcements

Table 1 represents a similitude of the mechanical properties exhibited by synthetic and natural fibers.

Table 1 Similitude of mechanical properties of synthetic and natural fibers [11,12,13,14,15,16,17,18,19]

Table 1 represents a reduction in density values for natural fibers compared to their synthetic counterparts. These characteristics of natural fibers are beneficial for automotive applications where weight reduction and cost reduction are significant [11]. Furthermore, owing to the inherent benefits of natural fiber reinforcements such as low density (synthetic fibers like carbon and E-glass fibers exhibit density values ranging from 1.8 to 2.1 g/cm³ and 2.54 g/cm³, respectively as opposed to natural fibers spanning from 1.25 to 1.6 g/cm³) and higher acoustic damping in composites, they can be widely used in interior automotive applications [3]. Potential applications in the automotive sector include interior structures such as door panel inserts, armrests, seatback lining, seat bottoms, under-floor body panels.

However, most natural fibers are water-absorbing (hydrophilic), which tends to create issues with moisture absorption and swelling, and this can be alleviated through the treatment of natural fibers with certain chemical agents like sodium chloride (NaCl) solution and sodium hydroxide (NaOH). Furthermore, the bonding between the natural fibers and matrix is intensified by chemical treatments like bleaching and alkali acetylation. This mechanism functions by obstructing the moisture-absorbing tendency utilizing a layer of hydroxyl (-OH) groups in the fiber and by modifying the fiber's surface roughness by removing impurities from its surface [20, 21].

3 Matrix material

Figure 5 depicts the categorization of bio-polymers used as matrices in green composites based on the sources [4, 22, 23]:

Fig. 5

Bio-polymers used as matrix materials classified based on sources

Some bio-polymers (either biodegradable or non-biodegradable) acquired from renewable resources can serve as potential candidates for green composite matrix materials. However, the rising concern is associated with the degree of biodegradability and recyclability of these materials at disposal. For a stochastic entirely bio-polymer-based composite, techniques such as incineration are selected to recover energy even when direct recycling is not conceivable. There is no discharge of noxious gases during incineration, and through decomposition, there is no emanation of gases [24].

4 Green composites

4.1 Soy protein-based

4.1.1 Soy protein

Soy proteins extracted from soybeans form a class of plenitudinous and extensively used protein derived from crops. They comprise globulins, typically categorized as 2S, 7S, 11S, and 15S, appertaining to their sedimentation coefficient [25, 26]. In soybeans, 7S and 11S globulins account for almost sixty percent of the protein reserve. Soy proteins contain 20 amino acids that can incorporate several functional groups, including carboxyl, hydroxyl, amide, etc., at the ends of the backbone or the side chains [27]. Unlike most biodegradable polymers like Polyhydroxyalkanoates and Polylactic acid that are hydrophobic in nature, soy-protein-based resins are characterized by polar groups that exhibit reasonable adhesiveness along with the ability to fabricate composites with improved mechanical properties. Significant types of soy protein include soy flour, soy protein concentrate (SPC), and soy protein isolate (SPI), constituting almost 50%, 70%, and 90% protein, respectively [28].

4.1.2 Soy flour-based green composites

Soy flour-based biocomposites incorporating Indian grass fiber (belonging to the Poaceae family) reinforcements subjected to alkali treatment were developed by Liu et al. [29] by utilizing twin-screw extrusion accompanied by injection molding. Biocomposites with Indian grass fiber reinforcements given alkali treatment demonstrated substantial enhancement in impact strength by 30%, tensile strength by 60%, and flexural strength values by 40% compared to the raw Indian grass fiber reinforcements.

Mohanty et al. [30] prepared green composites based on soy flour-derived bioplastic and shredded industrial hemp fibers by utilizing twin-screw extrusion accompanied by injection molding. The biocomposites described above were fabricated using extrusion cooking technique (in conjunction with a plasticizer) ensued by blending with poly(ester amide), a biodegradable polymer. It was substantiated that with 30 weight percent of hemp fibers, the flexural modulus and flexural strength were enhanced 10- and 3 folds, respectively, while the tensile modulus and strength values were accentuated by 1.5 and 9 times in comparison with the bioplastics derived from soy protein. Furthermore, it was observed that the impact strength and heat distortion temperature (HDT) was intensified by 40% and 35 °C, respectively, for 30 weight percent of hemp fibers. Fibrillation of hemp fibers from the polymer matrix was responsible for enhancing various mechanical properties.

Chabba et al. [31] formulated crosslinked soy flour (CSF) based green composites incorporating flax yarn natural fiber reinforcements. It was ascertained that for a weight ratio of 60% flax yarn fiber reinforcements, the reinforced CSF green composites displayed substantial advancement in modulus of elasticity by 2000% and fracture stress by 700% along the longitudinal axis in contrast with the unreinforced CSF resin. Thus, these composites had a potential for application in the interior structural domain.

Liu et al. [32] prepared green composites from soy protein (flour) based bioplastics incorporating pineapple leaf fiber (PALF) by utilizing twin-screw extrusion accompanied by injection molding. These composites were manufactured by blending soy protein (flour) with poly(ester amide) integrated with PALF reinforcements. The inclusion of a compatibilizing agent (PEG-g-GMA: Polyester amide grafted glycidyl methacrylate) to 30 weight percent composite enhanced the impact strength by 20% in contrast with the composites devoid of compatibilizers. In contradistinction to soy protein-based bioplastics, it was inferred that the impact strength was accentuated by 50%. It could be deduced that the compatibilizer served to improve adhesion between the matrix-fiber interface by functioning as an interfacial agent. Moreover, the heat distortion temperature (HDT) also substantially improved around 55 °C with 30 weight percent fiber reinforcement.

4.1.3 Soy protein concentrate–based green composites

Kim et al. [33] developed green composites based on soy protein concentrate (SPC) resin constituting hemp-yarn reinforcements (unidirectional). The influence of pH level on different mechanical properties (tensile strength and interfacial shear strength) of the fabricated green composite and SPC resin was examined. It was observed that at a neutral pH level (i.e., pH value = 7), SPC resin exhibited the least values for moisture absorbency and fracture strain, as 15.7% and 31.5%, respectively, while the maximum values of 13.1 MPa and 357 MPa were noticed for fracture stress and modulus of elasticity respectively. Therefore, it was interpreted that for the SPC resin, with the rise in pH values, fracture stress and modulus of elasticity observed a declining trend while moisture absorbency and fracture strain witnessed a substantial increase. Furthermore, the fabricated composite exhibited a peak interfacial shear strength value of 28 MPa at pH 10, enhancing these values with increasing pH.

Chabba et al. [34] formulated entirely biodegradable green composites by modifying soy protein concentrate (SPC) resins with (i) glutaraldehyde (MSPC-1) and (ii) glutaraldehyde and polyvinyl alcohol (MSPC-2) incorporated with flax fabric and flax yarn to reduce moisture absorbency and enhance mechanical and thermal properties. From the thermogravimetric analysis (TGA) plots of SPC, MSPC-1, and MSPC-2 resins, it was interpreted that owing to the adscititious crosslinks provided to MSPC resins by the crosslinking agent (glutaraldehyde), the MSPC resins (MSPC-1 and MSPC-2) displayed higher thermal stability in contrast with the SPC resins. Moreover, the initial decomposition temperature showed greater values for MSPC resins than the SPC resins. Green composites with MSPC-1 resin reinforced with flax fabrics exhibited strength and modulus of elasticity values around 50–55 MPa and 1 GPa (along longitudinal direction), respectively. In comparison, the green composites comprising MSPC-2 with flax yarn reinforcements displayed strength values around 126 MPa and modulus of elasticity value of about 2.24 GPa (along longitudinal direction).

Another class of environmentally sustainable green composites was developed by modifying soy protein concentrate (SPC) resin with glutaraldehyde (GA) crosslinking agent (MSPC-G) integrated with natural fiber reinforcements (in the form of flax fabrics) by Chabba et al. [35]. Figure 6 illustrates a characteristic thermogravimetric analysis (TGA) plot for MSPC-G and SPC comprising 10% glycerin resin in an inert nitrogen atmosphere. The onset of thermal degradation for MSPC-G and MSPC resins was observed at 270 °C and 235 °C, respectively. However, the former displayed enhanced thermal stability in contrast with the latter due to soy protein resin’s cross-linking with glutaraldehyde. It was analyzed that MSPC-G resin exhibited significant thermal stability up to the temperature at which these composites were processed (120 °C) with a minimum weight loss. Moreover, the fabricated MSPC-G composites displayed substantial improvement in fracture stress and modulus of elasticity values by 20% and 35%, respectively, in contrast with the SPC resin.

Fig. 6

Thermogravimetric analysis plots for MSPC-G resins (with 10% glycerin and 40% GA) and SPC (with 10% glycerin). Reprinted with permission from [35]. Copyright 2005 Springer

4.1.4 Soy protein isolate–based green composites

Wang et al. [36] fabricated biodegradable soy protein isolate (SPI) green composites integrated with cellulose whiskers as the reinforcement using the hot-pressing technique. In contrast with the SPI sheet produced devoid of cellulose whiskers, the green composites based on SPI, including cellulosic whisker reinforcements, exhibited significant enhancement in the modulus of elasticity from 44 to 133 MPa and tensile strength from 5.8 to 8.1 MPa.

Processing methods for SPI films: To synthesize SPI films, the functional state of proteins that are appropriately assembled or folded must undergo denaturation to form new configurations by subjecting them to heat, radiation, external stress, organic solvents, or changes in pH [37].

Conventional techniques to fabricate SPI films include the wet process and dry process.

• i. Wet process:

  The wet process, also referred to as the solution casting method, involves the dispersal or dissolution of protein molecules in the desired solvent. This process is generally carried out at higher pH values to induce the unfolding of the protein molecules present in their native state prior to the pouring of the solution on the plain surface and drying. Subsequent to the drying step, the film is peeled away from the plain surface [38, 39]. The drying process can be accomplished by utilizing microwave energy, infrared energy, or hot air [40]. Figure 7 illustrates the wet process used for the fabrication of SPI films.

Fig. 7

Wet process for the formation of SPI films

• ii. Dry process

Extrusion is a dry process that is dependent on the characteristics of the bio-based polymers during plasticization and heating beyond their glass transition temperatures (Tg) [41]. Plasticized materials are used to manufacture components via compression molding and injection molding [42] techniques by forming protein networks utilizing amide bonds between the α-amino group and α-carboxyl groups of amino acids or through disulfide linkages. The molding temperature substantially influences the proteins’ crosslinking density, consequently impacting the mechanical properties of the fabricated components [37].

4.1.5 Other soy protein–based green composites

Biodegradable green composites consolidated with soy milk resin and jute fiber reinforcements (woven and non-woven) were formulated by Behera et al. [43] using the compression molding technique. Soy milk, a relatively inexpensive bio-based resin, is withdrawn from drenched soy seeds, comprising carbohydrates and proteins [44]. These inherent proteins serve to undergo crosslinking reactions with the cellulose moieties existing on jute fibers to fabricate stiff green composites. Composites fabricated with 60 weight percent of jute fiber reinforcements exhibited a tensile strength and modulus of 37.1 MPa and 1040 MPa, respectively. These composites could be used in the automotive and packing industries.

4.2 Polylactic acid based

4.2.1 Polylactic acid

Polylactide, also referred to as polylactic acid (PLA), is a thermoplastic polyester obtained by the fermentation of agricultural commodities like starch, corn, and beet [45,46,47]. PLA can be found to exist in an entirely amorphous state (comprised of about 50–93% L-lactic acid) or a semicrystalline state (consisting of higher than 93% L-lactic acid) [48]. Polylactide can be synthesized by ring-opening polymerization mechanism (coordination or ionic polymerization) of lactides [40] or by polycondensation (direct) reaction of lactic acid. Figure 8 represents the basic structure of polylactic acid.

Fig. 8

Basic structure of PLA

Green composites incorporating multitudinous natural fiber reinforcements including nanofibrillated cellulose [49, 50], flax [51], hemp [52], bamboo [53,54,55], jute [56, 57], kenaf [58,59,60], ramie [61], chicken feathers [62], chitin [63], keratin [64, 65], and pineapple leaf fibers [66] have been explored by various researchers.

4.2.2 Hemp fiber/PLA green composites

Hemp belonging to the Cannabis sativa family comprises around 2–5% lignin, 8–19% hemicellulose, and 55–72% cellulose. In addition, hemp fiber is bestowed with outstanding Young’s modulus value and mechanical properties [67]. Hence, incorporating hemp fibers with bio-based polymer matrices will substantially improve the impact strength and thermal stability.

Hu et al. [67] formulated biodegradable PLA-based green composites incorporating hemp fiber reinforcements by utilizing hot-pressing technique. The influence of fibers (subjected to alkali treatment) on the surface morphology (assessed through SEM) and mechanical properties of the prepared composites were examined. SEM analysis revealed that the hemp fiber devoid of alkali treatment was coated with a non-cellulosic film. Following alkali treatment, elimination of the non-cellulosic film was carried out, and as a consequence, adhesion between the matrix (PLA) -fiber (hemp) interface was enhanced due to direct bonding of single fibers with the matrix. Furthermore, it was estimated that with a hemp fiber proportion of 40%, the fibers that underwent alkali treatment demonstrated superior tensile modulus of 8.5 GPa, a flexural strength of 112.7 Mpa, and tensile strength of 54.6 Mpa.

PLA-based biodegradable green composites integrated with hemp fiber reinforcements (in the form of plain and twill-woven fabrics) were developed by Song et al. [68] by utilizing the film stacking method. In addition, coefficient of thermal expansion (CTE) measurements was carried out using a strain gauge to assess the fabricated composites’ thermal properties. It was ascertained that with a hemp fiber content of 20%, composites with twill fabric reinforcements displayed exceptionally low values for CTE which was suitable for generating green composites for aerospace and automotive sectors. Figure 9 depicts Young’s modulus and tensile strength values for hemp fiber (plain and twill-woven) / PLA composites.

Fig. 9

(i) Young’s modulus and (ii) tensile strength of hemp fiber (plain and twill-woven) / PLA composites Reprinted with permission from [68]. Copyright 2011 Elsevier

It was interpreted that twill woven hemp fibers exhibited an enhancement in the mechanical properties compared to plain-woven fabric and hence was a more appropriate reinforcement. Furthermore, the dynamic mechanical analysis (DMA) test revealed that superior loss tangent values were witnessed for the fabricated composites comprising a volume fraction of 20% twill-woven fabric reinforcements.

4.2.3 Sisal fiber / PLA green composites-

Sisal belonging to the family of Agave sisalana plant species contains around 10–14% lignin, 6% hemicellulose, and 50–64% cellulose [4]. Sisal fibers possess several benefits, such as lower density (1.3–1.5 g/cm³), good resistance to chemicals (acids and bases), good tensile strength (600–700 MPa), and modulus of elasticity values (38 GPa), thus making them befitting reinforcements for green composites.

Zou et al. [69] developed PLA-based green composites by incorporating short fibers of sisal reinforcements (SF) (i) subjected to chemical treatment and (ii) devoid of any chemical treatment by utilizing melt mixing technique with subsequent compression molding process. Table 2 represents the mechanical properties of PLA and SF/PLA (treated and untreated) green composites in terms of modulus of elasticity, elongation at break, and tensile strength.

Table 2 Mechanical properties of PLA and SF/PLA (treated and untreated) green composites. Reprinted with permission from [69]. Copyright 2012 Wiley

It could be ascertained that the modulus of elasticity and tensile strength values of the untreated SF/PLA was improved by 57.6% and 29.3%, in contrast with virgin PLA. SF/PLA green composites subjected to mercerization displayed enhancement in modulus of elasticity and tensile strength values by 13.5% and 18.7% compared to the composites devoid of any chemical treatment. In collation with U-SF/PLA green composites, the P-SF/PLA, S-SF/PLA, A-SF/PLA, and M-SF/PLA green composites exhibited an enhancement in impact strength values by 9%, 16%, 22%, and 34%, respectively. The water absorptivity and degree of biodegradation of PLA and SF/PLA green composites (treated and untreated) were of the order: U-SF/PLA > M-SF/PLA > P-SF/PLA > S-SF/PLA ∼ A-SF/PLA > Virgin PLA.

Polylactide composites integrated with sisal fiber reinforcements were developed by Li et al. [70] by using the injection molding technique. Mechanical properties of the fabricated composites were analyzed with respect to modification of sisal fiber reinforcements by introducing two distinct coupling agents, namely, Polylactide-graft-γ-methacryloxypropyltrimethoxysilane (MPS-g-PLA) and Polylactide-co-glycidyl methacrylate (PLA-co-PGMA). The PLA-based-green composites comprising sisal fiber reinforcements that underwent chemical treatment with Polylactide-co-glycidyl methacrylate and Polylactide-graft-γ-methacryloxypropyltrimethoxysilane coupling agents displayed a substantial improvement in tensile strength by 6.77% and 5.35% implying an effective transfer of stress between sisal fibers and PLA matrix due to good wettability and improved adhesion.

4.2.4 Ramie fiber / PLA green composites

Chen et al. [71] formulated laminated PLA-based biocomposites with ramie fiber reinforcements by utilizing compression molding method succeeded by melt crystallization (in the presence of heat treatment) or direct cooling (devoid of heat treatment). For the PLA/RF biocomposite with a volume fraction of ramie fibers by 24 weight percent, it was noticed that after heat treatment, the tensile strength, flexural modulus, and flexural strength were 76.07 MPa, 5069.41 MPa, and 116.22 MPa, respectively. In contrast, virgin PLA demonstrated an improvement in tensile strength from 29.37 to 54.52 MPa, flexural modulus from 3479.41 to 3603.19 MPa, and flexural strength from 41.15 to 88.59 MPa, respectively. Furthermore, it was observed that the heat-treated PLA/RF biocomposites comprising 24 weight percent fiber reinforcements displayed heat distortion temperature (HDT) of 149.3 °C with a maximum elevation of 90.7 °C.

4.2.5 Processing of PLA-based green composites

Typical processing techniques for green composites are similar to those used for traditional fiber-reinforced polymer composites, and they can be categorized into open mold process and closed mold process. The former includes spray layup, autoclave method, tape layup, filament winding, and hand layup, while the latter includes transfer molding, injection molding, and compression molding techniques [46].

Film stacking method

Rapid press consolidation of layups in film stacking is associated with the following steps: (i) pre-compression, (ii) contact heating in the presence of vacuum, (iii) quick transfer to a press for consolidation followed by cooling, (iv) withdrawal of finished component from the press. Plackett et al. [72] prepared PLA-based biodegradable green composites, comprising 40 weight percent jute fiber reinforcements, using the film stacking method. The pulverized PLA was transformed to a film that was 0.2 mm thick by adopting a single screw extruder. Layups were fabricated with a portion of jute fiber mat stacked up along with numerous layers of PLA film at either of the sides within a metal frame comprising Teflon sheets.

Carding and hot pressing

Carding method ensures homogeneous blending of the two fibers, with subsequent needle punching, pre-pressing, and hot-pressing steps to fabricate the biocomposite. Lee et al. [73] formulated polylactide-based biocomposites incorporating kenaf fiber reinforcements using carding method accompanied by chemical treatment with 3-glycidoxypropyl trimethoxy silane and hot pressing. The pre-pressed non-woven web of PLA/ kenaf fiber was subjected to chemical treatment in the presence of a silane coupling agent in the proportions of 1,3 and 5 PPH (parts per hundred) of the pre-pressed composite. Silane was permitted to permeate and undergo pre-reaction with the pre-pressed mat for a duration of two hours. Eventually, the pre-pressed mat subjected to silane treatment underwent hot pressing for a period of 5 min at 0.7 MPa pressure and 200 °C temperature. This technique facilitated the melting of polylactide and suitable impregnation to generate a consolidated formed sheet.

4.3 Starch-based

4.3.1 Starch

Starch is the principal repository polysaccharide in plants, and the most copious plant material available in the environment, followed by cellulose [6, 74,75,76]. It is predominantly obtained from crop seeds and staple food crops like potatoes, rice, wheat, maize, legumes, plant stalks, and roots [77,78,79]. Starch comprises α-glucose units held together by glycosidic bonds, and it consists of two components: amylose (water-soluble) and amylopectin (water-insoluble). Amylose that constitutes 20–30% of starch is a polymer that exhibits a high degree of branching and has a molecular weight (weight average), M_w spanning from 107 to 109 g/mol, whereas amylopectin constitutes around 70–80% of starch and is a linear polymer with M_w in the range of 105 to106 g/mol. Figure 10 represents the skeletal structure of starch depicting the amylose and amylopectin components.

Fig. 10

Skeletal structure of starch constituting (i) amylose and (ii) amylopectin

Starch can be used as a matrix material as well as a reinforcement (filler) in green composites [6, 80,81,82,83]. It is also extensively utilized as gum in textile industries and adhesive for wood due to its low cost. When used in the coarse-grained form, it was ascertained that starch enhanced the modulus values and decreased elongation at fracture [84].

Starch-based green composites [75, 76] incorporating a diverse array of reinforcements such as rice straw [85], microcrystalline cellulose/ nanocellulose [86], barley straw particles [87], beet pulp [88], and cellulose nanofibers [89] were explored by various researchers.

4.3.2 Cassava starch–based green composites

L Famá et al. [90] prepared biodegradable green composites by integrating cassava starch with wheat bran fiber reinforcements, including glycerol and potassium sorbate, by utilizing the casting process. Esculent films fabricated from cassava starch and glycerol demonstrated enhancement in mechanical properties; i.e., these films exhibited 70% higher strain during tensile testing. Composites were formulated with distinct proportions of wheat bran fiber, viz., 1.5 mg/g, 13.5 mg/g, and 27.1 mg/g of the matrix. Water vapor transmission tests revealed that water vapor permeability (WVP) and moisture amount declined with an increment in the proportion of wheat bran fiber. Furthermore, taking into account the hydrophilicity of the matrix, several attributes (elevation in hardening and storage modulus was observed) of these green composites were enhanced due to the lowering of WVP with an increasing proportion of fiber. Thus, these composites could be used to preserve food and refine their service life [91].

Thermoplastic cassava starch (TPCS)–based biocomposite integrated with Cogon grass fibers (CGF) was developed by Jumaidin et al. [92], and the influence of CGF on the biodegradability, thermal and mechanical properties of the fabricated composites were analyzed. It was seen that with an increment in CGF proportion, the fabricated composites showed reduced elongation from 20 to 2.9% and a substantial enhancement in tensile strength values from 1.9 to 5 MPa. Furthermore, the incorporation of CGF led to an elevation in tensile modulus from 128.8 to 317.2 MPa. The refinement in tensile modulus and strength values was associated with an effective stress transfer between CGF and TPCS matrix owing to good adhesion (confirmed by SEM image depicting fiber fracture) and resemblance of chemical structures of the matrix and fiber. Furthermore, the assessment of biodegradability through soil burial tests demonstrated that the integration of CGF resulted in a reduced biodegradation rate.

4.3.3 Potato starch–based green composites

Chen et al. [93] formulated biodegradable composites using potato starch integrated with bagasse fibers subjected to alkali treatment by using extrusion process. The influence of average fiber length and alkali-treated bagasse fiber proportions on water absorption, water solubility index, and tensile strength was examined. It was interpreted that the fabricated composite displayed the highest tensile strength of 24.5 MPa for 15 weight percent of alkali-treated fibers, and with the increase in the proportion of fibers from 0 to 15%, the tensile strength was enhanced. However, at 21.5 weight percent fiber content, the tensile strength had declined, and this reduction in tensile strength was similar to that for a non-bagasse fiber composite implying that the average fiber length of the composite described above was shorter than twofold the critical fiber length. Furthermore, it was noted that the water absorption and solubility declined with an increasing alkali-treated bagasse fiber proportion.

4.3.4 Wheat starch–based green composites

Zhang et al. [94] fabricated polylactic acid (PLA)/ wheat starch composites by blending polylactic acid with wheat starch subjected to compatibilization with maleic anhydride (MA) in a laboratory-scale co-extruder. To enhance the compatibility between PLA, wheat starch, and MA, (2,5-bis(tert-butylperoxy)-2,5 dimethylhexane) (L101) initiator was employed. Initiator led to substantial adhesion between the matrix (PLA) – fiber (wheat starch) interface. Figure 11 represents SEM images of PLA/wheat starch (55/45) subjected to and devoid of MA compatibilizer.

Fig. 11

SEM images depicting microstructures of PLA/wheat starch composites (i) Neat PLA/wheat starch (55/45), (ii) PLA/wheat starch/MA (55/45)99/1, (iii) PLA/wheat starch/MA (55/45)99/1 subjected to 10 wt % L101 initiator (MA basis). Reprinted with permission from [94]. Copyright 2004 American Chemical Society

Figure 11 (a) depicts the SEM image representing the fracture surface of PLA/wheat starch composite devoid of initiator (L101) and compatibilizer (MA), indicating distinctive features of an immiscible composite where a tensile strength of 30 MPa verified inadequate adhesion between the matrix-fiber interface. Figure 11 (b) represents the SEM micrograph for PLA/wheat starch composite comprising compatibilizer (MA) but devoid of initiator (L101). Detachment of few wheat starch granules from the PLA matrix was observed along with a tensile strength of 28.6 MPa. Figure 11 (c) illustrates the homogeneous dispersion of granular wheat starch in the PLA matrix. Tensile testing signified fracture of both the PLA matrix and uniformly dispersed wheat starch surfaces displaying improved adhesion between the matrix (PLA) – fiber (wheat starch) interface, eventually leading to an elevated tensile strength value of 52.4 MPa. PLA/wheat starch (55/45) composites treated with 10% L101 initiator, and 1% MA compatibilizer exhibited the greatest elongation (4.1%) and tensile strength (52.4 Ppa).

4.3.5 Processing techniques for starch-based green composites

Some conventional techniques used to prepare starch-based green composites are as follows:

Melt processing technique

The melt processing technique is associated with melting and mixing of polymers in pellet form at higher temperatures ensued by instantaneous cooling [95]. Gironès et al. [96] formulated thermoplastic starch (TPS) composites incorporated with hemp and sisal fiber reinforcements using melt processing technique. Hemp and sisal reinforcements were employed in differing proportions in the range of 5–20% weight per weight to fabricate the composite aforestated. Manual pre-mixing of the fundamental constituents of this matrix, i.e., cornstarch and glycerol, was carried out in polyethylene bags; i.e., the matrix material was admixed with 30% weight per weight of glycerol. The resultant blend obtained was subjected to subsequent processing, along with the equivalent quantity of natural fiber reinforcements for a duration of 6 min in a rheometer at 60 rpm and 120 °C. This was included in an amount of 2.5% weight per weight to the formulations comprising latex [97]. Post-processing of these composites was succeeded by granulation and thermo-pressing operations with an intent to achieve film plates.

Thermal compression

Silva-Guzmán et al. [87] developed green composites constituting thermoplastic corn starch with barley straw reinforcements by utilizing thermal compression methodology. Barley straw is used as a feedstock to reinforce matrices in green composites due to lignocellulose presence [98]. First, barley straw was subjected to desiccation followed by pulverization and screening operations to attain the desired size distribution (length of about 425 μcm). Next, blending of cornstarch with 30 weight percent glycerol (in plastic bags) was performed until a consistent admixture was obtained. Barley straw pieces were then included in varying ratios to fabricate green composites of the order of 5,10, and 15% with 0% polymer matrix used as the benchmark for comparison. These infusions were then devolved into a stainless steel mold for further processing of the composite. Processing parameters utilized included pressure of 100–150 bar, pressurization time of 50 min, and temperature of 160 °C.

Baking

The baking process involves the following two steps:

• i. Gelatinization and evaporation of water followed by expansion of the mixture and generation of foams.

• ii. Drying of foams up to ultimate moisture content of 2–4% [99].

L.R.P.F. Mello et al. [100] fabricated green composites based on cassava starch and agro-industrial reinforcements such as malt bagasse fiber. Biodegradable foam trays were made using the baking process with malt bagasse fiber content ranging from 0 to 20% weight per weight. Proper distribution of malt bagasse fibers was observed across the matrix, displaying an amorphous configuration. A rise in malt bagasse fiber content from 5 to 10% exhibited reduced water absorbing capacity. These biodegradable foam trays could serve as a substitute for packaging dry foods [84].

Soykeabkaew et al. [101] formulated starch-based composite foams (SCFs) by using baking technique. This technique was implemented by baking batters based on starch by integrating flax and jute fibers. SCFs were developed in a compression press using starch-based batters. Starch-based batters containing water were baked in heated secured molds. During this process, the starch grains coagulated to form a gel-like substance, and water evaporation resulted in the foaming of starch to conform to the configuration of the mold [37].

Casting/solvent evaporation

Solvent casting involves the dissolution of polymers into appropriate solvents accompanied by casting it on desirable substrates, resulting in thin films post evaporation of solvent [95]. Cynthia et al. [102] generated chitosan-starch-based green composites integrated with keratin (extracted from chicken feathers) reinforcements by casting/solvent evaporation technique. Composite films based on chitosan-starch were formulated by mixing chitosan solution (obtained by dissolution of 0.2 gm chitosan in 100 mL acetic acid solution, 1% v/v followed by agitation) with starch solution (obtained by blending 3.8 gm starch in 100 mL water, followed by the addition of sorbitol plasticizer, 1% v/v, heating beyond gel point at 90 °C for a duration of 10 min, agitation and cooling to 25 °C) in 5:95 chitosan:starch proportion. Three distinct natural fiber reinforcements (Keratin), viz., short bio-fibers, long bio-fibers, and rachis fragments ranging from 5 to 20 weight percent were incorporated in the polymer matrix (chitosan-starch), followed by the agitation at ambient temperature for 20 min. Finally, the resulting mixture was decanted into Polytetrafluoroethylene (PTFE) dishes to complete gelatinization accompanied by cooling to ambient temperature. The incorporation of keratin fibers led to the improved thermal stability of the fabricated green composite in contrast with the pure starch matrix. Furthermore, SEM studies demonstrated a homogeneous distribution of keratin fibers in the chitosan-starch matrix, implying proper compatibility between different phases [84].

Table 3 represents various matrix materials, reinforcing agents, and processing techniques used to fabricate starch-based green composites.

Table 3 Matrix materials, reinforcing agents, and processing techniques to fabricate starch-based green composites

4.4 Cellulose-based

4.4.1 Cellulose

Cellulose is a non-branched macromolecule comprising of D-anhydroglucopyranose units linked by β-1,4-glycosidic bonds. The degree of crystallinity of cellulose in natural fibers (like cotton, flax, and ramie) varies from around 65 to 70% [103]. The length of these β-(1,4) glucan chains is variable and is influenced by the source of cellulose. Lignocellulosic compounds displayed degree of polymerization (DP) values up to 10,000 [104]. The mechanical properties of natural fiber reinforcements used are dependent on the type of cellulose. Moreover, each variety possesses its characteristic cell geometry, which further ascertains its mechanical properties. Cellulose in the solid form gives rise to a microcrystalline structure comprising an amorphous state of lower order and crystalline state of higher order. Cellulose has good resistance towards oxidizing agents and strong alkalis (17.5 weight percent) but is readily susceptible to hydrolysis by the attack of acids and water-soluble sugars [105]. It can undergo dissolution in strong acid solutions like N-ethylpyridinium (ionic liquid) [106], lithium chloride/N,N-dimethylacetamide [107], and concentrated sulphuric acid. Figure 12 represents the molecular structure of cellulose.

Fig. 12

Molecular structure of cellulose

Cellulose [50, 108, 109, 109,110,111,112,113,114,115,116], one of the most copious biodegradable polymers, could be used as an eco-friendly matrix material or as an effective reinforcing agent to augment mechanical properties such as strength and stiffness while fabricating green composites [86, 89, 117,118,119,120].

4.4.2 Liquid crystalline cellulose fiber–based green composites

Netravali et al. [121] formulated completely biodegradable advanced green composites comprising soy protein-based resins reinforced with liquid crystalline (LC) cellulose fibers (high strength). The modified soy protein-based resins were also reinforced with synthetic fibers (E-glass and Kevlar) to collate the properties. Soy protein resins were modified by blending them with polysaccharides to generate an interpenetrating polymer network (IPN) like structure subjected to subsequent modifications with microfibrillated cellulose (MFC) and nanoclay to achieve substantial enhancement of mechanical properties. The SPC resins that underwent modification were used to develop advanced green composites incorporating LC cellulose fibers. It was ascertained that the LC cellulose fibers displayed fracture strain values that were 3.5-folds greater than that of E-glass and Kevlar fibers, eventually leading to greater toughness compared to the synthetic fiber reinforcements. The green composites integrated with LC cellulose fiber reinforcements exhibited 40% of the modulus of elasticity and 60% of the fracture stress values displayed by the composites reinforced with Kevlar fibers. This could be attributed to enhanced adhesion between SPC resin and LC cellulose fibers. Moreover, the LC cellulose fiber reinforced green composites demonstrated greater energy absorption than the composites integrated with Kevlar fiber reinforcements. Furthermore, the liquid crystalline cellulose fiber-based advanced green composites were compostable and found diverse applications spanning from electronics to structural housing components.

4.4.3 Microcrystalline cellulose–based green composites

Xiao et al. [122] formulated high-performance green composites using polylactic acid (PLA) reinforced with microcrystalline cellulose (MC) subjected to grafting with L-lactic acid oligomers (g-MC). It was ascertained that g-MC/PLA composites displayed substantial enhancement in tensile strength values in contrast with MC/PLA composites and virgin PLA with the increment in g-MC fiber proportion up to 30 weight percent. Maximum tensile strength of 70 MPa was observed at a g-MC proportion of 30 weight percent, which was almost 50% higher than the values exhibited by virgin PLA owing to good compatibility between the fiber (g-MC)—matrix (PLA) interface. The aforesaid green composites could be employed in packaging and biomedical domains.

Luo et al. [123] developed transparent green composite sheets by incorporating soy oil-based polyurethane (BioPU) matrix reinforced with microcrystalline cellulose (MCC) by utilizing compression molding technique. It was interpreted that MCC reinforcements on the BioPU matrix led to significant enhancements in impact strength, flexural modulus, and flexural strength values as per Table 4.

Table 4 Effect of MCC reinforcement on impact strength, flexural modulus, and flexural strength values of BioPU [123]

Moreover, at one weight percent MCC reinforcement, the storage modulus of the fabricated green composites was elevated by almost 1.4 times.

4.4.4 Recycled cellulose fiber–based green composites

Huda et al. [124] formulated polylactic acid (PLA)–based green composites integrated with recycled cellulose fiber reinforcements by utilizing extrusion technique ensued by an injection molding process to analyze the characteristics of recycled cellulose fiber as a possible reinforcement for green composites. Compared to the virgin PLA polymer, the fabricated green composites exhibited substantial enhancement in flexural and tensile modulus values owing to the greater modulus of reinforcing agents incorporated within the polymer matrix. Moreover, the dynamic mechanical analysis (DMA) test revealed an elevation in the storage modulus values of the composites with increasing cellulose fiber content, thus signifying an effective stress transfer from the PLA matrix to the reinforcement.

Wattanakornsiri et al. [125] formulated green composites constituting tapioca starch matrix subjected to plasticization with glycerol (30% weight by weight of glycerol to starch) incorporating cellulose fiber reinforcements (obtained from utilized newspaper and office paper). It was interpreted that the mechanical properties were considerably improved, i.e., the modulus of elasticity values was elevated up to 405% and 128% for TS-NF8 (Thermoplastic starch reinforced with 8 weight percent recycled newspaper cellulose fibers) and TS-OF8 composites (Thermoplastic starch reinforced with 8 weight percent recycled office paper cellulose fibers) respectively. In addition, the ultimate tensile strength (UTS) values were enhanced up to 213% for TS-NF8 and 76% for TS-OF8 composites. This improvement in mechanical properties was attributed to good interfacial adhesion between the matrix and reinforcement. Moreover, it was observed that TS-OF8 composites displayed the greatest thermal resistance, while TS-NF8 exhibited substantial enhancement in mechanical properties. Moreover, the aforesaid composites demonstrated complete biodegradation within a period of almost eight weeks.

4.4.5 All cellulose green composites

All cellulose composites (ACC) are constituted by cellulose, serving as both the matrix and reinforcing phase. Cellulose fibrils in the form of crystalline substances (fillers) reinforce the pertinently soft regenerated cellulose matrix. As a result, these composites function as entirely sustainable materials with plausible biocompatibility and improved mechanical properties due to good interfacial compatibility between the matrix and fibrils [126].

Processing of all cellulose composites

• i. Cellulose dissolution

  The most essential step in the fabrication of all cellulose composites (ACCs) is cellulose dissolution since cellulose is not prone to melting below its thermal decomposition temperature. Furthermore, cellulose is indissoluble in water and most organic solvents due to intramolecular and intermolecular hydrogen bonding, thus, hindering the permeation of solvent molecules among cellulose fragments. Hence, cellulose should be dissolved in suitable solvents for further processing of these materials [126].

  Cellulose dissolution in aqueous solvents like NaOH/CS₂/H₂O resulted in unstable byproducts with CS₂. NMMO (N-Methylmorpholine N-oxide)/H₂O solvent was availed to develop lyocell fibers (Tencel) [127]. Incorporation of urea with the NaOH system (aqueous) substantially intensified the dissolution of cellulose [128], and in congruence with this, LiOH/urea evolved as a cellulosic solvent [129].

  Some of the favorable organic solvents for cellulose include LiCl/dimethyl sulfoxide [130], LiCl/1-methyl-2-pyrrolidinone, LiCl/1,3-dimethyl-2-imidazolidinone [131], LiCl/N,N-(DMAc) dimethylacetamide.

  Ionic liquids (ILs) are extensively used as cellulose solvents owing to their relatively low volatility, superior stability towards chemicals, and the ability for re-utilization. ILs possessing 1-butyl-3-methylimidazolium cations could efficiently dissolve cellulose in greater amounts subsequent to heating or irradiation with microwave [132].

• ii. Fabrication of ACCs

ACCs preparation methods can be categorized into two types depending on the fabrication technique employed as one-phase preparation method and two-phase preparation method.

• a. One-phase preparation method

  Figure 13 represents the one-phase preparation method of ACCs. This technique involves the preferential dissolution of cellulose in a desirable (good) solvent subjected to subsequent regeneration by exposing cellulose to poor solvents (like water, ethanol, or methanol) [86, 87]. ACCs fabricated by this technique are interface-free.

  Fig. 13

  

  One-phase preparation method of ACCs. Reprinted with permission from [133]. Copyright 2008 Elsevier

• b. Two-phase preparation method

This technique necessitates combining both cellulose phases, including the filler (solid) and matrix (solution) phases. Nishino et al. [134] formulated all-cellulose green composites by impregnating ramie fibers with craft pulp (pre-treated) solubilized in N, N′-dimethylacetamide (DMAc) comprising 8 weight % solvent and 3 weight % cellulose ensued by regeneration and washing with methanol to eliminate the solvent. As a result, the fabricated composite possessed a configuration in which ramie fibers encompassed craft pulp matrix (regenerated).

Zhao et al. [135] developed all-cellulose nanocomposite films by incorporating cellulose nanofibrils (CNFs) with lithium chloride/N, N-dimethylacetamide (LiCl/DMAc) regenerated cellulose matrix. Figure 14 depicts the two-phase fabrication of all-cellulose nanocomposite films.

Fig. 14

Two-phase preparation method. Reprinted with permission from [135]. Copyright 2014 Elsevier

4.5 Chitin-based

4.5.1 Chitin

Chitin [136] is a copious natural polysaccharide, serving as the primary constituent of the exoskeleton in anthropods, outer shells of crustaceans, and cell walls in fungi [137]. Chitin comprises N-acetyl-D-glucosamine monomer units held together by β-(1–4)-linkages. Procuring chitin from shrimp shells or crab shells involves extracting proteins further treated with calcium carbonate to dissolve the shells. Chitin is characterized by hydrophobicity, and it also displays properties of chelation and proficiency to form films and polyoxysalts [138]. For industrial utilization, chitin extraction is achieved by being treated with acids to accomplish demineralization, decolorization, and deproteinization [137]. Figure 15 depicts the molecular structure of chitin.

Fig. 15

Molecular structure of chitin

Chitin, along with chitosan (deacetylated chitin), displays outstanding properties like non-toxicity, biocompatibility, and biodegradability, thus finding diverse applications in water treatment, biomedical sectors, etc. [139].

4.5.2 Chitin/chitosan green composites

Ifuku et al. [140] formulated green chitosan–based nanocomposite films (transparent) incorporated with surface-deacetylated chitin nanofiber (NF) reinforcing agents to improve the mechanical properties. The fabricated composites (NF-chitin/chitosan subjected to surface-deacetylation) displayed a superior transparency value of around 84% at 600 nm compared to NF-chitin films subjected to surface deacetylation but devoid of chitosan, and this could be attributed to chitosan being implanted in air pockets of the NF, thus, diminishing the intensity of light scattering in the film. Pure chitosan films displayed modulus of elasticity and tensile strength of values of 2640 MPa and 40 MPa, respectively. The integration of surface-deacetylated chitin nanofibers resulted in substantial enhancements in these values by 65% and 94%, respectively, implying uniform dispersion of reinforcing fibers. Moreover, the coefficient of thermal expansion (CTE) values had declined by 26% with the incorporation of 10% NF.

4.5.3 Chitin microparticles reinforced green composites

Hassan et al. [63] fabricated polylactic acid (PLA)–based green composites integrated with chitin microparticle reinforcements by utilizing the melt blending technique. TGA (thermogravimetric analysis) plots illustrated in Fig. 16 revealed that the thermal stability of the PLA matrix declined with the integration of chitin reinforcements.

Fig. 16

TGA plots of neat PLA and PLA with varying proportions (weight percent) of chitin reinforcement. Reprinted with permission from [63]. Copyright 2020 Springer

All the chitin reinforced PLA composites exhibited weight loss in 4 stages, while the pure PLA resin displayed a 3-stages weight loss. Furthermore, it was observed that the fabricated composites with 3 weight percent chitin reinforcements exhibited a minimal strain of 4.9% and the greatest stress of 49 MPa, while pure PLA resin demonstrated the greatest strain value of 8.8% and minimal stress of 41.7 MPa.

4.5.4 Chitin nanowhiskers reinforced green composites

Chitosan (CS)-chitin nanowhisker (CNW)–based green composites subjected to plasticization (in the presence of glycerol, di-, tri-, and tetra-glycerol, polyethylene glycol 600 (PEG) plasticizers) were prepared by Kelnar et al. [141] with an intent to develop flexible materials. Differential scanning calorimetry (DSC) analysis revealed that incorporating 30% tri-glycerol plasticizer with CNW caused the resultant film's glass transition temperature (T_g) to change from − 60.3 °C in the first scan to − 37.5 °C in the second scan, thus validating the plasticization effect displayed by the solvent. Furthermore, the incorporation of 30% tri-glycerol plasticizer with chitosan caused changes in T_g values from − 51.6 °C in the first scan to − 39.7 °C in the second scan. The fabricated composite films could be used to produce eco-friendly food packaging films.

Lu et al. [142] formulated environmentally benign thermoplastic soy protein isolate (SPI) based nanocomposites integrated with chitin whisker (procured from crab shells) reinforcements by utilizing the hot-pressing method. Pure SPI sheets displayed water absorption values of around 40%. At the same time, SPI/chitin nanocomposites demonstrated a decline in water absorption with an increment in the proportion of chitin whisker reinforcements (i.e., at 30 weight percent chitin reinforcements, SPI-30 composites showed a water absorption of 23%), signifying an improvement in water resistance. Furthermore, it was observed that with an increment in the ratio of chitin reinforcements (from 0 to 20 weight %), the modulus of elasticity and tensile strength values were elevated from 26.4 to 158 MPa and 3.3 to 8.4 MPa, respectively, while the elongation at fracture for these composites was reduced from 205 to 29%. This was attributed to the enhanced matrix-reinforcement (whiskers) interaction impeding the movement of the matrix.

4.5.5 Processing methods for nano chitin and chitin based composite films

Some conventional processing methodologies for nano chitin and chitin based films include the following:

Extrusion

Extrusion is a commonly employed processing method for the fabrication of nanocomposite films. The difficulties associated with this method involve acquiring optimum dispersion of reinforcing agents (nanocrystals) in the matrix because nanomaterials are prone to conglomeration during the drying stage. In addition, the pre-drying step did not seem to be propitious as chitin had a tendency to undergo hydrogen bonding once dried. Problems involved in the drying process can be alleviated by introducing the feeding of nanomaterials in a liquid state accompanied by appropriate processing aids (like water or plasticizing agents). When solvents are involved during processing, a co-rotating twin-screw extruder should be utilized to ensure effective steam expulsion [143].

Wet process

The wet process, also referred to as the solution casting method, deals with the dissolution of chitin in an appropriate (desirable) solvent prior to decantation of the solution upon a plain surface. Once drying is accomplished, the film is peeled away from the surface [143]. Even though this technique was relatively slow, it imparted eminent mechanical properties to the composites being fabricated owing to Brownian motion in the solution, thus leading to a realignment of nanoparticles [144].

5 Properties of green composites

Table 5 represents the mechanical properties of some green composites formulated from soy protein, PLA, starch, cellulose, and chitin.

Table 5 Mechanical properties of green composites formulated using soy protein, PLA, starch, cellulose, and chitin

6 Applications of green composites

Over the decades, the fabrication of green composites that minimize the reliance on resources derived from naphtha catering to ecology preservation has tremendously increased [145]. Biopolymers derived from various plant, animal, or bacterial sources find a diverse array of applications spanning from pH-sensitive films, hydrogels, aerogels, electromagnetic interference (EMI) shielding materials to biomedical, agricultural, and packaging sectors. Modified biopolymer nanocomposites being bestowed with exceptional magnetic and electrical properties are often used to produce medical equipment, solar cells, light-emitting diodes, sensors, etc. Present techniques emphasize developing environment-friendly, biocompatible, and innocuous engineered materials [145]. This section highlights various applications of green composites as EMI shielding materials, hydrogels, aerogels, and pH-sensitive films.

6.1 EMI shielding materials

EMI shielding effectiveness (EMI SE) of a component can be defined as the ratio of the transmitted power to incident power, given by the following equation:

$${SE}_{total}= -10log\frac{{P}_{t}}{{P}_{i}}$$

(1)

where, ${SE}_{total}$ = EMI shielding effectiveness (dB).

${P}_{t}$= Transmitted power.

${P}_{i}$= Incident power.

The resultant EMI SE of any material can be computed through the following equation:

$${SE}_{total}= {SE}_{R}+ {SE}_{A}+S{E}_{MR}$$

(2)

where the terms ${SE}_{R}$, $S{E}_{MR},$ and ${SE}_{A}$ represent contributions by reflection, multiple reflection, and absorption, respectively, towards resultant EMI SE [146].

Ultra-lightweight cellulose composite aerogels demonstrating superior conductivity were reported by Huang et al. [147]. Scaffold structure was developed from nanofibrilliar to nanosheet networks by regulating cellulose concentration in sodium hydroxide/ urea solution. The aforestated composite aerogel was stated as an EMI shielding material that demonstrated an EM SE of ~ 20.8 dB and specific EM SE of ~ 219 dB cm³/g, with microwave absorption being the prominent mechanism of electromagnetic interference shielding. This green composite aerogel served as a prospective candidate for lightweight shielding against EM radiation, essentially for automotive, space vehicles, and conveyable electronic devices.

Some researchers have reported natural wood-derived green composites possessing superior mechanical strength, thermal conductivity, and EMI SE for potential applications as thermal management materials and state-of-the-art electronic appliances [148, 149].

6.2 Hydrogels for removal of organic effluents

Hydrogels are water-absorbent materials crosslinked to exceedingly higher molecular weight polymeric networks that absorb and sustain a considerable amount of water. Some of the inherent advantages of hydrogels include the efficiency to tune and sustain physical properties reminiscent of biological tissue, the capability to capsulize hydrophilic drug moieties and preserve them from deterioration [150].

Hydrogels have been extensively used for various applications, including drug delivery, agriculture, and biological adsorbents for water purification. Hydrogels based on cellulose are propitious compared to traditional hydrogels due to the presence of several hydrophilic moieties capable of expediting the adsorption process of effluents utilizing hydrogen bonding and electrostatic interactions. Furthermore, cellulose-based hydrogels adopting the three-dimensional network structure possess several pores accountable for the rapid adsorption of dyes [151]. For instance, Wang et al. [152] reported cellulose-based green composite hydrogels incorporating clay (montmorillonite – MTM) that manifested remarkable adsorption of methylene blue with a maximum adsorption capacity of 277 mg/g. Physisorption was the crucial step involved in the adsorption of methylene blue, while adsorption mostly prevailed within the interior of the fabricated green composite hydrogel. By virtue of biodegradation and superior adsorption potential, the aforestated hydrogels were probable candidates for colored organic wastewater treatment.

In another study, Capanema et al. [153] reported environment-friendly carboxymethylcellulose (CMC) hydrogel adsorbents crosslinked with citric acid for adsorbing methylene blue dye molecules. These hydrogels demonstrated an adsorption efficacy greater than 90%, appertaining to the generation of polyelctrolytic dye complexes owing to the forces of attraction of cationic functional groups of MB molecules and anionic carboxymethylcellulose polymeric chains. The optimum removal capacity of the dye (MB) ranged from 5 to 25 mg/g depending on the dye concentration and was utilized as the model cationic organic pollutant. On the other hand, these CMC hydrogels repelled methylene orange dye utilized as the model anionic contaminant. Furthermore, these hydrogels were envisaged as favorable, cost-effective biological adsorbents for effluent treatment in environmental applications.

6.3 Aerogels for oil–water emulsion separation

Aerogels are characterized by poriferous solid networks possessing larger specific surface area, greater porosity (≥ 90%), and are nanostructured materials [154]. Cellulose aerogels have been the forerunners amongst various biopolymeric materials owing to several inherent advantages, including greater specific surface area, higher insulation, lower density, heat conductivity, bio-compatibility, renewability, and cost-effectiveness. Subsequently, cellulose-based aerogels can serve as probable candidates for packaging, aerospace industry, insulators, wastewater treatment, etc. [155].

Yu et al. [156] reported superhydrophobic cellulose/ poly(vinyl alcohol) (PVA) aerogels with regulatable porosity for enhanced oil–water separation. The fabricated aerogels manifested greater porosity values above 96%. The synergistic effects of chemical, physical crosslinking, and hydrogen bond networks between the polymeric chains bestowed the aerogels with remarkable recovery and compressibility with a maximum compressive stress of 490.7 kPa at 90% strain. Permeation flux values up to 7176.3 L/m²/h and resultant separation of 4550.6 L/m² with a separation efficacy of 98.5% for water in cyclohexane mixtures were obtained.

6.4 pH-sensitive films

Smart packaging technology is associated with food packaging possessing internal or external indicators to render particulars about food quality and freshness characteristics. It also monitors food shelf life, pH, gas permeability, storage temperature with the aid of indicators, sensors, barcodes, cross-contamination labels, etc. Several colorimetric films conforming to halochromic materials and coloring agents directly responding to pH changes have been extensively used to regulate food freshness. These materials function by changing color in response to changes in food pH such that customers can easily distinguish between fresh and stale food without opening the food packaging [157]. For instance, Wen et al. [158] reported pH-sensitive biocomposite films of PVA incorporating green tea extract (GTE) as a propitious antioxidant and evident pH-sensing food packaging materials. The fabricated biocomposite films exhibited superior antioxidant activities compared to neat PVA films. With pH values lower than 6, the biocomposite films were yellow-colored, and they transformed to dark red color with increasing pH values greater than 6, indicating a color change in response to changes in pH. The aforestated films could be used as an antibacterial and biodegradable packaging material for food packaging.

Some researchers also reported pH-sensitive intelligent biocomposite films based on cellulose, chitosan, starch, PVA incorporating anthocyanin extracts for regulating the freshness of meat [157, 159].

Table 6 depicts some applications of green composites formulated using soy protein, polylactic acid (PLA), starch, cellulose, and chitin.

Table 6 Applications of green composites

7 Environmental impact considerations

Environmental impact can be enhanced by lowering the reliance on the resources derived from naphtha. Any new product that we develop for the world should undergo a life cycle analysis (LCA) from birth to death. LCA is carried out based on certain norms and standards provided by ISO (International Organization for Standardization), including ISO 14040–14043. LCA process commences with the specification of scope, followed by inspection of inventory, impact analysis, and deciphering of the outcome [9].

Alves et al. performed an LCA study of glass fiber composites utilized to develop the front bonnet of an automotive for potential substitution of synthetic glass fibers with natural fiber reinforcements (jute) to reduce the environmental consequences associated with landfill and incineration. The incorporation of jute fibers in the front bonnet of the vehicle led to considerable improvement in the overall environmental performance of the automotive. Furthermore, even though the fuel economy declined due to weight reduction of the vehicle caused by jute fibers, few impact analyses associated with disposal, and reprocessing were ascertained [166].

8 Conclusion

This review outlines green composites fabricated from renewable and sustainable resources such as soy protein, PLA, starch, cellulose, and chitin. Over the years, green composites have accentuated the inquisitiveness of researchers in this field towards the progress of lightweight, biodegradable, ecologically safe composites with optimized mechanical properties to meet the performance requirements. Furthermore, these composites manifested the prospectives for substituting their synthetic counterparts to reduce the challenges dealt with plastic-derived waste. Green composites have been extensively used over a broad spectrum of applications spanning from hydrogels for effluent treatment, aerogels for oil–water separation, EMI shielding materials, pH-sensitive films to agricultural, packaging, and biomedical sectors. Hydrogels based on these biopolymers have been used on account of various hydrophilic moieties capable of enhancing the adsorption of effluents through electrostatic interactions and hydrogen bonding [151]. When used as aerogels, these biopolymers found various applications in aerospace, packaging sectors, effluent treatment, etc. [155]. As pH-sensitive materials, they are closely associated with the food packaging sector to monitor and regulate food quality and freshness with the help of indicators, sensors, etc., through direct pH changes [157].

However, green composites are not being exploited on a massive scale since they are expensive. This can be alleviated by commercializing the production of such green composites with optimum processing parameters as the cost will get compensated with the usage. Therefore, we can expect green composites (with enhanced characteristics) in various high-end applications in the near future once the problems associated with inadequate adhesion between the polymer matrix- fiber interface are overcome. Nanotechnology has paved the path for developing economic, biodegradable green composites with enhanced thermal stability and mechanical properties.