Cutting-edge green nanoclay nanocomposites—fundamentals and technological opportunities for packaging, dye removal, and biomedical sectors

Abstract

Nanoclays (layered silicates) have been applied as effective reinforcements for range of natural and synthetic polymeric matrices. Recent research has turned toward design and exploration of green nanocomposites using green polymers and nanoclay nanofillers. This state-of-the-art comprehensive overview debates design and performance prospects of green nanoclay nanocomposites. In this regard, numerous green polymers like poly(lactic acid), poly(vinyl alcohol), natural rubber, cellulose, starch, etc. have been considered. The effectiveness of green nanoclay nanocomposites has been analyzed through microscopic, electrical, mechanical, thermal, adsorption, and biomedical properties and wide span of applications such as packaging, dye removal, and biomedical sectors. Packaging based on cellulose/montmorillonite had very low water vapor transmission rate of 43 g/m².day, whereas poly(lactic acid)/cellulose/montmorillonite packaging performed better with high water vapor transmission rate of 512–1861 g/m².day. Poly(vinyl alcohol)/Cloisite Na possess optimum water vapor transmission rate of 533 g/m².day. Nanocellulose/nanoclay packagings have also been found ideal due to low water vapor permeability (6.3–13.3 g.μm/m².day.kPa) and oxygen permeability (0.07 cm³μm/m².day.kPa) values. In dye removal applications, poly(ethylene glycol)/montmorillonite revealed optimum dye adsorption capacities of 190–237 mg/g, where chitosan/montmorillonite had high dye adsorption capacity of 446.43 mg/g. Poly(lactic acid)/modified Cloisite 20 A systems also own high dye adsorption efficiency of 97%. Poly(ɛ-caprolactone) and poly(vinyl alcohol) systems with montmorillonite nanoclay have effective drug delivery, tissue engineering, and wound healing applications. Furthermore, dielectric, mechanical, non-flammability, and self-extinguishing features of cellulose/montmorillonite nanocomposite systems have been reported. Future of these nanomaterials definitely relies on innovative design, facile fabrication strategies, and overcoming related challenges.

Graphical Abstract

Keywords:

• Green
• nanoclay
• nanocomposite
• polymer
• fabrication
• packaging
• dye removal
• biomedical

1. Introduction

Nanoclays usually have platelet size in the range of 10–100 nm with nanolayered aluminosilicate nanostructures [1]. The nanoclay layers have been mostly constructed from silica (SiO₄^4–tetrahedra) and alumina ([AlO₃(OH)₃]^6– octahedra) based nanostructures [2, 3]. The nanolayers or nanosheets in nanoclays are held together through van der Waals interactions. Ion entrapment between the nanoclay layers has been performed through the nanostructural modification routes [4, 5]. In addition to unique nanostructure, the nanoclays possess fine structural or microstructural, strength, barrier, and thermal properties [6–8]. In polymers, nanoclay nanofillers have been reinforced in matrices to develop high-performance nanocomposites [9–11]. However, the polymer compatibilization with the non-modified nanoclays have been found difficult due to varying organic and inorganic nature of matrix and nanofiller [12, 13]. In this context, the alkyl cations and other modifying agents have been used to form organo-modified nanoclay [14–16]. The organo-modification has been used to attain compatible polymer/nanoclay nanocomposites [17–19]. Resulting polymer/nanoclay nanomaterials reveal superior heat and flame resistance, strength, and barrier characteristics [20, 21]. The polymer/nanoclay nanocomposites have revealed potential utilizations for wide range of industrial fields such as space, automobile, packages, civil, electronics, and many more [22–24]. The polymer and nanoclay based green nanocomposites have eco-friendliness and biodegradability properties [25]. In this context, various synthetic and natural green polymers have been used as significant matrices [26–28]. Consequently, the as-obtained green nanocomposites have been applied in important technological sectors [29–31]. For future progress in this field, further experimental and theoretical investigations need to be performed. Especially, computational studies on the advanced polymer/nanoclay nanocomposites can reveal future potential in several directions [32].

This comprehensive overview reports on the fundamentals of green polymers and nanoclay nanofillers derived nanocomposites. The matrices and nanoclay compatibility can be enhanced by using appropriate methods for the development of the advanced nanocomposites. Advanced features and applications of the green nanoclay nanocomposites have been obtained due to design optimization, compatibilization, and interactions between the matrix and nanofiller. Consequently, the methodological applications of these nanocomposites have been observed for the packaging, dye remediation, and biomedical relevance. The objective is to develop an all-inclusive, pioneering, and up-to-date overview on green polymer/nanoclay nanocomposites portraying indispensable aspects from synthesis/essential features to technical potentials. While developing this review article, novelty is especially considered in terms of the included literature, outlined topic, and variation of nanoclay types, and polymers used for the formation of green nanocomposites. Subsequently, this review is novel to elucidate the nanoclay derived nanocomposite fields by using recent literature collection, outline, and discussions. Although research articles have reported on polymer and nanoclay nanocomposites, feature review articles on green polymer/nanoclay nanomaterials have not been reported in literature before; especially, systematic explanation of nanoclay behavior with green polymers and explanation of results using experimental as well as computational studies. Novelty of this article also relies on the methodical presentation of applications of green nanocomposites in packaging, dye removal, and biomedical sector. To the best of our knowledge, such specific review on green polymer/nanoclay nanocomposites have not been reported in literature before and our review article is completely novel in terms of specific outline, recent literature reports, and technical analysis. Therefore, this review article is undoubtedly a ground-breaking contribution in the field of green nanomaterials aiming the advanced industrial applications. Motivation behind developing this review article is to present a novel review article on green polymer/nanoclay nanocomposites which throws light on the past, current, and expected future developments to benefit the field experts. In fact, forthcoming evolutions in this field are not conceivable for the researchers before getting prior knowledge of the accumulated literature and investigations regarding these nanomaterials. Thus, this overview may lead to several advancements in high-performance green polymer/nanoclay nanocomposites for future industrial applications.

2. Nanoclay

Polymers matrices have been reinforced with numerous inorganic or organic nanofillers to form the nanocomposite materials [33, 34]. Among inorganic nanofillers, nanoclays have been used as effective reinforcing agents [35, 36]. Nanoclays have significant advantages of low cost, easy availability, and biodegradability characteristics [37]. Nanoclays belong to the group of hydrous aluminum phyllosilicates such as different types of montmorillonite, kaolinite, bentonite, mica, etc. [38]. Figure 1(A) shows nanoclay nanostructure made up of two-dimensional nanosheets of silica tetrahedra and alumina octahedra. The layered nanosilicate nanostructure possesses ∼0.7–1 nm thickness. The nanoclay nanosheets or platelets are layered through weak van der Waals interactions and have interlayer spacing between them [39]. The nanoplatelets have hydrophilic properties due to alumina and silica based nanostructure [40]. In nanoclay platelets, the available space between the nanosilicate nanosheets is usually occupied with exchangeable molecules, ions, surfactants, etc. The cations like K⁺, Na⁺, Ca²⁺, Mg²⁺, etc. can be easily inserted between the nanoclay nanosheets because of inherent negative charge. For organic modification of nanoclays, long chain alkyl cations have been used to form organophillic nanoclays [41]. The organo-modified nanoclays possess large exchange capacity than the non-modified nanoclay. Consequently, the electrical conductivity of up to 100 mS/m has been observed for the nanoclay nanostructures [42]. Moreover, the organo-modified nanoclays have been used to enhance the miscibility with the polymeric chains [43, 44]. In this context, the matrices such as polystyrene, epoxy, polyurethane, polyamide, and many more have been used [45]. The designed polymer/nanoclay nanocomposites attained high electrical, strength, barrier, as well as flame and thermal retardancy properties [46].

Figure 1. (A) Structure of montmorillonite nanoclay [178]. (Source: Reproduced with permission from Springer.) (B) Transmission electron microscopy images of nanoclay modified isoprene latexes with 5 wt.% nanoclay; (C) 10 wt.% nanoclay; and (D) film formation of the natural rubber/nanoclay modified polyisoprene (NR/Clay-PIP) nanocomposite [68]. NR: natural rubber; PIP: polyisoprene. Source: Reproduced with permission from MDPI.

3. Green nanoclay nanocomposites

3.1. Natural rubber and nanoclay derived nanocomposites

Natural as well as synthetic polymers like starch, cellulose, chitin, poly(lactic acid), etc. have been applied for the formation of green nanocomposites [47]. In addition, ecological techniques have been applied for the formation of ecofriendly nanocomposites [48]. The naturally green polymers have been used for various applications such as membranes [49], coatings [50], biomedical sectors [51], and biodegradability [52]. Chitosan has been employed both as the green matrix and nanofiller [53, 54]. Synthetic green polymers include poly(vinyl alcohol), poly(ethylene glycol), polyester, etc. [55]. Consequently, these polymers have been reinforced with the nanoclays nanofillers [56]. Montmorillonite has been applied as an effective nanoadditive for the green polymers [57–59]. The resulting green nanomaterials possess superior antibacterial, degradation, thermal, mechanical, and other physical characteristics. Natural rubber is an important green matrix which can be nanocomposited with the silica and nanoclay nanoparticles for the formation of green nanocomposites [60, 61]. Due to fine exfoliation of nanoclay nanoparticles in natural rubber, fine dispersion, microstructure, and strength characteristics have been observed [62, 63]. Siririttikrai et al. [64] formed the natural rubber and nanoclay derived nanocomposite vulcanizates. The nanoclay nanoparticles have the tendency of coagulation or aggregation in the matrix. The polymer viscosity, nanoclay contents, as well as nanoclay modification influenced the nanocomposite properties. George et al. [65] developed the natural rubber and organically modified Cloisite nanoclay derived nanomaterials through the melt blending technique. The addition of 5 phr nanoclay led to superior strength and modulus properties of the nanomaterials. The property improvement was observed due to nanoparticle exfoliation in the natural rubber matrix [66]. Sookyung et al. [67] prepared the sodium montmorillonite nanoclay using octadecylamine as an organic agent. The modified nanoclay has better dispersion in the rubber matrix due to increased d-spacing to accommodate the polymeric chains. The enhanced dispersion, exfoliation, and interaction with polymeric chains resulted in the superior material characteristics. Chouytan et al. [68] fabricated the natural rubber/nanoclay modified polyisoprene nanocomposites. The materials were formed through starve-fed emulsion polymerization. Sodium dodecyl sulfate was used as a surfactant for better dispersion of nanofiller nanoparticles. Transmission electron microscopy revealed fine dispersion of nanoclay nanoparticles in the colloidal latexes (Figure 1(B) and (C)). The distinct microstructure was observed due to the anchoring effect of nanoclay nanoparticles to natural rubber and polyisoprene to develop nanoclay linked nanoparticles. Figure 1(D) shows the behavior of natural rubber and nanoclay modified polyisoprene before and after vulcanization for the formation of nanocomposite. It has been observed that the nanocomposite formation after vulcanization led to better polymer chain alignment with the nanoclay nanoparticles. Inclusion of 5 wt.% nanoclay contents led to better dispersion and reinforcing effects. However, adding 7 and 10 wt.% nanoclay decreased the homogeneous dispersion and reinforcing effects of the nanomaterial. In fact, dispersion mechanism of nanoclay nanoplatelets seems to be reliant on the exchange mechanism and modification modes used. Mechanism of nanoclay scattering depends on the hydrophobicity properties of the nanoplatelets due to organic cations. Consequently, the hydrophobicity of nanoclay allows the facile interaction with the polymers to form a well compatible nanocomposite.

3.2. Nanoclay-reinforced poly(lactic acid) nanocomposites

Poly(lactic acid) is applied as a useful matrix material for nanoclay nanofillers [69–71]. Darie et al. [72] developed the poly(lactic acid) and Cloisite nanoclay derived nanocomposites through solution intercalation technique. The poly(lactic acid)/Cloisite nanocomposite had antibacterial features for gram-negative and gram-positive bacterial strains. Salah et al. [73] fabricated the poly(lactic acid) and organically modified montmorillonite nanoclay derived nanocomposite through the melt extrusion method. The poly(lactic acid)/organically montmorillonite nanocomposite had dielectric properties in the range of 26–40 GHz. Mahani et al. [74] applied the 3D printing and injection molding methods to form the nanocomposites of poly(lactic acid) and starch matrices with nanoclay contents of up to 5 wt.%. The nanoclay loading led to increase in tensile strength from 10 to 20 MPa. The enhanced mechanical features have been observed due to fine interfacial adhesion between the matrix–nanofiller. In addition, superior biodegradation and barrier properties have been observed for the poly(lactic acid)/starch/nanoclay nanocomposites. Grigora et al. [75] reported on the poly(lactic acid) and Cloisite® 20 A derived nanocomposites. The nanoclay loading was included from 1 to 4 wt.% in the polymer matrix. Figure 2(A) depicts the chemical structures of poly(lactic acid), Joncryl, and modified Cloisite® 20 A. Figure 2(B) shows the finite element analysis derived stress–strain curves of the neat matrix and nanocomposite with nanoclay. Increasing nanoclay contents loading led to enhanced strength and modulus properties of poly(lactic acid)/modified Cloisite® 20 A nanocomposite owing to better reinforcement effects and matrix–nanofiller interactions. Mechanism of mechanical property improvement depends on the high surface area of the nanofiller and the interaction with the matrix. Consequently, more polymer chains or segments can be adsorbed on the nanofiller surface leading to high amount of vitrified polymer and strong enhancements in the mechanical properties. Ramos et al. [76] depicts the formation of nanobiocomposites of poly(lactic acid) with thymol and montmorillonite nanoclay. Figure 2(C) illustrates differential scanning calorimetry curves of the poly(lactic acid) nanobiocomposites. It has been observed that the pristine poly(lactic acid) material was amorphous in nature. However, after 7 days, the poly(lactic acid) nanobiocomposites had multiple endothermic peaks because of enthalpic relaxation involved. Mechanism of endothermic behavior includes hydrogen bond cleavage and creation of new bonds leading to a less ordered structure. Darie-Niță et al. [77] developed the poly(lactic acid) nanobiocomposites using melt method. The nanomaterial was prepared by employing the medicinal plant sage, coconut oil, and organo-modified montmorillonite nanoclay. Water contact angle studies were performed on these poly(lactic acid) nanobiocomposites (Figure 2(D)). Neat poly(lactic acid) matrix had a water contact angle of 97°. The poly(lactic acid) nanobiocomposite with sage and coconut oil revealed higher water contact angle due to enhanced hydrophobic properties of the materials having bio components. Mechanism of increase in hydrophobicity properties depends upon the organo-modified nature of montmorillonite nanoclay interacting with the polymer matrix. Bai et al. [78] fabricated the poly(lactic acid) and montmorillonite derived nanocomposite through laser sintering technique. This technique has used varying laser powers in the range of 15–17 W. The ensuing poly(lactic acid)/montmorillonite nanocomposite revealed flexural modulus of about 41%. Furthermore, the polycaprolactone and polyester matrices have also been applied for nanoclays to form the green nanoclay nanomaterials [79]. These nanocomposites have been suggested for the water remediation, dye removal, and antimicrobial features [80].

Figure 2. (A) Chemical structures of (a) PLA, (b) Joncryl, and (c) modified of Cloisite® 20 A; (B) curve fitting between experimental and FEA generated stress–strain curves of PLA and PLA/MMT [75]; (C) DSC thermograms, first heating scan of PLA based nanobiocomposite films after different composting times [76]; and (D) water contact angle for neat PLA and the PLA based biocomposites [77]. DSC: differential scanning calorimetry; FEA: finite element analysis; FEA PLA: finite element analysis derived poly(lactic acid); FEA PLA/MMT: finite element analysis derived poly(lactic acid)/montmorillonite; PLA: poly(lactic acid); PLA/CO: poly(lactic acid)/coconut oil; PLA/CO/S: poly(lactic acid)/coconut oil/sage; PLA/CO/S/1.31 PS: poly(lactic acid)/coconut oil/sage/sage coconut oil nanoclay; PLA/S: poly(lactic acid)/sage; WCA: water contact angle. Source: Reproduced with permission from MDPI.

3.3. Poly(vinyl alcohol) with nanoclays

Poly(vinyl alcohol) is an important hydrophilic polymer having fine affinity for water molecules [81–83]. The structural stability of poly(vinyl alcohol) has been attained through modification, grafting, crosslinking, and amalgamation with other matrices [84]. Mallakpour et al. [59] used ammonium salt of L-isoleucine amino acid for the modification of nanoclay to form Cloisite Na⁺. Compatible poly(vinyl alcohol)/Cloisite Na⁺ nanocomposites have been developed through the solution intercalation reaction. Gaume et al. [85] mixed the poly(vinyl alcohol) matrix with sodium modified montmorillonite nanoclay of up to 10 wt.%. The poly(vinyl alcohol)/montmorillonite nanocomposite has fine photochemical properties. Awad and Khalaf [86] formed poly(vinyl alcohol) using solution casting technique. Inclusion of 2 wt.% nanofiller led to fine heat stability upon 200 h UV irradiation. In the microstructural studies, no cracking or surface damages were observed. Tian et al. [87] reported on the poly(vinyl alcohol)/corn starch nanocomposite filled with montmorillonite using the melt method. Including 10 wt.% nanofiller developed fine hydrogen binding interactions to enhance the strength, barrier, and heat transport features. Allel et al. [88] fabricated the poly(vinyl alcohol) and montmorillonite-Na⁺ nanoclay derived nanocomposite membranes through solution route. Including 1–20 wt.% nanofiller led to enhanced pervaporation properties and separation efficiency for water/ethanol mixture. Gu et al. [89] fabricated the poly(vinyl alcohol) and nanoclay derived nanocomposites. Cellulose was used to induce crosslinking in the poly(vinyl alcohol) matrix. The crosslinking between poly(vinyl alcohol) and nanoclay led to the formation of hydrogel. Inclusion of nanoclay formed stable poly(vinyl alcohol) structure along with the development of pathways for water molecules passage. Figure 3(A) shows the formation of poly(vinyl alcohol) and cellulose based crosslinked structure and nanoclay filled nanocomposite through simple sonication and coating techniques. Figure 3(B) and (C) depicts water vapor transmission rates for the poly(vinyl alcohol) and poly(vinyl alcohol)/cellulose and nanoclay derived hydrogels. The water vapor transmission rates have been studied at 23 °C and 38 °C for 50% and 90% RH, respectively. It has been observed that the water vapor transmission rate of 512 g/m².day was observed at low temperature. Whereas, the water vapor transmission rate was enhanced to 1861 g/m².day with increase in temperature. Mechanism of increase in the water vapor transmission rate depends upon the increase in micro-pores in the structure allowing better exchange of water molecules in vapor form between the nanomaterial and the water medium and capillary action. The hydrogel possesses contact angle of 108° and hydrophobicity of up to >90°. These materials were suggested for efficient packaging applications. Shen et al. [90] designed the poly(vinyl alcohol) and alkyl ketene dimer based nanocomposite films with Cloisite Na nanoclay. The nanocomposites were developed in the form of buckypapers. The nanoclays were loaded from 3 to 10 wt. % in the buckypapers. The resulting pristine and nanocomposite papers were tested for the water vapor transmission rates and water contact angle analysis (Figure 4). Field emission electron microscopy was used to investigate the surface morphology of the poly(vinyl alcohol)/alkyl ketene dimer with dispersed nanoclay nanoparticles (Figure 4(A)). Owing to nanoplatelet dispersion, a porous microstructure was observed for the nanocomposites. Such nanostructure allowed the passage of liquid molecule through the nanocomposite. Improving the polymer film thickness was not found to decrease the porous nature of the nanomaterials. Hence, optimum permeability and barrier features have been observed for the nanocomposites. The water vapor transmission rates of unsized as well as sized papers have been reduced with time (Figure 4(B)). The results revealed that the poly(vinyl alcohol) paper developed better barrier effects due to fine film forming and surface properties. Accordingly, the water vapor transmission rate of 533 g/m².day was observed for the base paper, which was reduced to 1.3 g/m².day after layering the paper because of increased thickness and barrier effects. The water contact angle measurements of the pristine and nanocomposite papers are given in Figure 4(C). Here, the presence of alkyl ketene dimer caused hydrophobic effects to prevent the water penetration through the films. Consequently, the water contact angle of the poly(vinyl alcohol)/alkyl ketene dimer was affected. Addition of alkyl ketene dimer decreased the water contact angle with time due to the enhanced hydrophobicity. The mechanism behind increasing the hydrophobicity of the nanomaterial seems to depend upon the organic nature of alkyl ketene dimer and organic modification of the nanoclay platelets. On the other hand, nanoclay inclusion in the matrix caused increase in water contact angle due to mutual interactions between the poly(vinyl alcohol) and nanoclay nanoparticles. Thus, the poly(vinyl alcohol) and related nanoclay filled nanocomposite paper have appropriate barrier properties as the packaging materials.

Figure 3. (A) Scheme of coating application on paper surface with a Mayer bar. The WVTR of PVA and PVA gel coated paper at different temperatures and humidity: (B) 23 °C, 50% RH and (C) 38 °C, 90% RH [89]. PVA: poly(vinyl alcohol); PVA/NC: poly(vinyl alcohol)/nanoclay; RH: relative humidity; WVTR: water vapor transmission rates. Source: Reproduced with permission from MDPI.

Figure 4. (A) FE-SEM images of (a) the base paper, (b) PVA/AKD-S coated paper, (c) PVA/AKD/5% nanoclay-S coated paper, (d) PVA/AKD-D coated paper, and (e) PVA/AKD/5% nanoclay-D coated paper. (B) WVTR of papers; (C) WCA of the base paper and coated papers; and (D) elongations at break and tensile strengths of the base paper and coated papers [90]. BP: base paper; D: double coating; FE-SEM: field emission electron microscopy; PVA/AKD: poly(vinyl alcohol)/alkyl ketene dimer; S: single coating; WCA: water contact angle; WVTR: water vapor transmission rates. Source: Reproduced with permission from MDPI.

3.4. Cellulose and chitosan filled with nanoclay nanofillers

Using raw cellulose pulp, several chemical and enzymatic routes have been applied to obtain the cellulose polymer [91]. It is a hydrophilic and biodegradable material [92]. For nanocompositing, cellulose has been applied as an effective polymer matrix [93]. Like other nanofillers, cellulose has been used to form nanocomposites with kaolinite, montmorillonite, and other nanoclays [94]. The nanoclay nanofillers have been found to enhance the microstructure, mechanical, thermal, non-flammability, and gas barrier properties of the nanocomposites [95]. Barbi et al. [96] developed the ceramic nanoclay filled bacterial cellulose derived green nanomaterials. Biomedical applications have been observed due to the hydrophilicity of the membranes. Ming et al. [97] formed the nanofibrillated cellulose and montmorillonite nanoclay derived green nanocomposites. Figure 5(A) reveals the preparation and analysis stages of the nanofibrillated cellulose/nanoclay nanomaterials by using the mechanical stirring as well as ultra-sonication techniques. The nanocomposite with 0.5 wt.% nanoclay contents has fine dispersion properties. In addition, the unfilled nanofibrillated cellulose had vigorous burning tendency (8 s) and smoldering (32 s) effects. On the other hand, the nanocomposite sample film was not burnt extensively and showed self-extinguishing nature. The non-flammability effects were observed because of the inclusion of nanoclay nanoparticles. Subsequently, nanoclay has been found to enhance the flame barrier of the polymer matrix. Ferfera-Harrar and Dairi [98] developed the green nanocomposites of organo-functionalized montmorillonite like gelatin functionalized montmorillonite or chitosan functionalized montmorillonite. Triethyl citrate was applied as a plasticizer in these green nanocomposites. In the presence of plasticizer, the functional nanoclays were exfoliated to form the nanocomposites. The thermal stability of the green nanomaterials has been attained at very low loading level of 0.5 wt.%. Similarly, other green cellulose/nanoclay nanocomposites designs have been investigated for enhanced physical features [92].

Figure 5. (A) Schematic of transparent NFC monolayer/nanoclay nanoplatelet based hybrid films with self-extinguishing behavior (90% transparency at 600 nm). One-dimensional nanofibrillated cellulose was extracted from wood pulp through mechanical and chemical treatments and mixed with disperse nanoclay nanoplatelets uniformly in water. The nanofibrillated cellulose dispersed monolayer nanoclay nanoplatelets suspension was blended with 0.5 wt.% nanoclay nanoplatelet suspension using mechanical stirring and ultra-sonication methods [97]. (Source: Reproduced with permission from ACS.) (B) Mechanism of MMT stacking and arrangement of stacks in nanocellulose network: (a) MMT stack formation at high loading level which decrease the tortuous paths and (b) MMT stacks broken down using high pressure homogenization of nanocellulose/MMT suspension which increase the tortuous path [112]. (Source: Reproduced with permission from Elsevier.) (C) WVP of nanocellulose/MMT nanocomposites. The pristine nanocomposite sheets; nanocomposite sheets with high pressure homogenization step, and sonication step [112]. (Source: Reproduced with permission from Elsevier.) (D) WVTR of PLA and its nanocomposites regarding the nanoclay contents and normalized to 25 µm [113]. (Source: Reproduced with permission from MDPI.) (E) The adsorption efficiency of C20A, C20AM, and PLAC20AM 5% (methylene blue dye concentration = 200 mg/L, nanocomposite content = 20 mg/20 mL, T = 25 °C and 60 min) [123]. (F) The removal efficiency of MB for 48 h in the presence of electrospun CA and of CA/NC1, CA/NC2, and CA/NC3 nanocomposites [124]. C20 A: Cloisite 20 A; C20AM: Cloisite 20 A with 1,4-diaminobutane dihydrochloride; CA: cellulose acetate; CA/NC: cellulose acetate/nanoclay; MB: methylene blue; MMT: montmorillonite; NFC: nanofibrillated cellulose; PLA: poly(lactic acid); PLAC20AM: poly(lactic acid)/Cloisite 20 A/methylene blue; WVP: water vapor permeability; WVTR: water vapor transportation. Source: Reproduced with permission from MDPI.

3.5. Computational studies on polymer/nanoclays

Computational studies have also been performed regarding the nanoclay filled polymeric nanocomposites [99–101]. El Haouti et al. [102] performed theoretical as well as experimental studies on montmorillonite nanoclay linked cationic dyes such as toluidine blue and crystal violet. The density functional theory and molecular dynamic simulations have been used to study the molecular arrangements and interaction energies of the dye molecules and interactions with the nanocomposite. The optimized structures of isolated toluidine blue and crystal violet molecules were generated according to parameters such as electrophilicity power, electronic chemical potential, and electron energies of highest occopied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) (Figure 6(A)). At constant cell volume, atomic positions of full geometry relaxation have been studied for the nanoclay with dye molecules (Figure 6(B)). Structure of crystal unit cell and lattice energies were used to depict the vibrational frequencies and elastic constants, in algorithmithic compass force field. Variety of simulated orientations like parallel, basal, tilt, piling, etc. have been studied in the equilibrated structures of dye molecules (Figure 6(C)). With the nanoclay surface, the aromatic rings present in the dye molecules interacted through parallel interactions. Chemisorption process was observed between the nanoclay and dye molecules. However, at basal surface few dye molecules were oriented flat to the nanoclay surface. Hence, according to DFT calculations, dye molecules acted more electrophilic in nature and attractive toward nanoclay. Chowdhury and Wu [103] used cohesive zone model based finite element analysis to find out representative volume elements of nanoclay nanocomposites. Consequently, the node normal stress and opening displacements were studied (Figure 7(A) and (B)). The nanoclay nanoparticles were aligned in stacked or staggered configuration. The linearly elastic loading region and linearly elastic softening region were observed. During softening, node normal and shear forces were reduced to zero, whereas in the debonding process the node normal and shear stresses approached higher values. Relative to rectangular nanoclay nanoparticles, edge curved nanoclay nanoparticles have reduced stress in stress–strain relation. Representative volume element of polymer nanoclay nanocomposite was studied for different configurations (Figure 7(C)). In the debonding process, typical von Mises stress and displacement fields were observed (Figure 7(D)). At this point large normal stress was experienced in structure. Hence, the computational studies based on cohesive zone model based finite element analysis revealed controlled mechanical properties and processing parameters. Chen et al. [104] used computational approaches to study the basalt fiber and nanoclay-reinforced polymer. The resulting structure and interfacial bonding were studied using the modified Eshelby–Mori–Tanaka micromechanic model. Nanoclay was observed uniformly and densely dispersed in the matrix. The nanoclay caused bridging effect to prevent the crack propagation in the polymer. Hence, the mechanism of the mechanical property improvement was dependent upon the bridging of the nanoclay to nanoclay and nanoclay to polymer chains.

Figure 6. (A) Geometries of the lowest energy conformers of CV and TB optimized at the DFT/B3LYP/6-31G(d) level of theory; (B) (a) Front view and (b) side view of Na-MNC supercell lattice. ; (C) Equilibrium configurations of adsorption of TB onto MNC from NVT molecular dynamics for: (a) 33%; (b) 66%; and (c) 100% of surface loading rate. Polyhedron style was used to represent the Na-MNC for the top view [102]. CV: crystal violet; MNC: montmorillonite nanoclay; NVT: canonical ensemble; TB: toluidine blue. Source: Reproduced with permission from Elsevier.

Figure 7. (A) Schematic node normal and tangential stresses and displacements in CZM-based FEM modeling of debonding process of clay nanoparticles in PNCs. (B) Schematic σ_n–δ_n and σ_t–δ_t relations of a bilinear CZM. (C) RVE of a PNC reinforced with aligned, identical clay nanoparticles in a stack distribution configuration: (a) idealized identical stacking clay platelets; (b) a typical RVE; and (c) a quarter RVE for efficient simulation; and (D) von Mises stress and displacement fields of RVE of a PNC reinforced with a rectangle-shaped clay nanoparticle: (a) crack initiates; (b) crack propagates; and (c) crack propagates at final failure [103]. CZM: cohesive zone model; FEM: finite element analysis; PNCs: polymer nanoclay composites; RVE: representative volume element. Source: Reproduced with permission from MDPI.

4. Technical significance of green nanocomposites

4.1. Packaging

The packaging materials have been developed using natural as well as synthetic polymers such as starch, cellulose, poly(vinyl alcohol), poly(lactic acid), etc. for beverages and electronics industries [105, 106]. These materials have revealed appropriate disintegration and recyclability features [107]. In poly(lactic acid), triacetin plasticizer and Cloisite nanoclay were included to form the nanocomposite packaging through solvent casting technique [108]. Varying amounts of nanoclay nanofiller and plasticizer have been used to form these materials. Inclusion of up to 10 wt.% triacetin plasticizer and 3 wt.% Cloisite nanoclay led to a minimum water vapor permeability of 1.06 × 10⁻¹⁰ g/m.s Pa. In this way, promising packages have been achieved. Aulin et al. [109] investigated the nanocellulose nanofibrous packaging reinforced with vermiculite nanoclay nanoplatelets. The packages were developed using the high pressure homogenization. The nanocomposite packaging revealed optimum strength (257 MPa) and modulus (17.3 GPa) features. The nanocellulose nanofiber/vermiculite nanoclay packaging owns a low oxygen permeability of 0.07 cm³μm/m².day.kPa (50% relative humidity). The nanocomposite has been considered appropriate to form large barrier coatings for technical commercial applications. Farmahini-Farahani et al. [110] fabricated the cellulose and montmorillonite nanoclay derived packaging. The modified nanoclay i.e. sodium montmorillonite was applied as a nanoadditive for the formation of nanocomposite packaging materials. For these nanomaterials, a minimum water vapor transmission rate of 43 g/m².day was attained. De Souza et al. [111] developed the starch carvacrol and essential oil modified montmorillonite derivative packaging materials. The organic modification of montmorillonite led to superior interactions of the nanoclay with the matrix. Consequently, strong antibacterial effects have been experiential for Escherichia coli bacterial strain. Garusinghe et al. [112] formed the nanocellulose and montmorillonite nanoclay based nanocomposite membranes. These nanomaterials were studied for the water vapor permeability and tortuosity. Figure 5(B) shows the mechanism behind the nanocomposite membrane formation. The montmorillonite nanoclay platelets seemed to be stacked and arrange homogeneously in the nanocellulose network. The nanoclay nanoplatelets have been considered to restack in the matrix due to van der Waals or electrostatic interactions. The uniform nanoclay dispersion in nanocellulose/montmorillonite matrix has decreased the nanofiller stacking and enhanced the tortuous ways in the nanocomposite. Figure 5(C) depicts the water vapor permeability of the pristine nanocellulose/montmorillonite nanocomposite nanosheets prepared using simple mixing, sonication, and homogenization routes. It can be seen that the nanocomposite prepared with sonication step revealed better water vapor permeability than the pristine nanocomposite nanomaterial. Using high pressure homogenization with 16–23 wt.% nanoclay contents, the nanocomposite revealed lowest values for water vapor permeability i.e. in the range of 6.3–13.3 g.μm/m².day.kPa. Consequently, the design of the obtained nanomaterials has been found suitable for the advanced packaging. Oliver-Ortega et al. [113] prepared the poly(lactic acid) nanocomposites with nanocellulose and montmorillonite. The water vapor transportation investigations have been performed on the poly(lactic acid)/nanocellulose/montmorillonite nanocomposites (Figure 5(D)). The nanoclay contents were varied from 2 to 4 wt.% in the nanomaterials. The synergetic effects between the matrix and nanofillers led to better nanoclay dispersion as well as fine barrier effects against water vapor transportation. Hence, the green polymers and nanoclay derivative packaging revealed resistance to gases, moisture, and microbes and can be effectively utilized for food, beverages, electronics, and other advanced packaging applications. Here, the development of precise and well-designed systems has been desirable to attain efficient green nanoclay based green systems.

4.2. Dye removal

Efficient methods have been applied for the dyes and heavy metals remediations from waste water [114–116]. Consequently, continuous sustainable research efforts were observed for obtaining clean water by eradicating the dye impurities [117, 118]. Among effective methods, adsorbents have been effectively used for dye removal [119]. Nanoclays possess high efficiency to remove dyes and heavy metals and toxic ions from the contaminated water. Nanoclays have high specific surface area and cation exchange capacity of >800 m²/g and >120 cmol/kg, respectively [120]. For example, nanoclays can effectively remove Congo Red dyes in the range of 32–2436 mg/g. Similarly, lead metal can be adsorbed by nanoclays in the range of 9–100 mg/g. Çınar et al. [121] fabricated the chitosan/nanoclay nanocomposites for the adsorption of Ponceau S dye. The Langmuir, Freundlich, and Temkin adsorption models were applied to study the adsorption of Ponceau S. According to adsorption kinetics and thermodynamic parameters, the maximum adsorption capacity of 140.85 mg/g and dye concentrations of 150–400 mg/L were achieved. The linear pseudo-second-order kinetic model was observed and dye adsorption was endothermic as well as spontaneous. The green nanocomposites have been found useful for the waste water remediation. Sardi et al. [122] formed the poly(ethylene glycol) and montmorillonite derived nanocomposite packaging. The nanoclay was modified using the hexadecyltrimethylammonium bromide surfactant. The poly(ethylene glycol)/hexadecyltrimethylammonium bromide–montmorillonite nanocomposite has been used for the adsorption of dyes such as methylene blue and trypan blue dyes. Efficient dye adsorption was observed owing to the intercalation of surfactant between the nanoclay layers. The adsorption capacities of 237.22 and 190.81 mg/g, respectively, were attained for the methylene blue and trypan blue dyes. In addition, the nanocomposite had fine microbial effects against E. coli and Micrococcus luteus bacterial strains. Andrade-Guel et al. [123] prepared the poly(lactic acid) and Cloisite 20 A derived nanocomposites. Poly(lactic acid) was loaded with about 0.5–5 wt.% nanoclay nanoparticle contents. The Cloisite 20 A modified with 1,4-diaminobutane dihydrochloride was also used. It has been observed that the dye adsorption by Cloisite 20 A was found to be enhanced with time until 55% adsorption efficiency at 60 min (Figure 5(E)). On the other hand, 1,4-diaminobutane dihydrochloride modified Cloisite 20 A revealed enhanced dye absorption efficiency of 91% after 50 min. The Cloisite 20 A composited with poly(lactic acid) depicted further enhanced adsorption efficiency of about 97%. The enhanced adsorption efficiency was observed due to the chemical interactions between the amino functionalities of methylene blue dye. In addition, the dye adsorption mechanism seems to be relying on ion exchange capability of the nanoclays and also the interactions with polymers. Due to anionic nature, nanoclays act as efficient superadsorbent to remove the dyes of cationic nature. Tsekova and Stoilova [124] fabricated the Cloisite 20 A filled cellulose acetate nanofibers. Various nanoclay contents were used to attain desired properties of the resulting materials. The electrospun cellulose acetate/Cloisite 20 A nanofibers were tested for the removal of methylene blue dye (Figure 5(F)). Maximum methylene blue dye was absorbed by the nanofibers after 6 h. The results show fine dye removal efficiency of methylene blue using electrospun cellulose acetate and cellulose acetate/Cloisite 20 A nanocomposite nanofibers. It has been observed that after 48 h, the relative dye removal efficiency of the nanocomposite nanofibers (34%) was slightly decreased relative to the neat polymer nanofibers. It was probably due to the saturation of the methylene blue dye absorption on the nanocomposite nanofibers.

4.3. Biomedical sector

The polymeric scaffolds have been applied for drugs impregnation in drug delivery application [125]. The solubility, permeability, absorption, and pharmacological behaviors of drugs have been investigated for numerous polymer scaffolds [126]. In addition to polymers, nanocomposites have also been used as the drug delivery scaffolds [127]. Ferrández-Rives et al. [128] developed the poly(vinyl alcohol) nanofibrous hydrogel reinforced with nanoclay and applied for the drug delivery application. The absorption of bovine serum albumin drug on nanocomposite hydrogel was considered. The effect of drug encapsulation in nanoclay filled poly(vinyl alcohol) nanofibrous hydrogel has been studied. The nanoclay based systems revealed better bovine serum albumin absorption and release, relative to unfilled hydrogel. Hsu et al. [129] prepared a biocompatible system based on poly-DL-lactic acid, polycaprolactone, and laponite nanoclay derivative nanocomposite for drug delivery. The nanocomposite was used for drug delivery to cure type-2 diabetes. Ferrández-Rives et al. [128] designed the electrospun nanofibrous nanocomposite membranes of poly(vinyl alcohol) and montmorillonite nanoclay. The nanofibers were prepared through the electrospinning techniques and nanocomposite films were casted. The morphological and drug delivery or protein absorption/release studies on the poly(vinyl alcohol)/montmorillonite nanoclay nanocomposites were performed for bovine serum albumin. Figure 8(A) and (B) reveals the scanning electron microscopic images of pristine poly(vinyl alcohol) and 10 wt.% poly(vinyl alcohol)/nanoclay nanocomposite. The fine microstructure of nanofibers was attributed to the glutaraldehyde crosslinking agent. Inclusion of nanoclay in the nanofibers resulted in fine dispersion and no aggregation was observed. Figure 8(C) shows the accumulated delivery of bovine serum albumin from the nanofibers. The 20%–40% nanoclay additions were monitored and outcomes were normalized for the delivery time of 7 days. According to comparison between nanocomposites with various nanoclay contents, the negligible kinetics difference between samples was observed. Liu et al. [130] used the nanoclays of Cloisite nanoclay, i.e. Cloisite Na and organo-modified Cloisite 20. The melt extrusion technique was used to form the nanocomposite of Eudragit® RS matrix material and nanoclay nanofiller. The polymer/nanoclay nanocomposites were applied as drug delivery carriers for theophylline. Figure 8(D) shows that Cloisite Na nanoclay effectively controlled the theophylline drug release relative to Cloisite 20 nanoclay. The regulated release of drug was attributed to the interactions between the Eudragit® RS matrix and organically modified Cloisite Na nanoclay. In addition, the controlled nanoclay loading level has been found essential to control the drug release from the nanocomposite nanofibers. The mechanism of drug release depends upon the efficient drugs uptake and interaction with drug molecules through encapsulation, electrostatic interactions, ion exchange reaction, and immobilization. Subsequently, due to hydrogen bonding and electrostatic interactions, the drug is released from drug-nanoclay. Further efforts on montmorillonite nanoclay modification and amalgamation with green polymers can be advantageous toward developing advanced green drug delivery routes [131]. Morariu et al. [132] investigated the nanocomposites based on chitosan, poly(ethylene glycol), and Laponite® nanoclay. It was suggested that the nanoclay nanoparticles were dispersed among the polymers chains to form an ordered pattern (Figure 9). Scanning electron microscopy images revealed that poly(ethylene glycol) molecules were distributed between the chitosan chains due to hydrogen bonding in the blend matrix. The nanocomposite sample with the Laponite filled in the blend matrix revealed that the nanoclay particles formed interactions with chitosan as well as poly(ethylene glycol) chains for dispersion. A consistent nanoclay dispersion pattern was analyzed due to scattering of nanoplatelet between the polymer blend chains. Such nanoclay dispersion has been found responsible for the enhancement in the crystallinity properties of the nanocomposites. Such a dispersion pattern has been found effective for water diffusion mechanism in the nanocomposite and was used as support material for drug release. In addition, thermogravimetric analysis depicted two-stage weight loss behavior. The adsorbed water was lost at around 80 °C, whereas the decomposition of fragile polymer chains of poly(ethylene glycol) caused the nanocomposite degradation in the second stage at ∼225 °C–360 °C. The degradation of 79% at such a low temperature was due to the use of a low molecular weight polymer like poly(ethylene glycol). Hence, the low thermal stability and the formation of a weak physical network were experiential.

Figure 8. (A) 0 wt.% MMT; (B) 10 wt.% MMT morphology of crosslinked PVA electrospun nanocomposite mats with different contents of nanoclay; (C) BSA delivery from PVA electrospun mats for different contents of nanoclay: 0 wt.% (circle), 2 wt.% (square), 10 wt.% (triangle), and 40 wt.% MMT (diamond) [128]; (D) Dissolution profiles of 500 mg theophylline granules 30–35 mesh, 20 wt.% theophylline) in 900 mL phosphate buffer pH 6.8 using USP apparatus II at 75 RPM (n = 3), Cloisite Na nanocomposites of different nanoclay loadings [130]. BSA: bovine serum albumin; MMT: montmorillonite; PVA: poly(vinyl alcohol). Source: Reproduced with permission from MDPI.

Figure 9. (A) Schematic representation of film structures with various clay concentrations; (B) SEM microphotograph of chitosan PEG nanoclay; and (C) TGA curves of CS/PEG films with various amounts of Lap [132]. CS/PEG: chitosan/poly(ethylene glycol); PEG: poly(ethylene glycol); SEM: scanning electron microscopy; TGA: thermogravimetric analysis; C1, C3, C5 = 0%, 1%, and 1.5% nanoclay. Source: Reproduced with permission from MDPI.

The polymer/nanoclay nanomaterials have also been focused for the advanced tissue engineering applications, especially for healing or repairing or replacing the tissues [133]. These nanocomposites possess fine porosity, compatibility, and biodegradable properties to be employed as tissue engineering scaffolds [134]. In addition, polymer/nanoclay derived systems revealed high elastic and/or storage modulus for better performance as tissue engineering scaffolds [135]. These nanomaterials also possess fine surface-to-volume ratio and antimicrobial properties to enhance the interactions, biocompatibility, and biodegradation properties of the nanocomposites [136–138]. The polymer/nanoclay based three-dimensional scaffolds have also expanded the research curiosity in the tissue engineering sector [139–141]. In this regard, green polymers derived nanostructures reveal fine cell attachment and proliferation properties in organic environment. Pierchala et al. [142] formed the poly(lactic acid) and halloysite nanoclay derived nanocomposites. These materials have porous, antibacterial, heat stability, and strength features. The poly(lactic acid)/halloysite nanoclay was condensed with gentamicin and used for bone regeneration applications. Nouri et al. [143] formed the electrospun poly(ɛ-caprolactone) and nanoclay based nanocomposite nanofibers. Inclusion of nanoclay nanoparticles revealed fine cell adhesion, proliferation, and bioactivity for fibroblasts cells. The wettability and degradability of the poly(ɛ-caprolactone)/nanoclay nanocomposite nanofibers have also been found useful for the tissue regeneration. Jin et al. [144] developed the poly(ethylene glycol) diacrylate and laponite nanoclay derived three-dimensional hydrogel nanomaterial as tissue scaffolds. Bioprinting technology was used. The nanocomposites had fine modulus, biodegradation, and proliferation characteristics. Further design investigations have been found indispensable for future progress in the field of polymer/nanoclay based tissue engineering.

Green polymer/nanoclay nanocomposites have also found application for wound healing [145]. Owing to antimicrobial features of nanoclay, these nanomaterials have been widely used to prevent infections and resulting pain in the wounded areas. Bibi et al. [146] designed the green poly(vinyl alcohol) and nanoclay derived nanocomposites for applying drug penicillin on wounds. The nanomaterials have effective antimicrobial properties. Asthana et al. [147] developed the poly(vinyl alcohol)/nanoclay nanocomposites using solution technique. These nanomaterials resulted in low microbial activity upon application on the wounded areas. The nanocomposite was also used along with aloe vera gel to cure the wounds. Poly(vinyl alcohol) and kaolinite based nanomaterials have also been used in dressings for wound healing application [148]. Hence, green nanocomposites possess high biodegradability, chemical stability, and high infection prevention rate in wound healing relevance.

5. Challenges, future directions, and conclusions

Green polymeric nanocomposites have been developed using the nanoclay nanofillers (Table 1). Facile processing techniques including solution and melt routes have been adopted to form the green polymer/nanoclay nanocomposites. Both synthetic and natural polymeric matrices have been used to form the green nanomaterials. The material properties have been enhanced at low nanofiller loading level. However, nanocomposite processing and achievement of high-performance nanomaterials face several challenges that need to be fixed [149]. Most importantly, dispersion of pristine nanoclay in polymers has been found difficult due to non-modified nano-silicate layers. Pristine nanoclays are hydrophilic in nature and so incompatible with organic polymers. The nanoclay modification along with intercalation and exfoliation approaches has been applied for the generation of uniform dispersion in the nanomaterials. In this context, nanoclays have been modified using the long alkyl chains. Consequently, the organic modification of nanoclays led to fine matrix–nanofiller interactions and dispersion due to enhanced compatibility. The resulting microstructure, mechanical, thermal, non-flammability, antimicrobial, electrical, and barrier features have been improved [150]. Up till now, the ecological polymer/nanoclay nanocomposites have been applied for the formation of sustainable packing material, dye remediation from waste water, and biomedical sectors such as tissue engineering, drug delivery, and wound healing. Using traditional polymeric materials in commercial or industrial applications may cause ecological problems [151]. Consequently, the sustainability demands can be fulfilled if the materials or nanomaterials are green or biodegradable without using any toxic materials during synthesis or decomposition [152]. Thus, the green materials have less greenhouse emissions, relative to non-green materials or nanomaterials used. Advancements in the field of green materials may lead to the development of environmentally friendly space, automotive, civil, electronics, and environment related applications.

Table 1. Green nanoclay filled nanocomposite based systems for technical applications.

Polymer	Nanofiller	Processing	Property/application	Ref
Natural rubber	Organically modified Cloisite nanoclay	Melt blending technique	Strength and modulus increase	[65]
Natural rubber	Octadecylamine modified sodium montmorillonite	Solution	Dispersion, exfoliation, interaction	[67]
Natural rubber; polyisoprene	Montmorillonite modified polyisoprene	Vulcanization	Homogeneous dispersion; ion exchange mechanism	[68]
Poly(lactic acid)	Cloisite	Solution/intercalation technique	Antibacterial properties for gram-negative/gram-positive bacteria	[72]
Poly(lactic acid)	Montmorillonite	Melt extrusion method	Dielectric properties 26–40 GHz	[73]
Poly(lactic acid); starch	Montmorillonite	3D printing; injection molding	Interfacial adhesion; tensile strength 10–20 MPa	[74]
Poly(lactic acid)	Cloisite® 20 A	Solution	Finite element analysis; stress–strain	[75]
Poly(lactic acid)	Thymol; montmorillonite	Melt method	Multiple endothermic peaks due to enthalpic relaxation	[76]
Poly(lactic acid)	Montmorillonite	Melt method	Water contact angle 97°; hydrophobicity	[77]
Poly(lactic acid)	Montmorillonite	Laser sintering technique	Flexural modulus 41% increase	[78]
Poly(lactic acid)	Ammonium salt of L-isoleucine amino acid modified cloisite Na^+	Solution intercalation	Dispersion	[59]
Poly(lactic acid)	Sodium modified montmorillonite	Intercalation	Photochemical properties	[85]
Poly(lactic acid)	Montmorillonite	Solution casting	Heat stability 200 h on UV irradiation	[86]
Poly(vinyl alcohol)/corn starch	Montmorillonite	Melt	Strength, barrier, heat transport	[87]
Poly(lactic acid)	Montmorillonite-Na^+	Solution route	Pervaporation/separation efficiency; water/ethanol mixture	[88]
Poly(lactic acid); Cellulose	Montmorillonite	Sonication; coating	Water vapor transmission at 23 °C and 38 °C for 50% and 90% RH, respectively; water vapor transmission rate 512–1861 g/m^2.day; contact angle 108°; hydrophobicity > 90°	[89]
Poly(vinyl alcohol); alkyl ketene dimer	Cloisite Na	Buckypaper method	Water vapor transmission rate 533 g/m^2.day	[90]
Nanofibrillated cellulose	Montmorillonite	Mechanical stirring; ultra-sonication techniques	Non-flammability effects; self-extinguishing	[97]
Cellulose; triethyl citrate plasticizer	Gelatin functionalized montmorillonite	Solution	Non-flammability	[98]
Poly(lactic acid); triacetin plasticizer	Cloisite nanoclay	Solution	Water vapor permeability 1.06 × 10^–10 g/m.s Pa. In	[108]
Nanocellulose	Montmorillonite 16.7–23.1 wt.%	Solution method	Low water vapor permeability 6.3–13.3 g.μm/m^2.day.kPa; Packaging	[112]
Nanocellulose nanofiber	Vermiculite nanoclay	Solution method	Oxygen permeability 0.07 cm^3μm/m^2.day.kPa at 50% relative humidity; packaging	[109]
Cellulose	Montmorillonite	Solution method	Low water vapor transmission rate 43 g/m^2.day; packaging	[110]
Poly(ethylene glycol)	Montmorillonite; hexadecyltrimethylammonium bromide surfactant	Solution method	Methylene blue and trypan blue with adsorption capacities 237.22 and 190.81 mg/g, respectively; antimicrobial effects E. coli/M. luteus bacteria	[122]
Poly(lactic acid)	1,4-diaminobutane dihydrochloride modified Cloisite 20 A	Solution	Methylene blue dye; dye absorption efficiency 91%–97 % (1 h)	[123]
Cellulose acetate	Cloisite 20 A	Electrospinning	Methylene blue dye removal efficiency 34%	[124]
Poly-(DL-lactic acid)	Laponite	Solution method	Drug delivery; Type-2 diabetes curation	[129]
Poly(vinyl alcohol)	Montmorillonite	Electrospinning	20%–40 % nanoclay; normalized delivery time 7 days	[128]
Eudragit® RS	Cloisite 20	Melt extrusion	Drug delivery theophylline	[130]
Chitosan, poly(ethylene glycol)	Laponite® nanoclay	Solution	Water diffusion mechanism; support materials for drug release	[132]
Poly(lactic acid)	Halloysite nanoclay	Gentamicin; solution	Bone regeneration	[142]
Poly(ɛ-caprolactone)	Montmorillonite	Electrospinning	Tissue engineering; better cell adhesion and bioactivity	[143]
Poly(vinyl alcohol)	Montmorillonite	Solution method	Wound healing; penicillin drug to wounds	[146]
Poly(vinyl alcohol)	Montmorillonite	Solution method	Wound healing; high microbiological stability	[147]
Wood	Montmorillonite	Solution/melt method	Sustainable construction materials	[174]
Cellulose, starch, poly(lactic acid)	Montmorillonite	Solution/melt method	Packaging	[105,106,175]
Chitosan	Montmorillonite	Solution method	Rhodamin-6 dye adsorption capacity 446.43 mg/g	[176]
Poly(epicholorohydrin-dimethylamine)	Bentonite nanoclay	Solution	Dye adsorption capacity 13–31 mg/g	[177]

More precise future directions need to be defined for the green nanoclay based nanocomposites. Interfacial interactions and spatial alignment of the nanocomposites need to be resolved through employing advancement surfacial chemistry. The formation of covalently channeled interactions must be focused. Novel synthesis methods need to be developed to reduce hazardous solvent emissions and explosion risks. Membranes and coating materials based on green polymer/nanoclay must be focused in addition to packaging. Research insights must be thrown on extended and durable utilization of these nanocomposite coatings/membranes. Moreover, quantitative analysis data must be collected on nanocomposite anticorrosion, permeability, barrier properties, life cycle, and repeated usages. Moreso, construction, aerospace, automotives, electronics pharmaceuticals, and gas/oil sectors must be focused [153, 154]. Lastly, multifunctional green nanocomposites need to be investigated for wide spectrum of versatile applications in several unexplored fields [155–157].

For the commercial scale and industrial applications of green nanocomposites, it has been found indispensable to investigate the end-life of composites [158], recyclability [159], and compostability [160] competences. Green nanocomposites have been preferred in modern industries owing to low weight, unique features, durability, and erosion resistance [161]. Consequently, it has been found essential to explore the end-of-life of the composite materials [162]. Moreover, recycling of the materials using environmentally friendly strategies have been still challenging to efficiently reduce, recycle, reuse, and refurbish the materials [162, 163]. Here, improved circular economy technologies need to be developed. The circular economy approaches need to be adopted for end-of-life of composites for circularity in industries [164]. The applicability of composite material end-of-life has been used for waste management. For recyclability and sustainability, biodegradable and green polymers have been used [165–167]. Green materials have been used to reduce the greenhouse emissions during material production and utilization. Hence, green materials have been found safe during recycling and further industrial uses [168]. During recycling, recovery of important material phases must be attained [169]. Although these composites have vast application prospects, their use in circular economy is still challenging due to inadequate accessibility of end-of-life waste management skills [170]. For using the green materials in circular manner, efficient methods such as UV or irradiation tactics, plasma approaches, high power ultrasonic, surface activation, and safe chemical means must be used [171, 172]. However, still innovative methods need to be used in this regard. All the end-life assessment, recycling, and compositing methods must have decreased environmental impacts on circular economy [173]. Development and study of end-life, recycling and sustainability concepts are also essential for future progress in this field.

The foremost aim of this article is to highlight the impact of green nanoclay nanocomposites. Consequently, this overview presents the fundamentals and advances in the field of some important green nanoclay nanocomposites. Here, both the natural and synthetic matrix materials have been used to form the nanocomposites with nanoclays as green nanofillers. The resulting ecofriendly and degradable nanomaterials have been applied for versatile technical applications focusing packages, dye removal, drug delivery, and tissue engineering. This article may serve as a guide for upcoming field researchers to design and device new ways toward future ecofriendly commercial or industrial materials.

Author contributions

Kausar, A.: Conceptualization, data curation, writing of original draft preparation; Kausar, A., Ahmad, I., Aldaghri, O., Ibnaouf, K.H., Eisa, M.H., Lam, T.D.: review and editing. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research. The authors also appreciate the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) for supporting and supervising this project.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research through the project number IFP-IMSIU-2023132. The authors also appreciate the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) for supporting and supervising this project.