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Green polymer nanocomposites and their environmental applications
Green polymer nanocomposites and their environmental applications
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S.A. Bhawani*, A.H. Bhat†, F.B. Ahmad*, M.N.M. Ibrahim‡ *Universiti Malaysia Sarawak, Sarawak, Malaysia, †University Technology PETRONAS, Bandar Seri Iskandar, Malaysia, ‡Universiti Sains Malaysia, Penang, Malaysia
23.1
Introduction
The word green is referred to the biodegradable and renewable materials and thus offers a sustainable environment. The green materials have received a lot of attention because they are environmentally friendly and biodegradable. This is because of the demerits posed by the synthetic materials due to the degradation (chemical, physical, etc.) especially when used in human-related areas such as surgery, pharmacology, agriculture, and environment. This leads to open a new area of research to introduce the materials that are biodegradable and biocompatible and releases low toxicity degradation products. The use of plant-based materials (renewable sources) helps in minimizing the dependence on declining natural sources such as wood and oil for many applications. This research has provided an opportunity to improve the standard of human health [1]. Therefore, a paradigm shift takes place for the development of new materials from nonbiodegradable to completely biodegradable and will be easily disposed of or composited without harmful effects. These biodegradable materials are invaluable gifts to mankind from modern scientific technologies and are used in the products having single use and short life and being disposable. On the other hand, the introduction of composite materials in the polymer matrix has exhibited synergistic effects in their properties. The polymer composite materials have diverse applications and are used in pharmaceuticals, food packaging, transportation, entertainment, etc. The polymer nanocomposites have overcome the limitations of the performance of monolithic materials. Polymer nanocomposites are composed of nanoparticles or nanofillers disposed in the polymer matrix. These nanoparticles/ nanofillers have different shapes (platelets, fibers, and spheroids) with the size range of 1–50 nm. The introduction of nanoparticles in the polymer matrix has improved the properties of basic polymers in terms of mechanical properties (strength, elastic modulus, and dimensional stability) and permeability (gases and water). The polymer nanocomposites have offered new opportunities in the thermoplastic and rubber industry with enhanced properties. The major concern in the modern technological developments is the challenges/risks posed by the synthetic materials produced from aliphatic polyesters, polyester amides, polystyrene, etc. [2,3]. These challenges/risks Polymer-based Nanocomposites for Energy and Environmental Applications. https://doi.org/10.1016/B978-0-08-102262-7.00023-4 Copyright © 2018 Elsevier Ltd. All rights reserved.
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can be overcome by the use of polymer composites from renewable sources. These are the alternatives for the sustainable development of economically and ecologically green technologies [4]. The green polymer nanocomposites are expected to improve the speed of manufacturing, recycling, and compatibility [5]. The most interesting features of green polymer nanocomposites are eco-friendly, biodegradable, and sustainable. The basic necessity of green polymer nanocomposites is that the polymer should be green (renewable and biodegradable). The classification of biodegradable polymers is given in Table 23.1. The polymeric materials are divided into two major groups such as agricultural polymers and biopolyesters. These were further classified on the basis of their origin and the path of their production.
23.2
Processing methods for polymer nanocomposites
The processing methods for polymer nanocomposites are categorized into four major routes such as melt intercalation, template synthesis, exfoliation adsorption, and in situ polymerization intercalation [6-11]. The three different kinds of structures (exfoliated (Fig. 23.1), intercalated (Fig. 23.2), and unintercalated or aggregated (Fig. 23.3)) can be obtained on the basis of method and materials used.
23.2.1 Melt intercalation This method involved the annealing of polymer matrix at the higher temperature and then addition of filler and finally kneading the composite to obtain uniform distributions. This method is a standard approach for the production of thermoplastic polymer nanocomposites. It is considered as eco-friendly because no solvent is used in the process and is compatible with industrial processes like injection molding and extrusion. The processing conditions like surface modification of fillers, compatibility of filler, and polymer matrix played an important role in determining the quality of dispersion achieved. The relationship between the processing conditions and morphologies was well discussed by Alig et al. [12]. In the same way, Pavlidou and Papaspyrides [9] discussed the thermodynamics and effects of multiple conditions. The only disadvantage of this process is the use of higher temperature that can damage the surface modification of fillers. Therefore, the optimization of conditions is very important in order to achieve good dispersion and exfoliation. For example, operating at lower temperature or at thermally stable conditions can avoid degradation [7].
23.2.2 Exfoliation adsorption This method is based on the solvent in which polymer or prepolymer is solubilized and is also known as polymer or prepolymer intercalation from solution. This method is widely used for the synthesis of intercalated nanocomposites from water-soluble polymers with low or no porosity. In this method, the layered silicate is first swollen and dispersed in the solvent before mixing it with the polymer. After that, the polymer
Table 23.1
Classification of biodegradable polymers Biodegradable polymers Agricultural polymers
Polyesters/biopolyesters
Polysaccharides
Proteins
Lipids
From microbial synthesis
-Cellulose -Gums -Chitosan -Starch -Cotton -Wood, etc.
-Cassein -Whey -Zein -Soya -Gluten
-Cross-linked glycerides
-Polyhydroxyalkanoates (PHA) -Poly(hydroxybutyrate) -Bacterial cellulose -Xanthan -Curdlan -Pullulan
From chemical synthesis
From petrochemicals
-Polylactides -Polyacids -Poly(vinyl alcohols) -Polyesters
-Polycaprolactones -Homopolyesters -Aliphatic copolyesters -Aromatic copolyester
Fig. 23.1 Exfoliated structure.
Fig. 23.2 Intercalated structure.
chains intercalate, and the solvent is displaced from within the silicate interlayers, and eventually, a multilayered structure is obtained [6,8,11]. As compared with the melt intercalation, this method is considered as environmentally unfriendly because a large amount of solvent is used in this process.
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Fig. 23.3 Aggregated structure.
23.2.3 Emulsion polymerization In this method, monomers (styrene and methacrylic acid) are dispersed in water along with the emulsifier and different concentrations of silicates [6]. After the polymerization process, a nanocomposite is formed in the way that a part of silicate is embedded inside the polymer and a part is adsorbed on the particle surface.
23.2.4 In situ polymerization In this method, firstly, the filler is swelled in the liquid monomer or monomer solution because low-molecular-weight monomers seep in between the interlayers [6]. After that, polymerization of monomers takes place in between the interlayers to produce intercalated or exfoliated nanocomposites. This method is used for the preparation of thermoplastics and thermoset-based composites [9]. This method allows the grafting of polymer on filler surface that can improve the properties of final composite. The partially exfoliated structure can also be obtained by this method because of the good dispersion and intercalation of fillers in the polymer matrix. As stated by Abedi and Abdouss [10], it is the most suitable preparation method for the polyolefin/ clay nanocomposites because of the lack of rigorous thermodynamic requirement.
23.2.5 Template synthesis This is also known as sol-gel technology. This method involved the formation of inorganic filler in an aqueous solution or gel containing the polymer or the filler building blocks [6,7,9-11]. The polymer promotes the growth of the inorganic filler crystals and also serves as a nucleating agent. The nanocomposites are formed with the growth
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of crystals with polymers trapped within the layers. The double-layered hydroxidebased nanocomposites are mainly prepared by this method, and it is less developed for the synthesis of layered silicates. This process is not commonly used for the synthesis of nanocomposites because high temperature is used for the synthesis that degrades polymer and the aggregation tendency of the growing inorganic crystals [6,9].
23.2.6 Nontraditional methods Many nontraditional methods have also been used for the synthesis of nanocomposites because to facilitate the better dispersion of the filler in the polymer matrix in order to achieve improved properties of composites. Researchers have investigated different routes based on the traditional methods; for example, in situ polymerization can be customized to be redox [13,14], catalytic chain transfer [15], or photoinduced polymerization [16]. The other nontraditional methods used for the synthesis of nanocomposites are microwave-induced [17,18], one-pot synthesis [19-21], template-directed [22], electrochemical synthesis [23], self-assembly [24,25], and intermatrix synthesis [26,27]. In one-pot synthesis, a series of reactions are carried out in the same reactor. For example, synthesis of tin (Sn)-embedded carbon-silica polymer nanocomposites. The self-assembly involved the spontaneous arrangement of the existing components followed by the interaction among the components. Eventually, an ordered structure QQ can be obtained; for example, graphene-polymer composites were prepared by stacking. The intermatrix synthesis is used to prepare the polymer stabilized metal nanoparticles. In this method, the polymer matrix must have some functional groups capable of binding nanoparticles. The microwave-assisted synthesis has several advantages like rapid volumetric heating, high reaction time, increased reaction selectivity, and energy-saving behavior [17]. The cellulose’silver nanocomposites are prepared by using this method [17].
23.3
Different types of green polymer nanocomposites
23.3.1 Polylactic acid (PLA)-based green nanocomposites It is a thermoplastic and is derived from cornstarch by fermentation. Its basic unit is lactic acid that is polymerized to polylactic acid by step-wise polycondensation or via ring opening polymerization of a dilactide intermediate [28-31]. Several tests have been performed on the PLA and found that it is a suitable matrix for the production of nanocomposites. Several products are already established in the market; for example, Jacob winter (Satzung, Germany) produces biodegradable urns from flax and PLA by compression molding. Several papers have been published on the optimization of natural and man-made cellulose fiber-reinforced PLA composites. Kimura et al. [32] studied the tensile and bending strength and stiffness volume content (45%–65%) of ramie fiber-reinforced PLA. It was found that the large content of
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ramie would increase the strength considerably. Ochi [33] investigated the increase in tensile and bending strength and Young’s modulus up to 50% fiber content in case of kenaf/PLA composites, whereas Pan et al. [34] produced kenaf/PLA composites up to 30% content of fiber by melt mixing and injection molding and have improved tensile strength by 30%. Tokoro et al. [35] investigated three kinds of injection-molded bamboo fiber-reinforced PLA composites, and the highest bending strength was observed with steam-exploded fibers. The surface treatment (alkali, permanganate, peroxide, and silane) of fibers were studied for mechanical, thermal, and wear performance of jute/PLA composites. These treatments have resulted in the improvement of tensile and flexural properties and reduction in impact strength. It has been found that the silane-treated composites showed higher thermal stability [36]. The plasticizing PLA composites have markedly improved the mechanical properties. Masirek et al. [37] have prepared PLA composites with hemp fibers and PEG by compression molding. It was observed that Young’s modulus increased with the hemp content.
23.3.2 Biopolyester-based green composites This group includes the naturally occurring biodegradable polymers produced from a wide range of microorganisms such as poly (hydroxybutyrate-co-valerate) (PHBV) [38,39]. Their mechanical properties are comparable with those of traditional thermoplastics (polyethylene and polypropylene) [40,41]. The application of PHBV polymers is limited because of their high cost. The incorporation of filler or fibers is the possible way to make them affordable and will also improve their mechanical properties. Few studies are reported in the literature about the use of fillers such as clay, calcium carbonate [42], and wood fibers [43] to modify the properties of PHBV resins. The green composites of PHBV were prepared by combining pineapple fiber and PHBV with 20% and 30% weight content of fibers in a 0 degree/90 degree/ 0 degree fiber arrangement. It has been found that both the tensile strength and flexural strength were significantly higher as compared with PHBV virgin resin [44,45].
23.3.3 Starch-based nanocomposites Starch is natural biopolymer and is not a true thermoplastic but can be converted into a thermoplastic starch [46]. It is widely used to produce eco-friendly packing materials as an alternative material for petrochemical-based nonbiodegradable plastic materials. The thermoplastic starch behaves like a native starch with poor mechanical properties and is water-sensitive. The reinforcement of starch with nanoscale minerals has greatly improved properties without altering the biodegradability of the composites. The study conducted by De Carvalho et al. [47] and Park et al. [48] concluded that the better dispersion of clay in the thermoplastic starch matrix will produce better mechanical, thermal, and barrier properties. Park et al. [49] also reported that the tensile and water vapor barrier properties increased with the increased content of clay. In another attempt, Wilhelm et al. [50] synthesized nanocomposite films from reinforcement of glycerol-plasticized starch with Ca2+-hectorite clay and found that the storage modulus increased considerably above 25°C. Xu et al. [51] synthesized starch
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Polymer-based Nanocomposites for Energy and Environmental Applications
acetate nanocomposite foam by melt intercalation method with four organoclays and found that the glass transition temperature increased by 6–14°C depending on the type of clay. It was observed that the thermal stability and mechanical properties were also increased.
23.3.4 Cellulose-based nanocomposites Cellulose is naturally occurring biopolymer and is available abundantly. Cellulose is high-molecular-weight polymer and is highly crystalline. Cellulose is converted into derivative (cellulose ethers and cellulose esters) to make it more processable because of its infusibility. At present, cellulose acetate is widely used in a variety of applications ranging from fibers to films. Currently, interest has developed to synthesize nanocomposites with cellulose materials with enhanced mechanical, thermal, and permeability properties. It has been found that the films obtained from nanocomposite cellulose acetate and organic clay have showed a significant decrease in water vapor permeability [52]. The interesting results reported by Ruan et al. [53] that the regenerated cellulose/tourmaline composite films have showed antimicrobial activity against the Staphylococcus aureus. This could develop a potential use of these films for packaging materials.
23.3.5 Chitosan-based nanocomposites Chitosan is the second most abundant naturally occurring biopolymer after the cellulose. It is a partially deacetylated derivative of chitin. This has been extensively studied for its potential application in industry and packaging production because it is biodegradable, biocompatible, and nontoxic in nature, but the properties need to be enhanced by reinforcing with nanoparticles/nanofillers. Lin et al. [54] reported the preparation of nanocomposites of chitosan/montmorillonite (MMT) by solvent casting method. It has been observed that when chitosan was incorporated with potassium persulfate (KPS)-MMT, the tensile properties of nanocomposites largely depend on the amount of KPS incorporated in the MMT. The more MMT will be exfoliated along with the degradation of chitosan, which resulted in the increased Young’s modulus, but the tensile strength decreased. Xu et al. [55] also used the solvent casting method for the preparation of chitosan-based nanocomposite films with Na-MMT and Cloisite 30B. They found that the nanoclay exfoliated along with the chitosan matrix by addition of small amount of Na-MMT. It has been also reported that the tensile strength of these films increased and the elongation at the break decreased with the addition of clay. Wang et al. [56] used solvent intercalation method for the preparation of chitosan/MMT nanocomposites. It has been reported that an intercalated/exfoliated nanostructure was formed with the low content of MMT and also the thermal stability, hardness, and elastic modulus were improved by increasing the loading of clay in the matrix. Darder et al. [57] used solution intercalation method for the preparation of chitosan/MMT nanocomposites with varying amount of clay.
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23.3.6 Protein-based nanocomposites The soy protein is available widely and is least expensive as compared with other commercially available materials. It has been reported that the protein-based resins exhibited good adhesion with the plant-based fibers and produced composites with excellent mechanical properties as compared with the other hydrophobic biodegradable resins [58,59]. This is because of the presence of polar groups in the protein-based resins. Commercially, soy protein is available in different varieties such as soy protein isolate (SPI) (90%), soy protein concentrate (SPC) (70%), and soy flour (50%). Soy proteins are ductile and can undergo physical changes without any damage such as bending, torsional, and tensile deformations [60]. Soy proteins are able to form new materials with significantly improved mechanical and thermal properties after certain modifications [58,59,61-67]. Soy proteins can be modified by using various materials; for example, modification of SPI by stearic acid can enhance the moisture absorption resistance [58], and modification with glutaraldehyde and polyvinyl alcohol followed by fabrication of composites with flax yarns and fabrics can exhibit improved properties [64,65]. Otaigbe and Adams [68] used phosphate fillers for soy protein composites and achieved improved mechanical properties and water resistance. Rhim et al. [69] demonstrated that there was an increase in the tensile strength with enhanced water vapor permeability when SPI films were modified by bentonite or chitosan/MMT. An animal protein gelatin was used for the synthesis of biocomposites with MMT clay and has exhibited improved mechanical and water resistance.
23.3.7 Lipid-based nanocomposites Lipids are mainly used for the formation of edible films. The most commonly used lipid materials for the coating of food or drug surface are beeswax, carnauba wax, candelilla wax, triglycerides, fatty acids etc. These materials can provide moisture properties to the composite films. The composite films are composed of both lipid and hydrocolloid (proteins or polysaccharides). The advantages of such kind of films are as follows: When a water vapor barrier property is required, the lipid component can function, and when mechanical strength is required, then hydrocolloid component will do necessary function.
23.4
Applications of green polymer nanocomposites
Polymer nanocomposites introduces a new era of the polymer industry toward the sustainable environment. In other words, polymer nanocomposites from renewable sources have received an enormous attention from last two decades because of the environmental concerns and depleting energy sources [70]. On the other hand, green nanocomposites have overtaken the traditional composites in production, development, and usage applications. Green nanocomposites are widely used in a variety of applications in the development of products such as automotive parts, building block based on cellulose nanofibers, blades for vacuum cleaners, power tool housing
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[71], development of electronic devices [72], and packaging materials for food and pharmaceuticals [73,74], and PLA-based nanocomposites have been reported to be used in tissue engineering implant [75]. Some of the important applications are described briefly.
23.4.1 Biomedical applications Collagen has been widely used for many biomedical applications such as fermentation of organs and tissues. Collagen has been widely used as coating on Ti hard-tissue implants for the purpose of stimulating cellular response [76], increasing remolding [77], and improving bone growth and bone-implant contact [78]. Especially, films of collagen are mainly employed for the biomedical applications, for example, for the treatment of tissue infections (corneal tissues or liver tissues). In the form of a composite matrix, collagen can be used for the implant bone formation by combing with the recombinant human morphogenetic protein-2 [79]. Collagen sponges are useful for various types of wounds, mostly severe burns and are highly efficient materials for the recovery of skin and artificial skin incorporation of gelatin [80].
23.4.2 Tissue engineering Both synthetic and natural biodegradable polymers such as pullulan, collagen, and chitosan have been used for the tissue engineering [81,82]. For example, the conjugation of carboxymethyl pullulan with heparin can inhibit in vitro the proliferation of smooth muscle cells [83]. Suzuki et al. [84] have fabricated an artificial skin composed of a cellular bilayer in which silicon was as an outer layer and collagen sponges as an inner layer. Liu et al. [85] studied bionanocomposite films as scaffold materials based on chitosan with incorporated halloysite nanotubes. It was observed that these films exhibit a cytocompatible nature with a maximum of 10% loading of halloysite nanotubes. Hajiali et al. [86] prepared nanocomposite scaffold based on PHA incorporated with 10% bioglass nanoparticles for bone tissue engineering. Okamoto et al. [87] have extensively reviewed the contribution of synthetic biopolymers and their nanocomposites in tissue engineering applications.
23.4.3 UV protection It is a well-known fact that UV radiations are very harmful to all living beings. Many UV-ray protecting products have been fabricated such as sunscreen lotions/creams, sunglasses, window protectors, and cloths. Exposure to UV rays can have both short- and long-term effects. Many nonbiodegradable materials have been used for the preparation of various UV-block products but are available in very high costs [88,89]. Therefore, many bio-based materials have been used to produce products with low cost and biodegradable in nature. It has been reported that the cotton-based cellulosic fibers and lignin were used in natural organic coatings because they have certain levels of UV-absorbing capacity [90]. From the past few decades, many researchers have extracted nanocellulose from the natural sources (cotton, wood,
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etc.) due to its excellent strength, high surface area, and unique optical properties [91]. The films based on nanocellulose exhibit high transparency, outstanding UV protection, and biodegradable properties and are easily processable at higher temperatures as compared with plastic materials [92]. The nanocomposite films based on nanocellulose incorporated with ZnO nanoparticles have shown excellent UV protection, transparency, and sensitivity. But due to the high water-binding capacity of nanocellulose, the issues like dewatering problems and nanocellulose hybrid heterogeneous architect remained during the production process [92]. The bionanocomposites based on lignin have been reported to have excellent adhesive [93] and biodegradable properties and have been used as stabilizing agents [94]. The lignin-based nanocomposite films composed of CNCs have been used in various medical, biological, and electronic applications. The UV-absorbing capacity is an inherit property of lignin [90] and is therefore used in various coatings composed with cellulose at suitable cellulose-to-lignin ratio. Recently, chemically modified lignins such as lignosulfonates, Kraft lignin, and acetylated lignin have been developed [95] and are used for the fabrication of materials with improved mechanical strength, hydrophobicity, and oxygen barrier properties.
23.4.4 Food preservation Many synthetic polymers have been widely used in the packaging of foods because they exhibit permeability to various gases, while biopolymers showed high permeability to water vapor [96]. The incorporation of clay into the polymers produces a maze structure that greatly reduces the permeation of gases. Burdock [97] reported that the hydroxypropyl methylcellulose (HPMC) is a promising material for edible films or coatings. The HPMC matrix incorporated with chitosan nanofiller has shown improved properties such as mechanical strength, water vapor permeability, and reduced significantly oxygen permeability. Therefore, these HPMC-chitosan films have become promising food packaging films with good shelf life [97]. The films based on soy protein incorporated with MMT by ultrasonication have exhibited improved elastic modulus, tensile strength, and reduced water permeability [98]. The incorporation of rosemary oil in the chitosan/MMT nanocomposites produces active biomaterials for food packaging.
23.5
Conclusion
The development of eco-friendly materials with high performance is a dynamic process to produce materials with affordable costs. The importance of these materials has increased due to the decline of nonrenewable sources and most importantly their high costs and growing demands for clean environment. The unique characteristics such as biocompatibility and biodegradability of these materials have attracted many scientists, environmentalists and engineers to develop sustainable technologies for the production of green polymer nanocomposites. The production of novel materials with inherent eco-friendly nature such as renewability and biodegradability, a number
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polymers have been utilized ranging from thermoplastic starch, PLA, cellulose, chitosan, gelatin, etc. The biopolymer nanocomposites have been extensively used in various applications such as for the packaging of food, in biomedical science, and in tissue engineering. Smart and hygienic packaging materials have been produced to reduce the waste generation and enhance the shelf life of food products.
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[34] Pan P, Zhu B, Kai W, Serizawa S, Iji M, Inoue Y. Crystallization behavior and mechanical properties of bio-based green composites based on poly(lactide) and kenaf fiber. J Appl Polym Sci 2007;105(3):1511–20. [35] Tokoro R, Duc Minh V, Okubo K, Tanaka T, Fujii T, Fujiura T. How to improve mechanical properties of poly lactic acid with bamboo fibers. J Mater Sci 2008;43(2):775–87. [36] Goriparthi BK, Suman KNS, Rao NM. Effect of fiber surface treatments on mechanical and abrasive wear performance of polylactide/jute composites. Compos Part A Appl Sci Manuf 2012;43(10):1800–8. [37] Masirek R, Kulinski Z, Chionna D, Piorkowska E, Pracella M. Composites of poly (L-lactide) with hemp fibers: morphology and thermal and mechanical properties. J Appl Polym Sci 2007;105(1):255–68. [38] Chowdhury AA. Poly-β-hydroxybutters€aure abbauende Bakterien und Exoenzym. Arch Microbiol 1963;47(2):167–200. [39] Holmes PA, Collins SH, Wright LF. Process for separating nitrated phenolic compounds from other phenolic compounds. US Patent No. 4447654; 1984. [40] Barham PJ. Nucleation behaviour of poly-3-hydroxybutyrate. J Mater Sci 1984;19 (12):3826–34. [41] Barham PH, Leller A. The relationship between microstructure and mode of fracture in polyhydroxybutyrate. J Polym Sci Pol Phys 1986;24(1):69–77. [42] Holmes PA. Bassett DC, editor. Development in crystalline polymers II. London: Elsevier; 1988. p. 1–65. [43] Gatenholm P, Kubat J, Mathiasson A. Biodegradable natural composites. I. Processing and properties. J Appl Polym Sci 1992;45(9):1667–77. [44] Luo S, Netravali AN. Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrateco-valerate) resin. J Mater Sci 1999;34(15):3709–19. [45] Das K, Ray D, Banerjee C, Bandopadhyay NR, Sahoo S, Mohanty AK, et al. Physicomechanical and thermal properties of jute-nanofiber-reinforced biocopolyester composites. Ind Eng Chem Res 2010;49(6):2775–82. [46] Tomka I. Thermoplastic starch. Adv Exp Med Biol 1991;302:627–37. [47] De Carvalho AJF, Curvelo AAS, Agnellib JAM. A first insight on composites of thermoplastic starch and kaolin. Carbohydr Polym 2001;45(2):189–94. [48] Park H-M, Li X, Jin C-Z, Park C-Y, Cho W-J, Ha C-S. Preparation and properties of biodegradable thermoplastic starch/clay hybrids. Macromol Mater Eng 2002;287(8):553–8. [49] Park H-M, Lee W-K, Park C-Y, Cho W-J, Ha C-S. Environmentally friendly polymer hybrids. Part I: Mechanical, thermal, and barrier properties of thermoplastic starch/clay nanocomposites. J Mater Sci 2003;38(5):909–15. [50] Wilhelm H-M, Sierakowski M-R, Souza GP, Wypych F. Starch films reinforced with mineral clay. Carbohydr Polym 2003;52:101–10. [51] Xu Y, Zhou J, Hanna MA. Melt-intercalated starch acetate nanocomposite foams as affected by type of Organoclay. Cereal Chem 2005;82(1):105–10. [52] Park H-M, Liang X, Mohanty AK, Misra M, Drzal LT. Effect of compatibilizer on nanostructure of the biodegradable cellulose acetate/organoclay nanocomposites. Macromolecules 2004;37(24):9076–82. [53] Ruan D, Zhang L, Zhang Z, Xia X. Structure and properties of regenerated cellulose/ tourmaline nanocrystal composite films. J Polym Sci Part B Polym Phys 2003;42 (3):367–73. [54] Lin K-F, Hsu C-Y, Huang T-S, Chiu W-Y, Lee Y-H, Young T-H. A novel method to prepare chitosan/montmorillonite nanocomposites. J Appl Polym Sci 2005;98(5):2042–7.
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[76] Kim HW, Li LH, Lee EJ, Lee SH, Kim HE. Fibrillar assembly and stability of collagen coating on titanium for improved osteoblast responses. J Biomed Mater Res A 2005;75:629–38. [77] Rammelt S, Illert T, Bierbaum S, Scharnweber D, Zwipp H, Schneiders W. Coating of titanium implants with collagen, RGD peptide and chondroitin sulfate. Biomaterials 2006;27:5561–71. [78] Morra M, Cassinelli C, Meda L, Fini M, Giavaresi G, Giardino R. Surface analysis and effects on interfacial bone microhardness of collagen-coated titanium implants: a rabbit model. Int J Oral Maxillofac Implants 2005;20:23–30. [79] Murata M, Maki F, Sato D, Shibata T, Arisue M. Bone augmentation by onlay implant using recombinant human BMP-2 and collagen on adult rat skull without periosteum. Clin Oral Implants Res 2000;11:289–95. [80] Koide M, Osaki K, Konishi J, Oyamada K, Katakura T, Takahashi A, et al. A new type of biomaterial for artificial skin: dehydrothermally cross-linked composites of fibrillar and denatured collagens. J Biomed Mater Res 1993;27:79–87. [81] Chen G-Q, Wu Q. Microbial production and applications of chiral hydroxyalkanoates. Appl Microbiol Biotechnol 2005;67:592–9. [82] Williams SF, Martin DP, Horowitz DM, Peoples OP. PHA applications: addressing the price performance issue. I. Tissue engineering. Int J Biol Macromol 1999;25:111–21. [83] Na K, Shin D, Yun K, Park K-H, Lee KC. Conjugation of heparin into carboxylated pullulan derivatives as an extracellular matrix for endothelial cell culture. Biotechnol Lett 2003;25:381–5. [84] Suzuki S, Kawai K, Ashoori F, Morimoto N, Nishimura Y, Ikada Y. Long-term follow-up study of artificial dermis composed of outer silicone layerand inner collagen sponge. Br J Plast Surg 2000;53:659–66. [85] Liu M, Zhang Y, Wu C, Xiong S, Zhou C. Chitosan/halloysite nanotubes bionanocomposites: structure, mechanical properties and biocompatibility. Int J Biol Macromol 2012;51:566–75. [86] Hajiali H, Karbasi S, Hosseinalipour M, Rezaie HR. Preparation of a novel biodegradable nanocomposite scaffold based on poly(3-hydroxybutyrate)/bioglass nanoparticles for bone tissue engineering. J Mater Sci Mater Med 2010;21:2125–32. [87] Okamoto M, John B. Synthetic biopolymer nanocomposites for tissue engineering scaffolds. Prog Polym Sci 2013;38:1487–503. [88] Saravanan DUV. Protection textile materials. AUTEX Res J 2007;7:53–62. [89] Smith GJ, Miller IJ, Clare JF, Diffey BL. The effect of UV absorbing sunscreens on the reflectance and the consequent protection of skin. Photochem Photobiol 2002;75:122–5. [90] Hambardzumyan A, Foulon L, Chabbert B, Aguie-Beghin VR. Natural organic UV absorbent coatings based on cellulose and lignin: designed effects on spectroscopic properties. Biomacromolecules 2012;13:4081–8. [91] Song Q, Winter WT, Bujanovic BM, Amidon TE. Nanofibrillated cellulose (NFC): a high value co-product that improves the economics of cellulosic ethanol production. Energies 2014;7:607–18. [92] Jiang Y, Song Y, Miao M, Cao S, Feng X, Fang J, et al. Transparent nanocellulose hybrid films functionalized with ZnO nanostructures for UV-blocking. J Mater Chem C 2015;3:6717–24. [93] Hoareau W, Oliveira FB, Grelier S, Siegmund B, Frollini E, Castellan A. Fiberboards based on sugarcane bagasse lignin and fibers. Macromol Mater Eng 2006;291:829–39. [94] Cerrutti B, de Souza C, Castellan A, Ruggiero R, Frollini E. Carboxymethyl lignin as stabilizing agent in aqueous ceramic suspensions. Ind Crop Prod 2012;36:108–15.
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23
S.A. Bhawani*, A.H. Bhat†, F.B. Ahmad*, M.N.M. Ibrahim‡ *Universiti Malaysia Sarawak, Sarawak, Malaysia, †University Technology PETRONAS, Bandar Seri Iskandar, Malaysia, ‡Universiti Sains Malaysia, Penang, Malaysia
23.1
Introduction
The word green is referred to the biodegradable and renewable materials and thus offers a sustainable environment. The green materials have received a lot of attention because they are environmentally friendly and biodegradable. This is because of the demerits posed by the synthetic materials due to the degradation (chemical, physical, etc.) especially when used in human-related areas such as surgery, pharmacology, agriculture, and environment. This leads to open a new area of research to introduce the materials that are biodegradable and biocompatible and releases low toxicity degradation products. The use of plant-based materials (renewable sources) helps in minimizing the dependence on declining natural sources such as wood and oil for many applications. This research has provided an opportunity to improve the standard of human health [1]. Therefore, a paradigm shift takes place for the development of new materials from nonbiodegradable to completely biodegradable and will be easily disposed of or composited without harmful effects. These biodegradable materials are invaluable gifts to mankind from modern scientific technologies and are used in the products having single use and short life and being disposable. On the other hand, the introduction of composite materials in the polymer matrix has exhibited synergistic effects in their properties. The polymer composite materials have diverse applications and are used in pharmaceuticals, food packaging, transportation, entertainment, etc. The polymer nanocomposites have overcome the limitations of the performance of monolithic materials. Polymer nanocomposites are composed of nanoparticles or nanofillers disposed in the polymer matrix. These nanoparticles/ nanofillers have different shapes (platelets, fibers, and spheroids) with the size range of 1–50 nm. The introduction of nanoparticles in the polymer matrix has improved the properties of basic polymers in terms of mechanical properties (strength, elastic modulus, and dimensional stability) and permeability (gases and water). The polymer nanocomposites have offered new opportunities in the thermoplastic and rubber industry with enhanced properties. The major concern in the modern technological developments is the challenges/risks posed by the synthetic materials produced from aliphatic polyesters, polyester amides, polystyrene, etc. [2,3]. These challenges/risks Polymer-based Nanocomposites for Energy and Environmental Applications. https://doi.org/10.1016/B978-0-08-102262-7.00023-4 Copyright © 2018 Elsevier Ltd. All rights reserved.
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can be overcome by the use of polymer composites from renewable sources. These are the alternatives for the sustainable development of economically and ecologically green technologies [4]. The green polymer nanocomposites are expected to improve the speed of manufacturing, recycling, and compatibility [5]. The most interesting features of green polymer nanocomposites are eco-friendly, biodegradable, and sustainable. The basic necessity of green polymer nanocomposites is that the polymer should be green (renewable and biodegradable). The classification of biodegradable polymers is given in Table 23.1. The polymeric materials are divided into two major groups such as agricultural polymers and biopolyesters. These were further classified on the basis of their origin and the path of their production.
23.2
Processing methods for polymer nanocomposites
The processing methods for polymer nanocomposites are categorized into four major routes such as melt intercalation, template synthesis, exfoliation adsorption, and in situ polymerization intercalation [6-11]. The three different kinds of structures (exfoliated (Fig. 23.1), intercalated (Fig. 23.2), and unintercalated or aggregated (Fig. 23.3)) can be obtained on the basis of method and materials used.
23.2.1 Melt intercalation This method involved the annealing of polymer matrix at the higher temperature and then addition of filler and finally kneading the composite to obtain uniform distributions. This method is a standard approach for the production of thermoplastic polymer nanocomposites. It is considered as eco-friendly because no solvent is used in the process and is compatible with industrial processes like injection molding and extrusion. The processing conditions like surface modification of fillers, compatibility of filler, and polymer matrix played an important role in determining the quality of dispersion achieved. The relationship between the processing conditions and morphologies was well discussed by Alig et al. [12]. In the same way, Pavlidou and Papaspyrides [9] discussed the thermodynamics and effects of multiple conditions. The only disadvantage of this process is the use of higher temperature that can damage the surface modification of fillers. Therefore, the optimization of conditions is very important in order to achieve good dispersion and exfoliation. For example, operating at lower temperature or at thermally stable conditions can avoid degradation [7].
23.2.2 Exfoliation adsorption This method is based on the solvent in which polymer or prepolymer is solubilized and is also known as polymer or prepolymer intercalation from solution. This method is widely used for the synthesis of intercalated nanocomposites from water-soluble polymers with low or no porosity. In this method, the layered silicate is first swollen and dispersed in the solvent before mixing it with the polymer. After that, the polymer
Table 23.1
Classification of biodegradable polymers Biodegradable polymers Agricultural polymers
Polyesters/biopolyesters
Polysaccharides
Proteins
Lipids
From microbial synthesis
-Cellulose -Gums -Chitosan -Starch -Cotton -Wood, etc.
-Cassein -Whey -Zein -Soya -Gluten
-Cross-linked glycerides
-Polyhydroxyalkanoates (PHA) -Poly(hydroxybutyrate) -Bacterial cellulose -Xanthan -Curdlan -Pullulan
From chemical synthesis
From petrochemicals
-Polylactides -Polyacids -Poly(vinyl alcohols) -Polyesters
-Polycaprolactones -Homopolyesters -Aliphatic copolyesters -Aromatic copolyester
Fig. 23.1 Exfoliated structure.
Fig. 23.2 Intercalated structure.
chains intercalate, and the solvent is displaced from within the silicate interlayers, and eventually, a multilayered structure is obtained [6,8,11]. As compared with the melt intercalation, this method is considered as environmentally unfriendly because a large amount of solvent is used in this process.
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Fig. 23.3 Aggregated structure.
23.2.3 Emulsion polymerization In this method, monomers (styrene and methacrylic acid) are dispersed in water along with the emulsifier and different concentrations of silicates [6]. After the polymerization process, a nanocomposite is formed in the way that a part of silicate is embedded inside the polymer and a part is adsorbed on the particle surface.
23.2.4 In situ polymerization In this method, firstly, the filler is swelled in the liquid monomer or monomer solution because low-molecular-weight monomers seep in between the interlayers [6]. After that, polymerization of monomers takes place in between the interlayers to produce intercalated or exfoliated nanocomposites. This method is used for the preparation of thermoplastics and thermoset-based composites [9]. This method allows the grafting of polymer on filler surface that can improve the properties of final composite. The partially exfoliated structure can also be obtained by this method because of the good dispersion and intercalation of fillers in the polymer matrix. As stated by Abedi and Abdouss [10], it is the most suitable preparation method for the polyolefin/ clay nanocomposites because of the lack of rigorous thermodynamic requirement.
23.2.5 Template synthesis This is also known as sol-gel technology. This method involved the formation of inorganic filler in an aqueous solution or gel containing the polymer or the filler building blocks [6,7,9-11]. The polymer promotes the growth of the inorganic filler crystals and also serves as a nucleating agent. The nanocomposites are formed with the growth
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of crystals with polymers trapped within the layers. The double-layered hydroxidebased nanocomposites are mainly prepared by this method, and it is less developed for the synthesis of layered silicates. This process is not commonly used for the synthesis of nanocomposites because high temperature is used for the synthesis that degrades polymer and the aggregation tendency of the growing inorganic crystals [6,9].
23.2.6 Nontraditional methods Many nontraditional methods have also been used for the synthesis of nanocomposites because to facilitate the better dispersion of the filler in the polymer matrix in order to achieve improved properties of composites. Researchers have investigated different routes based on the traditional methods; for example, in situ polymerization can be customized to be redox [13,14], catalytic chain transfer [15], or photoinduced polymerization [16]. The other nontraditional methods used for the synthesis of nanocomposites are microwave-induced [17,18], one-pot synthesis [19-21], template-directed [22], electrochemical synthesis [23], self-assembly [24,25], and intermatrix synthesis [26,27]. In one-pot synthesis, a series of reactions are carried out in the same reactor. For example, synthesis of tin (Sn)-embedded carbon-silica polymer nanocomposites. The self-assembly involved the spontaneous arrangement of the existing components followed by the interaction among the components. Eventually, an ordered structure QQ can be obtained; for example, graphene-polymer composites were prepared by stacking. The intermatrix synthesis is used to prepare the polymer stabilized metal nanoparticles. In this method, the polymer matrix must have some functional groups capable of binding nanoparticles. The microwave-assisted synthesis has several advantages like rapid volumetric heating, high reaction time, increased reaction selectivity, and energy-saving behavior [17]. The cellulose’silver nanocomposites are prepared by using this method [17].
23.3
Different types of green polymer nanocomposites
23.3.1 Polylactic acid (PLA)-based green nanocomposites It is a thermoplastic and is derived from cornstarch by fermentation. Its basic unit is lactic acid that is polymerized to polylactic acid by step-wise polycondensation or via ring opening polymerization of a dilactide intermediate [28-31]. Several tests have been performed on the PLA and found that it is a suitable matrix for the production of nanocomposites. Several products are already established in the market; for example, Jacob winter (Satzung, Germany) produces biodegradable urns from flax and PLA by compression molding. Several papers have been published on the optimization of natural and man-made cellulose fiber-reinforced PLA composites. Kimura et al. [32] studied the tensile and bending strength and stiffness volume content (45%–65%) of ramie fiber-reinforced PLA. It was found that the large content of
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ramie would increase the strength considerably. Ochi [33] investigated the increase in tensile and bending strength and Young’s modulus up to 50% fiber content in case of kenaf/PLA composites, whereas Pan et al. [34] produced kenaf/PLA composites up to 30% content of fiber by melt mixing and injection molding and have improved tensile strength by 30%. Tokoro et al. [35] investigated three kinds of injection-molded bamboo fiber-reinforced PLA composites, and the highest bending strength was observed with steam-exploded fibers. The surface treatment (alkali, permanganate, peroxide, and silane) of fibers were studied for mechanical, thermal, and wear performance of jute/PLA composites. These treatments have resulted in the improvement of tensile and flexural properties and reduction in impact strength. It has been found that the silane-treated composites showed higher thermal stability [36]. The plasticizing PLA composites have markedly improved the mechanical properties. Masirek et al. [37] have prepared PLA composites with hemp fibers and PEG by compression molding. It was observed that Young’s modulus increased with the hemp content.
23.3.2 Biopolyester-based green composites This group includes the naturally occurring biodegradable polymers produced from a wide range of microorganisms such as poly (hydroxybutyrate-co-valerate) (PHBV) [38,39]. Their mechanical properties are comparable with those of traditional thermoplastics (polyethylene and polypropylene) [40,41]. The application of PHBV polymers is limited because of their high cost. The incorporation of filler or fibers is the possible way to make them affordable and will also improve their mechanical properties. Few studies are reported in the literature about the use of fillers such as clay, calcium carbonate [42], and wood fibers [43] to modify the properties of PHBV resins. The green composites of PHBV were prepared by combining pineapple fiber and PHBV with 20% and 30% weight content of fibers in a 0 degree/90 degree/ 0 degree fiber arrangement. It has been found that both the tensile strength and flexural strength were significantly higher as compared with PHBV virgin resin [44,45].
23.3.3 Starch-based nanocomposites Starch is natural biopolymer and is not a true thermoplastic but can be converted into a thermoplastic starch [46]. It is widely used to produce eco-friendly packing materials as an alternative material for petrochemical-based nonbiodegradable plastic materials. The thermoplastic starch behaves like a native starch with poor mechanical properties and is water-sensitive. The reinforcement of starch with nanoscale minerals has greatly improved properties without altering the biodegradability of the composites. The study conducted by De Carvalho et al. [47] and Park et al. [48] concluded that the better dispersion of clay in the thermoplastic starch matrix will produce better mechanical, thermal, and barrier properties. Park et al. [49] also reported that the tensile and water vapor barrier properties increased with the increased content of clay. In another attempt, Wilhelm et al. [50] synthesized nanocomposite films from reinforcement of glycerol-plasticized starch with Ca2+-hectorite clay and found that the storage modulus increased considerably above 25°C. Xu et al. [51] synthesized starch
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acetate nanocomposite foam by melt intercalation method with four organoclays and found that the glass transition temperature increased by 6–14°C depending on the type of clay. It was observed that the thermal stability and mechanical properties were also increased.
23.3.4 Cellulose-based nanocomposites Cellulose is naturally occurring biopolymer and is available abundantly. Cellulose is high-molecular-weight polymer and is highly crystalline. Cellulose is converted into derivative (cellulose ethers and cellulose esters) to make it more processable because of its infusibility. At present, cellulose acetate is widely used in a variety of applications ranging from fibers to films. Currently, interest has developed to synthesize nanocomposites with cellulose materials with enhanced mechanical, thermal, and permeability properties. It has been found that the films obtained from nanocomposite cellulose acetate and organic clay have showed a significant decrease in water vapor permeability [52]. The interesting results reported by Ruan et al. [53] that the regenerated cellulose/tourmaline composite films have showed antimicrobial activity against the Staphylococcus aureus. This could develop a potential use of these films for packaging materials.
23.3.5 Chitosan-based nanocomposites Chitosan is the second most abundant naturally occurring biopolymer after the cellulose. It is a partially deacetylated derivative of chitin. This has been extensively studied for its potential application in industry and packaging production because it is biodegradable, biocompatible, and nontoxic in nature, but the properties need to be enhanced by reinforcing with nanoparticles/nanofillers. Lin et al. [54] reported the preparation of nanocomposites of chitosan/montmorillonite (MMT) by solvent casting method. It has been observed that when chitosan was incorporated with potassium persulfate (KPS)-MMT, the tensile properties of nanocomposites largely depend on the amount of KPS incorporated in the MMT. The more MMT will be exfoliated along with the degradation of chitosan, which resulted in the increased Young’s modulus, but the tensile strength decreased. Xu et al. [55] also used the solvent casting method for the preparation of chitosan-based nanocomposite films with Na-MMT and Cloisite 30B. They found that the nanoclay exfoliated along with the chitosan matrix by addition of small amount of Na-MMT. It has been also reported that the tensile strength of these films increased and the elongation at the break decreased with the addition of clay. Wang et al. [56] used solvent intercalation method for the preparation of chitosan/MMT nanocomposites. It has been reported that an intercalated/exfoliated nanostructure was formed with the low content of MMT and also the thermal stability, hardness, and elastic modulus were improved by increasing the loading of clay in the matrix. Darder et al. [57] used solution intercalation method for the preparation of chitosan/MMT nanocomposites with varying amount of clay.
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23.3.6 Protein-based nanocomposites The soy protein is available widely and is least expensive as compared with other commercially available materials. It has been reported that the protein-based resins exhibited good adhesion with the plant-based fibers and produced composites with excellent mechanical properties as compared with the other hydrophobic biodegradable resins [58,59]. This is because of the presence of polar groups in the protein-based resins. Commercially, soy protein is available in different varieties such as soy protein isolate (SPI) (90%), soy protein concentrate (SPC) (70%), and soy flour (50%). Soy proteins are ductile and can undergo physical changes without any damage such as bending, torsional, and tensile deformations [60]. Soy proteins are able to form new materials with significantly improved mechanical and thermal properties after certain modifications [58,59,61-67]. Soy proteins can be modified by using various materials; for example, modification of SPI by stearic acid can enhance the moisture absorption resistance [58], and modification with glutaraldehyde and polyvinyl alcohol followed by fabrication of composites with flax yarns and fabrics can exhibit improved properties [64,65]. Otaigbe and Adams [68] used phosphate fillers for soy protein composites and achieved improved mechanical properties and water resistance. Rhim et al. [69] demonstrated that there was an increase in the tensile strength with enhanced water vapor permeability when SPI films were modified by bentonite or chitosan/MMT. An animal protein gelatin was used for the synthesis of biocomposites with MMT clay and has exhibited improved mechanical and water resistance.
23.3.7 Lipid-based nanocomposites Lipids are mainly used for the formation of edible films. The most commonly used lipid materials for the coating of food or drug surface are beeswax, carnauba wax, candelilla wax, triglycerides, fatty acids etc. These materials can provide moisture properties to the composite films. The composite films are composed of both lipid and hydrocolloid (proteins or polysaccharides). The advantages of such kind of films are as follows: When a water vapor barrier property is required, the lipid component can function, and when mechanical strength is required, then hydrocolloid component will do necessary function.
23.4
Applications of green polymer nanocomposites
Polymer nanocomposites introduces a new era of the polymer industry toward the sustainable environment. In other words, polymer nanocomposites from renewable sources have received an enormous attention from last two decades because of the environmental concerns and depleting energy sources [70]. On the other hand, green nanocomposites have overtaken the traditional composites in production, development, and usage applications. Green nanocomposites are widely used in a variety of applications in the development of products such as automotive parts, building block based on cellulose nanofibers, blades for vacuum cleaners, power tool housing
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[71], development of electronic devices [72], and packaging materials for food and pharmaceuticals [73,74], and PLA-based nanocomposites have been reported to be used in tissue engineering implant [75]. Some of the important applications are described briefly.
23.4.1 Biomedical applications Collagen has been widely used for many biomedical applications such as fermentation of organs and tissues. Collagen has been widely used as coating on Ti hard-tissue implants for the purpose of stimulating cellular response [76], increasing remolding [77], and improving bone growth and bone-implant contact [78]. Especially, films of collagen are mainly employed for the biomedical applications, for example, for the treatment of tissue infections (corneal tissues or liver tissues). In the form of a composite matrix, collagen can be used for the implant bone formation by combing with the recombinant human morphogenetic protein-2 [79]. Collagen sponges are useful for various types of wounds, mostly severe burns and are highly efficient materials for the recovery of skin and artificial skin incorporation of gelatin [80].
23.4.2 Tissue engineering Both synthetic and natural biodegradable polymers such as pullulan, collagen, and chitosan have been used for the tissue engineering [81,82]. For example, the conjugation of carboxymethyl pullulan with heparin can inhibit in vitro the proliferation of smooth muscle cells [83]. Suzuki et al. [84] have fabricated an artificial skin composed of a cellular bilayer in which silicon was as an outer layer and collagen sponges as an inner layer. Liu et al. [85] studied bionanocomposite films as scaffold materials based on chitosan with incorporated halloysite nanotubes. It was observed that these films exhibit a cytocompatible nature with a maximum of 10% loading of halloysite nanotubes. Hajiali et al. [86] prepared nanocomposite scaffold based on PHA incorporated with 10% bioglass nanoparticles for bone tissue engineering. Okamoto et al. [87] have extensively reviewed the contribution of synthetic biopolymers and their nanocomposites in tissue engineering applications.
23.4.3 UV protection It is a well-known fact that UV radiations are very harmful to all living beings. Many UV-ray protecting products have been fabricated such as sunscreen lotions/creams, sunglasses, window protectors, and cloths. Exposure to UV rays can have both short- and long-term effects. Many nonbiodegradable materials have been used for the preparation of various UV-block products but are available in very high costs [88,89]. Therefore, many bio-based materials have been used to produce products with low cost and biodegradable in nature. It has been reported that the cotton-based cellulosic fibers and lignin were used in natural organic coatings because they have certain levels of UV-absorbing capacity [90]. From the past few decades, many researchers have extracted nanocellulose from the natural sources (cotton, wood,
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etc.) due to its excellent strength, high surface area, and unique optical properties [91]. The films based on nanocellulose exhibit high transparency, outstanding UV protection, and biodegradable properties and are easily processable at higher temperatures as compared with plastic materials [92]. The nanocomposite films based on nanocellulose incorporated with ZnO nanoparticles have shown excellent UV protection, transparency, and sensitivity. But due to the high water-binding capacity of nanocellulose, the issues like dewatering problems and nanocellulose hybrid heterogeneous architect remained during the production process [92]. The bionanocomposites based on lignin have been reported to have excellent adhesive [93] and biodegradable properties and have been used as stabilizing agents [94]. The lignin-based nanocomposite films composed of CNCs have been used in various medical, biological, and electronic applications. The UV-absorbing capacity is an inherit property of lignin [90] and is therefore used in various coatings composed with cellulose at suitable cellulose-to-lignin ratio. Recently, chemically modified lignins such as lignosulfonates, Kraft lignin, and acetylated lignin have been developed [95] and are used for the fabrication of materials with improved mechanical strength, hydrophobicity, and oxygen barrier properties.
23.4.4 Food preservation Many synthetic polymers have been widely used in the packaging of foods because they exhibit permeability to various gases, while biopolymers showed high permeability to water vapor [96]. The incorporation of clay into the polymers produces a maze structure that greatly reduces the permeation of gases. Burdock [97] reported that the hydroxypropyl methylcellulose (HPMC) is a promising material for edible films or coatings. The HPMC matrix incorporated with chitosan nanofiller has shown improved properties such as mechanical strength, water vapor permeability, and reduced significantly oxygen permeability. Therefore, these HPMC-chitosan films have become promising food packaging films with good shelf life [97]. The films based on soy protein incorporated with MMT by ultrasonication have exhibited improved elastic modulus, tensile strength, and reduced water permeability [98]. The incorporation of rosemary oil in the chitosan/MMT nanocomposites produces active biomaterials for food packaging.
23.5
Conclusion
The development of eco-friendly materials with high performance is a dynamic process to produce materials with affordable costs. The importance of these materials has increased due to the decline of nonrenewable sources and most importantly their high costs and growing demands for clean environment. The unique characteristics such as biocompatibility and biodegradability of these materials have attracted many scientists, environmentalists and engineers to develop sustainable technologies for the production of green polymer nanocomposites. The production of novel materials with inherent eco-friendly nature such as renewability and biodegradability, a number
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polymers have been utilized ranging from thermoplastic starch, PLA, cellulose, chitosan, gelatin, etc. The biopolymer nanocomposites have been extensively used in various applications such as for the packaging of food, in biomedical science, and in tissue engineering. Smart and hygienic packaging materials have been produced to reduce the waste generation and enhance the shelf life of food products.
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