8.1 Introduction

Biopolymers can be divided into three different classes based on their production method, as shown in Fig. 8.1 (Petersen et al. 1999). Starch and cellulose derivatives, as well as, polylactic acid (PLA), and polyhydroxy alkanoates (PHA), are some of the most studied biopolymers suitable for packaging. The inferior property profiles of biopolymers compared to commercial thermoplastic polymers urge the need for property enhancements for various applications. A notable increase in the mechanical, thermal, rheological, and gas barrier properties of biopolymers have resulted from inclusion of a large variety of fillers like layered silicates, nanotubes, etc. (Mark 1996; Favier et al. 1997; Ray et al. 2003, 2007; Mittal 2007; Mittal 2008; Luecha et al. 2010). Different functional groups in biopolymers have distinct and divergent properties which affect the interaction with different nanofillers, and in turn results in different levels of property enhancements and diverse applications. The high surface area, high aspect ratio, and high strength of graphene provide it with the superior ability to surpass other fillers and significantly improve the functional properties of polymers.

Fig. 8.1
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Schematic overview of classification of biopolymers

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Therefore, the focus of this chapter will be to discuss the different aspects of biopolymer-graphene nanocomposite fabrication, characterization, and property enhancement for biodegradable packaging applications. In addition, in this chapter, there will be a brief overview of the different clay, polysaccharide-based nanoparticles, and antimicrobial nanoparticles, and how their addition and interaction contribute in improving the different properties of biopolymers. This chapter will help the readers by comparing all relevant information encompassing the complete dimension of research on biopolymer functionalization and will contribute in understanding the basics for smart, biodegradable environment-friendly packaging.

8.2 Basic Principles of Reinforcement

The basic principle of effective polymer nanocomposite fabrication and reinforcement lies in good geometrical and aspect ratio, abundant functional groups in the filler, matching polarity between polymer and filler, in addition to, optimal interfacial interaction (Terzopoulou et al. 2015). The method of nano-functionalization of biopolymers can be classified into three categories, and these are (1) solution intercalation, (2) melt intercalation, and (3) in situ polymerization. For solvent casting, the dispersibility of the nanofiller in the solvent is equally important as the compatibility of the functional groups of the nanofillers and the matrix. Uniform dispersion during fabrication is very important, and it is dependent on proper interaction and interfacial bonding between the nanomaterial and the biopolymer. During composite formation, nanomaterials undergo three different types of dispersions in the polymer matrix, which can be labeled as (i) Tactoid dispersion, (ii) Intercalated dispersion, and (iii) Exfoliated dispersion. The tactoid dispersion is mainly observed in macrocomposites, where the polymer and the filler remain immiscible, and thus, clusters of fillers distribute throughout the polymer matrix, which ultimately results in little or no improvement of properties of the macrocomposites (Luduena et al. 2007; Alexandre et al. 2009). Nanofillers and polymer matrices usually undergo the two other types of interaction. In the intercalated dispersion, the interlayer spacing of the nanofiller (clay, carbon nanotubes graphene derivative, etc.) is penetrated by the polymer chain during composite formation, by means of mixing or sonication, creating an alternating layer of polymer/inorganic layer maintaining a distance of a few nanometers (Weiss et al. 2006). The third type of interaction “Exfoliation” is rather an extreme type of intercalation where the nanofiller layers are completely separated and distributed throughout the polymer matrix, as shown in Fig. 8.2 (Luduena et al. 2007). Exfoliation results in the best property reinforcement as the nanofiller distributed throughout the matrix forms a continuous network, which enables it to transfer its mechanical energy to the polymer and prevent rupture. In addition to this, the network formed creates a barrier against volatile components during thermal combustion and delays the escape of gases like oxygen, nitrogen, and water vapor (Fig. 8.3) and thus improves mechanical, thermal, and permeability properties.

Fig. 8.2
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Types of composite derived from interaction between nanomaterials and polymers: a tactoid dispersion creating phase-separated microcomposite; b intercalated dispersion; and c exfoliated dispersion

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Fig. 8.3
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Schematic representation of tortuous pathway created by dispersion of nanofillers in polymer matrix and the delayed escape of water vapor from the nanocomposite film. Adapted from Adame and Beall (2009)

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Depending on origin, function, and properties, the nanomaterial used for reinforcement of biopolymer used in packaging can be divided into four categories as shown in Fig. 8.4. Each category with fabrication, reaction mechanism, characterization will be briefly discussed in the following sections.

Fig. 8.4
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Classification of nanomaterials used for biopolymer nanocomposite fabrication for packaging application

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8.3 Carbon Nanomaterial-Based Reinforcement

8.3.1 Graphene-Based Functionalization

Despite the fact that the historical backdrop of graphene follows back to 1840, room temperature stable graphene did not come into existence until 2004, when Geim and Novoselov produced room temperature stable graphene (Novoselov et al. 2004) using the simple scotch tape method, for the first time which led to a Nobel Prize in 2010. A comparison among the different derivatives of graphene like graphene oxide (GO), reduced graphene oxide (RGO), graphene nanoplatelets (GNP); GO has found more application in biopolymer nanocomposite fields, due to the rich number of functional groups like carboxyl, epoxide, hydroxyl, etc.

The unit structure of graphene consists of a hexagonal cell, and a repeating pattern of these hexagonal cells creates a honeycomb structure. Graphene was originally defined as a single carbon atom thick sheet (Boehm 2010), but now the definition has been expanded to include up to 10 carbon layers. However, when more than 10 layers of carbon atoms are stacked on top of each other, it is known as graphite nanoplatelet (GNP) or exfoliated graphite nanoplatelet (xGNP) (Chung 2016). There are also graphene nanosheets (GNS), which are a slightly thinner version of GNP (Shokrieh et al. 2013). GNP or GNS has a low number of carboxyl, epoxy, and hydroxyl groups and therefore do not participate in polar interactions or hydrogen bonding and therefore they are considered to be hydrophobic.

GO has proven to be very useful due to its numerous and diverse functional groups that easily bond with other materials, and therefore find numerous applications in nanocomposites. The Lerf–Klinowski model for the structure of GO shows that the hydroxyl and epoxy (1,2-ether) functional groups in the carbon plane of GO enable hydrogen bonding with hydrophilic polymers (Lerf et al. 1998). Aside from the Lerf–Klinowski model, there are other models like the Hoffman model, Ruess model, Nakajima–Matsuo model, etc., which describe the structure of GO, as depicted in Fig. 8.5.

Fig. 8.5
figure 5

Summary of proposed structural models of GO, including recent examples (top Lerf–Klinowski and Dékány models) as well as earlier examples (bottom Nakajima–Matsuo, Hofmann, Ruess, and Scholz–Boehm models). Reprinted with permission from Szabó et al. (2006). Copyright 2006, American Chemical Society

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8.3.1.1 Synthesis of Graphene Oxide and Derivatives

The fabrication techniques for pristine graphene include chemical vapor deposition (CVD) (Dervishi et al. 2011), adhesion tape assisted repeated peeling of graphene layers from graphite (Singh et al. 2011), unzipping of multi-wall carbon nanotubes (MWCNTs), micro-mechanical exfoliation (Jayasena et al. 2013), mechanical exfoliation with ultrasonication (Guan et al. 2016), growth on crystalline silicon carbide, etc. The two major processes for fabrication of graphene-based nanofillers are (i) Exfoliation of graphite (ii) Oxidation of graphite.

8.3.1.1.1 Exfoliation of Graphite

The first step includes increasing the interlayer spacing between graphite using a mixture of sulfuric and nitric acid. This step is called intercalation, and the product of this step is called graphite intercalated compound (GIC). The next step of intercalation includes accelerated heating or treatment with microwave, producing even larger interlayer spacing and creating expanded graphite (EG), which contains barely attached thin graphite platelets (30–80 nm) (Chen et al. 2004). But further exfoliation of EG is necessary as the layered structure and relatively low specific surface area (below 40 m2/g) (Celzard et al. 2000) prevent efficient performance as a composite filler (Zheng et al. 2004; Yasmin et al. 2006; Pötschke et al. 2009). Higher degree of exfoliation of EG produces GNP with thickness of 5 nm or lower (Jang and Zhamu 2008; Potts et al. 2011) (Fig. 8.6).

Fig. 8.6
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Schematic representation of the steps for exfoliation of graphite to obtain GNP. Adapted from Rouf and Kokini (2016)

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8.3.1.1.2 Oxidation of Graphite

The first step involves an oxidizing agent like potassium permanganate (William et al. 1958) or a combination of KClO4 with HNO3 (Brodie 1859; Staudenmaier 1898). The oxidation level of graphite with the different oxidants are very similar (C:O = 2:1 approx.) (Dreyer et al. 2010), and they all result in the destruction of the π-conjugation of the stacked graphene sheets, producing nanoscale graphitic sp2 domains, which are surrounded by highly disorganized oxidized domains (sp3 C\C) (Krishnan et al. 2012).

Hummer’s method is a very well-known technique for synthesis of GO, in order to use it in biopolymer nanocomposite fabrication (Yang et al. 2012; Yadav et al. 2013, 2014; He et al. 2014; Huang et al. 2014; Stanier et al. 2014; Nie et al. 2015); modified Hummer’s method (Pan et al. 2011; Wang and Qiu 2011; Yoon et al. 2011; He et al. 2012; Ma et al. 2013; Pinto et al. 2013a, b; Thakur and Karak 2013; Faghihi et al. 2014; Liu et al. 2014; Si et al. 2014; Stanier et al. 2014; Tian et al. 2014), the Staudenmaier’s method has also been used in some studies (Cao et al. 2010; Kim and Jeong 2010; Wang and Qiu 2012) as well as Brodie’s method (Tian et al. 2014).

During further exfoliation of the oxidized graphite, the phenol, hydroxyl, and epoxy groups are formed at the basal plane and carboxylic acid groups are formed at the edges, and GO is synthesized (Singh et al. 2011). GO, in spite of being chemically similar to graphite oxide, is structurally different due to its quasi-infinite interlayer spacing (Fig. 8.7).

Fig. 8.7
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Schematic representation of the steps for oxidation of graphite to obtain GO. Adapted from Rouf and Kokini (2016)

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Mechanical stirring of graphite oxide to produce GO is very time consuming; however, it results in large lateral dimensions (several hundred nanometers). On the other hand, ultrasonication offers fast yield but often fractures the GO sheets into smaller pieces (Park and Ruoff 2009).

8.3.1.2 Biopolymer-Graphene Nanocomposites for Packaging

Uniform dispersion of the nanoparticles is necessary to optimize functionalization of the polymer nanocomposite. Particle characterization is therefore a very important aspect of nanocomposite fabrication for understanding the internal changes of the nanocomposites as well as for quality assurance. Distribution of nanofillers in polymer matrix, substantial changes in bulk matrix, and any changes in the interface of polymer and particle are some of the main aspects of characterization. Spectroscopic techniques including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) have been used in different studies to understand the mechanism of enhancement of functional properties. Therefore, different graphene-biopolymer nanocomposites that are appropriate for packaging applications will be discussed, where the fabrication, different types of characterization, the synergy resulting from the use of different spectroscopic techniques as well as the different property enhancement will be explained.

8.3.1.2.1 Starch

Starch-based materials have shown the most potential to replace synthetic plastics as packaging materials because of their biodegradability, low cost, abundance, and renewability. However, starch suffers from inferior mechanical strength and thermal stability which have been reinforced with various derivatives of graphene like GO, RGO, and GNP. Polarity of solvents, plasticizer addition, temperature, etc., affects the degree of property improvement by graphene nanofillers in the biopolymer nanocomposites. Since starch is soluble in water, in most of the studies water is used as a solvent, but since it is a polar protic solvent, samples with more polar groups would have higher dispersibility, meaning that GO would have higher degree of dispersion than RGO (Dai et al. 2015). This was observed in a study of glycerol plasticized starch (PS), where better overall property improvement was observed for starch-GO nanocomposites, as compared to starch-RGO nanocomposites (Ma et al. 2013). Tensile strength increase for starch-GO nanocomposite was 66% at 2wt% GO loading and water vapor permeability improved for PS-4wt% GO nanocomposite. On the other hand, tensile strength improvement for starch-RGO nanocomposite was 67% for 6wt% addition of RGO; however, noticeable improvement for PS-RGO was not achieved even at 8wt% RGO loading. This comparative improvement of starch properties with GO over RGO is due to the comparatively larger number of polar groups for GO leading to better compatibility with PS. In addition, this resulted in better mechanical and water vapor barrier properties of starch-GO nanocomposites compared with starch-RGO composites, which may ultimately result in an advanced packaging material.

To study the other forms of graphene as well as effect of plasticizers on tensile strength and water vapor permeability improvement of starch nanocomposites, glycerol was used as a plasticizer for starch, and starch-grafted graphene (GN-Starch) was used as a nanofiller to prepare PS-graphene nanocomposites (Zheng et al. 2013). TEM images, as shown in Fig. 8.8a, b, confirmed successful conversion GO to GN-Starch. Oxidized graphite was observed to have a flat, transparent, and wrinkled structure, while GN-Starch had a darker appearance due to the presence of starch. On the other hand, the presence of starch component in the GN-Starch nanofiller was confirmed from the FTIR spectra of GN-Starch. Figure 8.9a shows peaks corresponding to C–O bond stretching of the C–O–H group, and the two peaks from C–O bond stretching of the C–O–C group, which are also seen at the fingerprint region of starch. The comparison of Raman spectra (Fig. 8.9b) of GO and GN-Starch showed that GN-Starch had higher intensity peaks corresponding to a disordered structure (D mode), this disorder was a result of creation of new graphitic domain in GN-Starch. GN-Starch loading of 0.28–1.7wt%, was enough to make significant improvements in mechanical and WVP properties, proving its efficiency as a nanofiller for starch to be used for packaging. The improvement in mechanical properties and barrier properties are summarized in Tables 8.1 and 8.2.

Fig. 8.8
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TEM images of a Graphene b GN-Starch; SEM images of c Graphene, and d GN-Starch. Reprinted with permission from Zheng et al. (2013). Copyright 2013, American Chemical Society

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Fig. 8.9
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a FTIR spectra of starch, GO, and GN-Starch. b Raman spectra of GO and GN-Starch. Reprinted with permission from Zheng et al. (2013). Copyright 2013, American Chemical Society

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Table 8.1 The effects of graphene on mechanical properties of different biopolymers that are suitable for packaging application
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Table 8.2 The effects of graphene on gas and vapor barrier properties of candidate biopolymers for packaging applications
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In another study on GO-functionalized-glycerol plasticized pea starch (Li et al. 2011), morphological tests conducted on the nanocomposites showed that hydrogen bonding occurs as a result of the solution casting between starch and GO. FTIR spectra of pea starch-GO nanocomposite shows gradual shifting of characteristic peaks of starch with gradual addition of GO, which is an indication of hydrogen bond formation. Similarly, XRD of starch-GO nanocomposites showed decrease in intensity of starch-GO nanocomposites compared to the intensity of characteristic starch peak offering another proof of H-bonding. Moreover, AFM images (Fig. 8.10) showed that plasticized pea starch-1% GO nanocomposites have lower roughness than pristine starch, but above 2% GO, blisters start to form indicating aggregate formation (Li et al. 2011). The uniform dispersion positively affected the overall properties of starch and only a loading of 2wt% GO was sufficient to effectively improve the tensile strength from 4.56 to 13.79 MPa, and the young’s modulus from 0.11 to 1.05 GPa. In addition, significant improvement in moisture uptake was observed at 1.5wt% GO, indicating good potential for packaging application.

Fig. 8.10
figure 10

a TEM images of pea starch-GO nanocomposite; AFM images of b pea starch, c pea starch-1wt% GO nanocomposite, and d pea starch-2wt% GO nanocomposites. Reprinted with permission from Li et al. (2011). Copyright 2011, Elsevier

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Starch-GNS and chitosan-GNS nanocomposites (Ashori 2014) showed similar results to the previous study on pea starch-GO (Li et al. 2011), where smoother surface was observed on addition of GNS to both starch and chitosan. However, above 3% GNS loading, significant roughness coupled with phase separation were observed, with starch showing a higher percentage of aggregation than chitosan. These findings also explain the comparatively better water vapor barrier properties of chitosan-GNS nanocomposites compared to starch-GNS. Also, a blend of chitosan and starch with graphene nanofiller gave a higher tensile strength increase compared to starch and chitosan individually (Ashori and Bahrami 2014), due to which much stronger hydrogen bonding is observed between NH3 of the chitosan backbone and OH of the starch. The blend also showed improved water vapor barrier properties, over the previous study (Ashori 2014), with a WVTR drop from 44.3 to 38.7 g/m2h, at only 2.5wt% graphene addition.

8.3.1.2.2 Cellulose

Cellulose has also gained popularity as an industrial biopolymer that offers biodegradable packaging opportunities. Some forms of cellulose like cellophane which is a regenerated form of cellulose has been commercially used for decades. However, cellulose’s limited mechanical property and thermal stability hinder its performance and limits its application. Therefore, different graphene nanomaterials have been applied to different forms of cellulose in various studies to see how graphene interacts with cellulose and reinforces it to enhance its various properties.

Solubilization of natural cellulose and subsequent regeneration, using electrospinning or solution casting, is called regenerated cellulose (RC). RC-graphene oxide nanosheets (GONS) nanocomposites were also investigated for packaging application (Huang et al. 2014), where NaOH–urea was used as a solvent that efficiently dissolved GONS throughout RC matrix. TEM images confirmed the complete exfoliation of GONS into individual nanosheets and ensured homogeneous dispersion. In addition, two-dimensional wide-angle X-ray diffraction (2D-WAXD) showing disappearance of the regular and periodic structure of graphite oxide was a clear indication that characteristic diffraction ring of GONS was absent in the cellulose nanocomposite. The efficient dispersion and subsequent reinforcement by only 1.64vol% GONS increased tensile strength by 67% and Young’s modulus by 68%, relative to the pristine RC (Fig. 8.11). The authors were able to prove, using the Halpin–Tsai model (Affdl and Kardos 1976), that GONSs were more parallelly aligned to the film surface, as result of gravitational forces and hot pressing during fabrication (Fig. 8.11). GONS also improved the barrier properties of RC as evidenced by the three times decrease in O2 permeability of the nanocomposite compared to the control, as shown in Table 8.2.

Fig. 8.11
figure 11

a Typical stress–strain curves of RC and the cellulose nanocomposite films with different GONS loadings b Young’s modulus of the samples as determined experimentally and the theoretical values obtained by Halpin–Tsai models based on the hypothesis that GONSs are randomly or unidirectionally distributed in the cellulose matrix. Reprinted with the permission from Huang et al. (2014). Copyright 2014, Royal Society of Chemistry

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There were two different studies focusing on RC–GNP nanocomposites, but with two different solvents 1-ethyl-3-methylimidazolium acetate (EMIMAc) solvent (Mahmoudian et al. 2012) and the other used N,N-dimethylacetamide (DMAC)–LiCl (Zhang et al. 2012) solvent. DMAC–LiCl solvent proved to be a better option for solvent casting of cellulose and graphene, as tensile strength and thermal decomposition temperature increased substantially compared to the study with EMIMAc solvent. The tensile strength improvement, thermal stability improvement, and gas barrier property enhancement found in this study as well as other studies are shown in Tables 8.1, 8.2, and 8.3.

Table 8.3 The effects of graphene on thermal properties of different biopolymers
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Melt blending, which is another well-known biopolymer-graphene nanocomposite fabrication technique, has also been used to make cellulose-graphene nanocomposite. In a study to improve the properties of cellulose acetate propionate (CAP), EG nanoplatelets were melt blended with CAP (Jeon et al. 2012), which resulted in strong interlocked network between graphene and CAP. This lowered oxygen permeation, improved dynamic storage modulus, and increased thermal degradation temperature of the CAP–EG nanocomposite. Electrical volume resistivity of CAP–EG composites declined from 1015 to 106 Ωcm, at only 5–7wt% loading.

8.3.1.2.3 Poly(Lactic Acid)

Another highly known biodegradable and biocompatible polymer is PLA, which has shown its versatile application as a packaging material. With the objective to overcome its poor mechanical property, low crystallization rate, and thermal resistance, different studies have graphene nanofiller in PLA matrix. Property enhancement of the pristine PLA, plasticized PLA nanocomposite, and unplasticized PLA nanocomposites were compared, where the plasticized nanocomposites showed 100% increase in tensile strength and Young’s modulus, but unplasticized nanocomposite films showed only a 15% increase in tensile strength and 85% increase in Young’s modulus at the same concentration of graphene (Pinto et al. 2013a). With increasing filler loading, glass transition temperature increased and permeability decreased, and maximum glass transition and lowest permeability were observed at for 0.4wt% loading of GO and 0.4wt% of GNP, in the PLA-graphene nanocomposites. Their oxygen and nitrogen permeability decrease were threefold and fourfold higher compared to pristine PLA. GNP was assumed to show better performance due to its planar configuration, which would ideally create a tortuous path and delay the process of escape of various gases, but due to the disordered orientation of GNP along the film plane, it was unable to create an efficient tortuous path and resulted in permeability similar to GO.

8.3.1.2.4 Poly(Hydroxyalkanoate)

Recently medium-chain-length poly(hydroxyalkanoate)s (PHAmcl), a new class of renewable and biodegradable biopolymers have emerged, and has shown great promise in packaging applications. With the objective of producing biodegradable packaging sensors and charge dissipating applications, three different types of PHAmcl polymers: poly(hrydroxyoctanoate) (PHO), poly(hydroxydecanoate) PHD, and poly(hydroxyoctenoate) (PHOe) were functionalized with thermally reduced graphene (TRG) nanoparticles, using solution casting technique, (Barrett et al. 2014; Barrett 2014), which showed that efficacy of reinforcement was affected by the chain packing length, ability of non-covalent interaction with TRG and covalent crosslinking. The results showed that the addition of up to 2.5vol% TRG to PHAmcl increased the Young’s modulus of PHO by 590%, PHD by 200%, and PHOe by 280%. The extent of hydrogen bonding was directly related to the length of the aliphatic extensions from the polyester backbone. Steric hindrance was a result of longer chains, which reduced the intensity of hydrogen bonding between the PHAmcl and TRG. PHO, which is the shortest chain PHAmcl polymer out of the three biopolymers, had the least hindrance to hydrogen bonding and therefore the highest modulus enhancement. TEM and SEM images of the PHAmcl showed TRG had the best dispersion in PHO, but experienced entanglement at only 0.5% loading, on the other hand, TRG had the worst dispersion in PHOe, but entanglements did not occur until 2.5% loading of TRG. The thermal properties of the PHA polymers were increased by TRG, as melting temperature improved by 1−3 ℃, and an increase over 7 orders of magnitude of the electrical conductivity was attained.

8.3.2 Carbon Nanotube-Based Functionalization

Carbon nanotubes (CNTs) are another member of the carbon family similar to graphene. Carbon nanotubes are one atom thick nano-sized tubes that may exist as a single unit, which is referred as single-wall nanotube (SWNT) or it may consist of a number of concentric tubes defined as multi-walled nanotubes (MWNT). CNTs have astonishingly high aspect ratio, with tensile strength as high as 200 GPa and elastic modulus of 1 TPa (Lau and Hui 2002; Zhou et al. 2004). CNTs have been used in various types of synthetic polymer nanocomposites, like poly(ethylene-2,6-naphtalene) (PEN) (Kim et al. 2008), PVA (Chen et al. 2005; Bin et al. 2006), and polyamide (Zeng et al. 2006) for packaging application. These nanocomposites showed remarkable improvements in mechanical, thermal, and barrier properties. CNTs have also been coupled with different biopolymers like PLA (Kuan et al. 2008; Chiu et al. 2008; Villmow et al. 2008; Yoon et al. 2009), starch (Ma et al. 2008), etc., for improving their mechanical, thermal, and electrical properties. They have shown great promise for being used as reinforcement for biodegradable packaging application, as they increased the flexural and tensile strength of PLA, by 17.5 and 27.2%, respectively, at only 4 phr (parts of CNT per hundred parts of PLA) loading (Kuan et al. 2008) and has decreased PLA’s WVTR by 200% (Brody 2006). MWNTs have also been used with PLA to fabricate conductive biopolymer vapor sensor which can act as a smart packaging material (Kumar et al. 2012).

8.4 Clay and Silicate Nanoclay-Based Reinforcement

The widely available, inexpensive, and relatively simple processing of layered inorganic solids like clays and silicates garnered a lot of attention as a filler material to improve polymer performance in the packaging industry. Nanoclays usually have a thickness of 1 nm and length of several microns, and when nanoclays are introduced to nanocomposite formulations, it results in increased tortuosity of the diffusive path for a penetrant molecule (Fig. 8.2), providing excellent barrier properties (Bharadwaj et al. 2002; Cabedo et al. 2004; Mirzadeh and Kokabi 2007).

Montmorillonite (MMT), which is a hydrated alumina-silicate-layered clay (Weiss et al. 2006), is most well-known clay filler in polymer-nanoclay research. The high specific surface area and large aspect ratio (50–100), makes MMT an exceptionally effective nanofiller for various polymers (Uyama et al. 2003). For the purpose of improving the food packaging application of various biopolymers, MMT has been investigated, especially for starch (Park et al. 2002; Avella et al. 2005; Huang et al. 2005; Chen and Evans 2005; Yoon and Deng 2006; Cyras et al. 2008). At only 5wt% addition, MMT has been reported to improve the tensile strength and strain of corn starch by 450 and 20%, respectively (Huang et al. 2006). MMT has even been used for reinforcing PLA (Sinclair 1996; Thellen et al. 2005) and various types of proteins (Dean and Yu 2005; Rhim et al. 2005; Chen and Zhang 2006; Yu et al. 2007). Zein, a protein extract from corn with, film-forming ability, biodegradability, and biocompatibility, finds application as a coating agent in food industry (Shukla and Cheryan 2001; Liu et al. 2005; Luecha et al. 2010, 2011). However, due to its high water vapor permeability and low tensile strength, it has been functionalized with MMT and has shown tensile strength increase of 150% using only 5wt% clay, and the water vapor permeability drop from 11.58 to 4.56 g mm/days m2 KPa at only 3wt% MMT loading (Luecha et al. 2010).

Kaolinite [Al2Si2O5(OH)4] is a layered silicate mineral, mainly used in the production of paper, ceramics, and cosmetics. Nanocomposites of thermoplastic starch and kaolin clay showed that at a loading of 50 phr, tensile strength increased up to 135% and Young’s modulus increased up to 50% (de Carvalho et al. 2001). Moreover, water vapor resistance significantly increased up to 20 phr loading of kaolinite, but no improvement in thermal stability was observed with increasing kaolinite concentration. In another study, chemically modified kaolinite was used to reinforce amorphous PLA (Cabedo et al. 2006), where kaolinite successfully improved the oxygen barrier property by 50%.

Another widely known form of MMT is Cloisite (Na-MMT), which has also been used in different biopolymer nanocomposite research for improved packaging application. A research has been conducted on reinforcement of plasticized starch using four different types of Cloisite (natural Cloisite: Na-MMT, and organically modified Cloisite: Cloisite 30B, Cloisite 10A, and Cloisite 6A) (Park et al. 2002), at 5% loading. XRD and TEM characterization of the four types of TPS-Cloisite nanocomposite showed 20% increase in tensile strength and 30% in Young’s modulus, as well as significant reduction in water vapor permeability. Compared to the thermal property of pristine TPS, all four nanocomposites showed notable enhancement. Four different organoclays Cloisite 30B, 10A, 25A, and 20A were used as filler for starch acetate, using melt blending, which resulted in substantial improvement in mechanical and thermal stability as the glass transition temperature increased by 6–14 ℃ and onset temperature for thermal decomposition increased by 15–25 ℃ (Xu et al. 2005).

Another notable hydrous silicate mineral, Tourmaline, [Na(Li,Al)3Al6(BO3)3Si6O18(OHF)4] was also tested to examine its reinforcing capabilities on RC. Unfortunately, it did not show any significant mechanical or thermal property improvement over pristine RC (Ruan et al. 2004). However, it showed great promise as a filler for functional packaging material because of its antibacterial property against Staphylococcus aureus, a common food-borne pathogen.

8.5 Cellulose Nanofiber, Starch Nanocrystal, and Chitosan Nanoparticle-Based Reinforcements

A popular polysaccharide-based nanomaterial, cellulose nanofiber (CNF), has garnered a lot of attention, due to their inexpensive pricing, biodegradability, high strength, and renewability (Helbert et al. 1996; Podsiadlo et al. 2005). The type of CNF consisting of elongated molecule bundles, stabilized by hydrogen bonding, is called cellulose nanofibers (Azizi Samir et al. 2005; Wang and Sain 2007), and the other type, which consists of the crystalline part of microfibrils, called cellulose whiskers, is obtained using acid hydrolysis (Dujardin et al. 2003; Samir et al. 2004). A more commercially available alternative of cellulose whiskers is microcrystalline cellulose (MCC), obtained from particles of hydrolyzed cellulose microcrystals (Petersson and Oksman 2006).

One of the governing factors that influence the final property of the nanocomposite of reinforced cellulose nanofibers is the aspect ratio of the fiber, which is a direct consequence of type of cellulose and fabrication conditions (Azizi Samir et al. 2005). The other governing factors are the dimensions of the fiber as well as the geometrical and mechanical percolation effects (Dubief et al. 1999; Hubbe et al. 2008), including the orientation distribution of the fillers (Jiang et al. 2007; Kvien and Oksman 2007). According to the percolation theory, properly dispersed filler that is present in sufficient number to create continuous network is the primary condition for maximum improvement by the nanofiller (Helbert et al. 1996; Ljungberg et al. 2005).

Different types of biopolymers have been considered with CNF, like fruits and vegetable puree (Azeredo et al. 2009), poly(styrene-co-butyl acrylate) (Helbert et al. 1996), poly(vinyl alcohol) (PVOH) (Zimmermann et al. 2004), starch (Angles and Dufresne 2000, 2001; de Souza Lima and Borsali 2004; Alemdar and Sain 2008). Mango puree films have been functionalized with cellulose nanofibers, to create highly efficient edible packaging films (Azeredo et al. 2009), and showed an increase in tensile strength from 4.09 to 8.76 MPa at 36% loading and decrease in water vapor permeability from 2.66 to 1.67 g mm/kPa h m2 at 10wt% loading. A study on straw cellulose whiskers showed that addition of the nanofiller at 30wt% with poly(styrene-co-butyl acrylate) latex film resulted in modulus improvements three orders of magnitude over control (Helbert et al. 1996). Compatible geometry and whisker stiffness, effective linkage of cellulose fibers through hydrogen bonds, fibril network formation within the polymer matrix, are some of the key elements for such an impressive reinforcing effect. The reinforcing capabilities of cellulose fibers are somewhat temperature dependent, especially at temperatures above the glass transition temperature (T g) of the matrix polymer, when CNF has noticeably improved strength and modulus of polymers (Dufresne and Vignon 1998; Dufresne et al. 2000). CNF has also been reported to decrease water uptake in starch (Dufresne and Vignon 1998; Dufresne et al. 2000) and has improved moisture barrier properties of many other biopolymers (Paralikar et al. 2008; Sanchez-Garcia et al. 2008; Svagan et al. 2009).

Some other notable biopolymer-based nanofillers are starch nanocrystals, which have reportedly improved the mechanical properties of PVOH (Chen et al. 2008), by adding up to 10wt%. Another nanoreinforcement is chitin whiskers or chitosan nanoparticles, which have successfully improved tensile strength and water resistance of soy protein isolates (SPI) (Lu et al. 2004), tensile strength and barrier properties of chitosan films (Sriupayo et al. 2005), and hydroxypropyl methyl cellulose (HPMC) (de Moura et al. 2009) and has proven to be a potential food packaging material for elongated storage stability.

8.6 Antimicrobial Nanomaterial

Nano-sized antimicrobial filler-functionalized food packaging materials have gained a lot of popularity, as they can reduce, obstruct, or defer the growth of pathogenic and spoilage microorganisms (Rhim and Ng 2007). Antimicrobial compounds in nanoscale range have proven to be superior to the micro-sized compounds as the larger surface area increased adhesion to biological materials (Luo and Stutzenberger 2008). Silver-based antimicrobial packaging materials are well known for their application as they have significant toxicity toward various microorganisms and have excellent thermal stability and minimal volatility (Liau et al. 1997). Silver-based nanocompounds act as an antimicrobial agent by releasing silver ions, which binds to the electron donor group from the pathogenic molecule (Kumar and Münstedt 2005). Poly(Amide 6)-2wt% Ag-NP has shown antimicrobial activity against E. coli, even after 100 days of storage (Damm et al. 2007). Moreover, Ag-NPs have shown promise in elongation of shelf stability of fruits and vegetables (Hu and Fu 2003), jujube (a Chinese fruit) (Li et al. 2009), asparagus (An et al. 2008), etc. Another notable antimicrobial nanomaterial is chitosan nanoparticles, which, in the presence of protonated amino groups, interact with the oppositely charged cell membranes of the pathogenic molecule causing leakage and eventual rupture of intracellular material (Qi et al. 2004). CNTs have also shown antibacterial properties, as aggregates of CNT, using its long and thin structure can puncture the microbial cells of pathogens like E. coli, (Kang et al. 2007). Also, water dispersible GO and RGO paper have shown significant antibacterial activity against E. coli, while showing very little cytotoxicity (Hu et al. 2010). Graphene has also shown antibacterial activity against Staphylococcus aureus (Akhavan and Ghaderi 2010).

8.7 Future Perspective and Limitations

There are several forms of graphene, but only the GO or RGO powder forms have been explored for packaging application. So other forms like graphene aerogel, CVD graphene can also be explored, where CVD graphene or graphene aerogel can be used in biopolymer-graphene sandwich structure to make smart biodegradable packaging that ensures quality control and elongated and safe storage for food and pharmaceuticals. Biopolymer-graphene nanocomposites can also be used as a mechanically strong yet flexible, biodegradable platform for antibody-antigen or synthetic detecting element (liposome, aptamer, and peptide)-based FET biosensors (Guo et al. 2015; Qian et al. 2015). Moreover, different biopolymer-graphene nanocomposites can function as a gate dielectric for enhanced current modulation throughout FET biosensors (Qian et al. 2015), for a more efficient recognition of different food toxins, allergen, and pathogenic bacteria.

Nowadays, advanced chemical sensors made of chemical selective coating use the principle of surface adsorption to detect a particular chemical or gas (Vanderroost et al. 2014). Various kinds of carbon nanomaterials like graphene, graphite, and nanotubes can be used as transducers in various chemical sensors, where it is used as a transducer as well as mechanical reinforcement for the biodegradable packaging material, thus detects various pathogenic and spoilage microorganisms to ensure product integrity, and tracks ingredients or products through the processing chain (Nachay 2007; De Azeredo 2009).

Another revolutionary technique in the area of smart packaging is the radiofrequency identification (RFID), which is an programmable recognition technique that applies wireless sensors to detect items and collect data, for application in quality control and supply chain management, classify and handle the flow of goods (Jones et al. 2004; Sarac et al. 2010; Ruiz-Garcia and Lunadei 2011). Commercially available RFID with tags can track the temperature, relative humidity, light exposure, pressure, and pH of products. “Water-based, inkjet-printed GO ink” has been successfully applied to develop used as the basis for design and development of inexpensive, autonomous, wireless sensor, which has good response to spoilage gases like ammonia gas (NH3) (Le et al. 2012). Graphene-based RFID tags can also be used, coupled with the biopolymer-graphene nanocomposite packaging, to ensure food quality and food safety.

For long term packaging application and shelf life stability, a massive challenge would be the intrinsic biodegradability of biopolymers, as controlled environment, like control of moisture, nutrients, or microorganisms, as well as temperature during various experimentation becomes a great hindrance (Kaplan 1998). In addition, “more random configuration of the repeating units in biopolymers and the resulting steric hindrance” (Rouf and Kokini 2016) experienced by the nanofillers, as compared to the more controlled and organized configurations of synthetic polymers, ultimately result in the weaker performance of biopolymers compared to synthetic polymers when different nanomaterials like graphene is added. Table 8.4 shows comparison of the young’s modulus of biopolymer-graphene nanocomposite with synthetic polymer-graphene nanocomposites which provide a clear difference between biopolymer and synthetic polymer-graphene nanocomposites.

Table 8.4 Comparison of highest reported young’s modulus increase of some common biopolymer-graphene and synthetic polymer-graphene nanocomposites
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Another challenging aspect of nanomaterial functionalization in biopolymer is the release of the nanomaterial from the host matrix. Some studies have reported release of silver (Ag), Cu, Ag-zeolites, montmorillonite nanoclay, from its host polymer matrix in some applications (Duncan and Pillai 2014); however, the release of graphene from its polymer/biopolymer host matrix has not yet been reported, so more investigation is required in this regard. The nanomaterial release can be prevented by adding an extra layer of biopolymer that comes in contact with the food material, as shown in Fig. 8.12. This will not only provide a barrier to the nanomaterial release (if any) but will also improve the permeability property of the packaging and provide extra mechanical support.

Fig. 8.12
figure 12

ENM release from nanocomposite into food material [Adapted from Duncan and Pillai (2014)]

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8.8 Conclusion

The need for high quality, long lasting, and safe food supply has catalyzed various novel techniques in packaging systems, with smart biodegradable packaging being the latest and most promising addition. From this chapter, it is clear that usually a very small amount (<5%) of graphene and carbon nanotubes is used in biodegradable packaging for improvement of their various properties as packaging material, compared to other nanomaterials like kaolinite (up to 50% loading), CNF (up to 36% loading), and therefore, these graphene nanocomposites have more eco-friendly impact in the various novel packaging applications. The property improvement of various biodegradable packaging materials using graphene derivatives depends on not only the form of nanofiller but also the reaction condition and fabrication technique, which is clearly noticeable from the comparison of the improvement in mechanical property of all the biopolymer nanocomposites. There has been extensive research for packaging application with polysaccharides, PLAs, and other biopolymers as discussed above but when it comes to packaging research, the field of proteins has been neglected. Solution casting and melt blending technique have been the major fabrication technique for biopolymer nanocomposite fabrication, whereas grafting technique has shown great potential for synthetic polymers (Fang et al. 2010; Song et al. 2013); therefore, grafting techniques for combining different nanomaterials to biopolymers require much consideration. In conclusion, nanomaterial-based biodegradable packaging has great potential as smart packaging to further consumer benefits and convenience, as well as ensures a healthier environment.