Keywords

4.1 Introduction

Of all the materials available for food packaging, plastics have increased exponentially over the past two decades, with an annual growth of approximately 5%. It is estimated that worldwide annual plastic production exceeds 300 million tonnes, and was about 59 million tonnes in Europe in 2014. In fact, nowadays, plastics represent almost 40% of the European packaging market (Muller et al. 2017a). Of the plastic materials, petroleum-based plastics, such as polyethylene (PE) , polypropylene (PP), polyamide (PA), are widely used as packaging materials due to their ready availability at relatively low cost, good mechanical and barrier properties, thermo-processing ability and chemical characteristics, which make them suitable for food packaging. However, despite their good properties, their use and accumulation imply serious environmental problems and a dependence on fossil fuels. Around 63% of the current plastic waste comes from packaging applications, and it is estimated that less than 14% is recyclable. Taking this scenario into account, and bearing in mind the growing environmental awareness, research has focused on the development of alternative bio-packaging materials, derived from renewable sources, which are biodegradable or compostable.

Biopolymers can be used for food packaging applications or food coating purposes, reducing the environmental impact and oil-dependence (Rivero et al. 2017; Emadian et al. 2017). They can be divided into three main categories, on the basis of their origin and biodegradable nature. Together with the conventional, non-biodegradable, oil-based plastics, there are biobased-non-degradable bioplastics (e.g. polyethylene terephthalate: PET), biobased-biodegradable bioplastics (e.g. polylactic acid: PLA, starch and other polysaccharides, or proteins) or fossil-based biodegradable bioplastics (e.g. polycaprolactone: PCL, polyvinyl alcohol: PVA, or polybutylene succinate: PBS). So, biopolymers are biodegradable, biobased or both and can be classified as those directly obtained from biomass (polysaccharides and proteins), synthetic biopolymers from biomass or petrochemicals (e.g. PLA, PCL) or those obtained by microbial fermentation (polyhydroxyalcanoates: PHA and bacterial cellulose) (Nair et al. 2017). The former are directly extracted from biological and natural resources and they are hydrophilic and somewhat crystalline in nature, making an excellent gas barrier. Biodegradable polyesters (synthetic or biosynthesized) are more hydrophobic and constitute better barriers to water vapour. In general, the functional properties of biopolymer-based materials in terms of their mechanical and barrier properties need to be adapted to food requirements by using different strategies, such as physical or chemical modifications (crosslinking), blending with other components, fillers, plasticizers or compatibilizers (Ortega-Toro et al. 2017).

The industrial uses of biopolymers have been restricted because of their usually poor mechanical, barrier or thermal properties, and high price. The incorporation of micro- and nano-reinforcing agents into the matrix for the purposes of obtaining composites has been seen to improve their functional properties and so their competitiveness in the plastics market. Composites are made up of a continuous polymer matrix in which the filler particles are dispersed, thus contributing to a modification of the functional characteristics of the material (Azeredo 2009). Fillers differing in size, shape, amount, distribution and chemical nature have been used. Lignocellulosic or cellulosic materials obtained from agro-waste have been widely studied as organic micro-fillers (Gutiérrez and Alvarez 2017). Fibres from cotton (Ludueña et al. 2012), garlic straw (Kallel et al. 2016), rice husk (Johar et al. 2012), wheat straw (Berthet et al. 2015) or coffee silverskin (Sung et al. 2017), have been used as reinforcing agents in different biopolymer films. Micro-particles significantly improved the elastic modulus of composites while providing great thermal resistance to the matrices due to the presence of hydroxyl groups interacting with the biopolymer network (Ludueña et al. 2012; Berthet et al. 2015). Different organic nano-fillers can be obtained, mainly from cellulose (cellulose nanocrystals or nanofibres), chitin/chitosan nanocrystals from crustacean waste (Gutiérrez 2017) or starch nanoparticles. These nano-reinforcing agents improve the tensile strength and elastic modulus when they have a proper distribution, chemical affinity with the polymer and high aspect ratio. The crystalline structure of nanofillers enhances the tortuosity factor for the mass transport of gas molecules into the biopolymer matrix, contributing to the formation of a hydrogen-bonded network (Ng et al. 2015; Azeredo 2009; Azeredo et al. 2017). On the other hand, inorganic particles are relevant as filling agents in food packaging materials due to the enhancement of the mechanical and barrier properties (MgO, silicon carbide or nano-clays) Some of them also exhibited antimicrobial activity, such as Ag, TiO2 and ZnO nanoparticles (Gutiérrez et al. 2017; Azeredo 2009).

It is remarkable that biodegradation behaviour is a crucial factor in the development of composites. The biodegradation process takes place in aerobic conditions by the action of a microorganism, which identifies the polymer as a source of energy to produce organic residues from the packaging material. The incorporation of nano-fillers can affect the biodegradability of composites (Gutiérrez 2018). In this sense, cellulose nanocrystals (CNC) promoted the material’s water intake due to their hydrophilic nature, contributing to an acceleration of the biodegradation process (Ludueña et al. 2012; Luzi et al. 2016). Some inorganic nano-fillers could also affect the disintegration processes, such as what occurs with Ag nanoparticles (Ramos et al. 2014; Cano et al. 2016), or nano-clays, whose hydroxyl groups react with the chains of the polymer matrices (montmorillonite and fluorohectorite, Fukushima et al. 2013).

This chapter reviews the potential use of reinforcing agents of differing natures and sizes in biopolymer materials that are potentially useful for food packaging, analysing their effect on the mechanical and barrier properties and on the thermal resistance of the material. The surface properties and biodegradation behaviour were also analysed in different kinds of composites.

4.2 Bioplastics for Food Packaging

Over the last decade, several bioplastics, bio-based, biodegradable, or both, have been available as a suitable alternative to conventional plastics for food packaging applications (Fabra et al. 2014; Ortega-Toro et al. 2017). At least 90% of natural or synthetic biodegradable polymers decompose in less than 180 days (ASTM 2003). Figure 4.1 shows the main polymers of potential use in food packaging. Of the natural polymers, different polysaccharides and proteins and microbially-produced biopolymers have been extensively studied for food packaging applications. Starch is a promising polymer, suitable for processing by means of different techniques, such as the casting method (Moreno et al. 2017), compression moulding (Muller et al. 2017b), extrusion (Gutiérrez and Alvarez 2018) or injection moulding (López et al. 2015). Starch is naturally highly abundant, low cost and renewable. Cellulose, constituted by glucose units via β-1,4-glycosidic bonds, is the most naturally abundant carbohydrate (Xiao et al. 2014). This polymer is usually used as micro-filler or nano-filler (Shankar and Rhim 2016) in the composite formulation. These could be processed by compression moulding and injection moulding (Graupner et al. 2016) to obtain semi-rigid packaging (trays). On the other hand, agar is a fibrous polysaccharide obtained from marine algae, such as Gelidium sp. and Gracilaria sp., consisting of a mixture of agarose and agaropectin, which is slightly branched and sulphated. This is thermoplastic polysaccharide, biodegradable and biocompatible, which exhibits great mechanical strength with moderate water resistance (Giménez et al. 2013). Chitosan , the second most naturally abundant polysaccharide, has non-toxic, biodegradable, and antimicrobial characteristics, which are of great interest for packaging purposes (Leceta et al. 2013). Chitin , the precursor of chitosan, is a linear polymer of mainly β-(1 → 4)-2-acetamido-2-deoxy-d-glucopyranose units and low amounts of β-(1 → 4)-2-amino-2-deoxy-d-glucopyranose residues (Van den Broek et al. 2015). Others relevant natural polymers are proteins, such as gelatin or collagen, gluten proteins and dairy proteins. Gelatin is an animal protein obtained by the hydrolysis of the fibrous insoluble collagen from skins and bones of different animals. It is well known for its film-forming properties. It is abundantly available, low cost, and easily biodegradable and biocompatible (Kanmani and Rhim 2014). On the other hand, wheat gluten (constituted by gliadins and glutenin proteins) is an inexpensive protein from the milling process, which allows the production of membranes that are semipermeable to water vapour, oxygen and carbon dioxide molecules. This polymer can be applied as a food coating or edible film on different foods (Rocca-Smith et al. 2016), or processed by compression moulding (Zubeldía et al. 2015) and extrusion (Rombouts et al. 2013) for the purposes of developing flexible or semi-rigid packaging. Caseinates have been proposed as raw materials for food packaging development, since this protein exhibited good film-forming ability with good mechanical properties (Fabra et al. 2012; Arrieta et al. 2014a; Jiménez et al. 2013). The use of caseinates could be considered an alternative means of obtaining a high degree of protection from oxygen in modified atmosphere packaging (Arrieta et al. 2014a). Biopolymers obtained from several microorganisms, such as poly-hydroxyalkanoates -PHA-, poly-hydroxybutyrate -PHB-, or poly- hydroxybutyrate-co-hydroxyvalerate -PHBV-, are a family of biodegradable thermoplastic polymers. The polymer is produced in the microbial cells through a fermentation process and then collected by solvents, like chloroform. More than 100 PHA are identified, of which PHB is the most common (Peelman et al. 2013). PHB is a biopolymer produced from renewable sources and fermentation by certain micro-organisms, like Halomonas hydrothermalis and Burkholderia sp. and Chelatococcus daeguensis, inter alia (Bera et al. 2015). In addition to being biodegradable, PHB exhibits some properties similar to some synthetic polymers, especially polypropylene (PP) (Heitmann et al. 2016). PHBV is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid produced directly by microorganisms. This polymer is less brittle and more stretchable than PHB (Requena et al. 2017).

Fig. 4.1
figure 1

Main biopolymers with potential application in food packaging. Some molecular structures are included

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Other biodegradable polymers are obtained by synthesis from biobased monomers (e.g. PLA) and non-biobased monomers (e.g. PCL, PVA or PBS). PLA is the most common synthetic polymer obtained from biobased monomers. The synthetic routes to obtain PLA are through the ring-opening polymerisation of the esters of the acid and the direct condensation of the free acid (Cheng et al. 2009). PLA is of great potential to the packaging industry because of its mechanical, barrier and optical properties. It can be processed using readily available production technologies, and exhibits good thermal behaviour and water vapour barrier properties, although it is brittle and only a moderate gas barrier (Bonilla et al. 2013).

In the category of biodegradable synthetic petroleum-based polymers, PCL, PVA and PBS are the most representative. PCL is a linear, semicrystalline hydrophobic polyester, highly flexible, tough and thermally stable (Correa et al. 2017). In contrast, PVA is a synthetic, water soluble polymer which forms translucent films with good tensile strength, elongation at break and barrier properties (Dominguez-Martinez et al. 2017). PVA has been used in polymeric blends, with glycerol as a proper plasticiser because of its chemical affinity. PBS is another biodegradable thermoplastic polymer which has desirable melt processability and good mechanical properties, which are closely comparable to those of widely-used polyethylene (PE) or polypropylene (PP) (Mizuno et al. 2015).

4.3 Micro and Nano-Reinforcing Agents

An alternative means of improving the properties of biopolymers for food packaging applications, and reducing some of their drawbacks, is by the incorporation of micro- or nano-fillers to the matrix for the purposes of obtaining micro- and nano-composites. Composites have immiscible phases constituted by the polymer continuous network in which the filler particles are dispersed, thus generating new structures with different properties to those of the original polymer matrix. The filler can positively modify the functional characteristics of the material, depending on the filler polymer network interactions. Different kinds of fillers have been used, which modify the material characteristics as a function of the filler-matrix interactions. The size, shape, amount, distribution and chemical nature of the fillers are crucial factors in the final properties of the composite.

Many studies reported the use of different kinds of fillers. As is shown in Table 4.1, cellulose has been frequently used as a reinforcing agent in different forms, such as cellulosic fibres (Martino et al. 2015; Moustafa et al. 2016; Ludueña et al. 2012), bacterial cellulose (Fabra et al. 2016), nano-cellulosic fibres (Abdul Khalil et al. 2016) or cellulose nanocrystals (El-Hadi 2017; Fortunati et al. 2013a; Follain et al. 2013). In the polymer matrices, the incorporation of cellulosic fillers directly affects the mechanical and barrier properties depending on the particle size (micro or nano), which is a significant factor. As has been observed by various authors, particle size has differing effects on the mechanical properties. Cho et al. (2006) studied the effect of the particle size on the mechanical properties of polymeric composites with spherical micro (0.5 mm) and nano (15 nm) particles. They observed that, at nano-scale, Young’s modulus and tensile strength increased as the particle size decreased. As regards the barrier properties, it is expected that nano scale and a homogeneous distribution increases the tortuosity factor for the migration of molecules through the composite, decreasing the permeability of both water vapour and gases. On the other hand, the amount of filler included significantly affects the composite properties. At high concentrations, the polymer matrix could lose cohesiveness and continuity which could lead to a loss in functional properties (mechanical and barrier). In this sense, Maqsood et al. (2016) studied the reinforcing capacity of enzyme-hydrolysed longer jute micro-crystals in polylactic acid matrices. The elastic modulus and tensile strength increased by 40% and 28% respectively, once the filler loading rose to 5% with respect to neat PLA. However, a filler loading of 10% led to a decrease of 32% in the elastic modulus and 33% in tensile strength with respect to materials containing 5% of fillers. Other organic fillers are chitin nanocrystals (Herrera et al. 2016) from crustacean waste that improve the mechanical properties and transparency of neat PLA. Also, starch nanocrystals are frequently added to several biopolymers, improving the mechanical and barrier properties and decreasing the biodegradation time (Mukurubira et al. 2017; Le Corre and Angellier-Coussy 2014).

Table 4.1 Main inorganic and organic reinforcing agents used as fillers in biopolymer matrices and main provoked changes in the matrix
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Some inorganic particles have been extensively studied as fillers in food packaging materials. Some of these, such as MgO , (Sanuja et al. 2014); silicon carbide, (Dash and Swain 2013) and nano-clays (Cavallaro et al. 2013; Majeed et al. 2013; Abdollahi et al. 2013), enhanced the mechanical and barrier properties of the films. Additionally, other ones can provide antimicrobial activity to the material, as occurs with Ag nanoparticles (Carbone et al. 2016; Gutiérrez et al. 2017), TiO2 nanoparticles (He et al. 2015; Gutiérrez et al. 2017) and ZnO nanoparticles (Gutiérrez et al. 2017).

4.4 The Effect of Reinforcing Agents on the Functional Properties of Biopolymers

In the following sections, the effect that micro- and nano-fillers have on the tensile behaviour, barrier properties, thermal resistance and biodegradability of biopolymer matrices is discussed, taking the processing method into account.

4.4.1 Processing Methods

In general, thermomechanical processes (melt compounding, extrusion and compression moulding) with high shearing forces, temperature and adequate time are necessary to guarantee the convenient dispersion of the filler in the polymer matrix. However, the casting of polymer-filler dispersions is an alternative method to obtain nano-composites, due to the high aggregation tendency of nanoparticles, which are better maintained in liquid dispersions. Table 4.2 shows some recent studies on composite materials, including micro- or nano- fillers of differing characteristics , using different processing methods.

Table 4.2 Recent studies on composite materials including micro- or nano- fillers of different nature, applying different processing methods
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Berthet et al. (2015) studied the properties of PHBV composites containing wheat straw micro-fibres (10, 20 and 30 wt%). Compounding was carried out with a lab-scale twin-screw extruder, using a temperature profile from the polymer feeding to the die of 180–160 °C. Composite films were obtained by compression moulding at 170 °C. The mechanical and barrier properties of composites were poorer than those of neat PHVB, although the authors point that the obtained materials could be applied as packaging for respiring fresh products. Melt compounding using a Plastograph mixer (16 cm3) for 4 min at 160 °C was also applied to obtain PHBV composites with micro-particles of keratin at 0.5, 1, 3, 5, 10, 25 and 50 wt%. The composites exhibited improved mechanical and water vapour and oxygen barrier properties with only 1 wt% of micro-filler (Pardo-Ibáñez et al. 2014). Moustafa et al. (2016) studied the use of roasted coffee grounds as micro-reinforcing agent to produce high-quality biodegradable Polybutylene adipate co-terephthalate (PBAT) composites for food packaging applications. The composites were extruded at 160–165 °C with a screw speed of 100 rpm for 5 min mixing. The films were obtained by a special die attached to the mixing chamber. The authors observed an increase in the hydrophobicity and thermal stability compared to the control films without fillers. A compatibiliser was not necessary to obtain a filler-polymer matrix with suitable interfacial adhesion, especially at low filler content (<30%), which was attributed to the good grindability of roasted coffee, which improved compatibility and filler dispersion during processing.

As concerns the incorporation of nano-fillers in composites, casting is the most commonly used method due to the better dispersion (more limited aggregation) of nano-particles in a liquid medium. Casting is suitable for the obtaining of coatings, mulch films and flexible films. In some cases, this technique has been used as a preliminary test to study the filler effect before thermomechanical processing with actual industrial applications. Boumail et al. (2013) characterized trilayer antimicrobial films based on methylcellulose and PCL composites with 2% of cellulose nanocrystals (CNC). These were prepared under stirring before sonication at room temperature for 30 min and the subsequent casting of the PCL-CNC dispersion. The trilayer films were obtained by compression moulding at 120 °C. An increase in the matrix toughness and greater stability of active components was found by the filler addition. Fortunati et al. (2013b) incorporated the CNCs and Ag-NPs PLA films obtained by casting, leading to improved barrier properties and antimicrobial activity in the composites. Ag-NPs (20, 40 and 80 mg Ag-NPs) have also been included in other polymers, such as agar matrices; although a significant increase in the thermal stability of the material was obtained, with improved water vapour barrier properties and antimicrobial activity against Listeria monocytogenes and Escherichia coli O157:H7, the mechanical strength and stiffness of the composites slightly decreased with respect to filler-free polymers (Rhim et al. 2014). Chitosan composites with MgO nano-filler, obtained by casting, also exhibited antimicrobial properties. In addition, the metallic oxide improved the mechanical properties and reduced the water swelling capacity and solubility and water vapour permeability of the films, which became less transparent.

Other nanocomposites with organic (CNC) or inorganic nano-clays:Mnt) obtained by casting , using an alginate matrix, exhibited improved functional properties with respect to the net polymer matrix (Abdollahi et al. 2013). The addition of both nano-fillers improved the tensile properties of the material and promoted a decrease in water solubility and water vapour permeability, whereas the surface hydrophobicity increased with the use of CNC but decreased with nano-clays . In the same way, nanoparticles of silicon carbide increased the thermal stability and reduced the oxygen permeability in starch composites (Dash and Swain 2013). This material could be used as adhesive and coating in food applications. Halloysite nanotubes promoted a decrease in film wettability and improved the mechanical properties in composites of pectin and polyethylene glycol blends (Cavallaro et al. 2013).

4.4.2 The Effect on Tensile Properties

In this section, the changes in the tensile properties of some biopolymers caused by incorporating nano- and micro-fillers of differing characteristics are analysed, as summarized in Table 4.3. The main mechanical properties characterized in plastic packaging materials are elastic modulus (EM) and the tensile strength (TS) and elongation (ε) at break, which provide information about the rigidity and resistance to deformation and break of the material, respectively.

Table 4.3 Changes in tensile properties of some biopolymers by incorporating micro- or nano-fillers of different nature
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The changes in the polymer’s functional properties caused by filler addition are strongly associated with surface properties and polymer-filler interfacial interactions. In this sense, the effects caused by fillers on the mechanical properties of polymer are not always positive. The main reason is the interruption of the polymer matrix continuity, but this effect could be diminished if the polymer and fillers have chemical affinity or an interfacial agent is added into the composite formulation. As previously mentioned, Berthet et al. (2015) found a deterioration in the tensile properties of neat PHBV when wheat straw fibres were added to the matrix. Strain and stress at break decreased by 61% and 63%, respectively, with 30 wt% filler in the composite. However, Young’s modulus increased by 13% with 20 wt% filler. Moustafa et al. (2016) found different trends in the tensile strength with the variation in filler content, using differently roasted coffee ground (CG) in PBAT matrices. As compared with neat PHBV, the best tensile behaviour was obtained with 10% of filler roasted at the highest temperature (270 °C), while the worst behaviour was observed for 30% of non-roasted CG powder. As the authors mentioned in their study, roasted CG showed a better affinity with PBAT compared to untreated CG when obtaining green composites without the need for a compatibiliser. This is important because the greater chemical affinity among the components led to stronger structures and better tensile properties of the composites. The values of the strain at break were lower than in the net polymer films for every composite. Pardo-Ibáñez et al. (2014) also found a decrease in the elongation at break of PHBV composites in line with differing quantities of keratin microparticles, but the elastic modulus at low loading was significantly improved. The content of filler greatly affected the tensile behaviour of composites and, in general, low filler contents improved both tensile strength and elastic modulus.

As regards nano-fillers, different studies have shown the improvement in tensile strength and elastic modulus when nanoparticles are incorporated into the polymer matrices. This effect is enhanced by a good particle distribution, a chemical affinity between filler and polymer and a high aspect ratio of the particles and filler-polymer contact area (more interactions). However, the processing conditions and the filler-polymer ratio must be taken into account to optimize the composite properties. Excessively high filler content results in the polymer matrix interruption and formation of micro- nano-cracks. Herrera et al. (2016) studied the effect of the incorporation of CNCs and chitosan nano-crystals (ChNC) into PLA films obtained by extrusion and compression moulding using fast and slow cooling rates. The tensile properties of composites were affected by both the chemical nature of the filler and processing conditions. Strength at break was improved by CNC incorporation, both at fast and slow cooling rates, but ChNC only improved the film strength when processing at a slow cooling rate. Young’s modulus increased and elongation at break decreased after the addition of both nanoparticles at both processing cooling rates. Alves et al. (2015) studied the effect of CNC and gelatin on corn starch plasticised films. Nanocrystals were added at 0.44, 1.5, 2.56 and 3% with respect to the polymers. They found a significant increase in the film resistance when the CNC ratio rose, with a slight fall in the elongation values. Similar results were obtained in nanocomposites based on pectin/polyethylene glycol blends containing halloysite nanotubes (Cavallaro et al. 2013). The incorporation of nanotubes led to significantly more rigid films, but reduced the elongation capacity of the material.

Metallic oxide nanoparticles have been used as a suitable option to improve the mechanical properties of biopolymers. Sanuja et al. (2014) reported a significant increase in both the tensile strength and elongation at break of chitosan composites with 10% magnesium oxide, obtained by casting. Abdollahi et al. (2013) compared fillers of differing chemical characteristics (Mnt and CNC) added to alginate matrices at 1, 3 and 5%. The tensile strength values exhibited a constant growth as the CNC content increased but this parameter decreased when the content of Mnt was higher than 1%. Young’s modulus also behaved differently, depending on the filler. The composite stiffness was higher as the CNC content increased from 0 to 5%, but decreased when the Mnt content rose from 3 to 5%. However, the elongation values exhibited the same trend for both kinds of nanocomposites; decreasing when both CNC and Mnt contents increased.

4.4.3 The Effect on Barrier Properties

Some biodegradable materials need to improve the gas and water vapour permeability because it is a fundamental property in packaging (Ng et al. 2015). The incorporation of micro- and nano-fillers, of organic or inorganic nature, to biodegradable polymer matrices can modify the barrier properties (oxygen permeability: OP and water vapour permeability: WVP). The barrier properties of polymers containing fillers depend on their chemical nature, the particle size and shape of the particles, as well as other factors such as polarity, hydrogen bonding capacity and polydispersity (González et al. 2015; Pardo-Ibáñez et al. 2014). Table 4.4 summarizes the different effects several fillers were observed to have on the barrier properties of some biopolymer matrices. In every case, the addition of fillers can improve the oxygen permeability and the values of the control samples (without filler) were in the range of the corresponding composites or at the high end, especially when CNC fillers were used. Fabra et al. (2016) studied the incorporation of bacterial cellulose nanowhiskers (BCNW) into thermoplastic corn starch matrices (TPCS) and a 95% decrease in OP was obtained with 15% of filler with respect to the TPCS sample. The reinforced TPCS was assembled in multilayer PHB films to obtain more hydrophobic matrices and to improve the film performance. The best functional properties of multilayers were obtained with 15% BCNW-TPCS composite inner layer and PHB outer layers. Luzi et al. (2016) observed a reduction of 47% in OP with the incorporation of 3 wt% CNC from Carmagnola carded hemp fibres and a commercial surfactant into a PLA-PBS matrix. The addition of a surfactant could contribute to a better dispersion of the CNCs and PBS. They observed that the CNCs were well dispersed in the polymer matrices, through the Atomic Force Microscopy (AFM) analysis of surface roughness.

Table 4.4 Changes in barrier properties (oxygen and water vapour permeability) of some biopolymers by incorporating micro- or nano-fillers of different nature
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As regards values, analyses of WVP in composites provide similar tendencies to those OP values. In most cases, WVP was reduced after the incorporation of fillers, especially nano-size ones. Table 4.4 shows the different effect of micro- and nano-fillers on this barrier property for several biopolymer matrices. Raw lignocellulosic fibres (Berthet et al. 2015) or cellulose fibres obtained after chemical treatments (Ludueña et al. 2012) only slightly decreased, or did not affect, the WVP of some composites due to their hydrophilic nature. However, Pardo-Ibáñez et al. (2014) observed a 59% reduction in WPV of the PHVB matrices with 1 wt% of keratin micro-filler, while no significant differences were obtained with a filler load higher than 5 wt%. They detected a homogenous particle distribution in the matrix at 1 wt% filler, where the micro-particles are not aggregated, causing an increase in the tortuosity factor for the diffusion of gas molecules, which reduced the permeability values. On the other hand, the incorporation of nano-filler into different polymer matrices, especially CNCs, improved the WVP values . The crystalline structure of nanocrystals makes it difficult for certain molecules (O2, CO2, H2O) to diffuse into the biopolymer matrix because of the formation of a hydrogen-bonded network (Brinchi et al. 2013; Ng et al. 2015), which favours the development of a percolation network, as reported by Miranda et al. (2015); the incorporation of 1 wt% of CNC from gravata fibres into thermoplastic corn starch matrices, provoked a 30% decrease in WVP. The addition of 2.5 wt% of CNC from sugarcane bagasse into matrices made up of corn starch and CMC decreased WVP by 50% due to the impermeable crystalline structure of CNC and a good dispersion of CNCs, creating a highly tortuous path for water vapour transfer (El Miri et al. 2015).

The use of inorganic fillers had similar effects on barrier properties to those of organic nano-fillers. The improvement in the barrier properties is due to the increased tortuosity factor for the gas molecule mass transport in the matrix and the impermeable nature of fillers, as reported by Abdollahi et al. (2013) in alginate films with 5% of Mnt nano-clays, where WVP decreased by about 20%. Studying agar films reinforced with Ag-NPs, Rhim et al. (2014) observed that the dispersed phase of Ag-NPs in the polymer impeded the mobility of its chains, inducing an improvement in WVP of the composites.

4.4.4 The Effect on Thermal Properties

The effects of micro- and nano-fillers of differing characteristics on the thermal properties of some biopolymers have been studied by several authors. Table 4.5 shows the main calorimetric parameters obtained from Differential Scanning Calorimetry (DSC) and the thermal stability of different materials obtained by Thermogravimetric Analysis (TGA) for different biopolymers and composites. Information about glass transition temperature (Tg), crystallisation temperature (Tc), melting temperature (Tm), melting enthalpy (∆Hm), onset temperature (Tonset) and peak temperature (Tpeak) of thermodegradation are given in the Table 4.5.

Table 4.5 Changes in thermal behaviour (calorimetric parameters and thermal degradation) of some biopolymers by incorporating micro- or nano-fillers of different nature
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In general, the addition of micro- or nano-fillers can modify the Tg and crystallization/melting properties (Tc, Tm, ∆Hm) of polymer in line with the established interactions between particles and polymer chains. As expected, the addition of plasticizers to the filler-biopolymer blends decreases both the Tg and melting point (Tm). In this sense, Martino et al. (2015) analysed the effect of different plasticisers, such as ATBC (acetyltributyl citrate), GTA (glycerol triacetate) and PEG (polyethylen glycol) in PHVB films with 20 wt% of wheat straw fibres. Blends with ATBC showed the strongest Tg reduction due to its non-polar nature and great affinity with the polymer. Similar effects were observed in both polymer and composites. Cano et al. (2015) observed a ~5 °C reduction in Tg of pea starch-PVA (1:1) matrices after the addition of 3 wt% of CNC from MCC. This was related to the partial inhibition of the PVA crystallisation and to the lower mean molecular weight of the amorphous PVA fraction.

As regards the thermal degradation of materials , the addition of fillers generally improves the thermal stability of composites for both micro or nano fillers. The network of the matrices becomes more resistant to heat based on the inherently high heat resistance of organic and inorganic fillers. Lignocellulosic fillers , such as wheat straw fibres (Berthet et al. 2015; Martino et al. 2015), kenaf fibres (Moriana et al. 2011), garlic straw (Kallel et al. 2016), rice husk (Johar et al. 2012), sisal fibres (Santos et al. 2015), pineapple leaf fibres (Shih et al. 2014), soy hull (Flauzino Neto et al. 2013), rice straw (Boonterm et al. 2015), coconut husk fibres (Rosa et al. 2010) or banana peel waste (Hossain et al. 2016) decompose in the temperature range of 150–500 °C: specifically, hemicellulose decomposes mainly from 150 to 350 °C, cellulose at between 275 and 350 °C and lignin undergoes gradual decomposition in the range of 250–500 °C. This high/wide range of decomposition temperatures promotes the greater thermal resistance of composites. Moriana et al. (2011), found a Tpeak increase of 6% when natural micro-fibres (cotton, kenaf and hemp fibres) were incorporated into starch-based composites. The greatest increase was obtained with kenaf fibres , probably due to the better compatibility between this filler and the starch matrix. This was associated with the higher content of hemicellulose, which promotes the hydrogen bonding between the fibres and the matrix, improving the interfacial adhesion and thermal stability. However, with other biopolymer matrices, the addition of micro-fibres did not affect the thermal stability as described by Berthet et al. (2015) for PHBV-wheat straw micro-fibres blends. The presence of lignocellulosic micro-fibres could contribute to a reduction in the mean polymer molecular weight of the blend, reducing the overall thermal stability, as was also observed by Ludueña et al. (2012) in PCL-cotton micro-fibre films.

The particle size reduction from micro- to nano-scale of fibres (e.g. by means of alkali and bleaching treatments of lignocellulosic material and acid hydrolysis to obtain pure cellulose nanocrystals (Brinchi et al. 2013; Jonoobi et al. 2015; Zhou et al. 2016), implies a high yield in thermal resistance, as well as in the previously mentioned barrier and mechanical properties. The incorporation of CNCs into biopolymer composites improved their thermal stability due to the crystalline structure and compact chains present in the nanocrystals, which are not easily dissociated by heating, increasing the thermal stability (Ng et al. 2015). Arrieta et al. (2014a, b and 2015) reported greater thermal stability in PLA-PHB blends reinforced with 1 or 5 wt% of CNC, from commercial MCC, obtained by electrospinning or extrusion processes. Similar behaviour was observed by Cano et al. (2015) in PVA-starch matrices with 1, 3 and 5 wt% of the same reinforcing agent.

As concerns the influence of inorganic nano-fillers on the thermal properties of composites, they also enhanced the thermal stability of biopolymer matrices. Rhim et al. (2014) studied the use of Ag nanoparticles in glycerol plasticised agar matrices obtained by casting. The thermogravimetric analysis exhibited a high residual mass of the composite films due to the inclusion of the more thermically stable metallic nanoparticles. Cavallaro et al. (2013) obtained pectin-PEG blends with nano-clays, specifically hallosyte nanotubes, at 5, 10, 15, 20, 30 and 50 wt% by casting. The thermal degradation analyses reflected the fact that nano-composites had a high degree of thermal resistance in comparison with the control sample, which was attributed to the fact that the nano-clay lumen can encapsulate the pectin degradation products delaying the process. Moreover, the good dispersion of the nano-filler inside the polymer matrix improved the thermal stabilization of the biopolymer.

4.5 The Surface Properties of Micro- and Nano-Reinforced Polymers for Food Applications

In this section, recent studies into the effect of the addition of micro- and nano-fillers of differing characteristics on the surface properties of some biopolymers are analysed, and summarised in Table 4.6. The main changes in biopolymer functional properties caused by the addition of a filler are strongly associated with surface properties and the interfacial interactions between biopolymer and filler. Several methods have been used to characterise the morphology and the surface composition/structure of biomaterials, such as contact angle, electron spectroscopy for chemical analysis (ESCA) or X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), atomic force microscopy (AFM) (Gutiérrez et al. 2018). The surface properties of the composites can directly impact on the macroscopic observation of the material gloss, which can have notable influence on their practical applications. In this section, recent studies into the effect of different fillers on the surface hydrophobicity (contact angle), topographic analysis (AFM) or sample gloss are discussed and summarised in Table 4.6.

Table 4.6 Effect of fillers on surface properties of different composite films
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The contact angle (θ) of a liquid in contact with a solid material mainly depends on the balance between the adhesive liquid-solid forces and the cohesive forces of the liquid (Gutiérrez et al. 2018). Aqueous or organic solvents can be used on a determined material in order to characterise the relative affinity of the material for polar or non-polar systems, thus obtaining information about its respective wettability properties according to the hydrophobic-hydrophilic nature of the surface. The θ values vary according to the type of biopolymer and nature of the filler (organic/inorganic). The inorganic fillers, such as nano-clays, could negatively affect the surface hydrophobicity of matrices due to their great water affinity, as reported by Abdollahi et al. (2013) for matrices of alginate-Mnt nano-clays at 1, 3 and 5 wt%. The film’s surface was more hydrophilic than the control sample mainly due to the hydrophilic nature of the Mnt also present at surface level. However, the same authors observed an 87.5% increase in hydrophobicity when they used CNC from commercial MCC in the same matrices. Films with 5 wt% CNC exhibited the highest degree of hydrophobicity, which was associated with the high ratio of CNCs at surface level and their crystalline nature, with lower water affinity than the alginate matrix. Similar results are reported by Slavutsky and Bertuzzi (2014) for thermoplastic starch (TPS) matrices reinforced with CNCs from sugarcane bagasse. The water contact angle increased (rise in the surface hydrophobicity) when CNCs were incorporated into TPS, while strong interactions and the formation of hydrogen bonds between the starch chains and CNCs are expected. These strong internal bonds could also reduce the surface interactions between water molecules and the material. Cao et al. (2008) obtained nanocomposites with TPS and CNCs from hemp fibres and they also observed an increase in the water contact angle or surface hydrophobicity of the matrices. On the contrary, the incorporation of CNCs into hydrophobic polymer matrices, such as PBS, enhanced the water wettability of the films (decrease in contact angle). This could be expected from the surface presence of the cellulose hydroxyl groups, which favour water affinity at the surface (Fortunati et al. 2017).

From the AFM analyses , the presence of nanoparticles on the composite surface and their aggregation/isolation state can be assessed, while their effect on the surface roughness can be verified. Arrieta et al. (2014b) studied the surface properties of PLA-PHB matrices containing CNCs from MCC. The AFM analysis showed the presence of some agglomerated and individualised CNCs in matrices. The aggregation of nanoparticles was reduced by the use of surfactants, which allowed for a better polymer chain interaction with the cellulose nanostructure. Nevertheless, an opposite effect was deduced by Fortunati et al. (2016) from the AFM images for CNCs in wheat gluten matrices, probably due to the different kinds of interactions between cellulose and the amphiphilic protein chains. The tendency of nanocrystals to aggregate has been widely found in numerous studies (Brinchi et al. 2013; Ng et al. 2015; Zhou et al. 2016) due to the spontaneous tendency to reduce the interfacial free energy of the system, accumulated at the contact surface area. The aggregated nanocrystals can be successfully dispersed and homogenised by strong mechanical shearing effects into a homogeneous suspension (Ng et al. 2015) or with the incorporation of surfactants to achieve a good dispersion in the matrices (Hu et al. 2015; Kaboorani and Riedl 2015); all of this is dependent on the nature of the polymer and filler and the processing conditions.

The influence of fillers on the material gloss is related with the surface topography achieved in the composite. Materials with aggregated fillers exhibit greater surface roughness so that they are less bright than other homogeneous material with a smoother surface. Fortunati et al. (2016) studied the homogeneity of CNC and cellulose nano-fibril (CNF) dispersion in wheat gluten composites and observed changes in the material gloss as a function of the filler. In the case of bionanocomposites that are reinforced with CNC, the values of gloss increase as a function of the filler percentage while the opposite behavior was observed in the CNF-reinforced materials. This could be related with the presence of CNF aggregates on the composite surface , evidenced by optical microscopy, whereas in CNC nanocomposites, nanoparticles were homogeneously distributed in the matrix. Cano et al. (2015) observed that the addition of CNCs to pea starch-PVA matrices did not affect the gloss values, as compared with the control samples, which was attributed to the good CNC dispersion in the biopolymer blends, with strong adhesion forces between the filler and the matrix.

4.6 The Effect of Reinforcing Agents on the Material Biodegradability

The disintegration and biodegradation behavior of the materials is analyzed through their composting under controlled aerobic processes, designed to produce organic residues from the biodegradable parts of the material, by the action of microorganisms. In this sense, ISO standards establish methodologies, where specific disposal pathways, specific time frames and criteria are indicated in order to unify a proper composting analysis (Cano et al. 2016). The biodegradation behavior is a crucial factor for the purposes of developing environmentally-friendly packaging materials. Biodegradable polymers are able to decompose in the medium by the enzymatic action of microorganisms in a defined period (Nair et al. 2017). In the disintegration and biodegradation processes produced by the action of microorganisms (bacteria, fungi and algae), these identify the polymer as a source of energy to produce organic residues from the biodegradable materials. These chemically react under the microbial enzymatic action and the polymer chains are fragmented (Cano et al. 2016; Nair et al. 2017).

Table 4.7 shows the effect of some reinforcing agents on the composite disintegration or biodegradation| , using different composting conditions. The degradation rate of the materials varies according to the type of polymer and reinforcing agent. Ludueña et al. (2012) analysed the biodegradation behavior of PCL composites containing cotton fibres and CNC from cotton fibres at 20 °C and 40% relative humidity (RH), using compost material with the natural microflora present in soil (Pinocha type). The analysis was carried out throughout 6 months by controlling the average weight loss of the samples. They observed that the biodegradability of the reinforced material was enhanced with the addition of both kinds of fillers, which was attributed to the high hydrophilicity of the natural fibres, which promoted water transport and provided a rougher support for microbial growth. PCL is semicrystalline polyester and the reduction in the degree of crystallinity benefits the biodegradation process , since the amorphous regions are more quickly attacked by microorganisms. Similar conclusions were reported by Luzi et al. (2016), for PLA-PBS composites reinforced with CNC from Carmagnola carded hemp fibres submitted to composting in sawdust, rabbit food, compost inoculum, starch, sugar, oil and urea, at 58 °C and 50% HR throughout 90 days. The authors evaluated the degree of disintegration (D) and the physical changes and observed that the presence of CNC in the matrices benefited the biodegradation process. Likewise, the use of a hydrophilic surfactant improved the dispersion of cellulosic nano-fillers in the matrices and the D parameter. The biodegradation | in blends with PBS was retarded due to the more hydrophobic and semicrystalline nature of PBS. However, Fortunati et al. (2014) observed CNC from Phormium tenax leaves had the opposite effect on PLA composites. The presence of CNCs increased the crystallinity of the composites, limiting the water transport through the PLA matrices. When limonene was incorporated as plasticiser in the PLA composites, an increase in the biodegradation rate was observed due to an improvement in the chain mobility, which favoured the polymer erosion. Nevertheless, each sample was 90% disintegrated after 14 days of composting, which is within the limit defined by the ISO 20200. In PBS composites containing CNC from MCC, similar behavior was observed, but the hydrophobic nature of PBS and the degree of crystallinity slowed down the biodegradation process (Fortunati et al. 2017).

Table 4.7 Effect of reinforcing agents on the material biodegradability
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The addition of inorganic nano-fillers provoked similar effects to those brought about by organic fillers in the biodegradation process of composites . Ramos et al. (2014) studied the effect of Ag nanoparticles on the disintegrability of PLA composites at 58 °C, using a compost media made from sawdust, rabbit food, starch, oil and urea, throughout 35 days. They assumed that Ag atoms could catalyse the disintegration process and the synergies between the Ag-NPs and thymol could accelerate the hydrolysis process. The presence of homogeneously dispersed thymol in the PLA matrix could promote the polymer chain mobility and thus, diffuse the water molecules through the PLA structure. Cano et al. (2016) also observed an increase in the film disintegration rate when different ratios of Ag-NPs were incorporated in starch-PVA composites. Nevertheless, the generation of CO2 as the result of total carbon conversion was notably reduced when the Ag ratio increased, probably due to its antimicrobial effect on the microorganisms responsible for the biodegradation process. Fukushima et al. (2013) also observed an increase in the disintegration rate of PLA composites with Mnt and fluorohectorite nano-clays at 5 and 10 wt%. The biodegradation of PLA | matrices was enhanced by the catalytic effect of the hydroxyl groups of silicate layers.

It is remarkable that many factors can affect the degradation rate of composites. The environmental conditions have a significant impact on microbial growth and parameters, such as humidity, temperature, pH, salinity, oxygen pressure, and microbial nutrients, have a great influence on the microbial degradation of polymers. The biodegradation process also depends on the chemical and physical characteristics of the biopolymer. Nair et al. (2017) report that the enzymatic degradation implies the binding of the enzyme to the bioplastic surface, followed by hydrolytic split; biopolymers are degraded into low-molecular-weight oligomers, dimers, monomers and finally mineralised to CO2 and H2O. For instance, the biodegradation of PLA | starts with the hydrolysis of the polymer chains promoted by the water diffusion in the matrices. When the molecular weight reaches about 10,000–20,000 g mol−1, microorganisms, such as fungi and bacteria, can metabolise the macromolecules, converting them into carbon dioxide, water and humus (Luzi et al. 2016; Fortunati et al. 2014; Fukushima et al. 2013). Several microorganisms are able to decompose biomaterials, such as Tritirachium album, Amycolatopsis strain 41, Amycolatopsis sp. strain 3118, Kibdelosporangium aridum for PLA, Penicillium sp. strain 26–1 (ATCC 36507), Aspergillus sp. strain ST-01, Clostridium sp. for PCL or Pseudomonas sp., Bacillus sp., Streptomyces sp., Aspergillus sp. for PHB (Nair et al. 2017).

4.7 Final Remarks

The incorporation of reinforcing agents of different natures (organic or inorganic) and size (micro or nano-sized) represents a good strategy for the purposes of improving the functional properties of biopolymers. In general, improved barrier and mechanical properties can be achieved when compatible micro or nano particles are adequately dispersed in the polymer matrix. Nano-particles are generally more effective, but their natural tendency towards aggregation makes the dispersion process difficult, requiring carefully designed dispersion techniques. To a great extent, the surface interactions of the filler with the polymer matrix define the effectiveness of the reinforcement and the promotion of barrier properties. Therefore, if materials with optimal functionality are to be obtained, it is of relevance to make an adequate selection of both the filler for a determined polymer matrix and the processing conditions necessary to ensure high dispersion levels of the particles. Composite biodegradability is generally enhanced by the presence of the filler dispersed particles. However, the total conversion of carbon to CO2 through the action of microorganisms could be limited when the filler exhibits antimicrobial action.