4 Zinc Composite Materials and Food Packaging

4 Zinc Composite Materials and Food Packaging R. Venkatesan*, T. Thendral Thiyagu and N. Rajeswari Department of Printing Technology, Faculty of Science and Humanities, College of Engineering, Guindy, Anna University, Chennai 600025, Tamil Nadu, India

Abstract

Zinc is an inorganic compound widely used in different packaging applications because it shows good antibacterial properties and potential applications in food preservation. The ZnO is generally recognized as safe and it is listed in the FDA as a food additive. Zinc-based composites have increased packaging performance properties such as mechanical strength, oxygen barrier, water barrier, and antimicrobial activities. This chapter covers the importance of zinc-based composites for food packaging applications; various preparation methods of different polymers such as polyethylene (PE), poly lactic acid (PLA), and poly (butylene adipate-coterephthalate) (PBAT) with zinc composites; and the required characteristics for ideal food packaging. There are many metal oxides used in the field of packaging, but zinc oxide has wide applications in the field of food packaging. Keywords: Zinc oxide, Biodegradable polymer, Composites, Mechanical strength, Oxygen and Water barrier, Antimicrobial activity.

4.1 Introduction Food products and beverages are usually packed to ensure safety, quality, and wholesomeness from the time of production to its consumption. Packaging material that is intended for food packaging should withstand harsh conditions during transport, distribution, marketing, storage, and end use. Primarily, the package has to protect the food from chemical, *Corresponding author: [email protected] Giuseppe Cirillo, Marek A Kozlowski and Umile Gianfranco Spizzirri (eds.) Composites Materials for Food Packaging, (153–174) © 2018 Scrivener Publishing LLC

153

154 Composites Materials for Food Packaging biological, physical, and mechanical hazards. Besides protecting food, packaging material should offer traceability, convenience of handling, and tamper indication. But the ultimate aim of food packaging is to contain food in a cost-effective way that suits the requirement of industries and consumer preferences more importantly with minimum environmental pollution [14].

4.2 Food Packaging The use of improved packaging materials that can minimize the deterioration of the food has always been the main emphasis of the packaging industries. Increasing population has also increased the demand for the fresh and best quality food products and thus, more efforts have been made by researchers to meet the ever-increasing demands for safer food packaging. Food packaging offers the ability to increase the shelf-life of the products and also to change the sensory properties of the packaging material without harming the packed food. Various important methods used for extending the shelf-life are: • • • • • •

Reduced temperature Thermal processing Water reduction Chemical preservation Irradiation Modified atmospheric packaging

4.3 Polymers in Food Packaging Plastics are low cost, lightweight materials and durable in nature. It is molded into different shapes of products and used in a wide range of applications. At present, the usage of polymers is very huge in packaging particularly in the field of food packaging due to flexibility and barrier properties. On the other hand, recycling is the most important technique which is currently used to reduce the generation of packaging waste and it may be used for disposal but it generates air pollution. Hence, biodegradable plastics are an important area of many researchers focusing as an alternative material particularly in the area of food packaging. The common packaging material over the past few decades have been paper, paper and board, glass, metal, and plastics. Among all the materials,

Zinc Composite Materials and Food Packaging 155 the usage of plastics is very massive in the field of food, pharmaceutical, and other packaging applications. But the usage of polymer causes environment problems due to its non-degradability in nature. The biodegradable polymers are alternative to synthetic plastic packaging materials and it is used in various forms such as films, sachets, and flexible and rigid containers in the field of food packaging applications. Biodegradable polymers are materials that have the ability to be totally decomposed in nature by microorganisms in the soil. Advances in the production of biodegradable polymers and their processing led to the idea of green polymer packaging materials. Today, nearly most of the polymerbased packaging materials are produced from synthetic non-degradable materials. Replacement of synthetic polymers by biodegradable polymers has been taken seriously for a decade. Research and some early applications proved that biodegradable polymers have the potential to be used for packaging as blends with synthetic or natural polymers and composites. Although there is an extensive research and investment in the biodegradable polymer area, biopolymers are still not competitive to commonly used synthetic polymers [2, 4, 11]. Biodegradable polymers are classified into three categories according to their origin and method of production: • Polymers produced by microorganisms or genetically modified bacteria such as poly hydroxyalkonates (PHAs). • Polymers produced by chemical synthesis from renewable bio-derived monomers like poly (butylene adipate-co-terephthalate) (PBAT). • Polymers directly extracted from biomass such as proteins/ polysaccharides. Different biopolymers are prepared and are available in the market. These biopolymers are derived or extracted from natural sources, for example, polysaccharides such as starch and cellulose and proteins such as casein and gluten. The other classes of bio polymers namely PLA, PBAT, poly (ε-caprolactone) (PCL), and polyvinyl alcohol (PVA) are prepared by chemical synthesis using renewable bio-based monomers. But, there is a restriction by the use of these natural polymers or biodegradable polymers in food packaging because of their less mechanical and barrier properties. To use as a packaging material particularly for food packaging, it should have good mechanical properties and should also increase the shelf-life of food products by having good barrier properties. The different types of biodegradable polymer is shown in Figure 4.1.

156 Composites Materials for Food Packaging BIODEGRADABLE POLYMERS

Directly extracted from biomass

Synthesized from bio-derived

Proteins / Polysaccharides

PLA,PBAT

Product from microorganisms

PHB

Figure 4.1 Biodegradable polymer families to materials.

4.4 Nanotechnology Before proceeding toward the development of nanocomposites for food packaging, it is important first to define the term “nanotechnology.” Nanotechnology is the manipulation, arranging, or self-assembly of individual atoms, molecules, or molecular clusters smaller than 100 nm into structures to create materials and devices with new or vastly different properties. Lagaron et al. [9] defined nanotechnology in a precise way that it is the creation and utilization of structures with at least one dimension in the nanometer length scale that creates novel properties and phenomena otherwise not displayed by either isolated molecules or bulk materials. The word nanotechnology is generally tagged, when referring to materials with the size of 0.1–100 nm. However, there is no scientific reason in support of this specific upper limit [12]. In addition, nanotechnology is a multidisciplinary science, in which the concepts of physics, chemistry, and biology are to be applied.

4.5 Nano-Fillers Nano-fillers are particles which are added to the polymer to reduce the consumption or to improve the physical and chemical properties of the materials. They are relatively inexpensive, solid substances that are added into polymer matrices for adjusting the volume, weight, processing behavior, permeability, mechanical or thermal stability, etc. Fillers are classified mainly into two types: fillers that help in reducing cost are called inactive fillers and fillers which change the properties are called active fillers. The reinforcing filler can

Zinc Composite Materials and Food Packaging 157 be defined as the material capable of increasing the mechanical, thermal properties, abrasion resistance, etc. The properties of the composites are based on reinforcing fillers geometry, reinforcing characteristics, and orientation.

4.6 Classification of Nano-Fillers Nano-fillers are classified into three categories depending on their dimensions as follows. 1. Nanoclays: The particulates that are characterized by only one dimension in nanometer scale are nanoclays. For example, MMT clays. 2. Nanofibers: When two dimensions are in the nanometer scale and the third is larger, forming an elongated structure, they are generally referred to as nanofibers. For example, cellulose nanofibers. 3. Nanoparticles: When the three dimensions of particulates are in the order of nanometers, they are referred to as isodimensional nanomaterials or nanoparticles. For example, Ag, ZnO, etc.

4.7 ZnO Nanoparticles ZnO is non-toxic, affordable, widely available, and has good antibacterial activity. Due to thermal, binding energy, and antibacterial properties, it is used in a variety of applications namely cosmetics, coatings, and packaging. Zinc oxide nanoparticles are useful as antibacterial and antifungal agents when it is blended with plastic materials as fillers.

4.7.1 Advantages of ZnO Nanoparticles • As an additive for rubber and plastics. • As a stable, non-toxic platform for biomedical applications such as drug delivery. • The ZnO-based films are used for food packaging. • The nanoparticles are used in the packaging industry. • Improves stability and therapeutics index and reduce toxic effects. • Both active and passive drug targeting can be achieved by manipulating the particle size and surface characteristics of nanoparticles.

158 Composites Materials for Food Packaging

4.7.2 Limitations of ZnO Nanoparticles • Small size and large surface area can lead to particle aggregation. • Physical handling of nanoparticles is difficult in liquid and dry forms. • Limited drug loading. • Toxic metabolites may form.

4.8 Composites Composites are man-made material formed by the combination of matrix which is often called as continuous phase and fillers or fibers that are in dispersed phase. The composite materials have received tremendous importance since the overall properties of the composites are superior to those of the individual components that are very strong and stiff, light in weight, and have fatigue properties; do not corrode like steel, and have the ability to be used in high performance structural application and faster assembly. More and more researchers are now focusing their attention onto these materials due to their possible wide spectra of applications.

4.8.1 Classification of Composites Composite materials can be grouped into three major categories based on the matrix constituent - metal-matrix composites, ceramic-matrix composites, and polymer-matrix composites. The composites are mainly classified as shown in Figure 4.2.

Matrices

Metal Matrix Material

Ceramic Matrix Material

Thermoset material

Figure 4.2 Classification of composites.

Polymer Matrix Material

Thermoplastic material

Zinc Composite Materials and Food Packaging 159

4.8.1.1 Metal Matrix Composites In metal matrix composites, the matrix is the metal. Aluminum, magnesium, titanium, and zinc are metals generally used for making composites. The reinforcement phases in metal matrix composite systems are generally silicon carbide, carbon, boron, alumina, and glass.

4.8.1.2 Ceramic Matrix Composites Ceramic matrix composites are developed using alumina, calcium alumina silicate glass, glass-ceramic, and oxide ceramic as matrices. These matrices are reinforced by silicon carbide, silicon as fillers. Ceramics are strongly bonded materials that result in high strength and hardness. High temperature tolerance of super alloys is also offered by these composites.

4.8.1.3 Polymer Matrix Composites The polymer matrix composites are very common and it is considered as advanced composites. The properties such as strength and stiffness along with resistance to corrosion are provided by this material. These are characterized by low cost, high strength, and simple manufacturing principles. The materials with thermo-reversible cross-links are termed as thermoplastics. Thermoplastic polymers soften and eventually melt when heat is applied. Thermoplastics have a wide range of applications as they can be fabricated and re-fabricated in so many shapes. Some examples are food packaging, insulation, automobile bumpers, and credit cards. Commonly used commercial thermoplastics are polyethylene, poly (vinyl chloride), polypropylene, polystyrene, etc. Polymers that get cured irreversibly through chemical reaction, irradiation, or heating are called thermoset polymers. Thermosets are strong, durable, and primarily used in automobiles and construction. They are also used to make toys, varnishes, boat hulls, and glues. The major advantages of thermoplastic matrix composites over thermosets are the low processing cost, design flexibility, ease of molding complex parts of the former, etc.

4.8.2 Components of Composites 4.8.2.1 Matrix Matrix is generally the continuous phase that provides the composite system with toughness and ductility. The main role of the matrix in the composites is to transfer and distribute the applied stresses along the reinforcement

160 Composites Materials for Food Packaging phase. They protect the reinforcing phase from abrasion and degradation. They also adhere the dispersion phase together and cause them to act as a team in resisting failure or deformation under an applied load.

4.8.2.2 Fillers Fillers are fine particles added to polymer matrices to reduce the consumption or uptake of more expensive binding material or to improve some physical and chemical properties of the material. They are relatively inexpensive, solid substances that are added into polymer matrices to adjust volume, weight, cost, color, processing behavior, shrinkage, conductivity, permeability, mechanical properties, etc. Fillers are classified mainly into two types: fillers which help in reducing cost are called extenders or inactive fillers and the filler which changes the properties are called functional or active fillers so that the compound meets the requirements.

4.8.2.3 Nanocomposites Today, nanocomposites have emerged as new materials and have great interest and development in many areas of applications. This is due to their improved mechanical, thermal, and optical properties as compared to pure materials. In general, a nanocomposite is prepared by dispersing the inorganic or organic nanoparticles into a thermoplastic or thermoset polymer. A nanoparticle offers many advantages over usual macro- or microparticles due to their high surface area and adhesion between nanoparticles and the polymer. In the last 15 years, nanofillers provided an opportunity for the improvement of nanocomposite films [13]. Thus, the development of a nanocomposite has opened a new dimension in the field of material

Polymer molecules

Nanocomposites Nanoparticles

Figure 4.3 Schematic representations of nanocomposites.

Zinc Composite Materials and Food Packaging 161 science owing to their unique properties and potential applications in the automotive, construction, biomedical, and packaging industries. The nanocomposite preparation is shown in Figure 4.3.

4.8.3 Preparation of Nanocomposites The key to successful preparation of polymer nanocomposite films mainly depends upon the homogeneous dispersion of nanoparticles into the polymer matrix. A single method for the preparation of a polymer nanocomposite film is difficult due to the physical and chemical difference between each system. Each polymer system requires processing conditions based on the processing efficiency and characteristic properties of the materials. There are several processes to make polymer nanocomposites, namely solution casting, in situ polymerization, and melt blending. Every technique consists of a number of steps to get a polymer nanocomposite film. The development of polymer nanocomposites is driven by a different technique but all the techniques have its advantages and disadvantages. The methods of preparation of nanocomposite films are discussed as follows.

4.8.3.1 Solution Casting Solution casting is a liquid-state powder processing method that brings a good molecular level of mixing and is generally used in material preparation and processing. In this method, nanofillers are dispersed in the solvent in which the polymer is soluble. The polymer after swelling in the solvent

Ultra-sonication Polymer Dispersed Nanoparticles

Transient large aggregate

Dispersed solutions

Films

Figure 4.4 Schematic representation of solution casting.


Polymerization Initiator Monomer

Clay

A layer of clay

Polymer

Figure 4.5 Schematic representation of in situ polymerization.

Hot melt extrusion process Feeding polymer and API

Cooling Melting

Mixing

Homogeneous discharge

Pelletizing

Figure 4.6 Schematic representation of melt extrusion.

is then added to the nanofiller suspension and mixed well. Finally, the solvents are removed by evaporation under vacuum. The solvent casting technique is shown in Figure 4.4.

4.8.3.2 In Situ Polymerization Figure 4.5 shows the In-situ polymerization technique. In this method, the nanomaterials are first dispersed in a monomer solution. Polymerization is performed in the presence of nanoscale particles.

4.8.3.3 Melt Extrusion Figure 4.6 shows the melt extrusion processes. Melt extrusion is much commercially attractive than the other two methods, as both solvent casting and in situ polymerization are less environment friendly. This method involves direct inclusion of nanofillers into the molten polymer using a twin-screw extruder and adjusting the parameters such as screw speed, temperature, and time. The melt blending methods allow the use of polymers that were previously not suitable for in situ polymerization or solution casting.


4.8.4 Properties of Nanocomposites Polymer nanocomposites are a mixture of polymers with few wt% of inorganic fillers such as Ag, Zn, TiO2, SiO2, different nanoclays, etc. The nanocomposites can be produced by loading nanoscale range of inorganic fillers to enhance the mechanical, physical, and biological properties. These nanocomposites in packaging applications will play an important role in the selection of an appropriate polymer for applications and their properties are broadly discussed as follows.

4.8.4.1 Mechanical Properties The mechanical property is one of the primary important application performances for a packaging polymer. The crystalline and molecular weight of the polymer can influence the structural and morphological properties of the polymers. Tensile strength determines the strength required for the rupture of the packaging material and the ideal packaging polymer should have higher tensile strength with respect to the appropriate product selection for packaging.

4.8.4.2 Thermal Properties Polymers are widely used as packaging materials. However, many studies have been reported in order to improve the mechanical and thermal stability of the polymers used in packaging applications. Most of the biopolymers exhibit low thermal stability because of their weak chemical bonding. Incorporation of fillers in these polymers is a common approach to improve the mechanical and thermal properties. The addition of thermally stable fillers namely metal and metal oxides plays a major role in the nanocomposite film that are developed and finds suitable applications in the field of packaging based on the types of processing of food.

4.8.4.3 Barrier Properties A barrier property refers to the ability of a polymer to prevent the entry of undesired gases or water vapor and is characterized by solubility, permeability, and diffusivity across the barrier of the film. Additionally, the barrier properties are also dependent on the morphological properties of the material such as crystalline behavior, chain conformation, etc. In general, the crystalline nature increases the barrier properties particularly toward moisture. The enhanced barrier property ensures longer shelf-life of a package and also protects flavors or aromas that might be lost by permeation.


4.8.4.4 Antimicrobial Properties The requirements for antimicrobial plastic films are important in the field of packaging applications. The antimicrobial activities of the films are tested by the disk-diffusion method against Escherichia coli and Staphylococcus aureus. The inorganic filler-loaded plastic films exhibit distinctive microbial inhibition zones in the disk diffusion method for both the microorganisms and this may be recommended for antimicrobial food packaging applications.

4.8.5 Applications of Nanocomposites Nanocomposites represent as a promising alternative to conventional plastics owing to the dispersion of nanoparticles and their noticeable improvement toward mechanical, barrier, thermal, and other physical and chemical properties. Many industries such as automobile, construction, aerospace, medical, tissue engineering, drug delivery, and packaging sector have taken an initiative to capitalize in developing the new polymer nanocomposites for suitable applications.

4.8.6 ZnO-Based Composites in Food Packaging In this regard, zinc oxide (ZnO) nano-structured materials have gained much interest as it shows fascinating mechanical and optical properties that have led to a wide range of applications from packaging to food application [24]. Sawai et al. (2003) have synthesized the ZnO nanoparticles by the co-precipitation technique and it shows excellent antimicrobial activity [6]. Because of ZnO biocompatibility, it becomes a promising candidate for the substitution of ZnO-based nanoparticles in food packaging applications [20]. Different methods can be applied to synthesize ZnO including, sol–gel [5], hydrothermal (Suchanek 2009), and laser evaporation technique [15]. Among them, the sol–gel process is a simple and cost-effective method in which the high purity of the product can be obtained [8]. Biodegradable polymers are easily dissolved in suitable solvents, and the combination of ZnO and polymer results in improved mechanical and optical properties [1]. In this study, the effect of ZnO nanoparticles on the physical, mechanical, and antibacterial properties of polyethylene nanofilm was examined, which indicated that the addition of ZnO nanoparticles can enhance the properties of the film [23]. In a similar research, it was revealed that the insertion of ZnO nanoparticles resulted in improvements of physical, chemical, and antibacterial properties of polylactic acid (PLA) films [22].

PMMA/ZnO composites

ZnO/Poly(propylene carbonate) Solution blending composites

PVA/Zno nanocomposites

ZnO/biopolymer nanocomposites

Gelatin/ZnO nanocomposites

PLA/ZnO nanocomposites

PBAT/ZnO nanocomposites

2.

3.

4.

5.

6.

7.

8.

Solution casting

Melt compounding

Solution casting

Sono-chemical method

Solution casting

In situ emulsion polymerization

In situ preparation

ZnO/polyethylene composites

1.

Procedure

Composites

S. no

Table 4.1 Overview of ZnO-based Composites.

Food packaging

Packaging applications

Antimicrobial packaging

Self-cleaning textiles and foams

Food stuff packaging

Packaging applications

Packaging

Food packaging

Applications

R. Venkatesan and N. Rajeswari [24]

Roberto Pantani et al. [22]

Shiv Shankar et al. [25]

Sanoop et al. [18]

Ahangar et al. (2014)

Jongchul Seo et al. [7]

Maneesh Kumar et al. [17]

R. Tankhiwale and S.K. Bajpai [23]

Authors


166 Composites Materials for Food Packaging In another study, the mechanical, thermal and antibacterial properties of PBAT with the addition of ZnO NPs were recovered [24]. Although several reports on ZnO-based composites have appeared in the literature, it seems that a few of them focused on PBAT, which is a biodegradable polymer suitable for food packaging. Furthermore, the microstructure, mechanical, antibacterial, physical, and optical properties of PBAT nanocomposites were compared as a function of nano reinforced in filler loading contents. A short summary of ZnO-based composite materials with their applications has been given in Table 4.1.

4.8.6.1 Preparation of ZnO Composites Nowadays, there is an emerging interest in replacing non-renewable additives with biodegradable compounds. Biodegradable polymers play a very important role in this modification. Their low cost and low densities associated with high specific mechanical properties represent a good renewable and biodegradable alternative to the most commonly used synthetic reinforcement. The biodegradable composite films were developed by the film casting method using various biodegradable polymers with ZnO in different compositions. The film composites were characterized by mechanical, moisture absorption, water vapor permeability, oxygen permeability, and antimicrobial properties.

(a)

(b)

(d)

(e)

Figure 4.7 SEM images of the ZnO-based composite films.

(c)

(f)


4.8.6.2 Morphology of the ZnO Composites An observation of the morphology of composite films was performed by scanning electron microscopy. Figure 4.7 shows the micrographs of the ZnO-based PBAT composite films with different filler loadings. From Figures 4.3(b)–4.3(f) to 4.7(b)–4.7(f), it is evident that ZnO particles were distributed uniformly and individually in the PBAT/ZnO composite films with 1 to 10 wt.% ZnO content. However, from Figure 4.7(f), it can be observed that the ZnO particles were agglomerated at some places when the ZnO content was 10 wt.%. Further, voids do not appear in the composite films because the solvent completely evaporated in the composite films during the film casting. Also, no gaps appeared between the ZnO and PBAT due to the hydrophilic behaviour of filler and matrix.

4.8.6.3 Mechanical Properties of ZnO Composites Formation of composites with nano-ZnO has shown pronounced improvement in the mechanical properties with several polymers such as polyethylene, poly (ε-caprolactone), polylactic acid, poly (butylene adipate-co-terephthalate), etc., even with low filler loading. When nanoZnO was reinforced with nylon, it was found that the mechanical properties such as tensile strength, tensile modulus, and impact strength of the composite enhanced profoundly. The study on mechanical properties of polymer/ZnO composites revealed that the properties are dependent on Table 4.2 Mechanical properties of ZnO-based Composites. Tensile strength (MPa)

Elongation at break (%)

ZnO/polyethylene composites

40.9

26.7

2.

PMMA/ZnO composites

36.5

17.0

3.

ZnO/poly(propylene carbonate) composites

30.0

19.3

4.

PVA/ZnO nanocomposites

64.0

40.1

5.

ZnO/biopolymer nanocomposites

47.1

36.4

6.

Gelatin/ZnO nanocomposites

22.2

33.1

7.

PLA/ZnO nanocomposites

44.0

35.0

8.

PBAT/ZnO nanocomposites

45.1

18.0

S. no

Composites

1.

168 Composites Materials for Food Packaging 60

Tensile strength (MPa) Elongattion (%)

Mechanical properties

50

40

30

20

10

0 PE/ZnO

PMMA/ZnO ZnO/PPC

PVA/ZnO

ZnO/Bio Gelatin/ZnO PLA/ZNO PBAT/ZnO

ZnO Composites

Figure 4.8 Mechanical properties of the different ZnO composite films.

the filler content. Mechanical properties of the ZnO composite films were determined by a universal testing machine under the ASTM standards. In this study, mechanical characteristics of prepared films were mainly coming from the polypropylene base film. However, tensile properties of PP films were observed. In order to assess the mechanical performance of ZnO composite films, their tensile properties were studied. Fig. 4.8 shows the images of ZnO-based composite films. The effect of filler loading on the tensile (strength and elongation at break) properties of ZnO-filled composite films are presented in Table 4.2. From Table 4.2, it can be observed that the tensile strength increased with filler content up to 7 wt.% and decreased thereafter. This discontinuity could be attributed to increased filler quantity leading to a weaker fillermatrix interface and agglomeration of filler particles, which consequently decreases the strength. This is probably because of better interfacial adhesion between the filler and the matrix by the van der Waals or induction interactions. However, in all cases, the tensile strength of the ZnO composite films was found to be higher than that of the matrix. At the composition of 4 wt.% filler, the tensile strength of the composite films was found to be 64.0 MPa. From Table 4.1, it can be observed that the percentage elongation at break decreased with increasing filler loading. Increased filler loading in the matrix resulted in the stiffening and hardening of the composite films and reduced shape ductility.


4.8.6.4 Barrier Properties of ZnO Composites In several applications, such as packaging of oxygen-sensitive food products, it is desirable for the packaging material to have a high resistance to transmission of gases (such as oxygen and water vapor). Plastics having excellent barrier properties are required to get the desired characteristics in such packaging materials. However, packaging for respiring products such as fruits and vegetables must allow the transport of oxygen and carbon dioxide and hence should be permeable to these gases. Barrier property is inversely related to permeability, as lower value of permeability implies better barrier property. 4.8.6.3.1 Oxygen Transmission Rate This is a key factor in food packaging applications as the exposure of food products to oxygen can cause oxidation and undesirable changes in the food quality. The changes may include deterioration of odor, color, flavor, and nutrients. Hence, it is necessary to use food packaging films having high resistance to oxygen transmission to ensure quality and maintain long shelf-life. The standard methods for determining oxygen transmission rate (OTR) are ASTM D3985. The OTR is directly related to oxygen permeability, and is an important measure of barrier properties of the packaging film. Representative OTR trend for several ZnO composites is shown in Figure 4.9.

140

OTR (mL/m2 .24h)

120 100 80 60 40 20 0

ZnO/PPC

PLA/ZnO PVA/ZnO Gelatin/ZnO ZnO based composites

PBAT/ZnO

Figure 4.9 Oxygen transmission rate of the different ZnO composite films.

170 Composites Materials for Food Packaging 4.8.6.3.2 Water Vapor Permeability WVP is a critical parameter in food packaging applications as contact with water vapor may cause certain food items to lose texture. Polymer–water interaction, that is, hydrophilicity or hydrophobicity of the polymer, is a crucial factor affecting the WVP. In general, water would permeate preferentially through the hydrophilic portion of the film. Thus, films prepared from hydrophilic polymers are expected to allow a higher transmission rate of moisture than those prepared from hydrophobic polymers. However, increased hydrophobicity does not guarantee a higher resistance to moisture transmission. For instance, water vapor transmission rate (WVTR) for PBAT is much higher than that for PPC (see Fig. 4.10) although PBAT is more hydrophobic than PPC. Generally, the measurement of WVP is done by flowing air of controlled relative humidity on one side of the polymer film and flowing moisture-free nitrogen on the other side. The moisture level in leaving nitrogen stream is measured using a sensor. The standard methods for determining the WVTR are the ASTM F1249 standard method. Representative WVTR trend for several ZnO composites is shown in Figure 4.10. The comparison between OTR and WVTR indicates that ZnO-based composites are greatly effective in obstructing the oxygen permeation, but less effective in retarding the water vapor permeation. This result shows 70 60

WVTR (Wt. %)

50 40 30 20 10 0 ZnO/PPC

PVA/ZnO

Gelatin/ZnO

PLA/ZNO

ZnO based Composites

Figure 4.10 WVTR properties of the different ZnO composite films.

PBAT/ZnO

Zinc Composite Materials and Food Packaging 171 Table 4.3 Antimicrobial activity test results of ZnO composite films against S. aureus and E. coli. The zone of inhibition (mm) Strain

PE/ZnO

PPC/ZnO

PVA/ZnO

PLA/ZnO

PBAT/ZnO

E. coli

13.3

17.6

12.4

16.0

14.1

–

17.2

–

16.9

15.1

S. aureus

that these films may impede moisture transfer between the surrounding atmosphere and food, or between two components of a heterogeneous food product. This property is very much useful in packaging application. 4.8.6.3.3 Antimicrobial Properties of ZnO Composites The incorporation of antimicrobial compounds into food packaging materials has received considerable attention. Films with antimicrobial activity could help control the growth of pathogenic and spoilage microorganisms. An antimicrobial nanocomposite film is particularly desirable due to its acceptable structural integrity and barrier properties imparted by the nanocomposite matrix, and the antimicrobial properties contributed by the natural antimicrobial agents impregnated within [10]. Materials in the nanoscale range have a higher surface-to-volume ratio when compared with their micro-scale counterparts. ZnO materials have been investigated for antimicrobial activity so that they can be used as growth inhibitors. ZnO-based composites were tested for antimicrobial activity using E. coli and S. aureus. Table 4.3 shows the inhibition zone values that were formed by nanocomposites. The results are summarized and presented in Table 4.3. It is observed that a polymer that was used as a control matrix exhibited no antimicrobial activity when compared with other ZnO-based composite films. The observed results indicate that the composite films were highly active to kill all the tested pathogens, namely E. coli and S. aureus. The zone of inhibition was increased with the increase in the concentration of ZnO load from 1 to 10 wt.%. The ZnO-based composite materials displayed antimicrobial properties against the food pathogenic bacteria in E. coli and S. aureus.

4.9 Conclusions The composite films consisting of various biodegradable and different amounts of ZnO were prepared by the solution casting method. SEM results suggest that the strong interactions are formed between ZnO and

172 Composites Materials for Food Packaging biodegradable polymers. ZnO are dispersed homogeneously in the polymer matrix, according to the morphology of the surface due to compatibility and interactions between ZnO and biodegradable matrix. Tensile strength of the higher wt% ZnO-blended PVA composite film reached 64.0MPa compared with 22.2 MPa for gelatin/ZnO composites. Barrier properties of OTR and WVTR values are shown to be good when compared with pure polymers such as PE, PMMA, PVA, PLA, and PBAT. The improvement of barrier properties was attributed to the homogeneous and good dispersion of ZnO in a biodegradable polymeric matrix. In addition, the antimicrobial activity of ZnO-filled biodegradable composites is extraordinary against S. aureus and E. coli.

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Authors Profile Mr.R.Venkatesan is currently working as Research Scholar in the Department of Printing Technology, College of Engineering Guindy, Anna University and his Ph.D in the area of Biopolymer based Nanocomposites for Packaging applications. He received his UG in the discipline of Chemistry from Govt. Arts College, Tiruvannamalai, Thiruvalluvar University and PG in Polymer Chemistry from Department of Polymer Science, A.C. Tech, University of Madras. He is also working in the area of Polymer Science & Technology, Composites, and Food Packaging. His areas of interests are Biodegradable Polymers, Polymer Composites & Blends, and Nanostructure Materials. He published a lot of research papers in the International peer reviewed journals and presented many papers in both National and International conferences.