Chapter 1
Montmorillonite Composite Materials and Food Packaging

Aris E. Giannakas* and Areti A. Leontiou

Laboratory of Food Technology, Department of Business Administration of Food and Agricultural Enterprises, University of Patras, Agrinio, Greece

*Corresponding author: agiannakas@upatras.gr

Abstract

This chapter includes the recent trends in using montmorillonite (MMT)-based composite materials for food packaging applications. MMT is a naturally available phyllosilicate material that belongs to the group of smectites. Over the last few decades, it has found applications in many areas of nanotechnology such as catalysis, adsorption, and filtration. In recent years, it has also generated a wide range of applications in the food packaging industry. MMT has been used as an ideal nanofiller for polymer and biopolymer plastics, which leads to polymer and biopolymer nanocomposite films for food packaging with enhanced thermal and barrier properties. Incorporation of ions such as Ag+, Cu2+, and Zn2+ in clay platelets leads to nanocomposites with enhanced antimicrobial activity. Additionally, many strategies have been developed for immobilization of oxides, enzymes, essential oils, and other bioactive compounds in these platelets. This feature makes the MMT-based composite materials promising nanocarriers for smart and active packaging applications.

Keywords: Montmorillonite, oxides, essential oils, enzymes, antioxidant, antimicrobial, food packaging

1.1 Introduction

The word "nano" comes from the Greek for "dwarf" and denotes nanometer (10-9 m) [1]. The concept of nanotechnology was introduced by Richard Feynman in 1959 and the National Nanotechnology Initiative (Arlington, VA, USA), and involves the characterization, fabrication, and/or manipulation of structures, devices, or materials that have at least one dimension (or contain components with at least one dimension) that is approximately 1-100 nm in length. When particle size is reduced below this threshold, the resulting material exhibits physical and chemical properties that are significantly different from the properties of macroscale materials composed of the same substance [2]. Despite an explosion of growth in the area of nanotechnology, food nanotechnology is still a lesser known subfield of the greater nanotechnology spectrum, even among professional nanotechnologists. Potential uses of food nanotechnology include: (i) pesticide, fertilizer, or vaccine delivery; animal and plant pathogen detection; and targeted genetic engineering for agriculture, (ii) encapsulation of flavor or odor enhancers; food textural or quality improvement; new gelation or viscosifying agents for food processing, (iii) nutraceuticals with higher stability and bioavailability for nutrient supplements and (iv) pathogen, gas, or abuse sensors; anticounterfeiting devices; UV-protection and stronger more impermeable, antimicrobial, and antioxidant polymer films for food packaging. In order to enhance mechanical, barrier, antimicrobial, and antioxidant properties and to introduce sensor and UV protection ability in polymer and/or biopolymer films, various inorganic nanostructured materials [1] have been used including TiO2, ZnO nanoparticles, SiO2, carbon nanotubes, and nanoclays.

Nanoclays gathered the attention of the food packaging industry, due to their availability, low cost, significant enhancements, and relatively simple processability [1]. Clays and clay minerals belong to the phyllosilicate group (from the Greek "phyllon": leaf, and from the Latin "silic": flint). Clay minerals, that is, layered aluminum silicates, are the most abundant minerals of sedimentation basins (both marine and continental), weathering crusts, and soils [3]. Clay minerals are characterized by two-dimensional sheets of corner sharing SiO4 tetrahedra and/or AlO4 octahedra. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares three of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; that is, all of the unshared vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer, the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing toward each other and forming each side of the octahedral sheet. The most representative 2:1 clay mineral is bentonite that consists of 90% wt. montmorillonite (MMT) and is a weathering product of volcanic glass.

The structural unit of MMT consists of two tetrahedral sheets that cover one octahedral sheet in between (Figure 1.1). This micaceous clay structure has oxide anions at the tip of the tetrahedral subunits that are oriented toward silicone atoms, which are frequently substituted by aluminum, iron, and cations. However, the octahedral subunits contain aluminum ions that are substituted by silicon ions and surround the hydroxyl atoms present at the axial end of tetrahedral [3-5] planes. The MMT [(Na,Ca)0.33 (Al, Mg)2(Si4 O10)(OH)2nH2O] surface is slightly negatively charged because oxide anions dominate the charge-balancing anions (Si4+, Al3+, Fe2+, Fe3+, Mg2+) present in the interface and impart as light overall negative charge to the surfaces of the sheets clay minerals. The MMT particles are plate-shaped, typically 1 nm in thickness and 0.2-2 microns in diameter [6]. MMT has an excellent sorption property and possesses sorption sites available within its inter-layer space as well as on the outer surface and edges. Depending on the place of origin, MMT contains variable amounts of sodium and calcium along with water for hydration. Sodium montmorillonite (Na-MMT) hydrates more than calcium montmorillonite (Ca-MMT). Cation exchange capacity (cmol/kg), specific surface area (m2/g), and basal interlayer spacing are maximum for MMT compared to other clays such as illite, kaolinite, and muscovite-type layered silicate [6].

Figure 1.1 Structure of Montmorillonite (available online).

Most polymers are considered to be organophilic compounds. In order to render the layered silicates miscible with nonpolar polymers, one must exchange the alkali counter-ions with a cationic-organic surfactant [1, 7, 8]. Alkylammonium ions are mostly used, although other "onium" salts can be used, such as sulfonium and phosphonium. Surfactants can also be used to improve the dispersability of the clay. The surfactants were able to increase spacing between clay layers (d-spacing) to different extents, depending on the number of polar units in the copolymer molecule. The resulting clays are called organomodified layered silicates (OMLS) and in the case of montmorillonite, they are abbreviated as OMMT (organically modified MMT). Organoclays are cheaper than most other nanomaterials, since they come from readily available natural sources and are produced in existing, full-scale production facilities [8]. In Table 1.1, the most cited commercial OMMT are mentioned.

Table 1.1 Chemical composition of the main commercial montmorillonite cited.

The main advantages of MMT and OMMT nanoclays that make them ideal nanostructures for food packaging applications...