Introduction

Due to their superior physical, thermal, and mechanical properties, plastics have developed to be the most important class of packaging materials. In Europe, packaging is the largest market for plastics accounting for nearly half of all plastics processed. The main advantages of plastics as compared with other packaging materials are that they are lightweight and low cost and have good processability, high transparency and clarity, as well as good barrier properties with respect to water vapor, gases, and fats. It is estimated that global plastic packaging materials and products market will reach US$262.6 billion by 2015.
The huge use of plastics, however, brings in more and more environmental problems; in particular, plastic packaging consists largely of single-use, disposable items. Additionally, plastics are generally made from petroleum, which resources are finite and fast depleting. So, in order to reduce the negative impact to our environment, it is necessary to turn the packaging industry green by using more environment-friendly products.
Green packaging is broadly defined as packaging that is designed to lessen environmental impact throughout the whole life cycle; while maintaining accountable performance, it includes packaging with recycled content, reusable packaging, and degradable packaging. Figure 16 shows the green packaging demand by type (The Freedonia Group & Inc 2011) and Fig. 17 shows the green packaging demand by market (The Freedonia Group & Inc 2011).

Polymer Nanocomposites for Packaging Applications

Polymer nanocomposites, a new class of materials, have shown great potential to enhance the physical, thermal, mechanical, and processing characteristics at low filler loading. In the packaging industry, the use of polymer nanocomposites will not only increase the properties of the packaging polymer materials but also offer additional functions to the packaging.
Among the polymer nanocomposites for packaging, nanoclay is one of the nanofiller mostly used and studied. The nanoclay usually not only increases thermal and mechanical properties but also increases barrier properties to moisture, solvents, chemical vapors, gases such as O2, and flavors. Basilia et al. synthesized recycled polyethylene terephthalate (RPET)/organic modified nanoclay (OMMT) nanocomposites by direct melt intercalation method. The mechanical properties increased greatly with the nanofiller fraction as shown in Fig. 18 (Basilia et al. 2002).

Hamzehlou and Katbab (2007) also found that modified nanoclay can increase both tensile strength and tensile modulus of the recycled PET (RPET), and permeability of the thin films prepared from RPET/nanocomposites to oxygen gas was also reduced significantly compared with both virgin and neat RPET (Table 5).

Emamifar et al. (2011) prepared low-density polyethylene (LDPE) films containing Ag and ZnO nanoparticles by melt mixing in a twin screw extruder. The presence of the nanoparticles increases the antimicrobial activity of L. plantarum; reduced numbers of L. plantarum were observed (p < 0.05) in nanocomposite packages of orange juice containing nanosilver and nano-ZnO.
Alamri and Low (2012) reported on water absorption behavior of nanosilicon carbide-filled recycled cellulose fiber (RCF)-reinforced epoxy econanocomposites. Water absorption was found to decrease gradually due to the presence of n-SiC as shown in Fig. 19. It was believed that the high aspect ratio nature of the nanofiller enhances the barrier properties of the materials by creating tortuous pathways for water molecules to diffuse into the composites. Maximum water uptake of RCF/epoxy composites filled with 5 wt% n-SiC decreases by 47.5 % compared to unfilled RCF/epoxy composites.

Biopolymers are promising materials for green packaging applications. Biopolymers are polymers derived from renewable biomass sources, such as vegetable fats and oils, cornstarch, pea starch, or microbiota. Some biopolymers are designed to be biodegradable that are capable of being decomposed by bacteria or other living organisms in either anaerobic or aerobic environments. Most biodegradable polymers are actually designed to be compostable, which means it degrades to carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with known industrial composting conditions. Typical biopolymers from renewable resources and with biodegradable property are polylactic acid or polylactide (PLA), starch, and polyhydroxybutyrate (PHB).
There are many reports that nanofillers increase barrier, thermal, and mechanical properties of biopolymers which are usually have poor mechanical properties, high hydrophilicity, and poor processability (Tang and Alavi 2012; Dean et al. 2008; Park et al. 2004). Wu et al. (2013) reported grafting polymerization of polylactic acid (PLA) on the surface of nano-SiO2 (Fig. 20) and studied properties of PLA/PLA-grafted SiO2 nanocomposites. It was found that PLA-grafted SiO2 can accelerate the cold crystallization rate and increase the degree of crystallinity of PLA. Shear rheology testing indicated that PLA/PLA-grafted SiO2 nanocomposites have the typical homopolymer-like terminal behavior at low-frequency range even at a content of PLA-grafted SiO2 of 5 wt%.

Li and Sun (2011) prepared surface-grafted MgO (g-MgO) by in situ melt polycondensation of lactic acid and surface-hydroxylated MgO nanoparticles and then prepared poly(lactic acid) (PLA) nanocomposites through thermal compounding of PLA and g-MgO/MgO nanoparticles. It was found that PLA/g-MgO nanocomposites exhibited higher tensile strength than neat PLA and PLA/g-MgO nanocomposites with g-MgO fraction lower than 0.05 % show increased thermal stability.
Polymer foams, a type of lightweight materials, are very important packaging materials; it provides protection for the products with controllable performance. Generally, uniformity of cell sizes, surface quality, thermal and dimensional stability, and mechanical properties including strength and shock absorption are among the important properties determining the applications of the polymer foams. Using nanofillers into polymer foam will enhance significantly the performance of the foams as packaging materials.
Hu et al. (Liu et al. 2010) prepared nanoclay-filled biodegradable poly (e-caprolactone) (PCL) nanocomposites foam with chemical foaming agent. It was found that Young’s modulus of the nanocomposites increased with increasing clay fraction, and elongation at break of the nanocomposites increased with increasing clay fraction at low nanoclay fraction, but decreased at high nanoclay fraction higher than 10 wt% due to the agglomeration of the nanoclays (Fig. 21).

Istrate and Chen (2012) also studied nanoclay-filled poly(e-caprolactone) (PCL) foams with a blowing agent. The nanoclay was firstly treated with chemical blowing agents, and the polymer/treated nanoclay nanocomposites were prepared by solution mixing; the pores were foamed by thermal degradation of the blowing agent. The blowing agent played dual roles in this approach: formation of bubbles and facilitation of clay exfoliation. With nanoclay fraction as low as 2.2 and 2.9 wt%, the compressive modulus and stress at 10 % strain of the porous polymerwere found to increase by up to 152%and 177%, respectively. Strain improved by up to 69 %, while thermal degradation temperature was also greatly increased.
Lee et al. (2008) prepared tapioca starch–poly(lactic acid) nanocomposite foams with four different types of nanoclays (Cloisite 10A, Cloisite 25A, Cloisite 93A, and Cloisite 15A) by melt mixing with extrusion. It was found that the extent of intercalation depended greatly on the nanoclay types, and accordingly, the glass transition temperatures, melting temperatures, and unit density, bulk spring index, bulk compressibility, Young’s modulus, water absorption index, and water solubility index were all influenced significantly with the types of the nanoclays.

Summary

With the deteriorating environment and fast-depleting resources, traditional costeffective packaging materials are no longer a guaranteed competitive advantage. Polymer nanocomposites with much improved physical, thermal, mechanical properties and value-added functions will be among the main packaging materials in the future. Bio-based polymers, which are derived from renewable resources and biodegradable which possess the ability to degrade into small molecules upon bioactive environment exposure, are a promising polymer matrix for polymer nanocomposites in green packaging.