Over the past decades, polymers have replaced conventional materials (metals, ceramics, paper) in packaging applications due to their functionality, lightweight, ease of processing, and low cost. The use of synthetic polymers is ubiquitous in food packaging where they provide mechanical, chemical, and microbial protection from the environment and allow product display. Polymers most frequently used in food packaging are polyethylene, polypropylene, polystyrene, polyvinyl chloride (PVC), and polyethylene terephthalate (PET) (Bureau and Multon 1996; Marsh and Bugusu 2007). High-density polyethylene is used in applications such as milk bottles and bags. Low-density polyethylene is used for trays and general-purpose containers. Polypropylene has excellent chemical resistance, it is strong, and has the lowest density of the plastics used in packaging. It has a high melting point, making it ideal for hot-fill liquids. It is employed in film and microwavable containers (Adapted from Michaels 1995). PET is clear, tough, and has good gas and moisture barrier properties. Soft drink bottles are generally made of PET. It has good resistance to heat, mineral oils, solvents, and acids. Hence, it is becoming the packaging material of choice for many food products, especially beverages and mineral waters. The use of PET to make plastic bottles for carbonated drinks is increasing (van Willige and others 2002). However, despite their enormous versatility, a limiting property of polymeric materials in food packaging is their inherent permeability to gases and vapors, including oxygen, carbon dioxide, and organic vapors. Biopolymers are notorious for their high water vapor permeability. This has boosted interest in developing new strategies to enhance barrier properties and to carry out research aimed at the understanding of structure–barrier properties relationship. This review emphasizes the impact of nanostructures on barrier properties of polymers.
The most frequently used strategies to enhance barrier properties are the use of polymer blends, coating articles with high barrier materials, and the use of multilayered films containing a high barrier film. An effective high barrier material is aluminum foil. Thin coatings of aluminum can be applied to films and containers by several vapor deposition technologies. Multilayers are formed by embedding a thin layer of a high barrier material within layers of structural polymers. Coatings and multilayers are effective but their application is limited by the level of adherence between the materials involved. Polymers can also be added with suitable fillers to form composites of enhanced barrier properties.
Composites typically consist of a polymer matrix or continuous phase and a discontinuous phase or filler (Matthews and Rawlings 1994). Fibers, platelets, and particles, have been used for decades to form polymer composites with enhanced mechanical and thermal properties. A recent breakthrough in composite materials is the advancement of nanotechnology. Nanocomposites are materials in which the filler has at least one dimension smaller than 100 nm. Mechanical, thermal, and properties of nanocomposites often differ markedly from those of their component materials. Polymer nanocomposites promise a new crop of stronger, more heat resistant, and high barrier materials.