Plastics, relatively new in packaging materials, are used in many of the food-packaging applications that were once dominated by the more traditional packaging materials— glass, metal, and paper (6). Plastics are polymers or long-chain macromolecules that can be molded, extruded, and cast into various shapes such as films, sheets, and containers. Polymers are derived from petroleum, natural gas, or coal by polymerization processes. Most polymers used in food-packaging plastics have molecular weights between 50,000 and 150,000. In addition, most plastics contain small amounts of additives (such as plasticizers, lubricants, antioxidants, antistats, heat stabilizers, and ultraviolet stabilizers) that are used to facilitate processing or to impart desirable properties to the plastics. These additives can sometimes migrate into the food, causing the food to acquire undesirable flavors (see Packaging: part i— general considerations). Most packaging polymers are thermoplastics, which are characterized by the absence of cross-linking between polymer chains and can be softened repeatedly by heat and pressure to form different shapes (7-9).

Polymers can be formulated to provide a wide range of performance properties that are dependent on chemical structure, crystallinity, side-chain branching, molecular weight, molecular weight distribution, orientation, and so on (1). The chemical structures of some food-packaging polymers are shown in Figure 1. As a general rule, higher crystallinity results in greater strength, better gas-barrier properties, higher soften point, and lower clarity. The type of polymer used in a retail container can often be identified by the recycling code and lettering on the bottom.

The important considerations in selecting plastics for food-packaging applications are gas-barrier properties, strength, stiffness, chemical resistance, sealability, print-ability, formability, appearance, and cost. Plastics are unique in that they have a broad range of gas-barrier properties (Fig. 2), whereas glass and metal provide a total gas barrier, and paper provides virtually none. Thus plastics offer the versatility for packaging many different foods, including those that require a high gas barrier (eg, potato chips) and those that require a low gas barrier (eg, fresh produce).

The gas-barrier properties of polymers are often expressed in terms of permeability: the lower the permeability, the better the gas barrier. Permeability depends on the plastic material; permeant gas; temperature; and, sometimes, relative humidity. The permeabilities of some commonly used food-packaging polymers are listed in Table 2.

Following are general discussions of some commonly used food-packaging polymers. These polymers are used mostly as bottles, containers, bags, films, and so on. Because these polymers have some overlapping properties, more than one polymer can often meet the performance requirements of a food-packaging application. The specific properties of a polymer film, sheeting, or container are usually provided by the manufacturer.

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