the shelf-life of the products they protect. In spite of this deficiency, the light weight, low cost, ease of fabrication, toughness, and clarity of polymers have driven producers to convert from metal and glass to polymeric packaging. Polymers often provide considerable savings in raw materials, fabrication, and transportation, as well as improved safety for the consumer relative to glass; however, these advantages must be weighed against complex life-cycle issues now being addressed. The following discussion illustrates the current state of this technology, its problems, and future opportunities.
There are certain polymer molecular structures that provide good barrier properties; however, these structural features seem invariably to lead to other problems. For example, the polar structures of poly(vinyl alcohol), polyacrylonitrile, and poly(vinylidene chloride) make these materials extremely good barriers to oxygen or carbon dioxide under certain conditions, but each material is very difficult to melt fabricate for the same reason. The good barrier properties stem from the strong interchain forces caused by polarity that make diffusional jumps of penetrant molecules very difficult. To overcome these same forces by heating, so that the polymer chains can move in relation to one another in a melt, requires temperatures that cause these reactive materials to degrade chemically by various mechanisms. Thus neither poly(vinyl alcohol) nor polyacrylonitrile can be melt processed in its pure form. Resorting to solvent processing of these materials or using them to make copolymers compromises their value. Poly(vinyl alcohol), by virtue of its hydrogen bonding capability, is very hygroscopic, to the point of being water soluble, and this property prevents its use as a barrier material in the pure form even if it could be melt processed. In general, polarity favors good oxygen barrier properties but leads to poor water barrier properties. This is true for aliphatic polyamides (nylon). On the other hand, very nonpolar materials, such as polyethylene and polypropylene, are excellent barriers to water but not oxygen. This property-processibility trade-off has led to an interest in composite structures. The ''composites" can be at the molecular level (copolymers), microlevel (blends), or macrolevel (multilayers).
The attractive barrier characteristics of poly(vinyl alcohol) have been captured via copolymers, and this achievement has led to some important commercial products using clever molecular engineering and processes that minimize its shortcomings of water uptake and lack of melt processibility. Copolymers containing units of ethylene and vinyl alcohol are made commercially by starting with ethylene and vinyl acetate copolymers and then hydrolyzing them. By critically balancing the structure of these materials, melt processible products that are relatively good barriers with reduced moisture sensitivity can be achieved. These copolymers are incorporated into multilayer structures by coextrusion processes. For example, blow-molded bottles with five to seven layers in the side wall are in commercial use for marketing very sensitive foodstuffs. Lightweight, squeezable, fracture-resistant bottles for ketchup are now on the market. Layers of ethylene/vinyl alcohol copolymer provide the oxygen barrier