(32c) Barrier Membranes for Packaging
The goals of various barrier membranes or coatings are to keep food and drinks fresh, protect objects from corrosion and weathering, prevent unwanted water loss or water gain, slow the transport of pollutants or harmful agents, and generally minimize degradation processes. Nearly all of earth's terrestrial inhabitants are ?packaged? in an external membrane designed, at least in part, for controlling water loss and prohibiting invasion from the external environment. Whether naturally occurring, like our skin, or man-made, penetrant transport across a membrane barrier is controlled by four processes: (1) the solubility of the penetrant in the membrane material, (2) the mobility of the solute in the membrane material, (3) the tortuosity of the solute's diffusion path, and (4) reaction of the penetrant within the membrane. Strategies for designing effective barrier membrane involve altering one or more of these quantities. Approaches include lamination with a low solubility layer, and incorporation of less permeable and/or reactive fillers. Often, superior barrier effectiveness is achieved at a cost of poorer flexibility and toughness or with the loss of desired optical properties, such as transparency. Nanoscale fillers or appropriately designed block copolymer systems might allow permeability reduction with minimal effect on other properties. In addition to describing the current state of barrier materials, we will outline the theory of membrane performance as well as compare packaging materials with our skin.