The most important barrier properties with regard to the packaging of food products are gas, light and water vapour barriers.
Water vapour barriers
Fresh food products tend to dry out if left unpackaged over a period of time. Dried foods clump together and deep fried products such as snacks on the other hand lose their crispness if they take up water from their surroundings because of insufficient packaging. Good water vapour barrier properties are therefore vital if packaging materials are to protect the foods from undesirable loss of water to or absorption of water from their surroundings.
The different polymers have different water vapour barrier properties. Literature values for water vapour barriers can vary a great deal and depend on the thickness of the material and the conditions under which the tests were carried out. In general we can say that polyolefines such as polyethylene and polypropylene are relatively good water vapour barriers (< 10 g/m2 • day, measured at 23°C and 90% RH) and humidity sensitive polymers such as polyamide and ethylene vinyl alcohol are poorer water vapour barriers (> 40 g/m2 • day, measured at 23°C and 90% RH).
Since most food products are packed in laminates consisting of several types of polymer and most use polyethylene as the weld layer, this means in practice that most of the plastic materials currently in use have a sufficient water vapour barrier. For products requiring particularly good protection against water, metalized laminates or laminates with a layer of aluminium are often used.
