The prime objective of packaging is to protect the pack contents from undesirable interactions with the external environment. In most cases the requirements are for the prevention of oxidation or loss of volatiles, and contamination by foreign bodies.
The most differentiated barrier property between polymers is their permeability to moisture, gases, perfumes, fats and oils. Most polymers used in flexible packaging have broadly equal water vapour transmission rates (WVTR) but gas transmission rates vary widely. Experts in the packaging industry consider oxygen permeability to be the only important barrier parameter (because it is connected to food spoilage), however WVTR is also important to some critical applications such as pharmaceuticals and dry food products (for example bread products).
From all the barrier plastic materials the most common and usable ones are the following:
Pvdc (Polyvinylidene Chloride)
Pvdc as a coating to OPP or PET (alone or differentially with acrylic coatings) is (among others) a well established barrier material for the biscuits, confectionary, dairy and dry nuts market. It helps to preserve the organoleptic properties of the products from off-odors and moisture uptake or loss, and to prevent oxidation. Typical OTR value for PETpvdc is <12cm3/m2/day.
EVOH (Ethylene Vinyl Alcohol)
EVOH copolymer resins have very good barrier gas properties; however their weak resistance to humidity explains why they are coextruded between other films, e.g. PE/EVOH/PE. Most common structure are PET(or OPA)+PE/EVOH/PE for the dairy, dry nuts and meat market.
Depending on the type of EVOH and its thickness in the coex structure, OTR can range from 3 to <1cm3/m2/day. EVOH as well as PVOH (Poly Vinyl Alcohol), a polymer with similar properties, can be also used as a coating to OPP or PET.
Inorganic barriers used in packaging comprise aluminium foil, which is the classical barrier material used in flexible packaging offering unrivalled barriers to oxygen, moisture, light and gases, and vapour deposited layers (such as metallization, SiOx, AlOx). Both types are based on solid materials that should theoretically provide absolute barrier properties. In practice both types are subject to pinholes which could affect their properties.
Below an overview of barrier performance of laminates for packaging can be found.
