Permeability is the measure of the ease with which a material can be penetrated by a given gas or liquid. The property applies to the passage of matter through a membrane or thin film such as an adhesive. When permeability becomes a criterion in adhesive selection, it usually is with regard to either extreme:
* Very high permeability -- such as required for moisture vapor or gases to pass easily through breathable bandages, certain packaging materials, water resistant fabrics, etc., and
* Very low permeability -- such as required for barrier films, protective food packaging, pharmaceuticals, etc.
The packaging and medical / pharmaceutical industries are probably the largest industries to use adhesive systems having specific permeability requirements. Most often the requirement is for low permeability or barrier properties - to keep oxygen and moisture from penetrating the package and degrading the product.
Such barrier products are vital for packaging high-value perishables with longer storage times, from shelf-stable foods to medicines and vaccines.