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FLEXO Magazine : Sustainable Spring 2011
so that the platelets align correctly and close together, it offers a significant improvement in barrier performance. In effect, the diffusing gas has to find a pathway around the platelets. Since the distance the gas must travel is several times the thickness of the coating, the transmission rate of the diffusing gas is limited by the path it is forced to take. Consideration must be given to the aspect ratio of a plate- let, as shown in Figure 2. This is simply the ratio of the shortest to longest dimensions. A thickness of 1-3 nanometers is quite common in clay types, but it is those with one dimension ap- proaching 1-10 microns (i.e . an aspect ratio of 1,000 -10,000) that are most effective for use in barrier coatings. From an ap- plication standpoint, the aspect ratio will be compromised to achieve other desired barrier coating properties. The graph accompanying this article demonstrates how the aspect ratio and concentration of various particulates affect oxygen transmission. Oxygen barrier coating technology is based upon an op- timized ratio of ethylene vinyl alcohol copolymers and clays. The film forming polymer is very similar to EVOH extrusion polymers already used in barrier packaging. The clays have a platelet shape and enhance oxygen barrier performance. These platelets orientate themselves in the coating, as it dries, to create a tortuous path for gas transmission, resulting in higher barrier properties, at much lower film weights, than are usual with extruded EVOH. APPLICATION Barrier coatings can be applied to various substrates, such as PET, polyolefins, polyamides, metalized and ceramic coated films, PLAs, as well as cellulosic and starch based products. Moisture sensitivity, similar to that of EVOH, is designed for ‘sandwiching’ between two polymeric films in a laminate structure, such that the composite effect provides good oxygen and moisture barrier. Depending on the packaging specifications, oxygen barrier coatings can be applied to the surface of a substrate and then protected with a suitable, resistant overprint varnish. They provide protection against the ingress and egress of odors and aromas. In general, preparation and application is very simple. It involves a two component mix, resulting in a user friendly, press ready product. Although the product may contain some alcohol, depending on the application, it is eas- ily washed up with water. Once applied and dry, a smooth, homogenous, pinhole free layer can easily be overprinted with inks and laminated to a variety of secondary films using suitable adhesives. Bond strength is generally greater than 2N/15mm (330g/inch). Ap- www.flexography.org SPRING 2011 Sustainable FLEXO 11