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Flexo Sustainable : Spring 2013
High Oxygen Barrier Coatings Eliminating the Laminated Structure By Bob O’Boyle Printers engaged in the high oxygen barrier coatings market need options that are chlorine-free, enable light weighting, improve sustainability and the ability to recycle, remove metal, are transparent, and offer an alterna- tive to expensive barrier films. Since the 1990s, nanoparticulates have shown promise in coating formulations and recently have shown exciting properties when made into barrier formulations. In packag- ing applications, they most notably improve barrier to gases, aroma and UV light. Early problems with instabilities and poor coverage have been for the most part eliminated with im- proved formulations and press techniques. Functional oxygen barriers of less than .006 cm3/100 inch2 over 24 hours at 23° C and 50 percent RH on polyester are now routine. What are nanoparticles? Nano is a prefix meaning “dwarf ” in Greek; it also means one billionth. A nanometer is therefore one billionth of a meter. To provide a sense of scale, a tennis ball would be 100,000 ,000 nm. In practice, nanoparticles are finely dispersed nanoparticulate (intercalated/exfoliated) silicate mineral in a polymer solution/dispersion. As the nanoparticulates stack and arrange themselves on a flexible film substrate, they provide an obstacle to gases though the formation of a “tortuous path.” As seen in Figure 1, nanoparticulates move from agglom- erated clay to nanocomposite (exfoliated) coatings. As long as the distance d2 is greater than d1, a tortuous path exists and oxygen barrier performance is improved over an unmodi- fied polymer matrix. The uses for oxygen barrier coatings in the packaging industry are both economic and performance related. The barrier performance of these nanocomposite barriers are much improved over typical PVdC and EVOH barrier resins typically used in packaging films. While most commercial activity today is in the area of dry foods, prototypes have been made for liquid packaging, as well as chilled packaging. Ap- plication methods have varied widely with gravure, flexo and roller coatings—all being successful. In a typical example, a familiar three-ply laminate com- posed of PET/mPET/PE can be replaced with a barrier coated PET/PE structure. Beside the obvious reduction of the one film layer, there are economic benefits of using less adhesive and improving the visibility of the product. Barrier coatings of this type also offer an advantage in flex cracking, as they often are more flexible than mPET and oxide treated films. The flex cracking improvement can be quite dramatic. In Figure 2, various coatings on typical packaging films show they were subjected to a Gelbo flex tester to measure oxygen barrier before and after flexing. Another common example is moving from commercial two-ply laminate to a new two-ply laminate plus printable bar- Figure 1: Nanoparticulates move from agglomerated clay to nanocomposite (exfoliated) coatings. As long as the distance d2 is greater than d1, a tortuous path exists and oxygen barrier performance is improved over an unmodified polymer matrix. 6 Sustainable FLEXO SpriNg 2013 www.flexomag.com
Earth Day 2013