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FLEXO Magazine : March 2011
Technologies & Techniques remain in the inks after the drying process and can react with the isocyanate contained in the solvent-free adhesive and inhibit curing. This results in partially uncured adhesive and incomplete lamination bonding, which is why substrates printed with water-based inks and laminated to films using solvent-free adhesives are not known for reaching lamination bonds at the high end of the curve. In recognition of the poten- tial market in solvent-free adhesives, adhesive manufacturers, ink manufactures and machinery manufacturers have been working hard to solve this problem. Solvent-free lamination requires solid technology and state- of-the-art equipment for coating and lamination. The adhesive is coated at high viscosity, and it is paramount to maintain a constant coating weight during a production run to reach target bonds and optics. Laminating substrates by using adhesives with a significantly low shear resistance requires machinery built according to the highest standards in web- handling technology. These considerations are critical when laminating water-based printed films, especially water-based printed bio-degradable films. It has been proven that laminating-machine technol- ogy, once properly engineered, makes it possible to handle combinations of webs, inks and adhesives. A proper coating technology such as the patented 5 Roller System will guar- antee constant coating weight in both web directions. Of course, final bond results are in the hands of adhesive and ink manufacturers because the chemistry of both products is what makes it possible to achieve significant results. To testify to the importance of cooperation among play- ers within the flexo industry, Nordmeccanica and Dow have joined forces with their technical partners to create annual Earth Day events. The Earth Day events of 2009 and 2010, held at the Nordmeccanica location in Edgewood, N.Y., allowed participants from all over the U.S. to discuss and demonstrate the state of the art in the greenest packaging of all: bio-degradable substrates printed with water-based inks and laminated solvent-free with bio-based adhesives. Earth Day participants witnessed production runs on the Super Simplex laminator, which was developed expressly for solvent-free adhesives. The runs were completed using the latest substrates, adhe- sives and inks. To the surprise of some of the attendees, the run produced high-quality bonds, optics and conversion. A production-scale conversion process in such an extreme configuration is at hand. The data collected at the 2009 Earth Day event were dis- cussed in a paper presented at the Pack Expo conference in Las Vegas in Sept. 2009. The presentation had the greatest attendance at the Expo, indicating a strong industry interest in the technology. Since then, it has been shown that the process of laminating water-based printed substrates using solvent-free adhesive is not only accessible to most, but produces a level of bond considered standard in solvent-free lamination. Evolving TEchnology As previously stated, standard water-based film inks that perform well with either water- or solvent-based adhesive laminations cannot be used with most solvent-free adhesives because of their conflicting chemistries. Water-based ink con- tains hydroxyl and carboxyl functional groups, and hydroxyl groups interfere in the solvent-free adhesive reaction, result- ing in poor bonds, tacky adhesives and residual odor. Although the carboxylic acid groups found in most poly- mers do not adversely affect the reaction mechanism, most common press side additives used to improve the resolubility of the inks can interfere with the cure of the adhesive’s polyol component. For this reason, water-based inks must be com- pletely dry for their lamination to be successful. There are many obstacles to achieving a successful bond when combining the chemistries of water-based inks and solvent-free adhesives. For instance, the ink can have greater affinity for the adhesive than it does for the base, printed substrate or film, which often results in a decal. In ad- dition, the co-reactant of the adhesive package can re-wet the inks and cause poor bonds. Because solvent- free adhesives take time to cure completely, smearing can be an issue, as well. This phenomenon is most often observed as bleeding of the ink after lamination. Fortunately, all of these issues can be overcome. Early resin technology was limited, so it took several years to formulate a water-based ink system that can be used successfully with the solvent-free adhesives on the market today. Advancements in water-based resins, pigment dispersion technolo- gies and the solvent-free adhesives themselves have allowed for a greater latitude in formulation. The super simplex laminator. www.flexography.org march 2011 FLeXO 51 FLX_March11.indd 51 3/18/11 1:33 PM