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FLEXO Magazine : October 2013
soLventLess LAMinAtion Today, advancements in laminate technology have made it technically possible to control web handling and lamination accuracy in very thin and very elastic substrates while operat- ing at very high production speeds. Further design evolution has allowed for additional techni- cal developments like the ability to control a three-ply lamina- tion in solventless adhesives, as well as consistently control a lamination process involving thin foil. Those advancements have come with the evolution of solventless lamination. Solventless lamination is the process that covers high vis- cosity adhesive. Adhesive viscosity at the coating station is in the range of 1000 CPS and up. Adhesives are in a 100 percent solid formulation and are chemically cured. Based on Isocya- nate chemistry, the adhesive curing is triggered by the mixing of two components: a resin and a hardener. The process is depicted in Picture 04-1. There is no drying system or energy or thermal curing as- sociated with the process. As a result, it has the lowest energy consumption levels of all adhesive lamination types. The simplicity of the process and the cost of the equipment used help explain why it is the fastest-growing type of lamination in the industry. An example of a machine layout can be seen in Picture 04-2. The technology used in solventless lamination was born in the 1970s and saw commercially acceptable results in the 1980s. As it progressed, the adhesive was measured to the same benchmarks used for other forms of lamination: coating technology, web handling and temperature handling. When seeking to improve web handling, the goal was to keep a lamination of two substrates, each with specific me- chanical characteristics, under control without help from the adhesive. Evolutions in machine design and developments in motors and drive technology were what improved the reli- ability and accuracy of the web handling. Temperature handling is critical in a process that involves adhesives that are thermal sensitive due to viscosity variations as temperature changes. Machine design had to be tuned to solu- tions, allowing for consistent temperature control and process stability. Today, solutions implemented in commercial machin- ery are playing a very important role, despite being frequently misunderstood, in process handling. It is safe to say that the vast majority of lamination conversion, especially in flexible packag- ing, is now performed with solventless lamination. Substrates & Adhesives: Solventless lamination is compet- ing with dry and wet bond lamination for market share. The substrates involved in all of these are practically the same. Solventless adhesives are 100 percent solids; they contain no volatile carrier. They are polyester or polyether urethanes, usually two-component (but can be single-component), and are lower in molecular weight to allow them to be liquid at room temperature. Due to the lower molecular weight, the adhesives have a low shear strength after being applied, but they are also highly reactive, so that molecular weight builds quickly in the converted roll. The viscosity of these materials is 1500 to 4500 CPS at room temperature, and they are usually run at 400 degrees Celsius to further reduce viscosity. Due to the extreme reac- tivity of these products, they are fed to the coating head via a meter mix dispense pump and have a working life of 20 to 30 minutes. These adhesives are applied via a special lamina- tor that has smooth, roll-coating application heads and due to the low shear strength, the laminator must have excellent tension control. The lower initial molecular weight at lamination limits the application coating weight to about 1.5 pounds per ream to prevent adhesive from oozing out the sides of the roll and causing a blockage. Applications, performance levels and FDA clearances of solventless adhesives are the same as dry bond, except that they are not typically used in industrial applications, due to the high coating weights and film thicknesses required. Converters need to pay spe- cial attention to the mix ratio, as well as monitor coat weight and appearance. Inks: Trends in dry lamination inks are also true for solventless adhesives. The widest compatibility with adhesives and substrates are PUR inks in solventless adhesives. The highest bond strengths are achieved with PUR- and PVC-type inks for a given solventless adhesive. The same concerns need to be addressed when using solventless adhesive: Re- sidual solvents need to be minimized— especially alcohol-based solvents—and pigments need to be resistant to the chemistry of the solventless adhesive. The temperature stability of the pigments used in solventless adhesives is even WAX LAMinAtion useAges • Covering sticks of butter and margarine • Encasing certain cheeses and soaps • Serving as an adhesive for some chocolates • Wrapping different products • Packaging material for biscuits Temperature vs. viscosity data generated by Dow not to be construed as specification www.flexography.org OCTOBER 2013 FLeXo 47 46 FLeXo OCTOBER 2013 www.flexography.org © 2013 HARPER Bond development data generated byDownottobe construed as a specification