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FLEXO Magazine : October 2013
Water-based products usually come run ready at close to 50 percent solids, while the solvent-based adhesives usually require dilution to reach the appropriate solid level and vis- cosity. These products can be single- or two-component. The addition of the second component generally adds to the heat and chemical resistance of the product. When a second component is used, the reactive nature of the two components means that the viscosity, reactivity and adhesive quality/clarity need to be monitored, and that the mixed adhesive should be consumed within 8-10 hours. Typical end uses for dry bond adhesive run the gamut of food packaging, from snacks to retort packaging. Dry bond adhesives can also be used in demanding indus- trial applications that require heat, chemical and ultraviolet light resistance. These adhesives typically perform with all common packaging materials (films and foils) but care should be taken to ensure adequate surface treatment or cleanliness of the film. They generally comply with 21CFR 175.105, with highest-performance adhesives meeting 21CFR 177.1395 (hot fill) or 21CFR 177.1390 (retort). Additionally, dry bond adhesives exhibit a wide range of performance abilities, from a snack bag that has adequate bonds and allows the package to be heat sealed and holds relatively inert foodstuffs, to a pouch that can undergo retort with aggressive (fatty or oily) foodstuffs. Retort-grade adhe- sives have high heat and chemical resistance. Converters should ensure recommended application coating weights are used, proper mix ratios are met and urethane-grade solvents are used for diluting solvent-based adhesives. Adequate drying must be used and initial bond strength and cured bond strength should be measured to ensure that proper conversion is taking place. Inks: Dry lamination inks can be either solvent- or water- based. Typical solvent-based compounds are: • Nitrocellulose (NC) • Polyvinylbutyral (PVB) • Polyurethane (PUR) • Polyvinyl chloride (PVC) Most water-based lamination inks are based on acrylic chemistry. In general, bond strength increases through the following resin series: NC<PVB<PUR=PVC, for a given sub- strate and adhesive. Remember that the entire system—ink, adhesive, and substrate—is very sensitive to changes in any one component. The variety of adhesives that provide good performance on a given ink follows the following trend: NC<PVB=PVC<PUR. This means that more adhesives can be used successfully when using a PUR ink than an NC ink. The NC ink will have a smaller number of adhesives that provide good bond strengths. A similar trend can be drawn for the variety of sub- strates that will provide good adhesion and bond strength with a given adhesive. This general trend is as follows: NC<PVC<PVB<PUR. This means that for a given adhesive an NC ink will perform on fewer different substrates than a PUR ink. In general, NC inks require more careful selection of adhesive and substrate for optimum performance than a PVB- or PUR-based ink system. NC-based inks can be printed on both flexographic and rotogravure applications. They are generally recommended for laminations on paper, polyolefin (polyethylene and poly- propylene) and chemically treated polyethylene terephthalate (PET) print webs. NC inks provide excellent heat seal resis- tance. The combination of NC inks with special lamination white inks can be used for deep-freeze and pasteurization applications. NC inks are limited by their adhesion and bond strength when printed on some substrates like polypropylene. NC inks work better with solvent-based dry lamination adhe- sives than with water-based dry lamination adhesives. PVB inks can also be printed on both flexographic and rotogravure applications. They are usually recommended for use on oriented polypropylene, chemically and corona treated polyester, oriented polyamide and polyvinyl chloride substrates. PVB inks provide excellent bond strength values and adhesion on metallic film types like metalized polyester, metalized polypropylene and foil. PVB inks provide good per- formance with most dry lamination adhesives. When PVB inks are combined with specialized white inks, they can be used for pasteurization and sterilization applications. PVC inks can be printed only on gravure applications due to the solubility of the PVC resins. PVC resin is only soluble in non- polar solvents that are not compatible with flexographic plate materials. The inks are usually recommended for chemically and corona treated polyester, oriented polyamide and polyvinyl chloride substrates. PVC inks generally are not recommended for lamination structures based on oriented polypropylene. Lami- nates made with PVC inks can be used for pasteurization and sterilization up to 139 degrees Celsius. They are recommended for “hot-filling” structures, where the final package is filled at elevated temperatures. PVC lamination inks are also resistant to aggressive foodstuffs that might smear or discolor inks made from other chemistries. PVC inks perform best with solvent- based dry lamination inks. PUR inks can be printed on both flexographic and gravure applications. They are recommended for use on chemically and corona treated polyester, oriented polyamide, oriented polypropylene and polyethylene. PUR inks can be printed on structures for deep-freeze, pas- teurization and sterilization, also up to 139-degrees Celsius. PUR inks have a wide operating window of both solvent- and water-based adhesives. They are also resistant to aggressive foodstuffs and can be used for hot-filling applica- tions. PUR inks can cover the widest variety of applications for converter printing and laminating structures. Water-based acrylic inks for dry lamination tend to be very substrate and adhesive specific. There is a wide variety of acrylic resins that can be used to make water-based lamination inks. They range from very low glass transition temperature resins to those with glass transitions just below room temperature. They also vary with level and type of co-monomer. Each resin has a specific adhesive and substrate on which it performs best. For this reason, water-based acrylic lamination inks tend to be very selec- tive in their application. One ink may perform quite well with one adhesive and one substrate, but could perform very poorly on a different substrate or in combination with a different adhesive. Therefore, it is difficult to give general guidelines about their performance. It is common for water-based laminat- ing inks to be paired with water-based adhesive, both single-component and two-component adhesive types. They are also usually used on lower bond strength applications, like polyethylene to polyethylene or metalized polyester to polyethylene. However, there are some water-based inks that work quite well on corona treated polyester with the cor- rect adhesive selection. There are a few concerns that need to be considered when selecting an ink for use in dry bond laminations. First, residual solvents need to be minimized. Solvents with hydroxyl groups can inter- fere with the curing of dry bond adhe- sives. Inks must release alcohol-based solvents quickly under standard drying conditions. Also, when using water-based adhe- sives, pigment selection is critical. The pigments used in the inks must be resis- tant to water. Finally, when a structure is to be used in sterilization or pasteuriza- tion, the ink must be thermally stable. Again, pigments that will not fade under high temperatures must be used. Dry lamination is one of the most-used technologies in flexible packaging and is influenced by the reliability of adhe- sion at high temperatures. Because of this, packages for retort and packaging for sterilized and pasteurized applica- tions are almost exclusively converted in solvent-based dry lamination. Wet LAMinAtion Wet lamination is essentially a process in which two substrates are combined in a pressure nip while the adhesive layer is still wet. The combined compound is then run through a drying oven (See Picture 01-1). This process implies the use of low viscosity adhe- sives and uses water as a vehicle. Given the design of this process, the substrate has to be porous to allow the water to evaporate (See Picture 01-2). A part of the vehicle is also absorbed by one of the substrates that, also because of the design, has to be hydrophilic—for example, paper. Substrates & Adhesives: As men- tioned, paper is involved in the majority of inKs used in drY LAMinAtion • Nitrocellulose (NC) • Polyvinyl chloride (PVC) • Polyurethane (PUR) • Polyvinylbutyral (PVB) www.flexography.org OCTOBER 2013 FLeXo 43 42 FLeXo OCTOBER 2013 www.flexography.org © 2013 Harper Dow generated viscosity temperature data. Not to be construed as a specification