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FLEXO Magazine : October 2013
industrial wet lamination applications. The second substrate is, in most cases, foil. Wet bond adhesive chemistry is typically water-based and includes casein neoprene, vinyl acetate ethylene, acrylic and styrene-butadiene rubber. These are applied via smooth roll transfer coating process and are typically 2,000 to 6,000 CPS, depending on the type of porous substrate used. The coating process and the porous substrate require spe- cial rheology characteristics to allow for coating transfer and holdout on porous substrates. The adhesive must shear thin during the coating process and then rapidly rebuild viscosity to prevent diving into the porous substrate. These products are typically single-component, supplied application ready and are not externally crosslinked; work life or pot life is not a concern. Typical end uses for wet bond laminations include: • Carton packaging • Composite cans • Microwave susceptors • Multi-wall bags • Beer labels • A variety of industrial applications Adhesives typically comply with FDA 21CFR 175.105 when used in food packaging. Performance attributes include: • High bonds or fiber tear • High flexibility to allow for making folded cartons • Low odor • Resistance to water when cured/dried • Heat resistance Converters should monitor application coating weights to maintain the targeted coat weight, as well as any drying or retained moisture and initial bonds. Inks: It is very rare to print inks that will be on the interior of a wet-bond lamination structure. It is much more com- mon to surface print the paper substrate used in wet-bond laminations. The printed paper is either pre-laminated to foil or metalized polyester (met-PET) second web or laminated after printing to foil or met-PET. These types of structures are very common in the production of tear-open pouches for dry powder products, like drink mixes (hot cocoa/chocolate or children’s non-carbonated beverages), soup mixes and pow- dered cheese sauces. In the cases where the ink is printed on the outside of the laminated structure, most common ink systems will work, provided they have the necessary physical properties. Inks need to have good color strength, good rub resistance and good heat resistance. Pigment selection is limited only by the need for heat-resistant and bleed-resistant pigments. These inks can be treated as if they are surface printing inks and require additives such as waxes to develop proper coefficient of friction (CoF), rub resistance and anti-block properties. Most importantly, inks that are printed on the outside of a lamination structure that will be wet laminated after print- ing must not form a barrier to moisture. The moisture will be absorbed by the paper and the laminate will appear to be “cured” or “dried,” however the moisture will instead be trapped in the paper. When the laminate is heat sealed, the trapped moisture can cause blistering or worse. The inks must allow for the moisture absorbed by the paper to escape from the laminate structure. Avoid complete coverage of an overprint varnish, as the coverage will not allow moisture to escape. In the rare cases that the ink will be printed on the inside of a wet-lamination structure, it is imperative that the ink not be moisture sensitive. Water soluble resins like soy, shellac and casein should not be used in the inks, as the water diluents in the adhesive will dissolve the ink resin and cause bleeding and smearing. Most solvent-based ink chemistries will have good perfor- mance and bond strengths in wet-lamination structures and they will not bleed during lamination. When the adhesive itself is not high in pH, acrylic water-based inks can be successfully used. Again, pigment selection is critical. The water diluents in the adhesive can affect some pigments; for example, some Red 52:1 Rubine pigments will become colorless when exposed to water. Products such as the inside wrap for cigarette packs are made with a wet lamination of paper to foil, while the main pack itself is a wet lamination of paper to film. Paper to foil wet lamination is also used for chewing gum wraps and other odor-sensitive applications. WAX-Hot MeLt LAMinAtion The technology used for laminating with wax-hot melt ad- hesives belongs in the thermoplastics category. It comprises adhesives that are solid at room temperature and have no vehicle to be removed. The process is based on melting the adhesive, coating the adhesive on a substrate, laminating the primary web to a secondary web and then cooling the laminated compound on a chill roller to bring the adhesive back to a solid state and activate the adhesion (See Picture 03-1). Adhesive melting can be performed in a reservoir or, to maximize energy savings and process efficiency, in con- tinuous melter-dispensers that provide adhesive on demand. Picture 03-2 displays a schematic machine layout. Coating heads for wax lamination have a configuration similar to those used for dry lamination, with the added responsibility of handling a thermo- plastic adhesive. Because of this, each component of the coating station will be heated and temperature controlled: the coating cylinder, the adhesive tray and the doctor blade. Substrates & Adhesives: Waxes for lamination include naturally occurring varieties such as carnauba wax, palm oil wax or paraffin, micronized poly- olefins (PE, EVA or PP) or combinations thereof. Typically these are formulated products so that the appropriate adhe- sion, application properties, rheology at temperature and end performance characteristics are achieved. These products are considered thermoplastics, solid at room tempera- ture, typically dusted for non-blocking, melted in a heated reservoir and sup- plied to the coating head. If used in food packaging, FDA compliances will need to be verified for end use of the laminat- ed structure. Converters should monitor coat weight and adhesion values to ensure full end quality of the structure. Wax-laminated packaging is used for wrapping products such as butter and margarine, as well as certain cheeses and soaps. In some packaging for choc- olate and biscuits, wax can be used as adhesive. The lamination is not used to protect the print, therefore inks used to print those packages do not come in contact with the wax. Wet LAMinAtion APPLicAtions • Multi-Walled Bags • Carton Packaging • Composite Cans • Labels for Beer Containers • Microwave Susceptors • Industrial Applications www.flexography.org OCTOBER 2013 FLeXo 45 44 FLeXo OCTOBER 2013 www.flexography.org © 2013 Harper