- April 30, 2004, Dr. Richard M. Podhajny, Ph.D. Contributing Editor
One of the major advances in flexible packaging was the introduction of film laminated structures.
Film lamination allowed different properties to be combined in various structures to provide unique packaging protection. Lamination could be done through adhesives, heat-sealable coatings, or extrusion lamination.
In such laminated structures, the printing could be “buried” in the package. In this way unique products could be created that provide high quality graphics with functional packaging characteristics. Moisture and oxygen barrier plus package protection could be designed into the construction.
Laminating inks suitable for these structures evolved as ink suppliers met the requirements of new films. Solvent-based inks were the first laminating inks developed and continue to dominate the flexible packaging industry. Today, water-based inks are being used on a variety of packaging structures.
Laminating inks typically are printed on the backside of a film and then “laminated” to another film through the use of either an adhesive, a thermally heat-sealable coating on the film, or by molten PE applied to one film and then pressed to another.
In the case of adhesives, the inks are exposed to the drying temperatures of the adhesive, the temperature of the heat seal coating, or the extrusion temperature of PE. Extrusion lamination creates the highest-temperature exposure to laminating inks.
Typically, once extrusion lamination is applied, the process involves cooling the web quickly and stabilizing the film dimensionally.
The first criteria of a good laminating ink is that it will have good adhesion to the substrate. Special resins are chosen that have excellent ink adhesion and maintain this bond through the laminating process. These resins not only require good adhesion to the substrate but must have high cohesion characteristics to provide high packaging integrity and high performance bond values.
All the ingredients of a laminating ink are chosen carefully to assure the laminated product is free from undesirable odors. The primary source of odors in solvent laminating inks has been the residual solvents that can be trapped within the laminated structure. Water-based laminating inks have very low or no solvents at all, but they can contain amines as well as residual monomers that can contribute to odor.
Odors associated with laminated packages can be due to residual solvent retained in the laminated structure, or less commonly, to secondary sources. Secondary sources include resin decomposition or post reactions within the laminated structure. Such unusual reactions are complicated and can involve other ingredients, such as certain pigments that can catalyze oxidation of some resins.
Corona treatment of films is a common method of increasing the surface tension of a film to improve the ink adhesion. In some cases, excessive corona treatment can oxidize and decompose film ingredients, such as anti-oxidants or anti-stats. Such oxidation and decomposition also can lead to discoloration and produce off-odor side products.
Unlike surface printing inks, laminating inks do not contain waxes or other lubricants, as they will lower laminated bond values. In addition, the inks do not have to be high gloss since they are reverse printed, and the film substrate provides excellent gloss and surface protection. Laminating inks do not require the same product resistance as they are “sandwiched” in a protective structure.
Laminating inks can contain other ingredients, such as cross-linking adhesion promoters. These are used to maintain high bond values but occasionally can be problematic, as some adhesion promoters can create discoloration and odor problems from their chemical reactivity.
Today's laminating inks provide excellent performance and reliability, thanks to the R&D conducted by domestic and international ink companies. Challenges ahead include digital and UV-curable laminating inks. The primary issues connected with UV laminating inks for flexible packaging applications are low odor and suitability for food packaging.
Laminating inks have had a long evolutionary history. As new film substrates are introduced, new performance demands arise. Water-based and solvent-based laminating inks have been developed for many applications, but they continue to evolve in pursuit of higher quality and performance.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org