With its improved graphics quality and increased productivity, inkjet technology is expanding rapidly into new applications like label printing and packaging graphics. With these new applications come new requirements not previously considered in the design of inkjet inks and media. This is particularly true for common UV-curable inkjet inks. They incorporate chemicals incompatible with food and pharmaceutical packaging and labeling. Here we will discuss the issues and some of the efforts to design inks and media that meet these new requirements.
UV curable inks are composed of numerous ingredients that must not diffuse from the cured, printed ink into food by contact or emit objectionable odors. Chemicals called oligomers, monomers, photo initiators, resins and sometimes organic solvents are incorporated into common UV-curable ink formulations.
After complete curing of the ink by UV radiation most of these react with one another, crosslinking to give a plastic-like ink layer. The concerns are not presented by the crosslinked polymer in the layer because it is a solid and will not migrate into food. Rather, the concerns are with unreacted materials and solvents that do not crosslink with byproducts of the reaction. These materials must not be allowed to migrate or to have an undesirable odor.
Food-grade materials can be incorporated because they are considered harmless. Standard tests must be performed to assure that limits on migration are not exceeded. Only those systems that pass these tests can be used on food packaging. Legislation in Europe has set standards for migration and they are growing in acceptance as a worldwide standard.
When the ink formulator designs the ink with materials that conform to the food safety requirements, the question remains: how can you be certain you have cured all the ink? Any uncured ink whatsoever that may remain in a finished print will retain chemicals that are unacceptable.
Keep in mind that a completely cured ink depends on the UV light exposure, the ink and the substrate onto which it is printed. So rather than referring to “low-migration inkjet inks” we should be looking at “low migration inkjet printing systems” including curing conditions and suitable substrates.
The Substrate and Specific Application
UV inks are particularly attractive since they work well on films and foils without coatings. On porous materials such as papers they do not perform as well. If the ink penetrates the paper prior to exposure to the UV light, complete curing of the liquid ink will not occur since ink inside the paper surface does not receive the UV light.
Unreacted liquid ink will migrate, and that is unacceptable. Consideration should also be given to processing of the printed packaging after printing. An ink formulation may be acceptable in a plastic film application but on a foil that will be heated as part of the processing of the product following printing may not.
UV inks used for printing signage are not suitable for many packaging applications, especially those involving food or pharmaceuticals. In my opinion, there are no “low migration UV-curable inkjet inks” there are only “low migration UV-curable inkjet systems” which, in addition to the ink itself, takes into consideration curing, substrate and post printing processing conditions. Each system must be optimized for the specific packaging application when food contact is an issue. The opportunities are growing in packaging and label printing but the applications can be much more demanding than for wide-format graphics.