In modern inkjet applications, radiation-cured chemistries provide an alternative to traditional solvent-based inks using thermal drying. Radiation-cure inks use UV or e-beam radiation to initiate a polymerisation reaction, turning the ink carrier to a solid, rather than using heat to remove a solvent by evaporation. The key distinction is that radiation-cure inks, once cured, leave all of the deposited material on the substrate, whereas a solvent (or water) based ink has a significant fraction of its volume removed on drying.
Radiation-cure (or radcure) inks have potential advantages such as high production speeds, good printing reliability, good adhesion onto a wide variety of substrates and reduced emissions, although there are questions about its safety for applications such as food packaging. Development of new materials is helping to address these questions and broaden the applications for radcure inks.
When formulating radcure inks, material choice is critical. For example in the main monomer package that forms the bulk of the ink (and hence also the bulk of the cured film), new materials are enabling a wide variety of potential properties to be available to the formulator. Photoinitiator (PI) chemistry is also critical to ink curing performance and potential toxicity, and again the choice of materials is becoming wider as new variants are released onto the market, giving new choices for ink developers.
An area of rapid technological development in recent years is the introduction of LED curing into production printing processes. LED curing lamps have potentially game-changing advantages over conventional mercury arc lamps, with much lower heat emission, fast start up, low power consumption and significantly reduced environmental impact. Conversely, LEDs can be limited in power, and work at a narrow range of wavelengths compared with arc lamps. This requires changes in the photoinitiator chemistry to capitalise on the LED output spectrum and overcome the tendency to suffer from increased oxygen inhibition when compared with arc lamp curing. Specific PI chemistry for LED curing is now available, including amine acrylated materials to enhance reactivity and overcome oxygen inhibition, as well as tertiary amines and secondary thiol monomers. New materials and blends for this application are being developed and released continuously.
One of the most important areas of application for radcure inks, and an area of safety concern, is in food packaging. Food safety is a challenging and broad ranging and is obviously essential to get right. While this relies on correct materials choice, ultimately it is a complete production process that must be developed, controlled and proved to be in compliance. A key issue in food safety is migration of ink components through the packaging material and into the food. This is clearly a complex issue depending on the ink components but also on printing and curing conditions, the substrate and any coatings as well as the storage and final usage conditions.
Although not the whole story, material suppliers have a part to play in developing food-safe package printing solutions. Low migration PI chemistry based on high molecular weight polymer backbones can help, but the drawback for inkjet inks is that these materials have higher viscosity, which limits formulation options. Further material developments have the potential to provide efficient PI products with characterised by-products that fit well into a digital packaging solution. The combination of low migration requirements with LED cure provides further challenges requiring novel chemistry to address.
Combining UV cure chemistry with water-based technology to provide a hybrid ink is a recent trend showing much promise. Hybrid inks are designed to have many of the advantages of radcure inks, while also providing lower coat weights and better compatibility with flexible substrates without the need for VOC-based solvents. Designing of hybrid inks with good performance and acceptable cost relies on new chemistry from the materials suppliers.
In summary, while radcure inks have been around for many years for digital applications, continuing advances in materials have enabled improved performance in existing applications as well as expansion into new applications.
Tracey Norton, IGM Resins
You can hear more about curable materials for inkjet inks at the IMI Europe Inkjet Ink Development Conference, 13-14 April in Lausanne, Switzerland.