UV curable inks are being applied to many different substrates -- particularly since the development of LED UV lamps -- which reduce the heat of conventional arc lamps, reduce energy consumption, do not generate ozone pollution and have a small form factor allowing them to be attached to the shuttle containing the printheads.
With all these good things, more and more companies are providing printers for new applications where other inkjet or analog printing technologies have been widely used. One area that has gained a lot of attention is fabric printing. Here we will explore the pros and cons of using UV LED inkjet in these applications.
Safety Considerations with Fabrics
UV LED inkjet inks require much more care in their use and disposal when compared to pigment or dye water based inks (including Latex inks) and even pigment based eco-solvent inks. These UV LED inks are made of reactive chemicals and in the uncured state pose chemical hazards.
Consulting a Material Safety Data Sheet for a set of UV-LED inks provided by the printer manufacturer, the hazards listed for the un-cured inks are concerning:
"May cause skin irritation and discoloration, may cause allergic reaction and will cause irritation in eyes, will cause irritation and mild central nervous system effects such as headache, dizziness, drowsiness and coughing in the respiratory system, may cause upset stomach, headache, dizziness, blurred vision, nausea, vomiting and discoloration of mouth, teeth and throat if ingested. Acute exposure will affect skin, central nervous system and eyes. Chronic exposure will affect skin, kidneys, liver, central nervous system and reproductive system."
So, what about the “cured ink”? Is it safe? Well, it depends. First, is it completely cured? Second, what bad actors may remain even if it is completely cured? What can be done to ensure no problem chemicals remain? This is particularly important if the fabric is used in apparel applications.
Direct-to-Garment Printing
To print a process color image on a T-shirt today, most printers use pigmented water-based inkjet inks, or they use dye-sublimation inks for direct- or transfer-printing. The former can be used for cotton and cotton blends but the latter can only be used on synthetics. Both may require pretreatments to hold out the ink and to control color bleed.
UV-LED inks could provide a desirable alternative. It would not depend on fabric type and should provide very good wash-fastness and rub-fastness. Aside from the safety concerns, typical UV-LED inks do not stretch sufficiently to meet the needs of apparel applications.
However, at a recent trade show I saw a UV-LED T-shirt printer for the first time. The printer manufacturer sourced the inks, which when cured, can stretch with the fabric, even fabric with spandex. So, what about UV curing and any potential bad actors that may remain?
UV-LED Fabric Printing
UV inks are very low in viscosity when printed and will penetrate the fabric. To receive curing radiation from the UV-LED lamp the ink must be exposed directly to the UV light. To adhere to the fabric the inks must penetrate deep into the fabric's surface.
It's possible that some of the ink could be shielded from direct UV light exposure by the overlapping fibers of the fabric. Without full UV light exposure, the reactive chemicals (monomers and oligomers) remain reactive and thus could pose an irritation threat to the person wearing the garment.
I posed this concern to the folks selling the printer at the trade show where I first saw it, and they recommend a post-print heat treatment in a heat press to insure complete reaction of the ink. They indicated that their inks, in finished T-shirts after heat treatment, passed a CPSIA (Consumer Product Safety Improvement Act) textile compliance test for skin irritation. That sounds great.
I consulted with a supplier of UV-curing ink ingredients and asked if this claim was reasonable. Not knowing the formulation or the test procedure he could not render a judgement. He did mention that other ingredients that are normally added to UV inks for stabilizing the ink would remain and that the heat in a heat press may not be sufficient to cause a reaction for monomers remaining in the ink.
After contacting the printer manufacturer, I learned that the ink formula—in addition to containing the typical light-sensitive photo initiators that trigger the curing reaction when exposed to UV light—also includes heat-sensitive thermal initiators that further trigger the reaction when exposed to heat from the heat press. This would cure any inks that did not get exposed to the UV light and eliminate the possibility of un-cured ink hiding in the fibers.
Conclusion
For soft signage applications and the like, UV LED inks are a proven solution without concerns about unreacted chemistry. Care should be taken when using UV-curing technologies to print onto wearable fabric since penetration and adhesion issues may result. Remember to follow manufacturer's guidelines and always follow up with post-print heat press treatment. Remember, UV inkjet can print onto anything, but for best results, the ink, printer and substrate should be designed to work well together on the application for which it is intended.
UV LED advancements in inkjet technology are opening up many applications not possible with other ink technologies. You should expect more printers specifically designed to take advantage of UV LED ink. Just keep in mind these are reactive chemicals and must be handled carefully to protect both the printer operator and the consumer.