Another interesting presentation at the 21st European Ink Jet conference run by IMI Europe was from FUJIFILM Speciality Ink Systems. Jon Harper-Smith described a new hybrid UV ink technology that they have been developing.
A trend in ink jet is towards higher resolution printheads, which tend to require low viscosity inks. At the same time the range of applications for ink jet is growing, and with it the range of substrates that users wish to print on, requiring increased functionality of the ink which tends to increase the ink viscosity.
Conventional UV-curable inks consist of monomers, polymer/oligomers, and other additives. Basically the functionality comes from the polymer/oligomer and the viscosity from the monomer. It is hard to balance these to achieve the required performance. In addition all of the volume of the ink is left on the substrate surface, which can lead to undesirably thick layers for some applications.
The new ink consists of solvent, to adjust the viscosity, and a special hybrid polymer to create the functionality. After the drop reaches the surface, the solvent (which can be aqueous or a volatile organic solvent) evaporates to leave a smooth even film. This is then cured using a UV light source as usual.
The ink is free of monomer, which is important for food industry applications. Because the image is flatter than normal UV-curable inks the print quality is claimed to be higher, and this should suit consumer applications such as labels.
Conventional wisdom has taught us that thermal ink jet (TIJ) printing requires aqueous inks. After all, water has quite unique properties that are well suited to the technology. This was perhaps underscored by Hewlett Packard’s clever but complicating use of aqueous latex inks for printing on vinyl substrates. Surely if HP can’t find a simpler solution for its very own TIJ technology, there must not be one!
But recently ImTech (Corvallis, Oregon) was granted a patent (USP07763668) for a UV curable TIJ ink, claiming the use of alcohols, esters, or ketones as the driver fluid. Most of the examples incorporate about 24% methanol with conventional UV curing monomers, oligomers and photoinitiation packages. One of several suggested surfactants is included as protection against kogation and one of several black pigment dispersions as colorant.
ImTech offers two such inks for sale through distributors, in new HP-45A cartridges. Both inks are black; one is optimized for conventional UV lamps, the other for UV-LED systems. Suggested applications are in coding and addressing, and in printing on plastic cards.
The patent suggests (but does not claim) the use of similar driver fluids in non-aqueous TIJ inks other than UV curable ones.
In fairness to HP, there are other reasons to stick with aqueous inks than simple “conventional wisdom.” The driver fluids mentioned are emitted as VOCs (Volatile Organic Compounds) at a minimum and may also have low TLVs (Threshold Limit Values) for human exposure. UV curable inks for piezo printing and aqueous inks in general avoid these environmental issues.
It should also be noted that Xennia (Letchworth, UK) offers an aqueous UV curable ink suitable for TIJ printing, under the trade name XenInx Peridot.