The key to a successful inkjet project is to consider the ink and the printhead in parallel at the start, and then test for feasibility. There are many inkjet heads on the market and lots of these can handle a wide range of chemistry types, but each and every one will have a guideline viscosity target. Trying to shoe-horn a UV ink into a MEMs head that does not have a heater, or to put a water-based 3D binder through a system that works best at > 10cP tend to lead to compromises, in either performance of the ink itself or the stability of the system.
Since formulation descriptions are rather too detailed a subject for a blog, we will consider instead some of the motivations for using one chemistry over another using the wide-format graphics market as a case study. It is instructive because the market has embraced a multitude of inks over the years, from solvent to hotmelt and UV-curable to water-based.
Solvent
Ink based on organic solvents with dissolved resins and pigment dispersions present some of the simplest formulations to prepare. Resin properties ensure water resistance and the use of pigments give colour fade resistance for outdoor use. Ideal for small format and wide format printers alike, and easy to tune to low viscosity for use in cost-effective printheads, such solvent inks enabled a rapid growth of the inkjet print market into non-office applications. Beyond signage, one of the more industrial uses has been vehicle wraps, which has revolutionised advertising on buses for example.
The main limitation of solvent inks is the production of signs by direct printing onto rigid materials, as the difficulty in controlling the ink drying as the ink is deposited leads to ink bleed. As the requirement for more user-friendly inks moved suppliers from using “harder” solvents like cyclohexanone to more “eco” solvents this only became more difficult, since drying speed was impacted.
Hotmelt
These inks work by undergoing phase-change under temperature. One big advantage is that they have excellent pot-life and are typically very stable in the inkjet nozzles, having almost no volatile content. Contrary to solvent inks, the biggest issue was the flexibility of the resulting print, since the waxes could often be quite brittle. This limited use for more challenging applications, like the thick vinyl used on the side of heavy goods vehicles, for example. Nevertheless, there have been some well-known applications of hot-melt chemistry in the graphics sector, which is made possible by careful material selection.
UV-cured
The most often used phrase for UV inks is that you can “print on anything”. This is probably true, but the reality of the situation is that it is much harder to make it stick to everything! Like hotmelt, standard UV-curable inks are effectively 100% solids since no part of ink evaporates, making them great for printhead performance, as long as those heads are not exposed accidentally to UV light. The biggest advantage of UV inks is their very high level of tunability to different requirements, due to the incredible choice of materials available. In our graphics example, this means everything from 300% stretchable thermoforming inks for vehicle wrapping to highly scratch-resistant inks for glass printing have been formulated and are used by leading OEMs.
One big challenge for UV inks is health and safety, as more and more materials are classified as hazardous under REACH. This has led to constant reformulation to avoid risks to printer operators, as well as the costs of distribution due to the resultant labelling. A big factor for UV formulation is the viscosity requirement of the printhead, since even the thinnest UV monomers are much more viscous than water.
Water-Based
Probably the most famous implementation of water-based inkjet into graphics is the HP Latex technology, but it is not alone in terms of binder-containing inkjet inks that are available for different substrates. Like the solvent inks we already described, the key properties come from the polymer, only this is now an emulsion (latex) rather than a solution polymer. Such materials are normally post-heated to become a resilient film.
The main perceived advantages of water-based are in the health and safety and the cost. Water is regarded as safe & quite cheap compared to a UV monomer, so that should mean the inks are enviromentally friendly and cheap! Unfortunately life is not that simple, since the co-solvents required to get a good stable ink in the head may still be hazardous and those crucial polymers are often highly tailored and can be very expensive.
The other most-common family of water-based inks being used, especially indoors where UV odour can be noticeable, are dispersed dye inks. These are applied directly to polyester substrates and heat processed with a calendar, or applied to paper and transferred by sublimation. Similar inks are also used in textiles for polyester-based sportswear, for example.
Hybrid Inks
The last few years have seen the launch of a number of products that are clever combinations of the above types, like solvent-UV, hotmelt-UV (or UV-gel) and aqueous-UV. Each time the aim is to try and mitigate a downside of one ink type against another to create the best combination of cost and performance.
You can learn more about inkjet inks and the formulating principle for each chemistry platform at my course on Inkjet Inks and Applications. Covering uses of inkjet ranging from ceramic tiles to OLED TVs and from mobile phone covers to cardboard boxes, the course will be running at both the IMI Europe Winter Workshop in Bilbao, January 2020 and the IMI Innovation Academy in the US in February 2020.
Dr Mark Bale, DoDxAct
Find out more about this at the Inkjet Inks: Materials & Applications course being held at IMI Europe’s events.