In our previous articles focused on ink technology, we have approached the subject from the ink’s platform chemistry perspective, such a water-based or UV. In 2021, we are going to be looking at the ink chemistry requirements and capabilities from an application standpoint. We kick this approach off with a brief insight into the topic of security inks, which is a growing area for inkjet.
What’s “Security” Printing?
Actually, the term gets used for a wide range of different applications, but the overriding theme is the role of the print in terms of ensuring some level of quality, or compliance with legislation (IDs, tax stamps). One of the most familiar examples of security printing is ID cards and passports and so it will be interesting to kick things off by having a closer look at the driving license that I recently got renewed. Just like the new passport I got a couple of years ago, there are an amazing number of different features being used in parallel from microtext to holographic seals as shown in the photo.
However, it is the lenticular feature on the reverse that caught my attention as a potential “inkjet-printable” effect. Traditional lenticular technology usually involves a laminated film layer sitting above a carefully prepared “Jekyll and Hyde” hybrid print comprised of successive stripes from the orginal images. The important thing is to get alignment correct. Inkjet can do this easily since it can print the graphic part in CMYK and then overprint the lenticular structure on the same machine. Due to the flexibility of inkjet, the lenticular design can also be more than just a single axis view seen on the driving license. The appearance of visual depth or “3D” can be created using hemispherical lenses.
Despite those 2.5D possibilities, the most common examples of security print implemented by inkjet are predominantly 2D and involve not just the identity documents mentioned and well-known financial instruments (currency, cashier/travellers’ cheques, bank cards) but also tags and labels of luxury products for brand protection. We shall look at a few examples.
Much of the 2D printing done to produce tax stamps and currency still relies on high-speed non-inkjet printing techniques such as litho or flexo . As a result, where inkjet has been adopted there is a dominance of continuous (CIJ) technology because, as discussed in our recent print head webinar, these heads can typically print faster than most drop-on-demand (DOD) inkjet.
Other than speed, there are two critical challenges in implementing a good “covert” security print that influences the chemistry. The first is to have a solvent that can dissolve the colorant as a dye. CIJ is good for this since it is possible to formulate it with a high level of alcohol (for example) without causing latency issues. The second is to make the marking truly invisible to casual inspection under normal lighting conditions. In some early examples this meant eliminating the visible nature of the special IR or UV tag using overprinted hiding layers. Without those though, good “invisibility” requires a very thin layer with a gloss appearance matched to the substrate layer, meaning that the solid content is very low (e.g. US5837042). CIJ works well since it can work well in such a low viscosity regime.
Despite the early head start from CIJ, there are now actually quite a large number of covert inks available for thermal and piezo print heads, including HP cartridges used in low-cost coding systems . Indeed, they appear as options for many machines. Among these inks there are versions that use dyes and versions that use pigments. For an explanation between these, refer back to our ink matters article from 2018. Even more recent innovation has led to the use of quantum dot technology.
Some printer vendors even offer fluorescent UV inks ; however, since these typically print as a quite thick layer, it is perhaps necessary to consider how the print can be disguised by other image content. One way is to have two inks, one which can fluoresce and one which cannot but is otherwise identical. The security feature is encoded by mixing data between the two at the pixel level in a way that the printed info is hidden by the overall pattern, which is a little more sophisticated than the common method of simply printing an opaque hiding layer over a less covert ink.
Variation in matte/gloss finish and height (haptic) are also possible process-related mechanisms for making prints more secure by making it harder to duplicate the specific processes used. Many of these are not new concepts. HP has a patent on gloss variation dating back to the early 2000s (US6607267B2) although no specific chemistry is disclosed. Well known Swiss supplier of security inks, Sicpa, has a more recent European patent on making reflective layers using inkjet where they describe printing using metallic nanoparticles delivery in a water-based ink (EP2021 185B1).
Just writing the above words as a section break makes the author think of the intro of the Moody Blues song In the Beginning: “You’re magnetic ink… I’m more than that, I know I am, At least, I think I must be..” . 1970s prog rock aside, one of the most common security inks in the market is for Magnetic Ink Character Recognition (MICR) for document validation, especially cheques. The most common material used for this application in toner-based printers and thermal transfer ribbons is iron oxide, Fe3O4, which when used as larger particles for inkjet is difficult to keep dispersed against sedimentation.
In analogy with the successful ceramics tile market, this settlement property can be managed more easily in more complex ink delivery systems and using modern recirculating print heads, but such equipment is not common in document printing. To work reliably in thermal print heads and non-recirculating piezo heads, an alternative is required. Historically this has involved traditional milling of the pigment to reduce particle size to <1um and then careful selection of dispersant and binder resins to help prevent print head clogging. More recently, however, the approach has shifted to using engineered nanoparticles with particles < 100nm (e.g. x-nano, VersaInk), and which can include material other than iron oxide as described by Xerox (EP2295508) and Nanum Nanotech (US8815393).
DOD is also the dominant inkjet technology to produce passports and ID cards and other forms of photo ID since the speed of production is typically lower. Printing of UV inks onto plastic cards is quite well established by several manufacturers. One of the challenges in this market, which involves a lot of localised production, is making the inkjet printers as compact and cost-effective as their thermal transfer counterparts. This can be achieved by making small multi-pass systems, rather than having multiple expensive single-pass capable print heads. Such an approach can also increase the potential print resolution. The HID system uses solvent inks in thermal print heads to address this need .
For passports, many of which are still printed onto paper, standard CMYK aqueous inks are then used for the “personalisation”, often in combination with laser engraving . One of the more difficult challenges in multi-layered security devices is obtaining the reliable detection of an intentionally hidden layer. One patent application from Mühlbauer (US2020180347A1) discloses using only CMY inkjet ink so that black parts of the image are printed with composite color, thereby avoiding the IR absorption produced by carbon black found in black inks.
Beyond on these few familiar examples, there are many more interesting uses of inkjet that have been proposed, from etched metal foils through to inkjet-jetable formulations for holograms. There is clearly scope for continued inkjet ink innovation for the efficient and/or localised production of more secure documents and IDs and beyond. Of course, where such technology is more accessible, it is important for successful anti-counterfeiting to have supporting software that ensures production is only made possible by authorised parties.
Check out our article from Pete Basiliere on security printing for transactional print, direct mail and books
 John Corral, FM Print Summit October 2020; https://www.youtube.com/watch?v=9p8YG7HzSw8  InkSummary (polytij.co.nz)  Domino Digital Printing Solutions is launching a new fluorescent ink for its K600i piezo drop on demand ink jet printer at Labelexpo Europe 2017 (domino-printing.com)  https://youtu.be/AukFsBv2oDY  https://www.hidglobal.com/sites/default/files/resource_files/si-fargo-ink1000-printer-ds-en.pdf  https://www.muehlbauer.de/products/epassport/epassport-personalization-issuance/