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Ink and Equipment: The Yin and Yang of Industrial Inkjet Printing.

Digital inkjet printing has escaped the traditional confines of the graphic arts market and exploded into the realm of industrial applications. Digital printing enables the customization and personalization of printed products that today's consumer demands, quickly and efficiently.

The emergence of digital printing into the industrial arena has presented the industry with a two-pronged opportunity: both the inks and the equipment currently available for graphic arts applications must be improved and revamped to enable this new market--hence, the yin and yang of industrial inkjet printing.

According to Chinese philosophy, there are two fundamental principles, one negative, dark, passive, cold, wet and feminine (yin, and in this example ink) and the other positive, bright, active, dry, hot and masculine (yang, and for this scenario equipment). These forces must interact and strike a balance in the universe, or at the very least on the production floor, if digital inkjet is indeed destined to take industrial printing to the next level of innovation.

UV Curable Inkjet Inks

UV curable inkjet inks are particularly well suited to the industrial arena, as compared to solvent or aqueous inkjet inks. Their instantaneous curing allows for in-line processing on industrial presses and they typically exhibit better durability and abrasion resistance than solvent or aqueous inks. Additionally, they are compatible with a wide range of substrates and their environmental impact is less significant than aqueous or solvent inks, as there is no solvent handling or recovery involved. Also, they enjoy a lower energy cost of drying, especially when compared with water-based inks. There are some limitations that UV curable inks may present, and in these cases a palette of ink types may be considered, or a blend of technology to produce a hybrid system.

In general, however, although UV curable inkjet inks have superior properties as compared to solvent-and aqueous-based inks, the UV curable inks presently available for the graphic arts markets are not designed to withstand the multitude of demands that industrial ink faces.

In spite of the cosmic challenges facing the acceptance of industrial inkjet, it is penetrating the printing market rather aggressively. The total value of the entire global printing market is approximately 467 billion [euro] and projected to increase to 553 billion [euro] in 2010. Inkjet printing is expected to swell from 3.2 percent of all print in 2005 to 4.4 percent of all print in 2010. The UV inkjet market segment is still relatively new (374 million [euro], 0.08 percent of the print market in 2005), but is forecasted to show steady growth over the next five years.

Table 1 lists the market segments that UV inkjet inks participate in, as well as their expected average CAGR (compound annual growth rate). The packaging portion (corrugated, flexible material, cartons, etc.) is expected to show the greatest amount of growth as more and more companies customize their packaging to target specific demographics.

One facet of the packaging market that UV inkjet has been slow to penetrate is the food packaging segment, as small molecules such as photoinitiators can migrate to the surface of the print and be extracted, making these inks non-FDA approved for food contact applications. Electron beam curing may be used to circumvent this problem by using formulas that do not contain photoinitiators, thus providing the benefits of UV inks while still being acceptable for food contact. Electron beam curing units have become much less cost-prohibitive in recent years and also more lightweight, making

them a feasible curing alternative in many respects. However, oxygen inhibition still requires that inks cured by electron beam be inerted, which adds cost and complication into the equipment design.

An alternate method to make UV curable inks FDA approved is to use oligomeric photoinitiators, thus networking them into the cured film and preventing them from being extracted.

The "Other Industrial" category includes things like decorative prints on various items, as well as functional prints where the ink serves more than just an aesthetic purpose, such as printed electronics and 3-D printing. This category is fairly wide open and new possibilities are constantly emerging.

Industrial versus Graphic Arts Inkjet Markets: Dissonance and Concordance in the Printing Universe

The requirements of the graphic arts market are fairly well established. Although new hardware is continually being introduced by OEMs to improve the printing and curing speeds and better, more robust inks are constantly being released, the needs and demands of the segment are generally well known. There will always be an interest in faster production speeds, better lightfastness, wider color gamuts, adhesion to difficult substrates and lower costs.

In this market the substrates are typically various low surface energy polyolefins and plastics, such as PVC, polypropylene, polyethylene, polycarbonate, polyethylene terephlatate and vinyl as well as some paper stock.

Also, the path to market for ink manufacturers is relatively universal, whereby the OEM purchases the ink from the ink supplier and repackages it under their brand to resell to their customers.

Another approach to this market is to sell the ink directly to the end-user as a third party ink. While this is a direct approach that allows more players to participate in the market, it ultimately drives down the overall ink price and is therefore not supported by the OEMs or printhead manufacturers. This path is actively discouraged by OEMs by several hardware modifications, such as RFID chips that monitor the amount of ink consumed by the printer so that a new chip from the OEM is required along with the introduction of more ink. Also, some printhead manufacturers will only warranty their product if certified inks are used, thus providing incentive for printers to use ink from the OEMs.

The industrial market is a new and continuously evolving beast that differs in many ways from the graphic arts market. The demands are different for each industrial application, so ink sets must be custom formulated for various substrates and end uses. The substrates used in the industrial inkjet market are many and varied, from metals to glass to formable plastics and beyond. These printed materials will be used for many applications other than signage and decoration, hence the need for robust ink sets.

Also, off the shelf printers that were designed to deposit one ink set onto flat stock (in sheets or roll-to-roll) generally do not work for industrial printing.

Many of the objects printed for the industrial market are 3-D, requiring sensors to guide the printheads over the surface. Alternatively, the flat pieces may be formed after printing to achieve the contours.

The curing properties for a significant amount of the inks developed for industrial applications are different than the standard graphic arts inks, requiring UV bulbs with different spectral outputs, as well as some thermal curing capabilities (for example an IR lamp could be used inline to aid in curing). These equipment and ink requirements make it impossible to make an industrial printer and ink set to fit every need. This impacts the market path as well, since specialty integrators must be enlisted to design the equipment on a per customer basis.

In this market, third party inks are less of an option, due to the required customization. Generally, the ink will be fine tuned in parallel with equipment development, similar to the OEM/ink partner relationship for the graphic arts market, but on a more individual basis.

Formulating for Industrial Applications While Keeping The Yin in Delicate Balance

As inkjet printing begins to enable more and more industrial printing processes, inkjet formulators are faced with increasing challenges. While digital inkjet will probably never completely replace traditional printing techniques such as screen printing, flexography or lithography, it will enhance and at times actually overtake many print processes in the future, provided the digital ink properties can match or exceed those of the existing inks.

Post-forming images is one area of printing that has traditionally been dominated by screen or other processes. This type of vacuum thermoforming is used often in the automotive industry. The ink must be able to withstand drastic stretching of up to >300 percent elongation without displaying cracking and while maintaining adhesion and opacity.

UV curable inkjet inks are inherently rather brittle due to the crosslinked network that results from polymerization. This property is not easily overcome for digital inkjet inks, due to the viscosity requirements imposed by the printhead. The viscosity must be approximately 8-14 cps at the jetting temperature (many printheads have the ability to be heated, some as much as 70[degrees]C). This limits the formulator's choice of high molecular weight oligomers with low glass transition temperatures (Tg) that would give the cured film more elasticity.

The ability to lower crosslink density without compromising cure speed and physical properties while still maintaining a crack-free finish after thermoforming is a real challenge to the formulator. However, it is also important to monitor the abrasion and solvent resistance of the coating, as lowering the crosslink density will lower the film's durability. This may be addressed by using a formable laminate to provide the required durability. Lamination may be added as an in-line process after printing and curing, but before forming.

Figures 1 and 2 show examples of the elongation properties of highly flexible UV curable inkjet ink, HexiFlex, formulated by Hexion Specialty Chemicals. These inks were printed on vinyl and cured using a standard mercury vapor bulb. The samples were then laminated and thermoformed. The digital inkjet ink showed excellent elongation with no cracking. This is useful for many applications, such as fleet graphics for vehicle wraps.

[FIGURES 1-2 OMITTED]

Adhesion to many difficult substrates is another challenge for formulators targeting the industrial arena. Traditional free-radical based acrylate chemistry is often unsuited for substrates such as glass or metal. One approach is to use cationic chemistries to allow adhesion to many substrates. Figure 3 shows an example of a UV cur able digital inkjet ink, HexiLok, formulated by Hexion Specialty Chemicals, printed on glass. This image passed a crosshatch adhesion tape test using 610 tape with 100% adhesion.

[FIGURE 3 OMITTED]

An important feature for both the industrial and graphic arts market is faster cure speeds for improved productivity. Drop on demand (DOD) piezoelectric printheads are being developed with ever increasing print speeds; however, curing speed often remains the critical factor. The combination of low-viscosity acrylate oligomers, monomers and specific photoinitiator blends can dramatically increase cure speed and give excellent cured film properties. Hexion Specialty Chemicals has developed a line of low-dose curing inks that have a tack-free surface and fully developed film properties after exposure to just 65 mJ/[cm.sup.2] from a standard mercury vapor bulb.

Yet another ink development that is useful in both the graphic arts and industrial market is the emergence of a white UV curable inkjet ink. This has many uses, especially for second surface printing for the CMYK colors on transparent substrates.

The biggest challenge to formulators is overcoming the sedimentation that occurs when a dense particle such as Ti[O.sub.2] is dispersed in a low viscosity fluid. Ti[O.sub.2] is the white pigment of choice primarily because its refractive index (which dictates its hiding power and thus opacity) is considerably higher than other white pigments. Table 2 lists the refractive indices of several white pigments.

Figures 4 and 5 show the sedimentation profiles for two different UV curable white inkjet ink formulations. These profiles were collected with a Turbiscan LabXpert Sedimentometer. For this experiment, a vial of each ink was prepared and the amount of backscattering was measured (the purple line in each profile, shown on the Y-axis in time) along the length of the vial (shown on the X-axis in mm). The vials were stored in an oven set to 60[degrees]C and rescanned weekly for four weeks (30 days). The HexiJet White ink in Figure 4 had much better resistance to settling than the ink in Figure 5, which was not formulated with the patent-pending chemical method of suspension. This ink does not require any onboard agitation and can sit unused in the printhead for up to two weeks.

[FIGURES 4-5 OMITTED]

Application Methods (Ensuring The Yang is Present and Balanced)

The most well-formulated and robust ink is useless without an application method. The equipment portion of the industrial inkjet market is as equally important as creating the perfect ink chemistry. It is imperative that the chemistry and the hardware be well-matched so that they may work together for maximum performance, similar to way the ink supplier works with the OEM in the graphic arts market.

However, in the industrial arena the ink is not the only portion of the equation that must be fine-tuned. The hardware also must meet harsh demands. This equipment is often used in-line with the other processes such as caustic washing, die-cutting, stamping, embossing and forming. It must be robust enough to withstand a production environment. Material handling is a critical issue that must be addressed, as 3-D objects are decorated and flat sheets are moved from station to station while registration is maintained so that print quality does not suffer.

Grey-scale technology has drastically improved print quality and appearance and MEMS printheads are allowing wider print widths to be achieved with a single printhead, so in some cases printhead technology is catching up to the needs of the industrial print market.

Single-pass systems are desirable for high speed processes, but are limited by the need for nozzle redundancy to cover failed jets and maintain image quality. New printhead technology is making strides towards single-pass systems by allowing self-recovery of lost nozzles.

Another hardware consideration involves the curing conditions required by some of the industrial inks. In some cases IR lamps or heat tunnels must be incorporated into the process line in addition to the UV curing lamps. Also, the RIP software must be able to handle large amounts of variable data quickly. All of these details, and many more, must be considered as the hardware and the chemistry are blended together into one harmonious system. These systems rapidly become very complicated and since many of them are custom, one-of-a-kind presses, they are fairly expensive. However, the savings in terms of labor and production steps, as well as the benefit of customization, tend to make the transition to digital a cost-effective investment.

It is to the benefit of the industrial end-user to seek their perfect system from a total solution provider that can pull together all of the required components, from the best-suited printhead technology, to the best performing ink formula, to the correct curing equipment, to the material handling portion and the software, among other items.

Several companies, including Hexion Specialty Chemicals, have taken this approach. The central point of contact does not actually manufacture the equipment, instead they bring together all of the required components so the end-user is not forced into the role of the middleman.

Conclusions

The industrial inkjet market is a new and exciting area ripe with growth opportunities. As digital printing expands into more traditional print areas, formulators and equipment designers will be faced with increasing challenges to make and apply robust inks that can meet the demands of each application. As more and more traditional printing processes are supplemented and even replaced by digital printing, end-users will have a greater number of established choices for ink and equipment.

Until then, it is up to the innovators on the forefront of this market to maintain the balance of ink and equipment development.

Table 1 was reprinted from "The Future of UV Ink jet Printing," Dr. Sean Smyth, 2005, with permission from Pira International, Ltd. Contact stephen.hill@pira-international.com for more information.

Paul Lindquist graduated from the University of Wisconsin--Eau Claire in 1987 with a degree in chemistry with business emphasis. Mr. Lindquist has worked in the specialty chemical industry throughout the 19-year span of his career in business development. In 2004, he joined Hexion Specialty Chemicals' UV Curable Group to expand its product portfolio into new markets, including UV ink jet graphic arts and industrial printing. Hexion has quickly become a leader in UV curable ink jet inks market, introducing three new product lines, HexiFlex, HexiLok and FlashCure, in Q4 2006.

Editor's Note: "Ink and Equipment: The Yin and Yang of Industrial Inkjet Printing" was presented during uv.ebWest 2007, held March 6-7, 2007 in Los Angeles, CA.

By P.A. Lindquist and S.E. Edison Hexion Specialty Chemicals, Inc.

Sara Edison received her bachelor's in chemistry in 2000 from Bellarmine University. She completed her Ph.D. in inorganic chemistry from Michael Baldwin at the University of Cincinnati in 2004. Since then, she has been a member of Hexion Specialty Chemicals UV Curables Group, focusing on the development and sale of UV curable ink jet inks.
Table 1. UV inkjet markets, 2005 and 2010 ([euro] million)

                    2005     2010     Av. CAGR
                                      2005-2010 (%)

Packaging           34.9     1209.1   103
Signage             318.8    3423.7   61
Textiles            5.3      48.4     56
Other Industrial    15       345.7    87.5
Total               374      5,027    68

Source: Pira International.

Table 2. Refractive indices of white pigments.

White Pigment                         RI

Rutile Ti[O.sub.2]                   2.73
Anatase Ti[O.sub.2]                  2.55
Antimony Oxide                     2.09-2.29
Zinc Oxide                           2.02
Basic Carbonate, White Lead        1.94-2.09
Lithopone                            1.84
Clay                                 1.65
Magnesium Silicate                   1.65
Barytes (BaS[O.sub.4])               1.64
Calcium Carbonate (CaC[O.sub.3])     1.63
COPYRIGHT 2007 Rodman Publishing
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Author:Lindquist, P.A.; Edison, S.E.
Publication:Ink World
Date:Jun 1, 2007
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