The Evolution of UV-Curable Ceramic Decals

Recent developments in UV printing technology are providing ceramic decorators with the potential to obtain smaller quantities of higher-quality decals at a lower cost.



Radiation-curable printing inks, also called ultraviolet (UV)-curable inks, have been replacing solvent-based inks in general graphic printing applications for the past 20 years. Compared to their solvent-based counterparts, UV inks raise fewer concerns about potential health risks or environmental emissions. Additionally, their fast drying capabilities—from fractions of a second to a few seconds—have made possible the use of multicolor in-line printing, and UV inks also offer improved gloss, surface protection and print consistency.

Because of these and other benefits, UV inks have also become popular for direct printing applications on automotive and container glass. Only recently, however, has the ceramic industry begun to capitalize on the advantages of UV-curable inks in ceramic decals.

How UV-Curable Inks Work

UV-curable inks typically consist of pigments, viscous components, thinning constituents, photoinitiators and additives. While the pigments are basically the same as those used in solvent-based inks, surface-treated pigments are often required to provide storage stability.¹ The viscous components are inert (non-reactive, preformed) polymers and viscous oligomers; and the liquid, thinning constituents are monomers and low-viscosity oligomers. Molecules called photoinitiators are a vital part of a UV ink formulation and assist with the curing process. Other properties, such as surface tension and rheology, are controlled through essentially the same additives used for solvent-based inks.

The main difference between drying a solvent-based ink and curing a UV ink is that the latter is based on a chemical reaction, called photopolymerization, rather than on a physical evaporation process. Two main types of photopolymerization exist—one is promoted by “radicals,” a very reactive chemical species that typically reacts quickly and does not require heat; while the other, less common, type relies on a charged species called “cations,” which propagate a slower reaction that requires some heat and usually takes longer to complete. For this reason, the terms “radical cure” and “cationic cure” are both used to describe the photopolymerization process. Both types share the same trigger, which is UV radiation.

In the first step of a UV cure, high energy radiation converts the photoinitiators into radicals or cations, which then further react with the the bulk of the liquid—the monomers and oligomers—contained in the ink. After reacting with the activated photoinitiator, these “building blocks” become the reactive end of the growing “chain,” which forms a polymer when the reaction is terminated through recombination, disproportion or a chain transfer step.^2,3 The resulting polymeric product is the binder for the pigments in the ink.

A mercury vapor discharge process in an arc or microvave light bulb typically provides the radiation required for UV curing.⁴Visible light and IR radiation are also generated during the curing process (see Figure 1). Other than the bulb and the electric and electronic components needed to run the bulb, the curing unit consists of a conveyor belt to pass the substrate underneath the bulb, a reflector to shine as much UV light as possible on the substrate, and a cooling device for the bulb and possibly the substrate as well.⁴

Applying UV Technology to Ceramic Decals

Given that UV inks have been used in printing and screen printing applications for more than two decades, it seems surprising that they have found commercial use on screen printed ceramic decals only during the past three to four years. In fact, many patents concerning UV printed ceramic decals date back to the 1980s.^5,6,7 The patented ink and medium formulas for these early products were mostly derived from UV inks for direct printed applications, such as automotive glass or container glass decoration, or from standard inks for graphic applications. The inventors claimed that the formulas worked for ceramic decals and thus indirect application; however, while printing and curing were satisfactory, the decals produced inconsistent fired results, and they had little or no shelf life.

A problem with the lacquer also had to be overcome. Decal application requires a transfer lacquer that ensures the dimensional and structural integrity of the decal during transfer and is fired off later, ideally without any effect on the colors and/or precious metal decoration. The early UV lacquers, however, were not flexible enough for proper decal transfer, and they usually lost what little bit of flexibility they did have during storage.

Improvements have since been made on UV technology for ceramic decals. But even today’s sufficiently flexible UV lacquers suffer from two major drawbacks: 1) they inherently fire off only at temperatures considerably higher than conventional solvent-based covercoats, so they often interfere with the decoration during firing (see Figure 2); and 2) because of the raw materials used, they are much more expensive than conventional lacquers.

Conventional, solvent-based covercoats can be used on top of UV inks since solvent-based products fire off at lower temperatures than the UV medium.* However, UV inks shrink upon cure, and since the curing process is very fast, mechanical stress from shrinkage is not released. When the inks are covercoated, the uptake of solvent makes the ink soft and enables stress relaxation, causing a dimensional change of the printed layer. This change can lead to loss of adhesion and, consequently, a wrinkled, unusable print.

One solution would be to design a solvent-based covercoat containing solvents that do not swell the UV print. However, while this solution is technically feasible, the necessary raw materials would be costly, and it would be difficult to place new covercoats in a market saturated with good and cheap standard products.

Another challenge to using UV technology on ceramic decals is that ceramic inks, in which the particulate content has to be split between pigment and inorganic binder (glass), are inherently weaker than standard graphic inks, in which the solid particulate content is almost exclusively pigment. The easiest way to overcome this problem is to increase the layer thickness of the inks; however, using thicker layers with coarse mesh screens can cause some of the print definition to be lost. Overprinting several layers is feasible, but more prints increase the printing cost, and this additional cost can seldom be conveyed to customers.

Yet another problem is the heat sensitivy of many decal papers. Paper for waterslide decals is coated with a water-swellable compound that releases the decal when the print is immersed in water for a period of time. This compound contains a certain amount of water even when “dry.” The paper is usually factory-equilibrated to a certain combination of temperature and humidity that defines the conditions for a dynamic equilibrium, where evaporation from and condensation onto the paper occur at an identical rate (i.e., 23?C, 55% relative humidity). Before use in a print shop, the paper is again equilibrated to the actual climate in the print shop, which should be as close to the factory conditions as possible.

When the paper passes beneath a UV source with a distance of 15-25 cm, the paper is exposed not only to UV radiation and visible light, but also to heat. (The surface of a UV bulb is at 800-900?C.) The heat drives the humidity out of the paper, causing it to shrink and lose its planarity. In most UV curing applications, faster curing requires more heat, with an optimum around 80?C.¹ However, waterslide decal paper begins to shrink at 35-40?C (on the paper surface). This fact explains why UV formulations suitable for standard graphic screen printing will not work for ceramic decals. To maintain the perfect registration, a peak temperature of 38?C on the paper surface should not be exceeded.**

To ensure the successful application of UV technology to ceramic decals, new inks, media and curing technologies have had to be developed specifically for ceramic applications.

Overcoming Technological Challenges

Media Requirements
The ideal solution to increasing the color intensity of ceramics inks would be to increase the color-to-medium ratio, but this could not be done with conventional media. (A simplified composition of a conventional medium is given in Figure 3.) Given that the flexibility of a decal is primarily a function of the volume ratio of rigid (color) to flexible (organic solids) constituents in the dried print, it became evident that specially formulated UV media would provide an advantage (see Figure 4). Theoretically, only half the amount of a UV medium would be needed to produce a decal that has the same color-to-solid organics ratio as a conventionally printed decal.

To make full use of this advantage, rheology also had to be considered. If the UV and conventional medium have the same viscosity, then using only half the amount of medium results in a barely printable, dilatant paste. Conventional media contain high viscosity (or, more precisely, high molecular weight) solid polymers. The viscosity is adjusted with solvent that is lost upon drying and thus does not contribute to the solid organics content.

UV media, on the other hand, can theoretically contain any polymer content between 0 and the standard polymer content of a conventional medium, and their viscosity is adjusted with monomers that are not lost upon cure. Instead, the monomers are converted into a binder and fully contribute to the organic solids content of the decal. For this reason, the viscosity of a UV medium can easily be adjusted to accommodate much higher powder loads than a conventional medium and still produce a printable paste, as well as a flexible decal. The viscosity benefit can also be used to produce a paste that has the same color-to-medium ratio—and, hence, color intensity—as a conventional paste, but is more liquid and allows for higher printing speeds.

Curing Requirements
While high color-to-medium ratios and/or thick layers are required for ceramic colors, they can set severe limitations on UV curing since they decrease the chance of UV light passing all the way through the print to give a good through-cure. Prints that are not fully cured will wrinkle upon covercoating, and they can also become tacky over time because of residual monomer migration. To overcome these challenges, a UV curing unit was needed that would optimize the UV intensity-to-heat ratio.

Since approximately half of the radiation reaches the substrate through a reflector rather than directly, metal oxide-coated glass reflectors (dichroic reflectors), which are transparent in the IR radiation wavelength range and reflect UV light, were used to accomplish part of this task. Quartz plates placed between the lamp and the substrate also help eliminate heat while transmitting most of the UV light.⁴

The shape of the reflector, which results in either a diffused or focused reflection, was another factor. Diffused light keeps the peak temperature and UV intensity lower, thereby keeping the thermal impact low. However, it also keeps the peak UV intensity at an identical UV dose low compared to a focused reflector. For thick printed pastes with a high powder load and/or a high extinction coefficient, such as those sometimes used for ceramic applications, the peak intensity achieved with a diffused reflection may not be sufficient.

Several years ago, researchers discovered that dotating a UV bulb with elements such as Fe, Pb and Ga shifts the spectral output to a higher wavelength, enabling the radiation to penetrate deeper into matter.⁴ As in standard UV curing systems, the UV source provides high-energy, short-wavelength UV-C radiation that is of very limited use for ceramic processes but generates ozone;*** the UV-B radiation that is responsible for surface cure (a few microns); and the UV-A radiation that penetrates deeply into the printed layer and provides the through-cure. The respective wavelengths are depicted in Figure 5.

To further prevent heat damage, the optimized curing units also feature a post-cure cooling stage. The water in the paper begins to evaporate as soon as the decal paper is heated by the UV lamp. However, the speed of evaporation is dictated by the mobility of water in the high viscosity environment of the gelled release compound layer of the paper. Cooling the paper right after UV irradiation prevents humidity loss and stops the evaporation process before the decal becomes damaged. Additionally, the time the sheet is exposed to the UV environment when traveling from the stack in front of the printing machine to the stack behind the drier has also been reduced from ~45 minutes to ~30 seconds. A curing unit with the shortened cure time and active post-cure cooling is shown in the photo at left.

As a result of these measures, the dimensional stability of decal paper in today’s UV processes is better than in conventional process, provided that a suitable curing unit and UV medium have been used. While the number of suppliers for such curing units is still very limited compared to the total number of UV curing equipment suppliers, decal printers and decorators still have a reasonable choice of suppliers with worldwide sales and service organizations.⁹

The UV Absorption of Ceramic Colors
The UV absorption of a graphic UV ink depends on the types of pigments and fillers used, the level of pigmentation and the absorption characteristics of the liquid component (the medium). The ink is typically supplied ready-mixed, and the medium can be adjusted to the requirements of the individual pigments. Ideally, each ink should have its own “customized“ medium; however, since pigments can be classified in terms of UV absorption, only a limited set of media are required to make all the inks of a color range.

Ceramic decorative colors have to be treated somewhat differently. Besides pigments and a medium, they must also contain inorganic glass, which functions as the permanent inorganic binder for the pigments (as opposed to a temporary, pyrolysable organic medium). Like the pigments, the glass adds to the overall UV absorption; however, while the pigments act mainly by scattering, the glass exhibits specific absorption that requires a thorough spectral analysis before the correct photoinitiator can be selected. Since the glass compositions used cover the whole firing range, from low-temperature enamel or glass to high-temperature hard porcelain, the extinction coefficients vary considerably. The lead-free alternatives available in most of today’s decorative color series have brought even more glass compositions into play. Each of these compositions affects the curing speed, so it is not surprising to find curing speed differences of a factor of 10+ when looking at different decorative colors pasted with one medium at one pasting ratio. Printers who work with these colors must therefore have a good understanding of the technology, as well as detailed technical documentation from the color and media suppliers.

Ideally, ready-mixed paste would be supplied, as in the graphic printing industry. But given the number of ceramic decorative colors, multiplied by the number of required printing rheologies—from liquid to highly thixotropic—it is obvious that this approach would lead to an indefinite number of pastes. However, ready-to-use pastes for the four-color process are an option and are already commercially available.

Logistic and Economic Aspects

As the tableware and other decal-consuming ceramic industries have come under more and more economic pressure over the last decade, the economic aspects of decals have become increasingly important. The general trend toward minimizing supply inventories has resulted in the average decal order rarely exceeding a couple of hundred printed sheets, and delivery times are getting progressively shorter. At the same time, the complexity of decoration has increased, so that more colors and, hence, more prints, are now required per decal. Today’s average wicket drier has a capacity of 2000 sheets and a drying time of 45 minutes, making it ideal for drying large orders with limited color palettes. But when the order comprises 200 sheets in eight different colors, the discrepancy in dryer capacity to order size causes inefficiencies and logistical problems in the print shop. With UV printing, the only drying step required is for the covercoat, significantly reducing drying time and complexity.

Production speeds can also be increased using UV printed decals. If color intensity is maintained at a conventional level, a minimum of a 20% printing speed increase is achieved when using UV mediums. Additionally, fewer prints are often required to reach a given color intensity.

These are only some of the economic and logistic benefits of UV technology. It is beyond the scope of this article to go into more detail, as many factors, including local print shop and decal-market structures, are involved. More detailed information is available from suppliers’ documentation.¹⁰

Current Status and Future Developments

Making ceramic decals often involves more than 100 organic “decoration auxiliaries,” including media for various firing and printing requirements, covercoats, adhesives, dispersants, thinners and cleaners, as well as a legion of precious metal preparations that further add to the complexity. Since UV technology for ceramic decals is relatively young, not all of these components are available. However, a great deal of progress has been made over the past several years.

In the past, the drawbacks associated with UV inks have included firing instability, lack of flexibility, a limited shelf life, the limited storage stability of the medium and paste, and an incompatibility with standard solvent-borne covercoats. Today, UV media offer firing stability equal to or better than conventional decals. When the decals are properly made, they are at least as flexible, and their shelf life is comparable to or better than their solvent-based counterparts. Most media are guaranteed for one year of storage, and factory-made pastes are guaranteed for six months. Additionally, the development of specially formulated UV media has eliminated the problem of shrinkage with standard, good quality covercoats.

UV technology also has some specific advantages for ceramic decals when compared to conventional solvent-based technology. UV curing is much faster than solvent-based drying, a benefit that becomes increasingly vital in times where average production runs are a couple of hundred sheets, as opposed to a couple of thousand sheets 10 years ago. Additionally, conventionally made prints that consist of or contain halftone areas often suffer from inconsistent color deposits caused by the evaporation of the solvent when the paste is on the screen. UV media, on the other hand, do not contain solvents. Very little monomer evaporates from the screen, so a very constant color deposit is created. This provides a quantum leap in production consistancy for prints carried out with fine mesh (140-180 thread) screens, and the effect is also noticeable with coarser screens.

Although today’s printers can implement UV technology using a combination of UV and conventional products, more new products will be required to meet the long-term goal of the “UV only” printshop. Among the most demanded are:

• Products for the heat release process
• UV bright golds
• UV covercoats (with some economical restrictions)

These will be brought to the market within the next couple of years. With the continued advancement of UV printing technology for ceramic decals, ceramic decorators will soon be able to obtain smaller quanitities of higher quality, more complex decals at a lower cost.

Author's Acknowledgements

Most UV developments in the ceramic decal field are process developments much more than they are sheer product developments. Close cooperation of curing unit manufactures, paper suppliers, printers and medium and color suppliers was neccessary to get to the point where we are today. Cooperation of experts in their respective areas has contributed to the success as much as individual products have.

We would like to thank K?hnhackl GmbH, Rosenthal AG, SPS®-Rehmus, IST Metz GmbH, Dr. H?nle AG, Tullis Russell Coaters and Hoffmann & Engelmann KG for their continued support in the development of this technology.

For More Information

For more information about UV-curable ceramic decals, contact Ferro GmbH, Gutleutstr. 215, 60327 Frankfurt am Main, Germany; (49) 69-271160; fax (49) 69-27116270; or visit http://www.ferro.com.

*Conversely, using a UV covercoat on top of conventional inks causes serious firing defects. For this reason, a UV ink or coating should never be used on top of a solvent-based product.
**Different decal papers react differently to heat impact. Some shrink even below 38?C and are hence not suitable for this application. The requirements for wax-coated, heat-release papers are less severe and depend on the melting point of the wax. Information on suitable papers can be obtained from major decal paper manufacturers.⁸
***All UV curing units generate ozone, although ceramic curing units produce it to an even lower extent than standard graphic curing units. In a properly working curing unit, the ozone is removed from the process through an air duct. Ozone is such a fragile molecule that it is converted back to harmless oxygen within seconds as it travels through the air duct, so no ozone ever reaches a worker or the environment if the setup works properly. Ozone has a pungent odor, so leaks can be detected quickly, and the curing unit can easily be shut down to repair the damage. Additionally, gas detectors can be used to automatically switch off machines in case a preset ozone level is exceeded (ozone is always present in ambient air; it is a question of concentration whether it is harmless, healthy or hazardous).