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.
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 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.
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.
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.
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.
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.
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
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:
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.