Patent Application
20070289459
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings(s) will be provided by the Office upon request and payment of the necessary fees.
It has now been surprisingly discovered that partially curing with actinic radiation at least one but not every energy curable flexographic ink layer before applying the next layer while printing at least 3 layers of inks results in making the printing more suitable for use in a wet trapping method for printing flexographic inks. This happens because of the increase in the viscosity of the partially cured ink layer. The ink layer to be partially cured is preferably the first layer applied on a substrate. Also preferably, all ink layers applied are energy curable. However, non-energy curable ink layers could also be used in the present invention.
Accordingly, the present invention relates to a novel process for implementing wet trapping of energy curable liquid flexographic inks in a flexographic printing environment, wherein the inks are preferably formulated using energy curable compositions containing an amount of photoinitiator(s) sufficient to cause only a partial curing and not a complete cure upon exposure to actinic radiation. Accordingly, such inks preferably contain only a limited amount of photoinitoators that are undesirable in food packaging products. Preferably, the amount of photoinitoators is less than about 10% by weight of the total weight of said energy curable liquid inks. Alternatively, the inks used in the present invention can be partially cured by being subjected to a reduced level or type of actinic radiation.
It has been now found that merely evaporating the non-reactive diluent as suggested by U.S. Pat. No. 6,772,683 to increase the ink viscosity may not be sufficient for proper performance. One such example is wet trapping over 100% coverage of white ink, in cases where the substrate is transparent, and the white ink is printed first to make the substrate look white and opaque. Thus, applying actinic radiation, preferably LED radiation, to said white energy-curable ink raises its viscosity more effectively that mere evaporation of an inert diluent such as water.
Specifically, 1% of Irgacure 819DW photoinitiator, which is a water-based dispersion of Irgacure 819 (BAPO [bis-acyl phosphine oxide]) were added to energy-curable White Uniqure ink also containing 7% water as inert diluent. The ink was printed on transparent OPP substrate and the water evaporated. Still, the ink was very soft to the touch. This softness negatively affects the printability over this ink on press.
The printed ink was then subjected to 0.5 sec exposure to 450 mW 395 nm LED array (UV Process Supply, part # A-160-008). The ink areas exposed to the 395 nm light were resistant to scratch while the unexposed areas scratched easily, indicating greater hardness and viscosity of the exposed areas.
The LED arrays are the preferred radiation sources to increase the viscosity of the wet-trappable inks as they emit very little heat so that heat sensitive substrates and equipment can be used. It is understood that LED arrays will not cause the complete curing of said energy-curable inks because of the limited amount of photoinitiator in said inks, but only cause partial curing, which, however, will beneficially increase their viscosity before the next ink layer is applied.
At the end of the present method, all the energy curable ink layers will be simultaneously fully cured, preferably by electron beam radiation.
These energy-curable liquid flexographic inks are single-, ternary- or quaternary-compositions that contain a resin that is neutralizable by acid or base, and a non-reactive diluent, such as an organic solvent, water or a combination thereof.
The compositions of this invention are energy curable. The term “energy-curable” composition, as used herein, is intended to mean compositions that are polymerizable or crosslinkable by the action of a radiant energy source of actinic radiation, such as ultraviolet radiation (UV) and electron beam radiation (EB). As used herein “actinic radiation” is intended to encompass radiation having a wavelength range from about 190 nm to about 400 nm, and preferably from about 240 nm to 400 nm. Actinic radiation of this type may be obtained from a variety of sources, e.g., mercury arc lamps, xenon arc lamps, light emitting diode, fluorescent lamps, monochromatic laser sources, and the like.
A “partially cured ink” for the purpose of the present invention is an ink that has been polymerized or crosslinked to a degree yet remains not completely fixed to the surface it is printed on and thus is not suited for commercial use.
In one embodiment, ink compositions suitable for use in the present invention contain a non-reactive diluent, preferably water. More preferably about 5 wt. % and 50 wt. % of said diluent comprises water. However compositions containing other non-reactive diluents, such as alcohol and mixtures of water and alcohol may be used. In practical terms, a water based ink composition is highly desirable, as its use complies with health and anti-pollution regulations that limit the amount of solvents permitted to escape in the environment. Therefore the present invention will be described using aqueous ink compositions, inasmuch as such compositions are the most likely to be used. Such limitation in the description of the invention is, however, not to be construed as limiting; and radiant energy curable inks of similar ink viscosity profiles and comparable non-reactive diluent evaporation characteristics are considered within the scope of the present invention.
The method provided by the present invention for applying multiple, at least partly superposed, ink layers on a substrate, relies on the rapid and relatively significant change in the viscosity of an energy curable liquid flexographic ink, after it has been deposited as a layer onto a substrate and subjected to partial curing by actinic radiation. Each ink layer is deposited onto the substrate in an inking station. There are as many inking stations as there are individual inks used in printing the color image. At each inking station, the ink is transferred from an ink reservoir through an anilox roll to a flexographic printing plate, such as a Cyrel.RTM. polymer printing plate produced by E. I. Dupont de Nemours and Company, Inc. The ink is then transferred from the printing plate onto a receiving substrate, such as a web or sheet of polyethylene terephthalate film, or any other substrate, which may be printed with a flexographic printing plate.
The initial viscosity of a liquid flexographic ink deposited onto the substrate is typically under 4000 cps, and preferably under 70 cps, although ink viscosities of 2,000 cps may be used, depending on the particular printing application. As discussed earlier, this very low viscosity is preferred in order to achieve good ink transfer from the ink reservoir through the anilox roll to the printing plate surface.
Once the ink has been deposited onto the substrate, it is subjected to actinic radiation causing partial but not complete curing and increasing its viscosity.
By the time the ink layer arrives at the next inking station, where another ink layer, typically of a different color, is deposited on the substrate and over at least portions, if not all, of the previously deposited ink layer, the ink viscosity of the deposited ink layer will have increased sufficiently to wet-trap that ink layer without back trapping the newly deposited ink, having a viscosity typically in the same range as that of the earlier deposited ink at the time of its deposition. Therefore, by partially curing energy curable inks, wet trapping of multiple ink layers can be implemented without the need to change ink viscosity by heating the ink, or chilling the substrate containing the ink layer, between inking stations, or completely curing the ink between inking stations.
According to the present invention, once all ink layers have been applied, a single curing step with a proper energy curing source such as an excessive dose of electron beam radiation is sufficient to fix all applied layers.
The present wet trapping process is not limited to the use of energy curable liquid flexographic inks, but may encompass the use of at least one layer of non-energy curable ink. For example, a layer of an energy curable liquid flexographic ink of the type disclosed above may be applied and the application of this layer to the substrate may be followed by the application of a layer of a non-energy curable liquid flexographic ink, this second layer having a viscosity that is less than the increased viscosity (through partial curing) of the first layer. Again, because of the viscosity differential, wet trapping may be implemented. If this second layer is the uppermost or last printed layer, all ink layers may be then be cured and dried via conventional drying means and methods, to simultaneously fix the deposited ink layers onto the substrate.
In yet another embodiment of the present invention, a number of energy curable and traditional ink layers may be inked in superposed fashion and still employ the wet-trapping technique of this invention. For example, as stated above, a first energy curable ink, having a first viscosity, may be applied as a first layer. A traditional ink layer, having a lower viscosity than the increased viscosity of the first ink layer, may then be applied over the increased viscosity layer at a subsequent inking station to form a second layer. A third layer may next be applied over the second layer using a second energy curable ink having a lower viscosity than the viscosity of the second layer. The viscosity of this layer will again increase by partial curing before reaching the next inking station. At the fourth inking station, a fourth layer may be applied over the third layer using yet another energy curable ink having a lower viscosity than the increased viscosity of the third layer. Drying of the conventional ink layer may be implemented if the conventional ink layer viscosity is so low that energy curable ink with lower viscosity is not available. Thus it is possible to implement the process of the present invention in what may be referred to as a “hybrid” process, whereby only a number of ink layers are implemented by viscosity gradient wet-trapping according to this invention, and wherein certain layers are dried or cured prior to the application of additional ink layers, using combinations of inks. Such a hybrid process, however, while possible and within the scope of the present invention, is less efficient than a process wherein all applied layers are an energy curable liquid flexographic ink.
Electron Beam (EB) curable white ink was printed on Central Impression (CI) flexo press manufactured by Ko-Pack, Japan utilizing photopolymer printing plate, 800 line/inch engraved anilox roller and clear OPP (oriented polypropylene) film. Then, four additional energy curable (EB curable) Unicure inks (from Sun Chemical), yellow (Y), magenta (M), cyan (C) and black (K) were printed on the top of this white in consecutive printing stations of the press. Thereafter, all 5 layers were energy cured with 3 Mrads of Electron Beam radiation. Print densities achieved for each color are reported in Table 1 below.
1% of Irgacure 819DW, free radical photoinitiator manufactured by Ciba was added to the white ink composition of Example 1 and the printing cycle described in Example 1 was repeated, this time with exposure of the white ink to UV irradiation generated by medium pressure UV lamp prior to application of the subsequent ink layers. Specifically, the printed white ink was subjected to 0.5 sec exposure to 450 mW 395 nm LED array (UV Process Supply, part # A-160-008). Print densities achieved for Y, M, C and K colors are presented in Table 1 below.
In this example, visual comparison of the screen area of the prints generated in Examples 1 and 2 was performed under ×20 magnification with an optical microscope. The quality of printing dots (uniformity and shape consistency) can be seen in
The invention has been described in terms of preferred embodiments thereof, but is more broadly applicable as will be understood by those skilled in the art. The scope of the invention is only limited by the following claims.
