WO2011025773A1

WO2011025773A1 - Conductivity control of ink composition

Info

Publication number: WO2011025773A1
Application number: PCT/US2010/046456
Authority: WO
Inventors: Richard M. Jones
Original assignee: Sun Chemical Corporation
Priority date: 2009-08-24
Filing date: 2010-08-24
Publication date: 2011-03-03
Also published as: IN2012DN01982A; AU2010286777A1; CN102549085A

Abstract

Systems and methods to control the printability of an ink comprising water are presented In exemplary embodiments of the present invention, monitoring the conductivity of the ink via a conductivity measurement device yields a signal which, for example, decreases strongly and linearly as water is removed from the ink In exemplary embodiments of the present invention a lower limit on this signal can be set to control the turning on of a pump, and/or the opening of a valve to allow flow of a makeup fluid into the ink In exemplary embodiments of the present invention the makeup fluid can, for example, be compnsed of water, a caustic substance, and an inhibitor Such a make-up fluid can restore the lost component(s) and correct for damage due to ink heating on press Similarly, in exemplary embodiments of the present invention, an upper limit on the conductivity signal can also be set to turn off the pump and/or close the valve, so as to prevent over-addition of the makeup fluid and avoid ink dilution and/or insoluble phase formation

Description

CONDUCTIVITY CONTROL OF INK COMPOSITION

CROSS-REFERENCE TO RELATED APPLICATIONS:

The present application claims priority to U.S. Provisional Patent Application No. 61/236,206, filed on August 24, 2009, the disclosure of which is hereby

incorporated herein by reference.

TECHNICAL FIELD:

The present invention relates to printing and related technologies, and in particular to a method for controlling the composition of an ink during a printing run using measured conductivity.

BACKGROUND OF THE INVENTION:

Conventionally, viscosity and pH control of ink composition have been used for aqueous ink dispersions in flexographic and gravure printing. Both work to maintain ink composition in a range as best suited for printing quality in the face of changes to the ink brought about by water loss from the ink through evaporation. However, tracking both response functions in inks where the water is dissolved in the binder (such as, for example, in Sun Chemical's UniQure™ inks) over time on-press shows that for a significant initial period of time these measures change little followed by a period in which they experience abrupt and rapid change. The problem, therefore, with using viscosity or pH to control such ink compositions is that while the ink is changing its composition during this initial period of time and concomitant printing defects are being created, neither viscosity nor pH measurement can forecast these changes. Accordingly, control systems based upon viscosity or pH measurement lack the precision to detect these important initial changes in ink composition, and thus fail to remediate in time. Similarly, viscosity control can also err on the side of over-control. For example, air entrainment and foam can cause a false high viscosity reading, which in turn triggers injection of fluid into the ink in an attempt to maintain the viscosity. In such an occurrence the ink would be over-diluted, as the high viscosity reading is caused by air entrainment bubbles, and not a significant change in air-free ink viscosity.

What is thus needed in the art is a method for the control of ink composition on- press that addresses these problems of the prior art.

SUMMARY OF THE INVENTION:

Systems and methods to control the printability of an ink comprising water are presented. In exemplary embodiments of the present invention, monitoring the conductivity of the ink via a conductivity measurement device yields a signal which, for example, decreases strongly and linearly as water is removed from the ink. In exemplary embodiments of the present invention a lower limit on this signal can be set to control the turning on of a pump, and/or the opening of a valve to allow flow of a makeup fluid into the ink. In exemplary embodiments of the present invention the makeup fluid can, for example, be comprised of water, a caustic substance, and an inhibitor. Such a make-up fluid can, for example, restore the lost component(s) and correct for damage due to ink heating on press. Similarly, in exemplary embodiments of the present invention, an upper limit on the conductivity signal can also be set to turn off the pump and/or close the valve, so as to prevent over-addition of the makeup fluid and avoid ink dilution and/or insoluble phase formation. In exemplary embodiments of the present invention the lower limit on the conductivity signal can, for example, be set at less than or equal to half the conductivity value which is associated with print defects or permanent ink damage for a given press, print density, and pigment used. In exemplary embodiments of the present invention setting of the aqueous liquid flow and of the off-on control upper and lower limits, respectively, can be determined, for example, for one or more of (i) each ink color, (ii) volume in the sump, (iii) press speed, (iv) print width, and (v) press design.

BRIEF DESCRIPTION OF THE DRAWINGS:

Fig. 1 is a plot of trapping percentage over yellow versus run time for exemplary magenta and cyan UniQureTM inks running on a Ko-Pack central impression (Cl) press at 100 m/min, 35-40° C without any corrective action and under low ink volume conditions (stress test: 3000 gm of ink, 15 inch print width, circulation of ink at 1.0 liter/min);

Fig. 2 is a plot of percentage water by weight versus run time for various ink colors under the stress test conditions of Fig. 1 (Yellow = open circles, Magenta = grey circles, and Cyan = filled circles);

Fig. 3 is a plot of the viscosity of the three inks of Fig. 1 (Yellow = open circles, Magenta = grey circles, and Cyan = filled circles) versus elapsed run time under the stress test conditions of Fig. 1 (viscosity measured after removal of air by allowing ink to sit undisturbed for 24 hour to clear);

Fig. 4 is a plot of pH meter response of the three inks of Fig. 1 (Yellow = open circles, Magenta = grey circles, and Cyan = filled circles) versus elapsed run time under the stress test conditions of Fig. 1 ;

Fig. 5 is a plot of the measured conductivity of cyan inks over a wide range of formulated and evaporated conditions; Fig. 6 is a plot of pH versus run time for an exemplary magenta ink under stress test conditions using three types of control variables; and

Fig. 7 is a plot of trapping percentage for magenta ink over yellow ink versus run time for the three control variables used in Fig. 6.

DETAILED DESCRIPTION OF THE INVENTION:

Conventionally, the use of simple conductivity to control water addition to an ink has neither been practiced nor proposed. As described below, such control is particularly applicable to inks where the water is dissolved in the binder. This is the case for, for example, Sun Chemical's UniQure™ inks used in the WetFlex™ wet-trapping process.

In exemplary embodiments of the present invention, in contrast to conventional systems that measure ink pH or viscosity and tie their control systems to such variables, conductivity measurement precisely and sensitively follows the ink composition change over run time, such that using measured conductivity to control, for example, a water feed rate results in a reduction or prevention of the print defects caused by evaporation of water from the ink.

The acceptability of an ink or process by a printer often hinges, in large part, on the durability (robustness) of the ink or process to changing print conditions (e.g., changes in temperature, nip pressure, or substrate) which can be seen, for example, even in one print session. At first, when a print run begins, excellent results can be obtained. However, within 2-3 hours the doctor-bladed chamber can fill with a congealed ink and ink transfer may be negatively affected. This is because without any modification of an ink while running, the recirculating ink is concentrating by evaporation. It reaches a condition when the pigment and polymer cause gellation, an instability that either has a pH drop as a root cause or some other change in the nature of the dispersant. The first indication of

-A- gellation is in the doctor-bladed chamber where it cannot be seen. It builds up, leading to worse flow in the chamber and even greater heating as the ink is not able to efficiency carry away the heat. Such congestion will eventually stop printing, and the first indication is lack of transfer.

In exemplary embodiments of the present invention, if these changes can be avoided by maintenance of the initial ink composition throughout a press run, on- press ink performance is improved and the market for such a product and process, or similar products and processes, can significantly increase.

As noted, conductivity control of waterbome inks in gravure and flexographic applications has not been described in the art. This can be due in part to the rarity of inks based on aqueous solution, such as, for example, UniQure™ ink. Inks based on water-continuous dispersions of solids would show almost no change in conductivity until significant water has evaporated, and even then they would likely increase in conductivity. However, measured conductivity for aqueous solution based inks is generally very sensitive to small amounts of water loss, inasmuch as the water dissolved in the oligomers contributes strongly to the mobility of ions. Thus, conductivity falls even though the concentration of dissolved ions increases. Organic inks (e.g., those based on radcure monomers) become more conductive when water is dissolved in them (and less conductive when water is removed from them, such as via evaporation). Such conductivity changes are due to the increased mobility of ions when water is present in an organic phase. Thus, the novel approach of the present invention to control exploits such properties of organic inks, such as, for example, UniQure™ type inks.

In exemplary embodiments of the present invention, the composition of an ink can be maintained at or near its initial state during a print run by the addition of an aqueous fluid at a rate which can be determined by a measurement of ink conductivity. In exemplary embodiments of the present invention an ink to which such control is applicable is preferably one where water is soluble in the ink vehicle to a substantial degree, such as, for example, between 5% to 80% by weight.

In exemplary embodiments of the present invention the range of tolerance of the conductivity can be set, for example, by examining the ink conductivity associated with the onset of printing defects in an ink that is not being maintained by aqueous fluid addition. In exemplary embodiments of the present invention a conductivity measuring device provided in the sump or return line of the ink can, for example, output a signal that can be used with or without amplification to operate a pump and/or a valve in a fluid injection system delivering such fluid into the sump or ink intake line. In exemplary embodiments of the present invention, any device that yields a signal proportional to the ink conductivity can be employed. In exemplary embodiments of the present invention the sensor can be, for example, easily cleaned by passage of ink or fluid. In exemplary embodiments of the present invention the conductivity probe can, for example, preferably be of the inductive type. In exemplary embodiments of the present invention the injection system can be, for example, similar to or the same as those sold commercially for viscosity or pH management.

In exemplary embodiments of the present invention, when the measured conductivity falls to a differential, preferably of no more than half that which is associated with print defects or permanent ink damage (a function of the press, the print density, and the pigment used, et al.) the pump can be, for example, activated and/or a valve can, for example, open allowing a flow of makeup aqueous fluid having a composition so as to match that which has been lost into the ink intake to the press. Then, once the initial conductivity has been restored, the pump can, for example, shut off and/or a valve can, for example, close. In exemplary embodiments of the present invention, in order to avoid unnecessary feedback, the conductivity sensor can, for example, precede, the controlled valve in the ink flow. Thus, placing the conductivity sensor up-stream from the injector avoids any impact of lack of good mixing, which can lead to jittery on/off signals being sent to the pump or valve.

In exemplary embodiments of the present invention, the signal coming from an exemplary conductivity probe can be amplified as may be necessary. It is noted that some inks will have a low water component, such that even a small amount of evaporation can generate a significant change in conductivity. Alternatively, other inks, with a larger water content, upon losing the same quantity of water via evaporation, will have a lesser, although measurable, change in conductivity. Thus, in exemplary embodiments of the present invention, the conductivity control signal can, for example, be calibrated to specific inks, or specific types of inks, or more generally, as may be appropriate.

In exemplary embodiments of the present invention a preferred aqueous makeup solution can contain, for example, a caustic or buffer to pH 10 or higher, and, for example, a dilute solution of a soluble inhibitor such as MEHQ. In such exemplary embodiments, the general idea is to replace what is lost while the ink is on-press, namely water, basicity and inhibitor. By so maintaining composition of the ink vehicle, printing defects such as, for example, loss of ink transfer can be avoided. The advantage of the present concept compared to the known control of ink composition via pH and/or viscosity is that neither of the latter ink measures show any differential in the early stages of printing, when, in fact, changes to the ink and prints are occurring.

Wet trapping of water-based UniQure-type flexographic inks has been described, for example, in U.S. Patent No. 6,772,683 to Laksin, et al. to depend on evaporation of a fluid (preferably water) and the subsequent increase in viscosity of the ink after printing on a substrate. However, if evaporation of water occurs earlier on-press or within the ink recirculation system, wet trapping can be lost. Either no transfer of ink, or back trapping of the previous color, can occur. This is depicted in Fig. 1 , which shows loss of trapping of magenta (M) (open circles) over yellow (Y) within one hour of operation for two consecutive stations on a Ko- Pack Cl press, while two colors separated by two other stations, cyan (C) (closed circles) and yellow (Y) continue to print properly (in this later case the Yellow ink has had more opportunity to dry before the Cyan was applied). The cause for this behavior is the alteration of the magenta ink rheology due to water loss, as shown in Fig. 2, leading, in this case, to poor ink transfer from the engraved cylinder for magenta.

In other cases, evaporation of water can lead, for example, to higher print density, back trapping, ink slinging, static electrical hairs and misting.

In exemplary embodiments of the present invention, such premature evaporation of water from an ink can be managed by the controlled addition of an aqueous fluid either to the sump or into the ink lines. Such a fluid is sometimes referred to herein as a "make-up fluid." As noted, typically, viscosity or pH measurement has been used to send a feedback signal to a pump and/or to a valve to control the rate of make-up fluid addition. Such measured responses are recorded in Figs. 3 and 4 for fluid viscosity (cone and plate) and pH for UniQure™ inks. Both responses show little change over the period of time when the M/Y trap print density seen in Fig. 1 is greatly decreasing, thus exposing their inherent defects as control mechanisms.

In contrast, ink conductivity, as shown in Fig. 5, is very sensitive to the drop in water content over the range of about 30 - 20% by weight, for example, where this print defect (i.e., IWY trap print density drop) occurs. While feedback from viscosity or pH would demand no adjustment over the first 1 to 2 hours of printing in this example, feedback from a conductivity measurement signal to open a controlled water injection system would prevent excessive water loss from the circulating ink as occurs in the first two hours on press at speed, as shown in Fig. 2.

For UniQure™ inks comprising epoxy acrylate oligomer, water-dispersed urethane polymer, polyglycol diacrylate monomer, water, dispersing aids, wetting agents, and defoamers in the binder and pigment at substantial levels (15-40 % depending on color), the concentration of water in the ink is typically 5% to 40% by weight, more preferably 10% to 30% by weight, and most preferably 20% to 30% by weight. For the exemplary cyan ink shown in Fig. 5, the ink contained about 28% water and displayed a conductivity of about 270 microsiemens/cm. Experience has shown that if the conductivity of this ink does not decrease below about 200 microsiemens/cm (which corresponds to the water not decreasing below about 25% by weight), very acceptable prints result. However, when the water content of such inks go below this level, the following sequence of events are observed with further water evaporation: drop in pH, increase in ink density in solid areas, loss of ink transfer in dots, loss of trapping, loss of ink density in solid areas, ink slinging, hairs and tails from solid structures, and a buildup of curds of destabilized ink in the doctor-bladed chamber. Any of these events could be highly detrimental to print quality and can, for example, further lead to work stoppages and/or delays required to remedy or remove the defective inks from the press.

Thus, in exemplary embodiments of the present invention, in order to avoid a loss of water that could lead to print defects or press problems, for example preferably avoiding going below 20% water, and more preferably to avoid going below 22% water, a signal based on conductivity measure can, for example, be used to activate an aqueous feed into the ink. This can be accomplished simply by pouring in water to a stirred sump or by an automatic injection system which is activated at or near a conductivity reading of, for this example, about 150 microsiemens/cm. In general, the optimum water content of an ink can be both ink and pigment specific. Thus, it is noted that there are some inks whose water content should not fall below 25%, and thus the addition of water to those inks according to exemplary embodiments of the present invention can be activated at or near a conductivity reading of, for example, about 220 microsiemens/cm.

In exemplary embodiments of the present invention, to avoid over-addition of water past the solubility limit (even locally), a rate of addition not to exceed about 120 ml of aqueous solution per hour per gallon of ink circulated can be used, for example. It is noted that the consequence of too high a rate of addition is the risk of phase separation and agglomeration of the pigment. Oddly, this is also the consequence of allowing too much water to evaporate, but the latter is convoluted with a pH drop that destabilizes the urethane and pigment dispersions as well. In exemplary embodiments of the present invention, the rate of make-up fluid addition can be set to substantially equal the evaporation rate.

In exemplary embodiments of the present invention, in addition to the use of water as a preferred aqueous fluid, the aqueous fluid may also contain other materials, either alone or in combination, that are either consumed as the ink runs or would otherwise improve overall on-press ink performance. One such material, for example, is a caustic substance to help keep the pH high. In general the dispersion stability of ink solids depends on pH. Ink can be flocculated at a pH below about 6, as can be demonstrated in the laboratory with a small drop of strong acid. The origin of the drop in pH is likely hydrolysis of acrylate esters, leading to the formation of acrylic acid salts that buffer pH below about 9. Oddly, high pH is catalytic for this reaction, but so is acidic (low) pH. Thus, in exemplary embodiments of the present invention, hydrolysis can be slowed, preferably by minimizing the degree of heating on press. In exemplary embodiments of the present invention this can, for example, be best accomplished with low viscosity ink maintained by aqueous make-up fluid addition at a correct pH for the dispersion stability.

Such an exemplary caustic substance in the make-up aqueous solution can be, for example, a strong inorganic base such as, for example, sodium hydroxide or, for example, buffered solutions to at least about pH 9, more preferably to about pH 10. When using certain radiation curable inks, for example, it can also be, for example, an organic base such as a tertiary amine, but the material would preferably not be a secondary or primary amine or ammonia, as these may lead (in these radiation curable inks) to Michael addition and ink gelation upon recovery from the press. In exemplary embodiments of the present invention such a caustic substance is preferably soluble in water to at least about 1 % by weight as it should be effective even at low levels of aqueous make-up fluid addition. However, depending on base strength and/or the buffer capacity, the actual make-up solution may only contain, for example, about 0.1 % of such a caustic material. A preferred solution, for example, is to use an inorganic base or buffer so as not to change the conductivity of the organic phase. This is important, inasmuch as it is desired to preserve the relationship between the water content of an ink and its measured conductivity. Addition of an organic base or buffer could, for example, raise the conductivity of the ink, but not change the water content, which could then generate a false control signal.

Press heating also causes loss of inhibitor and possible stability problems in the recovered ink. The need for a water-soluble inhibitor is preferably met by hydroquinone-type materials and the like. In exemplary embodiments of the present invention, MEHQ can be used. It has a solubility in ink and in basic water (pH >8). However, due to its strong effect on other long-term storage inhibitors, the level of MEHQ preferably should not rise above about 0.5% by weight in the ink. Also, since MEHQ anion is a strong contributor to organic conductivity, it is useful to keep its concentration to below 1 % in an exemplary make-up aqueous solution, for example. In exemplary embodiments of the present invention where MEHQ is used, the depletion rate of MEHQ from the ink can be estimated, and sufficient MEHQ provided in the make-up solution to match given the estimated duty cycle of the pump and/or valve (e.g., half on, one-third of the time on, etc.).

In exemplary embodiments of the present invention a conductivity probe can be, for example, virtually any type, including a simple two-electrode device, but more preferably can be, for example, the four-electrode, so-called inductive device. Several known dielectric monitors can also be used, for example, such as those manufactured or provided by B&C Electronics of Carnate, Italy, Mettler Toldeo and Vernier, for example. In exemplary embodiments of the present invention the probe can be, for example, self-cleaning. Moreover, the placement of the probe can preferably be, for example, in the sump or in the ink return line where the conductivity measures would be from well-mixed ink returning from the doctor-bladed chambers (where considerable shear - and thus mixing - develops). It is noted that the fact that these devices have not been used to control ink composition (and the fact that it is surprising to viscosity control manufacturers that conductivity control is desirable) may possibly be due to the uniqueness of the UniQure-type construction, where radiation curing oligomers with high water compatibility are used; however the inventive process of this application is not limited to UniQure-type inks.

In exemplary embodiments of the present invention, the injection system can be, for example, virtually any type, including those sold commercially for viscosity or pH management. As noted, in exemplary embodiments of the present invention, the point of injection of the make-up aqueous fluid is preferably, for example, in the ink pump intake line to take advantage of any mixing in the pump and in the double doctor-bladed chamber. Such placement also avoids feedback loops where excessive on-off of the injector occurs due to too close position of detector and injector. In exemplary embodiments of the present invention, the injector can, for example, preferably be driven with a conductivity signal, or a derived signal therefrom, from a well-mixed ink.

In exemplary embodiments of the present invention, the exact setting of the make-up flow and the off-on control upper and lower limits, respectively, can be determined for each ink color, volume in the sump, press speed, print width, and possibly even press design, etc.. The examples below illustrate typical settings appropriate for a small press operating with a relatively small quantity of ink. In exemplary embodiments of the present invention, actual numbers will vary in other applications. The following examples are for the purpose of illustrating the effectiveness of the method proposed only and can be varied or tailored to meet specific press demands, as may be relevant in given applications or contexts. EXAMPLES

Figs. 6 and 7 depict examples that were obtained in actual press trials of exemplary UniQure™ inks using a Ko-Pack International Cl narrow web press (six colors), ESI EZCure electron beam curing, and white polyethylene substrate at 60 m/min. The anilox cylinder was a 1000 line, 1.6 bcm, the printing plates were Esko CBU, and the sticky back tape was L5.4. The doctor blades were steel. The press was not tempered. The inks were stressed by continuous circulation of the test ink to a running press without web. 3000 gm of ink was used (except for the yellow ink, where 10 Kg was used), with a 15 inch print width and a 1.0 liter/minute ink circulation rate. Web was introduced after each hour of running and prints obtained followed by web removal and continued press running. This approach quickly damages the ink and print due to a cascade of factors involving, for example, loss of water, loss of inhibitor, and pH drop of the inks which in turn lead to higher press temperatures and printing defects. We thus term this test the "Stress Test." The example inks used in Figs. 6 and 7 were standard UniQure™ inks made with appropriate pigments. Yellow was run in large volume (four gallon) in the sump, which was previously shown to minimize ink damage by increasing the recirculation time. Magenta was run in one gallon containers, roughly half of which was in the fill-up of the ink lines and the doctor-bladed chamber. The inks were monitored in the sump and samples taken for examination in the laboratory. The print quality was determined by an XRite Densitometer. The aqueous makeup liquid used was 0.1 w/w % NaOH with 0.5 w/w % MEHQ. This liquid is referred to below simply as "caustic."

Fig. 6 shows the effect on pH as measured via a pH meter of on-press UniQure™ ink of various means used to achieve a controlled caustic addition. It is known that when the pH of such an ink is allowed to fall, the ink can be irreversibly damaged by urethane polymer precipitation. In the open circles, conductivity control at 220 ± 10 microsiemens/cm was employed to toggle on and off a 100 ml/hr/gal metered flow of aqueous make-up liquid. Under this control scheme, the ink pH (meter) remained in a stable region, approximately 6.7-7.2. The grey circles show the result of using pH (meter) itself as the feedback control signal to the caustic addition. The pH (meter) first has to fall to below 6.5 to trigger the make-up addition; as can be seen, due to the flow limitation, the pH only slowly recovers. The dark circles show the result of using viscosity control. Here, no control signal is obtained until after approximately one hour of operation, when the viscosity exceeds 0.5 Pa. s.. This delay means that the pH cannot recover within the limitation of make-up volume/hr that can be added and print defects are observed.

It is noted that the fluid flow limitation is key, inasmuch as the limiting factor of how much make-up fluid can be added is color strength. The color cannot be diluted very much. While the pH of the make-up fluid could be increased, that would raise a concern of its shocking the ink or other issues such as corrosion. Injection of any caustic does take time to have the intended effect since it must be mixed into the total ink volume to be effective. Otherwise, the ink pH would yo-yo up and down as local regions of different pH would be sensed and responsive corrections taken. Thus, pH control is simply not effective.

Fig. 7 shows the effect of differing methods of ink composition control on a magenta over yellow trap. The open circles are data points generated from using conductivity control at 220 ± 10 microsiemens/cm. As can be seen in Fig. 7, this method shows excellent control of this print result. The grey circles show the result of using pH control of caustic flow, as described above. Due to the ^ΛA - 1 hour delay in achieving a significant signal indicating a pH drop (lower limit set point 6.5), the trap is degraded over the first two hours of running and does not recover the original trap density until five hours have elapsed following onset of make-up fluid flow, a bad result. Similarly, the dark circles are data points from a viscosity control system for this example. In this case, the one hour delay due to no measured viscosity change results in a significant drop in trap magenta density. In fact, as shown, this print quality measure never returns to the required value even after five hours of printing with make-up caustic flow at 120 ml/hr/gal.

It is noted in this context that even if the viscosity sensitivity were increased, and calibrated for small changes, the method would still fail. Foam in the ink defeats viscosity control. In addition, pH adjustment cannot be quick when it is also desired to replace the evaporated water to avoid thickening. Only a certain amount of caustic can be contained in the replacement water, and it is not desired to allow pH to exceed 10. Fig. 1 shows UniQure™ ink running on the Ko-Pack central impression (Cl) press at 100 m/min, 35-40C without any corrective action and under low ink volume Stress Test conditions, described above. Magenta (M) is progressively failing to transfer in a trap over yellow (Y) within one hour of operation (open circles) while cyan (C) continues to trap normally over yellow (closed circles).

Fig. 2 shows the weight percent water measured by oven drying of weighed samples taken over elapsed time under Stress Test conditions as in Figure 1 (yellow = open circles, magenta = grey circles, and cyan = filled circles). The falloff is linear and only depends on pumping rate for all inks.

Fig. 3 shows the viscosity of the three inks (Yellow = open circles, Magenta = grey circles, and Cyan = filled circles) over elapsed run time under Stress Test conditions as in Fig. 1. Over approximately the first two hours of running, no change in viscosity is detected. But after approximately two hours, a linear increase in viscosity can be seen.

Fig. 4 shows the pH of these inks (Yellow = open circles, Magenta = grey circles, and Cyan = filled circles) over elapsed run time under Stress Test conditions as in Fig. 1. The values are meter readings and not corrected to real hydrogen ion activities. Magenta and yellow show early pH drops while cyan is delayed but still occurring prior to two hours running time.

Figure 5 shows the measured conductivity of cyan inks over a wide range of formulated and evaporated conditions. The open circles are inks that were dried in an oven to near 20% water. As noted, Fig. 6 shows the pH response of magenta ink under Stress Test conditions that has been related to ink damage from dispersion instability. The open circles were obtained under conductivity control (inventive) of caustic injection (0.1 % NaOH) and show no significant pH change occurring over six hours of continuous running. The grey circles show the result of using pH control of caustic injection leading to a lower pH on average as the signal to start injection is delayed by nearly one hour. The dark grey circles show the result of using viscosity control of caustic injection where significant pH drop occurs prior to a viscosity signal to the injection system.

Finally, as noted, Fig. 7 shows the magenta over yellow trap under Stress Test conditions. The open circles were obtained under conductivity control (inventive) of caustic injection (0.1 % NaOH) yielding stable trapping. The grey circles show the result of pH control of caustic injection leading to a loss of magenta transfer which barely recovers in five hours following the delayed start of injection. The dark grey circles show the result of using viscosity control of caustic injection leading to severe loss of wet trapping which does not recover in five hours.

It is noted that although Sun Chemical's UniQure™ inks have been used to describe an exemplary composition for this process improvement, other inks could also be used in this process, including, but not limited to, other inks containing dissolved water. The compositional details of UniQure™ inks are disclosed in, for example, the following patents assigned to Sun Chemical, which are hereby incorporated herein by this reference: US 7479511 , EP 1504067, EP 1792956, US 7226959 and EP 1392780.

The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements on this invention that fall within the scope and spirit of the invention.

Claims

WHAT IS CLAIMED:

1. A method of maintaining the composition of an ink during a print run, comprising:

adding an aqueous fluid to the ink,

wherein said adding occurs at a rate determined by an ink conductivity measurement.

2. The method of claim 1 , wherein the aqueous fluid is water and wherein it is injected into said ink at a rate of between 50 and 150 ml/hour/gal.

3. The method of any claim 1 , wherein the aqueous fluid is water, and wherein it is injected into said ink at a temperature between 25° C and 35° C.

4. The method of claim 1 , wherein said addition of an aqueous fluid is controlled using a feedback loop based on the conductivity of the ink.

5. The method of claim 1 , wherein the aqueous fluid is one of water, alcohol or a combination thereof.

6. The method of claim 1 , wherein the aqueous fluid is one of water, alcohol or a combination thereof, and the ink contains materials designed to emulsify the fluid into the ink.

7. The method of claim 6, wherein said materials are surfactants.

8. The method of claim 1 , wherein the ink comprises water.

9. The method of claim 1 wherein the aqueous fluid is a combination of water and at least one of a caustic and a buffer.

10. The method of any of claims 1-9, wherein the ink is maintained at or near its initial state during a print run.

11. The method of any of claims 1 -10, used to prevent print defects caused by at least one of (i) evaporation of water and (ii) change in pH.

12. The method of claim 11 , wherein said print defects include at least one of congealed ink, diminished ink transfer, no ink transfer, loss of wet trapping, back trapping of a previous color, higher print density, ink slinging, static electrical hairs, misting and press damage.

13. The method of any of claims 1 -12, wherein the conductivity of the ink is sensitive to small amounts of water loss.

14. The method of claim 1 , wherein water is soluble in the ink vehicle to a substantial degree.

15. The method of claim 14, wherein said water solubility is between 5% and 80% by weight.

16. The method of any of claims 1-15, wherein a conductivity measuring device is placed in a sump or return line of the ink.

17. The method of claim 16, wherein said conductivity measuring device outputs a signal proportional to the ink conductivity.

18. The method of claim 16, wherein said conductivity measuring device can be easily cleaned by passage of ink or fluid.

19. The method of claim 16, wherein said conductivity measuring device comprises a conductivity probe.

20. The method of claim 19, wherein said conductivity probe is one of a simple two-electrode device and a four-electrode inductive device.

21. The method of claim 16, wherein the conductivity measuring device outputs a signal that can be used with or without amplification to operate a pump and/or a valve in a fluid injection system.

22. The method of any of claims 1 -21 , wherein the aqueous fluid is added to at least one of a sump, the ink intake lines, the ink return lines and a doctor bladed chamber.

23. The method of any of claims 1 -22, wherein the range of tolerance of the conductivity is set by examination of the ink conductivity associated with the onset of printing defects in an ink that is not being maintained by aqueous fluid addition.

24. The method of any of claims 1-22, wherein the aqueous fluid is added via an injection system such as those sold commercially for viscosity or pH management.

25. The method of any of claim 21 , wherein when the conductivity falls to a defined value, the pump is activated and/or the valve opens allowing a flow of aqueous fluid into the ink.

26. The method of claim 25, wherein when the initial conductivity has been restored, the pump shuts off and/or the valve closes.

27. The method of either of claims 21 , 25 and 26, wherein the conductivity sensor precedes the controlled valve in the ink flow.

28. The method of any of claims 25-27, wherein said defined value is less than or equal to half the conductivity value which is associated with print defects or permanent ink damage for a given press, print density, and pigment used.

29. The method of any of claims 1-28, wherein the aqueous solution contains (i) a caustic or buffer to pH 10 or higher and (ii) a dilute solution of a soluble inhibitor.

30. The method of claim 29, wherein said soluble inhibitor is MEHQ.

31. The method of any of claims 1 -30, wherein a differential in the ink conductivity measurement is seen in the early stages of printing.

32. A system for maintaining the composition of an ink during a print run, comprising:

an injector;

a conductivity probe; and

a controller communicably connected to an output of said conductivity probe,

wherein, in operation, the conductivity probe measures the conductivity of the ink and the controller, in response thereto, controls the addition of an aqueous solution to the ink.

33. The system of claim 32, further comprising a pump and/or a valve.

34. The system of claim 33, wherein when the measured conductivity of the ink falls to a defined value the pump is activated and/or the valve opens allowing a flow of aqueous fluid into the ink.

35. The system of claim 32, wherein the conductivity probe is one of a simple two-electrode device and a four-electrode inductive device.

36. The system of claim 32, wherein the conductivity probe outputs a signal that can be used with or without amplification to operate a pump and/or a valve in a fluid injection system.

37. The system of claim 32, wherein when the conductivity falls to a defined value the pump is activated and/or the valve opens allowing a flow aqueous fluid.

38. The system of claim 37, wherein when the initial conductivity of the ink has been restored, the pump shuts off and/or the valve closes.

39. The system of claim 32, wherein the aqueous fluid is at least one of added (i) to a sump or (ii) into ink intake lines.

40. The system of claim 32, wherein setting of the aqueous liquid flow and the off-on control upper and lower limits, respectively, are determined for at least one of (i) each ink color, (ii) volume in the sump, (iii) press speed, (iv) print width, and (v) press design.

41. The system of claim 37, wherein said defined value is less than or equal to half the conductivity value which is associated with print defects or permanent ink damage for a given press, print density, and pigment used.