Tailored ink for piston driven electrostatic liquid drop modulator

The present invention relates to an ink composition including water, a solvent, a solvent-soluble dye, and a surfactant, where the ink exhibits a stable liquid microemulsion phase at a first temperature and a second temperature higher than the first temperature and has a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60, and methods of making such ink compositions. The present invention also relates to a method of making an ink composition for use in a microelectromechanical system-based fluid ejector. The method involves providing a solution or dispersion including a dye or a pigment and adding to the solution or dispersion an additive which includes a material that enhances dielectric permittivity and/or reduces conductivity under conditions effective to produce an ink composition having a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60.

FIELD OF THE INVENTION

The present invention relates to ink compositions for use in a piston-driven micromachined or microelectromechanical system (MEMS) based liquid drop modulator.

BACKGROUND

Liquid drop modulators have been developed for ink jet recording or printing. Ink jet printing systems offer numerous benefits, including extremely quiet operation when printing, high speed printing, a high degree of freedom in ink selection, and the ability to use low-cost plain paper. The so-called “drop-on-demand” drive method, where ink is output only when required for printing, is now the conventional approach. The drop-on-demand drive method makes it unnecessary to recover ink not needed for printing.

Liquid drop modulators for ink jet printing include one or more nozzles which allow the formation and control of small ink droplets to permit high resolution, resulting in the ability to print sharper characters with improved tonal resolution. In particular, drop-on-demand ink jet print heads are generally used for high resolution printers.

Drop-on-demand technology generally uses some type of pulse generator to form and eject drops. For example, in one type of print head, a chamber having an ink nozzle may be fitted with a piezoelectric wall that is deformed when a voltage is applied. As a result of the deformation, the fluid is forced out of the nozzle orifice as a drop. The drop then impinges directly on an associated printing surface. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies when printing directly to paper or alternative substrates. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.

Another type of print head uses bubbles formed by heat pulses to force fluid out of the nozzle. The drops are separated from the ink supply when the bubbles collapse. This type of droplet ejection system based upon thermally generated bubbles, commonly referred to as the “thermal ink jet” or “bubble jet” system, has high speed printing capability.

Yet another type of drop-on-demand print head incorporates an electrostatic actuator. This type of print head utilizes electrostatic force to eject the ink. Examples of such electrostatic print heads are disclosed in U.S. Pat. No. 4,520,375 to Kroll and Japanese Laid-Open Patent Publication No. 289351/90. In particular, U.S. Pat. No. 6,367,915 to Gooray et al. discloses electrostatically or magnetically driven piston structures whose movement ejects a drop or droplet of fluid.

While there are many ink compositions known to be suitable for use with piezoelectric devices or bubble jet device, a need remains for ink compositions suitable for use with electrostatic liquid drop modulator devices such as those disclosed in U.S. Pat. No. 6,367,915 to Gooray et al. Electrostatic liquid drop modulation requires a fluid that possesses very specific dielectric properties. For electrostatic actuation, the maximum force is realized at a maximum product of field and dielectric constant. High field requires a high dielectric strength (high breakdown voltage) in the fluid. The fluid must exhibit a sufficiently low electrolytic decomposition rate to support a stable field. A fluid must also possess a low conductivity to minimize power losses due to the migration of ions to the electrodes and maximize energy efficiency of the device. The use of non-ideal conductors raises the possibility of capacitively coupling of the field to solution and the need for low conductivity in the fluid to ensure sufficiently long field relaxation times. For neat fluids, conductivity generally scales with dielectric constant, demonstrating that simple solvent selection is not enough to guarantee a fluid with optimum properties. Additions of solvent and solute impact both the dielectric constant and the conductivity. Appreciable solvent and solute create a media of mixed dipoles, reduces hydrogen bonding in water and may generate a greater concentration of mobile ion, often resulting in decreases in dielectric constant.

Specifically, the piston driven microelectromechanical system (MEMS) based liquid drop ejector technology requires ink with special electrical properties. The ink needs to satisfy the low conductivity of less than 200 microS/cm and high dielectric constant greater than 60. Purified deionized water can satisfy the requirements. However, the addition of water soluble dye to the deionized water will increase the conductivity of the ink to well above 200 microS/cm. An oil based ink (or hydrocarbon based ink) will have low conductivity below 200 microS/cm and low dielectric constant below 10. The addition of colorants to the oil, e.g. oil soluble dyes, can satisfy the low conductivity requirements, but cannot satisfy the high dielectric constant requirements. Therefore, aqueous inks with water soluble dyes or oil based ink with oil soluble dyes cannot satisfy the piston-MEMS ink requirements.

The present invention is directed to overcoming these deficiencies in the art.

SUMMARY

The present invention relates to an ink composition including water, a solvent, a solvent-soluble dye, and a surfactant, where the ink exhibits a stable liquid microemulsion phase at a first temperature and a second temperature higher than the first temperature and has a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60.

Another aspect of the present invention relates to a method of making an ink composition for use in a microelectromechanical system-based fluid ejector. The method includes providing a solution including a solvent, a solvent-soluble dye, and a surfactant. Then, the solution is added to water under conditions effective to produce an ink composition which exhibits a stable liquid microemulsion phase at a first temperature and a second temperature higher than the first temperature and has a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60.

The present invention also relates to a method of making an ink composition for use in a microelectromechanical system-based fluid ejector. The method involves providing a solution or dispersion containing a dye or a pigment and adding to the solution or dispersion an additive which includes a material that enhances dielectric permittivity and/or reduces conductivity under conditions effective to produce an ink composition having a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60.

DETAILED DESCRIPTION

The inks of the present invention are developed to match the needs of electrostatic actuation. Specifically, the ink compositions of the present invention which are useful for piston driven microelectromechanical system (MEMS) based liquid drop ejector technology have a conductivity of less than 200 μS/cm and a dielectric constant greater than 60. In addition, the ink compositions of the present invention have a low viscosity of less than 10 cPs, more specifically, less than 5 cPs, and a surface tension of from about 25 to about 55 mN/m, more specifically, from about 30 to about 45 mN/m.

The present invention relates to an ink composition including water, a solvent, a solvent-soluble dye, and a surfactant, where the ink exhibits a stable liquid microemulsion phase at a first temperature and a second temperature higher than the first temperature and has a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60. In one embodiment of the present invention, the ink has a conductivity of at most about 150 μS/cm. In another embodiment of the present invention, the ink has a conductivity of at most about 10 μS/cm and a dielectric constant of at least about 70. An emulsion is defined as a dispersion of oil/solvent in water or water in oil/solvent with the dispersion being stabilized by a surfactant. A “microemulsion” is a thermodynamically stable dispersion of one liquid phase into another, stabilized by an interfacial film of surfactant. The dispersion may be either oil-in-water or water-in-oil. Essentially, a microemulsion is an emulsifiable concentrate diluted with water and the appropriate type and amount of surfactant, such that the emulsified oil droplets are, typically, too small to be visible. To the naked eye, a perfect microemulsion is a clear solution indistinguishable from an aqueous solution. Typically, the microemulsion droplet diameter is approximately 100 nanometers or less. The interfacial tension between the two phases is extremely low. Microemulsions are two phase systems, whereas micellar solutions are a colloidal aggregate of surfactant molecules that occur at a well-defined low concentration of surfactant and may be considered one phase. The ink of the present invention can include micellar solutions. In contrast, emulsions (or macroemulsions) are unstable, where the suspended droplets will eventually agglomerate and the dispersed phase will phase separate. Emulsion droplet sizes are much larger, typically one micron or more, resulting in a cloudy or milky dispersion. The nature of an emulsion may depend on the order of mixing of the ingredients and the amount of energy put into the mixing process, whereas, in microemulsions, the final microemulsion state will not depend on the order of mixing and energy input only determines the time it will take to reach the equilibrium state. Microemulsion ink compositions have been disclosed in U.S. Pat. No. 5,492,559 to Oliver et al., issued Feb. 20, 1996, U.S. Pat. No. 5,551,973 to Oliver et al., issued Sep. 3, 1996, and U.S. Pat. No. 5,643,357 to Breton et al., issued Jul. 1, 1997, which are hereby incorporated by reference in their entirety.

The present invention also relates to a method of making an ink composition for use in a microelectromechanical system-based fluid ejector. The method includes providing a solution containing a solvent, a solvent-soluble dye, and a surfactant. Then, the solution is added to water under conditions effective to produce an ink composition which exhibits a stable liquid microemulsion phase at a first temperature and a second temperature higher than the first temperature and has a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60. Specifically, the solvent-soluble dye can be dissolved in the solvent and the surfactant at 60° C. for an hour with stirring inside a beaker. Then, the solvent-soluble dye-solvent-surfactant mixture can be added dropwise at 60° C. to a sonified deionized water solution. The resulting microemulsion ink can be then sonified for 10 minutes with temperature maintained at around 60° C.

Phase diagrams of different microemulsion systems of the present invention such as the representative phase diagram shown inFIG. 1are constructed to identify one phase isotropic regions. The isotropic regions for each water/solvent/surfactant composition at different temperatures are maintained to prevent phase separating of the system at the operating temperature. Typically, the ink composition of the present invention exhibits a stable liquid microemulsion phase at a first temperature of about 22° C. and a second temperature of about 60° C., as well as at any temperature between about 22° C. and about 60° C. Specifically, the ink composition of the present invention exhibits a stable liquid microemulsion phase at a first temperature of about 22° C. and a second temperature of about 50° C. More specifically, the ink composition of the present invention exhibits a stable liquid microemulsion phase at a first temperature of about 22° C. and a second temperature of about 35° C.

The water of the ink composition of the present invention is typically purified deionized water that has a high dielectric constant (78.3 at 25° C.) and low conductivity (0.4 μS/cm). In one embodiment of the present invention, the water in the ink composition is present in an amount of from about 60 to about 95 percent by weight. More specifically, the water in the ink composition of the present invention is present in an amount of from about 70 to about 85 percent by weight. Water at too low a concentration can lower the dielectric constant.

Solvents with high dielectric constants and strong solvency for the dyes are selected for the ink composition of the present invention. In addition, solvents with high water solubility are selected to avoid phase separation. Examples of solvents include 1-methyl-2-pyrrolidinone (dielectric constant 32.5), propylene carbonate (dielectric constant 66.14), ethylene carbonate (dielectric constant 89.7), N-acetylethanolamide (dielectric constant 96.6), N-ethylformamide (dielectric constant 102), N-butylacetamide (dielectric constant 104), formamide (dielectric constant 111), N-propylpropanamide (dielectric constant 118), N-ethylacetamide (dielectric constant 135), N-methylpropanamide (dielectric constant 170), and N-methylformamide (dielectric constant 189). In one advantageous embodiment of the present invention, 1-methyl-2-pyrrolidinone is disclosed.

In another embodiment of the present invention, the solvent in the ink composition is present in an amount of from about 2 to about 30 percent by weight. More specifically, the solvent in the ink composition is present in an amount of from about 2 to about 10 percent by weight. A solvent at too high a concentration can lower the dielectric constant and increase the conductivity.

In another embodiment of the present invention, the surfactant is present in an amount of from about 1 to about 10 percent by weight. The surfactant at too high a concentration can increase the viscosity of the ink.

Dyes with good solubility in the solvent but poor solubility in the water are selected. In another embodiment of the present invention, the solvent-soluble dye is present in an amount of from about 1 to about 10 percent by weight. Examples of solvent-soluble dyes include Neopen Blue 807 (BASF), Savinyl Black NS (Clariant, Charlotte, N.C.), and Solvent Yellow 030 (Spectra, Kearny, N.J.). Other dyes having similar solubility parameters can also be used.

In addition to water, the solvent, the dye, and the surfactant, the ink composition of the present invention can also contain a humectant and/or other additives. Examples of humectants include tetramethylene sulfone, ethylene glycol, diethylene glycol, glycerol, 1-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, pentanol, benzyl alcohol, 1,2-pentanediol, 1,5-pentanediol, and other higher molecular weight diols and polyols.

In one advantageous embodiment of the present invention, the water, the solvent, and the surfactant are present in relative amounts of about 85 percent by weight water, about 10 percent by weight solvent, and about 5 percent by weight surfactant.

In another advantageous embodiment of the present invention, the ink composition can further include a humectant and the water, the solvent, the surfactant, and the humectant are present in relative amounts of about 80 percent by weight water, about 10 percent by weight solvent, about 5 percent by weight surfactant, and about 5 percent by weight humectant.

The present invention also relates to a method of making an ink composition for use in a microelectromechanical system-based fluid ejector. The method involves providing a solution or dispersion including a dye or a pigment and adding to the solution or dispersion an additive which includes a material that enhances dielectric permittivity and/or reduces conductivity under conditions effective to produce an ink composition having a conductivity of at most about 200 μS/cm and a dielectric constant of at least about 60.

Selective additives, in the form of co-solvents and solutes, can be used to produce artificial dielectric properties to increase the high voltage breakdown threshold of the ink compositions, decrease the electrolytic decomposition susceptibility, and increase the dielectric constant and minimize conductivity.

The additive added to the ink composition of the present invention can be a co-solvent. Co-solvents that have high dielectric constant and low conductivity are selected. Suitable co-solvents include ethylene glycol, diethylene glycol, glycerol, 1-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, N-acetylethanolamide, N-ethylformamide, N-butylacetamide, formamide, N-propylpropanamide, N-ethylacetamide, N-methylpropanamide, and N-methylformamide.

The additive added to the ink composition of the present invention can also be in the form of particles. The particles can be in surface-grafted or ungrafted forms. Suitable material for the particles include latex, titanium dioxide, alumina, and silica. For example, latex particles of sizes ranging from about 0.01 to about 1 micron can be added. Latex particles can be stabilized with a non-ionic surfactant. The impact of the latex is to enhance the dielectric strength of the fluid as measured by the maximum field necessary for breakdown. The latex also aids in the dry properties of the ink resulting in improved print quality and better smear resistance.

When the solution or dispersion including a dye or a pigment is aqueous, additives such as zwitterionic biological buffers or peptides can be used to increase dielectric permittivity. Examples of buffers include cyclohexylaminopropanesulfonic acid (CAPS), hydroxyethylpiperazine ethanesulfonic acid (HEPES), and hydroxyethylpiperazine propanesulfonic acid (HEPPS). Examples of peptides include diglycine and triglycine. These types of aqueous ink compositions with dielectric enhancement require semi conductive coatings on the electrodes of the fluid ejector. The coatings must also be compatible with the ink's aggressive characteristics. These ink compositions offer very good print quality latitude on plain papers.

Advantageously, embodiments include the pigment particle size being as small as possible to enable a stable colloidal suspension of the particles in the liquid vehicle and to prevent clogging of the ink channels when the ink is used in the printer. Specifically, particle average diameters are generally from about 0.001 to about 5 microns, and more preferably from about 0.1 to about 1 micron, although the particle size can be outside these ranges. The ink compositions of the present invention can have the pigment present in any effective amount to achieve the desired degree of coloration. Specifically, the pigment is present in an amount of from about 0.1 to about 8 percent by weight of the ink, and more specifically from about 2 to about 7 percent by weight of the ink, although the amount can be outside these ranges.

Examples of various additives that can be included in the ink composition of the present invention to address the drop ejector requirements such as latency, recoverability, or print quality, and drying, include surfactants, humectants, penetrants, dispersants, biocides, or fixatives. With respect to the surfactants, a non-ionic surfactant or a mixture of non-ionic surfactants can be used. Additives that enhance dielectric permittivity and/or reduce conductivity can be present in the ink composition of the present invention in an amount from about 0.05 to about 25 percent by weight. Table 1 illustrates the typical ranges of weight percentages of different components in a water-based ink composition of the present invention with a dielectric enhancer added.

When the solution or dispersion including a dye or a pigment is non-aqueous, additives can also be used to increase dielectric permittivity. Examples of additives include lecithin (monoaminophosphatide), lanolin, or linolenic acid. These types of oil (hydrocarbon) based inks with dielectric enhancement do not require coated parts. However, good print quality might be best achieved on coated papers or in transfix type of printing where the image is formed on an intermediate substrate from which excess vehicle can be removed prior to image transfer to paper. Although such inks would generally be pigment-based, dye-based compositions having dyes soluble in a more viscous or less volatile ink vehicle component could be formulated so that the colorant could be separated and concentrated away from the major liquid ink component.

In addition, modified thermal ink jet ink compositions—aqueous or solvent based—which are optimized for the MEMS electrostatic liquid drop modulator can be formulated with dielectric enhancement additives and conductivity reduction.

Specific embodiments of the invention is described in the following examples. The examples are intended to be illustrative, and the invention is not limited to the materials, conditions, or process parameters set forth in these embodiments.

EXAMPLES

Ink compositions of the present invention with the following ingredients (by weight) were prepared by dissolving the solvent dye in the solvent and the surfactant at 60° C. for an hour with stirring inside a beaker, followed by adding the soluble dye-solvent-surfactant mixed solution dropwise at 60° C. to a sonified deionized water solution, and sonifying the resulting microemulsion ink for 10 minutes with temperature maintained at around 60° C.:

Phase diagrams for Ink 1 at two different temperatures—room temperature (22° C.) and 60° C.—are shown inFIGS. 2A and 3A, respectively. Phase diagrams for Ink 2 at two different temperatures—room temperature and 60° C.—are shown inFIGS. 2B and 3B, respectively. Phase diagrams for Ink 3 at two different temperatures—room temperature and 60° C.—are shown inFIGS. 2C and 3C, respectively.

The prepared microemulsion ink compositions (Ink 1, Ink 2, and Ink 3) had dielectric constants of 70-72, conductivities of 90-149 μS/cm, viscosities of 2.0 to 5 cps, and surface tensions of 34-36 dyne/cm2. Thus, the oil-in-water microemulsion ink compositions satisfied the physical properties of the piston-MEMS ink requirements and, thus, were expected to show good print qualities using a piston-MEMS printer. Print tests of the ink on a Hewlett-Packard thermal ink jet printer showed good image quality with excellent waterfastness properties. An additional humectant, e.g. 5 g sulfolane, can also be added together with the surfactants in the microemulsion ink compositions to satisfy the piston-MEMS requirements.

Ink compositions of the present invention with the following ingredients (by weight) were prepared by dissolving the solvent dye in the solvent and the surfactant at 60° C. for an hour with stirring inside a beaker, followed by adding the soluble dye-solvent-surfactant mixed solution dropwise at 60° C. to a sonified deionized water solution, and sonifying the resulting microemulsion ink for 10 minutes with temperature maintained at around 60° C.:

Phase diagrams for Ink 4, Ink 5, and Ink 6 at room temperature (22° C.) are shown inFIGS. 4A,4B and4C, respectively.

The prepared microemulsion ink composition of Ink 6, for example, had a dielectric constant of 70.7, conductivity of 107.2 μS/cm, viscosity of 2.2 cps, and surface tensions of 37.5 dyne/cm2. Thus, the microemulsion ink composition satisfied the physical properties of the piston-MEMS ink requirements and, thus, were expected to show good print qualities using a piston-MEMS printer. Print tests of the ink on a Hewlett-Packard thermal ink jet printer showed good image quality with excellent waterfastness properties.

In order to develop ink compositions suitable for use in a piston driven microelectromechanical system (MEMS) based liquid drop ejector, i.e., ink compositions that have a conductivity of less than 200 μS/cm and a dielectric constant greater than 60, the conductivities and dielectric constants of different types of ink compositions with different dye or pigment concentrations were measured. As shown in Table 2, a normal aqueous dye based ink such as the R series (R-1 to R-5) having different concentrations of dye, Duasyn Acid Blue AE-SF VP344, did not satisfy the ink conductivity requirement at dye concentrations of more than 0.1% (see R-4 and R-5). Even though the dielectric constants were similar, the R series exhibited very high conductivity at 1% dye loading (see R-5), since the dye increases the conductivity of the medium. A special carbon black ink dispersion from Cabot, such as the C series (C-1 to C-3), having different concentrations of pigment, IJX 59, also did not satisfy the ink conductivity requirements, as shown in Table 2. (The dielectric constants were not measured due to the high conductivity.)

In contrast, as shown in Table 2, a specially surface treated carbon black pigment dispersion from Degussa, such as the B series (B-1 to B-7), having different concentrations of pigment, IDIS 18.1, at around 2% loading dispersed in an aqueous medium (see B-2) did satisfy the ink's conductivity and dielectric constant requirements. The aqueous ink compositions mixed with co-solvents like ethylene glycol, diethylene glycol, glycerol, or N-ethyl acetamide also exhibited high dielectric permittivity but low conductivity (see Table 2, B-5 to B-7). A specially surface treated carbon black pigment dispersion from Degussa, having pigment, IDIS 15, was also found to satisfy the ink's conductivity and dielectric constant requirements.

When carbon black is put into a water base, black pigmented inks are generated. The graphite particles are suspended by surfactants. The particle surfaces are modified by grafting ionic functionalities to the surface in the form of organosulfates, amides, and carboxylates.FIG. 5shows the effect of carbon black loading on ink conductivity and dielectric constant. The carbon black contributed only modestly to conductivity increases, while significantly raising the dielectric constant well above the value of 78 for water.