Waterfast Ink Composition for Printers

Reference is made to commonly assigned copending patent application Ser. No. 08/570,140 Docket No. E-407, filed herewith entitled "Fluorescent Red And Magenta Waterfast Ink Jet Inks" in the names of Judith Auslander and Shunichi Higashiyama.

The invention relates generally to inks, and more particularly to inks that may be used in an Ink jet printer.

Inks have been used in Europe and elsewhere in postage meters or franking machines for imprinting postal indicia. The image obtained with the red inks for the above application has to be permanent, i.e. good water fastness, smear fastness and light fastness i.e., greater than 95%. Also, the images on various substrates have to show enhanced contrast properties.

Postal indicia or franking machines have been developed that use digital printing technology to permit the use of variable information on the postal indicia and have more accurate printing and indicia print quality. The enhanced print quality will enable the postal indicia to be easily read by optical character recognition equipment, bar code readers and other types of technologies.

The introduction of digital printing and franking is conditioned by the existence of inks that are suitable for franking, but at the same time are functional with the specific ink jet technology. Currently, the closest inks that meet the above criteria are the magenta inks that are used in ink jet printers. The aforementioned magenta inks are accidentally fluorescent, since the choice of the magenta dyes is very narrow and the most brilliant available magenta dyes are fluorescent. One of the few available water soluble magenta dyes is the Acid Red 52. The Acid Red 52 has satisfactory solubility in water but a very low water fastness. Thus, a disadvantage of the magenta Acid Red 52 ink is that the ink bleeds under water exposure. Another disadvantage of using current magenta inks is that prints obtained with these inks offset on the back of neighboring envelopes when exposed to water.

Another disadvantage of prior art piezoelectric ink jet inks which are water fast is that the inks use different organic solvents instead of water. Various organic solvents, such as tripropylene glycol methyl ether or other glycol ethers, are used to improve the water fastness. The reason for the above is that the solvents dissolve or disperse colorants that are insoluble in water. Due to safety and compatibility requirements, with various plastic materials used in ink jet print heads, water is nevertheless desirable as the main solvent. The reason why water is desirable as a solvent when plastic materials are used is that water is much less aggressive than the organic solvents. The widely used plastic materials for ink jet printer parts are inker foam, cartridge material, glue, printer base, etc. The above parts may be made from Acrylonitrile styrene (AS), Polymethyl metaacrylate (PMMA), and Acrylonitrile butadiene styrene (ABS). Therefore, the disadvantage of using organic solvents is that they attack plastic materials. Once plastic materials are attacked, the printer would not work properly. A water based ink is desirable as the main solvent since it's inert to most ink jet printer components.

The ink that is going to be used in an ink jet printer, must have certain physical properties, such as a certain viscosity and a specified surface tension. The viscosity of the liquid inks used in current piezoelectric ink jet printers is 1.5-20 cps and in the thermal ink jet printer is very low (1-5 cps). The desirable surface tension of liquid ink jet printer inks should be between 30-45 dynes/cm.

A disadvantage of current water based ink jet inks is that the ink has a tendency to dry in the nozzles of the printer, during operation of the printer and between operations of the printer.

One of the properties of an ink that is used in an ink jet printer is the decap time, which is the length of time over which an ink remains fluid in a nozzle opening when exposed to air and capable of firing a drop. Precipitation of the solid in the ink is what causes failure or coagulation in the ink and is often due to evaporation of the solvent causing precipitation or crystallization of a solid at the air/liquid surface.

Another disadvantage of the water based inks is that they have to use "naked" (pure) dyes without resins in order to achieve good solubility. The dyes should be preferably direct, acid, basic or reactive.

If the solubility of the colorant material in the solvent is not good enough, the drops stability, and the print quality are poor. The long term solubility which affects the shelf life depends on the colorant solubility in the solvent in various environmental conditions of temperature and humidity. The dyes that exhibit good water solubility suffer from deficiency in water fastness and smear fastness on the generated prints. The foregoing occurs because of the prints redisolution in water.

Another disadvantage of dye based inks is that they produce prints with high edge roughness and poorly defined characters which is also called feathering. The foregoing disadvantage is more apparent at lower resolutions such as 240 dpi, where there is less overlap between the dots.

Therefore, pigments which are insoluble in water are a preferred alternative to dyes provided that pigments dispersions can be made stable to sedimentation and gelling.

The problem of using pigments is that they exhibit fragile equilibrium in dispersion. The dispersions are easily destabilized by changes in temperature, humidity and impurities.

Other typical liquid ink properties are the inks ability to penetrate paper and to dry fast by absorption. Another problem with fast penetrating inks is that the optical density decreases with the ink penetration. The above effect has to be compensated for in order to achieve good print quality. Good print quality may be achieved by using dyes with high absorbance values.

Another problem of prior art inks is that they do not give consistent print quality on a large variety of substrates. Postage meter inks have to be used on a large variety of paper envelopes. The components of the paper can have adverse effects on print quality and on the optical density of the prints. The wax based inks such as hot melt ink do not interact with the paper in the same manner as the liquid inks. Therefore, there is a need for a liquid franking ink that will have consistent fluorescence and optical density on a large variety of envelopes, while interacting with the paper.

Another problem of prior art waterfast inks is that the components of the ink that are responsible for waterfastness have difficulty in achieving long term solubility. The large temperature variation encountered by the ink during normal operating conditions may destabilize the equilibrium of the ink causing settling of particles in the ink, which may cause clogging of the nozzles of the ink jet printer.

This invention overcomes the disadvantages of the prior art by providing a ink jet ink that produces better print quality and image permanence, while fulfilling the United States Postal Service requirements for automation compatibility. The inks of this invention may be read by current Postal Service optical character recognition equipment and/or bar code reading systems. The inks of this invention are safe for use in an office environment. The inks also are non toxic and inert to the range of materials used in the print heads of current ink jet printers. Some of the materials in ink jet print heads are polycarbonate (PC), Polystylene (PS) and Polyphenyleneoxide. One of the advantages of the inks of this invention is that the inks provide a permanent image (water and light fast) on a large variety of substrates.

The inks disclosed herein are based on solutions of soluble toners in water/cosolvent systems. The prints obtained with the inks of this invention are resistant to water exposure either by drainage or by dipping in water. Also the inks do not offset while exposed to water and when they are in contact under pressure with other pieces of paper. The inks of this invention are based on water as a main solvent (greater than 35%) and other cosolvents which are water soluble. The water soluble organic solvents serve to prevent clogging at the nozzle through moisture retention (hygroscopicity) and at the same time improve the stability of the ink. The other role of the organic solvents is to decrease the surface tension of the ink and increase the paper spreading and drying time. The last role of the cosolvent is to improve the solubility of the toner for the range of various types of toners compatible with the vehicle. The toner is a solid solution of a dye in a resin that is soluble or partially soluble in water, or in a water soluble organic solvent.

The water soluble solvents have to be polar. Solvent polarity is the over-all solvation ability of a solvent which in turn is determined by the sum of all the molecular properties responsible for the interactions between the solvent and the solute. These intermolecular forces are: columbic, directional, inductive, dispersion, charge transfer and hydrogen bonding. The solvent polarity is defined quantitatively by physical parameters: .epsilon. the dielectric constant and .mu. the permanent dipole moment. The dielectric constant describes the ability of a solvent to separate electrical charges and orient its dipolar molecules. The permanent dipole moment of a molecule is given by the product of the charges of the dipole units and the distance separating the two dipole charges. Another empirical parameter used for defining over all solvent polarity is the standard absorption of organic compounds (E.sub.t) determined by the .lambda..sub.max of organic compounds in different solutions. Examples of the water-soluble organic solvents with low vapor pressure are: glycerol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, hexylene glycol, polyethylene and polypropylene glycol.

The water soluble organic cosolvents may be used alone or in combination of two or more of them. The cosolvents used in the ink formulations have to be totally miscible with water, good solvents for a broad range of polar resins, nonvolatile (vapor pressure at room temperature less than 0.1 mm/Hg) and inert to the commonly used plastic materials. Further, it is preferred for the ink composition of the present invention to further contain highly volatile monhydric alcohols such as ethanol, propanol and isopropanol, from the viewpoint of improving the quick drying and fixing of prints.

In the ink composition according to the present invention, when the water soluble organic solvents are used alone, the Hildebrand solubility parameter (SP value) of the water soluble organic solvent used is preferably 12 cal/cm.sup.3 or more. Also, when the water soluble organic solvents are used in combination of two or more of them, the SP value of the mixed organic solvent is preferably 12 cal/cm.sup.3 or more. The SP value of the mixed solvent can be determined from the following equation. ##EQU1## wherein .delta. represents an SP value of each solvent, X represents a molar fraction of each solvent in the mixed solvent, V represents a molar volume of each solvent and n is a subscript representing the kind of the solvent and an integer of 2 or more. Other solvents that can be used are: 2-pyrrolidone, N-methyl pyrrolidone, sulfolane, gamma butyrlactone, etc.

The hygroscopic characteristics of the organic cosolvents used in the ink formulations prevent drying at the ink jet nozzles and therefore improve the ink reliability in the print head. An additional advantage of the inks of this invention is the high water and cosolvent solubility of the soluble toner which improves the shelf life of the ink. The general characteristics of these solvents as expressed in partial Hansen solubility parameters (.delta..sub..delta. (dispersion), .delta..sub.p (polar), .delta..sub.h (hydrogen bonding)) are as follows. For the category of solvents with low dispersion solubility parameter .delta..sub..delta. (7-8.3) they should have .delta..sub.h >8.0, .delta..sub.p >5.0. Examples are: diethylene glycol (.delta..sub..delta. of 7.9), triethylene glycol (.delta..sub..delta. of 7.8). From the category of high .delta..sub..delta. (8.4-10.0) they should have .delta..sub.h >5.0, .delta..sub.p of 8.0-8.7. Some examples of the foregoing are: 2-pyrrolidone, N-methyl pyrrolidone, suffolane, gamma butyrlactone, etc. The water soluble organic solvent individually or in solution does not exceed a viscosity of 20 cps.

The increased solubility of the colorant of the inks of this invention in water or in the solvent system used, makes this ink very efficient; and provides efficient jetting ability. Hence, good print quality is obtained. To the water cosolvent solution which also may be a cosolvent mixture a glycol ether is added. The glycol ether serves as a dye stabilizer, wetting and as a coupling agent. The glycol ethers are intramolecularly hydrogen bonded. They can therefore act as bridging solvents and assume the polar or non polar nature of their surrounding environment. The glycol ethers may act as polar solvents with polar resins and with less polar resins the above glycol ethers may act as non polar solvents. Therefore, they serve to solubilize relatively non polar resins in water. Examples of glycol ethers are: diethylene glycol ethyl ether; triethylene glycol ethyl ether; diethylene glycol butyl ether; diethylene glycol propyl ether; and propylene glycol butyl ether. All of the glycol ethers should have a viscosity less than 20 cps. Another very important role of the glycol ether is that they are strong penetrants into paper.

The ink formulation of this invention consists of an aqueous carrier medium, a "water like" organic polar solvent, "a water soluble toner" containing a dye encapsulated in a resin matrix and cosolvent penetrants such as propylene glycol butyl ether, diethylene glycol, or triethylene glycol butyl ether.

In order to solubilize toners based upon acid resins non ionic surfactants have to be added. The non ionic surfactants have HLB values from 8-15. The surfactants belong to the class of nonyl phenoxy poly (ethyleneoxy) ethoxylates or decyl and tridecyl alcohol ethoxylates. Other additives, such as dimethyl hydantoin can be used for good decap performance. The resultant inks are water fast on a large variety of substrates/envelopes. They are also non toxic and inert to most of the materials used in the print head. These inks exhibit rapid drying, are storage stable and give images having good print quality, and permanence. In essence, the ink formulation of this invention is a water cosolvent based ink that uses toners that contain dyes encapsulated in a resin matrix instead of dyes. Hence, the major solvent of the ink formulation is water, while at the same time, the dye protected by the resin is not bleeding under water exposure and a water environment. Therefore, the resulting print is waterfast.

Water based inks may be formulated that contain between 30-60% water with additional cosolvents that are water soluble and water like, like glycols, diethylene glycol, triethylene glycol, tetraethylene glycol (which have a relatively low viscosity) and/or other polar solvents, such as 2 pyrrolidone or N-methyl pyrrolidone or gammabutylactone or sulfolane may be used. All of the above solvents are nonvolatile, hygroscopic, and at the same time dissolve well in water causing an increase in the viscosity. The above solvents have a high dipole moment which is greater than 2.3 but preferably greater than 3 and with high hydrogen bonding solubility parameters which are greater than 5. The combination of the toner, water, cosolvent, and peneterant have to be adjusted for a desired viscosity.

The aforementioned solvents have to comply with all government safety standards that are defined by applicable statues and regulations. Therefore, a safe solvent may be defined as a solvent with, mild or moderate skin and eye irritation index and a haft life lethal dose greater than 1 g/kg.

Another property of the ink formulation is that the using of an additive like a glycol ether such as propylene glycol, n- butyl ether or triethylene glycol butyl ether improves the water fastness and the speed of penetration of the ink into the paper. The above solvents also have a strong effect on the dye stabilization. The water fastness is improved by the fact that the penetrant helps the toner to sink deeper into the paper fiber thus, protecting the dye from being released from the resin.

The use of toners i.e., dyes encapsulated in a resin results in a performance soluble similar to a soluble dye in solution and as a pigment on paper.

The solubility of the colorant has to be high enough so as not to cause problems in the shelf life, or in the ejection stability. The desired shelf life of more than one year is achieved by using very soluble toners like dimethyl hydantoin/formaldehyde or by adding non ionic surfactants acid resins (polyester polyamide). The non ionic surfactants belong to tridecyl alcohol alkoylates or nonyl phenoxy poly (ethyleneoxy) ethanol with HLB between 8 and 13 at concentrations of trace to 3%. At the same time, when the ink hits the paper, it has to become insoluble in water or in any other material that may come in contact with the paper, like coffee, bleaching agents or other liquids that can be exposed to a print.

After the toner is laid on the paper and after the evaporation of the solvent, the properties of the toner are more similar to a pigment than to a dye, because of the protection by the resin. The water solubilization of the toner on the printed image is also hindered due to the fact that the dye is protected by the resin. Therefore, by using soluble toners, a dual effect is obtained i.e., the functioning as a dye in the ink solution in the cartridge and print head and as a pigment when printed on the paper. Other effects similar to pigmented inks include enhanced contrasts, high optical density, image permanence and superior edge definition. In addition to the toner/solvent mixtures "loose" dye is added to enhance the contrast. The dyes can belong to various categories such as: direct, acid and basic. Some examples are: C.I. Direct Yellow 86, C.I. Acid Yellow 210, C.I. Basic Yellow 40, C.I. Direct, C.I. Direct Yellow 142, Solvent Yellow 44, C.I. Basic Blue 7, C.I. Basic Blue 3, C.I. Basic Yellow 51, C.I. Basic Yellow 2 etc. Other dyes like magenta, violet, blue or green may be added in small concentrations. The pure dye added to the ink is soluble in the solvent mixture and belongs to the categories of acid, direct, basic or water soluble toning dyes. Preferably the dyes used are basic. The concentration used is trace to 8.0 (wt. 90).

The formulation of the ink of the instant invention provides magenta or red inks that are capable of being used in an ink jet printer and are inert to materials used in the printer. Composition of the ink, in accordance with the invention comprises: soluble toner or mixture thereof from 3-15%, water 35-55%, cosolvent which is a water soluble organic solvent from 15-47% or a mixture thereof and Glycol Ether as additive 3-15%. Other agents that can be used for solubilization or PH stabilization are low volatility alkali components, or acid components of food grade or for cosmetics. Specific preferred examples of the nonvolatile alkali components include alkanolamines, for examples, mono ethanolamine, N-N-dimethylethanolamine, N-N-diethylethanolamine, diethanolamine, triethanolamine, N-N-butyldiethanolamine and triisopropanolamine.

The ink composition of the invention is suitable employed with any ink jet printer for any drop on demand ink jet technology. However, it is particularly useful in piezoelectric ink jet technology.

Soluble toner is defined as a solid solution of a dye in a polar resin matrix or a soluble polar resin such as dimethyl hydantoin formaldehyde. The dye can be fluorescent or non fluorescent and can belong to various dye groups, such as direct dyes, solvent dyes, acid dyes, or basic dyes. The concentration of the dye in the toner ranges from 1-8%. It is contemplated that any of the dyes currently used in inks for ink jet printers may be used as dyes in the ink formulation of this invention. The resin used to dissolve any of the dyes belong to the type triazine formaldehyde sulfonamide and can be either urea formaldehyde, melamine formaldehyde or benzoguanamine formaldehyde which are cross linked with para toluene sulfonamide. Other resins that can be used are polyester/polyamide. The above resins have a low molecular weight less than 6,000 and a high acid number (greater than 50). The weight average molecular weight (Mn) is preferably between 200 and 1000. An alternative resin matrix is dimethyl hydantoin formaldehyde which has a good water solubility.

The water soluble solvents that are used can belong to the group of glycols, including propylene glycols, polypropylene glycols, ethylene glycols, polyethylene glycols and glycerols. Any, of the glycol families of ethers and mixtures thereof may also be used. Other safe "water like" solvents, with low viscosity that can be used are 2-pyrrolidone, gamma butyrlactone and sulfolane.

Hygroscopic properties of the water like solvents of the above type aids in the formation of the soft plug of ink in the orifice of the print head of the ink jet printer. The soft plug means that there is a very thin layer formed in the orifice that can be easily broken upon firing of the print head. The additives, added to this ink formulations are propylene glycol, n-butylether triethylene glycol butyl ether or diethylene glycol ethers that are strong penetrants and therefore improve the speed of penetration into the paper and also the water fastness of the prints. The long term solubility of the colorants in the solvent system required for a long shelf life (more than 12 months) is achieved by adding non ionic surfactants. The non ionic surfactants can have HLB from 8 to 13 in order to achieve solubilization of polyester/polyamide toner. The non ionic surfactants with HLB>8 are also less damaging to the plastic materials used in the printhead.

The general composition of the inks of this invention can be essentially the same as the composition of the fluorescent inks disclosed in copending U.S. patent application Ser. No. 08/570,140, Docket Number E-407, entitled "Fluorescent Red And Magenta Waterfast Ink Jet Inks", filed herewith. The foregoing may be accomplished by using fluorescent toners, which are commercially available and by increasing the optical density and quenching the fluorescence completely with a high absorbance dye. The soluble toners without dyes (resins) containing diemethyl-hydanton formaldehyde, polyester polyamide or triazine formaldehyde sulfonamide can be used in combinations with nonfluorescent dyes which are soluble in water. The preferred dyes are basic dyes with increased water fastness. Various colors and hues may be obtained by using the colored fluorescent toners: blue, green, red and then adding trace to 8 (Wt. %) of dyes of the same hue as the toner which are soluble in the solvent system. Black inks may be obtained by mixing the subtractive colors, cyan, magenta and yellow toners. It will be obvious to one skilled in the art that any dye (not fluorescent) can be added to the resin used in the ink formulations of this invention in order to obtain the desired color.

The general composition of the ink is as follows:

The above embodiments have been given by way of illustration only, and other embodiments of the instant invention will be apparent to those skilled in the art, from consideration of the detailed description. Accordingly, limitation on the instant invention are to be found only in the claims.