PROCESS FOR PREPARING INK COMPOSITIONS FOR INK-JET PRINTER

Abstract: A process for preparing an aqueous-based ink composi­ tion for use in ink-jet printers is disclosed. The process com­ prises (a) forming a solution of a dye having at least one neg­ atively charged functional group per molecule; (b) acidifying the solution; (c) cycling the solution through a reverse osmo­ sis membrane to form a concentrate and a permeate, the con­ centrate including a cation of the compound associated with at least one functional group on the dye and the permeate including a cation formerly associated with at least one func­ tional group; (d) adding water as necessary; (e) concentrat­ ing the dye by reverse osmosis; and (f) admixing the con­ centrated dye with at least one glycol ether. The pH of the ink composition is maintained below about 7. The formulation of the final ink composition comprises: (a) a vehicle of about 5 to 95% water and the balance at least one glycol ether, such as diethylene glycol; (b) a dye, such as Food Black 2, having at least one negatively charged func­ tional group per molecule, present in an amount up to about 10% of the vehicle composition; and (c) a cationic compound selected from the group consisting of alkanol ammonium compounds and cationic amide compounds, present in an amount such that there is at least one molecule of cationic compound for at least one of the negatively charged func­ tional groups on the dye, the pH of the ink composition being maintained in the acidic region below about 7. The ink composition prepared by the process of the in­ vention evidences minimal crusting and reduced evaporation loss of water. The solubility of acid dyes is increased in the acidic medium over that otherwise obtainable, due to the pre­ sence of the alkanol ammonium compound or cation amide compound. The ink composition prepared in accordance with the invention is suitably employed in any of the ink-jet printers commonly in use.

Description: This invention relates to a process for preparing an ink composition suitable for use in thermal ink-jet printers, without undue crusting or dye decomposition. The use of aqueous-based inks for ink-jet printers is well-known. Such compositions are relatively inexpensive and easy to prepare; typically, the ink comprises water and a glycol ether, usually diethylene glycol, and a dye. Commonly, the water and glycol ether are present in generally the same proportion and the dye, for example, Food Black 2, is present up to about 6% of the total composition, depending on the desired density of the print. For instance a process for the preparation of concentrated solutions of dyestuffs and of brighteners is known from FR-A-24 16 926 (=GB-A-20 15 018). However, a persistent problem associated with aqueous-based inks is their propensity to crust over a period of time, eventually leading to plugging of the orifice in the printer mechanism from which droplets of ink are expelled in the printing operation. The crusting problem arises from the evaporation of the water from the ink solvent (vehicle) and the consequent precipitation of the aye which has become substantially insoluble as a result of this water loss. The propensity for anionic dye precipitation also occurs if the pH is reduced below about 7, that is, if the ink composition becomes acidic. The lower the pH, the greater the likelihood of precipitation of such dyes. Attempts have been made to solve the crusting problem. Hygroscopic agents have been added to reduce the rate of water evaporation by their ability to pick up water vapor from the air. Examples of such hygroscopic agents include water-soluble polymers, alkanol amines, amides and polyhydric alcohols. While some improvement has been realized with these hygroscopic agents, a total solution to the crusting problem has not yet been achieved. Further, apparently no methods are known to prevent precipitation of these anionic dyes (acid dyes) in acidic aqueous-based inks. It is the main object of this invention to provide a process for preparing an ink in which the crusting problem is minimal. It is a further object of this invention to provide a process for preparing an aqueous-based ink for ink-jet printers in which the evaporation of water is reduced. It is another object of this invention to provide a process for preparing an aqueous-based ink for ink-jet printers in which the solubility of the dye in the ink solvent (vehicle) is increased. It is yet another object of this invention to provide a process for preparing an aqueous-based ink for ink-jet printers in which the solubility of anionic dyes in acidic media is increased. These objects of the invention are accomplished by the features of claim 1. Preferred embodiments are disclosed in the sub-claims. Unless otherwise stated, composition percentages in the claims and in the following description are by weight. The ink composition as prepared herein evidences minimal crusting and reduced evaporation loss of water. The solubility of anionic dyes is increased in acidic media over that otherwise obtainable, due to the presence of the alkanol ammonium cation or amide cation. The ink composition is suitably employed in any of the ink-jet printers commonly in use. Detailed Description of the Invention : The ink composition provided by the process of the invention comprises an aqueous-based vehicle and a dye; a hygroscopic solubilizing cation is added to the ink to reduce evaporation of water and to increase the solubility of the dye in the aqueous-based vehicle, especially in acidic media. The pH of the ink is maintained in the acidic region, or below about 7. The vehicle of the ink comprises water and at least one glycol ether, preferably of the type represented by the formula H[O(C n H 2n )] m OH, where n is an integer of 1 to 3 and m is an integer of 1 to 4. Examples of glycol ethers suitable in the practice of the invention include diethylene glycol, triethylene glycol and polyethylene glycols. The water is present in an amount ranging from 5 to 95%, the balance being the glycol ether or ethers. Preferably, the vehicle comprises about 50% water and the balance the glycol ether. To the vehicle is added up to about 10% of a dye, preferably an anionic dye (sometimes referred to as an acid dye). The amount of the dye added is a function of choice, being largely dependent upon the solubility of the dye in the vehicle (which limits the upper range of dye concentration) and the desired density of the print achieved with the ink (which limits the lower range of dye concentration - typically about 0.5%). Preferably, about 6% of the ink composition at this point comprises the dye. The dye to which the invention is suitably applied is an organic molecule having at least one negatively charged functional group per molecule. Since sulfonate (SO₃⁻) groups are especially benefitted from the practice of the invention, dyes having such groups are preferred. A convenient dye in this regard is Food Black 2 (FB2), which has four sulfonate groups per molecule. The negative charge of the sulfonate group is most commonly balanced by the presence of positively charged sodium (Na⁺). Other anionic dyes may also be suitably employed, since, as will be discussed in further detail below, the solubility of the anionic dyes is increased in acidic media by employing the teachings of the invention. Examples of other anionic dyes which may be employed in the ink composition disclosed herein include Direct Red 9, Direct Red 227, Acid Yellow 23, Direct Yellow 86, Acid Blue 9, Direct Blue 86 and Acid Blue 185. All such anionic dyes include sulfonate functional groups, and are not otherwise soluble in acidic media, tending to precipitate out as the pH is lowered. The ink composition produced by the process of the invention includes a cationic compound selected from the group of alkanol ammonium compounds and cationic amide compounds. Alkanol ammonium cations are represented by the formula HN⁺RR'R'', where at least one of R, R' and R'' comprises a carbon chain having from 1 to 5 carbon atoms, with an alcohol group (-OH) associated with at least one of the carbon atoms. To the extent that one or more of the R groups is not an alkanol group, that group may comprise an alkyl. The anion is commonly chloride (Cl⁻), although other anions may also be employed to form the alkanol ammonium compound; indeed, any anion may be suitably employed so long as it can effectively transport through a reverse osmosis membrane. Examples of such other anions include sulfate (SO₄⁻²), nitrate (NO₃⁻), acetyl (CH₃CO₂⁻) and phosphate (PO₄⁻³). While up to 5 carbon atoms are permitted, use of higher carbon-containing R groups tends to defeat the purpose of the invention, namely, solubilizing the dye. Further, although each of the carbon atoms in a given R group may be associated with at least one alcohol group, higher order alcohols have a higher viscosity than lower order alcohols, so that, for example, diols are preferred over triols. The viscosity of the final ink composition should be maintained at or below about 50 mPa·s at 25 o C. An example of an alkanol ammonium cation suitable in the practice of the invention is the triethanol ammonium cation, which has the formula HN⁺(C₂H₅OH)₃ (here, R=R'=R''= C₂H₅OH), referred to herein as TEAH⁺. This cation, which exemplifies the class of cations in the compounds in accordance with the invention, increases the solubility cnaracteristics of the dye by exchanging the positively charged sodium associated with the sulfonate group on the dye with the alkanol ammonium cation. The alkanol ammonium compounds employed in the practice of the invention reduce loss of water due to their hygroscopic nature. Further, the alkanol ammonium compound keeps the dye solubilized in acidic media, due to protonation of the amine cation. Other examples of alkanol ammonium cations suitably employed in the practice of the invention include the diethanol ammonium cation and the monoethanol ammonium cation. Cationic amide compounds may alternatively be employed in place of the alkanol ammonium compounds. Cationic amide compounds have the general formula R-C(O)-NH₃⁺. An example of a suitable cationic amide compound is the protonated form of formamide which has the formula HC(O)-NH₃⁺. Cationic amides and alkanol ammonium cations of the type described herein may be termed bifunctional amine cations. If the pH of the ink becomes sufficiently high to restrict protonation of the bifunctional amine cation, then the anionic dye may not sufficiently associate with this cation. This would diminish the effect that the cation has on enhancing dye solubility. Accordingly, the pH is desirably maintained below a value of about 7. The foregoing ink compositions are suitable for use without further considerations in most types of ink jet printers. However, for thermal ink-jet printers, there is an additional consideration. As noted above, FB2 has four sulfonate groups. If all four sodium cations associated all four sulfonate groups are replaced by bifunctional amine cations, then a process known as kogation occurs. Kogation, which is a coined term unique to thermal ink jet printing, is used to decribe the decomposition of the dye which results from heating to a high temperature by the hot resistor used to "fire" bubbles of ink toward the paper substrate. Kogation is highly dependent upon a number of variables, one of which is the degree of cation substitution using TEAH⁺ and FB2. Other cations, dyes and vehicles and other operating conditions will affect the dependence of kogation on cation substitution. For FB2, kogation has been found to be acceptable for approximately equal sodium cations and triethanol ammonium cations (TEAH⁺). Kogation is most apparent at a ratio of 0:4 and is less apparent at a ratio of 4:0 Na⁺:TEAH⁺. Kogation has been found for the TEAH⁺/FB2 system to be acceptable at ratios of 3:1 and less. Thus, although at least one sodium cation per dye molecule should be replaced by the bifunctional amine cation, preferably, not all four sodium cations per molecule should be so replaced. Preparation of the ink composition fabricated by the process of the invention is preferably accomplished by use of reverse osmosis (ultrafiltration). While ion exchange processes may be employed, such processes are far more labor intensive and require considerable quantities of the bifunctional amine cation to effect the ion exchange with sodium. Briefly, the preferred process involves forming a solution of the anionic dye with a bifunctional amine cation with chloride anion, such as TEAH⁺Cl⁻, in an acidic medium, such as provided by HCl, and applying reverse osmosis to the feed so as to drive excess sodium and chloride ions across the polymer membrane of the reverse osmosis apparatus. As is well-known, the material passing through the membrane is called the "permeate"; that which is left behind is called the "concentrate". The concentrate is cycled repeatedly through the reverse osmosis membrane to replace sodium cations on the dye molecules with bifunctional amine cations. Water is added as necessary. A solution of the dye in water may be preliminarily subjected to the reverse osmosis process in order to reduce the sodium cation concentration, since commercial grade dyes such as FB2 include impurity amounts of sodium chloride and sodium sulfate. In the reverse osmosis process as applied to the foregoing composition, the concentrate has dye anions/sodium cations, bifunctional amine (e.g., triethanol ammonium cations/chloride anions), sodium cations/chloride anions and water, while the permeate has primarily sodium and chloride ions, water and a trace amount of the other ions that are primarily retained in the concentrate. The process is continued until the dye has a concentration such that addition of one or more glycol ethers to the concentrate results in the desired ink composition (water, glycol ether, dye and bifunctional amine), as described above. The pH is then adjusted as appropriate with, e.g., HCl to make more acidic or with, e.g., NaOH to make more basic. However, in any event, the pH is maintained below about 7, as discussed above. In the reverse osmosis process, pressure is applied to force undesired species through a membrane. This membrane is typically a polymer, such as cellulose acetate, available under the tradename "Super 50" from Osmonics (Hopkins, MN), or polysulfone, available under the tradename G-50 from De-Sal (Escondido, CA). The applied pressure may range, from 1.4 to 14 or more bar (20 - 200 psi). However, while higher pressures result in higher efficiencies (as measured by the amount of dye remaining at the end of the process having at least one sodium cation replaced by at least one bifunctional amine cation), higher pressures also tend to force dye molecules through the molecular filter. Thus, pressure can be varied according to desired efficiencies of dye retention and time of processing required. Consistent with these considerations, a pressure in the range of 4.2 to 5.6 bar (60 - 80 psi) is preferably employed when the dye is FB2 and the bifunctional amine is triethanol ammonium cation. In the reverse osmosis process, the initial charge may comprise dye, e.g., FB2, and water, in order to minimize Na⁺ as much as possible, due to the presence of the impurity amounts of sodium-containing compounds in commercial grade FB2. The reverse osmosis process is operated for at least about 1 hour, and preferably about 3 hours. The pressure variables described above are operative here, so the time and pressure will depend on the dye and the desired efficiency. The bifunctional amine cation, e.g., TEAH⁺, is next added and the pH reduced below about 7, and preferably to about 3. The ratio of TEAH⁺ is based on the considerations discussed earlier. A suitable starting ratio consistent with these considerations is 1.25 to 1.5 times the Na⁺ concentration. Water is added from time to time as necessary to replace that lost in the process. Again, the pressure and time considerations discussed earlier are operative here as well. Typically, the time is 15 to 20 hours. Chloride is an appropriate anion to employ with the alkanol ammonium species, and hydrochloric acid is suitably used to reduce the pH. Finally, the dye is concentrated by the reverse osmosis process. The concentration of the dye is chosen such that upon addition of the glycol, the desired final concentration of the dye in the ink will be achieved. The concentration process usually takes less than about 1 hour. Following removal of the composition from the reverse osmosis process, the desired glycol, as described above, is added to the composition to produce the ink composition. The pH is adjusted as necessary to maintain the ink at a pH below about 7. The entire process is conveniently operated at room temperature. EXAMPLE The conversion of Food Black 2 dye from the Na₄⁺ salt to the [(EtOH)₃NH]₄⁺ salt was accomplished as follows: An in-situ exchange of Na⁺ ions with triethanol ammonium cation, also referred to as (EtOH)₃NH⁺, was done using an Osmonic 600 M.W. reverse osmosis membrane. The Na⁺Cl⁻ salt was removed from a 6% FB2 solution in deionized water. The concentration of Na⁺ and Cl⁻ ions was monitored and the process continued until the ion concentrations were in the mole ratio of 2:1 ion to dye. The time for this process took about 1 hour. At this point, the dye was apparently in the [Na₂H₂]⁺⁴ form. Verification of this dye form was made by comparing the the stoichiometric ratios of Na⁺ to dye using a Na⁺ ion specific electrode. Once the mole ratio of about 2:1 was achieved, a stoichiometric excess of triethanol ammonium compound was added such that the ratio of [(EtOH)₃NH]⁺:dye was about 5:1. Following this addition, the solution was adjusted to a pH of about 3 using HCl. Based on a K H =8x10⁻⁹ , the ratio of [(EtOH)₃-NH]⁺:(EtOH)₃N was about 10⁵:1, as calculated from the equations: K b = 1.25 x 10⁻⁶ = [OH⁻][TEAH⁺]/[TEA] K H = 8.00 x 10⁻⁹ = ([H⁺][TEA]/[TEAH⁺]) x ([TEAH⁺]/[TEA]) = [H⁺]/8.00x10⁻⁹ The foregoing process ensured that all of the triethanol ammonium species was ionized and available as a counter ion. Immediately following this addition of triethanol ammonium compound, the concentrations of both Na⁺ and Cl⁻ increased. The Cl⁻ concentration increase was due to the addition of HCl, used to adjust pH. The Na⁺ increase was due, it was assumed, to the "liberation" of previously bound Na⁺ ions by competitive ionic association of [(EtOH)₃NH]⁺ with the FB2 anion. The reverse osmosis process was carried out until the mole ratio of 2:1 of ion to dye was again reached. In contrast to the first purification step, however, this second purification step generated a lower offset concentration of Na⁺ and Cl⁻, because of the cation exchange for Na⁺ by [(EtOH)₃NH]⁺. This process may be iterated as many times as desired, until enough partitioning has occurred to remove virtually all the Na⁺ ion. In practicality, the economics of time constraints control the number of iterations. In the present example, the reverse osmosis process was terminated when the resultant mole ratio of Na⁺:FB2 was 1:1.4 and the mole ratio of Cl⁻ to FB2 was 1:333. Water was subsequently extracted from the solution containing the substituted dye until the resulting dye concentration reached 12 wt %, as determined by visible spectroscopy. (This process was accomplished by continually extracting water from the concentrate using the reverse osmosis process.) An equal amount (by weight) of DEG was added to the concentrated liquor resulting in a final substituted dye concentration of 6% and a 50% DEG-50% water vehicle. The tetraethanol ammonium form of FB2 evidenced a significant improvement with orifice crusting when compared to the tetrasodium form. In general, results of orifice crusting experiments indicated that as lower polarity cations were exchanged for Na⁺, the propensity for orifice crusting was diminished. In fact, substantial differences were even observed between the monoethanol ammonium cation and the triethanol ammonium cation. The Table below summarizes kogation data on a variety of vehicle concentrations and sodium cation replacement on FB2 dye. Solubility curves showed substantially enhanced solubilities for the triethanol ammonium cation in both water and diethylene glycol. Although it may be apparent that this enhanced solubility was responsible for the improvement in orifice crusting, it should be noted that additionally, a substantial amount of hygroscopicity was introduced with the triethanol ammonium compound, due, in large part, to the moiety of the hydroxides on the cation. The affinity that the dye had for water was so high that the dessicated dye powder rapidly became a tar when exposed to an ambient relative humidity of 30% for less than one hour. The Table below also summarizes crusting data for a variety of vehicle concentrations and sodium cation replacement on FB2 dye. The pH of all inks was maintained between 6 and 7. Thus, there has been disclosed a process for preparing an ink composition for use in ink-jet printers which evidences substantially no orifice crusting due to dye precipitation. The process utilizes a reverse osmosis membrane to solubilize an acid dye in a low pH environment without precipitation of the dye. Various modifications and changes will make themselves available to those of ordinary skill in the art, and all such changes and variances not deviating from the spirit and essence of the invention are intended to be covered by the appended claims.

Claims: A process for preparing an ink composition suitable for use in thermal ink-jet printers without undue crusting or dye decomposition comprising: (a) forming a solution of an anionic dye having at least one negatively charged sulfonate functional group per molecule and a cationic compound selected from the group consisting of alkanol ammonium compounds and cationic amide compounds: (b) acidifying said solution; (c) cycling said solution through a reverse osmosis membrane to replace sodium cations on the dye molecules with cations of said compound by forming a concentrate and a permeate, said concentrate including a cation of said compound ionically associated with at least one but not all of the functional groups of each molecule of said dye, and said permeate including a cation formerly associated with said at least one functional group; (d) adding water as necessary to said concentrate; (e) concentrating said dye by reverse osmosis; and (f) forming said solution of said concentrated dye with a glycol ether to form said ink composition, said glycol ether being selected from the group consisting of diethylene glycol, triethylene glcol and polyethylene glycols, wherein said ink composition comprises: (1) a vehicle comprising 5 % to 95 % water and the balance at least one glycol ether, (2) a dye having at least one negatively charged functional group per molecule, present in an amount up to about 10 % of the vehicle composition, and (3) said cationic compound present in an amount such that there is at least one molecule of cationic compound for at least one of the negatively charged functional groups on the dye, the pH of the ink being maintained in the acidic region below about 7. The process of Claim 1 wherein dye containing cation impurities is admixed with water and cycled through a reverse osmosis membrane to reduce said cation impurities prior to forming said solution of said dye and said compound. The process of Claims 1 or 2 wherein said reverse osmosis is operated at a pressure ranging from 1.4 to 14 bar (20 - 200 psi). The process of Claim 3 wherein said pressure ranges from 4.2 to 5.6 bar (60 - 80 psi). The process of Claim 2 wherein reverse osmosis is operated for a time of at least about one hour. The process of Claim 5 wherein said time is about three hours. The process of Claim 2 wherein said cation impurity comprises sodium ion. The process of one of the preceding Claims wherein the ratio of said compound to said at least one negatively charged functional group ranges from 1.25 to 1.5. The process of one of the preceding claims Claims wherein said compound includes chloride anions and said solution is acidified with hydrochloric acid. The process of one of the preceding claims Claims wherein said glycol ether has a formula given by H[O(C n H 2n )] m OH where n is an integer of 1 to 3 and m is an integer of 1 to 4. The process of one of the preceding claims wherein said glycol ether consists essentially of diethylene glycol. The process of Claim 1 wherein said solution comprises said concentrated dye, about 50% water and the balance diethylene glycol. The process of one of the preceding Claims wherein said dye is selected from the group consisting of Food Black 2, Direct Red 9, Direct Red 227, Acid Yellow 23, Direct Yellow 86, Acid Blue 9, Direct Blue 86 and Acid Blue 185. The process of Claim 13 wherein said dye consists essentially of Food Black 2. The process of one of the preceding Claims wherein said alkanol ammonium compound includes a cation represented by the formula HN⁺RR'R'', where at least one of R, R', and R'' comprises a carbon chain having from 1 to 5 carbon atoms, with an alcohol group associated with at least one of said carbon atoms. The process of Claim 15 wherein said alkanol ammonium compound includes triethanol ammonium cation having the formula HN⁺(C₂H₅OH)₃. The process of Claim 1 wherein said dye consists essentially of Food Black 2 and said compound includes triethanol ammonium cation, said compound present in an amount such that less than 4 sodium ions associated with sulfonate groups on said dye molecule are displaced by said triethanol ammonium cations.

Inventors: PALMER, DONALD J.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1986
Main classification: C09D11
IPC codes: C09B67, C09D11