Patent Publication Number: US-2007100022-A1

Title: Low corrosivity inks and ink systems and methods of making low corrosivity inks

Description:
BACKGROUND  
      The present disclosure relates to inkjet ink sets, where a ‘set’ comprises black and color inks, as well as a fixer fluid, and wherein the fixer fluid has low corrosivity towards metals (e.g., steel, aluminum). A fixer fluid can be printed along with standard inks to improve print quality and durability of the printed ink on a medium. While these fixers increase the waterfastness, smudgefastness, and/or light-fastness of the inks, they typically are corrosive to the printhead or pen that houses them.  
      Typical fixers rely on cationic polymers such as, for example, poly(ethylene imine) (PEI), polyamines, quaternized polyamides, polyguanidines, dicyandiamide resins and poly(diallyldimethyl ammonium) resins in an aqueous solution to complex with and insolubilize dyes and pigments in aqueous ink formulations. Commercially available polycations are typically supplied with chloride counterions. Chloride may cause pitting corrosion of aluminum components within the printhead. Based on literature, the chloride ion is known to accelerate the corrosion of aluminum by dissolving away the protective oxide layer on the aluminum surface. See, e.g., Gu, H. C.; Huang, S. J.; Liu, X. F.,  Corrosion, V 58, No. 10, 826-834, 2002 and McCafferty, E.,  J. Electrochemical Soc.,  150, 7, B342-347, 2003, both of which are incorporated herein in their entireties.  
      For example, a solution sample containing 1 wt % NaCl generates 50 times more anodic current than a sample containing 0.01 wt % NaCl, and 1500 times more anodic current than deionized water. Anodic current is directly proportional to corrosion rate.  
      Due to pen imperfections and pitting from the chloride ions in the fixer solutions, premature pen failure can result. In addition, fixers with high chloride content may require special handling during transportation due to their corrosive nature, adding to product cost, and perhaps making air-shipments impractical. Thus, it is desirable to have a method of reducing the negative impact of fixer formulations on printhead components, while maintaining their other desirable properties.  
     SUMMARY  
      Briefly described, embodiments of this disclosure include low corrosivity inkjet ink sets and ink systems, and methods of preparing low corrosivity inks. One exemplary low corrosivity inkjet ink system, among others, includes and ink formulation and a fixer formulation. One exemplary fixer formulation includes water, a polycationic fixing agent, and a counterion, the counterion being chosen to reduce the presence of halogen in the fixer.  
      One exemplary method, among other, of making an ink formulation substantially non-corrosive includes the steps of: dissolving a polycation with a non-halogen containing counterion in a solvent to form a fixing formulation in a first vehicle, and preparing an ink formulation in a second vehicle, the fixing formulation being used to undercoat or overcoat the ink formulation on a substrate.  
      One exemplary inkjet ink system with low corrosivity includes an ink, wherein the ink comprises a colorant, and a first aqueous vehicle for the colorant; and the inkjet ink system further including a substantially non-corrosive fixer, wherein the fixer comprises a polycationic fixing agent, a non-halogen counterion for the polycationic fixing agent, and a second aqueous vehicle for the fixer.  
      Other inkjet ink sets and ink systems, methods of preparing low corrosivity inks, and fixer formulations are described in more detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Many aspects of this disclosure and the embodiments of the invention described herein can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
       FIG. 1  illustrates a bar chart of the results of an official U.S. Department of Transportation (DOT) test method for corrosion as performed on various exemplary fixers for 1 week extrapolated to one year.  
       FIG. 2  illustrates a bar chart of the results of an official U.S. DOT test method for corrosion as performed on various exemplary fixers for 1 day extrapolated to one year. 
    
    
     DETAILED DESCRIPTION  
      In order to obtain images that are durable to smear, smudge, or water, inks containing pigments and/or anionic dyes are underprinted and/or overprinted with fixer fluids containing cationic polymers. The dye/polymer complex forms a durable mixture that is resistant to smearing and is waterfast. Fixers commonly operate by creating an adverse charge-charge interaction with a colorant in aqueous ink, thereby precipitating and fixing the colorant on the substrate surface. This mechanism is effective with aqueous inks because the colorant is typically stabilized to dispersion or solution by an ionic mechanism. A fixer with opposite charge effectively destabilizes and fixes the colorant. With pigment-based inks, the typically low pH of the fixer crashes the pigments, making them difficult to re-disperse in water.  
      The disclosed ink composition includes one or more colorant(s), an aqueous vehicle and, optionally, other ingredients such as surfactants, dispersants, binders, and/or other additives and adjuvants well known in the relevant art. The ink composition is applied to a substrate in combination with the fixer (fixing) fluid, the fixer fluid including a fixing agent in an aqueous vehicle. The fixer fluid can be applied to the substrate first, and then the ink printed on top of the applied fixer. Alternatively, the ink can be applied to the substrate first, and then the fixer fluid printed on top of the applied ink.  
      Colorant for Aqueous Ink  
      The colorant can be soluble or dispersed in an aqueous ink vehicle. Soluble colorants (e.g., dyes) are dissolved in the aqueous vehicle, while insoluble colorants (e.g., pigments) are stably dispersed.  
      Suitable dyes for inkjet applications are generally well known. A representative selection of such dyes can be found, for example, in U.S. Pat. No. 5,932,631 and U.S. SIR H1967, the disclosures of which are incorporated by reference herein in their entireties for all purposes as if fully set forth. The exact choice of dyes depends upon the color reproduction and print quality requirements of the application.  
      Dyes used in an aqueous ink vehicle are most commonly ionic in character, which means they form an ionic (anionic or cationic depending on the specific dye) chromaphore in aqueous solution. Frequently, these dyes are only slightly soluble in a nonaqueous vehicle.  
      Non-limiting examples of anionic dyes that are effective with the disclosed fixers are: direct black dyes, such as Direct Black 168 (DB168), Direct Black 19 (DB19), variants of Fast Black 2; phthalocyanine cyan dyes, such as ProJet Cyan 485; acid cyan dyes, such as Acid Blue 9 (AB9), Acid Blue 7 (AB7); mixtures of acid cyan and phthalocyanine cyan, such as AB9 and ProJet Cyan 485; gamma acid magenta dyes, such as Magenta 377 (M377); H-acid magenta dyes, such as ProJet Magenta 364 (M364); Xanthene magenta dyes, such as Acid Red 289 (AR289); mixtures of H-acid magenta and Xanthene magenta dyes, such as mixtures of ProJet Magenta 364 and AR289; direct yellow dyes, such as Direct Yellow 132 (DY132); acid yellow dyes such as Acid Yellow 23 (AY23); and mixtures of direct yellow dyes and acid yellow dyes, such as mixtures of DY132 and AY23.  
      Suitable pigments for inkjet applications are also generally well known. A representative selection of such pigments can be found, for example, in U.S. Pat. No. 5,026,427, U.S. Pat. No. 5,086,698, U.S. Pat. No. 5,141,556, U.S. Pat. No. 5,169,436 and U.S. Pat. No. 6,160,370, the disclosures of which are incorporated by reference herein in their entireties for all purposes as if fully set forth. The exact choice of pigment depends upon color reproduction and print quality requirements of the application. The pigment may be black, such as those based on carbon black, or may be colored such as those based on cyan (e.g., PB 15:3 and 15:4), magenta (e.g., PR 122 and 123), and yellow (e.g., PY 128, 74 and 120). Suitable pigments also include self-dispersing pigments (SDPs). SDPs for aqueous inks are well known.  
      It is desirable to use small pigment particles for maximum color strength and good jetting. The particle size may generally be in the range of from about 0.005 to about 15 microns, is typically in the range of from about 0.005 to about 1 micron, or from about 0.005 to about 0.5 micron, or from about 0.01 to about 0.3 micron.  
      The levels of pigment employed in the inks disclosed herein are those levels that are typically needed to impart the desired optical density (OD) to the printed image. Typically, pigment levels are in the range of from about 0.01% to about 10% by weight, based on the total weight of the ink.  
      Fixing Fluid  
      The fixing fluid comprises an aqueous vehicle and an effective amount of one or more fixing agents. A fixing agent is an ingredient that initiates a change in the solubility or stability of the colorant and fixes the colorant in place in the printed image. An “effective amount” of fixing agent is an amount that is effective in achieving an improvement in print quality, e.g., decreased strikethrough and bleed, increased optical density (OD), chroma, edge acuity, and improved drip and smear fastness, as compared to a print that has not been fixed.  
      One mechanism of fixation, though not necessarily the only one, is interaction of a polycation in the fixing agent with an anionic dye in the aqueous ink. Alternatively, for pigment-based inks, the fixer lowers the pH of ink whereby the pigments crash, rendering them insoluble in water. Fixers can include multivalent ions, salts, and acids thereof that are soluble in the aqueous vehicle to facilitate pigment crashing.  
      The fixing fluid can be formulated for high spread and quick penetration and drying. To achieve these properties, surfactants and/or penetrating solvents are typically employed. The surface tension can be less than about 40 mN/m. Typically, the fixer fluid can be about 0.1% to about 20%, more preferably about 1.0% to about 15%, fixing agent based on the total weight of the fixing fluid.  
      Inkjet inks with anionic colorants have been found to work well with fixer solutions including cationic polymers. The use of such cationic polymer fixer solutions increases durability and waterfastness of anionic colorant ink-printed images.  
      The fixer solution can be applied onto the media substrate by any method available to spread the fixer accurately onto the substrate surface to be printed. For example, the fixer solution can be filled in a thermal inkjet pen and the fixer applied on the media or substrate before and/or after the color inks are applied. As a non-limiting example, an HP Business Inkjet 2200 can be used.  
      To increase durability and waterfastness of the printed image on the medium or substrate, the cationic polymers used in the fixer are highly reactive to fix the anionic colorants in the printed image. In one embodiment, the cationic polymer is a polycationic fixing agent that has a structure such that the polycation is capable of complexing with an insolubilizing an anionic dye. For example, polyguanidines, polymonoguanidines, polyamines, quaternized polyamines, and/or polyethyleneimines (e.g., polyethyleneimine (PEI) or methylated PEI), dicyandiamide resins, and/or poly(diallyldimethyl ammonium) resins have been found to be effective cationic polymers for this purpose. By way of further example, but not for purpose of limitation, other polymers with quaternary nitrogen atoms or that have nitrogen atoms that can be rendered cationic are also employed to immobilize the dye.  
      In one embodiment, the cationic polymers are polymonoguanidines, for example but not limited to, poly (C 3-18 -hydrocarbyl monoguanidines). Examples of poly (C 3-18 -hydrocarbyl monoguanidines) that can be used in the disclosed fixer compositions include those, for example, disclosed in U.S. patent application Ser. No. 10/443,566, incorporated herein by reference in its entirety.  
      As noted previously, commercially available polycations have typically been supplied with chloride counterions. Chloride or other halogens may cause pitting corrosion of aluminum components within the inkjet printhead. It has been discovered that use of non-halogen counterions for the polycations of the fixer greatly reduces corrosion of the printhead. Use of a non-corrosive counterion in the fixer can increase the life of a typical printhead. Additionally, the use of non-corrosive counterions can reduce shipping restrictions for the fixer. For example, a fixer with reduced or minimized corrosivity as disclosed herein can pass the official U.S. Department of Transportation (DOT) test method for corrosion, ASTM G31-72, which is incorporated herein by reference in its entirety.  
      Exemplary counterions include, for example, an acetate, a nitrate, a sulfate, a phosphate, a propionate, a malonate, a mesylate, a lactate, a sulfonate, a phosphonate, a triflate, or any non-halogen anion, or combinations thereof.  
      The disclosed fixer may further include any one or any combination of the following: water, a water soluble cosolvent, a watermiscible surfactant, a polycationic fixing agent that contains less than about 500 ppm halogen per polymer solids, and/or an acid. For example, one exemplary fixer formulation can include the following: water in an amount of about 50-95 wt % of the fixer formulation; a water soluble cosolvent having a lower vapor pressure than water in an amount of about 5-35 wt % of the fixer formulation; a water miscible surfactant in an amount of about 0.01-2 wt % of the fixer formulation; the polycationic fixing agent in an amount of about 0.5-8 wt %, or about 2-5%, of the fixer formulation, wherein the polycationic fixing agent contains less than about 500 ppm halogen per polymer solids; and an acid in an amount of about 1-20 wt % of the fixer formulation, wherein the acid has a pKa of about 3-6. The amount of halogen should be lower than typical chloride-based polycationic fixing agent. For example, a conventional fixing agent at 7000 ppm chloride will not pass the DOT corrosivity test, described in more detail below.  
       FIG. 1  illustrates a bar chart of the results of the official U.S. Department of Transportation (DOT) test method for corrosion as performed on various exemplary disclosed fixers, compared to a prior art fixer, for one week extrapolated to one year. The official DOT method (ASTM G31-72) for laboratory immersion corrosion testing of metals was duplicated to predict the corrosion rates that would be attained with alternative, low-chloride fixers, for example, a fixer which includes the following active ingredients: SURFYNOLI® 465 0.9% (a nonionic surfactant from Air Products and Chemicals), ZONYL® FSN 0.1% (a fluorosurfactant from DuPont, available from Aldrich Chem. Co.), FLOQUAT® FL 2350 2.0% (a polyamine available from SNF Floerger), trimethylolpropane 8.0% (a co-solvent available from Aldrich), 4-methylmorpholine-4-oxide 13.6% (a solvent from BASF, available from Aldrich), methanesulfonic acid 8.7% (from Aldrich, used to titrate the pH of the fixer to 4.0), and deionized (DI) water 66.7%. Aluminum alloy 7075-T6 is the required alloy for corrosion testing according to the ASMT G-31-72 test method.  
      Both  FIGS. 1 and 2  show the dramatic difference in aluminum corrosion rate for two polycations (polyamine (FL series) and polyguanidine (FA series) respectively).  FIG. 2  illustrates an bar chart of the results of an official U.S. DOT test method for corrosion as performed on various exemplary fixers for one day extrapolated to one year.  
      Also disclosed are methods of preparing an ink with the disclosed fixer that is non-corrosive or has reduced corrosivity and that is non-halogenated or has a reduced number of halogens compared to conventional fixing agents. In one exemplary method, the disclosed polycation is dissolved with the disclosed counterion in water and/or a solvent. The polycation can be produced by, for example, polymerizing cationic monomer with no halogens, or a reduced number of halogens. The polycation can be produced by, for example, using dialysis to replace the halogen counterion with a non-halogen counterion. The polycation can be produced by, for example, ion-exchanging the halogen counterion with a non- (or less) corrosive, non-halogen counterion.  
      Aqueous Vehicle  
      “Aqueous vehicle” refers to water or a mixture of water and at least one water-miscible organic solvent (co-solvent). Selection of a suitable mixture depends on requirements of the specific application, such as desired surface tension and viscosity, the selected fixer, drying time of the fixer fluid, and the type of substrate onto which an aqueous fixer fluid will be printed. If a mixture of water and a water-soluble solvent is used, the aqueous vehicle typically will contain about 30% to about 95% water with the balance (e.g., about 70% to about 5%) being the water-soluble solvent. Compositions can contain about 60% to about 95% water, based on the total weight of the aqueous vehicle.  
      The aqueous vehicle can be made to be fast penetrating (rapid drying) by including surfactants or penetrating agents such as glycol ethers and 1,2-alkanediols. Glycol ethers include ethylene glycol monobutyl ether, diethylene glycol mono-n-propyl ether, ethylene glycol mono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-n-butyl ether, triethylene glycol mono-n-butyl ether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-iso-propyl ether, propylene glycol mono-n-butyl ether, dipropylene glycol mono-n-butyl ether, dipropylene glycol mono-n-propyl ether, and dipropylene glycol mono-isopropyl ether. 1,2-Alkanediols are preferably 1,2-C4-6 alkanediols, most preferably 1,2-hexanediol. Suitable surfactants include ethoxylated acetylene diols (e.g., Surfynol® series from Air Products), ethoxylated primary (e.g., NEODOL® series from Shell) and secondary (e.g., TERGITOL® series from Dow Chemical) alcohols, sulfosuccinates (e.g., AEROSOL® series from Cytec), organosilicones (e.g., SILWET® series from Witco) and fluoro surfactants (e.g., Zonyl® series from DuPont). The amount of glycol ether(s) and 1,2-alkanediol(s) added can be properly determined, but is typically in the range of from about 1 to about 15% by weight and more typically about 2 to about 10% by weight, based on the total weight of the fixer fluid. Surfactants may be used, typically in the amount of about 0.01 to about 5% or about 0.2 to about 2%, based on the total weight of the fixer fluid.  
      The amount of vehicle in the fixer fluid is typically in the range of about 70% to about 99.8%, or about 80% to about 99.8%, based on total weight of the fixer fluid.  
      Other Ingredients  
      Other ingredients may be formulated into the inks and fixer fluids disclosed herein, to the extent that such other ingredients do not interfere with the mutually interactive (fixing) mechanisms of the ink set or the stability and jettability of the ink. Such other ingredients are generally well known in the art and include one or more of a biocide, e.g., bactericide, fungicide, algicide and the like, sequestering agent, buffering agent, corrosion inhibitor, light stabilizer, anti-curl agent, thickener, defoamer, and the like, to improve various properties or function of the ink or fixer compositions as needed.  
      The amount of each ingredient must be properly determined, but is typically in the range of from about 0.05 to about 15% by weight, and more typically about 0.2 to about 10% by weight, based on the total weight of the ink.  
      Binders can also be used and can be soluble or dispersed polymer(s), added to the ink to improve the adhesion of a pigment. Examples of polymers that can be used include polyesters, polystyrene/acrylates, sulfonated polyesters, polyurethanes, polyimides and the like. When present, soluble polymer can be used at levels of at least about 0.3%, or at least about 0.6%, based on the total weight of the ink. Upper limits are dictated by ink viscosity or other physical limitations.  
      Inkjet Ink Systems  
      Also disclosed are inkjet ink systems that have reduced corrosivity compared to conventional inkjet systems. One exemplary system includes an ink and a fixer, the fixer being formed to minimize corrosivity. The ink includes a colorant as described herein or otherwise known, and an aqueous first vehicle for the colorant, as described herein or otherwise known. The fixer includes the disclosed polycationic fixing agent, the disclosed non- (or reduced-) halogen counterion for the polycationic fixing agent, and an aqueous second vehicle for the fixer as described herein or otherwise known. The counterion is selected to reduce or eliminate the amount of halogens in the fixer, thereby reducing the corrosivity of the inkjet ink system.  
      Many variations and modifications may be made to the above-described embodiments. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.