Printhead cleaning solution

Abstract

The present invention provides a printhead cleaning solution comprising: a carboxylic acid; a solvent, wherein the solvent is an organic solvent having a logPoctanol / water from −1.00 to 1.20; and wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution. The present invention also provides a method of removing ink from a printhead nozzle plate, the method comprising: (i) preparing a printhead cleaning solution of the present invention; and (ii) jetting the printhead cleaning solution onto the printhead nozzle plate to remove the ink.

Description

[0001] Printhead cleaning solution

[0002] The present invention relates to a printhead cleaning solution and, in particular, a printhead cleaning solution for removing ink from a printhead nozzle plate. The present invention also relates to a method of removing ink from a printhead nozzle plate.

[0003] In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printheads through narrow nozzles onto a substrate, which is moving relative to the printheads. The ejected ink forms an image on the substrate.

[0004] Printheads account for a significant portion of the cost of an entry level printer and it is therefore necessary to maintain the printheads. In particular, the nozzle plate containing the nozzles needs to be kept clean and any residual ink removed from the nozzle plate in order to prevent misfiring nozzles, which would adversely affect the printed image. Nozzle plates may contain a non-wetting coating (NWC) to prevent ink from collecting on the nozzle plate but some ink, such as mist from the jetting process, may still accumulate on the nozzle plate.

[0005] Typical printhead cleaning fluids that are ejected onto the nozzle plate offer poor removal of residual ink, resulting in the need to manually wipe the nozzle plate clear of residual ink. Manual cleaning is not an efficient process in many printer systems where automated maintenance is desirable.

[0006] The inventors of the present invention have found that printhead cleaning fluids containing water offer poor removal of residual ink. In this regard, the water present in the printhead cleaning fluids can be absorbed into the nozzle. The presence of this water can cause residual ink to gel inside the nozzle overtime, which leads to printhead nozzle clogging, which can be impossible to remove, even with manual cleaning. Further, if the printhead is recirculating, the water can be taken into the ink system and the entire ink delivery system can be negatively affected.

[0007] There is therefore a need in the art for an effective printhead cleaning fluid which can remove all residual ink from a nozzle plate in an automated process, without the need for manual wiping of the nozzle plate.

[0008] Accordingly, the present invention provides a printhead cleaning solution comprising: a carboxylic acid; a solvent, wherein the solvent is an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20; and wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution.

[0009] The present invention also provides a method of removing ink from a printhead nozzle plate, the method comprising: (i) preparing a printhead cleaning solution of the present invention; and (ii) jetting the printhead cleaning solution onto the printhead nozzle plate to remove the ink. The inventors have surprisingly found that the inclusion of a carboxylic acid in an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20, wherein there is less than 5% by weight of water present, based on the total weight of the printhead cleaning solution, provides a printhead cleaning solution that can remove all residual ink from a nozzle plate in an automated process, without the need for manual wiping of the nozzle plate. This increases the efficiency of the cleaning process and hence the overall productivity of the printing process. The nozzle plate is also left clean, preventing misfiring nozzles, preventing nozzle clogging and ensuring a high quality image. The printhead cleaning solution is particularly effective at maintaining nozzle plates containing an NWC without damaging it.

[0010] The present invention provides a printhead cleaning solution comprising: a carboxylic acid; a solvent, wherein the solvent is an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20; and wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution.

[0011] The printhead cleaning solution is a printhead cleaning solution for removing ink from a printhead nozzle plate. In this way, the printhead cleaning solution is a printhead nozzle plate cleaning solution. The printhead cleaning solution is liquid at ambient temperature.

[0012] The printhead cleaning solution is a solution in that the carboxylic acid is dissolved in the solvent.

[0013] The printhead cleaning solution of the present invention comprises a carboxylic acid.

[0014] By acid is meant a molecule capable of donating one or more protons. Acids typically have a pKa(or pKai for polyprotic acids) of less than 14 in water. The parameter pKais a measure of the strength of an acid in solution and a detailed discussion is not required. If the pKaof an acid is not known or the acid is insoluble, it may be theoretically calculated using software.

[0015] Preferably, the carboxylic acid of the present invention has a pKa(or pKai) of less than 6 in water. Preferably, the carboxylic acid has a pKa (or pKai) of more than 0 in water. In a preferred embodiment, the carboxylic acid has a pKa (or pKai) from 0 to 6 in water.

[0016] The carboxylic acid is responsible for fully removing residual ink from the nozzle plate. In the absence of the carboxylic acid, the solvent is incapable of fully removing residual ink from the nozzle plate.

[0017] The carboxylic acid is not limited other than it has to be soluble in the solvent. In a preferred embodiment, the carboxylic acid has an acid value of more than 10 mg KOH / g. By acid value is meant the mass of potassium hydroxide (KOH) in milligrams that is required to neutralise one gram of acid. The acid value of the carboxylic acid is a measure of the number of carboxylic acid groups in the acid. In a typical procedure, a known amount of sample is dissolved in an organic solvent and titrated with a solution of alcoholic KOH of known concentration using phenolphthalein as a colour indicator.

[0018] The acid value of the carboxylic acid is preferably determined using the following procedure: prepare a mixture of one part industrial methylated spirits to two parts toluene; mix 2.0 g of a sample with 75 mL of the mixture until the sample is fully dissolved; titrate the dissolved sample using 0.1 M alcoholic potassium hydroxide solution and four drops of 0.5% alcoholic phenolphthalein as an indicator; and the end point is when the indicator changes colour from colourless to pink.

[0019] The acid is a carboxylic acid, which has the following structure: carboxylic acid wherein R is preferably carboxyl, alkyl, cycloalkyl, alkenyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms, further substituted by heteroatoms and / or terminated by hydroxyl. Non-limiting examples of R include carboxyl, C1-18 alkyl, C3-18 cycloalkyl, C2-18 alkenyl, CB-IO aryl and combinations thereof, such as Cs- aryl- or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, further substituted by 1-10 heteroatoms and / or terminated by hydroxyl, with typical heteroatoms being oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. R may also be a polymer chain such as a polyether, polyester or polyurethane chain.

[0020] In a preferred embodiment, the acid is salicylic acid, oxalic acid, a carboxylic acid resin or a combination thereof.

[0021] By resin is meant that the substituents of the carboxylic acid resin are polymer chains. The carboxylic acid resin preferably has a viscosity of 150 mPas or above at 25°C. Viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 1 2° steel cone at 60°C with a shear rate of 25 s1.

[0022] Examples of preferred carboxylic acid resins are PRO22099 commercially available from Arkema, SB400 commercially available from Arkema and Photomer 4703 commercially available from IGM Resins. PRO22099, SB400 and Photomer 4703 have viscosities of 530 mPas, 10,000-50,000 mPas and 150-230 mPas, respectively. Preferably, the carboxylic acid is present in an amount of 1-10% by weight, more preferably 2-9% by weight, most preferably 3-8% by weight, based on the total weight of the printhead cleaning solution.

[0023] The printhead cleaning solution may comprise one or more additional acids, other than a carboxylic acid. The additional acid is not limited, other than it has to be soluble in the solvent.

[0024] Preferably, the additional acid, other than a carboxylic acid, has a pKa(or pKai) of less than 14 in water, more preferably less than 10 in water and most preferably less than 6 in water. Preferably, the additional acid, other than a carboxylic acid, has a pKa (or pKai) of more than 0 in water. In a preferred embodiment, the additional acid, other than a carboxylic acid, has a pKa (or pKai) from 0 to 14 in water, more preferably from 0 to 10 in water and most preferably from 0 to 6 in water.

[0025] In a preferred embodiment, the additional acid, other than a carboxylic acid, has an acid value of more than 10 mg KOH / g.

[0026] Preferably, the additional acid, other than a carboxylic acid, is a dihydrogen phosphate, a hydrogen phosphate, a sulfonic acid or a combination thereof. The dihydrogen phosphate, the hydrogen phosphate and the sulfonic acid have the following structures:

[0027] ° ° o P

[0028] HO-P-OH HO-P-OR s

[0029] OR ORR' '0Hdihydrogen phosphate hydrogen phosphate sulfonic acid wherein R is preferably carboxyl, alkyl, cycloalkyl, alkenyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms, further substituted by heteroatoms and / or terminated by hydroxyl. Non-limiting examples of R include carboxyl, C1-18 alkyl, C3-18 cycloalkyl, C2-18 alkenyl, CB-IO aryl and combinations thereof, such as Cs- aryl- or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, further substituted by 1-10 heteroatoms and / or terminated by hydroxyl, with typical heteroatoms being oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. R may also be a polymer chain such as a polyether, polyester or polyurethane chain.

[0030] In a preferred embodiment, the additional acid, other than a carboxylic acid, is a dihydrogen phosphate resin, a hydrogen phosphate resin or a combination thereof.

[0031] By resin is meant that the substituents of the dihydrogen phosphate resin, the hydrogen phosphate resin or a combination thereof are polymer chains. The dihydrogen phosphate resin, the hydrogen phosphate resin or a combination thereof preferably have a viscosity of 150 mPas or above at 25°C. Viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 s1.

[0032] An example of a preferred dihydrogen phosphate / hydrogen phosphate resin is SR9054 commercially available from Sartomer, Ebecryl 168 commercially available from Allnex and Ebecryl 170 commercially available from Allnex. SR9054 has the following structure:

[0033] Preferably, the additional acid, other than a carboxylic acid, is present in an amount of 1-10% by weight, more preferably 2-9% by weight, most preferably 3-8% by weight, based on the total weight of the printhead cleaning solution.

[0034] In a preferred embodiment, the total amount of all acid, including a carboxylic acid and any additional acid is present in a total amount of 1-10% by weight, more preferably 2-9% by weight, most preferably 3-8% by weight, based on the total weight of the printhead cleaning solution.

[0035] In a preferred embodiment, the carboxylic acid is the sole acid present in the printhead cleaning solution.

[0036] Accordingly, the printhead cleaning solution preferably comprises less than 5% by weight of additional acid, other than a carboxylic acid, based on the total weight of the printhead cleaning solution. Preferably, the printhead cleaning solution comprises less than 3% by weight of additional acid, other than a carboxylic acid, more preferably less than 2% by weight, more preferably less than 1 % by weight, and most preferably the printhead cleaning solution is substantially free of additional acid, other than a carboxylic acid, where the amounts are based on the total weight of the printhead cleaning solution.

[0037] By substantially free is meant that only small amounts will be present, for example some additional acid may be present as impurities in the components of the printhead cleaning solution, but such low levels are tolerated. In other words, no additional acid is intentionally added to the printhead cleaning solution. However, minor amounts of additional acid, which may be present as impurities in commercially available components, are tolerated. For example, the printhead cleaning solution may comprise less than 0.5% by weight of additional acid, more preferably less than 0.1 % by weight of additional acid, most preferably less than 0.05% by weight of additional acid, based on the total weight of the printhead cleaning solution. In a preferred embodiment, the printhead cleaning solution is free of additional acid. Accordingly, preferably the carboxylic acid is the sole acid present in the printhead cleaning solution. By additional acid is meant any acid, other than a carboxylic acid.

[0038] The printhead cleaning solution of the present invention comprises a solvent, wherein the solvent is an organic solvent having a log Poctanoi / water from -1 .00 to 1 .20. Therefore, the printhead cleaning solution of the present invention comprises a carboxylic acid and an organic solvent having a log Poctanoi / water from -1 .00 to 1 .20, wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution.

[0039] The solvent acts as a carrier for the carboxylic acid and dilutes it so that it removes residual ink from the printhead nozzle plate without damaging it.

[0040] The solvent is an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20. One or more organic solvents having a logPoctanoi / water from -1.00 to 1.20 may be present. If more than one organic solvent having a logPoctanoi / water from -1.00 to 1.20 is present, the two solvents must be miscible. The two solvents can be mixed together in any amounts as long as they remain miscible.

[0041] LogP is known in the art and a detailed description is not required. LogP is a measure of the polarity of a compound. The partition coefficient (P) is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium. Therefore, this ratio is a measure of the difference in solubility of the compound in these two phases. Both phases are usually solvents. The two solvents have different polarity and the most common solvents are water and 1 -octanol (octanol). Water is the polar solvent and octanol is the non-polar solvent.

[0042] LogP is the logarithm of the ratio of the concentrations of a solute between the two solvents, specifically for un-ionised solutes. Accordingly, logP value is a measure of the lipophilicity or hydrophilicity. When the two phases are water and octanol:

[0043] To measure the partition coefficient of ionisable solutes, the pH of the aqueous phase is adjusted such that the predominant form of the compound in solution is the un-ionised. A number of methods of measuring partition coefficients have been developed including the shake-flask, reverse phase HPLC and pH-metric techniques.

[0044] Partition coefficients are also widely available in the art and may also be theoretical and based on the structure of the compound. For example, partition coefficients may be calculated using software, such as ChemSketch, or online on websites, such as the Chemicalize website. The partition coefficients discussed herein are typically provided by the European Chemicals Agency (ECHA).

[0045] The logPoctanoi / water value defines the relative polarity of the organic solvent which may be used to dissolve the carboxylic acid in the present invention. Low logPoctanoi / water values, such as a log Poctanoi / water value of from ~1.00 to 1 .20, indicate a polar solvent. Accordingly, the printhead cleaning solution of the present invention comprises a polar solvent.

[0046] It has been surprisingly found that the polar solvent as claimed is required for an effective printhead cleaning solution. A printhead cleaning solution containing just a polar solvent may remove some residual ink from a printhead nozzle plate but not all. The addition of an carboxylic acid to the polar solvent as claimed and the dissolution of the carboxylic acid in the polar solvent as claimed, wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution, surprisingly provides an effective printhead cleaning solution which is capable of removing all residual ink from a printhead nozzle plate, without resorting to manual wiping.

[0047] The present invention therefore increases the efficiency of the cleaning process and hence the overall productivity of the printing process. The nozzle plate is also left clean, preventing misfiring nozzles, preventing nozzle clogging and ensuring a high quality image. The printhead cleaning solution of the present invention is particularly useful for maintaining nozzle plates containing an NWC. The NWC is non-polar and so the polar solvent of the printhead cleaning solution as claimed has a lower affinity for the NWC and thus can be removed from the NWC more easily.

[0048] In a preferred embodiment, the solvent is an organic solvent having a logPoctanoi / water from -0.90 to 1 .10 and most preferably from -0.80 to 1 .00.

[0049] The organic solvent having a logPoctanoi / water from -1 .00 to 1 .20 may be a single solvent or a mixture of two or more solvents. The organic solvent having a logPoctanoi / water from -1.00 to 1.20 can be selected from any organic solvent having a logPoctanoi / water from -1.00 to 1.20, such as isopropyl alcohol (IPA) having a log Poctanoi / water of 0.05, acetone having a log Poctanoi / water of -0.24, propylene carbonate having a log Poctanoi / water of -0.41 and y-butyrolactone having a logPoctanoi / water of -0.76.

[0050] Preferably, the organic solvent having a logPoctanoi / water from -1.00 to 1.20 is isopropyl alcohol, acetone, propylene carbonate, y-butyrolactone or a combination thereof. In a particularly preferred embodiment, the solvent having a logPoctanoi / water from -1 .00 to 1 .20 is propylene carbonate. The present inventors have found that the combination of propylene carbonate and a carboxylic acid is particularly preferred and provides easy removal of all residual ink from a printhead nozzle plate, without resorting to manual wiping. The solvent may further comprise a (meth)acrylate monomer having a logPoctanoi / water from -1 .00 to 1 .20, more preferably from -0.90 to 1.10 and most preferably from -0.80 to 1 .00.

[0051] (Meth)acrylate monomers are well known in the art and are preferably the esters of (meth)acrylic acid. A detailed description is therefore not required. Mixtures of (meth)acrylates may also be used.

[0052] As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers. Forthe avoidance of doubt, mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in a polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in a polymerisation reaction on curing.

[0053] Examples of (meth)acrylate monomers having a log Poctanoi / water from -1.00 to 1.20 include 2-(2- ethoxyethoxy) ethyl acrylate (EOEOEA) having a logPoctanoi / water of 1.151 and tetrahydrofurfuryl acrylate (THFA) having a logPoctanoi / water of 0.81.

[0054] In an alternative preferred embodiment, the solvent comprises an organic solvent having a log Poctanoi / water from -1 .00 to 1 .20 and a (meth)acrylate monomer having a logPoctanoi / water from -1 .00 to 1.20.

[0055] High logPoctanoi / water values, such as a log Poctanoi / water value of more than 1 .20, indicate a non-polar solvent. A printhead cleaning solution containing just a non-polar solvent is ineffective at removing residual ink from a printhead nozzle plate. However, unlike for a polar solvent, adding an acid to a non-polar solvent and dissolving the acid in the non-polar solvent does not improve the performance of the printhead cleaning solution. Instead, a printhead cleaning solution containing an acid dissolved in a non-polar solvent is equally ineffective at removing residual ink from a printhead nozzle plate.

[0056] Therefore, in a preferred embodiment, the printhead cleaning solution contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of a solvent having a log Poctanoi / water value of more than 1 .20, based on the total weight of the printhead cleaning solution.

[0057] By substantially free is meant that only small amounts will be present, for example as impurities in the components of the printhead cleaning solution. In other words, no solvent having a log Poctanoi / water value of more than 1.20 is intentionally added to the printhead cleaning solution. However, minor amounts of solvent having a logPoctanoi / water value of more than 1 .20, which may be present as impurities in the components of the printhead cleaning solution, are tolerated. For example, the printhead cleaning solution may comprise less than 0.5% by weight, more preferably less than 0.1 % by weight and most preferably less than 0.05% by weight of a solvent having a logPoctanoi / water value of more than 1 .20, based on the total weight of the printhead cleaning solution. In a preferred embodiment, the printhead cleaning solution is free of a solvent having a log Poctanoi / water value of more than 1 .20.

[0058] Examples of solvents having a logPoctanoi / water value of more than 1 .20 include butyl glycol acetate (BGA) having a logPoctanoi / water value of 1.51 , propylene glycol phenyl ether (PPh) having a logPoctanoi / water value of 1.41 and 3-methyl-1 ,5-pentanediol diacrylate (3-MPDA) having a log Poctanoi / water Value Of 2.76.

[0059] The printhead cleaning solution may be prepared by mixing the carboxylic acid with the organic solvent having a logPoctanoi / water from -1 .00 to 1 .20 and forming a solution.

[0060] The printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution.

[0061] The inventors of the present invention have found that printhead cleaning fluids containing water provide poor removal of residual ink. In this regard, the water present in the printhead cleaning fluids can be absorbed into the nozzle. The presence of this water can cause residual ink to gel over time, which leads to printhead nozzle clogging, which can be impossible to remove, even with manual cleaning. Further, if the printhead is recirculating, the water can be taken into the ink system and the entire ink delivery system can be negatively affected.

[0062] The inventors of the present invention have found however that a printhead cleaning solution of the present invention comprising a carboxylic acid in an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20, wherein there is less than 5% by weight of water present, based on the total weight of the printhead cleaning solution, provides a printhead cleaning solution that can remove all residual ink from a nozzle plate in an automated process, without the need for manual wiping of the nozzle plate. The nozzle plate is left clean, preventing misfiring nozzles, preventing nozzle clogging and ensuring a high quality image.

[0063] In a preferred embodiment, the printhead cleaning solution contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of water, based on the total weight of the printhead cleaning solution.

[0064] By substantially free is meant that only small amounts will be present, for example some water may be absorbed by the cleaning solution from the air and water may be present as impurities in the components of the printhead cleaning solution. In other words, no water is intentionally added to the printhead cleaning solution. However, minor amounts of water, which may be present as impurities in the components of the printhead cleaning solution, are tolerated. For example, the printhead cleaning solution may comprise less than 0.5% by weight, more preferably less than 0.1 % by weight and most preferably less than 0.05% by weight of water, based on the total weight of the printhead cleaning solution. In a preferred embodiment, the printhead cleaning solution is free of water.

[0065] In a preferred embodiment, the organic solvent having a log Poctanoi / water from -1.00 to 1.20 is the sole solvent present in the printhead cleaning solution.

[0066] Accordingly, the printhead cleaning solution preferably comprises less than 5% by weight of additional solvent, other than the organic solvent having a logPoctanoi / water from -1 .00 to 1 .20, based on the total weight of the printhead cleaning solution. Preferably, the printhead cleaning solution comprises less than 3% by weight of additional solvent, other than the organic solvent having a log Poctanoi / water from ~1.00 to 1.20, more preferably less than 2% by weight, more preferably less than 1 % by weight, and most preferably the printhead cleaning solution is substantially free of additional solvent, other than the organic solvent having a logPoctanoi / water from -1 .00 to 1 .20, where the amounts are based on the total weight of the printhead cleaning solution.

[0067] By substantially free is meant that only small amounts will be present, for example some additional solvent may be present as impurities in the components of the printhead cleaning solution, but such low levels are tolerated. In otherwords, no additional solvent is intentionally added to the printhead cleaning solution. However, minor amounts of additional solvent, which may be present as impurities in commercially available components, are tolerated. For example, the printhead cleaning solution may comprise less than 0.5% by weight of additional solvent, more preferably less than 0.1 % by weight of additional solid, most preferably less than 0.05% by weight of additional solid, based on the total weight of the printhead cleaning solution. In a preferred embodiment, the printhead cleaning solution is free of additional solvent. Accordingly, preferably the organic solvent having a logPoctanoi / water from -1.00 to 1.20 is the sole solvent present in the printhead cleaning solution. By additional solvent is meant any solvent, other than the organic solvent having a log Poctanoi / water from “1 .00 tO 1 .20.

[0068] The printhead cleaning solution preferably exhibits a desirable low viscosity. As such, the printhead cleaning solution preferably has a viscosity of less than 100 mPas, more preferably less than 50 mPas or less and most preferably less than 20 mPas at 25°C. Viscosity may be measured using a rotational viscometer fitted with a thermostatically controlled cup and spindle arrangement, running at 20 rpm at 25°C. The present invention also provides a method of removing ink from a printhead nozzle plate, the method comprising: (i) preparing a printhead cleaning solution of the present invention; and (ii) jetting the printhead cleaning solution onto the printhead nozzle plate to remove the ink.

[0069] The method of the present invention is a method of removing ink from a printhead nozzle plate. Preferably, the printhead nozzle plate comprises a non-wetting coating (NWC). The printhead cleaning solution of the present invention is particularly effective at removing residual ink from a printhead nozzle plate comprising an NWC without damaging it. The NWC is non-polar and so the polar solvent of the printhead cleaning solution has a lower affinity for the NWC and thus can be removed from the NWC more easily.

[0070] The method of the present invention comprises (i) preparing a printhead cleaning solution of the present invention. The printhead cleaning solution may be prepared by mixing the carboxylic acid with the organic solvent having a logPoctanoi / water from -1 .00 to 1 .20 and forming a solution.

[0071] The method of the present invention comprises (ii) jetting the printhead cleaning solution onto the printhead nozzle plate to remove the ink.

[0072] The printhead cleaning solution can be jetted onto the printhead nozzle plate by any means. For example, a container may be filled with the printhead cleaning solution and the printhead cleaning solution ejected onto the printhead nozzle plate in an automated process. The printhead cleaning solution may be jetted at a range of pressures and for any length of time.

[0073] The printhead cleaning solution removes residual ink from the printhead nozzle plate. The residual ink may be wet / uncured or dried / cured depending on the type of ink and conditions used. Dried / cured ink is harder to remove from the printhead nozzle plate than wet / uncured ink but the printhead cleaning solution is effective at removing both wet / uncured and dried / cured ink from the printhead nozzle plate.

[0074] It should be noted that the terms “dry” and “cure” are often used interchangeably in the art when referring to radiation-curable inks to mean the conversion of the ink from a liquid to solid by polymerisation and / or crosslinking of the radiation-curable material. Herein, however, by “drying” is meant the removal of the water by evaporation and by “curing” is meant the polymerisation and / or crosslinking of the radiation-curable material.

[0075] Preferably, the ink removed from the printhead nozzle plate is radiation-curable. The printhead cleaning solution is particularly effective at removing residual radiation-curable ink from the nozzle plate, without requiring manual wiping. The method of the present invention is effective at removing residual ink from the printhead nozzle plate without the need for manual wiping. The method of the present invention is therefore more efficient than cleaning processes using existing cleaning fluids. The printing process is also more productive because less time is spent manually wiping the printhead nozzle plate. The printhead nozzle plate, preferably comprising an NWC, is also protected from damage caused by manual wiping.

[0076] The removal of residual ink from the printhead nozzle plate prevents misfiring nozzles and hence facilitates the printing of a high quality image.

[0077] Further, as the printhead cleaning solution of the present invention comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution, less water is absorbed into the nozzle. This reduces the gelation of the residual ink over time which occurs in the nozzles when water is absorbed therein. Accordingly, printhead nozzle clogging is avoided, which can be impossible to remove, even with manual cleaning, and water is not taken into the ink system.

[0078] The used printhead cleaning solution can be removed by any means, for example using a vacuum system to suction the printhead cleaning solution or using a stream of compressed air to blow away the printhead cleaning solution. The method of the present invention preferably further comprises (iii) collecting the printhead cleaning solution. For example, the printhead cleaning solution can be collected using vacuum suction.

[0079] The invention will now be described with reference to the following examples, which are not intended to be limiting.

[0080] Examples

[0081] Example 1

[0082] Printhead cleaning solutions were prepared according to the formulations set out in Table 1 . The printhead cleaning solutions either contained just solvent (and hence are comparative printhead cleaning solutions) or 5% by weight of acid, based on the total weight of the printhead cleaning solution, was mixed with solvent (to provide printhead cleaning solutions of the present invention). Table 1 (printhead cleaning solutions)

[0083] IPA is isopropyl alcohol, BGA is butyl glycol acetate, PPh is propylene glycol phenyl ether and 3- MPDA is 3-methyl-1 ,5-pentanediol diacrylate. IPA, acetone, BGA, propylene carbonate, y- butyrolactone and PPh are all organic solvents. 3-MPDA is a difunctional (meth)acrylate monomer.

[0084] Salicylic acid and oxalic acid are carboxylic acids. PRO22099 is a carboxylic acid resin commercially available from Arkema. A drop of ink having the formulation set out in Table 2 was placed on a printhead nozzle plate material containing a non-wetting coating. The ink was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 2 (ink placed on nozzle plate)

[0085] 3-MPDA and DVE-3 are difunctional monomers, Genorad 16 is a stabiliser, UVP6600 is a multifunctional oligoamine resin, BAPO, Omnirad 2959 and Esacure KIP160 are photoinitiators and Byk-3762 is a surfactant.

[0086] The black pigment dispersion contains 40.0% of carbon black pigment, 29.1 % of DVE-3, 30.0% of a dispersant and 0.9% of a pigmentary synergist. The nozzle plate containing the ink was then placed at a 45° angle. 2 mL of a printhead cleaning solution of Table 1 was loaded into a pipette. The printhead cleaning solution was ejected from the pipette onto the ink-covered nozzle plate five times. The cleaning efficiency was recorded on a scale of 1-5 set out in Table 3 and the results are set out in Table 4. Table 3 (cleaning efficiency scale) Table 4 (printhead cleaning solution rating)

[0087] The results show that when a pure solvent is used (printhead cleaning solutions 1 , 4, 7, 11 , 14, 18 and 21), the ink can never be fully removed from the nozzle plate. The solvents with very low logPoctanoi / water values (printhead cleaning solutions 11 and 14) demonstrate some cleaning potential but do not fully remove the ink from the nozzle plate.

[0088] Although the addition of an acid to a solvent having a logPoctanoi / water higher than 1.20 may improve the cleaning performance, it does not result in printhead cleaning solutions that can fully clean the nozzle plate (printhead cleaning solutions 19 and 22).

[0089] From the results, it is clear that only the addition of an acid to a solvent having a log Poctanoi / water from -1 .00 to 1 .20 results in printhead cleaning solutions that can fully clean the nozzle plate (printhead cleaning solutions 2, 3, 5, 6, 12, 13 and 15-17).

Claims

Claims1 . A printhead cleaning solution comprising: a carboxylic acid; a solvent, wherein the solvent is an organic solvent having a logPoctanoi / water from -1 .00 to 1 .20; and wherein the printhead cleaning solution comprises less than 5% by weight of water, based on the total weight of the printhead cleaning solution.

2. A printhead cleaning solution as claimed in claim 1 , wherein the carboxylic acid has an acid value of more than 10 mg KOH / g.

3. A printhead cleaning solution according to claim 1 , wherein the carboxylic acid is salicylic acid, oxalic acid, a carboxylic acid resin, or a combination thereof.

4. A printhead cleaning solution as claimed in any preceding claim, wherein the carboxylic acid is present in an amount of 1 -10% by weight, based on the total weight of the printhead cleaning solution.

5. A printhead cleaning solution as claimed in any preceding claim, wherein the carboxylic acid is the sole acid present in the printhead cleaning solution.

6. A printhead cleaning solution as claimed in any preceding claim, wherein the solvent has a logPoctanoi / water from ~0.90 to 1 .10, preferably from -0.80 to 1 .00.

7. A printhead cleaning solution as claimed in any preceding claim, wherein the solvent having a logPoctanoi / water from -1.00 to 1.20 is isopropyl alcohol, acetone, propylene carbonate, y- butyrolactone or a combination thereof, preferably propylene carbonate.

8. A printhead cleaning solution as claimed in any preceding claim, wherein the printhead cleaning solution contains less than 2% by weight, preferably less than 1 % by weight, more preferably is substantially free of water, based on the total weight of the printhead cleaning solution.

9. A printhead cleaning solution as claimed in any preceding claim, wherein the organic solvent having a logPoctanoi / water from -1.00 to 1.20 is the sole solvent present in the printhead cleaning solution.

10. A method of removing ink from a printhead nozzle plate, the method comprising:(i) preparing a printhead cleaning solution as claimed in any preceding claim; and(ii) jetting the printhead cleaning solution onto the printhead nozzle plate to remove the ink.

11. A method as claimed in claim 10, wherein the printhead nozzle plate comprises a nonwetting coating.

12. A method as claimed in claims 10 or 11 , wherein the ink is radiation-curable.

13. A method as claimed in any of claims 10-12, wherein the method further comprises: (iii) collecting the printhead cleaning solution.

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