Inkjet printer and printing method

Abstract

To provide a thermal inkjet type inkjet printer which can inhibit occurrence of kogation, and to provide a printing method using the inkjet printer.SOLUTION: The present invention relates to a thermal inkjet type inkjet printer having an ink passage from an ink tank to a head and including a hollow fiber deaeration module capable of deaerating in the ink passage. The present invention also relates to a printing method in which the inkjet printer is used to make prints while deaerating the ink with the hollow fiber deaeration module.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thermal inkjet type inkjet printer and a printing method using the printer. [Background technology]

[0002] Inkjet printing is a method of printing by ejecting tiny droplets of ink from an inkjet recording head and depositing them on a target recording medium. The advantages of the inkjet method are that its mechanism is relatively simple, it is inexpensive, and it can form high-definition, high-quality images.

[0003] There are various types of inkjet recording methods, but the on-demand recording methods that have become mainstream in recent years are classified into the so-called piezo method, which uses a piezo element, and the thermal inkjet method, which uses thermal energy. The thermal inkjet method employs a method in which the solvent in the ink is boiled by heating the ink to approximately 300°C, which is far above the boiling point of the solvent, and the ink is ejected from the nozzle using the high pressure generated at that time. The thermal inkjet method has a large ink droplet ejection force, so ejection failure of the ink droplets in the ejection nozzle is unlikely to occur, but because heat is applied, polymer particles contained in the ink droplets are likely to adhere to the heater part of the thermal inkjet recording head, causing scorching, a phenomenon known as kogation.

[0004] In response to this, a polymer particle that suppresses scorching has been proposed (see Patent Document 1), in which a volatile base is used to excessively neutralize the anionic groups of the polymer for the methacrylic acid ester-derived structural units that make up the polymer particles during dispersion processing of the polymer particles and pigment in the ink droplets, and the volatile base is then removed after dispersion processing; however, this does not necessarily prevent the occurrence of kogation.

[0005] Furthermore, an ink for a thermal inkjet type inkjet printer has been proposed (Patent Document 2), which contains at least two types of ink compositions and a reaction liquid that reacts with the ink compositions to cause agglomeration, and which has excellent color balance and can form high-quality images by making the particle size of the pigment in the ink approximately the same before and after the reaction. However, even in this case, the occurrence of kogation could not be sufficiently suppressed.

[0006] A method has been proposed in which pigment-containing polymer particles are made of specific acrylic acid ester units and acrylic acid units having a carboxyl group that generates a negative charge, thereby generating electrostatic repulsion between the polymer particles and the negatively charged heater portion, thereby suppressing the deposition of polymer particles on the heater portion and suppressing kogation (see Patent Document 3). However, even in this case, it was not always possible to suppress the occurrence of kogation. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2010-138297 A [Patent Document 2] JP 2004-107453 A [Patent Document 3] JP 2014-218078 A Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a thermal inkjet type inkjet printer and a printing method using the same, which are capable of suppressing the occurrence of kogation. [Means for solving the problem]

[0009] The inventors of the present application further investigated in detail the mechanism by which kogation occurs in thermal inkjet type inkjet printers, and discovered that when the ink is rapidly heated in the heater portion of the thermal inkjet recording head, oxygen molecules in the ink oxidize the polymer particles, and therefore, that kogation can be suppressed by degassing the ink to reduce the dissolved oxygen concentration, which led to the solution of the present invention.

[0010] That is, the present invention relates to a thermal inkjet type inkjet printer having an ink path from an ink tank to a head, The inkjet printer is characterized in that the ink path is provided with a hollow fiber degassing module capable of degassing.

[0011] Furthermore, the present invention provides a printing method using a thermal inkjet type inkjet printer having an ink path from an ink tank to a head, comprising: The inkjet printer includes a hollow fiber degassing module in the ink path, the hollow fiber degassing module being capable of degassing the ink. The present invention relates to a printing method, characterized in that printing is carried out while degassing ink using the hollow fiber degassing module. Effect of the Invention

[0012] According to the present invention, it is possible to provide a thermal inkjet type inkjet printer and a printing method using the same, which are capable of suppressing the occurrence of kogation. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a schematic diagram of a degassing module. [Diagram 2] FIG. 1 is a layout diagram of the degassing module during degassing. [Diagram 3] FIG. 2 is a diagram showing an example of the configuration from an ink tank to a head according to the present invention. [Figure 4]FIG. 2 is a schematic diagram showing an example of an ink circulation path of the present invention. [Diagram 5] 3A to 3C are schematic diagrams showing examples of various heads. [Figure 6] FIG. 2 is a schematic diagram showing an example of the inside of a thermal inkjet head. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The inkjet printer of the present invention is a thermal inkjet type (thermal jet type) inkjet printer having an ink path from an ink tank to a head, and is characterized in that the inkjet printer is provided with a hollow fiber degassing module capable of degassing in the ink path.

[0015] Furthermore, the printing method of the present invention is a printing method using an inkjet printer of a thermal inkjet system having an ink path from an ink tank to a head, wherein the inkjet printer is provided with a hollow fiber degassing module capable of degassing in the ink path, and printing is performed while degassing the ink by the hollow fiber degassing module.

[0016] The inkjet printer of the present invention is not particularly limited as long as it has a conventionally known structure. For example, the inkjet printer of the present invention is an inkjet printer having an ink path from an ink tank to a head (thermal inkjet type recording head), and preferably comprises a hollow fiber degassing module capable of degassing ink midway along the ink path, a liquid delivery pump capable of adjusting the ink pressure at the ink inlet of the external reflux type hollow fiber degassing module during degassing, and an image recording means for ejecting ink using a thermal inkjet recording head to record an image.

[0017] The hollow fiber degassing module used in the present invention may be either an external reflux type or an internal reflux type, but an external reflux type is preferred because it allows the degassing process to be carried out under conditions where the pressure loss in the hollow fiber degassing module during degassing is low.

[0018] Hereinafter, an external reflux type hollow fiber degassing module will be described based on the schematic diagram shown in FIG. 1. As shown in FIG. 1, the external reflux type hollow fiber degassing module performs degassing by flowing ink 5 from the ink inlet 2 to the outside of the hollow fiber and reducing the pressure inside the hollow fiber 1 to a vacuum. Even if the ink has a high viscosity, it can be degassed without reducing the pressure loss during degassing and without reducing the efficiency by flowing the ink through a thick flow path and reducing the pressure inside the thin hollow fiber, and it is possible to degas the ink efficiently without impairing the dispersion stability of the pigment ink. Here, 3 is the ink outlet and 4 is a vacuum pump. As such an external reflux type hollow fiber degassing module, commercially available ones can be used, for example, SEPAREL EF-002A-P and SEPAREL EF-004P by DIC Corporation. FIG. 1(b) shows a schematic diagram of the state in which dissolved air in the ink 5 passes through the hollow membrane 6 and moves into the hollow fiber 1, which is reduced in pressure. FIG. 1(c) shows a schematic cross section of three hollow fibers in a direction perpendicular to the longitudinal direction.

[0019] In FIG. 1(c), it is preferable that the outer diameter 61 of the hollow fiber is 150 μm or more and 250 μm or less, and the inner diameter 62 is 50 μm or more and 180 μm or less. If the outer diameter 61 of the hollow fiber is less than 150 μm, it is difficult to manufacture in terms of strength. On the other hand, if the outer diameter 61 of the hollow fiber exceeds 250 μm, the specific surface area of ​​the hollow fiber becomes small, and the degassing efficiency tends to decrease. Also, if the inner diameter 62 is less than 50 μm, the degassing efficiency tends to decrease. On the other hand, if the inner diameter 62 exceeds 180 μm, it tends to be difficult to manufacture in terms of strength. Note that the thickness of the hollow fiber is preferably 30 μm or more from the viewpoint of strength. Here, the diameter refers to the diameter when the cross section is circular, and when the cross section is not circular, the diameter refers to the diameter when the cross section is replaced with a circle of the same area. It can be measured by observing the cross section with an electron microscope.

[0020] For convenience, Fig. 1(c) shows only a configuration for three hollow fibers, but the external reflux type hollow fiber degassing module of this embodiment has a configuration in which a large number of hollow fibers are arranged as shown in Fig. 1(a). Here, in the present invention, the number of hollow fibers in the hollow fiber degassing module may be any number and is not limited to this embodiment. In addition, when there are multiple hollow fibers, it is preferable that the inner diameter and outer diameter of all the hollow fibers fall within the above-mentioned range.

[0021] On the other hand, although not shown, an internal circulation type hollow fiber degassing module can perform degassing by flowing ink inside hollow fibers having an inner diameter of 100 μm to 200 μm and reducing the pressure or creating a vacuum outside the hollow fibers. Commercially available internal circulation type hollow fiber degassing modules can be used, such as SEPAREL PF-001D and SEPAREL PF-004D from DIC Corporation.

[0022] In both the internal reflux type and external reflux type, various resins are used as the material for the hollow fibers, particularly polyolefin resins such as polypropylene and poly-4-methylpentene 1 resin, and fluororesins such as polytetrafluoroethylene resin. These are preferred because their degassing performance does not deteriorate easily even when a large amount of ink is processed, and the surfactants and dispersants contained in the ink are unlikely to adhere to the outer surface of the hollow fibers, so that the ink components are unlikely to change.

[0023] The inkjet printer used in the present invention is preferably equipped with a liquid feed pump capable of adjusting the ink pressure at the ink inlet of the hollow fiber degassing module during degassing.

[0024] In the external reflux type, the pressure loss of the hollow fiber degassing module during degassing is preferably 0.2 MPa or less, more preferably 0.1 MPa or less, and even more preferably 0.05 MPa or less. On the other hand, in the internal reflux type, the pressure loss of the hollow fiber degassing module during degassing is preferably 1.0 MPa or less, more preferably 0.5 MPa or less, and even more preferably 0.025 MPa or less. In the present invention, the pressure loss of the hollow fiber degassing module during degassing means the pressure applied to the inlet of the hollow fiber degassing module, and can be measured by a pressure gauge inserted upstream of the hollow fiber degassing module.

[0025] The degree of deaeration in the present invention can be measured by the dissolved oxygen concentration. The dissolved oxygen concentration can be measured, for example, by the Ostwald method (see Experimental Chemistry Lecture 1 Basic Operations [I], p. 241, 1975, Maruzen), by mass spectrometry, by a simple oxygen concentration meter such as a galvanic cell type or a polarograph type, or by colorimetric analysis. The dissolved oxygen concentration can also be easily measured by using a commercially available dissolved oxygen concentration meter (DO-30A type, manufactured by Toa Denpa Kogyo Co., Ltd.).

[0026] The degree of degassing of the ink in the present invention is preferably 8.0 ppm or less, more preferably 6.0 ppm or less, even more preferably 4.0 ppm or less, and particularly preferably 2.0 ppm or less, in terms of dissolved oxygen concentration, in order to prevent the occurrence of cavitation and to prevent graininess during printing, and the lower limit is not particularly limited, but is preferably 0 ppm or more, and more preferably 1 ppb or more.

[0027] The hollow fiber degassing module may be placed anywhere between the ink tank and the head, but since the ink flow path has many air supply points such as thin resin pipes and joints, it is preferable to place it as close to the head as possible. An example of the configuration from the ink tank to the head is shown in Figure 3, but the configuration of the present invention is not limited to this.

[0028] In FIG. 3, ink 5 filled in an ink tank 8 is sent to a head 12 by a liquid feed pump 9, and on the way there, the ink passes through a filter 100 and is sent to the hollow fiber degassing module 10 shown in FIG. 1, which is connected to a vacuum pump 4. After being degassed here, the pressure is adjusted by a back pressure regulator 11, and the ink passes through the filter 100 and reaches the head 12.

[0029] In addition, the ink tank in this invention refers to a place where ink is stored, and when the ink is distributed through a bottle and only the liquid is poured into the printer, it is a reserve tank, and when the ink is distributed through an ink cartridge or bottle and the cartridge or bottle itself serves as an ink tank, it refers to the above.

[0030] Next, an inkjet printer having an ink circulation path that returns the inkjet ink discharged from the head to the head again, and a degassing step using the above-mentioned inkjet ink degassing method, midway along the ink circulation path, will be described.

[0031] 4 shows an example of the configuration of the ink circulation path, but needless to say, the present invention is not limited to this. When the ink 5 is introduced, the solenoid valve A is opened and the solenoid valve B is closed, and the ink is filled into the head 12 from the ink tank 8 through the filter 100 by the liquid supply pump 9, through the intermediate tank 30, the back pressure regulator 11, and the filter 100. At this time, the dissolved air in the ink 5 is degassed by the degassing module 10 shown in FIG. 1, which is arranged in the middle and connected to the vacuum pump 4. During head maintenance, the head cap 7 is tightly attached to the head 12, and then the liquid supply pump 9 is used to reduce the pressure inside the cap, and the ink is sucked out from the head 12 or the ink is ejected from the head 12 to the head cap 7. At this time, the solenoid valve A is closed and the solenoid valve B is opened at the same time, so that the ink in the head cap is degassed again by the degassing module 10 and then sent to the head 12, where it is used for ejection again.

[0032] Here, FIG. 4 shows an example in which ink is circulated from the head cap, but the present invention is not limited to the head cap, and ink may be circulated from, for example, a common ink chamber of the head or a gutter for discharging ink.

[0033] The degassing module may be located anywhere between the ink tank and the head, but since the ink flow path has many air supply points such as thin resin pipes and joints, it is preferable to locate it as close to the head as possible.

[0034] In an inkjet printer having an ink circulation path that returns the inkjet ink discharged from the head of the present invention to the head again, an inkjet printer having a degassing process using the above-mentioned inkjet ink degassing method in the middle of the ink circulation path enables inkjet recording with excellent ejection stability and high reliability, and with minimal waste of ink other than that used for recording. The present invention is particularly suitable for printing in industrial printing applications.

[0035] The shape of the head 12 in the above embodiment is not particularly limited, but several examples are shown in FIG.

[0036] In addition to a single head 12 equipped with multiple nozzles 41 as shown in (a) of Fig. 5, a head unit can be used in which multiple heads 12 are connected in the nozzle arrangement direction to form a single head as shown in (b) of Fig. 5. In particular, the form shown in (b) of Fig. 5 is particularly effective in the case of the line head described above.

[0037] As shown in FIG. 6, the head is equipped with a nozzle 41, a heater 51, and an ink supply port, and has a mechanism for boiling the solvent in the ink and ejecting the ink from the nozzle by utilizing the high pressure generated at that time.

[0038] In the thermal ink jet recording method of the present invention, the following conditions are preferably mentioned as the ink ejection conditions.

[0039] From the viewpoint of the efficiency of the thermal inkjet recording method and of significantly exerting the effects of the present invention, the surface temperature of the resistance element of the heater is preferably 150°C or more, more preferably 200°C or more, even more preferably 250°C or more, and is preferably 800°C or less, more preferably 700°C or less, even more preferably 600°C or less.

[0040] From the viewpoint of the efficiency of the thermal inkjet recording method and of significantly exhibiting the effects of the present invention, the applied voltage is preferably 1 V or more, more preferably 3 V or more, even more preferably 5 V or more, and is preferably 25 V or less, more preferably 20 V or less, even more preferably 18 V or less.

[0041] From the viewpoint of suppressing the occurrence of kogation, the driving frequency is preferably 0.1 kHz or more, more preferably 1 kHz or more, even more preferably 3 kHz or more, and is preferably 20 kHz or less, more preferably 15 kHz or less, even more preferably 10 kHz or less.

[0042] From the viewpoint of suppressing the occurrence of kogation, the pulse width is preferably 0.01 μs or more, more preferably 0.1 μs or more, even more preferably 0.3 μs or more, and preferably 3.0 μs or less, more preferably 2.0 μs or less, even more preferably 1.5 μs or less.

[0043] The amount of ink ejected droplets is preferably 0.5 pL or more, more preferably 1.0 pL or more, even more preferably 1.5 pL or more, and even more preferably 3.0 pL or more per droplet, from the viewpoints of avoiding the influence of air currents and maintaining the accuracy of the landing position of the ink droplets. Also, when printing small characters, from the viewpoints of avoiding the risk of the characters being blurred due to thickening and improving the obtained image quality, the amount is preferably 30 pL or less, more preferably 10 pL or less, and even more preferably 5.0 pL or less.

[0044] The average ejection speed of the ink droplets is preferably 6 m / s or more, more preferably 8 m / s or more, even more preferably 10 m / s or more, and is preferably 20 m / s or less, more preferably 18 m / s or less, even more preferably 15 m / s or less.

[0045] Next, the ink-jet ink used in the present invention will be described. The ink-jet ink used in the present invention is not particularly limited as long as it is an ink-jet ink used in a thermal ink-jet system, and examples thereof include an aqueous dispersion containing polymer particles containing a pigment.

[0046] The pigment used in the present invention is not particularly limited as long as it is usable in ink-jet inks, and may be, for example, any of known inorganic and organic pigments. If necessary, these pigments may be used in combination with an extender pigment.

[0047] Examples of inorganic pigments include carbon black, metal oxides, metal sulfides, and metal chlorides. Of these, carbon black is particularly preferred for black water-based inks. Examples of carbon black include furnace black, thermal lamp black, acetylene black, and channel black.

[0048] Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxane pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments. Specific examples of preferred organic pigments include one or more products selected from the group consisting of CI Pigment Yellow, CI Pigment Red, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of extender pigments include silica, calcium carbonate, talc, etc.

[0049] As the pigment, a so-called self-dispersing pigment can also be used. A self-dispersing pigment means a pigment that can be dispersed in an aqueous medium without using a surfactant or resin by bonding one or more anionic hydrophilic groups or cationic hydrophilic groups to the surface of the pigment directly or via other atomic groups. Here, examples of the anionic hydrophilic group include, in particular, a carboxy group (-COOM1), a sulfonic acid group (-SO3M 1 ) is preferable (wherein, M 1 is a hydrogen atom, an alkali metal, or ammonium), and the cationic hydrophilic group is preferably a quaternary ammonium group. The above pigments can be used alone or in combination of two or more.

[0050] <Polymer particles containing pigment> (Polymers Constituting Polymer Particles) The polymer constituting the polymer particles used in the present invention may be a known polymer, but is preferably one having a carboxy group that has an electrostatic repulsive force with the heater portion, which is negatively charged. For example, a copolymer having a structural unit derived from an acrylic acid ester and a structural unit derived from acrylic acid is included. Further, a copolymer in which, among all the structural units of the polymer, structural units derived from an acrylic acid ester having a hydrocarbon group having 3 to 8 carbon atoms account for 78% by mass to 88% by mass, and structural units derived from acrylic acid account for 22% by mass to 12% by mass.

[0051] The content of the structural units derived from the acrylic ester in all the structural units of the polymer is preferably 79 mass % or more, more preferably 80 mass % or more, and preferably 87 mass % or less, more preferably 86 mass % or less.

[0052] The content of structural units derived from acrylic acid in all structural units of the polymer is preferably 21 mass % or less, more preferably 20 mass % or less, and preferably 13 mass % or more, more preferably 14 mass % or more.

[0053] Examples of the hydrocarbon group having 3 to 8 carbon atoms as a substituent in the acrylic ester include an alkyl group and an aromatic group.

[0054] From the viewpoint of suppressing the occurrence of kogation, the number of carbon atoms in the alkyl group is preferably 4 or more and preferably 6 or less, more preferably 5 or less.

[0055] Specific examples of acrylic acid esters having an alkyl group include (iso)propyl acrylate, butyl acrylate, (iso- or tert-)butyl acrylate, (iso)amyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, (iso)octyl acrylate, etc., with butyl acrylate being preferred. Note that "(iso)" refers to both the case where this group is present and the case where this group is not present, and refers to normal when this group is not present.

[0056] The aromatic group preferably has 6 or more carbon atoms and 7 or less carbon atoms, from the viewpoint of suppressing the occurrence of kogation.

[0057] Specific examples of the acrylic acid ester having an aromatic hydrocarbon group include benzyl acrylate and phenoxyethyl acrylate, with benzyl acrylate being preferred.

[0058] The polymer used in the present invention may contain structural units other than the structural units derived from the acrylic acid ester and the structural units derived from acrylic acid within a range that does not impair the effects of the present invention, but it is preferable that the polymer is substantially composed of the structural units derived from the acrylic acid ester and the structural units derived from acrylic acid and does not contain other structural units. That is, the total amount of the structural units derived from the acrylic acid ester and the structural units derived from acrylic acid is preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 100% by mass of the total structural units of the polymer.

[0059] (Polymer manufacturing) The polymer used in the present invention can be produced by copolymerizing a monomer mixture consisting essentially of an acrylic acid ester having a hydrocarbon group having from 3 to 8 carbon atoms and acrylic acid by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these polymerization methods, the solution polymerization method is preferred.

[0060] The solvent used in the solution polymerization method is not particularly limited, but a polar organic solvent is preferred. When the polar organic solvent is miscible with water, it can be mixed with water for use. Examples of the polar organic solvent include aliphatic alcohols having 1 to 3 carbon atoms, such as methanol, ethanol, and propanol; ketones, such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; and esters, such as ethyl acetate. Among these, methanol, ethanol, acetone, methyl ethyl ketone, and methyl isobutyl ketone, or a mixed solvent of one or more of these with water is preferred.

[0061] In the polymerization, known radical polymerization initiators such as azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile) and organic peroxides such as t-butyl peroxyoctoate and dibenzoyl oxide can be used. do.

[0062] The amount of the radical polymerization initiator is, per mole of the monomer mixture, preferably 0.001 mole or more, more preferably 0.002 mole or more, even more preferably 0.004 mole or more, and preferably 2 moles or less, more preferably 1 mole or less, even more preferably 0.1 mole or less, still more preferably 0.05 moles or less, even more preferably 0.01 mole or less.

[0063] During the polymerization, a known polymerization chain transfer agent such as a mercaptan such as octyl mercaptan or 2-mercaptoethanol, or a thiuram disulfide may be further added.

[0064] The polymerization conditions of the monomer mixture cannot be determined in general because they vary depending on the type of radical polymerization initiator, monomer, solvent, etc. used. Usually, the polymerization temperature is preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 50°C or higher, and preferably 100°C or lower, more preferably 80°C or lower, and even more preferably 70°C or lower. The polymerization time is preferably 30 minutes or more, more preferably 1 hour or more, and even more preferably 2 hours or more, and preferably 20 hours or lower, more preferably 15 hours or lower, and even more preferably 10 hours or lower. The polymerization time is preferably 1 to 20 hours, and more preferably 2 to 10 hours.

[0065] The polymerization atmosphere is preferably an atmosphere of an inert gas such as nitrogen gas or argon. After the polymerization reaction is completed, the produced polymer can be isolated from the reaction solution by a known method such as reprecipitation, solvent distillation, etc. The obtained polymer can be purified by removing unreacted monomers, etc., by repeated reprecipitation, membrane separation, chromatography, extraction, etc.

[0066] (Weight average molecular weight of polymer) From the viewpoints of print density, gloss, and pigment dispersion stability of the resulting print, the weight average molecular weight of the polymer is preferably 5,000 or more, more preferably 10,000 or more, even more preferably 20,000 or more, still more preferably 30,000 or more, still more preferably 50,000 or more, and is preferably 400,000 or less, more preferably 300,000 or less, still more preferably 200,000 or less, still more preferably 100,000 or less, still more preferably 90,000 or less. The weight average molecular weight of the polymer can be measured by the method described in the Examples.

[0067] (Polymer Neutralization) The polymer is preferably neutralized with a neutralizing agent before use. When neutralizing with a neutralizing agent, the degree of neutralization is not particularly limited. In general, the pH of the finally obtained aqueous dispersion is preferably, for example, 4.5 or more and 10 or less. The pH can also be determined depending on the desired degree of neutralization of the polymer.

[0068] Examples of the neutralizing agent include bases such as lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonia, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, triethanolamine, tributylamine, etc. Among these, one or more selected from sodium hydroxide and ammonia are preferred.

[0069] The degree of neutralization of the salt-forming groups is preferably 210% or more, more preferably 230% or more, even more preferably 250% or more, and preferably 500% or less, more preferably 450% or less, even more preferably 400% or less.

[0070] The degree of neutralization can be calculated by the following formula: {[Weight of neutralizing agent (g) / Equivalent weight of neutralizing agent] / [Acid value of polymer (KOHmg / g) × Weight of polymer (g) / (56 × 1000)]} × 100

[0071] The acid value can be calculated from the constituent units of the polymer, or it can be determined by a method of dissolving the polymer in an appropriate solvent (e.g., methyl ethyl ketone) and titrating the polymer.

[0072] (Preparation of Water Dispersion of Pigment-Containing Polymer Particles) In the present invention, the polymer particles are used in the form of pigment-containing polymer particles in order to stably disperse the pigment. The method for producing the aqueous dispersion of the pigment-containing polymer particles is not particularly limited, but can be efficiently carried out by a method having the following steps (I) to (III).

[0073] Step (I): A step of obtaining a mixture containing a polymer, an organic solvent, a pigment, water, and, if necessary, a neutralizing agent. Step (II): A step of dispersing the mixture obtained in step (I) to obtain a dispersion of polymer particles containing a pigment. Step (III): A step of removing the organic solvent from the dispersion obtained in step (II) to obtain an aqueous dispersion of polymer particles containing a pigment.

[0074] In step (I), the polymer is first dissolved in an organic solvent, and then a pigment, water, and optionally a neutralizer, a surfactant, etc. are added to the resulting organic solvent solution and mixed to obtain an oil-in-water dispersion. In the resulting mixture, the pigment is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 20% by mass or less.

[0075] The organic solvent is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 70% by mass or less, more preferably 50% by mass or less, even more preferably 30% by mass or less.

[0076] The polymer content is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and is preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less.

[0077] The water content is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 40% by mass or more, and preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 65% ​​by mass or less.

[0078] As described above, the polymer is preferably neutralized with a neutralizing agent before use, but the polymer may be neutralized in advance.

[0079] Examples of the organic solvent include alcohol-based solvents such as ethanol, isopropanol, and isobutanol, ketone-based solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and diethyl ketone, and ether-based solvents such as dibutyl ether, tetrahydrofuran, and dioxane. The organic solvent has a dissolution amount in 100 g of water at 20° C. of preferably 5 g or more, more preferably 10 g or more, and even more preferably 15 g or more, and is preferably 50 g or less, more preferably 35 g or less, and even more preferably 30 g or less, and methyl ethyl ketone and methyl isobutyl ketone are particularly preferred. The organic solvent has a dissolution amount in 100 g of water at 20° C. of preferably 5 to 80 g, more preferably 10 to 50 g.

[0080] The method for dispersing the mixture in step (II) is not particularly limited. Although the polymer particles can be finely divided to a desired average particle size only by the main dispersion, it is preferable to pre-disperse the polymer particles, and then further apply shear stress to carry out the main dispersion, thereby controlling the average particle size of the polymer particles to a desired particle size.

[0081] The dispersion temperature in step (II) is preferably 5°C or higher, more preferably 7°C or higher, even more preferably 10°C or higher, and preferably 50°C or lower, more preferably 35°C or lower, even more preferably 25°C or lower.

[0082] When pre-dispersing the mixture, a commonly used mixing and stirring device such as an anchor blade can be used. Among the mixing and stirring devices, high-speed mixing and stirring devices such as Ultra Disper (trade name, Asada Iron & Steel Co., Ltd.), Ebara Milder (trade name, Ebara Corporation), and TK Homo Mixer (trade name, Primix Corporation) are preferred.

[0083] Examples of means for applying shear stress to the dispersion include kneading machines such as roll mills, bead mills, kneaders, and extruders, high-pressure homogenizers (Izumifu Food Machinery Co., Ltd., product name), homovalve-type high-pressure homogenizers such as Minilab 8.3H type (Rannie Co., Ltd., product name), and chamber-type high-pressure homogenizers such as Microfluidizer (Microfluidics Co., Ltd., product name) and Nanomizer (Nanomizer Co., Ltd., product name). A plurality of these devices can also be combined. Of these, high-pressure homogenizers are preferred from the viewpoint of reducing the particle size of the pigment.

[0084] Also, an ultrasonic homogenizer can be used. For example, the ultrasonic homogenizer has a frequency of 20 kHz or more and 2000 kHz or less, and a wattage per liter of the total reaction liquid volume is preferably 20 W or more, more preferably 35 W or more, and even more preferably 50 W or more, and preferably 1000 W or less, more preferably 900 W or less, and even more preferably 800 W or less. The wattage per liter of the total reaction liquid volume is preferably 20 to 1000 W, more preferably 50 to 800 W. Such ultrasonic dispersion machines are commercially available from Nippon Seiki Seisakusho Co., Ltd., Alex Co., Ltd., and the like. A plurality of these devices can also be combined.

[0085] In step (III), the organic solvent is removed from the dispersion obtained in step (II) by a known method to obtain an aqueous dispersion of polymer particles containing a pigment. The organic solvent in the obtained aqueous dispersion containing polymer particles is substantially removed, and the amount of the organic solvent is preferably 0.5% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less.

[0086] As described above, the aqueous dispersion obtained in the step (III) is a dispersion in which the pigment-containing polymer particles are dispersed in water as the main medium.

[0087] The average particle size of the pigment-containing polymer particles in the obtained aqueous dispersion is, from the viewpoints of dispersion stability and ejection property, preferably 50 nm or more, more preferably 70 nm or more, even more preferably 90 nm or more, and preferably 200 nm or less, more preferably 170 nm or less, even more preferably 150 nm or less, and still more preferably 120 nm or less.

[0088] The average particle size of the pigment-containing polymer particles can be measured by the method described in the Examples.

[0089] [Water dispersion for thermal ink-jet printing] The contents of the respective components in the water dispersion for thermal ink-jet printing of the present invention are as follows.

[0090] From the viewpoint of increasing the print density, the content of the pigment contained in the aqueous dispersion of the present invention is preferably 3% by mass or more, more preferably 6% by mass or more, even more preferably 9% by mass or more, still more preferably 12% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and still more preferably 20% by mass or less.

[0091] In the aqueous dispersion of the present invention, the mass ratio of the amount of pigment to the amount of polymer [pigment / polymer] is, from the viewpoint of dispersion stability, preferably 1.0 or more, more preferably 1.5 or more, even more preferably 1.7 or more, and is preferably 9.0 or less, more preferably 6.0 or less, even more preferably 5.0 or less, still more preferably 4.0 or less, and even more preferably 2.5 or less.

[0092] In the aqueous dispersion of the present invention, the molar amount of carboxyl groups of the polymer relative to the mass of the pigment [-COOH (mmol) / pigment (g)] is, from the viewpoint of improving the discharge rate, preferably 0.65 mmol / g or more, more preferably 0.70 mmol / g or more, even more preferably 0.80 mmol / g or more, and is preferably 1.40 mmol / g or less, more preferably 1.30 mmol / g or less, even more preferably 1.25 mmol / g or less.

[0093] The water content in the aqueous dispersion of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more. % by mass or more, more preferably 55% by mass or more, even more preferably 70% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less.

[0094] The surface tension (20°C) of the aqueous dispersion of the present invention is preferably 30 mN / m or more, more preferably 33 mN / m or more, even more preferably 35 mN / m or more, and is preferably 65 mN / m or less, more preferably 63 mN / m or less, even more preferably 60 mN / m or less.

[0095] The viscosity (20°C) of 20% by mass (solid content) of the aqueous dispersion of the present invention is preferably 1.5 mPa·s or more, more preferably 2 mPa·s or more, even more preferably 3 mPa·s or more, in order to provide a preferred viscosity when made into an aqueous ink, and is preferably 7 mPa·s or less, more preferably 6 mPa·s or less, even more preferably 5 mPa·s or less.

[0096] The viscosity can be measured by the method described in the examples. The aqueous dispersion of the present invention can be used as it is as an aqueous ink, but if necessary, it can be further adjusted by adding thereto a wetting agent, a penetrating agent, a dispersing agent, a viscosity modifier, an antifoaming agent, an antifungal agent, an antirust agent, and the like which are commonly used.

[0097] v. From the viewpoints of preventing clogging of printer nozzles and maintaining dispersion stability, the average particle size of the pigment-containing polymer particles in the obtained aqueous dispersion is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 170 nm or less, even more preferably 150 nm or less, and still more preferably 120 nm or less.

[0098] The average particle size of the pigment-containing polymer particles can be measured by the method described in the Examples.

[0099] [Water-based ink for thermal inkjet printing] The water-based ink for thermal ink-jet printing of the present invention contains the water dispersion for thermal ink-jet printing of the present invention.

[0100] The content of each component in the water-based ink of the present invention is as follows. From the viewpoint of increasing the print density of the water-based ink, the content of the pigment in the water-based ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, still more preferably 4% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and still more preferably 12% by mass or less.

[0101] In the water-based ink of the present invention, the mass ratio of the amount of pigment to the amount of polymer [pigment / polymer] is the same as in the water dispersion, and is preferably 1.0 or more, more preferably 1.5 or more, even more preferably 1.7 or more, and is preferably 9.0 or less, more preferably 6.0 or less, even more preferably 5.0 or less, still more preferably 4.0 or less, and even more preferably 1.25 or less.

[0102] In the water-based ink of the present invention, the molar amount of the carboxyl groups of the polymer relative to the mass of the pigment [-COOH (mmol) / pigment (g)] is the same as that of the water dispersion, from the viewpoint of improving the ejection speed, and is preferably 0.65 mmol / g or more, more preferably 0.70 mmol / g or more, even more preferably 0.80 mmol / g or more, and is preferably 1.40 mmol / g or less, more preferably 1.30 mmol / g or less, even more preferably 1.25 mmol / g or less.

[0103] The water content in the water-based ink is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 55% by mass or more, still more preferably 70% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0104] The surface tension of the water-based ink of the present invention (at 25°C) ensures good ejection from the ink nozzle. From this viewpoint, it is preferably 20 mN / m or more, more preferably 23 mN / m or more, even more preferably 25 mN / m or more, and preferably 45 mN / m or less, more preferably 43 mN / m or less, even more preferably 40 mN / m or less.

[0105] In order to maintain good jetting properties, the viscosity of the water-based ink of the present invention (20°C) is preferably 2.0 mPa·s or more, more preferably 2.3 mPa·s or more, even more preferably 2.5 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 11 mPa·s or less, even more preferably 10 mPa·s or less.

[0106] The water-based ink of the present invention can be adjusted, if necessary, by adding a wetting agent, a penetrating agent, a dispersing agent such as a surfactant, a viscosity adjusting agent such as hydroxypropyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, an antifoaming agent such as silicone oil, an antifungal agent, an antirust agent, and the like that are commonly used in water-based inks.

[0107] Specific examples of the wetting agent and penetrating agent include polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, glycerin, trimethylolpropane, and diethylene glycol diethyl ether, as well as their ethers and acetates, with glycerin and trimethylolpropane being preferred.

[0108] The surfactant may be a non-ionic surfactant such as an ethylene oxide adduct of acetylene glycol.

[0109] From the viewpoints of preventing clogging of printer nozzles and of dispersion stability, the average particle size of the pigment-containing polymer particles in the obtained water-based ink is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 170 nm or less, even more preferably 150 nm or less, and still more preferably 120 nm or less. EXAMPLES

[0110] In the following Production Examples, Preparation Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The weight average molecular weight of the polymer, the average particle size of the pigment-containing polymer particles, the solid content concentration and the ink viscosity were measured as follows.

[0111] (1) Measurement of weight average molecular weight of polymer The measurement was performed by gel permeation chromatography using N,N-dimethylformamide containing 60 mmol / L phosphoric acid and 50 mmol / L lithium bromide as the solvent, and polystyrene as the standard substance. Columns used were Tosoh Corporation (TSK-GEL, α-M x 2 columns), main body: Tosoh Corporation (HLC-8120GPC), flow rate: 1 mL / min.

[0112] (2) Measurement of the average particle size of polymer particles containing pigment The average particle size of the polymer particles containing the pigment was measured using a laser particle analysis system ELS-8000 (cumulant analysis) manufactured by Otsuka Electronics Co., Ltd. The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measured concentration was 5×10 -3 The smaller the average particle size, the better the dispersibility.

[0113] (3) Measurement of solids concentration 1 g of the aqueous dispersion of polymer particles containing a pigment was mixed uniformly with 10 g of sodium sulfate (mirabilite), and the mixture was placed in a 10.5 cm evaporating dish. 2 The dispersion was then dried under reduced pressure at 105° C. for 2 hours at −0.07 MPa, and the mass of the aqueous dispersion after drying was measured, and the solids concentration (mass %) was calculated using the following formula. Solid content (%) = (mass of aqueous dispersion after drying / mass of aqueous dispersion before drying) x 100

[0114] (4) Viscosity measurement The viscosity was measured using an E-type viscometer RE80 manufactured by Toki Sangyo Co., Ltd. at a measurement temperature of 20°C, a measurement time of 1 minute, a rotation speed of 100 rpm, and a standard rotor (1°34′×R24).

[0115] Production Example 1 (Production of Polymer Solution) Into a reaction vessel, 55 parts of methyl ethyl ketone (MEK), 0.08 parts of a polymerization chain transfer agent (2-mercaptoethanol), and 15 parts of acrylic acid and 85 parts of benzyl acrylate (each 10%) as monomers were mixed and thoroughly purged with nitrogen gas to obtain an initial monomer solution.

[0116] Meanwhile, the remaining 90% of the above reagents were charged into a dropping funnel, followed by adding 1.0 part of a polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by Wako Pure Chemical Industries, Ltd., product name: V-65) and mixing. The atmosphere was then thoroughly replaced with nitrogen gas to obtain a dropping monomer solution.

[0117] Under a nitrogen atmosphere, the initial monomer solution in the reaction vessel was heated to 60° C. while stirring, and the monomer solution in the dropping funnel was gradually dropped into the reaction vessel over 3 hours. After the dropping was completed, a solution in which 0.6 parts of the polymerization initiator was dissolved in 2.5 parts of methyl ethyl ketone was added to the reaction liquid to obtain a polymer solution.

[0118] The resulting polymer solution was dried under reduced pressure at 105° C. for 2 hours to remove methyl ethyl ketone, thereby obtaining a polymer having a weight average molecular weight of 70,000.

[0119] Preparation Example 1 (Preparation of Water Dispersion of Pigment-Containing Polymer Particles) 21.4 parts of the polymer obtained in Production Example 1 was dissolved in 66.9 parts of methyl ethyl ketone, and 6.3 parts of a 5N aqueous sodium hydroxide solution, 9.1 parts of a 25% aqueous ammonia solution, and 212 parts of ion-exchanged water were added thereto and stirred. Then, the mixture was diluted with "Ultra Disper" manufactured by Asada Iron Works Co., Ltd. The mixture was then treated with a dispersing blade at 1400 rpm for 15 minutes, after which 50 parts of carbon black (manufactured by Cabot Corporation, product name: Monarch 880) was added as a pigment, and the mixture was further treated at 8000 rpm and 15° C. for 1 hour. Next, high-pressure dispersion was performed at a pressure of 180 MPa for 20 passes using a high-pressure dispersing machine, Microfluidizer “M-140K”, manufactured by Microfluidics Corporation.

[0120] Next, the dispersion obtained by high-pressure dispersion was heated to 60°C under reduced pressure to distill off methyl ethyl ketone, the volatile base, and some of the water, yielding an aqueous dispersion of pigment-containing polymer particles. The pigment content in the aqueous dispersion obtained was 14%, the [pigment / polymer] mass ratio was 2.3, the viscosity (20°C) was 4.1 mPa s, and the average particle size of the pigment-containing polymer particles was 94.2 nm.

[0121] The obtained aqueous dispersion of polymer particles gave off no odor of methyl ethyl ketone or ammonia.

[0122] Preparation Example 2 (Production of Water-Based Ink) 7 parts of glycerin, 7 parts of trimethylolpropane, 5 parts of triethylene glycol, 0.5 parts of an ethylene oxide adduct of acetylene glycol (Kawaken Fine Chemicals Co., Ltd., acetylene glycol-based surfactant, product name: Acetinol E100, average number of moles added: 10), and 73.4 parts of ion-exchanged water were mixed and stirred at room temperature for 15 minutes to obtain a mixed solution.

[0123] Next, 25 parts of the aqueous dispersion of the pigment-containing polymer particles obtained in Preparation Example 1 (7.1 parts in terms of solid content, 5 parts in terms of pigment content) was added to the mixed solution while stirring with a magnetic stirrer, and filtered through a 1.2 μm filter (acetyl cellulose membrane, Fujifilm Corporation) to obtain an aqueous ink. The pigment content in the obtained aqueous ink was 5%, the [pigment / polymer] mass ratio was 2.3, and the average particle size of the pigment-containing polymer particles was 94.2 nm.

[0124] Examples 1 to 6 and Comparative Examples 1 to 2 The water-based inks obtained in Production Example 1, Preparation Examples 1 and 2 were used in Example 1. In Example 2, water-based inks were obtained in the same manner as in Production Example 1, Preparation Examples 1 and 2, except that the monomers and monomer ratios were changed as shown in Table 1. In Examples 3 and 4, water-based inks were obtained in the same manner as in Production Example 1, Preparation Examples 1 and 2, except that the degassing rate was changed to 50%. In Examples 5 and 6, water-based inks were obtained in the same manner as Production Example 1, Preparation Examples 1 and 2, except that the degassing rate was changed to 20%. In Comparative Examples 1 and 2, water-based inks were obtained in the same manner as Production Example 1, Preparation Examples 1 and 2, except that the degassing rate was changed to 0%.

[0125] The obtained water-based ink was observed and evaluated for kogation, and the average ejection speed of the ink droplets was calculated by the following methods. The results are shown in Table 1.

[0126] (1) Observation and evaluation of kogation A thermal inkjet ejection observation device manufactured by our company, which has an electronic balance capable of adjusting the applied voltage, pulse width, and drive frequency, was used as the evaluation device. The equipment was arranged as shown in Figure 3 as a thermal inkjet printer, and a degassing device was installed using a hollow fiber degassing module (DIC Corporation's "SEPAREL EF-AG5-B", the pressure loss of the hollow fiber degassing module during degassing was 0.005 MPa). A Hewlett-Packard (HP) ink cartridge "HP135" filled with each of the inks prepared as described above was loaded, and 1 × 10 ink was printed on commercially available plain paper (high-quality plain paper, XEROX Corporation's "XEROX4200", 279.4 mm long x 215.9 mm wide) under the following conditions. 7 Ink droplets were continuously ejected from the pass to perform solid printing.

[0127] Print head nozzle diameter: approx. 20 μm Applied voltage: 16.5V, pulse width: 1μs, driving frequency: 5kHz Ink droplet volume: Approx. 4 pL / drop Ink droplet ejection speed: 12m / s After the solid printing, the recording head was disassembled, the heater portion was gently washed with pure water, and the resultant was observed under an optical microscope and evaluated according to the following criteria.

[0128] (Evaluation criteria for kogation) A: No adhesion was observed. B: The deposits were present in 30% or less of the area of ​​the heater resistor surface. C: Deposits were present in more than 30% of the heater resistor surface area.

[0129] (2) Calculation of the average ejection speed of ink droplets Using the above thermal inkjet printer and an in-house manufactured thermal inkjet ejection observation device as the evaluation device, ink was ejected under the same conditions as those used for observing and evaluating kogation. A strobe and a camera were used to measure the time it took for the ink droplet to reach a distance of 0.5 mm from the nozzle and the time it took for the ink droplet to reach a distance of 1.5 mm, and the average ejection speed was calculated by dividing 1.0 mm (= 1.5 mm - 0.5 mm) by the difference between these values.

[0130] [Table 1] *1 Type of monomer (solid mass part) *2 CB: Carbon black *3 Average particle size of pigment-containing polymer particles [Explanation of symbols]

[0131] 1 Hollow fiber 2 Ink inlet 3 Ink outlet 4. Vacuum pump 5. Ink 6 Hollow fiber membrane 61 (hollow fiber) outer diameter 62 (hollow fiber) inner diameter V Reduced pressure / vacuum S Dissolved oxygen 7 Head Cap 8 Ink Tank 9. Pressure pump (liquid delivery pump) P Pressure Gauge F Flowmeter D Dissolved oxygen meter 10 Degassing module 11 Back pressure regulator 12 Head 30 Intermediate Tank 41 Nozzle 100 Filters A, B solenoid valve 51 Heater section 52 Ink supply port

Claims

1. In a thermal inkjet type inkjet printer having an ink path from an ink tank to a head, The inkjet printer includes a hollow fiber degassing module disposed in the ink path and capable of degassing, and an ink circulation path that returns the inkjet ink discharged from the head to the head via the hollow fiber degassing module, an ink-jet printer, characterized in that a pulse width for ejecting the ink-jet ink is set to be equal to or greater than 0.01 μs and equal to or less than 3.0 μs;

2. A printing method using a thermal inkjet type inkjet printer having an ink path from an ink tank to a head, comprising: The inkjet printer includes a hollow fiber degassing module disposed in the ink path and capable of degassing, and an ink circulation path that returns the inkjet ink discharged from the head to the head via the hollow fiber degassing module, Printing is performed while degassing the ink by the hollow fiber degassing module, and the ink discharged from the head is degassed by the hollow fiber degassing module and returned to the head; A printing method, characterized in that a pulse width for ejecting the ink-jet ink is 0.01 μs or more and 3.0 μs or less.

3. 3. The printing method according to claim 2, wherein printing is performed while degassing the ink to a dissolved oxygen concentration of 8.0 ppm or less by the hollow fiber degassing module.

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