Inkjet recording device

Abstract

To provide an inkjet recording device which is excellent in ink circulation stability and ink discharge amount stability.SOLUTION: An inkjet recording device includes ink, a circulation type recording head, a damper member for supplying the ink to the circulation type recording head, and an ink supply unit for supplying the ink to the damper member. The circulation type recording head discharges at least a part of the ink supplied from the damper member to a recording medium, and discharges the remainder of the ink to the ink supply unit. The ink supply unit has a relief mechanism. The relief mechanism controls the pressure of the ink supplied to the damper member so as not to become a predetermined value or more. The ink contains a pigment, binder resin particles, and an aqueous medium. A zeta potential of the binder resin particles is -60.0 mV or more and -45.0 mV or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an inkjet recording apparatus. [Background technology]

[0002] When inkjet recording is performed using ink that dries easily or ink in which pigments and the like tend to settle, an inkjet recording device equipped with a circulation type recording head as a recording head is suitable. An inkjet recording device equipped with a circulation type recording head is required to have ink circulation stability and ink ejection volume stability. Ink circulation stability is the ability to suppress the occurrence of ink aggregation and a decrease in the circulation flow rate during ink circulation and to circulate ink stably for a long period of time. In addition, ink ejection volume stability is the ability to suppress fluctuations in the ink ejection volume even when ink is circulated for a long period of time.

[0003] In response to such demands, for example, an inkjet recording device has been proposed that has a function of collecting aggregates generated in an ink circulation flow path by directing them into a flow path separate from the ink circulation flow path (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-171754 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, even the inkjet recording apparatus described in Patent Document 1 cannot fully satisfy the requirements for ink circulation stability and ink ejection amount stability.

[0006] SUMMARY OF THE PRESENT EMBODIMENTS The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an inkjet recording apparatus which is excellent in ink circulation stability and ink ejection volume stability. [Means for solving the problem]

[0007] The inkjet recording device according to the present invention includes an ink, a circulation type recording head, a damper member that supplies the ink to the circulation type recording head, and an ink supply unit that supplies the ink to the damper member. The circulation type recording head ejects at least a portion of the ink supplied from the damper member onto a recording medium, and discharges the remaining portion of the ink to the ink supply unit. The ink supply unit has a relief mechanism. The relief mechanism adjusts the pressure of the ink supplied to the damper member so that it does not exceed a predetermined value. The ink contains a pigment, binder resin particles, and an aqueous medium. The zeta potential of the binder resin particles is -60.0 mV or more and -45.0 mV or less. The zeta potential is measured at 25°C using a mixed liquid containing 10 parts by mass of the binder resin particles and 90 parts by mass of water as a measurement sample. The volume median diameter of the binder resin particles is 5 nm or more and 100 nm or less. The binder resin particles have a breaking elongation at 25° C. of 60% or less. Effect of the Invention

[0008] The inkjet recording apparatus according to the present invention is excellent in the ink circulation stability and the ink ejection amount stability. [Brief description of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a main part of an example of an inkjet recording apparatus according to an embodiment of the present invention. [Diagram 2] 2 is a diagram showing members related to the circulation of ink in the inkjet recording apparatus of FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described. In the following, the volume median diameter (D 50) is a value measured using a dynamic light scattering particle size distribution analyzer (e.g., "Zetasizer Nano ZS" manufactured by Malvern Instruments) unless otherwise specified. In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic".

[0011] The breaking elongation can be measured by a tensile test using a tensile tester at 25° C. in accordance with JIS (Japanese Industrial Standards) K7127-1999. In measuring the breaking elongation of the binder resin particles, the binder resin particles are first formed into a film to form a sample, and the above-mentioned tensile test is performed on the obtained sample.

[0012] Unless otherwise specified, the measured value of the zeta potential is a value (peak value) measured at 25° C. using a laser Doppler type zeta potential meter (for example, "ELSZ-1000" manufactured by Otsuka Electronics Co., Ltd.).

[0013] Unless otherwise specified, the measured value of the glass transition point (Tg) is a value measured according to "JIS (Japanese Industrial Standards) K7121-2012" using a differential scanning calorimeter (e.g., "DSC-60" manufactured by Shimadzu Corporation). In the endothermic curve measured by the differential scanning calorimeter (vertical axis: heat flow (DSC signal), horizontal axis: temperature, heating rate 5°C / min), the temperature of the inflection point due to the glass transition (specifically, the temperature of the intersection between the extrapolated line of the baseline and the extrapolated line of the falling line) corresponds to Tg (glass transition point).

[0014] <Inkjet recording device> An inkjet recording apparatus according to an embodiment of the present invention will be described below. The inkjet recording apparatus of this embodiment includes an ink, a circulation type recording head, a damper member that supplies the ink to the circulation type recording head, and an ink supply unit that supplies the ink to the damper member. The circulation type recording head ejects at least a portion of the ink supplied from the damper member onto a recording medium, and discharges the remaining portion of the ink to the ink supply unit. The ink supply unit has a relief mechanism. The relief mechanism adjusts the pressure of the ink supplied to the damper member so that it does not exceed a predetermined value. The ink contains a pigment, binder resin particles, and an aqueous medium. The zeta potential of the binder resin particles is -60.0 mV or more and -45.0 mV or less. The zeta potential is measured at 25°C using a mixed liquid containing 10 parts by mass of binder resin particles and 90 parts by mass of water as a measurement sample. The volume median diameter of the binder resin particles is 5 nm or more and 100 nm or less. The breaking elongation of the binder resin particles at 25°C is 60% or less.

[0015] The inkjet recording device of the present embodiment, having the above-mentioned configuration, is excellent in the ink circulation stability and the stability of the ink discharge amount. The reason for this is presumed to be as follows. In a known inkjet recording device, when ink containing binder resin particles is used, the ink may aggregate during the ink circulation. In particular, in a known inkjet recording device, the ink is likely to aggregate when the ink is exposed to a sudden change in pressure during the ink circulation. In response to this, the inkjet recording device of the present embodiment is provided with a damper member and a relief mechanism. The damper member and the relief mechanism each mitigate a sudden change in pressure (particularly a sudden increase in pressure) of the ink during circulation. As a result, the inkjet recording device of the present embodiment can suppress the pressure change of the ink during circulation and suppress the ink from aggregating during the ink circulation.

[0016] Furthermore, in the inkjet recording device of this embodiment, the binder resin particles have a relatively large absolute value of the zeta potential and a relatively small volume median diameter, and therefore are less likely to aggregate. Furthermore, the above-mentioned binder resin particles have a relatively small breaking elongation (are relatively hard). Therefore, the inkjet recording device of this embodiment can suppress the binder resin particles from collapsing during ink circulation and the collapsed binder resin particles from aggregating. As a result, the inkjet recording device of this embodiment can effectively suppress the ink from aggregating during ink circulation. As a result, the inkjet recording device of this embodiment has excellent ink circulation stability and ink discharge volume stability.

[0017] The inkjet recording apparatus of this embodiment is suitable for forming an image on a non-permeable recording medium. A non-permeable recording medium has poor ink permeability compared to a permeable recording medium. In a non-permeable recording medium, the absorption amount of an aqueous medium is, for example, 1.0 g / m 2 The following are examples of non-permeable recording media. Examples of the non-permeable recording media include resin recording media, metal recording media, and glass recording media. Examples of the resin recording media include resin sheets and resin films. The resin contained in the resin recording media is preferably a thermoplastic resin. Specific examples of the resin include polyethylene, polypropylene, polyvinyl chloride, and polyethylene terephthalate (PET). Examples of the resin recording media include OPP films and PET films. When an image is formed on a resin recording medium by the inkjet recording device of this embodiment, the surface (printing surface) of the recording medium may be subjected to a corona discharge treatment.

[0018] The ink of the inkjet recording apparatus of the present embodiment contains a certain amount of binder resin particles, and therefore, when an image is formed on a non-permeable recording medium, the inkjet recording apparatus of the present embodiment can easily ensure the abrasion resistance and adhesion of the formed image to the recording medium.

[0019] The ink of the present invention is preferably used for surface printing. Surface printing here refers to printing on the front side of a transparent recording medium (the side viewed by a viewer). When a viewer views a recording medium on which surface printing has been performed, the positional relationship is "viewer, image, recording medium," and the image is directly viewed by the viewer.

[0020] The inkjet recording device of this embodiment will be described below in detail with respect to the ink, and then the device will be described in detail. The inkjet recording device of this embodiment may be equipped with only one type of ink, or may be equipped with multiple types of ink (for example, four types of ink: yellow ink, cyan ink, magenta ink, and black ink). When the inkjet recording device of this embodiment is equipped with multiple types of ink, it is sufficient that at least one of the inks satisfies the following conditions, but it is preferable that all of the inks satisfy the following conditions. Each of the components described below may be used alone, or two or more may be used in combination.

[0021] [ink] The ink contains a pigment, binder resin particles, and an aqueous medium. The ink preferably further contains a pigment-coated resin and a surfactant.

[0022] (Pigments) In the ink, the pigment constitutes a pigment particle together with, for example, a pigment-coated resin. The pigment particle is constituted, for example, by a core containing a pigment and a pigment-coated resin that coats the core. The pigment-coated resin exists, for example, in a state of being dispersed in a solvent. From the viewpoint of optimizing the color density, hue, or stability of the ink, the volume median diameter of the pigment particle is preferably 30 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

[0023] Examples of pigments include yellow pigments, orange pigments, red pigments, blue pigments, purple pigments, and black pigments. Examples of yellow pigments include CI Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include CI Pigment Red (122 and 202). Examples of blue pigments include CI Pigment Blue (15, more specifically 15:3). Examples of purple pigments include CI Pigment Violet (19, 23, and 33). Examples of black pigments include CI Pigment Black (7).

[0024] In the ink, the pigment content is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 1.5% by mass or more and 4.5% by mass or less. By making the pigment content 0.5% by mass or more, the ink can form an image having a desired image density. In addition, by making the pigment content 10.0% by mass or less, the ink fluidity can be ensured.

[0025] (pigment coated resin) The pigment coating resin is a resin that is soluble in an aqueous medium. A part of the pigment coating resin is present, for example, on the surface of the pigment particle to optimize the dispersibility of the pigment particle. A part of the pigment coating resin is present, for example, in a dissolved state in the aqueous medium.

[0026] As the pigment coating resin, styrene-(meth)acrylic resin is preferred. The styrene-(meth)acrylic resin has a repeating unit derived from at least one monomer selected from among (meth)acrylic acid alkyl esters and (meth)acrylic acid, and a styrene unit. As the (meth)acrylic acid alkyl esters, for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate can be mentioned. As the styrene-(meth)acrylic resin, a copolymer (X) of styrene, methyl methacrylate, methacrylic acid, and butyl acrylate is preferred. The copolymer (X) is preferably neutralized with an equal amount of a base (e.g., potassium hydroxide and sodium hydroxide).

[0027] In all repeating units contained in the copolymer (X), the content of repeating units derived from styrene is preferably 10% by mass or more and 20% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from methyl methacrylate is preferably 10% by mass or more and 20% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from methacrylic acid is preferably 35% by mass or more and 45% by mass or less. In all repeating units contained in the copolymer (X), the content of repeating units derived from butyl acrylate is preferably 25% by mass or more and 35% by mass or less.

[0028] In the ink, the content of the pigment coating resin is preferably from 0.1% to 4.0% by mass, and more preferably from 0.5% to 1.5% by mass. By setting the content of the pigment coating resin to be from 0.1% to 4.0% by mass, the ejection stability of the ink can be optimized.

[0029] In the ink, the content of the pigment coating resin relative to 100 parts by mass of the pigment is preferably from 10 to 60 parts by mass, and more preferably from 25 to 40 parts by mass. By setting the content of the pigment coating resin to be from 10 to 60 parts by mass, the ejection stability of the ink can be optimized.

[0030] (Binder resin particles) The binder resin particles are present in a state of being dispersed in an aqueous medium. The binder resin particles contain a binder resin. Examples of the binder resin include urethane resin, (meth)acrylic resin, styrene-(meth)acrylic resin, (meth)acrylic-urethane resin, polyester resin, and modified polyolefin resin. The binder resin is preferably a urethane resin, (meth)acrylic resin, or polyester resin, and more preferably a urethane resin. The content of the binder resin in the binder resin particles is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass.

[0031] The urethane resin is, for example, a copolymer of a diol compound or a bisphenol compound with a polyisocyanate.

[0032] The (meth)acrylic resin is a resin having a repeating unit derived from a (meth)acrylic acid compound. Examples of the (meth)acrylic acid compound include (meth)acrylic acid and (meth)acrylic acid alkyl esters.

[0033] The polyester resin is obtained by polycondensation of a polyhydric alcohol and a polycarboxylic acid. Examples of the polyhydric alcohol for synthesizing the polyester resin include dihydric alcohols (e.g., diol compounds and bisphenol compounds) and trivalent or higher alcohols. Examples of the polycarboxylic acid for synthesizing the polyester resin include divalent carboxylic acids and trivalent or higher carboxylic acids. Note that, instead of the polycarboxylic acid, a polycarboxylic acid derivative capable of forming an ester bond by polycondensation (e.g., anhydride of a polycarboxylic acid and a polycarboxylic acid halide) may be used.

[0034] The zeta potential of the binder resin particles is -60.0 mV or more and -45.0 mV or less, preferably -60.0 mV or more and -50.0 mV or less, and more preferably -55.0 mV or more and -52.0 mV or less. The greater the absolute value of the zeta potential of the binder resin particles, the less likely the binder resin particles are to aggregate. Therefore, by setting the zeta potential of the binder resin particles to -60.0 mV or more and -45.0 mV or less, the binder resin particles can be prevented from aggregating during the circulation of the ink, and excellent circulation stability can be imparted to the ink. The zeta potential is measured at 25°C using a mixture containing 10 parts by mass of binder resin particles and 90 parts by mass of water as a measurement sample.

[0035] Volume median diameter of binder resin particles (D 50 ) is 5 nm or more and 100 nm or less, preferably 20 nm or more and 65 nm or less, and more preferably 35 nm or more and 45 nm or less. By setting the volume median diameter of the binder resin particles to 5 nm or more and 100 nm or less, aggregation of the binder resin particles during circulation of the ink can be suppressed, and excellent circulation stability can be imparted to the ink.

[0036] The breaking elongation of the binder resin particles at 25° C. is 60% or less, preferably 8% to 60%, and more preferably 40% to 55%. By making the breaking elongation of the binder resin particles at 25° C. 60% or less, the ink can exhibit excellent ink circulation stability and stability of the ink ejection amount.

[0037] The glass transition point of the binder resin is preferably 30° C. or higher and 110° C. or lower, more preferably 45° C. or higher and 90° C. or lower, and more preferably 45° C. or higher and 55° C. or lower. By setting the glass transition point of the binder resin to 30° C. or higher, it is possible to prevent the binder resin particles from softening at room temperature, and more effectively prevent the ink from coagulating during the ink circulation. By setting the glass transition point of the binder resin to 110° C. or lower, it is easy to ensure the scratch resistance and adhesion to the recording medium of the formed image.

[0038] The content of the binder resin particles in the ink is preferably 2.0% by mass or more and 10.0% by mass or less, and more preferably 3.5% by mass or more and 6.5% by mass or less. By making the content of the binder resin particles 2.0% by mass or more, the ink can optimize the scratch resistance of the formed image and the adhesion to the recording medium. By making the content of the binder resin particles 10.0% by mass or less, the ink circulation stability and the stability of the ink ejection amount can be optimized.

[0039] The binder resin particles are preferably any one of the binder resin particles (1) to (4) shown in Table 1. In Table 1, "resin type" indicates the type of binder resin contained in the binder resin particles. "Elongation at break" indicates the elongation at break at 25°C. 50 " indicates the volume median diameter. The numerical ranges in Table 1 indicate the numerical ranges of the physical properties of each binder resin particle. For example, "45 to 55" for the breaking elongation of binder resin particle (1) indicates that the breaking elongation at 25°C is 44% or more and 55% or less.

[0040] [Table 1]

[0041] (Surfactant) The surfactant optimizes the compatibility and dispersion stability of each component contained in the ink. The surfactant also optimizes the permeability (wettability) of the ink to the recording medium. Examples of the surfactant include nonionic surfactants.

[0042] Examples of nonionic surfactants include acetylene glycol surfactants (surfactants containing acetylene glycol compounds), silicone surfactants (surfactants containing silicone compounds), and fluorine surfactants (surfactants containing fluorine resins or fluorine-containing compounds). Examples of acetylene glycol surfactants include ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol. The ink preferably contains a silicone surfactant as a surfactant.

[0043] The content of the surfactant in the ink is preferably from 0.05% by mass to 2.0% by mass, and more preferably from 0.2% by mass to 0.5% by mass.

[0044] [Aqueous medium] The aqueous medium contained in the ink is a medium containing water. The aqueous medium may function as a solvent or as a dispersion medium. Specific examples of the aqueous medium include aqueous media containing water and a water-soluble organic solvent.

[0045] (water) The water content in the ink is preferably from 40.0% by mass to 80.0% by mass, and more preferably from 55.0% by mass to 65.0% by mass.

[0046] Examples of the water-soluble organic solvent include glycol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, glycerin, and dimethyl sulfoxide.

[0047] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, and tetraethylene glycol.

[0048] Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether and propylene glycol monomethyl ether.

[0049] Lactam compounds include, for example, 2-pyrrolidone and N-methyl-2-pyrrolidone.

[0050] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0051] An example of the acetate compound is diethylene glycol monoethyl ether acetate.

[0052] The water-soluble organic solvent is preferably a glycol compound or a glycol ether compound, more preferably 1,3-propanediol, propylene glycol, triethylene glycol monobutyl ether or 3-methyl-1,3-butanediol, and even more preferably a mixed solvent containing at least one of 1,3-propanediol and propylene glycol, and at least one of triethylene glycol monobutyl ether and 3-methyl-1,3-butanediol.

[0053] The content of the water-soluble organic solvent in the ink is preferably from 20.0% by mass to 50.0% by mass, and more preferably from 32.0% by mass to 36.0% by mass.

[0054] The total content of 1,3-propanediol and propylene glycol in the ink is preferably from 10.0% to 40.0% by mass, and more preferably from 20.0% to 27.0% by mass.

[0055] The total content of triethylene glycol monobutyl ether and 3-methyl-1,3-butanediol in the ink is preferably from 3.0% by mass to 15.0% by mass, and more preferably from 6.0% by mass to 10.0% by mass.

[0056] In the aqueous medium, the total content of water, 1,3-propanediol, propylene glycol, triethylene glycol monobutyl ether and 3-methyl-1,3-butanediol is preferably 90% by mass or more, more preferably 99% by mass or more, and even more preferably 100% by mass.

[0057] [Other ingredients] The ink may further contain known additives (for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity adjuster, a pH adjuster, and an anti-mold agent) as necessary.

[0058] [Ink manufacturing method] The ink can be produced, for example, by uniformly mixing a pigment dispersion containing a pigment, a resin emulsion containing binder resin particles, an aqueous medium, and other components (e.g., surfactants) that are blended as necessary, using a stirrer. In producing the ink, after the components are uniformly mixed, foreign matter and coarse particles may be removed using a filter (e.g., a filter with a pore size of 5 μm or less).

[0059] [Preferred composition] The ink preferably has any one of compositions (i-1) to (i-8) shown in Table 2. In Table 2, the numerical ranges indicate the content [mass %] of each component. For example, "2.5-3.5" in the pigment column for composition (i-1) indicates that the pigment content in composition (i-1) is 2.5 mass % or more and 3.5 mass % or less.

[0060] [Table 2]

[0061] [Device configuration] The configuration of the inkjet recording apparatus of this embodiment will be described in detail below with reference to the drawings. Note that the drawings are mainly and diagrammatically illustrated with respect to each component for ease of understanding, and the size, number, etc. of each component illustrated may differ from the actual size for the convenience of drawing.

[0062] FIG. 1 is a diagram showing a main part of an inkjet recording device 100, which is an example of an inkjet recording device of the present embodiment. As shown in FIG. 1, the inkjet recording device 100 mainly includes a conveying section 1 and four recording heads 11 (circulating recording heads) which are circulating line-type recording heads. The inkjet recording device 100 further includes a paper feed tray 2, a paper feed roller 3, a paper feed driven roller 4, a conveying belt 5, a belt driving roller 6, a belt driven roller 7, a discharge roller 8, a discharge driven roller 9, and a paper discharge tray 10. The conveying belt 5, the belt driving roller 6, and the belt driven roller 7 constitute a part of the conveying section 1. A paper feed tray 2 is provided at the left end of the inkjet recording device 100 in the figure. The paper feed tray 2 stores a recording medium M (for example, a recording sheet or a printing paper). A paper feed roller 3 and a paper feed driven roller 4 are provided at one end of the paper feed tray 2. The paper feed roller 3 takes out the stored recording media M one by one starting from the uppermost recording medium M, and conveys and feeds them onto the conveyor belt 5. The paper feed driven roller 4 is pressed against the paper feed roller 3 and is rotated by the paper feed roller 3.

[0063] A conveyor belt 5 is rotatably disposed downstream in the paper conveying direction of the paper feed roller 3 and the paper feed driven roller 4 (on the right side in FIG. 1). The conveyor belt 5 is stretched across a belt drive roller 6 and a belt driven roller 7. The belt drive roller 6 is disposed downstream in the paper conveying direction. The belt drive roller 6 drives the conveyor belt 5. The belt driven roller 7 is disposed upstream in the paper conveying direction. The belt driven roller 7 rotates following the belt drive roller 6 via the conveyor belt 5. As the belt drive roller 6 is driven to rotate clockwise, the recording medium M is conveyed in the conveying direction X indicated by the arrow.

[0064] Further, a discharge roller 8 and a discharge driven roller 9 are provided downstream in the conveying direction of the conveyor belt 5. The discharge roller 8 is driven in a clockwise direction in the figure, and discharges the recording medium M on which an image has been formed to the outside of the device housing. The discharge driven roller 9 is pressed against an upper portion of the discharge roller 8 and rotates in response. A paper discharge tray 10 is provided downstream of the discharge roller 8 and the discharge driven roller 9. The recording medium M discharged to the outside of the device housing is stacked on the paper discharge tray 10.

[0065] The four recording heads 11 include a first recording head 11C, a second recording head 11M, a third recording head 11Y, and a fourth recording head 11K. The first recording head 11C, the second recording head 11M, the third recording head 11Y, and the fourth recording head 11K are arranged above the conveyor belt 5 in this order from the upstream side to the downstream side in the conveying direction X of the recording medium M at approximately equal intervals. Each of the four recording heads 11 is supported at a height such that the distance from the upper surface of the conveyor belt 5 is a predetermined length. Each of the four recording heads 11 records an image on the recording medium M conveyed on the conveyor belt 5. Each of the four recording heads 11 is supplied with ink (first ink, second ink, third ink, and fourth ink) of four different colors (cyan, magenta, yellow, and black). Each of the four recording heads 11 performs inkjet ejection onto the recording medium M in a predetermined order. As a result, a color image is formed on the recording medium M. The four recording heads 11 are driven, for example, by a piezoelectric method.

[0066] Next, the inkjet recording apparatus 100 will be further described with reference to Fig. 2. Fig. 2 is a diagram showing members related to the circulation of ink in the inkjet recording apparatus 100.

[0067] As shown in FIG. 2, the inkjet recording device 100 includes a recording head 11 (more specifically, any one of the first recording head 11C, the second recording head 11M, the third recording head 11Y, and the fourth recording head 11K in FIG. 1), supply tubular members 13a to 13c, an ink supply unit 15, a damper member 16, and a control unit 101. The ink supply unit 15 supplies ink to the damper member 16. The damper member 16 supplies ink to the recording head 11 via the supply tubular members 13a to 13c. The control unit 101 controls the ink supply unit 15. The control unit 101 also controls the recording head 11.

[0068] Although omitted for illustrative purposes, the inkjet recording device 100 corresponds to the four recording heads 11 (the first recording head 11C, the second recording head 11M, the third recording head 11Y, and the fourth recording head 11K in FIG. 1) and is provided with a total of four sets of supply tubular members 13a-13c, ink supply units 15, and damper members 16.

[0069] The ink supply unit 15 includes an ink tank 151, a pump 152 for a sub tank, a sub tank 153 (ink storage section), a diaphragm pump 154 ​​(supply pump), a first pipe 161, a second pipe 162, a third pipe 163, a circulation pipe 164, a first valve 162a, a second valve 163a, a third valve 164a, a circulation pump 164b, a relief valve 170, and a relief pipe 171.

[0070] The ink tank 151 contains the ink I. The ink tank 151 is connected to the sub tank 153 via a first pipe 161. The first pipe 161 distributes the ink I from the ink tank 151 to the sub tank 153. The sub tank 153 stores the ink I supplied from the ink tank 151.

[0071] The sub-tank pump 152 is disposed in the first pipe 161. The sub-tank pump 152 supplies the ink I stored in the ink tank 151 to the sub-tank 153 in accordance with an instruction from the control unit 101, for example, when the liquid level of the ink I in the sub-tank 153 falls below a predetermined value.

[0072] In the inkjet recording apparatus 100, the ink I is supplied from the subtank 153 to the recording head 11 via the damper member 16. The recording head 11 then ejects at least a portion of the ink I supplied from the damper member 16 onto the recording medium M, and discharges the remainder of the ink I (ink that was not ejected) to the ink supply unit 15. Specifically, the ink I discharged from the recording head 11 is returned to the subtank 153. That is, the ink circulates mainly between the subtank 153, the damper member 16, and the recording head 11.

[0073] The sub tank 153 is connected to the diaphragm pump 154 ​​via a second pipe 162. The second pipe 162 distributes the ink I from the sub tank 153 to the diaphragm pump 154. The diaphragm pump 154 ​​is connected to the damper member 16 via a third pipe 163. The third pipe 163 distributes the ink I from the diaphragm pump 154 ​​to the damper member 16.

[0074] The diaphragm pump 154 ​​sucks the ink I stored in the sub tank 153 through the second pipe 162. The diaphragm pump 154 ​​discharges the ink I sucked from the sub tank 153 to the third pipe 163. Specifically, the diaphragm pump 154 ​​has a cylinder and a piston. The cylinder stores the ink I sucked from the sub tank 153. The cylinder is, for example, cylindrical. An inlet and an outlet are formed at the bottom of the cylinder. The inlet is connected to the second pipe 162. The outlet is connected to the third pipe 163.

[0075] The piston is inserted into the cylinder. The piston moves in a direction away from the bottom of the cylinder according to a command from the control unit 101. The piston also moves in a direction toward the bottom of the cylinder according to a command from the control unit 101.

[0076] When the piston moves in a direction away from the bottom of the cylinder, the ink I is sucked into the cylinder. Specifically, the ink I flows out from the subtank 153 to the second pipe 162, and the ink I flows into the cylinder via the second pipe 162.

[0077] When the piston moves in a direction approaching the bottom of the cylinder, the ink I flows out of the cylinder into the third pipe 163 , and the ink I is supplied to the damper member 16 via the third pipe 163 .

[0078] The first valve 162a is disposed in the second pipe 162. The second valve 163a is disposed in the third pipe 163. The first valve 162a and the second valve 163a open and close according to instructions from the control unit 101. Specifically, while the piston moves in a direction away from the bottom of the cylinder, the first valve 162a is in an open state and the second valve 163a is in a closed state. While the piston moves in a direction toward the bottom of the cylinder, the first valve 162a is in a closed state and the second valve 163a is in an open state.

[0079] The damper member 16 supplies the ink I to the recording head 11 via the supply tubular members 13a to 13c. The damper member 16 has a function of mitigating pressure changes in the ink I supplied to the recording head 11. The damper member 16 has at least an ink passing portion through which the ink I passes. The damper member 16 may further have an ink storage chamber that temporarily stores the ink I. In the damper member 16, for example, a part of the member that constitutes the ink passing portion and the ink storage chamber is an elastic member (for example, a resin film). Thereby, when the pressure of the ink I inside (the ink passing portion or the ink storage chamber) of the damper member 16 increases or decreases, the elastic member deforms, and the damper member 16 can mitigate the pressure change of the ink I. As a result, the pressure change of the ink I supplied to the recording head 11 is mitigated.

[0080] The circulation pipe 164 connects the recording head 11 and the subtank 153. The third valve 164a is disposed in the circulation pipe 164. The third valve 164a opens and closes according to instructions from the control unit 101. For example, the control unit 101 opens the third valve 164a when the inkjet recording apparatus 100 is forming an image, and closes the third valve 164a when the inkjet recording apparatus 100 is not forming an image. The third valve 164a is, for example, a backflow prevention valve for preventing the ink I from flowing backward when the inkjet recording apparatus 100 is not forming an image.

[0081] The circulation pump 164b is disposed in the circulation pipe 164. The circulation pump 164b, for example, pumps the ink I inside the circulation pipe 164 to the sub-tank 153. This makes the pressure inside the circulation pipe 164 lower than the pressure inside the recording head 11. As a result, the ink I is discharged from the recording head 11 to the sub-tank 153 via the circulation pipe 164.

[0082] The control unit 101 includes a storage device 102 and a processing device 103. The storage device 102 stores data and programs. The storage device 102 includes semiconductor memories such as a random access memory (RAM) and a read only memory (ROM). The storage device 102 may further include a storage device such as a hard disk drive (HDD). The processing device 103 includes a processor such as a central processing unit (CPU) or a micro processing unit (MPU). The processing device 103 controls the operation of each part of the inkjet recording apparatus 100 based on the programs stored in the storage device 102.

[0083] The relief valve 170 is disposed in the third pipe 163. The relief pipe 171 communicates the second pipe 162 and the third pipe 163 via the relief valve 170. The relief valve 170 and the relief pipe 171 constitute a relief mechanism. That is, the ink supply unit 15 has a relief mechanism. The relief valve 170 is, for example, an independent member that is not controlled by the control unit 101. The relief valve 170 is normally kept in a closed state by, for example, the force of a spring, and opens when the pressure of the ink I in the third pipe 163 becomes equal to or higher than a predetermined value. Thereby, when the pressure of the ink I in the third pipe 163 becomes equal to or higher than the predetermined value, the relief mechanism discharges the ink I in the third pipe 163 to the second pipe 162 via the relief pipe 171. As a result, the relief mechanism adjusts the pressure of the ink I in the third pipe 163 (i.e., the pressure of the ink I supplied to the damper member 16) so as not to become equal to or higher than a predetermined value.

[0084] The pressure of the ink I circulating between the subtank 153 and the recording head 11 varies depending on the type of image formed by the inkjet recording apparatus 100. Specifically, when the inkjet recording apparatus 100 forms an image with a high image density, the recording head 11 consumes a large amount of ink I. This, for example, causes the pressure of the circulating ink I to decrease. On the other hand, when the inkjet recording apparatus 100 forms an image with a low image density, the recording head 11 does not consume much ink I. This also causes the pressure of the circulating ink I to increase. The pressure change of the circulating ink I is mitigated to some extent by adjusting the operating speed of the piston of the diaphragm pump 154, but this is not sufficient. In a known inkjet recording apparatus, a sudden increase or decrease in the ink pressure (particularly a sudden increase in the ink pressure) can cause the ink to aggregate. In contrast, the ink I provided in the inkjet recording apparatus 100 is less likely to cause ink aggregation, as described above. Furthermore, the inkjet recording apparatus 100 can suppress a sudden increase or decrease in the pressure of the ink I by providing the relief mechanism and the damper member 16. As a result, the inkjet recording apparatus 100 can suppress the occurrence of ink aggregation.

[0085] Although an example of the inkjet recording apparatus of this embodiment has been described above, the inkjet recording apparatus of this embodiment is not limited to the one shown in FIGS.

[0086] 1 and 2, an inkjet recording apparatus 100 having four recording heads 11 corresponding to four colors of ink has been described as an example. However, the number of recording heads provided in the inkjet recording apparatus of this embodiment is not particularly limited, and can be, for example, 1 to 10, and preferably 3 to 5. In addition, the inkjet recording apparatus 100 ejects four colors of ink, cyan, magenta, yellow, and black, in that order, but the type, combination, and ejection order of the inks in the inkjet recording apparatus of this embodiment are not limited to this.

[0087] Furthermore, in the inkjet recording apparatus of this embodiment, only the ink, the circulation type recording head, the damper member, and the ink supply unit are required. Furthermore, the relief mechanism may have a configuration other than the relief valve and the relief piping, so long as it has the function of reducing the pressure of the ink to a predetermined value or less when the pressure of the ink supplied to the damper member exceeds a predetermined value. Furthermore, the supply pump may be a pump other than a diaphragm pump.

[0088] Furthermore, the inkjet recording apparatus of this embodiment may be a multifunction machine further having the functions of a scanner, a copier, a printer, or a facsimile. EXAMPLES

[0089] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0090] [Preparation of black pigment dispersion] A black pigment dispersion was prepared for use in preparing the ink. The components and amounts of the components contained in the black pigment dispersion are shown in Table 3.

[0091] [Table 3]

[0092] In Table 3, "Resin A-Na" refers to Resin A (pigment-coated resin) neutralized with sodium hydroxide (NaOH). "Black Pigment" is a black pigment ("MONARCH® 800" manufactured by Cabot Corporation).

[0093] [Preparation of Resin A] Resin A for obtaining "Resin A-Na" in Table 3 was prepared by the following method. Specifically, a stirrer, a nitrogen inlet tube, a condenser, and a dropping funnel were set in a four-neck flask. Next, 100 parts by mass of isopropyl alcohol and 300 parts by mass of methyl ethyl ketone were placed in the flask. The contents of the flask were heated to reflux at 70°C while bubbling nitrogen through them.

[0094] Next, solution L1 was prepared. Specifically, 40.0 parts by mass of styrene, 10.0 parts by mass of methacrylic acid, 40.0 parts by mass of methyl methacrylate, 10.0 parts by mass of butyl acrylate, and 0.4 parts by mass of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to obtain solution L1, which is a monomer solution. While the contents of the flask were heated to reflux at 70°C, solution L1 was dripped into the flask over 2 hours. After dripping, the contents of the flask were heated to reflux at 70°C for another 6 hours.

[0095] Next, solution L2 was prepared. Specifically, 0.2 parts by mass of AIBN and 150.0 parts by mass of methyl ethyl ketone were mixed to obtain solution L2. Solution L2 was added dropwise to the flask over 15 minutes. After the addition, the contents of the flask were heated to reflux at 70°C for another 5 hours. In this way, resin A (styrene-(meth)acrylic resin) was obtained. The obtained resin A had a mass average molecular weight (Mw) of 20,000 and an acid value of 100 mgKOH / g.

[0096] Here, the mass average molecular weight Mw of Resin A was measured using gel filtration chromatography ("HLC-8020GPC" manufactured by Tosoh Corporation) under the following conditions. Column: Tosoh Corporation's "TSKgel SuperMultiporeHZ-H" (4.6mm I.D. x 15cm semi-microcolumn) Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35mL / min Sample injection volume: 10 μL Measurement temperature: 40℃ Detector: IR detector

[0097] The calibration curve was prepared by selecting seven types of TSKgel standard polystyrene (manufactured by Tosoh Corporation): "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", and "A-1000", as well as n-propylbenzene.

[0098] The acid value of Resin A was measured in accordance with JIS (Japanese Industrial Standards) K0070-1992 (Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products).

[0099] [Preparation of black pigment dispersion] While heating resin A in a 70°C warm bath, an amount of sodium hydroxide solution necessary for neutralizing resin A was added to resin A. More specifically, an amount of sodium hydroxide solution 1.1 times the neutralization equivalent mass was added to resin A. In this way, an aqueous solution of resin A neutralized with sodium hydroxide (resin A-Na) was obtained. The pH of the aqueous solution of resin A-Na was 8.

[0100] The above aqueous solution containing 5 parts by mass of Resin A-Na, 15 parts by mass of black pigment, and water were placed in the vessel of a media-type disperser (Dyno (registered trademark) Mill manufactured by Willy & Bachofen Co., Ltd. (WAB)) in the amounts shown in Table 3, making the total amount 100 parts by mass. Water was added so that the mass of water was 80 parts by mass, including the mass of water contained in the aqueous sodium hydroxide solution used to neutralize Resin A and the mass of water generated by the neutralization reaction.

[0101] Next, media (zirconia beads with a diameter of 1.0 mm) were filled into the vessel so that the filling rate was 70 volume % relative to the volume of the vessel. The contents of the vessel were subjected to a dispersion treatment using a media-type dispersing machine. In this way, a black pigment dispersion was obtained.

[0102] The black pigment dispersion was diluted 300 times with water to obtain a diluted solution. The diluted solution was measured using a dynamic light scattering particle size distribution measuring device ("Zetasizer Nano ZS" manufactured by Malvern Instruments) to obtain the volume median diameter (D 50 It was confirmed that pigment particles having a volume median diameter in the range of 70 nm to 130 nm were dispersed in the black pigment dispersion.

[0103] [Resin emulsion] Resin emulsions (R-1) to (R-9) shown in Table 4 were prepared as resin emulsions used in preparing the inks. The resin emulsions (R-1) to (R-9) contained binder resin particles (r-1) to (r-9), respectively. The resin emulsion (R-1) was manufactured by NICCA Chemical Co., Ltd. The resin emulsion (R-2) was manufactured by Unitika Ltd. The resin emulsion (R-3) was manufactured by Japan Coating Resin Co., Ltd. The resin emulsions (R-4) to (R-9) were all manufactured by Daiichi Kogyo Seiyaku Co., Ltd. In Table 4, "Tg" indicates the glass transition point. "(R)" indicates a registered trademark. "PEs" indicates a polyester resin.

[0104] [Table 4]

[0105] [Zeta potential measurement] The zeta potential of the binder resin particles (r-1) to (r-9) shown in Table 4 was measured by the following method. First, the resin emulsions (R-1) to (R-9) were diluted with water so that the content ratio of the binder resin particles (r-1) to (r-9) in each resin emulsion was 10 mass%. Next, the zeta potential of each diluted solution (specifically, a diluted solution containing 10 mass parts of any of the binder resin particles (r-1) to (r-9) and 90 mass parts of water) was measured at a measurement temperature of 25°C using a laser Doppler zeta potential meter (Otsuka Electronics Co., Ltd.'s "ELSZ-1000") to obtain a zeta potential distribution. The zeta potential distribution was shown by a graph with the zeta potential (unit: mV) on the horizontal axis and the intensity on the vertical axis. From the zeta potential distribution, the value (peak value) of the zeta potential of the measurement target was obtained.

[0106] <Ink Preparation> Inks (I-1) to (I-13) were prepared by the following method.

[0107] [Ink (I-1)] Water, 20.0 parts by mass of black pigment dispersion (black pigment: 3.0 parts by mass, resin A-Na: 1.0 part by mass), resin emulsion (R-1), 25.0 parts by mass of propylene glycol, 8.0 parts by mass of triethylene glycol monobutyl ether, and 0.3 parts by mass of silicone surfactant ("Silface (registered trademark) SAG503A" manufactured by Nissin Chemical Industry Co., Ltd., polyether modified siloxane compound) were placed in a beaker. The amount of resin emulsion (R-1) added was such that the amount of binder resin particles (r-1) contained in the mixture in the beaker was as shown in Table 5 (5.0 parts by mass in Example 1). The amount of water added was such that the total amount of the mixture in the beaker was 100 parts by mass. The contents of the beaker were mixed at a rotation speed of 400 rpm using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Scientific Co., Ltd.) to obtain a mixed liquid. The mixed liquid was filtered using a filter (pore size: 5 μm) to remove foreign matter and coarse particles contained in the mixed liquid, thus obtaining the ink of Example 1.

[0108] [Ink (I-2)~(I-13)] Inks (I-2) to (I-13) were prepared in the same manner as ink (I-1), except that the types and amounts of each component were changed as shown in Tables 5 to 6.

[0109] In Tables 5 and 6, the numerical values ​​indicate the content ratio [mass %]. "1,3-PD" indicates 1,3-propanediol. "TEGMBE" indicates triethylene glycol monobutyl ether. "MBD" indicates 3-methyl-1,3-butanediol. "Surfactant" indicates the above-mentioned silicone surfactant.

[0110] [Table 5]

[0111] [Table 6]

[0112] <Evaluation> Inkjet recording devices of Examples 1 to 8 and Comparative Examples 1 to 6 were prepared by the following method. For each inkjet recording device, the ink circulation stability (suppression of ink aggregation during circulation, and stability of the ink circulation flow rate) and the stability of the ink ejection amount were evaluated. The evaluation results are shown in Table 7. The evaluation was performed at a temperature of 25°C and a humidity of 60% RH, unless otherwise specified.

[0113] [Device] A test device (a test machine manufactured by Kyocera Document Solutions Inc.) including at least a circulation type recording head and an ink circulation pump ("Circulation Pump CIMS" manufactured by Megnajet) was prepared. The ink circulation pump corresponds to the ink supply unit of the embodiment, and was set to supply ink to the circulation type recording head. The circulation type recording head was set to eject at least a part of the ink supplied from the ink circulation pump onto a recording medium as necessary, and to return the remaining part of the supplied ink to the ink circulation pump. The circulation type recording head was set to return the entire amount of ink supplied from the ink circulation pump to the ink circulation pump when it was not necessary to eject ink (when image formation was not being performed).

[0114] The test device was capable of mounting a damper member between the ink circulation pump and the circulation type recording head. When the damper member was mounted on the test device, the ink supplied from the ink circulation pump was supplied to the circulation type recording head via the damper member. The ink circulation pump was also capable of mounting a relief mechanism (a relief valve and a relief flow path). The relief valve had a function of discharging the ink to the relief flow path when the ink sent to the damper member reached a predetermined pressure or higher. The relief flow path was connected so that the ink discharged from the relief valve returned to the vicinity of the inlet of the ink circulation pump. As shown in Table 7, the test device equipped with the relief valve and the damper member was combined with any one of the inks (I-1) to (I-13) to prepare the inkjet recording devices of Examples 1 to 8 and Comparative Examples 1 to 3 and 5 to 6. The test device not equipped with either the relief valve or the damper member was combined with the ink (I-1) to prepare the inkjet recording device of Comparative Example 4. Note that no filter was present in the circulation flow path of each inkjet recording device.

[0115] [Prevention of ink aggregation during circulation] Using each inkjet recording device, the set flow rate of the ink circulation pump was set to 60 (±5) mL / min, and the ink was circulated for 6 hours (first circulation test). The first circulation test was conducted in a state where the circulation type recording head was not capped (non-capped state). In the first circulation test, the circulation type recording head did not eject ink. That is, in each inkjet recording device, the ink sent out from the ink circulation pump was supplied to the circulation type recording head directly or indirectly via a damper member. Then, the ink supplied to the circulation type recording head was returned to the ink circulation pump without being ejected. After the first circulation test, 500 mL of ink was collected from each inkjet recording device. Next, the ink after the first circulation test was filtered through a filter with a pore size of 5 μm, and the volume of the ink after filtration was measured. The measured volume of the ink after filtration was used as an evaluation value for the inhibition of aggregation of the ink during circulation. If the ink aggregates during circulation, the volume of the ink after filtration decreases. Therefore, the more effectively an inkjet recording device can suppress the aggregation of the circulating ink, the larger the volume of the ink after filtration (closer to 500 mL). The suppression of aggregation of the circulating ink was evaluated according to the following criteria.

[0116] (Standard for preventing ink from coagulating during circulation) A (Good): The volume of the ink after filtration is 450 mL or more and 500 mL or less. B (Poor): The volume of the ink after filtration is 300 mL or more and less than 450 mL. C (particularly poor): The volume of the ink after filtration is less than 300 mL.

[0117] [Stability of ink circulation flow rate] Using each inkjet recording device, the set flow rate of the ink circulation pump was set to 60 (±3) mL / min, and the ink was circulated for 6 hours (second circulation test). The second circulation test was conducted in a state where the circulation type recording head was not capped (non-capped state). In the second circulation test, the circulation type recording head did not eject ink. That is, in each inkjet recording device, the ink sent out from the ink circulation pump was supplied to the circulation type recording head directly or indirectly via a damper member. Then, the ink supplied to the circulation type recording head was returned to the ink circulation pump without being ejected. Immediately after the start of the second circulation test and immediately before the end of the second circulation test, the actual flow rate (initial flow rate X and post-circulation flow rate Y) of the ink circulating in each inkjet recording device was measured. For the initial flow rate X and the post-circulation flow rate Y, the values ​​displayed on the control terminal (with control software installed) connected to the ink circulation pump were referenced. The rate of change in the flow rate of the ink due to the second circulation test was calculated by applying the initial flow rate X and the post-circulation flow rate Y to the following formula. The stability of the ink circulation flow rate was evaluated according to the following criteria. Flow rate change rate [%] = 100 × (YX) / Y

[0118] (Standard for stability of ink circulation flow rate) A (Good): Flow rate change rate is less than 15% B (bad): Flow rate change rate is 15% or more

[0119] [Stability of ink ejection volume] Using each inkjet recording device, the ink circulation pump was set to a flow rate of 60 mL / min, and the ink was circulated for 6 hours (third circulation test). The third circulation test was conducted without a cap on the circulation type recording head (non-capped state). Immediately after the start of the third circulation test, a solid image was formed on the recording medium using each inkjet recording device. At this time, the ink discharge amount (volume per ink drop) of the circulation type recording head was set to 6 pL. The formed solid image was used as the first evaluation image. As the recording medium, a PET film ("FE2001" manufactured by Futamura Corporation, one side corona discharge treatment) was used. More specifically, an image was formed on the corona discharge treated side of the PET film.

[0120] Just before the end of the third circulation test, a solid image was formed again on the above-mentioned recording medium using each inkjet recording device. At this time, the ink discharge amount (volume per ink droplet) of the circulation type recording head was set to 6 pL. The formed solid image was used as the second evaluation image.

[0121] The first evaluation image and the second evaluation image were each dried at 100° C. for 5 minutes immediately after formation. Next, the first evaluation image and the second evaluation image after drying were each left to stand at room temperature for 1 hour. Thereafter, the Lab values ​​(L value, a value, and b value) of the first evaluation image and the second evaluation image were each measured using a fluorescence spectrodensitometer ("FD-5" manufactured by Konica Minolta, Inc.).

[0122] Based on the Lab value of the first evaluation image (solid image formed immediately after the start of the third circulation test) and the Lab value of the second evaluation image (solid image formed immediately before the end of the third circulation test), the change in L value (ΔL), the change in a value (Δa), and the change in b value (Δb) were calculated. Next, the change in Lab value (ΔE) was calculated by applying ΔL, Δa, and Δb to the following formula. The stability of the ink ejection amount was judged based on ΔE according to the following criteria. ΔE = [(ΔL) 2 +(Δa) 2 +(Δb) 2 〕 1 / 2

[0123] (Standard for stability of ink ejection volume) A (Good): ΔE is 2.0 or less B (bad): ΔE is greater than 2.0

[0124] In Table 7, "Y" for the damper member and the relief valve indicates that the inkjet recording apparatus is equipped with a damper member and a relief valve. "N" for the damper member and the relief valve indicates that the inkjet recording apparatus is not equipped with either a damper member or a relief valve.

[0125] [Table 7]

[0126] As shown in Tables 3 to 7, the inkjet recording apparatus of Examples 1 to 8 included an ink, a circulation type recording head, a damper member that supplies ink to the circulation type recording head, and an ink supply unit that supplies ink to the damper member. The circulation type recording head ejected at least a part of the ink supplied from the damper member onto the recording medium, and discharged the remaining part of the ink to the ink supply unit. The ink supply unit had a relief mechanism. The relief mechanism adjusted the pressure of the ink supplied to the damper member so as not to exceed a predetermined value. The ink contained a pigment, binder resin particles, and an aqueous medium. The zeta potential of the binder resin particles was -60.0 mV or more and -45.0 mV or less. The zeta potential was measured at 25°C using a mixture containing 10 parts by mass of binder resin particles and 90 parts by mass of water as a measurement sample. The volume median diameter of the binder resin particles was 5 nm or more and 100 nm or less. The binder resin particles had a breaking elongation of 60% or less at 25° C. The inkjet recording devices of Examples 1 to 8 were capable of suppressing aggregation of the circulating ink, and were excellent in the stability of the circulating ink flow rate and the stability of the ink ejection amount.

[0127] On the other hand, in the inkjet recording devices of Comparative Examples 1 to 3, the strength of the binder resin particles contained in the ink was low (elongation at break exceeded 60%). In addition, in the inkjet recording device of Comparative Example 2, the volume median diameter of the binder resin particles contained in the ink exceeded 100 nm, and the zeta potential of the binder resin particles exceeded -45.0 mV. As a result, it is determined that the inkjet recording devices of Comparative Examples 1 to 3 suffered from the collapse of the binder resin particles contained in the ink during the circulation of the ink, causing the aggregation of the ink. In addition, in the inkjet recording devices of Comparative Examples 2 and 3, at least one of the stability of the ink circulation flow rate and the stability of the ink discharge amount was poor due to the aggregation of the ink.

[0128] The inkjet recording device of Comparative Example 4 was not provided with a damper member and a relief valve. It is determined that the inkjet recording device of Comparative Example 4 is prone to collapse of the binder resin particles contained in the ink due to the sudden pressure change occurring in the ink during the circulation of the ink. As a result, it is determined that the inkjet recording device of Comparative Example 4 caused ink aggregation during the circulation of the ink. Furthermore, the inkjet recording device of Comparative Example 4 was poor in stability of the ink circulation flow rate and stability of the ink discharge amount due to the ink aggregation.

[0129] In the inkjet recording device of Comparative Example 5, the volume median diameter of the binder resin particles contained in the ink exceeded 100 nm, and it is considered that when pressure was generated during circulation of the ink, the binder resin particles collapsed, causing the ink to aggregate.

[0130] It is considered that the binder resin particles in the ink of the inkjet recording device of Comparative Example 6 were prone to aggregation because the zeta potential of the binder resin particles in the ink exceeded -45 V. As a result, in the inkjet recording device of Comparative Example 6, ink aggregation occurred during ink circulation, and the stability of the ink circulation flow rate and the stability of the ink discharge amount were also poor. [Industrial Applicability]

[0131] The inkjet recording apparatus of the present invention can be used to form an image.

Claims

1. Ink and A circulating recording head; a damper member for supplying the ink to the circulation type recording head; an ink supply unit that supplies the ink to the damper member, the circulation type recording head ejects at least a portion of the ink supplied from the damper member onto a recording medium, and discharges the remainder of the ink into the ink supply unit; The ink supply unit has a relief mechanism, the relief mechanism adjusts the pressure of the ink supplied to the damper member so as not to exceed a predetermined value; The ink is Contains a pigment, binder resin particles, and an aqueous medium; The zeta potential of the binder resin particles is −60.0 mV or more and −45.0 mV or less, The zeta potential is measured at 25° C. using a mixed liquid containing 10 parts by mass of the binder resin particles and 90 parts by mass of water as a measurement sample. The volume median diameter of the binder resin particles is 5 nm or more and 100 nm or less, The binder resin particles have a breaking elongation of 60% or less at 25° C.

2. The binder resin particles contain a binder resin, 2. The inkjet recording apparatus according to claim 1, wherein the glass transition point of the binder resin is 45[deg.] C. or higher and 90[deg.] C. or lower.

3. The inkjet recording apparatus according to claim 2 , wherein the binder resin includes a urethane resin.

4. Further comprising a surfactant, The inkjet recording apparatus according to claim 1 , wherein the surfactant includes a silicone surfactant.

5. The inkjet recording apparatus according to claim 1 , wherein a content of the binder resin particles is from 2.0% by mass to 10.0% by mass.

6. The inkjet recording apparatus according to claim 1 or 2, which is used for front-side printing.

7. 3. The inkjet recording apparatus according to claim 1, which is used for forming an image on a non-permeable recording medium.

8. the ink supply unit further includes an ink storage section and a supply pump; the ink storage section stores the ink, The inkjet recording apparatus according to claim 1 , wherein the supply pump delivers the ink stored in the ink storage portion to the damper member.

9. The inkjet recording apparatus according to claim 8 , wherein the supply pump is a diaphragm pump.

10. The relief mechanism includes a relief valve and a relief pipe.

9. The ink jet recording apparatus according to claim 8, wherein the relief valve is opened when the pressure of the ink supplied to the damper member reaches or exceeds the predetermined value, and discharges the ink through the relief pipe.

Images