Inkjet inks and inkjet recording devices

Abstract

To provide an ink that combines ejection stability, abrasion resistance, and cleanability. [Solution] The ink contains a quinacridone pigment, a pigment dispersion resin, and an aqueous medium. The supernatant obtained by centrifuging the above ink at 140,000 rpm for 3 hours at 23°C is 25 times diluted, and the ultraviolet-visible light absorption spectrum of the supernatant has a maximum absorbance of 0.16 to 0.40 at wavelengths between 300 nm and 350 nm, and an absorbance of 0.002 or less at 429 nm. In the above ink, the number of coarse particles with a particle diameter of 500 nm or more is 4.0 × 10 8 The concentration is less than or equal to 1 particles / mL. The phosphorus concentration in a 50-fold dilution of the supernatant liquid is between 1.0 ppm and 16.0 ppm.

Description

[Technical Field]

[0001] The present invention relates to an inkjet ink containing a quinacridone pigment, and an inkjet recording apparatus. [Background technology]

[0002] Patent documents 1 and 2 disclose inkjet inks for recording images on recording media such as paper. Inkjet inks require ejection stability when ejected from the nozzles of the recording head. In the inkjet ink described in Patent Document 1, a pigment derivative is incorporated to improve ejection stability. In the inkjet ink described in Patent Document 2, ejection stability is improved based on the rate of increase in the average particle size of the colorant during concentration. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2000-273383 [Patent Document 2] Japanese Patent Publication No. 2002-167534 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, Patent Documents 1 and 2 do not consider the effects on ejection stability on coarse particles generated during the pigment dispersion process, intermediates produced during the pigment synthesis process, or catalysts used to synthesize the pigments. Furthermore, inkjet inks are required to have high scratch resistance in the image formed on the recording medium, as well as high redispersibility with cleaning fluid during maintenance of the ejection surface of the recording head.

[0005] In view of the above circumstances, the object of the present invention is to provide an inkjet ink that combines ejection stability, abrasion resistance, and cleanability. [Means for solving the problem]

[0006] To achieve the above objective, an inkjet ink according to one embodiment of the present invention contains a quinacridone pigment, a pigment dispersion resin, and an aqueous medium. The ultraviolet-visible light absorption spectrum of a 25-fold diluted supernatant obtained by centrifuging the above inkjet ink at 140,000 rpm for 3 hours at 23°C shows that the maximum absorbance at wavelengths of 300 nm to 350 nm is 0.16 to 0.40, and the absorbance at 429 nm is 0.002 or less. In the above inkjet ink, the number of coarse particles with a particle diameter of 500 nm or more is 4.0 × 10 8 The number of particles / mL is less than or equal to 1 / mL. The phosphorus concentration in a 50-fold dilution of the supernatant liquid is between 1.0 ppm and 16.0 ppm.

[0007] In this inkjet ink, by controlling the amounts of unadsorbed resin (not adsorbed to the quinacridone pigment in the pigment dispersion resin dispersed in the aqueous medium), intermediates produced during the synthesis of the quinacridone pigment, coarse particles with a particle size of 500 nm or more, and phosphorus-containing catalyst used to synthesize the quinacridone pigment, a configuration that combines ejection stability, abrasion resistance, and cleanability can be achieved.

[0008] An inkjet recording apparatus according to one embodiment of the present invention comprises a transport unit for transporting a recording medium and a line-type recording head for ejecting ink onto the recording medium. The above ink includes the above inkjet ink. [Effects of the Invention]

[0009] As described above, the present invention can provide an inkjet ink that combines ejection stability, abrasion resistance, and cleanability. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an inkjet recording device according to one embodiment of the present invention. [Figure 2] This diagram shows the underside of the recording head of the inkjet recording device described above.

[0011] Embodiments of the present invention will be described below.

[0012] [Inkjet ink] An inkjet ink according to one embodiment of the present invention (hereinafter also simply referred to as "ink") contains a quinacridone pigment, a pigment dispersion resin, and an aqueous medium. In the ink according to this embodiment, the pigment dispersion resin is composed of an adsorbed resin adsorbed onto the quinacridone pigment and an unadsorbed resin that is free in the aqueous medium without being adsorbed onto the quinacridone pigment, and the quinacridone pigment and the adsorbed resin constitute a pigment dispersion. In the ink according to this embodiment, by controlling the amounts of the unadsorbed resin, the intermediate produced in the process of synthesizing the quinacridone pigment (hereinafter also simply referred to as "intermediate"), coarse particles, and the phosphorus-containing catalyst used to synthesize the quinacridone pigment, a configuration that combines discharge stability, abrasion resistance, and cleaning properties can be achieved. In this invention, coarse particles refer to particles with a particle diameter of 500 nm or more.

[0013] The amount of unadsorbed resin in the ink according to this embodiment can be grasped by the ultraviolet-visible light absorption spectrum of a 25-fold dilution of the supernatant obtained by subjecting it to centrifugation at 140,000 rpm for 3 hours at 23°C (hereinafter, also simply referred to as "supernatant"). That is, it can be seen that the greater the maximum absorbance of the peak appearing in the range of wavelengths of 300 nm or more and 350 nm or less in this ultraviolet-visible light absorption spectrum, the greater the amount of unadsorbed resin. In the ink according to this embodiment, the amount of unadsorbed resin is controlled so that the maximum absorbance at wavelengths of 300 nm or more and 350 nm or less in this ultraviolet-visible light absorption spectrum is 0.16 or more and 0.40 or less. Further, in the ink according to this embodiment, it is preferable that the ratio of unadsorbed resin in the pigment dispersion resin is controlled to be 20% or more and 50% or less.

[0014] The amount of intermediate in the ink according to this embodiment can be grasped by the absorbance at 429 nm in the ultraviolet-visible light absorption spectrum of a 25-fold dilution of the supernatant. That is, it can be seen that the greater the absorbance at 429 nm in this ultraviolet-visible light absorption spectrum, the greater the amount of intermediate. In the ink according to this embodiment, it is necessary that the amount of intermediate is small, and it is controlled so that the absorbance at 429 nm in this ultraviolet-visible light absorption spectrum is 0.002 or less.

[0015] In the ink according to this embodiment, the fewer the number of coarse particles, the better. Examples of the coarse particles that can be included in the ink according to this embodiment include, for example, unadsorbed resin, intermediate, and phosphorus-containing catalyst aggregated in an aqueous medium. The number of coarse particles in the ink is counted based on the measurement result by a laser light scattering particle size distribution analyzer. In the ink according to this embodiment, the number of coarse particles is controlled to be 4.0×10 8 pieces / mL or less.

[0016] The amount of the phosphorus-containing catalyst in the ink according to this embodiment can be grasped by the phosphorus concentration in a 50-fold diluted solution of the supernatant liquid. That is, in the ink according to this embodiment, substantially all of the phosphorus can be regarded as being derived from the phosphorus-containing catalyst. In the ink according to this embodiment, the amount of the phosphorus-containing catalyst is controlled such that the phosphorus concentration in a 50-fold diluted solution of the supernatant liquid is 1.0 ppm or more and 16.0 ppm or less. When the phosphorus concentration exceeds 16.0 ppm, the discharge stability is likely to be impaired due to the adhesion of the phosphorus-containing catalyst to the discharge surface of the recording head and the inner wall of the nozzle holes. Further, when the phosphorus concentration is less than 1.0 ppm, sufficient redispersibility with respect to the cleaning liquid cannot be obtained, and the cleaning property is likely to be impaired.

[0017] [Quinacridone pigment] Examples of the quinacridone pigment that can be used in the ink according to this embodiment include quinacridone magenta, quinacridone red, quinacridone orange, quinacridone gold, quinacridone violet, and the like.

[0018] An example of a method for synthesizing a quinacridone pigment will be described. The quinacridone pigment is, for example, a compound represented by formula (D). The quinacridone pigment is synthesized, for example, by carrying out the reactions represented by reaction formulas (r-a), (r-b), and (r-c).

[0019] [Chemical formula]

[0020] In formulas (A), (B), (C), and (D), R A , R B , R 1 [[ID=**28**]], and R 2 It should be noted that there seems to be an error in the numbering in the original text where line 28 has an asterisk added in the translation for better understanding of the sequence. If you have any specific requirements regarding this or other aspects, please let me know.Each independently represents a monovalent group. Hereinafter, the "reactions represented by reaction formulas (r-a), (r-b), and (r-c)" may be described as "reactions (r-a), (r-b), and (r-c)", respectively. Also, the "compounds represented by formulas (A), (B), (C), and (D)" may be described as "compounds (A), (B), (C), and (D)", respectively. R 1 and R 2 When represents a methyl group, compound (D) becomes C.I. Pigment Red 122. R 1 and R 2 When represents a hydrogen atom, compound (D) becomes C.I. Pigment Violet 19.

[0021] In reaction (r-a), compound (B) is obtained from compound (A). Then, in reaction (r-b), 1 molar equivalent of compound (B) is reacted with 2 molar equivalents of an aniline derivative to obtain 1 molar equivalent of compound (C). Then, in reaction (r-c), the oxide of compound (C) is obtained by oxidizing compound (C). Then, in reaction (r-c), a hydrolyzate is obtained by hydrolyzing the oxide of compound (C). Then, in reaction (r-c), compound (D) is obtained by subjecting the hydrolyzate to a dehydration ring-closure reaction using a phosphorus-containing catalyst. The reaction temperature of the dehydration ring-closure reaction is, for example, 90°C or higher and 120°C or lower. The reaction time of the dehydration ring-closure reaction is, for example, 1 hour or longer and 2 hours or shorter.

[0022] Examples of the phosphorus-containing catalyst used in reaction (r-c) include at least one selected from the group consisting of polyphosphoric acid and its derivatives. Examples of the derivative of polyphosphoric acid include polyphosphoric acid esters, and more specifically, alkyl polyphosphoric acid esters. As the alkyl polyphosphoric acid ester, methyl polyphosphate is preferred.

[0023] After carrying out the above reaction (rc), the quinacridone pigment may be subjected to, for example, a solvent treatment step (hereinafter sometimes referred to as step A) and a post-treatment step (hereinafter sometimes referred to as step B). Step A is a step to refine the particle size, achieving high colorability and high saturation. Step B is a step to suppress the aggregation of quinacridone pigment particles caused by the refinement of the particle size in step A. By carrying out steps A and B, a fine quinacridone pigment with excellent dispersibility and storage stability can be obtained.

[0024] In step A, the quinacridone pigment is treated with a solvent. Before step A is performed, the quinacridone pigment is also called crude and consists of aggregated particles with a low degree of crystallinity. The quinacridone pigment before step A does not have sufficient coloring properties. Therefore, by performing step A, crystal growth and micronization of the quinacridone pigment are promoted. By promoting crystallization and micronization of the quinacridone pigment, the coloring properties and saturation of the quinacridone pigment are optimized. More specifically, by performing step A, the crude quinacridone pigment becomes β-type or γ-type crystals of the quinacridone pigment, improving the coloring properties and saturation of the quinacridone pigment. As a method for treating the quinacridone pigment, for example, one method is to knead the quinacridone pigment and solvent using a kneader (e.g., a salt milling kneader). The temperature and time for treating the quinacridone pigment are not particularly limited and should be set appropriately to achieve the desired particle size and particle size distribution of the quinacridone pigment. Step A may be carried out while heating the quinacridone pigment. In addition, an inorganic base (more specifically, sodium hydroxide and potassium hydroxide, etc.) may be added as a grinding aid in step A as needed. The kneaded quinacridone pigment obtained in step A may be washed with water or a solvent as needed to form, for example, a wet cake.

[0025] In step B, the quinacridone pigment mixture obtained in step A is post-treated. By performing step B, aggregation of the finely particulated quinacridone pigment is suppressed. By performing step B, in addition to the coloring and saturation imparted to the quinacridone pigment in step A, good dispersibility and storage stability can be imparted to the quinacridone pigment. As a post-treatment method, for example, a method of separating the quinacridone pigment by removing the solvent from the quinacridone pigment mixture can be considered. As a method of separating the quinacridone pigment, for example, filtration, drying, and solvent removal using a rotary evaporator can be considered. When the solvent is removed by distillation, the temperature of the solvent distillation is, for example, above the boiling point of the solvent.

[0026] Compounds (B) and (C) produced during the reactions (ra), (rb), and (rc) correspond to the above-mentioned intermediates. In ink, if such intermediates are present, relatively polar intermediates may electrostatically adhere to the ejection surface of the recording head and the inner wall of the nozzle hole. When the ink dries and thickens, the attached intermediates become aggregates, contributing to uneven ink ejection from the recording head. Therefore, in the ink according to this embodiment, the amount of intermediates is controlled so that the absorbance at 429 nm in the ultraviolet-visible light absorption spectrum of a 25-fold diluted supernatant is 0.002 or less.

[0027] (Pigment-dispersed resin) From the viewpoint of improving the quality of the image formed on the recording medium, the pigment dispersion resin is preferably made of fine particles of resin with a molecular weight of tens of thousands. Furthermore, the acid value of the pigment dispersion resin is preferably between 50 mg KOH / g and 300 mg KOH / g. The acid value of the pigment dispersion resin is obtained by measurement in accordance with JIS K 0070:1992 (Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products).

[0028] The pigment dispersion resin can be appropriately selected from known pigment dispersion resins. The pigment dispersion resin may be a random polymer or a block polymer. Specific examples of pigment dispersion resins include polyester, polyurethane, (meth)acrylic acid, styrene-(meth)acrylic acid copolymer, styrene-maleic acid copolymer, styrene-maleic acid half-ester copolymer, vinylnaphthalene-(meth)acrylic acid copolymer, or vinylnaphthalene-maleic acid copolymer. Styrene-(meth)acrylic acid copolymer is a resin containing units derived from styrene and units derived from (meth)acrylic acid, (meth)acrylic acid ester, or methacrylic acid ester. Examples of styrene-(meth)acrylic acid copolymers include styrene-(meth)acrylic acid-(meth)acrylic acid alkyl ester copolymer, styrene-methacrylic acid-methacrylic acid alkyl ester-(meth)acrylic acid alkyl ester copolymer, styrene-(meth)acrylic acid copolymer, styrene-maleic acid-(meth)acrylic acid alkyl ester copolymer, styrene-methacrylic acid copolymer, or styrene-methacrylic acid alkyl ester copolymer. Among these pigment dispersion resins, styrene-(meth)acrylic acid copolymer is preferred because it is easy to prepare and exhibits excellent dispersion effects for quinacridone pigments.

[0029] (aqueous medium) An aqueous medium is a medium containing water. An aqueous medium may function as a solvent or as a dispersion medium. Examples of aqueous mediums include those containing water and an organic solvent. To enhance compatibility with water, the organic solvent contained in the aqueous medium is preferably a water-soluble organic solvent. A water-soluble organic solvent is an organic solvent that is uniformly miscible with water in any proportion.

[0030] Examples of water-soluble organic solvents include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, γ-butyrolactone, thiodiglycol, and dimethyl sulfoxide.

[0031] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,2-pentanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, diethylene glycol, dipropylene glycol, trimethylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, 2-ethyl-1,2-hexanediol, and thiodiglycol. 3-methyl-1,5-pentanediol is preferred as the glycol compound.

[0032] Examples of triol compounds include glycerin, 1,2,3-butanetriol, and 1,2,6-hexanetriol. Glycerin is preferred as the triol compound.

[0033] Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monoethyl 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. Triethylene glycol monobutyl ether is preferred as the glycol ether compound.

[0034] Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

[0035] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0036] Examples of acetate compounds include diethylene glycol monoethyl ether acetate.

[0037] As an aqueous medium, a mixed solvent of water, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin is preferred.

[0038] In the ink according to this embodiment, the content of the water-soluble organic solvent is preferably 10% by mass or more and 40% by mass or less, and more preferably 20% by mass or more and 30% by mass or less. In the ink, the content of the aqueous medium is preferably 30% by mass or more and 95% by mass or less, and more preferably 70% by mass or more and 95% by mass or less.

[0039] (Other ingredients) The ink according to this embodiment may contain other components as needed. For example, a surfactant may be added to the ink according to this embodiment. Surfactants have the effect of improving the wettability of the ink to the storage medium and improving the compatibility and dispersion stability of each component contained in the ink. Examples of surfactants to be added to the ink according to this embodiment include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. The surfactant is preferably a nonionic surfactant. The nonionic surfactant is preferably a surfactant having an acetylene bond, and more preferably a surfactant having an acetylene glycol structure or an acetylene alcohol structure.

[0040] Furthermore, in addition to surfactants, the ink according to this embodiment may contain various additives as needed, such as penetrating agents, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0041] [Inkjet recording device] An inkjet recording apparatus 1 according to one embodiment of the present invention forms an image on a recording medium using the inkjet ink according to the above embodiment. Figure 1 is a diagram showing the inkjet recording apparatus 1. The X, Y, and Z axes shown in Figure 1 and Figure 2 (described later) define a common orthogonal coordinate system. Note that Figures 1 and 2 are schematic diagrams mainly showing the respective components for ease of understanding, and the size, number, etc. of each component shown may differ from the actual dimensions.

[0042] The inkjet recording device 1 comprises a paper feed unit 3, a first recording head 4C, a second recording head 4M, a third recording head 4Y, a fourth recording head 4K, a liquid storage unit 5, a first transport unit 6, a second transport unit 7, an ejection unit 8, and a maintenance unit 9. Hereinafter, when it is not necessary to distinguish between the first recording head 4C to the fourth recording head 4K, they may simply be referred to as "recording head 4".

[0043] The paper feeding unit 3 comprises a plurality of paper feeding cassettes 31, a plurality of pickup rollers 32, a plurality of transport rollers 33, and a pair of registration rollers 34. Recording media S are stacked and stored in the paper feeding cassettes 31. The pickup rollers 32 pick up the recording media S stored in the paper feeding cassettes 31 one by one. The transport rollers 33 transport the recording media S picked up by the pickup rollers 32. The pair of registration rollers 34 temporarily hold the recording media S transported by the transport rollers 33 and then supply them to the first transport unit 6 at a predetermined timing.

[0044] The recording head 4 is positioned above the first transport belt 63. The first to fourth recording heads 4C to 4K are arranged in this order in the transport direction D of the recording medium S. Each of the first to fourth recording heads 4C to 4K is positioned at the same height. The first to fourth recording heads 4C to 4K are each filled with four different inks (for example, cyan, magenta, yellow, and black). The ink filled in the second recording head 4M is the magenta ink according to this embodiment. Each of the recording heads 4 ejects ink onto the recording medium S. Of the recording heads 4, the second recording head 4M ejects the magenta ink according to the above embodiment onto the recording medium S. As a result, an image (for example, a color image) is formed on the recording medium S transported by the first transport belt 63.

[0045] In the inkjet recording device 1, since the ink according to the above embodiment is used, it has excellent stability in ink ejection from the second recording head 4M, abrasion resistance of the image formed on the recording medium, and redissolution of the ink in cleaning solution during maintenance of the ejection surface of the second recording head 4M.

[0046] The liquid storage unit 5 comprises a first ink tank 51C, a second ink tank 51M, a third ink tank 51Y, a fourth ink tank 51K, and a cleaning fluid tank 52. Hereinafter, when it is not necessary to distinguish between the first ink tank 51C to the fourth ink tank 51K, they may simply be referred to as "ink tank 51". The first ink tank 51C to the fourth ink tank 51K each contain four different colors of ink (for example, cyan, magenta, yellow, and black). The ink contained in the second ink tank 51M is the magenta ink according to this embodiment. The first ink tank 51C to the fourth ink tank 51K each supply ink to the first recording head 4C to the fourth recording head 4K. The cleaning fluid tank 52 supplies cleaning fluid to the liquid-impregnated body 91.

[0047] The first transport unit 6 is located downstream of the paper feeding unit 3 in the transport direction D of the recording medium S. The first transport unit 6 comprises a first driven roller 61, a first drive roller 62, and a first transport belt 63. The first drive roller 62 is located downstream of the first driven roller 61 in the transport direction D of the recording medium S. The first transport belt 63 is an endless belt stretched between the first driven roller 61 and the first drive roller 62. The first drive roller 62 is driven to rotate counterclockwise in Figure 1. This causes the first drive roller 62 to drive the first transport belt 63. As a result, the first transport belt 63 transports the recording medium S fed from the paper feeding unit 3 to the second transport unit 7 in the transport direction D. The first driven roller 61 rotates driven by the first drive roller 62 via the first transport belt 63.

[0048] The second transport unit 7 is located downstream of the first transport unit 6 in the transport direction D of the recording medium S. The second transport unit 7 comprises a second driven roller 71, a second drive roller 72, and a second transport belt 73. The second drive roller 72 is located downstream of the second driven roller 71 in the transport direction D of the recording medium S. The second transport belt 73 is an endless belt stretched between the second driven roller 71 and the second drive roller 72. The second drive roller 72 is driven to rotate counterclockwise in Figure 1. This causes the second drive roller 72 to drive the second transport belt 73. As a result, the second transport belt 73 transports the recording medium S transported from the first transport unit 6 to the discharge unit 8 in the transport direction D. The second driven roller 71 rotates driven by the second drive roller 72 via the second transport belt 73.

[0049] The discharge unit 8 is located downstream of the second transport unit 7 in the transport direction D of the recording medium S. The discharge unit 8 comprises a discharge tray 81, a discharge drive roller 82, and a discharge driven roller 83. The discharge drive roller 82 and the discharge driven roller 83 are pressed against each other at opposing positions. The discharge drive roller 82 is driven to rotate counterclockwise in Figure 1. The discharge driven roller 83 rotates in accordance with the rotation of the discharge drive roller 82. As a result, the discharge drive roller 82 and the discharge driven roller 83 discharge the recording medium S transported from the second transport unit 7 to the discharge tray 81. The discharged recording medium S is placed on the discharge tray 81.

[0050] The maintenance unit 9 comprises a liquid-impregnated body 91 and a cleaning member 92. The liquid-impregnated body 91 and the cleaning member 92 are each provided to be movable between a position below the second transport unit 7 and a position in contact with the ejection surface 42 (see Figure 2) of the recording head 4. The liquid-impregnated body 91 is impregnated with cleaning liquid. The liquid-impregnated body 91 contacts the ejection surface 42 (see Figure 2) of the recording head 4 and supplies cleaning liquid to the ejection surface 42. The liquid-impregnated body 91 is, for example, a sponge, nonwoven fabric, or absorbent sheet. The cleaning member 92 wipes the ejection surface 42 of the recording head 4. This cleans the ink adhering to the ejection surface 42. The cleaning member 92 is, for example, a rubber wiper.

[0051] Next, the recording head 4 will be further described with reference to Figure 2. Figure 1 shows the bottom surface of the recording head 4 shown in Figure 1.

[0052] As shown in Figure 2, the recording head 4 comprises a first nozzle row N1, a second nozzle row N2, and an ejection surface 42. For ease of understanding, in Figure 2, the first nozzle row N1 and the second nozzle row N2 are each enclosed by dashed lines. Each of the first nozzle row N1 and the second nozzle row N2 contains a plurality of nozzles 41. The nozzles 41 eject ink onto the recording medium S. The nozzles 41 open onto the ejection surface 42. The first nozzle row N1 and the second nozzle row N2 are arranged side by side in the transport direction D of the recording medium S. In each of the first nozzle row N1 and the second nozzle row N2, the plurality of nozzles 41 are spaced apart in a direction perpendicular to the transport direction D of the recording medium S. The recording head 4 is, for example, a line-type recording head.

[0053] The width 41w of each of the first nozzle row N1 and the second nozzle row N2 (i.e., the width of the area that can be recorded by the recording head 4) is equal to or wider than the width of the recording medium S. Therefore, the recording head 4 can record an image on the recording medium S being transported on the first transport belt 63 while remaining fixed. In other words, the inkjet recording device 1 employs a single-pass method, which does not involve shuttle motion. Because the inkjet recording device 1 of the second embodiment is equipped with such a recording head 4, it can print at a higher speed compared to an inkjet recording device equipped with a serial head.

[0054] The inkjet recording device 1 is not limited to the above configuration. The inkjet recording device 1 may employ a multi-pass method. In addition, in the inkjet recording device 1, the number of nozzles 41, the spacing between nozzles 41, and the positional relationship of the nozzles 41 in the first recording head 4C to the fourth recording head 4K can be appropriately set according to the specifications of the device. Furthermore, the arrangement of the first recording head 4C to the fourth recording head 4K in the inkjet recording device 1 is not limited to that shown in the diagram, and other arrangements are also possible. In addition, the number of recording heads 4 in the inkjet recording device 1 is not limited to 4, but may be 1 to 3, or 5 or more.

[0055] [Examples and Comparative Examples] Ink preparation and evaluation were performed as examples and comparative examples of the present invention. Note that the following examples merely illustrate one aspect of the present invention, and the present invention is not limited to the configurations of the following examples.

[0056] (Ink adjustment) In the examples and comparative examples, the pigment dispersion was prepared first. The pigment dispersion was prepared by blending quinacridone pigment, pigment dispersion resin, sodium hydroxide, Orphine® E1010, and water in the quantities shown in Table 1, and then performing a dispersion treatment.

[0057] [Table 1]

[0058] Sodium hydroxide was added as a neutralizing agent to neutralize the pigment dispersion resin. Olphine® E1010 was added as a dispersant to improve the dispersibility of quinacridone pigment in aqueous media and is a nonionic surfactant manufactured by Nisshin Chemical Industry Co., Ltd. In both the examples and comparative examples, CI Pigment Red 122 was used as the quinacridone pigment, and methacrylic acid-methyl methacrylate-butyl acrylate-styrene copolymer (molecular weight: 20,000, acid value: 100 mg KOH / g) was used as the pigment dispersion resin.

[0059] The pigment dispersion was prepared by mixing the above components using a wet dispersion process in a media-type wet disperser. Examples of media-type wet dispersers include wet dispersers (more specifically, "NanoGlenMill" manufactured by Asada Iron Works Co., Ltd., "MSC Mill" manufactured by Nippon Coke Industries Co., Ltd., and "DinoMill" manufactured by Shinmaru Enterprises, Ltd.). The dispersion process time was set to between 30 minutes and 120 minutes.

[0060] In wet dispersion using a media-type wet disperser, media (zirconia beads with a diameter of 0.5 mm) were set in the vessel, and the discharge rate was controlled to 300-400 g / min to adjust the average particle size of the pigment dispersion, in which the dispersant adhered to the quinacridone pigment dispersed in water, to 90-140 nm. The particle size distribution of the pigment dispersion was measured using a Zetasizer Nano manufactured by Sysmex Corporation, with the pigment dispersion diluted 300 times with deionized water.

[0061] To reduce the amount of unadsorbed resin in the ink, centrifugation and displacement processes were performed on the pigment dispersion as appropriate. In the centrifugation process, the pigment dispersion is separated by centrifugation to precipitate the pigment dispersion. On the other hand, unadsorbed resin free in the aqueous medium does not easily settle even when centrifuged, and most of it remains as the supernatant. Therefore, centrifugation can separate the adsorbed resin from the unadsorbed resin in the pigment dispersion. For example, the conditions for the centrifugation process can be a rotation speed of 10,000 rpm to 200,000 rpm and a centrifugation time of 12 hours to 48 hours.

[0062] In the displacement step, precipitates are isolated from the pigment dispersion and dispersed in another aqueous medium. The centrifugation and displacement steps allow for the obtaining of a pigment dispersion from which most of the unadsorbed resin that would otherwise be released into the aqueous medium has been removed. This reduces the amount of unadsorbed resin contained in the ink. Water is preferred as the aqueous medium used in this step. Furthermore, the centrifugation and displacement steps can also reduce the amount of intermediates and phosphorus-containing catalysts in the pigment dispersion.

[0063] Furthermore, ultrafiltration was performed on the pigment dispersion as appropriate to reduce the amount of phosphorus-containing catalyst in the ink. Ultrafiltration removes at least a portion of the phosphorus-containing catalyst, thereby reducing the phosphorus concentration in the pigment dispersion. To remove at least a portion of the phosphorus-containing catalyst, it is preferable to select an ultrafiltration membrane with a molecular weight cutoff that filters out at least a portion of the phosphorus-containing catalyst while leaving other components of the pigment dispersion untouched. To effectively reduce the phosphorus concentration, the ultrafiltration circulation time is preferably 0.4 hours or longer. To ensure redispersibility of the ink with respect to the cleaning solution, the ultrafiltration circulation time is preferably 2.0 hours or less. Through ultrafiltration, the liquid containing the phosphorus-containing catalyst is discharged as filtrate, and the liquid containing components of the pigment dispersion other than the phosphorus-containing catalyst is recycled as a recovered liquid. By adding the same amount of water as the discharged filtrate to the recovered liquid, the solid content concentration of the pigment dispersion can be maintained at a constant level.

[0064] Next, the inks for the examples and comparative examples were prepared. The inks for the examples and comparative examples were prepared by blending the above-mentioned pigment dispersion with Surfinol® 420, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, glycerin, and water in the quantities shown in Table 2. Surfinol® 420 is a surfactant manufactured by Nisshin Chemical Industry Co., Ltd. Triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin are water-soluble organic solvents.

[0065] [Table 2]

[0066] In the preparation of the inks in the examples and comparative examples, the components shown in Table 2 were added sequentially while stirring the solvent with a stirrer. Furthermore, each ink was filtered after stirring using a filter with a pore size of φ5 μm. This reduces the amount of foreign matter, dirt, and coarse particles contained in the ink. By varying the filtration conditions and number of times, the amount of coarse particles can be changed.

[0067] (Ink analysis) • Centrifugal processing Under conditions of 23°C, 2g of ink sealed in a container was centrifuged for 3 hours at a rotation speed of 140,000 rpm (equivalent to a centrifugal force of 1,050,000 G) using an ultracentrifuge (Eppendorf Hi-Mac Technologies Co., Ltd. "himac® CS150FNX", rotor: S140AT). This allowed the pigment dispersion contained in the ink to settle, and the supernatant liquid was obtained.

[0068] • Acquisition of ultraviolet-visible light absorption spectrum The supernatant was collected using a 1 mL syringe. The collected supernatant was diluted 25-fold with water and used as the measurement sample. The measurement sample, placed in a cell, was measured using a spectrophotometer (Hitachi High-Tech Science Co., Ltd. "U-3000") under the following conditions to obtain the ultraviolet-visible light absorption spectrum of the measurement sample.

[0069] The measurement conditions using the spectrophotometer were as follows: Measurement wavelength range: 200 nm to 800 nm Scan speed: 300nm / min Sampling interval: 1.00 nm Slit width: 1 nm Cell: A cell made of quartz glass. Optical path length: 10mm Beam configuration: Double beam Baseline measurement: Yes Reference: Deionized water

[0070] • Measurement of phosphorus concentration The supernatant was collected using a 1 mL syringe. The collected supernatant was diluted 50-fold with water and used as the measurement sample. The measurement sample was measured using an ICP (Inductively Coupled Plasma) mass spectrometer (Thermo Fisher Scientific "iCAP PRO ICP-OES Duo"). From the measured values, the phosphorus concentration was determined based on a calibration curve.

[0071] • Ratio of unadsorbed resin The supernatant liquid obtained from 2g of ink was dried under reduced pressure at 60°C for 24 hours. The mass of unadsorbed resin contained in 2g of ink was obtained by measuring the weight after reduced pressure drying. The ratio (mass %) of unadsorbed resin in the pigment dispersion resin was then determined from the obtained mass of unadsorbed resin and the theoretical value of the mass of pigment dispersion resin contained in 2g of ink.

[0072] (Ink evaluation) The inks used in the examples and comparative examples were evaluated for ejection irregularity, in-machine contamination, abrasion resistance, and cleanability. The evaluation of ejection irregularity and in-machine contamination corresponds to the evaluation of ejection stability.

[0073] • Evaluation method for discharge turbulence For evaluation, an inkjet recording device (a prototype manufactured by Kyocera Document Solutions Inc.) was used. This evaluation device consists of a transport unit and a piezo-type line head with nozzles (opening radius: 10 μm) as the recording head. Ink was loaded into the magenta ink recording head of the evaluation device. Plain paper (Fuji Xerox Co., Ltd. "C2", A4 size PPC paper) was used as the paper.

[0074] In the evaluation machine, the recording head temperature was set to 40°C, and the ink ejection amount per pixel was set to 3.5 pL. Using the evaluation machine, an image (20.5 mm x 29.0 mm) with image processing settings that ejected ink from all nozzles of the recording head was printed continuously on paper for one hour. The first image printed during continuous printing (initial image) and the last image printed during continuous printing (end-printed image) were observed with the naked eye. The amount of white streaks in the initial image and the end-printed image was then checked. White streaks are image defects caused by uneven ink ejection from the recording head. The unevenness of each ink was evaluated according to the following A and B criteria. For uneven ejection, inks with an evaluation of A are considered acceptable, and inks with an evaluation of B are considered unacceptable. A (Good): No change in the amount of white streaks is observed between the initial image and the print-saturated image. B (Defective): The print-tested image has more white streaks than the initial image.

[0075] • In-flight contamination Ink droplets with longer satellites ejected from the recording head are more likely to generate mist and cause in-flight contamination. Therefore, the likelihood of in-flight contamination by an ink was evaluated based on the length of the ink droplet's satellite. In the evaluation machine, the ink ejection amount per pixel was set to 5.5 pL, and the ink ejection speed was set to 8 m / s. Ten images of the ink's flight were captured from the recording head, and the length of the ink droplet's satellite was measured for each of the ten flight images. For each ink, the maximum satellite length was used as the evaluation value for in-flight contamination. The evaluation value for in-flight contamination for each ink was evaluated according to the following A and B criteria. Ink with an evaluation of A is considered acceptable for in-flight contamination, and ink with an evaluation of B is considered unacceptable. A (Good): Less than 0.3 mm B (defective): 0.3mm or more

[0076] ·Abrasion resistance A 4cm x 5cm solid image was formed on a single recording medium (Fuji Xerox Co., Ltd. "C2", A4 size PPC paper) using an evaluation machine. For solid image formation, the ink ejection rate was set to 11 pL per pixel. Next, a blank sheet of paper (the same recording medium without printing) was placed on the surface of the recording medium with the solid image (the solid image side). Then, while applying a 1kg load to the blank sheet using a weight, the solid image was rubbed back and forth five times with one side of the blank sheet. Afterward, the image density of the non-image-forming area of ​​the recording medium with the solid image was measured using a reflectivity densitometer (X-Rite "RD-19"). The maximum image density measured in the non-image-forming area was used as the evaluation value for scratch resistance. The scratch resistance evaluation value for each ink was evaluated according to the following A and B criteria. For scratch resistance, an A rating indicates a pass, and a B rating indicates a fail. A (Good): Less than 0.020 B (bad): 0.020 or more

[0077] • Cleanability In the evaluation of cleaning performance, a wiper with 0.3cc of ink on its tip was left for 10 minutes and slid along the ejection surface of the recording head in the forward direction (rightward) to the left side of the nozzle formation area to form an ink layer on the ejection surface. After drying the ink layer formed on the ejection surface by holding it at 45°C for 4 days, a nonwoven fabric (Asahi Kasei's "Bencott") impregnated with 3g of cleaning solution was pressed against the ejection surface and separated after 30 seconds. Then, 0.3cc of ink was ejected (purged) onto the ejection surface, and the wiper was slid along the ejection surface in the return direction (leftward) to the right side of the nozzle formation area. After that, the degree of ink adhesion on the ejection surface was visually observed. The cleaning performance of each ink was evaluated according to the following A and B criteria. For cleaning performance, inks with an evaluation of A are considered acceptable, and inks with an evaluation of B are considered unacceptable. A (Good): No ink smudges are visible. B (Defective): Ink stains are visible.

[0078] (Examples 1-9) In Examples 1-9, inks were prepared by varying the amounts of unadsorbed resin, intermediate, coarse particles, and phosphorus-containing catalyst. In Examples 1-9, the dispersion time, discharge rate, and ultrafiltration circulation time during the preparation of the pigment dispersion were varied. Table 3 shows the dispersion time, discharge rate, and ultrafiltration circulation time during the preparation of the pigment dispersion for Examples 1-9. Table 3 also shows the maximum absorbance at wavelengths between 300 nm and 350 nm, the absorbance at 429 nm, the number of coarse particles, the phosphorus concentration, the average particle size of the pigment dispersion, and the ratio of unadsorbed resin in the pigment dispersion resin in the ultraviolet-visible light absorption spectrum of a 25-fold diluted supernatant for the inks related to Examples 1-9.

[0079] [Table 3]

[0080] Table 4 shows the evaluation results for ejection irregularity, in-machine contamination, abrasion resistance, and cleanability of the inks used in Examples 1 to 9. All of the inks used in Examples 1 to 9 passed the evaluations for ejection irregularity, in-machine contamination, abrasion resistance, and cleanability.

[0081] [Table 4]

[0082] (Comparative Examples 1-6) In Comparative Examples 1 to 6, inks were prepared in which at least one of the following elements—unadsorbed resin, intermediate, coarse particles, and phosphorus-containing catalyst—differed from those in the above examples. In Comparative Examples 1 to 6, the dispersion treatment time, discharge rate, and ultrafiltration circulation time during the preparation of the pigment dispersion were varied. Table 5 shows the dispersion treatment time, discharge rate, and ultrafiltration circulation time during the preparation of the pigment dispersion for Comparative Examples 1 to 6. Table 5 also shows the maximum absorbance at wavelengths between 300 nm and 350 nm, the absorbance at 429 nm, the number of coarse particles, the phosphorus concentration, the average particle size of the pigment dispersion, and the ratio of unadsorbed resin in the pigment dispersion resin in the ultraviolet-visible light absorption spectrum of a 25-fold diluted supernatant for the inks related to Comparative Examples 1 to 6.

[0083] [Table 5]

[0084] The ink according to Comparative Example 1 differs from the ink according to the above example in that its phosphorus concentration is greater than 16.0 ppm. The ink according to Comparative Example 2 has a number of coarse particles of 4.0 × 10 8 The ink according to Comparative Example 3 differs from the ink according to the above example in that it has more than 4.0 × 10¹⁶ coarse particles per mL. 8 The ink in Comparative Example 4 differs from the ink in the above example in that it has more than 10 particles / mL and the ratio of unadsorbed resin in the pigment dispersion resin is less than 20% by mass. The ink in Comparative Example 4 differs from the ink in the above example in that the maximum absorbance at wavelengths of 300 nm to 350 nm in the ultraviolet-visible light absorption spectrum is more than 0.40. The ink in Comparative Example 5 differs from the ink in the above example in that the phosphorus concentration is less than 1.0 ppm and the ratio of unadsorbed resin in the pigment dispersion resin is more than 50% by mass. The ink in Comparative Example 6 has a number of coarse particles of 4.0 × 10 8 This differs from the ink according to the above example in that it has a concentration of more than 1 / mL.

[0085] Table 6 shows the evaluation results for ejection irregularity, internal contamination, abrasion resistance, and cleanability of the inks related to Comparative Examples 1 to 6. The ink related to Comparative Example 1, which had a large amount of phosphorus-containing catalyst, failed in terms of ejection irregularity. The ink related to Comparative Example 2, which had a large amount of coarse particles, failed in terms of ejection irregularity and internal contamination. The ink related to Comparative Example 3, which had a large amount of coarse particles and a low ratio of unadsorbed resin in the pigment dispersion resin, failed in terms of abrasion resistance. The ink related to Comparative Example 4, which had a large amount of unadsorbed resin, failed in terms of ejection irregularity. The ink related to Comparative Example 5, which had a small amount of phosphorus-containing catalyst and a high ratio of unadsorbed resin in the pigment dispersion resin, failed in terms of cleanability. The ink related to Comparative Example 6, which had a large amount of coarse particles, failed in terms of internal contamination.

[0086] [Table 6] [Explanation of Symbols]

[0087] 1… Inkjet recording device 4…Recording head 6…First Conveyor Unit S... Recording media

Claims

1. An ink containing a quinacridone pigment, a pigment dispersion resin, and an aqueous medium, The ultraviolet-visible light absorption spectrum of a 25-fold diluted supernatant obtained by centrifuging the ink at 140,000 rpm for 3 hours at 23°C shows that the maximum absorbance at wavelengths between 300 nm and 350 nm is 0.16 or more and 0.40 or less, and the absorbance at 429 nm is 0.002 or less. The number of coarse particles with a particle diameter of 500 nm or more is 4.0 × 10 ８ It is less than or equal to 1 / mL. The phosphorus concentration in the 50-fold diluted solution of the supernatant liquid is between 1.0 ppm and 16.0 ppm. Inkjet ink.

2. An inkjet ink according to claim 1, The pigment dispersion resin is composed of an adsorbed resin adsorbed onto the quinacridone pigment and an unadsorbed resin not adsorbed onto the quinacridone pigment. The ratio of the unadsorbed resin in the pigment dispersion resin is 20% by mass or more and 50% by mass or less. Inkjet ink.

3. It comprises a transport unit for transporting a recording medium and a line-type recording head for ejecting ink onto the recording medium, The ink includes the inkjet ink described in claim 1 or 2. Inkjet recording device.

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