Inkjet inks and inkjet recording devices

Abstract

This invention provides an inkjet ink containing quinacridone pigment and exhibiting excellent intermittent ejection properties. [Solution] The inkjet ink contains a quinacridone pigment, a pigment dispersion resin, and an aqueous medium. 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 weight-average molecular weight of the unadsorbed resin is 5,000 or less. The ratio of the unadsorbed resin to the pigment dispersion resin is 10% by mass or less. The phosphorus concentration in a 50-fold diluted solution of the supernatant obtained by centrifuging the ink at 140,000 rpm for 3 hours at 23°C is 0.5 ppm or less.

Description

Technical Field

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

Background Art

[0002] Inkjet inks are required to have ejection stability when ejected from nozzles of a recording head. Patent Document 1 discloses a technique for improving ejection stability in an inkjet ink using a quinacridone pigment.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of Invention

Problems to be Solved by the Invention

[0004] The inventors of the present application have found that in inks using quinacridone pigments, drying progresses inside the nozzles of the recording head, which tends to cause ejection failure, that is, it is difficult to ensure intermittent ejection performance.

[0005] In view of the circumstances as described above, an object of the present invention is to provide an inkjet ink that contains a quinacridone pigment and has excellent intermittent ejection performance.

Means for Solving the Problems

[0006] To achieve the above object, an inkjet ink according to an embodiment of the present invention contains a quinacridone pigment, a pigment dispersion resin, and an aqueous medium. The pigment dispersion resin is composed of an adsorbed resin adsorbed on the quinacridone pigment, and a non-adsorbed resin that is not adsorbed on the quinacridone pigment. The weight average molecular weight of the non-adsorbed resin is 5,000 or less. The ratio of the unadsorbed resin in the pigment-dispersed resin is 10% by mass or less. The phosphorus concentration in a 50-fold diluted solution of the supernatant obtained by centrifuging the ink at 140,000 rpm for 3 hours at 23°C is 0.5 ppm or less.

[0007] This inkjet ink achieves excellent intermittent ejection by controlling the weight-average molecular weight and ratio of unadsorbed resin that is not adsorbed to the quinacridone pigment in the pigment dispersion resin, and the residual amount of phosphorus-containing catalyst used to synthesize the quinacridone pigment.

[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 containing quinacridone pigment and exhibiting excellent intermittent ejection properties. [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, excellent intermittent ejection can be achieved by controlling the weight-average molecular weight and ratio of the unadsorbed resin and the remaining amount of the phosphorus-containing catalyst used to synthesize the quinacridone pigment.

[0013] Specifically, in the ink according to this embodiment, the weight-average molecular weight of the unadsorbed resin is 5,000 or less, and the ratio of the unadsorbed resin to the pigment-dispersed resin is 10% by mass or less. In other words, in the ink according to this embodiment, by reducing the amount of unadsorbed resin and also reducing the weight-average molecular weight of the remaining unadsorbed resin, the adhesion of the unadsorbed resin to the nozzle surface of the recording head can be effectively suppressed. As a result, good intermittent ejection performance can be obtained with the ink according to this embodiment.

[0014] Furthermore, in the ink according to this embodiment, substantially all of the phosphorus can be considered to originate from the phosphorus-containing catalyst, so the amount of phosphorus-containing catalyst can be determined from the phosphorus concentration. More specifically, in this embodiment, the amount of unadsorbed resin in the ink is determined by the phosphorus concentration in a 50-fold diluted solution of the supernatant liquid (hereinafter also simply referred to as "supernatant liquid") obtained by centrifuging the ink at 140,000 rpm for 3 hours at 23°C. In the ink according to this embodiment, the amount of phosphorus-containing catalyst remaining, which causes a decrease in intermittent discharge performance, is adjusted so that the phosphorus concentration in a 50-fold diluted solution of the supernatant liquid is 0.5 ppm or less.

[0015] [Quinacridone pigment] Examples of quinacridone pigments usable in the ink according to the present embodiment include quinacridone magenta, quinacridone red, quinacridone orange, quinacridone gold, quinacridone violet, and the like.

[0016] An example of the method for synthesizing the 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 reactions represented by reaction formulas (r-a), (r-b), and (r-c).

[0017]

Chemical Formula

[0018] In Formulas (A), (B), (C), and (D), R A , R B , R 1 , and R 2 each independently represent a monovalent group. Hereinafter, the "reactions represented by reaction formulas (r-a), (r-b), and (r-c)" may be respectively referred to as "reactions (r-a), (r-b), and (r-c)". In addition, the "compounds represented by Formulas (A), (B), (C), and (D)" may be respectively referred to as "compounds (A), (B), (C), and (D)". R 1 and R 2 each represent a methyl group, the compound (D) is C.I. Pigment Red 122. R 1 and R 2 each represent a hydrogen atom, the compound (D) is C.I. Pigment Violet 19.

[0019] In reaction (ra), compound (B) is obtained from compound (A). Then, in reaction (rb), 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 (rc), compound (C) is oxidized to obtain an oxide of compound (C). Then, in reaction (rc), the oxide of compound (C) is hydrolyzed to obtain a hydrolysate. Then, in reaction (rc), the hydrolysate is subjected to a dehydration and cyclization reaction using a phosphorus-containing catalyst to obtain compound (D). The reaction temperature for the dehydration and cyclization reaction is, for example, 90°C to 120°C. The reaction time for the dehydration and cyclization reaction is, for example, 1 hour to 2 hours.

[0020] The phosphorus-containing catalyst used in reaction (rc) is, for example, at least one selected from the group consisting of polyphosphate and its derivatives. Examples of polyphosphate derivatives include polyphosphate esters, and more specifically, alkyl polyphosphate esters. Among the alkyl polyphosphate esters, methyl polyphosphate is preferred.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] (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 about tens of thousands. Furthermore, the acid value of the pigment dispersion resin is preferably between 150 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).

[0025] 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.

[0026] In the ink according to this embodiment, it is preferable that the pigment dispersion resin content be 0.5% by mass or more in order to fully obtain the effect of the pigment dispersion resin described above. Furthermore, in the ink according to this embodiment, it is preferable that the pigment dispersion resin content be 7% by mass or less in order to obtain high dispersion stability.

[0027] (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.

[0028] 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.

[0029] 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.

[0030] Examples of triol compounds include glycerin, 1,2,3-butanetriol, and 1,2,6-hexanetriol. Glycerin is preferred as the triol compound.

[0031] 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.

[0032] Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

[0033] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0034] Examples of acetate compounds include diethylene glycol monoethyl ether acetate.

[0035] As an aqueous medium, a mixed solvent of water, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin is preferred.

[0036] In the ink according to this embodiment, the content of the water-soluble organic solvent is preferably 15% by mass or more and 35% by mass or less. In the ink according to this embodiment, the content of the aqueous medium is preferably 35% by mass or more and 55% by mass or less.

[0037] (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.

[0038] 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.

[0039] [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.

[0040] 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".

[0041] 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.

[0042] 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.

[0043] In the inkjet recording device 1, since the ink according to the above embodiment is used, the intermittent ejection of ink from the second recording head 4M is excellent.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] [Examples] As an embodiment of the present invention, ink preparation and evaluation were carried out. It should be noted that the following embodiment is merely an example of the present invention, and the present invention is not limited to the configuration of the embodiment described below.

[0054] (Ink preparation) In Examples 1-3, 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.

[0055] [Table 1]

[0056] Sodium hydroxide was added as a neutralizing agent to neutralize the pigment dispersion resin. Olphine® E1010 is a nonionic surfactant manufactured by Nisshin Chemical Industry Co., Ltd., and was added as a dispersant to improve the dispersibility of quinacridone pigment in aqueous media. In all of the examples, CI Pigment Red 112 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.

[0057] 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.).

[0058] In wet dispersion using a media-type wet disperser, media (e.g., zirconia beads with a diameter of 0.5 mm) were set in the vessel, and the discharge rate was controlled to adjust the average particle size of the pigment dispersion, in which the dispersant adhered to the quinacridone pigment dispersed in water, to 70-130 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. In wet dispersion, the amount of unadsorbed resin can be adjusted by changing the diameter of the media beads.

[0059] Ultrafiltration was performed on the pigment dispersion to reduce the amount of unadsorbed resin and phosphorus-containing catalyst in the ink. Ultrafiltration removes at least a portion of the unadsorbed resin and phosphorus-containing catalyst. Ultrafiltration was performed using a vibratory ultrafiltration method with a Bensil-type module UF membrane "AHP-0013D" (manufactured by Asahi Kasei Corporation). In ultrafiltration, the amount of unadsorbed resin and phosphorus-containing catalyst in the ink was adjusted by varying the circulation time. After ultrafiltration, the liquid containing the unadsorbed resin and phosphorus-containing catalyst is discharged as filtrate, and the liquid containing the components of the pigment dispersion other than the unadsorbed resin and phosphorus-containing catalyst is circulated again 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.

[0060] Next, the ink according to the example was prepared. The ink according to the example was prepared by blending the above-mentioned pigment dispersion, Surfinol® 420, triethylene glycol monobutyl ether, glycerin, and water in the amounts shown in Table 2. Surfinol® 420 is a surfactant manufactured by Nisshin Chemical Industry Co., Ltd. Triethylene glycol monobutyl ether and glycerin are water-soluble organic solvents.

[0061] [Table 2]

[0062] In the preparation of the inks according to the examples, the components shown in Table 2 were added in order while stirring the solvent with a stirrer. Furthermore, each ink after stirring was filtered 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.

[0063] (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 caused the pigment dispersion in the ink to precipitate, and the supernatant liquid was obtained.

[0064] • 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.

[0065] • Weight-average molecular weight of unadsorbed resin The weight-average molecular weight of the unadsorbed resin was measured using gel filtration chromatography (HLC-8020GPC, manufactured by Tosoh Corporation) under the following conditions. • Column: TSKgel SuperMultiporeHZ-H manufactured by Tosoh Corporation (semi-micro column with 4.6mm I.D. x 15cm) • Number of columns: 3 • Eluent: Tetrahydrofuran ·Flow rate: 0.35mL / min • Sample injection volume: 10 μL ·Measurement temperature: 40℃ • Detector: IR detector The calibration curve was created 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—along with n-propylbenzene.

[0066] • 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 by ICP (Inductively Coupled Plasma) emission spectrometry. From the measured values, the phosphorus concentration was determined based on a calibration curve. The calibration curve was created based on the results of measuring samples with phosphorus concentrations of 0.1 ppm, 0.5 ppm, and 1 ppm using phosphorus standard solution PO43-1000 (manufactured by Tomioka Wako Pure Chemical Industries, Ltd.) by ICP emission spectrometry. An "ICPS-8100" (manufactured by Shimadzu Corporation) was used as the ICP emission spectrometer.

[0067] (Ink evaluation) The intermittent ejection properties of the inks used in the examples were evaluated. In the evaluation of intermittent ejection properties, first, a recording head was used to adjust the ejection amount per nozzle to 3.5 pL using the ink to be evaluated on the recording medium, and a single-dot line image was formed. Then, using the recording head that had been left at 40°C for 3 hours, the same operation was performed again to form a single-dot line image on the recording medium using the ink to be evaluated. As a result, inks that formed an image of the same quality as the image formed before the waiting period were judged as passing (rating A), and inks that did not form an image of the same quality as the image formed before the waiting period were judged as failing (rating B).

[0068] (Example 1) In Example 1, ink samples 1-6 were prepared by varying the ultrafiltration circulation time during the preparation of the pigment dispersion, and the intermittent dispensing performance was evaluated for samples 1-6. For all samples 1-6, the dispensing rate during the preparation of the pigment dispersion was set to 150 g / min. Table 3 shows the ultrafiltration circulation time, weight-average molecular weight of unadsorbed resin, ratio of unadsorbed resin in the pigment dispersion resin, phosphorus concentration, and evaluation results for intermittent dispensing performance for samples 1-6. Samples 1-5, where the average molecular weight of the unadsorbed resin was 5,000 or less, the ratio of unadsorbed resin in the pigment dispersion resin was 10% by mass or less, and the phosphorus concentration was 0.5 ppm or less, all passed the intermittent dispensing performance evaluation. On the other hand, sample 6, where the ultrafiltration circulation time was short, the ratio of unadsorbed resin in the pigment dispersion resin was over 10% by mass, and the phosphorus concentration was over 0.5 ppm, failed the intermittent dispensing performance evaluation.

[0069] [Table 3]

[0070] (Example 2) In Example 2, ultrafiltration during the preparation of the pigment dispersion was performed in two stages: a first ultrafiltration and a second ultrafiltration performed after the first ultrafiltration. Samples 7-10 of the ink were prepared by varying the circulation times of the first and second ultrafiltrations, and the intermittent dispensing performance was evaluated for samples 7-10. Table 4 shows the circulation time of the first ultrafiltration, the circulation time of the second ultrafiltration, the weight-average molecular weight of the unadsorbed resin, the ratio of unadsorbed resin in the pigment dispersion resin, the phosphorus concentration, and the evaluation results of the intermittent dispensing performance for samples 7-10. Samples 7-9, in which the average molecular weight of the unadsorbed resin was 5,000 or less, the ratio of unadsorbed resin in the pigment dispersion resin was 10% by mass or less, and the phosphorus concentration was 0.5 ppm or less, all passed the intermittent dispensing performance test. On the other hand, sample 10, in which the weight-average molecular weight of the unadsorbed resin was greater than 5,000, failed the intermittent dispensing performance test. The evaluation results for intermittent discharge performance in Example 2 showed that when ultrafiltration was performed in two stages, it tended to be more difficult to remove unadsorbed resin with a large weight-average molecular weight. In particular, it was found that a short circulation time in the first ultrafiltration was disadvantageous from the standpoint of removing unadsorbed resin with a large weight-average molecular weight.

[0071] [Table 4]

[0072] (Example 3) In Example 3, ink samples 11-15 were prepared by varying the discharge volume during the preparation of the pigment dispersion, and the intermittent discharge performance was evaluated for samples 11-15. For all samples 11-15, ultrafiltration was performed only once during the preparation of the pigment dispersion, with a circulation time of 3.5 hours. Table 5 shows the evaluation results for discharge volume, weight-average molecular weight of unadsorbed resin, ratio of unadsorbed resin in the pigment dispersion resin, phosphorus concentration, and intermittent discharge performance for samples 11-15. Samples 11-14, where the average molecular weight of the unadsorbed resin was 5,000 or less, the ratio of unadsorbed resin in the pigment dispersion resin was 10% by mass or less, and the phosphorus concentration was 0.5 ppm or less, all passed the intermittent discharge performance evaluation. On the other hand, sample 15, with a high discharge volume (i.e., short dispersion time) and a ratio of unadsorbed resin in the pigment dispersion resin exceeding 10% by mass, failed the intermittent discharge performance evaluation.

[0073] [Table 5] [Explanation of Symbols]

[0074] 1… Inkjet recording device 4…Recording head 6…First Conveyor Unit S... Recording media

Claims

1. An inkjet ink containing a quinacridone pigment, a pigment dispersion resin, and an aqueous medium, The pigment dispersion resin is composed of an adsorbed resin adsorbed onto the quinacridone pigment and an unadsorbed resin that is not adsorbed onto the quinacridone pigment. The weight-average molecular weight of the unadsorbed resin is 5,000 or less. The ratio of the unadsorbed resin in the pigment dispersion resin is 10% by mass or less. The phosphorus concentration in a 50-fold diluted solution of the supernatant obtained by centrifuging the ink at 140,000 rpm for 3 hours at 23°C is 0.5 ppm or less. Inkjet ink.

2. 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. Inkjet recording device.

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