Inkjet recording method, and inkjet recording apparatus

Abstract

To provide an inkjet recording method capable of recording a high quality image for a prolonged period while suppressing reduction of an optical concentration of an edge of a black image adjacent to a color image even when using an inkjet recording apparatus including a high capacity ink-housing part having an openable and closable ink inlet.SOLUTION: An inkjet recording method includes: a process of recording an image using an inkjet recording apparatus including a first ink-housing part having an openable and closable ink inlet, a second ink-housing part, and a recording head for discharging an aqueous ink. The aqueous ink includes a black ink containing carbon black and an acrylic resin particle, and a color ink containing an organic pigment, and a content P1 (mass%) of carbon black in the black ink, and a content P2 (mass%) of the organic pigment in the color ink satisfy the relationship of the following formula (1). P1≤P2 (1).SELECTED DRAWING: None

Description

[Technical field]

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

[0002] In recent years, with the increase in remote work from home, there is a demand for inkjet recording devices that can record images with excellent color development on recording media such as plain paper with high productivity. In order to record images with excellent color development, for example, water-based pigment inks containing pigments as coloring materials are widely used. Among them, black inks containing carbon black as a coloring material and color inks containing colored organic pigments as coloring materials are widely used.

[0003] There is known an ink tank that is replaced when the remaining amount of ink contained therein becomes low. Also, in order to achieve high productivity, an inkjet recording device has been proposed that is provided with a large-capacity main tank as an ink storage section. As such a large-capacity main tank, it is being considered to use a tank that injects ink when the remaining amount of ink becomes low, rather than a replaceable tank as proposed in Patent Document 1. For example, an inkjet recording device has been proposed that is provided with an ink-injection type main tank that can be used with pigment ink (Patent Document 2).

[0004] For the main tank of the ink injection type, for example, a method of injecting ink from an ink bottle is adopted. That is, an ink injection port that can be opened and closed is provided in the main tank, and the timing and amount of ink injection can be appropriately adjusted. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-147984 A [Patent Document 2] JP 2017-001391 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors used an inkjet recording device equipped with an ink injection tank as proposed in Patent Document 2 to apply black ink containing carbon black and color ink containing organic pigments to a recording medium to record images for a long period of time. As a result, it was found that the optical density of the edge of the black image may decrease at the boundary between a black image and a color image recorded by applying black ink and color ink adjacent to each other. Furthermore, it was found that the decrease in optical density of the edge of the black image becomes more noticeable after recording images for a long period of time.

[0007] Therefore, an object of the present invention is to solve the problems that arise when recording images using an inkjet recording device equipped with a large-capacity ink storage unit having an openable and closable ink inlet. That is, an object of the present invention is to provide an inkjet recording method that can record high-quality images for a long period of time while suppressing a decrease in optical density at the edge of a black image adjacent to a color image, even when using such an inkjet recording device. Another object of the present invention is to provide an inkjet recording device used in this inkjet recording method. [Means for solving the problem]

[0008] That is, according to the present invention, there is provided an inkjet recording method comprising a step of applying the aqueous ink ejected from the ejection port to a recording medium using an inkjet recording device including a first ink holder having an openable and closable ink inlet through which aqueous ink can be refilled, a second ink holder connected to the first ink holder via a tube and holding the aqueous ink, and a recording head connected to the second ink holder and having an ejection port through which the aqueous ink supplied from the second ink holder is ejected, the inkjet recording method comprising the steps of: applying the aqueous ink ejected from the ejection port to a recording medium to record an image, the aqueous ink comprising a black ink containing carbon black and acrylic resin particles, and color inks containing an organic pigment, the carbon black content P1 (mass %) in the black ink and the organic pigment content P2 (mass %) in the color inks satisfying the relationship of the following formula (1): P1≦P2 (1) Effect of the Invention

[0009] According to the present invention, it is possible to solve the problems that arise when recording images using an inkjet recording device equipped with a large-capacity ink storage unit having an openable and closable ink inlet. That is, it is possible to provide an inkjet recording method that can record high-quality images for a long period of time while suppressing a decrease in optical density at the edge of a black image adjacent to a color image, even when such an inkjet recording device is used. Furthermore, according to the present invention, it is possible to provide an inkjet recording device used in this inkjet recording method. [Brief description of the drawings]

[0010] [Figure 1] 1 is a perspective view illustrating an embodiment of an inkjet recording apparatus of the present invention. [Diagram 2] FIG. 2 is a schematic diagram illustrating an example of an ink supply system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is present in the ink in the form of dissociation into ions, but for convenience, it is expressed as "containing a salt." In addition, water-based ink for inkjet printing may be simply referred to as "ink." Physical property values ​​are values ​​at room temperature (25°C) unless otherwise specified.

[0012] The present inventors first prepared an inkjet recording device equipped with a black ink containing carbon black but not acrylic resin particles, color ink containing an organic pigment, and a first ink storage section (main tank) having an openable and closable ink inlet. The black ink and color ink were then ejected using this inkjet recording device, and the optical density of the boundary between adjacently recorded black and color images was observed. As a result, it was found that bleeding occurred at the edge of the boundary on the black image side, resulting in a lighter black color than the original color. After that, when a black ink containing a larger amount of pigment than the color ink was used, it was found that the phenomenon of the black color at the edge of the black image becoming lighter was less likely to occur.

[0013] In an inkjet recording device equipped with a first ink storage section (main tank) and a second ink storage section (sub tank), it is easier to increase the overall capacity of the ink storage section compared to an inkjet recording device equipped with an ink storage section (ink cartridge) on a carriage. In addition, when a device equipped with a main tank having an openable ink inlet is used, the timing and amount of ink to be injected can be appropriately adjusted, so that concerns about running out of ink during recording can be minimized and productivity can be increased. However, an inkjet recording device equipped with a large-capacity main tank having an openable ink inlet is used for a long period of time while refilling ink from a bottle or the like. For this reason, the pigment settles in the main tank, and the pigment concentration in the ink tends to gradually increase, especially in the lower part of the tank.

[0014] Small inkjet recording devices used in offices and homes are often used to record web pages, text, etc., and the amount of black ink used tends to be greater than the amount of color ink used. For this reason, the main tank for black ink is often made larger than the main tank for color ink. However, due to restrictions on the size of the device, it is difficult to make the main tank for black ink larger until the refill frequency of black ink and color ink is almost the same. For this reason, the injection frequency of black ink tends to be higher than the injection frequency of color ink. That is, since black ink is frequently injected from the injection port of the main tank, the pigment is less likely to settle. On the other hand, since the injection frequency of color ink is low, the pigment is more likely to settle, and the pigment concentration in the ink tends to increase in the lower part of the main tank. Under such circumstances, when an image is recorded using an inkjet recording device for a long period of time, the pigment concentration in the ink gradually increases in the lower part of the main tank for color ink. It was found that when a black image and a color image are recorded adjacent to each other under such conditions, bleeding occurs at the edge of the black image side of the boundary, the black color becomes lighter than it should be, and the optical density decreases. This is a new problem that arises when an inkjet recording apparatus equipped with a first ink storage unit (main tank) and a second ink storage unit (sub tank) is used to record images over a long period of time.

[0015] The above problem occurs because the degree of pigment settling is significantly different between black ink and color ink when an inkjet recording device having an ink inlet in the main tank is used for a long period of time. If the pigment content in the black ink is increased in advance so as to exceed the increase in the pigment concentration of the color ink in the lower part of the main tank, the decrease in optical density occurring at the edge of the black image can be suppressed. However, if black ink with an increased pigment content is used, the pigment is likely to adhere to the recording head when the inkjet recording device is not in use, making it difficult to eject the ink normally. In particular, carbon black, which is the coloring material of black ink, has a shape (so-called "structure") in which primary particles are connected three-dimensionally to form secondary particles, unlike the organic pigments which are the coloring materials of color inks, and is prone to aggregation due to its relatively large specific surface area. Therefore, if the pigment content of the black ink is simply increased in order to suppress the decrease in optical density occurring at the edge of the black image, another problem is likely to occur, such as the pigment being more likely to adhere to the recording head.

[0016] As a result of investigation, the inventors have found that by adopting the following configuration, it is possible to suppress a decrease in optical density at the edge of a black image adjacent to a color image, even when using an inkjet recording device equipped with a main tank having an openable and closable ink inlet. That is, they have found that it is effective to add acrylic resin particles to the black ink and make the carbon black content P1 (mass%) in the black ink and the organic pigment content P2 (mass%) in the color ink satisfy the relationship of the following formula (1). P1≦P2 (1)

[0017] The inventors of the present invention closely observed an image recorded with a black ink containing carbon black but not containing acrylic resin particles, and an image recorded with a black ink containing carbon black and acrylic resin particles immediately after recording. Specifically, after the liquid component had almost completely permeated the recording medium, the aggregates of only carbon black and the aggregates formed of carbon black and acrylic resin particles were observed and compared. As a result, it was found that the aggregates of only carbon black had more liquid components held in the aggregates and had a higher degree of wetness than the aggregates formed of carbon black and acrylic resin particles. As described above, unlike organic pigments, carbon black has a structure. Then, due to the evaporation of water and the permeation of the liquid component into the recording medium, the carbon black particles having the structure aggregate with each other, and a bulky aggregate is formed on the recording medium. When such a bulky aggregate is formed, it is presumed that the liquid components remaining in the bulky aggregates increase the degree of wetness even after the liquid components have almost completely permeated the recording medium. It is believed that when an image is recorded so that black ink and color ink that form such aggregates are adjacent to each other, the color ink bleeds into the black ink, reducing the optical density at the edges of the black image.

[0018] In contrast, when a black ink containing carbon black and acrylic resin particles is used, the acrylic resin particles aggregate together with the carbon black to form an aggregate on the recording medium. As described above, carbon black has a structure, but it is considered that by aggregating with spherical acrylic resin particles, the acrylic resin particles enter between the bulky carbon black, forming an aggregate with a reduced bulk. The reduced bulk of the aggregate makes it difficult for the aggregate to retain liquid components, and the permeation rate of the liquid components into the recording medium is also increased compared to an aggregate formed only with carbon black. For the above reasons, it is considered that the use of black ink containing added acrylic resin particles suppresses the decrease in optical density of the edge of a black image adjacent to a color image.

[0019] <Inkjet recording method and inkjet recording apparatus> The inkjet recording method of the present invention is a recording method using an inkjet recording device including a first ink holder, a second ink holder, and a recording head. The first ink holder is a member having an ink inlet that can be opened and closed and that can be refilled with ink through the ink inlet. The second ink holder is a member that is connected to the first ink holder via a tube and that contains ink. The recording head is a member that is connected to the second ink holder and has an ejection port that ejects ink supplied from the second ink holder. The inkjet recording method of the present invention includes a step of applying ink ejected from the ejection port of the recording head to a recording medium to record an image. The ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment. The carbon black content P1 (mass%) in the black ink and the organic pigment content P2 (mass%) in the color ink satisfy the relationship of the following formula (1). P1≦P2 (1)

[0020] The inkjet recording device of the present invention includes a first ink storage section, a second ink storage section, and a recording head. The first ink storage section is a member having an openable and closable ink inlet through which ink can be refilled. The second ink storage section is a member that stores ink and is connected to the first ink storage section via a tube. The recording head is a member that is connected to the second ink storage section and has an ejection port formed therein for ejecting ink supplied from the second ink storage section. The ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment. The carbon black content P1 (mass%) in the black ink and the organic pigment content P2 (mass%) in the color ink satisfy the relationship of the following formula (1). P1≦P2 (1)

[0021] (Inkjet recording device) FIG. 1 is a perspective view showing a schematic diagram of an embodiment of an inkjet recording apparatus according to the present invention. The inkjet recording apparatus of this embodiment is a so-called serial type inkjet recording apparatus that performs a recording operation by reciprocating a recording head in an X direction (main scanning direction). A recording medium 101 is intermittently transported in a Y direction (sub-scanning direction) by a transport roller 107. A recording unit 102 mounted on a carriage 103 is reciprocally scanned in an X direction (main scanning direction) that is a direction perpendicular to the Y direction that is the transport direction of the recording medium 101. A recording operation is performed by transporting the recording medium 101 in the Y direction and reciprocating scanning of the recording unit 102 in the X direction. The recording unit 102 is composed of an inkjet type recording head 203 (FIG. 2) that ejects ink from an ejection port, and a subtank 202 (FIG. 2) as a second ink storage section, and is mounted on the carriage 103. The carriage 103 is supported so as to be movable along a guide rail 105 arranged along the X direction, and is fixed to an endless belt 106 that moves in parallel with the guide rail 105. The endless belt 106 reciprocates due to the driving force of a motor, thereby causing the carriage 103 to scan back and forth in the X direction.

[0022] A main tank 201 serving as a first ink storage section is stored inside the main tank storage section 108. The main tank 201 of the main tank storage section 108 and a sub-tank 202 of the recording unit 102 are connected by an ink supply tube 104 serving as an ink supply path. Ink is supplied from the main tank 201 to the sub-tank 202 (FIG. 2) via the ink supply tube 104, and then ejected from the ejection opening of the recording head 203. The main tank 201, ink supply tube 104, and sub-tank 202 can each be provided in numbers corresponding to the type of ink. It is preferable that the main tank 201 and the sub-tank 202 are connected by the ink supply tube 104 without going through another ink storage section.

[0023] An ink inlet 205 is provided at the top of the main tank 201 for injecting ink into the main tank 201 from outside the inkjet recording device. When using the inkjet recording device for the first time, or when the amount of ink in the main tank has decreased, ink is injected from an ink bottle into the main tank placed inside the inkjet recording device. In other words, the main tank is left inside the inkjet recording device and is not replaced by itself.

[0024] 2 is a schematic diagram showing an example of an ink supply system. Ink (shown by hatching) stored in a main tank 201 is supplied to a sub tank 202 via an ink supply tube 104, and then supplied to a print head 203. A gas introduction tube 204 serving as an air communication section is connected to the main tank 201. When printing is performed and ink is consumed, ink is supplied from the main tank 201 to the sub tank 202, and the ink in the main tank 201 decreases. Then, air is introduced into the main tank 201 from the gas introduction tube 204, one end of which is open to the atmosphere, so that the internal negative pressure for holding the ink is kept substantially constant in the ink supply system.

[0025] In order to increase the number of recordable sheets and thereby realize high productivity, it is preferable that the maximum ink capacity V1 (mL) of the main tank 201 is large. Specifically, the maximum ink capacity V1 (mL) of the main tank 201 is preferably 100 mL or more and 300 mL or less, and more preferably 100 mL or more and 200 mL or less. In addition, it is preferable that the initial ink filling amount of the main tank 201 is about 95% based on the maximum ink capacity.

[0026] It is also preferable that the subtank 202 has a large maximum ink capacity V2 (mL) in order to reduce the frequency of ink supply from the main tank 201 and to stably supply ink to the recording head 203. However, for example, assuming a serial system as shown in FIG. 1 in which the subtank 202 is mounted on the carriage 103, it is preferable that the maximum ink capacity V2 (mL) of the subtank 202 is not too large. In other words, if too much ink is stored in the subtank 202, the recording unit 102 becomes large, the moving speed of the carriage 103 decreases, and it becomes necessary to increase the strength of the endless belt 106 and the motor that move the carriage 103. Therefore, it is preferable that the maximum ink capacity V2 (mL) of the subtank 202 is 1 mL or more and 20 mL or less, and more preferably 2 mL or more and 10 mL or less.

[0027] The housings of the main tank 201 and the sub tank 202 are formed of thermoplastic resins such as polyester, polycarbonate, polyethylene, polypropylene, polystyrene, polyphenylene ether, and mixtures or modified versions of these. An ink absorber capable of generating negative pressure for retaining ink may be provided inside the housing. Compressed fibers of polypropylene, polyurethane, or the like are preferable as the ink absorber. Alternatively, the ink may be directly stored inside the housing without providing an ink absorber.

[0028] The recording unit 102 is composed of a recording head 203 and a subtank 202. The subtank 202 may be attached to the recording unit 102, which is a head cartridge in which the recording head 203 is built, and the recording unit 102 with the subtank 202 attached may be attached to the carriage 103. Furthermore, the recording unit 102 integrally composed of the subtank 202 and the recording head 203 may be attached to the carriage 103. Among these, it is preferable to adopt a configuration in which the recording unit 102 with the subtank 202 attached is set in the carriage 103 as shown in Figures 1 and 2.

[0029] The ink supply tube 104 is connected to a sub-tank 202 that constitutes the recording unit 102 mounted on the carriage 103. Therefore, the ink supply tube 104 is routed around inside the device following the reciprocating scanning of the carriage 103. Therefore, the material that constitutes the ink supply tube 104 must be selected to have flexibility that can withstand the frequent reciprocating scanning of the carriage 103. For this reason, the ink supply tube 104 is made of a resin material.

[0030] When the amount of ink contained in the main tank 201 decreases, the first valve 206 is operated to close the ink supply tube 104, and then the ink inlet 205 of the main tank 201 is opened to inject ink into the main tank 201. If the ink inlet 205 is opened without operating the first valve 206 and closing the ink supply tube 104, the negative pressure for holding the ink is lost, and ink leaks from the ejection port of the recording head 203. When injecting ink into the main tank 201, the second valve 207 can be operated to close the gas introduction tube 204, and then the ink inlet 205 can be opened. With this configuration, it is possible to block the flow of ink from the main tank 201 toward the gas introduction tube 204. In addition, by operating the first valve 206 and the second valve 207 in conjunction with each other, it is possible to more reliably suppress ink leakage.

[0031] The form in which ink is injected from the top of the main tank 201 shown in FIG. 2 is excellent in usability, but since the ink is stored in the main tank 201 for a long period of time, the pigment is likely to settle over time. Although the ink at the top of the main tank can flow due to convection caused by the injection of the ink, it does not stir up the pigment that has settled at the bottom and mix it to an extent that eliminates the pigment concentration gradient in the ink tank. From the viewpoint of suppressing pigment settling, it is also possible to make the main tank in the form of a replaceable cartridge. In this form, even if pigment settling occurs, measures can be taken such as shaking or replacing the cartridge, so issues caused by pigment settling are unlikely to occur.

[0032] In addition, in large inkjet recording devices for industrial printing and the like, some of them have a mechanism for preventing ink settling, since a printing stoppage has a large impact on output productivity. Examples of the mechanism for preventing settling include an agitation mechanism for agitating the ink in the ink storage section; an ink circulation mechanism for circulating the ink in a circulation path via an ink flow path; and a supply mechanism for supplying ink from the upper and lower parts of the ink storage section to cause the ink to flow. In recording devices equipped with such mechanisms, the problem that the present invention aims to solve does not occur. However, for small inkjet recording devices used in offices and homes, there is a high market demand for miniaturization and cost reduction of the main body. In order to meet these demands, recording devices used in offices and homes usually do not have a mechanism for preventing settling, and the inkjet recording device assumed in the present invention also does not have such a mechanism for preventing settling.

[0033] The ink ejection method of the recording head includes a method of ejecting ink by applying mechanical energy generated by a piezoelectric element or the like to the ink, and a method of ejecting ink by applying thermal energy generated by an electrothermal converter (heater) or the like to the ink. Either ink ejection method may be adopted.

[0034] (Recording process) The inkjet recording method of the present invention has a step of recording an image using the above-mentioned inkjet recording device (recording step). Specifically, in the recording step, ink discharged from the discharge port of a recording head is applied to a recording medium to record an image. Any recording medium may be used as the target for recording an image. In particular, it is preferable to use permeable paper such as a recording medium without a coating layer, such as plain paper or uncoated paper, and a recording medium with a coating layer, such as glossy paper or art paper. Other than using the above-mentioned inkjet recording device, the recording step may be a known one.

[0035] (Water-based ink) The inkjet recording method of the present invention includes a step of applying ink ejected from the ejection openings of a recording head to a recording medium to record an image. The inks used include black ink containing carbon black and acrylic resin particles, and color ink containing an organic pigment. The components constituting the ink are described below. When it is not necessary to distinguish between black ink and color ink, they are collectively referred to simply as "ink."

[0036] [Colorant] Carbon black is used as a coloring material for black ink. Carbon black has a "structure" in which multiple primary particles are connected three-dimensionally to form secondary particles. When carbon black aggregates on a recording medium, this structure forms a bulky pigment layer, which increases the optical density, making it suitable as a coloring material for black ink. The DBP oil absorption of carbon black can be cited as an index representing the size of the carbon black structure. The higher the DBP oil absorption, the larger the structure of the carbon black, and the bulkier the carbon black. The larger the structure of carbon black, the more likely it is that the degree of wetting of the aggregates will increase, and the effect of suppressing the decrease in optical density at the edge of a black image at the beginning of recording and after recording for a long period of time may decrease. In particular, the effect of suppressing the decrease in optical density is more likely to decrease after recording an image for a long period of time. For this reason, the DBP oil absorption of carbon black is preferably 180 mL / 100 g or less. There is no particular limit to the lower limit of the DBP oil absorption of carbon black, but it is preferably 50 mL / 100 g or more, and more preferably 120 mL / 100 g or more.

[0037] The BET specific surface area of ​​carbon black is 350m 2 / g or less. The larger the BET specific surface area, the smaller the primary particles of the carbon black. Carbon black with small primary particles tends to rapidly aggregate on a recording medium to form bulkier aggregates. The liquid components contained in the bulkier parts do not easily escape, resulting in a high degree of wetting, which may slightly reduce the effect of suppressing the decrease in optical density at the edges of black images at the beginning of recording and after long-term recording. There is no particular limit to the lower limit of the BET specific surface area of ​​carbon black, but it is generally within 150 m 2 It is preferable that the molecular weight is 1 / g or more.

[0038] As the coloring material of the color ink, an organic pigment (hereinafter, also simply referred to as "pigment") is used. Examples of the organic pigment include azo pigment, phthalocyanine pigment, quinacridone pigment, isoindolinone pigment, imidazolone pigment, diketopyrrolopyrrole pigment, dioxazine pigment, and perinone pigment. A dye may be used in combination for purposes such as color adjustment.

[0039] The content P1 (mass%) of carbon black in the black ink and the content P2 (mass%) of the organic pigment in the color ink need to satisfy the relationship of formula (1): P1 ≤ P2. Among them, it is more preferable to satisfy the relationship of formula (1'): P1 < P2. The content (mass%) of the pigment in the ink is preferably 0.5 mass% or more and 15.0 mass% or less, more preferably 1.0 mass% or more and 10.0 mass% or less, and particularly preferably 2.0 mass% or more and 8.0 mass% or less based on the total mass of the ink.

[0040] Examples of the pigment dispersion method include resin-dispersed pigments using a resin (resin dispersant) as a dispersant, and self-dispersed pigments in which a hydrophilic group is bonded to the particle surface of the pigment. Further, resin-bonded pigments in which an organic group containing a resin is chemically bonded to the particle surface of the pigment, microcapsule pigments in which the surface of the pigment particles is coated with a resin, etc. can be used.

[0041] As the resin dispersant for dispersing the pigment in an aqueous medium, it is preferable to use one that can disperse the pigment in the aqueous medium by the action of an anionic group. As the resin dispersant, the resins described later, particularly water-soluble resins, can be used. When using a resin-dispersed pigment, the content (mass%) of the pigment in the ink is preferably 0.2 times or more and 10.0 times or less in terms of the mass ratio to the content (mass%) of the resin dispersant.

[0042] As the self-dispersing pigment, an anionic group such as a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group can be used, which is bonded to the particle surface of the pigment directly or via another atomic group (-R-). The anionic group may be either an acid type or a salt type, and when it is a salt type, it may be either a partially dissociated state or a completely dissociated state. When the anionic group is a salt type, examples of the cation that serves as the counter ion include an alkali metal cation, ammonium, and organic ammonium. Specific examples of the other atomic group (-R-) include a linear or branched alkylene group having 1 to 12 carbon atoms; an arylene group such as a phenylene group or a naphthylene group; a carbonyl group; an imino group; an amide group; a sulfonyl group; an ester group; and an ether group. In addition, it may be a group that is a combination of these groups.

[0043] Amount of anionic groups in carbon black A P1 (mmol / g) is preferably 0.16 mmol / g or more and 0.80 mmol / g or less, and more preferably 0.16 mmol / g or more and 0.50 mmol / g or less. Pigment particles such as carbon black are dispersed in an aqueous medium due to the charge repulsion of the anionic group, while on the recording medium, they form aggregates with the evaporation of water and the penetration of liquid components into the recording medium. If the amount of anionic groups in carbon black is less than 0.16 mmol / g, when the conditions are such that the pigment concentration is likely to increase in the main tank, the carbon black will rapidly aggregate on the recording medium and will tend to form aggregates with a high degree of wetness. For this reason, the effect of suppressing the decrease in optical density at the edge of a black image after recording for a long period of time may be somewhat reduced. On the other hand, if the amount of anionic groups in carbon black is more than 0.80 mmol / g, the affinity with liquid components will be high and aggregates with a high degree of wetness will be easily formed. For this reason, the effect of suppressing the decrease in optical density at the edge of a black image may be somewhat reduced at the beginning of recording and after recording for a long period of time.

[0044] When carbon black is a self-dispersing pigment, the amount of anionic groups refers to the amount of anionic groups bonded directly or via other atomic groups to the particle surface of carbon black. When carbon black is a resin-dispersed pigment, the amount of anionic groups refers to the amount of anionic groups in the resin dispersant. When carbon black is a resin-dispersed carbon black, the amount of anionic groups in the carbon black can be adjusted by the ratio of carbon black to the resin dispersant, the acid value of the resin dispersant, etc. When carbon black is a self-dispersing carbon black, the amount of anionic groups can be adjusted by the amount of anionic groups.

[0045] The amount of anionic groups in the pigment and the resin particles described later can be measured by colloid titration. In the examples described later, an automatic potentiometric titrator (product name "AT-510", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) equipped with a streaming potential titration unit (product name "PCD-500", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) was used. The amount of anionic groups in the pigment and the resin particles was measured by colloid titration using potential difference. More specifically, the pigment and the resin particles were each diluted about 300 times (by mass) with pure water, and then the pH was adjusted to about 10 with potassium hydroxide as necessary, and potentiometric titration was performed using 5 mmol / L methyl glycol chitosan as a titration reagent. It is of course possible to measure the amount of anionic groups using pigment and resin particles extracted from the ink by an appropriate method. Of course, the measurement conditions and the measurement device are not limited to those described above.

[0046] This section describes a method for confirming the amount of anionic groups in a pigment for an ink that contains a water-soluble resin other than the resin dispersant in addition to a water-soluble resin (resin dispersant) for dispersing the pigment. The ink is concentrated or diluted to prepare a liquid with a solid content (pigment, water-soluble resin, resin particles, etc.) of about 10% by mass. This liquid is centrifuged at 12,000 rpm for one hour. This separates the liquid layer into one containing the water-soluble organic solvent and resin that does not contribute to dispersion, and one containing the pigment and resin that contributes to dispersion, and each is collected. The water-soluble resin that is mainly contained in the solid layer containing the pigment is the resin dispersant, and the resin that is mainly contained in the liquid layer is the water-soluble resin that does not contribute to the dispersion of the pigment.

[0047] The carbon black in the black ink is preferably a self-dispersed pigment in which an anionic group is bonded to the particle surface directly or via another atomic group. Compared with self-dispersed carbon black, resin-dispersed carbon black may have a higher degree of wettability due to a larger amount of liquid component remaining in the bulky part of the aggregate formed. For this reason, the effect of suppressing the decrease in optical density at the edge of the black image may be somewhat reduced at the beginning of recording and after recording for a long period of time. In addition, the organic pigment in the color ink is preferably a resin-dispersed pigment using a resin (resin dispersant) as a dispersant.

[0048] Cumulative 50% particle size D50 of the volume-based particle size distribution of carbon black P1 And the cumulative 50% particle size D50 of the volume-based particle size distribution of organic pigments P2 is preferably 10 nm or more and 300 nm or less, and more preferably 20 nm or more and 200 nm or less. The volume average particle diameters of carbon black and organic pigments can be measured using a dynamic light scattering particle diameter measuring device. The cumulative 50% particle diameter (D50) of the volume-based particle size distribution of organic pigments can also be measured. P2 ) is the cumulative 50% particle size (D50) of the volume-based particle size distribution of carbon black. P1) is preferable. The cumulative 50% particle size of the volume-based particle size distribution of carbon black and organic pigments can be measured using a method similar to the resin evaluation method described below. The measurement conditions can be the same as those of the resin evaluation method, except that the shape is aspheric and the refractive index is 1.80 (carbon black) and 1.51 (organic pigment).

[0049] [Resin particles] The black ink contains acrylic resin particles, which are resin particles formed from acrylic resin. Even if resin particles other than acrylic resin particles, such as wax resin particles, are used, it is considered that it is difficult for the resin particles to enter between the carbon black particles, and therefore it is difficult to form aggregates with reduced bulk. For this reason, it is not possible to obtain the effect of suppressing the decrease in optical density at the edge of the black image.

[0050] In this specification, "acrylic resin" refers to a resin having units derived from acrylic monomers such as (meth)acrylic acid and (meth)acrylate. The water-soluble acrylic resin is preferably a resin having hydrophilic units and hydrophobic units as constituent units. In the following description, "(meth)acrylic" means "acrylic" and "methacrylic", and "(meth)acrylate" means "acrylate" and "methacrylate".

[0051] In this specification, "resin particles" refers to a resin that does not dissolve in the aqueous medium that constitutes the ink, specifically, a resin that can exist in the aqueous medium in a state in which it forms particles whose particle diameter can be measured by dynamic light scattering. On the other hand, "water-soluble resin" refers to a resin that can dissolve in the aqueous medium that constitutes the ink, specifically, a resin that can exist in the aqueous medium in a state in which it does not form particles whose particle diameter can be measured by dynamic light scattering. "Resin particles" can also be referred to as "water-dispersible resin (water-insoluble resin)".

[0052] Whether or not a certain resin is a "resin particle" can be determined according to the following method. First, a liquid containing the resin to be determined (resin content: 10% by mass) is prepared. Next, the prepared liquid is diluted 10 times (volume basis) with ion-exchanged water to prepare a sample. Then, when the particle diameter of the resin in the sample is measured by dynamic light scattering, if a particle having a particle diameter is measured, the particle is determined to be a "resin particle" (water-dispersible resin). On the other hand, if a particle having a particle diameter is not measured, the resin is determined not to be a "resin particle" (it is a "water-soluble resin"). The measurement conditions at this time can be, for example, SetZero: 30 seconds, number of measurements: 3, measurement time: 180 seconds, shape: spherical, refractive index: 1.59, and density: 1.0. Of course, the measurement conditions are not limited to the above.

[0053] The resin particles may be a homopolymer in which a single monomer is polymerized, or a copolymer in which two or more monomers are polymerized. The copolymer may be a random copolymer or a block copolymer. Among them, resin particles obtained by using a monomer having an anionic group and a monomer not having an anionic group are preferable. Examples of the monomer having an anionic group include α,β-unsaturated carboxylic acid and its salt. Examples of the monomer not having an anionic group include an ester compound of α,β-unsaturated carboxylic acid and an α,β-ethylenically unsaturated compound having an aryl group. Specific examples of these monomers include the same monomers as those that become units constituting a resin that can be added to the ink, which will be described later.

[0054] Amount of anionic groups in acrylic resin particles A R1 (mmol / g), and the amount of anionic groups on the carbon black, A P1 (mmol / g) preferably satisfies the relationship of the following formula (4). R1 (mmol / g) and A P1When the carbon black and acrylic resin particles (mmol / g) satisfy the relationship of formula (4), the affinity of the aggregates formed by the carbon black and acrylic resin particles with the liquid component decreases. This is thought to accelerate the flow of the liquid component of the black ink on the recording medium, and the effect of suppressing the decrease in optical density at the edges of the black image can be further improved, not only at the beginning of recording, but also after recording for a long period of time. A R1 P1 (4)

[0055] The content (mass %) of acrylic resin particles in the black ink is preferably 0.5% by mass to 10.0% by mass, and more preferably 0.5% by mass to 6.0% by mass, based on the total mass of the black ink. If the content of acrylic resin particles is less than 0.5% by mass, it may be somewhat difficult to reduce the bulkiness of the carbon black aggregates on the recording medium, and the effect of suppressing the decrease in optical density at the edges of the black image may be somewhat reduced.

[0056] In the black ink, the content R1 (mass%) of the acrylic resin particles is preferably 0.2 to 1.3 times the mass ratio of the content P1 (mass%) of the carbon black. If the mass ratio is less than 0.2, it may be somewhat difficult to reduce the bulkiness of the carbon black aggregates on the recording medium. On the other hand, if the mass ratio is more than 1.3, the amount of the acrylic resin particles becomes relatively excessive, weakening the cohesive force, and the color ink may easily bleed into the black ink. Therefore, if the mass ratio is outside the range, the effect of suppressing the decrease in optical density at the edge of the black image at the beginning of recording and after recording for a long period of time may be somewhat reduced.

[0057] ​It is preferable that the carbon black content P1 (mass%) in the black ink, the acrylic resin particle content R1 (mass%) in the black ink, and the organic pigment content P2 (mass%) in the color inks satisfy the relationship of the following formula (2). By having P1, R1, and P2 satisfy the relationship of the following formula (2), it is possible to further improve the effect of suppressing the decrease in optical density at the edges of the black image not only at the beginning of recording but also after recording for a long period of time. P2<(P1+R1) (2)

[0058] Cumulative 50% particle size D50 of the volume-based particle size distribution of acrylic resin particles R1 From the viewpoint of ejection properties of the black ink, the particle diameter (nm) is preferably 50 nm or more and 300 nm or less. If the particle diameter of the acrylic resin particles is smaller than that of the carbon black, it may be difficult to reduce the bulkiness of the aggregates, and the effect of suppressing the decrease in optical density at the edge of the black image at the beginning of recording and after recording for a long period of time may be slightly reduced. The cumulative 50% particle diameter of the volume-based particle size distribution of the acrylic resin particles can be measured by the same method and conditions as the above-mentioned method for determining the resin.

[0059] In the ink storage section, particles with a larger particle diameter and a higher specific gravity have a higher sedimentation rate, so it is preferable that particles with a smaller specific gravity have a larger particle diameter. The cumulative 50% particle diameter of the volume-based particle size distribution of carbon black is called "D50 P1 The cumulative 50% particle size of the volume-based particle size distribution of organic pigments is called "D50 P2 Furthermore, the cumulative 50% particle diameter of the volume-based particle size distribution of the acrylic resin particles is defined as "D50 R1 (nm)". In this case, D50 P1 , D50 P2 , and D50 R1 It is preferable that the relationship of the following formula (3) is satisfied. This makes it possible to further reduce the difference in the settling speed of particles in the ink storage section, and further improves the effect of suppressing the decrease in optical density at the edge of a black image not only at the beginning of recording but also after recording for a long period of time. D50P1 <D50 P2 <D50 R1 (3)

[0060] [resin] The ink may contain a resin. The resin content (mass %) in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, and more preferably 0.5% by mass or more and 15.0% by mass or less, based on the total mass of the ink. This "resin content" includes the content of acrylic resin particles.

[0061] The resin can be added to the ink (i) to stabilize the dispersion state of the pigment, that is, as a resin dispersant or its auxiliary, and (ii) to improve various properties of the recorded image. The resin may be in the form of a block copolymer, a random copolymer, a graft copolymer, or a combination thereof. The resin may also be a water-soluble resin that can be dissolved in an aqueous medium.

[0062] Examples of the resin include acrylic resin, polyester resin, urethane resin, urea resin, polysaccharides, and polypeptides. Among them, acrylic resin is preferred from the viewpoint of ejection characteristics from the ejection port of the recording head. The acrylic resin is preferably one having a unit having an anionic group and a unit not having an anionic group as a constituent unit. Examples of the form of the acrylic resin include random copolymer, block copolymer, graft copolymer, and combinations thereof. As described above, the acrylic resin is a resin having a unit derived from an acrylic monomer such as (meth)acrylic acid or (meth)acrylate.

[0063] Examples of monomers that become units constituting the acrylic resin upon polymerization include monomers having an anionic group and monomers having no anionic group. Usually, a monomer having an anionic group becomes a hydrophilic unit upon polymerization, and a monomer having no anionic group becomes a hydrophobic unit upon polymerization.

[0064] Examples of monomers having an anionic group include α,β-unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid; anhydrides and salts thereof; and examples of cations constituting salts of α,β-unsaturated carboxylic acids include lithium cations, sodium cations, potassium cations, ammonium cations, and organic ammonium cations.

[0065] Examples of monomers not having an anionic group include α,β-ethylenically unsaturated compounds having an aryl group, such as styrene, α-methylstyrene, benzyl (meth)acrylate, 2-vinylpyridine, 4-vinylpyridine, and 1-vinylimidazole; and ester compounds of α,β-unsaturated carboxylic acids, such as ethyl (meth)acrylate, methyl (meth)acrylate, (iso)propyl (meth)acrylate, (n-, iso-, and t-)butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0066] Among them, acrylic resins having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of an α,β-ethylenically unsaturated compound having an aryl group and an ester compound of an α,β-unsaturated carboxylic acid are preferred. In particular, acrylic resins having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer of styrene and α-methylstyrene are preferred. These acrylic resins are suitable as resin dispersants for dispersing pigments in aqueous media because they easily interact with pigments.

[0067] The urethane resin can be obtained, for example, by reacting a polyisocyanate with a polyol. In addition, a chain extender may be further reacted with the urethane resin. Examples of the olefin resin include polyethylene and polypropylene.

[0068] [Aqueous medium] The ink is an aqueous ink containing at least water as an aqueous medium. The ink may contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. The ink preferably contains a water-soluble organic solvent. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water in the ink is preferably 50.0 mass% or more and 95.0 mass% or less based on the total mass of the ink. As the water-soluble organic solvent, any of those usable for inkjet inks such as alcohols, glycols, glycol ethers, and nitrogen-containing compounds can be used. The content (mass%) of the water-soluble organic solvent in the ink is preferably 3.0 mass% or more and 50.0 mass% or less based on the total mass of the ink, and more preferably 10.0 mass% or more and 30.0 mass% or less. In particular, it is particularly preferable that the content is 15.0 mass% or more and 25.0 mass% or less.

[0069] [Surfactants] The ink preferably further contains a surfactant. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. In particular, it is preferable to use nonionic surfactants such as acetylene glycol surfactants and polyoxyethylene alkyl ethers. The content (mass %) of the surfactant in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, and more preferably 0.1% by mass or more and 2.0% by mass or less, based on the total mass of the ink.

[0070] [Other ingredients] The ink may further contain various additives, such as an antifoaming agent, a pH adjuster, a viscosity adjuster, an antirust agent, an antiseptic, an antifungal agent, an antioxidant, and an antireducing agent, if necessary.

[0071] [Ink properties] The ink is a water-based ink applied to the inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property values. Specifically, the surface tension of the ink at 25°C is preferably 20 mN / m or more and 60 mN / m or less. In addition, the viscosity of the ink at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the ink at 25°C is preferably 7.0 or more and 9.5 or less, and more preferably 8.0 or more and 9.5 or less.

[0072] It is preferable that the surface tension γ1 (mN / m) of the black ink and the surface tension γ2 (mN / m) of the color inks satisfy the relationship of the following formula (5), which enhances the permeability of the color inks into the recording medium and makes it more difficult for the color inks to bleed into the black ink, thereby further suppressing the decrease in optical density at the edges of the black image. γ1>γ2 (5)

[0073] If the relationship of the above formula (5) is not satisfied, the effect of suppressing the decrease in optical density at the edge of the black image may be slightly reduced at the beginning of recording and after recording for a long period of time. In particular, the effect of suppressing the decrease in optical density may be more likely to decrease after recording for a long period of time. The surface tension of the ink is the "static surface tension" measured by the principle of the Wilhelmy method (plate method) or the like. EXAMPLES

[0074] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples without departing from the gist of the invention. "Parts" and "%" used to describe the amounts of components are based on mass unless otherwise specified.

[0075] <Method of measuring physical properties> (Amount of anionic groups) The amount of anionic groups in both the pigment and resin particles was measured using an automatic potentiometric titrator equipped with a streaming potential titration unit (product name "PCD-500", manufactured by Kyoto Electronics Manufacturing Co., Ltd.). Specifically, the amount of anionic groups was measured by potentiometric titration using 5 mmol / L methyl glycol chitosan as the titration reagent. The automatic potentiometric titrator used was an "AT-510" product (manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0076] (Average particle size (cumulative 50% particle size of volume-based particle size distribution (D50))) The average particle size (cumulative 50% particle size (D50) of the volume-based particle size distribution) of carbon black, organic pigment, and resin particles was measured as follows. The liquid containing the sample was diluted with pure water to obtain a measurement sample with a sample content of approximately 1.0%. The particle size (D50) of the particles in the measurement sample was then measured using a particle size measuring device. The measurement conditions are as follows. The particle size measuring device used was a particle size distribution meter using the dynamic light scattering method (product name "Nanotrac WAVEII-Q", manufactured by Microtrack Bell). [Measurement conditions] SetZero: 30 seconds Number of measurements: 3 Measurement time: 180 seconds Shape: non-spherical (carbon black and organic pigments), spherical (resin particles) Refractive index: 1.80 (carbon black), 1.51 (organic pigment), 1.59 (resin particles) Density: 1.0

[0077] (surface tension) The static surface tension of the ink was measured using an automatic surface tensiometer (product name "DY-300", manufactured by Kyowa Interface Science Co., Ltd.) that utilizes the Wilhelmy method.

[0078] <Preparation of pigment dispersion> (Pigment dispersions 1-11, 13) A solution of 5.0 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5°C, and the amount (g) of 4-aminophthalic acid shown in Table 1 was added. The container containing this solution was placed in an ice bath, and while stirring to keep the temperature of the solution below 10°C, a solution obtained by dissolving 1.8 g of sodium nitrite in 9.0 g of ion-exchanged water at 5°C was added. After stirring for 15 minutes, 6.0 g of carbon black having the properties (BET specific surface area and DBP oil absorption) shown in Table 1 was added under stirring, and the mixture was stirred for another 15 minutes to obtain a slurry. For pigment dispersions 2 and 3, the stirring time was controlled to adjust the average particle size (D50) of the pigment. The obtained slurry was filtered with filter paper (product name "Standard filter paper No. 2", manufactured by Advantec), the particles were thoroughly washed with water, and dried in an oven at 110°C. Then, the sodium ions were replaced with potassium ions by ion exchange to obtain a self-dispersing pigment in which -C6H3-(COOK)2 groups were bonded to the surface of the carbon black particles. An appropriate amount of water was added to adjust the pigment content, and pigment dispersions 1 to 11 and 13 with a pigment content of 15.0% were obtained.

[0079] (Pigment dispersion 12, 14-16, 18-21) A styrene-acrylic acid copolymer with an acid value of 90 mgKOH / g and a weight average molecular weight of 10,000 was neutralized with a 10% aqueous potassium hydroxide solution to prepare an aqueous solution of a resin dispersant. A mixture was obtained by mixing 15.0 parts of the pigment of the type shown in Table 1, the amount (parts) of the resin dispersant shown in Table 1, and the remaining amount of ion-exchanged water so that the total amount of the components was 100.0 parts. The mixture obtained was dispersed using a sand grinder for 1 hour, and then centrifuged to remove coarse particles. For pigment dispersions 18 to 21, the dispersion time was controlled to adjust the average particle size (D50) of the pigment. Furthermore, the dispersion was pressure-filtered with a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm to obtain pigment dispersions 12, 14 to 16, and 18 to 21, which contain the pigment dispersed by the resin dispersant. The pigment content in pigment dispersions 12, 14 to 16, and 18 to 21 was 15.0%, and the resin dispersant content was 4.5%.

[0080] (Pigment Dispersion 17) A commercially available pigment dispersion containing a self-dispersing pigment (product name "CAB-O-JET470Y", manufactured by Cabot, pigment content 15.0%) was used as pigment dispersion 17. The self-dispersing pigment in pigment dispersion 17 is an organic pigment (CI Pigment Yellow 74) in which an atomic group containing an anionic group is bonded to the particle surface.

[0081] TIFF2024174809000001.tif161170

[0082] <Synthesis of resin particles> (Resin particles 1-9) A four-neck flask equipped with a stirrer, a reflux condenser, and a nitrogen gas inlet tube was charged with potassium persulfate and ion-exchanged water in the amounts shown in Table 2, and nitrogen gas was introduced. Furthermore, the types and amounts of monomers shown in Table 2 were mixed to obtain a mixture. The obtained mixture was added dropwise to the four-neck flask under stirring over a period of 1 hour, and then reacted at 80°C for 2 hours. For resin particles 3, 4, and 7, the average particle diameter of the resin particles (D50 R1 After cooling the contents to room temperature, potassium hydroxide and an appropriate amount of ion-exchanged water were added to adjust the pH to 8.5, obtaining an aqueous dispersion of resin particles with a resin particle content of 15.0%. The abbreviations in Table 2 have the following meanings. BMA: n-Butyl methacrylate EMA: Ethyl methacrylate St: styrene ·MAA: Methacrylic acid EGdMA: Ethylene glycol dimethacrylate Aqualon KH-05: Trade name of reactive surfactant (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

[0083] TIFF2024174809000002.tif89170

[0084] (Resin particles 10) The urethane resin particles were synthesized according to the description of "Preparation of Polycarbonate-Modified Urethane Resin Emulsion A" in Patent Document 1. A reaction vessel equipped with a stirrer, a reflux condenser, and a thermometer was prepared. 1,500 g of polycarbonate diol, 220 g of dimethylolpropionic acid, and 1,347 g of a solvent (dipropylene glycol dimethyl ether) were added thereto, and the mixture was heated to 60°C under a nitrogen atmosphere to dissolve the components. As the polycarbonate diol, a reaction product of 1,6-hexanediol and dimethyl carbonate was used. Furthermore, 1,445 g of 4,4'-dicyclohexylmethane diisocyanate and 2.6 g of dibutyltin dilaurate were added and heated to 90°C, and a urethane reaction was carried out over 5 hours to synthesize an isocyanate-terminated urethane prepolymer. The mixture was cooled to 80°C, 149 g of triethylamine was added and stirred, and 4,340 g of the mixture was taken out. The mixture was added to a mixed solution of 5,400 g of water and 15 g of triethylamine while stirring vigorously. Next, 1,500 g of ice was added, and 626 g of 35% aqueous 2-methyl-1,5-pentanediamine solution was added to carry out a chain extension reaction. After that, the solvent was distilled off, and an appropriate amount of water was added to obtain an aqueous dispersion of resin particles 10 (resin particles formed of polycarbonate-modified urethane resin) with a resin particle content of 15.0%.

[0085] <Ink Preparation> Each of the components (unit: %) shown in Tables 3-1 to 3-3 and 4 was mixed and thoroughly stirred, and then pressure filtered using a polypropylene filter (manufactured by Pall) with a pore size of 2.5 μm to prepare each of the inks.

[0086] TIFF2024174809000003.tif174170

[0087] TIFF2024174809000004.tif176170

[0088] TIFF2024174809000005.tif188170

[0089] TIFF2024174809000006.tif133170

[0090] <Preparation of concentrated ink> A so-called continuous supply type inkjet recording device having a large-capacity main tank (first ink storage section) with an inlet has the advantage that it does not require replacement of ink cartridges. On the other hand, the quality of the recorded image tends to be easily degraded in the ink below the main ink due to an increase in the pigment content caused by the sedimentation of the pigment. As mentioned above, color inks tend to stay in the first ink storage section for a longer period of time than black ink. For this reason, the color inks were evaluated assuming a state after long-term use in which the pigment content is increased compared to black ink. Specifically, the black ink was evaluated by evaporating the liquid components to concentrate the ink and increasing the solid content (pigment, resin dispersant, and resin particles) by 20% from the composition shown in Table 3. The color inks were evaluated by evaporating the liquid components to concentrate the ink and increasing the solid content (pigment, resin dispersant, and resin particles) by 30% from the composition shown in Table 4.

[0091] <Inkjet recording device> A product name "PIXUS TS5130S" (manufactured by Canon) was used and modified to have the configurations shown in Table 5, to prepare inkjet recording apparatuses 1 to 3.

[0092] The inkjet recording device 1 has the configuration shown in Fig. 1, and is a type of inkjet recording device that replenishes ink through an inlet of a first ink holder when the remaining amount of ink is low. The maximum ink capacity of the first ink holder for black ink is 100 mL, and the maximum ink capacity of the second ink holder is 30 mL. The maximum ink capacity of the color inks is 58 mL, and the maximum ink capacity of the second ink holder is 10 mL.

[0093] The inkjet recording apparatus 2 is an inkjet recording apparatus of a type that does not have a first ink containing section mounted inside the apparatus, and has an inlet provided in a second ink containing section on a carriage.

[0094] The inkjet recording device 3 has a first ink storage section (without an inlet) and a second ink storage section, and is a type of inkjet recording device in which the first ink storage section is replaced when the remaining amount of ink decreases.

[0095] TIFF2024174809000007.tif62170

[0096] <Evaluation> Using the types of inkjet recording device, black ink, and color ink shown in Table 6, black and color solid images (3 cm x 2 cm) were recorded adjacent to the recording medium. Plain paper (product name "CS-680", manufactured by Canon) was used as the recording medium. The solid images were recorded under the conditions of approximately 22 ng of black ink and approximately 11 ng of color ink per unit area of ​​1 / 600 inch x 1 / 600 inch, at a temperature of 25°C and a relative humidity of 50%. After recording, the solid images were left in an environment at a temperature of 25°C and a relative humidity of 50% for one day, and then evaluated as follows. In the present invention, the evaluation criteria for each item shown below were "AA", "A", and "B" as acceptable levels, and "C" as unacceptable levels. The evaluation results are shown in Table 6. In Table 6, the case where the ink satisfies the "relationship P2<(P1+R1)" was marked as "○", and the case where it did not satisfy the relationship was marked as "×". In addition, when the ink was "D50 P1 <D50 P2 <D50 R1 If the above relationship is satisfied, it is marked as "○", and if it is not satisfied, it is marked as "×".

[0097] (Optical density at edge of black image (initial)) Based on the measured values ​​of the reflectance (Rmax) of the recording medium and the reflectance (Rmin) of the black image, a dynamic threshold was extracted as follows: an area (area 1) where the dynamic threshold was 60% or less, and an area (area 2) where the dynamic threshold was more than 60% and was less than 90%. The extraction of areas 1 and 2 was performed using a product name "Personal Image Quality Assessment System PIAS-II" (manufactured by QEA). On the other hand, a recorded solid image was saved as image data using an optical microscope (product name "ZEISS SteREO Discovery V8", manufactured by ZEISS, magnification: 8 times, with scale). The saved image data was read into an image processing software (product name "Adobe Photoshop CC", manufactured by Adobe Systems), and the Bk density (%) was read at any 30 points in each area using the eyedropper tool of the software, and the average value was calculated. The difference (Bk density difference (%)) between the average value of the Bk density (%) in area 1 and the average value of the Bk density (%) in area 2 was calculated, and the decrease in optical density at the edge of the black image was evaluated according to the following evaluation criteria. The AA:Bk concentration difference was less than 8%. A: The Bk concentration difference was more than 8% and was 14% or less. B: The Bk concentration difference was more than 14% and less than 20%. C: The Bk concentration difference exceeded 20%.

[0098] (Optical density at the edge of a black image (after long-term recording)) For the examples and comparative examples using the inkjet recording device 1, the optical density at the edge of the black image (after long-term recording) was evaluated in the same manner as in the above-mentioned "Optical density at the edge of the black image (initial)" except that concentrated ink was used. Note that, since the inkjet recording devices 2 and 3 do not have issues due to pigment sedimentation, evaluation after long-term recording was not performed.

[0099] TIFF2024174809000008.tif255157

[0100] The disclosure of this embodiment includes the following methods and configurations. (Method 1) A first ink holder having an openable and closable ink inlet through which water-based ink can be refilled; a second ink containing section that contains the water-based ink and is connected to the first ink containing section via a tube; a recording head connected to the second ink container and having an ejection port for ejecting the water-based ink supplied from the second ink container; An inkjet recording method comprising a step of applying the water-based ink ejected from the ejection orifice to a recording medium to record an image, the water-based ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment; an inkjet recording method, characterized in that a content P1 (mass %) of the carbon black in the black ink and a content P2 (mass %) of the organic pigment in the color inks satisfy the relationship of the following formula (1): P1≦P2 (1) (Method 2) The inkjet recording method according to Method 1, wherein the content R1 (mass %) of the acrylic resin particles in the black ink is 0.2 to 1.3 times, in mass ratio, the content P1 (mass %) of the carbon black in the black ink. (Method 3) The inkjet recording method according to Method 1 or 2, in which a content P1 (mass %) of the carbon black in the black ink, a content R1 (mass %) of the acrylic resin particles in the black ink, and a content P2 (mass %) of the organic pigment in the color inks satisfy the relationship of the following formula (2). P2<(P1+R1) (2) (Method 4) The cumulative 50% particle diameter D50P1 (nm) of the volume-based particle size distribution of the carbon black and the cumulative 50% particle diameter D50 of the volume-based particle size distribution of the organic pigment P2 (nm), and the cumulative 50% particle diameter D50 of the volume-based particle size distribution of the acrylic resin particles R1 The inkjet recording method according to any one of Methods 1 to 3, wherein (nm) satisfies the following formula (3): D50P1 <D50 P2 <D50 R1 (3) (Method 5) Amount A of anionic groups in the carbon black P1 5. The inkjet recording method according to any one of Methods 1 to 4, wherein (mmol / g) is 0.16 mmol / g or more and 0.80 mmol / g or less. (Method 6) Amount A of anionic groups in the acrylic resin particles R1 (mmol / g), and the amount A of anionic groups of the carbon black P1 6. The inkjet recording method according to any one of Methods 1 to 5, wherein (mmol / g) satisfies the relationship of the following formula (4): A R1 P1 (4) (Method 7) The BET specific surface area of ​​the carbon black is 350 m 2 7. The inkjet recording method according to any one of Methods 1 to 6, wherein the viscosity is 100 / g or less. (Method 8) The inkjet recording method according to any one of Methods 1 to 7, wherein the DBP oil absorption of the carbon black is 180 mL / 100 g or less. (Method 9) The inkjet recording method according to any one of Methods 1 to 8, wherein the carbon black is a self-dispersing pigment having an anionic group bonded to the particle surface directly or via another atomic group. (Method 10) The inkjet recording method according to any one of Methods 1 to 9, wherein the surface tension γ1 (mN / m) of the black ink and the surface tension γ2 (mN / m) of the color inks satisfy the relationship of the following formula (5). γ1>γ2 (5) (Configuration 1) A first ink holder having an openable and closable ink inlet through which water-based ink can be refilled; a second ink containing section that contains the water-based ink and is connected to the first ink containing section via a tube; a recording head connected to the second ink containing section and having an ejection port for ejecting the water-based ink supplied from the second ink containing section, ​the water-based ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment; an amount P1 (mass %) of the carbon black in the black ink and an amount P2 (mass %) of the organic pigment in the color inks satisfy the relationship of the following formula (1): P1≦P2 (1)

Claims

1. a first ink storage section having an openable and closable ink inlet through which water-based ink can be refilled; a second ink containing section that contains the water-based ink and is connected to the first ink containing section via a tube; a recording head connected to the second ink container and having an ejection port for ejecting the water-based ink supplied from the second ink container, An inkjet recording method comprising a step of applying the water-based ink ejected from the ejection orifice to a recording medium to record an image, the water-based ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment; The content P of the carbon black in the black ink １ (mass%), and the content P of the organic pigment in the color ink ２ (% by mass) satisfies the relationship of the following formula (1): P １ ≦P ２ ・・・(1)

2. The content R of the acrylic resin particles in the black ink １ (mass%) is the carbon black content P １ 2. The ink-jet recording method according to claim 1, wherein the mass ratio of the ink to the total amount of the ink (mass %) is 0.2 to 1.3 times.

3. The content P of the carbon black in the black ink １ (mass %), the content R of the acrylic resin particles in the black ink １ (mass%), and the content P of the organic pigment in the color ink ２ The ink jet recording method according to claim 1 , wherein (% by mass) satisfies the following formula (2): P ２ <(P １ +R １ ) ・・・(2)

4. The cumulative 50% particle diameter D50 of the volume-based particle size distribution of the carbon black Ｐ１ (nm), the cumulative 50% particle diameter D50 of the volume-based particle size distribution of the organic pigment Ｐ２ (nm), and the cumulative 50% particle diameter D50 of the volume-based particle size distribution of the acrylic resin particles Ｒ１ The ink jet recording method according to claim 1 , wherein (nm) satisfies the following formula (3): D50 Ｐ１ <D50 Ｐ２ <D50 Ｒ１ ・・・(3)

5. Amount A of anionic groups of the carbon black Ｐ１ 2. The ink-jet recording method according to claim 1, wherein (mmol / g) is 0.16 mmol / g or more and 0.80 mmol / g or less.

6. Amount A of anionic groups of the acrylic resin particles Ｒ１ (mmol / g), and the amount A of the anionic group of the carbon black Ｐ１ The ink-jet recording method according to claim 1 , wherein (mmol / g) satisfies the relationship of the following formula (4): A Ｒ１ <A Ｐ１ ・・・(4)

7. The BET specific surface area of ​​the carbon black is 350 m ２ The ink-jet recording method according to claim 1, wherein the viscosity is 100 / g or less.

8. 2. The ink-jet recording method according to claim 1, wherein the carbon black has a DBP oil absorption of 180 mL / 100 g or less.

9. 2. The ink-jet recording method according to claim 1, wherein the carbon black is a self-dispersing pigment having an anionic group bonded to the particle surface directly or via another atomic group.

10. The surface tension γ of the black ink １ (mN / m), and the surface tension γ of the color ink ２ The ink jet recording method according to claim 1 , wherein (mN / m) satisfies the relationship of the following formula (5): c １ >c ２ ・・・(5)

11. a first ink storage section having an openable and closable ink inlet through which water-based ink can be refilled; a second ink containing section that contains the water-based ink and is connected to the first ink containing section via a tube; a recording head connected to the second ink containing section and having an ejection port for ejecting the water-based ink supplied from the second ink containing section, the water-based ink includes a black ink containing carbon black and acrylic resin particles, and a color ink containing an organic pigment; The content P of the carbon black in the black ink １ (mass%), and the content P of the organic pigment in the color ink ２ (% by mass) satisfies the relationship of the following formula (1): P １ ≦P ２ ・・・(1)

Images