Inkjet recording method and inkjet recording apparatus

Abstract

This invention provides an inkjet recording method that offers good intermittent ejection stability and enables the recording of high-quality images by easily suppressing satellites. [Solution] An inkjet recording method is provided, which uses an inkjet recording device equipped with an inkjet recording head having an ejection port for ejecting aqueous ink, to eject the aqueous ink from the recording head and apply it to a recording medium to record an image. This inkjet recording method includes a humidification step of humidifying the vicinity of the ejection port. Furthermore, the aqueous ink used in this method contains carbon black and colloidal silica, and the volume-based cumulative 50% particle diameter of the colloidal silica is 20 nm or more.

Description

[Technical Field]

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

[0002] In recent years, inkjet printing methods have seen increased use not only for home image output but also in business settings such as offices. These applications often involve printing monochrome data such as documents, web pages, and emails, creating a demand for black ink capable of recording high-quality images with excellent black reproduction. Carbon black is primarily used as the colorant for black ink.

[0003] In addition to the above performance, it is also required that high image performance be consistently achieved. If the recording head does not recover and ink is not ejected from the recording head for a certain period of time (a paused state), the liquid components such as water in the ink evaporate from the ejection port of the recording head, causing the ink to thicken. When recording is then resumed, the ejection performance decreases due to the thickened ink, and as a result, the quality of the recorded image tends to deteriorate. The performance of the ink in relation to this phenomenon is called "intermittent ejection stability." There is a need for an inkjet recording method with excellent intermittent ejection stability that can maintain good ink ejection accuracy even when image recording is resumed from a paused state.

[0004] Various proposals have been made to meet the above-mentioned requirements. For example, an inkjet recording method has been proposed that improves the intermittent ink ejection stability by humidifying the space between the ejection port of an inkjet line head that ejects water-based ink and the recording medium (Patent Document 1). On the other hand, a recording method has been proposed that applies an ink containing carbon black and colloidal silica to a recording device that humidifies the area near the ejection port by supplying maintenance fluid to a cap portion that covers the surface on which the ejection port of the recording head is formed (ejection port surface) (Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2017-136846 [Patent Document 2] Japanese Patent Publication No. 2024-033789 [Overview of the project] [Problems that the invention aims to solve]

[0006] The present inventors, referencing the technology disclosed in Patent Document 1, used an ink containing carbon black as a pigment and performed recording using an inkjet recording method that includes a humidification step. As a result, it was confirmed that intermittent ejection stability was well maintained. On the other hand, upon detailed observation of the obtained images, it was found that multiple minute ink droplets were attached to positions offset from the main droplet, forming trails, indicating that there is still room to further improve image quality. These minute ink droplets are called satellites, and they are generated when ink is ejected from the ejection port of the recording head by splitting from the main droplet. In order to obtain higher quality images, it is necessary to suppress satellites so that they do not attach to positions offset from the main droplet and form trails. Furthermore, it was found that even when using the technology disclosed in Patent Document 2, it is not always possible to achieve both intermittent ejection stability and satellite suppression.

[0007] Therefore, an object of the present invention is to provide an inkjet recording method that has good intermittent ejection stability and can easily suppress satellites, enabling the recording of high-quality images. Another object of the present invention is to provide an inkjet recording apparatus that can be used in the above-mentioned inkjet recording method. [Means for solving the problem]

[0008] In other words, the present invention provides an inkjet recording method for recording an image using an inkjet recording apparatus equipped with an inkjet recording head having an ejection port for ejecting aqueous ink, wherein the aqueous ink is ejected from the recording head and applied to a recording medium, the method comprising a humidification step of humidifying the vicinity of the ejection port, the aqueous ink containing carbon black and colloidal silica, and the volume-based cumulative 50% particle diameter of the colloidal silica being 20 nm or more. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an inkjet recording method that exhibits good intermittent ejection stability and is capable of recording high-quality images by easily suppressing satellites. Furthermore, according to the present invention, it is possible to provide an inkjet recording apparatus that can be used in the above-mentioned inkjet recording method. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing an example of an ink circulation mechanism in an inkjet recording device that can circulate ink within a circulation path including the recording head. [Figure 2] This is a schematic diagram showing an example of a recording head and a humidification unit in an inkjet recording device capable of performing a humidification process. [Modes for carrying out the invention]

[0011] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "contains a salt." Also, water-based inkjet ink may be simply referred to as "ink." Unless otherwise specified, the physical properties are values ​​at room temperature (25°C) and normal pressure (1 atm).

[0012] In this disclosure, "volume-based cumulative 50% particle size" refers to the particle size that accounts for 50% of the volume-based particle size distribution (D 50 This means the cumulative 50% particle size (D) based on volume. 50 For measuring the particle size distribution (D), for example, a particle size distribution analyzer using dynamic light scattering can be used. In this disclosure, when simply referring to "average particle size," it means "volume-based cumulative 50% particle size (D 50 It means ")".

[0013] First, let's explain the formation of satellites. When ejection energy is applied, the ink is pushed out from the ejection port of the recording head and extends into a columnar shape. The columnar ink eventually becomes spherical from its tip due to the action of surface tension, and begins to form an ink droplet. This spherical part becomes the main droplet, but at this stage, the main droplet and the ejection port are connected by a columnar portion of ink (hereinafter sometimes referred to as the "liquid column portion"). This liquid column portion is gradually stretched in the direction of ink ejection, gradually becoming thinner, and eventually breaking off. At this time, the ink droplet leaves the ejection port and flies away. During the flight process, the ink droplet may split into a liquid droplet with a relatively large volume at the tip (main droplet) and a liquid columnar droplet that follows. Then, the liquid columnar droplet splits further into ink droplets with a smaller volume than the main droplet (satellites).

[0014] The properties of satellites are thought to be related to the length of the liquid column formed immediately after ejection. That is, if the liquid column is difficult to break and the liquid columnar droplet extends to a long length, the split ink droplets (satellites) will be distributed to positions far away from the main droplet. When these adhere to the recording medium, a low-quality image with a long trail from the main droplet is recorded. Conversely, if the liquid column of the ink breaks easily and the liquid columnar droplet is short, the split ink droplets (satellites) adhere to the recording medium near the main droplet, resulting in a high-quality image with a short trail from the main droplet. Therefore, in order to suppress satellites so that they adhere to positions offset from the main droplet and form a trail, it is important to make the liquid column of the ink ejected from the recording head easy to break and shorten the liquid columnar droplet.

[0015] As described above, in the inkjet recording method using an aqueous ink containing carbon black, in order to improve the intermittent ejection stability, when recording was performed with humidification near the ejection port, the image quality deteriorated due to the generation of satellites. The inventors speculate on the cause as follows. When the vicinity of the ejection port of the recording head is humidified, it becomes difficult for liquid components such as water in the ink to evaporate from the ink near the ejection port in the flow path. Thereby, an increase in the viscosity of the ink can be suppressed, and the intermittent ejection stability can be kept good. On the other hand, ink droplets that are discharged and do not reach the recording medium but adhere to the ejection port surface also become difficult to evaporate due to the influence of humidification and tend to remain on the ejection port surface. When recording is performed with such ink droplets adhering to a part of the edge of the ejection port, the ink droplets ejected from the ejection port and the ink droplets adhering to the edge of the ejection port come into contact and merge. As a result, the volume of the droplet increases, and the length of the droplet elongated in a liquid column shape becomes longer than when not humidified. Therefore, it is considered that the satellites are distributed to positions far from the main droplet.

[0016] The inventors have studied the composition of an ink that can control satellites so that the distribution range from the adhesion position of the main droplet is narrowed while maintaining good intermittent ejection stability by humidifying the vicinity of the ejection port of the recording head. As a result, it has been found that it is effective to contain carbon black and colloidal silica having a cumulative 50% particle diameter of 20 nm or more based on volume in the ink. The inventors speculate on the mechanism by which satellites are easily suppressed by using the above aqueous ink as follows.

[0017] Normally, particles dispersed in the ink are charged on the surface. The particles repel each other due to the electric double layer formed by this surface charge, and the dispersed state is kept stable. However, just before the droplet breaks, since the mobility of the particles is restricted in the thinly stretched droplet, particles that approach beyond the repulsion of the electric double layer appear. At this time, when the approaching particles are carbon black and colloidal silica, it is considered that the following specific interaction occurs between the two.

[0018] Carbon black is an aggregate of crystallites in which fused benzene rings are stacked in multiple layers, and it is rich in π electrons. Colloidal silica, on the other hand, is a particle consisting of a three-dimensional network structure of siloxane bonds (Si-O). The difference in electronegativity between silicon and oxygen atoms is large, and the shared electron pair is strongly attracted to the more electronegative oxygen atom, so the silicon atom is slightly electropositive.

[0019] When carbon black and colloidal silica come into close proximity in an ink droplet ejected from the recording head's nozzle, the abundant π electrons of the carbon black are attracted to the electron-deficient silicon atoms of the colloidal silica. This interaction is thought to form aggregates of multiple carbon black particles around the colloidal silica.

[0020] Furthermore, it is hypothesized that this aggregate acts to shorten the columnar portion of the ejected droplet, making it easier to suppress satellite formation. As the droplet elongates into a columnar shape, it becomes thinner, and eventually the surface of the pigment particles present in the ink appears at the gas-liquid interface. At this time, if the aggregate consisting of colloidal silica and carbon black described above is present in the ink, air bubbles that enter from the gas-liquid interface reach the interior of the columnar portion along the hydrophobic surface of the carbon black. Since air bubbles do not readily adhere to hydrophilic surfaces, they penetrate the interior of the columnar portion while avoiding the surface of the highly hydrophilic colloidal silica contained in the aggregate. As a result, paths that are easy for air bubbles to pass through and paths that are difficult to pass through are created inside the aggregate, resulting in an uneven flow of air bubbles, which is thought to make the columnar portion more easily broken.

[0021] However, it was found that the above effect does not occur with any colloidal silica, and that it is necessary to use colloidal silica with a volume-based cumulative 50% particle diameter (hereinafter sometimes simply referred to as "average particle diameter") of 20 nm or more. Colloidal silica with an average particle diameter of less than 20 nm has a large surface area per unit mass, resulting in high cohesiveness, and tends to aggregate with other colloidal silica particles just before the droplet breaks apart. Therefore, it is presumed that it is unable to interact with carbon black to form aggregates, and thus the effect of suppressing satellites was not obtained.

[0022] Furthermore, it was found that in order to obtain the effects of the present invention, it is necessary to include both carbon black and colloidal silica in the ink. For example, when particles with a hydrophobic surface, such as polystyrene particles, or organic pigments used in colored inks were used instead of carbon black, and when hydrophilic particles without siloxane bonds were used instead of colloidal silica, the effects were not obtained. In all of these cases, it is thought that the combination of the abundant π electrons of carbon black and the siloxane bonds with a large charge imbalance of colloidal silica was not present, making it impossible to form aggregates that can cut the liquid column into shorter pieces.

[0023] Furthermore, it was found that no effect was obtained when using inks containing compounds that have siloxane bonds but are not particulate (for example, silicone compounds such as silicone surfactants and silicone oils) and carbon black. Siloxane bonds have a high degree of freedom of rotation. Therefore, even if multiple carbon black particles are attracted to the silicon atoms of the siloxane bonds, the rotation of the bonds disrupts the hydrophobic surface connections between the carbon black particles, preventing air bubbles from penetrating into the liquid column and thus resulting in no effect. In contrast, with colloidal silica, where siloxane bonds are cross-linked to form a network, the bonds are less likely to rotate, and the connections between the carbon black particles gathered around the colloidal silica are less likely to break down, thus resulting in an effect.

[0024] <Inkjet recording method> The present invention relates to an inkjet recording method that uses an inkjet recording apparatus equipped with an inkjet recording head having an ejection port for ejecting aqueous ink, to eject aqueous ink from the recording head and apply it to a recording medium to record an image. This inkjet recording method includes a humidification step of humidifying the vicinity of the ejection port. Furthermore, this method uses an aqueous ink containing carbon black and colloidal silica with a cumulative 50% particle size by volume of 20 nm or more. In this inkjet recording method, the inkjet recording apparatus described later can preferably be used.

[0025] (humidification process) The inkjet recording method includes a step (humidification step) of humidifying the ejection port of the recording head in the inkjet recording device. The humidification step is preferably performed before applying ink to the recording medium and recording an image. In the humidification step, it is preferable to humidify to an extent that the evaporation of liquid components such as water in the ink near the ejection port in the flow path is suppressed to some extent in order to improve intermittent ejection stability. Specifically, in the humidification step, it is preferable to humidify the air near the ejection port of the recording head under conditions that create an atmosphere with an absolute humidity of 0.01 kg / kgDA or higher. Here, the above absolute humidity refers to mass absolute humidity, and as expressed by its unit (kg / kgDA), it represents the mass (kg) of water vapor contained in humid air relative to the mass (kg) of dry air. When the absolute humidity of the air near the ejection port of the recording head is 0.01 kg / kgDA or higher, the humidification effect is easily exerted, and intermittent ejection stability is more easily improved. It is preferable that the absolute humidity of the air near the ejection port is 0.09 kg / kgDA or lower. Furthermore, the temperature of the air near the discharge port is preferably between 15°C and 35°C.

[0026] One method of humidification is to supply humidified air between the ejection port of the recording head and the recording medium. Another method is to humidify the space inside the cap after performing a cap-contact operation (capping) that surrounds the ejection port of the recording head. Of these, the method of humidifying the space inside the cap after capping is preferred. Specifically, it is more preferable to use an inkjet recording device equipped with a cap that can form a closed space by covering the ejection port surface where multiple ejection ports of the recording head are arranged, and to perform a humidification process that includes capping and then humidifying the closed space inside the cap. As a method of humidifying the space inside the cap, one example is to circulate a humidifying fluid such as humidified air or humidifying liquid through a supply channel and a recovery channel provided to communicate with the inside of the cap. By capping the recording head to cover the ejection port surface and then humidifying the closed space inside the cap, the humidified air is less likely to diffuse, making it easier to maintain a humidified state near the ejection port, and thus improving the intermittent ejection stability.

[0027] The humidification process refers to the process of humidifying the ink to a degree that sufficiently suppresses the evaporation of liquid components such as water in the ink near the ejection port within the flow path. The process of applying a cleaning liquid to the ejection port surface of the recording head and wiping the ejection port surface with a rubber wiper or cloth wiper is not included in the humidification process because it does not sufficiently suppress the evaporation of liquid components in the ink within the ejection port.

[0028] Furthermore, if the capping device used in the process of capping the nozzle surface is a capping device that receives ink discharged during dry ejection, it is not included in the humidification process because it cannot sufficiently suppress the evaporation of liquid components such as water in the ink inside the nozzle. Here, "dry ejection" refers to the operation of discharging ink from the nozzle based on dry ejection data that is not used for image recording. However, even if capping is performed with a capping device that receives ink discharged during dry ejection, if this process is combined with a separate process that can suppress the evaporation of liquid components such as water in the ink from the nozzle, it is included in the humidification process.

[0029] (Ink circulation process) In one embodiment, the inkjet recording method of the present invention preferably includes an ink circulation step. The ink circulation step is a step of circulating ink in a circulation path that includes a supply channel for supplying ink from an ink storage section containing ink to a recording head, a recording head, and a recovery channel for recovering ink from the recording head to the ink storage section. When performing an inkjet recording method having an ink circulation step, an inkjet recording device is used that includes a recording head, the above-mentioned ink storage section, supply channel, and recovery channel. Circulating the ink can suppress the evaporation of liquid components such as water in the ink from the ejection port even when the inkjet recording device is in a idle state. As a result, the increase in ink viscosity is suppressed, and the effect of maintaining good intermittent ejection stability is easily obtained. The ink circulation flow rate (flow rate) when circulating the ink can be appropriately determined according to the configuration of the device, but for example, it is preferably 1 mL / min or more and 700 mL / min or less, and more preferably 1 mL / min or more and 50 mL / min or less.

[0030] <Inkjet recording device> The present invention relates to an inkjet recording device that includes an inkjet recording head having an ejection port for ejecting aqueous ink, and records an image by ejecting aqueous ink from the recording head and applying it to a recording medium.

[0031] The inkjet recording device includes an inkjet recording head that ejects aqueous ink and applies it to a recording medium, and a humidifying unit that humidifies the area near the ejection port of the recording head. The ink ejected from the recording head in this inkjet recording device contains carbon black and colloidal silica with a cumulative 50% particle size by volume of 20 nm or more. This ink is the same as the aqueous ink used in the inkjet recording method described above. Furthermore, the inkjet recording device can be preferably used in the inkjet recording method described above. In addition, the inkjet recording device may also include an ink and an ink storage unit for storing it, as well as an image recording unit for ejecting ink from the recording head and recording an image on the recording medium, etc.

[0032] (Recording head) As the recording head, serial-type recording heads (serial heads) or line-type recording heads (line heads) can be used. Among these, line heads are preferred. Line heads have ink ejection ports arranged across the entire width of the maximum recordable recording medium, enabling high-speed image recording. Methods for ejecting ink from an inkjet recording head include methods that impart mechanical energy to the ink and methods that impart thermal energy to the ink. Among these, it is preferable to employ a method that imparts thermal energy to the ink to eject it.

[0033] (Humidifying section) The humidification unit may be equipped with a mechanism (humidification mechanism) for humidifying the area near the discharge port of the recording head. The humidification mechanism can be a mechanism capable of supplying a humidifying fluid, such as humidified air or humidifying liquid, to the area near the discharge port. For example, the humidification unit may be equipped with a mechanism that applies ultrasonic vibrations to the humidifying liquid or a mechanism that sprays the humidifying liquid, and the humidifying fluid may be supplied to the area near the discharge port by a spraying method using these methods.

[0034] As mentioned above, it is preferable to cap the recording head using a cap that can form a closed space by covering the outlet surface where multiple outlets are arranged, and then humidify the closed space inside the cap. Therefore, it is preferable for the humidification unit to be equipped with the cap. In this case, it is even more preferable for the humidification unit to be equipped as a humidification fluid circulation unit as a humidification mechanism, which is capable of circulating the humidification fluid from a humidification fluid storage unit that contains the humidification fluid to the cap, and from the cap to the humidification fluid storage unit. The humidification fluid circulation unit can be configured, for example, to include a humidification fluid storage unit, a supply channel that supplies the humidification fluid from the humidification fluid storage unit to the cap, a recovery channel that recovers the humidification fluid from the cap to the humidification fluid storage unit, and a circulation pump that circulates the humidification fluid between them. With such a humidification fluid circulation unit, the humidification fluid can be circulated in a humidification fluid circulation path that includes the humidification fluid storage unit, the humidification fluid supply channel, the cap, and the humidification fluid recovery channel.

[0035] (Ink circulation section) Furthermore, it is preferable that the inkjet recording device includes an ink circulation unit capable of circulating ink from an ink storage unit to a recording head, and from the recording head back to the ink storage unit. The ink circulation unit can be configured, for example, to include an ink storage unit, a supply channel for supplying ink from the ink storage unit to the recording head, a recovery channel for recovering ink from the recording head back to the ink storage unit, and a circulation pump for circulating the ink between them. With such an ink circulation unit, ink can be circulated through a circulation path that includes the ink storage unit, the ink supply channel, the recording head, and the ink recovery channel.

[0036] The following describes the operation of ink circulation and humidification near the ejection port in an inkjet recording apparatus according to one embodiment of the present invention, with reference to the drawings. The following drawings are schematic diagrams showing a preferred example of the configuration of the inkjet recording apparatus of the present invention, and the inkjet recording apparatus of the present invention is not limited by the following description.

[0037] Figure 1 is a schematic diagram showing an example of an ink circulation mechanism in an inkjet recording device capable of circulating ink within a circulation path including the recording head. The ink contained in the ink storage unit 1 flows through the supply channel 3a in the direction of the arrow in the figure, via a circulation pump 2, which is a means of circulating the ink, passes through a filter 4, and is sent to the recording head 5. When ink is ejected from the recording head 5 based on image data, the ink flows through the ink channel 6 inside the recording head 5, and the ink is ejected from the ejection port formed on the ejection port surface 7, thereby recording an image on a recording medium (not shown). It is also possible to eject ink from the recording head 5 based on empty ejection data. Ink that is not ejected outside the device for image recording or empty ejection returns to the recovery channel 3b and is sent to the ink storage unit 1. In this way, the ink circulates between the ink storage unit 1 and the recording head 5, thereby circulating the ink within the circulation path including the recording head.

[0038] Figure 2 is a schematic diagram showing an example of a recording head and a humidification unit in an inkjet recording device capable of performing a humidification process. (a) is a schematic diagram of the recording head 5 viewed from the ejection port surface 7 side, (b) is a schematic diagram showing an example of a cap 10 covering the ejection port surface 7, and (c) is a schematic block diagram showing an example of a recording head 5 and a humidification unit 18 in an inkjet recording device. The recording head 5 includes a flow path (not shown) through which ink supplied from the ink storage unit 1 (see Figure 1) flows, and a plurality of ejection ports (not shown) that communicate with the plurality of flow paths and eject ink. The recording head 5 also has an ejection port surface 7 below it (on the recording medium side) in which an ejection port row 8, in which the plurality of ejection ports are arranged, is formed. When the cap 10 contacts the contact position 9 of the area including the ejection port surface 7, the ejection port surface 7 in which the ejection ports are arranged is covered by the cap 10, and a closed space is formed. By supplying a humidifying fluid such as humidified air or humidifying liquid to this closed space, humidification can be performed near the ejection port. The enclosed space formed when the discharge port surface 7 is covered by the cap 10 is sealed and therefore does not communicate with the atmosphere.

[0039] The cap 10 is provided with a humidifying fluid supply port 11a for supplying humidifying fluid from a humidifying fluid storage section 12, which will be described later, and a humidifying fluid recovery port 11b for recovering humidifying fluid from the cap 10. The humidifying fluid storage section 12 is a storage section for storing humidifying fluid. The humidifying fluid storage section 12 stores humidifying fluid containing moisture for humidifying the closed space formed when the cap 10 contacts the contact position 9 of the discharge port surface 7. The humidifying fluid storage section 12 is connected to the humidifying fluid supply port 11a of the cap 10 via a supply channel 13 (tube). A supply-side valve 14 is provided at an intermediate position in the supply channel 13 to start and stop the supply of humidifying fluid by opening and closing.

[0040] Furthermore, the humidifying fluid storage section 12 is connected to the humidifying fluid recovery port 11b of the cap 10 via a recovery channel 16 (tube). A recovery-side valve 17 is provided midway along the recovery channel 16, which starts and stops the recovery of the humidifying fluid by opening and closing.

[0041] The humidifying unit 18, which has a circulation path including a cap 10, a humidifying fluid storage section 12, a supply channel 13, and a recovery channel 16, is equipped with a pump 19 that circulates the humidifying fluid within the circulation path. By driving the pump 19 and circulating the humidifying fluid within the circulation path, the closed space formed when the cap 10 contacts the discharge port surface 7 is humidified, and evaporation of liquid components such as water in the ink from near the discharge port can be suppressed.

[0042] The inkjet recording device may include a humidifying fluid concentration adjustment operation that detects the amount of water evaporation from the humidifying fluid and supplies approximately the same amount of water that has evaporated from a water supply unit (not shown) into the circulation path of the humidifying unit 18 to maintain a constant concentration of the humidifying fluid.

[0043] Depending on the surrounding environment in which the inkjet recording device is installed, the temperature and humidity conditions may be as set by the humidification process described above. However, since the temperature and humidity of the external environment are constantly fluctuating, the desired temperature and humidity conditions are not always met. Therefore, performing the humidification process to set the conditions to the preferred temperature and humidity conditions described above remains effective in stably obtaining the effects of the present invention.

[0044] <ink> The ink used in the inkjet recording method and inkjet recording apparatus described above contains carbon black and colloidal silica with a cumulative 50% particle size of 20 nm or more by volume. The components of the ink are described in detail below.

[0045] (Carbon Black) The ink contains carbon black. The carbon black content (by mass) in the water-based ink is preferably 1.00% to 15.00% by mass, and more preferably 3.00% to 10.00% by mass, based on the total mass of the ink. Any carbon black suitable for inkjet inks can be used. Examples of carbon blacks include furnace black, lamp black, acetylene black, and channel black. These carbon blacks can be used individually or in combination of two or more. The ink may further contain dyes, pigments, etc., for purposes such as color matching.

[0046] The primary particle size of carbon black is preferably between 10 nm and 40 nm. Carbon black typically exists as multiple primary particles linked together three-dimensionally, like a bunch of grapes. Primary particle size refers to the particle size of the smallest unit of carbon black (primary particle) that forms a single pigment particle. The primary particle size of carbon black can be determined by observing and measuring the particle size of the smallest unit of carbon black forming the pigment particle at approximately 100 points using a transmission or scanning electron microscope, and then calculating the arithmetic mean of these measurements.

[0047] Cumulative 50% particle size (D) of carbon black by volume 50 The wavelength is preferably 50 nm to 200 nm, and more preferably 70 nm to 150 nm.

[0048] The carbon black used in the ink preferably has a DBP oil absorption of 50 mL / 100g or more, and more preferably 100 mL / 100g or more. Within this range, the structure of the carbon black develops and becomes complex, so the hydrophobic surfaces in the aggregate of carbon black tend to connect. Therefore, hydrophobic bubbles can easily penetrate into the liquid column, and the liquid column tends to break apart, thus making it easier to suppress satellite formation. There is no particular upper limit to the DBP oil absorption of carbon black, but it is preferably 200 mL / 100g or less, and more preferably 180 mL / 100g or less. The DBP (dibutyl phthalate) oil absorption (mL / 100g) of carbon black can be measured in accordance with ASTM D-2414.

[0049] The carbon black used in the ink has a BET specific surface area of ​​40 m². 2 / g or more 400m 2 It is preferable that it be less than / g, and 200m 2 / g or more 350m 2 It is even more preferable that the amount is less than or equal to / g. The BET specific surface area of ​​carbon black can be measured by methods in accordance with JIS K6217 or ASTM D-6556. These methods involve immersing the degassed pigment (carbon black) in liquid nitrogen and measuring the amount of nitrogen adsorbed on the surface of the pigment particles when equilibrium is reached.

[0050] Dispersion methods for carbon black include resin-dispersed carbon black, which uses a resin as a dispersant, and carbon black dispersed with a surfactant. Self-dispersing carbon black, in which hydrophilic groups are directly or via other atomic groups bonded to the surface of the carbon black particles, is also an option. These carbon blacks can be used individually or in combination of two or more types in inks, and it is also possible to use carbon blacks with different dispersion methods in combination.

[0051] Specific examples of resin-dispersed carbon black (hereinafter also referred to as "resin-dispersed carbon black") include the following: For example, resin-dispersed carbon black using a resin dispersant, microcapsule-type carbon black in which the surface of carbon black particles is coated with resin, and resin-bonded carbon black in which organic groups derived from resin are chemically bonded to the surface of carbon black particles. As the resin, it is preferable to use an acrylic resin having at least units having anionic groups such as (meth)acrylic acid and units without anionic groups such as monomers having aromatic rings or aliphatic groups. In this disclosure, when "(meth)acrylic acid" is written, it means "acrylic acid, methacrylic acid".

[0052] Examples of surfactants used in carbon black dispersed with surfactants (hereinafter also referred to as "surfactant-dispersed carbon black") include anionic surfactants, amphoteric surfactants, and nonionic surfactants. Examples of anionic surfactants include alkanesulfonates, α-olefin sulfonates, alkylbenzene sulfonates, alkylnaphthalene sulfonates, acylmethyl taurates, dialkyl sulfosucrates, alkyl sulfates, sulfated olefins, polyoxyethylene alkyl ether sulfates, and alkyl phosphates. Examples of amphoteric surfactants include alkyldimethyl betaine and alkylamine oxides. Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, polyoxyethylene alkylamides, glycerin alkyl esters, and sorbitan alkyl esters. These surfactants can be used individually or in combination of two or more.

[0053] Self-dispersing carbon black (hereinafter also referred to as "self-dispersing carbon black") can be defined as carbon black in which anionic groups are bonded directly to the particle surface or via other atomic groups (-R-). By using such a self-dispersing pigment, the addition of a dispersant to disperse the carbon black in the ink becomes unnecessary, or the amount of dispersant added can be reduced to a small amount.

[0054] Examples of anionic groups bonded directly to the particle surface of carbon black, or via other atomic groups, include carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, and phosphonic acid groups. These anionic groups may form salts. When anionic groups form salts, at least one proton of each of these groups is substituted with a cation. Examples of cations include alkali metal ions, ammonium ions, and organic ammonium ions. Examples of alkali metal ions include lithium, sodium, and potassium ions. Examples of organic ammonium ions include aliphatic amines such as mono- or trialkylamines; and cations or salts thereof of aliphatic alcohol amines such as mono- or trialcanolamines. The anionic groups are preferably in the form of alkali metal salts such as sodium or potassium, or ammonium salts, and are more preferably in the form of alkali metal salts such as sodium or potassium.

[0055] Anionic groups may be directly bonded to the surface of carbon black particles, or they may be bonded via other atomic groups (-R-). Examples of other atomic groups (-R-) include alkylene groups such as methylene, ethylene, and propylene; arylene groups such as phenylene, naphthylene, anthracenylene, phenantrenylene, and biphenylene; heteroarylene groups such as pyridylene, imidazoylene, pyrazolylene, pyridinylene, thienylene, and thiazoylene; carbonyl groups; ester groups such as carboxylic acid esters, sulfonic acid esters, phosphate esters, and phosphonic acid esters; imino groups; amide groups; sulfonyl groups; and ether groups. Combinations of these groups are also possible.

[0056] Methods for introducing functional groups to the particle surface of carbon black include oxidation treatment and chemical bonding treatment. For example, an oxidation treatment of carbon black involves treating the particle surface of the raw material carbon black with ozone, hypochlorous acid and its salts, and persulfuric acid and its salts to oxidize the carbon black. Hydrophilic groups introduced to the particle surface of the oxidized self-dispersing carbon black include anionic groups such as carboxylic acid groups, sulfonic acid groups, phosphate groups, and phosphonic acid groups. The counterion of the anionic group is preferably at least one selected from the group consisting of alkali metals, ammonium, and organic ammonium, with sodium ions, potassium ions, and ammonium ions being more preferred.

[0057] It is preferable that the carbon black contained in the ink be self-dispersing carbon black. Self-dispersing carbon black has advantages over resin-dispersed carbon black and surfactant-dispersed carbon black in situations where strong shear forces that cause droplets to break are applied. That is, even under such conditions, the anionic groups in self-dispersing carbon black are less likely to detach from the surface of the carbon black particles compared to the resin dispersant in resin-dispersed carbon black and the surfactant in surfactant-dispersed carbon black. Therefore, when using self-dispersing carbon black, aggregation of carbon black particles is less likely to occur even under the above conditions, and it interacts more easily with colloidal silica, making it easier to obtain a satellite suppression effect.

[0058] Furthermore, the carbon black to be contained in the ink is more preferably oxidized self-dispersible carbon black. In the oxidized self-dispersible carbon black, due to the oxidation treatment of carbon black during its production, the particle surface of the carbon black becomes rough and there are many hydrophobic portions. Therefore, hydrophobic bubbles easily enter into the interior of the ink along the hydrophobic surface of the oxidized carbon black aggregate, and it is easy to cut the liquid column portion. As a result, it is considered that the effect of suppressing satellites is easily obtained.

[0059] (Colloidal silica) The ink contains colloidal silica having a volume-based cumulative 50% particle diameter (average particle diameter) of 20 nm or more. One or more kinds of colloidal silica can be contained in the ink. Colloidal silica is a colloidal solution in which silicon dioxide (SiO2) particles are dispersed in a liquid medium such as water. In this specification, the silicon dioxide particles in a colloidal dispersion state are referred to as "colloidal silica".

[0060] The volume-based cumulative 50% particle diameter (D 50 ) of the colloidal silica is 20 nm or more, preferably 100 nm or less, and more preferably 20 nm or more and 60 nm or less. Hydrophilic colloidal silica easily adsorbs water on the particle surface. When the volume-based cumulative 50% particle diameter of the colloidal silica is 100 nm or less, the amount of water adsorbed on the surface of the colloidal silica is appropriately suppressed. Therefore, the amount of water brought into the aggregate of carbon black is suppressed to a small amount, the action of shortening the liquid column portion is enhanced, and the effect of suppressing satellites is easily obtained.

[0061] The content (mass%) of colloidal silica in the aqueous ink is preferably 0.50 mass% or more and 15.00 mass% or less based on the total mass of the ink. The above content of colloidal silica is more preferably 1.00 mass% or more and 5.00 mass% or less, and further preferably 2.00 mass% or more and 3.00 mass% or less.

[0062] Furthermore, the content (mass%) of colloidal silica in the aqueous ink is preferably 0.10 to 2.00 times the mass ratio of the carbon black content (mass%), and more preferably 0.20 to 1.00 times. When the above mass ratio is 0.10 times or more, colloidal silica is more easily incorporated into the carbon black aggregate. When colloidal silica is incorporated into the carbon black aggregate, hydrophobic bubbles penetrate into the interior of the liquid column portion, avoiding the hydrophilic surface of the colloidal silica. At this time, the uneven flow of bubbles that avoid the colloidal silica makes the liquid column portion easier to break, and the effect of suppressing satellite formation is more easily obtained. On the other hand, when the above mass ratio is 2.00 times or less, the amount of colloidal silica contained in the carbon black aggregate is appropriately suppressed. By moderately limiting the amount of colloidal silica, which has a hydrophilic surface and readily adsorbs water onto the particle surface, the adsorbed water molecules become less likely to connect with each other on the particle surface. This makes the liquid column more easily broken, and as a result, the effect of suppressing satellite formation becomes easier to achieve.

[0063] The volume-based cumulative 50% particle size of colloidal silica in aqueous ink is preferably 0.05 to 2.00 times, and more preferably 0.10 to 1.00 times, as a ratio to the volume-based cumulative 50% particle size of carbon black. When the above ratio is 0.05 or more, the size of the colloidal silica is not too small relative to the carbon black, so the bulky carbon black is less affected by steric hindrance and can approach the colloidal silica more easily. As a result, aggregates of carbon black are more easily formed, the effect of shortening the liquid column is more likely to occur, and the effect of suppressing satellites is more easily obtained. On the other hand, when the above ratio is 2.00 or less, the size of the colloidal silica is not too large relative to the carbon black, so connections of hydrophobic surfaces between carbon black particles surrounding the colloidal silica are more easily formed. As a result, the effect of shortening the liquid column is more likely to occur, and the effect of suppressing satellites is more easily obtained.

[0064] (aqueous medium) The ink contains at least water as an aqueous medium. It is preferable to use deionized water (ion-exchanged water). The water content (by mass) in the ink is preferably 30.00% to 90.00% by mass, based on the total mass of the ink. More preferably, the water content in the ink is 40.00% to 80.00% by mass, and even more preferably 60.00% to 70.00% by mass.

[0065] The ink may further contain a water-soluble organic solvent as an aqueous medium. Examples of water-soluble organic solvents include monohydric alcohols, polyhydric alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, and sulfur-containing polar solvents. Among these, alkylene glycol alkyl ethers and cyclic amide compounds, in which the terminal hydrogens of glycol ethers are alkylated, are preferred. Furthermore, triethylene glycol monobutyl ether, triethylene glycol monomethyl ether, and ε-caprolactam are more preferred. In addition, one or more water-soluble organic solvents can be used. The content (mass%) of the water-soluble organic solvent in the ink is preferably 3.00% by mass or more and 50.00% by mass or less, and more preferably 5.00% by mass or more and 30.00% by mass or less, based on the total mass of the ink.

[0066] (Betaine) The ink preferably contains betaine. Betaine is a compound that has non-adjacent positive and negative charges within the same molecule, and the molecule as a whole has no charge. When the ink contains betaine, it is thought that the dipoles generated by the charge imbalance within the betaine molecule further strengthen the polarization of the silicon and oxygen in the silanol group of colloidal silica. Therefore, carbon black, which has abundant π electrons, is more easily attracted to the surface of the colloidal silica particles, and aggregates of carbon black are more easily formed, which is thought to make it easier to suppress satellite formation.

[0067] The positively charged site of the betaine is preferably a quaternary ammonium cation. Examples of such betaines include trimethylglycine, γ-butyrobetaine, homarin, trigonelline, carnitine, homoserine betaine, valinebetaine, lysinebetaine, ornithinebetaine, alaninebetaine, stachydrine, and glutamate betaine. One of these may be used alone, or two or more may be used in combination. Among the betaines, trimethylglycine, γ-butyrobetaine, and carnitine are preferred, with trimethylglycine being more preferred. Trimethylglycine has a small molecular weight and a compact molecular shape, resulting in less steric hindrance and easier interaction with the negative charge of colloidal silica.

[0068] The betaine content (mass%) in the ink is preferably 2.00% by mass or more and 16.00% by mass or less, based on the total mass of the ink. Furthermore, the above betaine content is more preferably 4.00% by mass or more and 14.00% by mass or less, and even more preferably 6.00% by mass or more and 12.00% by mass or less. When the betaine content in the ink is within the above range, the effect on the silanol groups on the surface of the colloidal silica particles is sufficient, making it easier to obtain the effect of suppressing satellite formation.

[0069] The betaine content (mass%) in the aqueous ink is preferably 0.20 to 16.00 times the colloidal silica content (mass%), and more preferably 0.25 to 12.00 times. By having this mass ratio within this range, a more effective suppression of satellites can be achieved.

[0070] (Other ingredients) In addition to the components mentioned above, the ink may also contain, as necessary, water-soluble organic compounds that are solid at room temperature (25°C), such as polyhydric alcohols like trimethylolpropane and trimethylolethane, and urea derivatives like urea and ethylene urea. Furthermore, the ink may also contain, as necessary, various additives such as other resins, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation accelerators, chelating agents, and water-soluble resins.

[0071] (Ink properties) The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s, more preferably 1.0 mPa·s to 5.0 mPa·s, and even more preferably 2.0 mPa·s to 4.0 mPa·s. The viscosity of the ink is measured using a viscometer (for example, product name "RE80 type viscometer," manufactured by Toki Sangyo Co., Ltd.).

[0072] The surface tension (static surface tension) of the ink at 25°C is preferably between 10.0 mN / m and 50.0 mN / m. More preferably between 20.0 mN / m and 40.0 mN / m, and even more preferably between 20.0 mN / m and 35.0 mN / m. The static surface tension of the ink is measured using a Wilhelmie-type surface tensimeter (for example, product name "Automatic Surface Tensimeter CBVP-Z," manufactured by Kyowa Interface Science, etc.).

[0073] The pH of the ink at 25°C is preferably between 5.0 and 10.0, and more preferably between 7.0 and 9.5. The pH of the ink is measured using a general pH meter equipped with a glass electrode or the like.

[0074] <Ink Set> The ink used in the inkjet recording method and inkjet recording apparatus described above is a black ink containing carbon black. While the black ink may be used alone, it is preferable to use it as an ink set in combination with a color ink containing an organic pigment. Hereinafter, the ink containing carbon black and a specific colloidal silica will be referred to as "first ink," and the color ink containing an organic pigment will be referred to as "second ink." By using an ink set that includes color inks (such as cyan, yellow, and magenta) in addition to the black ink, full-color images can be recorded. The ink set may use one or more types each of the first and second inks.

[0075] The second ink preferably contains colloidal silica in addition to the organic pigment. In particular, the volume-based cumulative 50% particle size of the colloidal silica in the second ink is preferably larger than the volume-based cumulative 50% particle size of the colloidal silica in the first ink. By using an ink set that satisfies the above relationship, the intermittent ejection stability of each ink can be improved.

[0076] When the physical properties of each ink in an ink set are relatively similar, such as viscosity and surface tension, the ejection state tends to be more stable. When a recording head with integrated ejection ports for each ink is used, and the area near the ejection port is humidified, the colloidal silica present near the ink meniscus formed at the ejection port absorbs moisture due to its hydrophilicity, and water is incorporated into the ink. As a result, the concentration of water in the ink near the meniscus tends to increase locally. The carbon black contained in the first ink is relatively hydrophobic and tends to recede from the ejection port towards the back of the recording head's flow path. In contrast, the organic pigment contained in the second ink is relatively hydrophilic and does not recede easily. Therefore, as a result, viscosity differences tend to occur between the inks that make up the ink set.

[0077] When the average particle size of the colloidal silica in the first and second inks satisfies the above-mentioned relationship, viscosity differences between the inks become less likely. Specifically, if the average particle size of the colloidal silica in the first ink containing carbon black is small, less water is incorporated into the ink by the colloidal silica, and the retreat of the carbon black is easily suppressed. On the other hand, if the average particle size of the colloidal silica in the second ink containing organic pigment is large, more water is incorporated into the ink by the colloidal silica, and the retreat of the organic pigment is easily promoted. These factors work together to make it easier for the viscosity of each ink to become uniform after being left in a humidified state for a long period of time, further improving intermittent ejection stability.

[0078] The second ink used in the inkjet recording method and inkjet recording apparatus described above contains an organic pigment and colloidal silica, and the volume-based cumulative 50% particle size of the colloidal silica is larger in the second ink than in the first ink. Examples of organic pigments that are the colorants of the second ink include azo, phthalocyanine, quinacridone, isoindolinone, imidazolon, diketopyrrolopyrrole, and dioxazine. The second ink may contain one or more types of organic pigments. The components constituting the second ink other than the pigments can be appropriately selected from those similar to those of the first ink described above. [Examples]

[0079] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components indicated in "parts" and "%" are based on mass.

[0080] <Method for measuring physical properties> (Cumulative 50% particle size (D) of pigments and colloidal silica by volume basis) 50 )) For samples prepared by diluting dispersions of pigments such as carbon black and colloidal silica with pure water, the cumulative 50% particle size (average particle size, D) in the volume-based particle size distribution was determined by dynamic light scattering. 50 The particle size distribution (nm) was measured. A dynamic light scattering particle size distribution analyzer (product name "Microtrac WAVE", manufactured by Microtrac-Bell) was used for the measurement. The measurement conditions were: SetZero: 30 seconds, Number of measurements: 3, Measurement time: 120 seconds, Shape: Non-spherical, Refractive index (carbon black, organic pigment): 1.80, Refractive index (colloidal silica): 1.46.

[0081] <Preparation of Pigment Dispersion> Pigment dispersions 1 to 14 were prepared. Table 1 shows the properties of the pigments in these dispersions.

[0082] (Pigment dispersion 1) Based on the description in Example 1 of Japanese Patent Publication No. 2012-117020, a self-dispersing pigment was prepared by oxidizing the particle surface of carbon black using ozone gas. Specifically, the carbon black used was first prepared with a primary particle size of 16 nm and a BET specific surface area of ​​220 m². 2 Furnace black with a concentration of 100 mL / g and DBP oil absorption of 100 mL / g was used. This furnace black was pre-wetted with deionized water and then dispersed in a high-pressure homogenizer to obtain a carbon black dispersion. This carbon black dispersion was subjected to ozone treatment by colliding it with ozonated water in a high-pressure vessel. Next, potassium hydroxide was added to adjust the pH of the mixture to 8, and it was concentrated using an ultrafiltration membrane to obtain pigment dispersion 1 with a pigment (oxidized self-dispersing carbon black) content of 20.0%. The volume-based cumulative 50% particle size of the pigment in pigment dispersion 1 was 100 nm.

[0083] (Pigment dispersion 2) As carbon black, primary particle size: 15 nm, BET specific surface area: 260 m² 2The preparation procedure was the same as for pigment dispersion 1, except that gas black with a concentration of 140 mL / g and DBP oil absorption capacity was used. In this way, pigment dispersion 2 was obtained, which contained 20.0% pigment (oxidized self-dispersing carbon black). The volume-based cumulative 50% particle size of the pigment in pigment dispersion 2 was 90 nm.

[0084] (Pigment dispersion 3) As carbon black, primary particle size: 14 nm, BET specific surface area: 350 m² 2 The preparation procedure was the same as for pigment dispersion 1, except that furnace black with a DBP oil absorption capacity of 98 mL / g was used. In this way, pigment dispersion 3 was obtained with a pigment (oxidized self-dispersing carbon black) content of 20.0%. The volume-based cumulative 50% particle size of the pigment in pigment dispersion 3 was 80 nm.

[0085] (Pigment dispersion 4) As carbon black, primary particle size: 18 nm, BET specific surface area: 180 m² 2 The preparation procedure was the same as for pigment dispersion 1, except that furnace black with a DBP oil absorption capacity of 55 mL / g was used. In this way, pigment dispersion 4 was obtained with a pigment (oxidized self-dispersing carbon black) content of 20.0%. The volume-based cumulative 50% particle size of the pigment in pigment dispersion 4 was 55 nm.

[0086] (Pigment dispersion 5) As carbon black, it has a primary particle size of 50 nm and a BET specific surface area of ​​45 m². 2 The preparation procedure was the same as for pigment dispersion 1, except that furnace black with a DBP oil absorption capacity of 121 mL / g was used. In this way, pigment dispersion 5 was obtained, which contained 20.0% pigment (oxidized self-dispersing carbon black). The volume-based cumulative 50% particle size of the pigment in pigment dispersion 5 was 120 nm.

[0087] (Pigment dispersions 6-9) The procedure was the same as for the preparation of pigment dispersion 1, except that the dispersion conditions for carbon black using a high-pressure homogenizer after pre-wetting were changed so that the cumulative 50% particle size by volume of carbon black was the value shown in Table 1. In this way, pigment dispersions 6 to 9 with a pigment (oxidized self-dispersing carbon black) content of 20.0% were obtained.

[0088] (Pigment dispersion 10) A styrene / acrylic acid copolymer (composition (mass) ratio 26:74) was dissolved in deionized water using sodium hydroxide equimolar to the acid value to prepare an aqueous solution of resin dispersant with a resin content of 20.0%. The weight-average molecular weight of this copolymer was 10,000, and the acid value was 200 mgKOH / g. A mixture of 10.0 parts carbon black, 15.0 parts of the aqueous resin dispersant solution, and 60.0 parts water was placed in a sand grinder and dispersed for 1 hour. The carbon black had a primary particle size of 16 nm and a BET specific surface area of ​​220 m². 2 A furnace black with a concentration of 100 mL / g and DBP oil absorption of 100 mL / g was used. Afterward, coarse particles were removed by centrifugation, and the mixture was pressure-filtered through a 3.0 μm pore size microfilter (manufactured by Fujifilm). The mixture was then concentrated using an ultrafiltration membrane to prepare a dispersion with a pigment content of 20.0% and a resin content of 6.0%. Using the above method, a pigment dispersion 10 was obtained in which carbon black was dispersed in water by a resin dispersant. The volume-based cumulative 50% particle size of the pigment in the pigment dispersion 10 was 100 nm.

[0089] (Pigment dispersion 11) A solution of 5.0 g concentrated hydrochloric acid was prepared by dissolving 5.5 g of water in a solution, to which 1.6 g of 4-aminophthalic acid (treatment agent) was added while the solution was cooled to 5°C. Next, the container of this solution was placed in an ice bath and the liquid was stirred to keep the solution at a constant temperature of 10°C or below, and a solution of 2.2 g of sodium nitrite dissolved in 9 g of 5°C water was added. After stirring this solution for another 15 minutes, 6 g of carbon black was added under stirring. The carbon black had a primary particle size of 16 nm and a BET specific surface area of ​​220 m². 2Furnace black with a carbon black content of 100 mL / g and DBP oil absorption of 100 mL / g was used. The mixture was then stirred for a further 15 minutes. The resulting slurry was filtered through filter paper (product name "Standard Filter Paper No. 2", manufactured by Advantec), the particles were thoroughly cooled with water, and dried in an oven at 110°C. Water was then added to obtain a dispersion with a carbon black content of 20.0%. Sodium ions were then replaced with potassium ions by ion exchange. By the above method, a pigment dispersion 11 was obtained in which a pigment (self-dispersing carbon black) in which phthalate groups (-C6H3-(COOK)2) with potassium ions as counterions are bonded to the surface of carbon black particles was dispersed in water. The cumulative 50% particle size of the pigment in the pigment dispersion 11, based on volume, was 100 nm.

[0090] (Pigment dispersion 12) Except for changing the pigment to CI Pigment Red 122, the same procedure as for preparing Pigment Dispersion 1 was followed to obtain Pigment Dispersion 12 with a pigment content of 20.0%. The volume-based cumulative 50% particle size of the pigment in Pigment Dispersion 12 was 100 nm.

[0091] (Pigment dispersion 13) A commercially available pigment dispersion (product name "CAB-O-JET300", manufactured by Cabot) that had undergone ion exchange was used. Pigment dispersion 13 is a dispersion of self-dispersing carbon black in which a benzoic acid group (-C6H4-COOK) with a potassium ion as a counterion is bonded to the surface of the carbon black particles. Pigment dispersion 13 had a pigment content of 15.0% and a cumulative 50% particle size based on the volume of the pigment of 100 nm.

[0092] (Pigment dispersion 14) A commercially available pigment dispersion (product name "CAB-O-JET400", manufactured by Cabot) that had undergone ion exchange was used. Pigment dispersion 14 is a dispersion of self-dispersing carbon black in which atomic groups containing phosphonic acid groups (-PO(OK)2) with potassium ions as counterions are bonded to the surface of carbon black particles. Pigment dispersion 14 had a pigment content of 15.0% and a cumulative 50% particle size based on the volume of the pigment of 100 nm.

[0093] TIFF2026072071000001.tif104170

[0094] <Preparation of colloidal silica> Colloidal silicas 1-12 were prepared. The properties of these colloidal silicas are shown in Table 2.

[0095] (Colloidal silica 1, 6-11) Colloidal silicas 1, 6-11 were synthesized by the known water glass method, with reference to Japanese Patent Publication No. 61-158810. The water glass method involves ion-exchange of sodium silicate to prepare activated silicic acid, which is then added to an aqueous solution containing seed particles, whose pH has been adjusted with sodium hydroxide, under heating, to allow particle growth. By adjusting the amount of activated silicic acid added to the aqueous solution containing seed particles, the particles were grown to the cumulative 50% particle diameter based on volume shown in Table 2 to obtain colloidal silicas 1, 6-11. The properties of the obtained colloidal silicas are shown in Table 2.

[0096] (Colloidal silica 2-5 and 12) Colloidal silica 2-5 and 12 were commercially available products as shown in Table 2.

[0097] TIFF2026072071000002.tif92170

[0098] <Preparation of aqueous dispersion of resin particles> As the aqueous dispersion of resin particles, a commercially available aqueous dispersion of polymethyl methacrylate-based nanocrosslinked microparticles (product name "MX030W", manufactured by Nippon Shokubai, solid content: 10.0%, average particle size: 40 nm) was used.

[0099] <Preparation of resin aqueous solution> As an aqueous solution of water-soluble resin (resin aqueous solution), sodium polyacrylate with a weight-average molecular weight of 5,000,000 was diluted with deionized water and thoroughly stirred to prepare an aqueous solution of sodium polyacrylate with a resin content of 0.5%.

[0100] <Preparation of Silicone Compounds> A commercially available silicone compound (product name "BYK348," manufactured by Bic Chemie) was used.

[0101] <Ink preparation> Each ink was prepared by mixing the components (in %) shown in the middle section of Table 3 (Tables 3-1 to 3-6), stirring thoroughly, and then pressure filtering through a 3.0 μm pore size microfilter (manufactured by Fujifilm). The pigment dispersion and colloidal silica used were those of the type (number) shown in the upper section of Table 3. "Surfinol 104" and "Surfinol 465" shown in the middle section of Table 3 are trade names for acetylene glycol-based surfactants manufactured by Nisshin Chemical Industry Co., Ltd. "Orfin E1010" is a trade name for an acetylene glycol-based surfactant manufactured by Air Products Co., Ltd. "Proxel XL2" is a trade name for a preservative manufactured by Arcsarda Co., Ltd. The lower section of Table 3 shows the average particle size A(D) of the colloidal silica. 50 ;nm), average particle size B(D 50 The table shows the viscosity (nm), colloidal silica content C (%), carbon black content D (%), and betaine content E (%) in the ink. It also shows the A / B, C / D, and E / C ratios. A "-" in the lower row of Table 3 indicates that the component was not used. The viscosity of the prepared ink was in the range of 2.0 mPa·s to 4.0 mPa·s, and the surface tension was in the range of 20.0 mN / m to 35.0 mN / m.

[0102] TIFF2026072071000003.tif200170

[0103] TIFF2026072071000004.tif199170

[0104] TIFF2026072071000005.tif199170

[0105] TIFF2026072071000006.tif198170

[0106] TIFF2026072071000007.tif198170

[0107] TIFF2026072071000008.tif199170

[0108] <Preparation of maintenance fluid> The following components were mixed and thoroughly stirred, then pressure filtered through a 3.0 μm pore size microfilter (manufactured by Fujifilm) to prepare the maintenance solution. The preservative used was "Proxel XL2" (manufactured by Arcsarda). Glycerin: 24.9% • Preservatives: 0.3% • Ion-exchanged water: 74.8%

[0109] <Rating> (Ink evaluation: Examples 1-37 and Comparative Examples 1-12) For the evaluation of Examples 1-37 and Comparative Examples 1-12, an inkjet recording device (product name "PIXUS PRO-10S", manufactured by Canon) equipped with a recording head that ejects ink using thermal energy was used, modified as follows. Specifically, an ink circulation mechanism as shown in Figure 1 and a humidification mechanism as shown in Figure 2 were added. Water was used as the humidifying liquid. However, in Comparative Example 12, in the humidification mechanism shown in Figure 2, the maintenance liquid prepared above was supplied to the closed space formed by the ejection port surface and the cap covering the ejection port surface to humidify the closed space. In this example, the ink ejection amount per drop was set to 4 ng. Using each prepared ink, the following evaluations were performed using the above inkjet recording device. In each example, the ink number used, evaluation conditions, and evaluation results are shown in Table 4 (Tables 4-1 and 4-2). In this example, in the evaluation criteria for each evaluation item shown below, "AA", "A", and "B" were considered acceptable levels, and "C" was considered an unacceptable level. In Table 4, a "-" indicates that the humidification process or ink circulation process was not performed. The "Cap Present / Not Present" column indicates whether the area near the discharge port was covered with a cap during the humidification process ("Present") or not ("Not Present").

[0110] (Intermittent discharge stability) Using the inkjet recording device filled with each of the prepared inks, one drop of ink was ejected from each nozzle to confirm that ejection was normal. Next, the inkjet recording device was stopped for a predetermined time, and then one drop of ink was ejected again from each nozzle. The recorded images were visually observed, and the intermittent ejection stability was evaluated according to the evaluation criteria shown below. The results are shown in Table 4. AA: Even with a downtime of 10 minutes or more, ink was ejected normally from more than half of the nozzles. A: The downtime was between 7 and 10 minutes, and ink was ejected normally from more than half of the nozzles. B: The downtime was between 5 and 7 minutes, and ink was ejected normally from more than half of the nozzles. C: Even if the downtime is less than 5 minutes, ink is not ejected properly from more than half of the nozzles.

[0111] (Suppression of satellites) Each of the prepared inks was filled into the inkjet recording device described above and left for 24 hours in an environment of 35°C. Then, in this environment, vertical lines with a width of 1 dot were recorded on an A4-sized recording medium (product name "High-Quality Printing Paper HR-101," manufactured by Canon). The printer driver settings were set to "Paper Type: Plain Paper, Print Quality: Standard." The resulting recordings were observed with a 20x magnification magnifying glass and visually, and satellite suppression was evaluated according to the evaluation criteria shown below. The results are shown in Table 4. AA: Even when observed with a magnifying glass, there was almost no difference in the attachment positions of the main droplet and the satellite droplet. A: Visual inspection did not reveal any misalignment between the main droplet and the satellite droplet's attachment position, but observation with a magnifying glass revealed a slight misalignment in their attachment positions. B: Visual inspection did not reveal any misalignment between the main droplet and the satellite droplet, but observation with a magnifying glass revealed a significant misalignment. C: A visual inspection revealed a misalignment between the attachment positions of the main droplet and the satellite.

[0112] TIFF2026072071000009.tif221170

[0113] TIFF2026072071000010.tif91170

[0114] Both Examples 24 and 25 showed A-rank satellite suppression, but Example 24 was relatively inferior. Both Examples 26 and 27 showed A-rank satellite suppression, but Example 27 was relatively inferior. Both Examples 23, 28, and 29 showed A-rank satellite suppression, but Example 23 was relatively inferior compared to Examples 28 and 29.

[0115] (Evaluation of intermittent ink ejection stability of ink sets: Examples 38-40) The two types of inks shown in Table 5 were combined to form an ink set, which was then loaded into an inkjet recording device similar to the one used in "Ink Evaluation." This recording device is equipped with a recording head in which the ejection ports for each ink constituting the ink set are integrally formed on a single recording element substrate. Each ejection port row is covered together with a single cap, and a closed space is formed between the ejection port surface and the cap covering the ejection port surface. Using the above inkjet recording device filled with each ink constituting the ink set, one drop of each ink was ejected from each ejection port to confirm that ejection was performed normally. Next, after the inkjet recording device was stopped for a predetermined time, one drop of ink was ejected again from each ejection port. The recorded image was visually observed, and the intermittent ejection stability was evaluated according to the evaluation criteria shown below. The evaluation results are shown in Table 5. In this embodiment, both "AA" and "A" were considered acceptable levels according to the evaluation criteria shown below. AA: Even with a downtime of 20 minutes or more, ink was ejected normally from more than half of the nozzles. A: The downtime was between 15 and 20 minutes, and ink was ejected normally from more than half of the nozzles.

[0116] TIFF2026072071000011.tif47170

Claims

1. An inkjet recording method for recording an image using an inkjet recording apparatus equipped with an inkjet recording head having an ejection port for ejecting water-based ink, wherein the water-based ink is ejected from the recording head and applied to a recording medium, The process includes a humidification step that humidifies the vicinity of the discharge port, The aforementioned water-based ink contains carbon black and colloidal silica, An inkjet recording method characterized in that the volume-based cumulative 50% particle size of the colloidal silica is 20 nm or more.

2. The inkjet recording method according to claim 1, wherein the volume-based cumulative 50% particle size of the colloidal silica is 0.05 times or more and 2.00 times or less in ratio to the volume-based cumulative 50% particle size of the carbon black.

3. The inkjet recording method according to claim 1, wherein the cumulative 50% particle size by volume of the colloidal silica is 100 nm or less.

4. The inkjet recording method according to claim 1, wherein the content (mass%) of colloidal silica in the aqueous ink is 0.10 times or more and 2.00 times or less in mass ratio to the content (mass%) of carbon black.

5. The inkjet recording method according to claim 1, wherein the carbon black comprises oxidized self-dispersing carbon black.

6. The inkjet recording method according to claim 1, wherein the aqueous ink further contains betaine.

7. The inkjet recording method according to claim 6, wherein the betaine content (mass%) in the aqueous ink is 0.20 times or more and 16.00 times or less in mass ratio to the colloidal silica content (mass%).

8. The inkjet recording method according to claim 6, wherein the betaine is trimethylglycine.

9. The aqueous ink comprises a first ink containing carbon black and colloidal silica, and a second ink containing an organic pigment and colloidal silica. The inkjet recording method according to claim 1, wherein the volume-based cumulative 50% particle size of the colloidal silica in the second ink is greater than the volume-based cumulative 50% particle size of the colloidal silica in the first ink.

10. The inkjet recording method according to claim 1, wherein the humidification step provides an atmosphere with an absolute humidity of 0.01 kg / kgDA or higher in the air near the discharge port.

11. The inkjet recording apparatus includes a cap capable of forming a closed space by covering the ejection port surface on which the plurality of ejection ports of the recording head are arranged, The inkjet recording method according to claim 1, wherein the humidification step includes humidifying the enclosed space.

12. The inkjet recording apparatus comprises an ink storage section for storing the aqueous ink, a supply channel for supplying the aqueous ink from the ink storage section to the recording head, and a recovery channel for recovering the aqueous ink from the recording head to the ink storage section. The inkjet recording method according to claim 1, further comprising an ink circulation step of circulating the aqueous ink in a circulation path including the ink storage section, the supply channel, the recording head, and the recovery channel.

13. The inkjet recording method according to claim 12, wherein the circulation flow rate of the aqueous ink in the ink circulation step is 1 mL / min or more and 700 mL / min or less.

14. An inkjet recording apparatus comprising an inkjet recording head having an ejection port for ejecting water-based ink, wherein the water-based ink is ejected from the recording head and applied to a recording medium to record an image, The system includes a humidifying unit that humidifies the vicinity of the discharge port, The aforementioned water-based ink contains carbon black and colloidal silica, An inkjet recording apparatus characterized in that the volume-based cumulative 50% particle size of the colloidal silica is 20 nm or more.

Images