JP7171341B2 - Inkjet recording method and inkjet recording apparatus - Google Patents

Description

The present invention relates to an inkjet recording method and an inkjet recording apparatus.

2. Description of the Related Art In recent years, inkjet recording apparatuses are increasingly being used in the fields of commercial printing and office printing. In the fields of commercial printing and office printing, miniaturization of inkjet recording apparatuses is required. In order to reduce the size of the apparatus, it has been studied to shorten the conveying distance of the recording medium by using a recording head in which the ejection port surface of the recording head is arranged at an angle with respect to the direction of gravity (see Patent Document 1). ). In addition, instead of using a plurality of print heads corresponding to a plurality of inks such as cyan, magenta, yellow, and black, use of a print head having a plurality of ejection port arrays for ejecting a plurality of inks is under consideration. (See Patent Document 2).

JP-A-2005-342982 JP 2008-023989 A

Normally, even if the inks ejected from the recording head are mixed in color, by discharging the mixed inks by a recovery operation such as preliminary ejection or suction, it is possible to eject non-mixed inks when printing an image. However, in order to reduce the amount of waste ink discharged by recovery operations, it is important to be able to eject non-mixed ink even if the number of recovery operations is small. In this way, the ability to eject non-mixed ink with a small number of recovery operations is called color mixture recovery. The inventors of the present invention conducted studies using a print head having a plurality of ejection port arrays for ejecting a plurality of inks while the ejection port surface of the print head is inclined with respect to the direction of gravity. As a result, it has been found that when ink is ejected from the recording head, the ink tends to be mixed in color, and the color mixture recovery of the ink may be insufficient.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to achieve ink jet recording that is excellent in ink color mixture recovery even when ink is ejected from a recording head that is inclined with respect to the direction of gravity and has a plurality of ejection port arrays for ejecting a plurality of inks. It is to provide a method. Another object of the present invention is to provide an inkjet recording apparatus using the inkjet recording method.

In the present invention, a first ink and a second ink, which are aqueous inks containing a pigment; and a first ejection port array for ejecting the first ink and a second ejection port array for ejecting the second ink are formed. the first ejection port row and the second ejection port row are arranged adjacent to each other in order from the bottom in the gravitational direction, and the first ejection port row and the second ejection port row An inkjet recording method using an inkjet recording apparatus having a recording head arranged so that at least a part of it overlaps in the conveying direction of a recording medium, wherein the ejection port surface of the recording head and the direction of gravity form. a recording step of recording an image on a recording medium by ejecting the water-based ink from the recording head arranged so that the angle is 0° or more and less than 90°; The ink jet recording method, wherein the specific gravity is smaller than the true specific gravity of the pigment in the second ink.

In addition, the present invention further comprises a first ink and a second ink, which are water-based inks containing a pigment; The first ejection port array and the second ejection port array are arranged adjacent to each other in order from the bottom in the gravity direction, and the first ejection port array and the second ejection port surface are formed. An ink jet recording apparatus comprising recording heads arranged so that at least a part of the rows overlap in the conveying direction of the recording medium, wherein the ejection opening surface of the recording head and the direction of gravity form an angle of 0°. An image is recorded on a recording medium by ejecting the water-based ink from the recording head arranged so as to have an angle of 90° or more, and the true specific gravity of the pigment in the first ink is the same as that in the second ink. The present invention relates to an ink jet recording apparatus characterized in that the true specific gravity of the pigment is smaller than that of the pigment.

According to the present invention, an inkjet recording method that is excellent in color mixture recovery even when ink is ejected from a recording head that is inclined with respect to the direction of gravity and has a plurality of ejection port arrays for ejecting a plurality of inks. and an inkjet recording apparatus.

3A and 3B are diagrams showing the relationship between the ejection port surface of the recording head and the direction of gravity, FIG. is about 45°, and (c) is a view showing the case where the angle formed is 0°. 1A and 1B are diagrams schematically showing an example of a recording head, FIG. 1A being a schematic diagram of a recording element substrate, and FIG. 1B being a perspective view of the recording head; 1A and 1B are diagrams schematically showing an example of a line head, in which (a) is a schematic diagram of a line head in which recording element substrates are arranged in a zigzag pattern (non-adjacent arrangement) in the arrangement direction of a plurality of ejection port arrays; 1 is a schematic diagram of a line head in which recording element substrates are linearly arranged (adjacently arranged). FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows an example of an inkjet recording device typically, (a) is sectional drawing of the whole apparatus, (b) is an enlarged view of the periphery of a recording head.

Hereinafter, embodiments of the present invention will be described in detail. In the present invention, hereinafter, water-based ink may be referred to as "ink". Various physical property values are values at a temperature of 25° C. unless otherwise specified. "(Meth)acrylic acid" and "(meth)acrylate" shall mean "acrylic acid, methacrylic acid" and "acrylate, methacrylate", respectively.

1A and 1B are diagrams showing the relationship between the ejection port surface of the print head and the direction of gravity. FIG. is a diagram showing the case where the angle formed is approximately 45°, and (c) is a diagram showing the case where the formed angle is 0°. In FIG. 1, .theta. indicates the angle formed by the ejection port surface 8a of the recording head 8 and the direction of gravity (arrow G in FIG. 1). In a general inkjet recording method, as shown in FIG. 1A, from a recording head in which the angle between the ejection port surface of the recording head and the direction of gravity is 90°, that is, the ejection port surface is substantially perpendicular to the direction of gravity. An image is recorded by ejecting ink. However, in the ink jet recording method of the present invention, the angle between the ejection port surface of the recording head and the direction of gravity is 0° or more and less than 90°, that is, the ejection port surface is arranged at an angle with respect to the direction of gravity. Eject to record an image. As shown in FIG. 1C, the angle formed by the ejection port surface of the recording head and the direction of gravity may be 0°, that is, the ejection port surface may be substantially horizontal with respect to the direction of gravity.

An image was recorded using a recording head tilted with respect to the direction of gravity and having a plurality of ejection port arrays for ejecting a plurality of inks. In particular, it has been found that when images are continuously recorded and then left for a long period of time and images are continuously recorded again, the inks are mixed in color, and the color mixture recovery is insufficient. The reason is as follows.

When images are printed continuously, the ink that overflows during ink ejection tends to adhere to the periphery of the ejection port. Furthermore, in addition to the main ink droplets, small ink droplets (hereinafter referred to as “mist”) generated accompanying the ink droplets increase, and the mist tends to adhere to the periphery of the ejection port. When an inkjet recording apparatus with ink adhering to the periphery of the ejection port is left for a long period of time, the liquid component in the adhering ink evaporates, causing the pigment in the ink to precipitate, resulting in unevenness around the ejection port. likely to occur. When images are continuously printed again, the unevenness generated around the ejection port makes it even easier for the ink to adhere to the periphery of the ejection port, which had already been in a state where the ink easily adheres.

The phenomenon that ink tends to adhere to the periphery of the ejection port also occurs in a normal ink jet recording method in which an image is recorded by ejecting ink from a recording head whose ejection port surface is perpendicular to the direction of gravity. In that case, no ink color mixing occurred. Color mixture of inks is a problem that occurs when an image is printed using a print head in which the ejection port surface of the print head is inclined with respect to the direction of gravity.

Here, as an example, as shown in FIG. 1B, description will be made focusing on adjacent ejection port arrays I and II of a print head having an ejection port surface on which four ejection port arrays I to IV are formed. The ejection port groups forming each ejection port array are arranged substantially perpendicular to the conveying direction of the recording medium (arrow A in FIG. 1B). Hereinafter, one ejection port in the ejection port group forming the ejection port array I is referred to as an ejection port I, and one ejection port in the ejection port group forming the ejection port array II is referred to as an ejection port II. The recording medium is conveyed in the direction of arrow A, and ink is ejected onto the recording medium in the order of ejection port arrays I and II.

When an image is printed using a print head in which the ejection port surface of the print head is tilted with respect to the direction of gravity, the particles adhere toward the direction of the ejection port array I around the ejection ports forming the ejection port array II. A force in the direction of gravity is applied to the ink. As a result, ink tends to accumulate toward the direction of the ejection port array I around the ejection ports that constitute the ejection port array II. When images are printed continuously, ink further accumulates in the direction of the ejection opening array I around the ejection openings forming the ejection opening array II. Then, the meniscus of the ink formed at the ejection port II of the ejection port array II is destroyed, the ink overflows, and the ink ejected from the ejection port array II drips along the ejection port surface, resulting in the ejection port. Get into column I. If the inkjet recording apparatus is left for a long period of time with the ink entering the ejection port array I, the ink from the ejection port array I will diffuse into the ink of the ejection port array I, and the ink will be ejected from the ejection port array I. The mixed colors of the inks that are used are severe. In this way, when the color mixture of the ink becomes severe and the pigment in the ink of the ejection port II does not stay in the vicinity of the meniscus of the ink formed in the ejection port I, but goes deep into the ejection port, the color mixture recovery of the ink is reduced. is insufficient.

Furthermore, in a print head in which the ejection port array I and the ejection port array II are at least partially overlapped in the print medium conveying direction, the ink ejected from the ejection port array II tends to enter the ejection port array I. . As a result, the color mixture of the ink ejected from the ejection port array I becomes severe, and the color mixture recovery of the ink becomes insufficient.

In order to improve the recovery of ink color mixture, the present inventors have found that even if the device in which the ink of the ejection port array II has entered the ejection port array I is left for a long period of time, the pigment in the ink of the ejection port array II is removed. , it is important to keep the ink in the vicinity of the meniscus formed in the ejection port I. Therefore, attention was focused on the relationship between the true specific gravities of the pigments in the ink ejected from two adjacent ejection port arrays.

Among the inks ejected from the two adjacent ejection port arrays, when the ink containing the pigment with the smaller true specific gravity enters the ink containing the pigment with the larger true specific gravity, the pigment with the smaller true specific gravity is different from the pigment with the larger true specific gravity. In comparison, the pigment tends to move in the direction opposite to the direction of gravity. For this reason, pigments with a small true specific gravity are less likely to stay near the meniscus formed in the ejection port and more likely to enter deep into the ejection port. inadequate sexuality. Even if an ink containing a pigment with a large true specific gravity enters an ink containing a pigment with a small true specific gravity, the pigment with a large true specific gravity will move in the direction opposite to the direction of gravity compared to the pigment with a small true specific gravity. is difficult to move. Therefore, the pigment with a small true specific gravity tends to stay near the meniscus formed in the ejection port, and it is difficult for it to enter deep into the ejection port. improves.

In other words, among the inks ejected from the adjacent ejection port arrays, if the ink ejected from the ejection port arrays arranged lower in the gravitational direction is ink containing a pigment with a small true specific gravity, the color mixture recoverability is improved. .

<Inkjet recording method>
In the recording head used in the present invention, the first ejection port array and the second ejection port array for ejecting the first ink and the second ink are arranged adjacent to each other in order from the bottom in the direction of gravity. Furthermore, the true specific gravity of the pigment in the first ink is less than the true specific gravity of the pigment in the second ink. Even when the print head has a plurality of ejection port arrays (the third ejection port array and the fourth ejection port array), the relationship of the true specific gravity of the pigment in the ink ejected from each adjacent ejection port array is as follows. is preferably satisfied. This further improves the color mixture recoverability.

A third ejection port array for ejecting a third ink, which is water-based ink containing a pigment, is arranged at a position adjacent to the second ejection port array of the recording head. At least a portion of the ejection port array and the third ejection port array are arranged to overlap. In the print head, the first ejection port array, the second ejection port array, and the third ejection port array are arranged adjacent to each other in order from the bottom in the direction of gravity. The true specific gravity of the pigment in the second ink is preferably smaller than the true specific gravity of the pigment in the third ink.

A fourth ejection port array for ejecting a fourth ink, which is water-based ink containing a pigment, is arranged at a position adjacent to the third ejection port array of the recording head, and the third At least a part of the ejection port array and the fourth ejection port array are arranged to overlap. In the print head, the first ejection port array, the second ejection port array, the third ejection port array, and the fourth ejection port array are arranged adjacent to each other in order from the bottom in the direction of gravity. The true specific gravity of the pigment in the third ink is preferably smaller than the true specific gravity of the pigment in the fourth ink.

Also, the first ink and the second ink may have different hues or may have the same hue. If the first ink and the second ink have different hues, the color mixture in the recorded image is likely to be conspicuous. Even in such a case, the configuration of the present invention can suppress the color mixture in the image. The hue of the first ink and the second ink can be selected from black, cyan, magenta, yellow, and the like. In the case of different hues, the first ink and the second ink are preferably a combination of two inks selected from the group consisting of black ink, cyan ink, magenta ink, and yellow ink. When the hues are the same, the first ink and the second ink have a relationship of dark ink and light ink. The combination of the first ink and the second ink is a dark ink having a black hue (black ink), a light ink having a black hue (gray ink), a dark ink having a cyan hue (cyan ink), and a cyan hue. and a dark ink having a magenta hue (magenta ink) and a light ink having a magenta hue (light magenta ink). .

<Inkjet recording device>
2 to 4, the X direction indicates the horizontal direction, the Y direction indicates the depth direction of the inkjet recording apparatus, and the Z direction indicates the vertical direction.

FIGS. 2A and 2B are diagrams schematically showing an example of a printhead, where FIG. 2A is a schematic diagram of a print element substrate, and FIG. 2B is a perspective view of the printhead. The print head has an ejection port surface on which a plurality of ejection port arrays for ejecting a plurality of inks are formed. In particular, as shown in FIG. 2, it is preferable to use a print head having one print element substrate on which a plurality of ejection port arrays are arranged. FIG. 2(a) shows a recording element substrate H1110 having four ejection port arrays (100a to 100d) arranged in the Y direction. The ejection port surface of the printhead is the surface on which the printing element substrate H1110 having the ejection port array is provided. The recording element substrate may have a plurality of ejection port arrays. For example, when one recording element substrate has four ejection port arrays, four types of ink such as cyan, magenta, yellow, and black (CMYK) are ejected from the four ejection port arrays.

The distance (mm) between an ejection port array configured with ejection ports for ejecting a certain ink and an ejection port array configured with ejection ports for ejecting another ink is 0.1 mm or more and 1.5 mm or less. , and more preferably 0.1 mm or more and 1.0 mm or less. More preferably, the distance is 0.3 mm or more and 1.0 mm or less. Here, the distance between the ejection port arrays is the distance between a straight line connecting the centers of the ejection ports that eject a certain ink and a straight line connecting the centers of the ejection ports that eject another ink. That is. When there are a plurality of ejection port arrays that eject one type of ink, the distance between the ejection port arrays refers to the ejection port array that is the closest in the X direction and is composed of ejection ports that eject a certain ink. , is the distance between ejection port arrays composed of ejection ports for ejecting different inks.

If the distance between the ejection port arrays is short, the ejection port arrays can be arranged densely, so that a higher quality image can be printed. Problems arise remarkably. Even in such a case, by adopting the configuration of the present invention, the mixed color recoverability is improved.

The long diameter (μm) passing through the center of the ejection port of the print head is preferably 10 μm or more and 50 μm or less. Furthermore, the ejection amount (ng) per ink droplet ejected from the print head is preferably 8.0 ng or less. If the ejection amount exceeds 8.0 ng, the ink tends to overflow when ejected, and the amount of mist tends to increase. As a result, the inks tend to mix colors, and the color mixing recovery is lowered. More preferably, the discharge amount (ng) is 2.0 ng or more.

FIG. 2B shows a print head 8 having one print element substrate H1110. The recording head may have one recording element substrate, or may have a plurality of recording element substrates. When using a print head having a plurality of print element substrates, a print head in which a plurality of print element substrates are arranged along the Y direction in FIG. is preferred. When using a printhead having a plurality of printing element substrates, it is preferable to arrange the plurality of printing element substrates so that the ejection openings overlap in the conveying direction of the printing medium. As a result, although black streaks and white spots can be suppressed in the joints between the recording element substrates, overlapping the ejection ports makes it easy for ink to mix from one ejection port to another. The problem of color mixture recovery remarkably arises. Even in such a case, by adopting the configuration of the present invention, the mixed color recoverability is improved.

3A and 3B are schematic diagrams of a line head. FIG. 3A is a schematic diagram of a line head in which recording element substrates are arranged in a staggered manner (non-adjacent arrangement) in the arrangement direction of a plurality of ejection port rows, and FIG. 3B is a schematic diagram of a line head. 1 is a schematic diagram of a line head in which recording element substrates are linearly arranged (adjacently arranged); FIG. In FIG. 3, a plurality of recording element substrates H1110 are arranged on a support substrate. In order to reduce the size of the apparatus and suppress an increase in the length of the line head in the X direction in FIG. , it is preferable to use line heads arranged adjacently. That is, it is preferable to use a line head in which a plurality of recording element substrates H1110 are arranged linearly. Furthermore, the shape of the recording element substrate may be parallelogram, rectangle, trapezoid, or other shapes, but the parallelogram is preferable.

Methods for ejecting ink include a method of applying mechanical energy to ink, a method of applying thermal energy to ink, and the like. Among them, the method of ejecting ink is preferably a method of applying thermal energy to the ink.

4A and 4B are diagrams schematically showing an example of an inkjet recording apparatus, in which FIG. 4A is a cross-sectional view of the entire apparatus, and FIG. 4B is an enlarged view around the recording head. As shown in FIG. 4, in order to reduce the size of the apparatus, an inkjet printer that can record an image with a single recording head capable of ejecting multiple types of ink instead of recording images with multiple recording heads corresponding to multiple types of ink. A recording device is preferably used. When ink is ejected to record an image, as shown in FIG. 4, the angle formed by the ejection port surface 8a of the recording head 8 and the direction of gravity is 0° or more and less than 90°, and is tilted with respect to the direction of gravity. An image is recorded by ejecting ink from the recording head 8 .

When printing an image by ejecting ink, the difference between the angle between the recording medium S and the direction of gravity and the angle between the ejection port surface 8a of the recording head 8 and the direction of gravity should be ±5° or less. It is preferably 0°, more preferably 0°. That is, the difference between the distance between the ejection openings forming the ejection opening array I and the recording medium S and the distance between the ejection openings forming the ejection opening array IV and the recording medium S is ±1 mm or less. Preferably, it is more preferably 0 mm. Here, the distance between the ejection port and the recording medium S is the distance between the center of the ejection port and the position where the line intersects the recording medium when a line is extended from the center of the ejection port in the direction of gravity. It's about. In this way, by setting the transport direction of the recording medium when recording an image to the above conditions, the transport distance of the recording medium in the X direction is also shortened, and the size of the apparatus can be reduced. In order to shorten the conveying distance of the recording medium in the X direction, the angle formed by the ejection port surface of the recording head and the direction of gravity is preferably 10° or more and 80° or less, and preferably 30° or more and 60° or less. is more preferred.

An ejection port surface 8 a of the recording head 8 faces the platen 9 . In FIG. 4, the plane of the platen 9 is tilted at about 45° with respect to the direction of gravity, and the ejection port surface 8a of the recording head 8 is also tilted at about 45° with respect to the direction of gravity so that the distance from the platen 9 is kept constant. 45° tilted. When the inkjet recording apparatus is not performing a recording operation, the angle formed by the ejection port surface 8a of the recording head 8 and the direction of gravity is 90°.

Further, the conveying path of the recording medium when recording an image will be described. In FIG. 4B, the recording medium S is guided by the first guide 10, and the leading edge position of the recording medium S is detected by the paper sensor 11. As shown in FIG. The recording medium S is conveyed toward a recording area P between the recording head 8 and the platen 9 while being sandwiched between a first conveying roller 12 and a first pinch roller 13 constituted by a spring-biased spur or the like. be. In the print area P, ink is ejected toward the print medium S from a plurality of ejection port arrays (I to IV) of the print head 8 . The back surface of the recording medium S in the area to which ink is applied is supported by the platen 9, and the distance between the ejection port surface 8a and the recording medium S is kept constant. The recording medium S to which ink has been applied is conveyed by being guided by the second guide 16 while being nipped between the second conveying roller 14 and the second pinch roller 15 . The conveying direction of the recording medium when recording an image may be the direction opposite to the direction shown in FIG. 4(b), but is preferably the direction shown in FIG. 4(b). That is, the first ejection port array (ejection port array I) arranged on the upstream side in the recording medium transport direction is the second ejection port array (ejection port array I) disposed on the downstream side in the recording medium transport direction. It is preferably arranged below II) in the direction of gravity. Further, it is preferable that the direction in which the recording medium is conveyed when recording an image is a direction that intersects with the direction in which the ejection port arrays I to IV are arranged.

In order to suppress variations in the ink ejection amount, it is preferable to preheat the ink before ejecting the ink based on the image data. This preheating is heating by a heating element present in the vicinity of the recording element for ejecting ink. Heating the ink tends to lower the viscosity of the ink, so that the ink ejected from the ejection port tends to drip along the ejection port surface, and the ink tends to mix colors. As a result, the problem of color mixture recovery remarkably arises. Even in such a case, by adopting the configuration of the present invention, the mixed color recoverability is improved.

Furthermore, it is preferable that the ejection port surface of the recording head is water repellent. As a result, the contact angle between the ink adhering to the periphery of the ejection port and the ejection port surface becomes large, so that the ink droplets tend to be grainy. Therefore, it is difficult for the ink to drip in the gravitational direction along the ejection port surface, and the color mixture recoverability of the ink is further improved.

A method of applying a water-repellent material by spraying, a method of attaching a water-repellent material by vacuum deposition or plasma polymerization, or the like can be selected as a method for water-repellent treatment of the ejection port surface. The water repellency of the formed ejection port surface can be specified by measuring the contact angle of water droplets on the surface of the member. When the contact angle of water is 70 degrees or more, it can be said to have water repellency, and the case where the contact angle of water is 90 degrees or more is preferable. The contact angle with water can be measured using pure water (ion-exchanged water) using a general contact angle meter. Examples of such a contact angle meter include an automatic contact angle measuring machine (CA-W, manufactured by Kyowa Interface Science).

As the water-repellent material, for example, a fluororesin compound is preferably used. In particular, it is preferable that the water-repellent surface is formed as a uniform resin film made of a fluororesin compound, and it is preferable that this resin film does not contain a metal such as nickel. Examples of fluororesin compounds include polytetrafluoroethylene resins and fluororesins having a cyclic structure. Specifically, Polyflon PTFE (manufactured by Daikin Industries), Teflon (registered trademark) PTFE (manufactured by DuPont), Cytop (manufactured by Asahi Glass), and the like can be mentioned. Furthermore, other resins containing fluorine atoms, such as fluorinated epoxy resins, fluorinated polyimide resins, fluorinated polyamide resins, fluorinated acrylic resins, fluorinated urethane resins, fluorinated siloxane resins, modified resins thereof, etc. can be used. A compound containing a silicon atom or a silicone-based resin may also be used as the water-repellent material.

Among them, a condensate of a hydrolyzable silane compound having a fluoroalkyl group and a hydrolyzable silane compound having a cationically polymerizable group is used as the water-repellent material, since high water repellency and durability can be obtained. is preferred. A resin obtained by curing this condensate by irradiation with active energy rays such as ultraviolet rays may also be used. These hydrolyzable silane compounds have hydrolyzable groups in their molecular structures. An alkoxy group can be mentioned as a hydrolyzable group. Moreover, a cyclic ether group, a cyclic vinyl ether group, etc. can be mentioned as a cationic polymerizable group.

The inkjet recording apparatus may include means for applying a reaction liquid containing a reaction agent for aggregating the coloring material in the ink to the recording medium. Examples of means for applying the reaction liquid to the recording medium include means for applying the reaction liquid to the recording medium with a roller or the like, and means for ejecting the reaction liquid from an inkjet type recording head. The ink jet recording apparatus of the present invention does not need to be provided with an energy beam irradiation means.

<Ink>
Each component constituting the ink used in the present invention will be described in detail below. The ink used in the present invention may not contain a compound that polymerizes upon exposure to energy rays.

(pigment)
The ink contains pigments. The content of the pigment in the ink is preferably 0.1% by mass or more and 15.0% by mass or less, and preferably 1.0% by mass or more and 11.0% by mass or less, based on the total mass of the ink. more preferred.

As a method of dispersing the pigment, a resin-dispersed pigment using a resin as a dispersing agent, or a self-dispersing pigment having a hydrophilic group bonded to the particle surface of the pigment can be used. Further, a resin-bonded pigment in which an organic group containing a resin is chemically bonded to the surface of a pigment particle, or a microcapsule pigment in which the surface of a pigment particle is coated with a resin or the like can be used. Pigments with different dispersion methods can be used together. Even if the pigment dispersing method is different, the easiness of diffusion of the pigment in the ink when the inks are mixed is greatly affected by the true specific gravity of the pigment.

Specific examples of pigments include inorganic pigments such as carbon black and titanium oxide; and organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole and dioxazine.

[True specific gravity]
The true specific gravity (g/cm ³ ) of the pigment has a large effect on the easiness of diffusion of the pigment when the inks are mixed. The true specific gravity of a pigment is the specific gravity determined by the structure of the pigment. The true specific gravity of a pigment can be measured using a floating hydrometer (standard hydrometer, manufactured by Techjam). When the ink contains a plurality of types of pigments, it is sufficient to use a plurality of inks that satisfy the relationship of the true specific gravities of the pigments based on the true specific gravities of the pigments having the higher content. Specifically, the true specific gravity of the pigment is shown below. Regarding organic pigments, the values are described in Organic Pigment Handbook (edited by Color Office, Color Office, 2006), and the numbers in parentheses represent the true specific gravity of the pigment.

C. I. Pigment Yellow 74 (1.4), C.I. I. Pigment Yellow 128 (1.5), C.I. I. Pigment Yellow 155 (1.4), C.I. I. Pigment Red 122 (1.4), C.I. I. Pigment Red 149 (1.4), C.I. I. Pigment Red 202 (1.5), C.I. I. Pigment Violet 19 (1.5), C.I. I. Pigment Violet 23 (1.5), C.I. I. Pigment Blue 1 (1.8), C.I. I. Pigment Blue 15:3 (1.6), C.I. I. Pigment Blue 15:4 (1.7), C.I. I. Pigment Green 36 (2.9), Carbon Black (1.9).

(First water-soluble organic solvent)
The ink preferably contains a first water-soluble organic solvent having a dielectric constant of 20.0 or higher. By suppressing the precipitation of the pigment and reducing the unevenness around the ejection port, the ink is less likely to accumulate around the ejection port, and the ink is less likely to drip along the ejection port surface. As a result, it is possible to suppress color mixture of inks and further improve color mixture recovery. More preferably, the dielectric constant of the first water-soluble organic solvent is 45.0 or less. The vapor pressure of the first water-soluble organic solvent at a temperature of 25°C is preferably lower than that of water.

The dielectric constant of the water-soluble organic solvent can be measured at 10 kHz using a dielectric constant meter (eg, BI-870, manufactured by BROOKHAVEN INSTRUMENTS CORPORATION). The dielectric constant of a solid water-soluble organic solvent at a temperature of 25° C. can be calculated from the following formula (A) by measuring the dielectric constant of a 50.0 mass % aqueous solution.
ε _sol =2ε _50% -ε _water (1)
ε _sol : Relative permittivity of water-soluble organic solvent solid at 25° C. ε _50% : Relative permittivity of 50.0 mass % aqueous solution of solid water-soluble organic solvent at temperature 25° C. ε _water : Relative permittivity of water

The reason for calculating the dielectric constant of a solid water-soluble organic solvent at a temperature of 25° C. from the dielectric constant of a 50.0 mass % aqueous solution is as follows. Among water-soluble organic solvents that are solid at a temperature of 25° C., there are those that are difficult to prepare as high-concentration aqueous solutions exceeding 50.0% by mass, among those that can be used as constituents of ink. On the other hand, in a low-concentration aqueous solution of 10.0% by mass or less, the relative dielectric constant of water becomes dominant, and it is difficult to obtain a probable (effective) value of the relative dielectric constant of the water-soluble organic solvent. Therefore, as a result of investigation by the present inventors, most of the water-soluble organic solvents that are solid at a temperature of 25 ° C. used for ink can prepare an aqueous solution to be measured, and the calculated dielectric constant is It has been found to be consistent with the effects of the present invention. For the above reasons, in the present invention, the dielectric constant of a solid water-soluble organic solvent at a temperature of 25° C. is calculated from the dielectric constant of a 50.0% by mass aqueous solution and used. Even if the water-soluble organic solvent is solid at a temperature of 25 ° C., the solubility in water is low, and for a solvent that cannot be prepared into a 50.0% by mass aqueous solution, an aqueous solution with a saturated concentration is used to calculate the above ε _sol . The relative permittivity value calculated according to the case is used for convenience.

Specific examples of the first water-soluble organic solvent include monohydric alcohols having 1 to 4 carbon atoms such as methyl alcohol (33.1) and ethyl alcohol (23.8); 1,2-propanediol (28. 8), 1,3-butanediol (30.0), 1,4-butanediol (31.1), 1,5-pentanediol (27.0), 3-methyl-1,5-pentanediol ( 23.9) and other dihydric alcohols; 1,2,6-hexanetriol (28.5), glycerin (42.3), trimethylolpropane (33.7) and other polyhydric alcohols; ethylene glycol ( 40.4), diethylene glycol (31.7), triethylene glycol (22.7), alkylene glycols such as tetraethylene glycol (20.8); 2-pyrrolidone (28.8), N-methyl-2- Nitrogen-containing compounds such as pyrrolidone (32.0), urea (110.3), ethylene urea (49.7), triethanolamine (31.9); sulfur-containing compounds such as dimethylsulfoxide (48.9) is mentioned.

The content (% by mass) of the first water-soluble organic solvent is preferably at least 3.0 times the content (% by mass) of the pigment as a mass ratio (times). If the ratio is 3.0 times or more, the amount of the first water-soluble organic solvent is larger than that of the pigment. Ink is less likely to accumulate in the nozzle, and ink is less likely to drip along the ejection port surface. As a result, it is possible to suppress color mixture of inks and further improve color mixture recovery. More preferably, the ratio is 7.0 times or less.

The content (% by mass) of the first water-soluble organic solvent in the ink containing the pigment with a large true specific gravity is the content (% by mass) of the first water-soluble organic solvent in the ink containing the pigment with a small true specific gravity. is preferably 1.0 times or more as a mass ratio (times) to When the ratio is 1.0 times or more, deposition of the pigment around the ejection port for ejecting the ink containing the pigment having a large true specific gravity is suppressed, and unevenness around the ejection port is reduced. Therefore, the ink is less likely to accumulate around the ejection port, and the ink is less likely to drip along the ejection port surface. As a result, it is difficult for ink containing a pigment with a small true specific gravity to mix with an ink containing a pigment with a large true specific gravity, so that the color mixing recovery of the ink is further improved.

(aqueous medium)
The ink can contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. As water, it is preferable to use deionized water or ion-exchanged water. The water content (% by mass) in the water-based ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink.

As the water-soluble organic solvent, a water-soluble organic solvent other than the first water-soluble organic solvent (another water-soluble organic solvent) can be used in combination. Other water-soluble organic solvents are not particularly limited as long as they are water-soluble, and alcohols, glycols, glycol ethers, nitrogen-containing compounds, and the like can be used. In addition, one or more of other water-soluble organic solvents can be contained in the ink. The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink. This content is a value including the first water-soluble organic solvent. The content (mass%) of the first water-soluble organic solvent is a mass ratio (times) to the total content (mass%) of the water-soluble organic solvent, and is 0.5 times or more and 1.0 times or less. preferable.

Specific examples of water-soluble organic solvents include the following, including the specific water-soluble organic solvents mentioned above (the numbers in parentheses represent the dielectric constant at a temperature of 25°C). 1 to 4 carbon atoms such as methyl alcohol (33.1), ethyl alcohol (23.8), n-propyl alcohol, isopropyl alcohol (18.3), n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol the following monohydric alcohols; 1,2-propanediol (28.8), 1,3-butanediol (30.0), 1,4-butanediol (31.1), 1,5-pentanediol (27.0), 1, 2-hexanediol (14.8), 1,6-hexanediol (7.1), 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol (23.9) Dihydric alcohols. Polyhydric alcohols such as 1,2,6-hexanetriol (28.5), glycerin (42.3), trimethylolpropane (33.7) and trimethylolethane. Alkylene glycols such as ethylene glycol (40.4), diethylene glycol (31.7), triethylene glycol (22.7), tetraethylene glycol, butylene glycol, hexylene glycol and thiodiglycol. Glycol ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether (9.8). Polyalkylene glycols with a number average molecular weight of 200 or more and 1,000 or less, such as polyethylene glycol (11.5) with a number average molecular weight of 600, polyethylene glycol (4.6) with a number average molecular weight of 1,000, and polypropylene glycol. 2-pyrrolidone (28.8), N-methyl-2-pyrrolidone (32.0), 1,3-dimethyl-2-imidazolidinone, N-methylmorpholine, urea (110.3), ethylene urea (49 .7), nitrogen-containing compounds such as triethanolamine (31.9). Sulfur-containing compounds such as dimethylsulfoxide (48.9) and bis(2-hydroxyethylsulfone). The water-soluble organic solvent contained in the ink preferably has a dielectric constant of 3.0 or more and a vapor pressure lower than that of water at a temperature of 25°C.

(Other additives)
Various additives such as surfactants, pH adjusters, antifoaming agents, rust inhibitors, antiseptics, antifungal agents, antioxidants, reduction inhibitors, chelating agents, and resins are added to the ink as necessary. agents may be included. These additives generally have a considerably low content in the ink and have little influence on the effects of the present invention. Therefore, in the present invention, these additives are not included in the "water-soluble organic solvent" and are not subject to the calculation of the dielectric constant.

(physical properties)
In order to suppress color mixture of ink, it is important that the ink does not easily adhere around the ejection port when the ink is ejected. Therefore, since the time required from ink bubbling to ejection is several milliseconds, attention was focused on the dynamic surface tension of ink at 10 milliseconds as an extremely short life time that can be measured with high accuracy. The dynamic surface tension (mN/m) of the ink at a lifetime of 10 ms is preferably 35 mN/m or more.

When the dynamic surface tension is less than 35 mN/m, the tension that reduces the surface area of the ink surface is less likely to act, and when the ink is ejected, the ink tends to adhere around the ejection port. As a result, the ink ejected from the ejection port tends to run down along the ejection port surface, resulting in color mixture of the inks and deterioration of color mixture recoverability. More preferably, the dynamic surface tension is 48 mN/m or less.

The dynamic surface tension of ink is measured by the maximum bubble pressure method. In this method, a probe (capillary tube) is immersed in the liquid to be measured, and the surface tension is determined by measuring the maximum pressure required to release the bubble pushed out from the tip of the probe. In addition, the life time is the time when a bubble is formed from the tip of the probe, and the time when a new surface is formed after the bubble leaves and the maximum bubble pressure (the radius of curvature of the bubble and the radius of the tip of the probe are equal). means the time until

Furthermore, the static surface tension (mN/m) of the ink is preferably 30 mN/m or more and 40 mN/m or less. The static surface tension of ink is measured by the Wilhelmy method (plate method). The surface tension value can be appropriately adjusted depending on the type and amount of surfactant.

The viscosity of the ink at a temperature of 25° C. is preferably 1 mPa·s or more and 15 mPa·s or less.

The difference in specific gravity between the first ink and the second ink is preferably 0.05 or less, more preferably 0.03 or less. If the difference in specific gravity between the first ink and the second ink is large, the ink with high specific gravity stays in the vicinity of the meniscus of the ejection port, but the ink with low specific gravity penetrates into the interior of the ejection port, making it easier for the inks to mix. As a result, it may not be possible to obtain sufficient ink color mixture recovery.

EXAMPLES The present invention will be described in more detail below with reference to examples, comparative examples, and reference examples, but the present invention is not limited by the following examples as long as it does not exceed the scope of the invention. Note that "parts" and "%" regarding component amounts are based on mass unless otherwise specified.

<Preparation of pigment dispersion>
(Pigment dispersion liquid 1)
20.0 g of pigment, 8.0 mmol of treating agent, 8.0 mmol of nitric acid, and 200.0 mL of water were mixed. As a pigment, C.I. I. Pigment Yellow 74 (Hansa yellow 5GXB, manufactured by Clariant) was used, and p-aminophthalic acid was used as a treating agent. Mixing was performed using a Silverson mixer under conditions of a temperature of 25° C., 6,000 rpm, and 30 minutes. An aqueous solution of 8.0 mmol of potassium nitrite dissolved in a small amount of water was slowly added to the resulting mixture. The temperature of the mixture reached 60° C. with the addition of the aqueous solution. At a temperature of 60° C., the mixture was allowed to react for 1 hour. After that, the pH of the mixture was adjusted to 10 using a 1.0 mol/L potassium hydroxide aqueous solution. After 30 minutes, 20.0 mL of water was added to the mixture, and low-molecular substances were removed and desalted using a spectrum membrane. Furthermore, the mixture was diluted with water to obtain a pigment dispersion liquid 1 (pigment content: 10.0%) containing a self-dispersing pigment. Pigment Dispersion 1 contained a self-dispersed pigment with —C ₆ H ₃ —(COOK) ₂ groups attached to the particle surface.

(Pigment dispersion liquid 2)
In the preparation of Pigment Dispersion 1, the type of pigment was C.I. I. Pigment Yellow 128 (Cromophtal Yellow D0980J, manufactured by BASF) was used. Pigment dispersion 2 (pigment content: 10.0%) was obtained in the same manner as pigment dispersion 1 except for the above. Pigment Dispersion 2 contained a self-dispersing pigment with —C ₆ H ₃ —(COOK) ₂ groups attached to the particle surface.

(Pigment dispersion liquid 3)
In the preparation of Pigment Dispersion 1, the amount of the treating agent was 4.0 mmol, and the type of pigment was C.I. I. Pigment Violet 19 (Hostaperm Red Violet Er 02, manufactured by Clariant). Pigment Dispersion 3 (pigment content: 10.0%) was obtained in the same manner as Pigment Dispersion 1 except for the above. Pigment Dispersion 3 contained a self-dispersing pigment with —C ₆ H ₃ —(COOK) ₂ groups attached to the particle surface.

(Pigment dispersion liquid 4)
In the preparation of Pigment Dispersion 1, the amount of the treating agent was 4.0 mmol, and the type of pigment was C.I. I. Pigment Red 122 (Ink Jet Magenta E 02, manufactured by BASF). Pigment dispersion 4 (pigment content: 10.0%) was obtained in the same manner as pigment dispersion 1 except for the above. Pigment Dispersion 4 contained a self-dispersed pigment with —C ₆ H ₃ —(COOK) ₂ groups attached to the particle surface.

(Pigment dispersion liquid 5)
In the preparation of Pigment Dispersion 1, the amount of treatment agent was 1.6 mmol and the type of pigment was C.I. I. Pigment Blue 15:3 (Hostaperm Blue B2G, manufactured by Clariant). Pigment Dispersion 5 (pigment content: 10.0%) was obtained in the same manner as Pigment Dispersion 1 except for the above. Pigment Dispersion 5 contained a self-dispersing pigment with —C ₆ H ₃ —(COOK) ₂ groups attached to the particle surface.

(Pigment dispersion liquid 6)
To a solution of 5.0 g of concentrated hydrochloric acid dissolved in 5.5 g of water was added 1.6 g of 4-amino-1,2-benzenedicarboxylic acid at a temperature of 5°C. A solution of 1.8 g of sodium nitrite dissolved in 9.0 g of water was added to the solution obtained above while stirring in an ice bath in order to maintain the temperature at 10° C. or less. After stirring for 15 minutes, 6.0 g of carbon black having a specific surface area of 220 m ² /g and a DBP oil absorption of 105 mL/100 g was added and mixed. After further stirring for 15 minutes, the resulting slurry was filtered through a filter paper (standard filter paper No. 2, manufactured by Advantech), the carbon black was thoroughly washed with water, and dried in an oven at a temperature of 110°C. Water is added to the obtained carbon black to _prepare a pigment dispersion _{( pigment} containing amount of 10.0%). After that, using an ion exchange method, sodium ions in the pigment dispersion were replaced with potassium ions.

(Pigment dispersion liquid 7)
12.0 parts of pigment, 24.0 parts of resin-containing liquid, and 64.0 parts of deionized water were mixed. As a pigment, C.I. I. Pigment Blue 15:3 (Hostaperm Blue B2G, manufactured by Clariant) was used. As a liquid containing resin, a styrene-acrylic acid copolymer (Joncryl 680, manufactured by BASF) is neutralized with an aqueous potassium hydroxide solution of 0.85 equivalent to the acid value of the copolymer, and the resin content is was 20.0%. This mixture was dispersed for 3 hours while being cooled with water using a batch-type vertical sand mill (manufactured by Imex) filled with 85.0 parts of zirconia beads having a particle size of 0.3 mm. Thereafter, this dispersion liquid was subjected to centrifugal separation to remove coarse particles, and pressure filtration was performed using a cellulose acetate filter (manufactured by Advantech) having a pore size of 3.0 μm. Pigment Dispersion 7 (pigment content: 10.0%, resin content: 4.0%) in which the pigment was dispersed in water by the resin was obtained by the above method.

<Ink preparation>
Each component described in Table 1 was mixed and thoroughly stirred. Then, pressure filtration was performed with a cellulose acetate filter (manufactured by Advantec) having a pore size of 0.8 μm to prepare an ink. Acetylenol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals. The numbers attached to polyethylene glycol represent the number average molecular weight. The relative permittivity of the water-soluble organic solvent shown in parentheses is a value obtained at a frequency of 10 kHz using a permittivity meter (BI-870, manufactured by BROOKHAVEN INSTRUMENTS CORPORATION).

<Evaluation>
In the present invention, according to the following evaluation criteria, A or B was defined as an acceptable level, and C was defined as an unacceptable level. Evaluation results are shown in Table 3. Using the inkjet recording apparatus having the configuration shown in FIG. 4, ink was mounted on a recording head having one recording element substrate. As recording heads, recording heads 1 to 8 shown in Table 2 were used.

In Table 2, the ejection port arrays of the recording element substrate correspond to the ejection port arrays I to IV shown in FIG. The print element substrates of the print heads 1 to 6 and 8 have overlapping ejection port arrays in the print medium conveying direction, but the print element substrate of the print head 7 has an ejection port array in the print medium conveying direction. are not duplicated. Further, although the recording head 5 has the ejection port arrays I to III, ink is not ejected from the ejection port array II.

The print heads 1 to 8 had 1024 ejection ports per ejection port array, and the density of the ejection ports per ejection port array was 600 dpi. The length (μm) passing through the center of the ejection port was 20 μm, and the distance (mm) between adjacent ejection port rows was 0.7 mm. Further, the ejection port surface of the recording head is water-repellent treated with a condensate of a hydrolyzable silane compound having a fluoroalkyl group and a hydrolyzable silane compound having a cationic polymerizable group.

In this embodiment, an image printed under the condition that three 5.0 ng ink droplets are applied to a unit area of 1/600 inch×1/600 inch is defined as having a print duty of 100%. The conveying speed was 15 inches/second. When the recording medium is conveyed in the direction from ejection port array I to ejection port array II (“I→II” in Table 3), the recording medium is conveyed from bottom to top in the direction of gravity when printing an image. . When the recording medium is conveyed from ejection port array II to ejection port array I (“II→I” in Table 3), the recording medium is conveyed from top to bottom in the direction of gravity when printing an image. .

(color mixing recovery)
First, using each ink, a monochromatic solid image (approximately 10 cm in the conveying direction of the printing medium×approximately 4 cm in the depth direction of the apparatus) with a printing duty of 100% was printed. The obtained image was designated as image 1 for evaluation. After that, using each ink, a multi-color solid image (approximately 29 cm in the conveying direction of the recording medium×approximately in the depth direction of the apparatus) is obtained so that the recording duty of each ink is uniform and the total recording duty of the inks is 100%. 4 cm) was continuously recorded on 100 sheets. After leaving the apparatus for one week, all the inks were used again to produce a multi-color solid image (in the direction of transport of the recording medium) so that the recording duty of each ink was uniform and the total recording duty of the inks was 100%. Approximately 29 cm×approximately 4 cm in the depth direction of the device) was continuously recorded on 100 sheets.

After repeating this series of operations of continuous recording, leaving, and continuous recording seven times, each ink is used again to produce a monochrome solid image with a recording duty of 100% (approximately 10 cm in the conveying direction of the recording medium x the depth of the apparatus). 4 cm) was recorded. The obtained image was used as image 2 for evaluation. Plain paper (PPC paper PB Paper, manufactured by Canon Inc.) was used as the recording medium. In order to print a monochromatic solid image (approximately 10 cm in the conveying direction of the printing medium×approximately 4 cm in the depth direction of the apparatus), approximately 7000 droplets of ink are ejected from one ejection port.

Evaluation image 1 and evaluation image 2 were visually observed to evaluate how long the color mixture of evaluation image 2 would be eliminated after the start of ink ejection.
A: Color mixture was resolved when about 1,000 or less ink droplets were ejected from all ejection ports. B: Color mixture was resolved when more than about 1,000 droplets or less than about 5,000 droplets of ink were ejected from all ejection ports. Resolved C: Color mixture was resolved by ejecting more than about 5,000 droplets of ink from all ejection ports.

As Reference Example 12, an image was recorded using an inkjet recording apparatus having two recording heads corresponding to magenta ink 1 and yellow ink 1 in order from the upstream side in the conveying direction of the recording medium. An image was recorded in the same manner as in Comparative Example 6, except that two recording heads were used.

As Reference Example 13, an image was recorded using an inkjet recording apparatus having two recording heads corresponding to yellow ink 1 and magenta ink 1 in order from the upstream side in the conveying direction of the recording medium. An image was recorded in the same manner as in Example 1, except that two recording heads were used.

Claims (19)

A first ink and a second ink, which are water-based inks containing a pigment; and an ejection port surface on which a first ejection port array for ejecting the first ink and a second ejection port array for ejecting the second ink are formed. wherein the first ejection port row and the second ejection port row are arranged adjacent to each other in order from the bottom in the gravitational direction, and at least a part of the first ejection port row and the second ejection port row has An inkjet recording method using an inkjet recording apparatus comprising recording heads arranged so as to overlap in the conveying direction of a recording medium,
A recording step of recording an image on a recording medium by ejecting the water-based ink from the recording head arranged so that the angle between the ejection port surface of the recording head and the direction of gravity is 0° or more and less than 90°. has
An inkjet recording method, wherein the true specific gravity of the pigment in the first ink is smaller than the true specific gravity of the pigment in the second ink. 2. The inkjet recording method according to claim 1, wherein the angle formed by the ejection port surface of the recording head and the direction of gravity is 10[deg.] or more and 80[deg.] or less. 2. The inkjet recording method according to claim 1, wherein the angle formed by the ejection port surface of the recording head and the direction of gravity is 30[deg.] or more and 60[deg.] or less. 4. The inkjet recording method according to claim 1, wherein the recording head comprises one recording element substrate on which the first ejection port array and the second ejection port array are arranged. 5. The inkjet recording method according to claim 4, wherein said recording head comprises a plurality of said recording element substrates. 6. The inkjet recording method according to claim 5, wherein the plurality of recording element substrates provided in the recording head are arranged adjacent to each other in the direction in which the first ejection port array and the second ejection port array are arranged. The inkjet recording method according to any one of claims 1 to 6, wherein the recording medium is conveyed in a direction from the first ejection port array toward the second ejection port array. The inkjet recording method according to any one of claims 1 to 7, wherein the first ink and the second ink have hues different from each other. A third ejection port array for ejecting a third ink, which is water-based ink containing a pigment, is arranged at a position adjacent to the second ejection port array of the recording head, and
arranged such that at least a portion of the second ejection port array and the third ejection port array overlap in the conveying direction of the recording medium;
The inkjet recording method according to any one of claims 1 to 8, wherein the true specific gravity of the pigment in the second ink is smaller than the true specific gravity of the pigment in the third ink. A fourth ejection port array for ejecting a fourth ink, which is water-based ink containing a pigment, is arranged at a position adjacent to the third ejection port array of the recording head, and
arranged such that at least a portion of the third ejection port array and the fourth ejection port array overlap in the conveying direction of the recording medium;
10. The inkjet recording method according to claim 9, wherein the true specific gravity of the pigment in the third ink is smaller than the true specific gravity of the pigment in the fourth ink. 11. The inkjet recording method according to any one of claims 1 to 10, wherein the water-based ink contains a first water-soluble organic solvent having a dielectric constant of 20.0 or higher. 12. The inkjet recording method according to claim 11, wherein the dielectric constant of the first water-soluble organic solvent is 45.0 or less. 13. Any one of claims 1 to 12, wherein the pigment is at least one selected from the group consisting of carbon black, titanium oxide, azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole, and dioxazine. The inkjet recording method described in . The content (% by mass) of the pigment in the water-based ink is 0.1% by mass or more and 15.0% by mass or less based on the total mass of the ink. Inkjet recording method. 15. The ink jet recording method according to any one of claims 1 to 14, wherein the ejection port surface of the recording head is water repellent. 7. The ink jet recording method according to any one of claims 4 to 6 , wherein the shape of the recording element substrate is a parallelogram, a rectangle, or a trapezoid. 7. The ink jet recording method according to claim 4 , wherein the shape of the recording element substrate is a parallelogram. 18. The inkjet recording method according to any one of claims 1 to 17, wherein the recording head is a line head. A first ink and a second ink, which are water-based inks containing a pigment; and an ejection port surface on which a first ejection port array for ejecting the first ink and a second ejection port array for ejecting the second ink are formed. wherein the first ejection port row and the second ejection port row are arranged adjacent to each other in order from the bottom in the gravitational direction, and at least a part of the first ejection port row and the second ejection port row has An inkjet recording apparatus comprising recording heads arranged so as to overlap in the conveying direction of a recording medium,
recording an image on a recording medium by ejecting the water-based ink from the recording head arranged so that the angle between the ejection port surface of the recording head and the direction of gravity is 0° or more and less than 90°;
An inkjet recording apparatus, wherein the true specific gravity of the pigment in the first ink is smaller than the true specific gravity of the pigment in the second ink.

Images