JP6645449B2 - Ink set for inkjet recording, cartridge, and image forming method - Google Patents

Description

  The present invention relates to an ink set for inkjet recording, a cartridge, and an image forming method.

  When an image is formed by an inkjet method, a discharge surface of a recording head may be cleaned using a cleaning liquid. As a cleaning liquid, for example, a maintenance liquid described in Patent Document 1 is known.

JP 2013-146965 A

  In an image forming method using an ink jet method, first, ink for ink jet recording (hereinafter, sometimes simply referred to as “ink”) is discharged from a discharge surface of a recording head to a recording medium. A large number of nozzles are formed in the recording head, and each of the nozzles is open on the ejection surface. Therefore, the ink is discharged from the nozzle opening (discharge port) to the recording medium. Next, the cleaning liquid is supplied to the discharge surface. Further, the ink is pressurized and discharged from the discharge port (purge operation). Thereafter, the ejection surface is wiped using, for example, a blade (wiping operation).

  If the cleaning liquid is supplied to the ejection surface, the purge operation and the wiping operation are performed after the ejection of the ink, the ink fixed on the ejection surface (hereinafter, sometimes referred to as “fixed ink”) is removed from the ejection surface. it can. Accordingly, it is possible to prevent the fixed ink from preventing the ejection port, and thus it is possible to maintain the ink ejection performance.

  By the way, in the image forming method by the ink jet method, the wipe operation is performed after the ejection of the ink, the supply of the cleaning liquid to the ejection surface, and the purge operation. Therefore, when performing the wiping operation, the cleaning liquid and the ink discharged by the purge operation (hereinafter, referred to as “purge ink”) exist on the ejection surface. The ink used for image formation may be present on the ejection surface.

  When the wiping operation is performed in a state where the liquid is present on the ejection surface, the liquid existing on the ejection surface is swept by the blade to the peripheral portion of the ejection surface, and goes around from the peripheral portion of the ejection surface to the outer surface of the recording head. There is. The liquid that has flowed to the outer surface of the recording head may droop along the outer surface due to its own weight. In addition, the liquid dripping down to the ejection surface due to its own weight may wet and spread on the ejection surface. Thus, the ejection surface may be contaminated after the wiping operation. If the ejection surface is contaminated after the wiping operation, the ejection performance of the ink may decrease when the ejection of the ink is restarted after the wiping operation.

  The present invention has been made in view of such circumstances, and provides an ink set capable of preventing contamination of an ejection surface after a wiping operation, and a cartridge including an ink set capable of preventing contamination of an ejection surface after a wiping operation. And an image forming method using an ink set that can prevent contamination of the ejection surface after the wiping operation.

The ink set for inkjet recording according to the present invention includes an aqueous ink and an aqueous cleaning liquid. The aqueous cleaning solution contains a nonionic surfactant. The nonionic surfactant has an amino group in the molecule. The surface tension of the aqueous ink and the surface tension of the aqueous cleaning liquid satisfy the following expression (A).
1.1 × (surface tension of the aqueous ink) ≦ (surface tension of the aqueous cleaning liquid) ≦ 1.4 × (surface tension of the aqueous ink) Formula (A)

  The cartridge according to the present invention includes a first tank that stores the aqueous ink of the inkjet recording ink set having the above configuration, and a second tank that stores the aqueous cleaning liquid of the inkjet recording ink set having the above configuration. And a tank.

In the image forming method according to the present invention, the aqueous ink is ejected from an ejection surface of a recording head to a recording medium, an aqueous cleaning liquid is supplied to the ejection surface, and the aqueous ink is pressurized. The method includes a purging step of discharging from the discharge surface and a wiping step of wiping the discharge surface. The supply step and the purge step are each performed after the discharge step and before the wipe step. The aqueous cleaning solution contains a nonionic surfactant. The nonionic surfactant has an amino group in the molecule. The surface tension of the aqueous ink and the surface tension of the aqueous cleaning liquid satisfy the following expression (A).
1.1 × (surface tension of the aqueous ink) ≦ (surface tension of the aqueous cleaning liquid) ≦ 1.4 × (surface tension of the aqueous ink) Formula (A)

  According to the present invention, contamination of the ejection surface after the wiping operation can be prevented.

FIG. 1 is a diagram illustrating an example of a configuration of an image forming apparatus used in an image forming method according to the present invention. FIG. 3 is a diagram illustrating one process of an image forming method according to the present invention. FIG. 3 is a diagram illustrating one process of an image forming method according to the present invention.

An embodiment of the present invention will be described. In addition, the evaluation result (value indicating a shape or physical property) of the powder is a number average of values measured for a considerable number of particles unless otherwise specified. If the measured value of the volume median diameter (D ₅₀ ) of the powder is not specified, a laser diffraction particle size distribution analyzer (“Zetasizer nano-ZS” manufactured by Malvern) is used. It is a value measured using.

  Further, a compound and its derivative may be generically referred to by adding “system” after the compound name. When a polymer name is represented by adding “system” after the compound name, it means that the repeating unit of the polymer is derived from the compound or its derivative. In addition, acryl and methacryl may be collectively referred to as “(meth) acryl”.

  Further, among nonionic surfactants, an alcohol having an acetylene bond in a molecule is described as “acetylene-type surfactant”. Among the nonionic surfactants, a compound having an amino group in the molecule is referred to as “amine type surfactant”.

  Examples of a method for forming an image using the ink set for inkjet recording according to the present embodiment (hereinafter, may be simply referred to as “ink set”) include the following method, for example. More specifically, first, the aqueous ink of the ink set is discharged from a discharge surface of a recording head (a recording head provided in an inkjet recording apparatus) to a recording medium. Next, a purge operation is performed after the aqueous cleaning liquid of the ink set is supplied to the ejection surface. Thereafter, a wiping operation is performed using, for example, a blade.

[Ink set for inkjet recording according to this embodiment]
The ink set according to the present embodiment has an aqueous ink and an aqueous cleaning liquid. The aqueous cleaning solution contains a non-ionic surfactant. Nonionic surfactants have an amino group in the molecule. The surface tension of the aqueous ink and the surface tension of the aqueous cleaning liquid satisfy the following expression (A).
1.1 × (surface tension of aqueous ink) ≦ (surface tension of aqueous cleaning liquid) ≦ 1.4 × (surface tension of aqueous ink) Formula (A)

  As described above, in the ink set according to the present embodiment, since the aqueous cleaning liquid contains the amine-type surfactant, the fixed ink can be removed from the ejection surface. In addition, since the expression (A) is satisfied, it is possible to prevent the discharge surface from being contaminated after the wiping operation. If it is possible to prevent the discharge surface from being contaminated after the wiping operation, it is possible to prevent a decrease in the ink discharging performance even when the ink discharge is restarted after the wiping operation. For example, it is possible to prevent the ejection of the aqueous ink from becoming difficult. Further, it is possible to prevent the aqueous ink from being ejected in a direction different from the desired ejection direction. The surface tension of the aqueous cleaning liquid is preferably 1.10 to 1.30 times the surface tension of the aqueous ink, and more preferably 1.10 to 1.25 times the surface tension of the aqueous ink. Hereinafter, first, a preferable configuration of the aqueous ink and a process of forming the fixed ink will be sequentially described.

<Preferred configuration of aqueous ink>
The aqueous ink preferably has an aqueous solvent and a pigment dispersion. The pigment dispersion means a structure in which a plurality of pigment particles are dispersed in an aqueous medium. Each of the pigment particles preferably includes a pigment core containing a pigment and a coating resin provided on the surface of the pigment core.

  In aqueous inks, pigment particles are dispersed together. Specifically, in many cases, a resin salt is used as the coating resin. Here, the resin salt has an ionizable functional group (for example, a COONa group) in the molecule. Further, the aqueous ink contains a sufficient amount of an aqueous solvent. For these reasons, ionization easily occurs on the surface of the coating resin. Therefore, an electric double layer is formed on the surface of the coating resin. When the electric double layer is formed on the surface of the coating resin, the pigment particles repel each other, so that the pigment particles are mutually dispersed.

<Fixed ink forming process>
It is considered that the fixed ink is formed by the following process.
When the aqueous ink is ejected from the ejection surface of the recording head to the recording medium, the aqueous ink may adhere to the ejection surface. When the water-based ink adheres to the ejection surface, the water-based ink dries due to the contact between the water-based ink and air. When the aqueous ink dries, the coating resin tends to form a film.

  Specifically, when the water-based ink dries, the content of the water-based solvent in the water-based ink decreases, so that ionization is less likely to occur on the surface of the coating resin. This makes it difficult for the pigment particles to repel each other electrically, so that the pigment particles tend to aggregate with each other. When the pigment particles agglomerate with each other, the coating resins existing on the surfaces of the different pigment cores easily come into contact with each other. As a result, a film made of the coating resin (hereinafter, referred to as “resin film”) is easily formed. Thus, when the aqueous ink dries, the aggregate of the pigment core is covered with the resin film. In this way, a fixed ink is formed.

<Reason for obtaining the desired effect>
Next, a description will be given of possible reasons why a predetermined effect can be obtained by forming an image using the ink set according to the present embodiment. Here, the predetermined effects include an effect that the fixed ink can be removed from the ejection surface (effect 1) and an effect that the contamination of the ejection surface after the wiping operation can be prevented (effect 2).

(Effect 1)
Recently, the present inventors have found that if the aqueous cleaning liquid contains an amine-type surfactant, the fixed ink is easily removed from the ejection surface (see Example 1 and Comparative Examples 1 to 3 described later). . The following are conceivable reasons for obtaining such a result.

  Specifically, as described in the above <Fixed ink forming process>, in the fixed ink, the pigment particles are likely to aggregate with each other. However, in many cases, the aqueous cleaning liquid is supplied to the ejection surface during formation of the fixed ink. Therefore, in many cases, the aqueous cleaning liquid is supplied to the discharge surface before the pigment particles aggregate with each other.

  In the present embodiment, the aqueous cleaning solution contains an amine type surfactant. Here, since the amine type surfactant has an amino group in the molecule, it easily penetrates between adjacent pigment particles as compared with other surfactants. Therefore, the aqueous cleaning liquid in the present embodiment is more likely to penetrate between adjacent pigment particles than the aqueous cleaning liquid containing another surfactant. The “other surfactants” include nonionic surfactants different from amine surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants. .

  Furthermore, in this embodiment, the surface tension of the aqueous cleaning liquid is 1.40 times or less the surface tension of the aqueous ink (see Examples 1 to 6 and Comparative Example 5 described later). As a result, the aqueous cleaning liquid has a high affinity for the aqueous ink and also has a high affinity for the purge ink. Therefore, when the supply of the aqueous cleaning liquid to the ejection surface and the purging operation are performed, the purge ink easily enters between the adjacent pigment particles together with the aqueous cleaning liquid.

  As described above, in the present embodiment, the purge ink easily penetrates between adjacent pigment particles. Thus, when the aqueous cleaning liquid is supplied to the ejection surface and the purge operation is performed, the fixed ink is easily dissolved in the purge ink. Therefore, if the wiping operation is performed after the supply of the aqueous cleaning liquid to the ejection surface and the purging operation, the fixed ink can be removed from the ejection surface.

(Effect 2)
In the present embodiment, the surface tension of the aqueous cleaning liquid is at least 1.10 times the surface tension of the aqueous ink. Generally, the supply amount of the aqueous cleaning liquid to the discharge surface is larger than the discharge amount of the purge ink to the discharge surface. Therefore, when the wiping operation is performed, the amount of the aqueous cleaning liquid present on the ejection surface is larger than the amount of the purge ink. Therefore, the physical properties of the liquid existing on the ejection surface when performing the wiping operation (hereinafter simply referred to as “the liquid existing on the ejection surface”) are more likely to be governed by the physical properties of the aqueous cleaning liquid than the physical properties of the purge ink. . Therefore, if the surface tension of the aqueous cleaning liquid is at least 1.10 times the surface tension of the aqueous ink, the surface tension of the liquid present on the ejection surface tends to increase.

  If the surface tension of the liquid existing on the ejection surface is large, the liquid existing on the ejection surface is unlikely to wet and spread on the ejection surface. Therefore, when the wiping operation is performed, the liquid existing on the ejection surface is likely to hang down, for example, on a blade.

  When the liquid present on the ejection surface drops along the blade, it can be prevented that the liquid existing on the ejection surface goes around to the outer surface of the recording head. Thereby, it is possible to prevent the ejection surface from being contaminated after the wiping operation. Therefore, even when the ink discharge is restarted after the wiping operation, it is possible to prevent the ink discharge performance from being lowered.

  Further, if it is possible to prevent the discharge surface from being contaminated after the wiping operation, it is possible to prevent the discharge surface from being contaminated after each wiping operation even when performing the wiping operation a plurality of times. Accordingly, even when an image is formed using an image forming apparatus having a long mechanical life, the ink ejection performance can be maintained until the use period of the image forming apparatus reaches the mechanical life. Hereinafter, the ink set according to the present embodiment will be further described in comparison with a general ink set.

<Comparison between general ink set and ink set according to this embodiment>
In a general ink set, from the viewpoint that the fixed ink can be easily removed from the ejection surface, the surface tension of the aqueous cleaning liquid is made smaller than the surface tension of the aqueous ink to increase the affinity of the aqueous cleaning liquid for the aqueous ink. . However, as described above, the supply amount of the aqueous cleaning liquid to the ejection surface is larger than the discharge amount of the purge ink to the ejection surface. Therefore, if the surface tension of the aqueous cleaning liquid is smaller than the surface tension of the aqueous ink, the surface tension of the liquid existing on the ejection surface tends to be small.

  If the surface tension of the liquid existing on the ejection surface is small, the liquid easily spreads on the ejection surface. Therefore, when the wiping operation is performed, the liquid existing on the ejection surface is easily pushed by the blade to the peripheral portion of the ejection surface, and easily spills from the peripheral portion of the ejection surface to the outer surface of the recording head. Then, the liquid that has flowed to the outer surface of the recording head easily falls down to the ejection surface along the outer surface by its own weight, and is easily wetted and spread on the ejection surface. As described above, if the surface tension of the aqueous cleaning liquid is smaller than the surface tension of the aqueous ink, the ejection surface is likely to be contaminated after the wiping operation (Comparative Example 4 described later). Therefore, when the discharge of the ink is restarted after the wiping operation, the discharge performance of the ink may be reduced.

  On the other hand, if the surface tension of the aqueous cleaning liquid is higher than the surface tension of the aqueous ink, it is difficult to remove the fixed ink from the ejection surface (see Comparative Example 6 described later).

  However, in the present embodiment, the aqueous cleaning liquid contains the amine-type surfactant, and the formula (A) is satisfied. This makes it possible to remove the fixed ink from the ejection surface and prevent contamination of the ejection surface after the wiping operation (see Examples 1 to 6 described later).

  In Comparative Example 4 described below, the aqueous cleaning solution contained an amine-type surfactant. However, the surface tension of the aqueous cleaning liquid was 1.06 times the surface tension of the aqueous ink. Therefore, liquid contamination was confirmed on the outer surface of the recording head. The present inventors have found that if the surface tension of the aqueous cleaning liquid is less than one time the surface tension of the aqueous ink (the surface tension of the aqueous cleaning liquid <the surface tension of the aqueous ink), the liquid stain on the outer surface of the recording head becomes more remarkable. I am sure it will be.

[Cartridge according to this embodiment]
The cartridge according to the present embodiment includes the ink set according to the present embodiment, a first tank that stores the aqueous ink, and a second tank that stores the aqueous cleaning liquid. Accordingly, if the cartridge according to the present embodiment is loaded in an ink jet recording apparatus, image formation using the ink set according to the present embodiment can be easily performed. Therefore, the fixed ink can be removed from the ejection surface. Further, since the contamination of the ejection surface after the wiping operation can be prevented, the ink ejection performance can be maintained even when the ejection of the ink is restarted after the wiping operation.

[Image Forming Method According to the Present Embodiment]
The image forming method according to the present embodiment includes a discharge step, a supply step, a purge step, and a wipe step. In the ejection step, the aqueous ink is ejected from the ejection surface of the recording head to the recording medium. In the supply step, the aqueous cleaning liquid is supplied to the discharge surface. In the purging step, the water-based ink (purge ink) is pressurized and discharged from the ejection surface (purge operation). In the wiping process, the ejection surface is wiped (wiping operation). More specifically, in the wiping step, the ejection surface is wiped using a blade.

  The supply step and the purge step are each performed after the discharge step and before the wipe step. More specifically, the supply step may be performed before the purge step, may be performed after the purge step, or may be performed simultaneously with the purge step.

  The aqueous ink and the aqueous cleaning liquid to be used are as described in the above [Ink set for inkjet recording according to this embodiment]. Thus, in the image forming method according to the present embodiment, the fixed ink can be removed from the ejection surface. Further, since the contamination of the ejection surface after the wiping operation can be prevented, the ink ejection performance can be maintained even when the ink ejection is restarted after the wiping operation.

  If an image is formed using the ink jet recording apparatus loaded with the aqueous ink according to the present embodiment, the ejection step, the purge step, and the wipe step can be performed. Further, as described above, in the supply step, the aqueous cleaning liquid according to the present embodiment is supplied to the ejection surface. Examples of the method of supplying the aqueous cleaning liquid include discharge of the aqueous cleaning liquid by an inkjet method, application of the aqueous cleaning liquid using a roller, and spraying of the aqueous cleaning liquid.

  In the image forming method according to the present embodiment, an image may be formed by loading the cartridge according to the present embodiment into an inkjet recording apparatus, or an image may be formed by using separately prepared aqueous ink and aqueous cleaning liquid. May be.

[One Example of Image Forming Method According to Present Embodiment]
Hereinafter, an example of the image forming method according to the present embodiment will be specifically described with reference to FIGS. FIG. 1 is a diagram illustrating a configuration of an image forming apparatus used in the image forming method according to the present embodiment. FIG. 2 is a view for explaining one step of the image forming method according to the present embodiment, and more specifically, a view for explaining a supply step. FIG. 3 is a diagram illustrating another process of the image forming method according to the embodiment, and more specifically, a diagram illustrating a purge operation and a wipe operation. Here, the X axis, the Y axis, and the Z axis shown in FIGS. 1 to 3 are orthogonal to each other. FIGS. 2 and 3 are side views of main parts of the image forming apparatus 1 shown in FIG.

  First, the configuration of the image forming apparatus 1 shown in FIG. 1 will be described. The image forming apparatus 1 shown in FIG. 1 includes a paper feed unit 3, a recording head 4, a liquid storage unit 5, a paper transport unit 7, a discharge unit 8, and a maintenance unit 9.

  The paper supply unit 3 has a paper supply cassette 31 and a paper supply roller 32a. A plurality of recording media (for example, copy sheets) S are stored in the sheet cassette 31 in a stacked state.

  As shown in FIGS. 2 and 3, the recording head 4 has a nozzle 41, an ink inlet 43, and an ink outlet 45. Further, the recording head 4 has an ejection surface 47. The nozzle 41 is open at the ejection surface 47 and ejects the aqueous ink toward the recording medium S (see FIG. 1). The aqueous ink is contained in the first tank 52 (see FIG. 1). The aqueous ink flows from the first tank 52 through the ink inlet 43 to the recording head 4, and flows out of the recording head 4 through the ink outlet 45.

  As shown in FIG. 1, a cartridge 51 is provided in the liquid container 5. The cartridge 51 is detachably mounted on the image forming apparatus 1. The cartridge 51 includes the ink set according to the present embodiment, a first tank 52, and a second tank 53. The first tank 52 stores the aqueous ink according to the present embodiment. The second tank 53 stores the aqueous cleaning liquid in the present embodiment.

  The paper transport unit 7 includes a first transport unit 71 and a second transport unit 72. The discharge section 8 has a discharge tray 81.

  The maintenance unit 9 has a sponge 91 and a blade 92. Each of the sponge 91 and the blade 92 is movable between a position facing the ejection surface 47 (see FIGS. 2 and 3) and a position facing the second transport unit 72 (the position shown in FIG. 1). is there. As shown in FIG. 2, the sponge 91 is movable along each of the ascending direction D1 and the descending direction D2. The sponge 91 is impregnated with an aqueous cleaning liquid. The aqueous cleaning liquid is contained in the second tank 53 (see FIG. 1), and is supplied from the second tank 53 to the sponge 91. As shown in FIG. 3, the blade 92 is movable along each of the ascending direction D1, the descending direction D2, and the wiping direction D3. Here, the rising direction D1 means a direction approaching the ejection surface 47 along the Z-axis direction. The descending direction D2 means a direction moving away from the ejection surface 47 along the Z-axis direction. The wiping direction D3 means a direction along the ejection surface 47.

  When forming an image on the recording medium S using the image forming apparatus 1 shown in FIG. 1, first, the paper feed roller 32a takes out the recording medium S stored in the paper feed cassette 31 one by one from the top. Then, the taken out recording medium S is sent to the first transport unit 71. When the recording medium S reaches a position facing the ejection surface 47 (see FIG. 2), the aqueous ink is ejected from the ejection surface 47 (more specifically, the opening of the nozzle 41) to the recording medium S (ejection step). . After that, the recording medium S is sent to the second transport unit 72 and is discharged to the discharge tray 81.

  In the ejection step, the aqueous ink may adhere to the ejection surface 47. When the aqueous ink adheres to the ejection surface 47, a fixed ink (not shown) is formed due to the contact between the aqueous ink and the air. Therefore, after the discharge step, the supply step, the purge step, and the wipe step are performed.

  The supply step will be described with reference to FIG. In the supply step, first, the sponge 91 is impregnated with the aqueous cleaning liquid. Next, the sponge 91 moves to a position facing the ejection surface 47 (the position shown in FIG. 2), and then moves along the ascending direction D1 and is pressed against the ejection surface 47. At this time, it is preferable that the state in which the sponge 91 is pressed against the ejection surface 47 (hereinafter, referred to as “the pressed state of the sponge 91”) is maintained for a predetermined time. Further, an operation in which the sponge 91 moves along the rising direction D1 and an operation in which the sponge 91 moves along the descending direction D2 may be repeated while the pressed state of the sponge 91 is maintained. Alternatively, the sponge 91 may move in a direction along the ejection surface 47 (a direction along the X-axis direction shown in FIG. 2) while the pressed state of the sponge 91 is maintained.

  After the elapse of the predetermined time, the sponge 91 moves along the downward direction D2, and the pressed state of the sponge 91 is released. Thus, the supply step ends. After the supply step, a purge step is performed.

  The purge step will be described with reference to FIG. In the purging step, a purging operation is performed. In the purging operation, the recording head 4 performs a purging process. Thus, the purge ink Nf is forcibly discharged from the ejection surface 47 (more specifically, the opening of the nozzle 41). After the purge step, a wipe step is performed.

  The wiping step will be described with reference to FIG. In the wiping process, a wiping operation is performed. In the wiping operation, the blade 92 moves to a position facing the ejection surface 47 (the position shown in FIG. 3), and then moves along the ascending direction D1 and is pressed against the ejection surface 47. Then, the blade 92 moves in the direction along the discharge surface 47 (wiping direction D3 shown in FIG. 3) while the state in which the blade 92 is pressed against the discharge surface 47 is maintained. Thereby, the blade 92 removes the fixed ink. The image forming method according to the present embodiment has been described above with reference to FIGS. Hereinafter, the aqueous ink and the aqueous cleaning liquid will be further described.

[Water-based ink]
The aqueous ink is preferably ejected from the ejection surface of the recording head. Hereinafter, the surface tension of the water-based ink, the material constituting the water-based ink, and the preferred method of manufacturing the water-based ink will be described in order.

[Surface tension of water-based ink]
Preferably, the surface tension of the aqueous ink is from 25 mN / m to 33 mN / m. Thereby, the expression (A) is easily satisfied.

  Here, the surface tension of the aqueous ink means the surface tension of the aqueous ink measured at 25 ° C. The surface tension of the aqueous ink is measured, for example, according to the Wilhelmy method (plate method). In the measurement of the surface tension by the Wilhelmy method, for example, “Automatic Surface Tensiometer DY-300” manufactured by Kyowa Interface Science Co., Ltd. can be used as the surface tension meter.

[Examples of Materials Constituting Aqueous Ink]
Preferably, the aqueous ink contains a pigment dispersion and an aqueous solvent. More preferably, the aqueous ink contains a pigment dispersion, an aqueous solvent, and other components. The other component preferably contains at least one of a surfactant, a dissolution stabilizer, a humectant, a penetrant, and a pH adjuster.

<Pigment dispersion>
The pigment dispersion includes a plurality of pigment particles. The pigment particles each include a pigment core and a coating resin.

(Pigment core)
The pigment core contains a pigment. As the pigment, for example, a yellow pigment, an orange pigment, a red pigment, a blue pigment, a violet pigment, or a black pigment can be used. Examples of the yellow pigment include C.I. I. Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, or 193. Examples of the orange pigment include C.I. I. Pigment Orange 34, 36, 43, 61, 63, or 71. Examples of the red pigment include C.I. I. Pigment Red 122 or 202. Quinacridone magenta (PR122) can also be used as a red pigment. Examples of the blue pigment include C.I. I. Pigment Blue 15 or 15: 3. Examples of purple pigments include C.I. I. Pigment Violet 19, 23, or 33. Examples of the black pigment include C.I. I. Pigment Black 7. Carbon black can also be used as a black pigment.

  The content of the pigment core in the aqueous ink is preferably from 4% by mass to 8% by mass. When the content of the pigment core in the aqueous ink is 4% by mass or more, an image having a desired image density is easily obtained. When the content of the pigment core in the aqueous ink is 8% by mass or less, the fluidity of the aqueous ink is easily ensured. This also makes it easy to obtain an image having a desired image density. Further, the permeability of the aqueous ink into the recording medium is easily ensured.

The volume median diameter (D ₅₀ ) of the pigment core is preferably 30 nm or more and 200 nm or less. This improves the color density, hue, or stability of the aqueous ink. More preferably, the pigment core has a volume median diameter (D ₅₀ ) of 70 nm or more and 130 nm or less.

(Coating resin)
The coating resin is provided on the surface of the pigment core. The coating resin preferably has an anionic property, for example, styrene-acrylic acid-based resin, styrene-maleic acid copolymer, styrene-maleic acid half-ester copolymer, vinylnaphthalene-acrylic acid copolymer, and vinyl It is preferably at least one of a naphthalene-maleic acid copolymer. More preferably, the coating resin is a styrene-acrylic acid-based resin. If the coating resin is a styrene-acrylic acid resin, pigment particles can be easily produced. Further, the dispersibility of the pigment core can be improved.

  The styrene-acrylic acid-based resin is a resin including a unit derived from styrene and a unit derived from acrylic acid, methacrylic acid, an acrylate, or a methacrylate. Preferably, the styrene-acrylic acid-based resin is a copolymer of styrene, acrylic acid, and alkyl acrylate, a copolymer of styrene, methacrylic acid, alkyl methacrylate, and alkyl acrylate, and styrene and acrylic acid. At least one of a copolymer of styrene, maleic acid and an alkyl acrylate, a copolymer of styrene and methacrylic acid, and a copolymer of styrene and an alkyl methacrylate. is there. More preferably, the styrene-acrylic acid-based resin is a copolymer of styrene, methacrylic acid, alkyl methacrylate, and alkyl acrylate. More specifically, the styrene-acrylic acid resin is a copolymer of styrene, methacrylic acid, methyl methacrylate, and n-butyl acrylate.

  The content of the coating resin is preferably 15 parts by mass or more and 100 parts by mass or less based on 100 parts by mass of the pigment core. If the content of the coating resin is too small, strike-through may occur on the recording medium after image formation. On the other hand, if the content of the coating resin is excessive, a desired image density may not be obtained.

<Aqueous solvent>
The aqueous solvent preferably contains water, and more preferably contains ion-exchanged water. The content of water in the aqueous ink is preferably 20% by mass or more and 70% by mass or less. Thereby, an aqueous ink having an appropriate viscosity can be provided.

  For example, the aqueous solvent preferably contains ion-exchanged water, glycerin and / or glycol, and alcohol and / or glycol ether. When the aqueous ink contains glycerin and / or glycol, drying of the aqueous ink can be further prevented. When the aqueous ink contains alcohol and / or glycol ether, the permeability of the aqueous ink to the recording medium can be increased. As the alcohol, for example, a sugar alcohol represented by sorbitol can also be used.

  Examples of the glycol ether include diethylene glycol monoethyl ether, triethylene glycol mononormal butyl ether, triethylene glycol monoisobutyl ether, triethylene glycol monoisopropyl ether, and diethylene glycol mononormal butyl ether.

<Surfactant>
If the aqueous ink contains a surfactant, the wettability of the aqueous ink on the recording medium is improved. The surfactant contained in the aqueous ink is preferably a nonionic surfactant. The content of the nonionic surfactant in the aqueous ink is preferably from 0.05% by mass to 2.00% by mass. Thereby, the image density is improved while suppressing the image offset. More preferably, the aqueous ink contains an acetylene type surfactant.

(Acetylene surfactant)
Preferably, the content of the acetylene-type surfactant in the aqueous ink is 0.55% by mass or more and 0.70% by mass or less. Thereby, the surface tension of the water-based ink tends to be 25 mN / m or more and 33 mN / m or less. Therefore, equation (A) is easily satisfied. More preferably, the content of the acetylene surfactant in the aqueous ink is from 0.60% by mass to 0.70% by mass. When the aqueous ink contains two or more acetylene-type surfactants, the total content of the acetylene-type surfactant in the aqueous ink is 0.55% by mass or more and 0.70% by mass or less. Is preferred.

  The acetylene type surfactant is an alcohol having an acetylene bond in a molecule. The alcohol having an acetylene bond in the molecule is preferably, for example, at least one of a glycol having an acetylene bond in the molecule and a monohydric alcohol having an acetylene bond in the molecule. As the acetylene-type surfactant, a commercially available product can also be used. More specifically, the acetylene-type surfactant is preferably “Olfin (registered trademark) E1010” manufactured by Nissin Chemical Industry Co., Ltd. For example, "Olfin E1010" manufactured by Nissin Chemical Industry Co., Ltd. contains an ethylene oxide adduct of acetylene diol, and more specifically contains polyoxyethylene acetylenic glycol ether.

<Dissolution stabilizer>
If the aqueous ink contains a dissolution stabilizer, the components contained in the aqueous ink are easily compatible with each other, so that the dissolved state of the aqueous ink can be stabilized. The dissolution stabilizer is preferably at least one of 2-pyrrolidone, N-methyl-2-pyrrolidone, and γ-butyrolactone. The content of the dissolution stabilizer in the aqueous ink is preferably 1% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 15% by mass or less.

<Moisturizer>
If the water-based ink contains a humectant, volatilization of the liquid component from the water-based ink can be suppressed, so that the viscosity of the water-based ink can be stabilized. The humectant is preferably at least one of polyalkylene glycols, alkylene glycols, and glycerin. The polyalkylene glycols are preferably polyethylene glycol or polypropylene glycol. Alkylene glycols include ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, trimethylene glycol (1,3-propanediol), triethylene glycol, tripropylene glycol, 1,2,6-hexanetriol, and thiol glycol. It is preferably diglycol, 1,3-butanediol, or 1,5-pentanediol. The content of the humectant in the aqueous ink is preferably from 2% by mass to 30% by mass, and more preferably from 10% by mass to 25% by mass.

<Penetrant>
When the aqueous ink contains a penetrant, the permeability of the aqueous ink to the recording medium is improved. The penetrant is preferably 1,2-hexylene glycol, 1,2-octanediol, 2,4-diethyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, It is at least one of triethylene glycol monobutyl ether and diethylene glycol monobutyl ether. The content of the penetrant in the aqueous ink is preferably 0.5% by mass or more and 20.0% by mass or less.

<PH adjuster>
If the aqueous ink contains a pH adjuster, the aqueous ink tends to show weak basicity. For example, it is preferable to determine the content of the pH adjuster in the aqueous ink such that the pH of the aqueous ink is 8.5 or more and 10 or less.

  The pH adjuster is preferably an alkali metal salt. Thereby, the dispersibility of the pigment particles in the aqueous ink can be increased. The alkali metal salt is preferably an alkali metal hydroxide. More preferably, the alkali metal salt contains at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide.

[Preferred production method of aqueous ink]
A preferred method for producing an aqueous ink includes a step of preparing a pigment dispersion and a step of mixing the pigment dispersion with other ink components.

<Preparation process of pigment dispersion>
First, a coating resin is synthesized. Specifically, a monomer or prepolymer capable of synthesizing a coating resin by polymerization and a polymerization initiator are added to a predetermined solvent, and the mixture is heated and refluxed at a predetermined temperature. Thus, the coating resin is synthesized. More specifically, styrene, (meth) acrylic acid, an alkyl (meth) acrylate, and a polymerization initiator are added to a mixed solution of isopropyl alcohol and methyl ethyl ketone, and the mixture is heated and refluxed at 70 ° C. Thereby, a styrene-acrylic acid-based resin is synthesized.

  Next, the synthesized resin, the pigment core, and the aqueous solvent are kneaded using a media type disperser. In this way, a pigment dispersion containing a large number of pigment particles is obtained. By changing the particle size (for example, bead size) of the media used in the media type disperser, the degree of dispersion of the pigment particles, the amount of resin released into the pigment dispersion, or the particle size of the pigment particles can be adjusted. For example, the smaller the media particle size, the smaller the pigment particle size tends to be.

<Mixing step of pigment dispersion and other ink components>
The obtained pigment dispersion is mixed with other ink components. It is preferable to use a stirrer (for example, "Three One Motor (registered trademark) BL-600" manufactured by Shinto Kagaku Co., Ltd.) to mix the pigment dispersion with the other ink components. The other ink component preferably contains at least one of a surfactant, a dissolution stabilizer, a humectant, a penetrant, and a pH adjuster. More preferably, the other ink component contains an acetylene-type surfactant. After mixing the pigment dispersion and other ink components, filtration is performed as necessary. Thus, an aqueous ink is obtained.

[Aqueous cleaning liquid]
The aqueous cleaning liquid is preferably used for cleaning the ejection surface of the recording head. “Cleaning the ejection surface” includes removing the fixed ink from the ejection surface. Further, the aqueous cleaning liquid can be used not only for cleaning the ejection surface, but also for cleaning the blade used in the wiping operation or for cleaning the transport roller. Hereinafter, the surface tension of the aqueous cleaning liquid, the material constituting the aqueous cleaning liquid, and the preferred method of producing the aqueous cleaning liquid will be described in order.

[Surface tension of aqueous cleaning liquid]
Preferably, the surface tension of the aqueous cleaning liquid is from 34 mN / m to 40 mN / m. Thereby, the expression (A) is easily satisfied.

  Here, the surface tension of the aqueous cleaning liquid means the surface tension of the aqueous cleaning liquid measured at 25 ° C. The surface tension of the aqueous cleaning liquid is measured, for example, according to the Wilhelmy method (plate method). In the measurement of the surface tension by the Wilhelmy method, for example, “Automatic Surface Tensiometer DY-300” manufactured by Kyowa Interface Science Co., Ltd. can be used as the surface tension meter.

[Examples of materials constituting aqueous cleaning liquid]
As described above, the aqueous cleaning solution contains an amine type surfactant. The amine type surfactant will be described in the following <Amine type surfactant>.

  Preferably, the aqueous washing solution contains at least one of the aqueous solvent described in the above <aqueous solvent>, the dissolution stabilizer described in the above <dissolution stabilizer>, and the humectant described in the above <humectant>. Also contains. Accordingly, in the aqueous ink and the aqueous cleaning liquid, at least one material of the aqueous solvent, the dissolution stabilizer, and the humectant is the same or similar to each other. Therefore, the affinity of the aqueous cleaning liquid for the aqueous ink can be further increased. Therefore, penetration of the aqueous cleaning liquid between adjacent pigment particles is further facilitated. More specifically, the aqueous washing liquid preferably contains ion-exchanged water, sorbitol, triethylene glycol mono-normal butyl ether, 2-pyrrolidone, and 1,3-propanediol.

  Preferably, the aqueous cleaning solution further contains a pH adjuster. Thereby, the aqueous cleaning liquid tends to show weak basicity. Thereby, the affinity of the aqueous cleaning liquid for the aqueous ink can be further increased, and therefore, the penetration of the aqueous cleaning liquid between the adjacent pigment particles is further facilitated. As the pH adjusting agent contained in the aqueous cleaning solution, it is preferable to use the pH adjusting agent described in the above <pH adjusting agent>.

<Amine surfactant>
The amine type surfactant is preferably, for example, a polyoxyalkylene alkylamine. The polyoxyalkylene alkylamine preferably has a structure represented by the following formula (1-1). Examples of the polyoxyalkylene alkylamine include "Amit (registered trademark) 105A" manufactured by Kao Corporation, "Amate 320" manufactured by Kao Corporation, or "Puremir (registered trademark) EP-300S" manufactured by Sanyo Chemical Industries, Ltd. Can be used.

In the above formula (1-1), R ¹ represents a hydrocarbon group having 1 to 24 carbon atoms. Preferably, R ¹ represents an alkyl group or an alkenyl group having 1 to 24 carbon atoms. A ¹ O and A ² O each independently represent at least one of an oxyethylene group and an oxypropylene group. m1 and n1 each represent an integer of 0 or more, and represent an integer satisfying 1 ≦ (m1 + n1) ≦ 100. Preferably, m1 and n1 each represent an integer of 0 or more, and represent an integer satisfying 1 ≦ (m1 + n1) ≦ 40.

  The content of the amine-type surfactant in the aqueous cleaning solution is preferably 0.1% by mass or more and 20.0% by mass or less. Thereby, the amount of the aqueous cleaning liquid that enters between the adjacent pigment particles can be secured. Therefore, it is possible to secure the amount of the purge ink that enters between adjacent pigment particles.

  More preferably, the content of the amine type surfactant in the aqueous cleaning solution is 0.1% by mass or more and 10.0% by mass or less. Thereby, the surface tension of the aqueous cleaning liquid tends to be not less than 34 mN / m and not more than 40 mN / m. Therefore, equation (A) is easily satisfied. More preferably, the content of the amine-type surfactant in the aqueous cleaning solution is 0.5% by mass or more and 7.0% by mass or less.

  When the aqueous cleaning liquid contains two or more amine-type surfactants, the total content of the amine-type surfactants in the aqueous cleaning liquid is preferably from 0.1% by mass to 20.0% by mass. . More preferably, the total content of the amine type surfactant in the aqueous cleaning solution is 0.1% by mass or more and 10.0% by mass or less. More preferably, the total content of the amine type surfactant in the aqueous cleaning solution is 0.5% by mass or more and 7.0% by mass or less.

  Note that the amine type surfactant is a nonionic surfactant. Therefore, when the aqueous cleaning liquid in the present embodiment contains an amine type surfactant, the foaming property of the aqueous cleaning liquid can be suppressed low. In addition, an aqueous cleaning solution that is less harmful to the human body can be provided.

[Preferred production method of aqueous cleaning liquid]
A preferred method for producing the aqueous cleaning liquid includes a step of uniformly mixing a material (for example, an amine type surfactant) at a predetermined blending amount. It is preferable to mix the materials using a stirrer (for example, “Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.). After mixing the materials, filtration is performed if necessary.

  An embodiment of the present invention will be described. In addition, the evaluation result (the value indicating the shape or the physical property) of the powder including a plurality of particles is a number average of the values measured for a considerable number of particles, unless otherwise specified. In the evaluation in which an error occurs, a considerable number of measured values with which the error is sufficiently small were obtained, and the arithmetic average of the obtained measured values was used as the evaluation value.

  Using the ink sets A-1 to A-12 according to Examples and Comparative Examples, the wiping performance of the fixed ink on the nozzle surface, the presence or absence of liquid stain on the outer surface of the recording head, the landing accuracy of the aqueous ink, The stability of the pigment particles to the washing liquid was evaluated. Table 1 shows each configuration of the ink sets A-1 to A-12.

  Hereinafter, first, the method for producing the aqueous inks B-1 to B-8 and the method for producing the aqueous cleaning liquids C-1 to C-8 will be described in order. Next, evaluation methods and evaluation results of the ink sets A-1 to A-12 will be described in order.

[Method for producing aqueous ink]
Table 2 shows each constitution of the aqueous inks B-1 to B-5. Table 3 shows each constitution of the aqueous inks B-6 to B-8. The aqueous inks B-1 to B-8 are respectively an aqueous ink containing a cyan pigment (cyan ink), an aqueous ink containing a yellow pigment (yellow ink), an aqueous ink containing a magenta pigment (magenta ink), and black. And an aqueous ink containing a pigment (black ink). Hereinafter, a method for producing a cyan ink will be mainly described. When it is not necessary to indicate the color of the water-based ink, it is described as “water-based ink”.

  In Tables 2 and 3, "ether" means triethylene glycol mono-n-butyl ether. "Surfactant" means an acetylene-type surfactant, more specifically "Olfin E1010" manufactured by Nissin Chemical Industry Co., Ltd. The “NaOH aqueous solution” means an NaOH aqueous solution having a concentration of 0.1N. The configuration of the “pigment dispersion liquid L1” is as shown in Table 4.

  In Table 4, “Resin A-Na” means the resin A neutralized with an aqueous sodium hydroxide (NaOH) solution. “Pigment” means any one of a cyan pigment, a yellow pigment, a magenta pigment, and a black pigment.

<Method for producing pigment dispersion liquid L1>
(Synthesis of Resin A)
First, resin A was synthesized. Specifically, a stirrer, a nitrogen inlet tube, a condenser (stirrer), and a dropping funnel were set in a four-necked flask (capacity: 1000 mL). Next, the flask was charged with 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone. The mixture was heated to reflux at 70 ° C. while bubbling nitrogen through the contents of the flask.

  Also, 40.0 g of styrene, 10.0 g of methacrylic acid, 40.0 g of methyl methacrylate, 10.0 g of n-butyl acrylate, and 0.400 g of azobisisobutyronitrile (AIBN, And a polymerization initiator) to obtain a monomer solution. The monomer solution was dropped into the flask over a period of about 2 hours while heating and refluxing at 70 ° C. After the dropwise addition, the mixture was heated and refluxed at 70 ° C. for further 6 hours.

  A solution containing 0.200 g of AIBN and methyl ethyl ketone was added dropwise to the flask over 15 minutes. After the dropwise addition, the mixture was heated and refluxed at 70 ° C. for another 5 hours. Thus, resin A (styrene-acrylic acid-based resin) was obtained. In the obtained resin A, the mass average molecular weight (Mw) was 20,000, and the acid value was 100 mgKOH / g.

Here, the mass average molecular weight Mw of the resin A was measured using gel filtration chromatography ("HLC-8020GPC" manufactured by Tosoh Corporation) under the following conditions.
Column: “TSKgel SuperMultipore HZ-H” manufactured by Tosoh Corporation (4.6 mm ID × 15 cm semi-micro column)
Number of columns: 3 Eluent: tetrahydrofuran Flow rate: 0.35 mL / min Sample injection volume: 10 μL
Measurement temperature: 40 ° C
Detector: IR detector The calibration curve was obtained from TSKgel standard polystyrene manufactured by Tosoh Corporation, F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000. And n-propylbenzene were selected.

  The acid value of the resin A is described in "JIS (Japanese Industrial Standards) K0070-1992 (Test method for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable product of chemical products)". In accordance with the method sought.

(Preparation of pigment dispersion L1)
Next, a pigment dispersion liquid L1 was prepared using the synthesized resin A. Specifically, 6.0 mass% of resin A and 15.0 mass% of phthalocyanine were added to a vessel (0.6 L in capacity) of a media-type dispersing machine ("Dynomill" manufactured by Willy & Bacoffen (WAB)). Blue 15: 3 (cyan pigment, “Lionol (registered trademark) Blue FG-7330” manufactured by Toyo Ink Co., Ltd.), 0.5% by mass of 1,2-octanediol, and ion-exchanged water (remaining amount) I put it.

  Further, an aqueous solution of sodium hydroxide required for neutralizing the resin A was added to the vessel. Here, an aqueous NaOH solution was added to the vessel so that the pH of the vessel contents became 8. More specifically, an aqueous NaOH solution having a mass 1.1 times the neutralization equivalent was added to the vessel. The mass of Na to be added to the vessel was added to the mass of Resin A. The mass of water contained in the NaOH aqueous solution and the mass of water generated by the neutralization reaction were added to the mass of ion-exchanged water.

Media (zirconia beads having a diameter of 0.5 mm) was filled in the vessel so that the filling amount was 70% by volume with respect to the capacity of the vessel. While cooling the vessel with water at a temperature of 10 ° C. and a peripheral speed of 8 m / sec, a media-type dispersing machine is used so that the volume median diameter (D ₅₀ ) of the pigment particles is in the range of 70 nm to 130 nm. Was used to knead the contents of the vessel. Thus, a pigment dispersion L1 was obtained.

Here, the volume median diameter (D ₅₀ ) of the pigment particles is a value measured using a laser diffraction particle size distribution analyzer (“Zetasizer nano-ZS” manufactured by Malvern).

<Mixing of Pigment Dispersion Liquid L1 with Other Ink Components>
The materials described in Tables 2 and 3 were placed in beakers in the amounts shown in Tables 2 and 3. The contents of the beaker were stirred at a rotation speed of 400 rpm using a stirrer (“Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.) to uniformly mix the contents of the beaker. The obtained mixture was filtered using a filter (pore size: 5 μm) to remove foreign substances and coarse particles contained in the mixture. Thus, a cyan ink (cyan ink contained in each of the aqueous inks B-1 to B-8) was obtained.

  C.I. I. Pigment Yellow 74 was used to obtain a yellow ink (a yellow ink contained in each of the aqueous inks B-1 to B-8). In addition, a magenta ink (magenta ink contained in each of the aqueous inks B-1 to B-8) was obtained using quinacridone magenta (PR122) as a pigment. In addition, a black ink (a black ink contained in each of the aqueous inks B-1 to B-8) was obtained using carbon black as the pigment. Thus, aqueous inks B-1 to B-8 were obtained. The aqueous inks B-1 to B-8 all exhibited weak basicity. More specifically, the pH of each of the aqueous inks B-1 to B-8 was 8 or more and 10 or less.

[Method of measuring surface tension of water-based ink]
The surface tension of the obtained aqueous inks (aqueous inks B-1 to B-8) was measured according to the Wilhelmy method. The measurement temperature was 25 ° C. Tables 2 and 3 show the measurement results.

[Production method of aqueous cleaning liquid]
Table 5 shows each configuration of the aqueous cleaning liquids C-1 to C-4. Table 6 shows the configurations of the aqueous cleaning liquids C-5 to C-8.

  In Tables 5 and 6, “ether” means triethylene glycol mono-n-butyl ether. "Surfactants e to h" are as shown in Table 7. The “NaOH aqueous solution” means an NaOH aqueous solution having a concentration of 0.1N.

  The materials described in Tables 5 and 6 were placed in beakers in the amounts shown in Tables 5 and 6. The contents of the beaker were stirred at a rotation speed of 400 rpm using a stirrer (“Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.) to uniformly mix the contents of the beaker. The obtained mixture was filtered using a filter (pore size: 5 μm) to remove foreign substances and coarse particles contained in the mixture. Thus, aqueous cleaning liquids C-1 to C-8 were obtained. Each of the aqueous cleaning liquids C-1 to C-8 showed weak basicity. More specifically, the pH of each of the aqueous cleaning solutions C-1 to C-8 was 8 or more and 10 or less.

[Method of measuring surface tension of aqueous cleaning liquid]
According to the Wilhelmy method, the surface tension of the obtained aqueous cleaning liquid (aqueous cleaning liquid C-1 to C-8) was measured. The measurement temperature was 25 ° C. The measurement results are shown in Tables 5 and 6.

[Evaluation method of ink set]
<Evaluation of wiping performance of fixed ink on nozzle surface>
First, an evaluation machine was prepared. Specifically, an ink jet recording apparatus (prototype manufactured by Kyocera Document Solutions Co., Ltd.) equipped with four recording heads (each a line head) was used. The recording head is a piezo type head (manufactured by Konica Minolta) having a resolution of 360 dpi (= 180 dpi × 2 rows), 512 nozzles (= 256 × 2 rows), a droplet amount of 14 pL, and a driving frequency of 12.8 kHz. Met. The recording heads were arranged at intervals of 20 mm such that the longitudinal direction was perpendicular to the paper transport direction. The recording head was filled with the aqueous ink (the aqueous ink in Examples or Comparative Examples) manufactured by the above-described method. More specifically, four recording heads were filled with aqueous inks having different colors. In addition, the nozzle surface was cleaned and, therefore, was not contaminated with ink.

  A printing durability test was performed using the prepared evaluation machine. The printing durability test was performed under the conditions of a temperature of 25 ° C. and a humidity of 60% RH at a transport speed of 350 mm / sec. In the printing durability test, solid images having different colors (size 10 cm × 10 cm, printing rate 100%) are formed so as to overlap on a recording sheet (“C2” manufactured by Fuji Xerox Co., Ltd., A4 size plain paper). Printing was continuously performed on 5000 recording sheets.

  After the printing durability test, an aqueous cleaning liquid (the aqueous cleaning liquid in Examples or Comparative Examples) was supplied to the nozzle surface. Thereafter, a purge operation and a wipe operation were performed.

After the wiping operation, the nozzle surface was observed using an optical microscope. Then, the presence or absence of ink contamination on the nozzle surface was confirmed. The evaluation criteria for the wiping performance of the fixed ink on the nozzle surface are shown below. Table 8 shows the evaluation results.
Excellent (A): No ink stain was observed on the nozzle surface. Good (O): Almost no ink stain was observed on the nozzle surface. Bad (X): Ink stain was clearly observed on the nozzle surface.

<Evaluation of the presence or absence of liquid stain on the outer surface of the recording head>
According to the method described in the above <Evaluation of wiping performance of fixed ink on nozzle surface>, a printing durability test, a supply of an aqueous cleaning liquid, a purge operation, and a wiping operation were sequentially performed. After the wiping operation, each outer surface of the recording head was observed using an optical microscope. Then, the presence or absence of liquid stains (ink stains and cleaning liquid stains) on each outer surface of the recording head was checked. The evaluation criteria for the presence or absence of liquid contamination on the outer surface of the recording head are shown below. Table 8 shows the evaluation results.
Excellent (A): No liquid contamination was observed on each outer surface of the recording head. Good (O): Little liquid contamination was observed on each outer surface of the recording head. Liquid stains were clearly confirmed on each outer surface

<Evaluation of landing accuracy of water-based ink>
First, the initial landing accuracy (3σ value) was determined.
Next, the printing durability test, the supply of the aqueous cleaning liquid, the purge operation, and the wiping operation were sequentially performed according to the method described in <Evaluation of Wiping Performance of Fixed Ink on Nozzle Surface>. Immediately after the end of the wiping operation, the landing accuracy (3σ value) was determined. The landing accuracy (3σ value) thus obtained is referred to as the landing accuracy after printing (3σ value).

  If the difference between the initial landing accuracy (3σ value) and the landing accuracy after printing (3σ value) is less than 3, the landing accuracy of the aqueous ink immediately after the supply of the aqueous cleaning liquid was evaluated as good (○). . On the other hand, if the difference between the initial landing accuracy (3σ value) and the landing accuracy after printing (3σ value) is 3 or more, the landing accuracy of the aqueous ink immediately after the supply of the aqueous cleaning liquid is poor (×). evaluated. Table 8 shows the evaluation results.

  Further, in accordance with the method described in the above <Evaluation of the wiping performance of the fixed ink on the nozzle surface>, the printing durability test, the supply of the aqueous cleaning liquid, the purge operation, and the wiping operation were sequentially performed. Thereafter, the evaluation machine was left for three days. Thereafter, the landing accuracy (3σ value) was determined. The landing accuracy (3σ value) obtained in this way is referred to as landing accuracy after leaving (3σ value).

  If the difference between the initial landing accuracy (3σ value) and the landing accuracy after standing (3σ value) was less than 3, the landing accuracy of the aqueous ink after standing for 3 days was evaluated as good (○). On the other hand, if the difference between the initial landing accuracy (3σ value) and the landing accuracy after standing (3σ value) is 3 or more, the landing accuracy of the aqueous ink after standing for 3 days was evaluated as poor (×). . Table 8 shows the evaluation results.

  The landing accuracy (3σ value) was obtained by the following method. Specifically, in a state where the recording sheet is not conveyed (stamp type), one dot is ejected from all the nozzles of the four recording heads to the recording sheet. Next, for each dot on the recording sheet, the landing accuracy 3σ was measured using a general-purpose image processing system (“DA-6000” manufactured by Oji Scientific Instruments). The arithmetic average value of the obtained measurement data (the landing accuracy of each dot is 3σ) was defined as the landing accuracy (3σ value).

The landing accuracy 3σ means the landing accuracy represented by the equation “3σ = 3√ (σ _x ² + σ _y ² )”. In the equation, σ _x means the standard deviation of the amount of displacement in the X direction from the nozzle design position. Further, σ _y means the standard deviation of the amount of displacement in the Y direction from the nozzle design position.

<Evaluation of stability of pigment particles in aqueous cleaning liquid>
First, using a laser diffraction type particle size distribution analyzer (“Zetasizer nano-ZS” manufactured by Malvern Co., Ltd.) under a temperature of 25 ° C. in an aqueous ink (an aqueous ink in Examples or Comparative Examples) The volume median diameter (D ₅₀ ) of the pigment particles contained in was measured. The volume median diameter (D ₅₀ ) of the pigment particles thus measured is referred to as an initial volume median diameter (D ₅₀ ).

Next, a mixed solution containing the aqueous ink and the aqueous cleaning liquid (the aqueous cleaning liquid in Examples or Comparative Examples) was placed in a container (capacity: 300 mL). In the mixed liquid, (aqueous ink) :( aqueous cleaning liquid) = 1: 100 (volume ratio). Next, the container was placed in a thermostat set at a temperature of 60 ° C. for 24 hours. Subsequently, the container was taken out of the thermostat, and the container was allowed to stand until the temperature of the contents of the container dropped to 25 ° C. Thereafter, the volume median diameter (D ₅₀ ) of the pigment particles contained in the container was measured using a laser diffraction particle size distribution analyzer (“Zetasizer nano-ZS” manufactured by Malvern). In this manner, it referred to as a volume median diameter after mixing volume median diameter of the measured pigment particles _{(D 50) (D 50)} .

The increase rate of the volume median diameter (D ₅₀ ) of the pigment particles was calculated using the following equation. When the rate of increase of the volume median diameter (D ₅₀ ) of the pigment particles was less than 5%, the stability of the pigment particles to the aqueous cleaning solution was evaluated as good (○). On the other hand, if the increase rate of the volume median diameter (D ₅₀ ) of the pigment particles is 5% or more, the stability of the pigment particles with respect to the aqueous cleaning liquid was evaluated as poor (x). Table 8 shows the evaluation results.
Increase rate of volume median diameter (D ₅₀ ) of pigment particles = [volume median diameter after mixing (D ₅₀ ) −initial volume median diameter (D ₅₀ )] × 100 / initial volume median diameter (D ₅₀ )

[Evaluation results of ink set]
Table 8 shows the evaluation results of the ink sets A-1 to A-12. In Table 8, "Ratio" indicates the ratio of the surface tension of the aqueous cleaning liquid to the surface tension of the aqueous ink. Evaluation A shows the evaluation result of the wiping performance of the fixed ink on the nozzle surface. Evaluation B shows the evaluation result of the presence or absence of liquid stain on the outer surface of the recording head. Among the evaluation results of the landing accuracy of the aqueous ink, the evaluation results of the landing accuracy of the aqueous ink immediately after the supply of the aqueous cleaning liquid are shown in Evaluation C, and the evaluation results of the landing accuracy of the aqueous ink after standing for 3 days are shown in Evaluation D. Evaluation E shows the results of evaluating the stability of the pigment particles with respect to the aqueous cleaning liquid.

[Discussion]
The ink sets according to Examples 1 to 6 (ink sets A-1 to A-6) had an aqueous ink and an aqueous cleaning liquid. The aqueous cleaning solution contained an amine-type nonionic surfactant. The surface tension of the aqueous ink and the surface tension of the aqueous cleaning liquid satisfied Expression (A).

  In the ink sets according to Examples 1 to 6, the wiping performance of the fixed ink on the nozzle surface was excellent. Further, after the wiping operation, none of the aqueous ink, the aqueous cleaning liquid, and the purge ink had adhered to the outer surface of the recording head. Further, even when three days have elapsed since the aqueous cleaning liquid was supplied to the ejection surface, the landing accuracy of the aqueous ink could be maintained high. Further, even in a mixed state of the aqueous cleaning liquid and the aqueous ink, the expansion of the pigment particles could be prevented.

  The ink set according to the present invention can be used for forming an image in a color printer, for example.

4 Recording Head 47 Discharge Surface 51 Cartridge 52 First Tank 53 Second Tank S Recording Medium

Claims (5)

Having an aqueous ink and an aqueous cleaning liquid,
The aqueous cleaning solution contains a nonionic surfactant,
The nonionic surfactant has an amino group in the molecule,
And surface tension of the water-based ink, and the surface tension of said aqueous cleaning solution, meets the following formula (A),
The aqueous cleaning liquid has a surface tension of 34 mN / m or more and 40 mN / m or less,
The nonionic surfactant is a polyoxyalkylene alkylamine having a structure represented by the following formula (1-1),
The surface tension of the aqueous ink is from 25 mN / m to 33 mN / m,
The aqueous ink contains 0.55% by mass or more and 0.70% by mass or less of a nonionic surfactant,
The ink set for inkjet recording , wherein the nonionic surfactant contained in the aqueous ink is an alcohol having an acetylene bond in a molecule .
1.1 × (surface tension of the aqueous ink) ≦ (surface tension of the aqueous cleaning liquid) ≦ 1.4 × (surface tension of the aqueous ink) Formula (A)

In the formula (1-1), R ¹ represents a hydrocarbon group having 1 to 24 carbon atoms. A ¹ O and A ² O each independently represent at least one of an oxyethylene group and an oxypropylene group. m1 and n1 each represent an integer of 0 or more, and represent an integer satisfying 1 ≦ (m1 + n1) ≦ 100. The ink set for inkjet recording according to claim 1 , wherein the content of the nonionic surfactant in the aqueous cleaning liquid is from 0.1% by mass to 10.0% by mass. The aqueous ink is ejected from an ejection surface of a recording head,
The aqueous wash is used for cleaning the ejection face, the ink-jet recording ink set according to claim 1 or 2. A first tank that contains the aqueous ink of the ink set for inkjet recording according to any one of claims 1 to 3 ,
A second tank containing the aqueous cleaning liquid of the inkjet recording ink set according to any one of claims 1 to 3 ,
Including, cartridge. An ejection step of ejecting the aqueous ink from the ejection surface of the recording head to the recording medium,
A supply step of supplying an aqueous cleaning liquid to the discharge surface,
A purging step of discharging the water-based ink from the ejection surface by applying pressure,
A wiping step of wiping the discharge surface,
Including
The supply step and the purge step are each performed after the discharge step and before the wipe step,
The aqueous cleaning solution contains a nonionic surfactant,
The nonionic surfactant has an amino group in the molecule,
And surface tension of the water-based ink, and the surface tension of said aqueous cleaning solution, meets the following formula (A),
The aqueous cleaning liquid has a surface tension of 34 mN / m or more and 40 mN / m or less,
The nonionic surfactant is a polyoxyalkylenealkylamine having a structure represented by the following formula (1-1),
The surface tension of the aqueous ink is from 25 mN / m to 33 mN / m,
The aqueous ink contains 0.55% by mass or more and 0.70% by mass or less of a nonionic surfactant,
The image forming method , wherein the nonionic surfactant contained in the aqueous ink is an alcohol having an acetylene bond in a molecule .
1.1 × (surface tension of the aqueous ink) ≦ (surface tension of the aqueous cleaning liquid) ≦ 1.4 × (surface tension of the aqueous ink) Formula (A)

In the above formula (1-1), R ¹ represents a hydrocarbon group having 1 to 24 carbon atoms. A ¹ O and A ² O each independently represent at least one of an oxyethylene group and an oxypropylene group. m1 and n1 each represent an integer of 0 or more, and represent an integer satisfying 1 ≦ (m1 + n1) ≦ 100.

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