JP2018119078A - Ink set for ink jet recording, cartridge, and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink set capable of preventing contamination on a discharge face after wiping action.SOLUTION: An inkjet recording ink set has aqueous ink and aqueous cleaning liquid. The aqueous cleaning liquid contains a nonionic surfactant. The nonionic surfactant has an amino group in a molecule. A surface tension of the aqueous ink and a surface tension of the aqueous cleaning liquid satisfy following formula (A). 1.1×(surface tension of aqueous ink)≤(surface tension of aqueous cleaning liquid)≤1.4×(surface tension of aqueous ink)...formula (A)SELECTED DRAWING: None

Description

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

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

JP2013-146965A

  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 onto a recording medium. A large number of nozzles are formed in the recording head, and each of the nozzles opens at the discharge surface. Therefore, the ink is ejected from the nozzle opening (ejection 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 ink is ejected, the ink adhered to the ejection surface (hereinafter sometimes referred to as “fixed ink”) is removed from the ejection surface. it can. Thereby, since it is possible to prevent the fixed ink from preventing the ejection port, it is possible to maintain the ink ejection performance.

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

  When a wipe operation is performed in a state where there is liquid on the ejection surface, the liquid present on the ejection surface is pushed by the blade to the peripheral edge of the ejection surface and goes around from the peripheral edge of the ejection surface to the outer surface of the recording head. There is. The liquid that has entered the outer surface of the recording head may sag along the outer surface due to its own weight. In addition, the liquid that drips down to the discharge surface due to its own weight may spread wet on the discharge surface. Thus, the discharge surface may be contaminated after the wiping operation. If the ejection surface is contaminated after the wiping operation, the ink ejection performance may deteriorate when the ink ejection is resumed 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 the ejection surface after the wiping operation and a cartridge including the ink set capable of preventing contamination of the ejection surface after the wiping operation. And an image forming method using an ink set capable of preventing contamination of the ejection surface after the wiping operation.

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

  The cartridge according to the present invention stores a first tank that stores the water-based ink included in the ink set for ink-jet recording having the above-described configuration, and a second tank that stores the water-based cleaning liquid included in the ink set for ink-jet recording having the above-described configuration. And a tank.

An image forming method according to the present invention includes a discharge step of discharging a water-based ink from a discharge surface of a recording head onto a recording medium, a supply step of supplying an aqueous cleaning liquid to the discharge surface, and pressurizing the water-based ink. A purge step of discharging from the discharge surface, and a wiping step of wiping the discharge surface. The supply process and the purge process are each performed after the ejection process and before the wiping process. The aqueous cleaning liquid contains a nonionic surfactant. The nonionic surfactant has an amino group in the molecule. The surface tension of the water-based ink and the surface tension of the water-based cleaning liquid satisfy the following formula (A).
1.1 × (surface tension of the water-based ink) ≦ (surface tension of the water-based cleaning liquid) ≦ 1.4 × (surface tension of the water-based ink) Formula (A)

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

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. It is a figure explaining 1 process of the image forming method concerning this invention. It is a figure explaining 1 process of the image forming method concerning this invention.

An embodiment of the present invention will be described. In addition, the evaluation result (a value indicating the shape or physical properties) of the powder is the number average of the values measured for a considerable number of particles unless otherwise specified. The measured value of the volume median diameter (D ₅₀ ) of the powder is a laser diffraction particle size distribution measuring device (“Zetasizer nano-ZS” manufactured by Malvern, Inc.) unless otherwise specified. It is the value measured using.

  In addition, a compound and a derivative thereof may be generically named by adding “system” after the compound name. When the name of a polymer is expressed by adding “system” after the compound name, it means that the repeating unit of the polymer is derived from the compound or a derivative thereof. Acrylic and methacrylic are sometimes collectively referred to as “(meth) acrylic”.

  Further, among nonionic surfactants, an alcohol having an acetylene bond in the molecule is referred to as an “acetylene type surfactant”. Among 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 sometimes simply referred to as “ink set”) include the following methods. Specifically, first, the water-based ink included in the ink set is ejected from the ejection surface of the recording head (the recording head included in the inkjet recording apparatus) onto the recording medium. Next, a purge operation is performed after supplying the aqueous cleaning liquid of the ink set 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 includes an aqueous ink and an aqueous cleaning liquid. The aqueous cleaning liquid contains a nonionic 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 formula (A).
1.1 × (surface tension of water-based ink) ≦ (surface tension of aqueous cleaning liquid) ≦ 1.4 × (surface tension of water-based ink) Formula (A)

  Thus, 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 formula (A) is satisfied, contamination of the ejection surface after the wiping operation can be prevented. If contamination of the ejection surface after the wiping operation can be prevented, a decrease in ink ejection performance can be prevented even when ink ejection is resumed after the wiping operation. For example, it is possible to prevent water-based ink from becoming difficult to be ejected. Further, it is possible to prevent the water-based 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, more preferably 1.10 to 1.25 times the surface tension of the aqueous ink. In the following, first, a preferable configuration of the water-based ink and a formation process of the fixed ink will be described in order.

<Preferred configuration of water-based ink>
The aqueous ink preferably has an aqueous solvent and a pigment dispersion. A pigment dispersion means a composition 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 ink, pigment particles are dispersed with each other. Specifically, in many cases, a resin salt is used as the coating resin. Here, the resin salt has an ionizable functional group (for example, COONa group) in the molecule. The aqueous ink contains a sufficient amount of an aqueous solvent. Therefore, ionization is likely to occur 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 dispersed with each other.

<Fixed ink formation process>
It is considered that the fixing ink is formed by the following process.
When water-based ink is discharged from the discharge surface of the recording head onto the recording medium, the water-based ink may adhere to the discharge surface. When the water-based ink adheres to the ejection surface, the water-based ink is dried due to contact between the water-based ink and air. When the water-based ink is dried, the coating resin tends to form a film.

  Specifically, when the water-based ink is dried, the content of the water-based solvent in the water-based ink is reduced, so that ionization hardly occurs on the surface of the coating resin. Thereby, the pigment particles are less likely to be electrically repelled, and the pigment particles are likely to aggregate with each other. When the pigment particles are aggregated with each other, the coating resins present on the surfaces of the different pigment cores are likely to come into contact with each other. As a result, a film made of a coating resin (hereinafter referred to as “resin film”) is easily formed. As described above, when the water-based ink is dried, 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>
Subsequently, items that can be considered as a reason why a predetermined effect can be obtained by performing image formation using the ink set according to the present embodiment will be described. Here, the predetermined effect includes 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 can be considered as the reason why such a result was obtained.

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

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

  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). Accordingly, the aqueous cleaning liquid has a high affinity for the water-based ink, and thus has a high affinity for the purge ink. Therefore, when the aqueous cleaning liquid is supplied to the discharge surface and the purge operation is performed, the purge ink easily enters between adjacent pigment particles together with the aqueous cleaning liquid.

  Thus, in this embodiment, the purge ink is likely to enter 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, when the wiping operation is performed after the aqueous cleaning liquid is supplied to the ejection surface and the purge operation is performed, 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 1.10 times or more the surface tension of the aqueous ink. In general, the amount of aqueous cleaning liquid supplied to the ejection surface is larger than the amount of purge ink discharged to the ejection surface. Therefore, when the wiping operation is performed, the amount of aqueous cleaning liquid present on the ejection surface is larger than the amount of purge ink present. Therefore, the physical properties of the liquid present on the ejection surface when performing the wiping operation (hereinafter simply referred to as “liquid present on the ejection surface”) are more easily 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 1.10 times or more the surface tension of the aqueous ink, the surface tension of the liquid existing on the ejection surface tends to increase.

  If the surface tension of the liquid existing on the discharge surface is large, the liquid existing on the discharge surface is difficult to spread on the discharge surface. Therefore, when the wiping operation is performed, the liquid present on the ejection surface is likely to sag, for example, by passing through a blade.

  When the liquid existing on the discharge surface falls down by passing through the blade, it is possible to prevent the liquid existing on the discharge surface from flowing into the outer surface of the recording head. Thereby, it is possible to prevent the discharge surface from being contaminated after the wiping operation. Therefore, even when the ink ejection is resumed after the wiping operation, it is possible to prevent the ink ejection performance from being deteriorated.

  Further, if it is possible to prevent the discharge surface from being contaminated after the wiping operation, contamination of the discharge surface after each wiping operation can be prevented even when the wiping operation is performed a plurality of times. As a result, even when image formation is performed using an image forming apparatus having a long mechanical life, the ink ejection performance can be maintained until the service life 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.

<Contrast 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 amount of aqueous cleaning liquid supplied to the ejection surface is larger than the amount of purge ink discharged to the ejection surface. For this reason, 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 discharge surface is small, the liquid tends to wet and spread on the discharge surface. Therefore, when the wiping operation is performed, the liquid existing on the ejection surface is easily pushed away by the blade to the peripheral portion of the ejection surface, and easily flows from the peripheral portion of the ejection surface to the outer surface of the recording head. Then, the liquid that has entered the outer surface of the recording head tends to sag along the outer surface to the ejection surface due to its own weight, and easily spreads on the ejection surface. Thus, if the surface tension of the aqueous cleaning liquid is smaller than the surface tension of the aqueous ink, the ejection surface is easily contaminated after the wiping operation (Comparative Example 4 described later). Therefore, when ink ejection is resumed after the wiping operation, the ink ejection performance may deteriorate.

  On the other hand, if the surface tension of the aqueous cleaning liquid is larger 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 this embodiment, the aqueous cleaning liquid contains an amine surfactant and the formula (A) is satisfied. Accordingly, the fixed ink can be removed from the ejection surface, and contamination of the ejection surface after the wiping operation can be prevented (see Examples 1 to 6 described later).

  In Comparative Example 4 described later, 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. When the surface tension of the aqueous cleaning liquid is less than one times the surface tension of the aqueous ink (surface tension of the aqueous cleaning liquid <surface tension of the aqueous ink), the liquid stain on the outer surface of the recording head becomes more prominent. Confirm that 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 aqueous ink, and a second tank that stores aqueous cleaning liquid. Accordingly, if the cartridge according to the present embodiment is loaded into the 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 ink ejection is resumed after the wiping operation.

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

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

  The water-based ink and the water-based cleaning liquid to be used are as described above in [Ink set for inkjet recording according to this embodiment]. Thereby, 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 resumed after the wiping operation.

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

[One Example of Image Forming Method According to this 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 diagram for explaining one process of the image forming method according to the present embodiment, and more specifically for explaining the supply process. FIG. 3 is a diagram for explaining another process of the image forming method according to the present embodiment, and more specifically for explaining 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. 2 and 3 are views of the main part of the image forming apparatus 1 shown in FIG. 1 as viewed from the side.

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

  As shown in FIGS. 2 and 3, the recording head 4 is formed with a nozzle 41, an ink inlet 43, and an ink outlet 45. The recording head 4 has an ejection surface 47. The nozzle 41 is open at the discharge surface 47 and discharges water-based ink toward the recording medium S (see FIG. 1). The water-based ink is stored in the first tank 52 (see FIG. 1). The water-based 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 storage unit 5. The cartridge 51 is detachably attached to the image forming apparatus 1. The cartridge 51 includes an ink set according to the present embodiment, a first tank 52, and a second tank 53. The first tank 52 stores the water-based ink in 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 unit 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 discharge surface 47 (see FIGS. 2 and 3) and a position facing the second transport unit 72 (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 stored 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 an ascending direction D1, a descending direction D2, and a 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 away from the ejection surface 47 along the Z-axis direction. The wiping direction D3 means a direction along the ejection surface 47.

  When the image forming apparatus 1 shown in FIG. 1 is used to form an image on the recording medium S, first, the paper feeding roller 32a takes out the recording medium S stored in the paper feeding 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 water-based ink is ejected from the ejection surface 47 (more specifically, the opening of the nozzle 41) to the recording medium S (ejection process). . Thereafter, the recording medium S is sent to the second transport unit 72 and discharged to the discharge tray 81.

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

  The supplying process will be described with reference to FIG. In the supplying step, first, the sponge 91 is impregnated with the aqueous cleaning liquid. Next, after the sponge 91 has moved to a position (position shown in FIG. 2) facing the discharge surface 47, it moves along the rising direction D <b> 1 and is pressed against the discharge surface 47. At this time, it is preferable that the state in which the sponge 91 is pressed against the discharge surface 47 (hereinafter referred to as “the pressed state of the sponge 91”) is maintained for a predetermined time. Further, the operation of moving the sponge 91 along the upward direction D1 and the operation of moving the sponge 91 along the downward direction D2 may be repeated while the pressed state of the sponge 91 is maintained. Further, the sponge 91 may move in the direction along the ejection surface 47 (direction along the X-axis direction shown in FIG. 2) while the pressed state of the sponge 91 is maintained.

  When the predetermined time has elapsed, the sponge 91 moves along the descending direction D2, and the pressed state of the sponge 91 is released. Thereby, a supply process is complete | finished. A purge process is performed after the supply process.

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

  The wiping process 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 discharge surface 47 (position shown in FIG. 3), then moves along the rising direction D <b> 1 and is pressed against the discharge 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 where 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 water-based ink and the water-based cleaning liquid will be further described.

[Water-based ink]
The water-based 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 a preferable method for producing the water-based ink will be described in order.

[Surface tension of water-based ink]
Preferably, the surface tension of the water-based ink is 25 mN / m or more and 33 mN / m or less. Thereby, the formula (A) is easily satisfied.

  Here, the surface tension of the water-based ink means the surface tension of the water-based ink measured at 25 ° C. The surface tension of the water-based 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, an “automatic surface tension meter DY-300” manufactured by Kyowa Interface Science Co., Ltd. can be used as the surface tension meter.

[Examples of materials constituting water-based ink]
Preferably, the aqueous ink contains a pigment dispersion and an aqueous solvent. More preferably, the water-based 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. Each of the pigment particles includes 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 purple pigment, or a black pigment can be used. Examples of yellow pigments 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 red pigments include C.I. I. Pigment red 122 or 202. Quinacridone magenta (PR122) can also be used as the red pigment. Examples of blue pigments include C.I. I. Pigment Blue 15 or 15: 3. Examples of purple pigments include C.I. I. And CI Pigment Violet 19, 23, or 33. Examples of black pigments include C.I. I. And CI Pigment Black 7. Carbon black can also be used as the black pigment.

  The content of the pigment core in the water-based ink is preferably 4% by mass or more and 8% by mass or less. When the content of the pigment core in the water-based ink is 4% by mass or more, an image having a desired image density is easily obtained. If the content of the pigment core in the water-based ink is 8% by mass or less, the fluidity of the water-based ink is easily secured. This also makes it easy to obtain an image having a desired image density. Further, the permeability of the water-based ink to 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 water-based ink. More preferably, the volume median diameter (D ₅₀ ) of the pigment core is 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 resin, styrene-maleic acid copolymer, styrene-maleic acid half ester copolymer, vinylnaphthalene-acrylic acid copolymer, and vinyl. It is preferably at least one of naphthalene-maleic acid copolymers. More preferably, the coating resin is a styrene-acrylic acid 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 enhanced.

  The styrene-acrylic acid resin is a resin including a unit derived from styrene and a unit derived from acrylic acid, methacrylic acid, an acrylic ester, or a methacrylic ester. Preferably, the styrene-acrylic acid resin is a copolymer of styrene, acrylic acid, and alkyl acrylate, a copolymer of styrene, methacrylic acid, alkyl methacrylate, and alkyl acrylate, styrene and acrylic acid. A copolymer of styrene, maleic acid and alkyl acrylate, a copolymer of styrene and methacrylic acid, and a copolymer of styrene and alkyl methacrylate. is there. More preferably, the styrene-acrylic acid resin is a copolymer of styrene, methacrylic acid, a methacrylic acid alkyl ester, and an acrylic acid alkyl ester. 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 with respect to 100 parts by mass of the pigment core. If the content of the coating resin is too small, there may be a case where the recording medium after the image formation is exposed. 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, more preferably ion-exchanged water. The water content in the water-based ink is preferably 20% by mass or more and 70% by mass or less. Thereby, the water-based ink which has a suitable viscosity can be provided.

  For example, the aqueous solvent preferably contains ion-exchanged water, glycerin and / or glycol, and alcohol and / or glycol ether. If the water-based ink contains glycerin and / or glycol, the water-based ink can be further prevented from drying. If the water-based ink contains alcohol and / or glycol ether, the permeability of the water-based ink to the recording medium can be improved. As the alcohol, for example, a sugar alcohol typified by sorbitol can 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 water-based ink contains a surfactant, the wettability of the water-based ink with respect to the recording medium is improved. The surfactant contained in the water-based ink is preferably a nonionic surfactant. The content of the nonionic surfactant in the water-based ink is preferably 0.05% by mass or more and 2.00% by mass or less. Thereby, the image density is improved while suppressing the offset of the image. More preferably, the water-based ink contains an acetylene type surfactant.

(Acetylene type surfactant)
Preferably, the content of the acetylene type surfactant in the water-based ink is 0.55% by mass or more and 0.70% by mass or less. Thereby, the surface tension of water-based ink tends to be 25 mN / m or more and 33 mN / m or less. Therefore, the formula (A) is easily satisfied. More preferably, the content of the acetylene type surfactant in the water-based ink is 0.60% by mass or more and 0.70% by mass or less. When the water-based ink contains two or more acetylene-type surfactants, the total content of the acetylene-type surfactant in the water-based 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 the molecule. The alcohol having an acetylene bond in the molecule is preferably at least one of, for example, a glycol having an acetylene bond in the molecule and a monovalent alcohol having an acetylene bond in the molecule. A commercial item can also be used as an acetylene type surfactant. 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 water-based ink contains a dissolution stabilizer, the components contained in the water-based ink are easily compatible, so that the dissolved state of the water-based ink can be stabilized. The dissolution stabilizer is preferably at least one of 2-pyrrolidone, N-methyl-2-pyrrolidone, and γ-butyrolacton. The content of the dissolution stabilizer in the water-based ink is preferably 1% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 15% by mass or less.

<Humectant>
If the water-based ink contains a humectant, the 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 glycol is 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, thiol Diglycol, 1,3-butanediol, or 1,5-pentanediol is preferred. The content of the humectant in the water-based ink is preferably 2% by mass or more and 30% by mass or less, and more preferably 10% by mass or more and 25% by mass or less.

<Penetration agent>
If the water-based ink contains a penetrant, the penetrability of the water-based ink into 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, 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 water-based ink contains a pH adjuster, the water-based ink tends to exhibit weak basicity. For example, it is preferable to determine the content of the pH adjusting agent in the aqueous ink so 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 enhanced. The alkali metal salt is preferably an alkali metal hydroxide. More preferably, the alkali metal salt comprises at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide.

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

<Pigment dispersion preparation process>
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 to reflux at a predetermined temperature. In this way, the coating resin is synthesized. More specifically, styrene, (meth) acrylic acid, (meth) acrylic acid alkyl ester, and a polymerization initiator are added to a mixed solution of isopropyl alcohol and methyl ethyl ketone, and the mixture is heated to reflux at 70 ° C. Thereby, a styrene-acrylic acid 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 diameter (for example, bead diameter) of the media used in the media type disperser, the degree of dispersion of the pigment particles, the amount of resin released to the pigment dispersion, or the particle diameter of the pigment particles can be adjusted. For example, the smaller the particle diameter of the media, the smaller the particle diameter of the pigment particles.

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

[Aqueous cleaning solution]
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 cleaning the transport roller. Hereinafter, the surface tension of the aqueous cleaning liquid, the material constituting the aqueous cleaning liquid, and a preferred method for producing the aqueous cleaning liquid will be described in order.

[Surface tension of aqueous cleaning solution]
Preferably, the surface tension of the aqueous cleaning liquid is 34 mN / m or more and 40 mN / m or less. Thereby, the formula (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, an “automatic surface tension meter 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 liquid contains an amine type surfactant. The amine type surfactant will be described in the following <Amine type surfactant>.

  Preferably, the aqueous cleaning liquid contains at least one of the aqueous solvent described in <Aqueous solvent>, the dissolution stabilizer described in <Solubility stabilizer>, and the humectant described in <Moisturizer>. Furthermore, it contains. Thereby, in the water-based ink and the water-based cleaning liquid, at least one material of the water-based 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, it becomes easier for the aqueous cleaning liquid to enter between adjacent pigment particles. More specifically, the aqueous cleaning liquid preferably contains ion-exchanged water, sorbitol, triethylene glycol mononormal butyl ether, 2-pyrrolidone, and 1,3-propanediol.

  Preferably, the aqueous cleaning liquid further contains a pH adjuster. Thereby, an aqueous washing | cleaning liquid tends to show weak basicity. Thereby, since the affinity of the aqueous cleaning liquid with respect to the aqueous ink can be further increased, the infiltration of the aqueous cleaning liquid between the adjacent pigment particles is further facilitated. As the pH adjusting agent contained in the aqueous cleaning liquid, it is preferable to use the pH adjusting agent described in the above <pH adjusting agent>.

<Amine type 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 “Amate (registered trademark) 105A” manufactured by Kao Corporation, “Amate 320” manufactured by Kao Corporation, or “Puamyl (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 represents an integer of 0 or more, and represents an integer satisfying 1 ≦ (m1 + n1) ≦ 100. Preferably, m1 and n1 each represents an integer of 0 or more, and represents an integer satisfying 1 ≦ (m1 + n1) ≦ 40.

  The content of the amine surfactant in the aqueous cleaning liquid is preferably 0.1% by mass or more and 20.0% by mass or less. Thereby, the quantity of the aqueous washing | cleaning liquid which permeates between adjacent pigment particles is securable. Therefore, the amount of purge ink that enters between adjacent pigment particles can be secured.

  More preferably, the content of the amine surfactant in the aqueous cleaning liquid 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 34 mN / m or more and 40 mN / m or less. Therefore, the formula (A) is easily satisfied. More preferably, the content of the amine surfactant in the aqueous cleaning liquid 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 amine type surfactants in the aqueous cleaning liquid is preferably 0.1% by mass or more and 20.0% by mass or less. . More preferably, the total content of the amine type surfactant in the aqueous cleaning liquid 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 liquid is 0.5% by mass or more and 7.0% by mass or less.

  The amine type surfactant is a nonionic surfactant. Therefore, if the aqueous cleaning liquid in this embodiment contains an amine type surfactant, the foamability of the aqueous cleaning liquid can be kept low. In addition, an aqueous cleaning liquid that is less harmful to the human body can be provided.

[Preferred production method of aqueous cleaning liquid]
A preferable method for producing the aqueous cleaning liquid includes a step of uniformly mixing a material (for example, an amine type surfactant) in a predetermined blending amount. The materials are preferably mixed using a stirrer (for example, “Three-One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.). After mixing the materials, filter if necessary.

  Examples of the present invention will be described. Note that the evaluation result (a value indicating the shape or physical properties) of the powder containing a plurality of particles is the 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 measurement values with which the error is sufficiently small are obtained, and the arithmetic average of the obtained measurement values is 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 stains on the outer surface of the recording head, the landing accuracy of the aqueous ink, and the aqueous The stability of the pigment particles with respect to the cleaning liquid was evaluated. Table 1 shows the configurations of the ink sets A-1 to A-12.

  Below, first, the manufacturing method of water-based ink B-1 to B-8 and the manufacturing method of aqueous cleaning liquid C-1 to C-8 are demonstrated in order. Next, evaluation methods and evaluation results of ink sets A-1 to A-12 will be described in order.

[Method for producing water-based ink]
Table 2 shows the configurations of the water-based inks B-1 to B-5. Table 3 shows the configurations of the water-based inks B-6 to B-8. Water-based inks B-1 to B-8 are respectively water-based ink containing cyan pigment (cyan ink), water-based ink containing yellow pigment (yellow ink), water-based ink containing magenta pigment (magenta ink), and black And a water-based ink containing a pigment (black ink). Hereinafter, a method for producing cyan ink will be mainly described. When there is no need 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 mononormal butyl ether. “Surfactant” means an acetylene type surfactant, and 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 composition of “pigment dispersion L1” is as shown in Table 4.

  In Table 4, “resin A-Na” means resin A neutralized with an aqueous solution of sodium hydroxide (NaOH). “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 introducing tube, a condenser (stirrer), and a dropping funnel were set in a four-necked flask (capacity 1000 mL). The flask was then charged with 100 g isopropyl alcohol and 300 g methyl ethyl ketone. The flask was heated to reflux at 70 ° C. while bubbling nitrogen through the contents.

  Also 40.0 g styrene, 10.0 g methacrylic acid, 40.0 g methyl methacrylate, 10.0 g n-butyl acrylate, 0.400 g azobisisobutyronitrile (AIBN, And a polymerization initiator) to obtain a monomer solution. The monomer solution was added dropwise to the flask over about 2 hours in a state heated to reflux at 70 ° C. After the dropwise addition, the mixture was further refluxed at 70 ° C. for 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 further refluxed at 70 ° C. for 5 hours. Thus, Resin A (styrene-acrylic acid resin) was obtained. In the obtained resin A, the mass average molecular weight (Mw) was 20000, and the acid value was 100 mgKOH / g.

Here, the mass average molecular weight Mw of the resin A was measured under the following conditions using gel filtration chromatography (“HLC-8020GPC” manufactured by Tosoh Corporation).
Column: “TSKgel SuperMultipore HZ-H” manufactured by Tosoh Corporation (semi-micro column of 4.6 mm ID × 15 cm)
Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 mL / min Sample injection volume: 10 μL
Measurement temperature: 40 ° C
Detector: IR detector In addition, a calibration curve is F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000 from TSKgel standard polystyrene made from Tosoh Corporation. 7 types and n-propylbenzene were selected.

  The acid value of the resin A is described in “JIS (Japanese Industrial Standard) K0070-1992 (acid acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponified test method for chemical products)”. Required in accordance with the method.

(Preparation of pigment dispersion L1)
Next, using the synthesized resin A, a pigment dispersion L1 was prepared. Specifically, 6.0% by mass of resin A and 15.0% by mass of phthalocyanine were added to a vessel (capacity 0.6 L) of a media-type disperser (“Dynomill” manufactured by Willy et Bacofen (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 in.

  Further, an aqueous sodium hydroxide solution necessary 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 content was 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.

The vessel was filled with media (zirconia beads having a diameter of 0.5 mm) so that the filling amount was 70% by volume with respect to the vessel capacity. The media-type disperser is adjusted so that the volume median diameter (D ₅₀ ) of the pigment particles falls within the range of 70 nm or more and 130 nm or less while water-cooling the vessel under conditions where the temperature is 10 ° C. and the peripheral speed is 8 m / sec. The vessel contents were kneaded. 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 (Zetasizer Nano ZS)” manufactured by Malvern).

<Mixing of pigment dispersion L1 and other ink components>
The materials described in Table 2 and Table 3 were put in a beaker with the blending amounts described in Table 2 and Table 3. Using a stirrer (“Three-One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.), the contents of the beaker were stirred at a rotation speed of 400 rpm to uniformly mix the contents of the beaker. The liquid mixture obtained using a filter (pore size 5 μm) was filtered to remove foreign substances and coarse particles contained in the liquid mixture. In this way, cyan ink (cyan ink contained in each of water-based inks B-1 to B-8) was obtained.

  As a pigment, C.I. I. A yellow ink (yellow ink contained in each of water-based inks B-1 to B-8) was obtained using Pigment Yellow 74. Further, magenta ink (magenta ink contained in each of water-based inks B-1 to B-8) was obtained using quinacridone / magenta (PR122) as a pigment. Further, black ink (black ink contained in each of water-based inks B-1 to B-8) was obtained using carbon black as a pigment. In this way, aqueous inks B-1 to B-8 were obtained. All of the water-based inks B-1 to B-8 exhibited weak basicity. More specifically, the pH of each of the water-based inks B-1 to B-8 was 8 or more and 10 or less.

[Measurement method of surface tension of water-based ink]
According to the Wilhelmy method, the surface tension of the obtained water-based inks (water-based inks B-1 to B-8) was measured. The measurement temperature was 25 ° C. The measurement results are shown in Tables 2 and 3.

[Method for producing aqueous cleaning liquid]
Table 5 shows the configurations 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 mononormal 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 Table 5 and Table 6 were put in a beaker with the blending amounts described in Table 5 and Table 6. Using a stirrer (“Three-One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.), the contents of the beaker were stirred at a rotation speed of 400 rpm to uniformly mix the contents of the beaker. The liquid mixture obtained using a filter (pore size 5 μm) was filtered to remove foreign substances and coarse particles contained in the liquid mixture. In this way, aqueous cleaning liquids C-1 to C-8 were obtained. All of the aqueous cleaning liquids C-1 to C-8 exhibited weak basicity. More specifically, the pH of each of the aqueous cleaning liquids C-1 to C-8 was 8 or more and 10 or less.

[Measurement method of surface tension of aqueous cleaning liquid]
According to the Wilhelmy method, the surface tension of the obtained aqueous cleaning liquid (aqueous cleaning liquids 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 Inc.) having four recording heads (each line head) was used. Each of the recording heads has 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 (manufactured by Konica Minolta, Inc.). Met. The recording heads were arranged at an interval of 20 mm so that the longitudinal direction thereof was orthogonal to the paper transport direction. The recording head was filled with the water-based ink produced by the above-described method (water-based ink in Examples or Comparative Examples). More specifically, the four recording heads were filled with water-based inks having different colors. Also, the nozzle surface was washed 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 conveyance speed of 350 mm / second in an environment of a temperature of 25 ° C. and a humidity of 60% RH. In the printing durability test, solid images having different colors (size 10 cm × 10 cm, printing rate 100%) were 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 (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. And the presence or absence of ink stains on the nozzle surface was confirmed. Evaluation criteria for the wiping performance of the fixed ink on the nozzle surface are shown below. The evaluation results are shown in Table 8.
Excellent (◎): No ink stain was confirmed on the nozzle surface. Good (○): Little ink stain was confirmed on the nozzle surface. Bad (x): Ink stain was clearly confirmed on the nozzle surface.

<Evaluation of presence or absence of liquid contamination on outer surface of recording head>
In accordance with the method described in <Evaluation of wiping performance of fixed ink on nozzle surface>, a printing durability test, an aqueous cleaning liquid supply, a purge operation, and a wipe operation were sequentially performed. After the wiping operation, the outer surface of each recording head was observed using an optical microscope. And the presence or absence of the liquid stain | pollution | contamination (ink stain | pollution | contamination and washing | cleaning liquid stain | pollution | contamination) on each outer surface of the recording head was confirmed. Evaluation criteria for the presence or absence of liquid contamination on the outer surface of the recording head are shown below. The evaluation results are shown in Table 8.
Excellent (A): No liquid contamination was observed on the outer surface of each recording head. Good (O): Almost no liquid contamination was observed on the outer surface of each recording head. Liquid contamination was clearly confirmed on each outer surface

<Evaluation of landing accuracy of water-based ink>
First, initial landing accuracy (3σ value) was obtained.
Next, in accordance with the method described in <Evaluation of performance for wiping off fixed ink on nozzle surface>, a printing durability test, an aqueous cleaning liquid supply, a purge operation, and a wiping operation were sequentially performed. Immediately after completion of the wiping operation, the landing accuracy (3σ value) was obtained. The landing accuracy (3σ value) thus determined 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 aqueous ink immediately after the supply of the aqueous cleaning liquid is poor (×). evaluated. The evaluation results are shown in Table 8.

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

  If the difference between the initial landing accuracy (3σ value) and the landing accuracy after standing (3σ value) is less than 3, the landing accuracy of the water-based 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 water-based ink after leaving for three days was evaluated as poor (×). . The evaluation results are shown in Table 8.

  The landing accuracy (3σ value) was determined by the following method. Specifically, one dot was ejected from all nozzles of the four recording heads on the recording sheet in a state where the recording sheet was not conveyed (stamp type). 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 (landing accuracy of each dot 3σ) was defined as the landing accuracy (3σ value).

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

<Evaluation of pigment particle stability with respect to aqueous cleaning liquid>
First, using a laser diffraction type particle size distribution measuring apparatus (“Zetasizer nano-ZS (Zetasizer nano-ZS)” manufactured by Malvern) under a temperature of 25 ° C. environment, aqueous ink (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 described as the initial volume median diameter (D ₅₀ ).

Next, a mixed liquid containing an aqueous ink and an aqueous cleaning liquid (aqueous cleaning liquid in Examples or Comparative Examples) was placed in a container (capacity: 300 mL). In the mixed solution, (aqueous ink) :( aqueous cleaning solution) = 1: 100 (volume ratio). Next, the container was placed in a thermostatic bath whose temperature was set to 60 ° C. for 24 hours. Subsequently, the container was taken out from the thermostatic bath, and the container was allowed to stand until the temperature of the container contents 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 formula. When the increase rate of the volume median diameter (D ₅₀ ) of the pigment particles was less than 5%, the stability of the pigment particles with respect to the aqueous cleaning liquid was evaluated as good (◯). On the other hand, when the increase rate of the volume median diameter (D ₅₀ ) of the pigment particles was 5% or more, it was evaluated that the stability of the pigment particles with respect to the aqueous cleaning liquid was poor (x). The evaluation results are shown in Table 8.
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 )

[Ink set evaluation results]
Table 8 shows the evaluation results of 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 contamination on the outer surface of the recording head. Among the evaluation results of the landing accuracy of the water-based ink, the evaluation result of the landing accuracy of the water-based ink immediately after the supply of the aqueous cleaning liquid is shown in Evaluation C, and the evaluation result of the landing accuracy of the water-based ink after being left for 3 days is shown in Evaluation D. Evaluation E shows the evaluation results of 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 a water-based ink and a water-based cleaning liquid. The aqueous cleaning solution contained an amine type nonionic surfactant. The surface tension of the water-based ink and the surface tension of the water-based cleaning liquid satisfied the formula (A).

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

  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 (8)

Having water-based ink and water-based cleaning liquid,
The aqueous cleaning liquid contains a nonionic surfactant,
The nonionic surfactant has an amino group in the molecule,
The ink set for inkjet recording, wherein the surface tension of the water-based ink and the surface tension of the water-based cleaning liquid satisfy the following formula (A).
1.1 × (surface tension of the water-based ink) ≦ (surface tension of the water-based cleaning liquid) ≦ 1.4 × (surface tension of the water-based ink) Formula (A)   The ink set for inkjet recording according to claim 1, wherein the surface tension of the aqueous cleaning liquid is 34 mN / m or more and 40 mN / m or less.   The ink set for inkjet recording according to claim 2, wherein the content of the nonionic surfactant in the aqueous cleaning liquid is 0.1% by mass or more and 10.0% by mass or less. The ink set for inkjet recording according to any one of claims 1 to 3, wherein the nonionic surfactant is a polyoxyalkylene alkylamine having a structure represented by the following formula (1-1).

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 represents an integer of 0 or more, and represents an integer satisfying 1 ≦ (m1 + n1) ≦ 100. The water-based ink is ejected from the ejection surface of the recording head,
The ink set for inkjet recording according to claim 1, wherein the aqueous cleaning liquid is used for cleaning the ejection surface. The surface tension of the water-based ink is 25 mN / m or more and 33 mN / m or less,
The water-based 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 according to any one of claims 1 to 5, wherein the nonionic surfactant contained in the water-based ink is an alcohol having an acetylene bond in a molecule. A first tank that contains the water-based ink included in the ink set for inkjet recording according to any one of claims 1 to 6;
A second tank for storing the aqueous cleaning liquid included in the ink set for inkjet recording according to any one of claims 1 to 6;
Including the cartridge. A discharge step of discharging the water-based ink from the discharge surface of the recording head to the recording medium;
A supply step of supplying an aqueous cleaning liquid to the discharge surface;
A purge step of pressurizing and discharging the water-based ink from the ejection surface;
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 liquid contains a nonionic surfactant,
The nonionic surfactant has an amino group in the molecule,
The surface tension of the water-based ink and the surface tension of the water-based cleaning liquid satisfy the following formula (A).
1.1 × (surface tension of the water-based ink) ≦ (surface tension of the water-based cleaning liquid) ≦ 1.4 × (surface tension of the water-based ink) Formula (A)

Images