JP2016199684A - Ink set, inkjet recording device, and inkjet recording method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an image of high picture quality (for example, an image without color bleeding) while preventing the mist contamination of a recording head.SOLUTION: An ink set comprises plural kinds of inks. In the inks, a plurality of pigment particles each covered with a resin is dispersed. The ink set comprises a first ink with an acid number of the resin covering each pigment particle of 100 mgKOH/g or more and 180 mgKOH/g or less, and a second ink with an acid number of the resin covering each pigment particle of 60 mgKOH/g or more and 90 mgKOH/g or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an ink set, an ink jet recording apparatus, and an ink jet recording method.

  In order to form a high-quality image using ink, it is desired to suppress color bleeding. Color bleed is an image quality deterioration phenomenon that occurs at the boundary portion (between dots) where inks (dots) of different colors are adjacent in an image formed with ink. In an image where color bleeding has occurred, the pigment of one of the adjacent inks is mixed with the other ink, and the image becomes unclear.

  Patent Document 1 discloses an ink jet recording method in which when a color image is formed using two or more colors of ink, the ink permeability is changed for each color according to the printing order.

JP-A-6-226999

  However, in the inkjet recording method described in Patent Document 1, it is difficult to form a high-quality image while suppressing mist contamination of the recording head when printing at a high recording speed (for example, 100 sheets / min or more). It is.

  The present invention has been made in view of the above problems, and an object of the present invention is to form a high-quality image (for example, an image having no color bleed) while suppressing mist contamination of a recording head.

  The ink set according to the present invention includes a plurality of types of ink. In the ink, a plurality of pigment particles each covered with a resin are dispersed. The ink set according to the present invention includes, as the plurality of types of inks, a first ink in which an acid value of the resin is 100 mgKOH / g or more and 180 mgKOH / g or less, and an acid value of the resin is 60 mgKOH / g or more and 90 mgKOH / g or less. And the second ink.

  An ink jet recording apparatus according to the present invention includes a plurality of recording heads that respectively eject ink, and a transport unit that transports a recording sheet on the transport path while being sucked by a negative pressure below the transport path. The plurality of recording heads include a first recording head positioned on the most upstream side in the conveying direction of the recording sheet, a second recording head positioned on the most downstream side, the first recording head, and the second recording head. And other recording heads located between the two. The transport unit is configured to suck the recording sheet with a stronger negative pressure below each of the first recording head and the second recording head than below the other recording head. A plurality of pigment particles each covered with a resin are dispersed in the ink ejected by each of the plurality of recording heads. Each of the first recording head and the second recording head is configured to eject a first ink having an acid value of the resin of 100 mgKOH / g or more and 180 mgKOH / g or less. The other recording head is configured to eject a second ink having an acid value of 60 mgKOH / g or more and 90 mgKOH / g or less of the resin.

  The ink jet recording method according to the present invention performs recording on the recording sheet using the ink jet recording apparatus according to the present invention. The ink jet recording method according to the present invention includes a first step, a second step, and a third step. In the first step, recording is performed on the recording sheet by discharging a first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less by the first recording head. In the second step, after the first step, recording is performed on the recording sheet by discharging the second ink having an acid value of 60 mgKOH / g or more and 90 mgKOH / g or less by the other recording head. I do. In the third step, after the second step, recording is performed on the recording sheet by discharging the first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less by the second recording head. I do.

  According to the present invention, it is possible to form a high-quality image (for example, an image without color bleed) while suppressing mist contamination of the recording head.

It is a figure which shows an example of the inkjet recording device with which the ink set which concerns on embodiment of this invention is used. It is a figure which shows the image formation part (especially recording head) of the inkjet recording device shown by FIG.

  Hereinafter, embodiments of the present invention will be described. The ink set according to the present embodiment includes a plurality of types of ink. Ink included in the ink set according to the present embodiment (hereinafter referred to as ink according to the present embodiment) contains a solvent (for example, water) and a colorant. Further, the ink according to the present embodiment may further contain an arbitrary additive. For example, a dissolution stabilizer may be added to the ink in order to stabilize the dissolved state of the ink component in the solvent. Further, a surfactant may be added to the ink in order to increase the dispersion stability of particles (for example, pigment particles described later) in the ink.

  In the ink according to the present embodiment, a plurality of pigment particles each covered with a resin are dispersed. Since the pigment particles are covered with the resin, the dispersion stability of the pigment particles in the ink is improved. Hereinafter, an aggregate of a plurality of pigment particles dispersed in the ink is referred to as a pigment dispersion. In the ink according to this embodiment, the pigment dispersion corresponds to the colorant.

  In the ink set according to this embodiment, the acid value of the resin covering the pigment particles is 100 mgKOH / g or more and 180 mgKOH / g or less, and the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less. And the second ink. The method for measuring the acid value of the resin covering the pigment particles is a method according to “JIS (Japanese Industrial Standard) K0070-1992”. The acid value of the resin covering the pigment particles can be adjusted, for example, by changing the amount of monomer used when synthesizing the resin. For example, when a monomer having an acidic functional group (for example, carboxyl group) (for example, acrylic acid or methacrylic acid) is used for the synthesis of the resin, the amount of the monomer having an acidic functional group (and thus the whole material) The acid value of the resin can be increased by increasing the blending ratio). The first ink and the second ink may have the same component or different components with respect to the components other than the resin that covers the pigment particles. When the ink set includes a plurality of types of first inks, the first inks are resins having different acid values as long as the acid value of the resin covering the pigment particles is 100 mgKOH / g or more and 180 mgKOH / g or less. May be contained as a component of the pigment dispersion, or may have different compositions. When the ink set includes a plurality of types of second inks, the second inks are resins having different acid values as long as the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less. May be contained as a component of the pigment dispersion, or may have different compositions.

  In order to improve ink wiping properties, it is preferable that the ink (particularly, the first ink and the second ink) contain N-methyldiethanolamine. N-methyldiethanolamine is a compound having a relatively high boiling point among organic amine compounds. For this reason, by including N-methyldiethanolamine in the ink, it is possible to improve the wiping property of the ink attached to the recording head.

  Hereinafter, the solvent, the colorant, and the surfactant contained in the ink (first ink and second ink) according to the present embodiment will be described in order. However, unnecessary components (for example, surfactants) may be omitted depending on the use of the ink. Note that the evaluation result (a value indicating the shape or physical property) of the powder (more specifically, pigment particles or the like) is the number average of the values measured for a considerable number of particles unless otherwise specified. Further, the particle diameter of the powder is the equivalent-circle diameter of the primary particles (the diameter of a circle having the same area as the projected area of the particles) unless otherwise specified. Hereinafter, a compound and its derivatives 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”.

[solvent]
As the solvent contained in the ink, it is preferable to use a liquid containing water, and it is more preferable to use a mixed liquid containing water, glycerin and / or glycol, and alcohol and / or glycol ether. By containing glycerin and / or glycol in the ink, drying of the ink can be suppressed. Further, by including alcohol and / or glycol ether in the ink, it is possible to improve the permeability of the ink to the recording sheet. Preferable examples of the glycol ether include diethylene glycol monoethyl ether, triethylene glycol mononormal butyl ether, triethylene glycol monoisobutyl ether, triethylene glycol monoisopropyl ether, or diethylene glycol mononormal butyl ether. In order to prepare an ink having an appropriate viscosity, the amount of water in the ink is preferably 20% by mass to 70% by mass with respect to the total mass of the ink.

[Colorant]
In the ink according to this embodiment, a pigment dispersion is used as a colorant. The pigment dispersion includes a plurality of pigment particles. Each of the plurality of pigment particles is covered with a resin. Since the pigment particles are covered with the resin, aggregation of the pigment particles is suppressed. In addition to the resin adhering to the pigment particles (hereinafter referred to as coating resin), the ink not including the pigment particles (hereinafter referred to as dispersion resin) is present in the ink. Also good. In the ink, pigment particles that are not covered with the resin may be present in addition to the pigment particles that are covered with the resin. In order to suitably suppress aggregation of pigment particles, the amount of resin used in the pigment dispersion (total amount of coating resin and dispersion resin) is 15 parts by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the pigment. Preferably there is. In order to suppress mist contamination of the recording head, the ratio of the coating resin in the resin in the ink is preferably 95% by mass or more and less than 100% by mass.

(Pigment particles)
The pigment particles are substantially composed of a pigment. In order to improve the color density, hue, or stability of the ink, the volume median diameter (D ₅₀ ) of the pigment particles is preferably 30 nm or more and 200 nm or less. In order to improve the image density while suppressing the offset of the image formed by the ink, the amount of the colorant in the ink is preferably 4% by mass or more and 8% by mass or less with respect to the total mass of the ink.

  As the pigment constituting the pigment particles, for example, the following pigments 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) can be preferably used. Examples of the orange pigment include C.I. I. Pigment orange (34, 36, 43, 61, 63, or 71) can be preferably used. Examples of red pigments include C.I. I. Pigment Red (122 or 202) can be preferably used. Examples of the blue pigment include C.I. I. Pigment Blue (15: 3) can be preferably used. Examples of purple pigments include C.I. I. Pigment violet (19, 23, or 33) can be preferably used. Examples of the black pigment include C.I. I. Pigment black (7) can be preferably used. One pigment may be used alone, or two or more pigments may be mixed and used. In the ink, pigment particles of different colors may exist.

(resin)
In the first ink according to this embodiment, the acid value of the resin covering the pigment particles is 100 mgKOH / g or more and 180 mgKOH / g or less. In the second ink according to this embodiment, the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less. In order to synthesize a resin having such an acid value, at least one of the resins covering the pigment particles is composed of one or more repeating units derived from a styrene monomer and one or more repeating units derived from an acrylic acid monomer. A copolymer containing a unit (styrene-acrylic acid copolymer) is preferable. Suitable examples of the styrene-acrylic acid copolymer include styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-acrylic acid-alkyl acrylate copolymer, styrene-methacrylic acid-methacrylic. Acid alkyl ester-acrylic acid alkyl ester copolymer, styrene-maleic acid copolymer, styrene-maleic acid-acrylic acid alkyl ester copolymer, styrene-methacrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer Examples thereof include a polymer, a vinyl naphthalene-acrylic acid copolymer, and a vinyl naphthalene-maleic acid copolymer.

  In order to improve the quality of the image formed with the ink, it is preferable that the mass average molecular weight (Mw) of the resin covering the pigment particles is 15000 or more and 40000 or less. The measuring method of a mass average molecular weight (Mw) is the same method as the Example mentioned later, or its alternative method. The molecular weight of the resin can be adjusted by changing the polymerization conditions of the resin (more specifically, the amount of polymerization initiator used, the polymerization temperature, or the polymerization time). For example, if the polymerization temperature (reaction temperature during polymerization) is lowered, the amount of solvent used to dissolve the materials used for resin synthesis is reduced, or the amount of polymerization initiator used is reduced, the molecular weight of the resin is reduced. Can do. In addition, when the usage-amount of a polymerization initiator is reduced too much, a polymerization reaction may stop and a residual monomer (unreacted monomer) may increase.

[Surfactant]
The ink according to this embodiment may contain a surfactant. The surfactant is a compound having a hydrophilic group and a hydrophobic group in the molecule. In order to suppress mist contamination of the recording head, it is preferable to include a nonionic surfactant in the ink.

  When the ink contains a nonionic surfactant, the nonionic surfactant is selected from methyl (meth) acrylate and (meth) acrylic acid in order to improve the image density while suppressing the offset of the image formed by the ink. (Meth) acrylic acid alkyl ester having a hydrophilic group derived from one or more monomers selected from the group consisting of ethyl and polyethylene glycol di (meth) acrylate and an alkyl group having 12 to 20 carbon atoms ( More specifically, it preferably has a hydrophobic group derived from one or more monomers selected from the group consisting of dodecyl acrylate or octadecyl acrylate) and benzyl (meth) acrylate, and a nonionic interface The activators are polyethylene glycol diacrylate (PEGA), methyl methacrylate (MMA) and And particularly preferably a copolymer of benzyl acrylic acid (BA). As a suitable example of the nonionic surfactant contained in the ink, a nonionic surfactant ("Surfinol (registered trademark) 465" manufactured by Nissin Chemical Industry Co., Ltd., component: ethylene oxide adduct of acetylenic dialcohol) is used. Or a nonionic surfactant (“Olfin (registered trademark) E1010” manufactured by Nissin Chemical Industry Co., Ltd., component: acetylene glycol compound). In order to improve the image density while suppressing the offset of the image formed by the ink, the amount of the nonionic surfactant is 0.05% by mass or more and 2.0% by mass or less with respect to the total mass of the ink. Preferably there is.

[Ink production method]
Hereinafter, an example of a method for producing the ink according to the present embodiment having the above-described configuration will be described.

First, a pigment dispersion is prepared. Specifically, a material (for example, styrene, (meth) acrylic acid, and (meth) acrylic acid alkyl ester) for synthesizing a resin in a liquid (for example, a mixed liquid of isopropyl alcohol and methyl ethyl ketone), a polymerization initiator, And heating and refluxing at a predetermined temperature (70 ° C.) to synthesize a resin (for example, a styrene-acrylic acid resin). Subsequently, the obtained resin, the pigment, and the aqueous medium (for example, a mixed solution of alcohol and water) are kneaded using a media type dispersing machine to obtain a pigment dispersion containing a large number of pigment particles. The volume median diameter (D ₅₀ ) of the pigment particles contained in the pigment dispersion is preferably 70 nm or more and 130 nm or less. The measuring method of the volume median diameter (D ₅₀ ) is the same method as in the examples described later or an alternative method thereof. By changing the particle diameter (for example, bead diameter) of the media used in the media type disperser, the degree of pigment particle dispersion, the amount of free resin (dispersion resin), 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.

  Next, the obtained pigment dispersion is mixed with other ink components. Specifically, a pigment dispersion, a solvent (for example, a mixture of glycol ether, glycerin, alcohol, and 2-pyrrolidone), a nonionic surfactant (for example, an acrylic acid oligomer), N-methyldiethanolamine, ions Mix with exchange water. Then, the obtained liquid mixture is filtered as needed. As a result, ink is obtained.

[Inkjet recording apparatus]
The ink set according to the present embodiment can be suitably used in, for example, an ink jet recording apparatus including a long ink jet recording head (hereinafter referred to as a line head). Hereinafter, an example of an ink jet recording apparatus (printer 100) including a line head will be described with reference to FIG.

  The printer 100 forms an image by ejecting ink on a recording sheet based on, for example, image data received from an external computer and printing conditions (more specifically, whether or not double-sided printing is performed). The printer 100 is a color printer, for example.

  The printer 100 includes a paper feed cassette 101. In the paper feed cassette 101, paper P (for example, plain paper) is set as a recording sheet. A paper feeding device 102 is provided in the paper feeding cassette 101. The sheet feeding device 102 includes, for example, a roller 102a that is driven by a motor, and a roller 102b that is driven by pressure contact with the roller 102a. The paper feeding device 102 separates the paper P set in the paper feeding cassette 101 one by one and sends it to the transport unit 10.

  The transport unit 10 includes rollers 10a and 10b and an endless transport belt 10c. In the printer 100, the conveyance belt 10c constitutes a conveyance path. The conveyor belt 10c is stretched by being wound around rollers 10a and 10b that are separated from each other. The conveying belt 10c is provided with a number of suction vents (not shown). The conveyor belt 10c rotates according to the rotation of the rollers 10a and 10b located at both ends. In the present embodiment, of the rollers 10a and 10b, only the roller 10a on the downstream side (X2 side) in the transport direction is driven. The roller 10a is provided with an encoder 10d. The encoder 10d outputs a pulse train corresponding to the rotational displacement amount of the rotating shaft of the roller 10a.

  The transport unit 10 further includes a negative pressure generator 10e inside the transport belt 10c. The negative pressure generating unit 10e has a decompression case that houses a device that generates a negative pressure, such as a suction fan. The decompression case partitions the periphery of the suction fan so that the negative pressure generated by the suction fan acts on the transport belt 10c efficiently. By generating a negative pressure inside the conveyance belt 10c (under the conveyance path) by the negative pressure generating unit 10e, the paper on the conveyance belt 10c (on the conveyance path) is passed through a vent hole (not shown) of the conveyance belt 10c. P can be aspirated. The transport unit 10 transports the paper P on the transport belt 10c while sucking it with the negative pressure under the transport belt 10c. The paper P adsorbed on the transport belt 10c is transported from upstream (X1 side) to downstream (X2 side) in the transport direction of the paper P.

  An image forming unit 200 is provided above the transport unit 10 (Z1 side). The image forming unit 200 includes line heads 20a, 20b, 20c, and 20d. The image forming unit 200 includes a head cleaning device (not shown) that cleans the nozzle surfaces (ink ejection surfaces) of the line heads by wiping. The head cleaning device is configured to wipe off ink adhering to the nozzle surface of each line head, for example, with a cleaning roller that has absorbed the cleaning liquid. In the printer 100, when the transport unit 10 is transporting the paper P, ink is ejected from each of the line heads 20a, 20b, 20c, and 20d toward the paper P, and one side (recording surface) of the paper P is formed by the ink. An image is formed (recorded). Thereafter, each line head is cleaned by a head cleaning device. Further, the paper P after image formation is transported to the discharge device 103 by the transport unit 10 and discharged to the paper discharge tray 104 by the discharge device 103. The discharge device 103 is provided in the vicinity of the downstream end (X2 side end) of the transport unit 10. The paper discharge tray 104 is provided on the downstream side of the discharge device 103. The discharge device 103 includes, for example, a roller 103a that is driven by a motor, and a roller 103b that is pressed against and driven by the roller 103a. The discharged paper P is stacked on the paper discharge tray 104.

  Hereinafter, an example of the image forming unit 200 will be described mainly with reference to FIG. As shown in FIG. 2, the line heads 20a, 20b, 20c, and 20d are arranged in this order from the upstream (X1 side) to the downstream (X2 side) in the transport direction of the paper P. In this order, the ink is discharged from the line head. Discharged. Hereinafter, when it is not necessary to distinguish the line heads 20a, 20b, 20c, and 20d (when the common properties are described), each of the line heads 20a, 20b, 20c, and 20d is referred to as a line head 20.

  Since the image forming unit 200 includes the four line heads 20, it is possible to eject four different colors of ink. The line heads 20a, 20b, 20c, and 20d are filled with, for example, Y (yellow) ink, M (magenta) ink, C (cyan) ink, and Bk (black) ink. By ejecting these four colors of ink, a full color image can be formed (recorded) on the paper P. The color of the ink can be adjusted by changing the type of pigment contained in the ink.

  In the example of FIG. 2, each line head 20 has a plurality of discharge units 30 (nozzles) arranged in the Y direction. Each line head 20 ejects ink from each ejection unit 30 in accordance with the image signal. The ink ejection method may be a piezo method or a thermal ink jet method. Each line head 20 extends in a direction (Y direction) orthogonal to the transport direction (X direction) of the paper P. For example, 166 discharge units 30 are formed in each line head 20 (one head), and 664 discharge units 30 are formed in total (a total of four heads). The pitch between the discharge units 30 in each line head 20 is, for example, 150 dpi. Further, the dot density of the entire line head 20 (four heads) is, for example, 600 dpi.

  The example of the printer (printer 100) that can use the ink set according to the present embodiment has been described above. However, the ink according to the present embodiment can be used in an ink jet recording apparatus other than the above apparatus. For example, the ink set according to the present embodiment may be used in an ink jet recording apparatus that does not include a line head (more specifically, an ink jet recording apparatus that includes a serial head). The ink jet recording apparatus may include an electronic control device (for example, a CPU and a memory or other devices), an input unit (for example, a keyboard, a mouse, or a touch panel), and a communication device.

  Next, the ink jet recording apparatus according to this embodiment will be described. The ink jet recording apparatus according to the present embodiment has the following configurations (1) to (3).

(1) An ink jet recording apparatus includes a plurality of recording heads that discharge ink, and a transport unit that transports a recording sheet on the transport path while suctioning from below the transport path with negative pressure. The plurality of recording heads include a first recording head positioned at the most upstream position, a second recording head positioned at the most downstream position, and a position between the first recording head and the second recording head in the conveyance direction of the recording sheet. And other recording heads. For example, in the printer 100 shown in FIG. 1 and FIG. 2, the line head 20a corresponds to the first recording head, the line head 20d corresponds to the second recording head, and the line heads 20b and 20c respectively perform the other recording. Corresponds to the head. Further, the paper P corresponds to the recording sheet, the transport belt 10c corresponds to the transport path, and the transport unit 10 corresponds to the transport unit.

(2) The transport unit is configured to suck the recording sheet with a stronger negative pressure below each of the first recording head and the second recording head than below the other recording heads. The negative pressure is a pressure lower than the atmospheric pressure, and is represented by the expression “negative pressure = atmospheric pressure−absolute pressure”. The negative pressure in configuration (2) (hereinafter referred to as suction negative pressure) corresponds to the pressure (absolute value of the gauge pressure) measured between the recording head and the conveyance path (specifically, in the immediate vicinity of the recording head). . For example, in the printer 100 shown in FIGS. 1 and 2, the negative pressure generation unit 10 e is located below each of the line heads 20 a and 20 d (Z2 side) rather than below each of the line heads 20 b and 20 c (Z2 side). The recording sheet may be sucked with a stronger suction negative pressure. The negative pressure generator 10e may have a plurality of suction fans. A suction fan may be provided for each recording head. The strength of the suction negative pressure can be adjusted, for example, by changing the form of the decompression case in the negative pressure generating unit 10e or the rotation speed of the suction fan.

(3) A plurality of pigment particles each covered with a resin are dispersed in the ink ejected by each of the plurality of recording heads. Each of the first recording head and the second recording head is configured to eject a first ink whose acid value of the resin covering the pigment particles is 100 mgKOH / g or more and 180 mgKOH / g or less. The other recording head is configured to eject a second ink in which the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less.

  In order to form a high-quality image using the ink set according to the present embodiment, a recording sheet is formed by the inkjet recording method according to the present embodiment using the inkjet recording apparatus having configurations (1) to (3). It is preferable to perform recording. The ink jet recording method according to this embodiment includes the following first step, second step, and third step. In the first step, recording is performed on the recording sheet by discharging the first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less of the resin covering the pigment particles by the first recording head. In the second step, after the first step, recording is performed on the recording sheet by discharging the second ink in which the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less by another recording head. Do. In the third step, after the second step (when there are a plurality of other recording heads, after the ejection of ink by all of the other recording heads is finished), the second recording head uses a resin that covers the pigment particles. Recording is performed on a recording sheet by discharging a first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less. The first ink used in the first step and the first ink used in the third step have different acid values if the acid value of the resin covering the pigment particles is 100 mgKOH / g or more and 180 mgKOH / g or less. You may have, and you may have a mutually different composition. Further, when there are a plurality of other recording heads, the second ink discharged from these other recording heads has different acid values if the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less. May have different compositions from each other.

  For example, in the printer 100 shown in FIGS. 1 and 2, the line head 20a (first recording head), the line head 20b (other recording head), the line head 20c (other recording head), and the line head 20d (second recording head). Recording head) ejects ink in this order. Further, in the printer 100 having the configuration (2), the negative pressure generation unit 10e is located below each of the line heads 20a and 20d (Z2 side) rather than below each of the line heads 20b and 20c (Z2 side). The recording sheet is sucked with a stronger negative suction pressure. As a result, the paper P is attracted to the transport belt 10c even when only a part (the front end portion or the rear end portion) of the paper P is placed on the transport belt 10c (not sucked) below the line head 20a or 20d. It becomes possible. However, it is considered that a strong wind is generated below the line head 20a or 20d to increase the suction negative pressure and the ink is easily dried. When the ink mist adhering to the nozzle surface of the recording head dries and aggregates, it may not be wiped off. Also, the ink mist adhered to the nozzle surface may damage the nozzle.

  In order to suppress mist contamination of the recording head, it is effective to improve the pigment redispersibility of the ink. Ink excellent in pigment redispersibility is once dried, and even if pigment particles in the ink are deposited, the water is supplied again so that the pigment particles are easily redispersed in water. Further, ink excellent in pigment redispersibility is easy to wipe off by wiping. By removing the pigment particles from the nozzle surface by wiping, mist contamination of the recording head is suppressed. The inventor has found that the redispersibility of the ink can be improved by setting the acid value of the resin covering the pigment particles in the ink to 100 mgKOH / g or more and 180 mgKOH / g or less. In the ink set according to this embodiment, the first ink has high pigment redispersibility.

  However, when both inks ejected from adjacent recording heads have high pigment redispersibility, color bleeding tends to occur in the formed image. For this reason, in order to form a high-quality image, it is preferable to use an ink having a relatively low pigment redispersibility together with the first ink. In the second ink included in the ink set according to the present embodiment, the acid value of the resin covering the pigment particles is 60 mgKOH / g or more and 90 mgKOH / g or less. The inventors have found that such a second ink has moderate pigment redispersibility. For example, when one recording head of adjacent recording heads discharges ink whose acid value of the resin covering the pigment particles is 100 mgKOH / g or more, the other recording head is made of the resin covering the pigment particles. By discharging ink having an acid value of 90 mgKOH / g or less, color bleeding can be accurately suppressed.

  As described above, by using the ink set according to this embodiment, it is possible to form a high-quality image (for example, an image without color bleed) while suppressing mist contamination of the recording head. .

  In order to form a high-quality image while suppressing mist contamination of the recording head, in the inkjet recording method according to this embodiment, the suction negative pressure below each of the first recording head and the second recording head is 400 Pa or more. The suction negative pressure below other recording heads is preferably 300 Pa or less.

  The ink jet recording method according to this embodiment is effective for increasing the recording speed (throughput). For example, the ink jet recording method according to the present embodiment is effective for setting the recording speed for a recording sheet to 100 sheets / min or more. In addition, when the inkjet recording apparatus further includes a discharge unit (for example, the discharge apparatus 103 and the discharge tray 104 shown in FIG. 1) from which the recorded recording sheet is discharged, the inkjet recording method according to the present embodiment. Is the time from when the first ink ejected from the first recording head adheres to the recording sheet until the portion of the recording sheet to which the first ink has adhered contacts the discharge portion (for example, the roller 103a or 103b) ( Hereinafter, it is effective to set the discharge-discharge time to within 1 second.

  Examples of the present invention will be described below. Specifically, the ink set according to the example and the ink set according to the reference example are evaluated according to the following methods, respectively, and the ink set according to the example and the ink set according to the reference example are compared.

[Evaluation method]
(Image density)
As an evaluation machine, an inkjet recording apparatus (prototype evaluation machine manufactured by Kyocera Document Solutions Co., Ltd.) provided with four recording heads (each line head) was used. A sample (ink) was filled in the recording head of the evaluator, and the evaluator was adjusted so that the amount of ink ejected from the recording head was 11 pL per ink drop. Further, the suction negative pressure below each of the first recording head (the most upstream) and the second recording head (the most downstream) is 420 Pa, and other recording heads (between the first recording head and the second recording head). The evaluator was adjusted so that the suction negative pressure below each of the two recording heads positioned at 280 Pa was 280 Pa.

Using the above evaluation machine, on a recording sheet (“C ² ” manufactured by Fuji Xerox Co., Ltd., A4 size plain paper) under the condition of a temperature of 25 ° C. and a humidity of 60% RH at a recording speed of 150 sheets / minute. A solid image having a size of 10 cm × 10 cm was formed. Subsequently, the recording sheet on which the image was formed was allowed to stand for 24 hours in an environment of a temperature of 25 ° C. and a humidity of 60% RH. Then, the image density of 10 places in the same image was measured using the reflection densitometer (Sakata Inx Engineering Co., Ltd. sales "RD-19"). The arithmetic average value of 10 image densities measured was used as the evaluation value.

(First wipeability)
As an evaluation machine, an inkjet recording apparatus (prototype evaluation machine manufactured by Kyocera Document Solutions Co., Ltd.) having a cleaned recording head (line head) was used. The suction negative pressure below the used recording head was adjusted to 400 Pa. A sample (ink) 0.1 g was placed on the tip of the wiping blade of the evaluator and allowed to stand at room temperature (about 25 ° C.) and normal humidity (about 60% RH) for 10 minutes. Subsequently, wiping (outward wiping) was performed under the condition of a cleaning solution of 0.3 mL. As a result, the sample on the wiping blade adhered to the nozzle surface of the recording head, and a thin ink coating film was formed on the nozzle surface of the recording head. Subsequently, the recording head on which the ink coating film was formed was allowed to stand at a temperature of 40 ° C. and a humidity of 15% RH for a predetermined time (one day or three days). Subsequently, wiping (return wiping) was performed under the condition of a cleaning solution of 0.3 mL. Thereafter, the nozzle surface of the recording head is visually checked, and if the ink adhering to the nozzle surface of the recording head is wiped off, the wiping property of the sample (ink) is evaluated as “◯ (good)”. If the ink adhering to the nozzle surface was not wiped off, the wipeability of the sample (ink) was evaluated as “x (not good)”. For each sample, evaluation results were obtained for the case where the predetermined time (standing time) was set to 1 day and 3 days.

(Second wiping property)
As an evaluation machine, an inkjet recording apparatus (prototype evaluation machine manufactured by Kyocera Document Solutions Co., Ltd.) equipped with a recording head (line head) was used. A sample (ink) was filled in the recording head of the evaluator, and the evaluator was adjusted so that the amount of ink ejected from the recording head was 11 pL per ink drop. Further, the suction negative pressure below the used recording head was adjusted to a predetermined strength (200 Pa, 300 Pa, 400 Pa, or 500 Pa).

Using the above evaluation machine, on a recording sheet (“C ² ” manufactured by Fuji Xerox Co., Ltd., A4 size plain paper) under the condition of a temperature of 25 ° C. and a humidity of 60% RH at a recording speed of 150 sheets / minute. After continuous printing of 5000 sheets, wiping was performed under the condition of a cleaning solution of 0.3 mL. Then, the dirt on the nozzle surface of the recording head is visually observed, and if the ink adhering to the nozzle surface of the recording head is wiped off, the wiping property of the sample (ink) is evaluated as “good”, and the recording head If the ink adhering to the nozzle surface was not wiped off, the wipeability of the sample (ink) was evaluated as “x (not good)”. For each sample, evaluation results were obtained for each of the above-described predetermined strength (strength of suction negative pressure) of 200 Pa, 300 Pa, 400 Pa, and 500 Pa. It was.

(Third wipeability, color bleed resistance)
As an evaluation machine, an inkjet recording apparatus (prototype evaluation machine manufactured by Kyocera Document Solutions Co., Ltd.) provided with four recording heads (each line head) was used. The recording head of the evaluator was filled with a sample (ink) (see Table 6 for details), and the evaluator was adjusted so that the amount of ink ejected from the recording head was 11 pL per ink drop. Further, the suction negative pressure below each of the first recording head (the most upstream) and the second recording head (the most downstream) is 420 Pa, and other recording heads (between the first recording head and the second recording head). The evaluator was adjusted so that the suction negative pressure below each of the two recording heads positioned at 280 Pa was 280 Pa.

Using the above-mentioned evaluation machine, the recording sheet (“C ² ” manufactured by Fuji Xerox Co., Ltd., A4 size plain paper) under the conditions of a temperature of 10 ° C. and a humidity of 80% RH under a recording speed of 150 sheets / min. After continuous printing of 5000 solid images of 10 cm × 10 cm and a printing rate of 100% (total of 100% for each color 25%), wiping was performed under the condition of 0.3 mL of the cleaning solution. Then, by visually checking the nozzle surface of the recording head, if the ink adhering to the nozzle surfaces of all the recording heads is wiped off, the wiping property of the sample (ink) is evaluated as “good (good)”, and at least If the ink adhering to the nozzle surface of one recording head was not wiped off, the wiping property of the sample (ink) was evaluated as “× (not good)”. In addition, when the formed image is visually observed and the ink adjacent to each other is color-separated, the color bleed resistance of the sample (ink) is evaluated as “◯ (good)”, and the ink adjacent between the dots is evaluated. If mixed, the color bleed resistance of the sample (ink) was evaluated as “× (not good)”.

  Hereinafter, the production methods and evaluation results of the ink according to the example and the ink according to the reference example will be described. Note that the evaluation results (values indicating shape, physical properties, etc.) regarding the powder containing a plurality of particles (more specifically, pigment particles, etc.) are the values measured for a considerable number of particles unless otherwise specified. Number average. 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. Further, the particle diameter of the powder is a circle equivalent diameter of a particle (a diameter of a circle having the same area as the projected area of the particle) unless otherwise specified. Further, the measured value of the acid value is a value measured according to “JIS (Japanese Industrial Standard) K0070-1992” unless otherwise specified. Further, the measurement method of Mw (mass average molecular weight) is as follows unless otherwise specified.

<Measurement conditions of Mw>
As a measuring device, a GPC (gel permeation chromatography) device (“HLC-8020GPC” manufactured by Tosoh Corporation) was used. A sample (for example, resin) was injected into a measuring apparatus, and gel filtration chromatography was performed under the following measurement conditions to obtain a chromatogram. Based on the obtained chromatogram and a calibration curve prepared in advance, the Mw (mass average molecular weight) of the sample was determined. In addition, a calibration curve is F-40, F-20, F-4, F-1, A-5000, A-2500, A-1000, and n-propylbenzene from TSKgel standard polystyrene made from Tosoh Corporation. Eight types were selected and created.
(Measurement condition)
Column: “TSKgel SuperMultipore HZ-H” (4.6 mm ID × 15 cm semi-micro column) manufactured by Tosoh Corporation
・ Number of columns: 3 ・ Eluent: Tetrahydrofuran ・ Flow rate: 0.35 mL / min ・ Sample injection volume: 10 μL
・ Measurement temperature: 40 ℃
・ Detector: IR (infrared absorption analysis) detector

Evaluation 1
In Evaluation 1, the ink used in the ink set according to the example or the reference example was evaluated. Tables 1 to 3 show ink compositions according to Evaluation 1.

  The “pigment dispersion” in Table 1 is a pigment dispersion having the composition shown in Table 2. The “acrylic acid oligomer” in Table 1 is a copolymer of polyethylene glycol diacrylate (PEGA), methyl methacrylate (MMA) and benzyl acrylate (BA) (mass ratio of PEGA / MMA / BA: 50 / 20/30, molecular weight: 3000, solubility in water: yes). “Resin” in Table 2 is a resin that covers the pigment particles, and is one of resins A to G shown in Table 3.

  In the ink A-1 according to Evaluation 1, as a “pigment” in Table 2, a cyan pigment (CI Pigment Blue 15: 3) was used. Moreover, as “resin” in Table 2, a styrene-acrylic acid resin (resin A shown in Table 3) synthesized by the method shown below was used.

[Method for Producing Ink A-1]
(Preparation of pigment dispersion)
A stirrer, a nitrogen inlet tube, a condenser (stirrer), and a dropping funnel were set in a four-necked flask with a capacity of 1000 mL. Subsequently, 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone were placed in the flask. The flask contents were heated to reflux at a temperature of 70 ° C. while bubbling with nitrogen gas.

  Also, 40 g of styrene (ST), 10 g of methacrylic acid (MAA), 10 g of butyl acrylate (BA), 40 g of methyl methacrylate (MMA), and 0.4 g of azobisisobutyronitrile (AIBN) are mixed. To obtain a solution. Subsequently, the resulting solution was placed in a dropping funnel. Subsequently, the solution in the dropping funnel was added dropwise over 2 hours to the contents of the flask being heated to reflux. And after completion | finish of dripping, it heated and refluxed for 6 hours about the flask contents.

  Subsequently, methyl ethyl ketone containing 0.2 g of AIBN was dropped into the flask over 15 minutes. After completion of the dropping, the flask contents were further heated to reflux for 5 hours. As a result, a styrene-acrylic acid resin (resin A shown in Table 3) having a mass average molecular weight (Mw) of 20000 and an acid value of 40 mgKOH / g was obtained.

  15 parts by mass of cyan pigment (CI Pigment Blue 15: 3) in a 0.6 L vessel of a media-type disperser ("DYNO (registered trademark) -MILL" manufactured by Willy et Bacofen (WAB)) Then, 6 parts by mass of resin (resin A synthesized by the above-described procedure), 0.5 part by mass of 1,2-octanediol, and 78.5 parts by mass of water (ion-exchanged water) were added. Further, sodium hydroxide (NaOH) in an amount necessary for neutralizing the resin A was added into the vessel.

  In the neutralization of the resin A, an aqueous NaOH solution was added so that the pH of the vessel contents was 8. 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 calculated based on the mass of Resin A. The mass of water to be added to the vessel was calculated based on the sum of the mass of water contained in the NaOH aqueous solution and the mass of water generated by the neutralization reaction.

Subsequently, a medium (zirconia beads having a diameter of 0.5 mm) was filled in the vessel of the disperser so as to be 70% by volume with respect to the vessel capacity. Subsequently, the vessel contents were kneaded using a disperser filled with media at a temperature of 10 ° C. and a peripheral speed of 8 m / sec. As a result, a pigment dispersion A containing a large number of pigment particles was obtained. The volume median diameter (D ₅₀ ) of the pigment particles contained in the pigment dispersion A was 100 nm. The volume median diameter (D ₅₀ ) of the pigment particles contained in the pigment dispersion A is measured using a dynamic light scattering particle size distribution device (“Zetasizer Nano” manufactured by Sysmex Corporation) as a measuring device. A liquid obtained by diluting dispersion A 300 times with ion-exchanged water was used as a measurement target.

(Mixing of pigment dispersion with other components)
40 parts by mass of a pigment dispersion (pigment dispersion A prepared by the above procedure), 3 parts by mass of diethylene glycol monoethyl ether, 3 parts by mass of triethylene glycol mononormal butyl ether, 5 parts by mass of 2-pyrrolidone, and a nonionic interface 0.5 parts by mass of an activator (acrylic acid-based oligomer shown in Table 1), 0.4 parts by mass of N-methyldiethanolamine, 0.7 parts by mass of 1,2-octanediol, 10 parts by mass of glycerin, Stir uniformly 10 parts by mass of 1,3-propanediol and 27.4 parts by mass of ion-exchanged water using a stirrer (“Three-One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.) at a rotational speed of 400 rpm. Mixed. Subsequently, the obtained mixed solution was filtered using a filter having a pore diameter of 5 μm to remove foreign matters and coarse particles in the mixed solution. As a result, ink A-1 was obtained.

[Method for Producing Inks A-2 to A-7]
The production methods of the inks A-2 to A-7 are respectively “resins” in Table 2, and instead of the resin A (styrene-acrylic acid resin) shown in Table 3, the resins B to B shown in Table 3 are used. Except for using G (respectively styrene-acrylic acid resin), the production method was the same as that for ink A-1 (see Table 4 to be described later in detail). The synthesis method of the resins B to G is such that the blending ratio of the materials (monomers) used for the synthesis of the resin and the polymerization conditions (more specifically, so that the resin molecular weight and acid value are as shown in Table 3). The polymerization method was the same as that for Resin A except that the polymerization temperature, polymerization time, and amount of initiator used were adjusted.

[Evaluation results]
Table 4 shows the evaluation results of the image density and the first wiping property for each of the inks A-1 to A-7. Table 5 shows the evaluation results of the second wiping property for each of the inks A-3 and A-5.

  In each of Inks A-4 to A-6, the acid value of the resin covering the pigment particles was 100 mgKOH / g or more and 180 mgKOH / g or less. On the other hand, in each of inks A-1, A-2, A-3, and A-7, the acid value of the resin covering the pigment particles was less than 100 mgKOH / g and more than 180 mgKOH / g.

  As shown in Table 4, each of the inks A-4 to A-6 was excellent in wiping property not only after being allowed to stand for 1 day but also after being allowed to stand for 4 days in the evaluation of the first wiping property. . On the other hand, each of the inks A-2 and A-3 was excellent in wiping property after standing for 1 day in the evaluation of the first wiping property, but had sufficient wiping property after standing for 4 days. I did not. Also, any of the inks A-1 to A-7 could form an image having a high image density (specifically, an image density of 1.10 or higher).

  As shown in Table 5, the ink A-5 was excellent in wiping property when the suction negative pressure was any of 200 Pa, 300 Pa, 400 Pa, and 500 Pa in the second wiping property evaluation. On the other hand, in the evaluation of the second wiping property, the ink A-3 did not have sufficient wiping property when the suction negative pressure was set to 400 Pa or 500 Pa.

Evaluation 2
[Method for producing ink sets A to F]
Ink sets A to F according to Evaluation 2 are shown in Table 6. In Table 6, the unit of acid value is mgKOH / g. Regarding “color” in Table 6, Y represents a yellow pigment, M represents a magenta pigment, C represents a cyan pigment, and Bk represents a black pigment. Further, in Table 6, the four recording heads of the evaluation machine are described as a first head, another head 1, another head 2, and a second head in order from the recording head arranged upstream in the conveyance direction of the recording sheet. doing. The manufacturing method of each ink constituting the ink sets A to F was the same as the manufacturing method of the ink A-1, except that the resins and pigments shown in Table 6 were used. In each ink constituting the ink sets A to F, any of the resins B to F shown in Table 3 is used as the “resin” in Table 2 (see Table 6 for details). Examples of the “pigment” include a cyan pigment (CI Pigment Blue 15: 3), a yellow pigment (CI Pigment Yellow 74), a magenta pigment (CI Pigment Violet 19), and a black pigment (C.I. I. Pigment Black 7) was used (for details, see Table 6).

[Evaluation results]
Table 7 shows the results of evaluation of the third wiping property and color bleed resistance when an image was formed using each of the ink sets A to F shown in Table 6. In Table 7, “between colors 1-2” indicates the dot distance between the ink from the first head and the ink from the other head 1, and “between 2-3 colors” refers to the ink from the other head 1 and the other The distance between dots with ink by the head 2 is indicated, and “between 3-4 colors” indicates the distance between dots with ink by the other head 2 and ink by the second head.

  As shown in Table 6, ink sets D, E, and F are used in the ink jet recording method according to the present embodiment described above. On the other hand, in the ink set A, each of the other heads 1 and 2 uses ink containing a resin E having an acid value of 120. In the ink set B, the first head and the second head use ink containing the resin C having an acid value of 80. Further, in the ink set C, ink containing the resin C having an acid value of 80 is used in the second head.

  As shown in Table 7, the ink jet recording methods according to ink sets D to F were excellent in color bleeding resistance as compared with the ink jet recording methods according to each of ink sets A and C. In addition, each of the ink jet recording methods according to ink sets D to F was superior in wiping performance as compared with the ink jet recording methods according to each of ink sets B and C.

  The ink set, the ink jet recording apparatus, and the ink jet recording method according to the present invention are each suitable for use in image formation in a color printer or the like.

DESCRIPTION OF SYMBOLS 10 Conveyance unit 10a, 10b Roller 10c Conveyor belt 10d Encoder 10e Negative pressure generation part 20, 20a-20d Line head 30 Discharge unit 100 Printer 101 Paper cassette 102 Paper feeder 102a, 102b Roller 103 Ejector 103a, 103b Roller 104 Exhaust Paper tray 200 Image forming section P Paper

Claims (8)

An ink set including a plurality of types of ink,
In the ink, a plurality of pigment particles each covered with a resin are dispersed,
As the plurality of types of ink,
A first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less of the resin;
A second ink having an acid value of 60 mgKOH / g or more and 90 mgKOH / g or less of the resin;
Including ink set.   The ink set according to claim 1, wherein each of the first ink and the second ink contains N-methyldiethanolamine.   The resin of each of the first ink and the second ink is a copolymer including one or more repeating units derived from a styrene monomer and one or more repeating units derived from an acrylic acid monomer. The ink set according to claim 1 or 2.   The ink set according to any one of claims 1 to 3, wherein a mass average molecular weight of the resin of each of the first ink and the second ink is 15000 or more and 40000 or less.   Each of the first ink and the second ink is derived from one or more monomers selected from the group consisting of methyl (meth) acrylate, ethyl (meth) acrylate, and polyethylene glycol di (meth) acrylate. A hydrophobic group derived from one or more monomers selected from the group consisting of a hydrophilic group, a (meth) acrylic acid alkyl ester having an alkyl group having 12 to 20 carbon atoms, and benzyl (meth) acrylate The ink set according to any one of claims 1 to 4, further comprising a nonionic surfactant having: An inkjet recording apparatus comprising: a plurality of recording heads each for discharging ink; and a transport unit that transports a recording sheet on the transport path while sucking with a negative pressure from below the transport path,
The plurality of recording heads include a first recording head positioned on the most upstream side in the conveying direction of the recording sheet, a second recording head positioned on the most downstream side, the first recording head, and the second recording head. And other recording heads located between
The transport unit is configured to suck the recording sheet with a stronger negative pressure below each of the first recording head and the second recording head than below the other recording head,
In the ink ejected by each of the plurality of recording heads, a plurality of pigment particles each covered with a resin are dispersed,
The first recording head and the second recording head are each configured to eject a first ink having an acid value of the resin of 100 mgKOH / g or more and 180 mgKOH / g or less,
The other recording head is an ink jet recording apparatus configured to eject a second ink having an acid value of 60 mgKOH / g or more and 90 mgKOH / g or less of the resin. An inkjet recording method for recording on the recording sheet using the inkjet recording apparatus according to claim 6,
A first step of performing recording on the recording sheet by discharging a first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less by the first recording head;
After the first step, the second step of performing recording on the recording sheet by discharging the second ink having an acid value of the resin of 60 mgKOH / g or more and 90 mgKOH / g or less by the other recording head. ,
After the second step, a third step of performing recording on the recording sheet by discharging a first ink having an acid value of 100 mgKOH / g or more and 180 mgKOH / g or less by the second recording head. ,
An ink jet recording method comprising:   The inkjet recording according to claim 7, wherein the negative pressure below each of the first recording head and the second recording head is 400 Pa or more, and the negative pressure below the other recording head is 300 Pa or less. Method.

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