Inkjet recording ink, method for producing inkjet recording ink, inkjet recording ink set, image forming method and image forming apparatus

Abstract

The present invention addresses the problem of providing: an inkjet recording ink having improved re-dispersibility of a white pigment; a method for manufacturing an inkjet recording ink; an inkjet recording ink set; and an image forming method and an image forming device which use the inkjet recording ink.　The inkjet recording ink of the present invention comprises at least a white pigment, a dispersing agent and water, and is characterized in that the dispersing agent includes basic nitrogen.

Description

[Technical Field]

[0001] The present invention relates to an inkjet recording ink, a method for producing an inkjet recording ink, an inkjet recording ink set, an image forming method and an image forming apparatus using the inkjet recording ink, and more particularly to an inkjet recording ink having improved redispersibility of a white pigment. [Background technology]

[0002] Inkjet recording (hereinafter also referred to as "printing") is a recording method in which small droplets of ink for inkjet recording (hereinafter also simply referred to as "ink") are ejected from an inkjet head and landed on the surface of a recording medium such as paper. This method makes it possible to print high-resolution, high-quality images at high speed using a relatively inexpensive device.

[0003] In the inkjet recording method, the recording medium is not limited to white paper, and transparent recording media and colored recording media can also be used. When printing on these recording media, however, it is necessary to sufficiently conceal the transparency and color of the recording medium with ink, and therefore white ink is used.

[0004] However, the white pigment used in white ink has a higher specific gravity than the other components, so it is prone to settling and must be redispersed when used after long-term storage.

[0005] Patent Document 1 discloses a technology relating to a white ink containing a nonionic dispersant with an acid value of 100 mgKOH / g or less and an anionic dispersant with an acid value of 100 mgKOH / g or more. The anionic dispersant with an acid value of 100 mgKOH / g or more has an electrostatic interaction with the cationic fixing agent in the pretreatment liquid applied to the substrate. Therefore, when the white ink and the pretreatment liquid are mixed, the mixture aggregates on the substrate, allowing image formation.

[0006] This white ink is said to be able to suppress aggregation of the white pigment particles even when the content of the white pigment is relatively high, and to reduce accumulation of the white pigment inside the head or on the head surface in the inkjet recording device.

[0007] However, nonionic dispersants with an acid value of 100 mgKOH / g or less do not have sufficient adsorption power to white pigments, and there is room for further improvement in the dispersion stability and redispersibility of white pigments in the white ink.

[0008] Furthermore, Patent Document 2 discloses a technology relating to a white ink containing a short-chain anionic dispersant and a nonionic dispersant, each having a weight-average molecular weight in the range of 1,000 to 30,000. However, even in this technology, there is still room for improvement in the dispersion stability and redispersibility of the white pigment in the white ink. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Chinese Patent Application Publication No. 107406698 [Patent Document 2] Chinese Patent Application Publication No. 107429090 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in view of the above problems and circumstances, and an object of the present invention is to provide an inkjet recording ink having improved redispersibility of a white pigment, a method for manufacturing an inkjet recording ink, an inkjet recording ink set, and an image forming method and image forming apparatus that use the inkjet recording ink. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present inventors have investigated the causes of the above-mentioned problems, and as a result, have found that in an inkjet recording ink containing at least a white pigment, a dispersant, and water, the redispersibility of the white pigment is improved when the dispersant has basic nitrogen, and have arrived at the present invention. That is, the above-mentioned problems of the present invention are solved by the following means.

[0012] 1. An inkjet recording ink containing at least a white pigment, a dispersant, and water, The dispersant has a basic nitrogen. death, the inkjet recording ink further contains an anionic resin, The acid value of the anionic resin is within a range of 50 to 250 mgKOH / g, The weight average molecular weight of the anionic resin is within the range of 5,000 to 30,000. 1. An ink for ink-jet recording.

[0013] 2. The amine value of the dispersant is within the range of 5 to 100 mg KOH / g. 2. The ink for ink-jet recording according to claim 1,

[0014] 3. The content of the dispersant is in the range of 2.5 to 15% by mass relative to the total mass of the white pigment. 3. The ink for ink-jet recording according to item 1 or 2.

[0016] 4 The anionic resin is neutralized with a basic compound. Characterized by From Section 1 No. 3 term Any one of the items up to The ink for ink jet recording according to claim 1.

[0019] 5 The mass ratio of the content of the dispersant to the content of the anionic resin is within the range of 1:6 to 6:1. The first feature is 1 Section to section 4 Item 1. The ink for ink-jet recording according to any one of items 1 to 5.

[0020] 6 The white pigment is a white metal oxide. The first to second items are characterized by the 5 Item 1. The ink for ink-jet recording according to any one of items 1 to 5.

[0021] 7 The white metal oxide is rutile titanium oxide particles. The first feature is 6 Item 1. The ink for ink-jet recording according to item 1.

[0022] 8 The white pigment is at least alumina-treated. The first to second items are characterized by the 7 Item 1. The ink for ink-jet recording according to any one of items 1 to 5.

[0023] 9 The average dispersed particle size of the white pigment is within the range of 150 to 350 nm. The first to second items are characterized by the 8 Item 1. The ink for ink-jet recording according to any one of items 1 to 5.

[0025] 1 0 .No. 1 Section to section 9 A method for producing the inkjet recording ink according to any one of claims 1 to 5, comprising the steps of: preparing a pigment dispersion (I) containing the white pigment, the dispersant, and the water; and mixing the anionic resin with the pigment dispersion (I) to prepare a pigment dispersion (II). 1. A method for producing an ink for ink-jet recording, comprising:

[0026] 1 1 .No. 1 Section to section 9 A method for producing the inkjet recording ink according to any one of claims 1 to 5, comprising the steps of: preparing a pigment dispersion (III) containing the white pigment, the dispersant, the water, and the anionic resin. 1. A method for producing an ink for ink-jet recording, comprising:

[0027] 1 2 .No. 1 Section to section 9 An ink jet recording ink set comprising the ink jet recording ink according to any one of claims 1 to 3 and a treatment liquid, The treatment liquid contains a cationic compound. 1. An ink set for ink jet recording comprising:

[0028] 1 3 .1st 2 Item 1. An image forming method using the ink set for ink jet recording according to item 1, applying the treatment liquid to a recording medium; and applying the inkjet recording ink to the recording medium. An image forming method comprising:

[0029] 1 4 .1st 2 2. An image forming method using the ink set for ink jet recording and color inks according to claim 1, applying the treatment liquid to a recording medium; applying the inkjet recording ink to the recording medium; and applying the color ink to the recording medium. An image forming method comprising:

[0030] 1 5 .1st 2 2. An image forming method using the ink set for ink jet recording and color inks according to claim 1, applying the treatment liquid onto a recording medium; applying the ink for inkjet recording to the area to which the treatment liquid has been applied; and applying the color ink to the area where the ink for inkjet recording has been applied. An image forming method comprising:

[0031] 16 An image forming apparatus using an inkjet recording ink containing at least a white pigment, a dispersant, and water, No. 1 3 Section 1 to 1 5 The image forming method according to any one of claims 1 to 4, An image forming apparatus characterized by: [Effects of the Invention]

[0032] The above-described means of the present invention can provide an inkjet recording ink having improved redispersibility of a white pigment, a method for producing an inkjet recording ink, an inkjet recording ink set, and an image forming method and image forming apparatus using the inkjet recording ink.

[0033] The mechanism by which the effects of the present invention are manifested or the mechanism of action is not clear, but is speculated as follows.

[0034] Many of the white pigments used in white ink have a higher specific gravity than pigments of other colors and are prone to settling. Therefore, when using white ink after storing it for a long period of time, the white pigment in the white ink needs to be redispersed.

[0035] However, if the white ink is stored for a long period of time in a state in which the white pigment has settled, the white pigment particles may aggregate together, and even if a step of redispersing the white pigment is performed, the particle size of the white pigment particles cannot be reduced to the desired size, and the white pigment cannot be sufficiently redispersed. Furthermore, if a white ink in which the white pigment is not sufficiently redispersed is used, the white pigment will accumulate in the inkjet head and clog the nozzles, requiring maintenance of the inkjet head.

[0036] One of the causes of re-aggregation of white pigment particles dispersed during ink preparation is the intermolecular force (van der Waals force) between white pigment particles. The magnitude of the van der Waals force varies depending on the distance between particles; the smaller the distance between particles, the stronger the van der Waals force, making them more susceptible to re-aggregation.

[0037] As mentioned above, many white pigments have a higher specific gravity than pigments of other colors, and therefore are prone to settling during long-term storage. Settling occurs when the density of the dispersed phase (pigment particles in this case) is higher than the density of the dispersion medium (water and organic solvent in this case). Therefore, the greater the specific gravity of the dispersed phase (pigment particles), the more likely settling occurs.

[0038] The settled white pigment particles are deposited, for example, at the bottom of a storage container, and the distance between the white pigment particles becomes smaller than when they were dispersed. Therefore, the van der Waals forces acting between the white pigment particles become stronger than when they were dispersed, making them more susceptible to re-aggregation.

[0039] On the other hand, the surfaces of fine particles generally have an electric charge. For example, the surfaces of metal oxide particles used as white pigments are covered with hydroxyl groups, and these hydroxyl groups are dehydrogenated or hydrogenated depending on the pH value of the ink. Therefore, the surface charge of the metal oxide particles varies depending on the pH value of the ink. The zeta potential is a measurement of this electric charge.

[0040] (Formula 1) -M-OH + OH - → -MO - + H2O (Formula 2) -M-OH + H + → -M-OH2 +

[0041] When the surfaces of white pigment particles are charged, an electrical repulsive force acts between the white pigment particles, making them less likely to aggregate from an electrical standpoint. However, when the white pigment particles have settled, the van der Waals force acting between the white pigment particles is far greater than the electrical repulsive force, making the white pigment particles more likely to aggregate. In order to make settled white pigment particles less likely to aggregate, it is preferable to weaken the van der Waals force between the white pigment particles and to increase the distance between the white pigment particles.

[0042] The ink of the present invention contains a dispersant, and the dispersant has a basic nitrogen. In the present invention, "having a basic nitrogen" means that the nitrogen has basicity, that is, the nitrogen has a proton (H + The mechanism by which nitrogen attracts protons is that two electrons from the unshared electron pair on nitrogen enter the orbitals that make up the N-H bond, forming a coordinate bond and generating an ammonium cation.

[0043] Although details will be described later, dispersants generally have pigment adsorption groups (hydrophilic parts) and resin affinity parts (hydrophobic parts), and are adsorbed to the surfaces of hydrophilic white pigment particles via the pigment adsorption groups.

[0044] Furthermore, when the ink has a pH value greater than 7 and is alkaline, for example, the surfaces of the metal oxide particles are negatively charged as shown in the above formula 1, and the surfaces of the metal oxide particles and the positively charged ammonium cations are electrically attracted to each other, further increasing the degree of adsorption. Also, since the ammonium cations enter between the metal oxide particles, the distance between the metal oxide particles becomes relatively large, and the van der Waals forces acting between the metal oxide particles become relatively weak, which is thought to make it difficult for the metal oxide particles to aggregate.

[0045] Even when the white pigment is not a metal oxide, it is thought that the functional groups located on the surface give the surface of the pigment particles a negative charge, and ammonium cations are adsorbed to the pigment particles, making the pigment particles less likely to aggregate. [Brief explanation of the drawings]

[0046] [Figure 1] 1 is a schematic diagram illustrating an inkjet recording apparatus according to the present invention; [Figure 2] 1 is a bottom view of a carriage according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0047] The inkjet recording ink of the present invention contains at least a white pigment, a dispersant, and water, and is characterized in that the dispersant has a basic nitrogen. This feature is a technical feature common to or corresponding to the following embodiments.

[0048] From the viewpoint of storage stability of the ink and redispersibility of the white pigment, the amine value of the dispersant is preferably within the range of 5 to 100 mgKOH / g.

[0049] From the viewpoints of whiteness, ease of maintenance, and redispersibility of the white pigment, the content of the dispersant is preferably within the range of 2.5 to 15% by mass relative to the total mass of the white pigment.

[0050] From the viewpoint of image quality, it is preferable that the inkjet recording ink further contains an anionic resin.

[0051] From the viewpoint of image quality, it is preferable that the anionic resin is neutralized with a basic compound.

[0052] From the viewpoint of image quality and whiteness, the acid value of the anionic resin is preferably within the range of 50 to 250 mgKOH / g.

[0053] From the viewpoint of ease of maintenance, the weight average molecular weight of the anionic resin is preferably within the range of 5,000 to 30,000.

[0054] From the viewpoint of whiteness and ease of maintenance, the mass ratio of the content of the dispersant to the content of the anionic resin is preferably within the range of 1:6 to 6:1.

[0055] From the viewpoint of redispersibility of the white pigment, the white pigment is preferably a white metal oxide.

[0056] From the viewpoint of whiteness, the white metal oxide is preferably rutile-type titanium oxide particles.

[0057] From the viewpoint of redispersibility of the white pigment, it is preferable that the white pigment is at least treated with alumina.

[0058] From the viewpoint of whiteness, the average dispersed particle size of the white pigment is preferably within the range of 150 to 350 nm.

[0059] The method for producing an inkjet recording ink of the present invention is a method for producing an inkjet recording ink of the present invention, and is characterized by comprising a step of preparing a pigment dispersion (I) containing a white pigment, a dispersant, and water.

[0060] The method for producing the inkjet recording ink of the present invention is a method for producing the inkjet recording ink of the present invention, and is characterized by comprising the steps of preparing a pigment dispersion (I) containing a white pigment, a dispersant, and water, and mixing an anionic resin with the pigment dispersion (I) to prepare a pigment dispersion (II).

[0061] The method for producing an inkjet recording ink of the present invention is a method for producing an inkjet recording ink of the present invention, and is characterized by comprising a step of preparing a pigment dispersion (III) containing a white pigment, a dispersant, water, and an anionic resin.

[0062] The ink set for ink jet recording of the present invention is an ink set for ink jet recording containing the ink for ink jet recording of the present invention and a treatment liquid, and is characterized in that the treatment liquid contains a cationic compound.

[0063] The image forming method of the present invention is an image forming method using the ink set for inkjet recording of the present invention, and is characterized by comprising a step of applying a treatment liquid to a recording medium, and a step of applying inks for inkjet recording to the recording medium.

[0064] The image forming method of the present invention is an image forming method using the ink jet recording ink set and color inks of the present invention, and is characterized by comprising a step of applying a treatment liquid to a recording medium, a step of applying ink jet recording inks to the recording medium, and a step of applying color inks to the recording medium.

[0065] The image forming method of the present invention is an image forming method using the inkjet recording ink set and color inks of the present invention, and is characterized by comprising the steps of applying a treatment liquid onto a recording medium, applying inkjet recording inks to the areas to which the treatment liquid has been applied, and applying color inks to the areas to which the inkjet recording inks have been applied.

[0066] The image forming apparatus of the present invention is an image forming apparatus that uses an ink for ink jet recording containing at least a white pigment, a dispersant, and water, and is characterized by having means for carrying out the image forming method of the present invention.

[0067] The present invention, its components, and embodiments for carrying out the present invention will be described in detail below. In this application, the symbol "to" is used to mean that the numerical values ​​before and after it are included as lower and upper limits.

[0068] <<Outline of inkjet recording ink>> The inkjet recording ink of the present invention (hereinafter also referred to simply as "ink" or "white ink") is an inkjet recording ink containing at least a white pigment, a dispersant, and water, and is characterized in that the dispersant has a basic nitrogen.

[0069] The ink of the present invention has excellent redispersibility of the white pigment, so that even when the white pigment is redispersed and used after long-term storage, the white pigment is less likely to accumulate in the inkjet head, preventing nozzle clogging, i.e., improving ejection stability. Furthermore, because the white pigment is less likely to accumulate in the inkjet head, the frequency of maintenance can be reduced.

[0070] The ink of the present invention is an ink containing a white pigment, but it is believed that the application of the present invention to inks of other colors will also result in excellent redispersibility when using pigments that have a relatively high specific gravity and are prone to settling.

[0071] Generally, white inks are used not only to present a white color but also to conceal the transparency or color of a transparent or colored recording medium when such a recording medium is used. In this case, the color development of the recorded matter can be improved by applying a white ink onto the recording medium and then applying a color ink thereon, or by applying a color ink onto the recording medium and then applying a white ink thereon.

[0072] <Configuration of inkjet recording ink> The ink of the present invention is an ink jet recording ink containing at least a white pigment, a dispersant, and water, and is characterized in that the dispersant has a basic nitrogen. The composition of the ink of the present invention will be described below.

[0073] [1 Dispersant] The ink of the present invention contains a dispersant, and the dispersant is characterized by having a basic nitrogen. By including a dispersant, the white pigment can be dispersed in the ink.

[0074] The dispersion process for dispersing pigments consists of the following three steps: 1) wetting step, 2) crushing step, and 3) dispersion stabilization step. 1) Wetting process It wets (moists) the surface of the pigment aggregates and promotes the penetration of solvents and resins into the voids. 2) Crushing process The pigment agglomerates are broken down into primary particles or similar particles by mechanical force. 3) Dispersion stabilization It prevents re-agglomeration of pigment particles that have been crushed down to primary particles, stabilizing the dispersion state.

[0075] Generally, a "wetting agent" is used to further promote wetting, and a "dispersing agent" is used to further stabilize the dispersed state.

[0076] By adding a wetting agent, the surface tension of the resin solution relative to the pigment can be reduced, the contact angle can be brought closer to zero, and the wettability of the pigment to the resin solution can be improved. Generally, a surfactant having both a hydrophilic and a hydrophobic part in one molecule is used as a wetting agent.

[0077] In the present invention, the term "dispersant" refers to a substance that, when added to a pigment dispersion, prevents reagglomeration of pigment particles that have been disintegrated into primary particles and stabilizes the pigment dispersion. Dispersants adsorb to the pigment surface and maintain an appropriate space between the pigment particles by electrical repulsion and steric hindrance, thereby suppressing reagglomeration.

[0078] In the present invention, a dispersant that maintains an appropriate amount of space between pigment particles by steric hindrance and suppresses reagglomeration is preferred, and the structure of such a dispersant preferably has a pigment-adsorbing group and a resin-affinity moiety (hydrocarbon chain). By having a pigment-adsorbing group, the dispersant can be strongly and persistently adsorbed to the surface of pigments, many of which exhibit high polarity (hydrophilicity). Furthermore, by having a resin-affinity moiety (hydrocarbon chain), the pigment can be dispersed in a resin solution. Furthermore, the resin-affinity moiety acts as a steric hindrance, suppressing pigment reagglomeration.

[0079] In addition, since the pigment adsorption group is a highly polar group (hydrophilic part) and the resin affinity part is a low polarity / non-polar group (hydrophobic part), it also functions as a wetting agent. In this way, a dispersant that also functions as a wetting agent is called a "wetting dispersant." In the present invention, wetting dispersants are included in the "dispersant."

[0080] The dispersant according to the present invention also contains a basic nitrogen. The dispersant according to the present invention contains basic nitrogen, which generates ammonium cation sites in the ink, causing electrical attraction between the ammonium cation sites in the dispersant and the surfaces of the white pigment particles, thereby further increasing the degree of adsorption.

[0081] Whether or not a dispersant has basic nitrogen can be determined by measuring the amine value of the dispersant and determining whether the value is greater than 0.

[0082] (amine value) In the present invention, the term "amine value" refers to the number of milligrams of potassium hydroxide (KOH) equivalent to hydrochloric acid (HCl) required to neutralize the basic nitrogen in 1 gram of sample.

[0083] The basic nitrogen in the dispersant exists as an amino group, and the amino group here includes all of the primary amino group, secondary amino group, and tertiary amino group. That is, the amine value in the present invention refers to the number of milligrams of potassium hydroxide (KOH) equivalent to hydrochloric acid (HCl) required to neutralize all of the primary amino group, secondary amino group, and tertiary amino group.

[0084] The amine value can be measured by the following method in accordance with JIS-K7237. Accurately weigh 0.5 g of dispersant sample into a 100 mL beaker and dissolve in 50 mL of acetic acid. Using an automatic titrator equipped with a pH electrode, neutralize the sample solution with a 0.1 mol / L perchloric acid solution. The inflection point of the titration pH curve is used as the endpoint, and the amine value is calculated using the following formula. (Equation 3) Amine value [mgKOH / g] = (561 × V) / (W × S) However, the symbols are as follows: W: Dispersant sample weight [g] V: Titration volume at the end point [mL] S: Solid concentration of dispersant sample [mass%]

[0085] From the viewpoint of achieving excellent ink storage stability and white pigment re-dispersibility, the amine value of the dispersant is preferably within a range of 5 to 100 mgKOH / g, more preferably within a range of 10 to 80 mgKOH / g, and even more preferably within a range of 15 to 70 mgKOH / g. When the amine value is 5 mgKOH / g or greater, ammonium cationic moieties in the dispersant and the surfaces of the white pigment particles electrically attract each other, stabilizing the white pigment particles and suppressing reagglomeration of the white pigment particles. Furthermore, when the amine value is 100 mgKOH / g or less, the ammonium cationic moieties in the dispersant are adsorbed to the surfaces of the white pigment particles, but non-cationic moieties other than the ammonium cationic moieties in the dispersant cause moderate steric hindrance, suppressing reagglomeration of the white pigment particles.

[0086] From the viewpoint of achieving excellent whiteness, ease of maintenance, and re-dispersibility of the white pigment, the content of the dispersant is preferably within a range from 2.5 to 15% by mass, more preferably from 2.5 to 12.5% ​​by mass, and even more preferably from 2.5 to 10% by mass, relative to the total mass of the white pigment contained in the ink.

[0087] Examples of dispersants include salts of long-chain polyaminoamides and high molecular weight acid esters; salts of long-chain polyaminoamides and polar acid esters; polyester polyamines; stearylamine acetate; (partial) amine salts, (partial) ammonium salts, or (partial) alkylamine salts of (co)polymers of unsaturated carboxylic acids such as polyacrylic acid; unsaturated polyamides; long-chain polyaminoamide phosphates; polyethyleneimine-based compounds (e.g., amides which are reaction products of poly(lower alkyleneimines) and polyesters containing free carboxy groups); polyallylamine-based compounds (e.g., reaction products of polyallylamine and at least one compound selected from the group consisting of polyesters, polyamides, and polyesteramides (co-condensates of esters and amides) having free carboxy groups). Among these, compounds having a polyester polyamine skeleton or compounds having a polyethyleneimine skeleton are preferred, and compounds having a polyethyleneimine skeleton are more preferred.

[0088] The dispersant according to the present invention may have an acidic functional group in addition to an amino group, and the presence or absence of the acidic functional group can be determined by measuring the acid value, and if the acid value is greater than 0, it can be determined that the dispersant has an acidic functional group. The method for measuring the acid value will be described later.

[0089] Commercially available dispersants that can be used in the present invention are listed below. 1) Dispersants that have amino groups but no acidic functional groups (amine value greater than 0, acid value 0) For example, "DISPERBYK (registered trademark)-184, 185, 2014, 2018, 2019, 2055, 2061" (all manufactured by BYK-Chemie), "TEGO (registered trademark) Dispers 650" (all manufactured by Evonik), "Dispex (registered trademark) Ultra PA 4550, 4560, 4575, 4585", "EFKA (registered trademark) 6230" (all manufactured by BASF), "AFCONA (registered trademark)-4531" (all manufactured by AFCONA), and the like can be mentioned.

[0090] 2) Dispersants with both amino and acidic functional groups (both amine and acid values ​​greater than 0) For example, "DISPERBYK (registered trademark) -180, 187, 191, 2010, 2012, 2013", "ANTI-TERRA (registered trademark) -250" (all manufactured by BYK-Chemie), "TEGO (registered trademark) Dispers 660C, 760W" (manufactured by Evonik), "Dispex (registered trademark) Ultra PA4425" (all manufactured by BASF), "Florene G-700AMP, G-700DMAE" (all manufactured by Kyoei Chemical Industry Co., Ltd.), "Disparlon (registered trademark) AQ-360, 380" (all manufactured by Kusumoto Chemicals Co., Ltd.), "AFCONA (registered trademark) -4570, 5071, 6220, 6225, 6226" (all manufactured by AFCONA), and the like.

[0091] [2 White pigment] The ink of the present invention is characterized by containing a white pigment. By including a white pigment, the ink can be made into a white ink.

[0092] The white pigment is not particularly limited as long as it is a pigment that causes the cured film formed by curing an ink containing the white pigment to exhibit a white color. Examples of white pigments include titanium oxide, zinc oxide, zinc sulfide, calcium carbonate, calcium silicate, barium sulfate, aluminum hydroxide, antimony oxide, zirconium dioxide, silicas such as finely powdered silicic acid and synthetic silicates, talc, clay, white hollow resin particles, and white polymer particles. Among these, from the viewpoint of whiteness (color development), white metal oxides are preferred, and titanium oxide is more preferred. These may be used alone or in combination of two or more.

[0093] From the viewpoint of whiteness (color development), the particle size (average dispersed particle size) of the white pigment when dispersed is preferably within a range of 150 to 350 nm, more preferably within a range of 160 to 320 nm, and even more preferably within a range of 180 to 300 nm. Here, "when dispersing the white pigment" may refer to when the white pigment dispersion used in preparing the ink is prepared, or when the prepared ink is redispersed after long-term storage.

[0094] The average dispersed particle size can be measured using a particle size measuring device and a dynamic light scattering method. A slurry containing a white pigment is diluted with ion-exchanged water so that the white pigment content is 0.1% by mass, and the diluted slurry is irradiated with a laser beam. The intensity of the scattered light from the white pigment is measured over time in microseconds. The detected scattering intensity distribution caused by the white pigment is fitted to a normal distribution, and the Z-average particle size of the white pigment particles is determined by cumulant analysis. An example of a particle size measuring device is the Zetasizer Nano Z90 manufactured by Spectris, Inc. The particle size measuring device is equipped with data analysis software, which can automatically analyze the measurement data to calculate the Z-average particle size.

[0095] The content of the white pigment is preferably within a range of 2 to 20% by mass, more preferably within a range of 3 to 18% by mass, and even more preferably within a range of 5 to 15% by mass, relative to the total mass of the ink.

[0096] [2.1 White metal oxide] The white pigment according to the present invention is preferably a white metal oxide, which allows the amino group-containing dispersant to be easily adsorbed to the white pigment particles, resulting in superior redispersibility of the ink.

[0097] Specifically, as described above, the surfaces of metal oxide particles are covered with hydroxyl groups, and these hydroxyl groups are dehydrogenated or hydrogenated depending on the pH value of the ink, so the surface charge of the metal oxide particles varies depending on the pH value of the ink. The zeta potential is a measurement of this charge.

[0098] (Formula 1) -M-OH + OH - → -MO - + H2O (Formula 2) -M-OH + H + → -M-OH2 +

[0099] When the ink has a pH value greater than 7 and is alkaline, for example, the surfaces of the metal oxide particles are negatively charged as shown in the above formula 1, and the surfaces of the metal oxide particles and the positively charged ammonium cation sites are electrically attracted to each other, further increasing the degree of adsorption. Furthermore, since the ammonium cation sites are inserted between the metal oxide particles, the distance between the metal oxide particles becomes relatively large, and the van der Waals forces acting between the metal oxide particles become relatively weak, which is thought to make it difficult for the metal oxide particles to aggregate.

[0100] Examples of white metal oxides include titanium oxide, zinc oxide, antimony oxide, and zirconium dioxide.

[0101] [2.1.1 Titanium dioxide] The white pigment according to the present invention is preferably titanium oxide, which provides excellent whiteness (coloring and hiding power) and allows the particle size during dispersion to be adjusted within a suitable range, thereby improving image quality.

[0102] The crystalline form of titanium oxide may be any of rutile (tetragonal), anatase (tetragonal), or brookite (orthorhombic), but from the viewpoint of enabling a smaller particle size during dispersion, anatase, which has a relatively small specific gravity, is preferred. Furthermore, from the viewpoint of further enhancing the concealing properties of the formed image, rutile, which has a relatively large refractive index in the visible light region, is preferred. Furthermore, rutile exhibits photocatalytic activity. Among the titanium oxides having the above crystal forms, one type may be used alone, or two or more types of titanium oxides having different crystal forms may be used in combination.

[0103] From the viewpoint of dispersibility, it is preferable that the titanium oxide is surface-treated. The surface treatment may be carried out on at least a part or all of the particles. The surface treatment of the titanium oxide is not particularly limited, and examples thereof include surface treatment with at least one inorganic compound such as alumina, silica, zinc oxide, zirconia, and magnesium oxide, and surface treatment with an organic compound such as a titanium coupling agent, a silane coupling agent, and silicone oil.

[0104] From the viewpoint of high color development, it is preferable that the surface be treated with at least one of alumina, silica, and an organic compound, and among these, it is more preferable that the surface be treated with alumina. Because the white ink has high color development, when it is used for the purpose of concealing the transparency or color of a recording medium, it has excellent concealing properties and can enhance the color development of color inks applied over it, i.e., can improve image quality. It is preferable that the white pigments other than titanium oxide are also surface-treated.

[0105] Commercially available titanium oxide products include, for example, "CR-EL," "CR-50," "CR-50-2," "CR-58," "CR-80," "CR-90," "R-680," "R-930," "PF691," and "A-220" (all manufactured by Ishihara Sangyo Kaisha, Ltd.), "TCR-52," "R-310," "R-32," "A-190," and "A-197" (all manufactured by Sakai Chemical Industry Co., Ltd.), and "KR-310," "KR-380," and "KR-380N" (all manufactured by Titanium Industries Co., Ltd.).

[0106] [3 water] The ink of the present invention is characterized by containing water. By using an aqueous ink, the effects of the present invention can be achieved. Furthermore, compared to solvent inks, aqueous inks can reduce the content of organic solvents such as VOCs (volatile organic compounds). The water is not particularly limited and may be ion-exchanged water, distilled water, or pure water. The water content in the ink is preferably adjusted appropriately depending on the desired performance of the ink.

[0107] [4 Anionic Resins] The ink of the present invention preferably further contains an anionic resin. By including an anionic resin, the white pigment particles adsorbed onto the dispersant enter the network structure of the anionic resin, and the anionic resin and the ammonium cation moieties in the dispersant electrically attract each other, stabilizing the white pigment particles, allowing the white pigment particles to be dispersed stably and uniformly in the ink.

[0108] Furthermore, the ink of the present invention can be used as an ink set in combination with a treatment liquid containing a cationic compound, as will be described in detail later.

[0109] When used as an ink set, the anionic resin contained in the ink of the present invention and the cationic compound contained in the treatment liquid are electrically attracted to each other, which makes the white pigment more likely to aggregate. That is, by applying the ink of the present invention and the treatment liquid to a recording medium, the white pigment is more likely to aggregate, which suppresses ink bleeding on the recording medium and improves image quality. Therefore, image quality can be improved by using a set of the ink of the present invention and a coating liquid containing a cationic compound, rather than just the treatment liquid.

[0110] In the present invention, "anionic" refers to being negatively charged in water, and "anionic resin" refers to a resin having an anionic functional group (a functional group that is negatively charged in water). Examples of such anionic functional groups include acidic functional groups.

[0111] (acid number) The acid value of the anionic resin according to the present invention is preferably within the range of 50 to 250 mgKOH / g. The acid value of the anionic resin is preferably within a range of 50 to 250 mgKOH / g, more preferably within a range of 60 to 250 mgKOH / g, and even more preferably within a range of 70 to 240 mgKOH / g. By ensuring that the acid value is within this range, the white pigment particles can be stably and uniformly dispersed in the ink, and re-aggregation of the white pigment can be suppressed, thereby improving image quality and whiteness. Furthermore, by ensuring that the acid value is within this range, when a cationic treatment liquid is used, rapid aggregation of the white pigment can be suppressed and uniform aggregation can be achieved, thereby improving image quality and whiteness.

[0112] In the present invention, the term "acid value" refers to the number of milligrams of potassium hydroxide (KOH) required to neutralize the acidic components in 1 gram of sample.

[0113] The acid value can be measured by the following method in accordance with JIS-K0070. Accurately weigh 0.100 g of anionic resin sample into a 250 mL tall beaker, add 150 mL of a toluene / ethanol (3:1) mixture, and dissolve for 1 hour. Then, using a potentiometric titrator, perform potentiometric titration with a 0.1 mol / L potassium hydroxide ethyl alcohol solution. The factor of the potassium hydroxide ethyl alcohol solution can be determined using the potentiometric titrator. Next, as a blank test, perform titration in the same manner except that no anionic resin sample is used (only a toluene / ethanol (3:1) mixture) and calculate the acid value using the following formula.

[0114] (Formula 4) Acid value [mgKOH / g]=[(CB)×f×5.61] / S However, the symbols are as follows: B: Titration volume of potassium hydroxide ethyl alcohol solution in blank test [mL] C: Titration volume of potassium hydroxide ethyl alcohol solution in this test [mL] f: Factor of potassium hydroxide ethyl alcohol solution S: Amount of anionic resin sample [g]

[0115] The anionic resin is preferably neutralized with a basic compound. In the present invention, "neutralized with a basic compound" means that an anionic functional group and a basic compound are ionic bonded to each other.

[0116] In general, resins having polar groups (hydrophilic groups) become relatively more soluble in water, and by appropriately adjusting the type and number of polar groups, the resin can be made even more soluble in water.

[0117] However, even if the resin has a suitable type and number of polar groups, if the ionic dissociation property of each polar group is low, only a portion of the polar groups will dissociate into ions and the resin will be difficult to dissolve in water. Therefore, the ionic dissociation property of each polar group can be increased by neutralizing the resin in advance with a basic compound.

[0118] Therefore, it is preferable to appropriately adjust the type and amount of the basic compound used to neutralize the anionic resin depending on the acid value of the anionic resin. The amount of basic compound added for neutralization per gram of anionic resin is calculated by converting the acid value of the anionic resin into an equivalent amount of basic compound. The amount of basic compound added is preferably within a range of 20 to 100%, more preferably within a range of 30 to 90%, and even more preferably within a range of 40 to 80%, of the acid value [mgKOH / g] of the anionic resin.

[0119] (Weight average molecular weight) The weight-average molecular weight of the anionic resin is preferably within a range of 5,000 to 30,000, more preferably within a range of 7,000 to 25,000, and even more preferably within a range of 10,000 to 20,000. Having a weight-average molecular weight within the above range improves head maintainability; that is, even if dried ink adheres to the nozzle portion of the inkjet head, it can be removed relatively easily by wiping it off with a wiping member soaked in a maintenance liquid.

[0120] The weight average molecular weight according to the present invention is a value (styrene equivalent) measured by gel permeation chromatography (GPC).

[0121] The content of the anionic resin is preferably within a range of 2.5 to 15 mass %, more preferably within a range of 2.5 to 12.5 mass %, and even more preferably within a range of 2.5 to 10 mass %, relative to the total mass of the white pigment contained in the ink.

[0122] Furthermore, from the viewpoint of whiteness and ease of maintenance, the mass ratio of the dispersant content to the anionic resin content is preferably within a range of 1:6 to 6:1, more preferably within a range of 1:4 to 4:1, and even more preferably within a range of 1:2 to 2:1.

[0123] Examples of anionic resins include copolymers obtained by reacting one or more carboxyl group-containing unsaturated monomers (including acid anhydride group-containing unsaturated monomers that give a carboxyl group upon ring-opening) such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, maleic anhydride, maleic acid monoalkyl esters, citraconic acid, citraconic anhydride, and citraconic acid monoalkyl esters with one or more unsaturated monomers such as styrene-based monomers such as styrene, α-methylstyrene, and vinyltoluene, aralkyl methacrylates or acrylates such as benzyl methacrylate and benzyl acrylate, and alkyl methacrylates or acrylates (methacrylates or acrylates having an alkyl group having 1 to 18 carbon atoms are preferred) such as methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, lauryl methacrylate, methyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, and lauryl acrylate.

[0124] Specific examples of the copolymer include (meth)acrylic acid alkyl ester-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid alkyl ester copolymer, styrene-maleic acid-(meth)acrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer, styrene-maleic acid half ester-(meth)acrylic acid alkyl ester copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid alkyl ester-benzyl (meth)acrylate copolymer, styrene-α-methylstyrene-acrylic acid copolymer, styrene-α-methylstyrene-acrylic acid-acrylic acid ester copolymer, and the like.

[0125] In addition to the above, examples of the anionic resin include polycarboxylic acid, polyacrylic acid, and vinyl acetate-acrylic acid copolymer.

[0126] Commercially available anionic resins include "JONCRYL (registered trademark) 67, 586, 611, 678, 680, 682, 683, 690, 693, 819," "ECO675," "HPD671," and "HPD696" (all manufactured by BASF Japan Ltd.), "Polyacrylic acid sodium salt T-50, A-210," "Polyacrylic acid ammonium salt A-30," and "ARUFON (registered trademark) UF-5041" (all manufactured by Toagosei Co., Ltd.), and "AQUALIC (registered trademark) YS-100," "AQUALIC (registered trademark) TL-37," "AQUALIC (registered trademark) TL-213," "AQUALIC (registered trademark) GL-366," and "AQUALIC (registered trademark) GL-388" (all manufactured by Nippon Shokubai Co., Ltd.).

[0127] As the basic compound, a nitrogen-containing basic group-containing compound or a metal hydroxide can be used.

[0128] The nitrogen-containing basic group-containing compound is preferably ammonia or an organic amine, and more preferably an organic amine. The organic amine is preferably an amine having a hydroxy group, more preferably an alkanolamine, and particularly preferably a primary or tertiary alkanolamine. Use of such an organic amine as the basic compound improves the dispersibility and storage stability of the anionic resin in the ink.

[0129] Examples of organic amines include alkylamines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, and triethylamine; alkanolamines such as aminoethanol, methylaminoethanol, dimethylaminoethanol, ethylaminoethanol, diethylaminoethanol, diethanolamine, and triethanolamine; and amines having a nonionic group such as methoxypoly(oxyethylene / oxypropylene)-2-propylamine.

[0130] As the metal hydroxide, hydroxides of alkali metals or alkaline earth metals can be used.

[0131] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include beryllium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, etc.

[0132] Among these, alkali metal hydroxides are preferred, and lithium hydroxide or sodium hydroxide is more preferred.

[0133] [5 Resin] The ink of the present invention may further contain a resin, which can improve fixability to a recording medium.

[0134] The fixing resin for improving the fixing property is not particularly limited, and examples thereof include vinyl chloride resins, (meth)acrylic resins, urethane resins, polyether resins, polyester resins, etc. Furthermore, the following water-insoluble resin particles may also be used.

[0135] (Resin particle dispersion) The resin particles described below are water-insoluble resin particles, and a water-insoluble resin particle dispersion containing the water-insoluble resin particles exhibits solubility or affinity to the ink.

[0136] In the present invention, "water-insoluble resin microparticles" refer to resins that are inherently water-insoluble but have a form in which the resin disperses in an aqueous medium as microparticles, and are water-insoluble resins that are forcibly emulsified using an emulsifier or the like and dispersed in water, or water-insoluble resins that have hydrophilic functional groups introduced into their molecules and are self-emulsifiable to form a stable aqueous dispersion without the use of an emulsifier or dispersion stabilizer. These resins are usually used in a state of being emulsified and dispersed in water or a water / alcohol mixed solvent.

[0137] That is, by introducing a hydrophilic portion into a water-insoluble resin (hydrophobic portion) (by forced emulsification using an emulsifier or the like, or by introducing a hydrophilic functional group into the molecule), a structure having a hydrophilic portion and a hydrophobic portion in the same particle can be obtained, and the resin can be dispersed in an aqueous medium. Among these, a structure having a hydrophilic portion and a hydrophobic portion in the same molecule, that is, a water-insoluble resin that can be self-emulsified to form a stable aqueous dispersion by itself without using an emulsifier or dispersion stabilizer by introducing a hydrophilic functional group into the molecule, is preferred.

[0138] In the present invention, "water-insoluble" refers to a resin that, when dried at 105° C. for 2 hours and then dissolved in 100 g of water at 25° C., dissolution amounts of the resin are 10 g or less, preferably 5 g or less, and more preferably 1 g or less. However, if the resin has salt-forming groups, the dissolution amount refers to the amount dissolved when the salt-forming groups of the resin are 100% neutralized with acetic acid or sodium hydroxide, depending on the type of resin.

[0139] The resin particles are preferably any of acrylic resin particles, urethane resin particles, polyester resin particles, and composite resin particles of urethane resin and acrylic resin.

[0140] Furthermore, when a resin particle dispersion is mixed with the treatment liquid according to the present invention, the dispersion becomes unstable and the particles attract and aggregate, increasing the particle size. In this case, the average particle size (Z-average particle size) of the resin particles is preferably 1000 nm or more. Although the reason for this is unclear, using resin particles whose average particle size during aggregation is 1000 nm or more can inhibit excessive aggregation of the white pigment, improving the image quality of recorded materials.

[0141] The average particle size (Z-average particle size) of the fine particles can be measured by the following procedure using a particle size measuring device and dynamic light scattering method. The slurry containing the resin particles is diluted with ion-exchanged water so that the resin particle content is 0.1% by mass, and the diluted slurry is irradiated with laser light. The intensity of the scattered light from the resin particles is measured over time in microseconds. The detected scattering intensity distribution caused by the resin particles is fitted to a normal distribution, and the Z-average particle size of the resin particles is determined by cumulant analysis. An example of a particle size measuring device is the Zetasizer Nano ZS manufactured by Spectris, Inc. The particle size measuring device is equipped with data analysis software, which can automatically analyze the measurement data to calculate the Z-average particle size.

[0142] From the viewpoint of fixability, it is preferable to use resin particles having an average particle size (Z average particle size) of less than 1000 nm when aggregated.

[0143] The polyester resin, urethane resin, acrylic resin or composite resin particles of urethane resin and acrylic resin are preferably anionic or nonionic.

[0144] The glass transition temperature (Tg) of the resin particles is preferably within the range of -30 to 100°C. The glass transition temperature (Tg) can be determined by reading the glass transition temperature Tg from the endothermic peak when the temperature is increased at a rate of 10°C / min in the temperature range of -30 to 200°C using a DSC (differential scanning calorimeter).

[0145] Each resin will be described below. (polyester resin) The polyester resin having a polyester skeleton as the water-insoluble resin particles can be obtained by using a polyhydric alcohol component and a polycarboxylic acid component such as a polycarboxylic acid, a polycarboxylic acid anhydride, or a polycarboxylic acid ester.

[0146] Examples of polyhydric alcohol components include dihydric alcohols (diols), specifically alkylene glycols having 2 to 36 carbon atoms (ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexanediol, etc.), alkylene ether glycols having 4 to 36 carbon atoms (diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polybutylene glycol, etc.), and alicyclic diols having 6 to 36 carbon atoms. Examples of suitable alkylene oxides include alicyclic diols (1,4-cyclohexanedimethanol, hydrogenated bisphenol A, etc.), adducts of the above-mentioned alicyclic diols with alkylene oxides (ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO), butylene oxide (hereinafter abbreviated as BO)) having 2 to 4 carbon atoms (number of moles added: 1 to 30), and adducts of bisphenols (bisphenol A, bisphenol F, bisphenol S, etc.) with alkylene oxides (EO, PO, BO, etc.) having 2 to 4 carbon atoms (number of moles added: 2 to 30). These may be used alone or in combination of two or more.

[0147] Examples of polycarboxylic acid components include dicarboxylic acids (dicarboxylic acids), specifically alkane dicarboxylic acids having 4 to 36 carbon atoms (succinic acid, apidic acid, sebacic acid, etc.), alkenyl succinic acids (dodecenyl succinic acid, etc.), alicyclic dicarboxylic acids having 4 to 36 carbon atoms (dimer acids (dimerized linoleic acid), etc.), alkene dicarboxylic acids having 4 to 36 carbon atoms (maleic acid, fumaric acid, citraconic acid, mesaconic acid, etc.), and aromatic dicarboxylic acids having 8 to 36 carbon atoms (phthalic acid, isophthalic acid, terephthalic acid or derivatives thereof, naphthalenedicarboxylic acid, etc.). These may be used alone or in combination of two or more.

[0148] The number average molecular weight of the polyester resin is preferably within the range of 1,000 to 50,000, and more preferably within the range of 2,000 to 20,000.

[0149] As the polyester resin, commercially available products may be used, such as "Pesresin A-110F, A-640, A-647GEX" (all manufactured by Takamatsu Oil & Fat Co., Ltd.), "Vylonal (registered trademark) MD-1100, MD-1200, MD-1335, MD-1480, MD-1930, MD-2000" (all manufactured by Toyobo Co., Ltd.), and "Z-1100" (manufactured by Goo Chemical Co., Ltd.). These may be used alone or in combination of two or more types.

[0150] (urethane resin) As the urethane resin for the water-insoluble resin particles, those having hydrophilic groups can be used.

[0151] The urethane resin is preferably an aqueous dispersion of a self-emulsifying urethane having a water-soluble functional group dispersed in its molecule, or an aqueous dispersion of a forced-emulsifying urethane emulsified under strong mechanical shearing force in combination with a surfactant. The urethane resin in the aqueous dispersion can be obtained by reacting a polyol with an organic polyisocyanate and a hydrophilic group-containing compound.

[0152] Examples of polyols that can be used to prepare the aqueous dispersion of the urethane resin include polyester polyols, polyether polyols, polycarbonate polyols, and polyolefin polyols.

[0153] Examples of polyester polyols include low molecular weight polyols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2- and 1,3-propylene glycol, neopentyl glycol, 1,3- and 1,4-butanediol, 3-methylpentanediol, hexamethylene glycol, 1,8-octanediol, 2-methyl-1,3-propanediol, bisphenol A, hydrogenated bisphenol A, trimethylolpropane, and cyclohexanedimethanol; and condensates thereof with polycarboxylic acids such as succinic acid, glutaric acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, tetrahydrofuran acid, endomethinetetrahydrofuran acid, and hexahydrophthalic acid.

[0154] Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polyethylene polytetramethylene glycol, polypropylene polytetramethylene glycol, and polytetramethylene glycol.

[0155] Examples of polycarbonate polyols include those obtained by reacting a carbonic acid derivative such as diphenyl carbonate, dimethyl carbonate, or phosgene with a diol, such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2- and 1,3-propylene glycol, neopentyl glycol, 1,3- and 1,4-butanediol, 3-methylpentanediol, hexamethylene glycol, 1,8-octanediol, 2-methyl-1,3-propanediol, bisphenol A, hydrogenated bisphenol A, trimethylolpropane, and cyclohexanedimethanol.

[0156] Examples of organic polyisocyanates that can be used to prepare aqueous dispersions of urethane resins include aromatic isocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric MDI, xylylene diisocyanate (XDI), and tetramethylxylylene diisocyanate (TMXDI); aliphatic isocyanates such as hexamethylene diisocyanate (HMDI); and alicyclic isocyanates such as isophorone diisocyanate (IPDI) and 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI, H12MDI). These may be used alone or in combination of two or more.

[0157] Examples of hydrophilic group-containing compounds that can be used to prepare the aqueous dispersion of the urethane resin include carboxylic acid-containing compounds such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolbutyric acid, 2,2-dimethylolvaleric acid, and glycine, and derivatives thereof such as sodium salts, potassium salts, and amine salts; and sulfonic acid-containing compounds such as taurine (i.e., aminoethylsulfonic acid) and ethoxypolyethylene glycol sulfonic acid, and derivatives thereof such as sodium salts, potassium salts, and amine salts.

[0158] The urethane resin can be obtained by a known method. For example, a urethane prepolymer can be obtained by mixing the polyol, organic polyisocyanate, and hydrophilic group-containing compound and reacting them at 30 to 130°C for 30 minutes to 50 hours.

[0159] The urethane prepolymer is polymerized by extending the chain with a chain extender to form a urethane resin having hydrophilic groups. The chain extender is preferably water and / or an amine compound. By using water or an amine compound as the chain extender, the chain can react with free isocyanate in a short time, efficiently extending the isocyanate-terminated prepolymer.

[0160] Examples of the amine compound include aliphatic polyamines such as ethylenediamine and triethylenediamine, aromatic polyamines such as metaxylenediamine and toluylenediamine, and polyhydrazino compounds such as hydrazine and adipic acid dihydrazide. The amine compound may contain, together with the polyamine, a monovalent amine such as dibutylamine or methyl ethyl ketoxime as a reaction terminator to the extent that the polymerization is not significantly inhibited.

[0161] In addition, in the synthesis of the urethane prepolymer, a solvent that is inert to isocyanates and can dissolve the urethane prepolymer may be used. Examples of such solvents include dioxane, methyl ethyl ketone, dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, and propylene glycol monomethyl ether acetate. These hydrophilic organic solvents used in the reaction step are preferably finally removed.

[0162] In addition, in the synthesis of the urethane prepolymer, a catalyst such as an amine catalyst (e.g., triethylamine, N-ethylmorpholine, triethyldiamine, etc.), a tin-based catalyst (e.g., dibutyltin dilaurate, dioctyltin dilaurate, tin octoate, etc.), or a titanium-based catalyst (e.g., tetrabutyl titanate, etc.) may be added to promote the reaction.

[0163] The number-average molecular weight of the urethane resin is preferably increased as much as possible by introducing a branched structure or internal crosslinking structure, and a number-average molecular weight of 50,000 to 10,000,000 is preferable. By keeping the number-average molecular weight within this range, the urethane resin becomes less soluble in solvents, resulting in a coating film with excellent weather resistance and water resistance. The number-average molecular weight (Mn) is a value measured by gel permeation chromatography (GPC), and can be determined, for example, from a calibration curve prepared using a polystyrene standard sample using a Shimadzu Corporation "RID-6A" column (Tosoh Corporation "TSK-GEL" column, tetrahydrofuran (THF) solvent, and a column temperature of 40°C).

[0164] Alternatively, commercially available urethane resins may be used, such as "WBR-016U" (manufactured by Taisei Fine Chemical Co., Ltd.), "Superflex (registered trademark) 620, 650, 500M, E-2000" (all manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), "Permarin (registered trademark) UC-20" (manufactured by Sanyo Chemical Industries, Ltd.), and "Parasurf UP-22" (manufactured by Ohara Palladium Chemical Co., Ltd.).

[0165] (acrylic resin) The acrylic resin can be obtained by using a copolymer of an acrylic acid ester component, a methacrylic acid ester component, a styrene component, or the like. Examples of the acrylic acid ester component and the methacrylic acid ester component include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, and (meth)acrylic acid. 2-hydroxybutyl (meth)acrylate, benzyl (meth)acrylate, glycidyl (meth)acrylate, (meth)acrylic acid, (di)ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin di(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, acrylamide, and the like.

[0166] Examples of the styrene component include styrene, 4-methylstyrene, 4-hydroxystyrene, 4-acetoxystyrene, 4-acetylstyrene, styrenesulfonic acid, etc. These components may be used alone or in combination of two or more.

[0167] The number average molecular weight (Mn) of the acrylic resin is preferably 1000 to 50000, and more preferably 2000 to 20000. When the number average molecular weight (Mn) of the acrylic resin is 1000 or more, the cohesive force of the coating film is strong and the adhesion is improved, and when it is 50000 or less, the solubility in organic solvents is good and the particle size of the emulsion dispersion is facilitated to be miniaturized.

[0168] The number average molecular weight (Mw) is a value measured by gel permeation chromatography (GPC), and can be determined from a calibration curve prepared using a polystyrene standard sample, for example, using a Shimadzu RID-6A column (column: Tosoh TSK-GEL, solvent: tetrahydrofuran (THF), column temperature: 40°C).

[0169] In addition, commercially available products may be used as the acrylic resin, and examples thereof include acrylic emulsions such as "Movinyl (registered trademark) 6763" (manufactured by Japan Coating Resins Co., Ltd.), "RKW-620, UW-319SX, UW-600, UW-550CS" (all manufactured by Taisei Fine Chemical Co., Ltd.), and "2682, 2680, 2684, 2685, 2687" (all manufactured by Nissin Chemical Industry Co., Ltd.).

[0170] (Composite resin fine particles) The resin microparticles may be composite resin microparticles, preferably composite resin microparticles obtained by emulsifying an acrylic resin with a urethane resin, i.e., composite resin microparticles having an inner layer made of an acrylic resin and a surface layer made of a urethane resin.

[0171] Here, the urethane resin exists at the interface between the acrylic resin as the water-insoluble resin particles and the water as the continuous phase, and functions as a water-insoluble resin particle layer different from the resin that protects the water-insoluble resin particles.

[0172] By emulsifying the acrylic resin with the urethane resin to form composite resin particles, the compatibility with the urethane resin and the pigment flocculant can be suppressed, unlike when the acrylic resin is used alone. Furthermore, compared to when the acrylic resin and the urethane resin are emulsified and mixed separately, the physical properties of the image (coating film) can be improved, and the stability of the ink can also be improved.

[0173] In composite resin microparticles obtained by emulsifying an acrylic resin with a urethane resin, the mass ratio (U / A) of the urethane resin (U) to the acrylic resin (A) is preferably within the range of 40 / 60 to 95 / 5. When the proportion of the urethane resin (U) is within this range, compatibility with dispersants and solvent resistance are improved. Furthermore, when the proportion of the acrylic resin (A) is within this range, adhesion to acrylic films is excellent. In the above proportions, the mass ratio (U / A) of the urethane resin (U) to the acrylic resin (A) is preferably within the range of 40 / 60 to 80 / 20.

[0174] The total resin content of the acrylic resin and the urethane resin in the composite resin microparticles is not particularly limited, but is preferably 5.0% by mass or more, and more preferably in the range of 10.0 to 70.0% by mass, relative to the total mass of the composite resin microparticles. By being in this range, good fixation of the ink to the recording medium can be obtained.

[0175] In addition, when emulsifying the acrylic resin with the urethane resin, a surfactant that acts as an emulsifier can be used together with the urethane resin. By adding an emulsifier, the storage stability of the composite resin particles can be improved.

[0176] As the emulsifier, an anionic surfactant or a nonionic surfactant can be used, and in the present invention, it is more preferable to use both. The content of the emulsifier (surfactant) is preferably within the range of 1.0 to 20.0 mass% based on the total mass of the resins, including the acrylic resin and the urethane resin. By keeping the content within the above range, water resistance and solvent resistance can be improved.

[0177] The mass ratio (X / Y) of the anionic surfactant (X) to the nonionic surfactant (Y) is preferably within a range of 100 / 0 to 50 / 50. By keeping the ratio within this range, the emulsifiability and storage stability of the ink can be further improved.

[0178] Examples of anionic surfactants include alkyl sulfates, polyoxyethylene alkyl ether sulfates, sulfosuccinates, alpha-olefin sulfonates, N-acylamino acid salts, carboxylates, phosphates, etc. Among these, sulfosuccinates or alpha-olefin sulfonates are preferred. Examples of the type of salt include, but are not limited to, metal salts such as sodium salts, potassium salts, and magnesium salts, and triethanolamine salts.

[0179] Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkylamine ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, sucrose fatty acid esters, etc. Among these, polyoxyethylene alkyl ethers or polyoxyethylene alkylphenyl ethers are preferred.

[0180] From the viewpoint of improving fixability, the average particle size of the resin fine particles is preferably within a range of 10 to 500 nm, more preferably within a range of 10 to 300 nm, and even more preferably within a range of 10 to 200 nm. As described above, the average particle size can be measured using a commercially available particle size measuring device that uses dynamic light scattering, electrophoresis, or the like, but measurement by dynamic light scattering is simple and can accurately measure the particle size range.

[0181] Examples of commercially available resin particles that can be used in the ink according to the present invention are listed below. (polyester resin) "Pesresin A-110F, A-520, A-613D, A-615GE, A-640, A-645GH, A-647GEX" (all manufactured by Takamatsu Oil Co., Ltd.), "Vylonal (registered trademark) MD-1200, 1480, 2000" (all manufactured by Toyobo Co., Ltd.), "Elitell (registered trademark) KA-5034, KA-5071S, KA-1449, KA-0134, KA-3556, KA-6137, KZA-6034, KT-8803, KT-8701, KT-9204, KT-8904, KT-0507, KT-9511" (all manufactured by Unitika Ltd.)

[0182] (urethane resin) "NeoRez (registered trademark) R-967, R-600, R-9671" (all manufactured by Kusumoto Chemicals), "W-6061, W-5661, WS-4000" (all manufactured by Mitsui Chemicals), "Evaphanol (registered trademark) HA-560" (manufactured by Nicca Chemical)

[0183] (acrylic resin) "Movinyl (registered trademark) 6763, 6899D, 6969D, 6800, 6810" (all manufactured by Japan Coating Resins Co., Ltd.), "TOCRYL (registered trademark) W-7146, W-7150, W-7152" (all manufactured by Toyochem Co., Ltd.)

[0184] The resin content in the ink is preferably within the range of 2 to 15% by mass relative to the total mass of the ink.

[0185] [6 Organic solvents] The ink of the present invention may further contain an organic solvent, which can improve the wettability of the ink on the recording medium and the moisture retention of the ink.

[0186] Furthermore, because aqueous inks are primarily composed of water, it is difficult to adjust the viscosity required for stable ejection. For this reason, organic solvents are used to thicken the ink, but from the perspectives of reducing environmental impact and improving drying performance, it is desirable to keep the VOC content to 30% by mass or less.

[0187] The content of the organic solvent in the ink is not particularly limited, but is preferably in the range of 5 to 30% by mass relative to the total mass of the ink. A content of 5% by mass or more can prevent clogging of the ink head nozzles and foaming of the ink.

[0188] The organic solvent is preferably a water-soluble organic solvent, and examples thereof include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and 1,2-alkanediols having 4 or more carbon atoms.

[0189] Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, t-butanol, 3-methoxy-1-butanol, 3-methoxy-3-methylbutanol, 1-octanol, 2-octanol, n-nonyl alcohol, tridecyl alcohol, n-undecyl alcohol, stearyl alcohol, oleyl alcohol, and benzyl alcohol.

[0190] Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols having 5 or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycols having 4 or more propylene oxide groups, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, and thiodiglycol.

[0191] Examples of amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0192] Examples of amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0193] Examples of glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether.

[0194] Examples of 1,2-alkanediols having 4 or more carbon atoms include 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-heptanediol.

[0195] From the viewpoint of suppressing bleeding during high-speed printing, the organic solvent is preferably a polyhydric alcohol, and among these, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol are preferred. These organic solvents may be used alone or in combination of two or more.

[0196] [7 Additives] The ink according to the present invention may contain various known additives depending on the purpose, such as ejection stability, compatibility with print heads and ink cartridges, storage stability, image preservability, and the like.

[0197] Furthermore, from the viewpoint of achieving the effects of the present invention, it is preferable to adjust the pH value of the ink to greater than 7. There are no particular limitations on the pH adjuster, and examples thereof include amines such as diethanolamine and triethanolamine.

[0198] An example of an additive is a surfactant, which can improve the ejection stability of the ink and control the spread (dot diameter) of ink droplets that land on a recording medium.

[0199] The surfactant is not particularly limited, but when an anionic compound is included as a constituent of the ink, the ionicity of the surfactant may be anionic, nonionic (also called "nonionic"), or amphoteric (having both cationic functional groups and anionic functional groups), and the amphoteric surfactant is preferably a betaine type.

[0200] In the present invention, if an anionic surfactant contains an alkaline component, the resin particles contained as the fixing resin tend to aggregate, resulting in a decrease in fixing ability, and therefore, it is preferable that the surfactant is nonionic.

[0201] The surfactant is preferably a fluorine-based or silicone-based surfactant having a high static surface tension reducing ability, an anionic surfactant such as dioctyl sulfosuccinate having a high dynamic surface tension reducing ability, or a nonionic surfactant such as a relatively low molecular weight polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, acetylene glycol, Pluronic (registered trademark) surfactant, sorbitan derivative, etc. Note that a surfactant having a high static surface tension reducing ability and a surfactant having a high dynamic surface tension reducing ability may be used in combination.

[0202] The content of the surfactant in the ink is not particularly limited, but is preferably in the range of 0.1 to 5.0% by mass relative to the total mass of the ink.

[0203] In the ink according to the present invention, in addition to the surfactants described above, various known additives such as polysaccharides, viscosity modifiers, resistivity modifiers, film-forming agents, ultraviolet absorbers, antioxidants, anti-fading agents, anti-fungal agents, and anti-rust agents can be appropriately selected and used as needed. Examples of such additives include oil droplet fine particles such as liquid paraffin, dioctyl phthalate, tricresyl phosphate, and silicone oil; and the like; anti-fading agents described in JP-A Nos. 57-74192, 57-87989, 60-72785, 61-146591, 1-95091, and 3-13376; and fluorescent brightening agents described in JP-A Nos. 59-42993, 59-52689, 62-280069, 61-242871, and 4-219266.

[0204] <Physical properties of inkjet recording ink> The physical properties of the ink of the present invention are not particularly limited and can be appropriately selected depending on the purpose. For example, it is preferable that the viscosity, surface tension, pH, etc. are within the following ranges.

[0205] The viscosity of the ink at 25°C is preferably in the range of 1 to 40 mPa·s, and more preferably in the range of 2 to 10 mPa·s, from the viewpoints of improving print density and character quality and obtaining good ejection stability from the nozzles of the inkjet head.

[0206] The viscosity can be measured using, for example, a rotational viscometer "TVE-33LT" manufactured by Toki Sangyo Co., Ltd. under the following measurement conditions: 25°C, standard cone rotor (0.8° x R24), sample liquid volume 600 μL, and 1 minute.

[0207] The surface tension of the ink is preferably in the range of 1 to 55 mN / m at 25°C, more preferably 1 to 40 mN / m or less, and even more preferably in the range of 1 to 35 mN / m, in order to ensure that the ink is well leveled on the recording medium and the drying time of the ink is shortened.

[0208] The surface tension can be measured by the Wilhelmy method using a surface tensiometer such as "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd.

[0209] The pH value of the ink at 25° C. is preferably within the range of 6 to 10, and more preferably within the range of 7 to 9.5, from the viewpoint of achieving the effects of the present invention. Considering the presumed mechanism of action, the pH value of the ink is preferably greater than 7, and the effects of the present invention can be fully achieved by having a pH value greater than 7. However, in practice, the effects of the present invention can also be achieved when the pH value is within the range of 6 to 7.

[0210] <Method for producing inkjet recording ink> The method for producing an ink for inkjet recording of the present invention is a method for producing the ink for inkjet recording described above, characterized by comprising a step of preparing a pigment dispersion (I) containing the white pigment, the dispersant, and the water.

[0211] The method for producing an ink for inkjet recording of the present invention is a method for producing the ink for inkjet recording described above, and is characterized by comprising the steps of preparing a pigment dispersion (I) containing the white pigment, the dispersant, and the water, and mixing the anionic resin with the pigment dispersion (I) to prepare a pigment dispersion (II).

[0212] The method for producing an ink for inkjet recording of the present invention is a method for producing the ink for inkjet recording described above, and is characterized by comprising a step of preparing a pigment dispersion (III) containing the white pigment, the dispersant, the water, and the anionic resin.

[0213] The ink of the present invention is produced by preparing a pigment dispersion containing a white pigment, a dispersant, and water, and then mixing the pigment dispersion with a resin, water, an organic solvent, additives, etc. The method for mixing the materials is not particularly limited, and known methods can be used.

[0214] When the ink further contains an anionic resin, the ink is manufactured by preparing a pigment dispersion containing a white pigment, a dispersant, water, and an anionic resin, and then mixing the pigment dispersion, resin, water, an organic solvent, additives, etc. Here, the anionic resin may be mixed later or simultaneously with the white pigment particles, but from the viewpoint of efficiently adsorbing the dispersant to the surfaces of the white pigment particles, it is preferable to mix the anionic resin later.

[0215] Therefore, an ink that does not contain an anionic resin can be produced by preparing a pigment dispersion (I) containing a white pigment, a dispersant, and water, and then mixing the pigment dispersion, a resin, water, an organic solvent, additives, etc.

[0216] The ink containing an anionic resin can be produced by preparing a pigment dispersion (I) containing a white pigment, a dispersant, and water, then mixing the pigment dispersion (I) with an anionic resin to prepare a pigment dispersion (II), and then mixing the pigment dispersion, resin, water, an organic solvent, additives, etc.

[0217] Alternatively, the ink containing an anionic resin can be produced by simultaneously mixing the materials for a pigment dispersion containing a white pigment, a dispersant, water, and an anionic resin to prepare a pigment dispersion (III) containing a white pigment, a dispersant, water, and an anionic resin, and then mixing the pigment dispersion, resin, water, an organic solvent, additives, etc.

[0218] <Inkjet recording ink set> The ink set of the present invention is an ink set for ink jet recording that includes an ink for ink jet recording and a treatment liquid, and is characterized in that the treatment liquid contains a cationic compound.

[0219] Because the ink of the present invention is a water-based ink, bleeding between ink droplets and color unevenness occur when it is printed on a recording medium with low or no ink absorbency. For this reason, it is necessary to aggregate the white pigment in the ink and intentionally increase the viscosity of the ink, and by using the ink in combination with a treatment liquid containing an aggregating agent, bleeding between ink droplets and color unevenness can be suppressed.

[0220] From the viewpoint of agglomerating the white pigment in the ink and intentionally increasing the viscosity of the ink, it is preferable that the ink of the present invention further contains an anionic resin, and the treatment liquid of the present invention contains a cationic compound.

[0221] The treatment liquid according to the present invention is not particularly limited as long as it is a liquid that, when used in combination with an ink, allows the aggregating agent to aggregate the white pigment in the ink and improve the image quality of recorded matter, and may be either a pre-treatment liquid or a post-treatment liquid.

[0222] The term "pre-treatment liquid" refers to a treatment liquid that is applied to the surface of a recording medium before or simultaneously with the application of ink to the surface of the recording medium, and the term "post-treatment liquid" refers to a treatment liquid that is applied to the surface of a recording medium after the application of ink to the surface of the recording medium. An example of a treatment liquid that can be suitably used in the present invention will now be described. The method for producing the treatment liquid is not particularly limited, and any known method can be used.

[0223] [1 Flocculant] The treatment liquid according to the present invention contains an aggregating agent, which makes it possible to aggregate the white pigment in the ink, thereby improving the image quality of recorded material.

[0224] The flocculant is not particularly limited as long as it can flocculate the white pigment, and examples thereof include cationic compounds, and examples of cationic compounds include metal salts, cationic polymers, and acids. The flocculant may be used alone or in combination of two or more kinds.

[0225] As described above, under certain conditions in the ink, the surfaces of white pigment particles are negatively charged and exhibit anionic properties, which interact with the cationic compound, making the white pigment more likely to aggregate. Furthermore, when the ink of the present invention contains an anionic resin, the anionic resin and the cationic compound contained in the treatment liquid are electrically attracted to each other, making the white pigment more likely to aggregate. In other words, applying the ink of the present invention and the treatment liquid to a recording medium makes the white pigment more likely to aggregate, thereby suppressing ink bleeding on the recording medium, improving image quality, and further improving the whiteness of the ink in the image.

[0226] [1.1 Metal salts] (polyvalent metal salts) From the viewpoint of aggregating properties, among metal salts, polyvalent metal salts are preferred, which can aggregate anionic components such as the white pigment in the ink by salting out.

[0227] As the polyvalent metal salt, a salt of a metal having a valence of 2 or more can be used. The type of metal (cation) constituting the polyvalent metal salt is not particularly limited. For example, Ca 2+ , Cu 2+ , Ni2+ , Mg 2+ , Zn 2+ , Ba 2+ Divalent metal ions such as Al 3+ , Fe 3+ , Cr 3+ , Y 3+ Trivalent metal ions such as Zr 4+ and the like. From the viewpoint of water solubility, Ca 2+ , Mg 2+ , Zn 2+ or Al 3+ Preferably, Ca 2+ or Mg 2+ It is more preferable that:

[0228] The type of anion constituting the polyvalent metal salt is not particularly limited, and may be an inorganic ion or an organic ion. Examples of inorganic ions include carbonate ions, sulfate ions, nitrate ions, phosphate ions, chloride ions, and hydroxide ions, while examples of organic ions include organic acid ions such as borate ions and carboxylate ions.

[0229] Examples of polyvalent metal salts include calcium carbonate such as heavy calcium carbonate and light calcium carbonate, calcium nitrate, calcium chloride, calcium sulfate, magnesium sulfate, calcium hydroxide, magnesium chloride, magnesium carbonate, barium sulfate, barium chloride, zinc carbonate, zinc sulfide, aluminum silicate, calcium silicate, magnesium silicate, copper nitrate, calcium acetate, magnesium acetate, and aluminum acetate. These may be used alone or in combination of two or more.

[0230] Among these, magnesium sulfate, calcium nitrate, and calcium chloride are preferred from the viewpoint of good water solubility and reducing traces left by the treatment solution, i.e., making the traces less noticeable. These metal salts may contain water of hydration in the form of raw materials.

[0231] (monovalent metal salts) Examples of metal salts other than polyvalent metal salts include monovalent metal salts such as sodium salts and potassium salts, such as sodium sulfate and potassium sulfate.

[0232] The content of the metal salt is preferably 5% by mass or less, more preferably 0.1 to 3% by mass, and even more preferably 0.5 to 1.0% by mass, based on the total mass of the treatment liquid. By keeping the content within this range, anionic components such as the white pigment in the ink can be effectively aggregated, thereby achieving both high image quality and hot water resistance. The content of the metal salt can be measured by a known method such as ICP emission spectrometry.

[0233] In the processing liquid applied to the recording medium, the amount of metal salt applied is 0.1 to 20 g / m 2 It is preferable that the amount of the treatment liquid applied is adjusted so that the amount falls within the above range.

[0234] [1.2 Cationic resin] From the viewpoint of uniform dispersion in the treatment liquid, a water-soluble cationic resin is preferred. Examples of cationic resins include cationic urethane resins, cationic olefin resins, and cationic amine resins.

[0235] As the cationic urethane-based resin, commercially available products can be used, such as "Hydran (registered trademark) CP-7010, CP-7020, CP-7030, CP-7040, CP-7050, CP-7060, CP-7610" (all manufactured by Dainippon Ink and Chemicals, Inc.), "Superflex (registered trademark) 600, 610, 620, 630, 640, 650" (all manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and "Urethane Emulsion WBR-2120C, WBR-2122C" (all manufactured by Taisei Fine Chemical Co., Ltd.).

[0236] The cationic olefin resin is a resin having an olefin such as ethylene or propylene in the structural skeleton, and known resins can be appropriately selected and used. The cationic olefin resin may also be in an emulsion state dispersed in a solvent containing water or an organic solvent. Commercially available cationic olefin resins can be used, such as "Arrowbase (registered trademark) CB-1200, CD-1200" (manufactured by Unitika Ltd.).

[0237] The cationic amine resin may be any known one having an amino group in its structure, and may be appropriately selected and used. Examples include polyamine resins having an amino group in the main skeleton of the resin, polyamide resins having an amide group in the main skeleton of the resin, and polyallylamine resins having an allyl group in the main skeleton of the resin.

[0238] As the cationic polyamine resin, commercially available products can be used, and examples thereof include "Unisense KHE103L" (manufactured by Senka Corporation, hexamethylenediamine / epichlorohydrin resin, 1% aqueous solution having a pH of about 5.0, a viscosity of 20 to 50 (mPa·s), and a solids concentration of 50% by mass) and "Unisense KHE104L" (manufactured by Senka Corporation, dimethylamine / epichlorohydrin resin, 1% aqueous solution having a pH of about 7.0, a viscosity of 1 to 10 (mPa·s), and a solids concentration of 20% by mass).

[0239] Other examples include "FL-14" (manufactured by SNF), "Arafix (registered trademark) 100, 251S, 255, 255LOX" (all manufactured by Arakawa Chemical Co., Ltd.), "DK-6810, 6853, 6885; WS-4010, 4011, 4020, 4024, 4027, 4030" (all manufactured by Seiko PMC Co., Ltd.), "Papiogen (registered trademark) P-105" (manufactured by Senka Co., Ltd.), "Sumirez Resin 650 (30), 675A, 6615, SLX-1" (manufactured by Taoka Chemical Co., Ltd.), "Catiomaster (registered trademark) PD-1, 7, 30, A, PDT-2, PE-10, PE-30, DT-EH, EPA-SK01, TMHMDA-E" (manufactured by Yokkaichi Chemical Co., Ltd.), and "Jetfix 36N, 38A, 5052" (manufactured by Satoda Chemical Co., Ltd.).

[0240] Examples of polyallylamine resins include polyallylamine hydrochloride, polyallylamine amidosulfate, allylamine hydrochloride-diallylamine hydrochloride copolymer, allylamine acetate-diallylamine acetate copolymer, allylamine hydrochloride-dimethylallylamine hydrochloride copolymer, allylamine-dimethylallylamine copolymer, polydiallylamine hydrochloride, polymethyldiallylamine hydrochloride, polymethyldiallylamine amidosulfate, polymethyldiallylamine acetate, polydiallyldimethylammonium chloride, diallylamine acetate-sulfur dioxide copolymer, diallylmethylethylammonium ethyl sulfate-sulfur dioxide copolymer, methyldiallylamine hydrochloride-sulfur dioxide copolymer, diallyldimethylammonium chloride-sulfur dioxide copolymer, and diallyldimethylammonium chloride-acrylamide copolymer.

[0241] In addition to the above-mentioned acid salt types, free-type polyallylamine resins that are not neutralized with acid may also be used. The free-type form is preferable because, when used in a treatment liquid containing water, the pH of the treatment liquid can be easily adjusted from neutral to alkaline, which is advantageous for preventing the durability and corrosion of inkjet recording device components. Examples of free-type polyallylamine resins include "PAA-01, PAA-03, PAA-05, PAA-08, PAA-15, PAA-15C, and PAA-25" (all manufactured by Nittobo Medical Co., Ltd.).

[0242] The content of the cationic resin is preferably within the range of 1 to 5% by mass relative to the total mass of the treatment liquid.

[0243] [1.3 Acid] (organic acid) From the viewpoint of the storage stability of the treatment liquid and the ability to suppress blocking after the treatment liquid is applied and dried, organic acids are preferred among acids, as they can aggregate anionic components such as white pigments in the ink by changing the pH.

[0244] Examples of organic acids include formic acid, acetic acid, propionic acid, isobutyric acid, oxalic acid, fumaric acid, malic acid, citric acid, malonic acid, succinic acid, maleic acid, benzoic acid, 2-pyrrolidone-5-carboxylic acid, lactic acid, acrylic acid or a derivative thereof, methacrylic acid or a derivative thereof, acrylamide or a derivative thereof, and sulfonic acid derivatives. These may be used alone or in combination of two or more.

[0245] It is preferable to use an organic acid that is not completely neutralized with a base. In the present invention, "neutralized with a base" means that the acidic group of these acids is ionic bonded to another positively charged element or compound (for example, an inorganic compound such as a metal). Furthermore, "not completely neutralized" means that among the acidic groups possessed by the organic acid, there are acidic groups that do not form the above-mentioned ionic bond.

[0246] (Inorganic acid) Examples of acids other than organic acids include inorganic acids, such as sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. These may be used alone or in combination of two or more.

[0247] The acid content is preferably 5% by mass or less, more preferably 0.1 to 3% by mass, and even more preferably 0.5 to 1.0% by mass, based on the total mass of the treatment liquid. By keeping the acid content within this range, anionic components such as the white pigment in the ink can be effectively aggregated, thereby achieving both high image quality and hot water resistance. The acid content can be measured by a known method such as high performance liquid chromatography (HPLC).

[0248] In the treatment liquid applied to the recording medium, the amount of acid applied is preferably equal to or less than the neutralization equivalent of the anion component in the ink, and it is preferable to adjust the amount of treatment liquid applied so that it is within the above range.

[0249] [2 Wednesday] The treatment liquid according to the present invention contains water, and therefore the content of organic solvents such as VOCs (volatile organic compounds) can be reduced compared to solvent inks. The water is not particularly limited, and examples thereof include ion-exchanged water, distilled water, and pure water. The water content in the treatment liquid is preferably within a range of 50 to 90% by mass relative to the total mass of the treatment liquid.

[0250] [3 Organic solvents] The treatment liquid according to the present invention contains an organic solvent, which can improve the wettability of the treatment liquid on the recording medium and the moisturizing properties of the treatment liquid. The organic solvent is preferably a water-soluble organic solvent, and examples include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and 1,2-alkanediols having four or more carbon atoms, and the same organic solvents as those contained in the ink described above can be used. The content of the organic solvent in the treatment liquid is preferably within a range of 5 to 45% by mass relative to the total mass of the treatment liquid.

[0251] [4 Additives] The treatment liquid according to the present invention may contain other components such as surfactants, crosslinking agents, antifungal agents, and bactericides as appropriate, provided that the effects of the invention are not impaired.

[0252] Furthermore, for example, ultraviolet absorbers described in JP-A Nos. 57-74193, 57-87988 and 62-261476, anti-fading agents described in JP-A Nos. 57-74192, 57-87989, 60-72785, 61-146591, 1-95091 and 3-13376, and anions The ink may contain various known additives such as various ionic, cationic or nonionic surfactants, fluorescent brightening agents described in JP-A Nos. 59-42993, 59-52689, 62-280069, 61-242871 and 4-219266, antifoaming agents, lubricants such as diethylene glycol, preservatives, thickeners, antistatic agents and the like.

[0253] [5 Physical Properties] The physical properties of the treatment liquid according to the present invention are not particularly limited and can be appropriately selected depending on the purpose. For example, it is preferable that the viscosity, surface tension, pH, etc. are within the following ranges.

[0254] The viscosity of the treatment liquid at 25° C. is preferably within the range of 1 to 40 mPa·s, and more preferably within the range of 2 to 10 mPa·s, from the viewpoint of obtaining good ejection stability from the nozzles of the inkjet head.

[0255] The viscosity can be measured using, for example, a rotational viscometer "TVE-33LT" manufactured by Toki Sangyo Co., Ltd. under the following measurement conditions: 25°C, standard cone rotor (0.8° x R24), sample liquid volume 600 μL, and 1 minute.

[0256] The surface tension of the treatment liquid is preferably within a range of 1 to 55 mN / m at 25°C, more preferably 1 to 40 mN / m or less, and even more preferably within a range of 1 to 35 mN / m, from the viewpoints of ensuring favorable leveling of the ink on the recording medium and shortening the drying time of the ink.

[0257] The surface tension can be measured by the Wilhelmy method using a surface tensiometer such as "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd.

[0258] ≪Image forming method≫ The image forming method of the present invention is an image forming method using the ink set for inkjet recording, and is characterized by comprising a step of applying the treatment liquid to a recording medium, and a step of applying the ink set for inkjet recording to the recording medium.

[0259] The image forming method of the present invention is an image forming method using the inkjet recording ink set and color inks described above, and is characterized by comprising a step of applying the treatment liquid to a recording medium, a step of applying the inkjet recording inks to the recording medium, and a step of applying the color inks to the recording medium.

[0260] The image forming method of the present invention is an image forming method using the inkjet recording ink set and color inks described above, and is characterized by comprising the steps of: applying the treatment liquid onto a recording medium; applying the inkjet recording ink to an area where the treatment liquid has been applied; and applying the color ink to an area where the inkjet recording ink has been applied.

[0261] As described above, the ink of the present invention, when used in combination with the treatment liquid (used as the ink set), can improve the image quality of recorded matter on recording media with low or no absorbency.

[0262] Furthermore, as described above, the color development of the recorded matter can be improved by applying the ink of the present invention to a recording medium and then applying a color ink thereon, or by applying a color ink to a recording medium and then applying the ink of the present invention thereon.

[0263] [1 Recording medium] The recording medium that can be used in the present invention is not particularly limited, and may be a recording medium with high absorbency, low absorbency, or no absorbency, or may be a transparent recording medium or a colored recording medium. In the present invention, the term "absorbency" refers to the absorbency for aqueous ink.

[0264] Examples of highly absorbent recording media include plain paper ranging from thin paper to thick paper, medium-quality paper, fine paper, recycled paper, and the like.

[0265] The ink of the present invention, when used in combination with the treatment liquid (used as the ink set), can improve the image quality of recorded matter on recording media with low or no absorbency.

[0266] Examples of recording media with low or no absorbency include known plastic films, such as polyester films such as polyethylene terephthalate, polyethylene films, polypropylene films, polyamide films such as nylon, polystyrene films, polyvinyl chloride films, polycarbonate films, polyacrylonitrile films, and biodegradable films such as polylactic acid films.

[0267] Alternatively, the recording medium may be a multi-layer recording medium in which a layer such as a PVA (polyvinyl alcohol) coating is provided on the surface of a highly absorbent recording medium such as paper, thereby reducing or eliminating the absorbency of the recording area.

[0268] Furthermore, in order to impart gas barrier properties, moisture resistance, aroma retention, etc., it is also preferable to use a film coated on one or both sides with polyvinylidene chloride or a film vapor-deposited with a metal oxide. The film may be an unstretched film or a stretched film.

[0269] In the case of a plastic film, the thickness of the recording medium is preferably within the range of 10 to 120 μm, and more preferably within the range of 12 to 60 μm.

[0270] Metals such as tinplate for three-piece cans and tin-free steel plates (TFS plates, thickness within the range of 0.1 to 0.6 μm) are also preferably used as recording media with low or no absorbency, and are suitable for use as packaging materials for canned foods, for example, with a thermosetting resin coating layer. The packaging materials for canned foods typically use an epoxy-phenolic paint or polyester laminating agent on the food side to block air, moisture, and light and seal the food inside, and a polyester or acrylic thermosetting paint on the outside.

[0271] [2 color inks] In the present invention, the term "color ink" refers to ink that exhibits a color other than white, and is not particularly limited as long as it exhibits a color other than white. Black ink is also included in the color ink.

[0272] The color inks suitable for use in the present invention will be described below. Note that components used in the ink of the present invention may be used in the color inks to the extent that the effects of the present invention are not impaired.

[0273] The coloring material may be a dye or a pigment, but is preferably a pigment. The coloring material may be used alone or in combination of two or more kinds. Ink composed only of a coloring material that exhibits white does not fall under the category of color ink according to the present invention, but ink composed of a coloring material that exhibits white and a coloring material that exhibits a color other than white falls under the category of color ink according to the present invention.

[0274] The content of the coloring material is preferably in the range of 2 to 15% by mass, more preferably in the range of 3 to 13% by mass, based on the total mass of the color ink. By keeping the content within this range, a recorded product with excellent bleed resistance and color development can be obtained.

[0275] The black colorant used in the black ink is not particularly limited, but examples thereof include carbon black, such as No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B (all manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, and Raven 700 (all manufactured by Carbon Columbia), Rega1 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, and Monarch 1400 (manufactured by CABOT JAPAN KK), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, Special Black 4 (all manufactured by Degussa), and the like.

[0276] Furthermore, a colorant that is generally used in white ink may be used in combination with a colorant that exhibits a color other than white.

[0277] The colorant used in the yellow ink is not particularly limited, but examples thereof include CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 167, 172, and 180.

[0278] The colorant used in the magenta ink is not particularly limited, but examples thereof include CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, and 245, and CI Pigment Violet. Examples include 19, 23, 32, 33, 36, 38, 43, and 50.

[0279] The coloring material used in the cyan ink is not particularly limited, but examples thereof include CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, and 66, and CI Vat Blue 4 and 60.

[0280] Furthermore, coloring materials other than magenta, cyan, and yellow are not particularly limited, but examples thereof include CI Pigment Green 7, 10, CI Pigment Brown 3, 5, 25, 26, and CI Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, and 63.

[0281] The metallic coloring material used in the metallic ink includes metallic pigments, such as fine metal particles, and examples of the metal include aluminum, aluminum alloys, and silver. The term "metallic ink" refers to an ink that, when applied to a recording medium, forms a pattern that has a metallic luster, and the term "metallic pigment" refers to a pigment that exhibits a metallic luster.

[0282] When a pigment is used as a colorant, it is preferable to use the pigment in the form of a pigment dispersion in which the pigment is dispersed in water. Methods for dispersing the pigment include a method of dispersing the pigment in water using a dispersant, a method of introducing hydrophilic groups onto the surfaces of pigment particles using a chemical reaction to disperse the pigment in water as a self-dispersing surface-treated pigment, and a method of coating the pigment with a polymer and dispersing it in water.

[0283] The dispersant is not particularly limited, and examples thereof include polymer dispersants (glue, gelatin, casein, albumin proteins, gum arabic, tragacanth gum natural gums, saponin glucosides, alginic acid and propylene glycol ester, triethanolamine alginate, ammonium alginate alginate fermentation product methyl cellulose, carboxymethyl cellulose, ethyl hydroxycellulose cellulose derivatives, polyvinyl alcohols, polypyrrolidones, polyacrylic acid, acrylic acid-acrylonitrile copolymer, potassium acrylate-acrylonitrile copolymer, vinyl acetate-acrylic acid ester copolymer, acrylic acid-acrylic acid ester copolymer), and the like. styrene-acrylic resins such as styrene-acrylic acid copolymers, styrene-acrylic acid copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid-acrylate copolymers, styrene-m-methylstyrene-acrylic acid copolymers, styrene-maleic acid copolymers, styrene-maleic anhydride copolymers, vinyl naphthalene-acrylic acid copolymers, vinyl acetate-ethylene copolymers, vinyl acetate-fatty acid vinylethylene copolymers, vinyl acetate-maleic acid ester copolymers, vinyl acetate-croton copolymers, vinyl acetate-acrylic acid copolymers, and salts thereof), and surfactants (various anionic surfactants, nonionic surfactants, and amphoteric surfactants).

[0284] The self-dispersing surface-treated pigment can be dispersed or dissolved in water without using a dispersant by, for example, performing a surface treatment so that a carboxy group and a salt thereof are directly bonded to the surface of the pigment.

[0285] Specifically, it can be obtained by grafting functional groups or molecules containing functional groups onto the surface of the pigment through physical treatment using vacuum plasma or chemical treatment using an oxidizing agent such as sodium hypochlorite or ozone. A single or multiple types of functional groups may be grafted onto one pigment particle. The type and degree of grafting of functional groups are appropriately selected taking into consideration dispersion stability in the ink, color density, and drying properties at the front of the inkjet head.

[0286] The polymer-coated pigment is not particularly limited, but can be obtained, for example, by dispersing a pigment using a dispersant having a polymerizable group, and then carrying out emulsion polymerization in water using a monomer copolymerizable with the dispersant (copolymerizable monomer) and a photoradical polymerization initiator.

[0287] The polymer is preferably one obtained by polymerizing a monomer or oligomer having at least one double bond selected from the group consisting of an acryloyl group, a methacryloyl group, a vinyl group, and an allyl group according to a known polymerization method using a photoradical polymerization initiator. A general method can be used for emulsion polymerization, and polymerization proceeds by free radicals generated by thermal decomposition of a water-soluble photoradical polymerization initiator in the presence of an emulsifier.

[0288] The pigment and dispersant constituting the pigment dispersion may each be used alone or in combination of two or more.

[0289] The color ink according to the present invention preferably further contains a resin, water, an organic solvent, an additive, etc., and the resin, water, organic solvent, additive, etc. used in the ink according to the present invention can be used.

[0290] [3 Image forming method] The ink of the present invention is preferably used as part of the ink set for ink jet recording, and is further preferably used in combination with color inks.

[0291] When an image is formed using the ink set for inkjet recording, a layer formed from the white ink of the present invention (hereinafter also referred to as a "white ink layer") and a layer formed from the treatment liquid (hereinafter also referred to as a "treatment liquid layer") are laminated on the recording medium. When color inks are used in combination, a layer formed from the color inks (hereinafter also referred to as a "color ink layer") is further laminated.

[0292] The order in which each layer is stacked on the recording medium corresponds to the order in which each ink and treatment liquid are applied. For example, when the treatment liquid layer, white ink layer, and color ink layer are stacked in this order from the recording medium side, the treatment liquid, white ink, and color ink are applied to the recording medium in this order. The order in which each ink and treatment liquid are applied can be adjusted, for example, by the arrangement order of the carriages in the image forming apparatus. This will be described in more detail below.

[0293] By applying the treatment liquid onto a recording medium, the image quality of the recorded matter on the recording medium with low or no absorbency is improved, and the treatment liquid and the white ink of the present invention interact with each other, improving the image quality of the recorded matter formed with the white ink of the present invention. From this viewpoint, the order of lamination of the layers from the recording medium side is preferably a treatment liquid layer, a white ink layer, and a color ink layer.

[0294] Each layer may consist of only one layer or multiple layers. For example, from the recording medium side, the layers may be stacked in the following order: treatment liquid layer, white ink layer, color ink layer (1), white ink layer, and color ink layer (2). By stacking layers in this manner, a relatively bright color ink layer (2) can be formed with high color development on a relatively low-brightness color ink layer (1).

[0295] The steps of applying the treatment liquid, white ink, and color ink (methods of forming each layer) will be described below. As mentioned above, the order of the steps is not limited to this.

[0296] In the present invention, the "step of applying a treatment liquid onto a recording medium" refers to a step of applying a treatment liquid directly onto a recording medium. Furthermore, the "step of applying a treatment liquid onto a recording medium" is not limited to a step of applying a treatment liquid directly onto a recording medium, but also includes a step of applying a treatment liquid from above onto an area on a recording medium to which another ink or the like has been applied. The same applies to the white ink and the color ink.

[0297] Each step will be described below. While an example will be described in which the application steps are performed in the order of a treatment liquid application step, a white ink application step, and a color ink application step, the order of the steps may be reversed. For example, the steps may be performed in the order of a treatment liquid application step, a color ink application step, and a white ink application step. Furthermore, an ink heating and drying step, which will be described later, may be performed as appropriate.

[0298] 1) A step of applying a treatment liquid to a recording medium (treatment liquid application step) In the treatment liquid application step, although not particularly limited, it is preferable to apply the treatment liquid directly onto the recording medium. Note that in the treatment liquid application step, it is preferable to use the treatment liquid described above. Furthermore, during or after the treatment liquid application step, the method may further include a step of drying at least a portion of the applied treatment liquid.

[0299] The method for applying the treatment liquid is not particularly limited, but examples thereof include roller coating, curtain coating, spray coating, and inkjet coating. In the roller coating method, even when the viscosity of the treatment liquid is relatively high, the treatment liquid can be efficiently applied to the recording medium by using a roller coater, etc. The roller coater can be used in conjunction with an inkjet recording device.

[0300] On the other hand, the inkjet coating method can apply the treatment liquid only to the areas where the white ink is to be applied, and there is no need to apply the treatment liquid to areas where the white ink is not to be applied, so it is possible to prevent unreacted aggregating agent from being released and causing cloudiness in areas where the white ink is not to be applied.

[0301] As a method for applying the treatment liquid in the inkjet coating method, when the recording medium is metal, it is preferable to place the recording medium on a transport belt of the inkjet recording device and apply the treatment liquid while transporting the belt, or to fix the recording medium to a flatbed and apply the treatment liquid.

[0302] 2) A step of applying white ink to a recording medium (white ink application step) In the white ink application step, although not particularly limited, it is preferable to simultaneously apply the treatment liquid and the white ink onto the recording medium, or to apply the white ink directly onto the region of the recording medium to which the treatment liquid has been applied from above. In the white ink application step, the ink of the present invention is used. During or after the white ink application step, the method may further include a step of drying at least a portion of the treatment liquid and white ink applied to the recording medium.

[0303] When white ink is applied from above to an area where treatment liquid has been applied directly to the recording medium, the treatment liquid and white ink react directly, causing the white ink to quickly aggregate. Because the treatment liquid exerts its aggregation effect even if it is not completely dry, white ink can be applied immediately after the treatment liquid has been applied, allowing for faster recording speeds.

[0304] That is, after the treatment liquid application step, it is preferable to apply white ink to the area where the treatment liquid has been applied when the drying rate of the treatment liquid is 30% or less, and it is also preferable to apply white ink to the area where the treatment liquid has been applied within 10 seconds after the treatment liquid application step. The "drying rate of the treatment liquid" is defined by the following formula 5. (Equation 5) Drying rate of treatment liquid (%) = {1-(mass of processing solution after drying [g] / mass of processing solution before drying [g])} x 100

[0305] After the treatment liquid application process, by applying the white ink when the drying rate of the treatment liquid is 30% or less, the white ink and the treatment liquid are mixed appropriately, making it easier for the white pigment in the white ink to aggregate, thereby improving image quality. Furthermore, by applying the white ink within 10 seconds after the treatment liquid application step, it is possible to prevent the treatment liquid from penetrating into recording media with high absorbency and the treatment liquid from repelling onto recording media with low or no absorbency. After the treatment liquid application step, the drying rate of the treatment liquid can be adjusted to 30% or less by appropriately adjusting the time until the white ink is applied and the temperature of the recording medium.

[0306] From the viewpoint of image quality, the amount of white ink applied per unit area (also referred to as the "application amount") in the white ink application step is preferably in the range of 2 to 25 times, and more preferably in the range of 2.5 to 3.5 times, the amount of treatment liquid applied. The amount of white ink applied can be adjusted by the volume of the ink droplets ejected. The application amount here is expressed in mass [g].

[0307] Since the white ink of the present invention is an ink for inkjet recording, the white ink application step is carried out by an inkjet coating method. The inkjet coating method is not particularly limited and can be performed using a printer equipped with an inkjet head loaded with white ink. Specifically, white ink is ejected as droplets from the nozzles of the inkjet head based on a digital signal, and these droplets are caused to land on the treatment liquid layer of the recording medium, thereby applying the white ink and forming a white ink layer.

[0308] The inkjet head may be either an on-demand type or a continuous type. Examples of on-demand type inkjet heads include electro-mechanical conversion type inkjet heads, including single-cavity type, double-cavity type, bender type, piston type, shear mode type, and shared wall type inkjet heads, and electro-thermal conversion type inkjet heads, including thermal inkjet type and bubble jet type inkjet heads ("Bubble Jet" is a registered trademark of Canon Inc.). Among these, an inkjet head using a piezoelectric element as the electromechanical conversion element used in the electromechanical conversion method (also called a "piezo type inkjet head") is preferable.

[0309] The inkjet printer may be of either a scan type or a single pass type inkjet head, and in the case of a single pass type, it is preferably an inkjet head of a line head type.

[0310] The term "line head type inkjet head" refers to an inkjet head having a length equal to or greater than the width of the printing range. As a line head type inkjet head, a single head having a length equal to or greater than the width of the printing range may be used, or multiple heads may be combined to have a length equal to or greater than the width of the printing range.

[0311] Furthermore, a plurality of heads may be arranged in parallel so that the nozzles are arranged in a staggered pattern, thereby increasing the resolution of the heads as a whole.

[0312] The transport speed of the recording medium is preferably within the range of 1 to 120 m / min. The faster the transport speed, the faster the recording speed (image formation speed). The white ink of the present invention can produce high-resolution images even at a very high transport speed of 50 to 120 m / min, which is applicable to single-pass inkjet printers.

[0313] The thickness of the white ink layer formed by the white ink application step is preferably in the range of 0.3 to 3.0 μm, and more preferably in the range of 0.3 to 2.0 μm. Having a white ink layer thickness of 0.3 μm or more can improve the abrasion resistance of the image and its adhesion to the recording medium. Furthermore, having a white ink layer thickness of 3.0 μm or less can reduce the deformation stress applied to the white ink layer, making it less likely that the adhesion of the image to the recording medium will be impaired.

[0314] In the white ink application step, the surface temperature of the recording medium is preferably within the range of 10 to 65° C. From the viewpoint of excellent clogging reliability, it is more preferably 50° C. or less, and even more preferably 40° C. or less. On the other hand, from the viewpoint of increasing the recording speed, it is more preferably 20° C. or more, even more preferably 40° C. or more, and particularly preferably 50° C. or more.

[0315] 3) A step of applying color ink to the recording medium (color ink application step) In the color ink application step, although there are no particular limitations, it is preferable to apply white ink from above to the area where the treatment liquid has been applied directly onto the recording medium, and then apply color ink from above. In the color ink application step, it is preferable to use the above color inks and apply the color inks in the same manner as in the white ink application step.

[0316] When white ink is applied from above to an area where treatment liquid has been applied directly onto a recording medium, and then color ink is applied from above, the treatment liquid and white ink do not need to be completely dry. By applying color ink when the white ink is not completely dry, the treatment liquid and color ink can react, i.e., the color ink can be aggregated, even from above the white ink. Furthermore, even if the treatment liquid and white ink are not completely dry, it is possible to apply color ink while suppressing bleeding, and the recording speed can be further increased.

[0317] Before the color ink application step, a step of drying at least a portion of the treatment liquid and white ink applied to the recording medium may be included. In this drying step, it is preferable that the treatment liquid and white ink are not completely dried but are only partially dried. Furthermore, during or after the color ink application step, a step of drying the color ink applied to the recording medium may be included.

[0318] 4) A process of heating and drying the white ink or color ink (ink heating and drying process) The image forming method of the present invention may have an ink heating and drying step in addition to the treatment liquid applying step, the white ink applying step, and the color ink applying step.

[0319] The ink heating and drying step may be carried out only after the white ink application step, only after the color ink application step, or both after the white ink application step and the color ink application step. Furthermore, in the ink heating and drying step, the treatment liquid may also be dried in addition to the white ink and color ink, and the following will describe a case where both the ink and the treatment liquid are dried in the ink heating and drying step.

[0320] In the ink heating and drying process, the ink (here, "ink" may refer to only white ink, only color ink, or both white ink and color ink) applied to the recording medium, i.e., the area on the recording medium where the ink has been applied, is heated, thereby drying the ink and treatment liquid.

[0321] In the ink heating and drying step, the heating temperature of the area where the ink is applied is preferably within the range of 60 to 200° C. The heating time of the ink is preferably adjusted appropriately depending on the type of recording medium and the amount of ink applied.

[0322] Heating the ink-applied area can remove water and organic solvents, which are solvent components of the ink and treatment liquid, and can also improve the abrasion resistance and adhesion of the resulting image to the recording medium.

[0323] The heat drying may be carried out using a non-contact heating type drying device such as a drying oven or a hot air blower, or may be carried out using a contact heating type drying device such as a hot plate or a heat roller.

[0324] When a non-contact heating drying device such as a drying oven or a hot air blower is used, it is preferable to measure the drying temperature by measuring the ambient temperature such as the temperature inside the oven or the hot air temperature. When a contact heating drying device such as a hot plate or a heated roller is used, it is preferable to measure the temperature of the contact heating part or the surface temperature of the surface to be dried, and it is particularly preferable to measure the surface temperature of the surface to be dried.

[0325] <Image forming device> The image forming apparatus of the present invention is an image forming apparatus that uses an ink for ink jet recording containing at least a pigment, a dispersant, and water, and is characterized by having means for carrying out the image forming method described above. An example of the image forming apparatus of the present invention will be described below.

[0326] 1, the image forming apparatus 200 includes a paper feed unit 210, an image recording unit 220, a paper discharge unit 230, an ink circulation device (not shown) as an ink supply means, a control unit (not shown), etc. The image forming apparatus 200 transports a recording medium M stored in the paper feed unit 210 to the image recording unit 220, forms an image on the recording medium M in the image recording unit 220, and transports the recording medium M with the image formed thereon to the paper discharge unit 230.

[0327] The paper feed unit 210 has a paper feed tray 211 that stores the recording medium M, and a medium supply unit 212 that transports and supplies the recording medium M from the paper feed tray 211 to the image recording unit 220. The medium supply unit 212 has a ring-shaped belt supported on the inside by two rollers, and transports the recording medium M from the paper feed tray 211 to the image recording unit 220 by rotating the rollers with the recording medium M placed on this belt.

[0328] The image recording unit 220 includes a transport drum 221, a delivery unit 222, a heating unit 223, a carriage 224, a fixing unit 225, a delivery unit 226, and the like.

[0329] The transport drum 221 has a cylindrical surface, and its outer circumferential surface serves as a transport surface on which the recording medium M is placed. The transport drum 221 transports the recording medium M along the transport surface by rotating in the direction of the arrow in Fig. 1 while holding the recording medium M on the transport surface. The transport drum 221 also has claws and an air intake section (not shown), and holds the recording medium M on the transport surface by pressing down on the edges of the recording medium M with the claws and sucking the recording medium M toward the transport surface with the air intake section.

[0330] The transfer unit 222 is provided at a position between the medium supply section 212 of the paper feed section 210 and the conveying drum 221, and holds and picks up one end of the recording medium M conveyed from the medium supply section 212 with the swing arm section 222a, and transfers it to the conveying drum 221 via the transfer drum 222b.

[0331] The heating unit 223 is provided between the position where the delivery drum 222b is disposed and the position where the carriage 224 is disposed, and heats the recording medium M conveyed by the conveying drum 221 so that the temperature of the recording medium M falls within a predetermined temperature range. The heating unit 223 has, for example, an infrared heater or the like, and energizes the infrared heater based on a control signal supplied from a control unit (not shown) to cause the heater to generate heat.

[0332] In the present invention, the term "carriage" refers to a device that supports an inkjet head. There are no particular limitations on the carriage as long as it can support the inkjet head, and the carriage may or may not be equipped with an ink tank.

[0333] The inkjet head 100 disposed in the carriage 224 ejects ink onto the recording medium M at appropriate timing according to the rotation of the conveying drum 221 holding the recording medium M, based on image data, to form an image.

[0334] The carriages 224 are disposed at a predetermined distance such that their ink ejection surfaces face the transport drum 221. In the inkjet recording apparatus 200 of this embodiment, for example, five carriages 224 corresponding to the five colors of ink, white (W), yellow (Y), magenta (M), cyan (C), and black (K), are arranged at predetermined intervals from the upstream side in the transport direction of the recording medium M in the order of W, Y, M, C, and K. Furthermore, when a treatment liquid is used, a carriage 224 that ejects the treatment liquid may be disposed. For example, when the white ink of the present invention is used as a pretreatment liquid, the carriage 224 is disposed at the most upstream side in the transport direction of the recording medium M.

[0335] Furthermore, as described above, when the treatment liquid is applied by a coating method other than inkjet coating, an apparatus having a means for carrying out the treatment liquid application step may further be used.

[0336] The order in which each ink and treatment liquid is applied can be adjusted by appropriately adjusting the arrangement order of the carriages 224 corresponding to the white ink (white (W)), treatment liquid, and color inks (yellow (Y), magenta (M), cyan (C), and black (K)).

[0337] In the carriage 224, for example, as shown in FIG. 2, pairs of inkjet heads 100 adjacent to each other in the front-rear direction are arranged in a staggered pattern at different positions in the front-rear direction.

[0338] Furthermore, in the present invention, the image forming apparatus 200 is an image forming apparatus 200 that forms images by a one-pass drawing method using a line head, and the carriage 224 is used while its position relative to the rotation axis of the transport drum 221 is fixed when recording an image. However, the inkjet recording apparatus of the present invention may be of a scanning type, in which the carriage moves back and forth in the sub-scanning direction.

[0339] The fixing unit 225 dries and fixes the ink and treatment liquid ejected onto the recording medium M. The delivery section 226 has a belt loop 226b with a circular belt supported on the inside by two rollers, and a cylindrical transfer drum 226a that transfers the recording medium M from the transport drum 221 to the belt loop 226b. The recording medium M transferred from the transport drum 221 onto the belt loop 226b by the transfer drum 226a is transported by the belt loop 226b and sent to the paper discharge section 230.

[0340] The paper discharge unit 230 has a plate-shaped paper discharge tray 231 on which the recording medium P sent out from the image recording unit 220 by the delivery unit 226 is placed. [Example]

[0341] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these. In the examples, the terms "parts" and "%" are used, but unless otherwise specified, they represent "parts by mass" or "% by mass." In the following examples, unless otherwise specified, all operations were carried out at room temperature (25°C).

[0342] [Preparation of white ink] <Preparation of White Pigment Dispersion 1> The following ingredients were mixed and the resulting mixture was premixed. White pigment (CR-50-2, manufactured by Ishihara Sangyo Kaisha, Ltd., titanium oxide) 30.00 parts by mass Pigment dispersant (DISPERBYK-2019, manufactured by BYK-Chemie, solid content: 52% by mass) 20% by mass solution 7.50 parts by mass Propylene glycol 28.00 parts by mass Antifungal agent (Proxel GXL(S), manufactured by Lonza) 0.10 parts by mass Ion-exchanged water (balance, amount to make the total amount 100 parts by mass)

[0343] Thereafter, the mixture was dispersed using a bead mill filled with 0.3 mm zirconia beads at a volumetric rate of 60%, to prepare White Pigment Dispersion 1 with a pigment content of 30 mass %.

[0344] The average dispersed particle size of the pigment particles contained in White Pigment Dispersion 1 was 238 nm. The average dispersed particle size of the pigment particles was measured using a Zetasizer Nano S90 manufactured by Malvern Instruments.

[0345] <Preparation of White Pigment Dispersion 2> The following ingredients were mixed and the resulting mixture was premixed. White pigment (CR-50-2, manufactured by Ishihara Sangyo Kaisha, Ltd., titanium oxide) 30.00 parts by mass Pigment dispersant (DISPERBYK-2019, manufactured by BYK-Chemie, solid content: 52% by mass) 20% by mass solution 7.50 parts by mass Propylene glycol 28.00 parts by mass Antifungal agent (Proxel GXL(S), manufactured by Lonza) 0.10 parts by mass Ion-exchanged water (balance, amount to make the total amount 100 parts by mass)

[0346] Thereafter, the mixture was dispersed using a bead mill filled with 0.3 mm zirconia beads at a volumetric rate of 60%, to prepare white pigment dispersion 2 with a pigment content of 30 mass %. Then, a 20% by mass solution of the following anionic resin having a carboxy group neutralized with dimethylaminoethanol was added to and mixed with the above White Pigment Dispersion 2 to prepare White Pigment Dispersion 2. Anionic resin (BASF, Joncryl (registered trademark) 690) 20% by mass solution 7.50 parts by mass

[0347] The average dispersed particle size of the pigment particles contained in White Pigment Dispersion 2 was 235 nm. The average dispersed particle size of the pigment particles was measured using a Zetasizer Nano S90 manufactured by Malvern Instruments.

[0348] <Preparation of White Pigment Dispersions 3 to 24 and 30 to 32> White pigment dispersions 3 to 24 and 30 to 32 were prepared in the same manner as white pigment dispersion 2, except that the dispersants, anionic resins, and white pigments shown in Tables I to III were used. The average dispersed particle sizes of the pigment particles contained in white pigment dispersions 3 to 24 and 30 to 32 were as shown in Tables I to III, respectively.

[0349] <Preparation of White Dispersions 25 to 28> White Pigment Dispersions 25 to 28 were prepared in the same manner as White Pigment Dispersion 2, except that the dispersion time in the bead mill dispersion was changed and the average dispersed particle size of the white pigment particles was changed as shown in Table III.

[0350] <Preparation of White Pigment Dispersion 29> White Pigment Dispersion 29 was prepared in the same manner as White Pigment Dispersion 2, except that an anionic resin was added and mixed before dispersion in the bead mill, and then dispersion was performed using the bead mill. The average dispersed particle size of the pigment particles contained in White Pigment Dispersion 29 was as shown in Table III.

[0351] The dispersant, anionic resin and white pigment used are shown below. (dispersant) a: DISPERBYK (registered trademark) 2019, manufactured by BYK (Amine value: 22, Acid value: 0) b: DISPERBYK (registered trademark) 2018, manufactured by BYK (Amine value: 26, Acid value: 0) c: DISPERBYK (registered trademark) 2055, manufactured by BYK (Amine value: 40, Acid value: 0) d: TEGO® Dispers 650, manufactured by Evonik (Amine value: 50, Acid value: 0) e:EFKA (registered trademark) 6230, manufactured by BASF Japan Ltd. (Amine value: 100, Acid value: 0) f: DISPERBYK (registered trademark) 2010, manufactured by BYK-Chemie (Amine value: 20, Acid value: 20) g: DISPERBYK (registered trademark) 180, manufactured by BYK-Chemie (Amine value: 94, Acid value: 94) h: DISPERBYK (registered trademark) 190 (comparison example), manufactured by BYK-Chemie (Amine value: 0, Acid value: 10) i: TEGO® Dispers 655 (comparison example), manufactured by Evonik (Amine value: 0, Acid value: 100) j: DISPERBYK (registered trademark) 2061, manufactured by BYK-Chemie (Amine value: 3, Acid value: 0) k: Floren DOPA-100, manufactured by Kyoei Chemical Industry Co., Ltd. (Amine value: 135, Acid value: 15)

[0352] (anionic resin) t: JONCRYL (registered trademark) 690, manufactured by BASF Japan Ltd. (Acid value: 240, weight average molecular weight: 16500) u: JONCRYL (registered trademark) 819, manufactured by BASF Japan Ltd. (Acid value: 75, weight average molecular weight: 15000) v:3401MA, manufactured by Taisei Fine Chemical Co., Ltd. (Acid value: 27, weight average molecular weight: 27000) w: JONCRYL (registered trademark) 611, manufactured by BASF Japan Ltd. (Acid value: 53, weight average molecular weight: 8100) x: ARUFON (registered trademark) UF-5041, manufactured by Toagosei Co., Ltd. (Acid value: 260, weight average molecular weight: 7500) y: JONCRYL (registered trademark) 680, manufactured by BASF Japan Ltd. (Acid value: 215, weight average molecular weight: 4500) z: JONCRYL (registered trademark) 693, manufactured by BASF Japan Ltd. (Acid value: 205, weight average molecular weight: 6000)

[0353] (white pigment) α: CR-50-2, manufactured by Ishihara Sangyo Kaisha (Titanium oxide, rutile type, main treatment agent: alumina and organic matter) β: PF691, manufactured by Ishihara Sangyo Kaisha (Titanium oxide, rutile type, main treatment agents: alumina, silica and organic matter) γ:CR-58, manufactured by Ishihara Sangyo Kaisha (Titanium oxide, rutile type, main treatment agent: alumina) δ: A-220, manufactured by Ishihara Sangyo Kaisha, Ltd. (Titanium oxide, anatase type)

[0354] The composition of each white pigment dispersion is shown in Tables I to III below. In the table, "-" indicates that no additive was added. The units for amine value and acid value are [mgKOH / g], the unit for average dispersed particle size is [nm], and the unit for content is [parts by mass], where "DMAE" represents dimethylaminoethanol and "NaOH" represents sodium hydroxide. Furthermore, for the contents of the 20 mass% dispersant solution and the 20 mass% anionic resin solution, the figures in parentheses represent the actual contents [parts by mass] of the dispersant or anionic resin when the total amount of each white pigment dispersion is 100 parts by mass, and "dispersant:anionic resin" represents the mass ratio of the contents of the dispersant and the anionic resin.

[0355] [Table 1]

[0356] [Table 2]

[0357] [Table 3]

[0358] <Preparation of White Ink 1> The following components were mixed and the resulting mixture was filtered through a 1 μm filter to prepare White Ink 1. There was no substantial change in composition before and after filtration. White pigment dispersion 1 33.33 parts by mass (10 parts by mass as white pigment solids) Resin particle dispersion (Vylonal MD-2000, manufactured by Toyobo Co., Ltd., aqueous dispersion of polyester resin with a number average molecular weight of 18,000, solid content concentration: 40% by mass) 12.50 parts by mass Propylene glycol 21.00 parts by mass Diethylene glycol monobutyl ether 1.60 parts by mass Glycerin 5.00 parts by mass Surfactant (KF351A, manufactured by Shin-Etsu Silicone Co., Ltd.) 0.30 parts by mass Surfactant (E1010, manufactured by Nissin Chemical Industry Co., Ltd.) 1.00 parts by mass Antifungal agent (Proxel GXL(S), manufactured by Lonza) 0.10 parts by mass Ion-exchanged water (balance, amount to make the total amount 100 parts by mass)

[0359] <Preparation of White Inks 2 to 32> White inks 2 to 32 were prepared in the same manner as White ink 1, except that the white pigment dispersions shown in Tables IV to VI were used.

[0360] <Preparation of pretreatment solution> The following components were mixed, and the resulting mixture was filtered through a 5 μm membrane filter to obtain a pretreatment liquid. Propylene glycol 30.00 parts by mass Flocculant (calcium acetate, manufactured by Kanto Chemical Co., Ltd.) 3.00 parts by mass Silicone surfactant (BYK (registered trademark) 3450, manufactured by BYK-Chemie) 1.00 parts by mass Ion-exchanged water 66.00 parts by mass

[0361] <Evaluation> (1) Redispersibility (redispersibility of white pigment) 10 g of each white ink was placed in a 15 mL polypropylene centrifuge tube, and centrifugal force equivalent to 90 days was applied in a centrifuge to settle the white pigment. The tube was then left to stand for 15 to 20 hours. After standing, the centrifuge tube was gently tilted and the height of the sedimented white pigment was measured. The centrifuge tube was then fixed to a shaker and shaken to re-disperse the white pigment. After shaking for 60 minutes, the centrifuge tube was tilted again and the height of the sedimented white pigment was measured.

[0362] The volume height of the settled white pigment was evaluated according to the following criteria, with a score of △ or higher being considered acceptable. ◎: The height of the white pigment accumulation is 9 mm or less. Good: The height of the white pigment accumulation is more than 9 mm and less than 10 mm. △: The height of the white pigment accumulation is more than 10 mm and 11 mm or less. ×: The height of the white pigment accumulation is more than 11 mm.

[0363] (2) Storage stability (storage stability of ink) The average dispersed particle size of the white pigment particles in each ink and the viscosity of the ink were measured before and after heating each white ink for two weeks at 60°C. The average dispersed particle size of the white pigment particles in the ink was measured using a Malvern Zetasizer Nano S90, and the viscosity of the ink was measured using a Toki Sangyo TVE-33LT. The measurement conditions were 25°C, a standard cone rotor (0.8° x R24), a sample liquid volume of 600 μL, and 1 minute.

[0364] The rate of change between the average dispersed particle size of the white pigment particles in the ink and the viscosity of the ink before heating and the average dispersed particle size of the white pigment particles in the ink and the viscosity of the ink after heating was calculated using the following formulas.

[0365] (Equation 6) Change rate (%) of average dispersed particle size of white pigment particles in ink = {(average dispersed particle size of white pigment particles in ink after heating - average dispersed particle size of white pigment particles in ink before heating) / average dispersed particle size of white pigment particles in ink before heating} × 100 (Equation 7) Change rate of ink viscosity (%) = {(viscosity of ink after heating - viscosity of ink before heating) / viscosity of ink before heating} x 100

[0366] The rate of change in both the average dispersed particle size of the white pigment particles in the ink and the viscosity of the ink was evaluated according to the following criteria, with a score of △ or better being considered a pass. ○: The values ​​for both change rates are 5% or less. △: The value is 10% or less for both of the two rate of change (excluding cases where the value is 5% or less for both of the two rate of change, i.e., cases corresponding to ○). ×: At least one of the two change rates exceeds 10%.

[0367] (3) Image quality (uneven image) The pretreatment liquid and each white ink were combined to form an ink set, and printing was carried out in the following manner: A PET film (FE2001, thickness 50 μm, manufactured by Futamura Chemical Co., Ltd.) was prepared as a recording medium. A scanning printer equipped with two independently driven inkjet heads (360 npi, ejection volume of 6 pL or 14 pL, 1024 nozzles) manufactured by Konica Minolta was prepared, and the head to be recorded first was filled with pretreatment liquid, and the head to be recorded second was filled with white ink. An image with a resolution of 720 x 720 dpi was then divided into two in the scanning direction X and the transport direction Y to create four images (180 x 180 dpi), and printing was performed in one direction, always with the pretreatment liquid being recorded first, in a four-pass mode in which one printing area is printed four times.

[0368] The carriage transport speed was set to 300 mm / sec, and no drying process was performed between the printing of the pretreatment liquid and the white ink. The printing test was conducted in an environment of 25°C and 50% RH. The pretreatment liquid was applied in accordance with the ink image area, with a liquid volume of 6 pL and a maximum printing rate of 33%. The white ink was applied in accordance with the liquid volume of 14 pL and a maximum printing rate of 100%. With the above settings, the amount of pretreatment liquid applied to the solid area was 1.7 g / m. 2 The amount of white ink applied to the solid area is 11.9 g / m 2 The ratio of the amount of white ink applied to the amount of pretreatment liquid applied to the solid area (amount of white ink applied / amount of pretreatment liquid applied) was 7.0 times. In the above printing, after the white ink was applied, the PET film was placed in a dryer set at 90° C. and dried by heating for 5 minutes to obtain an image recording material.

[0369] An image recording material was prepared by recording a 5 cm x 5 cm solid image using the method described above, and the density unevenness of the solid portion was visually evaluated according to the following criteria. A score of △ or better was considered to be acceptable. ◯: No density unevenness is observed in the image when observed from a distance of 15 cm. △: When observed from a distance of 15 cm, density unevenness is observed in part of the image, but from a distance of 30 cm, density unevenness is not observed. ×: When observed from a distance of 30 cm, uneven density was observed in the image.

[0370] (4) Whiteness An image recording material was prepared by recording a 5 cm x 5 cm solid image using the method described above, and the whiteness of the solid area was measured using a Konica Minolta fluorescent spectrodensitometer "FD-7" and evaluated according to the following criteria. A score of △ or better was considered to be acceptable. ◯: Whiteness is 76 or more. △: Whiteness is 74 or more and less than 76. ×: Whiteness is less than 74.

[0371] (5) Discharge stability Each white ink was filled into an independently driven inkjet head (360 npi, ejection volume 6 pL, 1024 nozzles) manufactured by Konica Minolta, and a 30-minute continuous ejection test was carried out using a strobe-synchronized droplet observation device. The ejection stability was evaluated according to the following criteria. A grade of △ or better was considered a pass. ◯: Of the 256 nozzles evaluated, white ink was ejected normally from all nozzles (256 nozzles), and no ejection abnormalities were observed. △: Abnormal ejection was observed in 1 or more and 4 or less of the 256 nozzles evaluated. ×: Abnormal ejection was observed in 5 or more nozzles out of the 256 nozzles evaluated.

[0372] (6) Maintainability 0.05 g of each white ink was dropped onto a member used in the nozzle section of an independently driven inkjet head manufactured by Konica Minolta, Inc., and dried for 10 minutes at 50°C. The dried white ink was then wiped with a wiping member soaked in the following maintenance liquid to perform nozzle maintenance. The maintenance liquid was prepared by mixing the following ingredients. Dipropylene glycol monopropyl ether 5.00 parts by mass 2-amino-2-methyl-1-propanol 2.00 parts by mass Surfactant (E1010, manufactured by Nissin Chemical Industry Co., Ltd.) 0.05 parts by mass Antifungal agent (Proxel GXL(S), manufactured by Lonza) 0.10 parts by mass Ion-exchanged water (balance: amount that makes the total amount 100 parts by mass) The wiping member used was a Rubycell stick head (polyurethane) manufactured by AS ONE Corporation.

[0373] The number of times the nozzle was maintained (number of times it was rubbed) and the state of peeling of the dried material (dried white ink) were evaluated according to the following criteria, with a score of △ or better being considered a pass. ◎: Dried materials can be removed with maintenance work performed three times or less. Good: Dried materials can be removed with maintenance work performed 4 to 7 times. △: Dried matter can be removed with maintenance work performed 8 or more and 10 or less times. ×: The dried matter could not be removed even after performing the maintenance work 11 times.

[0374] The evaluation results are shown in Tables IV to VI below. No evaluation of the ink set (image quality and whiteness) was performed for Ink 1. In the table, "-" indicates that no evaluation was performed.

[0375] [Table 4]

[0376] [Table 1]

[0377] [Table 6]

[0378] A comparison of inks 1, 2, and 4 to 9 with inks 30 to 32 (comparative examples) shows that the amine value of the dispersant according to the present invention is greater than 0, i.e., the dispersant according to the present invention has basic nitrogen, thereby improving the redispersibility of the white pigment.

[0379] A comparison of inks 2 and 4 to 9 with inks 14 and 15 shows that the storage stability of the ink is improved when the amine value of the dispersant is within the range of 5 to 100 mgKOH / g.

[0380] Comparing inks 2, 10, and 11 with ink 13, it can be seen that whiteness and maintainability are improved when the dispersant content is within the range of 2.5 to 15 mass % relative to the total mass of the white pigment.

[0381] A comparison of Ink 2 and Ink 24 shows that inks with similar properties can be prepared by neutralizing the anionic resin with the basic compound dimethylaminoethanol or sodium hydroxide.

[0382] A comparison of inks 2, 3, 17, 19 and 20 with inks 16 and 18 shows that image quality is improved when the acid value of the anionic resin is within the range of 50 to 250 mgKOH / g.

[0383] A comparison of inks 2, 3, 16 to 18, and 20 with ink 19 shows that the storage stability and maintenance of the ink are improved when the weight average molecular weight of the anionic resin is within the range of 5,000 to 30,000.

[0384] Comparing inks 2, 10, and 11 with inks 12 and 13, it can be seen that the redispersibility, whiteness, and maintainability of the white pigment are improved when the mass ratio of dispersant to anionic resin is within the range of 1:6 to 6:1.

[0385] A comparison of Inks 2, 21, and 22 with Ink 23 shows that inks with similar properties can be prepared by using a white metal oxide as the white pigment and titanium oxide particles. Furthermore, the storage stability and whiteness of the ink are improved by using rutile-type titanium oxide particles that have been treated with alumina.

[0386] A comparison of inks 2, 26, and 27 with inks 25 and 28 shows that the storage stability, image quality, whiteness, and ejection stability of the ink are improved by having the average dispersed particle size of the white pigment within the range of 150 to 350 nm.

[0387] A comparison of Ink 2 and Ink 29 shows that in preparing a white pigment dispersion, whether the anionic resin is added and mixed after bead mill dispersion, or the anionic resin is added and mixed before bead mill dispersion and then dispersed using a bead mill, inks with similar properties can be prepared using either method. [Industrial Applicability]

[0388] By using the present invention, the redispersibility of the white pigment in the inkjet recording ink is improved, and an inkjet recording ink that can be stored for a long period of time can be provided. Furthermore, by using the ink of the present invention in combination with color inks, it is possible to provide recorded matter with improved color development. [Explanation of symbols]

[0389] 100 Inkjet head 111 Nozzle 200 Image forming device 210 Paper feed section 211 Paper tray 212 Media supply section 220 Image recording unit 221 Transport drum 222 Delivery Unit 223 Heating section 224 Carriage 225 Fixing section 226 Delivery Department 230 Paper output section 231 Paper output tray M Recording medium P Recording medium

Claims

1. An inkjet recording ink containing at least a white pigment, a dispersant, and water, The dispersant has a basic nitrogen, the inkjet recording ink further contains an anionic resin, the acid value of the anionic resin is in the range of 50 to 250 mg KOH / g; The weight average molecular weight of the anionic resin is within the range of 5,000 to 30,000.

1. An ink for ink-jet recording.

2. The amine value of the dispersant is in the range of 5 to 100 mgKOH / g.

2. The ink for ink-jet recording according to claim 1.

3. The content of the dispersant is in the range of 2.5 to 15% by mass with respect to the total mass of the white pigment.

3. The ink for ink-jet recording according to claim 1 or 2.

4. The anionic resin is neutralized with a basic compound. The ink for ink-jet recording according to any one of claims 1 to 3.

5. The mass ratio of the content of the dispersant to the content of the anionic resin is within the range of 1:6 to 6:

1. The ink for ink-jet recording according to any one of claims 1 to 4.

6. The white pigment is a white metal oxide. The ink for ink-jet recording according to any one of claims 1 to 5.

7. The white metal oxide is rutile titanium oxide particles.

7. The ink for ink-jet recording according to claim 6.

8. The white pigment is at least alumina-treated. The ink for ink-jet recording according to any one of claims 1 to 7.

9. The average dispersed particle size of the white pigment is in the range of 150 to 350 nm. The ink for ink-jet recording according to any one of claims 1 to 8.

10. A method for producing the inkjet recording ink according to any one of claims 1 to 9, comprising the steps of: a step of preparing a pigment dispersion (I) containing the white pigment, the dispersant, and the water, and a step of mixing the anionic resin with the pigment dispersion (I) to prepare a pigment dispersion (II).

1. A method for producing an ink for ink-jet recording, comprising:

11. A method for producing the inkjet recording ink according to any one of claims 1 to 9, comprising the steps of: preparing a pigment dispersion (III) containing the white pigment, the dispersant, the water, and the anionic resin; 1. A method for producing an ink for ink-jet recording, comprising:

12. An ink set for inkjet recording, comprising the ink for inkjet recording according to any one of claims 1 to 9 and a treatment liquid, The treatment liquid contains a cationic compound.

1. An ink set for ink jet recording comprising:

13. An image forming method using the ink set for inkjet recording according to claim 12, applying the treatment liquid to a recording medium; and applying the inkjet recording ink to the recording medium. An image forming method comprising:

14. An image forming method using the ink set for ink jet recording and the color inks according to claim 12, applying the treatment liquid to a recording medium; applying the inkjet recording ink to the recording medium; and applying the color ink to the recording medium. An image forming method comprising:

15. An image forming method using the ink set for ink jet recording and the color inks according to claim 12, applying the treatment liquid onto a recording medium; applying the ink for inkjet recording to the area to which the treatment liquid has been applied; and applying the color ink to the area where the ink for inkjet recording has been applied. An image forming method comprising:

16. An image forming apparatus using an inkjet recording ink containing at least a white pigment, a dispersant, and water, A device for carrying out the image forming method according to any one of claims 13 to 15. An image forming apparatus characterized by:

Images