Inkjet ink, ink set, and image recording method

Abstract

Provided are an ink-jet ink, an ink set, and an image-recording method, with which images having excellent hiding properties can be recorded and which can inhibit the ink-jet head from being deteriorated by wear. The ink-jet ink comprises water, titanium dioxide particles, and at least one silicon compound, wherein the titanium dioxide particles have an average primary-particle diameter of 100 nm or larger, the silicon compound is at least one compound selected from the group consisting of silicic acid salts and colloidal silica, and the content of the silicon compound is 0.0020 mass% or higher with respect to the amount of all the titanium dioxide particles, the ratio of the volume-average particle diameter of the colloidal silica to the average primary-particle diameter of the titanium dioxide particles being 0.04 or less.

Description

Technical Field

[0001] The present disclosure relates to an inkjet ink, an ink set, and an image recording method.

Background Art

[0002] In recent years, studies have been made on image recording using an inkjet ink containing a white pigment. For example, Patent Document 1 discloses a white pigment composition capable of improving sedimentation stability and whiteness in a well-balanced manner, having core particles and a shell layer covering the surface of the core particles and made of titanium dioxide, with an average particle size of 50 nm or more and 5000 nm or less, core-shell type titanium dioxide particles, silicon oxide particles with an average particle size of 3 nm or more and 100 nm or less, and a resin. Patent Document 1 also discloses using the above white pigment composition as an ink in an inkjet recording method. Further, Patent Document 2 discloses a recording method that is less likely to cause cracking while maintaining good image quality, having a white ink adhesion step of discharging white ink from a recording head and attaching it to a recording medium, a non-white ink adhesion step of discharging non-white ink containing a non-white colorant from a recording head and attaching it to a recording medium, and a pretreatment liquid adhesion step of attaching a pretreatment liquid containing a flocculant to the recording medium, wherein the white ink contains a white pigment and inorganic fine particles having a smaller volume average particle diameter than the white pigment.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] <管理番号 Image recording using inkjet ink containing titanium dioxide particles as a white pigment allows for the recording of images with excellent opacity. Here, opacity refers to the property of covering and concealing the underlying surface (e.g., the substrate, other images recorded on the substrate, etc.). However, through the inventors' research, it was found that in image recording using inkjet ink containing titanium dioxide particles as a white pigment, wear and deterioration of the inkjet head for ejecting the inkjet ink may occur. Here, wear and deterioration of the inkjet head refers to deterioration caused by the polishing of the contact area between the inkjet head and the inkjet ink by the titanium dioxide particles. Furthermore, through investigations by the inventors, it was found that when images are recorded under conditions designed to suppress wear and deterioration of the inkjet head, the opacity of the image may decrease.

[0005] An object of one aspect of this disclosure is to provide an inkjet ink, an ink set, and an image recording method that can record images with excellent opacity and suppress wear and deterioration of the inkjet head. [Means for solving the problem]

[0006] The following are examples of specific means for solving the problem: <1> It contains water, titanium dioxide particles, and silicon compounds. The average primary particle diameter of titanium dioxide particles is 100 nm or more. The silicon compound is at least one selected from the group consisting of silicates and colloidal silica. The silicon compound content relative to the total amount of titanium dioxide particles is 0.0020% by mass or more. The ratio of the volume-average particle diameter of colloidal silica to the average primary particle diameter of titanium dioxide particles is 0.04 or less. Inkjet ink. <2> Silicon compounds include silicates, The silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates. <1> The inkjet inks listed above. <3> Furthermore, it contains a polymer dispersant, The polymer dispersant contains a block polymer or a polymer having a crosslinked structure. <1> or <2> The inkjet inks listed above. <4> The silicon compound content relative to the total amount of titanium dioxide particles is 0.040% by mass to 2.0% by mass. <1> ~ <3> Inkjet ink as described in one of the following. <5> <1> ~ <4> White inkjet ink as described in one of the following, A color ink containing water and color pigments, An ink set including this. <6> If the silicon compound contained in the white ink is considered to be the first silicon compound, The color ink further contains a second silicon compound, which is at least one selected from the group consisting of silicates and colloidal silica. When the content of the first silicon compound relative to the total amount of white ink is X1 by mass, and the content of the second silicon compound relative to the total amount of color ink is X2 by mass, the X1 / X2 ratio is less than 1.0. <5> The ink set described above. <7> If the silicon compound contained in the white ink is considered to be the first silicon compound, The color ink further contains a second silicon compound, which is at least one selected from the group consisting of silicates and colloidal silica. The first silicon compound contains a silicate, The second silicon compound includes colloidal silica. <5> or <6> The ink set described above. <8> Furthermore, the pretreatment solution includes water and a coagulant. <5> ~ <7> The ink set listed in one of the following options. <9> An inkjet ink according to any one of <1> to <4>, and a pretreatment liquid containing water and a flocculant, and an ink set including the above. <10> An image recording method including an ink application step of applying an inkjet ink according to any one of <1> to <4> onto a non-permeable substrate by an inkjet method. Image recording method. <11> Before the ink application step, further including a pretreatment liquid application step of applying a pretreatment liquid containing water and a flocculant onto the non-permeable substrate. The ink application step is to apply the inkjet ink onto the area where the pretreatment liquid is applied on the non-permeable substrate where the pretreatment liquid is applied. The image recording method according to <10>. <12> Using an ink set according to any one of <5> to <9>, a color ink application step of discharging color ink from an inkjet head and applying it onto a non-permeable substrate, and using the ink set, discharging white ink from an inkjet head, and applying it onto the area where the color ink is applied on the non-permeable substrate where the color ink is applied, a white ink application step, and An image recording method including the above. <13> Before the color ink application step, further including a pretreatment liquid application step of applying a pretreatment liquid containing water and a flocculant onto the non-permeable substrate. The color ink application step is to apply the color ink onto the area where the pretreatment liquid is applied on the non-permeable substrate where the pretreatment liquid is applied. The image recording method according to <12>.

Effect of the Invention

[0007] According to one aspect of the present disclosure, there are provided an inkjet ink, an ink set, and an image recording method capable of recording an image with excellent concealment and suppressing wear and deterioration of an inkjet head.

Brief Description of the Drawings

[0008] [Figure 1] This is a character image used for evaluation in the example. [Modes for carrying out the invention]

[0009] In this disclosure, a numerical range represented by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. In this disclosure, the amount of each component in the composition means the total amount of any multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition. In the numerical ranges described in stages within this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages, or with the values ​​shown in the examples. In this disclosure, the term "process" includes not only independent processes but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved. In this disclosure, a preferred combination of embodiments is a more preferred embodiment.

[0010] In this disclosure, "image" means the entire film formed by applying ink, and "image recording" means the formation of the film. The concept of "image" in this disclosure also includes solid images.

[0011] [Inkjet ink] The inkjet ink disclosed herein (hereinafter also simply referred to as "ink") It contains water, titanium dioxide particles, and silicon compounds. The average primary particle diameter of titanium dioxide particles is 100 nm or more. The silicon compound is at least one selected from the group consisting of silicates and colloidal silica. The silicon compound content relative to the total amount of titanium dioxide particles is 0.0020% by mass or more. The ratio of the volume-average particle diameter of colloidal silica to the average primary particle diameter of titanium dioxide particles is 0.04 or less. That is, the inkjet ink of the present disclosure contains water, titanium dioxide particles, and a silicon compound, wherein the average primary particle diameter of the titanium dioxide particles is 100 nm or more, the silicon compound is at least one selected from the group consisting of silicates and colloidal silica, the content of the silicon compound relative to the total amount of titanium dioxide particles is 0.0020% by mass or more, and if the silicon compound includes colloidal silica, the ratio of the volume-average particle diameter of the colloidal silica to the average primary particle diameter of the titanium dioxide particles is 0.04 or less.

[0012] The inks of this disclosure enable the recording of images with excellent opacity, and suppress wear and deterioration of the inkjet head when recording images. The reason for this effect is presumed to be as follows:

[0013] The opacity of the image (i.e., the property of covering and concealing the background) is thought to be due to the fact that titanium dioxide particles have a high refractive index and that the average primary particle diameter of titanium dioxide particles is 100 nm or more.

[0014] It is believed that certain silicon compounds contained in the ink, specifically silicates and / or small-particle colloidal silica (specifically, colloidal silica whose average primary particle diameter is 0.04 or less compared to the average primary particle diameter of titanium dioxide particles; the same applies hereinafter), contribute to the effect of suppressing wear and deterioration of inkjet heads. More specifically, it is believed that the wear and tear of the inkjet head occurs because the contact area between the inkjet head and the ink is polished by titanium dioxide particles, which are hard particles contained in the ink. In the ink of this disclosure, a specific silicon compound (specifically, silicate and / or the small-particle colloidal silica) is thought to act as a cushion between the titanium dioxide particles and the contact portion of the inkjet head, thereby suppressing wear and deterioration of the inkjet head.

[0015] The fact that the average primary particle size of titanium dioxide particles is 100 nm or larger is thought to contribute to the effect of suppressing wear and deterioration of the inkjet head. This is thought to be because the average primary particle size of the titanium dioxide particles is not too small, which suppresses minute polishing scratches that have a significant impact on wear and degradation.

[0016] The following describes the components that may be contained in the ink of this disclosure.

[0017] <Water> The inks in this disclosure contain water. The water content is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and even more preferably 60% by mass or more, relative to the total amount of ink. The upper limit of the water content is determined appropriately depending on the content of other components. Examples of upper limits for water content include 90% by mass and 80% by mass.

[0018] <Titanium dioxide particles> The ink of this disclosure contains at least one type of titanium dioxide particle. Titanium dioxide particles have a high refractive index, which contributes to improving the opacity of images. In the inks of this disclosure, titanium dioxide particles preferably function as a white pigment. The ink of this disclosure is preferably a white ink.

[0019] The average primary particle size of the titanium dioxide particles contained in the ink disclosed herein is 100 nm or larger. This improves the opacity of the image. Furthermore, it also helps to suppress wear and degradation of the inkjet head. The average primary particle diameter of titanium dioxide particles is preferably 150 nm or more, and more preferably 200 nm or more. Furthermore, from the viewpoint of ink ejection performance, the average primary particle diameter of titanium dioxide particles is preferably 400 nm or less, and more preferably 300 nm or less.

[0020] In this disclosure, the average primary particle size of titanium dioxide particles is a value measured using a transmission electron microscope (TEM). A JEOL Ltd. 1200EX transmission electron microscope can be used for the measurement. Specifically, a Cu200 mesh (manufactured by JEOL Ltd.) with a carbon film attached is dripped with ink diluted 1,000 times and dried. Then, the circular equivalent diameter of 300 independent, non-overlapping titanium dioxide particles is measured from an image magnified 100,000 times using a TEM, and the measured values ​​are averaged to determine the average particle diameter.

[0021] The titanium dioxide particle content in the ink of this disclosure is preferably 1% to 20% by mass, more preferably 3% to 20% by mass, even more preferably 5% to 20% by mass, even more preferably 6% to 20% by mass, and even more preferably 7% to 15% by mass, based on the total amount of ink. When the titanium dioxide particle content in the ink is 1% by mass or more relative to the total amount of ink, the image opacity is further improved. When the content of titanium dioxide particles in the ink is 20% by mass or less of the total amount of ink, it is advantageous in terms of improving ink ejection performance and suppressing wear and deterioration of the inkjet head.

[0022] <Silicon compounds> The inks of this disclosure contain a silicon compound (hereinafter also referred to as the "specific silicon compound") which is at least one selected from the group consisting of silicates and colloidal silica. As mentioned above, specific silicon compounds contribute to suppressing wear and deterioration of inkjet heads. For specific silicon compounds, prior art such as Japanese Patent No. 5430316 may be referenced as appropriate. Furthermore, specific silicon compounds also contribute to suppressing the dissolution and degradation of inkjet heads. In this context, inkjet head dissolution degradation refers to the degradation that occurs when components in the parts of the inkjet head that come into contact with the ink dissolve into the ink. The effect of suppressing the dissolution degradation of inkjet heads is thought to be due to the silicon (Si) in specific silicon compounds. More specifically, it is thought that when dissolution degradation of the inkjet head occurs (more specifically, when components of the ink-contact portion of the inkjet head dissolve into the ink), a repair effect occurs in which the silicon (Si) in the specific silicon compound contained in the ink repairs the areas where dissolution degradation has occurred. This repair effect is particularly effective when at least a portion of the ink-contact portion of the inkjet head contains silicon. However, it is thought that the above repair effect can also be obtained even when the above portion does not contain silicon. Examples of parts of an inkjet head that come into contact with ink include the nozzle plate and the ink flow path.

[0023] (Silicate) As silicates that can constitute a specific silicon compound, water-soluble silicates are preferred.

[0024] In this disclosure, "water-soluble" means a substance whose solubility in 100g of distilled water at 25°C is 1g or more. Regarding "water solubility," it is preferable that the solubility in distilled water at 25°C is 2g or more, more preferably 5g or more, and even more preferably 10g or more.

[0025] The silicate may be a salt of metasilicic acid or a salt of orthosilicic acid. As the silicate, a commercially available compound (e.g., water glass) may be used, or one that is prepared by hand may be used.

[0026] Examples of silicates include: Alkali metal salts of silica, such as sodium silicate and potassium silicate; Alkaline earth metal salts of silica, such as calcium silicate and magnesium silicate; Ammonium salts of silicates; These are some examples. From the viewpoint of suppressing wear and deterioration of the inkjet head, it is preferable that the silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates.

[0027] From the viewpoint of suppressing wear and deterioration of the inkjet head, it is preferable that the specific silicon compound contains a silicate, and that the silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates.

[0028] (Colloidal silica) The colloidal silica that can constitute the specific silicon compound is colloidal silica whose volume-average particle diameter is 0.04 or less compared to the average primary particle diameter of titanium dioxide particles. This allows it to perform the cushioning role described above, and thus has the effect of suppressing wear and deterioration of the inkjet head. Furthermore, because the volume-average particle diameter of colloidal silica is 0.04 or less compared to the average primary particle diameter of titanium dioxide particles, the surface area of ​​colloidal silica is increased to a certain extent (i.e., the surface area of ​​colloidal silica is prevented from becoming too small), and as a result, the effect of suppressing the dissolution degradation of the inkjet head is also achieved.

[0029] There is no particular lower limit to the ratio of the volume-average particle diameter of colloidal silica to the average primary particle diameter of titanium dioxide particles (hereinafter also referred to as "particle diameter ratio [colloidal silica / titanium dioxide particles]") from the perspective of its effect in suppressing wear and degradation of the inkjet head. The lower limit of the particle size ratio [colloidal silica / titanium dioxide particles] may be, for example, 0.001, 0.05, or 0.01. The upper limit of the particle size ratio [colloidal silica / titanium dioxide particles] is 0.04, but may also be, for example, 0.03 or 0.02.

[0030] Colloidal silica is a colloid composed of fine particles of inorganic oxides containing silicon. Colloidal silica contains silicon dioxide (including its hydrate) as its main component, and may also contain aluminates as a small component. Examples of aluminates that may be included in small amounts include sodium aluminate and potassium aluminate. Furthermore, colloidal silica may contain inorganic salts such as sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium hydroxide, as well as organic salts such as tetramethylammonium hydroxide. These inorganic and organic salts act, for example, as colloidal stabilizers.

[0031] There are no particular restrictions on the dispersion medium in colloidal silica; it may be water, an organic solvent, or a mixture thereof. The organic solvent may be a water-soluble organic solvent or a non-water-soluble organic solvent, but a water-soluble organic solvent is preferred. Examples of water-soluble organic solvents include methanol, ethanol, isopropyl alcohol, and n-propanol.

[0032] There are no particular restrictions on the method of producing colloidal silica, and it can be produced using commonly used methods. For example, it can be produced by aerosil synthesis through the thermal decomposition of silicon tetrachloride or from water glass. Alternatively, it can be produced by liquid-phase synthesis methods such as hydrolysis of alkoxides (see, for example, "Textiles and Industry," Vol. 60, No. 7 (2004), p. 376).

[0033] The volume-average particle size of colloidal silica only needs to satisfy the particle size ratio [colloidal silica / titanium dioxide particles] of 0.04 or less. From the viewpoint of more effectively suppressing wear and deterioration of the inkjet head, the volume-average particle diameter of colloidal silica is preferably 200 nm or less, more preferably 100 nm or less, even more preferably 50 nm or less, even more preferably 25 nm or less, and even more preferably 20 nm or less. Examples of lower limits for the volume-average particle size of colloidal silica include 1 nm and 3 nm.

[0034] The volume-average particle size of colloidal silica is determined by dynamic light scattering. For measuring volume-average particle diameter using dynamic light scattering, for example, the NanoTrac UPA manufactured by MicroTrac is used.

[0035] The shape of the colloidal silica is not particularly limited, as long as it does not hinder the ink ejection performance. For example, it may be spherical, elongated, needle-shaped, or bead-shaped. Among these, a spherical shape is preferred from the viewpoint of ink ejection performance.

[0036] Colloidal silica may be manufactured or commercially available. Examples of commercially available products include: Ludox AM, Ludox AS, Ludox LS, Ludox TM, Ludox HS, etc. (all manufactured by EIDu Pont de Nemouvs & Co); Snowtex S, Snowtex XS, Snowtex 20, Snowtex 30, Snowtex 40, Snowtex N, Snowtex C, Snowtex O, etc. (all manufactured by Nissan Chemical Industries, Ltd.); Syton C-30, SytonZOO, etc. (all manufactured by Mons anto Co.); Nalcoag-1060, Nalcoag-ID21~64 (all manufactured by Nalco Chem Co.); Methanol sol, IPA sol, MEK sol, and toluene sol (all manufactured by Fuso Chemical Industries Co., Ltd.); Cataloid-S, Cataloid-F120, Cataloid SI-350, Cataloid SI-500, Cataloid SI-30, Cataloid S-20L, Cataloid S-20H, Cataloid S-30L, Cataloid S-30H, Cataloid SI-40, OSCAL-1432 (isopropyl alcohol sol), etc. (manufactured by JGC Catalysts & Chemicals); Adelite (manufactured by Asahi Denka Co., Ltd.); These are some examples. In addition, there are commercially available colloidal silica products in the form of beads, such as Snowtex ST-UP, PS-S, PS-M, ST-OUP, PS-SO, and PS-MO (all manufactured by Nissan Chemical Corporation).

[0037] (Content of specific silicon compounds) The content of specific silicon compounds relative to the total amount of titanium dioxide particles is 0.0020% by mass or more. This effectively suppresses wear and deterioration of the inkjet print head. The content of the specific silicon compound relative to the total amount of titanium dioxide particles is preferably 0.0030% by mass or more, more preferably 0.0050% by mass or more, even more preferably 0.010% by mass or more, even more preferably 0.020% by mass or more, and even more preferably 0.040% by mass or more.

[0038] There is no particular upper limit on the content of specific silicon compounds relative to the total amount of titanium dioxide particles. The upper limit may be, for example, 5.0% by mass, 4.0% by mass, 3.0% by mass, 2.0% by mass, or 0.050% by mass.

[0039] A preferred range for the content of specific silicon compounds relative to the total amount of titanium dioxide particles is, for example, 0.040% by mass to 2.0% by mass.

[0040] From the viewpoint of suppressing wear and deterioration of the inkjet head, the content of the specific silicon compound relative to the total amount of ink is preferably 0.0003% by mass or more, more preferably 0.0004% by mass or more, 0.001% by mass or more, and even more preferably 0.003% by mass or more. There is no particular upper limit on the content of specific silicon compounds relative to the total amount of ink. The upper limit may be, for example, 1.0% by mass, 0.50% by mass, or 0.30% by mass.

[0041] <Polymer dispersant> The inks of this disclosure may contain polymer dispersants. In this case, the polymer dispersant contained in the ink may be one type or two or more types. Polymer dispersants have the function of dispersing titanium dioxide particles in ink. Therefore, when ink contains a polymer dispersant, the dispersibility of titanium dioxide particles in the ink is further improved. If the ink contains a polymer dispersant, it is thought that the ink contains dispersed particles comprising titanium dioxide particles and a polymer dispersant that interacts with (e.g., adsorbs) the titanium dioxide particles.

[0042] Furthermore, if the ink of this disclosure contains, for example, self-dispersing titanium dioxide particles, the dispersibility of the titanium dioxide particles can be ensured even without containing a polymer dispersant. Therefore, from the viewpoint of the dispersibility of titanium dioxide particles, polymer dispersants are not essential components in the inks of this disclosure. However, from the viewpoint of further suppressing wear and deterioration of the inkjet head, it is preferable that the ink of this disclosure contains a polymer dispersant. This is thought to be because the polymer dispersant, like certain silicon compounds, acts as a cushion to mitigate the polishing of the inkjet head by titanium dioxide particles.

[0043] The method for manufacturing the inks of this disclosure is not particularly limited when it contains a polymer dispersant. In this case, the ink of the present disclosure can be manufactured by known methods, for example, by first preparing a dispersion containing a polymer dispersant, a dispersion medium, and titanium dioxide particles, and then preparing the ink using the obtained dispersion. Furthermore, the inks of this disclosure, when containing a polymer dispersant, may be manufactured by a phase inversion emulsification method.

[0044] In this disclosure, "polymer" means a compound having a weight-average molecular weight (Mw) of 1000 or more.

[0045] In this disclosure, weight-average molecular weight (Mw) refers to the value measured by gel permeation chromatography (GPC). Gel permeation chromatography (GPC) measurements were performed using an HLC®-8020GPC (manufactured by Tosoh Corporation) as the measuring instrument, with three TSKgel® Super Multipore HZ-H columns (4.6 mm ID × 15 cm, manufactured by Tosoh Corporation) and THF (tetrahydrofuran) as the eluent. The measurements were performed with a sample concentration of 0.45 mass%, a flow rate of 0.35 ml / min, a sample injection volume of 10 μL, and a measurement temperature of 40°C, using an RI detector. Calibration curves were prepared from eight samples of Tosoh Corporation's "Standard Samples TSK standard, polystyrene": "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", "A-1000", and "n-propylbenzene".

[0046] Any known polymer dispersant can be used without particular limitation. As polymer dispersants, for example, polymer dispersants described in prior art such as paragraphs 0080 to 0096 of Japanese Patent Publication No. 2016-145312 and paragraphs 0078 to 0108 of International Publication No. 2013 / 180074 can be used.

[0047] Acrylic resin is preferred as the polymer dispersant. In this disclosure, "acrylic resin" means a polymer (homopolymer or copolymer) of raw material monomers that includes at least one selected from the group consisting of acrylic acid, derivatives of acrylic acid (e.g., acrylic acid esters, etc.), methacrylic acid, and derivatives of methacrylic acid (e.g., methacrylic acid esters, etc.).

[0048] The weight-average molecular weight (Mw) of the polymer dispersant is preferably 3,000 to 100,000, more preferably 4,000 to 80,000, and even more preferably 5,000 to 60,000.

[0049] The polymer dispersant preferably contains structural units having an adsorption group and structural units having anionic groups.

[0050] Structural units containing anionic groups are structural units that contribute to dispersibility. The polymer dispersant may contain only one structural unit having anionic groups, or it may contain two or more. Examples of anionic groups include acidic groups and their salts. Examples of acidic groups include carboxyl groups, sulfol groups, phosphate groups, phosphonic acid groups, etc., with carboxyl groups being particularly preferred. As the salt of the acid group, alkali metal salts are preferred, and sodium salts or potassium salts are more preferred. As structural units having anionic groups, at least one of structural units derived from (meth)acrylic acid and structural units derived from salts of (meth)acrylic acid is preferred.

[0051] In this disclosure, a structural unit derived from compound A (e.g., (meth)acrylic acid) means a structural unit formed by the polymerization of compound A (e.g., (meth)acrylic acid).

[0052] In structural units containing adsorption groups, the adsorption groups have the function of adsorbing onto titanium dioxide particles. The polymer dispersant may contain only one type of structural unit having an adsorption group, or it may contain two or more types. The adsorbent group preferably includes at least one selected from the group consisting of aromatic ring structures, alicyclic structures, and alkyl groups having 6 or more carbon atoms, and more preferably includes at least one selected from the group consisting of aromatic ring structures and alicyclic structures. The structural unit having an adsorption group is preferably a structural unit derived from a (meth)acrylate having an adsorption group.

[0053] The polymer dispersant may be a random polymer (i.e., a random copolymer), a block polymer (i.e., a block copolymer), or a polymer having a crosslinked structure. The polymer dispersant preferably contains a block polymer or a polymer having a crosslinked structure. In this case, the polymer dispersant may contain both a block polymer and a polymer having a crosslinked structure. When the polymer dispersant contains a block polymer or a polymer with a cross-linked structure, wear and deterioration of the inkjet head can be further suppressed. This is thought to be because, when the polymer dispersant contains a block polymer or a polymer with a cross-linked structure, the titanium dioxide particles in the ink are more densely coated by the polymer dispersant, and as a result, the aforementioned cushioning function (i.e., the function of cushioning to mitigate the polishing of the inkjet head by the titanium dioxide particles) is exerted more effectively.

[0054] -Block Polymer- A block polymer, also known as a block copolymer, is a copolymer in which at least two polymers are bonded together within the molecule.

[0055] The block polymer preferably contains structural units derived from hydrophobic monomers and structural units derived from monomers containing anionic groups (hereinafter referred to as "anionic group-containing monomers").

[0056] The structural units derived from hydrophobic monomers contained in the block polymer may be one type or two or more types. The structural units derived from anionic group-containing monomers contained in the block polymer may be one type or two or more types.

[0057] Structural units derived from hydrophobic monomers include ethylenically unsaturated compounds having an aromatic ring structure or an alicyclic structure, and (meth)acrylates having an alkyl group with 1 to 20 carbon atoms.

[0058] The content of structural units derived from hydrophobic monomers is preferably 35% to 95% by mass, more preferably 50% to 95% by mass, and even more preferably 70% to 90% by mass, relative to the total amount of the block polymer.

[0059] From the viewpoint of adsorption with pigments, the hydrophobic monomer preferably contains an ethylenically unsaturated compound having an aromatic ring structure or an alicyclic structure, more preferably contains an ethylenically unsaturated compound having an alicyclic structure, and even more preferably contains an ethylenically unsaturated compound having an alicyclic structure with 6 or more carbon atoms.

[0060] The content of structural units derived from ethylenically unsaturated compounds having an aromatic ring structure or an alicyclic structure is preferably 10% to 90% by mass, more preferably 20% to 80% by mass, even more preferably 30% to 70% by mass, and still more preferably 30% to 60% by mass, based on the total amount of the block polymer.

[0061] The structural units derived from hydrophobic monomers may also preferably include (meth)acrylates having an alkyl group with 1 to 20 carbon atoms. The alkyl group may be linear or branched.

[0062] Examples of (meth)acrylates having an alkyl group with 1 to 20 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate.

[0063] The content of structural units derived from (meth)acrylates having an alkyl group with 1 to 20 carbon atoms is preferably 10% to 90% by mass, more preferably 20% to 80% by mass, even more preferably 30% to 70% by mass, and particularly preferably 40% to 60% by mass, based on the total amount of the block polymer.

[0064] In structural units derived from anionic group-containing monomers, examples of anionic groups include carboxyl groups, salts of carboxyl groups, sulfo groups, salts of sulfo groups, phosphate groups, salts of phosphate groups, phosphonic acid groups, and salts of phosphonic acid groups.

[0065] Counterions in salts include alkali metal ions such as sodium ions, potassium ions, and lithium ions; alkaline earth metal ions such as calcium ions and magnesium ions; and ammonium ions.

[0066] In particular, the anionic group is preferably a carboxyl group or a salt of a carboxyl group. Examples of anionic group-containing monomers include (meth)acrylic acid, β-carboxyethyl acrylate, fumaric acid, itaconic acid, maleic acid, and crotonic acid. In particular, the anionic group-containing monomer is preferably (meth)acrylic acid.

[0067] The content of structural units derived from anionic group-containing monomers is preferably 1% to 30% by mass, more preferably 2% to 25% by mass, and even more preferably 3% to 20% by mass, based on the total amount of the block polymer.

[0068] Whether or not the polymer contained in the ink is a block polymer can be determined, for example, by the following method. First, the polymer is separated from the ink using a separation method such as solvent extraction. The separated polymer is then analyzed using various analytical methods such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and thermal analysis, and its physical properties, such as the glass transition temperature, are measured to comprehensively determine whether or not it is a block polymer.

[0069] The weight-average molecular weight (Mw) of the block polymer is not particularly limited, but from the viewpoint of pigment dispersibility, it is preferably 3,000 to 100,000, more preferably 5,000 to 80,000, and even more preferably 10,000 to 60,000.

[0070] - A polymer with a cross-linked structure - A polymer having a cross-linked structure is not particularly limited as long as it has at least one cross-linked structure within its molecule. A polymer having a crosslinked structure (hereinafter also referred to as "crosslinked polymer") is formed, for example, by crosslinking an uncrosslinked polymer with a crosslinking agent. The uncrosslinked polymer is preferably a water-soluble polymer.

[0071] Examples of uncrosslinked polymers include polyvinyl, polyurethane, and polyester. Among these, polyvinyl is preferred as the uncrosslinked polymer.

[0072] The uncrosslinked polymer is preferably a polymer having functional groups that can be crosslinked by a crosslinking agent. Examples of crosslinkable functional groups include carboxyl groups or their salts, isocyanate groups, and epoxy groups. In particular, from the viewpoint of improving the dispersibility of titanium dioxide particles, the crosslinkable functional group is preferably a carboxyl group or a salt thereof, and a carboxyl group is especially preferred. That is, the uncrosslinked polymer is preferably a polymer containing a carboxyl group.

[0073] The uncrosslinked polymer is preferably a copolymer containing structural units derived from monomers containing carboxyl groups (hereinafter referred to as "carboxyl group-containing monomers"). The copolymer may contain only one type of structural unit derived from carboxyl group-containing monomers, or two or more types. The copolymer may be a random copolymer or a block copolymer, but a random copolymer is preferred.

[0074] Examples of monomers containing a carboxyl group include (meth)acrylic acid, β-carboxyethyl acrylate, fumaric acid, itaconic acid, maleic acid, and crotonic acid.

[0075] The carboxyl group-containing monomer is preferably (meth)acrylic acid or β-carboxyethyl acrylate, with (meth)acrylic acid being more preferred, from the viewpoint of crosslinkability and dispersibility.

[0076] The content of structural units derived from carboxyl group-containing monomers that may be contained in the uncrosslinked polymer is preferably 1% to 30% by mass, more preferably 2% to 25% by mass, and even more preferably 3% to 20% by mass, relative to the total amount of the block polymer.

[0077] The uncrosslinked polymer preferably contains structural units derived from hydrophobic monomers in addition to structural units derived from carboxyl group-containing monomers. The structural units derived from hydrophobic monomers may be one type or two or more types.

[0078] Structural units derived from hydrophobic monomers that may be contained in uncrosslinked polymers include structural units derived from ethylenically unsaturated compounds having an aromatic ring structure or an alicyclic structure, and structural units derived from (meth)acrylates having an alkyl group with 1 to 20 carbon atoms. Preferred embodiments of these structural units that may be contained in uncrosslinked polymers are similar to preferred embodiments of these structural units that may be contained in block polymers.

[0079] In the uncrosslinked polymer, the content of structural units derived from hydrophobic monomers is preferably 35% to 95% by mass, more preferably 50% to 95% by mass, and even more preferably 70% to 90% by mass, based on the total amount of the uncrosslinked polymer.

[0080] Similarly, the content of structural units derived from hydrophobic monomers in the crosslinked polymer is preferably 35% to 95% by mass, more preferably 50% to 95% by mass, and even more preferably 70% to 90% by mass, based on the total amount of the crosslinked polymer.

[0081] The uncrosslinked polymer and the crosslinked polymer may each contain, as structural units derived from hydrophobic monomers, (meth)acrylate units having a benzene ring and (meth)acrylate units having an alkyl group having 12 or more carbon atoms. The content of (meth)acrylate structural units having a benzene ring is preferably 20% to 60% by mass relative to the total amount of the polymer (i.e., uncrosslinked polymer or crosslinked polymer). The content of (meth)acrylate constituent units having an alkyl group with 12 or more carbon atoms is preferably 10% to 40% by mass relative to the total amount of the polymer (i.e., uncrosslinked polymer or crosslinked polymer).

[0082] The acid value of the uncrosslinked polymer is preferably 67 mg KOH / g to 200 mg KOH / g, and more preferably 67 mg KOH / g to 150 mg KOH / g, from the viewpoint of pigment dispersibility. In this disclosure, the acid value is the value measured by the method described in JIS K0070:1992.

[0083] Furthermore, from the viewpoint of suppressing image cracking and improving the image quality of the recorded image, the acid value of the crosslinked polymer is preferably 35 mg KOH / g to 185 mg KOH / g, more preferably 50 mg KOH / g to 150 mg KOH / g, and even more preferably 80 mg KOH / g to 130 mg KOH / g. When the acid value of the crosslinked polymer is 50 mg KOH / g or higher, the image quality of the recorded image is even better. Also, when the acid value of the crosslinked polymer is 150 mg KOH / g or lower, image cracking of the recorded image is further suppressed.

[0084] The weight-average molecular weight (Mw) of the uncrosslinked polymer is not particularly limited, but from the viewpoint of pigment dispersibility, it is preferably 3,000 to 100,000, more preferably 4,000 to 80,000, even more preferably 5,000 to 60,000, and still more preferably 10,000 to 60,000.

[0085] The preferred range for the weight-average molecular weight (Mw) of the crosslinked polymer is the same as the preferred range for the weight-average molecular weight (Mw) of the uncrosslinked polymer.

[0086] The crosslinking agent used when crosslinking an uncrosslinked polymer is preferably a compound having two or more reaction sites with the uncrosslinked polymer (for example, a polymer having a carboxyl group). One type of crosslinking agent may be used, or two or more types may be used.

[0087] A preferred combination of a crosslinking agent and an uncrosslinked polymer is a compound having two or more epoxy groups (i.e., a bifunctional or more epoxy compound) and a polymer having a carboxyl group. In this combination, a crosslinked structure is formed by the reaction between the epoxy groups and the carboxyl groups. It is preferable that the formation of the crosslinked structure by the crosslinking agent is carried out after the pigment has been dispersed by the uncrosslinked polymer.

[0088] Examples of bifunctional or more epoxy compounds include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and trimethylolpropane triglycidyl ether.

[0089] Among these, the preferred epoxy compounds with two or more functions are polyethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, or trimethylolpropane triglycidyl ether.

[0090] The crosslinking agent can be a commercially available product. Examples of commercially available products include Denacol EX-321, EX-821, EX-830, EX-850, and EX-851 (manufactured by Nagase ChemteX).

[0091] The molar ratio of the reaction sites in the crosslinking agent (e.g., epoxy groups) to the reaction sites in the uncrosslinked polymer (e.g., carboxyl groups) is preferably 1:1.1 to 1:10, more preferably 1:1.1 to 1:5, and even more preferably 1:1.1 to 1:3, from the viewpoint of crosslinking reaction rate and dispersion stability after crosslinking.

[0092] <Water-soluble organic solvents> The inks of this disclosure preferably contain at least one water-soluble organic solvent. This further improves the ink ejection performance from the inkjet head. The content of the water-soluble organic solvent is preferably 5% to 60% by mass, more preferably 10% to 40% by mass, and more preferably 15% to 30% by mass, based on the total amount of ink.

[0093] -Water-soluble organic solvents with a boiling point below 220°C- The water-soluble organic solvent may contain at least one water-soluble organic solvent with a boiling point of less than 220°C. This improves the drying time of the ink (i.e., the drying time of the recorded image). In this disclosure, boiling point means boiling point at 1 atmosphere (101,325 Pa).

[0094] Examples of water-soluble organic solvents with a boiling point below 220°C include 1,2-propanediol (also known as propylene glycol; PG) (boiling point 188°C), 1,3-propanediol (boiling point 213°C), propylene glycol monomethyl ether (boiling point 121°C), ethylene glycol (boiling point 197°C), ethylene glycol monomethyl ether (boiling point 124°C), propylene glycol monoethyl ether (boiling point 133°C), ethylene glycol monoethyl ether (boiling point 135°C), and propylene glycol monopropyl ether ( Examples include ethylene glycol monopropyl ether (boiling point 151°C), propylene glycol monobutyl ether (boiling point 170°C), ethylene glycol monobutyl ether (boiling point 171°C), 2-ethyl-1-hexanol (boiling point 187°C), dipropylene glycol monomethyl ether (boiling point 188°C), diethylene glycol dimethyl ether (boiling point 162°C), diethylene glycol diethyl ether (boiling point 188°C), and dipropylene glycol dimethyl ether (boiling point 175°C).

[0095] If the ink of this disclosure contains a water-soluble organic solvent with a boiling point of less than 220°C, the content of the water-soluble organic solvent with a boiling point of less than 220°C is preferably 1% to 50% by mass, more preferably 5% to 40% by mass, even more preferably 10% to 40% by mass, and even more preferably 15% to 35% by mass, based on the total amount of ink.

[0096] -Water-soluble organic solvents with a boiling point of 220°C or higher- The content of a water-soluble organic solvent with a boiling point of 220°C or higher (hereinafter also referred to as "high-boiling point solvent") in the ink of this disclosure is preferably 5% by mass or less. This further improves the drying properties of the ink (i.e., the drying properties of the recorded image). Here, "the content of water-soluble organic solvents with a boiling point of 220°C or higher in the ink is 5% by mass or less" means that the ink does not contain water-soluble organic solvents with a boiling point of 220°C or higher (i.e., the content of water-soluble organic solvents with a boiling point of 220°C or higher in the ink is 0% by mass), or, if it does contain them, the content of water-soluble organic solvents with a boiling point of 220°C or higher is 5% by mass or less of the total amount of ink. The content of water-soluble organic solvents with a boiling point of 220°C or higher in the ink is more preferably 3% by mass or less, even more preferably 2% by mass or less, even more preferably 1% by mass or less, and even more preferably 0% by mass.

[0097] Examples of water-soluble organic solvents with a boiling point of 220°C or higher include glycerin (boiling point 290°C), 1,2-hexanediol (HDO) (boiling point 223°C), diethylene glycol (boiling point 245°C), diethylene glycol monobutyl ether (boiling point 230°C), triethylene glycol (boiling point 285°C), dipropylene glycol (boiling point 232°C), tripropylene glycol (boiling point 267°C), trimethylolpropane (boiling point 295°C), 2-pyrrolidone (boiling point 245°C), tripropylene glycol monomethyl ether (boiling point 243°C), triethylene glycol monomethyl ether (boiling point 248°C), and the like.

[0098] <Resin particles> The inks of this disclosure may contain at least one type of resin particle. When the ink contains resin particles, the adhesion of the recorded image is improved. Here, resin particles are distinguished from polymer dispersants in that they are particles made of resin. Water-insoluble resins are preferred as the resins that make up the resin particles. When the ink contains resin particles, the increase in ink viscosity is more suppressed compared to when the ink contains the same mass of water-soluble resin. As a result, the ink ejection performance from the inkjet head (hereinafter also simply referred to as "ink ejection performance") when the ink is used as an inkjet ink is improved.

[0099] The definition and preferred range of "water-soluble" are as described above. "Water-insoluble" means that the amount that dissolves in 100g of distilled water at 25°C is less than 1g.

[0100] There are no particular restrictions on the glass transition temperature of the resin particles (i.e., the glass transition temperature of the resin in the resin particles). From the viewpoint of further improving image intensity, the glass transition temperature (Tg) of the resin particles is preferably 20°C or higher, more preferably 50°C or higher, and even more preferably 80°C or higher. From the viewpoint of suitability for manufacturing resin particles, the glass transition temperature (Tg) of the resin particles is preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 130°C or lower.

[0101] Preferably, the resin particles are made of acrylic resin (hereinafter also referred to as acrylic resin particles), polyester resin (hereinafter also referred to as polyester resin particles), polyurethane resin (hereinafter also referred to as polyurethane resin particles), or polyolefin resin (hereinafter also referred to as polyolefin resin particles).

[0102] In this disclosure, polyester resin means a polymer compound containing ester bonds in its main chain. Examples of polyester resins include polycondensates of polycarboxylic acids (e.g., dicarboxylic acids) and polyalcohols (e.g., diols). In this disclosure, polyolefin resin means a polymer (homopolymer or copolymer) of raw material monomers containing olefins. Examples of polyolefin resins include polymers of one type of olefin, copolymers of two or more types of olefins, copolymers of one or more types of olefins and one or more other monomers, etc. Examples of olefins include α-olefins having 2 to 30 carbon atoms. In this disclosure, polyurethane resin means a polymer compound containing urethane bonds.

[0103] From the viewpoint of further improving image adhesion and scratch resistance, it is preferable that the resin particles contained in the ink include acrylic resin particles. When the resin particles contained in the ink include acrylic resin particles, the ratio of acrylic resin particles to the total resin particles contained in the ink is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. When the ratio of acrylic resin particles to the total resin particles contained in the ink is 60% by mass or more, the adhesion of the image is further improved.

[0104] Self-dispersible resin particles are preferred as the resin particles. Examples of self-dispersing resin particles include self-dispersing polymer particles described in paragraphs 0062 to 0076 of Japanese Patent Publication No. 2016-188345 and paragraphs 0109 to 0140 of International Publication No. 2013 / 180074.

[0105] The resin in the resin particles preferably contains an alicyclic structure or an aromatic cyclic structure, and more preferably contains an alicyclic structure. As for the alicyclic structure, an alicyclic hydrocarbon structure having 5 to 10 carbon atoms is preferred, and a cyclohexane ring structure, dicyclopentanyl ring structure, dicyclopentenyl ring structure, norbornane ring structure, isobornane ring structure, norbornene ring structure, isobornene ring structure, or adamantane ring structure is preferred. The aromatic ring structure is preferably a naphthalene ring or a benzene ring, with a benzene ring being more preferred. The amount of alicyclic or aromatic ring structure is preferably 0.01 mol to 1.5 mol per 100 g of resin in the resin particles, and more preferably 0.1 mol to 1 mol.

[0106] In resin particles, it is preferable that the resin has ionic groups in its structure, from the viewpoint of further improving the water dispersibility of the resin particles. The ionic group may be an anionic group or a cationic group, but an anionic group is preferred. The anionic group is not particularly limited, but a carboxyl group, a salt of a carboxyl group, a sulfo group, or a salt of a sulfo group is preferred.

[0107] The resin in the resin particles is more preferably an acrylic resin comprising at least one selected from the group consisting of benzyl (meth)acrylate units, phenoxyethyl (meth)acrylate units, and alicyclic structure-containing (meth)acrylate units, and (meth)acrylic acid units. Even more preferably, the acrylic resin comprises at least one selected from the group consisting of benzyl (meth)acrylate units, phenoxyethyl (meth)acrylate units, and alicyclic structure-containing (meth)acrylate units, (meth)acrylic acid units, and alkyl (meth)acrylate units containing an alkyl group having 1 to 4 carbon atoms.

[0108] As the alicyclic structure-containing (meth)acrylate, at least one selected from alkyl (meth)acrylates having a cycloalkyl group with 3 to 10 carbon atoms (e.g., cyclohexyl (meth)acrylate), isobornyl (meth)acrylate, adamantyl (meth)acrylate, and dicyclopentanyl (meth)acrylate is preferred, and at least one selected from isobornyl (meth)acrylate, adamantyl (meth)acrylate, and dicyclopentanyl (meth)acrylate is more preferred.

[0109] The acid value of the resin in the resin particles is preferably 25 mg KOH / g to 100 mg KOH / g, more preferably 30 mg KOH / g to 90 mg KOH / g, and even more preferably 35 mg KOH / g to 80 mg KOH / g, from the viewpoint of self-dispersibility and aggregation during image recording.

[0110] The molecular weight of the resin in the resin particles is preferably 1,000 to 300,000, more preferably 2,000 to 200,000, and even more preferably 5,000 to 100,000. The weight-average molecular weight is measured by gel permeation chromatography (GPC). Details of GPC are as previously described.

[0111] The volume-average particle diameter of the resin particles is preferably 1 nm to 200 nm, more preferably 3 nm to 200 nm, and even more preferably 5 nm to 50 nm.

[0112] If the ink contains resin particles, the content of resin particles relative to the total amount of ink is preferably 1% to 25% by mass, more preferably 2% to 20% by mass, even more preferably 2% to 15% by mass, and still more preferably 2% to 10% by mass.

[0113] <Surfactants> The inks of this disclosure may contain at least one surfactant. Examples of surfactants include nonionic surfactants, cationic surfactants, anionic surfactants, and betaine surfactants.

[0114] A preferred surfactant is an acetylene glycol-based surfactant, which is a type of nonionic surfactant. As an acetylene glycol-based surfactant, for example, an acetylene glycol-based surfactant described in paragraphs 0070-0080 of International Publication No. 2017 / 149917 can be used. Examples of acetylene glycol-based surfactants include polyalkylene oxide adducts of 2,4,7,9-tetramethyl-5-decine-4,7-diol (preferably polyethylene oxide adducts), polyalkylene oxide adducts of 3,6-dimethyl-4-octin-3,6-diol (preferably polyethylene oxide adducts), polyalkylene oxide adducts of 2,5,8,11-tetramethyl-6-dodecine-5,8-diol (preferably polyethylene oxide adducts), and polyalkylene oxide adducts of 2,5-dimethyl-3-hexyn-2,5-diol (preferably polyethylene oxide adducts). Commercially available acetylene glycol-based surfactants include the Surfinol series (e.g., Surfinol 420, Surfinol 440, Surfinol 465, Surfinol 485), Olfin series (e.g., Olfin E1010, Olfin E1020), and Dynol series (e.g., Dynol 604) manufactured by Air Products Co., Ltd. or Nisshin Chemical Industry Co., Ltd.; and Acetyleneol, etc., manufactured by Kawaken Fine Chemical Co., Ltd. Commercially available acetylene glycol-based surfactants are also supplied by companies such as Dow Chemical and General Aniline.

[0115] The surfactant may include a fluorine-based surfactant. Commercially available fluorinated surfactants include Capstone FS-63, Capstone FS-61 (manufactured by Dupont), Futegent 100, Futegent 110, Futegent 150 (manufactured by Neos Co., Ltd.), and CHEMGUARD S-760P (manufactured by Chemguard Inc.).

[0116] Examples of surfactants include the compounds listed as surfactants on pages 37-38 of Japanese Patent Publication No. 59-157636 and in Research Disclosure No. 308119 (1989). Other examples include fluorine (alkyl fluoride) surfactants and silicone surfactants described in Japanese Patent Publication Nos. 2003-322926, 2004-325707, and 2004-309806.

[0117] If the ink contains a surfactant, the amount of surfactant in the ink is adjusted appropriately, taking into account the surface tension of the ink. The surfactant content in the ink is preferably 0.01% to 5% by mass, more preferably 0.05% to 3% by mass, and even more preferably 0.1% to 2% by mass, based on the total amount of ink.

[0118] <Other ingredients> The ink may contain other components besides those listed above. Other known additives include, for example, urea, urea derivatives, waxes, colorfastness inhibitors, emulsifying stabilizers, penetration enhancers, UV absorbers, preservatives, fungicides, pH adjusters, defoamers, viscosity modifiers, dispersion stabilizers, and chelating agents.

[0119] <Desired physical properties of ink> The viscosity of the ink of this disclosure is preferably 1.2 mPa·s or more and 15.0 mPa·s or less, more preferably 2 mPa·s or more and less than 13 mPa·s, and preferably 2.5 mPa·s or more and less than 10 mPa·s. Viscosity is a value measured using a viscometer at 25°C. As a viscometer, for example, the VISCOMETER TV-22 viscometer (manufactured by Toki Sangyo Co., Ltd.) can be used.

[0120] The surface tension of the inks of this disclosure is preferably 25 mN / m or more and 40 mN / m or less, and more preferably 27 mN / m or more and 37 mN / m or less. Surface tension is a value measured at a temperature of 25°C. Surface tension can be measured, for example, using an Automatic Surface Tentiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).

[0121] From the viewpoint of dispersion stability, the pH of the inks disclosed herein at 25°C is preferably pH 6 to 11, more preferably pH 7 to 10, and even more preferably pH 7 to 9. The pH of the ink at 25°C is measured using a commercially available pH meter.

[0122] [Ink Set] ≪First Embodiment≫ A first embodiment of the ink set of the present disclosure includes a white ink, which is the ink of the present disclosure as described above, and a color ink containing water and a color pigment.

[0123] According to the ink set of the first embodiment, a multi-color image can be recorded using white ink and color ink. Examples of multi-color images include: A multi-color image A is formed on a substrate in which a color pattern image (text, shapes, etc.) recorded with color ink and a solid white image recorded with white ink to cover the pattern image are arranged in this order; A multi-color image B is formed on a substrate in which a solid color image recorded with color ink and a white pattern image (text, shapes, etc.) recorded with white ink on top of the color image are arranged in this order; A multi-color image C is formed on a substrate in which a white pattern image (text, shapes, etc.) recorded with white ink and a solid color image recorded with color ink to cover the pattern image are arranged in this order; A multi-color image D is formed on a substrate in which a solid white image recorded with white ink and a color pattern image (text, shapes, etc.) recorded with color ink on top of this white image are arranged in this order; These are some examples. In multi-color images A and C, the pattern image is viewed through the substrate from the back side (i.e., the side where the image is not formed). This type of image recording is sometimes referred to as "reverse printing."

[0124] Since the ink set of the first embodiment includes the ink of the present disclosure as described above, it exhibits the same effects as those of the ink of the present disclosure.

[0125] In the first embodiment, the concept of "color" encompasses both chromatic colors such as cyan, magenta, yellow, red, blue, and green, and black.

[0126] <Color Ink> The ink set of the first embodiment includes water and a color ink containing a color pigment. The color ink in the ink set of the first embodiment may be one type or two or more types.

[0127] (water) Color inks contain water. The water content is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and even more preferably 60% by mass or more, relative to the total amount of color ink. The upper limit of the water content is determined appropriately depending on the content of other components. Examples of upper limits for water content include 90% by mass and 80% by mass.

[0128] (Color pigments) Color ink contains at least one color pigment. Here, "color ink" refers to any ink other than the white ink mentioned above. Examples of color pigments include chromatic pigments such as cyan, magenta, yellow, red, blue, green, violet, pink, orange, and brown, as well as black pigment.

[0129] Examples of color pigments include those described in "Dictionary of Pigments" edited by Seijiro Ito (published in 2000), "Industrial Organic Pigments" by W. Herbst and K. Hunger, "Handbook of Organic Pigments" by Isao Hashimoto (published in 2006), and Japanese Patent Publication No. 2002-12607, 2002-188025, 2003-26978, and 2003-342503.

[0130] The color pigment content is preferably 0.5% to 10% by mass, and more preferably 0.5% to 5% by mass, relative to the total amount of color ink.

[0131] (Silicon compounds) The color ink preferably contains a silicon compound, which is selected from the group consisting of silicates and colloidal silica. This further suppresses the dissolution and wear degradation of the inkjet head used for ejecting color ink.

[0132] Hereinafter, a silicon compound contained in white ink, which is at least one selected from the group consisting of silicates and colloidal silica, will be referred to as the first silicon compound, and a silicon compound contained in colored ink, which is at least one selected from the group consisting of silicates and colloidal silica, will be referred to as the second silicon compound.

[0133] A preferred embodiment of the second silicon compound is the same as a preferred embodiment of the first silicon compound.

[0134] In the ink set of the first embodiment, when the content of the first silicon compound relative to the total amount of white ink is X1 by mass and the content of the second silicon compound relative to the total amount of color ink is X2 by mass, the X1 / X2 ratio is preferably 5.0 or less, more preferably 4.0 or less, even more preferably 1.0 or less, even more preferably less than 1.0, even more preferably 0.9 or less, and even more preferably 0.5 or less, from the viewpoint of image detail. Image detail is further improved when the X1 / X2 ratio is 5.0 or less.

[0135] The effect of improving image resolution by limiting the X1 / X2 ratio to 5.0 or less is particularly effective in embodiments in which the image is recorded by applying the pre-treatment solution, color ink, and white ink described later to the substrate in that order. In detail, in the above embodiment, when the X1 / X2 ratio is limited to 5.0 or less, the content of silicon compounds in the white ink, which is applied after the color ink, is limited, thereby limiting the rise in pH of the white ink. As a result, the aggregation effect of the components in the white ink by the pretreatment solution can be exerted more effectively. Consequently, the image resolution is further improved in multi-color images using color ink and white ink. One example of the above embodiment is a method in which a pattern image such as characters or figures is recorded using colored ink, and a white image (for example, a solid white image) is recorded using white ink so as to cover the pattern image. In this case, the pattern image is visible through the substrate from the non-image-recorded side of the substrate.

[0136] There is no particular limit on the lower limit of the X1 / X2 ratio. The lower limit is preferably 0.01, and more preferably 0.02, from the viewpoint of more effectively exhibiting the effect of the first silicon compound in the white ink.

[0137] In the first embodiment, from the viewpoint of further improving image resolution, it is preferable that the first silicon compound contains a silicate and the second silicon compound contains colloidal silica. The effect of improving image resolution due to the first silicon compound containing a silicate and the second silicon compound containing colloidal silica is particularly effective in an embodiment in which an image is recorded by applying the pretreatment solution, color ink, and white ink described later to the substrate in that order. The reason for this is presumed to be as follows: In detail, in the image recording described above, the flocculant in the pretreatment solution is consumed by specific silicon compounds in the ink (i.e., colloidal silica and / or silicates). In this case, the amount of flocculant consumed by colloidal silica is considered to be less than the amount consumed by silicates. Therefore, by including colloidal silica as the second silicon compound in the color ink, it is possible to suppress the consumption of flocculant at the stage when the color ink is applied and leave some flocculant remaining. This ensures that there is enough flocculant to act on the white ink, which is applied after the color ink. As a result, the flocculation effect of the components in the white ink by the pretreatment solution can be exerted more effectively, and the image resolution is expected to be improved in multi-color images using color ink and white ink.

[0138] <Pretreatment solution> The ink set of the first embodiment preferably includes a pretreatment solution containing water and a coagulant. The function of the coagulant in the pretreatment solution is to coagulate the components in the ink. This allows for the recording of images with superior adhesion to the substrate, higher resolution, and other characteristics. In image recording using a pretreatment solution, the pretreatment solution is applied to the substrate before the color ink and white ink are applied. Then, the color ink and white ink are applied to the pretreatment area of ​​the substrate, respectively. This allows for the recording of, for example, the multi-color image described above.

[0139] (water) The pretreatment solution contains water. The water content is preferably 50% by mass or more, and more preferably 60% by mass or more, relative to the total amount of the pretreatment solution. The upper limit of the water content depends on the amounts of other components, but is preferably 90% by mass or less of the total amount of the pretreatment solution.

[0140] (Flocculant) The pretreatment solution contains at least one type of coagulant. A coagulant is a component used to coagulate the components in ink on a non-permeable substrate. As a flocculant, at least one selected from the group consisting of organic acids, organic acid salts, polyvalent metal compounds, metal complexes, and water-soluble cationic polymers is preferred.

[0141] -Organic acid- Examples of organic acids include organic compounds that have an acidic group. Examples of acidic groups include phosphate groups, phosphonic acid groups, phosphinic acid groups, sulfate groups, sulfonic acid groups, sulfinic acid groups, and carboxyl groups. From the viewpoint of the ink aggregation rate, the above-mentioned acidic group is preferably a phosphoric acid group or a carboxyl group, and more preferably a carboxyl group. Furthermore, it is preferable that at least a portion of the above-mentioned acidic group is dissociated in the pretreatment solution.

[0142] Preferred organic compounds having a carboxyl group include polyacrylic acid, acetic acid, formic acid, benzoic acid, glycolic acid, malonic acid, malic acid (preferably DL-malic acid), maleic acid, succinic acid, glutaric acid, fumaric acid, citric acid, tartaric acid, phthalic acid, adipic acid, pimelic acid, 4-methylphthalic acid, lactic acid, pyrrolidone carboxylic acid, pyrrone carboxylic acid, pyrrole carboxylic acid, furanic acid, pyridine carboxylic acid, coumaric acid, thiophene carboxylic acid, nicotinic acid, pimelic acid, and the like. These compounds may be used individually or in combination of two or more.

[0143] As for organic compounds having a carboxyl group, divalent or greater carboxylic acids (hereinafter also referred to as polyvalent carboxylic acids) are preferred from the viewpoint of ink aggregation rate. As for polycarboxylic acids, dicarboxylic acids or tricarboxylic acids are preferred, glutaric acid, malonic acid, succinic acid, adipic acid, pimelic acid, malic acid, maleic acid, fumaric acid, tartaric acid, or citric acid are more preferred, glutaric acid, malonic acid, succinic acid, adipic acid, pimelic acid, malic acid, fumaric acid, tartaric acid, or citric acid are even more preferred, and glutaric acid, malonic acid, succinic acid, adipic acid, or pimelic acid are even more preferred.

[0144] Organic acids are preferably those with a low pKa (e.g., 1.0 to 5.0). This allows for a reduction in the dispersion stability of particles such as pigments and polymer particles in inks, which are dispersed and stabilized by weakly acidic functional groups such as carboxyl groups, by bringing them into contact with organic acidic compounds with lower pKa values.

[0145] The organic acid is preferably a divalent or trivalent acidic substance that has a low pKa, high solubility in water, and a valency of 2 or higher, and more preferably has a high buffering capacity in a pH range lower than the pKa of the functional group (e.g., carboxyl group) that disperses and stabilizes the particles in the ink.

[0146] -Organic acid salt- Examples of organic acid salts include the salts of the organic acids exemplified above. Examples of organic acid salts include those containing alkaline earth metals from Group 2 of the periodic table (e.g., magnesium, calcium), transition metals from Group 3 of the periodic table (e.g., lanthanum), cations from Group 13 of the periodic table (e.g., aluminum), and lanthanides (e.g., neodymium). As for organic acid salts, organic acid salts containing alkaline earth metals are preferred, and organic acid salts containing calcium (e.g., calcium lactate, calcium acetate, etc.) or organic acid salts containing magnesium (e.g., magnesium lactate, magnesium acetate, etc.) are preferred.

[0147] -Polyvalent metal compounds- Examples of polyvalent metal compounds include salts (excluding organic salts) containing at least one selected from the group consisting of alkaline earth metals of Group 2 of the periodic table (e.g., magnesium, calcium), transition metals of Group 3 of the periodic table (e.g., lanthanum), cations from Group 13 of the periodic table (e.g., aluminum), and lanthanides (e.g., neodymium). Suitable polyvalent metal compounds include nitrates, chlorides, or thiocyanates. Particularly preferred polyvalent metal compounds are calcium or magnesium salts of nitric acid, calcium chloride, magnesium chloride, or calcium or magnesium salts of thiocyanate. It is preferable that the polyvalent metal compound dissociates into polyvalent metal ions and counterions in the pretreatment solution, at least a portion of which is present.

[0148] -Metal complex- As for the metal complex, a metal complex containing at least one element selected from the group consisting of zirconium, aluminum, and titanium is preferred. As for the metal complex, a metal complex comprising at least one selected from the group consisting of acetate, acetylacetonate, methylacetoacetate, ethylacetoacetate, octylene glycolate, butoxyacetylacetonate, lactate, lactate ammonium salt, and triethanolamine as a ligand is preferred.

[0149] Various metal complexes are commercially available, and in this disclosure, commercially available metal complexes may be used. In addition, various organic ligands, in particular various polydentate ligands that can form metal chelate catalysts, are commercially available. Therefore, metal complexes prepared by combining commercially available organic ligands with metals may also be used.

[0150] There are no particular restrictions on the amount of flocculant contained. From the viewpoint of ink aggregation rate, the content of the flocculant relative to the total amount of pretreatment solution is preferably 0.1% to 40% by mass, more preferably 0.1% to 30% by mass, even more preferably 1% to 20% by mass, even more preferably 1% to 10% by mass, and particularly preferably 2% to 8% by mass.

[0151] (resin) The pretreatment solution preferably contains at least one type of resin. If the pretreatment solution contains resin, the adhesion of the image will be further improved.

[0152] If the pretreatment solution contains a resin, the glass transition temperature (Tg) of the resin contained in the pretreatment solution is preferably 0°C or higher, more preferably 10°C or higher, even more preferably 20°C or higher, and even more preferably 30°C or higher. If the pretreatment solution contains a resin, the glass transition temperature (Tg) of the resin contained in the pretreatment solution is preferably 120°C or lower, more preferably 100°C or lower, even more preferably 80°C or lower, and even more preferably 70°C or lower.

[0153] In this disclosure, the glass transition temperature of the resin refers to the value measured using differential scanning calorimetry (DSC). The specific measurement of the glass transition temperature shall be carried out in accordance with the methods described in JIS K 7121 (1987) or JIS K 6240 (2011). In this disclosure, the glass transition temperature is the extrapolation glass transition onset temperature (hereinafter sometimes referred to as Tig). The method for measuring the glass transition temperature will be explained in more detail. To determine the glass transition temperature, the apparatus is held at a temperature approximately 50°C lower than the expected glass transition temperature of the resin until it stabilizes. Then, the temperature is heated at a heating rate of 20°C / min to approximately 30°C higher than the temperature at which the glass transition is completed, and a differential thermal analysis (DTA) curve or digital sensor cell (DSC) curve is created. The extrapolation glass transition onset temperature (Tig), i.e., the glass transition temperature in this disclosure, is determined as the temperature at the intersection of a straight line drawn by extending the low-temperature baseline of the DTA curve or DSC curve toward the high-temperature side, and a tangent line drawn at the point where the slope of the curve representing the stepwise transition portion of the glass transition is maximum.

[0154] If the pretreatment solution contains two or more types of resin, the glass transition temperature (Tg) of the resins in the pretreatment solution represents the weighted average of the glass transition temperatures of the individual resins.

[0155] Examples of resins that may be included in the pretreatment solution include acrylic resin, polyester resin, polyolefin resin, polyurethane resin, polyurea resin, polyamide resin, polycarbonate resin, and polystyrene resin. The resin that may be contained in the pretreatment solution preferably contains a polyester resin or an acrylic resin, and more preferably contains a polyester resin.

[0156] The resin that may be contained in the pretreatment solution may be a water-soluble resin or a water-insoluble resin, but a water-insoluble resin is preferred.

[0157] The pretreatment solution preferably contains resin particles. The resin particles are preferably made of a water-insoluble resin. Preferred resin particles include acrylic resin particles, polyester resin particles, a mixture of acrylic resin particles and polyester resin particles, or composite particles containing acrylic resin and polyester resin.

[0158] The weight-average molecular weight (Mw) of the resin in the resin particles is preferably 1,000 to 300,000, more preferably 2,000 to 200,000, and even more preferably 5,000 to 100,000.

[0159] The volume-average particle size of the resin particles is preferably 1 nm to 300 nm, more preferably 3 nm to 200 nm, and even more preferably 5 nm to 150 nm.

[0160] In this disclosure, the volume-average particle size of the resin particles refers to the value measured by a particle size distribution analyzer using light scattering (for example, the Microtrac UPA® EX150 manufactured by Nikkiso Co., Ltd.).

[0161] When preparing the pretreatment solution, commercially available aqueous dispersions of resin particles may be used. Commercially available aqueous dispersions of resin particles include Pesresin A124GP, Pesresin A645GH, Pesresin A615GE, Pesresin A520 (all manufactured by Takamatsu Oil & Fat Co., Ltd.), Eastek1100, Eastek1200 (both manufactured by Eastman Chemical Co., Ltd.), Pluscoat RZ570, Pluscoat Z687, Pluscoat Z565, Pluscoat RZ570, Pluscoat Z690 (all manufactured by Go-o Chemical Industry Co., Ltd.), Byronal MD1200 (manufactured by Toyobo Co., Ltd.), EM57DOC (manufactured by Daicel Finechem Co., Ltd.), and others.

[0162] If the pretreatment solution contains resin particles, the content of resin particles relative to the total amount of the pretreatment solution is preferably 0.5% to 30% by mass, more preferably 1% to 20% by mass, and particularly preferably 1% to 15% by mass.

[0163] (Water-soluble organic solvent) The pretreatment solution preferably contains at least one water-soluble organic solvent. Any known water-soluble organic solvent can be used without any particular restrictions. Examples of water-soluble organic solvents include glycerin, 1,2,6-hexanetriol, trimethylolpropane, alkanediols (e.g., ethylene glycol, propylene glycol (1,2-propanediol), 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2-butene-1,4-diol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, 1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol, 4-methyl-1,2-pentanediol, etc.), and polyalkylene glycols (e.g., diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, dipropylene glycol, polyoxyethylene polyoxypropylene glycol, etc.) and other polyhydric alcohols; Polyhydric alcohol ethers such as polyalkylene glycol ethers (e.g., diethylene glycol monoalkyl ether, triethylene glycol monoalkyl ether, tripropylene glycol monoalkyl ether, polyoxypropylene glyceryl ether, etc.); Alkyl alcohols having 1 to 4 carbon atoms, glycol ethers, 2-pyrrolidone, and N-methyl-2-pyrrolidone; These are some examples. In particular, from the viewpoint of suppressing the transfer of components, polyhydric alcohols or polyhydric alcohol ethers are preferred, and alkanediols, polyalkylene glycols, or polyalkylene glycol ethers are more preferred.

[0164] If the pretreatment solution contains a water-soluble organic solvent, the content of the water-soluble organic solvent relative to the total amount of the pretreatment solution is preferably 0.5% to 30% by mass, more preferably 1% to 20% by mass, and particularly preferably 1% to 15% by mass.

[0165] As for water-soluble organic solvents that can be contained in the pretreatment solution, water-soluble organic solvents with a boiling point of less than 220°C are also preferred. From the viewpoint of the drying properties of the pretreatment solution, it is preferable that the pretreatment solution does not contain organic solvents with a boiling point of 220°C or higher, or that the content of organic solvents with a boiling point of 220°C or higher is 5% by mass or less (more preferably 3% by mass or less, and even more preferably 1% by mass or less) of the total amount of the pretreatment solution. For specific examples of water-soluble organic solvents with a boiling point below 220°C and organic solvents with a boiling point above 220°C, please refer to the section "Preparation of the First Ink" described later.

[0166] (Other ingredients) The pretreatment solution may contain other components not listed above, if necessary. Other components that may be contained in the pretreatment solution include known additives such as surfactants, solid wetting agents, silicate compounds (e.g., colloidal silica), inorganic salts, anti-fading agents, emulsifying stabilizers, penetration enhancers, ultraviolet absorbers, preservatives, antifungal agents, pH adjusters, viscosity adjusters, rust inhibitors, chelating agents, and water-soluble polymer compounds other than water-soluble cationic polymers (e.g., water-soluble polymer compounds described in paragraphs 0026 to 0080 of Japanese Patent Application Publication No. 2013-001854). Other components that may be included in the pretreatment solution can be found in the components that may be included in the first ink, as described later.

[0167] (Physical properties of the pretreatment solution) From the viewpoint of ink aggregation rate, the pH of the pretreatment solution at 25°C is preferably 0.1 to 3.5. When the pH of the pretreatment solution is 0.1 or higher, the roughness of the resin substrate is further reduced, and the adhesion of the image area is further improved. When the pH of the pretreatment solution is 3.5 or lower, the aggregation rate is further improved, the coalescence of ink dots on the surface of the resin substrate is further suppressed, and the graininess of the image is further reduced. The pH of the pretreatment solution at 25°C is more preferably between 0.2 and 2.0.

[0168] If the pretreatment solution contains a coagulant, the viscosity of the pretreatment solution is preferably in the range of 0.5 mPa·s to 10 mPa·s, and more preferably in the range of 1 mPa·s to 5 mPa·s, from the viewpoint of the ink coagulation rate.

[0169] The surface tension of the pretreatment solution is preferably 60 mN / m or less, more preferably 20 mN / m to 50 mN / m, and even more preferably 30 mN / m to 45 mN / m.

[0170] ≪Second Embodiment≫ A second embodiment of the ink set of the present disclosure comprises the aforementioned ink of the present disclosure and a pretreatment solution containing water and a coagulant. The second embodiment includes a pretreatment solution, but is not limited to including color ink. In this respect, the second embodiment differs from the first embodiment. Apart from this point, the second embodiment is the same as the first embodiment, and preferred embodiments of the second embodiment (for example, preferred embodiments of the ink and pretreatment solution) are the same as preferred embodiments of the first embodiment.

[0171] Similar to the ink set of the first embodiment, the ink set of the second embodiment also includes the ink of the present disclosure as described above, and therefore exhibits the same effects as those of the ink of the present disclosure.

[0172] [Image recording method] The image recording method disclosed herein includes an ink application step of applying the ink disclosed herein to a substrate by an inkjet method. The image recording method disclosed herein may include other steps as necessary.

[0173] Since the image recording method of this disclosure uses the ink of this disclosure as described above, the image recording method of this disclosure produces the same effects as those produced by the ink of this disclosure.

[0174] <Ink application process> The ink application step is a step of applying the ink of the present disclosure onto a substrate by an inkjet method. This process records an image using the inks of this disclosure.

[0175] (base material) The substrate is not particularly limited, and any known substrate can be used. Examples of substrates include paper substrates, paper substrates laminated with resin (e.g., polyethylene, polypropylene, polystyrene, etc.), resin substrates, metal plates (e.g., plates of metals such as aluminum, zinc, and copper), paper substrates laminated or vapor-deposited with the aforementioned metals, resin substrates laminated or vapor-deposited with the aforementioned metals, and so on.

[0176] Another example of a base material is textile base material. Examples of materials for textile base materials include natural fibers such as cotton, silk, linen, and wool; chemical fibers such as viscose rayon and reocell; synthetic fibers such as polyester, polyamide, and acrylic; and mixtures of at least two selected from the group consisting of natural fibers, chemical fibers, and synthetic fibers. As textile base materials, textile base materials described in paragraphs

[0039] to

[0042] of International Publication No. 2015 / 158592 may also be used.

[0177] The base material is preferably a non-permeable base material. Here, a non-permeable substrate refers to a substrate whose water absorption rate (mass %, 24hr.) is less than 0.2 according to the ASTM D570 test method.

[0178] There are no particular restrictions on the non-permeable substrate, but a resin substrate is preferred. There are no particular restrictions on the resin substrate; for example, a thermoplastic resin substrate can be used. Examples of resin substrates include thermoplastic resins molded into sheet or film shapes. As the resin substrate, a substrate containing polypropylene, polyethylene terephthalate, nylon, polyethylene, or polyimide is preferred.

[0179] The resin substrate may be a transparent resin substrate. Here, "transparent" means that the transmittance of visible light with wavelengths of 400 nm to 700 nm is 80% or higher (preferably 90% or higher). A transparent resin substrate is suitable for the embodiment described above, in which the image is viewed through the substrate from the non-image recording side of the substrate.

[0180] The shape of the resin substrate is not particularly limited, but it is preferably a sheet-shaped resin substrate, and more preferably a sheet-shaped resin substrate that can be formed into a roll by winding. The thickness of the resin substrate is preferably 10 μm to 200 μm, and more preferably 10 μm to 100 μm.

[0181] The resin substrate may be surface-treated to improve its surface energy. Surface treatments include, but are not limited to, corona treatment, plasma treatment, flame treatment, heat treatment, abrasion treatment, light irradiation treatment (UV treatment), and fire treatment.

[0182] (Inkjet head) This process involves applying the ink of the present disclosure onto a substrate by an inkjet method. That is, the ink of this disclosure is ejected from the inkjet head and applied to the substrate.

[0183] The resolution of the inkjet head is preferably 300 dpi or higher, more preferably 600 dpi, and even more preferably 800 dpi. Here, dpi is an abbreviation for dots per inch, and 1 inch is equal to 2.54 cm.

[0184] Examples of ink ejection methods from an inkjet head include a charge control method that uses electrostatic attraction to eject ink, a drop-on-demand method (pressure pulse method) that uses the vibration pressure of a piezoelectric element, an acoustic inkjet method that converts an electrical signal into an acoustic beam, irradiates the ink with it, and uses the radiation pressure to eject ink, and a thermal inkjet (bubble jet®) method that heats the ink to form bubbles and uses the resulting pressure. Furthermore, as an ink ejection method from the inkjet head, for example, the method described in Japanese Patent Publication No. 54-59936, in which the ink, subjected to the action of thermal energy, undergoes a rapid volume change, and the force resulting from this state change ejects the ink from the nozzle can also be applied. Furthermore, the method described in paragraphs 0093 to 0105 of Japanese Patent Publication No. 2003-306623 can also be applied as the ink ejection method from the inkjet head.

[0185] As an inkjet head, an inkjet head is preferable in which at least a portion of the parts that come into contact with the ink (for example, the nozzle plate and the ink channel) contains silicon. In this embodiment, even if at least a portion of the part in contact with the ink dissolves into the ink, the effect of dissolving and degrading the inkjet head by the specific silicon compound (i.e., the repair effect by the specific silicon compound) is more effectively achieved. An example of a nozzle plate is a silicon nozzle plate having multiple discharge holes. This nozzle plate may also contain a silicon oxide film. For information on such inkjet heads, one can refer to prior art documents such as Japanese Patent Publication No. 5430316.

[0186] As an inkjet head, for example, an inkjet head having multiple ejection holes and having a liquid-repellent film on the ejection surface (for example, the ejection surface of the nozzle plate) can be used. The liquid-repellent film in the inkjet head preferably contains a fluorine compound, more preferably a compound containing an alkyl fluoride, and even more preferably a perfluoroalkyl ether. There are no particular restrictions on the thickness of the liquid-repellent film, but 0.2 nm to 30 nm is preferred, and 0.4 nm to 20 nm is more preferred.

[0187] Commercially available inkjet print heads may be used. Commercially available options include the Konica Minolta KM1800i inkjet head, the Kyocera KJ4A-AA inkjet head, and the Fujifilm Dimatix Samba G3L inkjet head.

[0188] From the viewpoint of obtaining an image with excellent opacity, the amount of ink droplets ejected from the ejection holes of the inkjet head is preferably 1.0 pL (picoliters) or more. The amount of ink droplets is more preferably 1.5 pL or more. The upper limit of the amount of ink droplets is preferably 10 pL, more preferably 6 pL.

[0189] In the inkjet head, the diameter of the ejection hole (i.e., nozzle diameter) for ejecting ink is preferably 20 μm or less, more preferably 18 μm or less. There is no particular limitation on the lower limit of the diameter of the ejection hole, and examples of the lower limit include 10 μm, 11 μm, 12 μm, etc.

[0190] The resolution of the inkjet head is preferably 300 dpi or more, more preferably 600 dpi, and even more preferably 800 dpi. Here, dpi is an abbreviation for dot per inch, and 1 inch is 2.54 cm.

[0191] (Ink application method) As the ink application method, either a single-pass method or a scan method may be used, but from the viewpoint of the image recording speed, the single-pass method is preferred. Here, the single-pass method is a method in which, as an inkjet head, a line head in which ejection holes (nozzles) are arranged corresponding to the entire area of one side of the substrate is used, the line head is fixedly arranged, and while the substrate is conveyed in a direction intersecting the arrangement direction of the ejection holes of the line head, ink is applied onto the conveyed substrate. On the other hand, the scan method is a method in which, as an inkjet head, a short serial head is used, and the short serial head is scanned over the substrate to apply ink.

[0192] In the ink application step, the ink applied onto the substrate may be heated and dried to obtain an image. Examples of the means for performing heat drying include known heating means such as heaters, known blowing means such as dryers, and means combining these. Methods for heat-drying ink include, for example, applying heat from a heater on the opposite side of the substrate from the side where the ink is applied, applying warm air or hot air to the side of the substrate from which the ink is applied, applying heat from an infrared heater on the opposite side of the substrate from the side where the ink or specific ink is applied, and methods combining several of these.

[0193] The heating temperature during heat drying is preferably 55°C or higher, more preferably 60°C or higher, and particularly preferably 65°C or higher. There is no particular upper limit to the heating temperature, but an upper limit of, for example, 100°C is possible, and 90°C is preferred. There are no particular restrictions on the time for heating and drying the ink, but 3 to 60 seconds is preferred, 5 to 60 seconds is more preferred, and 10 to 45 seconds is particularly preferred.

[0194] Alternatively, the substrate may be heated beforehand before applying the ink. The heating temperature can be set as appropriate, but it is preferable to set the substrate temperature to 20°C to 50°C, and more preferably to 25°C to 40°C.

[0195] <Other processes> The image recording method disclosed herein may include other steps besides the ink application step. Other processes include a pretreatment liquid application process, which is performed before the ink application process and involves applying a pretreatment liquid containing a coagulant and water onto the substrate. If the image recording method of the present disclosure includes a pretreatment solution application step, the ink application step involves applying ink to the area on the substrate to which the pretreatment solution has been applied. That is, it is preferable that the image recording method has a pretreatment solution application step and an ink application step in this order. Here, it is also preferable that the pretreatment solution application step is a step of applying a pretreatment solution containing water and a coagulant to a non-permeable substrate, and the ink application step is a step of applying inkjet ink to the area on the non-permeable substrate to which the pretreatment solution has been applied.

[0196] The pretreatment solution is as described in the ink set section above. For details regarding the pretreatment solution application process and the pretreatment solution itself, refer to publicly available documents such as International Publication No. 2019 / 004485 and International Publication No. 2019 / 163581 as appropriate.

[0197] The flocculant contained in the pretreatment solution is preferably at least one selected from the group consisting of polyvalent metal compounds, organic acids, metal complexes, and cationic polymers, and more preferably contains an organic acid.

[0198] The application of the pretreatment solution in the pretreatment solution application step can be carried out by applying known application methods such as coating, inkjet recording, or immersion. Known coating methods include those using bar coaters, extrusion die coaters, air doctor coaters, blade coaters, rod coaters, knife coaters, squeeze coaters, reverse roll coaters, and the like.

[0199] In the pretreatment solution application step, the pretreatment solution applied to the substrate may be heated and dried. The preferred method and conditions for heat drying are the same as the preferred method and conditions for heat drying of the ink that may be carried out in the ink application process.

[0200] <Color ink application process and white ink application process> The image recording method of this disclosure may be implemented using the ink set of this disclosure described above (i.e., the ink set of the first or second embodiment). The image recording method when using the ink set of the first embodiment is preferably as follows: A color ink application process in which color ink is ejected from an inkjet head and applied to a non-permeable substrate, A process of applying white ink by ejecting white ink from an inkjet head and applying it to areas on a non-permeable substrate that has been treated with color ink, Includes. According to the image recording method of this aspect, it is easy to manufacture an image recording material including a multi-color image A in which a color pattern image (characters, figures, etc.) recorded with color ink and a solid white image recorded with white ink so as to cover the pattern image are arranged in this order on a substrate.

[0201] The image recording method of the above aspect preferably includes a pretreatment liquid application step of applying the above-described pretreatment liquid onto a non-permeable substrate before the color ink application step. Thereby, an image with better fineness can be recorded. According to the image recording method of such a preferred aspect, the effects of limiting the X1 / X2 ratio to 5.0 or less and the effect of the first silicon compound containing a silicate and the second silicon compound containing colloidal silica described above are particularly effectively exhibited.

Example

[0202] Examples of the present disclosure will be shown below, but the present disclosure is not limited to the following examples. In the following, "water" means ion-exchanged water unless otherwise specified.

[0203] 〔Example 1〕 ≪Preparation of white ink≫ White ink W1 was prepared. Details are shown below.

[0204] <Synthesis of uncrosslinked polymer dispersant N1> 965 g of dipropylene glycol was added to a 5000 mL three-necked flask equipped with a stirrer and a cooling tube, and heated to 85°C under a nitrogen atmosphere. Solution I obtained by dissolving 640 g of benzyl methacrylate, 340 g of methacrylic acid, and 19.94 g of 2-mercaptopropionic acid in 370.28 g of dipropylene glycol, and Solution II obtained by dissolving 17.69 g of t-butylperoxy-2-ethylhexanoate (product name "Perbutyl O", manufactured by NOF Corporation) in 221.17 g of dipropylene glycol, and were each prepared. Solution I was added dropwise to the three-necked flask over 4 hours, and solution II over 5 hours. After the dropwise addition was complete, the reaction was allowed to continue for another 2 hours. The disappearance of monomers was observed. 1 The reaction was confirmed by 1H-NMR. The resulting reaction solution was heated to 70°C, 248.02 g of 50% potassium hydroxide aqueous solution was added, followed by 107.48 g of dipropylene glycol and 75.52 g of pure water, and the mixture was stirred to obtain a 37% solution of random polymer. This random polymer was used as the uncrosslinked polymer dispersant N1. The structural units that make up the obtained random polymer (i.e., uncrosslinked polymer dispersant N1) 1 The results were confirmed by 1H-NMR. The weight-average molecular weight (Mw) was also determined by GPC. The weight-average molecular weight (Mw) of the obtained uncrosslinked polymer dispersant N1 was 8400, and the acid value was 221.7 mgKOH / g.

[0205] <Preparation of white pigment dispersion N using uncrosslinked polymer dispersant N1> Uncrosslinked polymer dispersant N1 (150 parts by mass) was dissolved in water to prepare a polymer solution with a concentration of 25% by mass of uncrosslinked polymer dispersant N1. 96 parts by mass of the above polymer solution, 300 parts by mass of CI Pigment White 6 (product name "JR-405", titanium dioxide particles, manufactured by Teika Co., Ltd.), a white pigment, and 270 parts by mass of water were mixed to obtain a mixture. Potassium hydroxide aqueous solution was added to the obtained mixture to adjust the pH to 8.7 after neutralization. The pH was measured at 25°C using a pH meter (model: WM-50EG, manufactured by Toa DDK Co., Ltd.). Next, the mixture after neutralization was subjected to a dispersion treatment for 3 hours using a bead mill (bead diameter: 0.1 mmφ, zirconia beads). This yielded a white pigment dispersion N (uncrosslinked dispersion) in which the white pigment was dispersed by the uncrosslinked polymer dispersant N1. The pigment concentration of the uncrosslinked dispersion was 45% by mass, and the concentration of the uncrosslinked polymer dispersant N1 was 3.6% by mass.

[0206] <Preparation of white pigment dispersion CL using crosslinked polymer dispersant L1> To 136 parts by mass of a white pigment dispersion N (uncrosslinked dispersion), in which the white pigment is dispersed by an uncrosslinked polymer dispersant N1, 2.70 parts by mass of trimethylolpropane polyglycidyl ether (product name "Denacol EX-321", manufactured by Nagase ChemteX Corporation) and 29.5 parts by mass of an aqueous boric acid solution (boric acid concentration: 4% by mass) were added as crosslinking agents. The mixture was reacted at 70°C for 6 hours and then cooled to 25°C. This crosslinked the uncrosslinked polymer dispersant N1 in the dispersion to form a crosslinked polymer dispersant L1, and a white pigment dispersion (crosslinked dispersion) in which the white pigment is dispersed by the crosslinked polymer dispersant L1 was obtained. To the obtained cross-linked dispersion, deionized water was added to achieve a pigment concentration of 15% by mass. The cross-linked dispersion with added deionized water was ultrafiltered through an ultrafiltration apparatus (cross-flow type ultrafilter (UF), manufactured by Sartorius) equipped with a polyethersulfone (PESU) membrane (micropore size: 0.1 μm) at a flow rate of 600 mL per minute. At this time, the liquid temperature was adjusted to 25°C, and ultrafiltration was performed eight times, with each ultrafiltration cycle being 1x the volume of the initial liquid. Next, deionized water was added to achieve a white pigment concentration of 45% by mass. This yielded a white pigment dispersion CL. The acid value of the cross-linked polymer dispersant L1 contained in the white pigment dispersion CL was 105 mg KOH / g. The concentration of the cross-linked polymer dispersant L1 was 3.6% by mass. The crosslinked polymer dispersant L1 is a crosslinked polymer obtained by crosslinking the uncrosslinked polymer dispersant N1 with polyethylene glycol diglycidyl ether as a crosslinking agent. The white ink described later was prepared using a white pigment dispersion CL containing the crosslinked polymer dispersant L1.

[0207] <Preparation of a dispersion of resin particles P1> A dispersion of resin particles P1, one of the components in the ink, was prepared. Details are shown below. 560.0 g of methyl ethyl ketone was placed in a 2-liter three-necked flask (reaction vessel) equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, and the temperature was raised to 87°C. Then, while maintaining reflux in the reaction vessel (reflux was maintained until the end of the reaction), a mixed solution consisting of 220.4 g of methyl methacrylate, 301.6 g of isobornyl methacrylate, 58.0 g of methacrylic acid, 108 g of methyl ethyl ketone, and 2.32 g of "V-601" (polymerization initiator manufactured by Fujifilm Wako Pure Chemical Industries; dimethyl 2,2'-azobis(2-methylpropionate)) was added dropwise to the methyl ethyl ketone in the reaction vessel at a constant rate so that the addition was completed in 2 hours. After the addition was completed, the mixture was stirred for 1 hour, and then the procedure in step (1) below was performed on the solution after 1 hour of stirring. Step (1) ... A solution consisting of 1.16 g of "V-601" and 6.4 g of methyl ethyl ketone was added and stirred for 2 hours. Next, the above procedure (1) was repeated four times, and then a solution consisting of 1.16 g of "V-601" and 6.4 g of methyl ethyl ketone was added and stirring was continued for 3 hours (the operations up to this point are referred to as the "reaction"). After the reaction was complete, the solution was cooled to 65°C, 163.0 g of isopropanol was added, and the solution was allowed to cool to obtain a polymerization solution containing the copolymer (solid content concentration 41.0% by mass). Next, 317.3 g of the obtained polymerization solution was weighed, and 46.4 g of isopropanol, 1.65 g of 20% by mass maleic anhydride aqueous solution (a water-soluble acidic compound, equivalent to 0.3% by mass of maleic acid relative to the copolymer), and 40.77 g of 2 mol / L sodium hydroxide (NaOH) aqueous solution were added to it, and the temperature of the liquid in the reaction vessel was raised to 70°C. Next, 380g of distilled water was added dropwise to the liquid, which had been heated to 70°C, at a rate of 10mL / min to disperse it with water (dispersion step). Subsequently, under reduced pressure, the temperature of the liquid in the reaction vessel was maintained at 70°C for 1.5 hours to remove a total of 287.0 g of isopropanol, methyl ethyl ketone, and distilled water (solvent removal step). To the obtained liquid, 0.278 g of Proxel GXL(S) (manufactured by Arch Chemicals Japan Co., Ltd.) (440 ppm by mass as benzoisothiazolin-3-one relative to the polymer solids) was added. The obtained liquid was filtered through a 1 μm filter, and the filtrate was collected to obtain an aqueous dispersion of resin particles P1 (non-volatile content 23.2% by mass) consisting of a methyl methacrylate / isobornyl methacrylate / methacrylic acid / sodium methacrylate (=70 / 20 / 5 / 5 [mass ratio]) copolymer. The volume-average particle size of resin particles P1 was 5.0 nm, and the weight-average molecular weight (Mw) of resin particles P1 was 60,000.

[0208] <Preparation of White Ink W1> A white pigment dispersion CL containing the above-mentioned crosslinked polymer dispersant L1, a dispersion of the above-mentioned resin particles P1, and materials in the following composition were used to prepare a white ink W1 with the following composition. In the "Dispersion Method" column in Tables 1 and 2 described below, "crosslinking" means that a white pigment dispersion CL containing the crosslinking polymer dispersant L1 was used to prepare the white ink.

[0209] -Composition of White Ink W1- Titanium dioxide particles 1… 12% by mass • Crosslinked polymer dispersant L1… 0.96% by mass • Colloidal silica as a silicon compound (Nissan Chemical Corporation's "Snowtex XS")... 0.05% by mass as solid content • Propylene glycol (PG) (water-soluble organic solvent) ... 25% by mass ·Resin particles P1…5.0% by mass • Orphine E1010 (acetylene glycol-based surfactant manufactured by Nisshin Chemical Industry Co., Ltd.) ... 0.5% by mass • Water… Remaining amount totaling 100% by mass

[0210] <<Preparation of pretreatment solution>> A pretreatment solution was obtained by mixing the components with the following composition.

[0211] -Composition of pretreatment solution- • Malonic acid (manufactured by Fujifilm Wako Pure Chemical Industries; flocculant (organic acid)) …5.0% by mass • Sodium dodecylbenzenesulfonate (surfactant) (manufactured by Tokyo Chemical Industry Co., Ltd.) …1.0% by mass ·water ...The remaining amount will be 100% by mass in total.

[0212] ≪Image Record≫ Using the above ink, and a biaxially oriented polypropylene (OPP) film (40 μm thick, surface treatment: corona discharge treatment, manufactured by Futamura Chemical Co., Ltd.) (non-permeable substrate) as the substrate, image recording was performed as follows. (1) Recording method An inkjet recording device equipped with a transport system for transporting the substrate and the inkjet head described below was used to apply a solid image of ink to the corona discharge treated surface of the substrate by ejecting the ink from the inkjet head. The ink applied to the substrate was dried at 80°C for 30 seconds to obtain a solid image of white. The drying of the ink was performed by placing the substrate with the ink applied on a hot plate. As a result, an image recording material comprising a substrate and a solid image placed on the substrate was obtained. (2) Recording conditions Inkjet head: A 1200dpi x 1200dpi piezo head (42.69mm width), the "Samba G3L" inkjet head manufactured by FUJIFILM Dimatix. Ink droplet volume: 1.8 pL Drive frequency: 30kHz (Material transport speed: 635mm / sec) Ink application method: Single-pass method

[0213] <Measurement and Evaluation> The following measurements and evaluations were performed. The results are shown in Table 1.

[0214] <Deterioration of dissolution of the IJ head> On a 2 cm × 2 cm silicon substrate, a liquid-repellent film (SAM (Self-Assembled Monolayer) film) was formed using an alkylfluorosilane compound to obtain a test piece for evaluating an inkjet head (hereinafter also referred to as "IJ head"). The white ink W1 (30 mL) prepared above was put into a 50 mL wide-mouth bottle made of polypropylene (Iboy wide-mouth bottle 50 mL (manufactured by AS ONE Corporation)), and the test piece was immersed therein, and the ink was allowed to stand for 72 hours at an ink temperature of 60°C. After the elapse of time, the test piece was taken out from the white ink W1 and washed with ultrapure water, and the water contact angle of the surface on the liquid-repellent film side of the washed test piece was measured. The measurement of the water contact angle was carried out according to a conventional method using a contact angle measuring device (DM-500, manufactured by Kyowa Interface Science Co., Ltd.) and ultrapure water under an environment of 25°C and 50 RH%. Based on the measurement results of the water contact angle, the dissolution deterioration of the IJ head was evaluated according to the following evaluation criteria. Here, the water contact angle of the surface on the liquid-repellent film side of the test piece before being immersed in the white ink W1 was 80° or more. In the following evaluation criteria, the rank in which the dissolution deterioration of the inkjet head is most suppressed is AA.

[0215] -Evaluation criteria for dissolution deterioration of the IJ head- AA: The water contact angle was 80° or more. A: The water contact angle was 60° or more and less than 80°. B: The water contact angle was 40° or more and less than 60°. C: The water contact angle was 20° or more and less than 40°. D: The water contact angle was less than 20°.

[0216] <Wear deterioration of the IJ head> A test piece for evaluating the IJ head, which was the same as the test piece used for evaluating the dissolution deterioration of the IJ head, was prepared. Diethylene glycol monobutyl ether (20 parts by mass), diethylene glycol (10 parts by mass), and water (70 parts by mass) were mixed to obtain cleaning solution 1. The obtained cleaning solution 1 (97 parts by mass) and white ink W1 (3 parts by mass) were mixed to obtain diluted ink. A φ40mm rubber roller with a rotating mechanism was wrapped with a cloth (Toraysee manufactured by Toray Industries, Inc.), and the diluted ink described above was impregnated into the wrapped cloth. In this state, the roller was rotated, and a rubbing operation was performed on the surface of the test piece with the liquid-repellent film side using the cloth impregnated with the diluted ink. The rubbing operation (i.e., the rotation of the roller) was interrupted as needed, and the water contact angle of the liquid-repellent film side of the test specimen was measured. The water contact angle was measured in the same manner as the water contact angle measurement used in the evaluation of the dissolution degradation of the IJ head. The rubbing operation and water contact angle measurements were repeated, and based on the measurement results, the number of roller rotations required to achieve a water contact angle of less than 60° was determined. Based on the number of roller rotations at which the water contact angle is less than 60°, the wear and deterioration of the IJ head was evaluated according to the following evaluation criteria. In the evaluation criteria below, rank A represents the best suppression of inkjet head wear and deterioration.

[0217] -Evaluation criteria for wear and deterioration of the IJ head- A: The number of roller rotations at which the water contact angle is less than 60° is 1500 or more. B: The number of roller rotations at which the water contact angle is less than 60° is between 1000 and 1500. C: The number of roller rotations at which the water contact angle is less than 60° is between 800 and 1000. D: The number of roller rotations at which the water contact angle is less than 60° is less than 800.

[0218] <Concealing properties> The opacity of solid images in the above-mentioned image recordings was evaluated as follows. Separately from the image recordings mentioned above, four character images (2pt, 4pt, 6pt, and 8pt) were recorded on the substrate using black inkjet ink (Fujifilm "C-WP-QK") to obtain a substrate with character images. All four character images are the same as those shown in Figure 1. The above-mentioned image recording material and the substrate with the character image were stacked so that their non-image recording surfaces (surfaces without images) were in contact with each other to form a laminate. The resulting laminate was held up to a 30W fluorescent lamp with the solid image facing the evaluator, and the opacity of the solid image was evaluated by checking whether the details of each character image could be seen through the solid image, according to the evaluation criteria below. In this test, the distance between the evaluator's eye and the laminate was 20 cm, and the distance from the laminate to the fluorescent lamp was 2 m. In the evaluation criteria below, the rank that best represents the ability to conceal solid images is "A".

[0219] - Criteria for evaluating concealment - A: Details could not be clearly seen in the text images in 2pt, 4pt, 6pt, and 8pt sizes. While the details of the 8pt text image were visible, the details of the 2pt, 4pt, and 6pt text images were not. Details of the text images in 6pt and 8pt font sizes were visible, but details of the text images in 2pt and 4pt font sizes were not. Details of the text images in D:4pt, 6pt, and 8pt sizes were visible, but details of the 2pt text image were not visible. For text images in sizes E: 2pt, 4pt, 6pt, and 8pt, fine details were clearly visible.

[0220] [Examples 2-13, Comparative Examples 1-5] In white ink, Titanium dioxide particle size, Titanium dioxide content (mass%) relative to the total amount of white ink, Types of silicon compounds, Particle size when the silicon compound is colloidal silica, The same procedure as in Example 1 was followed, except that the content (mass%) of the silicon compound relative to the total amount of white ink and the combination of dispersion methods were changed as shown in Tables 1 and 2. The results are shown in Tables 1 and 2. Tables 1 and 2 also show the white ink numbers for each example and comparative example.

[0221] In the section on dispersion methods, "block" means that the following block polymer 1 was used as a polymer dispersant to disperse titanium dioxide particles. When the dispersion method column is set to "block," the white pigment dispersion was prepared as follows:

[0222] <Synthesis of polymer dispersant (block polymer 1)> Block polymer 1 was synthesized as a polymer dispersant, referring to Synthesis Example 8 in Japanese Patent Publication No. 2015-83688. Details are shown below. A reaction apparatus consisting of a 1 L separable flask equipped with a stirrer, a backflow condenser, a thermometer, and a nitrogen inlet tube, Diethylene glycol dimethyl ether (266 parts by mass; polymerization solvent), 2-Iodo-2-cyanopropane (6.2 parts by mass; polymerization initiation compound), Methyl methacrylate (MMA) (120 parts by mass; monomer), Acrylic acid (AA) (28.8 parts by mass; monomer), Cyclohexyl methacrylate (CHMA) (67.2 parts by mass; monomer), Azobisdimethylisovaleronitrile (7.9 parts by mass) and 2-t-butyl-4,6-dimethylphenol (0.7 parts by mass; catalyst) were added, and the mixture was stirred while flowing nitrogen. Next, the temperature of the mixture in the reaction apparatus (reaction temperature) was raised to 70°C, and polymerization was carried out for 3 hours to obtain polymerization solution A containing the MMA / AA / CHMA copolymer. After 3 hours, a sample of polymerization solution A was taken and its solid content was measured. It was found to be 42.0% by mass, confirming that most of the monomers had polymerized. Furthermore, when the molecular weight of the MMA / AA / CHMA copolymer was measured using GPC, the weight-average molecular weight (Mn) was found to be 7,500. The acid value of this MMA / AA / CHMA copolymer was 101.0 mgKOH / g.

[0223] Next, a mixture of benzyl methacrylate (BzMA) (35.2 parts by mass; monomer) and V-65 (0.3 parts by mass; radical generator) was added to polymerization solution A, and polymerization was carried out at 70°C for 3 hours to obtain polymerization solution B containing block polymer 1 as a polymer dispersant. Here, block polymer 1 is a block polymer comprising block A, which is an MMA / AA / CHMA copolymer, and block B, which is a BzMA homopolymer. When the solid content of the obtained polymerization solution B was measured, it was found to be 43.2% by mass, confirming that most of the monomers had polymerized. Furthermore, the Mw of block polymer 1 was 8,500, and its acid value was 89.3 mgKOH / g.

[0224] <Preparation of white pigment dispersion ("block")> The above-mentioned block polymer 1 (136.4 parts by mass), butyl carbitol (163.6 parts by mass), and CI Pigment White 6 (product name "JR-405", titanium dioxide particles, manufactured by Teika Co., Ltd.) (450 parts by mass) as a white pigment were blended and stirred with a disperser. Next, the white pigment was thoroughly dispersed using a horizontal media disperser to obtain an oil-based pigment dispersion. The average particle size of the white pigment dispersed in the oil-based pigment dispersion was 290 nm. The viscosity of the oil-based pigment dispersion was 86.3 m³ Pa·s. Next, while stirring the oily pigment dispersion (700 parts by mass) with a disperser, a mixture of potassium hydroxide (4.0 parts by mass) and water (341 parts by mass) was gradually added to neutralize the mixture. After that, the white pigment was thoroughly dispersed using a horizontal media disperser to obtain a pigment dispersion. Next, the obtained pigment dispersion was subjected to ultrafiltration using an ultrafiltration apparatus (cross-flow type ultrafilter (UF), manufactured by Sartorius) by flowing deionized water at a flow rate of 600 mL per minute. The liquid temperature was maintained at 25°C, and ultrafiltration was performed eight times, with each pass representing one volume of the initially charged liquid. Deionized water was added to obtain a white pigment dispersion "block" with a pigment concentration of 45% by mass and a pigment dispersant block polymer concentration of 3.6% by mass.

[0225] In the section on dispersion methods, "random" means that the following random polymer 1 was used as the polymer dispersant to disperse the titanium dioxide particles. When the dispersion method was set to "random," the white pigment dispersion was prepared as follows:

[0226] <Synthesis of Random Polymer 1 (Polymer Dispersant)> Solution I, which is a mixture of 234 g benzyl methacrylate, 120 g stearyl methacrylate, 84 g methacrylic acid, 162 g hydroxyethyl methacrylate, and 3.93 g 2-mercaptopropionic acid, Solution II, prepared by dissolving 6.2 g of t-butylperoxy-2-ethylhexanoate (Perbutyl O, manufactured by NOF Corporation) in 115 g of dipropylene glycol, Each of these was prepared. 605 g of dipropylene glycol was added to a three-necked flask equipped with a stirrer and a condenser, and heated to 85°C under a nitrogen atmosphere. Solution I was then added dropwise over 4 hours, followed by Solution II over 5 hours. After the dropwise addition was complete, the reaction was allowed to continue for another 2 hours, then the temperature was raised to 95°C and the mixture was heated and stirred for 3 hours to react all unreacted monomers. The disappearance of monomers was 1 Confirmed by 1H-NMR. The resulting reaction solution was heated to 70°C, 74 g of dimethylaminoethanol was added as the amine compound, and then 764 g of propylene glycol was added and stirred to obtain a solution of random polymer 1 (solid content: 30% by mass) with a weight-average molecular weight (Mw) of 30,000 and an acid value of 112 mg KOH / g. The constituent components of the obtained polymer1 Confirmed by 1H-NMR.

[0227] <Preparation of white pigment dispersion ("random")> 150 parts by mass of random polymer 1 was dissolved in water, and an aqueous solution of random polymer 1 was prepared using an aqueous potassium hydroxide solution to neutralize it so that the pH after neutralization was 9 and the concentration of random polymer 1 was approximately 25% by mass, thereby obtaining an aqueous solution of random polymer 1. The components were mixed according to the composition of the white pigment dispersion below. The resulting mixture was subjected to a dispersion treatment for 1.5 hours using a Labstar Mini LMZ015 (manufactured by Ashizawa Finetech Co., Ltd., bead diameter: 0.1 mmφ, zirconia beads, rotation speed: 10 m / sec) to obtain a white pigment dispersion (white pigment concentration 48% by mass, solid content concentration 50% by mass).

[0228] -Composition of the white pigment dispersion- • Aqueous solution of random polymer 1: 54 parts by mass • White pigment (CI Pigment White 6 (product name "JR-405", titanium dioxide particles, manufactured by Teika Co., Ltd.): 500 parts by mass • Propylene glycol: 100 parts by mass • Water: Remaining amount equals 1000 parts by mass

[0229] In the section on dispersion method, "self-dispersion" means that a white pigment dispersion of self-dispersing titanium dioxide particles ("self-dispersed") was used as the white pigment dispersion. When the dispersion method is "self-dispersion," the white pigment dispersion was prepared as follows.

[0230] <Preparation of a white pigment dispersion ("self-dispersing")> -White pigment dispersion (self-dispersion)- 100 g of titanium dioxide particles were added to 3,000 mL of 2.5 N sodium hypochlorite solution and stirred at 60°C and 300 rpm for 10 hours. Oxidation treatment yielded a reaction solution of white pigment in which carboxyl groups were attached to the surface of the titanium dioxide particles. The obtained reaction solution was filtered, and the filtered white pigment was neutralized with sodium hydroxide solution and ultrafiltration was performed. Next, ultrafiltration was performed using ion-exchanged water with a dialysis membrane, and then ultrasonic dispersion was performed using an ultrasonic disperser to obtain a white pigment dispersion ("self-dispersed") with a pigment concentration of 45% by mass.

[0231] [Table 1]

[0232] [Table 2]

[0233] As shown in Tables 1 and 2, in each example using an ink containing water, titanium dioxide particles, and a silicon compound, wherein the average primary particle diameter of the titanium dioxide particles is 100 nm or more, the silicon compound is at least one selected from the group consisting of silicates and colloidal silica, the content of the silicon compound relative to the total amount of titanium dioxide particles is 0.0020% by mass or more, and the particle size ratio [colloidal silica / titanium dioxide particles] (i.e., the ratio of the average primary particle diameter of colloidal silica to the average primary particle diameter of titanium dioxide particles) is 0.04 or less, images with excellent opacity could be recorded, and wear and deterioration of the IJ head could be suppressed. Furthermore, in each embodiment, the dissolution and degradation of the IJ head was also suppressed.

[0234] The results for each comparative example for each example were as follows. In Comparative Example 1, where the ink did not contain silicon compounds, wear and deterioration of the inkjet head could not be suppressed. In Comparative Examples 2 and 3, where the silicon compound content relative to the total amount of titanium dioxide particles in the ink was less than 0.0020% by mass, wear and deterioration of the inkjet head could not be suppressed. In Comparative Example 4, where the particle size ratio [colloidal silica / titanium dioxide particles] in the ink was greater than 0.04, wear and deterioration of the inkjet head could not be suppressed. In Comparative Example 5, where the average primary particle diameter of titanium dioxide particles in the ink was less than 100 nm, the image opacity decreased, and wear degradation of the inkjet head could not be suppressed.

[0235] The results from Examples 1, 4, 5, and 7-9 show that when the silicon compound in the ink contains a silicate, and the silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates (Examples 4, 5, and 7-9), wear and deterioration of the inkjet head can be further suppressed.

[0236] The results from Examples 7-13 show that when the ink contains a polymer dispersant, and the polymer dispersant contains a block polymer or a polymer with a cross-linked structure (Examples 7-10), wear and deterioration of the IJ head can be further suppressed. This is thought to be because, compared to cases where the polymer dispersant is a random polymer, the titanium dioxide particles can be densely coated with the polymer dispersant, thereby further suppressing polishing of the IJ head by the titanium dioxide particles.

[0237] The results from Examples 3 to 6 show that when the silicon compound content relative to the total amount of titanium dioxide particles is 0.040% to 2.0% by mass (Examples 4 to 6), wear degradation of the IJ head can be further suppressed.

[0238] [Examples 101-111] <<Preparing the ink set>> We prepared ink sets by combining the white inks of each No. shown in Table 3, the following color inks (common), and the following pre-treatment solutions (common). The white inks in each No. are the same white inks used in Examples 1 to 10.

[0239] <Preparation of colored inks> A white pigment dispersion CL containing the aforementioned crosslinked polymer dispersant L1, a dispersion of the above-mentioned resin particles P1, and materials in the following composition were used to prepare a color ink (specifically, cyan ink) with the following composition. Table 3 shows the types and content of silicon compounds in the following color inks.

[0240] -Composition of color inks- • Cyan pigment (CI Pigment Blue 15:3) …3.0% by mass • Crosslinked polymer dispersant L1 …1.4% by mass • Colloidal silica as a silicon compound (Nissan Chemical Corporation's "Snowtex XS") ... 0.05% by mass as solid content • Propylene glycol (PG) (water-soluble organic solvent) …25% by mass ·Resin particles P1…5.0% by mass • Orphine E1010 (an acetylene glycol-based surfactant manufactured by Nisshin Chemical Industry Co., Ltd.) …1.0% by mass ·water ...The remaining amount totals 100% by mass.

[0241] <Preparation of pretreatment solution> A pretreatment solution was obtained by mixing the components with the following composition.

[0242] -Composition of pretreatment solution- • Malonic acid (manufactured by Fujifilm Wako Pure Chemical Industries; flocculant (organic acid)) …5% by mass • Sodium dodecylbenzenesulfonate (surfactant) (manufactured by Tokyo Chemical Industry Co., Ltd.) …1.0% by mass ·water ...The remaining amount will be 100% by mass in total.

[0243] ≪Image Record≫ Using the above ink set and a substrate (non-permeable substrate) similar to that used in Example 1, image recording was performed as follows. (1) Recording method A transport system for conveying the substrate, and a wire bar coater for applying the pretreatment solution (coating amount 1.5g / m²). 2 Image recording was performed using an inkjet recording device equipped with multiple IJ heads. Multiple IJ heads were used, each similar to the IJ head used in Example 1. The multiple IJ heads were arranged in the direction of substrate transport. In detail, first, while the substrate was being transported, the pretreatment solution was applied to the corona discharge treated surface of the substrate and dried at 80°C for 3 seconds. Next, the above-mentioned color ink (specifically, cyan ink) was ejected from the IJ head on the upstream side in the substrate transport direction onto the area on which the substrate pretreatment liquid had been applied, to obtain a cyan character image. The cyan character image here was the same as the four character images used to evaluate opacity in Example 1 (i.e., the character images shown in Figure 1 in sizes of 2pt, 4pt, 6pt, and 8pt). Next, without drying the four cyan character images, the white ink was ejected from the IJ head downstream in the substrate transport direction, applying a solid white image that completely covered the four cyan character images. By drying the white ink applied to the substrate at 80°C for 30 seconds, a solid white image that completely covered the four cyan character images was obtained. The pretreatment solution and each ink were dried by placing the substrate on a hot plate. As a result, an image recording material was obtained comprising a substrate, four cyan character images placed on the substrate, and a solid white image placed over the entire region containing the four cyan character images. (2) Recording conditions The recording conditions, including the specifications of the IJ head, were the same as those used in the image recording in Example 1.

[0244] ≪Rating≫ <Image resolution> Four character images were visually observed through the substrate from the reverse side (i.e., the side without image recording) of the image recording material, and the image resolution was evaluated based on the following evaluation criteria. The results are shown in Table 3. In the evaluation criteria below, the rank that best represents image detail is "AA".

[0245] - Criteria for evaluating image resolution - AA: I was able to see the details of the text images in 2pt, 4pt, 6pt, and 8pt font sizes. A: While details could not be seen in the 2pt text image, details could be seen in the 4pt, 6pt, and 8pt text images. Details could not be seen in the 2pt and 4pt text images, but details could be seen in the 6pt and 8pt text images. While the details of the text images in C:2pt, 4pt, and 6pt were not visible, the details of the text image in 8pt were visible. Details could not be clearly seen in the text images for D:2pt, 4pt, 6pt, and 8pt font sizes.

[0246] [Example 111] The procedure was the same as in Example 101, except that the type of silicon compound in the color ink was changed as shown in Table 3. The results are shown in Table 3.

[0247] [Table 3]

[0248] As shown in Table 3, a certain level of image resolution was obtained in all of the embodiments.

[0249] In particular, the results from Examples 102-110 show that when the silicon compound contained in the white ink is designated as the first silicon compound, and the silicon compound contained in the color ink is designated as the second silicon compound, and the content of the first silicon compound relative to the total amount of the white ink is X1 mass%, and the content of the second silicon compound relative to the total amount of the color ink is X2 mass%, then when the X1 / X2 ratio is less than 1.0 (Examples 102-104, 107-110), the image resolution is superior.

[0250] Furthermore, the results from Examples 104 and 111 show that when the first silicon compound contained in the white ink contains a silicate and the second silicon compound contained in the color ink contains colloidal silica (Example 104), the image resolution is superior.

Claims

1. It contains water, titanium dioxide particles, and silicon compounds. The average primary particle diameter of the titanium dioxide particles is 100 nm or more. The silicon compound comprises a water-soluble silicate, The water-soluble silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates. A water-based white inkjet ink in which the content of the water-soluble silicate relative to the total amount of titanium dioxide particles is 0.040% by mass to 2.0% by mass.

2. Furthermore, it contains a polymer dispersant, The aqueous white inkjet ink according to claim 1, wherein the polymer dispersant comprises a block polymer or a polymer having a crosslinked structure.

3. A white ink which is a water-based white inkjet ink according to claim 1 or claim 2, A color ink containing water and color pigments, An ink set including this.

4. Furthermore, a pretreatment liquid containing water and a coagulant, The ink set according to claim 3, including the following:

5. A white ink which is a water-based white inkjet ink, A color ink containing water and color pigments, Includes, The aforementioned water-based white inkjet ink is It contains water, titanium dioxide particles, and silicon compounds. The average primary particle diameter of the titanium dioxide particles is 100 nm or more. The silicon compound comprises a water-soluble silicate, The water-soluble silicate is at least one selected from the group consisting of alkali metal silicates and ammonium silicates. The content of the water-soluble silicate relative to the total amount of the titanium dioxide particles is 0.0020% by mass or more. When the silicon compound contained in the white ink is the first silicon compound, The aforementioned color ink further contains a second silicon compound, which is at least one selected from the group consisting of water-soluble silicates and colloidal silica. When the content of the first silicon compound relative to the total amount of the white ink is X1% by mass, and the content of the second silicon compound relative to the total amount of the color ink is X2% by mass, the X1 / X2 ratio is less than 1.

0. Ink set.

6. The ink set according to claim 5, wherein the content of the silicon compound relative to the total amount of titanium dioxide particles is 0.040% by mass to 2.0% by mass.

7. When the silicon compound contained in the white ink is the first silicon compound, The aforementioned color ink further contains a second silicon compound, which is at least one selected from the group consisting of water-soluble silicates and colloidal silica. The first silicon compound comprises a water-soluble silicate, The second silicon compound includes colloidal silica. The ink set according to claim 5.

8. Furthermore, the pretreatment solution includes water and a coagulant. The ink set according to claim 5.

9. An image recording method comprising an ink application step of applying the aqueous white inkjet ink described in claim 1 or claim 2 onto a non-permeable substrate by an inkjet method.

10. Prior to the ink application step, the process further includes a pretreatment liquid application step in which a pretreatment liquid containing water and a coagulant is applied to the non-permeable substrate. The ink application step involves applying the aqueous white inkjet ink onto the area on the non-permeable substrate to which the pretreatment liquid has been applied. The image recording method according to claim 9.

11. A color ink application step in which the color ink is ejected from the inkjet head and applied to a non-permeable substrate using the ink set described in claim 3, A white ink application step, in which the white ink is ejected from the inkjet head using the ink set and applied to the area on the non-permeable substrate to which the color ink has been applied, Image recording method, including

12. Prior to the color ink application step, the process further includes a pretreatment solution application step in which a pretreatment solution containing water and a coagulant is applied to the non-permeable substrate. The color ink application step involves applying the color ink onto the area on the non-permeable substrate to which the pretreatment liquid has been applied. The image recording method according to claim 11.

13. A color ink application step in which the color ink is ejected from the inkjet head and applied to a non-permeable substrate using the ink set described in claim 5, A white ink application step, in which the white ink is ejected from the inkjet head using the ink set and applied to the area on the non-permeable substrate to which the color ink has been applied, Image recording method, including

14. Prior to the color ink application step, the process further includes a pretreatment solution application step in which a pretreatment solution containing water and a coagulant is applied to the non-permeable substrate. The color ink application step involves applying the color ink onto the area on the non-permeable substrate to which the pretreatment liquid has been applied. The image recording method according to claim 13.

Images