Dispersion for aqueous white ink and aqueous white ink composition

Abstract

To improve the (re)dispersibility of titanium oxide pigment not only in an aqueous white ink composition containing the titanium oxide pigment, but also in a pigment dispersion liquid, which is an intermediate composition for the production of the aqueous white ink composition. [Solution] An aqueous white ink dispersion containing a titanium oxide pigment, an anionic group-containing polymer dispersant, and water: the titanium oxide pigment contains titanium oxide and a surface treatment agent that treats the surface of the titanium oxide, and satisfies the condition that the mass of the titanium oxide in the titanium oxide pigment: the total mass of the surface treatment agents = 90:10 to 80:20; in the aqueous white ink dispersion, the median diameter of the average particle diameter of the titanium oxide pigment is 280 to 400 nm; the acid value of the anionic group-containing polymer dispersant is 470 to 830 mgKOH / g; the mass average molecular weight of the anionic group-containing polymer dispersant is 1,000 or more and 10,000 or less; and the aqueous white ink dispersion contains 1 to 5 parts by mass of the anionic group-containing polymer dispersant per 100 parts by mass of titanium oxide.

Description

[Technical Field]

[0001] The present invention relates to a dispersion for an aqueous white ink containing a titanium oxide pigment, and to an aqueous white ink composition using the same. [Background technology]

[0002] White ink compositions for aqueous inkjet printing often contain titanium oxide pigment as a white pigment. Because titanium oxide pigment has a high specific gravity, ink compositions containing it can experience sedimentation or aggregation of the titanium oxide pigment. In particular, ink compositions for aqueous inkjet printing must have a low viscosity to ensure ejection stability during printing, making them prone to sedimentation or aggregation; during storage of the ink composition or during pauses in printing, sedimentation or aggregation of the titanium oxide pigment can occur in the ink flow path of the inkjet printer. Furthermore, attempts to redisperse titanium oxide pigment that has settled or aggregated in the white ink composition can be difficult.

[0003] Therefore, attempts have been made to improve redispersibility while suppressing the sedimentation or aggregation of titanium oxide pigments. For example, Patent Document 1 discloses that, in a water-based ink for inkjet printing containing a rutile titanium oxide pigment and a polymer dispersant, a predetermined amount or more of the constituent units of the polymer dispersant are derived from an anionic group-containing monomer, thereby providing a water-based ink for inkjet printing that can be redispersed by a simple operation even if the titanium oxide pigment has settled (good redispersibility upon settling).

[0004] Furthermore, Patent Document 2 describes a method for storing an aqueous inkjet white ink composition containing a surface-treated titanium oxide pigment and a pigment dispersant in a tank, in which precipitation of the titanium oxide pigment is suppressed and redispersion is facilitated even if precipitation occurs by maintaining the temperature of the white ink composition at −5 to −35° C. Furthermore, Patent Document 3 describes that an aqueous inkjet printing white ink composition containing a specific surface-treated titanium oxide pigment and an anionic group-containing resin has excellent redispersibility during precipitation and storage stability.

[0005] There have also been attempts to improve the redispersibility of titanium oxide during sedimentation not only in aqueous white ink compositions for inkjet printing but also in titanium oxide-containing pigment dispersions used to produce such compositions (Patent Document 4).Patent Document 4 uses a titanium oxide pigment with an average primary particle diameter (D) of 100 nm or more and 190 nm or less. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6881836 [Patent Document 2] Patent Publication No. 2021-178908 [Patent Document 3] Japanese Patent Application Publication No. 2025-017693 [Patent Document 4] Japanese Patent Application Publication No. 2023-121392 Summary of the Invention [Problem to be solved by the invention]

[0007] When preparing a white ink composition containing a titanium oxide pigment, a pigment dispersion, which is a pigment concentrate, may be prepared and then diluted to obtain the white ink composition. Therefore, it is important to improve the (re)dispersibility of the titanium oxide pigment not only in the resulting white ink composition but also in the pigment dispersion, which is an intermediate composition for its production. However, the (re)dispersibility of the titanium oxide pigment in the white ink composition may be different from that in the pigment dispersion; therefore, there has been a need to improve the (re)dispersibility of the titanium oxide pigment in both cases.

[0008] Therefore, an object of the present invention is to improve the (re)dispersibility of titanium oxide pigment not only in a white ink composition but also in a pigment dispersion liquid, which is an intermediate composition for the production of the white ink composition, and also to improve the ink properties such as the jettability of the white ink composition obtained from the pigment dispersion liquid, as well as the hiding power (image density) of the coating film. [Means for solving the problem]

[0009] The present invention relates to the following aqueous white ink dispersion. [1] A water-based white ink dispersion containing a titanium oxide pigment, an anionic group-containing polymer dispersant, and water: the titanium oxide pigment contains titanium oxide and a surface treatment agent that treats the surface of the titanium oxide, and the ratio of the mass of the titanium oxide to the total mass of the surface treatment agents in the titanium oxide pigment satisfies the condition of 90:10 to 80:20; the median diameter (D50) of the average particle size of the titanium oxide pigment is 280 nm to 400 nm; the anionic group-containing polymer dispersant has an acid value of 470 mg KOH / g or more and 830 mg KOH / g or less; the anionic group-containing polymer dispersant has a mass-average molecular weight of 1,000 or more and 10,000 or less; and the water-based white ink dispersion contains 1 to 5 parts by mass of the anionic group-containing polymer dispersant per 100 parts by mass of the titanium oxide pigment. [2] The aqueous white ink dispersion according to [1], wherein the titanium oxide pigment is contained in an amount of 40 to 60% by mass relative to the aqueous white ink dispersion. [3] The aqueous white ink dispersion according to [1] or [2], wherein the ratio of the particle diameter D90 to the particle diameter D50 of the titanium oxide pigment (D90 / D50) is less than 2.0.

[0010] The present invention also relates to the following aqueous white ink composition. [4] An aqueous white ink composition for inkjet printing, comprising the aqueous white ink dispersion liquid according to any one of [1] to [3] above. [Effects of the Invention]

[0011] The aqueous white ink dispersion of the present invention contains a titanium oxide pigment, but the titanium oxide pigment is not prone to settling, and even if it does settle, it can be easily redispersed. The aqueous white ink dispersion of the present invention can be diluted to form an aqueous white ink composition for inkjet printing, and the titanium oxide pigment in this aqueous white ink composition is also not prone to settling, and even if it does settle, it can be easily redispersed. DETAILED DESCRIPTION OF THE INVENTION

[0012] [A. Dispersion for Water-Based White Ink] The aqueous white ink dispersion of the present invention contains a titanium oxide pigment, a polymer dispersant, and water, and may contain any other components. The aqueous white ink dispersion can be used as an aqueous white ink composition after being diluted, for example.

[0013] [A-1. Titanium oxide pigment] The titanium oxide pigment contained in the aqueous white ink dispersion contains a titanium oxide (TiO2) core and a treatment agent (surface treatment agent) for treating the surface of the titanium oxide core. The surface treatment agent for titanium oxide may be an inorganic oxide (or inorganic hydrate) such as alumina (Al2O3, etc.), silica (SiO2, etc.), zirconia (ZrO2, etc.), zinc oxide, magnesium oxide, or an organic substance such as stearic acid; however, an inorganic oxide (or inorganic hydrate) is preferred, and one or both of alumina and silica are more preferred. The surface treatment of titanium oxide can be carried out by a wet treatment in solution (e.g., aqueous treatment) or a dry treatment in solid phase (e.g., gas-phase treatment), as known to those skilled in the art.

[0014] The ratio of the "mass of titanium oxide" to the "total mass of surface treatment agents" in the titanium oxide pigment (mass of titanium oxide:total mass of surface treatment agents) is 90:10 to 80:20, preferably 87:13 to 80:20, and more preferably 85:15 to 80:20. The titanium oxide content in the titanium oxide pigment is preferably 90% by mass or less, more preferably 87% by mass or less, and even more preferably 85% by mass or less; on the other hand, it is preferably 80% by mass or more.

[0015] Increasing the total mass of the surface treatment agents relative to the mass of titanium oxide in a titanium oxide pigment improves the dispersibility (particularly sedimentation and redispersibility) of the titanium oxide pigment in an aqueous white ink dispersion (and aqueous white ink composition). The mechanism of this improvement is not particularly limited; however, it is believed that the titanium oxide pigment is more easily adsorbed to the anionic group-containing resin described below when a surface treatment agent is present on the surface of the titanium oxide pigment than when the titanium oxide is exposed. Furthermore, treating the surface of the titanium oxide with a surface treatment agent roughens the pigment surface, increasing its surface area, which may result in the titanium oxide pigment being more easily adsorbed to the anionic group-containing resin. If the total mass of the surface treatment agents relative to the mass of titanium oxide is excessively high (i.e., if the titanium oxide content is too low), the functionality of the titanium oxide pigment as a white pigment may be reduced, and the resulting printed matter may not have a sufficiently high white image density.

[0016] The amount of surface treatment agent in a titanium oxide pigment (i.e., the ratio of the "mass of titanium oxide" to the "total mass of surface treatment agents") can be measured by methods known to those skilled in the art. For example, the amount of inorganic treatment can be analyzed by X-ray fluorescence analysis (XRF), atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectroscopy (ICP), etc. to analyze the amount of inorganic surface treatment, such as alumina, silica, or zirconia, and quantify the amount of inorganic material adsorbed or attached to the titanium oxide surface. Furthermore, the particle surface of titanium oxide pigments can also be analyzed using X-ray photoelectron spectroscopy (ESCA, Electron Spectroscopy for Chemical Analysis), etc. Preferably, the amount of surface treatment agent in a titanium oxide pigment is determined by X-ray fluorescence analysis.

[0017] The particle size distribution of the titanium oxide pigment in the aqueous white ink dispersion has a median particle size (D50) in the range of 280 nm to 400 nm. This D50 is preferably 290 nm or more and 370 nm or less. Here, the particle size of the titanium oxide pigment does not refer to the particle size of the titanium oxide pigment as a raw material blended into the aqueous white ink dispersion, but rather to the particle size of the titanium oxide pigment dispersed in the aqueous white ink dispersion after preparation. When the particle size D50 of the titanium oxide pigment in the aqueous white ink dispersion is 280 nm or more, the storage stability of the dispersion tends to be improved, while when the particle size D50 is 400 nm or less, the sedimentation and redispersibility in the dispersion tends to be improved.

[0018] Furthermore, the particle size distribution of the titanium oxide pigment in the aqueous white ink dispersion is preferably such that the particle size D90 is 650 nm or less, more preferably 600 nm or less, even more preferably 550 nm or less, and may be 500 nm or less. The smaller the particle size D90 of the titanium oxide pigment in the aqueous white ink dispersion (for example, 600 nm or less), the greater the tendency for sedimentation and redispersibility in the dispersion to increase.

[0019] Furthermore, the particle size distribution of the titanium oxide pigment in the aqueous white ink dispersion is preferably such that the ratio of particle size D90 to particle size D50 (D90 / D50) is less than 2.0, more preferably less than 1.8, and even more preferably less than 1.6. In other words, a sharp particle size distribution of the titanium oxide pigment can make it easier to achieve both storage stability and sedimentation and redispersibility of the dispersion.

[0020] The particle size D50 and particle size D90 refer to the particle sizes corresponding to 50% and 90% cumulative percentages on a volume basis. The particle size distribution of the titanium oxide pigment in the aqueous white ink dispersion can be measured by dynamic light scattering. Measurements using the dynamic light scattering method can be performed using a Nikkiso Co., Ltd. product name "9340-UPA150."

[0021] The crystalline form of titanium oxide in the titanium oxide pigment is not particularly limited and may be rutile (tetragonal), anatase (tetragonal), brookite (orthorhombic), or the like, with rutile being preferred. Rutile titanium oxide tends to improve the stability of the crystals and the hiding power of the white ink composition. Titanium oxide can be produced by a gas phase method or liquid phase method known to those skilled in the art, but production by a gas phase method is preferred in order to enhance the crystallinity of titanium oxide.

[0022] The content of titanium oxide pigment in the aqueous white ink dispersion is preferably 40 parts by mass or more per 100 parts by mass of the aqueous white ink dispersion. The higher the content of titanium oxide pigment in the aqueous white ink dispersion, the more convenient it is as a production intermediate for an aqueous white ink composition. However, if the content of titanium oxide pigment in the aqueous white ink dispersion is high, the titanium oxide pigment is likely to settle and it is difficult to redisperse. Despite the high content of titanium oxide pigment (e.g., 40% by mass or more), the aqueous white ink dispersion of the present invention inhibits settling of the titanium oxide pigment and facilitates redispersion.

[0023] The content of titanium oxide pigment in the aqueous white ink dispersion is more preferably 45 parts by mass or more; and is preferably 60 parts by mass or less, and more preferably 55 parts by mass or less.

[0024] Titanium oxide pigments to be incorporated into aqueous white ink dispersions can be commercially available. Examples of titanium oxide pigments that satisfy the condition of "titanium oxide mass:total mass of surface treatment agent = 90:10 to 80:20" or titanium oxide pigments with "titanium oxide content of 80 to 90 mass%" include: TIPAQUE CR-85 (titanium oxide content 88%), TIPAQUE CR-90 (titanium oxide content 90%), TIPAQUE CR-93 (titanium oxide content 90%), TIPAQUE PFR209 (titanium oxide content 89%) (all from Ishihara Sangyo Kaisha); TiPure TS-6300 (titanium oxide content 82%), TiPure R-960 (titanium oxide content 90%) (all from Chemours); TITONE R-7E (titanium oxide content 85%), TITONE R-62N (titanium oxide content 89%), TITONE These include R-45M (titanium oxide content 88%) (both manufactured by Sakai Chemical Industry Co., Ltd.), JR-805 (titanium oxide content 88%) (both manufactured by Teika Corporation), KRONOS K2047 (titanium oxide content 88%), KRONOS K2043 (titanium oxide content 84%) (both manufactured by KRONOS Corporation), etc. The above titanium oxides may be used alone or in combination of two or more types.

[0025] The titanium oxide pigment to be blended as a raw material component in the aqueous white ink dispersion should be selected so that the particle size D50 in the prepared aqueous white ink dispersion is 280 nm to 400 nm. As a rough guide, the titanium oxide pigment selected as a raw material component should have an average primary particle size of 200 nm to 600 nm.

[0026] [A-2. Polymer Dispersants] The polymer dispersant contained in the aqueous white ink dispersion of the present invention includes an anionic group-containing resin. The anionic group-containing resin is a resin in which at least a portion of its monomer units contain an anionic group. The anionic group refers to an acidic group such as a carboxylic acid group, a sulfonic acid group, or a phosphate group. The anionic group contained in the anionic group-containing resin promotes the adsorption of the anionic group-containing resin to the titanium oxide pigment, thereby improving the dispersibility (particularly the sedimentation and redispersibility) of the titanium oxide pigment in the white ink composition.

[0027] In the anionic group-containing resin, the acid group may form a salt, such as a sodium salt or a potassium salt. When the acid group of the anionic group-containing resin forms a salt, the solubility of the anionic group-containing resin in water increases, and the anionic group-containing resin can be dissolved as a varnish in the white ink composition.

[0028] The anionic group-containing resin has an acid value of 470 mgKOH / g or more, more preferably 560 mgKOH / g or more, and even more preferably 650 mgKOH / g or more; on the other hand, it is 830 mgKOH / g or less, and more preferably 800 mgKOH / g or less. When the anionic group-containing resin has an acid value of 470 mgKOH / g or more, the presence rate of anionic groups increases, which facilitates adsorption to the titanium oxide pigment, thereby improving the sedimentation and redispersibility of the aqueous white ink dispersion. On the other hand, when the anionic group-containing resin has an acid value of 830 mgKOH / g or less, the image density of the white ink composition is likely to increase and the ejection properties of the white ink composition are likely to be stable.

[0029] The acid value of the anionic group-containing resin can be measured in accordance with JIS K 0070.

[0030] The mass average molecular weight of the anionic group-containing resin is 1,000 or more, preferably 2,000 or more, and more preferably 3,000 or more; on the other hand, it is 10,000 or less, preferably 8,000 or less, more preferably 6,000 or less, and may be 5,000 or less. When the weight average molecular weight of the anionic group-containing resin is 1,000 or more, the sedimentation and redispersibility of the aqueous white ink dispersion is easily improved. On the other hand, when the weight average molecular weight of the anionic group-containing resin is 10,000 or less, the storage stability of the aqueous white ink dispersion is easily stabilized.

[0031] Anionic group-containing resins can function as polymer dispersants for dispersing pigments, but it has generally been thought that the higher the molecular weight of the polymer dispersant, the easier it is to disperse the pigment. Therefore, for example, many attempts have been made to achieve dispersibility of titanium oxide pigments in white ink compositions by using polymer dispersants with high weight-average molecular weights (e.g., 10,000 or more). The present invention is based on the idea of ​​maintaining the dispersibility of titanium oxide pigments in compositions with a high titanium oxide content by reducing the molecular weight of the polymer dispersant and adjusting its amount.

[0032] The weight average molecular weight of the polymer dispersant may be a weight average molecular weight converted into polystyrene, and can be measured by gel permeation chromatography.

[0033] Examples of the anionic group-containing monomer that constitutes the anionic group-containing resin include acrylic acid, methacrylic acid, maleic acid, and sulfonic acid group-containing vinyl monomers (such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid).

[0034] Furthermore, the anionic group-containing resin preferably contains a hydrophobic monomer as a monomer unit together with the anionic group-containing monomer, and may further contain a nonionic monomer or the like, as long as the effects of the present invention are not impaired.

[0035] The hydrophobic monomer constituting the anionic group-containing resin may be an alkyl(meth)acrylate, an aromatic group-containing monomer, etc. The alkyl(meth)acrylate preferably has an alkyl group having 1 to 22 carbon atoms, preferably 6 to 18 carbon atoms; examples thereof include methyl(meth)acrylate, ethyl(meth)acrylate, (iso)propyl(meth)acrylate, (iso- or tertiary)butyl(meth)acrylate, (iso)amyl(meth)acrylate, cyclohexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, (iso)octyl(meth)acrylate, (iso)decyl(meth)acrylate, (iso)dodecyl(meth)acrylate [such as lauryl(meth)acrylate], and (iso)stearyl(meth)acrylate. The aromatic group-containing monomer is preferably a vinyl monomer having an aromatic group of 6 to 22 carbon atoms, which may have a substituent containing a hetero atom; examples of such monomers include styrene-based monomers (e.g., styrene, 2-methylstyrene, and divinylbenzene), and aromatic group-containing (meth)acrylates (e.g., benzyl (meth)acrylate, phenoxyethyl (meth)acrylate).

[0036] Examples of nonionic monomers that constitute the anionic group-containing resin include polyalkylene glycol (meth)acrylate. When the anionic group-containing resin contains a nonionic monomer as a monomer unit, the sedimentation and redispersibility of the aqueous white ink dispersion can sometimes be improved.

[0037] Examples of polyalkylene glycol (meth)acrylates include polyethylene glycol mono(meth)acrylate, methoxypolyethylene glycol mono(meth)acrylate, ethoxypolyethylene glycol mono(meth)acrylate, propoxypolyethylene glycol mono(meth)acrylate, butoxypolyethylene glycol mono(meth)acrylate, octoxypolyethylene glycol mono(meth)acrylate, and stearoxypolyethylene glycol mono(meth)acrylate.

[0038] The anionic group-containing resin can be obtained by copolymerizing predetermined monomers by a known polymerization method (e.g., solution polymerization) and neutralizing the anionic groups of the resulting copolymer with a neutralizing agent. The anionic group-containing resin may be a random copolymer or a block copolymer of the predetermined monomers.

[0039] The anionic group-containing polymeric dispersant contained in the aqueous white ink dispersion of the present invention may be a commercially available anionic group-containing resin; preferred examples include Aron T-50 (mass average molecular weight 6000, acid value 780 mg KOH / g), Aron A-210 (mass average molecular weight 2000, acid value 780 mg KOH / g) (Toagosei Co., Ltd.), Aqualic DL-40S (mass average molecular weight 3500, acid value 780 mg KOH / g), and Aqualic YS-100 (mass average molecular weight 5500, acid value 780 mg KOH / g) (Nippon Shokubai Co., Ltd.).

[0040] The content of the anionic group-containing resin in the aqueous white ink dispersion of the present invention is preferably 1 part by mass or more and 5 parts by mass or less per 100 parts by mass of the titanium oxide pigment. Generally, the greater the amount of anionic group-containing resin, which serves as a polymer dispersant, the easier it is to disperse the pigment. Therefore, many attempts have been made to improve the dispersibility of titanium oxide pigments in compositions containing titanium oxide pigments by increasing the content of the anionic group-containing resin (for example, 5 parts by mass or more per 100 parts by mass of the titanium oxide pigment). The present invention is based on the idea of ​​maintaining the dispersibility of titanium oxide pigments in compositions with a high titanium oxide content by reducing the content of the polymer dispersant and adjusting its molecular weight.

[0041] [A-3. Water] The water contained in the aqueous white ink dispersion of the present invention is not particularly limited, and may be ion-exchanged water, pure water, distilled water, purified water, industrial water, etc. The water content in the aqueous white ink dispersion may be set so that the titanium oxide pigment content falls within a predetermined range. As described above, the aqueous white ink dispersion of the present invention has good dispersion stability and excellent manufacturability, even when the titanium oxide pigment content is high.

[0042] [A-4. Other ingredients] The aqueous white ink dispersion of the present invention may contain any other components depending on the purpose. Examples of other components include basic compounds and the components contained in the aqueous white ink composition described below.

[0043] The basic compound may be any compound capable of dissolving the anionic group-containing polymeric dispersant in water. In other words, the basic compound may be a component for converting the anionic group-containing polymeric dispersant into a varnish. Examples of basic compounds include inorganic basic compounds such as sodium hydroxide and potassium hydroxide; and organic basic compounds such as ammonia, methylamine, ethylamine, monoethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N,N-dibutylethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. The basic compounds may be used alone or in combination. The proportion of the basic compound in the aqueous white ink dispersion may be sufficient to dissolve the anionic group-containing polymeric dispersant in the medium. However, a proportion of 0.05% by mass or more is generally preferred to enhance the dispersion stability of the pigment by the anionic group-containing polymeric dispersant, and a proportion of 1.0% by mass or less is preferred to enhance the water resistance of printed matter.

[0044] [A-5. Preparation of Water-Based White Ink Dispersion] The method for preparing (producing) the aqueous white ink dispersion is not particularly limited, and may involve adding and mixing the above-mentioned components in order or simultaneously. Examples include: (1) a method in which an aqueous resin varnish in which an anionic group-containing polymeric dispersant is dissolved in water in the presence of a basic compound, a titanium oxide pigment, and, if necessary, other components are mixed, and then an aqueous white ink dispersion (white ink base) is prepared using a variety of dispersing machines, such as a ball mill, attritor, roll mill, sand mill, or agitator mill; and (2) a method in which the pigment is dispersed by the above-mentioned method, and then an anionic group-containing polymeric dispersant is precipitated on the surface of the titanium oxide pigment using an acid precipitation method or the ion exchange method described in WO 2005 / 116147 to obtain a resin-coated pigment, and then the obtained resin-coated pigment is neutralized with a basic compound and re-dispersed in water using a variety of dispersing machines (such as a high-speed stirrer).

[0045] The particle size distribution of the titanium oxide pigment in an aqueous white ink dispersion can be adjusted by selecting the titanium oxide pigment and other ingredients and their amounts, but it can also be adjusted by the preparation conditions of the aqueous white ink dispersion. The Examples section below discloses aqueous white ink dispersions containing the titanium oxide pigment TiPure TS6300 (average primary particle size: 530 nm) (Dispersions 1 and 4-9 (Table 3), and Dispersions D-J (Table 4)). However, the particle size distributions (D50 and D90) of the titanium oxide pigment in each aqueous white ink dispersion vary. This shows that the particle size distribution of the titanium oxide pigment in an aqueous white ink dispersion is affected by the dispersion composition and preparation conditions.

[0046] [B. Water-based White Ink Composition] The aqueous white ink composition of the present invention is a diluted product containing the pigment dispersion described above in [A. Dispersion for Aqueous White Ink]. That is, the aqueous white ink composition can be obtained by diluting the pigment dispersion with a solvent (preferably water or a water-soluble organic solvent). The aqueous white ink composition may also contain other components (such as a surfactant or a water-dispersible resin) in addition to the pigment dispersion and the solvent.

[0047] [B-1. Solvent] The solvent for diluting the pigment dispersion preferably contains water or a water-soluble organic solvent. Examples of the water-soluble organic solvent include monoalcohols, polyhydric alcohols, lower alkyl ethers of polyhydric alcohols (glycol ethers, etc.), nitrogen-containing compounds, ketones, ethers, and esters; polyhydric alcohols and glycol ethers are preferred. One solvent may be used alone, or multiple solvents may be used in combination.

[0048] Examples of polyhydric alcohols include 1,2-alkanediols such as ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-pentanediol, and 1,2-hexanediol; diethylene glycol, polyethylene glycol, dipropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, glycerin, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol, and petriol.

[0049] The glycol ether may be an alkylene glycol monoalkyl ether, an alkylene glycol dialkyl ether, or the like. The alkyl group of the alkylene glycol monoalkyl ether may be either a straight chain or a branched chain. Specific examples of the alkylene glycol monoalkyl ether include ethylene glycol ethyl ether, ethylene glycol isopropyl ether, ethylene glycol propyl ether, ethylene glycol butyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol isopropyl ether, diethylene glycol isobutyl ether, diethylene glycol butyl ether, triethylene glycol methyl ether, dipropylene glycol butyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, and the like.

[0050] From the viewpoints of preventing the aqueous white ink composition from drying inside the inkjet nozzle and facilitating the formation of an ink layer (film) on a substrate, it is more preferable that the solvent used to dilute the pigment dispersion contains at least one selected from the group consisting of propylene glycol, 1,3-butanediol, glycerin, diethylene glycol, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and 3-methoxy-3-methyl-1-butanol.

[0051] The aqueous white ink composition obtained by diluting the pigment dispersion can contain 7 to 22 parts by mass of titanium oxide pigment per 100 parts by mass of the white ink composition, preferably 10 parts by mass or more, and more preferably 15 parts by mass or more. Generally, when the concentration of titanium oxide pigment in an aqueous white ink composition is high, the storage stability tends to decrease and the titanium oxide pigment tends to settle and be difficult to redisperse; however, in the aqueous white ink composition of the present invention, the storage stability and redispersibility can be improved even when the concentration of titanium oxide pigment is high.

[0052] [B-2. Surfactants] The surfactant contained in the aqueous white ink composition can further improve the dispersibility (particularly the sedimentation and redispersibility) of the titanium oxide pigment in the aqueous white ink composition. The surfactant can be a nonionic surfactant, a silicone surfactant, or the like. The content of the surfactant in the white ink composition can be appropriately set taking into consideration dispersibility and the like, but can be in the range of 0.1 to 2.0% by mass, preferably 0.2 to 1.5% by mass.

[0053] The nonionic surfactant may be an acetylene glycol surfactant or an alcohol ethoxylate surfactant.

[0054] Acetylene glycol surfactants are compounds that contain both acetylene alcohol or acetylene bonds and glycol units, and can be represented by the following structural formula 1. The acetylene bond site is a hydrophobic site, and the alcohol site or glycol site is a hydrophilic site, which can exhibit surfactant function. [ka]

[0055] In the above structural formula 1, R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 16 carbon atoms; R 3represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a phenyl group; R represents an alkylene group having 2 to 6 carbon atoms; m represents 0 to 40; and X represents a hydrogen atom or a group represented by the following structural formula 2. [ka]

[0056] In the above structural formula 2, R 4 and R 5 each independently represents a hydrogen atom or an alkyl group having 1 to 16 carbon atoms; R 6 represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a phenyl group; R represents an alkylene group having 2 to 6 carbon atoms; and n represents 0 to 40.

[0057] Specific examples of acetylene glycol surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, 2,5-dimethyl-3-hexyne-2,5-diol, and 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, and ethylene oxide adducts thereof.

[0058] Acetylene glycol surfactants are also commercially available and are available under the trade names SURFYNOL 104E, SURFYNOL 104H, SURFYNOL 104A, SURFYNOL 104BC, SURFYNOL 104DPM, SURFYNOL 104PA, SURFYNOL 104PG-50, SURFYNOL 420, SURFYNOL 440, SURFYNOL 465, SURFYNOL 485, Olfine 1004, Olfine E1004, Olfine E1010, Olfine E1020 (Nissin Chemical Industry Co., Ltd.), and the like.

[0059] Alcohol ethoxylate surfactants are compounds in which a polyoxyethylene chain and an alkyl group are bonded by an ether bond. A-O-(CH2CH2O) d It can be represented by the molecular formula "-H" and is sometimes called poly(oxyethylene) alkyl ether. In alcohol ethoxylate surfactants, the alkyl group (R A There are no limitations on the number of carbon atoms in the alkyl group (R ) and the average number of moles of ethyleneoxy chains (d) added. A ) preferably has about 11 to 15 carbon atoms, more preferably about 12 to 13 carbon atoms; and the average number of moles of ethyleneoxy chains added (d) is preferably about 3 to 15, more preferably about 5 to 10.

[0060] Examples of alcohol ethoxylate surfactants include those sold under the trade names Emulgen 705, Emulgen 104, Emulgen 106, Emulgen 707, Emulgen 1108, Emulgen 1118S-70, Emulgen 1135S-70, and Emulgen 1150S-60 (Kao Corporation); those sold under the trade names Adeka Toll LA-675B and Adeka Toll LA-775 (ADEKA); those sold under the trade name TRITON (registered trademark) HW1000 (Dow); and those sold under the trade name Sannonic SS70 (Sanyo Chemical Industries, Ltd.).

[0061] Silicone surfactants are dimethylpolysiloxanes (chemical formula: (C2H6OSi) n The polysiloxanes and their modified products are preferably polysiloxanes modified with polyether (polyether-modified silicone surfactants). Polyether-modified silicone surfactants can prevent excessive increases in ink viscosity.

[0062] Polyether-modified silicone surfactants have a structure in which the hydrocarbon groups on the side chains and / or terminals of silicone oil are substituted with polyether groups. Suitable polyether groups include polyethyleneoxy groups, polypropyleneoxy groups, and polyalkyleneoxy groups in which ethyleneoxy groups (EO) and propyleneoxy groups (PO) are added in a block or random manner. Compounds in which polyether groups are graft-bonded to a silicone main chain, compounds in which silicone and polyether groups are bonded in a block manner, etc. can be used.

[0063] Silicone surfactants can also be obtained from the market. Specific examples include KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF640, KF-642, KF643, KF-644, KF6020, KF6011, and the like (Shin-Etsu Chemical Co., Ltd. KF series); Silface SAG (Nissin Chemical Industry Co., Ltd.); BYK-345, 347, 348, 349, and the like (BYK series from BYK-Chemie); TEGOGLIDE 410, 450, and the like, and TEGO Twin 400, and the like (Evonik). These surfactants may be used alone or in combination of two or more types.

[0064] [B-3. Water-dispersible resin] The water-dispersible resin contained in the aqueous white ink composition is typically a resin emulsion, and may be a nonionic resin emulsion and / or anionic resin emulsion. The water-dispersible resin (resin emulsion) may be an acrylic resin (such as a styrene-acrylic resin), a urethane resin, a fluorene resin, a polyolefin resin, a rosin-modified resin, a terpene resin, a polyester resin, a polyamide resin, an epoxy resin, a vinyl chloride resin, a vinyl chloride-vinyl acetate copolymer, or an ethylene-vinyl acetate resin. The water-dispersible resin may be any of various resin emulsions, including polyurethane resin emulsions such as polyester-based polyurethane resin emulsions, polyether-based polyurethane resin emulsions, and polycarbonate-based polyurethane resin emulsions; acrylic resin emulsions such as styrene-acrylic resin emulsions; wax emulsions such as paraffin wax and polyethylene wax; and polyester resin emulsions. The water-dispersible resin particles may be used alone or in combination of two or more.

[0065] The water-dispersible resin may be an acrylic resin (such as a styrene-acrylic resin emulsion). A styrene-acrylic resin emulsion is an emulsion in which a styrene-acrylic resin is dispersed in water, and can be produced by emulsion polymerization, dispersion polymerization, suspension polymerization, grinding, or solution / bulk polymerization, followed by post-emulsification. Detailed information on this method and stabilizers is found in "Emulsion Polymerization and Emulsion Polymer" (PA Lovell, MS El-Aasser, John Wiley & Sons Ltd., England, 1977, incorporated herein by reference).

[0066] Commercially available acrylic resins (including styrene-acrylic resin emulsions) include JONCRYL 450, JONCRYL 734, JONCRYL 7600, JONCRYL 352, JONCRYL 390, JONCRYL 7100, JONCRYL 741, JONCRYL 74J, JONCRYL 511, JONCRYL 840, JONCRYL 775, JONCRYL HRC-1645, JONCRYL HPD-71, JONCRYL PDX-6102B, JONCRYL PDX-7741, and JONCRYL JDX-5050 (styrene-acrylic resin emulsions, BASF); UC-3900 (styrene-acrylic resin emulsion, Toagosei); and AP4710 (acrylic-silicone resin emulsion, Showa Polymer).

[0067] The water-dispersible resin may be a urethane-based resin. The urethane-based resin may be any of a polyether type containing an ether bond, a polyester type containing an ester bond, a polycarbonate type containing a carbonate bond, etc. The urethane resin may have a crosslinkable group.

[0068] Examples of crosslinkable groups possessed by urethane-based resins include isocyanate groups, silanol groups, carboxyl groups, and hydroxyl groups. Substituents in which the isocyanate group is chemically protected (capped or blocked) (blocked isocyanate groups) are preferred. The blocked isocyanate group is deprotected and activated by the application of heat, forming a crosslinked bond (e.g., a urethane bond, a urea bond, an allophanate bond, etc.). Furthermore, it is preferred that the urethane-based resin having crosslinkable groups has three or more crosslinkable groups per molecule, and in such cases, a crosslinked structure is formed by the reaction of the crosslinkable groups.

[0069] Examples of commercially available polyurethane resins (polyurethane resin emulsions) include Superflex 210, Superflex 130, Superflex 300, Superflex 500M, Superflex 460, Superflex 740, and Superflex 420NS (Dai-ichi Kogyo Seiyaku Co., Ltd.); Impranil DLP1380, Impranil DLP-R, Baybond PU407, Bayhydrol UH650, and Bayhydrol UH2606 (Sumika Covestro Urethanes Co., Ltd.); NeoRez R-966 and NeoRez R-967 (Covestro); Takelac W-6110 and Takelac WS-5000 (Mitsui Chemicals, Inc.); and DAOTANTW6491 and DAOTANTW6495 (Daicel-Allnex).

[0070] When the aqueous white ink composition of the present invention is used in inkjet textile printing (described below), it may be preferable that the water-dispersible resin is a urethane resin from the viewpoint of reducing stickiness of the coating film. In particular, a polycarbonate-type or polyether-type urethane resin having a crosslinkable group is preferable from the viewpoint of improving the rubbing fastness of the image of the printed textile.

[0071] The water-dispersible resin may be a polyolefin resin (polyolefin resin emulsion). Examples of polyolefin resins include polyethylene resin, polypropylene resin, polybutylene resin, and polyolefin resins obtained by copolymerizing two or more of ethylene, propylene, and butylene. The polyolefin resin may also be a modified polyolefin resin in which an amino group, a carboxyl group, a hydroxyl group, an acryloyl group, or other polymer chain is introduced into the polyolefin chain; an oxidized polyolefin resin in which a portion of the polyolefin chain is oxidized; or a halogenated polyolefin resin in which a portion of the polyolefin chain is treated with a halogen. The polyolefin resin emulsion may be used alone or in combination of two or more.

[0072] Examples of commercially available polyolefin resin emulsions include Chemipearl S100 (Mitsui Chemicals, Inc., polyethylene resin emulsion), Chemipearl XEP800H (Mitsui Chemicals, Inc., polypropylene resin emulsion), and Arrowbase TC-4010 (Unitika Ltd., polypropylene resin emulsion).

[0073] From the viewpoint of improving the drying properties of the coating film of the printed matter and the adhesion to the substrate, it is sometimes preferable that the water-dispersible resin has a glass transition temperature of 20° C. or lower. The glass transition temperature is determined by differential scanning calorimetry (DSC) and is usually calculated as the midpoint of the temperature range in which the glass transition occurs.

[0074] The content of the water-dispersible resin (resin emulsion) in the aqueous white ink composition, calculated as solid content, is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, relative to the total mass (100% by mass) of the ink; and is preferably 8% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less. In the aqueous white ink composition of the present invention, the content of the water-dispersible resin (resin emulsion) can contribute to the (re)dispersibility of the titanium oxide pigment, but if the content is too high, the ejection properties and storage stability may be reduced.

[0075] [B-4. Other ingredients] Various additives such as wetting agents, penetrating agents, dispersants, viscosity modifiers, antifoaming agents, antifungal agents, anticorrosive agents, and ultraviolet absorbers may be added to the aqueous white ink composition as needed.

[0076] [B-5. Preparation of Water-Based White Ink Composition] The procedure for producing the aqueous white ink composition is not particularly limited, but the composition can be obtained by mixing a dispersion containing a titanium oxide pigment (aqueous white ink dispersion of the present invention) with a solvent and, if necessary, various additives, and stirring the mixture.

[0077] [B-6. Physical Properties of Water-Based White Ink Composition] The aqueous white ink composition of the present invention can be used in inkjet printing. Therefore, it is preferable that the viscosity be set to a level that allows for inkjet printing and enhances dispersibility (particularly sedimentation and redispersibility). The viscosity of the aqueous white ink composition at 25°C is preferably 3.0 mPa·s or more, more preferably 4.5 mPa·s or more, and even more preferably 5.0 mPa·s or more; and is preferably 12.0 mPa·s or less, more preferably 9.0 mPa·s or less, even more preferably 7.5 mPa·s or less, and even more preferably 6.5 mPa·s or less. The viscosity of the white ink composition can be measured using an E-type viscometer.

[0078] The pH of the white ink composition is not particularly limited, but from the viewpoint of enhancing dispersibility, it is preferably 5.5 or higher, more preferably 6.0 or higher, and even more preferably 6.5 or higher; on the other hand, from the viewpoints of component resistance and skin irritation, it is preferably 11.0 or lower, more preferably 10.0 or lower, even more preferably 9.5 or lower, and even more preferably 9.0 or lower.

[0079] [C. Printing with Water-Based White Ink Composition] [C-1. Inkjet printing] The aqueous white ink composition of the present invention can be used for printing by an inkjet system. The aqueous white ink composition of the present invention has good storage stability and good ejection stability, and therefore, when inkjet printed, it is possible to form a high-quality white image with a high white density. Furthermore, because the aqueous white ink composition of the present invention has high sedimentation and redispersibility, even if the titanium oxide pigment settles in the inkjet printing device (for example, in an ink tank or ink flow path), it can be redispersed by a simple means, making it suitable for inkjet printing.

[0080] There are no particular limitations on the inkjet recording method for the aqueous white ink composition of the present invention. Examples of inkjet recording methods 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 electric signal into an acoustic beam and irradiates the ink with it, thereby ejecting the ink using radiation pressure, and a thermal inkjet method that heats the ink to form bubbles and uses the generated pressure. Inkjet recording methods also include a method that ejects a large number of low-concentration inks, known as photo inks, in a small volume, a method that uses multiple inks of substantially the same hue but different densities to improve image quality, and a method that uses a colorless, transparent ink. The white ink composition of the present invention is particularly suitable as an inkjet recording ink for a drop-on-demand method (pressure pulse method) using a piezoelectric element.

[0081] The aqueous white ink composition of the present invention may be used for underprinting (also referred to as "whiteout printing") on a substrate. "Whiteout printing" refers to printing a white ink composition in advance on a portion of a substrate where an ink composition of a desired color will be printed (or printing it solidly over the entire substrate); this masks the underlying substrate and prevents the color of the printed portion from mixing with the desired color. Whiteout printing is often performed particularly when a "light-colored ink composition" is printed on a substrate. The white ink composition of the present invention can form a printed layer with a high white image density, and therefore can more reliably mask the underlying substrate when used in whiteout printing.

[0082] The white ink composition of the present invention can be printed on any substrate. Examples of substrates include paperboard, resin films, corrugated cardboard, and fabrics. When the substrate is fabric, printing can be performed by, for example, the inkjet printing method described below.

[0083] [C-2. Inkjet printing] The aqueous white ink composition of the present invention can also be used in an inkjet textile printing method. The inkjet textile printing method is a method of printing an image on a fabric using a pretreatment liquid, a white ink composition, and a chromatic ink composition. That is, the inkjet textile printing method may include, in this order: 1) step A of preparing a fabric that has been pretreated with a pretreatment liquid; 2) step B of inkjet coating the pretreated fabric with a white ink composition; and 3) step C of inkjet coating chromatic ink compositions onto an ink layer formed by applying the white ink composition. The aqueous white ink composition of the present invention can be used as the white ink composition in step B. [Example]

[0084] The present invention will be described in more detail below with reference to examples, but the scope of the present invention should not be construed as being limited by the contents of the examples.

[0085] [1. White ink dispersion] White ink dispersions 1 to 10 and white ink dispersions A to J shown in Tables 3 and 4 were prepared.

[0086] [1-1. Materials used in preparing white ink dispersion] A white ink dispersion was prepared using the titanium oxide pigment (Table 1) and anionic group-containing polymer dispersant (water-based acrylic resin varnish) (Table 2) shown below. [Table 1] [Table 2]

[0087] Water-based acrylic resin varnishes 1 to 10 were each synthesized as follows. 20 parts by mass of anionic group-containing polymer dispersant 1 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 4000, acid value 750 mgKOH / g) was dissolved in a mixture of 15.0 parts by mass of potassium hydroxide and 65.0 parts by mass of water to obtain aqueous acrylic resin varnish 1 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 2 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 4000, acid value 500 mgKOH / g) was dissolved in a mixture of 10.0 parts by mass of potassium hydroxide and 70.0 parts by mass of water to obtain an aqueous acrylic resin varnish 2 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 3 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 4000, acid value 800 mgKOH / g) was dissolved in a mixture of 16.0 parts by mass of potassium hydroxide and 64.0 parts by mass of water to obtain aqueous acrylic resin varnish 3 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 4 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 2000, acid value 750 mgKOH / g) was dissolved in a mixture of 15.0 parts by mass of potassium hydroxide and 65.0 parts by mass of water to obtain aqueous acrylic resin varnish 4 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 5 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 8000, acid value 750 mgKOH / g) was dissolved in a mixture of 15.0 parts by mass of potassium hydroxide and 65.0 parts by mass of water to obtain aqueous acrylic resin varnish 5 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 6 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 4000, acid value 450 mgKOH / g) was dissolved in a mixture of 9.0 parts by mass of potassium hydroxide and 71.0 parts by mass of water to obtain aqueous acrylic resin varnish 6 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 7 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 4000, acid value 900 mgKOH / g) was dissolved in a mixture of 18.0 parts by mass of potassium hydroxide and 62.0 parts by mass of water to obtain aqueous acrylic resin varnish 7 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 8 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 500, acid value 750 mgKOH / g) was dissolved in a mixture of 15.0 parts by mass of potassium hydroxide and 65.0 parts by mass of water to obtain an aqueous acrylic resin varnish 8 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 9 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 12,000, acid value 750 mgKOH / g) was dissolved in a mixture of 15.0 parts by mass of potassium hydroxide and 65.0 parts by mass of water to obtain an aqueous acrylic resin varnish 9 with a resin solids content of 20% by mass. 20 parts by mass of anionic group-containing polymer dispersant 10 (acrylic acid / maleic acid / styrene copolymer: weight average molecular weight 15,000, acid value 650 mgKOH / g) was dissolved in a mixture of 13.0 parts by mass of potassium hydroxide and 67.0 parts by mass of water to obtain an aqueous acrylic resin varnish 10 with a resin solids content of 20% by mass.

[0088] [1-2. Preparation of White Ink Dispersion] Each white ink dispersion was prepared according to the formulation (the numerical values ​​in the formulation are in parts by mass) shown in Tables 3 and 4. Specifically, the components were mixed by stirring and then milled in a wet circulation mill to obtain a white ink dispersion.

[0089] [1-3. Measurement and Evaluation of White Ink Dispersion] For the resulting dispersions 1 to 10 (Table 3) and dispersions A to J (Table 4), the particle diameters D50 and D90 of the titanium oxide pigment in the dispersions were measured, and sedimentation redispersibility and storage stability were evaluated.

[0090] [1-3A. Particle size D50 and D90 of titanium dioxide pigment in white ink dispersion] The particle size distribution of the titanium oxide pigment in the obtained dispersions 1 to 10 (Table 3) and dispersions A to J (Table 4) was measured by dynamic light scattering using a product named "9340-UPA150" manufactured by Nikkiso Co., Ltd. D50 and D90 refer to the particle sizes corresponding to cumulative proportions of 50% and 90% on a volume basis.

[0091] [1-3B. Sedimentation and redispersibility of dispersion liquid] 100 g of the obtained dispersions 1 to 10 (Table 3) and dispersions A to J (Table 4) were filled into 100 mL sample bottles and allowed to stand for 4 weeks in an environment of 60°C. After that, the sample bottles were shaken up and down to redisperse the sediment (until no sediment was observed), the number of shakes required was counted, and the sediment redispersibility was evaluated according to the following criteria. ◎ Redispersed less than 50 times ○ Redispersed 50 to 100 times △ Redispersed between 100 and 200 times × Redispersion was not possible within 200 attempts

[0092] [1-3C. Storage Stability of Dispersion] In the sedimentation and re-dispersibility test of 1-3B, the viscosity (initial viscosity) of the white ink dispersion when filled into the sample bottle and the viscosity of the white ink dispersion after re-dispersing the sediment were measured, and the storage stability was evaluated according to the following criteria. ◎ The change rate from the initial viscosity is less than 5% ○ The rate of change from the initial viscosity is 5% or more but less than 10% △ The rate of change from the initial viscosity is 10% or more but less than 20% × The change rate from the initial viscosity is 20% or more

[0093] [Table 3]

[0094] [Table 4]

[0095] As shown in Table 3, dispersions 1 to 10, which contain titanium oxide pigment, aqueous acrylic resin varnishes 1 to 5, and water, and in which the titanium oxide content of the titanium oxide pigment contained therein is 80 to 90 mass % and the particle size D50 is in the range of 280 to 400 nm, all received sufficiently good evaluations for both sedimentation redispersibility and storage stability.

[0096] In contrast, Dispersion A in Table 4 contains a titanium oxide pigment (CR-80) with a high titanium oxide content, i.e., a small amount of surface treatment agent. It can be seen that the evaluation of sedimentation and redispersibility of Dispersion A was poor.

[0097] In Dispersions B and C in Table 4, the particle diameter D50 of the titanium oxide pigment in the dispersion was 256 nm and 428 nm, respectively, which is outside the range of 280 to 400 nm. In both cases, the evaluation of sedimentation and redispersibility tended to deteriorate.

[0098] In Dispersions D and E in Table 4, the content of the polymer dispersant (contained in the aqueous acrylic resin varnish) was 0.8% by mass and 7.0% by mass relative to the titanium oxide pigment, respectively, which are outside the range of 1 to 5% by mass. As a result, the sedimentation and redispersibility of Dispersion D deteriorated, and the storage stability of Dispersion E deteriorated.

[0099] Dispersions F and G in Table 4 contained polymer dispersants with acid values ​​of 450 mgKOH / g and 900 mgKOH / g, respectively, which are outside the range of 470 to 830 mgKOH / g. Dispersion F showed poor sedimentation and redispersibility, while dispersion G showed a tendency to have poor storage stability.

[0100] In Dispersions H and I in Table 4, the polymer dispersants contained therein have mass-average molecular weights of 500 and 12,000, respectively, which are outside the range of 1,000 to 10,000. Dispersion H exhibited poor sedimentation and redispersibility, and Dispersion I tended to have poorer storage stability. Dispersion J in Table 4 exhibited a polymer dispersant containing a mass-average molecular weight of 15,000, with a content of 10% relative to the titanium dioxide pigment. Dispersion J also tended to have poorer storage stability.

[0101] All of Dispersions 1 to 10 shown in Table 3 showed good results in terms of sedimentation redispersibility and storage stability, but the smaller the particle size D50 of the titanium oxide pigment, the better the sedimentation redispersibility tended to be. For example, comparing Dispersion 1 (D50: 313 nm) with Dispersion 3 (D50: 369 nm), Dispersion 1 had superior sedimentation redispersibility. On the other hand, the larger the particle size D50 of the titanium oxide pigment, the better the storage stability tended to be. For example, comparing Dispersion 1 (D50: 313 nm) with Dispersion 7 (D50: 291 nm), Dispersion 1 had superior storage stability.

[0102] In addition, in Dispersion 10 in Table 3, the ratio of the particle size D90 to the particle size D50 of the titanium oxide pigment in the dispersion exceeds 2.0 (2.02 = 699 / 346).The sedimentation and redispersibility of Dispersion 10 tended to be worse than that of Dispersion 1.

[0103] [2. Water-based white ink] Using the white ink dispersions shown in Tables 3 and 4, the aqueous white ink compositions shown in Tables 6 and 7 were prepared.

[0104] [2-1. Raw materials used in preparing the water-based white ink composition] To prepare the aqueous white ink compositions, the white ink dispersions (Dispersions 1 to 10 and Dispersions A to J) shown in Tables 3 and 4 were used, along with the following raw materials.

[0105] [Table 5]

[0106] Water-soluble organic solvents: propylene glycol, glycerin, 1,3-butanediol Surfactant: Olfine E1010 (Nissin Chemical) (Polyoxyethylene (10) Acetylenic Glycol Ether)

[0107] [2-2. Preparation of Water-Based White Ink Composition] Water-based white ink compositions with the formulations shown in Tables 6 and 7 (the numerical values ​​in the formulations are in parts by mass) were prepared using the white ink dispersions, water-dispersible resins, water-soluble organic solvents, and surfactants shown in Tables 3 and 4. Specifically, each water-based white ink composition was obtained by mixing the materials and stirring them with a disperser.

[0108] [2-3. Evaluation of Water-Based White Ink Composition] The aqueous white ink composition was evaluated for sedimentation and redispersibility, storage stability, ejection stability in an inkjet printing device, and image density and abrasion resistance of the printed film.

[0109] [2-3A. Sedimentation and redispersibility of aqueous white ink composition] 100 g of the aqueous white ink compositions of the Examples and Comparative Examples were filled into 100 mL sample bottles and allowed to stand in an environment of 60°C for 4 weeks. The sample bottle was then shaken up and down to redisperse any sediment (until no sediment was observable), and the number of shakes required was counted. The sediment redispersibility was evaluated according to the following criteria. ◎ Redispersed less than 50 times ○ Redispersed 50 to 100 times △ Redispersed between 100 and 200 times × Redispersion was not possible within 200 attempts

[0110] [2-3B. Storage stability of aqueous white ink composition] In the sedimentation and re-dispersibility test of 2-3A, the viscosity (initial viscosity) of the aqueous white ink composition when filled into the sample bottle and the viscosity of the aqueous white ink composition after re-dispersing the sediment were measured, and the storage stability was evaluated according to the following criteria. ◎ The change rate from the initial viscosity is less than 5% ○ The rate of change from the initial viscosity is 5% or more but less than 10% △ The rate of change from the initial viscosity is 10% or more but less than 20% × The change rate from the initial viscosity is 20% or more

[0111] [2-3C. Discharge Stability of Water-Based White Ink Composition] The aqueous white ink compositions of the Examples and Comparative Examples were subjected to continuous printing using an inkjet printer (an evaluation printer equipped with a printhead manufactured by SPECTRA), and the resulting white images were checked for missing nozzles, and the inkjet dischargeability was evaluated according to the following criteria. ◎ No ejection problems were observed in any of the nozzles during continuous printing for over an hour. ○ No ejection problems are observed in any nozzles during actual printing operations lasting 15 minutes or more but less than 1 hour. △: During continuous printing for 15 minutes or more but less than 1 hour, there are nozzles that become unable to eject ink. × All nozzles become unable to eject ink during continuous printing for 15 minutes or more but less than 1 hour.

[0112] [2-3D. Evaluation of Image Density (Concealment Rate) Using Water-Based White Ink Composition] The aqueous white ink compositions of the Examples and Comparative Examples were filled into cartridges for an Epson PX105 printer, and the ink was printed onto a PET film (E5100, 12 μm, manufactured by Toyobo) and dried in a constant temperature dryer at 70° C. for 3 minutes. Thereafter, tristimulus values ​​were measured using a spectrophotometer eXact (manufactured by xRite), and the hiding ratio was calculated and evaluated according to the following criteria. Concealment rate (%) = (tristimulus value of the solid white area on the shielding film) / (tristimulus value of the film itself) x 100 The "tristimulus values ​​of the solid white area on the shielding film" were determined by measuring the tristimulus values ​​at five locations and calculating the average tristimulus value. ◎ Concealment rate of 60% or more ○ Concealment rate is between 55% and 60% △ Concealment rate is between 50% and 55% × Concealment rate is less than 50%

[0113] [2-3E. Scratch Resistance of Image Coating Film Using Water-Based White Ink Composition] A cotton swab (product name: Cotton Stick, manufactured by Asai Shoji Co., Ltd.) was attached to a pencil hardness tester (product name: Wolf-Wilburn, manufactured by BYK-Gardner) and the printed surface of the print obtained in 2-3D was rubbed with a fixed pressure of 750 g.The printed surface was then visually inspected and evaluated according to the following criteria. ◎ The print did not fade even after rubbing the surface with a cotton swab 10 times. After rubbing the printed surface with a cotton swab 10 times, the color transferred to the cotton swab and the rubbed area faded slightly (less than 50%). After rubbing the printed surface with a cotton swab 10 times, the color transferred to the cotton swab and the majority of the rubbed area (more than 50%) faded. × After rubbing the printed surface with a cotton swab 10 times, the rubbed area completely peeled off.

[0114] [Table 6]

[0115] [Table 7]

[0116] As shown in Table 6, the aqueous white ink compositions 1 to 14 containing the dispersions 1 to 10 (see Table 3) all exhibited excellent sedimentation and redispersibility and storage stability, and also had practical jetting properties. Furthermore, the printing films produced by these compositions exhibited excellent hiding power and abrasion resistance.

[0117] The aqueous white ink composition A shown in Table 7 had a high titanium oxide content in the titanium oxide pigment contained therein, and the sedimentation and redispersibility was poor.

[0118] Water-based white ink compositions B and C shown in Table 7 contain pigment dispersions B and C, respectively, and the titanium oxide pigments contained therein have D50s of 256 nm and 428 nm, which are outside the range of 280 to 400 nm. Water-based white ink composition B showed a reduced image hiding rate, and water-based white ink composition C showed poor ejection properties and tended to show poor sedimentation and re-dispersion properties.

[0119] In water-based white ink compositions D and E shown in Table 7, the ratio of polymer dispersant to titanium oxide pigment was 0.8% and 7.0%, respectively, which are outside the range of 1 to 5%. Water-based white ink composition D exhibited poor sedimentation and redispersibility, and water-based white ink composition E exhibited slightly reduced storage stability.

[0120] The acid values ​​of the polymer dispersants contained in water-based white ink compositions F and G shown in Table 7 were 450 and 900 mgKOH / g, respectively, which are outside the range of 470 to 830 mgKOH / g. Water-based white ink composition F exhibited slightly worse sedimentation and redispersibility, and water-based white ink composition G exhibited slightly reduced storage stability.

[0121] The polymer dispersants contained in aqueous white ink compositions H and I shown in Table 7 have mass average molecular weights of 500 and 12,000, respectively, which are outside the range of 1,000 to 10,000. Furthermore, aqueous white ink composition H exhibited slightly deteriorated sedimentation and redispersibility, while aqueous white ink composition I exhibited deteriorated ejection properties and slightly reduced storage stability.

[0122] In aqueous white ink compositions J and K shown in Table 7, the polymer dispersant contained therein has a mass average molecular weight of 15,000, outside the range of 1,000 to 10,000. Aqueous white ink composition J contains 10% titanium oxide pigment, while aqueous white ink composition K contains 20% titanium oxide pigment. Water-based white ink composition J was sufficiently good in each evaluation, while water-based white ink composition K exhibited slightly reduced storage stability.

[0123] All of the aqueous white ink compositions 1 to 9 and 12 to 13 shown in Table 6 were sufficiently good in various evaluations, despite having a titanium oxide pigment content of 20% or more; therefore, it is clear that the aqueous white ink composition of the present invention can achieve well-balanced improvements in sedimentation and redispersibility, storage stability, and jetting properties, even when the pigment content is high.

[0124] Water-based white ink composition 11 shown in Table 6 has a titanium oxide pigment content of 15% by mass, and similarly to the case of a 20% by mass content, exhibits well-balanced improvements in sedimentation redispersibility, storage stability, and jetting properties. On the other hand, water-based white ink composition 10 has a titanium oxide pigment content of 7% by mass, and exhibits well-balanced improvements in sedimentation redispersibility, storage stability, and jetting properties, but the hiding power of the coating film is slightly reduced.

[0125] The aqueous white ink composition 14 shown in Table 6 contains the pigment dispersion 10, and the ratio of particle diameter D90 to particle diameter D50 of the titanium oxide pigment contained therein exceeds 2.0. The dischargeability of the aqueous white ink composition 14 is slightly reduced. [Industrial Applicability]

[0126] The pigment dispersion for aqueous white ink of the present invention and the aqueous white ink composition obtained using the same exhibit excellent (re)dispersibility of the titanium oxide pigment contained therein and excellent storage stability. Therefore, the pigment dispersion for aqueous white ink is highly convenient as an intermediate composition for producing an aqueous white ink composition; for example, the pigment dispersion for aqueous white ink of the present invention can be stored as a work-in-process product and then used to produce multiple types of aqueous white ink compositions.

Claims

1. A dispersion for an aqueous white ink containing a titanium oxide pigment, an anionic group-containing polymer dispersant, and water, the titanium oxide pigment contains titanium oxide and a surface treatment agent that treats the surface of the titanium oxide, and satisfies the condition that the mass of the titanium oxide: the total mass of the surface treatment agent in the titanium oxide pigment = 90:10 to 80:20; In the aqueous white ink dispersion, the titanium oxide pigment has an average particle size median diameter (D50) of 280 nm to 400 nm, the acid value of the anionic group-containing polymer dispersant is 470 mgKOH / g or more and 830 mgKOH / g or less; the mass average molecular weight of the anionic group-containing polymer dispersant is 1,000 or more and 10,000 or less; The aqueous white ink dispersion contains the anionic group-containing polymer dispersant in an amount of 1 to 5 parts by mass per 100 parts by mass of the titanium oxide pigment.

2. 2. The aqueous white ink dispersion according to claim 1, wherein the titanium oxide pigment is contained in an amount of 40 to 60% by mass relative to the aqueous white ink dispersion.

3. 3. The aqueous white ink dispersion according to claim 1, wherein the ratio of the particle diameter D90 to the particle diameter D50 of the titanium oxide pigment (D90 / D50) is less than 2.

0.

4. An aqueous white ink composition for inkjet printing, comprising the aqueous white ink dispersion according to claim 1 or 2.

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