Water-based white ink composition

Abstract

To provide an aqueous white ink for inkjet printing containing a titanium oxide pigment and a polymeric pigment dispersant, which ink exhibits improved suppression of pigment sedimentation and improved redispersibility even when the titanium oxide pigment content is high; and more preferably, to improve coating film properties (hiding rate, image density, or physical properties of the coating film). [Solution] The aqueous white ink composition for inkjet printing contains a titanium oxide pigment, an anionic group-containing polymer dispersant, a water-dispersible resin, a surfactant, 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 following condition: mass of titanium oxide:total mass of surface treatment agent = 90:10 to 80:20; the titanium oxide pigment has an average particle size D50 of 280 to 400 nm; the aqueous white ink composition contains more than 10 mass% and not more than 30 mass% of the titanium oxide pigment; the anionic group-containing polymer dispersant has an acid value of 470 to 830 mgKOH / g; the anionic group-containing polymer dispersant has a molecular weight of 1,000 to 10,000; and the anionic group-containing polymer dispersant is contained in an amount of 1 to 5 mass parts per 100 mass parts of the titanium oxide pigment.

Description

[Technical Field]

[0001] The present invention relates to an aqueous white ink composition, and more specifically to an aqueous white ink composition for inkjet printing that contains a titanium oxide pigment. [Background technology]

[0002] Aqueous white ink compositions 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, aqueous ink compositions for 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] Several solutions to these problems have been proposed. Patent Document 1 discloses a water-based ink for inkjet printing that contains a rutile titanium oxide pigment and a polymer dispersant; by using a predetermined amount or more of the constituent units of the polymer dispersant derived from an anionic group-containing monomer, the ink can be easily redispersed even if the titanium oxide pigment settles (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 the temperature of the white ink composition is maintained at -5 to -35°C, thereby suppressing precipitation of the titanium oxide pigment and facilitating re-dispersion even if precipitation does occur.

[0005] Furthermore, Patent Document 3 describes that an aqueous white ink composition for inkjet printing, which contains a titanium oxide pigment that has been subjected to a specific surface treatment and an anionic group-containing resin, has excellent redispersibility during sedimentation and storage stability. [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 Summary of the Invention [Problem to be solved by the invention]

[0007] As described in Patent Documents 1 to 3, techniques have been proposed for inhibiting sedimentation of titanium oxide pigment and improving redispersibility in aqueous white ink compositions for inkjet printing, which contain a titanium oxide pigment and a polymeric pigment dispersant. However, there is still a need for improvements in inhibiting sedimentation of titanium oxide pigment and improving redispersibility in such aqueous white ink compositions; in particular, there is a need for techniques for inhibiting sedimentation of titanium oxide pigment and improving redispersibility, even in aqueous white ink compositions with a high content of titanium oxide pigment.

[0008] Therefore, an object of the present invention is to provide an aqueous white ink composition for inkjet printing that contains a titanium oxide pigment and a polymeric pigment dispersant, and that has improved sedimentation and re-dispersibility of the titanium oxide pigment, even when the titanium oxide pigment content is high.An object of the present invention is to provide an aqueous white ink composition that preferably has improved storage stability of the ink composition and coating film physical properties (hiding rate, image density, or physical properties of the coating film). [Means for solving the problem]

[0009] That is, the present invention relates to the following aqueous white ink composition. [1] An aqueous white ink composition for inkjet printing, comprising: a titanium oxide pigment; an anionic group-containing polymer dispersant; a water-dispersible resin; a surfactant; and water; wherein the titanium oxide pigment comprises 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 is 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 titanium oxide pigment is contained in an amount of more than 10% by mass and 30% by mass or less relative to the aqueous white ink composition; the anionic group-containing polymer dispersant has an acid value of 470 mgKOH / g or more and 830 mgKOH / g or less; the molecular weight of the anionic group-containing polymer dispersant is 1,000 or more and 10,000 or less; and the anionic group-containing polymer dispersant is contained in an amount of 1 to 5 parts by mass per 100 parts by mass of the titanium oxide pigment.

[0010] Furthermore, the present invention relates to the following aqueous white ink composition as a preferred embodiment. [2] The water-based white ink composition according to [1], wherein the water-dispersible resin is contained in an amount of 5 mass % or less relative to the water-based white ink composition. [3] The water-based white ink composition according to [1] or [2], wherein the water-dispersible resin is an emulsion of an acrylic resin or a urethane resin. [4] The water-based white ink composition according to any one of [1] to [3], wherein the titanium oxide pigment is contained in an amount of less than 25 mass % relative to the water-based white ink composition. [5] The water-based white ink composition according to any one of [1] to [4], wherein the titanium oxide pigment is contained in an amount of 15 mass % or more relative to the water-based white ink composition. [Effects of the Invention]

[0011] The aqueous white ink composition of the present invention is suitable for inkjet printing because it inhibits sedimentation of the titanium oxide pigment and improves its redispersibility. Furthermore, the aqueous white ink composition of the present invention can increase the concentration of the titanium oxide pigment, making it possible to produce a coating film (printed matter) with high image density. Furthermore, the aqueous white ink composition of the present invention can also increase the physical strength (e.g., abrasion resistance) of the coating film (printed matter). DETAILED DESCRIPTION OF THE INVENTION

[0012] [1. Water-based white ink composition for inkjet printing] The aqueous white ink composition for inkjet printing of the present invention (hereinafter also simply referred to as "aqueous white ink composition") contains 1) a titanium oxide pigment, 2) an anionic group-containing polymer dispersant, 3) a water-dispersible resin, 4) a surfactant, and 5) a solvent containing water, and may further contain 6) other optional components.

[0013] [1-1. Titanium oxide pigment] The titanium oxide pigment contained in the aqueous white ink composition comprises 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 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 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.

[0018] The particle size distribution of the titanium oxide pigment in the aqueous white ink composition 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 composition, but rather to the particle size of the titanium oxide pigment dispersed in the aqueous white ink composition after preparation. When the particle size D50 of the titanium oxide pigment in the aqueous white ink composition is 280 nm or more, the storage stability of the ink composition tends to be improved, while when the particle size D50 is 400 nm or less, the sedimentation and redispersibility in the aqueous white ink composition tends to be improved.

[0019] The particle size of the titanium oxide pigment in the aqueous white ink composition is the particle size of the titanium oxide pigment dispersed in the aqueous white ink composition after preparation; however, when the aqueous white ink composition is prepared by diluting a pigment dispersion liquid (containing at least the titanium oxide pigment, an anionic group-containing polymer dispersant, and water) which is a pigment concentrate, the particle size of the titanium oxide pigment in the pigment dispersion liquid may be regarded as the particle size of the titanium oxide pigment in the aqueous white ink composition.

[0020] Furthermore, the particle size distribution of the titanium oxide pigment in the aqueous white ink composition 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 composition (for example, 600 nm or less), the greater the tendency for sedimentation and redispersibility in the aqueous white ink composition to be improved.

[0021] Furthermore, with regard to the particle size distribution of the titanium oxide pigment in the aqueous white ink composition, the ratio of particle size D90 to particle size D50 (D90 / D50) is preferably less than 2.0, more preferably less than 1.8, and even more preferably less than 1.6. In other words, if the particle size distribution of the titanium oxide pigment is sharp, it may be easier to achieve both storage stability and sedimentation and redispersibility of the aqueous white ink composition.

[0022] 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."

[0023] The content of the titanium oxide pigment in the aqueous white ink composition is more than 10 parts by mass, and preferably 15 parts by mass or more, per 100 parts by mass of the ink composition. 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 become difficult to redisperse. However, the aqueous white ink composition of the present invention can improve the storage stability and redispersibility even when the concentration of titanium oxide pigment is high. Furthermore, the content of titanium oxide pigment in the aqueous white ink composition is 30 parts by mass or less, and preferably 25 parts by mass or less, per 100 parts by mass of the ink composition. If the content of titanium oxide pigment is too high, the ejection properties in inkjet printing tend to decrease.

[0024] The titanium oxide pigment to be incorporated into the aqueous white ink composition 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 having a "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 composition should be selected so that the particle size D50 of the aqueous white ink composition after preparation 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] [1-2. Anionic group-containing polymer dispersants] The anionic group-containing polymer dispersant contained in the aqueous white ink composition 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 aqueous 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 is increased, and the anionic group-containing resin can be dissolved as a varnish in the aqueous 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 composition. 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 aqueous white ink composition is likely to be increased and the ejection properties of the aqueous 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 aqueous 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, as its monomer units, a hydrophobic monomer 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 then 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 composition 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 composition 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, even in compositions containing titanium oxide pigment, many attempts have been made to improve the dispersibility of the titanium oxide pigment 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). On the other hand, too much anionic group-containing resin tends to deteriorate the storage stability of the aqueous white ink composition. The present invention is based on the idea of ​​maintaining the dispersibility of the titanium oxide pigment in a composition with a high titanium oxide content by reducing the content of the polymer dispersant and adjusting its molecular weight.

[0041] [1-3. 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.

[0042] 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).

[0043] 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).

[0044] 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.

[0045] 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.

[0046] Commercially available examples of 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 DAOTAN TW6491 and DAOTAN TW6495 (Daicel-Allnex).

[0047] 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.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] The content of the water-dispersible resin (resin emulsion) in the aqueous white ink composition, calculated as solids, is preferably less than 8% by mass, more preferably 5% by mass or less, and even more preferably 4% by mass or less, relative to the total mass (100% by mass) of the ink composition; on the other hand, it is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. 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; however, if the content is too high (for example, if it exceeds 5% by mass of the ink composition), the ejection properties and storage stability may be reduced.

[0052] [1-4. 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 3 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; 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 SURFYNO L 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] [1-5. Water-containing solvents] The solvent contained in the aqueous white ink composition of the present invention preferably contains at least water and further contains a water-soluble organic solvent. The water contained in the aqueous white ink composition 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.

[0065] 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, esters, etc. Polyhydric alcohols, glycol ethers, etc. are preferred. One solvent may be used alone, or multiple solvents may be used in combination.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] [1-6. Other ingredients] Various additives such as basic compounds, 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.

[0070] 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 of two or more. 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, the proportion of the basic compound is preferably 0.05% by mass or more to enhance the dispersion stability of the pigment by the anionic group-containing polymeric dispersant, and preferably 1.0% by mass or less to enhance the water resistance of printed matter.

[0071] [1-7. Physical properties of aqueous 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 also 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.

[0072] The pH of the aqueous 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.

[0073] 2. Preparation of aqueous white ink composition The procedure for producing the aqueous white ink composition is not particularly limited; however, it can be preferably obtained by adding a solvent and, if necessary, various additives to a pigment dispersion containing a titanium oxide pigment and a polymer dispersant, mixing the mixture, and stirring the mixture.

[0074] The pigment dispersion is a pigment concentrate containing a high concentration of titanium oxide pigment, and contains titanium oxide pigment, a polymeric dispersant, water, and may contain other optional components, each of which is as described above as a component of the aqueous white ink composition.

[0075] The content of titanium oxide pigment in the pigment dispersion is preferably 40 parts by mass or more per 100 parts by mass of the aqueous white ink dispersion. A higher content of titanium oxide pigment in the pigment dispersion is more convenient as a production intermediate for the aqueous white ink composition. On the other hand, if the content of titanium oxide pigment in the pigment dispersion is too high, the titanium oxide pigment in the pigment dispersion is likely to settle and become difficult to redisperse.

[0076] 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).

[0077] [3. Printing with aqueous white ink composition] [3-1. 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.

[0078] 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 radiation pressure to eject the ink, 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 multiple 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 aqueous 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.

[0079] 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 aqueous 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.

[0080] The aqueous 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.

[0081] [3-2. 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]

[0082] 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.

[0083] [A. Preparation of Pigment Dispersion] In preparing the aqueous white ink compositions, first, pigment dispersions, which are pigment concentrates, were prepared (Pigment Dispersions 1 to 10 and Pigment Dispersions A to J). The materials used in preparing the pigment dispersions are shown below.

[0084] [Table 1] [Table 2]

[0085] Water-based acrylic resin varnishes 1 to 10 were each synthesized as follows. 20 parts by mass of anionic group-containing resin 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 resin 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 resin 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 resin 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 resin 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 resin 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 resin 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 resin 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 resin 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 resin 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.

[0086] Each pigment dispersion (Dispersions 1 to 10 and Dispersions A to J) was prepared according to the formulation (the numerical values ​​in the formulation are in parts by mass) shown in Tables 3 and 4. Specifically, each component was stirred and mixed, and then milled in a wet circulation mill to obtain a pigment dispersion.

[0087] [A-1. Evaluation of pigment dispersion] The resulting pigment dispersions (Dispersions 1 to 10 (Table 3) and Dispersions A to J (Table 4)) were evaluated for pigment sedimentation and redispersibility, and dispersion storage stability.

[0088] [A-1a. 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 left to stand in an environment of 60°C for 4 weeks. After that, the sample bottles were shaken up and down to redisperse the sediment (until no sediment was observed), and the number of shakes required was measured. 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

[0089] [A-1b. Storage stability of dispersion] In the above-mentioned A-1a sedimentation re-dispersibility test, 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

[0090] [Table 3]

[0091] [Table 4]

[0092] Generally, dispersions 1 to 10 in Table 3 showed a well-balanced improvement in sedimentation redispersibility and storage stability, whereas dispersions A to J in Table 4 showed a decrease in either sedimentation redispersibility or storage stability.

[0093] For example, 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. This shows that the titanium oxide content of the titanium oxide pigment is related not only to the sedimentation and redispersibility and storage stability of the aqueous white ink composition, but also to the sedimentation and redispersibility and storage stability of the pigment dispersion.

[0094] Dispersions B and C in Table 4 had poor sedimentation and redispersibility. Water-based white ink compositions B and C, described below, contain dispersions B and C, respectively; the particle diameters D50 of the pigments in these compositions are 256 nm and 428 nm, respectively, which are outside the range of 280 to 400 nm. Therefore, it is presumed that the particle diameters (D50) of the pigments in dispersions B and C were either too large or too small, resulting in poor sedimentation and redispersibility, but the reason for this is not particularly limited.

[0095] 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, 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. This shows that the content of the polymer dispersant relative to the titanium oxide pigment is related not only to the sedimentation and redispersibility and storage stability of the aqueous white ink composition, but also to the sedimentation and redispersibility and storage stability of the pigment dispersion.

[0096] In Dispersions F and G in Table 4, the acid values ​​of the polymer dispersants contained therein were 450 mgKOH / g and 900 mgKOH / g, respectively, which are outside the range of 470 to 830 mgKOH / g. There was also a tendency for the sedimentation and redispersibility of Dispersion F to deteriorate, and for the storage stability of Dispersion G to deteriorate. This shows that the acid value of the polymer dispersant is related not only to the sedimentation and redispersibility and storage stability of the aqueous white ink composition, but also to the sedimentation and redispersibility and storage stability of the pigment dispersion.

[0097] In Dispersions H and I in Table 4, the weight-average molecular weights of the polymer dispersants contained therein were 500 and 12,000, respectively, which are outside the range of 1,000 to 10,000. Thus, a tendency was observed for the sedimentation and re-dispersibility of Dispersion H to deteriorate, and the storage stability of Dispersion I to deteriorate. Furthermore, in Dispersion J in Table 4, the weight-average molecular weight of the polymer dispersant contained therein was 15,000, and the content thereof was 10%. Thus, a tendency was observed for the storage stability of Dispersion J to deteriorate. Thus, it can be seen that the molecular weight of the polymer dispersant is related not only to the sedimentation and re-dispersibility and storage stability of the aqueous white ink composition, but also to the sedimentation and re-dispersibility and storage stability of the pigment dispersion.

[0098] B. Preparation of Water-Based White Ink Composition The aqueous white ink compositions (aqueous white ink compositions 1 to 15 and A to N) shown in Tables 6 and 7 (the numerical values ​​in the formulations are in parts by mass) were prepared using the pigment dispersions shown in Tables 3 and 4. The following materials were used in addition to the white ink dispersions to prepare the aqueous white ink compositions. [Table 5]

[0099] Water-soluble organic solvents: propylene glycol, glycerin, 1,3-butanediol Surfactant: Olfine E1010 (Nissin Chemical) (Polyoxyethylene (10) Acetylenic Glycol Ether)

[0100] Using the white ink dispersions, solvents, water-dispersible resins, and surfactants shown in Tables 3 and 4, aqueous white ink compositions were prepared with the formulations shown in Tables 6 and 7. Specifically, each material was mixed and stirred with a disperser to obtain each aqueous white ink composition.

[0101] [B-1. Evaluation of Water-Based White Ink Composition] The aqueous white ink composition was evaluated for pigment particle size distribution (D50), sedimentation and redispersibility, storage stability, ejection stability in an inkjet printing device, and image density and abrasion resistance of the printed film.

[0102] [B-1a. Particle size D50 and D90 of titanium oxide pigment in aqueous white ink composition] The particle size distribution of the titanium oxide pigment in the aqueous white ink compositions of the Examples and Comparative Examples 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.

[0103] [B-1b. 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 bottles were 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

[0104] [B-1c. Storage stability of aqueous white ink composition] In the sedimentation and re-dispersibility test of B-1b above, 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

[0105] [B-1d. Discharge stability of aqueous 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.

[0106] [B-1e. Evaluation of Image Density (Concealment Rate) Using Aqueous 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 film" were calculated 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%

[0107] [B-1f. 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 B-1e 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.

[0108] [Table 6]

[0109] [Table 7]

[0110] As shown in Table 6, all of the aqueous white ink compositions 1 to 15 containing dispersions 1 to 10 (see Table 3) 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.

[0111] 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.

[0112] The titanium oxide pigment D50 of aqueous white ink compositions B and C shown in Table 7 was 256 nm and 428 nm, respectively, which are outside the range of 280 to 400 nm. The image hiding rate of aqueous white ink composition B decreased, and aqueous white ink composition C showed a tendency for its ejection properties to deteriorate as well as its sedimentation and redispersibility to deteriorate.

[0113] 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% by mass, respectively, which is outside the range of 1 to 5% by mass. Water-based white ink composition D exhibited poor sedimentation and redispersibility, and water-based white ink composition E exhibited slightly reduced storage stability.

[0114] 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.

[0115] 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.

[0116] In aqueous white ink compositions J and K shown in Table 7, the polymer dispersant contained therein has a molecular weight of 15,000, outside the range of 1,000 to 10,000; aqueous white ink composition J has a titanium oxide pigment content of 10 mass%, while aqueous white ink composition K has a titanium oxide pigment content of 20 mass%. While aqueous white ink composition J was sufficiently good in various evaluations, aqueous white ink composition K had slightly reduced storage stability and worsened dischargeability. The coating hiding power of aqueous white ink composition J tended to be slightly lower than in the examples.

[0117] The titanium oxide pigment contents of aqueous white ink compositions L and M shown in Table 7 were 5.0% by mass and 32.0% by mass, respectively, relative to the aqueous white ink composition, outside the range of more than 10% by mass and 30% by mass or less. Water-based white ink composition L tended to exhibit reduced hiding power, while water-based white ink composition M tended to exhibit poorer ejection properties and reduced sedimentation and redispersibility.

[0118] The water-based white ink composition N shown in Table 7 did not contain a water-dispersible resin, and the scratch resistance of the coating film deteriorated.

[0119] All of the aqueous white ink compositions 1 to 9 and 12 to 15 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.

[0120] 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.

[0121] Water-based white ink composition 14 shown in Table 6 contains pigment dispersion 10, and the ratio of particle size D90 to particle size D50 of the titanium oxide pigment contained therein exceeds 2.0. Water-based white ink composition 14 exhibited slightly reduced jetting properties. [Industrial Applicability]

[0122] The aqueous white ink composition of the present invention exhibits excellent (re)dispersibility and storage stability, even when the titanium oxide pigment content is increased, making it possible to form a coating film with high image density. Furthermore, the coating film also has high abrasion resistance, making it a particularly effective aqueous white ink composition when it is desired to form a white coating film with high hiding power.

Claims

1. A water-based white ink composition for inkjet printing, comprising a titanium oxide pigment, an anionic group-containing polymer dispersant, a water-dispersible resin, a surfactant, 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; the titanium oxide pigment has an average particle size median diameter (D50) of 280 nm to 400 nm; the titanium oxide pigment is contained in an amount of more than 10% by mass and 30% by mass or less relative to the water-based white ink composition, the acid value of the anionic group-containing polymer dispersant is 470 mgKOH / g or more and 830 mgKOH / g or less; the molecular weight of the anionic group-containing polymer dispersant is 1,000 or more and 10,000 or less; The aqueous white ink composition 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. The water-based white ink composition according to claim 1 , wherein the water-dispersible resin is contained in an amount of 5% by mass or less relative to the water-based white ink composition.

3. The water-based white ink composition according to claim 1 or 2, wherein the water-dispersible resin is an emulsion of an acrylic resin or a urethane resin.

4. The water-based white ink composition according to claim 1 or 2, wherein the titanium oxide pigment is contained in an amount of less than 25 mass % relative to the water-based white ink composition.

5. The water-based white ink composition according to claim 1 or 2, wherein the titanium oxide pigment is contained in an amount of 15 mass % or more relative to the water-based white ink composition.

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