JP6828672B2 - White ink for inkjet recording - Google Patents

Description

The present invention relates to a white ink for inkjet recording.

As the pigment particles contained in the white ink for inkjet recording, titanium dioxide particles have been conventionally used for the purpose of improving the whiteness and hiding power of an image (for example, Patent Document 1).

Re-table 2013-021633

However, when titanium dioxide particles are used, the storage stability of the white ink may decrease due to the large specific gravity of titanium dioxide, and the metal ions of titanium dioxide elute and precipitate as a metal near the inkjet nozzle. As a result, the ejection stability of the ink droplets may decrease.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a white ink for inkjet recording, which is excellent in storage stability and ejection stability.

The white ink for inkjet recording according to the present invention includes an aqueous medium, resin particles, white pigment particles, and a dispersant. The density of the white pigment particles is 3.0 g / cm ³ or less. The dispersant contains an anionic resin and a water-insoluble non-polar resin. The white pigment particles are dispersed as a white pigment particle dispersion. In the white pigment particle dispersion, the surface of the white pigment particles is coated with the water-insoluble non-polar resin, and further coated with the anionic resin. The acid value of the anionic resin is 50 mgKOH / g or more. The mass average molecular weight of the anionic resin is 10,000 or more.

The white ink for inkjet recording according to the present invention is excellent in storage stability and ejection stability.

An embodiment of the present invention will be described. In the following, the evaluation results (values indicating the shape or physical properties, etc.) of the aggregate of particles (for example, a plurality of particles in the powder or a plurality of particles in the dispersion liquid) are equivalent unless otherwise specified. It is the number average of the values measured for a number of particles.

In addition, the compound name may be followed by "system" to comprehensively refer to the compound and its derivative. When the polymer name is represented by adding "system" after the compound name, it means that the repeating unit of the polymer is derived from the compound or its derivative. In addition, acrylic and methacrylic may be collectively referred to as "(meth) acrylic".

The white ink for inkjet recording (hereinafter, may be referred to as white ink) according to the present embodiment is excellent in storage stability and ejection stability. The reason is presumed as follows. Since the density of the white pigment particles is 3.0 g / cm ³ or less, it is considered that the white pigment particles are hard to settle in the white ink, have excellent dispersibility, and have excellent storage stability. The dispersant contains an anionic resin and a water-insoluble non-polar resin, the acid value of the anionic resin is 50 mgKOH / g or more, and the mass average molecular weight of the anionic resin is 10,000 or more. Therefore, the white pigment particles are dispersed as a white pigment particle dispersion. The white pigment particle dispersion has a structure in which the surface of the white pigment particles is coated with a water-insoluble non-polar resin, and the coating layer of the water-insoluble non-polar resin is coated with an anionic resin (hereinafter, referred to as a coating structure). May be). Since the white pigment particles have a coating structure, they are considered to be less likely to aggregate in the white ink, have excellent dispersibility, and have excellent storage stability. Further, since the white pigment particles have a coating structure, the surface of the white pigment particles is not easily exposed to the aqueous medium. Therefore, it is difficult for the constituent components of the white pigment particles to elute as metal cations, and it is difficult for the constituent components to elute in the vicinity of the nozzle of the inkjet head. Therefore, it is considered that the discharge stability is excellent. From the above, the white ink according to the present embodiment is excellent in storage stability and ejection stability.

The white ink according to this embodiment is recorded on a recording medium. The white ink according to the present embodiment may be used for the purpose of recording a white image on a recording medium, may be used for the purpose of erasing the color of a non-white recording medium, and lower the transparency of a colored image. It may be used for the purpose. In any case, it is preferable that the white ink according to the present embodiment is ejected from the recording head of the inkjet recording device toward the recording medium. As the recording medium, for example, plain paper, copy paper, recycled paper, thin paper, thick paper, glossy paper, or OHP sheet can be used. The recording medium may be made of plastic, metal, or glass. The recording medium may be one processed using fibers (for example, cloth).

The white ink only needs to be able to record a color called "white" on a recording medium according to the conventional wisdom. More specifically, in the present embodiment, the lightness (L ^* ) of the ink measured by the following measuring method satisfies the following formula (1), and the chromaticity (a ^* ,) of the ink measured by the following measuring method is satisfied. When b ^* ) satisfies the following formulas (2) and (3), the ink is regarded as white ink.
70 ≤ L ^* ≤ 100 ... Equation (1)
-3.5 ≤ a ^* ≤ 1.0 ... Equation (2)
-5.0 ≤ b ^* ≤ 1.5 ... Equation (3)

<Measurement method of brightness (L ^* ) and chromaticity (a ^* , b ^* )>
A solid image of 100% duty is recorded on the surface of a recording medium (“Epson genuine photo paper <gloss>” manufactured by Seiko Epson Corporation). Using a spectrophotometer (“Spectrolino” manufactured by X-Rite), the brightness (L ^* ) and chromaticity (a ^* , b ^* ) of the ink recorded on the recording medium are measured under the measurement conditions shown below.

(Measurement condition)
Light source: D50
Observation field of view: 2 °
Concentration measurement conditions: DIN NB filter White Base: absolute
Filter: No
Measurement mode: Reflectance

(Duty)
The duty (the amount of ink applied) is expressed by the following formula. In the following formula, the "actual recording dot number" means the actual recording dot number per unit area. When the duty is 100%, it means that the amount of the monochromatic ink applied to the pixels is maximum.
duty (unit:%) = {(number of actual recording dots) / (image resolution)} x 100

[Structure of white ink]
The white ink according to the present embodiment contains an aqueous medium, resin particles, white pigment particles, and a dispersant. The plurality of resin particles and the plurality of white pigment particles are dispersed in an aqueous medium. The white ink may further contain additives as needed.

(Aqueous medium)
The aqueous medium preferably contains water. The water is preferably pure water or ultrapure water, and more preferably sterilized pure water or ultrapure water. By using sterilized pure water or ultrapure water, it is possible to prevent the growth of mold and bacteria in the white ink for a long period of time. The pure water is preferably at least one selected from the group consisting of, for example, ion-exchanged water, ultrafiltration water, reverse osmosis water, and distilled water. The sterilization treatment is preferably at least one selected from the group consisting of, for example, ultraviolet irradiation and hydrogen peroxide addition.

(Resin particles)
The resin particles function as a binder resin on the recording medium. Specifically, the resin particles hold the white pigment particles on a recording medium. Examples of the resin containing the resin particles include styrene-acrylic acid-based resin, acrylic acid-based resin, urethane resin, and polyester resin. Among these resins, a resin having an acidic functional group (more specifically, a styrene-acrylic acid-based resin or the like) is preferable from the viewpoint of improving the dispersibility of the resin particles in the white ink. The styrene-acrylic acid-based resin contains a repeating unit derived from a styrene-based monomer and a repeating unit derived from an acrylic acid-based monomer.

Examples of the styrene-based monomer include styrene, alkylstyrene, hydroxystyrene, and halogenated styrene. Examples of the alkyl styrene include α-methyl styrene, m-methyl styrene, p-methyl styrene, p-ethyl styrene, and 4-tert-butyl styrene. Examples of hydroxystyrene include p-hydroxystyrene or m-hydroxystyrene. Examples of the halogenated styrene include α-chlorostyrene, o-chlorostyrene, m-chlorostyrene, and p-chlorostyrene. A prepolymer of such a styrene-based monomer can also be used. For example, such a styrene-based monomer dimer can also be used.

Examples of the acrylic acid-based monomer include (meth) acrylic acid, (meth) acrylamide, (meth) acrylonitrile, (meth) acrylic acid alkyl ester, and (meth) acrylic acid hydroxyalkyl ester. Examples of the (meth) acrylic acid alkyl ester include methyl (meth) acrylic acid, ethyl (meth) acrylic acid, n-propyl (meth) acrylic acid, iso-propyl (meth) acrylic acid, and n (meth) acrylic acid. -Butyl, iso-butyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, tridecyl (meth) acrylate, palmityl (meth) acrylate , (Meta) oleyl acrylate, stearyl (meth) acrylate, cyclohexyl (meth) acrylate, isobornyl (meth) acrylate, or benzyl (meth) acrylate. Examples of the (meth) acrylic acid hydroxyalkyl ester include (meth) acrylic acid 2-hydroxyethyl, (meth) acrylic acid 3-hydroxypropyl, (meth) acrylic acid 2-hydroxypropyl, and (meth) acrylic acid 4 -Hydroxybutyl can be mentioned. A prepolymer of such an acrylic acid-based monomer can also be used. For example, a dimer of such an acrylic acid-based monomer can also be used.

The content of the resin particles in the white ink is not particularly limited. However, if the content of the resin particles is too small, an image having a desired image density may not be obtained. Therefore, it may not be possible to record a white image having a desired image density on a recording medium. In addition, it may not be possible to effectively erase the color of a recording medium that is not white. In addition, it may not be possible to effectively reduce the transparency of a colored image. If the content of the resin particles is too large, the permeability of the white ink to the recording medium may not be ensured. Further, if the content of the resin particles is too large, the fluidity of the resin particles may not be ensured in the white ink. This may also make it impossible to obtain an image having a desired image density. For example, the content of the resin particles in the white ink is preferably 4% by mass or more and 8% by mass or less.

(White pigment particles)
The white pigment particles are dispersed in the white ink as a white pigment particle dispersion. The density of the white pigment particles is 3.0 g / cm ³ or less, and preferably 2.7 g / cm ³ or less. When the density of the white pigment particles is 3.0 g / cm ³ or less, the white pigment particles are less likely to settle in the white ink and have excellent dispersibility, so that the white ink is excellent in storage stability.

The white pigment particles are preferably at least one selected from the group consisting of calcium carbonate, talc, mica and kaolin.

The volume average particle size of the white pigment particle dispersion is preferably 250 nm or more and 800 nm or less. When the volume average particle diameter of the white pigment particle dispersion is 250 nm or more, the white pigment particles easily block visible light in the image, so that an image having a desired hiding property can be easily formed. Further, when the volume average particle diameter of the white pigment particle dispersion is 800 nm or less, the white pigment particle dispersion is unlikely to settle in the white ink, so that the deterioration of the storage stability of the white ink can be suppressed. Further, when the volume average particle diameter of the white pigment particle dispersion is 250 nm or more and 800 nm or less, the white pigment particle dispersion is almost the same as the wavelength region of visible light, so that the white pigment particles coated with the resin in the image or Visible light is likely to be scattered in the white pigment particles, and it is easy to form an image having desired whiteness and concealment.

The volume average particle diameter indicates the harmonic mean particle diameter (diameter) based on the scattered light intensity standard. The volume average particle size is determined using a dynamic light scattering type particle size distribution measuring device (“Zetasizer (registered trademark) Nano ZS” manufactured by Sysmex Co., Ltd.). The volume average particle size is determined by the method described in ISO 13321: 1996 (Particle size analogy-Photon colony spectroscopy) or a method similar thereto. Specifically, first, the white pigment particle dispersion is diluted 1000 times with ion-exchanged water. A diluted white pigment particle dispersion is used as a measurement sample. Then, the dynamic light scattering method is used to detect a temporal change in the scattered light intensity from the measurement sample. Next, the autocorrelation function of the scattered light intensity is obtained by using the temporal change of the scattered light intensity. Subsequently, the autocorrelation function of the scattered light intensity is analyzed by the cumulant method. In this way, the volume average particle diameter of the white pigment particle dispersion can be obtained.

The polydisperse index of the white pigment particle dispersion is preferably 0.30 or more and 0.35 or less. The multi-dispersion index indicates a dimensionless index indicating the spread of the particle size distribution. The smaller the polydispersity index, the smaller the spread of the particle size distribution. Further, the larger the polydisperse index, the larger the spread of the particle size distribution. For example, the smaller the polydisperse index of the white pigment particle dispersion, the sharper the particle size distribution of the white pigment particle dispersion. The larger the polydisperse index of the white pigment particle dispersion, the broader the particle size distribution of the white pigment particle dispersion. The polydisperse index of the white pigment particle dispersion can be obtained by the same method as the volume average particle diameter.

(Dispersant)
The dispersant forms a white pigment particle dispersion together with the white pigment particles in the white ink to disperse the white pigment particles. The dispersant includes an anionic resin and a water-insoluble non-polar resin. The dispersant disperses the white pigment particles in the white ink by forming the white pigment particles and the white pigment particle dispersion. The coating structure is a structure in which the surface of the white pigment particles is coated with a water-insoluble non-polar resin and further coated with an anionic resin. The coating structure can be formed by a phase inversion emulsification method described later.

(Anionic resin)
The anionic resin is a resin having an anionic functional group. Examples of the anionic functional group include a metal salt of an acidic functional group (more specifically, a carboxyl group, a sulfo group, etc.) (more specifically, an alkali metal salt, an alkaline earth metal salt, etc.) or ammonium. Salt is mentioned. Examples of the anionic resin include a neutralizing resin (more specifically, a neutralizing acrylic acid resin and the like). Examples of the neutralizing acrylic acid-based resin include ammonium ion-neutralizing acrylic acid-based resin and alkali metal ion-neutralizing acrylic acid-based resin (more specifically, sodium ion-neutralizing acrylic acid-based resin or Potassium ion neutralized acrylic acid-based resin, etc.). In the ammonium ion-neutralized acrylic acid-based resin, the repeating unit derived from the acrylic acid-based monomer (more specifically, the repeating unit derived from acrylic acid or the repeating unit derived from the derivative of acrylic acid) has an ammonium salt of a carboxyl group. .. In the neutralized resin, a part of the plurality of acidic functional groups may be a salt.

The acrylic acid-based resin is a polymer of acrylic acid-based monomers. The acrylic acid-based resin may contain a repeating unit derived from another vinyl-based monomer in addition to the repeating unit derived from the acrylic acid-based monomer. Examples of the acrylic acid-based monomer include (meth) acrylic acid. A prepolymer of such an acrylic acid-based monomer can also be used. For example, a dimer of such an acrylic acid-based monomer can also be used.

The acid value of the anionic resin is preferably 50 mgKOH / g or more, and preferably 70 mgKOH / g or more and 250 mgKOH / g or less. When the acid value of the anionic resin is 50 mgKOH / g or more, it is easy to form a coating structure of white pigment particles together with the water-insoluble non-polar resin, and the white pigment particles are satisfactorily dispersed in the white ink.

The acid value of the anionic resin is measured by a method conforming to JIS (Japanese Industrial Standards) K0070-1992. The acid value of the anionic resin can be adjusted by the amount of acidic functional groups. For example, when the anionic resin has a repeating unit derived from a monomer having an acidic functional group, the acid value can be increased by increasing the proportion of the repeating unit derived from the monomer having an acidic functional group in the anionic resin. Can be done.

The mass average molecular weight Mw of the anionic resin is 10,000 or more, preferably 10,000 or more and 16,000 or less. When the mass average molecular weight Mw of the anionic resin is 10,000 or more, it is easy to form a coating structure of white pigment particles together with the water-insoluble non-polar resin, and the white pigment particles are well dispersed in the white ink.

The mass average molecular weight Mw of the anionic resin is measured by the following method. Using gel filtration chromatography (“HLC-8020GPC” manufactured by Tosoh Corporation), the mass average molecular weight Mw of the anionic resin is determined under the following conditions.

(Measurement condition)
Column: "TSKgel SuperMultipore HZ-H" manufactured by Tosoh Corporation (4.6 mm ID x 15 cm semi-micro column)
Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 mL / min Sample injection volume: 10 μL
Measurement temperature: 40 ° C
Detector: IR detector

The calibration curves are from TSKgel standard polystyrene manufactured by Tosoh Corporation, 7 types of F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000 and n-. Made by selecting propylbenzene.

The anionic resin may disperse the resin particles.

(Water-insoluble non-polar resin)
The water-insoluble non-polar resin has a total content of repeating units containing polar functional groups and hydrophilic repeating units of 10% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less. Refers to the resin group that is. Examples of the polar functional group include a hydroxyl group, a carboxyl group, a sulfonic acid group, and an amino group. Examples of the repeating unit containing a polar functional group include repeating units derived from polar monomers such as allylamine, allyl alcohol, (meth) acrylic acid, 2-hydroxyethyl acrylate, and 2- (dimethylamino) ethyl methacrylate. Be done. Examples of the hydrophilic repeating unit include a repeating unit derived from a hydrophilic monomer such as ethylene oxide, propylene oxide, vinylpyrrolidone, N, and N-dimethylacrylamide. For example, water-insoluble non-polar resins include ethylene, propylene, styrene, 4-methylstyrene, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, ethylhexyl (meth) acrylate, and butadiene. , N-Phenylmaleimide and the like, which is a polymer of a monomer containing a non-polar compound. Both the water-insoluble non-polar resin and the anionic resin have the property of forming a coating structure of white pigment particles. The water-insoluble non-polar resin has higher hydrophobicity than the anionic resin. The water-insoluble non-polar resin has a higher affinity for white pigment particles and anionic resin in white ink than in an aqueous medium. Examples of the water-insoluble non-polar resin include styrene-acrylic acid-based resins. The styrene-acrylic acid-based resin contains a repeating unit derived from a styrene-based monomer and a repeating unit derived from an acrylic acid-based monomer.

Examples of the styrene-based monomer include styrene, alkylstyrene, hydroxystyrene, and halogenated styrene. Examples of the alkyl styrene include α-methyl styrene, m-methyl styrene, p-methyl styrene, p-ethyl styrene, and 4-tert-butyl styrene. Examples of hydroxystyrene include p-hydroxystyrene or m-hydroxystyrene. Examples of the halogenated styrene include α-chlorostyrene, o-chlorostyrene, m-chlorostyrene, and p-chlorostyrene. A prepolymer of such a styrene-based monomer can also be used. For example, such a styrene-based monomer dimer can also be used.

Examples of the acrylic acid-based monomer include (meth) acrylic acid, (meth) acrylamide, (meth) acrylonitrile, (meth) acrylic acid ester, and (meth) acrylic acid hydroxyalkyl ester. Examples of the (meth) acrylic acid ester include methyl (meth) acrylic acid, ethyl (meth) acrylic acid, n-propyl (meth) acrylic acid, iso-propyl (meth) acrylic acid, and n-propyl (meth) acrylic acid. Butyl, iso-butyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, tridecyl (meth) acrylate, palmityl (meth) acrylate, Examples thereof include oleyl (meth) acrylate, stearyl (meth) acrylate, cyclohexyl (meth) acrylate, isobornyl (meth) acrylate, or benzyl (meth) acrylate. Examples of the (meth) acrylic acid hydroxyalkyl ester include (meth) acrylic acid 2-hydroxyethyl, (meth) acrylic acid 3-hydroxypropyl, (meth) acrylic acid 2-hydroxypropyl, and (meth) acrylic acid 4 -Hydroxybutyl can be mentioned. A prepolymer of such an acrylic acid-based monomer can also be used. For example, a dimer of such an acrylic acid-based monomer can also be used.

(Additive)
Additives include, for example, penetrants, surfactants, polyhydric alcohols, pH regulators, or other additives.

(Penetrating agent)
The penetrant can increase the permeability of the white ink to the recording medium. This is because the wettability of the white ink to the image forming surface of the recording medium can be improved. The penetrant is preferably at least one selected from the group consisting of alkanediols and glycol ethers. The content of the penetrant in the white ink is preferably 1% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 10% by mass or less.

The alkanediol is preferably a 1,2-alkanediol having 4 or more and 8 or less carbon atoms. More specifically, the alkanediol is selected from the group consisting of 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-heptanediol, and 1,2-octanediol. It is preferable that the amount is at least one. The alkanediol is more preferably 1,2-alkanediol having 6 or more and 8 or less carbon atoms.

The glycol ethers are preferably compounds in which the hydroxyl groups (-OH groups) at one end or both ends of the glycols are replaced with lower alkyl groups. Examples of glycol ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monoethyl ether. Examples thereof include triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, and tripropylene glycol monomethyl ether.

(Surfactant)
Surfactants can enhance the permeability of white ink to recording media. This is because the wettability of the white ink to the printing surface of the recording medium can be improved. Examples of the surfactant include an anionic surfactant, a cationic surfactant, and an amphoteric surfactant. Of these surfactants, nonionic surfactants are preferred. The nonionic surfactant is preferably at least one selected from the group consisting of an acetylene glycol type surfactant and a polysiloxane type surfactant. The content of the surfactant in the white ink is preferably 0.01% by mass or more and 5.00% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

The acetylene glycol type surfactant is, for example, 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5. It is preferably at least one selected from the group consisting of −dimethyl-1-hexyne-3-ol and 2,4-dimethyl-5-hexyne-3-ol. Examples of commercially available acetylene glycol-type surfactants include "Orphine (registered trademark) E1010", "Surfinol (registered trademark) 82", "Surfinol (registered trademark) 104" and "Surfinol (registered trademark) 104" manufactured by Nissin Chemical Industry Co., Ltd. "Surfinol (registered trademark) 465" and "Surfinol (registered trademark) 485" can be mentioned. "Orphine® E1010" contains an ethylene oxide adduct of acetylenediol. "Surfinol® 82" contains 3,6-dimethyl-4-octyne-3,6-diol. "Surfinol® 104" contains 2,4,7,9-tetramethyl-5-decine-4,7-diol. "Surfinol® 465" and "Surfinol® 485" each contain an ethylene oxide adduct of tetramethyldecinediol.

The polysiloxane-type surfactant is preferably, for example, a polyether-modified siloxane. Examples of commercially available polysiloxane-type surfactants include "BYK-347" and "BYK-348" manufactured by Big Chemie Japan. "BYK-347" and "BYK-348" contain a polyether-modified siloxane.

(Multivalent alcohol)
Since the polyhydric alcohol can suppress the drying of the white ink, it is possible to prevent the recording head from being clogged. The content of the polyhydric alcohol in the white ink is preferably 0.1% by mass or more and 30.0% by mass or less, and more preferably 0.5% by mass or more and 20.0% by mass or less.

Polyhydric alcohols include, for example, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, propylene glycol, butylene glycol, 1,2,6-hexanetriol, thioglycol, hexylene glycol, glycerin, trimethylolethane, etc. It is preferably at least one selected from the group consisting of trimethylolpropane and sorbitol.

(PH regulator)
The pH adjuster can adjust the pH of the white ink. The content of the pH adjuster in the white ink is preferably 0.01% by mass or more and 10.00% by mass or less, and more preferably 0.1% by mass or more and 2.0% by mass or less. The pH regulator is preferably a tertiary amine.

(Other additives)
The white ink may further contain at least one selected from the group consisting of fixing agents, fungicides, preservatives, antioxidants, UV absorbers, chelating agents, and oxygen absorbers. Examples of the fixing agent include water-soluble rosin. Examples of the fungicide include sodium benzoate. Sodium benzoate can also function as a preservative. Examples of the antioxidant include alohanates.

(Manufacturing method of white ink)
The white ink can be produced by a phase inversion emulsification method. First, a white pigment particle dispersion is prepared. Specifically, the white pigment particles, the anionic resin, the water-insoluble non-polar resin, and the organic solvent are mixed to prepare a mixed solution. Examples of the organic solvent include methyl ethyl ketone. An aqueous solvent is added to the mixed solution to invert the phase from the organic solvent to the aqueous solvent. As a result, a dispersion in which droplets of the organic solvent are dispersed in the aqueous solvent is prepared. The organic solvent is removed from the dispersion under reduced pressure conditions to obtain a white pigment particle dispersion. A substance having an affinity in the droplets in the process of gradually removing the organic solvent from the droplets of the organic solvent containing the white pigment particles, the anionic resin, the water-insoluble non-polar resin, and the organic solvent. When they are close to each other, a white pigment particle dispersion is finally formed in the white ink.

The resin particle suspension is added to the white pigment particle dispersion. Further, additives may be added if necessary. This is mixed with a stirrer or the like to obtain a white ink. After stirring, filtration may be performed if necessary.

Examples of the present invention will be described. Table 1 shows the white inks WA-1 to WA-5 and WB-1 to WB-5 according to Examples or Comparative Examples.

Hereinafter, first, each production method of the white pigment particle dispersion liquids A-1 to A-5 and B-1 to B-5, and a method for measuring the physical property value will be described in order. Next, the manufacturing method, the evaluation method, and the evaluation result of the white inks WA-1 to WA-5 and WB-1 to WB-5 will be described in order. The evaluation results (values indicating the shape or physical properties, etc.) of the plurality of particles (specifically, white pigment particles and resin particles) in the dispersion liquid are the values measured for a considerable number of particles unless otherwise specified. It is a number average. In the evaluation in which an error occurs, a considerable number of measured values in which the error is sufficiently small are obtained, and the arithmetic mean of the obtained measured values is used as the evaluation value.

[Manufacturing of white pigment particle dispersion]
<Manufacturing of white pigment particle dispersion liquid A-1>
60 parts by mass of calcium carbonate (“Gerton 50” manufactured by Shiraishi Calcium Co., Ltd.) as white pigment particles and ammonium ion-neutralized acrylic acid resin (“Alfon UC-3920” manufactured by Toa Synthetic Co., Ltd.) as an anionic resin, acid value 240 mgKOH / g, mass average molecular weight Mw15,500) 20 parts by mass and 20 parts by mass of a styrene-acrylic acid resin (“Reseda GP-210S” manufactured by Toa Synthetic Co., Ltd.) as a water-insoluble non-polar resin are used as an organic solvent. The solution was obtained by dissolving in 100 parts by mass of methyl ethyl ketone (MEK). Then, 300 parts by mass of ion-exchanged water was further added to the solution to invert the phase. Then, MEK was desolvated under reduced pressure conditions. As a result, a white pigment particle dispersion liquid A-1 was obtained. The solid content concentration of the white pigment particle dispersion liquid A-1 was 35% by mass. The volume average particle diameter of the white pigment particle dispersion A-1 contained in the white pigment particle dispersion A-1 was 300 nm, and the polydispersion index of the white pigment particle dispersion A-1 was 0.30. The volume average particle diameter and the polydispersion index of the white pigment particle dispersion A-1 were determined by the method described in the embodiment. The same applies hereinafter.

<Manufacturing of white pigment particle dispersion A-2>
Same as white pigment particle dispersion A-1 except that talc (“Nanoace D-600” manufactured by Nippon Tarku Co., Ltd.) was used instead of calcium carbonate (“Gerton 50” manufactured by Shiraishi Calcium Co., Ltd.) as white pigment particles. White pigment particle dispersion liquid A-2 was prepared by the production method of. The solid content concentration of the white pigment particle dispersion liquid A-2 was 35% by mass. The volume average particle diameter of the white pigment particle dispersion A-2 contained in the white pigment particle dispersion A-2 was 750 nm, and the polydispersion index of the white pigment particle dispersion A-2 was 0.35.

<Manufacturing of white pigment particle dispersion liquid A-3>
Similar to white pigment particle dispersion A-1 except that kaolin (“ASPG-90” manufactured by Hayashi Kasei Co., Ltd.) was used instead of calcium carbonate (“Gerton 50” manufactured by Shiraishi Calcium Co., Ltd.) as white pigment particles. White pigment particle dispersion A-3 was prepared by the production method. The solid content concentration of the white pigment particle dispersion liquid A-3 was 36% by mass. The volume average particle diameter of the white pigment particle dispersion A-3 contained in the white pigment particle dispersion A-3 was 200 nm, and the polydispersion index of the white pigment particle dispersion A-3 was 0.11.

<Manufacturing of white pigment particle dispersion A-4>
Ammonium ion-neutralized acrylic acid resin as an anionic resin (“Alfon UC-3920” manufactured by Toa Synthetic Co., Ltd., acid value 240 mgKOH / g, mass average molecular weight Mw15, 500) White pigment particle dispersion by the same production method as the white pigment particle dispersion liquid A-1, except that (“Alfon U-3000” manufactured by Toa Synthetic Co., Ltd., acid value 74 mgKOH / g, mass average molecular weight Mw 10,000) was used. Liquid A-4 was prepared. The solid content concentration of the white pigment particle dispersion liquid A-4 was 34% by mass. The volume average particle diameter of the white pigment particle dispersion A-4 contained in the white pigment particle dispersion A-4 was 550 nm, and the polydispersion index of the white pigment particle dispersion A-4 was 0.32.

<Manufacturing of white pigment particle dispersion A-5>
Acrylic acid resin ("Aron (registered trademark) NW-400" manufactured by Toa Synthetic Co., Ltd.) instead of styrene-acrylic acid resin ("Reseda GP-210S" manufactured by Toa Synthetic Co., Ltd.) as a water-insoluble non-polar resin. ) Was used, and the white pigment particle dispersion liquid A-5 was prepared by the same production method as the white pigment particle dispersion liquid A-1. The solid content concentration of the white pigment particle dispersion liquid A-5 was 34% by mass. The volume average particle diameter of the white pigment particle dispersion A-5 contained in the white pigment particle dispersion A-5 was 270 nm, and the polydispersion index of the white pigment particle dispersion A-5 was 0.30.

<Manufacturing of white pigment particle dispersion liquid B-1>
Similar to white pigment particle dispersion A-1 except that titanium oxide (“MT-100AQ” manufactured by Teika Co., Ltd.) was used instead of calcium carbonate (“Gerton 50” manufactured by Shiraishi Calcium Co., Ltd.) as white pigment particles. White pigment particle dispersion B-1 was prepared by the production method. The solid content concentration of the white pigment particle dispersion liquid B-1 was 36% by mass. The volume average particle diameter of the white pigment particle dispersion B-1 contained in the white pigment particle dispersion B-1 was 250 nm, and the polydispersion index of the white pigment particle dispersion B-1 was 0.28.

<Manufacturing of white pigment particle dispersion B-2>
Acrylic acid resin (Starlight PMC Co., Ltd.) instead of ammonium ion-neutralized acrylic acid resin (“Alphon UC-3920” manufactured by Toa Synthetic Co., Ltd., acid value 240 mgKOH / g, mass average molecular weight Mw15, 500) as an anionic resin White pigment particle dispersion B-2 was prepared by the same production method as white pigment particle dispersion A-1 except that "VS-1057" manufactured by "VS-1057", acid value 40 mgKOH / g, mass average molecular weight Mw19,000) was used. did. The solid content concentration of the white pigment particle dispersion liquid B-2 was 35% by mass. The volume average particle diameter of the white pigment particle dispersion B-2 contained in the white pigment particle dispersion B-2 was 500 nm, and the polydispersion index of the white pigment particle dispersion B-2 was 0.32.

<Manufacturing of white pigment particle dispersion B-3>
Acrylic acid resin (Toa Synthetic Co., Ltd.) instead of ammonium ion neutralizing acrylic acid resin (“Alphon UC-3920” manufactured by Toa Synthetic Co., Ltd., acid value 240 mgKOH / g, mass average molecular weight Mw15, 500) as an anionic resin The white pigment particle dispersion B-3 was prepared by the same production method as the white pigment particle dispersion A-1 except that "Alfon U-3000" manufactured by Alfon U-3000, acid value 108 mgKOH / g, mass average molecular weight Mw 4,600) was used. Prepared. The solid content concentration of the white pigment particle dispersion liquid B-3 was 35% by mass. The volume average particle diameter of the white pigment particle dispersion B-3 contained in the white pigment particle dispersion B-3 was 280 nm, and the polydispersion index of the white pigment particle dispersion B-3 was 0.29.

<Manufacturing of white pigment particle dispersion B-4>
White pigment particles except that ammonium ion-neutralized acrylic acid resin (“Alphon UC-3920” manufactured by Toa Synthetic Co., Ltd., acid value 240 mgKOH / g, mass average molecular weight Mw15,500) was not added as an anionic resin. A white pigment particle dispersion B-4 was prepared by the same production method as the dispersion A-1. In the white pigment particle dispersion liquid B-4, the white pigment particle dispersion B-4 could not be obtained because the solvent phase was not dispersed at the time of phase inversion. Therefore, the white ink according to Comparative Example 4 could not be prepared, and the whiteness, concealment property, ejection stability, and storage stability could not be evaluated.

<Manufacturing of white pigment particle dispersion B-5>
White pigment by the same production method as white pigment particle dispersion A-1 except that styrene-acrylic acid resin (“Reseda GP-210S” manufactured by Toa Synthetic Co., Ltd.) as a water-insoluble non-polar resin was not added. A particle dispersion B-5 was prepared. The solid content concentration of the white pigment particle dispersion liquid B-5 was 34% by mass. The volume average particle diameter of the white pigment particle dispersion B-5 contained in the white pigment particle dispersion B-5 was 320 nm, and the polydispersion index of the white pigment particle dispersion B-5 was 0.30.
The measurement results are shown in Table 1.

[Manufacturing of white ink]
<Manufacturing of white ink WA-1>
Styrene acrylic acid resin emulsion (“Aron® NW-7060” manufactured by Toa Synthetic Co., Ltd.) (10 parts by mass) as resin particles was mixed with white pigment particle dispersion A-1 (25 parts by mass). The solid content was 5% by mass. Next, propylene glycol (special grade reagent manufactured by Nacalai Tesque Co., Ltd.) (15 parts by mass), sorbitol (special grade reagent manufactured by Nacalai Tesque Co., Ltd.) (10 parts by mass), and an appropriate amount of surfactant (Nisshin Kagaku). "Surfinol (registered trademark) 104") manufactured by Kogyo Co., Ltd. and an appropriate amount of ion-exchanged water were added and diluted. As a result, white ink WA-1 was obtained. The concentration of the white pigment in the white ink WA-1 was 5% by mass, the viscosity at 25 ° C. was 4 mPa · s, and the surface tension of the white ink at 25 ° C. was 35 mN / m. The blending amount of the surfactant (“Surfinol (registered trademark) 104” manufactured by Nisshin Chemical Industry Co., Ltd.) was adjusted so that the surface tension of the ink at 25 ° C. was 35 mN / m. More specifically, the blending amount of the surfactant (“Surfinol (registered trademark) 104” manufactured by Nisshin Chemical Industry Co., Ltd.) was set to about 0.05 parts by mass.

<Manufacturing of white inks WA-2 to WA-5, WB-1 to WB-3 and WB-5>
With the white ink WA-1, except that any of the white pigment particle dispersions A-2 to A-5, B-1 to B-3, and B-5 was used instead of the white pigment particle dispersion A-1. White inks WA-2 to WA-5, WB-1 to WB-3 and WB-5 were obtained by the same production method, respectively. Table 1 shows the measurement results of the white ink. The "volume average particle size" in the white pigment particle column indicates the volume average particle size of the white pigment particle dispersion.

[Ink evaluation method]
<Evaluation of whiteness>
First, a white ink (more specifically, each of the white inks WA-1 to WA-5 and WB-1 to WB-3 and WB-5) is applied to an inkjet printer ("PX-045a" manufactured by Seiko Epson Corporation). ) The ink chamber of the dedicated cartridge was filled. Next, the cartridge was attached to the inkjet printer. Subsequently, a solid image having a duty of 100% was formed on an OHP film (an OHP film for inkjet by 3M Japan Ltd.) using an inkjet printer. The obtained printed matter was placed on a black mount (OD value: 2.0). Then, using a reflection densitometer (“SpectroEye®” manufactured by X-Rite), the brightness (L ^* ) of the ink recorded on the OHP film was measured under the following measurement conditions.
(Measurement condition)
Light source: D50
Observation field of view: 2 °

The evaluation criteria for whiteness are shown below. The evaluation results are shown in Table 2.
(Evaluation criteria for whiteness)
Evaluation A (good): The brightness (L ^* ) of the white ink is 70 or more.
Evaluation B (defective): The brightness (L ^* ) of the white ink is less than 70.

<Evaluation of concealment rate>
The concealment rate was evaluated using the printed matter obtained in the above-mentioned <evaluation of whiteness>. Specifically, first, the printed matter was set in the sample room of an ultraviolet-visible spectrophotometer (“U-3010” manufactured by Hitachi High-Tech Science Corporation). Next, monochromatic light in the visible region (wavelength: 400 nm or more and 800 nm, wavelength interval: 1 nm) is incident on the image forming portion of the printed matter, and the ratio (transmittance) of the incident light having each wavelength is transmitted through the image forming portion of the printed matter. Rate T (unit:%)) was measured. Then, the average value of the transmittance was calculated from the transmittance T of the image forming portion of the printed matter at the wavelengths of 400 nm, 500 nm, 600 nm, 700 nm, and 800 nm of the incident light. The obtained average value was used as the concealment rate (unit:%).

The evaluation criteria are shown below. The evaluation results are shown in Table 2. A small concealment rate means a low transmittance T. Therefore, a small concealment rate means that the concealment effect is sufficiently exhibited. A large concealment rate means a high transmittance T. Therefore, a large concealment rate means that the concealment effect is not sufficiently exhibited or that the concealment effect is difficult to be exhibited.
(Evaluation criteria for concealment rate)
Evaluation A (good): The concealment rate is 10% or less.
Evaluation B (defective): The concealment rate is more than 10%.

<Discharge stability>
White ink (more specifically, white ink WA-1 to WA-5, WB-1 to WB-3, and WB-5, respectively) is used exclusively for inkjet printers ("PX-045a" manufactured by Seiko Epson Corporation). The ink chamber of the cartridge was filled. Next, the cartridge was attached to the inkjet printer. Subsequently, a solid image having a duty of 100% was formed on an OHP film (an OHP film for inkjet by 3M Japan Ltd.) using an inkjet printer. Such images were continuously formed on 30 transparencies. Next, a nozzle check pattern was created, and the number of ink droplets not ejected (the number of missing nozzles) was counted from the total number of nozzles of the inkjet head. From the obtained counts, the ejection stability of the white ink was evaluated based on the following evaluation criteria.

The evaluation criteria are shown below. Table 2 shows the evaluation results.
(Evaluation criteria for discharge stability)
Evaluation A (good): The number of missing nozzles is less than 10.
Evaluation B (defective): The number of missing nozzles is 10 or more.

<Evaluation of storage stability>
First, the white ink (more specifically, each of the white inks WA-1 to WA-5, WB-1 to WB-3, and WB-5) was diluted 10-fold with ion-exchanged water. The obtained diluted solution was put into a cell and set in a sample room of an ultraviolet-visible spectrophotometer (“U-3010” manufactured by Hitachi High-Tech Science Corporation). Visible light (wavelength: 500 nm) was incident on the diluent, and the rate at which the incident light was absorbed by the diluent (absorbance Abs) was measured. The measured absorbance Abs was defined as the absorbance before storage.

Next, 50 mL of the white ink (more specifically, each of the white inks WA-1 to WA-5, WB-1 to WB-3, and WB-5) is placed in a closed container (capacity: 100 mL). , The closed container was allowed to stand in an environment at a temperature of 50 ° C. for 1 week. Then, the closed container was shaken by hand, and the white ink in the closed container was lightly stirred. Using ion-exchanged water, 10 mL of 50 mL of white ink in a closed container was diluted 10-fold to prepare a diluted dispersion. 10 mL of the diluted dispersion was placed in a measuring cylinder (volume: 10 mL), and the measuring cylinder was allowed to stand in an environment of a temperature of 25 ° C. and a humidity of 50% RH for 1 week. Then, the supernatant (2 mL) was collected from the diluted dispersion in the measuring cylinder, and the collected supernatant was diluted 1000-fold with ion-exchanged water to obtain a supernatant diluted solution. The obtained supernatant diluent was placed in a cell and set in a sample chamber of an ultraviolet-visible spectrophotometer (“U-3010” manufactured by Hitachi High-Tech Science Corporation). Visible light (wavelength: 500 nm) was incident on the supernatant diluent, and the ratio of the incident light absorbed by the supernatant diluent (absorbance Abs) was measured. The measured absorbance Abs was used as the absorbance after storage.

From the obtained absorbance before storage and the absorbance after storage, the storage stability was calculated based on the following formula.
Storage stability (unit:%) = {(absorbance after storage) / (absorbance before storage)} x 100

The evaluation criteria are shown below. The evaluation results are shown in Table 2.
(Evaluation criteria for storage stability)
Evaluation A (good): Storage stability is 60% or more.
Evaluation B (defective): Storage stability is less than 60%.

[Ink evaluation result]
Table 2 shows the evaluation results of the white inks (more specifically, the white inks WA-1 to WA-5 and WB-1 to WB-5, respectively).

The white inks WA-1 to WA-5 (white inks according to Examples 1 to 5) each had the basic configurations shown below. Specifically, as shown in Table 1, the white inks WA-1 to WA-5 each contained an aqueous medium, resin particles, a white pigment, and a dispersant. The density of the white pigment was 3.0 g / cm ³ or less. The dispersant contained an anionic resin and a water-insoluble non-polar resin. The surface of the white pigment was coated with a water-insoluble non-polar resin, and further coated with an anionic resin. The anionic resin had an acid value of 50 mgKOH / g or more and a mass average molecular weight of 10,000 or more.

As shown in Table 2, the evaluation results of the ejection stability and the storage stability of the white inks WA-1 to WA-5 were all evaluated as A (good).

On the other hand, none of the white inks WB-1 to WB-5 had at least one of the above-mentioned basic configurations. Specifically, in the white ink WB-1 (white ink according to Comparative Example 1), the density of the white pigment was 4.2 g / cm ³ . In the white ink WB-2 (white ink according to Comparative Example 2), the acid value of the anionic resin was 40 mgKOH / g. In the white ink WB-3 (white ink according to Comparative Example 3), the mass average molecular weight of the anionic resin was 4600. The white ink WB-4 (white ink according to Comparative Example 4) did not contain an anionic resin. The white ink WB-5 (white ink according to Comparative Example 5) did not contain a water-insoluble non-polar resin.

As shown in Table 2, with the white inks WB-2, WB-3 and WB-5, the evaluation result of the ejection stability was evaluation B (defective). Further, with the white inks WB-1, WB-2 and WB-5, the evaluation result of the storage stability was evaluation B (defective). As described above, in the white inks WB-1 to WB-3 and WB-5, at least one of the evaluation results of ejection stability and storage stability was evaluation B (defective).

As shown in Tables 1 and 2, it is clear that the white inks of Examples 1 to 5 are superior in ejection stability and storage stability to the white inks of Comparative Examples 1 to 5. Therefore, the white ink according to the present invention is excellent in storage stability and ejection stability.

In addition, the present inventor has each brightness (L ^* ) of the white inks WA-1 to WA-5 measured by the above-mentioned <Measuring method of brightness (L ^* ) and chromaticity (a ^* , b ^* )>. Was confirmed to satisfy the above-mentioned equation (1). In addition, the present inventor has determined the respective chromaticity (a ^* ) of the white inks WA-1 to WA-5 measured by the above-mentioned <Measuring method of brightness (L ^* ) and chromaticity (a ^* , b ^* )> ^. , B ^* ) was confirmed to satisfy the above equations (2) and (3). That is, the present inventor has confirmed that each of the white inks WA-1 to WA-5 is white ink. Further, the white inks of Examples 1 to 5 were excellent in the evaluation results of whiteness and concealing property. It is clear that the white ink according to the present invention can form an image having desired whiteness and hiding power.

The white ink according to the present invention is suitable for use in forming an image in, for example, a color printer.

Claims (4)

A white ink for inkjet recording containing an aqueous medium, resin particles, white pigment particles, and a dispersant.
The density of the white pigment particles is 3.0 g / cm ³ or less.
The dispersant contains an anionic resin and a water-insoluble non-polar resin.
The white pigment particles are dispersed as a white pigment particle dispersion,
In the white pigment particle dispersion, the surface of the white pigment particles is coated with the water-insoluble non-polar resin, and further coated with the anionic resin.
The acid value of the anionic resin is 50 mgKOH / g or more, and the acid value is 50 mgKOH / g or more.
A white ink for inkjet recording in which the mass average molecular weight of the anionic resin is 10,000 or more. The white ink for inkjet recording according to claim 1, wherein the white pigment particles are at least one selected from the group consisting of calcium carbonate, talc, mica and kaolin. The white ink for inkjet recording according to claim 1 or 2, wherein the anionic resin is an ammonium ion-neutralized acrylic acid-based resin, and the water-insoluble non-polar resin is a styrene-acrylic acid-based resin. The white ink for inkjet recording according to any one of claims 1 to 3, wherein the volume average particle diameter of the white pigment particle dispersion is 250 nm or more and 800 nm or less.