JP6958066B2 - 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 or resin particles having a hollow structure (hollow resin particles) have been conventionally used. 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. Further, when hollow resin particles are used, if the hollow structure disappears for some reason, it is difficult to secure the refractive index of the resin particles. Therefore, an image having a desired whiteness may not be obtained. Based on these, in recent years, the use of resin particles having a high refractive index has been proposed (see, for example, Patent Document 1 below).

Japanese Unexamined Patent Publication No. 2014-189558

When resin particles are formed using a thermosetting resin material having a high refractive index, the particle size of the resin particles may be non-uniform. Therefore, the storage stability of the white ink may decrease. Further, when the resin particles are formed by using a thermoplastic resin material having a high refractive index, the physical stability or the chemical stability of the white ink may decrease.

The present invention has been made in view of these points, and provides a white ink for inkjet recording which is excellent in storage stability, physical stability and chemical stability, and can form an image having a desired whiteness. The purpose is to provide.

The white ink for inkjet recording according to the present invention includes an aqueous medium and a plurality of resin particles dispersed in the aqueous medium. The average particle size of the resin particles is 500 nm or more and 1000 nm or less. Each of the resin particles contains a unit derived from a thermoplastic resin and a unit derived from a monomer of a thermosetting resin. In each of the resin particles, the unit derived from the thermoplastic resin is crosslinked with the unit derived from the monomer of the thermosetting resin. The content of the monomer of the thermosetting resin is 10% by mass or more with respect to the total content of the monomer of the thermosetting resin and the monomer of the thermoplastic resin. The monomer of the thermosetting resin is alkylene glycol dimethacrylate.

The white ink for inkjet recording according to the present invention is excellent in storage stability, physical stability, and chemical stability. Further, if recording is performed on a recording medium using the white ink for inkjet recording according to the present invention, an image having a desired whiteness can be formed.

An embodiment of the present invention will be described. In the following, the evaluation result (value indicating the shape, physical properties, etc.) of the powder is the average number of values measured for a considerable number of particles, unless otherwise specified.

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, acrylics and methacryl may be collectively referred to as "(meth) acrylics".

The white ink for inkjet recording according to the present embodiment (hereinafter, may be simply referred to as “white ink”) 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 lowers 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 apparatus 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 number of actual recorded dots" means the number of actual recorded dots 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:%) = 100 x (number of actual recorded dots) / (image resolution)

In white ink, a plurality of resin particles are dispersed with each other in an aqueous medium. In white ink, the resin particles function as pigment particles. The term "average particle size" used with respect to resin particles means a harmonic mean particle size (diameter) based on scattered light intensity. Further, the term "multi-dispersion index" used for resin particles means a dimensionless index indicating the spread of particle size distribution. The smaller the polydispersity index, the smaller the spread of the particle size distribution. Further, the larger the polydispersity index, the larger the spread of the particle size distribution. That is, the smaller the polydispersity index of the resin particles, the sharper the particle size distribution of the resin particles. The larger the polydispersity index of the resin particles, the broader the particle size distribution of the resin particles.

The average particle size of the resin particles and the polydispersity index of the resin particles are determined by the method described in ISO 13321: 1996 (Particle size analysis-Phototron spectroscopy spectrum) or a method similar thereto. Specifically, the average particle size of the resin particles and the polydispersity index of the resin particles are each obtained by analyzing the autocorrelation function of the scattered light intensity obtained by the dynamic light scattering method by the cumulant method. More specifically, first, a change in the scattered light intensity from the resin particles over time is detected by a dynamic light scattering method. 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 average particle size of the resin particles and the polydispersity index of the resin particles can be obtained.

[Structure of white ink]
The white ink according to the present embodiment includes an aqueous medium and a plurality of resin particles dispersed in the aqueous medium. The average particle size of the resin particles is 500 nm or more and 1000 nm or less. Each resin particle contains a unit derived from a thermoplastic resin and a unit derived from a monomer of a thermosetting resin. In each of the resin particles, the units derived from the thermoplastic resin are crosslinked with the units derived from the monomer of the thermosetting resin. The content of the monomer of the thermosetting resin is 10% by mass or more with respect to the total content of the monomer of the thermosetting resin and the monomer of the thermoplastic resin. The monomer of the thermosetting resin is alkylene glycol dimethacrylate.

Since the average particle size of the resin particles is 500 nm or more, light is likely to be scattered on the surface of the resin particles. As a result, the concealment effect is likely to be exhibited. Therefore, if the average particle size of the resin particles is 500 nm or more, an image having a desired whiteness can be formed. The larger the average particle size of the resin particles, the easier it is for the hiding effect to be exhibited. However, if the average particle size of the resin particles is too large, the precipitation of the resin particles in the white ink becomes remarkable. When the average particle size of the resin particles is 1000 nm or less, the sedimentation of the resin particles in the white ink can be effectively prevented, so that the storage stability of the white ink can be prevented from being lowered. From the above, when the average particle size of the resin particles is 500 nm or more and 1000 nm or less, an image having a desired whiteness can be formed while maintaining the storage stability of the white ink for a long period of time.

Each resin particle contains a unit derived from a thermoplastic resin and a unit derived from a monomer of a thermosetting resin. In each of the resin particles, the unit derived from the thermoplastic resin is crosslinked with the unit derived from the monomer of the thermosetting resin. In this way, each resin particle is composed of a crosslinked resin. Thereby, by controlling the synthesis condition of the crosslinked resin or the production condition of the resin particles, it is possible to prevent the particle size from becoming non-uniform in the produced resin particles. That is, the polydispersity index of the resin particles can be reduced by controlling the synthetic conditions of the crosslinked resin or the production conditions of the resin particles. This also prevents the white ink from being deteriorated in storage stability. The present inventors have confirmed that the storage stability of the white ink can be maintained for a long period of time when the polydispersity index of the resin particles is 0.25 or less. Further, the present inventors have confirmed that when the polydispersity index of the resin particles is 0.20 or less, it is easy to maintain the storage stability of the white ink for a long period of time. Therefore, the polydispersity index of the resin particles is preferably 0.25 or less, more preferably 0.20 or less. Even if the synthetic conditions of the crosslinked resin or the production conditions of the resin particles are controlled, it is difficult to set the polydispersity index of the resin particles to less than 0.01.

When the resin particles are composed of the crosslinked resin, the mechanical strength of the resin particles can be easily secured, so that it is possible to prevent the physical stability of the white ink from being lowered. Further, if the resin particles are composed of the crosslinked resin, the chemical stability of the resin particles can be easily ensured, so that it is possible to prevent the chemical stability of the white ink from being lowered. For example, if recording is performed on a recording medium using the white ink according to the present embodiment, the physical resistance and chemical resistance of the image recorded on the recording medium can be maintained for a long period of time.

In the resin particles, the content of the monomer of the thermosetting resin is described as the total of the content of the monomer of the thermosetting resin and the content of the monomer of the thermoplastic resin (hereinafter, "the total content of the monomers"). ), 10% by mass or more. As a result, the crosslink density of the resin constituting the resin particles tends to be high, so that it is possible to effectively prevent the white ink from being lowered in physical stability and chemical stability. The higher the monomer content of the thermosetting resin, the higher the crosslink density of the resin tends to be, but the synthesis of the crosslinked resin becomes difficult. The present inventors have confirmed that the crosslinked resin can be synthesized when the monomer content of the thermosetting resin is 40% by mass or less with respect to the total monomer content. More preferably, the content of the monomer of the thermosetting resin is 20% by mass or less with respect to the total content of the monomers.

In the resin particles, the monomer of the thermosetting resin is alkylene glycol dimethacrylate (hereinafter, may be referred to as “AGDM”). When the monomer of the thermosetting resin is an aliphatic compound, the cross-linking reaction is more likely to occur than when the monomer of the thermosetting resin is an aromatic compound. Further, when the monomer of the thermosetting resin is an aliphatic compound, unlike the case where the monomer of the thermosetting resin is an aromatic compound, a cross-linking reaction in an aqueous system is possible. From these facts, if the monomer of the thermosetting resin is AGDM, the crosslink density of the resin constituting the resin particles tends to be high. This also makes it possible to effectively prevent the white ink from being deteriorated in physical stability and chemical stability. Here, the "crosslinking reaction" includes a reaction between a monomer of a thermosetting resin and a monomer of a thermoplastic resin.

The number of carbon atoms of the alkylene group contained in AGDM is preferably 1 or more and 15 or less. If the alkylene group contained in AGDM has too many carbon atoms, the cross-linking reaction may be difficult to occur. Examples of AGDM include ethylene glycol dimethacrylate (alkylene group carbon number: 2), hexamethylene glycol dimethacrylate (alkylene group carbon number: 6), or dodecamethylene glycol dimethacrylate (alkylene group carbon number: 12). Can be mentioned. A commercially available product may be used as the AGDM.

In the resin particles, the chemical structure of the thermoplastic resin is not particularly limited. Methyl methacrylate is known as a monomer having a high refractive index. Therefore, if the thermoplastic resin is a copolymer or copolymer of methyl methacrylate, the refractive index of the resin particles tends to be high, so that the whiteness of the formed image can be further increased. Therefore, the thermoplastic resin is preferably a copolymer or copolymer of methyl methacrylate. When the thermoplastic resin is a copolymer of methyl methacrylate, the chemical structure of the monomer (other monomer) other than methyl methacrylate is not particularly limited. The other monomer may be a (meth) acrylic acid-based monomer excluding methyl methacrylate, or a monomer having an aromatic ring in the molecule (for example, a styrene-based monomer). Examples of the (meth) acrylic acid-based monomer excluding methyl methacrylate include ethyl (meth) acrylate or butyl (meth) acrylate.

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.

[Material contained in white ink]
Hereinafter, the material contained in the white ink will be specifically described.
As described above, in the white ink, a plurality of resin particles are dispersed with each other in an aqueous medium. The resin particles are as described above. The white ink may further contain at least one selected from the group consisting of penetrants, surfactants, polyhydric alcohols, and pH regulators.

<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, ultrafiltered 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.

<Penetrating agent>
When the white ink contains a penetrant, the wettability of the white ink to the printing surface of the recording medium can be enhanced, so that the permeability of the white ink to the recording medium can be enhanced. 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. More preferably, the alkanediol is a 1,2-alkanediol having 6 or more and 8 or less carbon atoms. More specifically, the alkanediol is more preferably at least one selected from the group consisting of 1,2-hexanediol, 1,2-heptanediol, and 1,2-octanediol. When the alkanediol is a 1,2-alkanediol having 6 or more and 8 or less carbon atoms, the permeability of the white ink to the recording medium can be further enhanced.

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. More specifically, the 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 mono. It is preferably at least one selected from the group consisting of ethyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, and tripropylene glycol monomethyl ether. More preferably, the glycol ethers are triethylene glycol monobutyl ethers. Image quality can also be improved by using triethylene glycol monobutyl ether.

<Surfactant>
When the white ink contains a surfactant, the wettability of the white ink to the printing surface of the recording medium can be enhanced, so that the permeability of the white ink to the recording medium can be enhanced. The surfactant may be an anionic surfactant, a cationic surfactant, or an amphoteric surfactant, but is preferably a nonionic surfactant. 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.

Acetylene glycol-type surfactants include, for example, 2,4,7,9-tetramethyl-5-decyne-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 104", and "Surfinol 465" manufactured by Nissin Chemical Industry Co., Ltd. And "Surfinol 485". "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-decyne-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" each contain a polyether-modified siloxane.

<Multivalent alcohol>
When the white ink contains a polyhydric alcohol, the white ink can be prevented from drying, so that the recording head can be prevented 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. And at least one selected from the group consisting of trimethylol propane.

<pH adjuster>
If the white ink contains a pH adjuster, the pH of the white ink can be easily adjusted. 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 adjuster is preferably a tertiary amine. More specifically, the pH adjuster is preferably triethanolamine.

<Other additives>
The white ink preferably further contains at least one selected from the group consisting of fixing agents, fungicides, preservatives, antioxidants, UV absorbers, chelating agents, and oxygen absorbers. The fixing agent is preferably, for example, a water-soluble rosin. The fungicide is preferably, for example, sodium benzoate. Sodium benzoate can also function as a preservative. The antioxidant is preferably, for example, alohanates. The white ink may further contain sorbitol.

[Preferable manufacturing method of white ink]
First, a dispersion liquid of resin particles is prepared. Specifically, a monomer or prepolymer capable of synthesizing a crosslinked resin by polymerization and a polymerization initiator are added to a predetermined solvent. The resulting mixture is kept at a predetermined temperature for a predetermined time. In this way, a dispersion liquid of resin particles is obtained.

The predetermined solvent preferably contains an aqueous medium and a surfactant. The aqueous medium is as described in <Aqueous medium> described above. As the surfactant, for example, an anionic surfactant can be used. The smaller the content of the surfactant, the larger the average particle size of the obtained resin particles tends to be. The higher the content of the surfactant, the smaller the average particle size of the obtained resin particles tends to be.

The monomer or prepolymer capable of synthesizing the crosslinked resin is as described in [Structure of white ink] described above. The polymerization initiator is preferably at least one selected from the group consisting of potassium persulfate, benzoyl peroxide, and azobisisobutyronitrile.

When the monomer or prepolymer and the polymerization initiator are added to a predetermined solvent in a short time, the resin particles tend to grow rapidly, so that the polydispersity index of the obtained resin particles tends to increase. When the monomer or prepolymer and the polymerization initiator are added to a predetermined solvent over time, the resin particles tend to grow slowly, so that the polydispersity index of the obtained resin particles tends to decrease.

The longer the time (holding time) for keeping the mixed solution at a predetermined temperature, the more the resin particles tend to aggregate, so that the polydispersity index of the obtained resin particles tends to increase. Further, as the holding time is longer, the amount of unreacted monomers remaining in the dispersion tends to be smaller, so that the crosslink density of the resin constituting the resin particles tends to be higher. The shorter the holding time, the more difficult it is for the resin particles to aggregate, so the polydispersity index of the obtained resin particles tends to be smaller.

Next, the obtained dispersion liquid is mixed with other ink components. A stirrer (for example, "Three One Motor (registered trademark) BL-600" manufactured by Shinto Kagaku Co., Ltd.) can be used to mix the dispersion liquid with other ink components. Other ink components may contain at least one selected from the group consisting of penetrants, surfactants, pH regulators, and polyhydric alcohols. Other ink components may further contain at least one selected from the group consisting of fixers, fungicides, preservatives, antioxidants, UV absorbers, chelating agents, oxygen absorbers, and sorbitol. .. After mixing the dispersion liquid with other ink components, filtration is performed if necessary. In this way, white ink is obtained.

Examples of the present invention will be described. Table 1 shows inks WA-1 to WA-7 and WB-1 to WB-5 according to Examples or Comparative Examples. In Table 1, “particle size” indicates the average particle size of the resin particles. "Monomer A" means a monomer of a thermoplastic resin. "MMA" means methyl methacrylate. "EA" means ethyl acrylate. "BA" means butyl acrylate. "Monomer B" means a monomer of a thermosetting resin. "EGDM" means ethylene glycol dimethacrylate. "HGDM" means hexamethylene glycol dimethacrylate. "DDGDM" means dodecamethylene glycol dimethacrylate. "DVB" means divinylbenzene.

Hereinafter, first, a method for producing each of the dispersions of the resin particles R-1 to R-12 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 inks WA-1 to WA-7 and WB-1 to WB-5 will be described in order. Unless otherwise specified, the evaluation result (value indicating the shape or physical properties) of the powder containing a plurality of particles is the average number of values measured for a considerable number of particles. 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 method of dispersion of resin particles]
<Manufacturing of dispersion of resin particles R-1>
First, a three-necked flask (capacity: 1 L) equipped with a thermometer and a stirring blade was set in a water bath. A flask was filled with 720 g of ion-exchanged water and 50 g of an anionic surfactant (“Neoperex® G-25” manufactured by Kao Corporation, composition: 25% by mass dodecylbenzene sulfonic acid aqueous solution). A water bath was used to keep the temperature inside the flask at 80 ° C. With the temperature inside the flask kept at 80 ° C., the first liquid and the second liquid were added dropwise to the flask over 2 hours, respectively. The first solution contained 180 g of methyl methacrylate (MMA) and 20 g of ethylene glycol dimethacrylate (EGDM). The second solution was obtained by dissolving 0.5 g of potassium persulfate in 30 mL of ion-exchanged water. Then, the temperature in the flask was kept at 80 ° C. for 2 hours. While the temperature inside the flask was kept at 80 ° C., the contents of the flask reacted (polymerization reaction). In this way, a dispersion liquid of the resin particles R-1 (solid content concentration: 20% by mass) was obtained.

<Manufacturing of dispersion of resin particles R-2>
750 g of ion-exchanged water and 20 g of anionic surfactant were used. Further, the dropping time of each of the first liquid and the second liquid was set to 1 hour. Except for these matters, a dispersion liquid of resin particles R-2 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-3>
As the first liquid, a liquid containing 160 g of methyl methacrylate and 40 g of ethylene glycol dimethacrylate was used. Except for this, a dispersion liquid of resin particles R-3 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-4>
As the first liquid, a liquid containing 120 g of methyl methacrylate, 40 g of ethyl acrylate (EA), 20 g of butyl acrylate (BA), and 20 g of ethylene glycol dimethacrylate was used. Except for this, a dispersion liquid of resin particles R-4 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-5>
Hexamethylene glycol dimethacrylate (HGDM) was used instead of ethylene glycol dimethacrylate. Except for this, a dispersion liquid of resin particles R-5 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-6>
Dodecamethylene glycol dimethacrylate (DDGDM) was used instead of ethylene glycol dimethacrylate. Except for this, a dispersion liquid of resin particles R-6 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-7>
The dropping time of each of the first liquid and the second liquid was set to 4 hours. Moreover, the time for keeping the temperature in the flask at 80 ° C. after dropping was changed from 2 hours to 3 hours. Except for these matters, a dispersion liquid of resin particles R-7 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-8>
680 g of ion-exchanged water and 100 g of anionic surfactant were used. Except for this, a dispersion liquid of resin particles R-8 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-9>
760 g of ion-exchanged water and 10 g of anionic surfactant were used. Further, the dropping time of each of the first liquid and the second liquid was set to 1 hour. Except for these matters, a dispersion liquid of resin particles R-9 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-10>
Divinylbenzene (DVB) was used instead of ethylene glycol dimethacrylate. Except for this, a dispersion liquid of resin particles R-10 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-11>
As the first liquid, a liquid containing 190 g of methyl methacrylate and 10 g of ethylene glycol dimethacrylate was used. Except for this, a dispersion liquid of resin particles R-11 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

<Manufacturing of dispersion of resin particles R-12>
The dropping time of each of the first liquid and the second liquid was set to 1 hour. Except for this, a dispersion liquid of resin particles R-12 (solid content concentration: 20% by mass) was obtained according to the method for producing a dispersion liquid of resin particles R-1.

[Measurement method of physical property values of resin particles]
The average particle size and the polydispersity index of the resin particles contained in the dispersion were measured using a dynamic light scattering type particle size distribution measuring device (“Zetasizer (registered trademark) Nano ZS” manufactured by Sysmex Co., Ltd.). Specifically, using ion-exchanged water, the dispersion liquid of the resin particles (more specifically, each of the dispersion liquids of the resin particles R-1 to R-12) is diluted 1000 times or more and 5000 times or less. Diluted to the chosen dilution rate (more specifically 1000 times). The obtained diluted solution was used as a measurement target. The average particle size and polydispersity index of the object to be measured were determined using a dynamic light scattering particle size distribution measuring device based on the method described in ISO 13321: 1996 (Particle size analysis-Phototron spectroscopy spectroscopy). The measurement results are shown in Table 1.

[Ink manufacturing method]
15 parts by mass of propylene glycol (special grade reagent manufactured by Nacalai Tesque Co., Ltd.) in 35 parts by mass of dispersion liquid of resin particles (more specifically, each of the dispersion liquids of resin particles R-1 to R-12) , 10 parts by mass of sorbitol (special reagent product manufactured by Nacalai Tesque Co., Ltd.), an appropriate amount of surfactant ("Surfinol 104" manufactured by Nisshin Chemical Industry Co., Ltd.), and ion-exchanged water (remaining amount). added. In this way, inks (more specifically, inks WA-1 to WA-7 and WB-1 to WB-5, respectively) were obtained. In the obtained ink, the pigment concentration was 5% by mass, the viscosity of the ink at 25 ° C. was 4 mPa · s, and the surface tension of the ink at 25 ° C. was 35 mN / m. The blending amount of the surfactant (“Surfinol 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 104” manufactured by Nisshin Chemical Industry Co., Ltd.) was set to about 0.05 parts by mass.

[Ink evaluation method]
<Evaluation of whiteness>
First, the ink (more specifically, each of the inks WA-1 to WA-7 and WB-1 to WB-5) is applied to the ink of the cartridge dedicated to the inkjet printer ("PX-045a" manufactured by Seiko Epson Corporation). The room was filled. Next, the cartridge was attached to the inkjet printer. Subsequently, using an inkjet printer, a 100% duty solid image was recorded on an OHP film (an inkjet OHP film manufactured by 3M Japan Ltd.). The obtained printed matter was placed on a black mount (black 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 are shown below. The evaluation results are shown in Table 2.
Good: The lightness (L ^* ) of the ink was 70 or more.
Defective: The lightness (L ^* ) of the ink was less than 70.

<Evaluation of concealment>
The concealment degree 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 a spectrophotometer (Hitachi High-Tech Science Corporation "U-3010"). Next, visible light (wavelength: 400 nm or more and 800 nm, wavelength interval: 1 nm) is incident on the printed matter, and the ratio (transmittance T (unit:%)) of the incident light having each wavelength is transmitted through the printed matter. It was measured. Then, the average value of the ratio of the incident light having wavelengths of 400 nm, 500 nm, 600 nm, 700 nm, and 800 nm transmitted through the printed matter was calculated. The obtained average value was used as the concealment degree (unit:%).

The evaluation criteria are shown below. The evaluation results are shown in Table 2. A small degree of concealment means that the transmittance T is low. Therefore, a small degree of concealment means that the concealment effect is sufficiently exhibited. A large degree of concealment means a high transmittance T. Therefore, a large degree of concealment means that the concealment effect is not sufficiently exhibited or that the concealment effect is difficult to be exhibited.
Good: The degree of concealment was 10% or less.
Defective: The degree of concealment was over 10%.

<Evaluation of physical resistance>
The physical resistance was evaluated using the printed matter obtained in the above-mentioned <evaluation of whiteness>. Specifically, first, the printed surface and the film surface of the OHP film were brought into close contact with each other. Next, using a Gakushin type friction fastness tester (manufactured by Tester Sangyo Co., Ltd.), the surface on the opposite side of the OHP film (the surface of the OHP film located on the opposite side of the film surface) while applying a load of 50 gf. The friction element was reciprocated 500 times in. Subsequently, the printed surface and the film surface of the OHP film were peeled off. ^{Then, the brightness (L *} ) of the ink on the printing surface was measured according to the method described in <Evaluation of whiteness> described above. The thus measured lightness (L ^*) was first lightness after the test (L ^*). Then, the physical resistance was calculated based on the following formula. The "brightness before the test (L ^* )" in the following formulas correspond to ^{the lightness (L *} ) of the ink obtained in the above-mentioned <evaluation of whiteness>.
Physical resistance (unit:%) = 100 x [brightness after the first test (L ^* )] / [brightness before the test (L ^* )]

The evaluation criteria are shown below. The evaluation results are shown in Table 2.
Good: Physical resistance was 80% or more.
Defective: Physical resistance was less than 80%.

<Evaluation of chemical resistance>
The chemical resistance was evaluated using the printed matter obtained in the above-mentioned <evaluation of whiteness>. Specifically, first, using a Gakushin-type friction fastness tester (manufactured by Tester Sangyo Co., Ltd.), the friction element was reciprocated 100 times on the printed surface while applying a load of 50 gf. At this time, the tip of the friction element (the portion of the friction element that contacts the printed surface) was covered with a cloth (cotton gold width No. 3) impregnated with ethyl acetate. ^{Then, the brightness (L *} ) of the ink on the printing surface was measured according to the method described in <Evaluation of whiteness> described above. The thus measured lightness (L ^*) is a second brightness after the test (L ^*). Then, the degree of chemical resistance was calculated based on the following formula. The "brightness before the test (L ^* )" in the following formulas correspond to ^{the lightness (L *} ) of the ink obtained in the above-mentioned <evaluation of whiteness>.
Chemical resistance (unit:%) = 100 x [brightness after the second test (L ^* )] / [brightness before the test (L ^* )]

The evaluation criteria are shown below. The evaluation results are shown in Table 2.
Good: The chemical resistance was 70% or more.
Poor: Chemical resistance was less than 70%.

<Evaluation of storage stability>
First, the ink (more specifically, each of the inks WA-1 to WA-7 and WB-1 to WB-5) was diluted 10-fold with ion-exchanged water. The obtained diluted solution was set in a sample room of a spectrophotometer (Hitachi High-Tech Science Corporation "U-3010"). 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, 10 mL of the diluted solution was placed in a measuring cylinder (volume: 10 mL), and the measuring cylinder was allowed to stand in an environment of room temperature 25 ° C. and humidity 50% RH for 1 week. Then, the supernatant (2 mL) was collected from the diluent in the measuring cylinder, and the collected supernatant was diluted 1000-fold with ion-exchanged water. The obtained diluted solution was set in a sample room of a spectrophotometer (Hitachi High-Tech Science Corporation "U-3010"). 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 used as the absorbance after storage. Then, the storage stability was calculated based on the following formula.
Storage stability (unit:%) = 100 x (absorbance after storage) / (absorbance before storage)

The evaluation criteria are shown below. The evaluation results are shown in Table 2.
Good: Storage stability was 60% or more.
Defective: Storage stability was less than 60%.

[Ink evaluation result]
Table 2 shows the evaluation results of the inks (more specifically, inks WA-1 to WA-7 and WB-1 to WB-5, respectively).

Each of the inks WA-1 to WA-7 (inks according to Examples 1 to 7) had the basic configuration shown below. Specifically, the inks WA-1 to WA-7 each contained an aqueous medium and a plurality of resin particles dispersed in the aqueous medium. The average particle size of the resin particles was 500 nm or more and 1000 nm or less. Each of the resin particles contained a unit derived from a thermoplastic resin and a unit derived from a monomer of a thermosetting resin. In each of the resin particles, the unit derived from the thermoplastic resin was crosslinked with the unit derived from the monomer of the thermosetting resin. The content of the monomer of the thermosetting resin was 10% by mass or more with respect to the total content of the monomers. The monomer of the thermosetting resin was alkylene glycol dimethacrylate. As shown in Table 2, the inks WA-1 to WA-7 each had a desired whiteness and a desired concealment. The inks WA-1 to WA-7 were excellent in physical stability, chemical stability, and storage stability, respectively.

On the other hand, none of the inks WB-1 to WB-5 had the above-mentioned basic configuration. Specifically, in the ink WB-1 (ink according to Comparative Example 1), the average particle size of the resin particles was less than 500 nm. Then, the ink WB-1 could not have the desired whiteness and could not have the desired concealment.

In the ink WB-2 (ink according to Comparative Example 2), the average particle size of the resin particles was more than 1000 nm. Ink WB-2 was inferior in storage stability to each of inks WA-1 to WA-7.

In the ink WB-3 (ink according to Comparative Example 3), the monomer of the thermosetting resin had an aromatic ring in the molecule instead of AGDM. Ink WB-3 was inferior in physical stability and chemical stability as compared with each of inks WA-1 to WA-7.

In the ink WB-4 (ink according to Comparative Example 4), the content of the monomer of the thermosetting resin was less than 10% by mass. Ink WB-4 was inferior in physical stability and chemical stability as compared with each of inks WA-1 to WA-7.

In the ink WB-5 (ink according to Comparative Example 5), the polydispersity index of the resin particles was remarkably large as compared with each of the inks WA-1 to WA-7. The ink WB-5 was inferior in storage stability to each of the inks WA-1 to WA-7.

The present inventor has previously described <lightness (L ^*) and chromaticity (a ^*, b ^*) of the measurement method> in the measured ink WA-1~WA-7 each lightness (L ^*) is It has been confirmed that the above equation (1) is satisfied. Further, the present inventor has previously described <lightness (L ^*) and chromaticity (a ^*, b ^*) of the measurement method> in the measured ink WA-1~WA-7 of each chromaticity (a ^*, It ^{has been confirmed that b *} ) satisfies the above equations (2) and (3). That is, the present inventor has confirmed that each of the inks WA-1 to WA-7 is a white ink.

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

Claims (2)

It contains an aqueous medium and a plurality of resin particles dispersed in the aqueous medium.
The average particle size of the resin particles measured based on the method described in ISO 13321: 1996 is 500 nm or more and 1000 nm or less.
Each of the resin particles contains a unit derived from a monomer of a thermoplastic resin and a unit derived from a monomer of a thermosetting resin.
In each of the resin particles, the unit derived from the monomer of the thermoplastic resin is crosslinked with the unit derived from the monomer of the thermosetting resin.
The content of the monomer of the thermosetting resin is 10% by mass or more and 20% by mass or less with respect to the total content of the monomer of the thermosetting resin and the monomer of the thermoplastic resin.
Monomers of the thermosetting resin, Ri dodecamethylene glycol dimethacrylate der,
The monomer of the thermoplastic resin is a white ink for inkjet recording containing methyl methacrylate. The white ink for inkjet recording according to claim 1, wherein the polydispersity index of the resin particles is 0.25 or less.