JP2013006890A - Water pigment dispersion for inkjet recording, water based ink for inkjet recording and method of manufacturing water pigment dispersion for inkjet recording - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide water based ink for inkjet recording which uses carbon black in which the number of coarse particles in a water pigment dispersion or the water based ink for inkjet recording is remarkably reduced and the discharge stability of the water based inke for inkjet recording is improved and to provide the water pigment dispersion for inkjet recording.SOLUTION: The water pigment dispersion for inkjet is prepared by mixing and stirring a pigment dispersion containing carbon black and a copolymer having an anion group and solid at normal temperature with a water based medium, wherein the ash content in the carbon black is ≤1.0 mass% and the ratio A/B of DBP oil absorption A (ml/100g) to BET specific surface area B (m/g) is ≤0.75.

Description

  The present invention relates to an aqueous pigment dispersion for inkjet recording, an aqueous ink for inkjet recording, and a method for producing the aqueous pigment dispersion for aqueous inkjet recording using carbon black.

Water-based ink for ink-jet recording has excellent characteristics such as oil-based inks that have no fire hazard and mutagenic toxicity, and can be greatly reduced. Has become the mainstream.
Conventionally, as a water-based ink for ink jet recording, a dye has been used as a colorant because it has high dissolution stability, has little nozzle clogging, has good color development, and enables high-quality printing. . However, there has been a problem that the use of dyes is inferior in water resistance and light resistance of images. Furthermore, recently, excellent water resistance, light resistance and long-term image quality stability have been demanded for home use, and in each application, conversion of colorants from dyes to pigments has been actively promoted. The ink using the pigment can be expected to have excellent water resistance and light resistance, but on the other hand, nozzle clogging due to sedimentation is likely to occur due to the remaining coarse particles of the pigment and the occurrence of aggregation. Therefore, various methods for dispersing a pigment in an aqueous medium using a polymer-based dispersant have been studied. For example, a mixture containing a pigment, a polymer dispersant, a wetting agent, and a basic compound and having a high solid content ratio. The pigment is dispersed by introducing a kneading step for kneading (see Patent Document 1). In this step, the dispersion is improved and the number of coarse particles is reduced by making the particles fine by pulverizing the pigment and preventing aggregation by coating the resin on the surface of the finely divided pigment.

Currently, the inkjet recording method is in the process of gradually replacing the printing field where conventional printing methods have been used, and in the process, continuous printing characteristics (ejection stability) and high-speed printing characteristics will be improved. It has become important. In particular, the coarse particles present in the ink cause clogging of the ink flow path, etc. Therefore, it is extremely important to improve the dispersion stability and reduce the number of coarse particles to improve the ejection stability. Yes. Above all, the amount of black ink used for text printing is much larger than that of color ink, and problems such as ink channel blockage become fatal defects in the product. Since carbon black is mainly used as the color material for black ink, the water-based ink for ink-jet recording using carbon black and the water-based pigment dispersion for ink-jet recording used for the production of the water-based ink are coarse. Various attempts have been made to reduce particles.
For example, a recording liquid in which the ratio of carbon black having a particle size of 0.6 μm or more to the total carbon black is 20% or less has been studied (see Patent Document 2). The number of coarse particles of carbon black is not sufficiently reduced.
In addition, a pigment dispersion for ink-jet recording characterized in that the number of coarse particles having a particle diameter of 0.5 μm or more is 1,000,000 / 5 μL or less has been studied (see Patent Document 3). Is only a dispersion method using beads, the carbon black surface may not be sufficiently coated with a resin, and there is a problem in dispersion stability in practice.
By dispersing for a long time with a dispersing device using media, the pigment particles of the ink for ink jet recording are miniaturized and the dispersed particle size is reduced. For this reason, the number of coarse particles can be reduced. However, the dispersion resin to be coated with carbon black tends to be released into the aqueous medium by the long-time dispersion. For this reason, viscosity increase and pigment aggregation tend to occur, and dispersion stability tends to decrease.
On the other hand, as for carbon black, carbon black having various chemical properties, physical properties, and shape properties suitable for various properties required for water-based ink for ink jet recording is used. However, when reducing the number of coarse particles is a problem, sufficient consideration has not necessarily been made on the selection of carbon black as a raw material from the viewpoint of solving the problem. In particular, an aqueous pigment dispersion for ink-jet recording prepared by dispersing a pigment dispersion that is solid at room temperature and then dispersing the pigment dispersion in an aqueous medium, or an aqueous ink for ink-jet recording prepared from the aqueous pigment dispersion. In contrast, selection of carbon black that is effective in suppressing coarse particles has not been sufficiently studied.
For example, with regard to the ash content of carbon black, the use of carbon black having an ash content of 0.05% by weight or less as a recording liquid is considered to reduce the precipitation of impurities on the electrode plate, which is a heating means of a heating type ink jet printer. However, the relationship with the number of coarse particles in the recording liquid has not been studied at all.
Or about DBP oil absorption and BET specific surface area, the value of (BET specific surface area (m < ² > / g) / DBP oil absorption (ml / 100g)) is 0.3-2.5, and OD value is used. However, the relationship with the number of coarse particles in the ink composition has not been studied at all.
In addition, the inks described in the cited documents 4 and 5 are all manufactured by a dispersing device using a bead mill, and carbon black, like the aqueous pigment dispersion for inkjet recording and the aqueous ink for inkjet recording in the present invention, is used. And a pigment dispersion which is solid at room temperature containing a resin having an anionic group, and is not produced through this.
Thus, the physical property values of carbon black are considered in relation to the number of coarse particles in an aqueous pigment dispersion for ink jet recording produced by dispersing these carbon blacks in an aqueous medium and the aqueous ink for ink jet recording. It has never been done before. In addition, with respect to an aqueous pigment dispersion for ink-jet recording and ink prepared by producing a solid pigment dispersion at room temperature, little studies have been made for the purpose of reducing the number of coarse particles.

JP2002-350377 JP-A-8-151546 JP 2010-261028 JP-A-10-081842 JP2007-056168

When the number of coarse particles in the aqueous pigment dispersion increases, the coarse particles are mixed into the final product, the aqueous ink for inkjet recording, unless the specific filtration step or centrifugal separation step is performed. When applied to, it causes a significant deterioration in properties. When the number of coarse particles increases, there is a possibility that the ink flow path may be blocked when the water-based ink for inkjet recording is adjusted and printed by the inkjet printer. In particular, when applied to thermal jet ink jet recording, coarse particles agglomerate and precipitate on the heater, degrading the heater function. Alternatively, there is a possibility that the nozzle portion is clogged by the precipitate thus deposited. Even if it is possible to remove these coarse particles by filtration or centrifugation in the ink-making process of the water-based ink for inkjet recording, it is difficult to selectively remove only the coarse particles, and these coarse particles As the number of coarse particles contained increases, the time spent for the removal of the process increases the time required for the removal, resulting in a significant reduction in production efficiency and production yield.
The problem to be solved by the present invention is a water-based ink for ink-jet recording using carbon black, and a water-based pigment dispersion for ink-jet recording for producing the water-based ink. The number of coarse particles in the water-based ink is greatly reduced, the storage stability and ejection stability of the water-based ink for inkjet recording are improved, and the production efficiency and production yield for producing the water-based ink are remarkably improved. It is to provide a water-based ink for inkjet recording, an aqueous pigment dispersion for inkjet recording, and a method for producing the aqueous pigment dispersion for inkjet recording.

The inventors knead a mixture containing carbon black and a copolymer having an anionic group to prepare a pigment dispersion, and the pigment dispersion is mixed in an aqueous medium and stirred to obtain an aqueous pigment dispersion for inkjet recording. As a result of studying various carbon blacks having different chemical characteristics and physical and shape characteristics, the present invention was found.
That is, the present invention is an aqueous pigment dispersion for inkjet recording obtained by mixing and stirring a solid pigment dispersion containing carbon black and a copolymer having an anionic group in an aqueous medium, The ash content of the carbon black is 1.0% by mass or less, and the ratio A / B between the DBP oil absorption amount A (ml / 100 g) and the BET specific surface area B (m ² / g) is 0.75 or less. An aqueous pigment dispersion for inkjet recording, and an ink composition for inkjet recording containing the aqueous pigment dispersion for inkjet recording are provided.
The present invention also includes a kneading step of kneading a mixture containing carbon black and a copolymer having an anionic group to produce a solid pigment dispersion at room temperature, and mixing the pigment dispersion in an aqueous medium. The carbon black has an ash content of 1.0% by mass or less, and the ratio A / B between the DBP oil absorption A (ml / 100 g) and the BET specific surface area B (m ² / g) is 0. The manufacturing method of the aqueous | water-based pigment dispersion for inkjet recording characterized by being below .75 is provided.

Carbon black is selected in advance based on the ash content described in the present invention and the ratio A / B between the DBP oil absorption A (ml / 100 g) and the specific surface area B (m ² / g), and then the kneading described in the present invention. By producing a water-based pigment dispersion for inkjet recording by a production process having a process, an aqueous pigment dispersion having a small number of coarse particles and an ink composition for ink-jet recording are obtained without accompanying a quality decrease such as an increase in viscosity. Therefore, production efficiency and production yield can be remarkably improved.

  The aqueous pigment dispersion for ink-jet recording of the present invention is produced by mixing and stirring a solid pigment dispersion containing carbon black and a copolymer having an anionic group in an aqueous medium. It is possible to greatly reduce the number of particles having a particle size of 0.5 μm or more in the dispersion. For this reason, when an ink composition for ink jet recording is prepared, there is very little possibility that coarse particles will settle even after long-term storage, and there is a risk of nozzle clogging even if continuous ejection is performed for a long time. Is low. Furthermore, the method for producing an aqueous pigment dispersion for inkjet recording of the present invention comprises a kneading step of kneading a mixture containing carbon black and a copolymer having an anionic group to produce a solid pigment dispersion at room temperature, It has a mixing step of mixing and stirring the pigment dispersion in an aqueous medium, and the number of coarse particles having a particle diameter of 0.5 μm or more in the aqueous pigment dispersion can be efficiently reduced in a very short time. Therefore, it is possible to produce an aqueous ink composition for ink jet recording from the aqueous pigment dispersion with high yield and production efficiency.

Coarse particles of 0.5 μm or more with respect to the ratio A / B of the DBP oil absorption A (ml / 100 g) and the BET specific surface area B (m ² / g) in the aqueous pigment dispersions prepared in Examples and Comparative Examples It is the figure which plotted the number.

  Hereinafter, various raw materials used in the production method of the present invention will be described in detail, and further, a production method using these raw materials will be described in detail.

The aqueous pigment dispersion for inkjet recording of the present invention is produced by mixing a solid pigment dispersion at room temperature containing carbon black having specific performance and a copolymer having an anionic group in an aqueous medium and stirring. An aqueous pigment dispersion for inkjet recording.
The characteristics of the carbon black used in the present invention will be described below.
The carbon black used in the present invention has an ash content of 1.0% by mass or less, and a ratio A / B of DBP oil absorption A (ml / 100 g) to BET specific surface area B (m ² / g) is 0.75 or less. It is characterized by being.

The ash content of the carbon black used in the present invention is 1.0% by mass or less, and more preferably 0.2% by mass or less. When the ash content of carbon black exceeded 1.0% by mass, a solid pigment dispersion containing carbon black and a copolymer having an anionic group at room temperature was mixed and stirred in an aqueous medium. The aqueous pigment dispersion tends to contain many carbon black particles having a particle size of 0.5 μm or more. The reason why the ash content in the carbon black thus affects the number of coarse particles in the aqueous pigment dispersion for inkjet recording is not necessarily clear. Usually, in the step of kneading the mixture containing carbon black and a copolymer having an anionic group, the coarse particles of carbon black are crushed and the surface of carbon black that has become fine particles is coated with a copolymer resin. Although it progresses, it is considered that the ash content on the surface of the carbon black has an influence on the resin coating on the surface, and aggregates on the pigment surfaces where the coating is insufficient, resulting in coarse particles in the subsequent manufacturing process. It is also conceivable that the ash present separately from the pigment particles on the surface of the pigment functions as an adhesive that binds the pigment particles to facilitate the formation of coarse particles.
In addition, as a measured value of the ash content in this invention, the value measured by the method based on JISK6128-2 is used.

In the carbon black used in the present invention, the ratio A / B between the DBP oil absorption amount A (ml / 100 g) and the BET specific surface area B (m ² / g) is 0.75 or less, but 0.5 or less. More preferably. Each value of DBP oil absorption A (ml / 100 g) and BET specific surface area B (m ² / g) of carbon black is not particularly limited, but the DBP oil absorption is usually preferably 40 ml / 100 g or more. 60 to 200 ml / 100 g is more preferable, and 80 to 150 ml / 100 g is more preferable. Although the BET specific surface area B ^(m 2 / g) is of conventional ^{30 m} 2 / g or more are preferably used, it is more preferable to use those 50~700m ² / g, the 100~600m ² / g More preferred is the use of
The reason why the A / B value affects the number of coarse particles in the aqueous pigment dispersion for inkjet recording or the aqueous ink for inkjet recording is not necessarily clear. However, since the DBP oil absorption is mainly related to the structure structure, the A / B calculated as the DBP oil absorption per unit BET specific surface area is the development of the network structure due to the formation of the structure. It is thought that the expression is made without being influenced so much by the particle size of the carbon black that is composed. In a kneading process in which a mixture containing carbon black and a copolymer resin is kneaded to produce a solid pigment dispersion at room temperature, the carbon black approaches and pressurizes under a high shearing force. Since the entanglement between the carbon blacks is promoted, the particles are not easily loosened. As a result, it is considered that the coarse particles of carbon black fall into a state where it is difficult to be crushed, and the carbon black surface is not sufficiently covered with the copolymer resin and aggregates into coarse particles.
In addition, in this invention, the value measured by the method based on JISK6217-4 is used for DBP oil absorption, and the value measured by the method based on JISK6217-2 is used for BET specific surface area.

The primary particle size of carbon black is usually preferably 60 nm or less, more preferably 40 nm or less, and still more preferably 20 nm or less. When carbon black having a primary particle size of less than 60 nm is used, the storage stability of the dispersion tends to be maintained well. On the other hand, the primary particle diameter of carbon black is preferably 5 nm or more. When the primary particle diameter is less than 5 nm, dispersion in an aqueous dispersion tends to be difficult. The primary particle diameter is indicated using an arithmetic average diameter of 30 values obtained by actually measuring the particle diameter by image processing from an electron micrograph.
Moreover, 3-11 are preferable and, as for pH of carbon black, 6-9 are more preferable. When carbon black having a volatile content and pH within the above ranges is used, the storage stability of the dispersion tends to be maintained well. Here, pH is a value measured by a method based on the old JIS standard JIS K6221.

  The pigment dispersion used in the aqueous pigment dispersion for inkjet recording and the aqueous ink for inkjet recording in the present invention contains carbon black having the above characteristics and a resin having an anionic group.

  The resin having an anionic group used in the present invention is preferably one that performs good adsorption and coating on the surface of carbon black in the kneading step performed by adding carbon black, and is stable in an aqueous medium after the mixing step. It is preferable to maintain the pigment surface coating and to have a stable water dispersibility by neutralizing the acid group with a basic compound. Therefore, it is preferable to use an acid value of 50 to 300 mgKOH / g. When the acid value is less than 50 mgKOH / g, the hydrophilicity is decreased and the dispersion stability of the pigment tends to be lowered. On the other hand, if the acid value is greater than 300 mgKOH / g, pigment aggregation tends to occur, and the water resistance of the water-based ink for inkjet recording using this dispersion tends to decrease. The acid value of the resin is more preferably 60 to 250 mgKOH / g, and still more preferably 70 to 200 mgKOH / g.

  The weight average molecular weight of the resin having an anionic group used in the present invention is preferably in the range of 7500 to 40000, and more preferably in the range of 7500 to 30000. Especially, it is especially preferable that it exists in the range of 10000-25000. When the weight average molecular weight is less than 7500, the storage stability of the aqueous pigment dispersion for inkjet recording tends to deteriorate, and precipitation due to aggregation of the pigment may occur. On the other hand, when the weight average molecular weight of the resin having an anionic group exceeds 40,000, the viscosity of the ink composition prepared from the aqueous pigment dispersion for inkjet recording using the resin increases, and the ejection stability of the ink is unstable. Tend to be. The weight average molecular weight of the styrene resin used in the present invention is a value measured by a gel permeation chromatogram (GPC) method and is a value converted to the molecular weight of polystyrene used as a standard substance.

Specific examples of the resin having an anionic group include styrene- (meth) acrylic ester- (meth) acrylic acid resin, styrene- (meth) acrylic acid resin, styrene-sulfonic acid resin, and vinylnaphthalene- (Meth) acrylic ester- (meth) acrylic acid resin, vinylnaphthalene- (meth) acrylic acid resin, (meth) acrylic ester- (meth) acrylic resin, (meth) acrylic resin, alkenyl ether Resin, pyrrolidone resin, urethane resin, urea resin, amide resin, ester resin, carbonate resin, ether resin, epoxy resin, alcohol resin, etc. Can be used.
Among these resins, a styrene (meth) acrylic acid resin having an anionic group due to an unsaturated aliphatic carboxylic acid having a double bond and containing a styrene monomer as a constituent component of the hydrophobic portion is preferable. In addition, in this specification, what is described as (meth) acryl shall mean methacryl and / or acrylic. Among the styrene (meth) acrylic acid resins, a monomer further containing 60% by mass or more of a styrene monomer and an unsaturated aliphatic carboxylic acid having a radical polymerizable double bond with respect to all monomer components, A copolymerized styrene acrylic acid resin is preferred. Furthermore, as described above, the styrene acrylic acid resin is preferably a resin having an acid value of 50 to 300 and a weight average molecular weight of 7500 to 40,000.

  A known compound can be used as the styrene monomer. For example, alkyl styrene such as styrene, α-methyl styrene, β-methyl styrene, 2,4-dimethyl styrene, α-ethyl styrene, α-butyl styrene, α-hexyl styrene, 4-chlorostyrene, 3-chlorostyrene, There are halogenated styrenes such as 3-bromostyrene, 3-nitrostyrene, 4-methoxystyrene, vinyltoluene and the like.

  The use ratio of the styrene monomer in all the constituent monomer units constituting the styrene acrylic resin is more preferably 60 to 90% by mass, and particularly preferably 70 to 90% by mass. When the use ratio of the styrene monomer is less than 60% by mass, the affinity of the styrene resin to the carbon black becomes insufficient, and the adsorption of the styrene acrylic resin to the carbon black surface in the kneading process becomes insufficient. A solid pigment dispersion cannot be produced at a good normal temperature. For this reason, the dispersion stability of the aqueous pigment dispersion for inkjet recording tends to decrease. Moreover, since the content of monomer units having an anionic group contributing to dispersion can be ensured when the total of styrene monomer units is 90% by mass or less of the total of all monomer units, the dispersion stability in an aqueous system, The risk of deterioration of long-term storage stability can be prevented. When the amount of the styrene monomer is within the above range, the dispersibility of the styrene acrylic resin in the aqueous medium can be improved, and the dispersibility and dispersion stability of the pigment in the aqueous pigment dispersion for inkjet recording can be improved. be able to.

  As the unsaturated aliphatic carboxylic acid having a radical polymerizable double bond to be copolymerized with a styrene monomer, a known compound can be used. For example, acrylic acid, methacrylic acid, α-ethylacrylic acid, crotonic acid, α-methylcrotonic acid, α-ethylcrotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid , Etc. Among them, it is preferable to use acrylic acid and methacrylic acid, and it is particularly preferable to use both in combination. By using acrylic acid and methacrylic acid together, the copolymerization at the time of resin synthesis is improved and the uniformity of the resin is improved. As a result, there is a tendency to obtain an aqueous pigment dispersion for inkjet recording having good storage stability.

  As the styrene acrylic resin, known monomers other than the styrene monomer and the unsaturated aliphatic carboxylic acid having a radical polymerizable double bond can be used. Examples of such monomers include methyl acrylate, methyl methacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, sec-butyl acrylate, tert-butyl acrylate, 2-ethylbutyl acrylate, 1,3-dimethylbutyl. Acrylic esters and methacrylic such as acrylate, hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, ethyl methacrylate, n-butyl methacrylate, 2-methylbutyl methacrylate, pentyl methacrylate, heptyl methacrylate, nonyl methacrylate Acid esters; 3-ethoxypropyl acrylate, 3-ethoxybutyl acrylate, dimethylaminoethyl acrylate, 2 Acrylic acid ester derivatives and methacrylic acid ester derivatives such as hydroxyethyl acrylate, 2-hydroxybutyl acrylate, ethyl-α- (hydroxymethyl) acrylate, dimethylaminoethyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate; phenyl acrylate Acrylic acid aryl esters and acrylic acid aralkyl esters such as benzyl acrylate, phenylethyl acrylate, phenylethyl methacrylate; monoacrylic acid of polyhydric alcohols such as diethylene glycol, triethylene glycol, polyethylene glycol, glycerin, bisphenol A Esters or monomethacrylates; dimethyl maleate, malee Mention may be made of maleic acid dialkyl esters such as diethyl acid, vinyl acetate and the like. One or more of these monomers can be added as monomer components.

  As a method for producing a styrene acrylic acid resin, a usual polymerization method can be adopted, and examples thereof include a method of performing a polymerization reaction in the presence of a polymerization catalyst such as solution polymerization, suspension polymerization, bulk polymerization and the like. Examples of the polymerization catalyst include 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobisisobutyronitrile, 1,1′-azobis (cyclohexane-1-carbonitrile), Examples thereof include benzoyl peroxide, dibutyl peroxide, butyl peroxybenzoate, and the amount used is preferably 0.1 to 10.0% by mass of the vinyl monomer component.

  The glass transition point of the styrenic resin used in the present invention is preferably 90 ° C. or higher. More preferably, it is 100 ° C. or higher and 150 ° C. or lower. When the glass transition point is 90 ° C. or higher, the thermal stability of the ink composition is improved. For this reason, even if the water-based ink composition for ink-jet recording prepared from the water-based pigment dispersion is used for thermal jet type ink-jet recording, it does not cause characteristic changes that cause defective discharge due to repeated heating. The glass transition point of the styrene resin used in the present invention is a value measured with a differential scanning calorimeter.

  The styrene acrylic acid resin used in the present invention may be a random copolymer as described above, but may also be a graft copolymer. As the graft copolymer, a copolymer of polystyrene or a nonionic monomer copolymerizable with styrene and styrene is used as a trunk or branch, and a copolymer of acrylic acid, methacrylic acid and other monomers including styrene is branched. Or the graft copolymer used as a trunk can be shown as the example. The styrene resin may be a mixture of the graft copolymer and the random copolymer.

  The pigment dispersion used in the present invention preferably contains a basic compound. The basic compound is for neutralizing the anionic group in the copolymer having an anionic group, and any known and conventional one can be used, for example, sodium hydroxide, potassium hydroxide, An inorganic basic substance such as ammonia or an organic basic substance such as triethylamine or alkanolamine can be used. Among them, it is preferable to use an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide or lithium hydroxide which has excellent thermal stability and is odorless. In particular, potassium hydroxide is preferred. Moreover, the addition amount of the basic compound is an amount corresponding to 0.8 to 1.2 times the amount necessary for neutralizing all anionic groups such as carboxyl groups of the copolymer having an anionic group. It is preferable that

  The basic compound is preferably added as an aqueous solution or an organic solvent solution. In this case, the concentration of the basic compound aqueous solution or the organic solvent solution is preferably 20% by mass to 50% by mass. Moreover, it is preferable to use alcohol solvents, such as methanol, ethanol, isopropanol, as a solvent which melt | dissolves a basic compound. Among these, in the present invention, it is preferable to use an aqueous solution.

  A water-soluble organic solvent as a wetting agent may be used in the aqueous medium used in the mixing step with respect to the carbon black solid pigment dispersion medium used in the present invention. Moreover, when performing a kneading | mixing process so that it may mention later, it is preferable to make a pigment dispersion which is a to-be-kneaded material contain a wetting agent. Known wetting agents can be used as the wetting agent used, such as glycerin, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,3-propanediol, Polyols such as 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2,6-hexanetriol, trimethylolpropane, pentaerythritol, 2-pyrrolidone, N-methyl-2 -Lactams such as pyrrolidone and ε-caprolactam, 1,3-dimethylimidazolidinone and the like.

  The above-mentioned wetting agent that can be used in the present invention does not have a strong dissolving power for a copolymer having an anionic group such as a styrene acrylic resin, and the wetting agent and the anionic group have a resin concentration of 25% by mass. A copolymer that does not become a homogeneous solution when the copolymer having s is stirred is preferred. Further, when 5 wt% or more of the wetting agent remains in the water-based ink composition for inkjet recording, it is preferable that the characteristics of the water-based ink composition are not deteriorated. The addition amount of the wetting agent is preferably 60 to 200 parts by mass with respect to 100 parts by mass of carbon black.

  In the aqueous pigment dispersion for inkjet recording of the present invention, the content of the copolymer having an anionic group with respect to 100 parts by mass of carbon black is preferably 10 to 50 parts by mass. More preferably. When the content of the styrene resin is less than 10 parts by mass, the dispersion stability of the pigment dispersion for inkjet recording is lowered, and the friction resistance of the image formed by the ink composition using the dispersion is lowered. When the amount exceeds 50 parts by mass, it is recognized that the viscosity of the liquid when ink for inkjet recording is produced tends to increase.

  Using the raw materials described above, a solid pigment dispersion is produced at room temperature, and the pigment dispersion is mixed and stirred in an aqueous medium to produce an aqueous pigment dispersion for inkjet recording or an aqueous inkjet recording water according to the present invention. The method for producing the ink will be described in detail below.

  In order to produce the aqueous pigment dispersion for ink-jet recording of the present invention, the carbon black described above, a copolymer having an anionic group, and a solid compound at room temperature containing a basic compound and / or a wetting agent as necessary. A pigment dispersion is prepared and the pigment dispersion is dispersed in an aqueous medium. In order to prepare the pigment dispersion, carbon black, a resin having an anionic group, and if necessary, a mixture containing a basic compound and a wetting agent are kneaded to form a solid colored kneaded solid pigment at room temperature. It can be carried out by a kneading step for producing a dispersion. The pigment dispersion can be dispersed in the aqueous medium through a mixing step of mixing and stirring the pigment dispersion, which is a solid colored kneaded product, in the aqueous medium. When kneading, it is preferable to knead the mixture under a high shearing force. By kneading under a high shearing force, the granular carbon black is pulverized, and the adsorption of the styrenic resin proceeds on the surface, whereby a pigment dispersion as a uniform solid colored kneaded product can be obtained.

  It is preferable that the solid content concentration of the pigment dispersion after completion of the kneading step dispersed in the aqueous medium is 40 to 80% by mass. Further, the solid content concentration in the pigment dispersion which is the colored kneaded product for inkjet ink in the kneading step is also preferably 40 to 80% by mass, and it is preferable to have a kneading step that proceeds at such a solid content concentration. . The solid content concentration of the pigment dispersion dispersed in the aqueous medium after completion of the kneading step is more preferably 60 to 80% by mass, and the colored kneaded product in the kneading step also has a solid content concentration of 60 to 80% by mass. It is more preferable that By setting the solid content concentration in such a range, a sufficient shearing force can be imparted to the kneaded product, and the kneaded product that is a uniform pigment dispersion can be obtained without insufficient pulverization of carbon black. be able to. The temperature at the time of kneading can be appropriately adjusted in consideration of the temperature characteristics of the styrenic resin so that a sufficient shearing force is applied to the kneaded product, but is lower than the glass transition point of the styrenic resin, and It is preferable that the temperature difference from the glass transition point is within a range smaller than 50 ° C. By kneading in such a temperature range, there is no shortage of shearing force due to a decrease in the viscosity of the kneaded product accompanying an increase in the kneading temperature, and solid content concentration is unlikely to increase due to evaporation of the liquid component in the kneaded product. Therefore, the kneaded material that is the pigment dispersion after the kneading is not a kneaded material having a very high solid content ratio. For this reason, it is not difficult to disperse the pigment dispersion in the aqueous medium.

  When producing a colored kneaded product for inkjet ink, it is preferable to use a closed type kneader having a stirring tank rather than using an open type kneader having no stirring tank such as a two-roll roll. When such a kneader is used, it is easy to prepare an aqueous pigment dispersion for inkjet recording through a mixing step of mixing and stirring the kneaded product as a pigment dispersion in an aqueous medium. Can be directly diluted in a kneader to produce an aqueous pigment dispersion for inkjet recording. A planetary mixer is preferable as a specific example of a kneader having a stirring tank. Planetary mixers can be kneaded in a wider range of viscosity than a two-roller, etc., and can also be added with an aqueous medium and distilled under reduced pressure, so the viscosity and load shear force during kneading can be adjusted. Is easy.

The aqueous medium used for dispersing and diluting the pigment dispersion in the aqueous medium by mixing and stirring the pigment dispersion, which is a colored kneaded product for inkjet ink, is an aqueous pigment dispersion for inkjet recording. A water-soluble organic solvent as a wetting agent may be included in order to prevent drying of the liquid and to adjust the viscosity at the time of carrying out the dispersion treatment, and the amount is combined with the wetting agent in the colored kneaded material for inkjet ink, The amount in the aqueous recording pigment dispersion is preferably 3 to 50% by mass, and more preferably 5 to 40% by mass. If it is less than this lower limit, the drying prevention effect tends to be insufficient, and if it exceeds the above upper limit, the dispersion stability of the dispersion tends to decrease. The wetting agent used in the aqueous medium when the kneaded product is produced and when it is diluted may be the same or different.
The aqueous medium is preferably added to the pigment dispersion in the mixing step continuously or intermittently while stirring the pigment dispersion to gradually reduce the viscosity of the pigment dispersion into a liquid state. In the mixing step involving the addition of such an aqueous medium, the aqueous dispersion medium is gradually added to the pigment dispersion in the same apparatus as that used in the kneading process with respect to the pigment dispersion produced in the kneading process. It can also be carried out by continuing the kneading operation, which is preferable in terms of production efficiency and production yield.

  An aqueous pigment dispersion for inkjet recording obtained by diluting and dispersing a pigment dispersion, which is a colored kneaded product for inkjet ink after kneading, in an aqueous medium through a step of mixing and stirring is further provided in a disperser. Thus, distributed processing can be performed. By performing the dispersion treatment, coarsely dispersed particles in the aqueous pigment dispersion for inkjet recording are further pulverized, and blockage of the ink flow path can be suppressed.

Examples of the disperser used in the dispersion treatment include a homogenizer, a high-pressure homogenizer, a paint shaker, a ball mill, a roll mill, a sand mill, a sand grinder, a dyno mill, a disper mat, a nano mill, an SC mill, and a nanomizer. One may be used alone, or two or more kinds of devices may be used in combination.
The pigment concentration of the aqueous pigment dispersion for inkjet recording thus prepared is preferably a high pigment concentration in consideration of the degree of freedom when adjusting the aqueous ink for inkjet recording, and 10 to 30 from the balance with dispersion stability. A pigment concentration of mass% is preferred.

In order to produce an aqueous ink for inkjet recording from the aqueous pigment dispersion prepared as described above, the aqueous pigment dispersion is further diluted with an aqueous medium, the necessary additive components are added, and the components are added. Can be adjusted. As the ink for ink jet recording, the pigment concentration is preferably about 2 to 10% by mass, and the pigment concentration is adjusted within this range.
The aqueous medium for diluting the aqueous pigment dispersion may be water alone, but in the ink for ink jet recording, a water-soluble organic solvent capable of exhibiting various functions such as drying prevention, viscosity adjustment, concentration adjustment may be blended. it can. As the water-soluble organic solvent, the same water-soluble organic solvent as that used for dispersing the above-mentioned kneaded product for an aqueous pigment dispersion can be added and used. In addition, it is preferable that a water-soluble organic solvent exhibiting permeability to the recording medium is blended to impart permeability to the pigment ink.
In the step of making an ink for finally producing ink, the number of coarse particles in the aqueous pigment dispersion can be further reduced by a coarse particle removal step such as filtration or centrifugation. However, the size of the coarse particles before the process directly affects the burden on the process and appears as a difference in production efficiency, and the relationship between the coarse particles before the process is maintained as it is after the process. This is reflected in the characteristics of the final ink for ink jet recording.
In addition to the aqueous medium and the aqueous pigment dispersion, for example, a known additive can be blended in the inkjet recording ink which is a pigment ink. As what can be mix | blended, an alkali agent, pH adjuster, surfactant, antiseptic | preservative, a chelating agent, a plasticizer, antioxidant, a ultraviolet absorber, an ultraviolet curable resin etc. can be illustrated, for example.
The ink for ink jet recording produced in this way is not particularly limited depending on the ink jet method to be applied, and is a continuous jet type (charge control type, spray type, etc.), on-demand type (piezo type, thermal type, electrostatic type). The present invention can be applied to various types of ink jet recording apparatuses such as a suction method. In particular, when applied to a thermal ink jet recording apparatus, in addition to dispersion stability and storage stability, the occurrence of kogation can be suppressed over a long period of time, and extremely stable ink ejection can be achieved. In addition, since the coarse particles are greatly reduced, a great effect is exhibited in an application field that requires long-term continuous stable discharge.
Furthermore, pigment settling does not occur even during long-term storage, and maintenance of the ink jet printer can be reduced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by a following example, unless the summary is exceeded. In the following examples, “parts” and “%” are based on mass unless otherwise specified.

<Styrene acrylic resin>
The styrene acrylic resin A used in this example is a powdery resin (diameter of 1 mm or less) prepared by solution polymerization. In the monomer composition ratio, styrene / acrylic acid / methacrylic acid = 77/10/13 ( Mass ratio), a molecular weight of 11,000, an acid value of 152 mgKOH / g, and a glass transition point of 107 ° C.
The weight average molecular weight in the present invention is a value measured by a GPC (gel / permeation / chromatography) method, and is a value converted to the molecular weight of polystyrene used as a standard substance. The measurement was carried out with the following equipment and conditions.
Liquid feed pump: LC-9A
System controller: SLC-6B
Auto injector: S1L-6B
Detector: RID-6A
Data processing software manufactured by Shimadzu Corporation: Sic480II data station (manufactured by System Instruments).
Column: GL-R400 (guard column) + GL-R440 + GL-R450 + GL-R400M (manufactured by Hitachi Chemical Co., Ltd.)
Elution solvent: THF
Elution flow rate: 2 ml / min
Column temperature: 35 ° C

Example 1
A mixture having the following composition was charged into a planetary mixer PLM-50V (manufactured by Inoue Seisakusho Co., Ltd.) having a capacity of 50 L, the jacket was heated to 60 ° C., and the rotational speed was 60 rpm and the revolution speed was 20 rpm. The mixture was stirred to obtain a kneaded product of carbon black and styrene acrylic resin.
(A) Carbon black a 50 parts (b) Styrene acrylic resin A 15 parts (c) 8N-potassium oxide aqueous solution 7 parts (d) Diethylene glycol 30 parts Carbon black a has an ash content of 0.05% by mass and DBP oil absorption A Was 59 ml / 100 g, BET specific surface area B was 128 m ² / g, and A / B was 0.47. After confirming the formation of the kneaded product, the kneading was continued for 3.5 hours, and then the mixing process was started. The kneading was further continued while gradually adding ion-exchanged water heated to 60 ° C. Finally, 150 parts of ion exchange water was added to obtain a dispersion having a carbon black concentration of about 20%. In order to obtain the dispersion, about 5 hours were required after starting the stirring of the raw materials. The black color dispersion taken out from the planetary mixer was further diluted with ion-exchanged water to obtain an aqueous pigment dispersion having a carbon black concentration of 14.5% (kneading production method).

(Example 2)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black b was used instead of carbon black a and 50 parts of diethylene glycol was used. A liquid was obtained. Carbon black b had an ash content of 0.04% by mass, a DBP oil absorption A of 101 ml / 100 g, a BET specific surface area B of 258 m ² / g, and an A / B of 0.39.

(Example 3)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black c was used instead of carbon black a and 47 parts of diethylene glycol was used. A liquid was obtained. Carbon black c had an ash content of 0.40%, a DBP oil absorption A of 94 ml / 100 g, a BET specific surface area B of 371 m ² / g, and an A / B of 0.25.

Example 4
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black d was used instead of carbon black a and 35 parts of diethylene glycol was used. A liquid was obtained. Carbon black d had an ash content of 0.74 mass%, a DBP oil absorption A of 70 ml / 100 g, a BET specific surface area B of 292 m ² / g, and an A / B of 0.24.

(Example 5)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black e was used instead of carbon black a and 35 parts of diethylene glycol was used. A liquid was obtained. Carbon black e had an ash content of 0.04% by mass, a DBP oil absorption A of 69 ml / 100 g, a BET specific surface area B of 108 m ² / g, and an A / B of 0.64.

(Comparative Example 1)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black f was used instead of carbon black a. Carbon black f had an ash content of 2.79% by mass, a DBP oil absorption A of 61 ml / 100 g, a BET specific surface area B of 302 m ² / g, and an A / B of 0.20.

(Comparative Example 2)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black g was used instead of carbon black a. Carbon black g had an ash content of 1.54% by mass, a DBP oil absorption A of 65 ml / 100 g, a BET specific surface area B of 320 m ² / g, and an A / B of 0.20.

(Comparative Example 3)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black h was used instead of carbon black a and 50 parts of diethylene glycol was used. A liquid was obtained. Carbon black h had an ash content of 0.01% by mass, a DBP oil absorption A of 96 ml / 100 g, a BET specific surface area B of 116 m ² / g, and an A / B of 0.83.

(Comparative Example 4)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black i was used instead of carbon black a and 65 parts of diethylene glycol was used. A liquid was obtained. Carbon black i had an ash content of 0.01% by mass, a DBP oil absorption A of 130 ml / 100 g, a BET specific surface area B of 146 m ² / g, and an A / B of 0.89.

(Comparative Example 5)
In Example 1, an aqueous pigment dispersion having a carbon black concentration of 14.5% was obtained in the same manner as in Example 1 except that carbon black j was used instead of carbon black a and 50 parts of diethylene glycol was used. A liquid was obtained. Carbon black g had an ash content of 0.01% by mass, a DBP oil absorption A of 102 ml / 100 g, a BET specific surface area B of 117 m ² / g, and an A / B of 0.87.

(Comparative Example 6)
A mixture having the following composition was placed in a 250 ml plastic container and stirred with a paint shaker for 4 hours together with zirconia beads having a diameter of 1.25 mm to obtain an aqueous pigment dispersion having a carbon black concentration of 14.5% (bead mill production method).
(A) Carbon black d 50 parts (b) Styrene acrylic resin A 15 parts (c) 8N-potassium oxide aqueous solution 7 parts (d) Diethylene glycol 35 parts (e) Ion exchanged water 238 parts

(Comparative Example 7)
In Comparative Example 6, carbon black was dispersed in the same manner as in Comparative Example 6 except that carbon black g was used instead of carbon black d. However, the dispersion of carbon black was insufficient, and an aqueous pigment dispersion could not be obtained.

The method for measuring the characteristics of the carbon black used in Examples 1 to 5 and Comparative Examples 1 to 7 and the measurement method for the measurement items performed on the aqueous pigment dispersions prepared in Examples and Comparative Examples are shown below. Table 1 shows the characteristics of the carbon black used in Examples and Comparative Examples measured by these measurement methods and the measurement results of the aqueous pigment dispersion.
(Measurement method of carbon black ash)
The ash content of carbon black was measured by a method based on JIS K6218-2. The unit of ash is mass%.

(Measurement of DBP oil absorption of carbon black)
The DBP oil absorption of carbon black was measured by a method based on JIS K6217-4. The unit of DBP oil absorption is “ml / 100 g”. In this example, the DBP oil absorption of carbon black is expressed as “A”.

(Measurement method of BET specific surface area of carbon black)
The BET specific surface area of carbon black was measured by a method based on JIS K6217-2. The unit of the BET specific surface area is “m ² / g”. In this example, the BET specific surface area of carbon black is expressed as “B”. The specific measurement method is as follows.
(1) A glass cell was filled with carbon black. At this time, the filling amount of carbon black was adjusted so that the total surface area of the object to be measured would be 1 to 50 m ² .
(2) The specific surface area of carbon black was measured by the BET method using a mountec BET specific surface area measuring device (Macsorb HM Model-1201).
(3) The measurement was performed twice and the average value of the measured values was defined as the BET specific surface area. At this time, it is necessary that the difference between the measured values does not exceed 10%. When the difference is exceeded, the measurement is further performed, and the measurement is repeated until the measured value does not exceed 10%.

(Calculation method of ratio A / B of DBP oil absorption A (ml / 100 g) and specific surface area B (m ² / g) of carbon black)
The measured value of DBP oil absorption measured by the above measuring method was divided by the measured value of BET specific surface area to calculate (A / B). In this example, the ratio between the DBP oil absorption A (ml / 100 g) and the specific surface area B (m ² / g) is expressed as “A / B”.

(Method for measuring the number of coarse particles in the dispersion)
Using a particle size measuring device (Accusizer 780 APS) manufactured by Particle Sizing Systems, the number of coarse particles having a diameter of 0.5 μm or more contained in the pigment dispersion diluted 500 to 1000 times with ion-exchanged water was measured. In the measurement, about 4 ml of a pigment dispersion is filled in an apparatus, and the number of particles passing through the measurement unit and the particle size are detected using optical means. At the time of measurement, the dilution ratio of the dispersion liquid and the weight of filling into the apparatus are input as parameters. By multiplying the obtained measurement result by the value of the dilution concentration, the number of particles contained in 1 ml of the pigment dispersion having a pigment concentration of 14.5% by mass produced in Examples and Comparative Examples was calculated, and this was calculated as the number of coarse particles. And The average value of the results obtained by performing the measurement twice was defined as the number of coarse particles. At this time, it is necessary that the difference between the measured values does not exceed 10%. When the difference is exceeded, the measurement is further performed, and the measurement is repeated until the measured value does not exceed 10%.

(Measuring method of average dispersed particle size of dispersion)
Using a Nikkiso Nanotrac particle size distribution analyzer (UPA150), the average dispersion particle size of the pigment dispersion diluted 2000 times with ion-exchanged water was measured. In the measurement, the particle diameter is measured by putting about 4 ml of the pigment dispersion into a measurement cell and detecting the scattered light of the laser beam. Parameters input at the time of measurement include viscosity (pigment dispersion viscosity after ion exchange water dilution), density of dispersed particles (true specific gravity of carbon black), and density of ion exchange water. The measurement temperature was 25 ° C. The volume average particle size (MV) obtained and displayed as a measurement result was defined as the value of the average dispersed particle size (unit: nm). The measurement was performed 3 times with a measurement time of 30 seconds, and the average value of the obtained results was determined and used as the average dispersed particle size described in Table 1 below.

(Storage stability test method for dispersion)
After the dispersion was sealed in a polypropylene container and stored at 60 ° C. for 4 weeks, the average dispersion particle size of the dispersion was measured by the same method as described above. The results were evaluated according to the following criteria.
○: Change rate of average dispersed particle size is within + 2% Δ: Change rate of average dispersed particle size is + 2% to 5%
X: Change rate of average dispersed particle size is + 5% or more

From Table 1 and FIG. 1, the ash content is lower than 1.0 mass%, and the ratio A / B of the DBP oil absorption A (ml / 100 g) and the specific surface area B (m ² / g) is 0.75 or less. It can be seen that the aqueous pigment dispersion for inkjet recording obtained by the production method using carbon black and having a kneading step has a small number of coarse particles (Examples 1 to 5). On the other hand, the aqueous pigment dispersion for inkjet recording using carbon black having an ash content higher than 1.0% by mass has a large number of coarse particles, and deterioration of storage stability is observed (Comparative Examples 1-2). Even if the ash content is lower than 1.0% by mass, the aqueous pigment dispersion for ink jet recording using carbon black having an “A / B” exceeding 0.75 has a large number of coarse particles and deteriorated storage stability. Is observed (Comparative Examples 3 to 5). In order to obtain an aqueous pigment dispersion for inkjet recording with a small number of coarse particles, the ratio A / B of ash, DBP oil absorption A (ml / 100 g) and specific surface area B (m ² / g) is determined according to the present invention. It is necessary to use carbon black that satisfies the stated conditions.
In FIG. 1, the three points showing a large value for the number of coarse particles in the region where A / B is small correspond to Comparative Examples 1, 2, and 6, which increase the number of coarse particles due to a large amount of ash. is doing.
However, the difference in the manufacturing method that the comparative example 6 uses the bead mill manufacturing method also causes the increase in the number of coarse particles. The reason why A / B shows a small value in these comparative examples is that carbon black having a small DBP oil absorption amount, that is, having suppressed structure growth, was used. Such carbon black uses a large amount of a modifier (alkali metal salt) to suppress the growth of the structure, and a large amount of the modifier remains as ash on the surface of the carbon black.

  Further, even when carbon black satisfying the above conditions is used, the aqueous pigment dispersion for inkjet recording obtained by the bead mill method has a low effect of reducing the number of coarse particles (Example 4, Comparative Example 6). When a dispersion treatment for a long time is performed to reduce the number of coarse particles by the bead mill method, the resin adsorbed on the pigment surface is peeled off, and conversely, the number of coarse particles increases and the viscosity of the dispersion increases. Further, carbon black outside the above range can be dispersed while covering with coarse particles in the kneading method, and an aqueous pigment dispersion can be prepared, but it cannot even be dispersed in the bead mill method (Comparative Example 7). For this reason, it is more effective to reduce the number of coarse particles more efficiently without increasing the viscosity of the dispersion by using the manufacturing method that has a kneading process to produce a solid pigment dispersion at room temperature described in the present invention. It can be seen that the storage stability of the dispersion can be ensured.

Further, the following tests were performed using the aqueous pigment dispersions prepared in Example 2 and Comparative Example 2.
(Discharge test method for water-based pigment ink for inkjet recording)
An aqueous pigment ink for ink jet recording having a pigment concentration of 3% with the following composition was prepared.
Aqueous pigment dispersion 20.7 parts 2-Pyrrolidinone 8 parts Triethylene glycol mono-n-butyl ether 8 parts Purified glycerin 3 parts Surfynol 440 (manufactured by Air Products) 0.5 parts Pure water 59.8 parts Prepared inkjet recording A water-based ink was filled into a black cartridge, and a nozzle check pattern was printed with an inkjet printer (EM-930C manufactured by EPSON). Furthermore, after printing a black solid pattern on one A4 sheet in monochrome mode, a nozzle check pattern was printed again. The defect state of the check pattern before and after the black solid pattern printing test was evaluated according to the following criteria.
○: Nozzle chipping does not increase Δ: Nozzle chipping increases by 1 to 5 or less ×: Nozzle chipping increases by 6 or more

When the ink discharge performance was evaluated by the discharge test method of the aqueous pigment ink for ink jet recording, for example, the evaluation result of the pigment dispersion liquid of Example 2 is ○, whereas the evaluation result of the pigment dispersion liquid of Comparative Example 2 is X. Therefore, when the number of coarse particles in the pigment dispersion liquid of Comparative Example 2 was reduced to the same level as in Example 2 by centrifugation and filtration, the evaluation result of the ejection test was “good”.
It can be seen that when the pigment dispersion in the comparative example is used, the ink ejection performance is reduced due to nozzle clogging due to coarse particles unless a coarse particle removal step such as specific filtration or centrifugation is introduced.

The aqueous pigment dispersion described in this example is stable by diluting with a diluent composed of a solvent, a surfactant, etc., without performing treatment for reducing coarse particles, particularly by centrifugation or filtration. A water-based pigment ink for ink-jet recording that can be ejected can be obtained, but by performing a coarse particle removal process such as specific filtration or centrifugal separation, the number of coarse particles in the aqueous pigment dispersion is reduced to achieve higher performance ink-jet recording. It can be used as a water-based pigment ink.
However, the carbon blacks listed in the examples and comparative examples have a clear difference in the time and yield required for the coarse particle removal step. For example, when the dispersions of Example 2 and Comparative Example 2 are treated using the same centrifuge, the number of coarse particles of the dispersion of Example 2 can be reduced to 500 × 10 ⁶ particles / ml in about 2 hours. In contrast, the dispersion of Comparative Example 2 could not reduce the number of coarse particles to 2000 × 10 ⁶ particles / ml or less even after treatment for about 4 hours. Further, when the change (yield) of the pigment weight in the dispersion before and after the centrifugal separation was compared, Example 2 was 95%, while Comparative Example 2 was 79%. Thereafter, both dispersion liquids were filtered with a filter having a rated accuracy of 0.5 μm. The dispersion liquid of Example 2 passed through the whole amount without any change in the differential pressure, whereas the dispersion liquid of Comparative Example 2 had a large difference in the differential pressure and the whole amount. Did not pass. Thus, the aqueous pigment dispersion prepared through the formation of a solid pigment dispersion at room temperature using carbon black having the characteristics defined in the present invention as shown in Example 2 has an adverse effect on production efficiency and yield. The number of coarse particles can be reduced without affecting the water, and an aqueous pigment ink for inkjet recording with high performance can be obtained.

  The aqueous pigment dispersion of the present invention can provide an aqueous pigment dispersion for ink-jet recording that has a small number of coarse particles and can significantly improve ejection stability when an aqueous ink for ink-jet recording is produced. Furthermore, according to the method for producing an aqueous pigment dispersion of the present invention, the aqueous pigment dispersion can be produced with high production efficiency and high production yield. Therefore, its industrial utility value is extremely high.

Claims (11)

Carbon black and an aqueous pigment dispersion for ink jet recording obtained by mixing and stirring a pigment dispersion solid at room temperature containing a copolymer having an anionic group in an aqueous medium, and the ash content of the carbon black is An aqueous pigment for ink jet recording, characterized in that the ratio A / B between the DBP oil absorption A (ml / 100 g) and the specific surface area B (m ² / g) is 0.75 or less. Dispersion. The aqueous pigment dispersion for inkjet recording according to claim 1, wherein the pigment dispersion contains a basic compound and a wetting agent and has a solid content ratio of 40 to 80% by mass. The aqueous pigment dispersion for inkjet recording according to claim 1 or 2, wherein the copolymer having an anionic group is a styrene acrylic copolymer having an acid value of 50 to 300 and a mass average molecular weight of 7500 to 40,000. The water-based pigment dispersion for inkjet recording according to claim 3, wherein the styrene-acrylic copolymer contains 60 to 90% by mass of a styrene monomer unit with respect to all monomer components. The ash content of the carbon black is 0.2% by mass or less, and the ratio A / B between the DBP oil absorption A (ml / 100 g) and the specific surface area B (m ² / g) is 0.5 or less. The aqueous pigment dispersion for inkjet recording according to any one of the above. 6. The aqueous pigment dispersion for inkjet recording according to claim 1, wherein the carbon black has a DBP oil absorption A (ml / 100 g) of 40 ml / 100 g or more. An aqueous ink for inkjet recording, comprising the aqueous pigment dispersion for inkjet recording according to any one of claims 1 to 6. A kneading step of kneading a mixture containing carbon black and a copolymer having an anionic group to produce a solid pigment dispersion at room temperature, and a mixing step of mixing and stirring the pigment dispersion in an aqueous medium The carbon black has an ash content of 1% by mass or less, and the ratio A / B of the DBP oil absorption A (ml / 100 g) to the specific surface area B (m ² / g) is 0.75 or less. A method for producing an aqueous pigment dispersion for inkjet recording. The method for producing an aqueous pigment dispersion for inkjet recording according to claim 8, wherein the pigment dispersion contains a basic compound and a wetting agent and has a solid content ratio of 40 to 80% by mass. The method for producing an aqueous pigment dispersion for inkjet recording according to claim 8 or 9, wherein the copolymer having an anionic group is a styrene acrylic copolymer having an acid value of 50 to 300 and a mass average molecular weight of 7500 to 40,000. The method for producing an aqueous pigment dispersion for inkjet recording according to claim 10, wherein the styrene-acrylic copolymer contains 60 to 90% by mass of a styrene monomer unit with respect to all monomer components.

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