JP6075931B2 - Method for producing aqueous pigment dispersion - Google Patents

Description

  The present invention relates to a method for producing an aqueous pigment dispersion that can be used as a raw material for ink jet recording inks.

Conventionally, an aqueous pigment dispersion in which a pigment is dispersed in a liquid medium mainly composed of water as an ink material having excellent durability and weather resistance and a small environmental load has been proposed. A composition has been manufactured. In producing an aqueous pigment dispersion, it is common to mix a pigment, water, an aqueous organic solvent, a resin, and a basic compound and perform a dispersion treatment of the pigment using a disperser. As a result, an aqueous pigment dispersion is obtained in which a particulate pigment coated with a resin or having a resin adsorbed on its surface is dispersed in a water-soluble solvent.
For example, when an aqueous pigment dispersion is prepared using a yellow pigment, an aqueous solution in which a water-soluble resin or a resin water-solubilized by addition of an alkali component is dissolved in water is prepared, and the yellow pigment is added thereto and stirred sufficiently. Furthermore, a method of obtaining an aqueous pigment dispersion by dispersing a pigment using a high-speed sand mill having a higher dispersion efficiency has been proposed. Ink for ink-jet recording using an aqueous pigment dispersion is significantly superior in light resistance and water resistance compared to conventional dye inks, but it is necessary to stably disperse pigment particles in an aqueous medium. Many studies have been made to achieve the desired dispersibility. Particularly in recent years, there has been an active use in photographic printing applications, and there has been an increasing demand for higher image quality such as higher gloss and excellent color development. It is becoming more and more necessary to have a complete set.

  For example, the above-described method for producing an aqueous pigment dispersion using a sand mill usually produces a dispersion using a low-viscosity liquid mixture having a small solid content ratio such as pigment. Therefore, it is difficult to apply a strong load to the pigment, and it takes time to pulverize the coarse particles of the pigment. Further, the aqueous pigment dispersion thus obtained contained a considerable amount of coarse particles having a particle diameter of 1 μm or more even after dispersion.

  In order to solve such a problem, for example, it is considered to knead a pigment and a polymer dispersant as a pretreatment before performing a dispersion treatment using a disperser. A method for producing a pigment dispersion by dispersing a dispersion in an aqueous medium after obtaining a dispersion with a 2-roll milling apparatus, and a pigment dispersion and a neutralizing agent prepared with a pigment and a polymer dispersant An aqueous pigment dispersion is obtained by mixing potassium hydroxide and water. (See Patent Document 1)

  However, in the above production method, the pigment is refined more than the unkneaded material by kneading in advance with a 2-roll milling device, and the dispersibility in the aqueous medium is improved. There is a problem that pigment aggregates generated when the body is dispersed in an aqueous medium cannot be completely prevented. In addition, this method has a problem that the dispersion time is long and the production efficiency is low. Furthermore, even in the aqueous pigment dispersion thus obtained, the dispersibility and dispersion stability are still insufficient.

  Accordingly, in order to solve the above-mentioned problems, a step of further blending a basic compound with a mixture containing a resin and a yellow pigment, kneading at a high viscosity, and producing a kneaded material that is solid or semisolid at room temperature, and the kneaded material There has been proposed a method for producing an aqueous pigment yellow dispersion, which comprises a step of producing an aqueous pigment dispersion by dispersing in an aqueous medium. (See Patent Document 2)

  By using the above-described production method for many pigments, the pigments are finer in the kneading step, and an aqueous pigment dispersion having good dispersibility can be produced. However, kneading for a long time with a high viscosity raises the temperature of the kneaded product, and in particular when the kneading apparatus is a closed system, depending on the pigment, the physical properties of the kneaded product may be affected. For example, C.I. which is a yellow pigment. I. In Pigment Yellow 74, when the above production method is used, the mechanical properties of the kneaded material are greatly changed due to the temperature rise during kneading, and the viscosity is lowered. Sufficient miniaturization does not progress. For this reason, even if an ink composition for ink jet recording is prepared from the aqueous pigment dispersion thus produced, there is a problem that good printing characteristics cannot be obtained.

Japanese Patent Laid-Open No. 6-157554 JP 2005-60411 A

  An object of the present invention is to obtain an aqueous pigment dispersion dispersed in a very small particle diameter, and an ink composition for ink jet recording produced using the dispersion as a main component gives an image having excellent gloss and color development. C. is possible and has high production efficiency. I. It is to provide a method for producing an aqueous pigment dispersion of CI Pigment Yellow 74.

In order to solve the above problems, the present inventors have developed C.I. I. As a result of examining the kneading process of Pigment Yellow 74, it is effective to suppress the temperature rise of the kneaded product during kneading by adding water, to prevent the viscosity from being lowered, and to maintain a high load on the kneaded product. The present invention has been completed. In particular, the pigment is C.I. I. In the case of Pigment Yellow 74, by using the method as described above, the efficiency of the kneading process is improved, and extremely fine dispersibility of the refined pigment can be achieved. For this reason, it has been found that substantially the same characteristics can be obtained without using a dispersion apparatus which has been used conventionally. That is, the present invention includes a kneading step of kneading a mixture containing a pigment, a styrene acrylic acid copolymer, a wetting agent, and a basic compound to produce a solid colored kneaded material at room temperature, and the colored kneaded material. A method for producing an aqueous pigment dispersion for inkjet recording, comprising a mixing step of adding an aqueous medium and stirring, wherein the pigment comprises C.I. I. Pigment Yellow 74, and in the kneading step, 10 to 50% by mass of water is added to the pigment, and a method for producing an aqueous pigment dispersion for inkjet recording is provided.
By adding 10 to 50% by mass of water to prevent the temperature of the kneaded product from rising, the above kneaded product is kneaded while maintaining a semi-solid or solid state, whereby the agglomerated pigment is physically crushed and simultaneously disintegrated. Since the styrene acrylic acid copolymer can be coated or adsorbed on the surface of the crushed pigment, it is extremely effective in reducing coarse pigments and improving pigment dispersion stability.

In the present invention, when the kneaded material is kneaded using a closed kneader such as a planetary mixer, the kneaded material is sheared, and a high load is applied to the kneaded material. A thing is heated and a viscosity fall occurs. Therefore, the viscosity of the kneaded material varies over a wide range with time.
In kneading in such a closed system, it is necessary to adjust the kneaded product to an appropriate viscosity during the kneading step. If the viscosity is too low, the kneaded product is not sufficiently loaded, so that the pigment is not refined. Further, if the viscosity is too high, there is a risk that a mechanical burden is applied to the kneader apparatus.

  In particular, C.I. I. In the kneaded product using Pigment Yellow 74, when the temperature of the kneaded product exceeds 100 ° C. during the kneading step, the viscosity is remarkably lowered, and the kneaded product is not sufficiently loaded, preventing the progress of the finer pigment. It is done. Therefore, it is preferable that the kneaded product is cooled during the kneading step, and the kneaded product is kneaded while maintaining a high viscosity state. However, in the present invention, the state of the kneaded product during the kneading step is a semi-solid or solid state, so a good adhesion state cannot always be maintained with the wall surface of the stirring tank, and effective heat transfer is performed. Is difficult. For this reason, it is difficult to effectively lower the temperature of the kneaded product even if the stirring tank is sufficiently cooled.

  The production method of the present invention includes a kneading step of kneading a mixture containing a pigment, a styrene acrylic acid copolymer, a wetting agent, and a basic compound to produce a solid colored kneaded material at room temperature, In the kneading step, since a specific amount of water is added to the pigment, a decrease in viscosity due to an increase in temperature of the kneaded product can be suppressed. Therefore, a sufficient shearing force is applied to the kneaded product, and an aqueous pigment dispersion containing a finely divided pigment having a narrow pigment particle size distribution can be produced. In addition, since the kneaded product is kept at a high viscosity, adhesion of the kneaded product to the wall surface of the stirring layer is also suppressed, and the manufacturing yield of the process is improved. Furthermore, since the pigment is effectively refined, the dispersion step using a known dispersion apparatus, which has been conventionally necessary after the kneading step, can be omitted, and the aqueous pigment dispersion can be achieved simply by adding an aqueous medium and stirring. A liquid can be produced. For this reason, the improvement of manufacturing efficiency is remarkable.

It is a figure which shows the temperature change of the kneaded material in the kneading | mixing process of an Example and a comparative example.

The method for producing the aqueous pigment dispersion of the present invention will be described in further detail below. First, various basic raw materials used in the production method of the present invention will be described in detail, and then more preferable selection and usage of those raw materials in the case of the present invention will be described, and the present invention using these materials will be described. The manufacturing method will be described in detail.
Pigment used in the present invention C.I. I. Pigment Yellow 74 is a monoazo yellow pigment characterized by strong tinting strength and clear hues, ranging from types with a small particle size to increase transparency to types with a large particle size to increase concealability. Since various products are available, they are widely used as printing color materials. For inkjet printing applications where a hue with high transparency is required, the particle diameter of the pigment affects the printing characteristics of the ink-jet recorded material. Therefore, the particle diameter is preferably 0.2 micrometers or less, and 0.1 micrometers. It is further desirable that C. I. Pigment Yellow 74 can be used in the pigment dispersion of the present invention regardless of the form such as dry powder, fruit granule, paste, and water-containing slurry.

  The styrene acrylic acid copolymer used in the aqueous pigment dispersion of the present invention contains styrene monomer units and at least one of acrylic acid monomer units and methacrylic acid monomer units as essential components.

Examples of the styrene monomer unit include styrene, α-methylstyrene, β-methylstyrene, 2,4-dimethylstyrene, α-dimethylstyrene, α-butylstyrene, α-hexylstyrene, alkylstyrene, 4-chloro. Examples thereof include halogenated styrene such as styrene and 3-bromostyrene, 3-nitrostyrene, 4-methoxystyrene, vinyltoluene and the like, and styrene is preferable.
In addition to the styrene monomer and the (meth) acrylic acid monomer, the styrene acrylic acid copolymer used in the aqueous pigment dispersion of the present invention has a vinyl group that is copolymerized with the styrene monomer and the (meth) acrylic acid monomer. Various derivatives can be used in combination.

  Examples of various derivatives having a vinyl group that can be used include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, outer acrylate acrylate, Benzyl acrylate, p-tolyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, stearyl methacrylate, benzyl methacrylate, p-methacrylate Tolyl, hydroxyethyl methacrylate, hydroxypropyl methacrylate, cyclohexyl methacrylate, dimethyl maleate, monoethyl maleate, diethyl maleate, di-n-butyl maleate, vinyl maleate (2-ethylhexyl), vinyl acetate, vinyl propionate, vinyl butyrate, vinyl hexanoate, vinyl benzoate, vinyl stearate, vinyl acrylate, vinyl methacrylate, allyl acetate, allyl propionate, allyl n-butyrate, heptanoic acid Unsaturated esters such as allyl, phenoxyacetate, and allyl methacrylate; acrylamide, N, N-dimethylacrylamide, N-isopropylacrylamide, Nt-butylacrylamide, N- (butoxymethyl) acrylamide, methacrylamide, N -Unsaturated amides such as [(3-dimethylamino) propyl] methacrylamide; Unsaturated nitriles such as acrylonitrile, methacrylonitrile, 3-hydroxypropionitrile; ethyl vinyl ether, propyl vinyl ether, n-butyl Unsaturated ethers such as butyl vinyl ether, t-butyl vinyl ether, cyclohexyl vinyl ether, 2-ethylhexyl vinyl ether, ethylene glycol vinyl ether, di (ethylene glycol) divinyl ether; 2-vinylpyrazine, 4-vinylpyridine, N-vinylcarbazole, N- And heterocyclic vinyl compounds such as vinylpyrrolidone; The selection of copolymerizable monomer components is not necessarily limited to the examples given here. Here, the (meth) acrylic acid monomer includes an acrylic acid monomer and a methacrylic acid monomer.

  In the composition of the styrene acrylic acid copolymer, the styrene monomer unit preferably occupies 50 to 90% by mass, more preferably 70 to 80% by mass of the total constituent monomers. When the ratio of the styrene monomer units to the total amount of all monomer units is 50% by mass or more, the styrene acrylic acid copolymer has C.I. I. The surface of Pigment Yellow 74 can be effectively adsorbed, and the storage stability of the aqueous pigment dispersion tends to be maintained well. In addition, when printing on paper such as plain paper using an ink composition for inkjet recording prepared from an aqueous pigment dispersion, it is possible to perform printing with high image recording density and excellent print quality, The water resistance of the image can be maintained well. When the styrenic monomer unit is 90% by mass or less, the advantage of occupying many component ratios of the styrene monomer is greatly exerted, and it is possible to blend an appropriate amount of a monomer having a polar group, There is a tendency that the dispersibility stability of the styrene acrylic acid resin in the aqueous medium can be satisfactorily maintained. Furthermore, the total of the styrene monomer unit, the acrylic acid monomer unit, and the methacrylic acid monomer unit is preferably 95% by mass or more based on the total amount of all monomers. 75-200 while further increasing the content of styrenic monomer units by making the total of styrene monomer units, acrylic acid monomer units, and methacrylic acid monomer units 95% by mass or more based on the total amount of all monomer units. Since the acid value can be secured, it is possible to impart sufficient hydrophilicity while maintaining good adsorption of the styrene acrylic acid copolymer to the pigment surface, and the styrene acrylic acid copolymer in an aqueous medium. The dispersibility of can be improved.

Furthermore, the total of hydrophilic monomer units other than acrylic acid and methacrylic acid is preferably 2% by mass or less of the total amount of all monomer units, and most preferably not substantially contained. Here, “substantially free of hydrophilic monomer” means that the amount is less than the amount that affects the characteristics of the styrene-acrylic acid copolymer by the hydrophilic monomer, and is about 0.5% by mass or less. It shall be shown.
In particular, the hydrophilic monomer whose use is preferably limited to the styrene acrylic acid copolymer of the present invention is a hydrophilic group such as a hydroxyl group, a sulfone group, an amino group, or a quaternary ammonium group in the chemical structure of the monomer. In addition to these substituents, those containing a repeating unit of ethylene oxide or propylene oxide can be mentioned. By using a styrene acrylic acid-based copolymer that does not contain these hydrophilic monomers, the water resistance of the inkjet printed image is further improved immediately after printing.

As the molecular weight of the styrene acrylic acid copolymer, a weight average molecular weight of 7,000 to 15,000 is selected. This is because C.I. I. When CI Pigment Yellow 74 is used, if the molecular weight is 15,000 or less, the viscosity at the time of preparation of the aqueous pigment dispersion is not excessively high, and ink jet discharge, particularly thermal ink jet discharge, is performed better. . On the other hand, when the molecular weight is 7,000 or more, the dispersion stability of the target pigment particles is further improved.
Here, the weight average molecular weight is a value measured by a GPC (gel permeation chromatography) method, and is a value converted to a molecular weight of polystyrene used as a standard substance. The weight average molecular weight of the styrene acrylic acid copolymer used in the present invention is measured by the following apparatus and conditions.
Liquid delivery pump: Shimadzu Corporation brand name “LC-9A”, System controller: Shimadzu Corporation brand name “SIL-6B”, Autoinjector: Shimadzu Corporation brand name “SIL-6B”, detector: Shimadzu Product name “RID-6A” manufactured by Seisakusho Co., Ltd. Data processing software: Product name “Sic480II Data Station” manufactured by System Instruments. Column: Hitachi Chemical Co., Ltd., trade name “GL-R400 (guard column)” + “GL-R440” + “GL-R450” + “GL-R400M”, elution solvent: THF, elution volume: 2 ml / min, column Temperature: 35 ° C.

  The acid value of the styrene acrylic acid copolymer of the present invention is preferably 75 to 200, more preferably 100 to 200. This is because the pigment is C.I. by setting the acid value to 200 or less. I. Since the amount of the hydrophobic component that is preferentially adsorbed on the surface of Pigment Yellow 74 is ensured, the coating with the styrene acrylic acid copolymer is performed better, and the dispersibility of the aqueous pigment dispersion is further improved. This is because there is a tendency. On the other hand, when the acid value is set to 75 or more, the effect of improving dispersibility when the acid group is neutralized by the base component is sufficiently exerted, so that the styrene-acrylic acid copolymer in the aqueous medium Dispersibility tends to be better.

  As for the mass ratio of the pigment and the styrene acrylic acid copolymer, it is sufficient that the amount necessary to coat the pigment surface in a stable capsule form is sufficient, and the content of the copolymer exceeding that is not included. Rather it is not preferable. When an excessive amount of a styrene acrylic acid copolymer is present, the free copolymer that is not adsorbed to the pigment in the dispersion increases, so that the copolymer is used particularly as an ink composition for ink jet recording. Adhering to an ink jet nozzle tends to cause ink discharge failure, and particularly in a thermal jet printer, there is a high risk of occurrence of this discharge failure problem. Therefore, in the production of the aqueous pigment dispersion kneaded product of the present invention, the styrene acrylic acid copolymer / pigment mass ratio in the aqueous pigment dispersion kneaded product is 1/10 to 2/1. Preferably, it is 1/10 to 6/10.

  The kneaded product for pigment dispersion needs to contain a styrene acrylic acid copolymer in an amount capable of uniformly covering the surface of the pigment. For this reason, when there are too few blend ratios of this copolymer, a pigment is not fully coat | covered and there exists a possibility that a dispersion stability and long-term storage stability may fall. However, if a large amount of excessive styrene acrylic acid copolymer exceeding the required amount is contained, the copolymer does not adhere to the pigment surface and exists in a particle or dissolved state in water or a wetting agent. An increase in the viscosity is not preferable because it causes an increase in viscosity.

Since the styrene acrylic acid copolymer used in the aqueous pigment dispersion of the present invention contains a carboxyl group derived from acrylic acid and methacrylic acid, it has a stable dispersibility in water by neutralization with a basic compound. Can be obtained. The basic compound used in the present invention is added at the time of kneading to neutralize the styrene acrylic acid copolymer and promote the transition to a swollen state in the presence of a wetting agent.
Examples of basic compounds used for such purposes include alkali metal hydroxides and low molecular organic amine compounds. In particular, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and alcohol amines such as triethanolamine, diethanolamine, and methyldiethanolamine can be used when an aqueous pigment dispersion is prepared into an inkjet ink. It is suitable from the viewpoints of decap characteristics of an ink jet printer, water resistance of printed matter, and the like. Among these basic compounds, alkali metal hydroxides typified by potassium hydroxide and sodium hydroxide contribute to lowering the viscosity of the pigment dispersion, and are particularly preferable from the viewpoint of ejection stability of the inkjet ink.

When neutralizing with a basic component, it is preferable to use a basic component having a neutralization rate of 50 to 130%, more preferably 50 to 110%, based on the acid value of the styrene acrylic copolymer of the present invention. It is further preferable to use a basic component having a neutralization ratio of from the viewpoint of PH management of the pigment dispersion and the composition of the ink-jet ink using the same.
In the present invention, the neutralization rate is a numerical value represented by the following formula.
Neutralization rate (%) = ((mass of basic compound (g) × 56 × 1000) / (acid value of styrene-acrylic copolymer × equivalent of basic compound × styrene-acrylic copolymer amount (g)) )) X 100

  The wetting agent used in the present invention is added in the kneading step in the preparation of the aqueous pigment dispersion kneaded product, and the kneading needs to proceed in the presence of the wetting agent. If the wetting agent is not present, the mixture cannot be sufficiently kneaded and the pigment surface is not wetted, so that the coating with the styrene acrylic acid copolymer may be insufficient.

Examples of the wetting agent include ethylene glycol, diethylene glycol, triethylene glycol, glycols such as tetraethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol (inserted reading); butanediol, pentanediol, hexanediol, and these Diols such as diols of the same family; Glycol esters such as propylene glycol laurate; Diethylene glycol ethers such as ethylene glycol monobutyl ether and carbitol; Diethylene glycol monoethyl, diethylene glycol monobutyl, diethylene glycol monohexyl, propylene glycol ether, dipropylene glycol ether And cello containing triethylene glycol ether Monoglycol ethers such as rubb; alcohols such as methanol, ethanol, isopropyl alcohol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, butyl alcohol, pentyl alcohol, and alcohols homologous thereto; or Lactones such as sulfolane, ester, ketone, and γ-butyl lactone can be used.
Since the wetting agent serves as a drying inhibitor in the aqueous pigment dispersion, polyhydric alcohols having a high boiling point, low volatility, and high surface tension are preferable, and glycols such as diethylene glycol and triethylene glycol are particularly preferable. preferable.
These water-soluble organic solvents can be used alone or in combination.

  The selection of the wetting agent depends on the styrene acrylic copolymer to be used, but a wetting agent having a certain degree of solubility is preferable, and the addition amount is adjusted by the solubility of the copolymer. Usually, 10-50 mass%, Preferably 20-40 mass% is mix | blended in a mixture. The addition amount is about 1/2 to 5 times that of the copolymer, and preferably about 1 to 4 times that of the copolymer. When the amount of the wetting agent is ½ or more of the amount of the copolymer, the copolymer can be sufficiently dissolved, partially dissolved, or swollen, and the dispersion stability of the yellow pigment may be lowered. is there. On the other hand, when it exceeds 5 times, the viscosity of the kneaded product is lowered and sufficient kneading cannot be performed, so that the dispersibility of the yellow pigment is lowered, and there is a possibility that the image quality such as ejection failure is deteriorated in the ink. In addition, as mentioned above, when what is derived from a basic compound etc. and plays the role of a solvent is mix | blended, it is preferable to determine the compounding quantity of a wetting agent in consideration of this.

  In addition, the wetting agent is blended in a mass ratio of 1/5 times or more, preferably 1/3 to 1 times that of the pigment. As a result, the kneading step proceeds while the styrene acrylic copolymer is always in a semi-dissolved or swollen state, and the surface of the pigment is satisfactorily coated. If the ratio is less than 1/5, the surface of the pigment cannot be sufficiently wet at the initial stage of kneading, or the copolymer cannot be dissolved, partially dissolved, or swollen, so that the effect may not be sufficiently obtained. There is.

Below, the manufacturing method of the aqueous pigment dispersion liquid of this invention using said raw material is demonstrated in detail.
(A) Kneading step In the production method of the present invention, a mixture containing a pigment, a styrene acrylic acid copolymer, a wetting agent, and a basic compound is kneaded to produce a solid colored kneaded material at room temperature. And a mixing step of adding and stirring an aqueous medium to the colored kneaded product, and further, in the kneading step, 10 to 50% by mass of water is added to the pigment.

In the present invention, a pigment and a styrene acrylic acid copolymer are kneaded in the presence of a wetting agent and a basic compound. For this reason, by kneading with the pigment in a state where the copolymer is dissolved, swollen or partially dissolved, the pigment is finely divided by crushing, the surface of the pigment is coated with the copolymer and Adsorption proceeds simultaneously.
In order to make the kneaded product into a clay state having a high viscosity at room temperature, the solid content ratio of the mixture to be kneaded is preferably 50 to 80% by mass. More preferably, it is 55-75 mass%. It is preferable that the solid content ratio is 50% by mass or more because the viscosity of the mixture is sufficiently high, so that the mixture can be kneaded under higher shearing force. Further, when the solid content ratio is 80% by mass or less, the mixture is more easily collected at the time of kneading, and better kneading tends to proceed.

  The temperature of the kneaded product in the kneading step can usually be about 60 to 120 ° C. I. It is preferable to keep the pigment yellow 74 at 100 ° C. or lower in order not to bring about a change in physical properties due to high temperature. In the production method of the present invention, by adding 10 to 50% by mass of water with respect to the pigment, the temperature of the kneaded product is lowered because the added water takes heat of vaporization at the time of evaporation. The amount of water to be added is more preferably 12 to 50% by mass, further preferably 15 to 50% by mass, more preferably 15 to 35% by mass, and still more preferably 20 to 35% by mass. Thus, during the kneading step, the temperature of the kneaded material can be suitably kneaded while maintaining the temperature at 100 ° C. or less, and more preferably, kneading can be performed while maintaining the temperature at 60 to 100 ° C.

The addition of water in the kneading step can be appropriately performed according to the temperature rise of the kneaded product accompanying the progress of the kneading step, and may be added during the kneading step, or may be added at the time of blending the mixture in the kneading step. May be. Furthermore, the water can be added at once, or can be divided into several times. However, the addition of the water is greatly impaired in the hardness of the mixture and the uniformity of the water distribution in the mixture. In this case, the pigment dispersion can proceed more favorably.
Although the solid content ratio of the mixture in the kneading step is reduced by the addition, the temperature rise of the mixture is suppressed by the cooling effect due to the heat of vaporization of water. The shearing force applied to the mixture increases with the start of the kneading process, and the load current of the kneading apparatus tends to gradually decrease after reaching a peak in 20 to 60 minutes. For this reason, it is preferable from the point of suppression of the temperature rise of a mixture to add 10-50 mass% of water with respect to a pigment before load current reaches a peak. Further, if the amount of water present in the stirring tank is within the above range, for example, through the kneading step, it is more effective in terms of suppressing the temperature rise, and the amount of water added to the mixture throughout the kneading step is within the above range. More preferably.

As the amount of water added to the pigment in the kneading step of the present invention, in addition to those explicitly added, those added as subcomponents of other additives are considered, that is, they are contained in the mixture of the kneading step. Any amount of water that contributes to a decrease in the temperature of the kneaded product in the kneading step by vaporization is taken into consideration. Therefore, in addition to water added to the kneaded product after the start of the kneading process, for example, water already added to the mixture as an aqueous solution of a metal alkali hydroxide before the start of kneading is included.
As described above, the “amount of water to be added” defined in the kneading step in the production method of the present invention includes the water blended in the mixture at the start of kneading, and water newly added to the mixture through the kneading step. This is the total amount. However, in order to reduce the rapid temperature rise at the start of kneading, it is preferable that the amount of water specified in the present invention is contained in the stirring tank from the beginning of the kneading, and the water in the stirring tank is present throughout the kneading process. More preferably, the amount is in the range of the amount defined in the present invention.

In the kneading step of the present invention, if the amount of water to be added is less than 10% by mass with respect to the pigment, the temperature of the kneaded product cannot be maintained at 100 ° C. or lower, and the viscosity of the kneaded product tends to decrease. For this reason, sufficient shearing force is not applied to the kneaded product, and the pigment is not easily refined. On the other hand, when the amount of water exceeds 50% by mass with respect to the pigment, the kneaded product becomes soft, and a high shearing force cannot be applied to the kneaded product in the kneading process, and the pigment is pulverized and refined. The surface of the pigment surface is likely to be insufficiently coated with the styrene acrylic acid copolymer.
By directly adding water to the kneaded product, the kneaded product can be kneaded at a suitable temperature condition of 100 ° C. or lower, more preferably 60 ° C. or higher and 100 ° C. or lower during the kneading step. Further, since the kneading is performed in a closed system, the evaporated water vapor is condensed on the inner wall of the stirring tank, and this is repeatedly used for cooling the kneaded product.

Adding water to the kneaded product in the kneading step means that a part of the neutralized styrene acrylic acid-based copolymer is dissolved in the water component, so that the coating and adsorption of the copolymer on the pigment surface are prevented. This may hinder and reduce the dispersibility or long-term dispersion stability of the pigment. Therefore, conventionally, kneading has been performed with a minimum amount of water derived from the base compound used for neutralization of the styrene acrylic acid copolymer.
However, C.I. I. In the present invention using Pigment Yellow 74 as a pigment, although there is a concern that some styrene acrylic acid-based copolymers are dissolved in an aqueous medium, the temperature lowering effect in a kneading step that compensates for this may be reduced. The effect of refining the pigment is shown.

The kneading apparatus used in the kneading step of the present invention preferably has a closed system structure so that water, a wetting agent and the like are not scattered outside during the kneading step. Therefore, it is preferable to use a kneader equipped with a sealable stirring tank and a uniaxial or multiaxial stirring blade. The number of stirring blades is not particularly limited, but two or more stirring blades are preferable in order to obtain a high kneading action.
In the kneading apparatus having such a configuration, since the added water is used for cooling without volatilization, efficient cooling can be performed. Usually, in such a closed-type kneading apparatus, it is difficult to diffuse the heat from the beginning, so it is attempted to suppress an excessive increase in the kneading temperature by cooling the wall surface.
However, in such a conventional method, since the thermal contact with the kneaded material is performed only through the wall surface of the stirring tank, it is difficult to sufficiently suppress the temperature increase that normally occurs with a high shearing force. In particular, the larger the capacity of the stirring tank, the smaller the ratio of the surface area of the wall surface to the volume of the kneaded product, so the cooling capacity decreases.
However, by using such a sealable kneading apparatus, water is directly added to the kneaded product, and cooling is performed with heat of vaporization from the surface or inside of the kneaded product, thereby effectively suppressing the kneading temperature rise. It is possible. The added water evaporates and removes the heat of vaporization to cool the kneaded product, and the water vapor cools and condenses on the wall surface of the kneading device, falls down along the wall surface and returns to the kneaded product, so it is considered to function repeatedly as a refrigerant. It is done.
Furthermore, when a kneader having such a configuration is used, a pigment dispersion kneaded material can be prepared and then diluted and dispersed directly with water and a wetting agent in the same stirring tank to produce a pigment dispersion. it can.

  Examples of the kneading apparatus that includes such a stirring tank and stirring blades and can be hermetically sealed include a Henschel mixer, a pressure kneader, a Banbury mixer, a planetary mixer, and the like, and a planetary mixer is particularly preferable. In the present invention, since the kneading is preferably performed in a state where the pigment concentration and the solid content concentration of the pigment and the styrene acrylic acid copolymer are high, the viscosity of the kneaded material is within a wide range depending on the kneading state of the kneaded material. Although it varies, the planetary mixer can cope with a wide range from a low viscosity to a high viscosity.

As the water-soluble organic solvent for lowering the temperature of the kneaded product, it is possible to use other than water as long as the heat of vaporization is high and the composition of the aqueous pigment dispersion is not adversely affected. It can also be used in combination with water.
As a method for adding water, if a large amount of water is added from the beginning of kneading, the kneaded product becomes soft, and a sufficient load is not applied to the kneaded product. A method in which water is sequentially added to adjust so that as much shear force as possible is applied to the kneaded product is preferable.

(B) Mixing step In the mixing step in the production method of the present invention, an aqueous medium is added to the colored kneaded material produced in the kneading step, and the mixed kneaded material is diluted, liquefied, and reduced in viscosity. To do. The necessary amount of water may be added at one time, but it is preferable to add the necessary amount continuously or intermittently while stirring and mixing because the generation of coarse particles can be effectively suppressed.

The kneaded product for an aqueous pigment dispersion prepared in the kneading step is usually a solid or solid kneaded product. Therefore, the preparation of the conventional aqueous pigment dispersion requires a dispersion step in which the kneaded product for aqueous pigment dispersion is diluted with an aqueous medium in the mixing step, stirred, and then dispersed using a known dispersion device such as nanomill. It was. By doing so, the pigment that has been crushed in the kneading step and coated with the styrene acrylic copolymer and loosely agglomerated is deagglomerated and the particle size becomes finer, so that the ink composition for inkjet recording Ink jet characteristics such as ejection stability of objects, printing density, and gloss are improved.

When starting the mixing step for the kneaded product after the kneading step, the kneaded product is not necessarily transferred to the device for the mixing step, and an aqueous medium is added and stirred in the stirring tank of the device used for the kneading step. Liquefaction may be performed. In the mixing step, water is added in the same manner as in the kneading step, but this addition is not a decrease in the temperature of the mixture, but the purpose is to lower the solid content ratio and liquefy it, so kneading with minimal water addition. Compared with the process, the addition rate and the addition amount are overwhelmingly large, and the solid content ratio of the mixture goes out of the range of 50 to 80% by mass, which is a preferable value for kneading, and continues to further decrease toward liquefaction.
For this reason, even when the mixing step is started using the same apparatus as the same kneading step, the difference between the two steps is usually clear.

  In the present invention, by performing the production method using the kneading step, the pigment C.I. I. The pigment yellow 74 is made finer and uniform coating or adsorption of the styrene acrylic acid copolymer onto the pigment surface proceeds extremely effectively. Therefore, for example, after performing a kneading step by the production method of the present invention in a kneader equipped with the above stirring tank and stirring blade to obtain a kneaded product, an aqueous medium is mixed with the kneaded product in this stirring tank, diluted, The aqueous pigment dispersion just stirred has superior dispersion characteristics compared to the aqueous pigment dispersion prepared by using the conventional kneading method without adding water and further to the dispersion step after the mixing step. It will be. And the temperature rise of a kneaded material is suppressed by addition of water. By carrying out the kneading step in the production method of the present invention, even if the aqueous pigment dispersion is prepared only by the mixing step of adding the aqueous medium after the kneading step, a dispersion step using a known dispersing device is further performed. The same particle size, dispersion stability, ink jet characteristics, and the like as the aqueous pigment dispersion prepared through the process can be obtained. Thus, since dispersion | distribution by a dispersion | distribution apparatus can be abbreviate | omitted after kneading | mixing, from manufacture of a kneaded material to viscosity adjustment can be performed continuously with one apparatus, and manufacturing efficiency can be improved.

  The aqueous medium used in the mixing step includes water used for the water-based inkjet ink, or a water-soluble organic solvent that contains water as a main component and is soluble in water. As the organic solvent that can be used, the organic solvent used as the solvent component of the aqueous pigment dispersion can be used. However, it is preferable to use the wetting agent exemplified in the kneading step, and the wetting actually used in the kneading step. It is preferable to use the same wetting agent as the agent.

  By diluting the kneaded material in an aqueous medium in this way, the anionic hydrophilic group in the styrene acrylic acid copolymer having the pigment surface coated in a capsule shape is gradually kept in a capsule state while gradually surrounding the surrounding aqueous medium. It is possible to orientate in the direction, and dispersibility in an aqueous medium is good and a stable coating state is possible.

  After the start of the mixing step, an aqueous medium is added to the colored kneaded product produced in the kneading step and stirred to perform the mixing step, and then dispersed in the aqueous medium using a dispersing device as in the past. Good. In this case, in the mixing step, viscosity adjustment suitable for the dispersing apparatus to be used and additives necessary for dispersion can be added. Dispersion using such a dispersing device is continued after the mixing step, so that the process itself becomes somewhat complicated and the production efficiency is lowered, but the colored kneaded material after kneading is dispersed in the aqueous medium only by the mixing step. The volume average particle diameter of the aqueous pigment dispersion and the ink composition for ink jet recording can be more reliably reduced, and the shape, composition, etc. of the raw material fluctuate due to various causes, and the desired characteristics can be obtained. Even when it is not obtained, the original characteristics can often be recovered by using a dispersion step.

In the dispersion step, a known dispersion device can be used. For example, in the case of using a medium, a paint shaker, a ball mill, an attritor, a basket mill, a sand mill, a sand glider, a dyno mill, a disperse mat, and an SC mill. , Spike mill, agitator mill, nano mill, pico mill and the like.
As the medium, known fine particles such as glass, ceramic, metal, metal oxide, and plastic can be used.
Examples of those that do not use media include ultrasonic homogenizers, nanomizers, resolvers, dispersers, and high-speed impeller dispersers. Among these, dispersers that use media are preferable because of their high dispersibility.
Depending on the type of dispersing device used, before dispersing with a disperser, a water-soluble solvent is added to the kneaded product for an aqueous pigment dispersion in the mixing step, mixed, diluted, and processed with the disperser. It is preferable to adjust to a viscosity suitable for the above.

(C) Centrifugation process Even if it performs operation which removes the coarse particle which performs a centrifugation process with respect to the aqueous pigment dispersion liquid obtained by passing through each said manufacturing process, and exists in an aqueous pigment dispersion liquid. good.
As the centrifugation conditions, a method of centrifuging at 10,000 G for 3 minutes or more is preferable, and it is more preferable to perform centrifugation at 15,000 to 21,000 G for 5 to 10 minutes. By carrying out the centrifugal separation, coarse particles having insufficient dispersion can be considerably removed, so that the sedimentation property of the pigment is greatly improved.

Below, the manufacturing method in the case of manufacturing the ink composition for inkjet recording using the aqueous pigment dispersion manufactured through each process of the said manufacturing method is demonstrated in detail.
The ink for inkjet recording can be produced by diluting the aqueous pigment dispersion produced by the production method of the present invention with an aqueous medium and adding an additive as necessary. The concentration of the pigment contained in the ink composition is preferably about 2 to 10% by mass.

When a wetting agent is blended in an aqueous medium used for diluting the aqueous pigment dispersion, the wetting agent exerts functions such as drying prevention, viscosity adjustment, and density adjustment after the production of the ink for inkjet recording. ,preferable. As the wetting agent, the same wetting agent as that used when dispersing the kneaded material in the aqueous medium in the mixing step can be used. Or the thing similar to the wetting agent mix | blended in the mixture in the kneading | mixing process and having performed the drying prevention function can be used. The content in the ink composition of a wetting agent mainly intended to prevent drying is preferably 3 to 50% by mass.
Also, in adjusting the ink composition for ink jet recording, the penetrability of the ink composition into the recording medium greatly affects the image quality through the adjustment of the dot diameter on the recording medium. Preferably it is done.
Examples of water-soluble organic solvents that can be used to adjust the permeability include lower alcohols such as ethanol and isopropyl alcohol; alkyl alcohol noethylene oxide adducts such as ethylene glycol hexyl ether and diethylene glycol butyl ether; and alkyls such as propylene glycol propylene ether. Examples include propylene oxide adducts of alcohol. The amount of penetrant added to the ink composition is preferably 0.01 to 10% by mass.

  When adjusting the ink composition for inkjet recording, a surfactant can be added to adjust ink characteristics such as surface tension. The surfactant that can be added is not particularly limited, and examples thereof include various anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Then, an anionic surfactant and a nonionic surfactant are preferable. Further, if necessary, an additive component for an ink jet recording ink such as an alkali agent, a pH adjuster, a preservative, a chelating agent, a plasticizer, an antioxidant, and an ultraviolet absorber is appropriately added, and the amount of the aqueous medium, The pigment concentration is adjusted.

  The ink composition for ink jet recording of the present invention prepared as described above basically has good dispersibility, dispersion stability, and ejection property, and is an on-demand type ink jet recording such as a piezo method or a thermal jet method. The ink composition of the present invention can be suitably used as an ink composition of a continuous jet type ink jet recording method. Moreover, the coating film formed on the recording medium by applying these ink compositions exhibits excellent color tone and gloss. For this reason, when used in combination with ink compositions for ink jet recording of other colors, a high-quality image can be formed together with the coating film formed by these ink compositions.

Hereinafter, the styrene acrylic acid copolymer used in Examples and Comparative Examples has a monomer composition ratio of styrene / acrylic acid / methacrylic acid = 77/10/13 (mass ratio), and an acid value of 153 to 156 mgKOH / g, A styrene acrylic acid copolymer having a weight average molecular weight of 11,000 was synthesized and used.
Example 1
Examples of the present invention will be described in detail below.
Unless otherwise specified, “part” is “part by mass” and “%” is “% by mass”.
Pigment Yellow 74
(Sanyo Color Co., Ltd., Fast Yellow 7413) 50 parts
Styrene-acrylic acid copolymer 15 parts (acid value 155 mg KOH / g weight average molecular weight 11000)
Diethylene glycol 24 parts 8N potassium hydroxide aqueous solution (solid content concentration = 34% by mass) 6.84 parts pure water 2.5 parts A mixture of the above composition (the amount of water with respect to the pigment at the time of compounding is 14%) was prepared. Put into a planetary mixer (PLM-V-50V manufactured by Inoue Seisakusho) kept at ℃ and stir for 5 minutes at 30 rpm rotation speed and 10 rpm rotation speed, then knead at 59 rpm rotation speed and 20 rpm rotation speed. Started. After 40 minutes of kneading, 2.5 parts of pure water was added and kneading was continued (at the time of addition, the amount of water relative to the pigment was 19%). Thereafter, the same operation was repeated after 70 minutes, 100 minutes and 162 minutes of kneading. The results of measuring the temperature of the kneaded product during kneading 7 to 8 times are shown in FIG. 1 (the amount of water relative to the pigment at the time of addition is 24, 29, and 34%, respectively). It can be seen that the temperature of the kneaded product does not exceed 100 ° C. and is stable at about 80 to 90 ° C. from about 30 minutes after kneading to the end of kneading. The current consumption during kneading was repeatedly increased and decreased according to the rotation period of the stirring blade. This is because the mixture is a semi-solid with extremely high viscosity and is not uniformly distributed inside the container, and a large force is applied to the stirring blade every time the mixture periodically shears the mixture. After 240 minutes, as a let-down operation in the mixing step, 87.5 parts of water was added to obtain a uniform mixture having a pigment concentration of 26.6% by mass.

The obtained mixture was transferred to a stainless drum, and 50.86 parts of ion-exchanged water, 23.97 parts of diethylene glycol and 0.248 parts of Proxel GXL were added and mixed by stirring. A beads mill (Nanomill NM manufactured by Asada Tekko Co., Ltd.) was added. -G-2L) and dispersed at a temperature of 19 ° C. and a residence time of 2.5 minutes to obtain a dispersion.
The dispersion is then passed through a continuous centrifuge (H-600S, 2 L capacity, manufactured by Kokusan Centrifuge Co.) and continuously centrifuged at a temperature of 33 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. Add 40.8 parts of water, and filter through a filter (Nexis NXA 0.5, manufactured by Nippon Pole Co., Ltd., using a cartridge length of 2 inches) to obtain an aqueous pigment dispersion having a pigment concentration of 14.4% by mass. It was.

(Preparation of ink)
Using the obtained aqueous pigment dispersion, an ink for inkjet recording having a pigment concentration of 4% was prepared by the following composition.
Aqueous pigment dispersion 5.54 parts 2-Pyrrolidinone 1.60 parts Triethylene glycol monobutyl ether 1.60 parts Acetylene glycol surfactant Surfynol 440
(Nissin Chemical Industry Co., Ltd.) 0.10 parts Glycerin 0.60 parts Ion-exchanged water 10.56 parts The obtained ink for ink jet recording was allowed to stand in a high-temperature tank at 60 ° C. for 8 weeks, but dispersed. The volume average particle size of the pigments that were present did not increase significantly.

(Comparative Example 1)
Pigment Yellow 74
(Sanyo Color Co., Ltd., Fast Yellow 7413) 50 parts Styrene-acrylic acid copolymer 15 parts (acid value 155 mg KOH / g weight average molecular weight 11000)
Diethylene glycol 24 parts 8N potassium hydroxide aqueous solution (solid content concentration = 34% by mass) 6.84 parts The above components are sequentially put into a planetary mixer (PLM-V-50V manufactured by Inoue Seisakusho) kept at 60 ° C. The mixture was stirred for 5 minutes at a rotational speed of 30 rpm and a rotational speed of 10 rpm, and kneading was started at a rotational speed of 59 rpm and a rotational speed of 20 rpm (the amount of water relative to the pigment was 9% at the time of addition). The current consumption during kneading was repeatedly increased and decreased according to the rotation period of the stirring blade. The result of measuring the temperature of the kneaded product during kneading 7 to 8 times is shown in FIG. It can be seen that the temperature of the kneaded product gradually increases from the start of kneading and reaches about 110 ° C. at the end.

Here, referring to FIG. 1, when the temperature rise of the kneaded material of Example 1 and Comparative Example 1 is compared, in Example 1, the kneaded material temperature is always 100 ° C. or less by adding water appropriately during kneading. It has changed. On the other hand, in Comparative Example 1 in which no water was added during the kneading, the temperature at the beginning of the kneading was almost the same as that of the example, but it can be seen that the temperature exceeded 100 ° C. after 1 hour of kneading.
The temperature of the kneaded product was measured by stopping kneading for a short time and inserting a thermometer directly into the kneaded product being kneaded.

After 240 minutes from the start of kneading of the mixture, 100 parts of water was added as a let-down operation to obtain a uniform mixture having a pigment concentration of 25.0% by mass.
The obtained mixture was transferred to a stainless steel drum, 35.44 parts of ion-exchanged water, 21.07 parts of diethylene glycol and 0.218 parts of Proxel GXL were added, and the mixture was stirred and mixed, and a bead mill (Nanomill NM manufactured by Asada Tekko Co., Ltd.) was added. -G-2L) and dispersed at a temperature of 14 ° C. and a residence time of 2.5 minutes to obtain a dispersion.
The dispersion was then passed through a continuous centrifuge (H-600S, 2 L capacity, manufactured by Kokusan Centrifuge Co.) and continuously centrifuged at a temperature of 28 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. 1.3 parts of water was added, and filter filtration (Nexis NXA 0.5 manufactured by Nippon Pole Co., Ltd., using a cartridge length of 2 inches) was performed to obtain an aqueous pigment dispersion having a pigment concentration of 17.4% by mass. .

(Preparation of ink)
Using the obtained aqueous pigment dispersion, an ink for inkjet recording having a pigment concentration of 4% was prepared by the following composition.
Aqueous pigment dispersion 4.60 parts 2-Pyrrolidinone 1.60 parts Triethylene glycol monobutyl ether 1.60 parts Acetylene glycol surfactant Surfynol 440
(Nissin Chemical Industry Co., Ltd.) 0.10 parts Glycerin 0.60 parts Ion-exchanged water 11.5 parts The obtained ink for ink jet recording was allowed to stand in a high-temperature tank at 60 ° C. for 8 weeks. The volume average particle size of the pigments that were present did not increase significantly.

(Example 2)
Pigment Yellow 74
(Sanyo Color Co., Ltd., Fast Yellow 7413) 50 parts
Styrene-acrylic acid copolymer 15 parts (acid value 155 mg KOH / g weight average molecular weight 11000)
Diethylene glycol 22.69 parts 8N potassium hydroxide aqueous solution (solid content concentration = 34% by mass) 6.84 parts pure water 2.5 parts Planetary mixer (Inoue Seisakusho Co., Ltd.) prepared as a mixture of the above composition and kept at 60 ° C. PLM-V-50V) and stirred for 5 minutes at a rotation speed of 30 rpm and a revolution speed of 10 rpm, and then kneading was started at a rotation speed of 70 rpm and a revolution speed of 24 rpm (the amount of water relative to the pigment at the time of addition) 14%). After 40 minutes of kneading, 2.5 parts of pure water was added and kneading was continued (at the time of addition, the amount of water relative to the pigment was 19%). Thereafter, the same operation was repeated after 70 minutes, 100 minutes, and 160 minutes after mixing (the amount of water relative to the pigment at the time of addition was 24%, 29%, and 34%, respectively). The current consumption during kneading was repeatedly increased and decreased according to the rotation period of the stirring blade.

After 240 minutes, as a let-down operation in the mixing step, 87.5 parts of water was added to obtain a uniform mixture having a pigment concentration of 26.9% by mass. The obtained mixture was transferred to a stainless drum, and 49.57 parts of ion-exchanged water, 24.17 parts of diethylene glycol and 0.238 parts of Proxel GXL were added and stirred to mix.
The dispersion is then passed through a continuous centrifuge (H-600S, 2 L capacity, manufactured by Kokusan Centrifuge Co.) and continuously centrifuged at a temperature of 15 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. 8.1 parts of water was added and filtered (Nexis NXA 0.5 manufactured by Nippon Pole Co., Ltd., using a cartridge length of 2 inches) to obtain an aqueous pigment dispersion having a pigment concentration of 15.7% by mass. .

(Preparation of ink)
Using the obtained aqueous pigment dispersion, an ink for inkjet recording having a pigment concentration of 4.1% was prepared by the following composition.
Aqueous pigment dispersion 5.26 parts 2-Pyrrolidinone 1.60 parts Triethylene glycol monobutyl ether 1.60 parts Acetylene glycol surfactant Surfynol 440
(Nissin Chemical Industry Co., Ltd.) 0.10 parts Glycerin 0.60 parts Ion-exchanged water 10.84 parts The obtained ink for ink-jet recording was allowed to stand for 8 weeks in a high-temperature tank at 60 ° C, but dispersed. The volume average particle size of the pigments that were present did not increase significantly.

(Example 3)
The aqueous pigment dispersion diluted with water and diethylene glycol after kneading in Example 2 was passed through a bead mill (Nanomill NM-G-2L manufactured by Asada Tekko Co., Ltd.) and dispersed at a temperature of 13 ° C. and a residence time of 2.5 minutes. Got.
The dispersion was then passed through a continuous centrifuge (H-600S, 2 L capacity, manufactured by Kokusan Centrifuge Co.), and continuously centrifuged at a temperature of 21 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. 7.7 parts of water was added, followed by filter filtration (Nexis NXA 0.5, manufactured by Nippon Pole Co., Ltd., using a cartridge length of 2 inches) to obtain an aqueous pigment dispersion having a pigment concentration of 14.8% by mass. .

(Preparation of ink)
Using the obtained aqueous pigment dispersion, an ink for inkjet recording having a pigment concentration of 4.1% was prepared by the following composition.
Aqueous pigment dispersion 5.50 parts 2-Pyrrolidinone 1.60 parts Triethylene glycol monobutyl ether 1.60 parts Acetylene glycol surfactant Surfynol 440
(Nissin Chemical Industry Co., Ltd.) 0.10 parts Glycerin 0.60 parts Ion-exchanged water 0.60 parts The obtained ink for ink jet recording was allowed to stand in a high-temperature tank at 60 ° C. for 8 weeks. The volume average particle size of the pigments that were present did not increase significantly.

(Reference Example 1)
Pigment Yellow 155
(Clariant, Ink Jet Yellow 4GP) 50 parts
Styrene-acrylic acid copolymer 10 parts (acid value 155 mg KOH / g weight average molecular weight 11000)
Diethylene glycol 14.50 parts 8N potassium hydroxide aqueous solution (solid content concentration = 34% by mass) 4.50 parts A 50 L capacity planetary mixer (PLM manufactured by Inoue Seisakusho) was prepared and kept at 60 ° C. -V-50V), and after stirring for 10 minutes at a rotation speed of 30 rpm and a revolution speed of 10 rpm, kneading was started at a rotation speed of 70 rpm and a revolution speed of 24 rpm. After 30 minutes of kneading, 0.50 part of diethylene glycol and 0.50 part of pure water were added, and the kneading was continued. Thereafter, the same operation was repeated 43 minutes, 53 minutes and 63 minutes after kneading. The increase / decrease was repeated according to the rotation period of the stirring blade during kneading. After 120 minutes, as a letdown operation in the mixing step, 13.5 parts of diethylene glycol and 118 parts of water were added to obtain a uniform mixture having a pigment concentration of 20.5% by mass.

The obtained mixture was transferred to a stainless steel drum, 46.41 parts of ion exchange water and 0.234 parts of Proxel GXL were added, and the mixture was stirred and mixed into a bead mill (Nanomill NM-G-2L manufactured by Asada Tekko Co., Ltd.). Through dispersion at a temperature of 21 ° C. and a residence time of 100 seconds, a dispersion was obtained.
The dispersion is then passed through a continuous centrifuge (H-600S, 2L capacity manufactured by Kokusan Centrifuge Co., Ltd.) and continuously centrifuged at a temperature of 23 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. Aqueous pigment dispersion having a pigment concentration of 14.7% by mass was obtained by adding 18.1 parts of water and performing filter filtration (Nexis NXA 0.5 manufactured by Nippon Pole Co., Ltd., using a cartridge length of 2 inches). .

(Reference Example 2)
Pigment Yellow 155
(Clariant, Ink Jet Yellow 4GP) 50 parts styrene-acrylic acid copolymer 10 parts (acid value 155 mg KOH / g weight average molecular weight 11000)
Diethylene glycol 14.5 parts 8N potassium hydroxide aqueous solution (solid content concentration = 34% by mass) 4.50 parts A 50-liter planetary mixer (PLM manufactured by Inoue Seisakusho Co., Ltd.) was prepared and kept at 60 ° C. -V-50V), and after stirring for 10 minutes at a rotation speed of 30 rpm and a revolution speed of 10 rpm, kneading was started at a rotation speed of 70 rpm and a revolution speed of 24 rpm. After 120 minutes, as a let-down operation in the mixing step, 15.5 parts of diethylene glycol and 120 parts of water were added to obtain a uniform mixture having a pigment concentration of 22.8% by mass.

The obtained mixture was transferred to a stainless drum, and 50.88 parts of ion-exchanged water and 0.241 parts of Proxel GXL were added, and the mixture was stirred and mixed into a bead mill (Nanomill NM-G-2L manufactured by Asada Tekko Co., Ltd.). Through dispersion at a temperature of 24 ° C. and a residence time of 100 seconds, a dispersion was obtained.
The dispersion is then passed through a continuous centrifuge (H-600S, 2L capacity manufactured by Kokusan Centrifuge Co., Ltd.) and continuously centrifuged at a temperature of 32 ° C., a centrifugal force of 18900 G, and a residence time of 10 minutes. Went. 19.6 parts of water was added, followed by filter filtration (Nexis NXA 0.5, manufactured by Nippon Pole Co., Ltd., cartridge length: 1 inch) to obtain an aqueous pigment dispersion having a pigment concentration of 15.0% by mass.

  The aqueous pigment dispersions obtained in the above Examples and Comparative Examples and the ink compositions prepared from the aqueous pigment dispersions were evaluated according to the following evaluation items, and the results obtained are shown in Table 1.

<Evaluation of particle size>
The aqueous pigment dispersions obtained in Examples, Comparative Examples and Reference Examples were diluted about 1,000 times with ion-exchanged water, and [Microtrac UPA150] (manufactured by Leeds & Northrup) was used. The volume average particle diameter measured at room temperature was used as the particle diameter of the aqueous pigment dispersion in each of Examples and Comparative Examples.

<Gloss / Saturation>
The ink for ink jet recording obtained in the above Examples and Comparative Examples was mounted on a black cartridge of an ink jet printer EM-930 (manufactured by EPSON), and Premium Photo Paper Glossy (manufactured by EPSON) was used as a recording medium. -100% image quality density is recorded, and the gloss of the obtained image is measured with a gloss value at an angle of 20 ° using “micro-TRI-gloss” (manufactured by BYK-Gardner), and the gloss within the range is measured. The highest value was taken as the gloss in each example and comparative example. In the saturation, “SpectroScan Transmission” (manufactured by GretagMacbeth) was used, and the value at an image quality density of 100% was defined as the saturation in the examples and comparative examples.
The results are shown below.

As can be seen from Table 1, in Examples 1 to 3 in which water was added during the kneading step, the particle size, compared to Comparative Example 1 in which water was not added during the kneading step, regardless of the presence or absence of the dispersion step. It is clear that it has excellent properties in both gloss and saturation. Comparing Example 1, Example 2 and Example 3, it can be seen that the characteristics can be sufficiently maintained even without the dispersion step. Since Example 1 was different from Example 2 in the number of revolutions of the kneading apparatus in the kneading step, Example 3 was added in which the number of revolutions was the same as Example 2.
Further, the pigment is added to C.I. I. In Reference Example 1 and Reference Example 2 performed by changing to Pigment Yellow 155, a decrease in the volume average particle diameter was observed in the same manner as in the Examples due to the addition of water during the kneading process. I. It can be seen that Pigment Yellow 74 has a much larger improvement effect.

Claims (7)

A kneading step of kneading a mixture containing a pigment, a styrene acrylic acid copolymer, a wetting agent, and a basic compound to produce a solid colored kneaded material at room temperature; and adding an aqueous medium to the colored kneaded material. A method for producing an aqueous pigment dispersion for inkjet recording, comprising a mixing step of stirring, wherein the pigment is C.I. I. Pigment Yellow 74, and 10 to 50% by mass of water is added to the pigment in the kneading step. The method for producing an aqueous pigment dispersion for inkjet recording according to claim 1, wherein 10 to 19% by mass of water is added to the pigment in the kneading step. The method for producing an aqueous pigment dispersion for inkjet recording according to claim 1 or 2 , wherein the temperature of the colored kneaded product in the kneading step is 60 to 100 ° C. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 3, wherein the colored kneaded material solid at normal temperature has a solid content ratio of 50 to 80% by mass. The styrene-acrylic acid copolymer, an acid value 75-200, relative to the total monomer components, according to claim 1 containing a styrenic monomer units of 50 mass% Of producing an aqueous pigment dispersion for inkjet recording. The method for producing an aqueous pigment dispersion for inkjet recording according to any one of claims 1 to 5, wherein the basic compound is an alkali metal hydroxide. The method for producing an aqueous pigment dispersion for inkjet recording according to any one of claims 1 to 6, wherein in the kneading step, kneading is performed using a sealable kneading apparatus having a stirring tank and stirring blades.

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