JP2012223975A - Set of ink and reaction liquid and image forming method using the set - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a set of an ink and a reaction liquid that can obtain and form a high-density image to be suppressed from being transferred to the other recording medium even if only a short time passes after image formation, and to provide an image forming method using the set.SOLUTION: Regarding the set of the ink and the reaction liquid, and the image forming method using the set, in the ink containing a pigment and an aqueous resin, and the set of the ink having a combination of the reaction liquid with buffer ability in an acid region and the reaction liquid, the pigment is a self-dispersing pigment whose surface charge amount obtained by colloid titration is 0.20 mmol/g or larger, and the aqueous resin is a copolymer having a unit derived from a (meth)acrylic acid.

Description

  The present invention relates to a set having a combination of an ink and a reaction liquid, and an image forming method for forming an image using the set.

  2. Description of the Related Art Conventionally, for the purpose of improving image performance, an image forming method for forming an image by preparing a so-called reaction liquid separately from an ink containing a color material and applying the reaction liquid and the ink to a recording medium Various proposals have been made. In this method, the aggregation of the coloring material in the recording medium and the permeability of the ink can be controlled by the reaction liquid, so that it is possible to achieve both the image density and the fixability compared to the image forming method that does not use the reaction liquid. It is becoming possible.

  For example, Patent Document 1 proposes an ink-jet ink set having a combination of a pigment ink containing an anionic dispersion resin and an acidic transparent treatment solution having a pH of 3.5 to 5.0 that does not contain a coloring material. . The acidic clear treatment solution contains a pH buffer, and the pH buffer has one carboxy group having a pKa of 3.7 to 6.5 and dissolves by 10% by mass or more with respect to water. In addition, a material that does not substantially volatilize at 20 ° C. under normal pressure is used.

  For the purpose of imparting water resistance, scratch resistance, and marker resistance to the formed image, it is common practice to include a water-soluble resin in the ink. Further, as another aspect, it is a general method to use a so-called resin-dispersed pigment obtained by stabilizing a water-insoluble colorant such as a pigment with a water-soluble resin so that the ink contains a water-soluble resin. In Patent Document 2, a first ink containing a pH-sensitive resin and a self-dispersing pigment and a second ink having a predetermined pH are brought into contact with each other in a recording medium, so that the pH-sensitive resin is insolubilized on the surface. It is disclosed to suppress blurring of an image by being present.

JP 2007-084607 A JP 2000-129184 A

  Compared with commonly used resin-dispersed pigments, inks containing self-dispersed pigments are excellent in reliability because they can easily maintain a stable pigment dispersion state even when pH changes. It can be said that. However, when an ink containing a self-dispersing pigment is applied to a two-component reaction system using a reaction solution having a buffering capacity in an acidic region, it is difficult to leave a color material on the surface of the recording medium, and a high image density can be obtained. There was a problem that it was not possible. This is because the stability of the self-dispersing pigment against pH change, which is one of the factors of high reliability in ink, is stable against a reaction solution having a buffering capacity in the acidic region, so that the color material does not react with the reaction solution. This is because it easily penetrates into the recording medium and it is difficult for the coloring material to remain on the surface of the recording medium. For this problem, as described in Patent Document 2, it is effective to coexist a pH-sensitive resin and a self-dispersing pigment in the ink. That is, with this configuration, the pH-sensitive resin in the ink is insolubilized by contact with the second ink having an appropriate pH and remains on the surface of the recording medium. Therefore, the self-dispersing pigment in the ink Can be left together on the surface of the recording medium. As described above, by using a self-dispersing pigment and a pH-sensitive resin in combination as an ink component, it is possible to obtain an image with a high image density even in a two-component reaction system using a reaction solution having a buffer capacity in an acidic region. become.

  However, as a result of the study by the present inventors, when the ink containing the self-dispersing pigment and the pH-sensitive resin described in Patent Document 2 is used in combination with a reaction solution having a buffering capacity in the acidic region, it is used as a set. It has been found that the following another problem arises. That is, it has been found that the set has a problem that the image is transferred to another portion when the image is overlapped when performing continuous high-speed recording, in particular, the fixability in a short time.

  Therefore, an object of the present invention is to use a set of an ink and a reaction liquid, which can form an image that can obtain a high image density and can suppress transfer even in a short time after image formation. An object is to provide an image forming method.

  The above object is achieved by the present invention described below. That is, the present invention is a set of an ink containing a combination of an ink containing a pigment and a water-soluble resin, and a reaction solution having a buffering ability in an acidic region, and the reaction solution, wherein the pigment is colloid titrated. A self-dispersing pigment having a surface charge amount calculated by the formula (ii) of 0.20 mmol / g or more, and the resin is a copolymer having a unit derived from (meth) acrylic acid; A set with a reaction solution and an image forming method using the set are provided.

  According to the present invention, an ink and a reaction liquid that can form an image in which an image having a high image density is obtained and an image is suppressed from being transferred to another portion even in a short time after the image is formed. A set with is provided. Furthermore, according to the present invention, an image forming method capable of forming the above-described excellent image by using the set is provided.

  Hereinafter, the present invention will be described in more detail with reference to preferred embodiments for carrying out the invention. In the present invention, various physical properties such as surface tension and pH are values at 25 ° C. In the present invention, the reaction solution “having a buffer capacity in the acidic region” means that the pH of a mixture obtained by mixing the reaction solution and the ink used in a set in equal amounts is pH 7.0. It means that there is substantially no change from the pH of the reaction solution in a pH range below. More specifically, in a pH range of less than 7.0, it means that the difference between the pH of the reaction liquid and the pH of the mixture obtained by mixing the ink and the reaction liquid in equal amounts is within 0.1. is doing. Further, “pKa” defined in the present invention is one of indices for quantitatively expressing the strength of acid, and is also called an acid dissociation constant or an acidity constant. Considering the dissociation reaction in which hydrogen ions are released from the acid, it is represented by the negative common logarithm pKa. A smaller pKa indicates a stronger acid.

  The inventors have improved the fixability in a short time, which occurs in a two-component reaction system using a reaction solution having a buffering capacity in an acidic region, particularly when images overlap when performing continuous high-speed recording. We studied to solve the problem that the image was transferred to another part. Specifically, when a water-soluble resin having a pH sensitivity necessary for increasing the resulting image density is added to an ink containing a self-dispersing pigment that is used in combination with the reaction liquid, the image can be printed in a short time. The conditions necessary for improving the fixability were examined. As a result, the above problems can be solved by using a specific ink containing a combination of the following pigment and resin in a set of ink and reaction liquid that has a buffer capacity in the acidic region and is used together with a reaction liquid that reacts with the ink. I knew it was possible. That is, as a constituent component of the ink, a water-soluble resin that is a copolymer having a self-dispersing pigment having a surface charge amount of 0.20 mmol / g or more determined by colloid titration and a unit derived from (meth) acrylic acid. It is a combination including. By using a set of the ink and the above reaction liquid, a high image density and a fixing property of the ink in a short time, in particular, when continuous high-speed recording is performed, transfer to another part of the image when the images overlap. The reliability in high-speed recording that has been required in recent years, such as a reduction in image quality, can be satisfied. About this reason, the present inventors are estimating as follows. In the following description, a water-soluble resin that is a copolymer having a unit derived from (meth) acrylic acid may be simply referred to as “water-soluble resin”. In addition, the resin in the following description is assumed to have a hydrophilic unit typified by a unit derived from (meth) acrylic acid and a hydrophobic unit.

  In the case of an ink in which a self-dispersing pigment having a surface charge amount determined by colloid titration of 0.20 mmol / g or more and the above-mentioned specific water-soluble resin coexist, it is considered that the following occurs. Since the self-dispersing pigment having a surface charge amount of 0.20 mmol / g or more has many ionic groups on the particle surface, these ionic groups are coexisting water-soluble resin (meth) acrylic. Repel each other with the ionicity of the carboxy group of the acid-derived unit. For this reason, the self-dispersing pigment and the water-soluble resin are present at a certain distance in the ink, and it is considered that interactions such as adsorption of the water-soluble resin to the surface of the pigment particles are unlikely to occur. Therefore, in the ink in which the self-dispersing pigment and the water-soluble resin coexist, the water-soluble resin and the pigment behave independently when contacting the reaction liquid. In other words, although the pigments do not aggregate, the coexisting resin is insolubilized by itself without involving the pigment because the solubility of the resin coexisting in the ink drops sharply when it comes into contact with the reaction solution. In the form of an emulsion, it is deposited on the surface of the recording medium. The pigment is also deposited on the surface of the recording medium so as to be taken in between the insolubilized resin thus deposited.

  As described above, in the case of an ink containing a self-dispersing pigment having a high surface charge amount, the water-soluble resin exists independently of the pigment particles, so that the water-soluble resin is quickly insolubilized. Since it does not exist, solid-liquid separation between the pigment and the liquid component also occurs quickly. As a result, a high image density can be obtained even for a reaction solution having a buffering capacity in an acidic region. Further, as described above, when the water-soluble resin is insolubilized rapidly when it comes into contact with the reaction liquid, solid-liquid separation between the resin and the liquid component in the ink is performed quickly. For this reason, also in the recording medium, the resin and pigment insoluble in water and the liquid component in other ink are quickly separated. From the above, it has become possible to reduce the fixability of ink in a short time, in particular, when images are overlapped when continuous high-speed recording is performed, the image is transferred to another part. Conceivable.

  On the other hand, when an ink is used in which a self-dispersing pigment whose surface charge obtained by colloid titration is less than 0.20 mmol / g and the above-mentioned specific water-soluble resin coexist, the following may occur. The self-dispersing pigment having a surface charge amount of less than 0.20 mmol / g has low ionic groups on the surface of the particles, and therefore the ionicity of the pigment particles is low, and repulsion as described above is unlikely to occur. The resin having the property exists close to the pigment particles. In addition, the self-dispersing pigment has many gaps on the surface of the pigment particles, that is, sites where ionic groups are not bonded, and a hydrophobic portion such as styrene which is one of the units constituting the resin is easily adsorbed here. . From the above, when a self-dispersing pigment having a small amount of ionic groups on the surface of the pigment and a resin having a (meth) acrylic acid unit coexist in the ink, the resin exists in a form that is adsorbed to the pigment. It is thought that there is. Therefore, the ink in which the self-dispersing pigment and the water-soluble resin coexist decreases the solubility of the resin abruptly when it comes into contact with the reaction liquid. At this time, a part of the resin is adsorbed on the surface of the pigment particles. Therefore, it is insolubilized while entraining the pigment particles. In this case, unlike the case where the surface charge amount is high as described above, the resin and the pigment are insolubilized as large aggregates and deposited on the surface of the recording medium. For this reason, pigment particles can be left on the surface of the recording medium, which makes it possible to obtain a high image density. However, on the other hand, since the aggregate remaining on the surface of the recording medium is large, the liquid component in the ink is not quickly separated, and the fixing property of the ink in a short time, particularly when continuous high-speed recording is performed. When the images overlap, it is likely that transfer to another part of the image will occur.

  As described so far, it is considered that the presence state of the pigment in the ink and the water-soluble resin is different due to the difference in the surface charge amount of the self-dispersing pigment. Due to the difference in the presence state of the resin, the behavior of the pigment and water-soluble resin after the reaction liquid having a buffering capacity in the acidic region and the ink contact each other is different, and the fixability of the ink in a short time is different. It is considered a thing.

<Set of ink and reaction liquid>
Hereinafter, each of the ink and the reaction liquid constituting the set of the present invention will be described in detail.
[Reaction solution]
The reaction solution constituting the present invention has a buffer capacity in the acidic region. In the present invention, the reaction between the reaction liquid and the ink is caused by acid precipitation of the water-soluble resin, which is a copolymer having units derived from (meth) acrylic acid in the ink, due to the buffer capacity in the acidic region of the reaction liquid. To be done. Since the reaction solution is used in combination with ink when forming an image, it is preferable that the reaction solution does not contain a colorant and is colorless and does not absorb in the visible range in consideration of the influence on the image. However, even if it shows absorption in the visible range, it may be light-colored as long as it does not affect the actual image. Hereinafter, each component which comprises a reaction liquid is demonstrated.

(Buffering agent)
The reaction solution constituting the present invention needs to have a buffer capacity in an acidic region, that is, a pH region having a pH of less than 7.0. In order to obtain a reaction solution having a pH buffering ability in an acidic region, it is preferable to contain a buffer in the reaction solution. As a buffering agent, any compound capable of having a buffering ability against pH change known in the art can be added to the present invention as long as it is a substance that can be buffered in an acidic region by adding it. Can be used. The content (mass%) of the buffering agent in the reaction solution only needs to satisfy that the reaction solution has the buffer capacity defined in the present invention. For example, 1.0% based on the total mass of the reaction solution. It is preferable that they are 1 mass% or more and 15.0 mass% or less.

  According to the study by the present inventors, considering the reactivity when the ink and the reaction liquid come into contact with each other in the recording medium, the buffer contained in the reaction liquid in order to provide a buffering capacity in the acidic region is a weak acid. It is preferred to use certain organic acids. Specific examples of such an organic acid include organic acids having a carboxy group. Since organic acids having a carboxy group are generally weak acids, the acid dissociation constant (pKa) can be applied as a measure of acid strength. As the pKa of the organic acid having a carboxy group, those having a pKa in water at 25 ° C. of 2.5 or more and 6.5 or less are particularly preferably used from the viewpoint of effectively improving the reactivity. If the pKa is less than 2.5, the acidity is too strong and the members constituting the recording apparatus may be easily corroded. On the other hand, if the pKa exceeds 6.5, the acidity is too weak and the water-soluble resin cannot be sufficiently insolubilized, and a high level image density may not be obtained. In addition, although polyvalent carboxylic acid more than bivalence shows multiple steps of ionization corresponding to the carboxy group of the valence, it is preferable that pKa of all the steps is contained in the said range.

  In the present invention, in order to provide the reaction solution with a buffering capacity, specifically, an organic acid having a carboxy group as described below is preferably contained as a buffering agent. For example, salts of monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid; phthalic acid, isophthalic acid, terephthalic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid , Dicarboxylic acid salts and hydrogen salts such as sebacic acid, dimer acid, pyromellitic acid and trimellitic acid; salts and hydrogen salts of tricarboxylic acid such as citric acid; hydroxy such as oxysuccinic acid, DL-malic acid and tartaric acid Examples include carboxylic acid salts. Examples of the cation forming the salt include alkali metal ions such as lithium, sodium and potassium.

  Among these, it is preferable to use a buffer having a high solubility in water. Such buffering agents include salts of monocarboxylic acids such as acetate and propionate, salts and hydrogen salts of polycarboxylic acids such as malonic acid, succinic acid, glutaric acid and citric acid, malic acid and tartaric acid. Examples thereof include salts of hydroxycarboxylic acid. Furthermore, since it is excellent in reactivity and has high solubility in water, it is preferable to use a salt of, for example, a dicarboxylic acid having at least two carboxy groups rather than a monocarboxylic acid. The dicarboxylic acid is less likely to change the pH of the mixture of the ink and the reaction liquid when the ink and the reaction liquid come into contact with each other in the recording medium, and can promote insolubilization of the water-soluble resin by the acid. Further, as the number of carboxy groups in the molecule increases, the reactivity with ink increases, but the solubility in water decreases. Further, according to the study by the present inventors, by using dicarboxylic acid, the buffer capacity in the acidic region becomes stronger and the water-soluble resin can be insolubilized particularly effectively. In the present invention, it is particularly preferable to use glutaric acid, a salt thereof or a hydrogen salt as the buffer.

  Further, as a result of the study by the present inventors, it has been found that the reaction solution preferably contains an organic acid having a carboxy group as a buffer, and the pH of the reaction solution is preferably 3.5 or more and 5.5 or less. It was. On the other hand, if the pH of the reaction solution is too low, the members constituting the recording apparatus may be easily corroded. On the other hand, if the pH of the reaction solution is too high, the reaction with the ink may not occur sufficiently, so it is preferable to set it to 5.5 or less. In order to adjust the reaction solution to have a pH of 3.5 to 5.5, for example, organic acids such as acetic acid and methanesulfonic acid, inorganic acids such as sulfuric acid and nitric acid, alkali metal hydroxides, etc. It is preferable to contain a pH adjusting agent such as a base.

(Aqueous medium)
The reaction solution constituting the present invention preferably contains water or a mixed solvent of water and a water-soluble organic solvent as an aqueous medium. As water, deionized water or ion exchange water is preferably used. The content (% by mass) of water in the reaction solution is 25.0% by mass or more and 95.0% by mass or less, further 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the reaction solution. Preferably there is. Moreover, as a water-soluble organic solvent, all the well-known things generally used for the reaction liquid for inkjets can be used, and 1 type, or 2 or more types of water-soluble organic solvents can be used. Specifically, monohydric or polyhydric alcohols, alkylene glycols having an alkylene group having about 1 to 4 carbon atoms, polyethylene glycols having an average molecular weight of about 200 to 2,000, glycol ethers, nitrogen-containing compounds And the like. These water-soluble organic solvents can be used alone or in combination of two or more as required. The content (% by mass) of the water-soluble organic solvent in the reaction solution is 3.0% by mass or more and 70.0% by mass or less, and further 3.0% by mass or more and 50.0% by mass based on the total mass of the reaction solution. % Or less is preferable.

(Other ingredients)
In addition to the above components, the reaction solution constituting the present invention contains a nitrogen-containing compound such as urea and ethylene urea, a water-soluble organic compound that is solid at room temperature such as trimethylolpropane and trimethylolethane, if necessary. May be. In addition to the above components, if necessary, a polymer compound, a surfactant, a pH adjuster, an antifoaming agent, an antirust agent, an antiseptic, an antifungal agent, an antioxidant, an antireduction agent, Various additives such as an evaporation accelerator and a chelating agent may be contained in the reaction solution.

[ink]
The ink constituting the set of the present invention together with the reaction liquid described above contains a pigment, and the pigment is a self-dispersing pigment. The hue of the ink is not particularly limited, and may be cyan, magenta, yellow, black, red, blue, green, and the like. Hereinafter, components constituting the ink will be described.

(Pigment)
The color material used for the ink constituting the present invention is a pigment. The kind of pigment that can be used in the present invention is not particularly limited, and any known inorganic pigment or organic pigment can be used. Specific examples include inorganic pigments such as calcium carbonate, titanium oxide, and carbon black, and organic pigments such as phthalocyanine and quinacdrine. The content (% by mass) of the pigment in the ink is 0.1% by mass or more and 15.0% by mass or less, and further 0.2% by mass or more and 10.0% by mass or less based on the total mass of the ink. preferable. The ink may contain other color materials such as known dyes in addition to the pigment.

  The self-dispersing pigment used in the ink constituting the present invention is required to have a surface charge amount determined by colloid titration of 0.20 mmol / g or more. That is, the amount of the hydrophilic group per gram of the pigment expressed as a surface charge amount obtained by colloid titration, in which a hydrophilic group is chemically bonded to the surface of the pigment particle directly or via another atomic group. Is required to be 0.20 mmol / g or more. The surface charge amount is 5.0 mmol / g or less, more preferably 2.0 mmol / g or less, and particularly preferably 1.8 mmol / g or less. As will be described later, the ink constituting the present invention contains a copolymer having a unit derived from (meth) acrylic acid as a water-soluble resin. However, in the present invention, the pigment is not expected to be dispersed only by the action of the water-soluble resin. That is, “self-dispersion” as used in the present invention means that a polymer compound such as a resin or a compound having surface activity is adsorbed on the surface of the pigment particles, and the pigment is basically removed only by the dispersing action of the pigment particles. It means not to be dispersed. In the self-dispersing pigment as used in the present invention, for example, a hydrophilic group is chemically bonded to the surface of the pigment particle directly or through another atomic group, and the action of the bonded hydrophilic group causes the pigment particle. It is possible to disperse. That is, unlike the so-called resin-dispersed pigment that achieves pigment dispersion only by adsorbing the resin (dispersant) to the surface of the pigment particles, the self-dispersing pigment used in the present invention uses the above-mentioned specific resin. It can be dispersed at least.

  The surface charge amount of the self-dispersing pigment required in the present invention is measured by a colloid titration method. In the Example mentioned later, it measured using the potentiometric automatic titration apparatus AT-510 (made by Kyoto Electronics Industry) equipped with the streaming potential titration unit (PCD-500). Further, the surface charge amount of the self-dispersed pigment in the pigment dispersion was measured by potentiometric titration using methyl glycol chitosan as a titration reagent.

On the surface of the pigment particles, the hydrophilic groups are chemically bonded directly or through another atomic group, for _{example, -COOM, -SO 3 M, -PO} 3 HM, -PO 3 M 2 ( Formula M represents a hydrogen atom, an alkali metal, ammonium, or organic ammonium. Examples of the other atomic group (—R—) include linear or branched alkylene groups having 1 to 12 carbon atoms, arylene groups such as phenylene groups and naphthylene groups, amide groups, sulfone groups, amino groups, and carbonyl groups. Group, ester group and ether group. Moreover, the group etc. which combined these groups are mentioned. Examples of the alkali metal represented as M include Li, Na, K, and the like. When the hydrophilic group forms a salt, the form of the salt in the ink may be in any dissociated state or all dissociated state.

  The self-dispersing pigment used in the present invention may be a self-dispersing pigment that has been subjected to surface oxidation treatment, and can be used as long as the surface charge obtained by colloid titration is 0.20 mmol / g or more. . Examples of such self-dispersed pigments include those obtained by methods such as oxidation with sodium hypochlorite, ozone treatment in water, and ozone treatment, followed by wet oxidation with an oxidizing agent and modification of the particle surface. It is done. In the present invention, other dispersion type pigments (resin dispersion pigments, microcapsule pigments, resin-bonded self-dispersion pigments, etc.) may be used in combination as long as the effects of the present invention are obtained.

(Water-soluble resin)
The ink constituting the present invention contains a copolymer having a unit derived from (meth) acrylic acid as a water-soluble resin. As described above, this water-soluble resin is not used as a dispersant for the above self-dispersing pigment. In the present invention, the fact that the resin is water-soluble means that the resin has no particle diameter when neutralized with an alkali equivalent to the acid value. A resin that satisfies such a condition is described as a water-soluble resin in the present invention. The content (% by mass) of the water-soluble resin in the ink is 0.1% by mass or more and 5.0% by mass or less, and further 0.3% by mass or more and 3.0% by mass or less based on the total mass of the ink. Preferably there is. The water-soluble resin preferably has a weight average molecular weight of 1,000 to 30,000, more preferably 3,000 to 15,000.

  The water-soluble resin that is a copolymer having a unit derived from (meth) acrylic acid as defined in the present invention is dissolved in an aqueous medium by the action of an anionic group contained in the (meth) acrylic acid structure. When the ink comes into contact with the reaction liquid in the recording medium, the solubility of the reaction liquid having a buffer capacity in the acidic region is drastically decreased and the copolymer is deposited on the surface of the recording medium. It is necessary to have a unit derived from (meth) acrylic acid.

  Specifically, a water-soluble resin having at least a hydrophilic unit and a hydrophobic unit as listed below as constituent units is preferable. In addition, the (meth) acryl in the following description shall show an acryl and methacryl.

  Examples of the monomer having a hydrophilic group that becomes a hydrophilic unit by polymerization include the following. For example, monomers having a carboxy group such as (meth) acrylic acid, itaconic acid, maleic acid and fumaric acid, monomers having a phosphonic acid group such as (meth) acrylic acid-2-phosphonic acid ethyl, Examples thereof include anionic monomers such as anhydrides and salts of acidic monomers, and monomers having a hydroxy group such as 2-hydroxyethyl (meth) acrylate and 3-hydroxypropyl (meth) acrylate. Examples of the cation constituting the salt of the anionic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium.

  In addition, as a monomer having a hydrophobic group that becomes a hydrophobic unit by polymerization, a monomer having an aromatic ring such as styrene, α-methylstyrene, benzyl (meth) acrylate, ethyl (meth) acrylate, Monomers having an aliphatic group such as methyl (meth) acrylate, (iso) propyl (meth) acrylate, (n-, iso-, t-) butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and the like. It is done.

  According to the study by the present inventors, it is preferable that the water-soluble resin is not too high in hydrophilicity of the insolubilized resin when it comes into contact with the reaction liquid while ensuring solubility in the ink. For this reason, it is preferable that the acid value of water-soluble resin is 100 mgKOH / g or more and 215 mgKOH / g or less. When the acid value is less than 100 mgKOH / g, the solubility of the water-soluble resin in the aqueous medium tends to be low, and ink ejection stability may not be sufficiently obtained. On the other hand, when the acid value exceeds 215 mgKOH / g, even if the water-soluble resin is insolubilized by contact with the reaction liquid, the water-soluble resin is too hydrophilic, so that it is separated from the liquid component in the ink. In some cases, it is not performed promptly and transfer resistance in a short time cannot be obtained sufficiently.

  In the present invention, the content (mass%) of the water-soluble resin in the ink is preferably 0.25 times or more and 0.75 times or less in terms of the mass ratio with respect to the pigment content (mass%). . In this case, the contents of the water-soluble resin and the pigment are values based on the total mass of the ink. If the mass ratio is less than 0.25 times, the amount of the resin insolubilized by contact with the reaction solution is too small, and the amount of resin deposited on the surface of the recording medium is also reduced. Since it may not be able to exist well, it is not preferable. In this case, an image having a sufficiently high image density may not be obtained. On the other hand, when the mass ratio exceeds 0.75 times, the amount of the resin insolubilized by contact with the reaction solution increases, the amount of resin deposited on the surface of the recording medium also increases, and the pigment is also present on the surface of the recording medium. There can be many. However, in this case, since the amount of the resin is too large, the pigment or insolubilized water-soluble resin and the liquid component in the ink are not quickly separated, and sufficient transfer resistance in a short time can be obtained. Since it may not be, it is not preferable.

(Aqueous medium)
The ink constituting the present invention preferably contains water or a mixed solvent of water and a water-soluble organic solvent as an aqueous medium. As water, deionized water or ion exchange water is preferably used. The content (% by mass) of water in the ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink. Moreover, as a water-soluble organic solvent, all the well-known things generally used for the ink for inkjets can be used, and 1 type, or 2 or more types of water-soluble organic solvents can be used. Specifically, monohydric or polyhydric alcohols, alkylene glycols having an alkylene group having about 1 to 4 carbon atoms, polyethylene glycols having a number average molecular weight of about 200 to 2,000, glycol ethers, nitrogen-containing compounds Examples thereof include compounds. These water-soluble organic solvents can be used alone or in combination of two or more as required. The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink.

(Other ingredients)
In addition to the above components, the ink used in the present invention may contain a water-soluble organic compound that is solid at room temperature, such as nitrogen-containing compounds such as urea and ethyleneurea, trimethylolethane, and trimethylolpropane, as necessary. Also good. In addition to the above components, various kinds of surfactants, pH adjusters, antifoaming agents, rust inhibitors, antiseptics, antifungal agents, antioxidants, reduction inhibitors, evaporation accelerators, chelating agents, etc. An additive may be contained.

  The pH of the ink is preferably 6.0 or more and 9.5 or less. If the pH is less than 6.0, the pigment dispersion stability tends to be low, and the storage stability of the ink may not be sufficiently obtained. On the other hand, if the pH exceeds 9.5, there may be a problem that ink wettability deteriorates, such as corrosion of members constituting the ink jet recording apparatus. The surface tension of the ink is preferably 25.0 mN / m or more and 38.0 mN / m or less. When the surface tension exceeds 38.0 mN / m, the penetration and diffusion of the ink into the recording medium are slow, and the ink drying time may be long. On the other hand, when the surface tension is less than 25.0 mN / m, the ink tends to permeate excessively into the recording medium, and the ink may reach the back surface of the recording medium and the back-through may easily occur.

<Image forming method>
The image forming method of the present invention includes a step of applying ink to a recording medium and a step of applying a reaction liquid to the recording medium, and an image is formed by bringing the ink and the reaction liquid into contact with each other on the recording medium. At this time, the set of the reaction liquid and ink of the present invention described above is used. In the present invention, the ink is preferably ejected from an ink jet recording head in order to obtain a high-definition image. However, the method of applying the reaction liquid to the recording medium is not limited to the ink jet method, and is applied with a roller or the like. You can also use the method. That is, it is only necessary that the reaction liquid is applied to the recording medium by some method, and the ink and the reaction liquid can be brought into contact with each other on the recording medium. Examples of the configuration of the apparatus for performing the image forming method of the present invention include an inkjet recording apparatus and an apparatus provided with a coating mechanism, and any known configuration can be employed. A recording head mounted on an ink jet recording apparatus has a method of discharging a liquid by the action of mechanical energy or thermal energy. In the present invention, a recording head of a method of discharging a liquid by the action of thermal energy is used. It is preferable.

  Examples of the order in which the ink and the reaction liquid are applied to the recording medium include a case where the ink is applied after the reaction liquid is applied, a case where the reaction liquid is applied after the ink is applied, or a combination thereof. . In view of the object of the present invention, it is preferable to include at least the case where the ink is applied after the reaction liquid is applied first.

  Further, from the viewpoint of dischargeability from an ink jet recording head, the viscosity of the ink and the reaction liquid is preferably 1 mPa · s to 15 mPa · s, and more preferably 1 mPa · s to 5 mPa · s. . Further, it is preferable that the reaction liquid reacts with the intended ink efficiently in the recording medium. For this reason, the surface tension of the reaction liquid is within a range that can be discharged from the recording head and is to be destabilized by the reaction liquid so that the reaction liquid does not bleed into a place different from the recording area with the desired ink. It is preferable to make it larger than that of the resulting ink.

The amount of the reaction liquid applied to the recording medium may be appropriately adjusted according to the buffer capacity of the reaction liquid and the configuration of the ink to be reacted therewith. In the present invention, from the viewpoint of the uniformity of the obtained image, the applied amount of the reaction solution is 0.5 g / m ² or more and 10.0 g / m ² or less, and more than 2.0 g / m ² and 5 0.0 g / m ² or less is preferable. In addition, when the area | region which applies a reaction liquid with respect to the magnitude | size (area: m < ² >) of a recording medium is only a certain part, it is assumed that it applied to the whole surface of a recording medium, and the application amount of a reaction liquid It is preferable that the value (g / m ² ) is obtained and this value satisfies the above range.

  Next, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not limited by the following Example, unless the summary is exceeded. In the text, “parts” and “%” are based on mass unless otherwise specified. Various physical properties are values measured at 25 ° C. A pH meter (F-21; manufactured by HORIBA, Ltd.) was used for pH measurement.

<Preparation of pigment dispersion>
(Surface charge of self-dispersing pigment)
First, a method for measuring the surface charge amount of the self-dispersing pigment will be described. The surface charge amount of the self-dispersed pigment in the pigment dispersion is determined using an automatic potentiometric titrator AT-510 (manufactured by Kyoto Denki Kogyo Co., Ltd.) equipped with a streaming potential titration unit (PCD-500). Measured by potentiometric titration.

(Pigment dispersion A)
To a solution of 5 g of concentrated hydrochloric acid dissolved in 5.5 g of water, 1.5 g of 4-aminophthalic acid was added while being cooled to 5 ° C. Next, the solution containing the solution is placed in an ice bath and the solution is stirred so that the solution is always kept at 10 ° C. or lower, and 1.8 g of sodium nitrite is added to 9 g of water at 5 ° C. The dissolved solution was added. After stirring this solution for another 15 minutes, 6 g of carbon black having a specific surface area of 220 m ² / g and a DBP oil absorption of 105 mL / 100 g was added with stirring. Thereafter, the mixture was further stirred for 15 minutes. After the obtained slurry was filtered with a filter paper (standard filter paper No. 2; manufactured by Advantech), the particles were sufficiently washed with water and dried in an oven at 110 ° C., and —C ₆ H ₃ — (COONa) was formed on the surface of the pigment particles. ) Self-dispersed pigment A having _two groups bonded thereto was prepared. Water was added to the obtained self-dispersing pigment A, and the mixture was dispersed so that the pigment content was 10.0% to prepare pigment dispersion A. The surface charge determined by colloid titration of self-dispersing pigment A was 0.41 mmol / g.

(Pigment dispersion B)
To a solution of 5 g of concentrated hydrochloric acid dissolved in 5.5 g of water, 1.55 g of p-aminobenzoic acid was added while being cooled to 5 ° C. Next, the solution containing the solution is placed in an ice bath and the solution is stirred so that the solution is always kept at 10 ° C. or lower, and 1.8 g of sodium nitrite is added to 9 g of water at 5 ° C. The dissolved solution was added. After stirring this solution for another 15 minutes, 6 g of carbon black having a specific surface area of 220 m ² / g and a DBP oil absorption of 105 mL / 100 g was added with stirring. Thereafter, the mixture was further stirred for 15 minutes. After the obtained slurry was filtered with a filter paper (standard filter paper No. 2; manufactured by Advantech), the particles were sufficiently washed with water and dried in an oven at 110 ° C., and a —C ₆ H ₄ —COONa group was formed on the surface of the pigment particles. Self-dispersing pigment B in which Pigment dispersion B was prepared by adding water to the obtained self-dispersion pigment B and dispersing the pigment so that the pigment content was 10.0%. The amount of surface charge obtained by colloid titration of the self-dispersing pigment B was 0.21 mmol / g.

(Pigment dispersion C)
To a solution of 2.5 g of concentrated hydrochloric acid dissolved in 5.5 g of water, 0.8 g of p-aminobenzoic acid was added while being cooled to 5 ° C. Next, the container containing the solution was placed in an ice bath and the solution was stirred to keep the solution constantly at 10 ° C. or lower, and 0.9 g of sodium nitrite was added to 9 g of water at 5 ° C. The dissolved solution was added. After the solution was further stirred for 15 minutes, 9 g of carbon black having a specific surface area of 220 m ² / g and a DBP oil absorption of 105 mL / 100 g was added with stirring. Thereafter, the mixture was further stirred for 15 minutes. After the obtained slurry was filtered with a filter paper (standard filter paper No. 2; manufactured by Advantech), the particles were sufficiently washed with water and dried in an oven at 110 ° C., and a —C ₆ H ₄ —COONa group was formed on the surface of the pigment particles. Self-dispersing pigment C in which is bound was prepared. Pigment dispersion C was prepared by adding water to the obtained self-dispersion pigment C and dispersing the pigment so that the pigment content was 10.0%. The amount of surface charge obtained by colloid titration of self-dispersing pigment C was 0.16 mmol / g.

(Pigment dispersion D)
As the pigment dispersion D, CAB-O-JET300 (manufactured by Cabot), which is a commercially available pigment dispersion containing a self-dispersing pigment having a carboxy group bonded to the surface of carbon black particles, was used. The pigment content in the pigment dispersion D is 15.0%. The pigment in the pigment dispersion D is referred to as self-dispersion pigment D. The surface charge obtained by colloid titration of self-dispersing pigment D was 0.098 mmol / g.

(Pigment dispersion E)
20 g of pigment, 7 mmol of ((4-aminobenzoylamino) -methane-1,1-diyl) bisphosphonic acid monosodium salt, 20 mmol of nitric acid, and 200 mL of pure water were mixed. At this time, carbon black having a specific surface area of 220 m ² / g and DBP oil absorption of 105 mL / 100 g was used as the pigment, and mixing was performed at 6,000 rpm at room temperature using a Silverson mixer. After 30 minutes, 20 mmol of sodium nitrite dissolved in a small amount of water was slowly added to the mixture. By this mixing, the temperature of the mixture reached 60 ° C., and the reaction was performed in this state for 1 hour. Thereafter, the pH of the mixture was adjusted to 10 using an aqueous sodium hydroxide solution. After 30 minutes, 20 mL of pure water was added, diafiltration was performed using a spectrum membrane, and water was added to disperse the pigment to a content of 10.0% to obtain a pigment dispersion E. Prepared. In this way, the self-dispersing pigment E having a ((4-aminobenzoylamino) -methane-1,1-diyl) bisphosphonic acid group, whose counter ion is sodium, is dispersed in water on the surface of the pigment particles. Pigment Dispersion Liquid E was obtained. The surface charge obtained by colloid titration of self-dispersing pigment E was 0.45 mmol / g.

<Preparation of water-soluble resin>
(Water-soluble resin A)
A styrene-n-butyl acrylate-acrylic acid copolymer (composition (molar ratio) 33:44:23) having a weight average molecular weight of 5,000 and an acid value of 120 mgKOH / g was synthesized by a conventional method. The polymer was neutralized with potassium hydroxide equivalent to the acid value of the polymer. Water was added thereto to prepare a resin aqueous solution A having a water-soluble resin A content of 10.0%.

(Water-soluble resin B)
A commercially available styrene-acrylic acid copolymer having a weight average molecular weight of 8,500 and an acid value of 215 mgKOH / g (Jonkrill 678; manufactured by BASF) is mixed with potassium hydroxide equivalent to the acid value of the copolymer. It was summed up. Thereafter, water was added to prepare a resin aqueous solution B having a water-soluble resin B content of 10.0%.

(Water-soluble resin C)
A commercially available styrene-acrylic acid copolymer having a weight average molecular weight of 4,600 and an acid value of 108 mgKOH / g (Joncrill 586; manufactured by BASF) was mixed with potassium hydroxide equivalent to the acid value of the copolymer. It was summed up. Water was added thereto to prepare a resin aqueous solution C having a water-soluble resin C content of 10.0%.

(Water-soluble resin D)
A commercially available styrene-acrylic acid copolymer having a weight average molecular weight of 8,000 and an acid value of 160 mgKOH / g (Jonkrill 683; manufactured by BASF) was mixed with potassium hydroxide equivalent to the acid value of the copolymer. It was summed up. Water was added thereto to prepare an aqueous resin solution D having a water-soluble resin D content of 10.0%.

(Water-soluble resin E)
A styrene-acrylic acid copolymer having a weight average molecular weight of 12,000 and an acid value of 230 mgKOH / g (composition (molar ratio) 66:34) synthesized by a conventional method is equivalent to the acid value of the copolymer. Neutralized with potassium hydroxide. Water was added thereto to prepare an aqueous resin solution E having a water-soluble resin E content of 10.0%.

(Water-soluble resin F)
A styrene-acrylamide-t-butylsulfonic acid copolymer having a weight average molecular weight of 8,000 and an acid value of 140 mgKOH / g, which is a copolymer having no unit derived from (meth) acrylic acid by a conventional method. A coalescence (composition (molar) ratio 48:52) was synthesized. Next, it neutralized with the potassium hydroxide equivalent to the acid value of the obtained copolymer, water was added, and resin aqueous solution F whose content of water-soluble resin F is 10.0% was prepared.

(Water-soluble resin G)
A benzyl methacrylate-methacrylic acid copolymer (composition (molar ratio) 40:60) having a weight average molecular weight of 10,000 and an acid value of 200 mgKOH / g synthesized by a conventional method was added to an acid value and an equivalent amount of potassium hydroxide. Neutralized. Water was added thereto to prepare a resin aqueous solution G having a water-soluble resin G content of 10.0%.

<Preparation of ink>
Each component (unit:%) shown in the upper part of Table 1 was mixed and sufficiently stirred, and then pressure filtration was performed with a micro filter (manufactured by Fuji Film) having a pore size of 3.0 μm to prepare each ink. In addition, acetylenol E100 in Table 1 is an ethylene oxide adduct of acetylene glycol, and is a nonionic surfactant manufactured by Kawaken Fine Chemicals. Polyethylene glycol having an average molecular weight of 600 was used. The lower part of Table 1 shows the surface tension value of the prepared ink. The surface tension of the ink was measured at a temperature of 25.0 ± 0.5 ° C. using a surface tension meter CBVP-A3 (manufactured by Kyowa Interface Science) equipped with a 10 mm × 24 mm platinum plate. Furthermore, the lower part of Table 1 also shows the water-soluble resin content [%] and pigment content [%], and the water-soluble resin content / pigment content value [times] in each ink. It was.

<Preparation of reaction solution>
Each component (unit:%) shown in the upper part of Table 2 was mixed, and an 8 mol / L potassium hydroxide aqueous solution was further added so that the pH shown in the lower part of Table 2 was obtained, followed by thorough stirring. Thereafter, pressure filtration was performed with a cellulose acetate filter having a pore size of 0.45 μm (manufactured by Advantech) to prepare each reaction solution. In addition, acetylenol E100 in Table 2 is a surfactant manufactured by Kawaken Fine Chemical, and polyethylene glycol having an average molecular weight of 600 was used. The pH of the reaction solution was measured at 25 ° C. using a pH meter (trade name: F-21; manufactured by Horiba, Ltd.).

<Evaluation>
The ink and reaction liquid obtained above were used in the combinations shown on the left side of Table 3 to make each set of ink and reaction liquid. In each set of Examples 1 to 17 and Comparative Examples 1 and 3 to 5, the difference between the pH of the reaction solution and the pH of the mixture obtained by mixing the ink and the reaction solution in equal amounts in the pH range of less than 7.0. Was within 0.1, and the reaction solution had a buffer capacity in the acidic region.

  Using these sets, evaluation was performed under the following conditions. For image formation, a modified ink jet recording apparatus (trade name: PIXUS Pro 9500; manufactured by Canon Inc.) equipped with a recording head that discharges liquid by the action of thermal energy was used. Ink and reaction liquid constituting the set were respectively filled in the cartridge, the reaction liquid cartridge was prepared in the photomagenta position, and two ink cartridges were prepared and set in the cyan and red positions.

  The recording condition is one-pass one-way recording in which an image corresponding to the arrangement width of the ejection opening of the recording head is recorded only by scanning starting from the home position of the recording head, and the reaction liquid is applied to the recording medium in the same pass. Then, the ink was applied so as to overlap. In this embodiment, 1/600 inch × 1/600 inch is defined as one pixel, and one pixel is divided into four in a lattice shape, and the reaction liquid and ink are applied as follows. Two drops of the reaction liquid are staggered on the four-divided grid, and four drops of ink are ejected from the ejection openings corresponding to the red position, and four ejections from the ejection openings corresponding to the cyan position. A total of 6 drops of 2 drops in a zigzag pattern were applied to the grid divided into 4 parts.

(Evaluation of image density)
Using each set shown in Table 3, a solid image of 1 cm × 2 cm was formed using PB PAPER GF-500 (manufactured by Canon Inc.) as a recording medium under the above conditions. One hour later, the image density of the solid image was measured using a spectrophotometer (Macbeth RD918; manufactured by Macbeth), and the image density was evaluated. The image density evaluation criteria are as follows. The evaluation results are shown on the right side of Table 3. In the present invention, the following evaluation criteria are set to a level that allows B or higher and a level that does not allow C.
A: The image density was 1.4 or more.
B: The image density was 1.2 or more and less than 1.4.
C: The image density was less than 1.2.

(Evaluation of short-term transfer resistance)
Using each set shown in Table 3, under the above conditions, an A4 size PB PAPER GF-500 (manufactured by Canon) was used as a recording medium, and the following images were formed. Specifically, a solid image of 1 cm × 2 cm was formed below the longitudinal direction, that is, near the rear end of the recording medium when the recording medium after image formation was discharged from the recording apparatus. At this time, the solid image was recorded at an angle of 45 degrees with respect to the longitudinal direction of the recording medium. Then, 5 seconds after the image is formed, the recording medium is folded so that the recording surface is inside so as to pass through the center of gravity of the solid image recorded obliquely, and the finger is slid in the vertical direction to make a crease, The part corresponding to the image was pressurized. Then, the recording medium was opened, and the solid image recorded obliquely was transferred to the corresponding portion when it was bent, and it was visually confirmed whether or not the transfer trace was formed, and the short-term transfer resistance was evaluated. . The same evaluation was performed 10 seconds and 15 seconds after the image was formed, and the short-term transfer resistance was similarly evaluated. Evaluation criteria for short-term transfer resistance are as follows. The evaluation results are shown on the right side of Table 3. In the present invention, the following evaluation criteria are set to a level that allows B or higher and a level that does not allow C.
A: The transfer trace of the solid image could not be confirmed.
B: Although the transfer trace of the solid image was confirmed slightly, no X shape was formed between the transferred portion and the solid image portion.
C: The transfer trace of the solid image could be confirmed, and an X shape was formed between the transferred portion and the solid image portion.

Note that a solid image formed under the same conditions as the short-term transferability evaluation was rubbed once with Sylbon paper on which a weight of 40 g / cm ² was placed 10 seconds after the image was formed. The conventional fixability was also evaluated by visually checking the degree of contamination. However, no transfer trace was confirmed in any of the sets of Examples and Comparative Examples.

  In addition, the following evaluation test was performed using an ink jet recording apparatus (trade name: PIXUS MX7600; manufactured by Canon) equipped with a recording head for ejecting ink by the action of thermal energy and a coating roller for applying a reaction liquid. Specifically, the reaction liquid 4 prepared above was set in the clear ink tank, and the inks constituting the sets of Examples 1 to 7 were set in the black position, respectively. In addition, short-time transferability was evaluated. As a result, even when the reaction solution was applied to the recording medium by the roller coating method, the same evaluation results as in Examples 1 to 7 when the reaction solution was applied by the ink jet method were obtained, and it was confirmed that the effect of the present invention was obtained. It was done.

Claims (6)

A set of an ink and a reaction liquid having a combination of an ink containing a pigment and a water-soluble resin and a reaction liquid having a buffering capacity in an acidic region,
The pigment is a self-dispersing pigment having a surface charge obtained by colloid titration of 0.20 mmol / g or more, and the water-soluble resin is a copolymer having a unit derived from (meth) acrylic acid. A set of ink and reaction liquid characterized by being.   The content (% by mass) of the water-soluble resin in the ink is a mass ratio with respect to the content (% by mass) of the pigment in the ink and is 0.25 to 0.75 times. Set of ink and reaction liquid.   The set of the ink and the reaction liquid according to claim 1 or 2, wherein an acid value of the water-soluble resin is 100 mgKOH / g or more and 215 mgKOH / g or less.   The ink and the reaction liquid according to any one of claims 1 to 3, wherein the reaction liquid contains an organic acid having a carboxy group and has a pH of 3.5 or more and 5.5 or less. Set.   The ink and reaction liquid set according to claim 4, wherein the organic acid has at least two carboxy groups.   An image forming method comprising: a step of applying ink to a recording medium; and a step of applying a reaction liquid to the recording medium, and forming an image by bringing the ink and the reaction liquid into contact with each other on the recording medium. An image forming method comprising using the reaction liquid and ink set according to claim 1 as the liquid and ink.