JP2012224740A - Ink, inkjet recording method, and ink cartridge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an ink which can satisfy at high level the storage stability of a self-dispersible organic pigment-containing ink and improves color-developing properties in an image recorded on a regular paper or the like; an ink cartridge using the ink; and an inkjet recording method.SOLUTION: The ink includes a pigment, a resin, and a salt. A self-dispersible organic pigment is used as the pigment, which has a surface charge amount measured by a colloidal titration of 0.9 to 1.5 μmol/m. The resin exhibits an acid value of 100 to 160 mg-KOH/g. The ink has a predetermined salt. The ink cartridge using the ink and the inkjet recording method are also provided.

Description

  The present invention relates to an ink suitable as an ink for ink jet, an ink cartridge using the ink, and an ink jet recording method.

  In the ink jet recording method, recording on various recording media is possible. And for the purpose of better image formation, for example, ink suitable for recording photographic image quality on glossy paper or ink suitable for recording documents on plain paper etc. Accordingly, various ink proposals have been made. In recent years, plain paper or the like is used as a recording medium, and the ink jet recording method is also used for printing business texts including characters and charts, and the frequency of use for such applications has increased remarkably. For this reason, pigments are increasingly used not only as black inks but also as color materials for color inks as inks suitable for recording characters and charts on recording media such as plain paper.

  However, a color ink containing a pigment as a color material has a problem in that the color developability of an image obtained is inferior to that of a dye, and various methods for solving this problem have been proposed. For example, there is a proposal for improving the color developability of an image recorded on plain paper by using a self-dispersing pigment that can be stably dispersed in ink without using a resin dispersant and an ink containing a specific salt. Yes (see Patent Document 1). Further, it has been proposed to improve the optical density of an image recorded on plain paper by using an ink containing a self-dispersing pigment and a specific resin (see Patent Document 2). Furthermore, it has been proposed to use a self-dispersing pigment with a specified surface treatment amount so as not to cause a change in viscosity even when the salt-containing ink is stored at a high temperature for a long period of time (see Patent Document 3). ).

Japanese Patent Laid-Open No. 2001-081378 JP 2000-169769 A JP 2002-080763 A

  The present inventors have studied to improve the color developability of an image recorded with a color ink containing a self-dispersing pigment with reference to the above-described techniques. However, when using an organic pigment having a lower surface functional group density (the amount of ionic groups bonded to or adsorbed on the surface of pigment particles per 1 g of pigment) than carbon black used in black ink, It has been found that it creates unique problems as described. That is, when a salt or resin is further added to an ink containing a self-dispersing organic pigment, the resulting color develops, but the viscosity and the particle size of the pigment increase when the ink is stored for a long period of time. Was confirmed to occur.

  Therefore, the object of the present invention is to satisfy the storage stability of the ink containing the self-dispersing organic pigment at a high level, and to improve the color development of the image when using plain paper as the recording medium. An object of the present invention is to provide an ink that can be made to flow. Another object of the present invention is to provide an ink cartridge and an ink jet recording method capable of stably obtaining an image having excellent color developability when plain paper or the like is used as a recording medium by using the ink. There is to do.

The above object is achieved by the present invention described below. That is, the ink according to the present invention is an ink containing a pigment, a resin, and a salt, and the pigment has a surface charge amount determined by colloid titration of 0.9 μmol / m ² or more and 1.5 μmol / m ^2. A self-dispersing organic pigment, wherein the resin exhibits an acid value of 100 mgKOH / g to 160 mgKOH / g, and the salt is selected from the group consisting of alkali metal ions, ammonium ions and organic ammonium ions At least one cation, and Cl ⁻ , Br ⁻ , I ⁻ , ClO ⁻ , ClO ₂ ⁻ , ClO ₃ ⁻ , ClO ₄ ⁻ , NO ₂ ⁻ , NO ₃ ⁻ , SO ₄ ²⁻ , CO ₃ ²⁻ , HCO ₃ ⁻ , HCOO ⁻ , (COO ⁻ ) ₂ , COOH (COO ⁻ ), CH ₃ COO ⁻ , C ₂ H ₄ (COO ⁻ ) ₂ , C ₆ H ₅ COO ⁻ , C ₆ H ₄ (COO ⁻ ) ₂ , PO ₄ ^3- HPO ₄ ^2-, and H ₂ PO ₄ ^- and at least one anion selected from the group consisting of is characterized in that which is configured by bonding. The present invention also provides an ink cartridge containing the ink in an ink containing portion, and an ink jet recording method for recording an image on a recording medium by ejecting the ink by an ink jet method.

  According to the present invention, the storage stability of an ink containing a self-dispersing organic pigment can be satisfied at a high level, and the color development of an image can be improved when plain paper or the like is used as a recording medium. Ink is provided. According to another embodiment of the present invention, there is provided an ink cartridge and an ink jet recording method capable of stably performing image recording excellent in color developability by using the ink.

Hereinafter, the present invention will be described in detail with reference to preferred embodiments. Note that various physical properties such as viscosity in the present invention are values at 25 ° C. unless otherwise specified.
First, the background to the present invention will be described. Among the commercially available self-dispersing organic pigments, the present inventors used Cab-O-Jet250C (manufactured by Cabot), which has the highest surface charge amount per unit area of pigment particles of 2.17 μmol / m ² . The following examination was conducted. Specifically, two types of inks, that is, an ink added with an appropriate amount of salt (potassium sulfate) together with this pigment and an ink not added, were prepared, and images were recorded with each ink using plain paper as a recording medium. As a result, as proposed in the past, the ink containing the salt clearly improves the color developability of the obtained image as compared with the ink not containing the electrolyte salt. I was able to confirm. Next, the above two types of ink were put in a sealed container and stored in an oven at a temperature of 60 ° C. for one month, and then the viscosity of the ink and the particle diameter of the pigment were measured. As a result, the viscosity of the ink containing no salt and the particle diameter of the pigment did not change from the values before storage, but the ink containing the salt gelled. In addition, the present inventors conducted the same examination as described above for commercially available self-dispersing organic pigments having different surface charge amounts (μmol / m ² ) per unit area of pigment particles as described below. Specifically, Cab-O-Jet260M (1.17 μmol / m ² ), Cab-O-Jet270Y (2.01 μmol / m ² ), Cab-O-Jet450C (1.69 μmol / m ² ), Cab-O -Jet465M (1.73 μmol / m ² ) and Cab-O-Jet470Y (1.82 μmol / m ² ) were evaluated in the same manner. As a result, as described above, when the ink containing a salt was used, it gelled after storage at a high temperature.

Therefore, the present inventors proceeded with studies for enhancing the dispersion stability of the self-dispersing organic pigment and suppressing the change in physical properties during storage. As a result, a self-dispersing organic pigment having a surface charge of 0.9 μmol / m ² or more and 1.5 μmol / m ² or less was used, and a resin having an acid value of 100 mgKOH / g or more and 160 mgKOH / g or less was added. And found that it is effective to use ink. That is, according to the study by the present inventors, the ink having such a configuration is a salt-containing ink, but does not cause a change in physical properties after storage. As described above, the present inventors presume the reason why the above-mentioned problems are solved by defining the surface charge amount of the pigment to be used and further adding a predetermined resin. . In the following description, the resin is assumed to have a hydrophilic unit represented by an acidic group that gives an acid value and a hydrophobic unit.

  First, the present inventors speculated that the following two types of actions would work in the ink by further adding a resin to the ink containing the self-dispersing organic pigment. The first action is that the hydrophobic part such as styrene, which is one of the units constituting the resin, is adsorbed to the gaps on the surface of the pigment particles, that is, the part where the hydrophilic group is not bonded. This increases the dispersion stability. As the second action, the addition of a resin having an acid value increases the electrolyte concentration in the ink and lowers the dispersion stability of the pigment. As a result of various studies, the present inventors have found that, in the ink in which the self-dispersing pigment and the resin satisfying the above-described conditions coexist, the action of increasing and decreasing the dispersion stability of the pigment is completely different. It came to the conclusion that the opposite actions work simultaneously.

  Therefore, the present inventors conducted the following investigation based on the above estimation. First, a self-dispersing pigment having a relatively low surface charge, a salt, a water-soluble organic solvent, a surfactant, an ink 1 containing water, and an ink 2 obtained by adding a resin having a low acid value to the ink 1 were prepared. . Furthermore, an ink 3 containing a self-dispersing organic pigment, a salt, a water-soluble organic solvent, a surfactant, water, and a resin having a high acid value having a relatively high surface charge amount was prepared. Next, an image was recorded using each ink. As a result, in the order of ink 3, ink 2, and ink 1, the obtained image was excellent in color developability. Next, these inks were put in a sealed container and stored in an oven at a temperature of 60 ° C. for one month, and then the viscosity of the ink and the particle diameter of the pigment were measured. As a result, although there was no problem in the inks 1 and 2, in the ink 3, both the viscosity of the ink and the particle diameter of the pigment increased by 5% or more with respect to the values before storage, and there was a problem in storage stability. It was confirmed.

  Based on the results, the inventors measured the metal ion concentration in the ink 2 and the ink 3 and the amount of resin adsorbed on the surface of the pigment particles. As a result, the metal ion concentration in the ink was relatively higher in the ink 3, and the amount of the resin adsorbed on the surface of the pigment particles was relatively higher in the ink 2.

  As described above, the present inventors presume the reason why the surface charge amount of the pigment and the acid value of the resin to be added have affected the color developability and storage stability of the image as follows. That is, when a resin having a high acid value is contained as in the ink 3, the electrolyte concentration in the ink is increased by the neutralizing agent and salt used when the resin is neutralized and dissolved. Further, since the hydrophobic portion is relatively reduced as the acid value of the resin is higher, the adsorption of the resin to the surface of the pigment particles hardly occurs. As a result, in such an ink, the effect of decreasing the dispersion stability of the pigment due to the increase in the electrolyte concentration in the ink is more effective than the effect of improving the dispersion stability of the pigment due to the adsorption of the resin to the surface of the pigment particles. Works great. For these reasons, the present inventors presume that although the color developability of the ink 3 is greatly improved, the dispersion state of the pigment cannot be kept stable for a long period of time.

  On the other hand, even when a resin with a low acid value is added as in the ink 2 described above, the electrolyte concentration in the ink increases due to the addition of salt or resin, but the electrolyte concentration is low because the acid value of the added resin is low. The value was lower than that of Ink 3. Furthermore, since the resin having a low acid value has a relatively large hydrophobic portion, the resin tends to be adsorbed on the surface of the pigment particles, and the dispersion stability of the pigment is increased. As a result, the effect of improving the dispersion stability of the pigment by adsorbing the resin to the surface of the pigment particles is more significant than the effect of reducing the dispersion stability of the pigment due to the increase in the electrolyte concentration in the ink. Therefore, although the color developability is slightly lower than that of the ink 3, it is presumed that the dispersed state of the pigment can be kept stable for a long time.

As a result of further detailed studies, the present inventors have found the following various things. First, the lower the surface charge amount of the pigment, the greater the amount of adsorption of the added resin to the surface of the pigment particles, so the effect of increasing the dispersion stability of the pigment increases. However, when the surface charge amount of the pigment is extremely low as less than 0.9 μmol / m ² , the dispersion stability of the pigment cannot be maintained even if the electrolyte concentration is slightly increased. For this reason, even if the resin has a low acid value in the range of 100 mgKOH / g or more and 160 mgKOH / g or less, the addition of the resin rather deteriorates the dispersion stability of the pigment. On the other hand, if the surface charge amount of the pigment is larger than 1.5 μmol / m ² , even if a resin having a low acid value is added, the amount of the resin adsorbed on the surface of the pigment particles is small. Almost no improvement effect can be obtained. As a result, due to the action of lowering the dispersion stability of the pigment due to the increase in the electrolyte concentration in the ink, the physical property change due to storage is caused, and the storage stability of the ink cannot be obtained.

  Furthermore, the degree of influence of the above two functions also varies depending on the acid value of the resin to be added. First, the lower the acid value of the resin, the lower the degree of increase in the electrolyte concentration in the ink, and the higher the amount of resin adsorbed on the surface of the pigment particles. However, when the acid value of the resin is lower than 100 mgKOH / g, the hydrophobicity becomes too high, and the resin molecules tend to aggregate in the ink rather than adsorb on the surface of the pigment particles. As a result, a resin amount sufficient to increase the dispersion stability of the pigment cannot be adsorbed on the surface of the pigment particles. On the other hand, when the resin acid value is higher than 160 mgKOH / g, not only the amount of resin adsorbed on the surface of the pigment particles is decreased, but also the electrolyte concentration in the ink is increased. As a result, rather than increasing the dispersion stability of the pigment, it is reduced. Therefore, even if a resin having an acid value lower than 100 mgKOH / g or a resin higher than 160 mgKOH / g is added in any case, the storage stability of the ink cannot be improved.

Based on the above detailed study, the present inventors have come to the following conclusion and achieved the present invention. That is, in the ink by adding salt to the self-dispersing organic pigment, the surface charge amount of the self-dispersing organic pigment in the range of 0.9μmol / m ² or more 1.5μmol / m ^2, and an acid value of 100 mg KOH / g or more 160mgKOH It is necessary to add a resin in the range of / g or less.

<Ink>
Hereinafter, components constituting the ink of the present invention will be described.
(Self-dispersing organic pigment)
The color material constituting the ink of the present invention is a self-dispersing organic pigment (referred to as a self-dispersing organic pigment in the present invention).
The kind of organic pigment that can be used in the present invention is not particularly limited, and any known organic pigment can be used. Specific examples include organic pigments such as phthalocyanine and quinacdrine. The content (% by mass) of the self-dispersing organic pigment in the ink is 0.1% by mass or more and 10.0% by mass or less, and further 1.0% by mass or more and 5.0% by mass or less based on the total mass of the ink. It is preferable that

The self-dispersing organic pigment constituting the ink of the present invention requires that the surface charge obtained by colloid titration is 0.9 μmol / m ² or more and 1.5 μmol / m ² . That is, the amount of the hydrophilic group per unit surface area 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 0.9 μmol / m ² or more and 1.5 μmol / m ² . As will be described later, the ink of the present invention contains a resin having an acid value of 100 mgKOH / g or more and 160 mgKOH / g or less. However, in the present invention, this resin is expected to have an effect of assisting the dispersion of the self-dispersing organic pigment, and is not expected to disperse the pigment only by the action of the 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. The self-dispersing organic pigment referred to in the present invention has, for example, a hydrophilic group chemically bonded to the surface of the pigment particle directly or through another atomic group, and the pigment particle is formed by the action of the bonded hydrophilic group. Is possible. 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 organic pigment used in the present invention has the above specific resin. It can be dispersed without using it.

  The surface charge amount of the self-dispersing organic pigment required in the present invention is measured by a colloid titration method. In the examples described later, the surface charge of the self-dispersed pigment in the pigment dispersion is determined by potentiometric titration using methyl glycol chitosan as a titration reagent using a potentiometric automatic titration apparatus equipped with a streaming potential titration unit (PCD-500). The amount was measured. Specifically, the measurement was performed using the above automatic potentiometric titrator using AT-510 manufactured by Kyoto Electronics Industry.

Examples of the hydrophilic group chemically bonded to the surface of the pigment particle directly or through another atomic group include -COOM ₁ , -SO ₃ M ₁ , -PO ₃ HM ₁ , -PO ₃ ( M ₁ ) ₂ (wherein M ₁ 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 ₁ include Li, Na, and K. 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 organic pigment used in the present invention may be a self-dispersing organic pigment that has been subjected to surface oxidation treatment, and the surface charge obtained by colloid titration is 0.9 μmol / m ² or more and 1.5 μmol / if m ^2, and can be used. Examples of such self-dispersing organic pigments include those obtained by methods such as oxidation with sodium hypochlorite, ozone treatment in water, wet oxidation with an oxidizing agent after ozone treatment, and modifying the particle surface. Is mentioned. In the present invention, in addition to the above, 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. .

(resin)
The ink of the present invention needs to contain a resin having an acid value in the range of 100 mgKOH / g to 160 mgKOH / g in addition to the above self-dispersing organic pigment. As described above, this resin is used in anticipation of the function of assisting the dispersion of the self-dispersing organic pigment. The resin used in the present invention is preferably water-soluble. In the present invention, “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.

  In the present invention, the content (% by mass) of the resin having an acid value in the specific range in the ink is preferably 1.0% by mass or more and 3.0% by mass or less based on the total mass of the ink. Furthermore, it is good to set it as 1.0 mass% or more and 2.0 mass% or less. Further, the content of the resin in the ink is preferably 0.25 times or more and 0.75 times or less in terms of a mass ratio with respect to the content of the self-dispersing organic pigment. In this case, the contents of the resin and the self-dispersing organic pigment are values based on the total mass of the ink. In the present invention, the inclusion of the above-described resin in the ink corresponds to an increase in the electrolyte concentration in the ink. Therefore, if the resin is contained more than necessary with respect to the pigment, the dispersion state of the pigment is not improved. Since it may be stabilized, it is not preferable.

  As a specific resin suitably used in the present invention, 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, Anionic monomers such as anhydrides and salts of acidic monomers, monomers having a hydroxy group such as (meth) acrylic acid-2-hydroxyethyl, (meth) acrylic acid-3-hydroxypropyl, methoxy ( Mono, di, tri, poly) monomers having an ethylene oxide group such as ethylene glycol (meth) acrylate. Examples of the cation constituting the salt of the anionic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium.

  Moreover, the following are mentioned as a monomer which has a hydrophobic group which becomes a hydrophobic unit by superposition | polymerization. For example, monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth) acrylate. Aliphatic groups such as ethyl (meth) acrylate, methyl (meth) acrylate, (iso) propyl (meth) acrylate, (n-, iso-, t-) butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, etc. And the like.

  The form of the resin used in the present invention is not limited, and examples thereof include random, block, and graft. Among them, in the present invention, a block copolymer is used to enhance the adsorptivity to the surface of the pigment particles. It is preferable. Further, the resin used in the ink of the present invention preferably has a weight average molecular weight of 3,000 or more and 10,000 or less. When the weight average molecular weight is less than 3,000, there is a case where the hydrophilicity is not sufficiently increased when adsorbed on the surface of the pigment particles, and the dispersed state of the pigment cannot be kept stable. On the other hand, if the weight average molecular weight exceeds 10,000, the resin may not be sufficiently adsorbed on the surface of the pigment particles due to the steric hindrance of the resin.

  As a result of the study by the present inventors, it was found that the amount of the resin adsorbed on the surface of the pigment particles is in the range of 10% to 40% based on the content of the pigment in the ink. Then, an image recorded with an ink containing a resin-dispersed pigment and salt within the above range of the resin absorption amount, and an ink of the present invention containing a self-dispersing organic pigment and salt within the above range of the resin absorption amount. As a result of comparison of the chromogenic properties, the latter showed higher chromogenic properties. The present inventors presume this reason as follows. That is, the dispersion stability of the self-dispersing pigment in the ink is brought about by the action of ionic repulsion of the hydrophilic group bonded to the particle surface directly or through another atomic group. On the other hand, the dispersion stability of the resin-dispersed pigment is brought about by the solubility of the resin adsorbed on the surface of the pigment particles and the action of steric repulsion. In this way, when the electrolyte concentration is increased by the addition of salt in two types of inks with different pigment dispersion methods, the effect of the electrolyte concentration is higher in the ink using the self-dispersing pigment in which the pigment is dispersed by ion repulsion. It is presumed that high color developability can be obtained because it is more susceptible to exposure to water.

(salt)
The ink of the present invention contains at least one cation selected from the group consisting of alkali metal ions, ammonium ions, and organic ammonium ions, Cl ⁻ , Br ⁻ , I ⁻ , ClO ⁻ , ClO ₂ ⁻ , ClO ₃ ⁻ , ClO ₄ ⁻ , NO ₂ ⁻ , NO ₃ ⁻ , SO ₄ ²⁻ , CO ₃ ²⁻ , HCO ₃ ⁻ , HCOO ⁻ , (COO ⁻ ) ₂ , COOH (COO ⁻ ), CH ₃ COO ⁻ , C ₂ H ₄ At least one anion selected from the group consisting of (COO ⁻ ) ₂ , C ₆ H ₅ COO ⁻ , C ₆ H ₄ (COO ⁻ ) ₂ , PO ₄ ³⁻ , HPO ₄ ²⁻ , and H ₂ PO ₄ ^−. And a salt formed by combining with each other. Examples of the alkali metal include Li, Na, and K. The form of the salt in the ink may be in a state in which a part thereof is dissociated or in a state in which all is dissociated.

  The salt content (% by mass) in the ink is preferably 0.3% by mass or more and 1.5% by mass or less based on the total mass of the ink. Furthermore, it is more preferable that it is 0.3 mass% or more and 1.0 mass% or less. If the content is less than 0.3% by mass, the effect of improving the color developability of an image recorded on a recording medium such as plain paper tends to be insufficient, such being undesirable. On the other hand, if the content exceeds 1.5% by mass, the dispersion state of the pigment may not be kept sufficiently stable regardless of the surface charge amount of the self-dispersing organic pigment or the acid value of the resin. Absent. The salt content in the ink is 0.075 to 0.40, more preferably 0.075 to 0.25, in terms of the mass ratio to the content of the self-dispersing organic pigment. preferable. In this case, the contents of the salt and the self-dispersing organic pigment are values based on the total mass of the ink. In particular, even if the content of the self-dispersing organic pigment is small, if the salt content is increased more than necessary, the dispersion stability of the pigment cannot be obtained sufficiently, and the storage stability of the ink tends to decrease. It is not preferable. For this reason, the salt content in the ink is preferably in the range of a suitable mass ratio with respect to the pigment content in the ink. The salt content in the ink of unknown composition can be calculated based on the amount of anion calculated by ion chromatography using the ink as a measurement sample.

(Aqueous medium)
The ink of 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. In addition, as the water-soluble organic solvent, conventionally, for example, any known one generally used for ink jet ink can be used, and one or two or more water-soluble organic solvents can be used. it can. Specifically, monohydric or polyhydric alcohols, alkylene glycols having an alkylene group of about 1 to 4 carbon atoms, polyethylene glycols having an average molecular weight of about 200 to 2,000, glycol ethers, nitrogen-containing compounds Can be used. 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 of the present invention may contain an organic compound that is solid at room temperature such as trimethylolethane or trimethylolpropane, or a nitrogen-containing compound such as urea or ethyleneurea, if necessary. In addition to the above components, if necessary, a surfactant, a pH adjuster, an antifoaming agent, a rust inhibitor, an antiseptic, an antifungal agent, an antioxidant, an anti-reduction agent, an evaporation accelerator, Various additives such as a chelating agent may be contained in the ink.

<Ink cartridge>
The ink cartridge of the present invention includes an ink storage portion that stores ink, and the ink storage portion stores the ink of the present invention described above. As for the structure of the ink cartridge, the ink storage portion includes an ink storage chamber that stores liquid ink and a negative pressure generation member storage chamber that stores a negative pressure generation member that holds the ink therein by a negative pressure. Things. Alternatively, the ink cartridge may be an ink container that does not have an ink storage chamber for storing liquid ink and is configured to hold the entire storage capacity by a negative pressure generating member. Further, the ink cartridge may be configured to have an ink storage portion and a recording head.

<Inkjet recording method>
The ink jet recording method of the present invention is a method of recording an image on a recording medium by ejecting the ink of the present invention described above with an ink jet recording head. Examples of a method for ejecting ink include a method for imparting mechanical energy to the ink and a method for imparting thermal energy to the ink. In the present invention, it is particularly preferable to employ an ink jet recording method that utilizes thermal energy. . Other than using the ink of the present invention, the steps of the ink jet recording method may be known.

  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.

<Preparation of water-soluble resin>
Using each monomer (unit: part) shown in the upper part of Table 1, it was copolymerized by a conventional method to synthesize water-soluble resins A to F, respectively. Each obtained resin was used for each, and each resin aqueous solution was prepared as follows. Specifically, a 10.0% sodium hydroxide aqueous solution is used to neutralize all acidic groups in the resin, and ion-exchanged water is further added so that the resin (solid content) content is 10.0. % Resin aqueous solutions A to F were prepared. The lower part of Table 1 shows the characteristics of the resin.

<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.

(Preparation of pigment dispersion A)
To a solution of 5 g of concentrated hydrochloric acid dissolved in 5.5 g of water, 1.0 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 to keep the solution at 10 ° C. or lower, and 1.2 g of sodium nitrite is dissolved in 9 g of water at 5 ° C. Solution was added. After stirring this solution for another 15 minutes, C.I. having a specific surface area of 130 m ² / g. I. 6 g of Pigment Red 122 (organic pigment) was added under 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. ) A self-dispersing organic pigment A having _two groups bonded thereto was prepared. Water was added to the obtained self-dispersing organic pigment A and dispersed so that the pigment content was 10.0% to prepare a pigment dispersion A. The surface charge obtained by colloidal titration of the self-dispersing organic pigment A in the pigment dispersion A was 0.9 μmol / m ² .

(Preparation of pigment dispersions B, C, D)
Pigment dispersions B, C, and D having a self-dispersing organic pigment content of 10.0% by the same procedure as in pigment dispersion A, except that the amounts used of p-aminobenzoic acid and sodium nitrite were changed. Were prepared respectively. The surface charge amounts determined by colloidal titration of the self-dispersing organic pigment in each dispersion thus obtained are 1.5 μmol / m ² for self-dispersing organic pigment B and 0.8 μmol for self-dispersing organic pigment C, respectively. / M ² , and the self-dispersing organic pigment D was 1.6 μmol / m ² .

(Preparation of pigment dispersion E)
The pigment dispersion E used in the comparative example was prepared by the following method. First, C.I. having a specific surface area of 130 m ² / g. I. Pigment Red 122 (organic pigment) 10.0 parts, weight average molecular weight 10,000, 10.0% aqueous solution of resin B (solid content) neutralized with potassium hydroxide equivalent to the acid value, ion 40.0 parts, ion 50.0 parts of exchange water was mixed. Next, this mixture was charged into a batch type vertical sand mill (manufactured by IMEX) and dispersed for 3 hours. Then, after performing pressure filtration with a micro filter (manufactured by Fuji Film) having a pore size of 3.0 μm, water is added, the organic pigment content is 10.0%, and the resin content is 4.0% by mass. To obtain a pigment dispersion E. The above pigment is not a self-dispersing type.

<Preparation of ink>
After mixing each component (unit:%) shown in the upper part of Table 2 and stirring sufficiently, pressure filtration is performed with a micro filter (manufactured by Fuji Film) having a pore size of 1.0 μm, and Examples, Comparative Examples and Reference Each ink of the example was prepared. Acetylenol E100 is a surfactant manufactured by Kawaken Fine Chemicals, and polyethylene glycol having an average molecular weight of 600 was used. The lower part of Table 2 shows the resin content [%], pigment content [%], salt content [%], resin content / pigment content value [times] in each ink. The salt content / pigment content value [times] was also shown.

<Evaluation>
(Evaluation of color development)
Each ink obtained above was loaded in an ink jet recording apparatus (trade name: PIXUS Pro 9500; manufactured by Canon Inc.) equipped with a recording head that is filled in an ink cartridge and ejects ink by the action of thermal energy. This recording apparatus has a resolution of 600 dpi × 600 dpi, and recorded a 5 cm × 5 cm solid image under the condition that six ink droplets of 3.5 pL per droplet are applied to a unit area of 1/600 inch × 1/600 inch. The printer driver settings at this time were paper type: plain paper, print quality: standard, and color adjustment: automatic. Further, PB PAPER GF-500 (manufactured by Canon), which is plain paper, was used as a recording medium. The obtained image was dried at a temperature of 23 ° C. and a relative humidity of 55% for 24 hours, and then solid using a spectrophotometer (trade name: Spectrolino; manufactured by Gretag Macbeth) under conditions of a light source: D50 and a field of view: 2 °. The optical density of the image was measured and the color developability of the image was evaluated. The evaluation criteria of color developability are as follows. The evaluation results are shown in Table 3. In the present invention, the following evaluation criteria are set to a level where B or higher is acceptable and C or lower is unacceptable.
AA: The optical density was 0.2 or more higher than the reference value.
A: The optical density was 0.1 or more higher than the reference value.
B: The optical density was higher than the reference value, but the degree exceeding the reference value was less than 0.1.
C: Reference value (image using the ink of Reference Example 1).
D: The optical density was lower than the reference value.

(Evaluation of storage stability)
For each of the inks obtained above, the viscosity of the ink and the particle size of the pigment before storage were measured. Each ink was then sealed in a polytetrafluoroethylene container. The container was stored in an oven at a temperature of 60 ° C. for 1 month, and then the ink was returned to room temperature. Thereafter, the viscosity of the ink after storage and the particle diameter of the pigment were measured. The particle diameter of the pigment was measured using a concentrated particle size analyzer FPAR-1000 (manufactured by Otsuka Electronics). Then, the storage stability of the ink was evaluated by comparing the viscosity of the ink before and after storage with the particle diameter of the pigment. The evaluation criteria for storage stability are as follows. The evaluation results are shown in Table 3. In the present invention, the following evaluation criteria are set to a level where B or higher is acceptable and C or lower is unacceptable.
AA: The viscosity of the ink and the particle diameter of the pigment did not change before and after storage.
A: The particle diameter of the pigment did not change before and after storage, but the viscosity of the ink after storage was less than 5% higher than before storage.
B: The viscosity of the ink after storage and the particle diameter of the pigment were both less than 5% higher than before storage.
C: The viscosity of the ink after storage and the particle diameter of the pigment were both 5% or more higher than before storage.
D: The ink gelated upon storage, and the viscosity of the ink and the particle diameter of the pigment could not be measured after storage.

  The evaluation results of the color developability shown in Table 3 are all “AA”, but the color developability of the image when the ink of Example 4 is used is slightly smaller than that of Examples 2, 3, 5 and 7. It was excellent. Further, the color developability of the images when the inks of Examples 1, 6, 8, 9, 10, 11, and 12 are used have the same “A” evaluation, but the inks of Examples 6, 8, and 9 are used. The color developability of the image was slightly better than that of the inks of Examples 1, 10, 11 and 12. Furthermore, although the storage stability evaluation results shown in Table 3 are all “B”, the ink of Example 4 is slightly more than the inks of Examples 1, 2, 3, 5 and 11. The rate of increase in viscosity was high. In addition, inks using various anion and cation salts defined in the present invention other than those used in the above examples were similarly evaluated. As a result, almost the same result as that of the control example was obtained in any salt.

Claims (7)

An ink comprising a pigment, a resin and a salt,
The pigment is a self-dispersing organic pigment having a surface charge amount determined by colloid titration of 0.9 μmol / m ² or more and 1.5 μmol / m ² or less,
The resin has an acid value of 100 mgKOH / g or more and 160 mgKOH / g or less,
The salt is at least one cation selected from the group consisting of alkali metal ions, ammonium ions and organic ammonium ions, and Cl ⁻ , Br ⁻ , I ⁻ , ClO ⁻ , ClO ₂ ⁻ , ClO ₃ ⁻ , ClO ₄ ^−. , NO ₂ ⁻ , NO ₃ ⁻ , SO ₄ ²⁻ , CO ₃ ²⁻ , HCO ₃ ⁻ , HCOO ⁻ , (COO ⁻ ) ₂ , COOH (COO ⁻ ), CH ₃ COO ⁻ , C ₂ H ₄ (COO ⁻ ) ₂ , C ₆ H ₅ COO ⁻ , C ₆ H ₄ (COO ⁻ ) ₂ , PO ₄ ³⁻ , HPO ₄ ²⁻ , and H ₂ PO ₄ ⁻ are combined with at least one anion. An ink characterized by being configured as described above.   The ink according to claim 1, wherein the content (% by mass) of the resin is 1.0% by mass or more and 3.0% by mass or less based on the total mass of the ink.   The ink according to claim 1 or 2, wherein a content (% by mass) of the salt is 0.3% by mass or more and 1.0% by mass or less based on the total mass of the ink.   The ink according to any one of claims 1 to 3, wherein the resin is a block copolymer. The self-dispersing organic pigment, which hydrophilic groups are coupled directly or through another atomic group to the surface of the pigment particles, the hydrophilic group _{is, -COOM 1, -SO 3 M 1} , - The PO ₃ HM ₁ and —PO ₃ (M ₁ ) ₂ (wherein M ₁ is at least one selected from the group consisting of a hydrogen atom, an alkali metal, ammonium and organic ammonium). The ink according to any one of 1 to 4.   An ink cartridge having an ink storage portion for storing ink, wherein the ink stored in the ink storage portion is the ink according to any one of claims 1 to 5.   6. An ink jet recording method for recording an image on a recording medium by ejecting ink from an ink jet recording head, wherein the ink is the ink according to claim 1. Inkjet recording method.