JP2007145928A - Inkjet ink, method for producing inkjet ink, inkjet printer, and inkjet recording method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet ink which, when applied to plain paper, causes less paper curling and less strike-through and has high ink dispersion stability and ink delivery stability, to provide a method for producing the ink, to provide an inkjet printer fitted with the ink, and to provide an inkjet recording method using the ink. <P>SOLUTION: The water-based ink includes essentially consisting of water, a pigment dispersion, and a water-soluble organic solvent, is the total amount of the water-soluble organic solvent being 50 to below 90 mass% based on the total ink, and includes portion of the water-soluble organic solvent which has an SP value of 16.5 to below 24.6 accounts for 30 mass% based on the entire ink. The method for producing the inkjet ink includes the water-based ink which is coated with the pigment particles constituting the pigment dispersion by a film-forming resin. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a water-based ink, in particular, a water-based ink used in an ink jet method, a method for producing a water-based ink, an ink jet printer, and an ink jet recording method. Furthermore, the present invention relates to a water-based ink capable of obtaining high image quality in plain paper printing, a method for producing a water-based ink, an ink jet printer, and an ink jet recording method.

  In recent years, the ink jet recording system can create images easily and inexpensively, and thus has been applied to various printing fields such as photographs, various printing, marking, and special printing such as color filters. In particular, ink jet recording devices that emit and control fine dots, ink with improved color reproduction gamut, durability, and emission suitability, ink absorbency, coloring material color development, surface gloss, etc. are dramatically improved. It is also possible to obtain image quality comparable to silver halide photography using special paper.

  However, in an inkjet image recording system that requires dedicated paper, there are problems in that the recording media that can be used are limited and the cost of the recording media is increased.

  On the other hand, in the office, there is an increasing need for a system that can perform full-color printing at high speed without being restricted by a recording medium (for example, plain paper, coated paper, art paper).

  In particular, it is desired to perform high-quality printing at high speed on plain paper (for example, PPC paper, non-coated paper for printing, etc.) that is inexpensive and easily available. When aqueous ink is used, there are problems such as character image quality, image bleeding, low reflection density, curling after printing, cockling, and the like. When oil-based ink is used, there is no problem with curling and cockling, but there are problems such as character image quality inferior to aqueous ink, intense bleeding, and low reflection density. In addition, when performing double-sided printing, back-through (print-through) is also a major problem. In order to solve the problem of printing on plain paper, various studies have been conducted including the composition of ink.

  As one of the improvement methods for water-based inks, a technique for improving image bleeding and ink back-through during printing on plain paper by using an ink containing a vinylpyrrolidone and an acrylic acid copolymer having a molecular weight of 80,000 to 250,000 is disclosed. (For example, refer to Patent Document 1). However, in this method, when the ink attached to the nozzle plate is wiped off by the wiping operation, the added ultra-high molecular weight polymer tends to stick to the nozzle plate, so that wiping residue is likely to occur. There has been a problem that image degradation is likely to occur due to a positional shift.

  As another method, a technique for reducing feathering, bleeding, and back-through during printing on plain paper using an ink containing alginic acid or alginate is disclosed (for example, see Patent Document 2). In this method, the alginic acid or alginate contained in the ink reacts with a polyvalent metal cation such as calcium or aluminum contained in the plain paper to cause gelation and thickening, thereby improving feathering, bleeding and strike-through. However, polyvalent metal ions such as calcium and aluminum in plain paper are not essential components, but are contained as impurities. The content varies depending on the brand of the paper, and some of the plain paper has extremely low content. Exists. Therefore, the magnitude of these effects depends on the brand of paper to be printed, and there is a problem that a sufficient effect cannot be obtained depending on the brand of paper.

  Further disclosed is a technique for reducing color bleed in plain paper printing by discharging a colorless liquid composition containing a cationic polymer having an average molecular weight of 400 to 1400 and a colored ink on separate papers and combining them on paper. (For example, see Patent Document 3). However, this method requires a head for discharging colorless liquid. Further, there is a problem that paper cockling and curling are likely to occur due to an increase in the total amount of liquid recorded on the paper by the amount of the colorless liquid composition.

  On the other hand, in an oil-based ink composition with almost no curling or cockling, it contains 10% or more of a solvent (nonpolar solvent) having a 50% distillation point of 280 ° C. or lower, and a solvent (polar solvent, ester) of 300 ° C. or higher. A technique for improving bleeding after recording by containing 20% or more of a (system solvent) is disclosed (for example, see Patent Document 4). However, even with this method, the reflection density, behind-the-scenes, and character reproducibility are not necessarily at sufficient levels.

  On the other hand, as essential problems in pigment ink, there are ink dispersion stability and ejection stability. A microencapsulated pigment ink has been proposed as an ink for ink jet recording that has excellent ink dispersion stability and nozzle clogging in a recording head, and excellent discharge stability (see, for example, Patent Document 5).

However, for example, problems remain in curling, cockling, and ejection stability when printing on plain paper.
Japanese Patent Laid-Open No. 10-1629 JP 2003-176432 A JP-A-10-95107 Japanese Patent Laid-Open No. 2004-2666 JP-A-8-183920

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide an ink jet ink having high ink dispersion stability and ejection stability with little show-through and curl when printed on plain paper, and the ink. Manufacturing method, an ink jet printer equipped with the ink, and an ink jet recording method using the ink.

  The above object of the present invention is achieved by the following configurations.

  1. It contains at least water, a pigment dispersion, and a water-soluble organic solvent, and the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total ink, and the SP value of the water-soluble organic solvent is 16. A water-based ink containing a water-soluble organic solvent of 5 to less than 24.6 in an amount of 30% by mass or more of the total ink, wherein the pigment particles constituting the pigment dispersion are coated with a film-forming resin. A method for producing an inkjet ink.

  2. 2. The method for producing an ink-jet ink as described in 1 above, wherein the film-forming resin has a glass transition temperature (Tg) of 50 ° C. or higher and 150 ° C. or lower.

  3. 3. The method for producing an ink-jet ink according to 1 or 2, wherein the film-forming resin has an acid group, and 40 mol% or more and 100 mol% or less of the acid group is neutralized in a base. .

  4). 4. The method for producing an inkjet ink according to any one of 1 to 3, wherein the film-forming resin has an acid value of 50 or more and 240 or less.

  5. An ink-jet ink produced by the method for producing an ink-jet ink according to any one of 1 to 4 above.

  6). An inkjet printer comprising an inkjet head having a nozzle diameter of 25 microns or less and emitting the inkjet ink described in 5 above from the nozzle.

  7). The nozzle has a line head arranged in a direction intersecting with the paper transport direction, and recording is performed by ejecting the ink-jet ink as described in 5 above from the nozzle without substantially scanning the head. 7. The ink jet printer as described in 6 above.

  8). 8. An ink jet recording method comprising printing on both sides of plain paper using the ink jet printer described in 6 or 7 above.

  INDUSTRIAL APPLICABILITY According to the present invention, an ink-jet ink having a high level of ink dispersion stability and ejection stability with little show-through and curl when printed on plain paper, a method for producing the ink, an ink-jet printer equipped with the ink, and the ink An ink jet recording method using can be provided.

  The best mode for carrying out the present invention will be described in detail below, but the present invention is not limited thereto.

  As a result of intensive studies in view of the above problems, the present inventors contain at least water, a pigment dispersion, and a water-soluble organic solvent, and the total amount of the water-soluble organic solvent is 50% by mass or more and 90% by mass of the total ink. % Of the water-soluble organic solvent, and an aqueous ink containing 30% by mass or more of the water-soluble organic solvent having an SP value of 16.5 or more and less than 24.6, and constituting the pigment dispersion Ink jet ink manufactured by the ink jet ink manufacturing method, in which pigment particles to be coated are coated with a film-forming resin, has low see-through and curl when printed on plain paper, and has high ink dispersion stability and ejection stability. It has been found that a water-based ink having a property can be realized, and has reached the present invention.

  When ink with a high water content in ink such as general water-based ink is recorded on plain paper, the penetration into the paper is not so great, but acceptable letter image quality and show-through are obtained, but curl is very Big.

  The reason why acceptable character image quality and behind-the-scenes are obtained is presumed to be that the penetration of the ink is suppressed because the water in the ink is coordinated to the cellulose constituting the plain paper.

  Also, the reason why the curl becomes very large is speculated that the paper forms a paper shape due to hydrogen bonds between the cellulose, but when the water in the ink is coordinated to the cellulose, the hydrogen bonds are broken. Then, the cellulose swells with water, so that the distance between the cellulose fibers in the portion where the ink is applied is separated, and curling occurs. Further, by the subsequent drying of water, hydrogen bonds are formed again and held at a place different from the hydrogen bonds formed before being driven. As a result, the plain paper as a whole is deformed into a different shape. This deformation appears as curl.

  On the other hand, when water-free, for example, oil-based ink is recorded on plain paper, the penetration of the ink into the plain paper is large, and the surface image density is lowered and the ink see-through is increased.

  As a reason for the decrease in surface image density and the increase in the ink see-through, it is speculated that the interaction between the oil-based solvent and the cellulose is small, so that the oil-based solvent easily diffuses the gaps between the cellulose and the ink. Since penetration increases, it is considered that the image density is lowered and the show-through is increased. On the other hand, oil-based solvents do not break hydrogen bonds between celluloses, so the paper structure before recording is maintained, and it is thought that curling is difficult.

  In the water-based ink of the present invention, the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total ink, and among the water-soluble organic solvents, the water-soluble organic solvent having an SP value of 16.5 or more and less than 24.6. The first characteristic is that the solvent is contained in an amount of 30% by mass or more based on the total ink.

  With the solvent composition described above, it is possible to obtain a print that is excellent in character image quality, has little show-through, and has little curl even when recorded on plain paper. Furthermore, the effect of achieving both character image quality, back-through and curl suppression is completely deviated from the trend of water-based ink and oil-based ink.

The SP value as used in the present invention refers to the solubility parameter of the solvent, and is a value represented by the square root of the molecular aggregation energy. It is described in Polymer Handbook (Second Edition) Chapter IV, Solubility Parameter Values, and the value was used. . The unit is (MPa) ^1/2 and refers to the value at 25 ° C. In addition, R.D. F. It can be calculated by the method described in Fedors, Polymer Engineering Science, 14, p147 (1967).

  As a mechanism for manifesting this effect, the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total ink, and the SP value of the water-soluble organic solvent is 16.5 or more and 24.6. By containing less than 30% by weight of a water-soluble organic solvent in the total ink, hydrogen bond breakage between celluloses is difficult to occur, and even if it occurs, it is difficult to swell cellulose, so the distance between cellulose fibers is difficult to separate. Curling is difficult to occur. On the other hand, since at least water is contained, the ink has a function of coordinating with cellulose.

  Further, it is more preferable to contain a water-soluble organic solvent having an SP value of 16.5 or more and less than 24.6 by 50% by mass or more of the total ink. Most preferably, it contains 60% by mass or more of the total ink.

  Moreover, it is preferable that water is 10-45 mass% in all the inks. The presence of 10% by mass or more of water is preferable for back-through, and the presence of 45% by mass or less of water is preferable from the viewpoint of curling suppression.

  Thus, although the improvement effect with respect to curling, conking, and back-through can be obtained with the ink having the above composition, the remaining problem is emission.

  This is because, as described above, by using an organic solvent having a relatively low SP value, it becomes easy to get wet on the nozzle surface of the inkjet head, and the pigment dispersion that does not adsorb to the pigment particles in the ink alone exists. It has been found that the agent may adhere to the nozzle surface of the ink jet head and deteriorate the emission property. This is because the SP value is a value represented by the square root of the cohesive energy of the molecule, and the low value is considered to be an index indicating the ease of wetting.

  In addition, recent inkjet printers have increased the number of printers that have line heads arranged in a direction intersecting with the paper transport direction for the purpose of speeding up printing, and the heads do not substantially scan. Response is being sought.

  In the ink of the present invention, although the solvent composition has a low SP value, by using a resin-coated pigment, both the plain paper recording performance and the emission stability due to the solvent composition having a low SP value can be achieved for the first time.

  Details of the present invention will be described below.

  The solvent composition of the present invention contains at least water and the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total ink, and the SP value of the water-soluble organic solvent is 16.5 or more and 24. Less than 6 water-soluble organic solvents are contained in an amount of 30% by mass or more based on the total ink. Furthermore, the total amount of the water-soluble organic solvent having an SP value of 16.5 or more and less than 22.5 is preferably 30% by mass or more of the ink.

  The ink of the present invention contains 30% by mass or more of a water-soluble organic solvent having an SP value of 16.5 or more and less than 24.6. When this addition amount is less than 30% by mass, curling and cockling during recording on plain paper become extremely large.

  If the SP value of the organic solvent is also less than 16.5, the compatibility with water is deteriorated and separation occurs. Conversely, an organic solvent having an SP value of 24.6 or more has an insufficient curl suppressing effect. The range of the SP value of the organic solvent is more preferably 20.0 or more and less than 22.5.

  Hereinafter, examples of water-soluble organic solvents having an SP value of 16.5 or more and less than 24.6 are shown together with the SP value. Needless to say, the present invention is not limited to this.

Ethylene glycol monomethyl ether (SP value: 24.5)
Ethylene glycol monoethyl ether (23.5)
Ethylene glycol monobutyl ether (22.1)
Ethylene glycol monoisopropyl ether (22.3)
Diethylene glycol monomethyl ether (23.0)
Diethylene glycol monoethyl ether (22.4)
Diethylene glycol monobutyl ether (21.5)
Diethylene glycol diethyl ether (16.8)
Triethylene glycol monomethyl ether (22.1)
Triethylene glycol monoethyl ether (21.7)
Triethylene glycol monobutyl ether (21.1)
Propylene glycol monomethyl ether (23.0)
Propylene glycol monophenyl ether (24.2)
Dipropylene glycol monomethyl ether (21.3)
Tripropylene glycol monomethyl ether (20.4)
1,3-dimethyl-2-imidazolidinone (21.8)
In the present invention, in addition to the water-soluble organic solvent having an SP value of 16.5 or more and less than 24.6, various water-soluble organic solvents are used in combination, and the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total amount of the ink. can do. Examples of water-soluble organic solvents are shown below.

  Examples of water-soluble organic solvents preferably used include alcohols (eg, methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, pentanol, hexanol, cyclohexanol, benzyl alcohol, etc.), Polyhydric alcohols (for example, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, 1,3 -Propanediol, 1,4-butanediol, 1,5-pentanediol, 1,2-pentanediol 1,2-hexanediol, 1,2,6-hexanetriol, etc.), amines (eg, ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine) , Diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, tetramethylpropylenediamine, etc.), amides (eg, formamide, N, N-dimethylformamide, N, N-dimethylacetamide, etc.), Heterocycles (eg, 2-pyrrolidone, N-methyl-2-pyrrolidone, cyclohexyl pyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone), sulfo Kishido compound (such as dimethyl sulfoxide, etc.), sulfones (e.g., sulfolane), urea, acetonitrile, and acetone.

  The ink-jet ink of the present invention is characterized by containing pigment particles coated with a film-forming resin.

  As the film-forming resin according to the ink of the present invention, any of the known and commonly used water-insoluble film-forming resins and resins modified to have self-water dispersibility can be used. Such a film-forming resin is not particularly limited, and examples thereof include acrylic acid resin, maleic acid resin, and polyester resin.

  Even if the glass transition temperature of this film-forming resin is less than 50 ° C., the character fixability after printing on the ink jet recording medium is good, but considering the further improvement of nozzle clogging and storage stability, the glass transition temperature The temperature is preferably 50 ° C. or higher and 150 ° C. or lower, and more preferably 60 ° C. or higher and 150 ° C. or lower is suitable for inkjet recording.

  The film forming resin according to the present invention preferably has an acid value in order to impart a self-water dispersible resin, and is preferably a resin having an acid value of 50 or more and 280 or less. When the acid value is less than 50, the dispersion stability of the obtained pigment particles is not sufficient, and when the acid value exceeds 280, the dispersible resin decomposed into an organic solvent is neutralized with a base. As an ink for inkjet recording, the acid value of the water-dispersible resin is more preferable because it is easy to cause aggregation in the ink, and when it is added to water, nozzle clogging is likely to occur due to dissolution of a part of the resin. It is in the range of 50 or more and 240 or less, particularly preferably in the range of 70 or more and 230 or less.

  As the film-forming resin according to the present invention, a styrene- (meth) acrylic acid resin is generally preferable. For example, substituted styrene such as styrene or α-methylstyrene, acrylic acid methyl ester, acrylic acid ethyl ester, At least one or more selected from acrylic acid butyl ester, acrylic acid ester such as acrylic acid 2-ethylhexyl ester, methacrylic acid methyl ester, methacrylic acid ethyl ester, methacrylic acid butyl ester, methacrylic acid 2-ethylhexyl etc. It is a copolymer containing a monomer unit and at least one monomer unit selected from acrylic acid and methacrylic acid. At least a part of these copolymers may be covalently crosslinked or ionically crosslinked with a polyvalent metal.

  When a film-forming resin having an acid value is used as a self-water dispersible resin, it is necessary that at least a part of the acid groups be neutralized with a base. A base, that is, an alkaline neutralizing agent, can be added to such an extent that the resulting self-water dispersible resin does not dissolve in water.

  Examples of the base include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide: basic substances such as ammonia, triethylamine and morpholine: alcohol amines such as triethanolamine, diethanolamine and N-methyldiethanolamine. Is mentioned.

  The neutralization rate with the base is not necessarily limited, but it is preferable to neutralize 40 mol% or more of the acid value of the film-forming resin having an acid value, more preferably 40 mol% or more, 100 mol%. It is as follows. A neutralization rate of 40 mol% or more is preferable because the obtained pigment resin particles have a small particle size and are excellent in dispersion stability.

  In addition, the neutralization rate with respect to the acid value of resin which the neutralizing agent used is represented by the following Formula (1).

Formula (1) Neutralization rate (%) = Wa × 5.611 × 106 / (A × Wp × M)
In the above formula (1), A is the acid value of the resin (KOHmg / g), Wp is the mass of the resin used (g), M is the molecular weight of the neutralizing agent, Wa is the mass of the added neutralizing agent (g) Represents.

  Although it does not specifically limit as a method of preparing the pigment particle coat | covered with the film forming resin which concerns on this invention, A more preferable method is a method which consists of the process of (1)-(3) shown below. .

  (1) A resin coloring step for obtaining a solid coloring compound by dispersing at least a pigment in a resin having an acid value.

  (2) A step of mixing at least water, an organic solvent for dissolving the resin, a base, and the solid coloring compound obtained in the resin coloring step to obtain a pigment suspension in which at least a part of the resin is dissolved by dispersion. .

  (3) A reprecipitation step of depositing a resin dissolved on the pigment surface in the pigment suspension obtained in the suspension step.

  (1) is a resin coloring step, in which at least a pigment is dispersed in a resin having an acid value, preferably a carboxyl group, to obtain a solid coloring compound. In this step, for example, a conventionally kneading apparatus such as a roll, a kneader, or a bead mill is used to uniformly disperse the pigment in the resin in a solution or heated and melted. It can be performed by taking it out as a compound.

  (2) is a suspending step, in which at least water, an organic solvent that dissolves the resin, a base, and the solid coloring compound obtained in the resin coloring step are mixed, and at least a part of the resin is dissolved by dispersion This is a step of obtaining a suspension. By adding the solid coloring compound obtained in the resin coloring step of (1) above as a dispersion medium to water, an organic solvent that dissolves the resin, and a mixed solvent in which a base is essential, and stirring to uniformly disperse, A resin containing the pigment is dissolved or self-emulsified with the help of an organic solvent and a base from the surface of the solid coloring compound, and in each case, a pigment suspension in which at least a part of the resin is dissolved can be obtained.

  (3) The above (3) is a reprecipitation step, which is a step of depositing a resin dissolved on the pigment surface in the pigment suspension obtained in the suspension step. The term “reprecipitation” as used in the present invention does not mean that the colorant adsorbed on the pigment surface by the pigment or the dissolved resin is separated and precipitated from the liquid medium of the suspension. Therefore, what is obtained in this step is not a simple mixture in which the solid component and the liquid component are clearly separated, but the precipitated resin is adsorbed on the pigment surface and stably dispersed in the liquid medium of the suspension. It is a pigment resin particle aqueous dispersion.

  By removing the coexisting organic solvent from the pigment resin particle aqueous dispersion thus obtained, a stable aqueous dispersion of pigment particles included in the film-forming resin can be obtained.

  Specific pigments that can be preferably used include the following pigments.

  Examples of the magenta or red pigment include C.I. I. Pigment red 2, C.I. I. Pigment red 3, C.I. I. Pigment red 5, C.I. I. Pigment red 6, C.I. I. Pigment red 7, C.I. I. Pigment red 15, C.I. I. Pigment red 16, C.I. I. Pigment red 48: 1, C.I. I. Pigment red 53: 1, C.I. I. Pigment red 57: 1, C.I. I. Pigment red 122, C.I. I. Pigment red 123, C.I. I. Pigment red 139, C.I. I. Pigment red 144, C.I. I. Pigment red 149, C.I. I. Pigment red 166, C.I. I. Pigment red 177, C.I. I. Pigment red 178, C.I. I. And CI Pigment Red 222.

  Examples of the pigment for orange or yellow include C.I. I. Pigment orange 31, C.I. I. Pigment orange 43, C.I. I. Pigment yellow 12, C.I. I. Pigment yellow 13, C.I. I. Pigment yellow 14, C.I. I. Pigment yellow 15, C.I. I. Pigment yellow 15: 3, C.I. I. Pigment yellow 17, C.I. I. Pigment yellow 74, C.I. I. Pigment yellow 93, C.I. I. Pigment yellow 128, C.I. I. Pigment yellow 94, C.I. I. And CI Pigment Yellow 138.

  Examples of the pigment for green or cyan include C.I. I. Pigment blue 15, C.I. I. Pigment blue 15: 2, C.I. I. Pigment blue 15: 3, C.I. I. Pigment blue 16, C.I. I. Pigment blue 60, C.I. I. And CI Pigment Green 7. Moreover, carbon black is mentioned as a pigment for black.

In addition to the above, when red, green, blue and intermediate colors are required, the following pigments are preferably used alone or in combination. I. Pigment Red; 209, 224, 177, 194,
C. I. Pigment Orange; 43,
C. I. Vat Violet; 3,
C. I. Pigment Violet; 19, 23, 37,
C. I. Pigment Green; 36, 7,
C. I. Pigment Blue; 15: 6,
Etc. are used.

  The average particle diameter of the pigment dispersion used in the recording liquid of the present invention is preferably 10 nm or more and less than 200 nm. When the average particle diameter of the pigment dispersion is less than 10 nm or 200 nm or more, the stability of the pigment dispersion tends to deteriorate, and the storage stability of the ink tends to deteriorate.

  The particle size of the pigment dispersion can be determined by a commercially available particle size measuring instrument using a dynamic light scattering method, an electrophoresis method, or the like. Accurate and often used.

  The pigment content is preferably 0.1% by mass or more and less than 12% by mass with respect to the total mass of the ink.

  As the water-soluble polymer dispersant that can be preferably used in the ink of the present invention, the following water-soluble resin is preferably used from the viewpoint of ejection stability.

  As the water-soluble resin, styrene-acrylic acid-acrylic acid alkyl ester copolymer, styrene-acrylic acid copolymer, styrene-maleic acid copolymer, styrene-maleic acid-acrylic acid alkyl ester copolymer are preferably used. Styrene-methacrylic acid copolymer, styrene-methacrylic acid-acrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer, vinyl naphthalene-acrylic acid copolymer, vinyl naphthalene-maleic acid copolymer It is a water-soluble resin such as

  The content of the water-soluble resin with respect to the total amount of the ink is preferably 0.1 to 10% by mass, and more preferably 0.3 to 5% by mass. Two or more of these water-soluble resins can be used in combination.

  Examples of the dispersing means that can be used in the present invention include various types such as a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet jet mill, a paint shaker, and a high-pressure homogenizer. A disperser can be used. It is also preferable to use a centrifugal separator or a filter for the purpose of removing the coarse particles of the pigment dispersion.

  In the ink of the present invention, in addition to the above description, if necessary, the output stability, print head and ink cartridge compatibility, storage stability, image storage stability, and other various performance improvement objectives are known. Various additives such as polysaccharides, viscosity modifiers, specific resistance regulators, film forming agents, ultraviolet absorbers, antioxidants, anti-fading agents, antifungal agents, rust preventives, etc. may be appropriately selected and used. For example, oil droplet fine particles such as liquid paraffin, dioctyl phthalate, tricresyl phosphate, silicon oil, ultraviolet absorbers described in JP-A-57-74193, 57-87988, and 62-261476, As described in Kaisho 57-74192, 57-87989, 60-72785, 61-146591, JP-A-1-95091 and 3-13376, etc. Anti-fading agents, fluorescent whitening agents described in JP-A Nos. 59-42993, 59-52689, 62-280069, 61-242871, and JP-A-4-219266 Can be mentioned.

  In the ink of the present invention having the above-described structure, the surface tension of the ink is preferably 25 to 40 mN / m at 25 ° C., more preferably 25 to 35 mN / m, still more preferably 30 to 35 mN / m. It is. The ink viscosity is preferably 1 to 40 mPa · s at 25 ° C., more preferably 5 to 40 mPa · s, and still more preferably 5 to 20 mPa · s.

  In addition, the dissolved oxygen concentration in the ink of the present invention is preferably 2 ppm or less at 25 ° C. By setting this dissolved oxygen concentration condition, the formation of bubbles can be suppressed, and the emission stability even in high-speed printing. An ink jet recording method having excellent properties can be realized. As a method for measuring the dissolved oxygen dissolved in the ink, for example, it can be measured using a dissolved oxygen measuring device DO-14P (manufactured by Toa Radio).

  The plain paper used in the ink jet recording method of the present invention is not particularly limited, and its constitution is mainly composed of chemical pulp, sizing agent and filler represented by LBKP and NBKP, and other paper making aids as necessary. Used and made by ordinary methods. As the pulp material used for the plain paper according to the present invention, mechanical pulp and recycled paper recycled pulp may be used in combination, and there is no problem even if these are used as the main material.

  Examples of the sizing agent internally added to the plain paper according to the present invention include rosin size, AKD, Alnicel succinic anhydride, petroleum resin-based size, epichlorohydrin, cationic starch, and acrylamide.

  Examples of the filler internally added to the plain paper according to the present invention include finely divided silicic acid, aluminum silicate, diatomaceous earth, kaolin, kaolinite, halloysite, nacrite, dickite, pyrophyllite, sericite, titanium dioxide, bentonite and the like. Is mentioned.

  In the image forming method using the recording liquid of the present invention, a recorded matter is obtained by ejecting the recording liquid as droplets from an ink jet head based on a digital signal and attaching it to a recording medium.

  When an image is formed by ejecting the ink of the present invention, the inkjet head to be used may be an on-demand system or a continuous system. Also, as a discharge method, an electro-mechanical conversion method (for example, a single cavity type, a double cavity type, a bender type, a piston type, a shear mode type, a shared wall type, etc.), an electro-thermal conversion method (for example, thermal ink jet) Any ejection method such as a mold, a bubble jet (registered trademark) mold, or the like may be used.

  Among them, in the ink jet recording method of the present invention, the recording liquid of the present invention is ejected from a piezo ink jet recording head having a nozzle diameter of 25 μm or less to perform recording on plain paper, and further, a nozzle diameter of 25 μm or less. It is characterized in that recording is performed on plain paper by being ejected from a piezo-type ink jet recording head having a line head.

  In particular, the ink jet recording system of the present invention is characterized in that image printing is performed on both sides of plain paper.

  Since the ink of the present invention has the above-mentioned characteristics, there is no show-through even when printing on both sides, so that a recorded matter with high density and excellent image quality can be obtained on either side. In addition, double-sided printing is often carried with the plain paper turned upside down after printing on one side and the printing side down, but the ink of the present invention has no deformation of the recording medium due to curling or cockling due to the above characteristics, There is no conveyance failure and the conveyance belt is not contaminated with ink.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In addition, although the display of "part" or "%" is used in an Example, unless otherwise indicated, "part by mass" or "mass%" is represented.

<Preparation of pigment particle dispersion>
<Preparation of pigment particle dispersion A>
Carbon black (MA-7, manufactured by Mitsubishi Chemical Corporation) 20 parts Styrene acrylic acid resin 1 (styrene / methyl methacrylate / butyl acrylate / acrylic acid / 2-ethylhexyl acrylate = 60/20/10/7/3, molecular weight: 20,000, acid value 55, glass transition point: 68 ° C.) 20 parts methyl ethyl ketone 60 parts glass beads 150 parts A mixture of the above additives is kneaded in a paint shaker for 4 hours, and 30 parts of methyl ethyl ketone and 40 parts of isopropyl alcohol are added. The contents were taken out to obtain 170 parts of a mill base solution.

  While adding triethanolamine in an amount corresponding to 90% of the neutralization ratio of the resin to 170 parts of the mill base solution and stirring, a mixed solution of 70 parts of glycerin and 200 parts of ion-exchanged water is slowly added to form a film-forming resin. Thus, a pigment particle dispersion coated with is obtained.

  Methyl ethyl ketone, isopropyl alcohol and a part of water were distilled off from this liquid to obtain a pigment particle dispersion A having a pigment concentration of 10% by mass.

<Preparation of pigment particle dispersion B>
In the preparation of the pigment dispersion A, the styrene acrylic resin 1 was changed to styrene acrylic resin 2 (styrene / methyl methacrylate / methacrylic acid / 2-ethylhexyl acrylate = 50/20/15/15, molecular weight 40,000. Pigment particle dispersion B was obtained in the same manner except that the acid value was changed to 20 parts at an acid value of 100 and a glass transition temperature of 48 ° C.

<Preparation of pigment particle dispersion C>
In the preparation of the pigment particle dispersion A, the styrene acrylic resin 1 was replaced with the styrene acrylic resin 3 (styrene / acrylic acid / methacrylic acid = 65/10/25, molecular weight 45,000, acid value 245, glass transition Pigment particle dispersion C was obtained in the same manner except that the temperature was changed to 20 parts at a temperature of 116 ° C.

<Preparation of pigment particle dispersion D>
In the preparation of the pigment particle dispersion A, the styrene acrylic acid resin 1 was changed to styrene-acrylic acid-methacrylic acid resin 4 (styrene / acrylic acid / methacrylic acid = 77/10/13, molecular weight 50,000, acid value 160, A pigment particle dispersion D was obtained in the same manner except that the glass transition temperature was changed to 20 parts at a glass transition temperature of 107 ° C.

<Preparation of pigment particle dispersion E>
In the preparation of the pigment particle dispersion A, the styrene acrylic resin 1 is replaced with the styrene acrylic resin 5 (styrene / acrylic / methacrylic acid = 55/15/30, molecular weight 40,000, acid value 238, glass transition temperature 65 ° C.). A pigment particle dispersion E was obtained in the same manner except that

<Preparation of pigment particle dispersion F>
A pigment particle dispersion liquid F was prepared in the same manner as in the preparation of the pigment particle dispersion liquid A, except that the styrene acrylic resin 1 was replaced with water.

<Preparation of ink>
(Preparation of ink 101)
Pigment particle dispersion A 30 parts Glycerin 2 parts Diethylene glycol 15 parts Diethylene glycol monobutyl ether 2 parts Olphine E1010 (Nisshin Chemical, nonionic surfactant, the same shall apply hereinafter) 0.2 parts After the above additives have been mixed, ion exchange Water was added to make the total amount 100 parts, and ink 101 (black ink) was obtained.

  Next, an ink 102 was prepared in the same manner except that the pigment particle dispersion A of the ink 101 was changed to the pigment particle dispersion B.

  Next, an ink 103 was prepared in the same manner except that the pigment particle dispersion A of the ink 101 was changed to the pigment particle dispersion C.

  Next, an ink 104 was prepared in the same manner except that the pigment particle dispersion A of the ink 101 was changed to the pigment particle dispersion D.

  Next, an ink 105 was prepared in the same manner except that the pigment particle dispersion A of the ink 101 was changed to the pigment particle dispersion E.

  Next, an ink 106 was prepared in the same manner except that the pigment particle dispersion A of the ink 101 was changed to the pigment particle dispersion F.

(Preparation of ink 107)
Pigment particle dispersion A 30 parts Triethylene glycol monomethyl ether 40 parts Ethylene glycol 10 parts Olfine E1010 0.2 parts After mixing the above additives, ion exchange water is added to make 100 parts in total to prepare ink 107 did.

(Preparation of ink 108)
Pigment particle dispersion A 30 parts Triethylene glycol monomethyl ether 25 parts Ethylene glycol 25 parts Orphine E1010 0.2 parts After mixing the above additives, ion exchange water is added to make 100 parts in total to prepare ink 108 did.

(Preparation of ink 109)
Pigment particle dispersion A 30 parts Triethylene glycol monomethyl ether 35 parts Ethylene glycol 25 parts Olphine E1010 0.2 parts After mixing the above additives, ion exchange water is added to make 100 parts in total to prepare ink 108 did.

(Preparation of ink 110)
Pigment particle dispersion B 30 parts Triethylene glycol monomethyl ether 60 parts Orphine E1010 0.2 part After the above additives were mixed, ion exchanged water was added to make the total amount 100 parts to prepare ink 110.

(Preparation of ink 111)
Pigment particle dispersion C 30 parts Triethylene glycol monomethyl ether 60 parts Orphine E1010 0.2 parts After mixing the above additives, ion exchange water was added to make the total amount 100 parts, and ink 111 was prepared.

(Preparation of ink 112)
Pigment particle dispersion D 30 parts Triethylene glycol monomethyl ether 60 parts Orphine E1010 0.2 parts After mixing the above additives, ion exchange water was added to make the total amount 100 parts, and ink 112 was prepared.

(Preparation of ink 113)
Pigment particle dispersion E 30 parts Triethylene glycol monomethyl ether 60 parts Orphine E1010 0.2 parts After mixing the above additives, ion-exchanged water was added to make the total amount 100 parts to prepare ink 113.

(Preparation of ink 114)
Pigment particle dispersion F 30 parts Triethylene glycol monomethyl ether 60 parts Orphine E1010 0.2 parts After mixing the above additives, ion exchange water was added to make the total amount 100 parts, and ink 114 was prepared.

  The inks 101 to 114 thus prepared were evaluated as follows and are listed in Table 1.

<< Preparation of image recording sample >>
The number of nozzles per head is 512, the resolution is 360 dpi (dpi represents the number of dots per 2.54 cm), the amount of droplets is 14 pl, and the driving frequency is 12 so that the arrangement of nozzles is perpendicular to the direction of travel on the belt conveyor. .8 kHz piezo-type heads (Konica Minolta IJ) are arranged, Konica First Class (manufactured by Konica Minolta Business Technologies, Inc.) is placed on a belt conveyor and conveyed at a speed of 350 mm / sec, and the above ink is used. Was printed.

(Evaluation of strikethrough resistance)
The back side of the sample printed on plain paper (Konica copy paper NR-A100) was visually observed and evaluated for see-through resistance according to the following criteria.

A: No ink bleed or show-through on the back side B: No ink bleed or show-through on the back side is acceptable and acceptable quality C: No ink bleed or show-through on the back side Slightly recognized, but not of concern when printing on the back side D: Ink bleed and back-through on the back side are quite acceptable and unacceptable quality for back-side printing E: Clear ink bleed or back-through on the back side Is not suitable for backside printing <Evaluation of curling characteristics after printing>
A solid image of 200 mm × 280 mm was printed at a resolution of 1440 dpi × 1440 dpi on A4 size plain paper using the inkjet printer under the environment of 23 ° C. and 30% RH. The printed plain paper was then allowed to stand for 1 week on a flat surface in an environment of 23 ° C. and 20% RH for 1 week, and then the heights of the four corners were measured. For the negative curl sample with the central part floating upside down, the sample was allowed to stand for another day after being turned upside down. The raised heights of the four corners were measured as described above, and the curl characteristics were evaluated according to the following criteria.

A: Almost flat, even at the most floating part at the four corners, the lifting height is less than 5 mm. O: The lifting height at the most floating part at the four corners is 5 mm or more and less than 10 mm. Δ: Most floating at the four corners. The height of the lifted portion is 10 mm or more and less than 20 mm. X: The height of the lifted portion at the four corners is 20 mm or more and less than 50 mm. XX: The height of the lifted portion of the four corners. Is more than 50mm or rounded into a cylindrical shape and cannot be measured. (Evaluation of ink dispersion stability)
After each ink was stored in a glass bottle, it was stored in an environment at 60 ° C. for 1 week, the change in viscosity before and after storage was measured, and the dispersion stability of the ink was evaluated according to the following criteria.

○: Change width of ink viscosity after storage is less than ± 10% of ink viscosity before storage ×: Change width of ink viscosity after storage is ± 10% or more of ink viscosity before storage (intermittent) Evaluation of printability)
Under the environment of 23 ° C. and 30% RH, a solid image having a width of 45 mm and a height of 40 mm was continuously printed on A4 size plain paper with a resolution of 1440 dpi in the main scanning direction using the above-described inkjet printer. At this time, ink droplets were printed continuously for 283 shots (about 0.27 seconds), ejection was interrupted for about 5.73 seconds, and then a pattern for ejecting 283 ink droplets was repeatedly printed. In this way, printing was continuously performed on 50 sheets of plain paper, the last 50th print was visually observed, and the suitability for intermittent printing was evaluated according to the following criteria.

◎: Image unevenness is not confirmed by visual observation, and it is a good image without chipping in the outline portion. ○: Extremely weak density unevenness that does not lead to streaks is observed by visual observation, but the contour portion is not chipped. O: No occurrence of the contour surface of the image at the front in the transport direction. X: One white streak is observed. XX: Two or more white streaks are generated (Evaluation of suitability for double-sided printing)
Immediately after the set-off evaluation, 200 sheets of the above-described solid print image evaluated for the set-off were set again on the paper feed tray of the inkjet printer so that the reverse side would be the print surface. Was printed continuously.

  The suitability for double-sided printing was comprehensively determined by checking whether there was a conveyance failure or contamination of the printer conveyance section during or after printing.

  The above evaluation results are shown in Table 1 below.

  It can be seen that the pigment dispersions A to F coated with the film-forming resin have a problem of practical problems due to poor print back-through and curl resistance after printing in any of the inks 101 to 106. . In particular, it can be seen that the ink 106 is poor in dispersion stability and emission. On the other hand, in the case of inks 109 to 113 using a pigment dispersion coated with a film-forming resin and an organic solvent having an SP value of 16.5 or more and less than 24.6 at 30% by mass or more, the back of 101 to 106 was observed. The drop and curl properties were greatly improved, and the dispersion stability and light emission were also improved, confirming the effects of the present invention. For the ink having the effect of the present invention, after printing a solid image, the paper on which the image was printed was quickly taken out and set so that the surface on which the image was not printed was the front side, and duplex printing was performed. The time from the completion of printing on the entire surface to the start of printing on the back surface was 3 to 4 seconds. Subsequently, using the printer that has finished double-sided printing as it is, ink was not discharged, the plain paper was passed through the printer, and the amount of ink transferred to the plain paper was evaluated for dirt in the printer. It was confirmed that there was no problem with the ink.

Claims (8)

It contains at least water, a pigment dispersion, and a water-soluble organic solvent, and the total amount of the water-soluble organic solvent is 50% by mass or more and less than 90% by mass of the total ink, and the SP value of the water-soluble organic solvent is 16. A water-based ink containing a water-soluble organic solvent of 5 to less than 24.6 in an amount of 30% by mass or more of the total ink, wherein the pigment particles constituting the pigment dispersion are coated with a film-forming resin. A method for producing an inkjet ink. 2. The method for producing an inkjet ink according to claim 1, wherein the film-forming resin has a glass transition temperature (Tg) of 50 ° C. or more and 150 ° C. or less. The ink-jet ink production according to claim 1 or 2, wherein the film-forming resin has an acid group, and 40 mol% or more and 100 mol% or less of the acid group is neutralized in a base. Method. The method for producing an inkjet ink according to claim 1, wherein the film-forming resin has an acid value of 50 or more and 240 or less. An ink-jet ink produced by the method for producing an ink-jet ink according to claim 1. An ink jet printer having an ink jet head having a nozzle diameter of 25 microns or less and emitting the ink jet ink according to claim 5 from the nozzle. 6. The nozzle has a line head arranged in a direction intersecting with the paper conveyance direction, and recording is performed by ejecting the ink-jet ink according to claim 5 from the nozzle without substantially scanning the head. The inkjet printer according to claim 6, wherein the inkjet printer is performed. An ink jet recording method comprising printing on both sides of plain paper using the ink jet printer according to claim 6.