JP2005263899A - Ink composition for inkjet recording and method for inkjet recording - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ink composition for inkjet recording which can ensure that an ink droplet can be always stably ejected in long-term inkjet recording, a high-quality image can be formed for a long period of time and, even when the recorded materials are superposed, the image-recorded materials do not adhere to each other in aging, and to provide a method for inkjet recording. <P>SOLUTION: The invention relates to the ink composition for inkjet recording comprising a dispersion medium and charged particles at least containing coloring material, wherein the charged particles are dispersed in the dispersion medium by a dispersing agent which is a graft polymer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an ink composition for ink jet recording and an ink jet recording method using the same.

  In recent years, various methods have been developed as means for outputting an image digital signal to a recording medium such as paper. The main methods include an electrophotographic method, a sublimation type and a melt type thermal transfer method, and an ink jet method. In the electrophotographic system, an electrostatic image is formed on a photoconductor by optical information, and a two-step transfer process including toner transfer by electrostatic force and subsequent transfer to paper is performed. Therefore, the apparatus is large and expensive. The thermal transfer system is a direct contact system of heat contact type and non-contact type, and since an ink ribbon is used, the support comes out as a waste material. On the other hand, the ink jet method is a direct inking system in which droplets of several pl to several tens of pl are ejected from nozzles and drawn directly on a recording medium such as paper, and the above-mentioned two methods are inexpensive and do not generate waste materials. It is better than the method.

  Inkjet systems use a system that rapidly heats ink and causes the ink to fly by the expansion and contraction of the generated bubbles, a system that ejects ink using ceramic that deforms when a voltage is applied, and an electrostatic field. In addition, there is an electrostatic ink jet method (see Patent Document 1 and Patent Document 2) that causes droplets containing charged particles to fly. In the method in which droplets are ejected by heat or mechanical pressure, the droplets cannot be landed at a desired position on the recording medium due to nozzle distortion or air convection. Also, since most of the volume of the droplet is occupied by the solvent, bleeding and image flow are likely to occur on the landed recording medium.

On the other hand, since the electrostatic ink jet method controls the landing point of a droplet by an electrostatic field, the droplet can be landed at a desired position on the recording medium. In addition, since the electrostatic force is hardly applied to the solvent, the droplets are ejected in the form of concentrated charged particles to which the electrostatic force is applied, and image flow and blurring after landing on the recording medium are unlikely to occur. Therefore, the electrostatic ink jet method can form precise dots and is effective as a means for forming a high-quality image. As the ink composition used for the electrostatic ink jet method, an ink composition containing charged particles including at least a colorant and a solvent is generally used (see, for example, Patent Document 3 and Patent Document 4).
US Pat. No. 6,158,844 Japanese Patent No. 3315334 US Pat. No. 5,952,2048 JP-A-8-291267

  However, the conventional ink composition has a wide particle size distribution and a variation in particle size. On the other hand, the charge amount on the particle surface depends on the surface area of the particle. In an electrostatic ink jet system, a strong electrostatic attraction is applied to large particles having a large particle surface area and a large amount of charge. For this reason, when conventional ink having a variation in particle size is used, particles are preferentially ejected from large particles, and particles with a small charge amount remain with the ejection time, making it difficult to eject droplets. For this reason, it has been difficult to stably discharge for a long time.

  Moreover, it is desirable that the dispersion medium used in the electrostatic ink jet system has a high electrical resistivity, and it is desirable to mainly use a low polarity solvent as the dispersion medium. In the dispersion medium, there is no thickness of the electric double layer formed around the charged particles, and the dispersion stabilization effect due to electrostatic repulsion is small. For this reason, in order to stably disperse the charged particles in the dispersion medium without aggregating the charged particles, it is presumed that it is necessary to disperse mainly due to the steric repulsion effect of the dispersant adsorbed or bonded to the particle surface. However, in the conventional ink composition, since the adsorptive power of the dispersant contained in the ink composition is not sufficient, it is considered that the dispersant is easily desorbed and sufficient steric repulsion cannot be imparted between the charged particles. For this reason, when the ink composition is stored for a long period of time, the particles are aggregated and the average particle diameter is greatly increased. When such ink is used in an electrostatic ink jet system, the head and piping are clogged and the color tone of the formed image is deteriorated. Therefore, there has been a demand for an ink having high dispersion stability in which charged particles do not aggregate even when stored for a long time.

  The present invention has been made for the purpose of always ejecting ink droplets stably in long-time inkjet recording, and further maintaining the dispersion stability of charged particles over a long period of time. The present invention provides an ink composition for ink jet recording which can be used for a long time and has long-term storage stability, and an ink jet recording method.

As a result of intensive studies to achieve the above object, the present inventor greatly depends on the dispersion stability of the particles containing the coloring material contained in the ink used and the uniformity of the particle size distribution. In addition, it has been found that long-term storage stability greatly depends on the dispersion stability of the particles containing the coloring material contained in the ink to be used, and furthermore, by using a specific dispersant, such dispersion stability is remarkably good. As a result, the present invention has been achieved.
That is, the present invention provides an ink composition containing charged particles containing at least a colorant and a dispersion medium, wherein the charged particles have at least one of an acidic group and a salt formed by neutralizing the acidic group as a main chain. An ink composition for ink jet recording, wherein the ink composition is dispersed in a dispersion medium by a graft polymer in a portion, and an ink jet recording method using an electrostatic field using the ink composition.

  By using the ink composition for ink-jet recording of the present invention, it is possible to stably eject ink droplets for a long period of time in long-term ink-jet recording, there is no ejection failure, and there is no decrease in density or ink bleeding. High-quality images can be formed. Moreover, the ink composition for inkjet recording of the present invention has long-term storage stability.

Hereinafter, the present invention will be described in further detail.
The ink composition of the present invention contains at least charged particles including at least a coloring material and a dispersion medium.

[Dispersion medium]
The dispersion medium used in the present invention is preferably a dielectric liquid having a high electrical resistivity, specifically an electrical resistivity of 10 ¹⁰ Ωcm or more. Use of a dispersion medium having a low electrical resistivity is not preferable in the present invention because electrical conduction is caused between adjacent recording electrodes. Further, the dielectric constant of the dielectric liquid is preferably 5 or less, more preferably 4 or less, and still more preferably 3.5 or less. By setting the relative dielectric constant in such a range, an electric field is effectively applied to charged particles in the dielectric liquid, which is preferable.

  Examples of the dispersion medium used in the present invention include linear or branched aliphatic hydrocarbons, alicyclic hydrocarbons, or aromatic hydrocarbons, halogen substitution products of these hydrocarbons, and silicone oil. For example, hexane, heptane, octane. Isooctane, decane, isodecane, decalin, nonane, dodecane, isododecane, cyclohexane, cyclooctane, cyclodecane, toluene, xylene, mesitylene, etc., specifically, Isopar C, Isopar E, Isopar G, Isopar H, Isopar L, Isopar M (isopar: trade name of Exxon), Shellsol 70, Shellsol 71 (shellsol: trade name of Shell Oil), Amsco OMS, Amsco 460 solvent (Amsco: trade name of Spirits), KF-96L (Trade name of Shin-Etsu Silicone Co., Ltd.) or the like can be used alone or in combination. The content of the dispersion medium with respect to the entire ink composition is preferably in the range of 20 to 99% by mass. In 20 mass% or more, the particle | grains containing a color material can be favorably disperse | distributed to a dispersion medium, and content of a color material can be satisfied in 99 mass% or less.

[Color material]
As the coloring material used in the present invention, known dyes and pigments can be used, and can be selected according to applications and purposes. For example, it is preferable to use a pigment from the viewpoint of the color tone of the recorded image recorded matter (printed matter) (for example, “Distribution Stabilization and Surface Treatment Technology / Evaluation” issued by the Technical Information Association, December 25, 2001 1) (hereinafter also referred to as “Non-Patent Document 1”). By changing the color material, four color inks of yellow, magenta, cyan, and black can be created. In particular, the use of offset printing inks or pigments used for proofing is preferable because the same color tone as that of offset printed matter can be obtained.

  Examples of pigments for yellow ink include C.I. I. Pigment yellow 1, C.I. I. Monoazo pigments such as CI Pigment Yellow 74; I. Pigment yellow 12, C.I. I. Disazo pigments such as CI Pigment Yellow 17; I. Non-benzidine azo pigments such as CI Pigment Yellow 180; I. Azo lake pigments such as CI Pigment Yellow 100; I. Condensed azo pigments such as CI Pigment Yellow 95; I. Pigment Yellow 115, etc., acidic dye lake pigments, C.I. I. Pigment yellow 18 and other basic dye lake pigments, anthraquinone pigments such as flavantron yellow, isoindolinone yellow 3RLT, C.I. I. Pigment Yellow 139 and other isoindolinone pigments, quinophthalone yellow and other quinophthalone pigments, isoindoline yellow and other isoindoline pigments, C.I. I. Nitroso pigments such as CI Pigment Yellow 153, C.I. I. And metal complex salt azomethine pigments such as CI Pigment Yellow 117.

  Examples of pigments for magenta ink include C.I. I. Monoazo pigments such as CI Pigment Red 3; I. Disazo pigments such as CI Pigment Red 38; I. Pigment red 53: 1 etc. and C.I. I. Azo lake pigments such as CI Pigment Red 57: 1; I. Condensed azo pigments such as CI Pigment Red 144; I. Acidic dye lake pigments such as C.I. Pigment Red 174; I. Basic dye lake pigments such as CI Pigment Red 81; I. Anthraquinone pigments such as CI Pigment Red 177; I. Thioindigo pigments such as CI Pigment Red 88; I. Perinone pigments such as CI Pigment Red 194; I. Perylene pigments such as CI Pigment Red 149; I. Quinacridone pigments such as CI Pigment Red 122; I. Isoindolinone pigments such as CI Pigment Red 180; I. And alizarin lake pigments such as CI Pigment Red 83.

  Examples of the pigment for cyan ink include C.I. Disazo pigments such as CI Pigment Blue 25, C.I. I. Phthalocyanine pigments such as CI Pigment Blue 15; I. Pigment Blue 24, etc., acidic dye lake pigments, C.I. I. Basic dye lake pigments such as CI Pigment Blue 1; I. Anthraquinone pigments such as CI Pigment Blue 60; I. And alkaline blue pigments such as CI Pigment Blue 18.

Examples of the pigment for black ink include organic pigments such as aniline black pigments and iron oxide pigments, and carbon black pigments such as furnace black, lamp black, acetylene black, and channel black.
Furthermore, processed pigments typified by microlith pigments such as Microlith-A, Microlith-K, and Microlith-T can also be suitably used. Specific examples thereof include Microlith Yellow 4G-A, Micro Resled BP-K, Microlith Blue 4G-T, and Microlith Black CT.
Various pigments can be used as necessary, such as calcium carbonate and titanium oxide pigments for white ink, aluminum powder for silver ink, and copper alloy for gold ink.

  Basically, it is preferable to use one kind of pigment for each color, from the viewpoint of simplicity of ink production. However, as a hue adjustment, for example, for black ink, carbon black is mixed with phthalocyanine, for magenta ink. In addition, it is also preferable to use two or more kinds in combination, such as mixing phthalocyanine with the disazo pigment CI Pigment Red. Alternatively, the pigment may be surface-treated by a known method such as rosin treatment (see Non-Patent Document 1 above).

  The pigment content relative to the entire ink composition is preferably in the range of 0.1 to 50% by mass. When the amount is 0.1% by mass or more, the amount of pigment is sufficient, and sufficiently good color development is obtained in the printed matter. When the amount is 50% by mass or less, the particles containing the coloring material can be favorably dispersed in the dispersion medium. More preferably, it is 1-30 mass%.

[Coating agent]
In the present invention, a color material such as a pigment may be directly dispersed (particulated) in a dispersion medium, or may be coated with a coating material and dispersed (particulated). Dispersing (particulate) in the state of being coated with a coating agent can shield the charge of the color material and impart desirable charge characteristics, and also cancel the difference in dispersion stability that varies depending on the type of color material. This is preferable in that the same dispersion stability can be imparted. Further, by covering the surface of the pigment that has a large surface area and easily forms higher-order aggregates, aggregation of particles can be suppressed, and polydispersion of the particle size distribution can be suppressed. Further, in the present invention, after ink jet recording on a recording medium, it is preferably fixed by a heating means such as a heat roller. At this time, the coating agent is melted by heat and can be fixed efficiently.

  Examples of coating agents include rosins, phenolic resins, rosin-modified phenolic resins, alkyd resins, (meth) acrylic polymers, polyurethane, polyester, polyether, polyamide, polyethylene, polybutadiene, polystyrene, polyvinyl acetate, and polyvinyl alcohol. Acetal-modified product, polycarbonate and the like. Among these, from the viewpoint of easy particle formation, the weight average molecular weight is in the range of 2,000 to 1,000,000, and the polydispersity (weight average molecular weight / number average molecular weight) is 1.0 to 5. Polymers that are in the range of 0 are preferred. Further, from the viewpoint of ease of fixing, a polymer having any one of a softening point, a glass transition point, and a melting point within a range of 40 ° C. to 120 ° C. is preferable.

  In the present invention, the polymer particularly preferably used as the coating agent is a polymer containing at least one of the structural units represented by the following general formulas (1) to (4).

In the formula, X ₁₁ represents an oxygen atom or —N (R ₁₃ ) —. R ₁₁ represents a hydrogen atom or a methyl group, and R ₁₂ represents a hydrocarbon group having 1 to 30 carbon atoms. R ₁₃ represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms. R ₂₁ represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms. R ₃₁ , R ₃₂ and R ₄₁ each independently represents a divalent hydrocarbon group having 1 to 20 carbon atoms. The hydrocarbon groups of R ₁₂ , R ₂₁ , R ₃₁ , R ₃₂ and R ₄₁ may contain an ether bond, amino group, hydroxyl group or halogen substituent.

  The polymer containing the structural unit represented by the general formula (1) can be obtained by radical polymerization of a corresponding radical polymerizable monomer by a known method. Examples of the radical polymerizable monomer used include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, hexyl (meth) acrylate, and (meth) acrylic. Octyl acid, 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, stearyl (meth) acrylate, cyclohexyl (meth) acrylate, phenyl (meth) acrylate, benzyl (meth) acrylate, (meth) (Meth) acrylic acid esters such as 2-hydroxyethyl acrylate, N-methyl (meth) acrylamide, N-propyl (meth) acrylamide, N-phenyl (meth) acrylamide, N, N-dimethyl (meth) Examples include (meth) acrylamides such as acrylamide.

  The polymer containing the structural unit represented by the general formula (2) can be obtained by radical polymerization of a corresponding radical polymerizable monomer by a known method. Examples of the radical polymerizable monomer used include ethylene, propylene, butadiene, styrene, and 4-methylstyrene.

  The polymer containing the structural unit represented by the general formula (3) can be obtained by dehydrating condensation of a corresponding dicarboxylic acid or acid anhydride and a diol by a known method. Examples of the dicarboxylic acid used include succinic anhydride, adipic acid, sebacic acid, isophthalic acid, terephthalic acid, 1,4-phenylenediacetic acid, diglycolic acid and the like. Examples of the diol used include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,10-decanediol, and 2-butene- Examples include 1,4-diol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 1,4-benzenedimethanol, and diethylene glycol.

  The polymer containing the structural unit represented by the general formula (4) is obtained by dehydrating and condensing a carboxylic acid having a corresponding hydroxyl group by a known method or by using a cyclic ester of a carboxylic acid having a corresponding hydroxyl group by a known method. Can be obtained by ring-opening polymerization. Examples of the carboxylic acid having a hydroxyl group or a cyclic ester thereof include 6-hydroxyhexanoic acid, 11-hydroxyundecanoic acid, hydroxybenzoic acid, and ε-caprolactone.

  The polymer containing at least one of the structural units represented by the general formulas (1) to (4) may be a homopolymer of the structural units represented by the general formulas (1) to (4). It may be a copolymer (copolymer) with these constituent components. In addition, these polymers may be used alone as a coating agent, or may be used in combination of two or more.

  The content of the coating agent with respect to the entire ink composition is preferably in the range of 0.1 to 40% by mass. When the amount is 0.1% by mass or more, the amount of the coating agent is sufficient, and sufficient fixability is obtained. When the amount is 40% by mass or less, particles containing the coloring material and the coating agent can be favorably formed.

[Dispersant]
The dispersant is a material for dispersing (particulating) a coloring material or a mixture of a coloring material and a coating material in a dispersion medium. In the present invention, the dispersing agent has long-term storage stability, In order to discharge ink droplets stably in ink jet recording at all times, it consists of a main chain part and a side chain part that is a graft chain as a dispersant, and neutralizes acidic groups and acidic groups in the main chain part. A major feature is the use of a graft polymer having at least one of salts.

  By having an acidic group or a salt formed by neutralizing an acidic group in the main chain part, strong acid-base interaction or electrostatic interaction is developed between the main chain part and the charged particle, and the main chain is charged with the charged particle. The adsorptive power of the chain part increases. Thereby, the dispersion stability of the charged particles is improved, and as a result, the long-term storage stability of the dispersion is improved.

  In addition, when an ink composition is prepared, an ink composition having a narrow particle size distribution can be obtained by using an appropriate amount of a graft polymer having an acidic group or a salt obtained by neutralizing an acidic group in the main chain as a dispersant. it can. In the present invention, the ink composition is prepared by mixing and dispersing the colorant, coating agent and dispersant by using a kneader, dissolver, mixer, high speed disperser, sand mill, roll mill, ball mill, attritor, bead mill, etc. at the same time. However, if the amount of the dispersant added is too large at this time, small particles are easily generated, and an ink having a polydispersed particle size distribution can be obtained.

  Since the charged amount on the surface of the charged particle depends on the surface area of the particle, such a polydisperse ink having a large particle size distribution has a large variation in the charged amount of the charged particle and is difficult to stably discharge for a long period of time. It becomes. According to the present invention, by using as a dispersant a graft polymer having an acidic group or a salt formed by neutralizing acidic groups in the main chain portion, the adsorptive power of the dispersant to charged particles is increased, and a small addition amount A desired particle diameter can be obtained. As a result, an ink composition having a uniform particle size distribution and a uniform charge applied to charged particles can be obtained, and ink droplets can be stably ejected for a long time.

  In the present invention, the graft polymer is a polymer having a graft chain as a side chain, preferably a polymer having a weight average molecular weight of 1,000 or more, including a graft chain as a side chain, which is a polymer component having a weight average molecular weight of 500 or more. It is.

In the present invention, the acidic group in the main chain portion represents a proton donating functional group having a pKa of 14 or less, and the salt formed by neutralizing the acidic group is an ion pairing or pairing of the acidic group with a base. The functional group neutralized by the component is shown.
Preferably, the functional group represented by the general formula (5) in which the conjugate base -Y ^r- and the cationic root Z ^{s +} form a covalent bond or an ion pair.

In the formula, —Y ^r — represents —SO ₃ ⁻ , —OSO ₃ ⁻ , —CO ₂ ⁻ , —C ₆ H ₄ —O ⁻ , —PO ₃ ²⁻ , or —OPO ₃ ²⁻ group. Z ^{s +} represents an onium ion or a heterocyclic compound ion such as hydrogen ion (H ⁺ ), alkali (earth) metal ion, ammonium ion, sulfonium ion, phosphonium ion, specifically sodium ion, potassium ion, Alkali (earth) metal ions such as lithium ion, calcium ion, barium ion, calcium ion, (mono, di, tri, tetra) methylammonium, (mono, di, tri, tetra) ethylammonium, (mono, di, Tri, tetra) n-butylammonium, (mono, di, tri, tetra) methoxyammonium, (mono, di, tri, tetra) phenylammonium, phenyldimethylammonium, diphenylmethylammonium, morpholinium, n-methylmorpholinium, Dimethylethanol ammonium Nitrogen, methyldiethanolammonium, (mono-, di-, tri) ethanolamine, trihydroxymethylamine, dihydroxymethylamine, monohydroxymethylamine, piperidine protonated ammonium, (mono, di, tri) methylsulfonium, triethyl Examples include sulfonium, tri-n-butylsulfonium, (mono, di, tri, tetra) methylphosphonium, tetraethylphosphonium, tetra n-butylphosphonium, (mono, di, tri, tetra) phenylphosphonium, pyridinium, and the like.

  The graft polymer having an acidic group or a salt formed by neutralizing an acidic group in the main chain part is a graft polymer containing the acidic group or a salt formed by neutralizing the acidic group in the main chain part, The main chain part may contain only one kind of the acidic group or a salt formed by neutralizing the acidic group, or may contain a plurality of kinds.

  The graft polymer that can be suitably used in the present invention has a weight average molecular weight of 1,000 or more and contains at least a structural unit represented by the following general formula (6) and a structural unit represented by the following general formula (7). It is a polymer.

The symbol ^-Yr-. ( ^{Zs +} ) _t in the general formula (6) has the same meaning as in the general formula (5). General formula (6) constitutes the main chain portion, and G in the general formula (7) constitutes the graft portion.
In the general formulas (6) and (7), R ₅₁ , R ₅₂ , R ₆₁ and R ₆₂ may be the same or different and each represents a hydrogen atom or a methyl group.
X ₅₁ represents a single bond or a hydrocarbon group having 1 to 100 carbon atoms which may have a substituent. The hydrocarbon group may have a branched structure, and the hydrocarbon group may contain an ether bond, an ester bond, an amide bond, or a carbamate bond. The functional group ^-Yr-. ( ^{Zs +} ) _t represented by the general formula (5) may be included in the structural unit alone or in plural. Moreover, the functional group ^-Yr-. ( ^{Zs +} ) _t represented by the general formula (5) may be included in the structural unit alone or in a plurality of types.
X ₆₁ represents a single bond or a divalent linking group having a total atom number of 50 or less composed of two or more atoms selected from C, H, N, O, S and P.
G represents a polymer component having a weight average molecular weight of 500 or more, or a polydimethylsiloxane group having a weight average molecular weight of 500 or more, which includes at least a structural unit represented by the following general formula (8).

In the general formula (8), R ₇₁ and R ₇₂ may be the same or different and each represents a hydrogen atom or a methyl group.
R ₇₃ represents a hydrogen atom or a hydrocarbon group having 1 to 30 carbon atoms which may have a substituent. The hydrocarbon group of R ₇₃ may contain an ether bond, ester bond, amide bond, carbamate bond, amino group, hydroxyl group or halogen substituent.
X ₇₁ represents a single bond or a divalent linking group having a total atom number of 50 or less consisting of two or more atoms selected from C, H, N, O, S and P.
The total number of R ₇₃ atoms is preferably larger than the total number of R ₅₃ atoms from the viewpoint of ink ejection stability.

  The graft polymer that can be suitably used in the present invention comprises a radical polymerizable monomer corresponding to the general formula (6) and a radical polymerizable macromonomer corresponding to the general formula (7), which are known radical polymerization initiators. Can be obtained by polymerization. Here, the monomer corresponding to the general formula (6) is a monomer represented by the following general formula (6M), and the macromonomer corresponding to the general formula (7) is a macro represented by the following general formula (7M). Monomer. One of the monomers selected from the general formula (6M) and the general formula (7M) may be copolymerized, or a plurality of monomers may be copolymerized.

  Here, each symbol in the general formulas (6M) and (7M) has the same meaning as in the general formulas (6) and (7).

  Examples of the monomer represented by the general formula (6M) include (meth) acrylic acid, 2-carboxyethyl (meth) acrylate, 2- (meth) acryloyloxyethyl phthalate, 2- (meth) acryloyloxyethyls. Radical polysynthetic monomers containing carboxyl groups such as succinate, styrene carboxylic acid, maleic acid, itaconic acid, fumaric acid, (meth) acrylamide-2-methylpropanesulfonic acid, 2- (meth) acryloyloxyethylsulfonic acid Radical polymerization monomers containing sulfonic acid groups such as styrene sulfonic acid, radical polymerizable monomers containing phosphoric acid groups such as ethylene glycol (meth) acrylate phosphate, styrene phosphoric acid, and acidic groups in the monomers Amines, sulfides, phosphines, heterocyclic compounds, etc. And a monomer obtained by neutralizing an acidic group in the monomer with a counter component such as an ammonium salt, a sulfonium salt, or a phosphonium salt.

  Examples of the amines, sulfides, phosphines, and heterocyclic compounds include (mono, di, tri) methylamine, triethylamine, tri n-butylamine, (mono, di, tri) phenylamine, phenyldimethylamine, Diphenylmethylamine, morpholine, n-methylmorpholine, dimethylethanolamine (DMEA), methyldiethanolamine, (mono-, di-, tri) ethanolamine, trihydroxymethylamine, dihydroxymethylamine, monohydroxymethylamine, piperidine, etc. Examples include heterocyclic compounds such as amines and pyridine.

  The above-mentioned counter components such as ammonium salt, sulfonium salt, phosphonium salt, etc. are alkali (earth) metal hydroxy such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide, calcium hydroxide, etc. (Mono, di, tri, tetra) methylammonium hydroxide, tetra n-butylammonium hydroxide, tetramethoxyammonium hydroxide, ammonium hydroxides such as (mono, di, tri, tetra) phenylammonium, (mono, Sulfonium hydroxide such as di, tri) methylsulfonium hydroxide, triethylsulfonium hydroxide, tri-n-butylsulfonium hydroxide, (mono, di, tri, tetra) methylphosphonium hydroxide And phosphonium hydroxides such as tetraethylphosphonium hydroxide, tetra n-butylphosphonium hydroxide, (mono, di, tri, tetra) phenyl phosphonium hydroxide, and the like.

The macromonomer represented by the general formula (7M) was obtained by polymerizing a radical polymerizable monomer of the following general formula (8M) corresponding to the general formula (8) in the presence of a chain transfer agent as necessary. It is a polymer having a radical polymerizable functional group at the terminal obtained by introducing a radical polymerizable functional group at the terminal of the polymer.
The weight average molecular weight of the macromonomer represented by the general formula (8M) is in the range of 500 to 500,000, and the polydispersity (weight average molecular weight / number average molecular weight) is 1.0 to 7. Macromonomers that are in the range of 0 are preferred. Further, the macromonomer represented by the general formula (8M) may be polydimethylsiloxane having a radical polymerizable functional group at the terminal.

  Here, each symbol in the general formula (8M) has the same meaning as in the general formula (8).

  Examples of the monomer represented by the general formula (8M) include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, hexyl (meth) acrylate, Octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, cyclohexyl (meth) acrylate, dodecyl (meth) acrylate, stearyl (meth) acrylate, phenyl (meth) acrylate, benzyl (meth) acrylate (Meth) acrylic acid esters such as 2-hydroxyethyl (meth) acrylate, and N-methyl (meth) acrylamide, N-propyl (meth) acrylamide, N-phenyl (meth) acrylamide, N, N- (Meth) acrylamides such as dimethyl (meth) acrylamide, styrene, methyl Styrene, chlorostyrene, styrene such as methoxystyrene, hydrocarbons such as 1-butene, and vinyl acetate, vinyl ethers, and vinyl pyridine and the like.

  The graft polymer containing a salt formed by neutralizing an acidic group in the main chain portion that can be suitably used in the present invention includes a radical polymerizable monomer containing an acidic group represented by the general formula (6M) and a general formula ( 7M) and a radically polymerizable macromonomer corresponding to 7M) using a known radical polymerization initiator and then ion-pairing an acidic group in the polymer with a base such as an amine or a heterocyclic compound, or It can also be obtained by neutralizing an acidic group in the polymer with a counter component such as an ammonium salt, a sulfonium salt, or a phosphonium salt.

  The graft polymer used for this invention may have only the structural unit shown by General formula (6) and (7), and may contain the other structural component. Specific examples of the graft polymer suitably used in the present invention include polymers [BZ-1] to [BZ-13] represented by the following structural formulas.

In the present invention, from the viewpoint of long-term storage stability and ink ejection stability of the ink composition, the dispersant has a weight average molecular weight in the range of 1,000 to 1,000,000 and a polydispersity (weight). The graft polymer is preferably an average molecular weight / number average molecular weight) in the range of 1.0 to 7.0.
Moreover, it is preferable that the weight average molecular weight of a graft polymer is 1.5 times or more with respect to the weight average molecular weight of a graft chain (preferably macromer component represented by the said General formula (10)).

  Further, the content of the acidic group contained in the main chain portion and the salt obtained by neutralizing the acidic group (functional group content / graft polymer weight) is preferably 0.001 mmol / g to 20 mmol / g, more preferably. Is from 0.01 mol / g to 2.0 mmol / g. The main chain part is strongly adsorbed to charged particles at a content of 0.001 mmol / g or more, and the desired solubility of the graft polymer in the solvent can be maintained at 20 mmol / g or less. Furthermore, the mass ratio between the units constituting the main chain and the units constituting the graft chain is preferably in the range of 30:70 to 95: 5. These polymers may be used alone as a dispersant, but may be used in combination of two or more.

  The content of the dispersant with respect to the entire ink composition is preferably in the range of 0.01 to 30% by mass. More preferably, it exists in the range of 0.01-15 mass%. More preferably, it is 0.01-5 mass%. When the amount is 30% by mass or less, ink particles do not aggregate and an ink composition having a uniform particle size distribution can be produced. In the present invention, the ink composition is prepared by mixing, dispersing (particles) by simultaneously using a kneader, dissolver, mixer, high speed disperser, sand mill, roll mill, ball mill, attritor, bead mill, etc. In this case, if the added amount of the dispersant exceeds 30% by mass, small particles are easily generated, and an ink having a polydispersed particle size distribution can be obtained. Since the charged amount on the surface of the charged particle depends on the surface area of the particle, such a polydisperse ink having a large particle size distribution has a large variation in the charged amount of the charged particle and is difficult to stably discharge for a long period of time. It becomes. In addition, at 0.01% by mass or more, ink particles are not aggregated in the ink discharge head or the circulation pump, and the clogging of the head or the pump can be prevented, and a desired particle diameter can be obtained. When discharging, it becomes easy to discharge.

  According to the present invention, by using a graft polymer having an acidic group or a salt formed by neutralizing an acidic group as a dispersant in the main chain portion, the adsorptive power of the dispersant to charged particles is increased, and the content is increased. Charged particles having a desired particle diameter can be obtained within a range of 0.01 to 30% by mass. This makes it possible to obtain an ink composition that has a uniform particle size distribution and that uniformly charges charged particles, and can stably discharge ink droplets for a long time.

  Here, “uniform particle size distribution” indicates a range in which ink droplets can be stably ejected for a long time. Specifically, the volume average particle diameter / number average particle diameter (Φv / φn) is 1. It means within the range of .0 to 5.0.

[Charge control agent]
In the present invention, a coloring material (preferably a mixture of a coloring material and a coating agent) is dispersed (particulated) in a dispersion medium using a dispersing agent, but a charge adjusting agent is used in combination to control the charge amount of the particles. More preferably.
Suitable charge control agents include metal salts of organic carboxylic acids such as zirconium naphthenate and zirconium octenoate, ammonium salts of organic carboxylic acids such as tetramethylammonium stearate, sodium dodecylbenzenesulfonate, dioctyl Metal salts of organic sulfonic acids such as magnesium sulfosuccinate, ammonium salts of organic sulfonic acids such as toluenebutyl tetrabutylammonium salt, polymers containing carboxylic acid groups modified with amines of copolymers of styrene and maleic anhydride, etc. Polymers having a carboxylic acid group in the side chain, polymers having a carboxylic acid anion group in the side chain such as a copolymer of stearyl methacrylate and tetramethylammonium methacrylate, side chains such as a copolymer of styrene and vinylpyridine Has a nitrogen atom Rimmer, butyl methacrylate and N- (2-methacryloyloxyethyl) -N, N, polymers having an ammonium group in the side chain of the copolymer of N- trimethylammonium tosylate. Among them, in the present invention, it is preferable to use a polymer charge control agent from the viewpoint of maintaining a stable charge for a long time. The charge applied to the particles may be positively charged or negatively charged.

  The content of the charge control agent with respect to the entire ink composition is preferably in the range of 0.0001 to 10% by mass, and more preferably 0.001 to 5% by mass. Within this range, the charge amount of particles necessary for ejection is satisfied, and the charge amount becomes appropriate.

[Other ingredients]
In the present invention, a preservative for preventing corruption, a surfactant for controlling surface tension, and the like can be further contained depending on the purpose.

[Create charged particles]
An ink composition can be prepared by dispersing (particulating) a color material (preferably a color material and a coating agent) using the dispersant of the present invention using the above components. Examples of the dispersion (particulate) method include the following.
1) A colorant and a coating agent are mixed in advance, and then dispersed (granulated) using a dispersant and a dispersion medium, and a charge adjusting agent is added.
2) Disperse (particulate) using a coloring material, a coating agent, a dispersant and a dispersion medium at the same time, and add a charge control agent.
3) Disperse (particulate) using a colorant, a coating agent, a dispersant, a charge control agent and a dispersion medium at the same time.

  Examples of the apparatus used for mixing and dispersing include a kneader, a dissolver, a mixer, a high-speed disperser, a sand mill, a roll mill, a ball mill, an attritor, and a bead mill.

[Physical properties of ink composition]
The ink composition prepared as described above is recorded on a recording medium by an inkjet recording method. In the present invention, since the ink droplets are always stably ejected in a long-time inkjet recording, the following conditions ( It is preferable to use an ink composition that satisfies all of A) to (D).
(A) The electric conductivity at 20 ° C. of the ink composition is in the range of 1 nS / m to 5000 nS / m.
(B) The electric conductivity of the charged particles is 20% or more of the electric conductivity of the ink composition.
(C) The volume average diameter of the charged particles is in the range of 0.2 to 5.0 μm.
(D) The viscosity of the ink composition at 20 ° C. is in the range of 0.5 to 50 mPa · s.

The electrical conductivity of the ink composition at 20 ° C. is preferably in the range of 1 nS / m to 5000 nS / m. When the electrical conductivity of the ink composition is 1 nS / m or more, ink droplet ejection is good, and when it is 5000 nS / m or less, conduction occurs in the head (ejection section) of the ink jet device, causing damage to the head. Absent. More preferably, it is 10 nS / m-1000 nS / m.
The electric conductivity of the particles is a value obtained by centrifuging the ink composition, measuring the electric conductivity of the supernatant liquid in which the particles are precipitated, and subtracting the electric conductivity of the ink composition.

In the present invention, the electrical conductivity of the particles is preferably 20% or more of the electrical conductivity of the ink composition. In the ink jet recording method using an electrostatic field, when ink droplets are ejected, the concentration of charged particles occurs. However, in the case of 20% or more, this concentration occurs, and as a result, when ink is recorded on the recording medium, There is no blurring. More preferably, it is 30% or more.
The charge amount of the particles is preferably in the range of 1 to 1000 μC / g. When the charge amount is 1 μC / g or more, the concentration is sufficient. When the charge amount is 1000 μC / g or less, the ink is not excessively concentrated and clogging of the ink at the head ejection port can be prevented. More preferably, it is in the range of 10 to 500 μC / g, still more preferably 15 to 100 μC / g.

  The volume average diameter of the particles can be measured by a centrifugal sedimentation method using an apparatus such as an ultracentrifugal automatic particle size distribution analyzer CAPA-700 (manufactured by Horiba, Ltd.). The average diameter may be a volume average or a number average depending on the calculation method. In the present invention, the volume average diameter of the charged particles is preferably in the range of 0.2 to 5.0 μm. When the thickness is 0.2 μm or more, the particles are sufficiently concentrated, and as a result, the ink can be prevented from bleeding when recorded on a non-recording medium. Further, when the thickness is 5.0 μm or less, the problem of clogging of the head ejection port does not occur. More preferably, it is 0.3-3.0 micrometers. The particle size distribution is preferably narrow and uniform, and the volume average particle diameter / number average particle diameter (Φv / φn) is desirably in the range of 1.0 to 5.0. More preferably, it is 1.0-4.0.

In the present invention, the viscosity of the ink composition is preferably in the range of 0.5 to 50 mPa · s. When the viscosity is 0.5 mPa · s or more, there is no problem that the ink composition leaks from the ink discharge port of the head, and when the viscosity is 50 mPa · s or less, ink droplets are ejected well. It is. More preferably, it is in the range of 0.8 to 10 mPa · s.
The surface tension of the ink composition is preferably in the range of 10 to 70 mN / m. When the surface tension is 10 mN / m or more, there is no problem that the ink composition spills from the ink discharge port of the head, and when the surface tension is 70 mN / m or less, ink droplets are discharged well. . More preferably, it is the range of 15-50 mN / m.

[Inkjet recording apparatus]
The ink composition described above is recorded on a recording medium by an ink jet recording method. In the present invention, it is preferable to use an ink jet recording method utilizing an electrostatic field. An ink jet recording method using an electrostatic field concentrates charged particles of an ink composition to an ejection position by an electrostatic force by applying a voltage between a control electrode and a back electrode on the back of a recording medium, and the recording medium from the ejection position It is a method to fly to. For example, when the charged particles are positive, the voltage applied between the control electrode and the back electrode is such that the control electrode is a positive electrode and the back electrode is a negative electrode. The same effect can be obtained by charging the recording medium instead of applying a voltage to the back electrode.

  As a method of flying ink, for example, there is a method of flying ink from a needle-like tip such as an injection needle. By using the ink composition of the present invention, recording in a short time is possible.

  On the other hand, ink is circulated in an ink chamber having an ejection opening, and is present in the ejection opening by applying a voltage to a control electrode formed at the periphery of the ejection opening, and an ink guide whose front end faces the recording medium side. In the method in which the concentrated ink droplets fly from the front end, the supply of charged particles by the circulation of ink and the meniscus stability at the ejection position can be achieved at the same time, so that recording can be performed stably for a long period of time. Further, in this method, since the portion where the ink is in contact with the outside air is very small with only the discharge opening, the evaporation of the solvent is suppressed and the ink physical properties are stabilized, so that it can be suitably used in the present invention.

A configuration example of an ink jet recording apparatus suitable for applying the ink composition of the present invention is shown below.
First, an outline of an apparatus that performs single-sided four-color printing on the recording medium shown in FIG. 1 will be described.
An ink jet recording apparatus 1 shown in FIG. 1 supplies ink to an ejection head 2 composed of ejection heads 2C, 2M, 2Y, and 2K for four colors for forming a full color image. A head driver 4 for driving the ejection head 2 by an output from an external device such as a computer (not shown) or a RIP, and a position control means 5. Further, the ink jet recording apparatus 1 includes a transport belt 7 stretched around three rollers 6A, 6B, and 6C, a transport belt position detection unit 8 including an optical sensor that can detect the position of the transport belt 7 in the width direction, An electrostatic adsorption unit 9 for holding the recording medium P on the conveyance belt, a static elimination unit 10 and a mechanical unit 11 for separating the recording medium P from the conveyance belt 7 after completion of image formation are provided. Upstream and downstream of the conveyance belt 7, the recording medium P is fed from a stocker (not shown) to the conveyance belt 7, the ink is fixed to the feed roller 12 and the guide 13, and the peeled recording medium P and conveyed to a paper discharge stocker (not shown). A fixing means 14 and a guide 15 are arranged. The ink jet printing apparatus 1 has a recording medium position detecting means 16 at a position facing the ejection head 2 with the conveying belt 7 interposed therebetween, and further collects solvent vapor generated from the ink composition. A solvent recovery unit comprising the exhaust fan 17 and the solvent vapor adsorbent 18 is disposed, and the vapor inside the apparatus is discharged outside the apparatus through the recovery unit.

  As the feed roller 12, a known roller can be used, and the feed roller 12 is arranged so as to increase the feeding ability to the recording medium. Further, since dirt, paper powder, or the like may adhere to the recording medium P, it is desirable to remove them. The recording medium P supplied by the feed roller is conveyed to the conveying belt 7 through the guide 13. The back surface (preferably metal back surface) of the conveyor belt 7 is installed via a roller 6A. The transported recording medium is electrostatically attracted onto the transport belt by the electrostatic attracting means 9. In FIG. 1, electrostatic adsorption is performed by a scorotron charger connected to a negative high voltage power source. The electrostatic adsorbing means 9 electrostatically adsorbs the recording medium 9 on the conveying belt 7 without floating, and uniformly charges the surface of the recording medium. Here, the electrostatic attraction means is also used as a charging means for the recording medium, but may be provided separately. The charged recording medium P is transported to the ejection head portion by the transport belt 7, and electrostatic ink jet image formation is performed by superimposing the recording signal voltage with the charging potential as a bias. The image-formed recording medium P is neutralized by the neutralizing unit 10, peeled off by the conveying belt 7 by the mechanical unit 11, and conveyed to the fixing unit. The peeled recording medium P is sent to the image fixing means 14 and fixed. The fixed recording medium P is discharged through a guide 15 to a discharge stocker (not shown). The apparatus also has a means for recovering the solvent vapor generated from the ink composition. The recovery means is composed of the solvent vapor absorbing material 18, and a gas containing solvent vapor in the apparatus is introduced into the adsorbent by the exhaust fan 17, and after the vapor is adsorbed and recovered, it is exhausted outside the apparatus. The apparatus is not limited to the above example, and the number, shape, relative arrangement, charging polarity, and the like of constituent devices such as rollers and chargers can be arbitrarily selected. In the above system, four-color drawing is described. However, a multicolor system may be combined with light color ink or special color ink.

  The ink jet recording apparatus used in the above ink jet printing method includes an ejection head 2 and an ink circulation system 3. The ink circulation system 3 further includes an ink tank, an ink circulation device, an ink concentration control device, an ink temperature management device, and the like. In addition, the ink tank may include a stirring device.

As the ejection head 2, a single channel head, a multi-channel head, or a full line head can be used, and main scanning is performed by the rotation of the transport belt 7.
An ink jet head suitably used in the present invention is an ink jet method in which charged particles in an ink flow path are electrophoresed to increase the ink concentration near the opening and discharge, and mainly a recording medium or a recording medium rear surface. Ink droplets are ejected by electrostatic attraction caused by the counter electrode disposed on the surface. Therefore, if the recording medium or the counter electrode is not facing the head, or if no voltage is applied to the recording medium or the counter electrode even at a position facing the head, the voltage is improperly applied to the ejection electrode. Ink droplets are not ejected even when applied or when vibration is applied, and the inside of the apparatus is not soiled.

  An ejection head suitably used for the ink jet apparatus is shown in FIGS. As shown in FIGS. 2 and 3, the inkjet head 70 includes an electrically insulating substrate 74 that forms the upper wall of the ink flow path 72 where the ink flow Q in one direction is formed, and the ink is directed toward the recording medium P. And a plurality of discharge portions 76 for discharging. Each of the ejection portions 76 is provided with an ink guide portion 78 that guides the ink droplet G flying from the ink flow path 72 toward the recording medium P, and the substrate 74 has an opening 75 through which the ink guide portion 78 is inserted. The ink meniscus 42 is formed between the ink guide portion 78 and the inner wall surface of the opening 75. The gap d between the ink guide portion 78 and the recording medium P is preferably about 200 μm to 1000 μm. Further, the ink guide part 78 is fixed to the support bar part 40 on the lower end side.

  The substrate 74 includes an insulating layer 44 that electrically isolates two discharge electrodes at a predetermined interval, a first discharge electrode 46 formed above the insulating layer 44, and an insulation that covers the first discharge electrode 46. It has a layer 48, a guard electrode 50 formed on the insulating layer 48, and an insulating layer 52 that covers the guard electrode 50. The substrate 74 includes a second ejection electrode 56 formed below the insulating layer 44 and an insulating layer 58 that covers the second ejection electrode 56. The guard electrode 50 is provided in order to prevent an electric field from being affected by the voltage applied to the first ejection electrode 46 and the second ejection electrode 56 in the adjacent ejection section.

Further, the ink jet head 70 constitutes the bottom surface of the ink flow path 72, and the ink flow is generated by the induced voltage that is constantly generated by the pulsed discharge voltage applied to the first discharge electrode 46 and the second discharge electrode 56. A floating conductive plate 62 that moves positively charged ink particles (charged particles) R in the path 72 upward (that is, toward the recording medium side) is provided in an electrically floating state. In addition, a coating film 64 that is electrically insulating is formed on the surface of the floating conductive plate 62 to prevent the physical properties and components of the ink from becoming unstable due to charge injection into the ink. The electric resistance of the insulating coating film is preferably 10 ¹² Ω · cm or more, and more preferably 10 ¹³ Ω · cm or more. In addition, the insulating coating film is desirably resistant to corrosion with respect to ink, and this prevents the floating conductive plate 62 from being corroded by ink. Further, the floating conductive plate 62 is covered with the insulating member 66 from below, and the floating conductive plate 62 is completely electrically insulated by such a configuration.

  There are one or more floating conductive plates 62 per head unit (for example, when there are four heads C, M, Y, and K, the number of floating conductive plates is at least one each, and C and M The floating conductive plate is not shared between the head units.

  As shown in FIG. 3, in order to fly ink from the inkjet head 70 and record on the recording medium P, the ink flow Q is generated by circulating the ink in the ink flow path 72, and the guard electrode 50. A predetermined voltage (for example, +100 V) is applied to. Further, a flying electric field that attracts the positive charged particles R in the ink droplet G that has been guided by the ink guide portion 78 and flew from the opening 75 to the recording medium P is generated by the first ejection electrode 46 and the second ejection electrode 56. A positive voltage is applied to the first ejection electrode 46, the second ejection electrode 56, and the recording medium P so as to be formed between the recording medium P and the recording medium P (when the gap d is 500 μm, 1 kV to 3. (The standard is to form a potential difference of about 0 kV).

In this state, when a pulse voltage is applied to the first ejection electrode 46 and the second ejection electrode 56 in accordance with the image signal, the ink droplet G having an increased charged particle concentration is ejected from the opening 75 (for example, initial charged particles). When the concentration is 3 to 15%, the charged particle concentration of the ink droplet G is 30% or more).
At that time, the ink droplet G is applied to the first ejection electrode 46 and the second ejection electrode 56 so that the ink droplet G is ejected only when the pulse voltage is applied to both the first ejection electrode 46 and the second ejection electrode 56. Adjust the voltage value.

  As described above, when a pulsed positive voltage is applied, the ink droplet G is guided by the ink guide 78 from the opening 75 and flies to adhere to the recording medium P, and the floating conductive plate 62 has the first ejection electrode. A positive induced voltage is generated by the positive voltage applied to 46 and the second ejection electrode 56. Even if the voltage applied to the first ejection electrode 46 and the second ejection electrode 56 is pulsed, the induced voltage is a substantially steady voltage. Therefore, the charged particles R that are positively charged in the ink flow path 72 are subjected to the upward movement force by the electric field formed between the floating conductive plate 62 and the guard electrode 50 and the recording medium P, and the substrate In the vicinity of 74, the concentration of charged particles R increases. As shown in FIG. 3, when the number of ejection units (that is, channels for ejecting ink droplets) to be used is large, the number of charged particles necessary for ejection increases, but the first ejection electrode 46 and the second ejection electrode to be used are used. Since the number of 56 increases, the induced voltage induced in the floating conductive plate 62 increases, and the number of charged particles R moving to the recording medium also increases.

  In the above, an example in which the colored particles are positively charged has been described. However, the colored particles may be negatively charged. In that case, all the above-mentioned charging polarities are reversed.

  In the present invention, it is preferable to fix the ink by an appropriate heating means after discharging the ink onto the recording medium. As the heating means used, a contact-type heating device such as a heat roller, a heat block, or a belt heating, and a non-contact type heating device such as a dryer, an infrared lamp, a visible light lamp, an ultraviolet lamp, or a hot air oven may be used. it can. These heating devices are preferably continuous with and integrated with the ink jet recording apparatus. The temperature of the recording medium at the time of fixing is preferably in the range of 40 ° C. to 200 ° C. for ease of fixing. The fixing time is preferably in the range of 1 microsecond to 20 seconds.

[Replenishment of ink composition]
In the ink jet recording method using an electrostatic field, charged particles in the ink composition are concentrated and discharged. Therefore, when ink is ejected for a long time, the amount of charged particles in the ink composition is reduced, and the electrical conductivity of the ink composition is lowered. Further, the ratio between the electric conductivity of the charged particles and the electric conductivity of the ink composition changes. Further, when discharging, charged particles having a larger diameter tend to be discharged preferentially than charged particles having a smaller diameter, so that the average diameter of the charged particles is reduced. Further, since the solid content in the ink composition changes, the viscosity also changes. Due to these changes in physical property values, as a result, ejection failure occurs, the optical density of the recorded image decreases, and ink bleeding occurs. Therefore, by replenishing the ink composition having a higher concentration (higher solid content concentration) than the ink composition initially charged in the ink tank, the reduction of charged particles is prevented, and the electrical conductivity of the ink composition is reduced. The ratio between the electrical conductivity of the charged particles and the electrical conductivity of the ink composition can be kept within a certain range. Also, the average particle diameter and viscosity can be maintained. Further, by maintaining the physical property value of the ink composition within a certain range, the ink discharge is stably and uniformly performed for a long time. The replenishment at this time is preferably performed mechanically or manually, for example, by detecting a physical property value such as the electrical conductivity or optical density of the ink liquid used and calculating the shortage. Further, the amount of the ink composition to be used may be calculated based on the image data, and may be formed mechanically or manually.

[Recording medium]
In the present invention, various recording media can be used depending on the application. For example, if paper, plastic film, metal, and paper on which plastic or metal is laminated or vapor-deposited, plastic film on which metal is laminated or vapor-deposited, etc., printed matter can be directly obtained by ink jet recording. The shape of the recording medium may be planar such as a sheet shape or three-dimensional such as a cylindrical shape.

[Fixing]
In the present invention, it is preferable to fix the ink by an appropriate heating means after the ink is discharged onto the recording medium. As the heating means to be used, a contact-type heating device such as a heat roller, a heat block, or a belt heating and a non-contact type heating device such as a dryer, an infrared lamp, a visible light lamp, an ultraviolet lamp, or a hot air oven may be used. it can. These heating devices are preferably integrated with an inkjet recording apparatus.

  The ink composition of the present invention described above has long-term storage stability, and can discharge ink droplets stably for a long time. Further, by using the above-described ink jet recording apparatus, a high-quality image recorded matter having no ink bleeding and high image density can be obtained for a long time.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.

<Materials used>
In this example, the following materials were used.

Cyan pigment (coloring material) phthalocyanine pigment C.I. I. Pigment Blue (15: 3) (Toyo Ink Manufacturing Co., Ltd., LIONOL BLUE FG-7350)
・ Coating agent [AP-1]
Graft polymer dispersant [BZ-1] to [BZ-5], [DZ-1] to [DZ-2]
-Charge control agent [CT-1]
・ Dispersion medium Isopar G (manufactured by Exxon Corporation),

  The structures of the coating agent [AP-1], the dispersing agents [BZ-1] to [BZ-5], the dispersing agents [DZ-1] to [DZ-2] and the charge adjusting agent [CT-1] are shown below. .

<Synthesis of coating agent [AP-1] and charge control agent [CT-1]>
The coating agent [AP-1] radically polymerizes styrene, 4-methylstyrene, butyl acrylate, dodecyl methacrylate and 2- (N, N-dimethylamino) ethyl methacrylate using a known polymerization initiator, Furthermore, it was obtained by reacting with methyl tosylate. The weight average molecular weight was 15,000, and the polydispersity (weight average molecular weight / number average molecular weight) was 2.7. The glass transition point (mid point) was 51 ° C., and the softening point by the strain gauge method was 46 ° C.
The charge control agent [CT-1] was obtained by reacting 1-octadecene and maleic anhydride copolymer with 1-hexadecylamine. The weight average molecular weight was 17,000.

<Synthesis of dispersant [BZ-1]>
The graft polymer dispersant [BZ-1] is a polymer of stearyl methacrylate having a methacryloyl group at its end by radical polymerization of stearyl methacrylate in the presence of 2-mercaptoethanol and further reacting with methacrylic anhydride. -1] (weight average molecular weight was 7,600), and obtained by radical copolymerization with styrene and p-vinylbenzoic acid. The weight average molecular weight was 110,000.

<Synthesis of dispersant [BZ-2]>
Dispersant [BZ-2] was obtained by radical polymerization of a stearyl methacrylate polymer [BP-1] having a methacryloyl group at the terminal, and methacrylic acid and n-hexyl methacrylate. The weight average molecular weight was 127,000.

<Synthesis of dispersant [BZ-3]>
Dispersant [BZ-3] is obtained by radical polymerization of methacryloxypropyl-terminated polydimethylsiloxane DMS-R18 (molecular weight 4,500-5,500) manufactured by Chisso Corporation and methacrylic acid, n-hexyl methacrylate. It was. The weight average molecular weight was 100,000.

<Synthesis of dispersant [BZ-4]>
Radical polymerization of a polymer of stearyl methacrylate having a methacryloyl group at its terminal [BP-1], methacrylic acid and n-hexyl methacrylate, and neutralizing the carboxyl group of the resulting polymer with tetramethylammonium hydroxide The dispersant [BZ-4] was obtained. The weight average molecular weight was 115,000.

<Synthesis of dispersant [BZ-5]>
Dispersant [BZ-5] was obtained by radical polymerization of stearyl methacrylate polymer [BP-1] having a methacryloyl group at the end, methacrylamide-2-methylpropanesulfonic acid, and n-hexyl methacrylate. The weight average molecular weight was 127,000.

<Example 1>
10 g of the cyan pigment and 20 g of the coating agent [AP-1] were put into a desktop kneader PBV-0.1 manufactured by Irie Shokai Co., Ltd., the heater temperature was set to 100 ° C., and the mixture was heated and mixed for 2 hours. 30 g of the obtained mixture was coarsely pulverized with a trio-brender manufactured by Trio Science Co., Ltd., and further finely pulverized with a SK-M10 type sample mill manufactured by Kyoritsu Riko Co., Ltd. 30 g of the resulting finely pulverized product was predispersed in a paint shaker manufactured by Toyo Seiki Seisakusho together with 7.5 g of the dispersant [BZ-1], 75 g of Isopar G, and glass beads having a diameter of about 3.0 mm. After removing the glass beads, together with the zirconia ceramic beads having a diameter of about 0.6 mm, the type KDL dynomill manufactured by Shinmaru Enterprises Co., Ltd. was used for 5 hours while maintaining the internal temperature at 25 ° C., followed by 45 ° C. for 5 hours. Dispersion (particle formation) was performed at a rotational speed of 000 rpm. Zirconia ceramic beads were removed from the obtained dispersion, and 316 g of Isopar G and 0.6 g of charge control agent [CT-1] were added to obtain an ink composition [EC-1]. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for two months, and an extremely clear image was obtained without ink bleeding and density reduction. . Further, when a storage stability test (results are shown in Table 3) was performed, the volume average particle diameter was not changed before and after storage, and high storage stability was confirmed.

<Inkjet recording experiment method>
The ink composition to be evaluated was filled in the ink tank connected to the head in the ink jet recording apparatus shown in FIGS. Here, a 150 dpi (three-row staggered arrangement with a channel density of 50 dpi) and 833 channel head of the type shown in FIG. 2 was used as the discharge head, and a silicon rubber heat roller incorporating a 1 kW heater was used as the fixing means. As the ink temperature control means, a throwing heater and a stirring blade were provided in the ink tank, the ink temperature was set to 30 ° C., and the temperature was controlled with a thermostat while rotating the stirring blade at 30 rpm. Here, the stirring blade was also used as a stirring means for preventing precipitation and aggregation. In addition, the ink flow path is partially transparent, and the LED light emitting element and the light detecting element are arranged between them, and the ink dilution liquid (Isopar G) or concentrated ink (the solid content concentration of the ink composition is determined by the output signal). The concentration was controlled by charging. A fine coated paper for offset printing was used as a recording medium. After removing dust on the surface of the recording medium by air pump suction, the ejection head is brought close to the recording medium to the image forming position, image data to be recorded is transmitted to the image data calculation control unit, and the recording belt is rotated by rotation of the conveying belt. The ink composition was discharged while sequentially moving the discharge head while transporting the recording medium, and an image was formed with a drawing resolution of 2400 dpi. As the conveyor belt, a metal belt and polyimide film bonded together are used, and a line-shaped marker is placed in the vicinity of one end of the belt along the conveyor direction, and this is optically read by the conveyor belt position detection means. An image was formed by driving the control means. At this time, the distance between the ejection head and the recording medium was kept at 0.5 mm by the output from the optical gap detector. Further, the surface potential of the recording medium was set to −1.5 kV at the time of ejection, and a pulse voltage of +500 V was applied (pulse width 50 μsec) at the time of ejection, and image formation was performed at a driving frequency of 15 kHz. Immediately after image recording, the image was fixed using a heat roller. The temperature of the coated paper at the time of fixing was 90 ° C., and the contact time with the heat roller was 0.3 seconds. In addition, for 10 minutes after the recording was completed, the head was stored in a cover filled with vapor of Isopar G after supplying the head with Isopar G instead of ink for cleaning.

<Example 2>
An ink composition [EC-2] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [BZ-2] was used as the dispersant. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for two months, and an extremely clear image was obtained without ink bleeding and density reduction. . Further, when a storage stability test (results are shown in Table 3) was performed, the volume average particle diameter was not changed before and after storage, and high storage stability was confirmed.

<Example 3>
An ink composition [EC-3] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [BZ-3] was used as the dispersant. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for two months, and an extremely clear image was obtained without ink bleeding and density reduction. . Further, when a storage stability test (results are shown in Table 3) was performed, the volume average particle diameter was not changed before and after storage, and high storage stability was confirmed.

<Example 4>
An ink composition [EC-4] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [BZ-4] was used as the dispersant. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for two months, and an extremely clear image was obtained without ink bleeding and density reduction. . Further, when a storage stability test (results are shown in Table 3) was performed, the volume average particle diameter was not changed before and after storage, and high storage stability was confirmed.

<Example 5>
An ink composition [EC-5] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [BZ-5] was used as the dispersant. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for two months, and an extremely clear image was obtained without ink bleeding and density reduction. . Further, when a storage stability test (results are shown in Table 3) was performed, the volume average particle diameter was not changed before and after storage, and high storage stability was confirmed.

<Synthesis of comparative dispersant [DZ-1]>
Dispersant [DZ-1] was obtained by radical polymerization of stearyl methacrylate polymer [BP-1] having a methacryloyl group at the terminal, methyl methacrylate, and n-hexyl methacrylate. The weight average molecular weight was 114,000.

<Synthesis of comparative dispersant [DZ-2]>
Stearyl methacrylate was radically polymerized in the presence of 3-mercaptopropionic acid to obtain a stearyl methacrylate polymer (dispersant [DZ-2]) having a carboxyl group at the terminal. The weight average molecular weight was 7,600.

<Comparative Example 1>
An ink composition [HC-1] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [DZ-1] was used as the dispersant. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for one month. However, the image after two months had bleeding and a decrease in density. Further, when a storage stability test (results are shown in Table 3) was conducted, there was a large difference in the volume average particle diameter before and after storage, and the storage stability was poor.

<Comparative example 2>
An ink composition [HC-2] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [DZ-1] was used as the dispersant and the addition amount was 33.0 g. Using the obtained ink, an ink jet recording experiment (results shown in Table 2) was performed. As a result, there was no need for maintenance work for one month. However, the image after two months had bleeding and a decrease in density. Further, when a storage stability test (results are shown in Table 3) was conducted, there was a large difference in the volume average particle diameter before and after storage, and the storage stability was poor.

<Comparative Example 3>
An ink composition [HC-3] was obtained in the same manner as the ink composition [EC-1] except that the dispersant [DZ-2] was used as the dispersant and the addition amount was 20.0 g. When an ink jet recording experiment (results are shown in Table 2) was performed using the obtained ink, the image after one week was slightly blurred and had a density decrease. Further, since the image obtained by continuous ejection for one month had severe blurring and density reduction, the ink jet recording test was stopped at this point. Further, when a storage stability test (results are shown in Table 3) was conducted, there was a large difference in the volume average particle diameter before and after storage, and the storage stability was poor.

  The physical properties of the ink compositions [EC-1] to [EC-5] and [HC-1] to [HC-3] were as follows (Table 1).

  Volume average diameter: The volume average diameter of the charged particles was measured with a CAPA-700 manufactured by Horiba, Ltd., at a rotation speed of 5000 rpm.

<Inkjet recording experiment results>
Table 2 shows the evaluation of the recorded image (printed material) after one week, one month and two months.

  In Examples 1 to 5, very clear images were obtained without ink bleeding and density reduction without the need for maintenance work for two months. In Comparative Examples 1 and 2, an image that was obtained by continuous ejection for two months was smudged, although there was no need for maintenance work for one month, and an extremely clear image was obtained with no ink bleed or density reduction. There was a decrease in concentration. In Comparative Example 3, the image after one week had slight blurring and density reduction, and the image obtained by continuous ejection for one month had severe blurring and density reduction. It is considered that the blur and concentration decrease in this experiment reflect the uniformity of particle size distribution (φv / φn) (described in Table 1).

<Long-term storage stability test method>
Ink compositions [EC-1] to [EC-5] and [HC-1] to [HC-3] were stored at 35 ° C. for one week, and the volume average particle diameters before and after storage were measured. The measurement was performed with a CAPA-700 manufactured by HORIBA, Ltd. at a rotational speed of 5000 rpm.

  Regarding Examples 1 to 5, there was no change in the volume average particle diameter before and after storage, and high storage stability was confirmed. As for Comparative Examples 1 to 3, there was a large difference in volume average particle diameter before and after storage, and the result that storage stability was poor was obtained.

It is a whole lineblock diagram showing typically an example of an ink jet printer used for the present invention. It is a perspective view which shows the structure of the inkjet head of the inkjet recording device of this invention (In order to make it intelligible, the edge of the guard electrode in each discharge part is not drawn). FIG. 3 is a side sectional view showing a distribution state of charged particles when the number of ejection units of the inkjet head shown in FIG. 2 is large (corresponding to an arrow XX in FIG. 2).

Explanation of symbols

G Injected ink droplet P Recording medium Q Ink flow R Charged particle 1 Inkjet recording apparatus 2 Discharge head 3 Ink circulation system 4 Head driver 5 Position control means 6A to 6C Conveying belt stretching roller 7 Conveying belt 8 Conveying belt position detecting means 9 Electrostatic adsorption means 10 Static elimination means 11 Mechanical means 12 Feed roller 13 Guide 14 Image fixing means 15 Guide 16 Recording medium position detection means 17 Discharge fan 18 Solvent vapor adsorbent 38 Ink guide 40 Support bar portion 42 Ink meniscus 44 Insulating layer 46 First ejection electrode 48 Insulating layer 50 Guard electrode 52 Insulating layer 56 Second ejection electrode 58 Insulating layer 62 Floating conductive plate 64 Coating film 66 Insulating member 70 Inkjet head 72 Ink flow path 74 Substrate 75, 75A, 75B Opening 76, 76A, 76B Discharge unit 78 Discharge unit

Claims (2)

  In an electrostatic ink jet recording ink composition containing a charged particle containing at least a colorant and a dispersion medium, the charged particle has at least one of an acidic group and a salt formed by neutralizing an acidic group as a main chain portion. An ink composition for electrostatic ink jet recording, wherein the ink composition is dispersed in a dispersion medium by a graft polymer.   An ink jet recording method, wherein an ink droplet is ejected by an ink jet recording method using an electrostatic field, using the ink composition according to claim 1.

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