JP2013018246A - Image forming method using inkjet recording apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming method that can suppress image irregularities when forming an image after ink is not be ejected from a recording head for a while, and can form the satisfactory image with a satisfactory print density, without causing a medium to be recorded to be contaminated by offset.SOLUTION: When the image is formed by a line head type inkjet recording apparatus including two or more recording heads where two or more inks are used, and an ejection part to eject the medium to be recorded, the image is formed by ejecting the inks having two or more colors in a specific order as a reference of a resin acid value, wherein the inks include pigment, resin, water, and organic solvent, and the resin acid value has a specific range.

Description

  The present invention relates to an image forming method using a line head type ink jet recording apparatus including two or more recording heads using two or more colors of ink.

  In recent years, rapid progress in recording technology has made it possible to obtain high-definition image quality comparable to silver halide photography, so inkjet recording apparatuses that form images by the inkjet recording method have been widely used as image forming apparatuses. ing.

  For such an ink jet recording apparatus, it is strongly desired to improve the image forming speed while maintaining the image quality. In order to increase the image forming speed, it is effective to use a line head type ink jet recording apparatus. However, when image formation is performed at high speed in a line head type ink jet recording apparatus, the recording medium is transported and discharged by the discharge roller before the ink penetrates the recording medium such as paper, and the ink is discharged. It may adhere (offset) to the roller, and in this case, image defects are likely to occur. In order to suppress the occurrence of offset, it is conceivable to reduce the amount of ink discharged or to increase the penetrability and dryness of the ink on the recording medium. In the latter case, when the image is formed after the ink has not been ejected from the recording head for a while, ink ejection failure may occur due to a change in the viscosity of the ink and the image may be disturbed.

  Under such circumstances, an image having excellent ink permeability to a recording medium such as paper, capable of forming an image having a good density, and being less likely to be disturbed when forming an image after the ink has not been ejected for a while. As a forming method, the water content is 10% by mass or more and less than 50% by mass with respect to the total ink mass, and at least one organic solvent is an aqueous solution having an SP value of 16.5 or more and less than 24.6. An ink set composed of an ink having an SP value of 16.5 or more and less than 24.6 and having a water-soluble organic solvent content of 30% by mass or more of the total ink mass, An image forming method using a line head type ink jet recording apparatus has been proposed in which the printing order is accelerated as the color material concentration in the ink is higher in the set (Patent Document 1).

JP 2007-145926 A

  According to the image forming method described in Patent Document 1, an image can be formed with a good print density. However, in the ink used in the image forming method described in Patent Document 1, a pigment dispersion in which a pigment is dispersed with a resin is used. The ease of occurrence of an offset problem and the ease of occurrence of image disturbance when forming an image after ink has not been ejected from the recording head for a while include the preparation conditions of the pigment dispersion and the pigment dispersion. It is greatly influenced by the properties of the resin used to prepare the body. For this reason, in the image forming method described in Patent Document 1, it is not always necessary to suppress image disturbance when forming an image after ink has not been ejected from the recording head for a while, and to suppress contamination of the recording medium due to offset. Cannot be resolved.

  The present invention has been made in view of such circumstances, and can suppress image disturbance when forming an image after ink has not been ejected from the recording head for a while. An object of the present invention is to provide an image forming method capable of forming a good image without causing contamination of a recording medium.

  When forming an image with a line head type ink jet recording apparatus including two or more recording heads using two or more colors of ink and a discharge unit for discharging a recording medium, the present inventors The above problem is solved by forming an image by ejecting ink of two or more colors containing water and an organic solvent and having a resin acid value in a specific range in a specific order based on the acid value of the resin. As a result, the present invention has been completed. More specifically, the present invention provides the following.

(1) An image forming method using a line head type ink jet recording apparatus including two or more recording heads using two or more colors of ink and a discharge unit for discharging a recording medium,
The ink of two or more colors includes a pigment, a resin, water, and an organic solvent,
In the two or more colors of ink, the acid value of the resin contained in the ink is 60 to 200 mgKOH / g or more,
The acid value of the resin contained in the ink discharged last is higher than the acid value of the resin contained in the ink discharged in the first color,
The image forming method, wherein the acid value of the resin contained in the ink ejected last is 100 to 200 mgKOH / g.

  (2) The image forming method according to (1), wherein the acid value of the resin contained in the ink ejected to the first color is 60 to 100 mgKOH / g.

  (3) (1) or (2) description, wherein the acid value of the resin contained in the ink ejected last from the second color is equal to or higher than the acid value of the resin contained in the ink ejected in the previous order. Image forming method.

  (4) (1) to (3), wherein the ink of two or more colors has a resin mass / pigment mass value of 0.3 or more and less than 1.0 for the resin and the pigment contained in the ink. ) Any one of the image forming methods.

  (5) Any of (1) to (4), wherein the time from when the last ejected ink landed on the surface of the recording medium to when the landed portion contacts the discharge portion is within 1 second The image forming method described.

  According to the present invention, it is possible to suppress image disturbance when forming an image after ink has not been ejected from the recording head for a while, and at a good print density without causing contamination of the recording medium due to offset. A good image can be formed.

FIG. 1 is a side sectional view showing a schematic configuration of a line head type ink jet recording apparatus. FIG. 2 is a plan view of the conveyance belt of the ink jet recording apparatus shown in FIG. 1 as viewed from above. FIG. 3 is a block diagram showing a configuration of a line head type ink jet recording apparatus. FIG. 4 is an enlarged plan view showing a part of a dot head formed on a line head and recording paper used in an ink jet recording apparatus of a line head type recording system.

  Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention. In addition, although description may be abbreviate | omitted suitably about the location where description overlaps, the summary of invention is not limited.

  The image forming method of the present invention is an image forming method using a line head type ink jet recording apparatus including two or more recording heads using two or more colors of ink and a discharge unit for discharging a recording medium. The image forming method of the present invention is an image forming method using an ink containing a pigment, a resin, water and an organic solvent, and the acid value of the resin contained in the ink ejected from each recording head is within a predetermined range. The acid value of the resin contained in the first color ink and the last ejected ink satisfies a predetermined relationship, and the acid value of the resin contained in the last ejected ink is a value within a specific range. It is what. Hereinafter, the ink, the ink manufacturing method, and the image forming method will be described in order with respect to the present invention.

〔ink〕
The ink used in the present invention contains a pigment, a resin, water, and an organic solvent. In addition, the ink used in the present invention includes a dissolution stabilizer that stabilizes the dissolution state of the components contained in the ink, and a humectant that stabilizes the viscosity of the ink by suppressing the volatilization of the liquid component from the ink, if necessary. May be included. In the ink used in the present invention, the pigment and the resin are contained as a pigment dispersion.

  In the present invention, inks of two or more colors are used, but the number of inks is not particularly limited as long as the object of the present invention is not impaired. Four colors of ink consisting of black ink, cyan ink, yellow ink, and magenta ink are used because images of various hues can be satisfactorily formed, the number of recording heads can be reduced, and the inkjet recording apparatus can be easily downsized. Is preferred.

  Hereinafter, regarding the ink used in the present invention, the pigment dispersion, water, organic solvent, dissolution stabilizer, and humectant will be described in order.

(Pigment dispersion)
The pigment that can be contained in the pigment dispersion is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected from pigments that have been conventionally used as colorants for inks for inkjet recording apparatuses. . Specific examples of pigments used for cyan ink include C.I. I. And blue pigments such as CI Pigment Blue 15. Specific examples of pigments used in yellow ink include C.I. I. Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, 193 and the like are listed. Examples of pigments used in magenta ink C. I. And red pigments such as CI Pigment Red 122 and 202. Examples of pigments used for black ink include C.I. I. And black pigments such as CI Pigment Black 7 (BK-7, carbon black).

  Specific examples of the other hue pigments include C.I. I. Pigment Orange 34, 36, 43, 61, 63, 71 and other orange pigments, C.I. I. And purple pigments such as CI Pigment Violet 19, 23 and 33. Two or more of these pigments can be used in combination to adjust the hue of the ink to a desired hue.

  For magenta ink, a good hue can be reproduced in a wide color gamut. I. More preferably, CI Pigment Red 122 (PR 122) is used.

  The pigment content in the ink is not particularly limited as long as the object of the present invention is not impaired. Typically, 6 to 8.5 mass% is preferable for cyan ink, 6 to 8.5 mass% is preferable for yellow ink, 7.5 to 8 mass% is preferable for magenta ink, and 6 to 7 mass for black ink. 0.5 mass% is preferable. When the content of the pigment in each color ink is in such a range, it is possible to form an image with a good print density and to suppress image disturbance when forming an image after the ink has not been ejected from the recording head for a while. Intermittent ejection properties (ink ejection performance when forming an image after ink has not been ejected for a while from the recording head) can be realized, and contamination of the recording medium due to offset is less likely to occur.

  The resin contained in the pigment dispersion is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected from various resins conventionally used in the production of pigment dispersions. Specific examples of suitable resins include styrene-acrylic acid-acrylic acid alkyl ester copolymers, styrene-acrylic acid copolymers, styrene-maleic acid copolymers, and styrene-maleic acid-acrylic acid alkyl ester copolymers. Styrene-methacrylic acid copolymer, styrene-methacrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer, vinyl naphthalene-acrylic acid copolymer, vinyl naphthalene-maleic acid copolymer, and the like. . Among these resins, since they are easy to prepare and have excellent pigment dispersion effects, styrene-acrylic acid-alkyl acrylate ester copolymers, styrene-acrylic acid copolymers, styrene-maleic acid-alkyl acrylates. Units derived from styrene, such as ester copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid alkyl ester copolymers, and units derived from acrylic acid, methacrylic acid, acrylic esters, or methacrylic esters A styrene-acrylic resin containing The above resin is obtained by radical polymerization.

  In the present invention, a resin having an acid value of 60 to 200 mgKOH / g is used as the resin used for preparing the pigment dispersion. The acid value of the resin can be adjusted by appropriately adjusting the amount of the monomer having an acidic functional group (for example, carboxy group) such as acrylic acid or methacrylic acid when the resin is synthesized. Specifically, the acid value can be increased by increasing the amount of the monomer having an acidic functional group. The acid value of the resin can be measured by titration according to the following method.

<Method for measuring acid value>
10 g of the resin is weighed into a 300 ml Erlenmeyer flask, and about 50 ml of a mixed solvent of ethanol: benzene = 1: 2 is added to dissolve the resin. Next, using a phenolphthalein indicator, titration was performed with a 0.1 mol / L potassium hydroxide ethanol solution that had been standardized in advance, and the acid value was calculated from the amount of the potassium hydroxide ethanol solution used for titration by the following formula (1). (Mg KOH / g) was determined. For resins that do not dissolve in about 50 ml of a mixed solvent of ethanol: benzene = 1: 2, select either 50 ml of ethanol or about 50 ml of a mixed solvent of ethanol / pure water = 1: 1. The others were titrated by the same operation.
Formula (1)
A = (B × f × 5.611) / S
In the formula, A is the acid value of the resin (mgKOH / g), B is the amount of 0.1 mol / L potassium hydroxide ethanol solution used for titration (ml), and f is the 0.1 mol / liter potassium hydroxide ethanol solution. Factor, S is the mass of the resin (g), 5.611 is the formula weight of potassium hydroxide (56.11 / 10).

  The weight average molecular weight (Mw) of the resin used for preparing the pigment dispersion is not particularly limited as long as the object of the present invention is not impaired, but is preferably 10,000 to 200,000, and more preferably 10,000 to 100,000. The weight average molecular weight (Mn) of the resin contained in the pigment dispersion can be measured by gel filtration chromatography. If the weight average molecular weight of the resin is too small, the recording medium is likely to be contaminated by offset, and it is difficult to form an image with a good density. If the weight average molecular weight of the resin is excessive, ink ejection failure is likely to occur when an image is formed after the recording head has not ejected ink for a while.

  The amount of resin used in the ink used in the present invention is not particularly limited as long as it does not impair the object of the present invention, but the mass of the resin and the mass of the pigment contained in the ink is the value of the mass of the resin / the mass of the pigment. As a result, it is preferably 0.3 or more and less than 1.0. By setting the masses of the resin and the pigment within such a range, it is possible to realize the formation of an image having a good print density and intermittent ejection properties.

  A method for producing a pigment dispersion containing a pigment and a resin is not particularly limited as long as the object of the present invention is not impaired, and can be appropriately selected from conventionally known methods. As a suitable method, for example, using a media type wet disperser such as Nano Glen Mill (manufactured by Asada Tekko Co., Ltd.), MSC Mill (manufactured by Mitsui Mining Co., Ltd.), Dino Mill (manufactured by Shinmaru Enterprises Co., Ltd.), water, etc. And a method of kneading a pigment and a resin in an appropriate liquid medium to form a pigment dispersion. In the treatment by the media type wet disperser, beads having a small particle diameter are used to pulverize and disperse the pigment. The particle diameter of the beads is not particularly limited and is typically a particle diameter of 0.5 to 1.0 mm. The material of the beads is not particularly limited, and a hard material such as zirconia is used.

(water)
The ink used in the present invention is a water-based ink and essentially contains water. The water contained in the ink is not particularly limited as long as it does not impair the object of the present invention, and water having a desired purity can be appropriately selected from water conventionally used for producing water-based inks. The water content in the ink is not particularly limited as long as the object of the present invention is not impaired. The content of water is appropriately changed according to the amount of other components to be described later. The typical water content in the ink is preferably 20 to 70% by mass, more preferably 30 to 60% by mass with respect to the total mass of the ink.

(Organic solvent)
The ink used in the present invention contains an organic solvent for the purpose of promoting the penetration of the ink into the recording medium. Specific examples of suitable organic solvents include alkylene glycol monoalkyl ethers such as ethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and ethylene glycol monomethyl ether. These organic solvents may be used alone or in combination of two or more. 10-60 mass% is preferable with respect to the total mass of an ink, and, as for content of the organic solvent in an ink, 20-50 mass% is more preferable.

(Dissolution stabilizer)
The dissolution stabilizer is a component that compatibilizes the components contained in the ink and stabilizes the dissolved state of the ink. Specific examples of the dissolution stabilizer include 2-pyrrolidone, N-methyl-2-pyrrolidone, and γ-butyrolactone. These dissolution stabilizers can be used in combination of two or more. When the ink contains a dissolution stabilizer, the content of the dissolution stabilizer is preferably 1 to 20% by mass and more preferably 3 to 15% by mass with respect to the total mass of the ink.

(Humectant)
The humectant is a component that stabilizes the viscosity of the ink by suppressing the volatilization of the liquid component from the ink. Specific examples of the humectant include polyalkylene glycols such as polyethylene glycol and polypropylene glycol; ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, 1,2,6-hexanetriol. Alkylene glycols such as thiodiglycol, 1,3-butanediol and 1,5-pentanediol; glycerin. Among these humectants, glycerin is more preferable because of its excellent effect of suppressing volatilization of liquid components such as water. Two or more kinds of humectants can be used in combination. When the ink contains a humectant, the content of the humectant is preferably 5 to 50% by mass and more preferably 10 to 40% by mass with respect to the total mass of the ink.

[Ink production method]
The ink production method is not particularly limited as long as ink components such as a pigment dispersion, water, and an organic solvent can be mixed uniformly. As a specific example of the method for producing the ink for an ink jet recording apparatus, there is a method in which each component of the ink is uniformly mixed by a mixer and then foreign matters and coarse particles are removed by a filter having a pore diameter of 10 μm or less. In addition, when manufacturing ink, components such as a dissolution stabilizer and a humectant, surfactants, antioxidants, viscosity modifiers, pH adjusters, antiseptic and antifungal agents, etc. Various additives that have been added to inks for inkjet recording devices can be added.

(Image forming method)
Hereinafter, with reference to the drawings, an image forming method using a line head type ink jet recording apparatus including two or more recording heads using two or more inks of the present invention and a discharge unit for discharging a recording medium will be described. To do. Here, as a preferable example, a line head type ink jet recording apparatus including four recording heads is used, and recording paper is used as a recording medium, and black ink, cyan ink, magenta ink, and yellow ink are used. A case where an image is formed using the above will be described. In such a case, various hues can be satisfactorily expressed with a small number of colors, and the ink jet recording apparatus can be miniaturized. FIG. 1 is a side sectional view showing a schematic configuration of an ink jet recording apparatus of a line head type recording system, and FIG. 2 is a plan view of a transport belt of the ink jet recording apparatus shown in FIG.

  As shown in FIG. 1, a paper feed tray 2 (paper feed unit) that accommodates recording paper P is provided on the left side of the ink jet recording apparatus 100, and is accommodated at one end of the paper feed tray 2. A recording roller P for conveying and feeding the recording paper P one by one from the highest recording paper P one by one to a conveying belt 5 to be described later, and a driven roller 4 that is pressed against the paper feeding roller 3 and driven to rotate are provided. ing.

  A transport belt 5 is rotatably disposed on the downstream side (right side in FIG. 1) of the paper transport direction X of the paper feed roller 3 and the driven roller 4. The conveyance belt 5 is stretched over a belt driving roller 6 disposed on the downstream side in the sheet conveyance direction X and a belt roller 7 disposed on the upstream side and rotated by the belt driving roller 6 via the conveyance belt 5. Thus, the recording paper P is conveyed in the direction of the arrow X by the belt driving roller 6 being rotated in the clockwise direction.

  Here, since the belt driving roller 6 is arranged on the downstream side in the paper transport direction X, the paper feeding side (upper side in FIG. 1) of the transport belt 5 is pulled by the belt driving roller 6, so that belt tension is applied. Therefore, the recording paper P can be stably conveyed. In addition, a sheet made of dielectric resin is used for the conveyance belt 5, and a (seamless) belt or the like having no seam is preferably used.

  Further, on the downstream side of the conveyance belt 5 in the sheet conveyance direction X, a recording sheet P on which an image is recorded by being driven clockwise in the drawing is discharged to the outside of the apparatus main body, and above the discharge roller 8a. A driven roller 8b that is pressed and rotated is provided, and a discharge tray 10 on which the recording paper P discharged outside the apparatus main body is stacked is provided downstream of the discharge roller 8a and the driven roller 8b. Yes.

  Since the driven roller 8b directly touches the printing surface of the recording paper P, the material forming the surface of the driven roller 8b is preferably a water repellent material. By forming the surface of the driven roller 8b with a water-repellent material, it is possible to suppress the ink that has not penetrated the recording paper P from adhering to the roller and easily suppress the occurrence of offset. Suitable water repellent materials include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, tetrafluoro Ethylene-vinylidene fluoride copolymer, tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer, polychlorotrifluoroethylene, chlorotrifluoroethylene-ethylene copolymer, chlorotrifluoroethylene-vinylidene fluoride copolymer Examples thereof include fluororesins such as polymers, polyvinylidene fluoride, and polyvinyl fluoride. Similar to the driven roller 8b, the surface of the member that contacts the printing surface of the recording paper P is preferably formed of a water repellent material.

  An image is recorded on the recording paper P that is supported above the conveying belt 5 at a height that forms a predetermined interval with respect to the upper surface of the conveying belt 5. Line heads 11a, 11b, 11c and 11d to be performed are arranged. In these line heads 11a to 11d, colored inks of four colors (cyan, magenta, yellow and black) whose acid value of the resin contained in the ink is 60 to 200 mg KOH / g are ejected from the first color line head 11a. The acid value of the resin contained in the ink is lower than the acid value of the resin contained in the ink ejected from the line head 11d of the fourth color, and the acid value of the resin contained in the ink ejected in the fourth color is 100. A color image is formed on the recording paper P by discharging the respective colored inks from the line heads 11a to 11d.

  The mass ratio between the resin and the pigment contained in the ink (resin / pigment mass ratio) is not particularly limited, but an image can be formed with a good print density as long as it is 0.3 or more and less than 1.0. Can be prevented from being disturbed when an image is formed after it has not been discharged for a while.

  If the acid value of the resin contained in the ink is 60 to 200 mgKOH / g, an image can be formed with a good print density, and the image is disturbed when the image is formed after the ink has not been ejected from the recording head for a while. And contamination of the recording paper P due to offset can be suppressed. When the acid value of the resin is too low, the print density of the formed image is lowered, and the recording paper P is easily contaminated by the offset. If the acid value of the resin is too high, the formed image is likely to be disturbed when an image is formed after the recording head has not ejected ink for a while. If the acid value of the resin is too low, the print density of the formed image may decrease.

  Note that the closer the line head is to the discharge portion 8, the easier the recording paper P reaches the discharge portion before the ink is sufficiently dried, and the contamination of the recording paper P due to the offset is likely to occur. Accordingly, since the line head 11d is closest to the discharge unit 8, when the four colors of ink are ejected to form an image, the fourth color of ink ejected from the line head 11d is most likely to contaminate the recording paper due to offset. Influence. For this reason, by setting the acid value of the resin contained in the ink discharged in the fourth color to a high value of 100 mgKOH / g or more, contamination of the recording paper P due to the offset is suppressed.

  In order to form a good image, it is preferable to eject ink that can form an image with a high print density for the first color. If the pigment concentration of the ink is increased, the image density of the formed image can be increased, but in this case, the intermittent ejection property may be impaired. However, if the acid value of the resin contained in the ink is lowered, the intermittent ejection property of the ink can be improved. Therefore, by containing the acid value of the resin contained in the ink ejected in the first color lower than the acid value of the resin contained in the ink ejected in the fourth color, the inclusion of the pigment of the ink ejected in the first color Even if the amount is increased, it is easy to maintain the intermittent ejection property of the ink ejected in the first color in a good state. In such a case, the acid value of the resin contained in the ink ejected to the first color is preferably 60 to 100 mgKOH / g. Thus, a good image can be formed according to the image forming method of the present invention.

  Furthermore, since the line head 11 a is far from the discharge unit 8, the ink is likely to dry before the landing point of the ink droplet on the recording paper P reaches the discharge unit 8. For this reason, even if the ink ejected from the line head 11a is an ink containing a resin having a low acid value that easily causes the recording paper P to be contaminated by the offset, the recording paper P is hardly contaminated by the offset.

  Resin acid contained in the ink ejected in the second and third colors from the line heads 11b and 11c, ie, the ink ejected between the first ejected ink and the last ejected ink. A value will not be specifically limited if it is 60-200 mgKOH / g. The line heads 11b and 11c are arranged at a position farther from the discharge unit 8 than the line head 11d that discharges the fourth color ink. For this reason, when the acid value is too low, even if the recording paper P tends to be contaminated by offset, if the acid value of the resin is 60 mgKOH / g or more, the ink discharged to the second and third colors is used. Contamination of the recording paper P due to offset is unlikely to occur.

  The higher the acid value of the resin, the less likely the recording paper P is contaminated by the offset, and the slower the ejection order, the easier the ink is discharged from the line head closer to the discharge section 8 and the more likely the recording paper P is contaminated. For this reason, in order to make it easier to suppress contamination of the recording paper P due to offset, the acid value of the resin contained in the ink ejected to the second to fourth colors is the resin contained in the ink ejected in the previous order. It is preferable that it is more than the acid value of.

  In the second embodiment, a method of forming an image using four colors of ink is described. However, the first ejected ink, the last ejected ink, and the middle of these inks are ejected. The relationship between the acid values of the resins contained in the ink is the same even when an image is formed by using three colors or five or more colors of ink.

  Among the line heads 11a to 11d, after the ink droplets discharged from the line head 11d have landed on the recording paper P, the ink landing location on the recording paper P consists of the discharge roller 8a and the driven roller 8b. The time required to reach the discharge unit 8 for discharging the recording paper P is preferably within 1 second in order to reduce the size of the apparatus. Even when the time is within 1 second, according to the image forming method of the present invention, the effect of suppressing the occurrence of offset during high-speed image formation can be sufficiently obtained.

Also, ejected from the line head 11a~11d the recording paper P, the total amount of ink applied to the recording paper P is preferably 10 g / ^{m 2} or less, more preferably 7 g / ^{m 2} or less. By setting the ink discharge amount to such an amount, it is easy to form an image at high speed while suppressing the occurrence of offset.

  As shown in FIG. 2, these line heads 11 a to 11 d include a nozzle row in which a plurality of nozzles are arranged in a direction (vertical direction in FIG. 2) orthogonal to the transport direction, and the width of the recording paper P being transported Having the above recording area, the image of one line can be collectively recorded on the recording paper P conveyed on the conveying belt 5.

  In the line head type ink jet recording apparatus, a plurality of nozzles are arranged in the longitudinal direction of a long head body formed to have a width dimension larger than that of the transport belt 5, thereby increasing the width of the recording paper P. However, the recording paper is conveyed by arranging a plurality of short head units each having a plurality of nozzles in the width direction of the conveying belt 5, for example. A line head that can record an image over the entire width in the P width direction may be used.

  Further, as the ink ejection method of the line heads 11a to 11d, for example, ink droplets are ejected using pressure generated in the liquid chambers of the line heads 11a to 11d using a piezoelectric element (piezo element) (not shown). Various methods such as a piezoelectric element method and a thermal ink jet method in which bubbles are generated by a heating element and ink is ejected under pressure can be applied. The ink ejection method is preferably a piezoelectric element method because the ejection amount can be easily controlled.

  FIG. 3 is a block diagram showing a configuration of a line head type ink jet recording apparatus. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The inkjet recording apparatus 100 includes a control unit 20. An interface 21, ROM 22, RAM 23, encoder 24, motor control circuit 25, line head control circuit 26, voltage control circuit 27, and the like are connected to the control unit 20. Has been.

  The interface 21 transmits / receives data to / from a host device such as a personal computer (not shown). The control unit 20 converts the image signal received via the interface 21 into image data by performing scaling processing or gradation processing as necessary. And a control signal is output to the various control circuits mentioned later.

  The ROM 22 stores a control program or the like for recording the image by driving the line heads 11a to 11d. The RAM 23 stores the image data subjected to scaling processing or gradation processing by the control unit 20 in a predetermined area.

  The encoder 24 is connected to a belt driving roller 6 on the paper discharge side that drives the conveyance belt 5, and outputs a pulse train according to the rotational displacement amount of the rotating shaft of the belt driving roller 6. The control unit 20 calculates the rotation amount by counting the number of pulses transmitted from the encoder 24, and grasps the paper feed amount (paper position). Then, the control unit 20 outputs a control signal to the motor control circuit 25 and the line head control circuit 26 based on the signal from the encoder 24.

  The motor control circuit 25 drives the recording medium transport motor 28 by an output signal from the control unit 20. The recording medium conveying motor 28 is driven to rotate the belt driving roller 6 and rotate the conveying belt 5 clockwise in FIG. 1 to convey the sheet in the direction of the arrow X.

  The line head control circuit 26 transfers the image data stored in the RAM 23 to the line heads 11a to 11d based on the output signal from the control unit 20, and the line heads 11a to 11d from the line heads 11a to 11d based on the transferred image data. Controls ink ejection. By this control and the control of the conveyance of the sheet by the conveyance belt 5 driven by the recording medium conveyance motor 28, the recording process on the sheet is performed.

  The voltage control circuit 27 generates an alternating electric field by applying a voltage to the belt roller 7 on the paper feeding side based on an output signal from the control unit 20, and electrostatically attracts the sheet to the transport belt 5. The electrostatic adsorption is released by grounding the belt roller 7 or the belt driving roller 6 based on an output signal from the control unit 20. Here, the voltage is applied to the belt roller 7 on the paper feed side, but the voltage may be applied to the belt drive roller 6 on the paper discharge side.

  A method of forming dots using a line head type ink jet recording apparatus will be described in detail with reference to FIG. In FIG. 4, the line head 11 a is described as an example among the line heads 11 a to 11 d illustrated in FIGS. 1 and 2, but the other line heads 11 b to 11 d are also described in the same manner.

  As shown in FIG. 4, the line head 11a is provided with nozzle rows N1 and N2 each including a plurality of nozzles arranged in parallel in the conveyance direction (arrow X direction) of the recording paper P. That is, as nozzles for forming each dot row in the conveyance direction of the recording paper P, each nozzle row N1 and N2 is provided with a total of two nozzles (for example, nozzles 12a and 12a ′ in the dot row L1). . For convenience of explanation, only 16 nozzles of 12a to 12p and 12a ′ to 12p ′ corresponding to the dot rows L1 to L16 are shown in the nozzles constituting the nozzle rows N1 and N2. In reality, however, it is assumed that a larger number of nozzles are arranged in a direction perpendicular to the conveyance direction of the recording paper P.

  Then, an image is formed on a recording paper P as a recording medium by using the nozzle arrays N1 and N2 sequentially. For example, while the recording paper P is moved in the conveyance direction of the recording paper P, ink is ejected from the nozzle array N1 for one dot row D1 in the width direction (left-right direction in the figure) of the recording paper P (solid arrow in the figure). After that, the next dot row D2 for one row is formed by ejecting ink from the nozzle row N2 (broken line arrow in the figure), and the next dot row D3 for the next row is again ink from the nozzle row N1. It is formed by discharge. Hereinafter, the dot rows D4 and thereafter are similarly formed by alternately using the nozzle rows N1 and N2.

  According to the image forming method described above, it is possible to suppress image disturbance when forming an image after ink has not been ejected from the recording head for a while, and contamination of the recording medium due to offset at a good print density. It is possible to form a good image without causing any problems. For this reason, the image forming method of the present invention is suitably used in various ink jet recording apparatuses that employ a line head system.

  Hereinafter, the present invention will be described more specifically with reference to examples. In addition, this invention is not limited at all by the Example.

[Preparation Example 1]
A stirrer, a nitrogen inlet tube, a condenser, and a dropping funnel were attached to a four-necked flask having a capacity of 1000 ml, and 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone were added to the flask. Subsequently, it heated until the solvent in a flask began to reflux, carrying out nitrogen bubbling. In a dropping funnel, 80 g of styrene (St), 68 g of methyl methacrylate (MMA), 40 g of butyl acrylate (BA), 3.7 g of methacrylic acid (MAA), and an initiator (azobisisobutyronitrile (AIBN)) A monomer and an initiator were added dropwise in a state in which a solution consisting of 0.3 g was added and the four-necked flask was heated to 70 ° C. and refluxed. After completion of the dropwise addition, the mixture was further heated and refluxed for 6 hours. After refluxing for 6 hours, methyl ethyl ketone containing 0.1 g of AIBN was added dropwise over 15 minutes. After completion of dropping, the mixture was further heated and refluxed for 5 hours to obtain a styrene-acrylic resin (resin 1) having a weight average molecular weight of 40000. The weight average molecular weight of the resin 1 was measured according to the following method. Moreover, it was 30 mgKOH / g when the acid value of the resin 1 was measured.

  From the production method of the resin 1, the amount of methacrylic acid (MAA) used and the production conditions were appropriately changed to prepare resins 2 to 7 having a weight average molecular weight and an acid value different from those of the resin 1. The acid value of the resin can be adjusted by adjusting the ratio of the mass of MAA to the mass of all monomers. For example, if the ratio of the mass of MAA to the mass of all monomers is doubled as compared with the case of producing resin 1, the acid value of the resulting resin will be about 60. The weight average molecular weight of the resin can be lowered by, for example, increasing the reaction temperature during polymerization, and can be increased by reducing the amount of solvent used during polymerization and the amount of initiator used. However, when reducing the amount of initiator used, care must be taken because the polymerization reaction may stop and residual monomer may increase.

<Weight average molecular weight measurement conditions>
Column: TSKgel, Super Multipore HZ-H (manufactured by Tosoh Corporation, 4.6 mm ID × 15 cm)
Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 ml / min Sample injection volume: 10 μl
Measurement temperature: 40 ° C
Detector: IR detector calibration curves are F-40, F-20, F-4, F-1, A-5000, A-2500, A from standard samples (TSK standard, polystyrene, manufactured by Tosoh Corporation). -1000 and 8 types of n-propylbenzene were selected and prepared.

[Reference Example 1]
(Preparation of cyan inks 1 to 7 (C1 to C7))
According to the following method, cyan inks C1 to C7 having a pigment content of 6% by mass, a resin / pigment mass ratio in the ink of 0.3, and a resin acid value of 30 to 240 were prepared.

  Styrene-acrylic resin (resins 1-7) 4.5% by mass, cyan pigment (P.B-15: 3) 15% by mass, surfactant (surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical) Manufactured by Kogyo Co., Ltd.)) After charging 0.5% by mass and the remaining amount of ion-exchanged water into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.) and filling dynomill vessel with 0.5 mm zirconia beads. Dispersion treatment was carried out to prepare 7 types of pigment dispersions having acid values of 30, 70, 90, 100, 150, 200, and 240 mgKOH / g.

  The styrene-acrylic resin was neutralized with an aqueous NaOH solution 1.1 times the neutralization equivalent. Further, the mass of Na was calculated as the mass of the resin, and the mass of water contained in the NaOH aqueous solution or the water generated by the neutralization reaction was calculated as the mass of ion-exchanged water. After preparing the pigment dispersion, it was confirmed that the average particle diameter of the pigment was in the range of 70 to 130 nm. Measurement of the average particle diameter of the pigment was performed with Zeta Sizer Nano (manufactured by Sysmex Corporation) using a liquid obtained by diluting the pigment dispersion 300 times with ion-exchanged water as a sample.

  Using a pigment dispersion containing an acid value resin described in Table 2, 40% by mass of the pigment dispersion, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0 0.5 mass%, triethylene glycol monobutyl ether 5 mass%, 2-pyrrolidone 5 mass%, 1,2-octanediol 0.5 mass part, and the total of glycerin and ion-exchanged water is 49 mass% These components were mixed uniformly with a stirrer and then filtered with a filter having a pore size of 5 μm to obtain C1 to C7 inks. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Moreover, according to the following method, the printing density of the formed image, intermittent discharge property, and the offset of the formed image when C1 to C7 ink was used were evaluated. Table 2 shows the resin / pigment mass ratio of the C1 to C7 ink, the type of resin, and the acid value of the resin, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image.

<Print density evaluation method>
Plain paper (A4, PPC paper) was used as the recording medium, and a solid image of 10 cm × 10 cm was printed so that the amount of ink ejected from the recording head was 15 g / m ² . After the plain paper on which the image is formed is stored in a room temperature and humidity environment for a day and night, the print density of the image portion is measured with a portable reflection densitometer RD-19 (manufactured by Gretag Macbeth Co., Ltd.). The average value of the print density was defined as the print density with C1 to C7 inks. A printing density of 1.0 or more was judged as ◯, and a printing density of less than 1.0 was judged as x.

<Intermittent ejection evaluation method>
Using a heater capable of keeping heat inside the head and a recording head having a temperature detection function, the heat retention set temperature was set to 25 ° C., and the intermittent discharge performance was evaluated in an environment of 10 ° C. and 15% RH.
Specifically, after printing the line image in the longitudinal direction of the head, after passing through any non-printing section of 21 mm or more, the line image in the longitudinal direction of the head is printed again, and the printed state of the line image is observed with a microscope. Intermittent discharge was evaluated. When the line image is formed across a non-printing section (420 mm) corresponding to the A3 vertical line, the intermittent ejection performance is quantified assuming that the line image is not disturbed as 100%. For example, when there is no disturbance in the line image provided with the 420 mm non-printing section with the previously formed line image and the line image provided with the 441 mm non-printing section is disturbed, the numerical value of the intermittent ejection property is 100. If the line image provided with the non-printing section of 336 mm is not disturbed and the line image provided with the non-printing section of 357 mm is disturbed, the numerical value of the intermittent discharge property is 80%. The case where the intermittent discharge property was 100% or more was determined as “good”, and the case where the intermittent discharge property was less than 100% was determined as “poor”.

<Evaluation of offset>
The recording head closest to the discharge roller (corresponding to 11d in FIG. 1) was filled with ink, and excess liquid coming out from the nozzle forming surface was scraped off with a wipe blade. The distance between the nozzle surface of the recording head and the recording paper P was fixed to 1 mm, and the conveyance speed of the recording paper P from the recording paper P feeding section to the discharging section was set to 846.7 mm / sec. As the recording paper P, a paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut to A4 size is used and ejected from the recording head so that the amount of ink applied to the recording paper P is 15 g / m ^2. Ten continuous images of 10 cm × 10 cm were printed. The recording paper P printed on the 10th sheet was evaluated for stains on the recording paper P due to offset according to the following method.

First, an image offset portion of the recording paper P on which an image is formed (a portion where paper smear is generated by ink attached to the roller) is read by an image scanner (GT-X820 (manufactured by Seiko Epson Corporation)), and a threshold value 220 is read. Binarization was performed. Based on the number of black pixels in the binarized image and the total number of pixels, an offset area ratio serving as an index for occurrence of offset was calculated based on the following equation. The higher the offset area ratio, the more severe the recording paper P is stained by the offset. Since the offset area ratio at which the recording paper P can be visually confirmed by the offset is greater than 0.030, the case where the offset area ratio exceeds 0.030 is determined as x, and the offset area ratio is 0.030 or less. It was determined as “good”.
(Offset area ratio calculation formula)
Offset area ratio (%) = 100 × number of black pixels / total number of pixels

  According to Table 2, when the resin acid value is more than 30 mgKOH / g and less than 240 mgKOH / g, preferably in the range of 60 to 200 mgKOH / g, good print density and intermittent discharge can be realized, and the recording medium by offset It can be seen that the contamination is less likely to occur.

[Reference Example 2]
(Preparation of cyan inks 8 to 13 (C8 to C13))
According to the following method, cyan inks C8 to C13 having a pigment content of 6% by mass, a resin acid value of 100 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.25 to 1.0 were obtained. Prepared.

  Styrene-acrylic resin (resin 4) 3.75 to 15% by mass, cyan pigment (P.B-15: 3) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical) Manufactured by Kogyo Co., Ltd.)) After charging 0.5% by mass and the remaining amount of ion-exchanged water into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.) and filling dynomill vessel with 0.5 mm zirconia beads. Dispersion treatment was performed to prepare six types of pigment dispersions having a resin / pigment mass ratio of 0.25, 0.3, 0.7, 0.8, 0.9, and 1.0.

  C8 to C13 inks were obtained in the same manner as in Reference Example 1 except that the pigment dispersion having a resin / pigment mass ratio shown in Table 3 was used as the pigment dispersion. In addition, as in Reference Example 1, the printing density, intermittent ejection performance, and offset of C8 to C13 inks were evaluated. Table 3 shows the resin / pigment mass ratio of the C8 to C13 ink, the resin type, and the acid value of the resin, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image.

  According to Table 3, when the resin / pigment mass ratio is 0.3 or more and less than 1.0, it is possible to realize a good printing density and intermittent ejection property, and it is difficult to cause contamination of the recording medium due to offset. .

[Reference Example 3]
(Preparation of cyan inks 14 to 18 (C14 to C18))
According to the following method, cyan inks C14 to C18 having a pigment content of 4 to 9% by mass, an acid value of the resin of 100 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 were prepared. .

  Styrene-acrylic resin (resin 4) 4.5% by mass, cyan pigment (P.B-15: 3) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical Co., Ltd.) (Made by company)) 0.5% by mass and the remaining amount of ion-exchanged water was charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.), and 0.5 mm diameter zirconia beads were charged into a Dynomill vessel and dispersed. Treatment was performed to prepare a pigment dispersion.

  Pigment dispersion 26.7-60% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0.5% by mass, triethylene glycol monobutyl ether 5% by mass, 2-Pyrrolidone 5% by mass, 1,2-octanediol 0.5 part by mass, and the remaining amount of glycerin and ion-exchanged water are uniformly mixed with a stirrer, and then filtered through a filter having a pore size of 5 μm to obtain a pigment. C14 to C18 inks having a content of 4, 6, 8, 8.5, and 9% by mass were obtained. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. In addition, as in Reference Example 1, the print density of the formed image, the intermittent ejection property, and the offset of the formed image when C14 to C18 ink was used were evaluated. Table 4 shows the evaluation results of the pigment content, resin / pigment mass ratio, resin type, and resin acid number, the print density of the formed image, the intermittent ejection property, and the offset of the formed image of the C14 to C18 inks. To

  According to Table 4, in the case of cyan ink, if the pigment content is 6 to 8.5% by mass, it is easy to achieve good printing density and intermittent ejection, and the recording medium is not easily contaminated by offset. I understand that. However, as can be seen from Table 2, contamination of the recording medium due to offset can be made difficult by increasing the resin acid value, and as can be seen from Table 3, the resin acid value can be lowered. The intermittent discharge property can be improved by the method, and the print density can be increased by a method such as increasing the pigment content. Thus, the pigment content of the cyan ink is not limited to 6 to 8.5% by mass.

[Reference Example 4]
(Preparation of yellow inks 1 to 7 (Y1 to Y7))
According to the following method, yellow inks Y1 to Y7 having a pigment content of 6% by mass, an acid value of the resin of 30 to 240 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 were prepared. .

  Styrene-acrylic resin (resins 1-7) 4.5% by mass, yellow pigment (P.Y-74) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical Co., Ltd.) (Made by company)) 0.5% by mass and the remaining amount of ion-exchanged water was charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.), and 0.5 mm diameter zirconia beads were charged into a Dynomill vessel and dispersed. Treatment was carried out to prepare 7 types of pigment dispersions having an acid value of 30, 70, 90, 100, 150, 200, and 240 mgKOH / g.

  Using a pigment dispersion containing an acid value resin listed in Table 5, 40% by mass of pigment dispersion, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0 0.5 mass%, triethylene glycol monobutyl ether 5 mass%, 2-pyrrolidone 5 mass%, 1,2-octanediol 0.5 mass part, and the total of glycerin and ion-exchanged water is 49 mass% These components were mixed uniformly with a stirrer and then filtered through a filter having a pore size of 5 μm to obtain inks Y1 to Y7. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent ejection property, and the offset of the formed image were evaluated when Y1 to Y7 inks were used. Table 5 shows the resin / pigment mass ratio of the inks Y1 to Y7, the type of resin, and the acid value of the resin, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image.

  According to Table 5, if the resin acid value is more than 30 mg KOH / g and less than 240 mg KOH / g, preferably in the range of 60 to 200 mg KOH / g, good print density and intermittent discharge can be realized, and the recording medium due to offset can be realized. It can be seen that contamination is less likely to occur. In addition, as a characteristic of hue, yellow ink is difficult to be recognized as a smudge of an image even if the ink adheres to a recording medium due to an offset.

[Reference Example 5]
(Preparation of yellow ink 8-12 (Y8-Y12))
According to the following method, yellow inks Y8 to Y12 having a pigment content of 4 to 9% by mass, a resin acid value of 100 mgKOH / g, and a resin / pigment mass ratio of 0.3 in ink were prepared. .

  Styrene-acrylic resin (resin 4) 4.5% by mass, yellow pigment (PY-74) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.) )) 0.5% by mass and the remaining amount of ion-exchanged water was charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.), 0.5 mm-diameter zirconia beads were filled into the Dynomill vessel, and then the dispersion treatment was performed. A pigment dispersion was prepared.

  Pigment dispersion 26.7-60% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0.5% by mass, triethylene glycol monobutyl ether 5% by mass, 2-Pyrrolidone 5% by mass, 1,2-octanediol 0.5 part by mass, and the remaining amount of glycerin and ion-exchanged water are uniformly mixed with a stirrer, and then filtered through a filter having a pore size of 5 μm to obtain a pigment. The ink of Y8-Y12 whose content is 4, 6, 8, 8.5, and 9 mass% was obtained. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent discharge property, and the offset of the formed image when using Y8 to Y12 inks were evaluated. Table 6 shows the pigment content, the resin / pigment mass ratio, the resin type, and the resin acid value, the evaluation result of the print density of the formed image, the intermittent ejection property, and the offset of the formed image of the inks Y8 to Y12. To

  According to Table 6, in the yellow ink, when the pigment content is 6 to 8.5% by mass, it is easy to realize a good print density and intermittent ejection property, and contamination of the recording medium due to the offset hardly occurs. I understand that. However, the pigment content of yellow ink is not limited to less than 6 to 8.5% by mass for the same reason as cyan ink.

[Reference Example 6]
(Preparation of magenta inks 1 to 4 (M1 to M4))
According to the following method, magenta inks M1 to M4 having a pigment content of 7% by mass, a resin acid value of 30 to 240 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 were prepared. .

  Styrene-acrylic resin (resins 1, 2, 6, and 7) 4.5% by mass, magenta pigment (PR-122) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol ( Nissin Chemical Industry Co., Ltd.)) 0.5% by mass and the remaining amount of ion-exchanged water are charged into Dynomill (Shinmaru Enterprises Co., Ltd.), and 0.5 mm diameter zirconia beads are added to the Dynomill vessel. After filling, dispersion treatment was performed to prepare four types of pigment dispersions having an acid value of 30, 70, 200, and 240 mgKOH / g.

  Using a pigment dispersion containing an acid value resin described in Table 7, 46.7% by mass of the pigment dispersion, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) ) 0.5 mass%, triethylene glycol monobutyl ether 5 mass%, 2-pyrrolidone 5 mass%, 1,2-octanediol 0.5 mass part, and the total of glycerin and ion-exchanged water is 49 mass%. Thus, these components were mixed uniformly with a stirrer and then filtered through a filter having a pore size of 5 μm to obtain inks of M1 to M4. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent ejection property, and the offset of the formed image when using M1 to M4 inks were evaluated. Table 7 shows the resin / pigment mass ratio of the inks M1 to M4, the type of resin, and the acid value of the resin, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image.

  In M1 to M4, since a favorable hue can be reproduced in a wide color gamut, P.I. R-122 is used. For this reason, in Table 7, P.P. When R-122 was used, the image density was slightly inferior when the acid value of the resin was 70 mg / KOHg. However, P.I. Resin acid can be obtained by changing R-122 to a magenta color pigment with high coloring power, increasing the pigment concentration in the ink, and increasing the resin / pigment mass ratio, as confirmed with cyan ink. Even if the value is about 70 mgKOH / g, a magenta ink capable of forming an image with a good density can be prepared.

  Further, according to Table 7, if the resin acid value is in the range of more than 30 mg KOH / g to less than 240 mg KOH / g, preferably 60 to 200 mg KOH / g, good intermittent discharge can be realized, and the recording medium is contaminated by offset. It turns out that it is hard to occur.

[Reference Example 7]
(Preparation of magenta ink 5-10 (M5-M10))
According to the following method, magenta inks M5 to M10 having a pigment content of 6 to 9% by mass, a resin acid value of 100 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 were prepared. .

  Styrene-acrylic resin (resin 4) 4.5% by mass, magenta pigment (PR 122) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.) )) 0.5% by mass and the remaining amount of ion-exchanged water was charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.), 0.5 mm-diameter zirconia beads were filled into the Dynomill vessel, and then the dispersion treatment was performed. A pigment dispersion was prepared.

  Pigment dispersion 40-60% by mass, Surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0.5% by mass, triethylene glycol monobutyl ether 5% by mass, 2- 5% by mass of pyrrolidone, 0.5 part by mass of 1,2-octanediol, and the remaining amount of glycerin and ion-exchanged water are uniformly mixed with a stirrer, and then filtered through a filter having a pore size of 5 μm to contain the pigment. M5 to M10 inks were obtained with amounts of 6, 7, 7.5, 8, 8.5, and 9% by weight. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent ejection property, and the offset of the formed image when using M5 to M10 inks were evaluated. Table 8 shows the evaluation results of the pigment content, resin / pigment mass ratio, resin type, resin acid value, print density of the formed image, intermittent ejection property, and offset of the formed image of the inks of M5 to M10. To

  According to Table 8, in the case of magenta ink, if the pigment content is 7.5 to 8% by mass, it is easy to achieve good printing density and intermittent ejection, and the recording medium is not easily contaminated by offset. I understand that. However, the magenta ink is not limited to a pigment content of 7.5 to less than 8% by mass for the same reason as the cyan ink.

[Reference Example 8]
(Preparation of black inks 1 to 7 (K1 to K7))
According to the following method, black inks K1 to K7 having a pigment content of 6% by mass, a resin acid value of 30 to 240 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 were prepared. .

  Styrene-acrylic resin (resins 1-7) 4.5% by mass, black pigment (BK-7) 15% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical Co., Ltd.) (Made by company)) 0.5% by mass and the remaining amount of ion-exchanged water was charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.), and 0.5 mm diameter zirconia beads were charged into a Dynomill vessel and dispersed. Treatment was carried out to prepare 7 types of pigment dispersions having an acid value of 30, 70, 90, 100, 150, 200, and 240 mgKOH / g.

  Using a pigment dispersion containing an acid value resin described in Table 9, 40% by mass of pigment dispersion, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0 0.5 mass%, triethylene glycol monobutyl ether 5 mass%, 2-pyrrolidone 5 mass%, 1,2-octanediol 0.5 mass part, and the total of glycerin and ion-exchanged water is 49 mass% These components were uniformly mixed with a stirrer and then filtered with a filter having a pore size of 5 μm to obtain inks of K1 to K7. The content of glycerin and ion-exchanged water was adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent discharge property, and the offset of the formed image were evaluated when the inks K1 to K7 were used. Table 9 shows the resin / pigment mass ratio of the inks K1 to K7, the type of resin, and the acid value of the resin, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image.

  According to Table 9, if the resin acid value is in the range of more than 30 mgKOH / g to less than 240 mgKOH / g, preferably 60 to 200 mgKOH / g, good print density and intermittent ejection can be realized, and the recording medium by offset It can be seen that the contamination is less likely to occur.

[Reference Example 9]
(Preparation of black ink 8-26 (K8-K26))
According to the following method, black inks K8 to K13 having a pigment content of 5 to 8.5% by mass, an acid value of the resin of 100 mgKOH / g, and a resin / pigment mass ratio in the ink of 0.3 are obtained. Prepared. Also, black inks K14 to K19 having a pigment content of 5 to 8.5% by mass, a resin acid value of 90 mgKOH / g, and a resin / pigment mass ratio of 0.3 in ink were prepared. Furthermore, black inks K20 to K26 having a pigment content of 5 to 9% by mass, an acid value of the resin of 70 mgKOH / g, and a resin / pigment mass ratio of 0.3 in ink were prepared.

  Styrene-acrylic resin (resin 4, acid value 100) 4.5 mass%, black pigment (BK-7) 15 mass%, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (Nisshin Chemical) Manufactured by Kogyo Co., Ltd.)) After charging 0.5% by mass and the remaining amount of ion-exchanged water into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.) and filling dynomill vessel with 0.5 mm zirconia beads. The pigment dispersion A was prepared by performing a dispersion treatment. A pigment dispersion B was prepared in the same manner as the pigment dispersion A, except that the styrene-acrylic resin (resin 4) was changed to a styrene-acrylic resin (resin 3, acid value 90). Furthermore, a pigment dispersion C was prepared in the same manner as the pigment dispersion A, except that the styrene-acrylic resin (resin 4) was changed to a styrene-acrylic resin (resin 2, acid value 70).

  Pigment dispersion A 33.3 to 56.7% by mass, surfactant (Surfinol 465, ethylene oxide adduct of acetylenic diol (manufactured by Nissin Chemical Industry Co., Ltd.)) 0.5% by mass, triethylene glycol monobutyl ether 5 mass %, 2-pyrrolidone 5% by mass, 1,2-octanediol 0.5 part by mass, and the remaining amount of glycerin and ion-exchanged water were uniformly mixed with a stirrer, and then filtered through a filter having a pore size of 5 μm. Ink of K8 to K13 having a pigment content of 5, 6, 7, 7.5, 8, and 8.5% by mass was obtained.

  Further, the pigment content is 5, 6, 7, 8, 8.5, and 9% by mass in the same manner as the ink of K8 to K13 except that the pigment dispersion A is changed to the pigment dispersion B. , K14 to K19 inks were obtained. Further, except that the pigment dispersion A is changed to the pigment dispersion C and the usage amount of the pigment dispersion is changed to 33.3 to 60% by mass, the pigment content is 5 as in the case of the ink of K8 to K13. , 6, 7, 8, 8.5, 9, and 9.5% by mass, inks of K20 to K26 were obtained.

  When preparing inks of K8 to K26, the contents of glycerin and ion exchange water were adjusted so that the viscosity of the ink at 25 ° C. was 6 mPa.s. It adjusted so that it might become sec. Further, as in Reference Example 1, the print density of the formed image, the intermittent ejection property, and the offset of the formed image were evaluated when inks of K8 to K26 were used. Table 10 shows the pigment content, the resin / pigment mass ratio, the resin type, and the resin acid value, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image of the inks of K8 to K13. To Table 11 shows the pigment content, the resin / pigment mass ratio, the resin type, and the acid value of the ink, the print density of the formed image, the intermittent ejection property, and the evaluation results of the offset of the formed image of the inks of K14 to K19. To Table 12 shows the pigment content, the resin / pigment mass ratio, the resin type, and the resin acid value, and the evaluation results of the print density of the formed image, the intermittent ejection property, and the offset of the formed image of the inks of K20 to K26. To

  According to Table 10, in the black ink whose acid value of the resin is 100 mgKOH / g, if the pigment content is 6 to 7.5% by mass, it is easy to realize good print density and intermittent ejection properties. It can be seen that the recording medium is hardly contaminated by the offset. Further, according to Table 11, in the black ink in which the acid value of the resin is 90 mgKOH / g, if the pigment content is 6 to 8% by mass, it is easy to achieve good print density and intermittent ejection properties. It can be seen that the recording medium is hardly contaminated by the offset. Further, according to Table 12, in the case of the black ink having an acid value of 70 mgKOH / g, if the pigment content is 6 to 8.5% by mass, good print density and intermittent ejection can be realized. It is easy to see that the recording medium is hardly contaminated by the offset. However, the pigment content of the black ink (K8 to K26) is not limited to the above preferred range for the same reason as the cyan ink.

  Further, it is known that the higher the density of an image formed with black ink, the better in order to form a good image. For this reason, the density of an image formed with black ink is preferably 1.2 or more. According to Table 10, Table 11, and Table 12, it can be seen that as the acid value of the resin is lowered, the image density tends to increase. For black ink, the acid value of the pigment is set to 90 mgKOH / g or less. It can be seen that the desired image density can be easily realized.

[Reference Example 10]
(Confirmation of the effect of inkjet head position on offset)
Using the black ink K23 prepared in Reference Example 9, the position of the recording head is 11a (first color), 11b (second color), and 11c (third color) from the position corresponding to 11d (fourth color) in FIG. The offset was evaluated by changing to a position corresponding to. Table 14 shows the movement time of the recording medium from each recording head to the driven roller of the discharge unit, the offset area ratio, and the determination of the presence or absence of the offset.

  According to Table 13, the black ink of K23 tends to contaminate the recording medium due to the offset when ejected from the fourth color recording head, but when ejected from the first to third color recording heads, It can be seen that the recording medium is hardly contaminated by the offset. For this reason, if the ink has no problem with the print density and the intermittent ejection property, the recording head for the first to third colors may be used even if the ink has a slightly higher offset area ratio in the evaluation of the offset with the recording head for the fourth color. It can be seen that it can be used as ink ejected from the ink.

[Examples 1 to 5, Comparative Example 1 and Comparative Example 2]
Using the ink jet recording apparatus used in Reference Example 10, an offset was evaluated when printing was performed using four colors of ink, cyan ink, yellow ink, magenta ink, and black ink. The ink used in the examples was selected from inks having good printing density and good intermittent ejection properties among the inks prepared in the reference examples. The offsets in the examples and comparative examples were evaluated according to the following methods.

<Evaluation of offset>
Each color recording head (corresponding to 11a to 11d in FIG. 1) was filled with ink, and the excess liquid coming out from the nozzle forming surface was scraped off with a wipe blade. The distance between the nozzle surface of the recording head and the recording paper P was fixed to 1 mm, and the conveyance speed of the recording paper P from the paper supply unit to the discharge unit was set to 846.7 mm / sec. As the recording paper P, a paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut to A4 size is used, and the amount of ink of each color applied to the recording paper P from the recording head is 1.5 g / m ² (4 colors). A total of 6 g / m ² ) was discharged, and 10 solid images of 10 cm × 10 cm were continuously printed. The recording paper P printed on the 10th sheet was evaluated for stains on the recording paper P due to offset in the same manner as for the single color ink.

  In Examples 1 to 6, an ink containing a resin having an acid value of 60 to 200 mgKOH / g is used, and the acid value of the resin contained in the ink ejected to the fourth color is contained in the ink ejected to the first color. The acid value of the resin contained in the ink ejected in the fourth color is 100 mgKOH / g or higher. For this reason, in Examples 1 to 6, the recording medium is not easily contaminated by the offset.

  In the first comparative example, the first color line head 11a farthest from the discharge unit 8 ejects ink containing a resin having a high acid value that is unlikely to cause contamination of the recording medium due to the offset, and the fourth color closest to the discharge unit. Since the recording medium is easily contaminated by the offset from the line head 11d, the recording medium is contaminated by the offset because the ink containing the resin having a low acid value is discharged.

  Comparative Example 2 uses an ink containing a resin having an acid value of 60 to 200 mgKOH / g, and the acid value of the resin contained in the ink ejected to the fourth color is contained in the ink ejected to the first color Higher than the acid value of the resin. However, in Comparative Example 2, since the ink ejected to the fourth color is an ink containing a resin having an acid value of less than 100 that easily contaminates the recording medium due to the offset, contamination due to the offset of the recording medium occurs. Yes.

DESCRIPTION OF SYMBOLS 2 Paper feed tray 3 Paper feed roller 4 Driven roller 5 Conveyor belt 6 Belt drive roller 7 Belt roller 8 Ejection part 8a Ejection roller 8b Follower roller 10 Ejection tray 11a, 11b, 11c, 11d Line heads 12a-12p, 12a'- 12p 'nozzle 20 control unit 30 detection means 100 inkjet recording apparatus D1 to D4 dot row (row direction)
L1 to L16 dot row (transport direction)
N1, N2 Nozzle array P Recording paper

Claims (5)

An image forming method using a line head type ink jet recording apparatus including two or more recording heads using two or more colors of ink and a discharge unit for discharging a recording medium,
The ink of two or more colors includes a pigment, a resin, water, and an organic solvent,
In the two or more colors of ink, the acid value of the resin contained in the ink is 60 to 200 mgKOH / g or more,
The acid value of the resin contained in the ink discharged last is higher than the acid value of the resin contained in the ink discharged in the first color,
The image forming method, wherein the acid value of the resin contained in the ink ejected last is 100 to 200 mgKOH / g.   The image forming method according to claim 1, wherein an acid value of a resin contained in the ink ejected to the first color is 60 to 100 mgKOH / g.   The image forming method according to claim 1, wherein the acid value of the resin contained in the ink ejected last from the second color is equal to or higher than the acid value of the resin contained in the ink ejected in the previous order.   4. The ink according to claim 1, wherein the ink of the two or more colors has a resin mass / pigment mass value of 0.3 or more and less than 1.0 for the resin and the pigment contained in the ink. Image forming method.   The image forming method according to claim 1, wherein the time from when the last ejected ink has landed on the surface of the recording medium to when the landed portion comes into contact with the discharge portion is within 1 second. .

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