WO2013129145A1 - Ink jet image forming method - Google Patents

Abstract

Provided is an ink jet image forming method which suppresses beading and in-dot density unevenness even without an ink drying unit and can form at high speed an image excellent in fixability on a printing paper sheet having a coating layer. The ink jet image forming method includes applying ink to a printing paper sheet using a recording head of an ink jet system to form an image. The ink contains a self-dispersion pigment, a polymer particle having a glass transition temperature of 25°C or lower and water and has a viscosity of 14 mPa⋅s or less and a surface tension of 34 mN/m or less. The diameter of a dot formed by one ink droplet applied from the recording head is 20-40 μm. The average ink application amount per unit area in the dot is 0.09-0.30 μL/cm².

Description

DESCRIPTION

Title of Invention

INK JET IMAGE FORMING METHOD

Technical Field

[0001] The present invention relates to an ink jet image

forming method.

Background Art

[0002]An ink jet recording technology of ejecting ink by an ink jet system to form an image has been developed mainly in, formation of a photo image on a dedicated paper sheet or formation of an image on a plain paper sheet represented by a copier paper sheet for office use and for home use. In recent years, development in the field of digital commercial printing has been expected. Specifically, a technology which can record at high speed an image having high image quality comparable to that of offset printing and having scratch resistance on a printing paper sheet having a coating layer is required.

[ 0003 ] However, a printing paper sheet having a coating layer has a dense structure taking into consideration the whiteness, the smoothness, and the strike-through resistance, and thus, has an extremely low ability of absorbing ink. Therefore, when recording is performed at high speed using an ordinary ink jet printer on a printing paper sheet having a coating layer, there arises a problem in that the density within a dot becomes lighter and density unevenness occurs in the dot. In order to solve the above-mentioned problem, PTL 1 proposes a technology of recording an image with an ink which dries fast using an ink jet recording apparatus including an ink drying unit.

[000 ] However, in the technology proposed in PTL 1, it is indispensable to provide the ink drying unit in the ink jet recording apparatus, and thus, there is a problem that the structure of the ink jet recording apparatus is complicated. Further, study by the inventors of the present invention has found that, even when the

technology proposed in PTL 1 is used, suppression of in-dot density unevenness and improvement in fixability of an image are sometimes insufficient.

Citation List

Patent Literature

[0005] PTL 1: Japanese Patent Application Laid-open No. 2009- 226715

Summary of Invention

Technical Problem

[0006] The present invention has been made in view of the

problems of the conventional technology. Accordingly, it is an object of the present invention to provide an ink jet image forming method which suppresses beading and in-dot density unevenness even without an ink drying unit and can form at high speed an image

excellent in fixability on a printing paper sheet having a coating layer.

Solution to Problem

[0007] The above-mentioned problem is solved by an exemplary embodiment of the present invention described below. That is, according to the exemplary embodiment of the present invention, there is provided an ink jet image forming method including applying ink to a printing paper sheet using a recording head of an ink jet system to form an image, the ink containing a self-dispersion pigment, a polymer particle having a glass transition temperature of 25°C or lower, and water, the ink having a viscosity of 14 mPa-s or less and a surface tension of 34 mN/m or less, in which a diameter of a dot formed by one ink droplet applied from the recording head is 20 μιη or more and 40 μπι or less, and in which an average ink application amount per unit area in the dot is 0.09 μL/cm² or more and 0.30 L/cm² or less. Advantageous Effects of Invention

[0008] According to the ink jet image forming method of the present invention, beading and in-dot density

unevenness can be suppressed even without the ink drying unit and an image excellent in fixability can be formed at high speed on a printing paper sheet having a coating layer.

[0009] Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. Brief Description of Drawings

[0010] FIG. 1 is a schematic view illustrating an exemplary ink jet recording apparatus having a serial type recording head.

FIG. 2 is a schematic view illustrating an exemplary serial type recording head.

FIG. 3 is a schematic view illustrating an exemplary line type recording head.

FIG. 4 is a schematic view illustrating a part of an^■ ink jet recording apparatus used in image formation. FIG. 5 is a schematic view illustrating a state of arrangement of recording heads.

Description of Embodiments

[0011] The present invention is described in detail in the following by way of an exemplary embodiment . The inventors of the present invention have found that, as a result of study, beading and in-dot density

unevenness are significantly affected by the average ink application amount per unit area of a dot

(hereinafter also simply referred to as "in-dot average ink application amount"). The in-dot average ink application amount is an average of the ink application amounts per unit area of a dot, and is different from the volume of an ink droplet. The in-dot average ink application amount is calculated by dividing the amount of an ink droplet applied at a time by one nozzle in an ink jet system (ejected amount) by the area of a dot formed on a recording medium. The in-dot average ink application amount can be controlled by the amount of the ejected ink droplet, the components and the

physical properties of the ink, and the like.

[0012] Ink used in the ink jet image forming method according to the present invention is an ink which contains a polymer particle having a glass transition temperature of 25°C or lower, and which has a viscosity of 14 mPa · s or less and a surface tension of 34 mN/m or less. When this ink is used to perform ink jet recording on a printing paper sheet, after an ink droplet is applied to the printing paper sheet, liquid in the ink droplet is rapidly absorbed in the printing paper sheet.

Further, the polymer particle is molten to form a film. This is thought to abruptly increase the viscosity of the ink droplet to suppress in-dot density unevenness. Further, a pigment is less easy to move in an ink droplet after the application. This is thought to suppress beading and improve the scratch resistance of the image.

[0013]<Image Forming Method>

As described above, the in-dot average ink application amount means an average of the ink application amounts per unit area of a dot. The area of a dot means the area of a region (dot) formed by penetration of a liquid (solvent) component of the ink applied to the recording medium in the printing paper sheet and by spread of a coloring material (pigment) component on the surface of the recording medium. Further, the diameter of a dot which is necessary when the area of the dot is calculated means the diameter of the region (dot) formed by penetration of the liquid (solvent) component of the ink applied to the recording medium in the printing paper sheet and by spread of the coloring material (pigment) component on the surface of the recording medium. Note that, the smoothness of the surface of a coating layer is not always good.

Therefore, the shape in plan view of a dot may not be a perfect circle, and, in many cases, an ellipse. When the shape in plan view of a dot is an ellipse, the diameter of the dot is the average of the major

diameter and the minor diameter of the ellipse. The diameter of a dot varies to some extent depending on the performance of a recording head, the accuracy of the dimensions of the nozzles, the ejection speed, and the like. Therefore, according to the present

invention, twenty or more dots are selected which are independent of one another, have shapes relatively close to a circle in plan view, and contain fewer small-size ink droplets separated from main droplets (so-called "satellites") by observation with an optical microscope. The average of the major diameters and the minor diameters of the selected dots is defined as the "diameter of a dot (dot diameter)".

[0014] In the ink jet image forming method according to the present invention, ink is applied to a recording medium so that the diameter of a dot formed by one ink droplet is 20 μπί or more and 40 ym or less to form an image. If the diameter of a dot formed by one ink droplet is less than 20 μπι, the print density is lowered. On the other hand, if the diameter of a dot formed by one ink droplet is more than 40 i , image quality required in digital commercial printing cannot be obtained.

[0015] The in-dot average ink application amount can be

considered to be the average thickness of the ink on a recording medium when an ink droplet is applied. In the ink jet image forming method according to the present invention, the in-dot average ink application amount is 0.09 pL/cm² or more and 0.30 pL/cm² or less. If the in-dot average ink application amount is less than 0.09 L/cm², the absolute amount of the applied ink is small, and thus, it is difficult to suppress in- dot density unevenness. Further, there are cases in which the color of the coating layer on the printing paper sheet cannot be sufficiently covered, which may result in lowered image quality. Further, there are also cases in which penetration of the ink in the recording medium is insufficient and the scratch resistance of the formed image is lowered. On the other hand, if the in-dot average ink application amount is more than 0.3

the absolute amount of the ink is large, and thus, it takes much time for the ink to penetrate in the recording medium, and highspeed fixability is lost. Further, multiple ink droplets are liable to be brought into contact with one another when applied to the printing paper sheet, and thus, there are cases in which suppression of beading is difficult.

[0016] In the ink jet image forming method according to the present invention, insofar as both the dot diameter and the in-dot average ink application amount satisfy the above-mentioned conditions, no specific limitation is made to the amount of the ink droplet (amount of the ejected ink) applied onto the printing paper sheet. However, the amount of the ejected ink is preferably 0.6 pL or more and 3.0 pL or less, more preferably 0.8 pL or more and 2.8 pL or less, and particularly

preferably 1.0 pL or more and 2.6 pL or less.

[0017 ] Beading is a phenomenon in which multiple ink droplets merge with one another by being brought into contact with one another on a printing paper sheet in a liquid state and desired area coverage modulation cannot be performed. Therefore, beading is liable to occur in a portion in which the dot representation is dense with a printing duty of 20% or more. When an image is formed in a multi-pass method of dividedly applying ink, beading is less liable to occur. However, when an image is formed in a so-called one-pass method, beading is liable to occur. Therefore, the ink jet image forming method according to the present invention is particularly effective in forming an image in the one- pass method.

[0018] "One pass" means that, when an image is formed in a

predetermined region of the recording medium, the number of scans of the recording head for the

predetermined region is one. The predetermined region is substantially equal to the width of the recording head. Specific examples of preferred recording

apparatus for carrying out the ink jet image forming method according to the present invention include a serial type printer (see FIGS. 1 and 2) and a line type printer (see FIG. 3). For example, when the serial type printer is used, an image is formed while a

recording head for ejecting ink moves in a main scan direction (direction of the arrow in FIG. 2). When the recording head reaches an end of the printing paper sheet, the printing paper sheet is fed in a sub scan direction by the width of the recorded image. After that, an image is formed while the recording head moves in the main scan direction again (see FIG. 2).

[0019] Further, according to the present invention, ink may be dividedly applied insofar as a line image is formed by the one pass method. Specifically, inks of the same color or different colors may be dividedly applied from two or more nozzle arrays included in the recording head. Even when ink is dividedly applied in this way, interference between ink droplets can be suppressed to suppress beading. When ink is divided in multiple parts and applied, it is preferred that the time

difference between the first application and the last application be 200 msec or less. This leads to more remarkable effects of the present invention. Note that, if the time difference between the first application and the last application is more than 200 msec, there are cases in which beading can be suppressed even when the structure of the ink jet image forming method according to the present invention is not adopted.

[0020] <Ink>

Coloring Material

A coloring material to be incorporated into the ink to be used in the ink jet image forming method according to the present invention is a self-dispersion pigment. It is preferred that the self-dispersion pigment be an anionic self-dispersion pigment. In the case of the anionic self-dispersion pigment, an anionic functional group directly bonded to the pigment is likely to have an interaction with the coating layer of a printing paper sheet, as compared to an anionic polymer- dispersed pigment. Accordingly, in-dot density unevenness hardly occurs. The anionic self-dispersion pigment is suitable also because the anionic self- dispersion pigment obviates the need for the

incorporation of a water-soluble polymer and hence its solid-liquid separation with respect to water after application easily progresses, allowing an image excellent in scratch resistance to be formed. Note that, the anionic functional group means such a functional group that a half or more of hydrogen ions can be dissociated at a pH of 7.0. Specific examples of the anionic functional group may include a carboxyl group, a sulfo group, and a phosphonic acid group. Of those, a carboxyl group or a phosphonic acid group is preferred as the anionic functional group from the viewpoint of the suppression of in-dot density

unevenness .

[0021] An ink set for forming images with inks of multiple colors basically includes black, cyan, magenta, and yellow inks. Note that, red, blue, green, gray, pale cyan, and pale magenta inks, for example, may be added to the ink set. It is preferred that the pigments contained in the inks to be added be self-dispersion pigments .

[0022] he self-dispersion pigment is generally a pigment that has been dispersed and stabilized without requiring a dispersant by introducing a water-soluble functional group such as an anionic functional group to the

surface of the pigment directly or via another atomic group. As the pigment before the dispersion

stabilization, there may be used various hitherto known pigments such as those listed in International Patent WO2009/014242A.

[0023] As a method for the introduction of an anionic

functional group to the surface of the pigment, there may be given, for example, a method involving

performing oxidation treatment on carbon black.

Examples of the method involving performing oxidation treatment may include methods involving performing treatment with hypochlorite, ozone water, hydrogen peroxide, chlorite, nitric acid, or the like. Of those, self-dispersion carbon black to be obtained by

performing oxidation treatment on the surface of carbon black with sodium hypochlorite is preferred from the viewpoint of the suppression of image unevenness.

Further, other examples of the method involving

performing oxidation treatment may include surface treatment methods involving using a diazonium salt as described in Japanese Patent No. 3808504, Japanese

Patent Translation Publication No. 2009-515007, and Japanese Patent Translation Publication No. 2009-506196. A commercially available pigment having a water-soluble (hydrophilic) functional group such as an anionic functional group introduced into its surface may be specifically exemplified by the following trade names: C -1, CW-2, and CW-3 (all of which are manufactured by ORIENT CHEMICAL INDUSTRIES CO., LTD.); and CAB-O-JET 200, CAB-O-JET 300, and CAB-O-JET 400 (manufactured by Cabot Corporation). Note that, the CW-2 and the CAB-O- JET 300 are self-dispersion carbon blacks including ionized carboxyl groups in a certain proportion or more as the anionic functional groups and including sodium ions as counterions. That is, those carbon blacks are carbon blacks having -COONa. Other specific examples of the functional group to be introduced through surface treatment may include -S0₃H, -S0₂H, and - P(=0) (OH) 2 - Those functional groups are ionized in an aqueous medium in a certain proportion or more.

Accordingly, pigment particles are stably dispersed owing to repulsion of charges. Examples of the

counterions may include: alkali metal ions such as a lithium ion, a sodium ion, a potassium ion, a rubidium ion, and a cesium ion; an ammonium ion; and ions derived from amines such as methylamine, ethylamine, dimethylamine, 2-hydroxyethylamine , di(2- hydroxyethyl) amine, and tri ( 2-hydroxyethyl ) amine . The counterions are preferably a lithium ion, a sodium ion, a potassium ion, a rubidium ion, a cesium ion, and an ammonium ion, more preferably a potassium ion, a rubidium ion, a cesium ion, and an ammonium ion. As a method for the exchange of counterions of the self- dispersion pigment into desired counterions, there is, for example, a method involving adding, to the self- dispersion pigment, a salt which can provide

counterions in an amount more than the amount of anionic functional groups in the self-dispersion pigment. Further, there is a method, as described in Japanese Patent No. 4001922 and Japanese Patent

Application Laid-Open No. Hll-222573, of repeatedly conducting the steps of exchanging counterions by performing the addition of an aqueous solution

containing target counterions and desalting (ion exchange method) . The average particle diameter of the self-dispersion pigment is preferably 40 nm or more, more preferably 60 nm or more, particularly preferably 70 nm or more.

Further, the average particle diameter of the self- dispersion pigment is preferably 140 nm or less, more preferably 130 nm or less, particularly preferably 120 nm or less. The average particle diameter of the self- dispersion pigment may be measured by a measuring method involving utilizing the scattering of laser light. Specifically, the measurement may be performed with "FPAR-1000" (trade name, manufactured by Otsuka Electronics Co., Ltd., cumulant method analysis), trade name "Nanotrac UPA150EX" (manufactured by NIKKISO CO., LTD., a 50% cumulative value is used), or the like. Note that, the average particle diameter of the self- dispersion pigment in the present invention is a physical property value defined by a light scattering average particle diameter, and is determined by a dynamic light scattering method in a liquid.

[0024] As required, two or more kinds of pigments may be

incorporated in combination into one ink. The addition amount of the self-dispersion pigment in the ink is set to preferably 0.5 mass% or more, more . preferably 1 mass% or more, particularly preferably 1.5 mass% or more with respect to the total amount of the ink in order to provide sufficient color developability .

Further, the use of an ink containing an excess amount of the pigment may reduce the gloss of an image. In order to improve the gloss of an image, it is preferred that the height of a dot be reduced. To that end, the concentration of the pigment is set to preferably 8 mass% or less, more preferably 6 mass% or less, particularly preferably 5 mass% or less.

[0025] queous Medium

The ink to be used in the ink jet image forming method according to the present invention contains water as an essential component. The content of water in the ink is preferably 30 massl or more, more preferably 95 mass% or less with respect to the total mass of the ink. Further, it is preferred that water and a water-soluble compound be used in combination as an aqueous medium. Herein, the water-soluble compound means a compound having such a high hydrophilicity that, in a mixed liquid of the compound with water at a concentration of 20 mass%, the compound is mixed in water without

causing phase separation. Note that, a compound that easily vaporizes to an excessive degree is not

preferred as the water-soluble compound from the

viewpoint of the prevention of the clogging of the ejection orifice of the recording head. Therefore, the vapor pressure at 20°C of the water-soluble compound is preferably 0.04 mmHg or less.

It is preferred that the ink contain a water-soluble compound having a hydrophilicity-hydrophobicity

coefficient defined by the following equation (A) of 0.26 or more. Further, depending on the kind of paper sheet, it is preferred to use an ink in which a water- soluble compound having a hydrophilicity-hydrophobicity coefficient defined by the following equation (A) of 0.26 or more and less than 0.37 and a water-soluble compound having a hydrophilicity-hydrophobicity

coefficient of 0.37 or more are used in combination.

When an ink composition using the hydrophobic water- soluble compound having a hydrophilicity-hydrophobicity coefficient of 0.37 or more in combination is adopted, in-dot density unevenness is further suppressed and the scratch resistance of an image is further improved because of the acceleration of the vaporization of water .

Equation (A)

(Hydrophilicity-hydrophobicity coefficient) = ( (Water activity of 20-mass% aqueous solution) - (Molar fraction of 20-mass% aqueous solution) )/( 1- (Molar fraction of water in 20-mass% aqueous solution) )

[0027] he water activity value in the equation (A) is

represented by the following equation: water activity value= (water vapor pressure of aqueous solution) / (water vapor pressure of pure water) . The water activity value may be measured by various methods, and any one of the measuring methods may be employed. Of those, a chilled mirror dew point measuring method is suitable. The term "water activity value" as used herein refers to a value measured for a 20-mass% aqueous solution (25°C) of the water-soluble compound by the chilled mirror dew point measuring method with "Aqualab CX-3TE" (trade name, manufactured by DECAGON) .

[ 0028 ] According to Raoult ¹ s law, the rate of vapor pressure reduction of a dilute solution equals the molar

fraction of the solute irrespective of the kinds of the solvent and the solute, and hence the molar fraction of water in an aqueous solution equals the water activity value. However, when the water activities of aqueous solutions of various water-soluble compounds are measured, many of the water activities do not equal the molar fraction of water.

[0029] A water activity value of an aqueous solution lower than the molar fraction of water means that the water vapor pressure of the aqueous solution is smaller than the theoretically calculated value and the vaporization of water is suppressed by the presence of the solute. This indicates that the solute is a substance having a large hydration force. On the other hand, a water activity value of an aqueous solution higher than the molar fraction of water suggests that the solute is a substance having a small hydration force.

[ 0030 ] The inventors of the present invention have focused their attention on the fact. that the degree of

hydrophilicity or hydrophobicity of a water-soluble compound to be incorporated into an ink significantly affects the acceleration of solid-liquid separation between the self-dispersion pigment and the aqueous medium as well as various ink performances. Based on such focus of attention, the inventors of the present invention have defined the hydrophilicity- hydrophobicity coefficient represented by the equation (A) . The water activity value is measured for aqueous solutions of various water-soluble compounds at the same concentration of 20 mass%. Then, the measured values are converted by the equation (A) . Thus, a relative comparison of the degrees of hydrophilicity or hydrophobicity of water-soluble compounds can be performed even when the compounds have different molar fractions of water owing to differences in molecular weight of the solutes. Note that, the water activity value of an aqueous solution does not exceed 1, and hence the maximum value of the hydrophilicity- hydrophobicity coefficient is 1. Table 1 shows the hydrophilicity-hydrophobicity coefficients of various water-soluble compounds calculated by the equation (A) . Note that, the water-soluble compound is not limited to those shown in Table 1.

[0031]Table 1

[0032] It is preferred that a water-soluble compound having a desired hydrophilicity-hydrophobicity coefficient be selected from various compounds appropriate for the ink for ink jet recording and be used. A water-soluble compound having a hydrophilicity-hydrophobicity

coefficient of 0.26 or more to have a low hydrophilic tendency is preferred from the viewpoints of further suppressing beading and in-dot density unevenness and further improving fixability. Of such compounds, a compound which has a glycol structure and in which the number of carbon atoms to which hydrophilic groups are bonded is equal to or smaller than the number of carbon atoms to which hydrophilic groups are not bonded is preferred as the water-soluble compound. Such water- soluble compound. is considered to have a relatively low affinity for water and for a self-dispersion pigment but have a high affinity for a coating layer of a printing paper sheet. Accordingly, after the

application of an ink droplet on a printing paper sheet, such water-soluble compound tends to be rapidly

absorbed by the coating layer, resulting in rapid fixation of an image.

[0033] rimethylolpropane is particularly preferred as the

water-soluble compound having a hydrophilicity- hydrophobicity coefficient defined by the equation (A) of 0.26 or more and less than 0.37. Further, a

compound having a glycol structure with 4 to 7 carbon atoms is preferred as the water-soluble compound having a hydrophilicity-hydrophobicity coefficient of 0.37 or more. Of such compounds, .1 , 2-hexanediol , 1,2- pentanediol, and 1 , 6-hexanediol are preferred. 1,2- hexanediol is particularly preferred because it has a water activity value of 0.37 or more and a vapor

pressure at 20°C of 5.3 Pa or less. The content of the water-soluble compound in the ink is preferably 5.0 mass% or more, more preferably 6.0 mass% or more, particularly preferably 7.0 mass% or more with respect to the total mass of the ink. Further, the content of the water-soluble compound in the ink is preferably 40.0 mass% or less, more preferably 35.0 mass% or less, particularly preferably 30.0 mass% or less with respect to the total mass of the ink.

[0034] Polymer Particle

Polymer particles are incorporated into the ink to be used in the ink jet image forming method according to the present invention. The glass transition

temperature of the polymer particles is 25°C or lower. By using the ink containing such polymer particles, beading and in-dot density are suppressed, and an image excellent in fixability and scratch resistance can be formed. The glass transition temperature (Tg) of the polymer particles is limited to 25°C or lower because the average temperature of an in-room environment is assumed to be approximately 25 °C. When polymer

particles having a glass transition temperature higher than 25°C are used, the polymer particles are less easy to form a film even after an ejected ink droplet is applied to a printing paper sheet. This means that the viscosity of the ink droplet is not abruptly increased. Therefore, beading cannot be suppressed, and the fixability of the image is reduced. It is preferred that the glass transition temperature of the polymer particles be 15°C or lower. Further, the glass

transition temperature of the polymer particles is preferably -60°C or higher, more preferably -50°C or higher. If the glass transition temperature of the polymer particles is lower than -60°C, there are cases in which the strength of the formed film is too low. Note that, the glass transition temperature of the polymer particles can be measured by a general method. Specifically, the measurement can be made using a thermal analyzer such as a differential scanning calorimeter (DSC) .

It is preferred that the polymer particles have

satisfactory dispersibility in an aqueous medium. It is preferred that a polymer constituting the polymer particles be a hydrophilic acrylic polymer or a

hydrophilic urethane-based polymer. The hydrophilic acrylic polymer is a copolymer obtained by the

copolymerization of an acrylic monomer and any other monomer copolymerizable with the acrylic monomer.

Examples of the acrylic monomer may include an

unsaturated carboxylic acid monomer, an unsaturated sulfonic acid monomer, an acrylic acid ester monomer, a methacrylic acid ester monomer, and a crosslinkable acrylic monomer having two or more polymerizable double bonds .

[0036] Specific examples of the unsaturated carboxylic acid

monomer may include acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid. Specific examples of the unsaturated sulfonic acid monomer may include 3-sulfopropyl (meth) acrylate . Specific

examples of the acrylic acid ester monomer may include acrylic acid esters such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate,

isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, decyl acrylate, dodecyl acrylate, octadecyl acrylate, cyclohexyl acrylate, phenyl acrylate, benzyl acrylate, glycidyl acrylate, phenoxyethyl acrylate, and 2- hydroxyethyl acrylate.

[ 0037 ] Specific examples of the methacrylic acid ester monomer may include methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, decyl methacrylate, dodecyl methacrylate, octadecyl methacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzyl methacrylate, glycidyl

methacrylate, phenoxyethyl methacrylate, 2-hydroxyethyl methacrylate, polyethylene glycol monomethacrylate, and polypropylene glycol methacrylate.

Specific examples of the crosslinkable acrylic monomer having two or more polymerizable double bonds may include: diacrylate compounds such as polyethylene glycol diacrylate, triethylene glycol diacrylate, 1,3- butylene glycol diacrylate, 1, -butylene glycol

diacrylate, 1 , 6-hexanediol diacrylate, neopentyl glycol diacrylate, 1 , 9-nonanediol diacrylate, polypropylene glycol diacrylate, 2, 2 '-bis (4- acryloxypropyloxyphenyl ) propane, 2,2' -bis (4- acryloxydiethoxyphenyl) propane, and Ν,Ν'- methylenebisacrylamide; triacrylate compounds such as trimethylolpropane triacrylate, trimethylolethane triacrylate, and tetramethylolmethane triacrylate;

tetraacrylate compounds such as ditrimethylol

tetraacrylate, tetramethylolmethane tetraacrylate, and pentaerythritol tetraacrylate; hexaacrylate compounds such as dipentaerythritol hexaacrylate; dimethacrylate compounds such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1, 4-butylene glycol dimethacrylate, 1 , 6-hexanediol dimethacrylate,

neopentyl glycol dimethacrylate, dipropylene glycol dimethacrylate, polypropylene glycol dimethacrylate, polybutylene glycol dimethacrylate, and 2, 2 '-bis (4- methacryloxydiethoxyphenyl ) propane ; trimethacrylate compounds such as trimethylolpropane trimethacrylate and trimethylolethane trimethacrylate; and

methylenebisacrylamide .

Further, specific examples of the monomer

copolymerizable with an acrylic monomer may include: aromatic vinyl monomers such as styrene, a- methylstyrene, vinyltoluene, 4 -t-butylstyrene,

chlorostyrene , vinylanisole, vinylnaphthalene, and divinylbenzene; olefins such as ethylene and propylene; dienes such as butadiene and chloroprene; vinyl

monomers such as vinyl ether, vinyl ketone, and

vinylpyrrolidone; acrylamides such as acrylamide, methacrylamide, and N, ' -dimethylacrylamide; hydroxy group-containing monomers such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, and 2-hydroxypropyl methacrylate ; and unsaturated sulfonic acid monomers such as

styrenesulfonic acid and 2-acrylamide-2- methylpropanesulfonic acid.

The weight-average molecular weight of the polymer is preferably 100,000 or more and 50,000,000 or less from the viewpoints of the ejection characteristic of the ink, print density, and scratch resistance. Further, the weight-average molecular weight of the polymer is more preferably 200,000 or more, particularly

preferably 250,000 or more. Further, the weight- average molecular weight of the polymer is more

preferably 10, 000, 000. or less, still more preferably 8,000,000 or less. When the weight-average molecular weight of the polymer is less than 100,000, the scratch resistance of an image to be formed may be reduced. On the other hand, when the weight-average molecular weight of the polymer is more than 50,000,000, the ejection characteristic of the ink may be impaired.

[0038] The polymer particles are blended in the ink in, for

example, a state of a polymer emulsion in which the polymer particles are dispersed in a solvent. The content of the polymer emulsion in the ink is

preferably 0.1 mass% or more and 10.0 massl or less, more preferably 0.5 mass% or more and 5.0 mass% or less in terms of solid content with respect to the total mass of the ink. When the content of the polymer emulsion is less than 0.1 mass% in terms of solid content, the fixability of an image may be insufficient. On the other hand, when the content of the polymer emulsion is more than 10.0 mass% in terms of solid content, the dispersion stability of the self- dispersion pigment may be reduced.

[0039]Salt

It is preferred that the ink used in the ink jet image forming method according to the present invention contain salt. By using the ink containing a salt, precipitation and coagulation of the self-dispersion pigment can be promoted to further improve the in-dot density unevenness. As the salt, a water-soluble salt of an organic acid and/or an inorganic acid may be- used. Note that, a salt is dissociated in the ink and present as ions, but such state is herein expressed as

"containing salt" for the sake of convenience.

[0040] The organic acid is an acid having a molecular

structure constituted of carbon atoms and having one or more functional groups in its molecule from which a hydrogen ion is dissociated at around neutral pH, the functional groups being exemplified by a carboxyl group, a phosphonic acid group, and a sulfone group. Specific examples of the organic acid may include:

alkylcarboxylic acids such as citric acid, succinic acid, formic acid, acetic acid, propionic acid, oxalic acid, tartaric acid, gluconic acid, tartronic acid, maleic acid, malonic acid, and adipic acid;

arylcarboxylic acids such as benzoic acid, phthalic acid, and trimellitic acid; and alkylsulfonic acids such as methanesulfonic acid and ethanesulfonic acid. Of those, the following acids are preferred: an aqueous alkylcarboxylic acid such as formic, acid, acetic acid, or propionic acid; an alkylcarboxylic acid having multiple carboxyl groups such as succinic acid or tartaric acid; and an arylcarboxylic acid having

multiple carboxyl groups such as phthalic acid or trimellitic acid. Using an ink containing a salt

obtained by using any such organic acid can enhance the optical density of an image, and can prevent clogging of a nozzle.

[0041] Examples of the inorganic acid include mineral acids

such as hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, and phosphoric acid. Of those, sulfuric acid, carbonic acid, or phosphoric acid is preferred. Using an ink containing a salt obtained by using any such inorganic acid can enhance the optical density of the image, and can prevent clogging of the nozzles.

[0042] As a counter ion of the organic acid and the inorganic acid, hydrogen ion, alkali metal ion, ammonium ion, or organic ammonium ion is preferred. Specific examples of the alkali metal include Li, Na, K, Rb, and Cs .

Specific examples of the organic ammoinum include methylammonium, dimethylammonium, trimethylammonium, ethylammonium, diethylammonium, triethylammonium, monohydroxymethyl (ethyl) amine,

dihydroxymethyl (ethyl) amine, and

trihydroxymethyl (ethyl) amine . Of those, ammonium, K, Rb, and Cs are particularly preferred.

[0043] Surfactant

It is preferred that a surfactant be incorporated into the ink to be used in the ink jet image forming method according to the present invention. The incorporation of the surfactant can provide more balanced ejection stability of the ink. As the surfactant, a nonionic surfactant is preferred. Further, of the nonionic surfactants, ethylene oxide adducts of a

polyoxyethylene alkyl ether, acetylene glycol, and the like are preferred. Those nonionic surfactants have HLB values (hydrophile-lipophile balances ) of 10 or more. The content of the surfactant in the ink is set to preferably 0.1 mass% or more, more preferably 0.2 mass% or more, particularly preferably 0.3 mass% or more with respect to the total mass of the ink.

Further, the content of the surfactant in the ink is set to preferably 5.0 mass% or less, more preferably 4.0 mass% or less, particularly preferably 3.0 massl or less with respect to the total mass of the ink.

[0044] Other Additives

As required, various additives may be incorporated into the ink to be used in the ink jet image forming method according to the present invention for the purpose of, for example, adjusting physical property values of the ink. Specific examples of the additives may include a viscosity adjuster, an antifoaming agent, an antiseptic agent, an antifungal agent, an antioxidant, and a penetrant .

[0045] Surface Tension

The surface tension of the ink to be used in the ink jet image forming method according to the present invention is 34 mN/m or less, preferably 33 mN/m or less, more preferably 32 mN/m or less. Further, the surface tension of the ink is preferably 20 mN/m or more, more preferably 23 mN/m or more, particularly preferably 26 mN/m or more. By using an ink having a surface tension in the above-mentioned range, the effects of the present invention that beading and in- dot density unevenness are suppressed and an image excellent in fixability and scratch resistance can be formed at high speed are exerted at the maximum. If the surface tension of the ink is more than 34 mN/m, wetting by an ink droplet of the coating layer of the printing paper sheet is less liable to occur, and the absorption rate of the liquid component in the coating layer becomes lower to reduce the fixability of an image. Note that, the surface tension of the ink is measured using the ilhelmy plate method. Specific examples of measuring apparatus for measuring the surface tension of the ink include CBVP-Z (trade name, manufactured by Kyowa Interface Science Co., Ltd).

[0046] iscosity

The viscosity of the ink to be used in the ink jet image forming method according to the present invention is 14 mPa · s or less, preferably 10 mPa · s or less, more preferably 6 mPa-s. By using an ink having a viscosity in the above-mentioned range, an ink droplet can be ejected from a recording head of a recording apparatus such as an ink jet printer at a high frequency with ease, which facilitates application of the ink to the ink jet image forming method according to the present invention in which high speed printing is performed.

[0047] <Recording Apparatus>

The recording apparatus suitably used in the ink jet image forming method according to the present invention is one having a recording head mounted thereon for ejecting and applying ink onto a printing paper sheet. The method of ejecting ink of the recording head is not specifically limited insofar as the above-mentioned ink can be ejected therefrom. As the method of ejecting ink, a method in which pressure is applied to ink by deformation of a piezoelectric element provided in a pump or a flow path, a method in which thermal energy is given to ink to generate a bubble, an electrostatic suction method in which ink is charged to use

electrostatic suction force, or the like may be used. In the ink jet image forming method according to the present invention, any recording apparatus including a recording head which ejects ink by any one of such methods can be used. Further, as a method of

controlling the timing of ejecting ink, a continuous method in which ink is constantly ejected and

unnecessary ink is collected before the ink is applied to a printing paper sheet, an on-demand method in which ink is ejected only when the ink is desired to be applied to a printing paper sheet, or the like may be used. In the ink jet image forming method according to the present invention, any recording apparatus which controls the timing of ejecting ink by any one of such methods can be used.

[0048] FIG. 1 is a schematic view illustrating an exemplary ink jet recording apparatus having a serial type recording head. A recording head which ejects ink in an ink jet recording system is mounted on a carriage 20. The recording head includes multiple ink ejection orifices 211 to 215. As a method of applying ink while the recording head scans once, a method in which ink of one color is ejected from one nozzle array (ink

ejection orifices) or a method in which ink of one color or inks of multiple colors is ejected from

multiple nozzle arrays may be used. Ink cartridges 221 to 225 include a recording head, the ink ejection orifices 211 to 215, and an ink tank for supplying ink to these ink cartridges 221 to 225. A density sensor 40 is a reflection density sensor, and detects, in a state of being provided on a side surface of the

carriage 20, the density of a test pattern recorded on a printing paper sheet 24. A control signal and the like are transferred via a flexible cable 23 to the recording head.

[0049] The printing paper sheet 24 having a coating layer

passes a transport roller (not shown) and then is picked up by a delivery roller 25, and is conveyed in the direction of the arrow (sub scan direction) while a ^' conveyer motor 26 is driven. The carriage 20 is guided and supported by a guide shaft 27 and a linear encoder 28. The carriage 20 is driven by a carriage motor 30 via a drive belt 29 to reciprocate along the guide shaft 27 in the main scan direction. A heat generating element (electrothermal energy converter) for

generating thermal energy for ejecting ink or a

piezoelectric element (electropressure converter) is provided in the recording head (liquid path) . With the timing of reading of the linear encoder 28, the heat generating element or the piezoelectric element is driven based on a record signal, and an ink droplet is ejected onto the printing paper sheet 24 to be fixed thereto to form an image.

[0050]A recovery unit 32 including caps 311 to 315 is provided at a home position of the carriage 20 which is provided outside a recording region. When recording is not performed, the carriage 20 is moved to the home position and the ink ejection orifices 211 to 215 are hermetically sealed with the corresponding caps 311 to 315, respectively. This can prevent clogging caused by adhesion of ink due to evaporation of a liquid

component thereof or by adhesion of foreign matter such as dust and the like. Further, the capping function of the caps is used to prevent ejection failure and

clogging of an ink ejection orifice which is

infrequently used for recording. Specifically, the caps are used for blank ejection for preventing

ejection failure of the ink ejection orifices.. Further, the caps are used for recovery of ejection of an ink ejection orifice in which ejection failure has occurred, by sucking ink from the ink ejection orifice using a pump (not shown) .

An ink receiving portion 33 plays a role in receiving an ink droplet which is preliminarily ejected when the recording head passes thereover immediately before recording operation. Further, by providing a blade or a wiping member (not shown) at a location adjacent to the caps 311 to 315, the ink ejection orifices 211 to 215 can be cleaned. Adding a recovery unit of the recording head and other auxiliary units to the

structure of the recording apparatus is preferred, because the recording operation can be stabilized.

Specifically, it is preferred to add to the structure of the recording apparatus, for example, a capping unit, a cleaning unit, a pressurizing or suction unit, an electrothermal energy converter, other kinds of a heating element, or an auxiliary heating unit as a combination thereof, for the recording head. Further, it is also effective to provide a preliminary ejection mode for performing ejection other than ejection for recording in order to stabilize the recording operation. Further, a recording head of a cartridge type in which an ink tank is integrally provided may also be used.

Further, there may also be used a replaceable recording head of a chip type which can be electrically connected to a body of the recording apparatus and to which ink can be supplied from the recording apparatus by being mounted to the body of the recording apparatus.

[0052] FIG. 2 is a schematic view illustrating an exemplary

serial type recording head. FIG. 2 illustrates the recording head including the ink ejection orifices 211 to 215 illustrated in FIG. 1. Note that, in FIG. 2, the scan direction of the recording head (recording scan direction) is the direction of the arrow. The recording head includes the multiple ink ejection orifices 211 to 215 which are arranged in a direction substantially orthogonal to the recording scan

direction. The recording head ejects ink droplets at predetermined timing from the respective ejection orifices while the recording head moves and scans in the recording scan direction.. This forms an image on the printing paper sheet with a recording resolution based on the arrangement density of the ink ejection orifices (nozzle arrays). Note that, the recording operation of the recording head may be performed in any one of the recording scan direction and a direction opposite to the recording scan direction.

[0053] As the recording apparatus, a full line type recording apparatus including a recording head having a length corresponding to the width of the printing paper sheet may also be used. As the full line type recording head, for example, one having an increased length by

arranging serial type recording heads in a staggered manner or in parallel with one another so as to have an intended length may be used. The recording head may also be one recording head in which ink ejection orifices 216 to 220 (nozzle columns) which are originally long are integrally formed as illustrated in FIG. 3.

[005 ] <Recording Medium>

The recording medium to be used in the ink jet image forming method according to the present invention is a printing paper sheet having a coating layer mainly used in^" offset printing, gravure printing, or the like. The coating layer is a layer of a coating provided on a front surface and/or a rear surface of woodfree paper or medium quality paper, or a layer of a coating formed when the paper is made, for the purpose of enhancing the aesthetic appearance or the smoothness of the surface of the paper.

[0055] According to "Census of Manufactures" by Ministry of

Economy, Trade, and Industry and "Classification Table of Paper and Paperboard" in "Paper and Paperboard

Statistics Yearbook" by Japan Paper Association, a printing paper sheet having a coating layer is in the category of coated printing paper and lightly coated paper in "printing and communication paper sheets".

The former has a coating layer by applying a coating which is about 15 g to 40 g per 1 m² on a surface (both surfaces) of a paper sheet. The latter has a coating layer by applying a coating which is 12 g or less per 1 m² on a surface of the paper sheet. The coated printing paper is further broken down into art paper, coated paper, light weight coated paper, and other coated printing paper (cast-coated paper, embossed paper, and the like) in accordance with the amount of an applied coating, the method of surface treatment after the coating application, and the like. Further, according to the glossiness of the surface it is also classified into gloss paper, matte paper, dull paper, and the like. The printing paper sheet to be used in the ink jet image forming method according to the present invention may be any one of these printing paper sheets having a coating layer.

[0056] It is preferred that, when the entire printing paper sheet is measured by X-ray florescence analysis (XRF) , the content of the other elements than carbon and than oxygen be 10.0 mass% or more. Further, it is preferred that the proportion of calcium to the other elements than carbon and than oxygen be 5.0 mass% or more. By using such a printing paper sheet, interaction thereof with the above-mentioned ink improves the coagulation rate of the pigment to reduce the image unevenness. Note that, with XRF described above, the amounts of various kinds of elements existing in a paper sheet having a thickness of about 100 μπι can be measured with good reproducibility by only fixing a sample (paper) on a sample stage and applying X-rays thereto. XRF cannot detect hydrogen, helium, lithium, and a superheavy element which is uranium or a heavier element from the measuring principle thereof. However, it is almost impossible that helium, lithium, or a superheavy element which is uranium or a heavier element exists in the paper with a proportion which is not negligible. Therefore, the element proportion obtained by analysis of paper with XRF can be said to be substantially the proportion of the element with respect to all elements forming the paper except hydrogen.

[0057] The mainstream of printing today is offset printing using an oil-based ink. Therefore, the coating layer has such a structure that a coloring material and a liquid component (in particular, a hydrophilic liquid component) incorporated in the ink are difficult to penetrate therein. Therefore, it is preferred to use a printing paper sheet having a coating layer, the coating layer having pores with an average diameter of 0.1 μπι or less and a pore volume of 0.3 mL/g or less.

[0058] In the ink jet image forming method according to the present invention, as the printing paper sheet having a coating layer, the following printing paper sheets

(trade names) can be used.

Examples of the art paper may include: OK Ultra

Aquasatin, OK Kinfuji, SA Kinfuji, and Satin Kinfuji (all of which are manufactured by Oji Holdings

Corporation) ; Hyperpyrenee and Silverdia (both of which are manufactured by Nippon Paper Industries Co., Ltd.); Green Utrillo (manufactured by Daio Paper Corporation) ; Pearl Coat and New V Matte (both of which are

manufactured by MITSUBISHI PAPER MILLS LIMITED) ; Raicho Super Art (manufactured by Chuetsu Pulp & Paper Co., Ltd.); and Hi-Mckinley (manufactured by Gojo Paper MFG., Co. Ltd.). Examples of the coated paper may include OK Top Coat, OK Top Coat dull, OK Top Coat matte, OK

Trinity, and OK Casablanca (all of which are

manufactured by Oji Holdings Corporation); Aurora Coat, Silverdia, and Shiraoi matte (all of which are

manufactured by Nippon Paper Industries Co., Ltd.);

Green Utrillo (manufactured by Daio Paper Corporation) ; and Pearl Coat and New V Matte (both of which are manufactured by MITSUBISHI PAPER MILLS LIMITED) .

Examples of the lightweight coated paper may include: OK Coat L (manufactured by Oji Holdings Corporation); Aurora L, Easter DX, and Pegasus (all of which are manufactured by Nippon Paper Industries Co., Ltd.);

Utrillo Coat L (manufactured by Daio Paper

Corporation) ; Pearl Coat L (manufactured by MITSUBISHI PAPER MILLS LIMITED) ; Super Emine (manufactured by

Chuetsu Pulp & Paper Co., Ltd.); and Dream Coat

(manufactured by MARUSUMI PAPER CO., LTD.). Examples of the cast coated paper may include: Mirror Coat

Platinum and OK Chrome (both of which are manufactured by Oji Holdings Corporation); Esprit Coat (manufactured by Nippon Paper Industries Co., Ltd.); and Picasso Coat (manufactured by Daio Paper Corporation) . Further, examples of the lightly coated printing paper sheet may include: OK Ever Light, OK Crystal, and OK Prunus White (all of which are manufactured by Oji Holdings

Corporation) ; and Pyrenee DX and Aurora S (both of which are manufactured by Nippon Paper Industries Co., Ltd. ) .

Examples

[0060] Next, the present invention is described more

specifically by way of examples and comparative

examples. Note that, "part" and "%" in the following description are based on the mass unless otherwise noted. Further, the surface tensions of the inks were measured using a surface tension meter (trade name "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd. ) . Further, the viscosities of the inks were measured using a viscometer (trade name "RE-80

Viscometer" manufactured by TOKI SANGYO CO., LTD).

[0061] <Production of Hydrophilic Polymer Emulsion A>

Polymerization was performed according to an ordinary method using styrene/n-butyl acrylate/acrylic acid of 3.0/6.0/1.5 (mass ratio), sodium dodecyl sulfate of 0.25 (mass ratio), and potassium persulfate

(manufactured by Sigma-Aldrich) as an initiator. After the polymerization, the resultant was neutralized with a potassium hydroxide (KOH) aqueous solution, purified, and concentrated to obtain a hydrophilic polymer emulsion A having a solid content of 10%. The pH of the resultant hydrophilic polymer emulsion A was adjusted to 8.5. The polymer particles contained in the hydrophilic polymer emulsion A had an average particle diameter (D50) of 122 nm. Further, the polymer constituting the polymer particles had an acid value of 101 mgKOH/g and a glass transition temperature (Tg) of -3°C.

[0062] <Production of Hydrophilic Polymer Emulsion B>

A hydrophilic polymer emulsion B having a solid content of 10 mass% was obtained in the same manner as in

"Production of Hydrophilic Polymer Emulsion A"

described above except for using styrene/n-butyl

acrylate/acrylic acid of 7.0/2.0/1.5 (mass ratio). The pH of the resultant hydrophilic polymer emulsion B was adjusted to 8.5. The polymer particles contained in the hydrophilic polymer emulsion B had an average particle diameter (D50) of 130 nm. Further, the

polymer constituting the polymer particles had an acid value of 100 mgKOH/g and a glass transition temperature (Tg) of 58°C.

Preparation of inks (inks 1 to 18) >

Respective components constituting inks were mixed according to compositions shown in Tables 2-1 and 2-2 (total: 100 parts), and then stirred for 1 hour. Next, the mixtures were filtered through a filter having a pore diameter of 2.5 μπι to prepare the inks. Note that, "Water" in Tables 2-1 and 2-2 refers to ion-exchanged water, and "Acetylenol EH" is a trade name of a

nonionic surfactant (manufactured by Kawaken Fine

Chemicals Co., Ltd.). Further, aqueous dispersions of various anionic self-dispersion pigments shown below were used as "Kind of pigment" in Tables 2-1 and 2-2. •CW-1S: black pigment manufactured by Orient Chemical Industries Co., Ltd. (trade name "BONJET BLACK CW-1S") ^•COJ400: black pigment manufactured by Cabot (trade name "CAB-O-JET 400")

64]Table 2-1

Ink 1 Ink 2 Ink 3 Ink 4

Kind CW-IS CW-IS CW-IS CW-IS

Pigment

Amount 2 2 2 2

Polymer Emulsion A

(Glass Transition 2 2 - - Temperature: -3°C)

Polymer Emulsion B

(Glass Transition - - 2 - Temperature: 58 °C)

Glycerol - - - -

Trimethylolpropane 5 5 5 5

1 , 2-Hexanediol 15 15 15 15

1, 6-Hexanediol - - - -

Acetylenol EH 0.5 0.5 0.5 0.5

Salt (potassium - 0.5 - - phthalate)

Water Remainder Remainder Remainder Remainder

Viscosity (mPa-s) 2.6 2.6 2.6 2.0

Surface tension

32 32 32 32 (mN/m)

[0065]Table 2-2

[0066]<Image Forming Method I (Examples 1 and 2 and

Comparative Examples 1 and 2)>

Prepared inks 1 to 4 and recording apparatus A were used to form an image on a printing paper sheet having a coating layer (trade name "OK Top Coat" manufactured by Oji Holdings Corporation). Specifically, an ink tank filled with ink was mounted on a black ink head portion of a recording apparatus (printer) . Next, an image corresponding to 1,200 dpi*l,200 dpi was printed using uniform print patterns with a duty of 10% and a duty of 20%. Note that, all the target images were printed by a one pass method. Table 3 shows the recording apparatus, the used inks, the dot diameters, the in-dot average ink application amounts, and the results of evaluation of the images. Further, the types of the used recording apparatus are as follows.

[ 0067 ] Recording apparatus. -Recording apparatus A: trade name "PIXUS iP480"

(manufactured by Canon Inc., recording head: nine ejection orifice arrays>512 nozzles, ink ejection amount: 1.0 pL (constant quantity), recording density: 9,600 dpi (transverse direction) *2, 00 dpi (machine direction) )

-Recording apparatus B: a drum fixing type ink jet recording apparatus including six compressive type piezoelectric single nozzle heads (aperture diameter: 8 μιη, ink ejection amount: 1.0 pL (constant quantity)). As illustrated in FIGS. 4 and 5, recording heads 1 to 6 were arranged so that the distances in the transverse direction between adjacent ejection orifices were 10.6 μπι (2,400 dpi). Note that, the recording medium was attached to a drum 10 and the rotation speed of the drum 10 and the frequency of the recording heads 1 to 6 were adjusted so that the image was formed with

intervals of 10.6 μιη (2, 400 dpi).

-Recording apparatus C: a recording apparatus formed by modifying Recording apparatus A so that the ink

ejection amount was 0.5 pL (constant quantity).

-Recording apparatus D: a recording apparatus formed by modifying Recording apparatus A so that the ink

ejection amount was 0.6 pL (constant quantity).

-Recording apparatus E: trade name "BJF950"

(manufactured by Canon Inc., ink ejection amount: 2.0 pL (constant quantity) )

-Recording apparatus F : trade name "PIXUS PRO 9500" (manufactured by Canon Inc., ink ejection amount: 3.0 pL (constant quantity) )

-Recording apparatus G: trade name "PIXUS 6500i"

(manufactured by Canon Inc., ink ejection amount: 5.0 pL (constant quantity) )

easurement of Dot Diameter

A dot pattern portion having a duty of 10% was visually observed with an optical microscope. Twenty dots were selected which were independent of one another, had shapes relatively close to a circle, and contained less satellites (small ink droplets separated from the main droplets), and the major diameters and the minor diameters thereof were measured. The average of the measured major diameters and the measured minor

diameters of the dots was determined to be "dot

diameter" .

[0069] <Evaluation Method of Image>

In-dot Density Unevenness

Printed images (dot images) were visually observed with an optical microscope, and the in-dot density

unevenness was evaluated in accordance with the

following evaluation criteria. Note that, in the following evaluation criteria, "A" and "B" are

acceptable levels, and "C" is an unacceptable level. A: In-dot density unevenness was not at all recognized, and a satisfactory image was formed.

B: In-dot density unevenness was slightly recognized, but a satisfactory image was formed.

C: In-dot density unevenness was caused, and a low quality image was formed.

[0070] Beading

A dot pattern portion having a duty of 20% was visually observed_ with an optical microscope. Beading of dots formed by overlapping of adjacent dots of the entire dots in an arbitrary field of view was evaluated in accordance with the following evaluation criteria.

Note that, in the following evaluation criteria, "A" and "B" are acceptable levels, and "C" is an

unacceptable level.

A: Adjacent dots maintained their respective single dot shapes .

B: Adjacent dots were slightly merged with each other. C: Adjacent dots were substantially merged with each other into one. [0071] Fixability

After a lapse of 20 seconds after a solid image was printed, the solid image was rubbed once with a lens- cleaning paper sheet with a weight of 360 g mounted thereon. The extent of fading of the ink in the solid image was visually observed, and the fixability was evaluated in accordance with the following evaluation criteria. Note that, in the following evaluation criteria, "A" and "B" are acceptable levels, and "C" is an unacceptable level.

A: No fading was recognized, and the lens-cleaning paper sheet was not dirty.

B: Fading was slightly recognized.

C: Fading was clearly recognized.

[0072]Table 3

[0073] As shown in Table 3, when Ink 1 containing a polymer

particle formed of a polymer having a glass transition temperature of -3°C was used, in-dot density unevenness and beading were suppressed, and it was possible to record images excellent in fixability (Example 1). In addition, when ink 2 in which a salt was added was used, in-dot density unevenness was able to be further

suppressed (Example 2) . On the other hand, when Ink 3 containing a polymer particle formed of a polymer having a glass transition temperature of 58°C was used, the fixability of the image was conspicuously reduced (Comparative Example 1) . Further, when Ink 4 which did not contain a polymer particle was used, the in-dot density unevenness was not suppressed, and the

fixability of the image was conspicuously reduced

(Comparative Example 2).

[0074]<Image Forming Method II (Examples 3 to 5 and

Comparative Examples 3 and 4)>

An image was formed (printed) similarly to the case of the above-mentioned "image forming method (I)" except that prepared Inks 5 to 9 and Recording apparatus B were used. Table 4 shows the recording apparatus, the used inks, the dot diameters, the in-dot average ink application amounts, and the results of evaluation of the images.

[0075]Table 4

[0076] As shown in Table 4, when Ink 8 having a high viscosity was used, the fixability of the image was reduced

(Comparative Example 3) . Further, with regard to an image recorded using an ink having a high surface tension, beading was not suppressed, and the fixability was conspicuously reduced (Comparative Example 4).

[0077]<Image Forming Method III (Examples 6 to 12 and

Comparative Examples 5 and 6)>

An image was formed (printed) similarly to the case of the above-mentioned "image forming method (I)" except that prepared Inks 10 and 11 and Recording apparatus A and C to G were used. Table 5 shows the recording apparatus, the used inks, the dot diameters, the in-dot average ink application amounts, and the results of evaluation of the images.

[0078]Table 5

[0079] As shown in Table 5, when the in-dot average ink

application amount was in the predetermined range, in- dot density unevenness and beading were suppressed, and an image excellent in fixability was able to be

recorded (Examples 6 to 12) . In particular, when the in-dot average ink application amount was in the range of 0.1 to 0.2 pL/cm², beading was able to be further suppressed (Examples 6 to 8 and 12) . On the other hand, when the in-dot average ink application amount was too large, beading was not suppressed and the fixability of the image was reduced (Comparative Example 5) . Further, when the in-dot average ink application amount was too small, in-dot density unevenness was not suppressed (Comparative Example 6) .

[0080] hile the present invention has been described with reference to exemplary embodiments, it is to be

understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest

interpretation so as to encompass all such

modifications and equivalent structures and functions.

[0081] This application claims the benefit of Japanese Patent Application No. 2012-041710, filed February 28, 2012, which is hereby incorporated by reference herein in its entirety .

Claims

[Claim l]An ink jet image forming method comprising applying ink to a printing paper sheet using a recording head of an ink jet system to form an image,

wherein the ink contains a self-dispersion pigment, a polymer particle having a glass transition temperature of 25 °C or lower and water, and has a viscosity of 14 mPa*s or less and a surface tension of 34 mN/m or less,

wherein a diameter of a dot formed by one ink droplet applied from the recording head is 20 μιη or more and 40 μπι or less, and

wherein an average ink application amount per unit area in the dot is 0.09 L/cm² or more and 0.30

L/cm² or less.

[Claim 2]An ink jet image forming method according to claim 1, wherein the ink contains a salt.

[Claim 3]An ink jet image forming method according to claim 1, wherein a content of other elements than carbon and than oxygen in the printing paper sheet measured by X-ray florescence analysis is 10.0 mass% or more, and

wherein a proportion of calcium to the other elements is 5.0 mass% or more.