Patent Publication Number: US-2005143492-A1

Title: Ink composition for inkjet recording and inkjet recording method

Description:
FIELD OF THE INVENTION  
      The present invention relates to an ink composition for inkjet recording and an inkjet recording method.  
     BACKGROUND OF THE INVENTION  
      As an image recording method of forming an image on a recording medium, for example, paper, based on image data signals, there are an electrophotographic system, a sublimation or melting thermal transfer system and an inkjet system. The electrophotographic system is a complex system and an apparatus therefor is expensive because it requires such a process that an electrostatic latent image is formed on a photoreceptor drum through charge and exposure. The thermal transfer system involves a high running cost and generation of waste materials due to the use of an ink ribbon, although an apparatus therefor itself is inexpensive. In the inkjet system, on the other hand, image formation is carried out with an inexpensive apparatus in such a manner that an ink is directly ejected to only a necessary image area on a recording medium, and thus the ink can be used efficiently to reduce the running cost. Further, the inkjet system causes less noise, and thus it is excellent as the image recording method.  
      The inkjet recording system includes, for example, a system of flying ink droplets by pressure of vapor generated by heat from a heat generator, a system of flying ink droplets by mechanical pressure pulses generated by a piezoelectric element, and a system of flying ink droplets containing charged particles by utilizing an electrostatic field (refer to Patent Document 1 and Patent Document 2) The system of flying ink droplets with vapor or mechanical pressure cannot control a flying direction of ink droplet, and there are some cases where ink droplet is difficult to be accurately reached to the desired position on a printing medium due to distortion of ink nozzle and air convection.  
      On the contrary, the system utilizing an electrostatic field controls the flying direction of ink droplet with the electrostatic field to enable ink droplet to be accurately reached the desired position, and thus it is advantageous in that an imaged material (printed material) with high image quality can be produced.  
      As an ink composition for use in the inkjet recording system utilizing an electrostatic field, an ink composition comprising a dispersion medium and charged particles containing at least a colorant is ordinarily employed (refer to Patent Document 3 and Patent Document 4). The ink composition containing a colorant can form inks of four colors, i.e., yellow, magenta, cyan and black, by changing the colorant, and can also form special color inks of gold and silver. Accordingly, the ink composition is useful for producing a color imaged material (printed material). In order to produce stably color imaged materials (printed materials) while maintaining high speed and high image quality, however, it is necessary that ink particles (charged particles) are concentrated at a tip of an ejection part with high electrophoretic speed. For such a purpose, it is confirmed that ink particles must be provided with a sufficient amount of charge. Thus, liquid developers comprising toner particles having a particle size of 0.5 μm or less ordinarily used in the electrophotographic system are difficult for use in the inkjet system utilizing an electrostatic field. It is necessary to increase a diameter of the ink particle to an extent that image quality is not degraded (form 0.8 to 2.0 μm) and to eliminate minute particles having a diameter of 0.2 μm or less, thereby increasing a charge amount per ink particle. Further, it is necessary to reduce particle size distribution in order to prevent increase of ink particles having a minute size accumulated during running inkjet recording. However, particle formation techniques do not yet reach a satisfactory level in the present state. 
      Patent Document 1; Japanese Patent 3,315,334     Patent Document 2: U.S. Pat. No. 6,158,844     Patent Document 3: JP-A-8-291267 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)     Patent Document 4: U.S. Pat. No. 5,952,048    

     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide an ink composition for inkjet recording and an inkjet recording method capable of ejecting ink droplets constantly and stably in the inkjet recording for a long period of time.  
      Another object of the invention is to provide an ink composition for inkjet recording and an inkjet recording method enabling formation of high quality image without blur for a long period of time.  
      As a result of intensive investigations to achieve the above-described objects, the inventor has found that the ink composition for inkjet recording enabling formation of high quality image without blur for a long period of time can be obtained by incorporating a polyester satisfying specific physical property conditions or a specific polyethylene derivative into a charged particle to complete the invention.  
      Specifically, the present invention includes the following items.  
      (1) An ink composition comprising a dispersion medium and a charged particle containing at least a colorant, wherein the charged particle contains a polyester satisfying the following physical property conditions a) to d); 
      a) Ester content ratio (meq./g) of from 5 to 15;     b) Molecular weight (GPC weight average) of 30,000 or less;     c) Glass transition point (Tg) of from 30 to 100° C.; and     d) Dynamic elastic modulus at 50° C. (G′) of 10 4  Pa or more.    

      (2) The ink composition as described in item (1) above, wherein the polyester comprises a combination of a diol monomer represented by formula (I) shown below and a dicarboxylic acid represented by formula (II) shown below.  
                 
 
 wherein m and n each independently represents an integer of from 1 to 22, provided that m+n≦23; a, a′, b and b′ each independently represents a hydrogen atom or a hydrocarbon group, or may be combined with each other to form a ring structure; Y represents a substituent; and q represents an integer of from 0 to 4. 
 
      (3) An ink composition comprising a dispersion medium and a charged particle containing at least a colorant, wherein the charged particle contains a polyethylene derivative having at least one group selected from an alkyl group having from 4 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms and a bridged cyclic hydrocarbon group having up to 22 carbon atoms.  
      (4) The ink composition as described in item (3) above, wherein the polyethylene derivative comprises repeating units represented by formula (III) or (IV) shown below.  
                 
 
 wherein m, n and l each independently represents a positive integer, provided that m+n=100% by mole in formula (III) and that m+n+l=100% by mole in formula (IV); a 1 , a 2 , and b 1  each independently represents a hydrogen atom or a hydrocarbon group, or may be combined with each other to form a ring structure; L represents a single bond or a divalent linking group comprising two or more atoms selected from C, H, N, O, S and P and having a total number of atoms of 50 or less; and Q represents an alkyl group having from 4 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms or a bridged cyclic hydrocarbon group having up to 22 carbon atoms. 
 
      (5) The ink composition as described in any one of items (1) to (4) above, wherein the charged particle is a charged particle prepared by a mechanical media dispersion method and a content of the charged particles having a diameter corresponding to ⅕ or less of a volume average diameter of the charged particles by means of a centrifugal sedimentation method is 1% by volume or less based on the total charged particles.  
      (6) An inkjet recording method comprising a step of flying an ink composition as an ink droplet by utilizing an electrostatic field, wherein the ink composition is the ink composition as described in any one of items (1) to (5) above.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an overall construction view schematically showing an example of an inkjet recording apparatus used in the invention.  
       FIG. 2  is a perspective view showing a constitution of an inkjet head of the inkjet recording apparatus used in the invention. For the sake of easy understanding, an edge of guard electrode in each ejection part is not shown.  
       FIG. 3  is a side cross sectional view along with a line X-X in  FIG. 2  showing a distribution state of charged particles where the number of ejection parts in the inkjet head shown in  FIG. 2  is large. 
      G: Ink droplet     P: Recording medium     Q: Ink flow     R: Charged particle      1 : Inkjet recording apparatus      2 ,  2 Y,  2 M,  2 C,  2 K: Ejection head      3 : Ink circulation system      4 : Head driver      5 : Position controlling means      6 A,  6 B,  6 C: Roller      7 : Conveying belt      8 : Conveying belt position detecting means      9 : Electrostatic adsorption means      10 : Static eliminating means      11 : Mechanical means      12 : Feed roller      13 : Guide      14 : Image fixing means      15 : Guide      16 : Recording medium position detecting means      17 : Exhaust fan      18 : solvent vapor absorbent      38 : Ink guide      40 : Supporting bar      42 : Ink meniscus      44 : Insulating layer      46 : First ejection electrode      48 : Insulating layer      50 : Guard electrode      52 : Insulating layer      56 : Second ejection electrode      58 : Insulating layer      62 : Floating electroconductive plate      64 : Coating film      66 : Insulating member      70 : Inkjet head      72 : Ink flow channel      74 : Substrate      75 ,  75 A,  75 B: Opening      76 ,  76 A,  76 B: Ejection part      78 : Ink guide part   
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      According to the invention, by employing the polyester satisfying the specific physical property conditions or the specific polyethylene derivative, a particle size and particle size distribution of ink particles can be controlled in the preferred ranges and thus, electrophoretic speed of the ink particle increases and concentration of the ink particles is sufficiently conducted in the ejection part so that images of high quality without blur can be obtained. Further, in the case of conducting running of the inkjet recording, stable drawing is made possible for a long period of time.  
      In a first embodiment of the ink composition according to the invention, the charged particle is characterized by containing a polyester satisfying the following physical property conditions a) to d): 
      a) Ester content ratio (meq./g) of from 5 to 15;     b) Molecular weight (GPC weight average) of 30,000 or less;     c) Glass transition point (Tg) of from 30 to 100° C.; and     d) Dynamic elastic modulus at 50° C. (G′) of 10 4  Pa or more.    

      In the invention, the ester content ratio is preferably 6 to 14, and more preferably from 7 to 13. The molecular weight (GPC weight average) means a weight average molecular weight measured by GPC method and calculated in terms of polystyrene. The molecular weight is preferably from 2,000 to 25,000, and more preferably from 3,000 to 20,000. The glass transition point (Tg) is preferably from 30 to 90° C., more preferably from 40 to 80° C. The dynamic elastic modulus at 50° C. (G′) is preferably from 10 4  to 10 9  Pa, and more preferably from 10 5  to 10 8  Pa.  
      In order to pulverize and disperse a pigment-polymer mixture by applying shear force as in the system of the first embodiment of the invention, it is necessary that the polymer is not swellable in a solvent. In the case of using a low-polar solvent (a hydrocarbon) as in the invention, it is requested that the ester content ratio, which is an index for indicating polarity of polyester, is relatively high. When the ester content ratio is lower than the above-described range, the polyester is swellable in the solvent, and shearing force is unable to function properly at the dispersion. As a result, it is difficult to form particles by mechanical dispersion with media. When the molecular weight of the polymer is higher than the above-described range, cohesion of the polymer becomes high and unevenness of the cohesion in the pigment-polymer mixture is apt to occur. As a result, it is difficult to perform uniform particle formation, resulting in formation of particles having a broad particle size distribution.  
      Further, it is important a physical property corresponding to hardness/softness of the polymer in the system of the first embodiment of the invention. When the polymer is too soft, the sear force is hard to transmit to the pigment-polymer mixture. On the other hand, when it is too hard, although the sear force sufficiently transfers, a problem occurs in that the polymer is pulverized into too small particles. Therefore, it is necessary that the glass transition point (Tg) and dynamic elastic modulus (G′) be controlled in the above-described ranges. When the glass transition point (Tg) is lower than 30° C., since brittleness necessary for being broken is deficient and also the force from media is hard to reach to the polymer, it is difficult to form particles by mechanical dispersion with media. When the dynamic elastic modulus (G′) is less than 10 4 , since the polymer exhibits physical properties of rubber elastic region in a temperature range of the pulverization and dispersion step, the sear force does not work well at the dispersion and it is also difficult to form particles by mechanical dispersion with media.  
      Now, each constituting component of the ink composition according to the invention is described below.  
      The polyester, which is used as a coating agent (coating polymer) for a colorant in the invention, is not particularly restricted as far as it satisfies the above-described specific physical property conditions. Preferably, the polyester comprises a combination of a diol monomer represented by formula (I) shown below and a dicarboxylic acid represented by formula (II) shown below.  
                 
 
 wherein m and n each independently represents an integer of from 1 to 22, provided that m+n≦23; a, a′, b and b′ each independently represents a hydrogen atom or a hydrocarbon group, or may be combined with each other to form a ring structure; Y represents a substituent; and q represents an integer of from 0 to 4. 
 
      In formula (I), a, a′, b and b′, which may be the same or different, each represents a hydrogen atom or a hydrocarbon group. The hydrocarbon group for a, a′, b or b′ represents an alkyl group having from 1 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aromatic group having up to 22 carbon atoms or a bridged cyclic hydrocarbon group having up to 22 carbon atoms, and each of the groups may be substituted.  
      Preferred examples of the hydrocarbon group for a, a′, b or b′ include an alkyl group having from 1 to 22 carbon atoms, which may be substituted, (for example, methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, 2-chloroethyl, 2-bromoethyl, 2-cyanoethyl, 2-methoxycarbonylethyl, 2-methoxyethyl or 3-bromopropyl), an alkenyl group having from 4 to 18 carbon atoms, which may be substituted, (for example, 2-methyl-1-propenyl, 2-butenyl, 2-pentenyl, 3-methyl-2-pentenyl, 1-pentenyl, 1-hexenyl, 2-hexenyl, 4-methyl-2-hexenyl, decenyl, dodecenyl, tridecenyl, hexadecenyl, octadecenyl or linolenyl), an aralkyl group having from 7 to 12 carbon atoms, which may be substituted, (for example, benzyl, phenethyl, 3-phenylpropyl, naphthylmethyl, 2-naphthylethyl, chrolobenzyl, bromobenzyl, methylbenzyl, ethylbenzyl, methoxybenzyl, dimethylbenzyl or dimethoxybenzyl), an alicyclic group having from 5 to 8 carbon atoms, which may be substituted, (for example, cyclohexyl group, 2-cyclohexylethyl group or 2-cyclopentylethyl group), or an aryl group having from 6 to 12 carbon atoms, which may be substituted, (for example, phenyl, naphthyl, tolyl, xylyl, propylphenyl, butylphenyl, octylphenyl, dodecylphenyl, methoxyphenyl, ethoxyphenyl, butoxyphenyl, decyloxyphenyl, chlorophenyl, dichlorophenyl, bromophenyl, cyanophenyl, acetylphenyl, methoxycarbonylphenyl, ethoxycarbonylphenyl, butoxycarbonylphenyl, acetamidophenyl, propionamidophenyl or dodecyloylamidophenyl).  
      Further, a, a′, b and b′ may be combined with each other to form a ring structure. The ring structure may contain a hetero atom (for example, an oxygen atom, a nitrogen atom or a sulfur atom). Further, the ring structure may be an onium form, for example, a cyclic ammonium group or a cyclic sulfonium group. Examples of the cyclic ammonium group include groups formed by quaternarization of a morpholino group, a piperidino group, a pyridyl group, an imidazolyl group, a quinolyl group or an oxazolyl group. Examples of the cyclic sulfonium group include groups formed by quaternarization of a thiophene group, a tetrahydrothiophene group, a benzothiophene group or a thiazole group.  
      The substituent represented by Y in formula (II) preferably includes a cyano group, a nitro group, a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom or a fluorine atom), —OR 1 , —COOR 1 , —CONHR 1  or —CONR 1 R 2 .  
      In the above formulae, R 1  and R 2 , which may be the same or different, each represents, for example, an aliphatic group having from 1 to 10 carbon atoms, which may be substituted, (for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, propenyl, butenyl, heptenyl, hexenyl, octenyl, decenyl, 2-hydroxyethyl, 2-hydroxypropyl, 2-methoxyethyl, 2-(methoxyethyloxy)ethyl, 2-(N,N-dietylamino)ethyl, 2-methoxypropyl, 2-cyanoethyl, 3-methyloxypropyl, 2-chloroethyl, benzyl, phenethyl, dimethoxybenzyl, methylbenzyl or bromobenzyl), or an alicyclic group having from 5 to 8 carbon atoms, which may be substituted, (for example, cyclohexyl, cyclopentyl, cyclooctyl, chlorocyclohexyl, methoxycyclohexyl, 2-cyclohexylethyl group or 2-cyclopentylethyl group).  
      The polyester, which satisfies the above-described specific physical property conditions according to the invention, can be obtained, for example, by condensation polymerization of a diol (preferably a compound represented by formula (I)) with a dicarboxylic acid (preferably a compound represented by formula (II)) by a known method.  
      In the first embodiment of the ink composition according to the invention, the polyesters satisfying the above-described specific physical property conditions may be used individually or in combination of two or more thereof, as a coating agent.  
      In a second embodiment of the ink composition according to the invention, the charged particle is characterized by containing a polyethylene derivative having at least one group selected from an alkyl group having from 4 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms and a bridged cyclic hydrocarbon group having up to 22 carbon atoms.  
      In the invention, a molecular weight (GPC weight average) of the polyethylene derivative is specifically from 1,000 to 200,000, preferably from 2,000 to 150,000, and more preferably from 3,000 to 120,000. A melting point (Tm) of the polyethylene derivative is specifically from 30 to 100° C., preferably from 40 to 90° C., and more preferably from 40 to 80° C. A glass transition point (Tg) of the polyethylene derivative is specifically from −70 to 100° C., preferably from −50 to 90° C., and more preferably from −40 to 80° C.  
      In order to pulverize and disperse a pigment-polymer mixture by applying shear force as in the system of the second embodiment of the invention, it is first of all necessary that the polymer is broken due to force generated from a dispersing machine. However, in the case of a soft polymer, for example, a conventional polyethylene, which includes a crystalline portion and a noncrystalline portion and in which the noncrystalline portion has a small rotation barrier of main chain, since rubber elastic remarkably appears and thus sear force for pulverizing the polymer is adsorbed and distributed, good dispersibility cannot be obtained. Accordingly, for the purpose of reducing the rubber elastic, a monomer having a large rotation barrier of main chain is copolymerized to restrain crystallization and to increase the rotation barrier of main chain in the noncrystalline portion, thereby making the polymer hard. As a result, the polymer is efficiently pulverized. Further, by introducing a functional group having high affinity to a low-polar solvent (a hydrocarbon) as in the invention into the copolymerizable monomer, the polymer pulverized is more stably dispersed in the low-polar solvent so that the desired pigment-polymer mixture particles can be obtained.  
      The polyethylene derivative, which is used as a coating agent (coating polymer) for a colorant in the second embodiment of the invention, is not particularly restricted as far as it has the above-described specific group. Preferably, the polyethylene derivative comprises repeating units represented by formula (III) or (IV) shown below.  
                 
 
      In formulae (III) and (IV), m, n and l each independently represents a positive integer, provided that m+n=100% by mole in formula (III) and that m+n+l=100% by mole in formula (IV).  
      a 1 , a 2 , and b 1  each independently represents a hydrogen atom or a hydrocarbon group, or may be combined with each other to form a ring structure. L represents a single bond or a divalent linking group comprising two or more atoms selected from C, H, N, O, S and P and having a total number of atoms of 50 or less. Q represents an alkyl group having from 4 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms or a bridged cyclic hydrocarbon group having up to 22 carbon atoms.  
      In formulae (III) and (IV), a 1 , a 2 , and b 1 , which may be the same of different, each represents a hydrogen atom or a hydrocarbon group. The hydrocarbon group for a 1 , a 2 , or b 1  preferably represents an alkyl group having from 1 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms or a bridged cyclic hydrocarbon group having up to 22 carbon atoms, and each of the groups may be substituted.  
      More preferred examples of the hydrocarbon group for a 1 , a 2 , or b 1  include an alkyl group having from 1 to 22 carbon atoms, which may be substituted, (for example, methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, 2-chloroethyl, 2-bromoethyl, 2-cyanoethyl, 2-methoxycarbonylethyl, 2-methoxyethyl or 3-bromopropyl), an alkenyl group having from 4 to 18 carbon atoms, which may be substituted, (for example, 2-methyl-1-propenyl, 2-butenyl, 2-pentenyl, 3-methyl-2-pentenyl, 1-pentenyl, 1-hexenyl, 2-hexenyl, 4-methyl-2-hexenyl, decenyl, dodecenyl, tridecenyl, hexadecenyl, octadecenyl or linolenyl), an aralkyl group having from 7 to 12 carbon atoms, which may be substituted, (for example, benzyl, phenethyl, 3-phenylpropyl, naphthylmethyl, 2-naphthylethyl, chrolobenzyl, bromobenzyl, methylbenzyl, ethylbenzyl, methoxybenzyl, dimethylbenzyl or dimethoxybenzyl), an alicyclic group having from 5 to 8 carbon atoms, which may be substituted, (for example, cyclohexyl group, 2-cyclohexylethyl group or 2-cyclopentylethyl group), or an aryl group having from 6 to 12 carbon atoms, which may be substituted, (for example, phenyl, naphthyl, tolyl, xylyl, propylphenyl, butylphenyl, octylphenyl, dodecylphenyl, methoxyphenyl, ethoxyphenyl, butoxyphenyl, decyloxyphenyl, chlorophenyl, dichlorophenyl, bromophenyl, cyanophenyl, acetylphenyl, methoxycarbonylphenyl, ethoxycarbonylphenyl, butoxycarbonylphenyl, acetamidophenyl, propionamidophenyl or dodecyloylamidophenyl).  
      Further, a 1 , a 2 , and b 1  may be combined with each other to form a ring structure. The ring structure may contain a hetero atom (for example, an oxygen atom, a nitrogen atom or a sulfur atom). Further, the ring structure may be an onium form, for example, a cyclic ammonium group or a cyclic sulfonium group. Examples of a cyclic ammonium group include groups formed by quaternarization of a morpholino group, a piperidino group, a pyridyl group, an imidazolyl group, a quinolyl group or an oxazolyl group. Examples of a cyclic sulfonium group include groups formed by quaternarization of a thiophene group, a tetrahydrothiophene group, a benzothiophene group or a thiazole group.  
      In formulae (III) and (IV), L represents a single bond or a divalent linking group comprising two or more atoms selected from C, H, N, O, S and P and having a total number of atoms of 50 or less. L preferably represents a linking group of —COO—, —OCO—, —(CH 2 ) k —OCO—, —(CH 2 ) k —COO—, —CONH— or —CONR 1 —. In the above formulae, k represents an integer of from 1 to 3, and R 1  represents an aliphatic group having from 1 to 10 carbon atoms, which may be substituted, (for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, propenyl, butenyl, heptenyl, hexenyl, octenyl, decenyl, 2-hydroxyethyl, 2-hydroxypropyl, 2-methoxyethyl, 2-(methoxyethyloxy)ethyl, 2-(N,N-dietylamino)ethyl, 2-methoxypropyl, 2-cyanoethyl, 3-methyloxypropyl, 2-chloroethyl, benzyl, phenethyl, dimethoxybenzyl, methylbenzyl or bromobenzyl), or an alicyclic group having from 5 to 8 carbon atoms, which may be substituted, (for example, cyclohexyl, cyclopentyl, cyclooctyl, chlorocyclohexyl, methoxycyclohexyl, 2-cyclohexylethyl group or 2-cyclopentylethyl group).  
      In formulae (III) and (IV), Q represents an alkyl group having from 4 to 22 carbon atoms, an alkenyl group having up to 22 carbon atoms, an aralkyl group having up to 22 carbon atoms, an alicyclic group having up to 22 carbon atoms, an aryl group having up to 22 carbon atoms or a bridged cyclic hydrocarbon group having up to 22 carbon atoms, and each of the groups may be substituted. Preferred examples of the group for Q include an alkyl group having from 4 to 22 carbon atoms, which may be substituted, (for example, butyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, stearyl, 4-chlorobutyl, 4-bromohexyl, 6-cyanohexyl, 4-methoxycarbonylbutyl, 6-methoxyhexyl or 6-bromohexyl), an alkenyl group having from 4 to 18 carbon atoms, which may be substituted, (for example, 2-methyl-1-propenyl, 2-butenyl, 2-pentenyl, 3-methyl-2-pentenyl, 1-pentenyl, 1-hexenyl, 2-hexenyl, 4-methyl-2-hexenyl, decenyl, dodecenyl, tridecenyl, hexadecenyl, octadecenyl or linolenyl), an aralkyl group having from 7 to 12 carbon atoms, which may be substituted, (for example, benzyl, phenethyl, 3-phenylpropyl, naphthylmethyl, 2-naphthylethyl, chrolobenzyl, bromobenzyl, methylbenzyl, ethylbenzyl, methoxybenzyl, dimethylbenzyl or dimethoxybenzyl), an alicyclic group having from 5 to 8 carbon atoms, which may be substituted, (for example, cyclohexyl group, 2-cyclohexylethyl group or 2-cyclopentylethyl group), or an aryl group having from 6 to 12 carbon atoms, which may be substituted, (for example, phenyl, naphthyl, tolyl, xylyl, propylphenyl, butylphenyl, octylphenyl, dodecylphenyl, methoxyphenyl, ethoxyphenyl, butoxyphenyl, decyloxyphenyl, chlorophenyl, dichlorophenyl, bromophenyl, cyanophenyl, acetylphenyl, methoxycarbonylphenyl, ethoxycarbonylphenyl, butoxycarbonylphenyl, acetamidophenyl, propionamidophenyl or dodecyloylamidophenyl). Examples of the bridged cyclic hydrocarbon group include a norbornyl group or an adamantyl group.  
      In formulae (III) and (IV), m, n and l each independently represents a positive integer, provided that m+n=100% by mole in formula (III) and that m+n+l=100% by mole in formula (IV).  
      Preferably, m represents an integer of a range of from 20 to 95% by mole, n represents an integer of a range of from 5 to 80% by mole, and 1 represents an integer of a range of from 1 to 20% by mole.  
      In the second embodiment of the ink composition according to the invention, the above-described specific polyethylene derivatives may be used individually or in combination of two or more thereof, as a coating agent.  
      A volume average diameter of the charged particles can be measured by a centrifugal sedimentation method using, for example, a super-centrifugal type automatic particle size distribution measuring apparatus (CAPA-700, manufactured by Horiba, Ltd.). The volume average diameter of the charged particles is preferably in a range of from 0.8 to 2.0 μm in the invention. In the above-described range, a sufficient amount of charging per ink particle is made possible on the surface of ink particle and a stable ink in which variation of charge is prevented with the lapse of time can be obtained.  
      According to the invention, it is preferred that the charged particles are prepared by a mechanical media dispersion method and that a content of the charged particles having a diameter corresponding to ⅕ or less of a volume average diameter of the charged particles measured by means of the centrifugal sedimentation method is 1% by volume or less, more preferably 0.5% by volume or less, based on the total charged particles. The mechanical media dispersion method means a method wherein kinetic energy of beads, for example, glass or zirconia is converted to shearing impact force by the collision of beads to pulverize a dispersion to smaller particles, thereby obtaining the desired dispersion. As an apparatus for moving the beads, for example, a paint shaker, a Dyno-mill is employed.  
      The ink composition according to the invention contains a dispersion medium and a charge particle containing at least a colorant. Each of the components will be described in detail below.  
      Dispersion Medium  
      The dispersion medium is preferably a dielectric liquid having a high electric resistance, specifically 10 10  Ωcm or more. A dispersion medium having a low electric resistance is not suitable for the invention since such a dispersion medium causes electric conduction between recording electrodes adjacent to each other. The dielectric liquid preferably has a specific dielectric constant of 5 or less, more preferably 4 or less, and still more preferably 3.5 or less. To control the specific dielectric constant of dielectric liquid in such a range is preferred since an electric filed is efficiently applied to the charged particles in the dielectric liquid.  
      Examples of the dispersion medium used in the invention include a straight chain or branched aliphatic hydrocarbon, an alicyclic hydrocarbon, an aromatic hydrocarbon, halogen-substituted products of these hydrocarbons, and a silicone oil. Specific examples thereof include hexane, heptane, octane, isooctane, decane, isodecane, decalin, nonane, dodecane, isododecane, cyclohexane, cyclooctane, cyclodecane, toluene, xylene, mesitylene, Isopar C, Isopar E, Isopar G, Isopar H, Isopar L and Isopar M (“Isopar” is a brand name of Exxon Corp.), Shellsol 70 and Shellsol 71 (“Shellsol” is a brand name of Shell Oil Co.), Amsco OMS and Amsco 460 solvent (“Amsco” is a brand name of American Mineral Spirits Corp.), and KF-96L (brand name of Shin-Etsu Silicone Co., Ltd.), which may be used individually or as a mixture.  
      The content of the dispersion medium in the whole ink composition is preferably in a range of from 20 to 99% by weight. The particles containing a colorant can be well dispersed in the dispersion medium with the content of the dispersion medium of 20% by weight or more, and the content of a colorant is sufficient with the content of the dispersion medium of 99% by weight or less.  
      Colorant  
      Known dyes and pigments can be used as the colorant for use in the invention, and are appropriately selected depending on use and purpose. For instance, from the standpoint of color tone of a recorded image material (printed material), a pigment is preferably used (as described, for example, in “Ganryo Bunsan Anteika to Hyomen Shori Gijutu-Hyoka” (Pigment Dispersion Stabilization and Surface Treatment Technique and Evaluation), First Edition, published by Gijutsu Joho Kyokai Co., Ltd. (Dec. 25, 2001), which is hereinafter sometimes referred to as Non-patent Document 1). Inks of four colors, i.e., yellow, magenta, cyan and black, can be prepared by changing the colorant. In particular, pigments that are used in offset printing inks or proofs are preferably used, because color tones similar to offset printed materials can be obtained.  
      Examples of the pigment for a yellow ink include a monoazo pigment, for example, C.I. Pigment Yellow 1 or C.I. Pigment Yellow 74, a disazo pigment, for example, C.I. Pigment Yellow 12 or C.I. Pigment Yellow 17, a non-benzidine azo pigment, for example, C.I. Pigment Yellow 180, an azo lake pigment, for example, C.I. Pigment Yellow 100, a condensed azo pigment, for example, C.I. Pigment Yellow 95, an acidic dye lake pigment, for example, C.I. Pigment Yellow 15, a basic dye lake pigment, for example, C.I. Pigment Yellow 18, an anthraquinone pigment, for example, Flavanthrone Yellow, an isoindolinone pigment, for example, Isoindolinone Yellow 3RLT, a quinophthalone pigment, for example, Quinophthalone Yellow, an isoindoline pigment, for example, Isoindoline Yellow, a nitroso pigment, for example, C.I. Pigment Yellow 153, a metallic complex azomethine pigment, for example, C.I. Pigment Yellow 117, and an isoindolinone pigment, for example, C.I. Pigment Yellow 139.  
      Examples of the pigment for a magenta ink include a monoazo pigment, for example, C.I. Pigment Red 3, a disazo pigment, for example, C.I. Pigment Red 38, an azo lake pigment, for example, C.I. Pigment Red 53:1 or C.I. Pigment Red 57:1, a condensed azo pigment, for example, C.I. Pigment Red 144, an acidic dye lake pigment, for example, C.I. Pigment Red 174, a basic dye lake pigment, for example, C.I. Pigment Red 81, an anthraquinone pigment, for example, C.I. Pigment Red 177, a thioindigo pigment, for example, C.I. Pigment Red 88, a perynone pigment, for example, C.I. Pigment Red 194, a perylene pigment, for example, C.I. Pigment Red 149, a quinacridone pigment, for example, C.I. Pigment Red 122, an isoindolinone pigment, for example, C.I. Pigment Red 180, and an alizarin lake pigment, for example, C.I. Pigment Red 83.  
      Examples of the pigment for a cyan ink include a disazo pigment, for example, C.I. Pigment Blue 25, a phthalocyanine pigment, for example, C.I. Pigment Blue 15, an acidic dye lake pigment, for example, C.I. Pigment Blue 24, a basic dye lake pigment, for example, C.I. Pigment Blue 1, an anthraquinone pigment, for example, C. Pigment Blue 60, and an alkali blue pigment, for example, C.I. Pigment Blue 18.  
      Examples of the pigment for a black ink include an organic pigment, for example, an aniline black pigment, an iron oxide pigment, and a carbon black pigment, for example, furnace black, lamp black, acetylene black and channel black.  
      A processed pigment represented by a Microlith pigment, for example, Microlith-A, -K or -T, can also be preferably used. Specific examples thereof include Microlith Yellow 4G-A, Microlith Red BP-K, Microlith Blue 4G-T and Microlith Black C-T.  
      Various kinds of other pigments may be used, if desired, for example, calcium carbonate or titanium oxide as a pigment for a white ink, aluminum powder for a silver ink, and a copper alloy for a gold ink.  
      It is preferred that only one kind of a pigment is essentially used for one color from the standpoint of simplicity in the production of ink, but in some cases, two or more kinds of pigments are preferably used in combination. For instance, phthalocyanine is mixed with carbon black to produce a black ink. The pigment may be used after subjecting to a surface treatment by a known method, for example, a rosin treatment (as described in Non-patent Document 1 above).  
      The content of the pigment in the whole ink composition is preferably in a range of from 0.1 to 50% by weight. The pigment amount is sufficient to provide good coloration on printed material with the content of 0.1% by weight or more, and the particles containing the colorant can be dispersed in the dispersion medium in good condition with the content of 50% by weight or less. The content of the colorant is more preferably from 1 to 30% by weight.  
      Coating Agent  
      In the invention, it is preferred that the colorant, for example, a pigment is dispersed (reduced to particles) in the dispersion medium in the state coated with a coating agent rather than the colorant is directly dispersed (reduced to particles) therein. The charge owned by the colorant can be shielded by coating with the coating agent, whereby the desired charging characteristics can be imparted. Further, in the invention, after the inkjet recording onto a recording medium, the image thus recorded is fixed with heating means, for example, a heat roller, and at that time the coating agent is melted by heat to fix the image efficiently.  
      As the coating agent, in addition to the polyester satisfying the specific physical property conditions or the specific polyethylene derivative described above, for example, a rosin compound, a rosin-modified phenol resin, an alkyd resin, a (meth)acrylic polymer, polyurethane, polyester, polyamide, polyethylene, polybutadiene, polystyrene, polyvinyl acetate, an acetal-modified product of polyvinyl alcohol or polycarbonate may be used together. Among these, a polymer having a weight average molecular weight of from 2,000 to 1,000,000 and a polydispersion degree (weight average molecular weight/number average molecular weight) of from 1.0 to 5.0 is preferred in view of easiness in particle formation. Furthermore, a polymer having any one of a softening point, a glass transition point and a melting point of from 40 to 120° C. is preferred from the standpoint of easiness in fixation.  
      Examples of the polymer used as the coating agent, together with the polyester satisfying the specific physical property conditions or the specific polyethylene derivative in the invention include a polymer containing at least one of constituting units represented by the following formulae (1) to (4).  
                 
 
      In the formulae, X 11  represents an oxygen atom or —N(R 13 )—; R 11  represents a hydrogen atom or a methyl group; R 12  represents a hydrocarbon group having from 1 to 30 carbon atoms; R 13  represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms; R 21  represents a hydrogen atom or a hydrocarbon group having from 1 to 20 carbon atoms; and R 31 , R 32  and R 41  each independently represents a divalent hydrocarbon group having from 1 to 20 carbon atoms. The hydrocarbon group represented by any one of R 12 , R 21 , R 31 , R 32  and R 41  may contain an ether bond, an amino group, a hydroxy group or a halogen atom.  
      The polymer having the constituting unit represented by formula (1) can be obtained by radical polymerization of a corresponding radical polymerizable monomer according to a known method. Examples of the radical polymerizable monomer include a (meth)acrylate, for example, methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, hexyl (meth)acrylate, octyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl(meth)acrylate, stearyl (meth)acrylate, cyclohexyl(meth)acrylate, phenyl (meth)acrylate, benzyl(meth)acrylate or 2-hydroxyethyl (meth)acrylate, and a (meth)acrylamide, for example, N-methyl(meth)acrylamide, N-propyl(meth)acrylamide, N-phenyl(meth)acrylamide or N,N-dimethyl(meth)acrylamide.  
      The polymer having the constituting unit represented by formula (2) can be obtained by radical polymerization of a corresponding radical polymerizable monomer according to a known method. Examples of the radical polymerizable monomer include ethylene, propylene, butadiene, styrene and 4-methylstyrene.  
      The polymer having the constituting unit represented by formula (3) can be obtained by dehydration condensation of a corresponding dicarboxylic acid or acid anhydride with a diol according to a known method. Examples of the dicarboxylic acid and acid anhydride include succinic anhydride, adipic acid, sebacic acid, isophthalic acid, terephthalic acid, 1,4-phenylenediacetic acid and diglycolic acid. Examples of the diol include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,10-decanediol, 2-butene-1,4-diol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 1,4-benzenedimethanol and diethylene glycol.  
      The polymer having the constituting unit represented by formula (4) can be obtained by dehydration condensation of a corresponding carboxylic acid having a hydroxy group according to a known method, or by ring-opening polymerization of a cyclic ester of a corresponding carboxylic acid having a hydroxy group according to a known method. Examples of the carboxylic acid having a hydroxy group and cyclic ester thereof include 6-hydroxyhexanoic acid, 11-hydroxyundecanoic acid, hydroxybenzoic acid and ε-caprolactone.  
      The polymer containing at least one constituting unit represented by any one of formulae (1) to (4) may be a homopolymer of the constituting unit represented by any one of formulae (1) to (4), or may be a copolymer with other constituting component. The polymers may be used in combination of two or more thereof.  
      The content of the coating agent in the whole ink composition is preferably in a range of from 0.1 to 40% by weight. The amount of the coating agent is sufficient to provide satisfactory fixing property with the content of 0.1% by weight or more, and particles containing the colorant and the coating agent can be produced in good condition with the content of 40% by weight or less.  
      In the case of using the polyester satisfying the specific physical property conditions or the specific polyethylene derivative described above together with other coating agent, a content of the other coating agent does not exceed 40% by weight, preferably not exceed 30% by weight, based on the total amount of the coating agent.  
      Dispersing Agent  
      According to the invention, a mixture of the colorant and the coating agent is preferably dispersed (reduced to particles) in the dispersion medium. It is more preferred to use a dispersing agent for the purpose of controlling the particle diameter and preventing the precipitation of particles.  
      Preferred examples of the dispersing agent include a surface active agent represented by a sorbitan fatty acid ester, for example, sorbitan monooleate, and a polyethylene glycol fatty acid ester, for example, polyoxyethylene distearate. Examples thereof further include a copolymer of styrene and maleic acid and an amine-modified product thereof, a copolymer of styrene and (meth)acrylic compound, a (meth)acrylic polymer, a copolymer of ethylene and (meth)acrylic compound, rosin, BYK-160, 162, 164 and 182 (brand names of polyurethane polymers, produced by BYK Chemie GmbH), EFKA-401 and 402 (brand names of acrylic polymers, produced by EFKA Additives B.V.), and Solsperse 17000 and 24000 (brand names of polyester polymers, produced by Zeneca PLC) According to the invention, such a polymer having a weight average molecular weight of from 1,000 to 1,000,000 and a polydispersion degree (weight average molecular weight/number average molecular weight) of from 1.0 to 7.0 is preferably used from the standpoint of storage stability of the ink composition for a long period of time. A graft polymer and a block polymer are most preferably used.  
      Particularly preferred examples of the polymer used as the dispersing agent in the invention include a graft polymer comprising a polymer component containing at least one of constituting units represented by formulae (5) and (6) shown below and a polymer component containing at least a graft chain containing a constituting unit represented by formula (7) shown below.  
                 
 
      In the formulae, X 51  represents an oxygen atom or —N(R 53 )—; R 51  represents a hydrogen atom or a methyl group; R 52  represents a hydrocarbon group having from 1 to 10 carbon atoms; R 53  represents a hydrogen atom or a hydrocarbon group having from 1 to 10 carbon atoms; R 61  represents a hydrogen atom, a hydrocarbon group having from 1 to 20 carbon atoms, a halogen atom, a hydroxy group or an alkoxy group having from 1 to 20 carbon atoms; X 71  represents an oxygen atom or —N(R 73 )—; R 71  represents a hydrogen atom or a methyl group; R 72  represents a hydrocarbon group having from 4 to 30 carbon atoms; and R 73  represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms. The hydrocarbon group represented by any one of R 52  and R 72  may contain an ether bond, an amino group, a hydroxyl group or a halogen atom.  
      The graft polymer can be obtained in such a manner that a radical polymerizable monomer corresponding to formula (7) is polymerized, preferably in the presence of a chain transfer agent, a polymerizable functional group is introduced into a terminal of the resulting polymer, and the macromonomer thus formed is then copolymerized with a radical polymerizable monomer corresponding to any one of formulae (5) and (6).  
      Examples of the radical polymerizable monomer corresponding to formula (5) include a (meth)acrylate, for example, methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, hexyl (meth)acrylate, cyclohexyl(meth)acrylate, phenyl (meth)acrylate, benzyl(meth)acrylate or 2-hydroxyethyl (meth)acrylate, and a (meth)acrylamide, for example, N-methyl(meth)acrylamide, N-propyl (meth)acrylamide, N-phenyl(meth)acrylamide or N,N-dimethyl(meth)acrylamide.  
      Examples of the radical polymerizable monomer corresponding to formula (6) include styrene, 4-methylstyrene, chlorostyrene and methoxystyrene.  
      Examples of the radical polymerizable monomer corresponding to formula (7) include hexyl(meth)acrylate, octyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, dodecyl(meth)acrylate and stearyl(meth)acrylate.  
      Specific examples of the graft polymer include polymers represented by the following structural formulae.  
                 
 
      The graft polymer comprising a polymer component containing at least one of constituting units represented by formulae (5) and (6) and a polymer component containing at least a graft chain containing a constituting unit represented by formula (7) may contain only the constituting units represented by formulae (5) and/or (6) and formula (7), and may also contain other constituting component. The ratio of the polymer component containing the graft chain and the other polymer component is preferably in a range of from 10/90 to 90/10 by weight. The range is preferred since formation of particles can be attained in good condition and the desired particle diameter can be easily obtained. The polymers may be used individually or in combination of two or more thereof, as the dispersing agent.  
      The content of the dispersing agent in the whole ink composition is preferably in a range of from 0.01 to 30% by weight. Within such a range, the formation of particles can be attained in good condition and the desired particle diameter can be obtained.  
      Charge Controlling Agent  
      According to the invention, a mixture of the colorant and the coating agent is preferably dispersed (reduced to particles) in the dispersion medium using the dispersing agent. It is more preferred to use together a charge controlling agent for the purpose of controlling the charge amount of particles.  
      Preferred examples of the charge controlling agent include metal salts of organic carboxylic acids, for example, zirconium naphthenate and zirconium octenate, ammonium salts of organic carboxylic acids, for example, tetramethylammonium stearate, metal salts of organic sulfonic acids, for example, sodium dodecylbenzenesulfonate and magnesium dioctylsulfosuccinate, ammonium salts of organic sulfonic acids, for example, tetrabutylammonium toluenesulfonate, polymers having carboxylic acid groups in the side chains thereof, for example, a polymer containing carboxylic acid groups obtained by modification of a copolymer of styrene and maleic anhydride with an amine, polymers having carboxylic acid anion groups in the side chains thereof, for example, a copolymer of stearyl methacrylate and tetramethylammonium methacrylate, polymers having nitrogen atoms in the side chains thereof, for example, a copolymer of styrene and vinyl pyridine, and polymers having ammonium groups in the side chains thereof, for example, a copolymer of butyl methacrylate and N-(2-methacroyloxyethyl)-N,N,N-trimethylammonium tosilate. The charge to be applied to the particle may be positive charge or negative charge. The content of the charge controlling agent in the whole ink composition is preferably in a range of from 0.0001 to 10% by weight. In the above-described range, electric conductivity of the ink composition can be easily controlled in a rage of from 10 to 300 nS/m. Further, electric conductivity of the charged particle can be easily controlled 50% or more of the electric conductivity of the whole ink composition.  
      Other Components  
      According to the invention, other components, for example, an antiseptic agent for preventing decomposition or a surface active agent for controlling surface tension may further be incorporated into the ink composition depending on purposes.  
      Preparation of Charged Particles  
      The ink composition containing the charged particles according to the invention can be prepared by dispersing (reducing to particles) the colorant and preferably the coating agent, if desired, together with the above-described components. Examples of the method for dispersing (reducing to particles) include the following methods.  
      (1) The colorant and the coating agent are mixed, the mixture is dispersed (reduced to particles) by using the dispersing agent and the dispersion medium, and then the charge controlling agent is added to the resulting dispersion.  
      (2) The colorant, the coating agent, the dispersing agent and the dispersion medium are simultaneously dispersed (reduced to particles), and then the charge controlling agent is added to the resulting dispersion.  
      (3) The colorant, the coating agent, the dispersing agent, the charge controlling agent and the dispersion medium are simultaneously dispersed (reduced to particles).  
      Examples of an apparatus for use at the mixing or dispersing include a kneader, a disolver, a mixer, a high-speed disperser, a sand mill, a roll mill, a ball mill, an attritor and a beads mill (as described in Non-patent Document 1 described above).  
      Inkjet Recording Apparatus  
      According to the invention, the ink composition described above is used for recordation on a recording medium by an inkjet recording system. In the invention, it is preferred to use an inkjet recording system utilizing an electrostatic field. In the inkjet recording system utilizing an electrostatic field, a voltage is applied between a control electrode and a back electrode positioned on the back side of the recording medium, whereby the charged particles in the ink composition are concentrated at an ejection position through an electrostatic force to cause the ink composition to fly from the ejection position to the recording medium. With respect to the voltage applied between the control electrode and the back electrode, in case of using the charged particles having positive charge, for example, the control electrode acts as a positive electrode and the back electrode acts as a negative electrode. The same effect can be obtained by charging the recording medium instead of the application of voltage to the back electrode.  
      Examples of the method for flying an ink include a method of flying an ink from a tip of a member having a needle shape such as an injection needle, which can be used for recordation with the ink composition according to the invention. In the method, however, replenishment of the charged particles after the concentration of charged particles and ejection is difficult, and thus it is difficult to stably conduct the recordation for a long period of time. Since the charged particles are forcedly supplied in the method, the ink is overspilled from the tip of the injection needle in the case of circulating the ink. Accordingly, the meniscus shape at the tip of the injection needle at the ejection position is not stabilized to make stable recordation difficult. Therefore, the method is suitable for recordation for a short period of time.  
      On the contrary, a method in which the ink composition is circulated without spillover of the ink composition from an ejection opening is preferably used. For instance, a method wherein an ink is circulated in an ink chamber having an ejection opening and a voltage is applied to a control electrode formed around the ejection opening to cause concentrated ink droplets to fly from a tip of an ink guide disposed in the ejection opening and directed to a recording medium simultaneously satisfies both the replenishment of the charged particles by circulation of the ink composition and the stabilization of the meniscus at the ejection position. Thus, the method is capable of perform stable recordation for a long period of time. Furthermore, since the ink comes in contact with the outside air only at a significantly small area, i.e., the ejection opening, the solvent can be prevented from being evaporated to stabilize the physical property of the ink composition. Accordingly, the method is preferably used in the invention.  
      An example of a construction of an inkjet recording apparatus suitable for application of the ink composition according to the invention will be described below.  
      An apparatus for performing four color printing on one side of a recording medium as shown in  FIG. 1  will be described below. The inkjet recording apparatus  1  shown in  FIG. 1  has an ejection head  2  for conducting full color image formation constituted by ejection heads  2 C,  2 M,  2 Y and  2 K for four colors, an ink circulation system  3  for supplying an ink to the ejection head  2  and recovering the ink from the ejection head  2 , a head driver  4  for driving the ejection head  2  based on output from an external device, for example, a computer or RIP, which is not shown, and a position controlling means  5 . The inkjet recording apparatus  1  also has a conveying belt  7  stretched with three rollers  6 A,  6 B and  6 C, a conveying belt position detecting means  8  constituted by an optical sensor or the like capable of detecting the position in the width direction of the conveying belt  7 , an electrostatic adsorption means  9  for retaining a recording medium P on the conveying belt  7 , and a static eliminating means  10  and a mechanical means  11  for releasing the recording medium P from the conveying belt  7  after the completion of image formation. A feed roller  12  and a guide  13  for feeding the recording medium P from a paper stock, which is not shown, to the conveying belt  7  are disposed on the upstream side of the conveying belt  7 , and an image fixing means  14  and a guide  15  for fixing the ink on the recording medium P after releasing and conveying the recording medium P to a paper stocker, which is not shown, are disposed on the downstream side of the conveying belt  7 . The inkjet recording apparatus  1  has a recording medium position detecting means  16  at a position opposite to the ejection head with respect to the conveying belt  7 , and a solvent recover part containing an exhaust fan  17  and a solvent vapor adsorbent  18  for recovering a solvent vapor generated from the ink composition, by which the vapor inside the apparatus is exhausted to the exterior of the apparatus through the solvent recover part.  
      The feed roller  12  is disposed to improve feeding capability of the recording medium. As the feed roller, a known roller may be used. Since the recording medium P often has dusts and paper powder attached thereon, it is desired to remove these materials. The recording medium P thus fed by the feed roller  12  is conveyed to the conveying belt  7  through the guide  13 . The back surface (preferably a metallic back surface) of the conveying belt  7  is disposed through the roller  6 A. The recording medium thus conveyed is electrostatically adsorbed on the conveying belt with the electrostatic adsorption means  9 . In the embodiment shown in  FIG. 1 , the electrostatic adsorption is attained by a scorotron charging device connected to a negative high voltage electric source. The recording medium P is electrostatically adsorbed on the conveying belt  7  without space and is uniformly charged over the surface thereof by the electrostatic adsorption means  9 . While the electrostatic adsorption means is also used as a charging means of the recording medium in this embodiment, these means may be separately provided. The recording medium P thus charged is conveyed by the conveying belt  7  to the position of the ejection head, and recording signal voltage is superposed on the charged potential as bias to attain electrostatic inkjet image formation. The recording medium P having the image thereon is subjected to elimination of static by the static eliminating means  10  and released from the conveying belt  7  by the mechanical means  11 , followed by being conveyed to the fixing part. The recording medium P thus released is delivered to the image fixing means  14  for fixing. The recording medium P thus fixed is delivered to the paper stocker, which is not shown. The apparatus has a recovery means for the solvent vapor generated from the ink composition. The recovery means has the solvent vapor adsorbent  18 . The gas containing the solvent vapor inside the apparatus is introduced into the absorbent by the exhaust fan  17 , and after adsorbing and recovering the solvent vapor, the gas is exhausted to the exterior of the apparatus. The apparatus is not limited to the above-described embodiment, and the numbers, shapes, relative positions and charging polarities of the constituting devices including, for example, the roller and the charging device, can be appropriately selected. Further, while the four-color printing is attained in the above-described system, multi-color systems exceeding four colors may be constituted by combining a light-color ink and a special color ink.  
      The inkjet recording apparatus used in the inkjet printing system has the ejection head  2  and the ink circulation system  3 . The ink circulation system  3  has an ink tank, an ink circulation device, an ink concentration controlling device, an ink temperature controlling device and the like, and the ink tank may contain a stirring device therein.  
      As the ejection head  2 , a single channel head, a multi-channel head and a full-line head may be used, and the main scanning is carried out by movement of the conveying belt  7 .  
      An inkjet head that can be preferably used in the invention is one for such an inkjet system that the charged particles is electrophoresed in an ink flow channel to increase the ink concentration in the vicinity of the opening, so as to eject the ink, and the ejection of ink droplets is carried out mainly through an electrostatic attraction force caused by the recording medium or a counter electrode disposed on the back side of the recording medium. Therefore, in the case where the recording medium or the counter electrode does not face the head and in the case where no voltage is applied to the recording medium or the counter electrode even though they face the head, ink droplets are not ejected even when the voltage is accidentally applied to the ejection electrode or vibration is applied to the head, whereby the interior of the apparatus is prevented from being contaminated.  
      An ejection head that is preferably used in the above-described inkjet apparatus is shown in  FIGS. 2 and 3 . As shown in  FIGS. 2 and 3 , an inkjet head  70  has a substrate  74  electrically insulating and constituting an upper wall of an ink flow channel  72  forming a unidirectional ink flow Q, and plural ejection parts  76  ejecting the ink toward the recording medium P. The ejection part  76  is provided with an ink guide part  78  for guiding an ink droplet G flying from an ink flow channel  72  toward the recording medium P, and the substrate  74  has openings  75  through which the ink guide parts  78  penetrate, respectively. An ink meniscus  42  is formed between the ink guide part  78  and an inner wall of the opening  75 . A gap d between the ink guide part  78  and the recording medium P is preferably from about 200 to about 1,000 μm. The ink guide part  78  is fixed at the lower end thereof to a supporting bar 40.  
      The substrate  74  has an insulating layer  44  electrically insulating two ejection electrodes with a prescribed distance, a first ejection electrode  46  provided on the upper side of the insulating layer  44 , an insulating layer  48  covering the first ejection electrode  46 , a guard electrode  50  provided on the upper side of the insulating layer  48 , and an insulating layer  52  covering the guard electrode  50 . The substrate  74  also has a second ejection electrode  56  provided on the lower side of the insulating layer  44 , and an insulating layer  58  covering the second ejection electrode  56 . The guard electrode  50  is provided for preventing the adjacent ejection parts from the influence on electric field due to a voltage applied to the first ejection electrode  46  or the second ejection electrode  56 .  
      The inkjet head  70  also has a floating electroconductive plate  62  constituting a bottom surface of the ink flow channel  72  in an electrically floating state. The floating electroconductive plate  62  also works to electrophorese the positively charged ink particles (charged particles) in the ink flow channel  72  upward (i.e., toward the recording medium) with an induced voltage steadily generated by a pulsewise injection voltage applied to the first ejection electrode  46  and the second ejection electrode  56 . The floating electroconductive plate  62  has formed on the surface thereof a coating film  64  electrically insulating for preventing the physical property and the composition of the ink from being destabilized due to charge injection into the ink. The electrically insulating coating film preferably has an electric resistance of 10 12  Ω·cm or more, and more preferably 10 13  Ω·cm or more. The electrically insulating coating film is preferably corrosion resistant to the ink, whereby the floating electroconductive plate  62  is prevented from being corroded by the ink. The floating electroconductive plate  62  is covered from underneath with an insulating member  66 . According to the constitution, the floating electroconductive plate  62  is in a completely electrically insulating state.  
      At least one floating electroconductive plate  62  is provided on each of the unit heads. For example, in the case where four unit heads of C, M, Y and K are used, the unit heads each has at least one floating electroconductive plate, and the unit heads C and M, for example, do not have one floating electroconductive plate in common.  
      In order to fly the ink from the inkjet head  70  to record on the recording medium P, as shown in  FIG. 3 , a prescribed voltage (for example, +100 V) is applied to the guard electrode  50  in such a state that the ink is circulated in the ink flow channel  72  to form an ink flow Q. Further, a positive voltage is applied to the first ejection electrode  46 , the second ejection electrode  56  and the recording medium P to form such a flying electric field, among the first ejection electrode  46 , the second ejection electrode  56  and the recording medium P, that the positive charged particles R in the ink droplets G flying from the opening  75  as guided with the ink guide part  78  are attracted by the recording medium P. For example, in the case where the gap d is 500 μm, the voltage may be applied such an extent that a potential difference of from about 1 to about 3.0 kV is formed.  
      In the above-described state, a pulse voltage is applied to the first ejection electrode  46  and the second ejection electrode  56  according to the image signal, whereby the ink droplets G with an increased charge particle concentration are ejected from the opening  75 . For example, in the case where the initial charged particle concentration is from 3 to 15%, the charged particle concentration of the ink droplets G is 30% or more.  
      At that time, the voltage applied to the first ejection electrode  46  and the second ejection electrode  56  is previously adjusted in such a manner that the ink droplets G are ejected only when the pulse voltage is applied to both the first ejection electrode  46  and the second ejection electrode  56 .  
      Upon applying the pulsewise positive voltage, the ink droplets G fly from the opening  75  as guided by the ink guide part  78  to attach on the recording medium P, and at the same time, a positive induction voltage is generated in the floating electroconductive plate  62  by the positive voltage applied to the first ejection electrode  46  and the second ejection electrode  56 . Even in the case where the voltage applied to the first ejection electrode  46  and the second ejection electrode  56  has a pulsewise form, the induction voltage is a substantially steady voltage. Therefore, the positively charged particles R in the ink flow channel  72  receive a force of moving them upward by the electric field formed among the floating electroconductive plate  62 , the guard electrode  50  and the recording medium P, whereby the concentration of the charged particles R is increased in the vicinity of the substrate  74 . In the case where the number of the ejection parts (i.e., channels for ejecting ink droplets) used is large as shown in  FIG. 3 , the number of charged particles required for ejection is also increased. In such a case, the numbers of the first ejection electrodes  46  and the second ejection electrodes  56  used are also increased to generate a higher induction voltage in the floating electroconductive plate  62 , whereby the number of the charged particles R moving toward the recording medium is increased.  
      While the case where the colored particles are positively charged is described in the above embodiment, the colored particles may be negatively charged. In the later case, the charging polarities are all inverted.  
      It is preferred in the invention that after ejecting the ink on the recording medium, the ink is fixed by an appropriate heating means. Examples of the heating means used include a contact heating device, for example, a heating roller, a heating block and a heating belt, and a non-contact heating device, for example, a dryer, an infrared ray lamp, a visible ray lamp, an ultraviolet ray lamp and a hot air oven. The heating device is preferably provided continuously to the inkjet recording apparatus and integrated thereto. The temperature of the recording medium at the fixing is preferably in a range of from 40 to 200° C. from the standpoint of easiness of fixing. The period of time for fixing is preferably in a range of from 1 μsec to 20 seconds.  
      Replenishment of Ink Composition  
      In the inkjet recording system utilizing an electrostatic field, the charged particles in the ink composition is concentrated and ejected. Therefore, the amount of the charged particles in the ink composition is reduced after ejecting the ink composition for a long period of time to lower the electric conductivity of the ink composition. The ratio of the electric conductivity of the charged particles to the electric conductivity of the ink composition is also changed. Further, there is such a tendency that the charged particles having larger diameter are ejected before the charged particles having smaller diameter, and thus the average particle diameter of the charged particles is decreased. Moreover, the content of the solid matters in the ink composition is changed to vary the viscosity thereof.  
      The changes in physical properties of the ink composition result in ejection failure, and decrease in optical density and blur of ink occur in the image thus recorded. Accordingly, an ink composition having a higher concentration (a higher solid content concentration) than an ink composition initially charged in the ink tank is replenished to prevent decrease in the amount of the charged particles, whereby the electric conductivity of the ink composition and a ratio of the electric conductivity of the charged particles to the electric conductivity of the ink composition can be maintained within certain ranges, respectively. The average particle diameter of the charged particles and the viscosity of the ink composition can also be maintained. Furthermore, since the physical properties of the ink composition are maintained within certain ranges, the ejection of ink can be performed stably and uniformly for a long period of time. The replenishment is preferably carried out mechanically or by humans after the physical properties of the ink composition, for example, the electric conductivity or the optical density, are detected to calculate the necessary replenishing amount. The replenishment may also be carried out mechanically or by humans after calculation of an amount of the ink composition to be used based on an image data.  
      Recording Medium  
      In the invention, various kinds of recording media may be used depending on use. For example, a printed material can be directly obtained by inkjet recording on paper, a plastic film, a metal, paper having a plastic or a metal laminated or deposited thereon, or a plastic film having a metal laminated or deposited thereon. An offset printing plate can be obtained by using a metallic support, for example, aluminum, having a roughened surface. A flexographic printing plate and a color filter for a liquid crystal display can be obtained by using a plastic support. The recording medium may have a flat shape, for example, a sheet form, or a stereoscopic shape, for example, a cylindrical form. The invention can also be applied to the production of a semiconductor device and a printed circuit board by using a silicon wafer and a circuit board as the recording medium.  
      By using the ink composition of the invention, image recorded materials having a high image density and high image quality without blur of ink can be stably obtained for a long period of time.  
      The invention will be described in more detail with reference to the following examples, but the invention should not be construed as being limited thereto.  
      Preparation of Polyester  
      Polyester 1  
      In a three-necked flask were charged 58.15 g (0.35 mol) of m-phthalic acid (manufactured by Wako Pure Chemical Industries, Ltd.), 41.36 q (0.35 mol) of 1,6-hexanediol (manufactured by Wako Pure Chemical Industries, Ltd.), 121 g of xylene (manufactured by Wako Pure Chemical Industries, Ltd.) and 0.67 g of p-toluenesulfonic acid monohydrate (manufactured by Wako Pure Chemical Industries, Ltd.), and the mixture was refluxed in an oil bath to react by an azeotropic dehydration method for 24 hours. Then, the reaction solution was cooled to room temperature, and poured into 3 liters of methanol to purify by reprecipitation, followed by drying in vacuum to obtain 60 g of crystalline polyester. The resulting polyester had a weight average molecular weight (Mw) of 8,000 and a glass transition point of 65° C. Other physical properties thereof are shown in Table 1 below.  
      Polyesters 2 to 11 and a to d  
      Various polyesters as shown in Table 1 were prepared in the same manner as in Polyester 1 except for using equimolar amounts of carboxylic acid components and alcohol components as shown in Table 1, respectively. Physical properties of the resulting polyesters are also shown in Table 1.  
                                   TABLE 1                                          Ester   Molecular               Polyester Composition   Content   Weight                                         Polyester   Carboxylic Acid   Alcohol   Ratio   (GPC weight   Tg   G′       No.   Component   Component   (meq./g)   average)   (° C.)   (×10 4  Pa)                                                 1   m-Phthalic acid   1,6-Hexanediol   8.06   8,000   65   800       2   m-Phthalic acid   1,3-Propanediol   10.7   10,000   70   1,000       3   m-Phthalic acid   1,10-Decanediol   7.13   13,000   50   2,000       4   m-Phthalic acid   1,4-Cyclohexanedimethanol   7.90   5,000   72   500       5   p-Phthalic acid   1,6-Hexanediol   8.06   12,000   75   5,000       6   p-Phthalic acid   1,4-Butanediol   9.81   11,000   66   6,000       7   o-Phthalic acid   1,6-Hexanediol   8.06   4,500   53   300       8   m-Phthalic acid   1,6-Octanediol/1,4-   8.94   8,500   62   1,200               Butanediol = 50/50 mol %       9   m-Phthalic acid   1,6-Octanediol/1,3-   8.71   10,000   71   3,600               Butanediol/1,10-               Decanediol = 35/35/30 mol %       10   m-Phthalic acid   1,6-Octanediol/1,4-   7.98   9,000   67   4,500               Cyclohexanedimethanol =               50/50 mol %       11   m-Phthalic acid   1,6-Octanediol/1,3-   8.94   7,000   60   3,300               Butanediol/1,4-               Cyclohexanedimethanol =               35/35/30 mol %       a   Sebacic acid   1,4-Cyclohexanedimethanol   6.44   32,000   68   10,000       b   Succinic acid   1,4-Benzenedimethanol   9.81   12,000   22   1,300       c   Sebacic acid   1,2-Octadecanediol   4.38   20,000   78   18,000       d   1,2-   1,6-Octanediol   7.74   22,000   38   0.7           Cyclohexanedicarboxylic           acid                  
 
     EXAMPLE 1  
      Materials Used  
      The following materials were used in Example 1. 
      Cyan pigment (colorant): Phthalocyanine pigment, C.I. Pigment Blue (15:3) (LIONOL BLUE FG-7350, manufactured by Toyo Ink Mfg. Co., Ltd.)     Coating agent: [Polyester 1]    Dispersing agent: [BZ-2]    Charge controlling agent: [CT-1]    Dispersion medium: Isopar G (manufactured by Exxon Corp.)    

      The structures of Dispersing agent [BZ-2] and Charge controlling agent [CT-1] are shown below.  
                 
 
      Dispersing agent [BZ-2] was obtained by conducting radical polymerization of stearyl methacrylate in the presence of 2-mercaptoethanol, reacting the resulting polymer with methacrylic anhydride to obtain a stearyl methacrylate polymer having a methacryloyl group at the terminal thereof (having a weight average molecular weight of 7,600), and conducting radical polymerization of the polymer with styrene. Dispersing agent [BZ-2] had a weight average molecular weight of 110,000.  
      Charge controlling agent [CT-1] was obtained by reacting a copolymer of 1-octadecene and maleic anhydride with 1-hexadecylamine. Charge controlling agent [CT-1] had a weight average molecular weight of 17,000.  
      Preparation of Ink composition [EC-1] 
      In a desktop kneader (PBV-0.1, manufactured by Irie Shokai Co., Ltd.) were charged 10 g of the cyan pigment and 20 g of Coating agent [Polyester 1], and the components were mixed under heating at a heater temperature set at 100° C. for 2 hours. Thirty grams of the mixture thus obtained was coarsely pulverized in a trio blender (manufactured by Trio Science Co., Ltd.), and then finely pulverized in a sample mill (Model SK-M10, manufactured by Kyoritsu Riko Co., Ltd.). Thirty grams of the finely pulverized product thus obtained was preliminary dispersed in a paint shaker (manufactured by Toyo Seiki Seisako-Sho, Ltd.) together with 7.5 g of Dispersing agent (BZ-2), 75 g of Isopar G and glass beads having a diameter of about 3.0 mm. After removing the glass beads, the mixture was further dispersed (reduced to particles) together with zirconia ceramic beads having a diameter of about 0.6 mm in a Dino-mill (Type KDL, manufactured by Shinmaru Enterprises Corp.) at a rotation number of 2,000 rpm for 5 hours while maintaining an inner temperature at 25° C. and then further for 5 hours at 45° C. The zirconia ceramic beads were removed from the resulting dispersion liquid, then 316 g of Isopar G and 0.6 g of Charge controlling agent [CT-1] were added thereto to obtain Ink composition [EC-1].  
      Inkjet Recording/Image Evaluation  
      Ink composition [EC-1] described above was charged in an ink tank of an inkjet recording apparatus as shown in  FIG. 1 . An ejection head used was a 833-channel head of 150 dpi (three rows having a channel density of 50 dpi arranged in a stagger pattern) having a structure as shown in  FIG. 2 , and a fixing means used was a heat roller made of silicone rubber having a built-in 1 kW heater. An immersion heater and stirring blades were provided as an ink temperature controlling means in the ink tank, and the ink temperature was set at 30° C., which was controlled with a thermostat while rotating the stirring blades at 30 rpm. The stirring blades were also used as stirring means for preventing precipitation and aggregation. A part of the ink flow channel was made transparent, at outside of which an LED light emitting element and a photodetector element were provided, and based on the output signals therefrom, a diluent for ink (Isopar G) or a concentrated ink composition (which had twice the solid content concentration of the ink composition described above). Fine coated paper for offset printing was used as a recording medium. After removing dusts on the surface of the recording medium by suction with an air pump, the ejection head was moved to an image forming position closely to the recording medium. Image data to be recorded were transmitted to an image data operating and controlling part, and the ink composition was ejected with sequential movement of the ejection head while the recording medium was conveyed through rotation of a conveying belt so as to form an image with a drawing resolution of 2,400 dpi. The conveying belt used was a belt prepared by laminating a metallic belt and a polyimide film, and a linear marker was provided in the conveying direction near one side of the belt. The marker was optically read out by a conveying belt position detecting means, and a position controlling means was driven to conduct the image formation. The distance between the ejection head and the recording medium was maintained at 0.5 mm based on output from an optical gap detecting device. The surface potential of the recording medium at ejection was set at −1.5 kV, and a pulse voltage of +500 V (with a pulse width of 50 μsec) was applied at ejection to conduct the image recording with a driving frequency of 15 kHz. With the resulting gray scale image recorded product (printed material), degrees of streak unevenness and ink blur were evaluated (evaluation of inkjet drawing image).  
     COMPARATIVE EXAMPLE 1  
      Ink composition [RC-1] was prepared in the same manner as in Example 1 except for changing [Polyester 1] to [Polyester a] as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
     COMPARATIVE EXAMPLE 2  
      Ink composition [RC-2] was prepared in the same manner as in Example 1 except for changing [Polyester 1] to [Polyester b] as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
     COMPARATIVE EXAMPLE 3  
      Ink composition [RC-3] was prepared in the same manner as in Example 1 except for changing [Polyester 1] to [Polyester c] as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
     COMPARATIVE EXAMPLE 4  
      Ink composition [RC-4] was prepared in the same manner as in Example 1 except for changing [Polyester 1] to [Polyester d] as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
      The results obtained in Example 1 and Comparative Examples 1 to 4 are shown in Table 2 below.  
                           TABLE 2                                      Particle Size and Particle Size               Distribution                                     Content of                   Particles Having               Diameter               Corresponding to   Evaluation               ⅕ or Less of   of Inkjet           Volume Average   Volume Average   Drawing           Diameter (μm)   Diameter (%)   Image 1)                                           Example 1   0.98   0.6   A       Comparative   3.62   4.5   C       Example 1       Comparative   Unable to form   —   —       Example 2   particles       Comparative   Unable to form   —   —       Example 3   particles       Comparative   Unable to form   —   —       Example 4   particles                   1) Evaluation of Inkjet Drawing Image            The degrees of blur and streak unevenness in the drawing image was visually evaluated according to the following criteria:            A: Neither blur nor streak unevenness occurred.            B: Blur and streak unevenness somewhat occurred.            C: Blur and streak unevenness definitely occurred.             
 
      As is apparent from Example 1, by incorporating [Polyester 1] according to the invention into charge particles, the particle size and particle size distribution of the charged particles can be controlled in the preferred ranges. Thus, an image of good quality free from blur can be obtained according to the inkjet drawing system of the invention because electrophoretic speed of the charged particle is fully high so that concentration of the charged particles sufficiently occurs at the ejection part of the inkjet recording device. Further, in the case of conducting running, stable drawing is possible for a long period of time.  
      On the contrary, in Comparison Example 1 since [Polyester a] having a high molecular weight is used as the coating agent, cohesion of the polymer becomes high and unevenness of the cohesion in the pigment-polymer mixture is apt to occur. As a result, it is difficult to perform uniform particle formation, resulting in formation of particles having a broad particle size distribution.  
      In Comparison Example 2, since [Polyester b] having a low Tg, which is soft, is used as the coating agent, brittleness necessary for being broken is deficient and also the force from media is hard to reach to the polymer. As a result, it is unable to form particles by mechanical dispersion with media.  
      In Comparison Example 3, since [Polyester c] having a low ester content ratio, which is more swellable in a solvent, is used as the coating agent, the polyester is swollen in the solvent, and shearing force is unable to function properly at the dispersion. As a result, it is unable to form particles by mechanical dispersion with media.  
      In Comparison Example 4, since [Polyester d] having a low dynamic elastic modulus (G′) and exhibits physical properties of rubber elastic region in a temperature range of the pulverization and dispersion step is used as the coating agent, the sear force does not work well at the dispersion and thus, it is unable to form particles by mechanical dispersion with media.  
      As described above, in the case of using the polyester, which does not satisfy the specific physical property conditions according to the invention, the desired charged particles cannot be obtained. Therefore, since electrophoretic speed of the charged particle is so slow that concentration of the charged particles does not sufficiently occur at the ejection part of the inkjet recording device, blur occurs and an image of good quality can be obtained.  
     EXAMPLES 2 TO 26  
      Ink Compositions [EC-2] to [EC-26] were prepared in the same manner as in Example 1 except for changing the coating agent used in the preparation of Ink Composition [EC-1] in Example 1 to each of those shown in Table 3 below.  
                       TABLE 3                           Ink               Composition   Coating Agent                  Example 2   EC-2   Polyester 2       Example 3   EC-3   Polyester 3       Example 4   EC-4   Polyester 4       Example 5   EC-5   Polyester 5       Example 6   EC-6   Polyester 6       Example 7   EC-7   Polyester 7       Example 8   EC-8   Polyester 8       Example 9   EC-9   Polyester 9       Example 10   EC-10   Polyester 10       Example 11   EC-11   Polyester 11       Example 12   EC-12   Polyester 1/[Butyl methacrylate/methyl methacrylate (75/25 wt %)               copolymer (manufactured by Sigma-Aldrich Co.)] = 75/25 wt %       Example 13   EC-13   Polyester-1/polystylene bimodal (manufactured by Sigma-Aldrich               Co.) = 75/25 wt %       Example 14   EC-14   Polyester 1/polyester 2 = 75/25 wt %       Example 15   EC-15   Polyester 1/polyester 2 = 50/50 wt %       Example 16   EC-16   Polyester 1/polyester 2 = 25/75 wt %       Example 17   EC-17   Polyester 1/polyester 3 = 50/50 wt %       Example 18   EC-18   Polyester 1/polyester 4 = 50/50 wt %       Example 19   EC-19   Polyester 1/polyester 5 = 50/50 wt %       Example 20   EC-20   Polyester 6/polyester 11 = 50/50 wt %       Example 21   EC-21   Polyester 7/polyester 10 = 50/50 wt %       Example 22   EC-22   Polyester 1/polyester 6/Polyester 8 = 50/25/25 wt %       Example 23   EC-23   Polyester 3/polyester 6/Polyester 9 = 50/25/25 wt %       Example 24   EC-24   Polyester 4/polyester 7/Polyester 11 = 50/25/25 wt %       Example 25   EC-25   Polyester 5/polyester 8/Polyester 10 = 40/30/30 wt %       Example 26   EC-26   Polyester 6/polyester 9/Polyester 11 = 40/30/30 wt %                  
 
      The ink compositions thus-obtained were evaluated in the same manner as in Example 1. As a result, it has been found that by using the coating agents as shown in Examples 2 to 26, the particle size and particle size distribution of the charged particles can be controlled in the preferred ranges. Thus, an image of good quality free from blur can be obtained according to the inkjet drawing system of the invention because electrophoretic speed of the charged particle is fully high so that concentration of the charged particles sufficiently occurs at the ejection part of the inkjet recording device. Further, in the case of conducting running, stable drawing is possible for a long period of time.  
      Preparation of Polyethylene  
      Polyethylene 1  
      In a three-necked flask were charged 100 g of polyethylene-methacrylic acid copolymer (Nucrel N-699, manufactured by Dupont-Mitsui Polychemical Co., Ltd.; containing 0.04% by mole of methacrylic acid) and 1,000 g of toluene (manufactured by Wako Pure Chemical Industries, Ltd.), and the mixture was refluxed in an oil bath. Then, the reaction solution was cooled to 60° C., 15 g of thionyl chloride (manufactured by Sigma-Aldrich Co.) was gradually added thereto, followed by reacting for one hour, and then 45 g of dodecyl alcohol (manufactured by Wako Pure Chemical Industries, Ltd.) was added thereto, followed by reacting at 65° C. for 20 hours. After the reaction, the reaction solution was poured into 3 liters of methanol to purify by reprecipitation, followed by drying in vacuum to obtain 120 g of the desired polyethylene. The resulting polymer had a weight average molecular weight (Mw) of 66,000 and a melting point (Tm) of 65° C.  
      Polyethylenes 2 to 16  
      Various polyethylenes as shown in Table 4 below were prepared in the same manner as in Polyethylene 1 except for using base polyethylene components and alcohol components as shown in Table 4, respectively. Physical properties of the resulting polyethylenes are also shown in Table 4.  
                               TABLE 4                                          Molecular               Polyethylene Composition   Weight                                 Poly-   Base       (GPC           ethylene   Polyethylene   Alcohol   weight   Tg       No.   Component   Component   average)   (° C.)                                         1   Nucrel N-699   Dodecyl alcohol   66,000   65       2   Nucrel N-699   Stearyl alcohol   68,000   70       3   Nucrel N-699   2-Ethylhexyl alcohol   62,000   50       4   Nucrel N-699   Cyclohexyl alcohol   60,000   72       5   Nucrel N-699   Benzyl alcohol   61,000   75       6   Nucrel N-699   p-Octylphenol   65,000   66       7   Nucrel N-699   tert-Butyl alcohol   59,000   63       8   Nucrel N-699   Dodecyl alcohol/Benzyl   63,000   62               alcohol = 50/50 mol %       9   Nucrel N-699   Dodecyl alcohol/Benzyl   60,000   71               alcohol/tert-Butyl alcohol =               35/35/30 mol %       10   Nucrel N-925   Dodecyl alcohol   27,000   67       11   Nucrel N-925   Stearyl alcohol   31,000   60       12   Nucrel N-925   Cyclohexyl alcohol   26,000   58       13   Nucrel N-925   Benzyl alcohol   28,000   68       14   Nucrel N-925   p-Octylphenol   30,000   60       15   Nucrel N-925   Dodecyl alcohol/Benzyl   33,000   67               alcohol = 50/50 mol %       16   Nucrel N-925   Dodecyl alcohol/Benzyl   34,000   56               alcohol/tert-Butyl alcohol =               35/35/30 mol %                 Note:            Nucrel N-925 is polyethylene-methacrylic acid copolymer.             
 
     EXAMPLE 27  
      Materials Used  
      The following materials were used in Example 27. 
      Cyan pigment (colorant): Phthalocyanine pigment, C.I. Pigment Blue (15:3) (LIONOL BLUE FG-7350, manufactured by Toyo Ink Mfg. Co., Ltd.)     Coating agent; [Polyethylene 1]    Dispersing agent: [BZ-2] described above     Charge controlling agent: [CT-1] described above     Dispersion medium: Isopar G (manufactured by Exxon Corp.) 
 
 Preparation of Ink composition [EC-27]
   

      In a desktop kneader (PBV-0.1, manufactured by Irie Shokai Co., Ltd.) were charged 10 g of the cyan pigment and 20 g of Coating agent [Polyethylene 1], and the components were mixed under heating at a heater temperature set at 100° C. for 2 hours. Thirty grams of the mixture thus obtained was coarsely pulverized in a trio blender (manufactured by Trio Science Co., Ltd.), and then finely pulverized in a sample mill (Model SK-M10, manufactured by Kyoritsu Riko Co., Ltd.). Thirty grams of the finely pulverized product thus obtained was preliminary dispersed in a paint shaker (manufactured by Toyo Seiki Seisako-Sho, Ltd.) together with 7.5 g of Dispersing agent (BZ-2), 75 g of Isopar G and glass beads having a diameter of about 3.0 mm. After removing the glass beads, the mixture was further dispersed (reduced to particles) together with zirconia ceramic beads having a diameter of about 0.6 mm in a Dino-mill (Type KDL, manufactured by Shinmaru Enterprises Corp.) at a rotation number of 2,000 rpm for 5 hours while maintaining an inner temperature at 25° C. and then further for 5 hours at 45° C. The zirconia ceramic beads were removed from the resulting dispersion liquid, then 316 g of Isopar G and 0.6 g of Charge controlling agent [CT-1] were added thereto to obtain Ink composition [EC-27].  
      Inkjet Recording/Image Evaluation  
      Ink composition [EC-27] of described above was charged in an ink tank of an inkjet recording apparatus as shown in  FIG. 1 . An ejection head used was a 833-channel head of 150 dpi (three rows having a channel density of 50 dpi arranged in a stagger pattern) having a structure as shown in  FIG. 2 , and a fixing means used was a heat roller made of silicone rubber having a built-in 1 kW heater. An immersion heater and stirring blades were provided as an ink temperature controlling means in the ink tank, and the ink temperature was set at 30° C., which was controlled with a thermostat while rotating the stirring blades at 30 rpm. The stirring blades were also used as stirring means for preventing precipitation and aggregation. A part of the ink flow channel was made transparent, at outside of which an LED light emitting element and a photodetector element were provided, and based on the output signals therefrom, a diluent for ink (Isopar G) or a concentrated ink composition (which had twice the solid content concentration of the ink composition described above). Fine coated paper for offset printing was used as a recording medium. After removing dusts on the surface of the recording medium by suction with an air pump, the ejection head was moved to an image forming position closely to the recording medium. Image data to be recorded were transmitted to an image data operating and controlling part, and the ink composition was ejected with sequential movement of the ejection head while the recording medium was conveyed through rotation of a conveying belt so as to form an image with a drawing resolution of 2,400 dpi. The conveying belt used was a belt prepared by laminating a metallic belt and a polyimide film, and a linear marker was provided in the conveying direction near one side of the belt. The marker was optically read out by a conveying belt position detecting means, and a position controlling means was driven to conduct the image formation. The distance between the ejection head and the recording medium was maintained at 0.5 mm based on output from an optical gap detecting device. The surface potential of the recording medium at ejection was set at −1.5 kV, and a pulse voltage of +500 V (with a pulse width of 50 μsec) was applied at ejection to conduct the image recording with a driving frequency of 15 kHz. With the resulting gray scale image recorded product (printed material), degrees of streak unevenness and ink blur were evaluated (evaluation of inkjet drawing image).  
     COMPARATIVE EXAMPLE 5  
      Ink composition [RC-5] was prepared in the same manner as in Example 27 except for changing [Polyethylene 1] to Nucrel N-699 as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
     COMPARATIVE EXAMPLE 6  
      Ink composition [RC-6] was prepared in the same manner as in Example 27 except for changing [Polyethylene 1] to a low-density polyethylene (manufactured by Wako Pure Chemical Industries, Ltd.) as the using material. Using the ink composition, evaluations of particle size, particle size distribution and inkjet drawing image were conducted.  
      The results obtained in Example 27 and Comparative Examples 5 to 6 are shown in Table 5 below.  
                           TABLE 5                                      Particle Size and Particle Size               Distribution                                     Content of                   Particles Having               Diameter               Corresponding to   Evaluation               ⅕ or Less of   of Inkjet           Volume Average   Volume Average   Drawing           Diameter (μm)   Diameter (%)   Image 1)                                           Example 27   0.98   0.6   A       Comparative   4.43   3.8   C       Example 5       Comparative   Unable to form   —   —       Example 6   particles                   1) Evaluation of Inkjet Drawing Image            The degree of blur in the drawing image was visually evaluated according to the following criteria:            A: Neither blur nor streak unevenness occurred.            B: Blur and streak unevenness somewhat occurred.            C: Blur and streak unevenness definitely occurred.             
 
      As is apparent from Example 27, by incorporating [Polyethylene 1] according to the invention into charge particles, the particle size and particle size distribution of the charged particles can be controlled in the preferred ranges. Thus, an image of good quality free from blur can be obtained according to the inkjet drawing system of the invention because electrophoretic speed of the charged particle is fully high so that concentration of the charged particles sufficiently occurs at the ejection part of the inkjet recording device. Further, in the case of conducting running, stable drawing is possible for a long period of time.  
      On the contrary, in Comparison Example 5 since Nucrel N-699 is used as the coating agent, cohesion of the polymer becomes high and unevenness of the cohesion in the pigment-polymer mixture is apt to occur. As a result, it is difficult to perform uniform particle formation, resulting in formation of particles having a broad particle size distribution.  
      In Comparison Example 6, since the low-density polyethylene (manufactured by Wako Pure Chemical Industries, Ltd.) having significant rubber elasticity is used as the coating agent, brittleness necessary for being broken is deficient and also the force from media is hard to reach to the polymer. As a result, it is unable to form particles by mechanical dispersion with media.  
      As described above, in the case of using the polyethylene other than the specific polyethylene according to the invention, the desired charged particles cannot be obtained. Therefore, since electrophoretic speed of the charged particle is so slow that concentration of the charged particles does not sufficiently occur at the ejection part of the inkjet recording device, blur occurs and an image of good quality can be obtained.  
     EXAMPLES 28 TO 57  
      Ink Compositions [EC-28] to [EC-57] were prepared in the same manner as in Example 27 except for changing the coating agent used in the preparation of Ink Composition [EC-27] in Example 27 to each of those shown in Table 6 below.  
                       TABLE 6                           Ink               Composition   Coating Agent                  Example 28   EC-28   Polyethylene 2       Example 29   EC-29   Polyethylene 3       Example 30   EC-30   Polyethylene 4       Example 31   EC-31   Polyethylene 5       Example 32   EC-32   Polyethylene 6       Example 33   EC-33   Polyethylene 7       Example 34   EC-34   Polyethylene 8       Example 35   EC-35   Polyethylene 9       Example 36   EC-36   Polyethylene 10       Example 37   EC-37   Polyethylene 11       Example 38   EC-38   Polyethylene 12       Example 39   EC-39   Polyethylene 13       Example 40   EC-40   Polyethylene 14       Example 41   EC-41   Polyethylene 15       Example 42   EC-42   Polyethylene 16       Example 43   EC-43   Polyethylene 1/[Butyl methacrylate/methyl methacrylate (75/25 wt %)               copolymer (manufactured by Sigma-Aldrich Co.)] = 75/25 wt %       Example 44   EC-44   Polyethylene 1/Polystylene bimodal (manufactured by Sigma-               Aldrich Co.) = 75/25 wt %       Example 45   EC-45   Polyethylene 1/Polyethylene 2 = 75/25 wt %       Example 46   EC-46   Polyethylene 1/Polyethylene 2 = 50/50 wt %       Example 47   EC-47   Polyethylene 1/Polyethylene 2 = 25/75 wt %       Example 48   EC-48   Polyethylene 1/Polyethylene 3 = 50/50 wt %       Example 49   EC-49   Polyethylene 1/Polyethylene 4 = 50/50 wt %       Example 50   EC-50   Polyethylene 1/Polyethylene 5 = 50/50 wt %       Example 51   EC-51   Polyethylene 6/Polyethylene 11 = 50/50 wt %       Example 52   EC-52   Polyethylene 7/Polyethylene 10 = 50/50 wt %       Example 53   EC-53   Polyethylene 1/Polyethylene 6/Polyethylene 8 = 50/25/25 wt %       Example 54   EC-54   Polyethylene 3/Polyethylene 6/Polyethylene 9 = 50/25/25 wt %       Example 55   EC-55   Polyethylene 4/Polyethylene 7/Polyethylene 11 = 50/25/25 wt %       Example 56   EC-56   Polyethylene 5/Polyethylene 8/Polyethylene 10 = 40/30/30 wt %       Example 57   EC-57   Polyethylene 6/Polyethylene 9/Polyethylene 11 = 40/30/30 wt %                  
 
      The ink compositions thus-obtained were evaluated in the same manner as in Example 27. As a result, it has been found that by using the coating agents as shown in Examples 28 to 57, the particle size and particle size distribution of the charged particles can be controlled in the preferred ranges. Thus, an image of good quality free from blur can be obtained according to the inkjet drawing system of the invention because electrophoretic speed of the charged particle is fully high so that concentration of the charged particles sufficiently occurs at the ejection part of the inkjet recording device. Further, in the case of conducting running, stable drawing is possible for a long period of time.  
      The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth herein.  
      While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.