Inkjet inks and inkjet recording devices

Abstract

To provide inkjet ink which can suppress occurrence of discharge deflection of ink from a recording head, enables formation of an image excellent in scratch resistance, and is excellent in re-melting property.SOLUTION: Inkjet ink contains a quinacridone pigment, a resin, and an aqueous medium. Phosphorus concentration in 50 times dilution liquid of a supernatant obtained by subjecting the inkjet ink at 1,050,000 G for 3 hours to centrifugal treatment is 1.0 ppm or more and 6.0 ppm or less. In the ultraviolet visible light absorption spectrum of the supernatant, first absorbance of a first peak is 0.16 or more and 0.40 or less, and the first peak is a maximum peak in a wavelength range of 300 nm or more and 340 nm or less. In the ultraviolet visible light absorption spectrum of the supernatant, second absorbance of a second peak is 0.15 or more and 0.80 or less, and the second peak is a maximum peak in a wavelength range of 400 nm or more and 490 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to inkjet ink and inkjet recording apparatus. [Background technology]

[0002] Inkjet ink is ejected from the recording head of an inkjet recording device. Inkjet ink requires stable ejection from the recording head. To address this requirement, Patent Document 1 describes, for example, an inkjet recording solution containing an aqueous pigment dispersion. This aqueous pigment dispersion comprises an aqueous liquid, a quinacridone pigment dispersed in the aqueous liquid, a water-soluble quinacridone derivative adsorbed on the surface of the quinacridone pigment, and an unadsorbed water-soluble quinacridone derivative. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2000-273383 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, the inkjet recording fluid described in Patent Document 1 is insufficient in terms of suppressing ejection distortion from the recording head of an inkjet recording device, forming an image with excellent scratch resistance, and resolubility.

[0005] The present invention has been made in view of the above-mentioned problems, and its purpose is to provide an inkjet ink that can suppress the occurrence of ejection distortion from the recording head, can form an image with excellent scratch resistance, and has excellent re-dissolvability, and an inkjet recording apparatus using this inkjet ink. [Means for solving the problem]

[0006] The inkjet ink according to the present invention contains a quinacridone pigment, a resin, and an aqueous medium. The phosphorus concentration in a 50-fold diluted solution of the supernatant obtained by centrifuging the inkjet ink at 1,050,000 G for 3 hours is 1.0 ppm or more and 6.0 ppm or less. In the ultraviolet-visible light absorption spectrum of the supernatant, the first absorbance of the first peak is 0.16 or more and 0.40 or less, and the first peak is the maximum peak in the wavelength range of 300 nm or more and 340 nm or less. In the ultraviolet-visible light absorption spectrum of the supernatant, the second absorbance of the second peak is 0.15 or more and 0.80 or less, and the second peak is the maximum peak in the wavelength range of 400 nm or more and 490 nm or less.

[0007] The inkjet recording apparatus according to the present invention comprises a transport unit for transporting a recording medium and a recording head for ejecting ink onto the recording medium. The ink is the inkjet ink described above. [Effects of the Invention]

[0008] The inkjet ink according to the present invention can suppress the occurrence of ejection distortion from the recording head, can form images with excellent scratch resistance, and has excellent re-dissolvability. Furthermore, the inkjet recording apparatus according to the present invention can suppress the occurrence of ejection distortion from the recording head, can form images with excellent scratch resistance, and has excellent re-dissolvability, and therefore has excellent cleaning performance because it uses inkjet ink. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows an example of the ultraviolet-visible light absorption spectrum of the supernatant liquid. [Figure 2] This figure shows an example of an inkjet recording apparatus according to a second embodiment of the present invention. [Figure 3] This figure shows the underside of the recording head shown in Figure 2. [Figure 4] This diagram illustrates the operation of supplying the cleaning fluid. [Figure 5] This diagram illustrates the purging and wiping operations.

Embodiments for Carrying out the Invention

[0010] Hereinafter, embodiments of the present invention will be described. First, the terms used in this specification will be explained. The volume median diameter (D 50 ) is a value measured using a dynamic light scattering particle size distribution measuring device (Malvern "Zetasizer Nano ZS") if not otherwise specified. The acid value is a value measured in accordance with "JIS (Japanese Industrial Standards) K0070:1992" if not otherwise specified. The mass average molecular weight (Mw) is a value measured using gel permeation chromatography if not otherwise specified. In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic" in some cases. "Independently of each other" in the description of the formula means that the same group or different groups may be represented. Each component described in this specification may be used alone or in combination of two or more.

[0011] [First Embodiment: Ink for Inkjet] Hereinafter, the ink for inkjet of the first embodiment of the present invention (hereinafter may be simply referred to as ink) will be described.

[0012] The ink of the first embodiment contains a quinacridone pigment, a resin, and an aqueous medium. The phosphorus concentration in a 50-fold dilution of the supernatant obtained by centrifuging the ink at 1,050,000 G for 3 hours is 1.0 ppm or more and 6.0 ppm or less. In the ultraviolet-visible light absorption spectrum of the supernatant, the first absorbance of the first peak is 0.16 or more and 0.40 or less, and the first peak is the maximum peak in the wavelength range of 300 nm or more and 340 nm or less. In the ultraviolet-visible light absorption spectrum of the supernatant, the second absorbance of the second peak is 0.15 or more and 0.80 or less, and the second peak is the maximum peak in the wavelength range of 400 nm or more and 490 nm or less.

[0013] Hereinafter, the "supernatant obtained by centrifuging the ink at 1,050,000 G for 3 hours" may be simply referred to as the "supernatant". The "phosphorus concentration in a 50-fold dilution of the supernatant obtained by centrifuging the ink at 1,050,000 G for 3 hours" may be referred to as the "predetermined phosphorus concentration". Also, the "first absorbance of the first peak in the ultraviolet-visible light absorption spectrum of the supernatant" may be referred to as the "first absorbance of the supernatant". Further, the "second absorbance of the second peak in the ultraviolet-visible light absorption spectrum of the supernatant" may be referred to as the "second absorbance of the supernatant".

[0014] By having the above-described configuration, the ink of the first embodiment can suppress the occurrence of ink ejection unevenness from the recording head, can form an image excellent in abrasion resistance, and is excellent in redissolution property. The reason is speculated as follows.

[0015] First, for the purpose of assisting understanding, an outline of the synthesis method of quinacridone pigment will be described. The quinacridone pigment is, for example, a compound represented by formula (2). The quinacridone pigment is synthesized, for example, by carrying out the reactions represented by reaction formulas (r-a), (r-b), (r-c), and (r-1).

[0016] [Chemical formula]

[0017] In formulas (A), (B), (C), (1), and (2), R A , R B , R 1 , and R 2 each independently represents a monovalent group. Hereinafter, the "reactions represented by reaction formulas (r-a), (r-b), (r-c), and (r-1)" may be each referred to as "reactions (r-a), (r-b), (r-c), and (r-1)". Also, the "compounds represented by formulas (A), (B), (C), (1), and (2)" may be each referred to as "compounds (A), (B), (C), (1), and (2)". R 1 and R 2When it represents a methyl group, compound (2) becomes C.I. Pigment Red 122. R 1 and R 2 When it represents a hydrogen atom, compound (2) becomes C.I. Pigment Violet 19.

[0018] In the process of carrying out reactions (r-a), (r-b), and (r-c), intermediates, namely compounds (B) and (C), are formed. Further, in reaction (r-1), compound (2) is obtained by reacting compound (1) in the presence of a catalyst. The catalyst used in reaction (r-1) includes, for example, a phosphorus-containing catalyst. The above is an explanation of the outline of the method for synthesizing quinacridone pigments.

[0019] Intermediates (more specifically, compounds (B) and (C)) generated in the process of carrying out reactions (r-a), (r-b), and (r-c), and the phosphorus-containing catalyst used in reaction (r-1) may remain as impurities in the quinacridone pigment. When such a quinacridone pigment is contained in the ink, the ink also contains the intermediates and the phosphorus-containing catalyst. The polarity of the intermediates and the phosphorus-containing catalyst is relatively high. Therefore, when an image is formed using an ink containing a quinacridone pigment, the intermediates and the phosphorus-containing catalyst may adhere electrostatically to the ejection surface of the recording head and the inner wall of the nozzle holes. The adhered intermediates and phosphorus-containing catalyst contribute to ink dripping from the recording head.

[0020] In the ink of the first embodiment, the predetermined phosphorus concentration is 1.0 ppm or more and 6.0 ppm or less. If the predetermined phosphorus concentration is 6.0 ppm or less, there are relatively few phosphorus-containing catalysts, which are impurities, so that the occurrence of ink dripping from the recording head can be suppressed. On the other hand, although the phosphorus-containing catalyst is an impurity, it has the advantage that the redissolution property of the ink is improved when it is contained in the ink in a trace amount. Therefore, if the predetermined phosphorus concentration is 1.0 ppm or more, the redissolution property of the ink is improved. In this specification, the redissolution property of the ink means the property that the dried ink adhered to the ejection surface of the recording head can be easily dissolved in one or both of the cleaning liquid and the purge ink.

[0021] Furthermore, in the ink of the first embodiment, the second absorbance of the supernatant is 0.15 or more and 0.80 or less. The second peak is a peak derived from, for example, an intermediate for synthesizing quinacridone pigment (more specifically, compounds (B) and (C), etc.). If the second absorbance of the supernatant is 0.80 or less, there are relatively few intermediates, which are impurities, so the occurrence of ink ejection irregularities from the recording head can be suppressed. On the other hand, although intermediates are impurities, they have the advantage that when included in the ink in trace amounts, the resolubility of the ink improves. Therefore, if the second absorbance of the supernatant is 0.15 or more, the resolubility of the ink improves.

[0022] Furthermore, in the ink of the first embodiment, the first absorbance of the supernatant liquid is 0.16 or more and 0.40 or less. The first peak is, for example, a peak derived from unadsorbed resin. Details of the unadsorbed resin will be described later. The unadsorbed resin, like the intermediate and phosphorus-containing catalyst described above, may electrostatically adhere to the ejection surface of the recording head and the inner wall of the nozzle hole. The adhered unadsorbed resin is one of the causes of ink ejection irregularities from the recording head. If the first absorbance of the supernatant liquid is 0.40 or less, the content of unadsorbed resin in the ink is relatively small, so the occurrence of ink ejection irregularities from the recording head can be suppressed. On the other hand, if the first absorbance of the supernatant liquid is 0.16 or more, the content of unadsorbed resin in the ink does not become too small, and the adhesion of the ink to the recording medium is improved. As a result, images with excellent scratch resistance can be formed using the ink.

[0023] The above explains why the ink of the first embodiment can suppress the occurrence of ink ejection irregularities from the recording head, form images with excellent scratch resistance, and exhibit excellent re-dissolvability. The ink of the first embodiment will now be described in more detail.

[0024] <Specified phosphorus concentration> As already mentioned, the predetermined phosphorus concentration is 1.0 ppm or more and 6.0 ppm or less. To further improve the resolubility of the ink, the predetermined phosphorus concentration is preferably 2.0 ppm or more. To further suppress the occurrence of ink ejection irregularities from the recording head, the predetermined phosphorus concentration is preferably 5.0 ppm or less.

[0025] The predetermined phosphorus concentration is, for example, the concentration of phosphorus atoms derived from the phosphorus-containing catalyst for synthesizing the quinacridone pigment contained in the supernatant liquid. That is, the predetermined phosphorus concentration is, for example, the concentration of phosphorus atoms contained in the phosphorus-containing catalyst contained in the supernatant liquid.

[0026] The phosphorus-containing catalyst for synthesizing quinacridone pigments is, for example, at least one selected from the group consisting of polyphosphate and its derivatives. Therefore, the predetermined phosphorus concentration is, more specifically, the concentration of phosphorus atoms contained in the supernatant liquid, which is at least one selected from the group consisting of polyphosphate and its derivatives. Hereinafter, "at least one selected from the group consisting of polyphosphate and its derivatives" may be referred to as "polyphosphate, etc."

[0027] Among polyphosphates, examples of polyphosphate derivatives include polyphosphate esters, and more specifically, alkyl polyphosphate esters. Methyl polyphosphate is preferred as the alkyl polyphosphate ester. The catalyst used in the above reaction (r-1) may further contain a metal catalyst such as tin in addition to the phosphorus-containing catalyst.

[0028] At least a portion of the phosphorus-containing catalyst remaining in the quinacridone pigment is removed, for example, by ultrafiltration, thereby lowering the predetermined phosphorus concentration. For example, the predetermined phosphorus concentration can be adjusted by changing the circulation time of the pigment dispersion during ultrafiltration. The longer the circulation time of the pigment dispersion, the lower the predetermined phosphorus concentration tends to be. The predetermined phosphorus concentration can be measured, for example, by the method described in the examples.

[0029] <First absorbance and second absorbance> The first and second absorbances will be explained below with reference to Figure 1. Figure 1 shows the ultraviolet-visible light absorption spectra of the supernatant obtained by centrifuging five reference inks at 1,050,000 G for 3 hours. Note that the five reference inks are different from the inks in the examples and comparative examples described later, but are shown as examples to explain the first and second absorbances. Each of the five reference inks contains at least quinacridone pigment (CI Pigment Red 122), resin (resin (RA) described later), and an aqueous medium. The vertical axis of Figure 1 shows absorbance, and the horizontal axis shows wavelength (unit: nm). In Figure 1, 0 minutes, 30 minutes, 60 minutes, 90 minutes, and 120 minutes represent the circulation time of the ultrafiltration treatment of the pigment dispersion, respectively. For the preparation of the five reference inks, pigment dispersions with ultrafiltration circulation times of 0 minutes, 30 minutes, 60 minutes, 90 minutes, and 120 minutes were used, respectively. The ultraviolet-visible light absorption spectra of the supernatant liquids obtained from five reference inks were measured using the method described later in the examples, and the obtained spectra are shown in Figure 1. In Figure 1, P1 and P2 represent the first peak P1 and the second peak P2, respectively. P1 and P2 are attached only to the spectra of the reference inks using pigment dispersions with an ultrafiltration circulation time of 0 minutes; P1 and P2 are omitted for the other reference inks.

[0030] In Figure 1, the first peak P1 is the maximum peak in the wavelength range of 300 nm to 340 nm. The second peak P2 is the maximum peak in the wavelength range of 400 nm to 490 nm. The maximum peak refers to the peak (i.e., the vertex) of the convex curve of the spectrum that appears in a given wavelength range that has the highest absorbance.

[0031] In the example shown in Figure 1, among the spectral peaks appearing in the wavelength range of 300 nm to 340 nm, the peak with the maximum absorbance is observed at a wavelength of 320 nm. Therefore, the peak at 320 nm is the first peak P1. The first absorbance can be determined by reading the absorbance of the first peak P1 from the ultraviolet-visible light absorption spectrum.

[0032] Furthermore, in the example shown in Figure 1, among the spectral peaks appearing in the wavelength range of 400 nm to 490 nm, the peak with the maximum absorbance is observed at a wavelength of 429 nm. Therefore, the peak at wavelength 429 nm is the second peak P2. The second absorbance can be determined by reading the absorbance of the second peak P2 from the ultraviolet-visible light absorption spectrum.

[0033] In the example shown in Figure 1, the spectrum shows a downward curve and does not exhibit a convex curve in the wavelength range below 310 nm. Therefore, in the example shown in Figure 1, it is determined that there are no peaks in the convex curve of the spectrum in the wavelength range below 310 nm. As shown in Figure 1, the longer the circulation time of the ultrafiltration treatment, the lower the first and second absorbances of the supernatant tend to be. The first and second absorbances of the supernatant have been explained above with reference to Figure 1. The first and second absorbances of the supernatant will be explained further below.

[0034] As already mentioned, the first absorbance of the supernatant is between 0.16 and 0.40. To form an image with excellent scratch resistance, the first absorbance of the supernatant is preferably 0.20 or higher. To suppress the occurrence of ink ejection irregularities from the recording head, the first absorbance of the supernatant is preferably 0.35 or lower. The first peak is, for example, a peak derived from unadsorbed resin contained in the ink. The amount of unadsorbed resin can be adjusted, for example, by changing the resin content ratio in the ink. Furthermore, the first absorbance of the first peak can be adjusted by changing the amount of unadsorbed resin in the pigment dispersion and the amount of unadsorbed resin in the ink containing that pigment dispersion. More specifically, the amount of unadsorbed resin can be adjusted by changing at least one of the following: the discharge amount of the pigment dispersion treatment, the number of passes of the activated carbon treatment of the pigment dispersion, and the circulation time of the ultrafiltration treatment of the pigment dispersion, as described in the examples. The higher the discharge rate of the pigment dispersion during dispersion treatment, the greater the amount of unadsorbed resin, and the higher the first absorbance of the supernatant tends to be. The more passes the pigment dispersion undergoes activated carbon treatment, the more unadsorbed resin is removed, and the lower the first absorbance of the supernatant tends to be. The longer the circulation time of the pigment dispersion during ultrafiltration treatment, the more unadsorbed resin is removed, and the lower the first absorbance of the supernatant tends to be.

[0035] As already mentioned, the second absorbance of the supernatant is between 0.15 and 0.80. To improve the resolubility of the ink, it is preferable that the second absorbance of the supernatant be 0.20 or higher. To suppress the occurrence of ink ejection irregularities from the recording head, it is preferable that the second absorbance of the supernatant be 0.70 or lower. For example, the second peak is a peak derived from an intermediate for synthesizing the quinacridone pigment contained in the ink. More specifically, the second peak is a peak derived from the intermediate compounds (B) and (C) contained in the ink. Therefore, the second absorbance of the second peak can be adjusted by changing the amount of the intermediate in the pigment dispersion and the amount of the intermediate in the ink containing the pigment dispersion. The amount of the intermediate can be changed, for example, by changing at least one of the number of passes of the activated carbon treatment of the pigment dispersion and the circulation time of the ultrafiltration treatment of the pigment dispersion, as described later in the examples. The more passes the pigment dispersion undergoes activated carbon treatment, the more intermediates are removed, and the lower the secondary absorbance of the supernatant tends to be. The longer the circulation time during ultrafiltration of the pigment dispersion, the more intermediates are removed, and the lower the secondary absorbance of the supernatant tends to be.

[0036] <Quinacridone pigment> Examples of quinacridone pigments contained in the ink include CI Pigment Violet (19 and 42), CI Pigment Red (122, 202, 206, 207, and 209), and CI Pigment Orange (48 and 49).

[0037] Examples of commercially available quinacridone pigments that can be used include "TRM-11" from Dainichi Seika Kogyo Co., Ltd., "Cinquasia® Magenta D4550" and "Cinquasia® Pink D4450" from BASF, "Inkjet Magenta ES" from Clariant, "HOSTAPERM PINK E 02" from Clariant, "HOSTAPERM RED E3B" from Clariant, and "HOSTAPERM RED E5B 02" from Clariant.

[0038] The quinacridone pigment content in the ink is preferably 1% by mass or more and 12% by mass or less, and more preferably 4% by mass or more and 8% by mass or less. By setting the quinacridone pigment content to 1% by mass or more, the image density of the image formed by the ink can be optimized. Furthermore, by setting the quinacridone pigment content to 12% by mass or less, the fluidity of the ink can be optimized. The ink may contain only quinacridone pigment as the pigment. In addition, to adjust the hue of the ink, the ink may further contain other pigments in addition to quinacridone pigment as the pigment.

[0039] Although an overview of the quinacridone pigment synthesis method has already been described, the following will explain the quinacridone pigment synthesis method in more detail.

[0040] In the above equations (A), (B), and (C), R A and R B Examples of monovalent groups represented by R include alkyl groups and aryl groups. A and R B The monovalent group represented is preferably an alkyl group, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an ethyl group.

[0041] In the above equations (C), (1), and (2), R 1 and R 2 Examples of monovalent groups represented by R include hydrogen atoms, alkyl groups, and halogen atoms. 1 and R 2 A chlorine atom is preferred as the halogen atom represented by R. 1 and R 2 The alkyl group represented is preferably an alkyl group having 1 to 6 carbon atoms, and a methyl group is more preferred.

[0042] In reaction (ra), compound (B) is obtained from compound (A). Then, in reaction (rb), 1 molar equivalent of compound (B) is reacted with 2 molar equivalents of an aniline derivative to obtain 1 molar equivalent of compound (C). Then, in reaction (rc), compound (C) is oxidized to obtain an oxide of compound (C), and then compound (1) is obtained by hydrolysis of the oxide of compound (C).

[0043] Next, in reaction (r-1), compound (2) is obtained by reacting compound (1) in the presence of a catalyst. The catalytic reaction (r-1) is a dehydration ring-closing reaction. For example, in reaction (r-1), compound (1) is pressurized at a first predetermined temperature for a predetermined time in the presence of a catalyst (e.g., a phosphorus-containing catalyst, more specifically, polyphosphate, etc.). The first predetermined temperature is, for example, 85°C to 200°C. The predetermined time is, for example, 1 hour to 3 hours. In reaction (r-1), an organic solvent such as acetone may be used in addition to compound (1) and the catalyst. Alternatively, instead of compound (1), an alkali salt obtained by saponifying compound (1) may be used in reaction (r-1). Alternatively, instead of compound (1), an oxide of the above-mentioned compound (C) may be used in reaction (r-1).

[0044] After reaction (r-1), a β-type quinacridone pigment can be obtained by alkali treatment of compound (2) as needed. Alkali treatment can be carried out, for example, by heating the wet cake of compound (2) and alkali in an autoclave at a second predetermined temperature. The second predetermined temperature is, for example, 120°C to 200°C. Organic solvents may also be used in the alkali treatment. Examples of organic solvents that can be used in the alkali treatment include N,N-dimethylformamide and glycol.

[0045] The method for synthesizing quinacridone pigment by reaction (r-1) does not require the removal of residual sulfonic acid groups or sulfonic acid chloride groups from the resulting quinacridone pigment using sulfuric acid, unlike the synthesis method that uses an acid containing a sulfonic acid group as a cyclizing agent. Therefore, the method for synthesizing quinacridone pigment by reaction (r-1) can be easily implemented.

[0046] <Resin> The resin contained in the ink is adsorbed onto, for example, the quinacridone pigment. Because the resin is hydrophilic, the adsorbed resin on the surface of the quinacridone pigment disperses the quinacridone pigment in the aqueous medium. Such resins are also called pigment-dispersing resins.

[0047] The resin comprises an adsorbent resin and an unadsorbed resin. That is, the ink contains an adsorbent resin and an unadsorbed resin as the resin. The adsorbent resin is adsorbed onto the quinacridone pigment. The adsorbent resin, for example, together with the quinacridone pigment constitutes a pigment particle. The pigment particle has, for example, a core and a coating layer covering the core. The core of the pigment particle contains the quinacridone pigment. The coating layer of the pigment particle contains the resin. On the other hand, the unadsorbed resin is not adsorbed onto the quinacridone pigment. The unadsorbed resin is free in the aqueous medium.

[0048] The proportion of unadsorbed resin in the total resin is preferably 20% by mass or more and 50% by mass or less. Hereinafter, "proportion of unadsorbed resin in the total resin" may be referred to as "unadsorbed resin ratio". If the unadsorbed resin ratio is 20% by mass or more, an image with excellent scratch resistance can be formed using ink. If the unadsorbed resin ratio is 50% by mass or less, the occurrence of ink ejection irregularities from the recording head can be suppressed. The unadsorbed resin ratio can be measured by centrifuging the ink using a centrifuge. The unadsorbed resin ratio can be calculated using the formula "Unadsorbed resin ratio = 100 × mass of unadsorbed resin / mass of resin = 100 × mass of unadsorbed resin / (mass of unadsorbed resin + mass of adsorbed resin)". For example, the unadsorbed resin ratio tends to increase as the ejection volume of the dispersion process described later increases.

[0049] Examples of resins include (meth)acrylic resin, styrene-(meth)acrylic resin, styrene-maleic acid resin, and urethane resin. From the viewpoint of stably dispersing the quinacridone pigment, styrene-(meth)acrylic resin is preferred as the resin.

[0050] The styrene-(meth)acrylic resin has at least repeating units derived from at least one styrene and its derivatives, and at least one repeating unit derived from at least one (meth)acrylic acid. Preferably, the styrene-(meth)acrylic resin further has repeating units derived from at least one (meth)acrylic acid ester.

[0051] Examples of first monomers capable of forming repeating units derived from styrene or its derivatives include styrene, α-methylstyrene, and vinyltoluene. Styrene is preferred as the first monomer. The content of repeating units derived from styrene or its derivatives relative to the total repeating units of the resin is preferably 25.0% by mass or more and 60.0% by mass or less.

[0052] Examples of secondary monomers capable of forming repeating units derived from (meth)acrylic acid include acrylic acid and methacrylic acid. Methacrylic acid is preferred as the secondary monomer. The content ratio of repeating units derived from (meth)acrylic acid to the total repeating units of the resin is preferably 4.5% by mass or more and 15.0% by mass or less, and more preferably 8.0% by mass or more and 11.0% by mass or less. When the resin has repeating units derived from both acrylic acid and methacrylic acid, the content ratio of repeating units derived from (meth)acrylic acid is the total content ratio of repeating units derived from acrylic acid and methacrylic acid.

[0053] Examples of third monomers capable of forming repeating units derived from (meth)acrylic acid esters include alkyl (meth)acrylates. Preferably, alkyl (meth)acrylates have an alkyl alkyl group with 1 to 8 carbon atoms in the alkyl group; more preferably, alkyl (meth)acrylates have an alkyl group with 1 to 4 carbon atoms; even more preferably, methyl (meth)acrylate and butyl (meth)acrylate; and particularly preferably, methyl methacrylate and butyl acrylate. The content of repeating units derived from (meth)acrylic acid esters relative to the total repeating units of the resin is preferably 35.0% by mass or more and 70.0% by mass or less, and more preferably 40.0% by mass or more and 70.0% by mass or less. If the resin has repeating units derived from two or more (meth)acrylic acid esters, the content of repeating units derived from (meth)acrylic acid esters is the total content of repeating units derived from the two or more (meth)acrylic acid esters.

[0054] The resin preferably has repeating units derived from at least one type of (meth)acrylic acid, repeating units derived from at least one type of alkyl (meth)acrylate, and repeating units derived from styrene. More preferably the resin has repeating units derived from one type of (meth)acrylic acid, repeating units derived from one or two types of alkyl (meth)acrylate, and repeating units derived from styrene. It is particularly preferable that the resin has repeating units derived from methacrylic acid, repeating units derived from methyl methacrylate, repeating units derived from butyl acrylate, and repeating units derived from styrene. The content of repeating units derived from at least one type of (meth)acrylic acid (preferably the content of repeating units derived from one type of (meth)acrylic acid, more preferably the content of repeating units derived from methacrylic acid) is preferably 8.0% by mass or more and 11.0% by mass or less of the total repeating units of the resin.

[0055] The acid value of the resin is preferably 60 mg KOH / g or more and 300 mg KOH / g or less, more preferably 80 mg KOH / g or more and 150 mg KOH / g or less, and even more preferably 100 mg KOH / g or more and 130 mg KOH / g or less. If the acid value of the resin is 60 mg KOH / g or more, the pigment particles are suitably dispersed in the aqueous medium, and the color development and coloring power of the ink are optimized. On the other hand, if the acid value of the resin is 300 mg KOH / g or less, the storage stability of the ink is optimized.

[0056] The mass-average molecular weight of the resin is preferably between 10,000 and 50,000, and more preferably between 15,000 and 30,000. A mass-average molecular weight of 10,000 to 50,000 of the resin optimizes the viscosity of the ink.

[0057] The ratio of the mass of resin to the mass of pigment (hereinafter sometimes referred to as the resin / pigment ratio) is preferably 0.50 or less. In order to suppress the occurrence of ink ejection irregularities from the recording head and to appropriately disperse the pigment particles in the aqueous medium, the resin / pigment ratio is preferably 0.35 or more and 0.50 or less. The resin / pigment ratio can be calculated using the formula "resin / pigment ratio = mass of resin / mass of pigment".

[0058] The resin content in the ink is preferably 0.5% by mass or more and 8.0% by mass or less, and more preferably 1.5% by mass or more and 4.0% by mass or less. By setting the resin content to 0.5% by mass or more, aggregation of quinacridone pigment can be suitably suppressed. By setting the resin content to 8.0% by mass or less, nozzle clogging of the recording head can be suitably suppressed.

[0059] <Aqueous medium> The aqueous medium contained in the ink is a medium containing water. The aqueous medium may function as a solvent or as a dispersion medium. Specific examples of aqueous mediums include aqueous mediums containing water and water-soluble organic solvents.

[0060] Examples of water-soluble organic solvents include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, γ-butyrolactone, thiodiglycol, and dimethyl sulfoxide.

[0061] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,2-pentanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, diethylene glycol, dipropylene glycol, trimethylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, 2-ethyl-1,2-hexanediol, and thiodiglycol. 3-methyl-1,5-pentanediol is preferred as the glycol compound.

[0062] Examples of triol compounds include glycerin, 1,2,3-butanetriol, and 1,2,6-hexanetriol. Glycerin is preferred as the triol compound.

[0063] Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether. Triethylene glycol monobutyl ether is preferred as the glycol ether compound.

[0064] Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone. 2-pyrrolidone is preferred as the lactam compound.

[0065] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0066] Examples of acetate compounds include diethylene glycol monoethyl ether acetate.

[0067] As the aqueous medium, a mixed solvent of water, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin is preferred.

[0068] The water-based medium content in the ink is preferably 30% by mass or more and 95% by mass or less, and more preferably 70% by mass or more and 95% by mass or less.

[0069] <Surfactants> The ink preferably further contains a surfactant. The surfactant optimizes the compatibility and dispersion stability of each component contained in the ink. Furthermore, the surfactant optimizes the penetration of the ink into the recording medium. A nonionic surfactant is preferred as the surfactant.

[0070] Examples of nonionic surfactants include acetylenediol and ethylene oxide adducts of acetylenediol. Examples of acetylenediols include 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol. Preferred nonionic surfactants are ethylene oxide adducts of acetylenediol and ethylene oxide adducts of acetylene glycol. The HLB value of the nonionic surfactant is preferably 4 to 14, more preferably 4 to 8 or 10 to 14. If the ink contains a surfactant, the surfactant content in the ink is preferably 0.01% by mass or more and 1.0% by mass or less.

[0071] <Other ingredients> The ink may further contain known additives as needed (more specifically, dissolving stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents, etc.).

[0072] <Ink manufacturing method> The ink manufacturing method of the first embodiment includes, for example, a step of preparing a pigment dispersion and a mixing step. Furthermore, if necessary, the ink manufacturing method preferably further includes one or both of an activated carbon treatment step and an ultrafiltration step. The activated carbon treatment step and the ultrafiltration step are preferably performed after the pigment dispersion preparation step and before the mixing step.

[0073] (Preparation process of pigment dispersion) In the preparation step of the pigment dispersion, quinacridone pigment, resin, and aqueous medium are mixed to obtain a pigment dispersion. To ensure sufficient dispersion of pigment particles, the pigment dispersion may further contain a surfactant. In the pigment dispersion, the pigment particles composed of quinacridone pigment and resin are D 50 Preferably, the wavelength is between 70 nm and 130 nm.

[0074] The quinacridone pigment content in the pigment dispersion is preferably 5% to 25% by mass, and more preferably 10% to 20% by mass. The resin content in the pigment dispersion is preferably 2% to 10% by mass, and more preferably 4% to 8% by mass. If the pigment dispersion contains a surfactant, the surfactant content in the pigment dispersion is preferably 0.1% to 2% by mass, and more preferably 0.3% to 1% by mass.

[0075] Pigment dispersions can be prepared by wet-dispersing the components contained in the aforementioned pigment dispersion using a media-type wet-disperser. Examples of media-type wet-dispersers include bead mills (more specifically, the "NanoGlen Mill" manufactured by Asada Iron Works Co., Ltd., the "MSC Mill" manufactured by Nippon Coke Industries Co., Ltd., and the "Dino® Mill" manufactured by Willy E. Bakkofen).

[0076] In wet dispersion using a media-type wet disperser, small-particle beads (e.g., beads with a diameter of 0.5 mm to 1.0 mm) are used as the media. By changing the diameter of the beads, it is possible to change, for example, the degree of dispersion of pigment particles, the first absorbance of the supernatant, and the ratio of unadsorbed resin. The smaller the diameter of the beads, the more the pigment particles are dispersed. 50 The ratio tends to decrease. The smaller the diameter of the beads, the easier it is for the core containing the quinacridone pigment to be coated with resin, and the primary absorbance of the supernatant liquid and the ratio of unadsorbed resin tend to decrease. The material of the beads is not particularly limited, but hard materials (e.g., glass and zirconia) are preferred. The discharge rate of the media-type wet disperser is, for example, 200 g / min to 600 g / min. The higher the discharge rate of the media-type wet disperser, the higher the primary absorbance of the supernatant liquid and the ratio of unadsorbed resin tend to be.

[0077] (Activated carbon treatment process) In the activated carbon treatment process, the pigment dispersion is treated with activated carbon. The activated carbon treatment removes unadsorbed resin and at least a portion of the intermediates. By including such a pigment dispersion in the ink, the first absorbance and the second absorbance of the supernatant obtained from the ink can be easily adjusted to values ​​within a desired range. The activated carbon treatment is carried out, for example, by circulating the pigment dispersion through an activated carbon filter. The flow rate of the circulating pigment dispersion is, for example, 100 g / min to 300 g / min. The circulation time of the pigment dispersion is, for example, 5 minutes to 30 minutes. If one pass is defined as 1000 g of pigment dispersion passing through the activated carbon filter, the number of passes in the activated carbon treatment is, for example, 5 times or less.

[0078] (Ultrafiltration process) In the ultrafiltration process, the pigment dispersion is ultrafiltered. Ultrafiltration removes at least a portion of the phosphorus-containing catalyst, unadsorbed resin, and intermediates. By including such a pigment dispersion in the ink, the predetermined phosphorus concentration of the ink, and the first and second absorbances of the supernatant obtained from the ink, can be easily adjusted to values ​​within a desired range. To remove at least a portion of the phosphorus-containing catalyst, unadsorbed resin, and intermediates, it is preferable to select a molecular weight cutoff of the ultrafiltration membrane used for ultrafiltration such that the phosphorus-containing catalyst, unadsorbed resin, and intermediates are filtered out, while other components of the pigment dispersion are not filtered out. Furthermore, it is preferable that the molecular weight cutoff of the ultrafiltration membrane used for ultrafiltration is greater than or equal to the molecular weight of the phosphorus-containing catalyst, unadsorbed resin, and intermediates. To adjust the predetermined phosphorus concentration to a value within a desired range, the ultrafiltration circulation time is preferably 0.4 hours or more. To reduce manufacturing costs, it is preferable that the ultrafiltration circulation time is 2.0 hours or less. Ultrafiltration removes a filtrate containing at least a portion of the phosphorus-containing catalyst, unadsorbed resin, and intermediates, while the remaining liquid containing the pigment dispersion components is recycled as a recovered liquid. By adding the same amount of water as the discharged filtrate to the recovered liquid, the solid content concentration of the pigment dispersion can be maintained at a constant level.

[0079] (Mixing process) In the mixing process, the pigment dispersion after either or both activated carbon treatment and / or ultrafiltration is mixed with components as needed (e.g., further added aqueous media and surfactants) using a stirrer. After mixing the ink components, foreign matter and coarse particles may be removed using a filter (e.g., a filter with a pore size of 5 μm or less).

[0080] The proportion of pigment dispersion in the total raw materials of the ink is, for example, 25% by mass or more and 60% by mass or less. The ink of the first embodiment can be suitably used, for example, in an inkjet recording device described later.

[0081] [Second Embodiment: Inkjet Recording Device] Next, an inkjet recording apparatus according to a second embodiment of the present invention will be described. The inkjet recording apparatus of the second embodiment comprises a transport unit for transporting a recording medium and a recording head. The recording head ejects the ink of the first embodiment described above onto the recording medium. The details of the inkjet recording apparatus of the second embodiment will be described below with reference to the drawings. Note that the drawings shown are schematic representations mainly of each component for ease of understanding, and the size, number, etc. of each component shown may differ from the actual dimensions.

[0082] Figure 2 shows the configuration of the inkjet recording apparatus 1 according to the second embodiment. The X, Y, and Z axes shown in Figure 2 and Figures 3 to 5 described later are orthogonal to each other.

[0083] The inkjet recording device 1 shown in Figure 2 comprises a paper feed unit 3, a first recording head 4C, a second recording head 4M, a third recording head 4Y, a fourth recording head 4K, a liquid storage unit 5, a first transport unit 6, a second transport unit 7, an ejection unit 8, and a maintenance unit 9. Hereafter, when it is not necessary to distinguish between the first recording head 4C to the fourth recording head 4K, they may simply be referred to as "recording head 4".

[0084] The paper feeding unit 3 comprises a plurality of paper feeding cassettes 31, a plurality of pickup rollers 32, a plurality of transport rollers 33, and a pair of registration rollers 34. Recording media S are stacked and stored in the paper feeding cassettes 31. The pickup rollers 32 pick up the recording media S stored in the paper feeding cassettes 31 one by one. The transport rollers 33 transport the recording media S picked up by the pickup rollers 32. The pair of registration rollers 34 temporarily hold the recording media S transported by the transport rollers 33 and then supply them to the first transport unit 6 at a predetermined timing.

[0085] The recording head 4 is positioned above the first transport belt 63. The first to fourth recording heads 4C to 4K are arranged in this order in the transport direction D of the recording medium S. Each of the first to fourth recording heads 4C to 4K is positioned at the same height. The first to fourth recording heads 4C to 4K are each filled with four different colors of ink (for example, cyan, magenta, yellow, and black). The ink filled in the second recording head 4M is the magenta ink of the first embodiment. Each of the recording heads 4 ejects ink onto the recording medium S. Of the recording heads 4, the second recording head 4M ejects the magenta ink of the first embodiment onto the recording medium S. As a result, an image (for example, a color image) is formed on the recording medium S transported by the first transport belt 63.

[0086] Because the ink of the first embodiment is used, for the same reasons as described in the first embodiment, the inkjet recording apparatus 1 of the second embodiment can suppress the occurrence of ink ejection irregularities from the second recording head 4M and can form images with excellent scratch resistance. Furthermore, because the ink of the first embodiment has excellent resolubility, the inkjet recording apparatus 1 of the second embodiment using the ink of the first embodiment also has excellent cleaning properties.

[0087] The liquid storage unit 5 comprises a first ink tank 51C, a second ink tank 51M, a third ink tank 51Y, a fourth ink tank 51K, and a cleaning fluid tank 52. Hereinafter, when it is not necessary to distinguish between the first ink tank 51C to the fourth ink tank 51K, they may simply be referred to as "ink tank 51". The first ink tanks 51C to the fourth ink tanks 51K each contain four different colors of ink (for example, cyan, magenta, yellow, and black). The ink contained in the second ink tank 51M is the magenta ink of the first embodiment. The first ink tanks 51C to the fourth ink tanks 51K each supply ink to the first recording heads 4C to the fourth recording heads 4K. The cleaning fluid tank 52 supplies cleaning fluid to the liquid-impregnated body 91.

[0088] The first transport unit 6 is located downstream of the paper feeding unit 3 in the transport direction D of the recording medium S. The first transport unit 6 comprises a first driven roller 61, a first drive roller 62, and a first transport belt 63. The first drive roller 62 is located downstream of the first driven roller 61 in the transport direction D of the recording medium S. The first transport belt 63 is an endless belt stretched between the first driven roller 61 and the first drive roller 62. The first drive roller 62 is driven to rotate counterclockwise in Figure 2. This causes the first drive roller 62 to drive the first transport belt 63. As a result, the first transport belt 63 transports the recording medium S fed from the paper feeding unit 3 to the second transport unit 7 in the transport direction D. The first driven roller 61 rotates driven by the first drive roller 62 via the first transport belt 63.

[0089] The second transport unit 7 is located downstream of the first transport unit 6 in the transport direction D of the recording medium S. The second transport unit 7 comprises a second driven roller 71, a second drive roller 72, and a second transport belt 73. The second drive roller 72 is located downstream of the second driven roller 71 in the transport direction D of the recording medium S. The second transport belt 73 is an endless belt stretched between the second driven roller 71 and the second drive roller 72. The second drive roller 72 is driven to rotate counterclockwise in Figure 2. This causes the second drive roller 72 to drive the second transport belt 73. As a result, the second transport belt 73 transports the recording medium S transported from the first transport unit 6 to the discharge unit 8 in the transport direction D. The second driven roller 71 rotates driven by the second drive roller 72 via the second transport belt 73.

[0090] The discharge unit 8 is located downstream of the second transport unit 7 in the transport direction D of the recording medium S. The discharge unit 8 comprises a discharge tray 81, a discharge drive roller 82, and a discharge driven roller 83. The discharge drive roller 82 and the discharge driven roller 83 are pressed against each other at opposing positions. The discharge drive roller 82 is driven to rotate counterclockwise in Figure 2. The discharge driven roller 83 rotates in accordance with the rotation of the discharge drive roller 82. As a result, the discharge drive roller 82 and the discharge driven roller 83 discharge the recording medium S transported from the second transport unit 7 to the discharge tray 81. The discharged recording medium S is placed on the discharge tray 81.

[0091] The maintenance unit 9 comprises a liquid-impregnated body 91 and a cleaning member 92. The liquid-impregnated body 91 is impregnated with cleaning liquid. The liquid-impregnated body 91 contacts the ejection surface 42 (see Figure 3) of the recording head 4 and supplies cleaning liquid to the ejection surface 42. The liquid-impregnated body 91 is, for example, a sponge, nonwoven fabric, or absorbent sheet. The cleaning member 92 wipes the ejection surface 42 of the recording head 4. This cleans the ink adhering to the ejection surface 42. The cleaning member 92 is, for example, a rubber wiper.

[0092] Examples of cleaning solutions include water, polyhydric alcohol, triethylene glycol monobutyl ether, lactam, and a mixture of polyhydric alcohols. In the cleaning solution, the water content, polyhydric alcohol content, triethylene glycol monobutyl ether content, lactam content, and polyhydric alcohol content are preferably 60% to 70% by mass, 10% to 15% by mass, 5% to 10% by mass, 5% to 10% by mass, and 1% to 5% by mass, respectively. An example of a cleaning solution having such a composition is the head cleaning solution for the TASKalfa Pro 15000c manufactured by Kyocera Document Solutions Inc.

[0093] Next, the recording head 4 will be further described with reference to Figure 3. Figure 3 shows the bottom surface of the recording head 4 shown in Figure 2.

[0094] As shown in Figure 3, the recording head 4 comprises a first nozzle row N1, a second nozzle row N2, and an ejection surface 42. For ease of understanding, in Figure 3, the first nozzle row N1 and the second nozzle row N2 are each enclosed by dashed lines. Each of the first nozzle row N1 and the second nozzle row N2 contains a plurality of nozzles 41. The nozzles 41 eject ink onto the recording medium S. The nozzles 41 open onto the ejection surface 42. The first nozzle row N1 and the second nozzle row N2 are arranged side by side in the transport direction D of the recording medium S. In each of the first nozzle row N1 and the second nozzle row N2, the plurality of nozzles 41 are spaced apart in a direction perpendicular to the transport direction D of the recording medium S. The recording head 4 is, for example, a line head.

[0095] The width 41w of each of the first nozzle row N1 and the second nozzle row N2 (i.e., the width of the area that can be recorded by the recording head 4) is equal to or wider than the width of the recording medium S. Therefore, the recording head 4 can record an image on the recording medium S being transported on the first transport belt 63 while remaining fixed. In other words, the inkjet recording device 1 employs a single-pass method, which does not involve shuttle motion. Because the inkjet recording device 1 of the second embodiment is equipped with such a recording head 4, it can print at a higher speed compared to an inkjet recording device equipped with a serial head.

[0096] Next, the cleaning operation by the maintenance unit 9 will be described with reference to Figures 4 and 5. The cleaning operation includes the supply of cleaning fluid, the purging operation, and the wiping operation. Figure 4 is a diagram illustrating the supply of cleaning fluid. Figure 5 is a diagram illustrating the purging operation and the wiping operation. Although the nozzle 41 is not visible in the side view of the recording head 4, its position is indicated by a dashed line in Figures 4 and 5 for ease of understanding.

[0097] As shown in Figure 4, the recording head 4 is further equipped with an ink inlet 43 and an ink outlet 44. Ink flows from the ink tank 51 through the ink inlet 43 to the recording head 4 and flows out of the recording head 4 through the ink outlet 44.

[0098] As shown in Figure 2, the liquid-impregnated body 91 of the maintenance unit 9 is positioned below the second conveyor belt 73. The cleaning member 92 of the maintenance unit 9 is positioned below the liquid-impregnated body 91. The liquid-impregnated body 91 and the cleaning member 92 are each movable between a position facing the second conveyor unit 7 and a position facing the ejection surface 42 of the recording head 4. Furthermore, as shown in Figure 4, the liquid-impregnated body 91 is movable in both the upward direction D1 and the downward direction D2. As shown in Figure 5, the cleaning member 92 is movable in both the upward direction D1, the downward direction D2, and the wiping direction D3. The "upward direction D1" is the direction approaching the ejection surface 42 in the Z-axis direction. The "downward direction D2" is the direction away from the ejection surface 42 in the Z-axis direction. The "wiping direction D3" is the direction along the ejection surface 42. The liquid-impregnated body 91 and the cleaning member 92 are each moved by a known drive mechanism (not shown).

[0099] At this point, ink adhering to the ejection surface 42 may dry and harden. A cleaning operation is performed to clean off this dried ink.

[0100] First, the cleaning liquid supply operation of the cleaning process will be explained. The cleaning liquid is impregnated into the liquid-impregnated body 91. Next, as shown in Figure 4, the liquid-impregnated body 91 moves to a position facing the discharge surface 42, and then moves upward in the direction D1. Then, the liquid-impregnated body 91 is pressed against the discharge surface 42. In this way, the cleaning liquid impregnated into the liquid-impregnated body 91 adheres to the discharge surface 42. It is preferable that the state in which the liquid-impregnated body 91 is pressed against the discharge surface 42 is maintained for a predetermined time. The predetermined time is preferably 1 second or more and 5 minutes or less. After the predetermined time has elapsed, the liquid-impregnated body 91 moves downward in the direction D2. Then, the state in which the liquid-impregnated body 91 is pressed against the discharge surface 42 is released.

[0101] Next, the purging operation will be explained. As shown in Figure 5, ink is purged from the recording head 4. In Figure 5, the purged ink (purged ink) is indicated by the label "Nf". Specifically, the ink is forcibly discharged from the nozzle 41 by pressurizing the recording head 4. This clears any clogging of the nozzle 41, and the purged ink Nf adheres to the discharge surface 42 of the recording head 4.

[0102] Next, the wiping operation will be described. The cleaning member 92 moves to a position facing the ejection surface 42 (the position shown in Figure 5), and then moves in the upward direction D1. The cleaning member 92 is then pressed against the ejection surface 42. While the state in which the cleaning member 92 is pressed against the ejection surface 42 is maintained, the cleaning member 92 moves in a direction along the ejection surface 42 (the wiping direction D3 shown in Figure 5). As a result, the cleaning member 92 wipes the ejection surface 42. Consequently, ink (e.g., dried ink and purge ink Nf) and cleaning fluid adhering to the ejection surface 42 are removed. This cleans the ejection surface 42 of the recording head 4. Next, the cleaning member 92 moves in the downward direction D2. The state in which the cleaning member 92 is pressed against the ejection surface 42 is then released.

[0103] The ink of the first embodiment has excellent resolubility. Therefore, even if the ink of the first embodiment dries and adheres to the ejection surface 42 of the recording head 4, the dried ink is easily dissolved in the purge ink Nf and cleaning fluid. Accordingly, the inkjet recording apparatus 1 of the second embodiment using the ink of the first embodiment has excellent cleaning properties for the ejection surface 42 of the recording head 4.

[0104] The above describes an inkjet recording device 1, which is an example of an inkjet recording device according to the second embodiment. However, the inkjet recording device of the second embodiment is not limited to the inkjet recording device 1. The inkjet recording device of the second embodiment may employ a multi-pass method. In addition, the number of nozzles 41, the spacing between nozzles 41, and the positional relationship of the nozzles 41 in the first recording head 4C to the fourth recording head 4K can be appropriately set according to the specifications of the device. In addition, the cleaning fluid supply operation may be discharge of cleaning fluid by the inkjet method, application of cleaning fluid using a roller, or spraying of cleaning fluid. Furthermore, the cleaning fluid supply operation, purging operation, and wiping operation may each be repeated. Furthermore, the order in which the cleaning fluid supply operation and purging operation are performed is not limited. In addition, the cleaning member 92 may move back and forth in the direction along the discharge surface 42. For example, while the cleaning member 92 is maintained in contact with the discharge surface 42, it may move in a first direction along the discharge surface 42 (wiping direction D3 shown in Figure 5), and then move in a second direction opposite to the first direction along the discharge surface 42 (the direction opposite to the wiping direction D3 shown in Figure 5). [Examples]

[0105] The following describes embodiments of the present invention. However, the present invention is not limited to the following embodiments. In the following embodiments, ion-exchanged water will be simply referred to as water.

[0106] [Preparing the resin] First, resins (RA) and (RB) used in the preparation of the ink were prepared. Table 1 shows the types and percentages of repeating units present in resins (RA) and (RB), their mass-average molecular weight, and their acid value.

[0107] [Table 1]

[0108] The terms used in Table 1 are as follows: "MAA" indicates repeating units derived from methacrylic acid. "MMA" indicates repeating units derived from methyl methacrylate. "BA" indicates repeating units derived from butyl acrylate. "ST" indicates repeating units derived from styrene. "Percentage" indicates the proportion of each repeating unit relative to the total mass of repeating units in the resin.

[0109] <Measurement of the acid value of resins> The acid value of each resin was measured in accordance with "JIS (Japanese Industrial Standards) K0070:1992".

[0110] <Measurement of mass-average molecular weight of resin> The mass-average molecular weight of each resin was measured using gel permeation chromatography (HLC-8020GPC, manufactured by Tosoh Corporation) under the following measurement conditions. Calibration curves were created using TSKgel standard polystyrenes F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000, manufactured by Tosoh Corporation, and n-propylbenzene.

[0111] (Measurement conditions for mass-average molecular weight) • Column: TSKgel SuperMultiporeHZ-H manufactured by Tosoh Corporation (semi-micro column with 4.6mm I.D. x 15cm) • Number of columns: 3 • Eluent: Tetrahydrofuran ·Flow rate: 0.35mL / min • Sample injection volume: 10 μL ·Measurement temperature: 40℃ • Detector: RI (refractive index) detector

[0112] [Preparation of inks (I-1), (I-3), (I-5), (I-7), (I-8), and (I-12)] The inks (I-1), (I-3), (I-5), (I-7), (I-8), and (I-12) shown in Table 2 were prepared by the following method. The conditions for dispersion treatment, activated carbon treatment, unadsorbed resin ratio, first absorbance, second absorbance, and predetermined phosphorus concentration for these inks are shown in Table 2. Ultrafiltration was not performed in the preparation of these inks.

[0113] [Table 2]

[0114] In Table 2, "-" indicates that activated carbon treatment was not performed. In Table 2 and Table 5 described later, "320nm" indicates that the first peak was observed at a wavelength of 320nm, and "429nm" indicates that the second peak was observed at a wavelength of 429nm.

[0115] <Preparation of ink (I-1)> (Preparation of pigment dispersion) Pigment dispersions were prepared to match the formulation da shown in Table 3.

[0116] [Table 3]

[0117] The terms used in Table 3 are as follows: "Pigment PR-122" refers to quinacridone pigment (CI Pigment Red 122). "Orphine E1010" refers to a nonionic surfactant (Orphine® E1010 manufactured by Nisshin Chemical Industry Co., Ltd., content: ethylene oxide adduct of acetylenediol, active ingredient concentration: 100% by mass, HLB value: 13.5±0.5).

[0118] First, 6.0 parts by mass of resin (RA) was mixed with an aqueous sodium hydroxide solution. The aqueous sodium hydroxide solution contained a predetermined amount of sodium hydroxide. The "determined amount" of sodium hydroxide added, shown in Table 3, represents 1.05 times the amount required for equivolent neutralization of resin (RA). In this way, resin (RA) was neutralized with an equal amount (strictly speaking, 105% equivalent) of sodium hydroxide to obtain aqueous solution I containing resin (RA).

[0119] The entire volume of the obtained aqueous solution I, along with 15.0 parts by mass of quinacridone pigment (CI Pigment Red 122), 0.5 parts by mass of nonionic surfactant (Orphine® E1010, manufactured by Nisshin Chemical Industry Co., Ltd.), and the remaining water were placed into a vessel. The contents of the vessel were mixed using a media-type wet disperser (DYNO®-MILL, manufactured by Willy E. Bakkofen (WAB)) to obtain mixed solution II.

[0120] Note that the "residual amount" of water added in Table 3 refers to the amount that makes Mixture II 100.0 parts by mass. The residual amount of water shown in Table 3 is the total amount of water added to the vessel as described above, plus the water contained in Aqueous Solution I (specifically, the water contained in the sodium hydroxide aqueous solution used to neutralize the resin, and the water produced by the neutralization reaction between the resin and sodium hydroxide).

[0121] Next, the contents of the vessel described above were dispersed using zirconia beads (particle size 0.5 mm) as a media and a bead mill ("Nanograin Mill" manufactured by Asada Iron Works Co., Ltd.). The conditions for dispersion using the bead mill were a temperature of 10°C, a peripheral speed of 8 m / sec, and a discharge rate of 700 g / min. This yielded pigment dispersion III.

[0122] It was confirmed that pigment particles with a median volume diameter in the range of 70 nm to 130 nm were dispersed in pigment dispersion III. The median volume diameter of the pigment particles was measured using a dynamic light scattering particle size distribution analyzer (ZetaSizer Nano ZS, manufactured by Spectris Co., Ltd.) with a diluted solution of pigment dispersion III diluted 300 times with water as the measurement sample.

[0123] (Activated carbon treatment) The obtained pigment dispersion III was treated with activated carbon. Specifically, 1800g of pigment dispersion III was passed through an activated carbon filter (YCC-1L, manufactured by Nippon Filter Co., Ltd.; type of activated carbon: coconut shell granular activated carbon) while circulating the pigment dispersion III at a flow rate of 190g / min. Hereafter, the passage of 1000g of pigment dispersion III through the activated carbon filter will be referred to as one pass. Since the flow rate was 190g / min, the time required for one pass was 5 minutes and 15 seconds (=(1000g) / (190g / min)). At the time of one pass, the circulation of pigment dispersion III was stopped, and pigment dispersion IV after activated carbon treatment was obtained. Therefore, the number of passes in the activated carbon treatment was 1.

[0124] (Ink preparation) Ink (I-1) was prepared to match the formulation ia shown in Table 4.

[0125] [Table 4]

[0126] The terms used in Table 4 are as follows: "Surfinol 420" refers to a nonionic surfactant (Surfinol® 420, manufactured by Nisshin Chemical Industry Co., Ltd., content: ethylene oxide adduct of acetylene glycol, active ingredient concentration: 100% by mass, HLB value: 4).

[0127] First, water was added to a flask equipped with a stirrer ("Three One Motor® BL-600" manufactured by Shinto Kagaku Co., Ltd.). While stirring the contents of the flask at a stirring speed of 400 rpm using the stirrer, the activated carbon treated pigment dispersion IV, a nonionic surfactant ("Surfinol® 420" manufactured by Nisshin Chemical Industry Co., Ltd.), triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and glycerin were added to obtain mixture V. The amount of each raw material added is shown in Table 4. The "residual amount" shown in Table 4, which is the amount of water added, is the amount that makes mixture V 100.0 parts by mass. Mixture V was filtered using a filter with a pore size of 5 μm to remove foreign matter and coarse particles from mixture V. This obtained ink (I-1).

[0128] <Preparation of inks (I-3), (I-5), (I-7), (I-8), and (I-12)> Inks (I-3), (I-8), and (I-12) were prepared in the same manner as ink (I-1), except that the discharge volume for dispersion treatment and the number of passes for activated carbon treatment were set as shown in Table 2 above. Inks (I-5) and (I-7) were prepared in the same manner as ink (I-1), except that the discharge volume for dispersion treatment was set as shown in Table 2 above, and activated carbon treatment was not performed.

[0129] [measurement] The samples to be measured (inks (I-1), (I-3), (I-5), (I-7), (I-8), and (I-12)) were centrifuged using the following method. For each supernatant obtained, the first absorbance, second absorbance, predetermined phosphorus concentration, and unadsorbed resin ratio were measured. The measurement results are shown in Table 2 above.

[0130] <Centrifugation> Under conditions of 23°C, a 2g sample of the material, sealed in a container, was centrifuged for 3 hours using an ultracentrifuge (Eppendorf Himac Technologies Ltd. "himac® CS150FNX", rotor: S140AT) at a rotation speed of 140,000 rpm (equivalent to a centrifugal force of 1,050,000 G). This caused the pigment particles contained in the sample to precipitate.

[0131] <Measurement of the first and second absorbances> The entire volume of the supernatant liquid contained in the sample after centrifugation was collected using a syringe. The collected supernatant liquid was diluted 25 times with water and used as the measurement sample. The measurement sample placed in the cell was measured using a spectrophotometer (Hitachi High-Tech Science Co., Ltd. "U-3000") under the following conditions to obtain the ultraviolet-visible light absorption spectrum of the measurement sample (i.e., the supernatant liquid). The first and second absorbances of the supernatant liquid were determined from the ultraviolet-visible light absorption spectrum.

[0132] (Measurement conditions for absorbance) • Measurement wavelength range: 200 nm to 800 nm • Scan speed: 300nm / min • Sampling interval: 1nm • Slit width: 1nm • Cell: Quartz glass cell ·Optical path length: 10mm • Beam system: Double beam • Baseline measurement: Yes • Reference: Water

[0133] <Measurement of specified phosphorus concentration> One milliliter of the supernatant liquid from the sample after centrifugation was collected using a syringe. The collected supernatant liquid was diluted 50-fold with water and used as the measurement sample. The measurement sample was measured using an ICP (Inductively Coupled Plasma) mass spectrometer (Thermo Fisher Scientific "iCAP PRO ICP-OES Duo"). From the obtained values, the phosphorus concentration (in ppm) of the 50-fold diluted supernatant liquid was determined. A calibration curve created using samples with known phosphorus concentrations was used to determine the phosphorus concentration.

[0134] <Measurement of unadsorbed resin ratio> The entire supernatant liquid contained in the sample after centrifugation was collected. Next, the entire collected supernatant liquid was placed in a disposable cup and dried under reduced pressure at 60°C for 24 hours to obtain the residue. The mass (WA) of the residue was measured. The mass (WA) of the residue was considered to be the mass of unadsorbed resin.

[0135] Based on the resin (RA) content B (=6.0 mass%) in the pigment dispersion, which can be read from Table 3, and the pigment dispersion content C (=40.0 mass%) in the ink, which can be read from Table 4, the mass of resin (WD) contained in 2g of the sample was calculated according to the following formula. WD = 2 × (C / 100) × (B / 100)

[0136] Then, the unadsorbed resin ratio was calculated from the mass of the residue (WA) obtained from 2g of the sample and the mass of the resin (WD) contained in 2g of the sample, according to the following formula. Unadsorbed resin ratio [mass%]=100×WA / WD

[0137] [Preparation of inks (I-2), (I-4), (I-6), (I-9), (I-10), (I-11), (I-13), and (I-14)] The inks (I-2), (I-4), (I-6), (I-9), (I-10), (I-11), (I-13), and (I-14) shown in Table 5 were prepared by the following method. The conditions for dispersion treatment, ultrafiltration treatment, unadsorbed resin ratio, first absorbance, second absorbance, and predetermined phosphorus concentration for these inks are shown in Table 5. Activated carbon treatment was not performed in the preparation of these inks.

[0138] [Table 5]

[0139] In Table 5, "-" indicates that ultrafiltration treatment was not performed.

[0140] <Preparation of ink (I-2)> Ink (I-2) was prepared in the same manner as ink (I-1), except that ultrafiltration treatment was performed using the following method instead of activated carbon treatment.

[0141] (Ultrafiltration treatment) The pigment dispersion III obtained in the "Preparation of Pigment Dispersion" described above was filtered by vibratory ultrafiltration. For details, the ultrafiltration membrane was Asahi Kasei Corporation's "UF Bensil Type Module AHP-0013D," with hollow fiber membrane material: polyacrylonitrile, membrane inner diameter: 0.8 mm, and effective membrane area: 170 cm². 2 Using an ultrafiltration system, 480 g of pigment dispersion III was circulated at a flow rate of 390 g / min for 60 minutes under the condition that the internal pressure on the ultrafiltration membrane was 50 kPa. That is, the circulation time was 60 minutes. By ultrafiltration, the liquid containing at least a portion of the unadsorbed resin, intermediates, and phosphorus-containing catalyst from pigment dispersion III was discharged as filtrate, and the remaining liquid was circulated again as recovered liquid. The solid content concentration of the pigment dispersion was kept constant by adding the same amount of water as the discharged filtrate to the recovered liquid. In this way, pigment dispersion VI after ultrafiltration was obtained. Pigment dispersion VI was used in place of pigment dispersion IV for the "Ink Preparation" described above.

[0142] <Preparation of inks (I-4), (I-10), and (I-13)> Inks (I-4), (I-10), and (I-13) were prepared in the same manner as ink (I-2), except that the ultrafiltration circulation time was set as shown in Table 5 above.

[0143] <Preparation of ink (I-6)> Ink (I-6) was prepared in the same manner as ink (I-2), except that CI Pigment Violet 19 was used instead of CI Pigment Red 122 as the quinacridone pigment.

[0144] <Preparation of ink (I-9)> Ink (I-9) was prepared in the same manner as ink (I-2), except that ultrafiltration was not performed and a manufacturing lot of CI Pigment Red 122 with a low intermediate content and a high phosphorus content was used as the quinacridone pigment.

[0145] <Preparation of Ink (I-11)> Ink (I-11) was prepared in the same manner as ink (I-2), except that ultrafiltration was not performed and a manufacturing lot of CI Pigment Red 122 with a high intermediate content and low phosphorus content was used as the quinacridone pigment.

[0146] <Preparation of Ink (I-14)> Ink (I-14) was prepared in the same manner as ink (I-2), except that resin (RB) was used instead of resin (RA) as the resin.

[0147] [measurement] Using the same method as described above for the measurements of inks (I-1), (I-3), (I-5), (I-7), (I-8), and (I-12), the supernatant liquid obtained from each of the target inks (I-2), (I-4), (I-6), (I-9), (I-10), (I-11), (I-13), and (I-14) was measured for the first absorbance, second absorbance, predetermined phosphorus concentration, and unadsorbed resin ratio. The measurement results are shown in Table 5 above.

[0148] [evaluation] For each of the inks (I-1) to (I-14), the presence or absence of ink ejection irregularities from the recording head, the scratch resistance of the image formed using the ink, and the resolubility of the ink were evaluated using the following method. The evaluation results are shown in Tables 6 and 7 below.

[0149] For these evaluations, an inkjet recording device (a prototype manufactured by Kyocera Document Solutions Inc.) was used as the evaluation machine. This evaluation machine was equipped with a piezo-type line head with nozzles (aperture radius: 10 μm) and a wiper as the recording head. The evaluation target (any of the inks (I-1) to (I-14)) was set in the magenta ink recording head of the evaluation machine. Plain paper (Fuji Xerox Co., Ltd. "C2", A4 size PPC paper) was used as the paper.

[0150] <Evaluation of discharge turbulence> The evaluation of ink ejection irregularities from the recording head was performed in an environment with a temperature of 25°C and a humidity of 60% RH. The recording head temperature was set to 40°C. The ink ejection amount per pixel was set to 3.5 pL. Using the evaluation machine, an image (20.5 mm x 29.0 mm) with image processing settings that ejected ink from all nozzles of the recording head was printed continuously on paper for one hour. The first image printed during continuous printing (initial image) and the last image printed during continuous printing (end-printed image) were observed with the naked eye. The presence or absence of white streaks in the initial image and end-printed image was checked. White streaks are image defects caused by ink ejection irregularities from the recording head. Ink ejection irregularities from the recording head were judged according to the following criteria.

[0151] (Criteria for discharge swirl) Good (A): In the print-saturated image, white streaks are less prevalent than in the initial image. Defect (B): In the print-saturated image, white streaks are more prevalent than in the initial image.

[0152] <Evaluation of abrasion resistance> The abrasion resistance was evaluated under conditions of 25°C and 60% RH. The ink ejection rate per pixel was set to 11 pL. Using the evaluation machine, a solid image (4 cm x 5 cm) was printed on one sheet of paper (hereinafter sometimes referred to as Paper A). Next, the abrasion test described below was performed. In the abrasion test, an unused sheet of paper (hereinafter sometimes referred to as Paper B) was placed on top of the solid image printed on Paper A. Next, a 1 kg weight was placed on Paper B. Then, Paper B and the weight were moved together so that only the weight of the weight itself was applied, causing Paper B to rub against the solid image five times. After the abrasion test, the image density of areas on Paper A where no solid image was formed was measured at 224 locations using a reflectance densitometer (X-Rite "RD-19"). The average value of the measured image density was used as the evaluation value. A lower evaluation value indicates less color transfer due to abrasion and a more abrasion-resistant image. The scratch resistance of images formed using ink was determined according to the following evaluation criteria.

[0153] (Standards for abrasion resistance) A (Good): The evaluation score is less than 0.020. B (Poor): The evaluation value is 0.020 or higher.

[0154] <Evaluation of resolubility> 0.3 mL of ink was placed on the tip of the wiper equipped with the evaluation machine and left for 10 minutes in an environment of 25°C and 60% RH humidity. Next, the ink-covered wiper was used to wipe the ejection surface of the recording head in the forward direction (opposite to the wiping direction D3 in Figure 5), spreading the ink across the ejection surface. The spread ink was dried at 45°C for 4 days to form dried ink on the ejection surface.

[0155] After the dry ink had formed, a cleaning operation was performed using the evaluation machine. Specifically, a nonwoven fabric impregnated with 3g of cleaning solution was pressed against the ejection surface of the recording head for 30 seconds (corresponding to the cleaning solution supply operation). As the cleaning solution, the head cleaning solution for the inkjet color production printer "TASKalfa Pro 15000c" manufactured by Kyocera Document Solutions Inc. was used. As the nonwoven fabric, a cut piece of "Bencott® M-3II" manufactured by Asahi Kasei Corporation was used. Next, the nonwoven fabric was separated from the ejection surface of the recording head. Next, 0.3 mL of ink was forcibly discharged (purged) from the recording head (corresponding to the purging operation). Next, the ejection surface of the recording head was wiped in the return direction (wiping direction D3 in Figure 5) using a wiper (corresponding to the wiping operation). This removed the dry ink adhering to the ejection surface of the recording head, along with the cleaning solution and purged ink. The details of the cleaning operation performed in this test are generally the same as the cleaning operation described with reference to Figures 4 and 5. Next, the ejection surface of the recording head was visually inspected to check for any remaining dried ink that could not be cleaned. Note that the more easily the dried ink dissolves in the cleaning solution and purge ink, the easier it is to remove the dried ink adhering to the ejection surface of the recording head. The resolubility of the ink was determined according to the following evaluation criteria.

[0156] (Criteria for resolubility) Good (A): No dried ink is observed on the ejection surface of the recording head. Defect (B): Dried ink is observed on the ejection surface of the recording head.

[0157] For further understanding, the unadsorbed resin ratio, first absorbance, second absorbance, and predetermined phosphorus concentration shown in Tables 2 and 5 are again shown in Tables 6 and 7 below.

[0158] [Table 6]

[0159] [Table 7]

[0160] As shown in Table 7, the first absorbance of the supernatant obtained from ink (I-7) was greater than 0.40. Ink (I-7) was unable to suppress the occurrence of ink ejection irregularities from the recording head.

[0161] As shown in Table 7, the first absorbance of the supernatant obtained from ink (I-8) was less than 0.16. Images formed using ink (I-8) had poor scratch resistance.

[0162] As shown in Table 7, the predetermined phosphorus concentration of ink (I-9) was greater than 6.0 ppm. Ink (I-9) failed to suppress the occurrence of ink ejection irregularities from the recording head.

[0163] As shown in Table 7, the specified phosphorus concentration of ink (I-10) was less than 1.0 ppm. Ink (I-10) had poor resolubility.

[0164] As shown in Table 7, the second absorbance of the supernatant obtained from ink (I-11) was greater than 0.80. Ink (I-11) was unable to suppress the occurrence of ink ejection irregularities from the recording head.

[0165] As shown in Table 7, the second absorbance of the supernatant obtained from ink (I-12) was less than 0.15. Ink (I-12) had poor resolubility.

[0166] On the other hand, as shown in Table 6, the predetermined phosphorus concentrations of inks (I-1) to (I-6) and (I-13) to (I-14) were 1.0 ppm to 6.0 ppm, the first absorbance of the supernatant was 0.16 to 0.40, and the second absorbance of the supernatant was 0.15 to 0.80. Inks (I-1) to (I-6) and (I-13) to (I-14) were able to suppress the occurrence of ink ejection irregularities from the recording head, form images with excellent scratch resistance, and exhibit excellent re-solubility.

[0167] Therefore, the inks of the present invention, which include inks (I-1) to (I-6) and (I-13) to (I-14), are judged to be able to suppress the occurrence of ink ejection irregularities from the recording head, form images with excellent scratch resistance, and have excellent re-dissolvability. Furthermore, the inkjet recording apparatus of the present invention using such inks is judged to be able to suppress the occurrence of ink ejection irregularities from the recording head, form images with excellent scratch resistance, and have excellent cleaning performance. [Industrial applicability]

[0168] The ink and inkjet recording apparatus of the present invention can be used to form an image. [Explanation of symbols]

[0169] 1: Inkjet recording device 4: Recording head 6: First Conveyor Unit 41: Nozzle 42:Discharge surface 92: Cleaning parts S: Recording medium

Claims

1. An inkjet ink comprising a quinacridone pigment, a resin, and an aqueous medium, The phosphorus concentration in a 50-fold diluted solution of the supernatant obtained by centrifuging the aforementioned inkjet ink at 1,050,000 G for 3 hours is between 1.0 ppm and 6.0 ppm. The supernatant liquid is diluted 25 times with water to obtain the measurement sample, and in the ultraviolet-visible light absorption spectrum of the measurement sample, the first absorbance of the first peak is 0.16 or more and 0.40 or less, and the first peak is the maximum peak in the wavelength range of 300 nm to 340 nm. In the ultraviolet-visible light absorption spectrum of the measured sample, the second absorbance of the second peak is 0.15 or more and 0.80 or less, and the second peak is the maximum peak in the wavelength range of 400 nm to 490 nm, wherein the inkjet ink is described above.

2. The resin comprises an adsorbed resin that is adsorbed onto the quinacridone pigment and an unadsorbed resin that is not adsorbed onto the quinacridone pigment. The inkjet ink according to claim 1, wherein the proportion of the unadsorbed resin in the resin is 20% by mass or more and 50% by mass or less.

3. The resin comprises an adsorbed resin that is adsorbed onto the quinacridone pigment and an unadsorbed resin that is not adsorbed onto the quinacridone pigment. The inkjet ink according to claim 1 or 2, wherein the first peak is a peak derived from the unadsorbed resin.

4. The inkjet ink according to claim 1 or 2, wherein the second peak is a peak derived from an intermediate for synthesizing the quinacridone pigment.

5. The inkjet ink according to claim 1 or 2, wherein the phosphorus concentration is the concentration of phosphorus atoms derived from the phosphorus-containing catalyst for synthesizing the quinacridone pigment.

6. The inkjet ink according to claim 1 or 2, wherein the phosphorus concentration is the concentration of phosphorus atoms contained in at least one selected from the group consisting of polyphosphate and its derivatives.

7. The inkjet ink according to claim 1 or 2, wherein the resin comprises repeating units derived from at least one (meth)acrylic acid, repeating units derived from at least one alkyl (meth)acrylate, and repeating units derived from styrene.

8. A transport unit that transports the recording medium, It comprises a recording head that ejects ink onto the recording medium, An inkjet recording apparatus wherein the ink is the inkjet ink described in claim 1 or 2.

9. The inkjet recording apparatus according to claim 8, wherein the recording head is a line head.

10. The recording head has an ejection surface through which the nozzle for ejecting the ink opens. The inkjet recording apparatus according to claim 8, further comprising a cleaning member for wiping the discharge surface.

Images