CN112094539B

CN112094539B - Aqueous ink, ink cartridge, and ink jet recording method

Info

Publication number: CN112094539B
Application number: CN202010542986.0A
Authority: CN
Inventors: 泷口修平; 西野勇辉; 齐藤秀行; 七宫隆树
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2019-06-18
Filing date: 2020-06-15
Publication date: 2022-06-21
Anticipated expiration: 2040-06-15
Also published as: CN112094539A

Abstract

The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method. An aqueous ink for inkjet which can record an image having a good color tone and having a high optical density is provided. There are aqueous ink-jet inks containing pigments and fluorescent brighteners. The fluorescent whitening agent is at least one compound selected from the group consisting of a bisstyryl biphenyl-based compound having an anionic group, a diaminostilbene-based compound, and a coumarin-based compound. The proportion (%) of the content (% by mass) of the fluorescent whitening agent based on the content (% by mass) of the pigment is 0.010% or more and 3.000% or less.

Description

Aqueous ink, ink cartridge, and ink jet recording method

Technical Field

The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method.

Background

An inkjet recording method is known in which an image is recorded by applying ink ejected from an inkjet recording head to a recording medium. According to the inkjet recording method, images can be recorded on various recording media. Thus, various proposals have been made for recording images in a good state according to the purpose on various recording media.

In recent years, an inkjet recording method has also been used for printing commercial documents including characters and charts on a recording medium such as plain paper, and the frequency of use for such use has been significantly increased. In such applications, high levels of image color developability and fastness (resistance to light, ozone gas, or water) are required, and therefore, pigment inks using pigments as coloring materials are often used. The pigment ink is required not only to record a high-density image but also to adjust the color tone of the image. In response to a demand for improvement in vividness in hue, for example, japanese patent application laid-open No. h08-239610 discloses an ink for recording containing a pigment and a coloring resin, and proposes an ink for recording in which a mixing ratio of the pigment and the coloring resin (pigment/coloring resin) is set to be within a predetermined range.

Disclosure of Invention

However, it was found that even when the recording ink proposed in japanese patent application laid-open No. h08-239610 is used, the optical density of an image is insufficient and there is room for improvement.

Accordingly, an object of the present invention is to provide an aqueous ink for inkjet which can record an image having a good color tone and having a high optical density. Another object of the present invention is to provide an ink cartridge and an ink jet recording method using the aqueous ink.

The above object is achieved by the present invention described below. That is, according to the present invention, there is provided an aqueous ink for inkjet comprising a pigment and a fluorescent brightener, wherein the pigment is carbon black, and the fluorescent brightener is at least one compound selected from the group consisting of a bisstyryl-biphenyl-series compound having an anionic group, a diaminostilbene-series compound, and a coumarin-series compound, and a proportion (%) of a content (% by mass) of the fluorescent brightener based on a content (% by mass) of the pigment is 0.010% or more and 3.000% or less.

According to the present invention, an aqueous ink for inkjet that can record an image having a good color tone and having a high optical density can be provided. Further, according to the present invention, an ink cartridge and an inkjet recording method using the aqueous ink can be provided.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Drawings

Fig. 1 is a sectional view schematically showing an exemplary embodiment of the ink cartridge of the present invention.

Fig. 2A is a perspective view of a main part of an inkjet recording apparatus used in the inkjet recording method of the present invention.

Fig. 2B is a perspective view of a head cartridge of an inkjet recording apparatus used in the inkjet recording method of the present invention.

Detailed Description

Hereinafter, the present invention will be described in more detail with reference to exemplary embodiments. In the present invention, in the case where the compound is a salt, the salt is dissociated into ions in the ink, but is expressed as "containing a salt" for convenience. Further, the aqueous ink for inkjet may be simply described as "ink". Unless otherwise stated, the physical property values are values at ordinary temperature (25 ℃ C.).

First, in order to obtain an image having a good color tone, the inventors of the present invention conducted a study by adding resin particles dyed with a fluorescent whitening agent disclosed in japanese patent application laid-open No. h08-239610 to an ink containing carbon black. As a result, the hue of the image was found to be good, but the optical density of the image had room for improvement. The inventors of the present invention speculate that the reason for this is that the resin particles dyed with the fluorescent whitening agent aggregate with the pigment in the vicinity of the surface of the recording medium and block the light absorbed by the pigment.

Therefore, the inventors of the present invention conducted studies on a technique for improving the color tone of an image and increasing the optical density. As a result, it was found that the use of a specific fluorescent whitening agent in a predetermined ratio based on the pigment can improve the hue of an image and increase the optical density. Specifically, as the fluorescent whitening agent, at least one compound selected from the group consisting of a bisstyryl-biphenyl-based compound having an anionic group, a diaminostilbene-based compound, and a coumarin-based compound is used. Further, the proportion (%) of the content (% by mass) of the specific fluorescent whitening agent in the ink based on the content (% by mass) of the pigment is required to be 0.010% or more and 3.000% or less. The inventors of the present invention presume the following reason why the above-described effects can be obtained by such a configuration.

At least one compound selected from the group consisting of a distyrylbenzene-based compound having an anionic group, a diaminostilbene-based compound, and a coumarin-based compound used as a fluorescent brightener has high water solubility (is soluble in water). Therefore, as the liquid component penetrates into the recording medium, the specific fluorescent whitening agent penetrates from the vicinity of the surface of the recording medium and is fixed to a position that sinks in the thickness direction. On the other hand, the pigment hardly sinks into the recording medium, and thus remains near the surface thereof. In this way, since the pigment and the fluorescent whitening agent are present at different positions in the thickness direction of the recording medium, the pigment efficiently absorbs light and the yellow color of the recording medium is cancelled by the blue fluorescence of the fluorescent whitening agent. Therefore, the recording medium looks white, and the contrast with black of carbon black is increased. In addition, the yellow color of carbon black is also offset by the blue fluorescence. Therefore, it is presumed that by using an ink containing a specific fluorescent whitening agent, the absorption of light by the pigment and the blue fluorescence of the fluorescent whitening agent are combined to record an image having a good hue and a high optical density.

Further, when the proportion (%) of the content (% by mass) of the specific fluorescent whitening agent in the ink based on the content (% by mass) of the pigment is required to be 0.010% or more and 3.000% or less, an image having a good hue and a high optical density can be recorded. When the ratio is less than 0.010%, the intensity of fluorescence emission of the fluorescent whitening agent is too low with respect to the absorption intensity of the pigment, and thus it is difficult to obtain an image having a good color tone. On the other hand, when the ratio is more than 3.000%, the intensity of fluorescence emission of the fluorescent whitening agent is excessively high relative to the absorption intensity of the pigment, and thus the absorption of light by the pigment is blocked. As a result, it is considered that it is difficult to obtain an image having a high optical density.

Some recording media contain fluorescent whitening agents. For the reason that the above-described effects can be obtained, it is conceivable to use a recording medium containing a large amount of fluorescent whitening agent as the recording medium. However, according to the ink of the present invention, more fluorescent whitening agent may be surely present in the vicinity of the pigment. For this reason, the yellow color of the ink-applied portion of the recording medium can be effectively offset regardless of the presence or absence of the fluorescent whitening agent in the recording medium, and an image having a good hue and a high optical density is recorded.

< ink >

The ink of the present invention is an aqueous ink for inkjet containing a pigment and a fluorescent brightener. However, since the ink of the present invention does not need to be an active energy ray-curable type, it does not need to contain a monomer having a polymerizable group. Hereinafter, the components constituting the ink of the present invention and the physical properties of the ink will be described in detail.

(pigment)

Carbon black is used as a pigment as a coloring material of ink. That is, the ink of the present invention is a black ink (achromatic ink). As the pigment, any known carbon black that can be used for ink jet inks can be used. Examples of the carbon black include furnace black, lamp black, acetylene black and channel black. Coloring materials other than carbon black may be used together for toning and the like. However, from the viewpoint of more reliably obtaining the effect of color tone adjustment by a specific fluorescent whitening agent, other coloring materials other than carbon black may not be used together.

When pigments are classified according to a dispersion method, there are self-dispersible pigments in which an anionic group is bonded to the particle surface of the pigment directly or through other atomic groups, and resin-dispersed pigments dispersed by means of a resin dispersant. The resin-dispersed pigment includes a microcapsule type pigment in which the surface of pigment particles is covered with a resin, and a resin-bound type pigment in which an organic group containing a polymer is chemically bonded to the surface of pigment particles. Pigments having different dispersion methods may also be used in combination.

Examples of the anionic group in the self-dispersible pigment include a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, and a phosphonic acid group. Further, the other atomic group has a function as a spacer between the surface of the pigment particle and the anionic group, and preferably has a molecular weight of 1,000 or less. Examples of the above-mentioned other atomic groups include: alkylene having 1 to 6 carbon atoms; arylene groups such as phenylene and naphthylene; an ester group; an imino group; an amide group; a sulfonyl group; and ether groups. Further, a group obtained by combining these groups may be used. The anionic group may be either of an acid type (H type) or a salt type, and the salt type is preferable. As the counter ion in the case of the salt type, at least one selected from the group consisting of alkali metal, ammonium, and organic ammonium is preferable. As the counter ion, at least one selected from the group consisting of sodium ion, potassium ion, and ammonium ion is more preferable.

As the resin dispersant in the resin-dispersed pigment, an acrylic resin having a hydrophilic unit and a hydrophobic unit as constituent units is preferably used, and a water-soluble acrylic resin is more preferably used. In the present specification, "the resin is water-soluble" means that, in the case where the resin is neutralized with an alkali in an amount equimolar to the acid value, the particles are present in the aqueous medium in a state where the particles having a particle diameter measurable by a dynamic light scattering method are not formed. Hereinafter, each unit constituting the acrylic resin will be described. In the present specification, "(meth) acryloyl group" means acryloyl group and methacryloyl group, and "(meth) acrylate" means acrylate and methacrylate.

The hydrophilic unit is formed by polymerizing a monomer having a hydrophilic group such as an anionic group, a hydroxyl group, and an oxyethylene group. Examples of the monomer having a hydrophilic group include: acidic monomers having a carboxylic acid group such as (meth) acrylic acid, itaconic acid, maleic acid, and fumaric acid; acidic monomers having phosphonic acid groups such as ethyl 2-phosphonate (meth) acrylate; anionic monomers such as anhydrides and salts of these acidic monomers; monomers having a hydroxyl group such as 2-hydroxyethyl (meth) acrylate and 3-hydroxypropyl (meth) acrylate; and monomers having an oxyethylene group such as methoxy (mono, di, tri, poly) ethylene glycol (meth) acrylate. Examples of the cation constituting the salt of the anionic monomer include lithium ion, sodium ion, potassium ion, ammonium ion, and organic ammonium ion.

The hydrophobic unit is formed by polymerizing a hydrophobic monomer having no hydrophilic group such as an anionic group, a hydroxyl group, and an oxyethylene group. Examples of hydrophobic monomers include: monomers having an aromatic ring such as styrene, α -methylstyrene and benzyl (meth) acrylate; and monomers having an aliphatic group such as ethyl (meth) acrylate, methyl (meth) acrylate, (iso) propyl (meth) acrylate, (n-, iso-, t-) butyl (meth) acrylate, and 2-ethylhexyl (meth) acrylate.

As the pigment, a self-dispersible pigment is preferably used from the viewpoint of further improving the color tone of an image. The present inventors speculate that the reason is as follows. When a resin-dispersed pigment is used as the pigment, a part of the fluorescent whitening agent may adhere to the resin and aggregate with the pigment. Therefore, it is presumed that the effect of improving the color tone of an image is slightly reduced because a part of the fluorescent whitening agent is shielded by the aggregated pigment and the efficiency of fluorescent light emission is lowered.

The content (% by mass) of the pigment in the ink is preferably 0.10% by mass or more and 15.00% by mass or less, and more preferably 1.00% by mass or more and 10.00% by mass or less, based on the total mass of the ink.

(fluorescent whitening agent)

The fluorescent whitening agent is a compound that absorbs light in an ultraviolet wavelength region of 280nm or more and less than 400nm and emits fluorescence in a wavelength region of 400nm or more and 500nm or less on the short wavelength side of the visible light region. That is, the fluorescent whitening agent is a compound that absorbs ultraviolet light and emits blue fluorescence. As the fluorescent whitening agent, those having a maximum value of absorbance in a wavelength region of 320nm or more and 380nm or less and a maximum value of fluorescence emission intensity in a wavelength region of 400nm or more and 460nm or less are preferable. In general, optical brighteners have a white to single yellow hue in the solid state.

As the fluorescent whitening agent, at least one compound selected from the group consisting of a bisstyryl biphenyl-based compound having an anionic group, a diaminostilbene-based compound, and a coumarin-based compound is used. As described above, these compounds (fluorescent whitening agents) are highly water-soluble (readily soluble in water), and permeate and fix from the vicinity of the surface of the recording medium to a position that sinks in the thickness direction. On the other hand, the pigment hardly sinks into the recording medium, and thus remains near the surface thereof. In this way, since the pigment and the fluorescent whitening agent are present at different positions in the thickness direction of the recording medium, the pigment efficiently absorbs light and the yellow color of the recording medium is cancelled by the blue fluorescence of the fluorescent whitening agent. Therefore, the recording medium looks white, and the contrast with black of carbon black is increased. In addition, the yellow color of carbon black is also offset by the blue fluorescence. Therefore, it is presumed that by using an ink containing a specific fluorescent whitening agent, the absorption of light by the pigment and the blue fluorescence of the fluorescent whitening agent are combined to record an image having a good hue and a high optical density.

The distyrylbiphenyl compound having an anionic group is a compound having a distyrylbiphenyl structure (fluorescent whitening agent) having an anionic group. The bisstyrylbiphenyl is represented by the following formula (1). Examples of the bisstyrylbiphenyl compound having an anionic group include compounds having the following structures: has a structure in which some or all of the hydrogen atoms in the distyrylbiphenyl are substituted with an anionic group, or a structure in which some or all of the hydrogen atoms in the distyrylbiphenyl are substituted with an anionic group and other atoms or groups. It is preferable that the anionic group in the distyrylbiphenyl compound having an anionic group is bonded as a substituent of a hydrogen atom in the benzene ring of the styryl group contained in the distyrylbiphenyl. Examples of anionic groups include carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, and phosphonic acid groups. The anionic group may be any of an acid type (H type) and a salt type, and in the case of the salt type, the counter ion is preferably an alkali metal, and more preferably a sodium ion and a potassium ion. Examples of the bisstyrylbiphenyl compound having an anionic group include c.i. fluorescent brightener 351.

The diaminostilbene-based compound is a compound having a diaminostilbene structure (fluorescent whitening agent). The diaminostilbene is represented by the following formula (2). Diaminostilbene is described as "amino" for convenience, but the structure of diaminostilbene also includes structures in which a hydrogen atom in an amino group in the structure is substituted to form an "imino". Further, examples of the diaminostilbene-based compound include compounds having a structure in which some or all of the hydrogen atoms in the diaminostilbene are substituted with other atoms or groups. Examples of the diaminostilbene-based compound include c.i. fluorescent brightener 9, c.i. fluorescent brightener 24, c.i. fluorescent brightener 28, c.i. fluorescent brightener 32, c.i. fluorescent brightener 71, c.i. fluorescent brightener 134, c.i. fluorescent brightener 154, c.i. fluorescent brightener 205, c.i. fluorescent brightener 220, and c.i. fluorescent brightener 260.

The coumarin compound is a compound having a coumarin structure (fluorescent whitening agent). Coumarin is represented by the following formula (3). Further, examples of the coumarin-based compound include compounds having a structure in which some or all of the hydrogen atoms in coumarin are substituted with other atoms or groups. Examples of the coumarin-based compound include c.i. fluorescent brightener 52 and c.i. fluorescent brightener 140.

In the specific examples of the above-described fluorescent brightener, at least one selected from the group consisting of c.i. fluorescent brightener 351, c.i. fluorescent brightener 24, c.i. fluorescent brightener 28, c.i. fluorescent brightener 71, c.i. fluorescent brightener 134, c.i. fluorescent brightener 220, c.i. fluorescent brightener 260, c.i. fluorescent brightener 52, and c.i. fluorescent brightener 140 is preferable. In addition, among the above-mentioned fluorescent whitening agents, a bisstyrylbiphenyl compound having an anionic group is preferable, and among them, the c.i. fluorescent whitening agent 351 is particularly preferable. Since the fluorescent whitening agent is particularly high in water solubility, easily and efficiently permeates into a recording medium, and is easily fixed in a wide range, an image having a good hue and a high optical density can be easily obtained by using an ink containing the fluorescent whitening agent.

As the mode of use of the fluorescent whitening agent, it is preferable to use a fluorescent whitening agent that can be present in a state dissolved in an aqueous medium in the ink, instead of being used as the resin particles dyed with the fluorescent whitening agent. The reason for this is that, in the case where resin particles or the like dyed with a fluorescent whitening agent are used, the fluorescent whitening agent is hardly fixed to the recording medium and aggregates together with the pigment in the vicinity of the surface of the recording medium, and the effect of improving the optical density of an image tends to be low.

Whether the ink contains the fluorescent whitening agent can be determined according to the following method. First, ink was dropped in a dot shape onto a filter paper using a capillary tube. As the filter paper, a general filter paper mainly made of cellulose fibers can be suitably used. Next, a portion from one end of the filter paper to a position where the ink was dropped was immersed in the developing solution in the closed vessel. As the developing solution, a mixed solvent of water and an organic solvent which does not dissolve the pigment but can dissolve the fluorescent brightener is used. Examples of such an organic solvent include ethanol and acetone, and the mixing ratio (volume ratio) with water is preferably such that the amount of the organic solvent increases. Then, when the developing solution was developed upward by about 5cm in the direction of gravity, the filter paper was taken out of the developing solution and dried. If the filter paper is irradiated with black light (wavelength of 375nm) of the LED light source and blue fluorescence is confirmed at a position different from the position where the ink is dropped, it can be judged that the ink to be judged contains the fluorescent whitening agent. Of course, the determination method is not limited to the above. Note that even in the case of an ink containing a fluorescent whitening agent, when the fluorescent whitening agent is dyed on the resin particles, the fluorescent whitening agent does not move from the position where the ink is dropped.

Further, the proportion (%) of the content (% by mass) of the fluorescent whitening agent in the ink based on the content (% by mass) of the pigment is required to be 0.010% or more and 3.000% or less. That is, the value of { content F (mass%)/content P (mass%) } × 100 (%) of the fluorescent whitening agent is 0.010% or more and 3.000% or less. As described above, when the ratio is less than 0.010%, the intensity of fluorescence emission of the fluorescent whitening agent is excessively low with respect to the absorption intensity of the pigment, and thus it is difficult to obtain an image having a good color tone. Further, when the ratio is more than 3.000%, the intensity of fluorescence emission of the fluorescent whitening agent is too high with respect to the absorption intensity of the pigment, and thus the absorption of light by the pigment is blocked. As a result, it is considered that it is difficult to obtain an image having a high optical density.

The content (mass%) of the fluorescent whitening agent in the ink is preferably 0.0003 mass% or more and 0.0900 mass% or less based on the total mass of the ink. The content (% by mass) of the fluorescent whitening agent is more preferably 0.0010% by mass or more and 0.0900% by mass or less, and is still more preferably 0.0050% by mass or more and 0.0500% by mass or less.

(aqueous Medium)

The ink may contain an aqueous medium as water or a mixed solvent of water and a water-soluble organic solvent. As the water, deionized water or ion-exchanged water is preferably used. The content (% by mass) of water in the ink is preferably 10.00% by mass or more and 95.00% by mass or less, and more preferably 50.00% by mass or more and 95.00% by mass or less, based on the total mass of the ink. Further, the content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.00% by mass or more and 50.00% by mass or less, and more preferably 3.00% by mass or more and 40.00% by mass or less, based on the total mass of the ink. As the water-soluble organic solvent, any of alcohols, (poly) alkylene glycols, glycol ethers, nitrogen-containing compounds, sulfur-containing compounds, and the like, which can be used in ink jet inks, can be used.

(other Components)

In the ink, in addition to the above components, if necessary, a water-soluble organic compound which is solid at ordinary temperature, for example, a polyhydric alcohol such as trimethylolpropane or trimethylolethane, or a urea derivative such as urea or ethylene urea, may be used. Further, various additives such as a resin, a pH adjuster, an antifoaming agent, a rust inhibitor, a preservative, a mildewproofing agent, an antioxidant, a reduction inhibitor, and an evaporation accelerator may be added to the ink, if necessary. However, since the fluorescent whitening agent is easily adsorbed to resin particles (resin whose particle diameter can be measured by a dynamic light scattering method in the ink), if the resin particles are excessively contained in the ink, the effect of the present invention may be slightly weakened. Therefore, in the case where the resin particles are contained in the ink, the content is preferably small (1.00 mass% or less, preferably 0.50 mass% or less), and an ink containing no resin particles is more preferable.

The ink may contain a surfactant. Examples of the kind of the surfactant include known anionic surfactants, nonionic surfactants, and cationic surfactants. In particular, it is preferable to include a nonionic surfactant in the ink. As the nonionic surfactant, for example, those having various structures such as hydrocarbon-based, silicone-based, fluorine-based, and the like can be used. Among them, a hydrocarbon-based nonionic surfactant is preferably used. In the case where the surfactant is contained in the ink, the content (% by mass) of the surfactant in the ink is preferably 0.01% by mass or more and 5.00% by mass or less, and more preferably 0.01% by mass or more and 3.00% by mass or less, based on the total mass of the ink.

(physical Properties of ink)

In the ink of the present invention, the surface tension of the ink at 25 ℃ is preferably 32mN/m or more. This is because the tone of the image is further improved. The present inventors speculate that the reason is as follows. When the surface tension of the ink is less than 32mN/m, a part of the pigment sinks together with the fluorescent brightener in the thickness direction of the recording medium. In this case, it is presumed that the effect of improving the color tone is slightly reduced because the optical density is slightly reduced, and a part of the fluorescent whitening agent is shielded by the aggregated pigment and the efficiency of fluorescent light emission is reduced. The surface tension of the ink at 25 ℃ is preferably 60mN/m or less. The surface tension of the ink can be measured by the Wilhelmy method (plate method).

From the viewpoint of reliability of ink applied to an inkjet system, it is preferable to appropriately control the physical properties of the ink. Specifically, the viscosity of the ink at 25 ℃ is preferably 1.0mPa · s or more and 10.0mPa · s or less. Further, the pH of the ink at 25 ℃ is preferably 7.0 or more and 9.5 or less, and more preferably 8.0 or more and 9.5 or less. The viscosity of the ink can be measured with a rotational viscometer or the like.

< ink Cartridge >

An ink cartridge of the present invention includes ink and an ink storage portion storing the ink. The ink contained in the ink storage portion is the aqueous ink of the present invention described above. Fig. 1 is a sectional view schematically showing an exemplary embodiment of an ink cartridge of the present invention. As shown in fig. 1, an ink supply port 12 for supplying ink to the recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is an ink storage portion for storing ink. The ink reservoir portion includes an ink reservoir chamber 14 and an absorber reservoir chamber 16 that communicate with each other via a communication port 18. Further, the absorber storage chamber 16 communicates with the ink supply port 12. The ink reservoir chamber 14 stores liquid ink 20, and the absorber reservoir chamber 16 stores

absorber bodies

22 and 24 that hold ink in an impregnated state. The ink reservoir portion may be configured not to have an ink reservoir chamber for storing liquid ink, and to hold the entire amount of ink stored by the absorber. Further, the ink reservoir may have a form in which the entire amount of ink is stored in a liquid state without having an absorber. Further, the ink cartridge may be configured to have an ink storage portion and a recording head.

< ink jet recording method >

The ink jet recording method of the present invention is a method of recording an image on a recording medium by ejecting the aqueous ink of the present invention described above from an ink jet recording head. Examples of the method of ejecting ink include a method of applying mechanical energy to ink and a method of applying thermal energy to ink. In the present invention, a method of applying thermal energy to ink to eject ink is particularly preferably employed. The steps of the inkjet recording method may be known other than using the ink of the present invention. However, it is not necessary to perform a step of curing the ink or the image by irradiation with an active energy ray or the like.

Fig. 2A and 2B are diagrams schematically showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, fig. 2A is a perspective view of a main part of the inkjet recording apparatus, and fig. 2B is a perspective view of a head cartridge. The inkjet recording apparatus includes a conveying unit (not shown) for conveying the recording medium 32 and a carriage shaft 34. A head cartridge 36 may be mounted on the carriage shaft 34. The head cartridge 36 includes recording heads 38 and 40, and is configured such that an ink cartridge 42 is provided therein. While the head cartridge 36 is conveyed along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, an image is recorded on the recording medium 32 by conveying the recording medium 32 in the sub-scanning direction by a conveying unit (not shown). The recording medium 32 is not particularly limited, and a recording medium using a paper base material, for example, a recording medium having no coating layer such as plain paper or a recording medium having a coating layer such as glossy paper or matt paper, is preferably used.

[ examples ]

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. However, the present invention is not limited to the following examples unless the gist thereof is exceeded. Components described as "parts" and "%" are based on mass unless otherwise indicated.

< preparation of pigment Dispersion >

(pigment Dispersion 1)

A solution obtained by dissolving 5.0g of concentrated hydrochloric acid in 5.5g of water was cooled to 5 ℃, and 1.6g of 4-aminophthalic acid (treating agent) was added in this state. The vessel containing the solution was placed in an ice bath, and a solution obtained by dissolving 1.8g of sodium nitrite in 9.0g of ion-exchanged water at 5 ℃ was added while stirring and maintaining the temperature of the solution below 10 ℃. After stirring for 15 minutes, 6.0g of carbon black (specific surface area 260 m) were added with stirring²/g), and the mixture was further stirred for 15 minutes to obtain a slurry. After the obtained slurry was filtered through a filter paper (trade name "standard filter paper No. 2", manufactured by Advantech), the particles were sufficiently washed with water and dried in an oven at 110 ℃. After the counter ion is replaced from sodium ion to potassium ion by an ion exchange method, an appropriate amount of ion exchange water is added to adjust the pigment content. Thereby, a pigment dispersion liquid 1 was obtained, which contained a self-dispersible pigment having a pigment content of 10.0% and having a phthalic acid group as a counter ion of potassium ion bonded to the particle surface of carbon black.

(pigment Dispersion 2)

Ion-exchanged water and carbon black (specific surface area: 260 m)²/g) are placed in a vessel equipped with an ozone generator and the carbon black is predispersed. Thereafter, the pH of the mixture was adjusted to about 7 by adding potassium hydroxide, and ozone treatment was performed for 6 hours while stirring. Subsequently, ozone treatment was performed for 3 hours while circulating the mixture through a liquid-liquid collision type disperser. Specifically, the method is carried out with reference to the method of example 3 in Japanese patent application laid-open No. 2003-535949. After completion of the reaction, purification was performed by ultrafiltration using hydrogen and oxygenPotassium hydroxide adjusts the pH to a predetermined value, and ion-exchanged water is used to adjust the content of the pigment. Thereby, a pigment dispersion liquid 2 containing a self-dispersible pigment having a pigment content of 10.0% and having a carboxylic acid group as a counter ion of potassium ion bonded to the particle surface of carbon black was obtained.

(pigment Dispersion 3)

Carbon black (specific surface area: 260 m)²/g) was added to ion-exchanged water and sufficiently stirred. Sodium hypochlorite (effective chlorine concentration: 4%) 21.0% was added dropwise thereto, and the mixture was stirred at 100 ℃ for 10 hours. After completion of the reaction, purification was performed by ultrafiltration, pH was adjusted to a predetermined value using potassium hydroxide, and the content of the pigment was adjusted using ion-exchanged water. Thereby, a pigment dispersion liquid 3 containing a self-dispersible pigment having a pigment content of 10.0% and having a carboxylic acid group as a counter ion of potassium ion bonded to the particle surface of carbon black was obtained.

(pigment Dispersion 4)

A mixture was obtained by mixing 15.0 parts of carbon black, 30.0 parts of an aqueous solution of a resin dispersant, and 55.0 parts of ion-exchanged water. As the aqueous solution of the resin dispersant, an aqueous solution was used in which a styrene-acrylic acid copolymer as a water-soluble resin was dissolved in ion-exchanged water using an equimolar amount of sodium hydroxide with respect to the acid value of the copolymer and the resin content was 20.0%. The styrene-acrylic acid copolymer had a composition (molar) ratio of styrene to acrylic acid of 33:67, a weight average molecular weight of 10,000 and an acid value of 200 mgKOH/g. The resulting mixture was placed in a sand mill and dispersed for 1 hour. After removing coarse particles by centrifugation, the mixture was pressure-filtered with a microfilter (manufactured by fujifilm corporation) having a pore diameter of 3.0 μm. The pigment content was adjusted by adding an appropriate amount of ion-exchanged water. In this way, a pigment dispersion liquid 4 was obtained, which had a pigment content of 10.0% and a resin dispersant content of 6.0%, and contained carbon black dispersed in the resin dispersant.

(pigment Dispersion 5)

A pigment dispersion liquid 5 having a pigment content of 10.0% and a resin dispersant content of 6.0% was obtained in the same manner as in the preparation of the pigment dispersion liquid 4, except that carbon Black was changed to a Black organic pigment (trade name "Chromo Fine Black a 1103", manufactured by Dainichiseika Color & Chemicals mfg.co., ltd.).

< Properties of fluorescent whitening agent >

[ Table 1]

Table 1: structure of whitening agent

< preparation of resin pellets dyed with fluorescent whitening agent >

(resin particle 1)

49.0 parts of ion-exchanged water, 2.5 parts of ammonium lauryl sulfate, 40.0 parts of methyl acrylate, 5.0 parts of acrylonitrile, 3.0 parts of methacrylonitrile, and 0.5 part of potassium persulfate were added to a reaction vessel equipped with a reflux tube, and the temperature was raised to 80 ℃ under a nitrogen stream while stirring. The reaction was continued for 3 hours while maintaining the temperature. Thereafter, the reaction was completed by cooling to room temperature to obtain an emulsified polymer having a resin (solid content) content of 48.0%. To 100.0 parts of the emulsified polymer, a mixture of 5.0 parts of a fluorescent whitening agent, 2.0 parts of sodium β -naphthalenesulfonate and 88.0 parts of ion-exchanged water was added at normal temperature, and these were heated to 90 ℃ over 2 hours, and the temperature was maintained for 1 hour, and then cooled to 25 ℃. As the fluorescent whitening agent, c.i. fluorescent whitening agent 351 (distyrylbiphenyl structure having an anionic group, trade name "Tinopal CBS-X", manufactured by BASF) was used. Thereafter, pressure filtration was performed with a filter (trade name "HDCII", manufactured by Nihon Pall Manufacturing ltd.) having a pore diameter of 1.2 μm, and an appropriate amount of ion-exchanged water was further added. Thus, an aqueous dispersion of the resin particles 1 dyed with the fluorescent whitening agent was prepared. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 9.4%.

(resin particle 2)

Instead of the fluorescent whitening agent used for preparing the resin particles 1, c.i. fluorescent whitening agent 363 (imidazole structure, trade name "Uvitex BAC", manufactured by BASF) was used. Except for this, an aqueous dispersion of resin particles 2 dyed with a fluorescent whitening agent was prepared in the same manner as in the preparation of the aqueous dispersion of resin particles 1. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 9.4%.

(resin particles 3)

2.0 parts of a powder of a fluorescent whitening agent and 8.0 parts of a powder of a styrene-acrylic acid copolymer were mixed and dissolved in 40.0 parts of ethyl acetate to obtain a solution. Examples of the fluorescent whitening agent include c.i. fluorescent whitening agent 184 (oxazole structure, trade name "Uvitex OB", manufactured by BASF). As the styrene-acrylic acid copolymer, a styrene-acrylic acid copolymer (trade name "Joncryl 611", manufactured by BASF) having a weight average molecular weight of 8,100 and an acid value of 53mgKOH/g was used. The above solution was added to a solution in which 0.15 parts of sodium lauryl sulfate was dissolved in 90 parts of ion-exchanged water, and the mixture was stirred. Thereafter, the mixture was emulsified at an amplitude of 50% for 10 minutes using an ultrasonic homogenizer (trade name: "Advanced Digital Sonifier 250 DA", manufactured by BRANSON). Ethyl acetate was distilled off from the emulsion using an evaporator, and the mixture was cooled, filtered under pressure through a filter (trade name "HDCII", manufactured by Nihon Pall Manufacturing ltd.) having a pore diameter of 1.2 μm, and further diluted with ion-exchanged water. Thus, an aqueous dispersion of the resin particles 3 dyed with the fluorescent whitening agent was prepared. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 20.0%.

(resin particle 4)

Instead of the powder of the fluorescent brightener used to prepare resin particles 3, a powder of c.i. fluorescent brightener 135 (oxazole structure, manufactured by Tokyo Chemical Industry co., ltd.) was used. Except for this, an aqueous dispersion of resin particles 4 dyed with a fluorescent whitening agent was prepared in the same manner as in the preparation of the aqueous dispersion of resin particles 3. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 20.0%.

(resin particle 5)

Instead of the powder of the fluorescent whitening agent used for preparing the resin particles 3, a powder of c.i. fluorescent whitening agent 367 (oxazole structure, trade name "HOSTALUX KCB", manufactured by Clariant Japan) was used. Except for this, an aqueous dispersion of resin particles 5 dyed with a fluorescent whitening agent was prepared in the same manner as in the preparation of the aqueous dispersion of resin particles 3. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 20.0%.

(resin particles 6)

Instead of the powder of the fluorescent brightener used for preparing the resin particles 3, a powder of c.i. fluorescent brightener 162 (naphthalimide structure, trade name "Mikawhite AT", manufactured by Nippon Kayaku co., ltd.) was used. Except for this, an aqueous dispersion of resin particles 6 dyed with a fluorescent whitening agent was prepared in the same manner as in the preparation of the aqueous dispersion of resin particles 3. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the fluorescent whitening agent to the resin particles was 20.0%.

< preparation of resin pellets dyed with coloring Material other than fluorescent whitening agent >

(resin particles 7)

Instead of the powder of the fluorescent whitening agent used for preparing the resin particles 3, a powder of c.i. solvent Blue 70 (trade name "Orasol Blue 855", manufactured by BASF) as a Blue dye was used. Except for this, an aqueous dispersion of resin particles 7 dyed with a blue dye was prepared in the same manner as in the preparation of the aqueous dispersion of resin particles 3. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the blue dye to the resin particles was 20.0%.

(resin particle 8)

Instead of the fluorescent whitening agent used for preparing the resin particles 1, c.i. solvent yellow 160:1 was used as a yellow fluorescent dye. Except for this, an aqueous dispersion of the resin particles 8 dyed with a yellow fluorescent dye was prepared in the same manner as in the preparation of the aqueous dispersion of the resin particles 1. The content of the resin particles in the aqueous dispersion was 10.0%, and the ratio of the yellow fluorescent dye to the resin particles was 9.4%.

< preparation of ink >

The components (unit:%) shown in the upper part of tables 2 and 3 were mixed, sufficiently stirred and dispersed, and then filtered under pressure through a membrane filter (trade name "HDCII filter", manufactured by Nihon pall manufacturing ltd.) having a pore diameter of 2.5 μm to prepare each ink. In table 2, the amount of ion-exchanged water used marked with "-" means the residual amount at which the total composition became 100.0000%. Pyranin 120 (manufactured by LANXESS) is a green fluorescent dye, and rhodamine F4G (manufactured by BASF) is a red fluorescent dye, and neither is an optical brightener. Acetyleneol E100 is a nonionic surfactant manufactured by Kawaken Fine Chemicals Co., Ltd. The lower part of table 2 shows the characteristics of the ink. The surface tension was measured at 25 ℃ using an automatic surface tensiometer (trade name "DY-300", manufactured by Kyowa Interface Science, Inc.).

< evaluation >

Using the prepared ink, the following items were evaluated. For image recording, an inkjet recording apparatus (trade name "PIXUS TS 5130", manufactured by Canon inc.) equipped with a recording head for ejecting liquid by the action of thermal energy is used. In the present embodiment, a recording job of a solid image recorded under the condition that two ink droplets each having a mass of 11ng ± 10% were applied to a unit area of 1/600 inches × 1/600 inches was defined as 100%. In the present invention, "AA", "a", and "B" are set to acceptable levels, and "C" is set to unacceptable levels, based on evaluation criteria of each item shown below. Table 4 shows the evaluation results.

(color tone)

Three solid images of 2cm × 2cm having a recording duty of 100% were recorded on three recording media (trade name "CS-680 a 4", manufactured by Canon inc.). One day after recording, b (chroma) in the L a b color system specified by the international commission on illumination (CIE) was measured for solid images. Using a fluorescence spectrophotometer (trade name "FD-7", manufactured by KONICA MINOLTA, inc.) under illumination conditions: m1(D50), observation light source: d50, and field of view: the measurement was performed under the condition of 2 °. Then, the color tone of the image was evaluated from the average value of b in the obtained three recording media according to the following evaluation criteria. b is an index representing the hue from yellow to blue, and a larger value means yellow and a smaller value means the hue of blue. In the case of an image (black image) recorded using an ink in which the coloring material is carbon black, it can be said that b is preferably small because it is considered to be blacker black.

AA: b is 3.90 or less on average.

A: b is greater than 3.90 and 4.00 or less on average.

B: b is greater than 4.00 and less than 4.10 on average.

C: b is greater than 4.10 on average.

(optical Density)

Three solid images of 2cm × 2cm having a recording duty of 100% were recorded on three recording media (trade name "CS-680 a 4", manufactured by Canon inc.). One day after recording, the mixture was measured under illumination conditions using a fluorescence spectrophotometer (trade name "FD-7", manufactured by KONICA MINOLTA, inc.): m1(D50), observation light source: d50, and field of view: the optical density of the solid image was measured under the condition of 2 °. Then, the optical density of the image was evaluated from the average value of the optical densities in the obtained three recording media according to the following evaluation criteria.

AA: the average value of the optical density is 1.26 or more.

A: the average value of the optical density is 1.22 or more and less than 1.26.

B: the average value of the optical density is 1.18 or more and less than 1.22.

C: the average value of the optical density is less than 1.18.

[ Table 4]

Table 4: evaluation results

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

1. An aqueous ink for inkjet, comprising:

a pigment; and

a fluorescent whitening agent,

characterized in that the pigment is carbon black,

the fluorescent whitening agent is at least one compound selected from the group consisting of a bisstyryl biphenyl-based compound having an anionic group, a diaminostilbene-based compound, and a coumarin-based compound, and

the content of the fluorescent whitening agent in mass% is 0.010% or more and 3.000% or less in% based on the content of the pigment in mass%.

2. The aqueous ink according to claim 1,

wherein the surface tension of the aqueous ink at 25 ℃ is 32mN/m or more.

3. The aqueous ink according to claim 1, wherein,

wherein the pigment is a self-dispersible pigment in which an anionic group is bonded to the particle surface of the pigment directly or through another atomic group.

4. The aqueous ink according to claim 1, wherein,

wherein the fluorescent brightener is at least one selected from the group consisting of c.i. fluorescent brightener 351, c.i. fluorescent brightener 24, c.i. fluorescent brightener 28, c.i. fluorescent brightener 71, c.i. fluorescent brightener 134, c.i. fluorescent brightener 220, c.i. fluorescent brightener 260, c.i. fluorescent brightener 52, and c.i. fluorescent brightener 140.

5. The aqueous ink according to claim 1, wherein the fluorescent whitening agent is the bisstyrylbiphenyl compound having an anionic group.

6. The aqueous ink according to claim 1, wherein the fluorescent brightener is c.i. fluorescent brightener 351.

7. The aqueous ink according to claim 1, wherein a content of the fluorescent brightener in mass% is 0.0003 mass% or more and 0.0900 mass% or less based on the total mass of the ink.

8. An ink cartridge, comprising:

ink; and

an ink storage portion that stores the ink,

characterized in that the ink is an aqueous ink according to any one of claims 1 to 7.

9. An ink jet recording method for recording an image on a recording medium by ejecting ink from an ink jet recording head,