JP2020019858A - Aqueous ink, ink cartridge and inkjet recording method - Google Patents

Abstract

To provide an ink capable of suppressing unevenness of an image by enhancing ejection stability of an ink and recording an image having high image clarity in an ink discharged by a thermal system.SOLUTION: There is provided an aqueous ink used for an inkjet recording method for recording an image on a recording medium by discharging an ink from a recording head by the action of thermal energy. The aqueous ink contains carbon black, a resin dispersant for dispersing carbon black, a first urethane resin, a second urethane resin and a nonionic fluorine-based surfactant. Both the first urethane resin and the second urethane resin have a unit derived from a polyether polyol. The ratio of a weight average molecular weight Mwof the first urethane resin to a weight average molecular weight Mwof the second urethane resin is 0.2 time or more and 0.8 time or less and the weight average molecular weight Mwof the second urethane resin is 20000 or more and 70000 or less.SELECTED DRAWING: None

Description

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

  In recent years, inks containing pigments (so-called pigment inks) have been widely used as coloring materials for inks used in ink jet recording methods. In photographic printing and graphic art printing, images with higher definition and excellent glossiness are required. There is also a high demand for high-speed recording.

  Inks applied to such applications often contain a so-called resin-dispersed pigment in which a pigment is dispersed by a resin dispersant. The resin-dispersed pigment physically adsorbs to the particle surface of the pigment by the hydrophobic unit and disperses the essentially hydrophobic pigment in the aqueous medium by the hydration power of the hydrophilic unit. The hydrophobic pigment is stably dispersed in the ink by the action of the resin dispersant. However, if the dispersion state of the pigment becomes unstable and affects the ejection stability of the ink from the recording head, unevenness of an image or the like is likely to occur. The term “image unevenness” as used herein refers to non-uniform density unevenness that can be formed into a solid image when a solid image is continuously printed with a small number of passes for the purpose of high-speed printing.

  Further, in general, the image clarity of an image recorded on a recording medium having a glossy surface (so-called glossy papers) is insufficient in the pigment ink due to the fact that the pigment is a particle, as compared with the dye ink. There is a problem that there is. Here, the term "image clarity" refers to the sharpness of an image when the image is projected on the surface of the image, and when the image clarity is low, the image looks blurry, and when the image clarity is high, The image is clearly visible.

  As a technique for eliminating unevenness of an image by improving the ejection stability of the ink and improving the glossiness of the image, an ink containing a resin-dispersed pigment and two kinds of urethane resins having different weight average molecular weights is used. This is proposed in Patent Document 1. Patent Literature 2 proposes an ink containing a self-dispersion pigment and two types of urethane resins.

JP 2017-036432 A JP-T-2005-515289

  In the case of a thermal method in which ink is ejected from a recording head by the action of thermal energy, kogation derived from a pigment may be deposited on a heater of the recording head for applying thermal energy. Accordingly, for example, even when the ink is repeatedly ejected from a certain ejection port, the ejection state does not always become the same every time, so that the ejection stability may be reduced depending on the kogation accumulation state. . Similarly, a recording head has a large number of ejection ports, and the ejection state from these ejection ports does not always become the same. Therefore, if ejection stability is reduced due to the state of kogation, image unevenness is caused. In particular, in the case of a pigment having high heat resistance such as carbon black, since the kogation deposited on the heater adheres firmly, the kogation does not detach even when the discharge energy is applied at the next discharge, and foaming is not generated. As a result, the ink becomes unstable and the ejection stability of the ink decreases.

  On the other hand, organic pigments are pigments having lower heat resistance than carbon black. In the case of such a pigment having low heat resistance, the kogation adhered to the heater due to the application of energy for ejecting ink is peeled off when the next ejection energy is applied. Therefore, when an ink containing a pigment having low heat resistance such as an organic pigment is used, unevenness of an image due to a decrease in ejection stability that appears when an ink containing carbon black is used is less likely to occur. In the case of a so-called piezo method in which ink is ejected by deformation of a piezo element, kogation does not occur, so that problems such as a decrease in ejection stability and image unevenness due to kogation do not occur.

  As a result of the study by the present inventors, it has been found that the technique proposed in Patent Document 1 does not lower the ink ejection stability and does not cause image unevenness. It is considered that when a specific organic pigment is used, the adhesion of the above-mentioned pigment-derived kogation is not strong, and the kogation is peeled off every time the ink is ejected.

  On the other hand, the present inventors performed continuous solid image recording with a small number of passes using an ink containing carbon black as a coloring material and evaluated it. No effect was obtained with respect to the properties, and it was found that image unevenness occurred. Further, the present inventors have conducted studies with reference to Patent Literature 2, and found that although the image clarity was improved, image unevenness due to a decrease in ejection stability occurred.

  Therefore, an object of the present invention is to increase the ejection stability of ink in ink ejected by the thermal method, thereby suppressing image unevenness and recording an image with high image clarity. To provide ink. It is another object of the present invention to provide an ink cartridge using the ink and an ink jet recording method.

The above object is achieved by the present invention described below. That is, according to the present invention, there is provided an aqueous ink used in an ink jet recording method for recording an image on a recording medium by discharging ink from a recording head by the action of thermal energy, wherein carbon black is used for dispersing the carbon black. It contains a resin dispersant, a first urethane resin, a second urethane resin, and a nonionic fluorinated surfactant, wherein the first urethane resin and the second urethane resin are all polyether polyols. Having a unit derived from, the weight average molecular weight Mw1 of the _first urethane resin is 0.2 times or more and 0.8 times or less in a ratio to the weight average molecular weight Mw2 of the _second urethane resin, said second weight average molecular weight Mw ₂ of urethane resin, an aqueous ink, characterized in that at least 20,000 70,000 It is subjected.

  ADVANTAGE OF THE INVENTION According to this invention, in the ink ejected by a thermal system, it is possible to improve the ejection stability and suppress the unevenness of an image, and to provide an ink capable of recording an image with high image clarity. be able to. Further, according to the present invention, an ink cartridge and an ink jet recording method using the above ink can be provided.

FIG. 2 is a cross-sectional view schematically illustrating an embodiment of the ink cartridge of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically illustrating an example of an inkjet recording apparatus used in an inkjet recording method of the present invention, wherein (a) is a perspective view of a main part of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge.

  Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is dissociated into ions in the ink, but is expressed as "contains a salt" for convenience. Further, the water-based ink for inkjet may be simply described as “ink”. Physical properties are values at normal temperature (25 ° C.) unless otherwise specified.

  The present inventors have studied using an ink containing carbon black, a resin dispersant, and a water-soluble urethane resin in order to record an image with high image clarity. As a result of the examination, it was found that although image clarity was effective, unevenness of the image due to a decrease in ejection stability occurred. Water-soluble urethane resin improves the wettability between multiple dots by increasing the surface energy of the dots in the recording medium to which the ink is applied, improves the smoothness of the pigment layer surface, and improves the image clarity of the image. Has the effect of causing.

  In addition, carbon black generally used in aqueous inks for inkjet generally has a smaller primary particle size and a higher surface hydrophobicity than organic pigments, so that aggregation tends to be strong. Usually, the surface of the carbon black particles dispersed by the resin dispersant is covered with the resin dispersant, and the dispersed state is stably maintained. However, in the thermal method, when thermal energy for discharge is applied, the dispersing state becomes unstable because the particle surface is exposed due to the detachment of the resin dispersant from the particle surface of some carbon black particles. Aggregation occurs. As a result, it is considered that the aggregated carbon black accumulates as kogation on the heater, the ink ejection stability becomes insufficient, and image unevenness occurs.

  Further, the carbon black-derived kogation that has been burnt onto the heater of the recording head is unlikely to be decomposed due to the high heat resistance of the carbon black, and therefore is not easily peeled off from the heater. In other words, the kogation does not detach even when the ejection energy is applied, and the foaming of the ink is affected by the kogation of the heater, and the ejection stability becomes unstable every time the ejection energy is applied. It is presumed to occur.

  Therefore, the present inventors formed a layer in which the kogation derived from carbon black was easily detached by the heater when the ink was ejected by applying thermal energy, thereby suppressing the strong adhesion of the kogation and stabilizing the ejection. It was considered that the unevenness of the image was suppressed by enhancing the property.

As a result of investigations by the present inventors, it has been found that by using the following aqueous ink, ejection stability can be improved, image unevenness can be suppressed, and an image with high image clarity can be recorded. That is, an aqueous ink containing carbon black, a resin dispersant for dispersing carbon black, first and second urethane resins having the following specific configuration, and a nonionic fluorine-based surfactant is used. Both the first urethane resin and the second urethane resin have a unit derived from polyether polyol. The weight average molecular weight Mw ₁ of the first urethane resin is 0.2 to 0.8 times the weight average molecular weight Mw ₂ of the second urethane resin, and the weight of the second urethane resin is The average molecular weight Mw ₂ is from 20,000 to 70,000.

  The mechanism by which the use of the above-described ink can suppress unevenness of an image due to improvement in ejection stability and enhance image clarity is considered as follows. The first urethane resin and the nonionic fluorinated surfactant are a hydrophilic portion of each of the polyether portion of the polyether polyol of the first urethane resin and the nonionic unit of the fluorinated surfactant, Interact. The hydrophobic group having a fluorine atom (such as a perfluoroalkyl group) of a nonionic fluorine-based surfactant shows extremely strong hydrophobicity among various surfactants. This has a very high orientation ability to the interface. Therefore, in the recording medium to which the ink is applied, the first urethane resin interacting with the fluorine-based surfactant is present near the surface of the dot due to the orientation of the fluorine-based surfactant to the surface. It is considered that the image showed higher image clarity.

  Further, since the fluorine-based surfactant has low heat resistance, when heat energy for ejection is applied, the fluorine-based surfactant is burnt and adheres to the heater before carbon black having high heat resistance. At that time, not only the fluorine-based surfactant but also the first urethane resin interacting therewith is burned. The kogation derived from carbon black having high heat resistance is formed on the kogation layer made of the first urethane resin and the fluorine-based surfactant. That is, the kogation derived from carbon black adheres to the heater via the kogation of the first urethane resin and the fluorine-based surfactant. Since the kogation layer made of the fluorine-based surfactant and the first urethane resin has lower heat resistance than the kogation made of carbon black, when heat energy for discharge is applied again, the kogation layer is decomposed and easily peeled off from the heater. Therefore, when thermal energy for discharge is applied, the kogation layer made of the first urethane resin and the fluorine-based surfactant is easily peeled off from the heater, and accordingly, the kogation layer derived from carbon black formed thereon Is considered to be peeled off. For the above reasons, it is presumed that even when the ejection is repeated, the adhesion of the carbon black kogation is suppressed, thereby improving the ejection stability of the ink and suppressing the unevenness of the image.

  Since the second urethane resin has a relatively large molecular weight than the first urethane resin, the molecule has high hydrophobicity, and the second urethane resin interacts with the hydrophobicity of the carbon black particle surface to stabilize the dispersion state. Since carbon black is dispersed in the ink without interacting with the first urethane resin or the fluorine-based surfactant, even if heat energy for ejection is applied, the carbon black is dispersed by the first urethane resin and the fluorine-based surfactant. Burns occur, which preferentially adhere to the heater.

  On the other hand, if only one urethane resin is contained in the ink, the carbon black and the urethane resin interact with each other, but the fluorosurfactant is present alone. Therefore, the kogation layer formed by the urethane resin and the fluorosurfactant is used. Is not formed. Therefore, the carbon black kogation adheres to the heater, and since the kogation is hard to peel off, the foaming is not stable, and it is considered that the effect of suppressing the unevenness of the image due to the decrease in the ejection stability and improving the image clarity cannot be obtained. Can be

<Ink>
The aqueous ink of the present invention is used for an ink jet recording method for recording an image on a recording medium by discharging ink from a recording head by the action of thermal energy. This ink contains carbon black, a resin dispersant for dispersing the carbon black, a first urethane resin, a second urethane resin, and a nonionic fluorinated surfactant. Hereinafter, components constituting the ink of the present invention and physical properties of the ink will be described in detail. The present invention is not limited by the following description unless it exceeds the gist.

(Carbon black)
The ink of the present invention contains carbon black as a pigment used for a coloring material. As the carbon black, furnace black, lamp black, acetylene black, channel black, and the like can be used, and one or more of these can be used.

  The content (% by mass) of carbon black in the ink is preferably from 0.20% by mass to 10.00% by mass based on the total mass of the ink. When the content of carbon black is less than 0.20% by mass, the color developability of an image may be reduced. On the other hand, when the content of carbon black is more than 10.00% by mass, the viscosity of the ink is excessively increased, and it may be difficult to discharge the ink.

  The volume average particle size of the carbon black in the ink is preferably from 10 nm to 300 nm, more preferably from 20 nm to 200 nm. The volume average particle size of the carbon black can be measured, for example, using a dynamic light scattering type particle size measuring device.

(Resin dispersant)
The ink of the present invention contains a resin dispersant for dispersing carbon black in the ink. As the resin dispersant, it is preferable to use a resin having a hydrophilic unit and a hydrophobic unit as constituent units. In the following description, “(meth) acryl” means “acryl” and “methacryl”, and “(meth) acrylate” means “acrylate” and “methacrylate”.

  The hydrophilic unit can be formed, for example, by polymerizing a monomer having a hydrophilic group, and preferably contains a unit derived from the monomer having a hydrophilic group. Specific examples of the monomer having a hydrophilic group include acidic monomers having a carboxy group such as (meth) acrylic acid, itaconic acid, maleic acid, and fumaric acid, and anionic monomers such as anhydrides and salts of these acidic monomers. Monomer; Monomer having a hydroxy group such as 2-hydroxyethyl (meth) acrylate and 3-hydroxypropyl (meth) acrylate; Ethylene oxide such as methoxy (mono, di, tri, poly) ethylene glycol (meth) acrylate A monomer having a group; One or more of these monomers can be used for the hydrophilic unit.

  Examples of the cation constituting the salt of the acidic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. The resin dispersant preferably has an acid value. Therefore, the hydrophilic unit more preferably includes a unit derived from the above-described anionic monomer. Preferably, the resin dispersant becomes water-soluble by being neutralized by a neutralizing agent such as a hydroxide of an alkali metal (such as lithium, sodium, or potassium) or aqueous ammonia.

  The hydrophobic unit can be formed, for example, by polymerizing a monomer having a hydrophobic group, and preferably includes a unit derived from the monomer having a hydrophobic group. Specific examples of the monomer having a hydrophobic group include monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth) acrylate; ethyl (meth) acrylate, methyl (meth) acrylate, (iso) propyl ( Examples thereof include monomers having an aliphatic group such as (meth) acrylate, (n-, iso-, t-) butyl (meth) acrylate, and 2-ethylhexyl (meth) acrylate. One or more of these monomers can be used for the hydrophobic unit.

  The content (% by mass) of the resin dispersant in the ink is preferably 0.10% by mass or more and 10.00% by mass or less in terms of resin content (solid content) based on the total mass of the ink. The content is more preferably 20% by mass or more and 5.00% by mass or less.

(Urethane resin)
The ink of the present invention contains a first urethane resin and a second urethane resin having different weight average molecular weights. By including such a urethane resin in the ink, a decrease in ejection stability is suppressed, so that unevenness of a recorded image can be suppressed. When only one urethane resin is used, as described above, the carbon black and the urethane resin interact with each other, but since the fluorine-based surfactant is present alone, a kogation layer is formed by the urethane resin and the fluorine-based surfactant. However, a kogation firmly attached to the heater is formed. Therefore, it is not possible to suppress a decrease in the ejection stability of the ink, and it is not possible to suppress the unevenness of the image. Note that the ink may contain three or more urethane resins as long as the ink contains at least two types of urethane resins satisfying the relationship described below. In the present invention, for convenience, a resin having a relatively small weight average molecular weight is referred to as a "first urethane resin", and a resin having a relatively large weight average molecular weight is referred to as a "second urethane resin". In the present invention, the weight average molecular weight of the urethane resin is a value obtained by gel permeation chromatography (GPC) in terms of polystyrene.

The weight average molecular weight Mw ₁ of the first urethane resin is a ratio (Mw ₁ / Mw ₂ ) to the weight average molecular weight Mw ₂ of the second urethane resin, and needs to be 0.2 times or more and 0.8 times or less. . When the above ratio is less than 0.2 times, there is a difference in the relative size of the molecular weight, so that even if the first urethane resin interacting with the fluorine-based surfactant is kogated to the heater, the layer is sufficiently formed. Not formed. As a result, kogation derived from carbon black adheres to the heater and is hardly peeled off, so that image unevenness due to a decrease in ejection stability is likely to occur. Further, since the amount of the first urethane resin existing on the surface of the dot formed on the recording medium is reduced, the image clarity tends to be low. On the other hand, when the above ratio is more than 0.8 times, it becomes difficult for each urethane resin to selectively interact with the fluorine-based surfactant or carbon black. Therefore, the kogation of the urethane resin and the fluorine-based surfactant does not preferentially adhere to the heater, and it is difficult to suppress a decrease in the ejection stability of the ink, and the image tends to be uneven.

Furthermore, the weight average molecular weight Mw ₂ of the second urethane resin needs to be 20,000 or more and 70,000 or less. When the weight average molecular weight Mw ₂ of the second urethane resin is less than 20,000, the effect of dispersion stability with respect to carbon black is reduced, and the effect of suppressing unevenness in image due to suppressing a decrease in ejection stability is reduced. It is difficult to obtain. When the weight average molecular weight Mw ₂ of the second urethane resin is more than 70,000, the viscosity of the recording medium increases, the dot smoothness decreases, and the image clarity tends to decrease. The weight-average molecular weight Mw ₂ of the second urethane resin is preferably 25,000 or more and 60,000 or less from the viewpoint of further improving ejection stability and further reducing image unevenness.

The weight average molecular weight Mw ₁ of the first urethane resin is preferably from 5,000 to 30,000, and more preferably from 10,000 to 20,000, from the viewpoint of interaction with the fluorine-based surfactant. Is more preferable. When the weight average molecular weight Mw1 of the _first urethane resin is within the above range, the strength of the interaction with the fluorine-based surfactant and the speed of the surface orientation become particularly good. As a result, the ejection stability can be further improved, the effect of suppressing image unevenness can be further improved, and the image clarity can be further improved.

  The content (% by mass) of the second urethane resin in the ink is preferably from 0.02% by mass to 2.00% by mass, and more preferably from 0.02% by mass to 0.2% by mass, based on the total mass of the ink. More preferably, it is at most 80% by mass. The content (% by mass) of the first urethane resin in the ink is preferably 0.02% by mass or more and 2.00% by mass or less based on the total mass of the ink. The content (% by mass) of the first urethane resin in the ink is more preferably from 0.10% by mass to 1.20% by mass, and is preferably from 0.20% by mass to 0.80% by mass. Is more preferable. The total content (% by mass) of the urethane resin in the ink is preferably from 0.10% by mass to 2.00% by mass, and more preferably from 0.20% by mass to 1.00% by mass, based on the total mass of the ink. % Is more preferable.

  The urethane resin can be obtained by reacting a polyisocyanate with a polyol including a polyether polyol. In addition, it may be obtained by further reacting a chain extender.

[Polyisocyanate]
Polyisocyanate is a compound having two or more isocyanate groups in its molecular structure. As the polyisocyanate, an aliphatic polyisocyanate, an aromatic polyisocyanate, or the like can be used, and one or more of them can be used. The proportion (mol%) of units derived from polyisocyanate in the urethane resin is preferably from 10.0 mol% to 80.0 mol%, and preferably from 20.0 mol% to 60.0 mol%. It is more preferred that there be. Further, the proportion (% by mass) of units derived from polyisocyanate in the urethane resin is preferably from 10.0% by mass to 80.0% by mass.

  Specific examples of the aliphatic polyisocyanate include tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, and 2-methyl Polyisocyanates having a chain structure such as pentane-1,5-diisocyanate and 3-methylpentane-1,5-diisocyanate; isophorone diisocyanate, hydrogenated xylylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4- Has a cyclic structure such as cyclohexane diisocyanate, methylcyclohexylene diisocyanate, 1,3-bis (isocyanatomethyl) cyclohexane Polyisocyanates; and the like can be mentioned.

  Specific examples of the aromatic polyisocyanate include tolylene diisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,4′-dibenzyl diisocyanate, 1,5 -Naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, α, α, α ′, α′-tetramethylxylylene diisocyanate, and the like. be able to.

[Polyol, polyamine]
A polyol is a compound having two or more hydroxy groups in its molecular structure. As the polyol, a polyether polyol is used. In addition, a polyol having no acid group, such as a polyester polyol and a polycarbonate polyol, and a polyol having an acid group can be used. The polyamine is a compound having two or more “amino groups and imino groups” in its molecular structure. The proportion (mol%) of units derived from polyols and polyamines in the urethane resin is preferably from 10.0 mol% to 80.0 mol%, and preferably from 20.0 mol% to 60.0 mol%. Is more preferable.

(Polyol having no acid group)
Examples of the polyether polyol include an addition polymer of an alkylene oxide and a polyol; a glycol such as a (poly) alkylene glycol; and the like. Examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide, and α-olefin oxide. Examples of polyols that undergo addition polymerization with alkylene oxide include 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, Methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, neopentyl glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4-dihydroxyphenylpropane Diols such as 4,4-dihydroxyphenylmethane, hydrogenated bisphenol A, dimethylolurea and derivatives thereof; glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol, Trimethylolmelamine and its Conductors, triols such as polyoxypropylene triol; and the like. Examples of glycols include hexamethylene glycol, tetramethylene glycol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, (poly) tetramethylene glycol, and the like. (Poly) alkylene glycol; ethylene glycol-propylene glycol copolymer; and the like.

  Among these, glycols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol are preferred. The proportion of the units derived from glycols in the units derived from the polyol having no acid group in the urethane resin is preferably 90.0% by mass or more. The ratio is more preferably 100.0% by mass, that is, all the units derived from the polyol having no acid group are more preferably units derived from glycols.

  The units derived from the polyether polyol in the first urethane resin and the second urethane resin are preferably the following combinations. That is, the first urethane resin preferably has a unit derived from polypropylene glycol, and the second urethane resin preferably has a unit derived from polytetramethylene glycol. This is thought to be related to the hydrophilicity / hydrophobicity of the polyol and the fact that the polyol has a branched chain.

  The urethane resin chain has a large number of urethane bonds (—NH—CO—O—), and forms a hydrogen bond between the urethane bonds. Therefore, the larger the molecular weight, the more hydrogen bonds are formed between the molecules, and the larger the molecular weight. The first urethane resin needs to have a hydrophilic interaction with the nonionic portion of the fluorinated surfactant. By making the structure of the polyether portion of the first urethane resin similar to the structure of the nonionic portion of the fluorine-based surfactant, the interaction is further enhanced. Since the fluorinated surfactant is composed of a straight carbon chain, the first urethane resin has a low steric hindrance and is unlikely to interact. From this, it is considered that the urethane resin having a relatively small weight average molecular weight, in which the hydrogen bonds between the urethane resins are small, selectively interacts with the fluorine-based surfactant. Therefore, it is preferable that the first urethane resin has a unit derived from polypropylene glycol. Further, when the first urethane resin has a unit derived from polypropylene glycol, the branched chain of the polypropylene glycol prevents hydrogen bonds between urethane bonds. Therefore, the steric hindrance of the first urethane resin is reduced, and the first urethane resin easily interacts with the fluorinated surfactant, so that the ejection stability is further improved and the unevenness of the image can be further reduced.

  On the other hand, the second urethane resin interacts with the high hydrophobicity of the carbon black particle surface. Therefore, by further increasing the hydrophobicity of the polyether portion of the second urethane resin, the interaction is further strengthened. When the second urethane resin has a unit derived from polytetramethylene glycol, the hydrophobicity is higher than that of polypropylene glycol, and since it is linear, the hydrogen bond between urethane bonds is not hindered, and steric hindrance is increased. Take a structure that becomes larger. Therefore, a fluorine-based surfactant that is not very high in hydrophobicity is kept away from the surface, and selectively interacts with carbon black having high hydrophobicity, so that the dispersion stability of carbon black is further improved.

  Examples of the polyester polyol include an acid ester. Examples of the acid component constituting the acid ester include aromatic dicarboxylic acids such as phthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, and tetrahydrophthalic acid; alicyclic dicarboxylic acids such as hydrogenated products of these aromatic dicarboxylic acids; Acid, succinic acid, tartaric acid, oxalic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, alkyl succinic acid, linoleic acid, maleic acid, fumaric acid, mesaconic acid, citraconic acid, itaconic acid, etc. Aliphatic dicarboxylic acid; and the like. These anhydrides, salts, derivatives (alkyl esters, acid halides) and the like can also be used as the acid component. Examples of the component that forms an ester with an acid component include polyols such as diols and triols; and glycols such as (poly) alkylene glycol. Examples of the polyols and glycols include those exemplified above as components constituting the polyether polyol.

  As the polycarbonate polyol, a polycarbonate polyol produced by a known method can be used. Specific examples include alkanediol-based polycarbonate diols such as polyhexamethylene carbonate diol. In addition, polycarbonate diols obtained by reacting carbonate components such as alkylene carbonate, diaryl carbonate, dialkyl carbonate and the like and phosgene with an aliphatic diol component can be exemplified.

  The ratio (mol%) of the unit derived from the polyol having no acid group to the total amount of the unit derived from the polyol in the urethane resin is preferably as follows. That is, it is preferably from 5.0 mol% to 50.0 mol%, more preferably from 10.0 mol% to 30.0 mol%. The proportion (% by mass) of units derived from a polyol having no acid group in the urethane resin is preferably from 5.0% by mass to 60.0% by mass.

  As the polyol having no acid group, a polyol having a number average molecular weight of about 450 to 4,000 is preferably used.

(Polyol having an acid group)
Examples of the polyol having an acid group include a polyol having an acid group such as a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, and a phosphonic acid group in its structure. Since these groups are hydrophilic groups, the urethane resin having these hydrophilic groups in its molecular structure can be more stably present in the aqueous ink. In particular, it is preferable to use a urethane resin having a unit derived from a polyol having an acid group such as dimethylolpropionic acid and dimethylolbutanoic acid. The anionic group may form a salt. Examples of the cation constituting the salt include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. For each urethane resin, it is preferable that 90.0% by mass or more of the units providing the acid value are units derived from a polyol having an acid group. In particular, it is particularly preferable that all units that give the acid value of each urethane resin are units derived from a polyol having an acid group.

(Polyamine)
Polyamines include monoamines having a plurality of hydroxy groups such as dimethylolethylamine, diethanolmethylamine, dipropanolethylamine and dibutanolmethylamine; ethylenediamine, propylenediamine, hexylenediamine, isophoronediamine, xylylenediamine, diphenylmethanediamine, hydrogen Bifunctional polyamines such as added diphenylmethanediamine and hydrazine; and trifunctional or higher polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, polyamidepolyamine and polyethylenepolyimine; and the like. For convenience, compounds having a plurality of hydroxy groups and one “amino group, imino group” are also listed as “polyamine”. The proportion (mol%) of units derived from polyamine in the urethane resin is preferably 10.0 mol% or less, more preferably 5.0 mol% or less. The ratio (mol%) of units derived from polyamine in the urethane resin may be 0.0 mol%.

[Cross-linking agent, chain extender]
When synthesizing the urethane resin, a crosslinking agent or a chain extender can be used. Usually, a crosslinking agent is used in the synthesis of a prepolymer, and a chain extender is used in performing a chain extension reaction on a prepolymer synthesized in advance. Basically, the cross-linking agent or chain extender can be appropriately selected from water, polyisocyanate, polyol, polyamine and the like depending on the purpose such as cross-linking and chain extension. As the chain extender, those capable of crosslinking a urethane resin can also be used. Each urethane resin contained in the ink of the present invention is preferably not crosslinked.

(Nonionic fluorinated surfactant)
The ink of the present invention contains a nonionic fluorinated surfactant (which may be simply referred to as “fluorinated surfactant” in this specification). The nonionic fluorine-based surfactant is preferably a perfluoroalkylethylene oxide adduct having a perfluoroalkyl group having 6 or less carbon atoms (fluorine-added carbon atoms). When the number of carbon atoms in the perfluoroalkyl group is 6 or less, it is considered that the hydrophobicity of the perfluoroalkyl group becomes appropriate, and the adsorption of the pigment to the particle surface due to the hydrophobic interaction is suppressed, thereby improving the ejection stability. , It is easy to obtain the effect of suppressing image unevenness.

  The structure of the perfluoroalkylethylene oxide adduct may be linear or branched, and is preferably linear. In the case of a straight chain, the intermolecular force is reduced due to a small difference in polarity between a perfluoroalkyl group that is a hydrophobic part and an ethylene oxide group that is a hydrophilic part, as compared with a branched case. It becomes difficult to adsorb to the carbon black particle surface. Therefore, it is easy to sufficiently obtain the effect of suppressing image unevenness due to the improvement in ejection stability.

  As the nonionic fluorine-based surfactant, a commercially available product can be used. As specific examples, MegaFac: F470, F444 (all made by DIC), Surflon: S-141, S-145 (all made by Asahi Glass), FS-3100, FSO-100 (all made by DuPont) Manufactured).

  The content (% by mass) of the nonionic fluorosurfactant based on the total mass of the ink is 0.05 to 5.00 times the mass ratio of the first urethane resin to the content (% by mass). The following is preferred. When the mass ratio is 0.05 times or more, the first urethane resin interacts with the fluorine-based surfactant to increase the alignment speed at the time of alignment at the surface of the dot or at the interface of foaming, and It is easy to sufficiently obtain the effect of improving image quality and the effect of suppressing image unevenness due to the improvement of ejection stability. On the other hand, when the mass ratio is 5.00 times or less, a fluorine-based surfactant that does not interact with the urethane resin hardly remains, and foaming hardly occurs. Therefore, foaming is stabilized, and discharge stability is improved. It is easy to sufficiently obtain the effect of suppressing image unevenness.

  The content (% by mass) of the nonionic fluorosurfactant in the ink is preferably 0.01% by mass or more and 0.10% by mass or less based on the total mass of the ink. When the content of the nonionic fluorosurfactant is 0.01% by mass or more, there is sufficient nonionic fluorosurfactant oriented at the interface, and the image is improved by improving the image clarity and the ejection stability. The effect of suppressing unevenness is easily obtained. On the other hand, when the content of the nonionic fluorine-based surfactant is 0.10% by mass or less, the amount of the nonionic fluorine-based surfactant becomes appropriate, and foaming is less likely to occur, so that foaming is stabilized, It is easy to obtain the effect of suppressing image unevenness by improving the ejection stability.

(Aqueous medium)
The ink of the present invention is an aqueous ink containing at least water as an aqueous medium. The aqueous medium may further contain a water-soluble organic solvent. As the water, it is preferable to use deionized water (ion exchange water). The content (% by mass) of water in the ink is preferably from 10.00% by mass to 90.00% by mass based on the total mass of the ink.

  The water-soluble organic solvent is not particularly limited as long as it is water-soluble. As the water-soluble organic solvent, for example, alcohol, polyhydric alcohol, polyglycol, glycol ether, nitrogen-containing polar solvent, sulfur-containing polar solvent and the like can be used. The content (% by mass) of the water-soluble organic solvent in the ink is preferably from 3.00% by mass to 50.00% by mass, and more preferably from 5.00% by mass to 40.00% by mass, based on the total mass of the ink. It is more preferred that the content be not more than mass%. If the content of the water-soluble organic solvent in the ink is out of the above range, ejection failure may occur.

(Other resins)
The ink of the present invention may further contain a resin (other resin) other than the above-described resin dispersant and urethane resin. Other types and forms of the resin may be any as long as they can be stably present in the aqueous ink. Examples of other resins include (meth) acrylic resins, polyamide resins, polyester resins, polyvinyl alcohol resins, and polyolefin resins. A base may be added to improve the solubility of these resins. Examples of the base include organic amines such as monoethanolamine, diethanolamine, triethanolamine, amine methylpropanol, and N, N-dimethylethanolamine; and inorganic bases such as potassium hydroxide and sodium hydroxide. .

(Other additives)
The ink of the present invention contains a water-soluble organic compound which is solid at normal temperature, such as polyhydric alcohols such as trimethylolpropane and trimethylolethane, and urea and its derivatives, as necessary, in addition to the above-described components. May be. Further, the ink of the present invention may contain, if necessary, other surfactants, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation promoters, and chelating agents. For example, various additives may be contained.

<Ink cartridge>
The ink cartridge of the present invention includes ink and an ink storage unit that stores the ink. The ink contained in the ink containing section is the ink of the present invention described above. FIG. 1 is a sectional view schematically showing an embodiment of the 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 unit for storing ink. The ink storage section includes an ink storage chamber 14 and an absorber storage chamber 16, which communicate with each other through a communication port 18. The absorber housing chamber 16 communicates with the ink supply port 12. The ink storage chamber 14 stores liquid ink 20, and the absorber storage chamber 16 stores absorbers 22 and 24 that hold the ink in an impregnated state. The ink storage section may be configured to have no ink storage chamber for storing liquid ink, and to hold the entire amount of ink stored by the absorber. Further, the ink storage section 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 unit and a recording head.

<Inkjet recording method>
The inkjet recording method of the present invention is a method of recording an image on a recording medium by discharging the above-described ink of the present invention from an inkjet recording head. As a method of discharging ink, a method of applying thermal energy to ink is used. Other than using the ink of the present invention, the steps of the inkjet recording method may be known.

  2A and 2B are diagrams schematically illustrating an example of an ink jet recording apparatus used in the ink jet recording method of the present invention, wherein FIG. 2A is a perspective view of a main part of the ink jet recording apparatus, and FIG. 2B is a perspective view of a head cartridge. It is. The inkjet recording apparatus is provided with a transport unit (not shown) for transporting the recording medium 32 and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 includes recording heads 38 and 40, and is configured so that the ink cartridge 42 is set. While the head cartridge 36 is conveyed in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the image is recorded on the recording medium 32 by the recording medium 32 being transported in the sub-scanning direction by a transport unit (not shown).

  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 it exceeds the gist thereof. Components described as "parts" and "%" are based on mass unless otherwise specified.

<Synthesis of urethane resin>
In a four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a condenser tube, use amounts (unit: parts) of polyols and polyisocyanates having no acid group and dibutyltin dilaurate in the amounts shown in Table 1 (unit: parts). 007 parts were added and reacted at 100 ° C. for 5 hours in a nitrogen gas atmosphere. After cooling to 65 ° C. or lower, the used amounts of the polyol having an acid group and the chain extender shown in Table 1 and 150.0 parts of methyl ethyl ketone were added and reacted at 80 ° C. After cooling to 40 ° C., the reaction was stopped by adding 20.0 parts of methanol. After adding an appropriate amount of ion-exchanged water, a 10.0% aqueous potassium hydroxide solution required for neutralizing the anionic groups of the resin was added while stirring with a homomixer. Methyl ethyl ketone and unreacted methanol were distilled off under reduced pressure under heating to obtain aqueous solutions of urethane resins 1 to 22 having a resin (solid content) content of 10.0%. The weight average molecular weight of the urethane resin was adjusted by appropriately changing the reaction time at 80 ° C.

  Hydrochloric acid was added dropwise to the obtained aqueous solution, and the precipitated urethane resin was separated and dried under vacuum at 40 ° C. overnight to obtain a solid urethane resin. A solution obtained by dissolving the obtained urethane resin in tetrahydrofuran was used as a sample to be measured for an acid value and a weight average molecular weight. The acid value of the urethane resin was measured by potentiometric titration with a potassium hydroxide / ethanol titrant using an automatic potentiometric titrator (manufactured by Kyoto Electronics Industry). The weight average molecular weight of the urethane resin was determined by gel permeation chromatography (GPC) as a value in terms of polystyrene. GPC (trade name “Alliance GPC 2695”, manufactured by Waters) was used as a measuring device. The column used was a quadruple column with the trade name “Shodex KF-806M” (manufactured by Showa Denko KK). As the detector, an RI (refractive index) detector (trade name “2414 Detector”, manufactured by Waters) was used. PS-1 and PS-2 (manufactured by Polymer Laboratories) were used as standard polystyrene samples. Table 1 shows the acid value and weight average molecular weight of the urethane resin.

The meanings of the abbreviations of the monomers in Table 1 are shown below. The numerical value attached to the abbreviation is the number average molecular weight of the polyol.
PPG: Polypropylene glycol PTMG: Polytetramethylene glycol PEG: Polyethylene glycol PCD: Polyhexamethylene carbonate diol IPDI: Isophorone diisocyanate DMPA: Dimethylolpropionic acid NPG: Neopentyl glycol

(Preparation of urethane resin 23)
According to the method for synthesizing PU1 in JP-T-2005-515289 (Patent Document 2), a urethane resin is synthesized, an appropriate amount of ion-exchanged water is added, and the content of the resin (solid content) is 10.0%. An aqueous dispersion of a certain urethane resin 23 was obtained. The weight average molecular weight of the urethane resin 23 was 15,000.

(Preparation of urethane resin 24)
According to the method for synthesizing PU2 in JP-T-2005-515289 (Patent Document 2), a urethane resin is synthesized, an appropriate amount of ion-exchanged water is added, and the content of the resin (solid content) is 10.0%. An aqueous dispersion of a certain urethane resin 24 was obtained. The weight average molecular weight of the urethane resin 24 was 30,000.

<Synthesis of acrylic resin>
In a four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a condenser tube, 200.0 parts of methyl ethyl ketone was charged, and the reaction system was replaced with nitrogen. On the other hand, benzyl acrylate 25.0 parts, styrene 35.0 parts, 2-ethoxyethyl acrylate 5.0 parts, methyl methacrylate 15.0 parts, acrylic acid 20.0 parts, and t-butyl peroxide (polymerization initiator) 2.0 parts of the mixture were prepared. The temperature of the reaction system was raised to 80 ° C., the prepared mixture was added dropwise over 2 hours, and the mixture was reacted at 80 ° C. for 14 hours to obtain an acrylic resin. After adding a necessary amount of 10.0% aqueous potassium hydroxide solution to neutralize the anionic groups of the resin, an appropriate amount of ion-exchanged water is added to reduce the resin (solid content) to 20.0%. Was obtained. The acid value of the acrylic resin was 156 mgKOH / g, and the weight average molecular weight was 33,000. The acid value and weight average molecular weight of the acrylic resin were measured using the obtained aqueous solution as a sample for measurement under the same conditions as in the case of the urethane resin.

<Preparation of pigment dispersion>
(Pigment dispersion liquid 1)
A mixture was obtained by mixing 20.0 parts of a pigment (carbon black, trade name “No. 900”, manufactured by Mitsubishi Chemical Corporation), 50.0 parts of a resin aqueous solution, and 30.0 parts of ion-exchanged water. The “resin aqueous solution” is an aqueous solution of a resin (resin dispersant) used for dispersing a pigment. As the resin aqueous solution, an aqueous solution (content (solid content): 20.0%) of an acrylic resin (trade name “John Krill 683”, manufactured by BASF) was used. The mixture was put into a media-type dispersing machine (trade name "Beads Mill LMZ2", manufactured by Ashizawa Finetech) having a filling rate of zirconia beads of 80%, dispersed at a peripheral speed of 12 m / s, and then rotated at 5,000 rpm. For 30 minutes to remove aggregated components. Next, a pigment dispersion 1 was prepared by adding an appropriate amount of ion-exchanged water. The content of the pigment in the pigment dispersion 1 was 20.0%, and the content of the resin was 10.0%.

(Pigment dispersion liquid 2)
The pigment is C.I. I. Pigment Blue 15: 3 (trade name “Hostaperm Blue B2G”, manufactured by Clariant), and a pigment dispersion 2 was prepared in the same procedure as the preparation of the pigment dispersion 1. The content of the pigment in the pigment dispersion liquid 2 was 20.0%, and the content of the resin was 10.0%.

(Pigment dispersion 3)
According to the method for preparing a self-dispersion pigment described in JP-T-2005-515289 (Patent Document 2), a self-dispersion pigment (self-dispersion carbon black) was prepared and the concentration was adjusted to obtain a pigment dispersion liquid 3. The content of the pigment (carbon black) in the pigment dispersion 3 was 20.0%.

<Preparation of ink>
After mixing each component (unit:%) shown in the middle row of Table 2 (Tables 2-1 to 2-5) and sufficiently stirring, a cellulose acetate filter having a pore size of 1.2 μm (trade name “Minisart”, manufactured by Sartorius) Each was filtered under pressure to prepare each ink. The number average molecular weight of the polyethylene glycol used was 1,000. The lower part of Table 2 shows the pigment type, the content C ₁ (%) of the first urethane resin, and the content C ₂ (%) of the second urethane resin. The lower part of Table 2 shows the values (times) of the weight average molecular weight Mw ₁ of the first urethane resin, the weight average molecular weight Mw ₂ , and Mw ₁ / Mw ₂ of the second urethane resin. Further, the lower part of Table 2 shows the content C ₃ (%) of the nonionic fluorine-based surfactant and the value (times) of C ₃ / C ₁ . The details of the components used in Table 2 and the abbreviations of the pigment types in Table 2 are shown below.
-FS-3100 (trade name of "Zonil", manufactured by DuPont): Nonionic fluorinated surfactant (perfluoroalkylethylene oxide adduct having 6 carbon atoms in the perfluoroalkyl group)
-FSO-100 (trade name of "Zonil", manufactured by DuPont): Nonionic fluorinated surfactant (perfluoroalkylethylene oxide adduct having 8 carbon atoms in the perfluoroalkyl group)
・ F-410 (trade name of “Megafac”, manufactured by DIC): Anionic fluorinated surfactant (perfluoroalkyl group-containing carboxylate)
Acetylene E100 (trade name, manufactured by Kawaken Fine Chemical): nonionic acetylene glycol-based surfactant Proxel GXL (S) (trade name, manufactured by Arch Chemicals): Preservatives CB: carbon black PC: phthalocyanine pigment

<Evaluation>
The following evaluation was performed using an ink jet recording apparatus (trade name “PIXUS PRO-1000”, manufactured by Canon Inc.) equipped with a recording head that ejects ink by thermal energy. In the above-described ink jet recording apparatus, an image recorded under the condition that resolution is 600 dpi × 600 dpi and 8 drops of 3.8 ng ink are applied to a unit area of 1/600 inch × 1/600 inch at a recording duty of 100% Defined to be. In the present invention, "A" and "B" were set to acceptable levels, and "C" was set to an unacceptable level based on the following evaluation criteria. Table 3 shows the evaluation results.

(Discharge stability)
The prepared ink was filled in an ink cartridge and set in the above-described ink jet recording apparatus. Then, with four printing passes, ten 20 cm × 30 cm solid images with a printing duty of 25% were printed on a printing medium (trade name “Canon Photo Paper / Glossy Gold”, manufactured by Canon) for 10 sheets. The print path is related to the feed length in which the print medium is fed in the sub-scanning direction after the print head scans once in the main scanning direction, and the feed length is the length in the ejection port array direction of the print head. It is a value obtained by dividing the number by the number of passes. Therefore, a desired image is usually recorded on the recording medium while the recording head reciprocates on the recording medium by the number of passes. The state of unevenness in the recorded image was visually checked, and the ejection stability was evaluated according to the following evaluation criteria.
A: There was no unevenness in the image.
B: There was unevenness in the image, but there was no white spot without ink applied.
C: The image was uneven, and there was a portion that was whitened without ink being applied.

(Mapping)
The prepared ink was filled in an ink cartridge and set in the above-described ink jet recording apparatus. Then, in the default mode, a 20 cm × 30 cm solid image having a recording duty of 100% was recorded on a recording medium (trade name “Canon Photo Paper / Glossy Gold”, manufactured by Canon). For the recorded image, two fluorescent lamps arranged at an interval of 10 cm were used as observation light sources, and the fluorescent lamps were projected onto the image from a position 2 m away. The image clarity of the image was evaluated according to the following evaluation criteria by visually confirming the shape of the fluorescent lamp projected on the image under the conditions of an illumination angle of 45 degrees and an observation angle of 45 degrees.
A: Two fluorescent lamps were clearly projected on the image.
B: The edges of the two projected fluorescent lamps were slightly blurred.
C: The boundary between the two projected fluorescent lamps was not found.

Claims (8)

A water-based ink used in an ink jet recording method for recording an image on a recording medium by discharging ink from a recording head by the action of thermal energy,
Carbon black, containing a resin dispersant for dispersing the carbon black, a first urethane resin, a second urethane resin, and a nonionic fluorine-based surfactant,
Both the first urethane resin and the second urethane resin have units derived from polyether polyol,
The weight average molecular weight Mw1 of the _first urethane resin is 0.2 times or more and 0.8 times or less in a ratio to the weight average molecular weight Mw2 of the _second urethane resin,
A water-based ink, wherein the weight-average molecular weight Mw ₂ of the second urethane resin is from 20,000 to 70,000. The water-based ink according to claim 1, wherein the first urethane resin has a weight average molecular weight Mw ₁ of 5,000 to 30,000. The first urethane resin has a unit derived from polypropylene glycol,
3. The aqueous ink according to claim 1, wherein the second urethane resin has a unit derived from polytetramethylene glycol.   The content (% by mass) of the nonionic fluorine-based surfactant based on the total mass of the ink is 0.05 times or more as a mass ratio to the content (% by mass) of the first urethane resin. The aqueous ink according to any one of claims 1 to 3, wherein the aqueous ink is not more than 00 times.   The content (% by mass) of the nonionic fluorine-based surfactant is 0.01% by mass or more and 0.10% by mass or less based on the total mass of the ink. Water-based ink.   The aqueous ink according to any one of claims 1 to 5, wherein the nonionic fluorine-based surfactant is a perfluoroalkylethylene oxide adduct having a perfluoroalkyl group having 6 or less carbon atoms. An ink cartridge including ink and an ink storage unit that stores the ink,
An ink cartridge, wherein the ink is the aqueous ink according to any one of claims 1 to 6. An ink jet recording method for recording an image on a recording medium by discharging ink from a recording head by the action of thermal energy,
An ink jet recording method, wherein the ink is the aqueous ink according to any one of claims 1 to 6.

Images