JP2018197284A - Aqueous ink, ink cartridge, and inkjet recording method - Google Patents

Abstract

Disclosed is a water-based ink that can suppress a drop in ejection stability when ink is ejected over a long period of time and can record an image having excellent optical density and marker resistance. An aqueous inkjet ink containing a pigment, a water-soluble urethane resin, and urea. The pigment is a self-dispersing pigment in which an anionic group is bonded to the carbon black particle surface directly or via other atomic groups, and the urethane resin is a polyisocyanate, a polyol having no acid group, and a polyol having an acid group Units derived from each of the above. [Selection figure] None

Description

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

  In recent years, with the improvement of image quality and recording speed, inkjet recording apparatuses have been increasingly used in the business field. The performance required for ink jet ink used in the business field includes ink reliability (e.g. ejection stability), image quality (e.g. high optical density and feathering resistance), and image robustness (e.g. scratch resistance, Marker resistance, water resistance, etc.).

  Among these performances, in order to improve the optical density of images, self-dispersing pigments are used rather than using resin-dispersed pigments, and inks containing urethane resins are being studied in order to improve the marker resistance of images. (Patent Document 1). Patent Document 1 describes that the marker resistance of an image is improved by an ink containing a urethane resin having an acid group and a self-dispersing pigment.

JP 2005-515289 A

  The inventors of the present invention caused an ink containing a self-dispersing pigment and a urethane resin to be ejected over a long period of time using an ink jet recording apparatus, and the solidified ink adhered to the ejection port surface of the recording head. It was recognized that the ink ejection stability was reduced.

  Accordingly, an object of the present invention is to provide a water-based ink capable of suppressing the decrease in ejection stability when ink is ejected over a long period of time and recording an image excellent in optical density and marker resistance. There is to do. Another object of the present invention is to provide an ink cartridge and an ink jet recording method using the water-based ink.

  The above object is achieved by the present invention described below. That is, according to the present invention, a water-based inkjet ink containing a pigment, a water-soluble urethane resin, and urea, wherein the pigment is an anion directly on the particle surface of carbon black or through another atomic group. There is provided a water-based ink, wherein the urethane resin has a unit derived from a polyisocyanate, a polyol not having an acid group, and a polyol having an acid group. The

  According to the present invention, it is possible to provide a water-based ink that can suppress a decrease in ejection stability when ink is ejected over a long period of time and can record an image excellent in optical density and marker resistance. Can do. In addition, according to the present invention, an ink cartridge using the water-based ink and an ink jet recording method can be provided.

It is sectional drawing which shows typically one Embodiment of the ink cartridge of this invention. FIG. 2 is a diagram schematically illustrating an example of an ink jet recording apparatus used in the ink jet recording method of the present invention, in which (a) is a perspective view of a main part of the ink jet 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 it is expressed as “contains a salt” for convenience. In addition, water-based ink for inkjet may be simply referred to as “ink”. Physical property values are values at normal temperature (25 ° C.) unless otherwise specified. The “unit” of the resin means a repeating unit derived from one monomer.

  When the ink containing the self-dispersing pigment and the urethane resin is ejected over a long period of time by the ink jet recording apparatus, the solidified ink adheres to the ink ejection port surface of the recording head. It has been recognized that the discharge stability of the ink drops. As a result of analyzing the cause of this phenomenon, it was found that solidified ink was deposited on the cap provided in the ink jet recording apparatus and adhered to the discharge port surface, thereby reducing the discharge stability. It was. The cap is a member that covers the ejection port of the recording head, and is provided for a problem caused by drying of the ink.

  When ink is ejected over a long period of time with an ink jet recording apparatus, a very small amount of ink is ejected (preliminary ejection) into the above-described cap in order to maintain intermittent ejection stability of the ink. The cap is provided with an absorber composed of a sponge or porous material, and a very small amount of ink ejected to the cap thickens by evaporating at the cap and losing water, depending on the recording environment. It solidifies but flows out by a very small amount of ink applied thereafter. Furthermore, since the suction operation is also performed using the cap, as a result, maintenance is performed so that ink does not accumulate on the cap.

  However, it is considered that the ink containing the urethane resin and the self-dispersing pigment investigated by the present inventors was not washed away by the applied ink and deposited when the ink was solidified by the cap.

  Regarding the above-described washing-off effect of newly applied ink, it is considered that the newly applied ink is washed away by re-dissolving (re-dispersing) the solidified ink. Therefore, the present inventors considered that the solidified ink is redissolved with newly applied ink, thereby solving the problem of the present invention.

  When the ink is evaporated and finally solidified, the solidified product is mainly composed of a urethane resin and a self-dispersing pigment. When the ink which is difficult to re-dissolve is solidified, it is considered that a strong aggregate composed of a urethane resin and a self-dispersing pigment is formed. In contrast, the ink applied thereto re-dissolves the solidified ink by loosening the strong aggregates. As a result of investigations based on this idea, the present inventors have found that, by adding urea to the ink, the accumulation of ink on the cap is alleviated, the adhesion of the ink to the ejection port surface of the recording head is suppressed, and stable. Has been found that a consistent recording state can be maintained.

  The present inventors consider the reason why such an effect is obtained by including urea in the ink as follows. The above-mentioned strong aggregates are mainly composed of a urethane resin and a self-dispersing pigment, but it is considered that urea is taken into the aggregates in the ink to which urea is added. The reason for this is that urea has two urea bonds (—NHCO—) in its structure, so it easily interacts with the urethane bond sites and urea bond sites contained in urethane, and is easily incorporated into aggregates. it is conceivable that. Furthermore, urea has a small molecular size, which is considered as a reason why it is easily incorporated into aggregates. The urea thus taken into the aggregate not only weakens the cohesive force of the aggregate, but also has excellent hygroscopicity. Therefore, when ink is applied, the water in the applied ink is drawn. This is considered to improve the re-solubility of the aggregate. As a result, it is considered that ink accumulation at the cap is suppressed.

<Ink>
The ink of the present invention is a water-based ink containing a pigment, a urethane resin, and urea. The pigment is a self-dispersing pigment in which an anionic group is bonded to the surface of carbon black particles directly or via other atomic groups. The urethane resin has units derived from polyisocyanate, a polyol having no acid group, and a polyol having an acid group. Hereafter, each component which comprises the ink of this invention is demonstrated in detail.

(Pigment)
As the pigment, a self-dispersing pigment in which an anionic group is bonded to the surface of carbon black particles directly or via another atomic group is used. In the ink, the anionic group may be in a state in which a part thereof is dissociated or in a state in which all of the anionic group is dissociated.

  The content (% by mass) of the pigment in the ink is preferably 0.1% by mass or more and 15.0% by mass or less, and 1.0% by mass or more and 10.0% by mass or less based on the total mass of the ink. More preferably.

  Any carbon black conventionally used in ink jet inks can be used. Specific examples include furnace black, acetylene black, channel black, thermal black, and lamp black.

  Examples of the anionic group chemically bonded to the carbon black particle surface directly or through other atomic groups (—R—) include carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, and phosphones. An acid group etc. can be mentioned. Among these, a phosphonic acid group is preferable. Examples of the counter ion of the anionic group include cations such as hydrogen atom, alkali metal, ammonium and organic ammonium. Among these, alkali metal cations such as lithium, sodium, and potassium are preferable. Other atomic groups (—R—) are not particularly limited, and examples thereof include alkylene groups, arylene groups, amide groups, sulfonyl groups, imino groups, carbonyl groups, ester groups, ether groups, and groups obtained by combining these groups. And so on.

(Urethane resin)
As the urethane resin, a urethane resin having units derived from polyisocyanate, a polyol having no acid group, and a polyol having an acid group is used. As described above, regarding the problem of ink depositing on the cap, the effect of alleviating ink deposition on the cap appears by adding urea to the ink. However, when a water-dispersible urethane resin is used, when the ink evaporates, not only does it form a stronger aggregate with the pigment than when a water-soluble urethane resin is used, but also the aggregate. It is difficult for urea to be taken in, and the effect of urea does not appear. Therefore, the urethane resin needs to be water-soluble.

Whether or not the urethane resin is water-soluble can be determined according to the following method. First, a liquid (resin solid content: 10% by mass) containing a resin neutralized with an alkali corresponding to an acid value (sodium hydroxide, potassium hydroxide, etc.) is prepared. Next, the prepared liquid is diluted 10 times (volume basis) with pure water to prepare a sample solution. And when the particle diameter of resin in a sample solution is measured by a dynamic light scattering method, when the particle | grains which have a particle diameter are not measured, it can be judged that the resin is water-soluble. The measurement conditions at this time can be as follows, for example.
[Measurement condition]
SetZero: 30 seconds Number of measurements: 3 times Measurement time: 180 seconds

  As the particle size distribution measuring apparatus, a particle size analyzer (for example, trade name “UPA-EX150”, manufactured by Nikkiso) using a dynamic light scattering method can be used. Of course, the particle size distribution measuring apparatus and measurement conditions to be used are not limited to the above.

[Polyisocyanate]
Polyisocyanate is a compound having two or more isocyanate groups in its molecular structure. As the polyisocyanate, aliphatic polyisocyanate, aromatic polyisocyanate, and the like can be used. The proportion (% by mass) of units derived from polyisocyanate in the urethane resin is preferably 10.0% by mass or more and 50.0% by mass or less.

  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-methylpentane. Aliphatic polyisocyanates having a chain structure such as -1,5-diisocyanate and 3-methylpentane-1,5-diisocyanate; isophorone diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane-4,4′-diisocyanate, , 4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, 1,3-bis (isocyanatomethyl) cyclohexane, etc. Aliphatic polyisocyanate having a cyclic structure; and the like.

  Examples of the aromatic polyisocyanate include tolylene diisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,4′-dibenzyldiisocyanate, 1,5- Naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, α, α, α ′, α′-tetramethylxylylene diisocyanate Can do.

  Said polyisocyanate can be used individually by 1 type or in combination of 2 or more types. Among the above polyisocyanates, aliphatic polyisocyanates are preferable, and isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate are more preferable. Of the aliphatic polyisocyanates, it is preferable to use an aliphatic polyisocyanate having a cyclic structure, and it is more preferable to use isophorone diisocyanate.

[Polyol]
A polyol is a compound having two or more hydroxy groups in its molecular structure. Examples of the polyol include a polyol having no acid group such as a polyether polyol, a polyester polyol, and a polycarbonate polyol; a polyol having an acid group; Moreover, as a polyol which does not have an acid group, the compound which has a some hydroxy group and one "amino group and imino group" in molecular structure can also be mentioned. These polyols can be used singly or in combination of two or more. The proportion (mol%) of units derived from polyol in the urethane resin is preferably 10.0 mol% or more and 80.0 mol% or less, and preferably 20.0 mol% or more and 60.0 mol% or less. More preferably.

[Polyol having no acid group]
Examples of polyether polyols include addition polymers of alkylene oxides and polyols; glycols such as (poly) alkylene glycols; and the like. Examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide, α-olefin oxide and the like. Examples of polyols to be addition-polymerized with alkylene oxide include 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl- 1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4-dihydroxyphenylpropane, 4,4-dihydroxy Diols such as phenylmethane, hydrogenated bisphenol A, dimethylolurea and derivatives thereof; glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol, trimethylolmelamine and derivatives thereof , Polyoxypropylene Triols such as polyol; and the like. Examples of glycols include tetramethylene glycol, hexamethylene glycol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, (poly) tetramethylene glycol, (Poly) alkylene glycol such as neopentyl glycol; ethylene glycol-propylene glycol copolymer;

  Examples of the polyester polyol include acid esters. 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, linolenic acid, maleic acid, fumaric acid, mesaconic acid, citraconic acid, itaconic acid, etc. An 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 diol and triol; glycols such as (poly) alkylene glycol; Examples of the polyols and glycols include those exemplified as the 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. Moreover, the polycarbonate diol etc. which are obtained by making carbonate components, such as alkylene carbonate, a diaryl carbonate, a dialkyl carbonate, and phosgene, and an aliphatic diol component react can be mentioned.

  Examples of the compound having a plurality of hydroxy groups and one “amino group, imino group” in the molecular structure include monoamines having a plurality of hydroxy groups. Specific examples thereof include dimethylolethylamine, diethanolmethylamine, dipropanolethylamine, and dibutanolmethylamine.

  The polyol having no acid group preferably has 10 or more carbon atoms. Moreover, it is preferable that the number average molecular weight of the polyol which does not have an acid group is 600 or more and 4,000 or less. If the number average molecular weight of the polyol having no acid group is less than 600, the flexibility of the formed film tends to be lowered, and the effect of improving the marker resistance of the image may be slightly insufficient. On the other hand, when the number average molecular weight of the polyol having no acid group is more than 4,000, the flexibility of the formed film tends to be high, and the effect of improving the marker resistance of the image may be slightly insufficient.

  The proportion (mol%) of the unit derived from the polyol having no acid group in the total amount of the units derived from the polyol in the urethane resin is from 1.0 mol% to 99.0 mol%. preferable. Further, this ratio (mol%) is more preferably 5.0 mol% or more and 50.0 mol% or less. The proportion (mass%) of the unit derived from the polyol having no acid group in the urethane resin is preferably 5.0 mass% or more and 60.0 mass% or less. This ratio (% by mass) is more preferably 10.0% by mass or more and 60.0% by mass or less.

  The said polyol which does not have an acid group can be used individually by 1 type or in combination of 2 or more types. Among the polyols having no acid group, polyether polyol is preferably used, (poly) alkylene glycol is more preferably used, and polypropylene glycol is further preferably used.

[Polyol having an acid group]
Examples of the polyol having an acid group include polyols having an acid group such as a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, and a phosphonic acid group. The acid group is preferably a carboxylic acid group. Examples of the polyol having a carboxylic acid group include dimethylolacetic acid, dimethylolpropionic acid, dimethylolbutanoic acid, and dimethylolbutyric acid. Of these, dimethylolpropionic acid and dimethylolbutanoic acid are preferable. The acid group of the polyol having an acid group may be in a salt form. Examples of the cation forming the salt include ions of alkali metals such as lithium, sodium and potassium; cations of organic amines such as ammonium ion and dimethylamine. Since the molecular weight of the polyol having a general acid group is about 400 at most, the unit derived from the polyol having an acid group basically becomes a hard segment of the urethane resin. The acid value of the urethane resin can be adjusted by the amount of the polyol having an acid group.

  The proportion (mol%) of units derived from polyol having an acid group in the total amount of units derived from polyol in the urethane resin is preferably 1.0 mol% or more and 99.0 mol% or less. . Further, this ratio is more preferably 30.0 mol% or more and 95.0 mol% or less, and further preferably 50.0 mol% or more and 90.0 mol% or less. The proportion (mass%) of the unit derived from the polyol having an acid group in the urethane resin is preferably 5.0 mass% or more and 60.0 mass% or less. This ratio (mass%) is more preferably 5.0% by mass or more and 50.0% by mass or less, and further preferably 5.0% by mass or more and 30.0% by mass or less.

[Polyamine]
The urethane resin preferably further has units derived from polyamine. A polyamine is a compound having two or more amino groups in its molecular structure. Examples of polyamines include bifunctional polyamines such as ethylenediamine, propylenediamine, hexylenediamine, isophoronediamine, xylylenediamine, diphenylmethanediamine, hydrogenated diphenylmethanediamine, and hydrazine; diethylenetriamine, triethylenetetramine, tetraethylenepentamine, polyamide And polyamines having three or more functional groups such as polyamine and polyethylene polyimine. Since the molecular weight of polyamine is about 400 at most, the unit derived from polyamine is basically a hard segment of urethane resin. The proportion (mol%) of units derived from polyamine in the urethane resin is preferably 10.0 mol% or less, and more preferably 5.0 mol% or less. The proportion (mol%) of units derived from polyamine in the urethane resin may be 0.0 mol%.

[Crosslinking agent, chain extender]
In synthesizing the urethane resin, a crosslinking agent or a chain extender can be used. Usually, the crosslinking agent is used in the synthesis of the prepolymer, and the chain extender is used in performing the chain extension reaction on the prepolymer synthesized in advance. Basically, the crosslinking agent or chain extender can be appropriately selected from water, polyisocyanate, polyol, polyamine and the like according to the purpose such as crosslinking and chain extension. As the chain extender, those capable of crosslinking the urethane resin can also be used.

  The urethane resin preferably has a structure crosslinked with a polyamine. In this case, the urethane resin has units derived from trifunctional or higher polyamines as mentioned above. Of the trifunctional or higher polyamines, the crosslinked structure in the urethane resin is preferably derived from at least one of diethylenetriamine and triethylenetetramine.

[Characteristic]
The acid value of the urethane resin is preferably 40 mgKOH / g or more and 140 mgKOH / g or less. When the acid value is less than 40 mgKOH / g, the hydrophilicity of the urethane resin is lowered, and ink ejection stability may not be sufficiently obtained. If the acid value exceeds 140 mgKOH / g, the hydrophilicity of the urethane resin is high, and the marker resistance of the image may not be sufficiently obtained.

  The content (% by mass) of the urethane resin in the ink is preferably 0.1% by mass or more and 10.0% by mass or less, and 0.5% by mass or more and 5.0% by mass based on the total mass of the ink. The following is more preferable. By setting the content of the urethane resin to 0.5% by mass or more, the marker resistance of the image can be obtained at a higher level. On the other hand, when the content of the urethane resin is 5.0% by mass or less, the ink ejection stability can be obtained at a higher level.

  The content (mass%) of the urethane resin in the ink is preferably 0.05 times or more and 2.00 times or less, and 0.10 times or more and 1. More preferably, it is 00 times or less. By setting the mass ratio to 0.10 times or more, the marker resistance of the image can be obtained at a higher level. On the other hand, by setting the above-mentioned mass ratio to 1.00 times or less, ink ejection stability can be obtained at a higher level.

  The weight average molecular weight (Mw) in terms of polystyrene measured by gel permeation chromatography (GPC) of the urethane resin is preferably 10,000 or more and 60,000 or less. When the Mw of the urethane resin is less than 10,000, the strength of the urethane resin becomes low, and the effect of improving the marker resistance of the image may not be sufficiently obtained. On the other hand, if the Mw of the urethane resin is more than 60,000, the viscosity of the ink tends to increase, and the effect of improving the ink ejection stability may not be sufficiently obtained.

[Synthesis method]
The urethane resin can be synthesized according to a conventional general method for synthesizing a urethane resin. For example, a urethane resin can be synthesized according to the method shown below. The polyisocyanate, the polyol having no acid group, and the polyol having an acid group are reacted in an equivalent ratio such that the isocyanate group becomes excessive in the presence or absence of an organic solvent having a boiling point of 100 ° C. or less. As a result, a urethane prepolymer having an isocyanate group at the molecular end is synthesized. Next, neutralizing agents are used to neutralize acidic groups such as carboxylic acid groups and sulfonic acid groups in the synthesized urethane prepolymer. Thereafter, the urethane prepolymer neutralized with acidic groups is allowed to react in an aqueous solution containing a chain extender, and then the organic solvent remaining in the system is removed as necessary to obtain a urethane resin. be able to.

  Examples of neutralizers include alkanolamines such as N, N-dimethylethanolamine, N, N-diethylethanolamine, diethanolamine and triethanolamine; inorganic bases such as sodium hydroxide, potassium hydroxide and ammonia; trimethylamine and triethylamine An organic base such as can be used. Especially, it is preferable to use the neutralizing agent containing alkali metals, such as sodium hydroxide and potassium hydroxide. When neutralizing acidic groups in the urethane prepolymer with a neutralizing agent containing an alkali metal, excellent ink ejection stability can be obtained as compared with the case of neutralizing with a neutralizing agent such as amines. The neutralizing agent is preferably used in an amount of 0.5 mol or more and 1.0 mol or less, more preferably 0.8 mol or more and 1.0 mol or less, relative to 1 mol of acidic groups in the urethane prepolymer. When the use amount of the neutralizing agent is out of the above range, the workability may be deteriorated because the liquid containing the urethane resin becomes unstable or the viscosity increases.

[Analysis method]
The composition of the urethane resin can be analyzed by the following method. First, a method for extracting urethane resin from ink will be described. In order to extract the urethane resin from the ink, an excess acid (such as hydrochloric acid) is added to the supernatant obtained by centrifuging the ink at 80,000 rpm to precipitate the resin. When chloroform is added to the precipitated resin, the urethane resin is dissolved, so that the urethane resin can be extracted from the liquid phase. Although the urethane resin can be analyzed even in the ink state, analysis of the urethane resin extracted from the ink is preferable because the measurement accuracy can be improved.

(1) Composition of urethane resin A urethane resin is dissolved in deuterated dimethyl sulfoxide (deuterated DMSO) and analyzed by proton nuclear magnetic resonance ( ¹ H-NMR). And from the position of the obtained peak, the kind of polyisocyanate, the polyol which does not have an acid group, the polyol which has an acid group, and a polyamine can be confirmed. Moreover, the kind of polyisocyanate, the polyol which does not have an acid group, the polyol which has an acid group, and the polyamine can also be confirmed by analyzing the dried urethane resin by pyrolysis gas chromatography. Furthermore, the composition ratio can be calculated from the ratio of the integrated values of the peaks.

(2) Acid value of urethane resin Dissolve urethane resin in tetrahydrofuran (THF), and use potentiometric automatic titrator (trade name “AT510”, manufactured by Kyoto Electronics Industry Co., Ltd.), using potassium hydroxide ethanol titrant. By titrating, the acid value of the urethane resin can be measured.

(3) Weight average molecular weight of urethane resin The weight average molecular weight of the urethane resin can be measured by GPC. An example of GPC measurement conditions is as follows. In the examples described later, a urethane resin dissolved in THF was used as a measurement sample, and the weight average molecular weight of the urethane resin was measured under the following conditions.

Apparatus: Alliance GPC 2695 (manufactured by Waters)
Column: 4-column column of Shodex KF-806M (Showa Denko)
・ Mobile phase: Tetrahydrofuran (special grade)
・ Flow rate: 1.0 mL / min
・ Oven temperature: 40.0 ℃
-Sample solution injection volume: 0.1 mL
・ Detector: RI (refractive index)
Polystyrene standard sample: PS-1 and PS-2 (manufactured by Polymer Laboratories, molecular weight: 7,500,000, 2,560,000, 841,700, 377,400, 320,000, 210,500, 148,000 , 96,000, 59,500, 50,400, 28,500, 20,650, 10,850, 5,460, 2,930, 1,300, 580)

(urea)
The ink of the present invention contains urea. The content (% by mass) of urea in the ink is preferably 0.1% by mass or more and 15.0% by mass or less based on the total mass of the ink. The content (% by mass) of urea in the ink is a mass ratio with respect to the content (% by mass) of the urethane resin in the ink, and is preferably 1.0 to 5.0 times. When the mass ratio is less than 1.0, the amount of urea taken into the agglomerates described above is small, so the effect of the present invention is reduced. On the other hand, if the mass ratio is more than 5.0 times, there is urea that is not taken into the aggregate, and the water in the ink applied to the cap is not taken into the aggregate. It will act preferentially on urea, and the effect of the present invention will be reduced.

(Aqueous medium)
The ink of the present invention is an aqueous ink containing an aqueous medium containing water. As water, it is preferable to use deionized water (ion exchange water). The content (% by mass) of water in the ink is preferably 10.0% by mass or more and 90.0% by mass or less, preferably 50.0% by mass or more and 90.0% by mass or less, based on the total mass of the ink. More preferably.

  The aqueous medium can contain other water-soluble organic solvents in addition to urea. As the water-soluble organic solvent, monohydric alcohol, polyhydric alcohol, (poly) alkylene glycol, glycol ether, nitrogen-containing polar solvent, sulfur-containing polar solvent and the like can be used, and one of these can be used alone or Two or more kinds can be used in combination. The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink. This content is a value containing urea.

(Other additives)
In the ink of the present invention, a surfactant, a pH adjuster, a rust inhibitor, an antiseptic, an antifungal agent, an antioxidant, an anti-reduction agent, an evaporation accelerator, a chelating agent, and a water-solubility, as necessary You may contain various additives, such as resin.

<Ink cartridge>
The ink cartridge of the present invention includes ink and an ink storage unit that stores the ink. And the ink accommodated in this ink accommodating part is the ink of this invention demonstrated above. FIG. 1 is a cross-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 portion for storing ink. The ink storage portion is composed of an ink storage chamber 14 and an absorber storage chamber 16, which communicate with each other via 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 unit may have a form in which the entire amount of ink stored is held by an absorber without having an ink storage chamber for storing liquid ink. Further, the ink storage portion may have a form that does not have an absorber and stores the entire amount of ink in a liquid state. Further, the ink cartridge may be configured to have an ink storage portion and a recording head.

<Inkjet recording method>
The ink jet recording method of the present invention is a method of recording an image on a recording medium by discharging the ink of the present invention described above from an ink jet recording head. Examples of the method for ejecting ink include a method for imparting mechanical energy to the ink and a method for imparting thermal energy to the ink. In the present invention, it is particularly preferable to employ a method in which thermal energy is applied to the ink and the ink is ejected. Other than using the ink of the present invention, the steps of the ink jet recording method may be known.

  2A and 2B are diagrams schematically showing an example of an ink jet recording apparatus used in the ink jet recording method of the present invention. 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 ink jet recording apparatus is provided with a conveying means (not shown) for conveying 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 such that an ink cartridge 42 is set. 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, the recording medium 32 is conveyed in the sub-scanning direction by a conveying unit (not shown), whereby an image is recorded on the recording medium 32.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further in detail, this invention is not limited at all by the following Example, unless the summary is exceeded. What is described as “parts” and “%” with respect to the component amounts is based on mass unless otherwise specified.

<Preparation of pigment dispersion>
(Pigment dispersion 1)
A solution obtained by dissolving 5 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5 ° C., and 1.5 g of 4-amino-1,2-benzenedicarboxylic acid (treatment agent) was added. A container containing this solution was placed in an ice bath and cooled to 10 ° C. or lower, and a solution obtained by dissolving 1.8 g of sodium nitrite in 9.0 g of water at 5 ° C. was added. After stirring for 15 minutes, 6.0 g of carbon black (trade name “Black Pearls 880”, manufactured by Cabot) was added with stirring, and stirring was further performed for 15 minutes to obtain a slurry. The obtained slurry was filtered with a filter paper (trade name “filter paper for standard No. 2”, manufactured by Advantech), and the obtained particles were sufficiently washed with water. The washed particles were dried in an oven at 110 ° C. to obtain a self-dispersing pigment. After ion exchange of sodium ions to potassium ions by a conventional method, water was added so that the pigment content was 10.0%. Thus, Pigment Dispersion Liquid 1 was obtained in which a self-dispersing pigment having —C ₆ H ₃ — (COOK) ₂ groups bonded to the surface of carbon black particles was dispersed in water.

(Pigment dispersion 2)
Mix 20.0 g of carbon black, 11.0 mmol of treatment agent, 20.0 mmol of nitric acid, and 200 mL of pure water, and mix for 30 minutes at 6,000 rpm under room temperature using a Silverson mixer. Obtained. As the treating agent, ((4-aminobenzoylamino) -methane-1,1-diyl) bisphosphonic acid sodium salt was used. As the carbon black, a trade name “Black Pearls 880” (manufactured by Cabot) was used. To the resultant mixture, an aqueous solution in which 20.0 mmol of sodium nitrite was dissolved in a small amount of water was slowly added and mixed. By this mixing, the temperature of the mixture reached 60 ° C., and the reaction was performed in this state for 1 hour. Thereafter, the pH of the mixture was adjusted to 10 using an aqueous sodium hydroxide solution. After ion exchange of sodium ions to potassium ions by a conventional method, water was added so that the pigment content was 10.0%. In this way, Pigment Dispersion Liquid 2 was obtained in which a self-dispersed pigment having —C ₆ H ₄ —CONH—CH (PO ₃ K ₂ ) ₂ groups bonded to the surface of carbon black particles was dispersed in water.

(Pigment dispersion 3)
A styrene-acrylic acid copolymer having an acid value of 200 mg KOH / g and a weight average molecular weight of 10,000 was neutralized with a 10% aqueous potassium hydroxide solution. Then, 10.0 parts of carbon black (trade name “Black Pearls 880”, manufactured by Cabot), 20.0 parts of a styrene-acrylic acid copolymer (solid content), and 70.0 parts of water were mixed. This mixture was dispersed using a sand grinder for 1 hour, and then centrifuged to remove coarse particles, followed by pressure filtration with a micro filter (manufactured by Fuji Film) having a pore size of 3.0 μm. By the above method, Pigment Dispersion Liquid 3 was obtained in which the pigment content was 10.0%, the resin content was 20.0%, and carbon black was dispersed in water with the resin.

<Synthesis of urethane resin>
A four-necked flask equipped with a stirrer, a thermometer, a nitrogen gas introduction tube, and a reflux tube was prepared. In this four-necked flask, the polyisocyanate of the kind and amount shown in Table 1, a polyol having no acid group, and a polyol having an acid group were added, and 400 parts of methyl ethyl ketone was further added as a solvent. And it was made to react at 75 degreeC under nitrogen gas atmosphere for 1 hour, and the reaction liquid was obtained. After cooling the obtained reaction liquid to 60 degreeC, potassium hydroxide aqueous solution was added. After further cooling to 40 ° C., ion exchange water was added, and a potassium hydroxide aqueous solution was added while stirring at high speed with a homomixer to obtain a liquid. Thereafter, the types and amounts of polyamines shown in Table 1 were added, and the chain extension reaction was carried out at 30 ° C. for 12 hours. When the presence of isocyanate groups is no longer confirmed by FT-IR, methyl ethyl ketone is distilled off from the liquid obtained by heating and depressurization, and the content of urethane resin (solid content) is 20.0%. Urethane resin 1 A liquid containing ˜4 was obtained respectively. All of urethane resins 1 to 4 had a weight average molecular weight of 30,000 as measured by GPC. The acid value of the urethane resin is shown in Table 1. The acid value of the urethane resin was measured by potentiometric titration using a potassium hydroxide-methanol titrant. For a sample obtained by diluting an aqueous solution of urethane resins 1 to 4 with pure water (volume basis) using a particle size analyzer (trade name “UPA-EX150”, manufactured by Nikkiso) by a dynamic light scattering method, SetZero: 30 The particle size was measured under the conditions of second, number of measurements: 3 times, measurement time: 180 seconds. As a result, the particle diameter of any urethane resin was not measured, and it was confirmed that it was water-soluble. Moreover, the detail of each component in Table 1 is shown below.

IPDI: isophorone diisocyanate HDI: hexamethylene diisocyanate PPG: polypropylene glycol (number average molecular weight: 2,000)
DMPA: dimethylolpropionic acid EDA: ethylenediamine DETA: diethylenetriamine TETA: triethylenetetramine

<Preparation of ink>
Each component (unit:%) shown in the upper part of Table 2 was mixed and sufficiently stirred, and then pressure filtration was performed with a microfilter (manufactured by Fuji Film) having a pore size of 3.0 μm to prepare each ink. Acetylenol E100 (trade name) is an ethylene oxide adduct of acetylene glycol manufactured by Kawaken Fine Chemicals. The lower part of Table 2 shows the urea content C ₁ (%), the urethane resin content C ₂ (%), and the value of C ₁ / C ₂ (times).

<Evaluation>
Each evaluation shown below was performed using the ink obtained above. In the present invention, “AA”, “A”, and “B” are acceptable levels and “C” is not acceptable according to the following evaluation criteria. The evaluation results are shown in Table 3.

(Optical density)
Each ink obtained above was filled in an ink cartridge, and set in an ink jet recording apparatus (trade name “PIXUS iP3100”, manufactured by Canon Inc.) that discharges ink from the recording head by the action of thermal energy. In this embodiment, the recording duty of a solid image recorded under the condition that one ink droplet having a mass per droplet of 28 ng ± 10% is applied to a unit area of 1/600 inch × 1/600 inch is 100. It is defined as%. Using this ink jet recording apparatus, a solid image of 2 cm × 2 cm having a recording duty of 100% was recorded on four types of recording media (plain paper). The four types of recording media include Canon brand name “PB PAPER”, Xerox brand name “4024”, Hured Packard brand name “Bright White”, and International Paper brand name “Hammer Mill Jet”. "Print" was used. One day after recording, the optical density of the solid image was measured using a reflection densitometer (trade name: Macbeth RD-918; manufactured by Macbeth), and the optical density of the image was evaluated according to the following evaluation criteria.
AA: The average optical density of the images of the four types of recording media was 1.45 or more.
A: The average optical density of the images of the four types of recording media was 1.40 or more and less than 1.45.
B: The average optical density of the images of the four types of recording media was 1.30 or more and less than 1.40.
C: The average optical density of the images of the four types of recording media was less than 1.30.

(Marker resistance)
Using an ink jet recording apparatus similar to that used in the above optical density evaluation, a vertical ruled line having a thickness of 1/10 inch was recorded on a recording medium (plain paper, trade name “PB PAPER”, manufactured by Canon). . After 5 minutes and 1 day after recording, the vertical ruled line was marked with a yellow line marker (trade name “OPTEX2”, manufactured by Zebra), and immediately after that, a non-recorded portion of the recording medium was marked. Then, the marking state of the marker nib and the non-recording portion was visually confirmed, and the marker resistance of the image was evaluated according to the following evaluation criteria.
AA: After 5 minutes and 1 day, the pen tip of the marker was not soiled, and the marking on the non-recording area was not soiled.
A: After 5 minutes, the marker nib was soiled, but the markings on the non-recorded area were hardly soiled. After one day, the pen tip was not soiled, and the markings on the non-recording area were not soiled.
B: After 5 minutes and 1 day, the pen tip of the marker was slightly dirty, but the marking on the non-recorded area was hardly dirty.
C: After 5 minutes and 1 day, the marker pen tip was dirty, and the marking on the non-recording area was also dirty.

(Discharge stability)
Each ink obtained above was filled in an ink cartridge, and set in an ink jet recording apparatus (trade name “iB4030”, manufactured by Canon Inc.) that ejects ink from the recording head by the action of thermal energy. In the present embodiment, the recording duty of a solid image recorded under the condition that one ink droplet having a mass per droplet of about 22 ng is applied to a unit area of 1/600 inch × 1/600 inch is 100%. Define. A solid image with a recording duty of 5% was recorded on the entire surface of an A4 size recording medium (plain paper, product name “PB PAPER”, manufactured by Canon) in an environment of a temperature of 30 ° C. and a relative humidity of 10%. Thereafter, a cycle of stopping recording for 240 seconds was repeated. In this cycle, ink is preliminarily ejected into the cap when recording is resumed after a recording pause of 240 seconds. This cycle was repeated until the total number of recorded sheets reached 10,000. Thereafter, vertical ruled lines were recorded on the recording medium. Next, the ink accumulation state and the vertical ruled lines in the cap were visually confirmed, and the ejection stability was evaluated according to the following evaluation criteria.
A: The vertical ruled lines were not disturbed, and no ink was deposited on the cap.
B: The vertical ruled line was not disturbed, and the ink was slightly deposited on the cap, but the ink deposit was not adhered to the ejection port surface.
C: Vertical ruled lines were disordered, ink was deposited on the cap, and ink deposits were attached to the ejection port surface.

Claims (8)

An aqueous inkjet ink containing a pigment, a water-soluble urethane resin, and urea,
The pigment is a self-dispersing pigment in which an anionic group is bonded to the surface of carbon black particles directly or via another atomic group,
The water-based ink, wherein the urethane resin has units derived from polyisocyanate, a polyol having no acid group, and a polyol having an acid group.   The water-based ink according to claim 1, wherein the urethane resin further has a unit derived from polyamine.   The water-based ink according to claim 2, wherein the polyamine is at least one selected from the group consisting of ethylenediamine, diethylenetriamine, and triethylenetetramine.   The water-based ink according to any one of claims 1 to 3, wherein the urethane resin has a structure crosslinked with a polyamine.   The water-based ink according to claim 4, wherein the crosslinked structure in the urethane resin is derived from at least one of diethylenetriamine and triethylenetetramine.   The content (mass%) of the urea is 1.0 to 5.0 times in mass ratio with respect to the content (mass%) of the urethane resin. Water-based ink. An ink cartridge comprising ink and an ink containing portion for containing the ink,
An ink cartridge, wherein the ink is the water-based ink according to any one of claims 1 to 6. An inkjet recording method for recording an image on a recording medium by discharging ink from an inkjet recording head,
An ink jet recording method, wherein the ink is the water-based ink according to any one of claims 1 to 6.

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