JP2020019178A - Inkjet recording method and inkjet recording apparatus - Google Patents

Abstract

To provide an inkjet recording method capable of suppressing a variation of a discharge speed of ink and recording an image having a good black appearance.SOLUTION: There is provided an inkjet recording method for recording an image on a recording medium by heating ink and discharging the heated ink from a recording head by the action of thermal energy from a recording element. A recording element having a smaller width in a direction orthogonal to an ink supply direction than the width in the ink supply direction ios used for the recording element. An aqueous ink containing carbon black, an acrylic resin for dispersing carbon black and a urethane resin, in which the ratio of a weight average molecular weight Mwof the urethane resin to a weight average molecular weight Mwof the acrylic resin is 2.0 times or more and 5.0 times or less, is used for the ink.SELECTED DRAWING: None

Description

  The present invention relates to an inkjet recording method and an inkjet recording device.

  In recent years, it has become possible to record high-definition, high-color images, such as those realized by silver halide photography and offset printing, by an inkjet recording method. In response to a demand for higher image quality, there is a method of improving the resolution of a recording head, that is, increasing the array density of ejection ports (nozzles) for ejecting ink. As the array density of the ejection ports is increased, as a factor affecting the resolution in arranging the ejection ports, a recording element (hereinafter, referred to as a heater and a heater) for applying thermal energy to the ink stored in the foaming chamber is considered as a factor affecting the resolution. ) May be very large. When the arrangement density of the ejection ports is further increased, the size of the heater itself affects the arrangement of the ejection ports, and it becomes difficult to pass wires through the heater. For this reason, a technique has been adopted in which the heater has a shape in which the width in the direction perpendicular to the ink supply direction is smaller than the width in the ink supply direction, thereby leaving a distance between the heaters and facilitating the passage of wiring.

  In addition, fluctuations in the viscosity of the ink directly affect the size of the discharged liquid droplets and the discharging speed of the liquid droplets, and deteriorate the image quality. Therefore, the fluctuations in the viscosity are maintained by keeping the temperature of the ink as constant as possible. It is necessary to control. However, a temperature distribution may be generated in the print head, which may cause a change in ink viscosity. Accordingly, the discharge speed of the droplets varies, causing image degradation. Therefore, a method has been adopted in which a constant temperature is applied to the recording head to keep the temperature of the ink in the foaming chamber constant, thereby suppressing the fluctuation in the viscosity of the ink.

  On the other hand, as a coloring material used in an ink jet recording ink, a pigment ink containing a pigment as a coloring material is widely used for the purpose of further improving the robustness of an image such as gas resistance and light resistance. It has become. In photographic printing and graphic art printing, higher definition images are required. 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, in general, the color developability of an image recorded with a pigment ink is inferior to an image recorded with a dye ink, especially when photographic glossy paper is used. Further, in particular, in a monochrome image recorded with an ink containing carbon black as a pigment, the actual apparent black density (black appearance) may be reduced and the color developability may be impaired.

  The ink jet recording ink has the above-described problems. As a method for stabilizing the ejection property of the ink and suppressing image unevenness to solve such problems, an ink containing a color pigment, two kinds of urethane resins, and zirconium oxide has been proposed (Patent Document 1). ). In addition, an ink containing a coloring material, an acrylic resin, and a urethane resin has been proposed for ejection stability (Patent Document 2).

JP 2017-036432 A JP 2017-095554 A

  The present inventors have studied the suppression of variation in the ejection speed and the appearance of black (coloring properties) of a recorded image using the above-described conventional ink using carbon black as a coloring material. As a result, it was found that the appearance of black in the image was improved, but it was found that, with any of the inks, the appearance of black in the image and the suppression of variation in the ejection speed were not compatible at a high level. During further study, when an image was recorded using an ink containing carbon black dispersed by a resin, image unevenness occurred. When checked in detail, it was found that bubbles remained in the ink flow paths (nozzles) near the discharge ports after the discharge of the ink, and that a new problem that did not occur normally occurred during the next discharge. .

  Therefore, an object of the present invention is to provide an ink jet recording method capable of suppressing variations in the ink ejection speed and recording an image with good black appearance. Another object of the present invention is to provide an ink jet recording apparatus that can be used in the above ink jet recording method.

The above object is achieved by the present invention described below. That is, according to the present invention, while heating the ink, the heated ink is recorded by the action of the thermal energy of the recording element having a smaller width in the direction perpendicular to the ink supply direction than the width in the ink supply direction. An inkjet recording method for recording an image on a recording medium by discharging from a head, wherein the ink is a water-based ink containing carbon black, an acrylic resin for dispersing the carbon black, and a urethane resin, An ink jet recording method is provided, wherein the weight average molecular weight Mw _{U of the} urethane resin is 2.0 times or more and 5.0 times or less as a ratio to the weight average molecular weight Mw _A of the acrylic resin.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to suppress the dispersion | variation of the ink discharge speed, the inkjet recording method which can record the image with favorable black appearance, and the inkjet recording apparatus which can be used for it Can be provided.

FIG. 1 is a perspective view of an inkjet recording apparatus according to an embodiment of the present invention. FIG. 2 is a perspective view illustrating a structure in which a part of a recording head in the inkjet recording apparatus illustrated in FIG. 1 is cut away. FIG. 3 is a cross-sectional view illustrating an internal structure of a main part of the recording head illustrated in FIG. 2 when viewed in a discharge direction.

  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.

<Inkjet recording device>
FIG. 1 is a perspective view of an inkjet recording apparatus IJRA according to an embodiment of the present invention. The inkjet recording apparatus IJRA shown in FIG. 1 includes a recording head IJH that ejects ink by the action of thermal energy. Further, the ink jet recording apparatus IJRA includes a carriage HC, and the carriage HC has an ink cartridge IJC mounted thereon. In the present embodiment, as the ink cartridge IJC, a recording head IJH and an ink tank IT are integrally built and formed. In the present embodiment, the ink cartridge IJC has the recording head IJH and the ink tank IT formed as an integral type. However, the present invention is not limited to this, and each is formed to be separable and assembled at the time of use. It may be of a format that can be used. The carriage HC is supported by the guide rail 5003 so as to be movable in the direction in which the guide rail 5003 extends. The carriage HC performs recording by discharging droplets onto the recording medium while reciprocating in the directions of arrows a and b in FIG. 1 along the guide rail 5003.

  Further, the ink jet recording apparatus IJRA is provided with a mechanism for heating the ink as a means for controlling the temperature of the ink in the foaming chamber. When printing a high-definition image, there is a case where the temperature distribution is generated in the print head IJH and the viscosity of the ink fluctuates. However, the temperature of the ink is controlled by heating the ink by a mechanism for heating the ink. By doing so, it is possible to suppress fluctuations in the viscosity of the ink. As a mechanism for heating the ink, a heater for adjusting the temperature is arranged so as to be in contact with the recording head IJH, a heater for heating is arranged in the vicinity of the ejection port array of the recording head IJH, and the recording described later is performed. Use of a heater as a recording element of the head IJH may be used. The heating of the ink by the heater is performed by applying a voltage that does not cause the ink to be ejected and causing the heater to generate heat. The mechanism for heating the ink may be provided separately from the recording head IJH, or may be a part of the recording head IJH.

  Next, the recording head IJH according to the present embodiment will be described with reference to FIGS. FIG. 2 is a perspective view in which a part of the print head IJH in the ink jet printing apparatus IJRA shown in FIG. 1 is cut away to show a part of the internal structure of the print head IJH. FIG. 3 is a cross-sectional view showing the inside of the recording head IJH of a main part of the recording head IJH shown in FIG. 2 as viewed from a direction in which ink droplets are ejected.

  The recording head IJH includes a bubbling chamber 200 for ejecting ink to the recording head IJH, a recording element 400 for applying thermal energy to the ink stored in the bubbling chamber 200, and a mechanism for heating the ink. ing. In the print head IJH, film boiling can be caused in the ink by the thermal energy generated by the print element 400. The ink is ejected from the ejection port 100 by giving kinetic energy to the ink by bubbles generated by the film boiling. In the recording head IJH, by using such a method, an image to be recorded is made higher in density and higher in definition.

  The recording head IJH is formed by joining the recording element substrate 150 and the flow path forming member 111 to each other. The printing element substrate 150 is provided with a plurality of heaters 400 as printing elements. A plurality of flow paths through which ink flows are formed in the flow path forming member 111, and are stacked and joined to the main surface of the recording element substrate 150.

  As shown in FIG. 2, the flow path forming member 111 has a plurality of nozzles 300 and an ink supply port 500 that supplies ink to each of the nozzles 300. A common liquid chamber 700 is formed between the ink supply port 500 and the nozzle 300. In addition, a bubbling chamber 200, a heater 400 as a recording element, and an ink flow path 600 are provided for each nozzle 300. The opening of the nozzle 300 in the flow path forming member 111 is the discharge port 100. The ejection port 100 is formed at a position corresponding to the heater 400 on the printing element substrate 150.

As shown in FIG. 3, the recording element (heater) 400 in the printhead IJH, rather than the width H _Y in the ink supply direction Y, and is formed in a shape the width H _X in the direction X is less perpendicular to the ink supply direction . By using the recording element 400 having such a configuration, the resolution of the recording head can be increased, and a high-definition image can be recorded.

  However, when an image was recorded by discharging the conventional ink using the inkjet recording apparatus IJRA having the recording head IJH having the above-described configuration, the following problems were found to occur. In other words, a small bubble remains inside the foaming chamber 200 after the ink is ejected, and even when a voltage is applied to the heater at the next ejection, the bubble is disturbed by the small bubble and does not foam properly. An issue has occurred.

  The present inventors have studied in detail the factors that caused the above-mentioned problems, and as a result, have come to think that the factors are the following three points. The first is that the ink is heated. When the ink is heated, the viscosity of the ink decreases, the foaming resistance decreases, and foaming may increase.

  Second, an aqueous ink containing carbon black dispersed in a resin (hereinafter sometimes referred to as “resin dispersed pigment”) is used. Since the dispersed state of the resin-dispersed pigment is stabilized, even if heat energy is applied, the dispersed state is not easily destabilized, so that kogation hardly occurs and the kogage adhered to the heater is small. Further, carbon black, which is an inorganic pigment, has a unique property that heat resistance is higher than that of an organic pigment. For this reason, even when the kogation adheres to the heater even in a small amount, the kogation has high heat resistance so that heat is easily stored, and nucleate boiling is easily caused by the kogation, so that foaming is liable to be specifically increased. In this way, when the ink contains the carbon black dispersed by the resin, it is considered that foaming at the time of ejection is specifically increased.

  Since organic pigments have lower heat resistance than carbon black, kogation is likely to occur even when dispersed with a resin, and kogation adhering to the heater increases. In this case, kogation derived from the organic pigment adheres so as to cover a wide area of the heater, forming a film on the heater, and since the heat resistance of the kogé is low, it is difficult to store heat. Film boiling occurs in the same manner as when not performed. That is, regardless of the presence or absence of kogation, the foaming state does not change much. Therefore, it is considered that the above-described problem in the case of using an ink containing carbon black dispersed by a resin hardly occurs.

  The third is the shape of the recording element (heater) of the recording head. The use of a recording element (heater) having a smaller width in the direction perpendicular to the ink supply direction than the width in the ink supply direction facilitates the expansion of the foam into an elliptical shape. At this time, if the foaming is specifically increased, the bubble is divided and a small bubble is generated at the end of the elliptical bubble in the long side direction at the time of defoaming after ink ejection. It is considered that the small bubbles generated in this manner tend to remain at the end of the foaming chamber 200 in the ink supply direction Y (the side farthest from the ink supply port).

  It is presumed that due to the above three factors, the foaming increased, and the foam separated at the time of defoaming remained in the foaming chamber, and the next foaming was not performed normally, resulting in a variation in ejection.

  The present inventors made an investigation using a technique proposed in Patent Document 1 to prepare an ink using carbon black instead of an organic pigment. As a result, it was found that no problem occurred because the shape of the recording element (heater) used in the study was not smaller in the direction perpendicular to the ink supply direction than in the ink supply direction.

  It has been found that even if the technique proposed in Patent Document 2 is used, it is not possible to suppress variations in ejection. Urethane resin has low heat resistance and is easily burnt, and burnt easily adheres to the heater. However, since the kogation derived from the urethane resin is easily peeled off from the heater, the kogation does not increase to such an extent as to cover the heater, and foaming does not become small. Further, even when the urethane resin was used, it was not possible to record an image with good black appearance. The reason is considered as follows.

  Here, the “black appearance” of a monochrome image will be described. In a monochrome image recorded with an ink containing carbon black as a pigment, it is necessary to improve the apparent black density (black appearance), and an image with a low black appearance is perceived as having impaired color developability. Black appearance is a performance that is hard to appear in indices such as optical density and lightness, but measures are required. For example, in a dark environment where a spotlight is applied to an image posted on a black wall, such as an art museum, an image that should be monochrome appears whitish, and coloring properties are significantly impaired. This is because, in the monochrome image, scattered light due to irregularities on the surface of the pigment layer constituting the image and voids inside the pigment layer is generated as diffused light of the image, and the diffused light is visually recognized as white light. The “diffuse light” is light excluding regular reflection light (light having a reflection angle equal to the incident angle). The light seen in a dark environment as described above is greatly affected by diffused light, so the black appearance is noticeable compared to a bright environment in which the image posted on a white wall is exposed to the light of a fluorescent lamp. descend.

  Since the molecular chain of the urethane resin proposed in Patent Document 2 is short, the urethane resin has permeated into the recording medium without remaining in the voids inside the pigment layer constituting the image. As described above, if the voids inside the pigment layer are not filled, the image looks whitish in a dark environment as described above, and the black appearance is reduced.

  According to the study of the present inventors, carbon black, an acrylic resin, and a urethane resin having a weight average molecular weight Mw of 2.0 to 5.0 times the Mw of the acrylic resin are included in the ink. It has been found that both improvement in black appearance and suppression of ejection variation can be achieved. The present inventors have considered the mechanism by which such an effect is obtained as follows. In the following description, “hydrophilic part of resin” refers to a unit having a hydrophilic group such as an acidic group, and “hydrophobic part of resin” refers to a unit having no hydrophilic group such as an acidic group.

Since carbon black is a highly hydrophobic pigment, the hydrophobic part of the acrylic resin used as a resin dispersant interacts hydrophobically with the particle surface of the carbon black, and the hydrophilic part of the acrylic resin hydrates to form an aqueous solution. Dispersed in the medium. In addition, the weight average molecular weight Mw _U of the urethane resin is less than 5.0 times 2.0 times or more in a ratio (Mw _U / Mw _A) to the weight average molecular weight Mw _A of the acrylic resin, the hydrophobic portion of the urethane resin And the hydrophobic portion of the acrylic resin interact hydrophobically. Due to this hydrophobic interaction, the acrylic resin and the urethane resin contributing to the dispersion stabilization of the pigment are in a pseudo cross-linked state (hereinafter, may be referred to as “pseudo cross-linked”).

  Furthermore, the pseudo-crosslinked urethane resin is pseudo-crosslinked with an acrylic resin that contributes to the dispersion stabilization of another pigment (carbon black) particle, so that a plurality of pigment particles approach each other and the dispersion state is unstable. Become The pigment whose dispersion state has been destabilized easily generates kogation when heat energy for discharge is applied thereto by the heater, so that kogage adhering to the heater increases. In this way, the kogation adhered so as to cover the heater is affected by the low heat resistance of the urethane resin because the acrylic resin and the urethane resin contributing to the dispersion stabilization of the pigment are pseudo-crosslinked. Strongly adhere to the heater. The kogation firmly adhered to the heater contains both an acrylic resin and a urethane resin that contribute to the stabilization of the dispersion of the pigment. For this reason, it is possible to make it difficult for the thermal energy for ejection to be transmitted to the ink due to the low thermal conductivity of the urethane resin, and it is possible to suppress a specific increase in foaming. It is considered that normal foaming is possible due to an increase in kogation adhering to the heater and a decrease in thermal energy transmitted to the ink, so that the residual foam in the foaming chamber is reduced, and the variation in ejection is suppressed.

  Further, it is considered that the acrylic resin and the urethane resin contributing to the dispersion stabilization of the pigment are pseudo-crosslinked in the recording medium when the image is recorded. Therefore, the urethane resin remains near the pigment, the voids formed when the plurality of pigment particles aggregate are filled with the urethane resin, and the generation of diffused light is suppressed, so that the image may appear whitish even in a dark environment. It is estimated that the appearance of black has improved.

When the weight average molecular weight Mw _U of the urethane resin is less than 2.0 times the weight average molecular weight Mw _A of the acrylic resin, the hydrophobic part where the urethane resin and the acrylic resin interact with each other is reduced and the pseudo part is reduced. Crosslinking is less likely to occur. Therefore, in this case, in the pigment dispersed by the acrylic resin, dispersion instability due to the approach of the plurality of pigment particles described above hardly occurs. In the heater, urethane resin not used for pseudo-crosslinking is preferentially kogated due to low heat resistance and adheres to the heater. However, kogation derived from the urethane resin is easily peeled off from the heater, so kogation does not increase. In addition, since the pigment dispersed in the acrylic resin is dispersion-stabilized, it is difficult to burn, and the burn adheres to the heater in a small amount. Further, due to the high heat resistance characteristic of carbon black, nucleate boiling occurs starting from a small amount of kogation adhered to the heater, foaming increases, and small bubbles remain to cause variation in ejection. Further, even in the recording medium on which the image is recorded, since the urethane resin does not pseudo-crosslink with the acrylic resin, the urethane resin permeates the recording medium without remaining in the voids of the pigment layer, so that generation of diffused light is not suppressed. , Black appearance does not improve.

On the other hand, when the weight average molecular weight Mw _{U of} the urethane resin is more than 5.0 times the weight average molecular weight Mw _A of the acrylic resin, the acrylic resin and the urethane resin that have contributed to the dispersion stabilization of the pigment are pseudo-crosslinked. However, pseudo-crosslinking is performed so that the urethane resin surrounds the acrylic resin. Therefore, the acrylic resin that has contributed to the dispersion stabilization of the pigment is close to a state in which the urethane resin is dispersed, and the acrylic resin is not exposed. As a result, the acrylic resin that has contributed to the dispersion stabilization of the pigment particles and the urethane resin that has been pseudo-crosslinked cannot pseudo-crosslink with the acrylic resin that has contributed to the dispersion stabilization of another pigment particle. And the effect of dispersion instability is less likely to occur. Therefore, when thermal energy for ejection is applied by the heater, the dispersion of the pigment does not become unstable, making it difficult to generate kogation, and the kogation does not exert an effect of suppressing the occurrence of large foaming. For this reason, when large foaming occurs, small bubbles generated by the division remain in the foaming chamber, and the next foaming is not performed normally, resulting in variation in ejection. Further, in a recording medium on which an image is recorded, since the urethane resin is pseudo-crosslinked so as to enclose the acrylic resin that has contributed to the dispersion stabilization of some pigment particles, the fluidity of the urethane resin is low. Then, it becomes difficult for the urethane resin to fill the voids in the pigment layer, and the image looks whitish in a dark environment, so that the black appearance is reduced.

<Inkjet recording method>
As described above, in the inkjet recording method of the present invention, the ink is heated, thermal energy is applied to the heated ink, and the ink is ejected from the recording head by the action of the thermal energy to form an image on the recording medium. Record In this method, a recording element that has a smaller width in the direction perpendicular to the ink supply direction than the width in the ink supply direction is used as a recording element that applies thermal energy to the ink to eject the ink. In this method, carbon black, an acrylic resin for dispersing the carbon black, and a weight average molecular weight Mw of 2.0 to 5.0 times the weight average molecular weight Mw _{A of} the acrylic resin are used. An aqueous ink containing a urethane resin having _U is used. The method includes a mechanism for heating the ink described above, and a recording head that discharges the heated ink by the action of thermal energy, the recording head having the foaming chamber and a recording element having a specific shape. The provided ink jet recording apparatus can be suitably used. Hereinafter, the aqueous ink used in the inkjet recording apparatus and the inkjet recording method of the present invention will be described in detail.

<Ink>
The water-based ink is 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. This aqueous ink contains carbon black, an acrylic resin for dispersing the carbon black, and a urethane resin. Hereinafter, components constituting the ink and physical properties of the ink will be described in detail.

(Carbon black)
The ink contains carbon black (which may be simply referred to as “pigment” in this specification) as a pigment used for a coloring material. Examples of the carbon black contained in the ink include furnace black, acetylene black, channel black, thermal black, and lamp black. One or more of these can be used. More specifically, commercially available carbon black can be used, and newly prepared carbon black can also be used.

  The dispersion method of carbon black in the ink is a resin dispersion method using a resin as a dispersant. Resin dispersion methods include resin-dispersed pigments using resin dispersants, microcapsule pigments in which the surface of pigment particles is coated with resin, and resin-bonded pigments in which organic groups containing resin are chemically bonded to the pigment particle surfaces. Is mentioned. Above all, a method in which the resin dispersant is physically adsorbed on the pigment by utilizing the hydrophobic interaction by the hydrophobic part of the resin dispersant is preferable. In the ink, an acrylic resin described below for dispersing carbon black is used as a resin dispersant.

The carbon black content _CP (% by mass) in the ink is preferably 0.20% by mass or more and 10.00% by mass or less based on the total mass of the ink. When the content C _P of the carbon black is less than 0.20 wt%, color development of the image may be decreased. On the other hand, when the content C _P of the carbon black is at 10.00 mass percent, increased viscosity of the ink is excessively, there is a case where the discharge is lowered slightly.

(Acrylic resin)
The acrylic resin is contained in the ink as a resin dispersant for dispersing carbon black. The acrylic resin includes a monomer having an acryloyl group and a monomer having a methacryloyl group such as acrylic acid and methacrylic acid, and a resin (polymer) containing a unit derived from at least one of alkyl acrylate and alkyl methacrylate. Say. “(Meth) acryl” means “acryl” and “methacryl”, and “(meth) acrylate” means “acrylate” and “methacrylate”. The acrylic resin preferably has a hydrophilic unit and a hydrophobic unit as constituent units.

  The hydrophilic unit can be formed, for example, by polymerizing a 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; The hydrophilic unit can include a unit derived from one or more of these monomers.

  Examples of the cation constituting the salt of the acidic monomer include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. Since the resin dispersant preferably has an acid value, the hydrophilic unit preferably contains 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. 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. Among these, monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth) acrylate are preferred. By having an aromatic ring in the hydrophobic unit of the acrylic resin, the interaction with the aromatic ring structure of carbon black becomes stronger, which contributes to further stabilization of dispersion. Further, the acrylic resin strongly interacts with the carbon black by pseudo-crosslinking with the urethane resin, so that the effect of making the pigment easier to approach is increased, and the effect of suppressing the dispersion of the discharge and the effect of improving the appearance of black are enhanced.

The content C _A (% by mass) of the acrylic resin used as the resin dispersant in the ink is preferably 0.10% by mass or more and 10.00% by mass or less based on the total mass of the ink. The content C _A (% by mass) of the acrylic resin is more preferably 0.20% by mass to 5.00% by mass, and more preferably 0.40% by mass to 2.00% by mass. More preferred.

The acrylic resin content C _A (% by mass) based on the total mass of the ink is 0.20 times or more in mass ratio (C _A / C _P ) to the carbon black content C _P (% by mass). It is preferably 1.00 or less. When the mass ratio (C _A / C _P ) is 0.20 or more, the acrylic resin easily and sufficiently disperses the carbon black. Therefore, pseudo-crosslinking between the acrylic resin and the urethane resin that has contributed to stabilization of the dispersion of the carbon black increases, and a large amount of kogation derived from the carbon black adheres to the heater. In addition, in the recording medium, since the pseudo-crosslinking of the acrylic resin and the urethane resin which contributed to the stabilization of the dispersion of the carbon black (pigment) is sufficient, the urethane resin tends to remain near the pigment and easily fill the voids in the pigment layer. , Easy to improve black appearance. On the other hand, when the mass ratio (C _A / C _P ) is 1.00 or less, it contributes to the stabilization of carbon black dispersion, which can occur when the amount of the acrylic resin is large relative to the amount of carbon black. It is possible to suppress the presence of an acrylic resin that is not used. Therefore, it becomes easy to secure pseudo-crosslinking by the acrylic resin and the urethane resin that contributed to the stabilization of the dispersion of the carbon black. Thereby, the dispersion instability of the pigment due to the above-described pseudo-crosslinking is promoted, and a large amount of kogation easily adheres to the heater, so that bubbling is likely to be reduced and variation in ejection is easily suppressed. Further, in the recording medium, the urethane resin interacting with the acrylic resin that has contributed to the dispersion stabilization can easily fill the voids in the pigment layer, so that the appearance of black tends to be improved.

The weight average molecular weight Mw _A of the acrylic resin is preferably 12,000 or less, more preferably 6,000 or more and 12,000 or less. When the weight average molecular weight Mw _A of the acrylic resin is 6,000 or more, the effect of stabilizing the dispersion of the pigment is enhanced, the cohesiveness of the pigment on the recording medium is low, and the appearance of black tends to be improved. When the weight average molecular weight Mw _A of the acrylic resin is 12,000 or less, when the acrylic resin is adsorbed on the pigment and dispersed and recorded, steric repulsion occurs due to the large molecular weight of the acrylic resin in the recording medium. And the voids in the pigment layer are unlikely to be large. For this reason, the voids are easily filled with the urethane resin near the pigment by pseudo-crosslinking, and the appearance of black is easily improved. The weight average molecular weight Mw _A of the acrylic resin can be measured by gel permeation chromatography (GPC) according to JIS Handbook Chemical Analysis K0124. The acid value of the acrylic resin is preferably from 100 mgKOH / g to 300 mgKOH / g. The acid value of the acrylic resin can be determined by potentiometric titration.

Whether or not the acrylic 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 potassium hydroxide equivalent to an acid value is prepared. Next, the prepared liquid is diluted 10-fold (by volume) with pure water to prepare a sample solution. When the particle size of the particles in the sample solution is measured, and the particles having the particle size are not measured, it can be determined that the resin is water-soluble. The measurement conditions at this time can be, for example, as follows.
[Measurement condition]
SetZero: 30 seconds Number of measurements: 3 Measurement time: 180 seconds

  As the particle size distribution measuring device, a dynamic light scattering type particle size analyzer (for example, trade name “UPA-EX150”, manufactured by Nikkiso Co., Ltd.) or the like can be used. Of course, the particle size distribution measuring device and measuring conditions to be used are not limited to the above.

(Urethane resin)
The urethane resin is a resin having a urethane bond, and is usually synthesized by reacting a polyisocyanate with a polyol. Since a general-purpose polyisocyanate has no acid group, a unit derived from the polyisocyanate becomes a hydrophobic portion of the urethane resin. The hydrophobic part of the urethane resin has a hydrophobic interaction with the hydrophobic part of the acrylic resin. For this reason, when the urethane resin is contained in the ink together with the carbon black dispersion-stabilized with the acrylic resin, the acrylic resin and the urethane resin are pseudo-crosslinked, and the urethane resin can be present in the vicinity of the pigment particles. . As a result, the irregularities on the surface of the pigment layer formed by the ink and the voids generated inside can be effectively filled with the urethane resin. It is presumed that the above operation makes it possible to record an image having excellent coloring properties.

  The urethane resin is preferably “water-soluble”. Specifically, whether or not the urethane resin is water-soluble can be determined by the same method as the above-described method of determining whether or not the acrylic resin is water-soluble. The present inventors have studied inks using urethane resin particles instead of the water-soluble urethane resin. As a result, the ink when the water-soluble urethane resin is used as the urethane resin is easier to suppress the variation in the ejection and the black appearance of the image is further improved as compared with the ink when the urethane resin particles are used. It turned out to be easy. Compared to urethane resin in the form of resin particles, water-soluble urethane resin is more likely to form pseudo-crosslinks with the acrylic resin, and is therefore more likely to remain near the carbon black, and to easily fill voids in the pigment layer, resulting in a black appearance. It is presumed that it is easier to improve. Further, the water-soluble urethane resin is more likely to pseudo-crosslink with the acrylic resin than the urethane resin particles. From this, it is presumed that dispersion instability due to the approach of a plurality of pigment particles caused by the pseudo-crosslinking easily occurs, so that kogation is likely to occur and kogage adhering to the heater is more likely to increase. Furthermore, the pseudo-crosslinking produces kogation containing both the acrylic resin and the urethane resin that have contributed to the dispersion stabilization. When the kogation adheres to the heater, thermal energy for ejection is less likely to be transmitted to the ink. Therefore, normal foaming is possible, so that the remaining foam in the foaming chamber is reduced, and variation in ejection is more easily suppressed. It is presumed.

The content C _U (% by mass) of the urethane resin in the ink is preferably from 0.01% by mass to 5.00% by mass based on the total mass of the ink. The content C _U (% by mass) of the urethane resin is more preferably 0.02% by mass or more and 2.00% by mass or less, and further preferably 0.05% by mass or more and 1.00% by mass or less. preferable.

The content C _U (% by mass) of the urethane resin in the ink is 0.05 times or more as a mass ratio (C _U / C _A ) to the content C _A (% by mass) of the acrylic resin in the ink. It is preferably at most 00 times. When the above mass ratio (C _U / C _A ) is 0.05 times or more, the black appearance of the recorded image is more likely to be further improved. This is because pseudo-crosslinking by the acrylic resin and urethane resin contributing to the stabilization of the dispersion of the pigment becomes sufficiently easy to occur, so that a large amount of kogation derived from the pigment adheres to the heater and fills the voids of the pigment layer in the recording medium. It is considered that the urethane resin becomes a sufficient amount. On the other hand, when the mass ratio (C _U / C _A ) is 1.00 or less, it is easy to further suppress the variation in ejection. This can occur when there is too much urethane resin, urethane resin that does not contribute to the pseudo-crosslinking, it is easy to preferentially suppress kogation and adhere to the heater from the low heat resistance, the pseudo-crosslinked resin It is considered that a large amount of kogation is easily attached to the heater.

  The urethane resin can be obtained, for example, by reacting a polyisocyanate with a polyol. Further, a product obtained by further reacting a chain extender may be used. Further, a hybrid resin in which a urethane resin and another resin are combined may be used.

[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. It is preferable to use a polyisocyanate having no acid group. 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 polyol having no acid group, such as a polyether polyol, 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 to be subjected to addition polymerization with alkylene oxide include 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 3-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 1,4-dihydroxyphenylmethane, hydrogenated bisphenol A, dimethylol urea and derivatives thereof; glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol and trimethylol Melamine and its induction , Triols such as polyoxypropylene triol; 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, and the like. (Poly) alkylene glycol; ethylene glycol-propylene glycol copolymer; and the like.

  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. The urethane resin preferably has a unit derived from poly (oxytetramethylene) or a unit derived from poly (oxypropylene). That is, it is preferable to use a polyether-based urethane resin containing a poly (oxytetramethylene) structure or a poly (oxypropylene) structure as the urethane resin. When such a polyether-based urethane resin is used, pseudo-crosslinking with an acrylic resin can be strengthened, thereby suppressing ejection variations and further improving the appearance of black in an image. The poly (oxytetramethylene) structure or the poly (oxypropylene) structure exhibits stronger interaction with the acrylic resin. For this reason, it is considered that pseudo-crosslinking with the acrylic resin can be strengthened to effectively fill the voids formed inside the pigment layer, and the appearance of black can be further improved.

(Polyol having an acid group)
Examples of the polyol having an acid group include a polyol having an acid group such as a carboxy group, a sulfonic acid group, and a phosphonic acid group in the structure. In particular, it is preferable to use a urethane resin synthesized using a polyol having an acid group such as dimethylolpropionic acid or dimethylolbutanoic acid. The acid groups may be in the form of a salt. Examples of the cation constituting the salt include ions such as lithium, sodium, potassium, ammonium, and organic ammonium.

  The ratio (mol%) of the units derived from the polyol having an acid group to the total amount of the units derived from the polyol in the urethane resin is preferably as follows. That is, the content is preferably from 30.0 mol% to 90.0 mol%, more preferably from 50.0 mol% to 90.0 mol%.

(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. Especially, it is preferable to use a polyhydric alcohol as a chain extender. When a urethane resin synthesized using a polyhydric alcohol as a chain extender is used, the color developability of an image can be more effectively improved. Further, among the polyhydric alcohols, neopentyl glycol is particularly preferable.

  It is considered that a urethane resin synthesized using a polyvalent amine or a polyvalent imine as a chain extender is likely to aggregate because it has a cationic portion in its structure. On the other hand, a urethane resin synthesized using a polyhydric alcohol such as neopentyl glycol as a chain extender has no cationic portion in its structure. For this reason, the smoothness of the surface of the pigment layer constituting the image to be recorded is hardly reduced, and the appearance of black can be particularly effectively improved.

[Physical properties of urethane resin]
The weight average molecular weight Mw _U of the urethane resin is preferably 30,000 or less, more preferably 12,000 to 30,000. When the weight average molecular weight Mw _U of the urethane resin is 12,000 or more, it is considered that the urethane resin has many hydrophobic parts which pseudo-crosslink due to hydrophobic interaction with the acrylic resin, and pseudocrosslinking is likely to occur. Therefore, it is considered that a large amount of kogation derived from the pigment, the acrylic resin, and the urethane resin adheres to the heater, lowers the heat energy transmitted to the ink, reduces foaming, and easily suppresses unevenness in ejection. Further, in the recording medium on which the image is recorded, it is considered that the sufficient pseudo-crosslinking facilitates the urethane resin to fill the voids in the pigment layer, thereby improving the black appearance of the image. On the other hand, when the weight average molecular weight Mw _{U of} the urethane resin is 30,000 or less, it is considered that the acrylic resin that has contributed to the dispersion stabilization of the carbon black is unlikely to be pseudo-crosslinked so as to surround the urethane resin. Therefore, it is considered that the flowability of the urethane resin in the pigment layer is secured, the effect of filling the voids of the pigment layer with the urethane resin is easily exerted, and the black appearance of the image is likely to be improved. The acid value of the urethane resin is preferably from 40 mgKOH / g to 100 mgKOH / g.

(Aqueous medium)
The ink used in the inkjet recording method of the present invention contains at least water as an aqueous medium. The ink may contain an aqueous medium that is a mixed solvent of water and 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 10.00% by mass to 95.00% by mass, and more preferably 50.00% by mass to 90.00% by mass, based on the total mass of the ink. Is more preferable.

  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%.

(Other pigments)
The ink may contain other pigments in addition to the carbon black described above. Such pigments are less likely to kog than urethane resins, but even if kogated by a heater, they are decomposed and peeled off by applying thermal energy for the next ejection, and do not affect the effects of the present invention.

(Other resins)
In addition to the acrylic resin and the urethane resin described above, the ink may contain other resins. Such a resin may be used as a dispersant for further stably dispersing a pigment or the like in an aqueous medium, or may be added to the ink for another purpose.

(Other components)
The ink may contain a water-soluble organic compound that is solid at room temperature, such as urea, a urea derivative, trimethylolpropane, and trimethylolethane, if necessary, in addition to the above-described components. The content (% by mass) of such a water-soluble organic compound in the ink is preferably from 0.1% by mass to 20.0% by mass, and more preferably 1.0% by mass, based on the total mass of the ink. More preferably, it is at least 15.0% by mass. In addition, the ink may contain various additives such as a surfactant, a pH adjuster, a rust inhibitor, a preservative, a fungicide, an antioxidant, and a reduction inhibitor, as necessary.

(Viscosity of ink)
The viscosity of the ink may be within a usual range used for the ink jet recording method. The viscosity of the ink at 25 ° C. is preferably 5.0 mPa · s or less. The viscosity of the ink at 25 ° C. is more preferably 3.0 mPa · s or less, and even more preferably 2.5 mPa · s or less. Accordingly, when thermal energy for discharging the ink is applied, the foaming resistance of the ink is reduced and foaming is increased, so that the effect of the present invention can be more easily obtained. The viscosity of the ink at 25 ° C. is preferably 1.0 mPa · s or more. The viscosity of the ink at 25 ° C. can be measured with a general rotary viscometer. For example, it may be measured at 100 rpm using an E-type viscometer (trade name “RE-80L”, manufactured by Toki Sangyo).

  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 acrylic resin>
An acrylic resin was synthesized according to the following procedure. Into a four-necked flask equipped with a stirrer, a reflux cooling device, and a nitrogen gas introducing tube, 100.0 parts of ethylene glycol monobutyl ether was added, and nitrogen gas was introduced into the reaction system, and the temperature was raised to 110 ° C under stirring. Was. Next, a mixture of monomers of the amounts and types shown in Table 1 and a solution of 1.3 parts of t-butyl peroxide (polymerization initiator) in ethylene glycol monobutyl ether were added dropwise to the reaction system over 3 hours. After aging for 2 hours, ethylene glycol monobutyl ether was removed under reduced pressure to obtain a solid water-soluble resin. The obtained water-soluble resin was neutralized and dissolved at 80 ° C. by adding an equivalent of an acid value to potassium hydroxide and an appropriate amount of ion-exchanged water. Thus, a liquid containing an acrylic resin having a resin (solid content) content of 20.0% was obtained. The liquid containing the obtained acrylic resin was used as a sample for measurement, under the same conditions as in the case of the urethane resin described later, it was measured acid value and weight average molecular weight Mw _A of the acrylic resin. Table 1 shows the acid value and the weight average molecular weight Mw _A of the acrylic resin.

The meanings of the abbreviations of the monomers in Table 1 are shown below.
St: styrene αMSt: α-methylstyrene BzMA: benzyl methacrylate LMA: lauryl methacrylate nBA: n-butyl acrylate AA: acrylic acid MAA: methacrylic acid

<Synthesis of urethane resin>
In a four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a condenser tube, 0.007 parts of polyols and polyisocyanates having no acid group, and dibutyltin dilaurate of the kind and amount shown in Table 2 And reacted at 100 ° C. for 5 hours under a nitrogen gas atmosphere. After cooling to 65 ° C. or lower, the kind and amount of polyol having an acid group shown in Table 2 and a chain extender, 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 an aqueous urethane resin solution 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. The solution obtained by dissolving the obtained urethane resin in tetrahydrofuran was used as a sample to be measured for the acid value and the weight average molecular weight Mw _U. 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 Mw _U of the urethane resin was determined as a value in terms of polystyrene by gel permeation chromatography (GPC). 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). 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 2 shows the acid value and the weight average molecular weight Mw _U of the urethane resin.

The meanings of the abbreviations of the monomers in Table 2 are shown below. The numerical value attached to the abbreviation is the number average molecular weight of the polyol.
PTMG: polytetramethylene glycol PPG: polypropylene glycol PCD: polyhexamethylene carbonate diol PBA: polybutylene adipate IPDI: isophorone diisocyanate HDI: hexamethylene diisocyanate TDI: 2,6-toluene diisocyanate DMPA: dimethylolpropionic acid DMBA: dimethylol Butanoic acid NPG: Neopentyl glycol TMP: Trimethylolpropane DETA: Diethylenetriamine

  The particle diameter of a sample obtained by diluting the liquid containing an acrylic resin and the liquid containing a urethane resin obtained above with pure water so that the content of the resin (solid content) becomes 1.0% was measured. For the measurement of the particle diameter, a particle size analyzer (trade name “UPA-EX150”, manufactured by Nikkiso Co., Ltd.) using a dynamic light scattering method was used. The measurement conditions were SetZero: 30 seconds, the number of measurements: 3, and the measurement time: 180 seconds. As a result of measuring the particle diameter of each of the samples, the particle diameter was not measured for any of the resins, and it was confirmed that the resin was water-soluble.

<Preparation of pigment dispersion>
(Pigment dispersion liquids 1 to 13)
Liquids containing pigments and resins of the types and amounts shown in Table 3, ion-exchanged water, and 85.00 parts of 0.3 mm zirconia beads were mixed to obtain a mixture. The obtained mixture was placed in a batch-type vertical sand mill (manufactured by Imex), dispersed for 3 hours while cooling with water, and then centrifuged to remove coarse particles. Further, the mixture was filtered under pressure with a cellulose acetate filter (manufactured by Advantech) having a pore size of 3.0 μm, to prepare Pigment Dispersions 1 to 13 containing the pigment dispersed in a resin dispersant.

(Pigment dispersion liquid 14)
A solution prepared by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of water was cooled to 5 ° C., and 1.5 g of 4-aminophthalic acid was added. The container containing this solution was placed in an ice bath, and stirred to maintain the temperature of the solution at 10 ° C. or lower, while dissolving 1.8 g of sodium nitrite in 9.0 g of 5 ° C. ion-exchanged water. added. After stirring for 15 minutes, 6.0 g of carbon black (specific surface area: 220 m ² / g, DBP oil absorption: 105 mL / 100 g) was added under stirring, and further stirred for 15 minutes to obtain a slurry. The obtained slurry was filtered through a filter paper (trade name “standard filter paper No. 2”, manufactured by Advantech), the particles were sufficiently washed with water, and dried in an oven at 110 ° C. Thus, carbon black -C surface of the particles ₆ H ₃ - (COONa) ₂ groups to give a self-dispersing pigment having attached. The pigment content was adjusted by adding an appropriate amount of water to obtain a pigment dispersion liquid 14 having a pigment content of 10.0%.

<Preparation of ink>
The respective components shown in the middle row of Table 4 (Tables 4-1 to 4-5) were mixed, sufficiently stirred, and filtered under pressure with a cellulose acetate filter (manufactured by Advantech) having a pore size of 0.8 μm to prepare each ink. did. Polyethylene glycol having a number average molecular weight of 1,000 was used. Acetyleneol E100 is a trade name of a surfactant manufactured by Kawaken Fine Chemicals. Proxel GXL (S) is a trade name of an antiseptic manufactured by Arch Chemicals. In the lower part of Table 4, as the properties of the ink, the respective contents of pigment, acrylic resin (resin dispersant), and urethane resin in the ink, C _P (%), C _A (%), and C _U ( %), C _A / C _P and C _U / C _A. Furthermore, the values of the weight average molecular weights Mw _A and Mw _{U of} the acrylic resin and the urethane resin used in the ink, and Mw _U / Mw _A are shown.

<Evaluation>
Evaluation was performed using a modified recording head mounted on an inkjet recording apparatus (trade name “PIXUS PRO-1000”, manufactured by Canon Inc.) as described below. 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. The evaluation results are shown in Table 5 (Table 5-1 and Table 5-2).

[Configuration of recording head]
Bubble chamber temperature control: As a mechanism for heating the ink in the bubbling chamber of the recording head, a sub-heater was provided separately from the recording element (heater) of the recording head so as to surround the discharge port array of the recording head. Using this mechanism, the case where the temperature of the ink inside the foaming chamber was heated to 40 ° C. was indicated as “present”, and the case where the ink was not heated was indicated as “absent”.
Ejection energy: As the energy for ejecting the ink in the bubbling chamber of the recording head, the case where thermal energy by a heater is used is shown as "thermal", and the case where mechanical energy by a piezo element is used is shown as "piezo". In addition, when the mechanical energy by the piezo element is used, foaming does not occur upon ejection of the ink, but the “liquid chamber” is described as “foaming chamber” for convenience.
-Shape of foaming chamber and heater: The foaming chamber had any of the following configurations (shapes). When the heat energy from the heater was used, the shape of the heater was also set to a shape corresponding to the shape of the foaming chamber.
A shape in which the width in the direction perpendicular to the ink supply direction is smaller than the width in the ink supply direction to the foaming chamber;
A shape in which the width in the ink supply direction to the foaming chamber is equal to the width in the direction perpendicular to the ink supply direction; this is indicated as "square".
A shape in which the width in the direction perpendicular to the ink supply direction is larger than the width in the ink supply direction to the foaming chamber;

(Dispersion of discharge speed)
Using the ink and the evaluation conditions shown in Table 5, the driving frequency was set to 21 kHz, and the ink was ejected using the above-described inkjet recording apparatus. During this time, the ejection state of the ink was photographed with a high-speed camera and converted to the ejection speed. The ejection speed of the ink from one ejection port was measured ten times, the standard deviation was calculated, and the variation in the ejection speed was evaluated according to the following evaluation criteria.
A: The standard deviation was less than 0.3.
B: The standard deviation was 0.3 or more and less than 0.7.
C: The standard deviation was 0.7 or more and less than 1.0.

(Black appearance)
Using the ink jet recording apparatus described above and the ink jet recording apparatus shown in Table 5, a recording medium (trade name “Canon Photo Paper / Glossy Gold”, manufactured by Canon Inc.) was used with a recording duty of 10% to 100%. Ten types of solid images changed at 10% intervals were recorded. The obtained solid image was pasted on a black wall surface, and the solid image was visually checked under the following two conditions. The condition 1 is a condition assuming a “bright environment” in which a spotlight is applied to the image from two directions behind the ceiling and the observer. Condition 2 is a condition assuming a “dark environment” in which a spotlight is applied from one direction of the ceiling toward the image. The black appearance of the image was evaluated according to the following evaluation criteria. As described above, a dark environment is easily affected by diffused light, and “black appearance” is more likely to be reduced than in a bright environment. Therefore, an image that does not appear whitish even in condition 2 has a good “black appearance”. I can say.
A: The appearance of the images under the conditions 1 and 2 was the same.
B: The appearance of the image under the conditions 1 and 2 was slightly different.
C: Compared to condition 1, the image under condition 2 looked whitish and black appearance was significantly reduced.

Claims (6)

While heating the ink, the heated ink is ejected from the recording head by the action of the thermal energy of the recording element having a smaller width in the direction perpendicular to the ink supply direction than the width in the ink supply direction, and is discharged onto the recording medium. An inkjet recording method for recording an image,
The ink is an aqueous ink containing carbon black, an acrylic resin for dispersing the carbon black, and a urethane resin,
An ink jet recording method, wherein the urethane resin has a weight average molecular weight Mw _U of 2.0 to 5.0 times the ratio of the acrylic resin to the weight average molecular weight Mw _A. The content C _A (% by mass) of the acrylic resin based on the total mass of the ink is 0.20 times or more and 1.00 times or less in a mass ratio to the content C _P (% by mass) of the carbon black. The inkjet recording method according to claim 1, wherein The content C _U (% by mass) of the urethane resin based on the total mass of the ink is 0.05 to 1.00 times in mass ratio to the content C _A (% by mass) of the acrylic resin. The inkjet recording method according to claim 1, wherein The inkjet recording method according to any one of claims 1 to 3, wherein the acrylic resin has a weight average molecular weight Mw _A of 12,000 or less. The inkjet recording method according to any one of claims 1 to 4, wherein the urethane resin has a weight average molecular weight Mw _U of 30,000 or less. An ink jet recording apparatus comprising: a mechanism for heating ink, and a recording head for discharging the heated ink by the action of thermal energy,
The recording head has a bubbling chamber for discharging the ink, and applies thermal energy to the ink stored in the bubbling chamber, and has a width in a direction perpendicular to the ink supply direction rather than a width in the ink supply direction. With a small recording element,
The ink contains carbon black, an acrylic resin for dispersing the carbon black, and a urethane resin, and the weight average molecular weight Mw _{U of the} urethane resin is a ratio to the weight average molecular weight Mw _A of the acrylic resin. An ink jet recording apparatus, which is an aqueous ink having a density of 2.0 times or more and 5.0 times or less.

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