JP2013059967A - Inkjet image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inkjet image forming method which makes the solid unevenness unnoticeable even if the recording medium is the printing paper that has a coating layer while the aqueous ink used maintaining high dispersion stability.SOLUTION: The inkjet image forming method is characterized in that the aqueous ink used contains self dispersion pigment and water, and pKa of the self dispersion pigment has a particular relationship. In addition, in the inkjet image forming method, the recording medium used is the printing paper that has a specific coating layer, and the content percentages of the elements other than carbon and oxygen, and the content percentage of calcium are specific content percentages measured by X-ray fluorescence analysis.

Description

  The present invention relates to an inkjet image forming method.

  Since the recording method using an ink jet can form an image on a wide range of recording media, its application range is rapidly expanding in recent years. For this reason, various improvements have been made to date for ink for inkjet printers. However, there are many problems to be overcome in order to form a stable image by an ink jet recording method on a printing paper having a coating layer having a low ink absorption rate.

  Patent Document 1 describes a technique of applying an ink having a non-volatile component of 10% by mass or more and less than 50% by mass in a droplet amount of 0.1 pL or more and 0.5 pL or less.

JP 2006-315363 A

  In this way, attempts have been made to evolve and develop the ink jet recording system by various methods. However, in the technique described in Patent Document 1, when the same color is printed on a relatively wide surface of a printing paper having a coating layer (commonly referred to as “solid printing”), a portion having a high ink recording density in a small area There was a case where a low part was formed, and the printed surface looked dirty (betamula was generated). This is because the ink absorption capability of the surface of the printing paper having the coating layer is lower than that of plain paper or inkjet paper.

  In order to solve this problem, it is necessary to improve the coagulation ability of the pigment dispersed in the ink. However, if the coagulation ability is too high, the dispersion stability of the pigment is lowered and the nozzle is clogged. Often occurs.

  Accordingly, it is an object of the present invention to provide an ink jet image forming method that improves the level of unevenness even when printing on printing paper having a coating layer while maintaining dispersion stability as an ink.

The present invention is an inkjet image forming method for forming an image by applying a water-based ink to a recording medium by an ink-jet recording method, wherein the water-based ink contains a self-dispersing pigment and water, PKa having a plurality of pKa values of 8 or less, pKa ₁ having the smallest value among the plurality of pKas being pKa ₁ , and pKa ₂ having the largest value among the plurality of pKas being pKa _{2. 1} and the pKa ₂ satisfy the relationship of the following formula (1), the pH of the water-based ink and the pKa ₂ satisfy the relationship of the following formula (2), and the recording medium is inorganic on the front surface and / or the back surface. A printing paper having a coating layer mainly composed of a material, the content of elements other than carbon and oxygen measured by fluorescent X-ray analysis is 10.0% by mass or more, and calcium An inkjet image forming method, wherein the chromatic rate is 5.0 mass% or more.
Formula (1) 2 ≦ pKa ₂ −pKa ₁
Formula (2) pKa ₂ −1.5 ≦ pH of water-based ink ≦ pKa ₂ +0.5

  The present invention can provide an image forming method that does not stand out even when the recording medium is a printing paper having a coating layer while maintaining high dispersion stability of the water-based ink.

It is a schematic diagram showing the dissociation or the non-dissociation state in the aqueous ink of the hydrophilic group which one kind of hydrophilic group introduce | transduced into the self-dispersion pigment surface has several pKa. It is a schematic diagram showing the state of solid-liquid separation of aqueous ink on a printing paper having a coating layer. It is a schematic diagram showing an example of a method for forming a recording dot by dividing and applying ink to printing paper twice. 1 is a schematic front view showing an outline of a serial type ink jet recording apparatus applicable to an embodiment of the present invention. 1 is a schematic diagram illustrating a configuration of a recording head of a serial type ink jet recording apparatus applicable to an embodiment of the present invention.

  Hereinafter, the present invention will be described in more detail with reference to preferred embodiments.

<Recording medium>
The recording medium used in the present invention is a printing paper having a coating layer mainly used for offset printing, gravure printing and the like.

  The coating layer is a layer of paint applied to the front and / or back of high-quality paper or medium-quality paper, or a layer of paint applied to the surface layer during paper making, in order to enhance the aesthetics and smoothness of the paper surface. It is.

In the present invention, a printing paper having a coating layer is used as a recording medium. According to the Ministry of Economy, Trade and Industry's “Industrial Survey Statistics” and the “Paper and Board Statistics” in the Japan Paper Association “Paper and Board Statistics”, printing papers with coating layers are among “Printing and Information Papers”. It is classified into coated printing paper and fine coated printing paper. Those coated printing paper is intended coating layer is coated with a paint before and after 40g from the front and rear 15g in 1 m ² per both sides to the surface, coated printing paper that is coated with the following coating material 12g in both per 1 m ² It is. Further, the coated printing paper is classified into art paper, coated paper, lightweight coated paper, and others (cast coated paper, embossed paper), etc., depending on the coating amount of the paint and the surface treatment method after coating. Also, depending on the glossiness of the surface, it may be classified into gloss, matte, dull, etc. The printing paper having a coating layer used in the present invention refers to the above-described coating printing paper and fine coating printing paper.

  The paint used for forming the coating layer is mainly composed of an inorganic material and a binder material for binding them. When the inventors examined the element distribution in the cross section of the printing paper having the coating layer, it was found that most of the elements other than carbon and oxygen were localized on the front surface and / or the back surface.

  In general, the main components of the paint applied to the front or back surface of printing paper having a coating layer are mostly kaolin (white clay) and calcium carbonate. Therefore, in order to sufficiently obtain an image that is not conspicuous, which is an effect of the present invention, the entire recording medium is used as a recording medium to be used, which contains elements other than carbon and oxygen, which are measured by XRF (fluorescence X-ray analysis). The rate needs to be 10.0% by mass or more. In addition, it is necessary that the calcium content is 5.0% by mass or more. Paper that does not have a coating layer (for example, non-coated printing paper or PPC paper that is a kind of information paper) has a content of elements other than carbon and oxygen of 10.0% by mass or less, and the entire paper The composition ratio and structure are greatly different, such as being uniformly dispersed. Of course, those satisfying the above-mentioned content ratio in the state of being marketed by a paper manufacturer are included in the printing paper used in the present invention. Also, other printing papers that are pre-processed before printing and modified to satisfy the above-described content are included in the printing paper used in the present invention.

  If the above-mentioned XRF method is used, if the paper has a film thickness of about 100 μm, the amount of various elements contained in the printing paper can be reproducibly simply by fixing the paper (sample) to the sample stage and irradiating X-rays. Can be measured. From the measurement principle of the XRF method, hydrogen, helium, lithium, and superheavy elements higher than uranium cannot be detected. However, helium, lithium and uranium or higher superheavy elements cannot be present in a non-negligible ratio on the printing paper. Therefore, the element ratio obtained by measuring the printing paper by the XRF method can be said to be substantially the ratio of all elements constituting the printing paper excluding hydrogen.

  The mainstream of current printing is offset printing with oil-based ink. For this reason, the coating layer of the printing paper has a structure in which the color material and liquid component contained in the oil-based ink for offset printing, in particular, the hydrophilic liquid component does not easily penetrate inside. Therefore, the physical characteristics of the coating layer are preferably those having an average pore diameter of 0.1 μm or less and a pore volume of 0.3 mL / g or less.

  Note that inkjet glossy paper is classified as information paper in the aforementioned “printing / information paper”. This inkjet glossy paper is obtained by coating an ink receiving layer on the surface of a base paper. For this reason, it is similar to the recording medium used in the present invention in that it has a coating layer.

  However, the main component of the coating material forming the ink receiving layer of glossy paper for inkjet is fine particles of alumina hydrate or xerogel (for example, porous silica). Since the average pore diameter of the ink receiving layer is 0.004 to 0.04 μm, it is the same value as that of the printing paper having the coating layer. However, since the pore volume of the ink receiving layer is 0.4 to 0.6 mL / g, it greatly exceeds the pore volume of the printing paper having the coating layer, which is clearly the recording medium used in the present invention. Different.

Specific examples of the printing paper having a coating layer include paper having the following trade names.
Art paper includes OK Ultra Aqua Satin, OK Kinto, SA Kinto, Satin Kinto (Oji Paper); Hyper Pyrene, Silver Dia (Nihon Paper); Green Utrillo (Daiou Paper); Examples include pearl coat, New V mat (Mitsubishi Paper), Raiktori Super Art (Made in Chuetsu Pulp), Hi McKinley (Gojo Paper), and the like.

  As coated paper, OK top coat, OK top coat dull, OK top coat mat, OK trinity, OK Casablanca (above, made by Oji Paper); Aurora coat, silver diamond, Shirai mat (above, made by Nippon Paper Industries); Green Utrillo (above, made by Daio Paper); pearl coat, new V mat (above, made by Mitsubishi Paper Industries), and the like.

  Lightweight coated papers include OK Coat L (above, Oji Paper); Aurora L, Easter DX, Pegasus (above, Nippon Paper Industries); Utrillo Coat L (above, Daio Paper); Pearl Coat L (above, Mitsubishi) Super Emine (manufactured by Chuetsu Pulp); Dream Coat (manufactured by Marusumi Paper Co., Ltd.) and the like.

  Others (cast coated paper etc.) include mirror coated platinum, OK chrome (above made by Oji Paper); espri coat (above made by Nippon Paper Industries); Picasso coat (above made by Daio Paper).

  Examples of the fine coated printing paper include OK Everlite, OK Crystal, OK Planas White (Oji Paper); Pyrene DX, Aurora S (Nippon Paper).

  A printing paper having these coating layers is a recording medium having a content of elements other than carbon and oxygen of 10.0% by mass or more as measured by fluorescent X-ray analysis.

<Water-based ink>
(Self-dispersing pigment)
The aqueous ink used in the present invention contains a self-dispersing pigment and water. The self-dispersing pigment has a plurality of pKas (acid dissociation constants) having a value of 8 or less, the pKa having the smallest value among the plurality of pKas is pKa ₁ , and the most among the plurality of pKas Let pKa ₂ having a large value be pKa ₂ . At this time, the pKa ₁ and the pKa ₂ satisfy the relationship of the following formula (1). Further, the pH and pKa ₂ of the water-based ink used in the present invention satisfy the relationship of the following formula (2).
Formula (1) 2 ≦ pKa ₂ −pKa ₁
Formula (2) pKa ₂ −1.5 ≦ ink pH ≦ pKa ₂ +0.5

  Self-dispersing pigments basically do not require a dispersant, but can introduce a hydrophilic group directly or indirectly to the pigment surface via other atomic groups and stably disperse it in an aqueous medium. A pigment that can be produced. The hydrophilic group introduced into the pigment surface may be directly bonded to the pigment surface, or may be indirectly bonded to the pigment surface by interposing another atomic group between the pigment surface and the hydrophilic group. Good.

  In the hydrophilic group bonded directly or indirectly through another atomic group to the surface of the self-dispersing pigment, a proton as a dissociating group in the hydrophilic group is dissociated and anionized under a specific pH. As a result, the self-dispersing pigment can be stably dispersed in the ink without using a dispersant such as a resin or a surfactant.

The self-dispersing pigment contained in the aqueous ink used in the present invention has a plurality of pKa values of 8 or less. Then, pKa ₁ the least value is less pKa among the plurality of pKa, when the highest value is greater pKa among the plurality of pKa is defined as pKa _2, the difference of the pKa ₁ and the pKa ₂ 2 This is the self-dispersing pigment. By adjusting the pH of the aqueous ink containing such a self-dispersed pigment from pKa ₂ -1.5 to pKa ₂ +0.5, an aqueous ink having good pigment dispersion stability can be obtained. Furthermore, it can be set as the water-based ink which can form the image which is not conspicuous when printing on the printing paper which has a coating layer with the content rate of elements other than carbon and oxygen mentioned above, and the content rate of calcium.

  As described above, the pKa of the self-dispersing pigment is in the above relationship, the content of elements other than carbon and oxygen, and the content of calcium measured by the fluorescent X-ray analysis method are as described above. The reason for the content rate is considered as follows. As described above, in the printing paper having the coating layer, the color material and the hydrophilic liquid component do not easily penetrate inside. Therefore, the pigment moves along with the flow of the liquid when the ink component is absorbed and evaporated on the surface of the coating layer, and is fixed in a locally uneven state. . Therefore, in order to prevent the occurrence of bethamra, it is necessary to increase the aggregation and precipitation rate of the pigment.

When the self-dispersing pigment has a plurality of pKa, there are the following two modes. One is an embodiment in which the self-dispersing pigment has one type of hydrophilic group, and the one type of hydrophilic group has a plurality of pKa. The other is an embodiment in which the self-dispersing pigment has two or more types of hydrophilic groups, and each hydrophilic group has pKa. As described above, examples of the hydrophilic group used in an embodiment in which one kind of hydrophilic group has a plurality of pKa include the following. For example, —PO ₃ (M) ₂ , —Ph (COOM) _n , wherein Ph is a phenyl group, and M is independently selected from a hydrogen atom, an alkali metal, ammonium, and organic ammonium. 1 represents 1 type, and n represents 2 or 3. Specific examples include phosphonic acid, phthalic acid, and 1,2,3-benzenetricarboxylic acid. The pKa of phosphonic acid is 7.20 and 2.15. The pKa of phthalic acid is 5.41 and 2.95. The pKa of 1,2,3-benzenetricarboxylic acid is 5.87, 4.20, and 2.80.

  When the above hydrophilic group is introduced on the pigment surface, there is a difference between the pKa of the pigment alone and the pKa of the pigment introduced with the hydrophilic group. Therefore, it is necessary to actually measure the pKa of the pigment alone. The pKa can be estimated from the pKa of the introduced hydrophilic group. Among those expressed as “M” in the hydrophilic group, specific examples of the alkali metal include Li, Na, K, Rb and Cs. Specific examples of organic ammonium include methylammonium, dimethylammonium, trimethylammonium, ethylammonium, diethylammonium, triethylammonium; monohydroxymethyl (ethyl) amine, dihydroxymethyl (ethyl) amine, and trihydroxymethyl (ethyl) amine. Etc.

  Moreover, the following combinations are mentioned as a combination of the hydrophilic group used in the aspect which has two or more types of hydrophilic groups as mentioned above, and each hydrophilic group has pKa. For example, combinations of methanesulfonic acid and acetic acid, methanesulfonic acid and benzoic acid, benzenesulfonic acid and benzoic acid, and benzenesulfonic acid and acetic acid can be mentioned. The pKa of methanesulfonic acid is -1.2, the pKa of acetic acid is 4.76, the pKa of benzenesulfonic acid is -2.5, and the pKa of benzoic acid is 4.2. Like the above combinations, a combination of hydrophilic groups in which the pKa of each hydrophilic group is 2 or more is preferable.

FIG. 1 is a schematic diagram showing a dissociation or non-dissociation state in a water-based ink of a hydrophilic group having one kind of hydrophilic group introduced into the surface of a self-dispersing pigment and having a plurality of pKa. FIG. 1 shows a state where the hydrophilic group X has pKa ₁ and the hydrophilic group Y has pKa ₂ . It is considered that the hydrophilic group introduced into the self-dispersing pigment in the aqueous ink exists in any one of the states 1 to 3 shown in FIG. 1 depending on the pH of the aqueous ink.

  State 1 in FIG. 1 is a state in which both the hydrophilic group X and the hydrophilic group Y are not dissociated (protonated), and the pigment cannot be stably dispersed in the aqueous ink and aggregates. End up. State 2 is a case where the hydrophilic group X is dissociated (anionized) while the hydrophilic group Y is protonated, and has the following characteristics. First, since the hydrophilic group X is dissociated, the pigment can be stably dispersed in the aqueous ink. When this ink lands on the coating layer surface of the printing paper, the anionized hydrophilic group X binds to calcium ions eluted from the coating layer surface into the aqueous ink liquid, and the pigment in the ink rapidly Aggregates and precipitates. This calcium ion is derived from calcium carbonate contained in a large amount in the coating layer of the printing paper having the coating layer. When the water-based ink droplets land and come into contact with the coating layer, it is considered that a small amount of calcium ions is released from the coating layer into the ink droplets. Calcium carbonate is sparingly soluble in water, but if the content ratio of calcium by the XRF method is 5.0% by mass or more, an amount sufficient to cause aggregation and precipitation of the pigment when the water-based ink lands. Calcium ions can be released.

After the water-based ink has landed on the surface of the coating layer where the water absorption rate of the liquid component of the water-based ink is slow, the ink liquid component may remain on the surface of the coating layer for several seconds to several tens of seconds. However, even in this case, the pigment is fixed on the spot without largely moving from the landing position due to the aggregation and precipitation of the pigment due to the binding of the hydrophilic group X and the calcium ion, and as a result, a high-quality recorded image with less betham is obtained. . This state is schematically shown in FIG. 2A. Here, the ink 1 lands on the coating layer 2 containing calcium carbonate, and after several hundred msec, the liquid component 4 of the ink spreads on the surface of the coating layer 2, but the pigment aggregate 3 does not move much from the landing position. It is formed. For this reason, after the liquid component is evaporated or absorbed, an image is formed as the solid print portion 5 having good solid uniformity as it is. When the present inventors observed, the aggregate 3 is mutually independent. The density is 10 or more per 10 μm ² , and when viewed with an optical microscope, the individual black spots appear to be independent, but the color material appears to spread evenly with the naked eye and does not cause deterioration in image quality.

  On the other hand, in the state 3 where both the hydrophilic group X and the hydrophilic group Y are dissociated (anionized), the pigment can be dispersed very stably in the aqueous ink. However, since the dispersion stability of the pigment is high, the aggregation and precipitation rate of the pigment is inferior to the state 2 even when calcium ions are eluted into the ink droplet when the ink lands on the recording medium. Therefore, the pigment moves with the flow of the liquid when the liquid component of the ink is absorbed and evaporated on the surface of the coating layer, and is fixed in a locally uneven state. This state is schematically shown in FIG. Here, the ink 1 ′ is landed on the coating layer 2 ′, and after several hundred msec, the water-based ink 6 is evaporated and concentrated to some extent and spreads on the surface of the coating layer 2 ′ including the pigment before aggregation and precipitation. At this time, the water-based ink 6 generates a liquid flow from the center toward the periphery due to the evaporation of water from the surface, and the pigment rides on the flow and diffuses to the peripheral portion. As a result, after the liquid component is evaporated or absorbed, the solid print portion 5 ′ has a small amount of pigment near the center and a large amount in the peripheral portion (generally referred to as “coffee stain phenomenon”), resulting in an image with poor solid uniformity. It is formed.

As described above, the present inventors have found that the presence of the self-dispersing pigment having the hydrophilic group in the state 2 in FIG. 1 at a certain ratio makes it possible to achieve both dispersion stability and high recording density. I found it. Specifically, the pH of the water-based ink used in the present invention is adjusted so as to satisfy the relationship of the following formula (2).
Formula (2) pKa ₂ −1.5 ≦ pH of water-based ink ≦ pKa ₂ +0.5

When the pH of the aqueous ink is lower than pKa ₂ -1.5, the pigment in the aqueous ink is dominated by the pigment having the hydrophilic group in the state 1 shown in FIG. Sedimentation may occur. On the other hand, when the pH of the aqueous ink exceeds pKa ₂ +0.5, the pigment having the hydrophilic group in the state 3 shown in FIG. 1 is dominant in the pigment in the aqueous ink, and the image density tends to decrease. Further, in order to ensure dispersion stability when the pH of the aqueous ink is adjusted to pKa ₂ -1.5, the pH of the aqueous ink needs to be sufficiently higher than pKa ₁ . Therefore, it is necessary to satisfy the relationship of 2 ≦ pKa ₂ −pKa ₁ . Further, in order to achieve both the dispersion stability of the pigment and the recording density of the recorded image, the pH of the aqueous ink is preferably neutral or acidic. Therefore, pKa ₂ in the present invention needs to be 8 or less.

  The hydrophilic group and the pigment may be directly bonded or may be indirectly interposed with another atomic group. Specific examples of the other atomic groups indirectly interposed include a linear or branched alkylene group having 1 to 12 carbon atoms, a substituted or unsubstituted phenylene group, and a substituted or unsubstituted naphthylene group. . As a substituent of a phenylene group and a naphthylene group, a C1-C6 linear or branched alkyl group is mentioned, for example.

  The average particle size of the self-dispersing pigment used in the present invention is an average particle size determined by a dynamic light scattering method in an aqueous ink, preferably 60 nm or more, more preferably 70 nm or more, and even more preferably 75 nm. That's it. Moreover, it is preferably 145 nm or less, more preferably 140 nm or less, and further preferably 130 nm or less. As a specific method for measuring the average particle diameter, FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd., cumulant method analysis) or Nanotrac UPA 150EX (manufactured by Nikkiso Co., Ltd., 50% integrated value) using laser light scattering is used. ) Etc.

  The amount of the self-dispersing pigment added to the aqueous ink is preferably 0.5% by mass or more, more preferably 1% by mass or more, and further preferably 2% by mass or more with respect to the total mass of the ink. Further, it is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 8% by mass or less.

  The method for adjusting the pH of the water-based ink is not particularly limited. For adjustment to the acidic side, for example, organic carboxylic acids such as acetic acid, propionic acid, phthalic acid, and benzoic acid; methanesulfonic acid, ethanesulfonic acid And organic sulfonic acids such as benzenesulfonic acid; inorganic acids such as phosphoric acid, sulfuric acid, hydrochloric acid, and nitric acid, or mixtures thereof can be used. When adjusting to the alkaline side, for example, hydroxides of alkali metals such as Li, Na, K, Rb and Cs; ammonia, methylammonium, dimethylammonium, trimethylammonium, ethylammonium, diethylammonium, triethylammonium, monohydroxymethyl Organic ammonium such as (ethyl) ammonium, dihydroxymethyl (ethyl) ammonium, trihydroxymethyl (ethyl) ammonium, triethanolammonium, or a mixture thereof can be used.

(Organic acids and / or inorganic acids and their salts)
The aqueous ink used in the present invention preferably contains an organic acid, an inorganic acid, or a salt thereof having a pKa lower than the pH of the aqueous ink. Thereby, the solid uniformity of the recorded image obtained can be improved. The reason is considered as follows.

  Inks containing organic acids, inorganic acids, or salts thereof (hereinafter referred to as organic acids) are self-dispersing pigments and organic acids synergistically after the ink is driven into printing paper having a coating layer. And solid-liquid separation occurs remarkably between the self-dispersing pigment and the aqueous medium. As a result, the pigment is quickly fixed on the surface of the coating layer on the spot, and the solid uniformity is increased. The pKa of the organic acid and inorganic acid to be added is preferably lower than the pH of the water-based ink.

  Examples of the inorganic acid include hydrochloric acid, sulfuric acid, nitric acid and the like. When the organic acid or the like is a salt, alkali metal, ammonium, organic ammonium, or the like can be used as the counter ion to be a salt, as in the case of the counter ion of the self-dispersing pigment. Specific examples of the alkali metal as the counter ion include Li, Na, K, Rb and Cs. Specific examples of organic ammonium include methylammonium, dimethylammonium, trimethylammonium, ethylammonium, diethylammonium, triethylammonium, monohydroxymethyl (ethyl) ammonium, dihydroxymethyl (ethyl) ammonium, trihydroxymethyl (ethyl) ammonium. Triethanolammonium and the like can be used. Of these, ammonium is particularly preferable.

  The pKa of the organic acid and inorganic acid constituting the salt is preferably lower than the pH of the aqueous ink. Examples of organic acids include citric acid, succinic acid, benzoic acid, acetic acid, propionic acid, phthalic acid, oxalic acid, tartaric acid, gluconic acid, tartronic acid, maleic acid, malonic acid, adipic acid, and other organic carboxylic acids. Can be mentioned. Of these, acetic acid, phthalic acid and benzoic acid are preferred.

  The amount of the organic acid or the like added to the aqueous ink is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and further preferably 0.2% by mass or more with respect to the total mass of the ink. . Moreover, Preferably it is 3 mass% or less, More preferably, it is 2 mass% or less, More preferably, it is 1 mass% or less.

  The water-based ink used in the present invention contains water. The content of water in the ink is preferably 30% by mass or more with respect to the total mass of the ink. Moreover, it is preferable that it is 95 mass% or less.

(Aqueous medium)
The water-based ink used in the present invention preferably further contains a water-soluble compound having a hydrophilicity / hydrophobicity coefficient defined by the following formula (3) of 0.26 or more. Further, depending on the paper type, it is preferable to have both a water-soluble compound having a hydrophilicity / hydrophobicity coefficient defined by the following formula (3) of 0.26 or more and less than 0.37 and a water-soluble compound of 0.37 or more. In this case, it may become a more preferable aspect when it is set as the form which contains 2 or more types of water-soluble compounds whose hydrophilicity / hydrophobicity coefficient is 0.37 or more.

上記式（３）中の水分活性値とは、
水分活性値＝（水溶液の水蒸気圧）／（純水の水蒸気圧）
で示されるものである。水分活性値の測定方法は、様々な方法があり、いずれの方法にも特定されないが、中でもチルドミラー露点測定法は、本発明で使用する材料測定に好適である。

The water activity value in the above formula (3) is
Water activity value = (water vapor pressure of aqueous solution) / (water vapor pressure of pure water)
It is shown by. There are various methods for measuring the water activity value, and none of the methods is specified, but the chilled mirror dew point measurement method is particularly suitable for the material measurement used in the present invention.

  The water activity value in this specification is determined by measuring a 20% aqueous solution of each water-soluble compound at 25 ° C. using a water activity measuring device (trade name “AQUARB CX-3TE”, manufactured by DECAGON) using a chilled mirror dew point measurement method. It is what. According to Raoul's law, the vapor pressure drop rate of dilute solutions is equal to the solute mole fraction and is independent of the solvent and solute type, so the water mole fraction and water activity value in the aqueous solution are equal. . However, when the water activity value of aqueous solutions of various water-soluble compounds is measured, the water activity value often does not coincide with the molar fraction of water. When the water activity value of the aqueous solution is lower than the molar fraction of water, the water vapor pressure of the aqueous solution is smaller than the theoretical calculation value, and the evaporation of water is suppressed by the presence of the solute. From this, it is considered that the solute is a substance having a high hydration power. Conversely, when the water activity value of the aqueous solution is higher than the molar fraction of water, the solute is considered to be a substance having a low hydration power.

  The present inventors have found that the degree of hydrophilicity or hydrophobicity of a water-soluble compound contained in an ink has a great influence on the promotion of solid-liquid separation between a self-dispersing pigment and an aqueous medium and on various ink performances. I focused on. From this, the “hydrophilicity coefficient” shown in the above formula (3) was defined. The water activity value is measured for various aqueous solutions at a uniform concentration of 20% by mass of the water-soluble compound, but by converting the above formula (3), the molecular weight of the solute is different and the molar fraction of water is Even if they are different, relative comparison of the degree of hydrophilicity or hydrophobicity of various solutes is possible. Moreover, since the water activity value of the aqueous solution does not exceed 1, the maximum value of the hydrophilicity / hydrophobicity coefficient is 1. Table 1 shows the hydrophilicity / hydrophobicity coefficient of the water-soluble compound obtained by the above formula (3). However, the water-soluble compound containing the water-based ink used in the present invention is not limited to these.

  As a result of examining the relationship between water-soluble compounds and various ink performances when water-soluble compounds having different hydrophilicity / hydrophobicity coefficients are contained in the ink, the present inventors have obtained the following knowledge. In the case of the ink containing the self-dispersing pigment used in the present invention, the lower-case printing characteristics such as bleeding between two colors and the thickening of characters are obtained by using a water-soluble compound having a hydrophilicity coefficient of 0.26 or more and a small hydrophilic tendency. When used, it was very good. Among them, a class of glycol structure having a carbon number not substituted with a hydrophilic group beyond the number of carbon atoms substituted with a hydrophilic group in the glycol structure was particularly preferable. These water-soluble compounds have a relatively low affinity with water, self-dispersing pigments, cellulose fibers, etc. after the ink has landed on the printing paper, and have a role in strongly promoting solid-liquid separation of the self-dispersing pigments. Conceivable. Of these, trimethylolpropane is particularly preferable as the water-soluble compound having a hydrophilicity / hydrophobicity coefficient defined by the above formula (3) of 0.26 or more and less than 0.37. Furthermore, as a water-soluble compound having 0.37 or more, those having a hydrocarbon glycol structure having 4 to 7 carbon atoms are preferable, and among them, 1,2-hexanediol and 1,6-hexanediol are particularly preferable. And when using 2 or more types of water-soluble compounds with a hydrophilicity-hydrophobicity coefficient of 0.37 or more, it is preferable that each hydrophilicity-hydrophobicity coefficient has a difference of 0.1 or more.

(Surfactant)
The total content of the water-soluble compounds in the ink is preferably 5% by mass or more, more preferably 6% by mass or more, and still more preferably 7% by mass or more with respect to the total mass of the ink. Further, it is preferably 40% by mass or less, more preferably 35% by mass or less, and further preferably 30% by mass or less.

  The aqueous ink used in the present invention preferably contains a surfactant in the aqueous ink in order to obtain a more balanced ejection stability. Among these, it is preferable to contain a nonionic surfactant. Among the nonionic surfactants, polyoxyethylene alkyl ethers and ethylene oxide adducts of acetylene glycol are particularly preferable. These nonionic surfactants have an HLB value (Hydrophile-Lipophile Balance) of 10 or more. The content of the surfactant used in this manner is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass or more with respect to the total mass of the ink. Moreover, Preferably it is 5 mass% or less, More preferably, it is 4 mass% or less, More preferably, it is 3 mass% or less.

(Other additives)
In addition to the above-described components, the water-based ink used in the present invention may have, as necessary, a viscosity modifier, an antifoaming agent, an antiseptic, and an antifungal agent, in order to obtain an ink having a desired physical property value. Additives, antioxidants, penetrants and the like can be added.

(surface tension)
The surface tension of the water-based ink used in the present invention is preferably 20 mN / m or more and 60 mN / m or less, and more preferably 25 mN / m or more and 50 mN / m or less. When the surface tension exceeds 60 mN / m, the penetration of the ink into the coating layer becomes extremely slow, and a short time after landing even if the pigment in the aqueous ink used in the present invention is affected by the calcium ions in the coating layer. Insufficient solid-liquid separation. As a result, the pigment collects in the droplets (or around the solid print portion) and generates a sticky image, which is inappropriate.

(viscosity)
The viscosity of the water-based ink used in the present invention is not particularly limited as long as it is a value suitable for the ink jet recording apparatus described below.
For example, when using an apparatus for ink jet recording by using thermal energy, it is preferably 6.0 mPa · s or less. If the viscosity is higher than this, ink may not be supplied to the nozzles in time, and an unclear image may be recorded. The viscosity of the ink is more preferably 5.0 mPa · s or less, and still more preferably 4.0 mPa · s or less.

<Image forming method>
The present invention is an inkjet image forming method in which ink is applied to a recording medium by an inkjet recording method. The total amount of ink applied to the recording medium is preferably 3.0 μL / cm ² or less. If the amount is larger than this, the ink applied to the coating layer will not be able to penetrate and will overflow, and even if the pigment in the aqueous ink used in the present invention is affected by the calcium ions in the coating layer, it will be sufficient in a short time after landing. In some cases, solid-liquid separation may not be possible. As a result, the pigment collects around the droplets (or around the solid print portion), resulting in the occurrence of solids.

The application amount of the water-based ink may be a fixed amount or a variable amount.
The fixed amount of ink means ink ejected without changing the structure of the nozzles constituting the recording head between the nozzles and changing the drive energy to be applied. That is, in such a state, even if there is a slight variation in ejection due to manufacturing errors of the apparatus, the amount of ink applied is quantitative. By determining the amount of ink to be applied, the penetration depth of the ink into the recording medium is stabilized, the image density of the recorded image is high, and the image uniformity is improved. In particular, in a high-duty portion of a recorded image, since it is difficult for variations to occur, the image uniformity is improved.

  On the other hand, the variable amount of ink means ink ejected by changing the drive energy applied to each nozzle and changing the amount to the intended amount for each nozzle. In such a state, the image density of the recorded image can be changed by controlling the ejection amount. For this reason, when it is desired to reduce the image density of the recorded image, it can be dealt with by reducing the ink discharge amount, and it is not necessary to reduce the duty, so that it is difficult to feel the graininess.

<Inkjet recording apparatus>
The ink jet recording method of the present invention can eject ink onto a recording medium using an ink jet recording apparatus. The ink jet recording apparatus will be described below.
As a preferable ink jet recording apparatus used in the present invention, a recording head for applying ink to a recording medium is mounted. The recording head of the ink jet recording apparatus used in the present invention may be anything as long as it makes the ink protrude in some manner. As a discharge method, a method in which pressure is applied to ink by deformation of a pump or a piezoelectric element provided in a flow path, a method in which thermal energy is applied to the ink to generate bubbles, a method in which ink is charged and electrostatic suction is performed An electrostatic attraction method using force can be considered. Any system is suitable as the system used in the present invention. In addition, as a method for printing ink on a recording medium, a continuous method in which ink is always discharged and unnecessary ink is collected before landing on the recording medium, and ink is discharged only when it is desired to land on the recording medium. Divided into on-demand methods. As the method used in the present invention, either method is suitable.

  FIG. 4 is a schematic front view showing an outline of a serial type ink jet recording apparatus applicable to the embodiment of the present invention. A plurality of ink jet recording type recording heads 211 to 215 are mounted on the carriage 20. The recording heads 211 to 215 have a plurality of ink ejection openings for ejecting ink. In one aspect of the recording head configured to apply black ink in two parts in one pass, 211, 212, 213, 214, and 215 are black (K), cyan (C), magenta (M), yellow ( Y) and black (K).

The ink cartridges 221 to 225 are composed of recording heads 211 to 215 and ink tanks for supplying ink to these.
The density sensor 40 is a reflection type density sensor, and is configured to detect the density of the test pattern of the ink recorded on the recording medium while being installed on the side surface of the carriage 20.

Control signals and the like to the recording heads 211 to 215 are transferred via a flexible cable (not shown).
A recording medium 24 such as printing paper is sandwiched between paper discharge rollers 25 via a conveyance roller (not shown), and is conveyed in the direction of the arrow (sub-scanning direction) shown in FIG. 4 as the conveyance motor 26 is driven.
The carriage 20 is guided and supported by the guide shaft 27 and the linear encoder 28. The carriage 20 is reciprocated in the main scanning direction along the guide shaft 27 via the drive belt 29 by driving the carriage motor 30.
Inside the ink discharge ports (liquid passages) of the recording heads 211 to 215, a heating element (electric / thermal energy converter) that generates thermal energy for ink discharge is provided. In accordance with the reading timing of the linear encoder 28, the heat generating element is driven based on the recording signal, and ink droplets are ejected and adhered onto the recording medium to form an image.

  A recovery unit (not shown) having cap portions 311 to 315 is installed at the home position of the carriage 20 arranged outside the recording area. When recording is not performed, the carriage 20 is moved to the home position, and the ink discharge port surfaces of the recording heads 211 to 215 are sealed with caps 311 to 315 respectively corresponding thereto. As a result, it is possible to prevent clogging due to ink adhering to the recording head or adhesion of foreign matters such as dust due to evaporation of the ink solvent. Further, the capping function of the cap part is used to eliminate ejection defects and clogging of ink ejection ports with low recording frequency. Specifically, the cap part is used for idle ejection for preventing ejection failure that causes ink to be ejected to the cap part in a state separated from the ink ejection port. Further, the cap portion is used for recovering the discharge of the discharge port that has caused a discharge failure by sucking ink from the ink discharge port by a pump (not shown) in the capped state.

  The ink receiving portion (not shown) serves to receive ink droplets that are preliminarily ejected when the recording heads 211 to 215 pass through the upper part immediately before the recording operation. In addition, by disposing a blade and a wiping member (not shown) at a position adjacent to the cap portion, it is possible to clean the ink discharge port forming surface of the recording heads 211 to 215.

  As described above, it is preferable to add ink recovery means, preliminary means, and the like for the recording head to the configuration of the recording apparatus because the recording operation can be further stabilized. Specific examples of these means include a capping means for the recording head, a cleaning means, a pressurizing or suction means, an electrothermal converter, a heating element different from this, or a preheating means using a combination thereof. Etc. It is also effective for performing stable recording that the recording head has a preliminary ejection mode in which ejection is performed separately from recording.

  Furthermore, a cartridge type recording head in which an ink tank is provided integrally with the recording head itself described in the above embodiment may be used. In addition, a replaceable chip-type recording head that can be electrically connected to the apparatus main body and supplied with ink from the apparatus main body by attaching the recording head to the apparatus main body may be used.

  FIG. 5 is a schematic diagram showing a configuration of a recording head of a serial type inkjet recording apparatus applicable to the embodiment of the present invention. In FIG. 5, the recording scanning direction of the recording heads 211 to 215 is the direction indicated by the arrows in the figure. Each of the recording heads 211 to 215 is provided with ejection openings for a plurality of nozzles arranged in a direction substantially orthogonal to the recording scanning direction. The recording head ejects ink droplets from each ejection port at a predetermined timing while moving and scanning in the recording scanning direction in the figure. Thus, an image is formed on the recording medium with a recording resolution corresponding to the nozzle arrangement density. At this time, the recording head may perform the recording operation in any direction of the recording scanning direction. Further, the recording operation may be performed either in a reciprocal manner.

  Further, the above embodiment is a serial type recording apparatus that performs recording by scanning the recording head, but as another embodiment, a full line using a recording head having a length corresponding to the width of the recording medium. It may be a type of recording device. As a full-line type recording head, there is a configuration in which serial type recording heads are arranged in a staggered manner or in parallel so as to be elongated to have a desired length. Alternatively, a configuration may be adopted in which one recording head is integrally formed so as to have a nozzle row that is elongated from the beginning.

  The serial type or line type recording apparatus uses four-color inks (Y, M, C, and K) that are independent or integrally formed. Furthermore, in this embodiment, a head having a five-ejecting port array (or nozzle array) is provided for each of the black ink 211 nozzle and the black ink 215 nozzle in order to apply only the black ink in two. Further, as a preferable aspect when the number of times of division application is set to about 2 to 12 using the four discharge port arrays (or nozzle arrays), at least one of the four color inks (Y, M, C, and K) is A type in which the same color ink is mounted on a plurality of ejection port arrays (or nozzle arrays) is also preferable. For example, an 8 discharge port array (or nozzle array) configuration in which 2 to 3 heads of 4 discharge port arrays (or nozzle arrays) are connected in an overlapping manner, a 12 discharge port array (or nozzle array) configuration, and the like are also included.

The ink jet recording apparatus used in the present invention forms an image having a portion in which a total application amount of ink is 3.0 μL / cm ² or less in a basic matrix for forming an image with 80% duty or more. In addition, it is preferable to apply the ink in two or more times. In addition, it is preferable that the amount of each ink applied in the divided application of ink is 0.7 μL / cm ² or less. The ink jet recording apparatus used in the present invention preferably has a control mechanism for performing the divisional application of the ink. By this control mechanism, it is possible to control the operation of the recording head and the timing of the paper feeding operation of the printing paper, and to perform the divisional application of the ink.

  The number of times of division application when applying ink can be set according to desired recording conditions. FIG. 3 is a schematic diagram illustrating an example of a method of forming recording dots by dividing ink into printing paper twice. In the present embodiment, the resolution of the basic matrix is 1200 dpi (horizontal) × 1200 dpi (vertical), and a 100% duty portion of the image is formed. In FIG. 3, the landing position of the first ink is shown as the first ink, and the landing position of the second ink is shown as the second ink. Each of the first ink and the second ink is quantitative.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated concretely, this invention is not limited to the following Example, unless the summary is exceeded. In the following description, “parts” and “%” are based on mass unless otherwise specified. The surface tension of the water-based ink was measured using a fully automatic surface tension meter (trade name “CBVP-Z”, manufactured by Kyowa Interface Science). The average particle size of the self-dispersing pigment was measured using a dynamic light diffusion particle size / particle size distribution measuring device (trade name “Nanotrack UPA-150EX”, manufactured by Nikkiso).

<Self-dispersing pigment A>
As the self-dispersing pigment, self-dispersing pigment A contained in a commercially available self-dispersing pigment dispersion (trade name: CAB-O-JET400, manufactured by Cabot) was used. The CAB-O-JET400 containing this self-dispersing pigment A was subjected to neutralization titration using 798MPT Titrino (manufactured by Metrohm), and pKa measurement was performed. Specifically, potassium hydroxide was added to CAB-O-JET400 containing self-dispersing pigment A to adjust the pH to 10, and then neutralization titration was performed using 0.1 M hydrochloric acid. As a result, the acid dissociation constant (pKa) indicated pH 2.5 (pKa ₁ ) and 6.1 (pKa ₂ ).

  Next, methods for preparing water-based inks of Examples and Comparative Examples of the present invention will be described. Water used was ion exchange water.

<Preparation of water-based ink 1>
A total of 100 parts of the following components were mixed for 2 hours, and then filtered using a filter having a pore size of 2.5 μm to obtain an aqueous ink 1. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm. The pH of the water-based ink was adjusted to 5.6 with acetic acid.
-Self-dispersing pigment A: 5 parts-Trimethylolpropane (hydrophobicity coefficient 0.31): 15 parts-1,2-hexanediol (hydrophobicity coefficient 0.97): 5 parts-Isopropyl alcohol: 1 part- Ethylene oxide adduct of acetylene glycol (trade name: Olphine E1010, manufactured by Nissin Chemical Industry, HLB value of 10 or more): 1 part, water: remainder

<Preparation of water-based ink 2>
A water-based ink 2 was obtained in the same manner as the water-based ink 1 except that the pH of the water-based ink 1 was adjusted to 6.6 with acetic acid. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm.

<Preparation of water-based ink 3>
Aqueous ink 3 was obtained in the same manner as aqueous ink 1 except that the pH of aqueous ink 1 was adjusted to 5.6 with hydrochloric acid. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm.

<Preparation of water-based ink 4>
A total of 100 parts of the following components were mixed for 2 hours, followed by filtration using a filter having a pore size of 2.5 μm to obtain water-based ink 4. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm. The pH of the water-based ink was adjusted to 5.6 with hydrochloric acid.
-Self-dispersing pigment A: 5 parts-Ammonium benzoate: 0.7 parts-Trimethylolpropane (hydrophobicity coefficient 0.31): 15 parts-1,2-hexanediol (hydrophobicity coefficient 0.97): 5 parts-Isopropyl alcohol: 1 part-Ethylene oxide adduct of acetylene glycol (trade name: Orphine E1010, manufactured by Nissin Chemical Industry, HLB value of 10 or more): 1 part-Water: remaining part

<Preparation of water-based ink 5>
A water-based ink 5 was obtained in the same manner as the water-based ink 4 except that the salt added to the water-based ink 4 was changed from 0.7 part of ammonium benzoate to 0.36 part of sodium sulfate. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm. The pH of the water-based ink 5 was adjusted to 5.6 with hydrochloric acid.

<Preparation of water-based ink 6>
A water-based ink 6 was obtained by the same treatment as that of the water-based ink 1 except that no pH adjusting agent was added. The pH of the aqueous ink was 8.2, the surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm.

<Preparation of water-based ink 7>
A water-based ink 7 was obtained in the same manner as the water-based ink 1 except that the pH of the water-based ink 1 was adjusted to 4.5 with acetic acid. The surface tension was 30.0 mN / m, and the particle size of the self-dispersing pigment was 120 nm.

<Examples 1-5, Comparative Examples 1 and 2>
(Evaluation of dispersion stability)
Using the water-based inks 1 to 7, the dispersion stability of the pigment in the water-based ink was evaluated under the following conditions. As an evaluation method, the ink was stirred and then allowed to stand, and the state of the aqueous ink after 30 minutes was visually determined according to the following criteria. Table 3-1 shows some of the ink prescriptions and the determination results.
A: Aggregation and sedimentation of the pigment was not observed.
X: Aggregation and sedimentation of the pigment was observed.
In addition, since the pigment of the water-based ink 7 aggregated and settled as shown in Table 3-1, it was difficult to perform printing with an ink jet printer. Therefore, regarding the water-based ink 7, the evaluation result of Table 3-2 described later is “−”.

(Evaluation of Betamra)
Using water-based inks 1 to 6, a recorded image was formed under the conditions shown below, and the printed matter was evaluated. As an evaluation method, a solid print portion of 2 cm × 2 cm was formed on the following recording medium by the following inkjet recording apparatus and image forming method, and the solid portion was visually determined according to the following criteria. The determination results are shown in Table 3-2.
(Double-circle): Betamra was not seen at all and the favorable image was formed.
◯: A slight image of bethamra was observed, but a good image was formed.
(Triangle | delta): Although an image is slightly worrisome, the image which does not have a problem practically was formed.
X: Betamla occurred and an image with low image quality was formed.

Inkjet recording apparatus: F930 (manufactured by Canon, recording head: 6 ejection port arrays, 512 nozzles each, ink application amount 4.0 pL (fixed), resolution maximum 1200 dpi (horizontal) × 1200 dpi (vertical))
Image forming method (applied once): Each type of ink was mounted on the black ink recording head portion of the printer, and printing was performed without dividing the solid image. The ink application amount per time was 1.0 μL / cm ² .
Recording medium: Six brands in Table 2 which are printing paper having a coating layer were used. What was stored in a general environment with a room temperature of 20 to 25 ° C. and a humidity of 30 to 60% was loaded and printed on an ink jet recording apparatus as it was without pretreatment.

“Contents other than C and O” in Table 2 indicate the contents of elements other than carbon and oxygen with respect to elements other than superheavy elements such as hydrogen, helium, lithium, and uranium contained in the printing paper. Furthermore, “Ca content” indicates the content of calcium. Both of these contents were calculated from values obtained by measurement using a fluorescent X-ray analyzer (XRF) and ZSX100e (manufactured by Rigaku).
“Pore diameter” was the average pore diameter of the coating layer portion of the printing paper, and “pore volume” was the cumulative pore volume, both of which were calculated from the values obtained by the mercury intrusion analysis.

  In Example 4, when the vicinity of the solid print portion boundary printed on the recording medium 3 is magnified with a microscope, the individual pigment aggregates are scattered independently on the surface of the coating layer. Could not be identified, and the entire surface was uniform and solid.

Regarding the dispersion stability of the inks, Examples 1 to 5 and Comparative Example 1 in which the pH of the aqueous ink was adjusted to 5.5 or 6.5, which is pKa ₂ −1.5 or more, are obtained in the aqueous ink. It was found that the dispersion stability was good without aggregation. On the other hand, the water-based ink 7 of Comparative Example 2 in which the pH of the water-based ink was adjusted to 4.5, which is pKa ₂ −1.5 or less, may have poor dispersion stability due to the aggregation and precipitation of pigments observed in the water-based ink. all right. This is presumably because the pigment having the hydrophilic group in the state 1 shown in FIG. 1 became dominant because the pH of the water-based ink was pKa ₂ −1.5 or less.

Regarding the bethamra, it was confirmed that the bethamra of the recorded image was greatly improved by adjusting the pH of the water-based ink to pKa ₂ −1.5 or more and pKa ₂ +0.5 or less. By adjusting the pH of the water-based ink to pKa ₂ −1.5 or more and pKa ₂ +0.5 or less, the pigments having the hydrophilic groups in the states 2 and 3 shown in FIG. As a result, as shown in FIG. 2 (a), when the water-based ink coating layer lands, the aggregation of the pigment proceeds rapidly using calcium ions contained in the coating layer of the printing paper as a trigger, and the pigment is applied at the landing position. This is thought to be due to fixing on the surface of the work layer.

  When examined for each recording medium, the recording medium 1 (fine coated paper) tended to have high solid uniformity in almost any ink from Examples 1 to 5. This is because fine coated paper has a small coating amount and the coating layer thickness is as thin as 1 to 5 μm, so that cellulose fibers are partially exposed on the surface of the coating layer, and the water absorption capability of the ink liquid component is high. This is thought to be because the time that ink droplets remain on the surface of the coating layer is relatively short. On the other hand, with the recording medium 6 (cast coated paper), although there was no practical problem, a slight difference in image quality was observed depending on the ink. This is because cast-coated paper has a coating layer thickness of 20 μm and the surface is smoothed, so the water absorption capacity of the ink component is low and the ink droplets remain on the coating layer surface. Is considered to be relatively long.

In addition, as can be seen from the comparison between Examples 1 and 2, there was a difference in image quality between the recording media 2 (lightweight coated paper) and 6 (cast coated paper) when the aqueous ink had a lower pH. The pH of the aqueous ink used in Example 1 is adjusted to pKa ₂ −0.5 (5.6 in this example), and the ratio of the pigments in states 2 and 3 shown in FIG. 1 In contrast, the pH of the ink of Example 2 is adjusted to pKa ₂ +0.5 (6.6 in this example), and the ratio of states 2 and 3 shown in FIG. 1 is about 1: 4. . In the pigment having a hydrophilic group in the state 2 shown in FIG. 1, the aggregation of the pigment proceeds rapidly upon landing of the coating layer of the aqueous ink, and the pigment is uniformly fixed on the coating layer surface. On the other hand, in the pigment in the state 3, the aggregation of the pigment is delayed, and the pigment moves along with the flow of the liquid when the liquid component of the ink is absorbed and evaporated on the surface of the coating layer, so that a slight amount of stickiness is generated. In the aqueous ink of Example 2, the ratio of the pigment having the hydrophilic group in the state 3 was high, and hence it is considered that the wet ink was more likely to occur than the aqueous ink of Example 1.

  Further, as can be seen from a comparison between Examples 1 and 4 and 5, recording media 1 (finely coated paper) and 2 (lightweight coated paper) were obtained by adding an inorganic or organic salt to water-based ink even at the same pH. There was a difference in image quality between recording media other than). This is presumably because the acid promotes the solid-liquid separation of the self-dispersing pigment, and it becomes more difficult to generate betalam. Note that the presence of salt in the recording medium 1 (fine coated paper) did not appear as a significant difference in image quality because the time for ink droplets to remain on the surface of the coating layer was short as described above. it is conceivable that.

DESCRIPTION OF SYMBOLS 1, 1 ': Water-based ink 2, 2': Coating layer 3: Aggregate mass 4: Liquid component 5 of ink, 5 ': Solid printing part 6: Water-based ink 20: Carriage 24: Recording medium 25: Paper discharge roller 26 : Transport motor 27: Guide shaft 28: Linear encoder 29: Drive belt 30: Carriage motor 40: Density sensor 211: Black ink (K)
212: Cyan ink (C)
213: Magenta ink (M)
214: Yellow ink (Y)
215: Black ink (K)
221 to 225: ink cartridges 311 to 315: cap

Claims (5)

An inkjet image forming method for forming an image by applying an aqueous ink to a recording medium by an inkjet recording method,
The water-based ink contains a self-dispersing pigment and water,
The self-dispersing pigment has a plurality of pKa values of 8 or less, the pKa having the smallest value among the plurality of pKas is pKa ₁ , and the pKa having the largest value among the plurality of pKas is pKa _2. The pKa ₁ and the pKa ₂ satisfy the relationship of the following formula (1), and the pH of the water-based ink and the pKa ₂ satisfy the relationship of the following formula (2):
The recording medium is a printing paper having a coating layer mainly composed of an inorganic material on the front surface and / or the back surface,
The content of elements other than carbon and oxygen in the recording medium measured by X-ray fluorescence analysis is 10.0% by mass or more, and the content of calcium is 5.0% by mass or more. An inkjet image forming method.
Formula (1) 2 ≦ pKa ₂ −pKa ₁
Formula (2) pKa ₂ −1.5 ≦ pH of water-based ink ≦ pKa ₂ +0.5   The inkjet image forming method according to claim 1, wherein the water-based ink further contains an organic acid, an inorganic acid, or a salt thereof having a pKa lower than the pH of the water-based ink. The inkjet image forming method according to claim 1, wherein the water-based ink further contains a water-soluble compound having a hydrophilicity / hydrophobicity coefficient defined by the following formula (3) of 0.26 or more.

The inkjet image forming method according to any one of claims 1 to 3, wherein the total amount of the water-based ink applied to the basic matrix for forming the image is 3.0 μL / cm ² or less.   The average pore diameter of the said coating layer is 0.1 micrometer or less, and the pore volume of the said coating layer is 0.3 mL / g or less, The inkjet image formation of any one of Claims 1-4 Method.

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