JP2014025008A - White ink composition, ink set for multi-layer formation, image-forming method and printed matter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a white ink composition giving good concealability to a formed image and allowing formation of an image excellent in film strength and blocking resistance.SOLUTION: A white ink composition contains (a) a polymeric compound comprising a repeating unit (a-1) having a partial structure represented by the general formula (1) and a repeating unit (a-2) having a hydrophilic group, (b) 10 mass% or more of a white pigment having a secondary average particle size of 200 nm to 500 nm relative to the total amount of the ink composition, and (c) water. In the general formula (1), Rand Rrepresent hydrogen atoms or alkyl groups having a carbon number of 1 to 4, at least one of Rand Rrepresents an alkyl group having a carbon number of 1 to 4. Rand Rmay be bound to each other to form a 4 to 6-membered alicyclic structure. * represents a binding site with a main chain or a side chain in the polymeric compound.

Description

  The present invention is formed by a white ink composition applicable to inkjet recording, a multi-layer forming ink set having the white ink composition, an image forming method using the white ink composition, and a white ink composition Related to prints.

As an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic method, a sublimation type and a melt type thermal transfer method, an ink jet method and the like.
The inkjet method is an inexpensive device and has the advantage that the ink can be used efficiently and the running cost is low because the ink is ejected only on the required image portion and the image is formed directly on the recording medium. Furthermore, there is little noise and it is excellent as an image recording method.

Among the ink compositions used for image recording by the inkjet method, the active energy ray-curable aqueous ink composition is used for image printing, pretreatment for imparting printability to the recording medium, and protection of the printed image. -It can be used suitably for decoration post-processing, etc., and since it has water as the main component, it has excellent safety, such as being able to be applied to high-density inkjet recording due to its low viscosity, It is a technology with potential.
In recent years, an ink image formed on a resin recording medium by an ink composition has been increasingly used for applications such as large-area advertisements and outdoor advertisements.
The greatest advantage of the ink jet method is that it is suitable for producing a wide print. Inkjet drawing methods include a single pass method and a shuttle scan method. The former is a system in which printing is performed by preparing an ink-jet head having a paper width and moving a recording medium underneath. The merit of this method is that high-speed printing is possible, but the disadvantage is that an ink-jet head needs to be arranged in the width when drawing a wide width, and the apparatus cost is high. On the other hand, the shuttle scan method is a method of drawing while moving a narrow inkjet head perpendicularly to the paper traveling direction. Although not suitable for applications that require high-speed drawing such as the single-pass method, this is an effective drawing method because the apparatus cost can be reduced when drawing on a wide substrate. Such a shuttle scan drawing method is mainly used in the sign and display fields.

Particularly in the case of outdoor advertising, high durability is required for images. In addition, when a large area print or a large number of prints is formed continuously, a process for removing stains on the printing apparatus with an organic solvent is required, but the organic solvent used for cleaning is scattered and formed. If the ink adheres to the ink image, the image with low solvent resistance dissolves and removes the solvent adhering and can cause image failure such as white spots, so the image has both high water resistance and solvent resistance. There is a need for an ink composition that can form.
For use in signage and display applications, it has become necessary to be able to draw on various substrates. Examples of the plastic substrate include polyvinyl chloride (PVC), polyethylene terephthalate (PET), various polyesters, and polyolefins such as polyethylene and polypropylene. One effective printing method for sign and display applications is a printed material using a transparent substrate. However, the printed material drawn on the transparent substrate is light-transmitting itself, and it is difficult to obtain a sufficient image density.
Therefore, in order to further improve the sharpness of an image formed on a transparent substrate or a light-colored substrate, a white ink composition is applied in advance to a region to which the colored ink composition is applied on the recording medium. It is preferable to perform so-called white pressing, and such a white ink composition is required to have high concealability.

  Examples of basic constituent materials of the active energy ray-curable water-based ink composition include water, a polymerizable substance, a polymerization initiator that generates radicals by active energy rays and starts polymerization, and a coloring material (pigment or dye). be able to. Among these, a polymerizable substance and a polymerization initiator may be prepared in an emulsion state, or may be present in a solution state given water solubility by an appropriate substituent.

Examples of the active energy ray-curable aqueous ink composition in which the polymerizable substance and the polymerization initiator are water-soluble include, for example, the ink composition described in Patent Document 1, and according to the ink composition, It is said that an ink composition for ink jet recording can be obtained in which a film having excellent adhesion and the like can be obtained by light irradiation.
Patent Documents 2 and 3 describe ink compositions containing an active energy ray polymerizable substance having a specific maleimide structure.

JP 2005-307199 A JP 2007-119449 A Japanese Patent Laid-Open No. 2000-160086

However, in the technique described in Patent Document 1 using a water-soluble polymerization initiator, the degradation of the added polymerization initiator and the unreacted residue remain in the cured film, which adversely affects the film properties and the printed matter. Therefore, there is still room for improvement in the water resistance of the printed image.
Further, in Patent Document 2, no study has been made regarding the blocking resistance of the formed image. In Patent Document 3, it is a solvent-based ink composition, which is applied to or formed in water-based ink. There has been no study on the blocking resistance of the obtained images.
Usually, white pigments used in white ink compositions often have high specific gravity such as titanium oxide, and it has been difficult to prepare a white ink composition having both concealability and film strength. Furthermore, by selecting the polymer compound or solvent contained in the ink composition, the formed image will have a certain degree of solvent resistance, but the blocking resistance of the image applied to a large area such as white press. It is difficult to improve the current situation.

The present invention has been made in light of the above circumstances, and includes a white ink composition capable of forming an image excellent in concealability and blocking resistance, and the white ink composition and a colorant having a hue other than white. It is an object of the present invention to provide an ink set for forming a multilayer having a color ink composition.
Another object of the present invention is to provide an image recording method for forming an image having excellent concealability and blocking resistance, and a printed matter having an image having excellent concealability and blocking resistance. .

Specific means for solving the above problems are as follows.
<1> (a) a polymer compound comprising a repeating unit (a-1) having a partial structure represented by the following general formula (1) and a repeating unit (a-2) having a hydrophilic group; b) A white ink composition containing a secondary pigment having a secondary average particle diameter of 200 nm to 500 nm in an amount of 10% by mass or more based on the total amount of the ink composition, and (c) water.

(In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure, and * represents a bonding site with a main chain or a side chain in the polymer compound.
<2> (b) The white ink composition according to <1>, wherein the white pigment is titanium oxide.
<3> The content ratio of (a) polymer compound and (b) white pigment [(a) polymer compound / (b) white pigment] is 1/4 to 1/1 on a mass basis, <1> Or the white ink composition as described in <2>.
<4> (b) The white ink composition according to any one of <1> to <3>, wherein the content of the white pigment with respect to the total amount of the ink composition is 25% by mass to 40% by mass.
<5> The white ink composition according to any one of <1> to <4>, wherein the viscosity at 25 ° C. is 15 mPa · s to 30 mPa · s.
<6> The white ink composition according to any one of <1> to <5>, wherein (a) the polymer compound further includes a repeating unit (a-3) having a hydrophobic group.
<7> Any one of <1> to <6>, wherein the repeating unit (a-1) having a partial structure represented by the general formula (1) is a repeating unit represented by the following general formula (2). Item 2. The white ink composition according to item.

(In General Formula (2), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may combine with each other to form a 4- to 6-membered alicyclic structure, R ^c represents a hydrogen atom or a methyl group, Z represents a single bond, —COO— * * Or -CONR ^d -**, R ^d represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and ** represents a bonding position with X. X represents a divalent organic group. )
<8> The repeating unit (a-2) having a hydrophilic group is a repeating unit having at least one hydrophilic group selected from alcoholic hydroxyl groups, alkyl-substituted carbamoyl groups, carboxyl groups, sulfo groups, and salts thereof. The white ink composition according to any one of <1> to <7>.
<9> The white as described in any one of <1> to <8>, wherein the repeating unit (a-3) having a hydrophobic group is a repeating unit derived from an alkyl (meth) acrylate having 4 to 22 carbon atoms. Ink composition.

<10> In any one of <1> to <9>, the repeating unit (a-2) having a hydrophilic group is a repeating unit having at least one hydrophilic group selected from a carboxyl group and a salt thereof. The white ink composition described.
<11> (a) The white color according to any one of <1> to <10>, wherein the solubility parameter of the polymer compound in an unneutralized state is 20.7 MPa ^{1/2 to} 23.0 MPa ^1/2 Ink composition.
<12> (a) The white color according to any one of <1> to <10>, wherein the solubility parameter of the polymer compound in an unneutralized state is 21.5 MPa ^{1/2 to} 22.5 MPa ^1/2 Ink composition.
<13> The white ink composition according to any one of <1> to <12>, wherein the content of the organic solvent having a boiling point of 250 ° C. or less is 1% by mass or less.
<14> The white ink composition according to any one of <1> to <13>, which is for inkjet recording.
<15> (a) a polymer compound comprising a repeating unit (a-1) having a partial structure represented by the following general formula (1) and a repeating unit (a-2) having a hydrophilic group; b) A white pigment having a secondary average particle diameter of 200 nm to 500 nm, 10% by mass or more based on the total amount of the ink composition, and (c) a white ink composition containing water and a colorant having a hue other than white. And an ink composition for forming a multilayer.

(In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure, and * represents a bonding site with a main chain or a side chain in the polymer compound.
<16> An ink application step of applying the white ink composition according to any one of <1> to <14> onto a recording medium, and an active energy ray to the white ink composition applied onto the recording medium. An image forming method including an irradiation step of irradiating.
<17> The ink application step of applying the white ink composition according to any one of <1> to <14> onto a recording medium, and the ink composition applied onto the recording medium (c) An image forming method comprising: an ink drying step of drying and removing at least a part of water; and an irradiation step of irradiating the ink composition applied and dried on the recording medium with active energy rays.
<18> The white ink composition according to any one of <1> to <14> is formed on the recording medium, or the image forming method according to <16> or <17>. Printed image with an image.

According to the present invention, a white ink composition capable of forming an image having good concealability of the formed image and excellent in film strength and blocking resistance, and the white ink composition and a color other than white Provided is an ink set for forming a multilayer having a color ink composition containing a hue colorant.
In addition, according to the present invention, an image recording method for forming an image excellent in concealing property, solvent resistance and blocking resistance, and a printed matter having an image excellent in concealing property, solvent resistance and blocking resistance. Is provided.

1 is an external perspective view showing an example of an ink jet recording apparatus suitably used in the present invention. FIG. 2 is a perspective plan view schematically showing a paper conveyance path of the ink jet recording apparatus shown in FIG. 1. It is a perspective view which shows the structural example of the light source moving part which moves the ultraviolet irradiation part shown in FIG. It is explanatory drawing which shows the structural example of the inkjet head and ultraviolet irradiation part for forming a laminated image.

Hereinafter, the white ink composition, the ink set for forming a multilayer, the image forming method, and the printed matter of the present invention will be described.
In addition, in this specification, when mentioning the amount of each component in the composition, when there are a plurality of substances corresponding to each component in the composition, the present in the composition unless otherwise specified. It means the total amount of multiple substances.
In the ink composition of the present invention, the solid content means the total mass of all components excluding the solvent among the components contained in the ink composition at 25 ° C. The solid content in the present specification includes liquid components such as low molecular weight components other than the solvent.
In the present specification, a numerical range indicated using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In this specification, the term “process” is not limited to an independent process, and is included in the term if the intended action of the process is achieved even when it cannot be clearly distinguished from other processes.

  In the present specification, the value of the solubility parameter may be referred to as “SP value”. The SP value referred to in the present invention is a solubility parameter calculated by the Okitsu method (Adhesion 38, No. 6, page 6 (1994) Polymer publication society), and the molar attractive constant, mol for each unit in the molecular structure. The estimated value proposed by Okitsu who can give the volume is shown. In calculating the SP value of the polymer, the carboxylic acid in (meth) acrylic acid is calculated in an unneutralized state, and the SP value in this specification is a value calculated by this method. ing.

[White ink composition]
The white ink composition of the present invention comprises (a) a repeating unit (a-1) having a partial structure represented by the following general formula (1) and a repeating unit (a-2) having a hydrophilic group. A white ink composition containing 10% by mass or more of (b) a white pigment having a secondary average particle size of 200 nm to 500 nm, and (c) water.

(In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure, and * represents a bonding site with a main chain or a side chain in the polymer compound.

  In the present specification, the present invention includes the repeating unit (a-1) having a partial structure represented by the general formula (1) and the repeating unit (a-2) having a hydrophilic group. The (a) polymer compound may be referred to as “(a) specific copolymer”.

The ink composition of the present invention can form an image having excellent concealability, film strength, and blocking resistance by containing each of the above components.
In the present invention, the image concealment means that the formed image cannot be visually observed from the back surface (the non-image forming surface side). Further, the anti-blocking property means that undesired adhesion between the images or between the image and the back surface of the recording medium is suppressed when the prints on which the images are formed are laminated. One major characteristic of the image produced by the white ink composition of the present invention is that excellent blocking resistance can be achieved even under high temperature conditions of 80 ° C to 100 ° C.
Here, “not visually observable from the back” means that the colored ink layer cannot be visually recognized from the plastic substrate side when a white ink layer is provided on the plastic substrate and a colored ink layer is provided on the white ink layer. Refers to that. When only the white ink layer is formed on the plastic substrate, the object placed on the white ink layer is observed through the white ink layer when viewed from the plastic substrate side. It refers to the state that is not.

Although the action mechanism of the present invention is not clear, the present inventors presume as follows.
The specific copolymer (a) according to the present invention includes a repeating unit (a-1) having a partial structure represented by the general formula (1), thereby forming a partial structure represented by the general formula (1). Both the resulting crosslinking effect and the insolubilization effect due to the improvement in polarity can be achieved. Furthermore, by containing the repeating unit (a-2) having a hydrophilic group, the SP value of the (a) specific copolymer is controlled within a preferable range, whereby the solvent resistance of the formed image is improved. It is considered that the anti-blocking property of the formed image is also improved by the crosslinking effect resulting from the partial structure (a-1) represented by the general formula (1).
In addition, an image formed by including a white pigment having a specific secondary particle diameter has high concealing properties and includes a partial structure (a-1) represented by the general formula (1) (a) specific The interaction with the white pigment having a specific particle size of the partial structure (a-1) in the specific copolymer (a) is controlled by the function of the copolymer. For this reason, it is suitable for the ink jet recording method and has a secondary effect that, when the white ink composition is applied onto a recording medium, it has an appropriate viscosity, and thus blurring of an image is suppressed. It is considered to be.

  The ink composition of the present invention contains (a) a specific copolymer, (b) a white pigment having a specific secondary particle diameter, and (c) water, as long as the effects of the present invention are not impaired. If necessary, various liquid or solid compounds may be included as additives.

  Hereinafter, the white ink composition of the present invention (hereinafter also simply referred to as “ink composition”) will be described in detail.

[(A) Specific copolymer]
The physical properties of (a) each repeating unit contained in the specific copolymer and (a) the specific copolymer contained in the ink composition of the present invention will be described in detail.

(Repeating unit (a-1) having a partial structure represented by the general formula (1))
(A) The specific copolymer includes a repeating unit (a-1) having a partial structure represented by the following general formula (1).

In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b represents an alkyl group having 1 to 4 carbon atoms. Represent. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure. * Indicates a binding site to the main chain or side chain in the polymer compound.

In general formula (1), the C1-C4 alkyl group represented by R ^<a> or R ^<b > may be a linear structure or a branched structure. Specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, and a t-butyl group. Among these alkyl groups, an alkyl group having 1 to 2 carbon atoms (methyl group and ethyl group) is preferable, and an alkyl group having 1 carbon atom (methyl group) is particularly preferable.

In general formula (1), the alkyl group represented by R ^a or R ^b may or may not have a substituent, but preferably does not have a substituent.

R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure.

As R ^a and R ^b , an aspect in which both are alkyl groups having 1 to 4 carbon atoms, or R ^a and R ^b are bonded to each other to form a 4- to 6-membered alicyclic structure. It is more preferable that both R ^a and R ^b are alkyl groups having 1 to 2 carbon atoms, and it is still more preferable that both R ^a and R ^b are alkyl groups having 1 carbon atom.

  Specific examples of the partial structure represented by the general formula (1) are shown below, but the present invention is not limited to these.

  (A) The specific copolymer may be an embodiment having a plurality of partial structures represented by the general formula (1) in the side chain.

  The repeating unit (a-1) having a partial structure represented by the general formula (1) is preferably a repeating unit represented by the following general formula (2).

In General Formula (2), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b represents an alkyl group having 1 to 4 carbon atoms. Represent. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure. R ^c represents a hydrogen atom or a methyl group. Z represents a single bond, —COO — **, or —CONR ^d — **, R ^d represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and ** represents a bonding position with X. X represents a divalent organic group.

In General Formula (2), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b represents an alkyl group having 1 to 4 carbon atoms. Represent. R ^a and R ^b may combine with each other to form a 4- to 6-membered ring structure.
^{R a} and ^{R b} in the general formula (2), the have each the same meaning as ^{R a} and ^{R b} in the general formula (1), and preferred ranges are also the same.

In the general formula (2), R ^c represents a hydrogen atom or a methyl group. R ^c is preferably a methyl group.
In General Formula (2), Z represents a single bond, —COO — **, or —CONR ^d — **, R ^d represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and ** represents X Represents the binding position. Z is preferably -COO-**.

Further, R ^d in the —CONR ^d — ** represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. The alkyl group having 1 to 4 carbon atoms represented by R ^d may have a linear structure or a branched structure. Specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, and a t-butyl group. R ^d is preferably a hydrogen atom or an alkyl group having 1 to 2 carbon atoms, that is, a methyl group or an ethyl group, and particularly preferably a hydrogen atom. If R ^d represents an alkyl group, the alkyl group may have a substituent, may not have a substituent, but preferably does not have a substituent.

  In the general formula (2), X represents a divalent organic group. Examples of the divalent organic group include an alkylene group or an aralkylene group, and an alkylene group having 2 to 20 carbon atoms or an aralkylene group having 6 to 12 carbon atoms is preferable. X is more preferably an alkylene group.

  When X represents an alkylene group, the alkylene group preferably has 2 to 20 carbon atoms, more preferably 2 to 12 carbon atoms, and still more preferably 2 to 8 carbon atoms. When the number of carbon atoms of the alkylene group represented by X is within this range, (a) the mobility of the partial structure represented by the general formula (1) existing at the end of the side chain in the specific copolymer is improved, The effect of the present invention is further improved.

  The alkylene group represented by X may have a linear structure, a branched chain in the alkylene chain, or a cyclic structure. In addition, the alkylene group may contain a bond selected from —O—, —COO—, —OC (═O) —, and —CONH— in the alkylene chain. The alkylene group may be substituted with an alkyl group having 4 or less carbon atoms, a hydroxyl group, or a chlorine atom.

As the repeating unit represented by the general formula (2), R ^a and R ^b are each independently an alkyl group having 1 to 2 carbon atoms, R ^c is a methyl group, and Z is —COO — **. And X is preferably an alkylene group having 2 to 12 carbon atoms.

  (A) The specific copolymer may contain only one type of repeating unit (a-1) having a partial structure represented by the general formula (1), or may contain two or more types.

The repeating unit (a-1) having a partial structure represented by the general formula (1) is copolymerized using a monomer having a partial structure represented by the general formula (1) as one of the copolymerization components. By doing so, it can be introduced into the specific copolymer (a). When the repeating unit (a-1) represented by the general formula (1) is a repeating unit represented by the general formula (2), the following general formula (2) A monomer represented by ') can be used.
The partial structure (a-1) represented by the general formula (1) can also be introduced by a method using a polymer reaction. Examples of such a method include a method obtained by reacting a prepolymer having a primary amino group with a corresponding anhydride, a functional group that reacts with a functional group in the prepolymer to form a bond and a general formula ( Examples thereof include a method of reacting a compound having a partial structure represented by 1) with a prepolymer.

In the general formula ^{(2 '), R a,} R b, R c, Z and X, ^R a in the formula ^{(3), R b, R} c, are respectively the Z and X synonymous, preferable ranges It is the same.

  Preferable examples of the monomer represented by the general formula (2 ′) include monomers (2′-1) to (2′-11) shown below. It is not limited.

Monomers containing a partial structure represented by the general formula (1) represented by the monomers (2′-1) to (2′-11) include, for example, JP-A-52-988, It can be produced with reference to the method described in JP-A-4-251258.
(A) The content of the repeating unit (a-1) having a partial structure represented by the general formula (1) contained in the specific copolymer, preferably the repeating unit represented by the general formula (2) The cured film (image) formed by the composition is appropriately selected according to the intended properties. Especially, as content of the repeating unit (a-1) represented by General formula (1), from the viewpoint of the intensity | strength of the formed image and a softness | flexibility, (a) To the total mass of a specific copolymer. On the other hand, it is preferably 20% by mass to 70% by mass, more preferably 30% by mass to 70% by mass, and further preferably 40% by mass to 60% by mass.
If the content is too large or too small, the image strength tends to decrease. In addition, when forming an ink image excellent in flexibility, it is preferable that the content is small in the above range.

(Repeating unit having a hydrophilic group (a-2))
The specific copolymer (a) in the present invention includes a repeating unit (a-2) having a hydrophilic group.
The hydrophilic group is not particularly limited as long as it is a group having a function of enhancing the hydrophilicity of the specific copolymer (a), and may be a nonionic hydrophilic group or an ionic hydrophilic group (for example, , An anionic hydrophilic group or a cationic hydrophilic group).

  The nonionic hydrophilic group is not particularly limited. For example, a residue obtained by removing one hydrogen atom from a heterocyclic structure containing a nitrogen atom or an oxygen atom, an amide group, a carbamoyl group, an alkyl-substituted carbamoyl group, an alcoholic hydroxyl group, Or nonionic hydrophilic groups, such as group which has a polyalkyleneoxy structure, are mentioned.

  Examples of the heterocyclic structure in the residue obtained by removing one hydrogen atom from a heterocyclic structure containing a nitrogen atom or an oxygen atom include lactones such as γ-butyrolactone; cyclic ureas such as 2-pyrrolidone and ethylene urea; ethylene carbonate, And cyclic carbonates such as propylene carbonate, and cyclic ethers such as tetrahydrofuran and 1,4-dioxane;

  As the amide group, an amide group having 2 to 10 carbon atoms is preferable, and a hydrogen atom is preferably bonded to a nitrogen atom in the amide group.

  The alkyl-substituted carbamoyl group is a monoalkylcarbamoyl group in which a hydrogen atom bonded to a nitrogen atom of a carbamoyl group is substituted with an alkyl group, or two hydrogen atoms bonded to a nitrogen atom of a carbamoyl group are substituted with an alkyl group And a dialkylcarbamoyl group formed. The alkyl group may further have a substituent such as a hydroxyl group. Among these alkyl-substituted carbamoyl groups, a monoalkylcarbamoyl group substituted with an alkyl group having 1 to 8 carbon atoms or an alkyl group having 1 to 4 carbon atoms substituted with a hydroxyl group is preferable.

  Although it does not specifically limit as group which has a polyalkyleneoxy structure, The polyalkyleneoxy structure which has a C1-C4 alkyleneoxy group in a repeating unit is preferable. One type of alkyleneoxy group in the polyalkyleneoxy structure may be used, or a plurality of types of alkyleneoxy groups may be combined. The terminal group of the polyalkyleneoxy structure is preferably a hydroxyl group or an alkoxy group, more preferably a hydroxyl group or a methoxy group.

  Although it does not specifically limit as an ionic hydrophilic group, For example, ionic hydrophilic groups, such as a carboxyl group, a sulfo group, a phosphoric acid group, a phosphonic acid group, a phenolic hydroxyl group, or a quaternary ammonium group, are mentioned. The ionic hydrophilic group may form a salt.

  When the ionic hydrophilic group forms a salt, examples of the counter salt include onium salts such as alkali metal salts (Li, Na, K, etc.), ammonium salts, pyridinium salts, and phosphonium salts. Among these, alkali metal salts (Li, Na, K, etc.) or ammonium salts are preferable.

  Among these hydrophilic groups, an amide group, a carbamoyl group, an alkyl-substituted carbamoyl group, an alcoholic hydroxyl group, a group having a polyalkyleneoxy structure, a carboxyl group, a sulfo group, and a salt thereof are preferable. A carbamoyl group, a carboxyl group, a sulfo group, and salts thereof are more preferable. Particularly preferred are carboxyl groups and salts thereof.

  The repeating unit (a-2) having a hydrophilic group is preferably a repeating unit represented by the following general formula (3).

In general formula (3), R ^cy represents a hydrogen atom or a methyl group. Z ^y represents —COO — ***, —CONR ^dy — ***, or a single bond, and R ^dy represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. R ^y represents a group selected from the group consisting of a single bond, an alkylene group, an arylene group, and an aralkylene group. A represents a hydrophilic group. *** represents a position where Z ^y is bonded to R ^y .

The general formula (3) will be described in detail.
In general formula (3), R ^cy represents a hydrogen atom or a methyl group.

In the general formula ^{(3), Z} y ^{is, -COO - ***, - CONR dy} - ***, or a single bond, -COO - is preferably ***. Incidentally, *** is a position ^{where Z y} is bonded to ^{R y.}

R ^dy represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. The alkyl group having 1 to 4 carbon atoms may have a linear structure or a branched structure. Specifically, it represents a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, or a t-butyl group. R ^dy is preferably a hydrogen atom or an alkyl group having 1 to 2 carbon atoms (that is, a methyl group or an ethyl group), and particularly preferably a hydrogen atom.

R ^dy may or may not have a substituent, but preferably does not have a substituent. Examples of the substituent that R ^dy may have include an aryl group having 6 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, a hydroxyl group, a carboxyl group, and a halogen atom (F, Cl, Br, I, etc.). Etc.

In General Formula (3), R ^y represents a single bond or a group selected from the group consisting of an alkylene group, an arylene group, and an aralkylene group, an alkylene group having 1 to 20 carbon atoms, and an arylene having 6 to 20 carbon atoms. It is preferably a group or an aralkylene group having 7 to 20 carbon atoms.
When ^Ry is a group selected from the group consisting of an alkylene group, an arylene group, and an aralkylene group, these groups may or may not have a substituent. The alkylene group, arylene group, and aralkylene group represented by R ^y may have an ether bond, an ester bond, an amide bond, or a urethane bond in the structure.
In the general formula (3), R ^y is preferably a single bond.

When ^Ry is a group selected from the group consisting of an alkylene group, an arylene group, and an aralkylene group, these groups may further have a substituent. Examples of the substituent include an aryl group having 6 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, a hydroxyl group, a carboxyl group, a halogen atom (F, Cl, Br, I, etc.) and the like.

When ^Ry is an alkylene group having 1 to 20 carbon atoms, the alkylene group may have a linear structure, a branched structure, or a cyclic structure. When ^Ry is an alkylene group, the number of carbon atoms is more preferably 2 to 12, and further preferably 2 to 8. Specific examples of the alkylene group for R ^y include —CH ₂ —, —C ₂ H ₄ —, —C (CH ₃ ) ₂ —CH ₂ —, —CH ₂ C (CH ₃ ) ₂ CH ₂ —, —C. _{6 H 12 -, - C 4} H 7 (C 4 H 9) C 4 H 8 -, C 18 H 36 -, 1,4-trans- cyclohexylene _{group, -C 2 H 4 -OCO-C} 2 H 4 _{-, - C 2 H 4 -OCO} -, - C 2 H 4 -O-C 5 H 10 -, - CH 2 -O-C 5 H 9 (C 5 H 11) -, - C 2 H 4 -CONH _{-C 2 H 4 -, - C} 4 H 8 -OCONH-C 6 H 12 -, - CH 2 -OCONHC 10 H 20 -, - CH 2 CH (OH) CH 2 -, and the like.

When ^Ry is an arylene group having 6 to 20 carbon atoms, the arylene group preferably has 6 to 18 carbon atoms, more preferably 6 to 14 carbon atoms, and particularly preferably 6 to 10 carbon atoms. . Specific examples of the arylene group represented by R ^y include a phenylene group, a biphenylene group, —C ₆ H ₄ —CO—C ₆ H ₄ —, a naphthylene group, and the like.

When ^Ry is an aralkylene group having 7 to 20 carbon atoms, the aralkylene group preferably has 7 to 18 carbon atoms, more preferably 7 to 14 carbon atoms, and particularly preferably 7 to 10 carbon atoms. . Specific examples of the aralkylene group represented by R ^y _{is, -C 3 H 6 -C 6 H} 4 -, - C 2 H 4 -C 6 H 4 -C 6 H 4 -, - CH 2 -C 6 H _{4 -C 6 H 4 -C 2 H} 4 -, - C 2 H 4 -OCO-C 6 H 4 - , and the like.

  Examples of the hydrophilic group represented by A in the general formula (3) include the hydrophilic groups described above, and preferred ranges thereof are also the same.

(A) When specific copolymer has a repeating unit represented by General formula (3), (a) Content of the repeating unit represented by General formula (3) in a specific copolymer is as follows. It is.
When the hydrophilic group A in the general formula (3) is an ionic hydrophilic group, the content of the repeating unit represented by the general formula (3) is (a) the hydrophilic group contained in the specific copolymer. It is preferable that it is 5 mass%-50 mass% in the repeating unit (a-2) which has this, 5 mass%-40 mass% are still more preferable, and 5 mass%-20 mass% are especially preferable.
When the hydrophilic group A in the general formula (3) is a nonionic hydrophilic group, (a) 20% by mass to 95% in the repeating unit (a-2) having a hydrophilic group contained in the specific copolymer. It is preferable that it is mass%, 30 mass%-80 mass% are still more preferable, and 30 mass%-70 mass% are especially preferable.

  The repeating unit represented by the general formula (3) is derived from the monomer represented by the following general formula (3 ′), and (a) specific by including these monomers as a copolymerization component. The repeating unit (a-2) is introduced into the copolymer.

In the general formula ^{(3 '), R cy,} Z y, R y, and A, ^R cy in Formula ^{(3), Z y, R} y, and A and have the same meanings, the preferred range is also the same is there.

  Preferable examples of the monomer represented by the general formula (3 ′) include the following monomer compounds, but the present invention is not limited thereto.

  Methoxypolyethylene glycol (meth) acrylate, polyethylene glycol (meth) acrylate, polypropylene glycol (meth) acrylate, poly (ethylene glycol-co-propylene glycol) (meth) acrylate, 2-hydroxyethyl (meth) acrylate, glycerol (meth) Acrylate (meth) acryloyloxyethyl ethylene urea, vinylpyrrolidone, 3- (meth) acryloyloxy-γ-butyrolactone, acrylamide, tert-butylacrylamide, N, N-dimethyl (meth) acrylamide, diacetone acrylamide, (meth) Sodium acrylate, potassium (meth) acrylate, tetrabutylammonium (meth) acrylate, mono (meth) acryloyloxyethyl succinic acid, Sodium (meth) acryloyloxyethyl succinate, sodium mono (meth) acryloyloxyethyl phthalate, (meth) acryloyloxyethyl acid phosphate, sodium 2-acrylamido-2-methylpropanesulfonate, 2-acrylamide- Examples include 2-methylpropane sulfonic acid, styrene sulfonic acid, sodium styrene sulfonate, and vinyl benzoic acid.

  As the monomer represented by the general formula (3 ′), a commercially available compound can be used, and it can be produced by a generally known and commonly used method.

  In the present invention, in addition to the monomer represented by the general formula (3 ′), unsaturated dicarboxylic acids such as maleic acid, maleic anhydride, and fumaric acid, and anhydrides thereof, and further derived therefrom. Dicarboxylic acid salts can also be preferably used.

(A) The content of the repeating unit having a hydrophilic group in the specific copolymer differs depending on the type of the hydrophilic group, but (a) the specific copolymer is water-soluble. Is preferred.
Here, (a) the specific copolymer being water-soluble means that (a) the specific copolymer can be dissolved in 3% by mass or more in water at 25 ° C.

(A) The content of the repeating unit (a-2) in the specific copolymer is preferably 8% by mass to 25% by mass, preferably 10% by mass to 23% by mass, and 10% by mass to 20%. More preferably, it is mass%.
When the content of the repeating unit (2-a) is within the above range, the polarity of the (a) specific copolymer according to the present invention is appropriately maintained, and suitable water resistance is obtained.
(A) The specific copolymer may contain only one type of repeating unit (a-2) having a hydrophilic group, or may contain two or more types.

(Repeating unit having a hydrophobic group (a-3))
(A) The specific copolymer may include a repeating unit (a-3) having a hydrophobic group. By including a repeating unit having a hydrophobic functional group, (a) the polarity of the specific copolymer is appropriately maintained, and the ink image formed by the ink composition is excellent in water resistance and non-absorbing. The adhesiveness to the recording medium is excellent.
As the repeating unit (a-3) having a hydrophobic group, a homopolymer having only a repeating unit (a-3) and having a weight average molecular weight of 10,000 or more has a solubility in water at 25 ° C. of 1. Any repeating unit derived from a vinyl monomer that is less than 0% by mass can be used without limitation, and among them, a repeating unit derived from a monomer selected from alkyl esters of (meth) acrylic acid and aralkyl esters is preferably used.
Especially, from the viewpoint of adjusting the polarity of the specific copolymer to an appropriate range, the hydrophobic group is preferably an alkyl group having 5 to 22 carbon atoms, and an alkyl group having 8 to 22 carbon atoms. Is more preferable, and an alkyl group having 8 to 14 carbon atoms is more preferable. That is, in the case of alkyl acrylate, it is an ester having an alkyl group having 4 to 19 carbon atoms, and an ester having an alkyl group having 4 to 13 carbon atoms is preferred. In the case of alkyl methacrylate, it is an ester having an alkyl group having 4 to 18 carbon atoms, and an ester having an alkyl group having 4 to 10 carbon atoms is preferred.
The alkyl group may be linear, branched, or cyclic. Also, aralkyl esters such as benzyl group and phenoxyethyl group may be used.

  Although the repeating unit (a-3) which (a) specific copolymer based on this invention may contain is illustrated below, (a-3) of this invention is not limited to these. Specific examples of (a-3) include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) ) Acrylate, tert-butyl (meth) acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, stearyl (meth) acrylate And cyclohexyl methacrylate, among others, n-propyl methacrylate, isopropyl methacrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, tert-butyl (meth) acrylate n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, benzyl methacrylate, phenoxyethyl (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, acetoacetoxyethyl (meth) acrylate and the like ( (Meth) acrylic acid esters, styrenes such as styrene, α-methylstyrene, 4-methylstyrene, vinyl ethers such as chloroethyl vinyl ether, etc., among others, substituted with an alkyl group having 5 to 22 carbon atoms. (Meth) acrylic acid esters, n-propyl methacrylate, isopropyl methacrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, tert-butyl (meth) acrylate, n-hex Sil (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, stearyl (meth) acrylate, cyclohexyl methacrylate, etc. are preferred, n-propyl methacrylate, isopropyl Methacrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, tert-butyl (meth) acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, benzyl methacrylate, phenoxy More preferred is ethyl (meth) acrylate. Further, n-butyl methacrylate, isobutyl methacrylate, tert-butyl (meth) acrylate, cyclohexyl methacrylate, isobornyl methacrylate, 2-ethylhexyl methacrylate, and phenoxyethyl (meth) acrylate are particularly preferable.

(A) When a specific copolymer contains a repeating unit (a-3), it may contain only 1 type and may contain 2 or more types.
From the viewpoint of reducing the polarity of the copolymer, the repeating unit (a-3) having a hydrophobic group has a range of 5% by mass to 72% by mass when included in the specific copolymer (a). Preferably, 20 mass%-65 mass% is more preferable, and 25 mass%-60 mass% is the most preferable.

((A) SP value of specific copolymer)
According to the present invention (a) the specific copolymer is preferably SP value of ^{20.7MPa 1/2 ~23.0MPa 1/2,} is 21.2MPa ^1/2 ~22.8MPa 1/2 More preferably, 21.5 MPa ^{1/2 to} 22.5 MPa ^1/2 is particularly preferable.

The SP values of typical monomer units used in the present invention are described below. The numerical value described in parentheses after the monomer name is the SP value, and the unit is MPa ^1/2 .
The following monomer 1 (24.41), the following monomer 2 (23.22), the following monomer 3 (22.75), methacrylic acid (24.02), 2-hydroxyethyl methacrylate (22.89), 2-hydroxyethyl Acrylamide (32.87) methyl methacrylate (19.5), n-butyl methacrylate (18.33), isobutyl methacrylate (17.76), tert-butyl methacrylate (17.97), 2-ethylhexyl methacrylate (17.30) ), Stearyl methacrylate (17.08), 2-hydroxyethyl methacrylate (22.89), tetrahydrofurfuryl methacrylate (21.29), cyclohexyl methacrylate (18.79), benzyl methacrylate (20.21), 2-phenoxy Echi Methacrylate (20.23), isobornyl methacrylate (18.09)

(A) When using a non-crosslinkable polymer in which the specific copolymer does not have the repeating unit (a-1) having the partial structure represented by the general formula (1), the water resistance and the resistance to resistance by adjusting the polarity are used. Solvent compatibility is difficult. However, since the ink composition of the present invention contains (a) the repeating unit (a-1) having a partial structure represented by the general formula (1) in the specific copolymer, the crosslinking effect and insolubilization due to polarity. It can play an effect together. Therefore, both effects of water resistance and solvent resistance can be achieved. Furthermore, in a preferred embodiment, (a) by controlling the polarity (SP value) of the specific copolymer within a specific range, an ink composition having excellent adhesion to a recording medium could be obtained.
(A) Of the repeating unit (a-1), the repeating unit (a-2) having a hydrophilic group, and the repeating unit (a-3) having a hydrophobic group optionally contained in the specific copolymer The preferred content is as described, and among each preferred content, the SP value is preferably in the above preferred range, and the total content of each repeating unit is preferably selected to be 100% by mass. .

The weight average molecular weight of the specific copolymer (a) contained in the ink composition of the present invention is preferably in the range of 5,000 to 150,000 from the viewpoint of water resistance. More preferably, it is in the range of 1,000,000 to 100,000.
The weight average molecular weight is measured by gel permeation chromatography (GPC). GPC uses HLC-8020GPC (manufactured by Tosoh Corporation), TSKgel SuperHZM-H, TSKgel SuperHZ4000, TSKgel SuperHZ200 (4.6 mm ID × 15 cm, Tosoh Corporation) as columns and THF (tetrahydrofuran) as an eluent. And the column oven was set at a set temperature of 40 ° C. Standard polystyrene was used for calculation of molecular weight.

  Specific examples of the specific copolymer (a) that can be used in the present invention will be described below by describing the repeating unit contained in the specific copolymer (a), the content based on its mass, and the weight average molecular weight. However, the present invention is not limited to this. In addition, the following "SP" shows the SP value of a copolymer and has each described the value obtained by the measuring method mentioned above.

  Specific examples (a) of the specific copolymer [Exemplary compounds (A-1) to (A-24)] which can be used in the present invention are composed of repeating units contained in the specific copolymer (a). Although it shows by describing the weight average molecular weight of the raw material monomer to be processed, the content rate of the mass reference | standard, and the obtained (a) specific copolymer, this invention is not limited to this. In addition, "SP value" described in the following Table 1 shows the SP value of (a) specific copolymer, and has each described the value obtained by the measuring method or calculating method mentioned above.

In Table 1, abbreviations are as follows.
MAA: Methacrylic acid (Wako Pure Chemical Industries, Ltd.)
MAA-Na: Methacrylic acid sodium salt (generated by neutralizing a polymer copolymerized with methacrylic acid)
BMA: n-butyl methacrylate (manufactured by Wako Pure Chemical Industries, Ltd.)
iBMA: Isobutyl methacrylate (manufactured by Wako Pure Chemical Industries, Ltd.)
tBMA: tert-butyl methacrylate (Wako Pure Chemical Industries, Ltd.)
IBOMA: Isoboronyl methacrylate (Wako Pure Chemical Industries, Ltd.)
C18MA: Octadecyl methacrylate (Wako Pure Chemical Industries, Ltd.)
CyHMA: cyclohexyl methacrylate (manufactured by Wako Pure Chemical Industries, Ltd.)
PEMA: Phenoxyethyl methacrylate (NK ester PHE-1G, manufactured by Shin-Nakamura Chemical Co., Ltd.)
EHMA: 2-ethylhexyl methacrylate (Wako Pure Chemical Industries, Ltd.)
BnMA: benzyl methacrylate (manufactured by Wako Pure Chemical Industries, Ltd.)

  The specific copolymer (a) according to the present invention has, for example, a monomer for forming the repeating unit (a-1) having a partial structure represented by the general formula (1) and a hydrophilic group. A monomer for forming the repeating unit (a-2) and a monomer for forming the repeating unit (a-3) having a hydrophobic group are polymerized by a known polymerization method. Accordingly, it can be obtained by neutralizing the acidic group with an alkali metal hydroxide or the like. Specifically, for example, a method according to the polymerization method described in JP-A-52-988, JP-A-55-154970, Langmuir Vol. 18, No. 14, pages 5414-5421 (2002), etc. (A) A specific copolymer can be produced.

[(B) White pigment having a secondary average particle size of 200 nm to 500 nm]
The white ink composition of the present invention contains a white pigment having a secondary average particle diameter of 200 nm to 500 nm in an amount of 10% by mass or more based on the total amount of the ink composition.
Specific examples of the white pigment include basic lead carbonate (2PbCO ₃ Pb (OH) ₂ , so-called silver white), zinc oxide (ZnO, so-called zinc white), titanium oxide (TiO ₂ , so-called titanium white), Strontium titanate (SrTiO ₃ , so-called titanium strontium white) or the like can be used.
Here, titanium oxide has a smaller specific gravity than other white pigments, a large refractive index, and is chemically and physically stable. Excellent durability against other environments. Therefore, it is preferable to use titanium oxide as the white pigment. Of course, other white pigments (may be other than the listed white pigments) may be used as necessary.

The white pigment that can be used in the present invention has a secondary average particle size of 200 nm to 500 nm. When the secondary average particle diameter is less than 200 nm, the concealment property of the image formed by the white ink composition is lowered, and when it exceeds 500 nm, uniform dispersion becomes difficult, and unevenness of the image surface due to particle aggregates, etc. Are likely to occur, both of which are not preferred. The secondary average particle size is preferably in the range of 200 nm to 450 nm, more preferably in the range of 200 nm to 400 nm.
Moreover, it is preferable that the primary average particle diameter of the white pigment that can be used in the present invention is 100 nm to 300 nm, and more preferably 200 nm to 250 nm.
When the primary average particle size is less than 100 nm, when a white ink composition is used, it is difficult to achieve sufficient shielding properties, and the surface energy of the primary particles increases, so that the particle size of the secondary aggregate, that is, Control of the secondary average particle size becomes difficult. On the other hand, if it exceeds 300 nm, coarse particles are easily formed as a by-product, which makes it difficult to prepare a uniform ink composition having excellent inkjet suitability.
In addition, the secondary average particle diameter and primary average particle diameter in the present invention are obtained by directly photographing a pigment at a magnification of 100,000 by electron microscopy (manufactured by JEOL Ltd., JEM-1200FX). An arithmetic average value obtained by observing and measuring the size of 100 and measuring 100 sizes for one image is employed. Here, the particle diameter is calculated by the equivalent circle diameter, that is, the diameter when the particle is assumed to be a circle.
For example, when the particles have an elliptical shape, the diameter may be calculated by calculating the average value by measuring the diameters of the vertical and horizontal slant, etc., and calculating the diameter converted into a circle.

The white pigment is preferably contained in the ink composition as a dispersion prepared using a dispersant.
For the dispersion of the white pigment, for example, known dispersion apparatuses such as a ball mill, a sand mill, an attritor, a roll mill, a jet mill, a homogenizer, a paint shaker, a kneader, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, and a wet jet mill. Can be used.
When dispersing, a known high molecular weight or low molecular weight dispersant such as a surfactant or an acrylic copolymer may be added.
As the dispersant, acrylic copolymers, specifically, for example, commercially available products manufactured by BYK, DISPERBYK-2010 and DISPERBYK-2015 are preferably used from the viewpoint of dispersion stability.
The dispersant can be added in an amount of 1 to 50 parts by weight with respect to 100 parts by weight of the white pigment.
Further, if necessary, a synergist according to the white pigment to be used can be used as a dispersion aid.

In order to control the secondary average particle diameter of the white pigment within the above range, parameters such as dispersion conditions, specifically, dispersion time, temperature, and rotation speed may be adjusted as appropriate.
The content of the white pigment in the white ink composition of the present invention is required to be 10% by mass or more based on the total amount of the white ink composition. When the content is less than 10% by mass, it is difficult to obtain desired concealability. Although there is no restriction | limiting in particular in an upper limit, It is preferable that it is 50 mass% or less when the balance with the solvent containing a sclerosing | hardenable component and water is considered. As content of a white pigment, it is more preferable that it is the range of 20 mass%-40 mass%, and it is still more preferable that it is the range of 25 mass%-35 mass%.
The content ratio of (a) polymer compound to (b) white pigment [(a) polymer compound / (b) white pigment] is 1/4 to 1/1 on a mass basis. In view of the above, it is preferably 1 / 3.8 to 1 / 1.5, more preferably 1 / 3.5 to 1/3.

[(C) water]
The ink composition of the present invention contains water.
(C) As water, it is preferable to use ion-exchange water, distilled water, etc. which do not contain an impurity.
The water content in the ink composition of the present invention is preferably 10% by mass to 97% by mass, more preferably 30% by mass to 95% by mass, and 50% by mass to 85% by mass. More preferred.

(D) Water-soluble organic solvent The ink composition of the present invention contains water as a main solvent, but may further contain a water-soluble organic solvent.
Here, the water-soluble organic solvent means an organic solvent having a solubility in water at 25 ° C. of 10% by mass or more.

Examples of water-soluble organic solvents that can be used in the present invention include the following.
Alcohols (eg, methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, pentanol, hexanol, cyclohexanol, benzyl alcohol, etc.),
Polyhydric alcohols (for example, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, 2- Methylpropanediol, etc.)
Polyhydric alcohol ethers (for example, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, tripropylene Glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol dimethyl ether, ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, ethylene glycol Monophenyl ether, propylene glycol monophenyl ether, etc.),

-Amines (eg, ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentane Methyldiethylenetriamine, tetramethylpropylenediamine, etc.),
Amides (eg, formamide, N, N-dimethylformamide, N, N-dimethylacetamide, etc.), methoxypropionamide, N-methylmethoxypropionamide, N, N-dimethylmethoxypropionamide, n-butoxypropionamide, N-methyl n-butoxypropionamide, N, N-dimethyl n-butoxypropionamide, etc.), heterocycles (for example, 2-pyrrolidone, N-methyl-2-pyrrolidone, cyclohexylpyrrolidone, 2-oxazolidone, 1, 3-dimethyl-2-imidazolidinone, γ-butyrolactone, propylene carbonate, ethylene carbonate, ethylene urea, etc.),
・ Sulphoxides (for example, dimethyl sulfoxide),
-Sulfones (for example, sulfolane),
・ Others (urea, acetonitrile, acetone, etc.)

  Preferable water-soluble organic solvents include polyhydric alcohol ethers and heterocyclic rings, and these may be used in combination.

Among the polyhydric alcohol ethers, so-called glycol ethers are preferable, specifically, tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol dimethyl ether are preferable, and 2-dipropylene glycol monomethyl ether is more preferable.
As the heterocyclic ring, 2-pyrrolidone, γ-butyrolactone, propylene carbonate, ethylene urea and the like are preferable, and 2-pyrrolidone and γ-butyrolactone are particularly preferable.
In particular, a solvent having a high boiling point can be preferably used, and the boiling point at normal pressure is preferably 120 ° C. or higher, more preferably 150 ° C. or higher.

  One or more water-soluble organic solvents may be used in combination. The addition amount of the water-soluble organic solvent in the ink composition is preferably 1% by mass to 60% by mass, and more preferably 2% by mass to 35% by mass when added.

  When the white ink composition of the present invention is applied on a recording medium, it may affect the curability of the image if it contains a volatile organic solvent, so in the white ink composition of the present invention, It is preferable that the organic solvent is not substantially contained, more specifically, the content of the organic solvent having a boiling point of 250 ° C. or less is preferably 5% by mass or less, more preferably 1% by mass or less, Except for inevitable impurities, it is most preferable not to contain such an organic solvent.

[Other additives]
In addition to the above-mentioned (a) specific copolymer, (b) a white pigment having a specific secondary average particle diameter, and (c) water, the ink composition of the present invention has the effects of the present invention. As long as they are not impaired, known additives can be used in combination.
Known additives include, for example, polymerization inhibitors, sensitizers, co-sensitizers, ultraviolet absorbers, antioxidants, anti-fading agents, conductive salts, solvents, polymer compounds, basic compounds, and surface active agents. Agents, leveling additives, matting agents, polyester resins for adjusting film properties, polyurethane resins, vinyl resins, acrylic resins, rubber resins, waxes and the like. About these additives, it describes in Unexamined-Japanese-Patent No. 2009-185186, Each compound described here can be selected suitably and used in this invention.

(Surfactant)
A surfactant can be added to the white ink composition of the present invention.
As surfactants preferably used, anionic surfactants such as dialkylsulfosuccinates, alkylnaphthalenesulfonates, fatty acid salts, polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, Nonionic surfactants such as polyoxyethylene / polyoxypropylene block copolymers, and cationic surfactants such as alkylamine salts and quaternary ammonium salts. In particular, an anionic surfactant and a nonionic surfactant can be preferably used.

In the present invention, a polymer surfactant can also be used, and the following water-soluble resins are mentioned as preferred polymer surfactants. As the water-soluble resin, styrene-acrylic acid-acrylic acid alkyl ester copolymer, styrene-acrylic acid copolymer, styrene-maleic acid-acrylic acid alkyl ester copolymer, styrene-maleic acid copolymer are preferably used. Polymer, styrene-methacrylic acid-acrylic acid alkyl ester copolymer, styrene-methacrylic acid copolymer, styrene-maleic acid half ester copolymer, vinyl naphthalene-acrylic acid copolymer, vinyl naphthalene-maleic acid copolymer Etc.
In the present invention, a silicone surfactant having a polyalkylsiloxane and a fluorine surfactant having a fluorinated alkyl group can also be preferably used.

  In the white ink composition of the present invention, when a surfactant is used, the addition amount is preferably 0.1% by mass or more and 5% by mass or less in terms of solid content. It is particularly preferable that the solid content addition amount is 0.5% by mass or more and 2% by mass or less.

(Sensitizing dye)
In the present invention, a known sensitizing dye can be used in combination, and a sensitizing dye is preferably used in combination. The solubility is preferably 0.5% by mass or more, more preferably 1% by mass or more, at room temperature in distilled water. Those that dissolve 3% by mass or more are particularly preferable. Further, as the sensitizing dye, a photopolymerization initiator in which a water-insoluble polymerization initiator is dispersed can also be used.

  Examples of known sensitizing dyes that can be used in combination include N- [2-hydroxy-3- (3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy) propyl] -N, N, N. -Trimethylaluminum chloride, benzophenone, thioxanthone, anthraquinone derivatives and 3-acylcoumarin derivatives, terphenyl, styryl ketone and 3- (aroylmethylene) thiazoline, camphorquinone, eosin, rhodamine and erythrosine, and their modified forms And dispersions thereof. Moreover, the compound represented by the general formula (i) described in JP-A 2010-24276 and the compound represented by the general formula (I) described in JP-A-6-107718 can also be suitably used. .

  In addition to the components described above, the white ink composition of the present invention, as necessary, is intended to improve ejection stability, print head and ink cartridge compatibility, storage stability, image storage stability, and other various performances. Depending on the type, various known additives, such as viscosity modifiers, surface tension modifiers, specific resistance modifiers, film formers, preservatives, dispersants, surfactants, UV absorbers, antioxidants, antifade Agent, anti-corrosive agent, rust preventive agent, solid wetting agent, silica fine particles and the like can be appropriately selected and used. For example, oil droplet fine particles such as liquid paraffin, dioctyl phthalate, tricresyl phosphate, silicone oil, and the like UV absorbers described in JP-A-57-74193, JP-A-57-87988, and JP-A-62-261476, JP-A-57-74192, JP-A-57-879. No. 9, JP-A-60-72785, JP-A-61-146591, JP-A-1-95091, JP-A-3-13376, etc. No. 42993, JP-A-59-52689, JP-A-62-280069, JP-A-61-228771 and JP-A-4-219266, etc., Examples include pH adjusters such as sulfuric acid, phosphoric acid, citric acid, sodium hydroxide, potassium hydroxide, and potassium carbonate.

<Method for Preparing Ink Composition>
The method for preparing the white ink composition of the present invention is not particularly limited, and each component may be a medium such as a ball driving machine mill such as a ball mill, a centrifugal mill or a planetary ball mill, a high speed rotating mill such as a sand mill, or a stirring tank mill. It can be prepared by stirring, mixing and dispersing with a simple dispersing machine such as a stirring mill or a disper. About the addition order of each component, it is arbitrary. Preferably, the azo pigment, the polymer dispersant and the organic solvent are premixed and then subjected to a dispersion treatment, and the obtained dispersion is mixed with the resin and the organic solvent. In this case, at the time of addition or after addition, the mixture is uniformly mixed with a simple stirrer such as a three-one motor, a magnetic stirrer, a disper, or a homogenizer. You may mix using mixers, such as a line mixer. Further, in order to make the dispersed particles finer, they may be mixed using a dispersing machine such as a bead mill or a high-pressure jet mill. Depending on the type of pigment or polymer dispersant, a resin may be added during premixing before pigment dispersion.

The ink composition of the present invention preferably has a surface tension at 25 ° C. of 40 mN / m to 200 mN / m. The surface tension is measured under the condition of 25 ° C. using an Automatic Surface Tensiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).
The viscosity is preferably 15 mPa · s to 30 mPa · s, and more preferably 18 mPa · s to 30 mPa · s. The viscosity of the ink composition is measured under a condition of 25 ° C. using VISCOMETER TV-22 (LT: manufactured by TOKI SANGYOCO. LTD).
The viscosity of the ink composition of the present invention is relatively higher than that of a normal ink composition, and the viscosity at 25 ° C. is preferably 15 mPa · s or higher. In the white ink composition used in the image forming method of the present invention, the ink composition deposited on the recording medium is dried to increase the viscosity, thereby suppressing landing interference and obtaining a good image. The method is used.
As described above, by using an ink composition having a viscosity condition of 15 mPa · s or more from the beginning, liquid movement is suppressed by evaporation of a small amount, and an image without blur is formed in a short time. On the other hand, when a general ink composition having a viscosity of about 5 mPa · s is used, it is necessary to remove a large amount of solvent before reaching a viscosity at which movement is suppressed. It takes a long time to hit the ink. For this reason, when forming an image for white pressing with the white ink composition of the present invention, it is necessary to quickly form an image of a certain area. Viscosity.
However, the viscosity at the discharge temperature (preferably 25 ° C. to 80 ° C., more preferably 25 ° C. to 50 ° C.) may be a lower viscosity, for example, 3 to 15 mPa · s from the viewpoint of discharge stability. Even in this case, if the temperature falls upon landing on the surface of the recording medium, a preferable viscosity range before heating is obtained.

[Ink set for layer formation]
The ink composition for layer formation of the present invention comprises (a) a repeating unit (a-1) having a partial structure represented by the general formula (1) and a repeating unit (a-2) having a hydrophilic group. A high molecular compound containing, (b) a white pigment having a secondary average particle size of 200 nm to 500 nm in an amount of 10% by mass or more based on the total amount of the ink composition, and (c) a white ink composition containing water and other than white And at least one ink composition (hereinafter, also referred to as other colored ink composition) containing a colorant having a hue of.
That is, with the white ink composition of the present invention, first, a white ink composition layer containing a white pigment is formed on a recording medium, and then an image is formed on the surface of the white ink composition with another colored ink composition. By forming the image, for example, even when a transparent or light-colored recording medium is used, a sharp image with excellent color developability is formed.
The white ink composition used here is the white ink composition of the present invention described above.
The ink composition containing a colorant having a hue other than white and used in combination with the white ink composition may be used in any combination of known colored ink compositions.
Other colored ink compositions usually have a yellow ink composition, a magenta ink composition, a cyan ink composition, and a black ink composition, but may further contain other color ink compositions and are necessary. Depending on the, you may have only one of these.

Furthermore, the ink composition for layer formation of the present invention may have a clear ink composition in addition to the white ink composition and other colored ink compositions. The clear ink composition is a transparent ink composition substantially free of a colorant.
The clear ink composition may be used as an undercoat layer of a recording medium, or may be used in the formation of an overcoat layer (protective layer) after forming a laminate having a white ink image and a colored ink image. . When the clear ink composition is used, the printed matter formed by the ink set preferably has a three-layer structure.

(Colorant with a hue other than white)
The colorant contained in the ink composition used in combination with the white ink composition of the present invention in the layered ink set of the present invention will be described.
As the colorant, any colorant can be used as long as the hue is other than white, and examples thereof include dyes and pigments, and are pigments from the viewpoint of durability such as heat resistance, light resistance, and water resistance. It is preferable.

  When a pigment is used as the colorant, the pigment can be contained in the ink composition as a pigment dispersion. Use of a pigment dispersion as a colorant is preferable from the viewpoint of improving solvent resistance. As the pigment dispersion, a self-dispersed pigment can be used in addition to a pigment dispersed with a pigment dispersant.

(Pigment)
As the pigment used as the colorant, generally used organic pigments, inorganic pigments, and those obtained by dyeing resin particles with a dye can be used. Usually, any commercially available pigment can be used, and further, a pigment pretreated with a commercially available pigment dispersion or surface treatment agent, for example, a pigment dispersed in an insoluble resin or the like as a dispersion medium, Or what grafted resin to the pigment surface etc. can be used unless the effect of this invention is impaired.
Examples of these pigments include, for example, “Pigment Dictionary” (2000), edited by Seijiro Ito. Herbst, K.M. Hunger “Industrial Organic Pigments”, JP 2002-12607 A, JP 2002-188025 A, JP 2003-26978 A, and JP 2003-342503 A3.

  Examples of organic pigments and inorganic pigments that can be used in the present invention include C.I. I. Pigment Yellow 1 (Fast Yellow G, etc.), C.I. I. A monoazo pigment such as C.I. Pigment Yellow 74; I. Pigment Yellow 12 (disaji yellow, etc.), C.I. I. Pigment yellow 17, C.I. I. Pigment yellow 97, C.I. I. Pigment yellow 3, C.I. I. Pigment yellow 16, C.I. I. Pigment yellow 83, C.I. I. Pigment yellow 155, C.I. I. Disazo pigments such as C.I. Pigment Yellow 219; I. Azo lake pigments such as C.I. Pigment Yellow 100 (eg Tartrazine Yellow Lake); I. Pigment yellow 95 (condensed azo yellow, etc.), C.I. I. Pigment yellow 93, C.I. I. Pigment yellow 94, C.I. I. Pigment yellow 128, C.I. I. Condensed azo pigments such as C.I. Pigment Yellow 166; I. Acidic dye lake pigments such as C.I. Pigment Yellow 115 (such as quinoline yellow lake); I. Basic dye lake pigments such as C.I. Pigment Yellow 18 (Thioflavin Lake, etc.); I. Anthraquinone pigments such as CI Pigment Yellow 24 (Flavantrone Yellow), isoindolinone pigments such as Isoindolinone Yellow 3RLT (Y-110), C.I. I. Quinophthalone pigments such as C.I. Pigment Yellow 138 (Kinophthalone Yellow); I. An isoindoline pigment such as C.I. Pigment Yellow 139 (isoindoline yellow); I. Nitroso pigments such as C.I. Pigment Yellow 153 (nickel nitroso yellow, etc.); I. Metal complex salt azomethine pigments such as C.I. Pigment Yellow 117 (copper azomethine yellow, etc.); I. Pigment yellow 120 (benzimidazolone yellow) C.I. I. Pigment yellow 151, C.I. I. Pigment yellow 154, C.I. I. Pigment yellow 175, C.I. I. Pigment yellow 180, C.I. I. Pigment yellow 181, C.I. I. An acetolone pigment such as C.I. Pigment Yellow 194; I. And nickel azo pigments such as CI Pigment Yellow 150. Among these, C.I. I. Pigment yellow 74, C.I. I. Pigment yellow 120, C.I. I. Pigment yellow 150, C.I. I. Pigment yellow 151, C.I. I. Pigment yellow 154, C.I. I. Pigment yellow 155, C.I. I. Pigment Yellow 180 or the like is preferably used.

  Examples of those exhibiting red or magenta color include C.I. I. Monoazo pigments such as CI Pigment Red 3 (Toluidine Red, etc.); I. Pigment red 1, C.I. I. Pigment red 4, C.I. I. B-naphthol pigments such as C.I. Pigment Red 6; I. Disazo pigments such as C.I. Pigment Red 38 (Pyrazolone Red B, etc.); I. Pigment Red 53: 1 (Lake Red C, etc.) and C.I. I. Pigment Red 57: 1 (Brilliant Carmine 6B etc.), C.I. I. Pigment red 52: 1, C.I. I. Azo lake pigments such as C.I. Pigment Red 48 (B-oxynaphthoic acid lake, etc.); I. Pigment red 144 (condensed azo red, etc.), C.I. I. Pigment red 166, C.I. I. Pigment red 220, C.I. I. Pigment red 214, C.I. I. Pigment red 221, C.I. I. Condensed azo pigments such as C.I. Pigment Red 242, C.I. I. Pigment Red 174 (Phloxine B lake, etc.), C.I. I. Acidic dye lake pigments such as C.I. Pigment Red 172 (Erythrosin Lake, etc.); I. Basic dye lake pigments such as C.I. Pigment Red 81 (Rhodamine 6G 'lake, etc.); I. Anthraquinone pigments such as C.I. Pigment Red 177 (eg, dianthraquinonyl red); I. Thioindigo pigments such as C.I. Pigment Red 88 (Thioindigo Bordeaux, etc.); I. Perinone pigments such as CI Pigment Red 194 (perinone red, etc.)

C. I. Pigment red 149 (perylene scarlet, etc.), C.I. I. Pigment red 179, C.I. I. Pigment red 178, C.I. I. Pigment red 190, C.I. I. Pigment red 224, C.I. I. Pigment red 123, C.I. I. Perylene pigments such as C.I. Pigment Red 224; I. Pigment violet 19 (unsubstituted quinacridone), C.I. I. Pigment red 122 (quinacridone magenta, etc.), C.I. I. Pigment red 262, C.I. I. Pigment red 207, C.I. I. Quinacridone pigments such as CI Pigment Red 209, and quinacridone pigments which are solid solutions of the plurality of quinacridone pigments; I. Isoindolinone pigments such as CI Pigment Red 180 (isoindolinone red 2BLT, etc.); I. Alizarin lake pigments such as C.I. Pigment Red 83 (Madalake, etc.); I. Pigment red 171, C.I. I. Pigment red 175, C.I. I. Pigment red 176, C.I. I. Pigment red 185, C.I. I. Naphtholone pigments such as C.I. Pigment Red 208; I. Naphthol AS lake pigments such as C.I. Pigment Red 247; I. Pigment red 2, C.I. I. Pigment red 5, C.I. I. Pigment red 21, C.I. I. Pigment red 170, C.I. I. Pigment red 187, C.I. I. Pigment red 256, C.I. I. Pigment red 268, C.I. I. Naphthol AS pigments such as C.I. Pigment Red 269; I. Pigment red 254, C.I. I. Pigment red 255, C.I. I. Pigment red 264, C.I. I. And diketopyrrolopyrrole pigments such as CI Pigment Red 27. Among these, C.I. I. Pigment violet 19 (unsubstituted quinacridone), C.I. I. Pigment red 122 (quinacridone magenta, etc.), C.I. I. Pigment red 262, C.I. I. Pigment red 207, C.I. I. A quinacridone pigment such as CI Pigment Red 209 and a quinacridone pigment which is a solid solution containing a plurality of these quinacridone pigments are preferred.

  Examples of pigments exhibiting blue or cyan colors include C.I. I. Disazo pigments such as C.I. Pigment Blue 25 (Dianisidine Blue, etc.); I. Pigment blue 15, C.I. I. Pigment blue 15: 1, C.I. I. Pigment blue 15: 2, C.I. I. Pigment blue 15: 3, C.I. I. Pigment blue 15: 4, C.I. I. Pigment blue 15: 6, C.I. I. Phthalocyanine pigments such as C.I. Pigment Blue 16 (phthalocyanine blue, etc.); I. Acidic dye lake pigments such as C.I. Pigment Blue 24 (Peacock Blue Lake, etc.); I. Basic dye lake pigments such as C.I. Pigment Blue 1 (Viclotia Pure Blue BO Lake, etc.); I. Anthraquinone pigments such as C.I. Pigment Blue 60 (Indantron Blue, etc.); I. And alkali blue pigments such as CI Pigment Blue 18 (Alkali Blue V-5: 1). Among these, C.I. I. Pigment blue 15, C.I. I. Pigment blue 15: 1, C.I. I. Pigment blue 15: 2, C.I. I. Pigment blue 15: 3, C.I. I. Pigment blue 15: 4, C.I. I. Copper phthalocyanine pigments such as CI Pigment Blue 15: 6 are preferred.

Examples of the pigment exhibiting green include C.I. I. Pigment green 7 (phthalocyanine green), C.I. I. Phthalocyanine pigments such as C.I. Pigment Green 36 (phthalocyanine green); I. Pigment green 8 (nitroso green), C.I. I. And azo metal complex pigments such as CI Pigment Green 10.
Examples of orange pigments include C.I. I. An isoindoline pigment such as C.I. Pigment Orange 66 (isoindoline orange); I. Anthraquinone pigments such as C.I. Pigment Orange 51 (dichloropyrantron orange); I. Pigment orange 2, C.I. I. Pigment orange 3, C.I. I. Β-naphthol pigments such as C.I. Pigment Orange 5; I. Pigment orange 4, C.I. I. Pigment orange 22, C.I. I. Pigment orange 24, C.I. I. Pigment orange 38, C.I. I. Naphthol AS pigments such as CI Pigment Orange 74; I. Isoindolinone pigments such as CI Pigment Orange 61; I. Perinone pigments such as C.I. Pigment Orange 43; I. Pigment orange 15, C.I. I. Disazo pigments such as CI Pigment Orange 16; I. Pigment orange 48, C.I. I. Quinacridone pigments such as CI Pigment Orange 49; I. Pigment orange 36, C.I. I. Pigment orange 62, C.I. I. Pigment orange 60, C.I. I. Pigment orange 64, C.I. I. An acetolone pigment such as C.I. Pigment Orange 72; I. Pigment orange 13, C.I. I. And pyrazolone pigments such as CI Pigment Orange 34.
Examples of the pigment exhibiting brown include C.I. I. Pigment brown 25, C.I. I. And naphthron pigments such as CI Pigment Brown 32.

  Examples of pigments exhibiting black color include carbon black, titanium black, C.I. I. Indazine pigments such as CI Pigment Black 1 (aniline black); I. Pigment black 31, C.I. I. And perylene pigments such as CI Pigment Black 32. Among these, carbon black is preferable.

Since the pigment having a hue other than white used here is more excellent in color developability as the average particle size is smaller, if applied to the pigment dispersion, the average particle size of the pigment contained in the pigment dispersion is The thickness is preferably about 0.01 μm to 0.4 μm, more preferably 0.02 μm to 0.3 μm.
The maximum particle diameter of the pigment is 3 μm or less, preferably 1 μm or less. The particle size of the pigment can be adjusted by selecting a pigment, a dispersant, a dispersion medium, setting dispersion conditions, and filtering conditions.

(Dispersant)
When a pigment is used as the colorant, a pigment dispersant may be used as necessary when preparing the pigment particles. Examples of the pigment dispersant that can be used include higher fatty acid salts and alkyl sulfates. , Alkyl ester sulfate, alkyl sulfonate, sulfosuccinate, naphthalene sulfonate, alkyl phosphate, polyoxyalkylene alkyl ether phosphate, polyoxyalkylene alkyl phenyl ether, polyoxyethylene polyoxypropylene glycol, glycerin Activators such as esters, sorbitan esters, polyoxyethylene fatty acid amides, amine oxides, or styrene, styrene derivatives, vinyl naphthalene derivatives, acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, Mar acids, block copolymers of two or more monomers selected from fumaric acid derivatives, random copolymers and can be exemplified salts thereof.

  A self-dispersing pigment can also be used in the ink composition of the present invention. The self-dispersing pigment as used in the present invention refers to a pigment that can be dispersed without a dispersant, and particularly preferably pigment particles having a polar group on the surface.

The pigment particles having a polar group on the surface as used in the present invention are a pigment whose surface is directly modified with a polar group, or an organic substance having an organic pigment mother nucleus, which is bonded directly or via a joint. (Hereinafter referred to as pigment derivative).
Examples of the polar group include a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a boric acid group, and a hydroxyl group, preferably a sulfonic acid group and a carboxylic acid group, and more preferably a sulfonic acid group.

  Examples of a method for obtaining pigment particles having a polar group on the surface include, for example, WO 97/48769, JP-A-10-110129, JP-A-11-246807, JP-A-11-57458, 11-189739, JP-A-11-323232, JP-A-2000-265094, etc., oxidize the pigment particle surface with an appropriate oxidizing agent, so that at least a part of the pigment surface has a sulfonic acid group or The method of introduce | transducing polar groups, such as the salt, is mentioned. Specifically, carbon black is oxidized with concentrated nitric acid, and in the case of color pigments, it is oxidized with sulfamic acid, sulfonated pyridine salt, amidosulfuric acid, etc. in sulfolane or N-methyl-2-pyrrolidone. Can be prepared. In these reactions, the pigment dispersion can be obtained by removing and purifying the substance that has been excessively oxidized and becomes water-soluble. Moreover, when a sulfonic acid group is introduced onto the surface by oxidation, the acidic group may be neutralized with a basic compound as necessary.

  As other methods for obtaining pigment particles having polar groups on the surface, the pigment particles described in JP-A-11-49974, JP-A-2000-273383, JP-A-2000-303014, etc. are treated with a treatment such as milling. Examples include a method of adsorbing to the surface, a method described in Japanese Patent Application Nos. 2000-377068, 2001-1495, and 2001-234966, and a method of crystallizing in a poor solvent after dissolving the pigment derivative together with a solvent. In any method, pigment particles having a polar group on the surface can be easily obtained.

  The polar group on the surface of the pigment may be free or in a salt state, or may have a counter salt. Examples of the counter salt include inorganic salts (lithium, sodium, potassium, magnesium, calcium, aluminum, nickel, ammonium) and organic salts (triethylammonium, diethylammonium, pyridinium, triethanolammonium, etc.), preferably 1 A counter salt having a valence.

  In the ink composition for layer formation of the present invention, the content of the colorant (colorant having a hue other than white) with respect to the total amount of the other ink composition used in combination with the white ink composition is 0.5% by mass to 10%. % By mass is preferable, and 0.5% by mass to 5% by mass is more preferable.

[Image forming method]
The image forming method of the present invention includes an ink applying step of applying the ink composition onto a recording medium, and an irradiation step of irradiating the applied ink composition with active energy rays. By performing these steps, an image of the ink composition fixed on the recording medium is formed.

(Ink application process)
Hereinafter, the ink application process in the image forming method of the present invention will be described.
The ink application process in the present invention is not limited as long as it is a process of applying the white ink composition of the present invention onto a recording medium.

  As an embodiment for applying the white ink composition of the present invention on the recording medium, an embodiment in which the white ink composition is applied on the recording medium by an inkjet method is particularly preferable.

  In the image forming method of the present invention, the ink jet recording apparatus used when the ink applying process ink jet method is applied is not particularly limited, and a known ink jet recording apparatus capable of achieving a target resolution is arbitrarily selected. Can be used. That is, any known inkjet recording apparatus including a commercially available product can discharge the ink composition onto the recording medium in the image forming method of the present invention.

Examples of the ink jet recording apparatus that can be used in the present invention include an apparatus including an ink supply system, a temperature sensor, and a heating unit.
Examples of the ink supply system include an original tank containing the ink composition of the present invention, a supply pipe, an ink supply tank immediately before the inkjet head, a filter, a piezo-type inkjet head, and the like. The piezo-type inkjet head preferably has a multi-size dot of 1 to 100 pl, more preferably 8 to 30 pl, preferably 320 × 320 to 4,000 × 4,000 dpi (dot per inch), more preferably 400 × 400. ˜1,600 × 1,600 dpi, more preferably 720 × 720 dpi. The dpi referred to in the present invention represents the number of dots per 2.54 cm (1 inch).

In the ink application step, it is desirable that the discharged ink composition has a constant temperature. Therefore, the ink jet recording apparatus preferably includes a means for stabilizing the ink composition temperature. The parts to be kept at a constant temperature are all the piping systems and members from the ink tank (intermediate tank if there is an intermediate tank) to the nozzle ejection surface. That is, heat insulation and heating can be performed from the ink supply tank to the inkjet head portion.
The temperature control method is not particularly limited, but for example, it is preferable to provide a plurality of temperature sensors in each piping portion and perform heating control according to the flow rate of the ink composition and the environmental temperature. The temperature sensor can be provided near the ink supply tank and the nozzle of the inkjet head. Moreover, it is preferable that the head unit to be heated is thermally shielded or insulated so that the apparatus main body is not affected by the temperature from the outside air. In order to shorten the printer start-up time required for heating or to reduce the loss of thermal energy, it is preferable to insulate from other parts and reduce the heat capacity of the entire heating unit.

  The temperature of the ink composition during ejection is preferably constant, and the control range of the temperature of the ink composition is more preferably ± 5 ° C. of the set temperature, more preferably ± 2 ° C. of the set temperature, and most preferably set. A temperature of ± 1 ° C. is appropriate.

In the present invention, the recording medium is not particularly limited, and a known recording medium can be used as a support or a recording material. Examples of the recording medium include paper, paper laminated with plastic (eg, polyethylene, polypropylene, polystyrene, etc.), metal plate (eg, aluminum, zinc, copper, etc.), plastic film (eg, polyvinyl chloride resin, two Cellulose acetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, polyvinyl acetal, etc.), paper or plastic on which the above metals are laminated or deposited A film etc. are mentioned.
Among them, since the ink composition of the present invention is excellent in adhesion, it can be suitably used for a non-absorbent recording medium as a recording medium, and a plastic substrate such as polyvinyl chloride, polyethylene terephthalate, polyethylene is preferable, A polyvinyl chloride resin base material is more preferable, and a polyvinyl chloride resin sheet or film is more preferable.

(Irradiation process)
Hereinafter, the irradiation step in the image forming method of the present invention will be described.
The irradiation step in the present invention is not limited as long as it is a step of irradiating the ink composition applied on the recording medium with active energy rays.

  Examples of active energy rays that can be used in the irradiation step include ultraviolet rays (hereinafter also referred to as UV light), visible light glands, electron beams, and the like, and UV light is preferably used.

  Although the peak wavelength of UV light depends on the absorption characteristics of the sensitizing dye used as necessary, it is preferably, for example, 200 nm to 405 nm, more preferably 220 nm to 390 nm, and 220 nm to 350 nm. More preferably. In this invention, when not using a sensitizing dye and a photoinitiator together, it is preferable that it is 200 nm-310 nm, and 200 nm-280 nm are more preferable.

UV light is the illumination intensity on the exposed surface is, for ^{example, 10mW / cm 2 ~2,000mW / cm} 2, ^is preferably a suitably be irradiated with ^{20mW / cm 2 ~1,000mW / cm 2} .

  As the UV light source, a mercury lamp, a gas / solid laser or the like is mainly used, and a mercury lamp, a metal halide lamp and a UV fluorescent lamp are widely known. In addition, replacement with GaN-based semiconductor ultraviolet light-emitting devices is very useful industrially and environmentally. LED (UV-LED) and LD (UV-LD) are small, have long life, high efficiency, and low cost. Yes, it is expected as a UV light source. In the present invention, when a sensitizing dye and a photopolymerization initiator are used in combination, a metal halide lamp, a high pressure mercury lamp, a medium pressure mercury lamp, a low pressure mercury lamp, and a UV-LED are preferable, and a sensitizing dye and a photopolymerization initiator are used in combination. If not, a medium pressure mercury lamp or a low pressure mercury lamp is preferred, and a low pressure mercury lamp is particularly preferred.

In the irradiation step, the ink composition of the present invention applied on the recording medium is irradiated with such UV light, for example, for 0.01 seconds to 120 seconds, preferably 0.1 seconds to 90 seconds. Is appropriate.
As the irradiation conditions and the basic irradiation method, the irradiation conditions and the irradiation method disclosed in JP-A-60-132767 can be similarly applied to the present invention. Specifically, a method in which a light source is provided on both sides of a head unit including an ink ejection device and the head unit and the light source are scanned by a so-called shuttle method, or a method in which a separate light source without driving is used is preferable. The irradiation with the active energy ray is performed after a certain period of time (for example, 0.01 seconds to 120 seconds, preferably 0.01 seconds to 60 seconds) after ink landing and heat drying.

(Ink drying process)
The image forming method of the present invention preferably further includes an ink drying step (also referred to as “heat drying step”) after the ink application step and before the irradiation step.
In the heat drying step, the ink composition discharged onto the recording medium is preferably fixed by evaporating (b) water and a water-soluble organic solvent used in combination as necessary.

A process of heating and drying and fixing the discharged ink composition of the present invention (heat drying process) will be described.
The heating means is not limited as long as (c) water and a water-soluble organic solvent used in combination can be dried, if necessary. Heat drum, hot air, infrared lamp, heat oven, heat plate heating, etc. Can be used.
The heating temperature is preferably 40 ° C or higher, more preferably about 40 ° C to 150 ° C, and still more preferably about 40 ° C to 80 ° C. The drying / heating time can be appropriately set in consideration of the composition and printing speed of the ink composition to be used.

  The ink composition fixed by heating is irradiated with an active energy ray in an irradiation step as necessary, and further photofixed. As described above, in the irradiation step, fixing with UV light is preferable.

Next, an image forming method using the layer forming ink set will be described.
When forming a laminated image using the ink set of the present invention, an image forming step of forming an image by discharging another colored ink composition by an ink jet recording method, and a white ink by applying a white ink composition A white ink layer applying step of forming a composition layer. A white ink composition layer may first be formed on a recording medium, and then another colored ink composition may be applied. The white ink composition may be applied by ejection from a nozzle by an inkjet recording method, or may be applied by coating, and the application method is not particularly limited.
There is no restriction | limiting in particular as an apparatus used for the said application | coating, A well-known coating apparatus can be suitably selected according to the objective. Examples include air doctor coaters, blade coaters, lot coaters, knife coaters, squeeze coaters, impregnation coaters, reverse roll coaters, transfer roll coaters, gravure coaters, kiss roll coaters, cast coaters, spray coaters, curtain coaters, extrusion coaters, etc. It is done.

Moreover, what is necessary is just to select the process order of a colored ink composition provision process and a white ink composition layer provision process suitably with a desired image.
Specifically, in order to obtain a printed material for observing an image from the recording medium side in the transparent recording medium, the other colored ink composition application step and the white ink layer application step may be performed in this order. When obtaining a printed material for observing an image from the image side, it is preferable to first apply a white ink composition layer on the recording medium and then apply another colored ink composition. In this case, it is also preferable to form a protective layer after image formation with the colored ink composition.

In the present invention, an inkjet recording method that is particularly preferably used includes a first nozzle row in which a plurality of nozzles for discharging other colored ink compositions are arranged, and a plurality of nozzles for discharging a white ink composition. A scanning step of reciprocally moving an inkjet head having a plurality of nozzle rows including a second nozzle row in a first direction with respect to the recording medium; and the recording medium in the first direction with respect to the inkjet head A relative movement step in which the nozzle array (first nozzle array and second nozzle array) is divided into a plurality of regions in the second direction, and each of the divided divisions. A discharge control step for controlling the ink discharge of the ink jet head for each unit of the nozzle region, and the ink jet head is discharged by the discharge control step. An actinic ray irradiating step of irradiating the ink adhered on the recording medium with an actinic ray, and the actinic ray irradiating step includes a plurality of actinic ray irradiation ranges corresponding to the divided nozzle regions. It is preferable that the ink jet recording method is a step of irradiating the actinic ray by dividing the region into regions and controlling the light quantity of the divided divided irradiation regions for each region.
Hereinafter, it will be described in detail with reference to the drawings.

(Overall configuration of inkjet recording apparatus)
FIG. 1 is an external perspective view showing an example of an ink jet recording apparatus suitably used in the present invention. The ink jet recording apparatus 10 is a wide format printer that forms a color image on a recording medium 12 using ultraviolet curable ink (UV curable ink). A wide format printer is a device suitable for recording a wide drawing range such as a large poster or a commercial wall advertisement. Here, the one corresponding to A3 Nobi or higher is called “wide format”.

  The ink jet recording apparatus 10 includes an apparatus main body 20 and support legs 22 that support the apparatus main body 20. The apparatus main body 20 includes a drop-on-demand type inkjet head 24 that ejects ink toward the recording medium (medium) 12, a platen 26 that supports the recording medium 12, and a guide mechanism as a head moving unit (scanning unit). 28 and a carriage 30 are provided.

  The guide mechanism 28 is disposed above the platen 26 so as to extend along a scanning direction (Y direction) perpendicular to the conveyance direction (X direction) of the recording medium 12 and parallel to the medium support surface of the platen 26. ing. The carriage 30 is supported so as to reciprocate in the Y direction along the guide mechanism 28. An ink jet head 24 is mounted on the carriage 30.

  The medium to which ink is applied by the inkjet head 24 is then transported to an ultraviolet irradiation device 34 provided on the downstream side, irradiated with ultraviolet rays as a curing light source, and the ink is completely cured to form an image.

The inkjet head 24 disposed on the carriage 30 moves integrally with the carriage 30 along the guide mechanism 28. The reciprocating direction (Y direction) of the carriage 30 may be referred to as a “main scanning direction”, and the conveyance direction (X direction) of the recording medium 12 may be referred to as a “sub scanning direction”. The Y direction corresponds to the “first direction”, and the X direction corresponds to the “second direction”.
As the recording medium 12, various media such as paper, non-woven fabric, vinyl chloride, synthetic chemical fiber, polyethylene, polyester, and tarpaulin can be used regardless of the material, regardless of permeable medium or non-permeable medium. it can. The recording medium 12 is fed in a roll paper state (see FIG. 1) from the back side of the apparatus, and after printing is wound up by a winding roller (not shown) on the front side of the apparatus. Ink droplets are ejected from the inkjet head 24 to the recording medium 12 conveyed on the platen 26, and ultraviolet rays are emitted from the ultraviolet irradiation device 34 provided on the downstream side to the ink droplets attached on the recording medium 12. Is irradiated.

  In FIG. 1, an attachment portion 38 for an ink cartridge 36 is provided on the left front surface of the apparatus main body 20. The ink cartridge 36 is a replaceable ink supply source (ink tank) that stores ultraviolet curable ink. The ink cartridge 36 is provided corresponding to each color ink used in the inkjet recording apparatus 10 of this example. Each color-specific ink cartridge 36 is connected to the inkjet head 24 by an ink supply path (not shown) formed independently. When the remaining amount of ink for each color is low, the ink cartridge 36 is replaced.

(Description of recording medium transport path)
FIG. 2 is an explanatory diagram schematically showing a recording medium conveyance path in the inkjet recording apparatus 10. As shown in FIG. 2, the platen 26 is formed in an inverted bowl shape, and its upper surface serves as a support surface (medium support surface) of the recording medium 12. A pair of nip rollers 40 that are recording medium conveying means for intermittently conveying the recording medium 12 are disposed on the upstream side in the recording medium conveying direction (X direction) in the vicinity of the platen 26. The nip roller 40 moves the recording medium 12 on the platen 26 in the recording medium conveyance direction.

  The recording medium 12 sent out from the supply-side roll (feed-out supply roll) 42 constituting the roll-to-roll type medium transport means is provided at the entrance of the printing unit (upstream side of the platen 26 in the recording medium transport direction). The pair of nip rollers 40 are intermittently conveyed in the recording medium conveyance direction. The recording medium 12 that has reached the printing unit immediately below the ink jet head 24 is printed by the ink jet head 24 and is taken up by the take-up roll 44 after printing. A guide 46 for the recording medium 12 is provided on the downstream side of the printing unit in the recording medium conveyance direction.

  A temperature adjusting unit 50 for adjusting the temperature of the recording medium 12 during printing is provided on the back surface (the surface opposite to the surface supporting the recording medium 12) of the platen 26 at a position facing the inkjet head 24 in the printing unit. Is provided. When the recording medium 12 at the time of printing is adjusted to a predetermined temperature, the physical properties such as the viscosity of the ink droplets that have landed on the recording medium 12 and the surface tension become the desired values, and the desired dot diameter is set. Can be obtained. In addition, as needed, the pre temperature control part 52 may be provided in the upstream of the temperature control part 50, and the after temperature control part 54 may be provided in the downstream of the temperature control part 50.

(Description of inkjet head)
FIG. 3 is a plan perspective view showing an example of an arrangement form of the ink jet head 24 arranged on the carriage 30.
The inkjet head 24 includes yellow (Y), magenta (M), cyan (C), black (K), light cyan (LC), light magenta (LM), clear (transparent) ink (CL), and white (white). Nozzle arrays 61Y, 61M, 61C, 61K, 61LC, 61LM, 61CL, and 61W are provided for each color of ink (W) to eject ink of the respective colors. In FIG. 4, the nozzle rows are indicated by dotted lines, and individual illustrations of the nozzles are omitted. In the following description, the nozzle rows 61Y, 61M, 61C, 61K, 61LC, 61LM, 61CL, and 61W may be collectively referred to by the reference numeral 61 to represent the nozzle rows.

  The ink color type (number of colors) and the color combination are not limited to the present embodiment. For example, a form in which the LC and LM nozzle arrays are omitted, a form in which the CL nozzle array is omitted, a form in which a metal ink nozzle array is added, and a form in which a nozzle array for discharging special color ink is added are possible. . Further, the arrangement order of the nozzle rows for each color is not particularly limited.

  By forming a head module for each color nozzle row 61 and arranging them, it is possible to form an inkjet head 24 capable of color drawing. For example, a head module 24Y having a nozzle row 61Y that discharges yellow ink, a head module 24M having a nozzle row 61M that discharges magenta ink, a head module 24C having a nozzle row 61C that discharges cyan ink, and black ink A head module 24K having a nozzle row 61K for discharging, and head modules 24LC, 24LM, 24CL, 24W having nozzle rows 61LC, 61LM, 61CL, 61W for discharging ink of each color of LC, LM, CL, W, respectively. A mode in which the carriage 30 is arranged at equal intervals so as to be aligned along the reciprocating direction of the carriage 30 (main scanning direction, Y direction) is also possible. The module group (head group) of the head modules 24Y, 24M, 24C, 24K, 24LC, and 24LM for each color may be interpreted as an “inkjet head”, or each module may be interpreted as an “inkjet head”. It is. Alternatively, a configuration in which an ink flow path is formed separately for each color inside one inkjet head 24 and a nozzle array that discharges a plurality of colors of ink with one head is also possible.

  In each nozzle row 61, a plurality of nozzles are arranged in a row (linearly) along the recording medium conveyance direction (sub-scanning direction, X direction) at regular intervals. In the inkjet head 24 of this example, the arrangement pitch (nozzle pitch) of the nozzles constituting each nozzle row 61 is 254 μm (100 dpi), the number of nozzles constituting one nozzle row 61 is 256 nozzles, and the total length of the nozzle row 61 Lw (the total length of the nozzle row) is about 65 mm (254 μm × 255 = 64.8 mm). Further, the discharge frequency is 15 kHz, and three types of discharge droplet amounts of 10 pl, 20 pl, and 30 pl can be distinguished by changing the drive waveform.

  As an ink ejection method of the inkjet head 24, a method (piezo jet method) in which ink droplets are ejected by deformation of a piezoelectric element (piezo actuator) is employed. In addition to a configuration using an electrostatic actuator (electrostatic actuator method) as a discharge energy generating element, a heating element (heating element) such as a heater is used to heat ink to generate bubbles and to eject ink droplets with that pressure. It is also possible to adopt a form (thermal jet system). However, since the ultraviolet curable ink generally has a higher viscosity than the solvent ink, when using the ultraviolet curable ink, it is preferable to adopt a piezo jet method having a relatively large ejection force.

(About drawing mode)
In the inkjet recording apparatus 10 shown in this example, multi-pass drawing control is applied, and the print resolution can be changed by changing the number of print passes. For example, three types of drawing modes, a high production mode, a standard mode, and a high image quality mode, are prepared, and the printing resolution is different in each mode. The drawing mode can be selected according to the printing purpose and application.

In the high production mode, printing is executed with a resolution of 600 dpi (main scanning direction) × 400 dpi (sub-scanning direction). In the high production mode, a resolution of 600 dpi is realized by two passes (two scans) in the main scanning direction. In the first scan (outward path of the carriage 30), dots are formed with a resolution of 300 dpi. In the second scan (return pass), dots are formed such that the middle of the dots formed in the first scan (forward pass) is interpolated at 300 dpi, and a resolution of 600 dpi is obtained in the main scan direction.
On the other hand, in the sub-scanning direction, the nozzle pitch is 100 dpi, and dots are formed at a resolution of 100 dpi in the sub-scanning direction by one main scanning (one pass). Therefore, a resolution of 400 dpi is realized by performing interpolation printing by four-pass printing (four scans). The main scanning speed of the carriage 30 in the high production mode is 1270 mm / sec.

In the standard mode, printing is performed at a resolution of 600 dpi × 800 dpi, and a resolution of 600 dpi × 800 dpi is obtained by 2-pass printing in the main scanning direction and 8-pass printing in the sub-scanning direction.
In the high image quality mode, printing is executed with a resolution of 1,200 × 1,200 dpi, and a resolution of 1,200 dpi × 1,200 dpi is obtained with four passes in the main scanning direction and 12 passes in the sub-scanning direction.

<About swath width by single ring scanning>
In the drawing mode of the wide format machine, drawing conditions for single ring (interlace) are determined for each resolution setting. Specifically, since the inkjet head discharge nozzle row width Lw (nozzle row length) is divided by the number of passes (number of scan repetitions) to produce a single ring, the nozzle row width of the ink jet head and the main scan The swath width differs depending on the number of passes in the direction and the sub-scanning direction (the number of divisions to be interlaced). Details of the single-pass drawing by the multi-pass method are described in, for example, Japanese Patent Application Laid-Open No. 2004-306617.

  As an example, the relationship between the number of passes and the swath width by shingling drawing when a QS-10 head (100 dpi, 256 nozzles) manufactured by FUJIFILM Dimatix is used is as shown in Table 2 below. The swath width assumed by the image formation is a value obtained by dividing the nozzle row width to be used by the product of the number of passes in the main scanning direction and the number of passes in the sub scanning direction.

(Arrangement of UV irradiation part)
As shown in FIG. 1, the recording medium to which ink is applied by the inkjet head 24 is conveyed into an ultraviolet irradiation device including a curing light source provided on the downstream side in the recording medium conveyance direction (X direction). The ink image applied by the inkjet head 24 is cured by moving the irradiation area of the curing light source in the recording medium conveyance direction (X direction).

  Others ejected from the nozzles (nozzles included in the nozzle arrays 61Y, 61M, 61C, 61K, 61LC, and 61LM) for the colored ink composition (other colored ink compositions) of the inkjet head 24 and landed on the recording medium 12 The droplets of the colored ink composition are sequentially applied. The ink droplets on the recording medium 12 that have passed through the print area of the inkjet head 24 along with the intermittent conveyance of the recording medium 12 pass through the ultraviolet irradiation device 34 that includes a curing light source provided downstream in the recording medium conveyance direction. At that time, ultraviolet rays for curing are irradiated. In this way, the ink droplets are in a cured state, the interaction between the droplets and the medium is promoted, and the adhesion between the recording medium and the ink image can be increased.

On the other hand, since the white ink composition layer formed from the white ink composition serves as an overcoat layer and / or an undercoat layer of the color image layer, dot resolution as high as that of the color image layer is not required. Further, since the white ink composition of the present invention has a relatively high viscosity, landing interference is unlikely to occur, and the white ink composition layer is formed with high productivity.
In the present embodiment, the ultraviolet irradiation device 34 also applies an ultraviolet ray to the ink image of the white ink composition ejected from the nozzle for the white ink composition (the nozzle included in the nozzle row 61W) and landed on the recording medium. Irradiation is performed.

(Description of image forming process)
The ink jet recording apparatus 10 shown in the present embodiment includes a color image layer formed of another colored ink composition (Y, M, C, K, LC, LM, etc.) and a white ink formed of a white ink composition. The composition layer is laminated to form an image having a layer structure. Further, the ultraviolet irradiation amount is controlled according to the order of layer formation and the ultraviolet absorption characteristics (ink curing characteristics) of the ink.

  For example, since a white ink composition contains titanium oxide, zinc oxide, or the like as a pigment, it has a lower ultraviolet transmittance than other colored ink compositions or clear ink, and other colored ink compositions. When the same amount of ultraviolet light is irradiated per unit volume as clear ink, the curing time becomes longer. In order to eliminate the difference in curing characteristics due to the ultraviolet transmission characteristics between the white ink composition and other colored ink compositions, the amount of ultraviolet irradiation per unit time for the white ink composition is higher than that of the other colored ink compositions. Ultraviolet irradiation is controlled so as to increase. A specific example of such image formation will be described later.

  Black ink compositions are classified as inks with a long curing time from the viewpoint of ultraviolet ray transmission, but they are used for the formation of color image layers and are temporarily cured immediately after droplet ejection to prevent droplet ejection interference. Therefore, it is classified into other colored ink compositions.

[Printed matter]
The printed matter of the present invention is characterized by having an image formed using the white ink composition of the present invention or an image formed by the image forming method of the present invention. The printed matter of the present invention is a printed matter excellent in blocking resistance and concealment of the formed image. In particular, a printed matter having a laminated image formed by an ink set having a white ink composition and an ink composition containing a colorant of another hue has no image blur and a transparent recording medium is used. However, it can be said that a sharp image having sufficient concealability is formed and the effect of the present invention is remarkable.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, “part” and “%” are based on mass.

[Examples 1 to 3, Comparative Examples 1 to 5]
(Preparation of white ink composition)
(White pigment dispersion: Preparation of WB-1 to WB-6)
TITANIX JR405 (titanium oxide, isoelectric point pH 8.1, manufactured by Teika Co., Ltd., primary particle size 0.21 μm) 500 g, 2-pyrrolidone 83 g, 2-methyl-1,3-propanediol (Tokyo Chemical Industry Co., Ltd.) 167 g, ion exchange water 235 g, and DISPERBYK-2015 (BYK) 15 g as a dispersant were mixed, and then 4000 rpm, temperature in an Eiger mill (product name: Eiger Japan). Dispersion was carried out at 35 ° C. for 5 hours to obtain a white pigment dispersion WB-1 having an average particle size of 485 μm. At this time, one bead was 1 mm.
White pigment dispersions WB-2 to WB-6 having different secondary average particle diameters were prepared by adjusting the Eiger mill dispersion conditions.

(Preparation of white ink composition: W-1)
(A) 286 g of a specific copolymer (Exemplary Compound A-1: the following structure) solution (Exemplary Compound A-1: 35 wt%, 2-pyrrolidone: 35 wt%, ion-exchanged water: 30 wt%), b) 600 g of the white pigment dispersion described above, (d) 17 g of high-boiling organic compound, surfactant cspstone FS-34 (24%: manufactured by DuPont Co., Ltd.) The obtained solution was stirred at room temperature at 5,000 rpm for 20 minutes using a mixer (Silverson L4R) to prepare a white ink composition W-1.
The resulting white ink composition had a viscosity (25 ° C.) of 18 mPa · s and a surface tension of 23 mN / m. The secondary average particle diameter D50 of the white pigment in the produced white ink composition was 485 nm.

(White ink composition: Preparation of W-2 to W-6)
The white pigment dispersion WB-1 used for the preparation of the white ink composition W-1 is replaced with WB-2 to WB-5 as shown in Table 3 below, and the white ink composition: W-2 to W-5 is used. Was prepared. Moreover, in the white ink composition W-6, a dispersion (EB-6) of TOTANXJA-1 (titanium oxide, manufactured by Teika Co., Ltd., primary particle size 0.18 μm: secondary average particle size 180 nm) was used as a white pigment. used.
Details of the white ink composition are shown in Table 3 below.

[Preparation of other colored ink compositions]
1. Preparation of cyan ink (a) Specific copolymer: 429 g of Exemplified Compound A-1 solution (Exemplary Compound A-1: 35 wt%, 2-pyrrolidone: 35 wt%, ion-exchanged water: 30 wt%), (b ) Cyan pigment dispersion: C dispersion Projet Cyan APD 1000 (manufactured by FUJIFILM Imaging Colorant Co., Ltd.): 133 g of pigment concentration 2%, (d) High boiling point organic compound: 2-pyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.) 50 g, (d) 100 g of MDP, surfactant cspstone FS-3424%: manufactured by DuPont Co., Ltd.) 17 g of (f) ion-exchanged water 271 g added to make 1000 g, a mixer (L4R manufactured by Silverson) Was used and stirred at 5,000 rpm for 20 minutes at room temperature to prepare cyan ink C-1. The prepared ink had a viscosity of 18 mPa · s and a surface tension of 23 mN / m.

Using the same commercially available pigment dispersions as shown below, magenta ink M-1, yellow ink Y-1, black ink K-1, light cyan ink Lc-1, Light magenta ink Lm-1 was produced. In Table 4 below, “-” indicates that the component is not contained.
Each obtained crude ink composition is packed in a plastic disposable syringe and filtered through a polyvinylidene fluoride (PVDF) pore size 5 μm filter (Millex-SV manufactured by Millipore, diameter 25 mm) to obtain a finished product. An ink composition was obtained.

M dispersion: Magenta pigment dispersion Projet Magenta APD 1000 (Fuji Film Imaging Colorant Co., Ltd.)
K dispersion: Black pigment dispersion Projet Black APD 1000 (Fuji Film Imaging Colorant Co., Ltd.)
Y dispersion: Yellow pigment dispersion Projet Yellow APD 1000 (manufactured by Fuji Film Imaging Colorant Co., Ltd.)

[Preparation of comparative latex ink]
(A) Specific copolymer used in the white ink composition W-1: Example compound A-1 and (a) specific copolymer used in the other colored ink composition used in the ink composition for layer formation of the present invention A white ink composition W- was similarly prepared except that an equivalent amount of latex (B-01) as a comparative polymer compound shown in Table 5 below was used instead of the exemplified compound (A-1) as a copolymer. 0 and other colored ink compositions, C-0, M-0, Y-0, K-0, Lc-0, and Lm-0, were prepared, respectively.

-Preparation of self-dispersing polymer particles B-01-
In a 2-liter three-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 560.0 g of methyl ethyl ketone was charged and the temperature was raised to 87 ° C. While maintaining the reflux state in the reaction vessel (hereinafter referred to as reflux until the end of the reaction), 220.4 g of methyl methacrylate, 301.6 g of isobornyl methacrylate, 58.0 g of methacrylic acid, 108 g of methyl ethyl ketone, and “V-601” (Wako Pure) A mixed solution consisting of 2.32 g (manufactured by Yakuhin Co., Ltd.) was added dropwise at a constant speed so that the addition was completed in 2 hours. After completion of the dropwise addition, the mixture was stirred for 1 hour, (1) a solution consisting of 1.16 g of “V-601” and 6.4 g of methyl ethyl ketone was added, and the mixture was stirred for 2 hours. Subsequently, the step (1) was repeated four times, and a solution consisting of 1.16 g of “V-601” and 6.4 g of methyl ethyl ketone was added, and stirring was continued for 3 hours. After completion of the polymerization reaction, the temperature of the solution was lowered to 65 ° C., and 163.0 g of isopropanol was added and allowed to cool. The weight average molecular weight (Mw) of the obtained copolymer was 63000, and the acid value was 65.1 (mgKOH / g).

Next, 317.3 g of the obtained polymerization solution (solid content concentration 41.0%) was weighed, 46.4 g of isopropanol, 1.65 g of a 20% maleic anhydride aqueous solution (based on the water-soluble acidic compound and copolymer). 40.77 g of 2 mol / L NaOH aqueous solution was added, and the temperature in the reaction vessel was raised to 70 ° C. Next, 380 g of distilled water was added dropwise at a rate of 10 ml / min to disperse in water (dispersing step).
Thereafter, under reduced pressure, the temperature in the reaction vessel was kept at 70 ° C. for 1.5 hours, and 287.0 g in total of isopropanol, methyl ethyl ketone and distilled water were distilled off (solvent removal step),
0.278 g of Proxel GXL (S) (manufactured by Arch Chemicals Japan Co., Ltd.) (440 ppm as benzoisothiazolin-3-one with respect to polymer solid content) was added. Thereafter, filtration was performed with a 1 μm filter, and the filtrate was recovered to obtain an aqueous dispersion of self-dispersing polymer particles B-01 having a solid content concentration of 26.5%. The obtained self-dispersing polymer fine particles were diluted with ion-exchanged water, and the physical properties of a liquid of 25.0% were measured. It was 2.8 nm.

<Measurement of glass transition temperature Tg>
It was 160 degreeC when the glass transition temperature of the obtained self-dispersing polymer particle (B-01) was measured with the following method.
The polymer solution after the polymerization was dried under reduced pressure at 50 ° C. for 4 hours, in particular, to obtain a solid content of 0.5 g. Using the obtained polymer solid content, Tg was measured with a differential scanning calorimeter (DSC) EXSTAR 6220 manufactured by SII Nanotechnology. The measurement condition was that a sample amount of 5 mg was sealed in an aluminum pan, and the DSC peak top value of the measurement data at the second temperature increase was defined as Tg under the following temperature profile in a nitrogen atmosphere.
30 ° C to -50 ° C (cooled at 50 ° C / min)
−50 ° C. → 120 ° C. (temperature rising at 20 ° C./min)
120 ° C to -50 ° C (cooling at 50 ° C / min)
−50 ° C. → 120 ° C. (temperature rising at 20 ° C./min)

<Measurement of volume average particle diameter Mv>
The obtained aqueous dispersion of self-dispersing polymer particles is appropriately diluted to a concentration suitable for measurement (loading index is in the range of 0.1 to 10), and ultrafine particle size distribution analyzer Nanotrac UPA-EX150 (Nikkiso Co., Ltd.) The volume average particle diameter of each aqueous dispersion was measured under the same measurement conditions by the dynamic light scattering method. That is, the measurement was performed under the conditions of particle permeability: transmission, particle refractive index: 1.51, particle shape: non-spherical, density: 1.2 g / cm ³ , solvent: water, cell temperature: 18-25 ° C.

[Evaluation of ink set]
Using the obtained white ink composition W-1 of the present invention and other colored ink compositions C-1, M-1, Y-1, K-1, Lc-1, and Lm-1, Formation was performed.
A platen heater having the same width as the drawable width was installed directly under the head carriage moving part of Accuracy LED 1600 (manufactured by Fuji Film Co., Ltd.). In addition, a dry air outlet was installed at the top of the head carriage.
Further, as shown in FIG. 1, an ultraviolet irradiation device 34 provided with an ultraviolet lamp (curing light source) having the same width as the drawable width was installed at a portion 50 cm below the paper discharge section.
Each head is filled with C-1, M-1, Y-1, K-1, Lc-1, Lm-1, which are other colored ink compositions, and W-1, which is the white ink composition of the present invention. did. However, the clear ink was not filled.

An image was formed on the recording medium using the filled ink.
As a recording medium, Viewful TP188 (manufactured by KIMOTO), which is a transparent PET film, was used.
The drawing mode is a mode of 600 dpi × 500 dpi, and the droplet size is 35 pl for each of the other colored ink compositions C-1, M-1, Y-1, K-1, Lc-1, and Lm-1, and white ink. Composition W-1 was 60 pl. The head temperature during drawing was 43 ° C. The drawn image is printed in two layers, a white ink composition layer is drawn so as to be applied at 28 g / m ² as ink, and another colored ink composition is added on top of that at a network percentage of 300% in total for each color. Drawn.
The platen temperature during image formation was 50 ° C., and the drying air temperature was 50 ° C.
For example, when an image is created with 1200 × 1200 (dots), 1440000 dots are formed, but when ink is deposited on all the points, the dot% becomes 100%, and half of the ink is 72,000. Is set to 50%, and a screen percentage of 300% indicates a state in which ink is ejected to three times the dot.

A comparative ink composition prepared by using latex instead of C-1, M-1, Y-1, K-1, Lc-1, Lm-1, and W-1 as the ink composition, C Each head was filled with -0, M-0, Y-0, K-0, Lc-0, Lm-0 and W-0.
In place of the ultraviolet lamp mounted on the apparatus used in the first embodiment, a fixing roller (fixing temperature 80 ° C./fixing time 1 second) is installed, and the same manner as in the preparation of the ink drawing sample of the first embodiment. Then, image formation was performed.
This change is because the ink composition of the present invention is cured by ultraviolet rays, whereas the comparative ink composition is cured by heat and fixed.

(Evaluation of blocking resistance of images)
The image-formed prints were stacked so that the ink images formed by applying the ink composition were in contact with each other.
As a simple test for blocking, weigh 200 kg / m ² and store it in a constant temperature bath at 80 ° C., 90 ° C. and 100 ° C. for 4 hours, then peel off and visually observe the surface state of the ink image. The following evaluation criteria were used for evaluation.
(Evaluation criteria)
1: No change is observed in the surface state of the ink image.
2: Although there is a change in surface gloss, peeling of the film surface is not recognized.
3: White spots (a state in which ink partially escapes from the printed matter) due to peeling of a small area on each surface are observed.
4: The film surface peels in an area of 1 mm square or more.
In addition, the evaluation standard 2 or more is a level with no problem in product quality.
The results are shown in Table 6 below.
In the above evaluation, it is assumed that the ambient temperature of 80 ° C. is placed on the dashboard of a car in summer, and 90 ° C. and 100 ° C. evaluate the robustness of the printed matter.

(Evaluation of image concealment)
The white ink was ejected on the Viewful TP188 (the substrate described above) in the drawing mode 600 dpi × 500 dpi mode. The droplet size was 60 pl, the drawing temperature was 43 ° C., and drawing was performed so that 28 g / m ² was applied as an ink on the substrate. On the substrate on which the white ink was discharged, the colored ink composition of C was drawn in a half of the total area of the substrate with a net percentage of 300% in total. The density difference between the drawn portion where the colored ink was drawn and the undrawn portion where the colored ink was not drawn was measured with a reflection densitometer from the opposite side of the surface on which the white ink and the colored ink were discharged.
A: The density difference between the drawn portion and the non-drawn portion is 0.01 or less, and sufficient shielding properties are obtained.
B: The density difference between the drawn portion and the non-drawn portion is more than 0.01 and less than or equal to 0.05, and the shielding property is acceptable.
C: The density difference between the drawn portion and the non-drawn portion is more than 0.05 and 0.1 or less, and the shielding limit is allowable.
D: The density difference between the drawn part and the non-drawn part exceeds 0.1, and the shielding property is not allowed.

  Examples 1 to 3 using the layer forming ink set having the white ink composition of the present invention did not block at all at 80 ° C., 90 ° C., and 100 ° C., and had sufficient image concealment properties. It can be seen that an image of a level having no practical problem is formed. On the other hand, when an ink set having a white ink composition (W-2, W-3) as a comparative product and a white ink composition (W-0) using latex, which is outside the scope of the present invention, is used. It turns out that it is inferior to the high temperature blocking aptitude. In Comparative Example 4 using the white ink composition (W-6) in which the secondary average particle diameter of the pigment was too small, there was no problem with blocking, but the image concealing property was not sufficient.

[Examples 4-1 to 4-2]
In the white ink composition (W-1) of the present invention used in Example 1, the content of titanium oxide, which is a white pigment, was changed as shown in Table 6 below, and was the same as in Example 1. The white ink compositions (W-1a) and (W-1b) were prepared and used as the ink composition for layer formation of Examples 4-1 to 4-2 similar to Example 1. Evaluation of the concealability of the image formed in the same manner as described above was performed according to the same criteria as described above.
The results are also shown in Table 6 below.

As described above, a concealing property having no practical problem can be obtained even if the titanium oxide content is lowered. On the other hand, in order to improve the shielding property, it is possible to increase the coating amount of the white ink composition per unit area (m ² ) without increasing the white pigment content, but the coating amount is increased. In such a case, the drying and curability of the image decreases as the amount of ink increases, so that wind pattern unevenness and banding occur, making it difficult to form a uniform white layer.

[Examples 5-1 to 5-5]
In the white ink composition W-1 of the present invention used in the ink set of Example 1, the content of titanium oxide as a white pigment and the content of (a) the specific copolymer (exemplary compound: A-1) The white ink composition was prepared in the same manner as in Example 1 except that the mass ratio was adjusted as described in Table 7 below, and the concealability evaluation of the ink set was performed in the same manner as in Example 1. It was.
The results shown in Table 7 below were obtained.

[Examples 6 to 13]
The same as Example 1 except that (a) the specific copolymer (Exemplary Compound: A-1) used in the white ink composition W-1 of Example 1 was changed to Exemplified Compounds A-2 to A-9. A white ink composition was prepared and evaluated in the same manner as in Example 1. As a result, both ink sets had good blocking resistance and concealment, and the results were almost the same as in Example 1. was gotten.
The structure of exemplary compound (A-1)-(A-9) used for Example 1- Example 13 is shown with a weight average molecular weight and SP value.

[Examples 14-1 to 14-2]
Instead of the white pigment TITANIX JR605 used in the white ink composition W-1 of Example 1, zinc oxide (ZnO, so-called zinc white: secondary average particle size: 300 nm), calcium carbonate (secondary average particle size: The white ink compositions of Examples 14-1 to 14-2 were prepared in the same manner as in Example 1 except that 300 nm was used, and evaluated in the same manner as in Example 1. As a result, any ink set was formed. Both the anti-blocking property and the hiding property at 80 ° C. and 90 ° C. of the image were good. However, although the blocking resistance at 100 ° C. was evaluated 2, it is at a level having no practical problem.

In addition, when the stability of the ink compositions of Examples 1 to 14-2 was evaluated according to the following criteria, it was confirmed that all were A ranks and excellent in stability.
<Ink stability>
After the ink composition is stored at 60 ° C. for 1 week, the dispersion state of the pigment in each composition is visually evaluated, and the viscosity of each composition is measured to measure the rate of change from the viscosity before storage. The ranking was based on the following criteria. The evaluation results are shown in Table 3.
-Evaluation criteria-
A: There is no generation of precipitates, and the level of increase in viscosity is less than 10% and the stability is high.
B: There is no generation of precipitates, and the increase in viscosity is 10% or more and less than 20%, and there is no problem with discharge properties. , A practically problematic level

DESCRIPTION OF SYMBOLS 10 Inkjet recording apparatus 12 Recording medium 20 Apparatus main body 22 Support leg 24 Inkjet head 26 Platen 28 Guide mechanism 30 Carriage 34 Ultraviolet irradiation apparatus 36 Ink cartridge 40 Nip roller 42 Supply side roll 44 Winding roll 46 Guide 50 Temperature control part, 52 Pre temperature Adjustment unit, 54 After temperature adjustment unit 61, 61C, 61M, 61Y, 61K, 61LC, 61LM, 61CL, 61W Nozzle array 61-1 Upstream region, 61-2 Downstream region

Claims (18)

(A) a polymer compound comprising a repeating unit (a-1) having a partial structure represented by the following general formula (1) and a repeating unit (a-2) having a hydrophilic group, (b) A white ink composition comprising a white pigment having a next average particle size of 200 nm to 500 nm in an amount of 10% by mass or more based on the total amount of the ink composition, and (c) water.

(In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure, and * represents a bonding site with a main chain or a side chain in the polymer compound.   The white ink composition according to claim 1, wherein the (b) white pigment is titanium oxide.   The content ratio of the (a) polymer compound to the (b) white pigment [(a) polymer compound / (b) white pigment] is 1/4 to 1/1 on a mass basis. The white ink composition according to claim 2.   The white ink composition according to any one of claims 1 to 3, wherein a content of the (b) white pigment with respect to the total amount of the ink composition is 25% by mass to 40% by mass.   The white ink composition according to any one of claims 1 to 4, wherein the viscosity at 25 ° C is from 15 mPa · s to 30 mPa · s.   The white ink composition according to claim 1, wherein the polymer compound (a) further includes a repeating unit (a-3) having a hydrophobic group. The repeating unit (a-1) having a partial structure represented by the general formula (1) is a repeating unit represented by the following general formula (2). 2. The white ink composition described in 1.

(In General Formula (2), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may combine with each other to form a 4- to 6-membered alicyclic structure, R ^c represents a hydrogen atom or a methyl group, Z represents a single bond, —COO— * * Or -CONR ^d -**, R ^d represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and ** represents a bonding position with X. X represents a divalent organic group. )   The repeating unit (a-2) having a hydrophilic group is a repeating unit having at least one hydrophilic group selected from alcoholic hydroxyl groups, alkyl-substituted carbamoyl groups, carboxyl groups, sulfo groups, and salts thereof. The white ink composition according to any one of claims 1 to 7.   The white ink according to any one of claims 1 to 8, wherein the repeating unit (a-3) having a hydrophobic group is a repeating unit derived from an alkyl (meth) acrylate having 4 to 22 carbon atoms. Composition.   The repeating unit (a-2) having the hydrophilic group is a repeating unit having at least one hydrophilic group selected from a carboxyl group and a salt thereof. White ink composition. The white ink composition according to any one of claims 1 to 10, wherein the solubility parameter of the (a) polymer compound in an unneutralized state is 20.7 MPa ^{1/2 to} 23.0 MPa ^1/2. object. 11. The white ink composition according to claim 1, wherein the solubility parameter of the (a) polymer compound in an unneutralized state is 21.5 MPa ^{1/2 to} 22.5 MPa ^1/2. object.   The white ink composition according to any one of claims 1 to 12, wherein the content of the organic solvent having a boiling point of 250 ° C or less is 1% by mass or less.   The white ink composition according to any one of claims 1 to 13, which is for inkjet recording. (A) a polymer compound comprising a repeating unit (a-1) having a partial structure represented by the following general formula (1) and a repeating unit (a-2) having a hydrophilic group, (b) A white pigment having an average particle size of 200 nm to 500 nm and a white ink composition containing 10% by mass or more based on the total amount of the ink composition, and (c) water;
And an ink set for forming a multilayer comprising at least one ink composition containing a colorant having a hue other than white.

(In General Formula (1), R ^a and R ^b each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and at least one of R ^a and R ^b is an alkyl group having 1 to 4 carbon atoms. R ^a and R ^b may be bonded to each other to form a 4- to 6-membered alicyclic structure, and * represents a bonding site with a main chain or a side chain in the polymer compound. An ink application step of applying the white ink composition according to any one of claims 1 to 14 onto a recording medium;
An irradiation step of irradiating the white ink composition applied on the recording medium with active energy rays;
An image forming method comprising: An ink application step of applying the white ink composition according to any one of claims 1 to 14 onto a recording medium;
(C) an ink drying step of drying and removing at least a portion of water contained in the ink composition applied on the recording medium;
An irradiation step of irradiating the ink composition applied and dried on the recording medium with active energy rays;
An image forming method comprising:   An image formed by the white ink composition according to any one of claims 1 to 14 or an image formed by the image forming method according to claim 16 or 17 on a recording medium. Prints that have.