JP2017226786A - Ink set and image formation method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a white image having a high degree of whiteness while preventing particle sedimentation in an ink composition.SOLUTION: An ink set includes a first ink composition containing a hollow resin particle (A) and a second ink composition containing a metallic compound particle (B). The first ink composition includes a coagulant (C) configured to aggregate the metallic compound particle (B) when making contact with the second ink composition.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an ink set and an image forming method using the ink set.

  In recent years, in the field of commercial printing, an image may be formed on a recording medium (base material) other than white, such as a transparent or translucent film or colored paper. In such a case, it is desirable that a white image can be formed on the recording medium.

  As a white ink composition for forming a white image, there has been proposed an ink composition using small particles of titanium oxide particles (metal compound particles) and large particles of hollow particles (Patent Document 1). ). In Patent Document 1, precipitation of titanium oxide in the white ink composition is suppressed by reducing the particle diameter of the titanium oxide particles that have a large specific gravity and are likely to settle. Further, Patent Document 1 compensates for the degree of whiteness that has been reduced by reducing the particle diameter of titanium oxide by adding hollow particles that have a small specific gravity and are difficult to settle.

JP 2014-95058 A

  However, as described in Patent Document 1, by simply mixing and using metal compound particles having a small particle size and hollow particles having a large particle size, sedimentation of particles such as pigment particles in the ink composition can be suppressed. Was insufficient.

  Accordingly, the present invention has been made in view of the above problems, and an object thereof is to form a white image having a high degree of whiteness while suppressing sedimentation of particles in an ink composition.

  An ink set according to one aspect of the present invention is an ink set having a first ink composition containing hollow resin particles and a second ink composition containing metal compound particles, wherein the first ink The composition includes an aggregating agent that aggregates the metal compound particles when contacted with the second ink composition.

  According to the present invention, a white image with a high degree of whiteness can be formed while suppressing sedimentation of particles in the ink composition.

It is a figure which shows typically each process of the image forming method which concerns on this embodiment.

  Hereinafter, the present invention will be described in detail with an example of a preferred embodiment of the present invention, but the present invention is not limited to this embodiment.

[Ink set]
The ink set according to the present embodiment is an ink set having a first ink composition (ink composition 1) and a second ink composition (ink composition 2). The ink set can form an image by mixing the first ink composition and the second ink composition at a predetermined ratio R on the substrate. The first ink composition contains hollow particles (A), and the second ink composition contains metal compound particles (B). Furthermore, the first ink composition includes an aggregating agent (C) that aggregates the metal compound particles (B) when coming into contact with the second ink composition.

  Hereinafter, each component of the ink set according to the present embodiment will be described in detail.

<First ink composition>
(Hollow resin particles (A))
The first ink composition contains hollow resin particles (A). The hollow resin particle (A) is not particularly limited as long as the outer shell is made of resin and has a cavity inside.

  Examples of the resin constituting the outer shell of the hollow resin particles (A) include acrylic resins such as polymethyl methacrylate, styrene resins, and acrylic resins such as styrene-acrylic resins, urethane resins, maleic resins, Etc. As commercially available hollow resin particles, for example, highly crosslinked hollow particles manufactured by JSR Corporation, high hollowness hollow particles, and the like can be used.

  The hollow resin particles (A) preferably have an outer shell that is liquid permeable. Since the outer shell has liquid permeability, a liquid component such as an aqueous medium of the first ink composition passes through the outer shell of the hollow resin particles (A), and the internal cavity is filled with the liquid component. Therefore, the apparent specific gravity of the entire hollow resin particle (A) can be made close to the specific gravity of components other than the hollow resin particle (A) of the first ink composition. As a result, sedimentation of the hollow resin particles (A) in the first ink composition can be suppressed. Thereby, the storage stability and ejection stability of the first ink composition can be enhanced.

  Further, when the first ink composition is discharged onto the substrate, the liquid components inside the hollow resin particles (A) are removed during drying, and the inside of the hollow resin particles (A) becomes hollow. Since the hollow resin particles (A) contain air inside, the hollow resin particles (A) form a resin layer (outer shell) and an air layer (cavity) having different refractive indexes, and effectively scatter incident light. Therefore, white color can be exhibited.

  The average particle diameter of the hollow resin particles (A) is preferably 50 nm or more and 1000 nm or less, and preferably 100 nm or more and 700 nm or less. The inner diameter of the outer shell of the hollow resin particle (A), that is, the diameter of the cavity of the hollow resin particle (A) is preferably 100 nm or more and 400 nm or less, and preferably 150 nm or more and 350 nm or less. The porosity of the hollow resin particles (A) is preferably 20% or more and 90% or less.

  The average particle diameter of the hollow resin particles (A) can be calculated from, for example, a particle size distribution obtained by measurement by a light scattering particle size distribution measurement method. At this time, the average particle diameter of the hollow resin particles (A) is a median diameter d50 calculated from the volume-based particle size distribution. As the light scattering type particle size distribution measuring device, for example, a particle size distribution meter (for example, Nanotrack UPA-EX150 (manufactured by Nikkiso Co., Ltd.)) based on the dynamic light scattering method can be used.

  The content of the hollow resin particles (A) in the first ink composition is preferably 1% by mass or more and 20% by mass or less when the total amount of the first ink composition is 100% by mass. More preferably, it is 5 mass% or more and 10 mass% or less. By setting the content of the hollow resin particles (A) in the first ink composition to 20% by mass or less, it is possible to improve inkjet discharge properties such as suppressing occurrence of clogging of the inkjet recording head. Moreover, the whiteness of the film | membrane formed can be improved because the said content shall be 1 mass% or more.

(Flocculant (C))
The first ink composition contains a flocculant (C). The aggregating agent (C) is an aggregating agent capable of aggregating the hollow resin particles (A) and the metal compound particles (B) described later. The type of the flocculant (C) is not particularly limited, but preferably metal ions can be used.

  Specific examples of the metal ion include monovalent metal ions such as sodium ion and potassium ion; divalent metal ions such as calcium ion, copper ion, nickel ion, magnesium ion, zinc ion, strontium ion and barium ion. Metal ions: Examples include, but are not limited to, trivalent metal ions such as aluminum ions, iron (III) ions, chromium (III) ions, and yttrium ions. Among these metal ions, a polyvalent metal ion that is a divalent or trivalent metal ion is preferably used because of its high ability to aggregate the metal compound particles (B).

  In order to contain these metal ions in the first ink composition, it is preferable to use a metal salt. The metal salt is a salt composed of the above-described metal ions and an anion that binds to these ions, but is required to be soluble in water. Preferred anions for forming a salt include, for example, chloride ion, nitrate ion, iodide ion, oxalate ion, chlorate ion, sulfate ion, carbonate ion, acetate ion, formate ion, benzoate ion, and lactic acid. Examples include, but are not limited to, ions, hydroxide ions, phosphate ions, and hydrogen phosphate ions. Specific examples of the metal salt include metal salts containing monovalent metal ions such as sodium chloride; magnesium chloride, magnesium sulfate, magnesium hydrogen phosphate, magnesium hydroxide, magnesium nitrate, magnesium lactate, magnesium phosphate, aluminum acetate, Aluminum sulfate, aluminum lactate, aluminum hydroxide, aluminum nitrate, aluminum chloride, aluminum sulfate, zinc acetate, zinc chloride, zinc sulfate, zinc lactate, zinc nitrate, zirconium sulfate, zirconium chloride, calcium chloride, calcium acetate, calcium benzoate, Examples include, but are not limited to, metal salts containing polyvalent metal ions such as calcium hydrogen phosphate, calcium hydroxide, calcium nitrate, barium chloride, barium acetate, and barium hydroxide.

  As the flocculant (C), one type of metal ion may be used alone, or two or more types of metal ions may be used in combination.

  The concentration of the flocculant (C) in the first ink composition is preferably set to a concentration that does not cause the hollow resin particles (A) to aggregate and cause precipitation. The concentration of the flocculant (C) in the mixed liquid when the first ink composition and the second ink composition are mixed at a ratio R is a concentration that causes the metal compound particles (B) to aggregate to cause precipitation. It is preferable that The concentration of the flocculant (C) in the mixed liquid when the first ink composition and the second ink composition are mixed at a volume ratio of 1: 1 is the content of the metal compound particles (B) in the mixed liquid. When it is 100% by mass, it is more preferably 0.5% by mass or more and 10% by mass or less. By doing in this way, in the state before mixing the 1st ink composition and the 2nd ink composition, sedimentation of hollow resin particles (A) in the 1st ink composition is controlled, and both In the state after mixing, the metal compound particles (B) can be aggregated.

  As will be described in detail later, it is preferable that the metal compound particles (B) in the second ink composition have a small particle size in order to suppress sedimentation in the second ink composition. Although the sedimentation of the metal compound particles (B) can be suppressed by reducing the particle diameter, even if an image is formed with the metal compound (B) having the same particle diameter, the degree of whiteness of the image is reduced. However, in this embodiment, the first ink composition contains the flocculant (C), and the first ink composition and the second ink composition are mixed at a predetermined ratio R to form an image. To do. Thereby, metal compound particle (B) aggregates and an aggregate is formed, and the apparent particle diameter of metal compound particle (B) increases. As a result, the degree of whiteness of the formed image can be improved.

  It should be noted that the concentration of the flocculant (C) is “concentration that does not cause aggregation by causing aggregation” or “concentration that causes aggregation to cause precipitation” as described above. This can be confirmed by the method described in the item “Evaluation of aggregating action”.

  The content of the flocculant (C) in the mixed liquid when the first ink composition and the second ink composition are mixed at a predetermined ratio R is the content of the metal compound particles (B) in the mixed liquid. When it is 100% by mass, it is preferably 0.5% by mass or more and 10% by mass or less. Moreover, it is more preferable that they are 1.0 mass% or more and 5.0 mass% or less. When the content of the flocculant (C) in the mixed liquid exceeds 10% by mass in the mixed liquid, the hollow resin particles (A) may be aggregated in the first ink composition. Moreover, when content of a coagulant | flocculant (C) is less than 0.5 mass%, there exists a possibility that a metal compound particle (B) cannot fully be aggregated.

  The content of the flocculant (C) in the first ink composition is 0.5% by mass or more when the content of the hollow resin particles (A) in the first ink composition is 100% by mass. It is preferable that it is 10 mass% or less. When the content of the flocculant (C) in the first ink composition exceeds 10% by mass, the hollow resin particles (A) may be aggregated in the first ink composition. Further, when the content of the flocculant (C) in the first ink composition is less than 0.5% by mass, when the first ink composition and the second ink composition are mixed, the metal compound particles ( B) may not be sufficiently agglomerated.

(Water (D))
The first ink composition is a composition in which each component contained in the first ink composition is dissolved or dispersed in an aqueous medium. As the aqueous medium, water (D) or a mixed solvent of water (D) and a water-soluble organic solvent (E) is used. As the water (D), it is preferable to use ion exchange water, ultrafiltration water, reverse osmosis water, ultrapure water, or the like. In addition, the content of water (D) in the first ink composition is preferably 50% by mass or more and 90% by mass or less when the total amount of the first ink composition is 100% by mass.

(Water-soluble organic solvent (E))
The first ink composition may further contain a water-soluble organic solvent (E). As the water-soluble organic solvent (E), those having an effect of preventing drying of the ink composition are preferable. Specific examples include alkyl alcohols having 1 to 4 carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol; dimethylformamide, dimethyl Amides such as acetamide; Ketones or ketoalcohols such as acetone and diacetone alcohol; Ethers such as tetrahydrofuran and dioxane; Polyalkylene glycols such as polyethylene glycol and polypropylene glycol; Ethylene glycol, propylene glycol, butylene glycol, and triethylene 2 alkylene groups such as glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol, diethylene glycol Alkylene glycols containing up to 6 carbon atoms; lower alkyl ether acetates such as polyethylene glycol monomethyl ether acetate; glycerin; ethylene glycol monomethyl (or ethyl) ether, diethylene glycol methyl (or ethyl) ether, triethylene glycol monomethyl (or Lower alkyl ethers of polyhydric alcohols such as ethyl) ether; polyhydric alcohols such as trimethylolpropane and trimethylolethane; N-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone Etc. These water-soluble organic solvents can be used singly or in combination of two or more.

  When the first ink composition contains a water-soluble organic solvent (E), the content of the water-soluble organic solvent (E) in the first ink composition is 100% by mass based on the total amount of the first ink composition. When it is, it is preferable that they are 1 mass% or more and 30 mass% or less. Moreover, it is more preferable that they are 5 mass% or more and 15 mass% or less.

(Surfactant (F))
The first ink composition may further contain a surfactant (F). As the surfactant (F), surfactants such as anionic surfactants, nonionic surfactants, and amphoteric surfactants can be used. By adding these, wettability and permeability to a recording surface such as a substrate (recording medium) can be improved. Among these, it is preferable to add an acetylene glycol surfactant. Examples of the acetylene glycol surfactant include polyoxyethylene acetylene glycol. Moreover, a commercial item can also be utilized for acetylene glycol surfactant, for example, acetylenol E100 (above, Kawaken Fine Chemical Co., Ltd. product) is mentioned.

  When the first ink composition contains a surfactant (F), the content of the surfactant (F) in the first ink composition is 100% by mass based on the total amount of the first ink composition. Sometimes, the content is preferably 0.01% by mass or more and 5% by mass or less. Moreover, it is more preferable that they are 0.1 mass% or more and 1 mass% or less.

(Other)
The first ink composition may further contain various additives such as a humectant, an antifoaming agent, a preservative, and an antifungal agent in addition to the above-described components.

<Second ink composition>
(Metal compound particles (B))
The second ink composition contains metal compound particles (B). The metal compound particle (B) is not particularly limited as long as it is a metal compound particle that can be used as a pigment. Examples thereof include metal oxides, barium sulfate, and calcium carbonate that have been conventionally used as white pigments. Examples of the metal oxide include titanium oxide (titanium dioxide), barium titanate, zirconium oxide, zinc oxide, silicon dioxide (silica), and aluminum oxide (alumina) magnesium oxide.

  Among these, titanium oxide particles are preferable as the metal compound particles (B) from the viewpoint of the high degree of whiteness. Examples of commercially available titanium oxide particle dispersions include neutral titania sol “TTO-W-5” manufactured by Ishihara Sangyo Co., Ltd. and weak alkaline slurry “STS-21”.

  The average particle size of the metal compound particles (B) is preferably 10 nm or more and 100 nm or less. By setting the average particle diameter of the metal compound particles (B) to 100 nm or less, the precipitation of the metal compound particles (B) in the second ink composition can be suppressed. In addition, by setting the average particle size of the metal compound particles (B) to 10 nm or more, the particle size of the aggregate formed by the aggregation of the metal compound (B) by the flocculant (C) can be increased. As a result, the degree of whiteness of the formed film can be improved.

  The average particle diameter of the metal compound particles (B) can be calculated from, for example, a particle size distribution obtained by measurement by a light scattering particle size distribution measuring method. At this time, the average particle diameter of the metal compound particles (B) is the median diameter d50 calculated from the volume-based particle size distribution. As the light scattering type particle size distribution measuring device, for example, a particle size distribution meter (for example, Nanotrack UPA-EX150 (manufactured by Nikkiso Co., Ltd.)) based on the dynamic light scattering method can be used.

  The content of the metal oxide particles (B) in the second ink composition is preferably 1% by mass or more and 20% by mass or less when the total amount of the second ink composition is 100% by mass, More preferably, it is 2.5 mass% or more and 10 mass% or less. By setting the content of the metal compound particles (B) in the second ink composition to 20% by mass or less, it is possible to improve inkjet discharge properties such as suppressing occurrence of clogging of the inkjet recording head. Moreover, the whiteness of the film | membrane formed can be improved because the said content shall be 1 mass% or more.

  The concentration of the metal compound particles (B) in the mixed liquid when the first ink composition and the second ink composition were mixed at a volume ratio of 1: 1 was 100% by mass based on the total amount of the mixed liquid. Sometimes, it is preferable that it is 1 mass% or more and 10 mass% or less. In this way, the degree of whiteness of the formed film can be improved.

(Water (D))
The second ink composition is a composition in which each component contained in the second ink composition is dissolved or dispersed in an aqueous medium. As the aqueous medium, water (D) or a mixed solvent of water (D) and a water-soluble organic solvent (E) is used. As the water (D), it is preferable to use ion exchange water, ultrafiltration water, reverse osmosis water, ultrapure water, or the like. Further, the content of water (D) in the second ink composition is preferably 50% by mass or more and 90% by mass or less when the total amount of the second ink composition is 100% by mass.

(High molecular organic compound (G))
The second ink composition may further contain a polymer organic compound (G). The polymer organic compound (G) functions as a binder that binds the hollow resin particles (A) and the metal compound particles (B) on the base material and improves the mechanical strength of the film. Alternatively, the polymer organic compound (G) has a function as a dispersant that improves the dispersion stability of the metal compound particles (B) in the second ink composition.

  Specific examples of the polymer organic compound (G) as the binder include resins such as acrylic resins, styrene resins, styrene-acrylic resins, and fluororesins. Moreover, it is preferable that the 2nd ink composition contains the high molecular organic compound (G) as a binder as a resin emulsion. As the high molecular organic compound (G) as a binder, for example, it is preferable to use an acrylic resin emulsion “AE986B” manufactured by E-Tech Co., Ltd.

  As the high molecular organic compound (G) as the dispersant, those that can be used in ordinary pigment ink compositions can be used without particular limitation, and examples thereof include an anionic dispersant and a nonionic dispersant. . Examples of the anionic dispersant include polyacrylic acid and styrene-acrylic acid copolymer. Examples of nonionic dispersants include polyvinyl pyrrolidone and polypropylene glycol. Among these, it is preferable to use a styrene-acrylic acid copolymer in that the dispersion stability of the metal compound particles (B) can be improved. Moreover, these dispersing agents may have a function as a binder.

  When the second ink composition contains the polymer organic compound (G), the solid content of the polymer organic compound (G) in the second ink composition is based on the total amount of the second ink composition. When the blending amount of the white pigment is 1 on a mass basis, it is preferably 0.25 or more and 2 or less. Further, it is more preferably 0.5 or more and 1 or less. By setting the ratio to 2 or less, the viscosity of the second ink composition can be kept low. Moreover, the mechanical strength of the film | membrane formed can be made high by the said ratio being 0.25 or more.

(Other ingredients)
The second ink composition may contain other components as necessary. Examples of other components that may be included in the second ink composition include a water-soluble organic solvent (E), a surfactant (F), and other various additives. Since these are the same as those described in the first ink composition, description thereof is omitted.

[Image forming method]
In the image forming method according to the present embodiment, the first ink application step of applying the first ink composition to the base material 10 (recording medium) and the second ink composition of the base material 10 (recording medium). And a second ink application step applied to the image forming method. Thereby, the first ink composition and the second ink composition are brought into contact with each other on the base material 10 to form an image on the base material 10. The “image” in this specification includes a solid pattern in which a certain range is filled with a single color.

  FIG. 1 is a diagram schematically showing each step of the image forming method according to the present embodiment. As shown in FIG. 1, in the first ink application step (FIG. 1 (a)) and the second ink application step (FIG. 1 (c)), each ink composition is ejected as droplets by an ink jet method. It is preferable that it is the process of making it adhere on the base material 10. Thereby, the resolution of the image to be formed can be increased. There is no particular limitation on the method of flying ink as droplets by the inkjet method, and for example, a thermal jet method in which droplets are ejected by applying thermal energy to the ink composition can be used. Alternatively, it is possible to use a piezo jet ink jet that ejects liquid droplets by applying mechanical energy to the ink composition.

  In the image forming method according to the present embodiment, it is preferable that the first ink composition and the second ink composition are adhered to the same region on the substrate 10 in an overlapping manner. The order in which the first ink composition and the second ink composition are attached to the same region on the substrate 10 is not particularly limited. For example, after the first ink composition is deposited on the substrate 10 (FIG. 1B), the first ink composition droplets and the second ink composition deposited on the substrate 10. The droplets of the second ink composition may be attached so that at least a part of the droplets overlap (FIG. 1 (c)). Alternatively, after the second ink composition is deposited on the substrate 10, at least part of the droplets of the second ink composition and the droplets of the first ink composition deposited on the substrate 10 A droplet of the first ink composition may be attached so as to overlap. Alternatively, the first ink composition droplet and the second ink composition droplet may land on the substrate 10 simultaneously.

  Among these, as shown in FIG. 1, after the first ink composition is deposited on the substrate 10, the first ink composition droplets and the second ink composition deposited on the substrate 10. It is preferable to attach the second ink composition droplets such that at least a part of the droplets of the object overlap. That is, the second ink application step is based on the second ink composition so that at least a part of the droplets of the first ink composition applied to the substrate 10 by the first ink application step overlaps. A step of applying to the material 10 is preferable. Thereby, the degree of whiteness of the formed image can be further increased.

  In the present embodiment, as described above, the metal compound particles (B) contained in the second ink composition are aggregated by bringing the first ink composition and the second ink composition into contact with each other on the substrate 10. (FIG. 1 (d)). Thereby, an aggregate of metal compound particles (B) is formed, and an image with a high degree of whiteness can be formed.

  In addition, in the image forming method according to the present embodiment, in addition to the ink set according to the present embodiment, a conventionally known ink containing another type of color material may be used in combination. Specifically, a full-color image including white can be formed by using each of the inks containing color materials of colors such as yellow, magenta, cyan, and black together with the ink set according to the present embodiment. it can. Or you may form the image of colors other than white on the white image formed using the ink set which concerns on this embodiment using the color ink containing another kind of coloring material.

  The type of the substrate 10 that is a recording medium is not particularly limited, and paper, polymer materials such as vinyl chloride and PET, metal, wood, fabric, glass, ceramics, and the like can be used. In addition, the shape of the base material 10 is not specifically limited, A film, a board may be sufficient, and another solid thing may be sufficient.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, the technical scope of this invention is not limited to a following example. Note that “parts” and “%” used below are all based on mass unless otherwise specified.

<Preparation of ink composition>
The following constituent materials were blended in the ratios shown in Table 1 and stirred to obtain ink compositions (inks 1 to 7). In addition, the unit of the numerical value described in Table 1 is mass%, and all are a solid content conversion density | concentration about a titanium oxide particle, a hollow particle, and a resin emulsion.

[Constituent materials]
(Hollow resin particles (A))
(A-1) Hollow particles Polymer hollow particles having an outer diameter of 400 nm and an inner diameter of 200 nm were used.

(Metal compound particles (B))
(B-1) Titanium oxide particle dispersion Commercially available “TTO-W-5” (manufactured by Ishihara Sangyo Co., Ltd.) was used. TTO-W-5 is an aqueous sol of rutile-type titanium oxide particles having an average particle diameter of several tens of nanometers, and was a neutral sol (pH 6 to 8) having a solid content concentration of about 30% by mass.

(Flocculant (C))
(C-1) Calcium ion (calcium nitrate)

(Water (D))
(D-1) Ion exchange water

(Surfactant (F))
(F-1) Acetylenol E100 (manufactured by Kawaken Fine Chemical Co., Ltd.)

(High molecular organic compound (G))
(G-1) AE986B (acrylic resin emulsion, manufactured by Etec Corporation)

<Image formation using ink set>
Example 1
Ink 1 was used as the first ink composition, and ink 6 was used as the second ink composition.

  First, 10 μL of ink 1 as the first ink composition was dropped on a 100 μm thick PET film (Tetron HL92W, manufactured by Teijin DuPont Films) using a micropipette. Next, 10 μL of the ink 6 as the second ink composition was dropped using a micropipette so as to overlap the ink 1 on the PET film. That is, the first ink composition and the second ink composition were mixed at a volume ratio of 1: 1 (ratio R = 1). Thereafter, the film was dried at room temperature for one day and night to form an image (ink film) having a film thickness of about 7 μm.

(Example 2)
Image formation was performed in the same manner as in Example 1 using Ink 2 as the first ink composition and Ink 7 as the second ink composition.

(Example 3)
An image was formed in the same manner as in Example 1 using Ink 3 as the first ink composition and Ink 7 as the second ink composition.

(Comparative Example 1)
An image was formed in the same manner as in Example 1 using Ink 4 as the first ink composition and Ink 7 as the second ink composition.

(Comparative Example 2)
An image was formed in the same manner as in Example 1 using Ink 5 as the first ink composition and Ink 7 as the second ink composition.

<Evaluation of coagulation action by coagulant>
10 ml of each prepared ink composition was put in a glass sample bottle with a capacity of 30 ml, sealed, and allowed to stand at room temperature (25 ° C.). After leaving still for 24 hours, each sample bottle was tilted slowly and the presence or absence of the deposit of the bottom of a sample bottle was confirmed. As a result, no precipitate was observed for any of the ink compositions of Inks 1 to 7. Therefore, it was confirmed that the concentration of the flocculant in the inks 1 to 3 and the ink 5 is a concentration that does not cause the hollow resin particles to aggregate and cause precipitation.

  In addition, the same evaluation was performed for the ink sets of each Example and Comparative Example 2. That is, 10 mL of the first ink composition was placed in a glass sample bottle having a capacity of 30 mL, and 10 mL of the second ink composition was added and mixed from above, and sealed and allowed to stand at room temperature (25 ° C.). After leaving still for 24 hours, each sample bottle was tilted slowly and the presence or absence of the deposit of the bottom of a sample bottle was confirmed. As a result, precipitates were observed for each ink set of each Example and Comparative Example 2. Therefore, in the ink sets of Examples and Comparative Example 2, the concentration of the flocculant in the mixed liquid when the first ink composition and the second ink composition are mixed at 1: 1 (volume ratio) is the metal The concentration was confirmed to agglomerate compound particles to cause precipitation.

<Evaluation of degree of whiteness of image>
The degree of whiteness was evaluated by visual evaluation, lightness (L * value) measurement, and concealment rate (Opacity) measurement. The brightness was measured by a SCI method including specularly reflected light using a spectrocolorimeter (manufactured by Konica Minolta, CM-2600d).

  The concealment rate was evaluated by an evaluation method based on ISO 2471: 2008. ISO 2471: 2008 is a standard relating to an opacity test method for paper and paperboard. In this example, the image concealment rate was evaluated based on this standard.

  In ISO 2471: 2008, a white plate and a black plate are each backed on paper to be tested, and the reflectance is measured. Then, the ratio (R0 / R∞ * 100) of the reflectance (R∞) when the white plate is backed to the paper to be measured and the reflectance (R0) when the black plate is backed is determined by the opacity. Or evaluation as a concealment rate (Opacity).

In this example, according to this method, the reflectance (Y _{Bg — white} ) was measured by backing an alumina plate, which is a white plate, on a film on which an image was formed. Further, the reflectance (luminous reflectance Y _{Bg_air} ) was measured such that an air layer having a sufficient thickness was disposed on the back side of the film instead of backing the black plate. This is because the concealability is measured only for the reflectance derived from the image without being influenced by the ground (film) or the back surface of the image to be evaluated. Using these reflectances, the concealment rate of the image to be measured was calculated from the equation (1). Note that the reflectance measurement conditions are a D50 light source used for the evaluation of printed matter, a measurement range of 3 mmφ, and the reflectance is measured by the SCI method for measuring the luminous reflectance of total reflected light including a regular reflection component. did.
Concealment rate (Opacity) = Y _{Bg_air} / Y _{Bg_white} * 100 Formula (1)

Here, in the formula (1), Y _{Bg_air} is a luminous reflectance measured in such a manner that an air layer having a sufficient thickness is disposed on the back side of the film on which the image is formed, and Y _{Bg_white} forms the image. It is the luminous reflectance measured by backing a white plate on the back side of the film. Table 2 shows the evaluation results of the degree of whiteness.

<Summary of results>
As compared with the images formed in Comparative Examples 1 and 2, the images formed in Examples 1 to 3 had high brightness (L * value) and concealment rate (Opacity), and a high degree of whiteness. This is because in Examples 1 to 3, the titanium oxide particles as the metal compound particles (B) are aggregated by the calcium ions as the aggregating agent (C), and the formed ink film contains a large number of aggregates of the titanium oxide particles. This is thought to be because of As a result, it is considered that light can be reflected or scattered more efficiently than the presence of titanium oxide particles having a small particle diameter, and the degree of whiteness has been improved. Further, not only is the light reflected or scattered by the aggregates of the titanium oxide particles, but also the light is scattered by the voids of the hollow particles, so that the lightness (L * value) and concealment rate (Opacity) have increased. Conceivable.

  Since Comparative Example 1 is a combination that does not include a metal salt, an aggregate of titanium oxide particles is not generated. It is considered that the brightness and the concealment rate are reduced because the whiteness of titanium oxide particles having a small particle diameter is insufficient.

  Since Comparative Example 2 is a combination that does not include hollow particles, voids are unlikely to be formed in the ink film, and the size of the voids formed is considered to be very small. It is considered that the degree of whiteness was not sufficiently high only with the aggregate of titanium oxide particles, and the brightness and the concealment rate were reduced.

Claims (13)

An ink set having a first ink composition containing hollow resin particles and a second ink composition containing metal compound particles,
The ink set comprising: an aggregating agent that aggregates the metal compound particles when the first ink composition comes into contact with the second ink composition. The flocculant is capable of aggregating the hollow resin particles,
The ink set according to claim 1, wherein the concentration of the flocculant in the first ink composition is a concentration that does not cause the hollow resin particles to aggregate and cause precipitation.   The ink set according to claim 1, wherein the flocculant is a metal ion.   The ink set according to claim 1, wherein the flocculant is a polyvalent metal ion.   The ink set according to any one of claims 1 to 4, wherein an average particle diameter of the metal compound particles in the second ink composition is 10 nm or more and 100 nm or less.   The ink set according to any one of claims 1 to 5, wherein an average particle diameter of the hollow resin particles in the first ink composition is 50 nm or more and 1000 nm or less.   The content of the flocculant in the mixed liquid when the first ink composition and the second ink composition are mixed at a volume ratio of 1: 1 is the content of the metal compound particles in the mixed liquid. The ink set according to any one of claims 1 to 6, wherein the ink set is 0.5 mass% or more and 10 mass% or less when the content is 100 mass%.   The content of the metal compound particles in the mixed liquid when the first ink composition and the second ink composition are mixed at a volume ratio of 1: 1 is 100% by mass based on the total amount of the mixed liquid. 8. The ink set according to claim 1, wherein the ink set is 1% by mass or more and 10% by mass or less.   The ink set according to claim 1, wherein the flocculant is calcium ions.   The ink set according to claim 1, wherein the metal compound particles are titanium oxide particles. A first ink application step of applying a first ink composition containing hollow resin particles to a substrate;
A second ink application step of applying a second ink composition containing metal compound particles to the substrate, the image forming method comprising:
The image forming method, wherein the first ink composition includes an aggregating agent that aggregates the metal compound particles when the first ink composition comes into contact with the second ink composition. The first ink application step is a step of applying the first ink composition to the substrate by flying as a droplet by an inkjet method,
The image forming method according to claim 11, wherein the second ink application step is a step of applying the second ink composition as droplets by an ink jet method to the substrate.   In the second ink application step, the droplets of the second ink composition are at least partially combined with the droplets of the first ink composition applied to the substrate by the first ink application step. 13. The image forming method according to claim 11, wherein the second ink composition is applied to the base material so as to overlap each other.