Metallic ink for inkjet printers and inkjet recording method

Abstract

To provide metallic ink for inkjet in which tabular metal particles can be stably ejected together with resin particles as metallic ink for inkjet, and can improve abrasion resistance of a printed area, and an inkjet recording method using the metallic ink.SOLUTION: A metallic ink has tabular metal particles and resin particles, where a volume-based median diameter (DP50) of the tabular metal particles is less than 500nm, an average thickness of the tabular metal particles is 50nm or less, and the volume-based median diameter (DP50) of the tabular metal particles / the volume-based median diameter (D50) of the resin particle is 4.0 or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to metallic ink for inkjet. The present invention also relates to an inkjet recording method using the metallic ink.

Background Art

[0002] Conventionally, inks containing metal pigments such as brass, aluminum, and silver have been used to form images having metallic luster in printed materials such as packages, labels, and photographs. Further, as a method for forming such an image, analog printing techniques such as offset printing, screen printing, and gravure printing have been used. In addition, foil stamping using a metal foil is also known.

[0003] In recent years, with the development of digital printing technology, a method for forming an image having metallic luster by an inkjet method has been proposed. Further, from the viewpoints of the global environment and safety to the human body, it has been developed into an inkjet recording method using an aqueous ink containing only a small amount of an organic solvent.

[0004] In the above analog printing technology, an aluminum pigment having a flat plate shape has been used as one of the pigments capable of expressing metallic luster. As an example of applying a flat aluminum pigment to an inkjet recording method, Patent Document 1 can be cited. In Patent Document 1, it aims to provide a high metallic specular gloss showing numerical values of 200, 200, and 100 or more for the specular gloss at 20 degrees, 60 degrees, and 85 degrees, respectively. Therefore, when the major axis on the plane of the flat metal particles is X, the minor axis is Y, and the thickness is Z, the average particle diameter R50 of the equivalent circle diameter obtained from the area of the X-Y plane of the particles is 0.5 to 3 μm, and a pigment dispersion is used as ink satisfying the condition of R50 / Z>5. On the other hand, for metallic ink for inkjet, which is representative of digital printing technology, an example has also been proposed in which granular metal nanoparticles, particularly silver nanoparticles, are prepared and used for printing in order to eject the ink from a fine nozzle.

[0005] Although a binder resin was used because metal nanoparticles could not achieve sufficient adhesion to the substrate (also called a recording medium), particularly non-absorbent substrates, sufficient metallic luster could not be obtained. To obtain sufficient metallic luster, Patent Document 2 proposes an inkjet ink containing metal nanoparticles and emulsion resin particles, wherein the average particle size of the metal nanoparticles and the average particle size of the emulsion resin particles are adjusted to a predetermined relationship. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-94217 [Patent Document 2] International Publication No. 2018 / 181080 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The flat metal particles used in Patent Document 1 have a 50% average particle diameter R50 of 0.5 to 3 μm. In inkjet printing, nozzles such as ink channels and ejection ports in the printing apparatus are miniaturized to form more precise images, and printing may not be possible with the flat metal particles described in Patent Document 1. Furthermore, in the case of flat metal particles, their adhesion to the substrate has not been sufficiently studied, and it is unclear how the abrasion resistance, which indicates the degree of adhesion of the flat metal particles, will change when resin particles such as those in Patent Document 2 are used in combination.

[0008] The object of the present invention is to provide an inkjet metallic ink that can stably eject flat metal particles together with resin particles as an inkjet metallic ink, thereby improving the abrasion resistance of the printed area. Another object of the present invention is to provide an inkjet recording method using the metallic ink. [Means for solving the problem]

[0009] According to one aspect of the present invention, a plate-shaped metal particle and a resin particle are included. For inkjet It is metallic ink, The volume-based median diameter (DP50) of the aforementioned flat metal particles is 300 nm or more and less than 500 nm. The average thickness of the aforementioned flat metal particles is 50 nm or less. The content of the flat metal particles is 0.2% by mass or more and 10% by mass or less relative to the total mass of the inkjet metallic ink. The content of the resin particles is 0.2% by mass or more and 10% by mass or less relative to the total mass of the inkjet metallic ink. The median diameter (DP50) of the flat metal particles based on volume / the median diameter (D50) of the resin particles based on volume is 4.0 or greater. 20.0 or less A metallic inkjet ink is provided, characterized by the following:

[0010] Furthermore, the present invention relates to an inkjet recording method for printing on a recording medium using the above-mentioned metallic ink for inkjet. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an inkjet metallic ink that can stably eject flat metal particles together with resin particles as an inkjet metallic ink, thereby improving the abrasion resistance of the printed area. Furthermore, according to the present invention, it is possible to provide an inkjet recording method using the metallic ink. [Modes for carrying out the invention]

[0012] The metallic inkjet ink of the present invention (hereinafter also simply referred to as "ink") contains flat metal particles and resin particles.

[0013] <Flat metal particles> The plate-shaped metal particles used in this invention are particles made of metal that have a flat surface and a thin thickness in the direction intersecting the direction of the surface. In other words, they are flattened particles with a high aspect ratio, where the diameter of the circle-like part of the flat surface is large relative to the thickness.

[0014] The pigments commonly found in inks are spherical. Furthermore, these pigments may adhere to the recording medium through penetration, physical adsorption, and chemical interactions, or they may have an auxiliary effect on adhesion to the recording medium, or they may have been processed to improve their adhesion. On the other hand, the flat metal particles contained in the metallic ink used in this invention have the characteristic of being less likely to adhere to the recording medium due to their shape.

[0015] In the present invention, a metallic ink containing resin particles and liquid is prepared to improve the fixability of the plate-shaped metal particles. Then, the metallic ink is ejected onto a liquid-absorbent recording medium, such as paper, by an inkjet method. As a result, the liquid in the metallic ink that adheres to the recording medium is absorbed by the recording medium, and the plate-shaped metal particles and resin particles level out on the recording medium. Since the resin particles have a smaller median diameter by volume than the plate-shaped metal particles, the plate-shaped metal particles are fixed onto the recording medium so that the plane of the metal particles is parallel to the surface of the recording medium.

[0016] The size of the aforementioned flat metal particles can be measured and analyzed using a wet particle image analyzer (for example, product name: FPIA-3000, manufactured by Sysmex Corporation, or product name: IF-3200, manufactured by Jusco International, etc.). The sizes of the flat metal particles measured using the aforementioned device, based on a volume-based particle distribution of the diameter equivalent to a circle on a flat surface, are described below at 50% integration (median diameter (DP50)) and 99% integration (median diameter (DP99)).

[0017] The flat metal particles used in the present invention have a DP50 of less than 500 nm, preferably 400 nm or less. Further, the DP99 is preferably 1 μm or less, more preferably 900 nm or less. When the DP50 is 500 nm or more or the DP99 exceeds 1 μm, clogging occurs in the ink flow path or near the ejection port in an inkjet printing apparatus, and stable ejection becomes impossible. From the relationship with the resin particles described later, the DP50 is preferably 300 nm or more.

[0018] The thickness of the flat metal particles of the present invention is in the range of 50 nm or less, preferably 40 nm or less. Further, from the viewpoint of maintaining the shape on the flat plate, it preferably has a thickness of 10 nm or more, more preferably 20 nm or more. When the thickness of the flat metal particles is greater than 50 nm, when forming a film in the process of producing the flat metal particles, unevenness occurs on the surface of the film-forming material, and the light reflectivity of the flat metal particles obtained by processing from the film-forming material decreases. Further, it takes time to reach the above thickness. Also, if the thickness of the flat metal particles becomes too thin, unevenness (dot-like or island-like) is partially formed in the film formation. As a result, when the film formation is processed into flat metal particles, the average particle size may deviate from the desired value. Consequently, sufficient fixing properties may not be obtained when ejected onto the recording medium. That is, by setting the thickness of the flat metal particles within the above range, the fixing property during inkjet printing becomes good. The thickness of the flat metal particles can be measured, for example, by cutting a place where the flat metal particles are fixed after printing on a recording medium with a microtome at room temperature or cooled to expose the cross section and observing and measuring the length with an electron microscope.

[0019] The flat metal particles are not limited in material as long as the desired metallic luster can be formed by the inkjet method, but those having light reflection without coloring when reflecting light on a flat surface are suitable, and aluminum or its alloy is preferably used from the viewpoint of cost. In addition, single materials such as titanium, nickel, chromium, tin, zinc, platinum, silver, etc., or alloys and mixtures thereof are included.

[0020] The method for producing the flat metal particles is not particularly limited as long as flat metal particles having a substantially uniform thickness can be obtained. From the viewpoint of easily obtaining metal particles with a uniform thickness, a production method by a film formation method is preferable. For example, a release layer is formed on the surface of a sheet-like resin substrate, and then a metal or alloy layer is laminated on the release layer by vacuum evaporation or sputtering. Thereafter, the release layer and the metal or alloy layer are separated in an organic solvent, and the metal or alloy layer is crushed to adjust the particle size, thereby obtaining the flat metal particles of the present invention. It is preferable to use the flat metal particles produced in the above steps after performing the surface treatment described in the present invention. As described above, aluminum or an alloy thereof is preferably used as the metal material. In that case, in order to prevent aluminum or an alloy thereof from reacting with moisture and becoming aluminized and whitened, it is preferable to provide a surface coating layer on the surface of aluminum or an alloy thereof. The surface coating layer may contain inorganic compounds such as phosphoric acid, silica, chromium, carbon, and organic compounds such as silane coupling agents. Whether aluminum has a surface treatment layer can be confirmed by a method such as X-ray diffraction.

[0021] The flat metal particles in the ink are preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less based on the total mass of the ink.

[0022] <Resin particles> The resin particles used in this invention are granular materials made of resin. Furthermore, the resin particles used in this invention are not limited by material and include, for example, acrylic resin, urethane resin, polyolefin resin, polyester resin, vinyl chloride resin, epoxy resin, polysiloxane resin, fluororesin, styrene resin, butadiene resin, etc. In particular, water-soluble compounds containing materials that exhibit good adhesion to plate-shaped metal particles and recording media are preferred. Examples include saturated polyester resins containing terephthalic acid, acrylic-modified polyester resins, vinyl chloride-acrylic mixed resins, urethane polyol resins, etc., and composites or mixtures thereof. These are used in inks where they are stably dispersed in the form of polymer resins, emulsions (including core-shell types), colloids, etc. The particle size of resin particles can be measured and analyzed using a dynamic light scattering device (e.g., product name: NanoTrac WAV, manufactured by MicroTrac; product name: Dynamic Light Scattering Photometer, manufactured by Otsuka Electronics, etc.). Using the above device, the particle size of resin particles based on a 50% cumulative (median diameter (D50)) in the volume-based particle distribution is described below. The resin particles used in this invention have a median diameter (DP50) of the volume of the flat metal particles divided by the median diameter (D50) of the resin particles that is 4.0 or greater, and preferably 5.0 or greater. Furthermore, the median diameter (D50) of the resin particles is preferably 10 nm or more and 80 nm or less, and more preferably 20 nm or more and 60 nm or less. The resin particles are preferably in an amount of 0.1% to 20% by mass, and more preferably 0.2% to 10% by mass, relative to the total mass of the ink.

[0023] <Components other than metal particles and resin particles> The components of the metallic inkjet ink in this invention, other than metal particles and resin particles, are described below.

[0024] In the present invention, when using an aqueous ink as the metallic ink for inkjet printing, it is preferable to use pure water or ultrapure water such as ion-exchanged water, ultrafiltered water, reverse osmosis water, or distilled water for the aqueous ink. Furthermore, it is preferable to sterilize these waters by ultraviolet irradiation or hydrogen peroxide addition, as this can suppress the growth of mold and bacteria. In addition, it is preferable that the amount of water used in the aqueous ink is 0.2% by mass or more and 99% by mass or less of the total mass of the ink.

[0025] The aqueous ink in the present invention may contain a water-soluble organic solvent that has excellent compatibility with water. Examples of water-soluble organic solvents include amides such as dimethylformamide and dimethylacetamide; ketones such as acetone; ethers such as tetrahydrofuran and dioxane; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; alkylene glycols containing 2 to 6 carbon atoms in an alkylene group, such as ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol, and diethylene glycol; glycerin; lower alkyl ethers of polyhydric alcohols such as ethylene glycol monomethyl (or ethyl) ether, diethylene glycol monomethyl (or ethyl) ether, and triethylene glycol monomethyl (or ethyl) ether; cyclic amide compounds such as N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, triethanolamine, sulfolane, dimethylsulfoxide, 2-pyrrolidone, and ε-caprolactam, and imide compounds such as succinimide. These can be used individually or in combination of two or more as needed. Furthermore, the content of the water-soluble organic solvent is preferably 1% to 40% by mass, and more preferably 3% to 30% by mass, relative to the total mass of the ink. Furthermore, the ink according to the present invention can also be a water-free ink. In that case, various organic solvents, particularly polar organic solvents, can be used as the liquid medium. Examples of polar organic solvents include alcohols (e.g., methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, isopropyl alcohol, or fluorinated alcohol), ketones (e.g., acetone, methyl ethyl ketone, or cyclohexanone), carboxylic acid esters (e.g., methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, or ethyl propionate), or ethers (e.g., diethyl ether, dipropyl ether, tetrahydrofuran, or dioxane). Furthermore, as a water-free ink, one or more of the following can be used as a liquid medium: polyalkylene glycols such as polyethylene glycol and polypropylene glycol; alkylene glycols containing 2 to 6 carbon atoms in an alkylene group, such as ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol, and diethylene glycol; glycerin; and lower alkyl ethers of polyhydric alcohols, such as ethylene glycol monomethyl (or ethyl) ether, diethylene glycol monomethyl (or ethyl) ether, and triethylene glycol monomethyl (or ethyl) ether.

[0026] The metallic inkjet ink of the present invention may contain a surfactant, and for example, any of the various surfactants conventionally used in the preparation of inkjet inks can be used. Examples of surfactants include anionic surfactants such as dialkyl sulfosuccinates, alkylnaphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants such as alkylamine salts and quaternary ammonium salts; and silicone-based and fluorine-based surfactants. These surfactants may be used individually or in combination of two or more as appropriate. The surfactant content is preferably 0.001% by mass or more and 2% by mass or less based on the total mass of the ink.

[0027] A dispersant may be used in the metallic inkjet ink of the present invention. The dispersant preferably has both a hydrophobic function that has an affinity for aluminum within its molecule and a hydrophilic function that enables dispersion in water, and it is preferable that the hydrophilicity is achieved by both ionic and nonionic functions. For example, a dispersant having hydrophobic and hydrophilic (ionic and nonionic) functions, or a combination of a dispersant having hydrophobic and hydrophilic (ionic) functions and a dispersant having hydrophobic and hydrophilic (nonionic) functions can be used. Specifically, such a dispersant can be a hydrophilic resin or surfactant that has both hydrophobic and hydrophilic functions, or a combination thereof, and a typical material is JONCRYL RESIN (manufactured by BASF). The content of the dispersant is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less, based on the total mass of the pigment in the ink.

[0028] In the metallic ink of the present invention, linear or branched C5-C9 alkanediols can be used to improve continuous ejection. Linear alkanediols are particularly preferred, and 1,2-diols are more preferred. Specific examples of preferred linear compounds include 1,2-pentanediol, 1,2-hexanediol, 1,2-heptanediol, 1,2-octanediol, 1,2-nonanediol, 2-methyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, and 2,4-diethyl-1,5-pentanediol. Such alkanediols are preferably present in an amount of 1% by mass or more and less than 20% by mass, and more preferably 1% by mass or more and 10% by mass or less, relative to the total mass of the ink.

[0029] The inkjet recording method in the present invention is a method of recording an image on a recording medium by ejecting the metallic ink described above from an inkjet recording head. In the inkjet recording method of the present invention, it is preferable to employ a method of imparting thermal energy to the ink as the method of ejecting the metallic ink. Aside from the use of metallic ink in the present invention, the steps of the inkjet recording method may be those of known origin.

[0030] Furthermore, in the inkjet recording method of the present invention, the metallic ink according to the present invention can be used in combination with conventional chromatic inks. Since the metallic ink according to the present invention has high opacity due to its flat metal particles, it is preferable to apply the metallic ink according to the present invention first, and then apply the chromatic ink.

[0031] <Recording medium> There are no particular restrictions on the recording medium, and examples include inkjet glossy paper, inkjet matte paper, inkjet raster paper, and offset coated paper. Resin films such as polyvinyl chloride and polyethylene terephthalate, glass, and printed circuit boards can also be used. The recording medium may be pre-treated before image formation. By adjusting the surface tension of the recording medium through pre-treatment and controlling its wettability to the ink, the dot area per drop of ink ejected onto the recording medium can be adjusted. Examples of pre-treatment include corona treatment, plasma treatment, and flame treatment. Furthermore, when using water-based ink, it is preferable that the surface of the recording medium has moisture-absorbing properties. This can be done by using a material such as paper that itself has liquid (moisture) absorbency, or by providing a liquid-absorbing layer on a substrate that does not have liquid absorbency.

[0032] The recording medium preferably has a surface roughness Ra of 2 μm or less. When the surface roughness is 2 μm or less, the approximately flat metal particles on the recording medium tend to be oriented in the same direction, resulting in increased specular reflection and a higher metallic luster. The surface roughness is measured using a laser microscope (product name: VK9710, manufactured by Keyence Corporation) to determine the arithmetic mean roughness (Ra) of a 1.4 mm × 1.1 mm area.

[0033] <Recording device> The recording apparatus used in the inkjet recording method of the present invention, particularly the structure around the recording head that ejects ink, is not limited as long as it can stably eject the metallic ink according to the present invention.

[0034] Here, if the diameter of the nozzle opening of the recording head of the inkjet recording device is Yμm and the opening of the mesh filter provided in the ink introduction path is Zμm, it is preferable that the relationship between the nozzle diameter Yμm and the opening of the mesh filter Zμm satisfies Y≧5Z. When using the metallic ink of this embodiment in an inkjet recording device that satisfies such a relationship, it is preferable that the condition Z≧2X is satisfied, where the median diameter (DP99) of the metal particles is X. This prevents clogging of the nozzle opening with flat metal particles and ensures stable ink ejection. [Examples]

[0035] The present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0036] [Example 1] (Preparation of a dispersion of flat metal particles) A solution of cellulose acetate butyrate (CAB, butylation rate 35-39%, manufactured by Eastman Chemical) adjusted to 3% by mass was uniformly coated onto a PET film with a thickness of 100 μm using the bar coating method, and dried at 60°C for 10 minutes to form a 30 μm thick film (release layer). Next, using a vacuum deposition apparatus (product name: EME-400 vacuum deposition apparatus, manufactured by ULVAC), aluminum was deposited onto the exfoliation layer as a raw material for flat metal particles to a thickness equivalent to 20 nm, as measured by a film thickness gauge. Subsequently, the exfoliation layer with the deposited aluminum was placed in an alkylene glycol monoether solvent and subjected to ultrasonic irradiation (product name: ASU-20M, manufactured by AS ONE Corporation) for 10 minutes to remove the material. Next, after removing the PET from the solvent, a 10% by mass aqueous solution of disodium hydrogen phosphate was added as a surface treatment agent, and the mixture was filtered using a SUS filter with a permeable particle size of 10 μm to remove coarse particles. Then, an ultrasonic disperser (product name: PR-1, manufactured by Thinky Co., Ltd.) was used to perform ultrasonic irradiation (24 hours, 40°C) to prepare a dispersion. Next, the dispersion was subjected to centrifugation (10,000 rpm, 30 minutes) to recover the precipitate consisting of aluminum flakes, and pure water was added to disperse the precipitate until it reached 10% by mass. The above steps of centrifugation, precipitate recovery, and dispersion with pure water were repeated a total of three times. After that, the surface treatment agent was removed to obtain an aqueous dispersion of aluminum flakes replaced with water. At this time, a portion of the precipitate was separately recovered, and X-ray diffraction using CuKα rays was performed on the surface of the aluminum flakes using an X-ray diffractometer (product name: Empyrean, manufactured by Malvern Panalytical). As a result, a diffraction peak with a diffraction angle of 25° and a full width at half maximum of 6.0° was measured, and the material on the surface of the aluminum flakes was identified as an amorphous form of a phosphate compound. This result confirmed that the surface of the aluminum flakes had been surface-treated with a phosphate compound. Furthermore, the particle size of the aluminum flakes in the aqueous dispersion was measured using a wet particle image analyzer (product name: FPIA-3000, manufactured by Sysmex Corporation).

[0037] (Resin particles) A saturated polyester resin containing terephthalic acid (product name: Pluscoat Z-446, manufactured by Go-O Chemical Co., Ltd.) was used as the resin particles. The resin particles were diluted with pure water to a concentration of 0.1% by mass, and the particle size of the resin particles was measured using a dynamic light scattering device (product name: NanoTrac WAV, manufactured by MicroTrac Corporation).

[0038] (Preparation of metallic ink) The metallic ink used in this example was prepared with the composition described below. 10 parts by mass of an aqueous dispersion containing 10% by mass of the aluminum flakes prepared above. Resin particles (saturated polyester resin) 1 part by mass 5 parts by mass of glycerin Surfactant (acetylene glycol) 1 part by mass (Product name: Acetyleneol E100, manufactured by Kawaken Fine Chemical Co., Ltd.) Triethylene glycol 5 parts by mass Pure water 78 parts by mass

[0039] (Measurement of particle size of aluminum flakes and resin particles after ink preparation) The metallic ink prepared above was centrifuged (10,000 rpm, 30 minutes) to separate it into a precipitate consisting of aluminum flakes and a supernatant consisting of other components. The precipitate was redispersed in a solution in which the same mass% of pure water as the aluminum flakes in the metallic ink of this embodiment was replaced, and the particle size of the aluminum flakes was measured using the wet particle image analyzer. As a result, the particle size was the same as that of the aluminum flakes in the aqueous dispersion before the preparation of the ink. Furthermore, the supernatant was removed, and the particle size of the resin particles was measured using the dynamic light scattering apparatus. The result was that it was the same as the particle size of the resin particles in the aqueous dispersion before the ink was prepared as described above. These results indicate that the aluminum flakes and resin particles used in this embodiment did not change in particle size even when included in the ink as a component of the metallic ink using the method described in this embodiment.

[0040] (Printing using an inkjet recording device) In this embodiment, the metallic ink obtained was used to print a solid A4 image onto an A4-sized recording medium (product name: Canon Photo Paper Gloss Gold, manufactured by Canon Inc.) under normal temperature and humidity conditions using an inkjet recording device (product name: PIXUS Pro-10, manufactured by Canon Inc.). At this time, the nozzle diameter (discharge port diameter) of the inkjet recording device was 15 μm, and the opening of the mesh filter installed in the flow path was 3 μm.

[0041] (Measurement of the thickness of aluminum flakes contained in the printed area) A portion of the printed area was cut out, sandwiched between PET films, and fixed in place. The cross-section was then formed and exposed using a microtome. The exposed cross-section was treated with a conductive agent according to standard procedures, and then observed and measured using a high-resolution scanning electron microscope (product name: SU-8030, manufactured by Hitachi High-Technologies Corporation).

[0042] (Evaluation of scratch resistance of printed images) The abrasion resistance of the printed image (solid color image) obtained above was evaluated using a Japan Society for the Promotion of Science (JSPS) type friction tester (product name: AB-301 device, manufactured by Tester Sangyo Co., Ltd.). Specifically, the friction element was brought into contact with the printed image, a load (200g) was applied to the friction element, and the test was performed by reciprocating motion (10 times) in the horizontal direction. At that time, a visual evaluation was performed according to the following criteria. A: There are almost no friction marks on the printed image area, and there is no peeling. B: The print image shows traces of the friction element moving back and forth, but there is no peeling. C: Part of the printed image peels off, exposing the recording medium in the peeled area. D: The printed image is completely removed, exposing the recording medium. E: Unable to eject, and unable to print on the recording medium.

[0043] (Evaluation of metallic luster) The metallic luster of the printed image obtained above was evaluated. The evaluation criteria are as follows. In this invention, A and B are considered desirable levels, and C is considered an unacceptable level in the evaluation criteria below. A: Upon visual observation, a metallic sheen and the outline of the reflected image can be confirmed. B: Upon visual inspection, it has a metallic sheen, but the outline of the reflected image cannot be confirmed. C: Upon visual observation, the reflected image lacks a metallic sheen, and the outline cannot be confirmed.

[0044] [Example 2] A metallic ink was prepared using the method and ink composition described in Example 1, except that Grade 701 of Vinibran (registered trademark) manufactured by Nisshin Chemical Co., Ltd., which has a vinyl chloride-acrylic mixed resin as the resin particles, was used. Using the prepared ink, the particle size of the resin particles and the abrasion resistance of the printed image were evaluated in the same manner as in Example 1.

[0045] [Example 3] A metallic ink was prepared using the method and ink composition described in Example 1, except that "PESRESIN A-645GH," a product manufactured by Takamatsu Oil & Fat Co., Ltd., which has an acrylic-modified saturated polyester resin backbone, was used as the resin particles. Using the prepared ink, the particle size of the resin particles and the abrasion resistance of the printed image were evaluated in the same manner as in Example 1.

[0046] [Example 4] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 1, except that the thickness of the aluminum flakes in Example 1 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0047] [Example 5] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 2, except that the thickness of the aluminum flakes in Example 2 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0048] [Example 6] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 3, except that the thickness of the aluminum flakes in Example 3 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0049] [Example 7] In Example 1, the aluminum dispersion was subjected to ultrasonic irradiation using the ultrasonic disperser for 12 hours, and the median diameter (DP50) of the aluminum flakes was set to 400 nm. Otherwise, a metallic ink was prepared using the method, ink composition, and resin particles described in Example 1. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0050] [Example 8] In Example 2, the aluminum dispersion was subjected to ultrasonic irradiation using the ultrasonic disperser for 12 hours, and the median diameter (DP50) of the aluminum flakes was set to 400 nm. Otherwise, a metallic ink was prepared using the method, ink composition, and resin particles described in Example 2. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0051] [Example 9] In Example 3, the aluminum dispersion was subjected to ultrasonic irradiation using the ultrasonic disperser for 12 hours, and the median diameter (DP50) of the aluminum flakes was set to 400 nm. Otherwise, a metallic ink was prepared using the method, ink composition, and resin particles described in Example 3. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0052] [Example 10] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 7, except that the thickness of the aluminum flakes in Example 7 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0053] [Example 11] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 8, except that the thickness of the aluminum flakes in Example 8 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0054] [Example 12] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 9, except that the thickness of the aluminum flakes in Example 9 was set to 40 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0055] [Example 13] (Preparation of a dispersion of flat metal particles) A solution of cellulose acetate butyrate (CAB, butylation rate 35-39%, manufactured by Eastman Chemical) adjusted to 3% by mass was uniformly coated onto a PET film with a thickness of 100 μm using the bar coating method, and dried at 60°C for 10 minutes to form a 30 μm thick film (release layer). Next, using a vacuum deposition apparatus (product name: EME-400 vacuum deposition apparatus, manufactured by ULVAC), aluminum was deposited onto the exfoliation layer as a raw material for flat metal particles to a thickness equivalent to 20 nm, as measured by a film thickness gauge. Subsequently, the exfoliation layer with the deposited aluminum was placed in an alkylene glycol monoether solvent and subjected to ultrasonic irradiation (product name: ASU-20M, manufactured by AS ONE Corporation) for 10 minutes to remove the material. Next, after removing the PET from the solvent, the material was filtered through a SUS filter with a permeability particle size of 10 μm without adding any surface treatment agent to remove coarse particles. Then, the dispersion was prepared by ultrasonic irradiation (12 hours, 40°C) using the ultrasonic disperser. Next, the dispersion was centrifuged (10,000 rpm, 30 minutes) to recover a precipitate consisting of aluminum flakes, and diethylene glycol monomethyl ether (manufactured by Nisshin Chemical Co., Ltd.) was added to disperse the precipitate to a concentration of 10% by mass. The particle size of the aluminum flakes in the dispersion was measured using the wet particle image analyzer.

[0056] (Resin particles) As the resin particles, we used "Pluscoat RZ-105," a product manufactured by Go-O Chemical Co., Ltd., which has a polyester resin backbone. The resin particles were diluted with diethylene glycol monomethyl ether to a concentration of 0.1% by mass, and the particle size of the resin particles was measured using the dynamic light scattering apparatus.

[0057] (Ink preparation) A metallic ink was prepared with the composition described below. Dispersion containing 10% by mass of aluminum flakes prepared above 10% by mass Resin particles 1% by mass Glycerin 5% by mass Surfactant 1% by mass (Product name: Acetinol E100, manufactured by Kawaken Fine Chemical Co., Ltd.) Diethylene glycol monomethyl ether 40% by mass Dipropylene glycol monobutyl ether 40% by mass Triethylene glycol 3% by mass

[0058] (Measurement of particle size of aluminum flakes and resin particles after ink preparation) When the median diameter (DP50) of the aluminum flakes and the median diameter (D50) of the resin particles were measured using the same method as in Example 1 with the ink prepared above, the values ​​were the same as those before the ink was incorporated as a component of the metallic ink. (Evaluation of the adhesion of aluminum flakes) Using the prepared ink, the abrasion resistance of the printed image was evaluated in the same manner as in Example 1.

[0059] [Example 14] A metallic ink was prepared using the method and ink composition described in Example 13, except that Grade 701 of Vinibran (registered trademark) manufactured by Nisshin Chemical Co., Ltd., which contains polyvinyl chloride resin as the resin particles, was used. Using the prepared ink, the particle size of the resin particles and the abrasion resistance of the printed image were evaluated in the same manner as in Example 1.

[0060] [Example 15] A metallic ink was prepared using the method and ink composition described in Example 13, except that "PESRESIN S-180," a product manufactured by Takamatsu Oil & Fat Co., Ltd., which has a saturated polyester resin backbone, was used as the resin particles. Using the prepared ink, the particle size of the resin particles and the abrasion resistance of the printed image were evaluated in the same manner as in Example 1.

[0061] [Comparative Example 1] A metallic ink was prepared using the method and ink composition described in Example 9. However, the aluminum was deposited to achieve a film thickness equivalent to 70 nm. Furthermore, ultrasonic irradiation was performed for 15 hours using the ultrasonic disperser, and the aluminum flakes were prepared to have a median diameter (DP50) of 400 nm. Using the prepared ink, the abrasion resistance of the printed image was evaluated in the same manner as in Example 1.

[0062] [Comparative Example 2] A metallic ink was prepared using the method and ink composition described in Example 10, except that Grade 715 of Vinibran (registered trademark) manufactured by Nisshin Chemical Co., Ltd. was used as the resin particles. Using the prepared ink, the particle size of the resin particles and the abrasion resistance of the printed image were evaluated in the same manner as in Example 1.

[0063] [Comparative Example 3] The aluminum vapor-deposited film prepared by the method described in Example 1 was subjected to ultrasonic irradiation for 2 hours using the ultrasonic disperser described in Example 10, and the median diameter (DP50) of the aluminum flakes was set to 600 nm. Otherwise, the metallic ink was prepared using the method and ink composition described in Example 1. The scratch resistance of the printed image was evaluated using the prepared ink in the same manner as in Example 1.

[0064] [Comparative Example 4] A metallic ink was prepared using the method, ink composition, and resin particles described in Example 13. However, the aluminum was deposited to a thickness equivalent to 70 nm. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0065] [Comparative Example 5] A metallic ink was prepared using the method and ink composition described in Example 14, except that VB-715 manufactured by Nisshin Chemical Co., Ltd., which contains polyvinyl chloride resin as resin particles, was used. The scratch resistance of the printed image was evaluated using the prepared ink, in the same manner as in Example 1.

[0066] [Comparative Example 6] For aluminum flakes, the ultrasonic disperser was used according to the method described in Example 13, with an ultrasonic irradiation time of 2 hours, and the median diameter (DP50) of the aluminum flakes was set to 600 nm. Otherwise, a metallic ink was prepared using the method described in Example 13, with the same ink composition and resin particles. Using the prepared ink, the abrasion resistance of the printed image was evaluated in the same manner as in Example 1.

[0067] Table 1 shows the particle size of aluminum and resin particles and the results of the evaluation of the scratch resistance of the printed image in the inks prepared in each example and comparative example.

[0068] [Table 1]

[0069] In Examples 1-15, the median diameter (DP50) of the flat metal particles was less than 500 nm, the average thickness was 50 nm or less, and the relationship between the median diameter (DP50) of the flat metal particles and the median diameter (D50) of the resin particles was 4.0 or greater was satisfied. When a metallic ink prepared to satisfy this relationship was used, the scratch resistance of the printed image was evaluated as A or B, indicating that good aluminum fixation was obtained. Furthermore, the metallic luster of the printed image was evaluated as A or B, indicating that sufficient luster was obtained. On the other hand, Comparative Examples 1 and 4 used metallic inks containing flat metal particles with an average thickness greater than 50 nm. In this case, the scratch resistance of the printed image was evaluated as C, indicating a decrease in the fixation of aluminum. This is thought to be because when the flat metal particles with an average thickness greater than 50 nm were fabricated by vacuum deposition, irregularities occurred on their surface, resulting in poor contact with the recording medium and resin particles. Furthermore, in Comparative Examples 2 and 5, metallic inks were used in which the ratio of the median diameter of the flat metal particles to the median diameter of the resin particles (DP50 / D50) was less than 4.0. In this case, the scratch resistance of the printed image was evaluated as D, and it became clear that the adhesion of aluminum was further reduced. This is thought to be because the particle size of the resin particles became larger than a certain size relative to the particle size of the flat metal particles, preventing the flat metal particles from being stably fixed so that their planes were parallel to the recording medium. Furthermore, in Comparative Examples 3 and 6, the median diameter (DP50) of the flat metal particles was 500 nm or larger, and the median diameter (DP99) also exceeded 1 μm. In this case, even though the relationship (DP50) / (D50) of 4.0 or larger was satisfied, ejection was not possible due to nozzle clogging, and printing was not possible. Therefore, metallic luster was not evaluated in Comparative Examples 3 and 6. In comparative examples 2, 4, and 5, the metallic luster of the printed images was evaluated as C, clearly indicating that the desired metallic luster could not be obtained.

Claims

1. A metallic ink for inkjet printers containing flat metal particles and resin particles, The volume-based median diameter (DP50) of the aforementioned flat metal particles is 300 nm or more and less than 500 nm. The average thickness of the aforementioned flat metal particles is 50 nm or less. The content of the flat metal particles is 0.2% by mass or more and 10% by mass or less relative to the total mass of the inkjet metallic ink. The content of the resin particles is 0.2% by mass or more and 10% by mass or less relative to the total mass of the inkjet metallic ink. A metallic inkjet ink characterized in that the median diameter (DP50) of the flat metal particles based on volume / the median diameter (D50) of the resin particles based on volume is 4.0 or more and 20.0 or less.

2. The metallic ink for inkjet according to claim 1, wherein the volume-based median diameter (DP50) of the flat metal particles is 400 nm or less.

3. The metallic ink for inkjet according to claim 1 or 2, wherein the volume-based median diameter (DP99) of the flat metal particles is 1 μm or less.

4. The metallic ink for inkjet printers according to claim 3, wherein the volume-based median diameter (DP99) of the flat metal particles is 900 nm or less.

5. The metallic ink for inkjet according to claim 3 or 4, wherein the volume-based median diameter (DP99) of the flat metal particles is 800 nm or more.

6. The metallic ink for inkjet according to any one of claims 1 to 5, wherein the average thickness of the flat metal particles is 10 nm or more.

7. The metallic ink for inkjet according to any one of claims 1 to 6, wherein the flat metal particles are aluminum particles.

8. The metallic ink for inkjet according to any one of claims 1 to 7, wherein the volume-based median diameter (D50) of the resin particles is 10 nm or more and 80 nm or less.

9. The median diameter (DP99) of the aforementioned flat metal particles is 1 μm or less based on volume. The average thickness of the aforementioned flat metal particles is 10 nm or more. The metallic ink for inkjet according to claim 1 or 2, wherein the volume-based median diameter (D50) of the resin particles is 10 nm or more and 80 nm or less.

10. The metallic ink for inkjet according to any one of claims 1 to 9, wherein the resin particles are polyester resin particles.

11. The inkjet metallic ink according to any one of claims 1 to 10, wherein the inkjet metallic ink is an aqueous ink containing water.

12. The metallic ink for inkjet according to any one of claims 1 to 11, wherein the flat metal particles are surface-treated with a phosphoric acid compound.

13. The metallic ink for inkjet according to any one of claims 1 to 12, wherein the median diameter (DP50) of the flat metal particles based on volume / the median diameter (D50) of the resin particles based on volume is 5.0 or more.

14. An inkjet recording method for printing an image on a recording medium using an inkjet metallic ink according to any one of claims 1 to 13.

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