Plate-like alumina-containing ink coating

Abstract

The purpose of the present invention is to provide an ink coating having excellent hiding power. Specifically, the ink coating is composed of an ink composition containing plate-like alumina particles, and has voids adjacent to the plate-like alumina particles in the coating.

Description

Plate-like alumina-containing ink coating

[0001] The present invention relates to platy alumina-containing ink coatings.

[0002] Conventionally, ink compositions containing, for example, a colorant and a solvent such as an organic solvent or water have been known, and pigment inks containing a pigment as the colorant have been widely used. Among these, those containing titanium oxide as a white pigment, which has excellent hiding power and clarity, are widely used.

[0003] For example, in ink compositions for writing instruments, alumina is sometimes added together with a colorant such as titanium oxide (Patent Documents 1 and 2). For example, the addition of alumina can reduce wear on the ball seat (referring to the contact point between the ball and the ball holder) of a ballpoint pen tip, which has a ball and a ball holder, due to friction generated by the rotation of the ball during writing. Furthermore, when writing long distances, the frictional force between the ball and the ball seat is smaller than the frictional force between the paper surface and the ball, preventing skipped lines.

[0004] JP 2015-091944 A JP 2020-203969 A

[0005] Machida, S. etc. al. CrystEngComm, 2022, 24, 5405-5409

[0006] However, alumina has low hiding power, and when added as an additive, there is a risk of reducing the inherent hiding power of the ink composition for a writing instrument.

[0007] In Non-Patent Document 1, a plate-like NaNbO ３ It has been shown that alumina has high hiding power, but even when alumina is similarly made to the submicron order, the improvement in hiding power is insufficient.

[0008] An object of the present invention is to provide an ink coating film having excellent hiding power.

[0009] As a result of extensive research, the present inventors have found that voids contained in an ink coating film improve hiding power, and that voids adjacent to alumina particles in particular have a significant effect on improving properties, leading to the completion of the present invention.

[0010] (1) A coating film made of an ink composition containing plate-like alumina particles, the coating film having voids adjacent to the plate-like alumina particles.

[0011] (2) The ink coating film according to (1) above, wherein the porosity of the coating film is 2% to 10%.

[0012] (3) The ink coating film according to (1) or (2) above, wherein the plate-like alumina particles have a silicon dioxide layer on the surface thereof.

[0013] (4) The ink coating film according to claim 1, wherein the plate-like alumina particles have a volume-based median diameter D50 of 1 μm or more and 5 μm or less, and an aspect ratio of 10 or more and 50 or less, as measured by laser diffraction particle size distribution measurement.

[0014] According to the present invention, by providing voids between adjacent alumina particles contained in an ink coating, it is possible to provide an ink coating having excellent hiding power.

[0015] Hereinafter, embodiments of the present invention will be described in detail.

[0016] [Ink Coating Film] The ink coating film of the present invention contains alumina particles and is characterized by having voids between adjacent alumina particles.

[0017] "Having voids adjacent to alumina particles" means that an air layer is formed between the alumina particles and the resin in the ink coating, and the alumina particles and the resin are not in direct contact with each other. The air layer may be formed so as to cover the entire alumina particles, or may be formed so as to cover only a portion of the alumina particles. When the air layer is formed so as to cover only a portion of the alumina particles, it is preferably formed on the surface side (the surface side of the coating) when the coating is observed in a cross-sectional view.

[0018] By having voids adjacent to the alumina particles, high hiding power can be obtained. Specifically, normally, the light scattering effect of a coating film can only occur at the interface between the alumina particles and the resin, but by having voids (i.e., air layers), two types of light scattering occur: between the resin and the air layer, and between the air layer and the alumina particles. Because the air layer has a low refractive index, the difference in refractive index between the resin and the alumina particles increases, which increases light scattering and is thought to improve hiding power.

[0019] The voids adjacent to the alumina particles preferably account for at least half of all the voids, and particularly preferably at least 70% of all the voids are in contact with the alumina particles.

[0020] The porosity of the ink coating is preferably 2% to 10%, and more preferably 3% to 9%. A porosity within this range is preferred because it improves the hiding power of the coating.

[0021] The presence or absence of voids, their location, and porosity can be determined from cross-sectional images of the coating film observed under an electron microscope. The presence or absence of voids can be confirmed using software that distinguishes differences in brightness. The porosity can be calculated as the ratio of the void area to the total cross-sectional area of ​​the coating film.

[0022] [Ink Composition] The ink coating of the present invention is formed from an ink composition containing alumina particles.

[0023] (Alumina Particles) The alumina particles are not particularly limited, and commercially available alumina particles may be used, such as YFA02050 (plate-shaped alumina, manufactured by Kinseimatec Co., Ltd.). Also, known conventional manufacturing methods may be used, and the alumina particles may be manufactured by manufacturing methods such as a hydrothermal method or a flux method. A manufacturing method for more simply obtaining the desired alumina particles includes the method described in JP 2016-222501 A, in which an aluminum compound is calcined in the presence of a molybdenum compound and a shape control agent.

[0024] The alumina particles are aluminum oxide and may be transition alumina of various crystalline forms such as γ, δ, θ, and κ, or may contain alumina hydrate in transition alumina. However, the α crystalline form (α type) is basically preferred in terms of superior stability.

[0025] The aspect ratio, which is the ratio of the average particle diameter (D50) to the thickness of the alumina particles, is from 10 to 50, preferably from 12 to 45, and more preferably from 15 to 40. When the aspect ratio of the alumina particles is within the above range, good hiding power can be obtained when the alumina particles are formed into an ink coating film.

[0026] The D50 of the alumina particles is from 1 μm to 5 μm, and more preferably from 1.1 μm to 4 μm. When the D50 is within this range, the dry hide effect is easily exhibited when a metal oxide layer, which will be described later, is provided, and the hiding power of the ink coating can be improved.

[0027] Furthermore, the inventors have discovered that by setting D50 to a specific particle size, the location and content of voids can be further controlled, improving the hiding power of the coating film. Although the details are not clear, it is believed that when the D50 of the alumina particles is within the above range, the specific surface area increases, and the amount of resin and solvent contained in the ink composition that is adsorbed to the alumina particles increases, making it easier to form voids near the alumina particles. Furthermore, it is believed that when the D50 of the alumina particles is within the above range, the number of alumina particles contained in the ink composition increases, making it possible to form more voids in the coating film.

[0028] In this specification, D50 means a value calculated as a volume-based median diameter from a volume-based cumulative particle size distribution measured by a laser diffraction particle size distribution analyzer.

[0029] The alumina particles may further contain molybdenum, and may also contain impurities derived from the raw materials or shape control agent, as long as the effects of the present invention are not impaired.

[0030] (Metal Oxide Layer) The alumina particles contained in the ink coating of the present invention preferably further have a metal oxide layer, more preferably one or more of zinc oxide, silicon dioxide, zirconium dioxide, and titanium oxide, and particularly preferably silicon dioxide. The presence of a metal oxide layer is preferred because it forms a porous structure, forming more voids in the areas adjacent to the alumina particles and increasing the difference in refractive index with the resin.

[0031] The metal oxide layer is preferably a single layer. A single layer provides a superior improvement in hiding power due to the difference in refractive index. Note that a single layer means that the layer is made of one type of component, and indicates that there are no two types of layers, such as a zinc oxide layer and a silicon dioxide layer.

[0032] The metal oxide layer may be formed on at least a portion of the surface of the alumina particle, but is more preferably formed on the entire surface of the alumina particle. Note that the "surface of the alumina particle" refers to the outer surface of the alumina particle. Therefore, it is clearly distinguished from the surface layer containing mullite or germanium formed on the inner surface of the alumina particle.

[0033] The thickness of the metal oxide layer is not particularly limited, but from the viewpoints of hiding power and cost, it is 0.1 nm or more, but is thin enough to be unmeasurable in an SEM image.

[0034] The amount of metal oxide in the metal oxide layer relative to the amount of alumina in the alumina particles is not particularly limited, but is preferably 5% by mass or less, more preferably 3% by mass or less, and more preferably more than 0% by mass. A content within this range is preferred because it provides excellent hiding power when used in an ink composition.

[0035] When used as a colorant, the content is preferably 1 to 15 mass % relative to the total mass of the ink composition, and more preferably 3 to 12 mass %. By being within this range, it is possible to provide both excellent hiding power and color development, which is preferable.

[0036] [Method for forming metal oxide layer] The method for forming the metal oxide layer is not particularly limited and can be formed by a known method. For example, in the case of silicon dioxide, a porous silicon dioxide layer can be formed by adding a solution of sodium silicate, adjusting the pH with a strong acid, and then drying.

[0037] (Titanium Oxide Particles) The ink composition of the present invention may further contain titanium oxide particles. The titanium oxide particles are not particularly limited, and either rutile or anatase type can be used, with the average particle size preferably being 0.01 μm or greater. For example, known titanium oxides can be used, such as Bayertitan R-FD-1, R-KB-3, and R-CK-20 (all manufactured by Bayer), TIPAQUE R-630, R-615, R-830, and the LPT series (all manufactured by Ishihara Sangyo Kaisha, Ltd.), Unitane OR-342 (manufactured by A.C.C.), Ti-pure R-900 and R-901 (manufactured by Chemours), and the Luxelenesilk series (manufactured by Sumitomo Chemical Co., Ltd.).

[0038] The amount added is preferably from 0.001 to 10% by mass, more preferably from 0.005 to 5% by mass, and particularly preferably from 0.02 to 4% by mass, relative to the total mass of the ink composition. When the amount is within this range, the resulting coating film has excellent hiding power and L value, and is also excellent in ink discharge performance, which is preferable.

[0039] (Organic Solvent) The ink composition may contain an organic solvent, for example, aromatic hydrocarbon solvents such as toluene, xylene, and ethylbenzene; aliphatic hydrocarbon solvents such as n-hexane, n-heptane, isoheptane, n-octane, and isooctane; cycloparaffin solvents such as methylcyclohexane and ethylcyclohexane; ketone solvents such as methyl isobutyl ketone and methyl ethyl ketone; glycol solvents such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, and glycerin; and glycol ether solvents such as ethylene glycol monomethyl ether and diethylene glycol monomethyl ether, which may be used alone or in combination of two or more. Glycol ether solvents are preferred from the viewpoints of water solubility, excellent writability, and safety.

[0040] The content of the organic solvent is not particularly limited, but from the viewpoint of the drying speed of the ink coating, it is preferably 10 to 90 mass % relative to the total mass of the ink composition, and more preferably 20 to 80 mass %.

[0041] (Resin) The ink composition may contain a resin, examples of which include polyvinyl butyral resin, ketone resin, polyacetal resin, polyvinyl alcohol resin, cellulose resin, terpene resin, alkyd resin, phenoxy resin, polyvinyl acetate resin, polyvinylpyrrolidone resin, ethylene oxide polymer, acrylic resin, styrene-acrylic resin, styrene-maleic acid resin, etc., and these may be used alone or in combination of two or more.

[0042] From the viewpoint of ink viscosity and ink leakage, the content of the resin is preferably 0.05 to 30% by mass, and particularly preferably 0.1 to 25% by mass, based on the total mass of the ink composition.

[0043] (Dispersant) The ink composition may contain a dispersant, and examples thereof include nonionic surfactants such as glycerin fatty acid esters, polyoxyethylene methyl ether, polyoxyethylene lanolin alcohol, polyoxyethylene alkylamines, and polyoxyethylene fatty acid amides; anionic surfactants such as alkyl sulfates, N-acylamino acid salts, polyoxyethylene alkyl ether acetates, and alkyl phosphates; cationic surfactants such as alkylamine salts and quaternary ammonium salts; amphoteric surfactants such as alkylbetaines, alkylamine oxides, and phosphatidylcholines; and polymeric surfactants such as acrylics.

[0044] The content of the dispersant is determined appropriately depending on the other components used, such as the solvent, but from the viewpoints of dispersion stability and ink viscosity, it is preferably 0.1 to 30 mass%, and particularly preferably 0.5 to 20 mass%, of the total mass of the ink composition.

[0045] (Other Additives) If necessary, additives such as colorants such as organic pigments, inorganic pigments, acid dyes, basic dyes, and direct dyes; rust inhibitors such as benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, diisopropylammonium nitrite, saponin, metal salt compounds, and phosphate ester compounds; preservatives such as carbolic acid, 1,2-benzthiazolin-3-one sodium salt, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl parahydroxybenzoate, and 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine; silicone-based, mineral oil-based, polyether-based, and fluorine-based antifoaming agents; antioxidants; stabilizers; and pH adjusters such as inorganic salts such as sodium carbonate, sodium phosphate, and sodium acetate, and organic basic compounds such as water-soluble amine compounds, may also be added.

[0046] [Method of Producing Ink Composition] The ink composition can be produced by a known method without any particular limitation. The ink composition can be produced by blending appropriate amounts of the above-mentioned components and mixing them using various stirrers such as a propeller stirrer, a homodisper, or a homomixer, or various dispersers such as a bead mill. When a pigment is added as a colorant, it is preferable to use a disperser such as a bead mill or a paint conditioner.

[0047] (Viscosity) The viscosity of the ink composition is not particularly limited, but is preferably at 20°C and a shear rate of 5 sec －１ The ink viscosity (at rest) is preferably 30,000 mPa·s or less, and more preferably 25,000 mPa·s or less. A viscosity below the upper limit of the above range is preferred for excellent ink ejection properties and writing comfort. Furthermore, from the viewpoint of suppressing ink leakage, the viscosity is preferably 500 mPa·s or more, and more preferably 1,000 mPa·s or more.

[0048] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to the following examples.

[0049] (Synthesis of Alumina) [Production Example 1] 142.3 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 1 μm), 3.7 g of silicon dioxide (manufactured by Kanto Chemical Co., Ltd., special grade), and 4.7 g of molybdenum trioxide (manufactured by Taiyo Koko Co., Ltd.) were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and heated to 1200°C at a rate of 5°C / min in a ceramic electric furnace, and then held at 1200°C for 10 hours for firing. After that, the temperature was lowered to room temperature at a rate of 5°C / min, and the crucible was removed, obtaining 95 g of a light blue powder.

[0050] Next, 50 g of the obtained pale blue powder was dispersed in 150 mL of 0.5% aqueous ammonia, and the dispersion was stirred at room temperature (25-30°C) for 0.5 hours. After filtering to remove the aqueous ammonia, the resulting mixture was washed with water and dried to remove molybdenum remaining on the particle surface, yielding 47 g of pale blue powder. The obtained powder was confirmed to have a plate-like shape by SEM observation. Furthermore, X-ray diffraction (XRD) measurement revealed sharp scattering peaks derived from α-alumina, and no alumina crystal peaks other than the α-crystalline structure were observed, confirming that the powder was plate-like alumina particles (A-1) with a dense crystalline structure. The α-conversion rate was 99% or more (almost 100%).

[0051] (Formation of Metal Oxide Layer) The obtained plate-like alumina particles (A-1) were dispersed in ion-exchanged water, and No. 3 sodium silicate (Fuji Chemical Co., Ltd.) was added dropwise with stirring. Silicon oxide was added in an amount of 3 parts by mass per 100 parts by mass of alumina. After the dropwise addition, the mixture was stirred for 10 minutes, and 1 M sulfuric acid was added dropwise to adjust the pH to 6.0. The mixture was then filtered, washed with ion-exchanged water, and dried at 150°C for 5 hours to obtain a light blue powder (A-2).

[0052] (Synthesis of Alumina) [Production Example 2] 50 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 12 μm), 0.65 g of silicon dioxide (manufactured by Kanto Chemical Co., Ltd., special grade), and 1.72 g of molybdenum trioxide (manufactured by Taiyo Koko Co., Ltd.) were mixed in a mortar to obtain a mixture. The resulting mixture was placed in a crucible and heated to 1200°C at a rate of 5°C / min in a ceramic electric furnace, and then held at 1200°C for 10 hours for firing. The temperature was then lowered to room temperature at a rate of 5°C / min, and the crucible was removed to obtain 34.2 g of a light blue powder. The resulting powder was crushed in a mortar until it passed through a 106 μm sieve.

[0053] Next, the obtained pale blue powder was dispersed in 150 mL of 0.5% aqueous ammonia, and the dispersion was stirred at room temperature (25-30°C) for 0.5 hours. After filtering to remove the aqueous ammonia, the resulting mixture was washed with water and dried to remove the molybdenum remaining on the particle surface, yielding 33.5 g of pale blue powder. SEM observation of the obtained powder confirmed that it had a plate-like shape and contained very few aggregates. Furthermore, XRD analysis revealed sharp scattering peaks derived from α-alumina, and no alumina crystal peaks other than the α-crystalline structure were observed, confirming that the particles were plate-like alumina particles (A-3) with a dense crystalline structure. The α-conversion rate was 99% or more (almost 100%).

[0054] (Formation of Metal Oxide Layer) The obtained plate-like alumina particles (A-3) were treated in the same manner as in Production Example 1 to prepare plate-like alumina particles (A-4) having a metal oxide layer formed thereon.

[0055] Commercially available plate-like alumina particles (Seraph YFA02050, manufactured by Kinsei Matec Co., Ltd.) were prepared as A-5.

[0056] For comparison, titanium oxide particles (Tipake R-830, manufactured by Ishihara Sangyo Kaisha, Ltd.) were prepared (T-1).

[0057] The fillers used in the examples and comparative examples are as shown in Table 1.

[0058]

[0059] (Production of ink composition) 5.5 parts by mass of Typec R-830 ​​(manufactured by Ishihara Sangyo Kaisha), 1.0 part by mass of Primal ASE-60 (manufactured by The Dow Chemical Company), 6.5 parts by mass of purified glycerin (manufactured by New Japan Chemical Co., Ltd.), 11.7 parts by mass of Joncryl 70J (manufactured by BASF), and 12.9 parts by mass of ion-exchanged water were mixed together. 4 parts by mass of a filler shown in Table 1 was added to the resulting mixture, and the mixture was further mixed to obtain a white ink composition.

[0060] [Evaluation of voids] The obtained white ink composition was used to form a coating film on colored paper (Koikura, manufactured by PLUS Corporation) using a bar coater RDS20. The coating film was dried at room temperature for one day and the cross section of the coating film was photographed at a magnification of 10,000 times using a JCM7000 (manufactured by JEOL Ltd.). The image of the obtained coating film cross section was analyzed using analysis software (WinRoof2018, manufactured by Mitani Shoji Co., Ltd.) to set a brightness threshold for voids using the automatic setting function, and the presence or absence of voids was confirmed. The image was visually confirmed to confirm that the voids and alumina particles were adjacent, and if they were adjacent, it was marked with ◯, and if they were not adjacent, it was marked with ×.

[0061] [Evaluation of Porosity] From the image of the cross section of the coating film obtained above, the quotient was calculated from the total area of ​​the regions corresponding to the voids and the total area of ​​the cross section of the coating film.

[0062] [Evaluation of hiding power] The obtained white ink composition was used to form a coating film on colored construction paper (Koikura, manufactured by Plus Corporation) using a bar coater RDS20. The coating film was dried at room temperature for one day and night, and the hiding power for the coated surface was evaluated by the L value. The L value was measured using a spectro-guide 45 / 0 gloss (manufactured by BYK-Gardner GmbH). An L value of 86 or more was evaluated as a good coating film that sufficiently hid the coated surface, and an L value below that was evaluated as an X, indicating insufficient hiding power.

[0063] The evaluation results are shown in Table 2.

[0064]

[0065] It was confirmed from Examples 1 to 3 and Comparative Example 1 that the use of alumina particles with a small particle size results in good hiding power. It is believed that the use of alumina particles with a small particle size increases the number of alumina particles contained in the coating film, and at the same time increases the number of voids adjacent to the alumina, thereby improving hiding power.

[0066] It was confirmed from Examples 1 to 3 and Comparative Example 2 that alumina particles with a small particle size have better hiding power than titanium oxide. It is believed that the hiding power is improved by the voids adjacent to the alumina particles in the coating film.

Claims

1. A coating film made of an ink composition containing plate-like alumina particles having a silicon dioxide layer on the surface thereof, An ink coating having voids adjacent to the platelet-shaped alumina particles in the coating.

2. 2. The ink coating according to claim 1, wherein the porosity of the coating is from 2% to 10%.

3. 2. The ink coating film according to claim 1, wherein the plate-like alumina particles have a volume-based median diameter D50 of 1 μm or more and 5 μm or less, and an aspect ratio of 10 or more and 50 or less, as measured by laser diffraction particle size distribution measurement.