Inkjet recording device

Abstract

The present invention provides an inkjet recording device that can suppress bleeding of non-white inks due to poor drying of white inks, and also improve the abrasion resistance of the ink after printing. [Solution] The inkjet recording device comprises at least a transport unit, a coating unit, a first drying unit, a recording unit, and a second drying unit. The first drying unit dries the white ink coated on the recording medium by the coating unit. The recording unit discharges non-white ink onto the recording medium after it has passed through the first drying unit. The second drying unit dries the non-white ink coated on the recording medium by the recording unit. The white ink contains first resin particles having a first minimum film-forming temperature of 40°C to 80°C. The non-white ink contains second resin particles having a second minimum film-forming temperature of 80°C or less. The drying temperature of the first drying unit is below the first minimum film-forming temperature, and the drying temperature of the second drying unit is at or above the first minimum film-forming temperature and the second minimum film-forming temperature.

Description

[Technical Field]

[0001] The present invention relates to an inkjet recording device that records an image by ejecting ink onto a recording medium. [Background technology]

[0002] Inkjet recording devices, such as fax machines, photocopiers, and printers, which eject ink to record images, are widely used because they can form high-resolution images.

[0003] In inkjet recording devices of this type, water-based pigment inks, which use pigments with excellent weather resistance as colorants, have become the mainstream. However, water-based pigment inks have a problem in that they tend to repel or bleed when ejected onto recording media with low wettability, such as plastic sheets.

[0004] Therefore, a method has been proposed in which a base layer is formed using a lower layer ink (white ink), and then an image is formed on top of it using an upper layer ink (non-white ink) (overprinting), thereby suppressing ink bleeding and creating an image with good durability.

[0005] For example, Patent Document 1 discloses an inkjet recording method comprising: a step of forming a first image layer on a recording medium by ejecting a first ink composition containing resin particles, an organic solvent, and water using an inkjet method; a drying step of evaporating 80% by mass or more of the water contained in the first ink composition in the first image layer; and a step of forming a second image layer on the first image layer after the drying step by ejecting a second ink composition containing an organic solvent that dissolves or swells the resin particles contained in the first ink composition, and water, using an inkjet method. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-15968 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the method described in Patent Document 1, the drying temperature after ejecting the second ink composition is 90°C. However, when using an OPP (biaxially oriented polyethylene) sheet as the recording medium (printing substrate), OPP is sensitive to heat, so drying must be done at 80°C or below to avoid damaging the substrate quality. Therefore, there was a problem in that the method described in Patent Document 1 could not be applied to OPP sheets.

[0008] In view of the above problems, the present invention aims to provide an inkjet recording apparatus that can suppress bleeding of non-white ink due to poor drying of white ink coated on a recording medium, and also improve the abrasion resistance of the ink after printing. [Means for solving the problem]

[0009] To achieve the above objective, the first configuration of the present invention is an inkjet recording apparatus comprising at least a transport unit, a coating unit, a first drying unit, a recording unit, and a second drying unit. The transport unit transports a recording medium. The coating unit coats white ink onto the recording medium transported by the transport unit. The first drying unit is located downstream of the coating unit with respect to the transport direction of the recording medium and dries the white ink coated onto the recording medium by the coating unit. The recording unit is located downstream of the first drying unit with respect to the transport direction of the recording medium and discharges non-white ink onto the recording medium. The second drying unit is located downstream of the recording unit with respect to the transport direction of the recording medium and dries the non-white ink coated onto the recording medium by the recording unit. The white ink contains first resin particles having a minimum film-forming temperature of 40°C or more and 80°C or less. The non-white ink contains second resin particles having a minimum film-forming temperature of 80°C or less. When the minimum film-forming temperatures for the first resin particles and the second resin particles are defined as the first minimum film-forming temperature and the second minimum film-forming temperature, respectively, the drying temperature in the first drying section is less than the first minimum film-forming temperature, and the drying temperature in the second drying section is equal to or greater than the first minimum film-forming temperature and the second minimum film-forming temperature. [Effects of the Invention]

[0010] According to the first configuration of the present invention, an inkjet recording apparatus is provided that can suppress bleeding of non-white ink due to poor drying of white ink coated on a recording medium, and also improve the abrasion resistance of the ink after printing. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic diagram showing the structure of an inkjet recording apparatus 100 according to the first embodiment of the present invention. [Figure 2] Enlarged view of the coating section 2, recording section 3, and the area around the first drying section 5 in Figure 1. [Figure 3] A schematic diagram showing the structure of an inkjet recording apparatus 100 according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described in detail below. In the following specification, the compound name may be followed by "system" to comprehensively refer to the compound and its derivatives. When the polymer name is followed by "system" to represent the polymer name, it means that the repeating unit of the polymer originates from the compound or its derivative. Also, acrylic and methacrylic may be comprehensively referred to as "(meth)acrylic". Also, acryloyl (CH 2 =CH-CO-) and methacryloyl (CH 2 =C(CH 3 The terms )-CO-) are sometimes collectively referred to as "(meth)acryloyl".

[0013] The ink used in the inkjet recording device of the present invention is an aqueous ink containing a pigment dispersion containing a pigment and a pigment dispersion resin, a binder resin, a water-soluble organic solvent, and water, and includes white ink and color inks other than white (hereinafter also referred to as non-white ink). The white ink and non-white ink used in the inkjet recording device of the present invention will be described in detail below.

[0014] [1. White Ink] The white ink used in the inkjet recording apparatus of the present invention forms a base image by being applied to a plastic sheet as a recording medium when an image is formed by the inkjet recording apparatus using non-white ink, and suppresses ink bleeding by adjusting the wettability of the sheet. The white ink contains at least a white pigment, a binder resin, and an aqueous medium. It is preferable that the white ink further contains a pigment dispersion resin.

[0015] (White Pigment) Examples of the white pigment include C.I. Pigment White 4, C.I. Pigment White 5, C.I. Pigment White 6, C.I. Pigment White 6:1, C.I. Pigment White 7, C.I. Pigment White 18, C.I. Pigment White 19, C.I. Pigment White 20, C.I. Pigment White 21, C.I. Pigment White 23, C.I. Pigment White 24, C.I. Pigment White 25, C.I. Pigment White 26, C.I. Pigment White 27, and C.I. Pigment White 28. Titanium oxide can also be used as the white pigment.

[0016] From the viewpoint of discharging the white ink well from the recording head 3e, the content of the white pigment is preferably 0.1% by mass or more and 30% by mass or less, and more preferably 10% by mass or more and 20% by mass or less with respect to the mass of the white ink.

[0017] (Pigment Dispersion Resin) The pigment dispersion resin (pigment coating resin) is a resin soluble in the aqueous medium of the ink. A part of the pigment dispersion resin exists, for example, on the surface of the pigment particles to optimize the dispersibility of the pigment particles. A part of the pigment dispersion resin exists, for example, in a dissolved state in the aqueous medium of the ink.

[0018] As the pigment dispersion resin, a styrene-(meth)acrylic resin is preferred. The styrene-(meth)acrylic resin has a repeating unit derived from at least one monomer of (alkyl (meth)acrylate and (meth)acrylic acid) and a styrene unit. Examples of the (alkyl (meth)acrylate) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate. As the styrene-(meth)acrylic resin, a copolymer of styrene, methyl methacrylate, methacrylic acid, and butyl acrylate is preferred.

[0019] In the white ink, the content ratio of the pigment dispersion resin is preferably 0.1% by mass or more and 4.0% by mass or less, more preferably 0.5% by mass or more and 1.5% by mass or less. By setting the content ratio of the pigment dispersion resin to 0.1% by mass or more and 4.0% by mass or less, the ejection stability when ejecting the white ink by an inkjet method can be ensured.

[0020] In the white ink, the content of the pigment dispersion resin with respect to 100 parts by mass of the pigment is preferably 10 parts by mass or more and 60 parts by mass or less, more preferably 20 parts by mass or more and 30 parts by mass or less. By setting the content of the pigment dispersion resin to 10 parts by mass or more and 60 parts by mass or less, the ejection stability of the white ink can be optimized.

[0021] (Binder resin) The binder resin is added for the purpose of fixing the white ink to the recording medium. The white ink contains resin particles (hereinafter referred to as the first resin particles) having a minimum film formation temperature (MFT) of 40 to 80°C as the binder resin. The measurement of the minimum film formation temperature was carried out in accordance with the ASTM D2354 test method using a measuring device (MFFTB90, manufactured by Rhopoint instulment).

[0022] If the minimum film-forming temperature of the first resin particles contained in the white ink (hereinafter also referred to as the first minimum film-forming temperature) is too low, film formation of the first resin particles occurs prematurely. As a result, the film of the first resin particles acts as a lid on the white ink, hindering the drying of the white ink. On the other hand, if the first minimum film-forming temperature is too high, in the drying process after the ejection of the non-white ink, it is necessary to heat the recording medium to a high temperature in order to form a film of the first resin particles contained in the white ink, which may impair the quality of heat-sensitive recording media such as OPP sheets. The first minimum film-forming temperature is preferably between 40°C and 80°C.

[0023] (Resin particles) The first resin particles added to the white ink are not particularly limited as long as they are resin particles with a minimum film-forming temperature of 40 to 80°C, but urethane resin particles are preferred. As urethane resin particles, anionic urethane resin particles, nonionic urethane resin particles, and cationic urethane resin particles can be used. Anionic urethane resin is a urethane resin that has anionic properties. Cationic urethane resin is a urethane resin that has cationic properties. Nonionic urethane resin is a urethane resin that does not have either anionic or cationic properties.

[0024] The content of urethane resin in the first resin particles is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass.

[0025] The median volume diameter of the first resin particles is preferably 10 nm to 100 nm, and more preferably 10 nm to 50 nm. By setting the median volume diameter of the first resin particles to 10 nm or more, the storage stability of the pretreatment solution can be optimized. By setting the median volume diameter of the first resin particles to 100 nm or less, the discharge stability of the pretreatment solution can be optimized.

[0026] The content of the first resin particles in the white ink is preferably 2.0% by mass or more and 15.0% by mass or less, and more preferably 4.0% by mass or more and 10.0% by mass or less. By setting the content of the first resin particles to 2.0% by mass or more, the adhesion of the image formed by the ink to the recording medium can be optimized. By setting the content of the first resin particles to 12.0% by mass or less, the coating properties of the white ink can be optimized. Furthermore, the ejection stability of the white ink when applying the white ink to the recording medium by the inkjet method can be optimized.

[0027] (Water-soluble organic solvent) Water-soluble organic solvents are added to improve compatibility with the binder resin and to retain moisture in the white ink. This suppresses the increase in viscosity of the white ink even if moisture evaporates in the white ink nozzle or coating device, thereby preventing nozzle sticking and misaligned ejection when applying white ink using an inkjet method, and thus maintaining ejection stability.

[0028] The water-soluble organic solvent added to the white ink is not particularly limited as long as it is compatible with the binder resin. Examples of water-soluble organic solvents that can be used in white ink include glycol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, 1,4-dioxane, thiodiglycol, glycerin, and dimethyl sulfoxide.

[0029] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, and tetraethylene glycol.

[0030] Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, dipropylene glycol methyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether.

[0031] Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

[0032] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0033] Examples of acetate compounds include diethylene glycol monoethyl ether acetate.

[0034] As the water-soluble organic solvent, glycol compounds or glycol ether compounds are preferred, with 1,3-propanediol, propylene glycol, triethylene glycol monobutyl ether (butyl triglycol), or dipropylene glycol methyl ether, and 1,4-dioxane being more preferred. In particular, the addition of 1,4-dioxane, which is water-soluble and has low polarity, is preferred because it increases compatibility with resin particles and improves the abrasion resistance of the printed material, as shown in the examples described later.

[0035] The amount of water-soluble organic solvent added is not particularly limited, but 5 to 40% by mass of the total white ink is preferred, and 10 to 35% by mass is more preferred. If the amount added is 5 to 40% by mass, the moisture content of the white ink can be sufficiently maintained, and the viscosity is not too high, so it can be effectively applied to the recording medium.

[0036] (Surfactants) White ink preferably contains a surfactant. The surfactant optimizes the compatibility and dispersion stability of each component in the ink. Furthermore, the surfactant optimizes the wettability of the ink to the recording medium. Various surfactants can be used in white ink, including silicone-based surfactants, nonionic surfactants, anionic surfactants, cationic surfactants, and betaine-based surfactants, but nonionic surfactants are preferred. These surfactants may be used individually or in combination of two or more.

[0037] Examples of nonionic surfactants in white ink include acetylene glycol-based surfactants (surfactants containing acetylene glycol compounds), silicone-based surfactants (surfactants containing silicone compounds), and fluorine-based surfactants (surfactants containing fluororesins or fluorine-containing compounds). Examples of acetylene glycol-based surfactants include ethylene oxide adducts of acetylene glycol and propylene oxide adducts of acetylene glycol. It is preferable that the ink contains a silicone-based surfactant.

[0038] The amount of surfactant added is preferably 0.001 to 5% by mass of the total white ink, and more preferably 0.05 to 2% by mass. An effect of adding surfactant can be obtained at 0.001% by mass or more. On the other hand, no further improvement can be expected by adding more than 5% by mass.

[0039] (water) The white ink used in this invention is aqueous and contains water as an essential component. The water contained in the white ink is not particularly limited as long as it does not hinder the objective of this invention, and water of a desired purity, such as purified water or deionized water, can be appropriately selected and used. The water content in the white ink is not particularly limited as long as it does not hinder the objective of this invention, and can be appropriately changed according to the amount of other components used as described above. The water content in the white ink is preferably 40 to 60% by mass relative to the total mass of the white ink.

[0040] Furthermore, the white ink may contain, as needed, known materials used in inks, such as anti-foaming agents, pH adjusters, preservatives and antifungal agents, rust inhibitors, dissolution stabilizers, drying inhibitors, antioxidants, and viscosity modifiers.

[0041] (Foam suppressant) Foam suppressants are added to suppress ink foaming. Generally, liquids with high surface tension, such as water, are less likely to foam because a force acts to minimize the surface area of ​​the liquid. However, liquids with low surface tension and high viscosity foam easily, and the foam that is generated is difficult to extinguish. The amount of foam suppressant added is preferably 0.01 to 10% by mass, and more preferably 0.02 to 5% by mass. A sufficient foam-suppressing effect can be obtained with an amount of 0.01% by mass or more. Also, if the amount is 10% by mass or less, the foam suppressant will not become unsoluble in the ink.

[0042] (pH adjuster) There are no particular restrictions on the pH adjuster, as long as it can adjust the pH to 6-10 without adversely affecting the ink; it can be selected appropriately depending on the purpose. If the pH is 6 or higher, corrosion of conveying components such as conveyor rollers that come into contact with the ink is less likely to occur.

[0043] Preferred pH adjusters include alcoholamines, alkali metal hydroxides, ammonium hydroxides, phosphonium hydroxides, and alkali metal carbonates. Examples of alcoholamines include diethanolamine, triethanolamine, and 2-amino-2-ethyl-1,3-propanediol. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of ammonium hydroxides include ammonium hydroxide and quaternary ammonium hydroxide. Examples of phosphonium hydroxides include quaternary phosphonium hydroxide. Examples of alkali metal carbonates include lithium carbonate, sodium carbonate, and potassium carbonate.

[0044] (preservative and fungicide) Examples of the antiseptic and antifungal agent include, for example, sodium dehydroacetate, sodium sorbate, sodium 2-pyridinethiol-1-oxide, sodium benzoate, sodium pentachlorophenol, sodium 1,2-benzisothiazolin-3-one, etc., which are preferably used.

[0045] (Rust inhibitor) Examples of the rust inhibitor include, for example, acid sulfite, sodium thiosulfate, ammonium thioglycolate, diisopropylammonium nitrite, dicyclohexylammonium nitrite, pentaerythritol tetranitrate, 1,2,3-benzotriazole, etc., which are preferably used.

[0046] By applying the white ink of the present invention obtained as described above to a recording medium before recording an aqueous ink (pigment ink) by an inkjet method, a base image is formed on the recording medium to improve the wettability (wetting tension) of the surface of the recording medium. Thereby, splashing and bleeding of the non-white ink ejected onto the recording medium can be suppressed.

[0047] There is no particular limitation on the coating amount (loading amount) of the white ink, but when the coating amount is less than 1 g / m 2 the wettability (wetting tension) of the surface of the recording medium cannot be sufficiently improved, and the effect of suppressing splashing and bleeding of the non-white ink becomes insufficient. On the other hand, when the coating amount exceeds 20 g / m 2 the wettability of the surface of the recording medium becomes too high, and the drying of the white ink is not in time, and there is a risk of bleeding of the non-white ink. Therefore, the coating amount of the white ink is preferably 1 g / m 2 or more and 20 g / m 2 or less, and more preferably 5 g / m 2 or more and 10 g / m 2 or less.

[0048] [2. Non-white ink] The non-white ink used in the inkjet recording apparatus of the present invention, like the white ink, contains at least a pigment, a binder resin, and an aqueous medium. Preferably, the ink further contains a pigment dispersion resin.

[0049] (Pigment) In non-white inks, the pigment, for example, constitutes pigment particles together with a pigment dispersion resin. Each pigment particle is composed of, for example, a core containing the pigment and a pigment dispersion resin coating the core. The pigment dispersion resin exists, for example, dispersed in a solvent. From the viewpoint of optimizing the color density, hue, or stability of the non-white ink, the median volume diameter of the pigment particles is preferably 30 nm to 200 nm, and more preferably 70 nm to 130 nm.

[0050] Examples of pigments include yellow pigments, orange pigments, red pigments, blue pigments, purple pigments, and black pigments. Examples of yellow pigments include CI Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include CI Pigment Red (122 and 202). Examples of blue pigments include CI Pigment Blue (15, more specifically 15:3). Examples of purple pigments include CI Pigment Violet (19, 23, and 33). Examples of black pigments include CI Pigment Black (7).

[0051] The pigment content relative to the non-white ink is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 1.5% by mass or more and 5.0% by mass or less. By setting the pigment content to 0.5% by mass or more, the non-white ink can form an image with the desired image density. Furthermore, by setting the pigment content to 10.0% by mass or less, the fluidity of the ink can be ensured.

[0052] (Binding resin) Non-white inks contain resin particles (hereinafter referred to as "second resin particles") as a binder resin, with a minimum film formation temperature (MFT) of 35 to 80°C. If the minimum film formation temperature of the second resin particles contained in the non-white ink (hereinafter also referred to as the second minimum film formation temperature) is too low, film formation of the resin particles will occur at room temperature, resulting in poor ejection of the non-white ink. On the other hand, if the second minimum film formation temperature is too high, it will be necessary to heat the recording medium to a high temperature in the drying process after ejection of the non-white ink in order to form a film of the resin particles contained in the non-white ink, which may impair the quality of heat-sensitive recording media such as OPP sheets. The second minimum film formation temperature is preferably 35°C or higher and 80°C or lower.

[0053] (Resin particles) The second resin particles added to the non-white ink are not particularly limited as long as they are resin particles with a minimum film-forming temperature of 35 to 80°C, but urethane resin particles similar to those added to the first resin particles in the white ink are preferred. Anionic urethane resin particles, nonionic urethane resin particles, and cationic urethane resin particles can be used as urethane resin particles. Anionic urethane resin is a urethane resin that has anionic properties. Cationic urethane resin is a urethane resin that has cationic properties. Nonionic urethane resin is a urethane resin that does not have either anionic or cationic properties.

[0054] The content ratio of urethane resin in the second resin particles is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass.

[0055] The content ratio of the second resin particles relative to the non-white ink is preferably 2.0% by mass or more and 15.0% by mass or less, and more preferably 4.0% by mass or more and 10.0% by mass or less. By setting the content ratio of the second resin particles to 2.0% by mass or more, the adhesion of the image formed by the non-white ink to the recording medium can be optimized. By setting the content ratio of the second resin particles to 12.0% by mass or less, the coating properties of the non-white ink can be optimized. Furthermore, the ejection stability of the non-white ink when applying it to the recording medium by the inkjet method can be optimized.

[0056] Other components of the non-white ink, such as the aqueous medium and pigment dispersion resin, and their appropriate addition amounts are the same as those for the white ink described above.

[0057] [3. Image Recording Method] Next, an image recording method using the inkjet recording apparatus of the present invention will be described. The inkjet recording apparatus used in the image recording method of the present invention is not particularly limited, and serial head type or line head type inkjet recording apparatus can be used. In particular, with inkjet recording apparatuses using aqueous pigment ink, ink repellency and bleeding tend to occur when a plastic sheet is used as the recording medium, but these problems are less likely to occur with the image recording method of the present invention.

[0058] Figure 1 is a schematic diagram showing the structure of an inkjet recording apparatus 100 according to the first embodiment of the present invention. Figure 2 is an enlarged view of the area around the coating section 2, recording section 3, and first drying section 5 in Figure 1. As shown in Figure 1, the inkjet recording apparatus 100 includes a sheet feeding section 1, a coating section 2, a recording section 3, a transport plate 4, a first drying section 5, a second drying section 6, a sheet winding section 7, a control section 10, and an operation panel 11.

[0059] The sheet S, which is a long, rectangular printing substrate (in this case, a plastic sheet), is installed in the sheet feeding unit 1, which is located upstream of the recording unit 3 in the transport direction (left side in Figure 1). The sheet feeding unit 1 includes a rotating shaft on which the roll-shaped sheet S is mounted, and a motor (neither shown) that rotates the rotating shaft in a predetermined direction. The sheet S is fed out downstream in the transport direction (right side in Figure 1) as the rotating shaft rotates due to the motor's operation.

[0060] The coating unit 2 is located downstream of the sheet feeding unit 1 (right side in Figure 1) in the sheet transport direction. The coating unit 2 is equipped with a first recording head 2a. The first recording head 2a has the same structure as the second recording heads 3a to 3d of the recording unit 3, which will be described later, and ejects the aforementioned white ink onto the sheet S fed from the sheet feeding unit 1 using an inkjet method.

[0061] The recording unit 3 is located downstream of the coating unit 2 (right side in Figure 1) in the sheet transport direction. The recording unit 3 is equipped with second recording heads 3a, 3b, 3c, and 3d. Four non-white inks (yellow, magenta, cyan, and black) stored in ink tanks (not shown) are supplied to the second recording heads 3a to 3d, one for each color. The recording unit 3 uses the second recording heads 3a to 3d to record an image on the sheet S, which is fed out from the sheet feeding unit 1 and coated with white ink by the coating unit 2.

[0062] A transport plate 4 is positioned below the coating unit 2 and the recording unit 3. The sheet S, which is fed out from the sheet dispensing unit 1, is supported by the transport plate 4 at a predetermined distance from the recording unit 2 and the coating unit 3.

[0063] A first drying unit 5 is positioned between the coating unit 2 and the recording unit 3. The first drying unit 5 is a known drying device, such as a hot air drying device (dryer) that blows out hot air. By blowing hot air onto the sheet S, which is supported and transported by the transport plate 4, the white ink coated on the sheet S by the coating unit 2 is dried.

[0064] In the drying process by the first drying section 5 after the application of white ink (first drying step), if the film formation of the first resin particles contained in the white ink occurs prematurely, it will form a lid on the surface of the white ink, hindering its drying. Therefore, the drying temperature of the white ink in the first drying section 5 is set below the first minimum film formation temperature of the first resin particles contained in the white ink. As a result, film formation of the first resin particles does not occur in the first drying step, and the drying of the white ink is not hindered.

[0065] A second drying unit 6 is located downstream of the recording unit 3 (to the right in Figure 1) in the sheet transport direction. The configuration of the second drying unit 6 is, for example, a hot air drying device similar to that of the first drying unit 5. The ink discharged onto the sheet S by the recording unit 3 is dried as it passes through the second drying unit 6, and the ink is fixed onto the sheet S.

[0066] In the drying process by the second drying section 6 after ink ejection (second drying step), the drying temperature in the second drying section 6 is set to be above the higher of the first minimum film-forming temperature of the first resin particles contained in the white ink and the second minimum film-forming temperature of the second resin particles contained in the non-white ink, i.e., above the first minimum film-forming temperature and the second minimum film-forming temperature. As a result, in the second drying step, film formation of the first resin particles contained in the white ink occurs simultaneously with film formation of the second resin particles contained in the non-white ink. Therefore, good adhesion and abrasion resistance of the ink to the sheet S can be obtained.

[0067] Furthermore, the drying temperature in the second drying section 6 is set below the temperature at which sheet S deteriorates. For example, if sheet S is a biaxially oriented polyethylene (OPP) sheet, setting the drying temperature in the second drying section 6 to below 80°C can suppress twisting (thermal deformation) of the OPP sheet and thus suppress quality deterioration.

[0068] When the first drying section 5 and the second drying section 6 are hot air drying devices, the wind speed of the hot air blown from the first drying section 5 and the second drying section 6 is preferably 0.5 m / sec to 1 m / sec. Furthermore, it is more preferable that the slit of the drying nozzle (the part from which the hot air is blown out) be 1 to 5 mm, and the distance from the printed material to the drying nozzle be 5 to 10 mm.

[0069] In this embodiment, a hot air dryer is used to dry the white ink and non-white ink in the first drying section 5 and the second drying section 6. However, other drying devices may be used as the first drying section 5 and the second drying section 6. For example, a configuration comprising a heater and a reflector may be used. As the heater, an infrared heater, halogen heater, ceramic heater, or induction heating (IH) type heater can be used.

[0070] After passing through the second drying section 6 and the ink has been fixed, the sheet S has its transport direction changed by the guide roller 7 and is then wound into a roll in the sheet winding section 8.

[0071] The control unit 10 controls the ejection of white ink in the coating unit 2 and the ejection of non-white ink in the recording unit 3 according to the image data received from an external computer. The control unit 10 also determines whether or not to eject white ink in the coating unit 2 and the amount to be ejected, as will be described later.

[0072] The control panel 11 functions as a touch panel for the user to make various settings and inputs for the inkjet recording device 100, and also displays the status of the inkjet recording device 100, image recording status, number of printed pages, etc.

[0073] Furthermore, the control unit 10 determines whether or not to apply white ink to the coating unit 2 and the amount to apply, according to the wetting tension of the sheet S. Specifically, a table that associates the type of sheet S with its surface tension (wetting tension) is stored in a storage unit (not shown) beforehand. Then, at the start of printing, the type of sheet S to be used is entered from the operation panel 11 of the inkjet recording device 100. The control unit 10 reads the surface tension corresponding to the entered type of sheet S from the table stored in the storage unit, and determines whether or not to apply white ink to the coating unit 2 and the amount to apply based on the read surface tension.

[0074] This makes it possible to adjust the wettability of sheet S to an optimal range, suppressing ink bleeding and repelling and enabling the acquisition of good images. In addition, if the wettability of sheet S is high, the white ink coating process is not performed, thus reducing the consumption of unnecessary white ink.

[0075] When the surface tension of sheet S is 45 mN / m or less, it is preferable to apply white ink to sheet S in the coating section 2. Furthermore, it is preferable to increase the amount of white ink applied as the surface tension of sheet S decreases.

[0076] According to the image recording method described above, a white ink containing first resin particles is applied to the sheet S before image formation using aqueous pigment ink with the inkjet recording device 100. This increases the wettability (wetting tension) of the sheet, suppressing the repulsion and bleeding of non-white ink and improving image quality. For this reason, the image recording method of the present invention can be suitably used in various inkjet recording devices that use aqueous ink.

[0077] Furthermore, by setting the drying temperature in the first drying section 5 below the first minimum film-forming temperature, premature film formation of the first resin particles in the white ink can be suppressed, thereby promoting the drying of the white ink. Additionally, by setting the drying temperature in the second drying section 6 to be higher than the higher of the first minimum film-forming temperature of the first resin particles and the second minimum film-forming temperature of the second resin particles, the first resin particles in the white ink and the second resin particles in the non-white ink can be reliably film-formed, ensuring good adhesion and abrasion resistance of the ink to the sheet S.

[0078] Figure 3 is a schematic diagram showing the structure of an inkjet recording apparatus 100 according to a second embodiment of the present invention. In this embodiment, a corona discharge apparatus 9 is provided for performing corona treatment on a sheet S. The configuration of other parts of the inkjet recording apparatus 100 is the same as that of the first embodiment shown in Figures 1 and 2.

[0079] The corona discharge device 9 is positioned upstream of the coating section 2 (left side in Figure 3) with respect to the sheet transport direction. The corona discharge device 9 adjusts the wetting tension of the sheet S by performing corona discharge treatment on the sheet S. By corona treating the sheet S, good adhesion and abrasion resistance can be obtained with respect to the pretreatment liquid discharged in the coating section 2.

[0080] Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. For example, Figures 1 and 3 describe an inkjet recording device 100 that feeds out a long sheet S wound in a roll at a predetermined speed and ejects ink, but the present invention is similarly applicable to an inkjet recording device that ejects ink onto a sheet cut into a predetermined shape.

[0081] Furthermore, in each of the above embodiments, the first recording head 2a is configured to eject white ink using an inkjet method. However, the method is not limited to an inkjet method, as long as it is possible to uniformly apply white ink to the sheet S. For example, it can also be applied by a spray method.

[0082] Furthermore, in each of the above embodiments, the first drying unit 5 is placed immediately downstream of the coating unit 2 that discharges the pretreatment liquid, and the second drying unit 6 is placed immediately downstream of the recording unit 3 that discharges the ink, but the invention is not limited to this. For example, the third drying unit may be placed downstream of the second drying unit 6, allowing the third drying process to be performed after the second drying process. Also, if the recording unit 3 has multiple recording heads 3a to 3d, the second drying unit 6 may be placed immediately downstream of each of the recording heads 3a to 3d.

[0083] Furthermore, the material of the recording medium is not limited to plastic film; various recording media can be used, such as plain paper, coated paper, natural fibers, synthetic fiber fabrics, knitted fabrics, or nonwoven fabrics. The effects of the present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0084] [Manufacturing Example 1] (Manufacturing of white ink) White inks I-1 to I-7 and non-white inks were prepared using the following method. First, a description of the commercially available ink resin particle dispersions used in the preparation of the white and non-white inks is given below. Details of each ink resin particle dispersion are shown in Table 1.

[0085] (1-1. Resin particle dispersion for ink) A-1: Superflex 820 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) A-2: Superflex 130 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) A-3: Superflex 420NS (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) A-4: Superflex 870 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) B-1: UC-6600 (manufactured by Saiden Chemical Co., Ltd.) C-1: Sepolion NE205 (manufactured by Sumitomo Seika Co., Ltd.)

[0086] [Table 1]

[0087] (1-2. Preparation of Pigment Dispersions) 75 g of pigment dispersion resin (DISPERBYK-190, manufactured by Bic Chemie Japan, non-volatile content: 40% by mass, dispersion medium: water) was diluted with 775 g of deionized water, and 150 g of white pigment (JR-804, manufactured by Teika Co., Ltd.) was added. Pre-mixing was then performed using a homodisper at a rotation speed of 5000 rpm for 1 hour. Subsequently, dispersion treatment was performed using a bead mill (manufactured by Nippon Coke Co., Ltd.) to obtain a white pigment dispersion. Zirconia beads (0.2 mmφ) were packed into the vessel at a packing density of 80% of the volume.

[0088] (1-3. Preparation of white ink) White inks I-1 to I-7 were prepared by mixing each component, including the resin particle dispersion shown in Table 1, in the proportions shown in Table 2, and stirring and mixing them using the same method as in Production Example 2 described later. In Table 2, "surfactant" refers to a polyether-modified silicone surfactant (SAG503A, manufactured by Nisshin Chemical Co., Ltd.). "PG" refers to propylene glycol, "BTG" refers to butyl triglyceride, and "DO" refers to 1,4-dioxane.

[0089] [Table 2]

[0090] [Manufacturing Example 2] (Manufacturing of non-white inks) (2-1. Preparation of Pigment Dispersions) A color (cyan) pigment dispersion was prepared using the same method as in Production Example 1, except that a blue pigment (pigment blue 15:03) was used instead of a white pigment.

[0091] (2-2. Ink Preparation) 30% by mass of propylene glycol, 10% by mass of butyl triglyceride, 1% by mass of surfactant (Surfinol 104, manufactured by Nisshin Chemical Industry Co., Ltd.), and 0.5% by mass of 1% sodium hydroxide aqueous solution were weighed into a beaker. Next, the cyan pigment dispersion obtained in 2-1 above and the resin particle dispersion (Superflex 130, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) shown in Table 1 A-2 were weighed so that the pigment dispersion and binder resin amounted to 5% by mass, and water was added to make a total of 100% by mass. The contents of the beaker were stirred at a rotation speed of 400 rpm using a stirrer (Three One Motor BL-600, manufactured by Shinto Kagaku Co., Ltd.) to uniformly mix the contents. The mixture was filtered using a filter (pore size 5 μm) to remove foreign matter and coarse particles contained in the mixture to obtain a non-white ink (cyan ink).

[0092] [Evaluation of ink bleeding, abrasion resistance, and substrate quality] The ink bleeding, image abrasion resistance, and substrate quality were evaluated when images were formed using the white inks I-1 to I-7 obtained in Manufacturing Example 1 and the non-white ink (cyan ink) obtained in Manufacturing Example 2. The test machine used for evaluation was an inkjet recording device as shown in Figure 1, equipped with a piezo-type inkjet head (KJ4B-1200, manufactured by Kyocera Corporation) that ejects white ink and non-white ink.

[0093] Using the above test machine, printing was performed with a head applied voltage of 21V, a drive frequency of 20kHz, an appropriate amount of ejected liquid of 3pl, a print head temperature of 32℃, a print head resolution of 1200dpi, and 1000 pre-ejection flushing cycles. As the printing substrate, a non-absorbent OPP film (Toray Industries, Inc.) was used.

[0094] <Evaluation Method> (Ink bleeding) By varying the drying temperatures in the first and second drying processes using white ink, a solid horizontal band image was printed on a printing substrate coated with white ink using a non-white ink, and printed materials of Inventions 1-9 and Comparative Examples 1-12 were produced. The vertical length (band width) of the bands in the printed materials of Inventions 1-9 and Comparative Examples 1-12 was measured. On the other hand, as a comparative example (reference image), the same solid band image was printed on a specially processed polyethylene terephthalate (PET) film (GT703, manufactured by Toyobo Co., Ltd.) that has fine pores and does not cause bleeding, and the vertical length of the band was measured in the same way. The rate of increase in the vertical length of the band compared to the reference image was evaluated as the degree of ink bleeding. (Evaluation Criteria) ○: The rate of increase in the vertical length of the band is less than 3%. △: The rate of increase in the vertical length of the band is 3% or more but less than 5%. ×: The rate of increase in the vertical length of the band is 5% or more but less than 10%.

[0095] (Abrasion resistance) The ink peeling of the printed materials of Inventions 1-9 and Comparative Examples 1-12 was visually confirmed after rubbing them once with a 1000g weight. (Evaluation Criteria) ◎: No ink peeling was observed. ○: Ink peeling of 5% or less can be observed. △: Less than 10% of the ink peeling is visible. ×: More than 10% of the ink peeling was observed.

[0096] (Base material quality) The quality of the printing substrate (OPP sheet) for the printed materials of Inventions 1-9 and Comparative Examples 1-12 was visually evaluated. (Evaluation Criteria) ○: The quality of the OPP sheet remains unchanged compared to before printing. △: The OPP sheet is slightly twisted compared to before printing. ×: The OPP sheet is significantly twisted compared to before printing.

[0097] Table 3 shows the evaluation results for ink bleeding, abrasion resistance, and substrate quality, along with the type of white ink and drying conditions.

[0098] [Table 3]

[0099] As is clear from Table 3, in Invention 1 to 9, in which urethane resin particles (first resin particles) with a minimum film-forming temperature of 40-80°C were added to white ink, the drying temperature in the first drying step was set lower than the minimum film-forming temperature of the first resin particles in the white ink, and the drying temperature in the second drying step was set to be equal to or higher than the minimum film-forming temperature of the first resin particles and the second resin particles in the non-white ink, the increase in the vertical length of the printed material's strip was less than 3% in all cases, there was no peeling of the ink when rubbed with a 1000g weight, and no deterioration in the quality of the OPP sheet used as the printing substrate was observed.

[0100] In contrast, in Comparative Examples 1 to 9, where the drying temperature in the first drying step was set to a temperature equal to or higher than the minimum film-forming temperature of the first resin particles in the white ink, the increase in the vertical length of the printed strip was 3% or more in all cases. This is thought to be because the film formation of the first resin particles in the white ink proceeded early, hindering the evaporation of moisture, etc., resulting in insufficient drying of the white ink during printing with non-white ink. Furthermore, in Comparative Examples 7 and 9, where the drying temperature in the second drying step was set to 90°C, the OPP sheet used as the printing substrate was significantly more twisted compared to before printing.

[0101] Furthermore, in Comparative Examples 10-12, where acrylic resin particles and polyamide elastomer resin particles were added to the white ink instead of urethane resin particles, the ink peeling rate exceeded 10% when the printed material was rubbed with a 1000g weight. This is likely because in Comparative Examples 10-12, the drying temperature in the second drying step was set below the minimum film-forming temperature of the resin particles in the white ink (first resin particles) and the resin particles in the ink (second resin particles). Due to insufficient heating, the resin could not form a film, and the coating quality could not be guaranteed, resulting in insufficient abrasion resistance.

[0102] From the results of Inventions 1-9 and Comparative Examples 1-12, it was confirmed that by adding first resin particles with a minimum film-forming temperature of 40-80°C to white ink, and second resin particles with a minimum film-forming temperature of 35-80°C to non-white ink, and by setting the drying temperature in the first drying step below the minimum film-forming temperature of the first resin particles, and setting the drying temperature in the second drying step to the higher of the minimum film-forming temperatures of the first and second resin particles, ink bleeding can be effectively suppressed and the abrasion resistance of the printed material can be improved. Furthermore, it was confirmed that by setting the drying temperature in the second drying step to 80°C or lower, twisting of the OPP sheet after printing can be suppressed even when the printing substrate is an OPP sheet that is prone to thermal degradation. [Industrial applicability]

[0103] The present invention is applicable to inkjet recording devices that eject ink onto a recording medium. By utilizing the present invention, it is possible to provide an inkjet recording device that can suppress bleeding of non-white ink due to poor drying of white ink coated on the recording medium, and also improve the abrasion resistance of the ink after printing. [Explanation of Symbols]

[0104] 1. Sheet dispensing unit (conveying unit) 2. Application area 2a First recording head 3. Records Section 3a~3d Second recording head (first recording head) 4. Conveyor board 5 1st drying section 6 Second drying section 8. Sheet winding section (conveying section) 9. Corona treatment device (corona processing unit) 10 Control Unit 11. Control Panel 100 Inkjet Recording Devices S Sheet (Recording medium)

Claims

1. A transport unit that transports the recording medium, A coating unit that applies white ink to the recording medium being transported by the transport unit, A first drying unit is positioned downstream of the coating unit with respect to the transport direction of the recording medium, and dries the white ink applied to the recording medium by the coating unit. A recording unit is positioned downstream of the first drying unit with respect to the transport direction of the recording medium, and ejects non-white ink onto the recording medium. A second drying unit is positioned downstream of the recording unit in the transport direction of the recording medium and dries the non-white ink applied to the recording medium by the recording unit, It has at least the following features: The aforementioned white ink contains first resin particles having a minimum film-forming temperature of 40°C or higher and 80°C or lower. The non-white ink contains second resin particles having a minimum film-forming temperature of 80°C or less. When the minimum film-forming temperatures for the first resin particles and the second resin particles are set to the first minimum film-forming temperature and the second minimum film-forming temperature, An inkjet recording apparatus characterized in that the drying temperature in the first drying section is less than the first minimum film formation temperature, and the drying temperature in the second drying section is equal to or greater than the first minimum film formation temperature and the second minimum film formation temperature.

2. The inkjet recording apparatus according to claim 1, characterized in that the second resin particles have a minimum film-forming temperature of 35°C or higher.

3. The inkjet recording apparatus according to claim 1, characterized in that the white ink contains 1,4-dioxane.

4. The inkjet recording apparatus according to claim 1, characterized in that the coating section has a first recording head for ejecting the white ink, and the recording section has one or more second recording heads for ejecting the non-white ink.

5. The recording medium is a sheet made of resin, The inkjet recording apparatus according to claim 1, characterized in that it has a corona processing unit which is arranged upstream of the coating unit with respect to the transport direction of the recording medium and performs corona discharge processing on the sheet.

6. The recording medium is a sheet made of biaxially oriented polypropylene. The inkjet recording apparatus according to any one of claims 1 to 5, characterized in that the drying temperature in the second drying section is 80°C or lower.

Images