Inkjet ink and inkjet printer

Abstract

To provide: an inkjet ink which can suppress occurrence of satellite and mist even when the distance from an inkjet head to a recording medium is large while maintaining stable dischargeability of ink, and can form high quality images even on a recording medium which has irregularity on its surface; an inkjet printer; and an inkjet recording method.SOLUTION: An inkjet ink according to an embodiment comprises a coloring agent and a resin, where a content ratio of water is less than 10 mass% and, when a dynamic surface tension measured by a maximum foam pressure method at 25°C at a surface lifetime of 0.015 second is denoted by γ1 and a dynamic surface tension at a surface lifetime of 1.2 second is denoted by γ2, a variation X of the dynamic surface tension per second represented by the following formula is 15 mN / m or more: X=(γ1-γ2) / (1.2-0.015).SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to inkjet inks and inkjet printers. [Background technology]

[0002] Rapid advances in inkjet image formation technology in recent years have made it possible to produce high-quality images comparable to photographs. Inkjet technology is characterized by its non-contact technology and ability to precisely deposit the right amount of ink onto the desired location, and its application is expanding. At the same time, the variety of recording media and inks available varies, depending on the application. In response to the unevenness of the recording medium surface, there is an increasing demand for greater distance between the inkjet head's nozzle surface and the recording medium surface, i.e., greater printing gap.

[0003] When printing at low speeds or with small gaps, the landing positions of the main droplets and the mist and satellites are generally the same when the ink lands on the surface of the recording medium, so misalignment of the landing positions of the mist and satellites relative to the landing position of the main droplets has not traditionally been a problem. However, as printing speeds increase and gaps become larger, larger misalignments occur between the landing positions of the main droplets and the mist and satellites, which can cause poor print quality.

[0004] One technique to improve this is to increase the static surface tension of the ink. It is generally known that increasing the static surface tension of the ink can reduce the occurrence of mist and satellites, i.e., the deviation of the landing position of the mist or satellites from the landing position of the main droplet. However, when attempting to reduce the occurrence of mist or satellites by increasing the static surface tension of the ink, there is a problem that droplets are more likely to fail to be ejected when the ink is continuously ejected. This phenomenon is thought to be caused by an unstable ejection state when ink droplets are ejected from the head due to the ink's static surface tension being too high.

[0005] Techniques for adjusting the dynamic surface tension of aqueous inkjet inks have been reported to improve the desired performance. However, no inkjet ink has been proposed that can maintain ink ejection stability and suppress the generation of mist or satellites due to misalignment of the ink landing position, even when the distance (gap) between the inkjet head and the recording medium surface is increased for recording media with uneven surfaces. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5478338 [Patent Document 2] Patent No. 6065271 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide an inkjet ink and an inkjet printer that can suppress the generation of mist and satellites while maintaining stable ink ejection even when the gap from the inkjet head to the recording medium is large, and that can form high-quality images even on recording media with uneven surfaces. [Means for solving the problem]

[0008] According to a first embodiment, there is provided an inkjet ink that contains a colorant and a resin, has a water content of less than 10% by mass, and has a change in dynamic surface tension per second, X, expressed by the following formula, of 15 mN / m or more, where γ1 is the dynamic surface tension at a surface life of 0.015 seconds and γ2 is the dynamic surface tension at a surface life of 1.2 seconds, as measured at 25°C by the maximum bubble pressure method: X = (γ1 - γ2) / (1.2 - 0.015)

[0009] According to a second embodiment, an inkjet printer is provided that includes an inkjet head that ejects ink toward a recording medium and a medium holding mechanism that holds the recording medium opposite the inkjet head, wherein the ink is the inkjet ink according to the above embodiment. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a perspective view showing a schematic configuration of an inkjet head provided in an inkjet printer according to the second embodiment. [Figure 2] FIG. 2 is an exploded perspective view showing a partial schematic configuration of the inkjet head. [Figure 3] FIG. 3 is an explanatory diagram showing the configuration of an inkjet printer according to the second embodiment. [Figure 4] FIG. 4 is a schematic top view for explaining a method for evaluating the state of mist or satellite generation on a recording medium in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] The embodiments will be described in detail below. Note that "mist" and "satellite" are sometimes used separately, but are sometimes used interchangeably. That is, "mist" refers to ink that has separated from a main droplet dropped from an inkjet printer head and become mist-like, and "satellite" refers to ink that has separated from the main droplet but still retains its droplet shape. As mentioned above, they are sometimes used interchangeably, but are often used as a general term for the above phenomena. In this specification, the terms "mist" and "satellite" are not strictly distinguished from each other, and the terms "mist" and "satellite" are used as general terms for droplets that land in a location other than the originally intended location, i.e., droplets that have separated from the main droplet and land at a location offset from the main droplet. <Inkjet ink> The inkjet ink according to the first embodiment is intended to be ejected from an inkjet head of an inkjet printer. The inkjet ink according to this embodiment is an inkjet ink in which the change in dynamic surface tension per second, X, as expressed by the following formula I, is 15 mN / m or more, where γ1 is the dynamic surface tension at a surface life of 0.015 seconds and γ2 is the dynamic surface tension at a surface life of 1.2 seconds, as measured at 25°C by the maximum bubble pressure method. X=(γ1-γ2) / (1.2−0.015) (Formula I)

[0012] Here, dynamic surface tension is a value measured at 25°C by the maximum bubble pressure method, and can be measured using, for example, a dynamic surface tensiometer (SITA pro line t15) (manufactured by SITA Messtechnik GmbH). The "surface life" is also called "bubble life time" (BLTime), and refers to the life of a bubble generated by the maximum bubble pressure method, and refers to the time from when a new interface is generated within the probe tip of the dynamic surface tensiometer until the maximum bubble pressure is reached.

[0013] The change in dynamic surface tension X calculated by formula (I) is the change in dynamic surface tension per second when the surface life is between 0.015 and 1.2 seconds, or in other words, the slope of the dynamic surface tension value when the surface life is between 0.015 and 1.2 seconds. Here, the surface life of "0.015 seconds" was selected from the perspective of the minimum surface life measurable by a dynamic surface tensiometer, for reasons explained in the next paragraph. On the other hand, the surface life of "1.2 seconds" was selected as the surface life at which the dynamic surface tension value approaches equilibrium over time and approaches a value close to the static surface tension value.

[0014] The reason why "0.015 seconds" was selected as the surface life is the minimum surface life that can be measured using a dynamic surface tensiometer is as follows. It is believed that the higher the surface tension of ink droplets ejected from an inkjet head, the easier they are to unite, reducing the occurrence of mist and satellites, which are small droplets scattering. Because ink is ejected from an inkjet head at a high driving frequency, forming and ejecting droplets in microseconds (μs), the instantaneous surface tension of droplets formed during ejection is expected to have a very short surface life, measured in μs, due to dynamic surface tension. It is believed that this momentary surface tension contributes to the aggregation of ejected droplets, but no dynamic surface tensiometer currently exists that can measure dynamic surface tension down to the μs level. For this reason, as mentioned above, we decided to convert the minimum measurable surface life (0.015 seconds) into a surface life measured in μs.

[0015] The dynamic surface tension was measured with surface lifetimes of 0.015 seconds and 1.2 seconds, and it was discovered that the slope (amount of change per second) of the surface tension value from static (1.2 seconds) to dynamic (0.015 seconds) can be used as a parameter for inkjet ink that suppresses the generation of satellites and mist and enables the formation of high-quality images, leading to the completion of the present invention. In other words, when this amount of change X is 15 mN / m or more, the surface tension of the ink droplets when ejected from the inkjet head is sufficiently high, making it easier for the droplets to coalesce into a single droplet and suppressing the generation of mist and satellites, which are caused by small amounts of droplets scattering.

[0016] In this embodiment, the change in dynamic surface tension X per second between the surface life of 0.015 seconds and 1.2 seconds is 15 mN / m or more as described above, and the higher the value, the better. The upper limit is a design factor that depends on the specific application of the technology, and is not specifically set here.

[0017] The inkjet ink according to this embodiment is capable of suppressing the generation of mist and satellites without increasing the static surface tension, and therefore has excellent ejection stability. In this embodiment, the static surface tension of the inkjet ink may be, for example, in the range of 20 mN / m to 40 mN / m. The static surface tension in this embodiment is a value measured at 25°C by the Wilhelmy method.

[0018] The components contained in the inkjet ink according to this embodiment will be described below. The inkjet ink according to this embodiment contains at least a colorant and a resin, and basically further contains a solvent. The inkjet ink according to this embodiment contains at least an organic solvent as the solvent, but may or may not contain water. Here, "water-free" means that the inkjet ink is manufactured without intentionally adding water. For example, water that is contained in the ink due to water vapor contained in the atmosphere or water contained in additives, which is not intended by the manufacturer, is not considered to be water as a solvent and is treated as "water-free." In this case, the water content is considered to be "0% by mass."

[0019] In the inkjet ink according to this embodiment, the water content is less than 10% by mass based on the total mass of the ink. Here, "less than 10% by mass" in relation to the water content includes "no water contained" in the above-mentioned sense, and therefore, in the inkjet ink according to this embodiment, the water content is 0% or more and less than 10% by mass. In the inkjet ink according to this embodiment, the water content is more preferably 0% or more and 5% by mass or less.

[0020] The components contained in the inkjet ink according to this embodiment can include colorants and resins commonly used in inkjet inks, as well as other optional components, as appropriate for the intended purpose, as long as the change in dynamic surface tension X, as expressed by the above-described formula I, is 15 mN / m or greater. The dynamic surface tension can be adjusted appropriately by selecting the components contained in the inkjet ink and adjusting their blending amounts. Generally, components with high surface tension tend to have a significant effect on the dynamic surface tension. Therefore, for example, since water has a higher surface tension than organic solvents and ultraviolet-curable resins, when using water, it is possible to adjust the dynamic surface tension by paying attention to the blending ratio.

[0021] (coloring agent) Pigments are preferably used as the colorant contained in the inkjet ink according to this embodiment. Any pigment may be used as the colorant, as long as it is a coloring material generally known as a pigment, has the desired optical coloring or coloring function, and is dispersible in the ink. The pigment may be, for example, an inorganic pigment, an organic pigment, or glass particles. Furthermore, the pigment may have properties such as a thermochromic pigment, or may exhibit other properties in addition to coloring and coloring properties, such as magnetism, fluorescence, conductivity, or dielectricity. In this case, various functions can be imparted to the image. Furthermore, powders that improve heat resistance or physical strength can also be added.

[0022] Examples of inorganic pigments that can be used include carbon black and those obtained as pulverized minerals, such as titanium oxide, calcium carbonate, kaolin, aluminum, ceramics, and glass. Synthetic inorganic pigments can also be used, such as iron oxide red, cadmium yellow, nickel titanium yellow, strontium yellow, hydrous chromium oxide, chromium oxide, cobalt aluminate, and synthetic ultramarine blue.

[0023] Examples of usable organic pigments include polycyclic pigments such as phthalocyanine pigments and anthraquinone pigments. Specific examples include isoindolinone, isoindoline, azomethine, anthraquinone, anthrone, xanthene, diketopyrrolopyrrole, perylene, anthraquinone (anthrone), perinone, quinacridone, indigoid, quinacridone, diketopyrrolopyrrole, anthraquinone, perylene, perinone, indigoid, dioxazine, quinacridone, perylene, indigoid, anthraquinone (anthrone), xanthene, phthalocyanine, anthraquinone, indigoid, phthalocyanine, azomethine, and perylene. Azo pigments, lake pigments, and fluorescent pigments can also be used.

[0024] Examples of usable glass particles include colored glass particles and colorless glass particles, and may be colored or colorless glass composition mixture particles that are vitrified by heat treatment.

[0025] The particle size of the pigment contained in the inkjet ink according to this embodiment is preferably as small as possible so that it can be ejected by inkjet and still exhibit its function. The average particle diameter of the pigment is, for example, preferably in the range of 0.01 μm to 5 μm, and more preferably in the range of 0.01 μm to 1 μm. Here, the average particle diameter of the pigment is the cumulant average particle diameter obtained by cumulant analysis. For example, the average particle diameter of the pigment can be determined by diluting an ink sample about 500 times with a solvent, measuring the particle diameter of the diluted sample by dynamic light scattering using a dynamic light scattering particle size analyzer, and then calculating the cumulant average particle diameter by cumulant analysis.

[0026] The pigment preferably has a volume cumulative particle diameter D90 of 1 μm or less, and a volume cumulative particle diameter D50 of 0.1 μm to 0.5 μm. Here, the volume cumulative particle diameter D90 is the particle diameter at which the total volume of particles, calculated from the smallest particle diameter to a certain particle diameter in the particle size distribution of the pigment, is 90% when expressed as a percentage of the total particle volume, and the volume cumulative particle diameter D50 is the particle diameter at which this percentage is 50%. The volume cumulative particle diameter D90 and volume cumulative particle diameter D50 are measured by dynamic light scattering.

[0027] As described above, the pigment contained in the inkjet ink according to this embodiment preferably has a cumulant average particle diameter in the range of 0.01 μm or more and 5 μm or less in one example, and in another example, 0.01 μm or more and 1 μm or less, a volume cumulative particle diameter D90 of 1 μm or less, or a volume cumulative particle diameter D50 of 0.1 μm or more and 0.5 μm or less, and more preferably satisfies all of these.

[0028] The pigment content is preferably in the range of 1% to 30% by mass based on the total mass of the ink. If the pigment content is less than 1% by mass, it may be difficult to ensure sufficient color density when used as a colorant in subsequent processing. On the other hand, if it exceeds 30% by mass, stability may decrease. The pigment content is more preferably in the range of 1% to 10% by mass based on the total mass of the ink. However, titanium oxide, which is generally used as a pigment in white ink, has a high specific gravity, so in that case the pigment may be formulated in a higher proportion than the above.

[0029] The inkjet ink according to this embodiment may contain a dye as an auxiliary component of the pigment in order to adjust the color. Examples of dyes that are typically used include azoic dyes, sulfide (building material) dyes, disperse dyes, fluorescent brighteners, and oil-soluble dyes, which have low acidity and basicity and high solubility in solvents. Of these, oil-soluble dyes such as azo dyes, triarylmethane dyes, anthraquinone dyes, and azine dyes are preferred.

[0030] (resin) The resin contained in the inkjet ink according to this embodiment is not particularly limited, and may be any resin that dissolves or disperses in the organic solvent (described below) blended into the ink, and may be selected appropriately depending on the type of medium. Examples of resins include acrylic resin, polyester resin, phenolic resin, polyamide, polyvinyl butyral, cellulose acetate butyrate, nitrocellulose resin, polyurethane, vinyl chloride-vinyl acetate copolymer, etc. These may be used alone or in combination of two or more.

[0031] In addition, in the inkjet ink according to this embodiment, an ultraviolet curable resin or a thermosetting resin may be used. The ultraviolet curable resin may be any radical polymerizable compound that polymerizes in the presence of a radical polymerization initiator when irradiated with ultraviolet light, such as acrylic acid oligomers, acrylic acid monomers, N-vinyl compounds, vinyl esters, acrylamides, aromatic vinyls, allyl compounds, etc. Specific examples of ultraviolet curable resins include urethane acrylate, acrylic resin acrylate, epoxy acrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-dimethylaminoethyl acrylate, and 2-hydroxyethyl acrylate.

[0032] Examples of the photopolymerization initiator include benzoin isopropyl ether, benzophenone, chlorothioxanthone, benzil dimethyl ketal, acetophenone diethyl ketal, α-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methyl-phenylpropane.

[0033] The thermosetting resin is not particularly limited as long as it is cured by heating, and examples thereof include an isocyanate compound, and a mixture of an epoxy compound with an amine or an amine derivative.

[0034] (organic solvent) The organic solvent contained in the inkjet ink according to this embodiment is not particularly limited, but it is preferable to use one that does not cause corrosion. Examples of such organic solvents include aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, organic acid esters, ethers, and ketones. From the standpoint of safety and solubility of resins, etc., organic acid esters are preferred as organic solvents. Specific examples include acetate esters such as methyl acetate, ethyl acetate, propyl acetate, butyl acetate, propylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethyl-3-ethoxypropionate, 3-methoxybutyl acetate, and 3-methyl-3-methoxybutyl acetate, which are readily available and suitable. Furthermore, water-soluble esters such as propylene glycol monomethyl ether acetate and ethylene glycol monoethyl ether acetate are suitable because they can be used in both oil-based and water-based inks.

[0035] (dispersant) In the inkjet ink according to this embodiment, a dispersant may be used as needed. For example, any dispersant used in conventionally known solvent-based ink compositions may be used as the dispersant. A polymer dispersant (resin) may be used as the dispersant. Such dispersants have a main chain made of a polyester, polyacrylic, polyurethane, polyamine, polycaprolactone, or the like, and have polar groups such as amino, carboxyl, sulfone, or hydroxyl groups as side chains.

[0036] (surfactant) The inkjet ink according to this embodiment may contain a surfactant, if necessary. Any of anionic, cationic, amphoteric, or nonionic surfactants may be used as the surfactant, and the surfactant may be appropriately selected depending on the purpose of addition. For example, a nonionic polyoxyethylene derivative that is liquid at room temperature and atmospheric pressure may be added to suppress evaporation of the solvent-based ink composition in devices such as the nozzle or tube, prevent solidification, or facilitate resolubilization after solidification.

[0037] (Other additives) The inkjet ink according to this embodiment may further contain conventionally known additives as needed, such as viscosity adjusters, pH adjusters, surface tension adjusters, dispersion aids, sensitizers, leveling agents, antifoaming agents, antioxidants, preservatives, antifungal agents, charge adjusters, and wetting agents.

[0038] The inkjet ink according to this embodiment must have a viscosity suitable for ejection from the head nozzles of an inkjet printer. For this reason, the inkjet ink according to this embodiment preferably has a viscosity during ejection of 5 mPa·s or more and 15 mPa·s or less, and 7 mPa·s or more and 11 mPa·s or less, for example.

[0039] <Inkjet printer> An inkjet printer according to a second embodiment will be described in detail below with reference to the drawings. Components that perform the same or similar functions are given the same reference numerals throughout the drawings, and duplicated descriptions will be omitted.

[0040] The inkjet printer according to this embodiment comprises an inkjet head that ejects ink toward a recording medium, and a media protection mechanism that holds the recording medium opposite the inkjet head, and uses the inkjet ink according to the first embodiment as the ink.

[0041] FIG. 1 is a perspective view showing the schematic configuration of an inkjet head provided in an inkjet printer according to a second embodiment. The inkjet head 1 shown in FIG. 1 is an on-demand type that is mounted on the head carriage of the inkjet printer. In the following explanation, a Cartesian coordinate system consisting of X, Y, and Z axes is used. For convenience, the direction indicated by the arrow in the figure is taken as the positive direction. The X-axis direction corresponds to the printing width direction. The Y-axis direction corresponds to the direction in which the recording medium is transported. The positive Z-axis direction faces the recording medium.

[0042] The inkjet head 1 includes an ink manifold 10, an actuator substrate 20, a frame 40, and a nozzle plate 50.

[0043] The actuator substrate 20 has a rectangular shape with its longitudinal direction aligned in the X-axis direction. Examples of materials for the actuator substrate 20 include alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), aluminum nitride (AlN), and lead zirconate titanate (PZT: Pb(Zr,Ti)O3).

[0044] The actuator substrate 20 is placed on top of the ink manifold 10 so as to close the open end of the ink manifold 10. The ink manifold 10 is connected to the ink cartridge via an ink supply pipe 11 and an ink return pipe 12.

[0045] A frame 40 is attached on the actuator substrate 20. A nozzle plate 50 is attached on the frame 40. A plurality of nozzles N are provided on the nozzle plate 50 at predetermined intervals along the X-axis direction so as to form two rows along the Y-axis.

[0046] 2 is an exploded perspective view showing a partial schematic configuration of the inkjet head 1, specifically, an exploded perspective view of the actuator substrate 20, the frame 40, and the nozzle plate 50. This inkjet head 1 is a so-called shear mode shared-wall side shooter type.

[0047] The actuator substrate 20 is provided with a plurality of ink supply ports 21 spaced apart along the X-axis direction so as to form a row in the center in the Y-axis direction. The actuator substrate 20 is also provided with a plurality of ink discharge ports 22 spaced apart along the X-axis direction so as to form rows in the positive and negative Y-axis directions relative to the row of ink supply ports 21.

[0048] A plurality of actuators 30 are provided between the central row of ink supply ports 21 and one row of ink discharge ports 22. These actuators 30 form a row extending in the X-axis direction. In addition, a plurality of actuators 30 are provided between the central row of ink supply ports 21 and the other row of ink discharge ports 22. These actuators 30 also form a row extending in the X-axis direction.

[0049] Each row of actuators 30 is composed of a first piezoelectric element and a second piezoelectric element stacked on an actuator substrate 20. Examples of materials for the first and second piezoelectric elements include lead zirconate titanate (PZT), lithium niobate (LiNbO3), and lithium tantalate (LiTaO3). The first and second piezoelectric elements are polarized in opposite directions along the thickness direction.

[0050] A laminate consisting of the first and second piezoelectric elements has a plurality of grooves, each extending in the Y-axis direction and aligned in the X-axis direction. These grooves open on the second piezoelectric element side and have a depth greater than the thickness of the second piezoelectric element. Hereinafter, the portion of this laminate sandwiched between adjacent grooves will be referred to as a channel wall. These channel walls each extend in the Y-axis direction and are aligned in the X-axis direction. The groove between two adjacent channel walls is the ink channel through which ink flows.

[0051] Electrodes are formed on the sidewalls and bottom of the ink channel, and these electrodes are connected to a wiring pattern 31 that extends along the Y-axis direction.

[0052] Except for the connection portion with a flexible printed circuit board (described later), a protective film (not shown) is formed on the surface of the actuator substrate 20 including the electrodes and wiring pattern 31. The protective film includes, for example, a multi-layer inorganic insulating film and an organic insulating film.

[0053] The frame 40 has an opening. This opening is smaller than the actuator substrate 20 and larger than the area of ​​the actuator substrate 20 where the ink supply port 21, the actuator 30, and the ink discharge port 22 are provided. The frame 40 is made of, for example, ceramics. The frame 40 is bonded to the actuator substrate 20 with, for example, an adhesive.

[0054] The nozzle plate 50 includes a nozzle plate substrate and a liquid-repellent film (not shown) provided on its medium-facing surface (the surface from which ink is ejected from the nozzles N). The nozzle plate substrate is made of a resin film such as a polyimide film.

[0055] The nozzle plate 50 is larger than the opening of the frame 40. The nozzle plate 50 is bonded to the frame 40 by, for example, an adhesive.

[0056] The nozzle plate 50 is provided with a plurality of nozzles N. These nozzles N are arranged in two rows corresponding to the ink channels. The diameter of the nozzles N increases as they move from the recording medium-facing surface toward the ink channels. The dimensions of the nozzles N are set to predetermined values ​​according to the amount of ink ejected. The nozzles N can be formed, for example, by laser processing using an excimer laser.

[0057] The actuator substrate 20, frame 40, and nozzle plate 50 are integrated as shown in FIG. 1 and form a hollow structure. The area surrounded by the actuator substrate 20, frame 40, and nozzle plate 50 is an ink flow chamber. Ink is supplied to the ink flow chamber from the ink manifold 10 through the ink supply port 21, passes through the ink channel, and circulates so that excess ink returns to the ink manifold 10 through the ink discharge port 22. A portion of the ink is ejected from the nozzle N while flowing through the ink channel and is used for printing.

[0058] A flexible printed circuit board 60 is connected to the wiring pattern 31 at a position on the actuator substrate 20 outside the frame 40. A drive circuit 61 that drives the actuator 30 is mounted on the flexible printed circuit board 60.

[0059] The operation of the actuator 30 will be explained below. Here, the operation will be explained focusing on the central ink channel among the three adjacent ink channels. The electrodes corresponding to the three adjacent ink channels are designated A, B, and C. When no electric field is applied in a direction perpendicular to the channel walls, the channel walls are in an upright state.

[0060] For example, a voltage pulse with a higher potential than that of the adjacent electrodes A and C is applied to the central electrode B, generating an electric field perpendicular to the channel wall. This drives the channel wall in a shear mode, deforming the pair of channel walls that sandwich the central ink channel so as to expand the volume of the central ink channel.

[0061] Next, voltage pulses with a higher potential than the central electrode B are applied to the adjacent electrodes A and C, generating an electric field in a direction perpendicular to the channel walls. This drives the channel walls in shear mode, deforming the pair of channel walls sandwiching the central ink channel so as to reduce the volume of the central ink channel. This action applies pressure to the ink in the central ink channel, causing it to be ejected from the nozzle N corresponding to this ink channel and land on the recording medium. For example, all the nozzles are divided into three groups, and the above-described drive operation is controlled in time division to perform three cycles to print on the recording medium.

[0062] FIG. 3 is a schematic diagram of an inkjet printer according to a second embodiment. The inkjet printer 100 shown in FIG. 3 includes a housing provided with a paper output tray 118. Inside the housing are cassettes 101a and 101b, paper feed rollers 102 and 103, pairs of conveying rollers 104 and 105, a pair of registration rollers 106, a conveying belt 107, a fan 119, a negative pressure chamber 111, pairs of conveying rollers 112, 113, and 114, inkjet heads 115C, 115M, 115Y, and 115Bk, ink cartridges 116C, 116M, 116Y, and 116Bk, and tubes 117C, 117M, 117Y, and 117Bk. Each of the inkjet heads 115C, 115M, 115Y, and 115Bk is the inkjet head 1 described with reference to FIGS. 1 and 2.

[0063] Cassettes 101a and 101b contain recording media P of different sizes. A paper feed roller 102 or 103 picks up a recording medium P corresponding to the selected size from cassette 101a or 101b and transports it to pairs of transport rollers 104 and 105 and a pair of registration rollers 106.

[0064] The conveyor belt 107 is tensioned by a drive roller 108 and two driven rollers 109. Holes are formed at predetermined intervals on the surface of the conveyor belt 107. A negative pressure chamber 111 connected to a fan 119 is installed inside the conveyor belt 107 to attract the recording medium P to the conveyor belt 107. Pairs of conveyor rollers 112, 113, and 114 are installed downstream of the conveyor belt 107 in the conveying direction. A heater for heating the printing layer formed on the recording medium P can be installed in the conveying path from the conveyor belt 107 to the paper discharge tray 118.

[0065] Four inkjet heads are arranged above the conveyor belt 107, which eject ink onto the recording medium P in accordance with image data. Specifically, an inkjet head 115C(1) that ejects cyan (C) ink, an inkjet head 115M(1) that ejects magenta (M) ink, an inkjet head 115Y(1) that ejects yellow (Y) ink, and an inkjet head 115Bk(1) that ejects black (Bk) ink are arranged in this order from the upstream side. As described above, each of these four inkjet heads is the inkjet head 1 described with reference to FIGS. 1 and 2. Hereinafter, the inkjet heads 115C, 115M, 115Y, and 115Bk may be collectively referred to as inkjet heads 1.

[0066] Above inkjet heads 115C(1), 115M(1), 115Y(1), and 115Bk(1), there are installed a cyan (C) ink cartridge 116C, a magenta (M) ink cartridge 116M, a yellow (Y) ink cartridge 116Y, and a black (Bk) ink cartridge 116Bk, each containing the corresponding ink according to this embodiment. These cartridges 116C, 116M, 116Y, and 116Bk are connected to inkjet heads 115C(1), 115M(1), 115Y(1), and 115Bk(1) by tubes 117C, 117M, 117Y, and 117Bk, respectively.

[0067] The inkjet printer 100 includes an inkjet head 1 and a medium holding mechanism that faces the inkjet head 1 and holds a recording medium P. The medium holding mechanism also functions as a recording paper moving mechanism that moves the recording medium P. The medium holding mechanism includes a conveyor belt 107, a drive roller 108, a driven roller 109, a negative pressure chamber 111, and a fan 119.

[0068] The image forming operation of this inkjet printer 100 will now be described. First, an image processing means (not shown) starts image processing for recording, generates an image signal corresponding to the image data, and generates control signals for controlling the operations of the various rollers, the negative pressure chamber 111, and the like.

[0069] Under the control of the image processing means, the paper feed roller 102 or 103 takes out recording media P of the selected size one by one from the cassette 101a or 101b and transports them to the pairs of transport rollers 104 and 105 and the pair of registration rollers 106. The pair of registration rollers 106 corrects the skew of the recording media P and transports the recording media P at a predetermined timing.

[0070] The negative pressure chamber 111 sucks in air through holes in the conveyor belt 107. Therefore, the recording medium P, while being adsorbed to the conveyor belt 107, is conveyed sequentially to positions below the inkjet heads 115C(1), 115M(1), 115Y(1), and 115Bk(1) as the conveyor belt 107 moves.

[0071] Under the control of the image processing means, the inkjet heads 115C(1), 115M(1), 115Y(1), and 115Bk(1) eject ink in synchronization with the timing at which the recording medium P is transported. As a result, a color image is formed at a desired position on the recording medium P.

[0072] In the inkjet printer 100, the distance (gap) between the recording medium P held by the medium holding mechanism and the inkjet head 1 may be, in one example, 2 mm or more, in another example, 3 mm or more, and in yet another example, 5 mm or more.

[0073] As the gap between the inkjet head and the recording medium widens, the accuracy of ink landing position decreases, making satellites and mist more likely to occur. Therefore, from the perspective of image quality, a wide gap generally cannot be set. For example, Patent Document 1 (Patent Document 1) describes setting the gap between the inkjet head and the recording medium to 1 mm (paragraph 0029). The inventors have confirmed that while inkjet inks that did not have problems with mist or satellites when the gap was 1 mm often experience noticeable mist and satellites when the gap exceeded 2 mm. According to this embodiment, the inkjet ink used has a dynamic surface tension X (expressed by the above formula (1)) of 15 mN / m or greater. This allows for a wide gap between the inkjet head and the recording medium without reducing the accuracy of ink landing position, resulting in printouts with excellent image quality. Therefore, the inkjet ink according to this embodiment and an inkjet printer using the inkjet ink are particularly excellent for printing on recording media with uneven surfaces. Examples of recording media with uneven surfaces include fabrics and wallpaper with rough surfaces, toys and plastic products with uneven patterns, and sheet-like materials with embossed or undulated surfaces.

[0074] Here, the gap between the inkjet head 1 and the recording medium P refers to the distance from the bottom surface of the inkjet head (the surface of the nozzle plate 50) to the position where a perpendicular line to this bottom surface intersects with the recording medium. When the recording medium has an uneven surface, the gap between the inkjet head 1 and the recording medium P refers to the shortest distance. In this embodiment, the gap between the inkjet head 1 and the recording medium P can be set appropriately depending on the unevenness of the surface of the recording medium P, and as described above, is set to 2 mm or more in one example, 3 mm or more in another example, and 5 mm or more in yet another example.

[0075] After the color image is formed, the pairs of conveying rollers 112, 113, and 114 discharge the recording medium P on which the image has been formed onto a paper output tray 118. If a heater is installed on the conveying path from the conveying belt 107 to the paper output tray 118, the printed layer formed on the recording medium P may be heated by the heater. Heating with a heater can improve the adhesion of the printed layer to the recording medium P, especially when the recording medium P is non-permeable.

[0076] With the inkjet ink according to this embodiment, even when the distance from the inkjet head to the recording medium is great, it is possible to suppress the generation of satellites and mist while maintaining stable ink ejection properties, and it is possible to form high-quality images even on recording media with uneven surfaces. [Example]

[0077] <Ink sample> Thirteen types of inks A to M shown in Table 1 below were prepared as ink samples. These inks were either UV-curable inks containing a pigment, an acrylic acid oligomer and / or an acrylic acid monomer, a UV polymerization initiator or sensitizer, and optionally a polymerization inhibitor and a dispersing aid, or solvent-based inks obtained by dissolving or dispersing a resin and a dispersing agent in an organic solvent, and then dispersing a pigment. Specific materials used for the pigment, acrylic acid oligomer, acrylic acid monomer, UV polymerization initiator, sensitizer, organic solvent, resin, and dispersing aid are materials commonly used in UV-curable and solvent-based inkjet inks.

[0078] The compounding ratios of the above components in inks A to M are also adjusted within general ranges. For reference, the compounding ratio of one type of UV-curable ink from inks A to M is shown below. The following raw material components were mixed and dispersed in the following ratio to prepare an ink. (Example of the blending ratio of raw material ingredients) Pigment 5 parts by mass Acrylic acid oligomer 25 parts by mass Acrylic acid monomer 55 parts by mass UV polymerization initiator 10 parts by mass Dispersion aid 5 parts by mass

[0079] [measurement] The static surface tension and dynamic surface tension of each ink sample were measured by the following method. <Static surface tension measurement> The static surface tension of each ink sample was measured using a Wilhelmy surface tensiometer (DY-500, manufactured by Kyowa Interface Science Co., Ltd.). The ink was placed in the measurement cell of the device, and a platinum plate was immersed in the ink surface at 25°C, causing the ink to wet onto the platinum plate. At this time, the force with which the platinum plate was drawn into the ink was read and measured as the static surface tension value. The measured values ​​are shown in Table 1.

[0080] <Measurement of dynamic surface tension> The dynamic surface tension of each ink sample was measured using the maximum bubble pressure method with a dynamic surface tensiometer (SITA pro line t15, manufactured by SITA Messtechnik GmbH). The needle of the device was placed in the ink in a container, and bubbles were generated from the needle at 25°C by varying the bubble generation rate (surface life) from 0.015 seconds to 1.2 seconds. The dynamic surface tension was measured from the maximum pressure of the bubbles corresponding to the surface life. Table 1 shows the dynamic surface tension γ1 at a surface life of 0.015 seconds, the dynamic surface tension γ2 at a surface life of 1.2 seconds, and the change in dynamic surface tension per second X, calculated by the following formula: X = (γ1 - γ2) / (1.2 - 0.015)

[0081] <Viscosity> The viscosity of each ink sample was measured. An "LVDV3T Cone Plate Type" (manufactured by Brookfield Engineering Laboratories, Inc.) was used to measure viscosity. Using a CPA-42Z cone spindle, measurements were taken at a rotation speed (rpm) that provided a torque of approximately 30-50% at the temperature during ejection. The measured values ​​are shown in Table 1.

[0082] [evaluation] An inkjet printer for evaluation was prepared, equipped with a share mode type inkjet head (product name "CF1", manufactured by Toshiba Tec Corporation, nozzle density (300) dpi), with a drop count of 1 to 7 drops and a drive frequency adjustable in the range of 4.8 to 13 kHz. Using this inkjet printer, the ejection stability and the generation of mist and satellites were evaluated using the following methods. The evaluation results are shown in Table 1.

[0083] <Discharge stability> The evaluation was carried out using super fine paper manufactured by EPSON as the recording medium. The printing conditions were a distance (gap) of 1 mm between the ejection nozzle surface of the inkjet head and the surface of the recording medium, and the driving voltage of the inkjet head was set to 42 pL (42 x 10) when ejecting 7 drops. 3 μm 3), ink was ejected from all nozzles (318 nozzles) for 5 minutes. Immediately after this, printing was performed on the recording medium, and the printed matter was observed for print defects, with one missing nozzle being counted as one missing line for each missing nozzle among the 318 nozzles. This was repeated five times, and the number of missing lines per 10 minutes was calculated statistically. That is, the number of missing lines was calculated by multiplying (total number of missing lines for five runs) by (10 / 25). The evaluation criterion was that a number of missing lines per 10 minutes of 1.0 or less was considered to be good ejection stability. The evaluation results are shown in Table 1.

[0084] <Mist / satellite generation status> The evaluation was carried out using glossy paper IJ-RC-UF170 manufactured by Mitsubishi Paper Mills as the recording medium. The printing conditions were a distance (gap) of 3 mm between the ejection nozzle surface of the inkjet head and the surface of the recording medium, and the driving voltage of the inkjet head was set to 42 pL (42 x 10) when ejecting 7 drops. 3 μm 3 ) and ejected ink in three patterns: one drop, two drops, and three drops. For each drop, the landing state of the mist / satellite droplets around the main droplet that landed on the printing medium was observed under a microscope and judged.

[0085] Figure 4 is a schematic top view illustrating the evaluation method for the occurrence of mist or satellite droplets on a recording medium. Comparing Figure 4(a) and Figure 4(b), Figure 4(a) shows fewer mist droplets or satellite droplets d2 that land at positions offset from the main droplet d1 than Figure 4(b), indicating superior image quality. In this evaluation method, a printout with no visible mist or satellite droplets d2 relative to the main droplet d1 was assigned a score of 100, with the score decreasing as the amount of mist or satellite droplets d2 increased. For each ink sample, the average scores for one, two, and three drops were calculated and evaluated according to the following criteria. A rating of A or B (average score of over 80 points) indicates suppression of mist and satellite droplets, resulting in superior image quality. The evaluation results are shown in Table 1. A: Average score is over 90 and under 100 B: Average score is over 80 and under 90 C: Average score is over 70 and below 80 D: Average score is over 60 and below 70 E: Average score is 60 or less

[0086] [Table 1]

[0087] The results shown in Table 1 show that the inkjet inks of Examples 1 to 5 according to this embodiment, in which the change in dynamic surface tension per second between an average lifespan of 0.015 seconds and 1.2 seconds is 15 mN / m or more, have excellent ejection stability and are capable of forming high-quality images with little generation of mist or satellites, even when the distance between the inkjet head and the recording medium is large.

[0088] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0089] 1...inkjet head, 10...ink manifold, 11...ink supply pipe, 12...ink return pipe, 20...actuator board, 21...ink supply port, 22...ink discharge port, 30...actuator, 31...wiring pattern, 40...frame, 50...nozzle plate, 60...flexible printed circuit board, 61...drive circuit, 100...inkjet printer, 101a, 101b...cassette, 102, 103...paper supply roller, 104, 105...conveyor roller pair, 10 6... registration roller pair, 107... conveyor belt, 111... negative pressure chamber, 112, 113, 114... conveyor roller pair, 115C(1), 115M(1), 115Y(1), 115Bk(1)... inkjet head, 116C, 116M, 116Y, 116Bk... ink cartridge, 117C, 117M, 117Y, 117Bk... tube, 118... paper output tray, 119... fan, N... nozzle, P... recording medium, d1... main droplet, d2... mist or satellite [Note] The inventions described in the original claims of this application are set forth below. [1] An inkjet ink containing a colorant and a resin, with a water content of less than 10% by mass, in which, when measured at 25°C by the maximum bubble pressure method, the dynamic surface tension at a surface life of 0.015 seconds is defined as γ1, and the dynamic surface tension at a surface life of 1.2 seconds is defined as γ2, the change in dynamic surface tension per second, expressed by the following formula, is 15 mN / m or more. X = (γ1 - γ2) / (1.2 - 0.015) [2] An inkjet ink according to appendix [1], having a static surface tension in the range of 20 mN / m or more and 40 mN / m or less. [3] The inkjet ink according to appendix [1] or [2], wherein the colorant contains at least one pigment selected from organic pigments, inorganic pigments, and glass particles, and the average particle size of the pigment is in the range of 0.01 μm or more and 5 μm or less. [4] An inkjet head that ejects ink toward a recording medium; a medium holding mechanism that holds the recording medium facing the inkjet head; An inkjet printer comprising: an ink jet printer, the ink being an ink jet ink according to any one of appendices [1] to [3]. [5] The inkjet printer according to appendix [4], wherein the medium holding mechanism holds the recording medium so that the distance between the inkjet head and the recording medium is 2 mm or more.

Claims

1. An inkjet ink comprising a colorant and a resin, having a water content of less than 10 mass %, a viscosity of 5 mPa·s to 15 mPa·s, a static surface tension of 20 mN / m to 40 mN / m, and a change in dynamic surface tension per second, X, expressed by the following formula, is 15 mN / m or more, where γ1 is the dynamic surface tension at a surface life of 0.015 seconds and γ2 is the dynamic surface tension at a surface life of 1.2 seconds, as measured at 25°C by a maximum bubble pressure method: X=(γ1-γ2) / (1.2-0.015)

2. 2. The inkjet ink according to claim 1, wherein the colorant contains at least one pigment selected from organic pigments, inorganic pigments, and glass particles, and the average particle diameter of the pigment is in the range of 0.01 μm or more and 5 μm or less.

3. an inkjet head that ejects ink toward a recording medium; a medium holding mechanism that holds the recording medium facing the inkjet head; An inkjet printer comprising: an inkjet ink according to claim 1 or 2.

4. 4. The inkjet printer according to claim 3, wherein the medium holding mechanism holds the recording medium so that the distance between the inkjet head and the recording medium is 2 mm or more.

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