Image forming method

Abstract

To provide an image forming method capable of preventing an increase in the internal pressure of a channel through which ink passes and improving the fixability of an image even when the flow rate of the ink supplied to an inkjet head is increased.SOLUTION: An image forming method includes the steps of supplying a water based ink to an inkjet head at a flow rate of 5 mL / min or more and 120 mL / min or less, and ejecting the supplied water based ink from a nozzle of the inkjet head. The water based ink contains a colorant and a water-insoluble resin having a glass transition temperature of 40°C or higher and 100°C or lower.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an image forming method.

Background Art

[0002] The inkjet method that enables digital printing without using a plate is used in various printing fields because it can form an image simply and inexpensively.

[0003] Inks used in the inkjet method include aqueous inks composed of water and a small amount of organic solvents, non-aqueous inks containing organic solvents but substantially free of water, hot melt inks that heat and melt a solid ink at room temperature for printing, and active ray curable inks that are cured by irradiating active rays after printing. There are multiple types of these inks, and they are used appropriately according to the application. Among these, aqueous inks generally have less odor and high safety.

[0004] In order to enhance the fixability (rub resistance) of an image formed using an aqueous ink, a method of adding a resin to the aqueous ink is known. When a resin-containing aqueous ink is landed on a substrate and then the substrate is heated or the ink is dried, the above resin forms a film, and an image with high fixability (rub resistance) can be obtained. Various methods are known as image forming methods using such inks.

[0005] For example, Patent Document 1 discloses an inkjet printing method using an ink set having a white ink and a yellow ink containing water, an organic solvent, a polysiloxane surfactant, and water-insoluble polyurethane resin particles. According to Patent Document 1, by including polyurethane resin particles and a polysiloxane surfactant in the ink, the above surfactant can be stabilized in the ink, suppressing phase separation of the ink and improving storage stability. Furthermore, in the yellow ink, by using pigment yellow 155 as a pigment and using the above surfactant and the above resin particles in combination, it is also said that rub resistance and solvent resistance can be improved.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] As in Patent Document 1, various image forming methods using an aqueous ink containing a water-insoluble resin are known.

[0008] The present inventors confirmed that the fixability of an image is improved by an image forming method using an aqueous ink containing a resin as described in Patent Document 1. By the way, when using an image forming apparatus having a circulation flow path or when using an inkjet head having a large size, it may be necessary to increase the flow rate of the ink supplied to the inkjet head.

[0009] In such a case, when attempting to form an image using the aqueous ink containing the water-insoluble resin described in Patent Document 1, a problem occurred in that the pressure in the flow path through which the ink passes increased over time, and the apparatus could not be driven steadily. Furthermore, when the flow rate of the above ink was increased, a problem also occurred in that the fixability of the image decreased.

[0010] The present invention has been made in view of the above circumstances, and an object thereof is to provide an image forming method capable of suppressing an increase in the pressure in the flow path through which the ink passes and improving the fixability even when the ink flow rate supplied to the inkjet head is increased.

Means for Solving the Problems

[0011] One embodiment of the image forming method according to the present invention for solving the above problems includes a step of supplying an aqueous ink to an inkjet head at a flow rate of 5 mL / min or more and 120 mL / min or less, and a step of applying the aqueous ink from the inkjet head to the surface of a recording medium. The aqueous ink includes a colorant and a water-insoluble resin having a glass transition temperature of 40°C or more and 100°C or less.

Effects of the Invention

[0012] According to the present invention, even when the ink flow rate supplied to the inkjet head is increased, it is possible to suppress an increase in the pressure in the flow path through which the ink passes and to improve the fixing property, and an image forming method is provided.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the following embodiments.

[0015] As described above, when using an image forming apparatus having a circulation flow path or when using an inkjet head having a large size, it may be necessary to increase the flow rate of the ink supplied to the inkjet head.

[0016] In an image forming apparatus having a circulation passage, ink that has not been ejected from an inkjet head is discharged from the inkjet head, and the discharged ink is circulated so as to be supplied to the inkjet head. By doing so, it is possible to suppress an increase in the ink viscosity near the nozzles due to sedimentation or aggregation of pigment particles or the like in the ink, and to easily enhance the ejection stability. In such an image forming apparatus, it is necessary to supply ink to the inkjet head at a larger flow rate in consideration of not only the amount of ink ejected from the inkjet but also the amount of ink discharged from the inkjet head.

[0017] Further, even in an image forming apparatus that does not have a circulation passage but uses a large-sized inkjet, since it is necessary to eject a large amount of ink at once, it is necessary to supply ink to the inkjet head at a larger flow rate.

[0018] As described above, in an image forming apparatus, when supplying ink containing a water-insoluble resin as described in Patent Document 1 to an inkjet head, if the flow rate of the ink is increased, the pressure in the passage through which the ink flows may increase over time. As a result, it may become difficult to adjust the flow rate at the time of ink ejection to a desired value. Further, in the ink described in Patent Document 1, when the flow rate of the ink is increased, the fixing property may also decrease. The reasons for these are considered as follows.

[0019] When the flow rate of the ink flowing through the narrow passage inside the inkjet head is increased, a frictional force is generated between the ink and the wall surface of the passage, and a slight amount of frictional heat is generated due to the frictional force. It is considered that the water-insoluble resin contained in the ink is softened by this slightly generated frictional heat, and the softened resin adheres to the wall surface of the passage. It is considered that the cross-sectional area of the passage decreases over time due to the resin adhering to the wall surface, resulting in a pressure increase. The ink described in Patent Document 1 is considered to be easily softened by the frictional heat when the flow rate is increased because the glass transition temperature (Tg) of the resin contained in the ink is low.

[0020] Furthermore, it is considered that when the resin adheres to the channel wall surface, the amount of resin discharged from the inkjet head and applied to the recording medium decreases, resulting in a decrease in fixability.

[0021] Therefore, as a result of intensive studies by the present inventors, it has been found that the above problems can be solved by supplying an ink containing a water-insoluble resin having a glass transition temperature (Tg) of 40°C or higher and 100°C or lower to the inkjet head.

[0022] Since the glass transition temperature (Tg) is 40°C or higher, even when a larger frictional heat is generated in the channel when the ink flow rate is increased, the resin is less likely to soften, so it is less likely to adhere to the wall surface of the channel. As a result, it is possible to suppress an increase in pressure due to a decrease in the channel cross-sectional area over time. Furthermore, since the resin is less likely to adhere to the wall surface, it is possible to suppress a reduction in the amount of resin in the ink applied to the recording medium, thereby improving the fixability. Also, since the glass transition temperature (Tg) is 100°C or lower, after the ink lands on the recording medium, the resin contained in the ink can be appropriately softened to facilitate film formation, so the fixability can be improved.

[0023] In the present invention, the above ink is supplied to the inkjet head at a flow rate of 5 mL / min or more and 120 mL / min or less. When the flow rate of the ink supplied to the inkjet head is 5 mL or more, the above-described problems of an increase in pressure in the channel through which the ink passes and a decrease in fixability occur. In the present invention, by including a water-insoluble resin having a glass transition temperature in the above range in the ink, the above problems can be solved.

[0024] And when the flow rate is 120 mL / min, when the ink flows through the narrow channel inside the inkjet head, it is possible to suppress the softening of the resin due to excessive generation of frictional heat and suppress the resin from adhering to the channel wall surface. As a result, it is possible to suppress an increase in the pressure in the ink channel over time due to a reduction in the channel cross-sectional area over time.

[0025] 1. Image forming apparatus FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus 100 that can be used to implement an image forming method according to an embodiment of the present invention.

[0026] The image forming apparatus 100 includes an inkjet head 101, an ink supply passage 102, a liquid feed pump 103, a pressure sensor 104, a filter 105, a supply damper 106, a flow meter 107, a discharge damper 108, an ink discharge passage 109, a first tank 110, and a second tank 111.

[0027] In the image forming apparatus 100, the ink stored in the first tank 110 is supplied to the inkjet head 101 through the ink supply passage 102 by the liquid feed pump 103. Then, the ink that has not been ejected from the inkjet head 101 is discharged into the ink discharge passage 109 by the liquid feed pump 103, and the discharged ink is returned to the first tank 110. Thereby, the ink that has not been ejected from the inkjet head 101 can be circulated from the ink discharge passage 109 toward the ink supply passage 102 and re-supplied to the inkjet head 101.

[0028] By circulating the ink in the image forming apparatus 100 in this way, it is possible to suppress an increase in the ink viscosity near the nozzles due to sedimentation or aggregation of pigment particles in the ink, and to easily improve the ejection stability.

[0029] According to the study by the present inventors, in the image forming apparatus 100, when the filter 105 through which the ink passes is installed in the passage through which the ink flows, it has been found that the problems of an increase in pressure in the passage through which the ink flows and a decrease in fixing property are more likely to occur. The reason is considered as follows.

[0030] When the ink passes through the filter, shear heat is generated. However, by increasing the ink flow rate, the shear heat becomes greater. Due to this shear heat, it is considered that the water-insoluble resin contained in the ink softens, and the softened resin adheres to the filter. It is considered that the adhesion of the resin to the filter causes a pressure increase in the ink flow path. Also, it is considered that the adhesion of the resin to the filter reduces the amount of resin contained in the ink applied to the recording medium and deteriorates the fixing property.

[0031] Regarding these problems, they can be solved by using an ink containing a water-insoluble resin having a glass transition temperature of 40°C or higher and 100°C or lower. Since the glass transition temperature is 40°C or higher, the resin is less likely to soften due to the shear heat generated during filter passage, and it is less likely to adhere to the filter. By suppressing the softening of the resin, it is possible to make it less likely for the above pressure increase to occur whether a single filter is installed or a plurality of filters are installed. Furthermore, since it is possible to suppress the reduction in the amount of resin in the ink applied to the recording medium when the resin passes through the filter, the fixing property can be improved.

[0032] Also, since the glass transition temperature (Tg) is 100°C or lower, after the ink lands on the recording medium, the resin contained in the ink can be moderately softened to facilitate film formation, so that the fixing property can be improved.

[0033] In the present embodiment, the image forming apparatus 100 has a filter 105 installed in the ink supply flow path 102 and filters 2c and 2d installed in the inkjet head 101. The image forming apparatus may have a plurality of filters in the flow path through which the ink flows (including the flow path inside the inkjet head 101) as in the present embodiment, or may have a filter only in either the ink supply flow path 102 or the inkjet head 101. The above-described adhesion of the resin can occur in any of the filters.

[0034] The materials of Filter 105, Filters 2c, and 2d are not particularly limited. For example, they can be metals such as stainless steel, copper, and nickel, or non-metals such as polyethylene, Teflon, and polyvinylidene fluoride. Among these, from the viewpoints of affinity and solubility in the ink solvent and durability, it is preferable that they are made of metals such as stainless steel, copper, and nickel.

[0035] The mesh number of Filter 105, Filters 2c, and 2d is preferably 80×700 or more in the vertical × horizontal direction, and more preferably 150×1200 or more. When the mesh number is 80×700 or more, resin particles with large sizes contained in the ink are less likely to pass through, so that a decrease in discharge stability due to nozzle clogging can be more effectively suppressed. On the other hand, when the mesh number is 80×700 or more, shear heat is likely to be generated when passing through the filter, so that the water-insoluble resin (described later) in the ink is likely to soften, and the problems of the above-described increase in pressure in the flow path and decrease in fixing property are likely to occur. Also, from the viewpoint of suppressing excessive collection of pigments and resins contained in the ink by Filters 2c and 2d, the mesh number is preferably 510×3600 or less. In this specification, the "mesh number" refers to the number of filter wires present in one inch.

[0036] Note that Filter 105, Filters 2c, and 2d may be the same filter or different filters. Also, Filter 105 may be installed in the ink discharge flow path 109.

[0037] The inkjet head 101 has a plurality of nozzles for ejecting ink droplets onto a recording medium M such as paper, which is the object to be printed.

[0038] As will be described in detail later, in the present embodiment, the inkjet head 101 has a filter disposed inside through which the supplied ink passes. More specifically, the inkjet head 101 has a filter through which the ink discharged from the inkjet head 101 but not ejected from the inkjet head 101 passes, and a filter through which the ink ejected from the inkjet head 101 passes.

[0039] Examples of inkjet heads used in the inkjet method include electro-mechanical conversion methods including single cavity type, double cavity type, vendor type, piston type, share mode type, and shared wall type, and electro-thermal conversion methods including thermal inkjet type and bubble jet ( "bubble jet" is a registered trademark of Canon Inc.) type, etc.

[0040] The inkjet head may be either a scanning type or a line type inkjet head, but a line type is preferred.

[0041] FIG. 2 is an exploded perspective view showing an outline of the inkjet head 101 used in the image forming apparatus 100. As shown in FIG. 2, the inkjet head 101 has a common ink chamber 2, a holding portion 3, a head chip 4, and a flexible wiring board 5.

[0042] Further, FIG. 3 is a cross-sectional view of the common ink chamber 2 taken along line A-A in FIG. 2.

[0043] The common ink chamber 2 is formed in a substantially rectangular parallelepiped shape with a hollow interior, and one surface facing the holding portion 3 is open. An ink supply port 2a for supplying ink and an ink discharge port 2b for discharging the ink not ejected from the inkjet head to the outside are provided on one surface of the common ink chamber 2 facing the opening.

[0044] The common ink chamber 2 has filters 2c and 2d inside. The filter 2c removes foreign matters from the ink supplied from the ink supply port 2a and heading toward the ink discharge port 2b without being discharged from the inkjet head 101. Also, the filter 2d removes foreign matters from the ink supplied from the ink supply port 2a and heading toward the discharge port (nozzle hole 11, to be described later) of the inkjet head 101.

[0045] The holding part 3 is formed in a substantially flat plate shape having an opening 3a at substantially the center, and is arranged to cover the opening of the common ink chamber 2. Thereby, the common ink chamber 2 is connected to one surface of the holding part 3 so as to cover the opening 3a. Also, the head chip 4 is connected to the other surface of the holding part 3 so as to cover the opening 3a. The holding part 3 communicates the common ink chamber 2 and the head chip 4 via the opening 3a.

[0046] An insertion hole 3b is provided in the outer peripheral part of the holding part 3. The flexible wiring board 5 is inserted into the insertion hole 3b. One end of the flexible wiring board 5 is connected to the wiring board 50 of the head chip 4, to be described later. Also, the other end of the flexible wiring board 5 is inserted from the other surface of the holding part 3 through the insertion hole 3b provided in the holding part 3 and drawn out to the common ink chamber 2 side.

[0047] The ink supply channel 102 is a channel for supplying ink from the liquid feed pump 103 to the inkjet head 101. Also, the ink discharge channel 109 is a channel for discharging the ink that has not been discharged from the inkjet head 101 from the ink discharge port 2b of the inkjet head 101.

[0048] The liquid feeding pump 103 is disposed in the ink supply channel 102, the ink discharge channel 109, and the ink replenishment channel 112, respectively. As described above, the liquid feeding pump 103 feeds the ink in the first tank 110 to the inkjet head 101, and feeds the ink discharged from the inkjet head 101 into the first tank 110. Further, the liquid feeding pump 103 disposed in the ink replenishment channel 112 feeds the ink in the second tank 111 to the first tank 110.

[0049] The pressure sensor 104 is a device that measures the pressure value in the channel. In the present embodiment, a plurality of pressure sensors 104 are installed in the ink supply channel 102. Then, by reading the numerical values of each pressure sensor 104, it is possible to determine whether a pressure increase has occurred in the channel between each pressure sensor 104 from the difference in pressure values.

[0050] The supply damper 106 is disposed in the ink supply channel 102 and can store the ink to be fed to the inkjet head 101. By means of the supply damper 106, the generation of pulsation in the ink supply channel 102 can be suppressed.

[0051] The discharge damper 108 is disposed in the ink discharge channel 109 and can store the ink discharged from the inkjet head 101. By means of the discharge damper 108, the generation of pulsation in the ink discharge channel 109 can be suppressed.

[0052] The supply damper 106 and the discharge damper 108 are each provided with a pressure sensor 104, and the pressure fluctuation in the supply damper 106 and the discharge damper 108 can be measured by the pressure sensor 104.

[0053] The flow meter 107 is installed in the ink discharge channel 109 and measures the ink flow rate flowing in the channel. The flow meter 107 may be installed in the ink supply channel 102.

[0054] The first tank 110 is a tank for storing ink for supplying to the inkjet head 101 and ink discharged without being ejected from the inkjet head 101. The second tank 111 is a tank for storing ink for supplying to the first tank 110 when the amount of ink stored in the first tank 110 decreases. In the present embodiment, the liquid feed pump 103 supplies ink from the second tank 111 to the first tank 110 via the ink supply passage 112.

[0055] The first tank 110 and the second tank 111 may have a stirring device (not shown). The first tank 110 and the second tank 111 can be appropriately determined according to the image forming performance, size, etc. of the image forming apparatus 100.

[0056] The conveyance device 113 is a device for conveying the recording medium M. The conveyance device 113 includes, for example, a belt conveyor 113a and a rotatable feed roller 113b.

[0057] In the present embodiment, the image forming apparatus 100 may have a dryer 114. The dryer 114 is a device for drying the ink applied to the recording medium M. The dryer 114 can use a known heater, an irradiator that irradiates an infrared lamp, etc.

[0058] The image forming apparatus 100 may further have a degassing module 115. In the present embodiment, the degassing module 115 is disposed in the ink supply passage 102. The degassing module 115 can degas and remove bubbles contained in the ink flowing through the ink supply passage 102.

[0059] In the present embodiment, the image forming apparatus 100 may be configured not to circulate the ink. For example, the ink discharge passage 109 may be removed from the inkjet head 101, and the ink discharge port 2b may be covered.

[0060] 1-2. Aqueous Ink The ink used in the image forming method according to this embodiment contains water, a colorant, and a water-insoluble resin having a glass transition temperature of 40°C or higher and 100°C or lower.

[0061] (Water) In this embodiment, the aqueous ink contains water. The type of water contained in the aqueous ink is not particularly limited. The content of water in the aqueous ink is preferably 50% by mass or more based on the total mass of the ink, and more preferably 60% by mass or more based on the total mass of the liquid components in the ink.

[0062] (Colorant) In this embodiment, the aqueous ink contains a colorant.

[0063] The colorant contained in the aqueous ink is not particularly limited, and is, for example, a dye or a pigment. From the viewpoint of enhancing the water resistance and light resistance of the formed image, the colorant is preferably a pigment.

[0064] Examples of dyes include yellow dyes, magenta dyes, cyan dyes, black dyes, and the like.

[0065] Examples of yellow dyes include C.I. Acid Yellow 7:1, 17, 19, 23, 25, 29, 38, 42, 49, 61, 72, 78, 110, 127, 135, 141, and 142 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0066] Examples of magenta dyes include C.I. Acid Red 8, 9, 14, 18, 26, 27, 35, 37, 51, 57, 82, 87, 92, 94, 111, 129, 131, 138, 186, 249, 254, 265, and 276 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0067] Examples of cyan dyes include C.I. Acid Blue 1, 7, 9, 15, 22, 23, 25, 40, 41, 43, 62, 78, 83, 90, 93, 103, 112, 113, and 158 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0068] Examples of black dyes include C.I. Acid Black 1, 2, 24, 26, 31, 52, 107, 109, 110, 119, and 154 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0069] Examples of pigments include yellow pigments, magenta pigments, cyan pigments, black pigments, white pigments, and the like.

[0070] Examples of yellow pigments include C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, 14, 15, 15:3, 17, 74, 93, 128, 94, 138, and 155 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0071] Examples of magenta pigments include C.I. Pigment Red 2, C.I. Pigment Red 3, 5, 6, 7, 15, 16, 48;1, 53;1, 57;1, 122, 123, 139, 144, 149, 150, 166, 177, 178, 184, 222, 238, etc.

[0072] Examples of cyan pigments include C.I. Pigment Blue 15, 15;2, 15;3, 15;4, 16, 60, 62, 66 (all manufactured by Tokyo Chemical Industry Co., Ltd.).

[0073] Examples of black pigments include carbon black, C.I. Pigment Black 7, 26, 28, etc.

[0074] White pigments include inorganic pigments and organic pigments. Examples of the inorganic pigments include heavy calcium carbonate, light calcium carbonate, titanium oxide, aluminum hydroxide, titanium white, talc, calcium sulfate, barium sulfate, zinc oxide, magnesium oxide, magnesium carbonate, amorphous silica, colloidal silica, white carbon, kaolin, calcined kaolin, delaminated kaolin, aluminosilicate, sericite, bentonite, smectite, and the like. Examples of the organic pigments include polystyrene resin particles, urea formalin resin particles, and the like.

[0075] When the aqueous ink contains a pigment, it preferably contains a pigment dispersant. The content of the pigment dispersant is not particularly limited, but is preferably 5% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 45% by mass or less, based on the total mass of the pigment.

[0076] The pigment dispersant is not particularly limited, and includes low molecular weight dispersants and high molecular weight dispersants. From the viewpoint of more stably dispersing the pigment by a steric repulsion effect, it is preferable to use a high molecular weight dispersant. In this specification, the "high molecular weight dispersant" refers to a pigment dispersant having a molecular weight of 1000 or more and 200,000 or less. The weight average molecular weight (Mw) can be measured by gel permeation chromatography using polymethyl methacrylate as a comparative standard substance.

[0077] The resin constituting the high molecular weight dispersant may be a homopolymer or copolymer of a hydrophilic monomer, or a copolymer of a hydrophilic monomer and a hydrophobic monomer. In this specification, the "hydrophilic monomer" refers to a monomer having a solubility of 1 g or more in 100 ml of water at 25°C, and the "hydrophobic monomer" refers to a monomer having a solubility of 1 g or less in 100 ml of water at 25°C.

[0078] Examples of hydrophilic monomers include monomers containing a carboxyl group or an acid anhydride group (unsaturated polyvalent carboxylic acids such as (meth)acrylic acid and maleic acid, and maleic anhydride), and ethylene oxide-modified (meth)acrylic acid ester monomers (such as ethylene oxide-modified (meth)acrylic acid alkyl esters).

[0079] Examples of hydrophobic monomers include (meth)acrylic acid ester monomers such as methyl (meth)acrylate and ethyl (meth)acrylate, propylene oxide-modified (meth)acrylic acid ester monomers, (meth)acrylic acid ester monomers having an alkyl group with 3 to 6 carbon atoms in the side chain, styrene-based monomers such as styrene, α-methylstyrene, and vinyltoluene, α-olefin-based monomers such as ethylene, propylene, and 1-butene, and vinyl carboxylate esters such as vinyl acetate and vinyl butyrate.

[0080] Examples of the above copolymers include styrene-acrylic acid copolymers, styrene-acrylic acid-acrylic acid alkyl ester copolymers, styrene-maleic acid copolymers, styrene-maleic acid-acrylic acid alkyl ester copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid-acrylic acid alkyl ester copolymers, and styrene-maleic acid half ester copolymers.

[0081] The above copolymers include random copolymers, alternating copolymers, block copolymers, comb-shaped copolymers, and comb-shaped block copolymers. Among these, from the viewpoint of enhancing the dispersibility of pigments, the above copolymer is preferably a comb-shaped block copolymer.

[0082] The comb-shaped block copolymer means a copolymer containing a linear polymer forming the main chain and another type of polymer graft-polymerized to the constitutional units derived from the monomers constituting the main chain.

[0083] Examples of commercially available pigment dispersants include DISPERBYK 182, DISPERBYK 184, DISPERBYK 190, DISPERBYK 194, DISPERBYK 2164, DISPERBYK 168, DISPERBYK N22024 (all manufactured by BYK Chemie), and the like.

[0084] The particle size of the pigment particles is not particularly limited. However, from the viewpoint of dispersing the pigment in the ink for a long period of time, the volume-based median diameter is preferably 50 nm or more and 200 nm or less. The above particle size can be determined by known particle size measuring instruments using the dynamic light scattering method or the electrophoresis method. From the viewpoint of simple and highly accurate measurement, measurement by the dynamic light scattering method is preferred.

[0085] The content of the colorant other than the white pigment contained in the ink is not particularly limited. However, it is preferably 2% by mass or more and 15% by mass or less, and more preferably 4% by mass or more and 12% by mass or less with respect to the total mass of the ink. When it is 2% by mass or more, the color reproducibility of the image formed by the ink can be improved, and when it is 15% by mass or less, the dispersion stability of the colorant in the ink can be further enhanced.

[0086] When the ink contains a white pigment as a colorant, the content of the white pigment is preferably 4% by mass or more and 15% by mass or less with respect to the total mass of the ink.

[0087] (Water-insoluble resin) In the present embodiment, the aqueous ink contains a water-insoluble resin having a glass transition temperature (Tg) of 40°C or more and 100°C or less.

[0088] In this specification, the "water-insoluble resin" refers to a resin having a solubility of 0.1 g or less in water at a temperature of 25°C and a pH of 7.

[0089] As described above, when the glass transition temperature (Tg) of the water-insoluble resin is 40°C or higher, even if greater frictional heat is generated in the flow path when the ink flow rate is increased, the resin is less likely to soften, so it is less likely to adhere to the wall surface of the flow path. Thereby, it is possible to suppress the pressure increase due to the reduction of the flow path cross-sectional area over time. Furthermore, since the resin is less likely to adhere to the wall surface, it is possible to suppress a reduction in the amount of the resin in the ink applied to the recording medium and improve the fixing property. Also, since the glass transition temperature (Tg) is 100°C or lower, after the ink lands on the recording medium, the resin contained in the ink can be moderately softened to facilitate film formation, so the fixing property can be improved. From the above viewpoints, the glass transition temperature (Tg) of the water-insoluble resin contained in the ink is preferably 60°C or higher and 90°C or lower, and more preferably 70°C or higher and 80°C or lower.

[0090] The glass transition temperature (Tg) can be measured by using a differential scanning calorimeter "Diamond DSC" (manufactured by PerkinElmer, Inc.) under heating and cooling conditions with a temperature increase / decrease rate of 10°C / min and a temperature increase range from 0°C to 150°C.

[0091] In the present embodiment, the type of the water-insoluble resin only needs to have a glass transition temperature (Tg) of 40°C or higher and 100°C or lower, and is not particularly limited. Examples of the water-insoluble resin include acrylic resins, vinyl chloride-acrylic resins, polyester resins, styrene acrylic resins, polyurethane resins, and the like. Among these, from the viewpoints of the water resistance and environmental adaptability of the film-forming product, acrylic resins, polyester resins, and styrene acrylic resins are preferable.

[0092] In this embodiment, the water-insoluble resin may be dispersed in an emulsion state. In this case, from the viewpoint of improving the injectability from the inkjet head, the particle size of the emulsion is preferably 500 nm or less. Further, from the viewpoint of increasing the volume of the resin and improving the fixing property, the particle size is preferably 10 nm or more. The particle size can be measured, for example, using a particle size distribution measuring instrument (Zetasizer 1000HS, manufactured by Malvern).

[0093] The content of the water-insoluble resin is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 8% by mass or less, and even more preferably 3% by mass or more and 6% by mass or less, based on the total mass of the aqueous ink. When the content is 1% by mass or more, after the ink lands on the recording medium, the film formation amount can be increased, and the fixing property can be improved more sufficiently. Further, when the content is 10% by mass or less, thickening of the ink can be suppressed, and the ejection stability of the ink can be further enhanced.

[0094] When a pigment is used as a colorant in the ink, the ratio (PB ratio) of the content (mass) of the pigment to the content of the water-insoluble resin is preferably 0.3 or more and 2.0 or less, and more preferably 0.5 or more and 1.5 or less. When the ratio is 0.3 or more, the color reproducibility of the image formed by the ink can be improved, and when it is 2.0 or less, the amount of pigment that cannot be sufficiently fixed by the resin can be sufficiently reduced, and the fixing property can be further improved.

[0095] The SP value of the water-insoluble resin is 1 / 2 15 (MPa) 1 / 2 or more and 25 (MPa) 1 / 2 or less, preferably 16 (MPa) 1 / 2 or more and 24 (MPa)

[0096] (water-soluble organic solvent) In this embodiment, the aqueous ink may contain an organic solvent. The type of the organic solvent is not particularly limited, but from the viewpoint of enhancing the compatibility with water, it is preferably a water-soluble organic solvent. In this specification, the "water-soluble organic solvent" refers to an organic solvent having a solubility of 10 g or more in water at 25°C.

[0097] Examples of the water-soluble organic solvent include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and the like. These organic solvents may be contained alone or in combination of two or more.

[0098] Examples of the above alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, and the like.

[0099] Examples of the above polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol having 5 or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, and the like.

[0100] Examples of the above amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0101] Examples of the above amides include formamide, N,N-dimethylformamide, N,N-dimethylacetamide, and the like.

[0102] The water-soluble organic solvent contained in the aqueous ink has an SP value of 16 (MPa) 1 / 2 or more and 28 (MPa) 1 / 2Preferably contains an organic solvent as follows, 18 (MPa) 1 / 2 or more and 26 (MPa) 1 / 2 More preferably, it contains an organic solvent that is as follows.

[0103] An organic solvent having an SP value within the above range is likely to have a value close to the SP value (approximately 15 to 25 MPa 1 / 2 ) of the resin contained in the aqueous ink, so it has high compatibility with the resin. After the ink containing such an organic solvent lands on the recording medium, as water evaporates, if the ratio of the above organic solvent in the ink increases, the resin contained in the ink and the above organic solvent are likely to be partially compatible. And when the surface of the resin particles softens moderately, the resins are likely to fuse together, so the resin is likely to form a film on the recording medium, and it is considered that the fixing property is likely to be improved.

[0104] By including a water-soluble organic solvent having an SP value within the above range in the ink together with a water-insoluble resin having a glass transition temperature (Tg) of 40°C or higher and 100°C or lower, the fixing property can be more significantly improved. To fuse and form a film of a resin having a high glass transition temperature on the recording medium to further improve the fixing property, a certain amount of heat may be required. However, as described above, the above water-soluble organic solvent makes it easier for the resin to fuse and form a film. Therefore, in the ink of the present embodiment, using a water-soluble organic solvent having an SP value within the above range is more effective in improving the fixing property.

[0105] Examples of water-soluble organic solvents having an SP value of 16 (MPa) 1 / 2 or more and 28 (MPa) 1 / 2 or less include 1,2-butanediol (SP value: 27.8 (MPa) 1 / 2 ), 1,3-butanediol (SP value: 29.1 (MPa) 1 / 2 ), 3-methyl-1,3-butanediol (SP value: 26.2 (MPa) 1 / 2 ), 1,2-pentanediol (SP value: 26.0 (MPa) 1 / 2 ), 1,5-pentanediol (SP value: 26.7 (MPa) 1 / 2) 2-Methyl-2,4-pentanediol (SP value: 25.2 (MPa) 1 / 2 ) 1,2-Hexanediol (SP value: 24.7 (MPa) 1 / 2 ) 1,6-Hexanediol (SP value: 25.5 (MPa) 1 / 2 ) Tripropylene glycol (SP value: 22.6 (MPa) 1 / 2 ) Diethylene glycol monoethyl ether (SP value: 21.8 (MPa) 1 / 2 ) Diethylene glycol monobutyl ether (SP value: 20.6 (MPa) 1 / 2 ) Triethylene glycol monobutyl ether (SP value: 21.2 (MPa) 1 / 2 ) Triethylene glycol monobutyl ether (SP value: 20.2 (MPa) 1 / 2 ) Dipropylene glycol monomethyl ether (SP value: 19.4 (MPa) 1 / 2 ) Tripropylene glycol monomethyl ether (SP value: 19.6 (MPa) 1 / 2 ) Dipropylene glycol dimethyl ether (SP value: 18.3 (MPa) 1 / 2 ) Dipropylene glycol propyl ether (SP value: 18.6 (MPa) 1 / 2 ) etc. are included.

[0106] Also, in this embodiment, the SP value of the entire organic solvent contained in the ink is 16 (MPa) 1 / 2 or more and 30 (MPa) 1 / 2 or less is preferable, 18 (MPa) 1 / 2 or more and 30 (MPa) 1 / 2 or less is more preferable, 18 (MPa) 1 / 2 or more and 27 (MPa) 1 / 2More preferably, the following is the case. As water contained in the ink evaporates after the ink lands on the recording medium, the SP value of the ink gradually approaches the SP value of the organic solvent contained in the ink. At this time, if an organic solvent having an SP value within the above range is contained in the ink, in the process of water evaporation, the SP value of the ink approaches the above range and also easily approaches the SP value of the above resin, and it is considered that the surface of the resin particles can be moderately dissolved (softened) to improve the fixing property. In the present embodiment, the SP value of the whole organic solvent can be obtained by adding the product of the SP value and the volume of the organic solvent and dividing by the volume of the whole organic solvent.

[0107] It is preferable that the difference between the SP value of the water-soluble organic solvent and the SP value of the water-insoluble resin is not too small. If the difference in the SP value is not too small and is within an appropriate range, it is possible to further suppress the SP value of the ink existing in the flow path, that is, the ink before being ejected from the inkjet head, from approaching the SP value of the resin too much. Thereby, it is possible to further suppress the resin from being excessively softened in the flow path and sticking to the flow path wall surface.

[0108] From such a viewpoint, the minimum value of the difference between the SP value of the water-soluble organic solvent and the SP value of the water-insoluble resin is 2 (MPa) 1 / 2 or more and 11 (MPa) 1 / 2 or less is preferable, and 2 (MPa) 1 / 2 or more and 9 (MPa) 1 / 2 or less is more preferable. When the minimum value of the difference in the SP value is 2 (MPa) 1 / 2 or more, it is possible to sufficiently suppress the SP value of the ink before being ejected from the inkjet head from approaching the resin and the resin from being excessively softened. Thereby, it is possible to sufficiently suppress the resin contained in the ink in the flow path from sticking to the flow path wall surface.

[0109] Also, when the difference in the SP value is 11 (MPa) 1 / 2 or less, after landing on the recording medium (after water evaporation of the ink), the SP value of the ink approaches the SP value of the resin, so that the effect of improving the fixing property due to softening of the resin particles becomes more remarkable.

[0110] Furthermore, the minimum value of the difference between the SP value of the entire organic solvent and the SP value of the water-insoluble resin is 2 (MPa) 1 / 2 or more, 11 (MPa) 1 / 2 or less is preferable, and 2 (MPa) 1 / 2 or more, 9 (MPa) 1 / 2 or less is more preferable.

[0111] The content of the organic solvent is preferably 1% by mass or more and 40% by mass or less, more preferably 2% by mass or more and 30% by mass or less, and even more preferably 3% by mass or more and 20% by mass or less with respect to the total mass of the aqueous ink. When the above content is 1% by mass or more, the wettability of the ink with respect to the recording medium can be improved, and the ink can be easily spread by wetting, so that the color developability can be improved. When the above content is 40% by mass or less, an increase in the viscosity of the ink can be suppressed, and the ejection stability can be improved.

[0112] Among the organic solvents, the content of the organic solvent having an SP value of 16 (MPa) 1 / 2 or more and 28 (MPa) 1 / 2 or less is preferably 1% by mass or more and 40% by mass or less, more preferably 2% by mass or more and 30% by mass or less, and even more preferably 3% by mass or more and 20% by mass or less with respect to the total mass of the ink. When the content of the organic solvent having an SP value in the above range is 1% by mass or more, after the ink lands on the recording medium, the resin contained in the ink can be softened more appropriately, making it easier to form a film on the recording medium and further improving the fixing property. Also, when the content of the organic solvent having an SP value in the above range is 40% by mass or less, before the water evaporates, that is, before the ink lands on the recording medium, the SP value of the ink approaches the above range, and the dissolution of the resin surface can be sufficiently suppressed. Thereby, it is possible to sufficiently suppress the excessive softening of the resin contained in the ink in the inkjet head and sufficiently suppress the adhesion to the flow path wall surface.

[0113] Occupying the organic solvent contained in the ink, the SP value is 16 (MPa)1 / 2 28 (MPa) or higher 1 / 2 The content ratio of the organic solvent being below is preferably 55 mass% or more and 100 mass% or less, and more preferably 70% or more and 100% or less. When it is 55 mass% or more, after the ink lands on the recording medium, the resin contained in the ink can be softened more moderately, making it easier to form a film on the recording medium and improving the fixing property more effectively.

[0114] (Others) In this embodiment, the ink may contain a surfactant, a pH adjuster, an ultraviolet absorber, oil droplet fine particles, a fluorescent brightener, a polysaccharide, a viscosity adjuster, a specific resistance adjuster, a film-forming agent, an antioxidant, a fungicide, a rust inhibitor, etc., according to the purpose. These components may be contained alone or in combination of two or more.

[0115] Examples of the above surfactant include anionic surfactants such as dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene - polyoxypropylene block copolymers; cationic surfactants such as alkyl amine salts and quaternary ammonium salts; and silicone - based and fluorine - based surfactants.

[0116] Examples of the above pH adjuster include known acids, bases, and buffers. Among these, ammonia, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, methylethylamine, monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, methylaminoethanol, and dimethylaminoethanol and their salts are preferred because they are less likely to inhibit the reaction between the cross - linked resin and the cross - linking agent.

[0117] Examples of the above ultraviolet absorbers include benzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, benzoate-based ultraviolet absorbers, and triazine-based ultraviolet absorbers.

[0118] (Physical properties) From the viewpoint of enhancing the ejection stability from the nozzles of the inkjet heads of the ink, the viscosity of the ink is preferably 1 cP or more and less than 100 cP. From the viewpoint of further enhancing the above ejection stability, the viscosity of the ink is preferably 1 cP or more and 50 cP or less, and more preferably 1 cP or more and 15 cP or less.

[0119] In the present embodiment, when the ink is passed through the filter at 100 L, the viscosity change rate before and after passing through the filter is preferably 5% or less. In this specification, the "viscosity change rate" refers to the ratio of the difference in viscosity before and after passing through the filter to the viscosity of the ink before passing through the filter, with a flow rate of 20 mL / min through a stainless-steel twill weave filter with a mesh number of 325×2300, a vertical wire diameter of 35 μm, and a horizontal wire diameter of 25 μm.

[0120] The more the resin contained in the ink clogs the filter, the more the solid content concentration in the ink after passing through the filter decreases, and the viscosity also decreases. Therefore, it can be understood that the smaller the change in viscosity before and after passing through the filter, the smaller the fluctuation in the solid content concentration of the ink, and the smaller the amount of resin adhering to the filter. From such a viewpoint, the viscosity change rate is preferably 5% or less, and more preferably 2% or less.

[0121] The viscosity change rate can be obtained, for example, by measuring the viscosity of the ink before being introduced into the first tank 110 and the viscosity of the ink remaining in the first tank after passing 100 L of the ink through the filter, respectively, with an E-type viscometer (TP-200E, Toki Sangyo Co., Ltd.). At this time, it is assumed that the ink is not supplied from the second tank 111 to the first tank 110 until 100 L of the ink has passed through the filter. Also, the fact that 100 L of the ink has passed through the filter can be determined from the flow rate of the ink and the time of passing.

[0122] From the viewpoint of enhancing the ejection stability of the ink from the nozzles of the inkjet head, the surface tension of the ink is preferably 20 mN / m or more and 50 mN / m or less. From the viewpoint of enhancing the wettability with respect to the substrate and making the formed image more high-definition, the surface tension of the ink is more preferably 20 mN / m or more and 35 mN / m or less. The surface tension of the ink can be adjusted to the above range by changing the type or amount of the surfactant and the organic solvent.

[0123] 1-3. Image Forming Method FIG. 5 is a flowchart of the image forming method according to the present embodiment.

[0124] As shown in FIG. 5, the image forming method according to the present embodiment includes a step of supplying ink to the inkjet head at a flow rate of 5 mL / min or more and 120 mL / min or less (step S20), and a step of ejecting the aqueous ink from the inkjet head (step S30). In the present embodiment, the image forming method can be implemented using the image forming apparatus 100.

[0125] (Step of passing the aqueous ink through the filter (step S10)) The image forming method according to the present embodiment may include a step of passing the aqueous ink through the filter.

[0126] In this embodiment, an aqueous ink is passed through a filter 105 installed in an ink supply channel 102, and the passed aqueous ink is supplied to an inkjet head 101 in step S20.

[0127] As described above, by passing the aqueous ink through the filter, the pressure increase due to the clogging of the filter is likely to occur. However, by using the above aqueous ink, the pressure increase due to the clogging of the filter can be suppressed.

[0128] In this embodiment, this step is performed before the step of supplying the aqueous ink to the inkjet head (step S20), but is not limited thereto. For example, this step may be performed between the step of supplying the aqueous ink to the inkjet head (step S20) and the step of discharging the ink (step S30). In this case, the aqueous ink is passed through a filter installed in the inkjet head 101. Further, for example, this step may be performed after the step of discharging the aqueous ink from the inkjet head (step S50). In this case, the aqueous ink is passed through a filter installed in the ink discharge channel 109.

[0129] (Step of supplying aqueous ink to the inkjet head (step S20)) In this step, the above-described aqueous ink is supplied to the inkjet head 101 at a flow rate of 5 mL / min or more and 120 mL / min or less.

[0130] In this embodiment, the aqueous ink stored in the first tank 110 is supplied to the inkjet head 101 through the ink supply channel 102 by the liquid feed pump 103. Then, the flow rate of the ink supplied to the inkjet head 101 can be adjusted by the liquid feed pump 103.

[0131] The flow rate of the aqueous ink supplied to the inkjet head 101 is 5 mL / min or more and 120 mL / min or less, preferably 7 mL / min or more and 100 mL / min or less, and more preferably 10 mL or more and 80 mL / min or less. When the flow rate is 7 mL / min or more, the flow rate of the ink supplied to the inkjet head 101 can be further increased, and the drying of the nozzles can be suppressed. On the other hand, the above-mentioned increase in pressure in the flow path and decrease in fixing property are more likely to occur. Also, when the flow rate is 10 mL / min or more, the flow rate of the ink can be further increased. On the other hand, the above-mentioned increase in pressure in the flow path and decrease in fixing property are more likely to occur. In the present embodiment, by using an ink containing a water-insoluble resin having a glass transition temperature of 40°C or more and 100°C or less, these problems can be solved.

[0132] When the flow rate is 100 mL / min or less, softening of the resin contained in the ink due to excessive shear heat generation in the flow path can be further suppressed, and adhesion of the resin to the flow path wall surface can be further suppressed. Also, when the flow rate is 80 mL / min or less, adhesion of the resin to the flow path wall surface can be further suppressed.

[0133] In the present embodiment, the flow rate supplied to the inkjet head 101 can be obtained by removing the ink supply flow path 102 from the inkjet head 101, measuring the weight of the ink per unit time discharged from the removed ink supply flow path 102, and dividing the measured value by the specific gravity of the ink. The specific gravity of the ink can be measured, for example, using a hydrometer (DMA 35 Basic, manufactured by Anton Paar).

[0134] (Step of discharging ink (Step S30)) In this step, an aqueous ink is discharged from the nozzles of the inkjet head 101.

[0135] The discharge amount of the aqueous ink from the inkjet head 101 is appropriately set according to the type of the inkjet head 101, and is, for example, 1.5 to 9 pL.

[0136] In this process, aqueous ink may be ejected from the nozzles of the inkjet head 101 and applied to a recording medium or an intermediate transfer member.

[0137] The type of the recording medium is not particularly limited. The recording medium may be, for example, a paper substrate with high water absorbency, or a non-water-absorbent substrate such as a film, a plastic board (soft vinyl chloride, hard vinyl chloride, acrylic plate, polyolefin, etc.).

[0138] (Drying step (step S40)) The image forming method according to this embodiment may include a step (step S30) of drying the aqueous ink applied to the recording medium.

[0139] The method for drying the aqueous ink is not particularly limited, and can be performed using, for example, a known heater, an infrared lamp, or the like.

[0140] From the viewpoint of further improving the fixing property, the temperature for drying the ink is preferably 60°C or higher, and from the viewpoint of energy-saving fixing, it is preferably 180°C or lower.

[0141] (Step of discharging the aqueous ink from the inkjet head (step S50)) The image forming method according to this embodiment may further include a step of discharging the aqueous ink that has not been ejected from the nozzles of the inkjet head 101 from the inkjet head 101. In this step, in step S20, the ink that has not been ejected from the inkjet head 101 is discharged from the inkjet head 101.

[0142] In this embodiment, the ink that has not been ejected from the inkjet head 101 can be discharged to the ink discharge flow path 109 by the liquid feed pump 103. The discharged ink is returned to the first tank 110 via the ink discharge flow path 109.

[0143] (Step of resupplying ink to the inkjet head (Step S60)) In this embodiment, it may further have a step of resupplying the aqueous ink discharged from the inkjet head 101 to the inkjet head. In this step, the aqueous ink discharged from the inkjet head 101 in Step S30 is resupplied to the inkjet head 101 again.

[0144] In this embodiment, in Step S30, the aqueous ink discharged from the inkjet head 101 and returned to the first tank 110 can be resupplied to the inkjet head 101 by the liquid feed pump 103 through the ink supply channel 102.

[0145] Note that in this step, a bypass channel may be provided to connect the ink discharge channel 109 and the ink supply channel 102, and the discharged aqueous ink may be resupplied to the inkjet head 101 through the bypass channel and the ink supply channel 102.

[0146] Step S40 and Step S50 may be performed after Step S10, or may be performed in parallel with Step S20 and Step S30.

[0147] With the image forming method in this embodiment, it is possible to improve the fixing property while suppressing the pressure increase in the channel that occurs when the flow rate of the ink supplied to the inkjet head 101 is increased.

Example

[0148] Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[0149] 1. Preparation of Aqueous Ink (Preparation of Pigment Dispersions 1 to 12) To 18.0 parts by mass of a cyan pigment (Pigment Blue 15:3, manufactured by Tokyo Chemical Industry Co., Ltd.), 13.5 parts by mass of a pigment dispersant (DISPERBYK 190, manufactured by BYK-Chemie Japan Co., Ltd., solid content 40% by mass), 20 parts by mass of propylene glycol, and 48.5 parts by mass of ion-exchanged water were added and mixed. Then, using a sand grinder filled with 50% by volume of zirconia beads having an average particle size of 0.5 mm, dispersion was carried out to prepare Pigment Dispersion Liquid 1 having a pigment content of 18.0% by mass.

[0150] Pigment Dispersion Liquids 2 to 9 were prepared in the same manner except that the propylene glycol used in the preparation of Pigment Dispersion Liquid 1 was changed to the solvents shown in Table 1. Also, Pigment Dispersion Liquids 10 to 12 were prepared in the same manner except that the cyan pigment used in the preparation of Pigment Dispersion Liquid 2 was changed to the pigments shown in Table 1. For the yellow pigment, Pigment Yellow 155 (manufactured by Tokyo Chemical Industry Co., Ltd.) was used, for the magenta pigment, Pigment Red 122 (manufactured by Tokyo Chemical Industry Co., Ltd.) was used, and for the black pigment, Carbon Black MA100 (manufactured by Mitsubishi Chemical Corporation) was used.

[0151] (Preparation of Pigment Dispersion Liquid 13) Into a 12 L flask equipped with a mechanical stirrer, thermometer, nitrogen inlet, drying tube outlet, and addition funnel, 3750 g of tetrahydrofuran (THF) and 7.4 g of p-xylene were placed. Next, a tetrabutylammonium m-chlorobenzoate catalyst (3.0 ml of a 1.0 M solution dissolved in acetonitrile) was added, and further 291.1 g of initiator 1,1-bis(trimethylsilyloxy)-2-methylpropene (1.25 M) was added. At this time, the tetrabutylammonium m-chlorobenzoate catalyst was added into the flask over 150 minutes from the start of addition.

[0152] Also, simultaneously with the addition of the tetrabutylammonium m-chlorobenzoate catalyst, the addition of 1976 g of trimethylsilyl methacrylate (12.5 M) was started and added into the flask over 35 minutes. After the addition of trimethylsilyl methacrylate was completed, a polymerization reaction was carried out for 180 minutes to react more than 99% of the monomer.

[0153] Next, 1772 g of butyl methacrylate (12.5 M) was added to the flask over 30 minutes. After a polymerization reaction for 400 minutes, 780 g of dry methanol was added to the flask and distillation was started. The distillation was carried out in two stages. In the first stage, the solvent having a boiling point of less than 55 °C was removed from the flask. In the second stage, the solvent having a boiling point of less than 76 °C was removed from the flask. During the second-stage distillation, 5100 g of isopropanol was added to the flask. By the two-stage distillation, a total of 8007 g of all solvents was removed.

[0154] By the above operations, a solution of a butyl methacrylate / methacrylic acid (BMA / MAA = 10 / 10) AB block polymer (molecular weight 2400) with a concentration of 52.5% dissolved in THF and isopropanol was obtained. The molecular weight of the above AB block polymer, the weight average molecular weight (Mw), was measured by gel permeation chromatography using polymethyl methacrylate as a comparative standard substance.

[0155] 19.05 g of the above AB block polymer solution was neutralized with 3.86 g of a 45.6% KOH solution (solvent: 36.7 g of deionized water) to obtain a 20% AB block polymer solution.

[0156] To 18 parts by mass of a cyan pigment (Pigment Blue 15:3, manufactured by Tokyo Chemical Industry Co., Ltd.), 27 parts by mass of the above AB block polymer solution (concentration 20%), 20 parts by mass of propylene glycol, and 35 parts by mass of ion-exchanged water were added and mixed. Then, it was dispersed using a sand grinder filled with 50% by volume of zirconia beads having an average particle size of 0.5 mm to prepare a pigment dispersion 13 having a pigment content of 18.0% by mass.

[0157] [Table 1]

[0158] (Preparation of Aqueous Ink) While stirring 22.2 parts by mass of the pigment dispersion 1, 15.6 parts by mass of propylene glycol (PG) as an organic solvent, 15 parts by mass of a polyester resin (MD2000, Toyobo Co., Ltd., solid content concentration 40% by mass, dispersion medium: water) as a resin, and 1.0 part by mass of a surfactant (KF351A, manufactured by Shin-Etsu Chemical Co., Ltd.) were added, and ion-exchanged water was further added so that the total was 100 parts by mass to prepare an ink composition. The above ink composition was filtered through a 0.8 μm filter to obtain an aqueous ink 1.

[0159] Regarding Experiments 1 to 50 described later, the types, amounts, types and amounts of organic solvents, types and amounts of resins of the pigment dispersion (described as "dispersion" in Tables 4 to 7) were changed so as to obtain the ink compositions shown in Tables 4 to 7, and the inks used in each experiment were prepared. Lists of the types of organic solvents and the types of resins used in Experiments 1 to 50 are shown in Tables 2 and 3, respectively. In Tables 4 to 7, the values of the resin concentration and the pigment concentration represent the values in terms of solid content. Also, in Tables 4 to 7, when two types of organic solvents are used, for example, when it is written as "1,2-HD / PG", the value of solvent SP represents the SP value of the entire mixed organic solvent, and the solvent amount "10 / 10" means that the addition amount of 1,2-HD is 10% by mass with respect to the total mass of the ink, and the addition amount of PG is 10% by mass with respect to the total mass of the ink.

[0160]

Table 2

[0161]

Table 3

[0162] 2. Experiment (Experiments 1 to 4, 6 to 40, 42 to 50) In the liquid tank (first tank) of an image forming apparatus having the same configuration as the image forming apparatus 100 shown in FIG. 1, the ink used in each experiment was filled, and the flow rate of the ink flowing into the piezo inkjet head (KM1024aLHG-RC, manufactured by Konica Minolta, Inc., 360 dpi, discharge amount 27 pL) was controlled by a liquid feed pump so as to be the values shown in Tables 4 to 7, and the ink was circulated through the ink flow path. As the filter 105, 2300 MESH (manufactured by Kodama Seisakusho Co., Ltd., reference passing particle value 5 μm) was used.

[0163] The measurement of the above flow rate was obtained by removing the pipe through which the ink flows into the inkjet head from the inkjet head, measuring the weight of the ink flowing out of the pipe within a predetermined time, and then dividing the measured value by the density of the ink. The density of the above ink was measured using a hydrometer (DMA 35 Basic, manufactured by Anton Paar).

[0164] (Experiment 5, 41) In the image forming apparatus 100, the ink discharge flow path was removed from the inkjet head, and the ink discharge port was covered. The liquid tank (first tank) of the image forming apparatus was filled with the ink used in each experiment, and the output of the pump was adjusted so that the flow rate of the ink flowing into the inkjet head was the value shown in Tables 4 to 7, and the ink was fed to the inkjet head while discharging the ink from the inkjet head.

[0165] The measurement of the above flow rate was performed in the same manner as in Experiments 1 to 4, 6 to 40, and 42 to 50.

[0166] (Experiment 26a, 26b) Experiment 26a was performed in the same manner as Experiment 26 except that the filter 105 in the ink supply flow path was changed to 1200 MESH (manufactured by Kodama Seisakusho Co., Ltd.). Also, Experiment 26b was performed in the same manner as Experiment 26 except that a 700 MESH filter (manufactured by Kodama Seisakusho Co., Ltd.) was used for the filter in the inkjet head 101.

[0167] 3. Evaluation (Pressure Rise Evaluation) In Experiments 1 to 50, in each image forming apparatus, the pressure increase value in the flow path due to the resin particles adhering to the ink flow path wall surface or the filter was calculated from the pressure value difference between two pressure sensors 104 installed in the ink supply flow path 102. Regarding the pressure increase value, evaluation was performed according to the following criteria, and 3 to 6 were regarded as passing. In the following evaluation, the fact that 100 L (300 L) of ink was passed through the flow path between the two pressure sensors was judged from the ink flow rate and the time of passing the ink.

[0168] 1 The pressure increase value after passing 100 L is 10 kPa or more 2 The pressure increase value after passing 100 L is 7 kPa or more and less than 10 kPa 3 The pressure increase value after passing 100 L is 5 kPa or more and less than 7 kPa 4 The pressure increase value after passing 100 L is 3 kPa or more and less than 5 kPa 5 The pressure increase value after passing 100 L is less than 3 kPa, and the pressure increase value after passing 300 L is 3 kPa or more 6 The pressure increase value after passing 300 L is less than 3 kPa

[0169] (Fixing property) Using the image forming apparatus and ink used in Experiments 1 to 50, a solid image was formed on coated paper (OK Top Coat, Oji Paper) under the conditions of a printing width of 100 nm × 100 nm and a resolution of 720 dpi × 720 dpi. Then, the solid image was dried under the conditions of a temperature of 75°C for 10 minutes. A tape peeling test by cross-cutting was performed on the formed solid image. Then, the fixing property of the image was evaluated according to the following criteria, and A to D were regarded as passing. A The area of the image peeled off by the tape is less than 1% of the area of the solid image B The area of the image peeled off by the tape is 1% or more and less than 3% of the area of the solid image C The area of the image peeled off by the tape is 3% or more and less than 5% of the area of the solid image D The area of the image peeled off by the tape is 5% or more and less than 10% of the area of the solid image The area of the image peeled off by the E-tape is 10% or more of the area of the solid image

[0170] (Viscosity change rate) The above viscosity change rate was obtained by measuring the viscosity of the ink before being introduced into the first tank 110 and the viscosity of the ink remaining in the first tank 110 after passing 100 L of the ink through the filter in the inkjet head at the flow rate in each experiment with an E-type viscometer (TP-200E, Toki Sangyo Co., Ltd.), respectively, and then calculating the viscosity change rate. At this time, it was assumed that the ink was not supplied from the second tank 111 to the first tank 110 until 100 L of the ink was passed through the filter. Also, passing 100 L of the ink through the filter was determined from the flow rate of the ink and the time of passing. The obtained viscosity change rates were evaluated according to the following criteria. A The viscosity change rate is 2% or less B The viscosity change rate is greater than 2% and 5% or less C The viscosity change rate is 5% or more

[0171] The evaluation results of each experiment are summarized in Tables 4 to 7. In Tables 4 to 7, the column of "resin-solvent SP value difference" represents the minimum value of the difference between the SP value of the entire organic solvent in the aqueous ink and the SP value of the water-insoluble resin when the aqueous ink contains a plurality of organic solvents.

[0172] [Table 4]

[0173] [Table 5]

[0174] [Table 6]

[0175] [Table 7]

[0176] (Ejection stability) Regarding Experiments 26, 26a, and 26b, after ejecting ink from the inkjet head 101 to form an image and one day has passed, a pattern image that can confirm whether ejection is possible from each of the 1024 nozzles is printed, and the ejection stability is evaluated according to the following criteria. The evaluation results are shown in Table 8. Less than 0.3% nozzle defect means that the number of nozzle defects is 1 or more and 3 or less out of 1024 nozzles, 0.3% or more and less than 0.5% nozzle defect means 4 or 5 out of 1024 nozzles, and 0.5% or more means 5 or more nozzle defects. C or above is considered a pass. A No nozzle defect was confirmed B A nozzle defect was confirmed, but it was less than 0.3% C A nozzle defect was confirmed, but it was 0.3% or more and less than 0.5% D A nozzle defect of 0.5% or more was confirmed

[0177]

Table 8

[0178] In Experiments 1 to 39, the evaluation of pressure rise and fixing property resulted in better results than in Experiments 40 to 50. This is considered to be because even when the ink flow rate is 5 mL / min or more and the glass transition temperature of the resin contained in the ink is 40°C or more, when the ink flow rate is increased, the softening of the resin due to frictional heat generated in the flow path and shear heat generated when passing through the filter is suppressed, making it difficult for the resin to adhere to the flow path wall surface and the filter. Also, since the above glass transition temperature is 100°C or less, it is easier to fix at a lower temperature, so the fixing property is considered to have improved.

[0179] In Experiments 1 to 7 where the glass transition temperature of the water-insoluble resin was 65°C or lower, the fixing property was better than that in Experiments 8 to 12. This is presumably because the decrease in the glass transition temperature made the resin easier to soften, thus making it easier to form a coating film. On the other hand, in Experiments 8 to 12 where the glass transition temperature was higher than that in Experiments 1 to 7, the evaluation of the pressure increase was better than that in Experiments 1 to 7. This is presumably because the softening of the resin due to the frictional heat generated in the flow path and the shear heat generated when passing through the filter was more suppressed, making it less likely for the resin to adhere to the flow path wall surface and the filter. For the same reason, it is considered that the pressure increase also increased in Experiments 13 to 25, 26b, 28 to 39.

[0180] The SP value is 16 (MPa) 1 / 2 or more 28 (MPa) 1 / 2 In Experiments 13 to 39 containing an organic solvent where the value is below, the evaluation of the fixing property was improved compared to Experiments 8 to 11 containing the same type of water-insoluble resin. This is because the organic solvent has high compatibility with the resin, and as water evaporates after the ink containing this lands on the recording medium, when the ratio of the organic solvent in the ink increases, it is considered that the resin contained in the ink and the organic solvent are more likely to be partially compatible. And because the surface of the resin particles softens moderately, it becomes easier for the resins to fuse with each other, so it is considered that the resin is more likely to form a film on the recording medium and the fixing property is more likely to be improved. Also, from the results of Experiments 13 to 18, 43, 47, the use of both the organic solvent and a water-insoluble resin with a glass transition temperature of 40°C or higher and 100°C or lower significantly showed an improvement in the fixing property. It is considered that because a resin with a high glass transition temperature and requiring a certain amount of heat or more for film formation is used, the effect of promoting the film formation of the organic solvent is more likely to appear.

[0181] In Experiments 13 to 24, the smaller the difference between the SP value of the water-insoluble resin and the SP value of the organic solvent, the better the fixation. This is thought to be because after the ink lands on the recording medium, in the process of water evaporating from the ink, the SP value of the ink approaches the SP value of the organic solvent and also approaches the SP value of the resin, which moderately dissolves the surface of the resin and makes it easier to form a film of the resin. Also, from Experiments 26, 27, and 28, it was found that when the difference in the SP values was less than 2 (MPa), the fixation was improved. 1 / 2 It was found that the above conditions resulted in a better evaluation of pressure rise. This is believed to be because the surface of the water-insoluble resin was prevented from excessively dissolving and adhering to the flow path walls and the filter before the ink landed on the recording medium.

[0182] In addition, the results of Experiments 29 to 35 showed that fixability was improved when the ratio of the pigment content to the resin content (PB ratio) was 0.5 or more and 1.5 or less. It is believed that fixability was improved by adding an appropriate amount of resin required for fixing the pigment relative to the amount of pigment. In addition, the results of Experiments 49 and 50 showed that fixability was significantly improved by using a water-insoluble resin with a glass transition temperature of 40°C or more and 100°C or less and adjusting the PB ratio to the above range. It is believed that the effect of improving fixability by adjusting the PB ratio is easily achieved because a resin with a high glass transition temperature and requiring a certain amount of heat for film formation is used. [Industrial Applicability]

[0183] By using the image forming method of the present invention, even if the flow rate of ink supplied to the inkjet head is increased, the pressure in the flow path through which the ink passes can be suppressed from increasing, and fixability can be improved. Therefore, the present invention is useful, for example, in an image forming method using a circulation flow path or an image forming method in which inkjet heads are connected. [Explanation of symbols]

[0184] 100 Image forming device 101, 201 Inkjet head 102 Ink supply channel 103 Liquid feed pump 104 Pressure sensor 105 Filter 106 Supply damper 107 Flow meter 108 Discharge damper 109 Ink discharge channel 110 First tank 111 Second tank 112 Ink replenishment channel 113 Conveyor 114 Dryer 115 Degassing module M Recording medium

Claims

1. A step of supplying an aqueous ink to an inkjet head at a flow rate of 5 mL / min or more and 120 mL / min or less; A step of discharging the supplied aqueous ink from a nozzle of the inkjet head; characterized by comprising: The aqueous ink contains a colorant and a water-insoluble resin having a glass transition temperature of 40°C or more and 100°C or less. An image forming method.

2. A step of discharging the aqueous ink that has not been discharged from the nozzle from the inkjet head; A step of resupplying the discharged aqueous ink to the inkjet head; characterized by comprising: The image forming method according to claim 1.

3. The method further includes a step of passing the aqueous ink through a filter, wherein the number of meshes of the filter is 80×700 or more. The image forming method according to claim 1 or 2.

4. The number of meshes of the filter is 150×1200 or more. The image forming method according to claim 3.

5. When 100 L of the aqueous ink is passed through a stainless steel twill weave filter having a mesh number of 325×2300, a vertical wire diameter of 35 μm, and a horizontal wire diameter of 25 μm at a flow rate of 20 mL / min, the viscosity change rate before and after passing through the twill weave filter is 5% or less. The image forming method according to any one of claims 1 to 4.

6. The aqueous ink has a solubility parameter (SP value) of 16 (MPa) １／２ or more and 28 (MPa) １／２ or less, and contains a water-soluble organic solvent, and the image forming method according to any one of claims 1 to 5

7. The SP value of the water-soluble organic solvent is 18 (MPa) １／２ or more and 26 (MPa) １／２ The image forming method according to claim 6, wherein the SP value is as described above.

8. The content of the water-soluble organic solvent is 1% by mass or more and 40% by mass or less based on the total mass of the aqueous ink. The image forming method according to claim 6 or 7.

9. The minimum value of the difference between the SP value of the water-insoluble resin and the SP value of the water-soluble organic solvent is 2 (MPa) １／２ or more and 11 (MPa) １／２ The image forming method according to any one of claims 6 to 8, wherein the image forming method is as described above.

10. The SP value of the entire organic solvent contained in the aqueous ink is 16 (MPa) １／２ or more and 30 (MPa) １／２ The image forming method according to any one of claims 6 to 9, which is below.

11. The minimum value of the difference between the SP value of the entire organic solvent contained in the aqueous ink and the SP value of the water-insoluble resin is 2 (MPa). １／２ or more and 11 (MPa). １／２ The image forming method according to any one of claims 6 to 10, wherein the value is 11 (MPa) or less.

12. The glass transition temperature of the water-insoluble resin is 60°C or more and 90°C or less. The image forming method according to any one of claims 1 to 9.

13. The content of the water-insoluble resin is 1% by mass or more and 10% by mass or less based on the total mass of the aqueous ink. The image forming method according to any one of claims 1 to 10.

14. The content of the colorant is 4% by mass or more and 15% by mass or less based on the total mass of the aqueous ink. The image forming method according to any one of claims 1 to 11.

Images