JP2015021023A - Aqueous ink composition and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aqueous ink composition that can give a high-quality image on various recording sheets while avoiding, as much as possible, use of consumable supplies such as a reaction solution except for an ink.SOLUTION: The ink composition comprises at least a colorant, an aqueous solvent, and an ABA type triblock copolymer dissolved in the aqueous solvent. The ABA type triblock copolymer includes an A-block having a monomer expressed by general formula (1) below as a structural unit and a B-block having a hydrophilic monomer having a radically polymerizable unsaturated bond as a structural unit. In general formula (1), Rrepresents either a hydrogen atom or a methyl group; Rrepresents an alkylene group having 6 to 18 carbon atoms; M represents either a hydrogen atom or a monovalent metal atom; and Xrepresents -NH-.

Description

  The present invention relates to a water-based ink composition used as a recording liquid in an inkjet image forming apparatus that forms an image by discharging a recording liquid such as ink from a head, and an image for image formation using the water-based ink composition. The present invention relates to a forming apparatus.

  When recording an image on recording paper using the inkjet method, characters and fine lines are blurred due to ink permeating along the recording paper fibers (feathering), and the color boundary at the two-color overlap portion Bleeding (bleeding), droplet coalescence (beading) due to insufficient permeation, and deformation (curl, wrinkle) of recording paper due to ink moisture in the case of water-based ink cause problems that impair image quality. Further, since ink jet ejects ink from fine nozzle holes, nozzle clogging is a problem that impairs reliability.

  In order to suppress bleeding and beading, it is a common practice to increase the permeability of the ink into the recording paper by adding a surfactant or the like to the ink. Feathering is more likely to occur when the ink permeability is increased, so it has been devised to use only black ink, which has low permeability, but in that case, the black ink is difficult to dry and high-speed printing is difficult. The problem remains. Therefore, in order to prevent bleeding such as feathering even in high-speed printing, a reaction liquid containing a polyvalent metal salt or an acidic compound that insolubilizes the colorant contained in the ink is applied to the recording paper in advance. By printing from the top with highly penetrating ink, it is possible to achieve both image quality and speed correspondence (see Patent Documents 1 to 3).

  For nozzle clogging, regular maintenance operations (empty discharge, forced suction) are required. On the other hand, Patent Document 4 and Patent Document 5 describe a method called an intermediate transfer method in which an ink image is formed on an ink non-permeable intermediate transfer body by ink jet and then the ink image is transferred from the intermediate transfer body to a recording sheet. Has been proposed.

  In the intermediate transfer method, the ink jet recording head can be arranged away from the recording paper, and nozzle clogging due to adhesion of paper dust on the recording paper can be suppressed. In addition, it is easy to maintain the gap between the head and the medium, which is difficult with the normal recording paper, and the intermediate transfer method is easy, so that the ink landing robustness is high and the paper type correspondence is good.

  However, in order to increase the ink transfer rate to the recording paper, an intermediate transfer material having a low surface energy is required, and there has been a problem that ink beading occurs vigorously on such an intermediate transfer member. In view of this, the intermediate transfer member is heated in advance, and when ink lands on the intermediate transfer member, moisture evaporates and the ink is thickened to suppress beading. However, in this method, there is a problem that the nozzle clogging of the ink is promoted by heating, and there is a problem that a large amount of energy is required for evaporation of a solvent such as moisture.

Therefore, a method of applying a reaction liquid that insolubilizes the colorant contained in the ink as described above onto the intermediate transfer member has been studied (see Patent Documents 6 to 10).
The reaction liquids described in these documents are effective for problems relating to image quality (feathering, bleeding, beading) on the intermediate transfer member or on the recording paper. However, this increases the size of the apparatus and increases costs, such as an increase in consumables in addition to ink and the need for a separate coating apparatus.

In view of the above circumstances, an ink composition capable of obtaining high-quality images on various recording papers without using consumables other than ink such as a reaction liquid, and a highly reliable inkjet image using the ink composition A proposal for a forming apparatus has been awaited. In addition, proposals have been awaited for an ink composition that does not require large heat energy to evaporate ink moisture on the intermediate transfer member, and an energy-saving inkjet image forming apparatus using the ink composition.
An object of the present invention is to provide a water-based ink composition having high image quality and high reliability without using a reaction liquid or thermal energy, and an image forming apparatus using the same.

The present inventor has found that the above-mentioned problems can be solved by using an aqueous ink composition containing at least a colorant, an aqueous solvent, and an ABA type triblock copolymer whose viscosity changes depending on pH, and has completed the present invention. did.
That is, the present invention relates to a water-based ink composition as described in the following (1).

(1) It contains at least a colorant, an aqueous solvent, and an ABA triblock copolymer dissolved in the aqueous solvent, and the ABA triblock copolymer includes a monomer represented by the following general formula (1) as a constituent unit. A water-based ink composition comprising a polymer having a block having A and a B block having a hydrophilic monomer having a radical polymerizable unsaturated bond as a constituent unit.
(In the general formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents a number 6-18 alkylene group having a carbon, M is a hydrogen atom and monovalent metal Represents any of the atoms, and X ¹ represents —NH—.)

  The present invention relates to an image forming means for forming an image in response to an image signal using an aqueous ink composition containing at least a colorant, an aqueous solvent, and an ABA triblock copolymer whose viscosity is changed by pH, an intermediate transfer member, An image provided with pH control means for changing the pH of the aqueous ink composition in order to change the viscosity of the aqueous ink composition on the intermediate transfer member, and transfer means for transferring an image from the intermediate transfer member to the recording paper By using the forming device, the thickening of the aqueous ink composition (recording solution) occurs on the intermediate transfer member without the above-described reaction solution, reaction solution coating device, and heating device, so that bleeding, feathering, It is possible to provide a high-quality printed matter that suppresses the cause of image defects such as beading. Further, by using the intermediate transfer member, it is possible to provide an image forming apparatus with improved landing robustness and paper type compatibility.

1 is a schematic front view according to an embodiment of an image forming apparatus to which the present invention is applied. FIG. 2 is a schematic diagram illustrating a state in which the aqueous ink composition (recording liquid) of the present invention is applied from an inkjet head to an intermediate transfer member in the image forming apparatus illustrated in FIG. 1. FIG. 2 is a conceptual diagram of an ABA type triblock copolymer dissolved in an aqueous ink composition of the present invention, and a conceptual diagram of a viscosity change (thickening) state after pH treatment. FIG. 2 is a conceptual diagram illustrating a state of a liquid column by a recording liquid formed between a cathode and an anode in the image forming apparatus illustrated in FIG. 1. FIG. 2 is a conceptual diagram for explaining that a change in viscosity of a recording liquid occurs according to pH in the image forming apparatus shown in FIG. 1. It is a schematic front view concerning the other Example of the image forming apparatus to which this invention is applied.

  EMBODIMENT OF THE INVENTION Below, the form for implementing this invention is demonstrated based on drawing. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description exemplifies an embodiment of the present invention, and does not limit the scope of the claims.

[Image forming apparatus]
First, the image forming apparatus of the present invention will be described with an example.
FIG. 1 shows an outline of an image forming apparatus to which the present invention is applied. The image forming apparatus 100 is a printer as an inkjet printer, and can perform full-color image formation. The image forming apparatus 100 performs an image forming process based on an image signal corresponding to image information received from the outside.
The image forming apparatus 100 can form an image on a sheet-like recording medium using not only plain paper generally used for copying, but also OHP sheets, cardboard, cardboard, cardboard, and envelopes. It is. The image forming apparatus 100 is a single-sided image forming apparatus capable of forming an image on one side of a transfer sheet S that is a recording medium, but may be a double-sided image forming apparatus capable of forming an image on both sides of the transfer sheet S.

  The image forming apparatus 100 can form an image as an image corresponding to each color separated into yellow, magenta, cyan, and black, and can serve as a recording liquid ejector that ejects a recording liquid as ink of that color. The recording heads 61Y, 61M, 61C, and 61BK are the ink heads.

  The heads 61Y, 61M, 61C, 61BK are arranged at positions facing the outer peripheral surface of the intermediate transfer member 37 as an intermediate transfer roller, which is an intermediate transfer drum disposed at a substantially central portion of the main body 99 of the image forming apparatus 100. 1 are arranged in the order as shown in FIG. In the drawing, Y, M, C, and BK added after the numerals of the reference numerals indicate members for yellow, magenta, cyan, and black.

  The heads 61Y, 61M, 61C, and 61BK are ink discharge devices that are recording liquid discharge devices as image forming units for forming yellow (Y), magenta (M), cyan (C), and black (BK) images, respectively. The devices 60Y, 60M, 60C, and 60BK are provided. Each of the heads 61Y, 61M, 61C, 61BK is a line head provided in the ink ejection devices 60Y, 60M, 60C, 60BK in a mode in which a plurality of heads 61Y, 61M, 61C, 61BK are arranged in a direction perpendicular to the paper surface of FIG.

  The intermediate transfer body 37 is rotated in the A1 direction, and is recorded in black, cyan, magenta, and yellow from each head 61BK, 61C, 61M, 61Y in an area facing each head 61BK, 61C, 61M, 61Y. The liquids are ejected and applied in such a manner that they are sequentially superimposed, and an image that is a primary image is formed on the primary image forming surface that is the surface thereof. As described above, the image forming apparatus 100 has a tandem structure in which the heads 61BK, 61C, 61M, and 61Y face the intermediate transfer body 37 and are arranged in parallel in the A1 direction.

  The recording liquid is ejected or applied to the intermediate transfer body 37 by the heads 61BK, 61C, 61M, and 61Y, so that the image areas of the respective colors of black, cyan, magenta, and yellow overlap the same position on the intermediate transfer body 37. The timing is shifted from the side toward the downstream side.

  As shown in FIG. 1, the image forming apparatus 100 includes ink discharge devices 60BK, 60C, 60M, and 60Y each including heads 61BK, 61C, 61M, and 61Y, and an intermediate transfer member 37 in the A1 direction. The transfer unit 10 as a sheet transfer unit that transfers the transfer sheet S in accordance with the rotation of the transfer sheet S, and a transfer unit 10 that can stack a large number of transfer sheets S and transfers only the uppermost transfer sheet S among the stacked transfer sheets S. A paper feed unit 20 that feeds the paper toward the printer, and a paper discharge tray 25 on which a large number of image-formed transfer papers S that have been transported by the transport unit 10 can be stacked.

  The image forming apparatus 100 is also disposed above the intermediate transfer body 37 so as to face the intermediate transfer body 37, and from the intermediate transfer body 37 after the recording liquid is transferred to the transfer paper S, onto the intermediate transfer body 37. That is, a cleaning device 40 as a cleaning unit for removing and cleaning the recording liquid remaining on the primary image forming surface, and a head support that integrally supports the heads 61BK, 61C, 61M, and 61Y. As a carriage 62.

  In the image forming apparatus 100, as shown in FIG. 2B, the liquid column of the recording liquid immediately after being ejected from the heads 61Y, 61M, 61C, 61BK is changed to the heads 61Y, 61M, 61C, 61BK, the intermediate transfer body 37, and the like. The liquid column is formed so that a potential difference is formed between the intermediate transfer body 37 and the heads 61Y, 61M, 61C, 61BK (or the ink cartridges 81Y, 81M, 81C, 81BK) in a state where they are temporarily bridged. As a control means including an energization means 33 for energizing the recording liquid in a state containing a current component resulting from an electrode oxidation reaction or an electrode reduction reaction, and a CPU, a memory, etc. (not shown) for controlling the overall operation of the image forming apparatus 100 Control unit 98.

  In addition to the intermediate transfer member 37, the transport unit 10 is disposed so as to face the intermediate transfer member 37, is driven to rotate by the intermediate transfer member 37, and the transfer sheet S passes through the transfer unit 31 between the conveyance unit 10 and the intermediate transfer member 37. In addition, a transfer roller 38 serving as a pressure roller serving as a transfer unit serving as a transfer recording unit that transfers and records the primary image of the recording liquid carried on the intermediate transfer body 37 onto the transfer sheet S is provided.

  The conveyance unit 10 also temporarily stops the transfer sheet 32 conveyed by the conveyance roller 32 and the conveyance roller 32 that conveys the transfer sheet S fed from the sheet feeding unit 20 toward the transfer unit 31. A registration roller 34 that feeds the transfer sheet S, which has been stopped according to the timing at which the image formed on the intermediate transfer body 37 reaches the transfer section 31 as the intermediate transfer body 37 rotates in the A1 direction, to the transfer section 31. And have.

  The transport unit 10 also guides the transfer paper S fed from the paper supply unit 20 to the transfer unit 31 and guides the transfer paper S that has passed through the transfer unit 31 to the paper discharge table 25; It has a motor or the like as a driving means (not shown) for driving the intermediate transfer member 37 in the A1 direction. As described above, the image forming apparatus 100 is an indirect image forming apparatus that indirectly performs image formation on the transfer sheet S using the intermediate transfer body 37.

  As shown in FIG. 2, the intermediate transfer member 37 includes a support 37a made of aluminum and a surface layer 37b made of conductive silicone rubber formed on the support 37a. The material of the support body 37a is not limited to aluminum, but must be highly conductive and have mechanical strength, and is formed of a metal or an alloy. Further, the intermediate transfer member 37 in the figure is shown as a roller, but it may be a belt.

  The surface layer 37b is not limited to the conductive silicone rubber, but may be formed of a smooth elastic material having high conductivity and water repellency. The conductivity of the surface layer 37b is formed between the intermediate transfer member 37 and the heads 61Y, 61M, 61C, 61BK (or ink cartridges 81Y, 81M, 81C, 81BK) by applying a voltage between them. This function is necessary for causing electrolysis of water in the recording liquid bridge.

  The water repellency of the surface layer 37b is an index of ease of transfer when the recording liquid is transferred from the surface layer 37b of the intermediate transfer body 37 to the transfer paper S. The higher the water repellency, the better the transfer rate. However, on the other hand, if the water repellency is high, there is a trade-off relationship that a recording liquid with insufficient viscosity is difficult to fix at the landing position on the surface layer 37b and causes beading. The elasticity of the surface layer 37b is a function required at the time of transfer. When the surface layer 37b is deformed along the fibers of the transfer paper S, the contact area is improved and a high transfer rate is achieved. In order to transfer at a low pressure, a material that is somewhat soft must be selected as the material of the surface layer 37b.

  Examples of materials that satisfy these functions of the surface layer 37b include rubber materials such as fluorosilicone rubber, phenylsilicone rubber, fluorine rubber, chloroprene rubber, nitrile rubber, nitrile butadiene rubber, and isoprene rubber, carbon black, carbon nanotubes, Examples thereof include conductive rubber mixed with metal fine particles such as gold and silver.

  In order to increase the conductivity, it is conceivable to increase the conductive fine particles, but this becomes a factor of lowering elasticity and water repellency. It is sufficient that the conductivity is in the film thickness direction of the support 37a and the surface layer 37b, and the anisotropic conductivity may be insulative in the surface direction. The size of the conductive fine particles needs to be sufficiently smaller than the dots of about 20 to 50 μm constituting the image, and there is no problem if it is 0.1 μm or less. Further, the surface layer 37b may be impregnated with silicon oil in order to increase water repellency, and may have either a single layer structure or a multilayer structure.

  The bulk physical properties and surface physical properties of the surface layer 37b are preferably as follows. The water repellency has a receding contact angle of water of 60 ° or more, preferably 80 ° or more, and the hardness is 60 or less, preferably 40 or less in JIS-A. The thickness of the surface layer is preferably about 0.1 to 1 mm, and preferably 0.2 to 0.6 mm. The electrical resistivity is 1000 Ω · cm or less, preferably 10 Ω · cm or less.

  As shown in FIG. 1, the paper feeding unit 20 transports only the uppermost transfer paper S among the paper feed tray 21 on which a large number of transfer papers S can be stacked and the transfer paper S stacked on the paper feed tray 21. A paper feed roller 22 as a feed roller for feeding toward the unit 10, a housing 23 supporting the paper feed tray 21 and the paper feed roller 22, and the paper feed roller 22 are arranged in the heads 61 Y, 61 M, 61 C, 61 BK. It has a motor or the like as a driving means (not shown) that rotates and feeds the transfer paper S so as to match the discharge timing of the recording liquid.

  The cleaning device 40 has a cleaning blade (not shown) as an insulating cleaning member that contacts the intermediate transfer member 37 and cleans the recording liquid on the intermediate transfer member 37. If the cleaning blade has a function of scraping off the recording liquid on the surface of the intermediate transfer body 37, specifically, the recording liquid remaining after the transfer, by contacting a part, that is, the tip of the cleaning blade. Good and wear resistant.

  The carriage 62 can be replaced with a new one when the heads 61Y, 61M, 61C, 61BK are deteriorated, and in order to facilitate maintenance, the heads 61Y, 61M, 61C, 61BK can be replaced. And can be attached to and detached from the main body 99. Each of the heads 61Y, 61M, 61C, 61BK can be independently attached to and detached from the main body 99 so that it can be replaced with a new one when deterioration or the like occurs and for easy maintenance. ing. This facilitates replacement work and maintenance work.

  The ink discharge devices 60Y, 60M, 60C, and 60BK have substantially the same configuration with respect to the other points although the colors of the recording liquid to be used are different. Each of the ink ejection devices 60Y, 60M, 60C, and 60BK has a plurality of heads 61Y, 61M, 61C, and 61BK arranged in parallel in the main scanning direction, and the ink ejection devices 60Y, 60M, 60C, and 60BK, and the image forming apparatus 100 are full. It is a line type.

  The ink ejection devices 60Y, 60M, 60C, and 60BK are ink cartridges 81Y, 81M, and 81C that are recording liquid cartridges as main tanks that store the recording liquids of the corresponding colors supplied to the plurality of heads 61Y, 61M, 61C, and 61BK. , 81BK, a pump (not shown) as a supply pump for pumping and feeding the recording liquid stored in the ink cartridges 81Y, 81M, 81C, 81BK toward the heads 61Y, 61M, 61C, 61BK, and a pump Is a distributor as an ink supply unit that is a recording liquid supply unit that distributes and supplies the recording liquid supplied from the ink cartridges 81Y, 81M, 81C, and 81BK to the heads 61Y, 61M, 61C, and 61BK. With distributor tank

  The ink ejection devices 60Y, 60M, 60C, and 60BK are also inks (not shown) as ink amount detection means that are recording liquid amount detection means for detecting the recording liquid amount in order to detect a shortage of the recording liquid amount in the distributor tank. An amount detection sensor, a pipe (not shown) that forms a feeding path for the recording liquid between the ink cartridges 81Y, 81M, 81C, 81BK and the distributor tank together with a pump; a distributor tank and each head 61Y, 61M, 61C; And a pipe (not shown) that forms a recording liquid feeding path between the unit and 61BK.

  The ink cartridges 81Y, 81M, 81C, 81BK are replaced with new ones when the internal recording liquid is consumed and the remaining amount is low or no longer used, and in order to facilitate maintenance. 99 is removable.

  The operation of the pump is controlled by the control unit 98. Specifically, when the recording liquid discharge by the heads 61Y, 61M, 61C, 61BK is stopped on condition that the ink amount detection sensor detects that the recording liquid amount in the distributor tank is insufficient. It is driven until shortage is no longer detected, and the recording liquid in the ink cartridges 81Y, 81M, 81C, 81BK is supplied to the distributor tank. In this respect, the control unit 98 functions as an ink supply control unit that is a recording liquid supply control unit. In addition, the control unit 98 controls the driving of the image forming apparatus 100 even when not specifically described.

  The recording liquid contains at least a colorant that is a colorant corresponding to yellow, magenta, cyan, and black, a solvent, and an ABA triblock copolymer that changes in viscosity depending on pH. The solvent is an aqueous solvent containing water from the viewpoint of safety and the conductivity from which electrolysis described later is caused, and the recording liquid is an aqueous ink composition. As the colorant, a dye or a dispersed pigment which is dissolved or dispersed in a solvent is used.

[Water-based ink composition]
Next, the water-based ink composition of the present invention will be described.
The aqueous ink composition of the present invention contains at least a colorant, an aqueous solvent, and an ABA type triblock copolymer dissolved in the aqueous solvent as components.

(ABA type triblock copolymer)
The ABA type triblock copolymer dissolved in the water-based ink composition comprises an A block having a monomer represented by the following general formula (1) as a structural unit and a hydrophilic monomer having a radical polymerizable unsaturated bond. Containing B block.

<A block>
The A block has a monomer represented by the following general formula (1) as a structural unit.
(In the general formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents a number 6-18 alkylene group having a carbon, M is a hydrogen atom and monovalent metal Represents any of the atoms, and X ¹ represents —NH—.)

The monovalent metal atom in M is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include sodium and potassium.
The alkylene group having 6 to 18 carbon atoms in R ² is not particularly limited and may be appropriately selected depending on the intended purpose. However, an alkylene group having 9 to 11 carbon atoms is preferable, and an alkylene group having 10 carbon atoms is preferable. More preferred. The alkylene group having 6 to 18 carbon atoms is preferably linear.

The combination of R ¹ , X ¹ , R ² , and M in the general formula (1) is not particularly limited and may be appropriately selected depending on the intended purpose. In a more excellent point, R ¹ is a hydrogen atom, X ¹ is an oxygen atom, R ² is an alkylene group having 9 to 11 carbon atoms, M is sodium, R ¹ is a hydrogen atom, A combination in which X ¹ is —NH—, R ² is an alkylene group having 9 to 11 carbon atoms, M is a hydrogen atom or sodium, R ¹ is a methyl group, X ¹ is an oxygen atom, R ² is an alkylene group having 9 to 11 carbon atoms, M is a combination of sodium, R ¹ is a methyl group, X ¹ is —NH—, and R ² is an alkylene group having 9 to 11 carbon atoms A combination in which M is a hydrogen atom or sodium is preferred. There.

Examples of the monomer represented by the general formula (1) include 11-acrylamide undecanoic acid, 8-acrylamide octanoic acid, and 12-acrylamide dodecanoic acid.
The monomer represented by the general formula (1) may be used alone or in combination of two or more.

In the ABA triblock copolymer, the monomer represented by the general formula (1) forms a repeating unit represented by the following general formula (1 ′).
(In the general formula (1 '), R ¹ represents a hydrogen atom or a methyl group, R ² represents a number 6-18 alkylene group having a carbon, M is a hydrogen atom and monovalent Represents ^one of metal atoms, and X ¹ represents either —NH— or an oxygen atom.)

  In the ABA type triblock copolymer, when the A block is represented by the general formula (1 ′), the water-based ink composition containing the ABA type triblock copolymer is acidified to increase the viscosity. To express. Specifically, as shown in FIG. 3, in P (A1), which is an A block in a state (carboxyl ion) in which the carboxylic acid at the end of the side chain in the general formula (1 ′) is deprotonated at alkaline pH. The ABA type triblock copolymer is dissolved in an aqueous solvent by carboxyl ions, but in P (A2), which is an A block in which the pH is acidic and the carboxylic acid at the end of the side chain is protonated, the hydrophobicity of the A block In the aqueous solvent, intermolecular hydrophobic association between the A blocks occurs in an aqueous solvent, and thus a thickening action according to pH change is exhibited.

The A block may have a monomer other than the monomer represented by the general formula (1) as a structural unit as long as the viscosity change effect due to pH is not inhibited. The other monomer is not particularly limited as long as it is a radical polymerizable monomer, and can be appropriately selected according to the purpose.
There is no restriction | limiting in particular as content of the said other monomer in the structural unit of the said A block, Although it can select suitably according to the objective, 5 mol% or less is preferable and 1 mol% or less is preferable.

  In the case where the ABA type triblock copolymer is an ABA type triblock copolymer that develops a thickening action under acidic conditions, the A block is composed of the monomer represented by the general formula (1) as a constituent unit. It is preferably contained in an amount of at least mol%, more preferably at least 99 mol%, particularly preferably at least 100 mol%.

<B block>
The B block has a hydrophilic monomer having a radical polymerizable unsaturated bond as a constituent unit.
There is no restriction | limiting in particular as said hydrophilic monomer, According to the objective, it can select suitably. Here, the hydrophilicity of the hydrophilic monomer means that 100 g or more of the monomer is dissolved in 1 L of water at 25 ° C., for example. Whether or not it has been dissolved can be judged by whether or not the water is transparent, and can be confirmed visually.

There is no restriction | limiting in particular as said hydrophilic monomer, Although it can select suitably according to the objective, The monomer represented by following General formula (2) is preferable.
(However, in the general formula (2), R ²¹ represents a hydrogen atom or a methyl group, R ²² and R ²³ each independently represents any of a hydrogen atom, a methyl group and an ethyl group .)

In the ABA triblock copolymer, the monomer represented by the general formula (2) forms a repeating unit represented by the following general formula (2 ′).
(In the general formula (2 '), R ²¹ represents a hydrogen atom or a methyl group, R ²² and R ²³ each independently represent a hydrogen atom, one of methyl and ethyl Represents.)

  Examples of the hydrophilic monomer include acrylamide (AA), methacrylamide (MA), N-methylacrylamide (MAA), N-methylmethacrylamide (MMA), N, N-dimethylacrylamide (DMA), N, N. -Dimethylmethacrylamide (DMMA), N, N-diethylacrylamide (DEAA), N, N-diethylmethacrylamide (DEMA), sodium 2-acrylamido-2-methylpropanesulfonate (AMPS), sodium p-styrenesulfonate (PSSNa), N-isopropylacrylamide (NIPAM), N-vinylpyrrolidone (NVP), vinyl alcohol (VA) and the like. These may be used individually by 1 type and may use 2 or more types together.

The B block may have a monomer other than the hydrophilic monomer as a structural unit as long as the effect of changing the viscosity due to pH is not inhibited. The other monomer is not particularly limited as long as it is a radical polymerizable monomer, and can be appropriately selected according to the purpose.
There is no restriction | limiting in particular as content of the said other monomer in the structural unit of the said B block, Although it can select suitably according to the objective, 5 mol% or less is preferable and 1 mol% or less is preferable.
The B block preferably contains 90 mol% or more, more preferably 95 mol% or more, more preferably 99 mol% or more, more preferably 100 mol%, as a structural unit of a hydrophilic monomer. Is particularly preferred. The structural unit does not include an initiator residue or a chain transfer agent residue during polymerization.

<Average polymerization degree>
The average polymerization degree [P (A)] of the A block and the average polymerization degree [P (B)] of the B block are not particularly limited and may be appropriately selected depending on the intended purpose. It is preferable that I) and the following formula (II) are satisfied in that the thickening action is more excellent.
3 ≦ [P (A)] ≦ 10 Formula (I)
20 ≦ [P (B)] / [P (A)] ≦ 300 Formula (II)
The average degree of polymerization [P (A)] is an average value of the degree of polymerization of the A blocks at both ends of the ABA triblock copolymer.

When the [P (A)] is 3 or more, the intermolecular hydrophobic associating force between the A blocks becomes strong, and sufficiently exhibits a thickening action according to pH change. When [P (A)] is 10 or less, a large amount of H ⁺ and OH ⁻ required for the expression of the thickening action is not required, and the responsiveness of the viscosity change is not lowered. The intermolecular hydrophobic association force is not too strong, phase separation due to polymer agglomeration does not occur, and a thickening action is favorably performed.

When [P (B)] / [P (A)] is 20 or more, a large amount of H ⁺ and OH ⁻ necessary for the expression of the thickening action is not required, and the response of the viscosity change Property does not deteriorate or polymer agglomeration does not occur. When [P (B)] / [P (A)] is 300 or less, the water solubility of the polymer does not become too high, and even if the A block is hydrophobized, a thickening action due to hydrophobic association hardly occurs. There will never be.
The average degree of polymerization can be measured by, for example, ¹ H-NMR.

The degree of polymerization [P (A ′)] and the average degree of polymerization [P (B)] of each A block in the ABA block copolymer satisfy the following formulas (XI) and (XII). However, it is preferable from the point that the thickening action is more excellent.
3 ≦ P (A ′) <10 Formula (XI)
20 ≦ P (B) / P (A ′) ≦ 300 Formula (XII)
The degree of polymerization [P (A ′)] of each A block can be measured, for example, by ¹ H-NMR and GPC.

It is more preferable that the average degree of polymerization [P (A)] and the average degree of polymerization [P (B)] satisfy the following formula (I ′) and the following formula (II ′).
4 ≦ P (A) ≦ 6 Formula (I ′)
100 ≦ P (B) / P (A) ≦ 200 Formula (II ′)

  By satisfying the formula (I ′) and the formula (II ′), the thickening action is remarkably excellent. As a result, it is possible to obtain a thickening action to such an extent that the aqueous ink composition containing the ABA type triblock copolymer is gelled. Here, “gelation” refers to a state in which the system has lost fluidity, for example, a state in which the shear viscosity when the shear rate is 1 / s is 10,000 mPa · s or more.

<Molecular weight distribution>
The molecular weight distribution (Mw / Mn) of the ABA type triblock copolymer is not particularly limited and can be appropriately selected according to the purpose. However, in terms of more excellent thickening action, 2.00 or less is preferable, 1.50 or less is more preferable. The lower limit is 1.00.
The molecular weight distribution can be measured by gel permeation chromatography (GPC).

There is no restriction | limiting in particular as an average molecular weight of the said ABA type | mold triblock copolymer, According to the objective, it can select suitably, For example, the average molecular weight etc. which are represented by following formula (III) are mentioned.
[P (A)] × Mn (A) × 2 + [P (B)] × Mn (B) Formula (III)
However, in the formula (III), [P (A)] and [P (B)] satisfy the formula (I) and the formula (II).
Here, Mn (A) is the molecular weight of the monomer represented by the general formula (1), and Mn (B) is the molecular weight of the hydrophilic monomer.

  When the average molecular weight is smaller than the range of the formula (III), the thickening action may be reduced when the concentration of the ABA triblock copolymer is low in the aqueous ink composition. When the average molecular weight is larger than the range of the formula (III), the viscosity of the water-based ink composition before exhibiting the thickening action becomes high, and it may be impossible to discharge from the inkjet nozzle.

The number average molecular weight of the ABA triblock copolymer is not particularly limited and may be appropriately selected depending on the intended purpose, but is preferably 10,000 to 300,000, more preferably 50,000 to 100,000. .
Here, the number average molecular weight can be measured by gel permeation chromatography (GPC).

  The ABA block copolymer may have an initiator residue or a chain transfer agent residue during polymerization. The initiator residue and the chain transfer agent residue may be present at the end of the ABA type block copolymer, and may be present at the connecting portion between the A block and the B block. You may have in B block.

The ABA type block copolymer is easily produced by a RAFT (Reversible Addition Fragmentation Chain Transfer) method using a dithioester or trithiocarbonate as a chain transfer agent, and has a narrow molecular weight distribution. This is preferable. That the ABA type block copolymer was produced by the RAFT method can be confirmed, for example, by the presence of a residue of a chain transfer agent in the polymer. The presence of the residue of the chain transfer agent can be confirmed, for example, by ¹ H-NMR.

<Method for producing ABA type triblock copolymer>
There is no restriction | limiting in particular as a manufacturing method of the said ABA type | mold triblock copolymer, According to the objective, it can select suitably, For example, living radical polymerization etc. are mentioned.
Examples of the living radical polymerization include an ATRP (Atom Transfer Radical Polymerization) method, a TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy radical) method, and the RAFT method. Is mentioned. Among these, the RAFT method is preferable because it is suitable for the polymerization of the monomer represented by the general formula (1), and a narrow molecular weight distribution is easily obtained and the polymerization is easily controlled.

-Production of ABA type triblock copolymer by RAFT method-
There is no restriction | limiting in particular as a manufacturing method of the said ABA type | mold triblock copolymer by the said RAFT method, According to the objective, it can select suitably, For example, a bifunctional chain transfer agent (A polymer chain is extended from two places. A three-step polymerization by a production method including an A block diploid synthesis step and a B block synthesis step, and a monofunctional chain transfer agent (polymerized so that the polymer chain extends from one place). And a production method by polymerization in the order of A block → B block → A block).
Hereinafter, the production method including the A-block diploid synthesis step is referred to as production method 1, and the production method based on the three-step polymerization is referred to as production method 2.

-Production method 1: A block diploid synthesis step-
The A block diploid synthesis step is not particularly limited as long as it is a step of polymerizing the monomer represented by the general formula (1) using a polymerization initiator in the presence of a bifunctional chain transfer agent. It can be appropriately selected depending on the purpose.
By the A block diploid synthesis step, an A block diploid in which two A blocks are bonded via a chain transfer agent is obtained.

  The bifunctional chain transfer agent is not particularly limited as long as it is a chain transfer agent that can be used in the RAFT method, and can be appropriately selected according to the purpose. Trithiocarbonate is preferred because it is suitable for polymerization of acrylic monomers and acrylamide monomers represented by the general formulas (1) and (2), and S, S-bis (α, α′dimethyl-α ″). -Acetic acid) trithiocarbonate and the like.

  The polymerization initiator is not particularly limited as long as it is an initiator capable of initiating radical polymerization, and can be appropriately selected according to the purpose. For example, an azo polymerization initiator or a peroxide polymerization initiator And persulfuric acid polymerization initiators. Examples of the azo initiator include 2,2′-azobis (isobutyric acid) dimethyl, 4,4′-azobis (4-cyanovaleric acid), 2,2′-azobis (isobutyronitrile), and the like. Can be mentioned. Examples of the peroxide polymerization initiator include benzoyl peroxide. Examples of the persulfuric acid polymerization initiator include potassium persulfate and ammonium persulfate.

  The A block diploid synthesis step can be performed in a solvent. The solvent is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include water, alcohol solvents, hydrocarbon solvents, ketone solvents, ester solvents, chloride solvents, aromatic solvents. Examples include solvents and aprotic polar solvents.

  Examples of the alcohol solvent include methanol, ethanol, 1-propanol, and isopropanol. Examples of the hydrocarbon solvent include hexane, heptane, octane, decane, and liquid paraffin. Examples of the ketone solvent include acetone and methyl ethyl ketone.

Examples of the ester solvent include methyl acetate, ethyl acetate, butyl acetate and the like.
Examples of the chloride solvent include methylene chloride, chloroform, carbon tetrachloride and the like.
Examples of the aromatic solvent include benzene and toluene.
Examples of the aprotic polar solvent include tetrahydrofuran, N, N-dimethylformamide, dimethyl sulfoxide, and the like.
As the reaction solvent as described above, a solvent having a boiling point higher than the radical generation temperature of the polymerization initiator is preferable.

There is no restriction | limiting in particular as superposition | polymerization temperature in the said A block diploid synthesis process, According to the objective, it can select suitably, For example, 50 to 100 degreeC etc. are mentioned.
There is no restriction | limiting in particular as superposition | polymerization time in the said A block diploid synthesis process, According to the objective, it can select suitably, For example, 30 minutes-24 hours etc. are mentioned.
The A block diploid synthesis step is preferably performed in an inert atmosphere. Examples of the inert atmosphere include argon.

--B block synthesis process--
The B block synthesis step is a step of polymerizing the hydrophilic monomer represented by the general formula (2) in the presence of the A block diploid and the polymerization initiator obtained in the A block diploid synthesis step. If there is, there is no restriction | limiting in particular, According to the objective, it can select suitably.

  The B block polymerization is continuously performed from the A block diploid synthesis step, that is, the reaction with the hydrophilic monomer necessary for the B block polymerization and an additional polymerization initiator as necessary is carried out without changing the reaction solvent and the polymerization temperature. One-pot polymerization may be performed simply by charging into a container, or a predetermined charge amount may be adjusted after purifying the A block diploid.

In the B block synthesis step, the bond between the A block and the chain transfer agent in the A block diploid is broken, and the hydrophilic monomer is radically added to the end of the A block by cleavage chain transfer. The ABA type triblock copolymer is obtained by the growth reaction.
The hydrophilic monomer is a monomer represented by the general formula (32) described in the ABA type triblock copolymer.

  There is no restriction | limiting in particular as said polymerization initiator, According to the objective, it can select suitably, For example, the said polymerization initiator etc. which were illustrated in the said A block diploid process are mentioned. The polymerization initiator used in the A block diploid step and the polymerization initiator used in the B block synthesis step may be the same polymerization initiator or different polymerization initiators. .

  The B block synthesis step can be performed in a solvent. There is no restriction | limiting in particular as said solvent, According to the objective, it can select suitably, For example, the said solvent etc. which were illustrated in the said A block diploid process are mentioned. The reaction solvent used in the A block diploid step and the reaction solvent used in the B block polymerization step may be the same solvent or different solvents.

There is no restriction | limiting in particular as polymerization temperature in the said B block synthetic | combination process, According to the objective, it can select suitably, For example, 50 to 100 degreeC etc. are mentioned.
There is no restriction | limiting in particular as polymerization time in the said B block synthesis | combination process, According to the objective, it can select suitably, For example, 30 minutes-24 hours etc. are mentioned.
The B block synthesis step is preferably performed in an inert atmosphere. Examples of the inert atmosphere include argon.
In addition, the ABA type triblock copolymer obtained by this method has a residue of a chain transfer agent at substantially the center in the B block and at both ends of the A block.

-Manufacturing method 2: Synthesis process of three-step polymerization-
As another synthesis method of the ABA type triblock copolymer, the above-mentioned three-step polymerization can be mentioned. That is, polymerization is performed in the order of A block → B block → A block. The polymerization process for the A block is not particularly limited as long as it is a process for polymerizing the monomer represented by the general formula (1) using a polymerization initiator in the presence of a monofunctional chain transfer agent. It can be appropriately selected according to the purpose.

The monofunctional chain transfer agent is not particularly limited as long as it is a chain transfer agent that can be used in the RAFT method, and can be appropriately selected according to the purpose. Trithiocarbonate is preferable in that it is suitable for polymerization of acrylic monomers and acrylamide monomers represented by the general formulas (1) and (2), and 2-methyl-2-[(dodecylsulfanylthiocarbonyl) sulfanyl] And propanoic acid.
As the polymerization initiator, polymerization solvent and the like, those exemplified above can be used.

  After completion of the polymerization of the A block, the B block polymerization to be performed next may be performed by polymerizing the B block by one-pot polymerization in which a hydrophilic monomer represented by the general formula (2) or an additional polymerization initiator is added if necessary. Alternatively, it may be a method of once purifying the A homopolymer and then adjusting the predetermined amount of charge.

After completion of the polymerization of the B block, the next A block polymerization may be performed by one-pot polymerization in which the monomer represented by the general formula (1) or an additional polymerization initiator is added as necessary. Alternatively, the AB diblock copolymer may be purified once and then a predetermined charge amount may be adjusted.
The ABA type triblock copolymer obtained by this method has a chain transfer agent residue at both A block ends.

  There is no restriction | limiting in particular as content of the ABA type | mold triblock copolymer in the said water-based ink composition, Although it can select suitably according to the objective, 1-5 mass% is preferable. When the content is 1% by mass or more, a thickening effect corresponding to the pH can be sufficiently obtained, and thus, bleeding prevention on the intermediate transfer member is favorably performed. When the content is 5% by mass or less, the viscosity of the aqueous ink composition before exhibiting the thickening action does not become too high, and it is not possible to discharge from the inkjet nozzle.

  The viscosity of the recording liquid during ejection is 1 to 20 mPa · s, preferably 2 to 8 mPa · s. The recording liquid has a viscosity increase at least 10 times, preferably 100 times, more preferably 1000 times or more of that due to the increase in viscosity due to pH change upon landing, which will be described later, and becomes a gel state. FIG. 5 shows a schematic view of the bridged liquid injection and thickening due to pH change. In addition, suitable ranges of physical properties of the recording liquid are a surface tension of 10 to 60 mN / m, preferably 20 to 50 mN / m, and an electrical conductivity of 0.1 to 5 S / m, preferably 1 to 3 S / m.

(Coloring agent)
Specific examples of the colorant component contained in the recording liquid include, for example, dyes classified into acid dyes, direct dyes, and food dyes in the color index.
More specifically, as acid dyes and food dyes, C.I. I. Acid Yellow 17, 23, 42, 44, 79, 142 C.I. I. Acid Red 1, 8, 13, 14, 18, 26, 27, 35, 37, 42, 52, 82, 87, 89, 92, 97, 106, 111, 114, 115, 134, 186, 249, 254, 289 C.I. I. Acid Blue 9, 29, 45, 92, 249 C.I. I. Acid Black 1, 2, 7, 24, 26, 94 C.I. I. Food Yellow 3, 4 C.I. I. Food Red 7, 9, 14 C.I. I. Food black 1, 2 etc.

  As a direct dye, C.I. I. Direct Yellow 1, 12, 24, 26, 33, 44, 50, 86, 120, 132, 142, 144 C.I. I. Direct Red 1, 4, 9, 13, 17, 20, 28, 31, 39, 80, 81, 83, 89, 225, 227 C.I. I. Direct Orange 26, 29, 62, 102 C.I. I. Direct Blue 1, 2, 6, 15, 22, 25, 71, 76, 79, 86, 87, 90, 98, 163, 165, 199, 202 C.I. I. Direct black 19, 22, 32, 38, 51, 56, 71, 74, 75, 77, 154, 168, 171 and the like.

  As reactive dyes, C.I. I. Reactive. Black 3, 4, 7, 11, 12, 17, C.I. I. Reactive. Yellow 1, 5, 11, 13, 14, 20, 21, 22, 25, 40, 47, 51, 55, 65, 67, C.I. I. Reactive. Red 1, 14, 17, 25, 26, 32, 37, 44, 46, 55, 60, 66, 74, 79, 96, 97, C.I. I. Reactive. Blue 1, 2, 7, 14, 15, 23, 32, 35, 38, 41, 63, 80, 95, etc. When recording with the method according to the present invention, high solubility, good color tone It is preferably used because of its good water resistance.

Other specific examples of the colorant component used in the recording liquid include inorganic pigments and organic pigments.
Examples of inorganic pigments include white pigments such as titanium oxide, zinc oxide, and barium sulfate, and black pigments such as iron oxide.
Organic pigments include azo pigments (including azo lakes, insoluble azo pigments, condensed azo pigments, chelate azo pigments), polycyclic pigments (eg, phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazines). Pigments, thioindigo pigments, isoindolinone pigments, quinofullerone pigments, etc.), dye chelates (for example, basic dye chelates, acidic dye chelates, etc.), nitro pigments, nitroso pigments, aniline black, and the like.
Further, carbon black produced by a known method such as a contact method, a furnace method, or a thermal method can be used.

  More specifically, C.I. I. Pigment Yellow 1 (Fast Yellow G), 3, 12 (Disazo Yellow AAA), 13, 14, 17, 24, 34, 35, 37, 42 (Yellow Iron Oxide), 53, 55, 81, 83 (Disazo Yellow HR) ), 95, 97, 98, 100, 101, 104, 408, 109, 110, 117, 120, 138, 153, C.I. I. Pigment orange 5, 13, 16, 17, 36, 43, 51, C.I. I. Pigment Red 1, 2, 3, 5, 17, 22 (Brilliant First Scarlet), 23, 31, 38, 48: 2 (Permanent Red 2B (Ba)), 48: 2 (Permanent Red 2B (Ca)), 48: 3 (Permanent Red 2B (Sr)), 48: 4 (Permanent Red 2B (Mn)), 49: 1, 52: 2, 53: 1, 57: 1 (Brilliant Carmine 6B), 60: 1, 63 : 1, 63: 2, 64: 1, 81 (Rhodamine 6G rake), 83, 88, 101 (Bengara), 104, 105, 106, 108 (Cadmium red), 112, 114, 122 (Quinacridone magenta), 123 146, 149, 166, 168, 170, 172, 177, 178, 179, 185, 190, 193, 209, 219, CI. Pigment violet 1 (rhodamine lake), 3, 5: 1, 16, 19, 23, 38, C.I. I. Pigment Blue 1, 2, 15 (phthalocyanine blue R), 15: 1, 15: 2, 15: 3 (phthalocyanine blue E), 16, 17: 1, 56, 60, 63, C.I. I. Pigment green 1, 4, 7, 8, 10, 17, 18, 36, and the like.

  When using a recording liquid containing a pigment as a colorant component, for example, carbon black having a carboxyl group introduced by an oxidation reaction, a radical generated from a diazonium salt containing a carboxyl group or a sulfonic acid group, and carbon black, phthalocyanine, Self-dispersing pigments made by reacting pigments such as quinacridone, self-dispersing pigments made by reacting radical initiators containing carboxyl groups and sulfonic acid groups with pigments such as carbon black, phthalocyanine, quinacridone, and pigment functionality And self-dispersing pigments obtained by reacting a group with a carboxylic acid anhydride.

  When a recording liquid in which a pigment is dispersed is used, the particle diameter of the pigment is not particularly limited, but it is preferable to use a pigment ink having a particle diameter of 20 to 150 nm in terms of the maximum frequency in terms of the maximum number. When the particle diameter exceeds 150 nm, not only the dispersion stability of the pigment as the recording liquid is deteriorated but also the discharge stability of the recording liquid is deteriorated, and the image quality such as the image density is lowered, which is not preferable. In addition, when the particle size is less than 20 nm, the storage stability of the recording liquid and the jetting characteristics in the printer are stable, and high image quality can be obtained. This is because the classification operation becomes complicated and it is difficult to produce the recording liquid economically.

  In the recording liquid, a high molecular type dispersant such as a high molecular dispersant or a low molecular type dispersant such as a surfactant can be used as a dispersant for dispersing the pigment. It is a polymer dispersant that has a multipoint adsorption to the pigment that has little influence on the thickening effect of the ABA type triblock copolymer.

  Another example of the colorant component used in the recording liquid is a recording liquid that uses a colored emulsion composed of colored resin fine particles. The colored resin fine particles are those obtained by coloring styrene-acrylic resin, polyester resin, polyurethane resin or the like with an oily dye, a disperse dye or a pigment.

(Other polymer components)
In addition to the ABA triblock copolymer, a hydrophilic polymer compound may be added to the recording liquid.
Examples of such hydrophilic polymer compounds include natural gums such as gum arabic, tragan gum, guar gum, karaya gum, locust bean gum, arabinogalactone, pectin, quince seed starch, alginates, carrageenan, agar, etc. Seaweed polymer, animal polymer such as gelatin, casein, albumin, collagen, microbial polymer such as xanthene gum, dextran, shellac, etc., semi-synthetic system, methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, strength Fibrous polymers such as ruboxymethylcellulose, starch polymers such as sodium starch glycolate and sodium starch phosphate, sodium alginate, propylene glycol alginate, etc. Seaweed polymers, pure synthetic systems such as polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl methyl ether and other vinyl polymers, non-crosslinked polyacrylamide, polyacrylic acid and alkali metal salts thereof, and acrylic resins such as water-soluble styrene-acrylic resins Water-soluble styrene-maleic acid resin, water-soluble vinyl naphthalene-acrylic resin, water-soluble vinyl naphthalene-maleic acid resin, polyvinyl pyrrolidone, polyvinyl alcohol, alkali metal salt of β-naphthalene sulfonic acid formalin condensate, and the like.

  A resin emulsion or latex that does not contain a colorant may be added to the recording liquid. Depending on the type of the resin emulsion, there is an advantage that the resin emulsion forms a film on the surface of the transfer paper S to improve the light resistance, water resistance, and abrasion resistance of the transfer paper S when an image is formed. Examples of the resin component in the suspension phase include acrylic resins, vinyl acetate resins, styrene-butadiene resins, vinyl chloride resins, acrylic-styrene resins, butadiene resins, and styrene resins. The particle diameter of these resin components is not particularly limited as long as an emulsion is formed, but is preferably about 150 nm or less, more preferably about 5 to 100 nm.

Examples of commercially available resin emulsions include Microgel E-1002, E-5002 (styrene-acrylic resin emulsion, manufactured by Nippon Paint Co., Ltd.), Boncoat 4001 (acrylic resin emulsion, manufactured by Dainippon Ink & Chemicals, Inc.). Boncoat 5454 (styrene-acrylic resin emulsion, manufactured by Dainippon Ink & Chemicals, Inc.), SAE-1014 (styrene-acrylic resin emulsion, manufactured by Nippon Zeon Co., Ltd.), Cybinol SK-200 (acrylic resin emulsion, Seiden) Chemical Co., Ltd.).
In the recording liquid, the resin emulsion is preferably added so that the resin component is 0.1 to 40% by weight of the recording liquid, and more preferably in the range of 1 to 10% by weight.
Pigment polymer dispersants, colored emulsions, latexes, water-soluble polymer compounds, etc., together with the ABA type triblock copolymer, are effective in improving the transfer rate from the intermediate transfer body 37 to the transfer paper S in the transfer process. Is.

(Humectant)
The recording liquid uses water as the main aqueous solvent. In order to make the recording liquid have the desired physical properties or to prevent clogging of the nozzle 61c described later due to the drying of the recording liquid, the water-soluble organic solvent described below is moisturized. It is preferable to use it as an agent component.

  Specific examples of the water-soluble organic solvent include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, and 1,4-butanediol. 1,5-pentanediol, 1,6-hexanediol, glycerin, 1,2,6-hexanetriol, 2-ethyl-1,3-hexanediol, 1,2,4-butanetriol, 1, Polyhydric alcohols such as 2,3-butanetriol and petriol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethyl Polyhydric alcohol alkyl ethers such as ethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monoethyl ether, polyhydric alcohol aryl ethers such as ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, N-methyl-2 -Pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethylimidazolidinone, nitrogen-containing heterocyclic compounds such as ε-caprolactam, formamide, N-methylformamide, N, N-dimethylformamide, etc. Amides, monoethanolamine, diethanolamine, triethanolamine, monoethylamine, diethylamine, triethylamine and other amines, dimethyl sulfoxide, sulfolane And sulfur-containing compounds such as thiodiethanol, propylene carbonate, ethylene carbonate, and γ-butyrolactone.

  In addition, as other moisturizing components, sugar alcohols such as sorbitol, polysaccharides such as hyaluronic acid, polymers such as polyethylene glycol, and natural moisturizing components such as urea, lactic acid, citrate, and amino acids can be used. is there. These solvents are used alone or in combination with water. Although there is no restriction | limiting in particular in content of these water-soluble organic solvents, Preferably it is 1-60 weight% of the whole ink, More preferably, it uses in the range of 10-40 weight%.

(Electrolytes)
In order to electrolyze the water in the recording liquid, it is necessary to add an electrolyte component for increasing the ionic conductivity of the recording liquid. As electrolyte components to be added to the recording solution, sodium chloride, potassium chloride, lithium chloride, rubidium chloride, sodium bromide, sodium iodide, sodium sulfate, sodium sulfite, sodium hydrogen sulfite, sodium thiosulfate, potassium sulfate, sodium nitrate, sulfite Inorganic alkali metal salts such as sodium nitrate, potassium nitrate, sodium phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, potassium acetate, sodium oxalate, sodium citrate, sodium hydrogen citrate, potassium citrate, potassium hydrogen citrate Organic alkali metal salts such as ammonium chloride, ammonium nitrate, ammonium sulfate, tetramethylammonium chloride, tetramethylammonium nitrate, and organic ammonium salts such as choline chloride .

  Since the divalent or higher polyvalent metal salt impairs the solubility or dispersibility of the ionic colorant or ABA triblock copolymer, it is preferably a monovalent metal salt. In particular, it is preferable to add a quaternary ammonium salt as an electrolyte component. Quaternary ammonium ions are dispersed in charge by alkyl groups bonded to the central element, and electrostatic interaction with ionic components such as colorants is small. Quaternary ammonium ions form clusters with water. It is difficult to remove hydration water necessary for dissolving or dispersing the ionic component such as the colorant, so that the influence on the solubility and dispersibility of the colorant is small. The electrical conductivity per unit molecular weight (molar ionic conductivity) is high for compounds having a small molecular weight, and among the quaternary ammonium salts, tetramethylammonium salts are particularly preferred. Counter ions include chloride ions, nitrate ions, sulfate ions, etc., but chloride ions may cause an electrode reaction at the anode to generate chlorine. Therefore, inactive nitrate ions and sulfate ions are preferable.

(PH adjuster)
Examples of other components contained in the recording liquid include a pH adjuster. As the acidic pH adjuster, boric acid, carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, acetic acid and other organic acids, ammonium chloride, and the like can be used. is there. Examples of alkaline pH adjusters include hydroxides of alkali metal elements such as lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, quaternary ammonium hydroxide, quaternary phosphonium hydroxide, lithium carbonate, carbonic acid It is possible to use alkali metal carbonates such as sodium and potassium carbonate, and amines such as diethanolamine and triethanolamine.
(Other additives)
In the recording liquid, other additives such as a pH buffer, a viscosity modifier, an antiseptic, an antioxidant, and a rust inhibitor may be used as necessary.

  The recording liquid thus prepared has a characteristic that the viscosity changes (thickens) depending on the pH without changing the color. That is, as shown in FIG. 3 described above, a network structure is formed in the aqueous ink composition by causing intermolecular hydrophobic association (physical crosslinking) due to the change in hydrophobicity from the hydrophilicity at both A block ends of the ABA type triblock copolymer. As a result, the viscosity of the recording liquid increases. Thus, the recording liquid is a pH-responsive ink composition that exhibits a change in viscosity in response to a change in pH. Thickening may occur with gelation.

  As shown in FIG. 2, each of the heads 61Y, 61M, 61C, and 61BK has a conductive orifice 61a that is a conductive member and an insulating orifice 61b that is an insulating member on the recording liquid discharge side facing downward in the drawing. The nozzle plate 61d is provided, and a minute nozzle 61c in which holes formed in the conductive orifice 61a and the insulating orifice 61b communicate with each other.

Although not shown, each head 61Y, 61M, 61C, 61BK is an actuator driven on the basis of an image signal for discharging the recording liquid from the nozzle 61c to be ejected and landed on the intermediate transfer body 37. Although the piezoelectric actuator has a piezo-type movable actuator that discharges the recording liquid from the nozzle 61c by applying pressure to the recording liquid in the liquid chamber by the displacement of the piezo element, the actuator may be another type of movable actuator. In addition, each of the heads 61Y, 61M, 61C, 61BK may use a heating film boiling method such as a thermal method in which the recording liquid is discharged from the nozzle 61c by applying pressure to the recording liquid in the liquid chamber with bubbles generated by heating the heater. good.
In any case, each of the heads 61Y, 61M, 61C, 61BK is driven in accordance with the image signal, and applies the recording liquid to the transfer sheet S via the intermediate transfer body 37 to form an image. A number of nozzles 61c are provided in each of the heads 61Y, 61M, 61C, 61BK, but only one of them is shown in the figure. The diameter of the nozzle 61c is 25 μm.

The conductive orifice 61a forms the surface of the heads 61Y, 61M, 61C, 61BK on the recording liquid discharge side. This conductive orifice functions as an electrode. Further, when the ink cartridge 81Y, 81M, 81C, 81BK is provided with an electrode as a cathode, the conductive orifice 61a is not necessary. Although not shown, in the case of the electrode built in the ink cartridge, a potential difference is provided between the electrode of the ink cartridge and the intermediate transfer member 37 by the energizing means 33.
The insulating orifice 61b is provided so as to be interposed between the recording liquid accommodated in the heads 61Y, 61M, 61C, and 61BK and the conductive orifice 61a. The material of the insulating orifice 61b is not particularly limited as long as it has insulating properties.

  In this embodiment, the conductive orifice 61a and the insulating orifice 61b are joined to each other. However, these are not necessarily joined, and other portions are provided between the insulating orifice 61b and the recording liquid. These layers may be interposed.

  As shown in FIG. 2, the energizing means 33 is realized as part of the function of the power supply 33a, an electric circuit (not shown) in which the power supply 33a is connected to the support 37a and the conductive orifice 61a, and the function of the control unit 98. Voltage application control means for controlling the application timing and application time of the voltage by 33a. The power source 33a has an anode connected to the support 37a and a cathode connected to the conductive orifice 61a by an electric circuit. Therefore, the energizing unit 33 includes the support 37a and the intermediate transfer body 37 as an anode, and the conductive orifice 61a (or an electrode built in the ink cartridges 81Y, 81M, 81C, and 81BK) as a cathode. As will be described later, the energizing means 33 applies a voltage for electrolyzing the recording liquid between the support 37a and the heads 61Y, 61M, 61C, 61BK (or electrodes built in the ink cartridges 81Y, 81M, 81C, 81BK). It functions as a voltage applying means for applying.

  In the image forming apparatus 100 having such a configuration, the intermediate transfer member 37 rotates in the A1 direction while facing the heads 61BK, 61C, 61M, and 61Y in response to the input of a predetermined signal indicating the start of image formation. In the process, black, cyan, magenta, and yellow recording liquids are transferred from the heads 61BK, 61C, 61M, and 61Y so that the image areas of each color of black, cyan, magenta, and yellow overlap with the same position on the intermediate transfer member 37. The images are ejected in such a manner that they are sequentially overlapped from the upstream side toward the downstream side in the A1 direction, and an image is temporarily carried on the intermediate transfer member 37.

At this time, the energization means 33 is driven by the control unit 98 as the voltage application control means, and a voltage is applied between the support 37a and the conductive orifice 61a from the power source 33a.
In this state, the recording liquid is applied onto the intermediate transfer member 37 from each of the heads 61BK, 61C, 61M, 61Y. At this time, first, as shown in FIG. 2A, the recording liquid forming the meniscus in the nozzle 61b is transferred from the heads 61BK, 61C, 61M, 61Y to the intermediate transfer as shown in FIG. The recording liquid moves toward the body 37, the tip of the recording liquid lands on the intermediate transfer body 37, and a bridge of a liquid column made of the recording liquid is temporarily formed between the nozzle 61c and the intermediate transfer body 37, and then As shown in FIG. 3C, the liquid column bridge made of the recording liquid is divided, so that the intermediate transfer body 37 holds the image, and an image of the recording liquid is formed on the intermediate transfer body 37. The diameter of the liquid column forming the bridge is about 10 μm, and the dot diameter of the recording liquid carried on the intermediate transfer body 37 after the bridge division is about 50 μm.

In the state where the liquid injection bridge made of the recording liquid is formed as shown in FIG. 2B, the electrolysis of water is caused by the energizing means 33, and the intermediate transfer body 37 is used as an anode. The recording liquid becomes acidic on the transfer body, and a thickening effect can be obtained with a recording liquid containing an ABA triblock copolymer composed of the monomer of the general formula (1). The electrolysis reaction of water occurring at the anode and cathode is shown below.
^{_{Cathode: 4H 2 O + 4e - →}} 2H 2 + 4OH - ··· reaction formula (1)
^{_{Anode: 2H 2 O → 4H + +}} O 2 + 4e - ··· reaction formula (2)
The time for forming the bridge of the recording liquid can be controlled by the peak voltage and pulse width of the electromagnetic pulse applied to the piezoelectric element in addition to the ink physical properties (viscosity / surface tension). The energized charge amount (total amount of protons and total amount of hydroxide ions) that is an integrated value of current can be adjusted.

Here, the bridge of the liquid column formed between the cathode and the anode will be described with reference to FIG.
Inside the bridge B of the liquid column, cations and anions move in the vicinity of the cathode C and the anode A, respectively. As a result, the surface of the cathode C and the anode A, the electric double layer E _C and E _A respectively is formed, the charging speed of the electric double layer E _C and E _A is the conductivity of the bridge B of the liquid column, recording It is almost determined by the concentration of ions contained in the liquid. At this time, when the voltage of the electric double layer E _A reaches several V, water the Faraday current flows electrolysis. As a result, the pH in the vicinity of both electrodes changes and the recording liquid thickens. In this way, a non-Faraday current corresponding to the formation of an electric double layer at the electrode interface and a Faraday current resulting from the electrolysis of water are generated.

  In order to form an image of the recording liquid on the intermediate transfer member 37, the voltage applied by the energizing means 33 is 1.23V, which is the theoretical decomposition voltage of water, or several V to tens, which is a general condition for electrolysis of water. Several volts are insufficient, and preferably several tens to several hundreds V. However, when several tens to several hundreds V, the reaction resistance of water electrolysis is so small that it can be ignored, and the solution resistance of the recording liquid is low. Become dominant. After the liquid column bridge B is formed, the liquid column is divided in the vicinity of the nozzle 61c, and the bridge state is finished in several μs to several tens μs, and the product of this time and the Faraday current is the amount of electric charge applied. As the value increases, the amount of hydrogen ion generation and hydroxide ion generation increases, so the viscosity change of the recording liquid may also increase.

  The thickening of the recording liquid on the intermediate transfer member 37 is indicated by the thickening portion G indicated by the dark color in FIG. Since the pH is most acidic or alkaline biased at the electrode interface of the intermediate transfer member 37, the thickening portion G is present in a large amount at and near the portion of the recording liquid that is in contact with the surface of the intermediate transfer member 37. Become. Due to the presence of the thickened portion G, the recording liquid dot is fixed to the intermediate transfer body 37, and the problem of beading such that the position of the recording liquid dot shifts due to the low surface energy of the intermediate transfer body 37 is suppressed.

  The energizing means 33 applies a voltage between the intermediate transfer member 37 and the conductive orifice 61a (or an electrode built in the ink cartridge (not shown)) with the bridge B formed, and is included in the recording liquid. A pH treatment for increasing the viscosity of the recording liquid is performed by electrolyzing water. The energization means 33 and the control unit 98 as voltage application control means for driving the energization means 33 function as pH control means for performing such pH treatment.

  In order to perform such electrolysis, it is essential to form the bridge B. Therefore, each of the heads 61Y, 61M, 61C, 61BK faces the intermediate transfer body 37 at a distance at which the bridge B can be formed with the intermediate transfer body 37 when the recording liquid is applied to the intermediate transfer body 37. Arranged. Specifically, the gap distance between the surface of the conductive orifice 61a and the surface of the intermediate transfer member 37 is set between 50 and 300 μm, preferably 100 to 200 μm, and in this embodiment, 100 μm. If it exceeds 200 μm, depending on the type of recording liquid and the ejection method, the liquid column is divided in the vicinity of the nozzle 61c due to the surface tension of the recording liquid before bridging and normal droplet formation occurs, and the conductive orifice 61a This is because electrolysis of water by the bridge of the recording liquid with the intermediate transfer member 37 becomes impossible. In addition, if the gap is small, advantages such as improved landing accuracy of the recording liquid can be obtained. However, if the gap is less than 50 μm, it is difficult to maintain the gap.

  In synchronization with the timing at which the leading edge of the image carried on the intermediate transfer body 37 reaches the transfer unit 31, a single sheet of transfer paper S fed from the paper supply unit 20 is supplied to the transfer unit 31, and the transfer roller 38. A transfer step of applying pressure to the transfer sheet S passing through the transfer unit 31 to bring it into close contact with the intermediate transfer body 37 and transferring the image carried on the intermediate transfer body 37 onto the surface of the transfer sheet S. Thus, an image is formed on the transfer paper S. As described above, the transfer roller 38 transfers the recording liquid whose viscosity has been changed by the pH treatment from the intermediate transfer body 37 to the transfer paper S. The transfer sheet S on which an image is formed by the transfer is guided to the paper discharge table 25 and stacked on the paper discharge table 25.

Since the recording liquid thus thickened is transferred onto the transfer paper S, even if the transfer paper S is plain paper, it is prevented or suppressed together with feathering, bleeding, and beading, and at a high speed, a high image Image formation with high density and high image quality is possible.
In order to perform high-speed image formation, since it is necessary to make the recording liquid dry quickly, it is common that the recording liquid is provided with a penetrating agent for increasing the absorbability to the transfer paper S. In this case, the recording liquid penetrates deeply into the transfer paper S, so-called back-through occurs, making it unsuitable for double-sided image formation. However, since the absorbability of the thickened recording liquid into the transfer paper S is reduced, the back-through is prevented or suppressed, which is suitable for double-sided image formation.

  In the image forming apparatus 100, since the recording liquid is thickened, it is considered that the recording liquid is less likely to penetrate into the transfer paper S and the quick drying property is reduced as compared with the case where there is no change in viscosity. However, the transfer roller 38 transfers the recording liquid from the intermediate transfer body 37 to the transfer paper S, and at the same time, applies a pressure between the recording liquid and the transfer paper S between the intermediate transfer body 37 to transfer as described above. An effect of spreading in the direction of the surface of the internal transfer paper S, not permeation inside the paper S, is given. Therefore, the image density can be improved. Further, it is necessary to apply pressure to the transfer sheet S, such as plain paper with unevenness, so that the intermediate transfer body 37 having elasticity increases the transfer rate of the recording liquid following the unevenness. As described above, the transfer roller 38 and the intermediate transfer member 37 function as a pressure applying unit that applies pressure to the recording liquid and the transfer sheet S.

  The transfer roller 38 is preferably provided with a water-repellent member having a low surface energy on the surface from the viewpoint of preventing contamination by the recording liquid. Therefore, the transfer roller 38 has a fluororesin such as tetrafluoroethylene resin, tetrafluoroethylene / perfluoroalkoxyethylene copolymer, fluorosilicone rubber, phenylsilicone rubber, fluororubber, chloroprene rubber, nitrile rubber on the surface. , Nitrile butadiene rubber, isoprene rubber, and other rubber materials, resin, metal, and rubber have a surface layer treated with fluorine.

  Regarding the physical properties of the transfer roller 38 as a surface member, the water receding contact angle is 60 ° or more, preferably 80 ° or more, and the hardness is 60 or more, preferably 80 or more, according to JIS-A. The thickness of the surface layer is preferably about 0.1 to 1 mm, and preferably 0.2 to 0.6 mm.

As a pressure at the time of pressurizing, 1-100 kgf / cm < ² > is preferable and 5-20 kgf / cm < ² > is more preferable. In addition to increasing the image density and transfer rate by pressurizing the recording liquid on the transfer paper S, it is possible to improve the smoothness of the dots of the recording liquid and to improve the glossiness of the image. .

  FIG. 6 shows another configuration example of the image forming apparatus 100. In this image forming apparatus 100, the same components as those provided in the image forming apparatus 100 shown in FIG. 1 are denoted by the same reference numerals and description thereof will be omitted as appropriate, and the image forming apparatus 100 shown in FIG. Differences will be mainly described. In the image forming apparatus 100 shown in FIG. 6, the control unit 98 is not shown.

  The image forming apparatus 100 illustrated in FIG. 6 does not include the intermediate transfer body 37 and the transfer roller 38 included in the image forming apparatus 100 illustrated in FIG. 1, and includes heads 61Y, 61M, 61C, 61BK, a guide plate 39, and the like. Is a direct type image forming apparatus that directly forms an image on the transfer sheet S at the recording liquid ejection portion 53 facing the surface.

  The image forming apparatus 100 illustrated in FIG. 6 does not include the energization unit 33 and does not energize the recording liquid. Therefore, as a pH control means, a pH processing unit 50 is provided that performs a pH process for changing the viscosity of the recording liquid applied to the transfer paper S on the transfer paper S before the recording liquid is applied. . Therefore, the image forming apparatus 100 shown in FIG. 6 performs pH treatment on the recording liquid before being applied to the transfer paper S simultaneously with the application of the recording liquid to the intermediate transfer body 37. It is different from 100.

  In the image forming apparatus 100 shown in FIG. 1, the downstream side of the paper feeding unit 20 and the upstream side of the transfer unit 31 in the transport direction of the transfer paper S, or the downstream side of the cleaning device 40 and the heads 61Y and 61M in the A1 direction. , 61C, 61BK may have a pH treatment unit 50 on the upstream side. In this case, the energization means 33 is omitted, and voltage application between the intermediate transfer member 37 and the heads 61Y, 61M, 61C, 61BK (or electrodes built in the ink cartridges 81Y, 81M, 81C, 81BK, respectively) is omitted. When such omission is performed, the conductivity of the conductive orifice 61a (or the electrode incorporated in the ink cartridge) and the intermediate transfer body 37 is not necessary. For example, the surface layer 37b is simply made of silicone. It can be made of rubber. However, the image forming apparatus 100 shown in FIG. 1 is advantageous in terms of cost and size as compared with a case where the image forming apparatus 100 is not provided with the pH processing unit 50.

  The image forming apparatus 100 shown in FIG. 6 does not include the intermediate transfer body 37 and the transfer roller 38 provided in the image forming apparatus 100 shown in FIG. I have. The image forming apparatus 100 illustrated in FIG. 1 may include a pressure application unit 70 on the downstream side of the transfer unit 31 and on the upstream side of the paper discharge table 25 in the transfer direction of the transfer paper S.

The pH processing unit 50 performs a chemical process of applying a pH adjusting agent to the transfer paper S surface. The application method may be any of roller application, blade application, wire bar application, mist application by inkjet, etc., but in this embodiment, mist application by inkjet is adopted.
Therefore, the pH processing unit 50 is disposed facing the guide plate 39 on the upstream side of the registration roller 34 and the downstream side of the transport roller 32 in the transport direction of the transfer paper S, and applies a pH adjuster to the transfer paper S. It has a head 51 as a pH adjusting agent applying means, and a pH adjusting agent storage section 52 as a pH adjusting agent supplying means for supplying a pH adjusting agent to the head 51.

  Examples of the acidic pH adjuster for acidifying the alkaline recording liquid include inorganic acids such as boric acid, carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, and ammonium chloride, and organic acids such as acetic acid, citric acid, and oxalic acid. As its component. When the pH adjuster is an aqueous solution, a humectant as described for the recording liquid may be added.

  Examples of the alkaline pH adjuster for making acidic recording liquid alkaline include, for example, hydroxides of alkali metal elements such as lithium hydroxide, sodium hydroxide and potassium hydroxide, ammonium hydroxide and quaternary ammonium hydroxide. It is possible to use alkali metal carbonates such as quaternary phosphonium hydroxide, lithium carbonate, sodium carbonate and potassium carbonate, and amines such as diethanolamine and triethanolamine.

  The pH adjusting agent storage unit 52 stores the pH adjusting agent and supplies the stored pH adjusting agent to the head 51. The head 51 discharges the pH adjusting agent supplied from the pH adjusting agent storage 51 toward the transfer sheet S at a timing when the transfer sheet S passes between the head 51 and the guide plate 39.

  The pressure application unit 70 is disposed on the downstream side of the discharge unit 53 and on the upstream side of the paper discharge tray 25 in the transfer direction of the transfer paper S. The pressure application unit 70 includes pressure rollers 71 and 72 that are in pressure contact with each other, and a motor (not shown) that rotationally drives the pressure roller 71 and rotates the pressure roller 72 following the pressure roller 71. Between the pressure rollers 71 and 72, the transfer sheet S to which the recording liquid is applied in the discharge section 53 is passed. The structure and physical properties of the materials provided on the surfaces of the pressure rollers 71 and 72, that is, film thickness, hardness, and surface energy are the same as those of the intermediate transfer body 37 and the transfer roller 38, respectively. The pressure between them is the same as the pressure between the intermediate transfer member 37 and the transfer roller 38.

  In the image forming apparatus 100 having such a configuration, one transfer sheet S fed from the sheet feeding unit 20 is fed toward the discharge unit 53 in response to the input of a predetermined signal indicating the start of image formation. . The transfer sheet S is applied with a pH adjusting agent by the pH processing unit 50 on the surface to which the recording liquid is applied in the discharge unit 53 after passing through the transport roller 32. Next, the transfer sheet S is timed by the registration roller 34 and supplied to the discharge unit 53. In the process of passing through the discharge unit 53, the image areas of yellow, magenta, cyan, and black are overlapped at the same position. As described above, yellow, magenta, cyan, and black recording liquids are ejected from each of the heads 61Y, 61M, 61C, and 61BK in such a manner that the recording liquid is sequentially overlapped from the upstream side to the downstream side in the transport direction. An image is formed.

When the recording liquid is applied to the transfer paper S, it reacts with the pH adjuster upon landing and penetration of the recording liquid to increase the viscosity by changing the pH, and in the same way as shown in FIG. Part. There are many thickening portions in the portion of the recording liquid in contact with the surface of the transfer paper S and in the vicinity thereof. Due to the presence of the thickened portion, the problem of beading such as dot enlargement or position shift is suppressed.
Further, by forming an image with the colorant in the thickened recording liquid, even when the transfer paper S is plain paper, feathering and bleeding are prevented or suppressed together with beading, and at a high speed. Image formation with high image density and high image quality is possible.

  In order to perform high-speed image formation, since the recording liquid needs to be quick-drying, the recording liquid generally has high absorbability to the transfer paper S. In this case, the recording liquid is deep in the transfer paper S. Penetrating up to 2%, so-called back-through occurs, making it unsuitable for double-sided image formation. However, the increase in the viscosity of the recording liquid reduces the absorbability of the recording liquid onto the transfer paper S, so that such back-through is prevented or suppressed, and is suitable for double-sided image formation.

In the image forming apparatus 100, since the recording liquid is thickened, the recording liquid is less likely to penetrate into the transfer paper S and the quick drying property may be reduced as compared with the case where there is no change in viscosity. However, when the transfer sheet S on which the image is formed in the discharge unit 53 passes through the pressure application unit 70, pressure is applied together with the recording liquid, so that the recording liquid spreads in the surface direction of the transfer sheet S. Therefore, it is possible to provide quick drying and improve image density. In addition to ensuring fast drying and improving image density, the application of such pressure not only improves the fixability of the recording liquid on the transfer paper S, particularly the colorant in the recording liquid, but also improves the smoothness of the dots of the recording liquid. This has the advantage of improving the glossiness of the image.
The transfer paper S that has passed through the pressure application unit 70 is guided to the paper discharge tray 25 and stacked on the paper discharge tray 25. Since the transfer sheet S is improved in fixability of the recording liquid by the pressure application unit 70, the transfer of the recording liquid to the back surface of the other transfer sheet S (set-back) when stacked on the paper discharge tray 25. Is prevented or suppressed.

Further, the present invention relates to the water-based ink composition described in the following (1), and includes the following (2) to (8) as embodiments.
(1) It contains at least a colorant, an aqueous solvent, and an ABA triblock copolymer dissolved in the aqueous solvent, and the ABA triblock copolymer includes a monomer represented by the following general formula (1) as a constituent unit. A water-based ink composition comprising a polymer having a block having A and a B block having a hydrophilic monomer having a radical polymerizable unsaturated bond as a constituent unit.
(In the general formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents a number 6-18 alkylene group having a carbon, M is a hydrogen atom and monovalent metal Represents any of the atoms, and X ¹ represents —NH—.)
(2) The aqueous ink composition according to (1), wherein the hydrophilic monomer having a radical polymerizable unsaturated bond is a monomer represented by the following general formula (2).
(However, in the general formula (2), R ²¹ represents a hydrogen atom or a methyl group, R ²² and R ²³ each independently represents any of a hydrogen atom, a methyl group and an ethyl group .)
(3) The aqueous ink composition according to (1) or (2), further containing an electrolyte.
(4) An image forming apparatus having an image forming means for forming an image by applying the water-based ink composition according to any one of (1) to (3) to a recording medium according to an image signal.
(5) An intermediate transfer body to which the aqueous ink composition is applied by the image forming means, and an aqueous ink composition for changing the viscosity of the aqueous ink composition applied to the intermediate transfer body by the image forming means. (4) characterized by comprising: pH control means for changing the pH of the product; and transfer means for transferring the water-based ink composition whose viscosity has been changed by the pH treatment from the intermediate transfer body to the recording medium. The image forming apparatus described.
(6) The image forming apparatus according to (5), wherein the pH control means is means for applying a pH adjusting agent to the transfer paper before the recording liquid is applied.
(7) When the aqueous ink composition is applied to the intermediate transfer body, the image forming means is at a distance capable of forming a bridge of the aqueous ink composition with the intermediate transfer body. Is arranged opposite to
The pH control means applies the voltage between the intermediate transfer member and the image forming means in a state where the bridge is formed to electrolyze water contained in the water-based ink composition, thereby performing the pH treatment. (5) The image forming apparatus according to (5), further including a voltage applying unit that performs the operation.

The present invention will be described below in more detail based on examples, but the technical scope of the present invention is not limited to the following examples.
Further, in the following description, unless otherwise specified,% indicates mass%.

[Synthesis example of ABA type triblock copolymer]
In this synthesis example, JEOL JNM-ECX500 FT NMR SYSTEM (manufactured by JEOL Ltd.) was used for ¹ H-NMR measurement.
GPC measurement was performed under the following conditions.

-Molecular weight measurement conditions for AaU (X) (only A block)-
Degassa: ERC 3215 Alpha (manufactured by Shodex)
Pump: PU-2085plus (manufactured by JASCO)
Column oven: U-620 TYPE30 (manufactured by Sugaikemi)
Refractive index detector: RISE-61 (manufactured by Showex)
column:
Shodex Asahipak GF-1G 7B (one guard column, manufactured by Shodex)
Shodex Asahipak GF-7M HQ (one analytical column, manufactured by Shodex)
Developing solvent: 0.1M LiClO ₄ methanol solution

-Conditions for measuring molecular weight of ABA type triblock copolymer-
Degassa: ERC 3215 Alpha (manufactured by Shodex)
Pump: DP-8020 (manufactured by Tosoh Corporation)
Column oven: CTO-10ASVP (Shimadzu Corporation)
Refractive index detector: RI-8020 (manufactured by Tosoh Corporation)
column:
Shodex OHpak SB-G (one guard column, manufactured by Shodex)
Shodex OHpak SB-804HQ (two analytical columns, manufactured by Shodex)
Developing solvent: acetonitrile: water (pH 8, 50 mM phosphoric acid) = 10: 90 (volume ratio)

<Synthesis of ABA-type triblock copolymer (1) [AaU (5) -DMA (900) -AaU (5)]>
ABA type triblock copolymer was synthesized by RAFT polymerization. The method is shown below.

<Synthesis of 11-acrylamide undecanoic acid (AaU)>
40.3 g of 11-aminoundecanoic acid and 36 g of sodium hydroxide were dissolved in 1.5 L of deionized water, to which 56.6 g of acryloyl chloride and 0.1 g of a polymerization inhibitor (2,6-di-oxide) were dissolved. -Tert-butyl-p-cresol) was added dropwise over about 15 minutes. Thereafter, the mixture was stirred at room temperature for 3 hours, and then the pH was adjusted to about 3 using 6N hydrochloric acid to obtain a precipitate.
The obtained precipitate was collected by suction filtration, and the collected product was dissolved in acetone, and further reprecipitated with hexane to obtain a precipitate. The obtained precipitate was again collected by suction filtration and dried under reduced pressure to obtain 11-acrylamide undecanoic acid (AaU).

<Synthesis of chain transfer agent (S, S-bis (α, α′dimethyl-α ″ -acetic acid) trithiocarbonate)>
A mixture of 27.4 g carbon disulfide, 107.5 g chloroform, 52.3 g acetone, 2.29 g tetrabutylammonium bromide and 120 mL mineral spirits was degassed with argon for 10 minutes. Subsequently, a 50% aqueous sodium hydroxide solution was dropped into the mixture after degassing in an ice bath under an argon atmosphere over about 90 minutes, and then stirred for about 9 hours. Subsequently, 900 mL of deionized water was added, and 120 mL of 12M hydrochloric acid was further added. Subsequently, the mixture was stirred in an ice bath for about 30 minutes under an argon atmosphere to obtain an ocherous precipitate. The resulting precipitate was collected by suction filtration. The crude product obtained by drying the collected product under reduced pressure at 50 ° C. is dissolved in acetone and further purified by reprecipitation with hexane, and the chain transfer agent S, S-bis (α, α 'Dimethyl-α ″ -acetic acid) trithiocarbonate was obtained.

-Synthesis of AaU decamer-
1.92 g of AaU obtained by the above synthesis, 0.21 g of chain transfer agent, and 0.084 g of 4,4′-azobis (4-cyanovaleric acid) as a polymerization initiator were dissolved in 7.5 mL of methanol. And degassed with argon for 30 minutes. The degassed methanol solution was heated to 70 ° C. and polymerized for 4.5 hours. After the polymerization, hexane was added to recover the separated methanol solution, which was dried under reduced pressure to obtain a polymer.
The obtained polymer was subjected to ¹ H-NMR measurement using heavy dimethyl sulfoxide as a solvent, and the average degree of polymerization was calculated by comparing the methyl peak of the chain transfer agent with the methylene peak derived from AaU. Thereby, the polymer was an AaU 10-mer “AaU (10)” having an average degree of polymerization of 10.
Moreover, molecular weight distribution (Mw / Mn) by GPC measurement was 1.15.

-Synthesis of ABA type triblock copolymer-
The obtained AaU (10) 0.567 g, dimethylacrylamide (DMA) 19.83 g, and polymerization initiator 2,2′-azobis (isobutyronitrile) 0.0131 g were dissolved in 100 mL of methanol, Degassed with argon for 30 minutes. The degassed methanol solution was heated to 60 ° C. and polymerized for 4.5 hours. After the polymerization, a methanol solution is put into a dialysis membrane (dialysis membrane 36/32, product number UC 36-32-100, manufactured by Aedia Co., Ltd.), dialyzed with water for 16 hours, adjusted to pH 9 and further treated with water for 6 hours. Dialysis was performed. The obtained solution was concentrated, and further ABA type triblock copolymer (1) was obtained by lyophilization.

The obtained ABA-type triblock copolymer (1) was subjected to ¹ H-NMR measurement using heavy water as a solvent, and the average degree of polymerization was calculated by comparing the DMA-derived methyl peak and the AaU-derived methylene peak. As a result, the obtained ABA type triblock copolymer (1) has an ABA type triblock copolymer having an average degree of polymerization of A block of 5 and an average degree of polymerization of B block of 900 [AaU (5) -DMA (900)- AaU (5)].
Moreover, molecular weight distribution (Mw / Mn) by GPC measurement was 1.17.
[AaU (5) -DMA (900) -AaU (5)] obtained in this synthesis example is used as the ABA triblock copolymer used in the following examples.

<Synthesis of ABA Triblock Copolymer (2) [AaH (25) -DMA (200) -AaH (25)]>
-Synthesis of AaH (50)-
6-acrylamidohexanoic acid (Tokyo Kasei Kogyo Co., Ltd., AaH) 2.011 g (10.86 mmol) and NaHCO ₃ 0.826 g (9.673 mmol) were dissolved in 4.3 mL of water, and the chain transfer agent S, S-bis. (Α, α′dimethyl-α ″ -acetic acid) trithiocarbonate 61.4 mg (0.217 mmol) and polymerization initiator 4,4′-azobis (4-cyanovaleric acid) 30.9 mg (0. 110 mmol) was dissolved in 1.1 mL of methanol and mixed with the aqueous solution. The methanol aqueous solution after deaeration with argon for 30 minutes was heated to 70 ° C. and polymerized for 30 minutes. After the polymerization, dialysis was performed for 6 hours with water, and a polymer was obtained by freeze-drying. The polymer obtained was subjected to ¹ H-NMR measurement using heavy water as a solvent, and the average degree of polymerization was calculated by comparing the methyl peak of the chain transfer agent with the methylene peak derived from AaH. As a result, the polymer was an AaH 10-mer “AaH (25)” having an average degree of polymerization of 25. The molecular weight distribution (Mw / Mn) was 1.09.

-Synthesis of AaH (25)-DMA (200)-AaH (25)-
AaH (50) 0.503 g (0.0508 mmol) obtained above, dimethylacrylamide (DMA) 1.001 g (10.097 mmol), and 4,4′-azobis (4-cyanovaleric acid as a polymerization initiator) ) 2.93 mg (0.0105 mmol) was dissolved in 10 mL of water, degassed with argon for 30 minutes, and then heated to 70 ° C. for polymerization for 60 minutes. After the polymerization, dialysis was carried out with water for 48 hours, and a polymer was obtained by lyophilization. The obtained polymer was subjected to ¹ H-NMR measurement using heavy water as a solvent. As a result, an ABA triblock copolymer “AaH (25) -DMA (200) -AaH (25)” with an average degree of polymerization of 200 was obtained. Met. The molecular weight distribution (Mw / Mn) was 1.12.

[Example 1]
Using the ABA type triblock copolymer (1), recording liquids of each color of black, yellow, magenta and cyan of Example 1 were prepared as follows.
<Black recording liquid>
-Sulfonic acid group-bonded carbon black pigment dispersion (CAB-O-JET-200, solid content 20%, manufactured by Cabot): 20.0%
・ Glycerin: 25.0%
・ Triethylene glycol: 15.0%
ABA type triblock copolymer: 2.0%
Propylene glycol monobutyl ether: 0.9%
・ Soda dehydroacetate: 0.1%
Distilled water: remaining amount Subsequently, the pH was adjusted to 9.1 with a 5% aqueous solution of lithium hydroxide, and pressure filtration was performed with a membrane filter having an average pore size of 0.8 μm.

<Yellow recording liquid>
-Sulfonic acid group-bonded yellow pigment dispersion (CAB-O-JET-270Y, solid content 10%, manufactured by Cabot): 30.0%
・ Glycerin: 25.0%
・ Triethylene glycol: 15.0%
ABA type triblock copolymer: 2.0%
Propylene glycol monobutyl ether: 0.9%
・ Soda dehydroacetate: 0.1%
Distilled water: remaining amount Subsequently, the pH was adjusted to 9.1 with a 5% aqueous solution of lithium hydroxide, and pressure filtration was performed with a membrane filter having an average pore size of 0.8 μm.

<Magenta recording liquid>
-Sulfonic acid group-bonded magenta pigment dispersion (CAB-O-JET-260M, solid content 10%, manufactured by Cabot): 30.0%
・ Glycerin: 25.0%
・ Triethylene glycol: 15.0%
ABA type triblock copolymer: 2.0%
Propylene glycol monobutyl ether: 0.9%
・ Soda dehydroacetate: 0.1%
Distilled water: remaining amount Subsequently, the pH was adjusted to 9.1 with a 5% aqueous solution of lithium hydroxide, and pressure filtration was performed with a membrane filter having an average pore size of 0.8 μm.

<Cyan recording liquid>
-Sulfonic acid group-bonded cyan pigment dispersion (CAB-O-JET-250C, solid content 10%, manufactured by Cabot): 30.0%
・ Glycerin: 25.0%
・ Triethylene glycol: 15.0%
ABA type triblock copolymer: 2.0%
Propylene glycol monobutyl ether: 0.9%
・ Soda dehydroacetate: 0.1%
Distilled water: remaining amount Subsequently, the pH was adjusted to 9.1 with a 5% aqueous solution of lithium hydroxide, and pressure filtration was performed with a membrane filter having an average pore size of 0.8 μm.

[Example 2]
A recording liquid was prepared in the same manner as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the following.
In the synthesis example described above, AaU (5) -DMA (400) -AaU (5) was obtained by adjusting the amount of DMA added when synthesizing the ABA type triblock copolymer (1). The molecular weight distribution (Mw / Mn) was 1.11.

[Example 3]
A recording liquid was prepared in the same manner as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the following.
In the above synthesis example, AaU (5) -DMA (200) -AaU (5) was obtained by adjusting the amount of DMA added when synthesizing the ABA type triblock copolymer. The molecular weight distribution (Mw / Mn) was 1.14.

[Example 4]
A recording liquid was prepared in the same manner as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the following.
In the synthesis example described above, the synthesis of the AaU decamer was changed to an AaU hexamer by adjusting the amount of AaU added. Moreover, AaU (3) -DMA (900) -AaU (3) was obtained by adjusting the DMA addition amount at the time of synthesizing the ABA type triblock copolymer. The molecular weight distribution (Mw / Mn) was 1.10.

[Example 5]
A recording liquid was prepared in the same manner as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the following. In the above synthesis example, AaU (10) -DMA (900) -AaU (10) was obtained by adjusting the amount of DMA added when synthesizing the ABA type triblock copolymer. The molecular weight distribution (Mw / Mn) was 1.17.

[Example 6]
A recording liquid was prepared in the same manner as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the following.
In the synthesis example described above, AaU (10) -DMA (200) -AaU (10) was obtained by adjusting the amount of DMA added when synthesizing the ABA type triblock copolymer. The molecular weight distribution (Mw / Mn) was 1.11.

[Comparative Example 1]
A recording liquid obtained by removing the ABA triblock copolymer from the recording liquid of Example 1 was used.

[Comparative Example 2]
The same procedure as in Example 1 except that the ABA type triblock copolymer (1) in Example 1 was changed to the ABA type triblock copolymer (2) [AaH (25) -DMA (200) -AaH (25)]. Thus, a recording liquid was prepared. The molecular weight distribution (Mw / Mn) was 1.12.

Table 1 shows the degree of polymerization and molecular weight distribution of the ABA triblock copolymers obtained in Examples 1 to 6 and Comparative Example 2.

[Image formation and evaluation test]
When each of the aqueous ink compositions (recording liquids) prepared above was subjected to a discharge test using the image forming tester 1, all the inks exhibited good discharge properties.
Of the aqueous ink composition (recording liquid) prepared above, the image forming performance was evaluated for each of the image forming apparatuses having the respective configurations shown in FIGS. 1 and 6 using black ink and yellow ink.

(Image formation test 1)
In this test, an image was formed under the following conditions using a commercially available inkjet printer (GX-5000 manufactured by Ricoh), which is a model having a structure corresponding to the image forming apparatus shown in FIG.
Each head of an ink jet printer was filled with a recording liquid of each color, and an image was formed on plain paper (type 6200, manufactured by Ricoh) whose pH was adjusted to pH 3 with a citric acid aqueous solution in advance, and the image quality was evaluated.

<Evaluation results>
Table 1 below summarizes the evaluation results of solid image density, feathering, and bleeding, which are judgment items, for the images on the paper obtained in Examples 1 to 6 and Comparative Examples 1 and 2.
The evaluation criteria for each judgment item are as follows.
(Solid image density)
Evaluation was performed according to the following criteria based on the solid image density value of black ink.
◎: 1.4 or more ○: 1.3 or more and less than 1.4 △: 1.2 or more and less than 1.3 ×: Less than 1.2

(Feathering)
The character bleeding of black ink was evaluated according to the following evaluation criteria.
○: Good △: Normal ×: Bad

(Bleeding)
The bleeding of characters in the yellow ink solid was evaluated according to the following evaluation criteria.
○: Good △: Normal ×: Bad

From Table 1 above, it is confirmed that the inclusion of the ABA type triblock copolymer in the recording liquid increases the solid image density and alleviates bleeding such as feathering and bleeding. Further, according to the result of Comparative Example 2, when the carbon number of R ² in the general formula (1) is 5, the hydrophobic association force of the A block is weak and a sufficient thickening effect cannot be obtained even as a 25-mer. It is considered a thing.

(Image formation test 2)
In this test, an image was formed under the following conditions using an ink jet printer which is a model having a structure corresponding to the image forming apparatus shown in FIG.
In the image forming apparatus, the intermediate transfer member 37 is an aluminum tube having a support 37a having a diameter of 20 mm and a length of 250 mm, and has a volume resistivity of 50 Ω · cm as a surface layer 37b on the outer periphery of the support 37a. A silicone rubber layer is formed with a thickness of 0.2 mm and is driven to rotate at 200 mm / s.
The transfer roller 38 is pressed against the intermediate transfer member 37 with a load of 20 kgf / cm ² , and rotates with the intermediate transfer member 37.
The heads 61Y, 61M, 61C, 61BK are line type recording heads used in the RICOH GX-5000 in which the nozzle plate 61d is made of metal, and the gap with the intermediate transfer body 37 is arranged at about 100 μm. .
The heads 61 </ b> Y, 61 </ b> M, 61 </ b> C, 61 </ b> BK can form recording liquid dots of an arbitrary pattern on the surface of the intermediate transfer body 37 by the piezoelectric element driving means configured by the control unit 98.
The dots formed on the intermediate transfer body 37 by the recording liquid pass through a plain paper (type 6200) as the transfer paper S between the intermediate transfer body 37 and the transfer roller 38, so that the surface of the transfer paper S is transferred. Transcript.
The recording liquid is ejected by the piezoelectric element driving means configured by the control unit 98 so that 10 pL of recording droplets per dot are overlapped three times, and a continuous belt-like region having a width of about 1 inch in the sub-scanning direction which is the A1 direction. A black ink halftone dot of 600 dpi was formed in both the main scanning direction and the sub-scanning direction. The current that flows when the recording liquid was discharged was also measured.

The head of the image forming apparatus was filled with the recording liquid of each color, and the image quality was evaluated by forming an image on plain paper (type 6200, manufactured by Ricoh) whose paper surface pH was adjusted to pH 3 with a citric acid aqueous solution in advance. .
As the black ink, the two types used in Example 1 and Comparative Example 1 were used. The driving of the intermediate transfer member 37 was stopped halfway, and the black ink dot on the intermediate transfer member 37 was observed with a digital microscope. The beading was evaluated.

<Evaluation results>
When the recording liquid containing no ABA triblock copolymer used in the comparative example was used, the beading occurred on the intermediate transfer member 37, whereas the recording liquid containing the ABA triblock copolymer used in the example was used. When used, beading was suppressed.

As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to this specific embodiment, Unless it is specifically limited by the above-mentioned description, this invention described in the claim is described. Various modifications and changes are possible within the scope of the gist of the invention.
For example, the pH may be adjusted with respect to the aqueous ink composition on the recording medium. However, the configuration in which the pH is adjusted when the water-based ink composition is landed as in the above-described embodiments is more advantageous for beading at the time of landing.
The intermediate transfer member may not be a roller but may be an endless belt.
The image forming apparatus to which the present invention is applied is not limited to the above-mentioned type of image forming apparatus, but other types of image forming apparatuses, that is, a copying machine, a single facsimile, or a complex machine thereof, or a monochrome machine related thereto. In addition, an image forming apparatus used for forming an electric circuit or an image forming apparatus used for forming a predetermined image in the biotechnology field may be used.
The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

DESCRIPTION OF SYMBOLS 10 Conveyance unit 20 Paper feed unit 21 Paper feed tray 22 Paper feed roller 23 Case 25 Paper discharge stand 31 Transfer part 32 Transfer roller 33 Current supply means 33a Power supply 34 Registration roller 39 Guide plate 37 Intermediate transfer body 37a Support 37b Surface layer 38 Transfer roller 39 Guide plate 40 Cleaning device 50 pH processing unit 51 Head 52 pH adjusting agent storage unit 53 Discharge unit 60Y, 60M, 60C, 60BK Ink discharge device 61Y, 61M, 61C, 61BK Head 61a Conductive orifice 62 Carriage 70 Pressure application Units 71 and 72 Pressure rollers 81Y, 81M, 81C, 81BK Ink cartridge 98 Control unit 99 Image forming apparatus main body 100 Image forming apparatus A Anode B Liquid column bridge C Cathode EA Electric double layer EC Electric double layer G Thickening part S transfer paper

Japanese Patent Laid-Open No. 11-10856 JP 2000-44855 A JP 2000-63719 A, etc. US Pat. No. 4,538,156 US 5099256 Specification JP 2003-246135 A JP 2002-370441 A JP 2005-170036 A JP 2003-82265 A JP 2008-019286 A

Claims (7)

At least a colorant, an aqueous solvent, and an ABA triblock copolymer dissolved in the aqueous solvent,
The ABA type triblock copolymer is a polymer having an A block having a monomer represented by the following general formula (1) as a structural unit and a B block having a hydrophilic monomer having a radical polymerizable unsaturated bond as a structural unit. A water-based ink composition characterized by the above.
(In the general formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents a number 6-18 alkylene group having a carbon, M is a hydrogen atom and monovalent metal Represents any of the atoms, and X ¹ represents —NH—.) The aqueous ink composition according to claim 1, wherein the hydrophilic monomer having a radical polymerizable unsaturated bond is a monomer represented by the following general formula (2).
However, In the general formula (2), R ²¹ represents a hydrogen atom or a methyl group, R ²² and R ²³ each independently represents any of a hydrogen atom, a methyl group and an ethyl group.   The aqueous ink composition according to claim 1 or 2, further comprising an electrolyte.   An image forming apparatus having an image forming means for applying the water-based ink composition according to any one of claims 1 to 3 to a recording medium in accordance with an image signal to form an image. An intermediate transfer body to which the aqueous ink composition is applied by the image forming means;
PH control means for changing the pH of the aqueous ink composition to change the viscosity of the aqueous ink composition applied to the intermediate transfer member by the image forming means;
The image forming apparatus according to claim 4, further comprising: a transfer unit that transfers the water-based ink composition whose viscosity has been changed by the pH treatment from the intermediate transfer body to a recording medium.   6. The image forming apparatus according to claim 5, wherein the pH control means is means for applying a pH adjusting agent to the transfer paper before the recording liquid is applied. The image forming unit is opposed to the intermediate transfer member at a distance capable of forming a bridge of the aqueous ink composition with the intermediate transfer member when the aqueous ink composition is applied to the intermediate transfer member. Arranged,
The pH control means applies the voltage between the intermediate transfer member and the image forming means in a state where the bridge is formed to electrolyze water contained in the water-based ink composition, thereby performing the pH treatment. The image forming apparatus according to claim 5, further comprising a voltage applying unit that performs the operation.