Inkjet recording method

Abstract

An inkjet recording method includes a process of circulating ink and ejecting the ink from an inkjet head to give it to a recording medium, the number of white pigment particles, i.e., white large particles, in the ink having a particle diameter in the range of 0.8 μm to 5 μm is 1.00 x 10 4 cm 3 The above, the circulation flow path includes a circulation pipe that returns the ink to the inkjet head, the following speed ratio (1) is 5.0 x 10 3 or less and 1.4 x 10 5 or more. Vc is the circulation flow rate (cm / s) of the ink in the connection portion of the circulation pipe to the inkjet head, Vslp is the settling velocity (cm / s) of the white large particles defined by a prescribed formula, and Pn is the number of white large particles (particles / cm 3 ). The speed ratio (1) = Vc / (Vslp x log(Pn)).

Description

Technical Field

[0001] This invention relates to an inkjet recording method. Background Technology

[0002] In recent years, research on inkjet recording using white ink has been ongoing.

[0003] For example, in Patent Document 1, a white ink for inkjet recording with good whiteness and excellent ejection stability is disclosed. This ink contains a white pigment composed of metal oxides with an average particle size of 200 nm or more and 400 nm or less, and satisfies the following formula (1): 0.5 × A ≤ V ≤ 1.3 × A…(1)

[0004] (In the above formula (1), A represents the content (mass%) of white pigment in the white ink for inkjet recording. And V represents the percentage (%) of the volume of white pigment in the total volume of the white ink for inkjet recording when the white pigment is completely precipitated.)

[0005] Patent Document 1: Japanese Patent Application Publication No. 2013-129708 Summary of the Invention

[0006] The technical problem to be solved by the invention

[0007] Through the research of the inventors, it has been found that in inkjet recording methods using inks containing water and white pigments as titanium dioxide particles, during long-term operation (i.e., in the case of continuous image recording), there may be a loss of color consistency of the recorded image (i.e., the color variation of the image becomes larger) and / or a decrease in ink ejection stability.

[0008] One embodiment of the present invention aims to solve the problem of providing an inkjet recording method that exhibits excellent color consistency and ink ejection stability during long-term operation in an inkjet recording method using ink containing water and white pigment as titanium dioxide particles.

[0009] means for solving technical problems

[0010] The present invention includes the following methods.

[0011] <1> An inkjet recording method using an inkjet recording apparatus comprising:

[0012] The inkjet head ejects ink; and

[0013] The circulation path is used to circulate the ink discharged from the inkjet head and return it to the inkjet head.

[0014] The inkjet recording method includes the steps of circulating ink in a circulation path and ejecting ink from the inkjet head to apply it to a recording medium.

[0015] The ink contains water and white pigment as titanium dioxide particles.

[0016] In inks, the number of white pigment particles with a diameter of 0.8 μm to 5 μm, i.e., large white particles, is 1.00 × 10⁻⁶. 4 pcs / cm 3 above,

[0017] The circulation path includes a circulation tube connected to the inkjet head, used to return ink to the inkjet head.

[0018] The speed ratio (1) defined by the following equation (1) is 5.0 × 10 3 Above and 1.4×10 5 the following.

[0019] Speed ​​ratio (1) = Vc / (Vslp×log(Pn))...Equation (1)

[0020] Vslp={(ρ-ρw)gR 2} / (18η)…Formula (A)

[0021] In equation (1),

[0022] Vc is the circulating flow rate of ink in the connection between the inkjet head and the circulation tube, expressed in cm / s; Vslp is the sedimentation rate of large white particles, expressed in cm / s, as defined by equation (A).

[0023] Pn represents the number of large white particles in the ink per cm³. 3 The number of units.

[0024] In formula (A),

[0025] ρ is 4.23.

[0026] ρw is the solvent in the ink in g / cm³ 3 Density in units

[0027] g is in cm / s 2 The acceleration due to gravity is expressed in units of 1.

[0028] R is the median particle size in cm for large white particles.

[0029] η is the viscosity of the solvent in the ink at 30°C, expressed in g / cm·s.

[0030] <2> according to <1> In the inkjet recording method described above, the speed ratio (2) defined by the following formula (2) is 1.5 × 10⁻⁶. 3 above.

[0031] Speed ​​ratio (2) = Speed ​​ratio (1) × (1-cosθ)... Equation (2)

[0032] In equation (2), θ is the contact angle of the ink relative to the inner surface of the circulation tube, in °.

[0033] <3> according to <1> or <2> The inkjet recording method, wherein,

[0034] Ink circulation occurs via ink tanks.

[0035] The speed ratio (3) defined by the following formula (3) is 10.0 or higher.

[0036] Speed ​​ratio (3) = Speed ​​ratio (1) × (1 / L)... Equation (3)

[0037] In formula (3), L is the length of the circulation tube from the ink tank to the inkjet head, in cm.

[0038] Invention Effects

[0039] According to one embodiment of the present invention, an inkjet recording method is provided that exhibits excellent color consistency and ink ejection stability during long-term operation in an inkjet recording method using ink containing water and white pigment as titanium dioxide particles. Attached Figure Description

[0040] Figure 1 This is a diagram that conceptually illustrates an example of an inkjet recording apparatus used to implement the inkjet recording method of the present invention. Detailed Implementation

[0041] In this specification, the numerical range represented by “~” indicates the range included by taking the values ​​before and after “~” as the minimum and maximum values, respectively.

[0042] Within the numerical ranges described in this specification, the upper or lower limit of a certain numerical range can be replaced with the upper or lower limit of other numerical ranges described in different periods. Furthermore, within the numerical ranges described in this specification, the upper or lower limit of a certain numerical range can be replaced with the values ​​shown in the embodiments.

[0043] In this specification, if a composition contains multiple substances corresponding to each component, unless otherwise specified, the amount of each component in the composition represents the total amount of the multiple substances present in the composition.

[0044] In this specification, a combination of two or more preferred methods is a more preferred method.

[0045] In this specification, the term "process" includes not only individual processes, but also processes that achieve the desired purpose of the process, even if they cannot be clearly distinguished from other processes.

[0046] In this specification, "image" refers to the entire film formed by sequentially applying a pretreatment liquid and ink, "inkjet recording" refers to image recording based on the inkjet recording method, and "image recording" refers to the formation of an image (i.e., the film).

[0047] The concept of "image" in this specification also includes solid images.

[0048] Unless otherwise specified in this specification, "amount given" refers to the amount given in g / m³. 2 Expressed in units, converted to per 1m 2 The mass (g) imparted by the area.

[0049] Unless otherwise specified, the term "solvent" in this specification refers to organic solvents.

[0050] In this specification, "(meth)acrylate" is a concept that includes both acrylates and methacrylates. Furthermore, "(meth)acrylic acid" is a concept that includes both acrylic acid and methacrylic acid.

[0051] [Inkjet recording method]

[0052] The inkjet recording method of the present invention (hereinafter also simply referred to as the "recording method") uses an inkjet recording apparatus, which includes:

[0053] The inkjet head ejects ink; and

[0054] The circulation path is used to circulate the ink discharged from the inkjet head and return it to the inkjet head.

[0055] This inkjet recording method includes the steps of circulating ink in a circulation path and ejecting ink from the inkjet head to apply it to the recording medium (hereinafter also referred to as the "ink application step").

[0056] The ink contains water and a white pigment (hereinafter also referred to as "white pigment") that is composed of titanium dioxide particles.

[0057] In inks, the number of white pigment particles with a diameter of 0.8 μm to 5 μm, i.e., large white particles (i.e., Pn in formula (1)), is 1.00 × 10⁻⁶. 4 pcs / cm 3 above,

[0058] The circulation path includes a circulation tube connected to the inkjet head, used to return ink to the inkjet head.

[0059] The speed ratio (1) defined by the following equation (1) is 5.0 × 10 3 Above and 1.4×10 5 the following.

[0060] The recording method of the present invention may also include other steps besides the ink application step, as needed.

[0061] Speed ​​ratio (1) = Vc / (Vslp×log(Pn))...Equation (1)

[0062] Vslp={(ρ-ρw)gR 2} / (18η)…Formula (A)

[0063] In equation (1),

[0064] Vc is the circulating flow rate of ink in the connection between the inkjet head and the circulation tube, expressed in cm / s; Vslp is the sedimentation rate of large white particles, expressed in cm / s, as defined by equation (A).

[0065] Pn represents the number of large white particles in the ink per cm³. 3 The number of units.

[0066] In formula (A),

[0067] ρ is 4.23.

[0068] ρw is the solvent in the ink in g / cm³ 3 Density in units

[0069] g is in cm / s 2 The acceleration due to gravity is expressed in units of 1.

[0070] R is the median particle size in cm for large white particles.

[0071] η is the viscosity of the solvent in the ink at 30°C, expressed in g / cm·s.

[0072] In the recording method of the present invention, the inkjet recording method using ink containing water and white pigment as titanium dioxide particles can improve the color consistency and ink ejection stability during long-term operation.

[0073] The reasons for this effect are speculated as follows.

[0074] Through the research of the inventors, we discovered

[0075] In inkjet recording methods using inks containing water and white pigments as titanium dioxide particles, the color consistency of the recorded image can sometimes be compromised during long-term operation (i.e., in the case of continuous image recording). This problem of color consistency during long-term operation is addressed by setting the number of large white particles in the ink to 1.00 × 10⁻⁶, with the aim of improving opacity caused by white images. 4 pcs / cm 3 This is especially evident in the above cases. That is, it is believed that the decrease in color consistency over long-term operation is due to the precipitation of large white particles.

[0076] Therefore, in the recording method of the present invention, ink circulation is performed during image recording, and the aforementioned speed ratio (1), which corresponds to the ratio of the ink circulation flow rate (Vc) to the deposition rate (Vslp) of white large particles as defined by formula (A), is set to 5.0 × 10⁻⁶. 3 Therefore, it is believed that the precipitation of large white particles can be suppressed, thereby suppressing the decline in color stability during long-term operation.

[0077] On the other hand, through the research of the inventors, it was discovered that by limiting the above-mentioned speed ratio (1) to 1.4 × 10 5 The following can suppress the decrease in ejection stability during long-term operation.

[0078] As described above, in the recording method of the present invention, by setting the speed ratio (1) to 5.0 × 10 3 Above and 1.4×10 5 The following methods can improve color consistency and ink ejection stability during long-term operation.

[0079] <Inkjet recording device>

[0080] In the recording method of the present invention, an inkjet recording device is used, the inkjet recording device comprising:

[0081] The inkjet head ejects ink containing water and white pigment, which is composed of titanium dioxide particles; and

[0082] The circulation path is used to circulate the ink that flows out of the inkjet head and returns to the inkjet head.

[0083] (Inkjet head)

[0084] The inkjet recording apparatus of the present invention comprises an inkjet head that ejects ink containing water and a white pigment as titanium dioxide particles (e.g., described later). Figure 1 The inkjet head IJ1 in the middle.

[0085] As for the inkjet head, there are no particular restrictions on using any known inkjet head.

[0086] There are no particular restrictions on the way ink is ejected from the inkjet head. It can be any of the known methods, such as charge control method that uses electrostatic induction to eject ink, on-demand inkjet method (pressure pulse method) that uses the vibration pressure of piezoelectric elements, acoustic inkjet method that converts electrical signals into sound beams that irradiate the ink and uses radiation pressure to eject ink, and thermal inkjet (Bubble Jet (registered trademark)) that heats the ink to form bubbles and uses the resulting pressure to eject ink bubbles.

[0087] As a recording method using an inkjet head, the recording method described in Japanese Patent Application Publication No. 54-59936 is particularly effective. In this recording method, the ink subjected to heat undergoes a rapid volume change, and the ink is ejected from the nozzle by the force of this state change. The method described in paragraphs 0093 to 0105 of Japanese Patent Application Publication No. 2003-306623 can also be applied as a recording method using an inkjet head.

[0088] Furthermore, the inkjet head can be configured in two ways: a reciprocating mode, in which a short strip-shaped serial head scans and records simultaneously along the width of the recording medium; and a linear mode, in which a linear printhead with recording elements arranged corresponding to the entire area of ​​one side of the recording medium.

[0089] In the inline method, by scanning the recording medium along a direction intersecting the arrangement direction of the recording elements, images can be recorded across the entire surface of the recording medium. The inline method eliminates the need for a transport system such as a carriage for scanning short, strip-shaped inkjet heads, as in the reciprocating method. Furthermore, compared to the reciprocating method, the inline method eliminates the need for complex scanning control of the carriage movement and the recording medium; only the recording medium needs to move. Therefore, the linear method achieves higher image recording speeds compared to the reciprocating method.

[0090] The inkjet head has a resolution of 300 dpi or higher (more preferably 600 dpi or higher, and even more preferably 800 dpi or higher).

[0091] Here, dpi is an abbreviation for dots per inch, and linch (1 inch) is 2.54 cm.

[0092] From the viewpoint of obtaining high-definition images, the amount of ink ejected from the inkjet head is preferably 1 pL (picoliter) to 10 pL, more preferably 1.5 pL to 6 pL.

[0093] The inkjet head ejects ink from the nozzle and discharges the ink for the purpose of ink circulation.

[0094] Ink ejection from the printhead can also be achieved by having the ink return to the printhead via a circulation tube, thus extruding ink from the printhead.

[0095] The ink ejection and ink discharge can be carried out simultaneously or at different times.

[0096] The inkjet head is connected to a circulation tube for ink circulation (e.g., ...). Figure 1 (Circulation pipes C1 and C2 in the middle).

[0097] The circulation tube connected to the inkjet head forms a circulation path for ink circulation, either alone or together with other components (e.g., ink tank, infusion pump, filter, heat exchanger, flow meter, etc.).

[0098] (Circular flow path)

[0099] The inkjet recording device of the present invention has a circulation path for ink circulation, which allows ink discharged from the inkjet head to flow and return to the inkjet head.

[0100] The circulation path includes at least a circulation tube connected to the inkjet head and returning ink to the inkjet head (e.g., described later). Figure 1 (C2 in the circulation tube).

[0101] The circulation path may also include a circulation tube connected to and discharging ink from the inkjet head (e.g., described later). Figure 1 (Circulation pipe C1 in the middle).

[0102] Furthermore, the circulation path can also be composed of a circulation pipe and other components besides the circulation pipe.

[0103] As components other than the circulation pipe, examples include:

[0104] Ink containers (e.g., main containers, buffer containers, etc.) used for storing ink;

[0105] Injection pumps used for conveying ink;

[0106] A filter used to remove foreign matter from ink;

[0107] Degassing module for degassing ink;

[0108] A heat exchanger used to regulate the temperature of the ink;

[0109] A flow meter used to measure ink flow rate;

[0110] Connector; and

[0111] Valve; etc.

[0112] The inkjet recording device of the present invention may also include an inkjet head and other components other than the circulation path.

[0113] There are no particular limitations on the appropriate application of components that can be installed in an inkjet recording device as other components.

[0114] (An example of an inkjet recording device)

[0115] Hereinafter, an example of an inkjet recording apparatus used in the recording method of the present invention will be described with reference to the accompanying drawings.

[0116] However, the inkjet recording device used in the recording method of the present invention is not limited to the following example.

[0117] Figure 1 This is a diagram that conceptually illustrates an example of an inkjet recording apparatus used in the inkjet recording method of the present invention.

[0118] like Figure 1 As shown, the inkjet recording apparatus involved in this example is an example of an inkjet recording apparatus equipped with a transport mechanism for transporting recording media in a roll-to-roll manner. The unwinding device W1 unwinds the recording medium A1, which is rolled into a long strip film shape, and transports the recorded medium A1 in the direction of the hollow arrow. It passes through the pretreatment liquid application device P1, the pretreatment liquid drying area DP1, the inkjet head IJ1 and the drying area D1 in sequence, and is finally wound up by the take-up device W2.

[0119] In this example, the pretreatment liquid is a liquid applied to the recording medium before the ink is applied.

[0120] The pretreatment solution may contain water and a coagulant.

[0121] The pretreatment solution will be explained later.

[0122] The recording method of the present invention is not limited to using a pretreatment liquid; ink can also be applied to the recording medium without using a pretreatment liquid.

[0123] In this case, the pretreatment liquid dispensing device P1 and the pretreatment liquid drying zone DP1 can be omitted.

[0124] Furthermore, due to Figure 1 This is a conceptual diagram, so the transport path of the recording medium A1 is simplified in the diagram. The recording medium A1 is transported in one direction, but in reality, the transport path of the recording medium A1 can also be curved, which goes without saying.

[0125] As a conveying method for the recording medium A1, various sheet conveying methods such as drums and rollers can be appropriately selected.

[0126] For the unwinding device W1 used for unwinding the recording medium A1, a pretreatment liquid application device P1, a pretreatment liquid drying zone DP1, an inkjet head IJ1 and a drying zone D1 are sequentially arranged on the downstream side (hereinafter also referred to as the "downstream side") of the recording medium A1 and the upstream side (hereinafter also referred to as the "upstream side") of the recording medium A1.

[0127] The pretreatment liquid is applied and the ink is applied through the pretreatment liquid application device P1 and the inkjet head IJ1, respectively.

[0128] At this time, at least one of the following can be performed: heating and drying of the pretreatment liquid in the pretreatment liquid drying zone DP1 and heating and drying of the ink in the drying zone D1.

[0129] In the drying zone D1, in addition to the heating and drying of the ink, the pretreatment liquid can also be substantially heated and dried.

[0130] Furthermore, if the recording medium is passed through each drying zone while the temperature of each drying zone is set to room temperature, the heating and drying process can be omitted.

[0131] A surface treatment section (not shown) for performing surface treatment (preferably corona treatment) on the recording medium A1 may also be provided on the upstream side relative to the pretreatment liquid application device P1.

[0132] Furthermore, a cooling area for cooling the recorded image can also be provided downstream of the drying area D1.

[0133] The inkjet head IJ1 can also be a reciprocating inkjet head, but from the viewpoint of speeding up image recording, it is preferable to have a linear head with multiple nozzles arranged in the width direction of the recording medium A1 in the shape of a long strip film.

[0134] One end of circulation tube C1 and one end of circulation tube C2 are connected to the inkjet head IJ1. The other ends of circulation tube C1 and circulation tube C2 are connected to the ink tank IT1.

[0135] In the inkjet recording apparatus described in this example, these structures enable ink circulation, in which ink is discharged from the printhead IJ1 to the circulation tube C1, and the discharged ink flows back to the printhead IJ1 through the circulation tube C2 (see reference). Figure 1 (Arrow F1 in the image).

[0136] Although the diagram is omitted, the circulation pipes C1 and C2 can also be equipped with a degassing module for ink degassing, an ink delivery pump, a filter for removing foreign matter, and a heat exchanger for regulating ink temperature.

[0137] Furthermore, ink tank IT1 can be either a main tank or a buffer tank. If ink tank IT1 is a buffer tank, a main tank (not shown) is connected to the buffer tank IT1.

[0138] The IJ1 inkjet head can be configured with only one or multiple heads.

[0139] For example, multiple inkjet heads IJ1 may be arranged along the transport direction of the recording medium A1. This allows for the application of both white ink (hereinafter also referred to as "white ink") and colored ink.

[0140] Here, "coloring ink" refers to ink of any color other than white. The concept of coloring ink includes not only colored inks such as cyan, magenta, and yellow, but also achromatic inks such as black ink (hereinafter also referred to as "black ink"). Similarly, the concept of "colored image" in this invention includes not only colored images but also black images (hereinafter also referred to as "black image").

[0141] When multiple inkjet heads IJ1 are configured, components for performing the above-mentioned ink circulation (e.g., circulation tubes C1 and C2, ink tank IT1, etc.) can be connected to each of the multiple inkjet heads IJ1.

[0142] In inkjet recording using the image recording device described in this example:

[0143] First, the recording medium A1, which is rolled into a long strip film shape, is unwound using the unwinding device W1;

[0144] The recording medium A1 is fed and unwound in the direction of arrow R1;

[0145] On the recording medium A1 being transported, pretreatment liquid is applied by the pretreatment liquid application device P1;

[0146] Next, the pretreatment solution is dried in the pretreatment solution drying area DP1 as needed;

[0147] Next, ink is ejected from the nozzles (not shown) in the inkjet head IJ1 onto the area that has been treated with the pretreatment liquid (see reference). Figure 1 (The dashed arrow in the middle);

[0148] Next, dry the ink in drying area D1 as needed.

[0149] Thus, an image record (i.e., a recording medium A1 with an image) can be obtained on the recording medium A1, on which an image derived from ink is recorded.

[0150] Next, the image is cooled as needed, and finally, the image recording medium (i.e., the recording medium A1 with the image) is wound up by the winding device W2.

[0151] In the inkjet head IJ1, ink is ejected (reference) Figure 1 Simultaneously with the dashed arrow in the image, ink is discharged into circulation pipe C1 (see reference). Figure 1 (Arrow F1 in the image).

[0152] The ink ejection and ink discharge can be carried out simultaneously or at different times.

[0153] The ink discharged into circulation pipe C1 is transported through circulation pipe C1 → ink tank IT1 → circulation pipe C2, and then returns to inkjet head IJ1 through circulation pipe C2.

[0154] In this way, in the inkjet recording device, ink is ejected from the inkjet head IJ1 (reference) Figure 1 Simultaneously with the dashed arrow in the image, ink circulation is performed (see reference). Figure 1 (Arrow F1 in the image).

[0155] Ink ejection and ink circulation can be performed simultaneously or at different times.

[0156] Ink circulation can be continuous or intermittent.

[0157] Furthermore, the ink circulation path can be alternated as needed.

[0158] Furthermore, as mentioned above, in the aforementioned example, the application of the pretreatment solution and the drying process can also be omitted.

[0159] In this case, the pretreatment liquid application device P1 and the pretreatment liquid drying area DP1 in the inkjet recording device involved in the above example can also be omitted.

[0160] <Ink application process>

[0161] The recording method of the present invention includes an ink application process.

[0162] The ink application process is the process of circulating ink through a circulation path, ejecting ink from the inkjet head, and applying it to the recording medium.

[0163] An image is obtained by spraying ink onto a recording medium.

[0164] (Recording medium)

[0165] There are no particular restrictions on the recording medium, and known recording media such as paper substrates and resin substrates can be used without particular limitations.

[0166] As a paper substrate, examples include so-called coated paper, which is commonly used in offset printing. Examples of coated paper include high-quality paper and neutral paper, which are mainly composed of cellulose and are usually not surface-treated, and which have a coating material applied to their surface.

[0167] There are no particular limitations on the resin substrate, but a resin substrate having a film shape (i.e., a sheet shape) is preferred.

[0168] The thickness of the resin substrate is preferably 12μm to 200μm, more preferably 12μm to 100μm, even more preferably 12μm to 60μm, and even more preferably 15μm to 60μm.

[0169] The resin substrate can be in the shape of a long strip film (i.e., a long strip sheet).

[0170] There is no particular limitation on the length of the resin substrate when it is in the shape of a long strip film, but it is preferably 5m or more, more preferably 10m or more, and even more preferably 100m or more.

[0171] There is no particular limit to the length of the resin substrate when it is in the shape of a long strip film. For example, 10,000m, 8,000m, 5,000m, etc. can be given as upper limits.

[0172] Examples of resin substrates include polyester substrates, polyethylene substrates, polypropylene substrates, and nylon substrates.

[0173] The resin substrate can be a stretched substrate or an unstretched substrate.

[0174] The resin substrate can also be a transparent, non-permeable substrate.

[0175] Here, transparency means that the transmittance of visible light with wavelengths of 400 nm to 700 nm is 80% or more (preferably 90% or more).

[0176] When the resin substrate is transparent, the image can be easily visually identified through the resin substrate from the non-recording side.

[0177] For example, when the resin substrate is a transparent resin substrate, when a pretreatment liquid, at least one coloring ink, and white ink (which is the ink in this invention) are sequentially applied to the resin substrate to record a color image, the image colored against a white image (e.g., a solid image) can be easily visually identified from the non-recording side of the resin substrate.

[0178] Surface treatment can be performed on resin substrates.

[0179] Examples of surface treatments include corona treatment, plasma treatment, frame treatment, heat treatment, wear treatment, light irradiation treatment (UV treatment), and flame treatment, but these are not limited to these.

[0180] (Ink circulation and ink ejection)

[0181] Regarding ink circulation and ink ejection in the ink application process, the details are as described above.

[0182] (Ink)

[0183] The ink that is circulated and sprayed in the ink application process is an ink (e.g., white ink) that is a white pigment containing water and titanium dioxide particles.

[0184] In inks, the number of white pigment particles with a diameter of 0.8 μm to 5 μm, i.e., large white particles (i.e., Pn in formula (1)), is 1.00 × 10⁻⁶. 4 pcs / cm 3 That's all. In other words, in the ink of this invention, every 1cm 3 The number of large white particles in the ink is 1.00 × 10⁻⁶. 4 More than one.

[0185] The number of large white particles (i.e., Pn in equation (1)) is 1.00 × 10⁻⁶. 4 pcs / cm 3 The above helps to improve the occlusion of the recorded images.

[0186] In this invention, opacity refers to one of the properties of an image, which means that the image can cover the substrate on which the image is recorded (e.g., a recording medium, a colored image recorded on a recording medium, etc.).

[0187] Generally speaking, the number of large white particles is 1.00 × 10⁻⁶. 4 pcs / cm 3 In the above situations, although the above-mentioned opacity is beneficial, large white particles in the ink are prone to sedimentation, resulting in impaired color stability during long-term operation.

[0188] However, according to the recording method of the present invention, the speed ratio (1) is set to 5.0 × 10. 3 In short (by relatively accelerating the ink circulation rate relative to the ink's sedimentation rate), the problem of large white particles settling can be solved, thereby addressing the issue of color consistency during long-term operation.

[0189] In this invention, the number of large white particles (i.e., particles with a diameter of 0.8 μm to 5 μm in white pigment) (i.e., Pn in formula (1)) represents the value measured using a particle size analyzer.

[0190] In the embodiments described later, a JASCO Corporation wet dispersion type injection-type image analysis particle size analyzer "IF-3200S" was used as the particle size analyzer to measure the number of large white particles in an aqueous solution obtained by diluting ink with water 5000 times. Based on the obtained measurement values, the number of particles per 1 cm was calculated. 3 The number of large white particles in the ink (unit: particles / cm) 3 ).

[0191] Preferred methods for the ink used in this invention will be described later.

[0192] (Heating and drying of ink)

[0193] The ink application process may include the process of heating and drying the ink applied to the recording medium.

[0194] There are no particular limitations on the methods for heating and drying inks. Examples include infrared (IR) drying, warm air drying, and heating and drying by means of a heating device (e.g., heater, hot plate, furnace, etc.).

[0195] As a method for heating and drying ink, it is also possible to combine two or more of these methods.

[0196] Heat drying can be performed by heating the ink from at least one side of the recording medium, either the image recording side or the non-recording side.

[0197] The heating temperature in the heat drying of ink is preferably 30℃~100℃, more preferably 35℃~90℃, and even more preferably 40℃~80℃.

[0198] There is no particular limitation on the heating time in the heating and drying of ink, but it is preferably 1 second to 180 seconds, more preferably 1 second to 120 seconds, and even more preferably 1 second to 60 seconds.

[0199] <Speed ​​Ratio (1)>

[0200] In the recording method of the present invention, the speed ratio (1) defined by the following formula (1) is 5.0 × 10 3 Above and 1.4×10 5 the following.

[0201] Speed ​​ratio (1) = Vc / (Vslp×log(Pn))...Equation (1)

[0202] Vslp={(ρ-ρw)gR 2} / (18η)…Formula (A)

[0203] In equation (1),

[0204] Vc is the circulating flow rate of ink in the connection between the inkjet head and the circulation tube, expressed in cm / s; Vslp is the sedimentation rate of large white particles, expressed in cm / s, as defined by equation (A).

[0205] Pn represents the number of large white particles in the ink per cm³. 3 The number of units.

[0206] In formula (A),

[0207] ρ is 4.23.

[0208] ρw is the solvent in the ink in g / cm³ 3 Density in units

[0209] g is in cm / s 2 The acceleration due to gravity is expressed in units of 1.

[0210] R is the median particle size in cm for large white particles.

[0211] η is the viscosity of the solvent in the ink at 30°C, expressed in g / cm·s.

[0212] In equation (1) which defines the speed ratio (1), the factor that improves color stability during long-term operation, namely “Vc” (the circulating flow rate of ink), is placed in the molecule, while the factors that inhibit color stability during long-term operation, namely “Vslp” (the sedimentation rate of large white particles) and “log(Pn)” (Pn is the number of large white particles), are placed in the denominator.

[0213] The speed ratio (1) is 5.0 × 10 3 The above methods can achieve color consistency over long-term operation.

[0214] On the other hand, the speed ratio (1) is 1.4 × 10 5 The following results can be obtained to achieve ink ejection stability during long-term operation.

[0215] The speed ratio (1) is preferably 1.0 × 10 4 Above and 1.0×10 5 Hereinafter, 1.0 × 10 is preferred. 4 Above and 9.3×10 4 Hereinafter, 1.0 × 10 is further preferred. 4 Above and 7.0×10 4 the following.

[0216] In equation (1), Vc is the circulation velocity of ink in the connection between the circulation tube and the inkjet head, expressed in cm / s.

[0217] Vc is calculated based on the flow rate of the ink in the aforementioned connecting part and the cross-sectional area of ​​the circulation pipe in the aforementioned connecting part.

[0218] The vitamin C content is preferably 5 to 100, and more preferably 10 to 80.

[0219] In equation (1), Pn represents the number of large white particles in the ink per cm³. 3 The number of units. That is, Pn is the number of units per 1cm. 3 The number of large white particles (i.e., particles with a diameter of 0.8 μm to 5 μm in white pigment) in the ink.

[0220] The method for determining Pn is as described above.

[0221] As mentioned earlier, Pn satisfies 1.00 × 10 9 pcs / cm 3 The values ​​above.

[0222] Pn is preferably 1.00 × 10⁻⁶. 9 pcs / cm 3 Above and 1.00×10 13 pcs / cm 3 Values ​​within the following range.

[0223] In equation (1), Vslp is the precipitation rate of large white particles in cm / s as defined by equation (A).

[0224] Vslp is preferably 1.00 × 10 -6 Above and 1.00×10 -3 the following.

[0225] Define Vslp's equation (A) as an equation based on Stokes' theorem.

[0226] In equation (A), ρ is 4.23.

[0227] Here, "4.23" (=ρ) represents rutile titanium dioxide in g / cm³. 3 Density in units of 1.

[0228] In formula (A), ρw is the solvent in the ink in g / cm³. 3 Density in units of 1.

[0229] In this case, the solvent in the ink contains at least water.

[0230] When ink contains organic solvents, the solvents in the ink include both water and organic solvents.

[0231] ρw is determined by the following method.

[0232] By measuring the ink under an acceleration of 10000G (Note: 1G = 9.80665m / s²), 2 Centrifuge for 60 minutes to separate the solvent from the ink. Measure the mass and volume of the obtained solvent, and calculate the solvent content in g / cm³ from the obtained mass and volume. 3 Density (ρw) in units.

[0233] In formula (A), ρw is 0.70 or more and 1.30 or less, preferably 0.80 or more and 1.20 or less, and more preferably 0.90 or more and 1.10 or less.

[0234] In equation (A), R is the median particle size (d50) of the large white particles in cm.

[0235] In this invention, the median particle size (d50) of large white particles (i.e., particles with a diameter of 0.8 μm to 5 μm in white pigment), expressed in cm, represents the value measured using a particle size analyzer.

[0236] In the embodiments described later, as a particle size analyzer, the median particle size (d50) of large white particles in an aqueous solution obtained by diluting ink with water 5000 times was determined using an injection-type image analysis particle size analyzer "IF-3200S" manufactured by Jasco Corporation as a wet dispersion type.

[0237] In formula (A), R is preferably 1.00 × 10⁻⁶. -5 Above and 1.00×10 -3 Hereinafter, 1.00 × 10 is preferred. -4 Above and 1.00×10 -3 the following.

[0238] In formula (A), η is the viscosity of the solvent in the ink at 30°C, expressed in g / cm·s.

[0239] Viscosity refers to the value measured using an E-type viscometer.

[0240] The η in equation (A) is determined as follows.

[0241] By applying ink at an acceleration of 10000G (Note: 1G = 9.80665m / s²), 2 Centrifuge for 60 minutes to separate the solvent from the ink, and measure the viscosity of the obtained solvent at 30°C in g / cm·s using an E-type viscometer.

[0242] In formula (A), η is preferably 1.00 × 10 -2 Above and 1.00×10 -3 Hereinafter, 0.30×10 is preferred.-2 Above and 0.70×10 -2 the following.

[0243] <Speed ​​Ratio (2)>

[0244] In the recording method of the present invention, the speed ratio (2) defined by the following formula (2) is preferably 1.2 × 10⁻⁶. 3 The above is preferred, and more preferably 1.5×10 3 above.

[0245] Speed ​​ratio (2) = Speed ​​ratio (1) × (1-cosθ)... Equation (2)

[0246] In equation (2), θ is the contact angle of the ink relative to the inner surface of the circulation tube that returns the ink to the inkjet head, in °.

[0247] θ is a contact angle (°) defined in the range of greater than 0 and less than 180. The larger the value in this range, the more beneficial it is to the color consistency during long-term operation.

[0248] That is, (1-cosθ) is a factor that improves the color constancy over long-term operation.

[0249] When the speed ratio (2) is 1.2 × 10 3 Under the above conditions, color consistency is further improved during long-term operation.

[0250] (1-cosθ) itself has no particular restrictions.

[0251] (1-cosθ) is, for example, 0.05 or more and 0.60 or less, preferably 0.10 or more and 0.20 or less.

[0252] From the viewpoint of further improving color consistency and ink ejection stability during long-term operation, the speed ratio (2) is preferably 1.2 × 10⁻⁶. 3 Above and 1.0×10 5 The following is more preferably 1.5 × 10 3 Above and 8.0×10 4 the following.

[0253] In this invention, regarding θ, the connection part with the inkjet head in the circulation tube that allows ink to return to the inkjet head is cut with a cutting machine, and stretched into a flat plate to make a contact angle measurement sample, and the contact angle measurement sample is used for measurement.

[0254] In detail, ink is dropped onto the surface of the sample used for contact angle measurement, which corresponds to the inner surface of the circulation tube, and the contact angle of the ink relative to the inner surface of the circulation tube is measured.

[0255] The contact angle was measured using a contact angle measuring device under the conditions of 25°C, 50% RH, a droplet size of 2μL, and a waiting time of 1000ms from droplet to measurement.

[0256] <Speed ​​Ratio (3)>

[0257] In the recording method of the present invention, when ink is circulated via an ink tank...

[0258] The speed ratio (3) defined by the following formula (3) is preferably 8.0 or more, and more preferably 10.0 or more.

[0259] Speed ​​ratio (3) = Speed ​​ratio (1) × (1 / L)... Equation (3)

[0260] In equation (3), L is the length (cm) of the aforementioned circulation pipe from the ink tank to the inkjet head (i.e., the circulation pipe that returns ink to the inkjet head).

[0261] The larger the value of L, the more it suppresses the effect of color constancy over long-term operation.

[0262] In equation (3) which defines the speed ratio (3), L is placed in the denominator.

[0263] Therefore, the larger the value of the speed ratio (3), specifically, when the speed ratio (3) is 8.0 or higher, the effect of color constancy during long-term operation is further improved.

[0264] From the viewpoint of further improving the effect of color constancy during long-term operation, the speed ratio (3) is more preferably 10.0 or higher.

[0265] From the viewpoint of further improving the color consistency during long-term operation and further improving the ink ejection stability during long-term operation, the speed ratio (3) is preferably 8.0 or more and 120.0 or less, more preferably 8.0 or more and 100.0 or less, and even more preferably 8.0 or more and 80.0 or less.

[0266] The L in equation (3) itself is not particularly restricted.

[0267] L is, for example, 200 or more and 5000 or less, preferably 300 or more and 3000 or less, more preferably 500 or more and 2400 or less, and even more preferably 600 or more and 2000 or less.

[0268] <Preferred Ink Methods>

[0269] The ink in this invention contains water and white pigment as titanium dioxide particles.

[0270] The preferred method of the ink in this invention will be described below.

[0271] (water)

[0272] The ink contains water.

[0273] The water content relative to the total amount of ink is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 50% by mass or more.

[0274] The upper limit of the water content relative to the total amount of ink can be appropriately determined based on the content of other components, for example, 99% by mass, preferably 95% by mass, and more preferably 90% by mass.

[0275] (Water-soluble organic solvent)

[0276] The ink contains water-soluble organic solvents.

[0277] This ensures the stability of the inkjet output.

[0278] The water-soluble organic solvent contained in the ink can be one type or two or more types.

[0279] In this invention, "water solubility" refers to the property of dissolving more than 1g of water at 25°C with 100g of water.

[0280] In this invention, "water insoluble" refers to the property that the amount of water that can be dissolved in 100g of water at 25°C is less than 1g.

[0281] There are no restrictions on the types of water-soluble organic solvents that can be contained in inks. For example, the following can be cited:

[0282] Monools with 1 to 4 carbon atoms;

[0283] Diols such as 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2-buten-1,4-diol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, 1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol, and 4-methyl-1,2-pentanediol;

[0284] Triols such as glycerol, 1,2,6-hexanetriol, and trimethylolpropane;

[0285] alkylene glycols such as ethylene glycol and propylene glycol;

[0286] Ethylene glycol monoalkyl ethers, propylene glycol monoalkyl ethers, and other alkylene glycol monoalkyl ethers;

[0287] Diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, dipropylene glycol, polyoxyethylene polyoxypropylene glycol, and other polyalkylene glycols;

[0288] Diethylene glycol monoalkyl ethers, triethylene glycol monoalkyl ethers, tripropylene glycol monoalkyl ethers, polyoxypropylene glycerol ethers, and other polyalkylene glycol ethers;

[0289] 2-Pyrrolidone, N-methyl-2-pyrrolidone; etc.

[0290] From the viewpoint of ejection stability, the water-soluble organic solvent in the ink preferably contains at least one selected from the group consisting of alkylene glycols and alkylene glycol monoalkyl ethers.

[0291] The content of water-soluble organic solvent relative to the total amount of ink is preferably 10% to 40% by mass, more preferably 15% to 30% by mass.

[0292] (White pigment)

[0293] The ink contains white pigment as titanium dioxide particles.

[0294] White pigments contain large white particles with a particle size of 0.8 μm to 5 μm.

[0295] The number of large white particles (Pn) is as described above.

[0296] White pigments are preferably those containing rutile titanium dioxide.

[0297] The proportion of rutile titanium dioxide in the total white pigment is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 80% by mass or more. The proportion of rutile titanium dioxide in the total white pigment can be 100% by mass or less than 100% by mass.

[0298] From the viewpoint of opacity, the average primary particle size of the white pigment is preferably 150 nm or more, more preferably 200 nm or more. Furthermore, from the viewpoint of ink ejectibility, the average primary particle size of the white pigment is preferably 400 nm or less, more preferably 350 nm or less.

[0299] In this invention, the average primary particle size of the white pigment is a value measured using a transmission electron microscope (TEM). Specifically, it is obtained by selecting any 50 white pigments present within the field of view observed by the TEM, measuring the primary particle size of each of the 50 pigments, and then averaging the results. The transmission electron microscope can be a JEOL Ltd. 1200EX transmission electron microscope.

[0300] From the viewpoint of image density and ejectibility, the content of white pigment in the ink is preferably 2% to 25% by mass relative to the total amount of ink, more preferably 5% to 25% by mass, and even more preferably 10% to 20% by mass.

[0301] (Pigment dispersion resin)

[0302] The ink may also contain at least one pigment dispersion resin.

[0303] In this invention, pigment dispersion resin refers to a resin that has the function of dispersing pigments.

[0304] Pigment dispersion resins can be random copolymers or block copolymers.

[0305] Furthermore, pigment dispersion resins can also have cross-linked structures.

[0306] Inks can also be prepared using pigment dispersions containing pigments and pigment dispersing resins.

[0307] Regarding pigment dispersion resins, for example, known polymer dispersants such as those described in paragraphs 0029 to 0106 of International Publication No. 2021 / 221069 can be used.

[0308] When the ink contains pigment dispersion resin, the ratio of the pigment content to the pigment dispersion resin content in the ink is preferably 1:0.04 to 1:3 by mass, more preferably 1:0.05 to 1:1, and even more preferably 1:0.05 to 1:0.5.

[0309] When the ink contains pigment dispersion resin, the content of pigment dispersion resin relative to the total amount of ink is preferably 0.1% to 10% by mass, more preferably 0.3% to 5% by mass, and even more preferably 0.5% to 2.5% by mass.

[0310] (Resin particles)

[0311] The ink may contain at least one type of resin particles.

[0312] As resin particles that may be contained in ink, examples can be the same resin particles that may be contained in pretreatment solutions.

[0313] However, the resin particles in the ink can be the same as or different from the resin particles in the pretreatment solution.

[0314] When resin is present, the ink preferably contains at least one type of resin particles.

[0315] The resin particles are preferably selected from at least one of the following groups: acrylic resin particles, ester resin particles, mixtures of acrylic resin particles and ester resin particles, composite particles containing acrylic resin and ester resin, and polyurethane resin particles.

[0316] The content of resin particles in the ink is preferably 1% to 20% by mass relative to the total amount of ink, more preferably 2% to 15% by mass, and even more preferably 2% to 10% by mass.

[0317] (additive)

[0318] Depending on the requirements, inks may also contain additives such as surfactants, water-soluble resins, co-sensitizers, UV absorbers, antioxidants, anti-fading agents, conductive salts, and alkaline compounds.

[0319] (physical properties)

[0320] From the viewpoint of improving ejection stability, the pH (25°C) of the ink is preferably 7 to 10, more preferably 7.5 to 9.5. The pH of the coloring ink can be determined using the same method as that used for the pH of the pretreatment solution.

[0321] The viscosity of the ink (at 25°C) is preferably 0.5 mPa·s to 30 mPa·s, more preferably 2 mPa·s to 20 mPa·s, more preferably 2 mPa·s to 15 mPa·s, and even more preferably 3 mPa·s to 10 mPa·s. The viscosity of the ink can be measured using the same method as that used for the viscosity of the pretreatment solution.

[0322] The surface tension of the ink (at 25°C) is preferably below 60 mN / m, more preferably 20 mN / m to 50 mN / m, and even more preferably 30 mN / m to 45 mN / m.

[0323] Surface tension can be measured using the same method as in the pretreatment solution.

[0324] <Coloring Inks>

[0325] In the recording method of the present invention, as the ink used in the present invention, in the case of using white ink containing water and white pigment, in addition to white ink, colored inks (e.g., black ink and / or colored inks) may also be used.

[0326] Coloring ink can be applied to the lower layer relative to white ink, the upper layer relative to white ink, or a different layer than the area where white ink is applied.

[0327] <Pretreatment solution>

[0328] In the recording method of the present invention, as shown in the aforementioned example, not only ink but also pretreatment liquid can be used.

[0329] The pretreatment solution preferably contains water and a coagulant.

[0330] (water)

[0331] The pretreatment solution preferably contains water.

[0332] The water content relative to the total amount of the pretreatment liquid is preferably 50% by mass or more, and more preferably 60% by mass or more.

[0333] The upper limit of water content also depends on the amount of other components, but it is preferably 90% by mass or less, more preferably 80% by mass or less, relative to the total amount of pretreatment liquid.

[0334] (Flocculant)

[0335] The pretreatment solution preferably contains at least one coagulant.

[0336] The coagulant in the pretreatment solution causes the components of the ink to agglomerate on the recording medium. This improves the image quality derived from the ink.

[0337] The coagulant is preferably selected from at least one of the group consisting of organic acids, polyvalent metal compounds, metal complexes and cationic polymers.

[0338] As a flocculant, the flocculant described in paragraphs 0122 to 0130 of International Publication No. 2020 / 195360 is preferred.

[0339] The preferred methods for organic acids, polyvalent metal compounds, metal complexes, and cationic polymers that can be used as flocculants are described below.

[0340] -Organic acids-

[0341] Organic acids can be exemplified by organic compounds that have acidic groups.

[0342] Examples of acidic groups include phosphate, phosphonic acid, hypophosphonic acid, sulfate, sulfonic acid, sulfinic acid, and carboxyl groups.

[0343] From the viewpoint of ink agglomeration speed, the acidic group is preferably a phosphate group or a carboxyl group, and more preferably a carboxyl group.

[0344] Preferably, at least a portion of the acidic groups dissociate in the pretreatment solution.

[0345] Examples of organic compounds containing a carboxyl group include (meth)acrylic acid, poly(meth)acrylic acid, acetic acid, formic acid, benzoic acid, ethylene glycol, malonic acid, malic acid (preferably DL-malic acid), maleic acid, succinic acid, glutaric acid, pimelic acid, adipic acid, fumaric acid, citric acid, tartaric acid, phthalic acid, 4-methylphthalic acid, lactic acid, pyrrolidone carboxylic acid, pyranone carboxylic acid, pyrrolic carboxylic acid, furan carboxylic acid, pyridine carboxylic acid, coumaric acid, thiophene carboxylic acid, and nicotinic acid.

[0346] From the viewpoint of ink agglomeration speed, organic compounds with carboxyl groups are preferably carboxylic acids with a valence of 2 or higher (hereinafter also referred to as polycarboxylic acids), and more preferably dicarboxylic acids.

[0347] Specifically, the polycarboxylic acid is preferably malonic acid, malic acid, maleic acid, succinic acid, glutaric acid, pimelic acid, adipic acid, fumaric acid, tartaric acid, 4-methylphthalic acid, or citric acid, and more preferably malonic acid, malic acid, tartaric acid, succinic acid, glutaric acid, pimelic acid, adipic acid, or citric acid.

[0348] The organic acid is preferably low in pKa (e.g., 1.0 to 5.0). Thus, by contacting with an organic acid with a lower pKa, the surface charge of particles such as pigments and resin particles in the ink, which are dispersed and stabilized by weakly acidic functional groups such as carboxyl groups, can be reduced, thereby reducing dispersion stability.

[0349] The organic acid preferably has a low pKa, high solubility in water, and a valence of 2 or higher. Furthermore, the organic acid is more preferably characterized by a high buffering capacity in a pH region lower than the pKa of the functional group (e.g., carboxyl group) that stabilizes the particle dispersion in the ink.

[0350] -Polyvalent metal compounds-

[0351] As examples of multivalent metal compounds, multivalent metal salts can be cited.

[0352] Examples of polyvalent metal salts include organic acid polyvalent metal salts and inorganic acid polyvalent metal salts.

[0353] As a polyvalent metal salt of an organic acid, the polyvalent metal salts of the above-mentioned organic acids (e.g., formic acid, acetic acid, benzoic acid, etc.) are preferred.

[0354] As polyvalent metal salts of inorganic acids, polyvalent metal salts of nitrates, polyvalent metal salts of hydrochloric acid, or polyvalent metal salts of thiocyanate are preferred.

[0355] Examples of polyvalent metal salts include salts of alkaline earth metals (e.g., magnesium, calcium) from Group 2 of the periodic table, salts of transition metals (e.g., lanthanum) from Group 3 of the periodic table, salts of metals (e.g., aluminum) from Group 13 of the periodic table, and salts of lanthanides (e.g., neodymium).

[0356] As a multivalent metal salt, calcium salt, magnesium salt, or aluminum salt are preferred, with calcium salt or magnesium salt being more preferred.

[0357] As a polyvalent metal compound, a polyvalent metal salt of an organic acid is preferred, and a calcium salt or a magnesium salt of an organic acid is more preferred.

[0358] Preferably, at least a portion of the polyvalent metal compound dissociates into polyvalent metal ions and counterions in the pretreatment solution.

[0359] -Metal complex-

[0360] The metal complex preferably contains at least one metal element selected from the group consisting of zirconium, aluminum and titanium.

[0361] The metal complex is preferably a metal complex containing at least one ligand selected from the group consisting of acetate, acetylacetone, methyl acetoacetate, ethyl acetoacetate, octanediol, butoxyacetylacetone, lactate, ammonium lactate and triethanolamine.

[0362] Metal complexes are commercially available. Various organic ligands (especially multidentate ligands capable of forming metal chelating catalysts) are commercially available. Therefore, metal complexes can be prepared by combining commercially available organic ligands with metals.

[0363] -Catonic polymer-

[0364] The cationic polymer is preferably a homopolymer, copolymer, or condensation polymer of cationic monomers having primary to tertiary amino groups or quaternary ammonium groups. As a cationic polymer, it can be used in either the form of a water-soluble polymer or a water-insoluble polymer (i.e., latex particles).

[0365] Examples of cationic polymers include polyvinylpyridine salts, polyalkylaminoethyl acrylates, polyalkylaminoethyl methacrylates, polyvinylimidazolium, polyethyleneimine, polybiguanidine, polyguanidine, polyallylamine, and their derivatives.

[0366] From the viewpoint of the viscosity of the pretreatment solution, a lower weight-average molecular weight of the cationic polymer is preferable. When the pretreatment solution is applied to the resin substrate by inkjet printing, a weight-average molecular weight of 1,000 to 500,000 is preferred, more preferably 1,500 to 200,000, and even more preferably 2,000 to 100,000. A weight-average molecular weight of 1,000 or higher is advantageous from the viewpoint of agglomeration rate. A weight-average molecular weight of 500,000 or lower is advantageous from the viewpoint of ejection reliability. However, this is not a limitation when the pretreatment solution is applied to the resin substrate by methods other than inkjet printing.

[0367] The content of coagulant in the pretreatment solution is preferably 0.1% to 40% by mass relative to the total amount of the pretreatment solution, more preferably 0.1% to 30% by mass, even more preferably 1% to 20% by mass, and even more preferably 1% to 10% by mass.

[0368] (Water-soluble organic solvent)

[0369] The pretreatment solution may contain at least one water-soluble organic solvent.

[0370] As a water-soluble organic solvent that may be contained in the pretreatment solution, examples include the same water-soluble organic solvents contained in the inks described later.

[0371] However, in this case, the water-soluble organic solvent in the pretreatment solution can be the same as or different from the water-soluble organic solvent in the ink.

[0372] The content of water-soluble organic solvent relative to the total amount of pretreatment liquid is preferably 20% by mass or less, more preferably 10% by mass or less.

[0373] The content of water-soluble organic solvents can be 0% by mass. That is, the pretreatment solution may not contain water-soluble organic solvents.

[0374] When the pretreatment solution contains a water-soluble organic solvent, the content of the water-soluble organic solvent relative to the total amount of the pretreatment solution is preferably 1% to 20% by mass, more preferably 3% to 10% by mass.

[0375] (Resin X)

[0376] The pretreatment solution may contain at least one resin other than a cationic polymer (hereinafter also referred to as "Resin X").

[0377] Examples of resin X include polyester resin, polyurethane resin, acrylic resin, polyamide resin, polyurea resin, polycarbonate resin, polyolefin resin, and polystyrene resin.

[0378] As resin X, at least one of acrylic resin, polyester resin and polyurethane resin is preferred.

[0379] In this invention, acrylic resin refers to a polymer (homopolymer or copolymer) comprising at least one raw material monomer selected from the group consisting of acrylic acid, derivatives of acrylic acid (e.g., acrylates), methacrylic acid, and derivatives of methacrylic acid (e.g., methacrylates).

[0380] The weight-average molecular weight (Mw) of resin X is preferably 1,000 to 300,000, more preferably 2,000 to 200,000, and even more preferably 5,000 to 100,000.

[0381] In this invention, unless otherwise stated, weight-average molecular weight (Mw) refers to the value determined by gel permeation chromatography (GPC).

[0382] For the determination by gel permeation chromatography (GPC), the apparatus used was an HLC-8020GPC (TOSOHCORPORATION), with three TSKgel Super Multipore HZ-H columns (4.6 mm ID × 15 cm, TOSOHCORPORATION) used as the column and THF (tetrahydrofuran) as the eluent. The determination conditions were set as follows: sample concentration 0.45% by mass, flow rate 0.35 ml / min, sample injection volume 10 μl, and determination temperature 40 °C, using an RI detector.

[0383] The calibration curves were prepared using eight samples from TOSOH Corporation: “TSKstandard, polystyrene”, namely “F-40”, “F-20”, “F-4”, “F-1”, “A-5000”, “A-2500”, “A-1000” and “n-propylbenzene”.

[0384] The preferred form of resin X in the pretreatment solution is resin particles.

[0385] That is, the pretreatment solution preferably contains resin particles as resin X.

[0386] In this case, resin X is preferably a water-insoluble resin.

[0387] The volume average particle size of the resin particles as resin X is preferably 1 nm to 300 nm, more preferably 3 nm to 200 nm, and even more preferably 5 nm to 150 nm.

[0388] In this invention, the volume average particle size refers to the value measured using a laser diffraction / scattering particle size analyzer.

[0389] As a measuring device, an example that can be cited is the particle size distribution measuring device "Microtrac MT-3300II" (manufactured by Nikkiso Co., Ltd.).

[0390] Resin particles as resin X

[0391] Preferred materials include acrylic resin particles, ester resin particles, mixtures of acrylic resin particles and ester resin particles, composite particles containing acrylic resin and ester resin, or polyurethane resin particles.

[0392] There are no particular restrictions on the content of resin X in the pretreatment solution.

[0393] The content of resin X relative to the total amount of pretreatment liquid is preferably 0.5% to 30% by mass, more preferably 1% to 20% by mass, and particularly preferably 1% to 15% by mass.

[0394] (surfactant)

[0395] The pretreatment solution may contain at least one surfactant.

[0396] There are no particular restrictions on the type of surfactant; it can be any of the following: anionic surfactants, cationic surfactants, betaine surfactants, and nonionic surfactants. Furthermore, examples of surfactants include acrylic surfactants, fluorinated surfactants, and silicone surfactants.

[0397] The surfactant content relative to the total amount of the pretreatment solution is preferably 0.1% to 5% by mass, more preferably 0.2% to 1% by mass.

[0398] (Other ingredients)

[0399] The pretreatment solution may contain other components besides those mentioned above, as needed.

[0400] Other components that may be contained in the pretreatment solution include known additives such as solid wetting agents, colloidal silica, inorganic salts, anti-fading agents, emulsion stabilizers, penetration enhancers, ultraviolet absorbers, preservatives, mildew inhibitors, pH adjusters, viscosity adjusters, rust inhibitors, chelating agents, and water-soluble polymers (e.g., the water-soluble polymers described in paragraphs 0026 to 0080 of Japanese Patent Application Publication No. 2013-001854).

[0401] (physical properties)

[0402] The pH of the pretreatment solution is preferably 2.0 to 7.0, more preferably 2.0 to 4.0. The pH is measured using a pH meter at 25°C, for example using a pH meter (model "HM-31") manufactured by DKK-TOA Corporation.

[0403] From the viewpoint of the coatability of the pretreatment solution, the viscosity of the pretreatment solution is preferably 0.5 mPa·s to 10 mPa·s, more preferably 1 mPa·s to 5 mPa·s. The viscosity is the value measured using a viscometer at 25°C. The viscosity is measured using a viscometer at 25°C, for example, using a TV-22 type viscometer manufactured by Toki Sangyo Co., Ltd.

[0404] The surface tension of the pretreatment liquid is preferably below 60 mN / m, more preferably 20 mN / m to 50 mN / m, and even more preferably 30 mN / m to 45 mN / m. The surface tension is a value measured at 25°C. The surface tension is measured using a surface tension meter at 25°C, for example, using an automatic surface tension meter (product name "CBVP-Z") manufactured by Kyowa Interface Science Co., Ltd., measured by the plate method.

[0405] Example

[0406] The present invention will be described in more detail below through embodiments, but the present invention is not limited to the following embodiments as long as it does not depart from its spirit.

[0407] [Examples 1-9, Comparative Examples 1-6]

[0408] <Preparation of Pretreatment Solution>

[0409] Mix the ingredients shown below to prepare a pretreatment solution.

[0410] -Composition of the pretreatment solution-

[0411] Glutaric acid (coagulant)

[0412] …6.1% by mass

[0413] Propylene glycol (PG) [other water-soluble organic solvents]

[0414] …20% by mass

[0415] • OLFINE F1010: (manufactured by Nissin Chemical Co., Ltd.) [surfactant]

[0416] …0.5% by mass

[0417] ·SUPERFLEX 500M (manufactured by DKS Co., Ltd.) [Aqueous dispersion of urethane resin particles]

[0418] …7.0% by weight

[0419] Triisopropanolamine (pH adjuster)

[0420] …0.2% by mass

[0421] • BYK024 (manufactured by BYK-Chemie GmbH) [Defoamer]

[0422] …0.01% by mass

[0423] Ultrapure water

[0424] …the remainder being 100% by mass in the total pretreatment solution.

[0425] Synthesis of Pigment Dispersant P1

[0426] Add 965 g of dipropylene glycol to a 5000 mL three-necked flask equipped with a stirrer and cooling tube, and heat to 85 °C under a nitrogen atmosphere.

[0427] Preparations were made separately: Solution I was obtained by dissolving 640 g of benzyl methacrylate, 340 g of methacrylic acid, and 19.94 g of 2-mercaptopropionic acid in 370.28 g of dipropylene glycol; and

[0428] Solution II was obtained by dissolving 17.69 g of tert-butyl peroxide-2-ethylhexanoate (product name "PERBUTYL 0", manufactured by NOF CORPORATI0N) in 221.17 g of dipropylene glycol.

[0429] Solution I was added dropwise to the three-necked flask over a period of 4 hours, and solution II was added dropwise over a period of 5 hours. After the additions were completed, the mixture was allowed to react for an additional 2 hours. 1 H-NMR confirmed the disappearance of the monomer.

[0430] The obtained reaction solution was heated to 70°C, and then 248.02 g of 50% potassium hydroxide aqueous solution was added. Next, 107.48 g of dipropylene glycol and 75.52 g of pure water were added and stirred to obtain a 37% (w / w) solution of the random polymer. This random polymer was used as pigment dispersant P1.

[0431] pass 1 ¹H-NMR confirmed the structural units constituting the obtained random polymer. Furthermore, the weight-average molecular weight (Mw) was determined by GPC. The obtained pigment dispersant P1 had a weight-average molecular weight (Mw) of 8400 and an acid value of 221.7 mg KOH / g.

[0432] <Preparation of White Pigment Dispersion (A)>

[0433] Pigment dispersant P1 (150 parts by mass) was dissolved in water to prepare a polymer solution with a pigment dispersant P1 concentration of 25% by mass.

[0434] A mixture was prepared by mixing 96 parts by mass of the polymer solution, 300 parts by mass of CI Pigment White 6 (trade name "JR-405", rutile titanium dioxide particles, manufactured by TAYCACvRPORATION) as a white pigment, and 270 parts by mass of water. A potassium hydroxide aqueous solution was then added to the obtained mixture to adjust the pH to 8.7 after neutralization.

[0435] Next, the neutralized mixture was dispersed for 3 hours using a bead mill (bead diameter: 0.1 mm φ, zirconia beads). This yielded a white pigment dispersant P1, dispersing the white pigment in a white pigment dispersion (uncrosslinked dispersion) PD1.

[0436] Next, the obtained white pigment dispersion (uncrosslinked dispersion) PD1 was subjected to ultrafiltration using a cross-flow ultrafiltration (UF) device (Sartorius) with ion-exchanged water flowing at a flow rate of 600 mL per minute. The liquid temperature was maintained at 25°C, and ultrafiltration was performed three times, with each cycle consisting of one volume of liquid added. Ion-exchanged water was then added to the ultrafiltered liquid to obtain an ultrafiltered dispersion with a white pigment concentration of 45% by mass and a pigment dispersant concentration of 3.6% by mass P1.

[0437] Relative to 136 parts by weight of the ultrafiltration dispersion, 1.35 parts by weight of trimethylolpropane polyglycidyl ether (product name "Denacol EX-321", manufactured by Nagase ChemteX Corporation) as a crosslinking agent and 14.5 parts by weight of boric acid aqueous solution (boric acid concentration: 4% by weight) were added. After reacting at 70°C for 6 hours, the mixture was cooled to 25°C. This caused the pigment dispersant P1 in the dispersion to crosslink, forming pigment dispersant P1a as a crosslinking polymer dispersant, resulting in a white pigment dispersion (crosslinked dispersion) in which the white pigment is dispersed by pigment dispersant P1a.

[0438] Deionized water was added to the obtained crosslinked dispersion to bring the pigment concentration to 15% by mass. The crosslinked dispersion with added deionized water was then passed at a flow rate of 600 mL per minute through an ultrafiltration apparatus (cross-flow ultrafiltration unit (UF), Sartorius) equipped with a polyethersulfone (PESU) membrane (micropore size: 0.1 μm). The liquid temperature was adjusted to 25°C, and ultrafiltration was performed three times, with each pass being equal to one volume ratio of the added liquid. Next, deionized water was added to bring the white pigment concentration to 45% by mass. This yielded a white pigment dispersion (A).

[0439] The white pigment dispersion (A) contains pigment dispersant P1a with an acid value of 144 mg KOH / g. Furthermore, the concentration of pigment dispersant P1a is 3.6% by mass.

[0440] <Preparation of White Pigment Dispersion (B)>

[0441] The diameter of the beads in the bead mill was changed to 0.3 mmφ. Otherwise, a white pigment dispersion (B) was obtained in the same manner as the preparation of white pigment dispersion (A).

[0442] <Preparation of White Ink>

[0443] As described below, inks W1 to W6, which are white inks, were prepared respectively.

[0444] (Preparation of ink W1)

[0445] The white pigment dispersion (A) was filtered through a 1-inch, 0.5μm filter (5EC4888389005J) manufactured by Pore Corporation of Japan, and the filtered white pigment dispersion (A) was mixed with other components shown below. The resulting mixture was then filtered through the aforementioned filter (5EC4888389005J) to obtain ink W1.

[0446] -Composition of Ink W1-

[0447] • White pigment dispersion (A)

[0448] …The content of white pigment is 10.0% by mass.

[0449] Propylene glycol (PG) [other water-soluble organic solvents]

[0450] …25.0% by mass

[0451] Propylene glycol monomethyl ether (PGmME) [water-soluble organic solvent]

[0452] …2.0% by mass

[0453] Neocryl A-1105 (manufactured by DSM Japan KK) [Acrylic resin particle dispersion]

[0454] …The content of resin particles is 5.0% by mass.

[0455] • OLFINE E1010 (manufactured by Nissin Chemical Co., Ltd.) [acetylene glycol surfactant]

[0456] …1.0% by mass

[0457] • BYK3450 (manufactured by BYK-Chemie GmbH) [Silicone-based surfactant]

[0458] …0.1% by mass

[0459] ·PVPK15 (Polyvinylpyrrolidone K15)

[0460] …0.15% by mass

[0461] ST-XS (manufactured by Nissan Chemical Corporation) [Colloidal silica dispersion]

[0462] …The content of colloidal silica particles is 0.05% by mass.

[0463] ·water

[0464] …becoming a 100% mass surplus in the overall ink composition.

[0465] <Preparation of Ink W2>

[0466] By replacing the white pigment dispersion (A) with the same mass of white pigment dispersion (B), ink W2 was obtained in the same manner as ink W1.

[0467] <Preparation of Ink W3>

[0468] By changing the amount of BYK3450 (0.1% by mass) to 1.5% by mass, ink W3 was obtained in the same manner as ink W1.

[0469] <Preparation of Ink W4>

[0470] The Nippon Palls Co., Ltd. filter (5EC4888389005J) with a length of 0.5 μm (1 inch) was replaced with the Nippon Palls Co., Ltd. filter (5EC4888389010J) with a length of 1.0 μm (1 inch) (5EC4888389010J) used in the filtration of the white pigment dispersion and the mixture, respectively. Otherwise, ink W4 was obtained in the same manner as ink W1.

[0471] <Preparation of Ink W5>

[0472] The amount of propylene glycol (PG) was changed from 25.0% by mass to 20.0% by mass.

[0473] The amount of propylene glycol monomethyl ether (PGmME) was changed from 2.0% by mass to 5.0% by mass.

[0474] Further addition of propylene glycol monopropyl ether (PGmPE) [a water-soluble organic solvent] (2.0% by mass)

[0475] In addition, ink W5 was obtained in the same manner as ink W4.

[0476] <Preparation of Ink W6>

[0477] After the preparation of ink W1 (i.e., after filtering the mixture with 5EC4888389005J), ink W6 is obtained by further filtering the obtained ink W1 with 5EC4888389005J.

[0478] <Preparation of Substrate>

[0479] As the substrate (i.e., the recording medium), a polyethylene terephthalate (PET) substrate “FE2001 (thickness 23 μm)” manufactured by FUTAMURA CHEMICAL CO., Ltd. was prepared.

[0480] The “FE2001 (thickness 23μm)” is a roller body that is rolled up in a roll shape with a thickness of 23μm, a width of 580mm and a length of 4000m for PET substrate.

[0481] <Preparation of Inkjet Recording Device>

[0482] As an inkjet recording device, Figure 1 The inkjet recording device shown has an inkjet recording device having a structure in which six inkjet heads IJ1 are arranged along the transport direction of the recording medium.

[0483] A concave plate coating machine was used as the pretreatment liquid application device P1.

[0484] The drying method in the pretreatment liquid drying zone DP1 is set to warm air drying.

[0485] Six inkjet heads IJ1 are connected to one end of circulation tube C1 and one end of circulation tube C2, respectively. The other end of circulation tube C1 and the other end of circulation tube C2 are connected to ink tanks IT1 corresponding to each color of ink.

[0486] Although the diagram is omitted, along each circulation pipe C2, starting from the ink tank IT1 side, there are sequentially arranged a degassing module for ink degassing, an ink delivery pump, a foreign matter removal filter, an ink temperature regulating heat exchanger, and an ink flow meter.

[0487] Although the illustration is omitted, an ink delivery pump and a foreign matter removal filter are sequentially arranged from the inkjet head IJ1 side along each circulation tube C1.

[0488] The circulation tubes shown in Tables 1 and 2 were used as circulation tubes C1 and C2.

[0489] The circulation tubes shown in Tables 1 and 2 are as follows.

[0490] ·LDPE1…Low-density polyethylene pipe (manufactured by Azusa Scientific Co., Ltd., “6-608-6”, inner diameter 0.6cm)

[0491] ·LDPE2…Low-density polyethylene pipe (manufactured by Azusa Techno Co., Ltd., “6-608-10”, inner diameter 1.0cm)

[0492] ·LDPE3…Low-density polyethylene pipe (manufactured by Azusa Techno Co., Ltd., “6-608-4”, inner diameter 0.4cm)

[0493] ·LDPE4…Low-density polyethylene pipe (manufactured by Azusa Techno Co., Ltd., “6-608-15”, inner diameter 2.1cm)

[0494] • Tygon…Tygon tube (made by Saint-Gobain KK, “Tygon2001”)

[0495] ·PP…Polypropylene pipe (AZWAN company “5-5666-44”, inner diameter 0.6cm)

[0496] Adjust the length L of the circulation tube C2 from the ink can to the length shown in Table 1 ("Length L of the circulation tube C2 between the can and the print head").

[0497] Hereinafter, the six inkjet heads IJ1 are sometimes referred to sequentially from the upstream inkjet head as head 1, head 2, head 3, head 4, head 5, and head 6. Also, the six inkjet heads IJ1 are sometimes referred to individually as each head.

[0498] Each connector is set to 1200dpi / 20inch width piezoelectric full-wire connector. Here, dpi is an abbreviation for dots per inch.

[0499] Each head is a linear head with nozzles arranged in a direction orthogonal to the conveying direction of the substrate (i.e., the width direction of the substrate).

[0500] The aforementioned heads were manufactured using Samba G3L (manufactured by FUJIFILM DIMATIX, Inc.).

[0501] <Inkjet Recording>

[0502] The above-mentioned substrate was surface treated by corona discharge under the conditions of a discharge gap of 1 mm, 100 W, and 4 m / min using a corona treatment machine (manufactured by Kasuga Denki Co., Ltd., TEC-4AX corona surface modification evaluation device).

[0503] The above-described image recording apparatus includes the surface-treated substrate, a pretreatment liquid, and white ink (i.e., any one of inks W1 to W6. For details, please refer to Tables 1 and 2.)

[0504] The white ink is set to be sprayed from the 5th nozzle.

[0505] While moving the substrate at a constant speed of 50 m / min, a pretreatment liquid was applied to the substrate using a wire bar coater.

[0506] The mass of the pretreatment solution was set at 1.5 g / m³. 2 .

[0507] Here, the mass of the pretreatment solution is the value obtained by dividing the mass of the pretreatment solution by the area of ​​the region to which the pretreatment solution is applied.

[0508] At the point where the pretreatment solution application ends, the pretreatment solution is dried using a dryer starting 2 seconds after application. The drying process is carried out at 60°C for 3 seconds.

[0509] Next, while moving the substrate at a constant speed of 50 m / min, white ink (i.e., any one of inks W1 to W6; see Tables 1 and 2 for details) is ejected from the fifth nozzle onto the surface of the substrate that has been treated with the pretreatment liquid, forming a solid image. At this time, the ink droplet volume is set to 3.4 ng, the duty ratio is set to 100%, and the ejection frequency is set to 39.37 kHz.

[0510] The white ink is ejected by operating an infusion pump (not shown) located in circulation pipe C1 and another infusion pump (not shown) located in circulation pipe C2, thereby... Figure 1 As shown, the process involves circulating white ink along the path from ink tank IT1 → circulation pipe C2 → inkjet head IJ1 → circulation pipe C1 → ink tank IT1. During this process, the ink circulation flow rate (mL / s) is adjusted to the values ​​shown in Tables 1 and 2, thereby adjusting the ink circulation speed Vc within the circulation pipe (specifically, the connection to the inkjet head) to the values ​​shown in Tables 1 and 2.

[0511] Furthermore, the white ink is degassed by a degassing module (not shown) located in the circulation pipe C2, then passed through a filter (not shown), and then regulated to 30°C by an ink temperature regulating heat exchanger (not shown) located in the circulation pipe C2 before being sprayed onto the substrate.

[0512] Next, two seconds after the white ink was applied to the substrate, infrared (IR) drying was performed using an infrared (IR) irradiation device (Noritake Co., Ltd. PLC-328) at a surface temperature of 75°C. Then, a dryer was used to perform warm air drying at 80°C for 20 seconds, thereby obtaining a solid image of white ink.

[0513] Through the above, an image recording material is obtained, which has a substrate and a solid white ink image disposed on the substrate.

[0514] <Contact angle θ of ink relative to the inner surface of the circulation tube>

[0515] The circulation tube C2 was cut open with a cutting machine and stretched into a flat plate to make a sample for contact angle measurement.

[0516] White ink was dropped onto the surface of the sample used for contact angle measurement, which corresponds to the inner surface of the circulation tube, and the contact angle of the ink relative to the inner surface of the circulation tube was measured.

[0517] The contact angle was measured using a contact angle measuring device (manufactured by Kyowa Face Science Co., Ltd., DM-).

[0518] The sample size (500) was measured using conventional methods at 25°C and 50% RH (droplet size was 2 μL, and the waiting time from droplet to measurement was 1000 ms).

[0519] The obtained contact angle θ (°) and “1-cosθ” are shown in Table 1 and Table 2.

[0520] <Speed ​​ratio (1), speed ratio (2) and speed ratio (3)>

[0521] Based on equations (1), (2) and (3), the speed ratios (1), (2) and (3) were calculated.

[0522] The results are shown in Tables 1 and 2.

[0523] <Evaluation>

[0524] The following evaluation was performed on the above inkjet records.

[0525] The results are shown in Tables 1 and 2.

[0526] (Color consistency over long-term operation)

[0527] The inkjet printer was continuously recorded at a speed of 50 m / min for 60 minutes to obtain image records. In the obtained image records, the opacity (coverage force) per 1000 m length was measured.

[0528] Opacity was measured using X-Rite, Inc.’s x-rite eXact Advanced, in the following order (see https: / / www.xrite.co.jp / allproduct / qa / 99-exact / 588-pqa0111.html).

[0529] First, the image recorder was placed on the black portion of the opacity test paper (JIS qualified product) manufactured by TPTECHNICAL LAB.CO., Ltd. with the image in the image recorder in contact with the black portion, and the opacity was measured.

[0530] Next, the image recorder on which the covering power on the black area was measured was placed on the white area of ​​the opacity test paper with the image in the image recorder in contact with the white area, and the covering power at the same position as the position on which the covering power on the black area was measured was measured.

[0531] The occlusion strength obtained per 1000m was compared (specifically, the occlusion strength of the area 1000m from the starting image recording area, the occlusion strength of the area 2000m from the starting image recording area, and the occlusion strength of the area 3000m from the starting image recording area), and the color constancy was evaluated.

[0532] The evaluation criteria are as follows.

[0533] In the following evaluation criteria, the highest level of color consistency over long-term operation is A.

[0534] -Evaluation criteria for color constancy during long-term measurements-

[0535] A: The standard deviation of the coverage is below 0.6%.

[0536] B: The standard deviation of the coverage is greater than 0.6% and less than 0.9%.

[0537] C: The standard deviation of the coverage is greater than 0.9% and less than 1.5%.

[0538] D: The standard deviation of the coverage is greater than 1.5%.

[0539] (Image occlusion)

[0540] In the evaluation of color constancy during long-term operation, the occlusion of the image was evaluated according to the following evaluation criteria based on the occlusion power of the part 1000m away from the part where the image was first recorded.

[0541] In the following evaluation criteria, the highest level of image occlusion is AA.

[0542] -Evaluation criteria for image occlusion-

[0543] AA: Coverage is 58% or more.

[0544] A: Coverage is 57% or more but less than 58%.

[0545] B: Coverage is 56% or more but less than 55%.

[0546] C: Opacity is 55% or higher but less than 54%.

[0547] D: Coverage is below 54%.

[0548] (Ejection stability during long-term operation)

[0549] The image record obtained during the evaluation of color constancy during long-term operation was placed on the black portion of the opacity test paper with the image in the image record in contact with the black portion, and the presence or absence of stripes in the image caused by poor ink ejection was observed.

[0550] In detail, the ejection stability during long-term operation was evaluated by comparing the number of stripes in the area where the initial image was recorded with the number of stripes in an area 3000m away from the area where the initial image was recorded, according to the following evaluation criteria.

[0551] In the following evaluation criteria, the highest level of ejection stability during long-term operation is A.

[0552] -Evaluation criteria for ejection stability during long-term operation-

[0553] A: The increase rate of the number of stripes in a region 3000m away from the region where the image was first recorded is less than 10%, relative to the number of stripes in the region where the image was first recorded.

[0554] B: The increase rate of the number of stripes in a region 3000m away from the region where the image was first recorded, relative to the number of stripes in the region where the image was first recorded, is greater than 10% but less than 50%.

[0555] C: The increase rate of the number of stripes in a region 3000m away from the region where the image was first recorded, relative to the number of stripes in the region where the image was first recorded, is greater than 50% but less than 100%.

[0556] D: The increase rate of the number of stripes in a region 3000m away from the region where the image was first recorded exceeds 100%, relative to the number of stripes in the region where the image was first recorded.

[0557]

[0558]

[0559] In Tables 1 and 2, “aEb” represents a × 10 b For example, "1.64E-04" and "1.23E+09" represent 1.64 × 10⁻⁴ respectively. -4 and 1.23×10 9 .

[0560] As shown in Tables 1 and 2, the number of large white particles, Pn, is 1.00 × 10⁻⁶. 4 pcs / cm 3 The above and the speed ratio (1) is 5.0 × 10 3 Above and 1.4×10 5 In the following Examples 1 to 9, the color consistency and ejection stability during long-term operation are excellent.

[0561] The results of the comparative examples regarding these embodiments are as follows.

[0562] When the speed ratio (1) is less than 5.0 × 10 3 In Comparative Examples 1, 3, and 4, the color consistency decreased during long-term operation.

[0563] When the speed ratio (1) exceeds 1.4 × 10 5 In comparative examples 2, 5 and 6, the ejection stability decreased during long-term operation.

[0564] The number of large white particles, Pn, is less than 1.00 × 10⁻⁶. 4 pcs / cm 3 In Comparative Example 6, the occlusion of the image also decreased.

[0565] Based on the results of Examples 5 and 9, it can be seen that when the speed ratio (2) is 1.5 × 10 3 Under the above conditions (Example 9), the color consistency during long-term operation is further improved.

[0566] As can be seen from the results of Examples 6 and 9, when the speed ratio (3) is 10.0 or higher (Example 9), the color stability during long-term operation is further improved.

[0567] [Examples 101-109, Comparative Examples 101-106] (No pretreatment solution)

[0568] No pretreatment solution was applied or dried. Otherwise, the same operations as in Examples 1-9 and Comparative Examples 1-6 were performed.

[0569] As a result, the same results as those in Examples 1-9 and Comparative Examples 1-6 were obtained in Examples 1-9 and Comparative Examples 1-6 in each of Examples 1-109 and Comparative Examples 1-106 (without pretreatment liquid).

[0570] The entire contents of the invention of Japanese Patent Application No. 2022-48837, filed on March 24, 2022, are incorporated herein by reference.

[0571] All documents, patent applications and technical standards described in this specification are incorporated herein by reference to the same extent as the specific documents, patent applications and technical standards which are incorporated herein by reference.

Claims

1. An inkjet recording method, comprising using an inkjet recording apparatus, said inkjet recording apparatus comprising: The inkjet head ejects ink; and A circulation path is used to circulate the ink discharged from the inkjet head and return it to the inkjet head. The inkjet recording method includes the steps of circulating the ink in the circulation path and ejecting the ink from the inkjet head to apply it to the recording medium. The ink contains water and a white pigment as titanium dioxide particles. In the ink, the number of white pigment particles with a particle size of 0.8 μm to 5 μm, i.e., large white particles, is 1.00 × 10⁻⁶. 4 pcs / cm 3 above, The circulation path includes a circulation pipe connected to the inkjet head, which is used to return the ink to the inkjet head. The speed ratio (1) defined by the following equation (1) is 5.0 × 10 3 Above and 1.4×10 5 the following, Speed ​​ratio (1) = Vc / (Vslp × log(Pn)) Equation (1) Vslp = { (ρ - ρw)gR 2 } / (18η) Equation (A) In equation (1), Vc is the circulation velocity of the ink in the connection part between the circulation tube and the inkjet head, expressed in cm / s. Vslp is the sedimentation rate of the large white particles as defined by equation (A), in cm / s, and Pn is the number of large white particles in the ink, in cm / s. 3 The number of units In formula (A), ρ is 4.

23. ρw is the solvent in the ink in g / cm³ 3 Density in units g is in cm / s 2 The acceleration due to gravity is expressed in units of 1. R is the median particle size in cm of the large white particles. η is the viscosity of the solvent in the ink at 30°C, expressed in g / cm·s.

2. The inkjet recording method according to claim 1, wherein, The speed ratio (2) defined by the following equation (2) is 1.5 × 10 3 above, Speed ​​ratio (2) = Speed ​​ratio (1) × (1-cosθ) Equation (2) In equation (2), θ is the contact angle of the ink relative to the inner surface of the circulation tube, in °.

3. The inkjet recording method according to claim 1 or 2, wherein, The ink circulation is carried out via an ink tank. The speed ratio (3) defined by the following formula (3) is 10.0 or higher. Speed ​​ratio (3) = Speed ​​ratio (1) × (1 / L) Equation (3) In formula (3), L is the length of the circulation tube in cm from the ink tank to the inkjet head.

Images