ink

Abstract

The present invention provides an ink that suppresses curling and offsetting while ensuring low viscosity, redispersibility, and intermittent dispensing. [Solution] The ink has a viscosity of 7.5 mPa·s or less at 25°C. The ink contains pigment a, polyethylene glycol b, a water-soluble solid additive c that is solid at 25°C, a penetrating agent d, a surfactant e, and water. The polyethylene glycol b includes a first polyethylene glycol b1 with a number average molecular weight of 150 or more and less than 450, and a second polyethylene glycol b2 with a number average molecular weight of 450 or more and less than 1050. In the ink, the content of the first polyethylene glycol b1 is 5% by mass or more and 10% by mass or less. In the ink, the content of the second polyethylene glycol b2 is 1% by mass or more and 5% by mass or less. The content of the water-soluble solid additive c is 3% by mass or more and 10% by mass or less.

Description

Technical Field

[0001] The present invention relates to an ink for forming an image by an inkjet method.

Background Art

[0002] An inkjet recording apparatus forms an image on a recording medium such as paper by repeatedly ejecting small droplets of ink from nozzles. In a recording medium such as paper composed of fibers, a phenomenon (curl) in which the shape is bent due to partial expansion and contraction of the fibers occurs easily due to the formation of an image by the ink. Patent Document 1 discloses a technique of blending polyethylene glycol in the ink in order to suppress the occurrence of curl.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In some inkjet recording apparatuses, an ink used for forming an image is required to have a low viscosity due to requirements based on the specifications of the recording head. In this regard, in an ink containing polyethylene glycol, the viscosity tends to increase, making it difficult to ensure a low viscosity. In addition, in an ink containing polyethylene glycol, drying progresses in the nozzles of the recording head as the viscosity increases, making it easy to become unable to eject, that is, it becomes difficult to ensure intermittent ejection performance. Furthermore, in an ink containing polyethylene glycol, adhesion to the paper discharge roller of the inkjet recording apparatus tends to occur, so that a phenomenon (offset) in which the subsequent recording medium is soiled through the paper discharge roller tends to occur. In addition, there is also a problem that it is difficult to ensure redispersibility in an ink having a low viscosity.

[0005] In view of the above circumstances, the object of the present invention is to provide an ink that can suppress the occurrence of curl and offset while ensuring low viscosity, redispersibility, and intermittent dispensing. [Means for solving the problem]

[0006] In one embodiment of the present invention, the viscosity at 25°C is 7.5 mPa·s or less. The ink contains pigment a, polyethylene glycol b, a water-soluble solid additive c that is solid at 25°C, a penetrating agent d, a surfactant e, and water. The polyethylene glycol b comprises a first polyethylene glycol b1 having a number average molecular weight of 150 or more and less than 450, and a second polyethylene glycol b2 having a number average molecular weight of 450 or more and less than 1050. In the ink, the content of the first polyethylene glycol b1 is 5% by mass or more and 10% by mass or less. In the aforementioned ink, the content of the second polyethylene glycol b2 is 1% by mass or more and 5% by mass or less. The content of the aforementioned water-soluble solid additive c is 3% by mass or more and 10% by mass or less.

[0007] In this ink, the redispersibility of pigment a is ensured by the action of the first polyethylene glycol b1, and the occurrence of curl and offset is suppressed by the action of the second polyethylene glycol b2 and the water-soluble solid additive c. In this ink, intermittent ejection is more easily ensured by using the second polyethylene glycol b2 and the water-soluble solid additive c in combination to suppress the occurrence of curl and offset. Furthermore, in this ink, the occurrence of offset can be further suppressed because drying is promoted by rapid penetration into the recording medium due to the action of the penetrating agent d and surfactant e.

[0008] The first polyethylene glycol b1 may contain polyethylene glycols with different number-average molecular weights. The second polyethylene glycol b2 may contain polyethylene glycols with different number-average molecular weights. These inks allow for more precise viscosity adjustment through the composition of polyethylene glycol b.

[0009] The second polyethylene glycol b2 may contain polyethylene glycol with a number average molecular weight of 550 or more. This ink can more effectively suppress curling.

[0010] In the aforementioned ink, the dynamic surface tension at a surface life of 10 milliseconds may be 40.0 mN / m or less. [Effects of the Invention]

[0011] As described above, the present invention provides an ink that can suppress the occurrence of curl and offset while ensuring low viscosity, redispersibility, and intermittent discharge. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described below.

[0013] [Ink composition] (Schematic configuration) An ink according to one embodiment of the present invention contains a pigment a, polyethylene glycol b, a water-soluble solid additive c, a penetrating agent d, a surfactant e, and water. The ink according to this embodiment is typically an aqueous ink that is ejected from the recording head of an inkjet recording device onto a recording medium to form an image on the recording medium. The recording medium on which the image is formed with the ink according to this embodiment is made of fibers such as cellulose fibers, and examples include plain paper, copy paper, recycled paper, thin paper, and thick paper.

[0014] In the ink according to this embodiment, polyethylene glycol b comprises a first polyethylene glycol b1 and a second polyethylene glycol b2. The first polyethylene glycol b1 is a polyethylene glycol with a number average molecular weight of 150 or more and less than 450. The second polyethylene glycol b2 is a polyethylene glycol with a number average molecular weight of 450 or more and less than 1050.

[0015] In the ink according to this embodiment, the viscosity at 25°C can be kept to 7.5 mPa·s or less by keeping the total amount of polyethylene glycol b relatively low. In this embodiment, the viscosity of the ink at 25°C is measured using an E-type viscometer TV-100EL (manufactured by Toki Sangyo Co., Ltd.). Furthermore, in the ink according to this embodiment, the redispersibility of pigment a can be ensured by the action of the first polyethylene glycol b1, which has a relatively small number-average molecular weight. Therefore, the ink according to this embodiment can achieve both low viscosity and redispersibility.

[0016] In the ink according to this embodiment, it is preferable to have a high content of first polyethylene glycol b1 in order to obtain high redispersibility, but it is undesirable for the content of first polyethylene glycol b1 to be too high in order to keep the viscosity at 25°C at 7.5 mPa·s or less. From these viewpoints, in the ink according to this embodiment, the content of first polyethylene glycol b1 is 5% by mass or more and 10% by mass or less.

[0017] Also, in the ink according to the present embodiment, due to the action of the second polyethylene glycol b2 having a relatively large number average molecular weight, when penetrating into the recording medium, the hydrogen bonds between the fibers constituting the recording medium are likely to be maintained. Further, in the ink according to the present embodiment, even when the hydrogen bonds between the fibers constituting the recording medium are broken, it is considered that the second polyethylene glycol b2 enters between the fibers where the hydrogen bonds are broken, making it difficult for the recording medium to deform. Therefore, the ink according to the present embodiment can suppress the occurrence of curl in the recording medium. In the ink according to the present embodiment, in order to more effectively suppress the occurrence of curl, it is preferable that the second polyethylene glycol b2 contains polyethylene glycol having a number average molecular weight of 550 or more.

[0018] Furthermore, in the ink according to the present embodiment, due to the action of the second polyethylene glycol b2 having a relatively large number average molecular weight, it is difficult for the ink to adhere to the paper discharge roller of the inkjet recording apparatus. Therefore, the ink according to the present embodiment can suppress the occurrence of offset.

[0019] In the ink according to the present embodiment, in order to more effectively suppress the occurrence of curl and offset, it is preferable that the content of the second polyethylene glycol b2 is large. However, as the content of the second polyethylene glycol b2 increases, it becomes more difficult to ensure intermittent ejection performance. Therefore, in the ink according to the present embodiment, while keeping the content of the second polyethylene glycol b2 at a level where intermittent ejection performance can be ensured, a water-soluble solid additive c having an action of suppressing the occurrence of curl and offset similar to that of the second polyethylene glycol b2 is blended to compensate for the shortage of the second polyethylene glycol b2.

[0020] In the ink according to the present embodiment, it is preferable that the content of the second polyethylene glycol b2 is large in order to more effectively suppress the occurrence of curl and offset, the viscosity at 25°C is kept at 7.5 mPa·s or less, and it is not preferable that the content of the second polyethylene glycol b2 is too large in order to ensure intermittent ejection property. From these viewpoints, in the ink according to the present embodiment, the content of the second polyethylene glycol b2 is 1% by mass or more and 5% by mass or less. Further, in the ink according to the present embodiment, in order to suppress the occurrence of curl and offset, the content of the water-soluble solid additive c is 3% by mass or more and 10% by mass or less.

[0021] Thus, in the ink according to the present embodiment, by the constitution of the polyethylene glycol b and the water-soluble solid additive c, it is possible to suppress the occurrence of curl and offset while ensuring low viscosity, redispersibility, and intermittent ejection property. In the ink according to the present embodiment, in order to more accurately adjust the viscosity, it is preferable to blend polyethylene glycols b having different number average molecular weights as the first polyethylene glycol b1, and it is also preferable to blend polyethylene glycols b having different number average molecular weights as the second polyethylene glycol b2.

[0022] Further, the ink according to the present embodiment becomes more likely to penetrate into the recording medium due to the action of the penetrant d. Furthermore, the ink according to the present embodiment becomes even more likely to penetrate into the recording medium due to the decrease in surface tension by the action of the surfactant e. Therefore, in the ink according to the present embodiment, it dries quickly due to rapid penetration into the recording medium, so that the occurrence of offset can be more effectively suppressed. In the ink according to the present embodiment, it is preferable that the dynamic surface tension at a surface lifetime of 10 milliseconds is 40.0 mN / m or less. In the present embodiment, the dynamic surface tension at a surface lifetime of 10 milliseconds of the ink is measured using a bubble pressure dynamic surface tensiometer ("BP-100" manufactured by KRUSS).

[0023] (Pigment a) In the ink according to this embodiment, pigment a can be, for example, a yellow pigment, an orange pigment, a red pigment, a blue pigment, a purple pigment, or a black pigment. Examples of yellow pigments include CI Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, or 193. Examples of orange pigments include CI Pigment Orange 34, 36, 43, 61, 63, or 71. Examples of red pigments include CI Pigment Red 122 or 202. Examples of blue pigments include CI Pigment Blue 15 or 15:3. Examples of purple pigments include CI Pigment Violet 19, 23, or 33. Examples of black pigments include CI Pigment Black 7.

[0024] In the ink according to this embodiment, the content of pigment a is preferably 4% by mass or more and 8% by mass or less. This makes it easier to obtain a high image density in the image formed on the recording medium and to ensure penetration into the recording medium. If the content of pigment a is less than 4% by mass, it becomes difficult to obtain a high image density in the image formed on the recording medium. On the other hand, if the content of pigment a exceeds 8% by mass, it becomes difficult to obtain penetration into the recording medium and it may be difficult to ensure the fluidity of the pigment particles in the solvent, making it difficult to obtain a high image density in the image formed on the recording medium.

[0025] Self-dispersing pigments can be prepared by physically or chemically surface-treating non-self-dispersing pigments using known methods. Examples of physical surface treatments include vacuum plasma treatment. Examples of chemical surface treatments include wet oxidation treatment using an oxidizing agent in water, and treatment to bond p-aminobenzoic acid to the pigment surface (in this treatment, carboxyl groups are bonded to the pigment surface via phenyl groups).

[0026] Commercially available self-dispersing pigments can also be used. Examples of commercially available self-dispersing pigments include the CAB-O-Jet® series from Cabot Corporation (e.g., CAB-O-Jet® 200, 300, 400, 250C, 260M, 270Y, 450C, 465M, 470Y, and 480M) and the BONJET® series from Orient Chemical Industries, Ltd. (e.g., BONJET® CW-3 and CW-4).

[0027] Examples of pigments that serve as raw materials for self-dispersing pigments (pigments that do not possess self-dispersibility) include yellow pigments, orange pigments, red pigments, blue pigments, purple pigments, and black pigments. Examples of yellow pigments include CI Pigment Yellow (1, 2, 3, 12, 13, 14, 16, 17, 55, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 120, 128, 129, 138, 139, 150, 151, 154, 155, 173, 180, 185, and 193).

[0028] Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71).

[0029] Examples of red pigments include CI Pigment Red (5, 7, 12, 48, 48 (more specifically 48:1), 57, 112, 122, 123, 146, 168, 184, and 202).

[0030] Examples of blue pigments include CI Pigment Blue (1, 2, 3, 15 (more specifically 15:3 and 15:4), 16, 22, and 60).

[0031] Examples of purple pigments include CI Pigment Violet (19, 23, 33, and 1960).

[0032] Examples of black pigments include No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B from Mitsubishi Chemical Corporation, Raven® (5750, 5250, 5000, 3500, 1255, and 700) from Columbia Chemical Corporation, Rega1® (400R, 330R, and 660R) from Cabot, Mogul® L, Monarch® (700, 800, 880, 900, 1000, 1100, 1300, and 1400), and BLACK Examples of conductive carbon blacks include PEARLS® 2000, VULCAN® XC-72, VULCAN® P, and STERLIB® C; and carbon blacks such as Color Black (FW1, FW2, FW2V, FW18, FW200, S150, S160, and S170) from Orion Engineered, Printex® (35, U, V, and 140U), and Special Black (4, 4A, 5, and 6).

[0033] (Polyethylene glycol b) Examples of first polyethylene glycol b1 with a number average molecular weight of 150 or more and less than 450 include polyethylene glycol 200, polyethylene glycol 300, and polyethylene glycol 400. Examples of second polyethylene glycol b2 with a number average molecular weight of 450 or more and less than 1050 include polyethylene glycol 500, polyethylene glycol 600, and polyethylene glycol 1000.

[0034] (Water-soluble solid additive c) In the ink according to this embodiment, by incorporating a water-soluble solid additive c that is solid at 25°C, it is possible to suppress the occurrence of curl and offset, as well as improve the scratch resistance of the image formed on the recording medium. Examples of water-soluble solid additive c to be incorporated into the ink according to this embodiment include sugar alcohols (sorbitol, maltitol), polyhydric alcohols (trimethylolpropane, neopentyl glycol, 1,6-hexanediol, trimethylethane, erythritol), betaine, and ε-caprolactam.

[0035] (Penetrating agent d) Examples of penetrating agents d to be incorporated into the ink according to this embodiment include triethylene glycol monobutyl ether (BTG), 1,2-hexanediol, 1,2-octanediol, 2-methyl-2,4-pentanediol, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether. In the ink according to this embodiment, it is preferable that the content of penetrating agent d is 1% by mass or more and 10% by mass or less.

[0036] (Surfactant e) Surfactant e has the effect of improving the drying properties of the ink, as well as improving the wettability of the ink on the storage medium and improving the compatibility and dispersion stability of each component contained in the ink. Examples of surfactant e to be incorporated into the ink according to this embodiment include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. Nonionic surfactants are preferred as the surfactant to be incorporated into the ink according to this embodiment, acetylene-based surfactants are more preferred, and acetylene glycol-based surfactants are even more preferred. Acetylene glycol-based surfactants have a small molecular weight and high symmetry, so they can quickly reduce the surface tension of the ink on the recording medium.

[0037] Acetylene-based surfactants are alcohols having an acetylene bond in their molecule. Preferably, the alcohol having an acetylene bond is at least one of, for example, a glycol having an acetylene bond and a monohydric alcohol having an acetylene bond. Commercially available products can also be used as acetylene glycol-based surfactants. More specifically, the acetylene glycol-based surfactant is preferably "Orphine® E1010" manufactured by Nisshin Chemical Industry Co., Ltd. For example, "Orphine E1010" manufactured by Nisshin Chemical Industry Co., Ltd. contains an ethylene oxide adduct of acetylenediol, and more specifically, contains polyoxyethylene acetylenic glycol ether.

[0038] In the ink according to this embodiment, it is preferable that the surfactant e content is 0.05% by mass or more and 2.00% by mass or less, from the viewpoint of suppressing offset and improving image density.

[0039] (water) In the ink according to this embodiment, for example, ion-exchanged water, purified water, or distilled water can be used as water. In the ink according to this embodiment, from the viewpoint of drying properties and ejection reliability, the water content is preferably 40% by mass or more and 80% by mass or less, more preferably 55% by mass or more and 75% by mass or less, and even more preferably 60% by mass or more and 70% by mass or less.

[0040] (Other ingredients) The ink according to this embodiment may contain components other than those listed above, as needed. For example, the ink according to this embodiment may contain a pigment dispersion resin to improve the dispersibility of pigment a in the solvent. The pigment dispersion resin is a fine particle of resin that suppresses the aggregation of pigment a by adsorbing onto the surface of pigment a. In an ink containing a pigment dispersion resin, pigment a and the pigment dispersion resin together constitute pigment particles (pigment dispersion). The pigment particles are composed of, for example, a core containing pigment a and a pigment dispersion resin coating the core. In the case of the pigment dispersion resin, a portion may be dispersed in the solvent without adsorbing onto the surface of pigment a. The pigment dispersion resin preferably has anionic properties.

[0041] The pigment dispersion resin can be appropriately selected from known pigment dispersion resins. Specific examples of pigment dispersion resins include styrene-acrylic resin, styrene-maleic acid copolymer, styrene-maleic acid half-ester copolymer, vinylnaphthalene-acrylic acid copolymer, or vinylnaphthalene-maleic acid copolymer. Styrene-acrylic resin is a resin containing units derived from styrene and units derived from acrylic acid, methacrylic acid, acrylic acid ester, or methacrylic acid ester. Examples of styrene-acrylic resins include styrene-acrylic acid-alkyl acrylate copolymer, styrene-methacrylic acid-alkyl methacrylate-alkyl acrylate copolymer, styrene-acrylic acid copolymer, styrene-maleic acid-alkyl acrylate copolymer, styrene-methacrylic acid copolymer, or styrene-alkyl methacrylate copolymer. Among these pigment dispersion resins, styrene-acrylic resin is preferred because it is easy to prepare and has excellent dispersion effect of pigment a, styrene-methacrylic acid-alkyl methacrylate-alkyl acrylate copolymer is more preferred, and methacrylic acid-methyl methacrylate-butyl acrylate-styrene copolymer is particularly preferred.

[0042] In the ink according to this embodiment, it is necessary to incorporate a certain amount of pigment dispersion resin in order to obtain the above-mentioned effects. On the other hand, in the ink according to this embodiment, it is undesirable to have too much pigment dispersion resin in order to obtain high permeability. From these viewpoints, in the ink according to this embodiment, it is preferable to incorporate 15 to 100 parts by mass of pigment dispersion resin per 100 parts by mass of pigment a.

[0043] Furthermore, the ink according to this embodiment may also contain various additives as needed, such as dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, neutralizing agents, and antifungal agents.

[0044] [Examples and Comparative Examples] Ink preparation and evaluation were carried out as examples and comparative examples of the present invention. Note that the following examples merely illustrate one example of the present invention, and the present invention is not limited to the configurations of the following examples.

[0045] (Ink preparation) First, a pigment dispersion was prepared by dispersing pigment a in water. The pigment dispersion resin was diluted with deionized water, pigment a was added, and then a preliminary dispersion treatment was performed using a homodisper. Subsequently, the main dispersion treatment was performed using a bead mill (manufactured by Nippon Coke Co., Ltd.) until the average particle size of the pigment particles reached 110 nm, thereby obtaining the pigment dispersion. In both the examples and comparative examples, CABOT's "Black Pearls 800" was used as pigment a. In both the examples and comparative examples, Bic Chemie's "DISPERBYK-190" was used as the pigment dispersion resin. The water contained in the pigment dispersion constitutes the water component in each ink.

[0046] Next, inks according to the examples and comparative examples were prepared. Pigment dispersion, polyethylene glycol b, water-soluble solid additive c, penetrating agent d, surfactant e, and deionized water were weighed and placed in a beaker. The contents of the beaker were stirred at a rotation speed of 400 rpm using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600") to uniformly mix the contents of the beaker and obtain ink. The ink was filtered using a filter (pore size 5 μm) to remove foreign matter and coarse particles contained in the ink.

[0047] (Evaluation method) The inks used in the examples and comparative examples were evaluated for viscosity, redispersibility, intermittent ejection, and resistance to offset and curling.

[0048] • Method for evaluating viscosity The viscosity of the inks was measured at 25°C using an E-type viscometer TV-100EL (manufactured by Toki Sangyo Co., Ltd.). The viscosity of each ink was evaluated according to the following criteria A to C. Ink with an evaluation of A or B was considered acceptable, while ink with an evaluation of C was considered unacceptable. A: 6.5mPa·s or less B: More than 6.5mPa·s and less than 7.5mPa·s C: More than 7.5mPa·s

[0049] • Method for evaluating redistribution Petri dishes containing 2g of each ink were placed in an incubator and maintained at 40°C for 72 hours. After maintenance, deionized water was added to each ink at a rate of 5ml / second, and the dispersion behavior of pigment particles in each ink was visually observed for 10 seconds after addition. The observation results were evaluated according to the following criteria A to C. In terms of redispersibility, inks with evaluations of A and B were considered acceptable, while inks with evaluations of C were considered unacceptable. A: The pigment particles dispersed quickly almost immediately after being dropped. B: Pigment particles dispersed in water over time after being dropped. C: Pigment particles did not disperse in water.

[0050] • Method for evaluating intermittent discharge Under conditions of 32°C and 10% RH, the recording head performed full nozzle ejection of ink, and after a predetermined time elapsed, the recording head formed a single dot line on the recording medium in the main scanning direction. The elapsed time from the completion of full nozzle ejection until the formation of a single dot line became impossible due to non-ejection was confirmed, and this elapsed time was used as the evaluation value for intermittent ejection performance. The evaluation value was assessed according to the following criteria A to C. For intermittent ejection performance, inks with evaluations of A and B were considered acceptable, while inks with evaluations of C were considered unacceptable. A: More than 2 minutes B: 1 minute or more but less than 2 minutes C: Less than 1 minute

[0051] • Evaluation method for resistance to curl formation A prototype evaluation unit manufactured by Kyocera Document Solutions Inc. was used as the inkjet recording device, and a recording head "KJ4B-QA" manufactured by Kyocera Document Solutions Inc. was used. A solid 10cm x 10cm image was formed on a recording medium (Fujifilm Business Innovation Co., Ltd. "C2" (A4 size)) with an ink ejection amount of 11pL per nozzle of the recording head. Immediately afterward, the recording medium with the formed solid image was placed on the top surface of a horizontal table so that the solid image on the recording medium and the top surface of the horizontal table faced each other. The height of the four corners of the recording medium from the top surface of the horizontal table was measured, and the average height of the four corners of the recording medium was used as the evaluation value for resistance to curling. For the evaluation value, when using a 0.8kPa suction fan, suction adsorption by a conveyor belt, and correction of the recording medium using a decal mechanism, the following criteria A to C were used to evaluate whether or not the recording medium could be adsorbed for the formation of an image on the second side (back side) of double-sided printing. In terms of resistance to curling, inks with an A or B rating are considered acceptable, while inks with a C rating are considered unacceptable. A: 10mm or less B: More than 10mm and less than 20mm C: More than 20mm

[0052] • Evaluation method for the likelihood of offset occurrence A prototype evaluation unit manufactured by Kyocera Document Solutions Inc. was used as the inkjet recording device, and a Kyocera Document Solutions Inc. "KJ4B-QA" recording head was used. First, ink was filled into the recording head closest to the paper output roller in the evaluation unit, and excess liquid flowing onto the nozzle surface was scraped off with a wipe blade. In the evaluation unit, the distance between the nozzle surface of the recording head and the recording medium was fixed at 1 mm, and the transport speed of the recording medium from the paper feed roller to the paper output roller was set to 846.7 mm / second. As the recording medium, Oji Paper Co., Ltd.'s "IJW" cut to A4 size was used. The amount of ink impregnated from the recording head to the recording medium by the evaluation unit was 15 g / m². 2 Ten consecutive 10cm x 10cm solid images were formed on recording media. For the tenth recording media, the area prone to smudging due to ink adhering to the paper output roller (offset area) was scanned using an image scanner (Seiko Epson Corporation "GT-X820") and binarized with a threshold of 220. The offset area ratio (%) (=100 x number of black pixels / total number of pixels) was calculated from the number of black pixels and the total number of pixels of the binarized image, and the offset area ratio was used as an evaluation value for the likelihood of offset occurring. For the evaluation value, the following criteria A to D were used as a standard to determine whether or not smudging on the recording paper due to offset could be confirmed by visual inspection. In terms of likelihood of offset occurring, inks with evaluations of A to C were considered acceptable, and inks with an evaluation of D were considered unacceptable. A: 0.020% or less B: More than 0.020% and less than 0.025% C: More than 0.025% and less than 0.030% D: More than 0.030%

[0053] (Examples 1-21) In Examples 1 to 21, two types of polyethylene glycol b were used from among PEG200, PEG400, PEG500, and PEG1000 manufactured by Tokyo Chemical Industry Co., Ltd. Furthermore, in all Examples 1 to 21, trimethylolpropane (TMP) manufactured by Tokyo Chemical Industry Co., Ltd. was used as the water-soluble solid additive c, triethylene glycol monobutyl ether (BTG) manufactured by Tokyo Chemical Industry Co., Ltd. was used as the penetrating agent d, and "Orphine E1010," an acetylene glycol-based surfactant manufactured by Nisshin Chemical Industry Co., Ltd., was used as the surfactant e. In all inks related to Examples 1 to 21, the pigment a content was 5% by mass, the pigment dispersion resin content was 2% by mass, the surfactant e content was 0.5% by mass, and the remainder was water. Table 1 shows the types and content (by mass) of polyethylene glycol b, water-soluble solid additive c, and penetrating agent d for the inks related to Examples 1 to 21.

[0054] [Table 1]

[0055] The inks from Examples 1 to 21 were evaluated for viscosity, redispersibility, intermittent discharge properties, and resistance to offset and curling. Table 2 shows the evaluation results for viscosity, redispersibility, intermittent discharge properties, and resistance to offset and curling for the inks from Examples 1 to 21.

[0056] [Table 2]

[0057] In the inks according to Examples 1 to 21, in which polyethylene glycol b and water-soluble solid additive c constitute the above embodiment, all passed the requirements for viscosity, redispersibility, intermittent discharge, and resistance to offset and curling. Furthermore, comparing Example 1 and Example 21, it can be seen that the ink according to Example 21, which has a higher content of penetrating agent d, is less prone to offset. This is thought to be because the ink according to Example 21 penetrated the recording medium more effectively, resulting in faster drying.

[0058] (Comparative Examples 1-7) In Comparative Examples 1 to 7, two types of polyethylene glycol b were used from among PEG100, PEG200, PEG400, PEG600, PEG1000, and PEG2000 manufactured by Tokyo Chemical Industry Co., Ltd. Furthermore, in all of Comparative Examples 1 to 7, trimethylolpropane (TMP) manufactured by Tokyo Chemical Industry Co., Ltd. was used as the water-soluble solid additive c, triethylene glycol monobutyl ether (BTG) manufactured by Tokyo Chemical Industry Co., Ltd. was used as the penetrating agent d, and "Orphine E1010" manufactured by Nisshin Chemical Industry Co., Ltd. was used as the surfactant e. In the inks of Comparative Examples 1 to 7, the pigment a content was 5% by mass, the pigment dispersion resin content was 2% by mass, the penetrating agent d content was 1% by mass, the surfactant e content was 0.5% by mass, and the remainder was water. Table 3 shows the types and content (by mass) of polyethylene glycol b and water-soluble solid additive c for the inks of Comparative Examples 1 to 7.

[0059] [Table 3]

[0060] The inks in Comparative Examples 1 and 2 differ from the inks in the above-mentioned Examples in that they contain an excess of the first polyethylene glycol b1 and the second polyethylene glycol b2, and do not contain the water-soluble solid additive c. The inks in Comparative Examples 3 and 4 differ from the inks in the above-mentioned Examples in that they contain an excess of the second polyethylene glycol b2, and do not contain the water-soluble solid additive c. The ink in Comparative Example 5 differs from the ink in the above-mentioned Examples in that it contains polyethylene glycol b with a number average molecular weight smaller than the first polyethylene glycol b1, and polyethylene glycol b with a number average molecular weight larger than the second polyethylene glycol b2. The ink in Comparative Example 6 differs from the ink in the above-mentioned Examples in that it contains an excess of the first polyethylene glycol b1, and polyethylene glycol b with a number average molecular weight larger than the second polyethylene glycol b2. The ink in Comparative Example 7 differs from the ink in the above-mentioned Examples in that it contains polyethylene glycol b with a number average molecular weight smaller than the first polyethylene glycol b1, and contains an excess of the second polyethylene glycol b2.

[0061] The viscosity, redispersibility, intermittent ejection properties, and resistance to offset and curling were evaluated for the inks related to Comparative Examples 1 to 7. Table 4 shows the evaluation results for viscosity, redispersibility, intermittent ejection properties, and resistance to offset and curling for the inks related to Comparative Examples 1 to 7.

[0062] [Table 4]

[0063] In all of Comparative Examples 1-7, which contained a high proportion of polyethylene glycol b with a large number-average molecular weight, the intermittent discharge performance failed. Furthermore, in Comparative Examples 5 and 7, which did not contain the first polyethylene glycol b1, the redispersibility also failed.

Claims

1. An ink with a viscosity of 7.5 mPa·s or less at 25°C, It contains pigment a, polyethylene glycol b, a water-soluble solid additive c that is solid at 25°C, a penetrating agent d, a surfactant e, and water. The polyethylene glycol b comprises a first polyethylene glycol b1 having a number average molecular weight of 150 or more and less than 450, and a second polyethylene glycol b2 having a number average molecular weight of 450 or more and less than 1050. The content of the first polyethylene glycol b1 is 5% by mass or more and 10% by mass or less. The content of the second polyethylene glycol b2 is 1% by mass or more and 5% by mass or less. The content of the water-soluble solid additive c is 3% by mass or more and 10% by mass or less. ink.

2. The ink according to claim 1, The first polyethylene glycol b1 comprises polyethylene glycols with mutually different number-average molecular weights. ink.

3. The ink according to claim 1, The second polyethylene glycol b2 comprises polyethylene glycols with mutually different number-average molecular weights. ink.

4. The ink according to claim 1, The second polyethylene glycol b2 contains polyethylene glycol with a number average molecular weight of 550 or more. ink.

5. An ink according to any one of claims 1 to 4, The dynamic surface tension at a surface lifetime of 10 milliseconds is 40.0 mN / m or less. ink.

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