Image formation method and liquid used therein

Abstract

To suppress discoloration of a medium.SOLUTION: An image forming method includes: a transport step for conveying a medium comprising a thermal layer containing a leuco dye that reacts with a color-developing agent at a first temperature and a coating layer containing a polymer; an adhesion step of applying a liquid containing a first organic solvent and a second organic solvent to the coating layer of the medium; and a drying step of drying the medium at a second temperature lower than the first temperature. The solubility parameter of the leuco dye is 8.0 or more and 9.0 or less. The solubility parameter of the polymer is 11.0 or less. The solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less. The solubility parameter of the second organic solvent is 9.0 or more and less than 12.0. The content of the first organic solvent relative to the total amount of the liquid is greater than the content of the second organic solvent relative to the total amount of the liquid. The sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This specification discloses a technique relating to an image forming method for forming an image on a medium having a heat-sensitive layer. [Background technology]

[0002] Patent Document 1 discloses an image forming method for forming an image on thermal paper. It is known that liquid sprayed onto the thermal paper can permeate the thermal paper, causing the thermal paper to discolor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP2000-25331 Public Relations Summary of the Invention [Problem to be solved by the invention]

[0004] The present specification provides a novel technology that can suppress discoloration of a medium when an image is formed by depositing a liquid on the medium having a heat-sensitive layer. [Means for solving the problem]

[0005] This specification discloses an image forming method. The image forming method includes a transporting step of transporting a medium having a thermosensitive layer and a coating layer on the thermosensitive layer, where the thermosensitive layer contains a color developer and a leuco dye that reacts with the color developer at a first temperature, and the coating layer contains a polymer; an attaching step of attaching a liquid to the coating layer of the transported medium, where the liquid contains a first organic solvent and a second organic solvent; and a drying step of drying the medium to which the liquid has been attached at a second temperature lower than the first temperature. The solubility parameter of the leuco dye is 8.0 or more and 9.0 or less. The solubility parameter of the polymer is 11.0 or less. The solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less. The solubility parameter of the second organic solvent is 9.0 or more and less than 12.0. The sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight, and the content of the first organic solvent relative to the total amount of the liquid is greater than the content of the second organic solvent relative to the total amount of the liquid.

[0006] According to the above configuration, the solubility parameter of the second organic solvent contained in the liquid is close to the solubility parameter of the polymer contained in the coating layer. Therefore, the liquid adhering to the coating layer is easily permeated into the coating layer due to the action of the second organic solvent. As a result, even when the medium is dried at a relatively low second temperature, the liquid on the coating layer can be dried appropriately. This makes it possible to suppress discoloration of the medium due to a reaction between the color developer and the leuco dye caused by the application of heat during the drying process.

[0007] On the other hand, the solubility parameter of the second organic solvent is close to that of the leuco dye. When these solubility parameters are close, the leuco dye can dissolve in the liquid. It is known that when the leuco dye dissolves in the liquid, the developer and the leuco dye react with each other, resulting in discoloration of the medium. However, with the above configuration, the liquid contains a first organic solvent whose solubility parameter is significantly different from that of the leuco dye, and the content of the first organic solvent is greater than the content of the second organic solvent. Therefore, the action of the first organic solvent can prevent the leuco dye from dissolving in the liquid. This can also prevent the leuco dye from dissolving in the liquid, resulting in discoloration of the medium due to a reaction between the developer and the leuco dye.

[0008] The liquids used to form images on media by the above-described imaging methods are also novel and useful.

[0009] UV ink is often used when printing images on thermal paper using an inkjet printer. However, if the UV ink does not cure sufficiently, the polymerization initiator contained in the ink can emit an odor. To avoid this problem, the inventors of the present application considered printing images on thermal paper using water-based ink instead of UV ink. However, because the main solvent of water-based ink is water, and the coating layer (so-called protective layer) disposed on the surface of the thermal paper is hydrophobic, it takes a relatively long time for the ink to penetrate the thermal paper. In other words, it takes a relatively long time for the ink adhered to the surface of the thermal paper to dry. Therefore, after the water-based ink is applied to the surface of the thermal paper, there is a risk that the undried ink will adhere to the transport mechanism. To avoid this, for example, heating the thermal paper with the water-based ink at a high temperature could be considered. However, heating at a high temperature could cause a reaction between the leuco dye and the color developer, resulting in discoloration of the thermal paper. Another possible solution is to add an organic solvent to the water-based ink, whose solubility parameter is close to that of the polymer contained in the coating layer. This is expected to increase the ink's wettability with respect to the coating layer of the thermal paper, increasing the surface area of ​​the ink exposed to external air and accelerating the drying of the ink. However, if the solubility parameter of the organic solvent added to the ink is close to that of the leuco dye in the thermal layer as well as to that of the polymer contained in the coating layer, the leuco dye may dissolve in the organic solvent, causing a reaction between the color developer and the leuco dye, resulting in discoloration of the thermal paper. This specification provides a technology for suppressing discoloration that may be caused by both heating temperature and the organic solvent contained in the ink, even when using aqueous ink to print images on thermal paper. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows the configuration of an image forming apparatus 10. [Figure 2] A cross section of thermal paper 2 is shown. DETAILED DESCRIPTION OF THE INVENTION

[0011] (Configuration of image forming apparatus 10) An image forming apparatus 10 according to an embodiment will be described with reference to the drawings. As shown in FIG. 1, the image forming apparatus 10 is an apparatus for forming an image on thermal paper 2. Specifically, the image forming apparatus 10 ejects ink toward the thermal paper 2 according to an inkjet recording method to form an image (hereinafter sometimes referred to as an "ink image") on the thermal paper 2. The image forming apparatus 10 can also form an image (hereinafter sometimes referred to as a "heated image") on the thermal paper 2 by selectively heating the thermal paper 2 on which an image has been recorded according to a thermal method. The image forming apparatus 10 is used while placed on the floor or a rack. However, in other embodiments, the image forming apparatus 10 may be used while placed on a table.

[0012] 1, the image forming apparatus 10 further includes a housing 12, a first holder 14, a second holder 16, a first tensioner 18, a second tensioner 20, two first conveyor roller pairs 22, two second conveyor roller pairs 24, a plurality of intermediate tensioners 26, and a control unit 28. The control unit 28 is communicatively connected to each part of the image forming apparatus 10 and controls the operation of each part. For ease of explanation, each component housed inside the housing 12 is illustrated in FIG. 1, but the components do not necessarily have to be located in the positions illustrated.

[0013] The first holder 14 supports the first roll 4 on which the thermal paper 2 is wound in a circular shape. Here, the thermal paper 2 is a long sheet. The first holder 14 is rotated by a conveying motor (not shown). As the first holder 14 rotates, the first roll 4 supported by the first holder 14 also rotates.

[0014] The first tensioner 18 is disposed above the first holder 14. The first tensioner 18 has an outer peripheral surface 18a against which the thermal paper 2 abuts. The thermal paper 2 pulled out from the first roll 4 curves along the outer peripheral surface 18a and is fed laterally (particularly to the right in FIG. 1).

[0015] Each of the two first conveying roller pairs 22 includes a first conveying roller 22a and a first pinch roller 22b. The first conveying roller 22a comes into contact with the first pinch roller 22b to form a nip 8. The nip 8 is located at a position generally equal to the upper end of the outer circumferential surface 18a of the first tensioner 18 in the vertical direction. Each of the two first conveying roller pairs 22 is rotated by a conveying motor (not shown). Each of the two first conveying roller pairs 22 rotates while nipping the thermal paper 2, thereby conveying the thermal paper 2 sent out from the first tensioner 18. The number and arrangement of the first conveying roller pairs 22 are not particularly limited.

[0016] Each of the two second conveying roller pairs 24 includes a second conveying roller 24a and a second pinch roller 24b. The second conveying roller 24a comes into contact with the second pinch roller 24b to form a nip 8. The nip 8 is located at a position generally equal to the upper end of the outer circumferential surface 20a of the second tensioner 20 in the vertical direction. Each of the two second conveying roller pairs 24 is rotated by a conveying motor (not shown). Each of the two second conveying roller pairs 24 rotates while nipping the thermal paper 2, thereby conveying the thermal paper 2 to the second tensioner 20. The number and arrangement of the second conveying roller pairs 24 are not particularly limited.

[0017] The second tensioner 20 is disposed above the second holder 16. The second tensioner 20 has an outer peripheral surface 20a against which the thermal paper 2 abuts. The thermal paper 2 conveyed by the second conveying roller pair 24 curves along the outer peripheral surface 20a and is sent downward.

[0018] The second holder 16 supports the second roll 6. The second holder 16 is rotated by a conveyance motor (not shown). As the second holder 16 rotates, the second roll 6 supported by the second holder 16 also rotates, and the thermal paper 2 on which the image is formed is wound up into a ring shape.

[0019] A transport path 100 along which the thermal paper 2 is transported is formed between the first holder 14 and the second holder 16. As the thermal paper 2 passes through this transport path 100, an image is recorded on the thermal paper 2.

[0020] In the above-described conveying path 100, a plurality of intermediate tensioners 26 are disposed between the two first conveying roller pairs 22 and the two second conveying roller pairs 24. Specifically, a first intermediate tensioner 26a, a second intermediate tensioner 26b, a third intermediate tensioner 26c, and a fourth intermediate tensioner 26d are disposed in this order from upstream to downstream of the conveying path 100. The four intermediate tensioners 26a to 26d form the conveying path 100, which curves in the front-to-rear and up-down directions, inside the housing 12.

[0021] As shown in FIG. 1, the image forming apparatus 10 further includes a head 30. The head 30 is disposed above a transport path 100. The head 30 includes a plurality of nozzles 32, and ink supplied from an ink tank is ejected to the outside through each nozzle 32 by driving a piezoelectric element (not shown) corresponding to each nozzle 32. In this manner, each nozzle 32 ejects ink toward the thermal paper 2 passing through the transport path 100, thereby forming an ink image on the thermal paper 2. The number and arrangement of the nozzles 32 are not particularly limited.

[0022] As shown in FIG. 1, the image forming apparatus 10 further includes a drying device 34. The drying device 34 is located downstream of the head 30. The drying device 34 is, for example, a hairdryer, a heat gun, an oven, or an IR heater. The control unit 28 controls the drying device 34 to dry the ink ejected onto the thermal paper 2 passing through the transport path 100. Note that one type of drying device 34 may be used alone, or two or more types may be used in combination.

[0023] As shown in FIG. 1, the image forming apparatus 10 further includes a thermal head 36. The thermal head 36 is disposed above the transport path 100. The thermal head 36 is disposed downstream of the drying device 34. The thermal head 36 includes a plurality of heating elements 38. The plurality of heating elements 38 are arranged on the lower surface of the thermal head 36 in a direction perpendicular to the plane of the paper in FIG. 1. In other words, the direction in which the plurality of heating elements 38 are arranged is perpendicular to the transport direction of the thermal paper 2 transported below the thermal head 36. The control unit 28 selectively heats the thermal paper 2 passing through the transport path 100 by selectively causing the plurality of heating elements 38 to generate heat, thereby forming a heated image on the thermal paper 2.

[0024] (thermal paper) Figure 2 shows a schematic cross section of thermal paper 2. As shown in Figure 2, the thermal paper 2 comprises a substrate K, a thermal layer L, and a coating layer M. The thermal layer L is disposed on the substrate K, and the coating layer M is disposed on the thermal layer L. The thermal paper 2 may further comprise a primer layer (not shown) between the substrate K and the thermal layer L to improve their adhesion, heat insulation, smoothness, etc.

[0025] The substrate K supports the thermosensitive layer L. Because heat is applied during thermal image formation on the thermosensitive layer L, the substrate K is made of a material with sufficient mechanical strength to handle heat without any problems. Examples of such substrates K include paper and plastic films. Examples of paper include various types of paper, processed paper, and synthetic paper. Specific examples include fine paper, coated paper, art paper, cast-coated paper, and cardboard, as well as paper impregnated with resin emulsions or synthetic rubber latex, and paper with synthetic resins. Examples of plastic films include polyolefin resin films, hard polyvinyl chloride films, polyester resin films, polystyrene films, polycarbonate films, polyacrylonitrile films, and polymethacrylate films. These plastic films may include not only transparent films but also white, opaque films formed by adding white pigments or fillers. The substrate K may be made of one of these materials alone, or a laminate of two or more materials.

[0026] The thermosensitive layer L contains a color developer P and a leuco dye Q. The leuco dye Q reacts with the color developer P at a predetermined temperature. The thermosensitive layer L may further contain a binder, a sensitizer, and a storage stabilizer. Additionally or alternatively, the thermosensitive layer L may further contain additives such as pigments, waxes, and antifoaming agents, and / or fillers for imparting a desired color to the thermosensitive layer L.

[0027] As the leuco dye Q, a colorless or pale-colored conventionally known leuco dye can be appropriately selected and used. Specific examples of the leuco dye Q include (1) triarylmethane compounds such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(p-dimethylaminophenyl)-3-(2-phenyl-3-indolyl)phthalide, 3-(p-dimethylaminophenyl)-3-(1,2-dimethyl-3-indolyl)phthalide, 3,3-bis(9-ethyl-3-carbazolyl)-5-dimethylaminophthalide, and 3,3-bis(2-phenyl-3-indolyl)-5-dimethylaminophthalide; (2) 4,4-bis(dimethylaminophenyl)-6-dimethylaminophthalide; (n) Diphenylmethane compounds such as benzhydrin benzyl ether and N-2,4,5-trichlorophenylleucoauramine, (3) 3-dibutylamino-6-methyl-7-bromofluoran, rhodamine-β-anilinolactam, 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-7-octylaminofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino)fluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-6-chloro-7-(β-ethoxyethylamino)fluoran, 3-diethylamino-6-chloro-7-(γ-chloropropylamino)fluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxyethylamino)- 6-Methyl-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-tolylamino)-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-7-(2-chloroanilino)fluoran, 3-dipentylamino-6-methyl-7-anilinofluoran (4) xanthene compounds such as fluoran, 3-piperidino-6-methyl-7-anilinofluoran, and 3-(4-anilino)anilino-6-methyl-7-chlorofluoran; (5) thiazine compounds such as benzoyl leucomethylene blue and p-nitrobenzoyl leucomethylene blue; (6) 3-methylspirodinaphthopyran, 3-ethylspirodinaphthopyran, 3-benzylspirodinaphthopyran, and 3-methylspirodinaphthopyran; Spiro compounds such as naphtho-(3-methoxybenzo)spiropyran, (6) and others such as 3,5',6-tris(dimethylamino)-spiro[9H-fluoren-9,1'(3'H)-isobenzofuran]-3'-one and 1,1-bis[2-(4-dimethylaminophenyl)-2-(4-methoxyphenyl)ethenyl]-4,5,6,7-tetrachloro(3H)isobenzofuran-3-one. Leuco dyes may be used alone or in combination of two or more.

[0028] Leuco dye Q has a solubility parameter (hereinafter referred to as "SP value") of 8.0 or more and 9.0 or less. This SP value is calculated based on the Fedors method (Polymer Engineering and Science, 1974, Vol. 14, No. 2, pp. 147-154). The unit of the SP value is (cal / cm 3 ) 0.5The lower limit of the SP value of the leuco dye Q is not particularly limited, but may be, for example, 8.0 or more, for example, 8.1 or more, for example, 8.2 or more, for example, 8.3 or more, or for example, 8.4 or more. The upper limit of the SP value of the leuco dye Q is not particularly limited, but may be, for example, 9.0 or less, for example, 8.95 or less, for example, 8.9 or less, for example, 8.85 or less, or for example, 8.8 or less. The range of the SP value can be set by appropriately combining the above upper and lower limits, but may be, for example, 8.0 to 9.0 or less, for example, 8.1 to 8.95 or less, for example, 8.2 to 8.9 or less, for example, 8.3 to 8.85 or less, or for example, 8.4 to 8.8 or less. The content of the leuco dye Q is not particularly limited, but may be, for example, 10% by weight to 35% by weight based on the total solid content of the thermosensitive layer L.

[0029] Examples of the color developer P include p-octylphenol, p-tert-butylphenol, p-phenylphenol, p-hydroxyacetophenone, α-naphthol, β-naphthol, p-tert-octylcatechol, 2,2'-dihydroxybiphenyl, bisphenol-A, 1,1-bis(p-hydroxyphenyl)butane, 2,2-bis(4-hydroxyphenyl)heptane, 2,2-bis-(3-methyl-4-hydroxyphenyl)propane, 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane, 2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, bis(3-allyl ...4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulf Examples of the developer P include phenolic developers such as bis(3,4-dihydroxyphenyl)sulfone, bis(3,4-dihydroxyphenyl)sulfone, 2,4'-dihydroxyphenyl sulfone, 1,1-bis(4-hydroxyphenyl)cyclohexane, bis(4-hydroxyphenyl)ether, bis[2-(4-hydroxyphenylthio)ethoxy]methane, 4-(4-isopropoxybenzenesulfonyl)phenol, dimethyl 4-hydroxyphthalate, butyl bis(4-hydroxyphenyl)acetate, benzyl p-hydroxybenzoate, and 3,5-di-tert-butylsalicylic acid; organic carboxylic acids such as benzoic acid; metal developers such as zinc salicylate; and anilide derivatives such as 2,4-dihydroxy-N-2'-methoxybenzanilide. One type of developer P may be used alone, or two or more types may be used in combination.

[0030] The heat-sensitive layer L can be formed, for example, by using water as a dispersion medium, mixing and stirring a color developer P and a leuco dye Q, and optionally a binder, a sensitizer, a storage stabilizer, etc., to prepare a coating liquid, which is then coated onto the substrate K by a known method such as roll coating, bar coating, gravure coating, gravure reverse coating, die coating, slide coating, or curtain coating.

[0031] The coating layer M enhances gloss and improves various durability properties such as heat resistance and plasticizer resistance. The coating layer M is sometimes called a protective layer because it prevents moisture, oil, organic solvents, etc. from adhering to the heat-sensitive layer L and dissolving the leuco dye Q and the color developer P in them, causing color development. The coating layer M contains a polymer (not shown). Examples of polymers include acrylic resin, methacrylic resin, polypropylene (PP), polyethylene (PE), polystyrene, polyethylene terephthalate (PET), and urethane. One type of polymer may be used alone, or two or more types may be used in combination.

[0032] The lower limit of the SP value of the polymer is, for example, 8.0 or more, for example, 8.5 or more, or for example, 9.0 or more. The upper limit of the SP value of the polymer is, for example, 11.0 or less, for example, 10.5 or less, or for example, 10.0 or less. The range of the SP value can be set by appropriately combining the above-mentioned upper and lower limits, for example, 8.0 or more and 11.0 or less, for example, 8.5 or more and 10.5 or less, or for example, 9.0 or more and 10.0 or less. The SP value of the polymer is a value calculated for the monomer that is the constituent unit of the polymer.

[0033] (ink) The ink contains resin particles, colorant, organic solvent, surfactant, and solvent (or dispersion medium). The ink is an aqueous ink in which the resin particles, colorant, and organic solvent are dissolved in a solvent or dispersed in a dispersion medium. However, the ink does not have to contain resin particles. Additionally or alternatively, the ink does not have to contain colorant.

[0034] The resin particles may contain, for example, at least one of methacrylic acid and acrylic acid as a monomer, and commercially available products may be used. The resin particles may further contain, for example, styrene, vinyl chloride, etc. as a monomer. The resin particles may be contained in, for example, an emulsion. The emulsion is composed of, for example, resin particles and a dispersion medium (e.g., water, etc.). The resin particles are not dissolved in the dispersion medium, but are dispersed in the dispersion medium within a specific particle size range. Examples of resin particles include acrylic acid-based resins, maleic acid-based ester resins, vinyl acetate-based resins, carbonate-based resins, polycarbonate-based resins, styrene-based resins, ethylene-based resins, polyethylene-based resins, propylene-based resins, polypropylene-based resins, urethane-based resins, polyurethane-based resins, polyester-based resins, and copolymer resins thereof.

[0035] The resin particles may be made of a resin having a minimum film forming temperature (MFT) in the range of 0° C. to 100° C. The minimum film forming temperature (MFT) of the resin particles may be, for example, 20° C. to 95° C., or, for example, 40° C. to 90° C., or, for example, 60° C. to 85° C.

[0036] The emulsion may be, for example, a commercially available product, such as "Superflex (registered trademark) 870" (MFT: 70°C) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., or "Hi-Loss-X (registered trademark) KE-1062" (MFT: 55°C) or "Hi-Loss-X (registered trademark) QE-1042" (MFT: 45°C) manufactured by Seiko PMC Corporation.

[0037] The resin particles have an average particle size of, for example, 30 nm to 200 nm. The average particle size can be measured as an arithmetic mean diameter using, for example, a dynamic light scattering particle size distribution analyzer "LB-550" manufactured by Horiba, Ltd.

[0038] The solid content of the resin particles relative to the total amount of ink is, for example, in the range of 0.1% by weight to 30% by weight, for example, in the range of 0.5% by weight to 20% by weight, or for example, in the range of 1.0% by weight to 15.0% by weight. One type of resin particle may be used alone, or two or more types may be used in combination.

[0039] Examples of colorants include resin-dispersed pigments and self-dispersed pigments. Resin-dispersed pigments are pigments in which a resin dispersant is adsorbed onto the pigment surface, imparting dispersion stability to the pigment in a solvent or the like. Examples of resin dispersants include common polymer dispersants (also referred to as pigment dispersion resins or resin dispersants), and these may be prepared in-house. Examples of resin dispersants include those containing at least one of methacrylic acid and acrylic acid as a monomer, and commercially available products may also be used. Examples of resin dispersants include hydrophobic monomers such as styrene, styrene derivatives, vinyl naphthalene, vinyl naphthalene derivatives, and aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids, or block copolymers, graft copolymers, or random copolymers composed of two or more monomers selected from acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, and fumaric acid derivatives, or salts thereof. Examples of methods for dispersing pigments using pigment dispersing resins include dispersing the pigment using a dispersing device. The dispersing device used to disperse the pigment is not particularly limited as long as it is a common dispersing machine, and examples thereof include a ball mill, a roll mill, and a sand mill (for example, a high-speed type).

[0040] Self-dispersing pigments are pigments that can be dispersed in water without the use of a dispersant, for example, by chemically bonding at least one hydrophilic functional group, such as a carbonyl group, a hydroxyl group, a carboxylic acid group, a sulfonic acid group, or a phosphate group, or a salt thereof, directly or via another group. Raw materials for self-dispersing pigments include carbon black, inorganic pigments, and organic pigments. Inorganic pigments include titanium oxide, iron oxide-based inorganic pigments, and carbon black-based inorganic pigments. Organic pigments include azo pigments, polycyclic pigments, dye lake pigments, nitro pigments, nitroso pigments, and aniline black daylight fluorescent pigments. Azo pigments include azo lake pigments, insoluble azo pigments, condensed azo pigments, and chelate azo pigments. Polycyclic pigments include phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments. The dye lake pigments include, for example, basic dye lake pigments and acid dye lake pigments.

[0041] The solid content of the colorant relative to the total amount of ink is not particularly limited and can be appropriately determined depending on, for example, the desired optical density or saturation. The solid content of the colorant is, for example, in the range of 0.1% by weight to 20.0% by weight, and for example, in the range of 1.0% by weight to 15.0% by weight. The solid content of the colorant is the weight of the pigment only and does not include the weight of the resin particles. One type of colorant may be used alone, or two or more types may be used in combination.

[0042] The solid content of the resin particles and colorant relative to the total amount of ink is, for example, in the range of 0.2% by weight to 20% by weight, or in the range of 1% by weight to 15% by weight, or in the range of 3% by weight to 14% by weight, or in the range of 6% by weight to 13% by weight.

[0043] The organic solvents include a first organic solvent and a second organic solvent. The first organic solvent has an SP value of 12.0 or more and 20.0 or less. Any first organic solvent can be used as long as its SP value is 12.0 or more and 20.0 or less. The lower limit of the SP value of the first organic solvent is not particularly limited, but is, for example, 12.0 or more, for example, 13.0 or more, for example, 14.0 or more, or for example, 15.0 or more. The upper limit of the SP value of the leuco dye Q is not particularly limited, but is, for example, 20.0 or less, for example, 19.0 or less, for example, 18.0 or less, or for example, 17.0 or less. The range of the SP value can be set by appropriately combining the above upper and lower limits, but is, for example, 12.0 or more and 20.0 or less, for example, 13.0 or more and 19.0 or less, for example, 14.0 or more and 18.0 or less, or for example, 15.0 or more and 17.0 or less. Examples of the first organic solvent include glycerin, 1,3-propanediol, propylene glycol, tripropylene glycol, dipropylene glycol, 1,2-hexanediol, diethylene glycol, triethylene glycol, polyethylene glycol 200, 1,2-butanediol, 1,3-butanediol, and 1,2-pentanediol. The second organic solvent has an SP value of 9.0 or more and less than 12.0. There are no particular limitations on the second organic solvent, and any second organic solvent can be used as long as its SP value is 9.0 or more and less than 12.0. The lower limit of the SP value of the second organic solvent is not particularly limited, but is, for example, 9.0 or more, for example, 9.25 or more, for example, 9.5 or more, for example, 9.75 or more, or for example, 10.0 or more. The upper limit of the SP value of the leuco dye Q is not particularly limited, but is, for example, 12.0 or less, for example, 11.75 or less, for example, 11.5 or less, for example, 11.25 or less, or for example, 12.0 or more.The range of the SP value can be set by appropriately combining the above-mentioned upper and lower limits, and is, for example, 9.0 to 12.0, for example, 9.25 to 11.75, for example, 9.5 to 11.5, for example, 9.75 to 11.25, or for example, 10.0 to 11.0. Examples of the second organic solvent include 2-pyrrolidinone, triethylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monopropyl ether, 1-butoxy-2-propanol, tripropylene glycol monomethyl ether, and tripropylene glycol monobutyl ether. Each of the first organic solvent and the second organic solvent may be used alone or in combination of two or more.

[0044] The content of the first organic solvent relative to the total amount of ink is, for example, 20% by weight or less, such as 19% by weight or less, such as 18% by weight or less, such as 17% by weight or less, such as 16% by weight or less, or such as 15% by weight or less. The content of the second organic solvent relative to the total amount of ink is, for example, 10% by weight or less, such as 7% by weight or less, such as 5% by weight or less, such as 4% by weight or less, such as 3% by weight or less, or such as 2% by weight or less. From the viewpoint of reducing the solubility of the leuco dye Q in organic solvents while ensuring the permeability of the ink into the coating layer M, the content of the second organic solvent is preferably 5% by weight or less.

[0045] The content of the first organic solvent relative to the total amount of ink may be greater than the content of the second organic solvent relative to the total amount of ink. The sum of the content of the first organic solvent relative to the total amount of ink and the content of the second organic solvent relative to the total amount of ink is, for example, less than 25% by weight, or, for example, 24% by weight or less, or, for example, 23% by weight or less, or, for example, 22% by weight or less, or, for example, 21% by weight or less, or, for example, 20% by weight or less.

[0046] The ratio of the content of the second organic solvent to the content of the first organic solvent is, for example, 0.5 or less, for example, 0.4 or less, for example, 0.3 or less, for example, 0.2 or less, for example, 0.15 or less, for example, 0.1 or less, or for example, 0.05 or less. From the viewpoint of reducing the solubility of the leuco dye Q in organic solvents and suppressing discoloration of the thermal paper 2, the ratio is preferably 0.2 or less.

[0047] The lower limit of the weighted average value of the SP values ​​of the first organic solvent and the second organic solvent is, for example, 14.0 or more, for example, 15.1 or more, or for example, 15.6 or more. The upper limit of the weighted average value of the SP values ​​of the first organic solvent and the second organic solvent is, for example, 19.0 or less, or for example, 18.8 or less. The range of the weighted average value can be set by appropriately combining the above-mentioned upper and lower limits, for example, 14.0 to 19.0 or less, for example, 15.1 to 18.0 or less, or for example, 15.6 to 17.0 or less. If the weighted average value is within the above range, the above-mentioned effects can be preferably obtained. Note that, from the viewpoint of reducing the solubility of the leuco dye Q in organic solvents and suppressing discoloration of the thermal paper 2, the weighted average value is preferably 15.1 or more. The weighted average value of the SP value of the first organic solvent and the SP value of the second organic solvent is the sum of the product of the SP value of the first organic solvent and the content of the first organic solvent and the product of the SP value of the second organic solvent and the content of the second organic solvent, divided by the sum of the content of the first organic solvent and the content of the second organic solvent.

[0048] The ratio of the content of the second organic solvent to the content of the resin particles is, for example, 0.02 or more, for example, 0.1 or more, for example, 0.2 or more, for example, 0.4 or more, for example, 0.6 or more, for example, 0.8 or more, or for example, 1.0 or more. From the viewpoint of improving the abrasion resistance of the ink image, the ratio is preferably 0.4 or more.

[0049] The vapor pressure of the first organic solvent and the second organic solvent are each lower than the vapor pressure of water. The vapor pressure of water at 20°C is approximately 20 mmHg. The vapor pressure of the first organic solvent and the second organic solvent at 20°C is, for example, 0.01 mmHg or higher and 15 mmHg or lower, such as 0.03 mmHg or higher and 10 mmHg or lower, or for example, 0.05 mmHg or higher and 5 mmHg or higher. If the vapor pressure of each organic solvent is within the above range, evaporation near the nozzles 32 in the image forming apparatus 10 can be suppressed, and adhesion of the nozzles 32 can be suppressed. However, in other embodiments, the vapor pressure of the first organic solvent and the second organic solvent can be higher than or equal to the vapor pressure of water.

[0050] The water is preferably ion-exchanged water or pure water. The water content of the ink is, for example, in the range of 15% to 95% by weight, or in the range of 25% to 85% by weight, based on the total weight of the ink. The water content may be, for example, the remainder of the other components.

[0051] The ink may further contain known additives as needed. Examples of additives include surfactants, pH adjusters, viscosity adjusters, surface tension adjusters, preservatives, antifungals, leveling agents, antifoaming agents, light stabilizers, antioxidants, nozzle drying inhibitors, polymer components such as emulsions, and dyes. The surfactant may further include cationic surfactants, anionic surfactants, or nonionic surfactants. These surfactants may be commercially available products. Examples of commercially available products include "Olfine (registered trademark) E1010," "Olfine (registered trademark) E1006," "Olfine (registered trademark) E1004," "Silface SAG503A," and "Silface SAG002" manufactured by Nissin Chemical Industry Co., Ltd. The surfactant content relative to the total amount of ink is, for example, 5% by weight or less, 3% by weight or less, or 0.1% by weight to 2% by weight. Examples of viscosity adjusters include polyvinyl alcohol, cellulose, and water-soluble resins.

[0052] The ink can be prepared, for example, by uniformly mixing resin particles, a coloring material, an organic solvent, water, and, if necessary, other additives, using a known method, and then removing any insoluble matter using a filter or the like.

[0053] (Image forming method) Next, a description will be given of an image forming method using the image forming apparatus 10. The image forming method includes a conveying step, an adhering step, a drying step, and a heating step.

[0054] (Transportation process) The control unit 28 executes each process in response to, for example, an external command to form an image input to the image forming apparatus 10. In the conveying process, the control unit 28 controls each part, such as the first conveying roller pair 22 and the second conveying roller pair 24, to convey the thermal paper 2 on the conveying path 100.

[0055] (Attachment process) In the adhesion process, the control unit 28 adheres ink to the surface (i.e., the coating layer M) of the thermal paper 2 being transported along the transport path 100. The control unit 28 controls piezoelectric elements (not shown) corresponding to the multiple nozzles 32, and thereby selectively ejects ink from each nozzle 32 toward the thermal paper 2 passing under the head 30. As a result, an ink image is formed on the surface of the thermal paper 2.

[0056] (drying process) In the drying process, the control unit 28 dries the thermal paper 2 with the ink attached thereto at a second temperature lower than the first temperature. The first temperature is the reaction temperature between the leuco dye Q and the color developer P, and in this embodiment, it is approximately 80°C. The second temperature is not particularly limited, but is, for example, approximately 60°C. The control unit 28 controls the drying device 34 to dry the ink attached to the surface of the thermal paper 2 as it passes below the drying device 34. In this embodiment, a dryer or an oven is used as the drying device 34. When a dryer is used as the drying device 34, the control unit 28 dries the ink by blowing gas at the second temperature toward the surface of the thermal paper 2 (i.e., the surface on which the ink image is formed).

[0057] In this image forming method, the time from the end of the application process to the start of the drying process is 10 seconds or less. This can be achieved, for example, by adjusting the distance between the head 30 and the drying device 34 and / or adjusting the transport speed of the thermal paper 2. If this time is 10 seconds or less, the ink is more likely to dry before it penetrates into the thermal layer L, effectively preventing discoloration of the thermal paper 2.

[0058] Generally, when thermal paper 2 with ink attached is heated to a temperature higher than the minimum film-forming temperature (MFT) of the resin particles, the resin particles in the ink become liquid or rubbery. Then, when the thermal paper 2 is cooled to a temperature lower than the minimum film-forming temperature, the liquid or rubbery resin particles solidify. This forms a film on the coating layer M of the thermal paper 2, improving the abrasion resistance of the ink image formed on the thermal paper 2. When resin particles are contained in ink, mixing with an organic solvent or the like tends to lower the minimum film-forming temperature of the resin particles. For example, if a certain amount of organic solvent with an SP value close to that of the resin particles is present (i.e., the ratio of the content of the second organic solvent to the content of the resin particles is 0.4), the resin particles swell due to the action of the organic solvent, reducing the distance between the resin particles. Therefore, resin particles can form a film even at temperatures lower than the minimum film-forming temperature of the resin particles. Therefore, even if the second temperature is lower than the minimum film-forming temperature of the resin particles (for example, 78 degrees), the abrasion resistance of the ink image formed on the thermal paper 2 can be improved.

[0059] (Heating process) In the heating step, the control unit 28 selectively heats the thermal paper 2 to a temperature equal to or higher than the first temperature. The control unit 28 selectively heats the thermal paper 2 passing under the thermal head 36 to a temperature equal to or higher than the first temperature by selectively causing the heating elements 38 to generate heat. As a result, the leuco dye Q in the thermal layer L reacts with the color developer P to develop color, and a heated image is further formed on the thermal paper 2.

[0060] The image forming method described above can be used, for example, when forming an image on a label to be attached to a product. Information that does not change depending on the manufacturing conditions (such as the product name and / or manufacturer information) is formed on the thermal paper 2 as an ink image using an inkjet recording method, while information that changes depending on the manufacturing conditions (such as the lot number and / or manufacturing date information) is formed on the thermal paper 2 as a heated image using a thermal method. In this way, the information that does not change depending on the manufacturing conditions is formed on the thermal paper 2 in advance as an ink image, and when the product is manufactured, information that changes depending on the manufacturing conditions can be formed on the thermal paper 2 as a heated image.

[0061] According to the above configuration, the SP value of the second organic solvent contained in the ink is close to the SP value of the polymer contained in the coating layer M. Therefore, the ink adhered to the coating layer M is easily permeated into the coating layer M due to the action of the second organic solvent. As a result, even if the thermal paper 2 is dried at a relatively low second temperature, the ink on the coating layer M can be dried appropriately. This makes it possible to prevent discoloration of the medium caused by a reaction between the color developer P and the leuco dye Q due to the application of heat in the drying process.

[0062] On the other hand, the SP value of the second organic solvent is close to that of the leuco dye Q. If these SP values ​​are close, the leuco dye Q may dissolve in the ink. It is known that if the leuco dye Q dissolves in the ink, the developer P and the leuco dye Q may react, causing discoloration of the thermal paper 2. However, the ink contains a first organic solvent whose SP value is significantly different from that of the leuco dye Q, and the content of the first organic solvent is greater than the content of the second organic solvent. Therefore, the action of the first organic solvent can prevent the leuco dye Q from dissolving in the ink. This also prevents the discoloration of the thermal paper 2 caused by the reaction between the developer P and the leuco dye Q due to the dissolution of the leuco dye Q in the ink.

[0063] Examples and Comparative Examples Next, the ink will be specifically described with reference to examples and comparative examples, although the present invention is not limited to these examples in any way.

[0064] (Ink Preparation) Inks 1 to 25 were each prepared by mixing the components shown in Tables 1-3. The numerical value in each component column in each table indicates the content of that component relative to the total amount of ink, expressed in weight percent. The content of resin particles and coloring materials is expressed as solid content.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] Using each ink prepared, the degree of ink staining, discoloration of the thermal paper 2, and abrasion resistance of the ink image-formed area were evaluated according to the methods described below. In the following evaluations, thermal synthetic paper manufactured by Lintec Corporation was used as the thermal paper 2. The reaction temperature between the leuco dye Q contained in the thermal layer L of the thermal paper and the color developer P was approximately 80°C.

[0069] (Ink stain test) Ink was applied to the coating layer M of thermal paper 2, and then the thermal paper 2 with the ink applied was dried under the conditions (drying temperature, drying method, time from the end of the application process to the start of the drying process) shown in Table 4-6 for each example to create each sample. Plain paper was then placed in contact with the ink-applied surface of these samples, and the ink staining properties were evaluated according to the following criteria. The evaluation results are shown in Table 4-6. ○: No ink adheres to plain paper ×: Ink adheres to plain paper

[0070] (Discoloration test) Ink was applied to the coating layer M of the thermal paper 2, and then the thermal paper 2 with the ink applied thereto was dried under the conditions for each example shown in Table 4-6 (drying temperature, drying method, time from the end of the application process to the start of the drying process), to create each sample. The L*a*b* values ​​of the image-forming surfaces of these samples were measured using a spectrophotometer (X-Rite eXact, (light source: D50, viewing angle: 2°, Status T, illumination conditions during measurement: M0 (no filter), backing: PANTONE white backing, geometric conditions of illumination and light reception: 45 / 0)). The CIEDE2000 color difference ΔE00 was calculated in accordance with JIS Z8781-6:2017 and evaluated according to the following criteria. For samples whose inks did not contain colorants (Comparative Examples 1 and 4), ΔE00 was calculated based on the L*a*b* values ​​of thermal paper 2. For samples whose inks contained colorants, ΔE00 was calculated based on the L*a*b* values ​​of an image formed using inks that did not contain the first organic solvent or the second organic solvent. The evaluation results are shown in Tables 4-6. A+: ΔE00 is less than 1.0 A: ΔE00 is 1.0 or more and less than 2.0 B+: ΔE00 is 2.0 or more and less than 2.5 B: ΔE00 is 2.5 or more and less than 4.5 B-: ΔE00 is 4.5 or more and less than 5.0 C: ΔE00 is 5.0 or more

[0071] (Abrasion resistance test) Ink was applied to the coating layer M of thermal paper 2, and then the ink-applied thermal paper 2 was dried under the conditions (drying temperature, drying method, and time from the end of the application process to the start of the drying process) shown in Table 4-6 to create each sample. The optical density (OD) values ​​of these samples were measured using a spectrophotometer (X-Rite eXact, light source: D50, viewing angle: 2°, Status T). The image-formed areas of these samples were then rubbed 10 times with a 200g load using the probe of a Gakushin-type rub fastness tester (Tester Sangyo Co., Ltd. AB-301). The OD values ​​of each sample after rubbing were then measured using the spectrophotometer, and the rub resistance of the image-formed areas was evaluated according to the following criteria. The evaluation results are shown in Table 4-6. A: The absolute difference in OD value before and after scraping is less than 0.1 B: The absolute value of the difference in OD value before and after scraping is 0.1 or more and less than 0.5 C: The absolute value of the difference in OD value before and after scraping is 0.5 or more.

[0072] [Table 4]

[0073] [Table 5]

[0074] [Table 6]

[0075] (Ink stain results) As shown in Tables 4-6, it was confirmed that Examples 1 to 28 had excellent ink smearing resistance. On the other hand, Comparative Examples 5 and 6 had poor ink smearing resistance. Ink 22 used in Comparative Example 5 was different from Ink 11 used in Example 12 in that the glycerin content was changed to 30 wt %. That is, it was confirmed that inks containing a large amount of the first organic solvent, which has a relatively large SP value, had poor ink smearing resistance. In Comparative Example 5, which had a high content of the first organic solvent, the total organic solvent content was high at 33 wt %, and the weighted average of the SP values ​​of the first organic solvent and the second organic solvent (i.e., equivalent to the SP value of the entire organic solvents) was high at 19.1. That is, this weighted average value deviated from the SP value of the polymer contained in the coating layer M. Therefore, in Comparative Example 5, the ink had low wettability to the coating layer M, making it difficult for the ink to penetrate into the coating layer M. Furthermore, ink 22 of Comparative Example 5 contains a large amount of glycerin, which has a relatively low vapor pressure, and therefore the evaporation rate of glycerin into the atmosphere is slow. For these reasons, it is believed that in Comparative Example 5, the ink attached to the coating layer M was difficult to dry, resulting in poor ink smearing resistance. In Comparative Example 6, the weighted average value was also relatively large at 16.7, which deviated from the SP value of the polymer contained in the coating layer M. This made it difficult for the ink to penetrate into the coating layer M, making it difficult for the ink attached to the coating layer M to dry, resulting in poor ink smearing resistance. In contrast, in Examples 1 to 28, the organic solvent was appropriately contained, and the weighted average value was within an appropriate range. This made it easy for the ink to penetrate into the coating layer M, and the ink on the coating layer M could be dried appropriately even at a relatively low drying temperature, resulting in excellent ink smearing resistance.

[0076] (Discoloration results) The smaller the ΔE00 of the evaluation sample, the less discoloration of the thermal paper 2, i.e., the more the reaction between the leuco dye Q and the color developer P is suppressed. On the other hand, the larger the ΔE00 of the evaluation sample, the greater the discoloration of the thermal paper 2, i.e., the more the reaction between the leuco dye Q and the color developer P is progressing. As shown in Tables 4 to 6, in Examples 1 to 28, in which the drying temperature was lower than the reaction temperature between the leuco dye Q and the color developer P (i.e., approximately 80°C), the ink smearing resistance described above was excellent and the reaction between the leuco dye Q and the color developer P was suppressed. In contrast, in Comparative Example 1, in which the drying temperature was higher than the reaction temperature between the leuco dye Q and the color developer P (i.e., approximately 80°C), the ink smearing resistance described above was excellent, but the reaction between the leuco dye Q and the color developer P progressed, resulting in significant discoloration of the thermal paper 2. In this way, it was found that even at a drying temperature lower than the reaction temperature between the leuco dye Q and the developer P (i.e., approximately 80 degrees), the ink on the thermal paper 2 can be dried and discoloration of the thermal paper 2 can be suppressed.

[0077] Furthermore, in Examples 1 to 28, in which the sum of the contents of the first and second organic solvents was less than 25% by weight, discoloration of the thermal paper 2 was suppressed. On the other hand, in Comparative Examples 2 to 4 and 6, in which the sum was 25% by weight or more, discoloration of the thermal paper 2 occurred. In Comparative Examples 2 to 4 and 6, in which the ink contained a large amount of organic solvent, a large amount of organic solvent penetrated into the thermal layer L. In this case, when the amount of the second organic solvent was large, the leuco dye Q was more likely to dissolve in the ink due to the action of the second organic solvent that penetrated into the thermal layer L. Therefore, in Comparative Examples 2 to 4 and 6, in which the ink contained a large amount of both organic solvent and second organic solvent, the leuco dye Q dissolved in the ink due to the action of the second organic solvent, which is thought to have facilitated the reaction between the leuco dye Q and the color developer P.

[0078] Examples 1, 2, and 14 were prepared using the same ink 1, but the only difference was the time from ink application to the start of drying (hereinafter referred to as the drying transition time). Comparing Examples 1, 2, and 14, the shorter the drying transition time, the more suppressed discoloration of the thermal paper 2 was. Similar results were also obtained when comparing Examples 8 and 16, which used the same ink 7 but changed only the drying transition time, and Examples 9 and 15, which used the same ink 8 but changed only the drying transition time. By shortening the drying transition time, the time it takes for the ink applied to the coating layer M to penetrate into the thermal layer L is shortened. Therefore, the ink begins to dry before the leuco dye Q contained in the coating layer M dissolves in the ink (second organic solvent). Thus, in the examples with shorter drying transition times, the reaction between the leuco dye Q and the color developer P, which occurs when the leuco dye Q dissolves in the ink, is thought to be more suppressed.

[0079] Furthermore, it was found that in Examples 1, 2, and 5 to 28, in which the ratio of the content of the second organic solvent to the content of the first organic solvent was 0.2 or less, discoloration of the thermal paper 2 was further suppressed. This is thought to be because the content of the first organic solvent was relatively large relative to the content of the second organic solvent, which effectively exerted the action of the first organic solvent contained in the ink that had permeated the thermal layer L, preventing the leuco dye Q from dissolving in the ink and, as a result, preventing the leuco dye Q from reacting with the color developer P.

[0080] Furthermore, it was found that in Examples 7 to 13 and 15 to 28, in which the weighted average of the SP values ​​of the first organic solvent and the second organic solvent was 15.1 or higher, discoloration of the thermal paper 2 was further suppressed. This is thought to be because the weighted average of the relatively large SP value of the first organic solvent and the relatively small SP value of the second organic solvent was moderately large, which allowed the second organic solvent to facilitate the penetration of the ink into the coating layer M of the thermal paper 2, and also prevented the leuco dye Q from dissolving in the liquid.

[0081] In Examples 17 to 27, discoloration was not evaluated because ink containing BK (carbon black) was used as the coloring material.

[0082] (Abrasion resistance results) The smaller the absolute value of the difference in OD values ​​of the evaluation sample before and after rubbing, the higher the abrasion resistance of the image recorded on the thermal paper 2. On the other hand, the larger the absolute value of the difference in OD values ​​of the evaluation sample before and after rubbing, the lower the abrasion resistance of the image recorded on the thermal paper 2. As shown in Tables 4 to 6, Examples 20, 21, and 27, in which the ratio of the content of the second organic solvent to the content of the resin particles was 0.4 or greater, exhibited superior abrasion resistance compared to Examples 17 to 19, in which the ratio was less than 0.4. Because the SP value of the second organic solvent is relatively close to the SP value of the resin particles (acrylic resin) contained in the ink, the resin particles swell due to the action of the second organic solvent. Therefore, in Examples 20, 21, and 27, in which the ratio was 0.4 or greater, the resin particles swell more easily, reducing the distance between the resin particles and promoting film formation by the resin particles, which is thought to have improved the abrasion resistance of the ink image compared to Examples 17 to 19, in which the ratio was less than 0.4.

[0083] Furthermore, Example 21, which used hot air drying, showed superior abrasion resistance compared to Example 22, in which only the drying method was changed to an oven. Even when Example 23, which used hot air drying, was compared to Example 24, in which only the drying method was changed to an oven, the abrasion resistance was superior when hot air drying was used as the drying method. This is thought to be because hot air drying allowed the ink to dry in a shorter time than drying with an oven.

[0084] Furthermore, Examples 21, 23, 25, and 26 were prepared using the same ink 17, but with different drying temperatures. Comparing these examples, it was found that the higher the drying temperature, the better the abrasion resistance. In particular, it was found that the closer the temperature was to the minimum film-forming temperature (78°C) of the acrylic resin, which is the resin particle contained in ink 17, the better the abrasion resistance. The minimum film-forming temperature of the acrylic resin was measured using a minimum film-forming thermometer (MFFTB-90, manufactured by Rhopoint Instruments).

[0085] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. Modifications of the above embodiments are listed below.

[0086] (Variation 1) In the above embodiment, the time from the end of the adhesion step to the start of the drying step does not have to be 10 seconds or less.

[0087] (Variation 2) In the above-described embodiment, in the drying step, instead of blowing gas having the second temperature (that is, hot air drying), for example, gas may be heated to the second temperature (for example, drying in an oven).

[0088] (Variation 3) In the above embodiment, the second temperature may be lower than the minimum film-forming temperature of the resin particles minus 30 degrees.

[0089] (Variation 4) In the above-described embodiment, the image forming apparatus 10 does not have to include the thermal head 36. That is, the image forming method does not have to include a heating step of selectively heating the medium to a temperature equal to or higher than the first temperature after the drying step. Alternatively, the thermal head 36 may be provided in a separate image forming apparatus. That is, after an ink image is formed on the thermal paper 2 in the image forming apparatus 10, a thermal image may be further formed on the thermal paper 2 on which the ink image has been formed in the separate image forming apparatus. In this case, after the thermal paper 2 on which the ink image has been formed in the image forming apparatus 10 is attached to a product, a thermal image may be further formed on the thermal paper 2 attached to the product in the separate image forming apparatus.

[0090] Furthermore, the technical elements described in this specification or drawings may exhibit technical utility either alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings may achieve multiple objectives simultaneously, and achieving one of those objectives is itself technically useful.

[0091] In the scope of the claims at the time of filing, even if each claim depends on only some of the claims, it is not limited to the fact that each claim can depend on only those some of the claims. To the extent that there is no technical contradiction, each claim can also depend on other claims that were not dependent at the time of filing. In other words, the technology of each claim can be combined in various ways as follows: (Item 1) An image forming method, comprising: a heat-sensitive layer; and of a conveying step of conveying a medium having a coating layer, the heat-sensitive layer including a color developer and a leuco dye that reacts with the color developer at a first temperature, and the coating layer including a polymer; a deposition step of depositing a liquid on the coating layer of the medium being transported, the liquid including a first organic solvent and a second organic solvent; a drying step of drying the medium to which the liquid has been attached at a second temperature lower than the first temperature; Equipped with the solubility parameter of the leuco dye is 8.0 or more and 9.0 or less; the solubility parameter of the polymer is 11.0 or less; the solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less; the solubility parameter of the second organic solvent is 9.0 or more and less than 12.0; a content of the first organic solvent relative to the total amount of the liquid is greater than a content of the second organic solvent relative to the total amount of the liquid; the sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight; Image forming method. (Item 2) Item 2. The image forming method according to item 1, wherein the time from the end of the adhesion step to the start of the drying step is 10 seconds or less. (Item 3) 3. The image forming method according to item 1 or 2, wherein the ratio of the content of the second organic solvent to the content of the first organic solvent is 0.2 or less. (Item 4) 4. The image forming method according to any one of items 1 to 3, wherein a weighted average value of the solubility parameter of the first organic solvent and the solubility parameter of the second organic solvent is 15.1 or more. (Item 5) 5. The image forming method according to claim 1, wherein the liquid further contains resin particles. (Item 6) 6. The image forming method according to any one of items 1 to 5, wherein the liquid further contains a colorant. (Item 7) Item 6. The image forming method according to item 5, wherein the ratio of the content of the second organic solvent to the content of the resin particles is 0.4 or more. (Item 8) 8. The image forming method according to claim 1, wherein the drying step includes blowing a gas having the second temperature. (Item 9) 8. The image forming method according to item 5 or 7, wherein the second temperature is equal to or higher than the minimum film-forming temperature of the resin particles minus 30 degrees. (Item 10) The image forming method further comprises: 10. The image forming method according to any one of items 1 to 9, further comprising, after the drying step, a heating step of selectively heating the medium to a temperature equal to or higher than the first temperature. (Item 11) Item 11. The image forming method according to any one of items 1 to 10, wherein the evaporation rate of the first organic solvent and the evaporation rate of the second organic solvent are each slower than the evaporation rate of water. (Item 12) A liquid used to form an image on a medium, the medium includes a heat-sensitive layer and a coating layer provided on the heat-sensitive layer; the heat-sensitive layer includes a color developer and a leuco dye that reacts with the color developer at a first temperature; the coating layer contains a polymer, the liquid includes a first organic solvent and a second organic solvent; the solubility parameter of the leuco dye is 8.0 or more and 9.0 or less; the solubility parameter of the polymer is 11.0 or less; the solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less; the solubility parameter of the second organic solvent is 9.0 or more and less than 12.0; a content of the first organic solvent relative to the total amount of the liquid is greater than a content of the second organic solvent relative to the total amount of the liquid; the sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight; liquid. [Explanation of symbols]

[0092] 2: thermal paper, 4: first roll, 6: second roll, 8: nip, 10: image forming device, 12: housing, 14: first holder, 16: second holder, 18: first tensioner, 20: second tensioner, 22: first conveying roller pair, 24: second conveying roller pair, 26: intermediate tensioner, 28: control unit, 30: head, 32: nozzle, 34: drying device, 36: thermal head, 38: heating element

Claims

1. An image forming method, comprising: a conveying step of conveying a medium including a thermosensitive layer and a coating layer on the thermosensitive layer, the thermosensitive layer including a color developer and a leuco dye that reacts with the color developer at a first temperature, and the coating layer including a polymer; a deposition step of depositing a liquid on the coating layer of the medium being transported, the liquid including a first organic solvent and a second organic solvent; a drying step of drying the medium to which the liquid has been attached at a second temperature lower than the first temperature; Equipped with the solubility parameter of the leuco dye is 8.0 or more and 9.0 or less; the solubility parameter of the polymer is 11.0 or less; the solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less; the solubility parameter of the second organic solvent is 9.0 or more and less than 12.0; a content of the first organic solvent relative to the total amount of the liquid is greater than a content of the second organic solvent relative to the total amount of the liquid; the sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight; Image forming method.

2. 2. The image forming method according to claim 1, wherein the time from the end of the adhering step to the start of the drying step is 10 seconds or less.

3. 3. The image forming method according to claim 2, wherein the ratio of the content of the second organic solvent to the content of the first organic solvent is 0.2 or less.

4. 4. The image forming method according to claim 3, wherein a weighted average value of the solubility parameter of the first organic solvent and the solubility parameter of the second organic solvent is 15.1 or more.

5. The image forming method according to claim 4 , wherein the liquid further contains resin particles.

6. The image forming method according to claim 5 , wherein the liquid further contains a coloring material.

7. 7. The image forming method according to claim 6, wherein the ratio of the content of the second organic solvent to the content of the resin particles is 0.4 or more.

8. The image forming method according to claim 7 , wherein the drying step includes blowing gas having the second temperature.

9. 9. The image forming method according to claim 8, wherein the second temperature is equal to or higher than the minimum film-forming temperature of the resin particles minus 30 degrees.

10. The image forming method further comprises: The image forming method according to claim 1 , further comprising a heating step of selectively heating the medium to a temperature equal to or higher than the first temperature after the drying step.

11. 2. The image forming method according to claim 1, wherein the vapor pressure of the first organic solvent and the vapor pressure of the second organic solvent are each lower than the vapor pressure of water.

12. A liquid used to form an image on a medium, the medium includes a heat-sensitive layer and a coating layer provided on the heat-sensitive layer; the heat-sensitive layer includes a color developer and a leuco dye that reacts with the color developer at a first temperature; the coating layer contains a polymer, the liquid includes a first organic solvent and a second organic solvent; the solubility parameter of the leuco dye is 8.0 or more and 9.0 or less; the solubility parameter of the polymer is 11.0 or less; the solubility parameter of the first organic solvent is 12.0 or more and 20.0 or less; the solubility parameter of the second organic solvent is 9.0 or more and less than 12.0; a content of the first organic solvent relative to the total amount of the liquid is greater than a content of the second organic solvent relative to the total amount of the liquid; the sum of the content of the first organic solvent relative to the total amount of the liquid and the content of the second organic solvent relative to the total amount of the liquid is less than 25% by weight; liquid.

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