Thermal recording medium

Abstract

The present invention provides an environmentally friendly thermal recording medium that uses kraft lignin as a color developer and has excellent head corrosion resistance when printed. [Solution] The thermal recording medium has a support and a thermal recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting color developer, and the thermal recording layer contains kraft lignin as an electron-accepting color developer, and the kraft lignin contains almost no hydrogen sulfide or at most 400 ppm.

Description

[Technical field]

[0001] This invention relates to a thermal recording medium that utilizes the color-developing reaction between a colorless or light-colored electron-donating leuco dye (hereinafter also referred to as "leuco dye") and an electron-accepting developer (hereinafter also referred to as "developer"), and that uses kraft lignin as the developer. [Background technology]

[0002] Generally, a thermal recording medium is prepared by coating a support such as paper, synthetic paper, film, plastic, etc. with a coating liquid containing a colorless or light-colored leuco dye and a color developer, and a recorded image is obtained by instantaneous chemical reaction caused by heating with a thermal head, hot stamp, heat pen, laser light, etc. Thermal recording media are widely used as recording media for facsimiles, computer terminal printers, automatic ticket vending machines, measurement recorders, receipts at supermarkets and convenience stores, etc. Generally, the developer used is a bisphenol, an alkylphenol, a novolac type phenolic resin, an aromatic carboxylic acid derivative or a metal salt thereof, a hydroxybenzoic acid ester, a sulfonylurea compound, or an activated clay. In addition, in order to provide a thermal recording medium using an environmentally friendly color developer, it is known to use isolated lignin as a color developer (Patent Document 1, etc.). Also, several measures are known against corrosion of the thermal head used in printing on the thermal recording medium (Patent Documents 2 to 4, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2020-104274 [Patent Document 2] Patent Publication No. 62-87362 [Patent Document 3] Patent Publication No. 10-264425 [Patent Document 4] Patent Publication No. 2000-79715 Summary of the Invention [Problem to be solved by the invention]

[0004] Among the printers for thermal recording paper, the thermal head is composed of a glaze layer, a heating element, electrodes, and a protective film on a base. Aluminum is generally used for the electrodes, which are known to corrode due to ionic substances such as sodium ions, chloride ions, and sulfide ions. If the electrodes corrode, printing will not be possible. On the other hand, when kraft lignin is used as a color developer to provide an environmentally friendly thermal recording medium, it has been found that, depending on the conditions of use, there is a problem of corrosion of the thermal head used for printing on the thermal recording medium (see Comparative Example 1, etc., described below). However, only countermeasures from the printing device side have been known so far regarding the corrosion of the thermal head (Patent Documents 2 to 4, etc.). An object of the present invention is to provide an environmentally friendly thermal recording medium which uses kraft lignin as a color developer and which exhibits excellent head corrosiveness when printed. [Means for solving the problem]

[0005] As a result of intensive research aimed at solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by using kraft lignin that is produced by adding an oxidation step to the production process, unlike conventional methods, and as a result has a reduced hydrogen sulfide content, thereby completing the present invention. That is, the present invention provides a thermosensitive recording medium having a support and a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer, the thermosensitive recording layer containing kraft lignin as an electron-accepting developer, the kraft lignin containing almost no hydrogen sulfide or containing at most 400 ppm of hydrogen sulfide. The present invention also provides a method for reducing corrosion of a thermal head when printing with a thermal head on a thermal recording medium whose thermal recording layer contains kraft lignin, the method including a step of printing on the thermal recording medium. Effect of the Invention

[0006] According to the present invention, it is possible to provide a thermal recording medium which uses kraft lignin as a color developer and which is excellent in thermal head corrosion resistance during printing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Natural lignin in plant cell walls is complexed with polysaccharides such as cellulose, making it difficult to isolate. Therefore, it is usually isolated by chemical modification. Isolated lignin is also called industrial lignin, and includes lignosulfonates, kraft lignin, soda lignin, soda-anthraquinone lignin, organolignin, explosive lignin, and sulfuric acid lignin. In the thermal recording medium of the present invention, kraft lignin is used as a color developer. Kraft lignin is also called thiolignin or sulfate lignin. Kraft lignin is a compound with hydroxyphenylpropane as a base unit and functional groups such as phenolic hydroxyl groups, alcoholic hydroxyl groups, and thiol groups. Kraft lignin is usually obtained by reacting a mixed aqueous solution of sodium hydroxide and sodium sulfide with wood, and is insoluble in water. As the form of use of kraft lignin, an alkaline solution of kraft lignin, powdered kraft lignin obtained by spray-drying an alkaline solution of kraft lignin to obtain powder, and acid-precipitated kraft lignin obtained by precipitating an alkaline solution of kraft lignin with an acid can be used.

[0008] The kraft lignin of the present invention is produced, for example, as described in JP-A-2013-527339 and obtained by a black liquor oxidation process. When refining kraft lignin, the black liquor is usually neutralized with carbon dioxide and filtered to purify the carbonic acid neutralized lignin, redispersed, extracted with sulfuric acid, and filtered and washed. The kraft lignin used in the present invention has an extremely low hydrogen sulfide content due to the addition of a process of oxidation with oxygen before the carbonic acid neutralization process. Such kraft lignin contains almost no hydrogen sulfide or at most 400 ppm, and even if it does contain hydrogen sulfide, the amount is preferably 200 ppm or less, more preferably 100 ppm or less. The lower limit is usually 50 ppm or less, for example about 10 ppm. The content of hydrogen sulfide in kraft lignin is measured, for example, by measuring the amount of hydrogen sulfide generated when kraft lignin is heated (200° C.) using a hydrogen sulfide concentration meter. The melting point of kraft lignin is about 170° C., and it can be measured by heating at 200° C. Moreover, the lignin content in such kraft lignin is preferably 90% by weight or more, and more preferably 92 to 100% by weight. The density of such kraft lignin at a concentration of 60% is preferably 0.4 to 0.5 g / cm 3 It is. By keeping the lignin content within the above range, the content of impurities such as sulfides is reduced, and the impact on the corrosion of the thermal head is reduced. In addition, compared to a density below the above range, the work efficiency during chemical dispersion is improved, and the amount of lignin required to achieve the same dispersion concentration can be reduced. An example of such kraft lignin is Amallin A, a product manufactured by West Fraser.

[0009] In the present invention, the above-mentioned kraft lignin is used as a color developer, but a color developer other than kraft lignin may be used in combination, which can bring about more excellent effects in terms of performance such as color development performance (print density). Examples of such color developers include inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate, 4,4'-isopropylidenediphenol, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenyl sulfide, hydroquinone monobenzyl ether, 4-hydroxybenzoate benzyl, 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl) sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl Sulfone, 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane, phenol condensation composition described in JP-A-2003-154760, aminobenzenesulfonamide derivative described in JP-A-8-59603, bis(4-hydroxyphenylthioethoxy)methane, 1,5-di(4-hydroxyphenylthio)-3-oxapentane, bis(p-hydroxyphenyl)butyl acetate, bis(p-hydroxyphenyl)methyl acetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, di(4-hydroxy-3-methylphenyl)sulfide, 2,2'-thiobis(3-tert-octylphenol), 2,Phenolic compounds such as 2'-thiobis(4-tert-octylphenol), diphenylsulfone-bridged compounds described in WO97 / 16420, compounds described in WO02 / 081229 or JP2002-301873A, N-(2-(3-phenylureido)phenyl)benzenesulfonamide, 3-(3-tosylureido)phenyl-p-toluenesulfonate, urea urethane compounds, 3-{[(phenylamino)carbonyl]amino}benzenesulfonamide, thiourea compounds such as N,N'-di-m-chlorophenylthiourea, p-chlorobenzoic acid, stearyl gallate, etc. Examples of the color developers include aromatic carboxylic acids such as bis[zinc 4-(n-octyloxycarbonylamino)salicylate] dihydrate, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, and 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, and salts of these aromatic carboxylic acids with polyvalent metals such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, and nickel, as well as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids. These color developers can be used alone or in combination of two or more kinds. In addition, metal chelate-type color-developing components such as higher fatty acid metal double salts and polyvalent hydroxy aromatic compounds described in JP-A-10-258577 can also be included.

[0010] Among these color developers, 4,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, or a color developer having a urea structure (-NHCONH-) is preferred as a color developer to be used in combination with kraft lignin, since it provides a good balance between color development performance (print density), storage stability, etc. Examples of color developers having a urea structure include the following color developers. i) N-(2-(3-phenylureido)phenyl)benzenesulfonamide (NKK1304, manufactured by Nippon Soda Co., Ltd., formula below) [ka] ii) A urea compound represented by the following general formula (Chemical Formula 2): [ka] In the general formula (Chemical Formula 2), R represents an alkyl group or an alkoxy group, preferably an alkyl group, and m represents an integer of 0 to 3, preferably 0 to 2, more preferably 0 to 1. The number of carbon atoms in this alkyl group is, for example, 1 to 12, preferably 1 to 8, more preferably 1 to 4. The positions of R in the benzene ring of the general formula (Chemical Formula 2) may be the same or different, and are preferably the 3-position, 4-position or 5-position, and more preferably the 4-position. An example of the urea compound is N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea.

[0011] iii) 3-(3-tosylureido)phenyl-p-toluenesulfonate (BASF, DP201, formula below) [ka] iv) Urea urethane compound (UU manufactured by Chemipro Chemical Co., Ltd., formula below) [ka] v) 3-{[(phenylamino)carbonyl]amino}benzenesulfonamide (SU727, manufactured by Mitsubishi Chemical Corporation, formula below) [ka]

[0012] In the present invention, the content (solid content) of kraft lignin in the thermosensitive recording layer is preferably 0.5 to 20% by weight, more preferably 1 to 15% by weight, and further preferably 2 to 12% by weight. In the present invention, when kraft lignin and a color developer other than kraft lignin are used in combination as a color developer, the weight ratio of the kraft lignin to the color developer other than kraft lignin (kraft lignin / color developer other than kraft lignin) is preferably 90 / 10 to 10 / 90, more preferably 80 / 20 to 20 / 80, and even more preferably 60 / 40 to 25 / 75.

[0013] The thermosensitive recording medium of the present invention can utilize the conventional structure except for the use of kraft lignin as a color developer. That is, the thermosensitive recording medium of the present invention essentially has a thermosensitive recording layer on a support, and may optionally have a protective layer on the thermosensitive recording layer, an undercoat layer between the support and the thermosensitive recording layer, or a backcoat layer on the opposite side of the support to the thermosensitive recording layer. In addition, a coating layer appropriately configured according to the purpose of use may be provided between these layers. The support can be appropriately selected from conventionally known supports such as paper, recycled paper, synthetic paper, film, plastic film, foamed plastic film, nonwoven fabric, etc., depending on the desired quality of the thermal recording medium, and is not particularly limited. A composite sheet combining these may also be used as the support.

[0014] The heat-sensitive recording layer of the present invention essentially contains a leuco dye in addition to the above-mentioned color developer, and may further contain, optionally, a sensitizer, a binder, a pigment, a crosslinking agent, an image stabilizer, and other components.

[0015] The leuco dye used in the present invention can be any known leuco dye in the field of conventional pressure-sensitive or heat-sensitive recording paper, and is not particularly limited, but triphenylmethane compounds, fluoran compounds, fluorene compounds, divinyl compounds, etc. are preferred. Specific examples of representative colorless or light-colored leuco dyes (dye precursors) are shown below. These leuco dyes may be used alone or in combination of two or more kinds.

[0016] <Triphenylmethane leuco dye> 3,3-Bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (also known as crystal violet lactone); 3,3-Bis(p-dimethylaminophenyl)phthalide (also known as malachite green lactone)

[0017] <Fluoran-based leuco dye> 3-Diethylamino-6-methylfluoran; 3-Diethylamino-6-methyl-7-anilinofluoran; 3-Diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran; 3-Diethylamino-6-methyl-7-chlorofluoran; 3-Diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran; 3-Diethylamino-6-methyl-7-(o-chloroanilino)fluoran; 3-Diethylamino-6-methyl-7-(p-chloroanilino)fluoran; 3-Diethylamino-6-methyl-7-(o-fluoroanilino)fluoran; 3-Diethylamino-6-methyl-7-(m-methylanilino)fluoran; 3-Diethylamino-6-methyl-7-n-octylanilinofluoran; 3-Diethylamino-6-methyl-7-n-octylaminofluoran; 3-Diethylamino-6-methyl-7-benzylaminofluoran; 3-Diethylamino-6-methyl-7-dibenzylaminofluoran; 3-Diethylamino-6-chloro-7-methylfluoran; 3-Diethylamino-6-chloro-7-anilinofluoran; 3-Diethylamino-6-chloro-7-p-methylanilinofluoran; 3-Diethylamino-6-ethoxyethyl-7-anilinofluoran; 3-Diethylamino-7-methylfluoran; 3-Diethylamino-7-chlorofluoran; 3-Diethylamino-7-(m-trifluoromethylanilino)fluoran; 3-Diethylamino-7-(o-chloroanilino)fluoran; 3-Diethylamino-7-(p-chloroanilino)fluoran; 3-Diethylamino-7-(o-fluoroanilino)fluoran; 3-Diethylamino-benzo(a)fluoran; 3-Diethylamino-benzo(c)fluoran; 3-Dibutylamino-6-methyl-fluoran; 3-Dibutylamino-6-methyl-7-anilinofluoran; 3-Dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran; 3-Dibutylamino-6-methyl-7-(o-chloroanilino)fluoran; 3-Dibutylamino-6-methyl-7-(p-chloroanilino)fluoran; 3-Dibutylamino-6-methyl-7-(o-fluoroanilino)fluoran;3-Dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran; 3-Dibutylamino-6-methyl-chlorofluoran; 3-Dibutylamino-6-ethoxyethyl-7-anilinofluoran; 3-Dibutylamino-6-chloro-7-anilinofluoran; 3-Dibutylamino-6-methyl-7-p-methylanilinofluoran; 3-Dibutylamino-7-(o-chloroanilino)fluoran; 3-Dibutylamino-7-(o-fluoroanilino)fluoran; 3-Di-n-pentylamino-6-methyl-7-anilinofluoran; 3-Di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluoran; 3-Di-n-pentylamino-7-(m-trifluoromethylanilino)fluoran; 3-Di-n-pentylamino-6-chloro-7-anilinofluoran; 3-Di-n-pentylamino-7-(p-chloroanilino)fluoran; 3-Pyrrolidino-6-methyl-7-anilinofluoran; 3-Piperidino-6-methyl-7-anilinofluoran; 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran; 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-xylamino)-6-methyl-7-(p-chloroanilino)fluoran; 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran; 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran; 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran; 3-cyclohexylamino-6-chlorofluoran; 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran; 2-(4-oxahexyl)-3-diethylamino-6-methyl-7-anilinofluoran;2-(4-Oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran; 2-Methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran; 2-Methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran; 2-Chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran; 2-Chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran; 2-Nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran; 2-Amino-6-p-(p-diethylaminophenyl)aminoanilinofluoran; 2-Diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran; 2-Phenyl-6-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran; 2-benzyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran; 2-hydroxy-6-p-(p-phenylaminophenyl)aminoanilinofluoran; 3-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran; 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran; 3-diethylamino-6-p-(p-dibutylaminophenyl)aminoanilinofluoran; 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran;

[0018] <Fluorene-based leuco dye> 3,6,6'-Tris(dimethylamino)spiro〔fluorene-9,3'-phthalide〕; 3,6,6'-Tris(diethylamino)spiro〔fluorene-9,3'-phthalide〕 <Divinyl leuco dye> 3,3-Bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrabromophthalide; 3,3-Bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide; 3,3-Bis-[1,1-bis(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrabromophthalide; 3,3-Bis-[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide

[0019] <Other> 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide; 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-azaphthalide; 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide; 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide; 3,6-bis(diethylamino)fluoran-gamma-(3'-nitro)anilinolactam; 3,6-bis(diethylamino)fluoran-gamma-(4'-nitro)anilinolactam; 1,1-Bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-dinitrileethane; 1,1-Bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2-β-naphthoylethane; 1,1-Bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-diacetylethane; Bis-[2,2,2',2'-tetrakis-(p-dimethylaminophenyl)-ethenyl]-methylmalonic acid dimethyl ester

[0020] Examples of sensitizers that can be used in the present invention include diphenylsulfone, fatty acid amides such as stearic acid amide and palmitic acid amide, benzyloxynaphthalene, 1,2-di-(3-methylphenoxy)ethane, di(p-methylbenzyl) oxalate, etc. These sensitizers may be used alone or in combination of two or more kinds. Of these sensitizers, it is preferable to use diphenyl sulfone in order to compensate for the color development performance (print density) when kraft lignin is used as the color developer.

[0021] Examples of binders that can be used in the present invention include polyvinyl alcohols such as fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, olefin-modified polyvinyl alcohol, nitrile-modified polyvinyl alcohol, pyrrolidone-modified polyvinyl alcohol, silanol-modified polyvinyl alcohol, cation-modified polyvinyl alcohol, and terminal alkyl-modified polyvinyl alcohol; cellulose ethers such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, and acetyl cellulose, and derivatives thereof; starch, enzyme-modified starch, thermochemically modified starch, oxidized starch, esterified starch, etherified starch (e.g., hydroxyethylated starch), and cationized starch. Examples of the starches include polyacrylamides, cationic polyacrylamides, anionic polyacrylamides, amphoteric polyacrylamides, and other polyacrylamides, urethane resins such as polyester polyurethane resins, polyether polyurethane resins, and polyurethane ionomer resins, acrylic resins consisting of (meth)acrylic acid and monomer components (excluding olefins) copolymerizable with (meth)acrylic acid, styrene-butadiene copolymers, styrene-butadiene-acrylonitrile copolymers, and styrene-butadiene-acrylic copolymers, and other styrene-butadiene resins, polyvinyl acetate, vinyl chloride-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, and other polyolefin resins, polyvinyl chloride, polyvinylidene chloride, polyacrylic esters, gum arabic, polyvinyl butyral, polystyrene and copolymers thereof, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These may be used alone or in combination of two or more.

[0022] Examples of pigments that can be used in the present invention include inorganic or organic fillers such as silica, calcium carbonate, kaolin, calcined kaolin, diatomaceous earth, talc, titanium oxide, aluminum hydroxide, etc. These may be used alone or in combination of two or more.

[0023] Examples of crosslinking agents that can be used in the present invention include glyoxal, methylol melamine, melamine formaldehyde resin, melamine urea resin, polyamine epichlorohydrin resin, polyamide epichlorohydrin resin, potassium persulfate, ammonium persulfate, sodium persulfate, ferric chloride, magnesium chloride, borax, boric acid, alum, and ammonium chloride agents.

[0024] In addition, in the present invention, 4,4'-butylidene (6-t-butyl-3-methylphenol), 2,2'-di-t-butyl-5,5'-dimethyl-4,4'-sulfonyldiphenol, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 4-benzyloxy-4'-(2,3-epoxy-2-methylpropoxy)diphenylsulfone, etc. can also be used as image stabilizers that exhibit oil resistance effects and the like of recorded images within a range that does not impair the desired effects for the above-mentioned problems. In addition, benzophenone-based or triazole-based ultraviolet absorbents, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc. may be used.

[0025] In the present invention, the developer is used in an amount of preferably 0.05 to 4.0 parts by weight, more preferably 0.1 to 3.0 parts by weight, per 1 part by weight of the leuco dye. The types and amounts of sensitizer, binder, pigment, crosslinker, image stabilizer, and other optional components are determined according to the required performance and recording suitability and are not particularly limited, but usually, 0.1 to 10 parts by weight of sensitizer, 0.5 to 50 parts by weight of pigment, 0.01 to 10 parts by weight of image stabilizer, and 0.01 to 10 parts by weight of other components are used, and the binder is suitably about 5 to 50 parts by weight in solid content per 100 parts by weight of the thermal recording layer (solid content). The content of the lubricant is suitably about 5 to 10 parts by weight in solid content per 100 parts by weight of the thermal recording layer (solid content).

[0026] The leuco dye, developer, and materials added as necessary are pulverized to a particle size of several microns or less using a grinding machine such as a ball mill, attritor, or sand grinder, or an appropriate emulsifying device, and a binder and various additives depending on the purpose are added to make a coating liquid. Water or alcohol can be used as the solvent for this coating liquid, and the solid content is about 20 to 40% by weight.

[0027] The protective layer can contain the binder, pigment, crosslinking agent, and other components that can be used in the above-mentioned thermosensitive recording layer, provided that the desired effects are not impaired. It is preferable for the protective layer to contain a binder and a pigment, and it may contain other components such as a surfactant and a viscosity adjuster as necessary.

[0028] As the binder used in the protective layer, polyvinyl alcohols and acrylic resins are preferred among the binders usable in the heat-sensitive recording layer described above. As the polyvinyl alcohols, fully saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, and acetoacetyl-modified polyvinyl alcohol are preferred. Examples of components (excluding olefins) that can be copolymerized with (meth)acrylic acid in acrylic resins include alkyl acrylate resins such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate, as well as epoxy resins, silicone resins, and modified alkyl acrylate resins such as alkyl acrylate resins modified with styrene or a derivative thereof, (meth)acrylonitrile, acrylic esters, and hydroxyalkyl acrylates. Examples of the alkyl acrylate ester include alkyl acrylate resins such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate, and epoxy resins, silicone resins, and modified alkyl acrylate resins such as alkyl acrylate resins modified with styrene or its derivatives, (meth)acrylonitrile, acrylic esters, and hydroxyalkyl acrylic esters. Particularly, (meth)acrylonitrile and / or methyl methacrylate are preferred. The acrylic resin is preferably a non-core-shell type acrylic resin.

[0029] In order to improve water resistance and the like, the protective layer may contain a carboxyl group-containing resin as a binder, and may further contain a polyamine / polyamide resin. Examples of the carboxyl group-containing resin include the above-mentioned carboxy-modified polyvinyl alcohol, acrylic resins, oxidized starch, and carboxymethyl cellulose. Examples of the polyamine / polyamide resin include polyamide urea resin, polyalkylene polyamine resin, polyalkylene polyamide resin, polyamine polyurea resin, modified polyamine resin, modified polyamide resin, polyalkylene polyamine urea formalin resin, polyalkylene polyamine polyamide polyurea resin, and the like.

[0030] In the present invention, when the protective layer contains a pigment, the water resistance and print running properties of the thermal recording medium are improved, and therefore, as the pigment, silica, kaolin, calcined kaolin, and aluminum hydroxide are preferred, and kaolin and aluminum hydroxide are more preferred. When the protective layer of the present invention does not contain a pigment, the blending amount of the binder in the protective layer is usually 70 to 100% by weight, preferably 85 to 100% by weight, in terms of solid content. On the other hand, when the protective layer contains a pigment, the total amount of the binder and pigment in the protective layer is usually 80 to 100% by weight, preferably 90 to 100% by weight, in terms of solid content, and it is preferable that the amount of the binder is about 30 to 300 parts by weight per 100 parts by weight of the pigment. The content of each of the components other than the binder, crosslinker and pigment in the protective layer does not exceed 15% by weight, preferably 10% by weight.

[0031] Each coating layer that is provided arbitrarily other than the heat-sensitive recording layer and the protective layer may contain the above-mentioned binders, pigments, crosslinking agents and other components within the range that does not impair the desired effects.

[0032] The coating method for the thermal recording layer, protective layer, and other coating layers is not particularly limited, and any conventionally known method can be used, such as curtain coating, air knife coating, bar blade coating, rod blade coating, bent blade coating, bevel blade coating, roll coating, or spray coating.

[0033] The coating weight of the thermal recording layer, protective layer, and other coating layers is determined according to the required performance and recording suitability, and is not particularly limited. For example, the general coating weight of the thermal recording layer is 2 to 12 g / m2 in terms of solid content. 2 The typical coating weight of the protective layer is 1 to 5 g / m2 in solids. 2 That's about it. Furthermore, after each layer is applied, various known techniques in the field of thermal recording media, such as smoothing treatment by supercalendering, may be applied as necessary. EXAMPLES

[0034] The present invention will be illustrated by the following examples, but is not intended to limit the present invention. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified. Each dispersion and coating solution was prepared as follows. The following mixture was stirred and dispersed to prepare a coating liquid for the undercoat layer. <Coating fluid for undercoat layer> Calcined kaolin (BASF product name: Ansilex 90) 100.0 copies Styrene-butadiene copolymer latex (manufactured by Zeon Corporation, Product name: ST5526, solid content 48%) 10.0 parts Water 50.0 parts

[0035] The kraft lignin dispersions (liquids A and B) were prepared by wet grinding with a sand grinder until the average particle size was 3.0 μm. Note that the kraft lignin used in liquid A of the kraft lignin dispersion went through a black liquor oxidation process in the manufacturing process, while liquid B did not go through an oxidation process.

[0036] Kraft lignin dispersion (liquid A) Kraft lignin (West Fraser, Amallin A, hydrogen sulfide content: 68 ppm, lignin content: 93%, density: 0.45 g / cm 3 6.0 parts of kraft lignin Completely saponified polyvinyl alcohol aqueous solution (Kuraray Co., Ltd., Product name: PVA117, solid content 10%) 5.0 parts 1.5 parts water Kraft lignin dispersion (liquid B) Kraft lignin (Meadwestbaco, Indulin AT, hydrogen sulfide content: 1100 ppm, lignin content: 87%, density: 0.35 g / cm 3(hereinafter referred to as "Kraft Lignin 2") 6.0 copies Completely saponified polyvinyl alcohol aqueous solution (Kuraray Co., Ltd., Product name: PVA117, solid content 10%) 5.0 parts 1.5 parts water

[0037] Next, developer dispersions (Liquids A1 to A4), leuco dye dispersion (Liquid B), and sensitizer dispersion (Liquid C) having the following compositions were prepared by wet grinding separately in a sand grinder until the average particle size became 0.5 μm. Developer dispersion liquid (A1 liquid) 4-Hydroxy-4'-isopropoxydiphenyl sulfone (Mitsubishi Chemical Corporation, product name: NYDS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water Developer dispersion liquid (A2 liquid) N,N'-Di-[3-(p-toluenesulfonyloxy)phenyl]urea (hereinafter referred to as "Urea Compound 1") 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water

[0038] Developer dispersion liquid (A3 liquid) 4,4'-Dihydroxydiphenyl sulfone (Konishi Chemical Industry Co., Ltd., product name: 44BPS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water Developer dispersion liquid (A4 liquid) N-(2-(3-phenylureido)phenyl)benzenesulfonamide (Nippon Soda Co., Ltd., product name: NKK1304) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water

[0039] Leuco dye dispersion (liquid B) 3-Dibutylamino-6-methyl-7-anilinofluoran (Yamamoto Chemical Industry Co., Ltd., product name: ODB-2) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water Sensitizer dispersion (liquid C) Diphenyl sulfone (Volant, trade name: DPS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water

[0040] Next, the dispersions were mixed in the following ratio and thoroughly stirred to prepare coating solutions 1 and 2 for the thermal recording layer. <Coating solution for thermal recording layer 1> Kraft lignin dispersion (liquid A) 1.5 parts Leuco dye dispersion (liquid B) 1.05 parts Sensitizer dispersion (liquid C) 1.2 parts Silica dispersion (manufactured by Mizusawa Chemical Industry Co., Ltd., product name: Mizukasil P-537 , solids content 25%) 5.0 parts Zinc stearate (manufactured by Chukyo Yushi Co., Ltd., product name: Hydrin L536, Solid content 40%) 5.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 20.0 parts

[0041] <Coating solution for thermal recording layer 2> Kraft lignin dispersion (liquid A) 0.75 parts Developer dispersion liquid (A1 liquid) 0.75 parts Leuco dye dispersion (liquid B) 1.05 parts Sensitizer dispersion (liquid C) 1.2 parts Silica dispersion (Mizukasil P-537) 5.0 parts Zinc stearate (Hydrin L536) 5.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 20.0 parts

[0042] [Example 1] Support (basis weight 47g / m 2 The coating solution for the undercoat layer is applied to one side of the paper (high-quality paper) at a coating weight of 10.0 g / m2 (solid content). 2 After coating by the bent blade method so that the above-mentioned coating was performed, the coated paper was dried to obtain a coated paper with an undercoat layer. On the undercoat layer of this undercoat layer coated paper, coating solution 1 for the thermal recording layer (craft lignin in the thermal recording layer = 10.0%, developer / leuco dye = 1.43) was applied in a coating amount of 6.0 g / m2 in terms of solid content. 2 After coating by the rod blade method so as to obtain the above-mentioned coating composition, the coating was dried and then processed with a super calendar so as to obtain a smoothness of 500 to 1000 seconds to prepare a thermal recording medium. [Example 2] A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the thermosensitive recording layer coating solution 1 was replaced with the thermosensitive recording layer coating solution 2 (kraft lignin in the thermosensitive recording layer=5.0%, developer / leuco dye=1.43).

[0043] [Example 3] A thermosensitive recording medium was prepared in the same manner as in Example 2, except that in the thermosensitive recording layer coating solution 2, the developer dispersion liquid (A1 liquid) was replaced with the developer dispersion liquid (A2 liquid). [Example 4] A thermosensitive recording medium was prepared in the same manner as in Example 2, except that in the thermosensitive recording layer coating solution 2, the developer dispersion liquid (A1 liquid) was replaced with the developer dispersion liquid (A3 liquid). [Example 5] A thermosensitive recording medium was prepared in the same manner as in Example 2, except that in the thermosensitive recording layer coating solution 2, the developer dispersion liquid (A1 liquid) was replaced with the developer dispersion liquid (A4 liquid).

[0044] [Comparative Example 1] A thermosensitive recording medium was produced in the same manner as in Example 1, except that in the thermosensitive recording layer coating solution 1, the kraft lignin dispersion solution (solution A) was replaced with the kraft lignin dispersion solution (solution B). [Comparative Example 2] A thermosensitive recording medium was produced in the same manner as in Example 2, except that in the thermosensitive recording layer coating solution 2, the kraft lignin dispersion (solution A) was replaced with the kraft lignin dispersion (solution B). [Comparative Example 3] A thermosensitive recording medium was produced in the same manner as in Example 3, except that in the thermosensitive recording layer coating solution 2, the kraft lignin dispersion (solution A) was replaced with the kraft lignin dispersion (solution B). [Comparative Example 4] A thermosensitive recording medium was prepared in the same manner as in Example 4, except that in the thermosensitive recording layer coating solution 2, the kraft lignin dispersion (solution A) was replaced with the kraft lignin dispersion (solution B). [Comparative Example 5] A thermosensitive recording medium was prepared in the same manner as in Example 5, except that in the thermosensitive recording layer coating solution 2, the kraft lignin dispersion (solution A) was replaced with the kraft lignin dispersion (solution B).

[0045] The prepared thermosensitive recording medium was evaluated as follows. <Color development performance (print density)> A checkered pattern was printed on the prepared thermal recording medium using a thermal printer (TH-PMD) manufactured by Ohkura Electric Co., Ltd. at applied energies of 0.27 mJ / dot and 0.35 mJ / dot and a printing speed of 50 mm / sec. The density of the printed area was measured using a Macbeth densitometer (RD-914, amber filter used). Table 1 below shows the density of the printed area for each applied energy. <Head corrosion> A solid pattern was printed on the prepared thermal recording medium using a thermal printer (TH-PMD) manufactured by Okura Electric Co., Ltd. under conditions of 23°C x 50% Rh and 40°C x 90% Rh at 0.35 mJ / dot and a printing speed of 50 mm / sec. After printing for 50 km, it was confirmed whether or not white streaks occurred in the image due to poor heating caused by thermal head corrosion, and head corrosion was evaluated according to the following criteria. ○: No occurrence ×: Occurs

[0046] The evaluation results are shown in the table below. The content of the color developer indicates the content (solid content, weight %) of the color developer in the thermosensitive recording layer. [Table 1] From this table, it is clear that the thermal recording medium using kraft lignin containing almost no hydrogen sulfide or containing only 400 ppm at most is excellent in head corrosiveness.

Claims

1. A thermosensitive recording medium comprising a support and a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer, the thermosensitive recording layer containing kraft lignin as the electron-accepting developer, the kraft lignin not containing hydrogen sulfide or containing at most 400 ppm of hydrogen sulfide.

2. 2. The thermosensitive recording medium according to claim 1, wherein the kraft lignin has a lignin content of 90% or more.

3. The density of the kraft lignin is 0.4 to 0.5 g / cm ３ 2. The thermosensitive recording medium according to claim 1,

4. 2. The thermosensitive recording medium according to claim 1, wherein the content (solid content) of kraft lignin in the thermosensitive recording layer is 0.5 to 20% by weight.

5. The thermosensitive recording medium according to any one of claims 1 to 4, wherein the thermosensitive recording layer contains an electron accepting developer other than the kraft lignin, and the electron accepting developer other than the kraft lignin is 4,4'-dihydroxydiphenylsulfone, 4-hydroxy-4'-isopropoxydiphenylsulfone, or a developer having a urea structure (-NHCONH-).

6. 6. The heat-sensitive recording medium according to claim 5, wherein the weight ratio of the developer to the electron-donating leuco dye is from 0.05 to 4.

0.

7. A method for reducing corrosion of a thermal head when printing with a thermal head on a thermal recording medium having a thermal recording layer containing kraft lignin, the method comprising a step of printing on the thermal recording medium according to any one of claims 1 to 4.

8. A method for reducing corrosion of a thermal head when printing with a thermal head on a thermal recording medium having a thermal recording layer containing kraft lignin, the method comprising the step of printing on the thermal recording medium according to claim 5.

9. 7. A method for reducing corrosion of a thermal head when printing with a thermal head on a thermal recording medium having a thermal recording layer containing kraft lignin, the method comprising the step of printing on the thermal recording medium according to claim 6.