JP2024085274A - Thermal recording medium - Google Patents

Abstract

[Problem] To provide a highly transparent thermosensitive recording medium using a paper base material, with the aim of reducing the environmental load and the amount of plastic used. [Solution] The thermosensitive recording medium has a base paper, a resin layer on at least one side of the base paper from the side closest to the base paper, and a thermosensitive recording layer containing a leuco dye and a color developer, and has a HAZE value of 80% or less as measured according to JIS K7136, and the resin layer is laminated by a melt extrusion method, or the base paper and the resin layer are bonded together using an adhesive. [Selected Figure] None

Description

The present invention relates to a thermal recording medium that utilizes a color-developing reaction between a leuco dye and a color developer.

Thermal recording media that utilize the color-developing reaction between leuco dyes and color developers are relatively inexpensive, the recording equipment is compact, and maintenance is easy, so they are widely used not only as recording media for facsimiles, various calculators, CAD plotters, etc., but also for food applications such as fresh foods, boxed lunches, and prepared dishes, medical applications such as labels for drug management, and labels for process management.

When used for food applications such as labels, top seals, and bands on transparent containers for salads and prepared foods, a highly transparent medium is often required so that the contents can be clearly seen. In such cases, a transparent thermal film based on a transparent PET film is generally used instead of a paper base.

In response to recent calls to reduce the environmental impact, there has been a move to switch some display labels from PET film to paper in order to reduce the amount of plastic used, but there remains a high demand for high transparency to improve the visibility of the contents.

As such a thermal recording medium, Patent Document 1 reports a thermal recording paper in which an organic solvent coating liquid in which a resin is dissolved is applied to one or both sides of glassine paper and then dried, and then a thermal recording layer containing a leuco dye and a color developer is provided on at least one side, and the organic solvent coating liquid contains 3 to 35% by weight of a pigment relative to the total solid content of the resin.

Patent Document 2 also discloses that, regarding a recording material having a recording layer provided on a support, the support is a support having a plastic film layer provided by a melt extrusion method on at least the surface of the base paper on which the recording layer is to be provided.

Furthermore, Patent Document 3 reports a gas barrier laminate formed by laminating a resin layer and a paper base layer, the paper base layer having a paper base material, the paper base material having a basis weight of 20 g/m ² or more and 40 g/m ² or less, and the total light transmittance of the paper base material being 70% or more, the ratio of the basis weight of the resin layer to the basis weight of the paper base material layer (resin layer/paper base material layer) being 1.00 or less, a heat seal layer containing a heat seal agent, a resin layer, and a paper base material layer being laminated in this order, the resin constituting the resin layer being at least one selected from the group consisting of polyolefin-based resins, polyester-based resins, polyamide-based resins, and biodegradable resins, and the paper base material layer and the resin layer being laminated by extrusion lamination, or the paper base material layer and the resin layer being bonded together using an adhesive.

Japanese Patent Application Laid-Open No. 8-142509 Japanese Patent Application Laid-Open No. 10-010675 Patent No. 7095792

The present invention aims to provide a highly transparent thermal recording medium using a paper base material, with the aim of reducing the environmental impact and the amount of plastic used.

In the present invention, the inventors found that the above-mentioned problems could be solved by using highly transparent glassine paper as the base paper, forming a resin layer on at least one side of the glassine paper by melt extrusion or by bonding the base paper and the resin layer with an adhesive, and then providing a thermal recording layer and a protective layer on the resin layer, in that order, and thus completed the present invention. That is, the present invention relates to the following thermal recording medium.

Item 1. A thermosensitive recording medium having a base paper, a resin layer on at least one surface of the base paper from the side closest to the base paper, and a thermosensitive recording layer containing a leuco dye and a color developer,
The HAZE value measured according to JIS K7136 is 80% or less,
The thermosensitive recording medium is formed by laminating the resin layer by a melt extrusion method, or by bonding a base paper and a resin layer together with an adhesive.
Item 2. The thermosensitive recording material according to item 1, further comprising a protective layer mainly containing an adhesive on the thermosensitive recording layer on the side opposite to the resin layer.
Item 3. The thermosensitive recording material according to item 1 or 2, which has an oxygen permeability of 10 mL/( ^m2 ·24 h·atm) or less at 23° C. and 50% RH as measured according to JIS K7126.
Item 4. The thermosensitive recording medium according to any one of Items 1 to 3, which has a water vapor transmission rate of 50 g/( ^m2 ·24 h) or less under conditions of 40° C. and 90% RH as measured according to JIS Z 0208.
Item 5. The thermal recording medium according to any one of Items 1 to 4, wherein the base paper is glassine paper and has a basis weight of 50 g/ ^m2 or less.
Item 6. The thermosensitive recording medium according to any one of Items 1 to 5, wherein the resin constituting the resin layer is at least one selected from the group consisting of polyethylene resin, polypropylene resin, polystyrene resin, polyethylene terephthalate resin, and biodegradable resin.
Item 7. The thermosensitive recording medium according to Item 6, wherein the biodegradable resin is at least one selected from the group consisting of polylactic acid (PLA), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and 3-hydroxybutanoic acid/3-hydroxyhexanoic acid copolymer (PHBH).
Item 8. The thermosensitive recording medium according to any one of Items 1 to 7, further comprising a resin layer on the base paper on the side opposite to the thermosensitive recording layer.
Item 9. The thermosensitive recording material according to any one of items 1 to 8, wherein the resin layer has a basis weight of 10 g/ ^m2 or more. Item 10. The thermosensitive recording material according to any one of items 1 to 9, wherein the total basis weight of layers other than the base paper is equal to or less than the basis weight of the base paper.
Item 11. The thermal recording medium according to any one of Items 1 to 10, wherein the developer contains at least one selected from the group consisting of 1,1-bis(4-hydroxyphenyl)-1-phenylethane, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, 5-(N-3-methylphenyl-sulfonylamido)-(N',N''-bis-(3-methylphenyl)-isophthalic acid diamide, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, and N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea.
Item 12. The thermosensitive recording medium according to any one of Items 1 to 11, wherein the thermosensitive recording layer contains an adhesive having an SP (solubility parameter) value of 7.5 to 9.5 (cal/cm ³ ) ^0.5 .
Item 13. The thermosensitive recording medium according to any one of Items 2 to 12, wherein the content of the adhesive in the protective layer is 80% by mass or more based on the total solid content.
Item 14. The thermosensitive recording material according to any one of Items 1 to 13, further comprising a thermosensitive recording layer partially on the resin layer.
Item 15. The thermosensitive recording medium according to any one of items 1 to 14, further comprising a printing layer.
Item 16. An image recording method for recording an image by irradiating the thermal recording medium according to any one of items 1 to 15 with laser light.

The thermal recording medium of the present invention uses a paper base material that can reduce the environmental impact and the amount of plastic used, and is highly transparent. In addition, the thermal recording layer has excellent adhesion to the base paper, gas barrier properties, light resistance, and heat resistance.

In this specification, the expression "comprise" includes the concepts of "comprise," "consist essentially of," and "consist only of."

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

Latex in the present invention includes the state of a gel or a dried film formed by drying a dispersion medium.

The thermal recording medium of the present invention is a thermal recording medium having a base paper, a resin layer on at least one surface of the base paper from the side closer to the base paper, and a thermal recording layer containing a leuco dye and a color developer,
The HAZE value measured according to JIS K7136 is 80% or less,
The resin layer is laminated by a melt extrusion method, or the base paper and the resin layer are attached to each other by using an adhesive.

In the present invention, gas barrier properties primarily refer to barrier properties against oxygen and water vapor, but may also have barrier properties against other gases.

In addition, base paper is made of a paper substrate, and the terms base paper and paper substrate are sometimes used interchangeably.

[Base paper]
The base paper in the present invention is made of a paper substrate.

From the viewpoints of ease of production and processing, strength, dimensional stability, and gas barrier properties, the basis weight of the base paper is preferably 20 g/ ^m2 or more, more preferably 25 g/ ^m2 or more, even more preferably 28 g/ ^m2 or more, and preferably 50 g/ ^m2 or less, more preferably 40 g/ ^m2 or less, even more preferably 36 g/ ^m2 or less, and particularly preferably 32 g/ ^m2 or less. The basis weight of the base paper is measured in accordance with JIS P 8124:2011.

In addition, from the viewpoints of ease of production and processing, strength, dimensional stability, and gas barrier properties, the thickness of the base paper is preferably 15 μm or more, more preferably 20 μm or more, even more preferably 25 μm or more, even more preferably 28 μm or more, and is preferably 50 μm or less, more preferably 45 μm or less, even more preferably 40 μm or less, and even more preferably 35 μm or less.

From the viewpoints of gas barrier properties and ease of production, the density (also called tightness) of the base paper is preferably 0.80 g/ ^cm3 or more, more preferably 0.90 g/ ^cm3 or more, even more preferably 0.95 g/ ^cm3 or more, and preferably 1.40 g/ ^cm3 or less, more preferably 1.30 g/ ^cm3 or less, even more preferably 1.20 g/cm3 or ^less . The density of the base paper is calculated from the basis weight and thickness of the base paper obtained by the above-mentioned measurement methods.

From the viewpoint of obtaining excellent gas barrier properties, the total light transmittance of the base paper is 70% or more, preferably 75% or more, more preferably 80% or more, and may be 100%, and from the viewpoint of easy availability, it is preferably 95% or less, more preferably 90% or less. The total light transmittance of the base paper is measured in accordance with JIS K 7375:2008.

In the present invention, from the viewpoint of improving transparency, the base paper preferably has an irregular freeness of 100 mL or more and 600 mL or less. Here, the irregular freeness is the freeness (freeness) measured by changing the pulp harvest amount from 3 g to 0.3 g and changing the JIS standard screen plate to 80 mesh wire in the Canadian standard freeness method specified in JIS P 8121:2012. If the irregular freeness of the pulp constituting the base paper is equal to or more than the lower limit, the dimensional stability of the base paper is high and it is difficult for lumps to occur, and if it is equal to or less than the upper limit, the transparency of the base paper can be maintained, which is preferable. The irregular freeness of the pulp fiber constituting the base paper is more preferably equal to or more than 150 mL and 500 mL or less, and even more preferably equal to or more than 200 mL and 400 mL or less. In order to prepare the irregular freeness, a known method can be used for beating the pulp. The irregular freeness of the pulp that constitutes the base paper can be measured by the above-mentioned method using pulp that has been disintegrated in accordance with JIS P 8220-1:2012 as a sample.

From the viewpoint of improving gas barrier properties, the air permeability of the base paper is preferably 30,000 seconds or more, more preferably 50,000 seconds or more, and even more preferably 99,999 seconds or more. The air permeability of the base paper is a value measured by the Oken method in accordance with JIS P 8117:2009.

There are no particular limitations on the type, shape, size, etc. of the base paper, but glassine paper is preferred. Among glassine papers, glassine paper that is particularly highly beaten and exhibits high total light transmittance is also called graphan paper, and it is more preferable to use graphan paper. In general, glassine paper contains softwood chemical pulp as the main component of the pulp raw material, is highly beaten, and is made into paper at an acidic or neutral pH, and is then compressed and finished using a super calendar or the like.

Specific examples of pulp include chemical pulp made from softwoods such as spruce and hemlock, but other types of pulp such as chemical pulp made from hardwoods, mechanical pulp, waste paper, and synthetic pulp may also be mixed.

[Resin layer]
The resin layer in the present invention may be a monolayer film made of a single resin, a monolayer or laminate film made of a plurality of resins, etc. Also, a laminated resin layer in which the above-mentioned resin is laminated on another substrate (metal, wood, paper, ceramics, etc.) may be used.

Preferred examples of resins constituting the resin layer include polyolefin resins (particularly polyethylene, polypropylene, etc.), polystyrene resins, polyester resins (particularly polyethylene terephthalate resins), polyamide resins (particularly nylon), biodegradable resins, etc.

From the viewpoint of excellent water vapor barrier properties, the resin constituting the resin layer is preferably a polyolefin resin, more preferably polyethylene and polypropylene, and even more preferably polypropylene.

The polyethylene may be low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), or high density polyethylene (HDPE).

The resin layer may be unstretched or may be uniaxially or biaxially stretched.

In addition, from the viewpoint of reducing the environmental impact, it is also preferable that the resin constituting the resin layer is a biodegradable resin. Examples of biodegradable resins include polylactic acid (PLA), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and 3-hydroxybutanoic acid/3-hydroxyhexanoic acid copolymer (PHBH).

The thermal recording medium of the present invention may also have a resin layer on the base paper opposite the thermal recording layer as a heat seal layer.

The ratio of the basis weight of the resin layer to the basis weight of the base paper (resin layer/base paper) is preferably 0.50 or more, more preferably 0.60 or more, and even more preferably 0.70 or more, from the viewpoints of obtaining appropriate strength (rigidity) as a thermal recording medium, water vapor barrier properties, and ease of manufacture.

The basis weight of the resin layer is not particularly limited, and is preferably 5 g/ ^m2 or more, more preferably 10 g/ ^m2 or more, even more preferably 15 g/ ^m2 or more, still more preferably 20 g/ ^m2 or more, and preferably 30 g/ ^m2 or less. The basis weight of the resin layer is measured in accordance with JIS P 8124:2011. When a resin layer is also present on the base paper on the side opposite to the thermosensitive recording layer, the basis weight here means the total basis weight of the resin layers.

The thickness of the resin layer is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 18 μm or more, and preferably 30 μm or less, from the viewpoints of the density of the resin constituting the resin layer, obtaining a suitable strength (rigidity) as a thermal recording medium, water vapor barrier properties, and ease of resin layer formation. The thickness of the resin layer is measured in accordance with JIS P 8118:2014. If there is also a resin layer on the base paper on the side opposite the thermal recording layer, the thickness here means the total thickness of each resin layer.

From the viewpoints of obtaining a suitable strength (rigidity), water vapor barrier properties, and availability, the density of the resin layer is preferably 0.70 g/ ^cm3 or more, more preferably 0.80 g/ ^cm3 or more, even more preferably 0.85 g/ ^cm3 or more, still more preferably 0.90 g/ ^cm3 or more, and is preferably 1.80 g/ ^cm3 or less, more preferably 1.50 g/ ^cm3 or less, and even more preferably 1.30 g/ ^cm3 or less. The density of the resin layer is calculated from the basis weight and thickness of the resin layer obtained by the above-mentioned measurement method.

The resin layer may contain additives such as fillers, antiblocking agents, antistatic agents, plasticizers, and oxidizing agents. These additives may be used alone or in combination of two or more.

The resin layer may be formed by extrusion laminating the resin onto the base paper, or the base paper and the resin layer (e.g., a resin film) may be bonded together using an adhesive.

When using an adhesive to bond the sheets together, there are no particular limitations on the adhesive used, and any of the following types may be used: solventless, organic solvent, water-based, etc. From the standpoint of ensuring the shape stability of the base paper, however, it is preferable to use an organic solvent-based adhesive or a solventless adhesive.

Examples of the main components constituting the adhesive include (meth)acrylic acid ester copolymers, α-olefin copolymers, ethylene-vinyl acetate copolymers, polyvinyl alcohol, polyurethane, styrene-butadiene copolymers, polyvinyl chloride, epoxy resins, melamine resins, silicone resins, natural rubber, casein, starch, etc. Among these, from the viewpoints of easy availability and obtaining good adhesive properties, (meth)acrylic acid ester copolymers and ethylene-vinyl acetate copolymers are preferred, and ethylene-vinyl acetate copolymers are more preferred.

Note that the resin layer may be coated with an adhesive and then laminated with the base paper, or the adhesive may be applied to the base paper and then laminated with the resin layer, or the adhesive may be applied to both the resin layer and the base paper and then laminated with the resin and base paper. Although there are no particular limitations, from the viewpoint of shape stability, it is preferable to apply the adhesive to the resin layer and then laminate the base paper.

The adhesive application method may be appropriately selected from conventionally known methods, and is not particularly limited, but examples include a roll coater, a die coater, and a spray coater.

The amount of adhesive applied is not particularly limited, but from the viewpoint of improving adhesion between the resin layer and the base paper, the amount applied (coating amount) after drying is preferably 1 g/ ^m2 or more, more preferably ² g/m2 or more, even more preferably 4 g/ ^m2 or more, and preferably 40 g/ ^m2 or less, more preferably 20 g/ ^m2 or less, even more preferably 10 g/ ^m2 or less.

[Thermal recording layer]
The heat-sensitive recording layer of the heat-sensitive recording medium of the present invention may contain various known colorless or light-colored leuco dyes. Specific examples of such leuco dyes are given below.

Specific examples of leuco dyes include blue-coloring dyes such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, and fluoran, and green-coloring dyes such as 3-(N-ethyl-N-p-tolyl)amino-7-N-methylanilinofluoran, 3-diethylamino-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluoran, and rhodamine B-anilinolactam. red color-forming dyes such as 3,6-bis(diethylamino)fluoran-γ-anilinolactam, 3-cyclohexylamino-6-chlorofluoran, 3-diethylamino-6-methyl-7-chlorofluoran, and 3-diethylamino-7-chlorofluoran; 3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, and 3-di(n-butyl)amino No-6-methyl-7-anilinofluoran, 3-di(n-pentyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-(N-isoamyl-N-ethylamino)-7-(o-chloroanilino)fluoran, 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino) N-(3-ethoxypropyl)-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-methylamino]-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-methylamino]-6-methyl-7-anilinofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-di(n-butylamino)-7-(2-chloroanilino)fluoran, 4,4'-bis-dimethylaminobenzhydrin benzyl ether, N-2,4,5-trichlorophenylleucoauramine, 3 -diethylamino-7-butylaminofluoran, 3-ethyl-tolylamino-6-methyl-7-anilinofluoran, 3-cyclohexyl-methylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-(β-ethoxyethyl)aminofluoran, 3-diethylamino-6-chloro-7-(γ-chloropropyl)aminofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N-isoamyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3 -dibutylamino-7-chloroanilinofluoran, 3-diethylamino-7-(o-chlorophenylamino)fluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-(p-toluidino)fluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-dimethylamino-6-methyl-7-anilinofluoran , 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 2,2-bis{4-[6'-(N-cyclohexyl-N-methylamino)-3'-methylspiro[phthalido-3,9'-xanthene]-2'-ylamino]phenyl}propane, 3-diethylamino-7-(3'-trifluoromethylphenyl)aminofluoran and other black coloring dyes, 3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylene-2-yl]-4,5 ,6,7-tetrachlorophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3-p-(p-dimethylaminoanilino)anilino-6-methyl-7-chlorofluoran, 3-p-(p-chloroanilino)anilino-6-methyl-7-chlorofluoran, 3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)phthalide, and other dyes having an absorption wavelength in the near infrared region. Of course, the present invention is not limited to these, and two or more compounds may be used in combination as necessary.

The content of the leuco dye is not particularly limited, and is preferably about 3 to 30% by mass, more preferably about 5 to 25% by mass, and even more preferably about 7 to 25% by mass, of the total solid content of the thermal recording layer. By making it 3% by mass or more, the color development ability can be improved and the print density can be improved. By making it 30% by mass or less, the heat resistance can be improved.

Specific examples of the color developer include 4-tert-butylphenol, 4-acetylphenol, 4-tert-octylphenol, 4,4'-sec-butylidenediphenol, 4-phenylphenol, 4,4'-dihydroxydiphenylmethane, 4,4'-isopropylidenediphenol, 4,4'-cyclohexylidene diphenyl, 4,4'-cyclohexylidene diphenol, 1,1-bis(4-hydroxyphenyl)-ethane, 1,1-bis(4-hydroxyphenyl)-1, Phenylethane, 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2'-bis[4-(4-hydroxyphenyl)phenoxy]diethyl ether, 4,4'-dihydroxydiphenyl sulfide, 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 2,2-bis(4-hydroxyphenyl)phenylmethane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2'-bis[4-(4-hydroxyphenyl)phenoxy]diethyl ether, 4,4'-dihydroxydiphenyl sulfide, 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, bis(p-hydroxyphenyl)-4-methylpentane, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, 4-hydroxy-4'-benzyloxydiphenyl sulfone, 3,3'-diallyl-4,4'-dihydroxydiphenyl sulfone, bis(p-hydroxyphenyl)butyl acetate, bis(p-hydroxyphenyl)methyl acetate, hydroquinone monobenzyl ether, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-allyloxy-4'-hydroxydiphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 4-hydroxybenzophenone, dimethyl 4-hydroxyphthalate, methyl 4-hydroxybenzoate, propyl 4-hydroxybenzoate, sec-butyl 4-hydroxybenzoate, phenyl 4-hydroxybenzoate, phenolic compounds such as 4-hydroxybenzoic acid benzyl, 4-hydroxybenzoic acid benzyl ester, 4-hydroxybenzoic acid tolyl, 4-hydroxybenzoic acid chlorophenyl, and 4,4'-dihydroxydiphenyl ether; or benzoic acid, p-chlorobenzoic acid, p-tert-butylbenzoic acid, trichlorobenzoic acid, terephthalic acid, salicylic acid, 3-tert-butylsalicylic acid, 3-isopropylsalicylic acid, 3-benzylsalicylic acid, 3-(α-methylbenzyl)salicylic acid, 3,5 -di-tert-butylsalicylic acid, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid zinc, and other aromatic carboxylic acids, and combinations of these phenolic compounds, aromatic carboxylic acids with, for example, zinc, magnesium, aluminum, calcium, titanium, manganese, tin, nickel, and salts with polyvalent metals such as zinc thiocyanate antipyrine complex, and organic acidic substances such as complex zinc salts of terephthalaldehydic acid and other aromatic carboxylic acids; N-p-toluenesulfonyl-N'-3-(p-toluenesulfonyloxy)phenylurea, N-p-toluenesulfonyl-N'-p-butoxycarbonylphenylurea, N-p-tolylsulfonyl-N'-phenylurea, 4,4'-bis(p-toluenesulfonylaminocarbonylamino)diphenylmethane, 4,4'-bis[(4 urea compounds such as N,N'-di-m-chlorophenylthiourea; organic compounds having an -SO 2 NH- bond in the molecule such as N-(p-toluenesulfonyl)carbamoyl acid p-cumylphenyl ester, N-(p-toluenesulfonyl)carbamoyl acid p-benzyloxyphenyl ester, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and N-(o-toluoyl)-p _- toluenesulfamide; and inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate.

Further examples include urea urethane derivatives such as 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, 4,4'-bis[(2-methyl-5-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, and 4-(2-methyl-3-phenoxycarbonylaminophenyl)ureido-4'-(4-methyl-5-phenoxycarbonylaminophenyl)ureidodiphenyl sulfone represented by the following general formula (1), diphenyl sulfone derivatives represented by the following general formula (2), 5-(N-3-methylphenyl-sulfonylamide)-(N',N''-bis-(3-methylphenyl)-isophthalic acid diamide, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, and N,N'-di[3-(p-toluenesulfonyl)oxy]phenyl urea. Of course, the present invention is not limited to these, and two or more compounds can be used in combination as necessary.

（式中、ｎは１～６の整数を表す。）

(In the formula, n represents an integer of 1 to 6.)

Among these, preferred color developers are 1,1-bis(4-hydroxyphenyl)-1-phenylethane, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, 5-(N-3-methylphenyl-sulfonylamide)-(N',N''-bis-(3-methylphenyl)-isophthalic acid diamide, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea, and the like, with 1,1-bis(4-hydroxyphenyl)-1-phenylethane, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea being more preferred.

The content of the developer is not particularly limited and may be adjusted according to the leuco dye used. In general, it is preferably 0.5 parts by weight or more per 1 part by weight of the leuco dye, more preferably 0.8 parts by weight or more, even more preferably 1 part by weight or more, even more preferably 1.2 parts by weight or more, and particularly preferably 1.4 parts by weight or more. The content of the developer is preferably 10 parts by weight or less per 1 part by weight of the leuco dye, more preferably 5 parts by weight or less, even more preferably 4 parts by weight or less, and particularly preferably 3.5 parts by weight or less. By making it 0.5 parts by weight or more, it is possible to improve the recording performance. On the other hand, by making it 10 parts by weight or less, it is possible to effectively suppress background fogging in a high-temperature environment.

In the present invention, a storage stability improver can be further incorporated into the thermal recording layer, mainly to further improve the storage stability of the color image. Examples of such storage stability improvers include 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol, and 4,4'-[1,3-phenylenebis(1-methylethylidene)]bisphenol. At least one selected from phenol compounds such as 4-benzyloxyphenyl-4'-(2-methyl-2,3-epoxypropyloxy)phenylsulfone, epoxy compounds such as 4-(2-methyl-1,2-epoxyethyl)diphenylsulfone, and 4-(2-ethyl-1,2-epoxyethyl)diphenylsulfone, and isocyanuric acid compounds such as 1,3,5-tris(2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl)isocyanuric acid can be used. Of course, the present invention is not limited to these compounds, and two or more compounds can be used in combination as necessary.

When a storage stability improver is used, the amount used should be an amount effective for improving storage stability, and is usually preferably about 1 to 30% by weight, and more preferably about 5 to 20% by weight, of the total solid content of the thermal recording layer.

A sensitizer can also be added to the thermal recording layer in the present invention. This can increase the recording sensitivity. Examples of sensitizers include stearic acid amide, methoxycarbonyl-N-stearic acid benzamild, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, behenic acid amide, methylene bisstearic acid amide, N-methylolstearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, diphenyl sulfone, benzyl p-benzyloxybenzoate, 1-hydroxy-2-phenyl naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, di-p-chlorobenzyl ester of oxalic acid, di-p-methylbenzyl ester of oxalic acid, dibenzyl ester of oxalic acid, p-tolyl biphenyl ether, di(p-methoxyphenoxyethyl)ether, 1,2-di(3-methylphenoxy)ethyl ... p-methylthiophenylbenzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, 1,2-diphenoxymethylbenzene, di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane, di-o-chlorobenzyl adipate, 1,2-bis(3,4-dimethylphenyl)ethane, 1,3-bis(2-naphthoxy)propane, diphenyl, benzophenone, and the like. These can be used together to the extent that no problems occur. The content of the sensitizer should be an amount effective for sensitization, and is usually preferably about 2 to 40% by mass, and more preferably about 5 to 25% by mass, of the total solid content of the thermal recording layer.

Other components that make up the thermal recording layer include adhesives, and if necessary, crosslinking agents, waxes, metal soaps, water-resistant agents, pigments, dispersants, colored dyes, fluorescent dyes, etc.

The adhesive used in the coating liquid for the thermal recording layer may be, for example, a water-soluble adhesive or a water-dispersible adhesive. Examples of the water-soluble adhesive include modified polyvinyl alcohols such as polyvinyl alcohol modified with carboxy, acetoacetyl, diacetone, and silicon-modified polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, methylcellulose, and ethylcellulose, sodium polyacrylate, polyvinylpyrrolidone, polyamide, diisobutylene-maleic anhydride copolymer salt, styrene-acrylic acid copolymer salt, styrene-maleic anhydride copolymer salt, ethylene-maleic anhydride copolymer salt, acrylic acid amide-acrylic acid ester copolymer, acrylic acid amide-acrylic acid ester-methacrylic acid copolymer, polyacrylamide, sodium alginate, gelatin, casein, and gum arabic. Examples of water-dispersible adhesives include emulsions of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid ester, vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, ethylene-vinyl acetate copolymer, and the like, or latexes of water-insoluble polymers such as styrene-butadiene copolymer and styrene-butadiene-acrylic copolymer. Among them, adhesives having an SP (solubility parameter) value of 7.5 to 9.5 (cal/cm ³ ) ^0.5 are preferred. By using an adhesive having an SP in such a range, the adhesion between the thermal recording layer and the resin layer can be improved. Examples of adhesives having an SP value of 7.5 to 9.5 (cal/cm ³ ) ^0.5 include styrene-butadiene copolymer latex and acrylic acid modified polypropylene resin. The solubility parameter is calculated in accordance with the solubility parameter calculation method according to the Hildebrand method. The adhesives can be used alone or in combination of two or more types. At least one type of adhesive is preferably blended in an amount of about 5 to 50% by mass, more preferably about 10 to 40% by mass, based on the total solid content of the heat-sensitive recording layer.

A crosslinking agent that hardens the adhesive of the thermal recording layer or other layers can be contained in the thermal recording layer. This can improve the water resistance of the thermal recording layer. Examples of crosslinking agents include aldehyde compounds such as glyoxal, polyamine compounds such as polyethyleneimine, epoxy compounds, polyamide resins, melamine resins, glyoxylates, dimethylol urea compounds, aziridine compounds, and blocked isocyanate compounds; inorganic compounds such as ammonium persulfate, ferric chloride, magnesium chloride, sodium tetraborate, and potassium tetraborate; boric acid, boric acid triesters, boron-based polymers, hydrazide compounds, and glyoxylates. These may be used alone or in combination of two or more. The amount of crosslinking agent used is preferably in the range of about 1 to 10 parts by mass per 100 parts by mass of the total solid content of the thermal recording layer. This can improve the water resistance of the thermal recording layer.

Examples of waxes include waxes such as paraffin wax, carnauba wax, microcrystalline wax, polyolefin wax, and polyethylene wax; higher fatty acid amides such as stearic acid amide and ethylene bisstearic acid amide, higher fatty acid esters, and derivatives thereof.

Examples of metal soaps include polyvalent metal salts of higher fatty acids, such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate. If necessary, various auxiliary agents such as oil repellents, defoamers, and viscosity regulators can be added to the thermal recording layer within a range that does not impair the effects of the present invention.

The thermal recording layer is generally formed on a base paper by dispersing a leuco dye and a developer, and optionally a sensitizer and a storage stability improver, together or separately, in water as a dispersion medium, together with water-soluble synthetic polymer compounds such as polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, styrene-maleic anhydride copolymer salt, and other surfactants, using various stirring and wet grinding machines such as ball mills, coball mills, attritors, and vertical and horizontal sand mills, to prepare a dispersion, and then dispersing the dispersion to an average particle size of 2 μm or less, and mixing with adhesives, auxiliaries, and the like, as necessary, to prepare a coating liquid for the thermal recording layer. The coating amount of the thermal recording layer is not particularly limited, and is preferably about 1 to 12 g/m ² in terms of dry mass, more preferably 2 to 10 g/m ² , even more preferably 2.5 to 8 g/m ² , and particularly preferably 3 to 5.5 g/m ² . If necessary, the heat-sensitive recording layer can be formed in two or more separate layers, and the composition and coating amount of each layer may be the same or different.

[Protective Layer]
In the thermal recording medium, a protective layer may be provided on the thermal recording layer on the side opposite to the resin layer, if necessary. The protective layer preferably contains an adhesive. Furthermore, the protective layer preferably contains a lubricant such as polyolefin wax or zinc stearate in order to prevent sticking to the thermal head, and may also contain an ultraviolet absorber and a pigment. Furthermore, by providing a glossy protective layer, the added value of the product can be increased.

The pigment contained in the protective layer is not particularly limited, and examples thereof include inorganic pigments such as amorphous silica, kaolin, clay, light calcium carbonate, heavy calcium carbonate, calcined kaolin, titanium oxide, magnesium carbonate, aluminum hydroxide, colloidal silica, and synthetic layered mica, and plastic pigments such as urea-formaldehyde resin filler. The pigment content is preferably about 1 to 20% by mass, more preferably about 1 to 10% by mass, and even more preferably about 1 to 5% by mass, of the total solid content of the protective layer.

The adhesive contained in the protective layer is not particularly limited, and a water-soluble or water-dispersible aqueous adhesive can be used. The adhesive can be appropriately selected from those that can be used in the thermal recording layer. Of these adhesives, various modified polyvinyl alcohols such as acetoacetyl-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and diacetone-modified polyvinyl alcohol are more preferably used. The content of the adhesive is preferably 80% by mass or more, more preferably about 90 to 100% by mass, and more preferably about 95 to 100% by mass, of the total solid content of the protective layer. By making the content 80% by mass or more, oxygen permeability can be suppressed and light resistance can be improved.

The protective layer is formed on the thermosensitive recording layer by, for example, applying a coating material for the protective layer prepared by mixing a pigment, an adhesive, and, if necessary, an auxiliary, etc., in water as a dispersion medium, and then drying the coating material. The amount of the coating material for the protective layer is not particularly limited, and is preferably about 0.3 to 15 g/ ^m2 in terms of dry mass, more preferably about 0.3 to 10 g/ ^m2 , even more preferably about 0.5 to 8 g/ ^{m2 ,} particularly preferably about 1 to 8 g/m2, and even more preferably about 1 to 5 g/ ^m2 . The protective layer can be formed in two or more layers as necessary, and the composition and coating amount of each layer may be the same or different.

[Printed layer]
In the present invention, a printing layer can be provided on at least one of the protective layer, between the thermosensitive recording layer and the protective layer, on the thermosensitive recording layer, between the resin layer and the thermosensitive recording layer, between the base paper and the resin layer, and on the surface of the base paper opposite to the thermosensitive recording layer. The printing ink used to form the printing layer includes inks for letterpress, lithographic, intaglio, and stencil printing, as classified by printing plate. In addition, inks for permeation drying, evaporation drying, oxidation polymerization drying, and photopolymerization drying (ultraviolet curing type) are classified by the drying mode. The printing layer can be formed by a printing method such as flexographic printing, offset printing, gravure printing, and screen printing. Digital printing methods such as plateless on-demand printing and direct printing can also be used. The color tone of the printing ink is not particularly limited, but it is preferable that the optical density of the printing layer is lower than the optical density of the recording part of the thermosensitive recording layer, especially when the thermosensitive recording is performed on the printing layer. The printing pattern is preferably a background pattern with an optical density of about 0.1 to 0.6, or fixed information that does not overlap with the thermosensitive recording.

[Other layers]
In the present invention, it is preferable that the base paper has an adhesive layer on at least one side. This can increase the added value of the thermal recording medium. For example, the adhesive layer can be made into adhesive paper, remoistened adhesive paper, delayed tack paper, etc. by applying coating processing with an adhesive, remoistened adhesive, delayed tack type adhesive, etc. on one side. In addition, the surface of the base paper opposite to the thermal recording layer can be given the function of thermal transfer paper, inkjet recording paper, carbonless paper, electrostatic recording paper, xeography paper, etc., to make a recording paper capable of double-sided recording. Of course, it can also be made into a double-sided thermal recording medium. In addition, a back layer can be provided to suppress the penetration of oil and plasticizer from the back side of the thermal recording medium, to control curling, and to prevent static electricity. It is also possible to make a linerless label that does not require a release paper by coating processing a release layer containing silicone on the protective layer and coating processing a pressure sensitive adhesive on one side.

[Thermal recording medium]
The thermosensitive recording medium can be manufactured by forming each of the above layers on the resin layer of the base paper. The method for forming each of the above layers on the resin layer may be any of known coating methods such as the air knife method, blade method, gravure method, roll coater method, spray method, dip method, bar method, curtain method, slot die method, slide die method, and extrusion method. Each coating material may be applied and dried one by one to form each layer, or the same coating material may be applied in two or more layers. Furthermore, simultaneous multi-layer coating may be performed in which two or more layers are applied simultaneously. After each layer is formed, or in any process after all layers are formed, a smoothing treatment may be performed using known methods such as a super calendar and a soft calendar. After a thermosensitive recording layer (and a protective layer) is formed on the resin layer, the base paper and the resin layer may be bonded together using an adhesive.

The lower the HAZE value of the thermal recording medium of the present invention, the higher the transparency, and therefore the more preferable. The HAZE value is 80% or less, preferably 75% or less, and more preferably 70% or less. The HAZE value is measured in accordance with JIS K7136.

The lower the oxygen permeability of the thermal recording medium of the present invention, the less oxygen is permeated, which is preferable. The oxygen permeability at 23°C and 50% RH (relative humidity) is preferably 10 mL/( ^m2 ·24h·atm) or less, more preferably 5 mL/( ^m2 ·24h·atm) or less, and even more preferably 2 mL/( ^m2 ·24h·atm) or less. The oxygen permeability of the thermal recording medium is measured under the above conditions using an oxygen permeability measuring device (OX-TRAN2/20, manufactured by MOCON Co., Ltd.) in accordance with JIS K7126.

The water vapor permeability of the thermosensitive recording medium of the present invention is preferably lower because less water vapor permeates, specifically, preferably 50 g/( ^m2 ·24h) or less, more preferably 30 g/( ^m2 ·24h) or less, even more preferably 20 g/( ^m2 ·24h) or less, and even more preferably 10 g/( ^m2 ·24h) or less. The water vapor permeability is measured in accordance with JIS Z 0208:1976 by preparing a moisture permeable cup so that the thermosensitive recording layer is on the outside (condition B: temperature 40±0.5°C, relative humidity 90±2%).

In the thermal recording medium of the present invention, it is preferable that the total basis weight of layers other than the base paper (resin layer, thermal recording layer, protective layer, etc.) is equal to or less than the basis weight of the base paper. In this way, by making the basis weight of the base paper larger than the sum of the basis weights of the other layers, the amount of plastic used can be reduced, and the environmental burden can be reduced.

In the thermosensitive recording medium of the present invention, a thermosensitive recording layer (and a protective layer) may be provided on the entire surface of the resin layer, or a thermosensitive recording layer (and a protective layer) may be provided partially on the resin layer.

The method for recording an image on the thermal recording medium of the present invention is not particularly limited and can be appropriately selected depending on the purpose. Examples of image recording methods that can be used include a thermal head printer and laser light (e.g., carbon dioxide laser, UV laser, semiconductor laser light, YAG laser light, fiber laser light, solid laser light, dye laser light, etc.). When laser light is used, the wavelength of the laser light is not particularly limited and can be appropriately selected depending on the purpose.

The thermal recording medium of the present invention is highly transparent, has excellent adhesion of the thermal recording layer to the base paper, gas barrier properties, light resistance, and heat resistance, and is therefore suitable for use in food applications, such as labels, top seal materials, and bands for transparent containers for salads, prepared foods, etc.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these. Unless otherwise specified, "parts" and "%" indicate "parts by mass" and "% by mass", respectively.

Example 1
Preparation of Solution A (Leuco Dye Dispersion) Solution A was obtained by pulverizing a composition consisting of 100 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 50 parts of a 20% aqueous solution of sulfone-modified polyvinyl alcohol (product name: Gohsenex L-3266, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), 10 parts of a 5% emulsion of a natural oil-based defoamer (product name: Nopco 1407H, manufactured by San Nopco Co., Ltd.) in a sand mill until the median diameter reached 0.5 μm as measured by a laser diffraction particle size distribution analyzer SALD2200 (manufactured by Shimadzu Corporation).

Preparation of Liquid B (Color Development Agent Dispersion Liquid) Liquid B was obtained by pulverizing a composition consisting of 100 parts of 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 40 parts of a 20% aqueous solution of sulfone-modified polyvinyl alcohol (product name: Gohsenex L-3266, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), 1 part of a 5% emulsion of a natural oil-based defoamer (product name: Nopco 1407H, manufactured by San Nopco Ltd.) using an Ultraviscomill until the median diameter reached 0.2 μm as measured with a dynamic light scattering particle size distribution analyzer LB-500 (manufactured by Horiba, Ltd.).

Preparation of coating liquid for thermal recording layer 36 parts of solution A, 47.8 parts of solution B, 33.5 parts of colloidal silica (product name: Snowtex N, average particle size 10-15 nm, manufactured by Nissan Chemical Industries, Ltd., solid content concentration 20%), 28.5 parts of styrene-butadiene-based latex (product name: Smartex PA9281, manufactured by Nippon A&L Co., Ltd., solid content concentration 48%, SP value 8.4), 38 parts of a 10% aqueous solution of diacetone-modified polyvinyl alcohol (product name: DF-17, manufactured by Nippon Vinyl Acetate & Poval Co., Ltd.) A coating liquid for a thermal recording layer was prepared by mixing and stirring a composition consisting of 10 parts of a 10% aqueous solution of methylcellulose (trade name: Metrose 60SH-03, manufactured by Shin-Etsu Chemical Co., Ltd.), 1 part of a polyethylene wax emulsion (trade name: Nopcoat PEM17, manufactured by San Nopco Ltd., solids concentration 40%), 6 parts of a 10% aqueous solution of dioctyl sulfosuccinic acid sodium salt (trade name: SN Wet OT-70, manufactured by San Nopco Ltd.), and 18 parts of water.

Preparation of Coating Liquid for Protective Layer A coating liquid for protective layer was prepared by mixing and stirring a composition consisting of 428.6 parts of a 14% aqueous solution of acetoacetyl-modified polyvinyl alcohol (GOHSENEX Z-200, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., fully saponified, polymerization degree 1100), 200.0 parts of an aromatic polyester-based polyurethane resin (product name: HYDRAN AP-30F, manufactured by DIC Corporation, solid content concentration 20%), and 100.0 parts of water.

Preparation of Support 1 Glassine paper (product name: Grafan, manufactured by Oji F-Tex Co., Ltd., basis weight 35 g/ ^m2 , thickness 35 μm, haze 72%, irregular freeness of disintegrated pulp 250 mL) was used as the base paper, and low-density polyethylene LC607K (manufactured by Japan Polyethylene Co., Ltd.) was extrusion laminated onto the base paper to a basis weight of 12 g/ ^m2 (thickness 13 μm) to prepare a single-sided laminated support 1.

Preparation of a thermosensitive recording medium A thermosensitive recording layer was formed by coating and drying a coating liquid for a thermosensitive recording layer on the laminate surface of the support 1 so that the coating amount after drying would be 4.0 g/ ^m2 . Further, a protective layer was formed by coating and drying a coating liquid for a protective layer on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 , thereby obtaining a thermosensitive recording medium.

Example 2
Preparation of Support 2 Low density polyethylene LC607K (manufactured by Japan Polyethylene Corporation) was extrusion laminated to the non-laminated surface (base paper surface) of Support 1 to a basis weight of 12 g/m ² (thickness 13 μm) to prepare a double-sided laminated product, Support 2.

Preparation of a thermosensitive recording medium On one side of a support 2, the coating liquid for a thermosensitive recording layer prepared in Example 1 was applied and dried so that the coating amount after drying would be 4.0 g/ ^m2 , forming a thermosensitive recording layer. Furthermore, the coating liquid for a protective layer prepared in Example 1 was applied and dried on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 , forming a protective layer, and a thermosensitive recording medium was obtained.

Example 3
Preparation of Support 3 Glassine paper (product name: Grafan, manufactured by Oji F-Tex Co., Ltd., basis weight 35 g/ ^m2 , thickness 35 μm, haze 72%, irregular freeness of disintegrated pulp 250 mL) was used as the base paper, and polypropylene resin PHA03A (manufactured by Sun Allomer Co., Ltd.) was extrusion laminated on both sides of the base paper to 12 g/ ^m2 (thickness 13 μm) on each side to prepare a double-sided laminated support 3.

Preparation of a thermosensitive recording medium The coating liquid for a thermosensitive recording layer prepared in Example 1 was applied and dried on the polypropylene laminate surface of Support ³ so that the coating amount after drying would be 4.0 g/m2 to form a thermosensitive recording layer. Furthermore, the coating liquid for a protective layer prepared in Example 1 was applied and dried on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 to form a protective layer, and a thermosensitive recording medium was obtained.

Example 4
Preparation of Support 4 An adhesive mixture of 10 parts of DIC Dry LX-500 (manufactured by DIC Corporation) and 1 part of DIC Dry KW-75 (manufactured by DIC Corporation) was applied to a transparent PET film of 12 g/ ^m2 (12 μm) to a thickness of 5 g/ ^m2 (thickness of 5 μm), and then this was dry laminated to the non-laminated surface (base paper surface) of Support 1 to prepare Support 4.

Preparation of a thermosensitive recording medium The coating liquid for a thermosensitive recording layer prepared in Example 1 was applied to the PET film side of the support 4 so that the coating amount after drying was 4.0 g/m2, and then dried to form a thermosensitive recording layer. Furthermore, the coating liquid for a protective layer prepared in Example ¹ was applied to the thermosensitive recording layer so that the coating amount after drying was 2.0 g/ ^m2 , and then dried to form a protective layer, and a thermosensitive recording medium was obtained.

Example 5
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of the protective layer coating liquid in Example 2, the amount of a 14% aqueous solution of acetoacetyl-modified polyvinyl alcohol (Gosenex Z-200, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., fully saponified, polymerization degree 1100) was changed from 428.6 parts to 342.9 parts, the amount of aromatic polyester polyurethane resin (product name: Hydran AP-30F, manufactured by DIC Corporation, solid content concentration 20%) was changed from 200 parts to 160.0 parts, and 100 parts of colloidal silica (product name: Snowtex O, average particle size 10 to 15 nm, manufactured by Nissan Chemical Industries, solid content concentration 20%) was added.

Example 6
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of the protective layer coating liquid in Example 2, the amount of a 14% aqueous solution of acetoacetyl-modified polyvinyl alcohol (Gosenex Z-200, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., fully saponified, polymerization degree 1100) was changed from 428.6 parts to 278.6 parts, the amount of aromatic polyester polyurethane resin (product name: Hydran AP-30F, manufactured by DIC Corporation, solid content concentration 20%) was changed from 200 parts to 130.0 parts, and 175 parts of colloidal silica (product name: Snowtex O, average particle size 10 to 15 nm, manufactured by Nissan Chemical Industries, solid content concentration 20%) was further added.

Example 7
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of solution B in Example 2, 100 parts of 1,1-bis(4-hydroxyphenyl)-1-phenylethane were replaced with 100 parts of N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea.

Example 8
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of solution B in Example 2, 100 parts of 1,1-bis(4-hydroxyphenyl)-1-phenylethane were replaced with 100 parts of N-[2-(3-phenylureido)phenyl]benzenesulfonamide.

Example 9
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of solution B in Example 2, 100 parts of 5-(N-3-methylphenyl-sulfonylamido)-(N',N''-bis-(3-methylphenyl)-isophthalic acid diamide was used in place of 100 parts of 1,1-bis(4-hydroxyphenyl)-1-phenylethane.

Example 10
A thermal recording medium was obtained in the same manner as in Example 2, except that in the preparation of solution B in Example 2, 100 parts of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate was used in place of 100 parts of 1,1-bis(4-hydroxyphenyl)-1-phenylethane.

Example 11
In Example 4, the coating liquid for the thermosensitive recording layer prepared in Example 1 was partially applied and dried on the PET film side of the support 4 so that the coating layer thickness after drying would be the same as the coating amount of 4.0 g/ ^m2 when coated on the entire surface, to form a thermosensitive recording layer, and further, the coating liquid for the protective layer was partially applied and dried on the thermosensitive recording layer so that the coating layer thickness after drying would be the same as the coating amount of 2.0 g/ ^m2 when coated on the entire surface, to form a protective layer, thereby obtaining a thermosensitive recording material.

Comparative Example 1
Glassine paper (product name: Grafan, manufactured by Oji F-Tex, basis weight 35 g/ ^m2 , thickness 35 μm, haze 72%, irregular freeness of disintegrated pulp 250 mL) was used as the base paper, and the coating liquid for thermosensitive recording layer prepared in Example 1 was applied and dried on the surface of the base paper so that the coating amount after drying would be 4.0 g/ ^m2 to form a thermosensitive recording layer. Furthermore, the coating liquid for protective layer prepared in Example 1 was applied and dried on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 to form a protective layer, and a thermosensitive recording material was obtained.

Comparative Example 2
Preparation of Support 5 Glassine paper (product name: Hyamone, manufactured by Oji F-Tex Co., Ltd., basis weight 75 g/ ^m2 , thickness 73 μm, haze 98%) was used as the base paper, and low-density polyethylene LC607K (manufactured by Japan Polyethylene Co., Ltd.) was extrusion laminated on both sides of the paper base material to give a basis weight of 12 g/ ^m2 (thickness 13 μm), to prepare Support 5.

Preparation of a thermosensitive recording medium A thermosensitive recording layer was formed by coating one side of the support 5 with a coating liquid for a thermosensitive recording layer so that the coating amount after drying would be 4.0 g/ ^m2 and then drying. Further, a protective layer was formed by coating a protective layer liquid on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 and then drying, and a thermosensitive recording medium was obtained.

Comparative Example 3
Preparation of Coating Liquid for Resin Layer 70 parts of vinyl chloride-vinyl acetate copolymer (product name: Kanevilac L-CN, weight ratio: vinyl chloride:vinyl acetate=1:1, solid concentration 37%, manufactured by Kaneka Corporation) was dissolved in 80 parts of toluene using a stirrer.

Preparation of Support 6 Glassine paper (product name: Grafan, manufactured by Oji F-Tex Co., Ltd., basis weight 35 g/ ^m2 , thickness 35 μm, haze 72%, irregular freeness of disintegrated pulp 250 mL) was used as the base paper, and the resin layer coating liquid was applied onto the paper base material and dried so that the coating amount after drying was 4.0 g/ ^m2 , to prepare Support 6.

Preparation of a Thermosensitive Recording Medium On one side of a support 6, the coating liquid for a thermosensitive recording layer prepared in Example 1 was applied and dried so that the coating amount after drying would be 4.0 g/ ^m2 , forming a thermosensitive recording layer. Furthermore, the coating liquid for a protective layer prepared in Example 1 was applied and dried on the thermosensitive recording layer so that the coating amount after drying would be 2.0 g/ ^m2 , forming a protective layer, and a thermosensitive recording medium was obtained.

The thermal recording media prepared in Examples 1 to 11 and Comparative Examples 1 to 3 were subjected to the following evaluations, and the results are shown in Table 1.

[Image Formation]
Examples 1 to 11, Comparative Examples 1 to 3
Using a thermal evaluation machine (product name: TH-TMD, manufactured by Okura Electric Co., Ltd.), solid printing of a 10 mm square was performed on each thermal recording medium at an applied energy of 0.24 mJ/dot and a printing speed of 4 ips to obtain a thermal recording print.

Example 12
Using a UV laser marker (product name: UV laser marker MD-U1000C, manufactured by Keyence Corporation), the thermal recording medium of Example 2 was irradiated with a laser from the thermal recording/protective layer coated surface at a wavelength of 355 nm, an output of 1.2 W, a scan speed of 3,000 mm/SEC, and a laser spot diameter of 14 μm, to perform solid printing of a 10 mm square, thereby obtaining a thermal recording print.

[Transparency evaluation]
The transparency of the non-printed areas of each thermal recording medium was measured with a HAZE meter (NDH-7000, manufactured by Nippon Denshoku Industries Co., Ltd.) (JIS K7136).
Haze value over 80%: When used as a food container, the contents cannot be seen. Haze value below 80%: When used as a food container, the contents can be seen.

[Evaluation of adhesion of coating layer to laminate layer]
An adhesive cellophane tape was applied onto the protective layer with a pressure of 100 g/cm ² and peeled off at a speed of 30 m/min. The degree of peeling of the thermosensitive recording layer from the support was visually evaluated according to the following criteria.
◯: The thermal recording layer does not peel off. ×: Peeling is observed in the thermal recording layer.

[Oxygen permeability]
The oxygen permeability of the thermal recording medium was measured in accordance with JIK K7126 using an oxygen permeability measuring device (OX-TRAN2/20, manufactured by MOCON) at 23° C. and 50% RH.
Oxygen permeability 2mL/( ^m2 ·24h·atm) or less: Very excellent as a gas barrier material Oxygen permeability over 2mL/( ^m2 ·24h·atm) and under 10mL/( ^m2 ·24h·atm): Excellent as a gas barrier material Oxygen permeability over 10mL/( ^m2 ·24h·atm): Not usable as a gas barrier material and unacceptable

[Water vapor permeability]
The water vapor permeability of the thermal recording medium was measured in accordance with JIS Z0208:1976 using a moisture permeable cup prepared with the thermal recording layer facing outward (Condition B: temperature 40±0.5° C., relative humidity 90±2%).
Water vapor permeability 30g/( ^m2 ·24h) or less: Very excellent as a gas barrier material Water vapor permeability over 30g/( ^m2 ·24h) and up to 50g/( ^m2 ·24h): Excellent as a gas barrier material Water vapor permeability over 50g/( ^m2 ·24h): Not usable as a gas barrier material and unacceptable

[Light resistance evaluation]
The printed surface of each thermal recording medium was irradiated for 15 hours using a xenon weather meter (manufactured by Suga Test Instruments Co., Ltd.) (black panel temperature 63°C, humidity 40% RH, irradiation illuminance 390 W/ ^m2 in the wavelength range of 300 to 700 nm), and then the recorded area was measured in visual mode with an X-rite spectrodensitometer (product name: X-rite530, manufactured by X-rite Co., Ltd.). The print retention rate of the recorded area was then calculated using the following formula.
Residual rate (%)=(recording density after processing/recording density before processing)×100
Remaining rate of 80% or more: The print is very clear and excellent. Remaining rate of less than 80% (65% or more): The print is clear and good. Remaining rate of less than 65%: The print is chipped and unusable.

[Heat resistance evaluation]
Each thermal recording print was treated in a dryer at 90° C. for 1 hour, and then the non-recorded areas were measured in visual mode with an X-rite spectrodensitometer (trade name: X-rite 530, manufactured by X-rite Corporation).
Density of non-recorded area: 0.15 or less: Very little coloring, excellent Density of non-recorded area: More than 0.15 and 0.20 or less: Little coloring, good Density of non-recorded area: More than 0.20: Strong coloring, practically unsuitable

[Evaluation of Thermal Head Wear]
A load of 900 g was applied to a brass ball having a diameter of 0.3175 cm, and the brass ball was run for 375 m on the protective layer surface of the thermal recording medium of Examples 1 to 11 and Comparative Examples 1 to 3. The wear depth of the brass ball was measured using a profile measuring device, Profile Measurement Unit VHX-S15, manufactured by Keyence Corporation. The smaller the wear depth, the better the head wear resistance.
◯: Wear depth less than 10 μm ×: Wear depth 10 μm or more

Claims (16)

A thermosensitive recording medium comprising a base paper, a resin layer on at least one surface of the base paper from the side closest to the base paper, and a thermosensitive recording layer containing a leuco dye and a color developer,
The HAZE value measured according to JIS K7136 is 80% or less,
The thermosensitive recording medium is formed by laminating the resin layer by a melt extrusion method, or by bonding a base paper and a resin layer together with an adhesive. The thermosensitive recording medium according to claim 1, which has a protective layer mainly containing an adhesive on the thermosensitive recording layer on the side opposite to the resin layer. 3. The heat-sensitive recording material according to claim 1, which has an oxygen permeability of 10 mL/( ^m2 ·24 h·atm) or less at 23° C. and 50% RH as measured in accordance with JIS K7126. 3. The heat-sensitive recording material according to claim 1, which has a water vapor permeability of 50 g/( ^m2 ·24 h) or less under conditions of 40° C. and 90% RH as measured in accordance with JIS Z 0208. 3. The thermosensitive recording medium according to claim 1, wherein the base paper is glassine paper having a basis weight of 50 g/ ^m2 or less. The thermal recording medium according to claim 1 or 2, wherein the resin constituting the resin layer is at least one selected from the group consisting of polyethylene resin, polypropylene resin, polystyrene resin, polyethylene terephthalate resin, and biodegradable resin. The thermal recording medium according to claim 6, wherein the biodegradable resin is at least one selected from the group consisting of polylactic acid (PLA), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and 3-hydroxybutanoic acid/3-hydroxyhexanoic acid copolymer (PHBH). The thermosensitive recording medium according to claim 1 or 2, which has a resin layer on the base paper on the side opposite to the thermosensitive recording layer. 3. The thermosensitive recording material according to claim 1, wherein the resin layer has a basis weight of 10 g/ ^m2 or more. The thermal recording medium according to claim 1 or 2, wherein the total basis weight of layers other than the base paper is equal to or less than the basis weight of the base paper. The thermal recording medium according to claim 1 or 2, containing as the developer at least one selected from the group consisting of 1,1-bis(4-hydroxyphenyl)-1-phenylethane, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, 5-(N-3-methylphenyl-sulfonylamido)-(N',N''-bis-(3-methylphenyl)-isophthalic acid diamide, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, and N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea. 3. The thermosensitive recording medium according to claim 1, wherein the thermosensitive recording layer contains an adhesive having an SP (solubility parameter) value of 7.5 to 9.5 (cal/cm ³ ) ^0.5. The thermal recording medium according to claim 2, wherein the adhesive content in the protective layer is 80% by mass or more of the total solid content. The thermosensitive recording medium according to claim 1 or 2, which has a thermosensitive recording layer partially on the resin layer. The thermal recording medium according to claim 1 or 2, further comprising a printing layer. An image recording method for recording an image by irradiating a laser beam onto the thermal recording medium according to claim 1 or 2.