JP6135913B2 - Coating agent for forming protective layer of thermal recording medium and thermal recording medium - Google Patents

Description

  The present invention relates exclusively to a coating agent capable of forming a protective layer constituting a thermosensitive recording medium.

  A heat-sensitive recording medium generally has a heat-sensitive color-developing layer containing a color-developing substance such as a leuco dye, a developer such as a phenolic compound, a binder resin such as polyvinyl alcohol on a substrate such as paper or a plastic film. The chromogenic substance reacts with the developer that has been melted by heating to develop a color, thereby forming a desired character or image.

  Since the thermal recording medium is generally compact and can be printed using a thermal printer with low noise during printing, currently, for example, facsimile paper, various receipts, train tickets, etc., foodstuffs and lunch boxes. It is used in a wide range of applications such as price tags, meter reading tables for gas and water charges.

  However, the conventional thermal recording medium has a large practical problem that it causes fading or disappearance of characters printed on the thermal recording medium over time due to external factors. It was necessary to be careful enough.

  Specifically, when a plastic product or the like is in contact with a colored portion such as a character printed on the thermal recording medium for a certain period, the plasticizer or the like contained in the plastic product is applied to the thermal coloring layer constituting the thermal recording medium. There is a problem that it causes a transition and causes discoloration of characters and the like. This problem may also be caused when a general food packaging wrap comes into contact with a food label made of a heat-sensitive recording medium for a certain period of time.

  Further, even when water, disinfecting ethanol, or the like adheres to a food label made of a heat-sensitive recording medium, it may cause discoloration of characters or the like.

  In addition, when a food container with a food label made of a heat-sensitive recording medium is heated using, for example, a microwave oven, the white portion of the food label is colored, and the printed characters cannot be discriminated. There is.

  As described above, with conventional thermal recording media, it is difficult to maintain vivid characters and the like for a practically sufficient period without causing discoloration of heat-printed characters and the like, and coloring of uncolored portions due to heating. there were.

  Methods for improving the above problem have been studied conventionally. For example, a method of providing a protective layer on a heat-sensitive color forming layer of a heat-sensitive recording medium is known. Specifically, the copolymer resin obtained by polymerizing methacrylamide and a vinyl monomer having a carboxyl group is substantially distributed on the surface of the resin particles obtained by polymerizing the vinyl monomer. A method of using an emulsion for a heat-sensitive recording material characterized in that for the protective layer is known (for example, see Patent Document 1).

  However, the heat-sensitive recording medium provided with the protective layer may still cause fading of heat-developed characters due to the influence of water or ethanol, and in particular, fading of characters or the like due to the influence of a plasticizer cannot be sufficiently prevented. There was a case. Further, since the heat-sensitive recording medium has insufficient heat resistance, the uncolored portion may be colored by heating with a microwave oven or the like.

  In this way, a protective layer with plasticizer resistance, water resistance, ethanol resistance, and heat resistance that can prevent the migration of plasticizer, water, ethanol, etc., and prevent fading of heat-printed characters, etc. Although development of a coating agent that can be formed is required, it has not yet been found.

JP 2001-270251 A

  The problem to be solved by the present invention is that it has a level of plasticizer resistance, water resistance and ethanol resistance that does not cause fading over time of heat-printed characters, etc., and uncolored portions develop color by heating. It is an object of the present invention to provide a coating agent for forming a protective layer that can be used in the production of a heat-sensitive recording medium having a level of heat resistance that can prevent this.

  The present inventors can solve the above-mentioned problems if the coating agent contains a vinyl polymer having either one or both of a tertiary amino group and a pyridinyl group and a vinyl polymer having an acid group. I found it.

That is, the present invention provides a protective layer for a thermal recording medium containing a vinyl polymer (A) having a tertiary amino group or a pyridinyl group, a vinyl polymer (B) having an acid group, and an aqueous medium (C). A coating layer, wherein the vinyl polymer (A) and the vinyl polymer (B) are a shell layer that is the vinyl polymer (B), and a core layer that is the vinyl polymer (A). The present invention relates to a coating agent for forming a protective layer of a heat-sensitive recording medium, characterized by forming composite resin particles (D) constituted by:

  The coating agent for forming a protective layer of the present invention has a level of plasticizer resistance, water resistance and ethanol resistance that does not cause discoloration over time of heat-printed characters, etc., and has excellent heat resistance. It can use suitably for manufacture of a medium.

  The coating agent for forming the protective layer of the heat-sensitive recording medium of the present invention includes a vinyl polymer (A) having a tertiary amino group or a pyridinyl group, a vinyl polymer (B) having an acid group, an aqueous medium (C), and It contains other components as necessary.

  As the coating agent for forming a protective layer of the heat-sensitive recording medium of the present invention, the vinyl polymer (A) and the vinyl polymer (B) are each independently dispersed in the aqueous medium (C), the vinyl A polymer (A) and a vinyl polymer (B) that form composite resin particles (D) and dispersed in an aqueous medium (C). The vinyl polymer (A) and the vinyl polymer (B) are aqueous. What was melt | dissolved in the medium (C) can be used. Among these, as the coating agent, it is preferable that the vinyl polymer (A) and the vinyl polymer (B) form composite resin particles (D) and are dispersed in an aqueous medium (C). The plasticizer property, water resistance, ethanol resistance and heat resistance can be further improved, and the film-forming property is excellent, and the generation of aggregates during production can be suppressed.

  The vinyl polymer (A) contained in the coating agent has one or both of a tertiary amino group and a pyridinyl group. The vinyl polymer (A) is, for example, a (meth) acrylic monomer having a tertiary amino group or a pyridinyl group, a polymerizable unsaturated monomer such as an olefin, and other polymerizable monomers as necessary. It is obtained by polymerizing saturation. The notation of “(meth) acryl” refers to one or both of methacryl and acryl.

  Since the tertiary amino group and the pyridinyl group are conventionally considered to cause discoloration over time of heat-printed characters and the like, such a functional group is usually added when forming a protective layer of a heat-sensitive recording medium. The fact is that trying to introduce it was avoided.

  In the present invention, by combining the vinyl monomer (A) having the tertiary amino group or pyridinyl group and the vinyl monomer (B) having an acid group, which will be described later, only fading over time such as characters. It was also found that fading of characters and the like due to plasticizer, water and ethanol can be prevented. Moreover, it discovered that the heat resistance of the level which can prevent that an uncolored part develops color by heating is provided.

  As the vinyl polymer (A), use of a tertiary amino group or a pyridinyl group having a tertiary amino group further improves the plasticizer resistance, water resistance, and ethanol resistance. In addition, it is preferable.

One or more types of functional groups selected from the group consisting of the amino group and the pyridinyl group are a total of 0.005 mmol to 0.7 mmol with respect to a total of 1 g of the vinyl polymer (A) and the vinyl polymer (B). It is preferable that the protective layer is present in the range of 0.06 mmol to 0.4 mmol to form a protective layer having further excellent plasticizer resistance, water resistance, ethanol resistance and heat resistance. More preferable. The substance amount of the functional group is a value calculated by the following calculation formula. [{(Mass of vinyl monomer having tertiary amino group / molecular weight of vinyl monomer having tertiary amino group) × (number of tertiary amino groups possessed by the vinyl monomer) + (vinyl having pyridinyl group] Monomer mass / molecular weight of vinyl monomer having pyridinyl group) × (number of pyridinyl groups possessed by the vinyl monomer)} / (used to produce vinyl polymer (A) and vinyl polymer (B) Total mass of vinyl monomers)] x 1000
All of the one or more functional groups selected from the group consisting of the amino group and pyridinyl group in the preferred range may be present in the vinyl polymer (A), and a part thereof is the vinyl polymer (B). Alternatively, it may be present in other vinyl polymers.

  Moreover, as said vinyl polymer (A), it is preferable to use what has an amide group other than an above described functional group, since the protective layer excellent in plasticizer resistance can be formed further.

The amide group is preferably present in a range of 0.1 mmol / g to 7 mmol / g with respect to 1 g in total of the vinyl polymer (A) and the vinyl polymer (B). The substance amount of the amide group is a value calculated by the following calculation formula. [{(Mass of vinyl monomer having amide group / Molecular weight of vinyl monomer having amide group) × (Number of amide groups of vinyl monomer)} / (Vinyl polymer (A) and vinyl weight) Total mass of vinyl monomers used in the production of coalescence (B))] × 1000
The preferred range of amide groups may all be present in the vinyl polymer (A), or a part thereof may be present in the vinyl polymer (B) or other vinyl polymers.

  For example, the vinyl polymer (A) is formed by the polymerization of the vinyl polymer (A) in the presence of a polymerization initiator and an aqueous medium (C) or in the presence of an aqueous dispersion of the vinyl polymer (B) described later. It can be produced by polymerizing vinyl monomers that can be used for production. Specifically, the vinyl polymer (A) includes at least one vinyl monomer selected from the group consisting of a vinyl monomer having a tertiary amino group and a vinyl monomer having a pyridinyl group, and necessary. Accordingly, a vinyl monomer mixture containing other vinyl monomers can be supplied in the presence of an aqueous dispersion of the vinyl polymer (B) and can be produced by radical polymerization.

  Examples of the vinyl monomer having a tertiary amino group include N, N-dimethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, (Meth) acrylic monomers such as N, N-dimethylaminopropyl (meth) acrylate and N, N-diethylaminopropyl (meth) acrylate can be used.

  The vinyl monomer having a tertiary amino group is 0.005 mmol / g with respect to 1 g in total of the vinyl monomer mixture used for the production of the vinyl polymer (A) and the vinyl polymer (B). It is preferable to use in the range which becomes -0.7 mmol / g, and it is more preferable to use in the range which becomes 0.06 mmol / g-0.4 mmol / g.

  Moreover, when manufacturing the said vinyl polymer (A), the vinyl monomer which has the said tertiary amino group and an amide group can be used.

  Examples of the vinyl monomer having a tertiary amino group and an amide group include (meth) acrylic monomers such as N, N-dimethylaminopropyl (meth) acrylamide and N, N-dimethylaminoethyl (meth) acrylamide. The body can be used.

  In the present invention, it is preferable to use a (meth) acrylic monomer having a tertiary amino group and an amide group, and it is even more excellent to use N, N-dimethylaminopropyl (meth) acrylamide. It is more preferable for forming a protective layer having plasticizer resistance.

  When a (meth) acrylic monomer having a tertiary amino group and an amide group is used, the vinyl monomer used in the production of the vinyl polymer (A) and the vinyl polymer (B) It is preferably used in a range of 0.005 mmol / g to 0.7 mmol / g, more preferably in a range of 0.06 mmol / g to 0.4 mmol / g with respect to 1 g of the total body mixture. preferable.

  Examples of the vinyl monomer having a pyridinyl group include N- [2- (meth) acryloyloxyethyl] piperidine, N- [2- (meth) acryloyloxyethyl] pyrrolidine, N- [2- (meta ) Acrylyloxyethyl] morpholine, 2-vinylpyridine, 4-vinylpyridine and the like can be used.

  When using a vinyl monomer having a pyridinyl group, for a total of 1 g of the vinyl monomer mixture used in the production of the vinyl polymer (A) and the vinyl polymer (B), It is preferable to use in the range of 0.005 mmol / g to 0.7 mmol / g, and it is more preferable to use in the range of 0.06 mmol / g to 0.4 mmol / g.

  The vinyl polymer (A) is an aqueous medium (C) or an aqueous dispersion of the vinyl polymer (B) described later, and the vinyl monomer and the polymerization initiator are separately or a mixture thereof. Can be produced by radical polymerization by supplying them in batches or divided.

  Examples of the polymerization initiator include radical polymerization initiators such as persulfate, organic peroxide, and hydrogen peroxide; 4,4′-azobis (4-cyanovaleric acid), 2,2′-azobis (2- Azo initiators such as amidinopropane) dihydrochloride can be used. The radical polymerization initiator may be used as a redox polymerization initiator in combination with a reducing agent described later.

  Examples of the persulfate that is a representative of the polymerization initiator include potassium persulfate, sodium persulfate, ammonium persulfate, and the like. Specific examples of the organic peroxide include benzoyl peroxide. Peroxyl such as diacyl peroxide such as lauroyl peroxide and decanoyl peroxide, dialkyl peroxide such as t-butylcumyl peroxide and dicumyl peroxide, t-butylperoxylaurate and t-butylperoxybenzoate Hydroperoxides such as ester, cumene hydroperoxide, paramentane hydroperoxide, t-butyl hydroperoxide, and the like can be used.

  Examples of the reducing agent include ascorbic acid and its salt, erythorbic acid and its salt, tartaric acid and its salt, citric acid and its salt, metal salt of formaldehyde sulfoxylate, sodium thiosulfate, sodium bisulfite, chloride Ferric iron or the like can be used.

  Moreover, as a vinyl polymer (B) used with the said coating agent, the vinyl polymer which has an acid group is used.

  As the acid group, for example, a carboxyl group, a sulfonic acid group, a carboxylate group or a sulfonate group in which a part or all of them are neutralized with a basic compound or the like can be used. Among them, the use of a carboxyl group or a carboxylate group can further improve the water dispersion stability of the composite resin particle (D) by imparting good water dispersibility to the vinyl polymer (B), and It is preferable for forming a protective layer having further excellent plasticizer resistance, water resistance and ethanol resistance.

  Examples of basic compounds that can be used for neutralizing the carboxyl group include organic amines such as ammonia, triethylamine, pyridine, and morpholine; alkanolamines such as monoethanolamine; metal bases including sodium, potassium, lithium, calcium, and the like Compounds. Of these, the use of ammonia is preferable for preventing the fading of the thermosensitive coloring layer and for forming a protective layer having further excellent water resistance.

  In the aqueous medium (C), the acid group is present in a range of 1.1 mmol / g to 7 mmol / g with respect to 1 g in total of the vinyl polymer (A) and the vinyl polymer (B). In order to form a good water dispersion stability of the vinyl polymer (A) and a protective layer having further excellent plasticizer resistance, water resistance, and ethanol resistance, it is preferably 1.1 mmol / g to 2 It is more preferable to use those present in the range of 1 mmol / g. The substance amount of the acid group is a value calculated by the following calculation formula. [{(Mass of vinyl monomer having acid group / Molecular weight of vinyl monomer having acid group) × (Number of acid groups of vinyl monomer)} / (Vinyl polymer (A) and vinyl weight) Total mass of vinyl monomers used in the production of coalescence (B))] × 1000

  Moreover, as said vinyl polymer (B), it is preferable to use what has a nitrile group in addition to the said acid group, since the protective layer excellent in plasticizer resistance and water resistance can be formed further. .

  The nitrile group is preferably present in a range of 0.1 mmol / g to 10 mmol / g with respect to 1 g in total of the vinyl polymer (A) and the vinyl polymer (B). The substance amount of the nitrile group is a value calculated by the following calculation formula. [{(Mass of vinyl monomer having nitrile group / molecular weight of vinyl monomer having nitrile group) × (number of nitrile groups possessed by the vinyl monomer)} / (vinyl polymer (A) and vinyl weight Total mass of vinyl monomers used in the production of coalescence (B))] × 1000

  Further, as the vinyl polymer (B), it is possible to use a polymer having a hydroxyl group, an amide group or the like in addition to the acid group, thereby forming a protective layer further excellent in plasticizer resistance and water resistance. This is preferable because it is possible.

  The vinyl polymer (B) can be produced by polymerizing a vinyl monomer that can be used for producing the vinyl polymer (B) in the presence of, for example, a polymerization initiator and an aqueous medium (C). it can. Specifically, the vinyl polymer (B) comprises a vinyl monomer mixture containing a vinyl monomer having an acid group and, if necessary, other vinyl monomers, a polymerization initiator and It can manufacture by supplying in presence of an aqueous medium (C) etc. and carrying out radical polymerization.

  Examples of the vinyl monomer having an acid group include (meth) acrylic monomers such as (meth) acrylic acid, allyl sulfonic acid, sulfopropyl (meth) acrylate, and (meth) acryloyloxynaphthalene sulfonic acid. Among them, it is preferable to use (meth) acrylic acid.

  The vinyl monomer having an acid group may be used in a range of 1.1 mmol / g to 7 mmol / g with respect to a total of 1 g of the vinyl polymer (A) and the vinyl polymer (B). Preferably, it is more preferable to use it in the range of 1.1 mmol / g to 2.1 mmol / g in order to further improve the plasticizer resistance and water resistance performance of the protective layer.

  Examples of other vinyl monomers that can be used in combination with the vinyl monomer having an acid group include a vinyl monomer having an amide group, a vinyl monomer having a nitrile group, and a vinyl monomer having a hydroxyl group. Etc. can be used.

  Examples of the vinyl monomer having an amide group include (meth) acrylamide, diethyl (meth) acrylamide, N-vinylpyrrolidone, N, N-dimethyl (meth) acrylamide, and N, N-diethyl (meth) acrylamide. N, N′-methylenebis (meth) acrylamide and the like can also be used.

  As the vinyl monomer having a nitrile group, (meth) acrylonitrile can be preferably used.

  The vinyl monomer having a nitrile group is 0.1 mmol / g to 10 mmol with respect to a total of 1 g of the vinyl monomer mixture used for the production of the vinyl polymer (A) and the vinyl polymer (B). It is preferable to use in the range which becomes / g.

  As the other vinyl monomer, in addition to those described above, methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate, isobutyl (meth) acrylate, Hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth) acrylate, isooctyl (meth) acrylate, n-nonyl (meth) acrylate, isononyl (meth) acrylate, n-decyl (meth) acrylate, Isodecyl (meth) acrylate, n-undecyl (meth) acrylate, n-dodecyl (meth) acrylate, n-tridecyl (meth) acrylate, n-tetradecyl (meth) acrylate, cyclohexyl (meth) acrylate, 2-hydroxy ester Ru (meth) acrylate, 2-hydroxypropyl (meth) acrylate, cyclohexylmaleimide, isopropylmaleimide, N-cyclohexylmaleimide, itaconimide, glycidyl (meth) acrylate, methylglycidyl (meth) acrylate, vinyl acetate, vinyl propionate, Vinyl laurate, styrene, chlorostyrene, chloromethylstyrene, α-methylstyrene, methyl vinyl ether, ethyl vinyl ether, isobutyl vinyl ether, N-methylaminoethyl (meth) acrylate, Nt-butylaminoethyl (meth) acrylate, amino It is possible to use methyl acrylate, aminoethyl acrylate, aminopropyl (meth) acrylate, amino-n-butyl (meth) acrylate, etc. Kill.

  As said other vinyl monomer, it is preferable to use an alkyl (meth) acrylate from the viewpoint of imparting further excellent water resistance to the protective layer, and in particular, n-butyl (meth) acrylate is used. Is preferred.

  The alkyl (meth) acrylate such as the n-butyl (meth) acrylate is in the range of 0.08 mmol / g to 7 mmol / g with respect to a total of 1 g of the vinyl polymer (A) and the vinyl polymer (B). Is preferably used.

  The vinyl polymer (B) is produced by radical polymerization by supplying the vinyl monomer and the polymerization initiator separately or in a batch or divided into the aqueous medium (C). can do.

  As said polymerization initiator, the thing similar to what was illustrated as what can be used when manufacturing the said vinyl polymer (A) can be used.

  The coating agent of the present invention may be produced by mixing a separately prepared aqueous dispersion of vinyl polymer (A) and an aqueous dispersion of vinyl polymer (B). The vinyl polymer (B) is produced by supplying a vinyl monomer or the like that can be used for the production of the vinyl polymer (A) to the aqueous dispersion of the polymer (B) and performing radical polymerization. The composite resin particles (D) constituted by the shell layer and the core layer which is the vinyl polymer (A) are preferable for obtaining a coating agent dispersed in the aqueous medium (C). The coating agent containing such composite resin particles (D) is excellent in water dispersion stability, and can form a protective layer having further excellent plasticizer resistance, water resistance, and ethanol resistance.

  As the vinyl polymer (A), use of a polymer having a weight average molecular weight in the range of 1,000 to 2,000,000 has further improved plasticizer resistance, water resistance and ethanol resistance. It is preferable when forming the provided protective layer. Moreover, as said vinyl polymer (B), it is preferable to use what has a weight average molecular weight of the range of 100,000-2,000,000, The range of 1,000,000-2,000,000 is used. In order to form a protective layer having even more excellent plasticizer resistance, water resistance and ethanol resistance, it is more preferable to use those having a weight average molecular weight of

  The vinyl polymer (A) and the vinyl polymer (B) obtained by the above method may be contained in the range of 1% by mass to 90% by mass with respect to the total amount of the protective layer forming coating agent of the present invention. preferable.

  Moreover, it is preferable that the said vinyl polymer (A) is contained in 0.001 mass%-15 mass% with respect to the total solid of the coating agent of this invention, and the said vinyl polymer (B) is It is preferably included in the range of 85% by mass to 99.999% by mass.

  Examples of the aqueous medium (C) used in the present invention include water, an organic solvent miscible with water, and a mixture thereof. Examples of the organic solvent miscible with water include alcohols such as methanol, ethanol, n- and isopropanol; ketones such as acetone and methyl ethyl ketone; polyalkylene glycols such as ethylene glycol, diethylene glycol and propylene glycol; alkyl ethers of polyalkylene glycol; And lactams such as N-methyl-2-pyrrolidone. In the present invention, only water may be used, a mixture of water and an organic solvent miscible with water may be used, or only an organic solvent miscible with water may be used. From the viewpoint of safety and load on the environment, water alone or a mixture of water and an organic solvent miscible with water is preferable, and only water is particularly preferable.

  The aqueous medium (C) is used from the viewpoint of suppressing a sudden increase in viscosity during production, and improving the productivity of the coating agent for forming the protective layer, the ease of coating, the drying property, and the like. It is preferable to use in the range of 5% by mass to 80% by mass with respect to the total amount of the coating agent for forming a protective layer of the invention.

  The coating agent for forming a protective layer of the present invention obtained by the above method is adjusted to a hydrogen ion concentration (pH) in the range of 8 to 13 in order to further improve the plasticizer resistance and ethanol resistance. Preferably, it is more than 8 and 13 or less.

  In addition to the above-described coating agent for forming a protective layer of the present invention, if necessary, for example, an inorganic filler, a lubricant, a thickening agent, a crosslinking agent, a dispersing agent, a film forming aid, a thixotropic agent, A pigment or the like can be used as long as the object of the present invention is not impaired.

  As the inorganic filler, for example, calcium carbonate, clay, talc, titanium oxide, magnesium carbonate, zinc oxide, aluminum silicate, silica, aluminum hydroxide and the like can be used. Among them, it is preferable to use clay, silica, aluminum hydroxide, and in particular, it is easy to disperse in the aqueous medium (B), does not inhibit color development of the thermosensitive coloring layer constituting the thermosensitive recording medium, and a plasticizer, etc. It is more preferable to use clay because it prevents the color shift and further improves the anti-fading property of heat-printed characters.

  The inorganic filler is preferably used in a range of 0.1 to 9 times the total amount of the vinyl polymer (A) and the vinyl monomer (B).

  As the lubricant, higher fatty acid metal salts such as higher fatty acid, higher fatty acid amide, zinc stearate, aluminum stearate, calcium stearate and other stearic acid amide emulsions are preferably used.

  Examples of the thickener include acrylic polymer and synthetic rubber latex that can be thickened by adjusting to alkaline, urethane resin that can be thickened by association of molecules, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, polyvinyl alcohol, Water-added castor oil, amide wax, polyethylene oxide, metal soap, dibenzylidene sorbitol and the like can be used. As the thickener, it is preferable to use an alkaline thickener which is alkaline and exhibits thickening, and more preferably an acrylic thickener.

  As said acrylic thickener, the vinyl polymer obtained by superposing | polymerizing a (meth) acryl monomer or its mixture can be used, for example, the vinyl monomer which has an acid group, and C1-C2 It is preferable to use an acrylic thickener (E) obtained by polymerizing a (meth) acrylic monomer mixture containing a (meth) acrylic monomer having an alkyl group.

  The vinyl monomer having an acid group is preferably used in the range of 20% by mass to 50% by mass with respect to the total amount of the (meth) acrylic monomer mixture. The (meth) acrylic monomer having an alkyl group having 1 to 2 carbon atoms may be used in the range of 50% by mass to 80% by mass with respect to the total amount of the (meth) acrylic monomer mixture. preferable.

  Examples of the vinyl monomer having an acid group include (meth) acrylic monomers such as (meth) acrylic acid, allyl sulfonic acid, sulfopropyl (meth) acrylate, and (meth) acryloyloxynaphthalene sulfonic acid. Among them, it is preferable to use (meth) acrylic acid.

  As the (meth) acrylic monomer having an alkyl group having 1 to 2 carbon atoms, methyl (meth) acrylate or ethyl (meth) acrylate can be used.

  As said (meth) acryl monomer mixture which can be used for manufacture of the said acrylic thickener (E), what contains other (meth) acryl monomers other than what was mentioned above can also be used. . However, the content of the (meth) acryl monomer having a tertiary amino group or the (meth) acryl monomer having a pyridinyl group is the (meth) acryl monomer mixture. It is preferable to use one that is 0% by mass to 1% by mass with respect to the total amount of the above, and using one that is 0% by mass maximizes the thickening effect of the acrylic thickener (E) This is more preferable.

  Examples of alkali components that can be used for thickening the thickener include organic amines such as ammonia, triethylamine, pyridine, and morpholine, alkanolamines such as monoethanolamine, sodium, potassium, lithium, and calcium. Metal base compounds containing etc. can be used. Among these, use of ammonia is preferable in order to prevent a decrease in image density of the heat-sensitive color developing layer and to form a protective layer having further excellent water resistance.

  The thickener is preferably used in the range of 0.01% by mass to 10% by mass with respect to the total mass of the vinyl polymer (A) and the vinyl polymer (B), and 0.01 It is more preferable to use 2% by mass to 2% by mass, and the viscosity may be adjusted to suit the coating method. The thickener is preferably used in combination with the inorganic filler.

  In addition, as the crosslinking agent, for example, a crosslinking agent such as a melamine compound, an epoxy compound, a glyoxal compound, a methylol urea compound, an aldehyde compound, a zirconium compound, an isocyanate compound, or an aziridine compound is used. Can do. Among these, it is preferable to use ammonium zirconium carbonate in order to further improve the plasticizer resistance, water resistance, and ethanol resistance of the protective layer.

  When using the said crosslinking agent, the said crosslinking agent should be used in 0.1 mass%-10 mass% with respect to the total mass of the said vinyl polymer (A) and the said vinyl polymer (B). Is preferred.

  Examples of the dispersant include low molecular weight surfactants such as di-2-ethylhexylsulfosuccinate, dodecylbenzenesulfonate, alkyldiphenyl ether disulfonate, alkylnaphthalenesulfonate, and hexametaphosphate; starch Water-soluble polymers such as polycarboxylates such as hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, modified polyvinyl alcohol, and ammonium salts of styrene-maleic acid monoalkyl ester copolymers can be used.

  The film-forming aid is not particularly limited. For example, an anionic surfactant (dioctylsulfosuccinate soda salt, etc.), a hydrophobic nonionic surfactant (sorbitan monooleate, etc.), a polyether-modified siloxane, Silicone oil etc. are mentioned.

  Examples of the thixotropy-imparting agent include fatty acid, fatty acid metal salt, fatty acid ester, paraffin, resin acid, surfactant, polyacrylic acid and the like surface-treated filler, polyvinyl chloride powder, hydrogenated castor oil, Fine powder silica, organic bentonite, sepiolite and the like can be mentioned.

  As the pigment, known and commonly used inorganic pigments and organic pigments can be used.

  As the inorganic pigment, for example, titanium oxide, antimony red, bengara, cadmium red, cadmium yellow, cobalt blue, bitumen, ultramarine, carbon black, graphite and the like can be used.

  Next, the thermal recording medium of the present invention will be described.

  The heat-sensitive recording medium of the present invention has at least a heat-sensitive color developing layer (c1) on a substrate, and a protective layer formed on the heat-sensitive color developing layer (c1) using the protective layer forming coating agent. (C2).

  The protective layer (c2) is formed by applying a coating agent for forming a protective layer of the thermosensitive recording medium on the thermosensitive coloring layer (c1) and then volatilizing the aqueous medium (B) contained in the applied layer. It is formed. The thermosensitive recording medium may have a layer other than the thermosensitive coloring layer (c1) and the protective layer (c2) as necessary.

  Examples of the substrate include paper, plastic film, and synthetic paper. Any of these may be used depending on the application for which the thermal recording medium is used.

  The thermosensitive coloring layer (c1) constituting the thermosensitive recording medium of the present invention generally contains a color-developing substance, a developer and a binder resin. The heat-sensitive color developing layer (c1), for example, develops color by the reaction between the color developer and the color developer heated and melted by a thermal head such as a heat-sensitive printer, and forms desired characters and pictures on the heat-sensitive recording medium. .

  Examples of the color former include 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide (crystal violet lactone), 3,3-bis (p-dimethylaminophenyl) phthalide, 3- (P-dimethylaminophenyl) -3- (1,2-dimethylindol-3-yl) phthalide, 3- (p-dimethylaminophenyl) -3- (2-methylindol-3-yl) phthalide, 3- (P-dimethylaminophenyl) -3- (2-phenylindol-3-yl) phthalide, 3,3-bis (1,2-dimethylindol-3-yl) -5-dimethylaminophthalide, 3,3 -Bis (1,2-dimethylindol-3-yl) -6-dimethylaminophthalide, 3,3-bis (9-ethylcarbazol-3-yl) -5 Dimethylaminophthalide, 3,3-bis (2-phenylindol-3-yl) -5-dimethylaminophthalide, and 3-p-dimethylaminophenyl-3- (1-methylpyrrol-2-yl)- Triarylmethane compounds (1) such as 6-dimethylaminophthalide, 4,4′-bis (dimethylaminophenyl) benzhydrylbenzyl ether, N-chlorophenylleucooramine, and N-2,4,5- Diphenylmethane compounds (2) such as trichlorophenyl leucooramine, rhodamine B anilinolactam, rhodamine Bp-chloroanilinolactam, 3-diethylamino-7-dibenzylaminofluorane, 3-diethylamino-7-octylamino Fluorane, 3-diethylamino-7-phenylfluorane, 3- Ethylamino-7-chlorofluorane, 3-diethylamino-6-chloro-7-methylfluorane, 3-diethylamino-7- (3,4-dichloroanilino) fluorane, 3-diethylamino-7- (2-chloroanilino) ) Fluorane, 3-diethylamino-6-methyl-7-anilinofluorane, 3-dibutylamino-6-methyl-7-anilinofluorane, 3-dipentylamino-6-methyl-7-anilinofluorane, 3- (N-ethyl-N-tolyl) amino-6-methyl-7-anilinofluorane, 3-piperidino-6-methyl-7-anilinofluorane, 3- (N-ethyl-N-tolyl) Amino-6-methyl-7-phenethylfluorane, 3-diethylamino-7- (4-nitroanilino) fluorane, 3-dibutylamino-6- Methyl-7-anilinofluorane, 3- (N-methyl-N-propyl) amino-6-methyl-7-anilinofluorane, 3- (N-ethyl-N-isoamyl) amino-6-methyl- 7-anilinofluorane, 3- (N-methyl-N-cyclohexyl) amino-6-methyl-7-anilinofluorane, and 3- (N-ethyl-N-tetrahydrofurfuryl) amino-6-methyl Xanthene compounds (3) such as -7-anilinofluorane, thiazine compounds (4) such as benzoyl leucomethylene blue and p-nitrobenzoyl leucomethylene blue, 3-methylspirodinaphthopyran, 3-ethylspirodinaphtho Pyran, 3,3′-dichlorospirodinaphthopyran, 3-benzylspirodinaphthopyran, 3-methylnaphtho- (3-methoxy Benzo) spiropyran, and 3-propyl-spiro-based compounds such as Spirobenzopyran (5), can be used in combination use of two or more in combination.

  Examples of the developer include 4-phenylphenol, 4-t-butylphenol, 4-hydroxyacetophenone, 2,2′-dihydroxydiphenyl, 2,2′-methylenebis (4-chlorophenol), 2, 2′-methylenebis (4-methyl-6-tert-butylphenol), 4,4′-ethylenebis (2-methylphenol), 1,1-bis (4-hydroxyphenyl) cyclohexane, 2,2-bis (4 '-Hydroxyphenyl) propane, 4,4'-cyclohexylidenebis (2-isopropylphenol), 4,4'-dihydroxydiphenylsulfone, 2,4'-dihydroxydiphenylsulfone, benzyl 4-hydroxybenzoate, 4- Dimethyl hydroxyphthalate, bis (4-hydroxyphenyl) acetate Phenyl compounds such as alkyl esters of gallic acid, novolac type phenol resin, phthalic acid monoanilide paraethoxybenzoic acid, parabenzyloxybenzoic acid, 3-5-di-t-butylsalicylic acid, 3-5-di-α-methyl Aromatic carboxylic acids such as benzylsalicylic acid, 3-methyl-5-t-butylsalicylic acid, 4-n-octyloxycarbonylaminosalicylic acid, 4-n-decyloxycarbonylaminosalicylic acid, and polyvalent metal salts thereof. Can be used.

  Examples of the binder resin of the thermosensitive coloring layer (c1) include polyvinyl alcohol, modified polyvinyl alcohol, starch, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, sodium polyacrylate, acrylic acid amide / acrylic acid ester Water-soluble adhesives such as polymers, acrylic acid amide / acrylic acid ester / methacrylic acid terpolymers, alkali salts of styrene / maleic anhydride copolymers, and alkali salts of ethylene / maleic anhydride copolymers, Latex such as polyvinyl acetate, polyurethane, polyacrylate, styrene / butadiene copolymer, acrylonitrile / butadiene copolymer, methyl acrylate / butadiene copolymer, and ethylene / vinyl acetate copolymer It can be used.

  The thermosensitive coloring layer (c1) preferably contains a sensitizer for the purpose of improving the coloring sensitivity.

  Examples of the sensitizer include stearic acid amide, N-hydroxymethyl stearic acid amide, behenic acid amide, N-hydroxymethyl behenic acid amide, N-stearyl stearic acid amide, and fatty acid amides such as ethylene bis stearic acid amide, Urea derivatives such as N-stearyl urea, naphthol derivatives such as 2-benzyloxynaphthalene, biphenyl derivatives such as m-terphenyl, 4-benzylbiphenyl, and 4-allyloxybiphenyl, 2,2′-bis (4-methoxy) Phenoxy) diethyl ether, polyether compounds such as bis (4-methoxyphenyl) ether, carbo such as diphenyl adipate, dibenzyl oxalate, di (4-chlorobenzyl) ester oxalate, dimethyl terephthalate, and dibenzyl terephthalate Acid esters, sulfonic acid esters such as benzenesulfonic acid phenyl ester, diphenyl sulfones such as bis (4-allyloxyphenyl) sulfone, diketones such as 4-acetylacetophenone, acetoacetic anilide, fatty acid anilide, and 1,2-bis ( 3,4-dimethylphenyl) ethane or the like can be used alone or in combination of two or more.

  The thermosensitive coloring layer (c1) preferably contains a pigment in order to improve whiteness.

  Examples of the pigment include clay, talc, heavy calcium carbonate, light calcium carbonate, silica, calcium silicate, aluminum silicate, titanium dioxide, aluminum hydroxide, zinc oxide, and other inorganic pigments, urea-formalin resin, phenol Organic pigments using resins, epoxy resins, melamine resins, guanamine resins, formalin resins, and the like can be used.

  The composition for forming a heat-sensitive color forming layer containing the color-developing substance, the developer, the binder resin, and other various additives that can be used for forming the heat-sensitive color developing layer (c1) is, for example, a ball mill or a sand. It is preferable to be dispersed in an aqueous medium using a dispersing machine such as a grinder. In that case, you may use a dispersing agent in the range which does not inhibit color development.

  Examples of the dispersant include low molecular weight surfactants such as di-2-ethylhexylsulfosuccinate, dodecylbenzenesulfonate, alkyldiphenyl ether disulfonate, alkylnaphthalenesulfonate, and hexametaphosphate; starch Water-soluble polymers such as polycarboxylates such as hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, modified polyvinyl alcohol, and ammonium salts of styrene-maleic acid monoalkyl ester copolymers can be used.

  The composition for forming a thermosensitive coloring layer may contain components other than those described above. As said other component, various resin components, such as a urethane resin, an acrylic resin, a polyester resin, an epoxy resin, can be used, for example.

  The heat-sensitive recording medium of the present invention is formed by applying the heat-sensitive color forming layer-forming composition on various substrates and then drying to form the heat-sensitive color developing layer (c1) on the substrate, and then the present invention. The coating agent for forming a protective layer can be produced by applying and drying on the thermosensitive coloring layer (c1) to form a protective layer (c2).

  The composition for forming a heat-sensitive color forming layer and the coating agent for forming a protective layer of the present invention can be applied using a coating apparatus such as an air knife coater, a blade coater, a curtain coater, or a bar coater.

The coating amount of the composition for forming a thermosensitive coloring layer is preferably in the range of ² g / m ^{2 to} 12 g / m ² in terms of nonvolatile content on the substrate. The coating amount of the protective layer forming coating agent of the present invention, on the heat-sensitive coloring layer (c1), is preferably in the range of ^1g / ^{m 2 ~10g} / m 2 non-volatiles.

  As a method of drying the thermosensitive color forming layer forming composition and the protective layer forming coating agent of the present invention after applying the method, there is a method using, for example, a dryer. The temperature in the drying step is preferably 70 ° C. or less from the viewpoint of preventing color development of the thermosensitive color forming layer constituting the thermosensitive recording medium of the present invention. Since the color development of the thermosensitive color-developing layer depends on the color-developing substance used and the developer, etc., as long as the heat-sensitive color developing layer does not develop color, drying at as high a temperature as possible improves the production efficiency of the thermosensitive recording medium. It is preferable from the viewpoint.

  The heat-sensitive recording medium of the present invention is provided between the heat-sensitive color developing layer (c1) and the base material for the purpose of improving thermal response and adhesion between the base material and the heat-sensitive color developing layer (c1). You may have a well-known and usual undercoat layer as needed.

  Further, the thermosensitive recording medium of the present invention is provided with a well-known conventional back on the back surface of the thermosensitive recording medium for the purpose of preventing the migration of the plasticizer and the like from the back surface of the substrate and suppressing the curvature of the thermosensitive recording medium. You may have a coat layer. Further, the back surface of the heat-sensitive recording medium of the present invention may have an adhesive layer.

  Hereinafter, the present invention will be specifically described by way of examples and comparative examples.

[Synthesis Example 1]
40 parts by mass (4.65 mmol / g) of methacrylic acid, 60 parts by mass (5.99 mmol / g) of ethyl acrylate and Lebenol WZ (manufactured by Kao Corporation, polyoxyethylene alkylphenyl ether sulfate, nonvolatile content 25% by mass ) A mixture of 12 parts by mass and 50 parts by mass of distilled water was designated as pre-emulsion X.

  151 parts by mass of distilled water was put into a separable flask equipped with a stirrer and a reflux condenser, and the inside of the separable flask was replaced with nitrogen gas, and then the temperature was raised to 50 ° C. Under stirring, 1 part by mass of 8 parts by mass of 10 parts by mass of ammonium persulfate aqueous solution as a part of the pre-emulsion X was supplied, and the temperature was raised to 80 ° C.

  Thereafter, the remaining amount of the pre-emulsion X and 10 parts by mass of 1% by mass of ammonium persulfate were dropped and polymerized in 60 minutes while maintaining the inside of the separable flask at 80 ° C. ± 2 ° C. After completion of dropping, the mixture was stirred at 80 ° C. ± 2 ° C. for 30 minutes.

  Thereafter, the contents were cooled and diluted with distilled water to obtain an acrylic thickener having a nonvolatile content of 23.5% by mass and a pH of 2.

[Example 1]
250 parts by mass of distilled water and 5 parts by mass of Aqualon KH-10 (reactive surfactant manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) are mixed in a separable flask equipped with a stirrer and a reflux condenser, and the inside of the separable flask is mixed. After replacing with nitrogen gas, the temperature was raised to 80 ° C., and 8 parts by mass of a 10% by mass ammonium persulfate aqueous solution was supplied.

  Next, the following vinyl monomer mixture 1 was dropped into the separable flask over 6 hours, and after further polymerizing at 80 ° C. for 2 hours, 0.1 part by weight of dimethylaminopropylacrylamide and 10% by weight of excess 0.5 parts by mass of an aqueous ammonium sulfate solution was supplied, polymerized at 80 ° C. for 2 hours, and cooled to room temperature.

  Thereafter, by supplying ion-exchanged water, a vinyl polymer composition (X-1) having a nonvolatile content of 20% by mass was obtained.

(Vinyl monomer mixture 1)
n-butyl acrylate; 30 parts by weight methyl methacrylate; 45 parts by weight methacrylic acid; 11 parts by weight styrene; 9 parts by weight acrylamide; 5 parts by weight

  Next, a dispersion obtained by dispersing 29 parts by mass of the vinyl polymer composition (X-1) and 13 parts by mass of UW-90 [manufactured by BASF, Kaolin clay] in 9 parts by mass of ion-exchanged water, Hydrin Z-7 -30 [manufactured by Chukyo Yushi Co., Ltd., 31.5 mass% aqueous dispersion of zinc stearate] 1.3 mass parts, 0.37 mass parts of the acrylic thickener obtained in Synthesis Example 1, By supplying ion-exchanged water and ammonia water, the hydrogen ion concentration (pH) is adjusted to 9, and the mixture is stirred for 15 minutes to thereby form a protective layer-forming coating agent I for the thermal recording medium (nonvolatile content 15% by mass, pH 9). Got.

[Example 2]
Instead of 0.1 parts by mass of dimethylaminopropylacrylamide, 7 parts by mass of vinylpyridine and 7 parts by mass of n-butyl acrylate were used. Instead of the vinyl monomer mixture 1, a vinyl monomer mixture 2 described later was used. A protective layer-forming coating agent II (nonvolatile content: 15% by mass, pH 9) was obtained in the same manner as in Example 1 except that it was used.

(Vinyl monomer mixture 2)
6 parts by weight n-butyl acrylate; 15 parts by weight methyl methacrylate; 13 parts by weight styrene; 18 parts by weight acrylic acid; 25 parts by weight methacrylonitrile; 8 parts by weight methacrylamide; 1 part by weight

[Example 3]
Instead of 0.1 parts by weight of dimethylaminopropylacrylamide, 1 part by weight of dimethylaminopropylacrylamide is used, instead of the vinyl monomer mixture 1, a vinyl monomer mixture 3 described later is used, and A protective layer-forming coating agent III (non-volatile) was prepared in the same manner as in Example 1 except that 25 parts by mass of a 4% by mass aqueous potassium persulfate solution was used instead of 8 parts by mass of the 10% by mass ammonium persulfate aqueous solution. 15% by weight, pH 9) was obtained.

(Vinyl monomer mixture 3)
n-butyl acrylate; 27 parts by weight methacrylic acid; 12 parts by weight 2-hydroxyethyl methacrylate; 5 parts by weight acrylonitrile; 50 parts by weight methacrylamide; 5 parts by weight

[Example 4]
Instead of 0.1 parts by mass of dimethylaminopropylacrylamide, 2 parts by mass of dimethylaminopropylacrylamide and 5 parts by mass of n-butyl acrylate were used, and instead of the vinyl monomer mixture 1, a vinyl monomer mixture described later was used. A protective layer-forming coating agent IV (nonvolatile content: 15% by mass, pH 9) was obtained in the same manner as in Example 1 except that No. 4 was used.

(Vinyl monomer mixture 4)
n-butyl acrylate; 15 parts by weight methyl methacrylate; 9 parts by weight styrene; 20 parts by weight methacrylic acid; 14 parts by weight 2-hydroxyethyl acrylate; 5 parts by weight acrylonitrile; 20 parts by weight acrylamide;

[Example 5]
250 parts by mass of distilled water and 5 parts by mass of Aqualon KH-10 (reactive surfactant manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) are mixed in a separable flask equipped with a stirrer and a reflux condenser, and the inside of the separable flask is mixed. After substituting with nitrogen gas, the temperature was raised to 80 ° C., and 0.8 mass parts of 4,4-azobis (4-cyanovaleric acid) was neutralized with 1.6 mass parts of 25 mass% ammonia. An aqueous solution containing 80 parts by mass of Japanese product and ion-exchanged water was supplied.

  Next, the following vinyl monomer mixture 5 was dropped into the separable flask over 6 hours, and further polymerized at 80 ° C. for 2 hours, then polymerized at 80 ° C. for 2 hours, and cooled to room temperature.

  Then, by supplying ion exchange water, a vinyl polymer composition (Y-5) having a nonvolatile content of 20% by mass was obtained.

(Vinyl monomer mixture 5)
Butyl acrylate; 25 parts by mass Acrylic acid; 50 parts by mass Acrylonitrile; 10 parts by mass Acrylamide; 5 parts by mass Next, 40 parts by mass of distilled water and Aqualon KH-10 (No. 1) were added to a separable flask equipped with a stirrer and a reflux condenser. 1 part by weight of a reactive surfactant (manufactured by Ichi Kogyo Seiyaku Co., Ltd.) and 10 parts by weight of dimethylaminoethyl methacrylate were mixed, the inside of the separable flask was replaced with nitrogen gas, the temperature was raised to 80 ° C., and 10 parts by weight. 5 parts by weight of an aqueous ammonium persulfate solution was supplied. After making it superpose | polymerize at 80 degreeC for 3 hours, it cooled to room temperature and obtained the polymer composition (Z-5).

  Then, the said polymer composition (Z-5) is mixed with the said polymer composition (Y-5), and a vinyl polymer composition (X-5 with a non volatile matter of 20 mass% is supplied by supplying ion-exchange water. )

Next, a dispersion obtained by dispersing 29 parts by mass of the vinyl polymer composition (X-5) and 13 parts by mass of UW-90 [manufactured by BASF, Kaolin clay] in 9 parts by mass of ion-exchanged water, Hydrin Z- 7-30 [manufactured by Chukyo Yushi Co., Ltd., 31.5 mass% aqueous dispersion of zinc stearate] 1.3 mass parts, and acrylic thickener obtained in Synthesis Example 1 0.37 mass parts Then, the hydrogen ion concentration (pH) is adjusted to 9 by supplying ion-exchanged water and ammonia water, and the coating agent V for forming the protective layer of the heat-sensitive recording medium (nonvolatile content: 15% by mass, pH 9) by stirring for 15 minutes. )

[Example 6]
Instead of using 0.1 parts by weight of dimethylaminopropylacrylamide, 5 parts by weight of dimethylaminopropylacrylamide, and using a vinyl monomer mixture 6 described later instead of the vinyl monomer mixture 1, In the same manner as in Example 1, a protective layer-forming coating agent VI (nonvolatile content: 15% by mass, pH 9) was obtained.

(Vinyl monomer mixture 6)
n-butyl acrylate; 32 parts by weight methacrylic acid; 11 parts by weight acrylonitrile; 6 parts by weight acrylamide; 46 parts by weight

[Comparative Example 1]
In a separable flask equipped with a stirrer and a reflux condenser, 250 parts by mass of distilled water and 0.3 part by mass of sodium dodecylbenzenesulfonate were mixed, the inside of the separable flask was replaced with nitrogen gas, and the temperature was raised to 70 ° C. 25 mass parts of 4 mass% potassium persulfate aqueous solution was supplied.

  Next, a vinyl monomer mixture 1 ′ described later was dropped into the separable flask over 4 hours, and then polymerized at 80 ° C. for 2 hours, cooled to room temperature, and a vinyl polymer having a nonvolatile content of 20% by mass. A composition (X′-1) was obtained.

(Vinyl monomer mixture 1 ')
Methyl methacrylate; 31 parts by weight Butyl methacrylate; 39 parts by weight Methacrylic acid; 7 parts by weight Acrylonitrile; 18 parts by weight Methacrylamide: 5 parts by weight

  Next, a dispersion obtained by dispersing 29 parts by mass of the vinyl polymer composition (X′-1) and 13 parts by weight of kaolin clay for papermaking [manufactured by BASF, UW-90] in 9 parts by mass of ion-exchanged water, hydrin Z-7-30 [manufactured by Chukyo Yushi Co., Ltd., 31.5 mass% aqueous dispersion of zinc stearate] 1.3 mass parts, and acrylic thickener obtained in Synthesis Example 1 0.37 mass The hydrogen ion concentration (pH) was adjusted to 9 by supplying ion exchange water and ammonia water to obtain a protective layer forming coating agent I ′ (nonvolatile content 15% by mass, pH 9) of the thermal recording medium. .

[Comparative Example 2]
A protective layer-forming coating agent II ′ (non-volatile content: 15 mass) in the same manner as in Comparative Example 1 except that a vinyl monomer mixture 2 ′ described later was used instead of the vinyl monomer mixture 1 ′. %, PH 9).

(Vinyl monomer mixture 2 ')
Butyl acrylate; 20 parts by weight Methyl methacrylate; 39 parts by weight Methacrylic acid; 20 parts by weight 2-hydroxyethyl methacrylate; 5 parts by weight Acrylonitrile; 12 parts by weight Acrylamide; 4 parts by weight

[Comparative Example 3]
Z-200 [acetoacetoxy group-modified polyvinyl alcohol manufactured by Nippon Synthetic Chemical Industry Co., Ltd.] was dissolved in warm water to prepare a 15% by mass aqueous solution. Thereafter, a dispersion obtained by dispersing 13 parts by mass of UW-90 [manufactured by BASF, Kaolin clay] in 9 parts by mass of ion-exchanged water, and Hydrin Z-7-30 [manufactured by Chukyo Yushi Co., Ltd., zinc stearate 31. 5 mass% aqueous dispersion] 1.3 parts by mass, 0.05 part by mass of glyoxal, and ion-exchanged water are supplied to form a coating agent III ′ for forming a protective layer of a thermal recording medium (non-volatile content: 15% by mass, pH 7) was obtained.

[Method for producing thermal recording medium]
The protective layer-forming coating agent obtained in the examples or comparative examples was dried on a thermal paper for fax machine (UG-0010A4 manufactured by Matsushita Electric Industrial Co., Ltd.) without a protective layer using No. 10 wire bar. After the coating was applied to 3 g / m ^{2, the} film was dried at 60 ° C. for 90 seconds using a dryer to obtain a thermosensitive recording medium having a protective layer formed on the surface.

[Method for evaluating plasticizer resistance]
A square (6 mm × 30 mm) black 1 cm solid image was printed on the heat-sensitive recording medium obtained above using a word processor (Shoin WD-220F) manufactured by Sharp Corporation to prepare a test piece. The black color density printed on each test piece was measured using a reflection color density meter D186 manufactured by Gretag Macbeth.

  Subsequently, a wrap made of vinyl chloride (polymer wrap manufactured by Shin-Etsu Polymer Co., Ltd.) was brought into close contact with the surface of the protective layer of the test piece so as to cover the printed portion of each test piece, and left at 40 ° C. for 24 hours. The color density of the printed part after standing was measured by the same method as described above. The heat-sensitive color density retention before and after standing was calculated according to the following formula, and the plasticizer resistance was evaluated based on this value.

Retention rate = [(thermal color density after standing) / (thermal color density before standing)] × 100
Those having a retention rate of approximately 90% or more were evaluated as having excellent plasticizer resistance.

[Water resistance evaluation method]
[Evaluation method of water blocking resistance]
Prepare two thermal recording media on which one type of protective layer is formed. After one drop (approximately 1 ml) of tap water is attached to the surface of the protective layer of one thermal recording medium, In a state where the protective layer of the heat-sensitive recording medium was placed in contact, a load of 5 kg was applied and the substrates were bonded for 10 seconds.

  Thereafter, the state of the protective layer when the two thermal recording media were peeled was visually evaluated.

  1: No tearing was observed on the surface of the protective layer, and no thermal paper tearing was observed.

  2: Although the surface of the protective layer was slightly blocked, the thermal recording medium was not broken.

  3: Some blocking occurred on the surface of the protective layer, and the protective layer was broken within 5% of the area of the surface of the protective layer where the load was applied, but the thermal recording medium was broken. I couldn't.

  4: Although tearing was observed in the range of 5% or more of the area of the surface of the protective layer where the load was applied, tearing of the thermal recording medium was not observed.

  5: Thermal recording medium was torn.

[Evaluation method for water rubbing resistance]
Of the protective layer surface of each heat-sensitive recording medium, the uncolored portion and the colored portion were rubbed 20 times with a 500 g load with a cotton swab soaked in water, and the surface condition was visually evaluated.

  1: There was no change before and after rubbing.

  2: After rubbing, dissolution of the protective layer was confirmed.

  3: After rubbing, it was confirmed that some of the colored portions were missing on the surface of the protective layer.

  4: After the rubbing, a remarkable omission of the colored portion was confirmed on the surface of the protective layer.

  5: After rubbing, the thermal paper broke.

[Evaluation method of ethanol resistance]
Of the surface of the protective layer of each heat-sensitive recording medium, the uncolored portion and the colored portion were applied to the surface of the protective layer by bringing a paintbrush soaked with ethanol into contact therewith.

  After coating, the surface state of the film was visually observed, and the ethanol resistance of the film was evaluated according to the following criteria.

[Evaluation criteria]
1: There was no change before and after ethanol application.

  2: After application of ethanol, it was confirmed that the color development on the surface of the protective layer was missing in a range of less than 5%.

  3: After applying ethanol, omission of a range of 5% or more and less than 30% of the colored portion on the surface of the protective layer was confirmed.

  4: After ethanol application, omission of a range of 30% or more and less than 50% of the colored portion on the surface of the protective layer was confirmed.

  5: After application of ethanol, a range of 50% or more of the colored portion on the surface of the protective layer was lost.

[Method for evaluating heat resistance]
The heat-sensitive recording medium prepared above was placed in a dryer at 70 ° C. for 24 hours, and the color change of the uncolored portion before being placed in the dryer was measured using a reflection color density densitometer D186 manufactured by Gretag Macbeth. . The lower the color density, the better the result.

Claims (9)

A coating agent for forming a protective layer of a thermal recording medium containing a vinyl polymer (A) having a tertiary amino group or a pyridinyl group, a vinyl polymer (B) having an acid group, and an aqueous medium (C). ,
Composite resin particles in which the vinyl polymer (A) and the vinyl polymer (B) are composed of a shell layer that is the vinyl polymer (B) and a core layer that is the vinyl polymer (A). A coating agent for forming a protective layer of a heat-sensitive recording medium, characterized by forming (D) .   One or more types of functional groups selected from the group consisting of the amino group and the pyridinyl group are a total of 0.005 mmol to 0.7 mmol with respect to a total of 1 g of the vinyl polymer (A) and the vinyl polymer (B). Wherein the acid group is present in a range of 1.1 mmol to 7 mmol with respect to a total of 1 g of the vinyl polymer (A) and the vinyl polymer (B). Item 2. A coating agent for forming a protective layer of a thermal recording medium according to Item 1.   The coating agent for forming a protective layer of a thermosensitive recording medium according to claim 1, wherein the acid group is a carboxyl group, a sulfonic acid group, or a carboxylate group or a sulfonate group obtained by neutralizing them.   The coating agent for forming a protective layer of a thermosensitive recording medium according to claim 1, wherein the acid group of the vinyl polymer (B) is formed by neutralizing a carboxyl group with ammonia.   The coating agent for forming a protective layer of a thermal recording medium according to claim 1, wherein the hydrogen ion concentration (pH) is in the range of 8 to 13.   The coating agent for forming a protective layer of a thermosensitive recording medium according to claim 1, wherein the vinyl polymer (A) or the vinyl polymer (B) further has an amide group. The thermosensitive recording medium according to claim 6 , wherein the amide group is present in a range of 0.1 mmol to 7 mmol with respect to 1 g in total of the vinyl polymer (A) and the vinyl polymer (B). Coating agent for forming a protective layer.   Furthermore, an acrylic thickener (E) obtained by polymerizing an acrylic monomer mixture containing a vinyl monomer having an acid group and a (meth) acrylic monomer having an alkyl group having 1 to 2 carbon atoms The coating agent for forming a protective layer of a thermosensitive recording medium according to claim 1, comprising: A protective layer (c2) having at least a thermosensitive coloring layer (c1) on a substrate, and formed on the thermosensitive coloring layer (c1) using the coating agent according to any one of claims 1 to 8. And a thermal recording medium.