JP2017125148A - Resin composition, aqueous coating liquid, protective layer, and thermal recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin composition useful for a protective layer of a thermal recording medium excellent in waterproofness and waterproof blocking.SOLUTION: The resin composition contains a carboxyl group-containing polyvinyl alcohol based resin and a polyamidepolyamine-epihalohydrin based resin, where the number n of structural units represented by the specified general formula (1) in the polyamidepolyamine-epihalohydrin based resin is 1.5 or more on average. (In the formula, X represents a halogen atom.)SELECTED DRAWING: None

Description

  The present invention relates to a resin composition containing a carboxyl group-containing polyvinyl alcohol resin and a polyamide polyamine / epihalohydrin resin, and further includes an aqueous coating solution containing such a resin composition, and such an aqueous coating solution. The present invention relates to a protective layer to be formed and a heat-sensitive recording medium containing such a protective layer.

Polyvinyl alcohol resin (hereinafter abbreviated as PVA resin) uses excellent water solubility, interface properties (dispersibility, protective colloid properties, etc.), film properties (film forming properties, strength, oil resistance, etc.), etc. Thus, they are widely used in dispersants, emulsifiers, suspending agents, fiber processing agents, paper processing agents, binders, adhesives, films and the like.
Further, by utilizing such water solubility, it is suitably used as a solvent-free aqueous coating liquid for a protective layer of a thermal recording medium.

The heat-sensitive recording medium has a heat-sensitive color forming layer formed on a supporting substrate by coating and drying an aqueous dispersion containing, for example, a leuco dye, a developer and a dispersant. The developer melts and mixes and reacts to develop color. Further, a protective layer is provided on the thermosensitive coloring layer to protect the thermosensitive coloring layer from dirt such as water and oil.
However, since the PVA resin is water-soluble, it has poor water resistance, and when applied to applications such as being exposed to water or placed under high humidity, the water resistance of the PVA resin is required. For this reason, various studies on water resistance of PVA-based resins have been conducted.

In order to improve the water resistance of the PVA resin, it is effective to add a crosslinking agent. In order to increase the reactivity with the crosslinking agent, various modified PVA resins have been proposed as PVA resins.
Especially, since it has high reactivity, the PVA-type resin containing a carboxyl group is used. In addition, since the carboxyl group-containing PVA resin reacts with an epoxy compound, a polyamide polyamine / epichlorohydrin resin is widely used as a crosslinking agent (see, for example, Patent Documents 1 and 2).

Japanese Patent Laid-Open No. 2015-013470 JP 2013-220589 A

However, the optimal polyamide polyamine / epihalohydrin resin has not been studied for the carboxyl group-containing PVA resin, and there is still room for improvement. Further water resistance and water blocking resistance (blocking when wet) There is a need to improve the performance of preventing such blocking.
Therefore, an object of the present invention is to provide a resin composition having excellent water resistance and water blocking resistance, which is useful for a protective layer of a thermal recording medium under such a background.

  However, as a result of intensive studies in view of the above circumstances, the present inventors, as a result, in the resin composition containing the carboxyl group-containing PVA resin and the polyamide polyamine / epihalohydrin resin, the structure of the specific structure in the polyamide polyamine / epihalohydrin resin. The present inventors have found that a film having excellent water resistance and water blocking resistance can be obtained by forming a crosslinked structure using a relatively large number of units as a crosslinking agent.

  That is, the gist of the present invention is a resin composition containing a carboxyl group-containing polyvinyl alcohol resin and a polyamide polyamine / epihalohydrin resin, and a structural unit represented by the following general formula (1) in the polyamide polyamine / epihalohydrin resin The present invention relates to a resin composition characterized in that the number n is 1.5 or more on average.

(In the formula, X represents a halogen atom, and n represents a natural number.)

  The present invention also provides an aqueous coating solution using the resin composition, a protective layer formed from the aqueous coating solution, and a thermosensitive recording medium having such a protective layer.

  The resin composition of the present invention is excellent in both water resistance and water blocking resistance when, for example, it is used as a protective layer for a heat-sensitive recording medium, so even if it becomes wet during use or storage, the protective layer Thus, it is possible to provide a heat-sensitive recording medium in which the recording surfaces do not stick to each other unless they dissolve.

  In the present invention, it is possible to form a dense cross-linked structure with a PVA resin by using a polyamide polyamine / epihalohydrin resin having many reactive sites in one molecule, and the above effects of the present invention can be obtained. It is guessed.

2 is an NMR chart of the polyamide polyamine / epichlorohydrin resin (1) of Example 1. FIG. 3 is an NMR chart of the polyamide polyamine / epichlorohydrin resin (2) of Example 2. FIG.

The description of the constituent requirements described below is an example (representative example) of an embodiment of the present invention, and is not limited to these contents.
Hereinafter, the present invention will be described in detail.

  The resin composition of the present invention contains a carboxyl group-containing PVA resin and a polyamide polyamine / epihalohydrin resin having a structural unit represented by the general formula (1). First, the carboxyl group-containing PVA resin will be described.

[Carboxyl group-containing PVA resin]
The carboxyl group-containing PVA resin used in the present invention has a structural unit having a carboxyl group, and as a production method, for example, (1) from an unsaturated monomer having a carboxyl group and a vinyl ester compound A method of saponifying the copolymer after obtaining the copolymer; (2) coexisting as a chain transfer agent an alcohol having a carboxyl group or a compound having a carboxyl group and a functional group such as aldehyde or thiol. Examples thereof include a method in which a vinyl ester compound is polymerized and then saponified with a catalyst such as an alkali metal hydroxide. The method (1) is practical from the viewpoint of resin production and performance.

  In the present invention, among the carboxyl group-containing PVA resins, maleic acid-modified PVA resins and itaconic acid-modified PVA resins are preferable because they are highly polymerizable with vinyl ester monomers. The maleic acid-modified PVA resin is preferred.

Hereinafter, the method (1) will be specifically described.
Examples of the unsaturated monomer having a carboxyl group include an ethylenically unsaturated dicarboxylic acid (maleic acid, fumaric acid, itaconic acid, etc.), or an ethylenically unsaturated carboxylic acid monoester (maleic acid monoalkyl ester, Fumaric acid monoalkyl ester, itaconic acid monoalkyl ester, etc.), or ethylenically unsaturated dicarboxylic acid diester (maleic acid dialkyl ester, fumaric acid dialkyl ester, itaconic acid dialkyl ester, etc.), or ethylenically unsaturated carboxylic acid anhydride Monomers (maleic anhydride, itaconic anhydride, etc.), monomers such as (meth) acrylic acid, and salts thereof, and ethylenically unsaturated carboxylic acid monoesters or salts thereof are preferably used. .
Of these, ethylenically unsaturated carboxylic acid monoesters are preferred in terms of reactivity with vinyl ester monomers, maleic acid monoalkyl esters and itaconic acid monoalkyl esters are preferred, and maleic acid monoalkyl esters are particularly preferred. .

  Examples of vinyl ester compounds include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl versatate, vinyl palmitate, and vinyl stearate, either alone or in combination. Although it can be used, vinyl acetate is particularly preferable from the viewpoint of practicality.

  In the present invention, in addition to the monomer having a carboxyl group and the vinyl ester compound as described above upon polymerization with an unsaturated monomer having a carboxyl group and a vinyl ester compound, an allyl ester of a saturated carboxylic acid ( Allyl stearate, allyl laurate, allyl coconut oil fatty acid, allyl octylate, allyl butyrate), α-olefin (ethylene, propylene, α-hexene, α-octene, α-decene, α-dodecene, α-hexadecene, α-octadecene, etc.), alkyl vinyl ethers (propyl vinyl ether, butyl vinyl ether, hexyl vinyl ether, octyl vinyl ether, decyl vinyl ether, dodecyl vinyl ether, tetradecyl vinyl ether, hexadecyl vinyl ether, octadecyl vinyl ether, etc.), Alkyl allyl ether (propyl allyl ether, butyl allyl ether, hexyl allyl ether, octyl allyl ether, decyl allyl ether, dodecyl allyl ether, tetradecyl allyl ether, hexadecyl allyl ether, octadecyl allyl ether, etc.), and (meth) 50 mol% or less, preferably 30%, of a monomer capable of copolymerization (with a vinyl ester) such as acrylamide, (meth) acrylonitrile, (meth) allyl sulfonate, ethylenically unsaturated sulfonate, styrene, vinyl chloride and the like. % Or less may be used for polymerization. The copolymerization is not particularly limited, and a known polymerization method can be arbitrarily used, but solution polymerization is usually performed using a lower alcohol such as methanol or ethanol as a solvent.

  In such a method, as a monomer charging method, first, the whole amount of the vinyl ester compound and a part of the carboxyl group-containing unsaturated monomer are charged, polymerization is started, and the remaining unsaturated monomer is removed during the polymerization period. Arbitrary means such as a method of adding continuously or in a divided manner and a method of batch charging may be used. The copolymerization reaction is performed using a known radical polymerization catalyst such as azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide, or lauroyl peroxide. The reaction temperature is selected from the range of about 50 ° C. to the boiling point.

  The copolymer obtained as described above is then saponified into a carboxyl group-containing PVA resin. In the saponification, the copolymer is dissolved in an alcohol, an acetate ester or a mixed solvent thereof in the presence of an alkali catalyst. Examples of the alcohol include methanol, ethanol, butanol and the like, and examples of the acetate ester include methyl acetate and ethyl acetate. The concentration of the copolymer in the alcohol is usually selected from the range of 20 to 50% by weight. As the saponification catalyst, for example, an alkali catalyst such as an alkali metal hydroxide or alcoholate such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate or potassium methylate is used. The amount of the catalyst used is usually 1 to 100 millimolar equivalents relative to the vinyl ester compound.

  Thus, a carboxyl group-containing PVA resin is obtained. The content of such carboxyl groups is preferably 0.1 to 20 mol%, more preferably 0.5 to 10 mol%, and particularly 1 to 3 mol%. Is preferred. If the carboxyl group content is too small, the water resistance tends to decrease. Conversely, if the carboxyl group content is too large, coating tends to be difficult when the coating solution is used.

  The saponification degree (based on JIS K 6726) of the carboxyl group-containing PVA resin is usually 70 to 100 mol%, preferably 75 to 99.9 mol%, particularly preferably 80 to 99.8 mol%. If the saponification degree is too small, the water solubility tends to decrease.

  The average polymerization degree (based on JIS K 6726) of the carboxyl group-containing PVA resin is usually 200 to 4000, preferably 300 to 3000, and particularly preferably 1000 to 2000. If the average degree of polymerization is too large, the viscosity of the coating solution tends to increase, and if it is too small, the water resistance tends to decrease.

[Polyamide polyamine, epihalohydrin resin]
The polyamide polyamine / epihalohydrin-based resin used in the present invention is characterized in that the number n of structural units represented by the general formula (1) is 1.5 or more on average. The number n of structural units is preferably 2 or more on average, more preferably 3 or more. If the number n of structural units is too small, it is difficult to obtain the effects of the present invention. The upper limit of the number n of structural units is usually about 200, preferably about 100.
About the structural unit represented by General formula (1), it is 10 mol% or more normally, Preferably it is 20-99 mol%, More preferably, it is 50-90 mol%.

In addition, the polyamide polyamine / epihalohydrin-based resin may generally have structural units represented by the following general formulas (2) and (3) in addition to the structural unit represented by the general formula (1).
About the structural unit represented by General formula (2), it is 0-90 mol% normally, Preferably it is 2-50 mol%, Most preferably, it is 3-20 mol%.
Moreover, the number m of structural units represented by the general formula (2) is usually 0 to 180 on average, preferably 0.5 to 50.

Furthermore, about the structural unit represented by General formula (3), it is 0-90 mol% normally, Preferably it is 2-50 mol%, Most preferably, it is 3-20 mol%.
The number of structural units 1 represented by the general formula (3) is usually 0 to 180 on average, preferably 0.5 to 50.

(In the formula, m represents a natural number.)

(In the formula, l represents a natural number.)

  The weight average molecular weight of the polyamide polyamine / epihalohydrin resin used in the present invention is usually 500 to 30000, preferably 800 to 20000, and particularly preferably 1000 to 10,000. If the weight average molecular weight is too large, the viscosity tends to increase and workability tends to decrease, and if it is too small, the water resistance tends to decrease.

The polyamide polyamine / epihalohydrin resin used in the present invention can be obtained by reacting an epihalohydrin with a polyamide polyamine obtained by reacting a polyalkylene polyamine and a dicarboxylic acid.
The polyamide polyamine / epihalohydrin-based resin having an average number of structural units n of the general formula (1) of 1.5 or more has more epihalohydrin than the polyamide polyamine, specifically, to 1 mol of amino groups of the polyamide polyamine. , About 0.01 to 2.0 moles, and produced by reacting.

The number of structural units of the general formula (1) in the polyamide polyamine / epihalohydrin resin is the content of the general formula (1), the general formula (2), and the general formula (3) from the measurement result using NMR. It can be calculated from the ratio.
A chart of ¹³ C-NMR (solvent: heavy water) is shown in FIG. The main attributes of each chart are as follows. The NMR measurement can be performed using “AVANCE III HD” manufactured by Bruker.

[ ¹³ C-NMR]
1, 2: 72.5-73 ppm
3: 58.5-59 ppm
4: 57.5-58 ppm
5: 59-59.5 ppm

  The polyalkylene polyamines only need to have at least two or more alkylene groups and two or more amino groups in the molecule, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, iminobispropylamine, etc. Among them, diethylenetriamine is preferable. These can be used alone or in combination of two or more. Further, in place of a part of polyalkylene polyamines, alkylene diamines such as ethylenediamine, propylenediamine or hexamethylenediamine; aminocarboxylic acids having 1 to 6 carbon atoms such as ε-aminocaproic acid; carbon atoms such as ε-caprolactam Lactams of aminocarboxylic acids of formulas 1 to 6 can also be used.

  The dicarboxylic acids may be those having two carboxyl groups in the molecule. For example, saturated or unsaturated aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, brassic acid, dodecanedioic acid, itaconic acid, maleic acid, fumaric acid, etc. Acids; aromatic dicarboxylic acids such as phthalic acid, isophthalic acid and terephthalic acid; acid anhydrides of the above acids; lower alcohols having 1 to 5 carbon atoms, particularly 1 to 3 carbon atoms (methyl alcohol, ethyl alcohol) , Propyl alcohol) and other dicarboxylic acid derivatives. Of these, glutaric acid, adipic acid, glutaric acid methyl ester, adipic acid methyl ester, and the like are preferable. These can be used alone or in combination of two or more.

  When synthesizing a polyamide polyamine, a reaction molar ratio of 0.8 to 1.5 mol of a polyalkylene polyamine is preferable with respect to 1.0 mol of a dicarboxylic acid, and when the reaction amount of the polyalkylene polyamine is too large, When the amount is too small, the amount of polyamide polyamine produced tends to decrease.

  The reaction between the polyalkylene polyamines and the dicarboxylic acids is preferably continued until the resulting polyamide polyamine reaches a viscosity within the range of 100 to 1000 mPa · s, based on the viscosity at 25 ° C. of an aqueous solution having a solid content of 50% by weight. .

  When reacting the amino groups of polyalkylene polyamines or polyalkylene polyamines with the carboxyl groups of dicarboxylic acids, use the heat of reaction generated during raw material charging, or heat from the outside to dehydrate and / or dealcoholize. I do. The reaction temperature depends on whether it is a free acid or a derivative such as an anhydride or an ester, but is usually 110 to 250 ° C, more preferably 120 to 180 ° C. In this case, as a catalyst for the polycondensation reaction, for example, sulfonic acids such as sulfuric acid, benzenesulfonic acid and paratoluenesulfonic acid, phosphoric acids such as phosphoric acid, phosphonic acid and hypophosphorous acid, and other known catalysts may be used alone or Two or more types can be used in combination. The amount to be used is preferably 0.005 to 0.1 mol, more preferably 0.01 to 0.05 mol, relative to 1 mol of the polyalkylene polyamine.

  The polyamide polyamine / epihalohydrin resin used in the present invention can be obtained by reacting the above-described polyamide polyamine with an epihalohydrin.

  Epihalohydrins include epichlorohydrin, epibromohydrin, methyl epichlorohydrin, and the like. Moreover, these can also be used in mixture of 2 or more types. Of these epihalohydrins, epichlorohydrin is particularly preferable.

  The reaction ratio of the epihalohydrin to the polyamide polyamine is preferably 0.01 to 2.0 mol, more preferably 0.05 to 1.5, and still more preferably 0.05 with respect to 1 mol of the amino group of the polyamide polyamine. ~ 1 mole. When the molar ratio of epihalohydrins is too large, the production of low-molecular organic halogen compounds tends to increase as a by-product of epihalohydrins. If the molar ratio of the epihalohydrins is too small, the water resistance of the resulting resin tends to decrease.

  The reaction between the polyamide polyamine and the epihalohydrins is preferably carried out at a reaction solution concentration of 15 to 80% by weight and a reaction temperature of 5 to 90 ° C. In particular, in order to increase the reaction efficiency between the polyamide polyamine and the epihalohydrins, the temperature when the epihalohydrins are added to the polyamide polyamine is in the range of 5 to 45 ° C, and in the subsequent reaction, the temperature is set to 45 to 90 ° C. It is preferable to increase the molecular weight of the resulting polyamide polyamine / epihalohydrin resin to a predetermined viscosity.

  The reaction between the polyamide polyamine and the epihalohydrin is within the range of 10 to 100 mPa · s, preferably 15 to 80 mPa · s, based on the viscosity of the resulting polyamide polyamine / epihalohydrin resin at 25 ° C. in an aqueous solution having a solid content of 15% by weight. By continuing the reaction until it has a viscosity of, a resin excellent in water resistance can be obtained. After the viscosity of the reaction solution falls within this viscosity range, the reaction is stopped by adding water to the reaction solution and cooling it to obtain an aqueous solution of a polyamide polyamine / epihalohydrin resin.

(Resin composition)
The resin composition of the present invention contains the carboxyl group-containing PVA resin and the polyamide polyamine / epihalohydrin resin.
In the resin composition of the present invention, the content of the polyamide polyamine / epihalohydrin resin is preferably 0.5 to 50 parts by weight, particularly 1 to 25 parts by weight based on 100 parts by weight of the carboxyl group-containing PVA resin. Parts, more preferably 2 to 15 parts by weight. If the content of the polyamide polyamine / epihalohydrin resin is too much or too little, the water resistance tends to decrease.

  The resin composition of the present invention may contain components other than the carboxyl group-containing PVA resin and the polyamide polyamine / epihalohydrin resin to such an extent that the effects of the present invention are not affected. For example, organic or inorganic fillers, acrylic resins Emulsions, surfactants, lubricants and the like can be mentioned.

  As a mixing method of the resin composition of the present invention, (i) a method of mixing a carboxyl group-containing PVA resin aqueous solution and a polyamide polyamine / epihalohydrin resin aqueous solution, and (ii) a solid polyamide in the carboxyl group-containing PVA resin aqueous solution Examples thereof include a method of blending a polyamine / epihalohydrin resin, (iii) a method of blending a solid carboxyl group-containing PVA resin in a polyamide polyamine / epihalohydrin resin aqueous solution, and the like. Among these, the method (i) is preferable because uniform mixing is easy.

[Water-based coating solution]
The aqueous coating solution of the present invention contains the above resin composition and water.
The solid content concentration of the aqueous coating solution is usually 0.1 to 50% by weight, preferably 1 to 20% by weight. If the concentration is too low, the effect of the present invention cannot be exhibited sufficiently. On the other hand, if the viscosity is too high, the viscosity of the coating solution increases, so that coating tends to be difficult.

  Further, the pH of the coating solution is preferably 10 or less, and more preferably 3 to 10. If the pH is too high, the hot water resistance and plasticizer resistance may decrease. In addition, since the carboxyl group-containing PVA-based resin is weakly acidic when made into an aqueous solution, such pH adjustment is usually not necessary in many cases, but if necessary, for example, inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, citric acid, The pH may be adjusted with an organic acid such as tartaric acid, succinic acid, or acetic acid, or a basic compound such as sodium hydroxide, potassium hydroxide, sodium bicarbonate, or ammonia.

In the present invention, a thermoplastic resin, a pigment, other additives, and the like can be further blended in the aqueous coating solution as necessary.
Examples of the thermoplastic resin include acrylic resins, polyester resins, polyvinyl chloride resins, saponified ethylene-vinyl acetate copolymers, and acrylic resins are preferred from the viewpoint of ink adhesion.
The blending amount of the thermoplastic resin is usually 10 to 70% by weight, preferably 20 to 60% by weight, based on the total solid content of the coating liquid.
Examples of the pigment include organic pigments such as nylon resin filler, urea / formalin resin filler, and starch particles. Examples of other additives include lubricants such as zinc stearate, calcium stearate, and paraffin wax, and interfaces. Activators, ultraviolet absorbers, fluorescent dyes, release agents, antioxidants and the like can be mentioned.

[Protective layer]
The protective layer of the present invention is formed from the above aqueous coating solution. A protective layer is formed by applying such an aqueous coating solution to various substrates to remove moisture. When removing water, it is usually dried after coating with an aqueous coating solution. As such drying conditions, it is usually 5 to 150 ° C., more preferably 30 to 120 ° C., particularly 50 to 110. It is preferable to carry out under the temperature condition of ° C., and it is preferable to carry out with a drying time of 0.01 to 60 minutes, further 0.1 to 30 minutes, especially 0.2 to 20 minutes.
The thickness of the protective layer is usually 0.01 to 100 μm, preferably 0.05 to 20 μm, particularly preferably 0.1 to 10 μm. If the thickness is too thick, drying takes time and tends to be inefficient, and if it is too thin, the water resistance tends to decrease.
As a coating amount, it is 0.1-20 g / m < ² > normally, Preferably it is 0.1-20 g / m < ² >, Most preferably, it is 0.1-20 g / m < ² >.

The protective layer of the present invention is suitably used as a protective layer for a heat-sensitive recording medium, and is formed by applying the aqueous coating liquid of the present invention onto a heat-sensitive color forming layer and drying it.
Although the resin composition of the present invention can be used for various applications, it is very useful as a protective layer for a thermal recording medium as described above, and the thermal recording medium will be described below.

[Thermosensitive coloring layer]
The thermosensitive recording medium is provided with a thermosensitive coloring layer on a substrate, and the thermosensitive coloring layer is further colored with a binder (for example, PVA resin, methylcellulose, carboxymethylcellulose, starches, latexes, etc.). It can be formed by obtaining an aqueous solution (coloring solution) containing a substance and a developer and then coating the aqueous solution on a substrate.
At this time, the color-developing substance and the developer are blocked in an aqueous solution and are pulverized to about 0.1 to 5 μm by a bead mill, ball mill, viscomill or the like.

  Examples of the color-forming substance include 3,3-bis (p-dimethylaminophenyl) -phthalide, 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide [crystal violet lactone], 3,3-bis (p-dimethylaminophenyl) -6-diethylaminophthalide, 3,3-bis (p-dimethylaminophenyl) -6-chlorophthalide, 3-dimethylamino-6-methoxyfluorane, 7-acetamino -3-diethylaminofluorane, 3-diethylamino-5,7-dimethylfluorane, 3-diethylamino-5,7-dimethylfluorane, 3,6-bis-β-methoxyethoxyfluorane, 3,6-bis- Examples include leuco bodies of triphenylmethane dyes such as β-cyanoethoxyfluorane.

  Further, the developer is one that reacts with the color-developing substance when heated and develops color, and is preferably liquefied or vaporized at room temperature or higher, preferably 70 ° C. or higher. For example, phenol, p-methylphenol, p-tertiary Butylphenol, p-phenylphenol, α-naphthol, β-naphthol, 4,4′-isopropylidene diphenol [bisphenol A], 4,4′-secondary butylidene diphenol, 4,4′-cyclohexylidene diphenol, 4 , 4′-isopropylidenebis (2-tertiarybutylphenol), 4,4 ′-(1-methyl-n-hexylidene) diphenol, 4,4′-isopropylidene dicatechol, 4,4′-pentylidene diene Phenol, 4,4-isopropylidenebis (2-chlorophenol), phenyl -4-hydroxybenzoate, salicylic acid, 3-phenylsalicylic acid, 5-methylsalicylic acid, 3,5-di-tert-butylsalicylic acid, 1-oxy-2-naphthoic acid, m-oxybenzoic acid, 4-oxyphthalic acid, gallic acid An acid etc. are mentioned.

In providing the heat-sensitive color developing layer, an aqueous solution containing the above binder, color developing material and developer is used as a roll coater method, air yarder method, blade coater method, bar coater method, size press method, gate roll method, curtain coater method. What is necessary is just to apply to a base material by arbitrary coating means etc., solid content of a coating liquid should just be about 10 to 60 weight%, and the coating amount of this aqueous solution is 0.1-20 g by dry weight. / M ² or so.

  Further, if necessary, an undercoat layer may be provided under the thermosensitive coloring layer. The undercoat layer is a known PVA resin, starch, methylcellulose, ethylcellulose, hydroxymethylcellulose, casein, acrylate ester copolymer. Copolymers, water-soluble and water-dispersible resins such as styrene-butadiene copolymer latex and the above-described pigments may be used alone or in combination of two or more. In the application of the undercoat layer, the same coating method, coating solution concentration and coating amount as the protective layer are employed.

[Thermal recording medium]
The heat-sensitive recording medium of the present invention has an arbitrary heat-sensitive coloring layer on a substrate, and is coated with the aqueous coating liquid for the protective layer of the present invention and dried.
The resulting thermosensitive recording medium has a layer structure of base material / (undercoat layer) / thermosensitive color developing layer / protective layer. In addition, an intermediate layer, a back layer, and the like may be provided.

  The base material is usually paper (manila balls, white balls, liner paper, etc., general high quality paper, medium quality paper, gravure paper, printing paper, upper / middle / lower grade paper, newsprint paper, release paper, carbon paper, etc. , Non-carbon paper, glassine paper, etc.) and plastic films (polyester film, nylon film, polyolefin film, polyvinyl chloride film, laminates thereof and the like).

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist.
In the examples, “parts” and “%” mean weight basis.

Example 1
[Production of carboxyl group-containing PVA resin]
A reaction vessel equipped with a reflux condenser, a dropping funnel and a stirrer was charged with 100 parts of vinyl acetate, 26 parts of methanol, and 0.1 part of monomethyl maleate (0.09 mol% based on the total amount of vinyl acetate) and stirred. Then, the temperature was raised to 60 ° C. under a nitrogen stream, and then t-butyl peroxyneodecanoate (the temperature at which the half-life was 1 hour was 65 ° C.) was 0.001 mol% (based on the total amount of vinyl acetate). And polymerization was started. Immediately after the start of polymerization, 2.2 parts of monomethyl maleate (2 mol% with respect to the total amount of vinyl acetate) and 0.008 mol% of t-butylperoxyneodecanoate (with respect to the total amount of vinyl acetate) were adjusted to the polymerization rate. When the polymerization rate of vinyl acetate reached 73%, 0.01 parts of 4-methoxyphenol and 58 parts of methanol for dilution / cooling were added to complete the polymerization. The amount of the remaining active catalyst at the end of the polymerization was 2 ppm with respect to the total amount of the reaction solution.
Subsequently, unreacted vinyl acetate monomer was removed out of the system by a method of blowing methanol vapor to obtain a methanol solution of the copolymer.
Next, the solution was diluted with methanol to adjust the concentration to 40%, and a 4% methanol solution of sodium hydroxide was mixed at a ratio of 30 mmol with respect to 1 mol of vinyl acetate structural units in the copolymer. The saponification reaction was performed at 40-50 ° C. for 25 minutes. The resin solidified by the saponification reaction was cut to obtain a carboxyl group-containing PVA resin. Drying at 70 ° C. yielded a carboxyl group-containing PVA resin. The vinyl acetate component of the PVA had a saponification degree of 99 mol%, an average polymerization degree of 1700, and a carboxyl group content of 2 mol%.

[Preparation of polyamide polyamine / epichlorohydrin resin aqueous solution]
The polyamide polyamine was reacted with epichlorohydrin to produce a polyamide polyamine / epichlorohydrin resin (1). As a result of the ¹³ C-NMR measurement of the obtained polyamide polyamine / epichlorohydrin resin (1), the average number of structural units n represented by the general formula (1) was 3.3. The NMR chart is shown in FIG.

[Preparation of aqueous coating solution]
100 parts of the 10% aqueous solution of the carboxyl group-containing PVA resin obtained above and 10 parts of the 10% aqueous solution of the polyamidepolyamine / epichlorohydrin resin (1) obtained above were mixed to prepare an aqueous coating solution. Produced.

[Production of cast film]
On the polyethylene terephthalate (PET) film on which a 10cm x 10cm mold was formed, the aqueous coating solution obtained above was poured and dried at 23 ° C and 50% RH for 3 days. A film was obtained.
The following evaluation was performed about the obtained cast film.

(Water resistance evaluation)
The obtained cast film was heat-treated at 70 ° C. for 5 minutes, then immersed in water at 23 ° C. for 1 hour, and the dissolution rate (%) of the film was measured. In calculating the dissolution rate (%), the dry weight of the film before water immersion (X ₁ ) and the dry weight of the film after water immersion (X ₂ ) (both in g) were obtained and eluted by the following formula: The rate (%) was calculated. The results are shown in Table 1.
Elution rate (%) = [(X ₁ −X ₂ ) / X ₁ ] × 100

[Preparation of aqueous coating liquid for protective layer]
100 parts of the 10% aqueous solution of the carboxyl group-containing PVA resin obtained above, 10 parts of the 10% aqueous solution of the polyamidepolyamine / epichlorohydrin resin obtained above, and 100 parts of kaolin as the inorganic filler are mixed to protect. A layer aqueous coating solution was prepared.

(Formation of protective layer)
A laminate in which the protective layer is formed by applying the aqueous coating liquid for protective layer obtained above to a copy paper so that the thickness of the protective layer is 5 μm, and drying with a dryer at 70 ° C. for 5 minutes. (Protective layer / copy paper) was obtained. The effect of the present invention is not affected by the heat-sensitive color developing layer. Therefore, the same effect as in this embodiment can be obtained even in a heat-sensitive recording medium provided with a heat-sensitive color developing layer.
The following evaluation was performed about the obtained laminated body.

[Water blocking resistance evaluation]
10 μl of water was dropped onto the protective layer surface of the obtained laminate, and another laminate was overlaid so that the protective layer forming surfaces were in contact with each other. A load of 10 g / cm ² is applied on the two laminates, left in an environment of 40 ° C. and 90% RH for 24 hours, the two laminates are peeled off, and the adhesion is visually observed. Evaluation was performed as follows. The results are shown in Table 1.
A: There is no sticking. B: There is sticking, but no damage to the laminate due to peeling is observed. C: There is sticking.

Example 2
The same procedure as in Example 1 was performed except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (2) (average of n = 1.6) in Example 1. Similar evaluations were made. The results are shown in Table 1. Moreover, the NMR chart of a polyamide polyamine epichlorohydrin resin (2) is shown in FIG.

Comparative Example 1
Example 1 was repeated except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (3) (average of n) = 1.2). The same evaluation was performed. The results are shown in Table 1.

Comparative Example 2
The same procedure as in Example 1 was conducted except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (4) (average of n = 0.8) in Example 1. Similar evaluations were made. The results are shown in Table 1.

Comparative Example 3
The same procedure as in Example 1 was conducted except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (5) (average of n = 0.7) in Example 1. Similar evaluations were made. The results are shown in Table 1.

  In Examples 1 and 2 using the aqueous coating solution for a protective layer of the present invention, the dissolution rate was small, the water resistance was excellent, and the water blocking resistance was also excellent. On the other hand, in Comparative Examples 1 to 3 using the polyamide polyamine / epichlorohydrin resin in which the number of structural units n represented by the general formula (1) is too small, water resistance and water blocking resistance were inferior.

  The resin composition of the present invention is excellent in both water resistance and water blocking resistance when, for example, it is used as a protective layer for a heat-sensitive recording medium, so even if it becomes wet during use or storage, the protective layer Thus, it is possible to provide a heat-sensitive recording medium in which the recording surfaces do not stick to each other unless they dissolve.

Claims (7)

A resin composition containing a carboxyl group-containing polyvinyl alcohol resin and a polyamide polyamine / epihalohydrin resin,
The resin composition, wherein the number n of structural units represented by the following general formula (1) in the polyamide polyamine / epihalohydrin resin is 1.5 or more on average.
(In the formula, X represents a halogen atom, and n represents a natural number.) 2. The resin composition according to claim 1, wherein the carboxyl group-containing polyvinyl alcohol resin is a maleic acid-modified polyvinyl alcohol resin. The resin composition according to claim 1, wherein the carboxyl group-containing polyvinyl alcohol resin is an itaconic acid-modified polyvinyl alcohol resin. The resin composition according to any one of claims 1 to 3, wherein the content of the polyamide polyamine / epihalohydrin resin is 0.5 to 50 parts by weight with respect to 100 parts by weight of the carboxyl group-containing polyvinyl alcohol resin. object. An aqueous coating liquid comprising the resin composition according to any one of claims 1 to 4 and water. A protective layer formed from the aqueous coating solution according to claim 5. A heat-sensitive recording medium comprising at least one protective layer according to claim 6.