JP6776537B2 - Resin composition, aqueous coating liquid, protective layer and thermal recording medium - Google Patents

Description

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

Polyvinyl alcohol-based resin (hereinafter abbreviated as PVA-based resin) utilizes excellent water solubility, interfacial properties (dispersibility, protective colloidal properties, etc.), film properties (film-forming properties, strength, oil resistance, etc.), etc. Therefore, it is 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 water-based coating liquid for a protective layer of a heat-sensitive recording medium or the like.

The heat-sensitive recording medium has a heat-sensitive color-developing layer formed by applying and drying an aqueous dispersion containing, for example, a leuco dye, a developer and a dispersant on a supporting base material, and the heat-sensitive recording medium is heated to form a leuco dye. The color developer melts and mixes, reacting and developing color. Further, a protective layer is provided on the heat-sensitive color-developing layer to protect the heat-sensitive color-developing layer from stains such as water and oil.
However, since PVA-based resin is water-soluble, it has poor water resistance, and when applied to applications such as being exposed to water or being placed under high humidity, the water resistance of PVA-based resin is required. Therefore, various studies have been conducted on the water resistance of PVA-based resins.

It is effective to add a cross-linking agent to make the PVA-based resin water resistant, and various modified PVA-based resins have been proposed as the PVA-based resin in order to enhance the reactivity with the cross-linking agent.
Among them, a PVA-based resin containing a carboxyl group is used because it has high reactivity. Further, since the carboxyl group-containing PVA resin reacts with the epoxy compound, a polyamide polyamine / epichlorohydrin resin is widely used as a cross-linking agent (see, for example, Patent Documents 1 and 2).

JP 2015-013470 Japanese Unexamined Patent Publication No. 2013-22589

However, the optimum polyamide polyamine / epihalohydrin resin for the carboxyl group-containing PVA resin has not been investigated, 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 to prevent such blocking.
Therefore, it is an object of the present invention to provide a resin composition having excellent water resistance and water blocking resistance, which is useful for a protective layer of a heat-sensitive recording medium under such a background.

However, as a result of diligent studies in view of the above circumstances, the present inventors have found that in a resin composition containing a carboxyl group-containing PVA-based resin and a polyamide polyamine / epihalohydrin-based resin, the structure of a specific structure in the polyamide polyamine / epihalohydrin-based resin. The present invention has been completed by finding that a coating film having excellent water resistance and water blocking resistance can be obtained by forming a crosslinked structure using a material having 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-based resin and a polyamide polyamine / epihalohydrin-based resin, and the structural unit represented by the following general formula (1) in the polyamide polyamine / epihalohydrin-based resin. The present invention relates to a resin composition, wherein the number n is 1.5 or more on average.

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

The present invention also provides a water-based coating liquid using the resin composition, a protective layer formed from the water-based coating liquid, and a heat-sensitive recording medium having such a protective layer.

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

In the present invention, by using a polyamide polyamine / epihalohydrin-based resin having many reaction points in one molecule, it is possible to form a dense crosslinked structure with the PVA-based resin, and the above-mentioned effect of the present invention can be obtained. It is presumed.

6 is an NMR chart of the polyamide polyamine / epichlorohydrin resin (1) of Example 1. 6 is an NMR chart of the polyamide polyamine / epichlorohydrin resin (2) of Example 2.

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

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

[Carboxylic acid-containing PVA resin]
The carboxyl group-containing PVA-based resin used in the present invention has a structural unit having a carboxyl group, and as a production method, for example, (1) an unsaturated monomer having a carboxyl group and a vinyl ester-based compound A method of saponifying the copolymer after obtaining the copolymer, (2) Coexistence of an alcohol having a carboxyl group or a compound having a functional group such as aldehyde or thiol as a chain transfer agent. A method of polymerizing a vinyl ester compound and then saponifying it with a catalyst such as an alkali metal hydroxide can be mentioned, but the method (1) is practical from the viewpoint of resin production and performance.

Further, in the present invention, among the carboxyl group-containing PVA-based resins, maleic acid-modified PVA-based resins and itaconic acid-modified PVA-based resins are preferable in that they are highly polymerizable with vinyl ester-based monomers, and further, handling points. A maleic acid-modified PVA-based resin is preferable.

Hereinafter, the method (1) will be specifically described.
Examples of the unsaturated monomer having a carboxyl group include ethylenically unsaturated dicarboxylic acid (maleic acid, fumaric acid, itaconic acid, etc.) or ethylenically unsaturated carboxylic acid monoester (maleic acid monoalkyl ester, etc.). 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 Examples thereof include products (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 preferable, and maleic acid monoalkyl esters and itaconic acid monoalkyl esters are preferable, and maleic acid monoalkyl esters are particularly preferable, in terms of reactivity with vinyl ester monomers. ..

Further, as the vinyl ester compound, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl versatic acid, vinyl palmitate, vinyl stearate and the like may be used 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 above-mentioned monomer having a carboxyl group and the vinyl ester compound at the time of polymerization with the unsaturated monomer having a carboxyl group and the vinyl ester compound, the allyl ester of the saturated carboxylic acid ( Allyl stearate, allyl laurate, allyl palm oil fatty acid, allyl octylate, allyl butyrate, etc.), α-olefin (ethylene, propylene, α-hexene, α-octene, α-decene, α-dodecene, α-hexadecene, α-Octadecene, etc.), alkyl vinyl ether (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, hexylallyl ether, octylallyl ether, decylallyl ether, dodecylallyl ether, tetradecylallyl ether, hexadecylallyl ether, octadecylallyl ether, etc.), as well as (meth) acrylamide, (meth) acrylonitrile, (meth). Even if the polymerization is carried out in the presence of 50 mol% or less, preferably 30% or less of a monomer that can be copolymerized (with a vinyl ester) such as allyl sulfonate, ethylenically unsaturated sulfonate, styrene, and vinyl chloride. good. The copolymerization is not particularly limited, and a known polymerization method is arbitrarily used, but solution polymerization usually using a lower alcohol such as methanol or ethanol as a solvent is carried out.

In this method, as a method for charging the monomer, first, the entire amount of the vinyl ester compound and a part of the unsaturated monomer containing a carboxyl group are charged, polymerization is started, and the remaining unsaturated monomer is used for the polymerization period. Any means such as a method of continuously or dividedly adding to the inside, a method of batch charging, etc. may be used. The copolymerization reaction is carried out using known radical polymerization catalysts such as azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide and 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 to form a carboxyl group-containing PVA-based resin. The saponification is carried out in the presence of an alkaline catalyst by dissolving the copolymer in an alcohol, an acetic acid ester or a mixed solvent thereof. Examples of the alcohol include methanol, ethanol, butanol and the like, and examples of the acetic acid ester include methyl acetate, ethyl acetate and the like. 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 metal hydroxide such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate, or an alkali catalyst such as alcoholate is used. The amount of such a catalyst used is usually 1 to 100 mmol equivalent with respect to the vinyl ester compound.

Thus, a PVA-based resin containing a carboxyl group can be obtained, and the content of the carboxyl group is preferably 0.1 to 20 mol%, more preferably 0.5 to 10 mol%, and particularly 1 to 3 mol%. Is preferable. If the content of the carboxyl group is too small, the water resistance tends to decrease, and if it is too large, the coating tends to be difficult when the coating liquid is used.

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

The average degree of polymerization (JIS K 6726 compliant) 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 liquid 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 of structural units n represented by the general formula (1) is 1.5 or more on average. The number of structural units n is preferably 2 or more on average, and more preferably 3 or more. If the number of structural units n is too small, it becomes difficult to obtain the effects of the present invention. The upper limit of the number of structural units n is usually about 200, preferably about 100.
The structural unit represented by the general formula (1) is usually 10 mol% or more, preferably 20 to 99 mol%, and more preferably 50 to 90 mol%.

Further, the polyamide polyamine / epihalohydrin-based resin usually may have structural units represented by the following general formulas (2) and (3) in addition to the structural units represented by the general formula (1).
The structural unit represented by the general formula (2) is usually 0 to 90 mol%, preferably 2 to 50 mol%, and particularly preferably 3 to 20 mol%.
The number of structural units m represented by the general formula (2) is usually 0 to 180, preferably 0.5 to 50 on average.

Further, the structural unit represented by the general formula (3) is usually 0 to 90 mol%, preferably 2 to 50 mol%, and particularly preferably 3 to 20 mol%.
The number of structural units l represented by the general formula (3) is usually 0 to 180, preferably 0.5 to 50 on average.

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

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

The weight average molecular weight of the polyamide polyamine / epihalohydrin-based resin used in the present invention is usually 500 to 30,000, preferably 800 to 20,000, and particularly preferably 1,000 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-based resin used in the present invention can be obtained by reacting the polyamide polyamine obtained by reacting polyalkylene polyamines with dicarboxylic acids with epihalohydrins.
The polyamide polyamine / epihalohydrin-based resin having an average number of structural units n in the general formula (1) of 1.5 or more has a large amount of epihalohydrin with respect to the polyamide polyamine, and specifically, with respect to 1 mol of the amino group of the polyamide polyamine. , 0.01 to 2.0 mol is blended and reacted to produce.

The number of structural units n of the general formula (1) in the polyamide polyamine / epihalohydrin-based resin is the content of the general formula (1), the general formula (2), and the general formula (3) from the measurement results using NMR. It can be calculated from the ratio of.
A chart of ¹³ C-NMR (solvent: heavy water) is shown in FIG. The main attributions of each chart are shown below. For NMR measurement, "AVANCE III HD" manufactured by Bruker can be used for measurement.

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

The polyalkylene polyamines may be those having at least two or more alkylene groups and two or more amino groups in the molecule, for example, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, iminobispropylamine and the like. Of these, diethylenetriamine is preferable. These can be used alone or in combination of two or more. Further, instead of some of the polyalkylene polyamines, an alkylenediamine such as ethylenediamine, propylenediamine or hexamethylenediamine; an aminocarboxylic acid having 1 to 6 carbon atoms such as ε-aminocaproic acid; and a carbon atom such as ε-caprolactam. Lactam and the like of aminocarboxylic acids of Nos. 1 to 6 can also be used.

The dicarboxylic acids may be those having two carboxyl groups in the molecule. Saturated or unsaturated aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelli acid, suberic acid, azelaic acid, sebacic acid, brasic acid, dodecanedioic acid, itaconic acid, maleic acid, fumaric acid 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 alcohols, ethyl alcohols). , A dicarboxylic acid derivative such as an ester of propyl alcohol). Among 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 polyalkylene polyamines to 1.0 mol of dicarboxylic acids is preferable, and if the reaction amount of such polyalkylene polyamines is too large, It tends to thicken, and if it is too small, the amount of polyamide polyamine produced tends to decrease.

Regarding the reaction between polyalkylene polyamines and dicarboxylic acids, it is preferable to continue until the produced polyamide polyamine reaches a viscosity in the range of 100 to 1000 mPa · s based on the viscosity of an aqueous solution having a solid content of 50% by weight at 25 ° C. ..

When the amino group of polyalkylene polyamines or polyalkylene polyamines is reacted with the carboxyl group of dicarboxylic acids, the heat of reaction generated during the preparation of raw materials is used, or the heat of reaction is heated 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. At this time, 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 are used alone or. Two or more types can be used in combination. The amount used is preferably 0.005 to 0.1 mol, more preferably 0.01 to 0.05 mol, based on 1 mol of the polyalkylene polyamine.

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

Examples of epichlorohydrins include epichlorohydrin, epichlorohydrin, methylepichlorohydrin and the like. In addition, two or more of these can be mixed and used. Among these epichlorohydrins, epichlorohydrin is particularly preferable.

The reaction ratio of epihalohydrins to the polyamide polyamine is preferably 0.01 to 2.0 mol, more preferably 0.05 to 1.5, still more preferably 0.05, based on 1 mol of the amino group of the polyamide polyamine. ~ 1 mol. If the molar ratio of epihalohydrins is too high, the production of low-molecular-weight organic halogen compounds as by-products of epihalohydrins tends to increase. If the molar ratio of epihalohydrins is too small, the water resistance of the obtained resin tends to decrease.

The reaction between the polyamide polyamine and the epihalohydrins is preferably carried out at a concentration of the reaction solution 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 at which the epihalohydrins are added to the polyamide polyamine is set in the range of 5 to 45 ° C., and the temperature is set to 45 to 90 ° C. in the subsequent reaction. It is preferable to increase the molecular weight of the polyamide polyamine / epihalohydrin resin to be obtained to a predetermined viscosity.

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

[Resin composition]
The resin composition of the present invention contains the above-mentioned carboxyl group-containing PVA-based resin and polyamide polyamine / epihalohydrin-based resin.
The content of the polyamide polyamine / epihalohydrin-based resin in the resin composition of the present invention 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-based resin. It is preferably 2 to 15 parts by weight. If the content of the polyamide polyamine / epihalohydrin resin is too high or too low, 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 the extent that the effects of the present invention are not affected. For example, an organic or inorganic filler or an acrylic resin may be contained. Emulsions, surfactants, lubricants and the like can be mentioned.

Examples of the method for mixing the resin composition of the present invention include (i) a method of mixing a carboxyl group-containing PVA-based resin aqueous solution and a polyamide polyamine / epihalohydrin-based resin aqueous solution, and (ii) a carboxyl group-containing PVA-based resin aqueous solution and a solid polyamide. Examples thereof include a method of blending a polyamine / epihalohydrin-based resin, and a method of blending a solid carboxyl group-containing PVA-based resin in a (iii) polyamide polyamine / epihalohydrin-based resin aqueous solution. Above all, the method (i) is preferable from the viewpoint that uniform mixing is easy.

[Aqueous coating liquid]
The aqueous coating liquid of the present invention contains the above resin composition and water.
The solid content concentration of the aqueous coating liquid is usually 0.1 to 50% by weight, preferably 1 to 20% by weight, and if the concentration is too low, the effect of the present invention tends not to be sufficiently exhibited. On the contrary, if it is too high, the viscosity of the coating liquid becomes high, which tends to make coating difficult.

The pH of the coating liquid is preferably 10 or less, more preferably 3 to 10. If the pH is too high, the temperature resistance and plasticizer resistance may decrease. Since the carboxyl group-containing PVA-based resin becomes weakly acidic when used as an aqueous solution, such pH adjustment is usually not necessary, but if necessary, for example, inorganic acids such as hydrochloric acid, sulfuric acid, and nitrate, citric acid, etc. The pH may be adjusted with an organic acid such as tartrate acid, oxalic acid or acetic acid, or a basic compound such as sodium hydroxide, potassium hydroxide, sodium hydrogencarbonate or ammonia.

In the present invention, a thermoplastic resin, a pigment, other additives and the like can be further added to the aqueous coating liquid, if necessary.
Examples of the thermoplastic resin include acrylic resins, polyester resins, polyvinyl chloride resins, and saponified ethylene-vinyl acetate copolymers. Among them, acrylic resins are preferable from the viewpoint of ink adhesion.
The blending amount of the above-mentioned 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 fillers, urea / formalin resin fillers, and starch particles, and examples of other additives include lubricants such as zinc stearate, calcium stearate, and paraffin wax, and interfaces. Examples include activators, ultraviolet absorbers, fluorescent dyes, release agents, antioxidants and the like.

[Protective layer]
The protective layer of the present invention is formed from the above-mentioned aqueous coating liquid. By applying such an aqueous coating liquid to various base materials and the like to remove water, a protective layer is formed. In the case of removing water, it is usually dried after applying an aqueous coating liquid, but such drying conditions are usually 5 to 150 ° C., further 30 to 120 ° C., particularly 50 to 110 ° C. The drying time is preferably 0.01 to 60 minutes, more preferably 0.1 to 30 minutes, and particularly preferably 0.2 to 20 minutes.
The thickness of such a protective layer is usually 0.01 to 100 μm, preferably 0.05 to 20 μm, and particularly preferably 0.1 to 10 μm. If the thickness is too thick, it takes a long time to dry and tends to be inefficient, and if it is too thin, the water resistance tends to decrease.
The coating amount is usually 0.1 to 20 g / m ² , preferably 0.1 to 20 g / m ² , and particularly preferably 0.1 to 20 g / m ² .

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

[Thermal color layer]
The heat-sensitive recording medium is provided with a heat-sensitive color-developing layer on a base material, and the heat-sensitive color-developing layer is further colored by a binder (for example, PVA-based resin, methyl cellulose, carboxymethyl cellulose, starches, latex, etc.). It can be formed by obtaining an aqueous solution (color-developing liquid) containing a substance and a color developer and then applying the aqueous solution to a substrate.
Since the color-developing substance and the color-developing agent at this time are blocked in the aqueous solution, they are pulverized to about 0.1 to 5 μm by a bead mill, a ball mill, a visco mill or the like.

Examples of the color-developing substance include 3,3-bis (p-dimethylaminophenyl) -phthalide, 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide [crystal violet lactone], and the like. 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 thereof include leuco-forms of triphenylmethane dyes such as β-cyanoethoxyfluorane.

The color developer is one that develops color by reacting with the color-developing substance when heated, and preferably liquefies or vaporizes at room temperature or higher, preferably 70 ° C. or higher. For example, phenol, p-methylphenol, p-tersalicy. Butylphenol, p-phenylphenol, α-naphthol, β-naphthol, 4,4'-isopropyridendiphenol [bisphenol A], 4,4'-secondary butylidendiphenol, 4,4'-cyclohexylidenediphenol, 4 , 4'-isopropyridenebis (2-terriary butylphenol), 4,4'-(1-methyl-n-hexylidene) diphenol, 4,4'-isopropyridene dicatechol, 4,4'-pendilidene Phenol, 4,4-isopropyridenebis (2-chlorophenol), phenyl-4-hydroxybenzoate, salicylic acid, 3-phenylsalicylic acid, 5-methylsalicylic acid, 3,5-ditersial butylsalicylic acid, 1-oxy- Examples thereof include 2-naphthoic acid, m-oxybenzoic acid, 4-oxyphthalic acid and gallic acid.

In providing the heat-sensitive color-developing layer, the above-mentioned binder, an aqueous solution containing a color-developing substance and a color-developing agent are used in a roll coater method, an airdactor method, a blade coater method, a bar coater method, a size press method, a gate roll method, and a curtain coater method. The base material may be coated by any coating means such as, and the solid content of the coating liquid may be about 10 to 60% by weight, and the coating amount of the aqueous solution is 0.1 to 20 g by dry weight. It is about / m ² .

Further, if necessary, an undercoat layer may be provided under the heat-sensitive color-developing layer, and the undercoat layer is a known PVA-based resin, as well as starch, methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, casein, and acrylic acid ester copolymer weight. Combined, styrene-butadiene copolymer system Water-soluble and water-dispersible resins such as latex and the pigments mentioned above may be applied individually or in combination of two or more. When coating the undercoat layer, the same coating method as the protective layer, the concentration of the coating liquid, and the coating amount are adopted.

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

As the base material, usually paper (paperboard such as Manila ball, white ball, liner, general high quality paper, medium quality paper, printing paper such as gravure paper, upper / middle / lower grade paper, newspaper paper, release paper, carbon paper , Non-carbon paper, glassin paper, etc.) and plastic films (polyester film, nylon film, polyolefin film, polyvinyl chloride film, laminates thereof, etc.) and the like.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.
In the example, "part" and "%" mean the weight standard.

Example 1
[Preparation of Carboxylic Acid-Containing PVA Resin]
In a reaction can equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts of vinyl acetate, 26 parts of methanol, and 0.1 part of monomethyl maleate (0.09 mol% with respect to the total amount of vinyl acetate) were charged and stirred. While raising the temperature to 60 ° C. under a nitrogen stream, 0.001 mol% (relative to the total amount of vinyl acetate) of t-butylperoxyneodecanoate (the temperature at which the half-life is 1 hour is 65 ° C.) as a polymerization catalyst is added. It was charged 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 part of 4-methoxyphenol and 58 parts of methanol for dilution / cooling were added to complete the polymerization. The amount of residual active catalyst at the end of polymerization was 2 ppm with respect to the total amount of the reaction solution.
Subsequently, the unreacted vinyl acetate monomer was removed from 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 to 1 mol of vinyl acetate structural units in the copolymer, and the temperature was adjusted. Was saponified at 40 to 50 ° C. for 25 minutes. The resin solidified by the saponification reaction was cut to obtain a carboxyl group-containing PVA-based resin. Drying at 70 ° C. gave a carboxyl group-containing PVA-based resin. The degree of saponification of the vinyl acetate component of PVA was 99 mol%, the average degree of polymerization was 1700, and the carboxyl group content was 2 mol%.

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

[Preparation of water-based coating liquid]
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 polyamide polyamine / epichlorohydrin resin (1) obtained above are mixed to prepare an aqueous coating solution. Made.

[Making cast film]
The aqueous coating solution obtained above was poured onto a polyethylene terephthalate (PET) film on which a 10 cm × 10 cm mold was formed, dried at 23 ° C. and 50% RH for 3 days, and cast with a film thickness of 100 μm. I got a film.
The cast film obtained was evaluated as follows.

[Water resistance evaluation]
The obtained cast film was heat-treated at 70 ° C. for 5 minutes and then immersed in water at 23 ° C. for 1 hour to measure the elution rate (%) of the film. In calculating the elution rate (%), the dry weight of the film before immersion in water (X ₁ ) and the dry weight of the film after immersion in water (X ₂ ) (both g) were calculated 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 ] x 100

[Preparation of water-based coating liquid for protective layer]
Protect by mixing 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 polyamide polyamine / epichlorohydrin resin obtained above, and 100 parts of kaolin as an inorganic filler. A water-based coating liquid for layers was prepared.

[Formation of protective layer]
The above-mentioned water-based coating liquid for protective layer is applied to copy paper so that the thickness of the protective layer is 5 μm, and dried in a dryer at 70 ° C. for 5 minutes to form a laminated body having a protective layer. (Protective layer / copy paper) was obtained. Since the heat-sensitive color-developing layer does not affect the effect of the present invention, the same effect as that of the present embodiment can be obtained even in the heat-sensitive recording medium provided with the heat-sensitive color-developing layer.
The obtained laminate was evaluated as follows.

[Evaluation of water resistance blocking resistance]
10 μl of water was dropped on the protective layer surface of the obtained laminated body, and another laminated body was laminated so that the protective layer forming surfaces were in contact with each other. A load of 10 g / cm ² was applied onto the two laminated bodies, and the mixture was left to stand in an environment of 40 ° C. and 90% RH for 24 hours, the two laminated bodies were peeled off, and the degree of sticking was visually observed. It was evaluated as follows. The results are shown in Table 1.
A: No sticking B: Sticking but no damage to the laminate due to peeling C: Sticking and peeling damages the laminate

Example 2
In Example 1, the same procedure was carried out except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (2) (mean of n = 1.6). A similar evaluation was performed. The results are shown in Table 1. Further, an NMR chart of the polyamide polyamine / epichlorohydrin resin (2) is shown in FIG.

Comparative Example 1
In Example 1, the same procedure was carried out 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 as was performed. The results are shown in Table 1.

Comparative Example 2
In Example 1, the same procedure was carried out except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (4) (average of n = 0.8). A similar evaluation was performed. The results are shown in Table 1.

Comparative Example 3
In Example 1, the same procedure was carried out except that the polyamide polyamine / epichlorohydrin resin (1) was replaced with the polyamide polyamine / epichlorohydrin resin (5) (average of n = 0.7). A similar evaluation was performed. The results are shown in Table 1.

In Examples 1 and 2 using the aqueous coating liquid for a protective layer of the present invention, the elution 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) was too small, both 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, for example, when it is used as a protective layer for a heat-sensitive recording medium. Therefore, even if it gets wet with water during use or storage, the protective layer It is possible to provide a thermal recording medium in which the recording surfaces do not stick to each other unless the recording surface is dissolved.

Claims (7)

A resin composition containing a carboxyl group-containing polyvinyl alcohol-based resin and a polyamide polyamine / epihalohydrin-based resin.
A resin composition having an average number of structural units n represented by the following general formula (1) in a polyamide polyamine / epihalohydrin-based resin of 1.5 to 3.3 .
(In the formula, X is a halogen atom and n is a natural number.) The resin composition according to claim 1, wherein the carboxyl group-containing polyvinyl alcohol-based resin is a maleic acid-modified polyvinyl alcohol-based resin. The resin composition according to claim 1, wherein the carboxyl group-containing polyvinyl alcohol-based resin is an itaconic acid-modified polyvinyl alcohol-based resin. The resin composition according to any one of claims 1 to 3, wherein the content of the polyamide polyamine / epihalohydrin-based resin is 0.5 to 50 parts by weight with respect to 100 parts by weight of the carboxyl group-containing polyvinyl alcohol-based resin. Stuff. An aqueous coating liquid containing the resin composition according to any one of claims 1 to 4 and water. A protective layer formed from the aqueous coating liquid according to claim 5. A heat-sensitive recording medium having at least one protective layer according to claim 6.