JP2007112902A - Polyvinyl acetal resin for use in heat-developable photosensitive material and heat-developable photosensitive material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyvinyl acetal resin for use in a heat-developable photosensitive material capable of preventing faults such as sagging and uneven coating and therefore providing improved productivity in coating. <P>SOLUTION: The polyvinyl acetal resin for use in a heat-developable photosensitive material is a polyvinyl acetal resin used in a heat-developable photosensitive material and comprises at least (1) vinyl alcohol units, (2) acetalized vinyl alcohol units, and (3) acetoacetyl side chain vinyl alcohol units. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a polyvinyl acetal resin for a heat-developable photosensitive material and a heat-developable photosensitive material that can prevent the occurrence of problems such as dripping and unevenness, and can improve productivity during coating.

Silver halide light-sensitive materials have been used in the field of image formation as a wide-range and high-quality material from the past due to their excellent photographic characteristics. However, in addition to the complicated development process and fixing process, when the development process is wet, the processing is complicated and a large amount of chemical waste liquid is discharged, which is a problem in terms of workability and environment. It was. On the other hand, in recent years, a heat-developable photosensitive material that can be developed by a dry heat treatment and does not generate waste liquid has been developed and put into practical use.

A heat-developable photosensitive material is usually obtained by coating an emulsion obtained by dissolving and dispersing an organic silver salt, a silver ion reducing agent, a photosensitive silver halide and the like in a solvent together with a binder resin onto a film such as PET as a base material. It is obtained by forming a photosensitive layer on a support by drying. For example, in Patent Document 1 or Patent Document 2, organic silver salts, silver ion reducing agents, photoreducible silver halides, and the like are used, such as polyvinyl butyral, polymethyl methacrylate, cellulose acetate, polyvinyl acetate, cellulose acetate propionate, acetic acid. There is disclosed a heat-developable photosensitive material in which a heat-developable coating layer made of an emulsion dispersed in a film-forming binder resin made of cellulose butyrate or the like is formed on a support.

However, such a heat-developable photosensitive material has a problem that, when an emulsion is applied to a substrate, if the coating speed is increased in order to improve productivity, problems such as dripping and unevenness occur. It was. On the other hand, it has been studied to use a solution whose viscosity has been increased in advance by simply increasing the resin concentration or increasing the degree of polymerization of the binder resin.
However, when such a solution is used, the time required for dissolution of the binder resin becomes long, or the pumpability of the solution is lowered, resulting in a problem that productivity is lowered.
Japanese Patent Publication No.43-4924 JP-A 49-52626

In view of the above-described situation, the present invention provides a polyvinyl acetal resin for a heat-developable photosensitive material and a heat-developable photosensitive material that can prevent the occurrence of defects such as dripping and unevenness and improve the productivity during coating. The purpose is to provide.

The present invention relates to a polyvinyl acetal resin used for a heat-developable photosensitive material, which has at least a structural unit represented by the following general formulas (1), (2) and (3). It is.

式中、Ｒは水素原子又は炭素数１〜２０のアルキル基を表す。
以下、本発明を詳細に説明する。

In the formula, R represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.
Hereinafter, the present invention will be described in detail.

As a result of intensive studies, the present inventors have found that when a polyvinyl acetal resin is used as a binder resin of a coating solution used for producing a heat-developable photosensitive material, problems such as dripping and unevenness occur when the coating speed is increased. It has been found that the cause of the occurrence is that the shape of the formed coating film cannot be maintained because the elastic modulus of the coating solution decreases under a high shear rate.
As a result of further intensive studies, the present inventors have used a polyvinyl acetal resin composed of a specific structural unit as a binder resin for the coating solution, so that the elasticity of the coating solution can be increased even under a high shear rate during coating. It has been found that the occurrence of problems such as dripping and unevenness can be prevented without lowering, and the present invention has been completed.

The polyvinyl acetal resin for heat-developable photosensitive material of the present invention (hereinafter also simply referred to as a polyvinyl acetal resin) comprises at least structural units represented by the following general formulas (1), (2) and (3).

式中、Ｒは水素原子又は炭素数１〜２０のアルキル基を表す。

In the formula, R represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.

When the polyvinyl acetal resin of the present invention is dissolved in an appropriate solvent to form a coating solution for forming a photosensitive layer of a heat-developable photosensitive material, the elasticity of the solution even at high shear when the coating speed is increased Generation | occurrence | production of malfunctions, such as a liquid dripping at the time of coating and a nonuniformity, can be prevented, without reducing a rate. Moreover, the polyvinyl acetal resin of the present invention does not require a long time for dissolution in an organic solvent, and the pumpability of the resulting solution does not deteriorate.

In the polyvinyl acetal resin of the present invention, the preferable lower limit of the content of the vinyl alcohol unit represented by the general formula (1) is 17 mol%, and the preferable upper limit is 35 mol%. If it is less than 17 mol%, the dispersibility of the organic silver salt may be lowered, and the sensitivity of the photothermographic material may be deteriorated. If it exceeds 35 mol%, moisture easily permeates, so fogging occurs and storage stability is deteriorated, and image density is lowered, so that the sensitivity of the heat-developable photosensitive material may be deteriorated. .

In the polyvinyl acetal resin of the present invention, the preferable lower limit of the content of the acetal unit represented by the general formula (2) is 30 mol%, and the preferable upper limit is 78 mol%. If it is less than 30 mol%, it becomes insoluble in an organic solvent, which hinders the production of a heat-developable photosensitive material. When it exceeds 78 mol%, the residual hydroxyl group is decreased, the toughness is impaired, and the coating strength at the time of preparing the photothermographic material may be lowered.

The acetoacetyl group in the acetoacetyl unit represented by the general formula (3) has a self-crosslinking property, and crosslinks with acetoacetyl groups and hydroxyl groups in other molecules by heating or heating under an acid catalyst. Forms a structure and partially creates a high molecular weight component. For this reason, it can prevent that the elasticity modulus of a coating solution falls also at the time of the high shear at the time of raising a coating speed.

A molecule having an acetoacetyl unit represented by the general formula (3) forms a complex with a metal between molecules. For this reason, the polyvinyl acetal resin of the present invention can form a complex with an organic silver salt such as silver behenate and silver halide, and can effectively disperse the organic silver salt, so that excellent image characteristics can be obtained. .

In the polyvinyl acetal resin of the present invention, the preferable lower limit of the content of the acetoacetyl unit represented by the general formula (3) is 0.5 mol%, and the preferable upper limit is 15 mol%. If the amount is less than 0.5 mol%, the effect of forming a crosslinked structure cannot be obtained sufficiently, and the elastic modulus decreases at the time of high shear, so that dripping or unevenness occurs during coating. There is. If it exceeds 15 mol%, the degree of crosslinking becomes so high that it becomes insoluble in an organic solvent, so that the coating properties may be lowered.

In the polyvinyl acetal resin of the present invention, the preferable upper limit of the amount of residual halide is 100 ppm. If it exceeds 100 ppm, it becomes a light-sensitive silver halide product, which reduces the storage stability of the coating solution, and may cause deterioration of storage stability of the resulting photothermographic material, fogging, and the like. Examples of the method of setting the upper limit of the residual halide amount to 100 ppm include, for example, a method of using a non-halogenated catalyst as a catalyst used for acetalization, and water, water / Examples include a method of purifying by a washing operation with a mixed solution of alcohol and the like and removing it to a specified amount or less. A more preferable upper limit is 50 ppm.

In the polyvinyl acetal resin of the present invention, the preferable upper limit of the water content is 3.0% by weight. If it exceeds 3.0% by weight, the pot life of the photothermographic material to be obtained is decreased, and it reacts with a crosslinking agent having an isocyanate group added for the purpose of strengthening the coating film of the photothermographic material. The strengthening of the coating film may be insufficient. A more preferred upper limit is 2.5% by weight.

In the polyvinyl acetal resin of the present invention, the preferable upper limit of the residual alkaline substance amount is 0.01% by weight. If it exceeds 0.01% by weight, the pot life of the coating solution may be lowered, or the storage stability of the resulting heat-developable photosensitive material may be lowered. As a method of setting the upper limit of the content of the remaining alkaline substance to 0.01% by weight, for example, in the acetalization, a method of washing excess alkali content after neutralization with water or a mixed solution of water / alcohol, Examples thereof include a method of using a volatile alkali and removing the alkali by drying to make the upper limit 0.01% by weight. A more preferred upper limit is 0.005% by weight.

The polyvinyl acetal resin of the present invention has a preferable lower limit of 200 and a preferable upper limit of 3000 for the degree of polymerization. By setting the degree of polymerization within the above range, the dispersibility, coating film strength, and coating property of the organic silver salt used together with the polyvinyl acetal resin of the present invention can be improved. Further, considering the balance of the dispersibility of the organic silver salt, the strength of the coating film, the coating property, etc., the more preferable lower limit of the degree of polymerization is 300, and the more preferable upper limit is 1000.

The polyvinyl acetal resin of the present invention can be produced by acetalizing a modified polyvinyl alcohol having an acetoacetyl group in the side chain.

The minimum with the preferable saponification degree of the said polyvinyl alcohol is 75 mol%. If it is less than 75 mol%, the water-solubility of the polyvinyl alcohol will decrease, and the acetalization reaction may be difficult. Further, when the amount of hydroxyl groups is small, the acetalization reaction itself may be difficult.

The acetalization method is not particularly limited, and a conventionally known method can be used. For example, an aqueous solution of polyvinyl alcohol, an alcohol solution, a water / alcohol mixed solution, a dimethyl sulfoxide (DMSO) solution in the presence of an acid catalyst. Examples thereof include a method of adding various aldehydes.

The aldehyde used for the acetalization is not particularly limited. For example, the aldehyde is not particularly limited. For example, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, amylaldehyde, hexylaldehyde, heptylaldehyde, 2-ethylhexylaldehyde, Examples include cyclohexyl aldehyde, furfural, glyoxal, glutaraldehyde, benzaldehyde, 2-methylbenzaldehyde, 3-methylbenzaldehyde, 4-methylbenzaldehyde, p-hydroxybenzaldehyde, m-hydroxybenzaldehyde, phenylacetaldehyde, β-phenylpropionaldehyde and the like. Among them, it is preferable to use acetaldehyde and butyraldehyde alone or to use acetaldehyde and butyraldehyde in combination.

When acetaldehyde and butyraldehyde are used in combination as the aldehyde used for the acetalization, the resulting heat-developable photosensitive material has improved dispersibility of the silver salt, sharpening of heat melting property, cooling curability, etc. Thus, it becomes possible to accurately control the nucleation of the organic silver salt, and as a result, the sharpness of the image and the gradation portion is excellent.

In the polyvinyl acetal resin of the present invention, the preferable lower limit of the degree of acetalization is 40 mol%, and the preferable upper limit is 78 mol%. If it is less than 40 mol%, it may be insoluble in an organic solvent and may not be used as a binder resin for a photosensitive layer of a heat-developable photosensitive material. When it exceeds 78 mol%, the toughness of the polyvinyl acetal resin obtained by reducing the amount of residual hydroxyl groups may be impaired, and the strength of the coating film may be lowered.
In this specification, the method for calculating the degree of acetalization is a method of counting two acetalized hydroxyl groups since the acetal group of the polyvinyl acetal resin is acetalized from two hydroxyl groups. Is used to calculate the mole percent of the degree of acetalization.

The acid catalyst is not particularly limited, and either an organic acid or an inorganic acid can be used. Examples thereof include acetic acid, paratoluenesulfonic acid, nitric acid, sulfuric acid, and hydrochloric acid. Of these, non-halogen acid catalysts are preferred. If a non-halogen acid catalyst is used, the residual halide amount can be easily adjusted to 100 ppm or less. Further, when a halogen acid catalyst is used, it is preferable to purify the obtained polyvinyl acetal resin by sufficiently washing it with water or a mixed solution of water / alcohol.

In the acetalization reaction between polyvinyl alcohol and an aldehyde, a hindered phenol system is used in the reaction system or the resin system to prevent oxidation of the aldehyde or to improve the oxidation resistance and heat resistance of the obtained polyvinyl acetal resin. A method of adding an antioxidant such as bisphenol or phosphoric acid is known. However, when producing the polyvinyl acetal resin of the present invention, it is preferable not to use the antioxidant. When an antioxidant is used, the antioxidant remains in the polyvinyl acetal resin, causing a decrease in pot life of the coating solution and a decrease in storage stability of the resulting heat-developable photosensitive material. The sharpness of the part may be impaired.

In order to stop the acetalization reaction, neutralization with an alkali is preferably performed. It does not specifically limit as said alkali, For example, sodium hydroxide, potassium hydroxide, ammonia, sodium acetate, sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate etc. are mentioned.
Moreover, it is preferable to wash | clean the polyvinyl acetal resin obtained using water etc. before and after the said neutralization process. In order to prevent contamination of impurities contained in the cleaning water, it is more preferable to perform the cleaning with pure water.

Although it does not specifically limit as a method to manufacture the said modified polyvinyl alcohol, For example, it can manufacture by saponifying the modified polyvinyl acetate which has an acetoacetyl group in a side chain by a conventionally well-known method.

When the modified polyvinyl alcohol is produced by such a production method, the polyvinyl acetal resin of the present invention may have a structural unit represented by the following general formula (4).

In the polyvinyl acetal resin of the present invention, the preferable upper limit of the content of the acetyl unit represented by the general formula (4) is 25 mol%. If it exceeds 25 mol%, blocking may occur in the resulting photothermographic material, and the image may become unclear. A more preferred upper limit is 15 mol%.

The polyvinyl acetal resin, organic silver salt, photosensitive silver halide, silver ion reducing agent, crosslinking agent, additive and organic solvent of the present invention are mixed to prepare a coating solution, and the resulting coating solution is supported. A photothermographic material in which a photosensitive layer is formed on a support can be produced by coating and drying on the substrate.
Such a heat-developable photosensitive material is also one aspect of the present invention.

The content of the polyvinyl acetal resin of the present invention in the photosensitive layer is a weight ratio with respect to the organic silver salt (polyvinyl acetal resin of the present invention: organic silver salt), and the preferable lower limit is 1:10 and the preferable upper limit is 10: 1. is there. If it is less than 1:10, the adhesion between the layers constituting the photothermographic material cannot be improved, and if it exceeds 10: 1, the image may become unclear. A more preferred lower limit is 1: 5, and a more preferred upper limit is 5: 1.

The organic silver salt is a colorless or white silver salt that is relatively stable to light, and reacts with a silver ion reducing agent when heated to 80 ° C. or higher in the presence of photosensitive silver halide to produce silver. It happens.

It does not specifically limit as said organic silver salt, For example, the silver salt of the organic compound which has a mercapto group, a thione group, or a carboxyl group, benzotriazole silver, etc. are mentioned. Specifically, a silver salt of a compound having a mercapto group or a thione group, a silver salt of 3-mercapto-4-phenyl1,2,4-triazole, a silver salt of 2-mercapto-benzimidazole, 2-mercapto-5 -Silver salt of aminothioazole, silver salt of 1-phenyl-5-mercaptotetrathiazole, silver salt of 2-mercaptobenzothiazole, silver salt of thioglycolic acid, silver salt of dithiocarboxylic acid such as silver salt of dithioacetic acid , Silver thioamide, silver salt of thiopyridine, silver salt of dithiohydroxybenzol, silver salt of mercaptotriazine, silver salt of mercaptooxadiazole, silver salt of aliphatic carboxylic acid: silver caprate, silver laurate, silver myristate, palmitic acid Silver, silver stearate, silver behenate, silver maleate, silver fumarate, silver tartrate, silver furoate, silver linoleate Silver oleate, silver hydroxystearate, silver adipate, silver sebacate, silver succinate, silver acetate, silver butyrate, silver camphorate, etc., aromatic carboxylic acid silver, thione carboxylic acid silver, aliphatic carboxylic acid having thioether group Acid silver, silver salt of tetrazaindene, silver S-2-aminophenylthiosulfate, metal-containing amino alcohol, organic acid metal chelate and the like.

Although it does not specifically limit as a particle diameter of the said organic silver salt, A preferable minimum is 0.01 micrometer and a preferable upper limit is 10 micrometers. By setting it within the above range, the covering power can be increased and the image density can be increased. A more preferable lower limit is 0.1 μm, and a more preferable upper limit is 5 μm.

The photosensitive silver halide is not particularly limited, and examples thereof include silver bromide, silver iodide, silver chloride, silver chlorobromide, silver iodobromide, and silver chloroiodide.
The photosensitive silver halide is preferably chemically sensitized with a sensitizer. The method for chemically sensitizing the photosensitive silver halide with a sensitizer is not particularly limited, and examples thereof include a sulfur sensitizing method, a selenium sensitizing method, and a tellurium sensitizing method. In addition, noble metal sensitization using sensitizers such as chloroauric acid, potassium chloroaurate, potassium aurithiocyanate, gold sulfide, gold selenide, platinum, palladium, iridium compounds, ascorbic acid, thiourea dioxide, etc. And a reduction sensitization method using these sensitizers.

The photosensitive silver halide is preferably spectrally sensitized by using a sensitizing dye in combination. By using the sensitizing dye in combination, the photosensitive silver halide can be spectrally sensitized to a desired absorption wavelength range according to the wavelength range of the light source.

The content of the photosensitive silver halide is preferably 0.0005 parts by weight and preferably 0.2 parts by weight with respect to 100 parts by weight of the organic silver salt. If the amount is less than 0.0005 part by weight, the image after development may become unclear, and if the amount exceeds 0.2 part by weight, the image after development may become cloudy. A more preferred lower limit is 0.01 parts by weight.

The silver ion reducing agent is not particularly limited as long as it is an arbitrary substance capable of reducing silver ions to metallic silver. For example, substituted phenols, bisphenols, naphthols, bisnaphthols, polyhydroxybenzenes , Di- or polyhydroxynaphthols, di- or polyhydroxynaphthalenes, hydroquinones, hydroquinone monoethers, ascorbic acid or derivatives thereof, reducing sugars, aromatic amino compounds, hydroxyamines, hydrazines, phenidones, hindered Examples include phenols. Among these, a photodegradable silver ion reducing agent is preferably used, but a thermally decomposable reducing agent can also be used. In addition, a compound that promotes photolysis can be used in combination, and a coating agent for inhibiting the reaction between silver halide and the reducing agent can be used in combination.

The content of the silver ion reducing agent is preferably 0.0001 part by weight and preferably 3.0 parts by weight with respect to 100 parts by weight of the organic silver salt. By setting it within the above range, the organic silver salt can be appropriately reduced. A more preferred lower limit is 0.01 part by weight, and a more preferred upper limit is 1.0 part by weight.

The crosslinking agent is not particularly limited as long as it reacts with the acetoacetyl group, and includes a polyfunctional isocyanate.

It does not specifically limit as said polyfunctional isocyanate, Tolylene diisocyanate, 4,4'- diphenylmethane diisocyanate, 4,4'-xylene diisocyanate, hexamethylene diisocyanate, 4,4'-methylenebis (cyclohexyl isocyanate), isophorone diisocyanate, etc. Is mentioned.

The content of the cross-linking agent is preferably 0.02 parts by weight and preferably 0.5 parts by weight with respect to 100 parts by weight of the organic silver salt. If the amount is less than 0.02 parts by weight, the effect of forming a crosslinked structure cannot be sufficiently obtained, and the elastic modulus decreases at the time of high shear, so that dripping or unevenness occurs during coating. There is. If it exceeds 0.5 parts by weight, the degree of crosslinking becomes too high and becomes insoluble in an organic solvent, so that the coating properties may be lowered. A more preferred lower limit is 0.05 parts by weight, and a more preferred upper limit is 0.2 parts by weight.

It does not specifically limit as said additive, For example, a colorant etc. are mentioned.
The color tone agent has a function of making the resulting silver image black by participating in the redox reaction between the organic silver salt and the reducing agent.

The colorant is not particularly limited, and examples thereof include imides, naphtholimides, mercaptans, N- (aminomethyl) aryldicarboximides, blocked pyrazoles, phthalazinone, phthalazinone derivatives, or metals of these derivatives. Salts, phthalazines, phthalic acid and 2,3-naphthalenedicarboxylic acid or o-phenylene acid derivatives and their anhydrides, quinazolinediones, benzoxazines, naltoxazine derivatives, benzoxazine-2, 4-diones, pyrimidines, tetra And azapentalene derivatives. Of these, phthalazine or phthalazinone is preferable.

The organic solvent is not particularly limited as long as it can dissolve the polyvinyl acetal resin of the present invention and has a low water content. Specific examples thereof include diethyl ketone, methyl ethyl ketone, and methyl isobutyl. Ketones such as ketones; esters such as methyl acetate, ethyl acetate and propyl acetate are preferred. When ethanol, normal propyl alcohol, isopropyl alcohol or the like is used as the organic solvent, it is preferable to use a sufficiently dehydrated one.

The support is not particularly limited. For example, polyolefins such as polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyethylene, and polypropylene; cellulose esters such as polyvinyl acetal, cellulose diacetate, and cellulose triacetate; nitrocellulose, vinyl chloride resin And a plastic film made of chlorinated polypropylene or the like, a metal plate made of glass, paper, aluminum or the like. Of these, polyethylene terephthalate is preferred.

As for the amount of silver coated on the support, a preferable lower limit is 0.1 g and a preferable upper limit is 5 g per 1 m ² of the support. If it is less than 0.1 g, the image density may be low, and even if it exceeds 5 g, no improvement in the image density is observed. A more preferable lower limit is 0.3 g, and an upper limit is 3 g.

The method for producing the photothermographic material of the present invention is not particularly limited. For example, the polyvinyl acetal resin, organic silver salt, photosensitive silver halide, silver ion reducing agent, additive and organic solvent of the present invention are ball milled. Mix and disperse to prepare a dispersion. Next, a method of producing a photothermographic material by coating the prepared dispersion on a support so that the organic silver salt becomes a predetermined amount and evaporating the organic solvent can be mentioned.

The photosensitive silver halide may be formed by causing a photosensitive silver halide forming component to act on the organic silver salt to make a part of the organic silver salt a photosensitive silver halide.

The organic silver salt and the silver ion reducing agent may be combined together in the polyvinyl acetal resin of the present invention and formed as a single photosensitive layer on the support, or the organic silver salt and the silver ion reducing agent may be combined. You may mix | blend a silver ion reducing agent with the polyvinyl acetal resin of this invention separately, and may form these in two layers separately on a support body layer.
The photosensitive layer may be formed only on one side of the support or may be formed on both sides of the support.

According to the present invention, when used in a heat-developable photosensitive material, excellent image characteristics can be obtained, the coating solution is not colored, and the occurrence of problems such as dripping and unevenness is prevented. It is possible to provide a polyvinyl acetal resin for a heat-developable photosensitive material and a heat-developable photosensitive material that can improve productivity during the process.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Example 1
100 g of modified polyvinyl alcohol having a polymerization degree of 500, a saponification degree of 98 mol%, and an acetoacetyl unit content of 5 mol% was dissolved in 700 g of distilled water by heating and then kept at 20 ° C., and 26 g of 70% nitric acid was added thereto. In addition, 16 g of butyraldehyde was added. Next, it was cooled to 10 ° C. and 70 g of butyraldehyde was added. After the resin was deposited, the mixture was kept for 30 minutes, and then 93 g of nitric acid was added and the temperature was raised to 35 ° C. and kept for 6 hours. After completion of the reaction, it was washed with distilled water for 10 hours, and the polyvinyl butyral resin after washing with water was dehydrated. Then, only particles passed through a 60-mesh sieve were dispersed in distilled water, and sodium hydroxide was added to the solution. The pH was adjusted to 8. The solution was held at 50 ° C. for 6 hours and then cooled.
Next, the solution was washed with distilled water for 2 hours, dehydrated, dried at 40 ° C. for 12 hours, passed through a 60 mesh sieve, and only the passed particles were washed with distilled water for 10 hours. After washing, dehydration and drying were performed to obtain a polyvinyl acetal resin. The obtained polyvinyl acetal resin had an acetyl unit content of 1.7 mol%, a residual hydroxyl group content of 28 mol%, and an acetoacetyl unit content of 5 mol%.

(Comparative Example 1)
A polyvinyl acetal resin was obtained in the same manner as in Example 1 except that polyvinyl alcohol having a polymerization degree of 500 and a saponification degree of 98 mol% and containing no acetoacetyl unit was used.
The resulting polyvinyl acetal resin had a residual acetyl group content of 1.7 mol% and a residual hydroxyl group content of 28 mol%.

(Evaluation)
The polyvinyl acetal resin obtained in Example 1 and Comparative Example 1 was evaluated by the following method. The results are shown in Table 1.

(1) Evaluation of viscoelasticity 30 g of the obtained polyvinyl acetal resin was dissolved in 70 g of methyl ethyl ketone, and stirred for 48 hours with a mix rotor to dissolve it. After dissolution, it was allowed to stand at 20 ° C. for 30 minutes. Thereafter, the elastic modulus (G ′) of a region having high shear stress (100 to 200 Pa) was measured using a rotary rheometer (BM type, manufactured by Tokimec).

(2) Coloring evaluation 5 g of the obtained polyvinyl acetal resin, 5 g of silver behenate and 40 g of methyl ethyl ketone were mixed with a ball mill for 24 hours. Further, 0.2 g of N-lauryl-1-hydroxy-2-naphthamide was added, and again with a ball mill. A coating solution was prepared by grinding. The obtained coating solution was placed under an indoor fluorescent lamp at room temperature for 3 days, and the presence or absence of coloring of the coating solution was evaluated according to the following criteria.
○: No change was observed as white.
X: Coloring was observed.

(3) The coating solution prepared in the unevenness evaluation (2) was coated on a polyester substrate so that the thickness after drying was 10 μm and dried. On this coated surface, a solution consisting of 0.5 g of N, N-dimethyl-p-phenylenediamine sulfate, 2 g of polyvinylpyrrolidone and 30 mL of methanol was applied and dried so that the thickness after drying was 1 μm, A photosensitive film was obtained.
The thickness of the obtained photosensitive film was measured, and unevenness was evaluated according to the following criteria.
○: Thickness variation was less than ± 5%.
X: Thickness variation was ± 5% or more.

(4) Image evaluation The photosensitive film obtained in (3) was exposed for 0.3 seconds at a distance of 20 cm with 250 watts of high-pressure mercury through a gradation pattern film, and then for 5 seconds using a 120 ° C. hot plate. A cyan pattern image was obtained by heating.
The obtained pattern image was exposed to white light, and the change in the contrast of the pattern image was evaluated according to the following criteria.
A: No change was observed.
○: Almost no change was observed.
X: Change was seen.

実施例１で得られたポリビニルアセタール樹脂を用いた溶液は、弾性率の低下が見られなかった。また、感光性フィルムとしたときの、着色、ムラ及び画像評価でも良好な結果が得られた。
比較例１で得られたポリビニルアセタール樹脂を用いた溶液は、弾性率の低下が見られた。

The solution using the polyvinyl acetal resin obtained in Example 1 did not show a decrease in elastic modulus. Also, good results were obtained in coloring, unevenness and image evaluation when a photosensitive film was obtained.
The solution using the polyvinyl acetal resin obtained in Comparative Example 1 showed a decrease in elastic modulus.

According to the present invention, it is possible to provide a polyvinyl acetal resin for a heat-developable photosensitive material and a heat-developable photosensitive material that can prevent the occurrence of problems such as dripping and unevenness and can improve the productivity during coating.

Claims (2)

A polyvinyl acetal resin used for a heat-developable photosensitive material,
A polyvinyl acetal resin for heat-developable photosensitive material, comprising at least a structural unit represented by the following general formulas (1), (2) and (3).

In the formula, R represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms. A photothermographic material comprising the polyvinyl acetal resin for a photothermographic material according to claim 1.