JP5114439B2 - Polyvinyl acetal and heat-developable photosensitive material - Google Patents

Description

The present invention relates to polyvinyl acetal and a heat-developable photosensitive material using the same.

Polyvinyl acetal uses, as a raw material, an aldehyde that is a reducing substance during production. Aldehyde remains in the obtained polyvinyl acetal. On the other hand, when polyvinyl acetal is used for a heat-developable photosensitive material, in order to maintain the preservability of the photosensitive material, a material having a low aldehyde content or a material containing no aldehyde is desired.

Patent Document 1 describes a method for producing polyvinyl acetal, characterized in that the particle diameter of polyvinyl acetal is reduced and the concentration of aldehyde is reduced in the purification step.

JP 2006-47974 A

An object of the present invention is to provide a polyvinyl acetal having a reduced content of aldehyde, particularly 2-ethyl-2-hexenal, and a heat-developable photosensitive material using the same.

The present invention is a polyvinyl acetal having a content of 2-ethyl-2-hexenal of 80 ppm or less.

A polyvinyl acetal having a 2-ethyl-2-hexenal content of 80 ppm or less can be obtained by reacting an aldehyde having a 3-hydroxy-2-ethylhexanal concentration of 2000 ppm or less with polyvinyl alcohol in the presence of an acid catalyst. .

An aldehyde having a 3-hydroxy-2-ethylhexanal content of 2000 ppm or less can be obtained by removing high-boiling components by distillation.

A polyvinyl acetal having a concentration of 2-ethyl-2-hexenal of 80 ppm or less is obtained.

Polyvinyl acetal can be obtained by an acetalization reaction of aldehyde and polyvinyl alcohol under an acid catalyst.

For example, polyvinyl alcohol having an average polymerization degree of 150 to 3500 and a saponification degree of 86 to 99.9 mol% can be used. Moreover, when synthesizing polyvinyl acetal by an aqueous medium method, it is preferable to use polyvinyl alcohol as an aqueous solution having a concentration of 1 to 15% by mass because handling becomes easy.

Polyvinyl alcohol may be saponified by copolymerizing another monomer copolymerizable with a fatty acid vinyl ester. Examples of other monomers copolymerizable with fatty acid vinyl esters include olefins (ethylene, propylene, etc.), (meth) acrylates (methyl (meth) acrylate, 2-hydroxy (meth) acrylate, glycidyl (meth)). Acrylates, etc.), allyl ethers (allyl glycidyl ether, etc.), vinyl halide compounds (vinyl chloride, vinylidene chloride, vinyl fluoride, etc.), vinyl ethers (alkyl vinyl ether, 4-hydroxy vinyl ether, etc.), and the like.
The blending amount of the monomer is preferably 0.5 to 25 parts by mass with respect to 100 parts by mass of the fatty acid vinyl ester. The characteristics of these monomers can be expressed while maintaining the characteristics of the obtained polyvinyl alcohol and the polyvinyl acetal obtained by acetalizing the same.

Conventionally known aldehydes can be used. For example, it has an aldehyde having a chain aliphatic hydrocarbon group such as formaldehyde, paraformaldehyde, acetaldehyde, paraacetaldehyde, propionaldehyde, butyraldehyde, 2-ethylhexylaldehyde, or a cyclic aliphatic hydrocarbon group such as cyclohexylaldehyde, furfural. Bifunctional aldehydes such as aldehyde, glyoxal, glutaraldehyde, benzaldehyde, 2-methylbenzaldehyde, 3-methylbenzaldehyde, 4-methylbenzaldehyde, p-hydroxybenzaldehyde, m-hydroxybenzaldehyde, phenylacetaldehyde, β-phenylpropionaldehyde, etc. There are aldehydes with aromatic groups. These compounds may be used alone or in combination of two or more.

Of these compounds, it is preferable to use butyraldehyde. When the obtained polyvinyl acetal is used as a heat-developable photosensitive material, there is an effect that image blurring is reduced. The amount of butyraldehyde used is preferably 20% by mass or more of the entire aldehyde.

It is preferable to use an aldehyde having a 3-hydroxy-2-ethylhexanal content of 2000 ppm or less, preferably 1000 ppm or less. The content of 3-hydroxy-2-ethylhexanal can be quantified by a general analysis method such as gas chromatography. In the case of a gas chromatographic method, the use of a capillary column facilitates quantification at the 1000 ppm level.
An aldehyde having a 3-hydroxy-2-ethylhexanal content of 2000 ppm or less can be obtained by general purification such as distillation. For example, by removing the high boiling point component in the aldehyde by distillation, the 3-hydroxy-2-ethylhexanal concentration can be reduced to 2000 ppm or less.

When the aldehyde obtained by purification is stored for a long time, it is better to store it in a cool and dark place to prevent an increase in impurities.

The blending amount of the aldehyde varies depending on the degree of saponification of polyvinyl alcohol, the acetalization method thereof, and the like, but when it is produced by an aqueous medium method, it is preferable to use 50 to 200 parts by mass with respect to 100 parts by mass of polyvinyl alcohol. The amount of residual hydroxyl groups in the obtained polyvinyl acetal can be set within the target range.

A conventionally well-known thing can be used for an acid catalyst. For example, there are hydrochloric acid, nitric acid, sulfuric acid, acetic acid, paratoluenesulfonic acid and the like. These compounds may be used alone or in combination of two or more.
Although the compounding quantity of an acid catalyst changes with compositions, the acetalization method, etc. of polyvinyl alcohol, it is good to use 5-90 mass parts with respect to 100 mass parts of polyvinyl alcohol. Necessary and sufficient acetalization reaction can proceed.

The average degree of polymerization of the polyvinyl acetal is preferably 150 to 3500 because handling becomes easy.

The content of the vinyl alcohol unit in the polyvinyl acetal is preferably in the range of 8 to 25% by mass. The water resistance of the obtained polyvinyl acetal can be maintained. In addition, since it is industrially difficult to make a vinyl alcohol unit smaller than 8 mass%, the obtained polyvinyl acetal may become expensive.

The content of the fatty acid vinyl ester unit in the polyvinyl acetal is preferably in the range of 0.1 to 15% by mass. The heat resistance of the obtained polyvinyl acetal can be maintained. In addition, since it is industrially difficult to make a fatty-acid vinyl ester unit smaller than 0.1 mass%, the obtained polyvinyl acetal may become expensive.

An antioxidant may be added to the reaction solution during the acetalization reaction to prevent aldehyde oxidation, or an antioxidant may be added to polyvinyl acetal to prevent the resulting polyvinyl acetal from being oxidized or to improve heat resistance. .
The polyvinyl acetal of the present invention can also use an antioxidant. The content of the antioxidant is preferably 100 ppm or less, more preferably 50 ppm or less. When the antioxidant is added in an amount of more than 100 ppm, when polyvinyl acetal is used as the heat-developable photosensitive material, the pot life of the coating solution may be lowered, or the raw storability of the raw film may be lowered.
Such a raw film may impair the fogging and the sharpness of the image / gradation part.

As the antioxidant, hindered phenols, bisphenols, phosphoric acids, and the like can be used. Of these, hindered phenols are preferred. Examples of hindered phenol antioxidants include butylhydroxytoluene (BHT), butylhydroxyanisole, 2,4-dimethyl-6-tert-butylphenol, and n-octadecyl-β- (4′-hydroxy3 ′, 5 ′. -Di-t-butylphenyl) propionate, styrenated phenol, styrenated cresol, tocopherol, 2,2'-methylenebis (4-ethyl-6-t-butylphenol), 2,2'-methylenebis (4-methyl-6) -Cyclohexylphenol), 2,2′-butylidene-bis (4-methyl-6-tert-butylphenol), 1,6-hexanediol bis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) ) Propionate], 1,3,5-trimethyl-2,4,6-tris (3,5-di-t-butyl) -4-hydroxybenzyl) benzene, tetrakis [methylene-3- (3 ', 5'-di-t-butyl-4-hydroxyphenyl) propionate] methane, propyl gallate, octyl gallate, lauryl gallate, 2,4,6 -Tri-t-butylphenol, 2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone, 4,4'-methylene-bis- (2,6-di-t-butylphenol), 1,3,5-trimethyl-2,4,6-tris- (3,5-di-t-butyl-4-hydroxybenzyl) benzene. These compounds may be used alone or in combination of two or more.

The polyvinyl acetal also includes a modified polyvinyl acetal having a modifying group in the side chain. Modified groups include -COOM, -CH2COOM, -SO3M, -OSO3M, -PO (OM) 2, -P (= O) (R) (OM), tertiary amine groups, and quaternary ammonium bases. Generally used. Here, M represents H, Li, Na, or K, and R represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms.

As a method for obtaining such a modified polyvinyl acetal, for example, as disclosed in JP-A-2-220221, a method in which a unit having a modified group is previously allowed to coexist in the main chain formation position, For example, a method of introducing a hydrophilic group using a hydroxyl group of an alcohol type unit after the formation of a chain is employed.

The ratio of the modifying group in the modified polyvinyl acetal is preferably in the range of 0.1 to 5 mol%. When the ratio of the modifying group is less than 0.01 mol%, the effect of introducing the modifying group may not be sufficiently obtained. If the ratio of the modifying group exceeds 5 mol%, the solubility in the solvent after acetalization may be lowered.

2-ethyl-2-hexenal contained in the polyvinyl acetal can be quantified by a general analysis method such as a gas chromatographic method. In the case of the gas chromatographic method, a sample in which polyvinyl acetal is dissolved in a solvent such as ethanol is introduced into the gas chromatograph device, or a volatile component generated when the polyvinyl acetal is heated in a sealed container using the head space method is gas chromatographed. It can be measured by introducing it into the device. In the case of a gas chromatograph, the use of a capillary column facilitates quantification at the level of several tens of ppm.

The polyvinyl acetal thus obtained can be suitably used, for example, as a photothermographic material. When producing a heat-developable photosensitive material, it may be carried out in the same manner as the production of a conventional heat-developable photosensitive material except that the polyvinyl acetal of the present invention is used. That is, an organic silver salt, a reducing agent, photosensitive silver halide, and other compounding agents may be blended with the polyvinyl acetal of the present invention. The polyvinyl acetal is preferably in a weight ratio of 1:10 to 10: 1, more preferably 1: 5 to 5: 1 with respect to the organic silver salt.

An organic silver salt is a colorless or white silver salt that is relatively stable to light and should react with a reducing agent to produce silver when heated to 80 ° C. or higher in the presence of light-sensitive silver halide. If it does not specifically limit.

Examples of the organic silver salt include a silver salt of an organic compound having a mercapto group, a thione group, and a carboxyl group, and benzotriazole silver. 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- Silver salt of 5-aminothiazole, silver salt of 1-phenyl-5-mercaptobenzothiazole, silver salt of thioglycolic acid, silver salt of dithiocarboxylic acid such as silver salt of dithioacetic acid, silver thioamide, silver thiopyridine Silver salt of hydroxybenzol, silver salt of mercaptooxadiazole, silver salt of mercaptotriazine, silver salt of mercaptooxadiazole, silver salt of aliphatic carboxylic acid,: silver caprate, silver laurate, silver myristate, palmitic Silver oxide, 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 Examples thereof include silver acid, silver salt of tetrazaindene, silver S-2-aminophenylthiosulfate, metal-containing amino alcohol, and organic acid metal chelate.

Of the organic silver salts, preferred is a silver salt of an aliphatic carboxylic acid, and more preferred is silver behenate. The particle size of the organic silver salt is preferably 0.01 to 10 μm in diameter, and more preferably 0.1 to 5 μm.

The organic acid silver salt may contact the photosensitive silver halide catalytically. For example, a method of forming a part of organic silver into silver halide by causing a silver halide-forming component to act on a solution or dispersion of an organic silver salt prepared in advance or a film material containing the organic silver salt. is there.

The photosensitive silver halide forming component is not particularly limited as long as it acts on an organic silver salt to form silver halide, and the photosensitive silver halide is 0.0005 to 100 parts by weight of the organic silver salt. 0.2 part by weight is preferably used, more preferably 0.01 to 0.2 part by weight.

A reducing agent is also used for the heat-developable photosensitive material. The reducing agent is appropriately selected according to the organic silver salt used together, for example, substituted phenols, bisphenols, polyhydroxybenzenes, di- or polyhydroxynaphthalenes, hydroquinone monoethers, ascorbic acid or its derivatives, reducing sugars. , Aromatic amino compounds, hydroxyamines, hydrazines and the like. Among these, a photodegradable reducing agent is preferably used, but a thermally decomposable reducing agent is also used in combination. A compound that promotes photolysis can be used in combination, or a coating agent for inhibiting the reaction between silver halide and the reducing agent can be used in combination.

As a method for producing a photothermographic material using polyvinyl acetal, for example, polyvinyl acetal, an organic silver salt, a reducing agent and a solvent are dispersed with a ball mill, and then, if necessary, silver halide or silver halide. What is necessary is just to add a formation component and various compounding agents and to disperse | distribute with a ball mill.

As the solvent, a solvent that dissolves polyvinyl acetal and has almost no water content is preferably used. Of these, ketones and esters are preferable, and diethyl ketone, methyl ethyl ketone, ethyl acetate, and the like are more preferable. When ethanol, normal propyl alcohol, isopropyl alcohol or the like is used as the solvent, it is preferable to use a dehydrated one.

The obtained dispersion can be applied on a support so that the organic silver salt has a prescribed amount, and the solvent can be evaporated to obtain a heat-developable photosensitive material. The organic silver salt and the reducing agent may be combined into polyvinyl acetal in a lump and formed on the support in a single layer. However, the organic silver salt and the reducing agent may be separately added to the polyvinyl acetal or the reducing agent. You may mix | blend with a modification | denaturation polyvinyl acetal and may form these in two layers separately on a support body.

Examples of the support include plastic films such as polyethylene terephthalate, polycarbonate, polyethylene, polyvinyl acetal, cellulose ester, cellulose acetate, nitrocellulose, and polyethylene naphthalate, and metal plates such as glass, paper, and aluminum plates.

The silver applied on the support is preferably used in an amount of 0.1 to 5 g per 1 m 2 of the support, and more preferably 0.3 to 3 g. If it is less than 0.1 g, the image density may be lowered. Even when 5 g or more is used, no improvement in image density is observed.

In the present invention, when a black image is formed with silver, a color toning agent may be blended as a blending agent. In the case of forming a color image, a color coupler, a leuco dye or the like may be added.

Polyvinyl acetal and heat-developable photosensitive material have the above-described configuration, thereby increasing the photosensitivity due to the generation of silver halide, suppressing the abnormal growth reaction of silver salt, reducing the pot life of the coating liquid due to impurities, Reduction of raw storage stability of the sheet can be suppressed.

In addition, when using polyvinyl acetal with an acetoacetal moiety introduced, the dispersibility of the silver salt is improved, the thermal meltability, the cooling curability, etc. are sharpened, and the nucleus growth of the silver salt can be accurately controlled. is there. For this reason, the sharpness of the image and the gradation portion is improved. When the modified polyvinyl acetal is used, the dispersibility of the silver salt is further improved by the action of the modifying group composed of the hydrophilic group, and the sharpness of the image and the gradation portion is improved.

The present invention will be specifically described below based on examples. The present invention is not limited only to these examples.

Example 1
(Synthesis of polyvinyl acetal)
A 5 L dissolution tank equipped with a stirrer was charged with 3600 g of pure water, 400 g of polyvinyl alcohol having an average degree of polymerization of 600 and a degree of saponification of 98.4 mol%, heated and dissolved to obtain a 10% by mass aqueous polyvinyl alcohol solution. A cylindrical glass container having a capacity of 1250 ml having four supply ports at the bottom of the reactor and one discharge port at the top of the reactor is prepared. The reactor is filled with pure water, stirred with a large blade, and the internal temperature Is maintained at 30 ° C., the polyvinyl alcohol aqueous solution is 4600 g / hr, 20 mass% hydrochloric acid 130 g / hr, the mixing weight ratio of acetaldehyde and butyraldehyde is 2 to 8, and the content of 3-hydroxy-2-ethylhexanal is 950 ppm of mixed aldehyde was introduced into the reactor from the lower part of the reactor at a feeding rate of 270 g / hr, and the reaction liquid was overflowed and discharged from the upper part of the reactor in parallel. The discharged reaction liquid was sent to a separately prepared 10 L aging tank (amount transferred: 4.2 L), and 450 g of 35% by mass hydrochloric acid was added to the aging tank, followed by aging reaction at 45 ° C. for 3 hours. The content of 3-hydroxy-2-ethylhexanal in the aldehyde was measured by an area ratio by gas chromatograph (GC-14B manufactured by Shimadzu Corporation, detector: FID, column: DB-624, 0.25 mm × 30 m). Asked.

Sodium hydroxide aqueous solution was mix | blended with the reaction material after ageing | curing | ripening, and it adjusted to pH7.5, and stopped the acetalization reaction. The reaction product after neutralization was cooled to room temperature and then filtered through a centrifuge. A part of this filter cake was taken and heated at 110 ° C. for 12 minutes, and the resin content was calculated by weight reduction. As a result, it was 78% by mass. Further, a part of this filter cake was dissolved in ethanol, and the content of 2-ethyl-2-hexenal was measured by the above-mentioned gas chromatograph. As a result, it was 50 ppm per resin content. The remaining filter cake was diluted with distilled water at a normal temperature of 20 times the resin content and filtered again twice, and then dried to obtain polyvinyl acetal.

The obtained polyvinyl acetal had a vinyl alcohol unit of 12.5% by mass, a vinyl acetate unit of 1.2% by mass, an acetalization degree of 86.2% by mass, and a 2-ethyl-2-hexenal content of 45 ppm. there were. The content of 2-ethyl-2-hexenal in the polyvinyl acetal was measured using a gas chromatograph (GC-14B manufactured by Shimadzu Corporation, detector: FID, column: DB-624, 0.25 mm × 30 m), head The area ratio was obtained by the space method.

(Storage stability of coating solution)
5 g of silver behenate, 5 g of polyvinyl acetal, and 40 g of methyl ethyl ketone were mixed with a ball mill for 24 hours, and further 0.2 g of N-lauryl-1-hydroxy-2-naphthamide was added and ground again with a ball mill to obtain a coating solution. . When the obtained coating solution was placed under a fluorescent lamp indoors at room temperature for 3 days and the color of the solution was observed, it remained white and did not change.

(Heat developability)
The coating solution was applied on a polyester substrate so that the thickness after drying was 10 μm and dried. On this coated surface, a solution composed 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. The photosensitive film thus obtained 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 developed by heating for 5 seconds using a 120 ° C. hot plate, A clear cyan pattern image without fogging was obtained.

(Image preservation)
The obtained image was exposed to white light for 24 hours, and the disturbance of the contrast of the image pattern was observed. However, the disturbance of the contrast was not observed.

(Odor)
The presence or absence of a specific odor of 2-ethyl-2-hexenal at the time of creating the photosensitive film and at the time of heat development was confirmed by a sensory test, but almost no odor was felt.

(Example 2)
A polyvinyl acetal was obtained in the same manner as in Example 1 except that the reaction was performed using a mixed aldehyde having a content of 3-hydroxy-2-ethylhexanal of 1900 ppm as a raw material. The obtained polyvinyl acetal has 13.0% by mass of vinyl alcohol units, 1.4% by mass of vinyl acetate units, 85.6% by mass of acetalization, and the content of 2-ethyl-2-hexenal is before washing. And 75 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was hardly colored, the photosensitive film was hardly fogged during heat development, the contrast after storage was hardly disturbed, and the odor was hardly felt.

(Example 3)
A polyvinyl acetal was obtained in the same manner as in Example 1 except that the reaction was performed using polyvinyl alcohol having an average polymerization degree of 2400 and a saponification degree of 98.5% as a raw material. The obtained polyvinyl acetal was 12.9% by mass of vinyl alcohol units, 1.8% by mass of vinyl acetate units, 85.3% by mass of acetalization, and the content of 2-ethyl-2-hexenal was before washing. And 43 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was not colored, the photosensitive film was not fogged during heat development, the contrast after storage was not disturbed, and almost no odor was felt.

Example 4
A polyvinyl acetal was obtained in the same manner as in Example 1 except that the reaction was performed using polyvinyl alcohol having an average polymerization degree of 300 and a saponification degree of 98.7% as a raw material. The obtained polyvinyl acetal has a vinyl alcohol unit of 12.0% by mass, a vinyl acetate unit of 1.1% by mass, an acetalization degree of 86.9% by mass, and the content of 2-ethyl-2-hexenal before washing. 53 ppm and 49 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was not colored, the photosensitive film was not fogged during heat development, the contrast after storage was not disturbed, and almost no odor was felt.

(Comparative Example 1)
A polyvinyl acetal was obtained in the same manner as in Example 1 except that the reaction was performed using a mixed aldehyde having a content of 3-hydroxy-2-ethylhexanal of 2300 ppm as a raw material. The obtained polyvinyl acetal was 13.2% by mass of vinyl alcohol units, 1.4% by mass of vinyl acetate units, 85.4% by mass of acetalization, and the content of 2-ethyl-2-hexenal was before washing. And 92 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was colored, and the photosensitive film was fogged during heat development, resulting in poor sharpness, the contrast after storage was disturbed, and odor was felt.

(Example 5)
Polyvinyl acetal was obtained in the same manner as in Example 1 except that butyraldehyde having a content of 3-hydroxy-2-ethylhexanal of 900 ppm was supplied at 340 g / hr and acetaldehyde was not used. The obtained polyvinyl acetal has a vinyl alcohol unit of 14.6% by mass, a vinyl acetate unit of 1.6% by mass, an acetalization degree of 83.8% by mass, and the content of 2-ethyl-2-hexenal before washing. And 57 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was not colored, the photosensitive film was not fogged during heat development, the contrast after storage was not disturbed, and almost no odor was felt.

(Example 6)
A polyvinyl acetal was obtained in the same manner as in Example 5 except that butyraldehyde having a content of 3-hydroxy-2-ethylhexanal of 1800 ppm was used as a raw material. The obtained polyvinyl acetal is 15.7% by mass of vinyl alcohol units, 1.2% by mass of vinyl acetate units, 83.1% by mass of acetalization, and the content of 2-ethyl-2-hexenal is before washing. And 76 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was hardly colored, the photosensitive film was hardly fogged during heat development, the contrast after storage was hardly disturbed, and the odor was hardly felt.

(Comparative Example 2)
A polyvinyl acetal was obtained in the same manner as in Example 5 except that butyraldehyde having a content of 3-hydroxy-2-ethylhexanal of 2400 ppm was used as a raw material. The obtained polyvinyl acetal was 15.4% by mass of vinyl alcohol units, 1.7% by mass of vinyl acetate units, 82.9% by mass of acetalization, and the content of 2-ethyl-2-hexenal was before washing. 95 ppm, and 87 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was colored, and the photosensitive film was fogged during heat development, resulting in poor sharpness, the contrast after storage was disturbed, and odor was felt.

(Comparative Example 3)
A polyvinyl acetal was obtained in the same manner as in Example 5 except that butyraldehyde having a content of 3-hydroxy-2-ethylhexanal of 5000 ppm was used as a raw material. The obtained polyvinyl acetal is 14.4% by mass of vinyl alcohol units, 1.5% by mass of vinyl acetate units, 84.1% by mass of acetalization, and the content of 2-ethyl-2-hexenal is before washing. 111 ppm and 103 ppm after washing. Using the obtained polyvinyl acetal, in the same manner as in Example 1, preparation of a coating solution, production of a photosensitive film, and evaluation were performed. The coating solution was remarkably colored, and the photosensitive film was severely fogged during heat development and poor in sharpness. The contrast after storage was greatly disturbed and the odor was strong.

As described above, the polyvinyl acetals obtained in Examples 1 to 6 have good storage stability of the coating solution, a clear image without fogging, good image storage stability, and almost no odor during handling. It was. On the other hand, the polyvinyl acetals obtained in Comparative Examples 1 to 3 have poor storage stability of the coating liquid, low image sharpness due to fogging, poor image storage stability, and odor during handling.

The polyvinyl acetal of the present invention has a low odor and a low concentration of the reducing substance, so that the storage stability of the coating liquid is excellent, a clear image is obtained, and the image storage stability is good. Can be used very suitably. Also, a polyvinyl acetal suitable for a heat-developable photosensitive material can be obtained without a special post-processing step.

Claims (2)

A photothermographic material having a 2-ethyl-2-hexenal content of 80 ppm or less in which an aldehyde having a content of 3-hydroxy-2-ethylhexanal of 2000 ppm or less and polyvinyl alcohol is acetalized under an acid catalyst. Method for producing polyvinyl acetal. 2. The method for producing a polyvinyl acetal for a heat-developable photosensitive material according to claim 1 , wherein the aldehyde has a content of 3-hydroxy-2-ethylhexanal of 2000 ppm or less by distillation.