JP2007137059A - Thermal transfer layer binder, thermal transfer ink sheet and thermal transfer overcoat sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermal transfer layer binder, a thermal transfer ink sheet and a thermal transfer overcoat sheet which can improve productivity in coating when used as a binder of an ink layer and an overcoat layer and can be used to manufacture the overcoat sheet being excellent in environmental resistance. <P>SOLUTION: The thermal transfer layer binder comprising polyvinyl acetal resin having at least a specific construction unit, is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention, when used as a binder for an ink layer and an overcoat layer, can improve the productivity during coating and can produce an overcoat layer excellent in environmental resistance. The present invention relates to a binder, a thermal transfer ink sheet, and a thermal transfer overcoat sheet.

Conventionally, a thermal transfer system has been used as one of means for forming and recording characters and images. As a basic principle of recording by such a thermal transfer method, first, an ink sheet containing a binder resin in which pigments or dyes of respective colors are dispersed is superposed on an image receiving sheet serving as an image forming medium. Next, the image information is separated into three primary colors of yellow, cyan, and magenta, each color is converted into heat through the thermal head as an electrical signal, and the thermal head is scanned from above the ink sheet while controlling the amount of heat. The pigment or dye is transferred from the ink sheet to the image receiving sheet. An image is formed by repeating such a transfer process for each color.
Further, at present, as disclosed in, for example, Patent Document 1 or Patent Document 2, it contains an ultraviolet absorber or binder resin called an overcoat layer after image formation in order to improve environmental resistance such as weather resistance. It is common practice to form a layer on an image receiving sheet.

Such thermal transfer systems are roughly classified into a melt type and a dye sublimation type. Among them, the melting type is used for card printers, barcode printers, ticket printers, and the like, and the dye sublimation type is used for photo printing, medical / analyzer output terminals, ID card photo printing, and the like. In particular, the dye sublimation type has been widely deployed as a photographic printing method to replace silver salt with the recent digitization, and future growth is expected.

Such a dye sublimation thermal transfer system has image characteristics comparable to silver salt photography for photographic printing. However, in recent years, it can cope with higher printing speed, weather resistance, and corrosion resistance. That are excellent in environmental resistance such as heat resistance, water resistance, and scratch resistance.

In order to cope with such high-speed printing, it is necessary to increase the amount of heat per unit time at the time of thermal transfer, an ink sheet having high sensitivity to heat and excellent transfer efficiency, and the like. Moreover, in order to make it excellent in environmental resistance, especially the intensity | strength of the binder resin component of an overcoat layer is required to be high.

On the other hand, in order to increase the heat resistance and strength of the ink sheet, the use of a binder resin having a high glass transition temperature or the use of a binder resin having a high degree of polymerization has been studied. However, if a binder resin having a high glass transition temperature is used, or if a binder resin having a high degree of polymerization is used, the time required for dissolution of the binder resin becomes longer, or the viscosity of the solution adjusted at the time of application to the substrate is increased. Therefore, there is a problem that the pumping performance is lowered and the productivity is lowered.

On the other hand, adjusting the viscosity by reducing the concentration of the binder resin in the solution has been studied. However, when the concentration of the binder resin in the solution is lowered, there is a problem that the elasticity at the time of coating is lowered, and problems such as dripping and unevenness occur, or the required film thickness cannot be obtained. .
Japanese Patent No. 2925699 Japanese Patent No. 3395090

In view of the above, the present invention, when used as a binder for an ink layer and an overcoat layer, can improve productivity during coating and can produce an overcoat layer excellent in environmental resistance. It is an object to provide a possible thermal transfer layer binder, thermal transfer ink sheet and thermal transfer overcoat sheet.

The present invention is a thermal transfer layer binder comprising a polyvinyl acetal resin having 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。
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 used polyvinyl acetal resin as a binder for a coating solution used for the production of a thermal transfer sheet. It has been found 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 inventors of the present invention have reduced the elastic modulus of the coating solution even under a high shear rate during coating by using a polyvinyl acetal resin composed of a specific structural unit as a binder of the coating solution. Thus, the present inventors have found that the occurrence of problems such as dripping and unevenness can be prevented without completing the present invention.

The thermal transfer layer binder of the present invention comprises at least a polyvinyl acetal resin having a structural unit 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 thermal transfer layer binder of the present invention is dissolved in a suitable solvent to form a coating solution for forming the ink layer and the overcoat layer of the thermal transfer sheet, the solution can be obtained 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 an elasticity modulus. The overcoat layer of the thermal transfer sheet obtained using the thermal transfer layer binder of the present invention is excellent in environmental resistance such as scratch resistance and weather resistance.

In the polyvinyl acetal resin, 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 cohesive strength of the resin is lowered, and the environmental resistance of the resulting overcoat layer may be deteriorated. If it exceeds 35 mol%, moisture tends to permeate, and fogging may occur, resulting in poor storage stability and image density.

In the polyvinyl acetal resin, the preferable lower limit of the content of the acetal unit represented by the general formula (2) is 40 mol%, and the preferable upper limit is 78 mol%. If it is less than 40 mol%, it becomes insoluble in an organic solvent, which hinders the production of a thermal transfer sheet. When it exceeds 78 mol%, the residual hydroxyl group is decreased, the toughness is impaired, and the coating strength at the time of producing the thermal transfer sheet 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 viscosity 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, when the thermal transfer layer binder of the present invention is used in the ink layer, it can interact with the dye and more effectively disperse and hold the dye, thereby improving the transfer efficiency. Moreover, when it uses for an overcoat layer, since it can form a complex with a ultraviolet absorber and an ultraviolet absorber can be disperse | distributed effectively, the outstanding weather resistance is obtained.

In the polyvinyl acetal resin, 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.

As for the said polyvinyl acetal resin, the minimum with a preferable glass transition temperature is 80 degreeC. If it is lower than 80 ° C., the coating strength of the solution using the thermal transfer layer binder of the present invention may be lowered.
In this specification, the glass transition temperature refers to a straight line equidistant in the vertical axis direction from a straight line obtained by extending the base lines on the low temperature side and the high temperature side in the DSC curve, and a step shape of the glass transition from the low temperature side. It means the temperature of the part where the curve of the change part intersects.

The polyvinyl acetal resin has a preferable lower limit of the polymerization degree of 200 and a preferable upper limit of 3000. By setting the degree of polymerization within the above range, the coating film strength and coating properties of the solution using the thermal transfer layer binder of the present invention can be improved. Furthermore, when considering the balance of the coating film strength, coating property, and the like, a more preferable lower limit of the degree of polymerization is 300, and a more preferable upper limit is 1300.

The polyvinyl acetal resin has a preferable lower limit of the degree of acetalization of 40 mol% and a preferable upper limit of 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 for the ink layer and overcoat layer of the thermal transfer sheet. 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 polyvinyl acetal resin can be produced by acetalizing polyvinyl alcohol having an acetoacetyl group in the side chain.

The lower limit of the saponification degree of the polyvinyl alcohol is preferably 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. Especially, it is preferable to use acetaldehyde alone or to use acetaldehyde and butyraldehyde in combination.

When acetaldehyde is used alone as the aldehyde, or when acetaldehyde and butyraldehyde are used in combination, the resulting thermal transfer sheet is sharp in terms of heat meltability, cooling curability, etc. It is more preferable because the ultraviolet absorber can be held and dispersed well in the overcoat layer.

When acetaldehyde and butyraldehyde are used in combination as the aldehyde, the preferred lower limit of the amount of acetaldehyde used is 50 mol%. If it is less than 50 mol%, the coating strength of the solution using the polyvinyl acetal resin may decrease, or the heat melting property, cooling curability, etc. of the resulting thermal transfer sheet may decrease.

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 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 may have a structural unit represented by the following general formula (4).

In the polyvinyl acetal resin, 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 ink layer and the image may become unclear. A more preferred upper limit is 15 mol%.

The thermal transfer layer binder of the present invention is mixed with an organic solvent, a dye, and the like to prepare a coating solution, which is coated on a support and dried to form an ink layer. A thermal transfer ink sheet comprising an ink layer containing the thermal transfer layer binder and dye of the present invention laminated thereon can be produced.
Such a thermal transfer ink sheet is also one aspect of the present invention.
In the thermal transfer ink sheet of the present invention, when applying the ink layer, the elastic modulus of the solution does not decrease even at high shear, and it is possible to prevent the occurrence of problems such as dripping and unevenness during coating. it can. In the thermal transfer ink sheet of the present invention, the ink layer is excellent in environmental resistance such as scratch resistance. Furthermore, an image having excellent image characteristics can be formed by using the thermal transfer ink sheet of the present invention.

The method for applying the ink layer is not particularly limited, and a general method can be used. Examples thereof include a bar coater, gravure printing, and screen printing.

The above dyes can be transferred between media by melting, diffusing or sublimating by heat, and are selected in consideration of sublimation temperature, hue, weather resistance, solubility in ink or binder, etc. The In general, for example, diallylmethane, triallylmethane, thiazole, methine, azomethine, xanthine, oxazine, thiazine, azine, acridine, azo, spirodipyran, isodolinospiropyran Typical examples include fluoran series, rhodamine lactam series and anthraquinone series. Specifically, for example, Sumikaron Violet RS (manufactured by Sumitomo Chemical Co., Ltd.), Dianix Fast Violet 3R FS (manufactured by Mitsubishi Chemical Co., Ltd.), Kayalon Polyol Blue NBGM, KST Black 146 (manufactured by Nippon Kayaku Co., Ltd.), etc. Anthraquinone dyes, Kayalon Polyol Blue Blue BM, Kayalon Polyol Dark Blue 2BM, KST Black KR (manufactured by Nippon Kayaku Co., Ltd.), Sumikaron Diazo Black 5G (Sumitomo Chemical H Azo dyes such as Direct Dark Green B (manufactured by Mitsubishi Chemical Industries), Dir Direct dyes such as ct Brown M and Direct Fast Black D (manufactured by Nippon Kayaku Co., Ltd.), etc. Acidic dyes represented by Kayanol Milling Cyanine 5R (manufactured by Nippon Kayaku Co., Ltd.) And basic dyes typified by Aizen Malachite Green (Hodogaya Chemical Co., Ltd.) and the like.

The preferred lower limit of the content of the dye is 10 parts by weight and the preferred upper limit is 120 parts by weight with respect to 100 parts by weight of the polyvinyl acetal resin. When the amount is less than 10 parts by weight, the density of the obtained image is not obtained, and when it exceeds 120 parts by weight, the dye may not be uniformly dispersed.

It does not specifically limit as said organic solvent, The thing which can melt | dissolve the thermal transfer layer binder of this invention, is excellent in the diffusibility of dye, and there is little water content is used suitably. Specific examples include ketones such as diethyl ketone, methyl ethyl ketone, and methyl isobutyl ketone; esters such as methyl acetate, ethyl acetate, and propyl acetate. When ethanol, normal propyl alcohol, isopropyl alcohol or the like is used as the organic solvent, it is preferable to use a dehydrated one. Moreover, these organic solvents may be used independently and may be used in mixture of 2 or more types.

An adhesive layer may be interposed between the ink layer and the support. By interposing such an adhesive layer, the ink layer can be prevented from being easily peeled off from the support.

The adhesive constituting the adhesive layer is not particularly limited as long as it is a material that improves the adhesiveness between the support and the ink layer. For example, polyvinyl acetal resin, acrylic resin, polyester resin, polyurethane Based resins and the like.

The preferable lower limit of the thickness of the ink layer is 0.2 μm, and the preferable upper limit is 6.0 μm. When the thickness is less than 0.2 μm, the density of the obtained image is not obtained, and when it exceeds 6.0 μm, the sublimation diffusibility of the dye is lowered, and the image quality may be lowered.

The support is not particularly limited, for example, paper such as condenser paper, glassine paper, polyester typified by polyethylene terephthalate, polystyrene, polysulfone, polyamide, polycarbonate, polyimide amide, polyimide typified by polyether imide, Polyvinyl alcohol, polyethylene, polypropylene, polyolefins typified by methylpentene polymer, cellulose esters typified by cellulose acetate, fluoropolymers typified by polyvinylidene fluoride, and polyethers typified by polyoxymethylene are used. It is done. In addition, these supports may be used alone or in combination of two or more.

The thickness of the support is appropriately adjusted from the viewpoints of material properties, strength when used as a sheet, and the like. In general, the preferable lower limit is 1.0 μm, and the preferable upper limit is 100 μm.

The thermal transfer layer binder of the present invention is mixed with an organic solvent to prepare a coating solution, which is coated on a support and dried to form an overcoat layer. A thermal transfer overcoat sheet comprising an overcoat layer containing the thermal transfer layer binder of the present invention laminated on can be produced.
Such a thermal transfer overcoat sheet is also one aspect of the present invention.
In the thermal transfer overcoat sheet of the present invention, when the overcoat layer is applied, the elastic modulus of the solution does not decrease even at high shear, and the occurrence of problems such as dripping and unevenness during coating is prevented. be able to. In the thermal transfer overcoat sheet of the present invention, the overcoat layer is excellent in environmental resistance such as scratch resistance. Furthermore, by using the thermal transfer overcoat sheet of the present invention, an overcoat layer having environmental resistance such as weather resistance and scratch resistance can be formed on the image.

As the method for applying the overcoat layer, the same method as that for producing the thermal transfer ink sheet of the present invention can be used.
As the organic solvent and the support, those similar to those used for producing the thermal transfer ink sheet of the present invention can be used.

The overcoat layer preferably contains an ultraviolet absorber. By including the ultraviolet absorber, excellent weather resistance and the like can be obtained.
The ultraviolet absorber is not particularly limited. For example, benzophenone-based, benzotriazole-based, oxalic acid anilide-based, cyanoacrylate-based, salicylate-based organic ultraviolet absorbers, zinc oxide, titanium oxide, cerium oxide, tin oxide, Examples include metal ultraviolet absorbers such as iron oxide.

The content of the ultraviolet absorber is preferably 5 parts by weight and preferably 50 parts by weight with respect to 100 parts by weight of the polyvinyl acetal resin. If the amount is less than 5 parts by weight, sufficient ultraviolet absorption ability cannot be obtained and the image may be deteriorated. If the amount exceeds 50 parts by weight, no further ultraviolet absorption effect can be obtained and image characteristics are deteriorated. Sometimes.

A release layer may be interposed between the overcoat layer and the support. By interposing such a release layer, the overcoat layer can be smoothly transferred from the support to the image receiving sheet without unevenness.

The releasable substance constituting the release layer is not particularly limited as long as it is a material that improves the releasability between the support and the binder used for the overcoat layer. For example, polyvinyl acetal resin, acrylic Resin, polyester resin, polyurethane resin and the like.
The release material may form a release layer with the release material alone, but may be dispersed in a binder.

The thickness of the overcoat layer is not particularly limited as long as it can exhibit the performance, but generally, the preferable lower limit is 2.0 μm and the preferable upper limit is 120 μm. If it is less than 2.0 μm, sufficient environmental resistance may not be obtained, and if it exceeds 120 μm, the image quality may be deteriorated.

In the thermal transfer ink sheet of the present invention and the thermal transfer overcoat sheet of the present invention, a sliding layer may be formed on the thermal head contact surface side. By forming such a sliding layer, it is possible to prevent the thermal transfer ink sheet of the present invention and the thermal transfer overcoat sheet of the present invention from adhering to the thermal head during thermal transfer.

A solid or liquid lubricant or a mixture thereof is used for the sliding layer.
Examples of the lubricant include polyvinyl stearate, beeswax, perfluorinated alkyl ester polyether, polycaprolactone, silicone oil, polytetrafluoroethylene, carbowax, polyethylene glycol and the like. Suitable binders for the sliding layer include, for example, polyvinyl acetal resin, polystyrene, polyvinyl acetate, cellulose acetate butyrate, cellulose acetate propionate, and the like.
The lubricant may form a sliding layer with a single lubricant, but may be dispersed in a binder.

As for the amount of the lubricant applied on the support, a preferable lower limit is 0.001 g and a preferable upper limit is 2 g / m ² per 1 m ² of the support.
When the lubricant is dispersed in the binder, the content of the lubricant is preferably 0.1% by weight and preferably 40% by weight with respect to the binder used.

In the thermal transfer ink sheet of the present invention and the thermal transfer overcoat sheet of the present invention, the ink layer and the overcoat layer may be formed on separate supports, respectively, and are sequentially arranged on the same support. It may be formed.

According to the present invention, by using it as a binder for an ink layer and an overcoat layer, even when the printing speed is increased, occurrence of problems such as dripping and unevenness is prevented, and productivity is improved. It is possible to provide a thermal transfer layer binder, a thermal transfer ink sheet, and a thermal transfer overcoat sheet that can produce an overcoat layer that is excellent in environmental resistance such as scratch resistance and weather resistance.

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 degree of polymerization, 98 degree saponification, 5 mole% acetoacetyl unit content 100 g of modified polyvinyl alcohol was dissolved in 700 g of distilled water by heating and kept at 20 ° C., and 26 g of 70% nitric acid was added thereto. 40 g of acetaldehyde was added. Next, it was cooled to 10 ° C. and 30 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 running water for 10 hours with distilled water, and after dehydrating the polyvinyl acetal resin after washing with water, only the 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. This solution was kept 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. A polyvinyl acetal resin obtained by dehydration and drying after washing was used as a thermal transfer layer binder. 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%.
Moreover, about the obtained polyvinyl acetal resin, a glass transition temperature is measured at a temperature increase rate of 10 ° C./min by a method according to JIS K 7121 using a differential scanning calorimeter (DSC-6200R, manufactured by Seiko Denshi Kogyo Co., Ltd.). The result was 90 ° C.

(Example 2)
100 degree of polymerization, 98 degree saponification, 5 mole% acetoacetyl unit content 100 g of modified polyvinyl alcohol was dissolved in 700 g of distilled water by heating and kept at 20 ° C., and 26 g of 70% nitric acid was added thereto. 40 g of acetaldehyde was added. Next, it cooled to 12 degreeC and 30 g of acetaldehyde 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 running water for 10 hours with distilled water, and after dehydrating the polyvinyl acetal resin after washing with water, only the 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. This solution was kept 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. A polyvinyl acetal resin obtained by dehydration and drying after washing was used as a thermal transfer layer binder. 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%.
Moreover, when the glass transition temperature was measured by the method similar to Example 1 about the obtained polyvinyl acetal resin, it was 110 degreeC.

(Comparative Example 1)
100 g of polyvinyl alcohol having a polymerization degree of 500 and a saponification degree of 98 mol% and containing no acetoacetyl units, instead of 100 g of modified polyvinyl alcohol having a polymerization degree of 500 and a saponification degree of 98 mol% and an acetoacetyl unit content of 5 mol% A polyvinyl acetal resin and a thermal transfer layer binder were obtained in the same manner as in Example 1 except that was used. The obtained polyvinyl acetal resin had an acetyl unit content of 1.7 mol% and a residual hydroxyl group content of 28 mol%.
Moreover, it was 90 degreeC when the glass transition temperature was measured by the method similar to Example 1. FIG.

(Comparative Example 2)
100 g of polyvinyl alcohol having a polymerization degree of 500 and a saponification degree of 98 mol% and containing no acetoacetyl units, instead of 100 g of modified polyvinyl alcohol having a polymerization degree of 500 and a saponification degree of 98 mol% and an acetoacetyl unit content of 5 mol% A polyvinyl acetal resin and a thermal transfer layer binder were obtained in the same manner as in Example 1 except that 40 g of butyraldehyde was used instead of 40 g of acetaldehyde. The obtained polyvinyl acetal resin had an acetyl unit content of 1.7 mol% and a residual hydroxyl group content of 28 mol%.
Moreover, it was 68 degreeC when the glass transition temperature was measured by the method similar to Example 1. FIG.

(Evaluation)
The polyvinyl acetal resins obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated by the following methods. The results are shown in Table 1.

(1) Evaluation of viscoelasticity 30 g of the obtained thermal transfer layer binder was dissolved in 70 g of methyl ethyl ketone, and was stirred for 48 hours with a mix rotor and dissolved. After dissolution, the mixture was allowed to stand at 20 ° C. for 30 minutes. Thereafter, using a rheometer (manufactured by Bohlin), the elastic modulus (G ′) of a region having a high shear stress (100 to 200 Pa) was measured.

(2) Evaluation of unevenness 10 g of the obtained thermal transfer layer binder, 10 g of a dye (Kayalon Polyol Blue N BGM, manufactured by Nippon Kayaku), 40 g of methyl ethyl ketone and 40 g of toluene were mixed in a ball mill for 24 hours to prepare a coating solution. The ink sheet was coated on the substrate so that the thickness after drying was 10 μm and dried to obtain an ink sheet.
The thickness of the obtained ink sheet was measured, and unevenness was evaluated according to the following criteria.
○: Thickness variation was less than ± 5%.
X: Thickness variation was ± 5% or more.

(3) Storage stability evaluation 10 g of the obtained thermal transfer layer binder, 10 g of a dye (CI disperse yellow 201), 40 g of methyl ethyl ketone and 40 g of toluene were mixed by a ball mill for 24 hours to prepare a coating solution. The ink sheet was obtained by coating and drying so that the thickness after drying was 10 μm.
The obtained ink sheet was allowed to stand in a constant temperature and humidity chamber at a temperature of 60 ° C. and a humidity of 90% for 1 week, and then the dispersion state of the dye was confirmed, and the storage stability was evaluated according to the following criteria.
○: Dye was not deposited.
X: Dye was precipitated.

(4) Evaluation of rubbing resistance 10 g of the obtained thermal transfer layer binder, 0.2 g of toluene diisocyanate, 40 g of methyl ethyl ketone and 40 g of toluene were dissolved and coated on a polyester substrate so that the thickness after drying was 10 μm. It dried and the thermal transfer overcoat sheet in which the overcoat layer was formed was obtained.
The surface of the obtained thermal transfer overcoat sheet on which the overcoat layer was formed was rubbed with a surface pad to confirm the presence or absence of scratches on the surface, and the rub resistance was evaluated according to the following criteria.
○: No scratch was observed.
X: Some scratches were observed.
XX: Scratches were seen.

実施例１及び２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、熱転写シートとしたときの、ムラ及び耐擦り性評価でも良好な結果が得られた。
比較例１及び２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 Examples 1 and 2 did not show a decrease in elastic modulus. Also, good results were obtained in the evaluation of unevenness and rubbing resistance when a thermal transfer sheet was obtained.
The solution using the polyvinyl acetal resin obtained in Comparative Examples 1 and 2 showed a decrease in elastic modulus.

According to the present invention, when used as a binder for an ink layer and an overcoat layer, productivity during coating can be improved and an overcoat layer having excellent environmental resistance can be produced. A thermal transfer layer binder, a thermal transfer ink sheet, and a thermal transfer overcoat sheet can be provided.

Claims (3)

A thermal transfer layer binder comprising at least a polyvinyl acetal resin having 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 thermal transfer ink sheet comprising: a support; and an ink layer containing a thermal transfer layer binder and a dye according to claim 1 laminated on the support. A thermal transfer overcoat sheet comprising a support and an overcoat layer containing the thermal transfer layer binder according to claim 1 laminated on the support.