JP2001121824A - Multi-printing thermal transfer sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal transfer sheet excellent in multi-printing performance. SOLUTION: In a multi-printing thermal transfer sheet in which a heat melting ink layer and a porous layer are formed in turn on one side of a base material, the porous layer has porous structure comprising a cyclic ester- modified cellulose derivative, and at least a surface layer contacting the ink layer of the porous layer contains at least part of a heat melting ink constituting the ink layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal transfer sheet, and more particularly, to a thermal transfer sheet having excellent printing performance for a large number of prints.

[0002]

2. Description of the Related Art Conventionally, various thermal transfer sheets have been used for thermal recording in thermal printers and facsimile machines. These thermal transfer sheets are obtained by providing a heat-meltable ink layer made of a heat-meltable ink on a base material such as a polyester film. However, the amount of ink actually transferred by one printing is small in area. Further, if all layers of the heat-fusible ink layer in the printed portion remain without being transferred in the thickness direction, it is possible to perform thermal transfer again. However, the thickness of the remaining hot-melt ink layer decreases with each printing, and when such a thermal transfer sheet is reused, the printing density of the previously used printing portion gradually decreases, and a sufficient desired printing density is obtained. I can't.
Even if a certain print density is obtained, print density unevenness occurs between the reuse section and the first print section. Therefore, from this, a normal thermal transfer sheet is discarded after one printing use without using most of the hot-melt ink, which is extremely uneconomical.

[0003] In view of the above, various types of thermal transfer sheets having a structure in which a heat-fusible ink is held in a porous structure have been proposed as a multiple-print thermal transfer sheet which can be printed many times and can be reused.
Have been tried. For example, JP-A-54-68253
In the publication, a resin component that forms a porous structure, a colorant component and a solid component that is solid at room temperature that constitute a hot-melt ink, and a solvent that dissolves and disperses the hot-melt ink to the resin component. A coating liquid composition comprising a volatile solvent component that can be applied to a substrate, and by drying and removing the volatile solvent component, a porous structure is formed by the resin component, and at the same time, in the porous structure. A multi-print thermal transfer sheet having an ink layer having a structure holding hot melt ink is disclosed.

Japanese Patent Application Laid-Open No. 55-105579 discloses a multi-print thermal transfer sheet having a structure in which a porous structure is formed in advance and then a hot-melt ink is impregnated to form an ink layer. That is, a resin composition forming a porous structure, a foaming agent component, a coating liquid composition comprising a solvent component and the like is applied to a substrate, the solvent component is dried, and the porous structure is first developed by the foaming agent component, Next, a hot-melt ink is applied to the porous structure, and the ink is impregnated into the porous structure to form an ink layer.

Japanese Patent Application Laid-Open No. Sho 60-234890 discloses a structure in which a porous structure is formed, a hot-melt ink is applied thereon, and the porous structure is impregnated with the ink to form an ink layer. However, the same publication discloses that
In the case of -105579, for example, it is necessary to impregnate the porous structure with a large amount of ink in order to increase the print density. In order to prevent this, a configuration in which an adhesive intermediate layer is provided between a substrate and a porous structure is disclosed.

In Japanese Patent Application Laid-Open No. 60-40293, a porous structure is formed separately. The order of formation is such that after forming a heat-meltable ink layer, a resin component forming a porous structure thereon is formed. There is disclosed a configuration in which a coating liquid composition comprising a foaming agent component, a solvent component, and the like is applied, the solvent component is dried, and the foaming agent component is foamed by heating to form a porous layer. In this configuration, at the time of thermal transfer, the ink penetrates into the porous layer and further passes through to reach the printing surface. JP-A-63-128989 discloses a heat-sensitive transfer ink sheet in which a heat-meltable ink layer is provided on a substrate, and a porous ink holding layer filled with the heat-meltable ink is laminated thereon. It has been disclosed. JP-A-7-314907 discloses that after a hot-melt ink layer is formed, a porous structure is formed thereon by coating a coating liquid with a coating liquid comprising cellulose acetate butyrate, a good solvent and a poor solvent. A method is disclosed in which a layer is formed and the porous film is submerged in the ink layer by heating.

According to these methods, a homogeneous porous structure can be formed to solve the above-mentioned problem, but sufficient performance cannot be obtained in terms of film strength, heat resistance, and the like at the time of printing many times. In particular, JP-A-7-314907 describes that the use of cellulose acetate butyrate improves the film strength (fatigue characteristics) at the time of printing a large number of times. Concentration and some improvement in film strength is observed. However, further improvement is required for practical use.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a thermal transfer sheet which is excellent in printing performance for a large number of times, with the object of further improving the invention of JP-A-907-907.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have provided a heat-fusible ink layer on a surface of a substrate,
A porous layer provided adjacently thereon, wherein the porous layer is formed by coating a coating solution obtained by dissolving a cyclic ester-modified cellulose derivative in a mixed solvent consisting of a good solvent and a poor solvent for the derivative. The present inventors have found that a porous layer having a porous structure, a part of which is a layer containing a heat-meltable ink, can solve such a problem, and have completed the present invention.

That is, the present invention relates to a multi-print thermal transfer sheet comprising a heat-fusible ink layer on one side of a base material and a porous layer provided adjacently on the ink layer. Has a porous structure comprising a cyclic ester-modified cellulose derivative, and at least a surface layer of the porous layer in contact with the heat-meltable ink layer is at least one of the heat-meltable inks constituting the heat-meltable ink layer. The present invention provides a multi-time printing thermal transfer sheet which is a layer including a part. In the above invention, the porous layer is coated with a coating solution in which the cyclic ester-modified cellulose derivative is dissolved in a mixed solvent composed of a good solvent and a poor solvent for the resin, and the good solvent and the poor solvent are evaporated when drying. Provided is a multi-time printing thermal transfer sheet which is a porous layer having a porous structure formed by utilizing a difference in speed. Further, the layer containing at least a part of the heat-meltable ink of the porous layer of the previous invention,
After the porous structure of the porous layer is formed, at least a part of the heat-fusible ink constituting the heat-fusible ink layer is formed by infiltration into the surface layer in contact with the heat-fusible ink layer of the porous layer by heating. To provide a thermally transferred heat transfer sheet having multiple layers. In the above invention, the cyclic ester-modified cellulose derivative is a multi-print thermal transfer sheet wherein the cyclic ester is obtained by ring-opening graft polymerization of a hydroxyl group-containing cellulose derivative, a multi-print thermal transfer sheet wherein the cyclic ester is ε-caprolactone, and a porous layer. Pore diameter of 0.5
A multi-print thermal transfer sheet having a thickness of ３3 μm is provided.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a longitudinal sectional view showing one embodiment of the multi-print thermal transfer sheet of the present invention. The multi-print thermal transfer sheet of the present invention illustrated in FIG. 1 is provided on a base material 1 and on one surface thereof (only one surface in FIG. 1), adjacent to a heat-fusible ink layer 2 thereon. A porous ink layer comprising at least a portion of the hot-melt ink constituting the hot-melt ink layer 2, the surface layer of the porous layer 3 being in contact with the hot-melt ink layer 2. 3 '. It should be noted that a known slip layer 4 may be provided on the other surface layer of the base material. Although not shown, a known easy-adhesion layer formed by forming a primer layer or the like may be provided on the slip layer side of the base material in order to improve the adhesion between the slip layer and the base material.

The base material 1 of the thermal transfer sheet of the present invention
Any known materials that have been conventionally used for thermal transfer sheets can be used as they are, and other materials can be used without any particular limitation. Preferred substrates include, for example, polyester, polypropylene, cellophane, cellulose acetate, polycarbonate, polyethylene, polyvinyl chloride, polystyrene, nylon, polyimide, polyvinylidene chloride, polyvinyl alcohol,
Plastic films such as fluororesins, chlorinated rubbers, and ionomers; papers such as condenser paper and paraffin paper;
There are nonwoven fabrics and the like, and a substrate obtained by combining these may be used. The thickness of the substrate 1 can be changed depending on the material so that its strength and thermal conductivity are appropriate.
Its thickness is preferably, for example, 2 to 25 μm.
As described above, the back surface of the substrate 1 may be provided with the slip layer 4 for preventing thermal fusion with the thermal head and improving the slipperiness.

In order to obtain a multi-print thermal transfer sheet of the present invention, for example, a heat-meltable ink layer is formed on the above-mentioned substrate 1 mainly comprising a colorant component and a binder component which disperses or dissolves the colorant component. A hot-melt ink layer 2 is formed by applying a coating liquid for use, and a porous layer 3 having a porous structure is formed thereon. And the hot-melt ink constituting the hot-melt ink layer 2 penetrates at least into the surface layer of the porous layer 3 which is in contact with the hot-melt ink layer 2 to form a porous ink layer 3 '. is there.

The colorant component of the heat-fusible ink layer 2 is a known organic or inorganic pigment or dye having good characteristics as a recording material, for example, having a sufficient printing density, Those which do not discolor by heat, temperature, etc. are preferred. Further, a substance which is colorless when not heated but develops a color when heated, or a substance which develops a color when it comes into contact with a substance applied to a material to be transferred may be used. In addition to the colorants that form cyan, magenta, yellow, and black, colorants of various other colors can also be used.

As the binder component, various known waxes are mainly used, and if necessary, mineral oil, vegetable oil,
Higher fatty acids such as stearic acid, plasticizers, thermoplastic resins,
What added various additives, such as a filler, is used. Examples of the wax include microcrystalline wax, ester wax, carnauba wax, and paraffin wax. Furthermore, various waxes such as Fischer-Tropsch wax, various low molecular weight polyethylene, wood wax, beeswax, whale wax, ibota wax, wool wax, shellac wax, candelilla wax, petrolactam, partially modified wax, fatty acid ester, fatty acid amide, etc. Is used. These waxes facilitate the sinking of the porous layer into the heat-meltable ink layer, so that the heat-meltable ink is transferred from the porous layer to the printing surface from the porous layer by appropriate printing energy, and a sufficient print density is obtained. Melting point 50-90 ° C
And the melt viscosity at 100 ° C. is 10 to 1000 m
Those in the range of Pa · s are preferred.

The binder component may be mixed with a thermoplastic resin having a relatively low melting point, if necessary, to adjust the fluidity and viscosity of the hot-melt ink, and to improve the flexibility and adhesion to the substrate and the material to be transferred. Etc. can be improved. Examples of such a thermoplastic resin include ethylene-vinyl acetate copolymer (EVA), ethylene-acrylate copolymer (EEA), polyethylene, polystyrene, polypropylene, polybutene, petroleum resin, vinyl chloride resin,
Vinyl chloride-vinyl acetate copolymer, polyvinyl alcohol, vinylidene chloride resin, methacrylic resin, polyamide, polycarbonate, fluororesin, polyvinyl formal, polyvinyl butyral, acetyl cellulose,
Nitrocellulose, polyvinyl acetate, polyisobutylene, ethylcellulose, polyacetal and the like are mentioned, and particularly those having a relatively low softening point conventionally used as a heat-sensitive adhesive, for example, those having a softening point of 50 to 80 ° C are preferable. .

Further, in order to give the heat-meltable ink good heat conductivity and heat-melt transferability, a heat-conductive substance may be blended as a filler for the binder component. Examples of such a filler include carbonaceous substances such as carbon black, and metals and metal compounds such as aluminum, copper, tin oxide and molybdenum disulfide.

In order to form the heat-meltable ink layer 2 on the base material 1, the colorant component and the binder component described above, and, if necessary, a heat component prepared by mixing and adjusting a solvent component such as water and an organic solvent. Coating using the fusible ink layer forming coating liquid,
Conventionally known methods such as hot melt coating, hot lacquer coating, gravure coating, gravure reverse coating, and roll coating are used. Also, there is a method of forming using an aqueous or non-aqueous emulsion coating liquid. Although the thickness of the heat-fusible ink layer 2 varies depending on the set number of times of printing, it is preferably about 5 to 30 μm. If it is less than 5 μm, the print density after the second printing decreases, and if it exceeds 30 μm, a large amount of printing energy is required, which is not preferable.

The porous ink layer 3 'is a layer in which the above-mentioned heat-meltable ink has penetrated into a porous layer having a porous structure having fine continuous holes. The fusible ink flows in the micropores and is transferred to the printing surface, so that the hot-melt ink is transferred. The resin forming the porous layer 3 is insoluble in the hot-melt ink, and has heat resistance and physical strength to the extent that structural loss does not occur at a temperature at which the hot-melt ink undergoes a flow transition, so that printing is performed. In this case, the toner is not retained on the printing surface but remains on the thermal transfer sheet. As a result of such a porous structure being formed on the surface of the thermal transfer sheet, the thermally fusible ink layer initially provided on the base material is controlled so that all layers are prevented from transferring in the thickness direction, and the porous layer is porous. A porous ink layer 3 'holding the ink is formed in the structure. In addition, the heat-fusible ink remaining as the lower heat-fusible ink layer is gradually consumed every time printing is performed, so that reprinting with a small change in print density can be performed.

In the present invention, a cyclic ester-modified cellulose derivative is used as a resin for forming such a porous layer 3. When a porous structure is developed by the method described below, the cyclic ester-modified cellulose derivative exhibits excellent performance in terms of heat resistance and film strength (fatigue properties) during multiple printing.

The cyclic ester-modified cellulose derivative is obtained by grafting a cyclic ester to a hydroxyl group-containing cellulose derivative. The hydroxyl group-containing cellulose derivative used as a raw material is a cellulose derivative having a remaining hydroxyl group, which is obtained by esterifying or etherifying a part of the hydroxyl group of cellulose.

Examples of the hydroxyl group-containing cellulose derivative include cellulose esters such as cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate phthalate, and cellulose nitrate, or ethyl cellulose, methyl cellulose, hydroxy cellulose, and hydroxypropyl. Examples include cellulose ethers such as methyl cellulose.

Of these, cellulose fatty acid esters are preferred for use in the present invention because they have good solubility in cyclic esters, are relatively inexpensive, and are easily available industrially. Cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate are preferred. Particularly preferably, the degree of acetyl group substitution is 1.5 to 2.8 (the degree of acetylation is 39 to 6).
0%) cellulose acetate; degree of propionyl group substitution 0.5 to 2.8 (propionylation degree 12 to 63%), degree of acetyl group substitution 0.5 to 2.8 (acetylation degree 16 to 60%)
Cellulose acetate propionate; or a butyryl group substitution degree of 0.5 to 2.5 (butyrylation degree of 14 to 67)
%), The degree of acetyl substitution 0.5 to 2.8 (the degree of acetylation 16 to
60%) of cellulose acetate butyrate.

The cyclic ester modified in the present invention, that is, the cyclic ester used for graft polymerization to the hydroxyl group of the hydroxyl group-containing cellulose derivative may be any one capable of ring-opening polymerization, for example, β-propiolactone, δ
-Valerolactone, ε-caprolactone, α, α-dimethyl-β-propiolactone, β-ethyl-δ-valerolactone, α-methyl-ε-caprolactone, β-methyl-ε-caprolactone, γ-methyl-ε -Caprolactone, 3,3,5-trimethyl-ε-caprolactone,
Lactones such as enantholactone; and cyclic dimers of hydroxycarboxylic acids such as glycolide and lactide. These may be used in combination. In particular, it is advantageous to use ε-caprolactone which is industrially available, is relatively inexpensive, and has excellent compatibility with cellulose derivatives such as cellulose acetate.

In the present invention, the ratio of the hydroxyl group-containing cellulose derivative to the cyclic ester is not particularly limited. However, in general, the cyclic ester modification is generally carried out in an amount of 1 to 85% by mass of the hydroxyl group-containing cellulose derivative (conventional weight% is SI unit). And 30 to 70% by mass of the cyclic ester 15 to
The ratio is preferably 99% by mass, more preferably 30 to 70% by mass. When the charging ratio of the hydroxyl group-containing cellulose derivative is more than 85% by mass, the viscosity of the reaction system becomes extremely high, and it becomes difficult to handle. If the proportion of the hydroxyl group-containing cellulose derivative is less than 1% by mass, the productivity is reduced. When the viscosity is particularly high, a reacting processing device using a twin-screw extruder or the like may be used together with a monomer vacuum distillation recovery device as necessary.

When the viscosity is high, if necessary, an organic solvent having no active hydrogen and having good compatibility with cellulose acetate and a cyclic ester is added as a third component, so that the viscosity of the system can be controlled in a range where it can be easily handled. It is also possible to make the reaction lower. Examples of these solvents include ketone and ester solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate and cellosolve acetate, and mixed solvents thereof.

In general, hydroxyl-containing cellulose derivatives include:
The catalyst used in the reaction for graft polymerization of cyclic esters is typically a catalyst used for ring-opening reaction of cyclic esters, that is, a derivative such as an alkali metal such as sodium or potassium and its alkoxide, and tetrabutyl titanate. Organometals such as alkoxide titanium compounds, tin octylate, dibutyltin laurate; and metal halides such as tin chloride, which are generally described in general literature. However, the preferred catalyst used in the present invention is tin octylate.

The reaction time varies depending on the type of the hydroxyl group-containing cellulose derivative and the cyclic ester, the charging ratio, the type and amount of the catalyst, the reaction temperature and the reaction apparatus, and is not particularly limited, but is preferably 1 hour. ~ 8 hours. In particular, when a reacting processing device such as a twin-screw extruder is used together with a vacuum distillation recovery device for unreacted monomers, the reaction time can be extremely short, such as 10 minutes or less, to achieve the object. .

In obtaining the graft polymer of the present invention, it is preferable that the raw materials used, nitrogen for purging the reactor, the reactor, and the like be sufficiently dried. The water content of the reaction system is 0.1% by mass or less, preferably 0.001% by mass or less. The polymerization temperature is usually preferably the temperature applied to ring-opening polymerization of a cyclic ester, and is preferably 100 to 210 ° C.
Is preferred.

The molecular weight of the thus obtained cyclic ester-modified cellulose derivative also depends on the molecular weight of the starting cellulose ester having a hydroxyl group and the type of the cyclic ester to be grafted. Mass average molecular weight (formerly referred to as weight average molecular weight) of 50,000 to 10
Those having a range of 100,000 are preferably used. Also, a raw material hydroxyl-containing cellulose derivative and a cyclic ester (for example, ε
-Caprolactone) within the above range, the average graft polymer is 1 to 50 moles, preferably 2 to 3 moles of ε-caprolactone per mole of glucose unit.
It has a structure obtained by addition polymerization of 0 mol, more preferably 2 to 20 mol.

As a method for forming a porous structure, a coating solution prepared by dissolving the resin in a mixed solvent of a good solvent having good solubility in the resin component and a poor solvent having poor or insoluble solubility. Is formed by coating. In preparing the mixed solvent, the good solvent and the poor solvent are those having mutual solubility at least within the range of preparation. Further, a solvent having no solubility may be referred to as a non-solvent, but here, a non-solvent including the non-solvent in this sense is also referred to as a poor solvent. In general, water is a solvent that is insoluble in other than specific resins, but the poor solvent here includes water. Water can also be used as long as it is mutually soluble with the other good and poor solvents used with water.

The solvent used in the above-described method for forming a porous structure is not only a combination of a good solvent and a poor solvent, but it is necessary that the combined good solvent has a higher evaporation rate than the poor solvent. Because, in the process of applying a coating solution in which the resin is dissolved in a mixed solvent and drying it,
When the good solvent evaporates more than the poor solvent from the applied liquid film that has not been dried, the resin and the poor solvent undergo phase separation, and the poor solvent forms fine droplets. When the solvent has completely evaporated, pores having the same size as the original separated poor solvent liquid remain in the coating film, and a porous structure is developed.

For such a mixed solvent system of a good solvent and a poor solvent, for the cyclic ester-modified cellulose derivative used in the present invention, specifically, for example, the following solvents are used.
Examples of good solvents include acetone, methyl ethyl ketone, methyl propyl ketone, cyclohexanone, isophorone, methyl acetate, ethyl acetate, tetrahydrofuran,
Examples include cimethylformamide, diacetone alcohol, and methylene chloride. On the other hand, as a poor solvent, for example,
Methyl n-amyl ketone, methyl isoamyl ketone, methyl isobutyl ketone, isobutyl isobutyrate, n-butyl acetate, isobutyl acetate, isopropyl acetate, methanol, ethanol, propanol, isopropyl alcohol, isobutanol, n-butanol, toluene, etc. .

The mechanism by which a porous structure is formed by applying a coating liquid composed of a resin and a good solvent and a poor solvent is that the resin dissolved in the mixed solvent is initially distributed almost homogeneously in the solution. However, as the solubility of the mixed solvent decreases in the drying process described above, the ratio of the interaction between the good solvent and the resin decreases, and the interaction between the molecules of the resin increases. The resin molecules tend to associate with each other, and the distribution of the resin molecules becomes heterogeneous. Then, coacervation occurs, the resin and the poor solvent undergo microphase separation, and the resin gels and becomes non-fluidized. When the solvent is completely evaporated, it is thought that pores having the same size as the original separated poor solvent liquid remain in the coating film, and the pores are connected to form an open-cell porous structure having an open structure. .
The mixing ratio between the resin, the good solvent and the poor solvent is appropriately adjusted depending on the strength of the interaction between the three, the evaporation rate of the good solvent and the poor solvent and the difference between them, and the intended porous structure. is there. However, a process is required in which microphase separation of the resin occurs as the solvent evaporates, and the resin gels and loses fluidity to form a porous structure through the gelation. If the compounding amount of the poor solvent is too small, resin incompatibility occurs due to an increase in resin concentration due to evaporation of the solvent without occurrence of microphase separation, resulting in a glass-like homogeneous coating film and no porous structure. In addition, when the amount of the poor solvent is increased, a porous structure having a large porous diameter is formed, so that the amount of thermal transfer increases and a thermal transfer sheet having a high print density can be obtained, but the number of times of printing that can be performed decreases, and The strength of the steel decreases. As described above, the mixing amounts of the resin, the good solvent, and the poor solvent are adjusted in consideration of the porous diameter, the strength of the porous layer, the number of times of printing, the printing density, and the like.

As described above, the formation and structure of the porous layer depend on the interaction between the solvents, the interaction between the solvent and the resin molecules, the interaction between the resin molecules, and the temporal relationship between these relative sizes. By controlling the change, the continuity and discontinuity of the pores of the porous structure, the size of the pores, the distribution of the pores in the film thickness direction, and the like are controlled.

Thus, an intermediate product of a thermal transfer sheet in which the porous layer 3 having a porous structure composed of open cells is formed on the heat-meltable ink layer 2 formed on the base material 1 is obtained. Next, the porous layer 3 is settled in the hot-melt ink layer. In order to settle, the heat transfer sheet is heated to bring the heat fusible ink layer 2 into a molten fluidized state, and the heat fusible ink penetrates into the porous layer 3 by a capillary phenomenon or the like, thereby thermally melting the porous layer. Into the ink layer 2. At this time, an appropriate pressure may be applied between the porous layer 3 and the heat-meltable ink layer 2 to cause the settlement to be performed more quickly. It should be noted that the purpose of transfer control is not achieved if the porous layer 3 has settled under the hot-melt ink layer 2, that is, inside, but such a phenomenon occurs as long as the porous layer 3 sinks by the capillary phenomenon. Absent.

In this manner, a porous ink layer 3 ′ containing at least a part of the hot-melt ink is obtained on the surface layer where the porous layer 3 is in contact with the hot-melt ink layer 2. The thickness of 3 ′ is appropriately adjusted depending on the porosity and the like,
It is preferably about 0.5 to 4.0 μm. If the thickness is too small, the physical strength of the porous ink layer 3 'is reduced. If the thickness is too large, a large amount of printing energy is required, and the printing density is reduced. Also,
The pore size of the pores into which the heat-fusible ink enters is 0.
5 to 3 μm, more preferably 0.5 to 2.5 μm,
This is preferable because the amount of transferred ink can be made uniform. If the hole diameter is too small, the transfer density due to ink outflow will decrease, and the print density will decrease. Conversely, if the hole diameter is too large, the number of prints will decrease,
The physical strength of the porous ink layer is undesirably reduced.

It is needless to say that the multi-print thermal transfer sheet of the present invention can be applied as a color print, and a multi-color multi-print thermal transfer sheet is also one embodiment of the present invention.
The thermal transfer printer can be applied to either a line or serial type.

As described above, in the multiple-print thermal transfer sheet of the present invention, after the heat-fusible ink layer 2 is formed on one surface of the substrate 1, the porous layer 3 having a porous structure is formed thereon. Then, the porous layer 3 is submerged in the hot-melt ink layer, whereby the hot-melt ink enters the porous structure from below the porous layer to form the porous ink layer 3 '. As a result, the heat fusible ink enters the porous structure from the back side of the porous layer 3, and air in the porous structure escapes without obstruction to the surface side. The air can be quickly and easily degassed, and the inside of the porous structure is sufficiently filled with the hot-melt ink uniformly.
A porous ink layer 3 'free of residual air is obtained. In order to form the porous structure, a resin and a good solvent and a poor solvent for the resin are formed by applying a coating liquid composed of these components.
By appropriately adjusting the compounding ratio and the coating conditions, the porous structure can be freely controlled, and as a result, a homogeneous and desirable porous structure is formed. In particular, by forming a porous structure using a cyclic ester-modified cellulose derivative, a desirable porous structure having excellent heat resistance and film strength (mechanical strength, fatigue properties) during multiple printing can be obtained. Further, by limiting the pore diameter of the pores of the porous layer to a specific range, the transfer amount of the ink becomes constant. The transfer amount control for gradually transferring the transfer amount of the hot-melt ink to the printing surface is performed by the porous ink layer thus obtained. In addition, there is a hot-melt ink layer that has sufficiently stored hot-melt ink at the bottom of the porous ink layer, so the hot-melt ink consumed for printing inside the porous ink layer is replenished from the bottom. And
Continuous multiple printing is possible.

[0040]

EXAMPLES The present invention will now be described specifically with reference to examples, but the present invention is not limited to these examples. Parts and percentages used in the examples are on a mass basis unless otherwise specified. Some production examples of the cyclic ester-modified cellulose derivative used in the present invention will be described. (Production Example 1) A cellulose acetate (manufactured by Daicel Chemical Industries, Ltd., manufactured by Daicel Chemical Industries, Degree of acetylation 55.2%, degree of substitution 2.
43) 100 parts were added at 180 ° C. under stirring and uniformly dissolved, and then 0.12 parts of tin octylate was charged and 3 parts were added.
The reaction was continued for hours. Thus, a slightly yellow transparent graft polymer was obtained. When the intrinsic viscosity of the graft polymer at 50 ° C. was determined using acetone, [η] = 0.7
It was 5 dl / g. This is designated as resin A.

(Production Example 2) In a sufficiently dried reactor equipped with a stirrer, a thermometer and a reflux condenser, and under a dry nitrogen atmosphere, 70 parts of ε-caprolactone was added to cellulose acetate (Daicel Made of chemical, degree of acetylation 55.2
%, Degree of substitution 2.43) at 180 ° C. while stirring.
And dissolved uniformly, then tin octylate 0.1
2 parts were charged and the reaction was continued for 3 hours. Thus, a slightly yellow transparent graft polymer was obtained. When the intrinsic viscosity of the graft polymer at 50 ° C. was determined using acetone, [η] = 0.50 dl / g. This is called resin B.

Example 1 A polyethylene terephthalate film having a thickness of 4.5 μm was used as a base material, and 10 parts of carnauba wax, 65 parts of paraffin wax (HNP-3 manufactured by Nippon Seiro Co., Ltd.) and 65 parts of ethylene・ 10 parts of vinyl acetate copolymer (Evaflex 41 manufactured by DuPont)
0), 15 parts of carbon black were 12
The mixture was dispersed for 2 hours while being heated to 0 ° C. to prepare a coating liquid for forming a heat-fusible ink layer. Apply this coating solution with a roll coater
g / m ² , and a resin A5
Parts, solvent (acetone: n-butanol = 1: 1.25)
A coating solution for forming a porous layer having a composition of 95 parts was applied by a gravure coater, and dried at a temperature of 80 ° C. for 3 minutes to remove a solvent component. The thickness was 2 μm (0.5 g / m ² ) and pores were removed. To form a porous layer having a pore size of 0.5 to 2.5 μm,
The porous layer was dropped into the heat-meltable ink layer by continuing the heating in the above, to obtain a thermal transfer sheet which was used as a porous ink layer containing the heat-meltable ink.

Example 2 A thermal transfer sheet was obtained in the same manner as in Example 1 except that the coating composition for forming a porous layer had the following composition. Coating liquid for forming porous layer: 5 parts of resin B, 95 parts of solvent (acetone: n-butanol = 1: 1.25).

(Comparative Example 1) A multi-time printing thermal transfer sheet of the present invention was obtained in the same manner as in Example 1 except that the coating solution for forming a porous layer had the following composition. Coating liquid for forming porous layer: 5 parts of cellulose acetate butyrate, solvent (acetone: n-butanol = 1: 1.2)
5) 95 parts. In this thermal transfer sheet, the porous film is broken by printing 7 to 8 times,
All the transfer occurred and the print density decreased rapidly. Using the thermal transfer sheets of the above examples and comparative examples, a printing test was performed on the same spot under the following conditions, and the printing density was measured for each printing count, and the results shown in Table 1 were obtained. Printing conditions Apparatus: Test printer equipped with thin-film thermal head Printing energy: 0.6 mJ / dot Transfer paper: Fine paper (Beck smoothness: 80 seconds)

[0045]

[Table 1]

[0046]

As described above, the multi-print thermal transfer sheet of the present invention is constructed as described above, so that the porous structure of the porous ink layer is excellent in mechanical strength, and the porous structure is broken or broken with printing. Transfer, and the like, and the porous structure is excellent in homogeneity, and a uniform print density without variation can be obtained. In addition, even when printing is repeated, the number of times that high density printing density is maintained is long, and excellent economic efficiency is exhibited.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of a multi-print thermal transfer sheet of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 base material 2 heat-meltable ink layer 3 porous layer 3 'porous ink layer 4 slip layer

Claims (6)

[Claims] 1. A multi-print thermal transfer sheet comprising a heat-fusible ink layer on one side of a substrate and a porous layer provided adjacently thereon, wherein the porous layer is a cyclic ester Having a porous structure composed of a modified cellulose derivative, and at least a surface layer of the porous layer in contact with the heat-meltable ink layer contains at least a part of the heat-meltable ink constituting the heat-meltable ink layer. A multi-print thermal transfer sheet characterized by being a layer. 2. The method according to claim 1, wherein the porous layer is coated with a coating solution in which the cyclic ester-modified cellulose derivative is dissolved in a mixed solvent of a good solvent and a poor solvent for the resin, and dried when the good solvent and the poor solvent are dried. The multi-print thermal transfer sheet according to claim 1, which is a porous layer having a porous structure formed by utilizing a difference in evaporation rate. 3. The method according to claim 1, wherein the layer containing at least a part of the hot-melt ink of the porous layer forms a porous structure of the porous layer.
3. A layer formed by permeating at least a part of the heat-fusible ink constituting the heat-fusible ink layer into a surface layer of the porous layer in contact with the heat-fusible ink layer by heating.
The heat transfer sheet printed multiple times as described. 4. The cyclic ester-modified cellulose derivative,
The multiple-print thermal transfer sheet according to any one of claims 1 to 3, wherein the cyclic ester is obtained by ring-opening graft polymerization of a hydroxyl group-containing cellulose derivative. 5. The multiple-print thermal transfer sheet according to claim 1, wherein the cyclic ester is ε-caprolactone. 6. The porous layer has a pore diameter of 0.5 to 3 μm.
The thermal printing sheet for multiple printing according to any one of claims 1 to 5, wherein