JP7725965B2 - Thermal transfer sheet - Google Patents

Description

This disclosure relates to a thermal transfer sheet.

Thermal transfer images are widely produced using the dye-sublimation thermal transfer method because it has excellent gradation of intermediate colors and can easily produce high-quality images equivalent to conventional full-color photographic images.

In the dye-sublimation thermal transfer method, for example, a thermal transfer sheet, which has a dye layer on one side of a substrate, is combined with a transferee such as a thermal transfer image receiving sheet, which has a receiving layer on one side of another substrate. Heat is applied to the back of the thermal transfer sheet to transfer the dye contained in the dye layer to the receiving layer, forming a thermal transfer image and obtaining a print.

Thermally transferred images formed on a receiving layer using a dye-sublimation thermal transfer method have excellent gradation. However, because the receiving layer on which the thermally transferred image is formed is located on the surface of the printed product, the thermally transferred image can have poor durability. To solve this problem, it has been proposed to use a thermal transfer sheet having a substrate and a protective layer that is releasable from the substrate, and to transfer the protective layer onto the receiving layer on which the thermally transferred image is formed (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 2008-238525

Conventional thermal transfer sheets comprise a substrate and a transfer layer having a protective layer and an adhesive layer, with the substrate, protective layer, and adhesive layer arranged in this order in the thickness direction. However, providing an additional adhesive layer on the protective layer can increase costs.

The present inventors have investigated ways to reduce the cost of thermal transfer sheets and improve their productivity by simplifying the configuration of the transfer layer in thermal transfer sheets that include a transfer layer with a protective layer. However, the present inventors have found that, for example, if the transfer layer does not include the above-mentioned adhesive layer, it tends to be difficult to achieve both the ability to easily separate the protective layer from the substrate and the adhesion of the protective layer to the transfer recipient.

The objective of this disclosure is to achieve both the ease of peeling the protective layer from the substrate and the adhesiveness of the protective layer to the transfer target when no adhesive layer is provided on the protective layer included in the transfer layer.

The thermal transfer sheet of the present disclosure comprises a substrate and a transferable protective layer provided on one surface of the substrate, and the surface tension (ST2) of the second surface of the transferable protective layer, which is the surface opposite the substrate, is greater than the surface tension (ST1) of the first surface of the transferable protective layer, which is the surface facing the substrate.

According to the present disclosure, when no adhesive layer is provided on the protective layer included in the transfer layer, it is possible to achieve both the ease of peeling the protective layer from the substrate and the adhesiveness of the protective layer to the transfer target.

FIG. 1 is a schematic cross-sectional view of a thermal transfer sheet according to one embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view of an embodiment of the thermal transfer sheet of the present disclosure. FIG. 3 is a schematic cross-sectional view of a thermal transfer sheet according to one embodiment of the present disclosure. FIG. 4 is a schematic cross-sectional view showing an example of a printed matter in a state where a protective layer is transferred onto a transfer-receiving body using the thermal transfer sheet of one embodiment of the present disclosure.

Embodiments of the present disclosure are described in detail below. The present disclosure can be implemented in many different forms and should not be construed as being limited to the description of the embodiments exemplified below. For clarity of explanation, the drawings may show the width, thickness, shape, etc. of each layer more schematically than in the embodiments, but these are merely examples and do not limit the interpretation of the present disclosure. In this specification and each figure, elements similar to those already described in the previous figures are designated by the same reference numerals, and detailed descriptions may be omitted where appropriate.

[Thermal transfer sheet]
The thermal transfer sheet of the present disclosure comprises:
A substrate;
and a transferable protective layer provided on one surface of the substrate.

The transferable protective layer refers to a protective layer that is peeled from the substrate and transferred onto a receiving material by applying energy such as thermal energy to the back surface of the thermal transfer sheet. In other words, the transferable protective layer constitutes a transfer layer in the thermal transfer sheet that can be peeled off by the application of energy. Hereinafter, the transferable protective layer will also be simply referred to as the "protective layer."

In one embodiment, the thermal transfer sheet of the present disclosure does not include an adhesive layer on the surface of the protective layer opposite the substrate. That is, in one embodiment, the protective layer comes into contact with the transfer recipient when the protective layer is transferred onto the transfer recipient. This simplifies the configuration of the transfer layer of the thermal transfer sheet, thereby improving the productivity of the thermal transfer sheet.

The thermal transfer sheet of the present disclosure includes a transfer layer having a protective layer. In one embodiment, the transfer layer is composed solely of the protective layer, for example, composed solely of a single protective layer. In other words, in one embodiment, the protective layer constitutes the surface layer of the print obtained when the protective layer is transferred onto a transfer recipient.

The thermal transfer sheet 1 shown in Figure 1 comprises a substrate 10 and a protective layer 20 provided on one side of the substrate 10. The thermal transfer sheet 1 shown in Figure 2 comprises a substrate 10, a protective layer 20 provided on one side of the substrate 10, and a backing layer 30 provided on the other side of the substrate 10. The thermal transfer sheet 1 shown in Figure 3 comprises a colorant layer 40 on one side of the substrate 10, in face-to-face order with the protective layer 20. The thermal transfer sheets 1 shown in Figures 1 to 3 may further comprise a release layer (not shown) located between the substrate 10 and the protective layer 20.

When the protective layer 20 is transferred onto the transferee 50 using the thermal transfer sheet 1 shown in Figures 1 to 3, a print 2 is obtained, as shown in Figure 4, which includes the transferee 50 and the protective layer 20 provided on the transferee 50. The protective layer 20 provides excellent protection for images (not shown) provided on the print 2.

<Base material>
The substrate can be any material without particular restrictions as long as it has heat resistance sufficient to withstand the thermal energy used when transferring the protective layer from the thermal transfer sheet onto the transfer recipient, and mechanical strength and solvent resistance sufficient to support the protective layer.

Examples of the substrate include resin films made of resin materials, such as polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyethylene terephthalate-isophthalate copolymer, and terephthalic acid-cyclohexanedimethanol-ethylene glycol copolymer; polyamides such as nylon 6 and nylon 6,6; polyolefins such as polyethylene, polypropylene, and polymethylpentene; vinyl resins such as polyvinyl chloride and vinyl chloride-vinyl acetate copolymer; (meth)acrylic resins such as poly(meth)acrylate and polymethyl(meth)acrylate; polyimides; polycarbonates; engineering resins such as polyarylate, polysulfone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyaramid, polyetherketone, and polyetheretherketone; styrene resins such as polystyrene, high-impact polystyrene, acrylonitrile-styrene copolymer, and acrylonitrile-butadiene-styrene copolymer; and cellulose resins such as cellophane, cellulose acetate, and nitrocellulose. Among resin films, resin films made of polyesters such as PET and PEN are preferred from the viewpoint of excellent heat resistance and mechanical strength.
The resin film can contain one or more types of resin materials.

In this disclosure, "(meth)acrylate" includes both "acrylate" and "methacrylate," and "(meth)acrylic" includes both "acrylic" and "methacrylic."

The substrate may be a laminate comprising multiple layers. The substrate may be a stretched film or an unstretched film, or, from the standpoint of improving strength, may be a film that has been stretched uniaxially or biaxially.

The thickness of the substrate is preferably 0.5 μm or more and 50 μm or less, more preferably 1 μm or more and 20 μm or less, and even more preferably 1 μm or more and 10 μm or less. This can, for example, further improve the balance between mechanical strength and the transferability of the protective layer.

<Protective layer>
The protective layer is a layer that protects the surface of the transfer recipient after it has been transferred from the thermal transfer sheet to the transfer recipient. In the thermal transfer sheet of the present disclosure, the surface of the protective layer facing the substrate is referred to as the first surface, and the surface of the protective layer opposite the substrate is referred to as the second surface. That is, the protective layer has a first surface and a second surface opposite the first surface. In Figures 1 to 4, the first surface is designated by the reference numeral 22, and the second surface is designated by the reference numeral 24.

One feature of the thermal transfer sheet disclosed herein is that the surface tension (ST2) of the second surface of the protective layer is greater than the surface tension (ST1) of the first surface of the protective layer. This allows, for example, when no adhesive layer is provided on the protective layer in the transfer layer, to maintain the peelability of the protective layer from the substrate during thermal transfer while improving the adhesion of the protective layer to the transfer recipient.

In this disclosure, "surface tension" refers to the "wet tension" measured in accordance with JIS K6768, "Plastics - Films and Sheets - Wet Tension Test Method." Measurements are performed in a standard laboratory atmosphere (see JIS K7100) at a temperature of 23°C and a relative humidity of 50%.

Surface tension (ST2) is measured on the second surface of the protective layer in the thermal transfer sheet state (e.g., second surface 24 in Figures 1 to 3). Surface tension (ST1) is measured on the first surface of the protective layer transferred from the thermal transfer sheet onto the transfer recipient (e.g., first surface 22 in Figure 4). The transfer conditions here are described below.

(Transfer conditions)
The thermal transfer sheet and genuine paper for SELPHY CP1300 (Canon Inc.) were placed opposite each other, and the protective layer of the thermal transfer sheet was transferred onto the genuine paper under the following test printer transfer conditions.

(Test printer transfer conditions)
Thermal head: KEE-57-12GAN2-STA (Kyocera Corporation)
・Average resistance of heating element: 3303 (Ω)
- Main scanning direction print density: 300 (dpi)
Sub-scanning direction print density: 300 (dpi)
Printing voltage: 18 (V)
Line period: 1.5 (msec./line)
・Printing start temperature: 35 (℃)
Pulse duty ratio: 85 (%)

The surface tension (ST2) of the second surface of the protective layer is preferably 46 mN/m or more and 70 mN/m or less, more preferably 48 mN/m or more and 68 mN/m or less, even more preferably 50 mN/m or more and 66 mN/m or less, and particularly preferably 50 mN/m or more and 60 mN/m or less. When the surface tension (ST2) is at least the above lower limit, for example, the adhesion of the protective layer to the transfer target can be improved. When the surface tension (ST2) is at most the above upper limit, for example, the protective layer can be easily peeled from the substrate.

The surface tension (ST1) of the first surface of the protective layer is preferably 25 mN/m or more and less than 46 mN/m, more preferably 30 mN/m or more and 45 mN/m or less, and even more preferably 35 mN/m or more and 45 mN/m or less. The surface tension (ST1) is usually the surface tension based on the material that constitutes the protective layer. When the surface tension (ST1) is within the above range, for example, the protective layer has excellent peelability from the substrate.

In this way, the protective layer has different surface tensions on the first and second surfaces. Such a protective layer combines the aforementioned foil-cutting ability and adhesiveness. While it is usually difficult to achieve both foil-cutting ability and adhesiveness by simply selecting the materials that make up the protective layer, it is possible to achieve both by increasing the surface tension of the second surface, for example, through surface treatment as described below.

The surface tension (ST2) can be adjusted, for example, by subjecting the second surface of the protective layer to a surface treatment. Examples of treatments that increase surface tension include discharge treatments such as corona discharge treatment and plasma discharge treatment (e.g., atmospheric pressure plasma discharge treatment, low pressure plasma discharge treatment; however, corona discharge treatment is excluded), flame treatment, ultraviolet treatment, and ozone treatment. Among these, corona discharge treatment is preferred because the energy supply source is electricity, which makes it easy to control, and it can be easily implemented and adopted industrially.

In corona discharge treatment, for example, high-energy electrons and ions generated by corona discharge collide with the second surface of the protective layer, generating radicals and ions. The generated radicals and ions react with surrounding ozone, oxygen, nitrogen, moisture, etc., introducing polar functional groups into the second surface of the protective layer. Examples of polar functional groups include carbonyl groups, carboxy groups, hydroxyl groups, and epoxy groups. In plasma discharge treatment, polar functional groups are also introduced into the second surface of the protective layer.

For example, in corona discharge treatment and plasma discharge treatment, the surface tension tends to increase in proportion to the input power (or discharge amount (W·min/m ² )). The discharge amount is the input power per unit time per area and is expressed by the following formula:

Discharge amount = P/(L x v)
P: Discharge power (W)
L: Discharge electrode length (m)
v: sheet speed (m/min)

The corona discharge treatment can be carried out using a known device.
Corona discharge treatment is typically carried out by discharging between an electrode and a dielectric roll (treatment roll) at a distance of several millimeters, and then passing the thermal transfer sheet between the electrode and treatment roll. Here, for example, the thermal transfer sheet is passed between the electrode and treatment roll with the protective layer facing the electrode and the substrate or backside layer in contact with the treatment roll. Since the surface tension can depend on, for example, the distance between the electrode and treatment roll, the electrode shape, the number of electrodes, the dielectric constant of the treatment roll, and the air thickness between the treatment roll and the thermal transfer sheet, it is desirable to adjust the amount of discharge so as to achieve the desired surface tension.

The discharge amount in the corona discharge treatment is preferably 5 W min/m ² or more and 80 W min/m ² or less, more preferably 10 W min/m ² or more and 60 W min/m ² or less, even more preferably 13 W min/m ² or more and 50 W min/m ² or less, even more preferably 15 W min/m ² or more and 30 W min/m ^{2 or} less, and particularly preferably 15 W min/m ² or more and 22 W min/m ² or less. This allows, for example, the surface tension (ST2) of the second surface of the protective layer to be adjusted within the above range. If the discharge amount is below the upper limit, for example, the main chain of the binder resin in the protective layer is three-dimensionally crosslinked, which can prevent the cohesive force of the protective layer surface from becoming too high and the foil tearability from being reduced.

Corona discharge treatment is usually carried out in the atmosphere, but nitrogen gas, helium gas, argon gas, oxygen gas, and polymerizable gases (e.g., ethylene) may be added to the atmosphere during corona discharge treatment.

The atmospheric pressure plasma discharge treatment can be carried out using a known device.
In atmospheric pressure plasma discharge treatment, for example, discharge is performed under an inert gas at a pressure of 0.8 to 1.2 atmospheres. A small amount of active gas is mixed into the inert gas. Examples of inert gases include helium gas, argon gas, and nitrogen gas. One or more inert gases may be used. Examples of active gases include oxygen gas, hydrogen gas, and carbon dioxide gas. One or more active gases may be used. The discharge amount in atmospheric pressure plasma discharge treatment is, for example, 5 W·min/ ^m2 to 150 W·min/ ^m2 .

The low-pressure plasma discharge treatment can be carried out using a known device.
In the low-pressure plasma discharge treatment, for example, discharge is performed under a gas pressure of 0.1 Pa or more and 1330 Pa or less. Examples of gases used in the low-pressure plasma discharge treatment include helium gas, neon gas, argon gas, nitrogen gas, oxygen gas, carbon dioxide gas, hydrogen gas, air, and water vapor. One or more gases may be used. The discharge amount in the low-pressure plasma discharge treatment is, for example, 5 W·min/ ^m2 or more and 150 W·min/m2 or ^less .

Usually, ultraviolet irradiation excites molecules on the second surface of the protective layer, causing photoreaction and photodecomposition, resulting in alteration of the second surface of the protective layer. The wavelength of the ultraviolet light used for ultraviolet irradiation is preferably 250 nm or more and 380 nm or less. Examples of light sources include high-pressure mercury lamps and ultra-high-pressure mercury lamps.

In ozone treatment, ozone is typically generated from oxygen by irradiation with short-wavelength ultraviolet light, and the ozone introduces polar functional groups onto the second surface of the protective layer, similar to corona discharge treatment. Examples of light sources include low-pressure mercury lamps and xenon excimer lamps.

In one embodiment, the protective layer contains a binder resin.
Examples of binder resins include (meth)acrylic resins; styrene resins; vinyl resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride, vinyl chloride-vinyl acetate copolymer, polyvinyl butyral, polyvinyl acetal, and polyvinyl pyrrolidone; polyesters such as polyethylene terephthalate and polyethylene naphthalate; cellulose resins such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, methyl cellulose, and cellulose acetate; polyamides; polyurethanes; polycarbonates; phenoxy resins; epoxy resins; silicone-modified resins of these resins; ultraviolet-absorbing resins; and ionizing radiation-curable resins.

In one embodiment, the protective layer contains a (meth)acrylic resin. Examples of the (meth)acrylic resin include a polymer of (meth)acrylic acid, a polymer of a (meth)acrylic acid ester, a copolymer of (meth)acrylic acid and another monomer, and a copolymer of a (meth)acrylic acid ester and another monomer. Specific examples of the (meth)acrylic resin include polymethyl(meth)acrylate, polyethyl(meth)acrylate, polypropyl(meth)acrylate, polybutyl(meth)acrylate, polyisobutyl(meth)acrylate, an ethyl(meth)acrylate-methyl(meth)acrylate copolymer, a butyl(meth)acrylate-methyl(meth)acrylate copolymer, an ethylene-methyl(meth)acrylate copolymer, and a styrene-methyl(meth)acrylate copolymer.
The protective layer can contain one or more (meth)acrylic resins.

The weight-average molecular weight (Mw) of the (meth)acrylic resin is, for example, 5,000 or more and 300,000 or less, preferably 10,000 or more and 150,000 or less, more preferably 15,000 or more and 100,000 or less, and even more preferably 20,000 or more and 60,000 or less. Mw is a polystyrene-equivalent value measured by gel permeation chromatography (GPC) in accordance with JIS K7252-1.

The glass transition temperature (Tg) of the (meth)acrylic resin is, for example, 30°C or higher and 120°C or lower, preferably 40°C or higher and 120°C or lower, and more preferably 50°C or higher and 120°C or lower. Tg is a value obtained by differential scanning calorimetry (DSC) in accordance with JIS K7121.

Examples of ionizing radiation curable resins include resins obtained by crosslinking and curing a radical polymerizable polymer or a radical polymerizable oligomer by irradiating it with ionizing radiation.Specific examples include resins obtained by adding a photopolymerization initiator to a radical polymerizable polymer or a radical polymerizable oligomer as needed, and then polymerizing and crosslinking the polymer by irradiating it with an electron beam or ultraviolet light.

The protective layer may contain one or more binder resins.
The content of the binder resin in the protective layer is preferably 80% by mass or more, and more preferably 85% by mass or more, based on the total mass of the protective layer, which can impart sufficient durability to the protective layer, for example.

The protective layer may contain additives. Examples of additives include release agents such as metal soaps such as zinc stearate, zinc stearyl phosphate, calcium stearate, and magnesium stearate; silicone oil, fluorine compounds, phosphate esters, fatty acid amides, and waxes such as polyethylene wax, carnauba wax, and paraffin wax; UV absorbers such as benzophenone-based UV absorbers, benzotriazole-based UV absorbers, benzoate-based UV absorbers, triazine-based UV absorbers, titanium oxide, and zinc oxide; light stabilizers such as hindered amine-based light stabilizers and Ni chelate-based light stabilizers; antioxidants such as hindered phenol-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, and lactone-based antioxidants; and fillers such as organic and inorganic particles.

The protective layer may contain one or more additives.
The content of the additive in the protective layer is preferably 1% by mass or more and 20% by mass or less, and more preferably 2% by mass or more and 15% by mass or less, based on the total mass of the protective layer.

The thickness of the protective layer is preferably 0.1 μm or more and 20 μm or less, more preferably 0.5 μm or more and 10 μm or less, and even more preferably 0.5 μm or more and 3 μm or less. This can, for example, prevent transfer defects of the protective layer and provide sufficient protective function to the transfer target.

The protective layer can be formed, for example, by applying a coating liquid containing the above-mentioned materials to the substrate or release layer by known means such as roll coating, reverse roll coating, gravure coating, reverse gravure coating, bar coating, or rod coating, and then drying.

<Release layer>
In one embodiment, the thermal transfer sheet of the present disclosure may include a release layer located between the substrate and the protective layer. The release layer is a layer that is optionally provided so that the protective layer can be easily peeled from the substrate during thermal transfer, and remains on the substrate side during thermal transfer.

In one embodiment, the release layer contains a binder resin and a release agent.
Examples of binder resins include (meth)acrylic resins, vinyl resins, polyurethanes, polyamides, melamine resins, polyol resins, silicone resins, silicone-modified (meth)acrylic resins, fluororesins, fluoro-modified resins, and cellulose resins.
The release layer may contain one or more binder resins.

Examples of the release agent include metal soaps such as zinc stearate, zinc stearyl phosphate, calcium stearate, and magnesium stearate, silicone oil, fluorine compounds, phosphate esters, fatty acid amides, polyethylene wax, carnauba wax, and paraffin wax.
The release layer may contain one or more types of release agents.

The release layer may contain one or more of the above additives.

The thickness of the release layer is preferably 0.5 μm or more and 3 μm or less, and more preferably 1 μm or more and 1.5 μm or less. This can improve the transferability of the protective layer in a thermal transfer sheet, for example.

The release layer can be formed, for example, by applying a coating liquid containing the above-mentioned materials to a substrate using known methods such as roll coating, reverse roll coating, gravure coating, reverse gravure coating, bar coating, or rod coating, and then drying the coating.

<Color material layer>
In one embodiment, the thermal transfer sheet of the present disclosure may include a colorant layer on one surface of the substrate, in face order with the protective layer. The colorant layer may be, for example, a sublimation transfer colorant layer to which a sublimable dye contained in the colorant layer migrates during thermal transfer, or a melt transfer colorant layer to which the colorant layer itself is transferred during thermal transfer.

In the thermal transfer sheet 1 shown in Figure 3, a single colorant layer 40 is provided on one surface of the substrate 10. Multiple colorant layers, such as a yellow dye layer, a magenta dye layer, a cyan dye layer, and, if necessary, a black dye layer, may be provided in face-sequential order on one surface of the substrate. When a colorant layer and a protective layer are considered to be "one unit," one unit may be repeatedly provided on one surface of the substrate.

By providing a colorant layer and a protective layer on the substrate, it is possible, for example, to use the same thermal transfer sheet to form a thermal transfer image on a transfer recipient and transfer the protective layer onto the formed thermal transfer image.

In one embodiment, the colorant layer contains a binder resin and a colorant.
Examples of binder resins include polyester, polyurethane, polyamide, polycarbonate, polyolefin, (meth)acrylic resin, vinyl resin, styrene resin, cellulose resin, phenoxy resin, and ionomer resin.
The colorant layer may contain one or more types of binder resin.

The coloring material may be a pigment or a dye, and the dye may be a sublimable dye.
Specific examples of colorants include carbon black, acetylene black, lamp black, black smoke, iron black, aniline black, silica, calcium carbonate, titanium oxide, cadmium red, cadmium phosphate red, chrome red, vermilion, red iron oxide, azo pigments, alizarin lake, quinacridone, cochineal lake perylene, yellow ochre, aureolin, cadmium yellow, cadmium orange, chrome yellow, zinc yellow, Naples yellow, nickel yellow, greenish yellow, ultramarine, rock ultramarine, cobalt, phthalocyanine, anthraquinone, and indicoid. , cinnabar green, cadmium green, chrome green, phthalocyanine, azomethine, perylene, aluminum pigments; and sublimable dyes such as diarylmethane dyes, triarylmethane dyes, thiazole dyes, merocyanine dyes, pyrazolone dyes, methine dyes, indoaniline dyes, acetophenoneazomethine dyes, pyrazoloazomethine dyes, xanthene dyes, oxazine dyes, thiazine dyes, azine dyes, acridine dyes, azo dyes, spiropyran dyes, indolinospiropyran dyes, fluoran dyes, naphthoquinone dyes, anthraquinone dyes, and quinophthalone dyes.
The colorant layer can contain one or more types of colorants.

The colorant layer may contain one or more of the above additives.

In one embodiment, the colorant layer is a sublimation transfer colorant layer.
The content of the binder resin in the sublimation transfer colorant layer is preferably 20% by mass or more relative to the total mass of the sublimation transfer colorant layer. There is no particular upper limit to the content of the binder resin, and it may be set appropriately depending on the content of the sublimation dye and any additives used.

The content of sublimation dye in the sublimation transfer colorant layer is preferably 50 to 350 parts by weight, and more preferably 80 to 300 parts by weight, per 100 parts by weight of binder resin. This can improve, for example, print density and storage stability.

The thickness of the colorant layer is, for example, 0.1 μm or more and 5 μm or less.

The colorant layer can be formed, for example, by applying a coating liquid containing the above materials to a substrate using known methods such as roll coating, reverse roll coating, gravure coating, reverse gravure coating, bar coating, and rod coating, and then drying the applied coating.

A dye primer layer may be provided between the substrate and the sublimation transfer colorant layer.
A release layer may be provided between the substrate and the melt-transfer colorant layer.

<Back layer>
In one embodiment, the thermal transfer sheet of the present disclosure may include a backing layer on the surface of the substrate opposite to the surface on which the protective layer is provided, which can, for example, prevent sticking and wrinkles from occurring during thermal transfer.

In one embodiment, the back surface layer contains a resin material, such as polyolefin, styrene resin, vinyl resin, polyester, polyurethane, polyether, polyamide, polyimide, polyamideimide, polycarbonate, cellulose resin, or silicone-modified versions of these.
The back layer can contain one or more resin materials.

In one embodiment, the back layer contains a cured resin obtained by curing a hydroxyl-containing resin with an isocyanate curing agent. This can, for example, prevent sticking and wrinkles from occurring during thermal transfer.

Examples of hydroxyl group-containing resins include polyvinyl alcohol, polyvinyl butyral, polyvinyl acetal, and (meth)acrylic polyol. The hydroxyl group value of the hydroxyl group-containing resin is preferably 5% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 20% by mass or less. This can improve the heat resistance of the back layer, for example, and reduce the thermal energy transmitted to the substrate and the protective layer, thereby further improving printability. "Hydroxyl group value" refers to the proportion of monomer structural units having hydroxyl groups in the resin, and is a value calculated as the proportion (mass%) of monomer structural units having hydroxyl groups to the mass of the entire resin.
The hydroxyl group-containing resins may be used alone or in combination.

Examples of isocyanate curing agents include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate, tolidine diisocyanate, p-phenylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, triphenylmethane triisocyanate, and tris(isocyanatophenyl)thiophosphate.
The isocyanate curing agent may be used alone or in combination of two or more.

The molar equivalent ratio (-NCO/-OH) of the isocyanate groups (-NCO) in the isocyanate curing agent to the hydroxyl groups (-OH) in the hydroxyl group-containing resin is preferably 1.5 or greater. This can improve the heat resistance of the backing layer, for example, and reduce the thermal energy transmitted to the substrate and protective layer, further improving printability.

The content of the cured resin in the back layer is preferably 50% by mass or more and 90% by mass or less, and more preferably 55% by mass or more and 80% by mass or less. This can further improve printability, for example.

The back layer may contain additives such as metal soaps such as zinc stearate, zinc stearyl phosphate, calcium stearate, and magnesium stearate; release agents such as silicone oil, fluorine compounds, phosphate esters, fatty acid amides, and waxes such as polyethylene wax, carnauba wax, and paraffin wax; organic particles such as fluorine resins; inorganic particles such as silica, clay, talc, and calcium carbonate; and organic/inorganic particles such as silicone resins, in order to improve slip properties, etc.

The content of the above additives in the back layer is preferably 10% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 45% by mass or less. This can, for example, further improve slip properties.

From the perspective of improving heat resistance and slip resistance, the thickness of the back layer is preferably 0.1 μm or more and 5 μm or less, and more preferably 0.3 μm or more and 2 μm or less.

The back layer can be formed, for example, by applying a coating liquid containing the above-mentioned materials to a substrate using known methods such as roll coating, reverse roll coating, gravure coating, reverse gravure coating, bar coating, and rod coating, and then drying the coating.

[Transferred object]
Examples of the object onto which the protective layer of the thermal transfer sheet of the present disclosure is transferred include a thermal transfer image receiving sheet comprising a substrate and a receiving layer provided on one side of the substrate, an intermediate transfer medium comprising a substrate and a receiving layer provided on one side of the substrate so as to be peelable from the substrate, paper substrates such as plain paper, high-quality paper, and tracing paper, plastic films, and plastic cards mainly composed of polyvinyl chloride, vinyl chloride-vinyl acetate copolymer, or polycarbonate. The object may have an image, may be colored, and may be transparent.

In the case of a thermal transfer image receiving sheet, a printed product can be obtained by forming a thermal transfer image on the receiving layer and then transferring a protective layer from the thermal transfer sheet of the present disclosure onto the thermal transfer image.In the case of an intermediate transfer medium, a printed product can be obtained by forming a thermal transfer image on the receiving layer, then transferring the receiving layer with the thermal transfer image formed on it onto any transfer target, and then transferring a protective layer from the thermal transfer sheet of the present disclosure onto this receiving layer.

There are no particular limitations on the method for transferring the transfer layer onto the transfer recipient; for example, transfer can be performed using a thermal transfer printer with a heating device such as a thermal head, or a heating means such as a hot stamp or heat roll.

The present disclosure relates to, for example, the following [1] to [3].
[1] A thermal transfer sheet comprising a substrate and a transferable protective layer provided on one side of the substrate, wherein the surface tension (ST2) of a second surface of the transferable protective layer, which is the surface opposite the substrate, is greater than the surface tension (ST1) of a first surface of the transferable protective layer, which is the surface facing the substrate.
[2] The thermal transfer sheet according to [1] above, wherein the surface tension (ST2) of the second surface is 46 mN/m or more and 70 mN/m or less.
[3] The thermal transfer sheet according to the above [1] or [2], wherein the transferable protective layer contains a (meth)acrylic resin.

The thermal transfer sheet of the present disclosure will be described below based on examples, but the thermal transfer sheet of the present disclosure is in no way limited to these examples. In the following description, "parts by mass" will simply be referred to as "parts."

[Examples and Comparative Examples]
A polyester film (Lumirror (registered trademark) #5A-F53, Toray Industries, Inc.) with a thickness of 5 μm or less was prepared as a substrate. The following back layer coating liquid was applied to one side of the polyester film and dried at 100°C for 2 minutes to form a back layer with a thickness of 1.2 μm. The following protective layer coating liquid was applied to the side of the polyester film opposite to the side on which the back layer was formed and dried at 100°C for 1 minute to form a protective layer with a thickness of 2 μm. The obtained sheet was passed between the electrode and treatment roll with the back layer in contact with the treatment roll so that the protective layer was located on the electrode side, and subjected to corona discharge treatment with the discharge amount shown in Table 1. In this manner, a thermal transfer sheet was obtained.

<Coating liquid for back layer>
Polyvinyl acetal (hydroxyl value = 12% by mass) 36 parts (S-LEC (registered trademark) KS-1, Sekisui Chemical Co., Ltd.)
Polyisocyanate (NCO=17.3% by mass) 25 parts (Burnoc (registered trademark) D750, DIC Corporation)
Silicone resin microparticles (average particle diameter = 4 μm, polygonal shape) 1 part (Tospearl 240,
Momentive Performance Materials Japan LLC)
10 parts of zinc stearyl phosphate (purified LBT1830, Sakai Chemical Industry Co., Ltd.)
Zinc stearate 10 parts (SZ-PF, Sakai Chemical Industry Co., Ltd.)
3 parts polyethylene wax (Polywax 3000, Toyo ADL Co., Ltd.)
7 parts ethoxylated alcohol-modified wax (Unitox 750, Toyo ADL Co., Ltd.)
・Methyl ethyl ketone 200 parts ・Toluene 100 parts

<Coating liquid for protective layer>
60 parts of (meth)acrylic resin A (NeoCryl (registered trademark) B-811, Kusumoto Chemicals Co., Ltd.)
30 parts of (meth)acrylic resin B (NeoCryl (registered trademark) B-891, Kusumoto Chemicals Co., Ltd.)
7 parts ultraviolet absorber (UV-329, Sun Chemical Co., Ltd.)
3 parts of phosphate ester type anionic surfactant (Plysurf (registered trademark) A208N, Dai-ichi Kogyo Seiyaku Co., Ltd.)
・200 parts methyl ethyl ketone ・200 parts toluene

[Print Creation]
The thermal transfer sheets of the Examples and Comparative Examples were placed opposite genuine paper for SELPHY CP1300 (Canon Inc.), and the protective layer was transferred onto the genuine paper under the following test printer transfer conditions to obtain a print.

(Test printer transfer conditions)
Thermal head: KEE-57-12GAN2-STA (Kyocera Corporation)
・Average resistance of heating element: 3303 (Ω)
- Main scanning direction print density: 300 (dpi)
Sub-scanning direction print density: 300 (dpi)
Printing voltage: 18 (V)
Line period: 1.5 (msec./line)
・Printing start temperature: 35 (℃)
Pulse duty ratio: 85 (%)

[surface tension]
The surface tension (wet tension) of the protective layer was measured in accordance with JIS K6768 "Plastics - Films and Sheets - Test Method for Wet Tension." The surface tension (ST2) is the surface tension of the second surface of the protective layer in the thermal transfer sheet, and was measured on the protective layer surface of the thermal transfer sheet. The surface tension (ST1) is the surface tension of the first surface of the protective layer in the thermal transfer sheet, and was measured on the protective layer surface of the print obtained by transferring the protective layer onto a transfer recipient.

[Protective layer adhesion]
A mending tape (Scotch (registered trademark) 810-3-18, 3M Japan Ltd.) was applied to the protective layer of the print and then peeled off, and it was visually observed whether or not the protective layer had peeled off.
AA: The area ratio of the peeled protective layer was less than 1%.
BB: The area ratio of the peeled protective layer was 1% or more and less than 10%.
CC: The area ratio of the peeled protective layer was 10% or more.

[Protective layer tearability]
The presence or absence of tailing of the protective layer at the edge of the print was visually observed. If there was no tailing or only a small amount of tailing, it can be said that the protective layer had excellent peelability from the substrate during thermal transfer.
AA: No tailing was observed, or tailing of 0.1 mm or more but less than 0.3 mm was observed.
BB: A trail of 0.3 mm or more and less than 2 mm occurred.
CC: A trail of 2 mm or more occurred.

DESCRIPTION OF SYMBOLS 1...thermal transfer sheet 2...printed matter 10...substrate 20...transferable protective layer 22...first surface of transferable protective layer 24...second surface of transferable protective layer 30...back surface layer 40...colorant layer 50...transferee

Claims (2)

A substrate;
A thermal transfer sheet comprising a transferable protective layer provided on one surface of the substrate,
a surface tension (ST2) of a second surface of the transferable protective layer that is the surface opposite to the substrate is greater than a surface tension (ST1) of a first surface of the transferable protective layer that is the surface on the substrate side;
The surface tension (ST2) of the second surface is 46 mN/m or more and 70 mN/m or less.
Thermal transfer sheet. The thermal transfer sheet according to claim 1 , wherein the transferable protective layer contains a (meth)acrylic resin.