Thermal transfer sheet

Abstract: A thermal transfer sheet having a base film, a heat resistant layer which is formed on one surface of the base film by coating a composition which contains (A) a compound having at least two (meth)acryloyl groups and (B) at least one polymer selected from the group consisting of a syrene-alkyl (meth)acrylate copolymer and an α-methylstyrene-alkyl (meth)acrylate copolymer and cured with an activation energy ray, and heat transfer coloring material layer on the other surface of the base film, which sheet has maintains good runnability and prevents deposition of a thermal head tailings.

Description: Field of the Invention The present invention relates to a thermal transfer sheet which is used in a recording system for thermally transferring a coloring material with heat-application means such as a thermal head. Description of the Related Art With the development of office automation, a thermal transfer recording system is used in various terminal equipments such as a facsimile machine or a printing machine. This recording system is advantageous over other recording systems such as electrophotographic type or ink jet type ones because of easy maintenance of the machine, easy operability, low costs of the machine and consumables. Since the thermal transfer sheet is heated with a thermal head at high temperature in the thermal transfer recording system, if a base film of the thermal transfer sheet has insufficient heat resistance, it is fusion-bonded to the thermal head, which results in generation of sticking noise or deposition of thermal head tailings. When the fusion bonding becomes severe, running of the thermal head is impossible so that no recording is possible. To solve this problem, Japanese Patent Kokai Publication No. 7467/1980 proposes the supply of a heat resistant resin layer such as a silicone resin or an epoxy resin on the base film, and Japanese Patent Kokai Publication No. 129789/1982 proposes the supply of a resin layer containing a surfactant which is in a solid or semisolid state at room temperature on the base film. However, these measures require a large amount of energy for heat-curing the resin. In addition, the base film suffers from deformation wrinkles caused by heat, so that the coloring material layer is irregularly coated on the base film and therefore printing quality deteriorates. When the surfactant is added to the resin layer, it will adhere to the thermal head to cause printing slips. With the increase of the energy applied to the thermal head due to recent high speed printing, the thermal transfer sheet receives large load. Then, it is difficult to improve the runnability of the thermal head by the methods disclosed in the above Japanese Patent Kokai Publications. As further methods for solving the above problem, Japanese Patent Kokai Publication No. 27289/1986 proposes the supply of a heat resistant layer of a UV-curable resin such as polyester acrylate on the base film and Japanese Patent Kokai Publication No. 207679/1987 proposes the supply of a cured layer of a silicone having radically polymerizable double bonds. While the base film having such layer is excellent in heat resistance, it has an insufficient slipping property, so that the runnability of the thermal head may not be improved, or the uncured silicone migrates on a surface of the base film on which the coloring material layer will be formed. Thereby, the coloring material layer is irregularly coated. In addition, the uncured silicone migrates in the coloring material layer so that the normal printing is interfered. To reduce recording cost in the thermal transfer recording system, a repeated use type thermal transfer sheet is being developed. Therefore, a heat resistant layer is required, which does not deteriorate the runnability of the thermal head even when it is repeatedly heated by the thermal head and prevents deposition of the thermal head tailings. Thermal transfer sheets comprising a base film, a heat resistant layer (which is formed on one surface of said base film by coating a composition which comprises a compound having at least two (meth)acryloyl groups), and a thermal transfer dye layer on the other surface of said base film are known from EP-A-0 458 538, EP-A-0 314 348 and EP-A-0 314 205. SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal transfer sheet which maintains good runnability of the thermal head even at high energy recording and keeps the thermal head clean after repeated printing. According to the present invention, there is provided a thermal transfer sheet comprising a base film, a heat resistant layer which is formed on one surface of said base film by coating a composition which comprises (A) a compound having at least two (meth)acryloyl groups, and a heat transfer coloring material layer on the other surface of said base film, which is characterized in that said coating composition for forming the heat resistant layer further comprises (B) at least one polymer selected from the group consisting of a styrene-alkyl (meth)acrylate copolymer and an α-methyl-styrene-alkyl (meth)acrylate copolymer and being cured with an activation energy ray. DETAILED DESCRIPTION OF THE INVENTION An example of the compound having at least two (meth)acryloyl group is a reaction product of a di- or polyhydric alcohol or its derivative and a compound having an acryloyl or methacryloyl group. A specific example is a reaction product of a polyhydric alcohol and (meth)acrylic acid or its halide or lower alkyl ester. Examples of the polyhydric alcohol are ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, tetraethylene glycol, trimethylolpropane, pentaglycerol, pentaerithritol, dipentaerithritol, glycerol and diglycerol. A preferred reaction product is a compound in which a group bridging the (meth)acryloyl groups is a hydrocarbon group having 20 or less carbon atoms, in particular 10 or less carbon atoms and no or one ether group, for example, trimethylolpropane tri(meth)acrylate, pentaglycerol tri(meth)acrylate, pentaerithritol tetra(meth)acrylate, dipentaerithritol hexa(meth)acrylate, ethyleneglycol (meth)acrylate, propyleneglycol di(meth)acrylate, and butyleneglycol di(meth)acrylate. Examples of the styrene-alkyl (meth)acrylate copolymer or α-methylstyrene-alkyl (meth)acrylate copolymer are copolymers of styrene or α-methylstyrene with methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert.-butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate or stearyl (meth)acrylate. Preferably, the styrene-alkyl (meth)acrylate copolymer or α-methylstyrene-alkyl (meth)acrylate copolymer has a number average molecular weight of 1000 to 10,000. A total amount of the polymer (B) is preferably from 0.1 to 30 % by weight, more preferably from 0.5 to 20 % by weight based on the whole weight of the heat resistant layer. When the total amount is less than 0.1 % by weight, the slipping property against the thermal head is not sufficient during thermal transfer recording. In the thermal transfer sheet of the present invention, the compound (A) and the polymer (B) are used in a weight (or molar) ratio of from 99.9:0.1 to 20:30, preferably from 99.5:0.5 to 30:20. In addition to the above two essential compounds, the heat resistant layer of the present invention may contain a copolymerizable monomer to improve the coating property. Specific examples of the copolymerizable monomer are acrylic acid, methacrylic acid and crotonic acid or their esters with an alcohol (e.g. methanol, ethanol, propanol, butanol, isopropanol, hexanol, 2-ethylhexanol, cyclohexanol, benzylalcohol, stearylalcohol, ethylene glycol, propylene glycol, diethylene glycol, and glycerol), glycidyl (meth)acrylate, allyl glycidyl ether, acrylonitrile, methacrylonitrile, vinyl acetate, styrene, α-methylstyrene, α-chlorostyrene, (meth)acrylamide, N-methylolacrylamide, N-butoxymethyl (meth)acrylamide, unsaturated polyester dimethacrylate, vinyltriethoxysilane, vinyltrimethoxysilane, acryloyloxypropyltriethoxysilane, methacryloyloxypropyltriethoxysilane, acryloyloxypropyltrimethoxysilane, and methacryloyloxytrimethoxysilane. An amount of the copolymerizable monomer preferably does not exceed 50 % by weight of the heat resistant layer. In addition, a photopolymerizable oligomer may be used. Examples of the photopolymerizable oligomer are epoxy (meth)acrylate, epoxidized oil (meth)acrylate, urethane acrylate, polyester (meth)acrylate, polyether (meth)acrylate, silicone (meth)acrylate, polybutadiene (meth)acrylate, polystyryl (meth)acrylate, and phosphazene base (meth)acrylate. The heat resistant layer of the present invention may contain an initiator such as a polymerization initiator or a photosensitizer. Examples of the polymerization initiator are 2,2-ethoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone, dibenzoyl, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, p-chlorobenzophenone, p-methoxybenzophenone, Michler's ketone, acetophenone, 2-chlorothioxanthone, anthraquinone, phenyldisulfide, and 2-methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1. They may be used independently or as a mixture. The amount of the polymerization initiator is from 0.05 to 5 parts by weight per 100 parts of the total amount of the materials contained in the heat resistant layer. Examples of the photosensitizer are tertiary amines (e.g. triethylamine, triethanolamine, and 2-dimethylaminoethanol), arylphosphines (e.g. triphenylphosphine), and thioethers (e.g. β-thioglycol). They may be used independently or as a mixture. The amount of the photosensitizer is from 0.05 to 5 parts by weight per 100 parts of the total amount of the materials contained in the heat resistant layer. In order to further improve the slipping property of the thermal transfer sheet against the thermal head, the heat resistant layer may further contain organic or inorganic particles. To prevent electrification of the thermal transfer sheet, the heat resistant layer may contain an electrically conductive powder such as a metal, a metal oxide or conductive carbon black, or an antistatic agent. If desired, the heat resistant layer may contain other additives such as a foam-inhibitor, a coating improver, and a tackifier. In the present invention, the heat resistant layer composition containing the above components is applied on one surface of the base film by a conventional method and then irradiated with the activation energy ray to form the heat resistant layer. Examples of the activation energy ray are UV light, visible light, electron ray, X-ray, α-ray, β-ray, and γ-ray. Usually, the activation energy ray is irradiated on the coated side, although it is possible to provide a reflection plate on the base film side to improve the adhesion of the heat resistant layer to the base film, or the activation energy ray is irradiated from the base film side. The thickness of the heat resistant layer is usually from 0.05 to 5 µm, preferably from 0.1 to 3 µm. As the base film, any of the conventionally-used films may be used. Examples of the base film are polyester film (e.g. polyethylene terephthalate film, polyethylene naphthalate film, and poly-1,4-cyclohexylenedimethylene terephthalate film), polyimide film, aromatic polyamide film, and polycarbonate film. Among them, a bi-axially-oriented polyester film is preferred in view of mechanical strength, dimensional stability, heat-resistance and cost. The thickness of the base film is usually from 2 to 15 µm. In the present invention, the coloring material layer may be formed by a conventional method. For example, in the case of a sublimation type thermal transfer sheet, a sublimable coloring material and a binder resin with good heat-resistance are dissolved or dispersed in a suitable solvent to prepare a paint. Then, the paint is coated on the other surface of the base film and dried. In the case of a melting-type thermal transfer sheet, a colorant such as a pigment or a dye is dissolved or dispersed in a heat-melting material, optionally using a solvent to prepare a paint, which is then coated on the other surface of the base film and dried. Examples of the sublimable coloring material are nonionic azo dyes, anthraquinone dyes, azomethine dyes, methine dyes, indoaniline dyes, naphthoquinone dyes, quinophthalone dyes, and nitro dyes. Examples of the binder resin are polycarbonate resins, polysulfone resins, polyvinylbutyral resins, polyarylate resins, polyamide resins, polyaramide resins, polyimide resins, polyetherimide resins, polyester resins, acrylonitrile-styrene resins, and cellulose resins (e.g. acetylcellulose, methylcellulose, and ethylcellulose, etc.). Examples of the solvent are aromatic solvents (e.g. toluene, and xylene), ketone solvents (e.g. methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone), ester solvents (e.g. ethyl acetate, and butyl acetate), alcohol solvents (e.g. isopropanol, butanol, and methylcellosolve), halohydrocarbon solvents, (e.g. methylene chloride, trichloroethylene, and chlorobenzene), ether solvents (e.g. dioxane, and tetrahydrofuran), and amide solvents (e.g. dimethylformamide, and N-methylpyrrolidone). Example of the colorant used in the melting type thermal transfer sheet are inorganic pigments (e.g. carbon black) and organic pigments (e.g. azo pigments, and fused polycyclic compounds). As the dye, for example, acid dyes, base dyes, metal complex dyes, and oil-soluble dyes are used. As the heating melting material, a solid or semisolid material having a melting point of 40 to 120°C is preferred. Examples of such material are carnauba wax, montan wax, microcrystalline wax, Japan wax, and fat-oil base synthetic wax. The coloring material layer may optionally contain, for example organic or inorganic particles, a dispersant, an antistatic agent, an anti-blocking agent, a foam-inhibitor, an antioxidant, and a viscosity regulator. To achieve the plural printings, the coloring material layer may have a stone wall structure in which heat resistant fine particles are filled, or a porous structure of a heat resistant resin. Between each of the layers and the base film, an intermediate layer such as an adhesion-enhancing layer, an antistatic layer, or a peeling layer may be provided. If desired, the surface or surfaces of the base film may be treated by, for example, corona discharge to enhance the adhesion. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be illustrated by the following Examples, in which "parts" are by weight. Example 1 On one surface of a biaxially-oriented polyethylene terephthalate film having a thickness of 5.5 µm, a coating paint having the following composition was gravure-coated, dried and cured by the irradiation with a high pressure mercury lamp having an energy of 120 W/cm from a distance of 150 mm for about 15 seconds to form a heat-resistant layer having a thickness of 0.5 µm: Component Parts Pentaerithritol triacrylate 30 Bisphenol-A type epoxy acrylate 10 Styrene-butyl acrylate copolymer (molecular weight of about 2700) 1 2-Methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1 2 Methyl ethyl ketone 100 Toluene 100 On the other surface of the above coated base film, a coloring material layer having the following composition was hot melt coated at a thickness of 4 µm to obtain a thermal transfer sheet: Component Parts Carbon black 20 Paraffin wax 40 Carnauba wax 30 Ethylene-vinyl acetate copolymer 10 Example 2 In the same manner as in Example 1 except that a coating paint for the heat resistant layer having the following composition was used, a thermal transfer sheet was produced: Component Parts Pentaerithritol tetraacrylate 30 Bisphenol A type epoxy acrylate 20 Styrene-butyl acrylate-methyl methacrylate copolymer (molecular weight of about 2900) 1 2-Methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1 2 Methyl ethyl ketone 100 Toluene 100 Comparative Example 1 In the same manner as in Example 1 except that a coating paint for the heat resistant layer having the following composition was used, a thermal transfer sheet was produced: Component Parts Pentaerithritol triacrylate 30 Bisphenol A type epoxy acrylate 10 2-Methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1 2 Methyl ethyl ketone 100 Toluene 100 Comparative Example 2 In the same manner as in Example 1 except that a coating paint for the heat resistant layer having the following composition was used, a thermal transfer sheet was produced: Component Parts Single end methacryloxypropyl group-containing polydimethylsiloxane (molecular weight of about 5000) 42 2-Methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1 2 Methyl ethyl ketone 100 Toluene 100 Comparative Example 3 In the same manner as in Example 1 except that a coating paint for the heat resistant layer having the following composition was used, a thermal transfer sheet was produced: Component Parts Pentaerithritol triacrylate 30 Bisphenol A type epoxy acrylate 10 Single end methacryloxypropyl group-containing polydimethylsiloxane (molecular weight of about 5000) 1 2-Methyl-[4-(methylthio)phenyl]-2-morpholinopropanone-1 2 Methyl ethyl ketone 100 Toluene 100 Each of the thermal transfer sheets prepared in Examples and Comparative Examples was subjected to the following evaluation tests. (1) Anti-sticking property and printing property Using a line-type thermal head, the anti-sticking property and the printing property were evaluated under the following recording conditions: Recording conditions Recording density: 4 dots/mm Recording power: 0.7 W/dot Head heating time: 4 ms Image receiving paper Fine quality paper The evaluation criteria are as follows: (i) Anti-sticking property ○: No sticking ▵: Slight sticking X: Heavy sticking to prevent running of the thermal head (ii) Printing property ○: No slip, blur or spreading (peripheral parts of the printed portion being transferred to cause fading of a printed image) ▵: Slight slip, blur or spreading X: Considerable slip, blur or spreading (2) Thermal head contamination After continuously (1 m long) printing under the same conditions as above, a surface of the thermal head was observed and evaluated according to the following criteria: ○: No head tailings ▵: A few head tailings X: Many head tailings The results are shown in the following Table. Example No. Anti-sticking property Printing property Thermal head contamination 1 ○ ○ ○ 2 ○ ○ ○ Comp. 1 X - - Comp. 2 ○ X X Comp. 3 ▵-X ▵ ▵ With the thermal transfer sheets produced in Examples 1 and 2, the thermal head and the thermal transfer sheet were not fused together, and the sheet ran smoothly and good images were transferred. In Comparative Example 1, the thermal transfer sheet sticked to the thermal head and did not run. In Comparative Example 2, though the thermal transfer sheet ran, the coloring material layer had coating irregularity due to the migration of chemicals from the heat-resistant layer, many slips were found in the coloring material layer after transfer, and the amount of head tailings was large. In Comparative Example 3, the thermal transfer sheet sticked to the thermal head so that it did not run smoothly. Example 3 In the same manner as in Example 1 except that a thickness of the heat resistant layer was increased to 1 µm and a coloring material paint having the following composition was used, a thermal transfer sheet was produced: Component Parts Sublimable dye (C.I. Solvent Blue 95) 5 Polysulfone resin 10 Chlorobenzene 85 Separately, a coating liquid containing a polyester resin (trade name: Vylon® 200 manufactured by Toyobo Co., Ltd.) (100 parts), an amino-modified silicone oil (trade name: AFL 40 manufactured by Nippon Unicar Co., Ltd.) (0.5 part), methyl ethyl ketone (15 parts) and toluene (15 parts) was coated on a polypropylene synthetic paper having a thickness of 100 µm and dried to form an image-receiving material having a dyeing layer of 5 µm in thickness. The thermal transfer sheet and the image-receiving material were laminated with the coloring material layer and the dyeing layer facing each other, and on the laminated sheet, an image was printed at a recording density of 4 dots/mm, a recording power of 0.8 W/dot and a head heating time of 8 ms. The laminated sheets ran smoothly and good transferred image was formed. After printing, the thermal head was observed but no head tailings were found. Example 4 In the same manner as in Example 1, except that the coloring material layer having the following composition was coated on the other surface of the base film through an adhesive layer containing a polyester resin, a thermal transfer sheet was produced: Component Parts Fatty acid amide 38 Paraffin wax 18 Black azo dye 18 Carbon black 4 Alumina 4 Acetone 400 The thickness of the coloring material layer was about 10 µm. Using the above thermal transfer sheet, an image was printed at a recording density of 4 dots/mm, a recording power of 0.4 W/dot and a head-heating time of 4 ms. As a recording paper, a Xerox® paper (Type PA4 manufactured by Kishu Paper Making Co., Ltd. having a Beck smoothness of 50 to 70 seconds) was used. After ten times repeated printings, the thermal transfer sheet ran smoothly and a good image was printed. No head tailings were found.

Claims: A thermal transfer sheet comprising a base film, a heat resistant layer which is formed on one surface of said base film by coating a composition which comprises (A) a compound having at least two (meth)acryloyl groups, and a heat transfer coloring material layer on the other surface of said base film, characterized in that said coating composition for forming the heat resistant layer further comprises (B) at least one polymer selected from the group consisting of a styrene-alkyl (meth)acrylate copolymer and an α-methyl-styrene-alkyl (meth)acrylate copolymer and being cured with an activation energy ray. The thermal transfer sheet according to claim 1, wherein said compound (A) is at least one selected from the group consisting of trimethylolpropane tri(meth)acrylate, pentaglycerol tri(meth)acrylate, pentaerithritol tetra(meth)acrylate, dipentaerithritol hexa(meth)acrylate, ethyleneglycol (meth)acrylate, propyleneglycol di(meth)acrylate, pentaerithritol tri(meth)acrylate and butyleneglycol di(meth)acrylate. The thermal transfer sheet according to claim 1, wherein said polymer (B) is a copolymer comprising styrene or α-methylstyrene and at least one (meth)acrylate selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert.-butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate and stearyl (meth)acrylate. The thermal transfer sheet according to claim 1, wherein said polymer (B) has a number average molecular weight of 1000 to 10,000. The thermal transfer sheet according to claim 1, wherein said heat resistant layer further contains a copolymerizable monomer. The thermal transfer sheet according to claim 1, wherein said heat resistant layer further contains an initiator. The thermal transfer sheet according to claim 1, wherein said heat resistant layer further contains an electrically conductive powder or an antistatic agent.

Inventors: OZAKI YOSHIHIDE, OZAKI, YOSHIHIDE
Applicants: DIAFOIL HOECHST CO LTD, DIAFOIL HOECHST CO., LTD
Application year: 1992
Main classification: B41M5
IPC codes: B41M5, B32B27