EP0613787A1 - Image receptor sheet for thermal transfer - Google Patents

Abstract

A thermal transfer image receptor sheet comprising a substrate and an image receptor layer composed mainly of a polycaprolactone whose main chain has the structure of the formula (1) and formed on the substrate.



        -(CH₂CH₂CH₂CH₂CH₂COO)-   (1)



   The above thermal transfer image receptor sheet shows excellent image receiving performance, and gives an image receptor product which is excellent in adhesion of the image receptor sheet to an image-forming ink composition, scratch resistance and weatherability, and which is free from contamination caused on the surface by dust, soot, etc.

Description

Field of the Invention
• The present invention relates to a thermal transfer image receptor sheet for use in heat-melting thermal transfer. More specifically, it relates to a thermal transfer image receptor sheet for use in thermal transfer, which has the property of receiving thermal transfer images, patterns, characters, etc. (to be simply referred to as image(s) hereinafter), and which gives an image transfer product excellent in adhesion to a transfer image, scratch resistance, abrasion resistance and weatherability.
Prior Art of the Invention
• A heat-melting thermal transfer device is recently widely used in a facsimile machine, a word processor, a computer terminal printer, and the like since it has features in that it is noise-free because of its non-impact system, maintenance-free, less expensive and small in size. A thermal transfer material is generally produced by forming a heat-melting ink layer composed mainly of a wax on one surface of a substrate formed of a thin plastic film such as a polyester film (e.g., polyethylene terephthalate or polyethylene naphthalate). An image is transferred as follows. While the above heat-melting ink layer is in contact with the surface of an image receptor such as general paper, part of the heat-melting ink is transferred to the image receptor by heating the other surface of the substrate, for example, by means of a thermal head.
• With a recent progress in the automatization of factories and stores, thermal transfer materials are increasingly used in the fields of labels and barcodes, and image-recorded sheets (e.g., labels and barcodes) are increasingly required to have various resistances such as scratch resistance and abrasion resistance. For this purpose, there has been developed a thermal transfer material having a heat-melting ink layer (transfer layer) composed mainly of a resin, and various materials such as coat paper, synthetic paper and a plastic sheet have begun to be used as image receptors depending upon purposes. When the thermal transfer material having a heat-melting ink layer composed mainly of a resin is used, it is difficult to transfer the heat-melting ink to general paper. The above heat-melting ink layer can be transferred to synthetic paper or a plastic sheet, while the adhesion of the heat-melting ink layer to the synthetic paper or the plastic sheet is insufficient, and the heat-melting ink layer is easily peeled off when a Cellophane tape is attached and peeled.
• As an image receptor which serves to improve the resistances of an image-recorded sheet, JP-A-63-137892 discloses an image receptor sheet produced by forming a layer of a thermoplastic resin having a melting point of 135°C or lower on a substrate. However, the thermoplastic resins disclosed in Examples of JP-A-63-137892 are polyolefins having a melting point of 100 to 135°C such as polyethylene, and the layers of these thermoplastic resins are hence poor in adhesion to a substrate other than paper, such as a polyethylene terephthalate film generally used as an OHP film. Further, these thermoplastic resins have relatively high melting points, and are therefore poor in adhesion to a transfer image when the transfer layer is formed of a resin-containing heat-melting ink layer.
• JP-A-1-120389 discloses an image receptor sheet produced by forming an image receptor layer composed mainly of a lubricant and a thermoplastic resin having Tg of 50 to 100°C on a substrate. However, when the lubricant is natural wax, synthetic wax or higher fatty acid metal salt, the image receptor layer is poor in surface gloss. Further, since the lubricant is poor in weatherability, the image receptor layer is liable to undergo oxidation or hydrolysis to deteriorate when the image-recorded sheet is used outdoors, and the image-recorded sheet practically discolors or deteriorates in gloss.
• For improving the image receptor sheet in the image receiving performance and adhesion to a transfer image, generally, a thermoplastic resin having a melting point or softening point of 100°C or lower is used for forming the image receptor layer. Of such thermoplastic resins, generally, those having a sharp melting point are polymers having a low-molecular weight (about 2,000 or less) or oligomers, and films formed of them show low film strength and have almost no weatherability. There is therefore a problem in practical use; When an image-recorded sheet including such an image receptor layer is used outdoors, the image receptor layer deteriorates for a short period of time. Thermoplastic resins which have no sharp melting point but show a softening point have tack (adhesion property) at a temperature between ordinary temperature (about 30°C) and a temperature around their softening points. Therefore, when the image-recorded sheet is used outdoors, the image receptor layer is softened to show tack due to an increase in temperature caused by sun light. As a result, dust may adhere or soot may be adsorbed to make the image-recorded sheet dark and dirty. Further, when the image receptor layer is formed of a thermoplastic resin having a softening point of 100°C or higher, the image receptor sheet show sufficient image-receiving performance or sufficient adhesion to a transfer image only when such high energy as will give an overload to a thermal head is charged to a thermal transfer material having a thermal transfer layer composed mainly of a resin. It has been therefore difficult to obtain a thermal transfer receptor sheet which has sufficient image-receiving performance, adhesion to an ink, scratch resistance and abrasion resistance and is excellent in outdoor weatherability for use with a thermal transfer material having a thermal transfer layer composed mainly of a resin.
Summary of the Invention
• It is an object of the present invention to provide a thermal transfer image receptor sheet having an image receptor layer which exhibits excellent film strength.
• It is another object of the present invention to provide a thermal transfer image receptor sheet having an image receptor layer which is excellent in outdoor weatherability, image receiving performance, adhesion to a transfer image, abrasion resistance and scratch resistance.
• The above objects and advantages of the present invention will be achieved by a thermal transfer image receptor sheet comprising a substrate and an image receptor layer composed mainly of a polycaprolactone whose main chain has the structure of the formula (1) and formed on the substrate.
  
  
  
          -(CH₂CH₂CH₂CH₂CH₂COO)-   (1)
  
  
Brief Description of Drawings
• Fig. 1 shows the cross section of a thermal transfer image receptor sheet according to the present invention.
• Fig. 2 shows the cross section of a thermal transfer image receptor sheet according to the present invention.
Detailed Description of the Invention
• The surface of the thermal transfer image receptor sheet of the present invention and the transfer surface of a thermal transfer material (produced by forming a thermal transfer layer composed mainly of a resin on one surface of a substrate) are brought into contact with each other, and part of the thermal transfer layer is transferred onto the image receptor layer by melting part of the image receptor layer and part of the thermal transfer layer under heat and pressure of a thermal head applied to the other surface (substrate side) of the thermal transfer material, whereby there can be obtained a thermal transfer recorded product which has an excellently transferred image and is excellent in the adhesion between the image receptor layer and the thermal transfer layer, scratch resistance, abrasion resistance and weatherability.
• The present invention will be explained hereinafter with reference to the drawings. Fig. 1 shows the cross section of the thermal transfer image receptor sheet of the present invention. The image receptor sheet of the present invention comprises a substrate such as a plastic sheet and an image receptor layer formed on the substrate as shown in Fig. 1, in which numeral (1) indicates a substrate and an image receptor layer (2) composed mainly of a polycaprolactone whose main chain has the structure of the formula (1),
  
  
  
          -(CH₂CH₂CH₂CH₂CH₂COO)-   (1)
  
  
  
  is formed on one surface the substrate. Further, the image receptor sheet of the present invention may have an adhesive layer (3) formed on the other surface of the substrate and a peel sheet (4) provided on the adhesive layer, as is shown in Fig. 2.
• The above polycaprolactone used in the present invention is a polyester whose main chain has the structure of the above formula (1) and which has excellent outdoor weatherability and high film strength. At the same time, the above polycaprolactone characteristically has a sharp melting point at 100°C or lower based on its high crystallizability although it has a high molecular weight (e.g., 10,000 to 100,000) as compared with such polyester as conventional polyethylene terephthalate. Therefore, it has excellent heat-sensitivity in that it maintains high film strength without showing any melting tendency until its melting point is reached and that it is sharply melted at a melting point. Due to these characteristics, the image receptor sheet to which an image (pattern, etc.) has been transferred is free from contamination caused on the sheet surface by dust, soot, etc., because of the softening of the image receptor layer, and it is excellent in weatherability, when used as an outdoor display. The above polycaprolactone is available as commercial products in the trade name of "Placcell" (trade name, supplied by Daicel Chemical Industries, Ltd.), which have a melting point of 60°C and have different molecular weight distributions.
• Further, for further improving the resistance to contamination outdoors, the image receptor may contain a fluorine-containing compound having a polyfluoro group or a silicone-modified resin having a polyorganosiloxane.
• The fluorine-containing compound preferably includes a compound in which a side chain of an acrylic resin bonds to a main chain of a fluorine resin, a graft polymer (which may be a low molecular weight oligomer) in which a side chain of a fluorine resin bonds to a main chain of an acrylic resin, and a copolymer (which may be a low molecular weight oligomer) formed from a polyfluoro group-containing vinyl monomer and other vinyl monomer. The content of the fluorine-containing compound in the image receptor layer is properly 0 to 50 % by weight depending upon the kind of the fluorine-containing compound, while it is preferably 1 to 20 % by weight. When this content exceeds the above upper limit, the image-receiving performance may decrease.
• The silicone-modified resin preferably includes polymers obtained by graft-modifying or block-modifying polyorganosiloxane as a main chain, and particularly preferred are a silicone-modified polyurethane resin obtained by an addition-reaction of a compound (or prepolymer) having at least two hydroxyl groups in the molecule, a polyoroganosiloxane having at least two hydroxy groups in the molecule and a compound (or prepolymer) having at least two isocyanate groups in the molecule, and a silicone-modified acrylic resin obtained by the polymerization of at least one acrylic monomer and a polyoroganosiloxane having a radical-polymerizable double bond. When a conventionally known silicone oil is used, undesirably, a so-called exudation such as bleeding may take place to cause a failure in transfer and a decrease in intimate adhesion of an transfer image.
• The content of the polyoroganosiloxane in the total constituents of the silicone-modified resin is preferably 10 to 70 % by weight, more preferably 20 to 50 % by weight. When this content is less than the above lower limit, undesirably, the silicone-modified resin scarcely shows its properties. When it exceeds the above upper limit, undesirably, the silicone-modified resin shows the properties similar to those of a silicone rubber formed of polyoroganosiloxane alone.
• The content of the silicone-modified resin in the image receptor layer is properly 0 to 50 % by weight depending upon the kind thereof, while it is preferably 1 to 20 % by weight. When the content of the silicone-modified resin exceeds the above upper limit, the image-receiving performance may decrease.
• Further, the image receptor layer may optionally contain at least one other thermoplastic resin in such an amount that the properties of the polycaprolactone are not impaired. The content of the thermoplastic resin in the image receptor layer is properly 0 to 50 % by weight, while it is preferred not to add any thermoplastic resin. When this content is large, the image receptor layer may cause a problem that it has dust sticking to it or it is turned black or dark due to soot when exposed outdoors. Further, the image-receiving performance may decrease.
• The above thermoplastic resin includes polyvinyl chloride, polyvinyl acetate, a vinyl chloride-vinyl acetate copolymer, polyethylene, polypropylene, polyacetal, an ethylene-vinyl acetate copolymer, an ethylene-(meth)acrylate copolymer, a styrene-(meth)acrylate copolymer, polystyrene, an acrylic resin, a polyamide resin, a cellulose derivative, an epoxy resin, a xylene resin, a ketone resin, a petroleum resin, a rosin or its derivative, a coumarone-indene resin, a polyester resin, chlorinated polyolefin, a styrene-maleic anhydride resin, polyvinylpyrrolidone, a vinyl pyrrolidone-vinyl acetate copolymer, styrene-butadiene rubber, polyvinyl butyral, nitrile rubber, acryl rubber, and ethylene-propylene rubber.
• The thermal transfer image receptor sheet is white or transparent, and further it can be colored in a variety of colors. That is, the image receptor layer can be formed on a colored plastic sheet such as a plastic sheet containing a pigment or a plastic sheet whose one surface is colored by printing or vapor deposition. Further, the image receptor layer containing a coloring material can be formed on a white or transparent plastic film. When white or transparent plastic sheets are used, there can be obtained thermal transfer image receptor sheets having various colors by incorporating coloring materials into image receptor layers.
• The coloring material includes those generally used for producing inks. That is, it is selected from pigments such as carbon black, aniline black, titanium oxide, phthalocyanine pigments, monoazo pigments, disazo pigments, nitro pigments, nitroso pigments, perylene pigments, isoindolinone pigments and quinacridone pigments, and dyes such as azo dyes, anthraquinone dyes and nigrosine dyes. For an image receptor product having excellent weatherability, particularly preferred are pigments such as carbon black, fast yellow, cadmium yellow, yellow iron oxide, chromophthal yellow, anthrapyrimidine yellow, isoindolinone yellow, copper azomethine yellow, benzoimidazolone yellow, quinophthalone yellow, nickel dioxine yellow, flavanthrone yellow, chrome yellow, titanium yellow, disazo yellow, benzimidazolone orange, pyranthrone orange, perynone orange, para red, lake red, naphthol red, pyrazolone red, permanent red, madder lake, thioindigo Bordeaux, red iron oxide, red lead, cadmium red, quinacridone magenta, perylene barmillion, perylene red, chromophthal scarlet, anthrone red, dianthraquinolyl red, perylene maroon, benzoimidazolone carmine, perylene scarlet, quinacridone red, pyranthrone red, manganese violet, dioxazine violet, phthalocyanine blue, iron blue, cobalt blue, ultramarine, indanthrone blue, phthalocyanine green, pigment green, nickelazo yellow, chromium oxide, viridian, benzoimidazolone brown, bronze powder, white lead, zinc white, lithopone, titanium oxide and a pearl pigment. A fluorescent pigment may be used as required.
• Further, for improving the weatherability, the image receptor layer may contain an ultraviolet light absorbent and an ultraviolet light shielding agent. Examples of the ultraviolet light absorbent include compounds which absorb light having a wavelength of 290 to 400 nm, such as benzophenone compounds, benzotriazole compounds, salicylic acid phenyl ester compounds, cyanoacrylate compounds, cinnamic acids and aminobutadiene compounds. Examples of the ultraviolet light shielding agent include fine particles of titanium oxide, zinc white, talc, kaolin, calcium carbonate and iron oxide.
• Further, for adjusting the coatability and coating properties, the image receptor layer may contain other additives such as a dispersing agent, an antistatic agent, a plasticizer and an antioxidant.
• Examples of the antistatic agent include polyoxyethylene alkylamine, polyoxyalkylamide, polyoxyethylene alkyl ether, glycerin fatty acid ester, sorbitan fatty acid ester, alkyl sulfonate, alkylbenzenesulfonate, alkylsulfate, alkylphosphate and quaternary ammonium sulfate. In particular, the antistatic agent has an effect on the prevention of electrostatically collected dust which causes drop-out (voids) in thermal transfer recording.
• Examples of the plasticizer include low molecular weight ester-containing plasticizers obtained from monohydric or polyhydric alcohol compounds and carboxylic acid compounds such as phthalic acid, isophthalic acid, tetrahydrophthalic acid, adipic acid, sebacic acid, maleic acid, fumaric acid, trimellitic acid and oleic acid, alkyd plasticizers and oxirane oxygen-containing epoxy type plasticizers.
• The substrate is preferably selected from 10 to 500 µm thick plastic sheets formed of synthetic paper, polyester, polyvinyl chloride, polyurethane, poly(meth)acrylate, polycarbonate, polyethylene, polypropylene, polyamide and cellulose. In view of weatherability, flexibility and aesthetically fine appearance, preferred are 50 to 500 µm thick plastic sheets formed of soft polyvinyl chloride, polyester and synthetic paper. Further, a primer layer may be formed between the image receptor layer and the substrate for improving the adhesion of the two members. The material for forming the adhesive layer is not specially limited.
• The image receptor layer can be formed on the substrate by a solvent coating method in which a composition for forming the image receptor layer is dissolved or dispersed in a solvent or water and the resultant solution or dispersion is coated and dried or by a hot melt coating method in which a composition for forming the image receptor layer is melted under heat to coat the image receptor layer. The thickness of the image receptor layer is preferably approximately 0.1 to 10 µm. For forming the image receptor layer having a small thickness, the solvent coating method is preferred, and for forming the image receptor layer having a large thickness, the hot melt coating method is preferred. It Is more preferred to form an image receptor layer having a thickness of approximately 0.2 to 2 µm by the solvent coating method.
• When the thermal transfer image receptor sheet according to the present invention is used, there can be obtained a thermal transfer product which is excellent in adhesion of the image receptor sheet to an image-forming ink composition, scratch resistance and weatherability. The thermal transfer image receptor sheet according to the present invention is excellent in image receiving performance. The image transfer product using the image receptor sheet according to the present invention is free from contamination caused on the surface by dust, soot, etc.
Examples
• The present invention will be explained more in detail hereinafter with reference to Examples, in which "part" stands for "part by weight".
Example 1
• An image receptor sheet 1 was prepared by gravure-coating one surface of a soft vinyl chloride sheet having a thickness of 100 µm with a coating solution having the following composition.

  Polycaprolactone ("Placcel H7", supplied by Daicel Chemical Industries, Ltd.)	10 parts
  Toluene	50 parts
  Methyl ethyl ketone	40 parts

Example 2
• Animage receptor sheet 2 was prepared by coating one surface of a polyethylene terephthalate sheet having a thickness of 100 µm with a coating solution having the following composition in the same manner as in Example 1.

  Polycaprolactone ("Placcel H7", supplied by Daicel Chemical Industries, Ltd.)	10 parts
  Fluorine-containing compound (Surflon S381, supplied by Asahai Glass Co., Ltd.)	0.5 parts
  Toluene	50 parts
  Methyl ethyl ketone	40 parts

Example 3
• Animage receptor sheet 3 was prepared by coating one surface of a polyethylene terephthalate sheet having a thickness of 100 µm with a coating solution having the following composition in the same manner as in Example 1.

  Polycaprolactone ("Placcel H7", supplied by Daicel Chemical Industries, Ltd.)	18 parts
  Silicone-modified urethane resin (Daiaromer SP2105, supplied by Dainichiseika Colour & Chemicals Mfg. Co., Ltd.)	2 parts
  Toluene	50 parts
  Methyl ethyl ketone	40 parts

Example 4
• Animage receptor sheet 4 was prepared by coating one surface of a soft vinyl chloride sheet having a thickness of 100 µm with a coating solution having the following composition in the same manner as in Example 1.

  Polycaprolactone ("Placcel H7", supplied by Daicel Chemical Industries, Ltd.)	15.5 parts
  Silicone-modified acrylic resin (Symac US-350, supplied by Toagosei Chemical Industry Co., Ltd.)	0.5 part
  Yellow pigment (Lionol Yellow 1308, supplied by Toyo Ink Manufacturing Co., Ltd.)	4 parts
  Toluene	40 parts
  Methyl ethyl ketone	40 parts

Comparative Example 1
• An image receptor sheet 5 was prepared by coating one surface of a soft vinyl chloride sheet having a thickness of 100 µm with a coating solution having the following composition in the same manner as in Example 1.

  Epoxy resin ("Epikote 1002" supplied by Yuka Shell Epoxy K.K.)	20 parts
  Toluene	40 parts
  Methyl ethyl ketone	40 parts

Comparative Example 2
• An image receptor sheet 6 was prepared by coating one surface of a soft vinyl chloride sheet having a thickness of 100 µm with a coating solution having the following composition in the same manner as in Example 1.

  Polyester resin ("Vylon 200", supplied by Toyobo Co., Ltd.)	20 parts
  Toluene	40 parts
  Methyl ethyl ketone	40 parts

• Thermal transfer materials were prepared as follows. A heat-resistant layer was formed on one surface of a polyethylene terephthalate film having a thickness of 6 µm, and a transfer layer of a heat melting ink composed mainly of an acrylic resin and a pigment was formed on the other surface of the polyethylene terephthalate film through a peeling layer.
• The image receptor sheets prepared in Examples and Comparative Examples were tested as follows. The image receptor layer surface and the heat melting ink surface were brought into contact, and these two sheets were heated from the heat-resistant layer surface with a thermal head to obtain an image transfer product. For the image-receiving performance of the image receptor, the resolution was visually evaluated. The adhesion of the image receptor sheet to the heat melting ink was evaluated by attaching a cellophane tape to the heat melting ink surface of the image transfer product and peeling the cellophane tape off rapidly. The scratch resistance was evaluated by a pencil hardness test (JIS K-5401) on the heat melting ink side. The abrasion resistance was evaluated by a coloring fastness to rubbing test (JIS L-0823) on the heat melting ink side. The outdoor weatherability was evaluated by fixing the image transfer product to an outdoor exposure tester, exposing it for 6 months and then assessing the degree of contamination.
• Table 1 shows the results.Table 1

  	Image-receiving performance	Adhesion	Scratch resistance	Abrasion resistance	Outdoor weatherability
  Ex.1	A	A	A	A	A
  Ex.2	A	A	A	A	A
  Ex.3	A	A	A	A	A
  Ex.4	A	A	A	A	A
  CEx.1	A	B	B	B	C
  CEx.2	B	B	A	A	A
  Ex. = Example, CEx. = Comparative Example A = Excellent B = Poor C = Defective

• As shown in Table 1, the image receptor sheets prepared in Examples 1 to 4 showed excellent results in all the tests. On the other hand, the image receptor sheet prepared in Comparative Example 1 showed excellent image receiving performance since the thermoplastic resin used in the image receptor layer had a sharp melting point (80°C), while the image transfer product was poor in adhesion, scratch resistance and abrasion resistance since it had low film strength, and further the image transfer product was weathered due to deterioration in the weatherability test since the thermoplastic resin had a low molecular weight and was hence poor in weatherability. The image transfer product using the image receptor sheet obtained in Comparative Example 2 was excellent in scratch resistance and abrasion resistance since the thermoplastic resin used in the image receptor layer had high film strength, while the image receptor sheet was poor in image receiving performance and adhesion since the thermoplastic resin had a high softening point (160°C), although the image transfer product was excellent in weatherability owning to the above high softening point.

Claims (8)

1. A thermal transfer image receptor sheet comprising a substrate having an image receptor layer formed thereon, wherein the image receptor layer comprises a polycaprolactone whose main chain has a structure of formula (1).
   
   
   
           -(CH₂CH₂CH₂CH₂CH₂COO)-   (1)
   
   
2. A thermal transfer image receptor sheet according to claim 1, wherein the polycaprolactone has a number average molecular weight of 10,000 to 100,000.
3. A thermal transfer image receptor sheet according to claims 1 or 2, wherein the image receptor layer further contains at least one compound selected from the group consisting of a fluorine-containing compound containing a polyfluoro group and a silicone-modified resin containing a polyorganosiloxane unit.
4. A thermal transfer image receptor sheet according to claim 3, wherein the silicone-modified resin is a silicone-modified polyurethane resin or a silicone-modified acrylic resin.
5. A thermal transfer image receptor sheet according to claims 3 or 4, wherein the silicone-modified resin contains 10 to 70 % by weight of a polyorganosiloxane unit.
6. A thermal transfer image receptor sheet according to any one of claims 3 to 5, wherein the image receptor layer contains 0 to 50 % by weight of the silicone-modified resin.
7. A thermal transfer image receptor sheet according to claim 3, wherein the image receptor sheet contains 0 to 50 % by weight of the fluorine-containing compound.
8. A thermal transfer image receptor sheet according to any preceding claim, wherein the image receptor layer further contains 0 to 50 % by weight of a thermoplastic resin.

Images