Cover Film for Sublimation Thermal-Transfer Hard Copy

1. Field of the Invention

The present invention relates to a cover film for a sublimation thermal transfer hard copy. The term "sublimation thermal transfer hard copy" is used in this specification to mean a hard copy of an image formed on an image receiving sheet by a sublimation thermal transfer method.

2. Related Background Art

Hitherto, an intense study has been made for developing methods for reproducing an image in the form of a hard copy similar to a photograph from electrical picture signals derived from, for example, a video camera, a still video camera, a television, a video disk and a photograph transmission system. Among these methods, a method called "sublimation thermal transfer (image) recording method" is now attracting attention.

Briefly, the sublimation thermal transfer recording method is a method in which electrical signals containing picture data are delivered to a thermal head having an array of electric heat generating elements arranged at a density of, for example, 4 to 16 dots (elements) per 1 mm, while an image-receiving sheet and an ink sheet superposed on the image-receiving sheet, the ink sheet being usually composed of a carrier sheet and a sublimation dyestuff layer on the carrier sheet, are moved together in contact with the head, so that the dyestuff is transferred in the form of a dot pattern from the ink sheet to the image-receiving sheet, whereby a hard copy is produced.

The thus obtained sublimation thermal transfer hard copies are usually stored in an office file, e.g., on a sheet of paper, in a notebook, a book, a scrap book, a card case, a letter case, and so forth, according to the uses of the hard copies.

When a sublimation thermal transfer copy is left for a long time with the image surface held in contact with an office file, the sublimation-type dyestuff sublimating from the image surface of the hard copy undesirably contaminates the portion of the office file contacting the hard copy.

In addition, discoloration or change in the color of the thus obtained hard copy image tends to be caused due to influence of heat, light or contact with gases, thus making it impossible to store the hard copy in good order.

In the cover film in accordance with the present invention, a layer of an anti-contamination material has a surface for contacting the image surface of the sublimation thermal-transfer hard copy, and a layer of a gas-impermeable material is provided to the other side of the anti-contamination layer. The anti-contamination layer may be constituted, for example, by a sheet of anti-contamination material or a coating of anti-contamination material formed on a carrier layer; and the gas-impermeable layer may be constituted, for example, by a sheet of gas-impermeable material or a coating of gas-impermeable material on a carrier layer.

According to the invention, coloring of the cover film is prevented even after a long covering of a sublimation thermal-transfer hard copy, by virtue of the layer of an anti-contamination material contacting the image surface of the sublimation thermal-transfer hard copy. Thus, the cover film may be used repeatedly for storing different copies. Furthermore, dye resublimated from the sublimation thermal-transfer hard copy does not penetrate the cover film, because the layer of a gas-impermeable material is provided to the other side of the anti-contamination layer. Consequently, contamination of an object which has happened to be brought into contact with the hard copy is avoided, as is invasion of external gases which tend to cause degradation of the image quality, such as steam, oxygen and ozone.

The anti-contamination material is preferably composed of a resin which has no affinity to the sublimation-type dyestuff, such as a polymeric material having a solubility parameter (referred to as "SP value", hereinafter) which is not greater than 8.5, and which is not smaller than 15.0.

The SP value is a value which is widely used as an index of the degree of chemical affinity or solubility between two or more substances. The smaller the SP value, the lower the affinity.

Examples of polymeric substances having SP values not greater than 8.5, suitable for use in the present invention, are silicone resin, Teflon resin, polyethylene and polypropylene. On the other hand, examples of polymeric substances having SP values not smaller than 15.0 are cellophane, hydrophilic resins such as polyvinylalcohol, and so forth.

Generally, resins having small permeability to steam and oxygen are suitably used as the gas-impermeable material in the invention. Such resins preferably have oxygen and steam permeability of 50 .times.10.sup.-13 cm.sup.3 .multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg or less, more preferably 30.times.10.sup.-13 cm.sup.3 .multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg or less. Examples of such resins are polyvinylidene chloride, polyester, polyvinylchloride, polyvinylfluoride and so forth.

The cover film of the present invention also is effective in preventing discoloration or change in color due to irradiation with light, by virtue of an ultraviolet-ray absorption agent contained in either one of the anti-contamination layer and the gas-impermeable layer. Preferably, the ultraviolet ray absorption agent is capable of absorbing ultraviolet rays of wavelengths ranging between 300 and 400 nm. Compounds of benzo-phenone type, benzotriazole type and salicylate type are suitably used as the ultraviolet ray absorption agent.

Various methods are usable for producing the cover film of the present invention. For instance, the cover film can be formed by applying a liquid containing a gas-impermeable component to an anti-contamination film, or by applying a liquid containing an anti-contamination component to a gas-impermeable film. It is also possible to obtain the cover film of the invention by bonding an anti-contamination film and a gas-impermeable film to each other. The ultraviolet ray absorption agent may be mixed beforehand in the film-making process or may be added during application of the anti-contamination or gas-impermeable material. The thickness of each of the anti-contamination layer and the gas-impermeable layer preferably ranges between 2 and 100 .mu.m, more preferably between 5 and 200 .mu.m. The total thickness of the cover film generally ranges between 5 and 200 .mu.m, preferably 20 and 100 .mu.m.

According to the present invention, it is possible to provide a writable layer (i.e., a layer for written notations) on the side thereof having the gas-impermeable material, so as to facilitate filing of the sublimation thermal-transfer hard copy.

The writable layer can be formed by applying, to the gas-impermeable layer, a coating liquid which contains a polymer and an inorganic filler such as titanium oxide, clay or the like, or an organic filler such as silicone resin, epoxy resin or the like. Specifications of the sublimation thermal-transfer hard copy, such as the content, date and so forth, can easily be written on the writable layer by means of a pencil, a ball-point pen, or the like.

Application of liquids in the process for producing the cover film of the present invention may be conducted by means of a reverse coater, roll coater or a gravure coater.

FIG. 1 is a longitudinal sectional view of a first embodiment of the cover film of the present invention, in a state in which the cover film composed of a gas-impermeable layer and a carrier layer of an anti-contamination substance is held in close contact with a sublimation thermal-transfer hard copy;

FIG. 2 is a longitudinal sectional view of a second embodiment of the cover film of the present invention, in a state in which the cover film composed of a carrier layer of a gas-impermeable substance and an anti-contamination layer is held in close contact with a sublimation thermal-transfer hard copy;

FIG. 3 is a longitudinal sectional view of a third embodiment of the cover film of the present invention, in a state in which the cover film composed of a layer of a gas-impermeable sheet material and a layer of an anti-contamination sheet material is held in close contact with a sublimation thermal-transfer hard copy;

FIG. 4 is an illustration of a sack formed of a cover film in accordance with the present invention;

FIG. 5 is an illustration of an example of use of the cover film of the present invention, wherein a sublimation thermal-transfer hard copy is sandwiched between the cover film and a base paper sheet to which is cover film is fixed; and

FIGS. 6, 7, 8, 9, 10 and 11 are longitudinal sectional views of cover films of fourth, fifth, sixth, seventh, eighth and ninth embodiments held in close contact with hard copies.

FIG. 1 shows a first embodiment of the cover film 1 of the present invention composed of a gas-impermeable layer 11 and a carrier layer 12 of an anti-contamination substance, the cover film being superposed on an image-receiving layer 21 of a sublimation thermal transfer hard copy 2 in contact therewith.

The illustrative sublimation thermal-transfer hard copy was formed by sublimation thermal transfer process conducted with an ink sheet and an image-receiving sheet 22 in accordance with the following procedure.

(Production of sublimation thermal-transfer hard copy)

(1) Preparation of ink sheet

A mixture liquid was prepared to have the following composition:

The mixture liquid was stirred in a bowl mill for 20 hours for dispersion, and was applied by means of a wire bar to a polyester film (carrier sheet) 6 .mu.m thick, so that a sublimation dyestuff layer 1 .mu.m thick was formed.

A silicone resin layer 0.5 .mu.m thick was formed as a heat-resistant layer on the side of the carrier sheet opposite to that having the sublimation dyestuff layer.

(2) Preparation of image-receiving sheet

A coating liquid was formed to have the following composition:

The coating liquid thus prepared was applied by a wire bar to a carrier sheet which was a synthetic paper (Corpo: produced by Ohji Yuka Kabushiki Kaisha, 130 .mu.m thick), followed by 5-hour drying at 80.degree. C., whereby an image-receiving sheet having a dyestuff image-receiving layer of 3 .mu.m was obtained.

(3) Thermal-transfer recording

The ink sheet mentioned above was superposed on the image-receiving sheet such that the sublimation dyestuff layer contacted the image-receiving layer, and a thermal head (0.2 W/dot) was pressed onto the ink sheet while electrical power was supplied to the thermal head for a period of 10 msec, whereby a red image having an image density of 1.6 was obtained. The thus obtained sublimation thermal-transfer copy is common to the embodiments of the invention described hereinafter.

To prepare the aforementioned cover film of FIG. 1, a coating liquid was applied by a wire bar to a sheet 12 of cellophane 40 .mu.m thick serving as a carrier layer of anti-contamination material. The coating was then dried to form a gas-impermeable layer 11 of 5 .mu.m thickness and capable of absorbing ultraviolet rays, whereby the cover film was obtained. The composition of the coating liquid was as follows:

The following experiment was conducted on this cover film.

(Anti-contamination and shelving tests)

The cover film 1 was laid on the sublimation thermal-transfer hard copy 2 such that the cellophane layer of the cover film 1 closely contacted the image-receiving layer 21 of the hard copy 2. Then, a sheet of white paper was laid on the gas-impermeable layer 11. The hard copy 2 with cover film 1 was shelved in an oven at a temperature of 50.degree. C. under application of a pressure of 10 g/cm.sup.2 on the white paper.

A test also was conducted in which the cellophane of the cover film 1 was held in close contact with the image-receiving layer 21 at 50.degree. C. and in an atmosphere of RH 80%. A test also was conducted in which the laminate of the hard copy 2 and the cover film 1 with the cellophane layer of the cover film 1 closely contacting the image-receiving layer 21 was exposed to outdoor solar light throughout one month (October). Change in the density of the sublimation thermal-transfer image density was examined after the shelving and exposure, and the results are shown in Table 1.

FIG. 2 shows a second embodiment of the cover film 1 of the present invention having an anti-contamination layer 14 formed on a carrier layer 13 of gas-impermeable material, the cover film 1 being held in close contact with the image-receiving layer 21 of the sublimation thermal-transfer hard copy 2.

A liquid having the following composition was prepared.

The liquid was applied by a wire bar to a polyvinyl chloride film (carrier layer of gas-impermeable substance) 50 .mu.m thick containing an ultraviolet ray absorbing substance, whereby a cover film 1 having an anti-contamination layer 14 of 5 .mu.m thickness was obtained. The thus-obtained cover film 1 was subjected to the same tests as those conducted on the first embodiment to obtain results as shown in Table 1.

FIG. 3 shows a third embodiment of the cover film 1 of the present invention having a layer 13 of a gas-impermeable material and a layer 12 of an anti-contamination material, the cover film 1 being laid on the image-receiving layer 21 of the sublimation thermal-transfer hard copy 2 in close contact therewith.

This cover film was prepared by bonding a cellophane sheet 12 of 40 .mu.m thickness to a polyvinyl-chloride film 13 having a thickness of 50 .mu.m and containing an ultraviolet ray absorbing agent. In this embodiment, bonding of the sheets 12 and 13 is not essential and these sheets may simply be superposed depending on use. This embodiment was not tested because satisfactory performance of this embodiment can obviously be expected from the results of tests conducted on the first and second embodiments.

Comparison examples were prepared as follows, for the purpose of comparison with the cover film 1 of the invention.

A nylon film 15 .mu.m thick (Harden: produced by Toyobo) having excellent gas-impermeability was used as the cover film and tested in the same manner as the first embodiment.

A polypropylene film of 20 .mu.m thick (Torayfan: produced by Toray) having excellent gas-impermeability was used as the cover film and tested in the same manner as the first embodiment.

The same tests as those conducted on the first embodiment were carried out without using a cover film, by way of comparison.

The results of tests on these comparison examples also are shown in Table 1.

Symbol .smallcircle. represents no coloring, .DELTA. represents slight coloring and X represents heavy coloring.

The density figures show the preservation characteristic and represent image density after preservation of the sublimation thermal-transfer hard copy which had a density of 1.6.

From Table 1, it will be seen that there was no coloring of the cover films of the first and second embodiments, nor of the paper contacting the cover films. Thus, the films are usable repeatedly, by virtue of the provision of the anti-contamination layer and the gas-impermeable layer. It will also be seen that these cover films enable images to be preserved in good order. The cover film of the first comparison example does not allow coloring of the paper but tends to be colored easily. This cover film, therefore, cannot be used for other image samples. The cover film of the second comparison example exhibits a reduced tendency of being colored but is inferior in the gas-impermeability and, hence, tends to allow the sublimating dyestuff to permeate therethrough, resulting in coloring of the paper contacting this film.

FIG. 4 is an illustration of the cover film of the invention in the form of a sack. The sack 40 of the cover film has an inner surface constituted by an anti-contamination layer. The outer surface of the sack is provided by a gas-impermeable layer or a carrier layer carrying a gas-impermeable substance. A writable layer 41 is provided on the outer surface of the sack.

FIG. 5 shows one manner in which a cover film is fixed to a base paper and the sublimation thermal-transfer hard copy is inserted between the base paper and the cover film.

The cover film 51 is fixed to the base paper 50 such that the anti-contamination layer faces the base paper. The other surface of the cover film 51 is presented by a gas-impermeable layer or a carrier layer carrying a layer of gas-impermeable substance. Although not shown, a writable layer 41 may be provided on the outer surface of this cover film 51.

Fourth to ninth embodiments of the present invention will now be described.

In the fourth to ninth embodiments of the cover film of the present invention, the anti-contamination material, which is used as the material of the layers denoted by 31e, 33a, 33b and 33c in FIGS. 6 to 11, is selected from a thin film of an inorganic material, a thin film of a metal and a thin film of an organic polymeric material having a glass transition point Tg not lower than 80.degree. C.

Examples of the inorganic materials are metal oxides such as silicon oxide, indium oxide, titanium oxide and aluminum oxide. Examples of the metal are Al, Cu, Ag, Ni and so forth. Thin films of these materials can be formed by evaporation deposition or sputtering. Examples of the organic polymeric material having a glass transition temperature Tg not lower than 80.degree. C. are polystyrene, polycarbonate, acrylic resin, polyamide, polyimide, polyether sulfone and polyphenylenesulfide.

In the fourth to ninth embodiments of the cover film of the present invention, the gas-impermeable material, which is used as the material of the layers denoted by 31b, 32a and 32b in FIGS. 6 to 11, is an organic polymeric material having an oxygen permeability not greater than 150 cc/m.sup.2 -24 hr-atm/25 .mu.m (=5.7.times. 10.sup.-13 cm.sup.3 .multidot.cm/cm.sup.2 .multidot.sec.multidot.cm Hg) and a steam permeability not greater than 100 g/m.sup.2 -24 hr/25 .mu.m (=3.8.times.10.sup.-13 cm.sup.3. cm/cm.sup.2 .multidot.sec.multidot.cm Hg). Examples of such an organic polymeric materials are polyvinylidene chloride, polyester, nylon, polyvinyl chloride, polyvinyl fluoride, polyacrylonitrile, and so forth.

The transparent carriers used in the fourth, sixth and eighth embodiments, denoted by 31a, 31c and 31d in FIGS. 6, 8 and 9, are generally made of plastic films and are not required to have anti-contamination or gas-impermeable characteristics.

The fourth to ninth embodiments of the cover film of the present invention can be produced by various methods. For example, the cover film can be formed by sequentially applying a solution containing a gas-impermeable material and a solution containing anti-contamination material to a surface of a transparent substrate so as to laminate the gas-impermeable layer and the anti-contamination layer.

The film also can be produced by bonding a layer of an anti-contamination sheet material to a layer made of a gas-impermeable sheet material, or by using a carrier sheet of either one of a gas-impermeable material and an anti-contamination material, while applying thereto a solution of the other material. (Production of Sublimation Thermal-transfer Hard Copy)

This was done in the same way as for the first to third embodiments.

This was done in the same way as for the first to third embodiments.

As in first to third embodiments, the ink sheet and the image receiving sheet were superposed such that the sublimation dye layer of the ink sheet and the dye image receiving layer of the image receiving sheet contacted each other, and a thermal head (0.22 W/dot) was pressed across the ink sheet. Electrical power was supplied for 10 msec to effect a thermal-transfer recording, thus obtaining a red color image having an image density of 1.7.

FIG. 6 shows the fourth embodiment of the cover film 1 held in close contact with the image receiving layer 21 of the sublimation thermal-transfer hard copy 2. The cover film 1 has a transparent carrier 31a, and an ultraviolet-absorbing gas-impermeable layer 32a and an organic polymeric anti-contamination layer 33a formed by application of solutions.

More specifically, the cover film 1 shown in FIG. 6 was formed by the following process.

A coating solution was prepared to have the following composition:

The coating solution thus prepared was applied by a wire bar on a polypropylene film (transparent carrier 31a) 40 .mu.m thick and was then dried to form an ultraviolet-absorbing gas-impermeable layer (ultraviolet-absorbing gas impermeable layer 32a) 2 .mu.m thick.

Then, a coating solution having the following composition was applied to the surface of the ultraviolet-absorbing gas-impermeable layer 32a and dried so as to form an anti-contamination layer (organic polymeric anti-contamination layer 33a) 2 .mu.m thick:

The cover film 1 was superposed on the sublimation thermal-transfer hard copy 2 such that the image receiving layer 21 of the hard copy 2 and the anti-contamination layer 33a of the cover film 1 closely contacted with each other. Then, a white paper was superposed on the polypropylene film (transparent carrier 31a) of the cover film and a load of 10 g/cm.sup.2 was applied to the white paper. The hard copy 2 and the cover film 1 were held in this state for 1 month in an oven maintaining an atmosphere of 50.degree. C. for the purpose of examination of the anti-contamination characteristic.

A shelving test also was conducted in which the sublimation thermal-transfer hard copy 2 and the cover film 1 were shelved for 1 month in an atmosphere of 50.degree. C., 80%RH, with their image receiving layer 21 and the anti-contamination layer 33a held in close contact with each other, as was a test in which the sublimation thermal-transfer hard copy 2 and the cover film 1 were held in contact with each other were subjected to 30-hour exposure by, a Fadeometer. The test results are shown in Table 2.

FIG. 7 shows a fifth embodiment of the cover film 1 held in close contact with an image receiving layer 21 of a sublimation thermal-transfer hard copy 2, the cover film 1 being composed of an organic polymeric anti-contamination layer 33a and an ultraviolet-absorbing gas-impermeable transparent carrier layer 31b. The cover film 1 shown in FIG. 7 was prepared by the following process.

A coating solution was prepared to have the following composition:

The coating solution was applied by a wire bar to a polyvinyl chloride film 50 .mu.m thick containing an ultraviolet-absorbing agent (ultraviolet-absorbing gas-impermeable transparent carrier 31b) and was dried to form an anti-contamination layer (organic polymeric anti-contamination layer 33a) 3 .mu.m thick.

This cover film was tested in the same manner as the fourth embodiment.

FIG. 8 shows a sixth embodiment of the cover film 1 of the present invention held in close contact with an image receiving layer 21 of a sublimation thermal-transfer hard copy 2. The cover film 1 has a transparent carrier layer 31c, and an ultraviolet-absorbing gas-impermeable layer 32a and a metallic thin film forming an anti-contamination layer 33b which were formed sequentially on the transparent carrier 31c by application of solutions.

The cover film 1 shown in FIG. 8 was prepared by forming, on a polyester film 38 .mu.m thick, an anti-contamination layer of Al (anti-contamination thin metallic film layer 33b) of 200 .ANG. by evaporation deposition. An ultraviolet-absorbing gas-impermeable layer 32a the same as that used in the fourth embodiment was formed on the opposite side of the polyester film. The thus-formed cover film was tested in the same way as the fourth and fifth embodiments.

FIG. 9 shows a seventh embodiment of the cover film 1 of the present invention. The cover film 1 of this embodiment was formed by providing an ultraviolet-absorbing gas-impermeable layer 32b formed on a nylon film (transparent carrier 31d) 40 .mu.m thick, and then forming, on the ultraviolet-absorbing gas-impermeable layer 32b, an anti-contamination layer (anti-contamination layer 33c of inorganic material) 1 .mu.m thick by vacuum evaporation. The ultraviolet-absorbing gas-impermeable layer 32b was formed in the same manner as that used in the fourth embodiment except that the 2-hydroxy-4-methoxy-benzophenone used in the fourth embodiment was substituted by 2(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzotriazole. The thus-formed cover film was tested in the same way as the preceding embodiments.

FIG. 10 shows an eighth embodiment of the present invention. The cover film 1 of this embodiment was produced by providing, on a polyphenylene, sulfide film (anti-contamination transparent carrier layer 31e) 10 .mu.m thick, an ultraviolet-absorbing gas-impermeable layer 32b formed in the same manner as that used in the seventh embodiment. The thus-formed cover film was tested in the same way as the preceding embodiments.

FIG. 11 shows the ninth embodiment of the cover film 1 of the present invention. The cover film 1 of this embodiment was produced by bonding a polyvinyl chloride film containing an ultraviolet-absorbing agent (ultraviolet-absorbing gas-impermeable transparent layer 31b) and having a thickness of 50 .mu.m with a polyphenylene sulfide film (anti-contamination transparent layer 31e) 10 .mu.m thick. These films need not always be bonded together. That is, in some uses, these films may be merely superposed without being bonded. The ninth embodiment of the cover film 1 was not tested because it was considered that the results of testing this film are predictable from the results of the tests of the fifth and eighth embodiments.

The following cover films were prepared as comparison examples, in addition to the first to third comparison examples previously discussed, for the purpose of comparison with the cover films of the fourth to eighth embodiments.

A polyester film of 25 .mu.m having a high gas-impermeability was used as a cover film and tested in the same way as the fourth embodiment.

A polyvinylalcohol film of 20 .mu.m having superior anti-contamination characteristic was used as a cover film and tested in the same way as the fourth embodiment.

Table 2 shows the results of the tests conducted for the purpose of examining the anti-contamination and shelving characteristics of the cover films of the fourth to eighth embodiments and the third to fifth comparison examples.

In Table 2, the same symbols are used to denote the same degrees of coloring contamination as were used in Table 1. The values of density show the shelving characteristic and indicate image density after preservation of the sublimation thermal-transfer hard copy which had a density value of 1.7.

As will be seen from Table 2, in the cases of fourth to eighth embodiments, coloring of the cover film was prevented and contamination of the paper contacting the sublimation thermal-transfer hard copy was avoided. In addition, the shelving characteristic of the image was superior.

From Table 2, it will also be appreciated that the cover film of the fourth comparison example provided a good anti-contamination characteristic; but this cover film itself was colored and, hence, cannot be used repeatedly for different sublimation thermal-transfer hard copies. Furthermore, the cover film of the fifth comparison example was permeable to steam so that it undesirably allowed image degradation under conditions of high-temperature and moisture, although the coloring of the film itself was not noticeable. The third and fourth comparison examples were inferior in anti-light characteristic, although they provided greater resistances to heat and moisture as compared with the case where no cover film was used, as in the fifth comparison example.

Thus, the fourth to ninth cover films of the present invention offer the following advantages.

Coloring of the cover film is prevented by virtue of the anti-contamination layer. This means that no trace of the sublimation thermal-transfer hard copy remains on the cover film so that the cover film can be used repeatedly for covering different sublimation thermal-transfer hard copies. In addition, the ultraviolet-absorbing gas-impermeable layer prevents dye re-sublimated from the sublimation thermal-transfer hard copy from penetrating the cover film, thus avoiding contamination of objects which have been brought into contact with the hard copy. In addition, fading of the image is suppressed because invasion by external gases such as steam, oxygen and ozone is suppressed and ultraviolet rays are absorbed.

The cover films, when put to practical use, are treated and processed in the manner shown in FIGS. 4 and 5, as in the cases of the first to third embodiments.

As has been described herein, it is possible according to the present invention to obtain a cover film which has anti-contamination and gas-impermeable characteristics, and which is also capable of absorbing ultraviolet rays, thus greatly facilitating storage and administration of sublimation thermal-transfer hard copies.

It is also possible to write data concerning the hard copy when a writable layer is provided, thus further facilitating administration and preservation of the hard copies.

The storage and administration of sublimation thermal-transfer hard copies are further facilitated when the cover film is formed as a sack or fixed to a base paper.