COVER FILM FOR HARD COPY PRINTING PAPER

Abstract: A cover film for hard copy printing paper, which is used in transfer-forming a colorless clear protection layer that inhibits the passage of ultraviolet rays onto the surface of a hard copy printing paper (4) such as color copy-printed paper obtained by transferring a transfer paper to which are applied sublimable dyestuffs onto a printing paper. The feature of the cover film resides in that at least a layer (2, 3) is formed on a substrate (1), the layer (2, 3) having a glass transition temperature of higher than 40°C, and a coefficient of water absorption of less than 2% (as evaluated in accordance with ASTMD 570 by immersing a film which is 0.01 in. thick in water at 23°C fo 24 hours). The protection layer formed by using the cover film of the present invention prevents adverse effects caused by the migration of dyes­ tuffs of the printing paper under high-temperature and high-humidity conditions.

Description: TECHNICAL FIELD The present invention relates to a cover film for hard copy printing paper which can easily form a colorless and transparent protecting film on the surface of a printing paper on which the printing was carried out. More particularly, this invention relates to a cover film for hard copy printing paper which can form, on the surface of a color copy that is made by transferring dye from a dye carrier paper made by using a sublimation dye to a printing paper, a protecting layer by heating and pressing a base material having thickness and strength relatively easy to handle to thereby transfer therefrom a thin and transparent resinuous layer impervious to ultraviolet rays which avoids color fading and less in curl. BACKGROUND ART A cover film for a colour hard copy printing paper is already known from EP-A-103 024. Said cover film comprising a heat-resistant base and a cover material layer including a colourless and transparent ultra-violet ray shielding layer which is formed on said heat-resistant base and has no adhesive property upon heating for said heat-resistant base and is melted and bonded to a surface of a printing paper by heating. Generally, the printing using a color hard copy printing paper is carried out, in which a dye carrier paper coated with an ink made of a sublimation dye is heated by a thermal print head to sublimate the dye formed on the necessary portion, whereby a color picture image is formed on the printing paper. However, the picture image formed by this sublimation transfer method has the following problems. Although at first the sublimated dye is adsorbed by the surface of the printing paper, it is not diffused well so that it is left as an aggregation of a part of the dye, and hence the sublimated dye can not be developed as it is expected to be. Further, the dye is again aggregated, crystallized, discolored or the dye is transferred to another material and thereby it is smudged To solve these problems, there are proposed in the art the following methods but these methods have their own defects. (a) As to the means to coat the printing paper with a resinous liquid, the picture image is blurred. Thus, the printing paper must be dried and the printing paper is difficult to provide a smooth surface. Further, the resinous liquid is troublesome to handle. (b) when such a method to laminate a polyethylene telephthalate film having a hot melt adhensive agent layer on the printing paper is employed, the film becomes easy to curl, to take in dusts, to be bent or the adhensive agent is apt to overflow. Further, if they are set incorrectly upon laminating, the laminating apparatus is smudged or the film is caught in the laminating apparatus and thus the handling of this film is cumbersome. (c) When such means to sandwich the color hard copy between the polyethylene telephthalate films having a hot melt adhensive agent layer is employed, there may occur problems such as the appearance of the film is damaged and a bubble is produced in the film by the water component contained in the paper. To solve these problems, there is proposed a transfer type thin cover film for color hard copy printing paper. However, since this cover film for color hard copy printing paper is thin, there occur troubles that when the printed color hard copy covered by the cover film is left in the inside of a car in summer, near a window in summer or is used under the conditions of high temperature and high humidity, the dye is moved to the surface of the cover film and thereby an album for preserving pictures and other color picture images are smudged. Although this phenomenon is not yet explained explicitely, it may be considered that a very small amount of dye dissolved or dispersed into the water component in the resin gets easy to move to the surface of the film together with water component or that the glass transition temperature of resin is lowered by the water component. In view of the above mentioned aspects, the present invention is intended to provide a cover film for hard copy printing paper which can remove a bad influence exerted by the transfer of the dye even under the condition of high temperature and high humidity. DISCLOSURE OF INVENTION According to the cover film for hard copy printing paper of the present invention, the cover film for hard copy printing paper formed on the surface of the hard copy printing paper printed by the sublimation transfer is designed to include at least one layer that is formed under the conditions that the glass transition temperature is selected to be higher than 40°C and that the water absorbing ratio (ASTM D570) is selected to be lower than 2% to thereby remove a bad influence exerted by the movement of the dye even at high temperature and high humidity. BRIEF DESCRIPTION OF DRAWINGS The single Figure is a cross-sectional view showing an embodiment of a cover film for hard copy printing paper according to the present invention. Tables I and II are useful for explaining the invention. BEST MODE FOR CARRYING OUT THE INVENTION [Example 1] First, a color print was formed as follows. A coating composition consisting of 24 parts by weight of internally-plasticized and saturated polyester resin (VYLON #200, manufactured by Toyobo Co., Ltd.), 6 parts by weight of super fine particle silica (NIPSIL E220A, manufactured by Nippon Silica Industrial Co., Ltd.) and 70 parts by weight of methy ethyl ketone solvent was coated on one surface of a free sheet having a measured weight of 170g/M² so as to have a coating weight of about 5g/M² after being dried, thus a printing paper for sublimation transfer type color hard copy being prepared. While, a magenta color ink represented by an ink consisting of 6 parts by weight of anthraquinone type dispersion dye (PTR63, manufactured by Mitsubishi Chemical Industries Co., Ltd.) having a sublimation property, 6 parts by weight of ethyl cellulose and 88 parts by weight of isopropyl alcohol solvent was made and other like cyan color ink, yellow color ink and black color ink were made similarly. Then, they were respectively coated on a paper having a measured weight of 40g/M² by using a gravure coater so as to have a coating weight of 5g/M² after being dried, thus the dye carrier papers of 4 colors being prepared. Then, when the dye carrier paper was heated from the back side thereof by a thermal print head heated at about 300°C, 4 colors were sequentially printed on the above mentioned printing paper, thus a color print 4 being made. Next, on a polyethylene telephthalate film 1, 30 µm thick, there were formed polysulfon resin (POLYSULFON P1700NT, manufactured by Nissan Chemical Industries Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 190°C and 0.3%) having a thickness of 5 µm as a first layer 2 and an internally-plasticized polyester resin (VYLON #300, manfactured by Toyobo Co., Ltd.) having a thickness of 10 µm as a second layer 3, thus a cover film for color hard copy printing paper of this example being made. In this case, the water absorbing ratio was measured according to the evaluation method of the ASTM D570 in which a film 0.01 inch thick was immersed into the water at 23°C for 24 hours. Then, the cover film for color hard copy printing paper was pressed on the color print 4 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 as the base material was peeled off therefrom. [Example 2] On a polyethylene telephthalate film 1, 30 µm thick, there were formed the first layer 2 by a polystyrene resin having a thickness of 5 µm (STYRON 660, manufactured by Asahi Dow Chemical Co., Ltd. and the glass transition temperature and water absorbing ratio of which were respectively selected to be 100°C and 0.05%) and the second layer 3 by nylon having a thickness of 10 µm (CM8000 having a glass transition temperature of 40°c and a water absorbing ratio of 3.5%), thus a cover film for color hard copy printing paper of this example being made. The resultant cover film for color hard copy printing paper was pressed on the color print formed same as that of the example 1 by a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. [Example 3] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there were formed the first layer 2 by a poly vinylidenechloride resin (SARAN R202 the glass transition temperature and water absorbing ratio of which were respectively selected to be 0°C and lower than 0.1%) having a thickness of 5 µm and the second layer 3 by an internally-plasticized polyester resin having a thickness of 10 µm (VYLON #103, manufactured by Toyobo Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 47° C and 0.8%), thus a cover film for hard copy printing paper in this example being made. The resultant cover film for hard copy printing paper was pressed on the color print formed same as that of the example 1 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. [Example 4] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there were formed the first layer 2 by cellulose acetate butyrate having a thickness of 5 µm (Cellidor BH, manufacture by Bayer A.G., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 120°C and 2.4%), the second layer 3 by polymethyl methacrylate resin having a thickness of 10 µm (Paralloid A-11, manufactured by Rhome and Hearth Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 100°C and 0.4%) and though not shown a third layer by polyester resin (VYLON #300, the glass transition temperature and water absorbing ratio of which were respectively selected to be 7°C and 0.8%), thus a cover film for color hard copy printing paper being made. The resultant cover film for hard copy printing paper was pressed on the color print formed same as that of the example 1 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. [Example 5] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there were formed the first layer 2 by phenoxy resin having a thickness of 5 µm (PHENO TOHTO YP-50, manufactured by Tohto Kasei Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 100°C and 1.5%) and the second layer 3 by ethyl cellulose resin (ETHYL CELLULOSE N, manufactured by Hercuries Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 43°C and 5%), thus a cover film for color hard copy printing paper in this example being made. The thus made cover film for color hard copy printing paper was pressed on the color print same as that of the example 1 by using the press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. To prove the effects of these examples, comparative examples 1, 2 and 3 were made. [Comparative example 1] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there were formed the first layer 2 by polyvinylidene chloride type resin having a thickness of 5 µm (SARAN RESIN R202, manufactured by Asahi Dow Chemical Co.,Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 0°C and lower than 0.1%) and the second layer 3 by internally-plasticized polyester resin having a thickness of 5 µm (VYLON #300, manufactured by Toyobo Co., Ltd., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 7°C and lower than 0.8%), thus a cover film for color hard copy printing paper of this comparative example being made. The resultant cover film for color hard copy printing paper was pressed on the same color print as that formed in the example 1 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. [Comparative example 2] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there was formed a nylon 6-line resin having a thickness of 5 µm (NYLON CM8000 manufactured by Toray Industries Inc., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 40°C and lower than 3.5%) thus a cover film for color hard copy printing paper in this comparative example being formed. The resultant cover film for color hard copy printing paper was pressed on the same color print as that formed in the example 1 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. [Comparative example 3] On the polyethylene telephthalate film 1 having a thickness of 30 µm, there was formed a layer made of cellulose acetate butyrate having a thickness of 5 µm (Cellidor BH, manufactured by Bayer A.G., and the glass transition temperature and water absorbing ratio of which were respectively selected to be 120°C and 2.4%) as the first layer 2 and a layer made of polyester having a thickness of 10 µm (VYLON #300 the glass transition temperature and water absorbing ratio of which were respectively selected to be 7°C and 0.8%) as the second layer 3, thus a cover film for color hard copy printing paper in this comparative example being formed. Then, the resultant cover film for color hard copy printing paper was pressed on the same color print as that formed in the example 1 by using a press plate heated at about 150°C and the polyethylene telephthalate film 1 was peeled off. Then, how much the dye was transferred to the above mentioned comparative example 1, comparative example 2, comparative example 3, and the example 1, example 2, example 3, example 4 and example 5 was evaluated by the method mentioned below. The treated surface of the printing paper was superposed on the surface of the cover film for color hard copy printing paper. Then, they were sandwiched between perforated plates made of vinyl chloride (the thickness was 2.5mm, the diameter of hole was 3mm and the arranging pitch of holes was 7mm) and thereby applied with a load of 30g/cm². Next, a product was subjected to the aging treatment by using bath (the temperature and relative humidity of which were selected to be 40°C and 95% and, 65°C and 80%) for one week. After the aging was ended, the treated surface of the printing paper, superposed on the cover film, was observed and checked whether the dye on the surface of the printing paper or the dye used for printing was moved up to the film surface through the cover film for hard copy printing paper or not by eyes. The results will be evaluated with reference to Tables I and II. In Tables I and II, marks ○ indicate the fact that such movement of the dye did not occur and that the cover film for hard copy printing paper could achieve the effect for preventing the dye from being moved. Whereas, marks X indicate the fact that the dye was moved up to the treating surface of the printing paper which was superposed for evaluating the movement of the dye. As will be clear from Tables I and II, since in the comparative example 1, comparative example 2 and comparative example 3, the resinous layer having the glass transition temperature higher than 40°C and the water absorbing ratio lower than 2% was not formed on the cover film for color hard copy printing paper even one layer, the dye was moved. While, since in the example 1, example 2, example 3, example 4 and example 5 the polysulfon resin, the polyethylene resin, the SARAN resin, the polymethyl methacrylate resin and the phenoxy resin respectively satisfied the conditions of the glass transition temperature higher than 40°C and the water absorbing ratio lower than 2%, it could be considered that the dye was not moved. The reason why the dye was moved in the cover film for color hard copy printing paper made by the example 3 in which the cover film for color hard copy printing paper was subjected to the aging treatment for one week under the conditions that the temperature was 65°C and the relative humidity 85% is that the glass transition temperature of the internally-plasticized polyester resin (VYLON #103, manufactured by Toyobo Co., Ltd) itself is 47°C, which is lower than the temperature for aging treatment. Although the cover film for color hard copy printing paper in this example has a little fear that the dye will be moved when it is used in the inside of a car in summer, it can be used in practice anyhow. As described above, according to the examples of the present invention, since the cover film for color hard copy printing paper is designed to include the layer having the glass transition temperature higher than 40°C and the water absorbing ratio lower than 2% (ASTM D570), it is possible to prevent the dye used for printing from being moved up to the surface of the cover film for color hard copy printing paper therethrough. Accordingly, in addition to the ordinary advantages of having a smooth surface, being easy to handle and being able to protect the surface of the printing paper with the thin film, the cover film for color hard copy printing paper has a further advantage that under the conditions of high temperature and high humidity, the dye can be prevented from being moved to the surface of the cover film. The polyethylene telephthalate film 1 as the base material may be a heat-resistant plastic film and can use films, such as polyester film, acetate film, polyimide film, polypropylene film and fluorine-line film. The thickness of the base material is preferally selected in a range from 3 to 100 µm. The thickness of the first layer 2 is preferably selected in a range from 1 to 100µm and preferably has no adhering property to the base material but the first layer may be treated so as to be peeled off from the base material at its surface on which it is oppposed to the base material. The second layer 3 is preferably made of a resin which has high absorbing property for the dye and the second layer can use resins, such as cellulose propionate and the like. Further, considering the inside of a car and the like in summer, it is more preferable to select the glass transition temperature thereof to be higher than 70°C. A discoloring agent, an ultra-violet ray absorbent and a phosphor whitener may be added thereto.

Claims: A cover film for hard copy printing paper formed on the surface of a hard copy printing paper on which a picture image is printed according to the sublimation transfer method, said cover film for hard copy printing paper being characterized in that said cover film includes at least one layer made of resinous material having a glass transition temperature higher than 40°C and a water absorbing ratio (ASTM D570) lower than 2%.

Inventors: ABE, TETSUYA SONY CHEMICALS CORPORATION, FUJIWARA, YOSHIO SONY CHEMICALS CORPORATION, KOBAYASHI, NAOTAKE SONY CHEMICALS CORPORATION, SHINOHARA, SATORU SONY CHEMICALS CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: B41M5
IPC codes: B41M7, B41M5