PLASTIC FILM FOR THERMAL TRANSFER RECORDING MEDIA

Abstract: A plastic film is suitable for the thermal transfer recording media and comprises a composition comprising 100 parts by weight of a polyolefin and 0.1 or more part by weight of a substance which can be crosslinked by irradiation, said com­ position having been treated for crosslinking with irradiation by a radiation.

Description: PLASTIC FILM FOR THERMAL TRANSFER RECORDING MEDIA This invention relates to a plastic film which is used as a support which is one component of a thermal transfer recording medium used in typewriters, copying machines, facsimiles, etc., having a thermal transfer recording system. Statement of Prior Arts There are already known thermal transfer recording media which are composed of a support and a normally solid colorant applied thereto and capable of softening and melting at 500c to 1400c and which are heated from the lower surface side of the support to allow said colorant to soften, melt and transfer to a recording body. The supports conventionally used in these recording media include plastic films such as polyethylene terephthalate, cellophane, polycarbonate, triacetylcellulose and nylon, and papers such as wood-free paper, glassine paper and tracking paper. However, plastic films have a drawback that because the support is in contact with a thermal head under pressure, it sticks to the thermal head by melting, i.e. the so-called sticking phenomenon occurs so that the thermal transfer recording medium cannot be fed smoothly. On the other hand, papers have a drawback that their dimensional stability is not sufficient because they are influenced by moisture. A proposal was made of a thermal transfer recording medium in which a water-insoluble, heat-resistant protective layer comprising a resin selected from the group consisting of silicone resins, epoxy resins, melamine resins, phenolic resins, fluorocarbon resins, polyimide resins, and nitrocellulose is applied to the lower surface of the support in order to improve the drawbacks of plastic films, such as sticking (see Japanese Patent Laid-Open No. 7467/1979). Further, another proposal was made of a thermal transfer record ing medium in which a water-soluble, heat-resistant protective layer comprising a member selected from the group consisting of cellulose derivatives such as hydroxyethylcellulose, ethylcellulose, methylcellulose, and carboxymethylcellulose, polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, starch and its derivatives, sodium alginate, casein, gelatin, shellac, styrene/maleic anhydride copolymers, etc., was applied to the lower surface of the support (see Japanese Patent Laid-Open No. 194893/1984). However, the provision of the above heat-resistant protective layer necessitates a step of coating or laminate coating, so that the production process for such a thermal transfer recording medium is complicated and has a drawback such as a high cost. On the other hand, when a water-insoluble, heatresistant substance in the form of an organic solvent solution is applied, it is necessary to recover the organic solvent, and when the substance is applied by laminate coating, there is a drawback that the reduction-of its film thickness is difficult. On the other hand, when a water-soluble heat-resistant substance is used, the support need be subjected to a surface treatment such as'corona discharge or vacuum glow-discharge for lack of its adhesion to the substance, so that there is, for example, a drawback that the production process becomes more complicated. Besides, when the heat-resistant protective layer is applied to the support, the total thickness of the support is increased, which adversely affect its thermal conductivity. Summary of the Invention It is an object of this invention to provide a heat-resistant plastic film which can be easily produced and undergoes no sticking phenomenon during its use. A plastic film of the invention is suitable for the thermal transfer recording media and comprises a composition comprising 100 parts by weight of a polyolefin and 0.1 or more part by weight of a substance which can be crosslinked by irradiation, said composition having been treated for crosslinking with irradiation by a radiation. It is preferable that said composition comprises from 0.1 to 50 parts by weight of said #substance. The plastic film of the invention is normally obtained by blending said polyolefin and said substance, moulding the blend into film and treating the film with irradiation. The substance which is crosslinkable with radiation is an ethylenically unsaturated compound or polymer. Thus, this invention provides a plastic film for thermal transfer recording media, which is characterized by being prepared by molding a composition obtained by mixing 100 parts by weight of a polyolefin resin with at least 0.1 part by weight of at least one compound which can form cross linkages by irradiation with a radiation and having cross linkages formed by irradiation with a radiation. The plastic film of this invention can be produced by a process in which a polyolefin resin is kneaded together with a compound or compounds which can form crosslinkages by irradiation with a radiation, the mixture is formed into a plastic film by a T die ex truder or an inflation extruder which is a general molding machine for plastic films and the formed film is irradiated with a radiation. Alternately, it can be produced by a process in which a mixture after kneading is irradiated with a small dose of a radiation, the irradiated mixture is formed into a film, and this film is irradiated again with a radiation, or a process in which a colorant layer is applied to the above unirradiated polyolefin film and this film is irradiated with a radiation, though the production process of the film of this invention is not limited thereto. However, because it is impossible to mold a resin composition having crosslinkages which are preformed to such a degree that the object of this invention can be attained into a thin film by a usual molding process, the production process in which the mixture after said kneading is formed into a film after it is sufficiently irradiated with a radiation is not suitable for practicing this invention. Examples of the polyolefin resins which can be used in this invention include homopolymers such as polyethylene, polypropylene, polybutene, polyisobutylene, polypentene, and polyhexene, and copolymers such as ethylene/propylene copolymers and ethylene/vinyl acetate copolymers, as well as graft polymers thereof and mixtures comprising at least two members selected from among the above homopolymers, copolymers and graft polymers. In addition, it is possible to use mixtures obtained by adding, if necessary, fillers such as reinforcements, lubricants, plasticizers, stabilizers, antioxidants, ultraviolet absorbers, antistatic agents, and pigments to the above polyolefin resins. The compounds which can form cross linkages by irradiation which can be used in this invention may be any compounds that have at least two ethylenically unsaturated bonds in the molecule, and examples of them include monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, tetramethylolmethane tetraacrylate, and tetramethylolmethane tetramethacrylate, polyacrylates or polymethacrylates such as epoxyacrylates resin, epoxymethacrylate resin, urethane acrylate resin, urethane methacrylate resin, novolac acrylate resin, novolak methacrylate resin, ester acrylate resin, ester methacrylate resin, amide acrylate resin, amide methacryalte resin, imide acrylate resin, and imide methacrylate resin, polyallyl compounds derived by replacing "acrylate" in the above compounds by "allyl", prepolymers such as styrene-modified products thereof, and polymers such as butadiene copolymers (or homopolymers), and isoprene copolymers (or homopolymers), and mixtures of these monomers, prepolymers and polymers can also be used. The reason why at least 0.1 part by weight is specified as the amount of the compound which can form cross linkages by irradiation with a radiation and is added as a radiation crosslinking agent to a polyolefin resin in this invention is that when this amount is smaller than the above, the degree of crosslinking by irradiation with a radiation is low and therefore the effect of radiation crosslinking can hardly be attained. With respect to the thickness of the plastic film for thermal transfer recording media of this invention, the thinner, the better, and it has a thickness of preferably about 3 ¯ 30 p. The radiation sources which can be used in this invention may be any ionizing radiations, and more particularly, radiations such as electron beams, neutron beams, a-rays, Rays, y-rays, X-rays, proton rays, and deuteron rays can be mentioned, for example. Among them, electron beams and X-rays are desirable. With respect to the dose of radiation, a dose of about 0.1 ¯ 50 Mrad will suffice. The plastic film of this invention is used as a support of a thermal transfer recording medium, while a heat-meltable colorant layer applied to the support may be formed from well-known, heat-meltable inks and are not particularly limited. Such a heatmeltable ink is a composition comprising a colorant, a binder, etc., and the thermal transfer recording medium is produced by forming an about liy 20 p-thick colorant layer on a support by hot melt coating of said heat-meltable ink or by solvent coating of a coating solution formed by dispersing said composition in a suitable solvent. The colorants which can be used include dyes and pigments, such as carbon black, which haveiheretofore been used in the field - of most of dry toners for copying machines. Examples of the binders include waxes such as Carnauba wax Japan wax, and bee wax. Further, the heat-sublimable solid colorant layer can be formed from well-known sublimable inks as such, and are not particularly limited. Such sublimable inks are compositions comprising a sublimable dye suitably selected from among monoazo dyes, anthraquinone dyes, nitrodiphenylamine dyes, etc., a binder, a dispersant, etc., and a thermal transfer recording medium is produced by forming a colorant about 1 ¯ 20 p-thick on a support by solvent coating of a coating solution formed by dispersing a sublimable ink in a suitable solvent. [ Example ] Examples and comparative examples of this invention will now be described in further detail, though this invention should not be limited to these examples. The degree of crosslinking formed is represented by the measured gel fraction of a polymer. This gel fraction was determined as follows. A polymer was extracted with xylene at 1000C for 24 hr, and withdrawn from the solvent and dried. The gel fraction òf the plastic film of this invention is preferably 50 % or above, particularly preferably 60 % or above. weight of sample after extraction gel fraction (%) = ------ x 100 weight of sample before extraction The plastic film for thermal transfer recording media of this invention has high heat resistance. A thermal transfer recording medium produced by using this film as a support undergoes no sticking phenomenon at a high surface temperature of a thermal head, because the heat resistance of the support is high. Therefore, it is. effective in that it can suitably cope with the speed-up of a thermal printer or the like. In the following examples, parts are all parts by weight. Ex=..pless 1 100 parts of high-density polyethylene (Hi-Zex 7000F, a product of Mitsui Petrochemical Industries, Ltd.) and 10 parts of a bisphenol methacrylate resin were kneaded together in a roll mill, ground, and fed to a T die extruder to obtain a 10 p-thick plastic film. This film was irradiated with electron beams (10 Mrad) from an electron beam irradiator (Curetron, a product of Nisshin High-Voltage Co., Ltd.) to obtain a plastic film for thermal transfer recording medium. A film not subjected to the electron beam irradiation was used as Comparative Example 1. Comparative Example 2 High-density polyethylene (Hi-Zex 7000F) was fed to a T die extruder to mold a 10 p-thick plastic film, and this film was irradiated with electron beams 11Q Mrad) from an electron beam irradiator to obtain a crosslinked polyethylene film. Example 2 100 parts of low-density polyethylene (NUC8012, Åa product of Nippon Unicar) and 15 parts of 1,2-polybutadiene resin (RB820, a product of Nippon Gosei Co., Ltd.) were mixed together and fed to an inflation molding machine to obtain an 8 p-thick plastic film. This film was irradiated with electron beams (10 Mrad) from an electron beam irradiator to obtain a plastic film for thermal transfer recording media. A film not subjected to the electron beam irradiation was used as Comparative Example 3. Example 3 100 parts of ethylene/vinyl acetate copolymer (NUC-3760, VA content of 18 %, a product of Nippon Unicar) and 5 parts of trimethylolpropane trimethacrylate were kneaded together, ground, and fed to a T die extruder to obtain a 12 p-thick plastic film. This film was irradiated with electron beams (10 Mrad) from an electron beam irradiator to obtain a plastic film for thermal transfer recording media. A film not subjected to the electron beam irradiation was used as Comparative Example 4. Comparative Examples 5 and 6 A 10% aqueous solution of polyvinyl alcohol, which was a water-soluble, heat-resistant resin, was applied to corona discharge-treated polyester films to form supports for thermal transfer recording media, having thicknesses of 0.5 p and 1.5 p, respectively. The 0.5 p-thick support was used as Comparative Example 5, and the 1.5 p-thick support was used as Comparative Example 6. Thermal transfer recording media (Examples 1 and 3, and Comparative Examples 1, 4, 5 and 6) were prepared by coating one side of each of the above Examples and Comparative Examples with a wax-type heat-meltable ink prepared by mixing the following components: carnauba wax 50 parts paraffin wax 50 microcrystalline wax 20 vinyl acetate/ethylene copolymer 10 carbon black 20 by means of a wire bar or a gravure roll so that the thickness might be about 4 p. On the other hand, thermal transfer recording media (Example 2 and Comparative Examples 2 and 3) were prepared by coating the films with a dye-type dispersant, used in place of the above wax-type heatmeltable ink, comprising the following components: dye (Kayaron Fast Orange GR, a product of Nippon Kayaks EMI14.1 50 parts methylcellulose 20 sodium dibutylnaphthalenesulfonate 45 (dispersant) water 385 by means of a wire bar or a gravure roll so that the amount of the adherent dye-type dispersant might 2 be 1.5 g/m The thermal transfer recording media prepared in this way were evaluated for their sticking by printing by transfer to commercially available wood free paper by using thermal transfer printers (NP-300, a product of Yokogawa Hqkushin Denki, and N5234-10, a product of Nippon Electric Co., Ltd.) Table 1 shows the results of the evaluation of the gel fractions and sticking of the plastic films. Table 1 Gel fraction Colorant layer Sticking (%) Ex. 1 73 wax-type good Ex. 2 87 dye-type good Ex. 3 65 wax-type good Comp. 1 0.2 wax-type poor Comp. 2 42 dye-type poor Comp. 3 0.1 dye-type poor Comp. 4 1.5 wax-type poor Comp. 5 - wax-type rather poor Comp. 6 - wax-type good

Claims: CLAIMS: 1. A plastic film, suitable for the thermal transfer recording media, which comprises a composition comprising 100 parts by weight of a polyolefin and 0.1 or more part by weight of a substance which can be crosslinked by irradiation, said composition having been treated for crosslinking with irradiation by a radiation. 2. A plastic film as claimed in Claim 1, in which said composition comprises from 0.1 to 50 parts by weight of said substance. 3. A plastic film as claimed in Claim 1, which has been obtained by blending said polyolefin and said substance, moulding the blend into film and treating the film with irradiation. 4. A plastic film as claimed in Claim 1, in which said substance is an ethylenically unsaturated compound or polymer. 5. A plastic film as claimed in Claim 1, which has a thickness of 3 to 30 microns. 6. A plastic film as claimed in Claim 1, which further comprises thereon a colorant layer.

Inventors: SAKAI, KOICHI, YAMAGUCHI, SHINICHIRO
Applicants: KAO CORPORATION
Application year: 1986
Main classification: C08J7
IPC codes: B29C71, B41M5, C08J3, C08J5