Heat-sensitive stencil sheets and process for producing the same

Abstract: A heat-sensitive stencil sheet consisting of a porous substrate and a thermoplastic film adhered to the substrate is provided without no use of an adhesive. The porous substrate comprises a screen cloth wholly or partially consisting of conjugate fibers (a sheath-core type or a side-by-side type) one exposed component of which has an affinity with the thermoplastic resin film and having a melting point lower than that of the other component of the conjugate fiber and lower than that of the thermoplastic resin, and the screen cloth is adhered to the thermoplastic resin film through the exposed component. The heat-sensitive stencil sheet can be produced by superposing the thermoplastic resin film on the substrate and heating under pressure the superposed film and substrate at the softening point or higher of the above exposed component of the conjugate fiber and at a temperature lower than the melting point of the other component of the conjugate fiber and lower than the melting point of the thermoplastic resin.

Description: The present invention relates to a heat-sensitive stencil sheet and a process for producing the same. Specifically, the present invention relates to a heat-sensitive stencil sheet which is excellent in a heat-sensitive and perforating property by a thermal head, a xenon process system, a flash valve system and others, and a process for producing the heat-sensitive stencil sheet. In the prior art, a heat-sensitive stencil sheet is composed of a thermoplastic resin film such as a polyester film and a porous substrate adhered on one surface of the film by an adhesive. The heat-sensitive stencil sheet is processed by bringing a thermal head in contact with the thermoplastic resin film of the sheet, thermally melting the film by the resulting heat and forming a corresponding opening portion to the image portion of any manuscript on the sheet. In such a stencil sheet, however, an adhesive layer is formed between the thermoplastic resin film and the porous substrate in order to achieve lamination. Therefore, there are the problems in that the film perforating property due to the heat from a thermal head printing, resulting in a lowering of the resistance to printing of the stencil sheet. In the case of producing the stencil sheet described above, there are also other problems in that when the coated quantity of adhesive is too great, the perforated portions in the porous substrate are buried by adhesive layers, the perforating sensitivity of the film is reduced, the ability for processing is lowered and the permeabilities of inks are decreased, and on the other hand, when the coated quantity of adhesive is too small, the adhesion strength is low, the resistance to printing becomes poor and it is difficult to control the production processes. JP-A-62184888 discloses a thermal stencil sheet comprising a tissue paper made of short conjugate fibers laminated to a high molecular weight resin film by an adhesive. EP-A-0439960 discloses a screen printing plate comprising a mesh fabric composed of conjugate fibers laminated with a photosensitive film. It is a main aim of this invention to solve the above-mentioned problems in the prior art and provide a heat-sensitive stencil sheet having an excellent film perforating property and resistance to printing by a thermal head, a xenon process system, a flash valve system and others, and further provide a process for producing a heat-sensitive stencil sheet by production processes which can be easily controlled. The present invention provides a heat-sensitive stencil sheet comprising a thermoplastic resin film adhered to a porous substrate, wherein said porous substrate comprises a screen cloth wholly or partially consisting of conjugate fibers consisting of two components having different melting points; characterized in that at least one exposed component of said conjugate fibers has an affinity with and a lower melting point than said thermoplastic resin film, and in that said substrate is adhered directly to said thermoplastic resin film through said exposed component. The present invention further provides a process for producing a heat-sensitive stencil sheet, comprising the steps of: providing a thermoplastic resin film and a porous substrate comprising a screen cloth wholly or partially consisting of conjugate fibers having at least one exposed component with a melting point lower than that of the other component of said conjugate fibers; superposing said thermoplastic resin film and said substrate; and adhering said substrate to said thermoplastic resin film; characterized in that said exposed component is composed of a resin having an affinity with said thermoplastic resin film and a melting point lower than that of said thermoplastic resin film, and in that said adhering step comprises heating under pressure said thermoplastic resin film and said substrate at or above the softening point of said exposed component and at a temperature lower than the melting point of said other component and lower than the melting point of said thermoplastic resin film to adhere said substrate directly to said thermoplastic resin film and fix the intersections of the wefts and warps of said screen cloth of said substrate. The above exposed component of the conjugate fiber is preferably a copolymerized polyester resin and the other component of the conjugate fiber is preferably a polyester resin having a higher melting point than said copolymerized polyester resin. The thermoplastic resin is preferably composed of the polyester resin. The above-mentioned conjugate fiber may be of a sheath-core type or a side-by-side type and can be obtained by melt-spinning two kinds of thermoplastic resins through a spinning device for conjugate spinning, and drawing the resulting fiber, if necessary. In order to adhere the thermoplastic film over a broader contact area on the substrate, it is preferable to use a sheath-core type conjugate fiber. According to the heat-sensitive stencil sheets of the present invention, since the screen cloth and the thermoplastic resin film are adhered to each other by lines or point contacts through the component of the conjugate fiber having an affinity with the thermoplastic resin, the following excellent effects can be obtained. (1) Since there is no adhesive layer between the film and the substrate, in contrast with the prior art, the perforation of the stencil sheet by the thermal head is improved. Further, since the smoothing property on the film surface of the superposed sheet can be enhanced, the contact of the sheet to the thermal head is improved at the time of perforation processing. Thereby, the thermal transfer efficiency is improved and both the perforating property and the process sensitivity can be significantly improved. (2) Since the sheet surface is smooth due to the absence of an adhesive layer, the stencil sheet can be made thinner and the film surface of the stencil sheet becomes more smooth and uniform. Thus, any printed letters are made more uniform by the uniformity of the pressure applied on the unit area of the sheet surface during printing so that more accurate images of manuscripts can be obtained. (3) Since the sheet strength is not influenced by the components in the ink, the resistance to printing can be improved. (4) Since the intersections between the wefts and warps of the screen cloth are rigidly fixed by the softening or fusion of the lower melting point component and the contact area between the film and the screen is enlarged, the rigidity of the sheet is improved and wrinkling of the sheet during transport can be prevented. (5) Since a step of coating an adhesive layer is not needed, control of the production processes becomes easier. As a thermoplastic resin film used in the present invention, it is possible to use any films applicable to normal heat-sensitive stencil sheets such as those of polyester, polyvinylidene chloride, polypropylene and the like. Their thicknesses are preferably in the range of 0.5 - 8 µm from the standpoint of thermal perforating property, film strength and others. A screen cloth used in the present invention comprises a cloth or web (non-woven cloth) consisting wholly or partially of conjugate fiber of a sheath-core or side-by-side type. The sheath component in the sheath-core structure or the at least one exposed side component in the side-by-side type structure, is composed of a resin having an affinity with the thermoplastic resin film. In the present invention, the component having an affinity with the thermoplastic resin film is a component that has an ample adhesion force with the thermoplastic resin film by heating under pressure the superposed film and screen cloth or by subjecting them to alternative means such as light irradiation, etc., and would not be easily peeled off. Such a component may be preferably of the same resin as that of the thermoplastic resin film. In order to adhere a screen cloth to the thermoplastic resin film by heating, the melting point of the exposed component, that is the sheath component in the sheath-core structure or the at least one side component in the side-by-side structure, is lower than that of the other component, that is the core component or the other side component. Further, the melting point of the exposed component should be lower than that of the thermoplastic resin film. The above exposed component having such a lower melting point will be referred to as the lower melting point component hereinafter. For example, when using a polyethylene terephthalate film as the thermoplastic resin film, a copolymerized polyester having a lower melting point than that of the polyethylene terephthalate film is used as the above-mentioned lower melting point component. The copolymerized polyester can be obtained by adding other monomer or reaction components such as polyethylene glycol at the time of preparing the polyethylene terephthalate. As the other monomer or reaction components, a dicarboxylic acid such as isophthalic acid, adipic acid or dimer acid, a lower molecular weight glycol such as ethylene glycol or butanediol, and polyalkylene glycols such as polyethylene glycol or polytetramethylene glycol are exemplified. Regarding the applied ratio of both components in the sheath-core type and side-by-side type conjugate fibers, it is preferable to use such an amount that the adhesion force is sufficient when the lower melting point component of the conjugate fiber is adhered to the thermoplastic film and that the meshes in the screen cloth are not damaged by deformation or thermal fusion of the conjugate fiber. Thus, the sectional area ratio of the sheath component and the core component (sheath component/core component) is preferably in the range of 5/95 - 70/30, more preferably in the range of 10/90 - 50/50. On the other hand, the sectional area ratio of the lower melting point component of the side-by-side type conjugate fiber (lower melting point component/higher melting point component) is preferably in the range of 5/95 - 70/30, more preferably in the range of 10/90 - 50/50. The sectional form of the conjugate fiber may be circular or a modified sectional surface. The sectional form of the core in the sheath-core structure and the number of core components are not limited to the above-mentioned one, and it is possible to select and decide upon them according to the particular use. As for the core component of the sheath-core type conjugate fiber and the other side component of the side-by-side type conjugate fiber (which will be referred to as the high melting point component hereinafter), there is no particular limitation as long as it is not melted or deformed by heating under pressure when the screen cloth and the thermoplastic resin film are adhered on each other. However, it is preferable to use a resin component having a lesser affinity with the ink. For example, it is preferable to use polyester, particularly polyethylene terephthalate from the standpoint of melting point and availability. The conjugate fiber can be obtained by a normal melt-spinning process using a known nozzle for conjugate spinning and the screen cloth used in the present invention can be obtained by weaving into a plain cloth or the like by means of a known weaving method using wholly or partially conjugate fiber (filament). The screen cloth may be constituted only by the conjugate fiber, or alternatively the conjugate fiber may be used for only a portion of the cloth, such as either the weft or warp, or may be used for example as every second or third weft or warp. As for other fibers beside the conjugate fiber, normal fibers consisting of polyester having the above higher melting point can for example be used. There is no particular limitation of the sieve opening (or mesh) in the screen cloth, but it is preferable from the standpoint of ink permeability and image property that the sieve opening is in the range of 70 mesh - 400 mesh and the thickness is in the range of 40 µm - 200 µm. The heat-sensitive stencil sheet of the present invention is prepared by superposing the thermoplastic resin film on the substrate and heating under pressure the superposed film and substrate at the softening point or higher of the above exposed component of the conjugate fiber but at a temperature lower than the melting point of the other component of the conjugate fiber and lower than the melting point of the thermoplastic resin film. The invention will specifically be described with reference to the following non-limiting Examples and Comparative Examples. In these Examples, the melting point of a resin was determined by a peak observed in the endothermic curve due to the crystal portion of the resin, which was measured by differential thermal analysis. A softening point of the resin can be also determined by differential thermal analysis. Example 1 A screen cloth (sieve opening 70 mesh and thickness 110 µm) composed of polyester conjugate multifilaments having a sheath-core structure (sectional area ratio of sheath component and core component: 50/50, core component: polyethylene terephthalate homopolymer, sheath component: polyethylene terephthalate-polyethylene glycol copolymer (m.p. 200°C)), and a polyethylene terephthalate film of 2 µm in thickness were superposed on each other, passed through between a metal roller heated at 120°C and a silicone rubber roller at a nip pressure of 1.8 kg/cm 2 so as to adhere the screen cloth and the film on each other to give a heat-sensitive stencil sheet. The surface of the fibers constituting the screen cloth was adhered uniformly to the film by thermal fusion. Dimethyl silicone oil was coated on the film surface of this sheet, which was then provided in an integrated type process printer (product of Riso Kagaku Kogyo Co., RISOGRAPH RC-115, registered trade mark) for printing. Good printed images were obtained. Example 2 A screen cloth (sieve opening 200 mesh and thickness 75 µm) composed of polyester conjugate monofilaments having a sheath-core structure (sectional area ratio of sheath component and core component: 60/40, core component: polyethylene terephthalate homopolymer, sheath component: polyethylene terephthalate-polyethylene glycol copolymer (m.p. 200°C)), was superposed on a polyethylene terephthalate film of 2 µm in thickness, passed through between a metal roller heated at 120°C and a silicone rubber roller at a nip pressure of 1.8 kg/cm 2 so as to adhere the screen to the film to give a heat-sensitive stencil sheet. Dimethyl silicone oil was coated on the film surface of this sheet, which was then provided in an integrated type process printer (Riso Kagaku Kogyo Co., RISOGRAPH RC-115) for printing. Good printed images were obtained. Example 3 Screen cloths (90, 70 and 60 µm in thickness) having respective sieve openings of 135, 200 and 420 mesh were prepared using polyester conjugate fibers (average fiber sizes: 45 µm, 45 µm and 30 µm, respectively) having a sheath-core structure (sectional area ratio of sheath component and core component: 10/90, core component: polyethylene terephthalate homopolymer, sheath component: polyethylene terephthalate-polyethylene glycol copolymer (m.p. 200°C)), as wefts, and normal polyester fibers (average fiber size 40 µm of monofilaments) as warps. Each of these screen cloths and a polyethylene terephthalate film of 2 µm in thickness were superposed on each other, passed through between a metal roller heated at 120°C and a silicone rubber roller at a nip pressure of 1.8 kg/cm 2 so as to adhere the respective screen cloth to the film, to give heat-sensitive stencil sheets. Dimethyl silicone oil was coated on the film surface of each sheet, which were then in turn provided in an integrated type process printer (Riso Kagaku Kogyo Co., RISOGRAPH RC-115) for printing. Good printed images were obtained. A screen cloth (sieve opening 200 mesh and thickness 72 µm) was prepared using side-by-side type conjugate fibers (average fiber size 45 µm of monofilaments) obtained by combining a lower melting point component (the same copolymerized polyester as in Example 1) and a higher melting point component (polyethylene terephthalate homopolymer) at the ratio (by weight) of 50/50 as wefts, and normal polyester fibers (average fiber size 45 µm of monofilaments) as warps. This screen cloth and a polyethylene terephthalate film of 2 µm in thickness were superposed on each other, passed through between a metal roller heated at 120°C and a silicone rubber roller at a nip pressure of 1.8 kg/cm 2 so as to adhere the screen to the film to give a heat-sensitive stencil sheet. The adhesion area between the film and the substrate layer was 45%, and the surface of the fibers constituting the screen cloth was uniformly adhered to the film by thermal fusion and exhibited a good adhesion strength. Dimethyl silicone oil was coated on the film surface of this sheet, which was then provided in an integrated type process printer (Riso Kagaku Kogyo Co., RISOGRAPH RC-115) for printing. Good printed images were obtained. Comparative Example 1 A screen cloth (a commercially available net, sieve opening 150 mesh) made of polyethylene monofilaments having a softening point of 105°C and a polyethylene terephthalate film of 2 µm in thickness were superposed on each other and heated under pressure in the same manner as in Example 1. However, the fibers comprising the screen cloth were melted into a film form and no ink could pass therethrough for printing. Comparative Example 2 A screen cloth (a commercially available net, sieve opening 150 mesh) made of polyethylene monofilaments having a softening point of 140°C and a polyethylene terephthalate film of 2 µm in thickness were superposed on each other and passed through between a metal roller heated at 150°C and a silicone rubber roller in the same manner as in Example 1. However, the fibers comprising the screen cloth were deformed. Further, since the adhesion strength between the screen cloth and the film was extremely weak, the screen cloth and the film could be peeled apart by slightly picking them up. In summary, with heat-sensitive stencil sheets in accordance with the present invention, an excellent film perforating property and resistance to printing can be obtained since a screen cloth and a thermoplastic resin film can be adhered to each other directly without forming an adhesive layer. In accordance with the process of the present invention, control of the production processes is easier since there is no need of a step for forming an adhesive layer between the screen cloth and the thermoplastic resin film.

Claims: A heat-sensitive stencil sheet comprising a thermoplastic resin film adhered to a porous substrate, wherein said porous substrate comprises a screen cloth wholly or partially consisting of conjugate fibers consisting of two components having different melting points; characterized in that at least one exposed component of said conjugate fibers has an affinity with and a lower melting point than said thermoplastic resin film, and in that said substrate is adhered directly to said thermoplastic resin film through said exposed component. A heat-sensitive stencil sheet according to claim 1, wherein said components of said conjugate fibers are each composed of a resin, the resin of said exposed component having a melting point lower than that of the resin of said other component and lower than that of said thermoplastic resin film. A heat-sensitive stencil sheet according to claim 2, wherein said thermoplastic resin film is composed of a polyester resin. A heat-sensitive stencil sheet according to claim 1, wherein said conjugate fibers are of a sheath-core or side-by-side type. A heat-sensitive stencil sheet according to claim 1, wherein said exposed component has a lower melting point than said other component and is adhered to said thermoplastic resin film by melt adhesion. A heat-sensitive stencil sheet according to claim 5, wherein said exposed component is composed of a copolymerized polyester resin, and said other component and said thermoplastic resin film are each composed of a polyester resin. A process for producing a heat-sensitive stencil sheet, comprising the steps of: providing a thermoplastic resin film and a porous substrate comprising a screen cloth wholly or partially consisting of conjugate fibers having at least one exposed component with a melting point lower than that of the other component of said conjugate fibers; superposing said thermoplastic resin film and said substrate; and adhering said substrate to said thermoplastic resin film; characterized in that said exposed component is composed of a resin having an affinity with said thermoplastic resin film and a melting point lower than that of said thermoplastic resin film, and in that said adhering step comprises heating under pressure said thermoplastic resin film and said substrate at or above the softening point of said exposed component and at a temperature lower than the melting point of said other component and lower than the melting point of said thermoplastic resin film to adhere said substrate directly to said thermoplastic resin film and fix the intersections of the wefts and warps of said screen cloth of said substrate. A process for producing a heat-sensitive stencil sheet according to claim 7, wherein said conjugate fibers are of a sheath-core or side-by-side type.

Inventors: CHONAN SATOSHI, IKEJIMA SHOICHI, KOISHIHARA TAKESHI, SHIBUYA KENICHI, YAMANAKA TETSU, CHONAN, SATOSHI, IKEJIMA, SHOICHI, KOISHIHARA, TAKESHI, SHIBUYA, KENICHI, YAMANAKA, TETSU, Chonan, Satoshi c/o Riso Kagaku Corporation, Ikejima, Shoichi, c/o Riso Kagaku Corporation, KOISHIHARA, TAKESHI,C/O RISO KAGAKU CORPORATION, Shibuya, Kenichi c/o Riso Kagaku Corporation, Yamanaka, Tetsu, c/o Riso Kagaku Corporation
Applicants: RISO KAGAKU CORP, RISO KAGAKU CORPORATION
Application year: 1993
Main classification: B41M1
IPC codes: B41N1