Stamp ink and stamp ink impregnation pad for stencil printing

Abstract: A stamp ink for stencil printing comprises a coloring-agent and a vehicle comprising resin and oil-based solvent, the stamp ink having a viscosity of from 20 to 250 poises at a shear rate of 20/sec and a thixotropy index T.I. of from 1.2 to 3.0 where the thixotropy index T.I. is defined by the expression: T.I. = (Viscosity at a shear rate of 0.5/sec)/ (Viscosity at a shear rate of 5.0/sec) .

Description: The present invention relates to a stamp ink and a stamp ink impregnation pad impregnated with the stamp ink for stencil printing. Stamp inks are generally impregnated in a porous ink pad which is made of paper, cloth, resin sponge, or other materials. When a user presses a rubber stamp against the surface of the ink pad, the ink pad can supply the stamp with the ink. The user presses the stamp onto a paper to imprint the stamp image with the ink. Stamp inks are used also in a stamp of an integrated-type. The integrated-type stamp has a porous-rubber member having numerous minute continuous pores. The porous-rubber member is formed with a raised portion representative of a stamp image. The porous-rubber member is impregnated with a stamp ink. By simply pressing the stamp against a paper, stamp ink oozes out of the porous-rubber member from its raised portion to imprint the stamp image on the paper. These stamp inks are generally made of dye, water-based resin, or glycol solvents, have a low viscosity, and exhibit a fluidity approximating Newtonian fluids. Accordingly, when printing continuously, the stamp ink will easily move and replenish the surface portion of the ink pad or of the porous-rubber member. The stamp ink will spread through the paper to speed the drying of the paper surface after printing. The diameter of the pores in the ink pad and in the porous-rubber member are particularly minute. Accordingly, when pressure is applied to the ink pad and to the porous-rubber member, a large quantity of the stamp ink will not ooze out in excess at one time. The minute pores also promote the capillary action in the ink pad and in the porous-rubber member. The stamp ink will therefore immediately move and replenish the surface portion of the ink pad and of the porous-rubber member. European Patent Application Publication No. 0 655 343 A1 and United States Patent Application Serial No.08/396,720 have proposed a stamp unit of a stencil-printing type. The stencil-printing type stamp unit is made from a combination of: an impregnation pad impregnated with stamp ink; and a stencil paper fixedly covering the surface of the impregnation pad. The stencil paper is made from a thermoplastic film. The stencil paper is thermally perforated in dots with a thermal head. By pressing the impregnation pad against paper via the stencil paper, stamp ink seeps out through the perforations to imprint an image. However, because the diameter of the perforations formed in the stencil paper are large, from 30 µm to 100 µm, the stencil-printing type stamp unit fails to provide satisfactory prints when impregnated with the above-described conventional stamp inks. That is, because the stamp inks have a low viscosity and have fluidity approximating Newtonian fluids, a large amount of ink is transferred during printing and becomes blurred on the print surface. The ink is slow to set or is slow to be dried on the surface of the paper. If rubbed, the ink will smudge the print and will stain the user's hands. With the passage of time, the stamp ink will drip down from the perforated portion of the stencil paper, staining the stamp and the paper. When the pad is leaned against something for a period of time, the stamp ink in the pad displaces and collects at the lower section due to gravity, so that there is no stamp ink at the upper section of the pad. If stamping is achieved in this condition, the stamp ink does not adhere to the paper and makes a faint imprint on the paper. A satisfactory print cannot be obtained. This tendency is noticeable particularly in the summer when the viscosity of the stamp ink drops considerably due to the high temperature. An object of the present invention is therefore to solve the above-described problems and to provide a stamp ink and a stamp ink impregnation pad in a stencil printing type stamp unit, in which the stamp ink does not displace to collect in a lower portion of the pad due to gravitational force, forms a sharp print with no blurring, and sets quickly, making possible the production of prints with a high print density and permitting even longer continuous printing. In order to attain the above and other objects, the present invention provides the viscosity characteristics of the ink and the density of the pad that can allow the stamp ink not to displace in the pad due to the gravitational force, to form sharp prints with no blurring, to set quickly, to produce prints with a high print density, and to perform longer continuous printing. The present invention therefore provides a stamp ink for stencil printing, which ink comprises a coloring agent mixed in a vehicle consisting of a resin and a solvent selected from hydrocarbon solvents with a boiling point of 300°C or higher, petroleum solvents, synthetic oils obtained through polymerization of unsaturated hydrocarbons, and animal and vegetable oils; the stamp ink having a viscosity of from 20 to 250 poises at a shear rate of 20/sec at 25°C and a thixotropy index T.I. of form 1.2 to 3.0 at 25°C where the thixotropy index T.I. is defined by the expression: T.I. = (Viscosity at a shear rate of 0.5/sec) / (Viscosity at a shear rate of 5.0/sec). The stamp ink is preferably made from the coloring-agent, the resin, and the oil-based solvent in amounts such that the stamp ink has a viscosity of 20 to 250 poises at a shear rate of 20/sec at 25°C and a thixotropy index T.I. of 1.2 to 3.0 at 25°C. The stamp ink may preferably be made by adding the coloring-agent to the resin and the oil-based solvents, breaking down and dispersing the mixture, and then adding to the mixture amounts of the resin and the oil-based solvents such that the stamp ink has a viscosity of 20 to 250 poises at a shear rate of 20/sec at 25°C and a thixotropy index T.I. of 1.2 to 3.0 at 25°C. The stamp ink may additionally comprise thixotropic agents in an amount such that the stamp ink has a viscosity of 20 to 250 poises at a shear rate of 20/sec at 25°C and a thixotropy index T.I. of 1.2 to 3.0 at 25°C. According to another aspect, the present invention provides a stamp ink impregnation pad (for supplying a stamp ink to a stencil paper), impregnated with a stamp ink of the invention, which pad has continuous pores and a density of from 20 kg/m 3 to 300 kg/m 3 . According to still another aspect, the present invention provides a stamp unit for stencil printing an ink image, the stamp unit comprising: a stamp ink impregnation pad of the invention; a stencil paper attached to said pad and formed with a perforation image to allow the stamp ink to seep out therethrough; and a holding member for holding said pad and the stencil paper in contact with each other. The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiment taken in connection with the accompanying drawings in which: Fig. 1 is a perspective view of a stamp unit of a preferred embodiment of the present invention; and Fig. 2 is a cross-sectional view of the stamp unit of Fig. 1 taken along a line II- II'. A stamp unit according to a preferred embodiment of the present invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals. Fig. 1 shows an external view of the stamp unit for stencil printing according to the present embodiment. Fig. 2 shows a cross-sectional view of the stamp unit. The stamp unit 1 includes a grip portion 2, a print face holding portion 3, and a print face portion 4. The print face portion 4 is constructed from a pad 5 impregnated with a stamp ink to be described later and covered with a heat-sensitive stencil paper 6. The pad 5 is attached to the bottom surface of the print face holding portion 3. The stencil paper 6 is attached at its ends to the sides of the print face holding portion 3 so as to cover the pad 5. Though the stencil paper 6 is in contact with both the pad 5 and the print face holding portion 3, the stencil paper 6 is illustrated separately in Fig. 2 for ease of understanding. The pad 5 is formed with numerous small continuous pores. Any of paper, cloth, rubber, resin sponge or other materials are suitable for the pad 5. From the point of view of cost and processing, a resin sponge, and in particular, polyurethane foam is preferred as the pad 5. The stencil paper 6 is constructed from a thermoplastic film attached to a porous carrier via an adhesive layer. The thermoplastic film is made of a thermoplastic synthetic resin film (for example, polyethylene terephthalate, polypropylene, vinylidene chloride-vinyl chloride copolymer) with a thickness of 1 to 4 µm, preferably 2 µm. The porous carrier is made of a thin sheet of porous paper made primarily of: a natural fiber such as Manila hemp, paper mulberry (Broussonetia kazinoki), and mitsumata (Edgeworthia papyrifera); a synthetic fiber such as polyethylene terephthalate, polyvinyl alcohol, and polyacrylonitrile; or a semi-synthetic fiber such as rayon. The stamp unit 1 is formed with a desired stamp image by a thermal head. That is, the stencil paper 6 provided to the stamp unit 1 is thermally perforated in dots by the thermal head so that a perforated dot image is formed on the stencil paper. For example, the stamp unit 1 is mounted to and perforated by a thermal perforating device. The thermal perforating device includes: a perforation mount portion in which the stamp unit 1 is detachably mounted; a thermal head for perforating the stencil paper 6; a keyboard for inputting characters; and a control unit for controlling the thermal head to perforate the inputted character array in the stencil paper. The thermal head is similar to that employed in a thermal printer. The thermal head is provided with, for example, ninety-six heating elements aligned in a row. The thermal head is mounted on a carriage. The control unit selectively drives the heating elements while driving the carriage to transport the thermal head along and in contact with the print face portion 4 of the stamp unit 1. Accordingly, the inputted character array is perforated on the stencil paper 6 of the stamp unit 1. By holding the grip portion 2 in the hand and pressing the stamp unit 1 on a paper or other printing surface, the pad 5 is compressed and stamp ink seeps out through the perforated portion of the stencil paper 6, making a print of the character array. For example, the stamp unit 1 can be constructed and perforated as described in the European Patent Application Publication No.0 655 343 A1 and the United States Patent No. 5 694 844. (Application Serial No. 08/396,720). It is noted that the stencil paper 6 can be perforated before being attached to the print face holding portion 3 of the stamp unit 1. The stamp ink, impregnated in the pad 5, is made of a coloring agent mixed in a vehicle formed from resin and an oil-based solvent. A thixotropic agent (or a plastic agent) can preferably be mixed in the stamp ink. Examples of materials utilized as the coloring agent are: inorganic pigments such as carbon black, red iron oxide, ocher, ultramarine blue, and Prussian blue; organic pigments such as insoluble azo dye, soluble azo dye, phthalocyanine, and quinacridon; and oil-soluble dyes. Examples of materials utilized as the resin are rosin, rosin ester, rosin-modified phenol resin, alkyd resin, petroleum resin, and rosin-modified maleic resin. Materials with a low volatility are preferable as the oil-based solvent. Examples include: hydrocarbon solvents with a boiling point of 300°C or higher; and petroleum solvents such as spindle oil, liquid paraffin, motor oil, gear oil, and machine oil. Further examples include synthetic oils obtained through the polymerization of unsaturated hydrocarbons such as ethylene, propylene, and butene. In addition, animal and vegetable oils such as castor oil, tall oil, soybean oil, olive oil, and whale oil can also be used. To produce the stamp ink of the present invention from the above-listed materials, the coloring agent is first added to all or a portion of the resin and of the solvents. The mixture is then broken down and dispersed in a dispersion mill such as a triple roll mill or a sand mill. Next, the remaining resin and solvents are added to the mixture so as to adjust the viscosity characteristics into a range as described below so that the produced stamp ink will come out from the pad 5 to the proper degree and so that the stamping operation will be properly attained. Thixotropic agents (or plastic agents) are added either during the preparation of the stamp ink or during the preparation of resin varnish. Examples include aluminum stearate, aluminum octylate, aluminum oleate, aluminum naphthenate, ethyl acetoacetate aluminum diisopropylate, alkyl acetoacetate aluminum diisopropylate, higher fatty acid amides, and extender pigments such as talc, kaolin, calcium carbonate, and barium sulfate. Further examples include silica gel and class four ammonium salts of montmorillonite. These thixotropic agents (or plastic agents) should preferably be added in the range of 1 to 5% of the weight of the resin. According to the present invention, the stamp ink has a viscosity of 20 to 250 poises at a shear rate of 20/sec. The stamp ink preferably has a viscosity of 50 to 200 poises at a shear rate of 20/sec at 25°C. More preferably, the stamp ink has a viscosity of 80 to 150 poises at a shear rate of 20/sec at 25°C. If the viscosity is lower than the values given above, then during printing the stamp ink comes out in excess, blurring the print. This makes the ink slow to set, and if rubbed, smudges the print and stains the hands. If the viscosity is higher than the values given above, then during printing the stamp ink comes out with difficulty, making the print image thin and faint. It then becomes necessary to increase the stamping load applied to the stamp unit 1 against the paper when stamping and The stencil paper 6 therefore is likely to becomes wrinkled at the print face. A thixotropy index T.I. is defined by the following expression: T.I. = (Viscosity at a shear rate of 0.5/sec) / (Viscosity at a shear rate of 5.0/sec). A fluid with a T.I. of 1.0 is a Newtonian fluid. As the T.I. increases, so does the thixotropy or plasticity of the fluid. According to the present invention, the stamp ink has a value of T.I. of 1.2 to 3.0 at 25°C. Preferably, the stamp ink has a value of T.I. of 1.3 to 2.5 at 25°C. More preferably, the stamp ink has value of T.I. of 1.4 to 2.0 at 25°C. If the T.I. value is lower than the values given above, then the stamp ink will displace and collect in a lower portion of the impregnation pad due to gravitational force. If the T.I. is higher than the values given above, then during printing the stamp ink comes out with difficulty, making the print image thin and faint. It then becomes necessary to increase the stamping load and the stencil paper 6 is likely to become wrinkled at the print face. Stamping durability suffers as a result. It is noted that various additive agents can be added to the stamp ink as necessary so long as the effects of the present invention are not impaired. Examples of the additive agents include dispersants, abrasion resistance improvers, antioxidants, and antisetoff agents. With the above-described viscosity characteristics of the stamp ink, the stamp unit 1 can obtain sufficiently high stamping quality. However, the stamp unit 1 can obtain even higher quality when the pad density of the pad 5 is selected as described below. The pad density is defined as the mass of the material forming the pad 5 per unit volume. As the pad density increases, there is a tendency for the continuous pores in the pad to become smaller, making it more difficult for the stamp ink to come out. When the pad density decreases, there is a tendency for the continuous pores of the pad to become larger, making it easier for the stamp ink to come out. The pad 5 of the present invention therefore has a pad density of 20 kg/m 3 to 300 kg/m 3 . Preferably, the pad 5 has a pad density of 50 kg/m 3 to 200 kg/m 3 . More preferably, the pad 5 has a pad density in the of 80 kg/m 3 to 150 kg/m 3 . If the pad density is lower than the values given above, during printing the stamp ink oozes out in excess, blurring the print. The ink is slow to set, and if rubbed, smudges the print and stains the hands. On the other hand, if the pad density is higher than the values given above, the stamp ink to comes out with difficutly. It becomes necessary to increase the stamping load and the stencil paper is likely to become wrinkled at the print face. This results in reduced stamping durability. The viscosity and T.I. value of the stamp inks can be set to any desired values in the above-described ranges by changing the manner of preparation, the kinds of materials, or the amounts of materials, especially, the amounts of the thixotropic agents (or plastic agents). An example of the stamp ink and the pad 5 of the present invention will be described below. In the examples, "parts" refers to parts by weight. (Varnish preparation example 1) First, 55 parts of motor oil and 45 parts of alkyd resin were prepared in a four-opening flask with agitator, and stirred for 30 minutes under a nitrogen air current at 180°C to dissolve the resin. Next, the mixture was cooled to 160°C, and 2.0 parts of aluminum stearate were added. The mixture was then stirred for one hour, producing resin varnish 1. (Varnish preparation example 2) Resin varnish 2 was produced through the same procedure as Varnish preparation example 1 except that the aluminum stearate was not added. (Ink preparation example) First, 13 parts of resin varnish 2, 7 parts of carbon black, 10 parts of motor oil were mixed and dispersed in a triple roll mill. Next, the mixture was further added with varnish 1 and varnish 2 and additional motor oil as appropriate and mixed with a disperser so that the viscosity, at a shear rate of 20/sec, ranged from 15 to 320 poise in seven steps and so that the T.I. ranged from 1.1 to 3.3 in five steps. In this way, a total of 35 individual stamp inks were obtained. The total amounts of the 35 individual stamp inks were 100 parts. During this step, the viscosity of the produced stamp inks was measured with a cone plate viscometer in a 25°C thermostatic chamber. The viscosity, at 2 rpm (0.5/sec) and 20 rpm (5.0/sec), of the produced stamp inks was measured with a BH-type viscometer also in the 25°C thermostatic chamber, and the T.I. of the stamp inks was calculated. (Printing test) Next, pads, made of urethane foam ("Urethane Color Foam CFS" produced by Inoac Corporation) with various pad densities in the range from 15 to 330 kg/m 3 , were impregnated with the stamp inks obtained above. Printing tests were carried out to observe: sharpness of ink prints (i.e., the degree how ink prints are obtained without being blurred); quickness to set; a stamping load; stamping durability; and stamp ink displacement. In a first test, pads with the same pad density of 120 kg/m 3 were used. Inks having various values of T.I. and viscosity were impregnated in the pads. The results are shown in Table 1. In a second test, inks having the same T.I. of 1.7 but having various values of viscosity were used. These inks were impregnated in pads with various values of pad density. The results are shown in Table 2. In a third test, inks having the same viscosity of 120 poises but having various values of T.I. were used. These inks were impregnated in pads with various values of pad density. The results are shown in Table 3. In each Table, the bold lines surround the items of the present invention. The data in each of the boxes in each Table have the following meanings. (1) Upper left --- sharpness of print (i.e., the degree how the print is obtained without blurring) under temperature of 25°C Good: ○ (not blurred) Fair: Δ (slightly blurred) Poor: × (very blurred) (2) Upper middle--- quickness to set under temperature of 25°C Good: ○ Fair: Δ Poor: × (3) Upper right --- stamping durability under temperature of 25 °C (Unit: number of times) The stamping durability was defined as the number of times a satisfactory image was obtained without becoming faint when stamped continuously at 10-second intervals. The print size of the used stamp unit 1 was 35 mm × 80 mm. The stencil paper 6 provided to the stamp unit 1 was perforated with a print face perforation rate of 20%. (4) Lower left--- stamping load under temperature of 10°C (Unit: kg) The stamping load was defined as the load applied on the single print face 4 of the stamp unit 1 during stamping. The size of the print face 4 was 35 mm × 80 mm. (5) Lower right--- ink displacement (Unit: number of times) The ink displacement was defined as the above-defined stamping durability obtained after the pad 5 was placed vertically and stored for one month at 35°C. The size of the pad was 15 mm × 80 mm. As described above, Table 1 shows the print results when the pad density was fixed at 120 kg/m 3 and the T.I. and the viscosity of the inks were varied. The stamp ink with a viscosity of 320 poises and a T.I. of 1.1 could not be prepared. A T.I. of 1.1 results in stamp ink displacement, while a T.I. of 3.3 results in slow setting and inferior drying. Further, a viscosity of 15 poises results in blurring, while a viscosity of 320 poises results in inferior drying. The most satisfactory results over all of the evaluation categories occurred at a T.I. of 1.7 and 2.2 and a viscosity of 120 poises. Table 2 shows the print results when the T.I. was fixed at 1.7 and the pad density and viscosity were varied. Table 3 shows the print results when the viscosity was fixed at 120 poises and the pad density and the T.I. were varied. It is clear that stamping durability suffers extremely at a pad density of 15 kg/m 3 and 350 kg/m 3 . The most satisfactory results over all of the evaluation categories occurred at a pad density of 70, 120, and 180 kg/m 3 . As described above, according to the viscosity characteristics of the stamp ink and the pad density of the impregnation pad for stencil printing, the stamp ink does not displace and collect in a lower portion of the pad due to gravitational force, makes no blurring, sets quickly, making possible the production of prints with a high print density and permitting even longer continuous printing.

Claims: A stamp ink for stencil printing, which ink comprises a coloring-agent mixed in a vehicle consisting of a resin and a solvent selected from hydrocarbon solvents with a boiling point of 300°C or higher, petroleum solvents, synthetic oils obtained through polymerization of unsaturated hydrocarbons, and animal and vegetable oils; the stamp ink having a viscosity from 20 to 250 poises at a shear rate of 20/sec at 25°C and a thixotropy index T.I. of from 1.2 to 3.0 at 25°C where the thixotropy index T.I. is defined by the expression: T.I. - (Viscosity at a shear rate of 0.5/sec)/(Viscosity at shear rate of 5.0/sec). An ink according to claim 1, wherein the ink further comprises a thixotrophic agent. An ink according to claim 2 wherein the amount of the thixotrophic agent ensures that the said viscosity is from 20 to 250 poises at a shear rate of 20/sec at 25°C and the said thixotropy index T.I. is from 1.2 to 3.0 at 25°C. An ink according to claim 2 or 3, wherein the thixotrophic agent comprises a material selected from aluminum stearate, aluminum octylate, aluminium oleate, aluminum naphthenate, ethyl acetoacetate aluminum diisopropylate, alkyl acetoacetate aluminium diisopropylate, higher fatty acid amides, extender pigments, silica gel and class four ammonium salts of montmorillonite. An ink according to claim 4, wherein the extender pigments are talc, kaolin, calcium carbonate and barium sulfate. An ink according to any of the preceding claims, wherein the said viscosity is from 50 to 200 poises at a shear rate of 20/sec at 25°C and the said thixotropy index T.I. is from 1.2 to 2.5 at 25°C. An ink according to claim 6, wherein the said viscosity is from 80 to 150 poises and the said thixotrophy index T.I. is from 1.2 to 2.0. An ink according to any of the preceding claims, wherein the coloring-agent comprises a pigment. An ink according to any one of the preceding claims, wherein the coloring-agent comprises a material selected from carbon black, red iron oxide, ochre, ultramarine blue, Purssian blue, insoluable azo dyes, soluble azo dyes, phthalocyanine, quinacridon and oil-soluable dyes. An ink according to any one of the preceding claims, wherein the resin comprises a material selected from rosin, rosin ester, rosin-modified phenol resin, alkyd resin, petroleum resin and rosin-modified maleic resin. An ink according to claim 1, wherein the petroleum solvents comprise spindle oil, liquid paraffin, motor oil, gear oil and machine oil, the synthetic oils are obtained through polymerization of one of ethylene, propylene and butene, and the animal and vegetable oils comprise castor oil, tall oil, soybean oil, olive oil, and whale oil. A process for the preparation of an ink as defined in claim 1, which process comprises adding the coloring agent to a portion of the resin and a portion of the oil-based solvent; breaking down and dispersing the resulting composition; and then adding to the resulting dispersion the remaining amounts of the resin and the oil-based solvent so as to ensure that the resulting ink has a said viscosity from 20 to 250 poises at a shear rate of 20/sec at 25°C and a said thixotropy index T.I. of from 1.2 to 3.0 at 25°C. A stamp ink impregnation pad, impregnated with a stamp ink according to any one of claims 1 to 11, which pad has continuous pores and a density of from 20 kg/m 3 to 300 kg/m 3 . A pad according to claim 13, wherein the density is from 50 kg/m 3 to 200 kg/m 3 . A pad according to claim 14, wherein the density is from 80 kg/m 3 to 150 kg/m 3 . A pad according to any one of claims 13 to 15, which is made from a resin sponge. A pad according to claim 16, wherein the resin sponge is made from a polyurethane foam. A stamp unit for stencil printing an ink image, the stamp unit comprising: a stamp ink impregnation pad according to any one of claims 13 to 17 a stencil paper attached to the said pad and formed with a perforation image to allow the stamp ink to seep out therethrough; and a holding member for holding the said pad and the stencil paper in contact with each other. A stamp unit according to claim 18, wherein the stencil paper comprises: a thermoplastic film; a porous carrier; and an adhesive layer between the thermoplastic film and the porous carrier.

Inventors: HASEGAEA HIDEKI, TAIRA HIROSHI, TAKAMI HIROSHI, YAMAMOTO MINORU, YUASA TAKAHIRO, HASEGAEA, HIDEKI, TAIRA, HIROSHI, TAKAMI, HIROSHI, YAMAMOTO, MINORU, YUASA, TAKAHIRO, Hasegaea, Hideki c/o Toyo Ink Manufact. Co., Ltd, TAIRA, HIROSHI, C/O BROTHER KOGYO K.K., TAKAMI, HIROSHI, C/O BROTHER KOGYO K.K., Yamamoto, Minoru, c/o Brother Kogyo K.K., Yuasa, Takahiro,c/o Toyo Ink Manufact. Co.,Ltd
Applicants: BROTHER IND LTD, BROTHER KOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: C09D11
IPC codes: B41K1, C09D11, B41L13