Recording medium and method of producing the same

Abstract: A recording medium for use in various types of cards required to accept printed and written marks has a writing layer exhibiting excellent ink absorption ability and high surface layer strength. The recording medium comprises a base material provided with a coating whose surface has irregular surface cracks and a center-line mean roughness Ra of 0.5 µm - 2.5 µm. The formation of the coating layer includes a drying step conducted in two or more stages using a drying temperature in the second stage that is 10 °C - 40 °C lower than that in the first stage.

Description: The invention relates to a recording medium and method of producing the same. It particularly relates to a recording medium usable for fabricating identification cards, driver's licenses, commuter's tickets, ATM cards and the like and especially adapted for enabling marks to be recorded on the back surface thereof by imprinting with a water-color ink stamp, writing with a water-color felt pen, water-color ball-point pen, fountain pen, pencil or other such writing instrument, or imprinting with cinnabar seal-ink. The characters, images etc. printed on the front surface of such a card or the like are produced by a well-known printing method such as offset printing, gravure printing or silk screen printing, or by a thermal dye transfer printing method such as disclosed in Japanese Utility Model Public Publication Hei 5-31975, a melt-transfer thermal printing method such as disclosed in Japanese Utility Model Application Hei 5-46191 or other similar means. For the rear surface of the card the practice has been to use, for example, high-grade paper, coated paper or other ordinary paper, a plastic film of polypropylene, polyethylene terephthalate, polyvinyl chloride, polystyrene, a foamed film of one of these or a film of one of these admixed with an inorganic pigment such as calcium carbonate, or synthetic paper obtained by coating a plastic film with a pigment coating layer. Among these, ordinary papers have the disadvantage of poor water resistance. While plastic films are highly resistant to water, they have almost no ability to absorb water-color or other type inks. Because of this, they are difficult to mark with a water-color felt pen, water-color ball-point pen, a fountain pen or the like. On the other hand, cinnabar ink impressed thereon with a seal takes around 48 hours to dry naturally. In the meantime, it usually experiences rubbing and smears and soils the surface. In short, none of the prior-art materials has much practical utility. Although synthetic paper has better writability than plastic film owing to the ink absorption ability of its pigment coating layer, a mark made thereon with a water-color ink stamp nevertheless takes 7 to 12 minutes to dry, while a drying time of more than 3 minutes is generally considered to cause problems. It is not practicable to reduce the drying time to under 3 minutes by including a large amount of pigment in the pigment coating layer because the increased pigment content lowers the strength of the coating layer, thus increasing the risk of the coating falling off when subjected to an impact such as by abrasion or by bending. A seal impression made on the synthetic paper with cinnabar seal-ink also takes more than 3 minutes to dry, generally at least 7 minutes, as in the case of a water-color ink stamp impression. This invention was accomplished in light of the aforesaid shortcomings of the prior art. A preferred embodiment may provide a mark-accepting material usable for identification cards, driver's licenses, commuter's tickets, ATM cards and the like and especially adapted for accepting marks on the back surface thereof by imprinting with a water-color ink stamp, writing with a water-color felt pen, water-color ball-point pen, fountain pen, pencil or other such writing instrument, or imprinting with cinnabar seal-ink, which mark-accepting material is not susceptible to soiling of its surface by smearing when rubbed, is excellent in water resistance, dries within 3 minutes of being marked by writing or stamping with a water-color ink stamp, fountain pen, water-color felt pen, a seal using cinnabar seal-ink or the like, and whose coating strength is strong enough to prevent the coating from coming off even when subjected to an abrasion or an impact such as by bending. Also preferred is to provide a mark-accepting material which when used for a card or the like provides a coating surface with good sliding property. EP-A-0634285 discloses a ink-jet recording paper and a method for producing it, said paper having a recording layer, the surface roughness by ten-point height of the recording layer ranging from 0.5 to 5 µm. The present invention provides a recording medium comprising a sheet having a writing layer on at least one side of a base material, the writing layer surface having irregular surface cracks and a center-line mean roughness Ra of 0.5 µm - 2.5 µm. Through studies conducted for overcoming the aforesaid problems, the inventor discovered that when a large number of cracks are produced in a pigment coating layer serving as a writing layer, ink applied to the writing layer is absorbed through the cracks, and since the desired short drying time can therefore be achieved with a coating layer containing only a small amount of pigment, it is also possible to obtain a coating of adequately high strength. The inventor further discovered that the abrasion resistance of the coating layer surface can be increased by ensuring that the center - line mean roughness of the coating layer surface falls within the prescribed value range mentioned above. This invention was achieved on the basis of these discoveries. The above and other features of the invention will become apparent from the following description made with reference to the drawings, in which: Figure 1 is a schematic view of the structure of a recording medium according to the invention. Figure 2 is a scanning electron micrograph showing cracks in the surface of a recording medium according to the invention. The invention will now be exemplified in detail. The recording medium according to this invention may be produced as follows. The structure of the recording medium is shown in Figure 1, in which reference numeral 1 designates a base material made of plastic film. While the type of plastic is not particularly specified, preferable examples include polyethylene terephthalate, polypropylene, polyvinyl chloride, polystyrene, a foamed film of one of these or a film of one of these admixed with an inorganic pigment such as calcium carbonate. It is also possible to use a laminated paper obtained by laminating a film of one of the foregoing plastics on one or both sides of ordinary paper. A pigment coating layer 2 is formed on the base material 1 as a writing layer. The pigment coating layer 2 consists basically of a binder and an inorganic or organic pigment. As the binder there is used a polymer exhibiting film forming property. For obtaining optimum water resistance, the binder composition (in terms of solid component) preferably consists mainly of a hydrophobic binder. Since a hydrophilic binder having a hydroxyl group, carboxyl group or the like lowers the water resistance of the coating, it should, if used at all, preferably be limited to less than 20 wt% of the total binder composition. Usable polyvinyl binders include, for example, polystyrene, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, saturated copolymerized polyester, polyvinyl chloride-polyvinyl acetate copolymer, alkyd resin, SBR (styrene-butadiene rubber), ABS (acrylonitrile-butadiene-styrene copolymer) and the like. Use of a hydrophilic binder is effective for improving printability and adhesion with water-color ink and the like. Preferable hydrophilic binders include, for example, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetal, polyethylene glycol, polyethyleneimine, carboxymethyl cellulose, starch, casein and the like. Preferable inorganic pigments include, for example, synthetic silica, clay, talc, diatomaceous earth, calcium carbonate, calcined kaolin, titanium oxide, zinc oxide, satin white and the like. Preferable organic pigments include, for example, polystyrene, poly(methyl methacrylate), styrene-acryl copolymer and the like. The grain diameter of the pigment is preferably 0.1 µm - 30 µm, more preferably 1 µm - 20 µm. The pigment coating layer 2 is formed by coating the base material 1 with a pigment coat-forming liquid in an amount to obtain a dried coating thickness generally of 5 µm - 50 µm and preferably of 10 µm - 40 µm, and drying the applied coating. The coating is applied using a conventional method such as roll coating, wire-bar coating, gravure coating or air knife coating. The reason for limiting the thickness of the pigment coating layer to within the aforesaid range is that when it is thinner than 5 µm, its ink absorption ability may be extremely poor and the time required for ink drying may exceed 3 minutes, and when it is thicker than 50 µm, its strength may become unacceptably low. The solid content weight R and the pigment solid content weight F of the coat-forming liquid may be adjusted to a ratio F/R of 0.3 - 2.0. Although a higher pigment content improves the ink liquid absorption ability, drying property and smear resistance of the pigment coating layer, it reduces the coating strength. In this invention, therefore, high coating strength is achieved by limiting the pigment content to within the foregoing range and the absorption ability is increased by producing fine cracks, as will now be explained. As shown in Figure 1 and the electron micrograph (x1000) of Figure 2, the surface of the pigment coating layer 2 formed in the aforesaid manner has irregular line-like cracks 3 too small to observe with the naked eye. Most of the cracks 3 extend as far as the surface of the base material 1. In other words, the pigment coating layer 2 is a film constituted of scales 4. The subdivisions defined by the fine, two-dimensional network of irregular cracks have areas (converted to the areas of equivalent squares) which generally fall in the range of 10 µm x 30 µm - 100 µm x 300 µm. The areas of the subdivisions can be varied substantially as desired within this range by appropriate adjustment of the coat-forming liquid, film formation conditions, and/or the drying conditions after coating. It is therefore possible to optimize the pigment coating layer 2 for its purpose of use. The crack width is in the approximate range of 1 µm - 10 µm. As will be better understood from Example 1 set out below, the formation of cracks can be promoted by conducting the drying in two or more stages and setting the drying temperature in the second stage 10 °C - 40 °C lower than in the first stage. When cracks are formed in the surface of the pigment coating layer in the foregoing manner and a pigment coating layer containing only a small amount of pigment (having an F/R ratio of not more than 2.0) is used, the gaps formed by the cracks make a large contribution to ink absorption. As a result, it is possible to realize an ink absorption ability sufficient for achieving the purpose of the invention, while, at the same time, realizing a high coating strength. Another feature of the invention is that the value of the center-line mean roughness Ra of the coating surface is controlled to 0.5 µm - 2.5 µm. Ra is defined as follows. The center-line mean roughness Ra, when the roughness curve has been expressed by y = f (x) , shall be a value, being expressed in micrometer (µm), that is obtained from the following formula, extracting a part of measuring length ℓ in the direction of its center - line from the roughness curve, and taking the center - line of this extracted part as X - axis and the direction of vertical magnification as Y - axis (JIS B0601-1982 Definitions and Designation of Surface Roughness). Ra = l ℓ ∫ ℓ 0 | f (x) | dx Since the surface projections are very easily removed by frictional impact when the value of Ra exceeds 2.5 µm, it is necessary for the value of Ra to be not more than 2.5 µm, preferably not more than 2.0 µm. Although a value of Ra smaller than 0.5 µm provides high resistance to frictional impact, the surface of the resulting cards or the like cannot easily be printed with a printer etc. because their excessive smoothness makes them difficult to feed and may cause two or more cards to be fed simultaneously. The value of Ra is therefore defined as not less than 0.5 µm, preferably not less than 1.0 µm. A value of Ra in the range of 0.5 µm - 2.5 µm can be achieved by reducing the pigment content so that the F/R ratio falls in the range of 0.3 - 2.0. The adhesion between the base material 1 and the pigment coating layer 2 can be effectively strengthened by providing the surface of the base material 1 with an undercoating or subjecting it to corona discharge treatment. To prevent generation of static electricity, the surface and/or interior of the pigment coating layer 2 can be subjected to antistatic treatment. The invention will now be explained with reference to examples. The parts or % mentioned in the examples and comparative examples are parts or % by weight, respectively. Example 1 As the base material there was used a polypropylene film including calcium carbonate (Yupo DFG-65, product of Ohji-Yuka Synthetic Paper Co., Ltd.). One surface of the base material was provided with an undercoating of acrylic binder (Movinyl 8020, product of Hoechst AG) having a dry film thickness of 1 µm. Next, a coat-forming liquid prepared by thoroughly mixing and dispersing 25 parts silica (Mizukasorb C-1, product of Mizusawa Industrial Chemicals, Ltd.) and 70 parts water in 100 parts of a saturated copolymerized polyester binder (solid content concentration of 30%; Vyronal MD-1200, product of Toyobo Co., Ltd.) was coated on the undercoating of the base material using a reverse roll coater. The result was dried by passage through a three-chamber drier (Temp. 1st chamber : 110 °C, 2nd chamber : 80 °C, 3rd chamber : 100 °C). As a result there was obtained a recording medium according to the invention having many surface cracks defining a large number of subdivisions measuring about 70 µm x 100 µm. It is preferable to establish a temperature gradient in the drier, with the temperature of the 1st chamber being 80 °C - 120 °C and the temperature of the 2nd chamber being 10 °C - 40 °C lower than that of the 1st chamber, and to drive out the residual solvent in the 3rd chamber. The pigment coating layer of the so-produced recording medium had a thickness of 20 µm and a center-line mean roughness of 1.6 µm. Characters stamped on the recording medium using a commercially available water-color ink stamp manufactured by Shachihata Co., Ltd. dried completely in 26 seconds. The pigment coating layer surface did not incur any surface layer removal in an abrasion test involving 1,000 reciprocations under a load of 1 kg, nor did bending cause any removal of the surface layer. Example 2 A foamed polyethylene terephthalate film (Crysper-100, product of Toyobo Co., Ltd.) used as the base material was coated on one side with an undercoating similar to that of Example 1. Next, a coat-forming liquid prepared by thoroughly mixing and dispersing 45 parts diatomaceous earth (Radiolight F, product of Showa Chemical Co., Ltd.) and 100 parts water in 100 parts of a polyvinyl chloride-polyvinyl acetate copolymer binder (solid content of 33%; Vinybran 240, product of Nisshin Chemical Industry Co., Ltd.) was coated on the undercoating of the base material using a wire-bar coater. The result was dried by passage through a three-chamber drier (Temp. 1st chamber : 120 °C, 2nd chamber : 75 °C, 3rd chamber : 110 °C). As a result there was obtained a recording medium according to the invention having innumerable surface cracks defining subdivisions measuring about 50 µm x 80 µm. The pigment coating layer of the so-produced recording medium had a thickness of 15 µm and a center-line mean roughness of 1.8 µm. Characters stamped on the recording medium using the same water-color ink stamp as in Example 1 dried completely in 53 seconds. The pigment coating layer surface did not incur any surface layer removal in the same surface abrasion test as in Example 1, nor did bending cause any removal of the surface layer. Comparative Example 1 A recording medium was produced in the same manner as in Example 1 except that the temperatures of the 1st to 3rd chambers of the drier were all set at 70 °C. The surface of the recording medium was found to be completely free of cracks. The pigment coating layer of the so-produced recording medium had a thickness of 20 µm and a center-line mean roughness of 1.2 µm. Characters stamped on the recording medium using the same water-color ink stamp as in Example 1 took 7 minutes and 45 seconds to dry completely. Comparative Example 2 A recording medium was produced in the same manner as in Example 1 except that the coat-forming liquid was prepared by adding 90 parts silica and 130 parts water to 100 parts binder. When the coat-forming liquid was applied to the base material and dried in the same manner as in Example 1, there was obtained a recording medium having surface cracks defining areas measuring about 120 µm x 400 µm. The pigment coating layer of the so-produced recording medium had a thickness of 24 µm and a center-line mean roughness of 3.6 µm. Characters stamped on the recording medium using the same water-color ink stamp as in Example 1 dried completely in 13 seconds. However, the pigment coating surface incurred surface layer removal in an abrasion test similar to that in Example 1 after only two reciprocations, and also suffered removal of the surface layer when bent. As is clear from the foregoing description, this invention realizes a recording medium whose pigment coating layer exhibits good ink absorption ability even when formed to have high strength and is also highly resistant to frictional impacts and the like, and is thus able to provide a recording medium with excellent performance as regards both ink absorption ability and strength of its pigment coating layer.

Claims: A recording medium comprising a base material and a writing layer constituted as a pigment coating layer on at least one side of the base material, the pigment coating layer surface having irregular surface cracks and a center-line mean roughness Ra of 0.5 µm - 2.5 µm. A recording medium according to claim 1, wherein the pigment coating layer contains a pigment having a solid content weight of F and a binder whose main component is a hydrophobic polymer and has a total solid content weight of R, the ratio F/R being in the range of 0.3 - 2.0. A recording medium according to claim 1 or 2 wherein the pigment coating layer contains a binder and at least one member selected from among the group consisting of an organic pigment and an inorganic pigment, the binder being constituted mainly of a hydrophobic binder consisting of one or more resins selected from among the group consisting of polystyrene, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, saturated copolymerized polyester, polyvinyl chloride-polyvinyl acetate copolymer, alkyd resin, SBR and ABS. A recording medium according to claim 3, wherein the binder contains less than 20 wt% of a hydrophilic binder consisting of one or more resins selected from among the group consisting of polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetal, polyethylene glycol, polyethyleneimine, carboxymethyl cellulose, starch and casein, the inorganic pigment is at least one member selected from among the group consisting of synthetic silica, clay, talc, diatomaceous earth, calcium carbonate, calcined kaolin, titanium oxide, zinc oxide and satin white, and the organic pigment is at least one member selected from among the group consisting of polystyrene, poly(methyl methacrylate) and styrene-acryl copolymer. A recording medium according to claim 4, wherein the grain diameter of the pigment is in the range of 0.1 µm - 30 µm. A recording medium according to claim 5, wherein the pigment coating layer has a dried coating thickness of 5 µm - 50 µm. A recording medium according to any preceding claim further comprising an undercoating between the base material and the pigment coating layer. A recording medium according to any preceding claim wherein the irregular line-like cracks define subdivisions with areas (converted to the areas of equivalue squares) in the range of 10 µm x 30 µm - 100 µm x 300 µm and the crack width is in the range of 1 µm - 10 µm. A recording medium according to claim 4 wherein the pigment coating layer contains a binder consisting of (a) at least 80 wt% of a hydrophobic polymer and (b) less than 20 wt% of polyvinyl acetal. A method of producing a recording medium of any preceding claim comprising an optional step of providing an undercoating on a base material and a step of forming a pigment coating layer on the base material or undercoating by coating it with a coat-forming liquid consisting mainly of a binder, a pigment and a solvent and/or dispersant and drying the coat forming liquid, the drying being conducted in two or more stages using a drying temperature in the second stage that is 10 °C - 40 °C lower than that in the first stage.

Inventors: SATO KAZUO, SATO, KAZUO, Sato, Kazuo, c/o Tokyo Plant
Applicants: NISSHIN SPINNING, NISSHINBO INDUSTRIES, INC.
Application year: 1996
Main classification: B41M5
IPC codes: B41M5, B42D15, B32B27, B32B3