MONOLAYER, SELF COLOR-FORMING, PRESSURE-SENSITIVE RECORDING SHEET

Abstract: A self color-forming pressure-sensitive sheet having on its surface a single recording layer containing color former-­ incorporating microcapsules and color developer-­ incorporating microcapsules as necessary components, with the recording layer containing a waxy material such as higher fatty acid, its water-insoluble metal salt or amide, can remark­ ably prevent undesirable color formation due to folding of paper sheets without reducing necessary printing color density.

Description: DESCRIPTION MONOLAYER TYPE SELF-COLORING PRESSURE-SENSITIVE RECORDING SHEET TECHNICAL FIELD This invention relates to improvement in a self-coloring recording sheet, more particularly improvement in a self-coloring pressure-sensitive sheet having, on a surface of a support such as paper or the like, a monolayer comprising a mixture of color-former-containing microcapsules and developer-containing microcapsules. BACKGROUND ART Self-coloring pressure-sensitive recording sheets (self-contained paper) are, as well known, recording sheets wherein on one surface of a support are coated an electron donor as a color former and an electron receptor as a color developer each of which is separated by a pressure-breakable wall, and giving printed letters on said surface by contact between them owing to printing pressure. As such recording sheets, there are known those of double. layer form which has, on the surface of a support, a layer of microcapsules containing a color former solution, and a layer of a color developer formed thereon, and those of monolayer form which have, on the surface of a support, a layer of a mixture of two kinds of microcapsules each containing a color former solution and a color developer solution (Japanese Patent Publication 16,096/72). The sheets of monolayer form are excellent in productivity. For example, the formation of the layer by coating is easy, so that the yield is good. They are advantageous in quality. For example, the color density of printed letters is high because the color former and the color developer are closer to each other. On the other hand, they are disadvantageous in that undesirable coloration is more liable to be caused by friction or folding. It is said that the above mentioned advantages and disadvantages are contrary to each other. In order to prevent coloration by friction or folding, it has been proposed to incorporate highmolecular-weight polyolefin powder into the layer (Japanese Patent Application Kokai (Laid-Open) 3,969/80). By this proposal, undesirable coloration has been improved, however there are inhibited diffusion of the color former solution and the color developer solution under printing pressure and hence contact between them, so that the color density of printed letters is not satisfactory. DISCLOSURE OF THE INVENTION The object of this invention is to provide a monolayer type self-coloring recording sheet which maintains the coloring property of conventional recording sheets, and undergoes minimized unintentional coloration by folding or the like. In this invention, the object described above can be achieved by replacing the conventional polyolefin powder by a wax material such as a wax, a higher fatty acid or its water-insoluble metal salt, amide or ester, a higher alcohol, a higher ketone, a higher amine or the like. BEST MODE FOR CARRYING OUT THE INVENTION Although the reason why the wax material used in this invention is effective to achieve the objects is not sufficiently clear, the wax material is considered to have a cushioning effect against the bending of the sheet and a lubricating effect while existing among the constituents of the layer such as pigments, adhesives, starch granules as capsule-protecting agent, and the like additives which are usually used together with colorformer-containing microcapsules and color-developercontaining microcapsules. The wax material used in this invention is a water-insoluble organic material which is solid at ordinary temperature, has a relatively low melting point (usually 2000C or lower), has a fatty touch, and is usually colorless or of a light color, and it includes ordinary waxes, for example, animal or vegetable waxes such as carnauba wax, rice wax, beeswax, shellac wax and the like, petroleum waxes such as paraffin wax, microcrystalline wax, petrolatum and the like, and synthetic waxes such as low-molecular-weight polyethylene wax and the like as well as higher fatty acids and their water-insoluble metal salts, amides and esters, higher alcohols, higher ketones, higher amines, etc. The wax material used in this invention may be either of an emulsion type or in the form of powder obtained by drying the emulsion. Although its particle size and form at the time of use are not critical, the particle size is preferably 20 um or less. Among the wax materials described above, higher fatty acids of from C16 to C18 and their water-insoluble metal salts or amides, etc. bring about the best results in this invention. Examples of these compounds include stearic acid; metal salts of stearic acid such as zinc stearate, calcium stearate, barium stearate, aluminum stearate, magnesium stearate and the like; metal salts of palmitic acid such as zinc palmitate, calcium palmitate and the like; and higher fatty acid amide derivatives such as stearamide, palmitamide, N,N'-ethylenebisstearamide and the like. These may be used alone or in combination. These wax materials should be used in a relatively large amount of 5 to 40% by weight based on the total amount of the constituents of the layer, and the amount thereof is determined within this range in consideration of both coloration by folding and the color density of printed letters. As the constituents of the monolayer of the monolayer type self-coloring recording sheet of this invention, conventional ones are used. As the color former, well-known electron-donative leuco dyes may be used, which are typified by crystal violet lactone and fluoran derivatives. As the color developer, there may be used oil-soluble acidic materials, for example, those known as novolak type phenol resins such as phenol-formaldehyde resin, phenol-acetaldehyde resin, phenol-acetylene resin and the like. As solvents for dissolving these color formers and developers, conventional high-boiling solvents for no-carbon paper are used. Capsule materials for enclosing these color formers and color developers are not critical and include polyurethanes, polyureas, epoxy resins, urea-formaldehyde resins, melamine-formaldehyde resins, gelatins, polyamides, etc. The microcapsules are obtained by the well-known interfacial polymerization method (Japanese Patent Publication 27,257/69, etc.) or the in situ method (Japanese Patent Publications 3,495/69 and 9,079/76, etc.). As adhesives for forming the layer, there are used, for example, hydroxyethyl cellulose, modified starch, polyvinyl alcohol and latexes which are usually used. As pigments, heretofore well-known clays, kaolin, zinc oxide, plastic pigments and the like may be used according to a conventional technique. As capsule-protecting agents, there may be used cellulose powder, starch granules, talc, calcium carbonate, calcined kaolin and the like which are usually used in pressure-sensitive recording paper, and in addition thereto, there may be used, depending on purposes, dispersing agents, waterproofing agents, anti-foaming agents and the like which are used for usual coating. Concrete examples of this invention are described below but are not by way of limitation but by way of illustration. In the examples, all parts are by dry weight. Referential Example 1 (Production of color-former-containing microcapsules) In 90 g of Hisol SAS N-296 (a diallyethane series high-boiling solvent manufactured by Nippon Petrochemicals Co., Ltd,) was dissolved 10 g of crystal violet lactone with heating to prepare an internal oil. Subsequently, the aforesaid internal oil was emulsified in 200 g of a 5% aqueous ethylene-maleic anhydride copolymer solution, and the pH of the system was adjusted to 4.0, To the emulsion thus obtained was added, with stirring, 100 g of an aqueous solution containing 10 g of urea and 1 g of resorcin, and 25 g of a 37% aqueous formaldehyde solution was further added, after which the temperature of the system was adjusted to 550C and subjected to reaction for 3 hours to obtain a dispersion of colorformer-containing capsules having a urea-formaldehyde resin film. The particle size of the capsules was 3.6 p on the average. Referential Example 2 (Production of color-former-containing microcapsules) The same internal material as in Referential Example 1 was emulsified in a 4% aqueous styrene-maleic anhydride copolymer solution. Ten grams of melamine and 25 g of a 37% aqueous formaldehyde solution were heated at pH 8 to obtain a melamine-formaldehyde precondensation product. This product was added to the emulsion obtained above with stirring, and there were maintained the pH of the system at 5.6 and its temperature at 700C for 2 hours to obtain a dispersion of color-former-containing capsules having a melamine-formaldehyde resin film. The average particle size of the capsules was 3.7 p. Referential Example 3 (Production of color-developercontaining microcapsules) A dispersion of color-developer- containing microcapsules was obtained in the same manner as in Referential Example 1, except that 50 g of POP resin (PR 26,298 manufactured by Sumitomo Duretz Co., Ltd., a paraoctylphenol-formaldehyde resin) was dissolved in 50 g of KMC-113 (an oil manufactured by Kureha Chemicals Co., Ltd., an alkylnaphthalene series high-boiling solvent) with heating to obtain an internal material. The average particle size of the microcapsules was 3.2 p. Referential Example 4 (Production of color-developercontaining microcapsules) By use of the same internal material as in Referential Example 3, a dispersion of color-developercontaining capsules was obtained in the same manner as in Referential Example 2. The average particle size of the capsules was 3.2 p. Comparative Example 1 (Production of monolayer type selfcoloring pressure-sensitive recording paper) The capsule dispersions described in above Referential Examples I to 4 were applied in the following mixing proportions to a plain paper having a weight per 2. 2 centare of 41 g/m2 in an amount of 8 g/m2. (Dry weight) Color former capsules 14 parts Color developer capsules 25 parts Starch granules 23 parts Kaolin clay 18 parts Aqueous solution of modified starch 20 parts (concn. 1083 100 parts Four kinds of monolayer type self-coloring pressure-sensitive recording papers were produced by combinations of the color former microcapsules and the developer capsules obtained in Referential Examples 1 to 4 (combinations of Referential Examples 1 and 3; 1 and 4; 2 and 3; and 2 and 4) and were tested for the degree of appearance of undesirable linear stains due to coloration by folding. All the test samples were highly colored in a line even by slightly folding (300 g/cm) them with the coated surface inside, and they were hardly resistant to folding. However, the color density of letters printed by means of a typewriter or the like was sufficient. Comparative Example 2 (Dry weight) Color former capsules 13.6 parts Color developer capsules 24.3 parts Starch granules 22.3 parts Kaolin clay 17.5 parts Aqueous solution of modified starch 19.4 parts (concn. 10%) Calcium stearate 3.0 parts (average particle size 3 Wm) - Total 100.1 parts When in the same manner as in Comparative Example 1, recording papers were produced by combinations of the color former microcapsules and the color developer microcapsules and tested for coloration by folding, they showed undesirable deep coloration as in Comparative Example 1 even by slight force applied by the folding. However, the density of coloration of printed letters was sufficient. Example 1 (Dry weight) Color former microcapsules 12.6 parts Developer microcapsules 22.6 parts Starch granules 20.7 parts Kaolin clay 16.1 parts Aqueous solution of modified starch 18.0 parts (concn. 10%) Calcium stearate 10.0 parts (average molecular size 3 um) Total 100.0 parts Comparative Example 1 was repeated, except that the composition described above were used. Almost no coloration was caused by folding, and only slight coloration was caused by stronger folding (1,000 g/cm). Thus, a marked improvement was observed. The color developability due to printing was not different from that in Comparative Examples 1 and 2. All the recording papers obtained in this example show excellent resistance to coloration due to folding in spite of different combinations of capsules. Example 2 (Dry weight) Color former microcapsules 11.0 parts Color developer microcapsules 20.1 parts Starch granules 18.5 parts Kaolin clay 14.2 parts Aqueous solution of modified starch 16.2 parts (concn. 10%) Calcium stearate 20.0 parts (average particle size 3 pm) Total 100.0 parts Comparative Example 1 was repeated, except that the compositions described above were used. No coloration was caused at all even by strong folding conducted in the same manner as in Example 1 and improvement in resistance to bending was thus very marked. On the other hand, there was obtained a sufficient color density of typed letters, which was quite equal to that in Comparative Examples 1 and 2 and Example 1. This effect was not influenced by the combinations of the microcapsules. Example 3 (Dry weight) The color former microcapsules of 11.0 parts Referential Example 2 The color developer microcapsules of 20.1 parts Referential Example 4 Starch granules 18.5 parts Kaolin clay 14.2 parts Aqueous modified starch solution 16.2 parts (concn. 10%) Wax material (particle size 0.1 - 20 fix Total 100.0 Comparative Example 1 was repeated, except that as the wax material, there was selected each of stearic acid, palmitic acid, lauric acid, paraffin wax, zinc stearate, and N,N'-ethylenebisstearamide, and that the combinations of the microcapsules was specified as shown in the above table. After the color developability due to printing was ascertained, appearance of an undesirable linear stain due to folding (300 g/cm) was tested. All the recording papers produced by using the respectiye wax materials showed a great improvement in resistance to folding coloration as compared with those produced by adding no wax. Almost no linear stain was observed. As shown in the above examples, the monolayer type self-coloring pressure-sensitive recording materials could be freed from linear stains due to folding by using, as one constituent, a wax material typified by stearic acid or derivatives thereof which show resistance to marked coloration by bending during coating.

Claims: WHAT IS CLAIMED IS: 1. A self-coloring pressure-sensitive recording sheet comprising a single recording layer which contains color-former-containing microcapsules and color-developercontaining microcapsules and is formed on a support, characterized in that the recording layer contains a water-insoluble wax material. 2. A self-coloring pressure-sensitive recording sheet according to Claim 1, wherein the content of the aforesaid wax material ranges from 5 to 40% by weight based on the whole solids in the recording layer. 3. A self-coloring pressure-sensitive recording sheet according to Claim 1, wherein the wax material is a powder of an animal or vegetable wax, a petroleum wax, a low-molecular-weight polyethylene wax, a higher fatty acid, its water-insoluble metal salt, amide or ester, a higher alcohol, a higher ketone or a higher amine. 4. A self-coloring pressure-sensitive recording sheet according to Claim 3, wherein the wax material is a higher fatty acid or its water-insoluble metal salt or amide. 5. A self-coloring pressure-sensitive recording sheet according to Claim 4, wherein the higher fatty acid has 16 to 18 carbon atoms. 6. A self-coloring pressure-sensitive recording sheet according to Claim 5, wherein the higher fatty acid is stearic acid. 7. A self-coloring pressure-sensitive recording sheet according to Claim 5, wherein the higher fatty acid is palmitic acid.

Inventors: IIDA, TOSHIZO
Applicants: MITSUBISHI PAPER MILLS, LTD.
Application year: 1983
Main classification: B41M5
IPC codes: B41M5