EP0120972B1

EP0120972B1 - Monolayer, self color-forming, pressure-sensitive recording sheet

Info

Publication number: EP0120972B1
Application number: EP19830903197
Authority: EP
Inventors: Toshizo Iida
Original assignee: Mitsubishi Paper Mills Ltd
Current assignee: Mitsubishi Paper Mills Ltd
Priority date: 1982-10-06
Filing date: 1983-10-05
Publication date: 1988-03-16
Also published as: EP0120972A1; EP0120972A4; JPS5964387A; DE3375984D1; WO1984001329A1

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 remarkably prevent undesireable color formation due to folding of paper sheets without reducing necessary printing color density.

Description

This invention relates to a self-coloring pressure-sensitive recording sheet comprising, on a surface of a support such as paper, a single recording layer which contains color-former-containing microcapsules and color-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 comprising two layers which have, 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 comprising a single recording layer 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 recording sheets comprising a single recording layer are excellent in productivity. For example, the formation of the recording 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 high-molecular-weight polyolefin powder into the recording 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.
US-A-4 352 855 relates to a transfer-onto-plain paper type pressure-sensitive copying sheet which can form a copy image on a plain paper. In US-A-4 352 855, a wax mixture comprising a saturated fatty acid amide having 14 or less carbon atoms is used to improve the transferability of a copy image from the pressure-sensitive copying sheet to a plain paper and to prevent coloration.
The object of this invention is to provide a self-coloring pressure-sensitive recording sheet comprising a single recording layer which contains color-former-containing microcapsules and color-developer-containing microcapsules and is formed on a support, said recording sheet maintaining the coloring property of conventional recording sheets, and undergoing minimized unintentional coloration upon friction or folding.
The object described above can be achieved by a recording sheet comprising a single recording layer which contains from 5 to 40 % by weight, based on the whole solids in the recording layer, of a water-insoluble wax material consisting of a higher fatty acid having 16 to 18 carbon atoms or its water-insoluble metal salt or amide.
Although the reason why the water-insoluble wax material used in this invention is effective to achieve the object is not sufficiently clear, the wax material is considered to have a cushioning effect against the bending of the recording sheet and a lubricating effect while existing among the constituents of the recording layer such as pigments, adhesives, starch granules as capsule-protecting agent, and the like additives which are usually used together with color-former-containing microcapsules and color-developer-containing microcapsules.
The water-insoluble wax material is solid at ordinary temperature, has a relatively low melting point (usually 200°C or lower), has a fatty touch, and is usually colorless or of a light color.
The water-insoluble wax material 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 pm or less.
Examples of the water-insoluble wax material include stearic acid; water-insoluble metal salts of stearic acid such as zinc stearate, calcium stearate, barium stearate, aluminum stearate and magnesium stearate; water-insoluble metal salts of palmitic acid such as zinc palmitate and calcium palmitate; and water-insoluble higher fatty acid amides such as stearamide, palmitamide and N,N'- ethylenebisstearamide. 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 recording 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 single recording layer of the self-coloring pressure-sensitive 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 and phenol-acetylene resin. As solvents for dissolving these colorformers 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 e.g. polyurethanes, polyureas, epoxy resins, urea-formaldehyde resins, melamine-formaldehyde resins, gelatins and polyamides.
The microcapsules are obtained by the well-known interfacial polymerization method e.g. (Japanese Patent Publication 27,257/69) or the in situ method (e.g. Japanese Patent Publications 3,495/69 and 9,079/76).
As adhesives for forming the recording 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 and plastic pigments may for example be used according to a conventional technique.
As capsule-protecting agents, there may be used e.g. cellulose powder, starch granules, talc, calcium carbonate and calcined kaoline which are usually used in pressure-sensitive recording paper, and in addition thereto, there may be used, depending on purposes, agents such as dispersing agents, waterproofing agents and anti-foaming agents which are used for usual coating.
Concrete examples of this invention are described below 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 55°C and subjected to reaction for 3 hours to obtain a dispersion of color-former-containing microcapsules having a urea-formaldehyde resin film. The particle size of the microcapsules was 3.6 pm 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 sytrene-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 70°C for 2 hours to obtain a dispersion of color-former-containing microcapsules having a melamine-formaldehyde resin film. The average particle size of the microcapsules was 3.7 pm.

Referential Example 3 (Production of color-developer-containing 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 alkyl naphthalene series high-boiling solvent) with heating to obtain an internal material. The average particle size of the microcapsules was 3.2 pm.

Referential Example 4 (Production of color-developer-containing microcapsules)

By use of the same internal material as in Referential Example 3, a dispersion of color-developer-containing microcapsules was obtained in the same manner as in Referential Example 2. The average particle size of the microcapsules was 3.2 pm.

Comparative Example 1 (Production of self-coloring pressure-sensitive recording paper comprising a single recording layer)

The microcapsule dispersions described in above Referential Example 1 to 4 were applied in the following mixing proportions in an amount of 8 g/m² to a plain paper having a weight per centare of 41 g_/m ² _.
Four kinds of self-coloring pressure-sensitive recording papers each comprising a single recording layer were produced by combinations of the color-former-containing microcapsules and the color-developer-containing microcapsules obtained in Referential Examples 1 to 4 (combinations of the 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

When in the same manner as in Comparative Example 1, recording papers were produced by combinations of the color-former-containing microcapsules and the color-developer-containing 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

Comparative Example 1 was repeated, except that the composition described above was 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 microcapsules.

Example 2

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

Comparative Example 1 was repeated, except that as the wax material, there was selected each of stearic acid, palmitic acid, 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 respective 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 self-coloring pressure-sensitive recording materials comprising a single recording layer could be freed from linear stains due to folding by using, as one constituent, a wax material typified by stearic acid or metal salts or amides thereof which show resistance to marked coloration by bending during coating.

Claims

1. A self-coloring pressure-sensitive recording sheet comprising a single recording layer which contains color-former-containing microcapsules and color-developer-containing microcapsules and is formed on a support, characterized in that the recording layer contains from 5 to 40% by weight, based on the whole solids in the recording layer, of a water-insoluble wax material consisting of a higher fatty acid having 16 to 18 carbon atoms or its water-insoluble metal salt or amide.

2. A self-coloring pressure-sensitive recording sheet according to claim 1, wherein the higher fatty acid is stearic acid.

3. A self-coloring pressure-sensitive recording sheet according to claim 1, wherein the higher fatty acid is palmitic acid.