GB2244728A - Pressure sensitive sheets - Google Patents

Abstract

A self-coloring, pressure-sensitive recording sheet formed of a substrate and a coating layer comprising microcapsules enclosing an electron-donating dye and an organic developer, characterized in that the coating layer contains a compound of the formula (I), <IMAGE> wherein each of R1 and R2 is independently an alkyl group having 4 to 12 carbon atoms. The recording sheet has excellent penetrability to a desensitizing ink, produces excellent desensitization effect immediately after printing and has excellent coloring properties in coloring rate and color density.

Description

The present invention relates to a single layer type, self-coloring, pressure-sensitive recording sheet, more specifically to a single layer type, selfcoloring, pressure-sensitive recording sheet which a desensitizing ink penetrates rapidly.

A self-coloring, pressure-sensitive recording sheet has been developed from a so-called "no-carbon" (carbon-free) pressure-sensitive recording sheet, and is also called a self-contained paper. A self-coloring, pressure-sensitive recording sheet is conventionally produced by applying microcapsules each of which encloses an electron-donating dye (to be referred to as "color former" hereinafter) dissolved in a highboiling solvent to form a first layer on one side of a substrate such as paper and applying an electronaccepting substance (to be referred to as "developer" hereinafter) to form second layer on the first layer, or by forming these two layers as one integrated layer.

A self-coloring, pressure-sensitive recording sheet, which is capable of giving a pressure-applied, colored image even with one paper sheet, has specific effectiveness in that printing can be effected without any typewriter ribbon, that a duplicate record can be obtained by laying a wood free paper on it and writing thereon with a ball-point pen, and the like. In recent years, therefore, there is an increasing demand for it.

The color former includes triallylmethanephthalides such as crystal violet lactone, 3,3-bis(pdimethylaminophenyl)naphthalide and 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)-6-dimethylaminophthalide; acyl derivatives of Methylene Blue such as benzoyl, anisoyl and pivaloyl derivatives of Methylene Blue; xanthenephthalides such as 3-diethylamino-6methyl-7-chlorofluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-methylcyclohexylamino-6-methyl-7-anilinofluoran and 3-ethyl-p-tolylamino-6-methyl-7-anilinofluoran; and the like.

These color formers are dissolved in a highboiling solvent when used in a self-coloring, pressuresensitive recording sheet. The high-boiling solvent includes aromatic hydrocarbons such as alkylnaphthalenes typified by diisopropylnaphthalene, diarylalkanes typified by l-phenyl-l-xylylethane, alkylbiphenyls typified by isopropylbiphenyl, triaryldimethanes, alkylbenzenes, benzylnaphthalenes, diarylalkylenes, arylindanes; carboxylic acid ester compounds typified by dibutyl phthalate, dioctyl maleate, diethyl azelate, etc; phosphoric acid ester compounds typified by tricresyl phosphate; plant oils such as caster oil, soybean oil, cotton seed oil and modified oils of these; high-boiling fractions of natural products (composed of aliphatic hydrocarbons) such as mineral oil; and the like.

The microcapsulation is -conventionally carried out by a coacervation method, in-situ polymerization method, interfacial polymerization method, etc.

The developer includes inorganic developers and organic developers. Examples of the inorganic developers are natural clay minerals such as acid clay, attapulgite, etc.; activated clay obtained by slightly or moderately treating acid clay which is montmorillonite type clay minerals with a mineral acid; semisynthetic solid acid disclosed in Japanese Patent Application Kokai No. S. 57-15996; etc. Examples of the organic developers are various phenolic compounds, novolak type phenolic resins, polyvalent metal salts of aromatic carboxylic acids, a polyvalent metallized substance of a salicylic resin proposed recently, etc.

The method for the production of a selfcoloring, pressure-sensitive recording sheet is as follows. One production method comprises dissolving the above color former in a high-boiling solvent and enclosing it with microcapsules, applying the microcapsules and a binder on a substrate such as paper in a proportion of 3 to 10 g/m2 (in terms of dry weight of solid content) by means of an air knife coater, etc., applying a finely milled organic developer and a binder in a proportion of 5 to 10 g/m2 (in terms of dry weight of solid content) by means of an air knife coater method, etc., and drying the resultant layer, whereby a so-called laminated self-coloring, pressure-sensitive recording sheet having a coloring layer (laminated selfcoloring, pressure-sensitive recording sheet) can be produced.Further, another method comprises mixing microcapsules enclosing the above color former and a finely milled organic developer together with a binder, applying the resultant mixture in a proportion of 4 to 15 g/m2 (in terms of dry weight of solid content) by means of to an air knife coater method, and drying the resultant layer, whereby a single layer type, selfcoloring, pressure-sensitive recording sheet having a coloring layer (single layer type, self-coloring, pressure-sensitive recording sheet) can be obtained.

These self-coloring, pressure-sensitive recording sheets are used as a writing paper sheet in such a manner that a wood free paper sheet is laid on them and a printing is formed thereon under pressure with a pencil or a dot printer.

When it is used as a writing paper sheet formed by laying wood free paper on it, coloring is inhibited by applying a desensitizing ink in a portion where no coloring is required (the degree of coloring inhibition is referred to as a desensitization effect hereinafter).

The desensitizing ink comprises a desensitizer compound as a main component. The desensitizer compound is often an organic compound being mostly in a liquid state, having a polyether chain in its molecule such as polyethylene oxide or polypropylene oxide and having a molecular weight of several hundred or more. In addition to the desensitizer, the desensitizing ink generally contains a conventionally known vehicle, binder, pigment, etc.

In general, the desensitizing ink is applied by a flexography, gravure, offset or letterpress method, and the drying of the ink is achieved when it penetrates a printed surface or paper. The penetration of the ink, however, is not achieved instantly after it is printed, and takes several tens of minutes or more.

However, a printing operator judges an amount of poured ink (coating weight of ink per unit area, ain2) with which sufficient desensitization effect is obtained by writing on it with a ball-point pen or spraying a so-called desensitization checker, which is a solution of the color former in a solvent, supplied from a bomb.

Since, however, the self-coloring, pressuresensitive recording sheet is formed of a laminated layer of a color former and a developer or is formed of a coloring layer as a single layer, it is difficult to determine an amount of poured ink with which a sufficient desensitization effect is obtained, i.e. a proper amount of poured ink, before the ink has penetrated the entire layer.

When an organic developer, which is less porous and has a smaller specific surface area than an inorganic developer is used, the ink takes a further longer time to penetrate it, and it requires several hours to determine an amount of poured ink with which a sufficient desensitization effect is obtained after the ink has penetrated the entire coloring layer.

Therefore, the measure to be taken is to use an excess amount of poured ink. When the amount of poured ink is increased, however, the following troubles are often caused. The sensitizing ink is sometimes transferred from a paper roll of a printing machine to the self-coloring, pressure-sensitive recording sheet, or the desensitizing ink gradually migrates to make it impossible to color a coloring-required portion while printed self-coloring, pressure-sensitive recording sheets are stored with one on another or in an taken-up state.

The organic developer is excellent in fluidity when dissolved or dispersed in a liquid, coloring density and lightfastness of an image, gives a high value to a product, and fascinates manufacturers.

Since, however, the organic developer hinders a desinsetizing ink from penetrating the recording sheet as described above, it has a serious problem for practical use in view of efficiency in printing operation.

Meanwhile, for the laminated recording sheet, it is required to carry out the application procedure twice as described above, and hence high thermal energy and electric power are required. And, since the laminated recording sheet is formed of two layers, one layer of microcapsules each of which encloses a color former and the other layer of a developer, the coloring rate thereof is low and the density is also low. On the other hand, the single layer type recording sheet has a great advantage in that it requires the application procedure only once and therefore permits improved operation efficiency. Moreover, as far as the quality thereof is concerned, the microcapsules and color former are adjacent to each other and positioned as a mixture, and hence, an excellent coloring rate and density can be obtained under low pressure.

It is therefore an object of the present invention to provide a single layer type, self-coloring, pressure-sensitive recording sheet containing an organic developer which sheet a desensitizing ink penetrates rapidly.

The present inventors have diligently made extensive experiments and studies, and have consequently developed a single layer type, self-coloring, pressuresensitive recording sheet containing an organic developer which sheet permits a desensitizing ink to penetrate the sheet rapidly and maintains excellent properties with regard to a coloring rate, density, etc.

According to the present invention, there is provided a single layer type, self-coloring, pressuresensitive recording sheet which comprises a substrate and a coating layer provided on the substrate, the coating layer comprising an organic developer and microcapsules enclosing an electron-donating color former, characterized in that the coating layer contains a compound of the formula (I):

wherein each of R1 and R2 is independently an alkyl group having 4 to 12 carbon atoms.

The present invention will be detailed hereinafter.

The recording sheet of the present invention comprises a substrate and a coating layer.

The substrate includes a paper, synthetic paper, film, etc., and a paper is mainly used.

The coating layer contains, as essential components, an organic developer, microcapsules enclosing an electron-donating color former and a compound of the above formula (I) (to be referred to as "Compound I" hereinafter). In general, the coating layer further contains a pigment and a binder.

Specifically, Compound I includes sodium dibutylsulfosuccinate, sodium diamylsulfosuccinate, sodium dihexylsulfosuccinate, sodium diheptylsulfosuccinate, sodium dioctylsulfosuccinate, sodium dinonylsulfosuccinate, sodium didecylsulfosuccinate, sodium diundecylsulfosuccinate, sodium dilaurylsulfosuccinate, and the like. These compounds can be used alone or in combination of two or more. Among them, particularly preferred are sodium diheptylsulfosuccinate, sodium dioctylsulfosuccinate and sodium dinonylsulfosuccinate.

The amount of Compound I for use per 100 parts by weight of the organic developer is preferably 0.05 to 3 parts by weight, more preferably 0.1 to 1 part by weight. When this amount is less than 0.05 part by weight, the effect on improvement in the penetrability s low. When it is more than 3 parts by weight, the coloring density is decreased.

The organic developer for use in the present invention includes novolak type phenolic resins, polyvalent metal salts of an aromatic carboxylic acid, polyvalent metal salts of salicylic acid, etc.

The novolak type phenolic resins obtained by polycondensation of phenol derivatives and lower aldehydes typified by formaldehyde are systematically described in Japanese Patent Application Kokoku No. S.

42-20144. Specific examples thereof are a p-octylphenolic resin, p-benzylphenolic resin, p-cumylphenolic resin, p-tert-butylphenolic resin and p-phenylphenolic resin.

The polyvalent metal salts of an aromatic carboxylic acid are disclosed in Japanese Patent Application Kokoku Nos. S. 49-10856, 51-25174, 52-1327, 52-7372, etc. Specific examples of the aromatic carboxylic acid are a-phenylbenzyl ester of 4hydroxybenzoic acid, phenethyl 4-hydroxybenzoate, cinnamyl ester of 4-hydroxybenzoic acid, m-fluorobenzyl ester of 4-hydroxybenzoic acid, 4,4'-dihydroxy-2,2'dibutylphenylsulfone, cinnamyl orsellinate, benzyl ester of 5,5'-methylenebis-2,4-dihydroxybenzylic acid, mphenylphenoxyethyl orsellinate, bisphenol A bis(phydroxybenzoic acid) ester, 2,4-dihydroxybenzoic acid pmethylbenzyl ester, 2,4-dihydroxy-6-methylbenzoic acid o-methylbenzyl ester, 4,4-diacetyldiphenylurea, 5-p-a methylbenzyl-a-methylbenzylsalicylic acid, 5-p-benzyl-amethylbenzylsalicylic acid, 3-xylyl-5-(a,a-dimethyl- benzyl)salicylic acid, 3-5-bis(a-methylbenzyl)salicylic acid, 2-hydroxy-l-a-ethylbenzyl-3-naphtoic acid, etc.

The polyvalent metal is selected from calcium, magnesium, copper, zinc, tin, barium, cobalt, etc., and zinc is particularly preferable.

The polyvalent metal salts of a salicylic resin are disclosed in Japanese Patent Application Kokai Nos. S. 63-186729 and 63-254124. Specific examples thereof are zinc salt of a resin produced by reacting 3,5-bis(4-methylbenzyl)salicylic acid with styrene, zinc salt of a resin produced by reacting 5-(4-methylbenzyl)salicylic acid with o-methylstyrene, zinc salt of a resin produced by reacting 3-phenylsalicylic acid with styrene, zinc salt of a resin produced by reacting 3,5bis(4-methylbenzyl)salicylic acid with benzyl chloride, zinc salt of a resin produced by reacting 5-(a,adimethylbenzyl)salicylic acid with p-methylbenzyl chloride, zinc salt of a resin produced by reacting 3tert-butyl-5-phenylsalicylic acid with 2,4-dimethylbenzyl chloride, and the like.

In the present invention, any color former which is generally used for pressure-sensitive recording materials can be used. Specifically, the following compounds may be mentioned: (i) Triarylmethane type compounds 3, 3-bis (p-dimethylaminophenyl ) -6-dimethyl- aminophthalide (Crystal Violet Lactone), 3,3-bis-(p dinethylaminophenyl )phthalide, 3-(p-dimethylamino phenyl)-3-(l,2-dimethylindole-3-yl)phthalide, 3-(pdimethylaminophenyl)-3-(2-methylindole-3-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-phenylindole-3-yl)phthalide, 3,3-bis(l,2-dimethylindole-3-yl)-5- dimethylaminophthalide, 3,3-bis(l,2-dimethylindole-3yl)-6-dimethylaminophthalide, 3,3-bis(9-ethylcarbazole 3-yl ) -5-dimethylaminophthalide, 3,3-bis(2-phenylindole3-yl)-5-dimethylaminophthalide, 3-p-dimethylaminophenyl 3-( l-methylpyrrole-2-yl)-6-dimethylaminophthalide, etc.

(ii) Diphenylmethane type compounds 4,4 '-bis (dimethylaminophenyl ) benzhydryl benzyl ether, N-chlorophenylleuccauramine, N-2,4,5-trichloro phenylleucoauramine, etc.

(iii) Xanthene type compounds Rhodamine B anilinolactam, Rhodamine B pchloroanilinolactam, 3-diethylamino-7-dibenzlaminofluoran, 3-diethylamino-7-octylaminofluoran, 3-diethylamino-7-phenylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino7-(3,4-dichloroanilino)fluoran, 3-diethylamino-7-(2chloroanilino)fluoran, 3-diethylamino-6-methyl-7anilinofluoran, 3-(N-ethyl-N-tolyl)amino-6-methyl-7anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-tolyl)amino-6-methyl-7-phenethylfluoran, 3diethylamino-7-(4-nitroanilino)fluoran, 3-dibutylamino6-methyl-7-anilinofluoran, 3-(N-methyl-N-propyl)amino-6 metyl-7-anilinofluoran, 3-(N-ethyl-N-isoamyl)amino-6methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexyl)amino6-methyl-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofuryl)amino-6-methyl-7-anilinofluoran, etc.

(iv) Triazine type compound Benzoyl Leucomethylene Blue, p-nitrobenzoyl Leucomethylene Blue, etc.

(v) Spiro type compound 3-methylspirodinaphthopyran, 3-ethylspirodinaphthopyran, 3,3'-dichlorospirodinaphthopyran, 3benzylspirodinaphthopyran, 3-methylnaphtho-(3-methoxybenzo)spiropyran, 3-propylspirobenzopyran, etc.

These compounds may be used alone or in combination of two or more.

The pigment for use in the present invention is specifically at least one selected from the group consisting of inorganic pigments such as kaolin, calcined kaolin, natural silica, titanium oxide, calcium carbonate, etc.; and organic pigments such as synthetic silica, a styrene-methacrylic acid copolymer, a ureaformaldehyde condensate, polystyrene resin, etc. A urea-formaldehyde condensate is particularly preferred.

The binder for use in the present invention specifically includes water-soluble polymers such as starch, phosphate esterified starch, casein, polyethylene oxide, polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, etc.; latex emulsions of hydrophobic pclymers such as those of styrenebutadiene type, acrylic type and vinyl acetate type; and the like.

In the present invention, a coating composition for a single layer type, self-coloring, pressure sensitive recording sheet is prepared from the above components and optionally, d dispersant, a capsule protecting agent, an antifoamer and an antiseptic. The thus prepared coating composition is applied to a substrate such as paper by means of a coating machine such as an air knife coater so that the amount of the resultant coating is preferably 4 to 15 g/m2 (dry weight of solid content), and the resultant coating is dried.

The amount of the coating is more preferably 5 to 10 g/m2.

Most preferred embodiments and excellent effects of the present invention will be explained by reference to the most typical examples, in which "part" stands for "part by weight" and "%" stands for "% by weight).

I. Production of single layer type, self-coloring recording sheet: Example 1 (1) Preparation of microcapsules enclosing 40% color former 5 Parts of Crystal Violet Lactone (CVL) as a color former was dissolved in 95 parts of phenylxylylethane (boiling point 2960C) as a high-boiling hydrocarbon oil under heat, and the resultant solution was added to 120 parts of a 5% aqueous solution of a styrene-maleic anhydride copolymer (pH 4.8). The mixture was emulsified at a liquid temperature 600C so as to obtain an emulsion having an average particle diameter of 4.5 urn.

12.5 Parts of melamine and 16.1 parts of a 37% formaldehyde aqueous solution were added to 60 parts of water and dissolved in the water at pH 9.5 under heat to obtain a melamine-formaldehyde initial condensate. The melamine-formaldehyde initial condensate was added to the above emulsion. And then the while resultant mixture was allowed to react at 750C for 2 hours with stirring. Subsequently, the reaction mixture was adjusted to pH 8.5 at a liquid temperature of not higher than 400C to give microcapsules enclosing 40% of the color former.

(2) Preparation of 25% developer dispersion (A) p-phenylphenolic resin 100 parts 10% sodium polyacrylate aqueous solution 30 parts Water 270 parts A mixture liquid of the above components was milled in a ball mill so as to prepare a 25% developer dispersion (A) having an average particle diameter of 3 urn.

(3) Preparation of 10% urea-formaldehyde condensate dispersion Urea-formaldehyde condensate 100 parts 10% sodium polyacrylate aqueous solution 50 parts Water 850 parts A mixture liquid of the above components was milled in a ball mill so as to prepare a 10% ureaformaldehyde condensate dispersion having an average particle diameter of 4 urn.

(4) Production of recording sheet 80 Parts of wheat starch was added to 342 parts of water, and then dispersed and mixed in the water. Thereafter, 200 parts of the 10% urea-formaldehyde condensate dispersion obtained in the above (3), 50 parts of the microcapsules enclosing 40% of the color former, obtained in the above (1), 40 parts of the 25% developer dispersion (A) obtained in the above (2), 0.5 part of a 1% sodium dioctylsulfosuccinate aqueous solution, 100 parts of a 10% polyvinyl alcohol aqueous solution and 20 parts of 50% carboxy-modified styrenebutadiene latex were saccessively added thereto and dispersed therein to obtain a coating composition.

This coating composition was coated on a wood free paper sheet having a basis weight of 40 g/m2 by means of an air knife coater so that the coating weight was 6 g/m2 (dry weight of solid content), whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Example 2 The same procedure as in Example 1 was repeated, except that the amount of water in (4) was decreased from 342 parts to 327 parts and the amount of a 1% sodium dioctylsulfosuccinate aqueous solution in (4) was increased form 0.5 part to 15 parts, whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Example 3 The same procedure as in Example 1 were repeated, except that the amount of water in (4) was decreased from 342 parts to 312 parts and the amount of a 1% sodium dioctylsulfosuccinate aqueous solution in (4) was increased from 0.5 part to 30 parts, whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Example 4 (2) Preparation of 25% developer dispersion (B) zinc 3,5-bis(a-methylbenzyl)salicylate 100 parts 10% sodium polyacrylate aqueous solution 30 parts Water 270 parts A mixture liquid of the above components was milled in a ball mill so as to prepare a 25% developer dispersion having an average particle diameter of 3 urn.

(4) Production of recording sheet 80 Parts of wheat starch was added to 327 parts of water, and dispersed and mixed in/with the water. Thereafter, 200 parts of the 10% ureaformaldehyde condensate dispersion obtained in (3) of Example 1, 50 parts of the microcapsules enclosing 40% of the color former, obtained in (1) of Example 1, 40 parts of the 25% developer dispersion (B) obtained in the above (2), 15 parts of a 1% sodium dioctylsulfosuccinate aqueous solution, 100 parts of a 10% polyvinyl alcohol aqueous solution and 20 parts of 50% carboxymodified styrene-butadiene latex were consecutively dispersed and mixed in/with the above dispersion to obtain a coating composition.

This coating composition was coated on a wood free paper sheet having a basic weight of 40 g/m2 with an air knife coater such that the coating weight was 6 g/m2 (dry weight of solid content), whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Example 5 (2) Preparation of 25% developer dispersion (C) zinc 3-tert-butyl-5-phenylsalicylate 100 parts 10% sodium polyacrylate aqueous solution 30 parts Water 270 parts A mixture liquid of the above components was milled in a ball mill so as to prepare a 25% developer dispersion (C) having an average particle diameter of 3 urn.

(4) Production of recording sheet The same procedure as in Example 4 was repeated, except that the 25% developer dispersion (B) was replaced with the same amount of the 25% developer dispersion (C), whereby a single layer type, selfcoloring, pressure-sensitive recording sheet was formed.

Example 6 The same procedure as in Example 1 was repeated, except that 0.5 part of the 1% sodium dioctylsulfosuccinate aqueous solution in (4) was replaced with the same amount of a 1% sodium dibutylsulfosuccinate aqueous solution, whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Example 7 The same procedure as in Example 1 was repeated, 0.5 part of the 1% sodium dioctylsulfosuccinate aqueous solution in (4) was replaced with the same amount of a 1% sodium dilaurylsulfosuccinate aqueous solution, whereby a single layer type, selfcoloring, pressure-sensitive recording sheet was formed.

Comparative Example 1 50 parts of activated clay was added to 255 parts of water containing 0.5 part of sodium pyrophosphate as a dispersant, and completely dispersed in the water. Thereafter, 80 parts of wheat starch, 200 parts of the 10% urea-formaldehyde condensate dispersion obtained in (3) of Example 1, 50 parts of the microcapsules enclosing 40% of the color former, obtained in (1) of Example 1, 100 parts of a 10% polyvinyl alcohol aqueous solution and 20 parts of 50% carboxy-modified styrene-butadiene latex were successively added thereto and dispersed therein to obtain a coating composition.

This coating composition was coated on a wood free paper sheet having a basis weight of 40 gm2 by means of an air knife coater so that the coating weight was 10 g/m2 (dry weight of solid content), whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Comparative Example 2 The same procedure as in Example 1 was repeated, except that 0.5 part ofthe 1% sodium dioctylsulfosuccinate aqueous solution was omitted in (4), whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Comparative Example 3 The same procedure as in Example 4 was repeated, except that the amount of water in (4) was increased from 327 parts to 342 parts and 15 parts of the 1% sodium dioctylsulfosuccinate aqueous solution was omitted, whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

Comparative Example 4 The same procedure as in Example 5 was repeated, except that the amount of water in (4) was increased from 327 parts to 342 parts and 15 parts of the 1% sodium dioctylsulfosuccinate aqueous solution in (4) was omitted, whereby a single layer type, selfcoloring, pressure-sensitive recording sheet was formed.

Comparative Example S The same procedure as in Example 1 was repeated, except that 0.5 part of the 1% sodium dioctylsulfosuccinate aqueous solution in (4) was replaced with the same amount of a 1% sodium dipropylsulfosuccinate aqueous solution, whereby a single layer type, selfcoloring, pressure-sensitive recording sheet was formed.

Comparative Example 6 The same procedure as in Example 1 was repeated, except that 0.5 part of the 1% sodium dioctylsulfosuccinate aqueous solution in (4) was replaced with the same amount of a 1% sodium ditridecylsulfosuccinate aqueous solution, whereby a single layer type, self-coloring, pressure-sensitive recording sheet was formed.

II. Evaluation (1) Evaluation on coloring rate and color density Each of the single layer type, self-coloring, pressure-sensitive recording sheets obtained (I) above was colored by passing them through a super calender, and measured with a color difference meter for color densities after 1 minute and 1 hour, and the following values were determined. The density after 1 minute was taken as a coloring rate, and that after 1 hour was taken as a color density. The smaller the value, the higher the color density.

Color Reflectance of colored portion = (%) x 100 density (%) Reflectance of background (2) Penetrability to desensitizing ink The penetrability was evaluated on the basis of a desensitization effect immediately after printing.

A desensitizing ink (TT-1) for letterpress printing, supplied by Mitsubishi, was printed on the single layer type, self-coloring, pressure-sensitive recording sheet by means of a printing machine, "Miyastar Model 18" supplied by Miyakoshi Seisakusho K.K. so that the amount of poured ink was 1.2 g/m2.

And, immediately thereafter, the recording sheet was laid on a wood free paper sheet (basis weight of 50 g/m2), and a printing was made thereon under pressure with a dot printer. The desensitization effect on a printed portion was evaluated immediately after and 2 hours after the printing by visual observation.

Tables 1, 2 and 3 show the results of coloring rates, color densities and penetrabilitiees to ink evaluated on the basis of desensitization effects with regard to Examples 1 to 7 and Comparative Examples 1 to 6.

Table 1

Coloring rate Color density Example 1 39.7 29.8 2 38.8 30.4 3 3 37.3 30.1 4 4 36.4 29.7 5 36.8 28.8 6 6 37.9 28.8 7 7 38.1 28.9 Comparative 57.0 43.2 Example 1 2 2 39.3 30.4 3 3 38.8 29.9 " 4 36.9 29.5 " 5 38.4 29.4 6 6 38.9 29.9 Table 2

Coloring rate Color density Example 1 0 0 2 2 0 0 3 3 0 0 " 4 0 0 5 5 0 0 " 6 0 0 7 7 0 0 Comparative x x Example 1 2 2 0 0 " 3 0 0 4 0 0 "5 0 0 "6 0 0 Table 3 (Evaluation)

Penetrability (Desensitization effect) Imediately after After 2 hours printing Example 1 0 0 2 2 0 0 3 3 0 0 4 4 0 0 5 5 0 0 6 6 0 0 7 7 0 0 Comparative x x Example 1 2 2 x O 3 x O x 4 A 5 6 A 0 0: good A: sligly poor X: poor As is clearly shown in Tables 1 to 3, in the present invention, since the coating layer comprising the color former-enclosing microcapsules and the organic developer contains the compound of the following formula (I), there is obtained a single layer type, selfcoloring, pressure-sensitive recording sheet which exhibits rapid penetrability to a desensitizing ink, which is excellent in desensitization effect immediately after printing, and which maintains excellent properties in coloring rate, color density, etc.

wherein each of R1 and R2 is independently an alkyl group having 4 to 12 carbon atoms.

Claims (5)

WHAT IS CLAIMED IS:

1. A single layer type, self-coloring, pressuresensitive recording sheet which comprises a substrate and a coating layer provided on the substrate, the coating layer comprising an organic developer and microcapsules enclosing an electron-donating color former, characterized in that the coating layer contains a compound of the formula (I):

wherein each of R1 and R2 is independently an alkylgroup having 4 to 12 carbon atoms.

2. A recording sheet according to claim 1, wherein the coating layer further contains a pigment and a binder.

3. A recording sheet according to claim 1, wherein the color former is dissolved in a high-boiling solvent.

4. A recording sheet according to claim 1, wherein the compound of the formula (I) is at least one member selected from the group consisting of sodium dibutylsulfosuccinate, sodium diamylsulfosuccinate, sodium dihexylsulfosuccinate, sodium diheptylsulfosuccinate, sodium dioctylsulfosuccinate, sodium dinonylsulfosuccinate, sodium didecylsulfosuccinate, sodium diundecylsulfosuccinate and sodium dilaurylsulfosuccinate.

5. A recording sheet according to claim 1, wherein the compound of the formula (I) is one member selected from the group consisting of sodium diheptylsulfosuccinate, sodium dioctylsulfosuccinate and sodium dinonylsulfosuccinate.