EP0160106B1

EP0160106B1 - Process for manufacturing color-developing sheet for image-recording material

Info

Publication number: EP0160106B1
Application number: EP19840903988
Authority: EP
Inventors: Takahiro Torii; Hirokazu Tsukahara
Original assignee: Mitsubishi Paper Mills Ltd
Current assignee: Mitsubishi Paper Mills Ltd
Priority date: 1983-10-31
Filing date: 1984-10-29
Publication date: 1990-07-18
Anticipated expiration: 2004-10-29
Also published as: EP0160106A4; JPS6096487A; DE3482767D1; EP0160106A1; AU3670884A; WO1985001917A1; AU560101B2

Abstract

A process for manufacturing a color-developing sheet from a combination of an electron-donating colorless dye such as no-carbon pressure-sensitive recording material, heat-developable image-recording material using heat-sublimable colorless dye, heat-sensitive, color-transferring image-recording material, etc. and an electron-accepting color-developing agent, which comprises preparing a coating solution containing at least an inorganic color-developing agent, calcium carbonate, and a bisphenol compound. This coating solution has a low viscosity and a good fluidity at high solids concentration in spite of the use of inorganic color-developing agent which usually causes troubles such as gelation. Thus, it facilitates coating work and provides good color-developing power of the color-developing sheet, thus being excellent in printing properties.

Description

TECHNICAL FIELD

This invention relates to a process for producing a color-developing sheet for image-recording materials. More particularly, the invention relates to a process for producing a color-developing sheet for image-recording materials, wherein, in production of a color-developing sheet for use in image-recording materials consisting of an electron-donating colorless dye and an electron-accepting color-developing agent, a preferable coating fluid is prepared to improve the production technique of said color-developing sheet for image-recording materials, said color-developing agent being particularly selected from inorganic color-developing agents such as an active clay (e.g. acid clay and activated clay) or a semi-synthetic solid acid produced by acid-treating a clay mineral having a layer structure consisting of regular tetrahedrons of silica so that the treated clay mineral has a Si0₂ content of 82 to 96.5% by weight on a dry basis (drying of 3 hr at 105°C), contacting the resulting clay mineral, in an aqueous medium, with a magnesium and/or aluminum compound at least partially soluble in said medium and, when the soluble compound is not a hydroxide, neutralizing the soluble compound with an alkali or acid so as to form a hydroxide, to introduce a magnesium and/or aluminum component into the acid-treated clay mineral, and, if necessary, drying the resulting clay mineral.

BACKGROUND ART

As an image-recording material utilizing a color development reaction based on a combination use of an electron-donating colorless (leuco) dye and an electron-accepting color-developing agent, there are, as well known, non-carbon (or carbonless) pressure-sensitive recording materials as described in, for example, USP No. 2505470, USP No. 2550471, USP No. 2730456 and USP No. 3418250. These pressure-sensitive recording materials have an established history and have already developed into a large industry. There are other image-recording materials utilizing the same color development theory, such as, for example, thermally color-developing image-recording materials using a heat-sublimable colorless (leuco) dye [Japanese Patent Application Kokai (Laid-open) No. 149123/1977, Japanese Patent Application Kokai (Laid-open) No. 53538/1979, etc.], heat-sensitive, thermal transfer, image-recording materials and the like.
Non-carbon pressure-sensitive recording materials basically consist of a colorless (leuco) dye donor sheet (upper sheet) and a color-developing sheet (lower sheet). (Some of these materials include a middle sheet further or consist of a self-contained sheet.) The colorless (leuco) dye is, in many cases, dissolved in a high boiling solvent and contained in microcapsules.
This encapsulation has been conducted by a coacervation method, an in-situ polymerization method, an interfacial polymerization method, etc. As the colorless (leuco) dye, there are used triarylmethane- phthalides such as Crystal Violet Lactone, 3-3-bis(p-dimethylaminophenyl)naphthalide and 3-(p-dimethylaminophenyl)-3-(2-methylindole-3-yl)-6-dimethylaminophthalide; acryl (benzoyl, anisoyl, pivaloyl, etc.) derivatives of Methylene Blue; xanthenephthalides such as 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-6-methylanilinofluoran, 3-methyl-cyclohexylamino-6-methyl-7-anilinofluoran and 3-ethyl-p-tolylamino-6-methyl-7-anilinofluoran; and other leuco dyes.
As the high boiling solvent often used in non-carbon pressure-sensitive recording materials, there are aromatic hydrocarbons such as alkylnapthalenes represented by diisopropylnaphthalene, diarylalkanes represented by 1-phenyl-1-xylylethane, alkylbiphenyls represented by isopropylbiphenyl triaryl- dimethanes, alkylbenzenes, benzylnaphthalenes, diarylalkylenes, arylindanes and the like; carboxylic acid ester compounds such as dibutyl phthalate, dicotyl maleate and the like; phosphoric acid ester type compounds represented by tricresyl phosphate; natural animal and vegetable oils and fats such as castor oil, soy bean oil, cotton seed oil, whale oil and the like as well as their modified products; high boiling fractions (composed of aliphatic hydrocarbons) of natural products such as mineral oils; and so forth.
With respect to the electron-accepting color-developing agent, there have been proposed and are already in actual use, for example, inorganic color-developing agents such as natural clay minerals (e.g. acid clay, attapulgite clay) and activated clay produced by simply subjecting an acid clay which is a montmorillonite type clay mineral, to mild or medium degree treatment with an inorganic acid (mineral acid) such as hydrochloric acid, nitric acid, sulfuric acid or the like, as well as organic color-developing agents such as pehol compounds, novolak type phenol resins, and aromatic carboxylic acids and their polyvalent metal salts.
Of these color-developing agents, novolak type phenol resins as well as substituted salicyclic acids (aromatic carboxylic acids) and their salts, which are all organic color-developing agents, are excellent in that they can be coated on a substrate sheet in the form of a coating fluid of high concentration and the resulting color-developing sheet gives a high density of developed color; however, they are slow in color-developing speed and the developed color image vanishes in contact with plasticizers for plastics or with oils. Further, they are poor in printability such as ink setting and are expensive because they are a product of organic synthesis. Meanwhile, inorganic color-developing agents are excellent in printability such as ink setting and inexpensive and accordingly attractive as an industrial material; however, they have a big drawback in production of color-developing sheets using them. That is, it is difficult to prepare a coating fluid containing an inorganic color-developing agent because, in most cases, the coating fluid becomes a gel and accordingly, coating of an inorganic color-developing agent at a high concentration on a substrate sheet becomes impossible.
Activated clay as an inorganic color-developing agent is produced, as described in Japanese Patent Publication No. 2373/1966, Japanese Patent Publication No. 7622/1966 and Japanese Patent Publication No. 8811/1967, by treating acid clay or other similar mineral clay with a mineral acid to dissolve and extract basic components soluble in the acid such as aluminum, iron and the like and to allow the treated clay to possess a surface area of 200 m²/g or larger. Activated clay is amorphous when observed through X-rays and has a large specific surface area and thus its properties are greatly different from those of clays used for coating of ordinary papers. In actual dispersion of activated clay in water, a large amount of water is required because of the unique particle shape and surface activity of activated clay.
Kaolin which is a typical clay used for paper coating, when dispersed in water, can retain fluidity until the solid content in the dispersion increases to 70% or higher. In contrast, activated clay, when dispersed in water, loses fluidity because of high viscosity and causes gelation already when the solid content in the dispersion reaches about 40%.
Coating of a coating fluid containing a color-developing agent at a concentration as high as possible is desired in view of productivity and energy saving. However, inorganic color-developing agents such as activated clay and the like, because preparation of a coating fluid containing such a color-developing agent at a high concentration is very difficult as mentioned above, is currently coated, in most cases, in the form of a low concentration fluid (for example, solid contnt: about 30%) by the use of an air knife coater.
As a method for producing a coating fluid containing an inorganic color-developing agent at a high concentration, there is described in Japanese Patent Application Kokai (Laid-open) No. 2608/1977, a method wherein calcium carbonate or kaolin is added to activated clay. According to the experiment by the present inventors, this method certainly provides a slightly higher concentration fluid; however, the fluid tends to cause gelation, has insufficient fluidity and accordingly is not suited for practical application.
In Japanese Patent Application Kokai (Laid-open) No. 97782/1980, there is described a method wherein a magnesium compound is added to activated clay. This method is not sufficient due to the same reasons as mentioned above. Also, there is described a method wherein a latex type binder is added to activated clay before the activated clay is mixed with water. According to the experiment by the present inventors, in order to obtain nearly complete dispersion of activated clay, a strong stirring force is required, which causes destruction of latex emulsion by a high viscosity and a temperature increase, reduction of surface strength of a coated sheet and deterioration of printability. Thus, this method cannot be put into practical application.
The object of the present invention is to develop a coating fluid for color-developing sheets using an inorganic color-developing agent which is low in viscosity, has good fluidity and enables high concentration coating, as well as to produce a color-developing sheet of excellent printability by using said coating fluid.
According to the invention, a process is provided for producing a color-developing sheet for image-recording materials which consist of an electron-donating colorless dye and an electron-accepting color-developing agent capable of allowing said colorless dye to develop into a colored dye, comprising the steps of:

preparing a coating fluid comprising at least calcium carbonate, a bisphenol selected from bis (4-hydroxyphenyl) sulfone and 4,4'-isopropylidenediphenol, and an inorganic color-developing agent which is a synthetic solid acid prepared by acid-treating a clay mineral having a layer structure consisting of regular tetrahedrons of silica so that the treated clay mineral has a Si0₂ content of 82 to 96.5% by dry weight (as measured after drying the treated clay material for 3 hours at 105°C), contacting the resulting clay mineral, in an aqueous medium, with a magnesium and/or aluminium compound at least partially soluble in said medium and, if the soluble compound is not a hydroxide, neutralizing the soluble compound with an alkali or acid to form a hydroxide thereby introducing a magnesium and/or aluminium component into the acid-treated clay mineral if necessary,
coating the coating fluid on a substrate sheet, and
drying the coated sheet.

By adding the inorganic color-developing agent with calcium carbonate and the bisphenol compound in combination and each in a relatively small amount, there can be prepared a coating fluid having a sufficiently reduced viscosity, a high solid content and good fluidity. The color-developing sheet produced by coating the coating fluid on a substrate has sufficient color developability and is excellent in printability, etc.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a rheological diagram of the coating fluid of Example of the present invention.
Fig. 2 is a rheological diagram of the coating fluid of Comparative Example 1.
Fig. 3 is a rheological diagram of the coating fluid of Comparative Example 2.
Fig. 4 is a rheological diagram of the coating fluid of Comparative Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention can be applied to all image-recording materials utilizing a color-developing reaction between an electron-donating colorless (leuco) dye and an electron-accepting color-developing agent, such as non-carbon pressure-sensitive recording materials, thermally color-developing image-recording materials using a heat-sublimable colorless (leuco) dye, heat-sensitive, thermal transfer, image-recording materials and the like. Accordingly, although the present specification described in detail mainly the effects on a production process of a color-developing sheet for non-carbon pressure-sensitive recording materials, the invention can of course be applied also to other image-recording materials. While the non-carbon pressure-sensitive recording materials consist basically of an upper sheet and a lower sheet, the present invention can be applied also to a middle sheet, a self-contained sheet, etc.
As the organic color-developing agent usable in the present invention, it is particularly recommended to use the semi-synthetic solid acid obtained by the production process proposed in Japanese Patent Application Kokai (Laid-open) No. 15996/1982, corresponding to EP-A1-0044645. This publication discloses the preparation of a semi-synthetic solid acid for use as an inorganic color-developing agent by acid treating a clay mineral having a layer structure consisting of regular tetrahedrons of silica so that the treated clay mineral has a Si0₂ content of 82 to 96.5% by weight, preferably 85 to 95% by weight on a dry basis (drying of 3 hr at 105°C), contacting the resulting clay mineral, in an aqueous medium, with a magnesium and/or aluminium compound at least partially soluble in said medium and, when the soluble compound is not a hydroxide, neutralizing the soluble compound with an alkali or acid so as to form a hydroxide, to introduce a magnesium and/or aluminum component into the acid-treated clay mineral, and, if necessary, drying the resulting clay mineral.
This color-developing agent is called "semi-synthetic solid acid" in the present specification.
The acid-treated clay mineral mentioned above, when measured by X-ray diffraction and electron rays diffraction, does not substantially show a diffraction pattern as given by a layer structure crystal consisting of regular tetrahedrons of silica, of the above mentioned clay mineral before acid treatment. Meanwhile, the acid-treated clay mineral into which a magnesium and/or aluminum component has been introduced, when measured by electron rays diffraction, shows a diffraction pattern as given by said layer structure crystal consisting of regular tetrahedrons of silica, but, when measured by X-ray diffraction, shows no such diffraction pattern as given by said layer structure crystal.
The semi-synthetic solid acid is a color-developing agent which can greatly improve the drawbacks of conventional color-developing agents of clay mineral type represented by acid clay and activated clay, namely, insufficient density of developed color and reduction of density of developed image under high humidity conditions.
The present inventors found that, when a coating fluid containing a low concentration of the semi-synthetic solid acid produced as above is prepared and a color-developing sheet is produced with the coating fluid, the color-developing sheet gives a higher density of developed color and lessens reduction of density of developed image under high humidity conditions. The present inventors also found that, when there is prepared a coating fluid containing a high concentration of the semi-synthetic solid acid, the coating fluid, as often seen in coating fluids containing conventional type activated clay, has a high viscosity, causes gelation and possesses no fluidity leading to low productivity.
Hence, the present inventors conducted extensive research in order to look for a substance which can improve the fluidity of a coating fluid containing a high concentration of one of inorganic color developers, a semi-synthetic solid acid. As a result, it was found that, by adding to a semi-synthetic solid acid as a color-developing agent calcium carbonate and a bisphenol compound in combination, there can be obtained a coating fluid containing a high concentration of a semi-synthetic solid acid which has a low viscosity and accordingly fluidity, and that the color-developing sheet produced with the coating fluid causes neither reduction of density of developed image under high humidity conditions nor vanishing of developed image in contact with plasticizers or oils and moreover has excellent printability. This novel knowledge has led to the present invention.
The calcium carbonate used in the present invention has no particular restriction. It is a white pigment called HAKUENKA (TRADE MARK) in some cases, used for paper coating and can be selected from heavy calcium carbonate and light calcium carbonate. If boldly said, light calcium carbonate gives a better result.
Calcium carbonate is added preferably in an amount of 2.5 to 40 parts by weight and particularly preferably in an amount of 5 to 30 parts by weight based on 100 parts by weight of inorganic color-developing agent. When it is added in an amount smaller than 2.5 parts by weight, the addition is not effective for reduction in viscosity of coating fluid. When it is added in an amount larger than 40 parts by weight, such addition reduces the density of developed color.
The bisphenol compound used in the present invention is selected from 4,4'-isopropylidenediphenol and bis(4-hydroxyphenyl) sulfone.
Particularly preferable is bis(4-hydroxyphenyl)sulfone.
The bisphenol compound is added preferably in an amount of 2.5 to 40 parts by weight and particularly preferably in an amount of 10 to 30 parts by weight based on 100 parts by weight of inorganic color-developing agent.
When the bisphenol compound is added in an amount smaller than 2.5 parts by weight, the addition is not effective for reduction in viscosity of coating fluid. When it is added in an amount larger than 40 parts by weight, the addition hinders color development and reduces the density of developed color.
The bisphenol compound is added to an inorganic color-developing agent after the compound has been ground and dispersed in a small amount of a dispersing agent by the use of a wet grinder such as a ball mill, an attritor, a sand grinder or the like.
The present invention is characterized in that a coating fluid is prepared by adding to an inorganic color-developing agent at least calcium carbonate and a bisphenol compound in combination. Addition of calcium carbonate or a bisphenol compound alone reduces the viscosity of the coating fluid slightly but insufficiently. In order to sufficiently reduce the viscosity of a coating fluid by addition of calcium carbonate or a bisphenol compound alone, the addition amount becomes very large, which induces the low content of an inorganic color-deveoping agent in the color-developing layer of the color-developing sheet produced with the coating fluid and this reduces the color developability of the sheet and impairs the printability; thus, such a coating fluid can not be used practically.
In contrast, when calcium carbonate and a bisphenol compound are added in combination, although the reason is not clarified yet, their addition each in a relatively small amount gives a coating fluid of sufficiently low viscosity, of good fluidity and yet of high solid content (for example, 40% or above). Further, the color-developing sheet produced with the coating fluid has sufficient color developability and is excellent in printability, etc.
According to the present invention, to an inorganic color-developing agent are added calcium carbonate and a bisphenol compound in combination; thereto are further added an inorganic dispersing agent (e.g. sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate, sodium silicate), an organic dispersing agent (e.g. a carboxylic acid type such as acrylic acid salt, a maleic acid type such as styrene maleic anhydride salt, a sulfonic acid type such as naphthalenesulfonic acid salt), a watersoluble binder (e.g. starch, a polyvinyl alcohol), an emulsion binder (e.g. a styrene-butadiene rubber latex, an acrylic type latex) and additives (a dusting-preventing agent, a defoamant) to obtain a high concentration coating fluid containing 40% or more of solid; and this coating fluid is coated on a substrate sheet such as a paper industrially desirably by the use of a blade coater and then dried to obtain a color-developing sheet for image-recording materials.
In the present invention, as necessary, there may further be added to the coating fluid a pigment such as clay, kaolin, magnesium carbonate, satin white, titanium oxide, zinc oxide, an urea resin pigment, a plastic pigment, talc, alumina, aluminum hydroxide and the like.
According to the present invention, the dispersibility and fluidity of an inorganic color-developing agent in water has been greatly improved, whereby preparation of a coating fluid of low viscosity as well as coating of an inorganic color-developing agent on a substrate sheet at a high concentration have become possible and accordingly coating operation, energy consumption and productivity have been improved. Moreover, the color-developing sheet produced with such a coating fluid possesses improved printability as represented by improved surface quality, good surface smoothness, lower smudge (lower staining by color development due to rubbing) and improved surface strength.
Incidentally, the addition order of chemical substances in preparation of a coating fluid is not restricted. Any coating fluid comprising at least an inorganic color-developing agent, calcium carbonate and a bisphenol compound is included in the coating fluids used in the present invention.
As mentioned previously, an inorganic color developing agent particularly recommended in the present invention is a semi-synthetic solid acid described in Japanese Patent Application Kokai (Laid-open) No. 15996/1982. Not only this agent but also other inorganic color-developing agents adsorb a colorless (leuco) dye on their surfaces and thereby develop a color. Accordingly, a large surface area per unit weight gives a higher efficiency of color development. In other words, the smaller the particle diameters of a color-developing agent, the better the color development. However, as the surface area gets larger (the particle diameters get smaller), the coating fluid has a higher viscosity and preparation of a coating fluid containing an inorganic color-developing agent at a high concentration becomes difficult.
The present invention exhibits the best effects when an inorganic color-developing agent having particle diameters as small as possible is used for improvement of color development efficiency and there is prepared a high concentration coating fluid having fluidity and causing no gelation for energy saving and productivity improvement. As the coater used at this time, a blade coater is best. Its use also allows high speed coating, safety and hygiene of working environments and good surface condition of coated sheet.
Conventional inorganic color-developing agents have ordinarily used particle diameters of 5 to 6 ₁₁m cut (particles which pass through a mesh of 5 to 6 um). While, in the present invention, there can be used an inorganic color-developing agent having particle diameters of 4 ₁₁m cut and, even with an inorganic color-developing agent having particle diameters of 3.5 pm, preparation of a coating fluid as well as its coating by a blade coater is possible.
Next, there is described a process for producing a color-developing (agent) sheet [colorless (lecuo) dye donor sheet] for blue color development which is used in the example of the present invention. All parts shown below are parts by weight.
Process for producing a color-developing (agent) sheet for blue color development.
3.5 parts of Crystal Violet Lactone which is a colorless (leuco) dye and 1 part of Malachite Green Lactone were dissolved in 100 parts of a diarylethane type organic solvent which is a high boiling solvent. The resulting solution was added to 160 parts of an aqueous solution containing 10% of an acid-treated gelatin (isoelectric point: 7.8) and the mixture was made an emulsion.
This emulsion was added to a solution produced by dissolving 20 parts of gum arabic in 1,500 parts of water. The mixture was adjusted to a pH of 8.5 with sodium hydroxide, kept at 50°C, adjusted to a pH of 4.4 with acetic acid, and cooled to 10°C. Thereto was added 20 parts of an aqueous solution containing 37% formalin, and the mixture was stirred for 24 hr. Then, the mixture was adjusted to a pH of 10 with sodium hydroxide to obtain microcapsules.
To 100 parts (as solid) of the microcapsules were added 50 parts of an aqueous solution containing 10% of a polyvinyl alcohol and 30 parts of wheat starch. The mixture was coated on a plain paper of 40 g/m^l by the use of an air knife coater so that the coated amount became 5 g/m² (as solid). The coated paper was dried to obtain a color-developing (agent) sheet for blue color development.
The preferable embodiment and excellent effects of the present invention will specifically be explained below by way of the most representative example.

Example

A 40% bisphenol compound dispersion was prepared by subjecting 100 parts of bis(4-hydroxyphenyl)-sulfone, 5 parts of sodium polymethacrylate and 145 parts of water (250 parts in total) to grinding by a ball mill for 2 days.
To 55 parts of water were added 2 parts of an aqueous solution containing 30% of sodium silicate and 100 parts of an aqueous solution containing 10% of an oxidized starch, and they were stirred. Thereto was added 10 parts of light calcium carbonate and it was dispersed therein. Then, 37.5 parts of the above prepared 40% bis(4-hydroxyphenyl)sulfone dispersion was added and the mixture was stirred. Thereafter, 100 parts of a semi-synthetic solid acid was slowly added and dispersed with stirring, after which 20 parts of a 50% styrene-butadiene rubber latex was added and the resulting mixture was stirred to obtain a coating fluid. This coating fluid had a solid content of 45% and good fluidity as stated later. When the coating fluid was coated on a plain paper of 40 g/m² by the use of a blader coater so that the coated amount became 5 g/m² (as solid) and dried, the coating and drying operations were smooth and a good color-developing sheet was obtained.

Comparative Example 1

To 45 parts of water were added 2 parts of an aqueous solution containing 30% of sodium silicate and 100 parts of an aqueous solution containing 10% of an oxidized starch, and they were stirred. Thereto was slowly added 100 parts of a semi-synthetic solid acid with stirring and it was dispersed. Then, 20 parts of a 50% styrene-butadiene rubber latex was added and the resulting mixture was stirred to obtain a coating fluid. This coating fluid was coated on a plain paper of 40 g/m² by the use of a blade coater so that the coated amount became 5 g/m² (as solid) and then dried to obtain a color-developing sheet. The coating fluid was very viscous and poor in fluidity; therefore, its coating was difficult.

Comparative Example 2

To 77 parts of water were added 2 parts of an aqueous solution containing 30% sodium silicate and 100 parts of an aqueous solution containing 10% of an oxidized starch. Thereto was added 25 parts of light calcium carbonate and it was dispersed. Further, 100 parts of a semi-synthetic solid acid was slowly added with stirring and dispersed. Thereafter, 20 parts of a 50% styrene-butadiene rubber latex was added and the resulting mixture was stirred to obtain a coating fluid. This coating fluid was coated on a plain paper of 40 g/m² by the use of a blade coater so that the coater amount became 5 g/m² (as solid) and then dried to obtain a color-developing sheet. The coating fluid had a high viscosity and poor fluidity; therefore, its coating was difficult.

Comparative Example 3

To 40 parts of water were added 2 parts of an aqueous solution containing 30% of sodium silicate and 100 parts of an aqueous solution containing 10% of an oxidized starch, and they were stirred. Thereto was added 50 parts of the 40% bis(4-hydroxyphenyl)sulfone dispersion prepared in Example and they were stirred. Then, 100 parts of a semi-synthetic solid acid was slowly added with stirring and dispersed. Further, 20 parts of a 50% styrene-butadiene rubber latex was added, and the resulting mixture was stirred to obtain a coating fluid. This coating fluid was coated on a plain paper of 40 g/m² by the use of a blade coater so that the coated amount became 5 g/m² (as solid) and then dried to obtain a color-developing sheet. The coating fluid had a high viscosity and poor fluidity and its coating was difficult.

Test method

The coating fluids and color-developing sheets thus obtained were subjected to measurements according to the following methods.

Coating fluid

0 Viscosity

A viscometer of B type manufactured by Tokyo Keiki was used. A value right after 1 min of rotating at 60 rpm by the use of a No. 4 rotor was measured. Figs. 1,2,3 and 4 show viscosity curves obtained using a Hercules II type high shear viscometer manufactured by Nihon Rigaku Kogyo.

0 Solid content

Drying was conducted for 16 hr at 105°C to measure the solid content of a coating fluid.

Color-developing sheet

0 Density of developed color

A color-developing sheet was superimposed on the previously mentioned color-forming (agent) sheet and they were passed through a calender to develop a color. Using a color difference meter, the density of the color as defined by the following formula was measured. When a density of developed color has a smaller value, the density is higher.

Density of developed color (%)

○ Smoothness

Measurement was made using a Beck smoothness tester. A larger value (expressed in seconds) means better smoothness.

○ Smudge (Staining by developed color due to rubbing)

A color-developing sheet was superimposed on the previously mentioned color-forming sheet, and they were rubbed against each other with a weight placed on them. The staining of the color-developing sheet was measured as a reflectance (%) using a color difference meter. Accordingly, a high smudge value means lower staining.

○ Surface strength (Printability test)

Measurement was made using an IGT tester. The strength obtained was expressed by O, Δ, or x (0 is good and x is poor).

Test results

○ Characteristics of coating fluids

As a obvious from Table 1, the coating fluid of the Example of the present invention, as compared with the coating fluids of Comparative Examples 1, 2 and 3, has about the same solid content (%) but a far smaller viscosity.
As is also obvious from Figs. 1, 2, 3 and 4, the coating fluid of Comparative Examples 1, 2 and 3, as compared with the coating fluid of Example, have higher viscosities at high rotation speeds and even at low rotation speeds, have higher viscosities and are in a gel state.
Coating of the coating fluids of Comparative Examples 1, 2 and 3 by a blade coater was very difficult due to high viscosity and gelation.

0 Characterstics of color-developing sheets

As a obvious from Tables 1 and 2 and Figs. 1, 2, 3 and 4, in production of a color-developing sheet using a semi-synthetic solid acid, use of calcium carbonate and a bisphenol compound in combination in a coating fluid cotaining said solid acid is very effective for the viscosity and fluidity of said coating fluid as well as for the smoothness, smudge and surface strength of a color-developing sheet produced with said coating fluid.

INDUSTRIAL APPLICABILITY

As stated above, the process for producing a color-developing sheet for image-recording materials according to the present invention can be used for production of a color-developing sheet for use in image-recording materials consisting of an electron-donating colorless dye and an electron-acception color-developing agent, particularly, an inorganic color-developing agent excellent in printability and advantageous in cost such as an active clay, namely, a so-called semi-synthetic solid acid (e.g. acid clay and activated clay). This color-developing sheet can effectively be used in non-carbon pressure-sensitive recording materials, thermally color-developing image-recording materials using a heat-sublimable colorless dye, heat-sensitive, thermal transfer, image-recording materials, etc.

Claims

1. A process for producing a color-developing sheet for image-recording materials which consist of an electron-donating colorless dye and an electron-accepting color-developing agent capable of allowing said colorless dye to develop into a colored dye, comprising the steps of:

preparing a coating fluid comprising at least calcium carbonate, a bisphenol selected from bis (4-hydroxyphenyl) sulfone and 4,4'-isopropylidenediphenol, and an inorganic color-developing agent which is a synthetic solid acid prepared by acid-treating a clay mineral having a layer structure consisting of regular tetrahedrons of silica so that the treated clay mineral has a Si0₂ content of 82 to 96.5% by dry weight (as measured after drying the treated clay material for 3 hours at 105°C), contacting the resulting clay mineral, in an aqueous medium, with a magnesium and/or aluminium compound at least partially soluble in said medium and, if the soluble compound is not a hydroxide, neutralizing the soluble compound with an alkali or acid to form a hydroxide, thereby introducing a magnesium and/or aluminium component into the acid-treated clay mineral, and drying the resulting clay mineral if necessary,

coating the coating fluid on a substrate sheet, and

drying the coated sheet.

2. A process for producing a color-developing sheet according to Claim 1, wherein the coating is conducted by a method which makes use of a blade coater.

3. A process for producing a color-developing sheet according to claim 1 or claim 2, wherein the inorganic color-developing agent is a powder having particle diameters substantially of 4 pm or less.

4. A process for producing a color-developing sheet according to any preceding claim, wherein the coating fluid has a solid content of 40% by weight or more.