Dichromatic thermo-sensitive recording paper

Abstract

Recording paper which forms, for example, a red color when scratched with a thermal pen or head at a certain temperature and which forms, for example, a blue color when scratched with a thermal head at a higher temperature, said dichromatic thermo-sensitive paper having first and second layers formed from a dispersion in a film-forming binder of discontinuous particles of a leuco base of triphenylmethane or fluoran type dye and a phenolic acidic substance or an organic acid; and, disposed therebetween an inter layer capable of being thermally fused to dilute as well as to exclude the color formed in the first layer.

Description

This invention relates to dichromatic thermosensitive paper for use in information terminal machines, e.g., facsimiles, telexes, electronic computers, etc., various metrological appliances and copying machines.

Recording materials, which have heretofore been utilized for the above-mentioned uses, are represented by conventional photographic recording papers, electrophotographic recording papers, electrostatic recording papers, electrolytic recording papers and diazo type recording papers. All these recording papers, however, have had such drawbacks that the supply of toners, developers, fixers and the like chemicals is necessarily required in the image-forming process. Furthermore, inks are used in recording machines employed at present. Accordingly, there have been such disadvantages for users that the recording papers are stained with the inks, or ink-flowing holes of pens are clogged with solids formed by vaporization of solvents of the inks, with the result that figures to be recorded are not recorded clearly or, in extreme cases, cannot be recorded. With an aim to overcome the above-mentioned drawbacks and disadvantages, pressure-sensitive papers have come to be used chiefly. At present, however, these recording papers are limited in uses to copying papers and teletyping papers which are used in place of carbon papers.

Heretofore, thermo-sensitive recording materials of various types have been proposed. Among these recording materials, those which have already been put into practical use and those which will be put into practical use in the near future are as follows:

1. A thermo-sensitive recording paper prepared by coating carbon or the like colored pigment or dye powder on a substrate, and then coating a white, opaque thermofusible substance on the resulting pigment or dye powder layer. This recording sheet is of such a type that the thermofusible substance on the surface is scratched with a thermal stylus to expose and make visible the lower colored layer. The recording material of this type has such drawbacks that scums are formed and multi-color recording is impossible.

2. A recording material which forms a color by forming a complex compound of an electron donor with an electron acceptor. This recording material is of such a type that ferric stearate and pyrogallol, for example, are independently dispersed in a binder, and the binder is softened by application of heat, thereby reacting the two compounds with each other to form a visible complex compound. The color formed in this case is dark brown.

3. A recording material of such a type that Crystal Violet Lactone and a phenolic substance are dispersed as discontinuous particles into a thermo-softening substance such as polyvinyl alcohol, and the thermo-softening substance is softened by application of heat to give a record. The color obtained in this case is brilliant cobalt blue. This type of recording material suggests the promising future of thermo-sensitive recording materials, but has a question in how to solve the recording of two or more colors.

As mentioned above, a recording material for use in information terminal machines is required to satisfy such conditions as that it can give a clear record and can be recorded by use of a recording machine which is simple in maintenance. Considering the said conditions, the thermo-sensitive recording material of the aforesaid item (3) is a preferable recording material since it can give a clear image by mere heating with a thermal head. With progress of society, however, high density informations have come to be required, and there is a strong need for a thermo-sensitive recording material which can give a two- or more-colored record.

The present invention provides a dichromatic thermo-sensitive paper which is free from the drawbacks of this kind of thermo-sensitive recording material and which satisfies the above-mentioned requirements.

That is, the invention provides a recording paper which forms, for example, a red color when scratched with a thermal pen or head at a certain temperature, and which forms, for example, a blue color when scratched with a thermal head at a higher temperature.

With reference to the drawings attached hereto,

FIG. 1 show the basic construction of the recording sheet of this invention;

FIG. 2A and 2B show the relation between reflection density and width of lines of image obtained in accordance with this invention;

FIGS. 3, 4 and 5 show the relation between temperature of recording stamp and reflection density of image obtained; and

FIGS. 6 A and 6 B and 7 A and 7 B show the relation between reflection density and width of lines of image obtained.

The thermo-sensitive recording paper of the present invention will be explained in detail below with reference to FIG. 1. As shown in FIG. 1, the recording paper is composed of a first color-forming layer 1, and inter layer 3, a second color-forming layer 2, and a base paper 4. In FIG. 1, 1.1 is a color-forming dye, 1.2 is a color-forming agent, 1.3 is a sensitizer, 2.1 is a color-forming dye different in hue from the color-forming dye 1.1, 2.2 is a color-forming agent, 2.3 is a sensitizer, and 3.1 is a crystalline organic substance.

In the first color-forming layer 1, the color-forming dye 1.1 and the color-forming agent 1.2 have dispersed separately from each other in a thermo-softening substance which is solid at normal temperature. However, when a certain quantity of heat is applied to the surface of the recording paper, the color-forming dye reacts with teh color-forming agent to form the first color. In this case, the sensitizer acts to promote the reaction of the color-forming dye with the color-forming agent so as to form the color quickly. In the second color-forming layer 2, the color-forming dye 2.1 and the color-forming agent 2.2 have dispersed separately from each other in a thermo-softening substance which is solid at normal temperature. However, when a certain quantity of heat is applied to the surface of the recording paper, the color-forming dye reacts with the color-forming agent to form a color different in hue from the first color. In this case, the sensitizer 2.3 acts to promote the reaction of the color-forming dye with the color-forming agent. Further, when a certain quantity of heat is applied to the surface of the recording paper, the crystalline organic substance contained in the inter layer fuses to dilute as well as to push aside the dye which has formed the first color in the first layer, and when heat is further applied, the organic substance so acts that the dye, which has formed the second color in the second layer, does not mix with the dye, which has formed the first color in the first layer.

The above-mentioned first and second color-forming layers have individually been well known as thermo-sensitive recording materials of this kind. However, the recording paper of the present invention is characterized in that the first layer, the inter layer and the second layer have been so combined that among the quantities of heat applied to the surface of the recording paper, the quantity of heat necessary to cause color formation of the first color-forming layer (hereinafter referred to as "heat Q.sub.1 "), the quantity of heat necessary to fuse the inter layer (hereinafter referred to as "heat Q.sub.2 "), and the quantity of heat necessary to cause color formation of the second color-forming layer (hereinafter referred to as "heat Q.sub.3 ") are in such a relation as to satisfy the inequality Q.sub.1 < Q.sub.2 .ltoreq. Q.sub.3, and in that the recording paper has been so designed that at a temperature attained when heat Q.sub.3 is applied to the paper surface, the first and second layers form colors, but the dye, which has formed the color in the first layer, is diluted and pushed aside by means of the inter layer and is surrounded by the dye, which has formed the color in the second layer, to be brought into such a state that, apparently, it has formed no color, and only the second layer is made visible, whereby the recording paper can form two brilliant colors.

The quantity of heat applied to each of the color-forming layers is dependent on the temperature of a thermal head contacted with the paper surface to give a record on the recording paper, the contact time of the thermal head with the paper surface, and the thermal conductivity of the recording paper from the surface to each layer. For example, in case the thermal head is applied from the above to the surface of recording paper having such structure as shown in FIG. 1, the quantity of heat applied to the first color-forming layer 1 is greater than the applied to the second color-forming layer, even if the temperature of the thermal head and the contact time thereof with the paper surface are definite. In this case, the temperature of the first color-forming layer is higher than that of the second color-forming layer, and hence is not always required to be made higher than a temperature at which the second color-forming layer forms a color.

Now, the color formation mechanism of the recording paper according to the present invention will be explained in detail below.

It has been clarified by O. Fischer, et al., [Berichte der Deutchen Chemischen Gesellschaft, 42 2934-2935 (1909)] that a leuco dye base of the triphenylmethane type, e.g., Crystal Violet Lactone, forms a color by reaction with a phenolic substance such as phenol or cresol and an organic acid. Further, a leuco dye base of the fluoran type also exhibits the same property as mentioned above. It is said that the color formation mechanism of the said reaction is such that the electron attractive acid attracts the electrons of amine of the leuco dye base to bring about ionization of the dye base. Generally, this kind of color-forming dye and color-forming agent are mixed or contacted with each other either as they are or in the form of solutions, whereby the color-forming dye forms a color due to electron transfer. However, no color is formed in case the said two components are dispersed individually in a non-solvent therefor by use of a binder and are then contacted with each other. This is ascribable to the fact that the binder separates the two components from each other to inhibit the contact thereof. If the binder which has separated the two components from each other is fused, or either the color-forming agent or the color-forming dye is fused to bring the two into contact with each other, there is formed a color.

Generally, when heat Q.sub.1 is applied to a recording paper having first and second color-forming layers containing the above-mentioned thermo-sensitive substances, the first color-forming layer forms a color according to the aforesaid color formation principle, and when heat Q.sub.2 is then applied to the recording paper, the second color-forming layer forms a color. However, if the color-forming dye in the first color-forming layer does not substantially differ in amount from that in the second color-forming layer, and if no inter layer has been disposed between the two layers, the color formed in the first color-forming layer is mixed with the color formed in the second color-forming layer to make it impossible to form two brilliant colors. For example, if the recording paper is subjected to a dot printer, two colors, which are to be formed, become mere shade and light, and thus the recording paper cannot be used as a two color recording paper. However, in case the color-forming dye in the first color-forming layer is smaller in amount than that in the second color-forming layer, and in case a thermofusible crystalline organic substance is present in the inter layer, the first and second color-forming layers form colors at the same time by application of heat Q.sub.3. In this case, the crystalline organic substance present in the inter layer fuses to dilute as well as to push aside the first color formed in the first layer, whereby the recording paper can form two brilliant colors. When the recording paper is used as a dichromatic recording paper for high efficiency recorder or printer, it is advantageous if the density of each color formed becomes maximum due to slight temperature difference. That is, the greater the value of .gamma., the more advantageous. (In the present invention, the value of .gamma. is defined by the formula .gamma. = (D.sub.2 -D.sub.1)/(T.sub.2 -T.sub.1), wherein T.sub.2 is a maximum density-reaching temperature; T.sub.1 is a temperature when "fog" is +0.1; and D.sub.2 and D.sub.1 are reflection densities at temperatures of T.sub.2 and T.sub.1, respectively. The greater the value of .gamma., the shorter the time to reach the maximum density.) The aforesaid color-forming dye and color-forming agent form a color according to color formation mechanism based on ionization of the color-forming dye due to contact of the said two components with each other. In preparing the recording paper by use of the said two color-forming components, the two components are separated from each other by means of a binder so as to inhibit the occurrence of color formation reaction at normal temperature. When heat is applied to the recording paper, the two components are swelled or activated and initiate to react with each other. In this case, the density of the formed color reaches maximum when the two components have completely mixed with each other. It is therefore ordinary that the color forming temperature T.sub.s is 90.degree.C., the maximum density-reaching temperature T.sub.D-max is 150.degree.C., and the value of .gamma. is small. When such recording paper is used for copying, it gives a half tone image high in gradation. However, when used in a line printer or the like machine required to be particularly high in speed, it is preferable that the recording paper is greater in .gamma. value. Further, if the value of .gamma. is small in the case of dichromatic recording paper, the resulting image is blurred due to thermal diffusion from the head, with the result that the image shows a somewhat stained appearance and no brilliant two colors can be formed.

In the present invention, the recording paper is made greater in .gamma. value by introduction of a third substance (referred to as "sensitizer" in the present invention) in addition to the color-forming dye and the color-forming agent, so that the recording paper can form brilliant two colors. The sensitizer is a crystalline organic substance having a sharp melting point, and has such action that when fused by application of heat, it dissolves at least one of the color-forming agent and the color-forming dye to quickly contact and mix the two with each other. Accordingly, the value of .gamma. can be made greater. Further, when a substance having a suitable melting point is selected as the sensitizer, it is possible to establish the temperature at which the color formation is initiated. The sensitizer used in the present invention is thermofusible by itself, so that it is not preferable to use as the sensitizer a substance which fuses at the storage temperature of the recording paper or which does not fuse at an ordinary color formation temperature of the color-forming components. In this sense, it is preferable that the sensitizer has a melting point in the range from 70.degree.C. to 180.degree.C.

Materials used in the present invention are exemplified below.

a. Color-forming dyes:

As the color forming dyes, there are used, in general, leuco bases of triphenylmethane type dyes represented by the general formula (1) or leuco bases of fluoran type dyes represented by the general formula (2). ##SPC1##

wherein R.sub.x, R.sub.y and R.sub.z are individually hydrogen, a hydroxyl group, a halogen, an alkyl group, an aryl group, a nitro group, an amino group, a dialkylamino group, a monoalkylamino group or an alkoxy group; and Z is an atom necessary to form a heterocyclic group, and represents O or S.

i. Compounds belonging to (1):

3,3-Bis(p-dimethylaminophenyl)-phthalide

3,3-Bis(p-dimethylaminophenyl)-6-dimethylaminophthalide

3,3-Bis(p-dimethylaminophenyl)-6-nitrophthalide

3,3-Bis(p-dimethylaminophenyl)-6-monoethylaminophthalide

3,3-Bis(p-dimethylaminophenyl)-6-chlorophthalide

3,3-Bis(p-dimethylaminophenyl)-6-ethoxyphthalide

3,3-Bis(p-dimethylaminophenyl)-6-diethylaminophthalide

ii. Compounds belong to (2):

3-Dimethylamino-6-methoxyfluoran

7-Acetamino-3-dimethylaminofluoran

3-Dimethylamino-5,7-dimethylfluoran

3,6-Bis-.beta.-methoxyethoxyfluoran

3,6-Bis-.beta.-cyanoethoxyfluoran

iii. Other lactam compounds:

9-p-nitroamino-3,6-(diethylamino)-9-xanthenoyl-6-benzoic acid lactam, (Rhodamine Lactam)

9-p-Nitroamino-3,6-bis(dimethylamino)-9-thioxanthenoyl-6-benzoic acid lactam

The above-mentioned dye bases are effective as the color-forming dyes. They are scarcely soluble in non-polar or substantially non-polar solvents such as, for example, n-hexane, xylene, ligroin, benzene and toluene, and in water, and can be pulverized to fine particles of less than 10 microns.

b. Color-forming agents:

Preferable as the color-forming agents, which form dyes by chemical reaction with the color-forming dyes mentioned in (a) are phenolic compounds or organic acids. It is desirable for dye formation that the color-forming agents are solid at normal temperature and are liquefied or gasified at above 50.degree.C.

i. Phenolic substances:

4-tertiary-Butylphenol

4-Phenylphenol

4-Hydroxy-di-phenol

.alpha.-Naphthol

Methyl-4-hydroxybenzoate

.beta.-Naphthol

4-Hydroxyacetophenone

4-tertiary-Octylcatechol

2,2'-Dihydroxydiphenol

2,2'-Methylenebis(4-chlorophenol)

4,4'-Isopropylidene-di-phenol=(Bisphenol A)

4,4'-isopropylidenebis(2-chlorophenol)

4,4'-Isopropylidenebis(2,6-dibromophenol)

4,4'-Isopropylidenebis(2,6-dichlorophenol)

4,4'-Isopropylidenebis(2-methylphenol)

4,4'-sec-Isobutylidenediphenol

4,4'-Cyclohexylidenediphenol

2,2'-Thiobis(4,6-dichlorophenol)

Hydroquinone

Pyrogallol

Phloroglucine

Phloroglucinolcarboxylic acid

ii. Organic acids:

p-Hydroxybenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, boric acid, tartaric acid, oxalic acid, maleic acid, succinic acid, gallic acid, 1-hydroxy-2-naphthenic acid and 2-hydroxy-p-tolyl acid.

c. Binders:

In case the aforesaid color-forming dye and color-forming agent are merely dispersed in water or in a non-polar or substantially non-polar organic solvent and the dispersion is coated on a substrate, the resulting layer is not only great in fog but also comes off and thus is low in practicality. It is therefore necessary to use a binder which can disperse and fix the two as discontinuous particles at normal temperature. Due to its inherent property, the binder softens or melts on heating, whereby the dye formation is promoted. Examples of the binder are as shown below.

i. Water soluble binders:

Polyvinyl alcohol, polyacrylic acid, hydroxyethyl cellulose, polyacrylamide, carboxymethyl cellulose, methoxy cellulose, polyvinyl pyrrolidone, gelatin and starch are preferable binders which provide tackiness when dissolved in or swelled with water.

ii. Binders soluble in non-polar or substantially non-polar organic solvents:

As this kind of binders, there are natural rubber, synthetic rubbers, chlorinated rubbers, alkyd resins, styrene-butadiene copolymers and polybutyl methacrylates. These binders can disperse and fix the color-forming dyes and color-forming agents as discontinuous particles at temperatures below the thermal recording temperature and, at the same time, play the role of adhering them onto substrates. Those which do not simply discolor are preferable.

d. Sensitizers and crystalline organic substances to be incorporated into the inter layer:

As the sensitizers, there are used colorless or pale-colored crystalline organic substances having a melting point of 70.degree. to 180.degree.C. which, when fused, can dissolve at least one of the color-forming dyes and color-forming agents. Effective as such substances are organic compounds having nitrogen in the molecules of nitro, amino and the like groups. Further, it has been found that acetamide, stearamide, phthalonitrile, m-nitroaniline and .beta.-naphthylamine are most suitable as the binders used in the present invention.

The substances to be incorporated into the inter layer may be the aforesaid phenolic substances or organic acids. Favorable results are obtained when the said substances are used together with the aforesaid sensitizers.

In addition thereto, talc, clay, titanium oxide, zinc oxide or calcium carbonate may be incorporated with an aim to improve the recording paper in whiteness and printability and to prevent the paper from adhering to a thermal head.

Procedures for examining and evaluating the effect of the recording paper according to the present invention were carried out in the following manner:

It is convenient, for the purpose of comparison, to express in terms of optical density (hereinafter abbreviated to "O.D.") an image formed by scratching a thermo-sensitive recording paper with a metal stamp head capable of being adjusted to and maintained at a definite temperature.

For evaluation of the present recording paper, the recording was conducted by use of a column-shaped stainless steel stamp having a tip diameter of 10 mm. which had a heating portion disposed therein, and a thermal head having a silicon resistor as the heating element. The O.D. value was measured by means of an automatic specular densitometer NLM-VI manufactured by Narumi Co., and a microdensitometer manufactured by Konishiroku Co., Ltd. The filter used was a gelatin filter produced by Kodak Co.

The present invention is illustrated in detail below with reference to examples.

EXAMPLE 1

35 Parts by weight of Rhodamine Lactam as a color-forming dye was mixed with 200 parts by weight of a 5 wt% aqueous solution of polyvinyl alcohol (PVA 107 produced by Kuraray Co., Ltd.), and the resulting mixture was ground for at least 2 hours in a ball mill having an inner volume of 500 liters to prepare a dispersion A. In the dispersion A, the color-forming dye had scarcely dissolved but had dispersed in the form of fine particles of less than several microns.

Separately, 35 parts by weight of p-hydroxybenzoic acid as a color-forming agent was mixed with 200 parts by weight of a 5 wt% aqueous polyvinyl alcohol solution, and the resulting mixture was ground for at least 2 hours in a ball mill to prepare a dispersion B. In the dispersion B, the color-forming agent had dispersed in the form of fine particles of less than several microns.

In the next place, 35 parts by weight of Crystal Violet Lactone as a color-forming dye was mixed with 200 parts by weight of a 5 wt% aqueous solution of polyvinyl alcohol (PVA 205 produced by Kuraray Co., Ltd.), and the resulting mixture was ground for at least 2 hours in a ball mill to prepare a dispersion C. The particle size of the color-forming dye after dispersion was less than several microns.

Further, 35 parts by weight of bisphenol A was mixed with 200 parts by weight of a 5 wt% aqueous solution of polyvinyl alcohol (PVA 205 produced by Kuraray Co., Ltd.), and the resulting mixture was ground for at least 2 hours in a ball mill to prepare a dispersion D. The particle size of the bisphenol A after dispersion was less than several microns.

Separately, 50 parts by weight of bisphenol A and 10 parts by weight of stearamide were mixed with 200 parts by weight of a 3 wt% aqueous solution of polyvinyl alcohol (PVA 205 produced by Kuraray Co., Ltd.), and the resulting mixture was ground for at least 2 hours in a ball mill to prepare a dispersion E. The particle size of the bisphenol A and stearamide after grinding was less than several microns.

Further, 20 parts by weight of stearamide was mixed with 200 parts by weight of a 3 wt% aqueous solution of polyvinyl alcohol (PVA 205 produced by Kuraray Co., Ltd.), and the resulting mixture was ground for at least 2 hours in a ball mill to prepare a dispersion F. The particle size of the stearamide after grinding was less than several microns.

Subsequently, 1 part by weight of the dispersion A and 15 parts by weight of the dispersion B were weighed and were mixed together by means of a homogenizing mixer. In the resulting mixed dispersion, the color-forming dye and the color forming agent had dispersed in the form of fine particles surrounded by the PVA. It may therefore be said that there was no direct contact between the two particles. The said mixed dispersion was coated by means of a wire bar on a white paper having a basis weight of 60 g/m.sup.2. The weight of the coated dispersion after air-drying was 4 g/m.sup.2. The thus coated paper was subjected to calendering to make the surface thereof uniform, and then the dispersion E was coated on said surface. The amount of the coated dispersion E after air-drying was 3 g/m.sup.2. On the thus formed layer of dispersion E was coated a mixed dispersion comprising 1 part by weight of the dispersion C, 25 parts by weight of the dispersion D and 15 parts by weight of the dispersion F. The amount of the coated mixed dispersion after air-drying was 1 g/m.sup.2. In the above-mentioned manner, a recording paper was obtained.

The thus obtained recording paper was subjected to recording by use of a recorder which had been so controlled that a definite quantity of heat could always be applied to the recording paper from a silicon semiconductor used as the heating element. As the result, a blue color was formed when the quantity of heat was 2 mj. (at a recording speed of 200 mm/sec.) while a brilliant red color was formed when the quantity of heat was 3.5 mj., and thus a two colored record could be obtained. Characteristics of the thus obtained record were as shown in FIGS. 2-A and 2 B. Densities of the two colors formed were measured by means of a microdensitometer manufactured by Konishiroku Co. In measuring the density of the first color (FIG. 2-A), the red component was made invisible by use of a red filter (Kodak Ratten No. 25), while in measuring the density of the second color (FIG. 2-B), the blue component was made invisible by use of a blue filter (Kodak Ratten No. 47B).

In FIG. 2-A, the solid line shows the formation state of the blue color, and the dotted line 2 shows that of the blue color at that time. In FIG. 2-B, the solid line 1 shows the formation state of the red color (measured by use of the blue filter), and the dotted line 2 shows that of the blue color at that time. FIG. 2-B indicates that in the formation of the second color (red), the blue component has substantially been pushed to both sides of the line and has been diluted in density.

The effect of the sensitizer used in the present invention is explained below with reference to Referential Example.

Referential Example 1

A 1:20 mixture of dispersions C and D prepared in Example 1 was coated on a white paper in a proportion of 4 g/m.sup.2, and was then sufficiently dried to prepare a paper (a). On the other hand, a 1:20 mixture of the dispersions C and E prepared in Example 1 was coated on a white paper in a proportion of 4 g/m.sup.2, and was then sufficiently dried to prepare a paper (b). The thus prepared papers (a ) and (b) were individually investigated in color formation characteristics by means of the aforesaid stamp type color formation tester under the conditions of a stamp contact pressure of 4 kg/cm.sup.2 and a contact time of 1 second. The results obtained were as shown in FIG. 3. In FIG. 3, a is the color formation characteristic curve of the paper (a), and b is that of the paper (b) incorporated with the sensitizer stearamide. That is, the color formation characteristics of the paper (a) are Ts=105.degree.C., D-max=1.05 and .gamma.=0.03, while those of the paper (b) are Ts=85.degree.C., D-max=1.0 and .gamma.=0.08. From the above results, it is understood that the use of the sensitizer increases the sensitivity of the paper to make the .gamma. value higher. As is clear from FIG. 3, the paper (b) incorporated with the sensitizer is small in difference between the color formation temperature and the maximum density-reaching temperature, and shows sufficient efficiency as a recording paper for high speed recording.

Subsequently, the paper (a) and the paper (b) incorporated with the sensitizer were individually recorded by means of a printer using a silicone resistor having 7 dots per line. As the result, the paper (b) incorporated with the sensitizer could be recorded at a speed of 95 cha./sec. to give a blue-free record. However, the paper (a) incorporated with no sensitizer could not give the same record as in the paper (b) unless it was recorded at a speed of 22 cha./sec.

EXAMPLE 2

A mixed dispersion comprising 3-dimethylamino-5,7-dimethylfluoran as a color-forming dye and p-hydroxybenzoic acid as a color-forming agent was prepared in the same manner as in Example 1. This mixed dispersion was coated on a white paper of 70 g/m.sup.2 in basis weight in a proportion of 7 g/m.sup.2, and was then sufficiently dried to form a layer on the paper. On the thus formed layer, a mixed dispersion comprising bisphenol A and phthalonitrile was coated in a proportion of 3 g/m.sup.2, and the dispersion was sufficiently dried to form an inter layer. On the thus formed inter layer, a mixed dispersion comprising Crystal Violet Lactone as a color-forming dye and bisphenol A as a color-forming agent was coated in a proportion of 1 g/m.sup.2, and the dispersion was sufficiently dried to obtain a recording paper (c). The thus obtained recording paper (c) is a dichromatic type recording paper having a first color-forming layer which forms a blue color (cobalt blue) by the reaction of Crystal Violet Lactone with bisphenol A, and a second color-forming layer which forms a red color by the reaction of 3-dimethylamino-5,7-dimethylfluoran with p-hydroxybenzoic acid.

On the other hand, a recording paper (d) was obtained in the same manner as above, except that stearamide having a melting point of 109.degree.C. was incorporated as a sensitizer into the first color-forming layer and phthalonitrile having a melting point of 141.degree.C. was incorporated as a sensitizer into the second color-forming layer.

Characteristics of these recording paper (c) and (d) were as set forth in Table 1.

Table 1 ______________________________________ Recording paper Characteristics (c) (d) ______________________________________ Color formation First color 95.degree.C. 82.degree.C. temperature Second color 125.degree.C. 95.degree.C. .gamma. First color 0.015 0.050 Second color 0.030 0.070 D-max *1 First color 0.30 0.60 Second color 1.05 1.00 Energy required for color forma- First color 6 mj. 5 mj. tion using a Second color 13 mj. 8 mj. thermal head *2 ______________________________________ *1 The D-max value of the first color is the density measured when the density of the second color became 0.1. (The densities of the first and second colors were measured by use of, respectively, the red and blue filters used in Example 1.) *2 The energy in Table 1 is an energy per dot which was required when eac recording paper was recorded at a speed of 45 cha./sec. by means of a thermal printer using a thermal head formed 7 dots per line.

FIGS. 4 and 5 show color formation characteristic curves of the recording papers (c) and (d), respectively, as measured by use of the aforesaid stamp type color formation tester under the conditions of a contact pressure of 4 kg/cm.sup.2 and a contact time of 1 second with varying stamp temperature. In each of FIGS. 4 and 5, the first color (blue) is represented by the curve 1, and the second color (red) is represented by the curve 2. For the measurement of reflection densities, a microdensitometer manufactured by Konishiroku Co. was used. In measuring the density of the first color, the red component was made invisible by use of a red filter (Ratten No. 25, a gelatin filter produced by Kodak), while in measuring the density of the second color, the blue component was made invisible by use of a blue filter (Ratten No. 47B, a gelatin filter produced by Kodak).

FIGS. 6 and 7 show color formation states per dot of the recording papers (c) and (d), respectively, when the recording papers were individually recorded by means of a thermal printer using as a head a silicon resistor having 7 dots (each 0.4 .times. 0.4 mm. in size) per dot. In each of FIGS. 6 and 7, A shows the color formation state at the time when the first color was formed in the first color-forming layer, and B shows the color formation state at the time when the second color was formed in the second color-forming layer, in which the curve 1 represents the color formation state of the blue component of the first color, and the curve 2 represents the color formation state of the red component of the second color.

As is clear from FIGS. 6 and 7, the color formation state of the recording paper (c) containing no sensitizers is such that at the time of color formation of the first color-forming layer, there is formed a blue color which has been mixed with the second color, i.e. red, (FIG. 6-A), and at the time of color formation of the second color-forming layer, there is formed a red color which has considerably been mixed with the first color, i.e. blue, (FIG. 6-B), whereas the color formation state of the recording paper (d) containing the sensitizers is such that at the time of color formation of each color-forming layer, there is formed a color which has not been mixed with the other component, with the result that a brilliant dichromatic recording can be obtained. Such effect of sensitizers is particularly marked in the case of a dichromatic type recording paper, and a clear record can be obtained by making great the difference in melting point between the two sensitizers to be incorporated into the individual layers.

EXAMPLE 3

Recording papers incorporated with sensitizers were prepared in the same manner as in Example 2, except that the ratio in amount of the color-forming dye in the first color-forming layer to that in the second color-forming layer was varied. These recording papers were compared in characteristics with each other to obtain the results as set forth in Table 2. The amount of the inter layer was made 3 g/m.sup.2, and the total amount of components constituting the thermo-sensitive layers was made definite to 9 g/m.sup.2. The recording was carried out according to Example 2, while applying input energies of 2 mj. and 4 mj., and the measurement of reflection density was conducted according to Example 2.

Table 2 __________________________________________________________________________ Input energy 2 mj. Input energy 4 mj. Areal ratio of Areal ratio of Dye ratio Density of first color Density of first color Sample No. First layer: first component to second component to Second layer color second color color second color component * component* __________________________________________________________________________ 1 1:1 0.95 95:5 0.65 75:25 2 1:4 0.90 90:10 0.84 50:50 3 1:5 0.85 90:10 1.05 25:75 4 1:7 0.80 90:10 1.10 20:80 5 1:10 0.75 85:15 1.15 20:80 6 1:15 0.70 80:20 1.15 15:85 7 1:20 0.58 70:30 1.20 15:85 8 1:30 0.35 65:35 1.20 10:90 __________________________________________________________________________ * The areal ratio of first color component to second color component is the ratio of the area surrounded by the density curve of the first color to the area surrounded by the density curve of the second color, as seen in FIG. 7.

This kind of dichromatic thermo-sensitive paper is such that in case a record of the first color is intended to be obtained by applying heat Q.sub.1 (2 mj. in this example) to the paper surface, the second color is also formed to a certain extent. Accordingly, it is necessary that the density of the first color and the ratio of the first color component to the second color component are within certain ranges, so that the first color formed can be visually distinguished from the second color formed by application of heat Q.sub.2 (4 mj. in this example). considering the above, it is desirable that the density of the first color is at least 0.50, and the areal ratio of the first color component to the second color component is greater than 70:30. Further, in case a record of the second color is intended to be obtained, the first color is necessarily formed as well. In this case, therefore, it is desirable that the ratio of the first color component to the second color component is smaller than 30:70. If the ratio is greater than said value and if the recording paper is subjected to a dot printer, the second color formed merely differs in shade and light from the first color. In view of the above, the color-forming dye ratio of the first layer to the second layer should be within the range from 1:5 to 1:20.

EXAMPLE 4

A thermo-sensitive dichromatic recording paper having a first layer comprising 3-dimethylamino-5,7-dimethylfluoran as a color-forming dye, 4-hydroxyacetophenone as a color-forming agent, and m-nitroaniline as a sensitizer; an inter layer comprising bisphenol A; and a second layer comprising 3,3-bis(p-dimethylaminophenyl)-6-aminophthalide as a color-forming dye and p-hydroxybenzoic acid as a color-forming agent was prepared. This recording paper was subjected to recording by use of the aforesaid recorder, whereby a red color could be recorded by application of an input energy of 1.8 mj., and a brilliant blue color could be recorded by application of an input energy of 2.8 mj.

EXAMPLES 5 - 10

Example 1 was repeated, except that the constructions of the first, inter and second layers were varied as set forth in Table 3, to prepare thermosensitive two color recording papers. The total amount of components constituting the layers was 8 g/m.sup.2, in which the amount of the inter layer was 2.5 g/m.sup.2. The thus prepared recording papers were subjected to recording by use of the same recorder as in Example 1 to obtain records having such colors as shown in Table 3.

Table 3 __________________________________________________________________________ Dye ratio First Second Construction Construction Construction of first color color Example of first of inter of second layer to formation formation layer layer layer second layer Color Energy Color Energy __________________________________________________________________________ 5 Crystal Rhodamine Violet Bisphenol A Lactam Lactone 1:10 Blue 1.6 mj. Red 3.0 mj. Naphthol Phthalo- Phlorogluci- nitrile nolcarboxylic Stearamide acid Rhodamine Stearamide Crystal 6 Lactam Violet Lactone 1:7 Red 1.5 mj. Blue 3.0 mj. Bisphenol A p-Hydroxy- Acetamide benzoic acid Rhodamine Bisphenol A Crystal 7 Lactam Violet Stearamide Lactone Bisphenol A 1:8 Red 1.6 mj. Blue 2.6 mj. Bisphenol A Stearamide .beta.-Naphthyl- amine Crystal Salicylic Rhodamine 8 Violet acid Lactam Lactone Hydroquinone 1:10 Blue 1.5 mj. Red 2.8 mj. 4-Hydroxy- Acetamide diphenol .beta.-Naphthyl- amine Acetamide 3,6-Bis-.beta.- Bisphenol A 9 methoxy- do. ethoxy- Acetamide Ye- fluoran 1:8 llow 1.6 mj. Red 2.8 mj. Bisphenol A Stearamide __________________________________________________________________________

Claims

What is claimed is:

1. A dichromatic thermo-sensitive recording paper comprising: a first layer formed from a dispersion in a film-forming binder of discontinuous particles of a leuco base of triphenylmethane or fluoran type dye and a phenolic substance or an organic acid, said first layer capable of forming a first color upon application of heat; a second layer formed from a dispersion in a film-forming binder of discontinuous particles of a leuco base of triphenylmethane or fluoran type dye and a phenolic substance or an organic acid, said second layer capable of forming a second color upon application of heat, said second color distinguishable from the first color formed in the first layer; and, disposed between the first and second layers, an inter layer capable of being thermally fused to dilute as well as to exclude the first color formed in the first layer from the second color formed in the second layer.

2. A dichromatic thermo-sensitive recording paper according to claim 1, wherein the heat capacity A necessary to form the first color of the first layer, the heat capacity B necessary to form the second color of the second layer, and the heat capacity C necessary to fuse the thermo-fusible inter layer, satisfy the inequality A < C .ltoreq. B.

3. A recording paper according to claim 1, wherein the inter layer comprises a dispersion in a film-forming binder of discontinuous particles of a phenolic substance or an organic acid.

4. A recording paper according to claim 1, wherein the inter layer comprises a dispersion in a film-forming binder of discontinuous particles of at least one member selected from the group consisting of acetamide, stearamide, phthalonitrile, m-nitroaniline and .beta.-naphthylamine.

5. A recording paper according to claim 4, wherein the dispersion additionally contains discontinuous particles of a phenolic substance or an organic acid.

6. A recording paper according to claim 1, wherein the first layer and/or the second layer additionally contain as a sensitizer at least one member selected from the group consisting of stearamide, acetamide, phthalonitrile, nitroaniline and .beta.-naphthylamine.

7. A recording paper according to claim 1, wherein the dye ratio of the first layer to the second layer is in the range from 1:5 to 1:20.

8. A recording paper according to claim 1, wherein said first layer, inter layer, and second layer are coated on the same side of a substrate.