GB2033594A - Heat-sensitive recording paper - Google Patents

Abstract

A heat-sensitive recording sheet comprises a support bearing a recording layer containing an electron-donating colorless dye and a bisphenol represented by the following general formula: <IMAGE> where R represents an alkyl group containing 2 to 12 carbon atoms.

Description

SPECIFICATION Heat-sensitive recording paper This invention relates to heat-sensitive recording paper capable of forming a dye image which lasts well under high temperature and high humidity conditions.

In heat-sensitive recording papers images are formed ultilizing physical or chemical changes induced in the recording materials by heat energy, and a large number of processes using heat-sensitive recording papers have been investigated.

One type of heat-sensitive recording paper which relies upon a heat-induced physical change is wax-type heat-sensitive recording paper which is well known and has been used for recording electrocardiograms and the like. Other types of heat-sensitive recording paper utilize a heat-induced chemical change to form images and a number of materials relying on various different color-forming mechanisms have been proposed; of these, a binary coloration system is most common.

A material relying on a binary coloration system is prepared by dispersing two heat-reactive compounds in fine particulate form in a binder or the like such that the binder separates them from each other, and coating the resultant mixture on a base paper.

A binary coloration system is prepared by dispersing two heat-reactive compounds as find particles in a binder or the like such that the binder separates them from each other, and coating the resultant mixture on a base paper. The thus prepared paper records images by heating it to melt one or both of the compounds such that they come into contact with each other, thus causing a color-forming reaction. These two heat-reactive compounds are generally referred to as electron-donating compounds and electron-accepting compounds, respectively. An extremely large number of combinations of them are known, and they are roughly classified as systems forming images of a metal compound and systems forming dye images.

Representative examples of systems forming images of a metal compound are those in which the electron-donating compounds are organic reducing agents such as phenols, chelating agents, sulfur compounds, or amino compounds and the electron-accepting compounds are organic metal salts. These two produce a metal, metal complex compound, metal sulfide, through the reaction therebetween upon being heated, thus providing a colored image. Specifically, there are a combination of thiourea and heavy metal salt (U. S. Patent 2,740,895), a combination of gallic acid and metal salt of stearic acid (U. S.- Patents 2,663,654, 2,663,655, 2,663,656, and 2,663,657), a combination of hydroquinone and silver behenate (U. S.

Patent 3,031,329), a combination of hexamethylenetetramine and tin compound (U. S. Patent 2,813,043).

On the other hand, examples of systems forming dye images are those in which the electron-donating compound is an electron-donating colorless dye, and the electron accepting compound is an acidic material such as phenol (Japanese Patent Publication No. 4,160/68 and U. S. Patent 3,451,338).

The binary coloration systems of heat-sensitive recording papers have many advantages as recording papers. For example (1) they are based on primary coloration and do not require development processing; (2) the paper quality is approximate to that of ordinary paper; and (3) they are handled with ease. In particular, those materials wherein a colorless dye is used as the electron-donating compound have the additional advantages that (4) they provide higher color density and (5) they enable one to easily obtain heat-sensitive recording papers forming different colors, thus being more valuable. Therefore, they are in the most use as heat-sensitive recording papers.

As the electron-donating colorless dyes, triarylmethane compounds, diphenylmethane compounds, xanthene compounds, thiazine compounds, spiropyran compounds, and the like have been used. Several examples thereof are illustrated below. As the triarylmethane compounds, there are 3,3-bis(pdimethylaminophenyl)-6-dimethylaminophthalide (or Crystal Violet lactone), 3,3-bis(p dimethylaminophenyl)-phthalide, 3-(p-dimethylaminophenyl)-3-(1 ,3-dimethylindol-3-yl)phthalide, 3-(p- dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide. As the diphenylmethane compounds, there are 4, 4'-bis-dimethylaminobenzhydrin benzyl ether, N-halophenyl-leucoAuramine, N-2,4,5-trichlorophenylleucoAuramine.As the xanthene compounds, there are Rhodamine-B-anilinolactam, Rhodamine-B-(pnitroanilino)-lactam, Rhodamine-B-(p-chloroanilino)lactam, 3-diethylamino-7-(dibenzylamino)fluoran, 3- diethylamino-7-phenylamino-fluoran, 3-diethylamino-6-methyl-7-anilinofluoran 3-diethylamino-7-(o- chloroanilino)fluoran, 3-diethylamino-7-(3,4-dichloroanilino)fluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-diethylamino-7-phenylfluoran. As the thiazine compounds, there are benzoyl leuco Methylene Blue, p-nitrobenzyl Leucomethylene Blue.As the spiro compounds, there are 3-methyl-spiro-dinaphthopyran, e-ethyl-spiro-dinaphthopyran, 3, 3'-dichloro-spiro-dinaphthopyran, 3-benzyl-spiro-dinaphthopyran, 3- methyl-naphtho-(3-methoxy-benzo)-spiropyran, 3-propyl-spiro-dibenzopyran. They may be used alone or in combination.

Specific examples of conventional electron-accepting compounds include phenolic compounds, organic acids or metal salts thereof, hydroxybenzoic acid esters. Of these compounds, phenolic compounds are favorably used because they have a melting point which is near the desired recording temperature (70 to 1200C) and they do not require the use of a low-melting compound or, if any, in only a small amount. They are described in detail in, for example, Japanese Patent Publication No. 29,830/76, and U. S. Patent 3,539,375.

To be specific, there are illustrated 4-t-butylphenol, 4-phenyiphenol, 4-hydroxydiphenoxide, a-naphthol, ss-naphthol, methyl-4-hydroxybenzoate, 2, 2'-dihydroxybiphenyl, 2,2-bis(4-hydroxyphenyl)propane (bisphe nol A), 4,4'-isopropyl idenebis(2-chlorophenol), 4,4'-isopropylidenebis(2-methyl-phenol), 4,4'-sec- isobutylidenediphenol. However, these phenolic compounds are not necessarily considered to be satisfactory heat-sensitive materials.

That is, they frequently suffer from one of the following defects: (1) inadequate color density is obtained when used in combination with an electron-donating colorless dye; (2) fog (i.e., color formation during storage before use) tends to be formed; (3) the color fastness of the images formed is not sufficient.

Specifically, 4,4'-isopropylidene-diphenol (2,2-bis(4-hydroxyphenyl)propane) most generally used as the phenolic compound tends to cause fog, though it provides the highest color density of the above-described electron-accepting compounds. When Crystal Violet lactone is used as an electron-donating colorless dye, the phenolic compound causes fog during storage under high temperature and high humidity (45"C, 80%RH) conditions and serious fading of colored images occurs.

In addition to the foregoing, in order to obtain sufficient color density at a color-forming temperature of about 100 C, a melting point-reducing agent must be used together with the phenolic compound due to the high melting point of the latter (156"C). The heat-senstivie recording sheets, particularly those presently used in facsimile recorders, are required to be recorded at recording temperature of about 80 to 1200C, and hence too high melting points are not desirable.

An object of the present invention is to provide a heat-sensitive recording sheet containing an electron-accepting phenol compound and which is free from the disadvantages discussed above that attend the use of the phenolic compounds currently employed.

According to the present invention the recording layer of a heat-sensitive recording sheet contains the combination of an electron-donating colorless dye and a phenolic compounds represented by the following general formula:

wherein R represents an alkyl group containing 2 to 12 carbon atoms.

The above phenolic compounds have not hitherto been used in heat-sensitive recording sheets. Their characteristic properties are as follows: (1) When used in combination with an electron-donating colorless dye, these phenolic compounds provide a colored image having high density and reduced fog.

(2) They provide colored images having a good stability which hardly suffer any fading with time under humid condition or upon exposure of light (3) Most of these phenol compounds have a melting point in the range of from about 60 to 1300C and hence they do not require the addition of a melting point reducing agent or the like for employment in a heat-sensitive recording sheet.

(4) They are stable and exhibit substantially no tendency to sublime.

(5) They can be easily synthesized, and highly pure products can be obtained in a high yield. In addition, starting materials for their synthesis are inexpensive.

Of the phenolic compounds, those wherein R is an alkyl group containing 5 to 8 carbon atoms and particularly a branched chain alkyl group are preferred such as a 1-ethylpentyl group and a 1-methylbutyl group. When the number of carbon atoms in R is not more than 1, the water solubility of the compound is so high that fog tends to occur during storage making such compounds unfavorable. On the other hand, when the number of carbon atoms in R exceeds 13, most of the compounds melt at about room temperature or lower, thus their usefulness is extremely low.

Representative examples of the phenolic compounds of the present invention are 1, 1 -bis(4- hydroxyphenyl)propane, 1,1 -bis(4-hydroxyphenyl )butane, 1,1 -bis(4-hydroxyphenyl )-pentane, 1,1 -bis(4hydroxyphenyl )hexane, 1, 1-bis(4-hydroxyphenyl)heptane, 1,1 -bis(4-hydroxyphenyl)octane, 1,1 -bis(4- hydroxyphenyl )-2-methyl-pentane, 1,1 -bis(4-hydroxyphenyl)-2-ethyl-hexane, 1,1 -bis(4- hydroxyphenyl)dodecane.

The phenolic compounds are easily synthesized by reacting excess phenol with aldehyde as disclosed in U. S. Patents 2,468,982 and 2,535,014.

A process of synthesizing the phenolic compound used in the present invention is exemplified below taking 1,1 -bis(4-hydroxyphenyl )-2ethyl-hexane, as an example. Other compounds can be synthesized analogously.

Synthesis Example 47 g of phenol, 4 mb of conc. hydrochloric acid, and 0.5 g of thioglycolic acid were introduced to a flask, and 12.8 g of 2-ethyl-hexylaldehydewere dropwise added thereto under stirring. After completion of the dropwise addition, 6 g of calcium chloride were added thereto and, after standing for 1 day at room temperature, excess phenol was removed by steam distillation, followed by extraction with benzene. Then, benzene was distilled off from the thus obtained benzene extract followed by recrystallization from a mixed solvent of benzene and hexane to obtain 16.3 g. of 1,1-bis(4-hydroxyphenyl)-2-ethyl-hexane having a melting point of 87"C.

As the electron-donating colorless dyes used in the present invention, those dyes discussed above and conventionally used for the purpose may be used. Of those described above 3-diethylamino-7-(o chloroanilino)fluoran, 3-(N-methyl-N-tolylamino)-6-methyl-7-anilinofluoran, and 3-(N-ethyl-N-tolylamino)-6methyl-7-anilinofluoran are preferred.

According to the most general process for preparing the heat-sensitive recording paper, the foregoing electron-donating colorless dye and the electron-accepting compound are dispersed respectively in an amount of about 1 to 10 wt % in separate water-soluble high polymer solutions using a ball mill, a sand mill, or the like, and the resulting dispersions are mixed with each other, followed by addition of an inorganic pigment such as kaolin, talc, calcium carbonate, to prepare a coating solution. If necessary, a paraffin wax emulsion, a latex binder, a sensitivity-improving agent, a metallic soap, an ultravoilet ray-absorbing agent, etc. may be added to the composition for effects well known in the art.However, when the above-defined phenolic compounds of the present invention are used, sensitivity-improving agents are not necessary and the amount of wax, metallic soap, uitraviolet ray-absorbing agent, etc. can be markedly reduced. The above-defined phenolic compounds form color images more stable to light as compared with commonly used bisphenol A, and sticking of the sensitivity-improving agent to a recording head scarcely occurs.

The support for the recording sheet of the present invention may be a plastics film (e.g., polyethylene terephthalate (PET), triacetyl cellulose (TAC) ), a paper or the like. In the case that a paper is used as a support, a paper having a hot water extraction pH of about 6 to 9 is preferable. The term "hot water extraction pH" as used herein means the pH of the extract obtained by extracting a sample piece of base paper with hot water as specified in JIS-P-8133-1976, (hereinafter abbreviated as extraction pH). 20 mt of the distilled water is added to about 1.0 g of sample pieces in a 100 me erlenmeyer flask. The sample pieces are uniformly immersed in the distilled water using a stirring rod. 50 me of distilled water is added to the obtained mixture and the mixture is stirred.The mixture is then placed in a flask equipped with a condenser and heated on a water bath maintained at 95 to 1000C for one hour while occasionally shaking the flask without boiling the water. The solution is cooled to a temperature of 20"C + 5 deg. The pH of the extract obtained at a temperature of 20"C + 5" is the extraction pH. The preferred thickness for the films is about 50 to 3001lem. The preferred thickness (base weight) for the paper is 40 to 200 iim (40 to 200 g/m2).

The coating solution is most generally coated on a base paper. The coating amount is generally in the range of about 2 to 10 g per m2 (as solids). The layer itself is generally about 1 to 10 cm thick. The lower limit is determined by the color density upon heating whereas the upper limit is determined mainly for economical reasons.

A preferred coating amount for the phenolic compounds is about 0.8 to 5 g/m2. A suitable ratios for the amount of phenolic compound to the amount of the electron-donating colorless dye is about 2:1 to 10:1.

The present invention will now be described in more detail by the following Example which, however, should not be construed as limiting.

EXAMPLE Preparation ofSamples 1-5 5 g of the electron donating colorless dye shown in Table 1 were dispersed in 50 g of a 5% polyvinyl alcohol (saponification degree: 99%; polymerization degree: 1,000) aqueous solution using a ball mill for one day and one night. On the other hand, 20 g of an electron-accepting compound (phenolic compound) similarly shown in Table 1 were dispersed in 200 g of a 5% polyvinyl alcohol aqueous solution using a ball mill for one day and one night. The resulting two dispersions were mixed with each other, and 20 g of kaolin (Georgia kaolin) were added thereto and well dispersed. Further,5 g of a 50% dispersion of paraffin wax emulsion (made by Chukyo Yushi Co.; Serozole &num;428) were added thereto to prepare coating solutions.

Each of the thus-obtained coating solutions was coated on a base paper having a basis weight of 50 g/m2, in an amount of 6 g/m2 (as solids), and dried for 1 minute at 600C, followed by super-calendering at a linear pressure of 60 kgW/cm to obtain coated papers.

The resulting coated papers were subjected to heat-coloration using a heated stamp under the conditions of 500 g/cm2 pressure and 1 second heating time while varying the temperature. The temperature at which the color density reached 1.00 as visual density was determined and used as the recording temperature.

The color images thus formed were stored for 1 week at 45'C and 80% RH, and the increase in fog in the uncolored areas and image-retention in the colored areas (i.e., the ratio of the density after storage to the density before storage) were determined to compare fastness. Further, light exposure properties were compared by exposing the images to 32,000 lux light for 10 hours. The results thus obtained are shown in Table 1.

Preparation of Comparative Samples C-i to C-3: Similar tests were conducted in the same manner as with Samples 1 - 5 but changing the electron accepting componds to compounds other than those used in the present invention are also shown in Table 1. The results thus obtained are also tabulated in Table 1.

TABLE 1 Light * Fastness Sample Electron- Electron- Fog Recording Fog Image- Image No. donating accepting Density Temperature Density Retention retention Colorless Dye Compound 1 Crystal Violet 1,1-Bis-(4- 0.07 85% 0.10 98% 100% lactone hydroxyphenyl)2-ethyl-hexane 2 " 1,1-Bis-(4- 0.08 86 0.10 100% 98% hydroxyphenyl)2-methyl-pentane 3 3-Diethylamino-7- 1,1-Bis-(4- 0.07 98 C 0.07 100% 100% (o-chloroanilino)- hydroxyphenyl)fluoran 2-ethyl-hexane 4 " 1,1-Bis-(4- 0.07 110 C 0.07 97% 100% hydroxyphenyl)2-methyl-pentane 5 " 1,1-Bis-(4- 0.08 108 C 0.10 96% 96% hydroxyphenyl) propane C-1 3-Diethylamino-7- 2,2-Bis-(4- 0.08 148 C 0.10 20% 95% (o-chloroanilino)- hydroxyphenyl) fluoran propane C-2 Crystal Violet " 0.13 132 C 0.18 66% 81% lactone C-3 " 1,1-Bis-(4- 0.14 88 C 0.22 72% 85% hydroxyphenyl) propane *45 C, 80% RH, 1 week.

In Table 1,fog densities higher than 0.13 seriously detract from the commercial value of the products. The colored image-retention is preferably not less than 90%. From this point of view, it is seen that the above-defined electron-accepting compounds of the present invention provide extremely excellent heat-sensitive recording sheets.

Claims (13)

1. A heat-sensitive recording sheet comprising a support and, on a surface thereof, a recording layer containing an electron-donating colorless dye and a bisphenol represented by the following general formula:

wherein R represents an alkyl group containing 2 to 12 carbon atoms.

2. A sheet as claimed in Claim 1, in which R is an alkyl group containing 5 to 8 carbon atoms.

3. A sheet as claimed in Claim 1 or 2, in which R is a branched alkyl group.

4. A sheet as claimed in Claim 1, in which said bisphenol is selected from 1,1 -bis(4hydroxyphenyl)propane, 1,1 -bis(4-hydroxyphenyl)butane, 1,1 -bis(4-hydroxyphenyl)-pentane, 1 ,1-bis(4 hydroxyphenyl)hexane, 1,1 -bis(4-hydroxyphenyl)heptane, 1,1 -bis(4-hydroxyphenyl)octane, 1,1 -bis-(4- hydroxyphenyl)-2-methylpentane, 1,1 -bis(4-hydroxyphenyl)-2-ethyl-hexane and 1,1 -bis(4- hydroxyphenyl)dodecane.

5. A sheet as claimed in Claim 1, in which said bisphenol has a melting point of from 60 to 1300C.

6. A sheet as claimed in any preceding Claim, in which the bisphenol is present in an amount of from 0.8 to 5.0 g/m2 on a dry basis.

7. A sheet as claimed in any preceding Claim, in which said electron-donating colorless dye is present in an amount of from 0.1 to 2.0 g/m2 on a dry basis.

8. A sheet as claimed in any preceding Claim, in which said electron-donating colorless dye is a triarylmethane compound, a diphenylmethane compound, a xanthene compound, a thiazine compound or a spiropyran compound.

9. A sheet as claimed in Claim 8, in which said colorless dye is selected from 3-diethylamino-7-(o chloroanilino)fluoran, 3-(N-methyl-N-tolylamino)-6-methyl-7-anilinofluoran and 3-(N-ethyl-N-tolylamino-6methyl-7-anilinofluran.

10. A sheet as claimed in Claim 1 and substantially as herein described.

11. A heat-sensitive recording sheet substantially as herein described with reference to any one of Samples 1 to 5 in the foregoing Example.

12. A sheet as claimed in any preceding Claim and bearing a coloured image produced by image wise exposure of the paper to heat.

13. The features as herein disclosed, or their equivalents, in any novel selection.