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(11) | EP 0 324 576 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Thermal recording material |
| (57) A thermal recording material having a thermal recording layer that utilizes a color
forming reaction between at least one basic dye and a color developing agent capable
of forming color in contact with said basic dye, said thermal recording layer containing
a dihydroxydiphenyl ether of the formula:
so as to enhance the color forming ability and degree of brightness of said thermal recording material. In order to enhance the image stability of this thermal recording material, the thermal recording layer may further contain at least one compound selected from the group consisting of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)-butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexyl)butane, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric acid and 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane. |
[0001] The present invention relates to a thermal recording material. More particularly, the present invention relates to a thermal recording material that is improved in color forming ability and other recording characteristics and which features a high degree of brightness and better image stability.
[0002] Various types of thermal recording materials have been proposed that utilize the thermal color-forming reaction between at least one colorless or pale-colored basic dye and color developing agents such as phenols and organic acids (see Japanese Patent Publication No. 45-14039) and some of them are in extensive commercial use. Applications of these thermal recording materials cover a wide range including recorders for measurements, terminal printers of computers and information communications systems, facsimiles and automatic ticket vending machines. These recording apparatus have heating elements such as thermal heads and heat pens (styli), which are heated to produce imagewise color formation. With the recent increase in the versatility of the use of thermal recording equipment and improvement of their performance, the requirements for the quality of thermal recording paper have become increasingly rigorous. For instance, recording on a sheet of A4-size (210 mm x 297 mm) with modern facsimiles has become possible within one minute, although several minutes were required previously. Under these circumstances, several methods including the addition of sensitizers have been proposed as techniques for improving the sensitivity of thermal recording paper. However, none of the methods so far proposed have successfully attained a balance between the qualities required of thermal recording materials including sensitivity to color formation, the stability of color image and recording adaptability.
[0003] In the thermal recording layer of thermal recording paper, both a color-forming material and a phenolic compound are dispersed as fine particles and it is generally held that upon heating, one or both of the components will fuse and come in intimate contact with each other to achieve color formation.
[0004] As a phenolic compound that is suitable for the purpose of color development, 2,2-bis(4′-hydroxyphenyl) propane (bisphenol A; m.p. 156°C) has been extensively used for several reasons including low cost and high stability of quality (see, for example, U.S. Patent No. 3,539,375). However, bisphenol A is slow in response to heat and has not been able to satisfy all of the requirements of the market for higher sensitivity of thermal recording paper that are adjunct to the recent tendency for faster operation or reduction in size of thermal recording equipment. With a view to solving this problem, many sensitizers have been proposed, including waxes (Japanese Patent Public Disclosure No. 48-19231), phthalic acid esters (Japanese Patent Public Disclosure Nos. 57-116690 and 58-98285), benzylphenyl ether derivatives (Japanese Patent Public Disclosure No. 59-155096), benzylbiphenyl (Japanese Patent Public Disclosure No. 60-82382), naphtholic derivatives (Japanese Patent Public Disclosure No. 58-87094) and carbonic acid diesters (Japanese Patent Public Disclosure No. 58-136489). However, combinations of these sensitizers with bisphenol A have so far been unable to produce satisfactory quality in such aspects as the degree of brightness, sensitivity for color formation and fog resistance. If one employs low-melting point monophenols such as α-naphthol (m.p. 95 - 96°C) and 4-tertiary butylphenol (m.p. 94 - 99°C) with a view to improving sensitivity for color formation, a strong "phenolic" smell results and the commercial acceptance of the recording paper is reduced. Japanese Patent Public Disclosure No. 56-144193 proposes the use of benzyl p-hydroxybenzoate or methylbenzyl p-hydroxybenzoate as a color developing agent for increasing the sensitivity of a thermal recording sheet. However, a problem with this idea is that the recording sheet is poor in image stability since white spots will appear in the image area during storage of the record.
[0005] As described above, none of the prior art thermal recording materials have been capable of fully satisfying the requirements of the market for higher performance with regard to such aspects as color-forming characteristics, the degree of brightness and image stability. A need, therefore, has existed for the development of a thermal recording material that has high sensitivity and high degree of brightness and which features better image stability.
[0006] An object of the present invention is to provide a thermal recording material which not only has a practical level of sensitivity for color formation that guarantees its use in a wide range of applications but which also features a high degree of brightness.
[0007] Another object of the present invention is to provide a thermal recording material which has the added advantage of better image stability.
[0008] These objects of the present invention can be attained by a thermal recording material that contains in a thermal recording layer at least one colorless or pale-colored basis dye and a dihydroxydiphenyl ether represented by the following general formula:
[0009] With a view to eliminating the several problems of the prior art techniques, the present inventors conducted intensive studies by screening a number of phenolic color-developing materials. As a result, the present inventors found that the dihydroxydiphenyl ether represented by the general formula set forth above is an effective color developing agent that ensures high sensitivity and high degree of brightness and which yet is free from the problem of white spots that has occurred in the use of benzyl p-hydroxybenzoate. The present invention has been accomplished on the basis of this finding.
[0010] The dihydroxydiphenyl ether having the general formula set forth above may be exemplified by 2,2′dihydroxydiphenyl ether (m.p. l2l°C), 3,3′-dihydroxydiphenyl ether (m.p. 98°C) and 4,4′-dihydroxydiphenyl ether (m.p. l66°C)
[0011] The reason why the dihydroxydiphenyl ether used in the present invention attains a higher degree of brightness and a higher sensitivity for color formation than the conventionally used bisphenol A is not completely clear but this would probably be due to the miscibility of this compound with basic dyes or sensitizers that may be used in combination with these compounds.
[0012] In the present invention, the dihydroxydiphenyl ether of formula (I) is normally used in an amount of 0.5 - 3 parts by weight, preferably 1 - 2 parts by weight, per part by weight of the color-forming basic dye.
[0013] By incorporating this amount of dihydroxydiphenyl ether in a thermal recording layer, a thermal recording material having a high degree of brightness and sensitivity for color formation is obtained and this recording material per se is normally suitable for practical use. However, if the thermal paper, after application of heat, is left to stand for 24 hours in a hot atmosphere (60°C x 20% R.H.) or in a humid atmosphere (40°C x 90% R.H.), the density of the image will decrease and the residual density may sometimes be reduced to 80% and below. In order to avoid this problem, the thermal recording material of the present invention is often required to have better image stability.
[0014] To meet this need, the present inventors conducted further studies and found that a thermal recording material that had better image stability without compromising the degree of brightness and sensitivity could be attained by using a color developing agent made of the dihydroxydiphenyl ether of the general formula (I):
in combination with at least one compound selected from the group consisting of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexyl)butane, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl isocyanuric acid, 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl) butane.
[0015] Among the compounds that may be used in combination with the dihydroxydiphenyl ether, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane is a substance represented by the formula (II):
[0016] 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexyl)butane is a substance represented by the formula (III):
[0017] 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanuric acid is a substance represented by the formula (IV):
[0018] 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane is a substance represented by the formula (V):
[0019] When these substances are used in combination with dihydroxydiphenyl ether of formula (I), the dihydroxydiphenyl ether is generally used in an amount of 0.5 - 3 parts by weight per part by weight of a color-forming basic dye, and one or more of the substances listed above are used in an amount of 0.1 - 20 parts by weight per part by weight of this color developing agent. Preferably, dihydroxydiphenyl ether is used in an amount of 1 - 2 parts by weight per part by weight of a color-forming basic dye and one or more of the substances of interest are used in an amount of 0.2 - 5 parts by weight per part by weight of dihydroxydiphenyl ether.
[0020] The addition of the substances listed above contributes to a marked improvement in image stability and in terms of the residual density of the image, the image stability of a thermal recording material using these substances is 10 - 20% better than in the case where dihydroxydiphenyl ether is employed in the absence of these substances. This improvement will guarantee the storage of the thermal recording material of the present invention either in a hot or humid atmosphere after recording has been effected.
[0021] The colorless or pale-colored basic dye for use in the formation of a thermal recording layer in the present invention may be selected from among those which are commonly employed for this purpose, and representative examples are listed below:
[0022] Triarylmethane dyes such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)phthalide, 3-(p-dimethylaminophenyl)-3-(1,2-dimethylindol-3-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl )phthalide, 3,3-bis(1,2-dimethylindol-3-yl)-5-dimethylaminophthalide, 3,3-bis(1,2-dimethylindol-3-yl)-6-dimethylaminophthalide, 3,3-bis(9-ethylcarbazol-3-yl)-6-dimethylaminophthalide, 3,3-bis(1,2-dimethylindol-3-yl)-5-dimethylaminophthalide, and 3-p-dimethylaminophenyl-3-(1-methyl-pyrrol-3-yl)-6-dimethylaminophthalide; diphenylmethane dyes such as 4,4′-bis-dimethylaminobenzhydrylbenzyl ether, N-halophenyl-leucoauramine and N-2,4,5-trichlorophenyl leucoauramine; thiazine dyes such as benzoyl leucomethylene blue and p-nitrobenzoyl leucomethylene blue; spiro dyes such as 3-methyl-spiro-dinaphthopyran, 3-ethyl-spiro-dinaphthopyran, 3-phenyl-spiro-dinaphthopyran, 3-benzyl-spiro-dinaphthopyran, 3-methyl-naphtho(6′-methoxybenzo)spiropyran and 3-propylspiro-dibenzopyran; lactam dyes such as rhodamine-B-anilinolactam, rhodamine(p-nitroanilino)lactam and rhodamine-(o-chloroanilino)lactam; and fluorans such as 3-dimethylamino-7-methoxy-fluoran, 3-diethylamino-6-methoxyfluoran, 3-diethylamino-7-methoxy-fluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-6,7-dimethylfluoran, 3-(N-ethyl-p-toluidino)-7-methylfluoran, 3-diethylamino-7-N-acetyl-N-methylamino-fluoran, 3-diethylamino-7-N-methylaminofluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-7-N-methyl-N-benzylaminofluoran, 3-diethylamino-7-N-chloroethyl-N-methylaminofluoran, 3-diethyl-amino-7-N-diethylaminofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-phenylaminofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-(p-toluidino)-fluoran, 3-diethylamino-6-methyl-7-phenylaminofluoran, 3-dibutyl-amino-6-methyl-7-phenylaminofluoran, 3-diethylamino-7-(2-carbomethoxy-phenylamino)fluoran, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-phenylaminofluoran, 3-pyrrolidino-6-methyl-7-phenylaminofluoran, 3-piperidino-6-methyl-7-phenylaminofluoran, 3-diethylamino-6-methyl-7-xylindinofluoran, 3-diethylamino-7-(o-chlorophenylamino)fluoran, 3-dibutylamino-7-(o-chlorophenylamino)fluoran, 3-pyrrolidino-6-methyl-7-p-butylphenylaminofluoran, 3-(N-methyl-N-n-amyl)-amino-6-methyl-7-phenylaminofluoran, 3-(N-ethyl-N-n-amyl)amino-6-methyl-7-phenylaminofluoran, 3-(N-ethyl-N-iso-amyl)amino-6-methyl-7-phenylaminofluoran, 3-(N-methyl-N-n-hexyl)amino-6-methyl-7-phenyl-aminofluoran, 3-(N-ethyl-N-n-hexyl)amino-6-methyl-7-phenylaminofloran, and 3-(N-ethyl-N-β-ethylhexyl)amino-6-methyl-7-phenylaminofluoran.
[0023] Auxiliary substances that are commonly incorporated in thermal recording materials may also be used in combination with the compounds described above. An exemplary additive is a sensitizer which may be illustrated by 1,1bis-(4-hydroxy-phenyl)-cyclohexane, parabenzyl biphenyl, dibenzyl terephthalate, phenyl-1-hydroxy-2-naphthoate, dibenzyl oxalate, di-o-chlorobenzyl adipate, 1,2-di(3-methylphenoxy)ethane and di-p-chlorobenzyl oxalate.
[0024] The support on which the thermal recording layer of the thermal recording material of the present invention is formed may be made of any suitable material that is selected from among paper, pigment-coated paper, paper made of synthetic fibers, synthetic paper, synthetic resin films, etc. Paper is normally preferred.
[0025] In preparing a coating solution from which a thermal color-forming layer is to be formed, the basic dye, dihydroxydiphenyl ether of formula (I) and a substance capable of improving image stability are dispersed in water by means of a grinder such as a ball mill, attritor or a sand grinder. The resulting coating solution contains 2 - 40 wt%, preferably 5 - 25 wt%, of the total solids content of a binder selected from among starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, gum arabic, polyvinyl alcohol, styrene, salts of maleic acid copolymers, styrene-butadiene copolymer emulsions, etc. Various auxiliary agents may be incorporated as required in the coating solution. Illustrative additives include: dispersants such as sodium dioctylsulfosuccinate, sodium dodecylbenzenesulfonate, sodium salt of laurylalcohol sulfate esters, and metal salts of aliphatic acids; antifoaming agents; fluorescent dyes; and pigment dyestuffs. With a view to reducing the chance of sticking of the thermal recording layer onto the thermal head or residue on the thermal head and in order to further improve its brightness, an inorganic pigment such as kaolin, clay, talc, calcium carbonate, calcined clay, titanium oxide, diatomaceous earth, fine particulate anhydrous silica or aluminum silicate may be incorporated in the recording layer. If desired, waxes such as dispersions or emulsions of stearic acid, polyethylene, carnauba wax, paraffin wax, calcium stearate, zinc stearate, ester waxes, etc. may also be incorporated in appropriate amounts in the coating solution.
[0026] The method of forming the recording layer of the thermal recording material of the present invention is not limited in any particular way. For instance, a thermal coating solution may be applied to the support with any suitable coating apparatus such as an air-knife coater or a blade coater.
[0027] The coating weight of the thermal color-forming layer is not limited to any particular value but it is generally formed in a dry weight of 1 - 15 g/m², the range of 3 - 8 g/m² being preferred.
[0028] If necessary, a variety of known modifications, such as the provision of an undercoat, may be applied to the thermal recording material of the present invention.
[0029] In accordance with the present invention, an improved thermal recording material is provided which not only has a practical level of sensitivity for color formation that guarantees its use in a wide range of applications but which also features a high degree of brightness.
[0030] The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting. In the following examples and comparative example, all "parts" and "%" are on a weight basis.
(1) Preparation of Solution A
[0031]
| 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-phenyl-aminofluoran | 40 parts |
| 10% Aqueous polyvinyl alcohol solution | 20 parts |
| Water | 20 parts |
(2) Preparation of Solution B
[0033]
| 2,2′-Dihydroxydiphenyl ether | 40 parts |
| 10% Aqueous polyvinyl alcohol solution | 20 parts |
| Water | 20 parts |
(3) Preparation of Solution C
(4) Preparation of Solution D
[0037]
| 1,1,3-Tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane | 40 parts |
| 10% Aqueous polyvinyl alcohol solution | 20 parts |
| Water | 20 parts |
EXAMPLE 1
[0039] Ten parts of Solution A, 20 parts of Solution B, 20 parts of Solution C, 30 parts of a 50% calcium carbonate dispersion, 8 parts of a 30% zinc stearate dispersion and 75 parts of a 10% aqueous polyvinyl alcohol solution were mixed with stirring to form a coating solution. This coating solution was applied to raw paper (50 g/m²) to give a dry weight of 7 g/m² and subsequently dried to form a thermal recording sheet.
EXAMPLE 2
[0040] A thermal recording sheet was prepared as in Example 1 except that Solution B had 2,2′-dihydroxydiphenyl ether replaced by the same amount of 4,4′-dihydroxydiphenyl ether.
EXAMPLE 3
[0041] A thermal recording sheet was prepared as in Example 1 except that Solution B had 2,2′-dihydroxydiphenyl ether replaced by the same amount of 3,3′-dihydroxydiphenyl ether.
EXAMPLE 4
[0042] Ten parts of Solution A, 20 parts of Solution B, 20 parts of Solution C, 5 parts of Solution D, 30 parts of a 50% calcium carbonate dispersion, 8 parts of a 30% zinc stearate dispersion and 75 parts of a 10% aqueous polyvinyl alcohol solution were mixed with stirring to form a coating solution. This coating solution was applied to raw paper (50 g/m²) to give a dry weight of 7 g/m² and subsequently dried to form a thermal recording sheet.
EXAMPLE 5
[0043] A thermal recording sheet was prepared as in Example 4 except that Solution D had 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)-butane replaced by the same amount of 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexyl)butane.
EXAMPLE 6
[0044] A thermal recording sheet was prepared as in Example 4 except that Solution D had 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)-butane replaced by the same amount of 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethyl) isocyanuric acid.
EXAMPLE 7
[0045] A thermal recording sheet was prepared as in Example 4 except that Solution D had 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)-butane replaced by the same amount of 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane.
COMPARATIVE EXAMPLE 1
[0046] A thermal recording sheet was prepared as in Example 1 except that Solution B had 2,2′-dihydroxydiphenyl ether replaced by the same amount of 2,2-bis(4′-hydroxyphenyl)propane (bisphenol A).
[0047] The eight samples of thermal recording sheet thus prepared were supercalendered and the degree of their brightness was measured with a Hunter brightness meter. After performing thermal recording on these sheets with Model UF-60 of Matsushita Graphic Communication Systems, Inc., the density of the image was measured with a densitometer, Model RD-100R of Macbeth Instrument Corporation.
[0048] The samples were thereafter subjected to a storage test and the residual density of the image was calculated by the following formula:
[0049] In the storage test, the record was left to stand for 24 hours in either a hot (60°C x 20% R.H.) or humid (40°C x 90% R.H.) atmosphere to evaluate its resistance to heat or moisture. The results are shown in the following table.
| Initial image density | Heat resistance | Moisture resistance | Brightness | |||
| Density | Residual density (%) | Density | Residual density (%) | |||
| Ex. 1 | 1.30 | 1.02 | 77 | 0.96 | 73 | 83.2 |
| 2 | 1.32 | 1.00 | 76 | 0.92 | 70 | 82.5 |
| 3 | 1.36 | 1.08 | 79 | 1.05 | 77 | 81.4 |
| 4 | 1.32 | 1.28 | 97 | 1.25 | 95 | 82.8 |
| 5 | 1.35 | 1.28 | 95 | 1.26 | 93 | 81.8 |
| 6 | 1.31 | 1.24 | 95 | 1.23 | 94 | 82.0 |
| 7 | 1.30 | 1.22 | 94 | 1.21 | 93 | 81.0 |
| Comp. Ex. 1 | 1.2 | 0.98 | 82 | 1.00 | 83 | 79.5 |
1. A thermal recording material having a thermal recording layer that utilizes a
color forming reaction between at least one basic dye and a color developing agent
capable of forming color in contact with said basic dye, said thermal recording layer
containing a dihydroxydiphenyl ether of the formula:
2. A thermal recording material according to Claim 1 wherein said thermal recording
layer further contains at least one compound selected from the group consisting of
1,1, 3-tris 2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,
1,1,3-tris(2-methyl-4-hydroxy-5-cyclcohexyl)butane, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric
acid and 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane.
3. A thermal recording material according to Claim 2 wherein said at least one compound
is used in the amount of 0.1 - 20 parts by weight per part by weight of dihydroxydiphenyl
ether.