JPH02249691A - Thermal recording material - Google Patents

Abstract

PURPOSE:To enhance long-term raw storage stability by using 2,2-bis(4- hydroxyphenyl)propane as a coupler and constituting a leuco dye of a mixture of specific compounds and further containing a specific sensitizer and a specific oxidation inhibitor. CONSTITUTION:2,2-bis(4-hydroxyphenyl) propane is used as a coupler and, as a leuco dye, a compound represented by formula I and a compound represented by formula II [wherein R1 and R2 are respectively a 1-8C alkyl group or a cycloalkyl group and may be bonded through a -(CH2)n-group and n is 4 or 5] are used in a mixed state and, further, a sensitizer represented by formula III (wherein R is a hydrogen atom, a halogen atom or a lower alkyl group) and an oxidation inhibitor represented by formula IV (wherein R1 is a t-butyl group or a cyclohexyl group and R2 is a 1-3C alkyl group) are contained. The mixing ratio of the leuco dye represented by the formula I and the leuco dye represented by the formula II is pref. 98/2-91/9 on a wt. basis.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a heat-sensitive recording medium, and more particularly, to a heat-sensitive recording medium that has excellent high-speed recording characteristics, and has high main paper stability and high stability of recorded images.

(Prior art) In general, a heat-sensitive recording material made by coating a support with a colorless or light-colored leuco dye and a color developer that causes the leuco dye to develop color when heated is disclosed in Japanese Patent Publication No. 14035/1983, etc. ,
It has been put into practical use. To record on this heat-sensitive recording medium, a thermal printer with a thermal head of Due to its features such as no maintenance, no maintenance, and relatively inexpensive equipment, it is widely used for computer output, facsimiles, printers such as calculators, measurement recorders, and ticket paper for automatic ticket vending machines.

In recent years, there has been a trend for these recording devices to become more compact and energy-saving, and the required characteristics for the thermal recording paper used in these devices are the ability to obtain high density color development with low applied energy, that is, the desire for high sensitivity. is increasing. Conversely, as equipment becomes more compact, the temperature in the space in which the recording paper is loaded increases due to heat accumulation in the recording head and heat generation from the electric circuit, which increases the background density of thermal recording paper. There is also a growing demand for thermal stability, which is easy to increase and has high sensitivity.

Methods for increasing sensitivity include lowering the melting point of the leuco dye and color developer, improving the compatibility of the two, and adding a sensitizer as a third component. It is being considered. For example, as a leuco dye,
6-52759, JP-A-59-120653, JP-A-60-208360, etc., disclose materials having a melting point of 180 DEG C. or less and excellent recording sensitivity.

As a color developer, p-hydroxybenzoic acid ester (JP-A-54-74762), diphenol or monophenol sulfone compound Th (JP-A-57-21)
0886, JP-A-59-96991), diphenol sulfide compound (JP-A-57-33187), diphenol disulfide compound (JP-A-59-52694)
) and other various studies are being conducted. In addition, as sensitizers, wax @ (Japanese Patent Application Laid-open No. 19231-1982), naphthol derivatives (Japanese Patent Application Publication No. 57-64593), naphthoic acid derivatives (Japanese Patent Application Publication No. 57-64592), and ether compounds (Japanese Patent Application Publication No. 60-1989) are used as sensitizers. Various materials such as 78780> are being considered.

However, although the heat-sensitive recording materials manufactured using these methods are relatively hard in terms of high sensitivity, the color fading of images is severe due to the strong sublimation properties of the color developer and sensitizer. The compatibility with leuco dyes is too good, causing fogging of the background, and the melting point drops excessively, resulting in thermal instability, and as of now, the properties are still unsatisfactory.

Furthermore, as the market size of thermal recording media expands worldwide, in addition to the problems of thermal stability and image stability mentioned above, there are also problems such as background fogging of recording paper and deterioration of printing performance over time during the transportation process and various storage environmental conditions. This is becoming an important issue. This is caused by the effects of heat, humidity, etc. over a long period of time, and water can dissolve the color developer and cause background fogging, and heat can cause the color developer and sensitizer to sublime and dissipate, reducing the color density. This is considered to be a phenomenon, and there are still high-sensitivity thermal recording materials that maintain the long-term storage stability of the thermal recording paper (main paper) before recording (hereinafter referred to as main paper storage stability) and have good image storage stability. The current situation is that it has not been developed.

<Problems to be Solved by the Invention> The present invention has been made in view of the above-mentioned problems, and specifically, it not only has good high-speed recording performance and excellent stability of the image area, but also has long-term durability. The present invention relates to a thermosensitive recording material with improved paper storage stability.

<Means for Solving the Problems> The present invention provides a heat-sensitive recording material in which a heat-sensitive color forming layer containing a leuco dye and a color developer that causes the leuco dye to develop color when heated is provided on a support as a main component. 2,2-bis(
4-hydroxyphenyl)propane, the leuco dye is a mixture of a compound represented by the following general formula (1) and a compound represented by the following general formula (2) having a melting point of 190°C or higher, and further This is a heat-sensitive recording material containing a sensitizer represented by the following general formula (3) and an antioxidant represented by the following general formula (2).

General formula (1) General formula (2-(in the formula, 81 and Rt each represent an alkyl group or a cycloalkyl group having 1 to 8 carbon atoms. R4 and R2 are -(C
It may also be formed by bonding through I and l groups. Here, n represents 4 or 5. ) General formula (3) (In the formula, R represents a hydrogen atom, a halogen atom, or a lower alkyl group, respectively.) General formula (4) (In the formula, R3 represents a t-butyl or cyclohexyl group, R
2 represents an alkyl group having 1 to 3 carbon atoms. ) The leuco dye of general formula (1) used in the present invention has a melting point of 180-1
The temperature is 84° C., and when used alone, it has a serious drawback in image stability. Melting point used in the present invention: 190°C
Examples of specific compounds of the leuco dye of the above general formula (2) include 3-diethylamino-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-
Anilinofluorane, 3-cyclohexylmethylamino-6-methyl-7-anilinofluorane, etc.
The weight ratio of the mixture of the leuco dye of general formula 2 (1) and the leuco dye of general formula (2) is preferably 98/2 to 9I/9. Outside this range, that is, if the leuco dye of general formula (2) is present in a large amount, there will be severe background fogging under high temperature and high humidity environments, and the main paper will lack long-term storage stability. This tends to cause problems such as lack of stability of recorded images.

The sensitizer represented by the general formula (3) used in the present invention is characterized by not only excellent recording sensitivity but also little damage to image stability or storage stability of main paper. Specific examples include oxalic acid dibenzyl ester, oxalic acid di(p-methylbenzyl) ester, and oxalic acid di(p-methylbenzyl) ester.
(p-chlorobenzyl) ester and the like.

Specific examples of the antioxidant of general formula (4) used in the present invention include 1,1.3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1.1.
Examples include 3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and it is preferable to add 0.2 parts by weight or more to 1 part by weight of the leuco dye. If the amount is outside this range, that is, if the amount added is small, the problem of poor image stability in high-temperature environments tends to occur.

The thermosensitive coloring layer in the thermosensitive recording material of the present invention contains inorganic and organic pigments as conventional fillers, such as heavy and light calcium carbonate, aluminum hydroxide, titanium oxide, zinc oxide, barium sulfate, talc, clay, satin. The clarity of colored images can be improved by adding white, kaolinite, polyolefin particles, urea-formalin resin particles, etc.

In addition, lubricants such as zinc stearate and calcium stearate are used to improve mating performance with the thermal recording head, and paraffin wax and microcrystalline wax are used to prevent pressure build-up when smoothing the surface of the recording paper. is added as necessary.

The thermosensitive coloring layer is provided on a support, and the support in this case may be paper, synthetic paper, resin film, or a composite thereof, with paper being preferred from the viewpoint of cost. Furthermore, for the purpose of improving the clarity of recorded images, it is also possible to provide an undercoat layer on the surface of the support and provide a thermosensitive coloring layer thereon.

In the heat-sensitive recording medium of the present invention, the coating material containing these substances is generally prepared by preparing a dispersion liquid using water containing a water-soluble polymer substance as a dispersant using a well-known and commonly used stirring/pulverization technique. It is prepared by mixing with a binder, various surfactants, antifoaming agents, etc. as necessary. As this binder, water-soluble or water-insoluble resin binders can be used, such as gelatin, casein, polyvinylpyrrolidone, polyvinyl alcohol, modified polyvinyl alcohol, starch, modified starch, isobutylene-maleic anhydride copolymer Resin, diisobutylene-maleic anhydride copolymer resin, styrene-maleic anhydride copolymer resin, polyacrylamide, modified polyacrylamide, carboxymethylcellulose, methylcellulose, hydroxyethylcetate/L10-su, vinyl acetate, acrylic acid Ester, vinyl chloride-vinyl acetate copolymer, 5B
Emulsions and latexes such as R1NBR are used alone or in combination. Moreover, it is also possible to add a waterproofing agent if necessary.

These prepared paints are formed as a heat-sensitive coloring layer by applying and drying the paint onto a support by conventionally well-known and commonly used coating techniques such as air knife coating, blade coating, roll coating, etc.

<Examples> The present invention will be described in detail below using Examples and Comparative Examples. All formulations are in parts by weight.

Example 1 A mixture having the following composition was subjected to a dispersion treatment using a sand grinder to prepare dispersions (A) to (E).

(A) Liquid 5% methylcellulose aqueous solution 40 parts water

20 parts (B) Liquid 5% methylcellulose aqueous solution [CD liquid 40 parts 20 parts Calcium carbonate 10% polyvinyl alcohol aqueous solution water CD) Liquid 100 parts 10 parts 140 parts 10% polyvinyl alcohol aqueous solution [E] Liquid 10 parts 50 parts 10 % polyvinyl alcohol aqueous solution 10 parts water
5
Using the (A3-(E) solution obtained as above), a heat-sensitive coloring layer paint having the following formulation was prepared, and the basis weight was 55 g/
After coating and drying on a high-quality paper of RR (dried coating amount: 6 g/rd), surface treatment was performed using a super calender to give a Bekk smoothness of 500 seconds to obtain a heat-sensitive recording material of the present invention.

(A) Liquid 47.5 parts CB
) solution 2.5 parts [CD solution
240 parts (D) liquid
75 parts [E] Solution
25 parts 10% polyvinyl alcohol aqueous solution 200 parts Example 2 In place of 3-methylcyclohexylamino-6-methyl-7-anilinofluorane in solution CB) of Example 1, 3-
A heat-sensitive recording material of the present invention was obtained in the same manner as in the example except that diethylamino-6-methyl-7-anilinofluorane (melting point: 195°C) was used.

Example 3 Di(p-methylbenzyl oxalate) was used instead of dibenzyl oxalate in the sensitizer (CD) solution of Example 1.
A thermosensitive recording material of the present invention was obtained in the same manner as in Example 1 except that ester was used.

Example 4 1. of liquid (E) which is the antioxidant of Example 1. ．． 1.3-
1.1.3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)-butane instead of
A thermosensitive recording material of the present invention was obtained in the same manner as in Example 1 except that methyl-4hydroxy-5-t-butylphenyl)butane was used.

Comparative Example 1 A comparative thermosensitive recording material was obtained in the same manner as in Example 1, except that liquid (A) was used instead of liquid CB) in Example 1, and no leuco dye was mixed.

Comparative Example 2 Using the dispersion of Example 1, the blending amount of liquid (A) was 42.5
Example 1 except that the amount of liquid (B) was 7.5 parts.
A comparative thermosensitive recording material was obtained in the same manner as described above.

Comparative Example 3 3-ethylisoamylamino-6-methyl-7-anilinofluorane was used in place of 3-dibutylamino-6-methyl-7-anilinofluorane in solution (A) of Example 1. A comparative thermosensitive recording material was obtained in the same manner as in Example 1 except for this.

Comparative Example 4 3-Methylpropylamino-6-methyl, a leuco dye other than the present invention, was used instead of 3-methylcyclohexylamino-6-methyl-7-anilinofluorane in place of [B] solution in Example 1. -7-anilinofluorane (melting point 17
A comparative heat-sensitive recording material was obtained in the same manner as in Example 1, except that a temperature of 8° C.) was used.

Comparative Example 5 In place of 2,2-bis(4-hydroxyphenylpropane) in solution (C) of Example 1, p-
A comparative heat-sensitive recording material was obtained in the same manner as in Example 1 except that benzyl hydroxybenzoic acid ester was used.

Comparative Example 6 A comparative heat-sensitive recording material was produced in the same manner as in Example 1 except that parabenzylbiphenyl, a sensitizer outside the present invention, was used instead of dibenzyl oxalate in solution (D) of Example 1. I got it.

Comparative Example 7 In place of 1.1.3-1-lis(2-methyl-4hydroxy-5-cyclohexylphenyl)butane in solution (E) of Example 1, 2,2, which is an antioxidant outside the present invention, was used. A comparative heat-sensitive recording material was obtained in the same manner as in Example 1 except that '-methylenebis(4-methyl-6-t-butylphenol) was used.

Comparative Example 8 A comparative heat-sensitive recording material was obtained in the same manner as in Example 1 except that liquid (E) was not blended.

The heat-sensitive recording materials of Examples and Comparative Examples obtained as described above were tested for recording density, image storage stability, and paper storage stability. The results are shown in Table 1. The test method is shown below.

(1) Recording density: 0.5 power consumption with thermal printing device manufactured by Matsushita Electronic Components Co., Ltd.
Printing was performed at a scanning line density of 8 x 7.7 dots/dot and 1.0 tasec during pulse, and the print density was measured using a Macbeth reflection densitometer RD-914.

(2) Image storage stability ■ Moisture resistance Samples printed under the above conditions were left in an environment of 30° C. and 90% RH for one week, and the residual rate was determined according to the following formula.

(2) Heat resistance The sample printed under the above conditions was left in a constant temperature bath at 60° C. for 24 hours, and the residual rate was determined in the same manner as in the previous section (2).

(3) Storage stability of paper The samples of Examples and Comparative Examples were placed in an environment of 50°C and 90% RH for 1
A monthly device was used to measure the skin density and evaluate the skin fog. In addition, the sample after the test was printed under the recording conditions in section (1), the recording density was measured, and the coloring performance relative to the density of the untested sample was determined according to the following formula.

As is clear from Table 1, the heat-sensitive recording material of the present invention not only has excellent recording density, but also has good image storage stability.
Furthermore, it can be seen that the main paper has a remarkable advantage in storage stability.

<Effects of the Invention> The heat-sensitive recording material of the present invention has excellent image storage stability while maintaining good recording density, and also has excellent main paper storage stability under harsh environments for long periods of time. It is an extremely reliable heat-sensitive recording medium.

Claims (1)

[Scope of Claims] (1) A heat-sensitive recording material provided with a heat-sensitive color-forming layer mainly composed of a leuco dye and a color developer that causes the leuco dye to develop color when heated on a support, wherein the color developer is 2, 2-bis(4-hydroxyphenyl)propane, and the leuco dye is a mixture of a compound represented by the following general formula (1) and a compound represented by the following general formula (2) with a melting point of 190 ° C. or higher, A heat-sensitive recording material further comprising a sensitizer represented by the following general formula (3) and an antioxidant represented by the following general formula (4). General formula (1) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ General formula (2) ▲ There are mathematical formulas, chemical formulas, tables, etc. In addition, R_1 and R_2
may be formed by bonding with a -(CH_2)_n- group. Here, n represents 4 or 5. ) General formula (3) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (In the formula, R represents a hydrogen atom, a halogen atom, or a lower alkyl group, respectively.) General formula (4) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (In the formula, R_1 is t-butyl or cyclohexyl group,
R_2 represents an alkyl group having 1 to 3 carbon atoms. )(2) A leuco dye represented by the general formula (1) and a melting point of 19
The heat-sensitive recording material according to claim 1, wherein the weight mixing ratio of the leuco dye represented by the general formula (2) at 0° C. or higher is from 98/2 to 91/9. (3) The heat-sensitive recording material according to claim 1, wherein the antioxidant represented by the general formula (4) is added in an amount of 0.2 parts by weight or more per part by weight of the leuco dye.