JPS6333479B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat-sensitive recording sheet, and more specifically, the present invention relates to a heat-sensitive recording sheet, and more specifically, a color-forming compound that is normally colorless or light-colored, a phenol compound or organic acid that causes the color-forming compound to develop color when heated, and a water-soluble polymer. The present invention relates to a heat-sensitive recording sheet containing an adhesive as a main component, which contains an imidazole derivative monomer in the recording layer in order to improve long-term storage stability. It is known from US Pat. No. 3,539,375 that a colorless or light-colored lactone compound, such as crystal violet lactone, reacts with a phenol compound to form a color. However, colored lines produced by thermal coloring are generally unstable with respect to light, temperature, and humidity, and printers for calculators and computer terminals, thermal pen recorders for medical measuring equipment, recorders for thermal facsimile machines, and station office labor equipment. Recording paper used for
Cards, tickets, and the like have the major disadvantage that if they are stored as images, the color will fade, and if the stored items are allowed to develop color over time, their ability to develop color will be reduced. Various measures have been taken to improve this drawback, but none have long-term storage stability, and even those that have some degree of storage stability,
In most cases, other properties such as background fogging, lees adhesion to the head, changes due to high temperature and humidity, and a decrease in whiteness occur, making it unsatisfactory. The present inventors conducted various studies to improve this drawback, and found that by adding an imidazole derivative to the recording layer in the form of a monomer, the long-term storage stability was significantly superior to that of a recording layer that did not contain the imidazole derivative. In addition, the inventors have found that other properties do not deteriorate at all, leading to the present invention. That is, the present invention provides a heat-sensitive recording sheet having a recording layer mainly composed of a colorless or light-colored color-forming compound, a phenol compound or organic acid that causes the color-forming compound to develop color when heated, and a water-soluble polymer adhesive. This is a heat-sensitive recording sheet with improved storage stability characterized by containing an imidazole derivative monomer in the recording layer. An example of using imidazoles as antifogging agents for heat-sensitive recording materials is described in, for example, Japanese Patent Application Laid-Open No. 137147/1983. However, in this case, imidazoles are used only in the form of polymers or copolymers, which is clearly different from the present invention, in which remarkable effects can be obtained when imidazoles are used in the form of monomers. It is something. The imidazole derivative used in the present invention is limited to monomers, and should be solid at room temperature and not strongly colored in powder form, with a melting point of 70°C to 300°C.
℃ is preferred. Those with a melting point of 70°C to 150°C act as a sensitivity regulator for the coloring system, and those with a melting point of 150°C to 300°C are more effective in increasing storage stability. Therefore, in the present invention, one type of imidazole derivative may be used, or two or more types of imidazole derivatives may be used depending on the purpose. The imidazole derivatives mentioned here include, for example, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole,
2-Undecylimidazole, 2-heptadecyl imidazole, 1-benzyl-2-methylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2- Undecylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4,5-di-1-cyanoethyl-2-methylimidazole trimellitate, 1-cyanoethyl-2-phenylimidazole trimellitate Melitate,
1-cyanoethyl-2-ethyl-4-methylimidazole trimellitate, 1-cyanoethyl-2
-undecyl-imidazole trimellitate,
2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-S-triazine, 2,4-
Diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')-ethyl-S-triazine, hydroxymethylimidazole, 2-mercaptobenzimidazole, 2-mercaptobenzimidazole zinc salt, 2-mercaptomethylbenzo imidazole, 2-methylimidazoline, 2-ethyl-4
-Methylimidazoline, 2-phenylimidazoline, 2-undecylimidazoline, 2-heptadecyl imidazoline, 2-ethylimidazoline, 2
Examples include, but are not limited to, -isopropylimidazoline, 2,4-dimethyl-imidazoline, and 2-phenyl-4-methylimidazoline. The amount of the imidazole derivative monomer suitable for the purpose of the present invention is preferably 1 to 20% by weight in the recording layer of the heat-sensitive recording sheet of the present invention, and if it is less than this, sufficient preservability may not be obtained. No improvement can be expected, and if the amount is too large, the storage stability will not improve significantly or in proportion to the rate of increase in the amount used. The colorless to light-colored color-forming compound used in the present invention is a substance that develops color by reacting with a phenol compound under heat, such as triphenylmethane type, triphenylmethane phthalide type, fluoran type, and leuco auramine type. , various leuco compounds such as spiropyrans. The phenol compound or organic acid is a substance that softens or melts and reacts with the above-mentioned leuco compound to develop a color when heated, such as 4-
Tertiary-butylphenol, 4-phenylphenol, 4-hydroxydiphenoxide, α-
Naphthol, β-naphthol, methyl-4-hydroxybenzoate, 4'-hydroxyacetophenone, 4-tertiaryoctylcatechol, 2,
2'-Hydroxydiphenyl, 2,2'-methylenebis(4-chlorophenol), 2,2'methylenebis(4-methyl-6-tertiarybutylphenol), 4,4'-isopyridene diphenol (bisphenol A), 4,4'-isopropylidene bis(2-chlorophenol), 4,4'-isopropylidene bis(2,6-dibromophenol),
4,4'-isopropylidene bis(2,6-dichlorophenol), 4,4'-isopropylidene bis(2-methylphenol), 4,4'-isopropylidene bis(2,6-dimethylphenol), 4 ，
4'-isopropylidene bis(2-tertiarybutylphenol), 4,4'-secondary isobutylidene diphenol, 4,4'-secondary isopropylidene bis(2-methylphenol), 4,
4'-cyclohexylidene diphenol, 4,4'-
Examples include cyclohexylidene bis(2-methylphenol), 2,2'-thiobis(4,6-dichlorophenol), benzoic acid, stearic acid, myristic acid, palmitic acid, salicylic acid, and succinic acid. It is also possible to use two or more of these phenolic compounds or organic acids in combination. The water-soluble polymer adhesive used in the present invention is one that separates and fixes the coloring agent dispersed in fine particles, the coloring aid and other additives, and the monomer of the imidazole derivative of the present invention. , such as polyvinyl alcohol, methylcellulose, carboxymethylcellulose, carboxyethylcellulose, polyacrylic acid, casein, gelatin,
Examples include starch and its derivatives, styrene-maleic acid aqueous copolymer, and the like. In the heat-sensitive recording sheet of the present invention, various additives including a sensitivity regulator may be added to the recording layer as necessary. Sensitivity can be controlled by the imidazole derivative monomer itself, but in addition, substances that promote low-temperature color development between a color-forming compound and a phenol compound or organic acid, such as a substance with a melting point of 60°C or higher, can be used to control the sensitivity.
Various waxes at 200°C and fatty acid amides are added as sensitivity adjusters. In addition, white pigments and fillers are added to improve the naturalness and writability of the recording layer, and lubricants such as stearic acid and metal salts of stearic acid are added to ensure smooth contact between the recording head and the recording layer. is added. Other waxes may also be used in combination with lubricants. The heat-sensitive recording sheet of the present invention is produced as follows. Color-forming compounds, phenolic compounds or organic acids, sensitivity modifiers and other additives, and imidazole derivative monomers are mixed in an aqueous medium containing an appropriate concentration of a binder using a pulverizer such as a ball mill.
A heat-sensitive composition is obtained by pulverizing and dispersing the particles to a particle size of 3μ or less. The heat-sensitive composition thus obtained is applied to a suitable support and dried to obtain the heat-sensitive recording sheet of the present invention. Examples of the support used in the heat-sensitive recording sheet of the present invention include papers such as wood-free paper, medium-quality paper, and coated paper, plastic films, film-laminated papers, and woven fabric sheets. The heat-sensitive recording sheet of the present invention does not fade even when stored as an image, and has excellent preservability without losing its coloring ability even when the stored sheet is allowed to develop color over time. The present invention will be explained in more detail with reference to Examples. Example 1 (a) Production of heat-sensitive recording composition A Liquid fluoran black coloring dye (product name PSD150, manufactured by Shinnitsaku Kako) 10 parts by weight Polyvinyl alcohol 10% aqueous solution 100 〃 Kaolin 10 〃 Stearic acid amide 15 〃 Water 20 〃 B Liquid Bisphenol A 10 parts by weight Polyvinyl alcohol 10% aqueous solution 100 〃 2-heptadecylimidazole 8 〃 Kaolin 10 〃 Water 10 〃 After dispersing A and B separately for 4 hours in a centrifugal ball mill, A and B were prepared. The liquids were mixed to prepare a heat-sensitive recording composition (paint). (b) This heat-sensitive recording composition was coated on 50 g/m ² of high-quality paper using a Mayer bar so that the coated amount after drying was 6 g/m ² to obtain heat-sensitive recording paper. (c) Next, the storage stability of this thermal recording paper was investigated. Storage stability is determined by (1) the density of the colored area and the background density immediately after coloring the thermal recording paper;
An image fading test that measures the density and background density of the colored area after time and compares the degree of discoloration before and after storage. After a certain period of time, the density of the colored area was measured and evaluated by a coloring ability test that compared the coloring ability before and after storage. The colored area and background density were measured using Macbeth RD514. Color development was carried out using a static color development tester at a pressure of 2 Kg/cm ² , a time of 1 second, and a temperature of 120°C. In addition, measurements after aging of heat-sensitive recording paper with or without color development were carried out on those that had been placed in a constant temperature bath at 40° C. and 90% RH for one month.
The temperature at which color development starts is determined by the Macbeth reflection density compared to the background density when creating a color development test piece using the above static color development tester at a pressure of 2 Kg/cm ² and a time of 1 second.
The temperature at which the concentration was 0.1 higher is shown in °C. The results are shown in Table 1. Example 2 Thermosensitive recording paper was obtained in exactly the same manner as in Example 1, except that 2-mercaptobenzimidazole was used in place of 2-heptadecyl imidazole in Solution B of Example 1, and storage stability was determined in the same manner. I conducted a test. The results are shown in Table 1. Comparative Example 1 For comparison, 2 from the heat-sensitive recording composition of Example 1
-Thermosensitive recording paper was obtained in exactly the same manner as in Example 1, except that heptadecylimidazole was omitted, and a storage stability test was conducted in the same manner. The results are shown in Table 1. Comparative Example 2 For comparison, 2 in the heat-sensitive recording composition of Example 1
-Thermosensitive recording paper was obtained in exactly the same manner as in Example 1, except that phenol resin (trade name: CKP-2400, manufactured by Showa Union) was used instead of heptadecylimidazole, and a storage stability test was conducted in the same manner. Summer. The results are shown in Table 1. Comparative Example 3 For comparison, 2- in the heat-sensitive recording composition of Example 1
A thermal recording paper was obtained in exactly the same manner as in Example 1, except that N-vinylimidazole vinyl acetate copolymer was used in place of heptadecylimidazole, and a storage stability test was conducted in the same manner. The results are shown in Table 1. 【table】

Claims (1)

[Claims] 1. In a heat-sensitive recording sheet having a recording layer mainly composed of a colorless or light-colored color-forming compound, a phenol compound or organic acid that causes the color-forming compound to develop color when heated, and a water-soluble polymer adhesive, in the recording layer. 1. A heat-sensitive recording sheet with improved storage stability, characterized in that it contains an imidazole derivative monomer.