JPS5853482A - Reversible heat-sensitive recording material - Google Patents

Abstract

PURPOSE:To obtain a reversible heat-sensitive recording material capable of color-disappearance in a short time and suitable for the protection of confidential document by using silicone or chlorinated rubber as a protective layer for a heat-sensitive recording material containing a substance with water of crystallization as a color developing component. CONSTITUTION:A heat-sensitive color developing layer composed of a substance capable of producing a color by losing its water of crystallization upon heating (e.g., hexamethylenetetramine-metal salt complex compounds, pyridine-cobalt arsenate (10H2O), etc.) and a binder is provided on a supporter (e.g., paper, plastic film etc.). The preferred amount of the binder to be used is 0.1-2.0pts.wt. on the basis of 1pt.wt. a substance with water of crystallization, and also the preferred amount of the heat-sensitive chromogenic layer to be adhered is about 4-10g/m<2>. Then, an organic solvent solution of silicone or chlorinated rubber is coated on the heat-sensitive color developing layer and dried to form a protective layer of a thickness of 1-10mu.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reversible heat-sensitive recording material that develops color when heated and disappears in a short period of time.

Normal heat-sensitive recording materials used for copying books, documents, etc., are heated by infrared rays, thermal heads, etc. to form semi-permanent colored images. There is a strong need for reversible recording materials that cannot be copied even if an image appears, or whose color fades after a certain period of time, such as after a meeting has ended, even if the image is faded.

By the way, although this kind of reversible recording material is not particularly intended, Japanese Patent Publication No. 14509/1989 discloses a technique that utilizes the phenomenon of color development due to the detachment of crystal water by heating a substance containing crystal water. A heat-sensitive recording material on which a colored image is formed is introduced. This heat-sensitive recording material has hexamethylenetetramine, a substance containing water of crystallization, on a support.
A heat-sensitive coloring layer and a diaphragm mainly composed of a metal salt complex compound, and if necessary, a protective layer of wax, styrene resin, or nitrocellulose is provided thereon to prevent moisture intrusion and improve wear resistance. It is. Among these heat-sensitive recording materials that do not have a protective layer, it was observed that the colored images formed fade after a certain period of time, especially those that use a water-soluble binder, but those that do have a protective layer This kind of phenomenon has not been recognized for...

The purpose of the present invention is to ensure that colored images are erased in a short period of about half a day to one day, despite having a wear-resistant protective layer.
Therefore, it is an object of the present invention to provide a reversible heat-sensitive recording material suitable for preventing secret leakage.

That is, the heat-sensitive recording material of the present invention is characterized by providing on a support a heat-sensitive coloring layer containing a light-colored substance containing water of crystallization that develops color when water of crystallization is lost, and a protective layer made of silicone or chlorinated rubber thereon. That is.

The present inventors have conducted various studies on materials for forming a protective layer that will cause the colored image to fade in a short period of time when a wear-resistant protective layer is provided on the heat-sensitive recording material without a protective layer described in the above-mentioned publication. As a result, it was found that silicone or chlorinated rubber is optimal.

That is, it has been found that the heat-sensitive recording material of the present invention, like the conventional heat-sensitive recording materials described above, develops color upon heating, but has reversibility such that the color reliably (or stably) disappears within about half a day to one day. The present invention is based on these findings. It should be noted that although the heat-sensitive recording material of the present invention has a protective layer, it is not clear why such good results were obtained, but the film formed of silicone or chlorinated rubber is not coated with wax or styrene. This is thought to be because, unlike a film formed of resin, it has an appropriate ability to promote moisture absorption. In other words, the crystal water-containing substance in the heat-sensitive coloring layer releases crystal water when heated to form a colored image, but if the lost crystal water-containing substance is left for about half a day to one day, the upper protective layer will be damaged. The substance containing water of crystallization used in the heat-sensitive coloring layer of the present invention is thought to absorb moisture in the air due to its moisture absorption promoting property and erase its color. Often, the specific examples are hexamethylenetetramine/metal salt complex compounds,
For example, hexamethylenetetramine/cobalt chloride 1O hydrate, hexamethylenetetramine/cobalt iodide decahydrate, hexamethylenetetramine/cobalt bromide 1ρ hydrate,
Hexamethylenetetramine/nickel chloride decahydrate, hexamethylenetetramine/nickel bromide decahydrate, other complex compounds of hexamethylenetetramine and cobalt nitrate or sulfate (containing 10 molecules of crystal water), hexamethylenetetramine Examples include a complex compound of nickel with nitrate or sulfate (containing 10 molecules of water of crystallization); dipyridine cobalt arsenate decahydrate; iron ammonium permanganate fluoride; cobalt phosphate octahydrate.

A binder is usually used in the thermosensitive coloring layer. As the binder, both water-soluble and water-insoluble binders can be used, but water-soluble ones are preferable because they promote decolorization even more than Li&Moisture.

Specific examples of such binders include casein, starch, dextrin, CMC, hydroxymethyl cellulose, PVA% vinegar 11 vinyl-maleic acid copolymer sodium salt, sodium alginate, styrene hemaleic acid copolymer ammonium salt, and polyglutamic acid sodium salt. , ethylene-maleic anhydride copolymer soda salt, carboxylic acid cellulose, polyaspartate sodium salt, butadiene-methacrylic acid copolymer soda salt, carboxymethyl starch, phosphate starch, ethyl cellulose, methyl cellulose, cellulose acetate and their derivatives , vinyl chloride resin, polyvinyl butyral, polyamide, styrene resin, silicone resin, phenol resin, and the like. The amount of these binders used is 0.1 part by weight of the crystal water-containing substance.
~2.0 parts by weight is preferred.

Silicone or chlorinated rubber is used for the protective layer, and examples of the silicone include silicone resins and silicone oils obtained by hydrolyzing and then polymerizing organochlorosilanes. The protective layer made of these materials has particularly excellent abrasion resistance during repeated use, has an appropriate ability to promote moisture absorption, and is also heat resistant, so it does not cause sticking or head contamination due to melting.

To produce the heat-sensitive recording material of the present invention, an aqueous solution in which a crystal water-containing substance and a binder are dissolved or dispersed is coated on a support such as paper, plastic film, or cured paper, and dried to form a heat-sensitive coloring layer. A protective layer may be provided by coating a solution of silicone or chlorinated rubber in an organic solvent and drying it. In addition, modifiers such as calcium carbonate, titanium oxide, zinc oxide, talc, and silica may be added to the heat-sensitive coloring layer.

The adhesion amount of the thermosensitive coloring layer is 4~LOg/v? The thickness of the protective layer should be 1 from the viewpoint of wear resistance, thermal sensitivity, etc.
~10μ is appropriate.

Examples are shown below. Note that all figures are based on weight.

Example 1 Hexamethylenetetramine Cobalt chloride decahydrate 5 parts pvum 2 parts water
A solution consisting of 83 parts was applied onto 5oti7i high-quality paper using an ear bar and dried to form a heat-sensitive coloring layer with an adhesion amount of 5.21 parts, and then a 15% xylene-butanol mixture of silicone resin was applied on top of the heat-sensitive coloring layer. Solvent solution (KR- manufactured by Shin-Etsu Silicone Co., Ltd.
5240) was applied and dried to form a protective layer with a thickness of 1 to 2 μm.

Next, the surface of the protective layer of the heat-sensitive recording material obtained in this way was heated and printed at a temperature of 80°C using a thermal printer equipped with a thermal head. A clear blue image was formed.

This image decreased in density to 0.10 when left for 24 hours. It was also confirmed that when the heat-sensitive recording material thus decolored was heated again to 80° C. in the same manner, the color developed, and if left for another 24 hours, the color decolored again.

Example 2 Chlorinated rubber (localized IIcR-20) as protective layer forming liquid
A heat-sensitive recording material was prepared in the same manner as in Example 1, except that a solution consisting of 15 parts of toluene and 80 parts of toluene was used, and the thickness of the protective layer was 2 to 3 microns.

Next, the protective layer surface of this recording material was heated and printed at a temperature of 85° C. using a thermal printer in the same manner as in Example 1, and a clear blue image with a density of 0.46 was formed. This image decreased in density to 0.11 when left for 24 hours.

It was also confirmed that when the same printing-leaving operation was repeated on the heat-sensitive recording material that had been color-decolored in this way, the color developed and disappeared again.

Example 3 A heat-sensitive recording material was prepared in the same manner as in Example 1, except that dipyridine cobalt arsenate decahydrate was used instead of hexamethylenetetramine cobalt chloride decahydrate in the heat-sensitive coloring layer forming liquid.

Next, when the protective layer surface of this recording material was heated and printed at a temperature of 90° C. using a thermal printer in the same manner as in Example 1, a clear blue image with a density of 0.43 was formed. This image decreased in density to 0.10 when left for 24 hours.

It was also confirmed that when the same printing-leaving operation was repeated on the heat-sensitive recording material that had been color-decolored in this way, the color developed and disappeared again.

Claims (1)

[Claims] 1. A reversible heat-sensitive recording material comprising a heat-sensitive coloring layer containing a light-colored crystal water-containing substance that develops color when water of crystallization is lost on a support, and a protective layer made of silicone or chlorinated rubber thereon.