JPH0243639B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to printed substrates in which leucomethylene blue dyes are stabilized against background formation by the addition of additive materials. BACKGROUND ART Leucomethylene blue dyes have found wide application in color printing techniques such as thermal printing, pressure printing and electrolytic printing. One problem with the various leuco dyes used in such techniques is that the leuco dyes are spontaneously converted to dye form by the surrounding conditions. A number of methods have been used to address this problem, including protection of the leuco dye, such as the use of encapsulation and support layers. Although such methods have met with some success in various printing techniques, they are not suitable for electronic coloring (where the mechanistic requirement is that the aqueous electrolyte is in intimate contact with the color forming agent). Electrochromic) cannot be applied to printing. It is in this electronic color printing that the present invention finds many particularly useful applications. Electronic color printing is well known and is currently covered by U.S. Patent No.
No. 4211616 and No. 4309255. However, there is no prior art disclosing the use of the additives that are the subject of the present invention. DISCLOSURE OF THE INVENTION In accordance with the present invention, a substrate for printing is prepared from a leucomethylene blue dye and citric acid, aspartic acid, malonic acid, oxalic acid, ascorbic acid, tartaric acid, monohydrated ketomalonic acid, glucose and methionine. by coating with sufficient additives to provide a stabilizing effect selected from the group consisting of: Of course, the usual printing substrate is paper. In accordance with the present invention, for typical electronic color printing applications, paper is treated with an aqueous solution containing about 0.5 to 5% by weight leuco dye. About 1% is the most preferred level. The solution contains a minimum amount of stabilizer equal to the molar weight of the dye. This stabilizer is about 5%
It may be present up to a concentration of 50% by weight. As a result of processing, the paper
Each standard size page measuring 21.59 cm x 27.94 cm contains approximately 10 mg of leuco dye. It has been observed that by using one of the additives mentioned above, the leucomethylene blue dye has less tendency to form background due to oxidation. The mechanisms by which these results occur are poorly understood. The ease with which a compound's electrons are removed is indicated by its oxidation potential. A high oxidation potential indicates high resistance to oxidation. Some of the materials that stabilize against background formation will increase the oxidation potential of the leuco dye, while others will not. Furthermore, although most useful additives are polyfunctional carboxylic acids, not all such materials are useful. Succinic acid in particular is not useful. Galactose reduces efficacy, maltose, lactose, d-fructose, dl-asparagine, glycine, l-cristine and dl-methionine are ineffective. However, effective additives have common physical and mechanical properties. All of these are solids that can be dissolved in water, and all contain two types of oxygen bonded to carbon atoms. Experimental results obtained using the additives of the present invention demonstrate that o-sulfobenzoylleucomethylene blue (o-
The results obtained using SBLMB) are summarized in the table below.

[Table] From the table above, it is clear that whatever the stabilizing mechanism that prevents background formation, this mechanism does not necessarily shift the oxidation potential of the leuco dye. be. Note that reducing sugars other than glucose do not give satisfactory results. Galactose has a slight effect, while lactose and d-fructose have no effect. The following leucomethylene blue dyes exhibit a shift to higher potentials when o-SBLMB and an equivalent amount of oxalic acid are present. 2-chloro-5-sulfur-BLMB and 2,
4-Dichloro-5-sulfo BLMB The most important point to emphasize is that the addition of these additives does not significantly interfere with the printing process and does not impair the quality of the prints obtained. On the contrary, the shelf life of the print is greatly increased. The most preferred additive is malonic acid. A typical treatment applied to Hammerhill Bond paper is as follows. Contains o-sulfobenzoylleucomethylene blue (0-SBLMB) (1%), oxalic acid (1%) and ascorbic acid (0.1%), with PH adjusted using phosphoric acid or potassium hydroxide as needed. One coat of solution adjusted to 2.3.
Three coats of a solution containing potassium bromide (20%), potassium dihydrogen phosphate (1.4%) and the pH adjusted to 11 as described above. Several identical formulations were prepared, spray coated, and background formation was monitored. When a stabilizer is present, the time for unacceptable background to occur approximately doubles. All these studies were performed at various pHs obtained using 1.4% potassium dihydrogen phosphate and oxalic acid. The following times (months) were obtained to reach unacceptable background levels.

【table】

Claims (1)

[Claims] 1 Leucomethylene blue dye and an additive in an amount sufficient to provide a stabilizing effect selected from the group consisting of citric acid, aspartic acid, malonic acid, oxalic acid, ascorbic acid, tartaric acid, monohydrated ketomalonic acid, glucose and methionine. Printing substrate including.