[0001] This invention relates to substrates for printing on which leucomethylene blue dyes
have been stabilized against background formation by the addition of an additive material.
[0002] Leucomethylene blue dyes find widespread application in color printing technologies
such as thermal printing, pressure printing and electrolytic printing. One common
problem with the different leuco dyes used in these technologies is their spontaneous
conversion to the dye form by ambient conditions. In order to circumvent this problem,
many methods involving protection of the leuco dyes have been used, such as encapsulation
and the use of stilt layers. Such methods have given some success in various printing
technologies, but they are not applicable to electrochromic printing, where the mechanism
requires an aqueous electrolyte layer to be in intimate contact with the colour-forming
agent. It is in electrochromic printing that the present invention finds its most
particularly useful application.
[0003] Electrochromic printing is well known in the art. The present state of electrochromic
printing is referenced by U.S. patents 4,211,616 and 4,309,255, along with the art
cited therein. No prior art of which we are aware, however, suggests the use of the
additives which are the subject matter of the present invention.
[0004] According to the invention there is provided a printing substrate comprising a leucomethylene
blue dye characterized in that the substrate also includes a stabilizing amount of
an additive selected from the group consisting of citric acid, aspartic acid, malonic
acid, oxalic acid, ascorbic acid, tartaric acid, ketomalonic acid monohydrate, glucose
and methionine.
[0005] In typical use for electrochromic printing, according to the present invention, paper
is treated with an aqueous solution containing from about .5 to 5% by weight of a
leuco dye. Generally about 1% is the most preferred level. The solution also contains
a stabilizer in an amount which is, at minimum, equal to the molar amount of the dye.
The stabilizer may be present up to about 5% by weight concentration. In most instances
the solution will also contain bromide ions in an amount of from 1% to 30%, generally
about 15%. As a result of the treatment, the paper contains approximately 10 mg of
leuco dye for each standard size page of 8 1/2 x 11 inch dimensions.
[0006] It has been observed that the use of one of the above-named additives render leucomethylene
blue dyes less prone to background formation by oxidation. The mechanisms by which
this result is obtained is not at all understood. The ease of electron removal of
a compound is indicated by its oxidation potential. A shift to higher potential is
indicative of greater resistance towards oxidation. Some of the substances which stabilize
against background formation raise the oxidation potential of the leuco dyes, but
some of them do not. Furthermore, although most of the useful additives are poly-functional
carboxylic acids, not all such materials work. Specifically, succinic acid is of no
help. Galactose shows decreased effectiveness while maltose, lactose, d-fructose,
dl-asparagine, glycine, 1-cystine and dl-methionine are ineffective.
[0007] The operative additives do, however, have a community of physical and mechanical
properties. They are all solids which are soluble in water and they all contain an
oxygen atom doubly bonded to a carbon atom. The experimental results obtained using
the additives of the present invention are summarized in the following table I, showing
results obtained with o- sulfobenzoylleucomethylene blue (o-SBLMB) as the substrate
leucodye.
[0008] As may be seen from inspection of the above table, whatever the mechanism involved
in stabilization against background formation, it obviously does not, of necessity,
involve shifting the oxidation potential of the leuco dye. It should also be noted
that reducing sugars other than glucose did not give satisfactory results. Galactose
has minimal effect while lactose and d-fructose have no effect. The following leucomethylene
blue dyes show a shift to higher potential in the presence of oxalic acid equal in
magnitude to that shown by o-SBLME: 2-chloro-5-sulfo-BLMB and 2,4-dichloro-5- sulfoBLMB.
[0009] Most importantly, it should be emphasized that the addition of these additives does
not significantly hinder the printing process, nor does it impair the quality of the
print obtained. Rather, the archival nature of the print is greatly increased.
[0010] The most preferred additive is malonic acid.
[0011] A typical formulation is the following applied to Hammermill Bond paper. One coat
of a solution containing o-sulfobenzoylleucomethylene blue (o-SBLMB) (1%), oxalic
acid (1%), and ascorbic acid (0.1%), with the pH adjusted to 2.3 using either phosphoric
acid or potassium hydroxide as needed. Three coats of a solution containing potassium
bromide (20%), and potassium hydrogen phosphate (1.4%), with the pH adjusted to 11
as above.
[0012] Several identical formulations were prepared, spray coated and monitored for background
formation. The time to an unacceptable background was approximately doubled when a
stabilizer was present. These studies were carried out at various pHs, all with 1.4%
potassium dihydrogen phosphate and oxalic acid. The following times, in months, to
reach an unacceptable background level were obtained with the first number corresponding
to the pH, the second to the condition with no stabilizer, then with stabilizer, and
finally the percent increase: 7, 1.3, 2.9, 223%; 11, 2.25, 4.1, 182%; 13,3.8, 7.2,
189%.
1. A printing substrate comprising a leucomethylene blue dye characterized in that
the substrate also includes a stabilizing amount of an additive selected from the
group consisting of citric acid, aspartic acid, malonic acid, oxalic acid, ascorbic
acid, tartaric acid, ketomalonic acid monohydrate, glucose and methionine.
2. A printing substrate as claimed in claim 1, wherein the stabilizer is present in
a molar amount approximately equal to the molar amount of the leuco dye.
3. A printing substrate as claimed in claim 1 or 2, wherein the substrate material
is paper.
4. A printing substrate as claimed in any one of claims 1 to 3, which also contains
bromide ions.
