Aqueous ink for an ink ejection process

An aqueous ink for an ink ejection process, particularly a blue ink for a process employing an ejection system which produces droplets, comprises (a) a water soluble triphenyl methane dye and (b) a buffer mixture of at least one organic hydroxy-containing carboxylic acid and at least one water soluble amine.

BACKGROUND OF THE INVENTION 
The present invention relates to an ink, and more particularly, to an 
aqueous ink for use in an ink ejection process. 
German Auslegeschrift No. 2,164,614, hereby incorporated by reference, for 
example, discloses an ink ejection process in which temporary pressures 
are produced in an ejection head leading to the discharge of ink from 
capillary nozzles and to interruption of this ink flow to form droplets. 
To have an ink which is suitable for such ejection processes, certain 
parameters must be present. Thus, a clean printed picture requires, among 
other things, uniformly sized ink drops. These depend, on the one hand, on 
the geometry of the nozzle and on the configuration of the break-off 
plane. On the other hand, the ink must have as high a surface tension as 
possible. The supply of ink in the ejection head, for example, from an ink 
reservoir is often effected without pressure through capillaries. In order 
to attain a high ejection speed, however, the ink employed must have a low 
viscosity. The selection of suitable inks is limited, on the one hand, by 
its drying behavior. When the ejection system is at rest, ink components 
must not crystallize out in the area of the ejection head after long 
periods of inactivity. On the other hand, no surface film must form in the 
nozzle opening as a result of the evaporation of water. A slight increase 
in viscosity of the ink in the area of the nozzle, however, generally will 
not lead to malfunction of the ejection system, but, in the past, a 
character printed first after a period of inactivity will appear 
imprecise. The replenishment of solvents, such as water in this case, into 
the area of evaporation of the nozzle opening takes place through long 
capillary systems. In this case, similar physical properties are inherent 
in the ink in the nozzle exit area after longer periods of inactivity as 
they are encountered by ink drops without the addition of solvents under 
normal environmental conditions, such as the influence of temperature and 
humidity. Thus, the ink must remain fluid even with significant 
evaporation and must not deposit solids. 
In order to avoid drying, moisturizing agents, such as glycerin, for 
example, are added to commercially available water-based inks. This 
additive, however, increases viscosity. The color intensity of such an ink 
is sufficient at most and its pH lies at 1.8 compared to a pH of 4 to 5 
which would be favorable for ejection processes as defined above. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide an aqueous ink 
which satisfies the required parameters for use in an ink ejection 
process. 
Additional objects and advantages of the present invention will be set 
forth in part in the description which follows and in part will be obvious 
from the description or can be learned by practice of the invention. The 
objects and advantages are achieved by means of the processes, 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the foregoing objects and in accordance with its purpose, the 
present invention, as embodied and broadly described, provides an aqueous 
ink for an ink ejection process which comprises: (a) a water soluble 
triphenyl methane dye and (b) a buffer mixture of at least one organic 
hydroxy-containing carboxylic acid and at least one water soluble amine. 
Preferably, the buffer components comprise citric acid and triethanolamine 
with which the pH of the ink can be adjusted, and enables the pH of the 
ink to be set to a pH of 4 to 5. It is further preferred to increase the 
ejection speed of the ink by adding a flow improving additive. Preferably, 
the flow improving additive is polyethylene glycol and the ink contains 
0.2 to 0.5% by weight of this additive. 
Further advantages of the present invention result from the fact that the 
pH of the ink can be set by means of the buffer solution and that the 
setting of the pH value produces maximum color intensity without an 
increase in the color concentration. Thickened ink can again be dissolved. 
Ink which thickens in the area of the nozzle reduces the problem of 
sealing of the nozzle openings, particularly in the nonoperative state of 
an ink ejection printer. The buffer system has no influence on the surface 
tension of the ink and the ink is hydrophilic. The ink of the present 
invention can be used in an ejection system which produces droplets. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary, but are not restrictive of 
the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
The aqueous ink of the present invention contains an organic dye as the 
coloring agent. The organic dye is water-soluble, and generally is a 
triphenyl methane dye, preferably one which imparts a blue color. The 
triphenyl methane dye "Ink Blue BJTN", sold by Farbwerke Hoechst AG, 
Frankfurt, is an example of a suitable dye that can be used in accordance 
with the present invention. The dye is present in the aqueous ink in 
conventional amounts, and, for example, can be present in amounts of 10 to 
20 grams per liter. The triphenyl methane dye can be added to the ink in 
amounts ranging from 5 to 50 g/l. 
Since color intensity as well as water solubility of dyes of the triphenyl 
methane type are heavily dependent on the pH, it has been found in 
accordance with the present invention that it is advisable, in order to 
maintain the best possible pH, to employ a buffer system which is 
compatible with the composition of the dye without neglecting the 
above-mentioned requirements for the ink. 
For technical and economic reasons, ink ejection heads are made of metallic 
materials. Consumption of protons as it may occur as a result of corrosion 
of the aqueous dye solution on metal surfaces or from metal oxides on 
record carriers adversely influences color intensity. The decisive factor 
is that corrosion over longer periods of time will not result in the 
formation of deposits of metal ions with the dye. A minimum amount of 
uniform surface abrasion is negligible in this connection. Since corrosion 
changes the pH and consequently increases the concentration of the leuco 
bases of the triphenyl methane dye which are only relatively difficultly 
soluble in water, the buffer system according to the invention produces a 
stabilization of the ink. The addition of an acid, which in the correct pH 
range produces a strong buffer effect, results in optimum color intensity. 
The buffer components of the aqueous ink of the present invention comprises 
a mixture of at least one organic hydroxy-containing carboxylic acid and 
at least one water-soluble amine. Examples of suitable organic 
hydroxy-containing carboxylic acids that can be used in the present 
invention include citric acid, malic acid, gluconic acid, tartaric acid, 
preferably citric acid. Examples of suitable water-soluble amines include 
diethanolamine, ethanolamine, ethylenediamine, triethanolamine, preferably 
diethanolamine. A factor to be considered in selecting the particular acid 
and amine that are used to buffer the aqueous ink is that all components 
of the ink should not contain any long aliphatic or aromatic groups which 
would greatly reduce surface tension. 
The carboxylic acid can be added to the ink in amounts ranging from 5 to 
200 g/l, preferably 20 to 50 g/l. The amine can be added to the ink in 
amounts ranging from 5 to 200 g/l, preferably 20 to 50 g/l. The exact 
amount of each of these two components can be varied to achieve a desired 
pH in the ink, such as a pH of 4 to 5. 
In forming the ink, the dye and buffer components are added to water, 
preferably with the dye being added first, followed by addition of the 
amine and then addition of the acid. 
In addition, for technical reasons, the capillary channels of the systems 
employed for the ejection process cannot be manufactured to produce 
optimum flow without major difficulties. In a preferred embodiment of the 
present invention, 0.2 to 0.5% by weight polyethylene glycol is added to 
the ink and produces a significant improvement of the flow properties of 
the ink.

The following example is given by way of illustration to further explain 
the principles of the invention. This example is merely illustrative and 
is not to be understood as limiting the scope and underlying principles of 
the invention in any way. All percentages referred to herein are by weight 
unless otherwise indicated. 
EXAMPLE 
The following ink mixture was prepared and produced a significant 
improvement over a similar known ink which was not buffered: 
a. 10 to 20 g "Tintenblau (ink blue) BJTN" dye per liter; 
b. 80 to 100 ml of 90% diethanol amine per liter; 
c. 80 to 100 g of citric acid hydrate per liter. 
The buffer solution produced by the addition of the citric acid hydrate and 
diethanol amine set the pH of the ink to 4 to 5. The ink was formed by 
first adding the dye to the water, followed by addition of the amine and 
then by addition of the acid. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.