Aqueous Ink Composition for Ink-Jet Recording

The present invention relates to an aqueous ink composition suitable for use as printing ink. More particularly, it relates to an aqueous ink composition for ink-jet recording, which is capable of yielding printed images with high quality (i.e., high clearness and sharpness, etc.), and which is also capable of providing stable ink ejection, without causing the plugging of the nozzles of the ink-jet recording apparatus, even if it is used continuously for a long period of time or if it is used intermittently after a period of non-use of the ink-jet recording apparatus.

Generally, an aqueous ink comprises as the main components a dye, water and a humectant. In particular, an aqueous ink for use in ink-jet recording should satisfy the following conditions in order to yield an excellent print:

First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, it is necessary that its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges.

Second, it is necessary that no precipitates be separated, due to coagulation, precipitation, chemical change of slightly soluble components or other causes, from the ink during an extended period of continuous use or storage, or during the periods when the apparatus is not in use. Nor should the physical properties of the ink otherwise change during the above-mentioned periods. If the separated solid components or viscous materials in the ink become affixed around the nozzles, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired recording quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.

Third, it is desired that the ink provide adequately high contrast, clearness and the desired color in the printed image, in particular, for use with full-color printers.

Fourth, it is necessary that the ink for ink-jet recording be such that the printed images are resistant to water, light and physical wear.

Fifth, it is necessary that the images printed by the ink dry rapidly.

So far, many proposals have been made for an ink for ink-jet recording, but from the viewpoint of practical use, a satisfactory ink which is capable of meeting the above-described requirements has not been found.

It is therefore an object of the present invention to provide an aqueous ink composition for ink-jet recording completely satisfying the above outlined conditions. That is to say, the object of the present invention is to provide an aqueous ink composition for ink-jet recording which does not result in plugging of the nozzles, does not change in quality or separate precipitates therefrom with time during storage, but exhibits excellent ink-ejection stability, and has good ink ejection response, shows slight changes in physical properties, if any, when used for a long period with continuous recirculation, or when used intermittently, and, yields printed images which are non-spreading, and have high sharpness and high image density.

The above object of the present invention is attained by an aqueous ink composition comprising an aqueous solution of a water-soluble dye having the following formula and a humectant dissolved in water, if necessary with addition of other additives thereto: ##STR2## wherein R.sup.1 and R.sup.2 each represent hydrogen, an alkyl group, an alkoxy group, halogen, a carboxyl group or a sulfonic acid group; R.sup.3 and R.sup.4 each represent a hydroxyl group, halogen, an alkoxy group, an unsubstituted or substituted amino group; X represents --O--, --CH.sub.2 --, --C.sub.2 H.sub.4 --, --CH.dbd.CH--, --S--, --SO.sub.2 -- or --; Ar.sup.1 and Ar.sup.2 each represent an unsubstituted or substituted phenyl group, an unsubstituted or substituted naphthyl group; M represents hydrogen ion, an alkali metal cation such as Li.sup.+, Na.sup.+ and K.sup.+, amine cation or an ammonium ion; and n is an integer of 1, 2 or 3.

An aqueous ink composition according to the present invention comprises a water-soluble dye of the above described formula, water and a humectant.

In the following table 1, there are listed representative examples of the water-soluble dyes having the above formula for use in the present invention: (1) ##STR3## (2) ##STR4## (3) ##STR5## (4) ##STR6## (5) ##STR7## (6) ##STR8## (7) ##STR9## (8) ##STR10## (9) ##STR11## (10) ##STR12## (11) ##STR13## (12) ##STR14##

The above dyes can be prepared by a conventional method. For example, the dye (5) can be prepared as follows: One mole of 4,4'-diaminostilbene 2,2'-disulfonic acid is dissolved in water. To this solution, (1) 2 moles of cyanuric chloride dissolved in acetone and (2) an aqueous solution of sodium carbonate are simultaneously added at temperatures below 0.degree. C., with the mixuture kept unalkaline, thereby allowing the added components to react for 2 hours. To the above reaction mixture is added a monoazo dye obtained by a conventional reaction of H-acid and o-toluidine. The monoazo dye is dissolved in the reaction mixture. To this reaction mixture, an aqueous solution of sodium carbonate is further added, with the reaction mixture kept unalkaline, so that the reaction mixture is allowed to react at room temperature for 10 hours. Thereafter, the temperature of the reaction mixture is gradually raised to 60.degree. C. and the reaction is continued for 1 hour. Aniline and sodium carbonate are then added to the reaction mixture and the temperature of the reaction mixture is raised to 90.degree. C., so that the reaction is further continued for 3 hours. After cooling the reaction mixture, the dye (5) is obtained by a salting-out technique using a 20% aqueous solution of sodium chloride.

In the present invention, it is preferable that the dye be contained in an amount of 0.1 to 20 parts by weight, more preferably 0.5 to 6 parts by weight, with respect to 100 parts by weight of the ink composition according to the present invention.

The humectant for use in the present invention, which is employed together with water, is for adjusting the physical properties of the ink composition so as to have the desired properties, to control the drying speed of the ink composition and to increase the solubility of the dye employed in the solvent of the ink composition.

Examples of such humectants for use in the aqueous ink composition according to the present invention are polyhydric alcohols, alkyl ethers of polyhydric alcohols, amine derivatives such as triethanol amine, amides such as dimethylformamide, and nitrogen-containing heterocyclic compounds such as N-methyl-2-pyrrolidone, 2-pyrrolidone and 1,3-dimethyl imidazolidinone.

Specific examples of the polyhydric alcohols are ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol and glycerin.

Specific examples of the alkyl ethers of polyhydric alcohols are ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether and triethylene glycol monoethyl ether.

Of the above mentioned humectants, the most preferable humectants are diethylene glycol, polyethylene glycol (200 to 600), triethylene glycol, ethylene glycol, glycerin and N-methyl-2-pyrrolidone, by which the solubility of the employed dye in the solvent of the ink composition can be increased and the evaporation of water from the ink composition can be appropriately controlled, so that the initial properties of the ink composition can be maintained even for an extended period of continuous use or storage, or during the periods when the apparatus is not in use, whereby reliable ink droplet stability and ink droplet ejection response of the ink composition, particularly after a prolonged period of non-use of the apparatus, are obtained.

In the present invention, it is preferable that the above humectant be contained in an amount of 5 to 80 parts by weight, more preferably 10 to 40 parts by weight, with respect to 100 parts by weight of the ink composition according to the present invention, from the viewpoint of obtaining an appropriate viscosity and drying speed of the ink composition.

In the present invention, in addition to the above humectants, additives, for instance, water-soluble preservative and anti-mold agents, pH adjustment agents, specific electric conductivity adjustment agents, chelating agents and anti-rusting agents, can be added to the ink composition.

As water-soluble preservative and anti-mold agents, for example, sodium dehydroacetate, sodium sorbate, 2-pyridine thiol-1-oxide sodium salt, sodium benzoate and sodium pentachlorophenol can be employed.

As pH adjustment agents, any materials can be used optionally, so long as they do not have an adverse effect on the ink composition and can control the pH of the ink composition within the range of pH 9.0 to 11.0.

Specific examples of such pH adjustment agents are amines, such as diethanolamine and triethanolamine; hydroxides of alkali metals, such as lithium hydroxide, sodium hydroxide and potassium hydroxide; ammonium hydroxide; and carbonates of alkali metals, such as lithium carbonate, sodium carbonate and potassium carbonate.

As specific electric conductivity adjustment agents, inorganic salts such as potassium chloride, ammonium chloride, sodium sulfate and sodium carbonate, and water-soluble amines such as triethanolamine can be employed.

As chelating agents, for example, sodium ethylenediaminetetraacetate, trisodium nitrilotriacetate, hydroxyethyl ethylenediamine trisodium acetate, diethylene triamino pentasodium acetate and uramil disodium acetate can be employed.

As rust preventing agents for the nozzles, for example, acid sulfites, sodium thiosulfate, ammonium thioglycolate, diisopropyl ammonium nitrite, pentaerythritol tetranitrate and dicyclohexyl ammonium nitrite can be employed.

Other additives, for example, water-soluble ultra-violet-ray-absorbing agents, water-soluble infrared-ray-absorbing agents, water-soluble polymeric compounds, solubility increasing agents for increasing the solubility of the dye dissolved in the solvent of the ink composition, and surfactants can be employed as thought necessary in specific embodiments of an aqueous ink composition for ink-jet recording according to the present invention.

Preferred embodiments of an aqueous ink composition for ink-jet recording according to the present invention will now be explained by referring to the following examples:

A mixture of the following components was heated to about 50.degree. C. and stirred until completely dissolved. The mixture was then filtered twice through a membrane filter with a 0.22 .mu.m mesh, whereby an aqueous ink composition No. 1 for ink-jet recording according to the present invention was prepared:

The properties of the thus prepared aqueous ink composition were as follows:

In the above, the water resistance of the ink composition indicated by fading ratio was measured as follows:

The aqueous ink composition No. 1 was diluted with ion-exchanged water to the extent that the concentration of the Dye (1) contained in the ink composition was 1 wt. %. The thus diluted ink composition was applied to a sheet of high quality paper by use of a doctor blade and was then dried at room temperature for one day to prepare a test sample. Then the density d.sub.0 of the applied ink composition on the paper was measured by a Macbeth densitometer. This test sample was immersed in water at a temperature of 30.degree. C. for one minute and was then taken out. Immediately after this, the density d.sub.1 of the ink composition applied on the immersed paper was measured by the Macbeth densitometer. From the above measured d.sub.0 and d.sub.1, the resistance to water of the ink composition was determined in accordance with the following formula: ##EQU1##

The result was that the water resistance of the ink composition No. 1 was 7.8% in terms of the above defined fading ratio.

Likewise, the light resistance of the ink composition No. 1 was measured as follows:

A test sample having an applied ink density d was prepared in the same manner as described above.

This test sample was exposed to the light of a carbon arc lamp by a fade meter at 63.degree. C. for 3 hours and the density d.sub.2 of the ink composition of the test sample was measured by the Macbeth densitometer. From the d.sub.0 and d.sub.2, the resistance to light of the ink composition No. 1 was determined by the following formula: ##EQU2##

The result was that the light resistance of the ink composition No. 1 was 11.6% in terms of the above defined fading ratio.

The aqueous ink composition No. 1 was then subjected to the following ink-jet performance tests:

The ink composition was caused to issue from a nozzle with an inner diameter of 30 .mu.m, with vibrations at a frequency of 100 KHz, by which vibrations the ink composition was ejected in a stream broken into individual drops, and was then caused to impinge on a sheet of commercially available high quality paper. As a result, clear images were obtained on each of the sheet. The time required for drying the printed image was not more than 10 seconds at normal room temperature and humidity.

Samples of the ink composition were tightly sealed in glass containers and subjected to the following storage tests:

a. Preserved at -20.degree. C. for one month;

b. Preserved at 4.degree. C. for one month;

c. Preserved at 20.degree. C. for one year; and

d. Preserved at 90.degree. C. for one week.

Separation of precipitates from the ink composition was not observed at all in storage. In addition, no changes were detected in the properties or color of the ink composition.

Ink-jet recording as was done in the above-described Image Clarity and Image Dryness Test was continuously carried out for 1,000 hours. There was no evidence of either clogging of the nozzle or change in ejection direction of the ink droplets; rather, stable recording was maintained.

After ink-jet recording was performed as outlined in (1), the apparatus and ink composition were allowed to stand at at room temperature and humidity for one month, after which they were used again to perform ink-jet recording under the same conditions as previously stated in (1). As in (3) above, there was no change in the ink droplet ejection stability. The above test was repeated in the same manner, except that the apparatus and ink were allowed to stand at 40.degree. C., 30% RH for one week, instead of being allowed to stand at room temperature and humidity for one month. The result was that again no change was observed in the ink droplet ejection stability.

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby an aqueous ink composition No. 2 for ink-jet recording according to the present invention was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby an aqueous ink composition No. 3 for ink-jet recording according to the present invention was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby an aqueous ink composition No. 4 for ink-jet recording according to the present invention was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby an aqueous ink composition No. 5 for ink-jet recording according to the present invention was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby a comparative aqueous ink composition No. 1 for ink-jet recording was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby a comparative aqueous ink composition No. 2 for ink-jet recording was prepared:

The procedure for Example 1 was repeated except that the formulation of Example 1 was replaced by the following formulation, whereby a comparative aqueous ink composition No. 3 for ink-jet recording was prepared:

The properties of the ink compositions No. 1 through No. 5 according to the present invention and the comparative ink compositions No. 1 to No. 3 are summarized in Table 2.

Aqueous ink composition No. 2 through No. 5 according to the present invention and comparative ink compositions No. 1 through No. 3 were also subjected to the same ink droplet ejection response tests as were done in Example 1.

With respect to aqueous ink compositions No. 2 through No. 5, the same excellent results were obtained as in Example 1. However, when comparative ink compositions No. 1 through No. 3 were employed, the nozzles became partially clogged when the apparatus and ink composition were allowed to stand at normal room temperature and humidity for one week, and when the apparatus and ink composition were allowed to stand at 40.degree. C./30% RH for three days, so that the direction of ejected ink droplets became extremely unstable and normal ink-jet recording was impossible.