GB2070049A - An ink composition for ink-jet recording - Google Patents

Abstract

An ink composition suitable for use in ink-jet recording is provided which comprises a water-soluble acid or direct dye and a polyethylene glycol with a polymerization degree of 4 to 25, said composition having a viscosity of 4 to 20 c.p.s. at 25 DEG C and a surface tension of 40 to 60 dyn/cm.

Description

SPECIFICATION An ink composition for ink-jet recording The present invention relates to an ink composition for ink-jet recording and more particularly, the present invention relates to an aqueous ink composition for ink-jet recording process, wherein ink droplets are jetted from an orifice of the print head by decreasing the volume of a pressure chamber responsive to electrical pulse signals.

Ink-jet recording systems of this kind, are known from, for example US Patent 3,946,398.

According to this, the recording process consists of the following two steps; a first step wherein the volume of the interior of the pressure chamber filied with ink is suddenly decreased by an electrical driving pulse and an ink drop is jetted from the orifice, so that a single drop of ink is transferred to the recording paper by a single driving pulse, and a second step to return the entire system to its original state to prepare for a repetition.Ink compositions used for this type of ink-jet printing process, as is the case for ink compositions used in other ink-jet recording systems should be such as not only to cause no serious clogging at the orifice, to give sufficient contrast to the jetted image, to cause no change in its physical or chemical properties, to cause no precipitation during storage, but also for the viscosity and the surface tension thereof to be maintained within an appropriate range.Particularly, the influence caused by the viscosity of the ink on ink-jet printing performance is great and in the case where ink having an inappropriate viscosity value, which is normally predetermined according to the particular printing apparatus, and the various electrical factors involved for the apparatus, is used, various troubles are likely to be caused in the jetting and the returning steps so that satisfactory recording becomes impossible. For example, if an ink having a viscosity lower than the proper viscosity range is used, it is impossible to compensate the speed deviation of each part of the ink column jetted from the orifice in the jet process, and as a result two or more of ink drops having different speeds are jetted for a single electrical driving pulse, which causes a considerable deterioration in the print quality.Further, in the returning step, problems may arise. Namely, after an ink drop is jetted, the meniscus of the ink falls for a while and, thereafter, on returning to its original position by capillary forces, if the viscosity of ink is too low, the action of the ink as a damper is small and therefore, and ink meniscus passes the orifice and returns again by surface tension, which causes vibration to the meniscus, and it takes a long time for this vibration to settle down to the original balanced position. On the other hand if the viscosity is too high, although such vibrations do not appear, none the less when the ink is returned by capillary forces, it takes a long time and, in both cases, the time required for the meniscus to return to the original state is long and consequently the printing speed is lowered.The proper viscosity range for ink in an ink-jet system of this type in which no such troublesome phenomena occur is 4 to 20 centipoise.

Similarly, the surface tension of the ink has a profound influence on an ink-jet recording system of this kind and when ink having a surface tension of inappropriate value is used, similar troubles arise in both the jetting process and the returning steps so that satisfactory recording becomes difficult. For example, when ink having a surface tension that is lower than the correct value is used, it is difficult to keep the shape of an ink drop constant and in some cases, two or more ink drops having different speeds are jetted for a single electrical driving pulse (so-called satellite), and further in the returning process, when the fallen ink meniscus is returned to its original position by capillary forces after an ink drop is jetted, the returning time is lengthened due to the fact that the capillary forces become small and, consequently, frequency characteristics deteriorate.

In the case where the surface tension is too high, on the contrary, in the returning step the returning speed is too high due to the large capillary forces and consequently the ink meniscus begins to vibrate. Therefore, again, it takes a long time to settle down to the original balanced position. The proper range for the surface tension of the ink for an ink-jet system of this kind within which no such troublesome phenomena occurs is about 40 to 60 dyn/cm.

Heretofore, various kinds of ink composition have been proposed for this purpose but nothing satisfies all above-mentioned requirements, although some of those requirements may be satisfied, and, therefore, the known ink compositions have some drawbacks. For example, it has been proposed that cellulose derivatives such as hydroxypropyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose and methyl cellulose etc. or a water-soluble thickener such as polyvinyl alcohol should be added to the ink composition in order to keep the desired viscosity, but such water-soluble thickeners change their viscosity during storage due to an interaction with a water-soluble dye as a colorant and, depending on the condition, precipitation may occur. Thus this kind of ink-jet recording system has had some drawbacks from a practical point of view.Further, in the case where a fine orifice structure (30--200 microns) is employed in the ink-jet system, which is usually the case, clogging often occurs due to the precipitation of a solid component in the ink composition by evaporation of solvent during storage of the ink inside the orifice; the response of the ink-jet to inner volume change induced by an electrical driving pulse wili be impeded and, consequently, ink may not be jetted; or the jetting condition changes with time and for one or more of the above-mentioned reasons stable recording becomes impossible.Further, it has been known that a water-soluble polyhydric alcohol can be added to the water-color ink composition for the purpose of preventing clogging of the orifice, as described in US Patents Nos. 3,705,043, 3,846,141, Japanese Patent Publications Open to Public Inspection No. 72712/1975 and 95008/1975 and West German OLS Patent No.2,258,804. All of these ink compositions consist of an aqueous solution comprising water-soluble dye and wetting agent, in the amount less than 40 weight percent, such as polyethylene glycol, polypropylene glycol, ethylene glycol, propylene glycol, diethylene glycol and glyceroi etc. as its basic component; even by the addition of such wetting agents it is almost impossible to maintain the viscosity range within 4-20 centipoise and the prevention of clogging in the orifice and of penetration of the air bubbles into the orifice can be achieved only imperfectly.

The object of the present invention is to eliminate these drawbacks and to provide an ink composition for ink-jet recording, whereby no serious clogging occurs at the orifice, no substantial changes in physical and chemical properties and no precipitate occurs during storage, and recorded images can have sufficient contrast, and, therefore stable and clear recording can be maintained in the recording for a long period and even after long storage.

The present invention provides an ink composition for ink-jet recording consisting of an aqueous solution comprising a water-soluble acid or direct dye and a polyethylene glycol with the polymerization degree of 4 to 25, said composition having a viscosity of 4 to 20 centipoise and a surface tension of 40 to 60 dyn/cm at250C.

As the water-soluble acid and direct dye to be used for the present invention, any of the ones having a solubility of more than 0.5 weight percent can be used and, in the case of monochromatic recording, it is preferable to use a black or blue dye to obtain high contrast on the recording paper. For example, C.l. Acid Blacks No. 2, 7, 24, 31, 52, 107, 118, 119, 1 56 and C.l. Direct Blacks No. 17, 32, 38, 51 can be given as black dyes and C.l. Acid Blues No. 9, 62, 102, 104, 113, 117, 120 and C.l.

Direct Blues No. 1,6, 15,25, 71,86 and 226 can be given as blue dyes; such dye can be used singly or in combination of two or more. The amount of these dyes is not necessarily an important factor, in particular when sufficient recording is obtained, however, in general, 0.5-8 weight percent is practically suitable and 1.0-5 weight percent is more preferable.

As polyethylene glycol to be used in the present invention, one having any average molecular weight consistent with the aforesaid polymerization degree can be used and, for example, polyethylene glycol (molecular weight 200), polyethylene glycol (molecular weight 300), polyethylene glycol (molecular weight 400), polyethylene glycol (molecular weight 600), and polyethylene glycol (molecular weight 1000) can be mentioned; in the present invention these compounds may be used singly or two or more in combination.

Such polyethylene glycols are all miscible with water and due to their wetting, low volatility and thickening properties they play an important role in the ink composition of the present invention. The polymerization degree of the polyethylene glycol can be from 4 to 25 and the polymerization degree of 4 to 10 is preferable. The amount of such polyethylene glycol may be decided depending on the predetermined viscosity, surface tension and in an amount sufficient to prevent the clogging of the orifice and penetration of bubbles; further the amount to be used varies depending upon the polymerization degree of the polyethylene glycol but it can be selected so long as the aforesaid various conditions are satisfied; the range of 30-60 weight percent is generally preferable.

In the present invention various kinds of additives, if necessary, can be used. For example in case the composition has to be stored for a long time, it is possible to add an antiseptic agent or antimold to the composition of the present invention so as to prevent or to decrease the growth of bacteria or mold therein. It has been known that various kinds of antiseptic agents are useful for this purpose and Bacillat 35 (13,5-hexahydro triazine derivative) sold by Hoechst AG, for example, is preferable. The amount of antiseptic agent or antimold is generally 1 weight percent or less based on the total amount of ink composition and the range of 0.01--0.5 weight percent is especially preferable.

Further, a surface active agent may be added in order to change the surface tension of the ink composition or to improve the so-called "wetting" of the ink in the ink passage. Preferred examples thereof are polyoxyalkylenes and alkylethers thereof for example the, siloxaneoxyalkylene copolymer (L-5340) sold by Union Carbide Co., and the fluorine surface agents (FC-430) sold by 3M, all of which are non-ionic surface active agents. The amount of these surface active agents is generally 1 weight percent or less based on the total amount of the ink composition and the range of 0.05 to 0.5 percent by weight is especially preferable.

Further, it is possible to add various kinds of inorganic or organic buffer so as to prevent a pH change caused mainly by absorption of carbon dioxide in the air during storage of the ink in the container or during the period of stay in the orifice. Carbonates such as sodium carbonate and potassium carbonate, for example, are preferred; 0.1 to 5 percent by weight based on the total amount of the ink composition may be suitable and more preferably 0.1 to 2 percent by weight is used.

Further, it is possible to add various kinds of chelating reagent so as to mask the metal and metallic ion of the composition. For example, sodium gluconate, ethylenediamine tetra acetic acid (EDTA), disodium ethylenediamine tetra acetate, trisodium ethylenediamine tetra acetate, tetrasodium ethylenediamine tetra acetate and sodium salt of diethylenetriamino penta acetic acid can be mentioned.

As stated above, the ink composition of the present invention is useful as an ink wherein no significant change in physical and chemical properties and no precipitate is caused during storage, no serious clogging at the orifice occurs and stable and clear recording can be maintained even for the recording for a long period of time and even after the ink has stayed for a long time in the ink chamber.

It is surprising that the ink composition of the present invention shows an excellent effect when it is used for ink-jet recording, since when other polyalkylene glycols that are similar to polyethylene glycol are used, the effects obtained by the present invention are not attained. For example, most of the polypropylene glycols do not dissolve in water and even if they dissolve, their viscosity is too high for practical use and with polymethylene glycol, for example, a stable and clear recording cannot be attained.

The present invention is illustrated hereinbelow with reference to the following examples.

EXAMPLE 1 Ink Composition % by weight C.l. Direct Black 17 2.5 Polyethylene glycol (molecular weight: 200 average 47.2 polymerization degree: 4.1) Distilled water 50.1 Potassium carbonate 0.2 Bacillat (antiseptic agent made by Hoechst AG) 0.1 The above-listed components for an ink composition were mixed under agitation to make a homogeneous aqueous solution and this solution was then filtered through a filter having a pore size of 1.0 micron.

The ink composition thus obtained has a viscosity value of 7.8 c.p.s. and surface tension of 58 dyne/cm at the normal temperature (25or).

The ink composition thus prepared was used in the inkjet recording apparatus device disclosed in Figs. 1 to 3 of U.S. Patent 3,946,398 and ink-jet recording was conducted under the conditions shown below.

Parameters for ink-jet printing operation: Printing speed 1750 dots/sec Static pressure 0.006 psi Peak pressure of the pulse 5.74 psi Pulse voltage 80 volts Pulse width 140 us Diameter of the orifice 0.0028 inches As the result of this experiment it was found that clear and excellent print was obtainable by the use of the ink composition according to the present invention and the quality of the printed image remained unchanged after continuous recording operation for 24 hours and, furthermore, also where the ink composition was stored for one month in the device.

EXAMPLE2 Ink Composition % by weight C.l. Direct Black 32 4.0 Polyethylene glycol (molecular weight: 400 and 47.7 average polymerization degree 8.7) Deionized water 48.0 EDTA-tetra sodium salt 0.2 Bacillat 35 (Hoechst A.G.) 0.1 Another aqueous ink composition consisting of the above shown components was prepared in the same manner as in Example 1. The viscosity and the surface tension of the ink composition thus prepared were 11.0 c.p.s. and 56 dyn/cm at normal temperature.

Using this ink, ink-jet recording was conducted using the same device and in the same manner as in Example 1 and, similarly as in Example 1, good results were obtained.

Further, even after a long period of storage thereof no growth of bacteria or mold was observed and no precipitation occurred.

EXAMPLE 4 Ink Composition % by weight C.l. Direct Blue 25 2.0 C.l. Direct Black 32 2.0 Polyethylene glycol (molecular weight: 600 and 45.0 average polymerization degree: 13.2) Distilled water 50.0 Sodium carbonate 0.2 EDTA-tertiary sodium salt 0.2 Bacillat 35 (Hoechst AG) 0.1 Another aqueous ink composition consisting of the above components was prepared in the same manner as in Example 1. The viscosity and the surface tension of the ink composition thus prepared were respectively 16.2 c.p.s. and 59 dyn/cm at the normal temperature (250C).

Using this ink the same experiments as in Example 2 were repeated and similarly good results were obtained.

EXAMPLE 5 Ink Composition % by weight C.l. Acid Black 109 3.5 Polyethylene glycol (molecular weight: 300 and 40.0 average polymerization degree: 6.4) Distilled water 56.2 EDTA-tetra sodium salt 0.2 Bacillat 35 (Hoechst AG) 0.1 Another aqueous ink composition consisting of the components shown above was prepared in the same manner as in Example 1. The viscosity and the surface tension of the ink composition thus prepared were 5.1 c.p.s. and 60 dyn/cm at the normal temperature (250C).

Using this ink, ink-jet recording was conducted in the same manner as Example 2, and similarly good results were obtained.

Claims (9)

1. An ink composition suitable for use in ink-jet recording which comprises a water-soluble acid or direct dye and a polyethylene glycol with a polymerization degree of 4 to 25, said composition having a viscosity of 4 to 20 c.p.s. at 250C and a surface tension of 40 to 60 dyn/cm.

2. An ink composition according to claim 1, wherein said dye is a blue or black dye having a solubility in water of more than 0.5% by weight.

3. An ink composition according to claim 2, wherein said dye is C.l. Acid Black No. 2, 7, 24, 52, 107,118, 119or 156, C.l. Direct Black No. 17,32, 38 or 51, C.l. Acid Blue No.9,62,102,104,113, 117 or 120 and Cl. Direct Blue No.1,6,15,25,71,86 or 226.

4. An ink composition according to claim 1, 2 or 3, wherein said dye is present in an amount from 0 > 5 to 8% by weight.

5. An ink composition according to any one of claims 1 to 4 wherein said polyethylene glycol is present in an amount from 30 to 60% by weight.

6. An ink composition according to any one of claims 1 to 5, wherein said polyethylene glycol has a polymerization degree of 4 to 10.

7. An ink composition according to any one of the preceding claims which contains a bactericide or fungicide in an amount from 0.01 to 0.5 weight percent.

8. An ink composition according to any one of the preceding claims which contains a non-ionic surface active agent in an amount from 0.05 to 0.5 weight percent.

9. An ink composition according to claim 1 substantially as described in any one of the Examples.