CA1157640A

CA1157640A - Ink composition for ink-jet recording

Info

Publication number: CA1157640A
Application number: CA000371564A
Authority: CA
Inventors: Masafumi Uehara; Mitsuyoshi Itano
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 1980-02-25
Filing date: 1981-02-24
Publication date: 1983-11-29
Also published as: GB2071130B; GB2071130A; JPS56118471A; DE3106208A1; US4381946A

Abstract

ABSTRACT OF THE DISCLOSURE

An ink composition for ink-jet recording which consists of an aqueous solution comprising a water-soluble acid dye or direct dye and a polyhydric alcohol containing 1 to 4 carbon atoms. This composition has a viscosity of 4 to 20 c.p.s., a surface tension of 40 to 60 dyn/cm and a specific resistance of 5 x 103 to 1 x 105 .OMEGA. .cm. With this composition, no serious clogging occurs at the orifice of a recording apparatus, no substantial changes take place in physical and chemical properties, there is no precipitate during storage and the recorded image has a good contrast.

Description

li~i76'~0 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 an inX-jet recording process, wherein ink droplets are jetted from an orifice of the print head by an impulsive decrease of the volume of a pressure chamber.
An ink-jet recording system of this kind is known.
Such a system is disclosed, for example, in U.S. Patent 3,946,398. According to this patent, the recording process consists of the following two steps, a step wherein the inner volume of the pressure chamber filled with ink is suddenly decreased by the formation of an electrical driving pulse and an ink drop is jetted from the orifice, thus a single drop of ink is transferred to the recording paper by a single driving pulse, and a step to return the entire system to its original state, thus preparing the system for another ink-jetting process. An ink composition used for this type of ink-jet printing process, as is the case for any ink com- ~ -position of other ink-jet recording systems, requires that the ink not only cause no serious clogging at the orifice, to give a sufficient contrast to a jetted image, no change in its physical or chemical properties, no precipitation during storage thereof, but also that the viscosity and the surface tension thereof be maintained within an adequate range. Particularly, the influence caused by the viscosity of the ink on ink-jet printing performance is great and in the case when ink having an improper viscosity value, which is normally predetermined according to the individual print-ing apparatus and the various electrical factors to be given to the apparatus, is used, various troubles are likely to be caused in the jetting and the returning steps and thus satisfactory recording would become impossible. For example, if the ink having a viscosity lower than the proper viscosity range is used, it is impossible to compensate for 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 ink drops having different speeds are jetted for a single electri-cal driving pulse, which greatly deteriorates the print quality. Further, in the returning step, there may be trouble. For example, after an ink drop is jetted, the meniscus of the ink drops for a while by reaction and, there-after, on returning to its original position by the action ofthe capillary force, if the viscosity of the ink is too low, the damper action of ink is small and therefore an ink meniscus passes the orifice and is again returned by surface tension, which causes the meniscus to vibrate and it takes a long time for this vibration to settle down in the original balanced position. On the other hand, when the viscosity is too high, such type of vibration does not appear, nonetheless when the ink is returned by a capillary force, 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 of the ink in the ink-jet system of this kind, in which no such troublesome phenomena influences the viscosity of the ink, would be within 4 to 20 centipoises.
Similarly, the surface tension of the ink has a big influence on the ink-~et recording system of this kind and when an ink having a surface tension of improper value is used, the same trouble happens in both the jetting and the returning steps and satisfactory recording becomes difficult.
For example, when an ink having a surface tension that is lower than the proper value is used, the shape of an ink drop is hardly kept constant and in some cases two or more ink 7 ~ ~

drops having different speeds are jetted for a single electrical driving pulse (so-called a satellite), and further in the returning step, when the fallen ink meniscus is returned to its original position by a capillary force after an ink drop is jetted, the return-ing time becomes long due to the fact that the capillary force becomes small and, consequently, the frequency characteristic is worsened.
In the case where the surface tension is too high, on the contrary, in the returning step resulting from capillary force, the returning speed thereof is too high due to a high capillary force 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 of the surface tension of the ink for the ink-jet system of this kind, within which no such troublesome phenomena occurs, is about 40 to 60 dyn/cm.
Further, the specific resistance of the ink can also have an effect, though the effect may be small, upon the jetting of the ink droplets in this type of ink-jet recording systems. However, this effect is very large on the ink supply system, storability of the ink and clogging at the orifice. In other words, in order for an ink composi-tion to have proper viscosity and surface tension, it has been known that a composition containing a water-soluble dye, water, polyhydric alcohols and in addition thereto a water-soluble solvent, is suitable. However, an ink com-position composed of an aqueous solution normally contains various kinds of inorganic salts and metallic ions due to the existence in the composition of the water-soluble dye or impurities in the water, which often interfere with the performance of an ink supply system or the characteristics d of the ink itself. As for the metals or metallic ions which are usually present in such an aqueous ink composition, sodium, calcium, magnesium, iron, copper, lead, etc. can be mentioned. The ink thus containing a lot of metallic ions often corrodes the portions which are connected to a glass orifice and to the ink supply system, as well as the metallic portion which is used as a terminal. In addition, metals such as calcium change into a carbonate or an oxide and then deposit in the ink, Clogging then tends to be caused at the orifice and the ink itself is deteriorated in its storage stability. Although these metallic ions have various di-s-advantageous e~fects on the orifice and on the ink supply system, nevertheless the reason why the existence of such metallic ions is necessary is that a dye itself exists in the solution in the form of a metallic salt so as to be soluble in water. Also, the other metall~c salts are in- -dispensable as stabilizers or buffers in order for the ~ -water-soluble dye to exist in a dissolved state while pro-viding a stable aqueous solution. As stated above, either too 20 much or no metallic ion in the ink will hinder the ink per-formance. In an ink composition comprising, as its main ingredients, an aqueous solution containing water-soluble dyes and polyhydric alcohols, it is possible to determine a suitable range of concentration of the metallic ion present in an ink composition by measuring the specific resistance of the solution. In other words, the preferable range of the specific resistance required for the ink composition of this type of ink-jet recording systems comprising a water-color ink, has been found to be from 5 x 103 to 1 x 105 I~-cm.
As for viscosity and surface tension of the ink composition, these requirements have been achieved by the ink composition described in Japanese Patent Publication ; . ' `

Open to Public Inspection No. 137,506/1976. More specifically, when an ink composition which comprises a water-soluble dye, a polyhydric alcohol, water and, if necessary, a very small amount of a surface active agent, is used, the viscosity and the surface tension may be adjusted within the range of 4 to 20 centipoises and 40 to 60 dyn/cm2 respectively. How-ever, in the heretofore known ink compositions, no attempt to adjust the specific resistance has been made.
The object of the present invention is to provide an ink composition for ink-jet recording, which has improved properties and wherein no serious clogging occurs at the orifice, no substantial changes in physical and chemical properties or no precipitate will take place during storage, and recorded images can have sufficient contrast, and therefore stable and clear recording can be maintained for a long period and even after long storage.
The present invention thus more specifically relates to an ink composition for ink-jet recording consisting of an aqueous solution comprising a water-soluble acid dye or direct dye and a polyhydric alcohol containing 1 to 4 carbon atoms, said composition having a viscosity of 4 to 20 c.p-s-, a surface tension of 40 to 60 dyn/cm and a specific resistance of 5 x 103 to 1 x 105 n cm.
The specific resistance value of the ink composi-tion for ink-jet recording heretofore available in the market has been less than 1000 Q ~cm and the values of specific resistance of normal ink composition comprising a water-soluble dye, a polyhydric alcohol and water does not usually exceed 5000 Q ~cm. When a jet test was conducted by using an ink composition having a low specific resistance, clogging was often observed at the orifices which were found to be clogged with ink even after the ink stayed there for a few 6~

days, and as the result further ink-jetting operations were no longer possible. To cope with this problem, if the concentrations of metals and metallic ions in the ink are decreased, then the occurrence of the cloggings at the orifices can also be decreased and the corrosion in the metallic parts of the ink supply system can also be pre-vented. In order to preserve the ink itself and to adapt it to an ink-jet recording apparatus, the desirable specific resistance should vary from 5 x 103 to 1 x 105, preferably from 5 x 103 to 2 x 104 ~cm.
In order to adjust the specific resistance of the ink composition, there are physical and chemical methods.
The physical method includes the addition of metallic salts, desalting or deionizing to purify a dye and a solvent. For example, there is the process wherein water-soluble dyes are dispersed in a small amount of distilled water and only salts contained therein are dissolved out, and then the dyes are filtrated; there is also the process wherein dyes are dissolved in a solvent which can dissolve only the dyes and then the salts are removed by filtration, there is also the process wherein dyes are put into a bag made of a CellophaneTM film for dialysis use and where desalting is carried out in distilled water. Generally speaking, de-salting and deionization are not satisfactory unless they are combined with a physical process and therefore a chemical process should preferably be combined with the above-mentioned physical process.
Examples of chemical processes include a process wherein the washing of dyes is carried out in a solvent containing a chelating agent, a process wherein metallic ions which are present in the composition are masked by simply adding a chelating agent to the ink, or a process B

~7~

wherein the purification is carried out under acidic con-ditions in which an acid is added to react with the metallic ions of the dyes.
Examples of water-soluble acids and direct dyes which can be used according to the present invention include the ones having a solubility of more than 0.5 weight percent:
in the case of monochromatic recording, it is preferable to use a black or blue dye to obtain a high contrast on the recording paper. For example, C.I. ACid Blacks No. 2, 7, 24, 31, 52, 107, 118, 119, 156 and C.I. Direct Blacks No.
17, 32, 38, 51 can be mentioned as black dyes and C.I. Acid Blues ~o. 9, 62, 102, 104, 113, 117, 120 and C.I. Direct Blues No. 1, 6, 15, 25, 71, 86 and 226 can be mentioned as blue dyes and such dye can be used independently or in combinations of two or more. The amount of dyes added is not necessarily an important factor, in particular when a sufficient recording is obtained, however, in general, 0.5-8 weight percent is suitable in practice and 1.0-5 weight percent is more preferable.
Examples of polyhydric alcohols having 2 to 6 carbon atoms according to the present invention include any arbitrarily selected polyhydric alcohol, for example, ethylene glycol, propylene glycol, trimethylene glycol, glycerol, 1,3-butane diol, 2,3-butanediol, 1,4-butanediol, diethylene glycol, l,5-pentanediol, hexylene glycol, triethylene glycol, di-propylene glycol and l,2,6-hexanetriol. These polyhydric alcohols may be used either singly or in optionally selected combinations.
Furthermore, if necessary, adequate water-miscible solvents such as dioxane, acetone, Cellosolve e.g., methylcellosolve, Carbitol TM alcohols e.g., methyl alcohol, pyridine and dimethylsulfoxide may be added to the ink com-6 L~

position of the present invention.
In the present invention various kinds of additives, if necessary, can be used. For example, if the composition has to be stored for a long time, it is possible to add an antiseptic agent or an antimold to the composition of the present invention to prevent or 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 TM (1,3,5-hexahydro triazine deri-vative) sold by Hoechst AG, for example, is preferred.
Furthermore, a surface active agent may be addedin 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 include polyoxyalkylene and alkylethers thereof which are sold as siloxane-oxyalkylene copolymer (L-5340) by Union Carbide Co., and fluorine surface ;
active agent (FC-430) sold by 3M, all of which are non-ionic surface active agents. The amounts of added surface active agents are generally 1 weight percent or less based on the total amount of the ink composition and between 0.05 to 0.5 percent by weight is especially preferred.
In addition, it is possible to add various kinds of chelating reagents 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 etc.
can be mentioned.
As stated above, the ink composition of the present invention is useful because no change in physical and chemical properties or no precipitate is caused during 7~

storage, no serious clogging occurs at the orifice and stable and clear recording can be maintained even for a long period of time and even after the ink has remained for ~ long period of 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, and that when other polyhydric alcohols are used, the effects obtained by the present in-vention will not be attainable. For example, when a polypropylene glycol having relatively high polymerization degree is used, the viscosity of the ink composition becomes too high for practical use and stable and clear recording cannot be attained.
The present invention is illustrated hereinbel~w with reference to the following examples.
Example 1:
Ink Composition% by weight Dye desalted by CellophaneTM
film for dialysis use C.I. Direct Black 172.5 Ethylene glycol 43.8 Diethylene glycol 10.0 Triethylene glycol 10.0 Distilled water 33-5 Bacillat 35 (antiseptic agent made by Hoechst AG) 0.2 The above components for an ink composition were mixed under agitation to provide a homogeneous aqueous solution and this solution was then filtered through a filter having a pore size of 0.6 microns.
The i~k composition thus obtained had a viscosity of 7.3 c.p.s-, a surface tension of 60 dyn/cm and a g _ B

,~ .

specific resistance of 8 x 103Q cm at normal temperature (25C) .
The ink composition thus prepared was used in the ink-jet recording apparatus illustrated in Figs. 1 to 3 of U.S. Patent 3 ,946,398 and an ink-jet recording was conducted under the conditions shown below.
Parameters for Ink-jet Printing Operation:
Printing speed 2000 dots/sec Static pressure 0.006 psi Peak pre'ssure of the pulse 4.83 psi Pulse voltage 80 volts Pulse width 140 us Diameter of the orifice 0.002~3 inches As a result of this experiment it was found that a clear and excellent print can be obtained by utilizing the ink composition according to the present invention and the quality of the printed image remained unchanged after con-tinuous recording operation for 24 hours and, furthermore, even in operation, wherein the ink composition was stored for one month in the device. .
Example 2:
Ink Composition % by weight Dye desalted with distilled water C.I. Direct Black 32 4.0 Ethylene glycol 64.2 Distilled water 31.2 EDTA-tetra sodium salt 0.2 Bacillat 35TM (Hoechst AG) 0.2 Another aqueous ink composition consisting of the above components was prepared in the same manner as in Example 1 . The viscosity, the surface tension and the ~r 11~76~

specific resistance of the ink composition thus prepared were '7.5 c.p.s., 56 dyn/cm and 1.5 x 104 Q ,cm respectively at normal temperature (25C).
Using this ink, an 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 3:
Ink Composition % by weight Dye desalted by recrystalli-zation C.I. Direct Black 32 2.0 Tertiary ethylene glycol 45.0 Distilled water 51.4 Potassium carbonate 0.3 EDTA-tetra sodium salt 0.2 Bacillat 35T (Hoechst AG) 0.1 Another aqueous ink composition consisting of the above shown components was prepared in the same manner as in Example 1. The viscosity, the surface tension and the specific resistance of the ink composition thus prepared were 6.6 c.p.s., 55 dyn/cm and 1.7 x 104~Lcm respectively at norrnal temperature (25C).
Using this ink, an 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.

B

6~

Example 4:
Ink Composition % by weight Dye desalted by recrystallization C.I. Direct Blue 25 2.0 Dye desalted by recrystallization C.I. Direct Blue 226 2.0 Ethylene glycol 61.5 Distilled water 33-9 .
Potassium carbonate 0.1 -.
EDTA-tetra sodium salt 0.3 Bacillat 35TM (Hoechst AG) 0.2 Another aqueous ink composition consisting of the ~ :
above components was prepared in the same manner as in Example 1. The viscosity, the surface tension and the specific resistance of the ink composition thus prepared were 6.0 c-p-s-, 58 dyn/cm and 1.0 x 104~~1.cm respectively at normal temperature (25C). Using this ink, an 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 precipita-tion occurred.
ExamPle 5:
Ink Composition% by weight Dye desalted by distilled water C.I. Direct Black 109 3.0 Diethylene glycol 45.0 Distilled water 51.4 Potassium carbonate 0.2 EDTA-tetra sodium salt 0-3 Bacillat 35 M (Hoechst AG) 0.1 }g ' ' , ...

liS7G~/~

Another aqueous ink composition consisting of the above components was prepared in the same manner as in Example 1. The viscosity, the surface tension and the specific resistance of the ink composition thus prepared were 5.1 c.p.s., 48 dyn/cm and 2.0 x 10 ~~L-cm respectively at normal temperature (25C).
Using this ink, an ink-jet recording was conducted using the same device and in the same manner as in Example 1 and, similarly as in Exa~ple 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.

B

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An ink composition for ink-jet recording which is an aqueous solution comprising a water-soluble acid dye, a water soluble direct dye, or mixtures thereof, and a compound selected from the group consisting of a polyhydric alcohol having 1-4 carbon atoms, hexylene glycol, triethylene glycol, di-propylene glycol and 1,2,6-hexane triol, said composition having a viscos-ity of from 4 to 20 cps, a surface tension of from 40 to 60 dynes/cm and a specific resistance of from 5 x 103 to 1 x 105 .OMEGA. cm.

2. An ink composition according to claim 1, wherein said dye is selected from 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 selected from a group consisting of C.I. Acid Blacks No. 2, 7, 24, 52, 107, 118, 119 and 156, C.I. Direct Blacks No. 17, 32, 38 and 51, C.I. Acid Blues No. 9, 62, 102, 104, 113, 117 and 120, and C.I. Direct Blues No. 1, 6, 15, 25, 71, 86 and 226.

4. An ink composition according to claim 1, 2 or 3, wherein said dye is incorporated in said composition in an amount of 0.5 to 8% by weight.

5. An ink composition according to claim 1, wherein said polyhydric alcohol is selected from the group consisting of ethylene glycol, propylene glycol, tri-methylene glycol, glycerol, 1,3-butanediol, 2,3-butanediol, glycol, glycerol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, hexylene glycol, triethylene glycol, dipropylene glycol and 1,2,6-hexanetriol.