WO1990001053A1

WO1990001053A1 - An electrically conductive ink for ink-jet printers

Info

Publication number: WO1990001053A1
Application number: PCT/DE1989/000467
Authority: WO
Inventors: Lutz Vite
Original assignee: Siemens Aktiengesellschaft
Priority date: 1988-07-22
Filing date: 1989-07-12
Publication date: 1990-02-08
Also published as: DE3825007A1

Abstract

An ink for ink-jet printers is proposed which, besides an organic solvent, contains at least one highly pure colorant and, where appropriate, further auxiliary materials and other organic additives to obtain a certain electrical conductivity which generate ions on being dissolved in at least one solvent. Good electrical conductivities are obtained with quaternary ammonium compounds, e.g. tetra-alkyl ammonium hydroxide or tetra-aryl ammonium hydroxide or other stable organic salts in the ink solution as additives for the ink. Here, the anions are so selected that the ink causes no corrosion in parts of the printer or parts to be printed which contain metals. The invention substantially extends the choice of solvents and colorants usable for inks and guarantees electrical conductivity even with highly purified raw materials and ink components.

Description

Electrically conductive ink for inkjet printers.

The invention relates to an ink for inkjet printers, which consists of at least one organic solvent, at least one highly purified dye and optionally other auxiliaries.

In ink jet printers, the color transfer is achieved by ejecting fine ink droplets from the nozzles of a print head onto the medium to be printed, which is usually paper. The nozzles, which are usually arranged in matrix form in the print head, are connected via fine ink channels to an ink jet generating device and further to an ink reservoir. In the piezo-jet method, the ink jet or the spinning off of an ink droplet is generated by a piezoelectric element which surrounds part of the ink channel in a ring. If such a piezoelectric element is addressed with an electrical pulse, there is a reduction in the cross section of the ink channel in the area of this element, as a result of which an ink droplet is ejected from the ink channel ending in a nozzle. In the bubble jet process, the kinetic energy for ejecting an ink droplet is generated by evaporating an ink component. The heat required for this is provided by a thermal transducer element, which is arranged in a specific area of the ink channel within the write head.

Corresponding requirements are placed on inks suitable for inkjet printers. For example, the viscosity and surface tension of the inks must reach certain values in order to achieve a good drip flight image and thus a good print image. These values must remain constant so that the print quality does not change even after long use of the ink changed or worsened. The ink solutions must therefore be both chemically and physically stable. There must be no deposits inside the print head that could clog the fine ink channels and nozzles. In the case of bubble jet printers, the ink must also be thermally stable in order to be able to withstand the short-term high temperatures at the thermal converter element. Since metals are generally also contained in the parts of the print head and the printer which come into contact with the ink, the inks should also not contain any ions which assist the corrosion of these metal parts. Above all, alkali chloride and alkali sulfate are dangerous and should be avoided. As a result of the ever increasing demands, the inks are provided in increasingly better and purer quality, since increasingly purer starting compounds are used.

However, this creates a new technical problem. The conductivity generated by the above-mentioned inorganic salts has hitherto been used for checking the liquid level in the ink reservoir. A plurality of electrodes located within the ink reservoir indicate the level when in contact with the ink by the flow of a current. With new ones

However, due to the lower content of charge carriers or ionic impurities, inks are no longer sufficient to ensure an exact level indicator.

The object of the present invention is therefore to provide an ink, in particular for piezo-jet printers, which has a required electrical conductivity without showing the disadvantages associated with the charge carriers.

This object is achieved according to the invention by an ink of the type mentioned above in that, in order to achieve a defined electrical conductivity of the ink, further organic additives are added which generate charge carriers (ions) when dissolved in the at least one solvent. Furthermore, it is within the scope of the invention that these organic additives are quaternary ammonium compounds of the general formula NH _4-a R _a A, where in which the radicals R independently of one another represent alkyl or aryl radicals, a is the number 1, 2 or 3 and A is a suitable anion which is selected such that the ammonium compound in the ink is stable without decomposition. Further refinements of the invention can be found in the subclaims.

Especially with inks for piezo-jet printers, which are almost water-free, it has been a problem to find suitable organic compounds which are compatible with the other ink constituents, which do not impair the properties of the ink with regard to stability and print quality, and which at the same time in the Ink solution to provide a sufficiently high concentration of charge carriers to produce sufficient electrical conductivity of the ink. The required conductivity can often not be guaranteed even when using ionic dyes.

From DE-OS 34 15 098 an electrically conductive writing or marking means is known, which can also be used in inkjet printers. Iodides or bromides of alkali or alkaline earth metals are added to this ink as an electrolytically active substance. With these purely inorganic additives, good electrical conductivity of the ink is achieved and corrosion of metal-containing printer components or printed materials is avoided, but one is limited to a small selection of usable organic solvents. To dissolve sufficient electrolyte, ketones or alcohols must be contained alone or in a mixture. In addition, iodides in particular are relatively expensive compounds.

According to the invention, purely organic additives are now used to achieve electrical conductivity. These can be used in inks with a variety of organic solvents and are therefore also suitable for special ink compositions. Compared to the iodides and bromides mentioned above, there is also a significant cost saving for the ink composition. In the mostly basic ink composition, the hydroxides of tetraalkyl or tetraarylammonium compounds are particularly suitable. These also form chemically stable solutions in basic. Monoalkyl, dialkyl and trialkylammonium or corresponding arylammonium compounds are also suitable in less basic ink solutions.

In a particularly preferred embodiment of the invention, ammonium compounds of the general formula are used as organic additives

NR ₄ ⁺ / XSO- ₃ used, where the radicals R independently of one another are monovalent alkyl, aryl or arylalkyl groups and X is an attached to the

Sulfur atom of the XSO ₃ group is an organic radical, for which at least the same selection applies as for R. Organic additives in which X comprises an at least partially halogenated group are also used with good success. For example, X may represent a trifluoromethyl group or a larger group containing such a trifluoromethyl group. Corresponding chlorinated or brominated organic groups are also possible. The following is a small selection of suitable compounds which, according to the invention, can increase the conductivity of inks as organic additives. These are only examples, so that the list cannot be exhaustive. The invention can be carried out in many embodiments in accordance with the general structural formulas given. Good results show, for example:

Tetramethylammonium hydroxide, tetraethylammonium hydroxide,

Trimethylphenylammonium hydroxide, benzyltrimethylammonium hydroxide, tetramethylammonium trifluoromethylsulfonate, tetraethylammonium trifluoromethylsulfonate, tetramethylammonium toluene-4-sulfonate and tetraethylammonium toluene-4-sulfonate.

The electrolytes or conductivity additives given by way of example are on the order of approximately 2 percent by weight mixed with the ink. The electrical that can be achieved

Conductivity clearly exceeds the technically required value of approx. 0.2 mS / cm (25ºC). The invention provides a way by which the range of dyes possible for ink compositions is significantly expanded. Zuifl example, solvent dyes can now also be used. The inks can be constructed on an anhydrous basis or, like newer ink compositions, can contain up to 10 percent by weight of water. Such more water-containing inks then contain a poly- or diethylene glycol as a viscosity-increasing additive. The ink according to the invention is particularly suitable for printers which operate according to the piezo-jet process, but can also be used for bubble-oet processes and other ink-jet processes.

5 claims

Claims

1. Ink for inkjet printers, consisting of at least one organic solvent, at least one highly purified dye and optionally other auxiliaries, characterized in that further organic additives are added to achieve a defined electrical conductivity of the ink, which charge carriers (ions.) When dissolved in the at least one solvent ) produce.

2. Ink according to claim 1, characterized in that the organic additives are quaternary ammonium compounds of the general formula NH _4-a R _a A, where the radicals R independently represent alkyl or aryl radicals, a is the number 1, 2 or 3 and A is a suitable anion that is selected so that the ammonium compound in the ink is stable without decomposition.

3. Ink according to claim 2, d a d u r c h g e k e n n z e i c h n e t that the organic additives are tetraalkyl or tetraarylammonium hydroxides.

4. Ink according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the organic additives have the general formula

NR ₄ ⁺ / XSO ₃ -, where the radicals R are independently monovalent alkyl, aryl or arylalkyl groups and X is an organic radical bonded to the sulfur atom of the XSO ₃ group, for which at least the same selection as for R applies .

5. Ink according to claim 4, d a d u r c h g e k e n n z e i c h n e t that bound to the organic radical X.

H atoms are at least partially replaced by the halogen atoms fluorine, chlorine or bromine.