EP0387264B1 - Inking ribbon - Google Patents

Abstract

An inking ribbon for thermographic printers contains components which can be decomposed by heat or radiation, in particular components which liberate a gas on decomposing. The ink-transfer regions (decomposition regions) of inking ribbons manufactured in this way are more sharply defined, the heat output required for printing is reduced, and adherence of the ink to the paper is improved.

Description

The invention relates to an ink ribbon for a transfer printing process in which dye and, if appropriate, further ink components are transferred from a carrier film to a material to be printed, in particular paper, by the targeted action of radiation and / or heat.

Such printing processes are used for example in thermal printers. Printer inks for known ribbons consist of a wax that contains the dye and a binder. This wax is applied to a carrier, which usually consists of a flexible plastic film, for example polybutylene terephthalate. For better heat conduction, an aluminum layer can still be present between the film and the wax layer. The ribbons for the printers are produced from foils coated in this way.

In the printing process, which is also called release technology, a print head, which can for example consist of several heatable elements arranged in the form of a matrix, transfers a certain amount of heat to the ink ribbon via these elements. As a result, the wax is melted and transferred to the medium to be printed, in particular paper, by the pressure applied at the same time. The wax layer containing the dye transferred onto the paper is relatively thick. Although this results in good color saturation, which is also independent of the surface of the paper, the dye adhesion on the paper and, as a result, the abrasion resistance can still be improved with this technique.

As the printing speed increases, the multilayer structure of the ink ribbon described is also disadvantageous. The wax layer must be melted during printing and therefore, like all other layers, consumes a certain amount Heating output that limits the maximum achievable printing speed. The mechanical stress on the ribbons also only allows a certain printing speed.

In addition, according to a rough estimate, the "thermal efficiency" for thermal printers with conventional printer ink compositions is only approximately 5%. This means that approx. 95% of the heating output is lost as waste heat and can no longer be used directly for ink transfer.

The object of the present invention is therefore to provide an ink ribbon for transfer printing processes which shows good color saturation, for example on paper, improved print quality with good ink adhesion and high abrasion resistance of the ink ribbon and which also requires less heating power for ink transfer.

This object is achieved according to the invention by means of a ribbon of the type mentioned at the outset, in that at least the dye is chemically unstable in a carrier phase, which breaks down chemically under the action of radiation and / or heat and at least chemically releases the dye , which is transferred to a mobile phase and transferred to the material to be printed. Furthermore, it is within the scope of the invention that in the preliminary stage the at least one releasable ink component or the dye is bound via an unstable group, upon the disintegration of which a gaseous compound is also released in addition to the ink component. Further refinements of the invention can be found in the subclaims.

The thermally labile chemical incorporation or connection of ink components in a carrier phase facilitates the release of these ink components. Release and mobilization mean the transfer of this ink component into a more mobile phase. This more mobile phase can be liquid or gaseous at the given temperature.

Releasable and thus transferable ink components to the material to be printed can be: one or more Dyes or pigments, binder material for the dye, a "wax" used for an intermediate or cover layer and transferable after melting, or a compound which acts as a solvent for other ink components and is liquid in the released state at the given temperature.

The transfer of the ink components of the ink ribbon according to the invention requires a lower energy input than with conventional ink ribbons. As a result of the chemical bonding of ink components according to the invention, the covering wax layer on the ink ribbon can be made thinner or can even be omitted entirely.

A gas released during the printing process in one embodiment of the invention supports the transfer of the ink components onto the medium to be printed. Due to the pressure generated when the gas is released, the ink components receive enough kinetic energy to penetrate deeply into the material to be printed (e.g. paper). This increases the dye adhesion on the paper.

This additional supportive effect can also be achieved by admixing another thermally unstable compound to the ink components. Such a compound which acts as a "blowing agent" is, for example, azodicarbonamide. This compound, which can be added to the ink components up to about 10 percent by weight, is preferred since it does not release any toxic gases. However, azo foamers can also be used, for example. With 2-t-butylazo-2'-cyanobutane, the decomposition temperature can be set to approx. 80 ° C. A blowing agent that thermally releases not only nitrogen but also carbon dioxide is 2,2'-diacetoxy-2,2'-azopropane.

The chemical disintegration reaction on the ink ribbon (even without any gas released) results in faster ink transfer than is possible with conventional ink ribbons, where the ink transfer is achieved only by melting a wax layer and applying pressure. Especially when the decay temperature of the labile compound is coordinated with the Melting point of the surface layer, if any Additional benefits are achieved with wax. The speed of the transfer of the ink components suddenly and steeply increases when the decay temperature is reached in the color ribbons according to the invention and thus enables a sharper print image on paper, for example.

Furthermore, the dye and other ink components can be bound to a carrier polymer. In a preferred embodiment, this polymer simultaneously represents the carrier film for the ink ribbon. A multi-layer structure of the ink ribbon can therefore be omitted in this case. With the printing speed remaining the same, the ink ribbon can now be made significantly thinner, since the risk of mechanical damage to an ink ribbon with a single-layer structure is significantly lower during operation than with a multi-layer structure. The amount of energy required for color transfer is also reduced in this embodiment. In addition to a small contribution to heating the film, only the heating power that is necessary to decompose the unstable groups has to be applied. The decomposition areas, that is, the areas in which a color transfer is to take place, can be defined more sharply with the novel printer ink than is possible with the conventional wax layer technology. This results in a sharper print image, with the ink penetrating into the deepest paper cavities and pores, adhering accordingly and also providing good ink coverage.

Another embodiment of the printer ink allows the dye components to be transferred in solution. For this purpose, in addition to the components containing, for example, dye, other ink components can also be bound to the carrier polymer or as a non-mobile compound via labile groups or bonds, which form a liquid phase on decomposition and are able to dissolve the ink components. By transferring the ink components in solution or liquid phase, an even better penetration of the color into the pores and voids of paper is achieved.

A further embodiment of the inventive concept relates to a Ribbon in which at least one ink component is released on a depolymerizable polymer with a low decomposition temperature. An example of this is poly-α-methylstyrene, which has a ceiling temperature of approx. 61 ° C. In the simplest case, the ink component, for example the dye, can be incorporated into such polymers by carrying out the polymerization in the presence of a dye. The dye is enclosed in the polymer matrix of the plastic like a filler. It is better to use dyes which carry a polymerizable functional group and can serve as monomers for copolymerization with the depolymerizable plastic, so that the dye is also chemically bound to and in the polymer.

There is a large selection of suitable dyes if the required conditions are met. For the embodiments of the invention which provide labile groups bonded to the dye components, the azo group and the carboxylate group which release the gases nitrogen and carbon dioxide on decay are suitable as labile groups. Both groups are easy for the chemist and accessible in a variety of reactions. In part, the representation of the labile group can be used simultaneously with the linking reaction of the dye components with the carrier polymer. For example, polyamines bearing free amino groups can easily be coupled with suitable dyes to form azo groups.

Both the azo group and the carboxylate group decompose in the heat, the decomposition temperature being able to be set within certain limits by chemical modification, as is the case, for example, for blowing agents from an article by D. Braun in Monnatshefte für Chemie 110, pages 699 to 713, ( 1979) is known. By suitable modification it is therefore also possible to incorporate different dyes in a carrier polymer so that they are released at different temperatures.

In principle, it is also possible to trigger the printing process by exposure to radiation. For the production of lithographic printing plates, for example, in US Pat. No. 3,962,513 Laser beams and a transfer film are used. The energy of the laser is absorbed by solid particles contained in a nitrocellulose binder layer on the transfer film. As a result, the binder decomposes in the irradiated areas and plastic applied over it can be transferred to a substrate suitable for a printing plate, for example aluminum.

A recording sheet is known from French patent FR 2 250 318. This also has a nitrocellulose layer in which certain areas are decomposed with a laser, an "image" being formed in the recording layer. The binder is subject to auto-oxidation and burns completely. Solid particles contained in the binder can be transferred to an adhesive tape.

One way of releasing the ink components of the ink ribbon according to the invention is to decompose the labile groups by radiation. For example, the azo group is unstable against UV light of 360 nm wavelength and disintegrates with the elimination of nitrogen. The decomposition necessary energy is about 120 kJ / mol. A similar amount of energy requires the decay of corresponding carboxylate groups, which in addition to heat can also be triggered by infrared radiation. This makes it possible to use the ink ribbon according to the invention in printers whose printheads do not transmit the ink from the ink ribbon to the paper with heat transfer, but with UV or IR radiation.

The invention is explained in more detail below with the aid of two exemplary embodiments.

1st embodiment

Das Monomer I wird anschließend nach bekannten Methoden in Lösung bei ca. 60° polymerisiert. Durch Mischen dieses Polymeren, zum Beispiel mit pulverförmigem Polyethylenvinylacetat, und anschließender Coextrusion werden daraus Folien von zum Beispiel 1 µm Stärke erzeugt und dann mit einer 2 µm dicken Polyethylenterephthalatfolie (Trägerfolie) verbunden. Auf dieser Schicht kann nun noch eine bis ca. 2 µm dicke Parafinschicht aus der Lösung abgeschieden werden. Nach dem Trocknen ist eine gebrauchsfertige Druckfolie für ein Farbband entstanden.A polymerizable olefinic monomer which carries an aromatic amine, for example benzyl p-aminomethacrylic acid, is diazotized and reacted with an alkylcyanoacetic acid ester of formula 1 bearing a dye X.

The monomer I is then polymerized in solution at about 60 ° by known methods. By mixing this polymer, for example with powdered polyethylene vinyl acetate, and subsequent coextrusion, films of, for example, 1 μm thick are produced therefrom and then bonded to a 2 μm thick polyethylene terephthalate film (carrier film). A layer of paraffin up to approx. 2 µm thick can now be deposited from the solution on this layer. After drying, a ready-to-use printing film for an ink ribbon is created.

A solution containing about 0.5 to 10 percent by weight of polymer in a suitable solvent can also be used for coating. In this case, a longer-chain alkyl radical R (for example R = hexyl) can serve as a solubilizer (when the layer is separated from solution). Instead of the monomer Alternatively, a poly-p-aminostyrene can be diazotized in a polymer-analogous reaction and reacted with the cyanoester.

In the finished ribbon, the dye is released when heated to approx. 120 ° C and transferred to the paper. It is particularly advantageous that, when the azo group decays, heat is released, which reduces the amount of energy required to decay or trigger the printing process.

Claims (10)

1. A ribbon for a transfer printing process in which dye and optionally other ink constituents, for instance binder, are transferred from a carrier film to a material which is to be printed, in particular paper, by selective action with radiation and/or heat, characterised in that on the carrier film at least the dye is bonded on or in chemically labile manner in a carrier phase which chemically decomposes under the action of radiation and/or heat and thereby chemically releases at least the dye, said dye being converted into a mobile phase and being transferred to the material to be printed.
2. A ribbon according to Claim 1, characterized in that in the carrier phase at least one ink constituent which can be released by radiation and/or heat or the dye is bonded by means of a labile group, upon the decomposition of which additionally a gaseous compound is released.
3. A ribbon according to claim 1 or 2, characterised in that at least the dye is bonded in chemically labile manner to a carrier polymer.
4. A ribbon according to Claim 3, characterised in that the carrier polymer at the same time represents the carrier film of the ribbon.
5. A ribbon according to one of Claims 1 to 4, characterised in that upon the chemical decomposition of the carrier phase at least one ink constituent which is liquid under the conditions given is released, which represents a solvent for the other ink constituents which are to be transferred.
6. A ribbon according to at least one of Claims 1 to 5, characterised in that the carrier phase is a polymer having a low depolymerisation temperature, which is obtained by polymerisation of monomers of a plastic in the presence of ink constituents at least comprising the dye.
7. A ribbon according to at least one of Claims 1 to 6, characterised in that the carrier phase contains a labile group which is an azo or carboxylate group.
8. A ribbon according to at least one of Claims 1 to 7, characterised in that two dyes which can be released at different temperatures are provided in the carrier phase.
9. A ribbon according to at least one of Claims 1 to 8, characterised in that an additional thermolabile compound is provided on the carrier film which upon decomposition releases at least one gas.
10. A ribbon according to Claim 9, characterised in that the additional thermolabile compound is azodicarbonamide.