EP0304672A1 - Process for manufacture of a thermally sensitive ribbon for use in thermal transfer printing and the printing ribbon obtained therefrom - Google Patents

Abstract

Process for the manufacture of a thermally sensitive ink ribbon for thermal transfer printing with a layer of a fusible ink on one side of a base film, where the fusible ink contains a wax and/or a wax-like substance, a colorant, a thermoplastic binder and, if desired, other additives, wherein an aqueous coating dispersion which contains the thermoplastic binder, the wax or wax-like substance and a fat-soluble dye in finely divided form is applied by a known method to the base film of the thermally sensitive ink ribbon, the aqueous portion of the dispersion is evaporated away and the wax or wax-like substance is melted by subsequent thermal treatment. The advantage of this process is that it can be carried out without environmentally disadvantageous solvents and gives to an end product which can be overwritten 5 to 15 times without deterioration of the print quality.

Description

The invention relates to a method for producing a thermal ink ribbon for thermal transfer printing with a layer of a melting ink on one side of a carrier film, the melting ink containing a wax and / or a wax-like substance, a colorant, a thermoplastic binder and optionally further additives, and one then available product.

Thermal ribbons have been known for a long time. They point to a film-like carrier which, for. B. can consist of paper or plastic, a layer of a melting ink, so in the form of a meltable wax-bound or plastic-bound colorant or carbon black layer. In these thermal ribbons, the melting ink is melted by means of a thermal print head and transferred to a recording paper or film. This is generally referred to as a thermal transfer ribbon or TCR ribbon ("Thermal Carbon Ribbon"). Thermal printers that emboss a heat symbol during the printing process are e.g. B. from DE-ASen 2 062 494 and 2 406 613 and DE-OS 3 224 445 known. The printing process is as follows:

The print head of a thermal printer presses the thermal ink ribbon onto the recording substrate. The printhead develops temperatures that can reach a maximum of around 400 ° C. The uncoated back of the thermal ribbon or the film-like carrier is in direct contact with the printhead or the thermal symbol formed thereon during the printing process. At the time of the actual printing process, the relative speed between the thermal ink ribbon and the printing paper or film is zero. Through the action of the print symbol, the melting ink in the form of the symbol to be printed is transferred from the thermal ink ribbon to the recording substrate by a melting process. When the thermal ink ribbon is detached from the recording substrate, the molten symbol adheres to it and solidifies.

In addition to the above-described thermal ribbons with simple film-like carriers, there are also those thermal ribbons in which the heat symbol is not generated by a thermal print head but by resistance heating of a specially designed film-like carrier. The melting color, which represents the actual "functional layer" during the printing process, also contains the materials already described above. In professional circles this is called an electro-thermal transfer process ("Electro Thermal Ribbon"). Such a thermal transfer printing system is described, for example, in US Pat. No. 4,309,117.

There are already known thermal ribbons that advertise several times (keyword: "multiuse"). Such thermal ribbons are described for example in EP-AO 063 000. The melting ink of the thermal ink ribbon is then a particulate material which is insoluble in the solvent of the coating liquid and does not melt below 100 ° C, and incorporates another particulate material with a melting point between 40 and 100 ° C. The particulate material not melting below 100 ° C should preferably be a metal oxide, a metal, an organic resin or carbon black.

This special particulate material is intended to give the layer of the melting ink, which is a solid mixture, a heterogeneous structure which can only consume a small amount of the molten colored material to be transferred in each individual printing operation.

The known methods for producing the above thermal ribbons show u. a. the main disadvantage that they rely on the use of environmentally harmful solvents. DE-OS 36 23 467 describes a method for thermal transfer recording, in which the thermal ribbon also does without the use of environmentally harmful solvents. However, it is only suitable for simple printing and not for the so-called "multiuse".

The invention was therefore based on the object of developing the method described at the outset in such a way that it precludes the need to use environmentally harmful solvents in its preparation.

According to the invention, this object is achieved in that an aqueous coating dispersion which contains the thermoplastic binder, the wax or the wax-like substance and a fat-soluble dye in finely divided form is applied in a manner known per se to the support of the thermal ribbon, the aqueous portion of the Dispersion evaporated and a thermal treatment is carried out with melting of the wax or the wax-like substance.

The essence of the invention is therefore that an aqueous coating dispersion containing the necessary fine-particle solids is applied to the carrier film and the aqueous portion of the dispersion evaporates below the melting point of the integrated wax particles or particles of the wax-like substance and these wax particles or particles of wax-like substance can be fused by thermal treatment. A technological explanation follows later.

For the purposes of the invention, any plastic films are suitable, which are also used as supports for conventional carbon ribbons of typewriters, but which also withstand the high temperatures mentioned during the briefly running printing process and also release the melting ink at these temperatures. The plastic film consists in particular of thermoplastic plastics with a higher glass transition temperature. The focus here is on the following materials: polyesters used in the prior art, in particular polyethylene terephthalates, polycarbonates, polyamides, polyvinyl compounds, in particular polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol and polyvinyl propionate, polyethylene, polypropylene and polystyrene. Among these materials, polyethylene terephthalates and polycarbonates are preferred.

The plastic film to be used according to the invention can also be a fabric laminated on one or both sides. Of course, similarly designed common composite films can be used within the scope of the invention.

In individual cases, it is advantageous to incorporate a plasticizer into the plastic carrier chosen in order to achieve improved flexibility. Furthermore, a substance that increases the thermal conductivity can also be incorporated. The thickness of the plastic film is determined according to the respective requirements. As a rule, however, it is relatively thin, e.g. B. about 3 to 6 microns to allow the necessary heat transfers to run optimally. This range can also be exceeded or fallen below to a greater or lesser extent.

The term "wax" used in connection with the invention is to be understood as far as possible. Such a material should generally have the following properties: not kneadable at 20 ° C, solid to brittle-hard, coarse to fine crystalline, translucent to opaque, but not glassy, meltable above 40 ° C without decomposition, but a little above the melting point relatively low-viscosity and non-stringy.

In the context of the invention, “wax-like substances” should be understood to mean those materials which largely resemble the waxes with regard to the physical and chemical properties. When carrying out the process according to the invention, it has been shown that the melting point of the wax or wax-like substance chosen in each case is preferably at least about 70.degree. The upper limit is preferably about 90 ° C.

The aqueous coating dispersion contains the mentioned solid particles, i.e. H. the thermoplastic, the wax or the wax-like substance and the fat-soluble dye and the pigment, preferably in a particle size of 0.5 to 100 micrometers and in particular of about 5 to 50 micrometers. Particularly good process products are obtained in this particle size range. The aqueous coating dispersion or suspension of these materials can be prepared in various ways. This can be done, for example, by suspending fine, solid particles of these materials or else by emulsifying in the melt and subsequent cooling, preferably with stirring to maintain the finest dispersion.

An essential component of the layer of the melting ink to be formed according to the invention is a thermoplastic. Thermoplastics are hard or even brittle plastics at normal temperature, which soften reversibly when heat is applied and become mechanically easily deformable in order to finally change to a viscous liquid state at high temperatures.

They go through a softening or melting range. Within the scope of the invention, preference is given to using thermoplastics which do not melt during the final thermal treatment, or which at most soften at most. Taking this requirement into account, it is easily possible for the person skilled in the art to select suitable thermoplastic binders. These include in particular polystyrene, polyvinyl acetate, polyvinyl acetal, polyvinyl chloride, polyethylene, copolymers of vinyl acetate and vinyl chlorides, polyvinyl ether, polyvinyl propionates, polyacrylates, ethylene / vinyl acetate copolymers.

The thermoplastic binders serve as a framework substance in the layer of the melting ink designed according to the invention. In order to control the hardness of this structural substance in the finished melting ink, suitable known plasticizers can also be incorporated into the thermoplastic binder. B. phthalic acid esters, such as di-2-ethylhexyl phthalate, di-isononyl phthalate and di-isodecyl phthalate, aliphatic dicarboxylic acid esters, such as those of adipic acid, in particular di-2-ethylhexyl adipate and diisodecyl adipate, phosphates, such as tricresyl phosphate and triphenyl phosphate, such as diethylene glycol, (2-ethyl butyrate) and the like. In individual cases it can also be advantageous to incorporate stabilizers into the thermoplastic binder.

The ratio of wax or wax-like substance to the thermoplastic binder in the aqueous coating dispersion can vary widely and is not critical for the purposes of the invention. The weight ratio can easily be between about 10: 1 to 1: 5. A weight ratio of approximately 5: 1 to 1: 1 is preferred.

The solids content of the coating dispersion or starting dispersion originally used can also vary within wide limits, for example. B. between about 20 and 80 wt .-%, preferably between about 30 and 60 wt .-%.

It is important for the successful practice of the invention that a special dye is used which is soluble in the wax or the wax-like substance. The so-called fat-soluble dyes or "fat dyes" meet this requirement. This class includes, for example, simply structured azo and anthraquinone dyes. B. the products sold by Bayer AG under the name "Ceres-Farstoffe". This also includes, in particular, the dyes Solvent Yellow 16, Solvent Yellow 29, Solvent Yellow 14, Solvent Red 1, Solvent Red 18, Solvent Red 25, Solvent Red 24, Solvent Red 19, Smoke Dye and those specified according to the Color Index Part I Solvent Blue 63, Solvent Blue 68, Solvent Green, Solvent Brown 1, Solvent Red 3, Solvent Green 3 and Solvent Black 3. This list is not intended to be limiting. Other suitable fat-soluble dyes can be found in the specialist literature. In addition to the dyes, pigments can also be used, such as carbon blacks, organic and / or inorganic color pigments, but also so-called fillers such as chalk, china clay, kaolin, clay, etc.

The advantages sought by the invention are achieved in particular when water is the only dispersing agent in the aqueous coating dispersion to be applied. Demzulfoge should not contain other polar and / or non-polar organic solvents if possible. In individual cases, however, it can also be harmless if, for example, small amounts of these solvents are present in the aqueous medium, for example small amounts of ethanol.

The aqueous coating dispersion can be applied to the support in any manner, for example with a doctor knife. The application technology is therefore not critical. You can also be evaporated or concentrated in any way, for. B. by transferring warm air.

However, the temperature during evaporation or concentration of the aqueous portion of the applied aqueous coating dispersion should preferably be chosen only so high that the wax particles or the particles of the wax-like substance are preferably not melted during the thermal treatment. In general, work can also be carried out at room temperature, whereby the thermal treatment or the passage of the air would require a longer time.

After the aqueous portion of the applied coating dispersion has evaporated, there follows a thermal treatment of the product obtained, which is at or above the melting temperature of the wax or the wax-like substance. As a result, the wax or the wax-like material is fused by this thermal treatment. This thermal treatment can be carried out using customary measures, for example by treatment with heated rollers, exposure to warm air and thermal radiation. Through this heat treatment, the fat-soluble dye is transferred into the phase of the fused wax or the fused wax-like substance. A particularly high color yield is therefore achieved when using the thermal ink ribbon in conventional writing systems.

The layer thickness of the melting ink should generally be between about 5 and 30 micrometers and preferably 10 to 20 micrometers (dry layer). If appropriate, an adhesion-promoting layer with a thickness of approximately 0.1 to 5 micrometers, preferably approximately 0.5 to 2 micrometers, can be arranged between the color layer and the carrier film. This preferably consists of polymeric materials of a known type.

The invention will be explained in more detail below with the aid of examples.

example 1

An aqueous dispersion was prepared using the following recipe: Ethylene-vinyl acetate copolymer (Ehaflex 222-Akzo Chemie) 12 parts by weight solid wax ester (Loxiol G 32 / Henkel) 10 " Streichclay (Colloid Clay Supreme / English China Clay Sales Co) 2.5 " Disazo dye solvent black 3 (Ceress black BN / Bayer AG) 0.5 " distilled water 15 " Silicone defoamer (contrafomit / Baumheier chemical factory) 1 "

The dispersion, which made up a total of 41 parts by weight, was applied by means of a doctor blade in a layer thickness of approximately 20 micrometers (based on the product which was subsequently dried) to a polyester support. By passing warm air at a temperature of 80 ° C, the aqueous portion of the applied dispersion was evaporated within a few minutes. A thermal treatment was then carried out with air at a temperature of about 100 ° C., the wax particles fusing together. The product obtained could be used directly as a thermal ribbon. It allowed an 8x multiuse.

Example 2

A thermal ribbon was produced using the following recipe: Ethylene-vinyl acetate interpolymer (Adkote 37 R 610 / Morton Chemie) 12 parts by weight Erucic acid amide (Loxamide E / Henkel) 6 " Carbon black (Regal 400 R / Cabot Corp.) 2.5 " Disazo dye solvent Red 18 (Ceresrot 3R / Bayer AG) 0.05 " Disazo dye solvent Black 3 (Neptune Black X 60 / BASF) 0.45 " distilled water 28 "

The 49 parts by weight of the dispersion obtained were applied to the support of a thermal ink ribbon in the manner described in Example 1 and dried. This is followed by a thermal treatment with hot air at a temperature of 100 ° C. to fuse the wax particles. The process product obtained also enabled an 8-fold multiuse.

Claims (6)

1. A method for producing a thermal ribbon for thermal transfer printing with a layer of a melting ink on one side of a carrier film, wherein the melting ink contains a wax and / or a wax-like substance, a colorant, a thermoplastic binder and optionally other additives, characterized in that an aqueous coating dispersion containing the thermoplastic binder, the wax or the wax-like substance and a fat-soluble dye in finely divided form, applied in a known manner to the support of the thermal ribbon, the aqueous portion of the dispersion evaporated and melting the wax or a thermal treatment is carried out on the wax-like substance. 2. The method according to claim 1, characterized in that the evaporation of the aqueous portion of the coating dispersion is carried out below the melting point of the wax or the wax-like substance. 3. The method according to claim 1 or 2, characterized in that the aqueous dispersant is free of further polar and / or non-polar solvents. 4. The method according to any one of claims 1 to 3, characterized in that the evaporation of the aqueous portion of the coating dispersion is carried out by passing warm air. 5. The method according to any one of claims 1 to 4, characterized in that the particle size of the solids contained in the aqueous coating dispersion in the form of the thermoplastic binder, the wax or the wax-like substance and the fat-soluble dye is about 0.5 to 10 microns. 6. Multiple rewritable thermal ribbon, obtainable by the method according to at least one of the preceding claims.