Thermal Transfer Ribbon Especially for Impressions on Rough Paper

Our present invention relates to a thermal transfer ribbon, also referred to as a thermal carbon ribbon in the art, for the thermal transfer of alphanumeric and other characters to a copy sheet, e.g. of paper, by partially melting a color-transfer layer on a substrate or support. The invention also relates to a method of making such a ribbon.

It is known that various polymeric materials, upon being dissolved in a solvent, are mutually incompatible. This incompatibility can have a variety of effects. For example, one polymer can flocculate out.

Alternatively, in certain systems, there is the possibility that both polymers will remain in solution but that there will be a liquid/liquid phase separation with mutually incompatible polymeric materials distributing themselves in different concentrations in the two phases.

Basic principles of such liquid/liquid phase separations and the relationship of the mutual incompatibility of the two polymers can be found in Dobry and Boyer-Kawenoki, Journal of Polymer Science, Vol. 2, No. 1, Pages 90-100, 1947.

Since the generation of such a system requires that the characteristics of three materials, namely, the two polymeric materials and the solvent, be taken into account, there has been no generalized technological use of such systems.

It is, however, possible, utilizing simple laboratory techniques, to determine suitable partners for the development of a liquid/liquid phase separation system. This will be evident from German patent document DE-OS No. 20 30 604 which describes a process for the formation of small polymer capsules.

According to the process of German patent document DE-OS No. 20 30 604, a liquid system of at least a first and a second hydrophobic polymer is formed in an organic solvent. The polymeric materials are so selected that they have, below a certain critical temperature, a homogeneous single-phase solution in the solvent. Above the critical temperature, a liquid/liquid phase separation occurs. In this case, particles of the core-forming material are dispersed in the liquid system. The system is agitated while its temperature is above the critical temperature to effect a wetting and coating of the dispersed particles of the core-forming substance by the separating phase to provide a liquid wall on each core particle. The capsules are thus grown in the liquid.

Preferably the first polymer is a halogenated rubber and the second polymer a polyethylenevinylacetate copolymer. The organic solvent is advantageously cyclohexane, toluene, xylene, carbon tetrachloride or methylisopropylketone. A similar process is described in German patent No. 1,212,497. From these disclosures, therefore, it will be evident that it is known to use such liquid/liquid phase-separated systems to produce microcapsules.

The Japanese patent publication JP 60-78 777A describes a thermocarbon ribbon which has an intermediate layer of its meltable transfer layer thermally expandable minute spheres or balls upon which the color layer is applied. This material has been found to have good printing and transfer quality even for rough papers.

However, this material is not suitable for multi-use (multi-strike) purposes and because it is necessary to provide a separate layer containing the microscopic balls, the cost of fabricating the material is high.

German patent No. 12 01 855 describes a carbon ribbon which has small droplets or microscopic balls and nonvolatile materials, a pigment or a dyestuff, distributed in a resin. This ribbon also is not suitable for multi-use or multi-strike purposes in a thermal transfer system.

It is the principal object of the present invention to provide a thermocarbon ribbon with a synthetic resin bonded melt-transfer color on a carrier which can be fabricated in a simple and economical manner, has especially good resolution and print quality on rough papers and has multi-use or multi-strike capabilities.

Another object of this invention is to provide an improved method of making such a thermocarbon ribbon.

These objects are attained, in accordance with the invention by providing a melt-transfer color layer for a thermal printing process utilizing a conventional thermal printing head, e.g. of the dot-matrix type, which contains meltable solid polymer spheres or balls, in a dispersed phases generally referred to as beads, in a continuous solid phase consisting essentially of at least one other polymer, a wax and/or a waxlike substance which is also meltable during the thermal printing process.

The method of the invention is carried out by providing in solution in a solvent the two different, mutually incompatible thermoplastic polymers which melt during the thermoprinting process to form a two-phase liquid/liquid phase separation system in the solution. The liquid/liquid phase separated system is then agitated while a coloring agent or pigment and a wax are supplied. The resulting color emulsion is applied to a carrier or support for a thermal carbon ribbon and the solvent of the emulsion is evaporated to form the solid color-transfer layer.

Hereinafter the word "pigment" may be used to refer to any coloring agent which can include carbon, particulate pigments or dyestuffs.

When a "wax" or a "waxlike" material is referred to in accordance with the invention, it will be understood that this material should be, at a temperature of 20.degree. C., not kneadable or plastically deformable, solid to brittlely hard, large crystalline to fine crystalline and transparent to opaque, but not glassy. Above 40.degree. C., the material should melt without decomposing and should have a low viscosity at a temperature close to but above its melting point, while being non-ropey, i.e. capable of melting in a manner which does not form strings, strands or other ropey structure.

Waxlike materials within the invention include those which fall into the category above but can have physical and chemical characteristics largely similar to those of waxes.

The wax materials which can be used according to the invention include, without limitation, paraffins, silicones, natural waxes such as carnauba wax, beeswax, ozocerite and paraffin wax, synthetic waxes such as acid waxes, ester waxes, partly saponified ester waxes, polyethylene waxes, as well as polyglycols. In practical terms, the ester waxes have been found to be most suitable and especially the waxes commercially marketed as Hoechstwachs E and E-wax marketed by BASF and derived from montan wax.

The solvent which can be used to make the color emulsion of the invention can be any solvent in which the two mutually incompatible and mutually nonmiscible thermoplastic polymers are soluble and which also can dissolve the wax or the waxlike material.

Especially aromatic and chlorinated solvents such as toluene, xylene, tetrahydronaphthalene, chlorinated hydrocarbons such as trichloroethane, trichlorethylene, carbon tetrachloride, perchlorethylene and mixtures thereof have been found to be useful. Other nonaromatic solvents such as ethanol, ethylacetate and methylethylketone can be mixed with the solvents previously mentioned, especially the aromatic solvents.

Other solvents may, of course, be used as long as they have equivalent properties, but in general the limitations on the solvents will be those posed by excessively high boiling points or toxicity. For these latter reasons we have found toluene to be best.

It is not the purpose of this application to generalize as to the chemical and physical parameters required for the mutual incompatibility and nonmiscibility of the two thermoplastic polymers, especially in the presence of a wax component, in the solution. As has already been stated, the ordinary worker in the art using simple tests can very readily determine mutually incompatible polymer pairs. By way of guidance, we note that the mutually incompatible polymers may be selected one from one group of groups I and II, while the other is selected from the other group. Group I consists of polystyrene (for example the commercial product polystyrene 143E of BASF AG), polyacrylates, polymethacrolates, polyamides, acrylonitrile-styrene copolymers, vinylidene-chloride-acrylonitrile copolymers, ethoxy resins and polyvinylformal. The polymers of Group II are ethylene-vinylacetate copolymer, polyvinylether, polyvinylester and polybutadiene.

The pigment or coloring agent can be any coloring agent which is commonly used in the melt-transfer color of thermocarbon ribbons. These agents include dyestuffs as well as solid pigments. The pigments can be carbon black or phthalocyanine. Dyestuffs include especially the azo dyes. True pigments are generally defined as coloring agents which are not soluble even in the binder while the dyestuff is soluble in the solvent and/or the binder (see Rompps Chemie-Lexikon, 8. Aufl. D 1.2, 1981, S. 1239).

Since this difference is not material for the present invention, where the term "pigment" is used in the claims, it will be understood to mean dyestuffs and other coloring agents which can be used for the purpose of the invention, including the insoluble pigments.

The pigment or coloring agent, whether dissolved or suspended, serves merely to provide a colored emulsion and in general will be present in both the beads and the continuous phase in which the beads are dispersed. The simultaneous presence of suspended and dissolved coloring agent can be ensured when a dyestuff is used in a supersaturated solution. The manner in which the coloring agent distributes itself in the beads and in the continuous phase will depend upon the type of coloring agent.

It has been found to be advantageous in forming the emulsion to supply additives to the system which promote emulsification, or otherwise improve the properties of the thermocarbon ribbon. We have found that especially emulsifiers and/or softeners for the polymers (i.e. the polymer from Group I and the complementary polymer of Group II are desirable additives). The softeners or plasticizers which are used can preferably be fatty acids, fatty acid esters and esters of phthalic acid andphtalic acid such as dioctylphthalate and tricresylphosphate. The emulsifiers such as the waxy emulsifier OSN (BASF), Emulan AF (BASF) and other Emulans.

The proportions of the substance forming the color-transfer layer can range widely and there are no critical limits, especially for the solvent proportion.

Too great excess of the solvent should be avoided because the evaporation of the solvent will then utilize too much energy.

As a rule the emulsion should contain from 5 to 30 parts by weight of the polymer of Group I, from 5 to 30 parts by weight of the polymer of Group II, from 10 to 40 parts by weight of the wax or the waxlike material and at least 60 parts by weight of the solvent, but in any event sufficient solvent to form the liquid/liquid phase-separation system.

When the polymer from Group I is polystyrene, polyacrylate or polyamide and the polymer from Group II is an ethylene-vinyl acetate copolymer, polyvinyl ether and/or polyvinyl ester, preferably 10 to 20 parts by weight of the polymer of Group I is used with 20 to 30 parts by weight of the polymer of Group II and 30 to 40 parts by weight of a wax, especially an ester wax. The solvent should be present in an amount of up to 400 parts by weight and in an amount of at least 9 parts by weight. Best results are obtained with 15 parts by weight of the polymer from Group I, 25 parts by weight of the polymer from Group II, 35 parts by weight of wax and at least 20 parts by weight and preferably 300 parts by weight of a solvent.

The amount of the coloring agent which is used will, of course, vary depending upon the desired intensity of the print to be transferred by the thermocolor layer. For satisfactory color intensities of the transferred image, the coloring agent should be present in an amount of from 5 to 30% by weight of the solidified melt-transfer color layer although the amount actually used can also deviate therefrom to the degree to which one desires a multi-use or multi-strike capability. The more uses or strikes that the ribbon must sustain, the greater will be the amount of the coloring agent to be supplied. Of course the color intensity will also depend upon the coloring agent which has been selected.

With respect to the amount of the solvent used, we note that in all cases it is important that the minimum amount of solvent be sufficient to dissolve both the polymer from Group I and the complementary polymer from Group II as well as the wax. From this minimum amount, one can deviate significantly to obtain the ideal coating characteristics of the emulsion on the foil forming the substrate.

Excess solvent, of course, results in an increase in the evaporation energy which must be supplied.

As a rule, the optimum solvent quantity will be two to three times the amount of the multiple solid substances which are to remain after the solvent has been evaporated. The other additives mentioned play a supporting role in this respect since they are usually present in an amount of 1 to 5% by weight of the solid melt-transfer layer.

The emulsion can be applied in a simple way to the carrier. Preferably it is applied by a doctor blade, a so-called "coater" and like apparatus commonly used for applying the color-transfer layer to a support strip of the ribbon.

The evaporation of the solvent from the emulsion is effected preferably by passing over it heated air at a temperature of about 60.degree. to 80.degree. C. Preferably the entire process is carried out in a continuous manner.

The carrier or support can be any ribbon support which has been found to be useful in thermocarbon ribbons up to now. Where multi-use or multi-strike porperties are desired, the ribbon is a synthetic resin foil and polyester or polycarbonate which has a thickness between 4 and 10 micrometers.

The thermocarbon ribbon of the invention has been found to have numerous advantages. Firstly, it has multi-strike capabilities allowing the same region of te ribbon to be typed over from 5 to 30 times. By contrast with conventional thermocarbon ribbons, the ribbon of the invention has especially good rendition and resolution in printing on rough paper. A sharply delineated print can be obtained to a paper roughness of 20 Bekksecs. Apparently the dispersed polymer microballs or beads, with other times of 5 to 100 micrometers, improve the partial and metered transfer of the color layer and thus contribute to the multi-use effect. Of course the process can be carried out quite simply. The components are readily transformed into a coating emulsion by the use of a simple mixer.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing, the sole FIGURE of which is a cross sectional view diagrammatically illustrating a thermal transfer ribbon of the invention.

In the drawing, the thermal transfer ribbon 1 is shown to comprise a carrier 2 upon which a color-transfer layer 3 is provided which contains the polymer balls or beads 4 dispersed in a continuous solid polymer phase 5.

The following example illustrates the application of the invention:

The materials are transformed into an emulsion in a conventional stirrer. The emulsion is then milled for 10 minutes in a ball mill to improve the distribution of the color pigment. The resulting emulsion is applied by a doctor blade in a thickness of 9 micrometers to a polyester support foil of a thickness of 6 micrometers. Air heated to 80.degree. C. is then passed over the coated foil to evaporate the solvent. The resulting solidified color-transfer layer was found to be useful for a minimum of five strikes utilizing a conventional thermal printing head on rough paper .