EP0785086A1 - Thermal transfer ribbon - Google Patents

Abstract

Thermal transfer ribbon has a layer (A) between a normal base and a layer (B) of thermal transfer ink with wax binder. Layer (A) is a resin-bound release layer for layer (B). Layer (B) contains wax with a melting point of 70-110 degrees C. A wax-soluble polymer is finely dispersed in both layers (A) and (B).

Description

The invention relates to a thermal transfer ribbon with a conventional carrier, a wax-bound layer of a thermal transfer ink formed on one side of the carrier and a further layer located between the carrier and the wax-bound layer.

Thermal transfer ribbons have been known for a long time. On a film-like carrier, for example made of paper, a plastic or the like, they have a thermal transfer color, in particular in the form of a plastic and / or wax-bound colorant or carbon black layer. In thermal printing technology, the thermal transfer ink is softened by means of a thermal print head and transferred to a recording paper or a printing paper. Thermal printers or thermal print heads that can be used for this process are known for example from DE-ASen 20 62 494 and 24 06 613 and DE-OS 32 24 445. In particular, e.g. as follows: On the thermal print head of the printer, a letter consisting of heated dots and to be printed on a paper sheet is formed. The thermal print head presses the thermal transfer ribbon onto a paper to be written on. The heated letter of the thermal print head with a temperature of up to about 400 ° C leads to the fact that the thermal transfer ink softens at the heated point and is transferred to the paper sheet in contact with it. The used part of the thermal transfer ribbon is then fed to a spool.

The thermal transfer ribbon can have different thermal transfer colors side by side. With the combination of the basic colors blue, yellow and red, colored print images can be produced. Compared to the usual color photography, there is no disadvantageous development and fixing. Thermal printers can be operated at high writing speeds and without annoying background noises. For example, a DIN A4 sheet can be printed in about 10 seconds.

So-called serial printers or line printers can be used for printing. The serial printers work with a relatively small movable print head approx. 1 cm ² . There are 1 or 2 dot rows perpendicular to the writing direction (dot = controllable heating point). The dot diameter is between about 0.05 to 0.25 mm. The number of dots per row of dots is between 6 and 64, which corresponds to a resolution of 2 to 16 dots / mm. Higher resolutions, eg 24 to 32 dots / mm, are expected in the near future. It is characteristic of the serial thermal head that it is moved horizontally to the transport direction of the paper during the printing process. In contrast to the serial print head, a line print head is a stationary head or a bar. Since the print bar is not movable, it must span the width of the substrate to be printed. Print bars are available in lengths of up to 297 mm. The resolution and dot size correspond to those of serial heads. The serial printers are used in particular in typewriters, video printouts, in the PC area, as well as in word processors and line printers, in particular in the case of barcode graphic printers, in the case of a computer output unit in the event of a large amount of data, in the facsimile, ticket printer, address printer, color copier and CAD areas / CAM systems used.

In addition to the thermal transfer ribbons described above, there are also those in which the heat symbol is not embossed by the action of a thermal print head, but by resistance heating of a specially designed film-like carrier. Resistance heating takes place in that the thermal transfer ink and / or its carrier contain electrically conductive materials. The thermal transfer ink, which is the actual "functional layer" during the printing process, also contains the materials already described above. This is also known as an ETR material ("Electro Thermal Ribbon"). A corresponding thermal transfer printing system is described, for example, in US Pat. No. 4,309,117.

In the systems of thermal transfer ribbons described above, the font sharpness and the optical density of the font produced depend, inter alia, on the adhesion of the thermal transfer ink to the paper. This is proportional to the adhesive surface and the adhesive force. Rough paper has a small adhesive area, since only the raised parts of the paper surface are wetted by the melted thermal transfer ink. In DE-A-35 07 097, a so-called "Filling-Läyer" is therefore formed on the layer of thermal transfer ink, which consists of a low-viscosity material in the molten state, which flows into the valleys of the rough paper surface during printing and so on the adhesive surface increases. The disadvantage here is that the melted filling layer with very smooth paper with a roughness of more than 200 Bekk can no longer penetrate the paper during the printing process, so that between Paper surface and layer of paint remain a layer. This layer therefore has the effect of a hold-off layer, as described in EP-A-0 042 954. However, this hold-off layer leads to poor document authenticity, since it prevents the thermal transfer ink from penetrating into the paper. A hold-off layer effect is undesirable for a document-correct layer.

In order to avoid the above-mentioned disadvantages of the prior art, in particular to advantageously enable writing on rough as well as smooth paper, EP-B-0 348 661 proposes that the hold-off layer or adhesive layer, also as Topcoat means incorporating a tackifying hydrocarbon resin embedded in a paraffin in finely divided form, the paraffin having a melting point of 60 to 95 ° C. The teaching according to EP 0 206 036 tries to avoid the need for such an adhesive layer or a top coat by forming a wax layer on the layer of a plastic-bound thermal transfer ink and the plastic-bound thermal transfer ink containing a thermoplastic plastic with a softening point of 60 to 140 ° C.

Although the above-described thermal transfer ribbons of the prior art are notable for advantages, it always means a procedural disadvantage to form a further layer on the actual layer of thermal transfer ink for the reasons mentioned above.

JP-1-38271 A discloses a thermal transfer ribbon in which a heat-resistant carrier is successively coated with a first color layer and a second color layer. The first color layer contains a color material and a wax-compatible thermoplastic resin. The second color layer consists of a continuous phase of wax-incompatible thermoplastic resin, in which a phase of wax and a wax-compatible thermoplastic resin is dispersed. A color material is finely dispersed in both phases. Both layers of color are transferred during the printing process.

JP-1-196380 A describes a thermal transfer ribbon with a support, an intermediate layer and a color layer. The intermediate layer consists of polyester resin, polyamide resin and carbon black. The color layer consists of a powder of soot particles coated with a water-soluble resin and a matrix component, the matrix component consisting of a colorant and wax. The water-soluble resin as an enveloping component for the in the Soot particles contained in the colored layer are evidently not soluble in wax due to their hydrophilic nature.

As a further problem, it has been found that the above-mentioned thermal transfer ribbons are unsatisfactory in individual cases with regard to the requirement for matt color printing on the substrate to be printed.

The invention was therefore based on the object of developing the thermal transfer ribbon described at the outset in such a way that the need for the formation of a top coat or a two-layer thermal transfer ink is not necessary and satisfactorily matte prints are obtained during the thermal printing process.

According to the invention, this object is achieved in that the further layer is a resin-bound separating layer A) for the wax-bound layer B) of the thermal transfer ink, the waxes of the wax-bound layer have a melting point of about 70 to 110 ° C. and in both layers A) and B) a wax-soluble polymer is finely dispersed.

A separating layer or release layer is understood in the present technical field to mean a layer which controls the delivery of the thermal transfer ink to the receiving substrate during the printing process, but is not itself transferred to the substrate. A separating layer does not melt during the printing process, but softens at most and also has high adhesion to the carrier.

The waxes used in layer B) within the scope of the invention follow the usual wax definition with the above restriction of the melting point to approximately 70 to 110 ° C. Waxes with a melting point of 75 to 90 ° C. are particularly preferred in the context of the invention. In the broadest sense, it is a material that is solid to brittle, hard, coarse to fine crystalline, translucent to opaque, but not glassy, melts above approx. 70 ° C, but is relatively low-viscosity and not stringy just above the melting point. Waxes of this type can be assigned to natural waxes, chemically modified waxes and synthetic waxes. Vegetable waxes in the form of carnauba wax, candelilla wax, mineral waxes in the form of higher-melting ceresin and higher-melting ozokerite (earth wax), petrochemical waxes, such as, for example, petrolatum, paraffin waxes and microwaxes, are particularly preferred among the natural waxes. Among the chemically modified waxes are especially montan ester waxes, hydrogenated castor oil and hydrogenated jojoba oil is preferred. Among the synthetic waxes, polyalkylene waxes and polyethylene glycol waxes and products made therefrom by oxidation and / or esterification are preferred. Amide waxes can also be used. Specifically, the following are particularly preferred: modified microcrystalline waxes.

The framework of the melting point to be observed according to the invention for the waxes used is critical. If the temperature falls below 70 ° C, this means that the mechanical anchoring is insufficient and thus color transfer and color resolution are unsatisfactory. Melting points higher than 110 ° C disadvantageously lead to increased energy expenditure during the printing process.

Preferably, "narrowly cut" waxes are used among the waxes used according to the invention, the melting and solidification points of which are close to one another. The temperature difference between the melting point and the solidification point is preferably less than approximately 10 ° C., in particular less than approximately 7 ° C. and very particularly preferably less than approximately 5 ° C. A good example of this is carnauba wax, which has a melting point of around 85 ° C and a solidification point of around 78 ° C. The indicated waxes lead to a desirable low cohesion of the thermal transfer ink during the printing process.

A variety of additives can be incorporated into the wax materials of the wax-bonded thermal transfer ink, such as, in particular, tackifiers in the form of terpene phenol resins (such as the commercial products Zonataclite 85 from Arizona Chemical) and hydrocarbon resins (such as the commercial products KW-Harz 61 B1 / 105 from VFT, Frankfurt).

The coloring can be done by any colorant. It can be pigments, such as, in particular, carbon black, but also solvent- and / or binder-soluble colorants, such as the commercial product Basoprint, organic color pigments and various azo dyes (Cerces and Sudan dyes). Carbon black is particularly suitable in the context of the present invention. The thermal transfer ink preferably contains the colorant, in particular color pigment, in an amount of about 10 to 20% by weight.

The thermal transfer color of the above-mentioned layer B) of the thermal transfer ribbon according to the invention, optionally with the additives described above, preferably has a viscosity determined with the Rheomat 30 rotary viscometer Rheograph (principle: rotary viscometer, see Bulletin T 304d-7605 from Contraves AG Zurich / CH) at a temperature of 100 ° C from about 50 to 200 mPa.s, in particular from 70 to 120 mPa.s. Falling below the value of about 50 mPa.s leads to blurring ("spreading"). If the value of 250 mPa.s is exceeded, the desired resolution may deteriorate.

A central feature of the thermal transfer ribbon according to the invention is that a wax-soluble polymer is contained in the two layers A) and B) discussed. “Wax-soluble” is understood here to mean that this polymer shows solubility in a liquid wax. These are not necessarily "real solutions", but mostly stable dispersions. As a result, when such a solution of the polymer in wax is cooled, no phase separation occurs or this polymer is compatible with the wax. The melt index MFI is 25 to 1000 g / 10 min, preferably 400 to 800 g / 10 min (DIN 53735 / ISO 1133, see also Römpp-Chemie Lexikon, Volume 5, 9th edition, p. 4036, r . Sp.). Wax-soluble polymers in the sense of the invention are distinguished by the fact that they are meltable below about 100 ° C. and show stickiness in the molten state. Suitable polymers are e.g. Ethylene-vinyl acetate copolymers, polyamides, ethylene-alkyl acrylate copolymer, ethylene-acrylic acid copolymers, polyvinyl ether, and polyisobutene and ionomer resins. Of these, ethylene-acrylic acid copolymers and ethylene-vinyl acetate copolymers (EVA) are particularly preferred.

The term "wax-soluble polymers" also includes those which show a certain stickiness even at room temperature, such as certain polyisobutenes with an oily, viscous to rubbery consistency. Products of this type are sold under the trade name Oppanol (BASF, Germany, cf. Römpp Chemie Lexikon 9th edition, vol. 4, p. 3121/3122). These wax-soluble polymers, which are sticky at room temperature, also include raw materials based on polyvinylethyl, methyl and isobutyl ether, which are sold under the trade name Lutonal (BASF, Germany, cf.Römpp-Chemie Lexikon, 9th edition, vol. 3 , P. 2566).

A special feature of the present invention is the incorporation of the wax-soluble polymer under discussion both in layer A) and in layer B). The wax-soluble polymers can be used individually or as a mixture with one another. The same or different wax-soluble polymers can be used in layer A) and in layer B). In layer A) the wax-soluble polymer is preferably in an amount of 10 to 60% by weight, in particular about 20 to 40% by weight and in layer B) in an amount of 2 to 20% by weight, in particular about 5 to 10% by weight. As can be seen, the proportion of the wax-soluble polymer in layer A), ie in the separating layer, is preferably higher than in layer B). The reason for this is that layer B) has a higher adhesion to layer A) and thus a better resolution is achieved.

The wax-soluble polymer in layer B) requires that the hard waxes used with particular preference in the context of the invention, in particular in the form of ester waxes, are plasticized and thus the brittleness or "splinter" is removed from the thermal transfer ink. Ester waxes are very hard or brittle waxes, i.e. they can be pulverized when cold. However, if these are mixed with the designated wax-soluble polymers, then elastic to highly elastic products are created which can no longer be pulverized.

The incorporation of the wax-soluble polymer in both layer A) and layer B) leads to a functional interaction of both layers during the printing process. On the one hand, the "releasability" of layer A) can be controlled, on the other hand, layer A) fulfills the function of a "matt layer". The matt layer plays a special role in the context of the invention. It leads to the fact that really matt prints are produced during the thermal printing process. This is due to the fact that during the printing process not only the thermal transfer ink becomes liquid and thus sticks to the substrate, in particular in the form of a paper acceptor, but also softens the separating layer and retains a noticeable adhesion to the ink layer, so that a completely flat transfer of, for example, printing symbols the paper acceptor is not possible. Rather, the surface of the printed symbols is roughened somewhat, so that the surface of the transmitted symbol appears matt as a result of light refraction / light diffusion.

The matting effect is further promoted if the layer B) contains a black pigment and the separating layer additionally contains carbon black, in particular in an amount of about 20 to 50% by weight, which leads to the fact that the thermal transfer tape written off offers adequate data protection. In this advantageous embodiment of the present invention, silica is preferably also incorporated into the separating layer. During the production of the layer, this means that the soot remains finely distributed in the layer and does not sediment out.

The thickness of layers A) and B) is not critical. Layer A) preferably has a thickness of approximately 0.2 to 5 μm, in particular approximately 1 to 3 μm, and layer B) a thickness of approximately 1.0 to 10 μm, in particular approximately 3 to 6 μm. Layer A) is a resin-bonded layer, the resin binder preferably being a solid resin with a softening range in the range from about 70 to 200 ° C. The resin is preferably an alkyd, epoxy, melamine, phenol, urethane and / or polyester or copolyester resin.

The carrier of the ribbon according to the invention is not critical. Polyethylene terephthalate (PETP) or capacitor papers are preferably used as the base film for thermal transfer ribbons. The selection parameters are the highest possible tensile elongation values and thermal stability with low film thicknesses. The PETP films are available down to about 2.5 µm, capacitor paper down to about 6 µm. During the printing process, the thermal print head reaches temperatures of up to 400 ° C, i.e. Temperatures that are above the softening point of PETP. When using PETP films, it is advisable to provide a layer that is particularly resistant to heat on the back of the film that comes into contact with the thermal head.

An advantageous further development of the idea according to the invention, in particular to achieve an advantageously sharp pressure, is based on the incorporation of the teaching of EP-B-0 133 638. Thereafter, a layer of a wax or a wax-like material is formed on the back of the carrier, in particular in a thickness of not more than about 1 micron and very particularly preferably in the form of a molecularly formed to about 0.01 micron thick layer. In this case, the coating material preferably consists of paraffin, silicone, natural waxes, in particular carnauba wax, beeswax, ozokerite and paraffin wax, synthetic waxes, in particular acid waxes, ester waxes, partially saponified ester waxes and polyethylene waxes, glycols or polyglycol, antistatic agents and / or surfactants. If such a coating on the back is provided, then there is an undisturbed heat transfer from the thermal print head to the thermal transfer ribbon, with the result that particularly neat prints are achieved.

The thermal transfer ribbon according to the invention described above can be produced in a variety of ways using customary application methods. This can be done, for example, by spraying on or printing on a solution or dispersion, be it with water or an organic solvent as the dispersion or solvent, by applying from the melt, which is particularly important for the wax-bound thermal transfer ink applies, or also by normal application by means of a doctor blade in the form of an aqueous suspension with the material to be applied finely divided therein. Concerning. From the environmental point of view, the following procedure has proven to be particularly advantageous: First, an aqueous suspension of the starting materials of the separating layer is applied to the support in a thin layer, which layer A) is formed when the water evaporates. After the formation of layer A), the application of an aqueous suspension of the starting material of the wax-bound thermal transfer dye follows, the water being evaporated off in the usual manner after the application of this material. The double-layer covering formed fulfills all the requirements that lie within the scope of the task. However, the thermal transfer ink can also be applied to the separating layer in the form of a melt using customary application technologies, for example using a doctor blade. The temperature of the respective melt should generally be about 100 to 130 ° C. After application, the applied materials are only allowed to cool.

For the practical or particularly advantageous implementation of the present invention, the following general conditions can be specified with regard to the application quantities of the individual layers or their thickness: thermal transfer color layer B) about 1 to 10 g / m ² , preferably about 3 to 6 g / m ² , Separating layer 0.2 to 5 g / m ² , preferably about 0.5 to 1.5 g / m ² , carrier film, in particular polyester film of a thickness of about 2 to 8 microns, in particular a thickness of about 4 to 5 microns, and the one mentioned Backside coating in a thickness of approximately 0.01 to 0.2 g / m ² , in particular approximately 0.05 to 0.1 g / m ² .

The low cohesion of the thermal transfer ink mentioned, taking into account the other features mentioned, in particular in the preferred embodiment in the form of "closely cut" waxes, leads to a mechanical anchoring of the thermal transfer ink on the printed substrate, in particular the paper. The simultaneous incorporation of the wax-soluble polymers into the thermal transfer ink and the separating layer makes it possible to control the "releasability" of the separating layer. This guarantees good edge sharpness, resolution and high optical density as well as the desirable matt and non-glossy print. It is surprising that without a "top coat" any paper, ie smooth as well as rough paper, can still be used with excellent print quality. The thermal carbon tapes according to the invention are particularly advantageous when used in fax machines with a relatively good resolution, for example Xeroxfax, etc. These thermal transfer ribbons can also be used with particular advantage in the areas of office printers, franking machines and label printers.

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

example 1

A material with the following recipe is applied to a customary carrier made of a polyester with a layer thickness of approximately 6 µm by means of a doctor blade to form the separating layer: Polyester resin 40 parts by weight wax soluble polymer (EVA) 30 parts by weight soot 29 parts by weight Silica 1 part by weight $\overline{\text{100 parts by weight}}$

The above material is applied in a solvent dispersion (about 15%, in toluene / isopropanol 80:20) at a dry thickness of about 1.0 μm. The solvent is evaporated by passing hot air at a temperature of around 100 ° C. The thermal transfer ink is then applied using the following recipe in the form of a melt at a temperature of approximately 105 ° C. using flexographic printing. Recipe: Microcrystalline wax 50 parts by weight Ester wax 25 parts by weight wax soluble polymer (EVA) 10 parts by weight soot 15 parts by weight $\overline{\text{100 parts by weight}}$

Example 2 :

Transferfarbschicht: Paraffinwachs 60 Gew.-Teile Esterwachs (Carnaubawachs) 17 Gew.-Teile wachslösliches Polymer (EVA) 8 Gew.-Teile Farbruß 15 Gew.-Teile $\overline{\text{100 Gew.-Teile}}$

Example 1 was repeated with the modification that the following recipes were used for the separating layer and the colored layer: Interface: Polyurethane resin 15 parts by weight Polyester resin 25 parts by weight wax soluble polymer (EVA) 30 parts by weight soot 28 parts by weight Silica 2 parts by weight $\overline{\text{100 parts by weight}}$

Transfer color layer: Paraffin wax 60 parts by weight Ester wax (carnauba wax) 17 parts by weight wax soluble polymer (EVA) 8 parts by weight Soot 15 parts by weight $\overline{\text{100 parts by weight}}$

Claims (13)

Thermal transfer ribbon with a conventional carrier, with a wax-bonded layer of a thermal transfer ink formed on one side of the carrier and with a further layer located between the carrier and the wax-bonded layer, characterized in that the further layer comprises a resin-bound separating layer A) for the wax-bound layer B) Thermal transfer ink is, the waxes of the wax-bound layer have a melting point of about 70 to 110 ° C and a wax-soluble polymer is finely dispersed in both layers A) and B). Thermal transfer ribbon according to claim 1, characterized in that the melting point of the waxes of the thermal transfer ink is between about 75 and 90 ° C. Thermal transfer ribbon according to Claim 1 or 2, characterized in that layer A) contains about 10 to 60% by weight, in particular about 20 to 40% by weight, of wax-soluble polymer. Thermal transfer ribbon according to Claims 1 to 3, characterized in that layer B) contains approximately 2 to 20% by weight, in particular approximately 5 to 10% by weight, of wax-soluble polymer. Thermal transfer ribbon according to one of claims 1 to 4, characterized in that the thermal transfer color of layer B) has a viscosity of about 50 to 250 mPa.s, in particular about 70 to 120 mPa.s, measured at 100 ° C with a rotary viscometer. Thermal transfer ribbon according to at least one of the preceding claims, characterized in that the waxes of layer B) natural waxes in the form of carnauba wax and candililla wax, chemically modified waxes or hard waxes in the form of modified, microcrystalline wax, ester waxes, Paraffin waxes and / or synthetic waxes in the form of Fischer-Tropfsch wax and polyethylene wax. Thermal transfer ribbon according to at least one of Claims 1 to 6, characterized in that the wax-soluble polymer is an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, a polyamide and / or an ionomer resin. Thermal transfer ribbon according to at least one of the preceding claims, characterized in that the resin of layer A) is a solid resin with a softening range of 70 to 200 ° C. Thermal transfer ribbon according to claim 8, characterized in that the resin is an alkyd, epoxy, melamine, phenol, urethane and / or polyester resin. Thermal transfer ribbon according to at least one of the preceding claims, characterized in that layer B) contains a black pigment and layer A) additionally contains conductive carbon black and silica. Thermal transfer ribbon according to at least one of the preceding claims 1 to 10, characterized in that layer A) has a thickness of approximately 0.2 to 5 µm, in particular approximately 1 to 3 µm. Thermal transfer ribbon according to one of Claims 1 to 11, characterized in that layer B) has a thickness of approximately 1 to 10 µm, in particular approximately 3 to 6 µm. Thermal transfer ribbon according to at least one of Claims 1 to 12, characterized in that the thermal transfer ink contains a colorant, in particular color pigment, in an amount of 5 to 20% by weight.