Thermo-transfer ribbon

Details Thermo-transfer ribbon Thermo-transfer ribbon

1999-05-06

2002-10-08

Hess, Bruce H. (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Discontinuous or differential coating, impregnation or bond

C428S488110, C428S484100

Reexamination Certificate

active

06461721

ABSTRACT:

The invention relates to a thermo-transfer ribbon with a carrier, having a thermo-transfer color on one side and with additional layers, if applicable, whereby the thermo-transfer color contains an amorphous polymer and a wax, in addition to a coloring substance.
A thermo-transfer ribbon of the above described type is known from DE 36 13 846. The thermo-transfer color of said known thermo-transfer ribbon contains an amorphous polymer, having an average molecular mass weight Mw of not more than 10000, a numerical molecular mass mean Mn of less than 5000 and a glass transformation temperature in the range from 50 to 80° C. The amorphous polymer constitutes at least 50% by weight of the thermo-transfer color in relation to its solid matter contents. This thermo-transfer ribbon provides clear color reproduction during the printing process and satisfies requirements with respect to resolution, absorption sensitivity, transmission- and fixation properties. These goals are attained in that the binding agent for the thermo-transfer color—which conventionally involves a crystalline binder on wax basis—is being replaced by an essentially amorphous, transparent polymer. In order to enhance the desired effect, a small amount of a “release agent” is bonded into the thermo-transfer color, resulting in higher image quality and, specifically, higher resolution. At the same time, excellent fixation is achieved by exploiting the polymer's intrinsic properties, i.e. its flexibility and scratch-resistance. The issue of obtaining scratch-resistant print-outs, which is one of the goals of the present invention, is not specifically addressed here.
The issue of obtaining scratch-resistant print-outs is specifically discussed in EP-B-0 380 920 and also DE 196 12 396 A1. EP-B-0 380 920 suggests for obtaining scratch-resistant print-outs that the thermo-transfer color contain, during the printing process, non-melting, coloring-substance-containing polymer pellets, which are fusible by a heat treatment following the printing process. The symbols obtained immediately after the printing process initially do not have the desired scratch resistance, which is achieved by supplying the symbols with additional heat. In doing so, a new structure develops with respect to the printed symbol. This suggestion has the drawback in that it requires a second heat treating step after the printing process itself.
DE 196 12 393 A1 provides a certain remedy. It suggests a thermo-transfer ribbon having a customary carrier with a layer of a thermo-transfer color formed on one side of the carrier, and a separation layer between the carrier and the layer of the thermo-transfer color. The separation layer is wax-bonded and contains waxes having a melting point of between approximately 70 to 110° C., including a polymer wax plasticizer with a glass temperature Tg of −30 to +70° C. In addition, the layer of the thermo-transfer color contains at least approximately 20% by weight of natural resin, modified natural resin and/or synthetic resin. This creates a thermo-transfer ribbon whose transferred thermo-transfer color, during printing, specifically during printing on paper labels, shows good adhesion as well as good rub-off resistance and scratch-resistance, and is deposited quickly and dot-accurately together with the subjacent separation- or release layer. The drawback, however, consists in that the formation of the described separation layer requires a certain expenditure, as a result of which we are dealing with a system having at least three layers.
Starting from the state of the art described in the preceding, the present invention is based on the object of further refining in such manner the initially identified thermo-transfer ribbon, so that no separation layer is needed, but obtaining, nevertheless, the benefits which are derived in relation to the description of the object of DE 196 12 393 A1.
The objective is to increase the rub-off resistance and scratch-resistance of printed symbols on labels, specifically also of printing on plastic labels, whereby, in this case, bar code labels are of particular significance. Moreover, adequate solvent-resistance shall also be provided.
According to the invention, this object is solved in that the thermo-transfer color contains, as a binding agent, a polar polyethylene wax and an amorphous polymer, which has a mol mass weight mean Mw of more than approximately 10000, and a numeric mol mass mean Mn of less than approximately 6000.
This proposed solution means that the thermo-transfer color of the thermo-transfer ribbon contains an amorphous polymer as a significant percentage of binding agent, specifically of at least approximately 50% by weight relative to the percentage of dry substance of the thermo-transfer color. When mentioning the term of “amorphous” polymer here, it shall have the meaning that from a roentgenographic aspect, its characteristic structural appearance is called amorphous.
Accordingly, the term “amorphous polymer” shall include also such oligomers and/or polymers which contain certain part-crystalline portions, for example up to approximately 30% by weight, specifically up to approximately 10% by weight. Contrary to the indicative specifications of DE 36 13 846 C2, the invention employs an amorphous polymer which has a mean mol mass weight Mw of over 10000. This comes as a surprise. The explanation that the mean mol mass weight Mw totals over 10000 is probably due to the fact that additionally, and of necessity, a polar polyethylene wax must be present. If the value falls below the Mw value of 10000, which would be in agreement with DE 36 13 846 C2, the adverse effect sets in that the adhesion of the thermo-transfer color vis-a-vis the carrier foil is too high and there is no assurance for homogenous color transfer during the printing process.
It is of particular benefit if the amorphous polymer has a mean mol mass weight Mw from 10000 to approximately 15000 and a numeric mol mass mean Mn of less than 5000, specifically approximately 2000 to 3000. The following statement can be made as a preferred quantitative basic specification relative to the ratio of polar polyethylene wax to amorphous polymer, namely that one part by weight of polar polyethylene wax corresponds to approximately 3 to 5 parts by weight, specifically approximately 4 parts by weight of amorphous polymer.
If the contents of amorphous polymer totals less than approximately 50% by weight, this may result in having a detrimental effect upon the desirable transparency of the thermo-transfer color and thus upon the capability of reproducing the color. In general, it is therefore preferred if the contents of amorphous polymer amounts to more than 50% by weight, specifically more than approximately 70% by weight, whereby, however, a ceiling value of approximately 80% by weight can be specified in order to still have available an adequate quantity of polar polyethylene wax.
It is not excluded, within the framework of the above specified basic quantity requirements, that some additional binding agents are present in small amounts in the thermo-transfer color , such as waxes customarily employed in thermo-transfer ribbons, for example paraffin wax, carnauba wax, montan wax, bees wax, vegetable wax, candelilla wax, including materials utilized as synthetic binding agents such as polyolefins having an average molecular weight of approximately 1000 to 10000, for example low-molecular weight polyethylene, polypropylene or polybutylene and similar. In some instances it may also be of advantage to include additional customary additives, which will enhance the properties of the ribbon. Within the framework of technical considerations, a person skilled in the art will hereby make the appropriate selection by which to achieve a desired effect.
Examples for utilizable amorphous polymers comprise homopolymers and co-polymers, styrol or its derivatives or substituted compounds of same (for example styrol, vinyl, toluol) acrylic acid ester, for example methyl-acrylate, ethylene-acrylate and butyl-acry

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