Thermo-transfer ribbon for luminescent letters

Details Thermo-transfer ribbon for luminescent letters Thermo-transfer ribbon for luminescent letters

1998-10-09

2002-04-23

Yamnitzky, Marie (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S195100, C428S484100, C428S488110, C428S488410, C428S522000, C428S690000, C428S913000, C428S914000

Reexamination Certificate

active

06376056

ABSTRACT:

The present invention relates to a thermo-transfer ribbon for luminescent letters or codings.
Modern sorting machines, as employed when sorting a multitude of objects, such as for example letters, react to luminescent coding which is not necessarily visible to the human eye. To that end, the pieces to be sorted are furnished, prior to the sorting process, with symbols which contain a luminescent substance. In growing measure, thermo-transfer (color) ribbons are used for said purpose, which have a layer of thermo-transfer color with a therein contained luminescent pigment. The luminescent transfer color, which is transferred to the substrate surface, is very thin and transparent for visual examination.
Luminescent colors have the property of absorbing ultraviolet light and visible light in the blue portion of the spectrum and to again radiate said absorbed portion at the lower end of the spectrum. From among the great number of organic compounds, which radiate visible light under the influence of short-wave rays, only such substances are suitable as luminescent pigments or luminogens which distinguish themselves in solid, non-dissolved state by intensive fluorescence. Those luminescent pigments which fluoresce in colors under day light, and which are employed as daylight fluorescent pigments, command the greatest technical interest. Soluble pigments of this type are for example rhodamine, eosine, brilliant sulfoflaven FF and the highly yellow-green fluorescent pyranine, also color pigments, for example 2,2-dihydroxy-alpha-naphthaldiazine and anthrapyrimidine.
Inasmuch as the pigments are organic in nature, it is necessary to dissolve them in an organic medium or carrier. Mostly tinted carrier substances are employed, for example pulverized polymerisates which are tinted with soluble pigments or finely distributed pigments. The type of material which corresponds to the requirements of a carrier or a matrix for the pigments is a transparent organic resin. By reacting acid polyester resins with basic pigments or by pulverizing solidified color substance solutions, one likewise obtains tinted carrier substances. Urea formaldehyde resins, acrylic resins and melamine resins are also used as carriers, onto which the pigments may be lacquered. Daylight fluorescent pigments are, in fact, organic plastic particles, which are tinted with fluorescent color substances. The physical structure of the pigment particles is mostly amorphous.
When luminescent material is precipitated on the surface of white paper, the whiteness of the paper serves as light reflector. The major portion of the incident light is reflected back by the paper through the imprinted luminescent material. The reflected light perceived by the observer contains both incident light as well as luminescent light.
If the luminescent material is transferred to the surface of a dark colored paper, a portion of the incident light, which has passed the luminescent layer, is absorbed by the paper. The amount of the light available from reflection is decreased. In addition, that percentage of light emitted by the luminescent color layer is being absorbed, which is radiated in the direction of the paper surface.
In order to compensate for the differences in intensity of luminescence, which are attributable to the type of carrier, DE-OS 30 42 526 proposes a fluorescent print ribbon, which is characterized by addition of a blocking material to the fluorescent pigment material, in order to block absorption of the incident light in the medium onto which the pigment and the blocking coating are transferred during printing. The blocking material is preferred as a second coating above the pigment coating. Both coatings are transferred in reverse order onto the substrate during the printing process. The blocking material contains reflecting metal particles or mother-of-pearl-type pigments.
DE-AS 12 22 725 discloses a transfer material for luminescent letters with a coating carrier of paper or foil with a luminescent color layer arranged thereon, whereby a pigmented, light-radiation reflecting coating is placed over the luminescent color layer, which participates in the writing process. The cover layer preferably contains titanium white and/or pressure-ground aluminum powder.
The known suggestions are aimed at avoiding absorption, in the substrate, of incident light passing through the luminescent layer, so that this portion is being reflected and once more passes through the luminescent color layer, in order to thus increase the overall excitation yield. It is, hereby, of drawback that the luminescent light perceived by the observer is always mixed with the reflected portion of incident light. The luminescent print-outs therefore always appear pale, i.e. they have low optical density.
If attempts are made to increase the optical density of the print-outs by adding a non-luminescent pigment to the layer of luminescent pigment, one notes that with an addition of extraneous pigments of more than 1%, fluorescence quality is significantly affected. With increasing addition amounts, there is growing impairment of the brilliance of the fluorescent pigments, the fluorescence power and color purity due to occurring interferences. Still higher addition amounts lead to almost total extinction of fluorescence. From the aspect of fluorescent output, an additional amount of 1% or less, however, would, only insignificantly increase the optical density.
The above described thermo-color ribbons are finding increasing entry in high speed printers, specifically in industry, whereby print heads are being employed of the so-called “Real-Edge” or “Corner” type. In these print heads a series of targetable heating points (dots) is arranged on a ceramic substrate, close to the edge or directly on the rim. The benefit of edge-type heads lies in shorter cooling-down times and correspondingly higher registration frequencies. Printing speeds of from 3 to 12″ per second can be achieved with them. Prints obtained with high-speed printing are subject to special requirements with respect to print quality, i.e. excellent edge definition, dissolution and optical density. A specific application field is the imprinting of paper and plastic labels. With respect to the latter, high scratch resistance of prints is desirable.
The thermo-transfer ribbons for luminescent codes provided thus far by the state of the art do not satisfy the addressed requirements to the desired extent. The invention is therefore based on the object of providing a thermo-transfer color ribbon for luminescent codes, whereby print-outs of high optical density could be obtained without affecting the luminescence output of the luminescent pigment and irrespective of substrate to be imprinted, including satisfactory print quality at high printing speeds. The luminescent symbols transferred during the printing process shall have good adhesion and good scratch resistance, specifically on paper and plastic labels.
According to the invention, this object is solved by a thermo-transfer ribbon with a carrier, a first thermo-transferable layer formed on one side of the carrier and a second thermo-transferable layer formed on the first thermo-transferable layer, whereby (i) the first thermo-transferable layer contains luminescent pigments, wax(es) having a melting point from approximately 70 to 110° C. and approximately 1 to 22% by weight of a polymer wax plasticizer with a glass temperature Tg of −30 to +70° C., (ii) the second thermo-transferable layer contains a non-luminescent pigment, a wax-compatible polymer binding agent and approximately 5 to 30% by weight of wax and/or wax-like substance, (iii) the second thermo-transferable layer has a melting enthalpy DH of approximately 10 to 80 J/g and (iv) there is, in the remission spectrum of the non-luminescent pigment in the wave length range of the light emitted by the luminescent pigment, a remission maximum or an ascending flank of remission.
In preferred specific embodiments, the second thermo-transferable layer contains approximately 5 to 40% by

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