Thermal transfer ribbon with adhesion layer

A thermocarbon ribbon having a foil-type carrier and a color transfer layer or melt color has its adhesion for the the paper substrate increased by an adhesion coating applied to the color transfer layer. The adhesion layer consists of a parafin wax matrix in which a sticky hydrocarbon resin is dispersed in finely divided form so that during printing, when the temperature of the adhesion layer is raised, the paraffin wax melts, dissolves the finely divided hydrocarbon resin and allows the sticky characteristic of the latter to increase adhesion of the color transfer material to the paper.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to the following commonly-owned copending 
applications: U.S. application Ser. Nos. 06/829,834 filed 14 Feb. 1986 and 
now abandoned; 07/109,489 filed 15 Oct. 1987 now U.S. Pat. No. 4,895,465; 
07/152,641 filed 5 Feb. 1988 and now abandoned; 7/154,651 filed 10 Feb. 
1988 now U.S. Pat. No. 4,998,486; 07/234,970 filed 19 Aug. 1988; 7/272,599 
filed 16 Nov. 1988; and 07/351,624 filed 12 May 1989. 
FIELD OF THE INVENTION 
Our present invention relates to a thermal-transfer ribbon, i.e. a ribbon 
which carries a color-transfer layer in a foil or like support and is 
capable, upon local heating and the application of pressure from a 
symbol-generating printhead, to cause a symbol formed by the transfer 
layer to adhere to a substrate, such as a paper sheet, which may be 
displaced past the printing location on a platen of a thermal printer or 
the like. More particularly, the invention relates to a thermocolor ribbon 
and especially a thermocarbon ribbon, having a melt color layer applied to 
one side of a carrier and which is designed to be locally melted and to 
adhere to the substrate to leave a symbol imprinted thereon. 
Background of the Invention 
Thermocolor ribbons have long been known. They comprise a foil-like 
carrier, hereinafter referred to also as a support strip, of paper, a 
plastic or the like and a melt color layer on this support strip, 
especially in the form of a plastic-bonded and/or wax-bonded coloring 
agent or carbon black layer. 
The melt color, during the use of a thermocolor ribbon, is melted by a 
heatable printing head and transferred to a writing or printing paper. 
Thermal printers or heated printing heads which can be used for this 
purpose are described, for example, in the German Published Applications 
DE-AS 2 062 494 and DE-AS 2,224,445. 
In the heated printing head of the printer, heated pins can generate 
symbols which can be formed on the receiving paper sheet in the form of 
alphanumeric characters or the like. The heated printing head presses 
against the thermocarbon ribbon and presses the latter against the paper 
which is to receive the imprint. The heated character of the of the 
printing head at a temperature of about 400.degree. C. causes a melting of 
the melt color in the heated region and, because this ribbon is in contact 
with the paper sheet, transfers an image of the symbol to the latter. The 
used part of the thermocolor ribbon can be taken up on a spool. 
The thermocolor ribbon can have a variety of melt colors lying next to one 
another and adapted to be imprinted in the same region. With the 
combination of the basic colors blue, yellow and red, for example, colored 
imprints with a full range of colorations may be made. Indeed, the use of 
such ribbons allows the formation of color images which have the advantage 
over color photography in that time consuming development and fixing steps 
can be eliminated. 
Thermal printers operate with high writing speeds and, for example, a DIN 
(German industrial standard) A4 sheet can be printed in about 10 seconds 
without serious noise pollution. 
Thermocolor ribbons may also be of the type in which a resistance heating 
brings about the melting of the melt color. In that case, the melt color 
and/or the carrier may be electrically conductive and the resistance heat 
serves to melt the function layer which is the melt color, to enable the 
transfer of symbols to the substrate. 
The ribbons of this type are known as ETR materials (electrothermal 
ribbons) and a thermal transfer printing system using these principles is 
described, for example, in U.S. Pat. 4,309,117. 
In both of the aforedescribed thermal transfer systems, the print sharpness 
(resolution) and the optical density of the transferred image depend, 
inter alia, upon the adhesion of the melt color to the paper. This 
adhesion to the paper is proportional to the adhesion surface area (i.e., 
the actual area of contact of the melt color and the paper) and the 
adhesion force. 
With rough paper, the adhesion area is small since practically only the 
peaks of the paper surface are wetted with the molten melt color. 
This problem has been recognized in the Europatent Publication EP-A-0 137 
532 and German Patent Document DE-A 3 507 097. These patent documents 
provide on the layer of the melt color, a so-called filling layer which, 
when the the melt color layer becomes molten, has an extremely low 
viscosity and during the print process is capable of flowing into the 
valleys of the rough paper surface, thereby increasing the adhesion area. 
This system has the drawback that with very smooth paper having a roughness 
in excess of 200 BEKK, the molten filling layer cannot penetrate into the 
paper during the printing process. Instead, the filling layer remains as 
layer between the paper surface and the color layer and thereby becomes a 
kind of hold-off layer as is described in Europatent Publication EP-A-0 
042 954. 
A hold-off layer has a detrimental effect on the permanence of the copy 
made, since it prevents permanent or long-term adherence of the melt color 
to the paper and any penetration of the melt color into the paper. 
Hold-off layers are desirable for correction-type thermocarbon ribbons. 
But, for permanent document-validating prints, a hold-off layer effect is 
highly undesirable. 
Objects of the Invention 
It, therefore, the principal object of the present invention to provide a 
thermal transfer ribbon whereby the aforedescribed drawbacks are avoided. 
Another object of the invention is to provide a thermal transfer ribbon 
which has improved adhesion both to rough and to smooth paper and is 
capable of making a permanent print, i.e. a print which cannot be erased 
or modified without trace. 
It is also an object of our invention to an improved method of making a 
thermal transfer ribbon with the advantages over the prior art, that the 
print is permanent and has greater adhesion both to rough and smooth 
paper. 
SUMMARY OF THE INVENTION 
These objects and others which will become more readily apparent 
hereinafter are attained, in accordance with the present invention, by 
providing on the color transfer layer, i.e. the melt color. An adhesion 
layer which promotes adhesion to the paper substrate and comprises a 
sticky hydrocarbon resin in finely divided form embedded in a paraffin. 
More particularly, the thermal transfer ribbon comprises: 
a support strip; 
a meltable color-transfer layer on a side of the support strip and adapted 
to be transferred to a substrate upon heating to imprint a symbol on the 
substrate; and 
an adhesion layer on the color-transfer layer and promoting adhesion of the 
color-transfer layer to the substrate, the adhesion layer comprising a 
sticky hydrocarbon resin embedded in finely divided form in a paraffin. 
In the following description, it will be understood that the support strip 
or carrier may be any of the carrier foils described in the aforementioned 
copending applications and the color transfer layers or melt color layers 
may be any of the melt colors described in the aforementioned copending 
applications. 
It is important for the invention that the adhesion layer, which comprises 
as a matrix phase, a paraffin, has a sticky hydrocarbon resin in finely 
divided form embedded and dispersed therein. 
The term hydrocarbon resin is a term describing a thermo-plastic, low 
molecular weight (molecular weight generally below 2000) polymers which 
have been used heretofore, among other things, for the modifying of 
adhesives or as tacky or sticky producing substrates for chewing gum (e.g. 
tarpene resins). 
Among the oldest of the hydrocarbon resins are the cumarone-indene resins 
derived from bituminous coal tar distillation. From crude oil cracking, 
petroleum resins are obtainable which can contain or consist of C.sub.4 
-C.sub.6 alkene fractions, C.sub.8 -C.sub.10 aromatic fractions (indene 
toluenes, vinyl toluenes, styrenes and homologs thereof) or the 
dicyclopentadiene fraction. 
The terpene resins can include dipentene, pinenene and limonene, etc. as 
well as terpene hydrocarbons which can be obtained, for example, in the 
production of terpene-free etheric oils or from cellulose as sulfate 
terpentines by cationic polymerization. 
Newer developments use pure monomers, mainly styrene or alkyl styrenes and 
similar compounds which are copolymerized. It will be self-understood that 
under the designated hydrocarbon resins, also fall aliphatic structures 
with a sticky texture. The hydrocarbon resins can also include aromatic, 
aliphatic, alicyclic and mixed hydrocarbons as are used as modifiers for 
adhesives and especially pressure adhesives and melt adhesives. 
It is apparent, therefore, that the above-described group of hydrocarbon 
resins is only given by way of example and should not be considered a 
limitation on the class definition. Indeed, there are a large number of 
hydrocarbons resins which, if they have a sticky texture can be dispersed 
in the paraffin and have their stickiness masked by the paraffin until the 
paraffin melts, can be used in accordance with the principles of the 
invention. 
The main component of the adhesion layer of the thermal transfer ribbon of 
the invention is a paraffin. The term "paraffin" is used in the sense of 
the invention can be a solid (at room temperature) mixture of purified, 
satisfied aliphatic-hydrocarbons which is colorless, odorless and 
tasteless easily soluble in ether and chloroforml and insoluble in water 
and 90% ethanol. It is a solid material having a hardening temperature in 
a range of about 50 to 110.degree. C., preferably between about 60 to 
95.degree. C. The paraffins preferably are microcrystalline waxes, cerasin 
waxes, petroleum wax and Fischer-Tropsch-waxes. 
Preferably, the adhesion layer contains the sticky hydrocarbon resin in an 
amount of about 10 to 45% by weight. The thickness of the adhesion layer 
can vary widely. Preferably, it is about 0.2 to 10 micrometers, especially 
1 to 3 micrometers. 
The lower limitation is determined by the practicalities of coating 
technology and cannot be lowered further without causing nonuniform 
coating. There is no value to be obtained by exceeding the upper limit set 
forth above and there is the disadvantage that it greatly increases the 
cost of production, requires higher energy for the printing process and 
generally gives rise to unsatisfactory results. Basically, the particular 
layer thickness of the adhesive layer is not critical within the range 
recited and to achieve the desired results of the invention. 
A variety of additives can be incorporated into the adhesion layer to 
obtain additional effects. For example, it can include coloring agents, 
other waxes, especially ester waxes which improve the writability of 
overhead foils, inert fillers and the like. 
The formation of the adhesion layer on the melt color of the thermal 
transfer ribbon of the invention can be brought about by doctoring a 
mixture of molten paraffin in which the sticky hydrocarbon resin is 
dispersed on the color transfer layer by conventional technology 
processes, for example a doctor blade. The temperature of the adhesion 
layer can be, as a rule, between 100 and 130.degree. C. and the coating 
material can then be cooled. 
While we do not wish to be bound to any particular theory for the unique 
effects of the invention, we believe that the key point to the invention 
is that the adhesion of the melt color utilizing the ribbon of the 
invention is not improved by increasing the adhesion surface, but rather 
by increasing the adhesion forces. 
Furthermore, since the thermal transfer ribbon must have a shelf life of 
the order of years, even when exposed to temperatures above 50.degree. C., 
the adhesion promoter cannot be activated during this long-term storage or 
by ambient temperatures. 
Normally, a sticky hydrocarbon resin cannot be used directly on the color 
transfer layer to increase the adhesion forces since this sticky material 
would cause the turns of the ribbon to stick together. 
We have found, to our surprise, that the paraffin apparently masks the 
sticky hydrocarbon resin of the adhesion layer so that even with long-term 
storage of the ribbon at temperatures above 50.degree. C., the sticky 
material cannot cause the turns of the ribbon to stick together. The 
masking effect appears to be achieved by embedding the particles of the 
sticky hydrocarbon resin in the paraffin matrix. 
Only when the adhesion layer is raised to a temperature above the melting 
point of the adhesion layer which is primarily determined by the melting 
point of the paraffin, can the sticky hydrocarbon resin by homogenously 
solubilized in the paraffin so that it can exert an adhesion effect only 
during the printing processes. 
Sticky hydrocarbon resins of the above-described type which, for example, 
are used in the manufacture of melt and tacky adhesives, when solubilized 
in paraffin have a temperature-dependent solubility equilibrium. This 
characteristic makes the invention possible. 
At temperatures above the melting point of the hydrocarbon resin/paraffin 
mixture, the hydrocarbon resin is homogenously dissolved in the paraffin. 
On cooling the hydrocarbon resin tends to be dispersed in the paraffin 
matrix before the hardening point is reached. 
Thus if the melt which forms the adhesion layer is applied to the color 
transfer layer, during cooling the hydrocarbon resin will disperse in 
finely divided form and become embedded in the paraffin matrix. The 
adhesion layer is thus neither tacky nor sticky and the adhesion effect of 
the hydrocarbon resin is effectively masked. 
Only during the printing process is the hydrocarbon resin redissolved and 
again enabled to exercise an adhesion effect upon the paper and the color 
transfer material to retain the latter with greater force upon the paper. 
Naturally, when considering the addition of the additives to the adhesion 
layer, one must be concerned with how they may effect the 
temperature-dependent solubility equilibrium of paraffin an hydrocarbon 
resin.

SPECIFIC DESCRIPTION 
The ribbon shown in the drawing comprises a support strip 10 of any of the 
foil support or carrier materials described above formed with a color 
transfer layer or melt color layer 11 and the adhesion layer 12 as 
described above and in the following examples. 
EXAMPLE 1 
A thermal transfer ribbon is made by applying to a polyester foil carrier a 
color transfer layer in a thickness of 5 micrometers of 45% by weight 
paraffin having a melting point of about 68.degree. C., 40% by weight 
ethylenevinylacetate and 15% by weight carbon black. 
To this color transfer layer an adhesion layer is applied of the following 
composition: 3.5 parts by weight paraffin with a melting point of about 
68.degree. C., 0.5 parts by weight of an ester Wax and 1.0 parts by weight 
of a hydrocarbon resin having a melting point of about 120.degree. C. (KW 
61 of the firm VfT (Verkaufsgesellschaft fur Teererzeugnisse mbH). The 
mixture has a hardening point of about 63.degree. C. while the cloud point 
of the melt upon cooling is about 97.degree. C. The material of the 
adhesion layer is melted and coated in a thickness of 2 micrometers with a 
roller on the color transfer layer. After cooling, a thermocarbon ribbon 
is obtained which has been found to be especially effective in the 
printing of rough paper with a BEKK smoothness of less than 50. 
Example 2 
The Example 1 is followed except that the composition of the adhesion layer 
was 3.0 parts by weight paraffin of a melting point of about 68.degree. 
C., 1.0 parts by weight ester wax and 1.0 parts by weight of a hydrocarbon 
resin with a melting point of 130.degree. C. (A 120 as marketed by 
Hercules). The cloud point was reached at about 110.degree. C. upon 
cooling. Similar excellent results as the thermocarbon ribbon were 
achieved.