WO2024078971A1

WO2024078971A1 - Heat-sensitive recording material

Info

Publication number: WO2024078971A1
Application number: PCT/EP2023/077632
Authority: WO
Inventors: Timo DR. STALLING; Ansgar DR. SEHLINGER; Uwe DR. BRASCH
Original assignee: Koehler Innovation & Technology Gmbh
Priority date: 2022-10-11
Filing date: 2023-10-05
Publication date: 2024-04-18
Also published as: DE102023127473A1

Abstract

The invention relates to a heat-sensitive recording material, comprising a carrier substrate which has a first face and a second face facing away from the first face; a heat-sensitive color-forming layer which is arranged on the first face of the carrier substrate, wherein the heat-sensitive color-forming layer comprises at least one color former, at least one color developer, and at least one stabilizer; and an adhesive layer which is arranged on the second face of the carrier substrate, said adhesive layer comprising at least one adhesive. The weight ratio of the stabilizer to the color developer in the heat-sensitive color-forming layer is selected such that the migration of components from the adhesive layer into the heat-sensitive color-forming layer leads to a remaining image permanence of the heat-sensitive color-forming layer of at least 35% after a period of time of two weeks according to the stability test defined in the description.

Description

HEAT-SENSITIVE RECORDING MATERIAL

TECHNICAL AREA

According to a first aspect, the present invention relates to a heat-sensitive recording material comprising a carrier substrate which has a first side and a second side facing away from the first side, a heat-sensitive color-forming layer which is arranged on the first side of the carrier substrate and comprises at least one color former, at least one color developer, and at least one stabilizer, and an adhesive layer which is arranged on the second side of the carrier substrate and comprises at least one adhesive.

The weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that migration of components from the adhesive layer into the heat-sensitive color-forming layer according to the stability test defined in the description leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35% after a period of two weeks.

According to a second aspect, the present invention relates to a method for producing a heat-sensitive recording material, comprising the following method steps: providing a carrier substrate which has a first side and a second side facing away from the first side, applying an application suspension to the first side of the carrier substrate, wherein the application suspension comprises at least one color former, at least one color developer, and at least one stabilizer, drying the application suspension in order to obtain a heat-sensitive color-forming layer arranged on the first side of the carrier substrate, and applying an adhesive layer to the second side of the carrier substrate. According to a third aspect, the present invention relates to a heat-sensitive recording material producible by a method according to the second aspect.

According to a fourth aspect, the present invention relates to a use of at least one stabilizer and at least one color developer in a heat-sensitive recording material comprising a carrier substrate which has a first side and a second side facing away from the first side, a heat-sensitive color-forming layer which is arranged on the first side of the carrier substrate and comprises at least one color former, at least one color developer, and at least one stabilizer, and an adhesive layer which is arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive.

TECHNICAL BACKGROUND

Heat-sensitive recording materials, also known as thermal papers, are used in a variety of applications, such as sales receipts in retail stores.

Back preparations are used in conjunction with heat-sensitive recording materials to obtain adhesive labels or thermal labels or to improve various application properties. The back preparation can be a self-adhesive layer and/or a coating (“back coat”) consisting essentially of polymeric binders and pigments.

Heat-sensitive recording materials with a self-adhesive layer on the back, so-called thermal labels, are known from the state of the art for direct thermal printing.

JPS 59 162 087 A, US 4 370 370 A and US 4 388 362 A describe thermal labels with a release paper. DE 19 757 589 B4 describes a release paper-free (“linerless”) thermal label with a protective layer over the heat-sensitive layer.

DE19 806 433 B4 discloses a linerless thermal label with a protective layer free of silicone compounds, which has been cured by means of actinic radiation.

EP 0600622 A1 and DE 19 724647 C1 describe a linerless thermal label with a protective layer which has been coated on the back with hotmelts.

EP 1 085 069 B1 and EP 2 474 963 B1 disclose thermal label materials in linerless design with a heat-activated adhesive, whereby thermal labels are described both with and without a protective layer.

EP 3 219 507 A1 claims a linerless thermal label without an actual protective layer, in which the surface of the heat-sensitive layer is designed to be dehesive towards adhesives.

Heat-sensitive recording materials equipped with so-called “back-coat” coatings to improve the printability of the back side with conventional printing processes or to minimize the curling of a carrier substrate under unfavorable humidity conditions are also known from the prior art.

The different shrinkage behavior of the two sides of the carrier substrate of heat-sensitive recording materials at different ambient humidity levels, i.e. at different water vapor absorption of the two sides/coatings, can be effectively improved by back coatings. US 6667 275 B2, for example, discloses a multi-layer back coating for heat-sensitive recording materials for advantageously influencing the curling tendency of the web of the carrier material.

JP 2018 167483 discloses ways to improve the durability of the thermally generated typeface in heat-sensitive recording materials, which have been coated on the back with oil-based printing inks or inks by applying coatings to the back. The formulations of these back coatings often contain aqueous emulsion polymers, such as styrene-butadiene or acrylate latices, as an essential component.

For example, JP 2003 175 671 claims back-coatings for heat-sensitive recording materials produced with aqueous latices, which form soft polymer films (glass transition temperature >-30°C). JPH 0 720 735 B2 and JP 2000 204 123 disclose back-coat formulations with acrylate emulsion polymers for heat-sensitive recording materials.

The publications EP 3 957 488 A1 and WO 2022/038242 A1 describe a heat-sensitive recording layer comprising phenol group-free organic color developers, a coating composition for producing the heat-sensitive recording layer and a heat-sensitive recording material, in particular thermal paper, comprising the heat-sensitive recording layer. The use of the heat-sensitive recording layer for producing the heat-sensitive recording material, a process for producing the heat-sensitive recording material and the use of the heat-sensitive recording material in thermal paper applications are also described. Due to its composition, the heat-sensitive recording material has particularly good recyclability and high environmental compatibility.

Heat-sensitive recording materials are also described in the publications US 2021/0060994, CA 3 149 562 A1 and WO 2021/041600 A2.

Furthermore, reference is made to the publications DE 10 2019 126 220 A1 , WO 2021/058661 A1 , KR 2022 070 021 A, and DE 20 2020 005 616 U1.

The colour-forming layer of heat-sensitive recording materials usually contains a colour former and a colour developer, which, under the influence of heat, react with each other and thus lead to color development. Inexpensive color developers containing phenol groups (bisphenol A, bisphenol S, etc.) are widely used. These can be used to produce thermal labels that have an acceptable performance profile for certain applications.

Thermal labels that contain a phenol group-free color developer in the heat-sensitive color-forming layer are also known from the state of the art. These were developed to improve the durability of the typeface, especially when the printed heat-sensitive recording material comes into contact with hydrophobic substances, such as plasticizer-containing substances or materials or oils. In addition to the technical advantages, public discussions about the toxic potential of (bis)phenolic chemicals have greatly stimulated interest in phenol group-free color developers.

Storage of thermally printed self-adhesive thermal labels or heat-sensitive recording materials with back-coatings over longer periods of time, particularly at high ambient temperatures and/or air humidity, can result in impairments in the application properties. In particular, heat-sensitive recording materials with such back-coatings may no longer achieve the specified values for the degree of blackness of the print (e.g. the barcode or the typeface) or the surface whiteness after storage.

However, a low degree of blackening of the print generally leads to a reduction in the reading contrast and is further worsened by the drop in background white. Since the readability of barcodes, for example, is particularly important in label applications, low contrast values have a detrimental effect on this important application-related property.

The cause of this phenomenon is said to be the adhesive layer on the back, from which substances migrate through the carrier substrate into the chemically reactive heat-sensitive recording layer over the course of storage and interact with it in a way that is harmful to the color complex. Substances with a relatively low molecular weight (< 10 kDa) contribute to the migration problem. Impairment of performance after storage can also occur when a heat-activated adhesive, ie one that is solid at room temperature, is used.

In particular, colour developers with more complex chemical structures than structurally relatively simple (bis)phenols, which have a large number of reactive sites in the molecule, as is often the case with colour developers free of phenol groups, can be affected by the problem of undesirable interaction with substances that can be released from heat-sensitive recording materials provided with a backside preparation.

To ensure the cohesive strength within the adhesive layer of pressure-sensitive adhesives and the adhesion strength (bonding) to the substrate, typical pressure-sensitive adhesive formulations use a base polymer as well as non-polymeric tacky resins (e.g. natural or carbon resins) and/or plasticizers, tackifiers and possibly other small-molecular additives such as crosslinkers, stabilizers, etc. If one takes into account that the base polymers contain residual monomers or oligomers from the synthesis process and can form further monomers due to the hydrolytic degradation promoted by high ambient humidity and temperatures, the migration potential of small-molecular substances inherent in an adhesive layer can easily be recognized.

In addition to the above-described possibilities of the presence of residual monomers in the latex, the presence of typical process-related substances such as surfactants, soaps, initiators during emulsion polymerization must be taken into account when evaluating the migration potential of the polymeric components of the back-coat coatings.

The state of the art offers different solutions to this problem for adhesive thermal labels: the application of an additional layer between the back of the carrier substrate and the adhesive layer (so-called back-coat), as in US 4 370 370 A and EP 2 474 963 B1, or the incorporation of special substances, e.g. carboxylated polyvinyl alcohols, into the adhesive layer, as described in JPS 59 162087 A. However, these solutions are economically disadvantageous because they require additional production steps and make the label material more complex overall, and are only able to minimize the migration problem of back coatings if they use more expensive, low-migration binders.

DESCRIPTION OF THE INVENTION

The object of the present invention is to optimize the property profile of a thermally printed heat-sensitive recording material which has an adhesive layer on the back, in particular to achieve the greatest possible limitation of the loss of image stability after storage of the heat-sensitive recording material, in particular when the heat-sensitive recording material is stored over a longer period of time.

The image stability is characterized by the image durability after storage of the heat-sensitive recording material, whereby the image stability can also be characterized in particular by the print contrast.

Thus, the present invention aims to improve the minimum shelf life of a heat-sensitive recording material, while at the same time maintaining the usual high requirements for such a heat-sensitive recording material, such as dynamic sensitivity, contrast, etc.

Surprisingly, it has now been found that the disadvantages of the prior art described above can be remedied by using a combination of at least one color developer with groups capable of forming hydrogen bonds, which are selected from the group consisting of -NHCO-, -CONH-, - NHCONH-, -CONHNHCO-, -NHCOCONH-, -SO2NH- and -NHSO ₂ , with 5-(N-3-methyl-phenyl-sulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic acid diamide (trade name PF425) as a stabilizer in the heat-sensitive layer of the heat-sensitive recording material. In particular, the use of this combination consisting of the corresponding at least one color developer and stabilizer in the heat-sensitive layer of the heat-sensitive recording material leads to a limitation of the loss of image stability after storage under demanding environmental conditions of heat-sensitive recording materials provided with backside preparations, so that the use of both components in the heat-sensitive layer leads to a synergistic effect.

The above-mentioned object is achieved according to the first aspect by a heat-sensitive recording material comprising a carrier substrate which has a first side and a second side facing away from the first side; a heat-sensitive color-forming layer which is arranged on the first side of the carrier substrate, wherein the heat-sensitive color-forming layer comprises at least one color former, at least one color developer, and at least one stabilizer; and an adhesive layer which is arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive; wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that migration of components from the adhesive layer into the heat-sensitive color-forming layer according to the stability test defined in the description after a period of two weeks leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35%.

According to the first aspect, the stabilizer comprises a compound of formula (I):

wherein R and R are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-alkyl, wherein Rg is selected from the group comprising Ci-Cs-alkyl, Cs-Cs-cycloalkyl; or a compound of formula (II)

wherein R2, R3, R4, Rs, and Re are independently selected from the group comprising hydrogen, Ci-Cs-alkyl, -NH-C(=O)-R?, and -C(=O)-NH-Ry, wherein R? is selected as Ci-Cs-alkyl or -C(=O)ORs, wherein Rs is selected as Ci-Cs-alkyl or halogen, or wherein R2 and R3, or R4 and Rs, or both, or wherein R3 and R4, or Rs and Re, or both, or wherein R2 and R3 and Rs and Re, together form a hydrocarbon group having three or four carbon atoms, and wherein Q comprises a single bond or Ci-Cs-alkylene, which may be branched or unbranched, and wherein the Ci-Cs-alkylene comprises a main chain having one or more oxygen atoms between two carbon atoms when the Ci-Cs-alkylene has more than two carbon atoms.

According to the first aspect, the at least one colour developer comprises a compound of formula (Nl):

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C4-alkyl, C1-C4-alkoxyl, C2-C4-alkenyl, C1-C4-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl, wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C4-alkyl, wherein R5 is selected as C1-C4-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or the at least one color developer comprises a compound of formula (1):

where Ri is selected from the group comprising unsubstituted or substituted phenyl, naphthyl and Ci-C2o-alkyl, where X is selected from the group comprising — C(=NH) — , — C(=S) — and — C(=O) — , where A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, Ci-Ci2-alkylene, and a unsubstituted or substituted heterocyclic group, where B is selected from the group comprising — O — SO2 — , — SO2 — O — , — NH — SO ₂ — , — SO ₂ — NH—, — S— SO ₂ — , — O— CO— , — O— CO— NH— , — NH— CO— , — NH— CO— O— , — S— CO— NH— , — S— CS— NH— , —CO— NH— SO ₂ — , — O— CO— NH— SO ₂ — , — NH=CH— , —CO— NH— CO— , — S— , —CO—, — O— , — SO ₂ — NH— CO — , — O — CO — O — and — O — PO — (OR2)2, and wherein R2 is selected from the group comprising unsubstituted or substituted aryl, benzyl and C1-C20-alkyl, with the proviso that when B is not a group of the formula — O — SO2 —, then R2 is unsubstituted or substituted phenyl, naphthyl or Ci-Cs-alkyl, and that when B is — O —, then R2 is not alkyl.

The combination of the stabilizer and the color developer in the heat-sensitive color-forming layer advantageously leads to the limitation of the loss of image stability and in particular also of the relative print contrast, whereby the image stability and/or the relative print contrast of the heat-sensitive recording material stored according to the stability test (as defined below) is always greater than is the value obtained using the same heat-sensitive recording material without the use of the stabilizer. This effect is particularly evident in the commercially available, critical medium current range of 9.00 mJ/mm ² .

Without being bound to theory, it is assumed that the stabilizer, due to the color complex-stabilizing interactions resulting from its functional groups, such as hydrogen bonds, TT-TT interactions and/or ionic interactions, contributes to the preservation of the color complex, in interaction with the color developer and color former, and of the typeface under the influence of destabilizing influences such as water, chemicals and critical climatic conditions.

In particular, the at least one stabilizer comprises a compound of formula (I) which does not comprise a hydroxy-substituted phenyl radical and can thus also be referred to in particular as a phenol group-free compound.

In particular, the at least one color developer comprises a compound of the formula (N-I) or (1) which does not comprise a hydroxy-substituted phenyl radical and can thus also be referred to in particular as a phenol group-free compound.

The heat-sensitive recording material is well suited for applications in which the heat-sensitive recording material is provided with an adhesive layer on the back, which is printed using the direct thermal process and must ensure a long storage life even under critical environmental conditions, with regard to the specified image stability and print contrast values.

The present invention is not subject to any significant restrictions with regard to the choice of adhesive applied to the back. Adhesives that are sticky at room temperature as well as those that only become sticky after activation, for example by heat, are possible. Both permanently adhesive and removable adhesives can be used. The application technology of the adhesive to the back of the heat-sensitive recording material does not restrict the scope of the invention in any way. Aqueous dispersions of the adhesive or adhesives dissolved or suspended in organic media, as well as adhesives applied in a molten state, so-called hotmelt adhesives, can be used.

In summary, it has surprisingly been found that it is possible to obtain heat-sensitive recording materials, in particular thermal labels, by using at least one color developer and the stabilizer according to the first aspect of the present invention, which are characterized by high image stability and good contrast values of the printed image (print contrast) during thermal printing after long-term storage under critical environmental conditions.

The heat-sensitive recording material according to the invention relates to the application-relevant case of a self-adhesive heat-sensitive recording material, i.e. a heat-sensitive recording material which has an adhesive layer on the side of the carrier substrate facing away from the heat-sensitive color-forming layer

In the context of the present disclosure, the stability test defined below comprises storing the heat-sensitive recording material for a period of two weeks between two glass plates at 60°C, at a pressure of 1350 N/m ² , at a relative humidity of 50% and under exclusion of light.

As part of the stability test according to the present disclosure, the image stability of the heat-sensitive recording material is determined based on the optical density once before the heat-sensitive recording material is stored and after the heat-sensitive recording material has been stored for two weeks. The two determined values of the optical density are related to one another as illustrated by the following formula A, with the scatter of the calculated % values being <±2 percentage points:

Image stability in % = (optical density after storage / optical density before storage) x 100 (Formula A)

As part of the stability test according to the present disclosure, the optical density was measured using a SpectroEye densitometer from X-Rite, with the measurement uncertainty of the measured value being estimated at <2%.

Here, the image stability of a heat-sensitive recording material according to the present invention tested according to the stability test, preferably at an energization energy of 9.00 mJ/mm ² , is at least 35% after the storage period of two weeks.

To carry out the stability test, a DIN A4 heat-sensitive recording material with an adhesive layer on the side of the carrier substrate facing away from the heat-sensitive color-forming layer is divided lengthwise into at least two 6 cm wide strips.

A strip of this is printed and measured to determine the optical density before storage. The unprinted strip is placed on the printed strip (simulating a thermal roll) and then the stack is stored for two weeks (or, according to some embodiments, for four weeks) between two glass plates at 60°C, at a pressure of 1350 N/m ² , at a relative humidity of 50% and in the absence of light.

After storage and conditioning to room temperature, the optical density is determined, averaged and related to the initial optical density values determined before storage to determine the image permanence according to Formula A above.

In addition to the image stability of the heat-sensitive recording material, the relative print contrast can also be taken into account as a further criterion for the image stability of the heat-sensitive recording material. The relative print contrast (DK in %) is calculated based on the value of the optical density of a thermally printed area, i.e. an optical density of a print pattern (oDs) and the optical density of a non-printed area (oDw) according to the following formula B, whereby the scatter of the calculated % values is <±2 percentage points:

DK in % = (oDs - oDw / oDs) x 100

(Formula B)

The relative print contrast of a heat-sensitive recording material examined according to the stability test, preferably at an energization energy of 9.00 mJ/mm ² , is at least 50% after the storage period of two weeks.

wherein R and R are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-AI- kyl, wherein Rg is selected from the group comprising Ci-Cs-alkyl, Cs-Ce-cycloalkyl; or a compound of formula (II)

In particular, the hydrocarbon group having three or four carbon atoms formed by R2 and R3 and Rs and Rs together includes trimethylene, tetramethylene, propenylene, 2-butenylene or 1,3-butadienylene.

Preferably, Rs is selected as a halogen, with Rs more preferably selected as chlorine.

Preferably, Q is selected from -CH2-, -CH2-CH2-, -CH2-O-, -CH2-CH2-O-, -CH2-CH2- CH ₂ -, -CH2-CH2-CH2-CH2-, -C(Me)H-, -C(Et)H-, -C(n-Pr)H-, -C(i-Pr)H-, -C(n-Bu)H-, -C(i-Bu)H-, -C(sec-Bu)H-, -C(tert-Bu)H-, -C(Me)HCH2-, -CMe2CH2-, where Q is more preferably selected from -CH2-, -CH2-CH2-, -CH2-CH2-O- or -C(Me)H-. Preferably, the selection “Ci-Ci8-alkyl” includes methyl, ethyl, propyl, isopropyl, N-butyl, isobutyl, sec-butyl, tert-butyl, amyl, tert-amyl (1,1-dimethylpropyl), 1,1,3,3-tetramethylbutyl, N-hexyl, 2-methylpentyl, neopentyl, N-heptyl, 2-ethylhexyl or N-octyl, N-nonyl, N-decyl, N-undecyl, N-dodecyl, N-tridecyl, N-tetradecyl, N-pentadecyl, N-hexadecyl, N-heptadecyl, or N-octadecyl.

Preferably, the selection “Ci-Cs-alkyl” comprises methyl, ethyl, propyl, isopropyl, N-butyl, isobutyl, sec-butyl, tert-butyl, amyl, tert-amyl (1,1-dimethylpropyl), 1,1,3,3-tetramethylbutyl, N-hexyl, 2-methylpentyl, neopentyl, N-heptyl, 2-ethylhexyl or N-octyl, or tridecyl, more preferably methyl, ethyl, N-propyl, isopropyl, N-butyl, isobutyl, sec-butyl, tert-butyl, 2-ethylhexyl or tridecyl.

Preferably, the selection “Ci-Cs-alkoxy” includes methoxy, ethoxy, N-propoxy, isopropoxy, N-butyloxy, N-pentyloxy, N-hexyloxy, N-heptyloxy, or N-octyloxy. More preferably, the selection “Ci-Cs-alkoxy” includes the selection “Ci-Cs-alkoxy”, even more preferably methoxy, ethoxy, N-propoxy, isopropoxy, N-butyloxy, or N-hexyloxy.

Preferably, the selection “Ci-Cs-alkoxy-Ci-Cs-alkyl” comprises methoxymethyl, ethoxymethyl, N-propoxymethyl, isopropoxymethyl, N-butyloxymethyl, N-pentyloxymethyl, N-hexyloxymethyl, N-heptyloxymethyl, N-octyloxymethyl, methoxyethyl, ethoxyethyl, N-propoxyethyl, isopropoxyethyl, N-butyloxyethyl, N-pentyloxyethyl, N-hexyloxyethyl, N-heptyloxyethyl, N-octyloxyethyl, methoxymethyl, 1-methoxyethyl, 2-methoxyethyl, 3-methoxy-n-propyl, 1-methoxy-2-propyl, 4-methoxy-n-butyl, 5-methoxy-n-pentyl, 6-methoxy-n-hexyl, 7-methoxy-n-heptyl, 8-methoxy or N-octyl, more preferably 2-methoxyethyl.

Preferably, the selection “Cs-Cs-cycloalkyl” comprises cyclopentyl or cyclohexyl, more preferably cyclohexyl.

Preferably, the selection “(R^N-Ci-Cs-alkyl” comprises 2-(dimethylamino)methyl, 2-(dimethylamino)ethyl, 2-(diethylamino)ethyl, 2-(diisopropylamino)ethyl, 2-(n-propylamino)ethyl, 3-(dimethylamino)propyl, or 3-(cyclohexylamino)propyl. Preferably, the selection “R2 and R3 together” comprises tetramethylene if R2 and R3, or R4 and R5 or both, or if R3 and R4, or R5 and Re or both, or if (R2 and R3) and (R5 and Re) together form a hydrocarbon group having three or four carbon atoms, such as in particular trimethylene, tetramethylene, propenylene, 2-butenylene, or 1,3-butadienylene.

Preferably, the compound of formula (II) is phenyl, benzyl, 3-methylphenyl, 2,6-diethylphenyl, o-isopropylphenyl, p-acetamidophenyl, 1-phenylethyl, 2-phenylethyl, 2-phenoxyethyl, 1-tetralino, or 2-tetralino.

According to one embodiment, the stabilizer comprises a compound of formula (la):

or the stabilizer comprises a compound of formula (Ib):

Preferably, the compound of formula (Ia) comprises at least one of the following compounds, 5-(N-benzyl-sulfonylamido)-(N',N"-dibenzyl)-isophthalic acid diamide, 5-(N-3-methylphenyl-sulfonylamido)-(N',N"-bis-(3-methylphenyl)-isophthalic acid diamide, 5-(N- 2,6-diethylphenyl-sulfonylamido)-(N',N"-bis-(2,6-diethylphenyl)-isophthalic acid diamide, 5-(N-phenyl-sulfonylamido)-(N',N"-bisphenyl)-isophthalic acid diamide, 5-(No-isopropyl- phenyl-sulfonylamido)-(N',N"-bis-(o-isopropylphenyl)-isophthalic acid diamide, 5-(Np- acetamido-phenyl-sulfonylamido)-(N',N”-bis-(p-acetamido-phenyl)-isophthalic acid diamide, 5-(N-1-tetralino-sulfonylamido)-(N',N”-bis-(1-tetralino)-isophthalic acid diamide, 5-(N- 3-methylphenyl-sulfonylamido)-(N',N”-bis-(3-methylphenyl)-isophthalic acid diamide, 5-(N- 1-phenylethyl-sulfonylamido)-(N',N”-bis-(1-phenylethyl)-isophthalic acid diamide, 5-(N-2- phenylethyl-sulfonylamido)-(N',N”-bis-(2-phenylethyl)-isophthalic acid diamide, 5-(N-2,6- diethylphenyl-sulfonylamido)-(N',N”-bis-(2,6-diethylphenyl)-isophthalic acid diamide, 5-(N- n-butyl-sulfonylamido)-(N',N”-di-n-butyl-isophthalic acid diamide, 5-(N-2-ethylhexyl- sulfonylamido)-(N',N”-di-2-ethylhexyl-isophthalic acid diamide, 5-(N-benzyl-sulfonylamido)- (N',N”-diphenyl)-isophthalic acid diamide, 5-(N-phenyl-sulfonylamido)-(N',N”-dibenzyl)- isophthalic acid diamide, 5-(N-benzylsulfonylamido)-(N',N”-bis-(3-methyl-phenyl)-isophthalic acid diamide, 5-(N-butyl-sulfonylamido)-(N',N”-bis-(3-methyl-phenyl)-isophthalic acid diamide, 5-(N-1-phenyl-ethyl-sulfonylamido)-(N',N”-bis-(3-methyl-phenyl)-isophthalic acid diamide, 5-(N-2-phenyl-ethyl-sulfonylamido)-(N',N”-bis-(3-methyl-phenyl)-isophthalic acid diamide, 5-(N-2-methoxy-ethyl-sulfonylamido)-(N',N”-bis-(3-methyl-phenyl)- isophthalic acid diamide, 5-(Nn-octyl-sulfonylamido)-(N',N”-bis-(3-methyl-phenyl)- isophthalic acid diamide, 5-(N-benzyl-sulfonylamido)-(N',N”-bis-(2,6-diethyl-phenyl)-isophthalic acid diamide, 5-(Nn- octyl-sulfonylamido)-(N',N”-bis-(2,6-diethyl-phenyl)-isophthalic acid diamide, and 5-(N-2-phenoxy-ethyl-sulfonylamido)-(N',N”-bis-(2,6-diethyl-phenyl)-isophthalic acid diamide.

In particular, the heat-sensitive marking material comprises a bisamide of the formula (Ic):

Preferably, the stabilizer is selected as a compound of formula (Ia) and the heat-sensitive recording material comprises a bisamide of formula (Ic), more preferably the amount of bisamide (Ic) is from 0.01 mol% to 10 mol% with respect to the compound of formula (Ia). According to one embodiment, the stabilizer comprises 5-(N-3-methylphenyl-sulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic acid diamide.

Here, the compound 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)isophthalic acid diamide comprises in particular three different polymorphic forms, comprising an α-polymorphic form with a melting point determined by DSC of 211.2 °C, a β-polymorphic form with a melting point determined by DSC of 192.2 °C and a γ-polymorphic form with a melting point determined by DSC of 215.6 °C.

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C10 alkyl, C1-C10 alkoxyl, C2-C10 alkenyl, C1-C10 fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl, wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C10 alkyl, wherein R5 is selected as C1-C10 alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4.

Preferably, the compound of formula (Nl) is a compound of formula (IV) or a compound of formula (V), wherein Ri and R3 are as defined for the compound of formula (Nl): (IV)

Preferably, the compound of formula (N-1) is a benzenesulfonamide compound.

Preferably, R1, R2 and/or R3 is selected from hydrogen, halogen, more preferably fluorine, chlorine, bromine or iodine, nitro, a straight, branched or cyclic Ci-Ce-alkyl group, more preferably methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, t-butyl, pentyl, isopentyl, neopentyl, hexyl, isohexyl, cyclopropyl, cyclobutyl, 2-methylcyclopropyl, cyclopropylmethyl, cyclopentyl, or cyclohexyl, a straight, branched or cyclic Ci-Ce-alkoxy group, more preferably methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, t-butoxy, pentyloxy, isopentyloxy, hexyloxy, cyclopropoxy, cyclobutoxy, 2-methylcyclopropoxy, cyclopropylmethoxy, cyclopentyloxy, or cyclohexyloxy; a C2-Ce-alkenyl group, more preferably a vinyl group, an allyl group, an isopropenyl group, a 1-propenyl group, a 2-propenyl group, a 1-butenyl group, a 2-butenyl group, a 3-butenyl group, a 1,3-butanedienyl group, or a 2-methyl-2-propenyl group, a Ci-Ce-fluoroalkyl group, more preferably a trifluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, a perfluorobutyl group, a perfluorohexyl group, or a perfluorocyclohexyl group, an N(R4)2 group, more preferably R4 being selected as hydrogen, phenyl, benzyl or Ci-Ce-alkyl, an NHCORs group, more preferably R5 being selected as C1-Ce-alkyl, an optionally substituted phenyl group; and as an optionally substituted benzyl group. Even more preferably, R1, R2 and/or R3 is selected as hydrogen or a straight Ci-Ce-alkyl group, most preferably R1 is hydrogen or methyl, and R2 and R3 are each hydrogen.

Even more preferably, the choices for the Ci-Ce-alkyl group selected according to R4 or R5 are the same as for the Ci-Ce-alkyl group selected according to R1.

Even more preferably, the optional substituents of the optionally substituted groups are selected from hydroxy, halogen, most preferably fluorine, chlorine, bromine or iodine, Ci-Ce-alkyl, most preferably methyl, ethyl, N-propyl, isopropyl, N-butyl, sec-butyl, T-butyl, N-pentyl, isopentyl, neopentyl, T-pentyl, N-hexyl, isohexyl, 1-methyl-pentyl, or 2-methyl-pentyl, and Ci-Ce-alkoxy, most preferably methoxy, ethoxy, N-propoxy, isopropoxy, N-butoxy, sec-butoxy, or T-butoxy.

Preferably, the compound of formula (N-1) is selected from 4-methyl-N-(2-(3-phenylureido)phenyl)benzenesulfonamide and N-(2-(3-phenylureido)phenyl)benzenesulfonamide, more preferably N-(2-(3-phenylureido)phenyl)benzenesulfonamide.

According to one embodiment, the at least one color developer comprises a compound of formula (N-1), wherein R1, R2, and R3 are selected as hydrogen.

According to one embodiment, the at least one color developer comprises N-(2-(3-phenylureido)phenyl)benzenesulfonamide, preferably the α-polymorph and/or the β-polymorph of N-(2-(3-phenylureido)phenyl)benzenesulfonamide.

In particular, the a-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (20/CuKa) of 5.8, 9.3, 13.2, 15.7, 17.3, 18.3, 18.7, 19.5, 20.3, 21.1, 21.9, 22.8, 23.3, 23.6, 24.4, 24.9, 25.6, 26.7, 27.8, 28.1, 29.3, 29.6, 30.2, 31.6, 32.3, 32.8 and/or a melting point of 158°C to 159°C (onset) determined by DSC.

In particular, the ß-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (26/CuKa) of 10.0, 11.0, 12.3, 12.7, 13.8, 14.9, 15.6, 16.8, 17.7, 18.5, 20.1, 20.9, 21.6, 22.0, 22.8, 23.0, 23.6, 24.3, 25.5, 26.7, 27.8, 28.4, 29.0, 29.8, 30.5, 31.1, 31.3 and/or a melting point of 173°C to 174°C (onset) determined by DSC.

The corresponding X-ray diffraction pattern was obtained by X-ray diffraction (XRD) measurement with a Bruker D2 Phaser, a Cu electrode, a voltage of 30 kV and a Lynxeye detector.

The corresponding melting point determined by DSC was measured by differential scanning calorimetry using a Netzsch DSC 200 F3 Maia® instrument, an Al crucible with a cold-welded, perforated lid, a heating rate of 10 K/min and a temperature range of 25°C to 200°C under ^ atmosphere.

The corresponding compound, N-(2-(3-phenylureido)phenyl)benzenesulfonamide, is also sold under the name NKK-1304.

Alternatively or in addition to the at least one colour developer according to the formula (N-1), according to the first aspect the at least one colour developer comprises a compound of the formula (1):

Preferably, Ri is selected as phenyl or naphthyl, which may be unsubstituted or substituted by, for example, Ci-Cs-alkyl, Ci-Cs-alkoxy or halogen.

More preferably, the substituents are selected from Ci-C4-alkyl, most preferably methyl or ethyl, Ci-C4-alkoxy, more preferably methoxy or ethoxy, or halogen, more preferably chlorine.

More preferably, Ri is selected as unsubstituted naphthyl.

More preferably, Ri is selected as substituted phenyl, most preferably the substituents are selected as one of the above-mentioned alkyl substituents.

Preferably, Ri is selected as unsubstituted or substituted Ci-C2o-alkyl, more preferably Ci-C8-alkoxy or halogen, even more preferably Ci-C4-alkoxy, most preferably methoxy or ethoxy, or halogen, most preferably chlorine.

More preferably, Ri is selected as unsubstituted Ci-C2o-alkyl.

Further preferably, R is unsubstituted phenyl or phenyl which is substituted by Ci-Cs-alkyl, Ci-Cs-alkoxy or halogen, with substituted phenyl being even more preferred. Most preferred is phenyl which is substituted by Ci-C4-alkyl, even more preferably methyl.

Preferably, X is a group of the formula -C(=S)- or -C(=O)-, more preferably a group of the formula -C(=O)-. Preferably, A is an unsubstituted phenylene or an unsubstituted naphthylene group, or a phenylene or a naphthylene group, which is further preferably substituted by Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy, halogen-substituted Ci-Cs-alkoxy, Ci-Cs-alkylsulphonyl, halogen, phenyl, phenoxy or phenoxycarbonyl.

Even more preferred are alkyl and alkoxy substituents which comprise 1 to 4 carbon atoms, with Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkyl-sulphonyl or halogen being most preferred.

More preferably, A is an unsubstituted naphthylene group.

Preferably, A is a heterocyclic group, which more preferably comprises unsubstituted pyrimidylene or pyrimidylene which is substituted by Ci-Cs-alkyl, even more preferably Ci-C4-alkyl.

Preferably A is a Ci-Ci2-alkylene group, more preferably Ci-Cs-alkylene, even more preferably Ci-C4-alkylene.

More preferably, A is unsubstituted phenylene or phenylene which is substituted by Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy, halogen-substituted Ci-Cs-alkoxy, Ci-Cs-alkylsulphonyl, halogen, phenyl, phenoxy or phenoxycarbonyl, even more preferably Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkylsulphonyl or halogen.

Most preferably A is unsubstituted phenylene or phenylene substituted by Ci-C4 alkyl or halogen, even more preferably unsubstituted phenylene.

Preferably, B is selected from — O — SO2 — , — SO2 — O — , — SO2 — NH — , — S — SO2 — , — O — , — O — CO — and — O — CO — NH — , more preferably from — O — SO2 — , — SO2 — O — and — SO2 — NH — , and most preferably from — O — SO2 — and — O— . Preferably R2 is aryl, more preferably phenyl or naphthyl, which is unsubstituted or substituted, even more preferably by Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy, halogen-substituted C1-Cs-alkoxy or halogen, most preferably alkyl and alkoxy substituents comprising 1 to 4 carbon atoms, with Ci-C4-alkyl and halogen being the even more preferred substituents.

Preferably R2 is naphthyl, which is more preferably unsubstituted.

Preferably, R2 is benzyl which is substituted by the substituents already mentioned for the selection of R2 as phenyl or naphthyl, with unsubstituted benzyl being further preferred.

Preferably R2 is Ci-C2o-alkyl, more preferably Ci-Cs-alkyl, even more preferably Ci-Cs-alkyl, which is unsubstituted or substituted by, for example, Ci-Cs-alkoxy, halogen, phenyl or naphthyl, most preferably unsubstituted alkyl groups, even more preferably Ci-C4-alkyl.

More preferably, R2 is Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, phenyl-substituted Ci-Cs-alkyl, naphthyl-substituted Ci-Cs-alkyl, unsubstituted phenyl, or phenyl which is substituted by Ci-Cs-alkyl, halogen-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy-substituted Ci-Cs-alkyl, Ci-Cs-alkoxy, halogen-substituted Ci-Cs-alkoxy or halogen, naphthyl and benzyl which is substituted by Ci-C4-alkyl or halogen.

Even more preferably, R2 is C1-C4-alkyl, halogen-substituted C1-C4-alkyl, phenyl which is unsubstituted or substituted by C1-C4-alkyl or halogen, naphthyl and benzyl which is unsubstituted or substituted by C1-C4-alkyl or halogen, most preferably phenyl which is unsubstituted or substituted by C1-C4-alkyl or halogen.

Preferably, Ri is phenyl which is substituted by Ci-C4-alkyl, more preferably methyl, X is -C(=O)-, A is phenylene which is unsubstituted or substituted by Ci-Cs-alkyl or halogen, with unsubstituted phenylene being further preferred, such as 1,3-phenylene, B is a group of the formula — O — SO2 — or — O — , and R2 is phenyl, naphthyl or benzyl which is unsubstituted or substituted by C1-C4-alkyl or halogen, with phenyl which is substituted by Ci-C4-alkyl being further preferred.

According to one embodiment, the at least one color developer of the formula (1) comprises 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, preferably the α-polymorph and/or the β-polymorph of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide.

In particular, the a-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (26/CuKa) of 8.5, 9.5, 11.8, 12.1, 12.2, 13.7, 14.1, 16.6, 17.1, 18.3, 18.6, 19.1, 19.3, 20.1, 20.4, 20.9, 21.3, 23.1, 24.2, 24.6, 25.0, 27.9, 28.6 and/or a melting point determined by DSC of 161 °C to 162 °C.

In particular, the ß-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (26/CuKa) of 10.3, 11.0, 12.9, 13.2, 15.4, 17.1, 18.0, 18.2, 19.4, 20.0, 20.7, 21.2, 23.0, 24.9, 25.3, 26.5, 26.8, 27.5, 30.7, 32.7 and/or a melting point determined by DSC of 166°C to 167°C.

According to one embodiment, the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6.

With the corresponding weight ratios of stabilizer to color developer in the heat-sensitive color-forming layer, the stability test defined in the description shows a particularly high image stability of the heat-sensitive marking material after a period of two weeks, and in particular also a high remaining print contrast.

In particular, the stabilizer is selected as 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)isophthalic acid diamide (trade name PF425) and the at least one color developer is selected as N-(2-(3-phenylureido)phenyl)benzenesulfonamide (trade name NKK-1304), and/or 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (trade name PF-201), wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6.

According to one embodiment, the stabilizer has a weight proportion of 1 wt.% to 10 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 2 wt.% to 8 wt.%, more preferably from 3 wt.% to 6 wt.%, and most preferably 4 wt.%.

With the corresponding weight proportion of the stabilizer in the heat-sensitive color-forming layer, the stability test defined in the description shows a particularly high remaining image stability of the heat-sensitive color-forming layer after a period of two weeks, and in particular also a high remaining print contrast.

In particular, the stabilizer is selected as 5-(N-3-methylphenyl-sulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic acid diamide (trade name PF425) and has the Stabilizer has a weight proportion of 1 wt.% to 10 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 2 wt.% to 8 wt.%, more preferably from 3 wt.% to 6 wt.%, and most preferably 4 wt.%.

According to one embodiment, the at least one color developer has a weight proportion of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 10 wt.% to 30 wt.%, more preferably from 20 wt.% to 30 wt.%, and most preferably 24 wt.%.

With the corresponding weight proportion of the at least one colour developer in the heat-sensitive colour-forming layer, a particularly high remaining image stability of the heat-sensitive colour-forming layer and, in particular, also a high remaining print contrast are shown after a period of two weeks within the framework of the stability test defined in the description.

In particular, the at least one color developer is selected from N-(2-(3-phenylureido)phenyl)benzenesulfonamide (trade name NKK-1304), and/or 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (trade name PF-201), wherein the at least one color developer has a weight proportion of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 10 wt.% to 30 wt.%, more preferably from 20 wt.% to 30 wt.%, and most preferably 24 wt.%.

According to one embodiment, the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%. A correspondingly high image fastness of the heat-sensitive color-forming layer remaining after a period of two weeks ensures a high image stability of the heat-sensitive recording material.

According to one embodiment, the adhesive comprises a pressure-sensitive adhesive and/or a heat-activatable adhesive, wherein the adhesive is preferably a permanently adhesive hot-melt adhesive based on styrene-isoprene and PVC copolymers, and/or wherein the adhesive is preferably a removable adhesive based on acrylate, and/or wherein the adhesive is preferably a permanently adhesive hot-melt adhesive based on synthetic rubber.

This achieves the technical advantage that a variety of different adhesive types can be used in the adhesive layer of the heat-sensitive recording material according to the invention, and yet the image stability remaining after a period of two weeks according to the stability test is at least 35%.

For example, a permanently adhesive hot melt adhesive based on styrene-isoprene and PVC copolymers is the S2200 adhesive from Avery Dennison.

For example, an acrylic-based removable adhesive is Avery Dennison's R5000N adhesive.

For example, a permanently adhesive hot melt adhesive based on synthetic rubber is the adhesive Technomelt PS 8746 from Henkel.

According to one embodiment, the adhesive comprises a permanently adhesive hot melt adhesive based on styrene-isoprene and PVC copolymers, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6. According to one embodiment, the color developer comprises the β-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide, and/or the color developer comprises the α-polymorphic form of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide.

This achieves the technical advantage that when using a permanently adhesive hot melt adhesive based on styrene-isoprene and PVC copolymers, when using the ß-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl]benzenesulfonamide (NKK-1304) and/or when using the α-polymorphic form of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (PF-201), particularly advantageous results are achieved in determining the image stability according to the stability test.

According to one embodiment, the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%.

According to one embodiment, the adhesive comprises a removable acrylic-based adhesive, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6.

In one embodiment, the color developer comprises the α-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide.

This provides the technical advantage that when using a removable acrylate-based adhesive, particularly advantageous properties are achieved when using the a-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide (NKK-1304). Results in determining image stability according to the stability test are achieved.

According to one embodiment, the adhesive comprises a permanently adhesive hot-melt adhesive based on synthetic rubber, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6.

According to one embodiment, the color developer comprises the α-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide, and/or the color developer comprises the α-polymorphic form of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide.

This achieves the technical advantage that when using a permanently adhesive hot melt adhesive based on synthetic rubber, when using the a-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide (NKK-1304) and/or when using the -polymorphic form of 4-methyl-N-(((3- (((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (PF201 ), particularly advantageous results are achieved in determining the image stability according to the stability test.

According to one embodiment, the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%.

According to one embodiment, the image stability is determined by thermally printing the heat-sensitive recording material with a print pattern using a printer at an energy level of 9.00 mJ/mm ² and a printing speed of about 100 mm/s, then measuring the optical density of the print pattern with a densitometer before storing the heat-sensitive recording material and measuring the optical density of the print pattern with the densitometer after a storage period of two weeks of the heat-sensitive recording material, wherein the image stability corresponds in particular to the quotient of the optical density of the print pattern after the storage period and the optical density of the print pattern before storage in %.

In particular, a temperature of 60°C and a humidity of 50% are maintained during the storage period of two weeks.

This achieves the technical advantage that a uniform stability test can be provided according to the present invention.

According to one embodiment, the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 35%, and that the print contrast of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 50%, preferably at least 60%, and more preferably at least 70%, and most preferably at least 80%.

This achieves the technical advantage that taking the print contrast into account in addition to the image stability enables a particularly effective characterization of the quality of the heat-sensitive recording material. According to one embodiment, the print contrast remaining after two weeks is determined by means of the following formula, DK in % = (oDs - oDw / oDs) x 100, where the parameter oDs corresponds to the optical density of the print pattern after a period of two weeks, and where the parameter oDw corresponds to the optical density of a non-printed area of the heat-sensitive recording material after a period of two weeks.

This provides the technical advantage of enabling effective determination of the print contrast.

According to one embodiment, the at least one color former is a dye of the triphenylmethane type, the fluoran type, the azaphthalide type and/or the fluorene type, preferably a dye of the fluoran type.

The use of the particularly preferred fluoran-type dye as at least one color former enables the provision of a heat-sensitive recording material with an advantageous price-performance ratio due to its availability and balanced application-related properties.

According to one embodiment, a fluoran-type dye is selected from the group comprising 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-4-toludin-amino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2, 4-dimethylanilino)fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-(cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-7-(3-trifluoromethylanilino)fluoran, 3-Nn-dibutylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(3-methylanilino)fluoran, 3-Nn-dibutylamino-7-(2-chloroanilino)fluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-dipentylamino- 6-methyl-7-anilinofluoran and mixtures thereof. According to one embodiment, the at least one color former has a weight proportion of 3 wt.% to 30 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 5 wt.% to 20 wt.% and particularly preferably from 8 wt.% to 15 wt.%.

According to one embodiment, the carrier substrate comprises paper, synthetic paper, and/or a plastic film.

The carrier substrate is not limited here. The carrier material preferably has a basis weight of 30 g/m ² to 100 g/m ² , in particular of 40 g/m ² to 80 g/m ² .

In particular, the carrier substrate comprises a paper substrate made of hardwood and/or softwood pulp.

According to one embodiment, the heat-sensitive recording material has at least one intermediate layer which is arranged between the carrier substrate and the heat-sensitive color-forming layer, wherein the intermediate layer preferably comprises at least one pigment, and more preferably organic hollow sphere pigments and/or calcined kaolins.

The organic hollow sphere pigments preferably comprise a styrene-acrylate copolymer. The organic hollow sphere pigments preferably have a glass transition temperature of 40 °C to 80 °C and/or an average particle size of 0.1 .m to 2.5 .m. In particular, the average particle size comprises the average particle size (D50).

The intermediate layer serves as a heat barrier between the carrier substrate and the heat-sensitive color-forming layer, and also makes it possible to improve the surface smoothness of the carrier substrate for the heat-sensitive color-forming layer. According to one embodiment, the heat-sensitive recording material has at least one protective layer and/or pressure support layer, which is arranged on the heat-sensitive color-forming layer on a side facing away from the adhesive layer, wherein the protective layer preferably comprises a binder and at least one pigment, and more preferably a binder and an inorganic pigment.

The protective layer and/or print support layer arranged on the outside of the heat-sensitive color-forming layer ensures effective protection or advantageous printability of the heat-sensitive color-forming layer.

According to one embodiment, the binder is selected from the group comprising water-soluble starches, starch derivatives, starch-based biolatices of the EcoSphere type, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, partially or fully saponified polyvinyl alcohols, chemically modified polyvinyl alcohols, such as acetoacetyl, diacetone, carboxy, silanol-modified polyvinyl alcohols, ethylene-vinyl alcohol copolymer (EVOH) or styrene-maleic anhydride copolymers, styrene-butadiene copolymers, acrylamide-(meth)acrylate copolymers, acrylamide-acrylate-methacrylate terpolymers, polyacrylates, poly(meth)acrylic acid esters, acrylate-butadiene copolymers, polyvinyl acetates, acrylonitrile-butadiene copolymers and mixtures thereof, with particular use being made of (meth)acrylamide-acrylic acid ester-based copolymers of the Bariastar ® type, which are preferably in Form of core-shell structures.

According to one embodiment, the binder has a weight proportion of 30% to 90% by weight based on the total solids content of the protective layer, wherein the weight proportion is preferably from 40% to 80% by weight.

According to one embodiment, the binder is designed as a crosslinked binder, wherein the crosslinked binder is preferably designed as a self-crosslinking binder, or wherein the protective layer comprises a crosslinker which is designed to react with the binder to obtain the crosslinked binder. In order to achieve specific application-related performance characteristics of a heat-sensitive recording material, the binder is preferably present in cross-linked form in the protective layer, with the optimum degree of cross-linking of the binder being established during a drying step of the coating process, in particular in the presence of a cross-linking agent.

According to one embodiment, the self-crosslinking binder comprises modified polyvinyl alcohols and/or modified acrylates.

Self-crosslinking binders, such as specially modified polyvinyl alcohols and/or modified acrylates, enable crosslinking without the addition of a crosslinker due to the reactive groups which are already part of the polymer of the self-crosslinking binder.

According to one embodiment, the crosslinker is selected from the group comprising polyhydric aldehydes, preferably glyoxal, dialdehyde starch, and/or glutaraldehyde, in particular alone or in admixture with boron salts, salts or esters of glyoxylic acid, ammonium zirconium carbonate, polyamidoamine epichlorohydrin resins, adipic acid dihydrazide, boric acid or salts thereof, polyamines, epoxy resins, formaldehyde oligomers, cyclic ureas, methylol urea, and melamine formaldehyde oligomers and mixtures thereof, wherein the crosslinker is further preferably selected from the group comprising ammonium zirconium carbonate and polyamidoamine epichlorohydrin resins.

According to one embodiment, the crosslinker has a weight fraction of 0.01 wt.% to 25.0 wt.% based on the total solids content of the protective layer, wherein the weight fraction is preferably from 0.05 wt.% to 15 wt.%.

According to one embodiment, the pigment comprises an inorganic pigment which is selected from the group comprising calcium carbonates, preferably synthetic, natural or precipitated calcium carbonates, aluminum oxides, aluminum hydroxides, silicas, precipitated and pyrogenic silicas, diatomaceous earths, magnesium carbonates, talc, kaolin, titanium oxide, bentonite and mixtures thereof, or wherein the pigment an organic pigment selected from the group comprising hollow pigments with a styrene/acrylate copolymer wall, urea/formaldehyde condensation polymers, and mixtures thereof.

According to one embodiment, the pigment has a weight fraction of 5% to 50% by weight based on the total solids content of the protective layer, wherein the weight fraction is preferably from 10% to 45% by weight.

According to one embodiment, the protective layer has at least one lubricant, wherein the lubricant is preferably selected from the group comprising fatty acid metal salts, preferably zinc stearate or calcium stearate, behenate salts or synthetic waxes, preferably in the form of fatty acid amides, more preferably stearic acid amide and behenic acid amide, fatty acid alkanolamides, preferably stearic acid methylolamide, paraffin waxes of various melting points, ester waxes of different molecular weights, polyethylene waxes, polypropylene waxes of different hardnesses, natural waxes, preferably carnauba wax or montan wax, and mixtures thereof.

According to one embodiment, the lubricant has a weight proportion of 1 wt.% to 30 wt.% based on the total solids content of the protective layer, wherein the weight proportion is preferably from 2 wt.% to 20 wt.%.

According to one embodiment, the protective layer comprises at least one brightener, wherein the brightener is preferably selected as a stilbene.

The brightener can be used to advantageously adjust the surface whiteness of the heat-sensitive color-forming layer.

According to one embodiment, the protective layer has a basis weight of 0.3 g/m ² to 5.0 g/m ² , preferably from 1.0 g/m ² to 3.0 g/m ² .

According to one embodiment, the protective layer has a thickness of 0.3 .m to 6.0 .m, preferably from 0.5 .m to 2.0 .m. According to one embodiment, the protective layer has a Bekk smoothness of 100 see to 3000 see, preferably 500 see to 2000 see, measured according to ISO 5267:1995-03.

A corresponding advantageous Bekk smoothness can be achieved by a smoothing process.

According to one embodiment, the protective layer has a surface roughness measured according to the standard ISO 8791-4:2008-05 of 0.5 .m to 2.50 .m, preferably of 1.00 |_im and 2.00 .m.

In addition to the at least one color former, the at least one color developer and the at least one stabilizer, the heat-sensitive color-forming layer according to one embodiment can further comprise at least one sensitizer, which has the advantage that the control of the thermal pressure sensitivity is easier to realize.

In general, crystalline substances whose melting point is between about 90°C and about 150°C and which, in the molten state, dissolve the color-forming components comprising the at least one color former and the at least one color developer without disrupting the formation of the color complex are advantageously considered as sensitizing agents. Sensitizing agents can be present alone or as mixtures.

The at least one sensitizer is preferably selected from the group comprising a fatty acid amide, in particular stearic acid amide, beheneamide or palmitamide, an ethylene-bis-fatty acid amide, in particular /V,/V'-ethylene-bis-stearic acid amide or /V,/V'-ethylene-bis-oleic acid amide, a fatty acid alkanolamide, in particular N-(hydroxymethyl)stearic acid amide, /V-hydroxymethylpalmitamide or hydroxyethylstearic acid amide, a wax, in particular polyethylene wax or montan wax, a carboxylic acid ester, in particular dimethyl terephthalate, dibenzyl terephthalate, benzyl 4-benzyloxybenzoate, di-(4-methylbenzyl)oxalate, di-(4-chlorobenzyl)oxalate or di-(4-benzyl)oxalate, an aromatic ether, in particular 1,2-diphenoxyethane, 1,2-di-(3-methylphenoxy)ethane, 2-benzyloxynaphthalene or 1,4-diethoxynaphthalene, an aromatic sulfone, in particular diphenylsulfone, an aromatic sulfonamide, in particular benzenesulfonanilide or ZV-benzyl-4-toluenesulfonamide, aromatic hydrocarbons, in particular 4-benzylbiphenyl and mixtures thereof.

According to one embodiment, the sensitizing agent has a weight proportion of 5 wt.% to 40 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 10 wt.% to 25 wt.%.

In addition to the at least one color former, the at least one color developer, the at least one stabilizer and the optionally present sensitizer, the heat-sensitive color-forming layer can, according to one embodiment, further comprise at least one further stabilizer.

The additional stabilizing agent serves as an anti-aging agent for the color complex in the heat-sensitive color-forming layer.

Preferably, the further stabilizing agent is selected from the group comprising sterically hindered phenols, more preferably 1,1,3-tris-(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, and/or 1,1-bis-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, urea-urethane compounds, ethers derived from 4,4'-dihydroxydiphenylsulfone, more preferably 4-benzyloxy-4'-(2-methylglycidyloxy)-diphenylsulfone, oligomeric ethers and mixtures thereof.

The further stabilizing agent preferably has a weight proportion of 0.2 wt.% to 1.0 wt.% based on the total proportion of the color developer in the heat-sensitive color-forming layer.

According to one embodiment, the heat-sensitive color-forming layer comprises at least one further binder, which is preferably selected from the group comprising water-soluble starches, starch derivatives, starch-based biolatices of the EcoSphere® type, methylcelluloses, hydroxyethylcelluloses, carboxymethylcellulose, partially or fully saponified polyvinyl alcohols, chemically modified polyvinyl alcohols, ethylene-vinyl alcohol copolymer (EVOH) or styrene-maleic anhydride copolymers, styrene-butadiene copolymers, acrylamide-(meth)acrylate copolymers, acrylamide-acrylate-methacrylate terpolymers, polyacrylates, poly(meth)acrylic acid esters, acrylate-butadiene copolymers, polyvinyl acetates, acrylonitrile-butadiene copolymers, and mixtures thereof, with particular use being made of (meth)acrylamide-acrylic acid ester-based copolymers of the Bariastar® type, which are preferably in the form of core-shell structures.

In order to achieve specific application-related performance characteristics of heat-sensitive recording materials, preferably of self-adhesive heat-sensitive recording materials, in particular of self-adhesive labels, the binder is preferably present in cross-linked form in the heat-sensitive layer, the optimum degree of cross-linking of the binder being established in the drying step of the coating process in the presence of a cross-linking agent.

Preferably, the binder is formed as a cross-linked further binder, wherein the cross-linked further binder is preferably formed as a self-cross-linking further binder, or wherein the heat-sensitive color-forming layer comprises a further cross-linker which is formed to react with the further binder in order to obtain the cross-linked further binder.

More preferably, the self-crosslinking additional binder comprises modified polyvinyl alcohols and/or modified acrylates.

More preferably, the further crosslinker is selected from the group comprising polyhydric aldehydes, preferably glyoxal, dialdehyde starch, and/or glutaraldehyde, in particular alone or in admixture with boron salts, salts or esters of glyoxylic acid, ammonium zirconium carbonate, polyamidoamine epichlorohydrin resins, adipic acid dihydrazide, boric acid or salts thereof, and mixtures thereof. More preferably, the further crosslinker has a weight proportion of 2.5 wt. % to 20 wt. % based on the total proportion of the crosslinkable further binder in the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 5 wt. % to 15 wt. %.

According to one embodiment, the carrier substrate, the heat-sensitive color-forming layer, and/or the adhesive layer are formed in one layer or in multiple layers.

According to one embodiment, the heat-sensitive color-forming layer has a release agent which is preferably selected from the group comprising fatty acid metal salts, more preferably zinc stearate or calcium stearate, behenate salts, synthetic waxes, in particular in the form of fatty acid amides, more preferably stearic acid amide and behenic acid amide, fatty acid alkanolamides, more preferably stearic acid methylolamide, paraffin waxes of various melting points, ester waxes of different molecular weights, polyethylene waxes, propylene waxes of different hardnesses, natural waxes, more preferably carnauba wax or montan wax and mixtures thereof.

According to one embodiment, the release agent has a weight proportion of 1 wt. % to 10 wt. % based on the total solids content in the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 3 wt. % to 6 wt. %.

According to one embodiment, the heat-sensitive color-forming layer has a further pigment, which has the advantage that these further pigments can fix the chemical melt created in the thermal printing process on their surface, and that the surface whiteness and opacity of the heat-sensitive color-forming layer and its printability with conventional printing inks can be controlled via the further pigments. In addition, the corresponding further pigments have an "extender function", for example for the relatively expensive color-imparting functional chemicals. The further pigment is preferably selected from the group comprising inorganic pigments, both synthetic and natural origin, precipitated or natural calcium carbonates, clays, aluminum oxides, aluminum hydroxides, silicas, precipitated and pyrogenic silicas, diatomaceous earths, magnesium carbonates, talc, but also organic pigments, more preferably hollow pigments with a styrene/acrylate copolymer wall, urea/formaldehyde condensation polymers and mixtures thereof.

The additional pigment preferably has a weight proportion of 15 wt.% to 50 wt.% based on the total solids content in the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 20 wt.% to 35 wt.%.

According to one embodiment, the heat-sensitive color-forming layer comprises at least one further brightener, wherein the further brightener is preferably selected as a stilbene.

The additional brightener can be used to control the surface whiteness of the heat-sensitive recording material.

According to one embodiment, the heat-sensitive color-forming layer comprises a rheology aid, preferably a thickener and/or a surfactant.

This has the advantage that certain coating properties of the heat-sensitive color-forming layer can be improved during the manufacturing process.

According to one embodiment, the application weight of the heat-sensitive color-forming layer is from 1 g/m ² to 10 g/m ² , preferably from 3 g/m ² to 5 g/m ² .

According to one embodiment, the heat-sensitive color-forming layer has a Bekk smoothness of 100 see to 1500 see, preferably 200 see to 750 see, measured according to ISO 5267:1995-03. According to one embodiment, the surface application weight of the adhesive layer is from 10 g/m ² to 150 g/m ² , preferably from 15 g/m ² to 30 g/m ² .

The above-mentioned object is achieved according to the second aspect by a method for producing a heat-sensitive recording material, comprising the following method steps:

Providing a carrier substrate having a first side and a second side facing away from the first side;

Applying an application suspension to the first side of the carrier substrate, wherein the application suspension comprises at least one color former, at least one color developer, and at least one stabilizer;

drying the coating suspension to obtain a heat-sensitive color-forming layer arranged on the first side of the carrier substrate;

Applying an adhesive layer to the second side of the carrier substrate, wherein the weight ratio of stabilizer to color developer in the application suspension is selected such that migration of components of the adhesive layer of the heat-sensitive recording material into the heat-sensitive color-forming layer of the heat-sensitive recording material according to the stability test defined in the description after a period of two weeks leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35%, wherein the stabilizer comprises a compound of formula (I):

wherein R and R1 are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-alkyl, wherein R9 is selected from the group comprising Ci-Cs-alkyl, Cs-Cs-cycloalkyl; or a compound of formula (II)

wherein R2, R3, R4, Rs, and Re are independently selected from the group comprising hydrogen, Ci-Cs-alkyl, -NH-C(=O)-R?, or -C(=O)-NH-R?, wherein R? is selected as Ci-Cs-alkyl or -C(=O)ORs, wherein Rs is selected as Ci-Cs-alkyl or halogen, or wherein R2 and R3, or R4 and Rs or both, or wherein R3 and R4, or Rs and Re or both, or wherein R2 and R3 and Rs and Re together form a hydrocarbon group having three or four carbon atoms, and wherein Q comprises a single bond or Ci-Cs-alkylene, which may be branched or unbranched, and wherein the Ci-Cs-alkylene comprises a main chain which has one or more oxygen atoms between two carbon atoms when the Ci-Cs-alkylene has more than two carbon atoms; wherein the at least one color developer comprises a compound of formula (NI):

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C4-alkyl, C1-C4-alkoxyl, C2-C4-alkenyl, C1-C4-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl; wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, or C1-C4-alkyl, wherein R5 is selected as C1-C4-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or wherein the at least one color developer comprises a compound of formula (1):

where Ri is selected from the group comprising unsubstituted or substituted phenyl, naphthyl or Ci-C2o-alkyl, where X is selected from the group comprising — C(=NH) — , — C(=S) — and — C(=O) — , where A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, Ci-Ci2-alkylene, and a unsubstituted or substituted heterocyclic group, where B is selected from the group comprising — O — SO2 — , — SO2 — O — , — NH — SO ₂ — , — SO ₂ — NH—, — S— SO ₂ — , — O— CO— , — O— CO— NH— , — NH— CO— , — NH— CO— O— , — S— CO— NH— , — S— CS— NH— , — CO— NH— SO ₂ — , — O— CO— NH— SO ₂ — , — NH=CH— , —CO— NH— CO— , — S— , —CO—, — O— , — SO ₂ — NH— CO — , — O — CO — O — and — O — PO — (OR2)2, and wherein R2 is selected from the group comprising unsubstituted or substituted aryl, benzyl and C1-C20-alkyl, with the proviso that when B is not a group of the formula — O — SO2 —, then R2 is unsubstituted or substituted phenyl, naphthyl or Ci-Cs-alkyl, and that when B is — O —, then R2 is not alkyl. By the method according to the second aspect, the heat-sensitive recording material can be advantageously obtained.

According to one embodiment, applying the adhesive layer to the second side of the carrier substrate may comprise applying, in particular laminating, an adhesive film to the second side of the carrier substrate.

According to an alternative embodiment, applying the adhesive layer to the second side of the carrier substrate may comprise applying an adhesive dispersion to the second side of the carrier substrate and subsequently drying the adhesive dispersion to obtain the adhesive layer arranged on the second side of the carrier substrate.

According to one embodiment, the drying of the adhesive layer is carried out at a temperature of 60 °C to 80 °C, preferably at 70 °C.

According to one embodiment, the method comprises the further method step, which is carried out after the application of the adhesive layer:

Applying a release paper to the adhesive layer, wherein the release paper is preferably designed as a siliconized release paper.

According to one embodiment, the application of the adhesive layer comprises applying an adhesive dispersion and subsequently drying the adhesive dispersion, the method comprising the further method step which is carried out before the application of the adhesive dispersion:

Applying another release paper to the second side of the carrier substrate, whereby the adhesive dispersion is applied to the additional release paper.

According to one embodiment, the application and drying of the application dispersion on the first side of the carrier substrate is carried out before the application of the adhesive layer on the second side of the carrier substrate, or the application of the adhesive layer to the second side of the carrier substrate is carried out before the application and drying of the application dispersion to the first side of the carrier substrate, wherein preferably the application and drying of the application dispersion to the first side of the carrier substrate is carried out before the application of the adhesive layer to the second side of the carrier substrate.

According to one embodiment, the application of the application dispersion to the first side of the carrier substrate is carried out by means of curtain coating or by means of a bar doctor on the coating side of a carrier substrate pre-coated with a pigment coating, wherein the pigmented pre-coat preferably comprises calcined kaolin and a binder based on styrene-butadiene and/or polyvinyl alcohol (PVA) and/or starch, or organic pigments in a mixture with inorganic pigments, wherein the application amount of the pigmented pre-coat is more preferably from 2 g/m ² to 12 g/m ² .

According to one embodiment, the application amount of the application dispersion is between 3 g/m ² and 5 g/m ² , preferably between 3.6 g/m ² and 4.8 g/m ² , and/or the application amount of the adhesive dispersion is between 10 g/m ² and 30 g/m ² , preferably 20 g/m ² .

According to one embodiment, the application suspension is produced in particular by grinding and preferably has an average grain size D<4.3) of 0.5 pm to 5.0 pm, particularly preferably of 0.50 pm to 1.50 pm and very particularly preferably of 1.5 pm to 2.50 pm.

The embodiments given for the heat-sensitive recording material according to the first aspect are also embodiments for the method for producing a heat-sensitive recording material according to the second aspect and vice versa. The above object is achieved according to the third aspect by a heat-sensitive recording material producible by a method according to the second aspect.

The embodiments given for the heat-sensitive recording material according to the first aspect and the method for producing a heat-sensitive recording material according to the second aspect are also embodiments for the heat-sensitive recording material producible by a method according to the third aspect.

The above-mentioned object is achieved according to the fourth aspect by using at least one stabilizer and at least one color developer in a heat-sensitive recording material, comprising a carrier substrate which has a first side and a second side facing away from the first side, a heat-sensitive color-forming layer which is arranged on the first side of the carrier substrate, wherein the heat-sensitive color-forming layer comprises at least one color former, the at least one color developer, and the at least one stabilizer, and an adhesive layer which is arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that migration of components of the adhesive layer of the heat-sensitive recording material into the heat-sensitive color-forming layer of the heat-sensitive recording material according to the stability test defined in the description after a period of two weeks leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35%, wherein the stabilizer comprises a compound of formula (I):

wherein R and R1 are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-alkyl, wherein R9 is selected from the group comprising Ci-Cs-alkyl, Cs-Cs-cycloalkyl; and a compound of formula (II)

wherein R2, R3, R4, Rs, and Re are independently selected from the group comprising hydrogen, Ci-Cs-alkyl, -NH-C(=O)-R?, and -C(=O)-NH-Ry, wherein R? is selected as Ci-Cs-alkyl or -C(=O)ORs, wherein Rs is selected as Ci-Cs-alkyl or halogen, or wherein R2 and R3, or R4 and Rs or both, or wherein R3 and R4, or Rs and Re or both, or wherein R2 and R3 and Rs and Re together form a hydrocarbon group having three or four carbon atoms, and wherein Q comprises a single bond or Ci-Cs-alkylene, which may be branched or unbranched, and wherein the Ci-Cs-alkylene comprises a main chain which has one or more oxygen atoms between two carbon atoms when the Ci-Cs-alkylene has more than two carbon atoms; wherein the at least one color developer comprises a compound of formula (NI):

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C4-alkyl, C1-C4-alkoxyl, C2-C4-alkenyl, C1-C4-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl; wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C4-alkyl, wherein R5 is selected as C1-C4-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or wherein the at least one color developer comprises a compound of formula (1):

where Ri is selected from the group comprising unsubstituted or substituted phenyl, naphthyl or Ci-C2o-alkyl, where X is selected from the group comprising — C(=NH) — , — C(=S) — or — C(=O) — , where A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, Ci-Ci2-alkylenes, and a unsubstituted or substituted heterocyclic group, where B is selected from the group comprising — O — SO2 — , — SO2 — O — , — NH — SO ₂ — , — SO ₂ — NH— , — S— SO ₂ — , — O— CO— , — O— CO— NH— , — NH— CO— , — NH— CO— O— , — S— CO— NH— , — S— CS— NH— , — CO— NH— SO ₂ — , — O— CO— NH— SO ₂ — , — NH=CH— , —CO— NH— CO— , — S— , —CO—, — O— , — SO ₂ — NH— CO — , — O — CO — O — and — O — PO — (OR2)2, and wherein R2 is selected from the group comprising unsubstituted or substituted aryl, benzyl and C1-C20-alkyl, with the proviso that when B is not a group of the formula — O — SO2 —, then R2 is unsubstituted or substituted phenyl, naphthyl or Ci-Cs-alkyl, and that when B is — O —, then R2 is not alkyl.

The embodiments given for the heat-sensitive recording material according to the first aspect and those given for the method for producing a heat-sensitive recording material according to the second aspect are also embodiments for the use of at least one stabilizer and at least one color developer in a heat-sensitive recording material according to the fourth aspect.

EXAMPLES OF IMPLEMENTATION

In the following detailed embodiments, a large number of heat-sensitive recording materials or thermal papers were produced by applying aqueous application suspensions and adhesive dispersions to form a composite structure on a carrier substrate.

Production of heat-sensitive recording materials

For example, the adhesive dispersion is applied with a doctor blade to the second side of a carrier substrate, in particular A4 paper, which bears the color-forming heat-sensitive layer on the first side, which is the front side, and dried with a hot air dryer at a maximum of 70°C in order to obtain the adhesive layer. To protect the adhesive layer during further processing, a siliconized release paper is laminated onto the adhesive layer, avoiding air pockets and wrinkles.

In the case of an “adhesive liner sandwich” consisting of a thin adhesive layer between two release papers, after removing one of the two liner papers, the sticky side of the adhesive layer is laminated to the second side, which forms the back of the carrier substrate, while avoiding air pockets and wrinkles. It is irrelevant whether, during the production of the heat-sensitive recording material, the adhesive layer is first applied to the second side of the carrier substrate and then the heat-sensitive recording layer is applied to the opposite first side of the carrier substrate bearing the adhesive layer, or vice versa.

The application of an aqueous application suspension for forming the heat-sensitive color-forming layer of a heat-sensitive recording material is carried out, in particular on a laboratory scale, by means of a doctor blade on the coating side of a paper pre-coated with a pigment coating and having a basis weight of 65 g/m ² and 72 g/m ² .

The composition of the pigmented primer is not critical. In particular, this coating consists of calcined kaolin and a binder based on styrene-butadiene and/or starch. In particular, primers with organic (hollow sphere) pigments, possibly mixed with inorganic pigments, are also possible. The application quantity of this pigmented layer is between about 2 and 10 g/m ² .

After drying the aqueous application suspension of the heat-sensitive coating material, the heat-sensitive color-forming layer is obtained. The application quantity of the heat-sensitive color-forming layer is between 3.8 g/m ² and 4.3 g/m ² .

By applying an adhesive layer to the opposite back of the substrate carrying the heat-sensitive layer, or the second side of the carrier substrate, a composite material suitable for use as a thermal label is obtained. The amount of adhesive applied is approximately 20 g/m ² .

The preparation of the dispersions (each for 1 part by weight) for the application suspensions is described below.

Color former dispersion A: Color former dispersion A was prepared by grinding 20 parts by weight of 3-/Vn-dibutylamine-6-methyl-7-anilinofluorane (ODB-2) with 33 parts by weight of a 12% aqueous solution of Gohsenx™ L-3266 (sulfonated polyvinyl alcohol, Nippon Gohsei) in a bead mill.

Stabilizer dispersion B:

Stabilizer dispersion B was prepared by grinding 40 parts by weight of (N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic acid diamide (GAS# 2375645-78-4, referred to as Compound I in the table below) together with 66 parts by weight of a 12% aqueous solution of Gohsenx™ L-3266 (sulfonated polyvinyl alcohol, Nippon Gohsei) in a bead mill.

Colour developer dispersions C:

The aqueous color developer dispersions C were prepared by grinding 40 parts by weight of the respective color developer 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (GAS# 232938-43-1, a-polymorph, referred to as compound II in the table below), 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (GAS# 232938-43-1, ß-polymorph, referred to as compound III in the table below), N-(2-(3-phenylureido)phenyl)benzenesulfonamide (GAS# 215917-77-4, a-polymorph, referred to as compound IV in the table below), or N-(2-(3-phenylureido)phenyl)benzenesulfonamide (GAS# 215917-77-4, ß-polymorph, referred to as Compound V in the table below) together with 66 parts by weight of a 12% aqueous solution of Gohsenx™ L-3266 (sulfonated polyvinyl alcohol, Nippon Gohsei) in a bead mill.

Sensitizing dispersion D:

Sensitizing dispersion D was prepared by grinding 40 parts by weight of 1,2-diphenoxyethane with 66 parts by weight of a 12% aqueous solution of Gohsenx™ L-3266 (sulfonated polyvinyl alcohol, Nippon Gohsei) in a bead mill. All dispersions A, B, C and D produced by grinding have an average grain size D(4.3) of 0.7 .m to 1.3 .m. The grain size distribution of the dispersions was measured by laser diffraction with a Coulter LS13320 device from Beckman Coulter.

Lubricant dispersion E:

The lubricant dispersion E is a 20% zinc stearate dispersion comprising 9 parts by weight of zinc stearate, 1 part by weight of Gohsenx™ L-3266 (sulfonated polyvinyl alcohol, Nippon Gohsei) and 40 parts by weight of water.

Pigment P:

Pigment P is a 72% coating kaolin suspension (Lustra® S, BASF).

Binder solution:

The binder solution consists of a 10% aqueous polyvinyl alcohol solution (Poval 28-99, Kuraray Europe).

The heat-sensitive application suspension was prepared by mixing, while stirring, 1 part by weight of the color former dispersion A, 1 part by weight of the stabilizer dispersion B, 1.5 parts by weight of the respective color developer dispersion C, 2.5 parts by weight of the sensitizing dispersion D, 1 part by weight of the lubricant dispersion E, 2.1 parts by weight of the pigment P and 2.5 parts by weight of the binder solution, taking into account the order of addition C, D, B, E, P, A, and brought to a solids content of about 25% with water. The application suspension was presented as an example for the color developer-stabilizer ratio of 1.5:1.

The heat-sensitive application suspensions thus obtained were used to produce composite structures consisting of a paper carrier and a heat-sensitive color-forming layer.

Adhesives In order to produce self-adhesive thermal labels, the following commercially available adhesives were used on the side of the paper carrier facing away from the heat-sensitive color-forming layer, or the second side of the carrier substrate:

- R5000N (Avery Dennison) is a removable acrylic-based adhesive.

- S2200 (Avery Dennison) is a permanent hot melt adhesive for deep-freeze applications based on styrene-isoprene and PVC copolymers.

- Technomelt PS 8746 (Henkel) is a permanent hot melt adhesive based on synthetic rubber.

The heat-sensitive recording materials thus obtained, equipped as self-adhesive thermal labels, were tested and evaluated as shown in the table below.

Measurement of optical density:

6 cm wide strips were obtained from the heat-sensitive recording materials produced in this way. These were thermally printed using a GeBE PrinterLab GPT-10000 test printer (GeBE Elektronik und Feinwerktechnik GmbH, Germany) with a Kyocera print bar of 305 dpi at an applied voltage of 24 V and with a checkerboard pattern at an energy level of 9.00 mJ/mm ² and a printing speed of around 100 mm/s. The area of one checker of the print pattern corresponds to 80 x 80 points. The optical density (oD) shown in the table below was measured with a SpectroEye densitometer from X-Rite, whereby the measurement uncertainty of the oD values is estimated at <2%. The scatter of the corresponding calculated % values is <±2 percentage points.

Measurement of relative pressure contrast

The relative print contrast was determined using the value of the optical density of a thermally printed area (oDs) and the optical density of a non-printed area (oDw) is calculated according to the following formula B, where s stands for a black area and w for a white area:

DK in % = (oDs - oDw / oDs) x 100

(Formula B)

Adhesive stability test of thermal label papers

A strip of the heat-sensitive recording material was measured according to the measurement described above and the optical density (oD) before storage was determined. The heat-sensitive recording material was then stored for two or four weeks between two glass plates at 60 °C, a pressure of 1350 N/m ² , a relative humidity of 50% and under exclusion of light.

After the end of the storage period and conditioning to room temperature, the optical densities of the printed (oDs, at an energization energy of 9.00 mJ/mm ² ) and non-printed (oDw) areas were measured, averaged and used to determine the relative print contrast according to formula B.

Using the optical densities before and after storage at an energization energy of 9.00 mJ/mm ^{2 ,} the remaining image stability was determined according to the following formula A:

Image stability in % = (optical density after storage / optical density before storage) x 100

(Formula A)

Evaluation

The table below describes the results of the described image stability and print contrast measurements for a variety of different heat-sensitive recording materials, which were tested on a a heat-sensitive color-forming layer on the first side of the carrier substrate and an adhesive layer on the side of the carrier substrate facing away from the first side.

The results obtained in the table are based on the analysis of the image stability and the print contrast of the respective heat-sensitive recording material as a function of a migration of components of the adhesive layer into the heat-sensitive color-forming layer, after the corresponding period of two weeks or four weeks.

According to the table, the adhesives tested for the adhesive layer were the previously mentioned adhesives R5000N from Avery Dennison, S2200 from Avery Dennison, and Technomelt PS 8746 from Henkel.

According to the table, the compound 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)isophthalic acid diamide (PF425) was investigated as a stabilizer of the heat-sensitive color-forming layer, which is designated as compound I in the table.

According to the table, the compound 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (PF-201) which comprises the α-polymorph as compound II and β-polymorph as compound III shown in the table was investigated as the color developer of the heat-sensitive color-forming layer.

According to the table, the compound N-(2-(3-phenylureido)phenyl)benzenesulfonamide (NKK-1304) which comprises the α-polymorph as compound IV and β-polymorph as compound V shown in the table was investigated as the color developer of the heat-sensitive color-forming layer.

The table is shown below. Tabel:

From the results shown in the table, it can be seen that the heat-sensitive recording material according to the present invention exhibits the following advantageous properties.

Firstly, it is emphasized that for all adhesives investigated (R5000, S2200, and Technomelt) in the corresponding comparative experiments the exclusive use of the stabilizer 5-(N-3-methylphenyl-sulfonylamido)-(N',N"-bis-{3-methylphenyl)- isophthalic acid diamide (PF425) according to the compound I shown in the table without the addition of a corresponding color developer (see compounds II, III, IV, V in the table) only leads to a corresponding extremely low remaining image stability after two weeks, which is therefore not in accordance with the invention.

According to the corresponding comparative experiments, the same applies to all adhesives investigated (R5000, S2200, and Technomelt) also for the exclusive use of the corresponding color developer N-(2-(3-phenylureido)phenyl)benzenesulfonamide (NKK-1304) according to the a-polymorph IV and ß-polymorph V shown in the table, and the corresponding color developer 4-methyl-N-(((3-(((4-methyl-phenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (PF-201) according to the a-polymorph II and ß-polymorph III shown in the table, without the addition of a corresponding stabilizer of compound I.

For example, when the α-polymorph IV or the β-polymorph V of N-(2-(3-phenylureido)phenyl)benzenesulfonamide (NKK-1304) is added alone, only a corresponding remaining extremely low image stability of 10% or 7% when using the R5000 adhesive is obtained after two weeks, which is significantly below the limit of the remaining image stability of at least 35% considered sufficient according to the present invention.

The same applies to the sole addition of compounds II or III, whereby the corresponding remaining extremely low image stability of 26% or 29% is recorded when using the adhesive R5000 after the corresponding period of two weeks.

From the corresponding comparative examples in the table, it can thus be seen that the exclusive use of the stabilizer alone or of the respective color developer alone is not sufficient to achieve a remaining image stability of at least 35%, which is considered to be in accordance with the invention. Secondly, it is emphasized that for all adhesives investigated (R5000, S2200, and Technomelt), the inventive combination of stabilizer with at least one color developer in all cases leads to a significant increase in the image stability remaining after the investigation period of two weeks compared to the use of stabilizer or color developer alone.

For example, for the adhesive R5000 in combination with the colour developer

III with the stabilizer I, depending on the weight ratio, an image stability of between 38% and 51% remaining after two weeks can be obtained, which is significantly higher than the corresponding image stability when using the stabilizer alone (16%) or compound III alone (29%).

A corresponding effect can be seen from the table for all adhesives and for all colour developers examined.

Thirdly, it is emphasized that for all adhesives studied (R5000, S2200, and Technomelt), it was found that an increase in the weight ratio of stabilizer to color developer leads in all cases to a significant increase in the image stability remaining after the two-week study period.

For example, for the adhesive R5000 in combination with the colour developer

IV with the stabilizer I for a weight ratio of 1.5 parts by weight of color developer IV with 1 part by weight of stabilizer I, after the corresponding period of two weeks a remaining image stability of 72% can be taken, which corresponds to an excellent value.

When using the S2200 adhesive, when combining the color developer IV with the stabilizer I for a weight ratio of 1.5 parts by weight of color developer IV with 1 part by weight of stabilizer I, a remaining image stability of as much as 75% can be obtained after the corresponding period of two weeks, which corresponds to an extremely excellent value. A corresponding optimization of the remaining image stability after a period of two weeks, based on the optimization of the weight ratio between the respective color developer and the stabilizer, can be taken from the table for all adhesives tested and for all color developers tested.

Thus, a synergistic effect with respect to the image stability remaining after two weeks can be demonstrated by the combination of stabilizer and at least one color developer in the heat-sensitive color-forming layer.

Fourthly, it is emphasized that the print contrast remaining after the two-week test period is also optimized by optimizing the weight ratio between the respective color developer and the stabilizer.

For example, for the adhesive R5000, when combining the colour developer IV with the stabiliser I for a weight ratio of 1.5 parts by weight of colour developer IV with 1 part by weight of stabiliser I, a relative print contrast of 91% can be obtained after the corresponding period of two weeks, which corresponds to an excellent value.

Thus, a synergistic effect in relation to the print contrast remaining after two weeks can be demonstrated by the combination of stabilizer and at least one color developer in the heat-sensitive color-forming layer.

Claims

EXPECTATIONS

1. Heat-sensitive recording material, comprising: a carrier substrate which has a first side and a second side facing away from the first side; a heat-sensitive color-forming layer which is arranged on the first side of the carrier substrate, wherein the heat-sensitive color-forming layer comprises at least one color former, at least one color developer, and at least one stabilizer; and an adhesive layer which is arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive; wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that migration of components from the adhesive layer into the heat-sensitive color-forming layer according to the stability test defined in the description after a period of two weeks leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35%; wherein the stabilizer comprises a compound of the formula (I):

wherein R and R are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-AI- kyl, wherein Rg is selected from the group comprising Ci-Cs-alkyl, Cs-Cs-cycloalkyl; or a compound of formula (II)

wherein R2, R3, R4, Rs, and Re are independently selected from the group comprising hydrogen, Ci-Cs-alkyl, -NH-C(=O)-R?, and -C(=O)-NH-Ry, wherein R? is selected as Ci-Cs-alkyl or -C(=O)ORs, wherein Rs is selected as Ci-Cs-alkyl or halogen, or wherein R2 and R3, or R4 and Rs or both, or wherein R3 and R4, or Rs and Re or both, or wherein R2 and R3 and Rs and Re together form a hydrocarbon group having three or four carbon atoms, and wherein Q comprises a single bond or Ci-Cs-alkylene, which may be branched or unbranched, and wherein the Ci-Cs-alkylene comprises a main chain which has one or more oxygen atoms between two carbon atoms when the Ci-Cs-alkylene has more than two carbon atoms; wherein the at least one color developer comprises a compound of the formula (N-I):

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C4-alkyl, C1-C4-alkoxyl, C2-C6-alkylene, C1-C4-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl, wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C4-alkyl, wherein R5 is selected as C1-C4-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or wherein the at least one color developer comprises a compound of formula (1):

where Ri is selected from the group comprising unsubstituted or substituted phenyl, naphthyl and Ci-C2o-alkyl, where X is selected from the group comprising — C(=NH) — , — C(=S) — and — C(=O) — , where A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, C1-C12-alkylene, and a unsubstituted or substituted heterocyclic group, where B is selected from the group comprising — O — SO2 — , — SO2 — O — , — NH — SO ₂ — , — SO ₂ — NH—, — S— SO ₂ — , — O— CO— , — O— CO— NH— , — NH— CO— , — NH— CO— O— , — S— CO— NH— , — S— CS— NH— , —CO— NH— SO ₂ — , — O— CO— NH— SO ₂ — , — NH=CH— , —CO— NH— CO— , — S— , —CO—, — O— , — SO ₂ — NH— CO — , — O — CO — O — and — O — PO — (OR2)2, and wherein R2 is selected from the group comprising unsubstituted or substituted aryl, benzyl and C1-C20-alkyl, with the proviso that when B is not a group of the formula — O — SO2 —, then R2 is unsubstituted or substituted phenyl, naphthyl or Ci-Cs-alkyl, and that when B is — O —, then R2 is not alkyl.

2. A heat-sensitive recording material according to claim 1, wherein the stabilizer comprises a compound of formula (Ia):

or wherein the stabilizer comprises a compound of formula (Ib):

Heat-sensitive recording material according to claim 1 or 2, wherein the stabilizer comprises 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic acid diamide. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color developer comprises a compound of formula (N1), and wherein R1, R2, and R3 are selected as hydrogen. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color developer comprises N-(2-(3-phenylureido)phenyl)benzenesulfonamide, preferably the α-polymorph and/or the β-polymorph of N-(2-(3-phenylureido)phenyl)benzenesulfonamide, wherein in particular the α-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (26/CuKa) of 5.8, 9.3, 13.2, 15.7, 17.3, 18.3, 18.7, 19.5, 20.3, 21.1 , 21.9, 22.8, 23.3, 23.6, 24.4, 24.9, 25.6, 26.7, 27.8, 28.1 , 29.3, 29.6, 30.2, 31.6, 32.3, 32.8 and/or a melting point determined by DSC of 158°C to 159°C. wherein in particular the ß-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (20/CuKa) of 10.0, 11.0, 12.3, 12.7, 13.8, 14.9,

15.6, 16.8, 17.7, 18.5, 20.1 , 20.9, 21.6, 22.0, 22.8, 23.0, 23.6, 24.3, 25.5, 26.7, 27.8, 28.4, 29.0, 29.8, 30.5, 31.1 , 31.3 and/or a melting point of 173°C to 174°C as determined by DSC.

6. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color developer comprises a compound of formula (1), wherein X is selected as - C(=O) - , wherein Ri is selected as substituted or substituted phenyl, preferably as Ci-Ca-alkyl-substituted phenyl, wherein R ² is selected as unsubstituted or substituted aryl, preferably unsubstituted phenyl, wherein B is selected as - O - SO2 - , and wherein R2 is selected as substituted or substituted aryl, preferably as Ci-Ca-alkyl-substituted phenyl.

7. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color developer comprises 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, preferably the α-polymorph and/or the β-polymorph of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, wherein in particular the α-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (20/CuKa) of 8.5, 9.5, 11.8, 12.1, 12.2, 13.7, 14.1, 16.6, 17.1, 18.3, 18.6, 19.1, 19.3, 20.1, 20.4, 20.9, 21.3, 23.1, 24.2, 24.6, 25.0, 27.9, 28.6 and/or a melting point of 161 °C to 162 °C determined by DSC, wherein in particular the ß-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (20/CuKa) of 10.3, 11.0, 12.9, 13.2, 15.4, 17.1, 18.0, 18.2, 19.4, 20.0,

20.7, 21.2, 23.0, 24.9, 25.3, 26.5, 26.8, 27.5, 30.7, 32.7 and/or a melting point of 166°C to 167°C as determined by DSC.

8. Heat-sensitive recording material according to one of the preceding claims, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6. Heat-sensitive recording material according to one of the preceding claims, wherein the stabilizer has a weight proportion of 1 wt. % to 10 wt. % based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 2 wt. % to 8 wt. %, more preferably from 3 wt. % to 6 wt. %, and most preferably 4 wt. %. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color developer has a weight proportion of 6 wt. % to 35 wt. % based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 10 wt. % to 30 wt. %, more preferably from 20 wt. % to 30 wt. %, and most preferably 24 wt. %. Heat-sensitive recording material according to one of the preceding claims, wherein the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75%, and most preferably at least 80%. Heat-sensitive recording material according to one of the preceding claims, wherein the adhesive comprises a pressure-sensitive adhesive and/or a heat-activatable adhesive, wherein the adhesive is preferably a permanently adhesive hot-melt adhesive based on styrene-isoprene and PVC copolymers, and/or wherein the adhesive is preferably a removable adhesive based on acrylate, and/or wherein the adhesive is preferably a permanently adhesive hot-melt adhesive based on synthetic rubber. Heat-sensitive recording material according to one of the preceding claims, wherein the adhesive comprises a permanently adhering hot melt adhesive based on styrene-isoprene and PVC copolymers, and wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6. Heat-sensitive recording material according to claim 13, wherein the color developer comprises the β-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl]benzenesulfonamide, and/or wherein the color developer comprises the α-polymorphic form of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide. A heat-sensitive recording material according to claim 13 or 14, wherein the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%. A heat-sensitive recording material according to any one of the preceding claims, wherein the adhesive comprises a removable acrylate-based adhesive, and wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6. A heat-sensitive recording material according to claim 16, wherein the color developer comprises the α-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl}benzenesulfonamide.

18. A heat-sensitive recording material according to claim 16 or 17, wherein the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%.

19. A heat-sensitive recording material according to any one of the preceding claims, wherein the adhesive comprises a permanently adhesive hot-melt adhesive based on synthetic rubber, and wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is from 1 to 11 to 1 to 1, preferably from 1 to 8 to 1 to 4, more preferably from 1 to 7 to 1 to 5, and most preferably 1 to 6.

20. A heat-sensitive recording material according to claim 19, wherein the color developer comprises the α-polymorphic form of N-{2-[(phenylcarbamoyl)amino]phenyl]benzenesulfonamide, and/or wherein the color developer comprises the α-polymorphic form of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide.

21. A heat-sensitive recording material according to claim 19 or 20, wherein the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, even more preferably at least 60%, even more preferably at least 65%, even more preferably at least 70%, even more preferably at least 75% and most preferably at least 80%.

22. A heat-sensitive recording material according to any one of the preceding claims, wherein the image stability is determined by thermally printing the heat-sensitive recording material with a print pattern by means of a printer at an energy level of 9.00 mJ/mm ² and a printing speed of about 100 mm/s, a subsequent measurement of the optical density of the print pattern with a densitometer before storage of the heat-sensitive recording material and a measurement of the optical density of the print pattern with the densitometer after a storage period of two weeks of the heat-sensitive recording material, wherein the image stability corresponds in particular to the quotient of the optical density of the print pattern after the storage period and the optical density of the print pattern before storage in %.

23. A heat-sensitive recording material according to claim 22, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that the image stability of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 35%, and that the print contrast of the heat-sensitive color-forming layer remaining after a period of two weeks is at least 50%, preferably at least 60%, and more preferably at least 70% and most preferably at least 80%.

24. A heat-sensitive recording material according to claim 23, wherein the print contrast remaining after two weeks is determined by means of the following formula, DK in % = (oDs - oDw / oDs) x 100, wherein the parameter oDs corresponds to the optical density of the print pattern after a period of two weeks, and wherein the parameter oDw corresponds to the optical density of a non-printed area of the heat-sensitive recording material after a period of two weeks.

25. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color former is a dye of the triphenylmethane type, the fluoran type, the azaphthalide type and/or the fluorene type, preferably a dye of the fluoran type.

26. A heat-sensitive recording material according to claim 25, wherein a fluoran-type dye is selected from the group comprising 3-diethylamino- 6-methyl-7-anilinofluoran, 3-(N-Ethyl-N-4-toludinamino)-6-methyl-7-anilinofluoran, 3-(N-Ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-Diethylamino-6- methyl-7-(2, 4-dimethylanilino)fluoran, 3-Pyrrolidino-6-methyl-7-anilinofluoran, 3- (Cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran, 3-Diethylamino-7-(3-trifluoromethylanilino)fluoran, 3-Nn-Dibutylamino-6-methyl-7-anilinofluoran, 3-Diethylamino-6-methyl-7-(3-methylanilino)fluoran, 3-Nn-Dibutylamino-7-(2-chloroani- lino)fluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropyl- lamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-dipentylamino-6-methyl-7-anilinofluoran and mixtures thereof.

27. Heat-sensitive recording material according to one of the preceding claims, wherein the at least one color former has a weight proportion of 3 wt.% to 30 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight proportion is preferably from 5 wt.% to 20 wt.% and particularly preferably from 8 wt.% to 15 wt.%.

28. A process for producing a heat-sensitive recording material, comprising the following process steps:

wherein R2, R3, R4, Rs, and Re are independently selected from the group comprising hydrogen, Ci-Cs-alkyl, -NH-C(=O)-R?, or -C(=O)-NH-Ry, wherein R? is selected as Ci-Cs-alkyl or -C(=O)ORs, wherein Rs is selected as Ci-Cs-alkyl or halogen, or wherein R2 and R3, or R4 and R5 or both, or wherein R3 and R4, or R5 and Re or both, or wherein R2 and R3 and R5 and Re together form a hydrocarbon group having three or four carbon atoms, and wherein Q comprises a single bond or Ci-Cs-alkylene, which may be branched or unbranched, and wherein the Ci-Cs-alkylene comprises a main chain which has one or more oxygen atoms between two carbon atoms when the Ci-Cs-alkylene has more than two carbon atoms; wherein the at least one color developer comprises a compound of formula (N-I):

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, Ci-Cs-alkyl, Ci-Cs-alkoxyl, C2-C6-alkylene, Ci-Cs-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl; wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, or Ci-Ce-alkyl, wherein R5 is selected as Ci-Ce-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or wherein the at least one color developer comprises a compound of formula (1):

where Ri is selected from the group comprising unsubstituted or substituted phenyl, naphthyl or Ci-C2o-alkyl, where X is selected from the group comprising — C(=NH) — , — C(=S) — and — C(=O) — , where A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, C1- Ci2-alkylene, and a non-substituted or substituted heterocyclic group, wherein B is selected from the group comprising — O — SO2 — , — SO2 — O — , — NH — SO ₂ — , — SO ₂ — NH—, — S— SO ₂ — , — O— CO— , — O— CO— NH— , — NH— CO— O— , — S— CO— NH— , — S— CS— NH— , — CO— NH— SO ₂ — , — O— CO— NH— SO ₂ — , — NH=CH— , —CO— NH— CO— , — S— , —CO—, — O— , — SO ₂ — NH— CO — , — O — CO — O — and — O — PO — (OR2)2, and wherein R2 is selected from the group comprising non-substituted or substituted aryl, benzyl and C1-C20-alkyl, with the proviso that when B is not a group of the formula — O — SO2 —, then R2 is unsubstituted or substituted phenyl, naphthyl or Ci-Cs-alkyl, and that when B is — O —, then R2 is not alkyl.

29. The method according to claim 28, wherein the drying of the adhesive layer is carried out at a temperature of 60 °C to 80 °C, preferably at 70 °C.

30. The method according to claim 28 or 29, wherein the method comprises the further method step which is carried out after the application of the adhesive layer:

31. Method according to one of claims 28 to 30, wherein the application of the adhesive layer comprises applying an adhesive dispersion and subsequently drying the adhesive dispersion, the method comprising the further method step which is carried out before the application of the adhesive dispersion:

32. Method according to one of claims 28 to 31, wherein the application and drying of the application dispersion on the first side of the carrier substrate is carried out before the application of the adhesive layer on the second side of the carrier substrate, or wherein the application of the adhesive layer on the second side of the carrier substrate is carried out before the application and drying of the application dispersion on the first side of the carrier substrate, wherein preferably the application and drying of the application dispersion on the first side of the carrier substrate is carried out before the application of the adhesive layer on the second side of the carrier substrate.

33. Method according to one of claims 28 to 32, wherein the application of the application dispersion to the first side of the carrier substrate is carried out by means of curtain coating or by means of a bar doctor on the coating side of a carrier substrate pre-coated with a pigment coating, wherein the pigmented pre-coat preferably comprises calcined kaolin and a binder based on styrene-butadiene and/or polyvinyl alcohol (PVA) and/or starch, or organic pigments in a mixture with inorganic pigments, wherein the application amount of the pigmented pre-coat is more preferably from 2 g/m ² to 12 g/m ² .

34. Method according to one of claims 28 to 33, wherein the application amount of the application dispersion is between 3 g/m ² and 5 g/m ² , preferably between 3.6 g/m ² and 4.8 g/m ² , and/or wherein the application amount of the adhesive dispersion is between 10 g/m ² and 30 g/m ² , preferably 20 g/m ² .

35. Heat-sensitive recording material producible by a process according to one of claims 28 to 34.

36. Use of at least one stabilizer and at least one color developer in a heat-sensitive recording material comprising a carrier substrate having a first side and a second side facing away from the first side, a heat-sensitive color-forming layer which the first side of the carrier substrate, wherein the heat-sensitive color-forming layer comprises at least one color former, the at least one color developer, and the at least one stabilizer, and an adhesive layer which is arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive, wherein the weight ratio of stabilizer to color developer in the heat-sensitive color-forming layer is selected such that a migration of components of the adhesive layer of the heat-sensitive recording material into the heat-sensitive color-forming layer of the heat-sensitive recording material according to the stability test defined in the description after a period of two weeks leads to a remaining image stability of the heat-sensitive color-forming layer of at least 35%, wherein the stabilizer comprises a compound of the formula (I):

wherein R and R are independently selected from the group comprising hydrogen, Ci-Cis-alkyl, Ci-Cs-alkoxy-Ci-Cs-alkyl, and (Rg^N-Ci-Cs-AI- kyl, wherein Rg is selected from the group comprising Ci-Cs-alkyl, Cs-Ce-cycloalkyl; and a compound of formula (II)

wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, Ci-Cs-alkyl, Ci-Cs-alkoxyl, C2-C6-alkylene, Ci-Cs-fluoroalkyl, N(R4)2, NHCORs, optionally substituted phenyl, and optionally substituted benzyl; wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and Ci-Cs-alkyl, wherein Rs is selected as Ci-Cs-alkyl, wherein n1 and n3 are independently selected from an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; and/or wherein the at least one color developer comprises a compound of formula (1):