Heat-sensitive recording material

Abstract

The 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 colour-forming layer which is disposed on the first side of the carrier substrate, wherein the heat-sensitive colour-forming layer comprises at least one colour former and at least one colour developer; wherein the heat-sensitive layer comprises at least one polymer binder and at least one inorganic pigment, wherein the at least one inorganic pigment comprises a precipitated calcium carbonate (PCC), wherein the precipitated calcium carbonate (PCC) has an average particle size (d90) of less than 50 μm, and wherein the precipitated calcium carbonate (PCC) comprises a screened precipitated calcium carbonate (PCC) and / or a ground precipitated calcium carbonate (PCC) and / or a filtered precipitated calcium carbonate (PCC).

Description

HEAT-SENSITIVE RECORDING MATERIAL TECHNICAL FIELD According to a first aspect, the present invention relates to a heat-sensitive recording material comprising a support substrate having a first side and a second side facing away from the first side, a heat-sensitive color-forming layer arranged on the first side of the support substrate and comprising at least one color former and at least one color developer, wherein the heat-sensitive layer comprises at least one polymeric binder and at least one inorganic pigment. The at least one inorganic pigment comprises precipitated calcium carbonate (PCC), wherein the precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 50 µm.and wherein the precipitated calcium carbonate (PCC) comprises sieved precipitated calcium carbonate (PCC) and / or ground precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC). According to a second aspect, the present invention relates to a method for producing a heat-sensitive recording material, comprising the following process steps: providing a support substrate having a first side and a second side facing away from the first side; applying a coating suspension to the first side of the support substrate, wherein the coating suspension comprises at least one color former and at least one color developer, wherein the coating suspension comprises at least one polymeric binder and at least one inorganic pigment, wherein the at least one inorganic pigment comprises precipitated calcium carbonate (PCC),wherein the precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 50 µm, and wherein the precipitated calcium carbonate (PCC) comprises sieved precipitated calcium carbonate (PCC) and / or ground precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC); and drying the application suspension to obtain a heat-sensitive ink-forming layer arranged on the first 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 the use of a heat-sensitive recording material according to the first or third aspect for direct thermal printing, for example as a receipt, label, or ticket. TECHNICAL BACKGROUND Heat-sensitive recording materials, which are also referred to as thermal papers,Thermal labels are used in a variety of applications, such as sales receipts in retail. Thermal labels, also known as heat-sensitive recording materials, are known from the prior art for direct thermal printing. Two types of thermal labels are distinguished, particularly for direct thermal printing. The first type comprises materials in which the printed image is created by a local heat-induced chemical reaction in a color layer, for example, between a color former, such as a leuco dye, and a color developer, such as bisphenol A or a phenol-free alternative. Typically, the color layer also contains a heat-sensitive solvent that melts under the influence of heat and consists, for example, of long-chain aliphatic alcohols, amides,It may contain esters or carboxylic acids, thus enabling the color reaction of color former and color developer. Furthermore, the ink layer may contain heat-sensitive sensitizers. The second type of heat-sensitive recording materials comprises heat-sensitive recording materials in which the printed image is created by, A heat-sensitive top layer becomes translucent through local application of heat, for example, by means of a thermal direct printer, so that an underlying color layer becomes visible. This technology is described or interpreted differently in the prior art, and such a heat-sensitive recording material is obtained through partially different compositions, porosities, and materials of the heat-sensitive top layer, optimized for thermal direct printing, and explained in more detail below. In the second type of heat-sensitive recording material, the heat-sensitive top layer should cover the underlying color layer as completely as possible. This is achieved essentially through light scattering, especially by means of scattering particles, and light absorption.The heat-sensitive top layer should have the highest possible contrast to the underlying color layer in order to produce a printed image readable by the human eye and / or a machine, such as a scanner, for example, white / black or blue / yellow. The present invention relates to heat-sensitive recording materials of the first type described above. Heat-sensitive recording materials, so-called thermal labels, for direct thermal printing are known in the prior art. JPS 59162087 A, US 4370370 A, and US 4388362 A describe thermal labels with a release paper. DE 19757589 B4 describes a release paper-free ("linerless") thermal label with a protective layer over the heat-sensitive layer. DE 19806433 B4 discloses a linerless thermal label with a protective layer free of silicone compounds, which was cured by actinic radiation.The EP 0600622 A1 and the DE 19724647 C1 describe a linerless thermal label with a protective layer, which is coated on the back with hot melts. EP 1085069 B1 and EP 2474963 B1 disclose linerless thermal label materials with a heat-activated adhesive, describing thermal labels both with and without a protective layer. EP 3219507 A1 claims a linerless thermal label without an actual protective layer, in which the surface of the heat-sensitive layer is designed to be non-adhesive. Heat-sensitive recording materials equipped with so-called "back-coat" coatings to improve reverse printing with conventional printing methods or to minimize the curling tendency of a carrier substrate under unfavorable humidity conditions are also known from the prior art. This allows for the different shrinkage behavior of the two sides of the carrier substrate of heat-sensitive recording materials at different ambient humidity levels, i.e.,Different water vapor absorption rates on the two sides / lines can be effectively improved by back-side coatings. US 6667275 B2, for example, discloses a multilayer back-side coating for heat-sensitive recording materials to advantageously influence the curl tendency of the substrate. JP 2018167483 discloses methods for improving the durability of thermally produced prints on heat-sensitive recording materials that have been printed on the reverse side with oil-based inks or dyes, by, among other things, applying coatings to the reverse side. The formulations of these back-side coatings often contain aqueous emulsion polymers, such as styrene-butadiene or acrylate latices, as a key component.For example, JP 2003175671 claims backside coatings produced with aqueous latices for heat-sensitive recording materials that form soft polymer films (glass transition temperature ≥ -30°C). JPH 0 720 735 B2. JP 2000204123 discloses back-coat formulations with acrylate emulsion polymers for heat-sensitive recording materials. Publications EP 3957488 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. Further described are the use of the heat-sensitive recording layer for producing the heat-sensitive recording material, a method for producing the heat-sensitive recording material, and the use of the heat-sensitive recording material in thermal paper applications.Due to its composition, the heat-sensitive recording material exhibits particularly good recyclability and high environmental compatibility. Heat-sensitive recording materials are also described in the publications US 2021 / 0060994, CA 3149562 A1 and WO 2021 / 041600A2. Reference is also made to the publications DE 10 2019126220 A1, WO 2021 / 058661 A1, KR2022070021 A, and DE 202020005616 U1. For all these conventionally used heat-sensitive recording materials, it is desirable to reduce the occurrence of white or light defects in correspondingly dark printed areas, which are perceived as disturbing to the viewer's eye and / or are detrimental to electronic reading devices, in order to improve the print image.In particular, it is desirable to at least maintain or further enhance the protection of heat-sensitive recording materials from external influences such as pressure, friction, humidity, liquids, and moisture. Furthermore, the functionality, properties, and economic manufacturability of conventional heat-sensitive recording materials should at least be maintained, and ideally even improved. to be improved, particularly with regard to optical density and the optical impression of the printed image on the heat-sensitive recording materials. In addition, conventional heat-sensitive recording materials often suffer from the problem of suboptimal surface smoothness or high surface roughness, which, due to the resulting abrasion, significantly limits the operating life of corresponding printers and is therefore disadvantageous. DESCRIPTION OF THE INVENTION Objective One object of the present invention is to provide a heat-sensitive recording material which, after printing, exhibits an advantageous printed image and, in particular, has a reduced number of white or light defects in correspondingly dark printed areas.A further object of the present invention is to provide a heat-sensitive recording material capable of ensuring the application-related functional properties (in particular, high optical density). A further object of the present invention is to provide a heat-sensitive recording material that ensures advantageous surface smoothness or low surface roughness. Surprisingly, it has now been found that the aforementioned disadvantages of the prior art can be overcome by using sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as an inorganic pigment in the heat-sensitive layer of the heat-sensitive recording material. By using sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as an inorganic pigment in the heat-sensitive layer, the number of white or light defects in correspondingly dark printed areas can be significantly reduced. Furthermore, the use of sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as an inorganic pigment in the heat-sensitive layer can achieve a favorable surface smoothness or low surface roughness of the heat-sensitive recording material.The use of sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as an inorganic pigment in the heat-sensitive layer ensures improved barcode readability and also results in a favorable print image. Sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) is available at a very low price, which reduces the production costs of corresponding heat-sensitive recording materials.Heat-Sensitive Recording Material The aforementioned tasks are solved according to the first aspect by a heat-sensitive recording material comprising: a support substrate having a first side and a second side facing away from the first side; a heat-sensitive color-forming layer arranged on the first side of the support substrate, wherein the heat-sensitive color-forming layer comprises at least one color former and at least one color developer; wherein the heat-sensitive layer comprises at least one polymeric binder and at least one inorganic pigment, wherein the at least one inorganic pigment comprises precipitated calcium carbonate (PCC), wherein the precipitated calcium carbonate (PCC) has a mean-. The precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 50 µm, and the precipitated calcium carbonate (PCC) comprises sieved precipitated calcium carbonate (PCC) and / or milled precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC). By using the sieved, milled, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as the inorganic pigment of the heat-sensitive layer according to the first aspect, it is possible to significantly reduce the number of white or light defects in dark printed areas. Within the scope of the present invention, the sieved, milled, and / or filtered precipitated calcium carbonate (PCC) can be mixed arbitrarily in the heat-sensitive color-forming layer.In particular, the at least one inorganic pigment comprises exclusively sieved precipitated calcium carbonate (PCC), or exclusively ground precipitated calcium carbonate (PCC), or exclusively filtered precipitated calcium carbonate (PCC). Alternatively, and in particular, the at least one inorganic pigment comprises a mixture of sieved precipitated calcium carbonate (PCC) and ground precipitated calcium carbonate (PCC), or a mixture of sieved precipitated calcium carbonate (PCC) and filtered precipitated calcium carbonate (PCC), or a mixture of ground precipitated calcium carbonate (PCC) and filtered precipitated calcium carbonate (PCC), or a mixture of sieved precipitated calcium carbonate (PCC) and ground precipitated calcium carbonate (PCC) and filtered precipitated calcium carbonate (PCC).In particular, the mean particle size (d90) of the sieved, and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by laser diffraction, specifically using a Coulter laser diffraction analyzer. Specifically, the mean particle size (d90) of the sieved, and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined according to ISO 13320. Specifically, the mean particle size (d90) of the sieved, and / or ground and / or filtered precipitated calcium carbonate (PCC) according to ISO 13320 is determined. 13320 the 90th percentile of the particle size distribution. If the mean particle size (d90) is 50 µm, this means that the proportion of particles smaller than 50 µm is 90%. Furthermore, the use of sieved, and / or ground, and / or filtered precipitated calcium carbonate (PCC) with a mean particle size (d90) of less than 50 µm as the inorganic pigment of the heat-sensitive layer, as described in the first aspect, ensures that no further disturbances occur in the printed image. A corresponding heat-sensitive recording material according to the present invention also exhibits an image quality of the printed image comparable to, and in particular improved to, that of conventional recording materials, which is characterized by the optical density of the heat-sensitive recording material.In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (D4,3) of 2.3 to 2.5 ^m, the mean particle size (D4,3) of the sieved precipitated calcium carbonate (PCC) being determined according to standard ISO 9276-2. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d10) of 1.1 to 1.3 µm, the mean particle size (d10) of the sieved precipitated calcium carbonate (PCC) being determined according to ISO 13320. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d50) of 2.2 to 2.3 µm, the mean particle size (d50) of the sieved precipitated calcium carbonate (PCC) being determined according to ISO 13320. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d90) of 3.8 to 4.1 µm, the mean particle size (d90) of the sieved precipitated calcium carbonate (PCC) being determined according to ISO 13320.In particular, the sieved precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment, according to the standard DIN EN ISO 787-7, in the range of 0.01 to 0.03%, especially approximately 0.02% or 0.03%. Specifically, the sieved precipitated calcium carbonate (PCC) can be produced by sieving precipitated calcium carbonate (PCC) using a sieve, which in particular has a pore size between 20 and 50 µm, especially 25 µm. Specifically, the filtered precipitated calcium carbonate (PCC) can be produced by filtering a dispersion of precipitated calcium carbonate (PCC) using a filter, which in particular has a pore size between 20 and 50 µm, especially 25 µm.In particular, the filtered precipitated calcium carbonate (PCC) has a mean particle size (D4,3) of 2.2 to 2.3 µm, the mean particle size (D4,3) of the filtered precipitated calcium carbonate (PCC) being determined according to ISO 9276-2. In particular, the filtered precipitated calcium carbonate (PCC) has a mean particle size (d10) of 1.0 to 1.1 µm, the mean particle size (d10) of the filtered precipitated calcium carbonate (PCC) being determined according to ISO 13320. In particular, the filtered precipitated calcium carbonate (PCC) has a mean particle size (d50) of 2.0 to 2.2 µm, the mean particle size (d50) of the filtered precipitated calcium carbonate (PCC) being determined according to ISO 13320.In particular, the filtered precipitated calcium carbonate (PCC) has a mean particle size (d90) of 3.6 to 3.8 µm, the mean particle size (d90) of the filtered precipitated calcium carbonate (PCC) being determined according to ISO 13320. Specifically, the filtered precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment in the range of 0.01 to 0.03%, as determined according to DIN EN ISO 787-7. In particular, the ground precipitated calcium carbonate (PCC) can be produced by grinding precipitated calcium carbonate (PCC) using a mill, in particular a bead mill, in particular a cooled bead mill, in particular a bead mill cooled to a temperature of less than or equal to 40°C, which in particular has a throughput of between 20 and 50 L / min, wherein the throughput is alternatively between 20 and 60 L / min, preferably between 40 and 55 L / min. In particular, the bead mill has a peripheral speed of 8.0 to 11.0 m / s. In particular, the bead mill has mill beads, or grinding beads, with a diameter between 0.8 and 1.0 mm, and / or the bead mill in particular has a fill level of greater than or equal to 50%.In particular, during the milling process, up to 1%, especially 0.1% to 0.5%, preferably 0.30% to 0.40%, of an aqueous solution of a sodium salt of polyacrylic acid is added as a dispersing agent, and / or the precipitated calcium carbonate (PCC) is diluted with water to a solids content of 45% to 55%. In particular, the milled precipitated calcium carbonate (PCC) has a mean particle size (D4,3) of 1.2 to 2.1 µm, wherein the mean particle size (D4,3) of the milled precipitated calcium carbonate (PCC) was determined according to ISO 9276-2. In particular, the ground precipitated calcium carbonate (PCC) has a mean particle size (d10) of 0.1 to 1.0 ^m, the mean particle size (d10) of the ground precipitated calcium carbonate (PCC) being determined according to ISO 13320.In particular, the ground precipitated calcium carbonate (PCC) has a mean particle size (d50) of 1.1 to 1.9 ^m, the mean particle size (d50) of the ground precipitated calcium carbonate (PCC) being determined according to ISO 13320. In particular, the ground precipitated calcium carbonate (PCC) has a mean particle size (d90) of 1.9 to 3.4 µm, the mean particle size (d90) of the ground precipitated calcium carbonate (PCC) being determined according to ISO 13320. Specifically, the mean particle size (d4.3) of the screened, and / or ground and / or filtered precipitated calcium carbonate (PCC) includes the De Brouckere mean or the volume-weighted mean of the particle size distribution according to ISO 9276-2. Specifically, the mean particle size (d10) of the screened, and / or ground and / or filtered precipitated calcium carbonate (PCC) according to ISO 13320 includes the 10th percentiles of the particle size distribution. If the mean particle size (d10) is, for example, 1.2 ^m, this means that the proportion of particles smaller than 1.2 ^m is 10%.In particular, the mean particle size (d50) of the sieved, and / or ground and / or filtered precipitated calcium carbonate (PCC) according to the ISO 13320 standard includes the 50% percentiles of the particle size distribution. If the mean particle size (d50) is, for example, 2.2 µm, this means that the proportion of particles smaller than 2.2 µm is 50%. In particular, the ground precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment in the range of 0.01 to 0.03%, according to the standard DIN EN ISO 787-7. In particular, the ground precipitated calcium carbonate (PCC) exhibits a sieve residue obtained after treatment in the range of 0.01 to 0.03%, in particular of approximately 0.02% or 0.03%, when filtered with a sieve with a pore size of 25 µm, according to the standard DIN EN ISO 787-7.In particular, the filtered precipitated calcium carbonate (PCC) exhibits a sieve residue of 0.01 to 0.01 obtained after treatment, in accordance with the standard DIN EN ISO 787-7. 0.03%, in particular approximately 0.02% or 0.03% when filtered with a sieve with a pore size of 25 µm. In particular, the sieved precipitated calcium carbonate (PCC) has a sieve residue of 0.01 to 0.03%, in particular approximately 0.02% or 0.03% when filtered with a sieve with a pore size of 25 µm, as obtained after treatment according to DIN EN ISO 787-7. According to one embodiment, the at least one inorganic pigment consists exclusively of precipitated calcium carbonate (PCC).According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 40 µm, preferably less than 30 µm, more preferably less than 25 µm, more preferably less than 20 µm, most preferably less than 10 µm, more preferably less than 7 µm, more preferably less than 5 µm, and more preferably less than 4.5 µm. In particular, the mean particle size (d90) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by laser diffraction, in particular by a Coulter laser diffraction analyzer. In particular, the mean particle size (d90) of the precipitated calcium carbonate (PCC) is determined according to ISO 13320.According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d50) of less than 50 µm, preferably less than 40 µm, more preferably less than 30 µm, even more preferably less than 25 µm, further still more preferably less than 20 µm, most preferably less than 10 µm, more preferably less than 5 µm, more preferably less than 3 µm, and more preferably less than 2.5 µm. In particular, the mean particle size (d50) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by laser diffraction, especially by a Coulter laser diffraction analyzer.In particular, the mean particle size (d50) of the sieved and / or milled and / or filtered precipitated calcium carbonate (PCC) is determined according to ISO 13320. According to one embodiment, the sieved and / or milled and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d10) of less than 50 µm, preferably less than 40 µm, more preferably less than 30 µm, more preferably less than 25 µm, more preferably less than 20 µm, most preferably less than 10 µm, more preferably less than 5 µm, more preferably less than 2 µm, and more preferably less than 1.5 µm.In particular, the mean particle size (d10) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by laser diffraction, especially using a Coulter laser diffraction analyzer. Specifically, the mean particle size (d10) of the precipitated calcium carbonate (PCC) is determined according to ISO 13320. According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (D4,3) of less than 50 µm, preferably less than 40 µm, more preferably less than 30 µm, even more preferably less than 25 µm, further still more preferably less than 20 µm, most preferably less than 10 µm, even more preferably less than 5 µm, and even more preferably less than 3 µm. In particular, the mean particle size (D4,3) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by laser diffraction, specifically using a Coulter laser diffraction analyzer. Specifically, the mean particle size (D4,3) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is determined by the De Brouckere mean or the volume-weighted mean of the particle size distribution according to ISO 9276-2. This achieves the technical advantage that the very small mean particle sizes (d90), (d50), (d10), or (D4,3) of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) ensure the advantageous properties of the heat-sensitive recording material.According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises scalenohedral precipitated calcium carbonate (s-PCC), wherein in particular the proportion of the scalenohedral precipitated calcium carbonate (s-PCC) in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, further preferably more than 70%, even more preferably more than 80%, most preferably more than 90%, and even more preferably more than 98%.According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises precipitated calcium carbonate (PCC) having an aragonite structure, wherein, in particular, the proportion of the precipitated calcium carbonate (PCC) with the aragonite structure in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, more preferably more than 70%, more preferably more than 80%, most preferably more than 90%, and more preferably more than 98%. According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises precipitated calcium carbonate (PCC) which... a prismatic structure, wherein in particular the proportion of the precipitated calcium carbonate (PCC) with the prismatic structure in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, further preferably more than 70%, even more preferably more than 80%, most preferably more than 90%, and even more preferably more than 98%.According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises precipitated calcium carbonate (PCC) having a rhombohedral structure, wherein, in particular, the proportion of the precipitated calcium carbonate (PCC) with the rhombohedral structure in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, more preferably more than 70%, even more preferably more than 80%, most preferably more than 90%, and even more preferably more than 98%. This achieves the technical advantage that the specific structures of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) ensure the advantageous properties of the heat-sensitive recording material.According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment of less than 1%, preferably less than 0.5%, more preferably less than 0.1%, and most preferably less than 0.05%, in accordance with the standard DIN EN ISO 787-7. This achieves the technical advantage that the sieve residue of the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) ensures the advantageous properties of the heat-sensitive recording material. According to one embodiment, the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is present in the heat-sensitive layer in an amount of 5 wt.% to 60 wt.%, preferably in an amount of 10 wt.% to 55 wt.%, more preferably in an amount of 10 wt.% to 50 wt.%, and more preferably in an amount of 15 wt.% to 50 wt.%, based on the total dry mass of the heat-sensitive layer. This achieves the technical advantage that the defined weight ranges for the precipitated calcium carbonate (PCC) ensure advantageous properties of the resulting heat-sensitive recording material.According to one embodiment, the at least one inorganic pigment comprises at least one further inorganic pigment selected from the group consisting of calcium silicate hydrate, barium sulfate, kaolinite, calcium silicate, calcium sulfate, sodium aluminum silicate, aluminum oxides, aluminum hydroxides, silicas, precipitated and pyrogenic silicas, diatomaceous earths, magnesium carbonates, silicon dioxide, talc, kaolin, in particular natural kaolin or calcined kaolin, titanium dioxide, bentonite and mixtures thereof, preferably silicon dioxide, kaolin, in particular natural kaolin or calcined kaolin and / or aluminum hydroxide. According to one embodiment, the at least one further inorganic pigment has a weight fraction of 0.1 wt.% to 20 wt.% based on the total solids content of the heat-sensitive layer, wherein the weight fraction is preferably from 1 wt.% to 10 wt.%.This achieves the technical advantage that the use of the additional inorganic pigment enables particularly advantageous properties of the heat-sensitive recording material. According to one embodiment, the heat-sensitive layer comprises at least one crosslinking agent, which is preferably selected from the group consisting of polyhydric aldehydes such as glyoxal, dialdehyde starch, glutaraldehyde, salts or esters of glyoxylic acid, crosslinkers based on ammonium zirconium carbonate, and organic compounds. Titanates, polyamidoamine-epichlorohydrin resins (PAAE resins), polyamide resins, polyamine resins, polyamidoamine resins, polyamide-polyurea resins, polyamine-polyurea resins, adipic acid dihydrazide (ADH), polyamidoamines, epoxy resins, formaldehyde oligomers, cyclic ureas, methylol urea, melamine-formaldehyde oligomers, oxazoline resins, carbodiimide, borate compounds and mixtures thereof, wherein the at least one crosslinking agent is particularly preferably selected as ammonium zirconium carbonate, glyoxal and / or polyamidoamine-epichlorohydrin resins (PAAE resin). According to one embodiment, the at least one crosslinking agent is present in an amount of 0.01 wt.% to 10.0 wt.%, preferably in an amount of 0.1 wt.% to 5.0 wt.%, and most preferably in an amount of 0.5 wt.% to 2.5 wt.% based on the total dry mass of the heat-sensitive layer,in the heat-sensitive layer. To achieve specific application-related performance characteristics of heat-sensitive recording materials, the polymeric binder present in the heat-sensitive layer is preferably in cross-linked form, with the optimal degree of cross-linking of the polymeric binder being established during the drying step of the coating process in the presence of a cross-linking agent. Ammonium zirconium carbonate and polyamidoamine epiclorhydrin resins (PAAE resins) are particularly preferred. Self-cross-linking binders, such as specially modified polyvinyl alcohols or acrylates, enable cross-linking without any cross-linking agent, thanks to the reactive, cross-linkable groups that are already incorporated into the binder polymer. According to one embodiment, the heat-sensitive layer has at least one sensitizing agent, which is preferably selected from the group comprising a fatty acid amide,particularly preferably stearamide, behenamide or palmitamide, an ethylenebis fatty acid amide, particularly preferably N,N'-ethylenebis-stearic acid amide or N,N'-ethylenebis-oleic acid amide, a wax, particularly preferably polyethylene wax or montan wax, a carboxylic acid ester, particularly preferably dimethyl terephthalate, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-(p-methylbenzyl)oxalate, di-(p-chlorobenzyl)oxalate or di-(p-benzyl)oxalate, an aromatic ether, particularly preferably 1,2-diphenoxyethane, 1,2-di-(3-methylphenoxy)ethane, 2-benzyloxynaphthalene or 1,4-diethoxynaphthalene, an aromatic sulfone, particularly preferably diphenylsulfone, and / or an aromatic sulfonamide, particularly preferably benzenesulfonanilide or N-Benzyl-p-toluenesulfonamide, o-toluenesulfonamide, , p-benzylbiphenyl (PBBP),1,2-bis-(phenoxy-methyl)-benzene, 4-(4-tolyloxy)-biphenyl, 1,2-bis-(3,4-dimethylphenyl)ethane and / or mixtures thereof. In general, crystalline substances with a melting point between about 90°C and about 150°C are advantageously considered as sensitizing agents 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 interfering with the formation of the color complex. Sensitizing agents can be present alone or as mixtures. According to one embodiment, the sensitizing agent has a weight fraction of 1 wt.% to 40 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 2 wt.% to 25 wt.%. According to one embodiment, the heat-sensitive layer has at least one optical brightener in the form of white toners, which is selected from the group comprising diaminostilbene disulfonic acid, distyrylbiphenyls,Benzoxazole derivatives, fluorescent substances in the form of daylight fluorescent pigments of different colors or fluorescent fibers and mixtures thereof, and / or the heat-sensitive layer comprises at least one aging inhibitor in the form of sterically hindered phenols, preferably 1,1,3-tris-(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1'-bis-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1'-bis-(4-hydroxyphenyl)cyclohexane and mixtures thereof. In particular, the at least one optical brightener has a weight fraction of 0.1 wt.% to 3.0 wt.% based on the total fraction of the color developer in the heat-sensitive color-forming layer and / or the at least one anti-aging agent has a weight fraction of 0.2 wt.% to 30.0 wt.%, preferably 5 wt.% to 25.0 wt.% based on the total fraction of the color developer in the heat-sensitive color-forming layer.According to one embodiment, the at least one polymeric binder of the heat-sensitive layer is selected from the group comprising water-soluble starches, starch-based biolatices of the Ecosphere type, starch derivatives, methylcellulose, hydroxyethylcellulose, carboxymethylcelluloses, partially or fully saponified polyvinyl alcohols, ethylene-vinyl alcohol copolymers, chemically modified polyvinyl alcohols 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 and / or acrylonitrile-butadiene copolymers and mixtures thereof, wherein the at least one binder is preferably selected as polyvinyl alcohol. In particular, the at least one binder comprises at least one binder of a synthetic or biogenic nature.The at least one binder of biogenic nature comprises, in particular, biogenic polymers based on modified, especially chemically and / or thermally modified, and unmodified starches, celluloses, proteins, chitin, chitosan, lignin, casein, gelatin, collagen, shellac, vegetable oil, lipids, polylactic acid (PLA), polyhydroxyalkanoates (PHA), and mixtures thereof. According to one embodiment, the at least one polymeric binder is present in the heat-sensitive layer in an amount of 0.5 wt.% to 30.0 wt.%, preferably in an amount of 2.5 wt.% to 20.0 wt.%, based on the total dry mass of the heat-sensitive layer. To achieve specific application-related performance characteristics of heat-sensitive recording materials, preferably self-adhesive heat-sensitive recording materials, especially self-adhesive labels, the binder is preferably present in cross-linked form within the heat-sensitive layer, with the optimal degree of cross-linking of the binder being established during the drying step of the coating process in the presence of a cross-linker. Preferably, the binder is configured as a cross-linked further binder, wherein the cross-linked further binder is preferably configured as a self-cross-linking further binder, or wherein the heat-sensitive ink-forming layer comprises a further cross-linker configured to react with the further binder to obtain the cross-linked further binder.The self-crosslinking binder more preferably comprises modified polyvinyl alcohols and / or modified acrylates. According to one embodiment, the heat-sensitive layer has at least one release agent, which is preferably selected as fatty acid metal salts, particularly preferably zinc stearate or calcium stearate, or also behenate salts, synthetic waxes, particularly preferably in the form of fatty acid amides, further particularly preferably stearic acid amide and behenic acid amide, fatty acid alkanolamides, particularly preferably stearic acid methylolamide, paraffin waxes of different melting points, ester waxes of different molecular weights, ethylene waxes, propylene waxes of different hardnesses and / or natural waxes, particularly preferably carnauba wax or montan wax. The release agent is preferably present in an amount of about 1 to about 30 wt.%, particularly preferably in an amount of about 2 to about 30 wt.%.-%, preferably in an amount of approximately 3 to approximately 25 wt.%, based on the total dry mass of the heat-sensitive layer. According to one embodiment, the heat-sensitive layer has an basis weight of 1 g / m² as determined by standard ISO 536. 2 up to 10 g / m² 2 on, preferably from 2 g / m² 2 up to 6 g / m² 2According to one embodiment, the heat-sensitive recording material, in particular the heat-sensitive layer, contains no organic pigments, especially no unexpanded and / or expanded hollow pigments. In particular, the heat-sensitive layer contains no styrene-acrylate copolymer hollow pigment and / or no styrene-butadiene solid spherical pigment. According to one embodiment, the heat-sensitive recording material has at least one intermediate layer arranged between the support substrate and the heat-sensitive layer, wherein the intermediate layer preferably comprises at least one binder and / or preferably at least one pigment.In particular, at least one binder of the intermediate layer is selected from the group comprising water-soluble starches, oxidized starch, Ecospher-type biolatex, starch derivatives, methylcellulose, hydroxyethylcellulose, carboxymethylcelluloses, partially or fully saponified polyvinyl alcohols, ethylene-vinyl alcohol copolymers, chemically modified polyvinyl alcohols 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 / or styrene-butadiene latex and mixtures thereof, wherein the at least one binder is preferred to polyvinyl alcohol and / or styrene-butadiene latex, further preferred. a mixture of polyvinyl alcohol and styrene-butadiene latex or styrene-butadiene latex.In particular, at least one pigment of the intermediate layer is selected from the group comprising calcium silicate hydrate, barium sulfate, kaolinite, calcium silicate, calcium sulfate, sodium aluminum silicate, calcium carbonates, preferably synthetic, natural. Precipitated or precipitated calcium carbonates, aluminum oxides, aluminum hydroxides, silicas, precipitated and pyrogenic silicas, diatomaceous earths, magnesium carbonates, silicon dioxide, talc, kaolin, in particular natural or calcined kaolin, titanium dioxide, bentonite, organic hollow pigments, in particular expanded or non-expanded organic hollow pigments and mixtures thereof, preferably silicon dioxide, kaolin, in particular calcined kaolin, calcium carbonate, and / or organic hollow pigments, in particular expanded or non-expanded organic hollow pigments. In particular, the expanded or non-expanded organic hollow pigments and mixtures thereof comprise thermally expanded or non-thermally expanded organic hollow pigments.The at least one thermally expanded hollow pigment is characterized in particular by having a closed cavity bounded by a polymer shell, in which a gas is present, wherein the gas present in the cavity is formed during the production of the thermally expanded hollow pigment by the application of heat, causing the polymer shell to expand. The at least one non-thermally expanded hollow pigment is characterized in particular by having a closed cavity bounded by a polymer shell, in which, in particular, predominantly air is present. The at least one non-thermally expanded hollow pigment comprises, in particular, a hollow pigment of the Ropaque® type. In particular, the polymer shell of the at least one non-thermally expanded hollow pigment and / or the thermally expanded hollow pigment is a thermoplastic polymer shell.In contrast to a non-thermally expanded hollow pigment, the gas present in the closed cavity of the thermally expanded hollow pigment limits the cavity. The polymer shell is subjected to pressure due to heat exposure during the manufacturing process, causing it to expand and resulting in a variety of different effects. In a non-thermally expanded hollow pigment, the polymer shell containing the cavity is not gas-tight, and the cavity is typically filled with air. There is no temperature-dependent change in the size of the hollow pigment. Among other things, the expansion of the polymer shell in a thermally expanded hollow pigment, as the name suggests, leads to a larger diameter compared to a non-thermally expanded hollow pigment, as well as a decrease in the thickness of the polymer shell with a corresponding increase in pigment diameter.The expansion of the polymer shell of a thermally expanded hollow pigment, in contrast to a non-thermally expanded hollow pigment, leads to an increased specific surface area and a reduced density. A thermally expanded hollow pigment is produced by applying heat to the pigment cavity, causing the gas within the cavity to expand and thus widening the polymer shell. This process yields the thermally expanded hollow pigment. Specifically, the gas present in the cavity of the thermally expanded hollow pigment does not contain air as its main component, and in particular, it does not contain any air at all. Specifically, the gas present in the cavity of the thermally expanded hollow pigment consists of hydrocarbon gas, which is inherent to the manufacturing process.For example, corresponding thermally expanded hollow pigments include at least one hollow pigment of the Expancel® type, which is currently distributed by the company Nouryon. Examples include thermally expanded hollow pigments in the paragraph

[0243] described in publication EP 4046813 A1, which refers to the specific Expancel® type 461WE20d36. This paragraph describes

[0056] EP 4046813 A1, in particular, the use of a readily evaporable liquid in which the corresponding hollow pigment evaporates upon heating, expands, and thereby expands the hollow pigments, so that the corresponding thermally expanded hollow pigments are obtained. The corresponding thermally expanded hollow pigments according to the first aspect are further characterized below in the context of further embodiments based on their relevant physical parameters. The composition of the pigmented primer or intermediate layer is not critical. In particular, this coating consists of calcined kaolin and a binder based on styrene-butadiene, polyvinyl alcohol, and / or starch. In particular, primers with non-expanded and / or expanded organic (hollow) pigments, possibly mixed with inorganic pigments, are also possible. The application quantity of this pigmented layer,The density of the intermediate layer is particularly between approximately 2 and 12 g / m². In particular, the at least one pigment of the intermediate layer comprises unexpanded and / or expanded organic hollow pigments, which preferably comprise a styrene-acrylate copolymer or polyacrylonitrile copolymer. Preferably, the unexpanded and / or expanded organic hollow pigments have a glass transition temperature of 40 °C to 100 °C and / or an average particle size of up to 20 µm. The intermediate layer serves, firstly, as a thermal barrier between the support substrate and the heat-sensitive color-forming layer, and secondly, it improves the surface smoothness of the support substrate for the heat-sensitive color-forming layer. According to one embodiment, the heat-sensitive recording material has at least one protective layer arranged on the heat-sensitive layer, wherein the protective layer preferably comprises at least one polymer and / or copolymer.particularly preferably comprising polyvinyl alcohols, modified polyvinyl alcohols, polyacrylates and polyurethanes, and / or at least one pigment and / or at least one crosslinking agent. The protective layer arranged on the outer surface of the heat-sensitive ink-forming layer ensures effective protection and / or advantageous printability of the heat-sensitive ink-forming layer. According to one embodiment, the at least one polymer of the protective layer 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, wherein (meth)acrylamide-acrylic acid ester-based copolymers of the Bariastar® type are used in particular, which are preferably in the form of core-shell structures, and / or wherein, in particular, chemically modified polyvinyl alcohols are used, which in particular comprise ethylene-modified polyvinyl alcohol and / or ethylene vinyl acetate. According to one embodiment, the at least one polymer of the protective layer has a weight fraction of 30 wt.% to 90 wt.% based on the total solids content of the protective layer, wherein the weight fraction is preferably from 35 wt.% to 80 wt.%, more preferably from 40 wt.% to 80 wt.%. According to one embodiment, at least one polymer of the protective layer is designed as a cross-linked polymer.wherein the crosslinked polymer is preferably configured as a self-crosslinking polymer, or wherein the protective layer comprises a crosslinker configured to react with the polymer to obtain the crosslinked polymer. To achieve specific application-related performance characteristics of a heat-sensitive recording material, the at least one polymer is preferably present in crosslinked form in the protective layer, wherein the optimal degree of crosslinking of the polymer is established during a drying step, or after the drying step of the coating process, particularly in the presence of a crosslinker. In particular, there is a post-maturation of the crosslinking reaction at ambient temperature or artificial climate.so that the optimal degree of cross-linking is achieved with a slight time delay after the drying step. According to one embodiment, the self-crosslinking polymer comprises modified polyvinyl alcohols and / or modified acrylates. Self-crosslinking polymers, such as specially modified polyvinyl alcohols and / or modified acrylates, enable cross-linking without the addition of a crosslinking agent due to the reactive groups that are already part of the polymer of the self-crosslinking polymer. According to one embodiment, the at least one cross-linking agent of the protective layer is selected from the group comprising polyhydric aldehydes, preferably glyoxylic acid, dialdehyde starch, and / or glutaraldehyde, in particular alone or in mixture with borate salts, salts or esters of glyoxylic acid, ammonium zirconium carbonate, organic titanates, polyamidoamine epichlorohydrin resins, adipic acid dihydrazide, boric acid, oxazoline resins, carbodiimide,Borate compounds and mixtures thereof or their salts, 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 at least one crosslinking agent of the protective layer 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 at least one pigment of the protective layer comprises at least one inorganic pigment 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 carbonate, talc, kaolin, in particular natural or calcined kaolin, titanium dioxide, bentonite, calcium silicate hydrate, and mixtures thereof. According to one embodiment, the at least one pigment of the protective layer has a weight fraction of 2.5 wt.% to 50 wt.% based on the total solids content of the protective layer, wherein the weight fraction is preferably from 10 wt.% to 45 wt.%. According to an alternative embodiment, the protective layer has no pigment. 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 different melting points,Ester waxes of different molecular weights, polyethylene waxes, polypropylene waxes of different hardnesses, polyolefin particles of the Chemipearl® type, natural waxes, preferably carnauba wax or montan wax, and mixtures thereof. According to one embodiment, the at least one lubricant of the protective layer has a weight fraction of 1 wt.% to 30 wt.% based on the total solids content of the protective layer, wherein the weight fraction is preferably from 2 wt.% to 20 wt.%. According to one embodiment, the protective layer has at least one brightener and / or at least one tinting dye, wherein the brightener is preferably selected as a stilbene. The brightener and / or tinting dye can advantageously adjust the surface whiteness and / or the color point of the protective layer. According to one embodiment, the protective layer has a basis weight of 0.3 g / m². 2 up to 5.0 g / m² 2 on, preferably from 1.0 g / m² 2up to 3.0 g / m² 2 According to one embodiment, the protective layer has a thickness of 0.3 m to 6.0 m, preferably 0.5 m to 3.0 m. According to another embodiment, the protective layer has a Bekk smoothness measured according to DIN 53107 of 100 sec to 3000 sec, preferably 500 sec to 2500 sec. A corresponding advantageous Bekk smoothness can be achieved by a smoothing process. According to one embodiment, the protective layer has a surface roughness of 0.5 µm to 2.50 µm, preferably 0.80 µm and 2.00 µm, as measured according to ISO 8791-4. According to one embodiment, the heat-sensitive recording material, in particular the heat-sensitive layer and / or the intermediate layer, comprises a biopolymeric material, preferably starch or other polysaccharides, cellulose, plant gums, proteins, and more preferably gelatin or whey protein. According to one embodiment, the heat-sensitive color-forming layer comprises at least one additional brightener, preferably a stilbene. The additional brightener allows the surface whiteness of the heat-sensitive recording material to be controlled. According to one embodiment, the heat-sensitive color-forming layer comprises a rheology aid, preferably a thickener and / or a surfactant. This offers 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 heat-sensitive color-forming layer comprises at least one defoamer. According to one embodiment, the heat-sensitive recording material has a basis weight of 20 g / m² as determined by ISO 536. 2 up to 100 g / m² 2 on, preferably from 35 g / m² 2 up to 90 g / m²2 According to one embodiment, the heat-sensitive color-forming layer has a Bekk smoothness of 150 sec to 1500 sec, preferably 250 sec to 1000 sec, as measured according to DIN 53107. According to another embodiment, the heat-sensitive recording material has an optical density (n.d.) of at least 0.75, preferably at least 0.9, and most preferably at least 1.0, as defined in the description, particularly at an energy level of 8.88 mJ / mm². 2 was measured. The optical density (o.D.) was measured, in particular using a Techkon SpectroDens densitometer, specifically at an energy level of 8.88 mJ / mm². The measurement uncertainty of the o.D. values ​​is approximately ≤2%. Specifically, 6 cm wide strips of the heat-sensitive recording materials were thermally treated using a GeBE PrinterLab GPT-10000 test printer (GeBE Elektronik und Feinwerktechnik GmbH, Germany), specifically with a Kyocera printhead of 305 dpi, particularly at an applied voltage of 24 V, and specifically with a bar pattern at an energy level of 8.88 mJ / mm². 2and in particular printed at a printing speed of approximately 100 mm / s. The area of ​​a bar of the printed pattern is, in particular, 53 x 12 mm. According to one embodiment, the heat-sensitive recording material has a roughness, measured according to DIN ISO 8791-4 using a Parker Print Surf tester, of less than 5 µm, preferably less than 4.5 µm, more preferably less than 4 µm, even more preferably less than 3 µm, and further still more preferably less than 2.5 µm. According to one embodiment, the heat-sensitive recording material is defined as a [material] as described above at an energy level of 8.88 mJ / mm². 2 or 10.32 mJ / mm 2A heat-sensitive recording material is produced by printing a barcode test pattern using a GeBE PrinterLab GPT-10000 test printer. The printed heat-sensitive recording material exhibits a barcode machine readability of grade 1 or higher, preferably 2 or higher, and most preferably 3 or higher, as defined in the description and evaluated according to ISO 15416. In particular, a barcode pattern printout according to the UPC-A code is printed lengthwise and crosswise on the heat-sensitive recording material. The evaluation of the barcode machine readability is performed using an REA VeriCube barcode verification device from REA Elektronik Deutschland according to ISO 15416, with the laser scanner operating at a wavelength of 660 nm. In particular, the evaluation is carried out using the software “TransWin32 V.1.2.0.2 / 16829”. Specifically, the barcode machine readability is assessed based on the scan reflection profile class in descending order of quality of the respective barcode: A (corresponds to a grade of at least 4), B (corresponds to a grade of 3), C (corresponds to a grade of 2), D (corresponds to a grade of 1), F (corresponds to a grade of 0). According to one embodiment, the at least one color developer comprises a compound of formula (I): where R and R1 are independently selected from the group comprising hydrogen, C1-C 18 -Alkyl, C1-C8-alkoxy-C1-C8-alkyl, and (R9)2N-C1-C8-alkyl, wherein R9 is selected from the group comprising C1-C8-alkyl, C5-C6-cycloalkyl; or a compound of formula (II) wherein R2, R3, R4, R5, and R6 are independently selected from the group comprising hydrogen, C1-C8-alkyl, -NH-C(=O)-R7, and -C(=O)-NH-R7, wherein R7 is selected as C1-C8 alkyl or -C(=O)OR8, wherein R8 is selected as C1-C8 alkyl or halogen, or wherein R2 and R3, or R4 and R5 or both, or wherein R3 and R4, or R5 and R6 or both, or wherein R2 and R3 and R5 and R6, together form a hydrocarbon group with three or four carbon atoms, and wherein Q comprises a single bond or C1-C8 alkylene, which may be branched or unbranched, and wherein the C1-C8 alkylene comprises a main chain having one or more oxygen atoms between two carbon atoms if the C1-C8 alkylene has more than two carbon atoms, wherein the compound of formula (I) preferably comprises a compound of formula (Ia): ; or wherein the compound of formula (I) preferably comprises a compound of formula (Ib): , and wherein the compound of formula (I) particularly preferably comprises 5-(N-3-methylphenylsulfonylamido)-(N',N'-bis-{3-methylphenyl)-isophthalic diamide. In particular, the compound of formula (Ia) comprises at least one of the following compounds: 5-(N-benzylsulfonylamido)-(N',N''-dibenzyl)-isophthalic diamide, 5-(N-3-methylphenylsulfonylamido)-(N',N''-bis-(3-methylphenyl)-isophthalic diamide, 5- (N-2,6-diethylphenyl-sulfonylamido)-(N’,N’’-bis-(2,6-diethylphenyl)-isophthalsäuredia- mid, 5-(N-phenyl-sulfonylamido)-(N’,N’’-bisphenyl)-isophthalsäurediamid, 5-(N-o-isop-ropyl-phenyl- sulfonylamido)-(N’,N’’-bis-(o-isopropylphenyl)-isophthalsäurediamid, 5-(N-p-acetamido-phenyl-sulfonylamido)-(N’,N’’-bis-(p-acetamido-phenyl)-isophthalsäuredia- mid, 5-(N-1-tetralino-sulfonylamido)-(N’,N’’-bis-(1-tetralino)-isophthalsäurediamid, 5-(N- 3-methylphenyl-sulfonylamido)-(N’,N’’-bis-(3-methylphenyl)-isophthalsäurediamid, 5-(N- 1-phenylethyl-sulfonylamido)-(N’,N’’-bis-(1-phenylethyl)-isophthalsäurediamid, 5-(N-2- phenylethyl-sulfonylamido)-(N’,N’’-bis-(2-phenylethyl)-isophthalsäurediamid, 5-(N-2,6- diethylphenyl-sulfonylamido)-(N’,N’’-bis-(2,6-diethylphenyl)-isophthalsäurediamid, 5-(N-n-butyl-sulfonylamido)-(N’,N’’-di-n-butyl-isophthalsäurediamid, 5-(N-2-ethylhexyl- sul-fonylamido)-(N’,N’’-di-2-ethylhexyl-isophthalsäurediamid, 5-(N-benzyl-sulfonylamido)- (N’,N’’-diphenyl)-isophthalsäurediamid,5-(N-phenyl-sulfonylamido)-(N’,N’’-dibenzyl)- isophthalsäurediamid, 5-(N-benzylsulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)-isoph- thalsäurediamid, 5-(N-butyl-sulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)-isophthalsäu- rediamid, 5-(N-1-phenyl-ethyl-sulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)-isophthal- säurediamid, 5-(N-2-phenyl-ethyl-sulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)-isoph- thalsäurediamid, 5-(N-2-methoxy-ethyl-sulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)- isophthalsäurediamid, 5-(N-n-octyl-sulfonylamido)-(N’,N’’-bis-(3-methyl-phenyl)-isoph- thalsäurediamid, 5-(N-benzyl-sulfonylamido)-(N’,N’’-bis-(2,6-diethyl-phenyl)-isophthal-säurediamid, 5-(N-n- octyl-sulfonylamido)-(N’,N’’-bis-(2,6-diethyl-phenyl)-isophthalsäu-rediamid, und 5-(N-2- phenoxy-ethyl-sulfonylamido)-(N’,N’’-bis-(2,6-diethyl-phenyl)-isophthalsäurediamid. Die Verbindung 5-(N-3-Methylphenyl-sulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic diamide is also marketed under the name Pergafast 425. The compound 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic diamide comprises, in particular, three different polymorphic forms, including a ^-polymorphic form with a melting point of 211.2 °C determined by DSC, a ^-polymorphic form with a melting point of 192.2 °C determined by DSC, and a ^-polymorphic form with a melting point of 215.6 °C determined by DSC. According to one embodiment, the at least one color developer comprises a compound of the formula (NI):, wherein R1, R2, and R3 are independently selected from the group comprising hydrogen, halogen, nitro, C1-C6 alkyl, C1-C6 alkoxyl, C2-C6 alkenyl, C1-C6 fluoroalkyl, N(R4)2, NHCOR5, optionally substituted phenyl, and optionally substituted benzyl, wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C6 alkyl, wherein R5 is selected as C1-C6 alkyl, wherein n1 and n3 are independently selected as an integer from 1 to 5, and wherein n2 is an integer from 1 to 4; wherein R1, R2, and R3 are preferably selected as hydrogen, wherein the compound of formula (NI) preferably comprises N-(2-(3-Phenylureido)phenyl)benzenesulfonamide, and further preferably the ^-polymorph and / or the ^-polymorph of N-(2-(3-Phenylureido)phenyl)benzenesulfonamide, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC. In particular, the compound of formula (NI) is a compound of formula (IV) or a compound of formula (V), where R1 and R3 are defined as for the compound of formula (NI). In particular, the compound of formula (NI) is a benzenesulfonamide compound. In particular, R1, R2 and / or R3 is selected as hydrogen, halogen, more preferably fluorine, chlorine, bromine or iodine, nitro, a straight, branched or cyclic C1-C6 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 C1-C6 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-C6 alkenyl group, further preferred, 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 C1-C6 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, wherein R4 is more preferably selected as hydrogen, phenyl, benzyl, or a C1-C6 alkyl group, an NHCOR5 group, wherein R5 is more preferably selected as a C1-C6 alkyl group, an optionally substituted phenyl group, and an optionally substituted benzyl group. In particular, R1, R2, and / or R3 is selected as hydrogen or an even C1-C6 alkyl group, wherein R1 is more preferably hydrogen or methyl.and R2 and R3 are each hydrogen. In particular, the selections for the C1-C6 alkyl group selected according to R4 or R5 are the same as for the C1-C6 alkyl group selected according to R1. In particular, the optional substituents of the optionally substituted groups are selected as hydroxy, halogen, most preferably fluorine, chlorine, bromine or iodine; C1-C6 alkyl, most preferably methyl, ethyl, N-propyl, isopropyl, N-butyl, sec-butyl, T-butyl, N-pentyl, isopentyl, neopentyl, T-pentyl, N-hexyl, isohexyl, 1-methylpentyl, or 2-methylpentyl; and C1-C6 alkoxy, most preferably methoxy, ethoxy, N-propoxy, isopropoxy, N-butoxy, sec-butoxy, or T-butoxy. Preferably the compound of formula (NI) selected as 4-methyl-N-(2-(3-phenylureido)phenyl)benzenesulfonamide and N-(2-(3-phenylureido)phenyl)benzenesulfonamide,Further preferably N-(2-(3-phenylureido)phenyl)benzenesulfonamide. According to one embodiment, the at least one color developer comprises N-(2-(3-phenylureido)phenyl)benzenesulfonamide, preferably the ^-polymorph and / or the ^-polymorph of N-(2-(3-phenylureido)phenyl)benzenesulfonamide. In particular, the corresponding X-ray diffraction pattern was obtained by X-ray diffraction (XRD) measurement using a Bruker D2 phaser, a copper electrode, a voltage of 30 kV, and a Lynxeye detector. In particular, the corresponding melting point determined by DSC was measured by dynamic differential calorimetry using a Netzsch DSC 200 F3 Maia® instrument, an aluminum 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 a nitrogen atmosphere. The corresponding compound, N-(2-(3-phenylureido)phenyl)benzenesulfonamide,It is also marketed under the name NKK-1304. According to one embodiment, the at least one color developer comprises a compound of formula (1): wherein R1 is selected from the group comprising unsubstituted or substituted phenyl, naphthyl and C1-C20 alkyl, wherein X is selected from the group comprising —C(=NH) —, —C(=S) — and —C(=O) —, wherein A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, C1-C12 alkylene and an unsubstituted or substituted heterocyclic group, wherein B is selected from the group comprising —O—SO2—, —SO2—O—, —NH—SO2—, —SO2—NH—, —S—SO2—, —O—CO—, —O—CO—NH—, —NH—CO—, —NH—CO—O—, —S—CO—NH—, —S—CS—NH—, —CO—NH—SO2—, —O—CO—NH—SO2—, —NH═CH—, —CO—NH—CO—, —S—, —CO—, —O—, —SO2—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-C 20-Alkyl, subject to the condition that if B is not a group of the formula —O—SO2—, then R2 is unsubstituted or substituted phenyl, naphthyl or C1-C8 alkyl, and that if B is —O—, then R2 is not alkyl; wherein X is preferably selected as —C(=O) —, wherein R1 is selected as substituted or substituted phenyl, preferably as C1-C3 alkyl substituted phenyl, wherein R 2is 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 C1-C3 alkyl substituted phenyl, and wherein the at least one color developer of formula (1) further preferably comprises 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, more preferably the ^-polymorph and / or the ^-polymorph of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC. The compound 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carboxy)benzenesulfonamide is also marketed under the name Pergafast 201. In particular, R1 is selected as phenyl or naphthyl, which may be unsubstituted or substituted by, for example, C1-C8 alkyl, C1-C8 alkoxy or halogen.In particular, the substituents are selected as C1-C4 alkyl, more preferably methyl or ethyl, C1-C4 alkoxy, even more preferably methoxy or ethoxy, or halogen, and further more preferably chlorine. In particular, R1 is selected as unsubstituted naphthyl. In particular, R1 is selected as substituted phenyl, wherein the substituents are more preferably selected as one of the above-mentioned alkyl substituents. In particular, R1 is selected as unsubstituted or substituted C1-C20 alkyl, preferably C1-C8 alkoxy or halogen, more preferably C1-C4 alkoxy, even more preferably methoxy or ethoxy, or halogen, most preferably chlorine. In particular, R1 is selected as unsubstituted C1-C20 alkyl. In particular, R1 is unsubstituted phenyl or phenyl which is modified by is substituted with C1-C8 alkyl, C1-C8 alkoxy or halogen, with substituted phenyl being further preferred.Phenyl is most preferred, which is substituted by C1-C4 alkyl, and even more preferably by methyl. In particular, X is a group of the formula -C(=S)- or -C(=O)-, preferably a group of the formula -C(=O)-. In particular, A is an unsubstituted phenylene or an unsubstituted naphthylene group, or a phenylene or a naphthylene group, which is preferably substituted by C1-C8 alkyl, halogen-substituted C1-C8 alkyl, C1-C8 alkoxy-substituted C1-C8 alkyl, C1-C8 alkoxy, halogen-substituted C1-C8 alkoxy, C1-C8 alkylsulfonyl, halogen, phenyl, phenoxy, or phenoxycarbonyl. In particular, alkyl and Alkoxy substituents comprising 1 to 4 carbon atoms, wherein C1-C8 alkyl, halogen-substituted C1-C8 alkyl, C1-C8 alkyl sulfonyl or halogen are preferred. In particular, A is an unsubstituted naphthylene group.In particular, A is a heterocyclic group, preferably comprising unsubstituted pyrimidylene or pyrimidylene substituted by C1-C8 alkyl, more preferably C1-C4 alkyl. In particular, A is a C1-C12 alkylene group, preferably C1-C8 alkylene, more preferably C1-C4 alkylene. In particular, A is unsubstituted phenylene or phenylene substituted by C1-C8 alkyl, halogen-substituted C1-C8 alkyl, C1-C8 alkoxy-substituted C1-C8 alkyl, C1-C8 alkoxy, halogen-substituted C1-C8 alkoxy, C1-C8 alkylsulfonyl, halogen, phenyl, phenoxy or phenoxycarbonyl, more preferably C1-C8 alkyl, halogen-substituted C1-C8 alkyl, C1-C8 alkylsulfonyl or halogen. In particular, A is unsubstituted phenylene or phenylene substituted by C1-C4 alkyl or halogen, preferably unsubstituted phenylene.In particular, B is selected as —O—SO2—, —SO2—O—, —SO2—NH—, —S—SO2—, —O—, —O—CO— and —O—CO—NH—, preferably as —O—SO2—, —SO2—O— and —SO2—NH—, and most preferably as —O—SO2— and —O—. In particular, R2 is aryl, more preferably phenyl or naphthyl, which is unsubstituted or substituted, further preferably by C1-C8 alkyl, halogen-substitutes C1-C8 alkyl, C1-C8 alkoxy-substitutes C1-C8 alkyl, C1-C8 alkoxy, halogen-substitutes C1-C8 alkoxy or halogen, wherein alkyl and alkoxy substitutes comprising 1 to 4 carbon atoms are most preferably selected, wherein C1-C4 alkyl and Halogens are even more preferred substituents. In particular, R2 is naphthyl, which is further preferably unsubstituted. In particular, 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.In particular, R2 is C1-C20 alkyl, more preferably C1-C8 alkyl, and even more preferably C1-C6 alkyl, which is unsubstituted or substituted by, for example, C1-C8 alkoxy, halogen, phenyl, or naphthyl, wherein unsubstituted alkyl groups are most preferred, and C1-C4 alkyl is even more preferred. In particular, R2 is C1-C6 alkyl, halogen-substituted C1-C6 alkyl, phenyl-substituted C1-C6 alkyl, naphthyl-substituted C1-C6 alkyl, unsubstituted phenyl, or phenyl substituted by C1-C8 alkyl, halogen-substituted C1-C8 alkyl, C1-C8 alkoxy-substituted C1-C8 alkyl, C1-C8 alkoxy, halogen-substituted C1-C8 alkoxy or halogen, naphthyl and benzyl, which is substituted by C1-C4 alkyl or halogen. In particular, 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. In particular, R1 is phenyl, which is substituted by C1-C4 alkyl, more preferably methyl, X is -C(=O)-, A is phenylene, which is unsubstituted or substituted by C1-C8 alkyl or halogen, wherein unsubstituted phenylene is more preferably, 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, wherein phenyl, which is substituted by C1-C4 alkyl, is more preferably. In particular, The corresponding X-ray diffraction pattern was obtained by X-ray diffraction (XRD) measurement using a Bruker D2 phaser, a Cu electrode, a voltage of 30 kV and a Lynxeye detector.In particular, the corresponding melting point determined by DSC was measured by differential scanning calorimetry using a Netzsch DSC 200 F3 Maia® instrument, an aluminum 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 a nitrogen atmosphere. According to one embodiment, the at least one color developer comprises a compound of formula (2), J1—K1—L1—N(H)—C(=O)—N(H)—L2—K2—J2, wherein J1 and J2 are independently selected as unsubstituted or substituted aryl, wherein K1 and K2 are selected as —O—SO2—, wherein L1 and L2 are independently selected as unsubstituted or substituted aryl, wherein J1 and J2 are preferably selected as unsubstituted or substituted phenyl, preferably as C1-C3 alkyl. substituted phenyl, and wherein L1 and L2 are preferably selected as unsubstituted phenyl, and. wherein the most preferably comprises at least one color developer according to the formula 2 N,N'-bis[3-[[(4-methylphenyl)sulfonyl]oxy]phenyl]urea. In particular, L1 and L2 are independently selected as unsubstituted or substituted phenyl, more preferably substituted phenyl, even more preferably C1-C8-alkyl-substituted phenyl, most preferably methyl-substituted phenyl. In particular, J1 and J2 are independently selected as unsubstituted or substituted phenyl, more preferably unsubstituted phenyl. According to one embodiment, the at least one color developer comprises N,N'-di-(3-(p-toluenesulfonyloxy)phenyl)urea, [3-(3-phenylureido)phenyl]-4-methylbenzenesulfonate, 4,4'-bis(N,N'-p-toluenesulfonyl-aminocarbonylaminophenyl)methane, N,N-p-toluenesulfonyl-aminocarbonylaminophenyl, n-butyl-4(3-(p-toluenesulfonyl)ureido)benzoate, N,N'-diphenylurea, bisphenol A, 4,4'-dihydroxy-diphenylsulfone, 2,4'-Dihydroxy-diphenyl sulfone, 4-Hydroxy-4'-iso-propoxy-diphenyl sulfone, Bis-(3-allyl-4-hydroxy-phenyl)sulfone, 4-Hydroxy-4'-benzyloxy-diphenyl sulfone, 4-Hydroxy-4'-n-propoxy-diphenyl sulfone, N-phenyl-p-hydroxyphenylsulfonamide, 4-Hydroxy-4'-allyloxydiphenyl sulfone, 2,4-Bis(phenylsulfonyl)phenol, N-(4-((4-(3-phenylureido)phenyl)sulfonyl)phenyl)benzenessulfonamide, 2'-(3'-phenylureido)phenyl 3-(3-phenylureido)benzenesulfonate, N-phenyl-N'[(phenylamino)sulfonyl]urea, lignin and / or a urea compound according to the following formula, in particular comprising 4,4'-bis[(4-methyl-3-phen-oxycarbonylaminophenyl)ureido]diphenylsulfone, 4,4'-bis[(2-methyl-5-phenoxycarbo[1]nylaminophenyl)ureido]diphenylsulfone, 4-(2-methyl-3-phenoxycarbonylami- nophenyl)ureido-4'-(4-methyl-5-phe[1]noxycarbonylaminophenyl)ureidodiphenylsulfone:, , and mixtures thereof. According to one embodiment, the at least one color developer comprises at least one stabilizer, which is in particular selected from the group comprising sterically hindered phenolic compounds or sterically hindered amine compounds, wherein the sterically hindered amine compounds, as electron-absorbing compounds, have a relatively low coloring activity and can only optionally be added to the heat-sensitive recording layer.In particular, the at least one stabilizer comprises 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidene bis(6-tert-butyl-2-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy[1]droxy-5-cyclohexylphenyl)butane, 4,4'-thiobis(6-tert-butyl-2-methylphenol), tetrabromobisphenol A, tetrabromobisphenol S, 4,4-thiobis(2-methylphenol), 4,4'-thiobis(2-chlorophenol), tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)-1,2,3,4-butane tetracarboxylate, Tetrakis(1,2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate, bis(2-chloroethyl)ether-4,4'-dihydroxydiphenylsulfone copolymer and mixtures thereof. According to one embodiment, the at least one color developer has a weight fraction of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, more preferably from 10 wt.% to 20 wt.%.In particular, the at least one color developer is selected as N-(2-(3-Phenylu-reido)phenyl)benzenesulfonamide (trade name NKK-1304), wherein the at least one color developer has a weight fraction of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, more preferably from 10 wt.% to 20 wt.%, and most preferably from 12 wt.% to 18 wt.%. In particular, the at least one color developer is selected as 4-Methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (trade name Pergafast 201), wherein the at least one color developer has a weight fraction. The at least one color developer is selected as N,N'-bis[3-[[(4-methylphenyl)sulfonyl]oxy]phenyl]urea, wherein the at least one color developer has a weight fraction of 6 wt% to 35 wt% based on the total solid content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably 10 wt% to 30 wt%, further preferably 10 wt% to 20 wt%, and most preferably 12 to 18 wt%. In particular, the at least one color developer is selected as 3-(3-phenylu- reido)phenyl]-4-methylbenzenesulfonate, wherein the at least one color developer comprises a weight fraction of 6 wt.% to 35 wt.%.-% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, further preferably from 10 wt.% to 20 wt.%, and most preferably from 12 wt.% to 18 wt.%. In particular, the at least one color developer is selected as 4-hydroxy-4'-isopropoxydiphenylsulfone, wherein the at least one color developer has a weight fraction of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, further preferably from 10 wt.% to 20 wt.%, and most preferably from 12 wt.% to 18 wt.%. In particular, the at least one color developer is selected as bis-(3-allyl-4-hydroxyphenyl)sulfone, wherein the at least one color developer has a weight fraction of 6 wt.% up to 35 kg.-% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, more preferably from 10 wt.% to 20 wt.%, and most preferably from 12 wt.% to 18 wt.%. In particular, the at least one color developer is selected as 4,4'-sulfonyldiphenol (trade name BPS), wherein the at least one color developer has a weight fraction of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably 10 wt.% to 30 wt.%, more preferably 10 wt.% to 20 wt.%, and most preferably 12 wt.% to 18 wt.%. According to one embodiment, the at least one color former is a dye of the triphenylmethane type, the fluorane type, the azaphthalide type, and / or the fluorene type, preferably a fluorane type dye. The use of the particularly preferred fluorane type dye as at least one color former enables, due to its availability and balanced application-related properties, the provision of a heat-sensitive recording material with an advantageous price-performance ratio.According to one embodiment, a fluorane-type dye is selected from the group comprising 3-diethylamino-6-methyl-7-anilinofluorane, 3-(N-ethyl-N-4-toludinamino)-6-methyl-7-anilinofluorane, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino)fluorane, 3-pyrrolidino-6-methyl-7-anilinofluorane, 3-(cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-7-(3-trifluoromethylanilino)fluorane, 3-n-dibutylamino-6-methyl-7-anilinofluorane, 3-diethylamino-6-methyl-7-(3-methylanilino)fluorane, 3-n-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 fraction of 2 wt.% to 20 wt.% based on the total solids content of the heat-sensitive color-forming layer, wherein the weight fraction is preferably 4 wt.% to 15 wt.%, most preferably 4 wt.% to 12 wt.%. According to one embodiment, the support substrate comprises paper, synthetic paper, and / or a plastic film, preferably paper. The support substrate is not limited in this respect. The support material preferably has a basis weight of 30 g / m² to 100 g / m², more preferably of 30 g / m² to 80 g / m², and even more preferably of 32 g / m² to 76 g / m². 2In particular, the carrier substrate comprises a paper substrate made from hardwood and / or softwood pulp. According to one embodiment, the heat-sensitive recording material comprises an adhesive layer arranged on the second side of the carrier substrate, wherein the adhesive layer comprises at least one adhesive, the adhesive preferably comprising an pressure-sensitive adhesive and / or a heat-activated adhesive and / or a permanently bonding hot-melt adhesive based on styrene-isoprene and PVC copolymers and / or a permanently bonding hot-melt adhesive based on synthetic rubber and / or a removable acrylate-based adhesive. For example, Avery Dennison's S2200 adhesive is a permanent hot-melt adhesive based on styrene-isoprene and PVC copolymers. For example, Avery Dennison's R5000N adhesive is a removable, acrylate-based adhesive.For example, a permanently adhesive hot melt adhesive based on synthetic rubber is the Technomelt PS 8746 adhesive from Henkel. Method, Product-by-Process, and Use: The aforementioned tasks are solved according to the second aspect by a method for producing a heat-sensitive recording material, comprising the following process steps: providing a carrier substrate which has a first side and a second side facing away from the first side; application. a coating suspension onto the first side of the support substrate, wherein the coating suspension comprises at least one color former and at least one color developer, wherein the coating suspension comprises at least one polymeric binder and at least one inorganic pigment, wherein the at least one inorganic pigment comprises precipitated calcium carbonate (PCC), wherein the precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 50 µm, and wherein the precipitated calcium carbonate (PCC) comprises sieved precipitated calcium carbonate (PCC) and / or milled precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC); drying of the coating suspension to obtain a heat-sensitive color-forming layer arranged on the first side of the support substrate.It is preferred to obtain the heat-sensitive recording material according to the invention using a method in which dispersions, particularly aqueous dispersions, comprising the starting materials of the individual layers, are successively applied to the carrier substrate, wherein the coating suspensions, particularly aqueous dispersions, have a dry matter content of 8 to 60 wt.%, and are applied using the blade coater coating process at an operating speed of the coating system of at least 200 m / min, particularly at least 850 m / min. Alternatively, the coating can also be carried out using a film press or curtain coating process. This method is particularly advantageous from an economic point of view and due to the uniform coating over the carrier substrate. If the dry matter content is 8 wt.%, the coating process can be carried out using a film press or curtain coating process.If the water content falls below -%, efficiency deteriorates because a large amount of water must be removed quickly through gentle drying, which negatively impacts the coating speed. Conversely, if the value exceeds 60% by weight, this only results in increased technical effort to ensure the stability of the coating curtain during the coating process and the drying of the applied film, as the machine must then run very quickly. In the curtain coating process, a freely falling curtain of coating dispersion is formed. The coating dispersion, which is in the form of a thin film (curtain), is "poured" onto a substrate by free fall, thus applying the coating dispersion to the substrate. DE 10196052 T1 discloses the use of the curtain coating process in the production of information recording materials, wherein multi-layered recording layers are realized by applying the curtain, consisting of several coating dispersion films, to substrates. Embodiments of the process according to the invention are also conceivable in which a "double curtain" is used. This means that two successive layers are applied directly one after the other.The application process involves applying the coatings so directly one after the other that the first layer has not yet dried before the next layer is applied. The application of the two layers is therefore preferably carried out "wet-on-wet." All definitions relating to the curtain coating process apply analogously to the double-curtain coating process. The advantage of a "wet-on-wet" application using a double-curtain coating process is that the two layers exhibit a stronger bond and, in particular, the need for intermediate adhesion promoters can be eliminated. In a preferred embodiment of the process according to the invention, the aqueous, deaerated coating suspension has a viscosity of approximately 50 to approximately 2500 mPas (Brookfield, 100 rpm, 20 °C). If the value falls below approximately 50 mPas, or if the viscosity is significantly lower, the coating process can be carried out using a different method.If the value exceeds approximately 2500 mPas, this leads to poor flowability of the coating compound on the coating unit. The viscosity of the aqueous, deaerated coating suspension is preferably between approximately 100 and 700 mPas. The viscosities of successive coatings in the double curtain should decrease from bottom to top. With incorrectly adjusted coatings, the viscosity increases. The probability of heel formation at the point of contact of the coating, as well as the occurrence of "wetting defects," is a key consideration. In a preferred embodiment, to optimize the process, the surface tension of the aqueous coating suspension can be adjusted to approximately 25 to approximately 70 mN / m, preferably to approximately 35 to approximately 60 mN / m, measured in accordance with the standard for bubble pressure tensiometry (ASTM D 3825-90), as described below. Improved control over the coating process is achieved by determining the dynamic surface tension of the coating and adjusting it precisely by selecting a suitable surfactant and determining the required amount of surfactant. The dynamic surface tension is measured using a bubble pressure tensiometer. This measures the maximum internal pressure of a gas bubble formed via a capillary in a liquid.According to the Young-Laplace equation, the internal pressure p of a spherical gas bubble (Laplace pressure) depends on the radius of curvature r and the surface tension σ. When a gas bubble is created at the tip of a capillary in a liquid, the curvature initially increases and then decreases, resulting in a pressure maximum. The greatest curvature, and therefore the highest pressure, occurs when the radius of curvature equals the capillary radius. Pressure profile during bubble pressure measurement, location of the pressure maximum: The radius of the capillary is determined using a reference measurement with a liquid of known surface tension, usually water. Once the radius is known, the surface tension can be calculated from the maximum pressure pmax. Since the capillary is immersed in the liquid, the hydrostatic pressure p0, which results from the immersion depth and the density of the liquid, must be subtracted from the measured pressure (this is done automatically in modern measuring instruments). This leads to the following formula for the bubble pressure method: The measured value corresponds to the surface tension at a specific surface age, the time from the start of bubble formation until the occurrence of the pressure maximum. By varying the bubble generation rate, the dependence of the surface tension on the surface age can be determined, resulting in a curve where the surface tension is plotted against time. This dependence plays an important role for the use of surfactants, as the equilibrium value of the interfacial tension is often not reached in many processes due to the sometimes low diffusion and adsorption rates of surfactants. The formation of the individual layers can be carried out online or offline in a separate coating process. In particular, to ensure that the layers described in detail above exhibit the aforementioned Bekk smoothing, the following process steps are preferably carried out.The substrate is preferably smoothed in a first cylinder. This high degree of smoothness on one or both sides, achieved through this process, already provides an advantage to the substrate. Additional satin finishing by a downstream calender, preferably before a first coating unit, can further improve the smoothness and / or contribute to good profiling. If a thickness coating, as defined above, is applied, this is preferably done using a film press before the intermediate layer is applied. The thickness on the reverse side is particularly advantageous to prevent the coating paint from bleeding through with the blade coater. The use of a blade coater gives the material a good base smoothness, which is crucial for the dynamic sensitivity of the final product. There is a correlation between final smoothness and dynamic sensitivity. The same applies to the protective layer. Alternatively, the protective layer can be printed or applied using a curtain coater. Protective layers that can be cured using actinic radiation are particularly suitable in terms of processing and technological properties. "Actinic radiation" refers to UV or ionizing radiation, such as electron beams. The heat-sensitive layer is preferably applied using curtain coating, as described above. If substrates, especially papers, are coated on one side, the resulting curl should be corrected. This is preferably done with a LAS (Liquid Applicator System).For this purpose, a film of water is applied to the less coated side and then dried. This restores the so-called flatness. Applying the water film slightly degrades the surface. A preferred option for protecting the surface would be a steam humidifier. With this method, steam is blown in instead of water being applied. This does not damage the surface. This is very suitable for applications where the highest surface quality is required. Another possibility would be a spray humidifier, which applies a water mist. All of the aforementioned layers can be applied in one or more layers. According to one embodiment, the method comprises applying an adhesive dispersion to the second side of the carrier substrate and drying the applied adhesive suspension to obtain an adhesive layer, wherein the application rate of the adhesive dispersion is preferably between 10 g / m²2 and 30 g / m² 2 , especially preferably 20 g / m² 2The drying temperature of the adhesive layer is [value missing]. 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 another embodiment, the method includes the further process 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 coating dispersion to the first side of the carrier substrate is carried out by means of a curtain brush or by means of a squeegee on the coating side of a carrier substrate pre-coated with a pigment coating, wherein the pigmented primer 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 rate of the pigmented primer is further preferably 2 g / m². 2 up to 12 g / m² 2 According to one embodiment, the application rate of the coating dispersion is between 2 g / m² and 5 g / m², preferably between 3.6 g / m² and 4.8 g / m², so that the surface weight of the heat-sensitive layer is between 2 g / m² and 5 g / m², preferably between 3.6 g / m². 2 and 4.8 g / m² 2The embodiments listed for the heat-sensitive recording material according to the first aspect are also embodiments for the method of producing a heat-sensitive recording material according to the second aspect, and vice versa. The aforementioned problems are solved according to the third aspect by a heat-sensitive recording material producible by a method according to the second aspect. The embodiments of the heat-sensitive recording material according to the first aspect and the embodiments of the method for producing a heat-sensitive recording material as described in the second aspect are also embodiments of the heat-sensitive recording material producible by a method according to the third aspect. According to a fourth aspect, the present invention relates to the use of a heat-sensitive recording material according to the first or third aspect for direct thermal printing, for example as a receipt, label, or ticket.The embodiments listed for the heat-sensitive recording material according to the first aspect and for the method for producing a heat-sensitive recording material according to the second aspect are also embodiments for use according to the fourth aspect. FIGURE 1 In Figure 1, a dark printed area is shown for the comparative examples V1, V2, V3, and V4 described below (see Figures A, C, E, and G) and for the following embodiments 1, 2, 3, and 4 (see Figures B, D, F, and H). This area contains light defects or white dots, which are disadvantageous and which are to be avoided or reduced in number within the scope of the present invention. EMBODIMENTS In the embodiments described in detail below, several heat-sensitive recording materials or...Thermal papers are produced by applying aqueous coating suspensions to form a composite structure. The samples were produced on a carrier substrate and examined and evaluated using various measurement methods. In all examples, a paper substrate made from hardwood and softwood pulp with a basis weight of 34 or 38 g / m² was used as the carrier substrate. Measurement methods: Measurement of optical density: 6 cm wide strips were obtained from the correspondingly produced heat-sensitive recording materials, in particular by cutting them. These strips were thermally printed using a GeBE PrinterLab GPT-10000 test printer (GeBE Elektronik und Feinwerktechnik GmbH, Germany) with a Kyocera printhead of 305 dpi at an applied voltage of 24 V and with a bar pattern at an energy level of 8.88 mJ / mm² and a printing speed of approximately 100 mm / s. The area of ​​one bar of the printed pattern corresponds to 53 x 12 mm. The optical density (oD) shown in the following table) was measured with a Techkon SpectroDens densitometer, with the measurement uncertainty of the oD values ​​estimated at ≤2%. Measurements of the occurrence of white dots or light defects in a dark print area: To assess the occurrence of white dots or light defects in a dark print area, an area of ​​approximately 7 x 2.5 cm of appropriately manufactured heat-sensitive recording materials was thermally printed over the entire surface using a standard Epson TM T-88VI thermal printer. The perception of white dots and their frequency can be qualitatively assessed using the following ratings (even if this is not shown, for example, in Figure 1): "--" (very poor), "-" (poor), "0" (average), "+" (good), "++" (very good). Measurement of barcode machine readability: A barcode sample printout longitudinal and transverse measurements according to the UPC-A code were taken at an energy level of 8.88 mJ / mm.2 or 10.32 mJ / mm 2Barcodes were generated using a GeBE PrinterLab GPT-10000 test printer on specially prepared heat-sensitive recording materials. Barcode machine readability was evaluated using an REA VeriCube barcode verification device from REA Elektronik Deutschland, in accordance with ISO 15416, with the laser scanner operating at a wavelength of 660 nm. The software "TransWin32 V.1.2.0.2 / 16829" was used for evaluation. Barcode machine readability was assessed based on the scan reflection profile class in descending order of quality for each barcode: A (corresponds to a score of at least 4), B (corresponds to a score of 3), C (corresponds to a score of 2), D (corresponds to a score of 1), F (corresponds to a score of 0). Bekk smoothness measurement: Bekk smoothness was determined according to DIN 53107. Roughness measurement: Roughness is determined using the airflow method according to the standard DIN ISO 8791-4 using a Parker Print Surf tester.Production of the heat-sensitive recording materials: The heat-sensitive recording materials described below, according to embodiments 1, 2, 3 and 4, as well as comparative examples V1, V2, V3 and V4, each have an intermediate layer applied to the substrate and a heat-sensitive recording layer applied to the intermediate layer. The aqueous coating suspensions for forming the intermediate layer of the heat-sensitive recording material were applied one-sided online in the paper machine using a film press and a coating weight of 2.5 g / m². 2 up to 3.0 g / m² 2 at an operating speed of 1600 m / min on a paper web with a basis weight of at least 34 g / m² (Exemplary 4 and Comparative Example V4) or 38 g / m² 2 (Examples 1, 2 and 3, as well as comparative examples V1, V2 and V3). The heat-sensitive layer is applied offline to the intermediate layer. The following describes the preparation of the dispersions for the coating suspensions of embodiments 1, 2, 3, and 4, as well as comparative examples V1, V2, V3, and V4. Colorant dispersion A1: Colorant dispersion A1 was prepared by milling 1.0 wt. parts of 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluorane (S-205) with 1.2 wt. parts of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) in a bead mill. Colorant dispersion A2: The colorant dispersion A2 was prepared by milling 9 parts by weight of 3-n-dibutylamine-6-methyl-7-anilinofluorane (ODB-2) with 11 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 2 and Comparative Example V2), or 16.5 parts by weight of 3-n-dibutylamine-6-methyl-7-anilinofluorane (ODB-2) with 22 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 2 and Comparative Example V2).-Parts of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary Examples 3 and 4 and Comparative Examples V3 and V4), or 19 wt. parts of 3-n-dibutylamine-6-methyl-7-ani-linofluorane (ODB-2) with 25 wt. parts of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary Example 1 and Comparative Example V1) produced in a bead mill. Color developer dispersions B: The aqueous color developer dispersions B were prepared by milling 35 parts by weight of the respective color developer 4,4'-sulfonyldiphenol (BPS) with 25 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 2 and Comparative Example V2), or 33 parts by weight of the respective color developer 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide (Pergafast 201, CAS# 232938-43-1, ^-polymorph) with 16 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 2 and Comparative Example V2).-parts of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplaries 3 and 4 and Comparative Examples V3 and V4), or 41 parts by weight of the respective color developer N-(2-(3-Phenylureido)phenyl)benzenesulfonamide (NKK-1304, CAS# 215917-77-4, ^-polymorph) with 20 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 1 and Comparative Example V1) in a bead mill. Sensitizing dispersions C: The sensitizing dispersions C were prepared by milling 7.5 parts by weight of 1,2-diphenylsulfone (DPS) with 5 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray company) (Exemplary 2 and Comparative Example V2), or by milling 7 parts by weight of diphenoxyethane (DPE) with 7 parts by weight.-Parts of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplaries 3 and 4 and Comparative Examples V3 and V4), or by milling 8 parts by weight of 1,2-diphenoxyethane (DPE) with 9 parts by weight of a 20% aqueous solution of Poval 4-85 (polyvinyl alcohol solution, Kuraray) (Exemplary 1 and Comparative Example V1) in a bead mill. Sensitizing dispersions D: The sensitizing dispersions D each comprise 50 parts by weight (Exemplary 2 and Comparative Example V2), 58 parts by weight (Exemplary 3 and 4 as well as Comparative Examples V3 and V4), or 70 parts by weight (Exemplary 1 as well as Comparative Example V1) of a 25% stearic acid amide dispersion. All dispersions A1, A2, B, C, and D produced by milling have a mean particle size D(4,3) of 0.7 µm to 1.3 µm.The particle size distribution of the dispersions was measured by laser diffraction using a Coulter LS13320 instrument from Beckman Coulter. Release agent dispersions E: The release agent dispersions E each comprise 21 parts by weight (Exemplary 2 and Comparative Example V2), 17.5 parts by weight (Exemplary 3 and 4 and Comparative Examples V3 and V4), or 21 parts by weight (Exemplary 1 and Comparative Example V1) of a 35% zinc stearate dispersion. Pigment dispersions P: The pigment dispersions P each comprise 187 parts by weight (Exemplary 2 and Comparative Example V2), 179 parts by weight (Exemplary 3 and 4 and Comparative Examples V3 and V4), or 205 parts by weight (Exemplary 1 and Comparative Example V1) of a 45% zinc stearate dispersion. Dispersion of precipitated calcium carbonate (PCC). Binder solutions: The binder solutions each comprise 126 parts by weight (Exemplary 2 and Comparative Example V2), or 128 parts by weight.-parts (Exemplary Models 3 and 4 and Comparative Examples V3 and V4), or 144 parts by weight (Exemplary Model 1 and Comparative Example V1) of a 9% aqueous polyvinyl alcohol solution (Poval 28-99, Kuraray Europe). Additives: Furthermore, the coating suspensions according to Exemplary Models 1, 2, 3 and 4, or Comparative Examples V1, V2, V3 and V4, comprise further additives, which are present in small quantities. Intermediate layer: The intermediate layer used in Exemplary Models 1 to 4, as well as in Comparative Examples V1 to V4, is based on a styrene-acrylate copolymer, contains calcium carbonate as a pigment, and was produced with a basis weight of between 2.5 g / m². 2 up to 3.0 g / m² 2applied to the carrier substrate. Exemplary embodiments 1, 2, 3 and 4, as well as comparative examples V1, V2, V3 and V4. As already described, the heat-sensitive recording materials are applied according to exemplary embodiments 1, 2, 3 and 4, as well as comparative examples V1, V2, V3 and V4, by applying an intermediate layer to the base paper, and by adhering to it. The respective heat-sensitive recording layer is subsequently applied to the intermediate layer. The application suspension used for embodiment 2 and for comparative example V2 to produce the heat-sensitive recording layer comprises the color-forming dispersion A1, the respective color-forming dispersion A2, the respective color-developer dispersion B, the respective sensitizing dispersion C, the respective sensitizing dispersion D, the respective release agent dispersion E, the respective pigment dispersion P, and the respective binder solution, as well as additives in small quantities. The basis weight of the heat-sensitive recording material according to comparative example V2 and according to embodiment 2 is 48 g / m² in each case.The application suspension used for embodiment 1 and for comparative example V1 to produce the heat-sensitive recording layer comprises the respective color-forming dispersion A2, the respective color-developer dispersion B, the respective sensitizing dispersion C, the respective sensitizing dispersion D, the respective release agent dispersion E, the respective pigment dispersion P, the respective binder solution, and additives in small quantities. The basis weight of the heat-sensitive recording material according to comparative example V1 and according to embodiment 1 is 48 g / m² in each case.The coating suspension used for embodiments 3 and 4, as well as for comparative examples V3 and V4, to produce the heat-sensitive recording layer comprises the respective color-forming dispersion A2, the respective color-developer dispersion B, the respective sensitizing dispersion C, the respective sensitizing dispersion D, the respective release agent dispersion E, the respective pigment dispersion P, the respective binder solution, and additives in small quantities. The compositions of the coating suspensions for comparative examples V3 and V4 for embodiments 3 and 4 do not differ. The only difference between comparative examples V3 and V4 and embodiments 3 and 4 lies in the basis weight of the heat-sensitive recording material. The basis weight of the heat-sensitive recording material according to comparative example V3 and according to embodiment 3 is 48 g / m² in each case. 2The basis weight of the heat-sensitive recording material according to comparative example V4 and according to embodiment 4 is 44 g / m² in each case. 2The embodiments 1, 2, 3, and 4 differ from the corresponding comparative examples V1, V2, V3, and V4 only in the type of pigment used, which is precipitated calcium carbonate (PCC). While comparative examples V1, V2, V3, and V4 use untreated precipitated calcium carbonate (PCC), embodiments 1, 2, 3, and 4 use milled precipitated calcium carbonate (PCC). The milling process described in embodiments 1, 2, 3, and 4 is carried out as follows: Before milling, the pigment dispersion P is diluted to a solids content of 40% or greater, and up to 1.0% sodium polyacrylate is added as a dispersing agent. This pigment dispersion is then pumped through a stirred ball mill. The mill is filled with grinding beads with a diameter of 0.8 to 1.0 mm and a fill level of 50% or greater.The residence time of the pigment dispersion in the mill is a maximum of 10 minutes, preferably 5 minutes. The rotor speed of the mill is greater than or equal to 10 m / s to keep the beads sufficiently in motion. The mill is cooled to ensure that the product temperature remains below 40°C. At the mill outlet, the product is cooled to 30°C or less via a plate heat exchanger. Mills from Bühler, model Centex T3, are used in particular. The treated pigment dispersion P is used directly in the coatings according to embodiments 1, 2, 3, and 4. The treatment according to the embodiments is monitored by particle size measurement and by determining the sieve residue of ground precipitated calcium carbonate (PCC).The ground precipitated calcium carbonate (PCC) according to embodiments 1, 2, 3 and 4 has a particle size distribution (d90) determined according to ISO 13320 from 1.9 ^m to 3.4 ^m, has a particle size distribution (d50) determined according to ISO 13320 from 1.1^m to 1.9 ^m, has a particle size distribution (d10) determined according to ISO 13320 from 0.1^m to 1.0 ^m, and has a particle size distribution (D4,3) determined according to ISO 9276-2 from 1.2^m to 2.1 ^m. The ground precipitated calcium carbonate (PCC) according to embodiments 1, 2, 3 and 4 has a sieve residue obtained after treatment according to standard DIN EN ISO 787-7 of between 0.01% and 0.03% when filtered with a sieve with a pore size of 25 µm.In comparison, the inorganic pigment of the heat-sensitive layer according to comparison examples 1, 2, 3 and 4 consists of untreated precipitated calcium carbonate (PCC), which has a particle size distribution (d90) determined according to standard ISO 13320 of 3.3 ^m to 4.3 ^m, which has a particle size distribution (d50) determined according to standard ISO 13320 of 2.00 ^m to 2.40 ^m, which has a particle size distribution (d10) determined according to standard ISO 13320 of 0.80 ^m to 1.40 ^m, and which has a particle size distribution (D4,3) determined according to standard ISO 9276-2 of 2.20 ^m to 2.60 ^m. The untreated precipitated calcium carbonate (PCC) according to embodiment 1 has a sieve residue of between 0.35% and 1.0% as obtained according to the standard DIN EN ISO 787-7 when filtered with a sieve with a pore size of 25 µm.Although this was not done in the present examples, it is also optionally possible to carry out the described milling of the precipitated calcium carbonate (PCC) of the respective pigment dispersion P in combination with another partial mass, for example the respective color developer dispersion B, the respective sensitizing dispersion C and / or the respective sensitizing dispersion D. The following table shows a comparison of the respective heat-sensitive recording materials according to embodiments 1, 2, 3 and 4, as well as comparison examples V1, V2, V3 and V4, with respect to the measured optical density (oD), the smoothness in Bekk seconds, the roughness in pps, and the barcode machine readability (Code 1 to Code 4) according to the corresponding grade: oD Smoothness Roughness Code 1 Code 2 Code 3 Code 4 Example V1 1.00 457 2.04 2.3 3.1 1.7 2.41 1.05 466 1.97 2.9 3.6 2.0 2.9 V2 0.86 591 1.75 2.1 3.0 1.5 2.12 0.91 618 1.67 2.6 3.2 1.7 2.7V3 1.00 446 2.28 2.5 3.1 1.8 2.73 1.06 526 2.05 2.7 3.3 2.4 3.1V4 1.01 494 2.26 2.1 2.9 1.1 2.04 1.08 587 2.07 2.2 3.2 1.1 2.6 Table: The column "Code 1" describes the barcode machine readability rating for a printed longitudinal grid with an energy dose of 8.88 mJ / mm² 2The column “Code2” describes the barcode machine readability rating for a printed longitudinal grid with an energy dose of 10.32 mJ / mm². 2 The column “Code 3” describes the barcode machine readability rating for a printed cross-scratch with an energy dose of 8.88 mJ / mm². 2 The column “Code 4” describes the barcode machine readability rating for a printed cross-hatch pattern with an energy dose of 10.32 mJ / mm². 2 . A comparison of the optical densities between the respective comparison examples V1, V2, V3 or V4 and the respective embodiments 1, 2, 3 or 4 reveals a clear improvement that can be achieved with minimal effort. The respective embodiments 1, 2, 3, and 4 and the respective comparative examples V1, V2, V3, and V4 are highlighted (for example, for the pair V3 / 3, 1.06 / 1.00 = 6%). The same applies to the other determined parameters such as smoothness, roughness, and barcode machine readability according to Code 1 to Code 4, whereby the corresponding improvement is attributable to the use of ground precipitated calcium carbonate (PCC). Compared to conventional, untreated precipitated calcium carbonate (PCC), the use of ground precipitated calcium carbonate (PCC) also has the further effect of significantly reducing the occurrence of white defects or white spots, as illustrated, for example, in Figure 1.Figure A of Figure 1 shows a test print on the heat-sensitive recording material according to comparative example V1, and Figure B of Figure 1 shows a test print on the heat-sensitive recording material according to embodiment 1. Furthermore, Figure C of Figure 1 shows a test print on the heat-sensitive recording material according to comparative example V2, and Figure D of Figure 1 shows a test print on the heat-sensitive recording material according to embodiment 2. Figure E of Figure 1 also shows a test print on the heat-sensitive recording material according to comparative example V3, and Figure F of Figure 1 shows a test print on the heat-sensitive recording material according to embodiment 3.Furthermore, Figure G of Figure 1 shows a test print on the heat-sensitive recording material according to comparative example V4, and Figure H of Figure 1 shows a test print on the heat-sensitive recording material according to embodiment 4. A comparison of Figures B, D, F, and H of Figure 1 reveals a significant reduction in the occurrence of white defects or white spots in embodiments 1, 2, 3, and 4 compared to comparative examples V1, V2, V3, and V4, as well as in Figures A, C, E, and G of Figure 1. Although not shown in the table and figures, filtered precipitated calcium carbonate (PCC) can be used instead of the ground precipitated calcium carbonate (PCC) used in embodiments 1, 2, 3, and 4 to achieve comparably positive results.A corresponding filtered precipitated calcium carbonate (PCC) is obtained by diluting the pigment dispersion P to a solids content of ≤ 40% to reduce its viscosity. The diluted pigment dispersion is then filtered through a 25 µm filter. Filtration can be carried out either under pressure or without pressure. Filter stockings, shaker sieves, or scraped pressure filters can be used, particularly nylon filter bags from Wolftechnik. The coarse particles are retained in the filter and discarded; the diluted pigment dispersion can be reused.The filtered precipitated calcium carbonate (PCC) has a particle size distribution (d90) determined according to ISO 13320 of 3.6 ^m to 3.8 ^m, a particle size distribution (d50) determined according to ISO 13320 of 2.0^m to 2.2 ^m, a particle size distribution (d10) determined according to ISO 13320 of 1.0^m to 1.10 ^m, and a particle size distribution (D4,3) determined according to ISO 9276-2 of 2.2^m to 2.3 ^m. The filtered precipitated calcium carbonate (PCC) according to embodiment 2 exhibits a sieve residue of 0.01 to 0.03% after treatment, in accordance with DIN EN ISO 787-7, and in particular approximately 0.02% or 0.03% when filtered with a sieve with a pore size of 25 µm. Although not shown in the table and figures, the ground precipitated calcium carbonate (PCC) of embodiments 1, 2, 3, and 4 can be used instead. A sieved precipitated calcium carbonate (PCC) can also be used to achieve comparably positive results. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (D4,3) of 2.3 to 2.5 µm, the mean particle size (D4,3) of the sieved precipitated calcium carbonate (PCC) being determined according to standard X. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d10) of 1.1 to 1.3 µm, the mean particle size (d10) of the sieved precipitated calcium carbonate (PCC) being determined according to standard ISO 13320. In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d50) of 2.2 to 2.3 µm, the mean particle size (d50) of the sieved precipitated calcium carbonate (PCC) being determined according to standard ISO 13320.In particular, the sieved precipitated calcium carbonate (PCC) has a mean particle size (d90) of 3.8 to 4.1 µm, the mean particle size (d90) of the sieved precipitated calcium carbonate (PCC) being determined according to ISO 13320. Specifically, the sieved precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment in the range of approximately 0.01% to approximately 0.03%, as determined according to DIN EN ISO 787-7.

Claims

1. Heat-sensitive recording material comprising: a support substrate having a first side and a second side facing away from the first side; a heat-sensitive color-forming layer arranged on the first side of the support substrate, wherein the heat-sensitive color-forming layer comprises at least one color former and at least one color developer; characterized in that the heat-sensitive layer comprises at least one polymeric binder and at least one inorganic pigment, wherein the at least one inorganic pigment comprises precipitated calcium carbonate (PCC), wherein the precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 50 µm, and wherein the precipitated calcium carbonate (PCC) comprises sieved precipitated calcium carbonate (PCC) and / or ground precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC). 2.Heat-sensitive recording material according to claim 1, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d90) of less than 40 µm, preferably less than 30 µm, further preferably less than 25 µm, more preferably less than 20 µm, most preferably less than 10 µm, more preferably less than 7 µm, more preferably less than 5 µm, and further preferably less than 4.5 µm.

3. Heat-sensitive recording material according to claim 1 or 2, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d50) of less than 50 ^m, preferably less than 40 ^m, further preferably less than 30 ^m, still more preferably less than 25 ^m, further still more preferably less than 20 ^m, most preferably less than 10 ^m, still more preferably less than 5 ^m, further still more preferably less than 3 ^m and additionally further still more preferably less than 2.5 ^m.4.Heat-sensitive recording material according to one of the preceding claims, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) has a mean particle size (d10) of less than 50 µm, preferably less than 40 µm, further preferably less than 30 µm, even more preferably less than 25 µm, further still more preferably less than 20 µm, most preferably less than 10 µm, even more preferably less than 5 µm, further still more preferably less than 2 µm and furthermore more preferably less than 1.5 µm.Heat-sensitive recording material according to one of the preceding claims, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises scalenohedral precipitated calcium carbonate (s-PCC), wherein in particular the proportion of the scalenohedral precipitated calcium carbonate (s-PCC) in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, further preferably more than 70%, even more preferably more than 80%, most preferably more than 90%, and even more preferably more than 98%.Heat-sensitive recording material according to one of the preceding claims, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) comprises precipitated calcium carbonate (PCC) having a prismatic, rhombohedral or aragonite structure, wherein in particular the proportion of precipitated calcium carbonate. (PCC) with the prismatic, rhombohedral or aragonite structure in the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is more than 50%, preferably more than 60%, further preferably more than 70%, even more preferably more than 80%, most preferably more than 90%, and even more preferably more than 98%.

7. Heat-sensitive recording material according to one of the preceding claims, characterized in that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is characterized by a sieve residue obtained after treatment according to DIN EN ISO 787-7 of less than 1%, preferably less than 0.5%, further preferably less than 0.1%, and most preferably less than 0.05%.

8. Heat-sensitive recording material according to one of the preceding claims, characterized in that,that the sieved and / or ground and / or filtered precipitated calcium carbonate (PCC) is present in the heat-sensitive layer in an amount of 5 wt.% to 60 wt.%, preferably in an amount of 10 wt.% to 50 wt.%, further preferably in an amount of 15 wt.% to 50 wt.% based on the total dry mass of the heat-sensitive layer.

9. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one inorganic pigment comprises at least one further inorganic pigment selected from the group comprising calcium silicate hydrate, barium sulfate, kaolinite, calcium silicate, calcium sulfate, sodium aluminum silicate, aluminum oxides, aluminum hydroxides, silicas, precipitated and pyrogenic silicas, diatomaceous earths, magnesium carbonates, silicon dioxide, talc, kaolin, in particular natural kaolin or calcined kaolin, titanium dioxide, bentonite and mixtures thereof, preferably silicon dioxide, kaolin,especially natural kaolin or calcined kaolin and / or aluminum hydroxide.

10. Heat-sensitive recording material according to one of the preceding claims, characterized in that the heat-sensitive layer comprises at least one sensitizing agent, which is preferably selected from the group comprising a fatty acid amide, particularly preferably stearamide, behenamide or palmitamide, an ethylene-bis fatty acid amide, particularly preferably N,N'-ethylenebis-stearic acid amide or N,N'-ethylenebis-oleic acid amide, a wax, particularly preferably polyethylene wax or montan wax, a carboxylic acid ester, particularly preferably dimethyl terephthalate, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-(p-methylbenzyl)oxalate, di-(p-chlorobenzyl)oxalate or di-(p-benzyl)oxalate, an aromatic ether, particularly preferably 1,2-diphenoxyethane, 1,2-di-(3-methylphenoxy)ethane, 2-benzyloxynaphthalene or 1,4-diethoxynaphthalene, an aromatic sulfone, especially preferred diphenylsulfone,and / or an aromatic sulfonamide, especially preferred benzenesulfonanilide or N-benzyl-p-toluenesulfonamide, o-toluenesulfonamide, p-toluenesulfonamide, p-benzylbiphenyl (PBBP), 1,2-bis-(phenoxymethyl)benzene, 4-(4-tolyloxy)biphenyl, 1,2-bis-(3,4-dimethylphenyl)ethane and / or mixtures thereof.

11. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one polymeric binder of the heat-sensitive layer is selected from the group comprising water-soluble starches, starch-based biolatices of the Ecosphere type, starch derivatives, methylcellulose, hydroxyethylcellulose, carboxymethylcelluloses, partially or fully saponified polyvinyl alcohols, ethylene-vinyl alcohol copolymers, chemically modified polyvinyl alcohols 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 and / or acrylonitrile-butadiene copolymers and mixtures thereof, wherein the at least one binder is preferably selected as polyvinyl alcohol.

12. Heat-sensitive recording material according to any of the preceding claims, characterized in that the heat-sensitive recording material has at least one intermediate layer arranged between the support substrate and the heat-sensitive layer, wherein the intermediate layer preferably comprises at least one binder and / or preferably at least one pigment.

13. Heat-sensitive recording material according to any of the preceding claims, characterized in that the heat-sensitive color-forming layer has a Bekk smoothness of 150 sec to 1500 sec, preferably 250 sec to 1000 sec, as measured according to DIN 53107.

14. Heat-sensitive recording material according to any of the preceding claims, characterized in that the heat-sensitive recording material has an optical density (o. D.) of at least 0.75, preferably at least 0.9, and most preferably at least 1.0, as defined in the description, particularly at an energy level of 8.88 mJ / mm².

215. Heat-sensitive recording material according to one of the preceding claims, characterized in that the heat-sensitive recording material has a roughness measured according to DIN ISO 8791-4 using a Parker Print Surf tester of less than 5 µm, preferably less than 4.5 µm, more preferably less than 4 µm, more preferably less than 3 µm, and more preferably less than 2.5 µm. 16.Heat-sensitive recording material according to one of the preceding claims, characterized in that the heat-sensitive recording material is designed as a heat-sensitive recording material, as defined in the description, printed with a barcode test pattern at an energy level of 8.88 mJ / mm² or 10.32 mJ / mm² using a GeBE PrinterLab GPT-10000 test printer, wherein the printed heat-sensitive recording material has a barcode machine readability of grade 1 or more, preferably 2 or more, and most preferably 3 or more, as defined in the description and evaluated according to ISO 15416.

17. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one color developer comprises a compound of formula (I):. where R and R1 are independently selected from the group comprising hydrogen, C1-C 18-Alkyl, C1-C8-alkoxy-C1-C8-alkyl, and (R9)2N-C1-C8-alkyl, wherein R9 is selected from the group comprising C1-C8-alkyl, C5-C6-cycloalkyl; or a compound of formula (II) wherein R2, R3, R4, R5, and R6 are independently selected from the group comprising hydrogen, C1-C8 alkyl, -NH-C(=O)-R7, and -C(=O)-NH-R7, wherein R7 is selected as C1-C8 alkyl or -C(=O)OR8, wherein R8 is selected as C1-C8 alkyl or halogen, or wherein R2 and R3, or R4 and R5 or both, or wherein R3 and R4, or R5 and R6 or both, or wherein R2 and R3 and R5 and R6, together form a hydrocarbon group with three or four carbon atoms, and wherein Q comprises a single bond or C1-C8 alkylene, which may be branched or unbranched, and wherein the C1-C8 alkylene comprises a main chain having one or more oxygen atoms between two carbon atoms if the C1-C8 alkylene has more than two carbon atoms, wherein the A compound of formula (I) preferably comprises a compound of formula (Ia): ; or wherein the compound of formula (I) preferably comprises a compound of formula (Ib): wherein the compound of formula (I) particularly preferably comprises 5-(N-3-methylphenylsulfonylamido)-(N',N"-bis-{3-methylphenyl)-isophthalic diamide.

18. Heat-sensitive recording material according to one of the preceding claims, characterized in that 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-C6 alkyl, C1-C6 alkoxyl, C2-C6 alkenyl, C1-C6 fluoroalkyl, N(R4)2, NHCOR5, optionally substituted phenyl, and optionally substituted benzyl, wherein R4 is selected from the group comprising hydrogen, phenyl, benzyl, and C1-C6 alkyl, wherein R5 is selected as C1-C6 alkyl, wherein n1 and n3 are independently selected as an integer from 1 to 5, and where n2 is an integer from 1 to 4; wherein R1, R2, and R3 are preferably selected as hydrogen, further preferably comprising the compound of formula (NI) N-(2-(3-phenylureido)phenyl)benzenesulfonamide, and even more preferably the ^-polymorph and / or the ^-polymorph of N-(2-(3-phenylureido)phenyl)benzenesulfonamide, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC.

19. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one color developer comprises a compound of formula (1):. wherein R1 is selected from the group comprising unsubstituted or substituted phenyl, naphthyl and C1-C20 alkyl, wherein X is selected from the group comprising —C(=NH) —, —C(=S) — and —C(=O) —, wherein A is selected from the group comprising unsubstituted or substituted phenylene, naphthylene, C1-C12 alkylene and an unsubstituted or substituted heterocyclic group, wherein B is selected from the group comprising —O—SO2—, —SO2—O—, —NH—SO2—, —SO2—NH—, —S—SO2—, —O—CO—, —O—CO—NH—, —NH—CO—, —NH—CO—O—, —S—CO—NH—, —S—CS—NH—, —CO—NH—SO2—, —O—CO—NH—SO2—, —NH═CH—, —CO—NH—CO—, —S—, —CO—, —O—, —SO2—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-C 20-Alkyl, subject to the condition that if B is not a group of the formula —O—SO2—, then R2 is unsubstituted or substituted phenyl, naphthyl or C1-C8 alkyl, and that if B is —O—, then R2 is not alkyl; wherein X is preferably selected as —C(=O) —, wherein R1 is selected as substituted or substituted phenyl, preferably as C1-C3 alkyl substituted phenyl, wherein R 2is 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 C1-C3 alkyl substituted phenyl, and wherein the at least one color developer of formula (1) preferably comprises 4-methyl-N-(((3-((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, more preferably the ^-polymorph and / or the ^-polymorph of 4-methyl-N-(((3-(((4-methylphenyl)sulfonyl)oxy)phenyl)amino)carbonyl)benzenesulfonamide, wherein in particular the ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 ^-polymorph is characterized by an X-ray diffraction pattern with Bragg angles (2^ / CuK^) 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 determined by DSC.

20. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one color developer comprises a compound of formula (2), J1—K1—L1—N(H)—C(=O)—N(H)—L2—K2—J2, wherein J1 and J2 are independently selected as unsubstituted or substituted aryl, wherein K1 and K2 are selected as —O—SO2—, wherein L1 and L2. are independently selected as unsubstituted or substituted aryl, wherein J1 and J2 are preferably selected as unsubstituted or substituted phenyl, preferably as C1-C3 alkyl substituted phenyl, and wherein L1 and L2 are preferably selected as unsubstituted phenyl, and wherein most preferably the at least one color developer according to the formula 2 N,N'-bis[3-[[(4-methylphenyl)sulfonyl]oxy]phenyl]urea.

21. Heat-sensitive recording material according to one of the preceding claims, characterized in that the at least one color developer comprises N,N'-di-(3-(p-toluenesulfonyloxy)phenyl)urea, [3-(3-phenylureido)phenyl]-4-methylbenzenesulfonate, 4,4'-bis(N,N'-p-toluenesulfonyl-aminocarbonylaminophenyl)methane, N,N'-p-toluenesulfonyl-aminocarbonylaminophenyl, n-butyl-4(3-(p-toluenesulfonyl)ureido)benzoate, N,N'-diphenylurea, bisphenol A, 4'-dihydroxy-diphenylsulfone, 2,4'-dihydroxy-diphenylsulfone,4-Hydroxy-4´-iso-propoxy-diphenyl-sulfon, Bis-(3-allyl-4-hydroxy-phe-nyl)-sulfon, 4-Hydroxy-4´-benzyloxy-diphenyl-sulfon, 4-Hydroxy-4´-n-propoxy-diphenyl-sulfon, N-Phenyl-p-hydroxyphenylsulfonamid, 4-Hydroxy-4´-allyloxydiphenylsulfon, 2,4-Bis(phenylsulfonyl) phenol, N-(4-((4-(3-Phenylureido)phenyl)sulfonyl)phenyl)benzolsul-fonamid, 2‘-(3‘-Phenylureido)phenyl 3-(3-phenylureido)benzolsulfonat, N-phenyl-N’[(phenylamino)sulfonyl]harnstoff, Lignin, und / oder eine Harnstoff-Verbindung gemäßnachfolgenden Formel umfasst, insbesondere umfassend 4,4’-bis[(4-methyl-3-phen-oxycarbonylaminophenyl)ureido]diphenylsulfon, 4,4’-bis[(2-methyl-5-phen- oxycarbo[1]nylaminophenyl)ureido]diphenylsulfon, 4-(2-methyl-3-phenoxycarbonylami- nophenyl)ureido-4’-(4-methyl-5-phe[1]noxycarbonylaminophenyl)ureidodiphenylsulfon:, , and mixtures thereof.

22. Heat-sensitive recording material according to any of the preceding claims, wherein the at least one color developer comprises a weight fraction of 6 wt.% to 35 wt.% based on the total solids content of the heat-sensitive comprising a color-forming layer, wherein the weight fraction is preferably from 10 wt.% to 30 wt.%, and more preferably from 10 wt.% to 20 wt.%.23.A method for producing a heat-sensitive recording material, comprising the following process steps: providing a support substrate having a first side and a second side facing away from the first side; applying a coating suspension to the first side of the support substrate, the coating suspension comprising at least one color former and at least one color developer, the coating suspension comprising at least one polymeric binder and at least one inorganic pigment, the at least one inorganic pigment comprising precipitated calcium carbonate (PCC), the precipitated calcium carbonate (PCC) having a mean particle size (d90) of less than 50 µm, and the precipitated calcium carbonate (PCC) comprising sieved precipitated calcium carbonate (PCC) and / or ground precipitated calcium carbonate (PCC) and / or filtered precipitated calcium carbonate (PCC); 24.Heat-sensitive recording material producible by a method according to claim 23.

25. Use of a heat-sensitive recording material according to one of claims 1 to 22 or 24 for thermal direct printing, for example as a receipt, label or ticket.

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