Use of N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea as a developer in thermal recording materials

Abstract

Use of N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea having an X-ray diffraction pattern with a Bragg angle (2θ / CuKα) 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 as a developer in a heat-sensitive recording material to limit the loss of image density and / or relative print contrast and / or a reduction in the amount of developer based on area, the heat-sensitive recording material comprising a carrier substrate, a A thermosensitive color-forming layer on one side and comprising at least one non-phenolic color developer and at least one color former, and an adhesive layer and / or a coating allowing back printing using conventional printing methods on the side of the carrier substrate opposite to the thermosensitive color-forming layer, wherein the image density of the thermosensitive recording material stored according to the migration test defined in the description is equal to at least 35% of the image density value before storage at a value of ≥1.20 optical density units and / or the relative printing contrast of the thermosensitive recording material stored according to the migration test is equal to at least 70% of the relative printing contrast value before storage and / or the amount of color developer based on area of ​​the thermosensitive recording material stored according to the migration test is equal to at least 30% of the amount of color developer based on area before storage.

Description

[0001] The present invention relates to the use of N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea as a developer in a thermosensitive recording material, wherein the thermosensitive recording material comprises a carrier substrate, a thermosensitive color-forming layer applied to one side of the carrier substrate and comprising N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea as at least one non-phenolic developer and at least one color former, and a so-called back coating formulation applied to the side of the carrier substrate opposite to the side bearing the thermosensitive layer, with the aim of limiting the loss of thermowriting properties of the non-thermosensitively printed recording material after long-term storage.

[0002] The back coating preparation is used in combination with a heat-sensitive recording material to obtain a self-adhesive label (heat-sensitive label) or to improve various application properties. Specifically, the back coating preparation may be a self-adhesive layer or a coating (back coating) consisting essentially of a polymer binder and a pigment.

[0003] Known from the prior art are thermosensitive recording materials which are provided with a self-adhesive layer on the back side for direct thermal printing (so-called thermosensitive labels).

[0004] Documents JP S59162087A, US Pat. No. 4,370,370A and US Pat. No. 4,388,362A describe thermal labels with release paper.

[0005] Document DE 197 57 589 B4 describes a release-paper-free (“linerless”) heat-sensitive label having a protective layer on the heat-sensitive layer.

[0006] Document DE 198 06 433 B4 discloses a linerless heat-sensitive label having a protective layer which is free of organosilicon compounds and which has been cured by means of actinic radiation.

[0007] Documents EP 0 600 622 A1 and DE 197 24 647 C1 describe a linerless heat-sensitive label having a protective layer and a hot melt applied on the back side.

[0008] Documents EP 1 085 069 B1 and EP 2 474 963 B1 disclose thermosensitive label materials with a heat-activatable adhesive in a linerless embodiment, wherein thermosensitive labels with and without a protective layer are described.

[0009] Document EP 3219507 A1 claims a linerless heat-sensitive label without an actual protective layer, wherein the surface of the heat-sensitive layer has been made non-adhesive.

[0010] Also known from the prior art are thermosensitive recording materials which are provided with a so-called “backcoat” coating in order to improve the printability on the reverse side using conventional printing methods or to minimize curling of the support substrate under adverse humidity conditions.

[0011] Therefore, the back coating can effectively improve the different shrinkage behaviors of the two sides of the heat-sensitive recording material carrier substrate under different ambient humidity (different water vapor absorption of the two sides / coating). For example, document US6667275B2 discloses a multilayer back coating for heat-sensitive recording material to favorably influence the curling of the substrate web.

[0012] Document JP2018167483 discloses a method for improving the hot printing resistance of heat-sensitive recording materials which, among other things, have been exposed to oil-based inks or printing inks on the back by applying a coating to the back. Typically, the formulation of these back coatings contains an aqueous emulsion polymer, such as SB or acrylate latex, as an essential component.

[0013] For example, document JP2003175671 claims a back coating prepared with an aqueous latex forming a soft polymer film (Tg≥-30°) for heat-sensitive recording materials. Documents JPH0720735B2 and JP2000204123 disclose back coating formulations with acrylate emulsion polymers for heat-sensitive recording materials.

[0014] In the color-forming layer of a heat-sensitive recording material, there are usually color formers and developers, which react with each other under the influence of heat, thereby causing color development. Cost-effective phenolic developers (bisphenol A, bisphenol S, etc.) are widely used. With these, heat-sensitive labels with acceptable performance profiles for certain applications can be obtained.

[0015] Thermal labels containing non-phenolic developers in the thermal color-forming layer are also known in the prior art. These were developed to improve the durability of the print, especially when the printed thermal recording material comes into contact with hydrophobic substances, such as plasticized substances and materials or oils. In addition to the technical advantages, the public discussion on the potential toxicity of (bis)phenolic chemicals has greatly stimulated the interest in non-phenolic developers in particular.

[0016] The storage of unprinted (“white”) self-adhesive heat-sensitive labels or heat-sensitive recording materials with a back coating over a longer period of time, in particular at elevated ambient temperature and / or humidity, may lead to a deterioration of the properties relevant to the application. In particular, heat-sensitive recording materials with such back coating formulations may no longer reach the specified values ​​for the blackness or surface whiteness of the printout (e.g. barcodes or text) after storage. However, a low blackness of the printout generally leads to a reduction in the reading contrast, which is further deteriorated by a reduction in the background whiteness. Since the readability of barcodes, for example, is particularly important for label applications, low contrast values ​​have a negative impact on the basic properties relevant to this application. The cause of this phenomenon is believed to be the adhesive layer on the back and / or back coating, from which substances migrate through the carrier substrate into the chemically reactive heat-sensitive recording layer during the storage time and interact with the components necessary for the color-forming reaction, in particular the color developer, in a manner that is detrimental to the subsequent heat-sensitive printing process. The greatest contribution to the migration problem is substances with a relatively low molar mass (<10 kDa). If heat-activatable adhesives, i.e. adhesives that are solid at room temperature, are used, a deterioration of the properties may also occur after storage.

[0017] Especially developers which have a more complex chemical structure than the relatively simple (bis)phenols and which have a large number of reactive sites in the molecule, which is generally the case with non-phenolic developers but especially with sulfonylurea developers (SU developers), such as N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea, may suffer from problems of undesired interactions with substances which may be released from heat-sensitive recording materials provided with a back coating.

[0018] In order to ensure the cohesive strength of the pressure-sensitive adhesive within the adhesive layer and the bonding strength (adhesion) to the substrate, typical pressure-sensitive adhesive formulations use non-polymeric tackifying resins (such as natural or carbon resins) and / or plasticizers, tackifiers and possibly other small molecule additives such as cross-linking agents, stabilizers, etc. in addition to the base polymer.

[0019] The migration potential of small molecules inherent to the adhesive layer can be easily identified if one considers that the base polymer contains residual monomers or oligomers from the synthesis process and that more monomers can be formed due to favorable hydrolytic degradation at high ambient humidity and temperature.

[0020] The same applies to SB and acrylate latexes which are often used as binders or barrier agents in backcoats and are produced by emulsion polymerization.

[0021] In addition to the above-mentioned possibility of residual monomers in the emulsion, the presence of typical process-related substances during emulsion polymerization, such as surfactants, soaps, and initiators, must also be taken into account when evaluating the migration potential of the polymer components of the backcoat.

[0022] The prior art provides different solutions to this problem of adhesive heat-sensitive labels: applying an additional layer (so-called back coating) between the back side of the carrier substrate and the adhesive layer, as described in documents US4370370A and EP2474963B1, or incorporating specific substances such as carboxylated polyvinyl alcohol into the adhesive layer, as described in document JP59162087A.

[0023] However, these solutions are economically disadvantageous because they require additional production steps and make the label material more complex overall; they can only minimize migration problems of the back coating if they use more expensive low-migration adhesives.

[0024] The object of the present invention is to optimize the performance profile of non-thermally printed thermosensitive recording materials, in particular by improving the minimum durability of thermosensitive recording materials which carry an adhesive layer and / or a functional coating on their back side. In this way, the loss of writing performance during thermosensitive printing after storage will be limited to the greatest extent possible, in particular if the non-thermally printed thermosensitive recording materials are exposed to longer storage times.

[0025] The writing performance is characterized by, among other things, relative print contrast and image density.

[0026] It is also intended to limit negative influences on important application-relevant properties of the self-adhesive recording material (or, for back printing, recording material provided with a back coating), which occur primarily due to migration of components from the adhesive layer, the back coating or the back printing ink under adverse storage conditions.

[0027] The inventors have recognized that this unfavorable migration, especially through substances with relatively low molar mass (<10 kDa), can occur in the following situations:

[0028] a) The recording material has a self-adhesive layer on the back side (migrated from the adhesive).

[0029] b) The recording material is printed immediately ("directly") on the reverse side (migrated from the printing ink).

[0030] c) The recording material has a back coating (from which migration occurs). In this case, the recording material is not necessarily printed; migration can also occur through storage in an unprinted state.

[0031] Surprisingly, it has now been found that the above-mentioned disadvantages can be overcome by using a specific polymorphic form of the non-phenolic developer N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea. In particular, this use brings about a limitation on the loss of writing properties of heat-sensitive recording materials with back-coat formulations under harsh environmental conditions.

[0032] This specific polymorphic form of the non-phenolic developer N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea is also referred to hereinafter as form β.

[0033] In addition to the β-form, there are two other known polymorphs:

[0034] 1) The α type was first described in WO 00 / 35679 and is also commercially available under the name Pergafast 201 or PF201 as a developer for heat-sensitive recording materials.

[0035] 2) A polymorph corresponding to the β-type and another polymorph are described for the first time in US 2005 / 221982 A1. The document also describes the use of a mixture of three polymorphs as a developer for a thermal recording material, and a thermal recording material containing a mixture of two other polymorphs or three polymorphs. The thermal recording material may also contain other known developers and may be prepared by using at least one developer, such as the β-type or the γ-type. The carrier material may have a protective layer, an adhesive layer or a magnetic layer on its back. However, it is not recognized here that the β-type is used to solve the problem according to the present invention, and the problem according to the present invention is not obvious on the basis of the general disclosure.

[0036] Document EP 3 109 059 A1 describes the use of the α form of the compound N-(p-toluenesulfonyl)-N′-(3-p-toluenesulfonyl-oxy-phenyl)urea (Pergafast 201) as a developer.

[0037] The above problems are solved by using N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea (β-PF201) having an X-ray diffraction pattern with a Bragg angle (2θ / CuKα) 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 as a developer in a heat-sensitive recording material, the heat-sensitive recording material comprising a carrier substrate, a substrate applied to one side of the carrier substrate, and a thermosensitive color-forming layer comprising at least N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyl-oxy-phenyl)urea as a non-phenolic developer and at least one color former, and an adhesive layer and / or a coating for improving the application-related properties (e.g. suitability for reverse printing using conventional printing methods and / or rollers) on the side of the carrier substrate facing away from the thermosensitive color-forming layer, wherein the relative print contrast of the stored thermosensitive recording material according to the migration test (defined below) is equal to at least 70% and / or the image density of the stored thermosensitive recording material according to the migration test is equal to at least 35% of the image density value before storage at a value of ≥1.20 optical density units and / or the developer amount based on area (mg / m2) of the stored thermosensitive recording material according to the migration test is equal to at least 1.20 optical density units. 2 ) is equal to at least 30% of the developer amount on an area basis before storage.

[0038] The use according to the invention relates in particular to the very practically relevant case of self-adhesive heat-sensitive recording materials, ie heat-sensitive recording materials which have an adhesive layer on the side of the carrier substrate facing away from the heat-sensitive color-forming layer.

[0039] The self-adhesive heat-sensitive recording material is preferably removable or permanently bonded to a surface.

[0040] Preferably, the stored paper has a relative print contrast according to the Migration Test (as defined below) of at least 80%.

[0041] Preferably, at values ​​of ≧1.20 optical density units, the image density of the stored paper according to the migration test is at least 40%, particularly preferably at least 50%.

[0042] Preferably, the amount of developer based on area according to the migration test in the stored paper is at least 30% of the amount of developer before storage.

[0043] In the context of this disclosure, storage refers to the heat-sensitive recording material being stored between two glass plates at 60°C, 1350 N / m 2 The samples were stored at 50% pressure, 50% relative humidity and no light for four weeks (migration test).

[0044] Migration tests (adhesive migration tests), determination of image density, determination of relative print contrast, and quantitative determination of the area concentration of color formers and developers are described as follows:

[0045] (1) Determination of whiteness

[0046] The whiteness of the side of the thermal label paper bearing the thermal coating (= top side) was determined according to ISO 2470 using an Elrepho 3000 spectrophotometer.

[0047] The percentage of whiteness reduction after storage was determined using Equation 1.

[0048]

[0049] (2) Determination of image density:

[0050] The image density (optical density, od) was measured using a SpectroEye densitometer from X Rite, where the measurement uncertainty of the od values ​​was estimated to be ≤ 2%. The dispersion of the % values ​​calculated according to (Equation 3) was ≤ ± 2 percentage points.

[0051] (3) Determination of relative printing contrast:

[0052] The relative contrast ratio is calculated from the optical density of the thermal printed area (oDs) and the optical density of the unprinted area (oDw) according to equation (2). (s = black area, w = white area):

[0053]

[0054] (4) Migration test (adhesive migration test):

[0055] The A4 thermal recording material with an adhesive layer on the side of the carrier substrate facing away from the thermal color-forming layer was divided into three 6 cm wide strips in the longitudinal direction. 2 Two strips were stored between two glass plates under pressure of 100 %, relative humidity of 50% and in the absence of light for 4 weeks, while one strip was printed according to (2) and measured (od, image density before storage).

[0056] After storage, once room temperature has been reached, two strips are printed according to (2) and the optical density is determined, averaged and set relative to the similarly determined image density values ​​of the unstored sample according to the formula (Equation 2).

[0057]

[0058] (5) Color formers and developers, especially quantitative determination of the area concentration of the developer:

[0059] The coating components (color former and developer) were quantified after HPLC separation using an Agilent 1200 series HPLC instrument with a DAD detector.

[0060] Sample preparation: Use a punch to punch out two circular areas (area 0.000402m 2 ). The paper sample is extracted with 3 ml acetonitrile (HPLC quality) in an ultrasonic bath for 30 minutes. If the extract is turbid, filter it through a 0.45 μm filter. As a standard, inject 10 μl.

[0061] HPLC separation of components: Using an autosampler, the above extract was loaded onto a separation column (Synergi 4μm Fusion RP80A, 250x3 mm, preceded by a SecurityGuard pre-column with a 4x2 mm cartridge) and eluted with a mobile solvent of acetonitrile:H2O (60:40 by volume) with 0.1% formic acid and an acetonitrile gradient.

[0062] The quantitative evaluation of the chromatogram is performed by comparing the area of ​​the sample peak assigned at time t with that of the standard curve determined using reference samples. The measurement error of the HPLC quantification is ±2%.

[0063] The preparation of polymorphic compounds (α and β forms) is carried out according to known methods.

[0064] Table 1 summarizes the most important measured properties of the investigated polymorphs of PF201, including the most intense reflections from the X-ray powder diffraction pattern (XRPD), characteristic FTIR bands and melting behavior (DSC).

[0065]

[0066] The choice of the support substrate for the heat-sensitive recording material is not critical. However, from an economic and environmental point of view, it is preferred if the support substrate comprises paper, synthetic paper and / or plastic foil, especially paper.

[0067] If necessary, at least one further intermediate layer (“precoat”) is present between carrier substrate and heat-sensitive layer, wherein this further intermediate layer has the task of improving the surface smoothness of the carrier for the heat-sensitive layer and ensuring a thermal barrier between carrier substrate and heat-sensitive layer.

[0068] Preferably, organic hollow sphere pigments and / or calcined kaolin are used as the pigment in the intermediate layer.

[0069] Furthermore, at least one protective layer and / or at least one layer promoting printability may be present in the heat-sensitive recording material according to the invention, which layers are applied to the heat-sensitive layer.

[0070] The heat-sensitive recording material may additionally comprise at least one conventional protective layer and / or a conventional non-stick coating (e.g. a silicone coating), in particular for so-called linerless applications. Here, too, the beta form has proven to be advantageous with regard to possible migration of substances from these layers. In principle, it can be assumed that the beta form makes the heat-sensitive recording material less sensitive to substances that can migrate from other coatings into the color-forming layer, in particular under customary, sometimes harsh and prolonged storage conditions.

[0071] Preferably, at least 80% by weight, particularly preferably at least 90% by weight of the beta form is used as developer (relative to the total amount of developer used). Particularly preferably, apart from unavoidable impurities, only the beta form is used as developer.

[0072] With regard to the choice of the coupler, the thermosensitive recording material is also not subject to any significant restrictions. Preferably, however, the coupler is a dye of the triphenylmethane, fluoran, azaphthalide and / or fluorene type. Very particularly preferred couplers are dyes of the fluoran type, since their availability and balanced application-related properties make it possible to provide recording materials with an attractive price / performance ratio.

[0073] Particularly preferred fluoran-type dyes are:

[0074] 3-Diethylamino-6-methyl-7-anilinofluoran,

[0075] 3-(N-ethyl-N-4-toluidino)-6-methyl-7-anilinofluoran,

[0076] 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluoran,

[0077] 3-Diethylamino-6-methyl-7-(2,4-dimethylanilino)fluoran,

[0078] 3-Pyrrolidine-6-methyl-7-anilinofluoran,

[0079] 3-(Cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran,

[0080] 3-Diethylamino-7-(3-trifluoromethylanilino)fluoran,

[0081] 3-Nn-dibutylamino-6-methyl-7-anilinofluoran,

[0082] 3-Diethylamino-6-methyl-7-(3-methylanilino)fluoran,

[0083] 3-Nn-dibutylamino-7-(2-chloroanilino)fluoran,

[0084] 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran,

[0085] 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran,

[0086] 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran,

[0087] 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran and / or

[0088] 3-Dipentylamino-6-methyl-7-anilinofluoran.

[0089] The couplers can be used as individual substances or as any mixture of two or more couplers, as long as the desired application-related properties of the recording material are not affected thereby.

[0090] The color former is preferably present in an amount of about 5 to about 30, particularly preferably about 8 to about 20, relative to the total solid content of the heat-sensitive layer.

[0091] The developer used was the non-phenolic developer with the polymorphic modification having the highest melting point (polymorph = β form in Table 1), alone or in admixture with other polymorphs of the same developer or with chemically different developers.

[0092] The amount of the developer is preferably about 3 to about 35 wt %, particularly preferably about 10 to about 25 wt %, relative to the total solid content of the heat-sensitive layer.

[0093] The weight ratio of developer to coupler is preferably at most 3:1, particularly preferably at most 2:1, very particularly preferably at most 1:1. Presumably, the β form can be used in smaller amounts than the α form to obtain improved or at least equal quality heat-sensitive recording materials.

[0094] In addition to at least one polymorphic modification (β-form) of the developer, one or more sensitizers, also called thermal solvents, may be present in the thermal color-forming layer, which has the advantage that the thermal printing sensitivity is more easily controlled.

[0095] Generally, crystalline substances which have a melting point between about 90 and about 150° C. and which dissolve the color-forming components (color former and developer) in the molten state without interfering with the formation of the color-forming complex are advantageously considered as sensitizers.

[0096] Preferably, the sensitizer is a fatty acid amide, such as stearamide, behenamide or palmitamide, an ethylenebisfatty acid amide such as N,N'-ethylenebisstearamide or N,N'-ethylenebisoleamide, a fatty acid alkanolamide such as N-(hydroxymethyl)stearamide, N-hydroxymethylpalmitamide or hydroxyethylstearamide, a wax such as polyethylene wax or montan wax, a carboxylic acid ester such as dimethyl terephthalate, dibenzyl terephthalate, 4-benzyloxy benzyl benzoate, di-(4-methylbenzyl) oxalate, di-(4-chlorobenzyl)-oxalate or di-(4-benzyl)-oxalate, aromatic ethers such as 1,2-diphenoxyethane, 1,2-di-(3-methylphenoxy)-ethane, 2-benzyloxynaphthalene or 1,4-diethoxynaphthalene, aromatic sulfones such as diphenyl sulfone, and / or aromatic sulfonamides such as benzenesulfonanilide or N-benzyl-4-toluenesulfonamide, or aromatic hydrocarbons such as 4-benzylbiphenyl.

[0097] The sensitizer is preferably present in an amount of about 10 to about 40, more preferably about 15 to about 25, relative to the total solid content of the heat-sensitive layer.

[0098] In a further preferred embodiment, in addition to the color former, the phenol-free developer and the sensitizer, at least one stabilizer (anti-aging agent) for the color-forming complex is optionally present in the heat-sensitive color-forming layer.

[0099] The stabilizer is preferably composed of hindered phenols, particularly preferably 1,1,3-tris-(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1-bis-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane.

[0100] Urea-urethane compounds (trade name UU) or ethers derived from 4,4'-dihydroxydiphenyl sulfone, such as 4-benzyloxy-4'-(2-methylglycidyloxy)-diphenyl sulfone (trade name NTZ Nippon Soda Co. Ltd.) or oligoether (trade name Nippon Soda Co. Ltd.) can be used as a stabilizer in the recording material according to the present invention.

[0101] The stabilizer is preferably present in an amount of 0.2 to 0.5 parts by weight relative to the at least one phenol-free developer.

[0102] In a further preferred embodiment, at least one binder is present in the thermosensitive color-forming layer. The binder is preferably water-soluble starch, starch derivatives, The present invention is composed of starch-based bio-latex, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, partially or completely saponified polyvinyl alcohol, chemically modified polyvinyl alcohol or styrene-maleic anhydride copolymer, styrene-butadiene copolymer, acrylamide-(meth)acrylate copolymer, acrylamide-acrylate-methacrylate terpolymer, polyacrylate, poly(meth)acrylate, acrylic acid-butadiene copolymer, polyvinyl acetate and / or acrylonitrile-butadiene copolymer.

[0103] In order to achieve specific application-related performance characteristics of the heat-sensitive recording material, preferably a self-adhesive heat-sensitive recording material, especially a self-adhesive label, the binder is preferably present in crosslinked form in the heat-sensitive layer, and in the presence of a crosslinking agent (crosslinker), an optimal degree of crosslinking of the binder is achieved during the drying step of the coating process.

[0104] The crosslinking agent can be a polyvalent aldehyde, such as glyoxal, dialdehyde starch, glutaraldehyde, etc., which may be mixed with a boron salt (borax), can be a salt or ester of glyoxylic acid, can be a crosslinking agent based on ammonium zirconium carbonate, can be a polyamidoamine epichlorohydrin resin (PAE resin), can be adipic acid dihydrazide (AHD), boric acid or its salt, or others.

[0105] Self-crosslinking adhesives, such as specially modified polyvinyl alcohols or acrylates, can be crosslinked without the use of any crosslinking agents at all, thanks to the reactive, crosslinkable groups already contained in the adhesive polymer.

[0106] The crosslinking agent is preferably present in an amount of about 0.05 to about 0.5, particularly preferably about 0.1 to about 0.2, relative to the crosslinkable binder component from the heat-sensitive layer.

[0107] In a further preferred embodiment, at least one release agent (non-stick agent) or lubricant is present in the heat-sensitive color-forming layer. These agents are preferably fatty acid metal salts such as zinc stearate or calcium stearate or also behenate, synthetic waxes: for example in the form of fatty acid amides, such as stearic acid amide and behenic acid amide, fatty acid alkanolamides such as stearic acid hydroxymethylamide; paraffin waxes with different melting points, ester waxes of different molecular weights, propylene waxes of different hardness, ethylene waxes, and / or natural waxes such as carnauba wax or montan wax.

[0108] The release agent is preferably present in an amount of about 1 to about 10, particularly preferably about 3 to about 6, relative to the total solid content of the heat-sensitive layer.

[0109] In another preferred embodiment, the thermosensitive color-forming layer comprises a pigment. One of the advantages of using pigments is that they can fix the chemical melt produced during the thermosensitive printing process on its surface. Pigments can also be used to control the surface whiteness and opacity of the thermosensitive color-forming layer and its printability with conventional printing inks. Finally, pigments have an "extended function", such as for relatively expensive coloring functional chemicals.

[0110] Particularly suitable pigments are inorganic pigments of synthetic and natural origin, preferably clays, precipitated or natural calcium carbonate, aluminum oxides, aluminum hydroxide, silicon dioxide, precipitated and fumed silica (e.g. Type), diatomaceous earth, magnesium carbonate, talc, and also organic pigments such as hollow pigments with styrene / acrylate copolymer walls or urea / formaldehyde polycondensates. These can be used alone or in any mixture.

[0111] The pigment is preferably present in an amount of about 20 to about 50% by weight, particularly preferably about 30 to about 40% by weight, relative to the total solid content of the heat-sensitive layer.

[0112] In order to control the surface whiteness of the heat-sensitive recording material, an optical brightener (brightener) may be added to the heat-sensitive color-forming layer. These are preferably stilbene derivatives.

[0113] In order to improve certain coating properties, it may be preferred in individual cases to add further ingredients, in particular rheology auxiliaries, such as thickeners and / or surfactants, to the essential ingredients of the heat-sensitive recording material.

[0114] The (dry) heat-sensitive layer is preferably applied at an area density of about 1 to about 10 g / m 2 , preferably from about 3 to about 5 g / m 2 .

[0115] The above-mentioned thermosensitive recording material can be obtained by a known production method.

[0116] It is advantageous if the smoothness is measured on the dried heat-sensitive color-forming layer. Here, it is advantageous to set the Bekk smoothness measured according to ISO 5627:1995-03 to about 100 to about 1000 seconds, preferably about 250 to about 600 seconds.

[0117] The surface roughness (PPS) according to ISO 8791-4:2008-05 is in the range of about 0.50 to about 2.50 μm, preferably in the range of 1.00 to 2.00 μm.

[0118] The thermal recording material is preferably phenol-free and is very suitable for applications in which thermal papers are provided with an adhesive layer on the back, which are printed using a direct thermal process and which must ensure a long shelf life with regard to specific writing properties, background whiteness and contrast values ​​even when stored under harsh environmental conditions.

[0119] About the selection of the adhesive applied to the back side, the present invention is not subject to any significant restrictions. The adhesive that is tacky at room temperature and the adhesive that only becomes tacky after activation (such as by heating) are all suitable. Permanent adhesive and removable adhesive can be used. The technology that is used for adhesive material to be applied to the back side of thermosensitive recording material does not limit the scope of the present invention in any way either. The aqueous dispersion of adhesive or the adhesive dissolved or suspended in the organic medium and the adhesive (hot melt adhesive) applied with molten state can be used.

[0120] The application area density of the dried adhesive layer is preferably from about 10 to about 150 g / m 2 , preferably from about 15 to about 30 g / m 2 .

[0121] The use according to the invention is further preferably characterized in that the relative print contrast of the paper stored according to the migration test is equal to at least 70% and / or the image density of the paper stored according to the migration test at a value of ≥ 1.20 od units is equal to at least 35% of the image density value before storage.

[0122] The use according to the invention is further preferably characterized in that the relative print contrast of the paper stored according to the migration test is equal to at least 70% and / or the developer amount (mg / m2) based on area of ​​the paper stored according to the migration test is equal to at least 70%. 2 ) is equal to at least 30% of the developer amount before storage.

[0123] The use according to the invention is further preferably characterized in that the image density of the stored paper according to the migration test is equal to at least 35% of the image density value before storage at a value of ≥ 1.20 od units and / or the developer amount (mg / m2) based on area of ​​the stored paper according to the migration test is equal to at least 35% of the image density value before storage. 2 ) is equal to at least 30% of the developer amount before storage.

[0124] In a further preferred embodiment, the use according to the invention is characterized in that the relative print contrast of the stored paper according to the migration test is equal to at least 70%, preferably at least 80%.

[0125] In another preferred embodiment, the use according to the invention is characterized in that the image density of the stored paper according to the migration test is at least 40%, preferably at least 50%, of the image density value before storage at values ​​≥ 1.20 od units.

[0126] In a further preferred embodiment, the use according to the invention is characterized in that the developer dosage (mg / m2) based on area of ​​the paper stored according to the migration test is 2 ) is equal to at least 25%, preferably at least 30% of the developer amount before storage.

[0127] In conclusion, it can be stated that it has surprisingly been shown that by using a specific polymorphic modification of the phenol-free β form of the developer, self-adhesive thermosensitive recording materials, in particular thermosensitive labels, can be obtained which are characterized by excellent durability of the surface whiteness, high writing properties and good contrast values ​​of the printed images in thermosensitive printing processes after long-term storage under adverse environmental conditions. Example:

[0128] Pergafast The low melting polymorphic modification of (α-polymorph, α form from Table 1) was used as comparative developer.

[0129] Finished thermal paper as self-adhesive labels

[0130] Apply adhesive layer to the back of A4 paper

[0131] a) The adhesive dispersion was applied using a doctor blade to the back of an A4 paper (thermal paper) bearing a heat-sensitive layer on the front side and dried using a heat gun at a maximum temperature of 70° C. To protect the adhesive layer during further processing, a siliconized release paper was laminated onto the adhesive layer, avoiding air bubbles and wrinkles.

[0132] b) In the case of "adhesive liner sandwich", consisting of a thin adhesive layer between two release papers, the adhesive layer (sticky side) is laminated to the back of the A4 thermal paper after removing one of the two liner papers, avoiding bubbles and wrinkles.

[0133] During the production of the thermosensitive labels it is immaterial whether the adhesive layer is applied first and then the thermosensitive recording layer is applied to the opposite side bearing the adhesive layer.

[0134] The aqueous coating suspension for forming the heat-sensitive color-forming layer of the heat-sensitive recording paper was applied on a laboratory scale by means of a doctor bar to a 72 g / m2 sheet pre-coated with a pigment coating. 2 Coated side of paper.

[0135] The composition of the colored pre-coat is not critical. Typically, the coating consists of calcined kaolin and a binder based on styrene-butadiene and / or starch. It is also common to use organic (hollow sphere) pigments, possibly mixed with inorganic pigments. The amount of application of the colored layer is between about 3 and 10 g / m2.

[0136] After the aqueous application suspension of the heat-sensitive coating material is dried, a heat-sensitive recording sheet is obtained. The application amount of the heat-sensitive color-forming layer is between 3.8 and 4.3 g / m2 By applying an adhesive layer to the opposite substrate side (back side) bearing the heat-sensitive layer according to one of the above methods a) or b), a composite material suitable for use as a heat-sensitive label is obtained. The amount of adhesive applied is about 20 g / m2.

[0137] Based on the above information, a heat-sensitive recording material or heat-sensitive paper was prepared in which the following formulation of an aqueous application suspension had been used to form a composite image on a carrier substrate as described above.

[0138] Preparation of the dispersion for application suspension (in each case 1 part by weight):

[0139] Aqueous dispersion A (developer dispersion) was prepared by grinding 20 parts by weight of 3-Nn-dibutylamino-6-methyl-7-anilinofluoran (ODB 2) with 33 parts by weight of a 15% aqueous solution of GhosenexTM L-3266 (sulfonated polyvinyl alcohol, Nippon Ghosei) in a bead mill.

[0140] Aqueous dispersion B (developer dispersion) was prepared by grinding 40 parts by weight of the developer and 66 parts by weight of a 15% aqueous solution of Ghosenex™ L-3266 in a bead mill.

[0141] Aqueous dispersion C (sensitizer dispersion) was prepared by grinding 40 parts by weight of the sensitizer with 33 parts by weight of a 15% aqueous solution of Ghosenex™ L-3266 in a bead mill.

[0142] The average particle size D of all dispersions produced by milling (4,3) The particle size distribution of the dispersions was measured by laser diffraction using a Coulter LS230 instrument from Beckman Coulter.

[0143] Dispersion D (lubricant dispersion) is a 20% zinc stearate dispersion consisting of 9 parts by weight zinc stearate, 1 part by weight Ghosenex™ L-3266, and 40 parts by weight water.

[0144] Pigment P is a 56% PCC suspension (PCC=precipitated calcium carbonate).

[0145] The adhesive consisted of a 10% aqueous solution of polyvinyl alcohol (Mowiol 28 to 99, Kuraray Europe).

[0146] Crosslinker V was a 42% aqueous solution of glyoxal.

[0147] Crosslinker V is a 42% aqueous solution of a glyoxal-borax based crosslinker (Cartabond TSI, Clariant).

[0148] Use of tetrasulfonylbenzene compounds A 31% aqueous solution of PT (Blankophor) was used as an optical brightener.

[0149] A heat-sensitive application suspension was prepared by mixing 1.6 parts of A, 1.5 parts of B, 1.5 parts of C, 70 parts of D, 188 parts of pigment P, 400 parts of binder solution, 4 parts of optical brightener and 14 parts of crosslinker solution V (all parts by weight) in the combined order of B, D, C, P, A, binder, optical brightener and V under stirring and brought to a solids content of about 25% with water.

[0150] The following commercially available adhesives are used to produce self-adhesive thermal labels:

[0151] R5000N (Avery Fasson) is a removable acrylate-based adhesive.

[0152] S2200 (Avery Fasson) is a permanent hot melt adhesive based on styrene-isoprene and PVC copolymer for deep freeze applications.

[0153] Technomelt PS 8746 (Henkel) is a permanent hot melt adhesive based on synthetic rubber.

[0154] The thermosensitive recording materials thus converted into self-adhesive thermosensitive labels were tested / evaluated as shown below (Table 2).

[0155] (1) Surface whiteness (whiteness)

[0156] The whiteness of the side of the thermal label paper bearing the thermal coating (= top side) was determined according to ISO 2470 using an Elrepho 3000 spectrophotometer.

[0157] The percentage of whiteness reduction after storage was determined using Equation 1.

[0158]

[0159] (2) Dynamic color density:

[0160] The thermal paper (6 cm wide) was thermally printed with a checkerboard pattern without energy scale using a Kyocera print bar of 200 dpi and 560 ohm at an applied voltage of 20.6 V and a pulse width determined by a preliminary test using an Atlantek 200 test printer (Atlantek, USA), the pulse width was selected to achieve an optical density of 1.20 ± 0.05. One square area of ​​the printed sample corresponds to 80x80 dots. The image density (optical density, od) was measured using a SpectroEye densitometer from X-Rite, and the measurement uncertainty of the od value was estimated to be ≤2%. The dispersion of the % value calculated according to (Equation 2) was ≤±2 percentage points.

[0161] (3) Relative printing contrast:

[0162] Relative contrast is calculated using the optical density (OD) of the thermal printed area. S ) and the optical density of the unprinted area (oD W ) is calculated according to equation (2). (s = black area, w = white area):

[0163]

[0164] (4) Adhesive migration test for thermal label paper:

[0165] A4 self-adhesive thermal label paper was divided into three 6 cm wide strips longitudinally. Two strips were stored between two glass plates at 60°C, a pressure of 1350 N / m2, a relative humidity of 50% and no light for 4 weeks, while one strip was printed according to (2) and measured (od, image density before storage).

[0166] After storage, once room temperature has been reached, two strips are printed according to (2) and the optical density is determined, averaged and set according to the formula (Equation 2) relative to the similarly determined image density value of the non-stored sample.

[0167]

[0168] Table 2 summarizes the evaluation of the produced recording materials.

[0169] (5) Quantitative determination of area concentration of color former and developer (Table 3):

[0170] The coating components (color former and developer) were quantified after HPLC separation using an Agilent 1200 series HPLC instrument with a DAD detector.

[0171] Sample preparation: Use a punch to punch out two circular areas (area 0.000402m2 ). The paper sample is extracted with 3 ml acetonitrile (HPLC quality) in an ultrasonic bath for 30 minutes. If the extract is turbid, filter it through a 0.45 μm filter pad. As a standard, inject 10 μl.

[0172] HPLC separation of components Using an autosampler, the extract was loaded onto a separation column (Synergi 4 μm Fusion RP80A, 250x3 mm, preceded by a SecurityGuard pre-column with a 4x2 mm cartridge) and eluted with an acetonitrile gradient (with 0.1% formic acid) of acetonitrile:H2O (60:40 by volume) with 0.1% formic acid.

[0173] The quantitative evaluation of the chromatogram is performed by comparing the area of ​​the sample peak assigned at time t with that of the standard curve determined using reference samples. The measurement error of the HPLC quantification is ±2%.

[0174] It can be seen from the above examples that the heat-sensitive self-adhesive labels of the present invention particularly exhibit the following advantageous properties (Table 2 and Table 3):

[0175] (1) The whiteness of the unprinted and stored self-adhesive thermal label paper with the β-form as the developer is higher than that of the comparative sample with the alternative polymorphic modification (α-form, Pergafast 201).

[0176] (2) The use of the β-type as a developer resulted in self-adhesive thermal labels that demonstrated significantly higher print densities than those using αPF 201 as a developer after long-term storage under harsh conditions.

[0177] (3) From (1) and (2), it can be seen that the thermal label paper using β-type as the color developer has obvious advantages in terms of performance comparison after storage.

[0178] (4) The chemical resistance to components that can migrate from the adhesive layer is significantly higher than that of the comparative example (Table 3).

[0179] (5) By using the β type, high-quality self-adhesive thermal labels can be obtained in important application-related aspects.

[0180]

Claims

1. With Bragg angle (2θ / CuK α ) is 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 as a developer in a heat-sensitive recording material to limit the loss of relative print contrast and / or image density and / or a reduction in the amount of developer based on area, the heat-sensitive recording material comprising a carrier substrate, a substrate applied to one side of the carrier substrate and comprising A thermosensitive color-forming layer comprising at least one non-phenolic developer and at least one color former, and an adhesive layer and / or a coating which allows reverse printing using conventional printing methods on the side of the carrier substrate facing away from the thermosensitive color-forming layer, wherein the image density of the thermosensitive recording material stored according to the migration test defined below equals at least 35% of the image density value before storage at a value of ≥ 1.20 optical density units and / or the relative print contrast of the thermosensitive recording material stored according to the migration test equals at least 70% of the relative print contrast value before storage and / or the amount of developer based on area of ​​the thermosensitive recording material stored according to the migration test equals at least 30% of the amount of developer based on area before storage; The migration test: The A4 thermal recording material having an adhesive layer on the side of the carrier substrate facing away from the thermal color-forming layer was longitudinally divided into three 6 cm wide strips and heated at 60°C and 1350 N / m 2 Two strips were stored between two glass plates for 4 weeks under pressure, 50% relative humidity and no light, while one strip was printed according to (2) and measured, od: image density before storage, After storage, once room temperature has been reached, two strips are printed according to (2), and the optical density is determined, averaged, and set relative to similarly determined image density values ​​of the unstored sample according to formula Equation 2; (2) Determination of image density: The image density, i.e., optical density, od, was measured using a SpectroEye densitometer from X Rite, where the measurement uncertainty of the od values ​​was estimated to be ≤ 2% and the dispersion of the % values ​​calculated according to Equation 3 was ≤ ± 2 percentage points; Determination of relative printing contrast: The relative contrast ratio is calculated from the optical density of the thermal printed area (oDs) and the optical density of the unprinted area (oDw) according to equation (2), s = black area, w = white area:

2. The use according to claim 1, characterized in that The carrier substrate comprises paper and / or plastic foil.

3. The method according to claim 1 or 2, characterized in that The at least one coupler is a dye of the triphenylmethane type, fluoran type, azaphthalide type and / or fluorene type.

4. The method according to claim 1 or 2, characterized in that At least one further intermediate layer is present between the carrier substrate and the heat-sensitive layer.

5. The use according to claim 4, characterized in that The intermediate layer contains organic hollow sphere pigment and / or calcined kaolin.

6. The method according to claim 1 or 2, characterized in that The color former is present in an amount of 5 to 30% by weight relative to the total solid content of the heat-sensitive layer.

7. The method according to claim 1 or 2, characterized in that The color former is present in an amount of 8 to 20% by weight relative to the total solid content of the heat-sensitive layer.

8. The method according to claim 1 or 2, characterized in that The developer is present in an amount of 3 to 35 wt % relative to the total solid content of the heat-sensitive layer.

9. The method according to claim 1 or 2, characterized in that The developer is present in an amount of 10 to 25% by weight relative to the total solid content of the heat-sensitive layer.

10. The method according to claim 1 or 2, characterized in that The adhesive layer comprises at least one pressure-sensitive adhesive and / or comprises a heat-activatable adhesive.

11. The method according to claim 10, characterized in that The pressure-sensitive adhesive is a rubber and / or acrylate based pressure-sensitive adhesive.

12. The method according to claim 1 or 2, characterized in that The image density of the thermosensitive recording material stored according to the migration test at a value of ≥1.20 optical density units is equal to at least 35% of the image density value before storage and the relative printing contrast of the thermosensitive recording material stored according to the migration test is equal to at least 70% of the relative printing contrast value before storage and the amount of color developer based on area of ​​the thermosensitive recording material stored according to the migration test is equal to at least 30% of the amount of color developer based on area before storage.

13. The use according to claim 1 or 2, characterized in that: The image density of the thermosensitive recording material stored according to the migration test at a value ≥ 1.20 optical density units is equal to at least 35% of the image density value before storage and the relative print contrast of the thermosensitive recording material stored according to the migration test is equal to at least 70% of the relative print contrast value before storage.

14. The use according to claim 1 or 2, characterized in that: The relative print contrast of the thermosensitive recording material stored according to the migration test is equal to at least 70% of the relative print contrast value before storage and the relative print contrast of the thermosensitive recording material stored according to the migration test is expressed in mg / m 2 The calculated developer amount based on area is equal to at least 30% of the developer amount based on area before storage.

15. The use according to claim 1 or 2, characterized in that: The image density of the thermosensitive recording material stored according to the migration test at a value ≥ 1.20 optical density units is equal to at least 35% of the image density value before storage and the developer amount based on area of ​​the thermosensitive recording material stored according to the migration test is equal to at least 30% of the developer amount based on area before storage.

16. The use according to claim 1 or 2, characterized in that: The image density of the thermosensitive recording material stored according to the migration test at a value of ≥1.20 optical density units is equal to at least 40% of the image density value before storage.

17. The use according to claim 1 or 2, characterized in that: The image density of the thermosensitive recording material stored according to the migration test at a value of ≥1.20 optical density units is equal to at least 50% of the image density value before storage.

18. The use according to claim 1 or 2, characterized in that: The relative print contrast of the thermosensitive recording material stored according to the migration test is equal to at least 70% of the relative print contrast value before storage.

19. The use according to claim 1 or 2, characterized in that: The relative print contrast of the thermosensitive recording material stored according to the migration test is equal to at least 80% of the relative print contrast value before storage.

20. The use according to claim 1 or 2, characterized in that: The developer amount on an area basis of the thermosensitive recording material stored according to the migration test is equal to at least 30% of the developer amount on an area basis before storage.

21. The use according to claim 1 or 2, characterized in that The developer amount on an area basis of the thermosensitive recording material stored according to the migration test is equal to at least 35% of the developer amount on an area basis before storage.

22. The use according to claim 1 or 2, characterized in that: The heat-sensitive recording material has an adhesive layer on the side of the carrier substrate facing away from the heat-sensitive color-forming layer, and is produced in the form of a self-adhesive label.

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