EP2953797B1 - Heat sensitive recording material - Google Patents

Description

The invention relates to a heat-sensitive recording material (thermal paper) with a flat support (thermal raw paper), a thermal reaction layer on at least one side of the flat support and optionally an intermediate layer (thermal insulation layer) formed between the flat support and the respective thermal reaction layer and optionally with further layers. The invention also relates to a method for producing such a heat-sensitive recording material and to the use thereof.

Heat-sensitive recording materials of the type described above are for example from the US-A-6,759,366 and the WO 2008/006474 A1 known.

The US-A-6,759,366 describes a heat-sensitive recording material which has a thermal reaction layer on the top and bottom of a carrier substrate. The carrier substrate is preferably based on cellulose and is thermally insulating. This ensures that the heat pulse generated during thermal printing is largely available for the development of the thermal reaction layer. A so-called primer layer is preferably located between the carrier substrate and the thermal reaction layer formed by which a better adhesion of the layers and the thermal insulation necessary for thermal printing is achieved.

The WO 2008/006474 A1 also discloses a heat-sensitive recording material with a flat support, a thermal reaction layer on at least one side of the flat support and an intermediate layer formed between the flat support and the respective thermal reaction layer, which contains hollow-sphere pigments embedded in a binder, and optionally with further layers and / or Top layers, the hollow-sphere pigments being in the form of a composite pigment, and wherein nanoscale pigment particles adhere to the surface of an organic hollow-sphere pigment. That from the WO 2008/006474 A1 known recording material shows in particular improved insulating properties. A material is applied as an intermediate layer which contains the pigments mentioned in a suitable binder. The binder is used in particular to connect the intermediate layers as well as possible to the flat carrier and to ensure an optimal connection of the subsequent layers. Synthetic and / or natural polymers are used as binders.

The DE 11 2007 002 203 T5 describes a thermal recording material comprising an intermediate layer and a thermal recording layer laminated on a support in that order, the intermediate layer being a layer formed by applying a coating liquid having a swellable starch and a pigment in a dispersed state in one Dispersing medium consisting of water as the main component is obtained, and the intermediate layer contains a heat-insulating organic pigment which is in the form of hollow or cup-shaped particles.

Binders are of great importance in heat-sensitive recording materials. They are used to fix pigments and other components such as color formers, co-reactants, sensitizers and lubricants as well as other additives. Binders also favor the connection of the different layers to one another. Starches are usually used as binders, Polyvinyl alcohol or synthetic binders, such as styrene / butadiene latices and stryol / acrylate latices, are used. Binding agents can be applied in pure form directly on one or two sides to the base paper as surface sizing or can be introduced into the paper in the so-called sump mode over the paper surface (impregnation).

However, the known heat-sensitive recording materials have various disadvantages, for example in terms of aging resistance, in particular when using synthetic binders. These adverse effects are particularly noticeable at elevated temperatures and high ambient humidity. Furthermore, the placement behavior of the known heat-sensitive recording materials can be critical, especially when using org. Hollow ball pigments in the thermal insulation coating. Finally, the synthetic binders commonly used in known heat-sensitive recording materials are expensive and have ecological disadvantages.

The object of the present invention is therefore to provide a heat-sensitive recording material which overcomes the disadvantages of the known heat-sensitive recording materials. In particular, heat-sensitive recording materials are to be provided which have improved properties with regard to the aging resistance and the laying behavior. Finally, it would be desirable to reduce manufacturing costs and use environmentally friendly materials.

According to the invention this object is achieved by a heat-sensitive recording material according to claim 1.

The cross-linked biopolymer material in the form of nanoparticles has a degree of swelling of less than 1. The degree of swelling was as in the DE 11 2007 002 203 T5 as described:
The degree of swelling relates to a volume expansion when the crosslinked biopolymer material swells in water in the form of nanoparticles. For this purpose, a sample of an anhydrous amount of 2 g is added to 200 ml of pure water, dispersed therein and immediately thereafter it is heated in a good-boiling water bath for 30 minutes and cooled to room temperature. The part of the water that has evaporated is added and the sample is redispersed and 100 ml of the dispersion are placed exactly in a measuring cylinder. The measuring cylinder is left for 24 hours at room temperature and a precipitate is measured visually for its amount (ml) and this value is taken as the degree of swelling.

The selection of the material of the flat carrier is not critical. However, it is preferred that the flat carrier is based on cellulose fibers, a synthetic paper carrier, the fibers of which, in particular, consist wholly or partly of plastic fibers, or a plastic film. The flat carrier is preferably used with a weight per unit area of approximately 20 to 600 g / m ² , in particular approximately 30 to 300 g / m ² .

There are also no special requirements for the choice of materials for the thermal reaction layer (s). Possible materials are color formers, color developers, other binders, pigments, melting aids, anti-aging agents and other additives, etc. The thermal reaction layer therefore contains the essential functional components that are ultimately responsible for the development of a font or an image.

When selecting the color former and the color developer for the thermal reaction layer (s) of the recording material according to the invention, there are no relevant restriction. Color formers in the form of 2-anilino-3-methyl-6-diethylamino-fluorane, 2-anilino-3-methyl-6-di-n-butylamino-fluorane, 2-anilino-3-methyl-6 are preferred - (N-ethyl-, Np-toluidino-amino) -fluorane, 2-anilino-3-methyl-6- (N-methyl-, N-propyl-amino) -fluorane, 2-anilino-3-methyl-6 - (N-ethyl-, N-isopentylamino) -fluorane and / or 3,3-bis- (4-dimethylamino-phenyl) -6-dimethylamino-phthalide are present and the color developers are in the form of phenol or Urea derivatives such as 2,2-bis (4-hydroxyphenyl) propane, bis (4-hydroxyphenyl) sulfone, 4-hydroxy-4 'isopropoxy diphenyl sulfone, bis (3-allyl-4- hydroxy-phenyl) -sulfone, 2,2-bis- (4-hydroxyphenyl) -4-methyl-pentane, N- (toluenesulfonyl) -N '- (3-p-toluenesulfonyl-oxy-phenyl) -urea and zinc salts of Derivatives of salicylic acid are used. As mentioned, the thermoreaction layer (s) can also contain various other substances or auxiliaries which promote the properties. These can be, for example, sensitizing melting aids, lubricants, rheology aids, fluorescent substances and the like.

The sensitizing melting aids are available, for example, in the form of 2-benzyloxynaphthalene (BON), p-benzylbiphenyl (PBBP), oxalic acid dibenzyl ester, oxalic acid di- (p-methylbenzyl) ester, 1,2-bis (phenoxy-methyl) -benzene, 4- (4-tolyloxy) biphenyl, ethylene glycol diphenyl ether, ethylene glycol m-tolyl ether and 1,2-bis (3,4-dimethylphenyl) ethane and the lubricants in the form of fatty acid amides, such as, for . B. stearic acid amide, fatty acid alkanolamides, such as. B. stearic acid methylolamide, ethylene-bis-alkanoylamides, such as. B. ethylene bisstearoylamide, synthetic waxes such. B. paraffin waxes of different melting points, ester waxes of different molecular weights, ethylene waxes, propylene waxes of different hardness or natural waxes, such as. B. carnauba wax and / or fatty acid metal soaps, such as. As zinc stearate, calcium stearate or behenate salts, the rheological aids in the form of water-soluble hydrocolloids, such as starches, starch derivatives, sodium alginates, polyvinyl alcohols, methyl celluloses, hydroxyethyl or hydroxypropyl methyl celluloses, carboxymethyl celluloses, poly (meth) acrylates, in the form of white whitening acrylates e.g. B. from the substance groups diaminostilbene-disulfonic acid, distyryl biphenyls, benzoxazole derivatives, the fluorescent substances in the form of daylight fluorescent pigments of different colors or fluorescent fibers, the anti-aging agents in the form of sterically hindered phenols, such as 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 and 1,1'-bis (4-hydroxyphenyl) cyclohexane.

The thermal reaction layer (s) with a weight per unit area of approximately 1 to 8 g / m ² , in particular approximately 2 to 6 g / m ² , are / are preferably used.

The usual intermediate layer (s) can also be used as the intermediate layer (s). The intermediate layer increases the image quality, prevents heat conduction into the base paper and supports the function and sensitivity properties of the thermal reaction layer. In particular, it also contributes to a sufficient fixation of the fusible components in the writing process and thus ensures good runnability in the thermal printer.

Suitable materials of the intermediate layer (s) are those which allow the thermal reaction layer to adhere to the flat support or which serve to protect or isolate the thermal reaction layer. In addition to the crosslinked biopolymer material in the form of nanoparticles, the usual materials used are further binders, pigments, rheology aids, dispersants, optical brighteners and surfactants. The binders are preferably in the form of synthetic and / or natural polymers. The pigments are preferably organic hollow sphere pigments or inorganic pigments, such as, for example, calcined kaolin. Mixtures of these pigments, but also CaCO ₃ or Ca silicates or others can be used.

The respective intermediate layer is preferably used with a weight per unit area of approximately 1 to 14 g / m ² and in particular approximately 2 to 9 g / m ² .

If desired, additional layers can be used. For example, an outer layer (top coat) can be applied, which has the function of a protective layer. One of these advantageously consists of film-forming polymers, such as polyvinyl alcohols, modified polyvinyl alcohols, polyacrylates and polyurethanes, into which pigments can also be incorporated, it being expedient to crosslink the film-forming polymer. The function of the protective layer is particularly favorable when the film-forming polymer is largely crosslinked. Crosslinking generally takes place by incorporating agents which promote crosslinking, during the drying of the coating slip used in the formation of the protective layer. There can also be another layer on the back (backcoat) that provides additional protection, for example when printing, laminating, etc.

The essence of the invention is that a crosslinked biopolymer material in the form of nanoparticles is used in at least the thermal reaction layer (s) and / or the intermediate layer (s), and particularly preferably in the intermediate layer (s) .

Such a material is for example from the US-B1-6,677,386 and the WO 2008/022127 known.

The cross-linked biopolymer material in the form of nanoparticles is according to the in the US-B1-6,677,386 described process, according to which a biopolymer material, such as starch, containing amylose and amylopectin or both, is mixed with a plasticizer. This mixture is mixed under the action of strong shear forces to plasticize the biopolymer material and form a thermoplastic melt phase, preferably in a co-rotating, fully intermeshing twin-screw extruder, as a result of which the crystalline structure of the biopolymer material is lost. In order to crosslink the nanoparticles, during the

Mixing process added a crosslinking agent. The nanoparticles leave the extruder as a strand, which is ground to a fine powder. The nanoparticles are present in agglomerated form in the powder and can be dispersed in an aqueous medium.

The biopolymeric material can be starch or other polysaccharides, such as cellulose and vegetable gums, as well as proteins (e.g. gelatin, whey protein). The biopolymeric material can be modified beforehand, e.g. B. with cationic groups, with carboxymethyl groups, by acylation, phosphorylation, hydroxyalkylation, oxidation or the like. Starches, starch derivatives and mixtures of other polymers containing at least 50% starch are preferred. The starch, either as a single component or in a mixture with other polymers, and the starch derivatives preferably have a molecular weight of at least 10,000 g / mol and are not dextran or dextrin. Waxy starches, such as, for example, waxy corn starch, are particularly preferred.

The bio-polymeric material preferably has a dry weight of at least about 50% by weight at the start of the process. The process is preferably carried out at at least about 40 ° C, but below the decomposition temperature of the biopolymer material, for example at about 200 ° C.

The shear forces can be such that 100 J specific mechanical energy per g biopolymer material acts. Depending on the equipment used, the minimum energy may be higher; even if non-gelatinized material is used, the specific mechanical energy can be higher, e.g. B. at least about 250 J / g, preferably at least about 500 J / g.

The plasticizer can be water or a polyol (for example ethylene glycol, propylene glycol, polyglycols, glycerol, sugar alcohols, urea, citric acid ester, etc.). The total amount of plasticizer is preferably between about 15 and 50%. A lubricant such as lecithin, other phospholipids or monoglycerides can if desired, be added, for example in an amount of about 0.5 to 2.5 wt .-%. An acid, preferably a solid or semi-solid organic acid, such as maleic acid, citric acid, oxalic acid, lactic acid, gluconic acid or a carbohydrate-degrading enzyme, such as amylase, can be present in an amount of about 0.01 to 5% by weight, based on that biopolymer material. The acid or enzyme help with the slight depolymerization, which is advantageous in the production of nanoparticles of a defined size.

The crosslinking is preferably reversible, and this can be partially or completely removed after mechanical processing. Suitable reversible crosslinking agents preferably include those that form chemical bonds at a low water concentration and dissociate or hydrolyze again in the presence of a higher water concentration. This type of crosslinking leads to a temporarily high viscosity during the process, followed by a lower viscosity after the process has ended. Examples of reversible crosslinking agents are dialdehydes and polyaldehydes, acid anhydrides and mixed anhydrides and the like (e.g. succinate and acetic anhydride). Suitable dialdehydes and polyaldehydes are glutaraldehyde, glyoxal, periodate oxidized hydrocarbons and the like. Glyoxal is a particularly suitable cross-linking agent.

The crosslinking agents can be used alone or as a mixture of reversible and non-reversible crosslinking agents. Conventional crosslinking agents such as epichlorohydrin and other epoxides, triphosphates, divinyl sulfone can be used as non-reversible crosslinking agents for biopolymer material based on polysaccharides. Dialdehydes, thiol reagents and the like can be used for protein-based biopolymers. The crosslinking can take place with acid or base catalysis. The amount of crosslinking agent can be between about 0.1 and 10% by weight, based on the biopolymer material. The crosslinking agent can be present at the start of the mechanical reaction, but in the case of a non-pregelatinized biopolymer material, such as granular starch, it is preferred that the crosslinking agent is added later, for example during the mechanical reaction.

The mechanically treated, crosslinked biopolymeric material is then preferably brought into the form of a latex by dispersing it in a suitable solvent, usually in water and / or another hydroxylic solvent such as alcohol, at a concentration between about 4 and 50% by weight .-%, particularly preferably between about 10 and 40 wt .-%. A cryogenic grinding process can be carried out before the dispersion, but stirring at a slightly elevated temperature can also be useful. This processing results in a gel that takes the form of a latex either spontaneously or after induction by water adsorption. This viscosity behavior can be used for the application of the particles, such as improved mixing behavior. If desired, the dispersed biopolymeric material can be further crosslinked using the same or different crosslinking agents. The extrudate is characterized in that it swells in an aqueous solvent, for example in water or a mixture which contains at least about 50% water together with a water-miscible solvent, such as an alcohol, and forms a dispersion of nanoparticles after a drop in viscosity .

Conjugates of the same can also be used as the cross-linked biopolymer material in the form of nanoparticles. This is the cross-linked biopolymeric material described above in the form of nanoparticles which are chemically or physically linked to another additive. Examples of additives are titanium dioxide, aluminum oxide, aluminum trihydrate, sodium aluminum phosphate, aluminum phosphate, sodium aluminum magnesium silicate, fly ash, zeolites, sodium aluminum silicate, tallow clay minerals, delaminated clay, calcined kaolin clay, montmorylonite clay, nano clay, silica. Particles, zinc oxide, calcium carbonate, optical brighteners, biocides, stabilizers, etc., as well as combinations thereof. Such conjugates are for example in the WO 2010/065750 A1 described.

As mentioned, the crosslinked biopolymer material is used in the form of nanoparticles in the thermoreaction layer (s) and / or in the intermediate layer (s). Its use in the intermediate layer (s) is particularly preferred, since the remaining line porosity increases the insulation and thus the thermal sensitivity to the reaction could be improved. In addition, this favors the absorption of fusible components in the writing process, which is advantageous in particular with heat-sensitive recording materials without a topcoat with regard to the behavior on the thermal strip.

In a preferred embodiment, the crosslinked biopolymer material in the form of nanoparticles is starch, a starch derivative or a polymer mixture with at least about 50% by weight of starch or starch derivative, starch and starch derivatives being particularly preferred. Starch is very particularly preferred, in particular a cross-linked starch that has not been modified in any other way.

The average mean particle size of the nanoparticles is preferably between approximately 10 nm and 600 nm, particularly preferably between approximately 40 nm and 400 nm and very particularly preferably between approximately 40 nm and 200 nm. For example, Ecosphere 2240 Biolatex Binder, Ecosphere 92240 can be used as the crosslinked biopolymer material , 92273, X282 Biolatex Binder and Ecosphere 2202 (all available from EcoSynthetix Inc.).

The biopolymer material in the form of nanoparticles is preferably in the respective layer (s) in an amount of about 1 to 50% by weight, particularly preferably in an amount of about 2 to 40% by weight, and particularly preferably in an amount of about 2 to 30% by weight, based on the total weight of the dry matter of the respective layer. Too low quantities have the disadvantage that the connection of the neighboring layers is unsatisfactory.

In a particularly preferred embodiment, the planar support has a basis weight of approximately 20 to 600 g / m ² , in particular approximately 30 to 300 g / m ² , the respective intermediate layer (s) has a basis weight of approximately 1 to 14 g / m ² , in particular from about 2 to 9 g / m ² and / or the thermal reaction layer (s) a basis weight of about 1 to 8 g / m ² , in particular from about 2 to 6 g / m ² .

In a further preferred embodiment, at least one further binder is also present in the layer or layers in which the crosslinked biopolymer material is in the form of nanoparticles. This has the advantage that the desired result can be further adapted to the requirements of the respective heat-sensitive recording material, in particular with regard to optical appearance, insulation behavior and / or other specific features, by combining different binders and their characteristics. The at least one further binder is preferably present in the respective layer in an amount of less than 20% by weight.

The invention is essentially free in the selection of the at least one further binder, provided that the properties of the heat-sensitive recording material are not impaired thereby. At least one further binder in the form of water-soluble starches, starch derivatives, hydroxyethyl celluloses, polyvinyl alcohols, modified polyvinyl alcohols, acrylamide / (meth) acrylate copolymers and / or acrylamide / acrylate / methacrylate terpolymers is preferred. Such materials result in a coating that is water soluble. On the other hand, there are also those that lead to a water-insoluble structure. These are, for example, latices, such as polymethacrylate esters, styrene / acrylate ester copolymers, styrene / butadiene copolymers, polyurethanes, acrylate / butadiene copolymers, polyvinyl acetates and / or acrylonitrile / butadiene copolymers and the like. It is in the professional consideration to use a particularly suitable binder or mixture of binders in individual cases. The use of polyvinyl alcohol is particularly preferred.

The at least one further binder can be present in all layers, preferably in the thermal reaction layer (s) and / or in the intermediate layer (s), its use in the intermediate layer (s) being particularly preferred, since this results in the desired Properties can be particularly improved.

A further binder is understood here to mean a binder which is used in addition to the crosslinked biopolymer material in the form of nanoparticles in the layer or layers in which the crosslinked biopolymer material is in the form of nanoparticles. It goes without saying that one or more conventional binders can be present in those layers in which the crosslinked biopolymer material in the form of nanoparticles is not used.

That is, in the heat-sensitive recording material according to the invention, one or more conventional binders can be completely or partially replaced by a crosslinked biopolymer material in the form of nanoparticles. This applies to all layers.

In a preferred embodiment, the heat-sensitive recording material according to the invention is a heat-sensitive recording material with a flat support, a thermoreaction layer on at least one side of the flat support and an intermediate layer formed between the flat support and the respective thermoreaction layer and optionally further layers, where as a binder a biopolymer material in the form of nanoparticles is used in at least one of the layers.

In a preferred embodiment, the heat-sensitive recording material comprises a flat support, a thermal reaction layer and an intermediate layer formed between the flat support and the thermal reaction layer, the interlayer in addition to the crosslinked biopolymer material in the form of nanoparticles, among other things, at least one Pigment, preferably at least one hollow spherical pigment, and at least one co-binder, preferably polyvinyl alcohol, latex or starch (this is a different starch than the starch that can be used as a crosslinked biopolymer material in the form of nanoparticles, for example natural enzymatic or oxidatively degraded starches, starch esters or starch ethers), particularly preferably polyvinyl alcohol. Instead of a wood ball pigment, an inorganic pigment or a mixture of the two can also be used. Particularly suitable hollow spherical pigments are styrene / acrylate copolymers. The crosslinked biopolymer material in the form of nanoparticles is preferably in an amount of about 1 to 40% by weight, particularly preferably in an amount of 2 to 30% by weight, the pigment (mixture) is preferably in an amount of about 50 to 95% by weight, particularly preferably in an amount of approximately 60 to 90% by weight, and the co-binder preferably in an amount of approximately 0 to 10% by weight, particularly preferably approximately 1 to 9% by weight %, in front.

The crosslinked biopolymer material in the form of nanoparticles can be obtained by a process in which a biopolymer material is plasticized using shear forces and in the presence of a crosslinking agent and, if appropriate, subsequently dispersed in a hydroxylic solvent, preferably water.

Various methods are available to the person skilled in the art for producing the heat-sensitive recording material according to the invention in accordance with the invention. For example, both sides of the carrier substrate can be provided simultaneously with the coating slip to form the intermediate layers on-line in the paper machine. It is also possible to first provide the one and then the other side of the carrier substrate with intermediate layers. The respective application process is therefore not subject to any restrictions and can be carried out in the usual way. The same also applies to the formation of the thermal reaction layer, in which an aqueous dispersion which contains the necessary and beneficial constituents is applied and applied in the usual way is dried. The specialist therefore does not need any further technical instructions.

Furthermore, a method for producing the heat-sensitive recording material described above is disclosed, in which a crosslinked biopolymer material in the form of nanoparticles, preferably as a powder, particularly preferably directly in the color formulation, is used.

This has the advantage that, compared to conventional cooking starches, larger amounts of the biopolymer material are used and that higher coating color solids contents can be made available without the rheological properties being adversely affected thereby.

The heat-sensitive recording material according to the invention can be used in many areas, for example as paper for fax printing, the printing of receipts or receipts, parking tickets, entrance tickets and tickets, medical examination protocols and barcode labels.

The findings and advantages associated with the present invention can essentially be summarized as follows:
Binding agents, or presumably especially their low-molecular accompanying substances from all layers, can impair the aging resistance. These negative effects increase with increasing storage time of the paper at elevated temperatures and increased ambient humidity, as is the case, for example, in the tropics. Migration processes, in particular the low-molecular accompanying substances, probably play a role here. The use of synthetic latices in particular has a negative impact on writing performance and writing stability.

The present invention, in particular the use of a crosslinked biopolymer material in the form of nanoparticles, leads to a heat-sensitive recording material whose aging resistance is significantly improved. The resistance to aging affects both the aging before labeling, ie the aging of the unprinted thermal paper, and the aging after labeling, that is, the aging of the thermal printing. The background whiteness of the heat-sensitive recording material according to the invention is also very favorable after aging.

The heat-sensitive recording material according to the invention also shows clearly positive effects with regard to the so-called deposit behavior on the thermal strip. This is an important property characteristic of thermal paper, which reflects the degree of contamination of a thermal strip in the application. When a thermal paper is heated in the thermal printer, it melts, and the melt that forms can lead to deposits on the thermal bar of the printer. It is of crucial importance whether the thermal melt is sufficiently fixed in the thermal function layers. The absorption capacity of the intermediate layer plays a central role here, a porous line structure being very helpful. The use of a crosslinked biopolymeric material in the form of nanoparticles in the intermediate layer leads to such a line porosity, whereby a reduced tendency for soiling of the thermal print head can be achieved, in particular when using a low-absorptive hollow sphere pigment as a pigment in the intermediate layer.

Finally, the heat-sensitive recording material according to the invention is less expensive to produce and the use of synthetic binders which have to be obtained from fossil raw materials can be reduced.

The invention is explained in detail below on the basis of non-restricted examples.

Examples Production of heat-sensitive recording materials:

An interlayer recipe according to Table 1 (Recipe 1) or an interlayer recipe according to Table 2 (Recipe 2) was applied with a dry application of about 3 g / m ² by means of a doctor blade onto a conventional flat carrier (thermal raw paper) with a respective basis weight of 44 g / m ² applied and dried.

The paper substrates produced in this way were then coated with a thermal coating slip in accordance with Table 3 (recipe 3). The line application was about 4.5 g / m ² (otro) using a doctor blade. The coating dispersion A mentioned there was ground by grinding 30 parts by weight of 2-anilino-3-methyl-6-di-n-butylamino-fluorane with 55 parts by weight of a 15% aqueous solution of polyvinyl alcohol in a ball mill to an average particle size of 1.5 µm. Coating dispersion B was prepared by grinding 65 parts by weight of 2,2-bis (4-hydroxyphenyl) propane together with 35 parts by weight of benzyl naphthyl ether, 75 parts by weight of a 15% strength aqueous polyvinyl alcohol solution and 90 Parts by weight of water in a mill to an average particle size of 1.5 microns. Table 1 Recipe 1 TG Wet mass 100% Oven dry (otro) component % G G water 5.50 --- Ropaque HP-1055 * ¹ 27 71.08 19.19 Styron latex * ² 48 14.66 7.04 PV-OH * ³ 20th 8.58 1.72 Rheology aids * ⁴ 31 0.18 0.06 100.00 28.00 pH = 8.2; Brookfield viscosity (100 rpm; spindle 3; 20 ° C) = 380 mPas
* ¹ hollow spherical pigment from Dow (styrene / acrylate copolymer)
* ² styrene / butadiene latex binders (from Styron)
* ³ low viscosity, highly saponified polyvinyl alcohol (Kuraray)
* ⁴ Rheocoat type from Coatex (acrylate copolymer) Recipe 2 TG Wet mass 100% Oven dry (otro) component % G G water 13.73 --- Ropaque HP-1055 * ¹ 27 70.43 19.02 Ecosphere 2240 95 7.35 6.98 PV-OH * ³ 20th 8.49 1.70 100.00 27.70 pH = 8.8; Brookfield viscosity (100 rpm; spindle 4; 20 ° C) = 1400 mPas
* ¹ hollow spherical pigment from Dow (styrene / acrylate copolymer)
* ² networked starch, EcoSphere® quality (Ecosynthetix)
* ³ low viscosity, highly saponified polyvinyl alcohol (Kuraray) Recipe 3 Wet mass 100% Oven dry (otro) component G G water 12.35 --- PVA highly viscous, highly saponified (10%) 10.44 1.04 Leukophore UO (31.3%) * ¹ 0.22 0.07 PCC slurry (55%) * ² 28.92 15.91 Dispersion B 25.52 10.72 Stearic acid amide dispersion * ³ 11.12 2.78 Zn stearate dispersion * ³ 4.84 1.45 Dispersion A 5.92 2.66 Rheology aids (25%) * ⁴ 0.67 0.16 100.00 34.8 pH: 8.3; Brookfield viscosity (100 rpm, spindle 3, 20 ° C) = 480 mPas; Surface tension (static ring method according to Du Noüy) 48 mN / m; Dry content about 35% by weight
* ¹ optical brightener (anionic stilbene derivative) (Clariant)
* ² d ₅₀ : 1.0 µ, calcite type,
* ³ Chukyo
* ⁴ Sterocoll type (from BASF) (copolymer of acrylic acid esters and carboxylic acids)

Aging after labeling

The heat-sensitive recording materials thus obtained were subjected to an aging test (aging after inscription) in two defined climates over a period of several weeks. The image stability was determined weekly.

For this purpose, a typeface was generated on a thermal printer and its optical density was determined before aging. Afterwards the material was aged freely hanging in different climates over a certain period of time. Climates were dry heat (50 ° C) and damp heat (40 ° C / 80% rh) over a period of 1, 2, 4, 6 and 9 weeks. After aging, the remaining optical density was measured and the drop in image stability was determined in%: (OD ^after / OD ^before -1) ^∗ 100. Furthermore, the background whiteness of the respective paper samples after aging was determined. The white measurement was carried out from the top using an Elrepho 3000 reflection photometer (from Datacolor). The degree of whiteness was determined with filter R 457 (ISO 2470) without UV filter.

The results are summarized in Table 4. Table 4 Image stability after aging Test duration % Drop in opt. Density after x weeks of aging Background white% after x weeks of aging Intermediate layer after: 0.25 mJ / dot 0.45 mJ / dot 50 ° C 40 ° C / 80% RH 50 ° C 40 ° C / 80% RH 50 ° C 40 ° C / 80% RH Recipe 1 1Where. -24.1 -25.5 -1.6 -3.9 77.2 81.3 2Where -28.4 -36.4 -6.6 -6.3 74.3 76.4 4Wo. -40.5 -42.7 -18.0 -11.8 72.2 72.2 6Wo. -46.6 -50.9 -27.0 -18.1 68.5 70.3 9Wo. -49.1 -56.4 -31.1 -19.7 67.3 69.5 Recipe 2 1Where. -17.6 -21.8 1.7 2.5 78.0 82.6 2Where -21.6 -22.8 -0.8 -0.8 76.5 82.7 4Wo. -28.4 -28.7 -6.8 -0.8 75.3 81.7 6Wo. -30.4 -39.6 -12.7 -5.8 71.6 80.4 9Wo. -35.3 -38.6 -14.4 -6.6 72.3 80.9

The results show a more stable aging behavior of the heat-sensitive recording material when using formulation 2 in comparison to a heat-sensitive recording material when using formulation 1. The increased stability of the background can be seen particularly when the storage time is longer. This trend is particularly evident under warm, humid climatic conditions.

Laying behavior:

The filing behavior was checked on two commercially available thermal printers (Epson TM-T88II and Mettler scales type L2-RT) and was rated with marks from 0 to 3 after visual inspection:
Table 5 shows the evaluation of the placement on the thermal bar: Table 5 comment Printer A Printer B Recipe 1 2 -3 2-3 Recipe 2 0.5-1 0.5-1 0 = no deposits, l = easy / recognizable, 2 = medium, 3 = strong

The heat-sensitive recording material with recipe 2 showed significantly better deposition behavior than the heat-sensitive recording material with recipe 1.

Aging before labeling:

To determine the storage stability, ie the stability of a heat-sensitive recording material before writing, a conventional thermal paper with its thermal reaction layer (reference paper) was brought into contact with a pure binder layer, which was applied to a base paper (counter paper). The reference paper was a standard POS paper (available from the August Koehler SE paper mill). The binder to be examined was provided as a solution or as a dispersion. The binder solution or dispersion was applied to a Thermal raw paper applied with a squeegee and dried. The application weight was in the range of 2 to 3 g / m ² (dry). The paper was then stored at 35 ° C / 75% RH between plexiglass plates at a defined pressure of 7kg. After defined intervals of 4, 8, 12, 16, 20, 28 weeks, a sample was taken in each case and printed on a thermal printer in order to determine the remaining writing performance. For this purpose, the optical density was measured before and after aging of the paper and the writing performance [(OD ^after / OD ^before ) ^∗ 100] was determined. This test methodology aims at the influence of the binder on the aging of the heat-sensitive recording material. The results can be seen in Table 6. It can be seen that the heat-sensitive recording material using a crosslinked biopolymer material in the form of nanoparticles (No. 2) has a significantly improved storage stability compared to heat-sensitive recording materials with known binders. Table 6 Aging before labeling Write performance [%] Write performance [%] No. binder 0.25 mJ / dot; 35 ° C / 75% RH 0.45 mJ / dot; 35 ° C / 75% RH 4 weeks 8 weeks 12 weeks 16 weeks 20 weeks 28 weeks 4 weeks 8 weeks 12 weeks 16 weeks 20 weeks 28 weeks 1 Reference without contact to the counter paper 97.0 92.9 99.0 93.9 92.9 93.9 97.7 97.7 100.8 95.5 99.2 98.5 2nd Reference paper in contact with Ecosphere 2240 97.0 91.9 99.0 89.9 90.9 93.9 99.2 94.7 99.2 94.0 92.5 92.5 3rd Reference paper in contact with SB-Latex 1 89.8 80.8 80.8 76.8 70.7 62.6 91.7 88.0 80.5 79.7 77.4 56.4 4th Reference paper in contact with SA-Latex 1 81.8 66.7 72.7 50.5 39.4 40.4 88.0 68.4 75.2 49.6 42.1 37.6 5 Reference paper in contact with SB-Latex 2 87.6 84.3 75.2 66.9 70.3 - 95.0 90.7 84.9 77.0 69.8 - 6 Reference paper in contact with SB-Latex 3 90.9 79.3 76.9 71.9 57.9 - 97.1 89.2 80.6 79.9 61.9 - 7 Reference paper in contact with SB-Latex 4 86.0 81.0 81.0 70.3 73.6 - 95.0 90.7 87.8 77.7 79.9 - 8th Reference paper in contact with SA-Latex 2 67.8 44.6 28.1 29.8 21.5 - 66.9 38.9 34.5 25.2 23.7 - 9 Reference paper in contact with PV-OH 92.9 86.9 88.9 79.8 77.8 74.7 97.7 87.2 91.9 85.0 75.2 77.4 SB-Latex 1 = XZ34946.01 styrene-butadiene copolymer (from Styron)
SB-Latex 2 = Synthomer 76M10 (from Synthomer)
SB-Latex 3 = Litex PX9366 (from Polymer Latex)
SB-Latex 4 = XZ9182.00 (Styron)
SA-Latex 1 = Makrovil SE348 (Indulor)
SA-Latex 2 = DAL 7294 (Styron)
PV-OH = polyvinyl alcohol, low viscosity, highly saponified (Kuraray)
Ecosphere 2240 = cross-linked starch, EcoSphere® quality (Ecosynthetix)

Claims (9)

1. A heat-sensitive recording material with a flat support, a thermal reaction layer on at least one side of the flat support and an intermediate layer, which is formed between the flat support and the respective thermal reaction layer, and selectively further layers, wherein a cross-linked biopolymer material in the form of nanoparticles is used as the binder, and wherein the cross-linked biopolymer material in the form of nanoparticles is obtainable by means of a method, in which a biopolymer material is plasticised using shear forces and in the presence of a cross-linking agent and then dispersed in a hydroxylic solvent, wherein the cross-linked biopolymer material in the form of nanoparticles is used in the thermal reaction layer(s) and/or the intermediate layer(s), and wherein the cross-linked biopolymer material in the form of nanoparticles has a degree of swelling, of less than 1, wherein the cross-linked biopolymer material in the form of nanoparticles is a starch, a starch derivative or a polymer mixture with at least 50% by weight starch or starch derivative, wherein the degree of swelling relates to a volume expansion when the cross-linked biopolymer material in the form of nanoparticles swells in water, wherein for this purpose, a sample of a water-free quantity of 2 g is added to 200 ml pure water, dispersed therein and directly thereafter heated in a water bath that is boiling well for 30 minutes and cooled to room temperature, and the part of the water that was evaporated is added and the sample is dispersed again and 100 ml of the dispersion are placed precisely in a measuring cylinder, and the measuring cylinder is allowed to stand for 24 hours at room temperature and a precipitate is measured visually with respect to its quantity (ml) and this value is taken as the degree of swelling.
2. A recording material according to any one of the preceding claims, characterised in that the cross-linked biopolymer material in the form of nanoparticles is used in the intermediate layer(s).
3. A recording material according to any one of the preceding claims, characterised in that the cross-linked biopolymer material in the form of nanoparticles is starch.
4. A recording material according to any one of the preceding claims, characterised in that the average mean particle size of the nanoparticles is between 10 nm and 600 nm, preferably between 40 nm and 400 nm, and quite especially preferably between 40 and 200 nm.
5. A recording material according to any one of the preceding claims, characterised in that the cross-linked biopolymer material in the form of nanoparticles is present in the respective layer(s) in a quantity of 1 to 50 % by weight, preferably in a quantity of 1 to 40 % by weight and especially preferably in a quantity of 2 to 30 % by weight, based on the total weight of the respective layer.
6. A recording material according to any one of the preceding claims, characterised in that the flat support has a weight per unit area of 20 to 600 g/m², especially of 30 to 300 g/m², the respective intermediate layer(s) has/have a weight per unit area of 1 to 14 g/m², especially of 2 to 9 g/m² and/or the thermal reaction layer(s) has/have a weight per unit area of 1 to 8 g/m², especially of 2 to 6 g/m².
7. A recording material according to any one of the preceding claims, characterised in that at least one further binder is additionally present in the layer(s), in which the cross-linked biopolymer material in the form of nanoparticles is present.
8. A recording material according to any one of the preceding claims, characterised in that it comprises a flat support, a thermal reaction layer and an intermediate layer formed between the flat support and the thermal reaction layer, wherein the intermediate layer contains in addition to the cross-linked biopolymer material in the form of nanoparticles starch or a starch derivative, a hollow sphere pigment or an inorganic pigment or a mix of the two and a co-binder, preferably polyvinyl alcohol, latex or a starch, which differs from the starch that can be used as a cross-linked biopolymer material in the form of nanoparticles, especially preferably polyvinyl alcohol.
9. Use of the heat-sensitive recording material according to any one of the preceding claims 1 to 8 as paper for fax printing, the printing of sales slips or receipts, car park tickets, entry and travel tickets, medical investigation programs and barcode labels.