Heat-sensitive recording material

Abstract

The invention relates to a heat-sensitive recording material, comprising a supporting substrate and a heat-sensitive color-forming layer, which contains at least one color former and at least one phenol-free color developer, characterized in that the at least one color developer is a compound of formula (I), wherein Ar is an aryl residue, a heteroaryl residue, or a benzyl residue and Y is an aryl residue, a heteroaryl residue, a benzyl residue, an aryloxy residue, a heteroaryloxy residue, a benzyloxy residue, an arylamino residue, a heteroarylamino residue, or a benzylamino residue. The invention further relates to a method for producing said heat-sensitive recording material. ##STR00001##

Claims

1. A heat-sensitive recording material, comprising a carrier substrate and a heat-sensitive colour-forming layer, which contains at least one colour former and at least one phenol-free colour developer, characterised in that the at least one colour developer is a compound of formula (I) ##STR00005## wherein Ar is an aryl group, a heteroaryl group, or a benzyl group and Y is an aryl group, a heteroaryl group, a benzyl group, an aryloxy group, a heteroaryloxy group, a benzyloxy group, an arylamino group, a heteroarylamino group, or a benzylamino group.

2. A heat-sensitive recording material according to claim 1, characterised in that Ar and/or Y contain at least one substituent.

3. A heat-sensitive recording material according to claim 2, characterised in that the at least one substituent is selected from the group comprising C.sub.1-C.sub.5 alkyl, a C.sub.2-C.sub.5 alkenyl, C.sub.2-C.sub.5 alkinyl, alkoxy (RO), halide, carboxyl (ROCO), cyanide, Ar.sub.1O.sub.2SO, nitro, NHCONHAr.sub.1 groups, wherein R is a C.sub.1-C.sub.5 alkyl, a C.sub.2-C.sub.5 alkenyl, a C.sub.2-C.sub.5 alkinyl or a phenyl group, and wherein Ar.sub.1 is an aromatic group.

4. A heat-sensitive recording material according to claim 2, characterised in that the at least one substituent is selected from the group comprising C.sub.1-C.sub.5 alkyl, a C.sub.2-C.sub.5 alkenyl, C.sub.2-C.sub.5 alkinyl, alkoxy (RO), halide, carboxyl (ROCO), cyanide, Ar.sub.1O.sub.2SO, nitro, NHCONHAr.sub.1 groups, wherein R is a C.sub.1-C.sub.5 alkyl, a C.sub.2-C.sub.5 alkenyl, a C.sub.2-C.sub.5 alkinyl or a phenyl group, and wherein Ar.sub.1 a phenyl group which optionally is substituted with one or more C.sub.1-C.sub.5 alkyl, C.sub.2-C.sub.5 alkenyl, and/or C.sub.2-C.sub.5 alkinyl groups.

5. A heat-sensitive recording material according to claim 1, characterised in that Ar is a phenyl group and Y is an arylamino group.

6. A heat-sensitive recording material according to claim 1, characterised in that Ar is a phenyl group and Y is a 4-methoxycarbonyl phenyl group.

7. A heat-sensitive recording material according to claim 1, characterised in that the phenol-free colour developer is selected from the group consisting of N-phenyl-N[(phenylamino)sulfonyl]urea, N-(4-methylphenyl)-N[(4-methylphenylamino)sulfonyl]-urea, N-(4-ethoxycarbonylphenyl)-N[(4-ethoxycarbonylphenylamino)sulfonyl]-urea, N-(1-naphthyl)-N[(1-naphthylamino)sulfonyl]-urea, N-[(phenylamino)sulfonyl]-benzamide, N-[(4-methoxycarbonylphenyl)aminosulfonyl]-benzamide, N-({2-[(phenylcarbamoyl)amino]phenyl}sulfamoyl)benzamide, N-[(4-nitrophenyl)aminosulfonyl]-benzamide and/or N-[2-[[(phenylamino)carbonyl]amino]phenyl]-benzenesulfonamide.

8. A heat-sensitive recording material according to claim 1, characterised in that the at least one colour former is a dye of the triphenylmethane type, of the fluoran type, of the azaphthalide type and/or of the fluorene type.

9. A heat-sensitive recording material according to claim 1, further comprising one or more further non-phenolic colour developers.

10. A heat-sensitive recording material according to claim 1, characterised in that at least two compounds of formula (I) are present as developer.

11. A heat-sensitive recording material according to claim 1, characterised in that approximately 0.5 to approximately 10 parts by weight of the compound of formula (I) are present, based on the colour former.

12. A heat-sensitive recording material according to claim 1, characterised in that the compound of formula (I) is present in an amount of from approximately 3 to approximately 35% by weight based on the total solids content of the heat-sensitive layer.

13. A heat-sensitive recording material according to claim 1, characterised in that the heat-sensitive colour-forming layer contains at least one of sensitising agents, stabilisers, binders, release agents, pigments and/or brighteners.

14. A heat-sensitive recording material according to claim 1, characterised in that the heat-sensitive colour-forming layer contains a urea-urethane compound of general formula (II) ##STR00006##

15. A method for producing a heat-sensitive recording material according to claim 1, characterised in that an aqueous suspension containing the starting materials of the heat-sensitive colour-forming layer is applied to a carrier substrate and dried, wherein the aqueous application suspension has a solids content of from approximately 20 to approximately 75% by weight, and is applied using the curtain coating method at an operating speed of the coating apparatus of at least approximately 400 m/min, and dried.

16. A heat-sensitive recording material obtainable in accordance with the method according to claim 15.

17. A heat-sensitive recording material according to claim 1, characterised in that, besides the compound of formula (I) 4-methyl-N-[[[3-[[(4-methylphenyl)sulfonyl]oxy]phenyl]amino]carbonyl]-benzenesulfonamide and/or N-[2-[[(phenylamino)carbonyl]amino]phenyl]-benzenesulfonamide is present.

18. A heat-sensitive recording material according to claim 1, characterised in that approximately 1.5 to approximately 4 parts by weight, of the compound of formula (I) are present, based on the colour former.

19. A heat-sensitive recording material according to claim 1, characterised in that the compound of formula (I) is present in an amount of from approximately 10 to approximately 25% by weight, based on the total solids content of the heat-sensitive layer.

20. A method for producing a heat-sensitive recording material according to claim 1, characterised in that an aqueous suspension containing the starting materials of the heat-sensitive colour-forming layer is applied to a carrier substrate and dried, wherein the aqueous application suspension has a solids content of from approximately 30 to approximately 50% by weight, and is applied using the curtain coating method at an operating speed of the coating apparatus of at least approximately 1500 m/min, and dried.

21. A heat-sensitive recording material according to claim 1, characterised in that the at least one colour former is a dye of the fluoran type.

Description

EXAMPLES

(1) On a laboratory scale, an aqueous application suspension for forming the heat-sensitive colour-forming layer of a heat-sensitive recording paper was applied by means of a doctor bar to one side of a synthetic base paper (Yupo FP680) of 63 g/m.sup.2 (coating formulations R1, R2) or of a paper of 45 g/m.sup.2 carrying a pre-coating (coating formulations R3 to R11), wherein the pre-coating was formulated with organic hollow bead pigments (of the Ropaque type). Once dried, a thermal recording sheet was obtained. The applied amount of the heat-sensitive colour-forming layer was between 4.0 and 4.5 g/m.sup.2.

(2) On a production scale, the application of the aqueous application suspension to a paper web having a weight per unit area of 43 g/m.sup.2 was carried out by means of the curtain coating method. The viscosity of the aqueous application suspension was 450 mPas (according to Brookfield, 100 rev/min, 20 C.) (in the deaerated state). The surface tension thereof was 46 mN/m (statistical ring method). The coating apparatus was arranged inline. The curtain coating method was operated at a speed of 1550 m/min.

(3) After the application of the aqueous application suspension, the operation of drying the coated paper carrier was carried out in the customary way. The application weight per unit area of the dry heat-sensitive layer was 4.0-4.5 g/m.sup.2.

(4) A heat-sensitive recording material or thermal paper was produced on the basis of the details given above, wherein the following formulations of aqueous application suspensions were used to form a composite structure on a carrier substrate and then the further layers, especially a protective layer, were formed in the customary way, which will not be discussed separately here.

(5) Production of the Dispersions (in Each Case for 1 Part by Weight) for the Application Suspensions:

(6) The aqueous dispersion A1 (colour former dispersion) is produced by grinding 20 parts by weight of 3-N-n-dibutylamino-6-methyl-7-anilinofluoran (ODB-2) with 33 parts by weight of a 15% aqueous solution of Ghosenex L-3266 (sulfonated polyvinyl alcohol, Nippon Ghosei) in a bead mill.

(7) The aqueous dispersion A2 (2-component colour former dispersion) is a mixture of two colour formers, which was produced by mixing a first dispersion, which was produced by grinding 12 parts by weight of 3-N-n-dibutylamino-6-methyl-7-anilinofluoran (ODB-2) with 20 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill, and a second dispersion, which was produced by grinding 8 parts by weight of 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluoran (S-205) with 14 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill.

(8) The aqueous dispersion B1 (colour developer dispersion) is produced by grinding 40 parts by weight of the colour developer together with 66 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in the bead mill.

(9) The aqueous dispersion B2 (2-component colour developer dispersion formed of FE I and FE II) was produced by mixing a first dispersion, which was produced by grinding 20 parts by weight of FE I with 33 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill, and a second colour developer dispersion, which was produced by grinding 20 parts by weight of FE II with 33 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill (reference is made to Tables 3 and 4 with regard to the definitions of FEI and FEII).

(10) The aqueous dispersion B3 (2-component colour developer dispersion formed of FE I and FE II) was produced by mixing a first dispersion, which was produced by grinding 28 parts by weight of FE I with 46 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill, and a second colour developer dispersion, which was produced by grinding 12 parts by weight of FE II with 20 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill.

(11) The aqueous dispersion C (sensitising agent dispersion) was produced by grinding 40 parts by weight of sensitising agent with 33 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill.

(12) The aqueous dispersion D (anti-ageing agent or stabiliser dispersion) was produced by grinding 12.5 parts by weight of UU (urea-urethane) with 10 parts by weight of a 15% aqueous solution of Ghosenex L-3266 in a bead mill.

(13) All dispersions produced by grinding have a mean particle size D.sub.(4.3) of 0.80-1.20 m.

(14) The dispersion E (lubricant dispersion) is a 20% zinc stearate dispersion, consisting of 9 parts by weight of Zn stearate, 1 part by weight of Ghosenex L-3266 and 40 parts by weight of water.

(15) Pigment P1 is a 72% coating kaolin suspension (Lustra S, BASF)

(16) Pigment P2 is a 56% PCC dispersion (precipitated calcium carbonate)

(17) Pigment P3 is a 56% aluminium hydroxide dispersion (Martigloss, Albemarle Corp.)

(18) Pigment P4 is obtained by dispersing 132 parts of a 56% aluminium hydroxide dispersion (Martigloss, Albemarle Corp.) in 31.5 parts of a precipitated silicic acid (Sipernat 350, Evonik).

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

(20) The application suspension is produced by mixing the dispersions, with stirring, in accordance with the amounts specified in Table 1 under consideration of the entry order B, E, C, D, P, A, binder, and is brought with water to a solids content of approximately 25%.

(21) The particle size distribution of the application dispersions was measured by laser diffraction using a Coulter LS230 apparatus from Beckman Coulter.

(22) Table 2 summarises the developers used in the example formulations.

(23) The water-sensitive coating suspensions thus obtained were used to produce composite structures formed of paper carrier and thermal reaction layer.

(24) TABLE-US-00001 TABLE 1 Summary of the formulations for the application dispersions (parts by weight) R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 A1 1 1 1 1 1 1 1 1 A2 1 1 1 1 B1 1 1 1 1 1 1 1 1 B2 1 1 1 B3 1 C 1 1 1 1 1 1 1 1 1 1 1 1 D 1 1 E 56 56 56 56 56 56 56 56 56 56 56 56 P1 146 146 P2 188 162 188 188 188 162 P3 188 132 188 132 P4 31.5 31.5 Binder 138 138 138 138 138 138 138 138 138 138 138

(25) TABLE-US-00002 TABLE 2 Classification of the colour developers (FE) Colour developer FE1 N-phenyl-N[(phenylamino)sulfonyl]urea FE2 N-(4-methylphenyl)-N[(4-methylphenylamino)sulfonyl]urea FE3 N-(4-ethoxycarbonylphenyl)-N[(4- ethoxycarbonylphenylamino)sulfonyl]urea FE4 N-(1-naphthyl)-N[(1-naphthylamino)sulfonyl]urea FE5 N-[(phenylamino)sulfonyl]benzamide FE6 N-[(4-methoxycarbonylphenyl)aminosulfonyl]benzamide FE7 N-({2-[(phenylcarbamoyl)amino]phenyl}sulfamoyl)- benzamide FE8 N-[(4-nitrophenyl)aminosulfonyl]benzamide FE9* 4-methyl-N-[[[3-[[(4-methylphenyl)sulfonyl]oxy- ]phenyl]amino]carbonyl]benzenesulfonamide FE10* N-[2- [[(phenylamino)carbonyl]amino]phenyl]benzenesulfonamide *non-phenolic developer of the prior art

(26) The thermal recording materials according to Tables 3, 4 and 5 were analysed as below.

(27) (1) The paper whiteness on the coating side was determined in accordance with DIN/ISO 2470 using an Elrepho 3000 spectral photometer.

(28) (2) Dynamic Colour Density:

(29) The papers (6 cm wide strips) were printed thermally using the Atlantek 200 test printer (Atlantek, USA) with a Kyocera printhead of 200 dpi and 560 ohm at an applied voltage of 20.6 V and a maximum pulse width of 0.8 ms with a chequered pattern with 10 energy stages. The image density (optical density, o.d.) was measured using a Macbeth densitometer RD-914 from Gretag.

(30) (3) Static Starting Point:

(31) The recording material sheet was pressed against a series of thermostatically controlled dies heated to different temperatures with a press-on pressure of 0.2 kg/cm.sup.2 and a contact time of 5 sec (thermal tester TP 3000QM, Maschinenfabrik Hans Rychiger AG, Steffisburg, Switzerland).

(32) The image density (optical density) of the images thus produced was measured using a Macbeth densitometer RD-914 from Gretag.

(33) The static starting point, according to definition, is the lowest temperature at which an optical density of 0.2 is achieved.

(34) (4) Storage Stability of the Unprinted Material:

(35) A sheet of recording paper is cut into three identical strips. One strip is dynamically recorded in accordance with the method of (2) and the image density is determined. The two other strips, in the unprinted (white) state, are exposed to a climate of 60 C. and 50% relative humidity for 4 weeks. After climate conditioning of the papers they are dynamically printed in accordance with the method of (2) and the image density is determined using the densitometer. The % change in the writing performance with printing of the stored specimens was calculated in accordance with the following equation (I).

(36) $\begin{array}{cc}\%\mathrm{change}\mathrm{in}o.d.=\left(\frac{\mathrm{image}\mathrm{density}\mathrm{after}\mathrm{test}}{\mathrm{image}\mathrm{density}\mathrm{before}\mathrm{test}}-1\right)*100& \left(I\right)\end{array}$

(37) (5) Plasticiser Stability of the Printed Image:

(38) A plasticiser-containing cling film (PVC film with 20-25% dioctyl adipate) was brought into contact with the sample of the thermal recording paper, which had been dynamically recorded in accordance with the method of (2), avoiding folds and inclusions of air, then rolled up into a roll and stored for 16 hours at room temperature (20-22 C.). After removal of the film, the image density (o.d.) was measured and set in relation to the corresponding image density values before the action of the plasticiser in accordance with formula (I).

(39) (6) A strip of transparent self-adhesive tape from Tesa (#57315) was adhered to the sample of the thermal recording paper, which had been dynamically recorded in accordance with the method of (2), avoiding folds and inclusions of air. After storage at room temperature (20-22 C.), the image density (o.d.) was measured after 3 hoursthrough the adhesive tapeand was set in accordance with the formula (I) in relation to the similarly determined image density values of a freshly adhered specimen.

(40) (7) Quantification of the coating components (colour former and colour developer) is effected after HPLC separation using a series 1200 HPLC apparatus from Agilent having a DAD detector.

(41) Sample preparation: Two circular areas are cut out from the paper specimen using a punch and weighed. The paper samples are extracted with 3 ml of acetonitrile (HPLC quality) in an ultrasonic bath for 30 minutes and the extract is filtered through a PTFE syringe filter (0.45 m).

(42) HPLC separation of the ingredients: Using an autosampler the above extract was applied to a separating column (Zorbax Eclipse XDB-C18) and eluted using the solvent acetonitrile:THF:H.sub.2O (450:89:200 parts by weight) with an acetonitrile gradient. Quantitative analysis of the chromatograms is carried out by comparing the areas of the sample peaks assigned by means of tr times with a calibration curve determined by means of the reference specimens. The measurement error in the HPLC quantification is 2%.

(43) Table 3 summarises the analysis of the recording materials manufactured with synthetic paper (Yupo FP680) as carrier; Table 4 summarises the analysis of the recording materials manufactured with a pre-coated carrier paper.

(44) The achieved maximum image densities (o.d. max.) of the fresh paper are presented in Table 5 with the corresponding values after thermal printing of the (unprinted) stored paper over 4 weeks at 60 C. and 50% relative humidity, and also the change in paper whiteness after storage for selected specimens.

(45) For the paper artificially aged by storage, a quantitative determination of the colour developer in the fresh and stored papers was also performed, and as control a corresponding determination of the colour former as coating component was performed, which from experience hardly changes at all over the storage period.

(46) Changes to o.d. of s 10% are tolerable and do not impair the practical suitability of the papers. For the change in artificial ageing, a change of 15% is tolerable.

(47) TABLE-US-00003 TABLE 3 Analysis of the laboratory specimen (Substrate: synthetic paper) Background % change o.d. whiteness Static starting Artificial ageing Stability of the printed image No. Formulation Developer (%) o.d. max point (C.) dry moist light TESA 3 h plasticiser 1 R1 FE1 88.6 1.29 85 2.4 2.3 11.5 21.1 5.6 2 FE2 89.0 1.26 83 7.9 6.5 19.0 22.5 28.0 3 FE3 88.4 1.35 85 7.4 5.3 12.0 10.3 4.4 4 FE4 88.4 1.22 99 7.6 13.6 15.3 17.4 51.5 5 FE5 88.5 1.05 86 1.0 5.7 31.3 17.7 54.1 6 FE6 89.9 1.26 96 1.7 3.3 18.9 12.4 10.2 7 FE7 87.9 1.25 86 2.4 2.4 22.4 18.9 22.2 8 R2* FE10 + FE1 88.2 1.31 78 1.6 1.6 18.3 26.0 5.3 9 FE5 + FE1 86.2 1.25 79 3.9 5.5 29.3 32.5 21.0 10 R1 FE9 88.2 1.29 77 2.5 7.1 18.8 13.2 3.3 (Comparison) 11 FE10 88.5 1.31 83 1.2 1.9 20.8 23.6 4.0 (Comparison) *No. 8, FEI is FE10, FEII is FE1 No. 9 FEI is FE5, FEII is FE1

(48) TABLE-US-00004 TABLE 4 Analysis of the laboratory specimen (Substrate: paper with pre-coating) Background % change of o.d. (best value after o.d. 2) whiteness Static starting Artificial change Stability of the printed image No. Formulation Developer (%) o.d. max point (C.) dry moist light TESA 3 h plasticiser 12 R3 FE1 87 1.37 87 1.0 16.9 10.2 29 9.2 13** FE10 87.5 1.34 86 0 1.6 11.6 26 6.1 14 R4 FE1 87.7 1.35 87 3.7 15.6 8.5 28 10.2 15** FE10 88 1.31 87 0 1.6 16.8 30 5.5 16 R5 FE1 87.4 1.36 85 3.1 14.0 16.1 27 6.8 17** FE10 87.8 1.36 86 0 2.2 19.1 29 3.0 18 R6 FE1 84.2 1.36 84 3.0 5.0 9.2 20 4.6 19** FE10 87.3 1.30 83 1.0 0 14.3 17 3.9 20 R12 FE1 + FE5 85.7 1.30 84 2.3 7.3 15.5 22 9.5 21 R7 FE1 85.3 1.37 84 3.0 14.8 12.4 31 5.8 22** FE10 86.6 1.32 80 0 1 19.1 26 3.9 23 R8 FE1 86.3 1.36 81 3.0 12.7 10.6 34 8.1 24** FE10 87.2 1.34 81 0 1.5 18.5 26 4.7 25 R9 FE1 86 1.42 81 2.1 11.5 12.5 27 4.4 26** FE10 86.9 1.35 80 0 0 18.7 25 3.0 27* R11 FE1 + FE10 85.5 1.34 78 1.0 1.5 12.2 25 4.7 28* FE5 + FE10 86.1 1.28 80 0 1.0 16.1 30 4.0 29* R10 FE1 + FE10 87.0 1.33 81 0 3.0 12.8 26 5.3 30* FE5 + FE10 87.5 1.29 86 0 2.4 18.9 27 3.3 *No. 27: FEI is FE1, FEII is FE10, No. 28: FEI is FE5, FEII is FE10, No. 29: FEI is FE1, FEII is FE10, No. 30: FEI is FE5, FEII is FE10 **Comparison

(49) TABLE-US-00005 TABLE 5 Analysis of selected specimens from Table 3 after storage* in the unprinted state Test Specimen 10 11 parameter (no.) 5 6 7 9 (comparison) (comparison) Surface fresh 88.5 89.9 87.9 88.2 88.2 88.5 whiteness 4 weeks 75.3 83.4 70.0 69.6 69.7 76.0 (%) storage % change 14.9 7.6 20.3 15.5 20.9 14.1 o.d. max. fresh 1.07 1.21 1.20 1.14 1.30 1.32 4 weeks 0.84 0.90 1.04 0.79 0.55 1.05 storage % change 21.5 25.6 13.3 30.7 42.3 20.5 FE fresh 608 673 561 637 516 554 (mg/m.sup.2) 4 weeks 552 664 541 387 284 520 storage % change 9.2 1.3 3.6 39.2 45.0 6.1 FBB fresh 308 300 294 337 329 301 (mg/m.sup.2) 4 weeks 307 310 280 319 307 295 storage % change 0.3 3.3 4.8 5.3 6.7 2.0 *Ageing 60 C./50% relative humidity

(50) The heat-sensitive recording material according to the invention demonstrates especially the following advantageous properties: (1) The recorded image of the heat-sensitive recording materials according to the invention with the colour developers according to the invention has a maximum print density, which corresponds to the print density of the comparison specimens (maximum o.d. values from Tables 3 and 4), is stable under the conditions of artificial ageing, and hardly declines after exposure to hydrophobic agents (adhesives, plasticisers), comparably to the performance of the known non-phenolic developer substances (Tables 3 and 4). (2) The surface whiteness of the heat-sensitive recording materials according to the invention is better or comparable to the comparison specimens with colour developers of the prior art, both in the fresh state and after storage under adverse climatic conditions (Table 5). (3) The temperature from which a visually noticeable greying of the heat-sensitive recording materials according to the invention occurs (static starting point, Tables 3, 4) is comparable or higher than in the comparison papers, even with use of colour developer mixtures. (4) The heat-sensitive recording material according to the invention demonstrates a slight drop in writing performance after four weeks of storage in the unprinted state under extreme storage conditions, which is comparable or better than that of the comparison papers (Table 5, o.d. max change, specimen nos. 5, 6, 7, 9 vs. 10 and 11). (5) The reduction of the colour developer concentration in the heat-sensitive colour-forming layer of the heat-sensitive recording materials according to the invention after storage is low (specimen nos. 5, 6 and 7 from Table 5 (10%)) and hardly affects the writing performance. By contrast, the use of known non-phenolic developers (specimen no. 10, Table 5) leads to considerable losses of the colour developer amount in the paper and to an unacceptable low writing performance after storage. (6) With customary anti-ageing agents, the image stability of the papers according to the invention with artificial ageing and compared to hydrophobic agents can be increased (Table 4, specimen nos. 18 and 20). (7) Mixtures of the colour developers in the heat-sensitive recording materials according to the invention with one another or with known non-phenolic developers do not lead to any disadvantages with respect to the surface whiteness or starting temperature and can be used advantageously in order to selectively improve the performance properties and/or control the economical efficiency of the colour development system of the heat-sensitive recording material according to the invention. (8) Especially, a heat-sensitive recording material that is of high quality in all key aspects of its application can be produced with the production method according to the invention under economically advantageous conditions. (9) Previously, it was indeed possible to improve individual properties important for the application, however the core point of the heat-sensitive recording material according to the invention lies in optimising all important properties.