Coating formulation for digital printing media

Abstract

The present invention relates to a coating formulation for digital printing media comprising a polymeric binder and a solid metal complex, wherein said solid metal complex comprises (i) a divalent or trivalent metal cation and (ii) a polydentate ligand. The invention further relates to a method for preparing the inventive coating formulation, to a digital printing medium comprising the inventive coating and a method for preparing same, and to corresponding uses.

Claims

1. A coating formulation for digital printing media comprising a polymeric binder and a solid metal complex having a solubility in water of less than 25 g/l at 23° C. determined according to ISO 787-3:2000, said solid metal complex comprising: (i) a divalent or trivalent metal cation; and (ii) a polydentate ligand; characterized in that the polydentate ligand is a molecule of general Formula I or a corresponding anion: ##STR00004## wherein X is N, C—H or C—OH; and R.sup.1 is selected from —CO.sub.2H or —CH(R.sup.2)CO.sub.2H, wherein R.sup.2 is H or a substituted or unsubstituted alkyl.

2. The coating formulation according to claim 1, characterized in that the solid metal complex has an ISO brightness R457(%) of at least 80.

3. The coating formulation according to claim 1, characterized in that the coating formulation is a coating formulation for inkjet printing media or flexographic printing media.

4. The coating formulation according to claim 1, characterized in that the solid metal complex is obtainable by a process comprising the following steps: (4-a) providing a metal salt comprising said divalent or trivalent metal cation and a counter anion; (4-b) providing said polydentate ligand; (4-c) contacting the metal salt provided in step (4-a) and the polydentate ligand provided in step (4-b) to obtain a mixture; (4-d) precipitating the solid metal complex from the mixture obtained in step (4-c); wherein said metal salt provided in step (4-a) and/or said polydentate ligand provided in step (4-b) is/are optionally provided in the form of an aqueous preparation.

5. The coating formulation according to claim 4, characterized in that the process defined in claim 4 further comprises a step of increasing the pH value of the mixture obtained in step (4-c) by adding a base.

6. The coating formulation according to claim 1, characterized in that the metal cation is a divalent metal cation.

7. The coating formulation according to claim 1, characterized in that the metal cation of the solid metal complex is selected from the group consisting of Al.sup.3+, Ba.sup.2+, Cr.sup.3+, Ca.sup.2+, Cd.sup.2+, Fe.sup.3+, Mg.sup.2+, Pb.sup.2+, Sr.sup.2+, Zn.sup.2+, and Zr.sup.2+.

8. The coating formulation according to claim 1, characterized in that the polymeric binder is selected from the group consisting of starch, modified starch, modified cellulose, proteins synthetic polymers carboxymethyl cellulose, casein, a styrene-butadiene-based copolymer, and polyvinyl alcohol.

9. The coating formulation according to claim 1, characterized in that the formulation has a Brookfield viscosity in the range of from 250 to 5000 mPa.Math.s.

10. The coating formulation according to claim 1, characterized in that the formulation has the following composition, based on the total solids content of the formulation: 0.1 to 60 wt % of the solid metal complex; 0.1 to 60 wt % of the polymeric binder; optionally 0.01 to 10 wt % of one or more additives; wherein the solid metal complex, the polymeric binder and the additives add up to 100 wt %, based on the total solids content of the formulation.

11. The coating formulation according to claim 1, characterized in that the formulation further comprises an inorganic pigment or a calcium carbonate-containing pigment.

12. The coating formulation according to claim 11, characterized in that the formulation has the following relative composition: 100 parts by weight of inorganic pigment; 0.1 to 30 parts by weight of the solid metal complex; 0.1 to 20 parts by weight of the polymeric binder; optionally 0.001 to 20 parts by weight of one or more additives.

13. The coating formulation according to claim 1, characterized in that the coating formulation has a total solids content in the range of from 25 to 85 wt %, based on the total weight of the coating formulation.

14. A method for preparing a coating formulation for digital printing media, the method comprising the following steps: (14-a) providing a polymeric binder; (14-b) providing a solid metal complex having a solubility in water of less than 25 g/l at 23° C. determined according to ISO 787-3:2000, said solid metal complex comprising: (i) a divalent or trivalent metal cation; and (ii) a polydentate ligand; (14-c) mixing the polymeric binder provided in step (14-a) with the solid metal complex provided in step (14-b) and, optionally, with an inorganic pigment; characterized in that the polydentate ligand is a molecule of general Formula I or a corresponding anion: ##STR00005## wherein X is N, C—H or C—OH; and R.sup.1 is selected from —CO.sub.2H or —CH(R.sup.2)CO.sub.2H, wherein R.sup.2 is H or a substituted or unsubstituted alkyl.

15. The method according to claim 14, characterized in that the solid metal complex has an ISO brightness R457(%) of at least 80.

16. The method according to claim 14, characterized in that the method further comprises the following process for preparing the solid metal complex provided in step (14-b): (16-a) providing a metal salt comprising said divalent or trivalent metal cation and a counter anion; (16-b) providing said polydentate ligand; (16-c) contacting the metal salt provided in step (16-a) and the polydentate ligand provided in step (16-b) to obtain a mixture; (16-d) precipitating the solid metal complex from the mixture obtained in step (16-c); wherein said metal salt provided in step (16-a) and/or said polydentate ligand provided in step (16-b) is/are optionally provided in the form of an aqueous preparation.

17. The method according to claim 16, characterized in that the metal salt provided in step (16-a) is a water-soluble metal salt.

18. The method according to claim 16, characterized in that the counter anion of the metal salt provided in step (16-a) is a monovalent or divalent anion.

19. The method according to claim 16, characterized in that the process for preparing the solid metal complex defined in claim 16 further comprises a step of increasing the pH value of the mixture obtained in step (16-c) by adding a base.

20. The method according to claim 16, characterized in that the process for preparing the solid metal complex defined in claim 16 further comprises a step of drying the solid metal complex obtained in precipitating step (16-d).

21. The method according to claim 16, characterized in that the process for preparing the solid metal complex defined in claim 16 further comprises a step of grinding the solid metal complex obtained in precipitating step (16-d).

22. A method for preparing a digital printing medium, the method comprising the following steps: (22-a) providing a printable substrate having at least one printable surface; (22-b) providing a coating formulation as defined in claim 1; (22-c) applying the coating formulation provided in step (22-b) onto said at least one printable surface of the printable substrate provided in step (22-a).

23. The method according to claim 22, characterized in that the coating formulation is applied onto said at least one printable surface by using a blade coater, a curtain coater, a rod coater or a size press.

24. The method according to claim 22, characterized in that the printable substrate is a precoated or uncoated cardboard substrate, a precoated or uncoated paper substrate, a polymer substrate, a textile-based substrate or a wooden substrate.

25. A digital printing medium obtainable by the method according to claim 22.

26. A digital printing medium according to claim 25 which is suitable for use in digitally printed paper products digitally printed cardboard products, stickers, labels, tags, tickets, posters, wallpapers, documents, passports, identification cards, banknotes, postage stamps, digitally printed textile products, garments or curtains.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1: SEM picture of calcium-NTA complex obtained from calcium chloride as calcium source.

(2) FIG. 2a: Printed reference paper.

(3) FIG. 2b: Printed paper of Example 4E.

(4) FIG. 2c: Printed paper of Example 4F.

(5) FIG. 2d: Printed paper of Example 4A.

EXAMPLES

(6) The scope and interest of the invention may be better understood on basis of the following examples which are intended to illustrate embodiments of the present invention.

(A) ANALYTICAL METHODS

(7) All parameters defined throughout the present application and mentioned in the following examples are based on the following measuring methods:

(8) Brightness (Powders)

(9) Brightness of solids is measured using an ELREPHO 450X from the company Datacolor according to ISO 2469:2014. The brightness measurement was performed immediately after preparing the tablet. The obtained values are reported as ISO brightness R457 in %.

(10) Water Solubility

(11) Solubility measurements are performed according ISO 787-3:2000. The solution was allowed for decantation for 24 h and the filtration was performed at 23° C. by using filters with 0.2 μm pores to ensure total filtration of small particles.

(12) Brookfield Viscosity

(13) The Brookfield viscosity is measured by a Brookfield DV III Ultra viscometer at 24° C.±3° C. at 100 rpm using an appropriate spindle of the Brookfield RV-spindle set and is specified in mPa.Math.s. Once the spindle has been inserted into the sample, the measurement is started with a constant rotating speed of 100 rpm. The reported Brookfield viscosity values are the values displayed 60 seconds after the start of the measurement. Based on his technical knowledge, the skilled person will select a spindle from the Brookfield RV-spindle set which is suitable for the viscosity range to be measured. For example, for a viscosity range between 200 and 800 mPa.Math.s the spindle number 3 may be used, for a viscosity range between 400 and 1 600 mPa.Math.s the spindle number 4 may be used, for a viscosity range between 800 and 3 200 mPa.Math.s the spindle number 5 may be used, for a viscosity range between 1 000 and 2 000 000 mPa.Math.s the spindle number 6 may be used, and for a viscosity range between 4 000 and 8 000 000 mPa.Math.s the spindle number 7 may be used.

(14) Total Solids Content

(15) The total solids content was measured with Smart System 5 from CEM Corporation (USA) with the following settings: temperature of 120° C., automatic switch off 3, standard drying, sample size 2 to 4 g.

(16) Particle Size Distribution

(17) The weight-based median particle size d.sub.50(wt) and top cut d.sub.98(wt) are measured by the sedimentation method, which is an analysis of sedimentation behaviour in a gravimetric field. The measurement is made with a Sedigraph™ 5120 of Micromeritics Instrument Corporation, USA. The method and the instrument are known to the skilled person and are commonly used to determine particle size distributions of fillers and pigments. The measurement is carried out in an aqueous solution of 0.1 wt % Na.sub.4P.sub.2O.sub.7. The samples are dispersed using a high speed stirrer and sonication.

(18) The volume-based median particle size d.sub.50(vol) and the volume-based top cut particle size d.sub.98(vol) are evaluated using a Malvern Mastersizer 2000 Laser Diffraction System (Malvern Instruments Plc., Great Britain). The raw data obtained by the measurement is analyzed using the Mie theory, with a particle refractive index of 1.57 and an absorption index of 0.005. The methods and instruments are known to the skilled person and are commonly used to determine particle size distributions of fillers and pigments. To determine the volume-based particle size distribution of the solid metal complex, the measurement is carried out in water as solvent, and the water is saturated with the complex to be measured. Then the measurement is started.

(19) Elemental Analysis

(20) Elemental analysis (CHN analysis) is performed on a Vario EL III from Elementar Analysensysteme GmbH. The method and the instruments are known to the skilled person and are commonly used to determine CHN percentages.

(21) Total Moisture Content (Karl Fischer Titration)

(22) The total moisture contents as defined herein are measured according to the Karl Fischer coulometric titration method, desorbing the moisture in an oven at 220° C. for 10 min and passing it continuously into a Karl Fischer coulometer (Mettler Toledo coulometric KF Titrator C30, combined with Mettler Toledo oven DO 0337) using dry nitrogen at 100 ml/min for 10 min. A calibration curve using water is recorded and a blank of 10 min nitrogen flow without a sample is taken into account.

(23) Coating Weight

(24) Coating weights of coated papers are analyzed according the normed method EN ISO 536:2012.

(25) Paper Humidity

(26) Paper humidity of manufactured papers is measured according the normed method EN ISO 287:2009.

(27) pH Measurement

(28) pH is measured at 25° C. using a Mettler-Toledo Seven Easy pH meter and a Mettler-Toledo InLab Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument is first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Aldrich). The reported pH values are the endpoint values detected by the instrument (signal differs by less than 0.1 mV from the average over the last 6 seconds).

(29) Ink Density

(30) Ink density is analyzed with a SpectroDens from the company Techkon GmbH according to DIN 16527-3:1993-11. The density is calculated from the average of 10 measurements after calibration on a white part of the sample paper. Used parameters: D50 light, angle 2°, ISO E.

(31) Microscopic Pictures of Metal Complexes

(32) Pictures are taken under the field emission scanning electron microscope (FESEM, Zeiss Sigma VP) using the secondary electron detector (SE2) to show the particle structure of the samples. The methods and the instruments are known to the skilled person.

(33) Microscopic Pictures of Printed Papers

(34) Pictures are taken with a PCE-MM200 digital microscope from PCE Instruments (UK Limited).

(B) MATERIALS

(35) All commercially available materials and reagents were purchased and used without further purification.

(36) Chemicals

(37) Nitrilotriacetic acid trisodium salt monohydrate (#72565) was obtained from Sigma-Aldrich. Anhydrous calcium chloride was obtained from IMPAG AG (#17033) and Sigma-Aldrich (#C1016) as granules. Sodium carbonate (#S6014) was obtained from Sigma-Aldrich. Citric acid (#C0759) was obtained from Sigma-Aldrich. Nitrilotriacetic acid (NTA) was obtained from Sigma-Aldrich (#N9877). Tricalcium citrate tetrahydrate (Powder N, #M7090) was purchased from Jungbunzlauer (Univar AG) and Sigma-Aldrich (#359734). Starch (C*Film™ 07311) was obtained from Cargill. The polyvinyl alcohol (PVA) used herein was a commercially available fully hydrolyzed BF-04 grade PVA. Rheocarb™ 121 was obtained from Coatex Arkema Group. Litex PX 9460 was purchased from Synthomer Deutschland GmbH. Catiofast BP was obtained from BASF.

(38) The following calcium carbonates were used as inorganic pigments: Calcium carbonate 1: Ground natural calcium carbonate, suspension, 72% solids content, d.sub.98(wt)=4 μm, d.sub.50(wt)=0.6 μm Calcium carbonate 2: Ground natural calcium carbonate, suspension, 60% solids content, d.sub.98(wt)=1.0 μm, d.sub.50(wt)=0.37 μm Calcium carbonate 3: Ground natural calcium carbonate, suspension, 60% solids content, d.sub.98(wt)=0.8 μm, d.sub.50(wt)=0.3 μm

(39) Paper Substrates Paper 1: Uncoated, Metsä Board Kemi Liner, 135 g/m.sup.2 Paper 2: Uncoated, Rieger SpreeWhite C, 125 g/m.sup.2 Paper 3: Pre-coated, Metsä Board Kemiart Brite 135 g/m.sup.2

(C) EXAMPLES

(40) The following examples are not to be construed to limit the scope of the claims in any manner whatsoever.

Example 1—General Procedures for Preparing Metal Complexes

(41) Different possibilities were tested to synthesize solid metal complexes used in the present invention.

Example 1A (Calcium-NTA Complex)

(42) Modified procedure of E. R. Souaya et al., Molecules 2000, 5, 1121-1129: Nitrilotriacetic acid (0.38 mol), ground natural calcium carbonate (0.38 mol, d.sub.50(wt)=1.7 μm, d.sub.98(wt)=5 μm) and sodium carbonate (0.19 mol, Sigma Aldrich #S6014) were added in this order to 3.8 litres of H.sub.2O. The suspension was heated to 90° C. for 3 h. The volume was concentrated by heating and the reaction mixture was then allowed to cool down to 53° C. Subsequently, 0.5 litres ethanol (96 vol %) were added. The mixture was cooled down to room temperature and the suspension was filtrated (Whatman, grade 5, pore size 2.5 μm, d=185 mm, #1005-185). The filter cake was washed twice with 400 mL ethanol (96 vol %) and dried at 105° C. over night to obtain the calcium complex as a white powder.

Example 1B (Calcium-NTA Complex)

(43) Nitrilotriacetic acid trisodium salt monohydrate (1 mole equivalent) was dissolved in water with at a solids content of 40 wt %. To this solution was added calcium chloride or calcium nitrate as a solution (1 to 3 mole equivalent, 35 wt % in H.sub.2O) under stirring and a white solid was formed. The obtained homogenous suspension was then used directly for preparing the coating formulations of the present invention, or the solid was filtered and dried in an oven at 110° C. for 24 h to obtain the calcium complex as a white powder.

(44) The solid may be ground with a ZM 200 machine from the company Retsch GmbH (Germany).

(45) A SEM picture of a calcium-NTA complex obtainable according to Example 1B (calcium chloride was used as calcium source) is shown in FIG. 1.

(46) Parameters of Different Metal Complexes

(47) The following metal complexes were analyzed exemplarily to confirm the chemical composition:

(48) TABLE-US-00001 Calcium-NTA Tricalcium citrate Origin Example 1A Commercial (#M7090) ISO brightness R457 (%) 95.4 97.0 Solubility (g/100 mL) 2.38 1.7 Particle size d.sub.98(vol) < 6 μm d.sub.90(νo1) < 70 μm Moisture (Karl Fischer) 2.11% 13.03% Elemental analysis Calcd. Found Calcd. Found C (%) 28.08 27.71 25.15 25.96 N (%) 5.46 5.33 — <0.2 H (%) 2.58 2.57 3.20 3.07

Example 2—Preparation of Coating Formulations

Example 2A

(49) Polyvinyl alcohol (BF-04; 465.0 g, 25%, 36.8 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic heating bath and Eurostar laboratory stirrer (1 000 rpm) from IKA Labortechnik. At the same time calcium-NTA complex was prepared by adding a CaCl.sub.2 solution (166.0 g, 35%, 18.1 wt % based on total amount of dry material) to a NTA trisodium solution (363.0 g, 40%, 45.1 wt % based on total amount of dry material). After formation of the white precipitate, the suspension was added to the polyvinyl alcohol and the solution was stirred at room temperature for 15 min before using it in the coating machine.

Example 2B

(50) Polyvinyl alcohol (BF-04; 115.0 g, 25%, 5.4 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 2 (803.0 g, 60%, 88.5 wt % based on total amount of dry material). At the same time, calcium-NTA complex was prepared by adding a Ca(NO.sub.3).sub.2 solution (44.0 g, 35%, 2.8 wt % based on total amount of dry material) to a NTA trisodium solution (45.0 g, 40%, 3.3 wt % based on total amount of dry material). After formation of the white precipitate, the suspension was added to the polyvinyl alcohol/calcium carbonate and the mixture was stirred at room temperature for 15 min before using it in the coating machine.

Example 2C

(51) Polyvinyl alcohol (BF-04; 116.0 g, 25%, 5.5 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 2 (808.0 g, 60%, 89.8 wt % based on total amount of dry material). At the same time, calcium-NTA complex was prepared by adding a CaCl.sub.2 solution (21.1 g, 35%, 1.4 wt % based on total amount of dry material) to a NTA trisodium solution (45.0 g, 40%, 3.3 wt % based on total amount of dry material). After formation of the white precipitate, the suspension was added to the polyvinyl alcohol/calcium carbonate and the mixture was stirred at room temperature for 15 min before using it in the coating machine.

Example 2D (Comparative)

(52) Polyvinyl alcohol (BF-04; 114.0 g, 25%, 5.3 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 3 (793.7 g, 60%, 86.1 wt % based on total amount of dry material). Commercial tricalcium citrate (12.4 g, 100%, 2.2 wt % based on total amount of dry material) was added under stirring. Finally, Catiofast (81.0 g, 44%, 6.4 wt % based on total amount of dry material) was added and the solution was stirred at room temperature for 15 min before using it in the coating machine.

Example 2E (Comparative)

(53) Polyvinyl alcohol (BF-04; 119.0 g, 25%, 5.3 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 3 (832.0 g, 60%, 86.1 wt % based on total amount of dry material). Commercial tricalcium citrate (50.0 g, 100%, 8.6 wt % based on total amount of dry material) was added under stirring. The solution was stirred at room temperature for 15 min before using it in the coating machine.

(54) Precoating Formulation Used for Coatings of Examples 2F/2G

(55) To calcium carbonate 1 (626 kg, 72%, 90.5 wt % based on total amount of dry material) was added Litex PX 9460 (90 kg, 45%, 9.0 wt % based on total amount of dry material). The solution was stirred at room temperature and Rheocarb™ 121 (9 kg, 25%, 0.5 wt % based on total amount of dry material) was added. The mixture was stirred for 15 min at room temperature before using it on the coating machine.

Example 2F (Comparative)

(56) Polyvinyl alcohol (BF-04; 181 kg, 25.5%, 54.6 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min. An aqueous suspension of tricalcium citrate (96 kg, 40%, 45.4 wt % based on total amount of dry material) was added to the polyvinyl alcohol under stirring condition. The solution was stirred at room temperature for 15 min before using it in the coating machine.

Example 2G (Comparative)

(57) Polyvinyl alcohol (BF-04; 42 kg, 25%, 5.7 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min. A tricalcium citrate suspension (11 kg, 40%, 2.3 wt % based on total amount of dry material) was added to calcium carbonate 1 (243 kg, 71.8%, 92.0 wt % based on total amount of dry material) previously added to polyvinyl alcohol. The resulting pH was 8.7 and therefore no adjustment was needed and the solution was stirred for 30 min at room temperature.

Example 2H (Comparative)

(58) Polyvinyl alcohol (BF-04; 110.0 g, 25%, 5.1 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 2 (808.0 g, 60%, 88.5 wt % based on total amount of dry material). At the same time, calcium EDTA complex was prepared by adding a CaCl.sub.2 solution (21.0 g, 35%, 1.3 wt.-% based on total amount of dry material) to an EDTA trisodium solution (69 g, 40%, 5.1 wt % based on total amount of dry material). The resulting mixture was added to the polyvinyl alcohol/calcium carbonate and the solution was stirred at room temperature for 15 min before using it in the coating machine.

Example 2I (Comparative)

(59) The composition of this coating formulation corresponds to a commercial coating formulation containing 2.5 parts by weight of calcium chloride per 100 parts calcium carbonate pigment as ink fixation agent.

(60) Polyvinyl alcohol (BF-04; 49.1 g, 25%, 5.3 wt % based on total amount of dry material) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 3 (341.3 g, 60%, 86.1 wt % based on total amount of dry material). Catiofast BP (34.9 g, 44%, 6.4 wt.-% based on total amount of dry material) and CaCl.sub.2 solution (14.6 g, 35%, 2.2 wt.-% based on total amount of dry material) were added subsequently in this order. The resulting mixture was stirred at room temperature for 15 min before using it in the coating machine.

(61) The chemical composition of the coating formulations prepared in Examples 2A-2I is summarized in Tables 1 and 2 hereinbelow.

(62) TABLE-US-00002 TABLE 1 Coating formulations (pigment-based coatings only) Parts by weight Ex. Ex. Ex. 2D Ex. 2E Ex. 2G Ex. 2H Ex. 2I 2B 2C (comp) (comp) (comp) (comp.) (comp.) Calcium carbonate 1 100 Calcium carbonate 2 100 100 100 Calcium carbonate 3 100 100 100 Tricalcium citrate 2.6 Tricalcium citrate 2.6 10 NTA 3.75 3.75 EDTA 5.75 CaCl.sub.2 1.5 1.5 2.5 Ca(NO.sub.3).sub.2 2.2 PVA (BF-04) 6.0 6.0 6.0 6.0 6.0 5.75 6 Catiofast BP 7.5 7.5

(63) TABLE-US-00003 TABLE 2 Coating formulations (all coatings) wt % based on total solids content Ex. Ex. Ex. Ex. 2D Ex. 2E Ex. 2F Ex. 2G Ex. 2H Ex. 2I 2A 2B 2C (comp) (comp) (comp) (comp) (comp.) (comp.) Calcium carbonate 1 92.0 Calcium carbonate 2 88.5 89.8 88.5 Calcium carbonate 3 86.1 86.1 86.1 Tricalcium citrate 45.4 2.3 Tricalcium citrate 2.2 8.6 Nitrilotriacetic 45.1 3.3 3.3 acid trisodium salt monohydrate (NTA salt) EDTA 5.1 CaCl.sub.2 18.1 1.4 1.3 2.2 Ca(NO.sub.3).sub.2 2.8 PVA (BF-04) 36.8 5.4 5.5 5.3 5.3 54.6 5.7 5.1 5.3 Catiofast BP 6.4 6.4

Example 3—Coating Properties and Trials

Examples 3A-3E, Example 3H (Comparative)

(64) The coating formulations prepared in Examples 2A-2E and 2H were first characterized and then applied to different paper substrates. Coating experiments where performed on a Webcoater from Durrer (Switzerland) equipped with a blade coat head. Blades were obtained from Durrer (Switzerland) and were characterized by a thickness of 0.3 mm with a pre-grinding angle of 20°. The machine is known to the skilled person and is commonly used to coat paper.

Examples 3F and 3G (Comparative)

(65) The coating formulations prepared in Examples 2F and 2G where first characterized and then applied to different paper substrates after applying the corresponding precoating formulation (see Example 2). Coating experiments where performed on a Valmet coating pilot machine at Keskuslaboratorio-Centrallaboratorium Ab (KCL), Espoo, Finland, equipped with a blade coat head. Blades used on the KCL machine were provided by Uddeholm and were characterized by a thickness of 0.381 mm with a pre-grinding angle of 40°. The machine is known to the skilled person and is commonly used to coat paper.

Example 3I (Comparative)

(66) The physical properties of the coating prepared in Example 2I were characterized as for the coatings of Examples 2A-2H.

(67) The coating properties and results of the coating trials of Examples 3A-3I are summarized in Table 3 hereinbelow.

(68) TABLE-US-00004 TABLE 3 Coating properties and results Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. 3A 3B 3C 3D (comp) 3E (comp) 3F (comp) 3G (comp) 3H (comp.) 3I (comp.) Paper substrate 2 2 2 3 3 1 1 1 — Pre-coating — — — — — (2F) (2G) — — Coating 2A 2B 2C 2D 2E 2F 2G 2H (comp.) 2I (comp.) Solids Content (%) 30.9 54.4 53.0 55.0 57.7 64.2 27.5 64.2 63.9 54.4 47.35 Viscosity (mPa .Math. s) 1950 900 1300 880 340 735 300 735 280 166 2500 pH 9.2 9.7 9.6 8.1 8.4 8.4 7.5 8.4 8.7 9.0 6.47 Temperature (° C.) 42 27 30 27 27 23 23 23 23 33 — Coater speed (m/min) 20 20 20 20 20 700 700 700 700 20 — Blade thickness (mm) 0.3 0.3 0.3 0.3 0.3 0.381 0.381 0.381 0.381 0.3 — Blade load (bar) 0.6 0.6 0.6 0.6 0.6 0.53 0.3 0.53 1.65 0.6 — Beam angle (°) 20 20 20 20 20 50 50 50 50 20 — Coating weight (g/m.sup.2) 3.4 10.5 9.5 7.7 6.7 10 3.0 10 6.0 6.5 — Final moisture (%) 5.7 5.7 5.6 6.8 6.8 5.5 5.2 5.5 5.5 6.2 —

(69) As can be gathered from Table 2 above, the inventive coating formulations used in Examples 3A-3G show viscosities within a range of from 280 to 1 950 mPa.Math.s and are thus well suitable for use as coating formulations. The coating formulation used in Example 3H (comparative) has a low viscosity of only 166 mPa.Math.s which may negatively affect the coating performance and coating results. The coating formulation characterized in Example 3I (comparative) shows a high viscosity although the solids contents is comparably low which may also affect the coating performance.

Example 4—Printing Trials

(70) The papers produced according to Examples 3A-3H were printed on HP Envy 5540 using original HP pigment based ink cartridges at a resolution of 4 800×1 200 dpi. The reference paper used for comparison was Zweckform Inkjet 2578.

(71) Ink Density

(72) The ink density measurements of Examples 4A-4H (printed papers of Examples 3A-3H) are summarized in Table 3 hereinbelow.

(73) TABLE-US-00005 Ex. Ex. Ex. Ex. 4D Ex. 4E Ex. 4F Ex. 4G Ex. 4H Ref. 4A 4B 4C (comp) (comp) (comp) (comp) (comp.) Paper Inkjet Ex. Ex. Ex. Ex. 3D Ex. 3E Ex. 3F Ex. 3G Ex. 3H 2578 3A 3B 3C (comp) (comp) (comp) (comp) (comp.) Cyan 1.11 0.94 0.90 0.91 1.10 1.04 1.08 0.78 0.96 Magenta 1.29 1.31 0.78 0.80 1.05 0.91 1.26 0.78 0.90 Yellow 1.33 1.18 0.78 0.79 1.04 0.87 1.22 0.72 0.81 Black 1.45 1.30 1.69 1.56 1.84 1.93 1.26 1.39 1.66

(74) Printing Results

(75) Some printing results are shown in FIG. 2a-2d at 200× magnification. Compared to the printed reference paper (FIG. 2a), the papers printed in Example 4E (FIG. 2b) and Example 4F (FIG. 2c) showed a higher ink fixation capacity.

(76) The paper in Example 4A was made from uncoated paper and the coating did not contain a calcium carbonate pigment. The ink density (FIG. 2d) was still comparable to the reference paper (FIG. 2a).

Example 5—Comparison Test

(77) The paper substrate used in this example was Paper 1. The coating compositions were prepared according to the procedures described above. Thus, for the precoating formulation, to calcium carbonate 1 was added Litex PX 9460. The solution was stirred at room temperature and Rheocarb™ 121 was added. The mixture was stirred for 15 min at room temperature. For the preparation of the respective compositions, polyvinyl alcohol (BF-04) was in a first phase dissolved in water at 90° C. for 30 min using a HB4 basic and Eurostar (1 000 rpm) from IKA Labortechnik. The solubilized polyvinyl alcohol was added subsequently to calcium carbonate 3. Sodium NTA or Sodium EDTA was added under stirring. Finally, CaCl.sub.2 solution was added and the solution was stirred at room temperature for 15 min before using it in the coating machine.

(78) TABLE-US-00006 TABLE 4 Coating formulations (pigment-based coatings) Recipes Precoating 5A 5B (comp) Solid (parts by (parts by (parts by Components content weight) weight) weight) Calcium carbonate 1 71.7 100.0 Litex PX 9460 50.0 10.0 Rheocarb 121 24.0 0.5 Calcium carbonate 3 60.0 100.0 100.0 PVA BF-04 25.0 6.0 6.0 Sodium NTA 40.0 5.65 Sodium EDTA 40.0 5.68 CaC12 35.0 3.21 1.57 Solid content 68.3 53.7 53.5 Viscosity @ 100 rpm 1860 2680 180 pH 8.5 9.0 8.9 Temperature (° C.) 27 28 29 Coater speed (m/min) 20 20 20 Blade thickness (mm) 0.30 0.30 0.30 Blade load (bar) 1.25 1.10 1.00 Beam angle (°) 20 20 20 Coating weight (g/m.sup.2) 10.7 10.5 9.6 Final moisture (%) 5.5 6.3 6.2

(79) The ink density of the corresponding coatings was analyzed with a SpectroDens from the company Techkon GmbH as described above. The results are shown in table 5 below.

(80) TABLE-US-00007 TABLE 5 Base paper Precoating V5A V58 (comp) Cyan 1.17 0.80 0.95 0.92 Magenta 1.44 0.68 0.88 0.89 Yellow 1.42 0.71 0.82 0.78 Black 1.56 1.94 1.79 1.71

(81) It becomes evident that the coating formulation containing the NTA-ligand provides better ink intensities than the corresponding formulation containing EDTA.