COATING FORMULATION FOR A DIGITAL PRINT MEDIUM

Abstract

The present invention relates to a cationic coating formulation, which contains beside water a specific anionic ultrafine ground calcium carbonate, a non-ionic polyvinyl alcohol, a water-soluble salt of a divalent metal ion, a dispersing agent and a cationic polymer. Said coating formulation according to the present invention can be used for manufacturing a digital print medium, preferably an inkjet print medium.

Claims

1. A cationic coating formulation consisting of: water, at least one anionic ultrafine ground calcium carbonate, at least one non-ionic polyvinyl alcohol, at least one water-soluble salt of a divalent metal ion, at least one dispersing agent, and at least one cationic polymer, wherein the at least one anionic ultrafine ground calcium carbonate, when in the form of a compacted bed, has a monomodal pore size distribution, a volume defined pore size polydispersity expressed as full width at half maximum (FWHM) from 40 to 80 nm, and a volume defined median pore diameter from 30 to 80 nm, and wherein the cationic coating formulation has a solid content in the range from 10 to 80 wt.-%, based on the total weight of the cationic coating formulation.

2. The cationic coating formulation according to claim 1, consisting of the following components 30-60 wt.-% of water, 30-60 wt.-% of the at least one anionic ultrafine ground calcium carbonate, 0.1-3.0 wt.-% of the at least one non-ionic polyvinyl alcohol, 0.1-4.0 wt.-% of the at least one dispersing agent, 0.1-4.0 wt.-% of the at least one water-soluble salt of a divalent metal ion, and 0.1-5.0 wt.-% of the at least one cationic polymer, based on the total weight of the cationic coating formulation, and wherein the total sum of the components in the cationic coating formulation is 100 wt.-%.

3. The cationic coating formulation according to claim 1, wherein the at least one water-soluble salt of a divalent metal ion is selected from the group consisting of CaCl.sub.2, CaBr.sub.2, CaI.sub.2, Ca(NO.sub.3).sub.2, MgCl.sub.2, MgBr.sub.2, MgI.sub.2, and Mg(NO.sub.3).sub.2.

4. The cationic coating formulation according to claim 1, wherein the at least one dispersing agent is selected from a polyacrylate-based dispersing agent, a polyphosphate-based dispersing agent, carboxymethyl cellulose or a salt thereof, or a mixture thereof.

5. The cationic coating formulation according to claim 1, wherein the at least one cationic polymer is polydiallyldimethylammonium chloride (PolyDadmac) or a polyethyleneimine.

6. The cationic coating formulation according to claim 1, wherein the at least one anionic ultrafine ground calcium carbonate is in the form of an aqueous suspension having a solids content in the range from 10 to 99.9 wt.-%, based on the total weight of the aqueous suspension.

7. The cationic coating formulation according to claim 1, wherein the cationic coating formulation has a solid content in the range from 20 to 70 wt.-%, based on the total weight of the cationic coating formulation.

8. The cationic coating formulation according to claim 1, consisting of the following components: 40-60 wt.-% of water, 30-56 wt.-% of the at least one anionic ultrafine ground calcium carbonate, 0.5-2.0 wt.-% of the at least one non-ionic polyvinyl alcohol, 0.3-1.0 wt.-% of the at least one dispersing agent, 0.1-1.5 wt.-% of the at least one water-soluble salt of a divalent metal ion, and 0.1-2.5 wt.-% of the at least one cationic polymer, based on the total weight of the cationic coating formulation, wherein the total sum of the components in the cationic coating formulation is 100 wt.-%, wherein the at least one dispersing agent is a polyacrylate-based dispersing agent, wherein the at least one water-soluble salt of a divalent metal ion is CaCl.sub.2, and wherein the at least one cationic polymer is polydiallyldimethylammonium chloride (PolyDadmac).

9. A process for the preparation of the cationic coating formulation according to claim 1 comprising the steps of: a) providing water, b) providing at least one anionic ultrafine ground calcium carbonate, c) providing at least one non-ionic polyvinyl alcohol, d) providing at least one water-soluble salt of a divalent metal ion, e) providing at least one dispersing agent, f) providing at least one cationic polymer, and g) combining the components of steps a) to f) to form a cationic coating formulation.

10. A process for preparing a digital print medium, comprising: i) providing a substrate having a first and a reverse side, ii) applying the cationic coating formulation according to claim 1 on at least one side of the substrate to form at least one coating layer.

11. The process according to claim 10, wherein the substrate is, an optionally pretreated and/or calendered, paper, paperboard, containerboard, textile, metal, concrete, wood or wood-based board, or a polymer.

12. The process according to claim 10, wherein the substrate is a pre-coated substrate.

13. The process according to claim 10, wherein the digital print medium comprises recycled material.

14. The process according to claim 13, wherein the recycled material is present in the digital print medium in an amount of 50 to 99.5 wt.-%, based on the total weight of the digital print medium.

15. The process according to claim 10, wherein the cationic coating formulation increases the brightness of the digital print medium by 10%, as measured according to ISO R457 (Tappi452) and DIN 6167.

16. The process according to claim 10, wherein no primer is added prior and/or after the application of the cationic coating formulation onto the digital print medium.

17. The cationic coating formulation according to claim 1, wherein the at least one water-soluble salt of a divalent metal ion is CaCl.sub.2.

18. The cationic coating formulation according to claim 1, wherein the at least one anionic ultrafine ground calcium carbonate is in the form of an aqueous suspension having a solids content in the range from 30 to 80 wt.-%, based on the total weight of the aqueous suspension.

19. The cationic coating formulation according to claim 1, wherein the at least one anionic ultrafine ground calcium carbonate is in the form of an aqueous suspension having a solids content in the range from 50 to 70 wt.-%, based on the total weight of the aqueous suspension.

20. The cationic coating formulation according to claim 1, wherein the cationic coating formulation has a solid content in the range from 30 to 60 wt.-%, based on the total weight of the cationic coating formulation.

21. The process according to claim 10, wherein the substrate is paper, paperboard, or containerboard.

22. The process according to claim 10, wherein the cationic coating formulation increases the brightness of the digital print medium by 50%, as measured according to ISO R457 (Tappi452) and DIN 6167.

23. The process according to claim 13, wherein the recycled material is present in the digital print medium in an amount of 70 to 98 wt.-%, based on the total weight of the digital print medium.

Description

DESCRIPTION OF THE FIGURES

[0174] FIG. 1 shows an inkjet print image on a digital print medium coated with the inventive cationic coating formulation (printed on HP Web Press T 1100S).

[0175] FIG. 2 shows an inkjet print image on a prior art digital print medium (printed on HP Web Press T 1100S).

EXPERIMENTAL SECTION

1. Measurement Methods

[0176] In the following, materials and measurement methods implemented in the examples are described.

Particle Size

[0177] The particle size distribution of the pigment particles was measured using a Sedigraph 5120 from the company Micromeritics, USA. The method and the instruments are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonics.

Solids Content of an Aqueous Suspension

[0178] The suspension solids content (also known as “dry weight”) was determined using a Moisture Analyser HR73 from the company Mettler-Toledo, Switzerland, with the following settings: temperature of 120° C., automatic switch off 3, standard drying, 5 to 20 g of suspension.

Brookfield Viscosity

[0179] The Brookfield viscosity of the liquid coating compositions was measured after one hour of production and after one minute of stirring at room temperature at 100 rpm by the use of a Brookfield viscometer type RVT equipped with an appropriate spindle.

Specific Surface (BET) Measurement

[0180] The specific surface area (in m.sup.2/g) of the mineral filler was determined using nitrogen and the BET method, which is well known to the skilled man (ISO 9277:2010). The total surface area (in m.sup.2) of the mineral filler was then obtained by multiplication of the specific surface area by the mass (in g) of the mineral filler. The method and the instrument are known to the skilled person and are commonly used to determine the specific surface of fillers and pigments.

pH Measurement

[0181] The pH was 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 was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Aldrich). The reported pH values were the endpoint values detected by the instrument (the endpoint was when the measured signal differs by less than 0.1 mV from the average over the last 6 seconds).

Formation of a Compacted Bed

[0182] A compacted bed or tablet formulation of a pigment was formed in a wet tablet press apparatus by applying a constant pressure (usually 15 bar) to the pigment suspension or slurry for several hours such that water is released by filtration through a fine 0.025 μm filter membrane resulting in a compacted bed or tablet of the pigment with a diameter of about 4 cm and a thickness of 1 to 1.5 cm. The obtained tablets can be divided and fashioned into suitable sample configurations for subsequent analysis. The apparatus used is shown schematically in Ridgway et al. “Modified calcium carbonate coatings with rapid absorption and extensive liquid uptake capacity” (Colloids and Surfaces A: Physiochem. and Eng. Asp. 2004, 236(1-3), 91-102). The tablets were removed from the apparatus and dried in an oven at 60° C. for 24 hours.

Porosity Measurements

[0183] Portions of a compacted bed or tablet formulation were characterized by mercury porosimetry for porosity, intruded total specific void volume, and pore size distribution using a Micromeritics Autopore IV mercury porosimeter. The maximum applied pressure of mercury was 414 MPa, equivalent to a Laplace throat diameter of 0.004 μm. The data were corrected using Pore-Comp (P. A. C. Gane et al. “Void Space Structure of Compressible Polymer Spheres and Consolidated Calcium Carbonate Paper-Coating Formulations” (Industrial and Engineering Chemistry Research 1996, 35 (5), 1753-1764) for mercury and penetrometer effects, and also for sample compression. By taking the first derivative of the cumulative intrusion curves the pore size distributions based on equivalent Laplace diameter, inevitably including pore-shielding, was revealed. Volume defined median pore diameter was calculated from the mercury intrusion curve, and volume defined pore size polydispersity as full width at half maximum (FWHM) is calculated from the pore size distribution curve.

Permeability Measurements

[0184] According to Ridgway et al. “A new method for measuring the liquid permeability of coated and uncoated papers and boards” (Nordic Pulp and Paper Research Journal 2003, 18(4), 377-381) for measuring the permeability, measurement samples were prepared by placing a cuboidal piece of a tablet (compacted bed) structure having an area of 15 mm×15 mm and a height of 10 mm into a PTFE-mould and pouring the resin Technovit 4000 (Heraeus GmbH, Wherheim/Ts, Germany) around it to produce a sample disk having a diameter of 30 mm. The quickly rising viscosity of the chosen curing resin results in a penetration of approximately 1 mm locally at the outer boundaries of the sample. This penetration depth is clearly visible because of the opacity change at the edge of the sample and can, therefore, be calibrated. The open area of the porous sample, i.e. that free from resin, is evaluated so that the permeable cross-sectional area can be established. The sample discs are placed in a dish containing the probe liquid in order to saturate the void network of the sample before placing in the apparatus. Hexadecane was used in the experiments with density, ρ=773 kgm.sup.−3 and viscosity, η=0.0034 kgm.sup.−1 s.sup.−1 to avoid any interaction with synthetic or natural binders if present. The sample disc is then placed in a specially constructed pressure cell. The cell design used for the pressurised permeability experiments is described in Ridgway et al. (Nordic Pulp and Paper Research Journal 2003, 18(4), 377-381). Gas over-pressure is supplied from a nitrogen bottle. The pressure cell is fixed over a Mettler Toledo AX504 microbalance and a PC samples the balance data by a software. A drop captor device in the base of the cell guides the permeated liquid drops to the outlet. The whole chamber below the position of the sample was pre-wetted with the liquid so that each drop leaving the sample causes a drop to fall into the sampling dish.

Absorption Rate Measurements

[0185] According to Schoelkopf et al. “Measurement and network modelling of liquid permeation into compacted mineral blocks” (Journal of Colloid and Interface Science 2000, 227(1), 119-131) for the measurement of the “absorption rate”, compacted bed samples were coated with a thin barrier line of silicone around the base of the vertical edges arising from the basal plane to reduce artefacts caused by the wetting of their outer surfaces. The remainder of the outer planes were not coated, to allow for the free movement of displaced air or liquid during absorption, and to minimise any interaction between the silicone and the absorbed liquid. Once the sample is lowered to contact the absorbing fluid source, the weight loss from the dish is continually recorded using an automated microbalance, namely a PC-linked Mettler Toledo AX504 balance with a precision of 0.1 mg, capable of 10 measurements per second, accounting for any evaporation if present. When the recorded weight is constant, indicative of absorption-saturation, the measurement is complete. Knowing the sample weight before and after the absorption measurement allows the intruded volume per gram of sample to be calculated. (Dividing the weight difference by the density of the liquid gives the volume intruded into the sample, and hence the volume per gram of sample).

Polyelectrolyte Titration (PET)

[0186] The polyelectrolyte content in the aqueous suspension is determined using a Memotitrator of the Mettler DL 5x or Tx series equipped with a Phototrode DP 660 or DP5 commercialised by Mettler-Toledo, Switzerland. The measurements of the polyelectrolyte content was carried out by weighing a sample of the dispersed calcium carbonate or the cationic coating formulation into a titration vessel and diluting said sample with deionized water up to a volume of approximately 40 ml. Subsequently, 10 ml of 0.01 M cationic poly(dimethyldiallylammonium chloride) (PolyDADMAC; obtained from Merck (former Sigma-Aldrich)) are slowly added under stirring into the titration vessel within 5 min. and then the content of the vessel is stirred for another 20 min. Afterwards the suspension is filtered through a 0.2 μm mix-ester membrane filter (∅ 47 mm) and washed with 5 ml of deionized water. The thus obtained filtrate is diluted with 5 ml of acetate buffer pH 4.65 (Sigma-Aldrich/Merck, Switzerland) and then 0.01 M of a potassium polyvinylsulfate (KPVS; obtained from SERVA Feinbiochemica, Heidelberg) solution is added slowly to the filtrate to titrate the excess of cationic reagent. The endpoint of titration is detected by a Phototrode DP660 or DP5, which is adjusted to 1200 to 1800 mV in deionized water, prior to such measurement. The charge calculation is carried out according to the following evaluation:

[00001]$Q_{\mathrm{atro}}=\frac{\left(\left(V_{PDADMAC}*t_{PDADMAC}\right)-V_{\mathrm{KPVS}}\right)*\left(-1000\right)}{m_{\mathrm{sample}}*FS}\left[\mu \mathrm{Val}/g\right]$$w_{atro}=\frac{Q_{atro}}{K_{DP}*100}\left[\%\right]$

[0187] Calculation of the exact weight-in quantity:

[00002]$m_P=\frac{60}{w_{DP}*K_{\mathrm{DP}}*FS}$

[0188] Correction calculation for consumption of 4 mL:

[00003]$m_{4\mathrm{ml}}=\frac{m_1*6}{\left(10-V_{\mathrm{KPVS},1}\right)}$

[0189] Abbreviations [0190] m.sub.P=Weight-in quantity of sample [g] [0191] w.sub.DP=Dispersing agent content in [%] [0192] K.sub.DP=Dispersing agent constant [μVal/0.1 mg dispersing agent] [0193] FS=Solids content [%] [0194] V.sub.PDADMAC=Volume PolyDADMAC [ml] [0195] V.sub.KPVS=Volume KPVS [ml] [0196] t.sub.PDADMAC=Titer PolyDADMAC [0197] Q=Charge [μVal/g] [0198] w.sub.atro=Dispersing agent content atro [%] [0199] m.sub.1 =Sample weight of experiment to be optimised [g] [0200] V.sub.KPVS,1=experimental consumption KPVS [ml] of experiment to be optimised

2. Examples

[0201] The following components were used to prepare the cationic coating formulation applied to the substrate.

[0202] Substrate: Recycled fluting of the company SAICA, Spain, sold under the name “Saica medium” with a basis weight of 127 g/m.sup.2.

[0203] Pigment 1: Naturally ground calcium carbonate (d.sub.50: 250 nm, BET: 24.8 m.sup.2/g)

[0204] Pigment 2: Naturally ground calcium carbonate (d.sub.50: 0.6 μm and a d.sub.98: 4 μm), commercially available from Omya AG, Switzerland

[0205] Binder: Polyvinyl alcohol BF-08, sold by the company CCP Taiwan

[0206] Water-soluble divalent metal ion salt: Calcium chloride, sold by the company Sigma-Aldrich

[0207] Dispersing agent: Sodium polyacrylate/sodium phosphate based dispersing agent, which is a blend of a partially neutralized sodium polyacrylate (with a molecular weight Mw equal to 12 000 Dalton, measured by GCP) and sodium phosphates, produced by mixing an aqueous solution of 40 wt.-% sodium polyacrylate and a phosphoric acid (85 wt.-% in H.sub.2O) in a ratio 2:1 in respect to active dry weight of each additive

[0208] Cationic polymer: Catiofast® BP, a water-soluble polyDADMAC, sold by the company BASF

[0209] Table 1 shows the properties of the pigment used to produce the cationic coating formulations.

TABLE-US-00001 TABLE 1 Pigment properties. intruded total Median specific specific void pore surface area Charge of volume diameter FWHM (BET) d.sub.50 pigment Pigment [cm.sup.3/g] [μm] [μm] [m.sup.2/g] [μm] Modality [μVal/g] Pigment 1 0.303 0.06 0.042 24.8 0.250 Mono- −45.3 (±0.5) (inventive) modal Pigment 2 0.234 0.10 0.038  8.7 0.6  Mono- −21.3 (±0.7) (prior art) modal

Preparation of Cationic Coating Formulations

[0210] The pigments 1 and 2 were used to prepare different cationic coating formulations (see Table 2) to demonstrate the invention.

TABLE-US-00002 TABLE 2 Cationic coating formulations. Coating formulation A Coating formulation B (prior art) (inventive) [wt.-%, based on total [wt.-%, based on total weight of the weight of the formulation] formulation] Pigment 1 — 54.4 Pigment 2 65.8 — Binder 1.1 1.1 Divalent metal ion 0.2 0.2 salt Dispersing agent 0.3 0.6 Cationic polymer 1.0 1.0 Water 31.6 42.7

[0211] The cationic coating formulations were prepared by adding the compounds into an appropriate container: [0212] 1. Water [0213] 2. Pigment [0214] 3. Dispersing agent [0215] 4. Binder [0216] 5. Cationic polymer, and [0217] 6. Divalent metal ion salt [0218] and stirring the resulting mixture with a Disperlux Pendraulik Dissolver ID100 at a speed of 4500 rpm at room temperature.

[0219] The properties of the prepared coating formulations are listed in Table 3.

TABLE-US-00003 TABLE 3 Properties of the coating formulations Coating Coating formulation A formulation B (prior art) (inventive) Charge (measured by PET) +138.4 (± 8.8) +113.2 (± 7.8) Solids content (wt.-%) 52.3 52.3

Preparation of Coated Paper

[0220] Substrate: Top ply sized testliner III with a basis weight (grammage) of 125 g/m.sup.2, commercially available from Hamburger Pitten GmbH & Co. KG, Austria.

[0221] The substrate was subjected to a softnip calandering to a Bendtsen roughness of 300 ml/min using an OptiCalender Soft of the company Valmet, Finland.

[0222] Then, the calendered substrate was pre-coated with the following pre-coating formulation.

TABLE-US-00004 TABLE 4 Composition of the pre-coating formulation Pre-coating formulation [wt.-%, based on total weight of the formulation] Pigment 2 44.82 Pigment 3 11.96 Binder 7.68 Rheology modifier (Lumiten l- 0.13 SC) Rheology modifier (Sterocoll 0.13 DF3x) Water 35.27 Solids content % 68.0

[0223] Binder: Litex PX 9464 (anionic carboxylated styrene/butadiene copolymer), commercially available from Synthomer Deutschland GmbH, Germany.

[0224] Pigment 3: TiO.sub.2; supplied by Hunstman as SR3 slurry,

[0225] Rheology modifiers: Sterocoll DF3x (acrylate copolymer) and Lumiten I-SC (Solution of sodium sulphosuccinate), both commercially available from BASF, Germany

[0226] The pre-coating was applied with an amount of 15 g/m.sup.2 by an OptiCoat layering curtain coater of the company Valmet, Finland. The application speed was 1000 m/min, and the resulting pre-coated substrate was air hood dried on the coating machine to and end moisture content of 7.5%.

[0227] Then, coating formulation A (prior art) or coating formulation B (inventive) was coated on the pre-coated substrate using an OptiCoat jet blade coater of the company Valmet, Finland. The respective coating formulation was applied in an amount of 15 g/m.sup.2. The application speed was 1000 m/min, and the resulting coated substrate (print medium) was air hood dried on the coating machine to an end moisture content of 7.5%.

Print Evaluation

[0228] The obtained coated substrate (print medium) was then subjected to inkjet printing using a HP Web Press T 1100S and standard HP printing inks.

[0229] The print quality of the obtained inkjet printed medium was analysed by visual inspection the presence or absence of feathering. Feathering is a printing defect characterized by ink spreading at the edges of a printed area, typically caused by irregularities in ink distribution and/or absorption on the surface of the paper.

[0230] First the obtained printed medium was scanned and 60-times magnified. The results of the magnification are shown in FIG. 1 (print medium coated with inventive coating formulation B) and FIG. 2 (print medium coated with prior art coating formulation A).

[0231] In view of the print images shown in FIG. 1. and FIG. 2, it becomes evident that the print medium coated with the inventive cationic coating formulation B does not show any feathering, whereas the print medium coated with the prior art coating formulation A shows feathering.