CALCIUM CARBONATE FOR ROTOGRAVURE PRINTING MEDIUM

Abstract

The present invention relates to the field of rotogravure printing, and more specifically to a rotogravure printing medium, a coating composition for rotogravure printing media, a method for producing such a rotogravure printing medium and an use of the rotogravure printing medium in a printing application, preferably in rotogravure printing using electrostatic assist (ESA).

Claims

1. Rotogravure printing medium comprising a) a substrate having a first side and a reverse side, and b) a coating layer being in contact with at least the first side of the substrate, wherein the coating layer consists of i) >50.0 to 100.0 parts by weight of at least one natural source of calcium carbonate, the at least one natural source of calcium carbonate comprises particles having a BET specific surface area of from 4.0 to 12.0 m.sup.2/g, measured by the BET nitrogen method, ii) 0.0 to 50.0 parts by weight of at least one further pigment suitable for papermaking, iii) 3.0 to 6.0 parts by weight of at least one synthetic binder, iv) 0.1 to 0.5 parts by weight of at least one stearate salt, v) optionally 1.0 to 1.5 parts by weight of at least one polysaccharide, vi) optionally 0.1 to 0.5 parts by weight of at least one thickener, and vii) optionally 0.2 to 3.0 parts by weight of at least one dispersing agent, wherein the sum of the at least one natural source of calcium carbonate and the at least one further pigment in the coating layer is 100.0 parts by weight.

2. Rotogravure printing medium according to claim 1, wherein the substrate is selected from paper, cardboard, containerboard, plastic, cellophane, textile, wood, metal, or concrete, preferably paper, cardboard, or containerboard.

3. Rotogravure printing medium according to claim 1, wherein the at least one natural source of calcium carbonate is dolomite and/or at least one natural ground calcium carbonate (NGCC), preferably the at least one natural ground calcium carbonate (NGCC) is selected from the group comprising marble, chalk, limestone and mixtures thereof.

4. Rotogravure printing medium according to claim 1, wherein the at least one natural source of calcium carbonate comprises particles a) having a BET specific surface area of from 5.0 to 10.0 m.sup.2/g, measured by the BET nitrogen method, and/or b) having a weight median particle size d.sub.50 of 2.5 m, preferably from 0.1 to 2.5 m, more preferably from 0.1 to 2.0 m, and most preferably from 0.5 to 2.0 m or from 0.2 to 1.5 m, as measured according to the sedimentation method or c) having a i) weight particle size d.sub.75 of 0.7 to 3.0 m, ii) weight median particle size d.sub.50 of 0.5 to 2.0 m, iii) weight particle size d.sub.25 of 0.1 to 1.0 m, as measured according to the sedimentation method, and/or d) consisting of calcium carbonate in an amount of 50.0 wt.-%, preferably of 90.0 wt.-%, more preferably of 95.0 wt.-% and most preferably of 97.0 wt.-%, based on the total dry weight of the natural source of calcium carbonate.

5. Rotogravure printing medium according to claim 1, wherein the at least one natural source of calcium carbonate consists of crumbles comprising dolomite and/or the at least one natural ground calcium carbonate (NGCC), and optionally the at least one further pigment suitable for papermaking.

6. Rotogravure printing medium according to claim 1, wherein a) the at least one further pigment suitable for papermaking is selected from the group comprising precipitated calcium carbonate (PCC), metal oxides such as titanium dioxide and/or aluminium trioxide, metal hydroxides such as aluminium tri-hydroxide, metal salts such as sulfates, silicates such as talc and/or kaolin and/or kaolin clay and/or mica, carbonates such as magnesium carbonate and/or gypsum, satin white and mixtures thereof, and/or b) the at least one synthetic binder is selected from the group comprising polyvinylalcohol, styrene-butadiene latex, styrene-acrylate latex, styrene-acrylic acrylonitrile latex, polyvinyl acetate latex and mixtures thereof, and is preferably a styrene-butadiene latex, and/or c) the at least one stearate salt is a stearate salt of a monovalent or divalent cation, preferably the stearate salt of a monovalent or divalent cation is selected from the group comprising sodium stearate, potassium stearate, calcium stearate, magnesium stearate, strontium stearate and mixtures thereof, more preferably the stearate salt of a monovalent or divalent cation is calcium stearate.

7. Rotogravure printing medium according to claim 1, wherein the coating layer consists of a) 51.0 to 100.0 parts by weight of the at least one natural source of calcium carbonate, and b) 0.0 to 49.0 parts by weight of the at least one further pigment suitable for papermaking.

8. Rotogravure printing medium according to claim 1, wherein the at least one polysaccharide and/or the at least one thickener and/or the at least one dispersing agent is/are present in the coating layer.

9. Rotogravure printing medium according to claim 1, wherein a) the at least one polysaccharide is selected from starch and/or guar, and/or b) the at least one thickener is selected from cellulosic derivatives, such as ethylhydroxylethyl cellulose and/or carboxymethyl cellulose, acrylic copolymers and mixtures thereof, and/or c) the at least one dispersing agent is a polyacrylate-based dispersing agent.

10. Rotogravure printing medium according to claim 1, wherein the coating layer has a coat weight from 1.0 to 50.0 g/m.sup.2, preferably from 2.0 to 40.0 g/m.sup.2, more preferably from 3.0 to 30.0 g/m.sup.2, and most preferably from 5.0 to 20.0 g/m.sup.2.

11. Rotogravure printing medium according to claim 1, wherein the rotogravure printing medium consists of the substrate and the coating layer being in contact with at least the first side of the substrate.

12. Coating composition for a rotogravure printing medium, the composition consisting of a) >50.0 to 100.0 parts by weight of at least one natural source of calcium carbonate, the at least one natural source of calcium carbonate comprises particles having a BET specific surface area of from 4.0 to 12.0 m.sup.2/g, measured by the BET nitrogen method, b) 0.0 to 50.0 parts by weight of at least one further pigment suitable for papermaking, c) 3.0 to 6.0 parts by weight of at least one synthetic binder, d) 0.1 to 0.5 parts by weight of at least one stearate salt, e) at least one aqueous solvent, f) optionally 1.0 to 1.5 parts by weight of at least one polysaccharide, g) optionally 0.1 to 0.5 parts by weight of at least one thickener, and h) optionally 0.2 to 3.0 parts by weight of at least one dispersing agent, wherein the sum of the at least one natural source of calcium carbonate and the at least one further pigment in the coating composition is 100.0 parts by weight.

13. The coating composition according to claim 12, wherein the coating composition has a solid content from 10.0 to 80.0 wt.-%, preferably from 30.0 to 75.0 wt.-%, more preferably from 40.0 to 70.0 wt.-%, and most preferably from 45.0 to 65.0 wt.-%, based on the total weight of the coating composition.

14. A method for producing a rotogravure printing medium comprising the steps of: a) providing a substrate having a first side and a reverse side as defined, and b) applying a coating composition as defined in claim 12 on at least the first side of the substrate to form a coating layer.

15. The method of claim 14, wherein the method further comprises step c) of drying the coating layer.

16. The method according to claim 14, wherein the coating composition is applied by high speed coating, metering size press, curtain coating, spray coating, or electrostatic coating, and preferably by high speed coating.

17. Use of a rotogravure printing medium as defined in claim 1 in a printing application, preferably in rotogravure printing using electrostatic assist (ESA).

Description

EXAMPLES

1. Measurement Methods

[0243] In the following, materials and measurement methods implemented in the examples are described.

[0244] Particle Size Distribution (Mass % Particles with a Diameter <X) and Weight Median Diameter (d.sub.50) of a Particulate Material

[0245] Weight grain diameter and grain diameter mass distribution of a particulate material were determined via the sedimentation method, i.e. an analysis of sedimentation behaviour in a gravitational field. The measurement was made with a Sedigraph 5120 or a Sedigraph 5100 of Micromeritics Instrument Corporation.

[0246] The method and the instrument are known to the skilled person and are commonly used to determine grain size 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 supersonics.

[0247] BET Specific Surface Area of a Material

[0248] Throughout the present document, the specific surface area (in m.sup.2/g) of a particulate material was determined using the BET method (using nitrogen as adsorbing gas), which is well known to the skilled man (ISO 9277:1995). The total surface area (in m.sup.2) of the particulate material is then obtained by multiplication of the specific surface area and the mass (in g) of the particulate material. The method and the instrument are known to the skilled person and are commonly used to determine the specific surface of particulate materials.

[0249] Solids Content of an Aqueous Suspension

[0250] 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.

[0251] Calcium Carbonate Content of a Particulate Material

[0252] For the measurement of the calcium carbonate content of a particulate material, about 10.000 grams of the dry sample (dried at 110 C. for 5 hours in an oven) were weighed in a flask/beaker and a small amount of demineralized water was added. Then, 40 mL of hydrochloric acid (25% p.a.) were added to the respective sample and after the CO.sub.2 development stopped, the mixture was boiled for about 5 min. After cooling down, the mixture was filtered through a 0.8 m cellulose-acetate filter and washed thoroughly. Then the filtrate was quantitatively rinsed to a volumetric flask with distilled water and filled up to 1000.0 ml at 20 C.

[0253] The thus obtained filtrate was then slowly titrated by pipetting 10.00 mL of the obtained filtrate (about 20 C.) into a Memotitrator-beaker and 1.0 g (0.2 g) of triethanolamine puris. and 3.0 g of MgSO.sub.47 H.sub.2O. The mixture was diluted with demineralized water up to 70 mL and then, just before the titration, 10.0 mL of 2N sodium hydroxide and 7 to 9 drops of a HHSNN-methanol solution (0.2 wt.-% of HHSNN in methanol) were added to the mixture. After the pre-dosing, the titrator stirred the mixture for 60 s and then the phototrode voltage was set to 900 to 1150 mV during titration. The calcium carbonate content was displayed in percent.

[0254] Moisture Content

[0255] The moisture content of the particulate material was determined by thermogravimetric analysis (TGA). TGA analytical methods provide information regarding losses of mass with great accuracy, and is common knowledge; it is, for example, described in Principles of Instrumental analysis, fifth edition, Skoog, Holler, Nieman, 1998 (first edition 1992) in Chapter 31 pages 798 to 800, and in many other commonly known reference works. In the present invention, thermogravimetric analysis (TGA) is performed using a Mettler Toledo TGA 851 based on a sample of 500+/50 mg and scanning temperatures from 25 C. to 350 C. at a rate of 20 C./minute under an air flow of 70 ml/min.

[0256] Alternatively, the moisture content of the particles was determined by the oven method.

[0257] Brookfield Viscosity

[0258] The Brookfield-viscosity of a slurry was determined with a Brookfield Viscometer type RVT equipped with a LV-4 spindle at a speed of 100 rpm or 20 rpm and room temperature (23.51 C.).

[0259] pH Measurement

[0260] 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).

[0261] Optical Print Density

[0262] Optical print density was measured using a SpectroDens spectrometer from Techkon GmbH, Germany, according to DIN 16527-3:1993-11. The deviation of the instrument is 0.2 points. The measuring was carried out with and without electrostatic assist (ESA).

[0263] Moisture Pick-Up Susceptibility

[0264] The term moisture pick up susceptibility in the meaning of the present invention refers to the amount of moisture absorbed on the surface of the calcium carbonate-containing particles and is determined in mg moisture/g of the dry crumbles after exposure to an atmosphere of 50% of relative humidity for 48 hours at a temperature of 23 C.

[0265] Pigment Whiteness, Aper Opacity, Light Scattering and CIELAB

[0266] Pigment whiteness R457 (or brightness), paper opacity and light scattering were measured using an ELREPHO 3000 from the company Datacolor according to ISO 2469:1994 (DIN 53145-2:2000 and DIN 53146:2000). The opacity and scattering were typically measured on the felt side (FS). The CIELAB L*, a*, b* coordinates were measured using an ELREPHO 3000 from the company Datacolor according to EN ISO 11664-4 and barium sulfate as standard.

[0267] Paper Gloss

[0268] Paper and print gloss were measured using LGDL-05.3-lab instrumentation from the company Lehmann Messsysteme GmbH, DE-Koblenz according to EN ISO 8254-1:2003, TAPPI 75 (%).

[0269] Roughness PPS

[0270] Paper roughness is measured using the PPS roughness tester from the company Lorentzen & Wettre to DIN ISO 8791-4 with a pressure of 1.0 mPa using soft component, PPS 1.0 (m).

[0271] Missing Dots Analysis

[0272] The missing dots analysis was carried out by using a verity instrument of Verity IA, USA. The settings for the missing dots analysis were as described in the following table 1.

TABLE-US-00001 TABLE 1 Settings Measured area: 20 mm 20 mm Threshold: 19 Color separation: Blue Dot sphericity minimum: 12.56* Dot sphericity maximum: 10 000.sup.# Average brightness: 0-255 Filter min: 0.01 mm.sup.2 Filter max: 0.5 mm.sup.2 Erosion: 0 Dilation: 2 No objective AOI (side objects are not measured) *refers to the sphericity of a perfectly round-shaped circle; .sup.#refers to the sphericity of a thin and long fiber

[0273] The software Prfbau Verity Print Target Version 4 was used to analyze the paper coating.

[0274] Evaluation of the Print Quality on Test Substrates

[0275] The following method was used to determine and quantify the print quality in rotogravure printing. The method can be divided into four steps, which are carried out successively.

[0276] 1. Digitization of a Printed Area

[0277] A scanner was used to digitize areas of 500500 pixel by applying a resolution of 1 200. The obtained images (five for each sample) were saved as jpeg files without data compression.

[0278] 2. Image Preparation

[0279] The obtained images were used to determine the number of printing dots in the chosen sample area as well as the area covered by the printing dots. For the analysis the obtained images in the form of RGB color prints were thus converted into grey scale images by using the free software IrfanView. The newly generated images were further analyzed with the free software GNU Octave.

[0280] 3. Image Analysis

[0281] The free software GNU Octave was used to analyze the newly generated images. Said software handles images as matrices and allows simple manipulations of these images. The printing dots were separated by a specific threshold level, which is calculated for each image separately. The threshold level is defined as the level at which the highest number of detected area containing more than 1 pixel is determined. An algorithm to find the threshold level was developed, and is shown in the following:

TABLE-US-00002 % Starting the function function missingdot; %Looking for files in a given directory dirlist=dir(pwd); %Define the results matrix results=[ ]; %Work through all files in the directory for l=3:length(dirlist)2 % reading the image img=imread(dirlist(l).name); % converting the image to gray scale i2=rgb2gray(img); % defining an intermediate results vector do determine optimal threshold rv=[ ]; %Starting a loop to check for the preferred threshold for i=1:255; %apply a threshold i3=im2bw(i2,i/255); % change black/white i4=~i3; %remove single and most probably wrong detected pixel i5=bwmorph(i4,clean); % label the detected areas i6=bwlabel(i5); % create intermediate result (threshold, counts, area) ri=[i,max(i6(:)),sum(i5(:))]; %create intermediate results vector rv=[rv;ri]; end % looking for the threshold level, given by the highest number of detected areas. [a,b]=max(rv(:,2)); % creating output matrix results=[results;rv(b,:,:)]; end %saving output matrix save RESULTS.txt results ascii endfunction

[0282] This algorithm is started from a user interface (GUI Octave) and returns a text file (RESULTS.txt in the working directory) with complied results for further analysis.

[0283] 4. Meaningful Preparation of Results

[0284] Excel was used to create tables of the single results. The highest number of printing dots as well as the highest area covered by the printing dots corresponds to the ideal image.

2. Examples

[0285] The following components were used to prepare the liquid coating compositions applied to the substrate. [0286] Substrate 1: Paper with a basis weight (grammage) of 37.6 g/m.sup.2, a thickness of 57 m, opacity-FS of 83.3% and scattering-FS of 52.65 m.sup.2/kg, commercially available from Stora Enso Kabel GmbH & Co KG, Germany. [0287] Substrate 2: Paper with a basis weight (grammage) of 39.1 g/m.sup.2, a thickness of 60 m, opacity-FS of 84.7% and scattering-FS of 53.67 m.sup.2/kg, commercially available from Stora Enso Kabel GmbH & Co KG, Germany. [0288] Pigment 1: Calcium carbonate in crumbled form having a solids content of 85.0 wt.-%, based on the total weight of the crumbles, and in which 60 wt.-% of the particles are <1 m and 90 wt.-% of the particles are <2 m, as measured by the sedimentation method. The calcium carbonate particles of the crumbles have a d.sub.50 of 0.8 m, a d.sub.98 of 2 to 7 m and a BET specific surface area of 6 to 7 m.sup.2/g. The crumbles have a brightness >94, a yellowness index of <1.5, Cielab a* of 0, a Cielab b* of 0.4 and a Cielab L* of 97. The particles of the crumbles were surface treated by using 0.5 wt.-% stearic acid, based on the total weight of the crumbles. [0289] Pigment 2: Kaolin, commercially available as Lustra S from BASF, Germany. [0290] Pigment 3: Talc, commercially available as Finntalc C10 from Mondo Minerals, Finnland. [0291] Binder: Acronal 5201 (acrylate copolymer), commercially available from BASF, Germany. [0292] Thickener: Sterocoll HT (acrylate copolymer), commercially available from BASF, Germany. [0293] Stearate: Ombrelub CD (calcium stearate), commercially available from Mnzing Chemie GmbH, Germany.

[0294] The foregoing pigments were used to prepare four different liquid coating compositions (see Table 2) to demonstrate the invention.

TABLE-US-00003 TABLE 2 Composition of coating compositions Coating Coating Coating Coating composition composition composition composition 1 2 3 4 (inventive) (inventive) (inventive) (reference) [pbw] [pbw] [pbw] [pbw] Pigment 1 100 75 75 50 Pigment 2 25 25 Pigment 3 25 25 Binder 5 5 5 5 Thickener 0.1 0.1 0.2 0.2 Stearate 1 1 1 1 pH 9.0 9.0 9.0 9.0 Viscosity 1 000-1 500 1 000-1 500 1 000-1 500 1 000-1 500 [mPas] pbw: parts by weight (d/d); coating compositions 1 to 4 further contained 0.4 parts by weight of a commercially available brightener.

[0295] The coating compositions 1 to 4 were prepared as aqueous slurries and have the properties as described in the following table 3.

TABLE-US-00004 TABLE 3 Properties of the coating compositions 1 to 4 Coating Coating Coating Coating compo- compo- compo- compo- sition 1 sition 2 sition 3 sition 4 (inventive) (inventive) (inventive) (reference) Pigment 1 100 75 75 50 Pigment 2 25 25 Pigment 3 25 25 Time start [min] 12.55 13.55 14.40 15.20 Time end [min] 13.40 14.30 15.05 15.50 Solids content 64.8 63.1 60.7 57.9 start [wt.-%] Solids content 63.7 63.0 60.5 58.1 end [wt.-%] Viscosity 100 rpm 810 930 620 510 start [mPas] Viscosity 100 rpm 720 960 640 460 end [mPas] Viscosity 20 rpm 2350 2680 1810 1240 start [mPas] Viscosity 20 rpm 2040 2760 1860 1350 end [mPas]

[0296] As regards table 3, it is to be noted that even though the coating compositions 1 to 4 were prepared by the same amount of pigment (100 parts by weight), the solids content of the slurries varied depending on the pigment or pigment mixture from which the corresponding coating composition was prepared. In particular, it can be gathered that the pigment 1 gave a slurry with the highest solids content (coating compositions 1), while the reference pigment mixture of pigments 1, 2 and 3 gave a slurry with the lowest solids content (coating composition 4).

[0297] The liquid coating compositions 1 to 4 (as described in table 2) were single-coated with a metering system at the pilot coating machine at BASF's paper technical center in Ludwigshafen, Germany using a stiff blade (compositions 1 to 3) with an amount of 7.5 g/m.sup.2 on the first side of the substrate and with an amount of 8.0 g/m.sup.2 on the second side of the substrate (substrate 1 for coating composition 1 and 2; substrate 2 for coating composition 3 and 4). The solids content of each liquid coating composition was as high as possible as described in table 3. The coating layers were dried on the coating machine by IR and airfoils to end moisture content of 5.0 to 5.5%.

[0298] The obtained samples were then optionally calendered to a paper gloss target of 52% at a top and bottom temperature of 90 C., a speed of 300 m/min and a number of 11 nips.

[0299] The obtained paper samples were tested with regard to opacity, light scattering and roughness. The results are outlined in the following tables 4a and 4b for the uncalendered and calendered samples.

TABLE-US-00005 TABLE 4a Paper characteristics for the uncalendered samples S 1 S 2 S 3 S 4 (sub* 1 + cc.sup.# 1) (sub* 1 + cc.sup.# 2) (sub* 2 + cc.sup.# 3) (sub 2* + cc.sup.# 4) (inventive) (inventive) (inventive) (reference) Grammage 52.6 55 54.4 54.5 [g/mm.sup.2] Thickness [m] 59 62 63 64 Density [g/cm.sup.3] 0.89 0.89 0.87 0.86 Opacity-FS [%] 92.5 93.2 93.7 93.6 Scattering-FS 78.13 76.32 80.33 77.05 [m.sup.2/kg] Roughness-FS 1.88 1.95 2.04 2.25 [m] Roughness-WS 2.18 2.21 2.45 2.76 [m] *substrate; .sup.#coating composition

TABLE-US-00006 TABLE 4b paper characteristics for the calendered samples S 1 S 2 S 3 S 4 (sub* 1 + cc.sup.# 1) (sub* 1 + cc.sup.# 2) (sub* 2 + cc.sup.# 3) (sub* 2 + cc.sup.# 4) (inventive) (inventive) (inventive) (reference) Grammage 53.6 54.6 53.8 54.1 [g/mm.sup.2] Thickness [m] 49 50 49 49 Density [g/cm.sup.3] 1.09 1.09 1.1 1.1 Opacity-FS [%] 91 91.3 91.2 91.2 Scattering-FS 63.96 62.44 61.66 59.51 [m.sup.2/kg] Roughness-FS 0.82 0.83 0.8 0.81 [m] Roughness-WS 0.88 0.84 0.84 0.84 [m] *substrate; .sup.#coating composition

[0300] It can be gathered from tables 4a and 4b that the calendered as well as uncalendered samples comprising pigment 1 in the coating layer obtain the highest light scattering, while the calendered as well as uncalendered samples comprising the reference pigment mixture of pigments 1, 2 and 3 gave lower light scattering. Furthermore, it can be gathered that the calendered as well as uncalendered samples comprising pigment 1 in the coating layer have well balanced optical and mechanical properties.

[0301] The effect of the coating compositions 1 to 4 on the optical density of black and color of a calendered coated paper product prepared therefrom is outlined in FIGS. 1 to 4. From FIGS. 1 to 4 it can be concluded that the coating compositions 1 to 3 gave sufficient results for black as well as colour inks. It has thus to be assumed that the coating compositions 1 to 3 impart positive effects on the optical and mechanical properties of paper end products comprising a coating layer prepared from such coating compositions.

[0302] The biggest issue with the print quality when coated with prior art compositions is the appearance of missing dots and the insufficient dot area. The effect of the coating compositions on missing dots and dot area of a coated paper product prepared from the coating compositions 1 to 4 are outlined in FIGS. 5 and 6. From FIGS. 5 and 6 it can be clearly gathered that the coating compositions 1 to 3 gave sufficient results as regards the dot area and the trial points for the missing dots. Thus, it has to be assumed that coating compositions 1 to 3 impart positive effects on the optical and mechanical properties of paper end products comprising a coating layer prepared from such coating compositions.