Stretchable coatings

Abstract

The present invention relates to aqueous coating compositions for stretchable coatings in paper applications comprising at least one acrylic polymeric binder having a T.sub.g of 3 C. or lower, and at least one inorganic filler having a BET specific surface area in the range of 1.0 to 30.0 m.sup.2/g, wherein the dry weight ratio of the at least one acrylic polymeric binder to the at least one inorganic filler is between 15:100 and 20:100, as well as their use as stretchable coatings in paper and board applications.

Claims

1. An aqueous coating composition for stretchable coatings in paper and board applications comprising: at least one acrylic polymeric binder having a T.sub.g of 15 C. or lower, and at least one inorganic filler having a BET specific surface area in the range of 1.0 to 30.0 m.sup.2/g, wherein (i) the dry weight ratio of the at least one acrylic polymeric binder to the at least one inorganic filler is from 15:100 to 20:100 and (ii) the at least one inorganic filler comprises a calcium carbonate containing material.

2. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder has a T.sub.g in the range of from 15 to 46 C.

3. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder has a T.sub.g in the range of from 15 to 40 C.

4. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder has a T.sub.g of 20 C. or lower.

5. The coating composition according to claim 1, wherein the at least one inorganic filler has a BET specific surface area in the range of 2.0 to 20.0 m.sup.2/g.

6. The coating composition according to claim 1, wherein the at least one inorganic filler has a BET specific surface area in the range of 4.0 to 15.0 m.sup.2/g.

7. The coating composition according to claim 1, wherein the at least one inorganic filler has a BET specific surface area in the range of 5.0 to 13.0 m.sup.2/g.

8. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder is selected from the group consisting of acrylic homopolymers, methacrylic homopolymers, and copolymers composed of at least two different monomers, one monomer having an acrylic or methacrylic functional group and the other monomer having a functional group selected from the group consisting of styrene, vinyl and allyl; and any mixture thereof.

9. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder is selected from the group consisting of an acrylic homopolymer, a vinyl-acrylic copolymer, a styrene-acrylic copolymer, and any mixture thereof.

10. The coating composition according to claim 1, wherein the at least one inorganic filler has a weight median particle size d.sub.50 in the range of 0.1 to 5.0 m.

11. The coating composition according to claim 1, wherein the at least one inorganic filler has a weight median particle size d.sub.50 in the range of 0.4 to 2.0 m.

12. The coating composition according to claim 1, wherein the at least one inorganic filler has a particle size d.sub.98 in the range of 1.0 to 20.0 m.

13. The coating composition according to claim 1, wherein the at least one inorganic filler has a particle size d.sub.98 in the range of 2.0 to 12.0 m.

14. The coating composition according to claim 1, wherein the at least one inorganic filler further comprises talc, kaolin, clay, titanium dioxide, satin white, bentonite, or any mixture thereof.

15. The coating composition according to claim 1, wherein the at least one inorganic filler further comprises clay, kaolin, or any mixture thereof.

16. The coating composition according to claim 1, wherein the at least one inorganic filler comprises natural ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), dolomite, or any mixture thereof.

17. The coating composition according to claim 1, wherein the at least one inorganic filler is natural ground calcium carbonate (GCC) selected from the group consisting of marble, limestone, chalk, and any mixture thereof.

18. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder and the at least one inorganic filler constitute at least 90 wt.-%, of the composition, based on the dry weight of the composition.

19. The coating composition according to claim 1, wherein the coating composition has a solids content in the range of 50 to 75 wt.-%, based on the total weight of the coating composition.

20. The coating composition according to claim 1, further comprising one or more additives selected from the group consisting of thickeners, lubricants, dispersants, milling aids, rheology modifiers, defoamers, optical brighteners, dyes, and pH controlling agents.

21. The coating composition according to claim 1, wherein the at least one inorganic filler is present in a range of 75 to 87 wt.-%, based on the dry weight of the coating composition.

22. The coating composition according to claim 1, wherein the at least one acrylic polymeric binder is present in a range of 12 to 17 wt.-%, based on the dry weight of the coating composition.

23. The coating composition according to claim 1, wherein at least one further additive is present in a range of 0.1 to 8 wt.-%, based on the dry weight of the coating composition.

24. The coating composition according to claim 1, wherein the at least one inorganic filler is natural ground calcium carbonate (GCC).

Description

DESCRIPTION OF THE FIGURES

(1) FIGS. 1a-d show a diagonal top view (FIG. 1a), side view (FIG. 1b), top view (FIG. 1c) and front view (FIG. 1d) of a 3D formability tester.

(2) FIGS. 2a-e, 3 and 4 show microscope images of samples using different binders.

(3) FIG. 5 shows microscope images of samples using different fillers.

(4) FIG. 6 shows microscope images and results of image analysis of samples having different binder levels.

(5) FIG. 7 shows microscope images and results of image analysis of samples using fillers having different particle sizes.

(6) FIG. 8 shows microscope images of samples using thickeners.

(7) FIG. 9 shows the results of CIE whiteness measurement of different coatings.

(8) FIG. 10 shows the results of smoothness measurements of different coatings.

(9) FIG. 11 shows the results of CIE whiteness measurement of different coatings having high binder levels.

(10) FIG. 12 shows microscope images of double-coated samples.

(11) FIG. 13 shows microscope images of double-coated samples using optical brightening agents in the top-coat.

(12) FIG. 14 shows microscope images of double-coated samples using different fillers.

EXAMPLES

(13) I. Measurement Methods

(14) In the following, measurement methods implemented in the examples are described.

(15) Particle Size Distribution

(16) The d.sub.50 and d.sub.98 values were measured using a Sedigraph 5120 from the company Micromeritics, USA. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurements were 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. For the measurement of dispersed samples, no further dispersing agents were added.

(17) Solids Content of an Aqueous Suspension

(18) The suspension solids content (also known as dry weight) was determined using a Moisture Analyser MJ33 from the company Mettler-Toledo, Switzerland, with the following settings: drying temperature of 160 C., automatic switch off if the mass does not change more than 1 mg over a period of 30 s, standard drying of 5 to 20 g of suspension.

(19) Specific Surface Area (SSA)

(20) The specific surface area was measured via the BET method according to ISO 9277 using nitrogen, following conditioning of the sample by heating at 250 C. for a period of 30 minutes. Prior to such measurements, the sample is filtered within a Bchner funnel, rinsed with deionised water and dried overnight at 90 to 100 C. in an oven. Subsequently the dry cake is ground thoroughly in a mortar and the resulting powder placed in a moisture balance at 130 C. until a constant weight is reached.

(21) CIE Whiteness

(22) CIE whiteness was determined via whiteness measurements according to ISO 11457.

(23) Parker Print Surfaces (PPS) Smoothness

(24) Surface smoothness given as Parker Print Surface was determined according to ISO 8791-4.

(25) II. Material

(26) 1. Substrate

(27) FibreForm 3D paper of 100% primary fibre; basis weight of 100 g/m.sup.2 (available from BillerudKorsns; Sweden). The paper is characterized by its high elongation at break.

(28) 2. Fillers

(29) Filler 1: natural ground calcium carbonate; d.sub.50=0.7 m; d.sub.98=5.0 m; BET SSA=11.5 m.sup.2/g; solids content 78 wt.-% (available from Omya, Switzerland) Filler 2: natural ground calcium carbonate; d.sub.50=1.5 m; d.sub.98=10.0 m; BET SSA=6.8 m.sup.2/g; solids content 78 wt.-% (available from Omya, Switzerland) Filler 3: natural ground calcium carbonate; d.sub.50=0.4 m; d.sub.98=2.0 m; BET SSA=18.0 m.sup.2/g; solids content 75 wt.-% (available from Omya, Switzerland) Filler 4: Clay No. 1 high brightness ultrafine clay; BET SSA=21 m.sup.2/g; solids content 73 wt.-% (available from Omya, Switzerland) Filler 5: Sachtleben R 320, rutile titanium dioxide; BET SSA=13 m.sup.2/g (available from Sachtleben Chemie GmbH, Germany
3. Binders Appretan E2100: pure acrylic dispersion; T.sub.g 30 C. (available from Archroma) Appretan E6200: styrene/acrylic dispersion; T.sub.g 20 C. (available from Archroma) Appretan E4250: vinyl/acrylic dispersion; T.sub.g 15 C. (available from Archroma) Primal 325 GB: styrene/acrylic dispersion; T.sub.g 25 C. (available from Dow Chemical Company) Primal P-308 MS: styrene/acrylic dispersion; T.sub.g+8 C. (available from Dow Chemical Company) Plextol D270: aqueous emulsion of a thermoplastic acrylic polymer; T.sub.g 42 C. (available from Synthomer, Germany) Plextol D 5240: acrylic ester copolymer dispersion; T.sub.g 43 C. (available from Synthomer, Germany) Plextol X 4427: aqueous emulsion of an acrylic copolymer; T.sub.g 40 C. (available from Synthomer, Germany) Litex P 5100: carboxylated styrene/butadiene copolymer dispersion; Tg2 C. (available from Synthomer, Germany) Litex SX 1024: styrene/buradiene copolymer dispersion; Tg15 C. (available from Synthomer, Germany) Litex S 7641: self-crosslinking styrene/butadiene copolymer dispersion; Tg44 C. (available from Synthomer, Germany)
4. Additives Rheocarb 101: steric rheology modifier (available from Coatex Arkema, France) Rheocarb 121: steric rheology modifier (available from Coatex Arkema, France) PVA BF-04: fully hydrolyzed Polyvinylalcohol (available from Chang Chun Petrochemical Co., Ltd., Taiwan)
III. Methods
1. Coating Preparation

(30) Different coating compositions were prepared and evaluated as described below. The respective filler slurries and binder slurries were combined in a beaker by gentle mixing resulting in coating compositions having initial solids contents given in the below tables. Subsequently, the aqueous coating composition was mixed under higher shear conditions without drawing air until the individual phases of the composition are visually homogenously mixed. For adjustment of a final solids content of the aqueous coating composition calculated amounts of water were added by mixing again under higher shear conditions without drawing air. All mixing steps were done with a Pendraulik Laboratory Dissolver, model LD 50.

(31) 2. Paper Coating

(32) The coatings were applied to a substrate as described below.

(33) 3. Stretchability Testing Method

(34) The coating layers applied to the stretchable paper were tested with a newly developed 3D formability tester that was developed by Omya and built by Norbert Schl fli Maschinen (Zofingen). Schematic drawings indicating major dimensions of the formability tester built of aluminium are shown in FIG. 1. The key element is a profiled wheel with a diameter of 125 mm and a width of 30 mm. The profile covers half of the circumference of the wheel, and develops like a membrane in bulge tests from a flat surface to a semi-circle. The stretch level develops continuously along the profile having a total testing length of 19.6 cm (wheel diameter*pi/2) from 0% (30 mm stretching length and 30 mm stretched material) at the starting point to 57% (30 mm stretching length and 47.1 mm stretched material) at the end point. The wheel is part of an upper body of the testing instrument and is connected to two parallel rails that are also part of the upper body and guide the wheel when pulled manually for testing. The surface of the upper body that is showing towards the lower body is planar with the un-profiled section of the wheel. The lower body of the testing instrument is a massive block of aluminium with a 30 mm wide groove with broken edges not to cut the paper during testing when the profile is pressed into the paper. In order to avoid slipping paper during forming sandpaper can be glued just above the edge to firmly hold the testing paper between the upper and the lower body of the testing instrument.

(35) For testing, paper is clamped between the upper and lower body of the testing instrument with the coated surface showing into the groove of the lower body. Due to the fact that papers e.g. FibreForm have a higher elongation at break in the machine direction (the direction the paper is produced, MD) the sample should be cut in the paper cross direction (CD) to use the higher stretchability in the MD, the wheel rolls in the CD and the stretch developed by the width of the wheel is applied in the MD, respectively. A trained person operates the testing instrument to ensure comparable results with regard to testing speed, clamping force and starting point of the measurement. The wheel rolls over the paper due to friction between paper and wheel surface and presses the profile into the paper. Obvious breaks of FibreForm material without coating as described above have stretch levels of ca. 35-40% or brake after ca. 12 cm testing length. Coated samples were tested after 10 cm testing length or 29% of stretch.

(36) To better visualize cracks, the coated surface is painted with Neocarmin W (MERCK) a testing liquid for colouring cellulose fibres that are visible at the cracks and gently cleaned with a soft tissue. Samples sufficiently large for microscopic evaluation are cut from the middle of the test area at a testing length of 10 cm and glued to a flat carton board. A stereo microscope is used to image the sample (Leica) at about 16 times magnification.

(37) These images can be used for qualitative evaluation or further analysed by image analysis means: see the exemplary source code for Octave below, returning the number of pixels associated with a detected crack in an input image below as well as providing an image highlighting the extracted cracks. For better comparison, one could scale the result value by the total number of pixels in an image.

(38) Exemplary Source Code for Octave:

(39) TABLE-US-00001 function scm ( ); dirlist=dir(pwd); Result=[ ]; for i=3:length(dirlist)2 dirlist(i).name img=imread(dirlist(i).name); d=edge(img,Prewitt); d=bwmorph(d,dilate,2); imgname=dirlist(i).name; imgname=imgname(:,1:5); imlabel=strcat(imgname,A,.jpg) imwrite(d,imlabel); d=d(3:size(d,1)3,3:size(d,2)3); res=sum(d(:)); Result=[Result;i,res]; end save RESULTS.txt Result -ascii endfunction
IV. Experiments

(40) The following experiments were carried out for investigating the stretchability of several coatings in 3D forming in terms of the formation of cracks and the influence on the colour density of printed coatings.

(41) Accordingly, a substrate was coated, and printed, respectively, and subjected to 3D forming. Subsequently, the formation of cracks and the colour density were evaluated.

(42) 1. Formation of Cracks

(43) The coating was applied to the substrate with a variable speed drawdown coater (K Control Coater 303 Model 625 available from Erichsen GmbH & Co. KG, Hemer, Germany; 12 speed steps increasing from 2 and 40 m/min and 10 application rods allowing increasing application weights at given speeds).

(44) The coated samples were stretched in the 3D formability tester as described above.

(45) Subsequently, the formation of cracks was investigated by the evaluation of microscope images.

(46) The microscope images show coated surfaces that were exposed to a stretch level of approx. 29%. The y-axis represents the paper machine direction, the x-axis the paper cross direction. The image edge length is about 4 mm.

(47) The first row of the respective images shows the microscope images of coated surfaces, the second row shows the results obtained by image analysis.

(48) 1.1. Evaluation of Binder Type

(49) For evaluating binder types useful in the present invention, the following coating compositions were prepared and investigated.

(50) The coating was applied to the substrate with a variable speed drawdown coater (K Control Coater 303 Model 625 available from Erichsen GmbH & Co. KG, Hemer, Germany; 12 speed steps increasing from 2 to 40 m/min and 10 application rods allowing increasing application weights at given speeds), and the samples analysed with the 3D formability testing method as described above.

(51) TABLE-US-00002 TABLE 1 Coating C1 C2 C3 C4 C5 Filler 1 (parts by weight) 100 100 100 100 100 Appretan E2100 (parts by weight) 20 Appretan E4250 (parts by weight) 20 Appretan E6200 (parts by weight) 20 Primal 325 GB (parts by weight) 20 Primal P-308 MS (parts by weight) 20 Rod #/Speed step 3/4 3/4 3/3 3/2 3/2 Coating Weight (g/m.sup.2) 20.2 20.6 19.4 20.5 20.2 Initial solids content (wt.-%) 69.7 71.2 70.6 71.0 71.0 Final solids content (wt.-%) 61.4 61.5 61.5 60.8 61.5

(52) TABLE-US-00003 TABLE 2 Coating A1 A2 A3 Filler 1 (parts by weight) 100 100 100 Plextol D 270 (parts by weight) 20 Plextol D 5240 (parts by weight) 20 Plextol X 4427 (parts by weight) 20 Rod #/Speed step 3/3 3/3 3/3 Coating Weight (g/m.sup.2) 19.0 20.5 19.8 Initial solids content (wt.-%) 71.3 71.3 70.0 Final solids content (wt.-%) 63.5 62.0 63.2

(53) Table 3 summarizes typical crack values, defined as the number of crack pixel divided by the total number of pixel in the image and gives a brief summary on the visual evaluation of the images shown in FIGS. 2 and 3:

(54) TABLE-US-00004 TABLE 3 Visual evaluation C1 No visible cracks C2 Local tiny cracks C3 No visible cracks C4 No visible cracks C5 Long dominant cracks A1 Nearly no cracks A2 Nearly no cracks A3 Nearly no cracks

(55) The T.sub.g of acrylic binders obviously plays an important role, as can be seen from FIGS. 2a-e and FIG. 3. For the inventive samples with low T.sub.g binders (C1, C3 and C4, and A1-A3) almost no cracks can be seen in the original image. For inventive sample C2 cracks are visible, but are very short and located very locally. The visibility of these cracks does not lead to an exclusion of the corresponding binder from further considerations. Comparative sample C5, however, shows long dominant cracks, which are clearly not desirable.

(56) Accordingly, it can be summarized that binders having low T.sub.g values are the binders of choice for stretchable coatings, wherein acrylic based binders appear to be especially advantageous.

(57) Furthermore, the stretchability of non-acrylic polymeric binders were investigated.

(58) Accordingly, the following comparative coating compositions using styrene butadiene based binders were prepared and evaluated.

(59) TABLE-US-00005 TABLE 4 Coating S1 S2 S3 Filler 1 (parts by weight) 100 100 100 Litex P 5100 (parts by weight) 20 Litex SX 1024 (parts by weight) 20 Litex S 7641 (parts by weight) 20 Rod #/Speed step 3/3 3/3 3/3 Coating Weight (g/m.sup.2) 19.0 20.5 19.8 Initial solids content (wt.-%) 71.3 71.3 70.0 Final solids content (wt.-%) 63.5 62.0 63.2

(60) It is evident from FIG. 4 that styrene butadiene binders do not show the required stretchability. The visual evaluation of the images shown in FIG. 4 is summarized in Table 5.

(61) TABLE-US-00006 TABLE 5 Visual evaluation S1 Heavy cracks S2 Heavy cracks S3 Obvious cracks
1.2. Evaluation of Filler Type

(62) The influence of clay as an example for fillers other than calcium carbonate containing material on the stretchability of coating colors was investigated, coating colors were prepared accordingly.

(63) The coatings were applied with a Durrer continuous lab coating machine, using rod metering (C23, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(64) Coated paper surface properties were evaluated with regard to optical properties (CIE whiteness).

(65) The 3D formability tests of the coated sheets were done as described above.

(66) TABLE-US-00007 TABLE 6 Coating K1 K2 K3 Filler 1 (parts by weight) 100 75 50 Filler 4 (parts by weight) 25 50 Appretan E6200 (parts by weight) 20 20 20 Rheocarb 101 (parts by weight) 0.1 0.1 Coating Weight (g/m.sup.2) 19.5 18.0 18.3 Initial solids content (wt.-%) 70.5 69.4 68.4 Final solids content (wt.-%) 60.0 59.9 59.0

(67) The microscope images in FIG. 5 indicate good stretchability of coating layers also for the clay containing samples. Only a few and tiny cracks can be observed. Table 7 summarizes the visual evaluation of the images in FIG. 5.

(68) TABLE-US-00008 TABLE 7 Visual evaluation K1 Nearly no cracks K2 Nearly no cracks K3 Nearly no cracks
1.3. Evaluation of Filler/Binder Ratio

(69) For evaluating filler/binder ratios useful in the present invention, the following coating compositions were prepared and investigated.

(70) The coating was applied to the substrate with a variable speed drawdown coater (K Control Coater 303 Model 625 available from Erichsen GmbH & Co. KG, Hemer, Germany; 12 speed steps increasing from 2 and 40 m/min and 10 application rods allowing increasing application weights at given speeds), and the samples analyzed with the 3D formability testing method as described above.

(71) TABLE-US-00009 TABLE 8 Coating T1 T2 T3 Filler 1 (parts by weight) 100 100 100 Appretan E2100 (parts by weight) 10 15 20 Rod #/Speed step 3/4 3/4 3/3 Coating Weight (g/m.sup.2) 20.4 19.2 19.1 Initial solids content (wt.-%) 73.2 71.3 69.7 Final solids content (wt.-%) 63.0 63.1 61.4

(72) Table 9 summarizes typical crack values, defined as the number of crack pixel divided by the total number of pixel in the image and gives a brief summary on the visual evaluation of the images shown in FIG. 6:

(73) TABLE-US-00010 TABLE 9 Crack Number Visual evaluation T1 0.0963 Clearly visible cracks T2 0.0580 Some cracks T3 0.0442 Almost no cracks

(74) FIG. 6 shows microscope images of coatings with different binder levels (T1-T3), the second row of images according results obtained from crack detection by image analysis.

(75) As can be taken from FIG. 6, increasing the binder levels decreases the number of cracks, wherein at a certain binder level no significant improvement can be detected any more.

(76) 1.4. Evaluation of Filler Particle Size

(77) For evaluating filler particle sizes useful in the present invention, the following coating compositions were prepared and investigated.

(78) The coating was applied to the substrate with a variable speed drawdown coater (K Control Coater 303 Model 625 available from Erichsen GmbH & Co. KG, Hemer, Germany; 12 speed steps increasing from 2 and 40 m/min and 10 application rods allowing increasing application weights at given speeds), and the samples analyzed with the 3D formability testing method as described above

(79) TABLE-US-00011 TABLE 10 Coating P1 P2 P3 Filler 2 (parts by weight) 100 Filler 1 (parts by weight) 100 Filler 3 (parts by weight) 100 Appretan E2100 (parts by weight) 20 20 20 Rod #/Speed step 3/2 3/3 3/2 Coating Weight (g/m.sup.2) 19.0 19.1 19.2 Initial solids content (wt.-%) 69.7 69.8 67.2 Final solids content (wt.-%) 63.2 61.4 56.8

(80) Microscope images of coatings made from particles with different particle size are shown in the first row of FIG. 7, according images with detected cracks in the second row. Table 11 gives a summary on the measured values.

(81) TABLE-US-00012 TABLE 11 Count Visual evaluation P1 0.0265 Nearly no cracks P2 0.0442 Some tiny cracks P3 0.0590 Many very tiny cracks

(82) As can be taken from FIG. 7, any one of the coatings provide good results. However, it appears that finer particles (P2, P3) lead to more cracks in the coating layer, whereas almost no cracks and an obviously dense coating layer can be found for coarser particles (P1).

(83) 1.5. Use of Thickeners

(84) The influence of thickeners for adjusting coating color rheology on stretchability was investigated.

(85) The coatings were applied with a Durrer continuous lab coating machine, using rod metering (C23, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(86) The 3D formability tests of the coated sheets were done as described above.

(87) TABLE-US-00013 TABLE 12 Coating R1 R2 Filler 1 (parts by weight) 100 100 Appretan E6200 (parts by weight) 20 20 Rheocarb 101 (parts by weight) 0.1 Rheocarb 121 (parts by weight) 0.1 Coating Weight (g/m.sup.2) 19.5 18.6 Initial solids content (wt.-%) 70.5 70.5 Final solids content (wt.-%) 60.0 60.1

(88) FIG. 8 shows that addition of thickeners have no effect on stretchability by showing the same crack-pattern at a very low and acceptable level. Table 13 summarizes the visual evaluation of the images shown in FIG. 8.

(89) TABLE-US-00014 TABLE 13 Visual evaluation R1 Nearly no cracks R2 Nearly no cracks
2. Further Paper Surface Properties

(90) The printing properties of the coatings according to the invention as well as changes in the printed image after paper forming were investigated by a continuous lab-scale coating and printing trial.

(91) The coatings were applied with a Durrer continuous lab coating machine, using rod metering (C23, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(92) Coated paper surface properties were evaluated with regard to optical properties (CIE whiteness) and smoothness (Parker Print Surfaces).

(93) The 3D formability tests of the coated sheets were done as described above.

(94) TABLE-US-00015 TABLE 14 Coating W1 W2 W3 Filler 1 (parts by weight) 100 100 100 Appretan E2100 (parts by weight) 15 Appretan E6200 (parts by weight) 15 Primal 325 GB (parts by weight) 15 Coating Weight (g/m.sup.2) 19.5 19.0 18.0 Initial solids content (wt.-%) 72.1 72.4 71.3 Final solids content (wt.-%) 58.3 58.7 58.1

(95) As expected, coating significantly improves the paper surface quality in terms of whiteness (FIG. 9) and smoothness (FIG. 10). Forming led to some tiny cracks in the coating layers clearly indicating the lower end of acceptable binder levels.

(96) For evaluating the upper binder level useful in the present invention, the following coating compositions were prepared and investigated.

(97) The coating was applied to the substrate with a variable speed drawdown coater (K Control Coater 303 Model 625 available from Erichsen GmbH & Co. KG, Hemer, Germany; 12 speed steps increasing from 2 and 40 m/min and 10 application rods allowing increasing application weights at given speeds), and the samples analyzed with the 3D formability testing method as described above

(98) TABLE-US-00016 TABLE 15 Coating B1 B2 B3 Filler 1 (parts by weight) 100 100 100 Appretan E6200 (parts by weight) 20 30 40 Rod #/Speed step 3/2 3/2 4/2 Coating Weight (g/m.sup.2) 19.8 19.0 19.1 Initial solids content (wt.-%) 70.6 68.1 66.1 Final solids content (wt.-%) 60.0 60.8 48.1

(99) FIG. 11 shows that coating becomes transparent at higher binder levels in the coating formulation. Thus, high binder levels are not beneficial for improving paper surfaces from an paper optical point of view.

(100) 3. Double Coating Experiments

(101) 3.1. Influence of Pre-Coat Weight

(102) A double coating concept with stretchable coatings was evaluated. In a first experiment the influence of pre-coat weight was examined, according coating composition is given in the following.

(103) The pre-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(104) TABLE-US-00017 TABLE 16 Coating V1 V2 Filler 2 (parts by weight) 100 100 Appretan E6200 (parts by weight) 20 20 Coating Weight (g/m.sup.2) 11.0 15.3 Initial solids content (wt.-%) 72.1 72.1 Rod type C15 C23 Final solids content (wt.-%) 54.1 57.0

(105) On the pre-coats a top-coat with the following coating composition was applied.

(106) TABLE-US-00018 TABLE 17 Coating D1 D2 Filler 1 (parts by weight) 100 100 Appretan E6200 (parts by weight) 20 20 Coating Weight (g/m.sup.2) 9.6 10.3 Initial solids content (wt.-%) 70.6 70.6 Final solids content (wt.-%) 55.4 55.4

(107) The top-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (C15, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(108) The 3D formability tests of the coated sheets were done as described above.

(109) As shown in FIG. 12 higher pre-coat weights with stretchable coatings are beneficial for the overall stretchability of stretchable double layer coatings. The visual evaluation of the images presented in FIG. 12 are summarized in Table 18.

(110) TABLE-US-00019 TABLE 18 Visual evaluation V1 + D1 Nearly no cracks V2 + D2 Nearly no cracks
3.2. Use of Optical Brightening Agents

(111) A double coating concept with stretchable coatings was evaluated. In an experiment the influence of optical brightening agents in the top-coat layer were evaluated.

(112) As the pre-coat layer coating composition V2 described above was used. The pre-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (C23, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(113) On the pre-coats a top-coat with the following coating composition was applied.

(114) TABLE-US-00020 TABLE 19 Coating O1 O2 Filler 1 (parts by weight) 100 100 Appretan E6200 (parts by weight) 20 20 PVA BF-04 (parts by weight) 0.2 Blancophor PT (parts by weight) 0.25 Coating Weight (g/m.sup.2) 10.3 9.4 Initial solids content (wt.-%) 70.6 70.4 Final solids content (wt.-%) 55.4 56.8

(115) The top-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (C15, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(116) The 3D formability tests of the coated sheets were done as described above.

(117) The images in FIG. 13 show that the use of OBA in the top-coat formulation of a double coating concepts does not influence the stretchability of the coating layer. Table 20 gives an overview on the visual evaluation of the images presented in FIG. 13.

(118) TABLE-US-00021 TABLE 20 Visual evaluation V2 + O1 Nearly no cracks V2 + O2 Nearly no cracks
3.3. Use of Different Fillers

(119) A double coating concept with stretchable coatings was evaluated. In an experiment the influence of additional titanium dioxide in the top-coat formulation was evaluated.

(120) As the pre-coat layer coating composition V2 described above was used. The pre-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (C23, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(121) On the pre-coats a top-coat with the following coating composition was applied.

(122) TABLE-US-00022 TABLE 21 Coating X1 X2 Filler 1 (parts by weight) 100 80 Filler 5 (parts by weight) 20 Appretan E6200 (parts by weight) 20 20 Coating Weight (g/m.sup.2) 10.3 10.0 Initial solids content (wt.-%) 70.6 69.6 Final solids content (wt.-%) 55.4 57.2

(123) The top-coating layers were applied with a Durrer continuous lab coating machine, using rod metering (C15, rod pressure of ca. 1 bar, rod revolution 12 rpm) at a coating speed of 20 m/min.

(124) The 3D formability tests of the coated sheets were done as described above.

(125) In FIG. 14 it can be seen that addition of TiO.sub.2 to the top-coat formulation results in tiny local cracks when stretching the coating layer which are considered as not disturbing and acceptable. Table 22 gives a summary on the visual evaluation of the images shown in FIG. 14.

(126) TABLE-US-00023 TABLE 22 Visual evaluation V2 + X1 Nearly no cracks V2 + X2 Local tiny cracks