GB2139606A

GB2139606A - Calcium carbonate

Info

Publication number: GB2139606A
Application number: GB08329759A
Authority: GB
Inventors: Manfred Arnold; Peter Belger; Ludwig Huggenberger; Dieter Strauch
Original assignee: Pluess Staufer AG
Current assignee: Omya AG
Priority date: 1983-05-09
Filing date: 1983-11-08
Publication date: 1984-11-14
Also published as: FR2545812A1; FI832279A0; FI832279L; ES8602544A1; IT1172304B; NO832355L; FI84184C; CA1219730A; JPH0259181B2; DE3316949C2; SE8303934D0; NL188281C; GB8329759D0; AU2093083A; AT381687B; FI84184B; AU558632B2; SE8303934L; ES527196A0; IT8348719A0

Abstract

A calcium carbonate, especially a natural calcium carbonate, is disclosed, which contains 50 to 70% by weight of particles smaller than 1 micron, less than 10% of particles smaller than 0.2 microns, and a BET specific surface area of less than 10 m<2>/g. This calcium carbonate, which is obtained by grinding and classifying, can be used especially as a high-solids coating pigment for gravure printing.

Description

SPECIFICATION Calcium carbonate especially natural calcium carbonate The invention relates to calcium carbonates, especially natural calcium carbonates, methods for the preparation thereof, and their preferential use as coating pigments on gravure printing papers.

Coated papers are coated with a composition containing essentially a binding agent and a pigment in the form of a mineral filler. A description of the components of paper coating materials and their use is given, for example, in the book, "Pulp and Paper Chemistry and Technology", by James P. Casey, Chapter XIX, Vol. 111(1961).

In the printing art, essentially three different printing methods are known, letterpress, offset, and gravure printing.

Letterpress printing is known as a relief process. As in the case of the rubber stamp, the image areas are raised above the rest of the printing plate. In printing, coated paper, which contains kaolin or calcium carbonate, is also used.

Offset printing is known as a flat printing process, i.e., the image and nonimage areas are in the same plane. They differ in that the image areas can accept oils (i.e., printing ink) and yield them, but they repel water, while the non page parts retain water but repel oil (ink). The rotating printing cylinder first passes the water-bearing cylinder and then the ink-bearing cylinder. However, it does not print directly on the paper, but on a rubber "blanket" stretched over a cylinder. It is from this that the paper, pressed by cylinders against the rubbber blanket, picks up the image. In this process both kaolin and calcium carbonate are used as paper coating pigments.

In gravure printing, the printing parts of the cylinder are below the polished surface. The cylinder passes first through the trough filled with liquid ink, then the doctor blade, which is a steel blade that takes off the excess ink, and the ink that is to do the printing is held in cells until the impression cylinder carrying the paper rolls over them and the ink is absorbed out of them.

For gravure printing, it is mostly kaolin that is used as a coating pigment in practice. Calcium carbonate is mentioned in the literature (DE-OS 3,132,841 and DE-OS 2,943,653) in conjunction with gravure printing papers. However, calcium carbonate actually has hardly ever been used in gravure printing, especially because the people of the art have been of the opinion that kaolin would have substantially better properties in coated gravure printing papers.

Furthermore, the person of the art is even expressly discouraged in the literature from using calcium carbonate asd a coating pigment in gravure printing papers, on the grounds that poor quality printing would result. Only for the sake of example, reference will be made herewith to two quotations from the literature in this regard.

1. "ECC International", an in-house periodical, 1981, "How developments in coating pigments affect paper printability", by Dr. Ken Beazley, Research and Development, pp. 1 and 2: In this publication it is literally stated that one of the facts of life is that ground calcium carbonate is a poorer coating pigment for gravure printing papers than kaolin. On page 2 it is again pointed out that calcium carbonate results in poor printability.

2. Tappi Coatings Proceedings 1979: ''Possibilities and Limitations of High Solids Colours", page 39: Under point 4, summary, No. 3, it is stated that the printing quality, when ground calcium carbonate is used, is poorer than when kaolin is used in equal and in higher solids concentration.

The use of kaolin as a coating pigment on gravure printing papers, however, entails a series of disadvantages, especially: -Poor rheological behaviour; consequently it is not possible to coat with a high solids content, resulting in high energy costs in the drying of the coating.

-Need for large amounts of binding agent.

-Uneconomical, due to high cost factor.

-it is therefore an object of the present invention to avoid these disadvantages, and especially to prepare a coating pigment for gravure printing papers, which can be applied in high solid concentrations with a low binding agent content, with which energy can be saved, which results in an gravure printing paper of high gloss and high opacity, and better strike-through and showthrough characteristics than the coating pigments of the prior art.

Brief Summary of the Invention It has now surprisingly been found, after years of extensive research and experimentation that, contrary to the opinion of the people of the art, calcium carbonates are used for the achievement of the above object, if they have the features specified in Claim 1.

The invention is therefore characterized by a very specific combination of three features, in which 50 to 70% of the particles are smaller than 1 micron, less than 10% (especially between 1 and 10%) of particles smaller than 0.2 microns are present, and the BET specific surface area is less than 10 m2/g. Preferably, the upper cross section of the products of the invention amounts to 3 to 10 microns.

It has been found that, in the scope of the invention, all of the alkaline earth carbonates, and their mixtures such as dolomite, can be used successfully. Precipitated calcium carbonates provide a very good solution of the problem stated above. The natural calcium carbonates, limestone, chalk and marble or mixtures thereof have proven outstanding.

Description of Preferred Embodiments Preferred embodiments of the invention are specified in Claims 2 to 5. All percentages stated are by weight. The particle sizes stated in microns correspond to a spherical diameter.

Other features and advantages of the invention will be found in the following description of embodiments.

Example 1-Preparation of the Experimental Products Natural calcium carbonate was preground dry in a manner that is known in itself. The fine grinding was performed wet in a sand mill, the grinding conditions being selected so that the percentages of particles smaller than 1 micron and smaller than 0.2 microns, as well as the specific surface area, could be varied greatly.

The measurement of the particle size distribution was performed by sedimentation analysis, and the measurement of the specific surface area by the BET method.

In the manner described above, 3 products were made having different percentages of particles smaller than 1 micron, for the purposes of demonstrating the effect of these percentages on the paper coating formulation.

The testing of these products was performed on a paper coating formulation for gravure printing.

Coating formulation Binding agent (copolymer containing acrylic acid) 4.5 wt.-parts, abs. dry Dispersant (sodium salt of a polycarboxylic acid) 0.6 wt.-parts, abs. dry Calcium stearate 0.8 wt.-parts Cationic polyethyleneimine 0.1 2 wt.-parts Anionic acrylate thickener 0.3 wt.-parts Coating pigment 100.0 wt.-parts The solids content of the coating color was adjusted to 65% Coating conditions: Coating base paper 36-37 g/m2 containing wood Coat weight: Felt side 7 g/m2 (Coating layer) Wire side 8 g/m2 Coating machine: Combi Blade Coater; operating speed 1000 m/min.

Drying: Air Foils 250"C Calendering: Superkalander, operating speed 200 m/min, line pressure 250 kp/cm, roll temp. 70"C Test Conditions: Gloss: Tappi T 480 os-78 Opacity: Per DIN Standard 53,146 Laboratory Gravure Printing: Printing press: Prüfbau gravure printing test press Printing cylinder Engraved Printing speed 5 m/s Ink: Rotary gravure ink (black) Printing quality was judged visually. A rating scale was established.

Print gloss: The gloss was measured by the Tappi method at a 75 angle of incidence and reflection on the full surface of the laboratory-made gravure print.

Strike-through and show-through: The laboratory gravure prints are used.

The remissivity of the coated, unprinted paper (= R) and the remissivity of the back of the printed full surface (= Rfr) are measured.

The computation is performed according to the formula, R-Rfr .100 R Tests in actual printing practice: Printing press Wifag Rotomaster Printing cylinder Engraved Cylinder revol. per h 15,000 Color sequence yellow, magenta, cyan, black All papers were printed without static printing aid.

The actual-practice tests were evaluated visually. A rating scale was established.

Characteristics: VP 50 VP 60 VP 70 Particles smaller than 1 micron 53% 62% 70%' Particles smaller than 0.2 micron 7% 9% 12% Specific surface area 8.0 m2/g 9.1 m2/g 10.2 m2/g Test results: VP 50 VP 60 VP Gloss 46% 50% 48% Print gloss 72% 74% 72% Opacity 90.8% 92.2% 91.4% Strike-through + show-through 13.1 10i6 11.9 Practical gravure print test (rating) 2 1 3 Product VP 60 with a content of 62% particles smaller than 1 micron gives the best results as regards gloss, print gloss, opacity and strike-through + show-through, and in the practical tests.

It is followed by products VP 70 and VP 50 with a content of 70 and 53%, respectively, particles smaller than 1 micron. The optimum content of particles smaller than 1 micron is, in accordance with the invention, between 50 and 70%, and preferably is 60%.

Example 2: In the manner described in Example 1, 2 products were prepared with the same percentages of particles smaller and larger than 1 micron, but with a high and low percentage of particles smaller than 0.2 microns and of high and low specific surface area, so as to point out the difference between the invention and a calcium carbonate which is incapable of solving the problem to which the invention is addressed.

These products were tested in a paper coating formulation for gravure printing.

Coating formulation: Binding agent (copolymer containing acrylic acid ester) 3.5 wt.-parts, abs. dry Dispersant (sodium salt of a 0.6 wt.-parts, abs. dry polycarboxylic acid) Calcium stearate 0.8 wt.-parts Cationic polyethyleneimine 0.12 wt.-parts Anionic acrylate thickener 0.3 wt.-parts Coating pigment 100.0 wt.-parts The solids content of the coating color was adjusted to: 65% Coating conditions: Coating base paper: 37 g/m2, containing wood Coat weight Felt side 8 g/m2 (Coating layer) Wire side 9 g/m2 Coating machine: Combi Blade Coater; 1000 m/min.

operating speed Drying: Air Foils 250'C Calendering: Superkalander, 200 m/min operat ing speed. 250 kp/cm line pressure and 70"C roll temperature Under the test conditions described in Example 1, the following characteristics were obtained: Characteristics: VP 5 VP 14 Content smaller than 1 micron 59% 60% Content smaller than 0.2 micron 5% 14% Specific surface area 8.9 mug 12.1 m2/g Test results: Gloss 55% 47% Print gloss 82% 72% Opacity 91.4% 90.8% Strike-through + show-through 11.8 1 2.6 Laboratory gravure print (rating) 1 2 While the product of the invention VP 5 solves the problem to which the invention is addressed, unsatisfactory results are obtained with product VP 14, which is also surprising because VP 5 is a coarser calcium carbonate than VP 14.

Example 3: By the manner described in Example 1, a product was prepared which is a preferred embodiment with regard to the figures for the percentages smaller than 1 micron, those smaller thn 0.2 micron, and the specific surface area.

The testing of this product was performed in comparison with a kaolin commonly used in practice, in a paper coating formulation for gravure printing, i.e., a product of the state of the art.

Coating formulation: VP 60/6 Kaolin Binding agent (copolymers 3.5 wt.parts 4.5 wt.parts containing acrylic acid) abs. dry abs. dry Dispersant 0.6 wt.-parts 0.3 wt.-parts abs. dry abs. dry Calcium stearate 0.8 wt.-parts 0.8 wt.-parts Anionic acrylate thickener 0.3 wt.-parts 0.3 wt.-parts Coating pigment 100.0 wt.-parts 100.0 wt.-parts The solids content of the coating color was adjusted to 65% 56% Coating conditions: Coating base paper: 36 g/m2 containing wood Coat weight Felt side 8 g/m2 (Coating laver) Wire side 9 g/m2 Coating machine: Combi Blade Coater, 1000 m/min. operating speed.

Drying: Air Foils 250"C Calendering: Supercalender; 200 m/min operat ing speed, 250 kp/cm line pressure and 70"C roll temp.

Under the test conditions described in Example 1, the following characteristics were obtained: Characteristics: VP 60/6 Kaolin Content smaller than 1 micron 60% 49% Content smaller than 0.2 micron 6% 1 7% Specific surface area 7.8 m2/g 13.0 m2/g Test Results: Gloss 63% 64% Print gloss 85% 85% Opacity 92.6% 90.0% Strike-through + show-through 10.7 13.5 Laboratory gravure print (rating) 1 2 Practical gravure print test (rating) 1 1 The calcium carbonate VP 60/6 used in this example, containing 60% of particles smaller than 1 micron and 6% of particles smaller than 0.2 micron and a specific surface area of 7.8 m2/g gives the same glass and print gloss values as the kaolin. In opacity and in strike-through and show-through, and in the laboratory print, the VP 60/6 gives better results than the kaolin.

In the practical print tests both products were rated equal.

These results show that, with a calcium carbonate corressponding to the product VP 60/6, a paper coating for gravure printing can be made, resulting in a coated paper which is equal to and in some ways superior to gravure printing papers produced with the use of kaolin.

From the coating compound formulas of Example 3 it can be seen that, with calcium carbonate, approximately 20% less binding agent could be used in achieving the same and in some respects better properties. The reduction of the binding agent content represents an important cost saving for the paper manufacturer.

The good rheological properties of calcium carbonate make it possible to produce coating formulations of higher solids content (high solids coating), which is not possible with kaolin. The result is lower energy costs in the drying of the coating, which again is a substantial cost saving for the manufacturer of paper.

Claims

1. Calcium carbonate, especially natural calcium carbonate, classified according to particle size,-and having: (a) 50-70% of particles smaller than 1 micron, (b) less than 10% of particles smaller than 0.2 micron, and (c) a specific BET surface area of less than 10 m2/g

2. Calcium carbonate of claim 1, having: (a) 55-65% of particles smaller than 1 micron, (b) less than 10% of particles smaller than 0.2 micron, and (c) a specific BET surface area of less than 10 m2/g.

3. Calcium carbonate of claim 1 m having: (a) 60% of particles smaller than 1 micron, (b) less than 10% of particles smaller than 0.2 microns, and (c) a specific BET surface area of less than 10 m2/g.

4. Calcium carbonate of claim 1, having (a) 60% of particles smaller than 1 micron, (b) 4-6% of particles smaller than 0.2 micron and (c) a specific Bet surface area of less than 10 m2/g.

5. Calcium carbonate of claim 1 having: (a) 60% of particles smaller than 1 micron, (b) 4-6% of particles smaller than 0.2 micron, and (c) a specific BET surface area of less than 8 m2/g.

6. Method of preparing the calcium carbonate of claim 1, comprising first grinding the same in a dry state and then classifying the ground calcium carbonate by means of an air separator to produce a resulting product having: (a) 50-70% of particles smaller than 1 micron, (b) less than 10% of particles smaller than 0.2 micron, and (c) a specific BET surface area of less than 10 m2/g.

7. Method of preparing the calcium carbonates of claim 1, comprising first grinding the same in a wet state and then classifying the ground calcium carbonate to produce a resulting product having: (a) 50-70% of particles smaller than 1 micron, (b) less than 10% of particles smaller than 0.2 micron, and (c) a specific BET surface area of less than 10 m2/g.

8. Use of the calcium carbonate of claim 1 as coating pigment for LWC-gravure printing papers.

9. Use of the calcium carbonates of claim 1 in suspensions having a solids content of 65-85% by weight for LWC-gravure printing papers (High solid Coatings).