EP0755989B1 - Coating slurry, and coating composition having an improved performance in intaglio printing - Google Patents

Abstract

A coating pigment mixt. with improved rotogravure properties contains discrete particles of finely divided CaCO3 (a) and finely divided, swelling, layered silicate particles (b1) and/or an acid activated layered silicate (b2) with a mean particle size of 2-10 mu m in a weight ratio of from 99.5:0.5 to 80:20 (a/b1) and/or 99:1 to 70:30 (a/b2).

Description

The invention relates to a coating pigment mixture with improved Suitability for gravure printing as well as one made from it Coating color.

The use of ground, natural calcium carbonate (CaCO ₃ ) in coating colors for paper and cardboard offers many advantages: high whiteness, good rheology and good economy. For example, combinations of finely divided CaCO ₃ with ultrafine kaolin are used in LWC offset coating inks (LWC = Light Weight Coating), while the carbonate content in the top coat for wood-free papers is up to 80% by weight.

In LWC gravure formulations, on the other hand, CaCO _{3 is} only around 10% - and this also limits the upper coating weight range. Low-gram LWC and ULWC gravure inks (ULWC = Ultra Light Weight Coating) are free of calcium carbonate. Special kaolins, kaolin mixtures or blends of kaolin with talc dominate here.

The reason for this is that CaCO _{3 is} not suitable for gravure printing. It is believed that this is due to the rhombohedral structure of the CaCO ₃ and the too rapid wettability by the gravure printing ink, which allow the ink to be removed too quickly.

Talc / carbonate mixtures are an alternative gravure pigment e.g. in "Wochenblatt für Papierfabrikation" 17 (1991), pages 662-665. Talk is not, however a swellable layered silicate because its surface is hydrophobic is. Such mixtures result in poorly printable Papers; these papers can be because of the sliding properties of the talk does not feed well into printing presses. Also For this reason, paper stacks tend to slip easily.

From EP-A-0 604 095 a process for the treatment of aqueous suspensions of particulate waste material from the paper industry is known, whereby an alkaline earth carbonate is precipitated in the aqueous suspension of the particulate material, so that the particulate material originally present in the alkaline earth carbonate precipitate is included. The product obtained can be used as a filler in papermaking or as a coating pigment. This material is co-aggregate, ie not discrete particles. No more than 10% of the alkaline earth carbonate particles should be larger than 0.5 µm , ie they are too small as components for coating pigments for gravure printing. On the other hand, the coaggregates for coating pigments are too large because they produce uneven strokes, which is why the printing ink easily penetrates.

From US-A-4 725 318 paper fillers are based on heavy calcium carbonate particles and other particles with an average particle size of 10 to 150 microns in weight ratio 100: 1-30 known. Such average particle sizes are too big for coating pigments.

From CA-A-814 304 are surface-modified with organosilanes Pigments are known primarily as fillers for synthetic resin, paper, paints and varnishes, as well can be used as coating pigments. The surface modified Pigments (e.g. layered silicates) are no longer swellable.

From EP-A-0 337 771 printing papers are known which are on a cellulose-containing sheets a first and a second hydrophilic, contain water-absorbing and porous pigment layer, being the first layer of a non-smectitic Pigment, such as kaolin, calcium carbonate, talc or calcium sulfate, and the second layer of a swellable smectic Pigment exists. There is no pigment mixture in the form of a single layer used. About the particle size of the smectite Pigments are not found.

From DE-C-43 12 463 a CaCO ₃ -talkum coating pigment slurry is known which consists of four cover-ground components: (a) 24-64% by weight CaCO ₃ , (b) 5-48% by weight talc , (c) 20-40% by weight H ₂ O and (d) an auxiliary combination of conventional grinding aids and dispersants, the pigment mixture having an average statistical particle diameter of 0.4-1.5 μm. The talc component is not swellable.

EP-A-0 572 037 describes a coating pigment which can be fixed onto paper and cardboard essentially without a binding agent and which results in coating surfaces which can be printed by deep and offset printing. This pigment consists of at least 30% by weight of a swellable layered silicate and has a swelling volume of 5 to 30 ml (based on a suspension of 2 g in 100 ml of distilled water). Minerals from the smectite group, preferably bentonite or synthetic hectorite, are primarily used as swellable layered silicates. The remaining 70% by weight of the coating pigment can consist of conventional coating pigments, such as kaolin, CaCO ₃ , etc. A summary of these coating pigments can be found in "Wochenblatt für Papierfabrikation" 6 (1994), pages 214-218.

The gravure suitability of the coating color produced from this coating pigment leaves something to be desired, however, since it still has a relatively high viscosity even at low solids contents and can therefore only be processed on special coating units, such as film presses, "speedsizers", etc. and the coating weight at most 6 g / m ² and side is limited.

The invention is based, the gravure suitability of calcium carbonate-containing coating pigment mixtures.

Surprisingly, it has now been found that adding significantly smaller amounts of finely divided, swellable layered silicate or acid-activated layered silicate than mentioned above to finely divided calcium carbonate significantly improves its suitability for gravure printing, so that the use of CaCO _{3 is} also increasingly possible in LWC gravure formulations.

The invention thus relates to a coating pigment mixture with improved suitability for gravure printing, comprising a discrete mixture of (a) finely divided calcium carbonate and at least (b) ₁ a finely divided swellable layered silicate and / or (b) ₂ an acid-activated layered silicate with an average particle size of about 2 to 10 μm in a weight ratio of approximately 99.5: 0.5 to 80:20 (a / b ₁ ) or 99: 1 to 70:30 (a / b ₂ ).

The coating pigment mixture according to the invention can be dry Powder mixture or in the form of an aqueous dispersion.

The weight ratio between finely divided calcium carbonate and finely divided swellable layered silicate or acid-activated layered silicate is approximately 99: 1 to 90:10 (a / b ₁ ) or 97: 3 to 80:20 (a / b ₂ ).

The finely divided calcium carbonate (a) and the swellable layered silicate (b ₁ ) or the acid-activated layered silicate (b ₂ ) have an average particle size of approximately 3 to 7 μm, preferably approximately 5 μm.

The average particle size (d50) is determined using a "Malvern Particle Sizer 2600 C "determined.

The swellable layered silicate is preferably an alkali or alkaline earth layered silicate or an alkaline activated Layered silicate.

A suitable layered silicate (b ₁ ) or acid-activated layered silicate (b ₂ ) is, for example, a smectitic layered silicate or is obtainable therefrom.

Smectitic layered silicates are e.g. Bentonite, montmorillonite, Hectorite, saponite or nontronite. Are preferred from This series uses bentonite and montmorillonite. As also Synthetic layered silicates have been very suitable, especially synthetic hectorite. The Layer silicates swellability in the case of alkali layer silicates larger than that of the alkaline earth layer silicates. As Swellable layered silicates can, for example, be the natural ones Alkali bentonites (e.g. Wyoming bentonite) can be used. The required swellability can also be caused by alkaline Activation of layered alkaline earth silicates (e.g. calcium bentonite) be generated. An excessive swelling capacity However, gives coating colors with high viscosity, so that the highly swellable layered silicates in general in less Shares are added. The less swellable Alkaline-earth layered silicates can be added in higher proportions become.

In addition to the swellable layered silicates, acid-activated layered silicates can advantageously be used, mineral acids such as hydrochloric acid and sulfuric acid preferably being used for the activation. Acid-activated bentonites are particularly preferred acid-activated sheet silicates. These materials, known per se as color developers in carbonless papers, have the advantage over the swellable layered silicates that the coating color has a lower viscosity, which allows a higher amount of CaCO _{3 to be added} without disturbing the rheological properties of the gravure printing ink.

The invention also relates to a process for the preparation of the coating pigment mixture according to the invention, which is characterized in that either (A) in a dispersion of finely divided calcium carbonate (a) at least one finely divided swellable layered silicate (b ₁ ) and / or an acid-activated layered silicate ( b ₂ ) stir in; or (B) an aqueous dispersion of finely divided calcium carbonate (a) mixed with an aqueous dispersion of at least one finely divided swellable layered silicate (b ₁ ) and / or an acidically activated swellable layered silicate (b ₂ ).

Process variant (A) is used in particular for the acid-activated layered silicates, these being added to the CaCO ₃ dispersion (slurry) with vigorous stirring. If necessary, the solids content of the slurry must be somewhat reduced in order to maintain the flow properties. A similar procedure can be used with the swellable layered silicates; however, it proves more expedient here to disperse the layered silicate separately from the calcium carbonate with high shear and then to combine the aqueous dispersions.

The layer silicates can of course just as well coating color production, e.g. dosed into the mixing chest become.

The invention also relates to an aqueous coating slip with improved gravure suitability, containing the above coating pigment mixture described and, if appropriate, per se known coating color additives.

The coating color preferably contains binders as further additives, Dispersants, retention aids, anti-foaming agents and / or thickeners.

The invention further relates to a gravure printing paper, in particular an LWC gravure printing paper, which is coated with the coating pigment mixture described above or with a dewatered coating ink. The coating weight of the coating pigment mixture is preferably about 4 to 12 g / m ² , in particular about 6 to 10 g / m ² and side.

In addition to improving the gravure printing properties of the CaCO ₃ , the swellable layered silicates, in particular the swellable bentonites, improve the running properties of the coating colors under high shear conditions, which means that synthetic thickeners can be dispensed with. This is important when reducing or avoiding the formation of beards or squeegees on the coating machine.

This property can be explained by the ability of Layer silicates, under high pressures - like those under the Streichblade prevail - to deliver interlamellar bound water and so to reduce viscosity.

The invention is by the following examples in not restrictive way explained.

Example 1 (a-c)

A calcium bentonite of Turkish origin with a montmorillonite content of 75% by weight (rest of coarser accompanying substances such as Quartz, kaolin, etc.) was down to a solids content of 20% by weight suspended in water and over several hydrocyclone stages cleaned up. Here was the Montmorilloniteteil raised to 90% by weight. This product was used in membrane filter presses thickened to a solids content of 40% by weight and in a twin-shaft mixer with 1% by weight (Example 1a), 3 % By weight (example 1b) and 5% by weight (example 1c) soda 5 min. activated to obtain a swellable sodium bentonite. The products were gently at 90 ° C to a water content of about 10 wt .-% dried and in a rotary rotor mill to an average particle size (d50; measured in "Malvern Particle Sizer 2600c ") of about 5 µm.

The alkaline-activated bentonite samples were dispersed in water at solids contents of 4-18% by weight and, after a maturing time of 6 hours, in amounts of 1 to 10% by weight of a coating carbonate slurry with an average particle size of 3.5 Am, ( measured in the "Malvern Particle Sizer 2600 C") or 90% <2 µm (measured in the Sedigraph 5000) ("Hydrocarb 90" from Omya) added. The viscosity of the blend slurry was adjusted to about 200 mPa.s by adding water. A gravure binder (DL 1010® from DOW, 5 parts per 100 parts of pigment) was then metered into the slurries, and the coating color thus obtained was applied to an LWC base paper (containing wood, using a hand knife at application weights of 6-7 g / m ² , 45g / m ² ) doctored. The coated papers were satin-finished in a laboratory calender and printed with black gravure ink from Kast and Ehinger GmbH in the "LTG" pressure device from Testa Color, a laboratory gravure printing machine for rolls and formats with a mileage of around 100 m / min. The pressure was assessed visually based on the halftone area. The gray scale produced by printing technology (well depth 0.8 mm with a screen density of 70 dots / cm ² ) was assessed according to cloudiness (calm / uneven print image) and missing screen dots (missing dots).

+++

sehr gutes, ruhiges Bildbild mit wenig "missing dots"

- --

unruhiges Druckbild, mit vielen "missings dots"

Here means:

+++

very good, quiet picture with few missing dots

- -

troubled printed image, with many "missings dots"

The results are summarized in Table I. Depending on the degree of activation, the bentonite stock slurry could be produced with a solids content of 18, 8 or 5% by weight. The blend ratios with CaCO ₃ were between 90/10 (example la) and 98/2 (example lc), the solids content of the coating colors and the viscosities being adjusted accordingly. The results of the gravure test were better for all blends than for an uncut CaCO ₃ coating color. It was found that the printability was a function of the amount of bentonite added and the higher degree of alkaline activation of the bentonites.

The Brookfield viscosities (spindles 2 and 3, 100 rpm) were determined. example Bentonite slurry% solid Blends Hydrocarb 90 / Bentonite Coating color Pressure assessment Solid% Brookf.Vis. mPa.s comparison - 100/0 66.3 270 + - 1a 18th 90/10 58.2 255 ++ 95/5 62.0 310 + 97/3 64.5 290 + 1b 8th 95/5 57.5 295 ++ 97/3 62.4 325 ++ 1c 5 96/4 56.5 310 ++ 98/2 62.5 295 +

Example 2 (a-c)

Bavarian calcium bentonite was slurried as in Example 1 and cleaned up using hydrocyclones. The hydrocyclone inlet was with 40 wt .-% (based on bentonite) Hydrochloric acid (30%) added and activated at 95 to 98 ° C for 10 h. The acid-activated bentonite thus obtained was still filtered hot, washed free of chloride and gentle at 80 ° C dried to a solids content of about 7% by weight. The product was placed on a medium sized rotor mill Diameter (d50; Malvern Particle Sizer 2600 c of about Grind 5 µm. The acid activated bentonite became a Hydrocarb® 90 slurry metered in amounts of 5-25 wt .-%, and the slurry was applied to a coating color as in Example 1 processed.

The results are summarized in Table II. Due to the low suspension viscosities of the acid-activated bentonites compared to the alkaline-activated bentonites, higher blending rates with CaCO _{3 are} possible without an overly serious decrease in the solids content. Very good printing results were obtained, especially with the higher blends 2a and 2b. example Blends Hydrocarb 90 / Bentonite Coating color Pressure assessment Solid% Brookf.Vis. mPa.s 2a 75/25 54.6 318 +++ 2 B 85/15 59.2 295 +++ 2c 90/10 61.5 332 ++ 2d 95/5 64.5 320 ++

Example 3

The Hydrocarb® 90 slurry was each mixed with 5% by weight of synthetic hectorite (from Süd-Chemie AG) (average particle size 4.5 μm) and, as in Example 1, processed and examined for a coating color. The results are shown in Table III. Here, too, a better printing result could be achieved than without the addition. example Layered silicate Coating color Pressure assessment Solid% Brookf.Vis. mPa.s 3rd Hectorite 59.6 305 ++

Example 4 (a-c)

The knowledge gained in Examples 1-3 was transferred to a practical gravure coating slip with the composition given in Table IV. example 4a 4b 4c Gravure kaolin Parts 80 80 80 "Hydrocarb 90" parts 20th 20th 20th Bentonite from Example 1c parts - 1 2 Gravure latex Parts 4.5 4.5 4.5 synthetic thickener Parts 1.0 0.6 -

The coating colors were rheologically characterized and coated with an application weight of 7.9 to 8.4 g / m ² in the "Dixon Coater" on LWC base paper of 36 g / m ² .

The results are shown in Table V. example 4a 4b 4c Solids content% by weight 50.4 50.3 50.3 PH value 8th 8.2 8.2 Brookfield 100 rpm mPa.s 2070 1330 650 Water retention (Warren) sec. 38 24th 17th Coating speed m / min 100 100 100 Coat weight g / m ² 7.9 8.2 8.4 Smoothness (Bekk)) sec 1460 1520 1550 Gloss (75 °)% 46.7 47.9 52.4 Whiteness (Elrepho R457)% 71.2 70.6 70.7 Opacity% 90.0 89.6 89.7 Printability + - + ++

According to Table V took with the coating pigment mixtures according to the invention 4b and 4c the smoothness, the shine and the Printability compared to the coating color 4a without Bentonite additive too.

The reduction or removal of the synthetic thickener from 4a to 4c resulted in lower water retention values in the static Warren measurement. In contrast, the capillary viscometer values according to Table VI in the coating pigment mixtures 4b and 4c according to the invention showed an increased reduction in viscosity in the maximum shear range. Such rheological behavior leads to good running behavior ("runnability") on high-speed doctor blade coaters without the risk of the feared phenomena of doctor strips or beard formation. example Viscosity Pa.s Shear rate 1 / sec 4a 4b 4c 100,000 0.18 0.12 0.10 200,000 0.10 0.09 0.08 400,000 0.08 0.07 0.04 600,000 0.06 0.05 0.02 800,000 0.06 0.04 0.02 1,000,000 0.055 0.038 0.015 2,000,000 0.03 0.01 0.006

Claims (11)

1. A coating pigment mixture with improved suitability for intaglio printing, containing discrete particles of (a) finely divided calcium carbonate and at least (b)₁ one finely divided layered silicate capable of swelling and/or (b)₂ an acid-activated layered silicate with a mean particle size of approximately 2 to 10 µm in a weight ratio of approximately 99.5:0.5 to 80:20 (a/b₁) and/or 99:1 to 70:30 (a/b₂)
2. A coating pigment mixture according to Claim 1, characterised in that the weight ratio between finely divided calcium carbonate and finely divided layered silicate and/or acid-activated swelling layered silicate is approximately 99:1 to 90:10 (a/b₁) and/or 97:3 to 80:20 (a/b₂)
3. A coating pigment mixture according to Claim 1 or 2, characterised in that the finely divided calcium carbonate (a) and the swelling layered silicate (b₁) and/or the acid-activated layered silicate (b₂) have a mean particle size of approximately 3 to 7 µm, preferably of approximately 5 µm.
4. A coating pigment mixture according to any one of Claims 1 to 3, characterised in that the swelling layered silicate constitutes an alkaline or alkaline earth layered silicate or an alkaline activated layered silicate.
5. A coating pigment mixture according to any one of Claims 1 to 4, characterised in that the swelling layered silicate (b₁) and/or the acid-activated layered silicate (b₂) constitutes a smectitic layered silicate or can be obtained therefrom.
6. A method for the preparation of the coating pigment mixture according to any one of Claims 1 to 5, characterised in that either (A) at least one finely divided swelling layered silicate (b₁) and/or an acid-activated layered silicate (b₂) is stirred into a dispersion of finely divided calcium carbonate (a); or (B) an aqueous dispersion of finely divided calcium carbonate (a) is mixed with an aqueous dispersion of at least one finely divided swelling layered silicate (b₁) and/or an acid-activated layered silicate (b₂).
7. A method according to Claim 6, characterised in that the dispersion mixture (A) or the dispersion of finely divided swelling layered silicate (b₁) and/or of the acid-activated layered silicate (b₂) undergoes a high-shearing dispersion treatment.
8. An aqueous coating ink with improved suitability for intaglio printing, containing a coating pigment mixture according to any one of Claims 1 to 5 or prepared according to Claim 6 or 7, as well as, optionally, per se known coating ink additives.
9. A coating ink according to Claim 8, characterised in that as further additives it contains binders, dispersants, retention agents, antifoaming agents and/or thickening agents.
10. An intaglio printing paper which is coated with a coating pigment mixture according to any one of Claims 1 to 5 or with a dehydrated coating ink according to Claim 8 or 9.
11. An intaglio printing paper according to Claim 10, characterised in that the coat weight of the coating pigment mixture is approximately 4 to 12 g/m², preferably approximately 6 to 10 g/m² per side.