EP1994220A1

EP1994220A1 - Production of singly and/or multiply coded substrates

Info

Publication number: EP1994220A1
Application number: EP07726593A
Authority: EP
Inventors: Benjamin Mendez
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2006-03-06
Filing date: 2007-03-02
Publication date: 2008-11-26
Also published as: WO2007101822A1; JP2009529076A; CN101432484A

Abstract

Coding composition for a curtain coater whereby a moving web of paper, card or other fibrous material is to be coated one or more times on one or both of the sides comprising lubricants as well as pigments and binders.

Description

Process for the preparation of mono- and / or multicoated substrates

description

The invention relates to a process for the production of mono- and / or multi-coated substrates, such as paper and cardboard, excluding photographic papers and self-adhesive label papers.

State of the art

The curtain coating process is a process known in the art for coating in the photographic industry. The emulsions and liquids used in the photographic industry have a low solids content and only a low viscosity, moreover the application rate is very slow and is less than 600 m / min. By contrast, in the production of graphic papers, pigmented suspensions of high solids content and high viscosities are used in comparison to the suspensions used in the photographic industry. Furthermore, graphic papers are mostly made by blade coating or film pressing at speeds well above 1000m / min. Both the blade application process and the film press application process had disadvantages that affect the quality of the coated paper. In the case of blade application methods, for example, the aggregation of particles, induced by the high shear rates under the blade, can lead to streaks on the paper coating, which adversely affect the paper and board quality. Furthermore, the coating colors used in the graphic industry claim the blade used so much that there is a relatively frequent replacement of this blade in order to ensure consistent line quality on the paper or cardboard.

In addition, the bar distribution on the paper or board surface is influenced by the unevenness of the paper substrate. Uneven bar distribution on the paper surface can lead to visual print unevenness. This quality defect is also referred to as mottling.

In the above-mentioned film press application method, there is usually a narrow operation window, which is determined by the surface properties, the porosity of the substrate to be processed or by the coating color solid content. Furthermore, for each web speed or for each line weight, the above-mentioned close operation window must be re-worked. Thus, in non-optimized film-press coating formulations, an uneven film splitting pattern may occur on the surface of the substrate to be coated. which in turn leads to poor printability of the same. Furthermore, the formation of small drops in the film press coating can occur, which in turn deposit on the substrate and represent a loss of quality of the coated substrate, be it paper, cardboard or cardboard.

The maximum application weight achievable with the film press application method is also lower than that for the knife method (blade method). This limitation is particularly pronounced at high application speeds on the substrate to be processed.

For the two application methods outlined above, the application weight between elevations (mountains) and depressions (valleys) of the substrate to be coated is distributed unevenly, so that the ink acceptance is irregular, which may lead to the mottling already mentioned above. Due to the high transfer speeds, both the film-press method and the knife method (blade method) are very widespread in the production of graphic papers.

From JP 94/89437, JP 93/31 1931, JP 93/177816, JP 93 131718 and EP 0 517 223 B1 and EP-A 1 249 533 is the use of the curtain coating method for coating paper with one or more pigmented coating colors known.

Thus, EP-A 517 223 B1 already discloses a method for producing coated printing paper. The produced spread paper is used in particular during printing, wherein a free-falling curtain curtain is produced from the coating liquid and the pressure base paper is coated with the deaerated coating liquid, so that the free-falling curtain curtain of the coating liquid impinges on the coating base paper. This runs continuously in a direction crossing the free-falling curtain. The coating liquid comprises at least one pigment and at least one binder, a concentration between 50% by weight and 70% by weight and a viscosity between 400 and 4000 mPa.s. The coating liquid is deaerated in an environment having a value of the vacuum of the saturated vapor pressure or below and under the condition that shearing is applied to the coating liquid. The deaerating ratio of the bubbles having a diameter between 0.01 mm and 0.5 mm in the coating liquid is 90% or more. The coating base paper has a primer layer applied by a coating method selected from the group including a blade type coating method or a roller type coating method. From EP 1 249 533 A1 a method for the production of multi-coated paper or cardboard is known. This method is used to produce multi-layer coated papers or cartons except photographic papers and self-adhesive label papers. The multi-coated papers or cartons are particularly suitable for printing, packaging and for labeling purposes, in which at least two liquids to be applied, selected from aqueous solutions or suspensions, are combined as a combined, free-falling curtain and a continuous web of base paper or green cardboard is coated with the combined coating fluid.

The use of the curtain coating process for the finishing of paper and board, as shown in EP 0 517 223 B1 and EP 1 249 533 A1, results in an improved coated surface structure compared to conventional application methods. In particular, increased application speeds are difficult to realize in the curtain coating process at low application weights, since the liquid curtain then becomes unstable. Further, upon impact of the coating color on the paper substrate, the coating color is deflected during free fall and accelerated to substrate speed. With this curtain locally very high shear and strain rates occur in the fluid. The fluid falling as a free curtain can be so heavily stressed that tearing of the fluid film by cavitation bubbles can occur. The risk of rupture increases with increasing substrate web speed, which is the upper limit at which the curtain coating process can be operated.

DE 10 2004 045 171 A1 relates to a process for producing mono- and / or multicoated substrates. These are coated paper or coated paperboard except photographic papers and self-adhesive label papers, which are particularly suitable for printing, packaging and labeling. The substrate, such as, for example, base paper or paperboard, is coated once or more than once with a free-flowing liquid curtain, the coating liquid in particular having an extensional viscosity of between 1 and 1000 Pa. s at a Hencky strain of between 1 and 15.

A coating medium to be used for curtain coating has to have special rheological properties, especially if it is used at very high machine speeds (> 1500 m / min) and / or very low coating weights (<8 g / m ² ) on rough or very rough substrates such as paper or cardboard with a roughness of PPS-10> 4 microns) should be applied. Up to now, it has not been possible in practice to apply line films below 9 g / m ² at high solids content (K> 65%) at speeds above 1800 to 2000 m / min, free of microskip coating with a curtain coater.

Coating paints are currently adjusted with anionic emulsions of the aqueous solution of a copolymer of acrylic acid and acrylamide provided with a protective oil phase. The copolymers cause a thickening to increase the viscosity of the aqueous phase, because they preferentially bind the water. An improvement in the water retention and extensibility of the liquid paint is due to the addition of small amounts of this adjuvant. Depending on the chosen molecular weight of the copolymers of acrylic acid and acrylamide, the thickening and stretching properties of the coating color, i. the viscoelasticity are influenced and controlled. The increase in the amount of this auxiliary (copolymer of acrylic acid and acrylamide) in the coating color, increases the thickening of the coating color and increases the dynamic tensile strength of liquid filaments or of liquid films. On the one hand, an overdose of the auxiliary can reduce or prevent the development of microskip coating. On the other hand, overdosing can also lead to the formation of streaks on the coated fibrous web or to various negative effects at the point of impact of the curtain on the fibrous web. By choosing the molecular weight of the copolymers, these effects can be corrected in individual cases but not completely eliminated in each case.

If the amount of thickener is greatly increased, periodic arcs or a thin "window" will form at the bottom of the curtain, which can also lead to quality deterioration or line scoring, the higher the machine speed or the lower the coating weight or the lower the curtain length is, the harder these problems are to control a satisfactory solution of this problem encountered in practice has not been found so far.

Presentation of the invention

The present invention has for its object to expand the application areas for the curtain coating process with pigmented coating colors.

Slip agents may be added to the coating composition in conjunction with the addition of an anionic emulsion of a copolymer of acrylic acid and acrylamide so that better flowability and stretchability of the coating color and thus better viscoelasticity can be achieved compared to coating compositions to which the above-mentioned copolymer is blended , In the present context, the term "lubricant" Hend understood substances that are added in particular coating colors for curtain coating to prevent the formation of agglomerates or high fluidity by a kind of "lubricating effect" between the components of the coating color essentially pigments, binders and auxiliaries. The "lubricant" also improves the properties of the line when calendering, because the line is also softer and more flexible.

The special properties of the "lubricant" used in conjunction with the properties of the mentioned thickeners, such as anionic copolymers of acrylic acid and acrylamide, according to the present invention for achieving a synergistic effect in the extensibility properties and thus in the processability of the coating color, especially in the curtain The added copolymer has the task of increasing the cohesion of the particles in the coating color by steric bonding (interlocking of the molecular structures) or by chemical bonding, thus increasing the dynamic yield strength of the liquid application medium and / or the coating color in the form of a liquid film, a liquid curtain or a liquid thread considerably increases.The yield strength of a coating composition can be shifted by adding synthetic and / or natural "lubricants". The term "yield strength" is understood to mean the maximum elongation rate and the maximum stretch factor under which a coating color is applied during the application process, in particular with the curtain coater. Exceeding the yield strength would lead to so-called Skip Coating or Microskip Coating. Under Microskip Coating the local tearing of the coating medium or the liquid film is understood, with microscopic holes in the applied coating or the applied layer on the substrate or the fibrous web are caused. These microscopically small uncovered spots on the coated fibrous web significantly deteriorate the printability properties of paper and board.

The stretching rate ε ^{* of} a coating composition is defined according to the invention by means of the following mathematical formula:

ε ^* = dV / ie

being under

ε ^* the ratio of the change of the speed to the change of the film thickness at the point of impact of the curtain, dV the speed of the substrate minus the impact speed of the curtain and below ie the applied film thickness on the paper minus the curtain thickness at the point of impact should be understood.

The stretching factor is also defined according to the invention, wherein

X ₉ the extension of the curtain by the gravitational acceleration = v _ιmp / v ₀ 'X ₀ the extension of the curtain by the acceleration to machine speed = V _B / V _ιmp , X ₁ the total _extension V _B / V ₀ , - V ₆ the machine _speed , Speed of the substrate or fibrous web,

V _{0 is} to be understood as the outflow velocity of the ink from the nozzle gap and Vi _{mp is} the impact velocity of the curtain on the substrate.

When applying or painting with the curtain coater, the following stretch factors and strain rates occur when the machine speed is in the range of 500 to 2500 m / min and the coating weights are applied in the range between 6 to 30 g / m ² at high solids contents.

2 <Xg ^(k) > 10

3 <X ₀ <20

10> X ₁ <60

1, 10 ⁴ <ε ^* <-1, 10 ⁶ (s ^"1 )

For example, lubricants incorporated with the coating composition of the present invention include stearin and similar lubricants such as PVA, starch and oleaginous additives in conjunction with said acrylic copolymer based thickeners. For the purposes of the present invention, the critical expansion rate and critical elongation factor are to be understood as those maximum values of the abovementioned variables, from which microskip coating and banding occur by overstretching of the released medium, for example in the form of the liquid curtain. Calcium stearate is particularly suitable for coating colors that are applied with a curtain coater. Calcium stearate is typically a compound of 94% vegetable stearin and 5% elemental calcium. Dispersions of calcium stearate are weakly alkaline and have a nonionic charge character. They are also emulsified well in water in any mixing ratio, which makes them particularly suitable for coating colors. In conjunction with the anionic thickeners listed below, such as, for example, ethyl acrylate and carboxylic acids (without or with plasticizers and without solvents). semittel), acrylic acid and acrylamide in an oil phase, acrylic acid, acrylate and acrylonitrile (with or without plasticizers and without solvents) and with other thickeners, such as CMC (carboxymethylcellulose) can be a particularly good stretchability and processability of pigmented coating colors, especially when achieve a set in a curtain commissioned work. The advantage of the admixture of a lubricant proposed according to the invention can be observed especially when extreme operating conditions prevail, such as machine speeds> 1500 m / min, line weights <8 g / m ² and if at the same time the solids contents of the coating colors> 65 % lie.

It has proven particularly expedient to carry out the addition of the calcium stearate before the addition of the copolymer when adding the lubricant, for example calcium stearate or stearin in conjunction with one or more anionic copolymers. Thereby, agglomeration can be suppressed and an improvement of the smoothness and the flowability as well as a higher stretchability of the coating composition can be achieved. The addition of further auxiliaries, for example an optical brightener, is preferably carried out after addition of the thickener.

The thickener or acrylic copolymer may be added together with the stearate or stearin. The stearin or the calcium stearate used thereby reduces or prevents the formation of coagulum in the coating colors. Calcium stearate further imparts to the coating composition a slightly water repellent character which can improve the water resistance of the coated substrate such as a paper or a cardboard. Stearates, such as calcium stearate, also have a mild defoaming effect. This additional property is desirable in the production of a coating color. The coating composition may be prepared including the "lubricant" in the following order:

1. dispersion of the pigments and preparation of the pigment mixture,

2. addition of dispersing agent, if necessary

3. addition of binders (styrene, butadiene or acrylic latex, starch, PVA),

4. Adjusting the pH to about 8.5 to 9 (with NaOH or ammonium hydroxide) 5. Addition of lubricant stearate (preferably calcium stearates or vegetable stearines),

6. addition of thickeners and extensional rheology regulators (under high shear), for example copolymers of acrylic acid and acrylamide,

7. Addition of additional aids (optical brightener). The dosage of the stearate or stearin depends on the selected pigment system and the thickener type. Good results can be achieved, for example, with 0.8 part of calcium stearate per 0.2 to 0.5 part of thickener (for example with the abovementioned acrylic polymers). It has proved to be particularly advantageous if coating colors based on pure carbonate, ie 100 wt .-% CaCO ₃ are prepared. This coating color may contain 0.5 to 0.8 parts of stearate and 0.3 to 0.6 parts of thickener to positively influence the rheological properties.

Another alternative is to produce mixtures of carbonate and kaolin. These may in some cases contain a higher proportion of stearate, for example 0.6 to 0.8 parts, and a higher amount (0.4-0.6 parts) of thickener, in order to be able to develop favorable rheological properties at high substrate delivery rates.

In addition to stearin or calcium stearate, other additives which positively influence the properties of the curtain coating composition can be suitable as lubricants. These include, for example, wax emulsions or wax powders based on natural or synthetic waxes, PTFE wax emulsions or powders, polyvinyl acetate / polyvinyl alcohol or polyvinylformamide / polyvinylamine with different saponification degrees. Furthermore, silicone resin emulsions or inorganic lubricants such as talc particles can be used. Furthermore, polymers such as polyethylene glycol, polytetrahydrofuran and polyvinylpyrrolidone can be used with different molecular weights. By a steeper curve of the viscosity η as a function of the strain rate γ, an increase in the viscoelasticity and an increase in the stretch limit of the color films can be achieved. The prerequisite for achieving this positive effect is sufficient compatibility with the other constituents of the coating composition for all substances so that no agglomerate or coagulum is formed. The avoidance of agglomeration or coagulum formation can be achieved by means of suitable aqueous formulations using suitable emulsifiers.

The substrate is coated one or more times with the coating liquid of a free-falling liquid curtain, wherein the coating liquid has an extensional viscosity, measured by the CaBER method between 1 and 10,000 Pa. s at a Hencky strain of between 1 and 15. The coating colors preferably used when using the method proposed according to the invention have the compositions listed below. All percentages given are based on dry weight fractions. A coating composition based on CaCO 3 is used, for example a 77% slurry of calcium carbonate with a particle size of 90% <2 μm (Hydrocarb 90 ME, available from OMYA, Oftringen, Switzerland), and a 74.6 % Premium Clay Slurry from Amazon Premium with a particle size of 98% <2 μm (Amazon Plus available from Kaolin International). Further, the coating colors may contain a binder A of styrene-butadiene latex (Styronal® D 536 available from BASF AG, Ludwigshafen), 50% in water. It is also possible to use various additives, for example an ASE thickener, available from BASF AG (additive C) and, alternatively or in combination, an additive A, polyacrylamide thickener (40 mol% acrylic acid, 60 mol% acrylamide, 20 million molecular weight) and an additive B, polyacrylamide thickener (40 mol% acrylic acid, 60 mol% acrylamide, 44 million molecular weight) admix. Furthermore, the coating colors comprise a surfactant in the form of an aqueous solution of sodium dialkylsulphosuccionates (Lumiten® I-DS 3525), also available from BASF AG. Finally, the coating colors used in the process proposed according to the invention may be admixed with an optical brightener, for example in the form of Blancophor® P, available from Bayer AG, Leverkusen.

The extensional viscosity of the coating liquid, i. the coating color is between 1 and 1000 Pa. s, measured by the Ca B ER method at a Hencky strain of between 1 and 15. Preferably, the extensional viscosity is between 5 and 500 Pa. s, measured by the CaBER method at a Hencky strain of between 1 and 12, and more preferably the extensional viscosity of the coating color is between 10 and 100 Pa. s, measured by the CaBER method at a Hencky strain of between 1 and 8. The shear viscosity (100 rpm Brookfield) of the coating liquid is between 0 and 5000 mPa.s, preferably between 0 and 2000 mPa.s, and particularly preferably the coating liquid has a shear viscosity (100 rpm Brookfield) between 0 and 1000 mPa.s.

The coating liquid may have a solids content of between 40% and 75%, preferably between 50% and 75%, and most preferably between 60% and 65%.

The free-falling liquid curtain comprises at least one binder selected from the group consisting of styrene-butadiene latex binder, ethylene acrylic acid waxes, polyethylene, polyester, styrene-alkylacrylate latex binder, styrene-butadiene-acrylonitrile latex binder, styrene-maleic anhydride binder, styrene-acrylate maleic anhydride binder, polysaccharides, proteins , Polyvinylpyrrolidones, polyvinylalcohol, polyvinylacetate, cellulose and cellulose derivatives. In addition, the free-falling liquid curtain contains organic and / or inorganic pigments selected from the group comprising kaolin, talc, calcium carbonate, precipitated calcium carbonate, titanium dioxide, satin white, synthetic polymer pigments, zinc oxides, barium sulphates, gypsum, silica and aluminum trihydrate.

Danben comprises the free-falling liquid curtain of coating color polyacrylamides having a molecular weight Mw of from 1 to 50 million, preferably a molecular weight Mw of from 5 to 45 million and more preferably the free-falling liquid curtain contains polyacrylamides having a molecular weight Mw of 20 to 40 million.

The Brookfied viscosity of the free-falling liquid curtain is between 20 to 5000 mPa.s, preferably between 20 mPa.s to 2000 mPa.s and more preferably between 20 mPa.s to 1300 mPa.s (spindle no. 2).

The coating weight of the coating color, based on the dry weight on the substrate, is in the range between 0.1 g / m ² to 50 g / m ² .

The pH of the pigmented coating formulations outlined above was adjusted to 8.7 by addition of 10% aqueous NaOH solution. The solids content of the coating formulations outlined above was adjusted by dilution with water.

Associative thickeners may be used in the additives added in coating colors. Associative thickeners are generally hydrophobically modified polymer thickeners with side by side hydrophilic and hydrophobic structural units. Important representatives of this thickener class are the polyurethane thickeners (= hydrophobically modified, ethoxylated urethanes HEUR or PU thickeners) and the HASE thickeners (= hydrophobically modified, alkali-swellable emulsions). The associative thickeners are able to adsorb via hydrophobic groups in the molecule on the surface of the binder particles and to form cell-like, associative complexes in the water phase. This makes it possible to specifically raise the viscosity of the coating colors at medium and high shear rates in binder-rich formulations.

In the case of cellulose ethers, too, hydrophobically modified types (HEER = hydrophobically modified cellulose ethers) are generally disseminated starting from HEC or EHEC. However, these thicken rather conventionally and usually show only a weak associative interaction with the binder particles. Polyurethane thickeners usually comprise polyethylene glycols, the isocyanates (for example hexamethylene diisocyanate) and hydrophobic long-chain alcohols having polymers which have a kind of triblock structure. In its center is the rather hydrophilic polyurethane block, whereas the chain ends are hydrophobically modified by the long-chain alcohol. Suitable thickeners for coating compositions or coating colors are, in addition to free-radical (co) polymers, customary organic and inorganic thickeners, such as hydroxyethyl cellulose or bentonite.

In addition, ionic or anionic polyacrylamides and polyvinylformamides can be used as additives.

The preparation of binder polymers is not limited to a particular method. Rather, all known processes for polymer production can be used. Preference is given to using the processes of emulsion polymerization, suspension polymerization, microemulsion polymerization or microsuspension polymerization which make use of free-radical polymerization.

In the inverse emulsion polymerization and the inverse microemulsion polymerization, water-in-oil emulsions of the polymer are generally obtained with solids contents of from 10 to 50% by weight, preferably from 20 to 40% by weight.

The thickeners are used individually, but it is quite possible to use thickener mixtures.

The associative thickeners or PAM's described above represent a selection of rheological additives which can be added to the coating composition proposed by the invention.

With the coating color proposed according to the invention, in particular, the coating effects can be considerably minimized in the context of an application method to a substrate to be coated, such as paper or paperboard, as shown in detail in the examples below.

The extensional viscosity of the coating color proposed according to the invention is determined in a so-called CaBER experiment, in which case a liquid thread or film is formed whose thickness subsequently decreases under the influence of the surface tension σ as a dominant force. The temporal decrease of the film thickness D _mιd (t) is measured. From this, the tensile viscosity η _E , a _{PP is} determined according to the following relationships.

2σ

X r =

D _mid (t) The resulting stretching rate ε (t) results according to:

_ε = __? _ (,) ^< »- <'>

D mid dt

The extensional viscosity r \ _{E app} = τ _E / ε adjusts itself accordingly:

σ

IE,, aapppp dD _mιd (t) dt

Design variant / examples

The proposed solution according to the invention allows in the experiment, the striking of 7.9 g / m ² at speeds up to 2500 m / min without the occurrence of Skip Coa- ting at a solids content of 64.4% and at a Brookfield viscosity of 1300 mPa. s. In the experiment it could also be shown that at 1800 m / min a coating weight of 6 g / m ² at a solids content of 68.4% and at a viscosity of 2650 mPa.s to the substrate with good quality and good runnability with a curtain Application work (Curtain Coater) could be applied. Coating paints with a pigment mixing ratio of calcium carbonate to kaolin at 70:30 and 60:40 at rates above 1500 m / min and even up to 2500 m / min were observed without microcoating occurring.

In Tables 1 to 4 reproduced below, the coating color formulations according to the invention, including a lubricant, are broken down with regard to their composition. Table 1

Table 2

Table 3

Table 4

In addition to the mentioned stearates or in addition to the stearin mentioned, other additives besides calcium stearate are used in the sense of the solution proposed according to the invention in order to positively influence the properties of the coating composition for curtain coating. These further additives include, for example, wax emulsions or powders based on natural or synthetic waxes, PTFE wax emulsions or powders, polyvinyl acetate / polyvinyl alcohol or polyvinylformamide / polyvinylamine with different degrees of saponification. Furthermore, the silicone resin emulsions or inorganic lubricants such as talc particles can be used. It is also possible to use polymers such as, for example, polyethylene glycol, polytetrahydrofuran or polyvinylpyrrolidone with different molecular weights. Due to the already mentioned steeper course of the viscosity η as a function of the elongation and shear rate, an increase in the maximum yield strength of the coating color is achieved, further improving the film-forming properties and optimizing the flowability, ie all the processability of the coating color favors properties and properties that the Quality of the substrate obtained, be it a film, be it a cardboard, be it significantly improve a paper web or the like. A prerequisite for the achievement of a positive effect is sufficient compatibility with the other constituents of the coating composition for all substances, so that no agglomerates or coagulates are formed. Agglomerate or coagulum formation can be achieved by means of suitable aqueous formulations using suitable emulsifiers.

The viscosity of the coating colors according to the respective formulations listed below was determined by means of a Brookfield Viscometer (RVT), available from Brookfield Engineering Laboratories, Inc., Stoughton, Massachusetts USA, at a temperature of 25 ° C. To measure the Brookfield viscosity, 600 ml of the dispersion was placed in a 1 liter receiving beaker and the viscosity was measured with spindle # 4 at spindle RPM of 100 rpm. The coating colors according to the following formulations were applied to the substrate (paper, cardboard) by curtain coating.

Elongation rheology measurement

To determine the extensional rheology of the coating colors, a Haake CaBER 1 device from ThermoElectron was used. The sample liquid (coating color) is applied between two punches. The diameter of the cylindrical punch is 6 mm, the gap between the punches 3 mm and the final gap height 11 mm. Within 20 ms, the sample liquid drop is stretched from 3 mm to 11 mm. This forms a liquid thread. The thread diameter (Dmid) is detected by means of a laser micrometer in the middle between the two punches. The extensional viscosity is determined by the following formulas.

Assuming the surface tension (σ) as the driving force for the tearing of the liquid film in the CaBER experiment results in an increasing shear stress according to the following relationships.

2σ

X r = ^■

_Dmιd {t)

The resulting strain rate ε (t) results according to:

e = - ± D ( ₍ ) ^aD - ' ^(t) mid dt The ^extensional viscosity ^{ε αpp E is} calculated as follows:

σ

IE,, aapppp dD _mιd (t) dt

As a measure of the stretching of the liquid thread or film, the so-called Hencky strain ε (t) is calculated:

For the calculation of the theological quantities on the basis of these equations, the change of the thread diameter δDmid (t) / δt is calculated numerically from the measured values Dmid (t).

Literature:

1. Entov, V.M. and Hinch, EJ. , J. Non-Newt. Fluid Mech., 72 (1), 31 (1997) 2. McKinley, G.H. and Tripathi, A., J. Rheol., 44 (3), 653 (2000).

3. Willenbacher, N., Proc. of the Int. Congress on Rheology, Seoul, Korea (2004)

FIGS. 1 and 2 show an alternative embodiment of a device for applying coating slip to a web-shaped substrate in accordance with the curtain coating method.

The illustration according to FIG. 1 shows an application device 1 with which the upper side of a web-shaped substrate 2 is coated. A film 3 emerging from an opening of the applicator 1 strikes the top of the web-like substrate 2 at an application point 4. The web-shaped substrate 2 is guided in the conveying direction 7 via a first deflection roller 5 and a second roller 6. In the area between the first deflecting roller 5 and the second roller 6, the point of impingement 4 of the film 3 lies on the upper side of the web-shaped substrate 2. FIG. 2 shows the application device 1 according to the illustration in FIG. 1 on an enlarged scale.

The application device 1 comprises a nozzle body 8, on the underside of which an outlet opening 9 is located. The coating ink stored in the nozzle body 8 according to the composition discussed in Examples 1 to 6 emerges from the outlet opening 9 in the form of a film 3, whereby the film 3 continuously tapers in the direction of the application site 4 and on the application site 4 onto the surface 10 of the web-shaped substrate 2 impinges. Before the film 3 exits the outlet opening 9, the film 3 is accelerated and forms on the underside of the outlet opening 9, perpendicular to the plane of the drawing, as a curtain extending over the width of the web-shaped substrate 2. After the exit of the film 3 from the outlet opening 9, this contracted and is deflected at the point of impact 4. The surface 10 of the web-shaped substrate 2 has a roughness 1 1; Corresponding to the roughness 11 of the surface 10 of the web-shaped substrate, a film thickness 12 of the coating color forms on the surface 10 of the web-shaped substrate 2. The sheet-like substrate 2 may be paper, cardboard or even plastic films or the like. An air scraper 13 serves to retain the air layer entrained by the substrate surface.

In the preparation of the coating color, an aqueous pigment dispersion is first prepared. For this purpose, pigments are mixed with supplied water until the desired solids content and the desired viscosity are reached. The viscosity of the slurry is preferably set very low for degassing. It is less than 500 mPa.s (Brookfield 100 rpm 20 ° C), preferably less than 200 mPa.s (Brookfield 100 rpm 20 ° C). As pigments, e.g. Calcium carbonate, kaolin, titanium dioxide or talc can be used. The binder may be added to the pigment dispersion in the container if it does not interfere with the subsequent degassing. Alternatively, the binder can also be admixed only after degassing. The degassing takes place within a degassing device, in which the supplied dispersion is sprayed at reduced pressure. In this case, the gases emerging from the dispersion, in particular air, are discharged from the container. In order for the degassable components to be removed from the dispersion, i. emerge from the coating color, the dispersion is distributed over a large surface at very low absolute pressure. Preferably, the enlargement of the surface of the coating color (dispersion) by spraying by means of nozzles; Alternatively, an enlargement of the O berfläche on the use of centrifugal plates would be conceivable.

The pigmented dispersion can then be admixed with the thickener and the additives in the absence of air. The degassing device can for

Example, two degassing stages connected in series, in which the coating degassing is carried out continuously in succession before the thickener and the additives are mixed under exclusion of air. Depending on the nature of the coating color, it is also possible for more than two, for example three or five degassing stages, to be connected in series. The degassing stages contain spray degasser with an evacuable container. For conditioning the coating color of the first degassing can be preceded by a tempering in which the desired temperature of the coating color can be adjusted by heating or cooling.

Bezugszeichenhste

1 application device 2 web-shaped substrate

3 movie

4 job site

5 first guide roller

6 second roller 7 conveying direction

8 nozzle body

9 outlet opening

10 surface substrate

1 1 Roughness 12 Film thickness

13 air scrapers

Claims

claims

A curtain coating composition for a curtain coater with which a running paper, board or other fibrous web, on one or both sides, is to be coated once or more than once, the coating composition comprising lubricants in addition to pigments and binders ,

2. coating composition according to claim 1, characterized in that the added lubricants are stearates, in particular stearin, preferably calcium stearate.

3. coating composition according to claim 1, characterized in that it additionally contains a thickener.

4. coating composition according to claim 3, characterized in that a copolymer is used as the thickener, which contains acrylic acid or acrylamide or acrylic acid and acrylamide.

5. coating composition according to claim 3, characterized in that the addition of lubricant to the coating composition before the addition of the thickener takes place.

6. coating composition according to the preceding claims, which is prepared in the following order:

1. dispersion of the pigments and preparation of the pigment mixture,

2. addition of dispersant,

3. addition of binders such as styrene butadiene, acrylic latex, starch, PVA,

4. Adjust the pH to about 8.5 to 9.0 with sodium hydroxide or with ammonium hydroxide,

5. addition of the lubricant, preferably calcium stearates or vegetable stearines,

6. Addition of thickeners and the extensional rheology regulators or copolymers of acrylic acid and acrylamide and 7. addition of further auxiliaries, such as optical brighteners.

7. coating composition according to claims 1 to 6, characterized in that the yield strength (maximum stretch from which a film breaks off) by addition of synthetic and / or natural lubricants is changed, wherein the strain rate ε ^* , which is a color film during the transfer the curtain film on the substrate, is defined by the following mathematical formula:

ε ^* = dV / dh with

ε ^* : ratio of the change of the speed to the change of the film thickness at the impact point of the curtain, dV: speed of the substrate minus impact speed of the curtain, ie: applied film thickness on the substrate minus curtain thickness at the point of impact.

8. coating composition according to claim 1, characterized in that the lubricant from the group of wax emulsions or wax powder based on natural or synthetic waxes, PTFE wax emulsions or

PTFE wax powder, polyvinyl acetate / polyvinyl alcohol or polyvinylformamide / polyvinylamine with different degrees of saponification is selected.

A coating composition according to claim 1, characterized in that the lubricant is a silicone resin emulsion or an inorganic lubricant containing talc particles.

10. coating composition according to claim 1, characterized in that the lubricant is selected from the group of polyethylene glycol, polytetrahydrofuran and polyvinylpyrrolidone having different molecular weights.

1 1. A process for the production of single- and / or multi-coated paper and / or cardboard, except photographic papers and self-adhesive label papers, which are particularly suitable for printing, packaging and labeling, wherein a substrate with the coating composition according to one or more of claims 1 to 10 of a free-falling liquid curtain is coated one or more times and the coating liquid has an extensional viscosity, measured by the Ca B ER method, between 1 and 10,000 Pa. s with a Hencky strain of between 1 and 15.

12. The method according to claim 11, characterized in that the extensional viscosity, measured by the Ca B ER method, between 5 and 500 Pa. s is at a Hencky strain between 1 and 12.

13. The method according to claim 11 or 12, characterized in that the extensional viscosity, measured by the CaBER method, between 10 and 100 Pa. s at a Hencky strain is between 1 and 8.

14. The method according to any one of claims 1 1 to 13, characterized in that the coating composition has a Brookfield viscosity between 0.1 and 5000 mPa.s.

15. The method according to claim 14, characterized in that the coating composition has a Brookfield viscosity between 0.1 and 2000 mPa.s.

16. The method according to claim 14 or 15, characterized in that the coating composition has a Brookfield viscosity between 0.1 and

1000 mPa.s has.

17. The method according to any one of claims 1 1 to 16, characterized in that the coating composition has a solids content of between 40% and 75%.

18. The method according to claim 17, characterized in that the coating composition has a solids content of between 50% and 75%.

19. Process according to claim 17 or 18, characterized in that the coating composition has a solids content of between 60% and 75%.

20. The method according to any one of claims 1 to 19, characterized in that the coating composition contains at least one binder.

21. The method according to any one of claims 1 1 to 20, characterized in that the coating composition comprises organic and inorganic pigments.

22. The method according to any one of claims 1 1 to 21, characterized in that the coating composition contains polyacrylamides having a molecular weight Mw of 1 to 50 million.

23. The method according to claim 22, characterized in that the coating composition contains polyacrylamides having a molecular weight Mw of 5 to 45 million.

24. The method according to claim 22 or 23, characterized in that the coating composition contains polyacrylamides, which particularly preferably have a molecular weight Mw of 20 to 40 million.

25. The method according to any one of claims 1 1 to 24, characterized in that the coating composition has a Brookfield viscosity between 20 to

5000 mPa.s, preferably 20 mPa.s to 2000 mPa.s and more preferably from 20 mPa.s to 1300 mPa.s.

26. The method according to claims 1 1 to 25, characterized in that the coating weight of the coating composition, based on the dry weight on the substrate, between 0.1 g / m ² to 50 g / m ² .

27. Process according to claim 21, characterized in that the pigment is selected from the group of clay, kaolin, talc, calcium carbonate, titanium dioxide, satin white, synthetic polymer pigments, zinc oxides, barium sulphates, gypsum,

Silica and aluminum trihydrate.

28. The method according to claim 20, characterized in that the binder is selected from the group consisting of styrene-butadiene latex binder, styrene-acrylate latex binder, styrene-butadiene-acrylonitrile-latex binder, styrene-maleic anhydride

Binders, styrene-acrylate maleic anhydride binders, polysaccharides, proteins, polyvinylpyrrolidones, polyvinyl alcohol, polyvinyl acetates, cellulose and cellulose derivatives.

29. The method according to any one of claims 1 to 28, characterized in that the coating composition comprises one or more polymers based on ethylene acrylic acid waxes, polyethylene, polyester, styrene-butadiene latex binder, styrene acrylate latex binder, styrene-butadiene-acrylonitrile latex binder, styrene maleic anhydride binder , Styrene acrylate maleic anhydride binders, polysaccharides, proteins, polyvinylpyrrolidones, polyvinyl alcohol, polyvinyl acetate, cellulose and

Contains cellulose derivatives and silicones.

30. substrate, in particular paper or paperboard, which is produced in a method according to claims 1 1 to 29.

31. Use of the coating composition according to claims 1 to 10 for use as a coating agent in a process according to one of claims 11 to 29, characterized in that its solids content is greater than 60% and for curtain applicators at machine speeds greater than 1200 m / min and line weights less than 30 g / m ^{2 is} used.