EP1914345A1 - Process for coating substrates with polyvinylacetal using curtain coating - Google Patents

Abstract

A method for coating a substrate with one or more layers containing polyvinyl acetal (I), by applying an aqueous solution or dispersion containing (I) as binder, is carried out by curtain coating.

Description

The invention relates to a process for coating substrates with one or more layers containing polyvinyl acetals by curtain coating.

State of the art

The coating of substrates such as paper or cardboard is being carried out industrially for the production of photographic papers, ink-jet printing papers or food packaging. These coatings are said to provide the substrate with improved receptivity to inks or pigments, or to inhibit the absorption of fats or oils by a barrier effect. The binder used for such layers is preferably polyvinyl alcohol or its derivatives. Binder systems of this type are for example in EP 0 905 313 . US 4,418,119 . US 5,587,204 or US 6,193,831 disclosed.

Coatings for papers must be applied in the paper industry for large areas of constant thickness at high paper web speeds. For this purpose, contact methods in which the substrate has mechanical contact with a tool and contactless methods in which this is avoided are used.

Contact methods are e.g. Processes with size presses, film presses, doctor blade processes, blade processes or contour coating processes. These processes have considerable disadvantages, especially at high paper web speeds and high layer thicknesses. Thus, under the contact of the applicator tool, the paper web may tear, the film coating may have defects, or the coated surface may be irregular due to so-called "misting".

Contactless methods are, for example, the slot die coating process, spray coating or curtain coating. The curtain coating process is particularly at high paper web speeds used and delivers uniform coating thicknesses in high quality. Curtain coating processes are, for example, in US 2003/0188839 A1 . US 2006/0099410 A1 or US 2003/0194501 A1 described.

The quality of the curtain coating process depends to a great extent on the consistency of the curtain of the liquid. In this case, only slight variations in the thickness of the curtain or the density of the liquid may occur, otherwise this would result in errors in the coating. Furthermore, the curtain must not tear off, which places high demands on the constancy of the viscosity of the liquids used for coating.

Accordingly, curtain coating processes are used only for the coating colors and surface applications of polyvinyl alcohols or latices established in the paper industry, optionally combined with further binders such as starch and / or with fillers or pigments. However, the binder systems mentioned have certain disadvantages in the barrier action against fats and oils, which are due to their chemical composition. Furthermore, they are unable to disperse selected pigments in the desired manner without having to add a barrier-resistant dispersant.

task

The object of the present invention was therefore to provide binder systems for the curtain coating process which have an improved barrier action against fats and oils and / or a sufficient dispersing action for pigments.

Surprisingly, it has been found that high-quality coatings with curtain coating processes can also be prepared with water-soluble polyvinyl acetals as binders.

Presentation of the invention

The present invention therefore provides a process for coating a substrate with one or more layers comprising polyvinyl acetals by applying aqueous solutions or dispersions which contain one or more polyvinyl acetals as binders by means of curtain coating.

The process according to the invention is used in particular for producing coatings having at least one (partial) layer with an oil-tight or grease-tight barrier effect. Despite the use of water-soluble polyvinyl acetals, the process according to the invention makes it possible to produce coatings with outstanding barrier action against oils or greases.

The applied layers or the solutions or dispersions used for this purpose comprise one or more polyvinyl acetals as binders and barrier formers and optionally pigments such as chalk, kaolin, talc, calcium carbonate, titanium dioxide, silica or aluminum oxide. It is possible to use additional binders, e.g. Latexes based on styrene butadiene, styrene-acrylate, pure acrylate, vinyl acetate, vinyl acetate copolymers or polyurethane, starch or cellulose derivatives. Furthermore, the applied layers or the solutions or dispersions used for this purpose may contain the additives customarily used in papermaking, such as optical brighteners, defoamers, if appropriate also crosslinkers, rheological additives or plasticizers.

The solutions or dispersions used preferably contain 10-100% by weight of polyvinyl acetal. In particular, solutions or dispersions containing pigments have a proportion of polyvinyl acetals of 10-50% by weight, preferably 10-25% by weight. Without pigments, the proportion of polyvinyl acetals can be between 100 and 50% by weight, in particular 100-75% by weight. All data relate to the dry mass of the solutions or dispersions. The remaining shares are possibly formed from the above-mentioned tools.

The method according to the invention makes it possible to produce a coating which has a barrier effect for oils or fats. The barrier effect can be determined according to TAPPI T 559 kit test and / or TAPPI T 454. Preferably, at least one layer has an oil or grease-tight barrier according to the TAPPI T 559 kit test greater than 5, particularly preferably from 6 to 12 and in particular from 7 to 12. Alternatively or additionally, at least one layer has an oil or grease-tight barrier of TAPPI T 454 greater than 950, in particular greater than 1050, preferably greater than 1100. Values greater than 1800 are usually no longer determined in the TAPPI T 454 test, whereby barrier effects of 3000 can be achieved in the process according to the invention.

The barrier effect of TAPPI T 454 and / or TAPPI T 559 kit test refers to a single layer (i.e., a single application) and can be achieved by multiple application e.g. be significantly improved by two or three layers, wherein the barrier effects of several layers does not behave additively. Due to the filler, the barrier effect of layers with pigments is naturally lower than that of layers on almost pure polyvinyl acetal.

The coating can be applied in several, in particular 2, 3 or 4 partial layers. The layers may have the same or a different composition. If the coating is composed of several partial layers, then at least one layer of the coating must have the required oil or grease-tight barrier effect.

In the context of the curtain coating process, in a multilayer structure, the oil- or grease-proof layer can be applied directly to the substrate and provided in the same operation with a further coating, which contains printable pigments, for example. Also conceivable is the first application of a conventional surface sizing agent, for example based on styrene-maleic anhydride, modified starch, styrene-acrylate or pure acrylate latex, followed by a coating based on polyvinyl acetal in the manner already described.

Preferred substrates for the process according to the invention are papers, in particular those having a weight per unit area of 10 to 100 g / m ² or board paper, in particular those having a basis weight of more than 100 g / m ² . Particularly preferably, the substrates have a surface of paper or cardboard.

The polyvinyl acetals used in the process according to the invention are preferably readily soluble in water in order to produce layers having a high solids content. Their water solubility, which is a function of the acetal content, should be at least high enough to obtain a homogeneous dispersion. The viscosities of the pure polyvinyl acetals in water range from 5-2000 mPa.s.

Except where otherwise noted, the viscosity data was determined using a rotational "Brookfield RTV" at 25 ° C and 100 rpm.

In order to obtain sufficient water solubility, the polyvinyl acetals used according to the invention preferably have a degree of acetalization of from 0.1 to 30 mol%. The proportion of vinyl acetate units is preferably 0.1 to 15 mol%.

The polyvinyl acetals which can be used in the process according to the invention are formally constructed as a terpolymer from the monomer units vinyl alcohol, vinyl acetate and vinyl acetal, but in addition olefin units, such as, for example, Include ethylene, propylene or butylene.

As the preferred polyvinyl acetal, polyvinyl butyral is used. Furthermore, the polyvinyl acetals used can have silanol or carboxylate groups.

With the method according to the invention, one or more layers of polyvinyl acetals can be applied to the substrate. The layers may be in the nature of the polyvinyl acetal, in the thickness or in the added additives such as pigments or the like. differ. That's how it works Substrate, for example, with several order layers (layers), each containing different polyvinyl acetals coated. For example, various polyvinyl acetals have different degrees of acetalization, degrees of hydrolysis, or degrees of polymerization and / or are chemically different by substitutions or comonomers.

The solutions or dispersions used according to the invention, without fillers or pigments, preferably have a viscosity of 3 to 150 mPa.s at a solids content of 5 to 20% by weight. If solutions or dispersions with fillers or pigments are used according to the invention, the viscosity can be between 80 and 2000 mPa.s at a total solids content of 10 to 65% by weight.

After application of the aqueous solutions of the polyvinyl acetals, they must be dried, for example, by means of infrared dryers, hot-air dryers, etc. In the normal dryer operation of finished paper and cardboard with a final moisture of 1-10% drying is not critical.

• Viskosität der Lösung/Dispersion ohne Pigmente 50 - 500 mPa.s, mit Pigmenten 80 - 2000 mPa.s
• Auftragsgewicht gesamt (ein- und mehrlagig additiv) 1-20 g/m²
• Bei zweilagigem Auftrag: Auftragsgewicht 1. Lage 1-5 g/m², 2. Lage 1-10 g/m²
• Geschwindigkeit des Substrats (Papier) 300-1000 m/min
• Feststoffgehalt der Lösung/Dispersion ohne Pigmente 5-20 Gew.%, mit Pigmenten 10 - 65 Gew.%

Suitable process parameters or tools for carrying out the curtain coating process, in particular for the application of coatings having a plurality of partial layers, are, for example, in US 2003/0194501 disclosed. In the process according to the invention, the following process parameters have proved successful:

• Viscosity of the solution / dispersion without pigments 50 - 500 mPa.s, with pigments 80 - 2000 mPa.s
• Total application weight (single- and multi-layer additive) 1-20 g / m ²
• For two-layer application: application weight 1. layer 1-5 g / m ² , 2nd layer 1-10 g / m ²
• Speed of the substrate (paper) 300-1000 m / min
• Solids content of the solution / dispersion without pigments 5-20% by weight, with pigments 10 - 65% by weight

Examples

Solutions or dispersions having the compositions described in Table 1 (with pigments) and 3 (without pigments) were prepared using the polyvinyl alcohols described in the following examples (comparative examples 1-4 not according to the invention) or polyvinyl acetals (inventive examples 5-16).

These solutions or dispersions were then applied in one or two layers to papers having a basis weight of 80 g / m ² by means of curtain coaters. The working width was 660 mm, the speed 300 m / min. The coated papers were heated at a temperature of max. Dried 200 ° C and then examined for their barrier effect. Tables 2 and 4 show the results.

It has been found that solutions or dispersions with polyvinyl acetals can be well processed by curtain coater and the coatings thus obtained have better barrier properties against fats and oils than the polyvinyl alcohols usually used. At the same time polyvinyl acetals have a good dispersing effect for pigments so that even dispersions with a high solids content can be used in curtain coater processes.

Polyvinyl alcohols (not according to the invention) Example 1: Carboxyl-containing PVA Mowiol 15-99 (Kuraray Europe GmbH) Example 2 Silanol Group-Containing PVA K Polymer Type KL-318 (Kuraray Europe GmbH) Example 3 Terpolymer of Vinyl Acetate, Vinyl Alcohol and Ethylene Units Exceval Type HR3010 (Kuraray Europe GmbH) Example 4: Cationized PVA R-Polymer Type 1130 (Kuraray Europe GmbH) Polyvinyl acetals (according to the invention) Example 5:

720 g of polyvinyl alcohol Mowiol 28-99 are dissolved in 6480 ml of water. After entry of 29.01 g of n-butyraldehyde is with 20% hydrochloric acid, the pH is adjusted to about 1. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 23 mPa.s for a 4 wt.% Solution in water and 302 mPa.s for an 8 wt.% Solution in water. The product has no turbidity and no precipitation point. The glass point (Tg) determined by DSC measurement is 78 ° C.

Example 6:

1080 g of polyvinyl alcohol Mowiol 15-99 are dissolved in 6,120 ml of water. After adding 60.8 g of n-butyraldehyde, the pH is adjusted to about 2 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted with 10% sodium hydroxide solution to a pH of 6-6.5 and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 15 mPa.s for a 4 wt.% Solution in water and 138 mPa.s for an 8 wt.% Solution in water. The solutions are already cloudy at room temperature. If heated, they will precipitate at about 76 ° C. The glass point (Tg) determined by DSC measurement is 83 ° C.

Example 7:

1440 g of polyvinyl alcohol Mowiol 4-98 are dissolved in 5760 ml of water. After entry of 172.02 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 5 mPa.s for a 4 wt.% Solution in water and 21 mPa.s for an 8 wt.% Solution in water. The solutions are already cloudy at room temperature. If you heat them, they fall out at about 30 ° C. The glass point (Tg) determined by DSC measurement is 79 ° C.

Example 8:

1440 g of polyvinyl alcohol Mowiol 5-88 are dissolved in 5760 ml of water. After entry of 92.52 g of n-butyraldehyde is with 20% hydrochloric acid, the pH is adjusted to about 1. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 5 mPa.s for a 4% strength by weight solution in water and 23 mPa.s for an 8% strength by weight solution in water. The solutions are cloudy at about 30 ° C. If you heat them up, they will fall out at about 35 ° C. The glass point (Tg) determined by DSC measurement is 72 ° C.

Example 9:

1080 g of polyvinyl alcohol Mowiol 15-99 are dissolved in 6,120 ml of water. After adding 60.8 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The viscosity according to Hoppler according to DIN 53015 is 14 mPa.s for a 4% strength by weight solution in water and 124 mPa.s for an 8% strength by weight solution in water. The solutions are already cloudy at room temperature. If you heat them, they fall out at about 70 ° C. The glass point (Tg) determined by DSC measurement is 81 ° C.

Example 10:

1080 g of polyvinyl alcohol Mowiol 26-88 are dissolved in 6,120 ml of water. After entry of 69.4 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 23 mPa.s for a 4 wt.% Solution in water and 328 mPa.s for an 8 wt.% Solution in water. The solutions are already cloudy at room temperature. If you heat them, they fall out at about 33 ° C. The glass point (Tg) determined by DSC measurement is 78 ° C.

Example 11:

1062.5 g of polyvinyl alcohol Exceval RS-2117 are dissolved in 7437.5 ml of water. After entry of 41.57 g of n-butyraldehyde is with 20% hydrochloric acid, the pH is adjusted to about 1. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The viscosity according to Hoppler according to DIN 53015 is 19 mPa.s for a 4% by weight solution in water and 261 mPa.s for an 8% strength by weight solution in water. The solutions are cloudy at about 41 ° C. If heated further, they precipitate at about 48 ° C. The glass point (Tg) determined by DSC measurement is 77 ° C.

Example 12:

900 g of polyvinyl alcohol Exceval HR-3010 are dissolved in 6300 ml of water. After entry of 43.34 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 12.6 mPa.s for a 4 wt.% Solution in water and 130.4 mPa.s for an 8 wt.% Solution in water. The solutions are already cloudy at room temperature. If you heat them, they fall out at about 30 ° C. The determined by DSC measurement glass point Tg) is 55 ° C.

Example 13:

720 g of polyvinyl alcohol K-polymer KL-318 are dissolved in 6480 ml of water. After entry of 28.71 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 22 mPa.s for a 4 wt.% Solution in water and 282 mPa.s for an 8 wt.% Solution in water. The solutions become cloudy at about 73 ° C. If you heat them up, they will not fall out. The glass point (Tg) determined by DSC measurement is 78 ° C.

Example 14:

900 g of polyvinyl alcohol R-polymer R-1130 are dissolved in 6300 ml of water. After entry of 21.67 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 20 mPa.s for a 4 wt.% Solution in water and 261 mPa.s for an 8 wt.% Solution in water. The solutions are cloudy at about 24 ° C. If heated further, they precipitate at approximately 53 ° C. The glass point (Tg) determined by DSC measurement is 80 ° C.

Example 15:

1080 g of polyvinyl alcohol C-polymer C-506 are dissolved in 6,120 ml of water. After entry of 37.71 g of n-butyraldehyde, the pH is adjusted to about 1 with 20% strength hydrochloric acid. The solution is stirred for 2 hours at the adjusted pH. Subsequently, the solution is adjusted to a pH of 6-8 with 10% sodium hydroxide solution and stirred for 1 hour. The Höppler viscosity according to DIN 53015 is 4 mPa.s for a 4 wt.% Solution in water and 13 mPa.s for an 8 wt.% Solution in water. The solutions are cloudy at about 38 ° C. If heated further, they fall out at about 44 ° C. The glass point (Tg) determined by DSC measurement is 63 ° C. Table 1: Composition of the solutions / dispersions with pigments, data in% by weight example 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Polyvinyl alcohol or acetal according to Example 1 6 2 13 3 12 4 14 1 6 2 13 3 12 4 14 Proportion of polyvinyl alcohol or polyvinyl acetal 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 Kaolin, 80% <2 μm, shape factor 15-25 65 65 65 65 65 65 65 65 Kaolin, 80% <2 μm, shape factor 50-60 65 65 65 65 65 65 65 65 glyoxal 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 Sodium bis (2-ethyl-hexyl sulfosuccinate) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Single application with dispersion according to example 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Application weight (g / m2) 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Fat density according to Tappi T 454 [s] 800 1010 920 1050 840 1100 820 950 900 990 980 1090 910 960 920 1020 KIT after Tappi 559 4 6 5 7 5 8th 5 7 5 7 5 8th 5 7 5 8th Double job, consisting of example and example 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 16 17 18 19 19 19 19 19 24 25 26 27 27 27 27 27 1. Order weight order (g / m2) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2. Order weight order (g / m2) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Fat density according to Tappi T 454 [s] 980 1120 1020 1210 1250 1190 1240 1270 1050 1150 1080 1140 1060 1090 1120 1190 KIT after Tappi 559 7 8th 7 9 7 9 7 8th 7 8th 7 10 7 9 7 9 example 32 33 34 35 36 37 38 39 40 41 42 Polyvinyl acetal according to Example 5 6 7 8th 9 10 11 12 13 14 15 Proportion of polyvinyl acetal 92 92 92 92 92 92 92 92 92 92 92 glyoxal 8th 8th 8th 8th 8th 8th 8th 8th 8th 8th 8th Surfynol SEF 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Single application with dispersion according to example 32 33 34 35 36 37 38 39 40 41 42 Application weight (g / m2) 5 5 5 5 5 5 5 5 5 5 5 Fat density according to Tappi T 454 [s] 1260 1440 1440 990 1440 1140 1200 1500 1200 1260 1260 KIT after Tappi 559 8th 7 8th 7 9 8th 7 7 7 9 7 Double order of two layers according to example 32 33 34 35 36 37 38 39 40 41 42 Order weight (g / m2) 1st order 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Application weight (g / m2) 2nd order 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Fat density according to Tappi T 454 [s] 1680 > 1800 > 1800 > 1800 > 1800 > 1800 > 1800 > 1800 > 1800 > 1800 > 1800 KIT after Tappi 559 10 9 10 8th 11 10 8th 8th 9 10 9

Claims (10)

Process for coating a substrate with one or more layers comprising polyvinyl acetals by application of aqueous coatings or dispersions containing as binder one or more polyvinyl acetals by curtain coating. A method according to claim 1, characterized in that at least one layer has an oil or grease-tight barrier effect. A method according to claim 1, characterized in that at least one layer is an oil or grease-tight barrier according to TAPPI T 559 kit test greater than 5. A method according to claim 1, characterized in that at least one layer is an oil or grease-tight barrier according to TAPPI T 454 greater than 950. Method according to one of claims 1 to 4, characterized in that the substrate has a surface of paper or cardboard. Method according to one of claims 1 to 5, characterized in that the polyvinyl acetal has a degree of acetalization of 0.1 to 30 mol%. Method according to one of claims 1 to 6, characterized in that the polyvinyl acetal has 0.1 to 15 mol% vinyl acetate units. Method according to one of claims 1 to 7, characterized in that the polyvinyl acetal is a terpolymer of vinyl acetate, vinyl alcohol, vinyl acetal and olefin units. Method according to one of claims 1 to 8, characterized in that the polyvinyl acetal silanol groups or carboxylate groups. Method according to one of claims 1 to 9, characterized in that the substrate is coated with a plurality of layers containing in each case different polyvinyl acetals.