EP1419298B1 - Method for production of coated paper with extreme whiteness - Google Patents

Abstract

Paper coated with a coating slip containing at least one optical brightener is produced by a process in which base paper or precoated paper is treated, before application of the coating slip containing optical brightener, with at least one substance which enhances the efficiency of optical brighteners.

Description

The invention relates to a novel process for the production of coated paper, which is characterized by a particularly high degree of whiteness. Furthermore, the invention relates to papers, which are produced by this method and the printing of papers, which are produced by this method.

Paper coatings consist essentially of a mostly white pigment, a polymeric binder and additives which influence, for example, the rheological properties of the coating color and the properties of the surface of the coated paper in the desired sense. Such additives are often referred to as "cobinders". By the binder, the pigments are fixed on the paper and ensures the cohesion in the resulting coating.

Coating with paper coating slips gives base papers a smooth, uniform white surface. The paper coatings also improve the printability of the paper. In order to obtain optimum qualities, papers are also often coated twice or three times, that is, a second or a third time a coating is applied to an already precoated or "pre-painted" paper.

The coating of paper with paper coating slips is well known today, see eg " The Essential Guide to Aqueous Coating of Paper and Board ", TWR Dean (ed.), Published by the Paper Industry Technical Association (PITA), 1997 ,

One of the most important goals of coating paper with coating colors is to increase the whiteness of the paper. The skilled person has the task by the coating of uncoated paper, which is also referred to as base paper or base paper, or pre-coated paper, which is hereinafter called "pre-coated paper", paper with improved properties, especially with higher whiteness To make available.

For this purpose, it is the coating, in particular those who want to form the top layer, so-called "whitener" (fluorescent or phosphorescent) or "optical brightener" to. These are dye-like fluorescent Compounds that absorb the short-wave, ultraviolet light invisible to the human eye and re-emit it as longer-wavelength blue light, thereby giving the human eye a higher whiteness, thus increasing the whiteness.

The optical brighteners used in the paper industry are usually 1,3,5-triazinyl derivatives of 4,4'-diaminostilbene-2,2'-disulfonic acid, which may carry additional sulfonic acid groups. An overview of such brighteners can be found, for example, in: Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, OPTICAL BRIGHTENERS - Chemistry of Technical Products. But there are also newer Aufhellertypen in question, z. B. Derivatives of 4,4'-distyrylbiphenyls, as also described in the aforementioned literature Ullmann's Encyclopedia of Industrial Chemistry.

However, the use of the optical brighteners in the coating material only leads to optimum success if they are present in the finished coating of the paper in an optimal structure, conformation and distribution, since e.g. in stilbenes only the trans-form is optically active and only fluoresces maximally when it is monomolecularly distributed and held in one plane (KP Kreutzer, Basic processes of paper production 2: Interfacial processes in the use of chemical aids, H.-G. Völkel and R Border (Ed.), PTS Munich, 2000, PTS manuscript: PTS-GPE - SE 2031-2). To achieve this, polymeric compounds which enhance the effect of the optical brightener in the coating slip and are referred to as "activator", "carrier" or "carrier" are added to the paper coating slip. So far, these activators have basically been mixed into the paper coating slip. An important function of the above-mentioned co-binder in coating colors is their brightener-activating effect. As suitable cobinder can be water-soluble polymers, for. Polyvinyl alcohol, carboxymethyl cellulose, anionic or nonionic degraded starches, casein, soy protein, water-soluble styrene-acrylate copolymers and acrylic ester-containing copolymers (see, e.g., K. P. Kreutzer, supra).

All of these compounds, hereinafter referred to as "activators", are polymeric compounds whose problem is that they increase the viscosity of the coatings. As a result, an increase in their application rate to get a higher whiteness, narrow limits. Particularly effective with respect to the activation of optical brighteners are paper coating slips with polymers and copolymers which copolymerizes N-vinylformamide as described in the German application with the file number 100 55 592.6.

This invention has for its object to develop a method by which the whiteness of coated paper can be increased.

A process has now been found for the production of paper coated with at least one optical brightener by treating raw paper or prepainted paper with at least one substance which enhances the effectiveness of optical brighteners prior to applying the optical brightener.

It is surprising that the strong increase in whiteness is also obtained when the coating composition containing the brightener itself contains no activator for the optical brightener.

It has furthermore been found that polymers and copolymers, referred to below as (co) polymers, which comprise at least one N-vinylcarboxamide, for example of the formula (I), in copolymerized form, bring about a particularly marked improvement in the properties of the coated paper when used in accordance with the invention Apply the raw paper or prepainted paper before coating the paper with a coating containing at least one optical brightener. In particular, brightness and / or whiteness are increased.

In formula I, R ¹ and R ² independently of one another are hydrogen or C ₁ to C ₂₀ -alkyl, where the alkyl radical may be straight-chain or branched.

Preferably, R ¹ and R ^{2 are} independently hydrogen or C ₁ to C ₁₀ alkyl, more preferably hydrogen or C ₁ to C ₄ alkyl, most preferably hydrogen or methyl and especially hydrogen.

R ¹ and R ² may also together form a straight chain or branched chain containing 2 to 8 carbon atoms, preferably containing 3 to 6 and more preferably containing 3 to 5 carbon atoms. Optionally, one or more carbon atoms may be used be replaced by heteroatoms, such as oxygen, nitrogen or sulfur.

Examples of the radicals R ¹ and R ² are methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-hexyl, n-heptyl, 2-ethylhexyl, n-octyl, n-decyl, n-undecyl, n-dodecyl, n-tetradecyl, n-hexadecyl, n-octadecyl or n-eicosyl.

Examples of radicals R ¹ and R ² which together form a chain are 1,2-ethylene, 1,2-propylene, 1,3-propylene, 2-methyl-1,3-propylene, 2-ethyl-1, 3-propylene, 1,4-butylene, 1,5-pentylene, 2-methyl-1,5-pentylene, 1,6-hexylene or 3-oxa-1,5-pentylene.

Examples of such N-vinylcarboxamides of the formula (I) are N-vinylformamide, N-vinylacetamide, N-vinylpropionamide, N-vinylbutyramide, N-vinylisobutyric acid amide, N-vinyl-2-ethylhexanoic acid amide, N-vinyldecanoic acid amide, N-vinyldodecanoic acid amide, N- Vinyl stearic acid amide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, N-methyl-N-vinylpropionamide, N-methyl-N-vinylbutyramide, N-methyl-N-vinylisobutyric acid amide, N-methyl-N-vinyl 2-ethylhexanoic acid amide, N-methyl-N-vinyldecanoic acid amide, N-methyl-N-vinyldodecanoic acid amide, N-methyl-N-vinylstearamide, N-ethyl-N-vinylformamide, N-ethyl-N-vinylacetamide, N-ethyl-N- Vinylpropionic acid amide, N-ethyl-N-vinylbutyramide, N-ethyl-N-vinylisobutyric acid amide, N-ethyl-N-vinyl-2-ethylhexanoic acid amide, N-ethyl-N-vinyldecanoic acid amide, N-ethyl-N-vinyldodecanoic acid amide, N-ethyl N-vinylstearic acid amide, N-isopropyl-N-vinylformamide, N-isopropyl-N-vinylacetamide, N-isopropyl-N-vinylpropionamide, N-isopropyl-N Vinyl butyric acid amide, N-isopropyl-N-vinylisobutyric acid amide, N-isopropyl-N-vinyl-2-ethylhexanoic acid amide, N-isopropyl-N-vinyldecanoic acid amide, N-isopropyl-N-vinyldodecanoic acid amide, N-iso Propyl-N-vinylstearic acid amide, Nn-butyl-N-vinylformamide, Nn-butyl-N-vinylacetamide, Nn-butyl-N-vinylpropionic acid amide, Nn-butyl-N-vinylbutyramide, Nn-butyl-N-vinylisobutyric acid amide, Nn-butyl N-vinyl-2-ethylhexanoic acid amide, Nn-butyl-N-vinyldecanoic acid amide, Nn-butyl-N-vinyldodecanoic acid amide, Nn-butyl-N-vinylstearic acid amide, N-vinylpyrrolidone or N-vinylcaprolactam.

Preference is given to N-vinylformamide, N-vinylacetamide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, N-vinylpyrrolidone or N-vinylcaprolactam; N-vinylformamide is particularly preferred.

Even if the pre-coated or uncoated paper with other than activators for optical brighteners known water-soluble compounds before coating with the coating, the at least contains an optical brightener treated according to the invention is obtained according to an increase in the brightness and whiteness of the coated paper.

Examples of such activators are polyvinyl alcohol, carboxymethyl cellulose, anionic or nonionic degraded starches, casein, soy protein, water-soluble styrene-acrylate copolymers and acrylic ester-containing copolymers.

For example, such polyvinyl alcohols can be used as activators having degrees of polymerization in the range of about 500 to 2500, corresponding to molar masses of about 20,000 to 100,000 g / mol. The degrees of hydrolysis of the polyvinyl alcohols which can be used according to the invention are generally at least 70 mol%; preferred polyvinyl alcohols have a degree of hydrolysis of either 98-99 or 87-89 mol% and, as mostly partially hydrolyzed polyvinyl acetates, have a residual content of acetyl groups of approx. 1 - 2 or 11 - 13 mol%.

The polyvinyl alcohols which can be used according to the invention predominantly have 1,3-diol units, the content of 1,2-diol units is generally below 2%, preferably below 1%.

Polyvinyl alcohol is here understood to mean a polymer which comprises at least 10% by weight, preferably at least 20% by weight and more preferably at least 50% by weight and in particular at least 90% by weight, based on the polymer, of vinyl acetate in copolymerized and optionally cleaved form contains.

Particularly suitable are those polyvinyl alcohols which are sold under the brand names Mowiol® (Clariant AG), Polyviol® (Wacker-Chemie GmbH), Rhodoviol® (Rhodia), Alcotex® (Revertex), Polivinol® (Rhodiatoce), Denka Poval® (Denki Kagaku Kogyo), Gohsenol® (Nippon Gohsei), Kurashiki Poval® (Kuraray), Shinetsu Poval® (Shinetsu Chem. Ind.), Unitika Poval® (Unitika), Elvanol® (Du Pont), Gelvatol® (Shawinigan Resins) and Lemol ® (Borden) are commercially available, particularly preferred are the Mowiol®, Polyviol® and Rhodoviol® brands.

As carboxymethylcellulose, such products can be used according to the invention as activators which have a molecular weight of 50,000 to 500,000 g / mol. The carboxymethylcellulose can be used as the sodium salt or as the free acid or as a mixture thereof, preferably as the sodium salt. The degree of substitution of carboxymethyl groups per anhydroglucose unit may be between 0.5 and 1.5.

As anionic or nonionic degraded starches as activators, for example, hydroxyethyl, hydroxypropyl, methyl, ethyl or carboxymethyl starches can be used according to the invention which have a molecular weight of between 50,000 and 2,000,000 g / mol.

Acrylic-containing copolymers are understood as meaning copolymers which contain at least 10% by weight, preferably at least 20% by weight and more preferably at least 50% by weight and in particular at least 70% by weight, based on the copolymer, of at least one acrylic acid ester polymerized form contain, for example, methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate. Acrylic acid, methacrylic acid, acrylonitrile, vinyl acetate, vinyl propionate, N-vinylformamide, allylacetic acid, vinylacetic acid, maleic acid, fumaric acid, N-vinylpyrrolidone or hydroxybutyl vinyl ether in copolymerized form can be contained as further monomers, for example. These acrylic ester-containing copolymers can be used, for example, in the form of their aqueous solutions or dispersions having a copolymer content of from 10 to 75% by weight, preferably from 20 to 60% by weight.

The Acrosol® grades from BASF AG are preferably used here, for example Acrosol® A30D, A40D, B37D, C50L or E20D, preferably Acrosol® C50L.

Polyvinyl alcohols and / or (co) polymers which contain N-vinylcarboxamides in copolymerized form are preferably used in accordance with the invention, particularly preferably (co) polymers which contain monomers of the formula (I) in copolymerized form.

The preparation of the activators suitable for the process according to the invention is known per se.

For example, the preparation of the polymers and copolymers of N-vinylformamide (R ¹ = R ² = H in (I)) which can be used for the process according to the invention, in EP-B1 71 050 described.

The synthesis of N-alkyl-N-vinylcarboxamides and their polymers and copolymers is known or carried out by known methods, see, for example Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Edition, Volume 24, J. Wiley & Sons, NY, 1995 , N-vinyl amide polymers, page 1070; Uchino, N., Machida, S., Japan. Kokai JP 51100188 (CA 86: 73393) or DE-A 42 41 117 ,

The preparation of polymers and copolymers of N-vinylpyrrolidone is known, for example Handbook of Water-Soluble Gums and Resins, Robert L. Davidson ed., McGraw-Hill, New York, 1980 ,

Polyvinyl alcohol has been produced on an industrial scale since 1939 and has been used in papermaking for many decades ( Handbook of Water-Soluble Gums and Resins, Robert L. Davidson ed., McGraw-Hill, New York, 1980 ).

1. a) 5 bis 100 Mol-% eines oder mehrerer N-Vinylcarbonsäureamide, z.B. der Formel (I),
2. b) 0 bis 95 Mol-% monoethylenisch ungesättigte Carbonsäuren mit 3 bis 8 C-Atomen und/oder deren Alkalimetall- und Ammoniumsalzen und gegebenenfalls
3. c) bis zu 30 Mol-% anderen monoethylenisch ungesättigten Verbindungen, die mit den Monomeren a) und b) copolymerisierbar sind und gegebenenfalls
4. d) bis zu 2 Mol-% an Verbindungen, die mindestens zwei ethylenisch ungesättigte nicht-konjugierte Doppelbindungen im Molekül aufweisen,

wobei die Summe immer 100 mol% beträgt, und gegebenenfalls anschließend teilweiser oder vollständiger Abspaltung der Carbonsäuregruppen aus den in das (Co)polymerisat einpolymerisierten N-Vinylcarbonsäureamide unter Bildung von Amin- bzw. Ammoniumgruppen.The (co) polymers which can be used according to the invention are obtainable, for example, by (co) polymerizing

1. a) 5 to 100 mol% of one or more N-vinylcarboxamides, for example of the formula (I),
2. b) 0 to 95 mol% of monoethylenically unsaturated carboxylic acids having 3 to 8 carbon atoms and / or their alkali metal and ammonium salts and optionally
3. c) up to 30 mol% of other monoethylenically unsaturated compounds which are copolymerizable with the monomers a) and b) and optionally
4. d) up to 2 mol% of compounds having at least two ethylenically unsaturated non-conjugated double bonds in the molecule,

wherein the sum is always 100 mol%, and if appropriate subsequently partial or complete removal of the carboxylic acid groups from the N-vinylcarboxamides copolymerized into the (co) polymer to form amine or ammonium groups.

Suitable monomers of group a) are, for example, the abovementioned N-vinylcarboxamides of the formula (I).

For the preparation of the copolymers, the monomers mentioned can be used either alone or in a mixture with one another. N-vinylacetamide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, N-vinylpyrrolidone or N-vinylcaprolactam and particularly preferably N-vinylformamide are preferably used from this group of monomers. The copolymers contain the monomers of group a) in amounts of 5 to 100, preferably 30 to 100 mol% in copolymerized form.

Suitable monomers of group b) are monoethylenically unsaturated carboxylic acids having 3 to 8 carbon atoms and the water-soluble salts of these monomers. This group of monomers includes, for example, acrylic acid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid, allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid and itaconic acid. From this group of monomers are preferably used acrylic acid, methacrylic acid, maleic acid or mixtures of said carboxylic acids, in particular mixtures of acrylic acid and maleic acid or mixtures of acrylic acid and methacrylic acid. The monomers of group b) can be used either in the form of the free carboxylic acids or in partially or completely neutralized form in the copolymerization. To neutralize the monoethylenically unsaturated carboxylic acids, for example, use is made of alkali metal, alkaline earth metal bases, ammonia or amines, e.g. Caustic soda, potassium hydroxide, soda, potash, sodium bicarbonate, magnesium oxide, calcium hydroxide, calcium oxide, ammonia, triethylamine, ethanolamine, diethanolamine, triethanolamine, morpholine, diethylenetriamine or tetraethylenepentamine. The copolymers contain at least one monomer from group b) in an amount of 95 to 0, preferably 70 to 0 mol% in copolymerized form.

The copolymers of the monomers a) and b) may optionally be modified by using in the copolymerization at least one other monoethylenically unsaturated compound which is copolymerizable with the monomers a) and b). Suitable monomers of group c) are, for example, the esters, amides and nitriles of the carboxylic acids indicated under a), for example methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4-hydroxybutyl acrylate, hydroxyethyl methacrylate, 2- or 3-hydroxypropyl methacrylate, hydroxyisobutyl acrylate, hydroxyisobutyl methacrylate, monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acrylamide, methacrylamide, N-dimethylacrylamide, N-tertiary butylacrylamide, acrylonitrile, methacrylonitrile, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate and the salts of the last-mentioned monomers with carboxylic acids or mineral acids and the quaternized products. Other suitable monomers of group c) are acrylamidoglycolic acid, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate and acrylamidomethylpropanesulfonic acid and phosphonic acid group-containing monomers such as vinyl phosphate, allyl phosphate and acrylamidomethanepropanephosphonic acid. Further suitable compounds of this group are N-vinyl-2-methylimidazoline, diallylammonium chloride, vinyl acetate and vinyl propionate. It is of course also possible to use mixtures of said monomers of group c), for example mixtures of acrylic esters and vinyl acetate, mixtures of various acrylic esters, mixtures of acrylic esters and acrylamide or mixtures of acrylamide and hydroxyethyl acrylate. Of the monomers of group c) are preferably used acrylamide, acrylonitrile, vinyl acetate, N-vinylimidazole or mixtures of these monomers, for example mixtures of acrylamide and vinyl acetate or mixtures of acrylamide and acrylonitrile. If the monomers of group c) are used to modify the copolymers, they are present in amounts of up to 30 mol% in copolymerized form in the copolymers, preferably in amounts of 1 to 20 mol%.

The copolymers of the monomers a) and b) and, if appropriate, c) can furthermore be modified by carrying out the copolymerization in the presence of at least one monomer of group d), which are compounds which have at least two ethylenically unsaturated nonconjugated Have double bonds in the molecule. The concomitant use of the monomers of group d) in the copolymerization causes an increase in the K values (see above) of the copolymers. Suitable compounds of group d) are, for example, methylenebisacrylamide, esters of acrylic acid and methacrylic acid with polyhydric alcohols, such as glycol diacrylate, glycerol triacrylate, glycol dimethacrylate, glycerol trimethacrylate and at least two times with acrylic acid or methacrylic acid esterified polyethylene glycols or polyols, such as pentaerythritol and glucose. Suitable crosslinkers are also divinylbenzene, divinyldioxane, pentaerythritol triallyl ether and pentaallylsucrose. From this group of compounds it is preferred to use water-soluble monomers, such as glycol diacrylate or glycol diacrylates, of polyethylene glycols having a molecular weight of up to 3,000. If the monomers of group d) are used to modify the copolymers, the amounts used are up to 2 mol%. In the case of their use, they are preferably contained in 0.01 to 1 mol% in copolymerized form in the copolymers.

1. a) 30 bis 100 Mol-% N-Vinylformamid, N-Vinylacetamid, N-Methyl-N-vinylformamid, N-Methyl-N-vinylacetamid, N-Vinylpyrrolidon oder N-Vinylcaprolactam oder deren Mischungen,
2. b) 70 bis 0 Mol-% Acrylsäure, Methacrylsäure und/oder deren Alkalimetall-, Erdalkalimetall-, Ammonium- oder Aminsalzen oder deren Mischungen und
3. c) 0 bis 30 Mol-% Acrylamid, Acrylnitril, Vinylacetat, N-Vinylimidazol oder deren Mischungen,

wobei die Summe immer 100 mol% beträgt, und gegebenenfalls anschließender partieller oder vollständiger Hydrolyse der einpolymerisierten N-Vinylcarbonsäureamideinheiten erhältlich sind.Preference is given to the use of compounds obtained by (co) polymerizing

1. a) from 30 to 100 mol% of N-vinylformamide, N-vinylacetamide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, N-vinylpyrrolidone or N-vinylcaprolactam or mixtures thereof,
2. b) 70 to 0 mol% of acrylic acid, methacrylic acid and / or their alkali metal, alkaline earth metal, ammonium or amine salts or mixtures thereof and
3. c) 0 to 30 mol% acrylamide, acrylonitrile, vinyl acetate, N-vinylimidazole or mixtures thereof,

the sum being always 100 mol%, and optionally subsequent partial or complete hydrolysis of the copolymerized N-vinylcarboxamide units.

Examples include homopolymers of N-vinylformamide, copolymers of N-vinylformamide, acrylic acid and acrylamide, copolymers of N-vinylformamide, acrylic acid and acrylonitrile, copolymers of N-vinylformamide, acrylic acid and vinyl acetate, copolymers of N-vinylformamide, acrylic acid and N-vinylpyrrolidone , Copolymers of N-vinylformamide, acrylic acid, acrylonitrile and vinyl acetate, copolymers of N-vinylformamide, acrylic acid, acrylamide and arynitrile. In the last-described copolymers, the acrylic acid may be wholly or partly replaced by methacrylic acid. Acrylic acid or methacrylic acid may be partially or completely neutralized with caustic soda, potassium hydroxide, calcium hydroxide or ammonia.

The preparation of the copolymers is carried out by known free-radical processes, e.g. solution, precipitation, suspension or emulsion polymerization using compounds which form radicals under the polymerization conditions.

The polymerization temperatures are usually in the range of 30 to 200, preferably 40 to 110, more preferably 40 to 100 ° C, optionally at reduced or elevated pressure. Suitable initiators include, for example, azo and peroxy compounds as well as the usual redox initiator systems, such as combinations of hydrogen peroxide and reducing compounds, e.g. Sodium sulfite, sodium bisulfite, sodium formaldehyde sulfoxilate and hydrazine. These systems may optionally additionally contain small amounts of a heavy metal salt.

The copolymers are preferably prepared by solution polymerization in water, wherein the monomers of group b) are preferably used in the salt form and the pH during the Polymerization between 4 and 10, preferably between 6 and 8 holds. In order to keep the pH constant during the copolymerization, it is expedient to add small amounts, for example 0.5 to 2 wt .-%, of a buffer, such as disodium hydrogen phosphate. The polymerization initiator used is preferably water-soluble azo compounds, such as 2,2'-azobis (2-methylpropionamidine) dihydrochloride, 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis (2) methyl-N-phenylpropionamidine) dihydrochloride, 2,2'-azobis (isobutyronitrile), 2,2'-azobis (2-amidinopropane) hydrochloride or 4,4'-azobis (4'-cyano) pentanoic acid).

The compounds mentioned are usually used in the form of aqueous solutions or dispersions, the lower concentration being determined by the amount of water acceptable in the (co) polymerization and the upper concentration by the solubility of the compound in question in water. In general, the concentration is 0.1 to 30 wt .-%, preferably 0.5 to 20 wt .-%, particularly preferably 1.0 to 10 wt .-%, based on the solution.

The amount of initiators is generally from 0.1 to 10 wt .-%, preferably 0.5 to 5 wt .-%, based on the (co) polymerizing monomers. It is also possible to use a plurality of different initiators in (co) polymerization.

As a solvent or diluent, e.g. Water, alcohols, such as methanol, ethanol, n- or iso-propanol, n- or iso-butanol, or ketones, such as acetone, ethyl methyl ketone, diethyl ketone or isobutyl methyl ketone.

To produce low molecular weight copolymers, the copolymerization is carried out in the presence of a regulator. Suitable regulators are, for example, secondary alcohols such as isopropanol and sec-butanol, hydroxylamine, formic acid and mercapto compounds such as mercaptoethanol, mercaptopropanol, mercaptobutanol, thioglycolic acid, thiolactic acid, tert-butylmercaptan, octylmercaptan and dodecylmercaptan. The regulators are usually used in amounts of 0.01 to 5 wt .-%, based on the monomers used. If secondary alcohols are used as regulators, the polymerization can also be carried out in the presence of substantially larger amounts, for example in amounts of up to 80% by weight, based on the monomers. In these cases, the secondary alcohols are also solvents for the monomers.

The (co) polymers thus obtainable have K values of 30 to 300, preferably 50 to 250. The K values are determined according to H.Fikentscher in 5% strength aqueous sodium chloride solution at pH 7, 25 ° C. and a polymer concentration of 0, 1% by weight.

However, the (co) polymerization can also be carried out in another manner known to the person skilled in the art, e.g. as solution, precipitation, water-in-oil emulsion or reverse suspension polymerization. The solution polymerization is preferred.

In the emulsion polymerization, ionic and / or nonionic emulsifiers and / or protective colloids or stabilizers are used as surface-active compounds.

Depending on the polymerization conditions, the (co) polymerization gives (co) polymers of a different molecular weight, which is characterized in this document by means of the K values according to Fikentscher. (Co) polymers with a high K value, e.g. above 80, are preferably prepared by (co) polymerizing the N-alkyl-N-vinylcarboxylic acid amide (I) in water. (Co) polymers having a high K value are also obtained, for example, by (co) polymerizing the monomers in the form of reverse suspension polymerization or by (co) polymerizing the monomers by the water-in-oil polymerization method.

In the reverse suspension polymerization and the water-in-oil polymerization, the oil phase used is saturated hydrocarbons, for example hexane, heptane, cyclohexane, decalin or aromatic hydrocarbons, such as benzene, toluene, xylene and cumene. The ratio of oil phase to aqueous phase in the reverse suspension polymerization is, for example, 10: 1 to 1:10.

(Co) polymer with a low K value, e.g. below 80 is obtained by carrying out the (co) polymerization in the presence of polymerization regulators or in a solvent which controls the (co) polymerization, e.g. Alcohols such as methanol, ethanol, n- or iso-propanol, or ketones such as acetone, ethyl methyl ketone, diethyl ketone or isobutyl methyl ketone.

(Co) polymers having low molecular weights and correspondingly low K values are furthermore obtained by means of the customary methods, ie use of relatively large amounts of polymerization initiator or use of polymerization regulators or combinations of the measures mentioned.

The molecular weight of the (co) polymers which can be used according to the invention is not limited. Preference is given to (co) polymers having K values between 30 and 110, with K values between 40 and 90 being particularly preferred.

The N-alkyl-N-vinylcarboxamides, for example of the formula (I) and especially N-vinylformamide, in copolymerized form containing (co) polymers can be used according to the invention both partially or fully cleaved as well as in un-cleaved form. A degree of hydrolysis is preferably between 0 and 30%, particularly preferably between 0 and 20% and very particularly preferably between 0 and 10%. The nature of the cleavage of the carboxylic acid or formyl group is not limited, it can be carried out for example in the presence of acid or base, preferably cleavage in the presence of bases, such as sodium hydroxide, potassium hydroxide, alkaline earth metal: l hydroxides, ammonia or amines. By partial hydrolysis, e.g. of a (meth) acrylate and N-alkyl-N-vinylcarboxamides, for example of the formula (I) in polymerized form-containing copolymer, amphoteric (co) polymers are formed.

It is particularly easy to obtain cationic copolymers of N-vinylformamide by hydrolytically cleaving homopolymers of N-vinylformamide with defined amounts of acid or base to the desired degree of hydrolysis, as in US Pat EP-B1 071 050 described. The resulting amino groups on the polymer chain are more or less protonated depending on the pH of the solution and thus give the polymer a more or less cationic character.

If a cleavage of the carboxylic acid group is desired, this can be carried out, for example, in water.

The cleavage of the formyl group in the hydrolysis takes place at temperatures in the range from 20 to 200, preferably 40 to 180 ° C, in the presence of acids or bases. The hydrolysis in the presence of acids or bases is preferably carried out in the temperature range of 70 to 90 ° C.

For each carboxylic acid group equivalent in the poly-N-alkyl-N-vinylcarboxamide, about 0.05 to 1.5 equivalents of an acid, such as hydrochloric, hydrobromic, phosphoric, sulfuric, are required for acid hydrolysis. The pH of the acidic hydrolysis is in the range of 2 to 0, preferably 1 to 0. The hydrolysis of N-vinylformamide proceeds much faster than that of (co) polymers of other N-alkyl-N-vinylcarboxamides, such , As the N-methyl-N-vinylformamide, and therefore can under gentler conditions, ie at lower temperatures and without a high excess of acids.

Moreover, the hydrolysis of the formyl groups of the poly-N-alkyl-N-vinylcarboxamide can also be carried out in alkaline medium, e.g. in the pH range of 11 to 14. This pH is preferably adjusted by the addition of sodium hydroxide solution or potassium hydroxide solution. However, it is also possible to use ammonia, amines and / or alkaline earth metal bases. For the alkaline hydrolysis use 0.05 to 1.5, preferably 0.4 to 1.0 equivalents of a base.

The cleavage can also be carried out at high temperatures, for example above 100 ° C., preferably 120 to 180 ° C., particularly preferably 140 to 160 ° C. in the presence of a solvent, e.g. Water, be carried out without acid or base. This is preferably carried out at conditions above the critical point, for example with supercritical water.

In the hydrolysis, i. the carboxylic acid group is split off in water in the presence of acids or bases from the poly-N-vinylcarboxamide, the by-product carboxylic acid, for example formic acid, or salts thereof is obtained.

The solutions thus obtained can be used without further work-up in the process according to the invention, but the hydrolysis or solvolysis products can also be separated off.

For separation, the solutions obtained are treated, for example, with ion exchangers. The residue separated from the hydrolysis products can then be incorporated into the coating compositions or used as an activator for the pretreatment.

The molecular weight of the present invention (co) polymers is, as stated above, not limited, but it should be adapted to the respective application method. For the order z. B. with a doctor, the molecular weight should be relatively high, while it should be relatively low for the order with a sprayer.

The application of the activators by the process according to the invention to the surface of the coating base paper or prepainted paper can be carried out according to the methods customary for the surface treatment of paper in the paper industry. You can use known order aggregates, such. B. film presses, size presses, various coating units with squeegees, scrapers (English blades) or air brushes, or spraying, as z. B. for the application of starch in the EP-A 373 276 or for the application of coating compounds from V. Nissinen, Wochenblatt für Papierfabrikation, 2001, 11/12, pp. 794-806 , to be discribed. The application of the activators can also be done in the calendering of the paper on the humidification.

According to the invention it is essential that the activator is applied to the raw or prepainted paper in one operation before applying the coating composition containing optical brighteners.

Particularly preferred is the application by means of a size press, or, in several coating operations, instead of a first or middle line, as a further step can be saved.

The usable for the pretreatment according to the invention, optionally already vorgeleimte, uncoated base paper usually has a water content of not more than 10% by weight, preferably not more than 8% by weight, more preferably between 3 and 8% by weight and especially between 5 and 8 wt %.

The prepainted paper which can be used for the pretreatment according to the invention has generally already undergone one or two brushing operations.

The activators can be applied to the paper from aqueous solution or as a solution in methanol, ethanol, isopropanol , n-propanol, n-butanol, ethyl acetate, acetone or N-methylpyrrolidone, preferably from aqueous solution, the concentration being chosen in this way should be that the respective application method can be optimally performed due, for example, the viscosity of the substance or solution to be applied.

Usual concentrations are between 10 and 60% by weight.

The activators can be applied to the paper as individual substances, but also in mixtures with each other. But you can also apply the activators on the paper surface in admixture with other paper chemicals that affect other paper properties. Such paper chemicals that can be applied to the paper together with the brightener activators according to the method of the invention are, for. As starch, cationic starch, other starch derivatives such. Hydroxyethyl, hydroxymethyl, methyl or ethyl starch ethers, others Polysaccharides such. Guar or guar derivatives, carboxymethyl, hydroxyethyl, hydroxymethyl, methyl or ethyl cellulose ethers or surface sizing agents.

Of course, the activator-containing composition in addition to at least one activator and optionally solvent at least one pigment, at least one known per se binder and optionally other paper-typical auxiliaries (s.u.).

However, optical brighteners are preferably applied first with a coating compound in a following step (see below).

The amount in which the activator is applied to the paper according to the method of the invention can vary within wide limits. In general, one should apply an amount of 0.05 g to 5 g per m ^{2 of} paper, but an amount of 0.1 g to 3 g is preferable.

After applying the activators to the raw or prepainted paper, e.g. by infrared radiator, to remove any solvent contained, and if desired, still calendered (satin) at a temperature between 15 and 100 ° C.

Furthermore, it has been found that the whiteness of the paper can often be further increased if, in addition to the pretreatment of the paper, a suitable activator for the optical brightener is added to the coating composition. This may be the same activator as used to pretreat the coating base paper or one of the previous lines, but also another activator. Suitable activators are, for example, those already mentioned above. Preferred are those (co) polymers containing N-vinylcarboxamides, e.g. the o.g. N-vinyl-N-alkylcarboxamides of the formula (I), in copolymerized form. Here, too, the (co) polymers containing N-vinylcarboxamides can be used both partially or completely cleaved as well as in un-cleaved form. A degree of hydrolysis is preferably between 0 and 30%, particularly preferably between 0 and 20% and very particularly preferably between 0 and 10%.

The amount of activator in the coating will be chosen so that the viscosity of the coating composition is in processing areas favorable. Usually, it is between 0.2 and 10% based on the pigment in the coating.

The amount of (co) polymers containing N-alkyl-N-vinylcarboxamides, e.g. of the formula (I) which can be added to the paper coating slip depends on the amount of the brightener in the coating slip.

Normally 0.2 to 2 parts by weight of optical brightener per 100 parts by weight of pigment is added to the coating. Of the (co) polymer is usually added the same to five times the amount of optical brightener to the coating composition, that is 0.2 to 10 parts by weight, preferably 0.5 to 8 and particularly preferably 1 to parts by weight.

The paper coating slip applied after the pretreatment according to the invention with an activator preferably contains at least one optical brightener.

In addition to the activator, the paper coating slips contain at least one white pigment and at least one binder.

The paper coating slips may contain further components known to the person skilled in the art. Consider, for example, Flow control agents, thickeners, wetting aids for the pigments, etc.

The optical brighteners that can be used in conjunction with the method of the invention are not limited. For example, such brighteners can be used as described in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, OPTICAL BRIGHTENERS - Chemistry of Technical Products.

Suitable optical brighteners include, for example, the group of distyrylbenzenes, for example cyano-substituted 1,4-distyrylbenzenes with cyano groups in positions 2 'and 3 "[CAS RegNo. 79026-03-2], or in positions 2' and 2" [ 13001-38-2], 3 'and 3 "[36755-00-7], 3' and 4" [79026-02-1] and 4 'and 4 "[13001-40-6], or amphoteric compounds, such as [84196-71-4], in the 2 'and 2 "position each one group

-O- (CH ₂ ) ₂ -N ⁺ (C ₂ H ₅ ) ₂ -CH ₂ COO ^-

4,4'-di (2-sulfostyryl) biphenyl disodium salt [27344-41-8], 4,4'-di (3-sulfostyryl) biphenyl disodium salt [51119-63-2], 4,4'-di (4-chloro-3-sulfostyryl) biphenyl disodium salt [42380-62-1], 4,4'-di (6-chloro-3-sulfostyryl) biphenyl disodium salt [60477-28-3], 4,4'-di (2-methoxystyryl) biphenyl [40470-68-6] or a 4,4'-di (styryl) biphenyl, which in position 2 on the styryl radical is a group

-O- (CH ₂ ) 2 -N ⁺ (CH ₃ ) (C ₂ H ₅ ) (C ₂ H ₅ ) ₂ · CH ₃ OSO ₃ ^-

carries [72796-88-4], the divinylstilbene group, for example, 4,4'-di (ethoxycarbonylvinyl) stilbene [60683-03-6] or 4,4'-di (cyanovinyl) stilbene [60682-87-3] , to the group of the triazinylaminostilbenes, for example 1,3,5-triazinyl derivatives of 4,4'-diamino-stilbene-2,2'-disulfonic acid, such as aniline derivatives which each carry the following radicals on the triazine rings in position 3: a methoxy radical ( CAS Registry No. [3426-43-5]), aminomethyl [35632-99-6], ethylamino [24565-13-7], hydroxyethylamino [12224-16-7], N -hydroxyethyl-N-methylamino [13863- 31-5], bis (hydroxyethyl) amino [4193-55-9], morpholino [16090-02-1], phenylamino [133-66-4], N-2-aminocarbonylethyl-N-2-hydroxyethylamino [68444 -86-0] or anilinosulfonic acid derivatives which carry the following radicals in position 3 at the triazine rings: N-hydroxyethylamino and additionally at the anilino group in position 5 of the triazine ring a sulfonic acid group in position 3 (CAS Reg. No. [61968- 74-9]), N-bis (hydroxyethyl) amino- and in addition to de An anilino group, a sulfonic acid group in position 3 (CAS RegNr. [12224-02-1]), N-bis (2-hydroxypropyl) amino and additionally at the anilino group a sulfonic acid group in position 4 (CAS RegNr. [99549-42-5]), N-bis (hydroxyethyl) amino - And in addition to the anilino group a sulfonic acid group in position 4 (CAS RegNr. [16470-24-9]), N-hydroxyethyl-N-methyl-amino- and additionally at the anilino group a sulfonic acid group in position 4 (CAS RegNr. [74228-28-7]), diethylamino and additionally at the anilino group sulfonic acid groups in position 2 and 5 (CAS RegNr. [83512-97-4]), N-bis (hydroxyethyl) amino- and in addition to the anilino sulfonic acid groups in position 2 and 5 (CAS RegNr. [76482-78-5]), or morpholino groups and in addition to the anilino group sulfonic acid groups in position 2 and 5 (CAS RegNr. [55585-28-9]), or to the group of Stilbenyl 2H-triazoles, eg stilbenyl 2H-naphtho [1,2-d] triazoles such as the sodium salt of 4- (2H-naphtho [1,2-d] triazol-2-yl) stilbene-2-sulfonic acid [6416 -68-8] or those in position 6 at the naphtholri and at position 2 of the stilbene skeleton carry a sulfonic acid [2583-80-4], or at the stilbene backbone in position 2 a cyano group and in position 4 'carry a chloro group [5516-20-1] or eg Bis (1,2,3 -triazol-2-yl) stilbenes such as 4,4'-bis (4-phenyl-1,2,3-triazol-2-yl) stilbene-2,2'-disulfonic acid dipotassium salt [52237-03-3] or 4,4'-bis (4- (4'-sulfophenyl) -1,2,3-triazol-2-yl) stilbene-2,2'-disulfonic acid tetrasodium salt [61968-72-7], or the group of Benzoxazoles, for example stilbenylbenzoxazoles, for example 5,7-dimethyl-2- (4'-phenylstilben-4-yl) benzoxazole [40704-04-9], 5-methyl-2- (4 '- (4 "-methoxycarbonyl) -phenylstilben-4-yl) benzoxazole [18039-18-4] or those other than the 4''position Heterocycles such as [64893-28-3], or bis (benzoxazoles), for example, ethylene, thiophene, naphthylene, phenylethylene or stilbene bridged bis-benzoxazoles, such as those having the CAS numbers [1041-00-5 ], [2866-43-5], [7128-64-5], [5089-22-5], [1552-46-1], [1533-45-5] or [5242-49-9].

Furans, benzo [b] furans and benzimidazoles, e.g. Bis (benzo [b] furan-2-yl) biphenyls, for example, sulfonated 4,4'-bis (benzo [b] furan-2-yl) biphenyls or cationic benzimidazoles, for example, 2,5-di (1-methylbenzimidazole-2 -yl) furan [4751-43-3], [72829-17-5], [74878-56-1], [74878-48-1] or [66371-25-3], or 1,3-diphenyl -2-pyrazolines, eg 1- (4-amidosulfonylphenyl) -3- (4-chlorophenyl) -2-pyrazoline [2744-49-2]; [60650-43-3], [3656-22-2], [27441-70-9], [32020-25-0], [61931-42-8] or [81209-71-4], and tertiary and quaternary amine salts of 1,3-diphenyl-2-pyrazoline derivatives, eg [106359-93-7], [85154-08-1], [42952-22-7], [63310-12-3], [12270-54-1] or [36086-26-7], and coumarins , such as 7-diethylamino-4-methylcoumarin [91-44-1] and [6025-18-9], [19683-09-1], [3333-62-8], [63660-99-1], [26867] 94-7] or [52725-14-1] and naphthalimides, such as 4-Acetylamino-N- (n-butyl) naphthalimide [3353-99-9], 4-methoxy-N-methylnaphthalimide [3271-05-4], [3271-05-4], [22330-48-9] , [25826-31-7], [26848-65-7] or [60317-11-5] and 1,3,5-triazin-2-yl derivatives, for example (4,6-dimethoxy-1,3, 5-triazin-2-yl) pyrene [3271-22-5] or 4,4'-di (4,6-diphenyl-1,3,5-triazin-2-yl) stilbene [6888-33-1] ,

Preferably can 4,4'-distyrylbiphenyl derivatives or stilbene derivatives which are substituted by up to 6, particularly preferably having 2, 4 or 6 sulphonic acid groups may be used, preferably the Blankophor® grades from Bayer AG, particularly preferred are Blankophor P ^® and Blankophor.RTM ^® PSG, preference is further given the Tinopal® grades from Ciba Specialty Chemicals, particularly preferably Tinopal ^® MC liquid, Tinopal ^® ABP-Z liquid, Tinopal ^® SPP-Z liquid and Tinopal ^® SK-B liquid and further preferred are the Leukophor® brands from Clariant AG, particularly preferably Leukophor® APN, UO, NS or SHR.

The pigments which can be used in the coating compositions are likewise not limited. For example, satin white (calcium sulfoaluminate), calcium carbonate in ground or precipitated (precipitated) form, barium sulfate in ground or precipitated form, kaolin (clay), calcined clay, talc, silicates or organic pigments, e.g. As plastics in particulate form, can be used.

The binders ((co) polymeric binders) which can be used in the coating slips according to the invention are likewise not limited. For example, casein, starch, soy protein, carboxymethyl cellulose, alginate and / or polyvinyl alcohol or dispersions containing acrylic acid, acrylic esters, vinyl acetate and / or styrene in copolymerized form, e.g. (Co) polymers of acrylic ester / styrene, styrene / butadiene or vinyl acetate may be used.

The paper coating slips may continue to be e.g. Contain dispersant. Suitable dispersants are polyanions, for example of polyphosphoric acids or of polyacrylic acids (polysalts), which are usually present in amounts of from 0.1 to 3% by weight, based on the amount of pigment.

The paper pond compounds are usually aqueous paper coating slips. The water content can be adjusted depending on the desired viscosity or flow properties.

The content of water in the paper coating slip is usually adjusted to 25 to 75% by weight, based on the total paper coating slip (including water).

The coating compositions are processed completely analogously to the processing of coating colors according to the prior art, eg according to "The Essential Guide to Aqueous Coating of Paper and Board", TWR Dean (ed.), Published by the Paper Industry Technical Association (PITA), 1997, the "Guide to the use of BASF products in paper and board coking", BASF Aktiengesellschaft, D-6700 Ludwigshafen, Federal Republic of Germany, B 376 d, 09.77 or Ullmann's Encyclopadie der Technischen Chemie, 4th Edition, Vol. 17, p. 603 ff ,

Optionally, a thickener may be added. Suitable thickeners besides free-radically (co) polymerized (co) polymers, customary organic and inorganic thickeners such as hydroxymethylcellulose or bentonite.

To prepare the paper coating composition, the ingredients can be mixed in a known manner. The paper coating slips are suitable for coating eg paper or cardboard. The paper coating slip can then be applied by conventional methods to the papers or cardboard to be coated.

The papers or paperboards coated with the paper coating compositions of the invention can be prepared by conventional methods, e.g. Offset, gravure or gravure printing processes or digital printing processes, e.g. Laser printing or ink-jet printing process can be printed.

The process according to the invention makes it easier for a person skilled in the art to work out a formulation for a coating composition which conventionally also contains the brightener-enhancing activator in addition to the many other components influencing the rheology and the coating quality. It is known that activators in addition to their brightener-enhancing effect also change the properties of the coating composition and even influence the rheological properties of the coating composition by their interaction with the optical brighteners. Since the coating compositions but in addition to one or more pigments and in addition to one or more Bindemitten and the optical brightener additionally thickener, line hardening agents, leveling agents, brightener, flow improvers, dispersants, wetting agents, lubricants, u.s.w. The expert is grateful for any component whose properties he does not consider and which he does not have to incorporate into the coating.

The following examples are intended to illustrate the process of the invention, but without limiting it to these examples.

As "parts",% and ppm are in this document, unless otherwise stated, "parts by weight", Gew% and Gew.ppm understood.

Examples Preparation of a polyvinylformamide Example A

In a stirred reactor with nitrogen inlet, reflux condenser and metering 715 g of distilled water, 1.92 g of phosphoric acid (75%) and 1.31 g of sodium hydroxide solution (25%) are introduced so that a pH of about 6.5 is reached. The reactor is stirred and heated to 75 ° C. A slight vacuum is applied (about 400 mbar) and then 204.2 g of N-vinylformamide are added over a period of one hour. At the same time, 1.84 g of 2,2'-azobis (2-amidinopropane) dihydrochloride dissolved in 50 g of water are metered in over a period of 3 hours. After the addition of the starter is polymerized for another three hours. The polymer solution has a solids content of 20.2% and a K value of 67.

Example B Hydrolysis of Example A:

In the same reactor from Example A, first 2.5 g of sodium bisulfite solution (40% strength) and then 22.9 g of sodium hydroxide solution (25% strength) are added. The reaction mixture is heated to 80 ° C and stirred for 3 hours, then cooled to 40 ° C and 13.1 g of hydrochloric acid (30%) was added, so that it reaches a pH of about 7. The slightly yellowish polymer solution has a degree of hydrolysis of 5% (enzymatic determination of the resulting formic acid).

Example C

In a stirred reactor with nitrogen inlet, reflux condenser and metering 971 g of distilled water, 3.37 g of phosphoric acid (75%) and 4.73 g of sodium hydroxide solution (25%) are introduced so that a pH of about 6.5 is reached. The reactor is stirred and heated to 75 ° C. A slight vacuum is applied (about 400 mbar) and then 360 g of N-vinylformamide are added over a period of one hour. At the same time, 6 g of 2,2'-azobis (2-amidinopropane) dihydrochloride, dissolved in 50 g of water, are metered in over a period of 3 hours. After the addition of the starter is polymerized for another three hours. The polymer solution has a solids content of 35.3% and a K value of 45.9.

Example D Hydrolysis of Example C:

In the same reactor of Example C are first added 2.5 g of sodium bisulfite solution (40%) and then 32.4 g of sodium hydroxide solution (25%). The reaction mixture is heated to 80 ° C and stirred for 3 hours then cooled to 40 ° C and 22.4 g of hydrochloric acid (30%) was added, so that it reaches a pH of about 7. The slightly yellowish polymer solution has a degree of hydrolysis of 5.4% (enzymatic determination of the resulting formic acid).

Example E

In a stirred reactor with nitrogen inlet, reflux condenser and metering 690 g of distilled water, 3.73 g of phosphoric acid (75%) and 5.25 g of sodium hydroxide solution (25%) are introduced so that a pH of about 6.5 is reached. The reactor is stirred and heated to 75 ° C. A slight vacuum is applied (about 400 mbar) and then 400 g of N-vinylformamide are added over a period of one hour. At the same time, 20 g of 2,2'-azobis (2-amidinopropane) dihydrochloride, dissolved in 135 g of water, are metered in over a period of 3 hours. After the addition of the starter is polymerized for another three hours. The Polymer solution has a solids content of 45.3% and a K value of 30.5.

Example F Hydrolysis of Example E:

In the same reactor of Example 1 are first 2.5 g of sodium bisulfite solution (40%) and then 45 g of sodium hydroxide solution (25%) to. The reaction mixture is heated to 80 ° C and stirred for 3 hours then cooled to 40 ° C and 30.8 g of hydrochloric acid (30%) was added, so that a pH of about 7 is reached. The slightly yellowish polymer solution has a degree of hydrolysis of 4.3% (enzymatic determination of the resulting formic acid).

example 1

A commercially available wood-free base paper with the optical properties given in Table 1 was coated with 15 g / m ^{2 of} a coating by means of a hand knife. The coating contained 33% water. The non-aqueous fraction consisted of 70 parts of calcium carbonate (Hydrocarb® 90 from Plüss-Staufer AG), 30 parts of kaolin (Amazon® 88, Kaolin International), 8 parts of a polymer dispersion based on styrene and butadiene as binder (Styronal® D 610 , BASF Aktiengesellschaft), 0.5 parts of a 1,3,5-triazinyl derivative of (4,4'-diaminostilbene-2,2'-disulphonic acid with 2 sulfonic acid groups Tinopal ^® MC liquid from Ciba Specialty Chemicals) as an optical brightener and, if appropriate, 2 parts of a polyvinylformamide from Example D which acts as a rheological assistant, binding force improver and activator for the brightener and has a K value of 45.9 and a degree of hydrolysis of 5.4%. With this optionally the coating color added amount of activator (2%) is still achieved a sufficient viscosity of the coating composition for processing.

The coated paper was dried according to the prior art, calendered and examined as follows:

The white R 457 of the paper was determined according to DIN 53 145, part 2.

The CIE whiteness of the paper was measured according to ISO 2469.

According to the process of the invention, a 10% strength aqueous solution of a polyvinylformamide having a K value of 45.9 and a degree of hydrolysis of 5.4% (see Example D) was applied to the coating base paper before coating with the coating composition in such a way that after drying, 2 g / m ^{2 of} the polyvinylformamide remain on the paper. The following papers were compared in Table 1: I: coating base II: Paper coated with the coating containing no activator. III: Paper coated with the coating composition which contains 2 parts of polyvinylformamide from Example D with a K value of 45.9 and a degree of hydrolysis of 5.4% as activator. IV: Paper according to the process of the invention pretreated with polyvinylformamide and coated with a coating composition containing no activator. V: Paper according to the process of the invention which has been pretreated with polyvinylformamide but which additionally contains in the coating composition 2 parts of polyvinylformamide from Example D with a K value of 45.9 and a degree of hydrolysis of 5.4% as activator.

From the table 1 the expert recognizes from the comparison of lines II and IV how much the paper whiteness increases by the application of the method according to the invention. From the comparison of lines III and IV it can be seen that the use of the activator in the coating composition does not give the high whiteness by far as in the process according to the invention. From the comparison of lines IV and V it can be seen that, according to the process of the invention, the whiteness of the paper can be further increased if, in addition to the pretreatment of the paper, a suitable activator is added to the coating composition. From the comparison of lines III and V can be seen again the great effect of the method according to the invention. Table 1 White R 457 White CIE I. coating base 90.50 98.3 II: Paper coated with coating (without activator) 91.19 100.4 III: paper coated with coating (including activator) 97.17 115.6 IV: paper pretreated with activator, coated with coating slip (without activator) (according to the invention) 100.62 124.4 V: Paper pretreated with activator, coated with coating slip (including activator) (according to the invention) 102.67 129.8

Example 2

The procedure was as in Example 1 with the difference that a 1,3,5-triazinyl derivative of 4,4'-diaminostilbene-2,2'-disulfonic acid with 4 sulfonic acid groups (Tinopal ^® ABP-Z liquid from Ciba Specialty Chemicals) was used as optical brightener.

From Table 2 it can be seen that the advantages of the method according to the invention also occur when using an optical brightener with 4 sulfonic acid groups. Table 2 White R 457 White CIE I: base paper 90.50 98.3 II: Paper coated with coating (without activator) 91.52 98.0 III: paper coated with coating (including activator) 99.14 117.7 IV: paper pretreated with activator, coated with coating slip (without activator) (according to the invention) 105.28 131.6 V: Paper pretreated with activator, coated with coating slip (including activator) (according to the invention) 107.07 136.1

Example 3

The procedure was as in Example 1 with the difference that a 1,3,5-triazinyl derivative of 4,4'-diaminostilbene-2,2'-disulfonic acid with 6 sulfonic acid groups (Tinopal ^® SPP-Z liquid from Ciba Specialty Chemicals) was used as an optical brightener, and that optionally 1.5 parts of a functioning as a rheology aid, binding force enhancer and activator for the brightener unhydrolyzed polyvinylformamide from Example C were added with the K value 45.9.

From Table 3 it can be seen that the advantages of the method according to the invention also occur when using an optical brightener with 6 sulfonic acid groups. Further, it can be seen that the presence of an additional activator, which differs from that with which the coating base paper was treated, can further increase the whiteness of the coated paper. Table 3 White R 457 White CIE I. coating base 90.50 98.3 II: Paper coated with coating (without activator) 94.02 107.3 III: paper coated with coating (including activator) 100.74 124.3 IV: paper pretreated with activator, coated with coating slip (without activator) (according to the invention) 105.98 136.8 V: Paper pretreated with activator, coated with coating slip (including activator) (according to the invention) 106.84 139.2

Example 4

• Polyvinylformamid mit dem K-Wert 45,9, unhydrolysiert
• Polyvinylformamid mit dem K-Wert 45,9 und dem Hydrolysegrad 5,4%.

A commercially available wood-free base paper with the optical properties given in Table 1 was coated with 15 g / m ^{2 of} a coating by means of a hand knife. The coating contains 33% water. The non-aqueous fraction consisted of 70 parts of calcium carbonate (Hydrocarb® 90, Pluess-Staufer AG), 30 parts of kaolin (Amazon® 88, obtained from Kaolin International), 8 parts of a polymer dispersion based on styrene and butadiene as binder (Styronal® D 610, BASF Aktiengesellschaft), 0.5 parts of a 1,3,5-triazinyl derivative of 4,4'-diaminostilbene-2,2'-disulfonic acid 4-sulfonic acid (Tinopal ^® ABP-Z liquid from Ciba Specialty Chemicals ) as optical brightener and each 1.0 part, 1.5 parts and 2.0 parts of the following activators:

• Polyvinylformamide with a K value of 45.9, unhydrolysed
• Polyvinylformamide having a K value of 45.9 and a degree of hydrolysis of 5.4%.

In accordance with the process of the invention, a 10% strength aqueous solution of a polyvinylformamide from Example D having a K value of 45.9 and a degree of hydrolysis of 5.4% (see above) was applied to the coating base paper before coating with the coating composition such that after drying, 2 g / m ^{2 of} the polyvinylformamide remained on the paper.

Table 4 compares papers coated with coating compositions containing the various activators in varying amounts. Table 4 All coating base papers pretreated according to the invention with activator White R 457 White CIE I: base paper without activator 105.28 131.6 II: coating composition with 1.0 part of polyvinylformamide unhydrolysed as activator 105.87 134.4 III: coating composition with 1.5 parts of polyvinylformamide unhydrolysed as activator 106.12 135.4 IV: coating composition with 2.0 parts of polyvinylformamide unhydrolysed as activator 106.54 136.6 V: coating composition with 1.0 part of polyvinylformamide, degree of hydrolysis 5.4% as activator 105.50 133.1 VI: coating composition with 1.5 parts of polyvinylformamide, degree of hydrolysis 5.4% as activator 105.97 134.1 VII: coating composition with 2.0 parts of polyvinylformamide, degree of hydrolysis 5.4% as activator 107.07 136.1

It can be seen from Table 4, Lines II to VII, that the whiteness of the paper can be increased if, in addition to the pretreatment of the base paper, a suitable activator to the coating composition.

Example 5

A commercially available wood-free base paper with the optical properties given in Table 1 was coated with 15 g / m ^{2 of} a coating by means of a hand knife. The coating contained 33% water. The non-aqueous fraction consisted of 70 parts of calcium carbonate (Hydrocarb® 90, Pluess-Staufer AG), 30 parts of kaolin (Amazon® 88, obtained from Kaolin International), 8 parts of a polymer dispersion based on styrene and butadiene as binder (Styronal® D 610, BASF Aktiengesellschaft) and 0.5 parts of a 1,3,5-triazinyl derivative of 4,4'-diaminostilbene-2,2'-disulfonic acid 6-sulfonic acid (Tinopal ^® SPP-Z liquid from Ciba Specialty Chemicals ) as an optical brightener.

The coating base paper used was either not pretreated or various activators were applied to the coating base paper according to the technique described in Example 1.

• Polyvinylformamid aus Beispiel D mit dem K-Wert 45,9 und dem Hydrolysegrad 5,4%
• Polyvinylformamid aus Beispiel C mit dem K-Wert 45,9, unhydrolysiert
• Polyvinylpyrrolidon (Lumiten® P-PR 8450, BASF Aktiengesellschaft)
• Polyvinylalkohol (Rhodoviol® R4/20, Firma Rhodia)

Tabelle 5 Streichrohpapier Weiße R 457 Weiße CIE ohne Vorbehandlung 95,92 110,6 mit Polyvinylformamid aus Beispiel D, Hydrolysegrad 5,4% 106,61 136,5 mit Polyvinylformamid aus Beispiel C, unhydrolysiert 107,64 139,3 mit Polyvinylpyrrolidon 103,42 133,8 mit Polyvinylalkohol 101,77 127,7 The raw papers were treated with the following activators before being coated with the activator-free coating slip:

• Polyvinylformamide from example D with the K value 45.9 and the degree of hydrolysis 5.4%
• Polyvinylformamide from Example C with the K value 45.9, unhydrolysed
• Polyvinylpyrrolidone (Lumiten® P-PR 8450, BASF Aktiengesellschaft)
• Polyvinyl alcohol (Rhodoviol® R4 / 20, Rhodia company)

Table 5 coating base White R 457 White CIE without pretreatment 95.92 110.6 with polyvinylformamide from Example D, degree of hydrolysis 5.4% 106.61 136.5 with polyvinylformamide from Example C, unhydrolyzed 107.64 139.3 with polyvinylpyrrolidone 103.42 133.8 with polyvinyl alcohol 101.77 127.7

It can be seen from Table 5 that many different optical brightener activators can be used in the process of this invention.

Example 6

The procedure was as in Example 1 with the difference that a derivative of 4,4'-distyrylbiphenyl (Tinopal ^® SK-B liquid from Ciba Specialty Chemicals) was used as an optical brightener, and that optionally 1.0 parts of a rheology aid , Binderkraftsverbesserer and activator for the brightener acting unhydrolyzed polyvinylformamide from Example C with a K value of 45.9 added to the coating slip. Table 6 White R 457 White CIE I: base paper 90.50 98.3 II: Paper coated with coating (without activator) 94,10 109.1 III: paper coated with coating (including activator unhydrolysed polyvinylformamide) 96.79 115.0 IV: paper pretreated with activator, coated with coating slip (without activator) (according to the invention) 99.75 121.6 V: paper pretreated with activator, coated with coating slip (including activator unhydrolysed polyvinylformamide) (according to the invention) 100.67 124.3

From Table 6 it can be seen that the inventive method is also advantageous if optical brighteners of other chemical nature, eg. Based on derivatives of 4,4'-distyrylbiphenyl.

Example 7

A paper already coated with a pigment-containing coating composition (precoated paper) was provided with a top coat by coating with a pigment-containing coating composition containing 33% of water. The non-aqueous content of the coating composition consists of 70 parts of calcium carbonate (Hydrocarb® 90 from Plüss-Staufer AG), 30 parts of kaolin (Amazon® 88, obtained from Kaolin International), 8 parts of a polymer dispersion based on styrene and butadiene as binder (Styronal® D 610 from BASF Aktiengesellschaft) and 0.5 part of a 1,3,5-triazinyl derivative of 4,4'-diamino-stilbene-2,2'-disulfonic acid having 6 sulfonic acid groups (Tinopal® SPP-Z liquid from Ciba Specialty Chemicals) as optical brightener. According to the process of the invention, the polymers specified in Table 7 are applied to the prepainted paper in a quantity of 1 g / m ² prior to coating the prepainted paper with the topcoat coating composition.

The topcoated paper is dried, calendered and tested according to the prior art. The white R 457 of the paper was determined according to DIN 53 145, part 2.

The CIE whiteness of the paper was measured according to ISO 2469. The opacity was determined according to DIN 53146. Table 7 Pre-painted paper treated White R 457 of paper with top coat White CIE of paper with top coat Opacity of the paper with top coat without pretreatment 93.39 105.9 89,70 with polyvinyl alcohol (Rhodoviol® R4 / 20) 99.56 122.9 89.88 with polyvinylpyrrolidone (Lumiten® P-PR 8450) 102.62 132.7 91.02 Polyvinylformamide from Example E, K value 30.5 105.40 134.9 90.87 Polyvinylformamide from Example F, K value 30.5 Hydrolysis degree 4.3% 104.13 131.6 90.78 Polyvinylformamide from Example C, K value 45, 9 106.42 137.6 90.94 Polyvinylformamide from Example D, K value 45.9 Hydrolysis degree 5.4% 103.94 131.3 91.19 Polyvinylformamide from Example A, K value 67 105.83 136.1 91.12 Polyvinylformamide from example B, K value 67 degree of hydrolysis 5.0% 101.97 126.4 91.10

From the Table 7 the skilled artisan recognizes that one also gets an enhancement of the optical brightening in the uppermost coating and an increase in the opacity when applying the activators on an already coated paper before the pigment-containing and whitener top coat is applied.

Claims (10)

1. A process for the production of paper coated with a coating slip comprising at least one optical brightener, wherein base paper or precoated paper is treated, before application of the coating slip comprising optical brightener, with at least one substance which enhances the efficiency of optical brighteners.
2. The process according to claim 1, wherein the coating slip comprises at least one pigment.
3. The process according to either of claims 1 and 2, wherein a polymer or copolymer which comprises at least one N-vinylcarboxamide in the form of polymerized units is used for the pretreatment.
4. The process according to any of claims 1 to 3, wherein a polymer or copolymer comprising N-vinylformamide, N-vinylacetamide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, N-vinylpyrrolidone or N-vinylcaprolactam in the form of polymerized units is used for the pretreatment of the paper.
5. The process according to claim 3 or 4, wherein the copolymer or polymer which comprises N-vinylcarboxamide in the form of polymerized units is partially cleaved.
6. The process according to either of claims 3 and 4, wherein from 0 to 30% of the copolymer or polymer which comprises N-vinylcarboxamide in the form of polymerized units have been cleaved.
7. The process according to either of claims 1 and 2, wherein polyvinyl alcohol, carboxymethylcellulose, an anionic or nonionic degraded starch, casein, soybean protein, a water-soluble styrene/acrylate copolymer or an acrylate-containing copolymer is used for the pretreatment of the paper.
8. The process according to any of claims 1 to 7, wherein the coating slip comprising an optical brightener comprises a (co)polymer which comprises at least one N-vinylcarboxamide in the form of polymerized units.
9. A coated paper obtainable by a process according to any of claims 1 to 8.
10. The use of paper according to claim 9 in a printing process.