Dispersions of optical brightening agents

The present invention relates to new dispersions of optical brighteners, their use in coating slips, for example for coating paper, and coating slips containing these dispersions.

Optical brighteners which are used in the paper industry are usually marketed in the form of solutions which mostly contain the sodium salt of the brightener and considerable amounts of a solubilizer, such as urea or a solvent, for example a polyalkylene glycol, to make the brightener complete is solved. In order to ensure the stability of the commercial form even at low temperatures, a concentration of about 22% active substance could not be exceeded so far if the content of solubilizer should remain in a reasonable ratio to the amount of brightener. Since these solubilizers are undesirable after application in the waste water, there was a need for a formulation which contains no or only small amounts of such substances.

It has now been found that stable aqueous dispersions can be produced from the optical brighteners of the formula (1) given below if the brighteners are present as free acid and the dispersions contain a polyhydroxy compound. Surprisingly, such dispersions are stable in storage for a long time and dissolve quickly enough when used in paper coating slips, even in the presence of small amounts of alkali, so that a perfect distribution is ensured, both when applied in bulk and when incorporated into paper coating slips the surface application in the line.

Surprisingly, when using fillers such as precipitated calcium carbonate or natural calcium carbonate, the addition of alkali can be dispensed with entirely.

The dispersions according to the invention contain no dispersants, which represents an important economic advantage.

The present invention relates to dispersions of optical brighteners, which are characterized in that they<ul id="ul0001" list-style="none" compact="compact"><li>a) 5 to 60% by weight of an optical brightener of the formula<chemistry id="chem0001" num="0001"><img file="EP0835906A2_D0001.tif" /></chemistry> which is present as a free acid, where X₁, X₂, Y₁ and Y₂ one NH each independently₂Group or a primary or secondary amine residue or an optionally branched alkoxy group or a phenoxy group,</li><li>b) 0.01 to 3% by weight of a polyhydroxy compound,</li><li>c) water and optionally</li><li>d) contain other additives.</li></ul>

The dispersions preferably contain less than 2% by weight, in particular less than 0.8% by weight, of electrolyte.

The dispersions preferably contain 10 to 50% by weight, in particular 20 to 50% by weight, especially 30 to 45% by weight, of brightener, based on the total weight of the dispersion.

Preferred dispersions are those which have a brightener of the formula<chemistry id="chem0002" num="0002"><img file="EP0835906A2_D0002.tif" /></chemistry> included, where X₃, X₄, Y₃ and Y₄ independently of one another each a phenylamino group which is represented by C₁-C₂-Alkyl can be mono- or disubstituted, the morpholino group, a mono-C₁-C₄-Alkylamino- or Di-C₁-C₄Alkylamino group, which can be substituted by one or more hydroxyl radicals, or a C₁-C₄-Alkoxy group.

Of particular interest are dispersions containing an optical brightener of the formula<chemistry id="chem0003" num="0003"><img file="EP0835906A2_D0003.tif" /></chemistry> where X₅ and X₆ independently of one another each a phenylamino group which is represented by C₁-C₂-Alkyl can be mono- or disubstituted, and Y₅ and Y₆ a morpholino group or a Di-C₁-C₄- Alkylamino group, which can be substituted by one or more hydroxyl radicals.

Among them, those in which Y₅ and Y₆ a Di-C₁-C₄-Alkylamino group, which can be substituted by one or more hydroxyl radicals.

The dispersions according to the invention preferably contain more than 25% by weight of brightener and less than 0.8% by weight of electrolyte, based on the total weight of the dispersion. In this context, electrolyte is understood to mean low molecular weight inorganic salts which are present as by-products from synthesis or the chemicals or solvents used, for example water. Above all, it concerns alkali halides, sulfates or hydrogen sulfates.

The optical brighteners of the formula (1) are known or can be prepared in a manner known per se. If the brighteners contain more than the desired amount of electrolyte after synthesis, this excess must be removed. This can be done using standard methods. In most cases, washing the press cake with demineralized water is sufficient.

The usual thickeners are suitable as polyhydroxy compounds. These are organic natural thickeners, organic modified natural substances, organic fully synthetic thickeners, such as. As polyvinyl alcohol, or inorganic thickeners such as montmorillonite or hectorite.

The dispersions according to the invention preferably contain, as the polyhydroxy compound, an anionic modified polysaccharide which is derived from cellulose, starch or heteropolysaccharides, and further monosaccharides such as mannose and glucuronic acid may be contained in the side chains. Examples of suitable anionic polyhydroxy compounds are sodium alginate, carboxymethylated guar, carboxymethyl cellulose, carboxyethyl cellulose, carboxymethyl starch, carboxymethylated locust bean gum and especially xanthan gum. Of course, mixtures containing two or more polyhydroxy compounds can also be used.

The amount of polyhydroxy compound in the dispersions according to the invention is 0.01 to 3% by weight, based on the weight of the dispersion, a range from 0.05 to 0.5% by weight being preferred and a range from 0.1 to 0% 3% by weight is particularly preferred.

If desired, the dispersions according to the invention may contain further additives, for example preservatives, e.g. B. 1, 2-benzisothiazolin-3-one or aldehydes such as formaldehyde or acetaldehyde, antifreezes or defoamers.

The dispersions according to the invention are obtained, for example, by intensively mixing the moist presscake or else the dry powder of an optical brightener of the formula (1) with water and, if appropriate, reducing the particle size to a value below 20 μm, preferably below 10 μm, by wet grinding. A polyhydroxy compound and, if appropriate, further additives are then added and mixed until the components are homogeneously distributed.

The dispersions obtained are stable in storage for a long time, for example for 6 months, even at temperatures from about 0 ° C. to 40 ° C.

The dispersions according to the invention are suitable for lightening paper pulps in papermaking, e.g. B. pulp, wood pulp (chemical and mechanical pulp) and for lightening the coating slips commonly used in the paper industry, specifically for lightening unpigmented, but in particular pigmented paper slips and coating slips.

The known coating slips contain, among other things, plastic dispersions based on copolymers of butadiene-styrene, acrylonitrile-butadiene-styrene, acrylic acid esters, ethylene-vinyl chloride or ethylene-vinyl acetate or on the basis of homopolymers, such as polyvinyl chloride, polyvinylidene chloride, polyethylene, polyvinyl acetate or polyurethanes. A preferred binder consists of styrene-butyl acrylate or styrene-butadiene-acrylic acid copolymers. Further polymer latices are described, for example, in US Pat. Nos. 3,265,654, 3,657,174, 3,547,899 and 3,240,740.

Aluminum silicates such as china clay and kaolin, barium sulfate, satin white, titanium dioxide or calcium carbonate (chalk) are usually used to pigment the coating slips.

Formulations of such known coating slips for paper are described, for example, in JP Casey "Pulp and Paper", Chemistry and Chemical Technology, 2nd Ed. Vol. III, pp. 1648 - 1649 and in Mc Graw-Hill "Pulp and Paper Manufacture", 2nd and 5th Ed. Vol. Ll, p. 497.

The coating slips according to the invention preferably contain 5 to 70% by weight of a white pigment. The binder is preferably used in an amount sufficient that the dry content of polymeric compound constitutes 1 to 30% by weight, preferably 5 to 25% by weight, of the white pigment. The amount of the brightener dispersion according to the invention is calculated in such a way that the brightener is present in amounts of 0.005 to 1% by weight, in particular 0.01 to 0.55% by weight, based on the white pigment.

The coating slip according to the invention can be prepared by mixing the components in any order at temperatures from 10 to 100 ° C., preferably 20 to 80 ° C. The components here also include the usual auxiliaries which can be used to regulate the rheological properties, such as viscosity or water retention, of the coating slips. Such tools are, for example natural binders, such as starch, casein, protein or gelatin, cellulose ethers, such as carboxyalkyl cellulose or hydroxyalkyl cellulose, alginic acid, alginates, polyethylene oxide or polyethylene oxide alkyl ethers, copolymers of ethylene oxide and propylene oxide, polyvinyl alcohol, polyvinylpyrrolidone, water-soluble condensation products of formaldehyde with urea or melamine polyacrylate, polyphosphate .

The dispersions according to the invention are either incorporated into the finished coating slip or into one of the components of the coating slip.

The coating slip according to the invention can be used for coating paper, wood, foils, such as cellulose, cellulose triacetate, textiles, etc. Use on paper and cardboard and photo paper is particularly preferred.

The coating slip can be applied to the substrate by any conventional method, for example using an air knife, a doctor knife, a brush, a roller, a squeegee or a rod, whereupon the coating is then applied, for example using an infrared dryer and / or hot air dryer at temperatures of the substrate surface Range from 70 to 200 ° C, preferably from 90 to 130 ° C, is dried to a residual moisture content of 3 to 6% by weight.

Through the use of the brightener dispersions according to the invention, the coatings obtained are distinguished by an optimal distribution of the optical brighteners over the entire surface and a consequent increase in the degree of whiteness as well as a high light fastness.

When applying the brightener dispersions according to the invention, an alkaline pH is favorable for the perfect solution of the optical brightener. In the case of coating slips, this pH value should be primarily in the range from 8 to 11 and in the sludge mass above all from 7.5 to 9. The desired alkaline pH can, for example can be adjusted by adding NaOH, this pH preferably being set in the fiber dispersion or the coating slip even before the brightener dispersion is added. When using alkaline fillers such as precipitated or natural calcium carbonate, the addition of alkali can be dispensed with.

Another object of the present invention is therefore the use of the dispersions according to the invention for the optical brightening of coating slips for the coating of paper and therefore the use of the dispersions according to the invention for the optical brightening of fiber dispersions in paper production.

Example 1:

To 3333 parts of moist filter cake with an electrolyte content of less than 0.3%, containing 1733 parts of water and 1600 parts of the optical brightener of the formula<chemistry id="chem0004" num="0004"><img file="EP0835906A2_D0004.tif" /></chemistry> in the form of the free acid, 312 parts of demineralized water are added with vigorous stirring and the mixture is stirred until the slurry is homogeneously distributed. Then 355 parts of an aqueous solution containing 1.5 parts of xanthan and 2.3 parts of a preservative (aqueous solution of 1,2-benzisothiazolin-3-one) are added and the mixture is stirred for a further 50 minutes until the components are homogeneously distributed.

A dispersion is obtained which shows no deposits even after storage at 25 ° C. for several weeks.

Example 2:

A fiber suspension consisting of 5 parts of fiber (1: 1 mixture of birch and pine kraft pulp; freeness 35 ° SR) in 200 parts of water with a hardness of 25 ppm CaO is mixed with 10% precipitated calcium carbonate, based on the fiber. Then 0.0275 parts of the brightener dispersion obtained according to Example 1 are added, the mixture is stirred slowly for 1 minute and 0.03%, based on the total weight of fiber and pigment, of a solution of a commercial cationic retention agent (modified polyacrylamide) is added. Then paper sheets are produced on a sheet former (System Rapid-Köthen). The moist paper sheets are pressed and dried between two filter paper sheets at 110 ° C. for 20 minutes.

This gives paper sheets which are just as good in terms of fluorescence and brightening effect as paper sheets which are prepared using commercially available formulations of the same brightener as a solution containing urea.

Example 3:

The procedure is as described in Example 2, but instead of precipitated calcium carbonate, the same amount of natural calcium carbonate is used.

Paper sheets are obtained which are just as good in terms of fluorescence and brightening effect as the paper sheets obtained according to Example 2.

Example 4:

The procedure is as described in Example 2, but 1.6 parts of 0.1 N NaOH are added after the brightener dispersion has been added. The mixture is then worked up as described in Example 2.

Paper sheets are obtained which are just as good in terms of fluorescence and brightening effect as the paper sheets obtained according to Example 2.

Example 5:

The procedure is as described in Example 3, but 1.6 parts of 0.1 N NaOH are added after the brightener dispersion has been added. The mixture is then worked up as described in Example 2.

Paper sheets are obtained which are just as good in terms of fluorescence and brightening effect as the paper sheets obtained according to Example 2.

Example 6:

100 parts of natural calcium carbonate, 18 parts of a 50% styrene-butadiene latex dispersion, 1 part of polyvinyl alcohol, 0.25 part of a synthetic thickener and 45 parts of water are stirred in with the addition of so much NaOH that a pH of 9.5 results until a homogeneous distribution is achieved.

0.55 part of the dispersion prepared according to Example 1 is added to this mass and the mixture is stirred for a further 5 minutes at room temperature.

With the coating slip obtained, a wood-free paper is made on a laboratory coating machine. which does not yet contain an optical brightener (90 g / m². Cobb30 = 12 g / m², ISO Weisse 82.4%). The weight of the top layer after conditioning at 23 ° C and 50% relative humidity is 9 g / m². It is dried in an infrared and then in a hot air dryer until a residual moisture of 7% is reached.

A paper is obtained which is just as good in terms of fluorescence and brightening effect as paper which is prepared using commercially available formulations of the same brightener as a solution containing urea.

Example 7:

If you work as described in Example 1, but use the brightener of the formula<chemistry id="chem0005" num="0005"><img file="EP0835906A2_D0005.tif" /></chemistry> in the form of the free acid, a dispersion is also obtained which shows no deposits even after storage at 25 ° C. for several weeks.

Example 8

:

If you work as described in Example 1, but use the brightener of the formula<chemistry id="chem0006" num="0006"><img file="EP0835906A2_D0006.tif" /></chemistry> in the form of the free acid, a dispersion is also obtained which shows no deposits even after storage at 25 ° C. for several weeks.

Example 9:

If you work as described in Example 1, but use the brightener of the formula<chemistry id="chem0007" num="0007"><img file="EP0835906A2_D0007.tif" /></chemistry> in the form of the free acid, a dispersion is also obtained which shows no deposits even after storage at 25 ° C. for several weeks.