EP0409028A1 - Optical bleaching process for paper coatings and optical bleaching compositions used for this process - Google Patents

Abstract

Combinations of cellulose powder (mean particle size less than 50 mu m) and anionic cellulose brighteners and also, if appropriate, coating pigments are outstandingly suitable for the whitening of paper coating compositions. The papers obtained with these show improved light-fastness in addition to an excellent brightening effect.

Description

For the production of coated papers and cardboard, aqueous coating slips are used to a very large extent, which in addition to the usual white pigments - especially china clay and calcium carbonate - contain plastic dispersions as binders.

For the optical brightening of these coating slips, substantive anionic whiteners are usually used, e.g. Alkali salts of bistriazinylaminostilbene disulfonic acids used. However, these whiteners only have very unsatisfactory brightening effects and a very low graying limit (whitener concentration, at which further addition of the whitener does not result in an increase or even a decrease in the whiteness). In addition, the white toner types mentioned have a very low light fastness in those coating slips.

• 1. Durch Zusatz von geringen Anteilen an hydrophilen Cobindern zur Streichmasse, auf welche die Weißtöner aufziehen können. Solche Cobinder sind z.B.:
  Stärke, Kasein, Carboxymethylcellulose, Alginate, Polyvinylalkohol, Polyacrylate, Melamin- oder Harnstoff-Formaldehydharze (vgl. "Das Papier" 36 (1982), 66).
• 2. Verwendung von speziellen wasserlöslichen Weißtö­nern, welche hydrophile Carrier enthalten. Solche Carrier können z.B. Polyglykole sein (vgl. DE-A-­35 02 038 und EP-A-43 790).

It is generally known to partially solve the difficulties mentioned in two ways:

• 1. By adding small proportions of hydrophilic co-binders to the coating slip, onto which the whiteners can apply. Examples of such cobinders are:
  Starch, casein, carboxymethyl cellulose, alginates, polyvinyl alcohol, polyacrylates, melamine or urea-formaldehyde resins (see "Das Papier" 36 (1982), 66).
• 2. Use of special water-soluble whiteners which contain hydrophilic carriers. Such carriers can be, for example, polyglycols (cf. DE-A-35 02 038 and EP-A-43 790).

In this way, the white effect can be significantly improved, but at the same time the water sensitivity of the coated papers is increased, which leads to difficulties with certain printing processes and with the use of the coated papers. In addition, these additives to the coating slip often give rise to rheological problems on modern, high-speed coating systems.

It is also known (cf. DE-A 3 112 435) to have water-soluble white toners applied to certain plastics such as urea or methylol resins and to add these lightened plastics as a dispersion of the paper coating slip. However, these proposals have so far not been able to prevail because of their far too high costs or because of rheological difficulties.

It has now surprisingly been found that coating slips for the paper coating can be lightened with the above-mentioned whiteners without the disadvantages mentioned, if the coating slips are brighteners, water-dispersible cellulose powder and optionally coating pigments or a brightener preparation consisting essentially of brighteners in any order Cellulose powder and optionally a coating pigment are added, the cellulose powder in each case having an average particle size of less than 50 μm.

Suitable cellulose powder is, for example, microcrystalline cellulose which is obtained from cotton linters or wood pulp mostly by a combination of hydrolytic degradation and mechanical wet comminution of the particles and by subsequent spray drying and which no longer has any fiber structure (cf. "Umschau" 77 (1977), 312).

Cellulose powders which have been produced by dry grinding of conventional wood pulp and which likewise have practically no fiber structure are also suitable.

Cellulose powders which have a high crystallinity of more than 30%, preferably more than 50%, and a reduced hydrophilicity and dispersibility in water up to a solids content of about 25% are particularly suitable.

Also suitable are water-insoluble substituted cellulose types e.g. Cellulose ethers such as methyl or ethyl cellulose or cellulose esters such as cellulose acetate, acetobutyrate or acetopropionate.

Preferred cellulose powders are those which have particle sizes of 25-35, in particular approximately 30 μm.

To produce the coating color, up to 20% cellulose powder (based on the total amount of pigment addition) can be used in the coating color.

Suitable cellulose whitening agents to be used according to the invention are bistriazolylstilbene, bisstilbene and especially bistriazinyl-aminostilbene disulfonic acids.

worin
X₁ Amino, Methylamino, Ethylamino, Dimethylamino, Di­ethylamino, 2-Hydroxy-ethylamino, 3-Hydroxypropyl­amino, Di-(2-hydroxy-ethyl)-amino, Di-(2-hydroxy­propyl)-amino, 2-Sulfo-ethylamino, Morpholino, Anilino, Chloranilino, Sulfoanilino, Methylanilino oder Disulfoanilino und
X₂ Hydroxy, Methoxy, Ethoxy, Methoxyethoxy, Chlor oder X₁ bedeuten,
sowie solche der Formel

worin
X₃ und X₄ Wasserstoff, Methyl, Ethyl, Phenyl oder Sulfo­phenyl bedeuten,
sowie der Formel

worin
X₅ Wasserstoff, Methyl, Ethyl, Methoxy, Ethoxy, Chlor oder Sulfo bedeuten, wobei in allen Fällen
Z ein Alkalimetall-, Amin- oder Ammoniumion ist.Examples of particularly suitable brighteners are those of the formula

wherein
X₁ amino, methylamino, ethylamino, dimethylamino, diethylamino, 2-hydroxy-ethylamino, 3-hydroxypropylamino, di- (2-hydroxy-ethyl) -amino, di- (2-hydroxypropyl) -amino, 2-sulfo-ethylamino, morpholino , Anilino, chloranilino, sulfoanilino, methylanilino or disulfoanilino and
X₂ is hydroxy, methoxy, ethoxy, methoxyethoxy, chlorine or X₁,
as well as those of the formula

wherein
X₃ and X₄ represent hydrogen, methyl, ethyl, phenyl or sulfophenyl,
as well as the formula

wherein
X₅ is hydrogen, methyl, ethyl, methoxy, ethoxy, chlorine or sulfo, in all cases
Z is an alkali metal, amine or ammonium ion.

The brighteners are preferably in the form of their sodium salts.

Suitable coating pigments are clay, kaolin, silica, aluminum oxide, aluminum silicate, talc and especially chalk. These pigments are preferably used as an aqueous slurry.

The brightener preparations according to the invention can be prepared in a simple manner by dispersing suitable cellulose powders in water and adding customary anionic paper or textile brighteners.

Surprisingly, the brightener is readily absorbed onto the dispersed cellulose powder even at low temperature, and the brightened cellulose powder shows no graying even with relatively high brightener additives.

If the white toner preparations according to the invention contain customary white pigments, the preparation can be carried out in a simple manner by stirring suitable cellulose powders and customary anionic paper or textile brighteners, possibly together with a little water, into commercially available filler slurries or else by adding them during the dispersing and grinding process in the production of the filler slurries.

The dispersions obtained can contain conventional anionic or nonionic dispersants, e.g. Salts of polycarboxylic acids, sulfated or unsulfated higher alkanol or alkylphenol polyglycol ethers, lignin sulfonates, carboxymethyl cellulose, hydroxyethyl cellulose and others.

Surprisingly, the addition of the cellulose powder, despite its high hydrophilicity, only leads to a slight increase in the viscosity of the filler slurries, which can therefore still have high solids concentrations of up to over 70%.

The cellulose powders can be used in the white toner preparations according to the invention in any ratio based on white pigment, the viscosity being able to be adjusted to a desired value by adding water at the same time. A content of up to 20% cellulose powder based on white pigment is preferred, it being possible to set high pigment concentrations in the slurry without a significant increase in viscosity occurring.

To produce the coating color, up to 20% cellulose powder (based on the total amount of pigment addition) can be used in the coating color.

The amount of brightener used depends on the amount of the cellulose powder present in the white toner preparations according to the invention. In general, you can Amounts up to 10% active substance based on cellulose powder are used. Higher quantities no longer have any additional effects since the graying limit is exceeded. Amounts of 2.5 to 7% based on dry cellulose powder are preferred. It is advantageous to use the brightener in the form of an approximately 10 to 30%, preferably 15 to 25%, commercially available liquid formulation.

The brightener preparations to be used according to the invention are new and therefore also the subject of the present invention.

The brightener preparation can be added to the coating color in dispersed form immediately after its preparation and can be processed on conventional coating machines.

The aqueous brightener preparations can also be processed into powder mass by customary process steps, such as filtration and drying.

Another variant according to the invention is to add the cellulose powder and the brightener in separate form when preparing the coating color, the brightener immediately drawing onto the cellulose powder used.

The amount of the brightener or the brightener preparation used depends on the desired brightening effect. In general, 0.01 to 0.5% by weight of pure active substance on brightener (based on the pigment of the paper coating slip to be lightened) is sufficient. A particular advantage is that, depending on the coating slip composition, the graying limit that can be achieved (up to which a further brightener additive brings additional effects) is extremely high.

It is particularly advantageous to use the brightener in the form of an approximately 10-30%, preferably 15-25%, commercially available liquid formulation (% =% by weight).

Example 1a

(als 25 %ige Flüssigeinstellung).250 parts of microcrystalline cellulose with an average particle size of 20 μm, for example Avicel PH 105® from Lehmann and Voss, Hamburg), are dispersed in 750 parts of water and 25 parts of an optical brightener of the formula (A)

(as 25% liquid setting).

This dispersion can be incorporated directly into the paper coating slip. An excellent brightening effect with a comparatively high light fastness is obtained.

Example 2a

(in Form einer 13%-igen Flüssigeinstellung).250 parts of cellulose powder having an average particle size of 30 μm (for example Arbocel BE 600 / 30® from Rettenmaier, Holzmühle via Ellwangen) are dispersed in 750 parts of water and 25 parts of a brightener of the formula (B)

(in the form of a 13% liquid setting).

This dispersion can be incorporated directly into the paper coating slip. The paper coating obtained shows excellent lightfastness.

Example 3a

(als 25%ige Flüssigeinstellung).250 parts of cellulose powder (for example Avicel PH 105® from Lehmann and Voss) are dispersed in 750 parts of water, and 50 parts of a brightener of the formula (C) are added

(as 25% liquid setting).

When incorporated into coating slips, a good lightening effect and a comparatively high lightfastness of the paper coating is achieved.

Example 4a

(als Pulvereinstellung mit 78% Reinaufheller).250 parts of cellulose powder having an average particle size of 30 μm (for example Technocel 30® from the Cellulose Filler Factory, Mönchengladbach, are dispersed in 750 parts of water, and 3.5 parts of a brightener of the formula (D)

(as powder setting with 78% pure brightener).

The dispersion obtained can be incorporated directly into coating slips. It also gives excellent results.

Example 5a

(als Pulvereinstellung mit 83% Wirkstoffgehalt).250 parts of cellulose powder with an average particle size of 30 μm (for example Arbocel BE 600/30 from Rettenmaier) are dispersed in 750 parts of water and 5 parts of a brightener of the formula (E)

(as powder setting with 83% active ingredient content).

The dispersion obtained is also extremely suitable for incorporation into coating slips.

Example 6a

250 parts of pulverulent cellulose acetobutyrate are dispersed in 750 parts of water, and 25 parts of a 25% liquid formulation of the optical brightener (A) are added. The dispersion can be incorporated directly into the paper coating slip. An excellent brightening effect with comparatively good light fastness is obtained.

Example 7a

By stirring together
60 parts of China Clay SPS
40 parts calcium carbonate
20 parts of an anionic plastic dispersion of a copolymer containing acylate with a solids content of approx. 50% (e.g. Acronal S 320 D® from BASF)
0.5 parts polyvinyl alcohol
5 parts of polyacrylic acid ester as co-binder (40%) (e.g. Acrosol 40 D® from BASF)
80 parts of water
a non-lightened paper coating slip with a solids content of approx. 55% is produced, the pH of which is adjusted to 9 with sodium hydroxide solution.

Example 8a (comparison not according to the invention)

The coating color from Example 6 is lightened by adding 1.6 parts of the brightener of the formula (A) according to Example 1, based on pigment.

Example 9a

By stirring together
60 parts china clay SPS
30 parts calcium carbonate
40 parts of the 25% lightened cellulose dispersion of Example 1
20 parts of an anionic plastic dispersion of a copolymer containing acrylic acid ester with a solids content of approx. 50% (e.g. Acronal S 320 D® from BASF)
0.5 parts polyvinyl alcohol
5 parts of polyacrylate binder (40%) (e.g. Acrosol 40 D® from BASF
50 parts water
a paper coating slip with a solids content of approx. 55% is prepared and adjusted to a pH of 9 with sodium hydroxide solution.

Examples 10a - 13a

A coating color
60 parts of China Clay SPS
30 parts calcium carbonate
10 parts of cellulose powder (Arbocel BE 600/30 from Rettenmaier)
20 parts of an anionic plastic dispersion of a copolymer containing acrylic acid ester with a solids content of approx. 50% (eg Acronal S 320 D from BASF)
0.5 parts polyvinyl alcohol
5 parts of polyacrylate binder (e.g. Acrosol 40 D from BASF)
80 parts of water
The following amounts of the optical paper brightener according to formula (A) of Example 1a are added and the pH is adjusted to 9 using sodium hydroxide solution:
Example 10a: 0.4 parts of brightener of the formula (A)
Example 11a: 0.6 part of brightener of the formula (A)
Example 12a: 0.8 parts of brightener of the formula (A)
Example 13a: 1.6 parts of brightener of the formula (A).

Example 14a

To produce the coated paper, the coating slips of Examples 7a-13a are applied to paper with the aid of a hand-held squeegee or a test coater and dried at 80.degree.

Table 1 shows the CIE whiteness of the papers after production and after exposure (1 week in daylight).

Papers which have been produced by the process according to the invention have both an increased degree of whiteness and a significantly improved light fastness even with a reduced brightener concentration. Table 1 Brightening (% -related to pigment) CIE whiteness after manufacture CIE whiteness after exposure Whiteness drop due to exposure Example 7a - 72.0 72.4 - Example 8a 1.6% brightener (A) 92.5 76.5 -16.0 Example 9a 1.0% brightener (A), fixed on cellulose powder 103.2 95.0 -8.2 Example 10a 0.4% brightener (A), fixed on cellulose powder 89.6 86.9 -2.7 Example 11a 0.6% brightener (A), fixed on cellulose powder 94.5 89.7 -4.8 Example 12a 0.8% brightener (A), fixed on cellulose powder 97.9 91.0 -6.9 Example 13a 1.6% brightener (A), fixed on cellulose powder 106.3 93.3 -13.0

Examples 15a-22a

A coating color is prepared analogously to Example 7a, part of the chalk being replaced by an equal amount of microcrystalline cellulose (Avicel PH 105), to which 2% of the brightener of the formula (A) or (B) has been fixed beforehand.

The coated papers were examined for their whiteness after production and after exposure (Table 2). Papers which have been produced by the process according to the invention have both an increased degree of whiteness and improved light fastness. Table 2 example Cellulose powder addition to the coating color Brightening addition to the coating color CIE whiteness after manufacture CIE whiteness after exposure Whiteness drop due to exposure 15a - 2% (A) 96.9 86.8 -10.1 16a 5% Avicel PH 105 2% (A) 105.7 95.8 -9.9 17a 10% Avicel PH 105 2% (A) 107.5 100.8 -6.7 18a 20% Avicel PH 105 2% (A) 120.0 113.0 -7.0 19a - 2 B) 99.3 94.7 -4.6 20a 5% Avicel PH 105 2 B) 105.2 101.1 -4.1 21a 10% Avicel PH 105 2 B) 107.2 103.4 -3.8 22a 20% Avicel PH 105 2 B) 112.2 109.2 -3.0

Example 1b

(als 25 %ige Flüssigeinstellung) sowie mit 33 Teilen Wasser.90 parts of microcrystalline cellulose with an average particle size of 20 μm (for example Avicel PH 105® from Lehmann and Voss, Hamburg) are dispersed in 900 parts of chalk slurry (for example Omyalite 90® from Omya with a solids content of 72%) and mixed 9 parts of an optical brightener of the formula (A)

(as 25% liquid setting) and with 33 parts of water.

This dispersion has a solids content of 72 and can be incorporated directly into the paper coating slip. An excellent brightening effect with a comparatively high light fastness is obtained.

Example 2b

(in Form einer 13%-igen Flüssigeinstellung) sowie mit 80 Teilen Wasser.116 parts of cellulose powder having an average particle size of 30 μm (for example Arbocel BE 600 / 30® from Rettenmaier, Holzmühle via Ellwangen) are dispersed in 900 parts of chalk slurry according to Example 1 and mixed with 9 parts of a brightener of the formula (B)

(in the form of a 13% liquid setting) and with 80 parts of water.

This dispersion with a solids content of 70% can be incorporated directly into the paper coating slip. The paper coating obtained shows excellent lightfastness.

Example 3b

(als 25%ige Flüssigeinstellung).90 parts of cellulose powder (for example Avicel PH 105® from Lehmann and Voss) are dispersed in 900 parts of chalk slurry according to Example 1 and mixed with 18 parts of a brightener of the formula (C)

(as 25% liquid setting).

When incorporated into coating slips, a good lightening effect and a comparatively high lightfastness of the paper coating is achieved.

Example 4b

(als Pulvereinstellung mit 78% Reinaufheller) sowie mit 60 Teilen Wasser.90 parts of cellulose powder having an average particle size of 30 μm (for example Technocel 30® from the Cellulose Filler Factory, Mönchengladbach, are dispersed in 900 parts of chalk slurry according to Example 1 and 1.2 parts of a brightener of the formula (D)

(as powder setting with 78% pure brightener) and with 60 parts of water.

The dispersion obtained can be incorporated directly into coating slips. It also gives excellent results.

Example 5

90 parts of cellulose powder having an average particle size of 30 μm (for example Arbocel BE 600 / 30® from Rettenmaier) are dispersed in 900 parts of chalk slurry according to Example 1, and 1.75 parts of a brightener of the formula (E)

This dispersion can be incorporated directly into coating slips. The paper coating obtained shows a good brightening effect and a comparatively high light fastness.

Example 6b

90 parts of cellulose powder having an average particle size of 30 μm (for example Arbocel BE 600 / 30® from Rettenmaier, Holzmühle above Ellwangen) are dispersed in 900 parts of kaolin slurry (for example Clay SPS®) and 9 parts of a brightener of the formula (A) (as 25% liquid setting) and with 250 parts of water.

This dispersion can be incorporated directly into the paper coating slip. An excellent brightening effect with a comparatively high light fastness is obtained.

Example 7b

By stirring together
60 parts of China Clay SPS®
40 parts calcium carbonate (corresponding to 55 parts chalk slurry according to Example 1)
20 parts of an anionic plastic dispersion of a copolymer containing acylate with a solids content of approx. 50% (e.g. Acronal S 320 D® from BASF)
0.5 parts polyvinyl alcohol
5 parts of polyacrylic acid ester as co-binder (40%) (e.g. Acrosol 40 D® from BASF)
65 parts of water
a non-lightened paper coating slip with a solids content of approx. 55% is produced, the pH of which is adjusted to 9 with sodium hydroxide solution.

Example 8b (comparison not according to the invention)

The coating color from Example 7 is lightened by adding 1.6 parts of the brightener of the formula (A) according to Example 1b, based on pigment.

Example 9b

By stirring together
60 parts of China Clay SPS®
55 parts of the 72% lightened chalk slurry of Example 1
20 parts of an anionic plastic dispersion of a copolymer containing acrylic acid ester with a solids content of approx. 50% (e.g. Acronal S 320 D® from BASF)
0.5 parts polyvinyl alcohol
5 parts of polyacrylate binder (40%) (e.g. Acrosol 40 D® from BASF)
65 parts of water
a paper coating slip with a solids content of approx. 55% is prepared and adjusted to a pH of 9 with sodium hydroxide solution.

Example 10b

To produce the coated paper, the coating slips of Examples 7b-9b are applied to paper with the aid of a hand-held squeegee or an experimental coating system and dried at 80.degree.

Table 3 shows the CIE whiteness of the papers after production and after exposure (1 week in daylight).

Papers which were produced by the process according to the invention have both an increased degree of whiteness and a significantly improved light fastness even when the brightener concentration is reduced. Table 3 Brightening (% -related to pigment) CIE whiteness after manufacture CIE whiteness after exposure Whiteness drop due to exposure Example 7b - 72.0 72.4 - Example 8b 1.6% brightener (A) 92.5 76.5 -16.0 Example 9b 1.0% brightener (A), fixed on cellulose powder 92.4 88.4 -4.0

Claims (10)

1. A process for tinting paper coating slips with substantive anionic white toners, characterized in that the coating slips in any order brightener, water-dispersible cellulose powder or a brightener preparation consisting essentially of brightener-containing cellulose powder and optionally coating pigment, in each case Cellulose powder has an average particle size of less than 50 microns. 2. The method according to claim 1, characterized in that white toners of the formula

wherein
Z is an alkali metal, amine or ammonium ion,
X₁ amino, methylamino, ethylamino, dimethylamino, diethylamino, 2-hydroxyethylamino, 3-hydroxy-propylamino, di- (2-hydroxy-ethyl) -amino, di- (2-hydroxy-propyl) -amino, 2-sulfo -ethylamino, morpholino, anilino, chloranilino, sulfoanilino, methylanilino or disulfoanilino and
X₂ is hydroxy, methoxy, ethoxy, methoxyethoxy, chlorine or X₁,
used. 3. The method according to claim 1, characterized in that the proportion of cellulose powder in the coating slips is up to 20 wt .-%, based on pigment. 4. White toner preparations consisting essentially of cellulose powder with an average particle size of less than 50 microns and a substantive anionic white toner. 5. White toner preparation consisting essentially of a) a substantive anionic white toner, b) water-dispersible cellulose powder with an average particle size of less than 50 microns and c) coating pigment. 6. white toner preparation according to claim 4 or 5, characterized in that it is an aqueous dispersion. 7. white toner preparation according to claim 4 or 5, characterized in that these white toner of the formula

wherein
Z is an alkali metal, amine or ammonium ion,
X₁ amino, methylamino, ethylamino, dimethylamino, diethylamino, 2-hydroxyethylamino, 3-hydroxy-propylamino, di- (2-hydroxy-ethyl) -amino, di- (2-hydroxy-propyl) -amino, 2-sulfo -ethylamino, morpholino, anilino, chloranilino, sulfoanilino, methylanilino or disulfoanilino and
X₂ is hydroxy, methoxy, ethoxy, methoxyethoxy, chlorine or X₁,
contains. 8. white toner preparation according to claim 5, characterized in that it contains chalk as a coating pigment. 9. White toner preparation according to claim 4 or 5, characterized in that it additionally contains dispersing agents. 10. A method for whitening paper coating slips with substantive anionic whiteners, characterized in that brightening preparations according to claim 4 or 5 are added to the coating slips.