HK1138030A - Aqueous solutions of optical brighteners - Google Patents

Description

Aqueous solutions of optical brighteners

The present invention relates to aqueous solutions of optical brighteners with polyvinyl alcohols which the papermaker can use directly and provide coated papers of high whiteness.

It is well known that the whiteness and thus the visual appeal of coated paper can be improved by adding optical brighteners to the coating composition. To meet the demand for coated papers of higher whiteness, more efficient optical brighteners are needed.

However, although polyvinyl alcohol (PVOH) is known to improve the performance of optical brighteners in pigmented coating compositions by acting as a carrier (see, for example, "Surface Application of Paper Chemicals" page 164 from Brander (Springer, 1997)), when the papermaker wants to use the alcohol, it has to be added separately to the coating composition, usually in the form of an aqueous solution, which leads to a higher water content in the coating composition and thus to a longer drying time. The problem of providing the papermaker with a fully satisfactory solution to use PVOH as a carrier for optical brighteners remains.

WO2005/056658 seeks to provide a solution by disclosing a method of preparing an aqueous concentrate of optical brightener/PVOH comprising the following sequential steps: (a) providing an aqueous brightener composition comprising water and an optical brightener active, wherein the optical brightener active is typically present in the aqueous brightener composition in an amount of from about 10% to about 25%; (b) mixing a polyvinyl alcohol resin with said optical brightener composition in an amount of about 1 part of dried polyvinyl alcohol resin per 0.25 to 10 parts of wet aqueous brightener composition to provide a freshly produced aqueous concentrate of polyvinyl alcohol resin and optical brightener; and (c) heating the aqueous concentrate to dissolve the solids (i.e., to provide an aqueous solution containing the optical brightener and 9.1-80% polyvinyl alcohol). The method allows the preparation of colored coating compositions having a lower water content without affecting whitening performance and color.

However, WO2005/056658 does not provide a satisfactory solution for the papermaker, who usually wish to meter the optical brightener/PVOH solution directly into the coating composition; aqueous solutions containing optical brighteners and more than 9% PVOH typically have a high viscosity that is difficult, if not pumpable. Papermakers cannot generally use liquids having a viscosity greater than 1000mpa.s not only because of the difficulty of pumping, but also because of the impact thickening when such high viscosity liquids are introduced into the coating composition.

WO98/42685 discloses in example 15 an aqueous coating composition comprising 40% by weight of water, 40 parts of clay, 60 parts of calcium carbonate and 0.2 part of polyvinyl alcohol, to which 0.2 or 0.4% of a fluorescent whitening agent is added. There is no suggestion in this document that higher concentrations of polyvinyl alcohol and optical brightener still give pumpable compositions for the papermaker.

US2003/0089888a1 discloses the preparation of aqueous brighteners which comprise as organic thickeners polyvinyl alcohols having a degree of hydrolysis of more than 70% and the preparation temperature is from 40 to 98 ℃.

The problem of providing an optical brightener/PVOH aqueous solution that combines excellent whitening properties with low viscosity remains to be solved.

It has now been found that it is possible to prepare low-viscosity optical brightener/PVOH solutions which can be used directly by the papermaker, which can be metered by pump into the coating composition and which provide coated papers of surprising whiteness.

Accordingly, the present invention provides an aqueous fluorescent whitening agent solution comprising:

(a) between 6 and 60% by weight of at least one fluorescent whitening agent of formula (1)

Wherein

M is hydrogen, an alkali metal atom, ammonium or a cation derived from an amine, preferably hydrogen or sodium, most preferably sodium,

n is 1 or 2, and

x is a natural or unnatural amino acid in which the hydrogen on the amino group is removed;

(b)0.5 to 9% by weight of polyvinyl alcohol having a degree of hydrolysis of 60 to 75% and a Brookfield viscosity of 2 to 40mPa.s (4% w/w aqueous solution at 20 ℃);

(c) and (3) water.

In the case of fluorescent whitening agents in which n is 1, SO₃The M group is preferably in the 4-position of the phenyl ring.

In the case of fluorescent whitening agents in which n is 2, SO₃The M groups are preferably in the 2, 5 position of the phenyl ring.

Examples of amino acids from which X can be derived are alanine, 2-aminobutyric acid, asparagine, aspartic acid, S-carboxymethylcysteine, cysteic acid, cysteine, glutamic acid, glutamine, glycine, iminodiacetic acid, isoleucine, leucine, methionine, N-methyltaurine, norleucine, norvaline, phenylalanine, 2-phenylaminoacetic acid, 2-piperidinoic acid, proline, sarcosine, serine, taurine, threonine and valine. Wherein the amino acids contain chiral centers, optical isomers may be used, or racemic mixtures.

Preferred amino acids are aspartic acid, glutamic acid and iminodiacetic acid.

The aqueous solution may contain up to 10% by weight of a salt, usually sodium chloride, which is formed as a by-product of the preparation of the optical brightener.

The aqueous solution may also contain one or more antifreezes, biocides, complexing agents or other additives, and by-products formed in the preparation of the optical brightener. The aqueous solution may also contain other carriers such as polyethylene glycol.

The polyvinyl alcohol preferably has a degree of hydrolysis in the range from 65 to 75% and a Brookfield viscosity of from 2 to 20mPa.s (4% solution in water at 20 ℃).

Preferably, the content of polyvinyl alcohol in the solution falls within the range of 1 to 5% by weight.

Preferably the concentration of the optical brightener in the solution is in the range of 10 to 50% by weight.

Optical brightener/PVOH solutions are typically prepared by adding solid polyvinyl alcohol to a stirred solution of optical brightener in water and heating to 90-95 ℃ until a clear solution is formed.

The pH of the aqueous solution is preferably from neutral to substantially alkaline, especially in the range from pH7 to pH 10. If desired, the pH can be adjusted by adding M-to the corresponding base, for example alkali metal hydroxides or carbonates, ammonia or amines.

The optical brightener/PVOH solutions of the present invention are storage-stable and can be used directly by metering them directly into the coating composition by means of a pump. It is therefore another object of the present invention to add a brightener/PVOH solution to the coating composition to obtain a coated, optically brightened paper.

The present invention therefore also provides a process for the preparation of coated paper which is optically brightened at least in the coating layer, wherein the coating composition as described above is applied to the paper after sheet formation.

The coating compositions are essentially aqueous compositions containing at least one binder and a white pigment, in particular an opaque white pigment, and may additionally contain other additives such as dispersants, defoamers and synthetic thickeners.

Although coating compositions can be prepared without white pigments, the best white substrates for printing are prepared with opaque coating compositions containing 10 to 70% by weight of white pigment. The white pigment is generally an inorganic pigment, for example aluminium silicate (kaolin, or otherwise as china clay), calcium carbonate (chalk), titanium dioxide, aluminium hydroxide, barium carbonate, barium sulphate or calcium sulphate (gypsum).

The binder may be any of those commonly used in the paper industry for preparing coating compositions and may consist of a single binder or a mixture of primary and secondary binders. The one-component or primary binder is preferably a synthetic latex, typically a styrene-butadiene, vinyl acetate, styrene acrylate, vinyl acrylate or ethylene-vinyl acetate polymer. The secondary binder may be, for example, starch, carboxymethyl cellulose, casein, legume polymers, polyvinyl alcohol, or a mixture of any of the foregoing.

The one-component or primary binders are generally used in amounts in the range from 5 to 25% by weight, based on the white pigment. The secondary binders are generally used in amounts in the range from 0.1 to 10% by weight, based on the white pigment.

The fluorescent whitening agents of the formula (1) are generally used in amounts in the range from 0.01 to 1% by weight, based on the white pigment, preferably in amounts in the range from 0.05 to 0.5% by weight, based on the white pigment.

Examples

The following examples are to be understood as a detailed explanation of the present invention. If not otherwise stated, "%" and "parts" represent weight and viscosity is measured with a Brookfield viscometer.

Preparation of example 1

Fluorescent whitening agent solution 1 is prepared by mixing

18.5 parts of a fluorescent whitening agent of the formula (2),

80.3 parts of water, and

1.2 parts of polyvinyl alcohol having a degree of hydrolysis of 72.5% and a Brookfield viscosity of 6.0mPa.s (4% strength aqueous solution at 20 ℃) were stirred together,

while it is heated to 90-95 ℃ until a clear solution is obtained, which remains stable after cooling to room temperature. The pH of the solution was adjusted to 9.0 with sodium hydroxide.

The viscosity of the solution was 10.4 mPas at 20 ℃ and 14.3 mPas at 10 ℃.

Preparation of example 2

Fluorescent whitening agent solution 2 is prepared by mixing

18.5 parts of a fluorescent whitening agent of the formula (2),

79.1 parts of water, and

2.4 parts of polyvinyl alcohol having a degree of hydrolysis of 72.5% and a Brookfield viscosity of 6.0mPa.s (4% strength aqueous solution at 20 ℃) were stirred together,

while heating it to 90-95 ℃ until a clear solution is obtained that remains stable when cooled to room temperature. The pH of the solution was adjusted to 9.0 with sodium hydroxide.

The viscosity of the solution was 31.3 mPas at 20 ℃ and 48.1 mPas at 10 ℃.

Preparation of example 3

Comparative example (without polyvinyl alcohol)

An optical brightening solution 3 is prepared by stirring together 18.5 parts of the optical brightener of formula (2) and 81.5 parts of water. The pH of the solution was adjusted to 9.0 with sodium hydroxide.

Application examples

A solution containing 500 parts chalk (commercially available under the trade name Hydrocarb 90 from OMYA), 500 parts clay (commercially available under the trade name Kaolin SPS from IMERYS), 370 parts water, 6 parts dispersant (commercially available as sodium polyacrylate salt from BASF under the trade name Polysalz), 200 parts 50% latex (commercially available as styrene butadiene copolymer from Dow under the trade name DL 921) and 400 parts 20% anionic potato starch (Perfectamyl a4692 from AVEBE) in water was prepared. The solids content is adjusted to 60% by adding water and the pH is adjusted to 8-9 with sodium hydroxide.

Optical brightener solutions 1, 2 and 3, prepared as described in preparative examples 1, 2 and 3 respectively, are added to the stirred coating composition at a concentration of from 0.4 to-1.0% by weight of dry solids. The brightener coating composition was then applied to a commercially available 75g/m using a standard speed setting and standard rod load automatic wire wound rod applicator²Neutral-sized paper substrate. The coated paper was then dried in a stream of hot air for 5 minutes. The dried paper can be moved to ambient temperature and then measured for CIE whiteness using an Elrepho spectrophotometer at calibration.

TABLE 1

("OBA" means "optical brightener")

The results clearly show the surprising superiority in whiteness of coated papers prepared using this solution.

Claims (8)

1. Aqueous solutions of optical brighteners from

(a) Between 6 and 60% by weight of at least one fluorescent whitening agent of formula (1)

Wherein

M is hydrogen, an alkali metal atom, ammonium or a cation derived from an amine,

n is 1 or 2, and

x is a natural or unnatural amino acid from which a hydrogen atom on the amino group has been removed;

(b) from 0.5 to 9% by weight of a polyvinyl alcohol having a degree of hydrolysis of from 60 to 75% and a Brookfield viscosity (4% strength aqueous solution at 20 ℃) of from 2 to 40 mPa.s; and

(c) and (4) water.

2. The solution according to claim 1, wherein

M is hydrogen or sodium, and M is hydrogen or sodium,

n is a number of 1 or 2,

x is aspartic acid, glutamic acid or iminoacetic acid and wherein the polyvinyl alcohol has a degree of hydrolysis in the range 65 to 75% and a Brookfield viscosity (4% in water at 20 ℃) in the range 2 to 20 mPa.s.

3. A solution according to claim 1 or 2, wherein the concentration of polyvinyl alcohol is from 1 to 5% by weight and the concentration of optical brightener is from 10 to 50% by weight.

4. Use of a solution according to any of the preceding claims for a paper coating composition.

5. A process for the preparation of coated paper, wherein a solution according to any one of claims 1 to 3 is added to a coating composition which is applied to the paper after sheet formation.

6. A method according to claim 5, wherein the coating composition contains from 10 to 70% by weight of one or more white pigments.

7. A method according to claim 5 or 6, wherein the coating composition comprises a primary binder based on a synthetic latex selected from styrene-butadiene, vinyl acetate, styrene acrylate, vinyl acrylate or ethylene-vinyl acetate polymers, or a secondary binder additionally selected from starch, carboxymethyl cellulose, casein, pulse polymers, polyvinyl alcohol or mixtures of any combination of the above.

8. A process according to any one of claims 5 to 7, wherein the primary binder is used in an amount of from 5 to 25% by weight, based on the white pigment, wherein the secondary binder is used in an amount of from 0.1 to 10% by weight, based on the white pigment, and wherein the fluorescent whitening agent of formula (1) is used in an amount of from 0.01 to 1% by weight, based on the white pigment, preferably in an amount of from 0.05 to 0.5% by weight, based on the white pigment.