IMPROVED OPTICAL BRIGHTENING COMPOSITIONS

The instant invention relates to compositions which provide superior optical brightening effects when applied to the surface of paper at the size-press.

BACKGROUND

A high level of whiteness is an important parameter for the end-user of paper products. The most important raw materials of the papermaking industry are cellulose, pulp and lignin which naturally absorb blue light and therefore are yellowish in color and impart a dull appearance to the paper. Optical brighteners are used in the papermaking industry to compensate for the absorption of blue light by absorbing UV-light with a maximum wavelength of 350 - 360 nm and converting it into visible blue light with a maximum wavelength of 440 nm.

In the manufacture of paper, optical brighteners may be added either at the wet end of the paper machine, or to the surface of paper, or at both points. In general, it is not possible to achieve the whiteness levels required of higher-quality papers by addition at the wet end alone.

A common method of adding optical brightener to the surface of paper is by application of an aqueous solution of the optical brightener at the size-press together with a sizing agent, typically a native starch or an enzymatically or chemically modified starch. A preformed sheet of paper is passed through a two-roll nip, the entering nip being flooded with sizing solution. The paper absorbs some of the solution, the remainder being removed in the nip.

In addition to starch and optical brightener, the sizing solution can contain other chemicals designed to provide specific properties. These include defoamers, wax emulsions, dyes, pigments and inorganic salts.

In order to reach higher whiteness levels, considerable effort has been put into the development of new optical brighteners. See, for example, Japanese Kokai 62-106965, PCT Application WO 98/42685, US Patent 5,873,913 and European Patent 1 ,763,519.

GB 1 239 818 discloses hexasulphonated optical brighteners derived from triazinylaminostilbenes. Examples 1 to 6 disclose their sodium salts. Magnesium is only

mentioned in a list of possible counterions for the hexasulphonated optical brighteners, starch as a component in a surface sizing composition is also only mentioned in a list of possible binding agents.

The demand remains for more efficient means of achieving high whiteness levels in paper.

DESCRIPTION OF THE INVENTION

Surprisingly, we have found that optical brighteners of formula (1 ) when applied to the surface of paper in combination with magnesium salts in a starch sizing composition give enhanced whitening effects. Parts mean parts by weight in the following, if not otherwise specified.

The present invention therefore provides a method for brightening paper in the size- press, characterised in that the sizing composition comprises

(a) at least one optical brightener of formula (1 )

in which Ri is hydrogen or SO ₃ M, R ₂ is hydrogen or SO ₃ M, R ₃ is hydrogen, C _1-4 alkyl, C _2-3 hydroxyalkyl, CH ₂ CO ₂ M, CH ₂ CH ₂ CONH ₂

Or CH ₂ CH ₂ CN,

R ₄ is Ci _-4 alkyl, C _2-3 hydroxyalkyl, CH ₂ CO ₂ M, CH(CO ₂ M)CH ₂ CO ₂ M or

CH(CO ₂ M)CH ₂ CH ₂ CO ₂ M, benzyl, or

R ₃ and R ₄ together with the neighbouring nitrogen atom signify a morpholine ring, and M is hydrogen, an alkali metal cation, ammonium, mono-methyl-di-C ₂ -

C ₃ -hydroxyalkyl ammonium, di-methyl-mono-C ₂ -C ₃ -hydroxyalkyl ammonium, ammonium which is mono-, di- or trisubstituted by a C ₂ - C ₃ hydroxyalkyl radical, or mixtures of said compounds,

(b) a magnesium salt; and

(c) a binding agent, which is selected from the group consisting of native starch, enzymatically modified starch and chemically modified starch;

0.1 to 15 parts of component (b) being present per part of component (a).

Preferred compounds of formula (1 ) are those in which R ₃ represents hydrogen, methyl, ethyl, n-propyl, isopropyl, β-hydroxyethyl, β-hydroxypropyl, CH ₂ CO ₂ M, CH ₂ CH ₂ CONH ₂ Or CH ₂ CH ₂ CN and R ₄ represents methyl, ethyl, n-propyl, isopropyl, 2- butyl, β-hydroxyethyl, β-hydroxypropyl, CH ₂ CO ₂ M, CH(CO ₂ M)CH ₂ CO ₂ M, CH(CO ₂ M)CH ₂ CH ₂ CO ₂ M or benzyl.

Optical brighteners of formula (2) and (3) are specific examples for the optical brighteners of formula (1 ), but the invention is not limited to these two specific examples.

The magnesium salt can be, for example, magnesium acetate, magnesium bromide, magnesium chloride, magnesium formate, magnesium iodide, magnesium nitrate, magnesium sulphate or magnesium thiosulphate. Preferably, the magnesium salt is magnesium chloride, magnesium sulphate or magnesium thiosulphate. Most preferably, the magnesium salt is magnesium chloride.

Preferably, 0.15 to 10 parts of component (b) are present per part of component (a). Most preferably, 0.4 to 5 parts of component (b) are present per part of component (a).

For the treatment of paper in the size-press, sizing compositions containing 0.2 to 30, preferably 1 to 15 grams per litre of the optical brightener, may be used. The sizing composition also contains a binding agent in a concentration of preferably 2 to 15% by weight, based on the total weight of the sizing composition. The pH is typically in the range 5-9, preferably 6-8.

The binding agent or size is selected from the group consisting of native starch, enzymatically modified starch and chemically modified starch. Modified starches are preferably oxidized starch, hydroxyethylated starch or acetylated starch. The native starch is preferably an anionic starch, an cationic starch, or an amphoteric starch. While the starch source may be any, preferably the starch sources are corn, wheat, potato, rice, tapioca or sago. One or more secondary binders may be present, preferably polyvinyl alcohol or carboxymethylcellulose.

Further subject of the invention is a process for the optical brightening of paper comprising the steps of a) applying the sizing composition to the paper, b) drying the treated paper. Preferably, a defoamer, a wax emulsion, a dye and/or a pigment is added to the sizing composition.

The following examples shall explain the instant invention in more details. If not indicated otherwise, "%" and "parts" are meant by weight.

EXAMPLE 1

Sizing compositions are prepared by adding an optical brightener of formula (2) in such an amount, that a range of final concentrations of from 2.5 to 12.5 g/l of optical brightener is achieved, to a stirred, aqueous solution of magnesium chloride (final concentration is 8 g/l) and an anionic oxidized potato starch (Perfectamyl A4692 from

AVEBE B.A.) (final concentration is 50 g/l) at 6O ⁰ C.

The sizing solution is allowed to cool, then poured between the moving rollers of a laboratory size-press and applied to a commercial 75g/m ² AKD (alkyl ketene dimer) sized, bleached paper base sheet. The treated paper is dried for 5 minutes at 7O ⁰ C in a flat bed drier. The dried paper is allowed to condition, then measured for CIE whiteness on a calibrated Elrepho spectrophotometer.

The Example is repeated both in the absence of magnesium chloride, i.e. only the sodium salt of the optical brightener is present, and with the magnesium chloride replaced by an equivalent amount of calcium chloride.

The results are summarized in Table 1 , and clearly demonstrate the advantage of using magnesium chloride over the use of calcium chloride and over the use only of the sodium salt of the optical brightener in order to reach higher whiteness levels. The surprising nature of the invention is further illustrated by the observation that chloride salts of other divalent Group Il metal ions, such as calcium chloride, even have a negative impact on the whitening effect of the optical brightener.

TABLE 1

EXAMPLE 2

Sizing solutions are prepared by adding an optical brightener of formula (3) in such an amount, that a range of final concentrations of from 2.0 to 10.0 g/l of optical brightener is achieved, to a stirred, aqueous solution of magnesium chloride (final concentration is 8 g/l) and an anionic oxidized potato starch (Perfectamyl A4692 from AVEBE B.A.) (final concentration 50 g/l) at 6O ⁰ C.

The sizing solution is allowed to cool, then poured between the moving rollers of a laboratory size-press and applied to a commercial 75g/m ² AKD (alkyl ketene dimer) sized, bleached paper base sheet. The treated paper is dried for 5 minutes at 7O ⁰ C in a

flat bed drier. The dried paper is allowed to condition, then measured for CIE whiteness on a calibrated Elrepho spectrophotometer.

The Example is repeated both in the absence of magnesium chloride, and with the magnesium chloride replaced by an equivalent amount of calcium chloride.

The results are summarized in Table 2, and clearly demonstrate the advantage of using magnesium chloride to reach higher whiteness levels in comparison to where the optical brightener is present only as the sodium salt.

TABLE 2

EXAMPLE 3

Sizing compositions are prepared by adding an optical brightener of formula (3) in such an amount, that a range of final concentrations of from 0 to 12.5 g/l of optical brightener is achieved, to stirred, aqueous solutions of magnesium chloride (final concentrations are 6.25 and 12.5g/l) and an anionic oxidized corn starch (final concentration 50 g/l) (Penford Starch 260) at 60 ⁰ C. Each sizing solution is allowed to cool, then poured between the moving rollers of a laboratory size-press and applied to a commercial 75 g/m ² AKD (alkyl ketene dimer) sized, bleached paper base sheet. The treated paper is dried for 5 minutes at 70 ⁰ C in a flat bed drier. The dried paper is allowed to condition, and then measured for CIE whiteness on a calibrated Auto Elrepho spectrophotometer. The results are shown in Table 3.

EXAMPLE 4

Sizing compositions are prepared by adding an optical brightener of formula (3) in such an amount, that a range of final concentrations of from 0 to 12.5 g/l of optical brightener is achieved, to stirred, aqueous solutions of magnesium thiosulphate hexahydrate (final concentrations are 10 and 20g/l) and an anionic oxidized corn starch (final concentration 50 g/l) (Penford Starch 260) at 60 ⁰ C. The sizing solution is allowed to cool, then poured between the moving rollers of a laboratory size-press and applied to a commercial 75 g/m ² AKD (alkyl ketene dimer) sized, bleached paper base sheet. The treated paper is dried for 5 minutes at 70°C in a flat bed drier. The dried paper is allowed to condition, and then measured for CIE whiteness on a calibrated Auto Elrepho spectrophotometer. The results are shown in Table 3.

TABLE 3

The results clearly demonstrate the advantage of using magnesium chloride or magnesium thiosulphate to reach higher whiteness levels in comparison to where the optical brightener is present only as the sodium salt.