DISPERSION AGENT COMPOSITION, ITS USE AND COATING COMPOSITION COMPRISING THE SAME AND METHOD FOR IMPROVING VISCOSITY PROPERTIES OF A COATING COMPOSITION OR SLURRY The invention relates to a dispersion agent composition and its use according to the preambles of the enclosed claims. The invention relates also to a coating composition comprising the dispersion agent composition and a method for improving viscosity properties of a coating composition or slurry. Papermaking industry uses aqueous paste compositions or slurries, which comprise pigment particles for coating of paper or board in order to improve the appearance and/or properties of the finished product. Calcium carbonate and kaolin are traditional mineral pigments that are used in coating compositions. In addition to these pigments, there is an increasing interest to use calcium sulphate in the coating of paper and board, either alone or together with other pigments. Calcium sulphate has plurality of advantageous properties, for example it improves the opacity, shine, water resistance, printing properties and whiteness of the produced paper or board. However, it is known that when different mineral pigment particles are mixed, there sometimes occurs an extremely rapid and considerable increase in the viscosity of the coating composition. The viscosity increase may even reach the solidification point of the coating composition, whereby a "viscosity shock" takes place. This phenomenon is especially pronounced if the coating composition comprises calcium sulphate as a pigment, as calcium in calcium sulphate is more readily dissolved into the aqueous phase of the composition. This "viscosity shock" has prevented or reduced the use of calcium sulphate in coating compositions, as the risk for composition solidifying during the coating process in the mill pipelines, to the screens or in the coating colour mixer has been too great. The risk for coating composition solidifying has also inh ibited rapid changes between coating compositions comprising different pigment constituents. Change from one coating composition to a second composition comprising calcium sulphate should nowadays be preceded by careful cleaning and washing of pipelines, screens and all coating equipment coming into contact with the coating composition. All this takes time and causes losses in production capacity.

Document US 5,109,090 discloses copolymers, which are useful as compatibility, dispersion and grinding agents for aqueous suspensions comprising a mixture of mineral pigments, at least one of which pigments is hydrated calcium sulphate. However, it has been noticed that when the concentration of calcium sulphate in the coating composition increases these copolymers do not necessarily function in satisfactory manner and the viscosity of the slurry is still increasing, even if no solidifying of the composition is taking place.

Document EP 0353 817 discloses a method for controlling the formation of scale deposits in aqueous systems at high pH and/or calcite saturation levels by adding an effective amount of an allyl sulphonate/maleic acid anhydride polymer.

An object of this invention is to minimise or even eliminate the disadvantages or problems existing in the prior art.

An object is this invention also to provide a dispersion agent composition, with which it is possible to prepare and use coating compositions comprising calcium sulphate in papermaking without risk of sudden viscosity increase.

A further object of this invention is to obtain a dispersion agent composition, which provides an effective and economically interesting alternative for prior art dispersion agents.

These objects are attained with a method and an arrangement having the characteristics presented below in the characterising parts of the independent claims.

Typical dispersion agent composition according to the present invention for dispersing pigment pastes, coating compositions and/or slurries comprising calcium sulphate and, optionally a second mineral material, and used in coating of paper or board, comprises

- a first component, which comprises a polycarboxylate based first dispersion agent constituent, and

- a second component, which comprises a water-soluble copolymer of monomers, which comprise sulphonic acid and a carboxylic acid groups, or a derivative of the said copolymer.

Typically the dispersion agent composition according to the present invention is used for improving viscosity properties of coating compositions and/or slurries comprising calcium sulphate dihydrate and used in coating of paper or board.

Typical coating composition according to the present invention for coating of paper or board comprises

- a first pigment which is calcium sulphate and a second different pigment,

- binder, and optionally known additives used in coating compositions, and a dispersion agent composition according to the invention.

Now it has been surprisingly found out that the dispersion effect for paper coating compositions comprising calcium sulphate is significantly improved when using a dispersion agent composition that comprises both a first component that has a good dispersion effect as such, and a second component, which comprises a copolymer of a first monomer comprising sulphonic acid group(s) and a second monomer comprising carboxylic acid group(s). When dispersion agent composition according to present invention is used, the viscosity of coating compositions comprising calcium sulphate and a second conventional pigment mineral, such as calcium carbonate, may be even lower than viscosity of conventional coating compositions, without calcium sulphate. Thus the dispersion agent composition according to the present invention is especially suitable for coating compositions and/or pigment slurries that comprise calcium sulphate dihydrate and at least one second pigment material, such as calcium carbonate or kaolin, which is different from calcium sulphate dihydrate. It has been noticed that the first and second component of the dispersion agent composition according to the present invention have an unexpected synergetic effect, clearly exceeding the expectations based on the properties of the individual components.

According to one embodiment of the invention the second component comprises a copolymer of a monomer comprising allyl sulphonic acid group or a monomer comprising styrene sulphonic acid group. The allyl sulphonic acid and styrene sulphonic acid groups are preferably in sodium salt form. Further, the second component may comprise a copolymer of unsaturated dibasic C4 - C8 carboxylic acid . Preferably the second component comprises a copolymer of unsaturated dibasic C4 - C5 carboxylic acid, preferably maleic acid, itaconic acid and/or fumaric acid. According to one preferred embodiment the second component is a copolymer of a first monomer comprising allyl sulphonic acid group(s) and a second monomer comprising maleic acid group(s), most typically a copolymer of sodium allyl sulphonate and maleic acid. The second component may also be a mixture of different copolymers comprising allyl sulphonic acid groups or styrene sulphonic acid groups.

According to one preferred embodiment of the invention the second component, which comprises a water-soluble copolymer of a first monomer comprising a sulphonic acid group and a second monomer comprising a carboxylic acid group, or a derivative of the said copolymer, is typically used for improving viscosity properties of coating compositions and/or slurries comprising calcium sulphate, preferably in mixture with other pigment mineral material, and used in coating of paper or board. This means that the second component may be added separately from the first component to the paper coating composition or slurry in order to improve its viscosity properties.

The first component may comprise any polycarboxylate based first dispersion agent constituent having a comb-like structure comprising of a backbone having pendant side chains, resembling the teeth of a comb. Preferably, the first dispersion agent constituent is a copolymer comprising an ethylene monomer having a phosphate or phosphonate functionality, an ethylene monomer comprising a carboxylic acid group and an ethylene monomer of the ester or substituted type. The ethylene monomer comprising a carboxylic acid group comprises acrylic acid, methacrylic acid, itaconic acid, and maleic acid. The ethylene monomer of the ester or substituted type includes esters of acrylic and methacrylic acid, such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, and butyl methacrylate. Suitable polycarboxylate based first dispersion agent constituents are disclosed in US 5,109,090, which is included by reference.

According to one embodiment of the invention the first component comprises also a second dispersion agent constituent, which is a polyacrylate based dispersion agent. The second dispersion agent constituent may be based on straight-chain polyacrylate, polymethacrylate, polyacrylate copolymer, such as a methacrylic and acrylic acid copolymer, or methacrylic and acryl ic acid copolymer, or their mixtures. Such dispersion agent constituents are known as such for a person skilled in the art. Usually the second dispersion agent constituent is in form of sodium salt. The first component may thus be a mixture of a first and a second dispersion agent constituents, and optionally third or further dispersion agent constituents. This means that the first component may be a mixture of plurality of dispersion agent constituents, whereby at least the mixture preferably comprises a polycarboxylate based dispersion agent constituent and a polyacrylate based dispersion agent constituent.

The dispersion agent composition dosage is 0.1 - 1 .5 pph, preferably 0.125 - 1 pph, more preferably 0.2 - 0.7 pph. According to one embodiment of the invention the total amount of the second component in the final slurry is in the range 0.01 - 0.15 pph, more typically in the range of 0.05 - 0.1 pph. Thus it is possible to obtain an improved d ispersion effect and low viscosity values for paper coating compositions by using relatively small dispersion agent composition dosage. According to one embodiment of the invention the weight proportion of the first component to the second component in the dispersion agent composition is 0.1 - 0.7, preferably 0.15 - 0.4. The dispersion agent composition is typically a liquid having dry solids content of 20 - 50 %, more typically 30 - 45 %, and Brookfield viscosity less than 500 mPas at 25°C. The dispersion agent composition may comprise, in addition to the first and second component, also additives, such as stabilising agents. The dispersion agent composition has normally a pH value in the range of 5 - 1 1 , typically 6 - 9, more typically 7 - 8.

The amount of the first component in the composition is 0.1 - 1 .0 part, preferably 0.2 - 0.7 part, most preferably 0.3 - 0.6 part. The amount of the second component in the composition is 0.05 - 0.2 part, preferably 0.07 - 0.15, most preferably 0.08 - 0.1 2 part, calculated to the total amou nt of pig ment as conventional in the art.

According to one preferred embodiment of the present invention the coating composition or slurry comprises, in addition to calcium sulphate, at least one second different pigment. Typical second pigments that may be used for preparation of coating composition and/or pigment slurry are calcium carbonate, kaolin, calcinated kaolin, talc, titanium dioxide, chalk, satine white, barium sulphate, sodium aluminium silicate, aluminium hydroxide or any of their mixture. Calcium carbonate may be ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC) or their mixture. Preferably the pigment is calcium carbonate or kaolin.

The dispersion agent composition is suitable to be used as a dispersion agent for pigment slurries which are to be used in preparation of coating compositions. In this context, pigment slurry means a semi-liquid composition comprising fine particles of one or several pigments having particle size D ₅₀ in the range of 2 - 5 μιτι. Brookfield 100 viscosity of a pigment slurry is typically < 500 mPas. A coating composition in this context is used for semi-liquid compositions comprising in addition to particles of one or several pigments also binding agents, such as latex and/or starch. Brookfield 100 viscosity of a coating composition is typically < 2500, typically 1000 - 2000 mPas.

According to one advantageous embodiment the dispersion agent composition according to the present invention is suitable for dispersing aqueous coating compositions or slurries comprising calcium sulphate dihydrate, which coating compositions or slurries are obtainable by mixing to aqueous liquid phase calcium sulphate particles having a particle size D ₅₀ which is 0.1 μιτι < D ₅₀ < 5.0 μιτι, more preferably 0.1 μιτι < D ₅ o < 4.0 μιτι, still more preferably 0.5 μιτι < D ₅ o < 2.0 μιτι, sometimes 0.5 μιτι < D ₅₀ < 1 .0 μιτι. This means that preferably over 80 % of the original calcium sulphate particles have a particle size which is less than 2 μιτι. Preferably, the width of the particle size distribution WPSD of the used calcium sulphate dihydrate is below 2.5, more preferably below 2.0, still more preferably below 1 .5. The width of the particle size distribution is given as WPDS = (D ₇₅ - D ₂ 5)/D ₅ o, and it describes the homogeneity of the particle size distribution. Still more advantageously compositions or slurries comprise in addition of calcium sulphate dihydrate one or several other pigment minerals as mentioned above.

In this application the amounts are given, if not otherwise stated, as conventional in the art, by giving the total amount of pigments value 100, and calculating the amounts of other components relative to the amount of the total pigment, in weight-%. Proportions of all components are given as dry parts.

The present invention relates also to a method of improving viscosity properties of coating compositions and/or slurries comprising calcium sulphate and used in coating of paper or board by adding to the coating composition a first component, which comprises a polycarboxylate based first dispersion agent constituent, and a second component, which comprises a water-soluble copolymer of monomers, which comprise sulphonic acid and a carboxylic acid groups or a derivative of the copolymer. The first and second component may be added to the aqueous coating composition or the pigment slurry separately from each other, either simultaneously or after each other. After the addition of the first and second component the composition or pigment slurry is carefully mixed. The first and second component can be mixed with each other before the addition to the coating composition, whereby a dispersion agent composition is obtained. This dispersion agent composition is then added to the coating composition or pigment slurry. EXPERIMENTAL

Experiment is carried out in laboratory scale. A reference composition is prepared according to table 1 , the reference composition comprising ground calcium carbonate and kaolin clay as pigment. A coating colour composition comprising ground calcium carbonate and calcium sulphate as pigment is prepared according to table 2. Both compositions are prepared using Diaf laboratory dissolver. Compositions are mixed 10 minutes at ambient temperature using mixing speed 3000 rpm.

Table 1 : Reference composition comprising ground calcium carbonate and kaolin clay.

Coating Trade name Dry Part "Dry" "Real" composition Content [pph] Dose Dose component [%] [g] [g]

Ground Calcium HC 90 77.9 70 2800 3594.4

Carbonate

(GCC)

Kaolin HG 90 73.2 30 1200 1639.3

CMC Finnfix 10 9.7 0.5 20 206.2

Whitening agent Blankophor® P FL 100 0.5 20 20.0

01

Styrene DL 966 50 1 1 440 880.0 butadiene latex

Synthetic Sterocoll FS 40 0.1 4 10.0 Thickener

Total 1 12.1 4484 6349.9

Water content 1865.9

(g)

Added water (g) 548.58 Table 2. Coating composition comprising ground calcium carbonate and calcium sulphate.

Both the reference composition comprising GCC/kaolin and the composition comprising GCC/CaSO4 are tested for various properties. The measured results are shown in Table 3. Viscosities are measured by using Brookfield viscometer, type DV-II, with speeds 100 and 50 rpm and using spindle 3. The sample size is 500 g of dry pigment, solid content of the slurry ca. 65 %. High shear viscosity is measured using Hercules Hi-Shear DV-10 rotational viscometer. Table 3. Measurement results for the composition of table 1 comprising GCC/kaolin and for the composition of table 2 comprising GCC/CaSO ₄ .

EXPERIMENT 1

Coating composition comprising calcium sulphate according to Table 2 is added in four different amounts to the reference composition according to Table 1 . After the addition of the composition comprising calcium sulphate the viscosity of the samples are measured by using Brookfield viscometer, type DV-II, with speed 100 rpm, spindle 3. Viscosity values for different samples are shown in Table 4. Viscosity values for samples comprising different amounts of calcium

It can be clearly seen from Table 4 that the addition of coating composition comprising calcium sulphate produces significant increase in viscosity of the final coating composition mixture. In practice samples 2 to 4 are unusable for coating of paper due to their viscosity values.

EXPERIMENT 2

A prior art dispersion agent comprising a polycarboxylate based dispersion agent is added at two levels, 0.3 and 0.5 part, to the reference composition according to Table 1 . After dispersion agent addition coating composition comprising calcium sulphate according to Table 2 is added in four different levels. After this the viscosity of the samples are measured by using Brookfield viscometer, type DV-II, with speed 100 rpm, spindle 3. Viscosity values for different samples are shown in Table 5. Table 5. Viscosity values for samples comprising prior art dispersion agent and different amounts of calcium sulphate.

It can be seen from Table 5 that the viscosities for the samples are lower than for the samples without dispersion agent addition. It is also possible to increase the amount of composition comprising calcium sulphate up to 10 pph. However, the measured viscosities are still high, probably invoking problems in real life coating situations.

EXPERIMENT 3

A dispersion agent composition according to the invention comprising both a first component comprising a polycarboxylate based first dispersion agent constituent, and a second component comprising a water-soluble copolymer of monomers, which comprise sulphonic acid and a carboxylic acid groups is added to the reference composition according to Table 1 . The first component is Fennodispo C 33 (from Kemira, Finland), comprising sodium salts of polycarboxylic acids, having a pH 7.5 - 8.5, density around 1 .27 kg/m ³ , being n liquid form and water soluble. The second component is Cyanmer P-80 (from Kemira, Finland), having a pH <1 , density around 1 .31 - 1 .34 kg/m ³ , being n l iquid form and water soluble. The dispersion agent composition A comprises 0.3 parts of the first dispersion agent component and 0.1 parts of the second dispersion agent component, whereby the total amount of the dispersion agent composition added is 0.4 parts. The dispersion agent composition B comprises 0.5 parts of the first dispersion agent component and 0.1 parts of the second dispersion agent component, whereby the total amount of the dispersion agent composition added is 0.6 parts. In both dispersion agent compositions A and B the pH of the solution was adjusted to pH 7 with KOH after addition of the second dispersion agent component.

After the addition of the dispersion agent composition, the coating composition comprising calcium sulphate according to Table 2 is added in three different levels. After this the viscosity of the samples are measured by using Brookfield viscometer, type DV-II, with speed 100 rpm, spindle 3. Viscosity values for different samples are shown in Table 6.

Table 6. Viscosity values for samples comprising dispersion agent composition according to the invention and different amounts of calcium sulphate.

It can be seen from Table 6 that the viscosities for the samples are dramatically lower than for the samples without dispersion agent addition, and clearly lower than for the samples with prior art dispersion agent. It can be further observed that by using the dispersion agent composition according to the present invention the viscosity values for coating compositions comprising calcium sulphate in mixture with other pigments are lower than for coating compositions comprising ground calcium carbonate and kaolin clay. Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.