EP4646464A1

EP4646464A1 - Polyvinyl alcohol as a co-dispersing agent for mineral dispersions

Info

Publication number: EP4646464A1
Application number: EP24700516.8A
Authority: EP
Inventors: Louise MCCULLOCH; Fabio Ippolito; Antti FINÉR; Jonas Hannus
Original assignee: Omya International AG
Current assignee: Omya International AG
Priority date: 2023-01-05
Filing date: 2024-01-02
Publication date: 2025-11-12
Also published as: WO2024146883A1; CN120476178A; KR20250133392A

Abstract

The disclosure relates to a use of polyvinyl alcohol as co-dispersing agent for an aqueous suspension comprising a mineral material and a dispersing agent, wherein the mineral material is selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, and the dispersing agent is a polyacrylate-containing dispersant. The disclosure further relates to methods for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent, and aqueous suspensions obtainable by said methods. The disclosure also relates to a dispersing agent composition comprising a dispersing agent in combination with a co-dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, and the co-dispersing agent is polyvinyl alcohol.

Description

Polyvinyl alcohol as a co-dispersing agent for mineral dispersions

The present invention relates to the use of polyvinyl alcohol as a co-dispersing agent for an aqueous suspension comprising a mineral material and dispersing agent, a method for producing such an aqueous suspension in combination with said co-dispersing agent, an aqueous suspension obtainable by said method, use of said aqueous suspension, and a dispersing agent composition comprising a dispersing agent in combination with polyvinyl alcohol as co-dispersing agent.

Mineral materials are among the main constituents in paints, plastic, paper or paper coating colours. Mineral materials such as calcium carbonate, may provide improvements, for example, in paper and painting quality and agriculture properties, notably relative to their optical properties.

For reasons of applicability, transport, storage and drying costs, it is especially useful to produce mineral materials in the form of high solids suspensions, i.e. suspensions where the minority in respect to total weight of the suspension is water. Such high solid suspensions usually require the addition of a dispersing agent or grinding aid in order to maintain the suspension stability, pumpability and/or to allow grinding of such suspensions.

However, the presence of dispersing agents may impair the material properties of the final products, for example, their optical properties may be deteriorated. Dispersing agents such as polyacrylates are also not biodegradable and it is therefore desirable to further reduce their use for environmental reasons. Moreover, dispersing agents may be a significant cost factor in the production of mineral dispersions.

EP0461635 A1 relates to a polyvinyl alcohol resin soluble in high solids aqueous paper coating compositions without external heating. Multifunctional poly(vinyl alcohol) binders for fine particle size calcium carbonate pigments are described in WO0134906 A1 .

Therefore, there is a continuous need in the art for alternative aqueous mineral dispersions which address the foregoing technical problems.

Accordingly, it is an object of the present invention to provide an aqueous mineral suspension which has a high solids content at a workable viscosity. Furthermore, it is desirable that the aqueous suspension contains a reduced amount of dispersing agent. The aqueous suspension may also have additional improved properties such as improved long term stability, improved wet-grindability, or improved optical properties.

It would also be desirable to provide an aqueous mineral suspension, which is suitable for paper production, especially paper coating applications, and may even improve the optical or mechanical properties of a paper substrate.

It is also an object of the present invention to provide an additive for an aqueous mineral suspension that can reduce the amount of dispersing agent, which is necessary to shift the viscosity of the mineral suspension to a workable range. Furthermore, it is desirable to provide an additive which can add additional functionalities to an aqueous mineral suspension such as, e.g., improved long term stability, improved wet-grindability, or improved optical properties.

Furthermore, it is an object of the present invention to provide an additive for an aqueous mineral suspension, which can be added during the production of said mineral suspension, for example, before grinding the mineral material or precipitating the mineral material or before or during the up-concentration of the mineral suspension to its final solids content .

The foregoing and other objects are solved by the subject-matter as defined herein in the independent claims.

According to one aspect, use of polyvinyl alcohol as co-dispersing agent for an aqueous suspension comprising a mineral material and a dispersing agent is provided, wherein the mineral material is selected from an alkaline earth mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, and the dispersing agent is a polyacrylate-containing dispersant.

According to another aspect, a method for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent is provided, comprising the steps of: i) providing a mineral material selected from an alkaline earth mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, ii) providing a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, iii) providing a co-dispersing agent, wherein the co-dispersing agent is a polyvinyl alcohol, iv) providing water, and v) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv).

According to still a further aspect, a method for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent is provided, wherein the mineral material is precipitated calcium carbonate, and the method comprises the following steps:

I) providing a calcium oxide-containing material,

II) providing a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant,

III) providing a co-dispersing agent, wherein the co-dispersing agent is a polyvinyl alcohol,

IV) preparing a milk of lime by mixing water, the calcium oxide containing material of step I) and the co-dispersing agent of step III),

V) carbonating the milk of lime obtained in step IV) to form an aqueous suspension of precipitated calcium carbonate, and

VI) mixing the aqueous suspension obtained in step V) with the dispersing agent of step II).

According to still a further aspect of the present invention, a dispersing agent composition comprising a dispersing agent in combination with a co-dispersing agent is provided, wherein the dispersing agent is a polyacrylate-containing dispersant, and the co-dispersing agent is polyvinyl alcohol.

According to still a further aspect of the present invention, an aqueous suspension obtainable by a method according to the present invention is provided.

According to still a further aspect of the present invention, use of an aqueous suspension according to the present invention in paper applications, packaging applications, polymer applications, or water treatment applications is provided.

Advantageous embodiments of the present invention are defined in the corresponding subclaims.

According to one embodiment the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%, preferably at least 85 mol-%, more preferably at least 90 mol-%, even more preferably at least 95 mol-%, and most preferably at least 98 mol-%, and/or the polyvinyl alcohol has an average molecular weight M_w from 10 000 to 150 000 g/mol, preferably from 12 000 to 100 000 g/mol, more preferably from 14 000 to 50 000 g/mol, and most preferably from 16 000 to 30 000 g/mol. According to a further embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 1 .8 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, even more preferably in an amount from 0.08 to 1 wt.-%, and most preferably in amount from 0.1 to 0.6 wt.-%.

According to one embodiment the dispersing agent is a polyacrylate homopolymer, a polyacrylate copolymer, or a mixture thereof, and preferably the dispersing agent is a polyacrylate homopolymer, preferably at least partially neutralized, preferably with sodium ions, potassium ions, lithium ions, ammonium ions, calcium ions, and mixtures thereof, more preferably with sodium ions, calcium ions, or mixtures thereof, and most preferably with sodium ions. According to a further embodiment the dispersing agent has a neutralization degree of at least 30 mol-%, preferably at least 40 mol-%, more preferably at least 60 mol-%, and most preferably at least 80 mol-%, and/or the dispersing agent has an average molecular weight M_w from 1000 to 15 000 g/mol, preferably from 2000 to 12 000 g/mol, more preferably from 3000 to 11 000 g/mol, and most preferably from 4000 to 10 000 g/mol. According to still a further embodiment the aqueous suspension comprises the dispersing agent in an amount of 3 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 2 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, and most preferably in an amount from 0.1 to 0.8 wt.-%.

According to one embodiment the mineral material is in the form of particles having a weight median particle size dso from 0.05 to 100 pm, preferably from 0.1 to 50 pm, more preferably from 0.15 to 25 pm, even more preferably from 0.25 to 10 pm, and most preferably from 0.5 to 2 pm, and/or having a weight determined top cut particle size dgs from 0.1 to 200 pm, preferably from 0.2 to 100 pm, more preferably from 0.3 to 50 pm, even more preferably from 0.25 to 20 pm, and most preferably from 1 to 4 pm. According to a further embodiment the aqueous suspension comprises the mineral material in an amount from 10 to 78 wt.-%, preferably from 20 to 75 wt.-%, and most preferably from 50 to 72 wt.-%, based on the total weight of the aqueous suspension.

According to one embodiment of the method for producing an aqueous suspension, the step v) comprises the steps of: a1) contacting the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv), and a2) adding the mineral material of step i) to the mixture obtained in step a1), or b1) contacting the dispersing agent of step ii), and the co-dispersing agent of step iii), and b2) adding the mineral material of step i) and water of step iv) to the mixture obtained in step b1), wherein the mineral material and the water are added separately or in combination, preferably in combination, or c1) contacting the dispersing agent of step ii), the mineral material of step i), and the water of step iv), and c2) adding the co-dispersing agent of step iii) to the mixture obtained in step c1), or d) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv) simultaneously. According to a further embodiment the method further comprises the step of grinding and/or dewatering the aqueous suspension during and/or after step v).

It should be understood that for the purpose of the present invention, the following terms have the following meaning:

A “calcium oxide-containing material” in the meaning of the present invention can be a mineral or a synthetic material having a content of calcium oxide of at least 50 wt.-%, preferably 75 wt.-%, more preferably 90 wt.-%, and most preferably 95 wt.-%, based on the total weight of the calcium oxide containing material.

The term “mineral” in the meaning of the present application encompasses natural or synthetic minerals, fillers and/or pigments, like calcium carbonate, chalk, dolomite, titanium dioxide, etc.

“Natural ground calcium carbonate” (GCC) in the meaning of the present invention is a calcium carbonate obtained from natural sources, such as limestone, marble, or chalk, and processed through a wet and/or dry treatment such as grinding, screening and/or fractionating, for example, by a cyclone or classifier.

“Precipitated calcium carbonate” (PCC) in the meaning of the present invention is a synthesised material, obtained by precipitation following reaction of carbon dioxide and lime in an aqueous, semi-dry or humid environment or by precipitation of a calcium and carbonate ion source in water. PCC may be in the vateritic, calcitic or aragonitic crystal form. PCCs are described, for example, in EP2447213 A1 , EP2524898 A1 , EP2371766 A1 , EP1712597 A1 , EP1712523 A1 , or WO2013142473 A1.

The “particle size” of the mineral herein is described by its weight-based distribution of particle sizes d_x. Therein, the value d_x represents the diameter relative to which x % by weight of the particles have diameters less than d_x. This means that, for example, the d2o value is the particle size at which 20 wt.% of all particles are smaller than that particle size. The dso value is thus the weight median particle size, i.e. 50 wt.-% of all particles are smaller than this particle size. For the purpose of the present invention, the particle size is specified as weight median particle size dso(wt) unless indicated otherwise. Particle sizes were determined by using a Sedigraph™ 5100 instrument or Sedigraph™ 5120 instrument of Micromeritics Instrument Corporation. The method and the instrument are known to the skilled person and are commonly used to determine the particle size of fillers and pigments. The measurements were carried out in an aqueous solution of 0.1 wt.-% Na4P2O?.

As used herein the term “polymer” generally includes homopolymers and copolymers such as, for example, block, graft, random and alternating copolymers, as well as blends and modifications thereof. The polymer can be an amorphous polymer, a crystalline polymer, or a semi-crystalline polymer, i.e. a polymer comprising crystalline and amorphous fractions. The degree of crystallinity is specified in percent and can be determined by differential scanning calorimetry (DSC). An amorphous polymer may be characterized by its glass transition temperature and a crystalline polymer may be characterized by its melting point. A semi-crystalline polymer may be characterized by its glass transition temperature and/or its melting point. The term “copolymer” as used herein refers to a polymer derived from more than one species of monomer. Copolymers that are obtained by copolymerization of two monomer species may also be termed bipolymers, those obtained from three monomers terpolymers, those obtained from four monomers quaterpolymers, etc. (cf. IUPAC Compendium of Chemical Terminology 2014, “copolymer”). Accordingly, the term “homopolymer” refers to a polymer derived from one species of monomer.

Unless indicated otherwise, the “moisture content” of a material refers to the percentage of moisture (i.e. water) which may be desorbed from a sample upon heating to 220°C. The moisture content may be measured according to the Karl Fischer coulometric titration method, desorbing the moisture in an oven at 220 °C for at least 10 min and passing it continuously into a Karl Fischer coulometer (Mettler Toledo coulometric KF Titrator C30, combined with Mettler Toledo oven DO 0337) using dry nitrogen at 100 ml/min for at least 10 min. A calibration curve using water may be recorded and a blank of 10 min nitrogen flow without a sample taken into account.

The “specific surface area” (expressed in m²/g) of a material as used throughout the present document can be determined by the Brunauer Emmett Teller (BET) method with nitrogen as adsorbing gas and by use of an ASAP 2460 instrument from Micromeritics. The method is well known to the skilled person and defined in ISO 9277:2010. Samples are conditioned at 100°C under vacuum for a period of 30 min prior to measurement. The total surface area (in m²) of said material can be obtained by multiplication of the specific surface area (in m²/g) and the mass (in g) of the material.

A “solution” as referred to herein is understood to be a single phase mixture of a specific solvent and a specific solute, for example a single phase mixture of a water-soluble salt and water. The term “dissolved” as used herein thus refers to the physical state of a solute in a solution.

A “suspension” or “slurry” in the meaning of the present invention comprises undissolved solids and water, and optionally further additives, and usually contains large amounts of solids and, thus, is more viscous and can be of higher density than the liquid from which it is formed.

The term “aqueous” suspension refers to a system, wherein the liquid phase comprises, preferably consists of, water. However, said term does not exclude that the liquid phase of the aqueous suspension comprises minor amounts of at least one water-miscible organic solvent selected from the group comprising methanol, ethanol, acetone, acetonitrile, tetrahydrofuran and mixtures thereof. If the aqueous suspension comprises at least one water-miscible organic solvent, the liquid phase of the aqueous suspension comprises the at least one water-miscible organic solvent in an amount of from 0.1 to 40.0 wt.-% preferably from 0.1 to 30.0 wt.-%, more preferably from 0.1 to 20.0 wt.-% and most preferably from 0.1 to 10.0 wt.-%, based on the total weight of the liquid phase of the aqueous suspension. For example, the liquid phase of the aqueous suspension consists of water.

For the purpose of the present invention, the “solids content” of a liquid composition is a measure of the amount of material remaining after all the solvent or water has been evaporated. If necessary, the “solids content” of a suspension given in wt.-% in the meaning of the present invention can be determined using a Moisture Analyzer HR73 from Mettler-Toledo (T = 160 °C, automatic switch off 3, standard drying) with a sample size of 2 to 10 g.

For the purpose of the present invention, the term “viscosity” or “Brookfield viscosity” refers to Brookfield viscosity. The Brookfield viscosity is for this purpose measured by a Brookfield DV-II+ Pro viscometer at 25 °C ± 1 °C at 100 rpm using an appropriate spindle of the Brookfield RV-spindle set and is specified in mPa s. Based on his technical knowledge, the skilled person will select a spindle from the Brookfield RV-spindle set which is suitable for the viscosity range to be measured. For example, for a viscosity range between 200 and 800 mPa s the spindle number 3 may be used, for a viscosity range between 400 and 1 600 mPa s the spindle number 4 may be used, for a viscosity range between 800 and 3200 mPa s the spindle number 5 may be used, for a viscosity range between 1 000 and 2 000 000 mPa s the spindle number s may be used, and for a viscosity range between 4 000 and 8 000 000 mPa s the spindle number 7 may be used.

Where an indefinite or definite article is used when referring to a singular noun, e.g., “a”, “an” or “the”, this includes a plural of that noun unless anything else is specifically stated.

Where the term “comprising” is used in the present description and claims, it does not exclude other elements. For the purposes of the present invention, the term “consisting of’ is considered to be a preferred embodiment of the term “comprising”. If hereinafter a group is defined to comprise at least a certain number of embodiments, this is also to be understood to disclose a group, which preferably consists only of these embodiments.

Terms like “obtainable” or “definable” and “obtained” or “defined” are used interchangeably. This, for example, means that, unless the context clearly dictates otherwise, the term “obtained” does not mean to indicate that, for example, an embodiment must be obtained by, for example, the sequence of steps following the term “obtained” though such a limited understanding is always included by the terms “obtained” or “defined” as a preferred embodiment.

Whenever the terms “including” or “having” are used, these terms are meant to be equivalent to “comprising” as defined herein above.

According to the present invention, use of polyvinyl alcohol as co-dispersing agent for an aqueous suspension comprising a mineral material and a dispersing agent is provided. The mineral material is selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, and the dispersing agent is a polyacrylate-containing dispersant.

In the following details and preferred embodiments of the inventive use will be set out in more detail. It is to be understood that these technical details and embodiments also apply to the inventive methods, the inventive dispersing agent composition, the inventive aqueous dispersion, and the inventive use of said dispersion.

Mineral material

According to the present invention, the aqueous suspension comprises a mineral material, wherein the mineral material is selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof.

According to one embodiment, the mineral material is in the form of particles having a weight median particle size cko from 0.05 to 100 pm, preferably from 0.1 to 50 pm, more preferably from 0.15 to 25 pm, even more preferably from 0.25 to 10 pm, and most preferably from 0.5 to 2 pm. The weight determined median particle size cko may be evaluated using a Sedigraph™ 5100 instrument or Sedigraph™ 5120 instrument of Micromeritics Instrument Corporation.

Additionally or alternatively, the mineral filler is in the form of particles having a weight determined top cut particle size dgs from 0.1 to 200 pm, preferably from 0.2 to 100 pm, more preferably from 0.3 to 50 pm, even more preferably from 0.25 to 20 pm, and most preferably from 1 to 4 pm. The volume determined top cut particle size dgs may be evaluated using a a Sedigraph™ 5100 instrument or Sedigraph™ 5120 instrument of Micromeritics Instrument Corporation.

Additionally or alternatively, the mineral filler is in the form of particles having a specific surface area in the range from 1 to 100 m²/g, preferably from 5 to 90 m²/g, more preferably from 10 to 80 m²/g, even more preferably from 15 to 70 m²/g, and most preferably from 17 to 60 m²/g, measured using nitrogen and the BET method according to ISO 9277:2010.

According to one embodiment the aqueous suspension comprises the mineral material in an amount from 10 to 78 wt.-%, preferably from 20 to 75 wt.-%, and most preferably from 50 to 72 wt.-%, based on the total weight of the aqueous suspension.

According to one embodiment the mineral material is selected from an alkaline earth metal mineral material.

The alkaline earth metal mineral material may be selected from the group consisting of alkaline earth metal carbonates, alkaline earth metal phosphates, alkaline earth metal sulphates, alkaline earth metal oxides, alkaline earth metal hydroxides, and mixtures thereof. According to a preferred embodiment, the alkaline earth metal mineral material is an alkaline earth metal carbonate, an alkaline earth metal hydroxide, or a mixture thereof.

According to one embodiment, the alkaline earth metal mineral material is selected from the group consisting of calcium carbonate, magnesium carbonate, calcium magnesium carbonate, calcium hydroxide, magnesium hydroxide, and mixtures thereof, preferably the alkaline earth metal mineral material is selected from the group consisting of ground calcium carbonate, precipitated calcium carbonate, dolomite, hydromagnesite, calcium hydroxide, magnesium hydroxide, and mixtures thereof.

Dolomite is a carbonatic calcium-magnesium-mineral having the chemical composition of CaMg(CC>3)2 (“CaCOs ■ MgCOs”). Dolomite minerals may contain at least 30.0 wt.-% MgCOs, based on the total weight of dolomite, preferably more than 35.0 wt.-%, more preferably more than 40.0 wt.-% MgCOs.

Clay refers to crystalline small particles of mainly hydrous silicates of aluminium, sometimes with magnesium and/or iron substitution for all or a part of the aluminium. The main groups of clay minerals are: kaolinite (the main constituent of kaolin), halloysite, illite, montmorillonite, and vermiculite. Calcined clay may be obtained by a heating a source of kaolin to between 650 and 750°C.

Titanium dioxide can have an anatase, rutile, or amorphous structure. Titanium dioxide particles are usually prepared in the chloride process, in which TiCU is oxidized to TiC>2 particles, or in the sulfate process, in which sulfuric acid and ore containing titanium are dissolved, and the resulting solution goes through a series of steps to yield the at least one titanium dioxide-containing particles. The titanium dioxide can be essentially pure titanium dioxide or may contain other metal oxides, such as silica, alumina, zirconia and the like, preferably silica. Such other metal oxides may be incorporated into the titanium dioxide particles by co-oxidizing or co-precipitating titanium compounds with other metal compounds during their preparation in the chloride or sulfate process. Calcium carbonate

According to one embodiment, the alkaline earth metal mineral material is calcium carbonate, optionally in combination with magnesium carbonate, calcium magnesium carbonate, calcium hydroxide, magnesium hydroxide, or a mixture thereof.

According to one embodiment, the calcium carbonate is selected from the group consisting of ground calcium carbonate, precipitated calcium carbonate, or mixtures thereof. According to a preferred embodiment the calcium carbonate is ground calcium carbonate.

Ground (or natural ground) calcium carbonate (GCC) is understood to be manufactured from a naturally occurring form of calcium carbonate, mined from sedimentary rocks such as limestone or chalk, or from metamorphic marble rocks, eggshells or seashells. Calcium carbonate is known to exist as three types of crystal polymorphs: calcite, aragonite and vaterite. Calcite, the most common crystal polymorph, is considered to be the most stable crystal form of calcium carbonate. Less common is aragonite, which has a discrete or clustered needle orthorhombic crystal structure. Vaterite is the rarest calcium carbonate polymorph and is generally unstable. Ground calcium carbonate is almost exclusively of the calcitic polymorph, which is said to be trigonal-rhombohedral. The term “source” of the calcium carbonate in the meaning of the present application refers to the naturally occurring mineral material from which the calcium carbonate is obtained. According to one embodiment of the present invention, the ground calcium carbonate is selected from the group consisting of marble, chalk, limestone and mixtures thereof. The source of the calcium carbonate may comprise further naturally occurring components such as magnesium carbonate, alumino silicate etc.

According to one embodiment of the present invention the GCC is obtained by dry grinding. According to another embodiment of the present invention the GCC is obtained by wet grinding and optionally subsequent drying.

In general, the grinding step can be carried out with any conventional grinding device, for example, under conditions such that comminution predominantly results from impacts with a secondary body, i.e. in one or more of: a ball mill, a rod mill, a vibrating mill, a roll crusher, a centrifugal impact mill, a vertical bead mill, an attrition mill, a pin mill, a hammer mill, a pulveriser, a shredder, a de-clumper, a knife cutter, or other such equipment known to the skilled man. In case the calcium carbonate comprising mineral material comprises a wet ground calcium carbonate comprising mineral material, the grinding step may be performed under conditions such that autogenous grinding takes place and/or by horizontal ball milling, and/or other such processes known to the skilled man. The wet processed ground calcium carbonate comprising mineral material thus obtained may be washed and dewatered by well-known processes, e.g. by flocculation, centrifugation, filtration or forced evaporation prior to drying. The subsequent step of drying may be carried out in a single step such as spray drying, or in at least two steps. It is also common that such a mineral material undergoes a beneficiation step (such as a flotation, bleaching or magnetic separation step) to remove impurities.

According to one embodiment of the present invention, the calcium carbonate comprises one type of ground calcium carbonate. According to another embodiment of the present invention, the calcium carbonate comprises a mixture of two or more types of ground calcium carbonates selected from different sources. “Precipitated calcium carbonate” (PCC) in the meaning of the present invention is a synthesized material, generally obtained by precipitation following reaction of carbon dioxide and calcium hydroxide in an aqueous environment or by precipitation of calcium and carbonate ions, for example CaCh and Na2COs, out of solution. Further possible ways of producing PCC are the lime soda process, or the Solvay process in which PCC is a by-product of ammonia production. Precipitated calcium carbonate exists in three primary crystalline forms: calcite, aragonite and vaterite, and there are many different polymorphs (crystal habits) for each of these crystalline forms. Calcite has a trigonal structure with typical crystal habits such as scalenohedral (S-PCC), rhombohedral (R- PCC), hexagonal prismatic, pinacoidal, colloidal (C-PCC), cubic, and prismatic (P-PCC). Aragonite is an orthorhombic structure with typical crystal habits of twinned hexagonal prismatic crystals, as well as a diverse assortment of thin elongated prismatic, curved bladed, steep pyramidal, chisel shaped crystals, branching tree, and coral or worm-like form. Vaterite belongs to the hexagonal crystal system. The obtained PCC slurry can be mechanically dewatered and dried.

According to one embodiment of the present invention, the precipitated calcium carbonate is precipitated calcium carbonate, preferably comprising aragonitic, vateritic or calcitic mineralogical crystal forms or mixtures thereof.

According to one embodiment of the present invention, the calcium carbonate comprises one precipitated calcium carbonate. According to another embodiment of the present invention, the calcium carbonate comprises a mixture of two or more precipitated calcium carbonates selected from different crystalline forms and different polymorphs of precipitated calcium carbonate. For example, the at least one precipitated calcium carbonate may comprise one PCC selected from S-PCC and one PCC selected from R-PCC.

Hydromagnesite

According to one embodiment, the alkaline earth metal mineral material is hydromagnesite, optionally in combination with calcium carbonate, magnesium carbonate, calcium magnesium carbonate, calcium hydroxide, magnesium hydroxide, or mixtures thereof.

Hydromagnesite or basic magnesium carbonate, which is the standard industrial name for hydromagnesite, is a naturally occurring mineral which is found in magnesium rich minerals such as serpentine and altered magnesium rich igneous rocks, but also as an alteration product of brucite in periclase marbles. Hydromagnesite is described as having the following formula Mgs(CO3)4(OH)2 ■ 4 H₂O.

It should be appreciated that hydromagnesite is a very specific mineral form of magnesium carbonate and occurs naturally as small needle-like crystals or crusts of acicular or bladed crystals. In addition thereto, it should be noted that hydromagnesite is a distinct and unique form of magnesium carbonate and is chemically, physically and structurally different from other forms of magnesium carbonate. Hydromagnesite can readily be distinguished from other magnesium carbonates by x-ray diffraction analysis, thermogravimetric analysis or elemental analysis. Unless specifically described as hydromagnesite, all other forms of magnesium carbonates (e.g. artinite (Mg2(CC>3)(OH)2 ■ 3 H2O), dypingite (Mgs(CO3)4(OH)2 ■ 5 H2O), giorgiosite (Mgs(CO3)4(OH)2 ■ 5 H2O), pokrovskite (Mg2(CC>3)(OH)2 ■ 0.5 H2O), magnesite (MgCOs), barringtonite (MgCOs ■ 2 H2O), lansfordite (MgCOs ■ 5 H2O) and nesquehonite (MgCOs ■ 3 H2O)) are not hydromagnesite within the meaning of the present invention and do not correspond chemically to the formula described above.

Besides the natural hydromagnesite, precipitated hydromagnesite (or synthetic magnesium carbonate) can be prepared. For instance, US1361324, US935418, GB548197 and GB544907 generally describe the formation of aqueous solutions of magnesium bicarbonate (typically described as “Mg(HCO3)2”), which is then transformed by the action of a base, e.g., magnesium hydroxide, to form hydromagnesite. Other processes described in the art suggest to prepare compositions containing both, hydromagnesite and magnesium hydroxide, wherein magnesium hydroxide is mixed with water to form a suspension which is further contacted with carbon dioxide and an aqueous basic solution to form the corresponding mixture; cf. for example US 5979461 .

It is appreciated that the hydromagnesite can be one type or a mixture of different types of hydromagnesite. In one embodiment of the present invention, the hydromagnesite comprises, preferably consists of, one type of hydromagnesite. Alternatively, the hydromagnesite comprises, preferably consists of, two or more types of hydromagnesites.

According to a preferred embodiment, the hydromagnesite is precipitated hydromagnesite.

Polyacrylate-containing dispersant

In addition to the mineral material, the aqueous suspension of the present invention comprises a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant.

The polyacrylate-containing dispersant may be a polyacrylate homopolymer, a polyacrylate copolymer, or a mixture thereof.

The polyacrylate homopolymer may be prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof. The polyacrylate copolymer may be prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof and at least one non-ionic compound selected from hydroxyethylacrylate, hydroxypropylacrylate, hydroxyethylmethacrylate, hydroxypropylmethacrylate, Ci - C5 esters of acrylic acid, C1-C5 esters of methacrylic acid, fumaric acid, itaconic acid, acrylamide, crotonic acid, maleic anhydride acid, isocrotonic acid, aconitic acid (cis or trans), mesaconic acid, sinapinic acid, undecylenic acid, angelic acid, canellic acid, hydroxyacrylic acid, acrolein, acrylonitrile, dimethylaminoethyl methacrylate, vinylpyrrolidone, vinylcaprolactam, ethylene, propylene, isobutylene, diisobutylene, vinyl acetate, styrene, a-methyl styrene, methyl vinyl ketone, and mixtures thereof.

The C1-C5 ester of acrylic acid and/or C1-C5 ester of methacrylic acid may be selected from alkyl esters of acrylic acid and/or alkyl esters of methacrylic acid. The alkyl esters may comprise further substituents, such as hydroxy groups and/or sulfonic acid groups. For example, the C1-C5 ester of acrylic acid is selected from alkyl esters of acrylic acid.

According to one embodiment, the C1-C5 ester of acrylic acid is selected from the group consisting of methyl, ethyl, propyl, hydroxypropyl, butyl, and pentyl acrylate, preferably the C1-C5 ester of acrylic acid is selected from the group comprising methyl, ethyl, propyl, and hydroxypropyl acrylate, and most preferably the ester of acrylic acid is hydroxypropyl acrylate. Additionally or alternatively, the C1-C5 ester of methacrylic acid may be selected from alkyl esters of methacrylic acid. According to one embodiment, the C1-C5 ester of methacrylic acid is selected from the group consisting of methyl, ethyl, propyl, hydroxypropyl, butyl, and pentyl methacrylate, preferably the C1-C5 ester of methacrylic acid is selected from the group comprising methyl, ethyl, and hydroxypropyl methacrylate, and most preferably, the ester of methacrylic acid is hydroxypropyl methacrylate.

According to one embodiment, the polyacrylate homopolymer is prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof, and/or the polyacrylate copolymer is prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof and at least one non-ionic compound selected from the group consisting of hydroxyethylacrylate, hydroxypropylacrylate, hydroxyethylmethacrylate, hydroxypropylmethacrylate, C1-C5 esters of acrylic acid, C1-C5 esters of methacrylic acid, fumaric acid, itaconic acid, acrylamide, crotonic acid, maleic anhydride acid, isocrotonic acid, aconitic acid (cis or trans), mesaconic acid, sinapinic acid, undecylenic acid, angelic acid, canellic acid, hydroxyacrylic acid, acrolein, acrylonitrile, dimethylaminoethyl methacrylate, vinylpyrrolidone, vinylcaprolactam, ethylene, propylene, isobutylene, diisobutylene, vinyl acetate, styrene, a-methyl styrene, methyl vinyl ketone, and mixtures thereof.

The polyacrylate homopolymer may be one kind of polyacrylate homopolymer, or a mixture of two or more kinds of polyacrylate homopolymers. For example, the polyacrylate homopolymer may be a mixture of two or three kinds of polyacrylate homopolymers. Preferably, the polyacrylate homopolymer is one kind of polyacrylate homopolymer.

The polyacrylate copolymer may be one kind of polyacrylate copolymer, or a mixture of two or more kinds of polyacrylate copolymers. For example, the polyacrylate copolymer may be a mixture of two or three kinds of polyacrylate copolymers. Preferably, the polyacrylate copolymer is one kind of polyacrylate copolymer.

In one embodiment of the present invention, the polyacrylate-containing polymer comprises, preferably consists of, one kind of polyacrylate homopolymer or one kind of polyacrylate copolymer. Alternatively, the polyacrylate-containing polymer is a mixture comprising, preferably consisting of, one kind of polyacrylate homopolymer and one kind of polyacrylate copolymer.

The carboxylic acid functions of the polyacrylate-containing polymer may be at least partially neutralized. According to one embodiment the dispersing agent has a neutralization degree of at least 30 mol-%, preferably at least 40 mol-%, more preferably at least 60 mol-%, and most preferably at least 80 mol-%. For example, the carboxylic acid functions of the dispersing agent may be at least partially neutralized by any suitable monovalent cation such as sodium ions, potassium ions, lithium ions, ammonium ions, calcium ions, and mixtures thereof, preferably with sodium ions, calcium ions, or mixtures thereof, and most preferably with sodium ions. According to one embodiment the carboxylic acid functions of the polyacrylate-containing polymer are at least partially neutralized sodium ions, potassium ions, lithium ions, ammonium ions, calcium ions, and mixtures thereof, preferably in an amount of at least 30 mol-%, preferably at least 40 mol-%, more preferably at least 60 mol-%, and most preferably at least 80 mol-%.

Unless indicated otherwise, the term “partially neutralized” as used herein indicates that the polyacrylate-containing polymer still contains non-neutralized carboxylic acid functions, i.e. free carboxylic acid functions are present. In contrast thereto, the term “totally neutralized” indicates that any carboxylic acid function present in the polyacrylate-containing polymer is neutralized, i.e. no free carboxylic acid functions are present. The methods of partially or totally replacing protons of carboxylic acid functions by monovalent cations are well known as well as the methods to determine whether a polymer is partially or totally neutralized.

According to one embodiment the dispersing agent has an average molecular weight M_w from 1000 to 15 000 g/mol, preferably from 2000 to 12 000 g/mol, more preferably from 3000 to 11 000 g/mol, and most preferably from 4000 to 10 000 g/mol.

According to one embodiment, the dispersing agent is a polyacrylate homopolymer being at least partially neutralized with sodium ions. Preferably said dispersing agent has a neutralization degree of at least 30 mol-% and an average molecular weight Mwfrom 1000 to 15 000 g/mol. According to an exemplary embodiment, the dispersing agent is a polyacrylate homopolymer being fully neutralized with sodium ions and having an average molecular weight M_w from 4000 to 10 000 g/mol.

The dispersing agent may be provided in the form of a solution in water or in solid form, and preferably in the form of an aqueous suspension.

According to one embodiment the aqueous suspension comprises the dispersing agent in an amount of 3 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 2 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, and most preferably in an amount from 0.1 to 0.8 wt.-%.

Polyvinyl alcohol

According to one embodiment, polyvinyl alcohol is used as co-dispersing agent for an aqueous suspension comprising a mineral material and dispersing agent as defined above.

The polyvinyl alcohol may be prepared by any method known in the art. For example, polyvinyl alcohol may be manufactured by polymerization of vinyl acetate monomers, followed by hydrolysis of the polyvinyl acetate. Alternatively, commercially available polyvinyl alcohols may be used, e.g. BF-05, BF-04, BF-14, BF-17, BP-04, or BP-05 of Chang Chun Petrochemical Co., Ltd.

The polyvinyl alcohol may be partially or fully hydrolyzed. According to one embodiment, the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%, preferably at least 85 mol-%, more preferably at least 90 mol-%, even more preferably at least 95 mol-%, and most preferably at least

98 mol-%. According to another embodiment, the polyvinyl alcohol has a degree of hydrolysis from 80 to 100 mol-%, and more preferably from 85 to 99 mol-%, for example, from 85 to 90 mol-% or 98 to

99 mol-%.

According to one embodiment the polyvinyl alcohol has an average molecular weight Mwfrom 10 000 to 150 000 g/mol, preferably from 12 000 to 100 000 g/mol, more preferably from 14 000 to 50 000 g/mol, and most preferably from 16 000 to 30 000 g/mol.

The polyvinyl alcohol may be one kind of polyvinyl alcohol, or a mixture of two or more kinds of polyvinyl alcohols. For example, the polyvinyl alcohol may be a mixture of two or three kinds of polyvinyl alcohol. Preferably, the polyvinyl alcohol is one kind of polyvinyl alcohol.

The polyvinyl alcohol may be provided in the form of a solution in water or in solid form. According to one embodiment, the polyvinyl alcohol is provided in solid form. For example, the polyvinyl alcohol may be provided in the form of a powder, or in the form of particles, beads, or granules. According to one embodiment, the polyvinyl alcohol is provided in the form of a powder and/or in the form of granules. According to one embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 1 .8 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, even more preferably in an amount from 0.08 to 1 wt.-%, and most preferably in amount from 0.1 to 0.6 wt.-%.

According to one embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, and the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%. According to another embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, and the polyvinyl alcohol has an average molecular weight M_w from 10 000 to 150 000 g/mol. According to still another embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%, and the polyvinyl alcohol has an average molecular weight Mwfrom 10 000 to 150 000 g/mol. Preferably the polyvinyl alcohol has a degree of hydrolysis of at least 85 mol-%, more preferably at least 90 mol-%, even more preferably at least 95 mol-%, and most preferably at least 98 mol-%.

According to one embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount from 0.1 to 0.6 wt.-%, based on the total weight of the aqueous suspension, and the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%. According to another embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount from 0.1 to 0.6 wt.-%, based on the total weight of the aqueous suspension, and the polyvinyl alcohol has an average molecular weight M_w from 12 000 to 100 000 g/mol. According to still another embodiment the aqueous suspension comprises the polyvinyl alcohol in an amount from 0.1 to 0.6 wt.-%, based on the total weight of the aqueous suspension, the polyvinyl alcohol has a degree of hydrolysis of at least 75 mol-%, and the polyvinyl alcohol has an average molecular weight Mwfrom 12 000 to 100 000 g/mol. Preferably the polyvinyl alcohol has a degree of hydrolysis of at least 85 mol-%, more preferably at least 90 mol-%, even more preferably at least 95 mol-%, and most preferably at least 98 mol-%.

The inventors of the present invention surprisingly found that using polyvinyl alcohol as a codispersing agent in an aqueous mineral material suspension comprising a dispersing agent can further reduce the viscosity of the mineral material suspension. This provides the possibility to reduce the amount of dispersing agent in the mineral material suspension and/or to provide a mineral suspension having a higher solids content in a workable viscosity range. Since dispersing agents such as polyacrylates tend to affect the optical properties in a negative way, for example, if they are included in paper coating, the reduction of dispersing agents may also have a positive impact on the optical properties of such products. Moreover, it was found that the addition of polyvinyl alcohol can further improve the optical properties of the mineral material suspension and products containing the same, and may increase e.g. light scattering.

Furthermore, it was found that polyvinyl alcohol can already be added during the production process of an aqueous mineral material suspension, and thus, additional processing time for the addition can be avoided. It was also found that polyvinyl alcohol reduces the viscosity during grinding and thus, the addition of specific grinding agents can be omitted. Moreover, it was found that the longterm stability of aqueous mineral material suspensions can be significantly improved. According to a further aspect of the present invention a dispersing agent composition comprising a dispersing agent in combination with a co-dispersing agent is provided, wherein the dispersing agent is a polyacrylate-containing dispersant, and the co-dispersing agent is polyvinyl alcohol.

Additional components

The aqueous suspension of the present invention may comprise further additives.

According to one embodiment, the aqueous suspension comprises cationic homopolymers based on monomer units, such as diallyl dialkyl ammonium salts or polyethylene imines, alkali polyphosphates, carboxymethylcellulose, or mixtures thereof.

Furthermore, the aqueous suspension may comprise a preservative, a grinding agent, pH stabilizer, a pH adjusting agent, a biocide, or mixtures thereof.

Examples of suitable preservatives are quaternary ammonium salts, peroxides, perchlorates, tributyl tin, zinc, biocidal enzymes, biocidal polypeptides, sulphonamides and mixtures thereof.

According to one embodiment, the aqueous suspension only consists of a mineral material selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, and co-dispersing agent, wherein the co-dispersing agent is a polyvinyl alcohol.

Production methods

According to one aspect of the present invention, a method for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent is provided, wherein the method comprises the steps of: i) providing a mineral material selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, ii) providing a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, iii) providing a co-dispersing agent, wherein the co-dispersing agent is a polyvinyl alcohol, iv) providing water, and v) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv).

The mineral material may be provided in any suitable liquid or dry form. For example, the mineral material may be in the form of a powder and/or a suspension. The suspension can be obtained by mixing the mineral material with a solvent, preferably water. The mineral material to be mixed with a solvent, and preferably water, may be provided in any form, for example, as a suspension, slurry, dispersion, paste, powder, moist filter cake or in pressed or granulated form, and preferably is provided as a powder.

According to one embodiment the mineral material is provided in the form of an aqueous suspension, preferably having a solids content of at least 20 wt.-%, preferably at least 40 wt.-%, more preferably at least 60 wt.-%, and most preferably at least 68 wt.-%, based on the total weight of the suspension. According to another embodiment the mineral material is provided in the form of an aqueous suspension having a solids content from 10 to 78 wt.-%, preferably from 20 to 75 wt.-%, and most preferably from 50 to 72 wt.-%, based on the total weight of the aqueous suspension.

The dispersing agent may be provided in the form of an aqueous solution.

The co-dispersing agent may be provided in the form of a solution in water or in solid form. According to one embodiment, the co-dispersing agent is provided in solid form, preferably in the form of a powder, or in the form of particles, beads, or granules, and most preferably in the form of a powder or in the form of granules.

For the purpose of the present invention, any type of water can be provided in method step iv). According to one embodiment, the water is tap water, deionized water, process water, or mixtures thereof. Preferably, the water is tap water.

According to one embodiment the water of step iv) is provided in combination with the mineral material of step i) and/or in combination with the dispersing agent of step ii) and/or in combination with the co-dispersing agent of step iii), preferably the water of step iv) is provided in combination with the mineral material of step i).

The contacting step v) may be carried out in any manner known to the skilled person. For example, the contacting of the mineral material of step i), the dispersing agent of step ii), the codispersing agent of step iii), and the water of step iv) may be carried out under mixing and/or homogenizing and/or particle dividing conditions. The skilled person will adapt these mixing and/or homogenizing and/or particle dividing conditions such as the mixing speed, dividing, and temperature according to his process equipment.

Suitable mixing methods are known to the skilled person. Examples of suitable mixing methods are shaking, mixing, stirring, agitating, ultrasonication, or inducing a turbulent or laminar flow by means such as baffles or lamellae. Suitable mixing equipment is known to the skilled person, and may be selected, for example, from stirrers, such as rotor stator systems, blade stirrers, propeller stirrers, turbine stirrers, or anchor stirrers, static mixers such as pipes including baffles or lamellae. According to an exemplary embodiment of the present invention, a rotor stator stirrer system is used. The skilled person will adapt the mixing conditions such as the mixing speed and temperature according to his process equipment.

Simultaneous mixing and homogenizing may take place by means of a ploughshare mixer. Ploughshare mixers function by the principle of a fluidized bed produced mechanically. Ploughshare blades rotate close to the inside wall of a horizontal cylindrical drum and convey the components of the mixture out of the product bed and into the open mixing space. The fluidized bed produced mechanically ensures intense mixing of even large batches in a very short time. Choppers and/or dispersers are used to disperse lumps in a dry operation. Equipment that may be used in the inventive process is available, for example, from Gebruder Lbdige Maschinenbau GmbH, Germany.

According to another embodiment of the present invention, method step v) is carried out in a milling device, preferably in a ball mill, preferably in combination with a cyclone device that recirculates agglomerates and/or aggregates formed during method step v) back to the inlet of the milling device. A cyclone device enables the separation of particulate material such as particles, agglomerates or aggregates, into fractions of smaller and larger particulate material based on gravity. According to one embodiment, during method step v) the mineral material particles are divided into smaller particles. The term “dividing” as used in the present invention means that particles are split into smaller particles. This may be done by grinding, e.g. using a ball mill, a hammer mill, a rod mill, a vibrating mill, a roll crusher, a centrifugal impact mill, a vertical bead mill, an attrition mill, a pin mill, a hammer mill, a pulveriser, a shredder, a de-clumper, or a knife cutter. However, any other device that is able to divide the mineral material during method step v) into smaller particles may be used.

According to a preferred embodiment, method step v) is carried out under mixing conditions. Thus, in method step v) the mineral material of step i), the dispersing agent of step ii), the codispersing agent of step iii), and the water of step iv) are mixed.

Method step v) may be carried out at room temperature, i.e. at a temperature of 20°C ± 2°C, or at other temperatures. According to one embodiment, method step v) is carried out at about 25°C. According to one embodiment of the present invention, method step v) is carried out for at least 1 s, preferably for at least 1 min, e.g. for at least 15 min, 30 min, 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, or 10 hours.

The components of the aqueous suspension may be contacted in any suitable order.

According to one embodiment step v) comprises the following steps: a1) contacting the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv), and a2) adding the mineral material of step i) to the mixture obtained in step a1).

According to another embodiment step v) comprises the following steps: b1) contacting the dispersing agent of step ii), and the co-dispersing agent of step iii), and b2) adding the mineral material of step i) and water of step iv) to the mixture obtained in step b1), wherein the mineral material and the water are added separately or in combination, preferably in combination.

According to still another embodiment step v) comprises the following steps: c1) contacting the dispersing agent of step ii), the mineral material of step i), and the water of step iv), and c2) adding the co-dispersing agent of step iii) to the mixture obtained in step c1). Preferably, the co-dispersing agent is added in the form of a solution in water.

According to still another embodiment step v) comprises the following step: d) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii) and the water of step iv) simultaneously.

According to one embodiment, the aqueous suspension formed in step v) has a solids content from 10 to 78 wt.-%, based on the total weight of the aqueous suspension, preferably from 20 to 75 wt.-%, and most preferably from 50 to 72 wt.-%.

The Brookfield viscosity of the obtained aqueous suspension may be from 10 to 5 000 mPa s at 25°C, preferably from 50 to 2 000 mPa s at 25°C, more preferably from 100 to 1000 mPa s at 25°C, and most preferably from 150 to 800 mPa s at 25°C.

According to one embodiment, the method further comprises the step of grinding and/or dewatering the aqueous suspension during and/or after step v). The grinding process may be undertaken by all the techniques and grinders well known to the person skilled in the art for wet grinding. The grinding step may be carried out with any conventional grinding device, for example, under conditions such that refinement predominantly results from impacts with a secondary body, i.e. in one or more of: a ball mill, a rod mill, a vibrating mill, a centrifugal impact mill, a vertical bead mill, an attrition mill, or other such equipment known to the skilled person. The grinding may be carried out in batch or continuously, preferably continuously.

According to one embodiment of the present invention, the grinding is carried out at a temperature from 30 to 110°C, preferably from 40 to 100°C. Alternatively, the grinding can be carried out at room temperature, i.e. at a temperature of 20°C ± 2°C.

After grinding, the mineral material may have a weight median particle size dso from 0.05 to 100 pm, preferably from 0.1 to 50 pm, more preferably from 0.15 to 25 pm, even more preferably from 0.25 to 10 pm, and most preferably from 0.5 to 2 pm.

Alternatively or additionally, the aqueous suspension may be dewatered during and/or after method step v). Thereby, the solids content of the suspension can be increased. The suspension may be partially or fully dewatered by a filtration, centrifugation or thermal separation process. For example, the suspension may be partially or fully dewatered by a filtration process such as nanofiltration or a thermal separation process such as an evaporation process under ambient, atmospheric pressure or at reduced pressure.

According to one embodiment step v) comprises the following steps: e1) contacting the dispersing agent of step ii), the mineral material of step i), and the water of step iv), e2) dewatering the aqueous suspension obtained in step e1), and e3) adding the co-dispersing agent of step iii) to the dewatered suspension obtained in step e2), preferably in the form of a solution in water, and e4) optionally dewatering the suspension obtained in step e3).

According to one embodiment, the solids content of the aqueous suspension obtained after dewatering it is from 55 to 80 wt.-%, preferably from 60 to 79 wt.-%, and more preferably from 65 to 78 wt.-%, based on the total weight of the aqueous suspension.

According to a further aspect, a method for producing an aqueous suspension comprising a mineral material, and a dispersing agent in combination with a co-dispersing agent is provided, wherein the mineral material is precipitated calcium carbonate and the method comprises the following steps:

I) providing a calcium oxide-containing material,

II) providing a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, or a mixture thereof,

IV) preparing a milk of lime by mixing water, the calcium oxide-containing material of step I) and the co-dispersing agent of step III),

VI) mixing the aqueous suspension obtained in step V) with the dispersing agent of step II). The calcium oxide-containing material of step I) can be obtained by calcining a calcium carbonate-containing material. Calcination is a thermal treatment process applied to calcium carbonate-containing materials in order to bring about a thermal decomposition resulting in the formation of calcium oxide and gaseous carbon dioxide. Calcium carbonate-containing materials which may be used in such a calcination process are those selected from the group comprising precipitated calcium carbonates, natural calcium carbonate containing minerals such as marble, limestone and chalk, and mixed alkaline earth carbonate minerals comprising calcium carbonate such as dolomite, or calcium carbonate rich fractions from other sources. It is also possible to subject a calcium carbonate-containing waste material to a calcination process in order to obtain a calcium oxide-containing material.

Calcium carbonate decomposes at about 1000°C to calcium oxide (commonly known as quicklime). The calcination step may be carried out under conditions and using equipment well-known to the person skilled in the art. Generally, calcination may be carried out in furnaces or reactors (sometimes referred to as kilns) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors.

The end of the calcination reaction may be determined, e.g. by monitoring the density change, the residual carbonate content, e.g. by X-ray diffraction, or the slaking reactivity by common methods.

According to one embodiment of the present invention, the calcium oxide-containing material of step I) is obtained by calcining a calcium carbonate-containing material, preferably selected from the group consisting of precipitated calcium carbonate, natural calcium carbonate minerals such as marble, limestone and chalk, mixed alkaline earth carbonate minerals comprising calcium carbonate such as dolomite, and mixtures thereof. For reasons of efficiency, it is preferred that the calcium oxide- containing material has a minimum calcium oxide content of at least 75 wt.-%, preferably at least 90 wt.-%, and most preferably 95 wt.-%, based on the total weight of the calcium oxide-containing material. According to one embodiment, the calcium oxide-containing material consists solely of calcium oxide.

The calcium oxide-containing material can consist of only one type of calcium oxide-containing material. Alternatively, the calcium oxide-containing material can consist of a mixture of two or more types of calcium oxide-containing materials.

The calcium oxide containing material can be used in the inventive process in its original form, i.e. as a raw material, for example, in the form of smaller and bigger chunks. Alternatively, the calcium oxide-containing material can be ground before use. According to one embodiment of the present invention, the calcium oxide-containing material is in the form of particles having weight median particle size cko from 0.1 to 1000 pm, and preferably from 1 to 500 pm.

In method step IV), a milk of lime is prepared by mixing water, the calcium oxide-containing material of step I), and the co-dispersing agent of step III).

The reaction of the calcium oxide-containing material with water results in the formation of a milky calcium hydroxide suspension, better known as milk of lime. Said reaction is highly exothermic and is also designated as “lime slaking” in the art. According to one embodiment of the present invention, the temperature of the water, which is used in mixing step IV), i.e. the temperature of the water that is used to slake the calcium oxide-containing material, is adjusted to be in the range from more than 0°C and less than 100°C. In other words, the water that is used to slake the calcium oxidecontaining material is adjusted to a temperature range, in which the water is in liquid form. Preferably, the temperature of the water, which is employed in mixing step IV) is adjusted to be from 1 °C to 70°C, more preferably from 2°C to 50°C, even more preferably from 30°C to 50°C, and most preferably from 35 to 45°C. It will be apparent to the skilled person that the initial temperature of the water is not necessarily the same one as the temperature of the mixture prepared in step IV) due to the highly exothermic slaking reaction and/or due to the mixing of substances having different temperatures.

According to one embodiment, step IV) comprises the steps of:

A1) mixing the calcium oxide-containing material of step I) with water, and

A2) adding the co-dispersing agent of step II) to the mixture of step A1).

Preferably, method step A2) is carried out after the reaction between the calcium oxide- containing material of step I) with water is completed, i.e. after the lime has been completely slaked.

According to another embodiment, step IV) comprises the steps of:

B1) mixing the co-dispersing agent of step III) with water, and

B2) adding the calcium oxide-containing material of step I) to the mixture of step B1).

According to still another embodiment, in step IV) the calcium oxide-containing material of step I), the co-dispersing agent of step III), and water are mixed simultaneously.

The co-dispersing agent of step III) may be added in step IV) in one portion or in several portions. According to one embodiment, in step IV) the co-dispersing agent is mixed with the water, and the calcium oxide-containing material by adding the co-dispersing agent in one portion or in two, three, four, five, or more portions.

Method step IV) may be performed at room temperature, i.e. at a temperature of 20°C ± 2°C, or at an initial temperature of 30 to 50°, preferably 35 to 45°C. Since the reaction is exothermic, the temperature typically raises to a temperature between 70 and 85°C during step IV). According to a preferred embodiment, process step IV) is performed with mixing, agitation, or stirring, for example, mechanical stirring. Suitable process equipment for mixing, agitation or stirring is known to the skilled person. The progress of the slaking reaction may be observed by measuring the temperature and/or conductivity of the reaction mixture. It can also be monitored by turbidity control. Alternatively or additionally, the progress of the slaking reaction can be inspected visually.

According to one embodiment, the calcium oxide-containing material and the water are mixed in a mass ratio from 1 :4 to 1 :15. According to one preferred embodiment, in step IV) the calcium oxide-containing material and the water are mixed in a mass ratio from 1 :5 to 1 :9.

According to one embodiment, the milk of lime of step IV) has a solids content from 5 to 25 wt.-%, based on the total weight of the milk of lime, preferably from 10 to 20 wt.-%, and most preferably from 10 to 15 wt.-%.

According to one embodiment, the milk of lime of step IV) has a Brookfield viscosity from 10 to 1000 mPa s at 25°C, more preferably from 20 to 800 mPa s at 25°C, and most preferably from 50 to 600 mPa s at 25°C. According to one embodiment, the Brookfield viscosity is measured at 100 rpm.

It is within the confines of the present invention that additional water may be introduced during the slaking reaction in order to control and/or maintain and/or achieve the desired solids content or Brookfield viscosity or temperature of the milk of lime. Method step IV) can be carried out in the form of a batch process, a semi-continuous or a continuous process.

In method step V) the milk of lime obtained from step IV) is carbonated to form an aqueous suspension of precipitated calcium carbonate. The carbonation is carried out by means and under conditions well-known by the person skilled in the art. The introduction of carbon dioxide into the milk of lime quickly results in the formation of the carbonate ion (CO3²), and thus, the requisite concentration for calcium carbonate to be formed. Particularly, the carbonation reaction can be readily controlled considering the reactions involved in the carbonation process. Carbon dioxide dissolves according to its partial pressure forming carbonate ions via the formation of carbonic acid (H2CO3), which, in such an alkaline solution, dissociates to its constituent hydrogen and carbonate ions. Once the ionic product of calcium carbonate is sufficiently greater than the solubility product, calcium carbonate precipitates. At the same time, hydroxide ions are neutralized by the dissociated hydrogen ions. As a result the ionic product for calcium hydroxide would therefore be less than the solubility product, and it would continue to dissolve. This occurs continually, so long as CO2 is bubbled into solution, until all calcium hydroxide is consumed, or is trapped in the calcium carbonate crystal structure. It is the inventors’ belief that the methods demonstrated in this patent application, by avoiding nucleation/growth near the calcium hydroxide surface, significantly minimize the trapping of calcium hydroxide, thus resulting in its near full consumption.

According to one embodiment of the present invention, in step V) the carbonation is carried out by feeding pure gaseous carbon dioxide or technical gases containing at least 10 vol.-% of carbon dioxide into the milk of lime. The progress of the carbonation reaction can be readily observed by measuring the conductivity, density, turbidity and/or pH. In this respect, the pH of the milk of lime before addition of carbon dioxide will be more than 10, usually between 11 and 12.5, and will constantly decrease until a pH of about 7 is reached. At this point the reaction can be stopped.

Conductivity slowly decreases during the carbonation reaction and rapidly decreases to low levels, when the precipitation is completed. The progress of the carbonation may be monitored by measuring the pH and/or the conductivity of the reaction mixture.

According to one embodiment of the present invention, the temperature of the milk of lime obtained from step IV), which is used in step V) is adjusted to be in the range from 5°C to 95°C. It will be apparent to the skilled person that the initial temperature of the milk of lime is not necessarily the same one as the temperature of the mixture prepared in step IV) due to the exothermic carbonation reaction and/or due to the mixing of substances having different temperatures. According to one embodiment of the present invention, step V) is carried out at a temperature from 5 to 95°C, preferably from 30 to 70°C, and more preferably from 40 to 60°C.

Method step V) can be carried out in form of a batch process, a semi-continuous or a continuous process. According to one embodiment, the method of the present invention involving the method steps I) to V) is carried out in form of a batch process, a semi-continuous or a continuous process. Method step V) may involve any suitable mixing method known to the skilled person, e.g. any of the mixing methods described above.

According to one embodiment, the method further comprises the step of dewatering the aqueous suspension during and/or after step V), as described in more detail above. According to a further embodiment, the method for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent according to the present invention may comprise a further step of sieving the aqueous suspension after step v) or step VI. Such sieving can be carried out with any conventional sieving means known to the skilled person. The sieving can be carried out using one or more mesh sizes. Suitable mesh sizes are, for example, 180 pm, 90 pm, 63 pm, or 45 pm.

Aqueous dispersion and applications

According to a further aspect of the present invention, an aqueous suspension obtainable by a method according to the present invention is provided.

According to one embodiment an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent is provided, wherein the mineral material is selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, the dispersing agent is a polyacrylate-containing dispersant, and the co-dispersing agent is polyvinyl alcohol, wherein the aqueous dispersion comprises the dispersing agent in an amount of 3 wt.-% or less, based on the total amount of the aqueous suspension, the co-dispersing agent in an amount of 2 wt.-% or less, based on the total amount of the aqueous suspension, and the mineral material in an amount from 10 to 78 wt.-%, based on the total weight of the aqueous dispersion.

The aqueous suspension obtainable by the methods of the present invention may be used in various materials and applications. According to one embodiment of the present invention, the aqueous suspension is used in paper applications, packaging applications, polymer applications, or water treatment applications. According to one embodiment, the aqueous suspension of the present invention is used in a paper coating composition. Accordingly, a paper coating composition is provided comprising the aqueous suspension according to the present invention. The inventors of the present invention surprisingly found that paper coating compositions comprising the aqueous suspension according to the present invention may improve the optical and/or mechanical properties of the coated paper products such as paper substrates or cardboards. In particular it was found that the water retention of the paper product can be improved as well as its mechanical surface strength.

The scope and interest of the invention will be better understood based on the following examples which are intended to illustrate certain embodiments of the present invention and are non- limitative. Examples

1. Methods

1.1. Particle size distribution

The weight determined median particle size cfeo(wt) was measured by the sedimentation method, which is an analysis of sedimentation behaviour in a gravimetric field. The measurement was made with a Sedigraph™ 5120 of Micromeritics Instrument Corporation, USA. The method and the instrument are known to the skilled person and are commonly used to determine particle size distributions of fillers and pigments. The measurement was carried out in an aqueous solution of 0.1 wt% N34P2O7. The samples were dispersed using a high speed stirrer and supersonicated. The processes and instruments are known to the skilled person and are commonly used to determine particle sizes of fillers and pigments.

1.2. Viscosity measurement

The Brookfield viscosity in Examples 1 to 10 was measured by a Brookfield DV-II+ Pro viscometer at 25°C ± 1 °C at 100 rpm using an appropriate spindle of the Brookfield RV-spindle set and is specified in mPa s. A spindle was selected from the Brookfield RV-spindle set which was suitable for the viscosity range to be measured. For a viscosity range of < 200 mPa s the spindle number 2 was used, for a viscosity range between 200 and 800 mPa s the spindle number 3 was used, for a viscosity range between 400 and 1 600 mPa s the spindle number 4 was used, and for a viscosity range between 800 and 3 200 mPa s the spindle number 5 was used.

The Brookfield viscosity in Example 11 was measured by a Brookfield DV-III Ultra viscometer at 24°C ± 3°C at 100 rpm using an appropriate spindle of the Brookfield RV-spindle set and is specified in mPa s. Once the spindle has been inserted into the sample, the measurement was started with a constant rotating speed of 100 rpm. The reported Brookfield viscosity values are the values displayed 60 seconds after the start of the measurement. For a viscosity range between 200 and 800 mPa s the spindle number 3 was used, for a viscosity range between 400 and 1 600 mPa s the spindle number 4 was used, and for a viscosity range between 800 and 3 200 mPa s the spindle number 5 was used.

1.3. ACAV viscosity

The ACAV viscosity measurement was conducted in an ACAV-2 viscosimeter.1 liter of coating composition was put into the probe chamber of the viscosimeter and the automatic routine was started. At the end of the measurement a graph was obtained that showed the viscosity in dependency of the shear rate. Shear rate is a measure used to describe the flow of liquid through a channel. More specifically, the shear rate is the rate at which the velocity of a fluid under shear changes through its thickness, also known as its velocity gradient. For the present application the ACAV viscosity at a shear rate of 500000 1/s was taken.

1.4. Water retention

The water retention was determined using an AA-GWR water retention meter (GRADEK™, DT Paper Science). It comprised a measurement chamber, into which a piece of test paper (test blotter paper) was placed, covered with a perforated plastic sheet (test filter PCTE), wherein both the paper and the sheet are commercially available from the retention meter manufacturer.

10 ml of the coating composition to be tested was added to the measurement chamber. The AA-GWR device exerted a certain amount of pressure on the coating composition, causing all or some of the water and water-soluble substances contained in the composition to pass through the perforated plastic sheet and penetrate the test paper. A pressure of 0.5 bar was applied for 90 seconds. The difference between the weight of the test paper before and after the experiment yielded the water retention value, which corresponded to the weight of the water and water-soluble substances contained within the composition that penetrated into the test paper during the experiment. The lower the water retention value, the better the printability of the resulting paper.

1.5. pH

The pH of a suspension or solution was measured at 25°C using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab® Expert Pro pH electrode. A three-point calibration (according to the segment method) of the instrument was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20°C (Sigma-Aldrich Corp., USA). The reported pH values are the endpoint values detected by the instrument (the endpoint was when the measured signal differed by less than 0.1 mV from the average over the last 6 seconds).

1.6. Gloss

Paper gloss was measured using LGDL-05.3-lab instrumentation (Lehmann Messsysteme GmbH, Germany) according to EN ISO 8254-1 :2009, TAPPI 75° (%).

1.7. Brightness and UV

The brightness was measured according to standard ISO 2470-2:2008 with an Elrepho 450 spectrometer (light: D65, view: 10°, Datacolor, Germany).

1.8. PPS roughness

Paper surface roughness was measured using the L&W PPS tester (Lorentzen & Wettre, Sweden) according to DIN ISO 8791-4 with a pressure of 1 .0 mPa using soft component, PPS 1.0 (pm).

1.9. Setoff optical ink density

The setoff optical ink density was measured according to the lab standard „Wegsch lagtest Offset¹¹ using the test device “Prufbau Multipurpose Printability Testing Instrument MZ II” (Prufbau, Germany).

For the testing a print was countered in a second print unit with a defined counter paper at certain times to see how fast the print was set/absorbed in the paper and will therefore with increasing delay reduce transfer of the ink to the counter paper.

Device setting: Ink unit temperature: 23° C

Contact pressure: 200 N/cm (800 N device setting)

Printing speed: 0.5 m/s Ink quantity: 300 mm³ Counter times: 15, 30, 60, and 120 s Printing ink: test colour (Prufbau, Germany)

Counter printing paper: Normkunstdruck APCO ll/ll 150 g/m²

The device settings were made on the test device. The timer was set to 5 seconds. The counter paper was clamped on the clamping bolt in such a way that the counter paper could be easily attached to the print sample carrier after proofing. A clean metal printing form was placed on the second printing unit. The print sample carrier with the paper to be tested was positioned in front of the first printing unit. The ink was applied to the first ink sector of the rubber roll and the distribution unit was started. After 30 seconds, the metal printing form was placed and inked for 30 seconds. The inked form was placed on the first printing unit and started. The stopwatch was switched on during the start release. The print sample carrier was caught by the timer so that the printing process could be stopped immediately.

Subsequently, the counter paper was inserted into the gap of the second printing unit together with the print sample carrier and the first counter pressure was applied after 15 seconds using the lubrication lever. This allowed 4 counter times to be reproduced in succession on one print (15, 30, 60, 120 s). To prevent possible picking or sticking, the test strip was separated from the counter printing paper immediately after reverse printing. A strip of the sample was then printed to check the printed image for errors and the required amount of ink. The distribution unit and the print form were recoloured for each subsequent print.

For evaluation, print and the various gradations on the counter paper were punched into 1 .9 x 2.9 cm strips using a punch and glued onto a table with 5 fields, ranging from the print to the counter time 120 s. To determine the results, the reverse prints were measured with an Epson Dual Lens Flatbed scanner and the image analysis software “Print Target” (set-off module, optical density) and the value obtained was indicated in the report for the respective reverse time.

1.10. Dry pick

The dry pick refers to the extent to which a paper will resist picking in the absence of moisture fountain solution. Pick resistance, also called surface strength, refers to the extent to which a paper can withstand a force applied at right angles to its surface (such as that generated by a sticky ink film during printing) without rupturing, or picking. Picking can include either a delamination of the plys of a paper and/or a partial or total removal of a paper coating.

The dry pick was measured according to the lab standard „Trockenrupfen” using the test device “Prufbau Multipurpose Printability Testing Instrument MZ II” (Prufbau, Germany).

Device setting: Ink unit temperature: 23° C

Contact pressure: 150 N/cm (600 N device setting)

Printing speed: 0 - 3 m/s (increasing) Ink quantity: 200 mm³

Counter times: 15, 30, 60, and 120 s

Printing ink: pick test colour (Prufbau, Germany): No. 1 (low tack), No. 2 (medium tack), and No. 3 (high tack)

Counter printing paper: Normkunstdruck APCO ll/ll 150 g/m² The device settings were made on the test device. The ink pipette was filled and the print sample carrier with the paper to be tested was positioned in front of the second printing unit. The ink was applied to the first inking sector of the rubber roll, wherein the distribution time was 30 s. Then the rubber blanket printing form was placed on said roll, and after a further 30 seconds, the inked rubber blanket printing form was placed on the second printing unit. Using the lubrication lever on the righthand side of the device, the sample carrier was guided under the rubber blanket printing form until there was contact between the rubber blanket printing form and the sample at the 0 point of the 200 mm printing path. The rubber blanket printing form was placed on the printing unit in such a way that the gap in the blanket elevator did not lead to any interruption in printing over the 200 mm long printing path. As soon as the rubber blanket printing form was placed on the printing surface, the printing unit drive (speed) was switched on and the plucking test was carried out.

Depending on the picking strength of the paper, picking test colors no. 1 - 3 with the appropriate tack were selected. For each test, the distribution sector and rubber blanket printing form were recoloured.

The final speed of 3 m/s in the plucking test with increasing speed referred to a pressure distance of 200 mm. The start of picking was the point on the printed paper strip where the first fibers or pigment particles were lifted or torn from the paper surface. This point was marked on the side with a pencil. The distance from the start of printing to the start of picking was measured and entered in the equation given below under the corresponding colour number in mm. The start of picking in m/s was calculated using the following equation: m/s

Start of picking [m/s]=0.015 x Distance start of printing to start of picking [mm] mm

The higher the dry pick value, the better the printability of the paper substrate.

1.11. Deltack

The deltack was measured according to lab standard “Messung der Oberflachenfestigkeit im Offset Druck“ using the test device “Prufbau Deltack” (Prufbau, Germany). The deltack reflects the surface strength of a printing substrate in offset printing and indicates the force at which parts of the substrate (e.g. fibres, pigments, etc.) are torn out of the substrate. For the testing a paper sample was attached to a load cell of a rotating cylinder, which passed a printing unit again and again until rupture was visible and force was measured.

Device settings:

Pressure sample carrier: Adapted to substrate (for paper) or blanket strips with 263 pm (for cardboard)

Colour quantity: 200 mm³ for paper and 300 mm³ for cardboard

System temperature: 23°C

Measuring unit selection: 1

Selection of printing unit: B Printing tension: 1000 N Measuring interval: 4 s

Number of measuring cycles: 20

Print speed: 1 m/s Minimum waiting time: 3 s

Pressure full turn: activated

Mean values range: 60 - 160 mm

The paper strip to be tested was taped at both ends so that the holes from the subsequent hole process were in the area of the adhesive tape. The cardboard strip to be tested was glued to the metal printing form 63.45 using double-sided adhesive tape. Two measurements were made for each sample. If the results deviated too much from each other, more repetitions were made in order to obtain a more representative result.

The paper sample to be tested was placed with the corresponding side facing upwards in the tensile measurement holder and on the compression sample carrier. It was ensured that the sample did not already trigger tensile forces on the measuring unit. A pressure sample carrier was used on the opposite measuring unit to smear the paint. This ensured that the amount of ink was reduced during initial printing and supplemented with continuous printing on the printing roller. This resulted in a more even increase in tack overtime/cycles. The required amount of ink was rubbed into the ink distribution system for 30 seconds. The rubber blanket printing form was then inked for a further 30 seconds. After inking, said printing form was placed on printing unit B. The pressure was applied using the "Start measurement" and "Press 1/2 turn" fields. After clicking on "Start", the test strip ran through the pressure section again and again at the same settings.

For the measurement of the cardboard samples the rubber blanket strip was placed with the darker side facing upwards in the tensile measurement holder and on the pressure sample carrier. It was ensured that the rubber blanket strip did not already trigger tensile forces on the measuring unit. The prepared metal printing form with the cardboard sample to be tested was placed on printing unit B. The required amount of ink was rubbed into the ink distribution system for 30 seconds. The rubber blanket printing form was then inked for a further 30 seconds. After inking, this printing form was placed on printing unit A. The pressure was applied using the "Start measurement" and "Press 1/2 turn" fields. The pressure was applied using the "Start measurement" and "Press 1/2 turn" fields. After clicking on "Start", the test strip ran through the pressure section again and again at the same settings.

After each completed rotation, the sample came to a standstill for a defined time. During this time, the surface was inspected for damage. The letters A - Z and the numbers 1 - 9 were used to clearly mark any damage detected during the corresponding pass. Once the damage was confirmed, the measurement was stopped.

While the sample was in motion, the increasing force curve was observed in the device software in order to optimize or adjust it for another stripe if necessary. Generally speaking, the more colour or the higher the printing speed, the higher the force (initial force and maximum force). The smaller the measuring interval was selected, the steeper (more progressive) the curve in the forcetime diagram rises and, as a result, less delta force was obtained from pass to pass, which led to a better detailed image. The deltack is expressed in Newton, based on a strip width of 5 cm [N/5 cm],

1.12. Bulk

The formula for bulk is thickness (mm) x basis weight (g/m²) x 1000. Using the value for the thickness and the basis weight, the bulk can be calculated. 2. Materials

Table 1 : Fillers.

Table 2: Dispersing agents.

Table 3: Polyvinyl alcohol (PVOH). All PVOH grades, except for PVA 8, were supplied by Chang Chun Petrochemical Co. Ltd. as granules or powders. PVA 8 was obtained from Sigma Aldrich.

Table 4: Materials for paper coating compositions.

3. Examples

Polyvinyl alcohol was used in the examples below in form of pre-made aqueous solutions having a PVOH concentration of 20 wt.-%, based on the total weight of the solution. The 20 wt.-% PVOH pre-made solutions were obtained by dissolving PVOH granules or powder in water (85-90°C) with stirring until no more particles were visible. The pre-made solutions were cooled to room temperature and used within one week of preparation.

In Examples 1-4 and 11 , the mineral material was provided in the form of an aqueous suspension having a solids content of approximately 50 wt.-%. Said suspensions were further diluted until they could be homogeneously mixed with 0.4-0.5 % dispersant. This was typically the case at a solids content of between 30 and 40 wt.-%.

Subsequently, water was evaporated either on a hot plate or using the evaporation apparatus until a concentration of either ca. 68 wt.-% or ca. 72 wt.-% was reached. Extra dispersant was added if required. The total amounts of dispersing agent within the final compositions are indicated in Tables 5 to 7 below. In a next step, PVOH was added using the pre-made solutions. Further evaporation was performed if a significant amount of water was added with the pre-made PVOH solutions, for example, it the solids content of the mineral suspensions changed by more than 1 wt.-%.

Viscosity was measured using the appropriate spindle on a Brookfield viscometer once the suspensions had cooled to room temperature. 3.1. Example 1

Table 5: Composition and viscosity of aqueous mineral suspensions prepared according to Example 1 (comp.: comparative).

Aqueous mineral material suspensions were prepared by adding 0.5 wt.-% PVA 1 , 0.75 wt.-%

PVA 2 or 0.4 wt.-% of a mixture of 2/7 PVA 1 and 5/7 PVA 2 to a 68 wt.-% Pigment B suspension with 0.4 or 0.5 wt.-% Dispersant A. In addition, comparative suspensions without PVOH were prepared. The compositions of the tested suspensions are listed in Table 5 above.

As can be gathered from Table 5 as well as Fig. 1 , the viscosity was reduced from 3036 mPa s for the Pigment B suspension without any PVOH to 105 mPa s for the Pigment B suspension with PVOH (PVA 1) and to 110 mPa s for the Filler B suspension with PVOH (PVA 2). The viscosity of Pigment B suspension with 0.4 wt.-% Dispersant A was so high that it could not be measured without the addition of PVOH. The viscosity of the final suspension (Pigment B suspension + mix of PVA 1 and PVA 2 + Dispersant A) was 156 mPa s. For comparison, the 0.75 wt.-% and 1 wt.-% Dispersant A suspensions (Pigment B suspension + Dispersant A) without PVOH had a viscosity of 1094 mPa s and 85 mPa s, respectively.

Increasing the amount of Dispersant A reduced the viscosity of the Pigment B suspension. It was surprisingly found that the viscosity of the Pigment B suspension could be reduced to a similar level when using PVOH and reducing the amount of Dispersant A by at least half. Different molecular weight PVOH can be used, both PVA 1 and PVA 2 are shown to work, and the combination of these (mixture) was also effective.

3.2. Example 2 - Longer chain length PVOH

Table 6: Composition and viscosity of suspensions prepared according to Example 2 (comp.: comparative).

Aqueous mineral material suspensions were prepared by adding 0.5 wt.-% PVA 3 or PVA 4 to a 68 wt.-% Pigment A suspension with 0.5 wt.-% Dispersant A. In addition, comparative suspensions without PVOH were prepared. The compositions of the tested suspensions are listed in Table 5 above.

As can be gathered from Table 6 as well as Fig. 2, the viscosity was reduced from 740 mPa s for the Pigment A suspension without any PVOH to 332 mPa s for the Pigment A suspension with PVOH (PVA 3) and 321 mPa s with PVOH (PVA 4). For comparison, a 1 wt.-% Dispersant A suspension (Pigment A suspension + Dispersant A) without PVOH has a viscosity of 77 mPa s.

Furthermore, the longer chain length PVOH could be used to reduce the viscosity replacing some of the dispersant. It was shown that a larger filler particle size requires less dispersant to reduce the viscosity. The viscosity was not reduced as much with the coarser particle size of Pigment A, however, the surprising viscosity reduction effect with PVOH was still visible.

3.3. Example 3 - Partially hydrolysed PVOH

Table 7: Composition and viscosity of suspensions prepared according to Example 3 (comp.: comparative).

An aqueous mineral material suspension was prepared by adding 0.5 wt.-% PVA 4 to a

68 wt.-% Pigment A suspension with 0.5 wt.-% Dispersant A. In addition, comparative suspensions without PVOH were prepared. The compositions of the tested suspensions are listed in Table 7 above.

Furthermore, an aqueous mineral material suspension was prepared by premixing a 20 wt.-% PVA 6 solution with Dispersant A in a 1 :1 weight ratio. The PVA 6-dispersant solution was added to a Filler A suspension prior to evaporation to 68 % in such an amount that a concentration of 0.5 wt.-% Dispersant A and 0.5 wt.-% PVA 6 was achieved in the final suspension.

As can be gathered from Table 7 as well as Fig. 3, the viscosity was reduced from 740 mPa s for the Pigment A suspension without any PVOH to 389 mPa s for the Pigment A suspension with PVOH (PVA 5). The viscosity in the test trial # 3.4 using a PVOH/dispersing agent premix was 207 mPa s. For comparison, a 1 wt.-% Dispersant A suspension (Pigment A suspension + Dispersant A) without PVOH had a viscosity of 77 mPa s.

It was shown that a larger particle size requires less dispersant to reduce the viscosity. The use of partially hydrolysed PVOH can also be used to replace part of the dispersant, although it was not as effective as for smaller particle sizes. In previous examples, the PVOH was added after the dispersant addition, however, as shown in this example with PVA 6, the PVOH can also be added at the same time as the dispersant.

3.4. Example 4 - thermal concentration

The suspensions obtained from test trials # 1 .1 to 1 .5 of example 1 were evaporated to a solids content of 72 wt.%.

As can be gathered from Fig. 4, the viscosity of (Pigment B suspension + mix of PVA 1 and PVA 2 + Dispersant A) was 367 mPa s . For comparison, a 0.5 wt.-%, 0.75 wt.-% and a 1 wt.-% Dispersant A slurry (Pigment B slurry + Dispersant A) without PVOH had a viscosity of 4290 mPa s, 351 m Pas and 278 mPa s, respectively.

In previous examples it has been shown that PVOH can be used to reduce the dispersant concentration required to reduce the viscosity of a suspension having a solids content of 68 wt.-%. This can be also achieved at higher slurry concentrations as was shown in this example for a solids content of 72 wt.-%.

3.5. Example 5 - Magnesium hydroxide

A 65 wt.-% suspension of Pigment C was prepared by adding water to Pigment C powder with stirring. 0.25 wt.-% PVOH (PVA 1) was added for the inventive example (grey line with squares). Dispersant B was added incrementally and the viscosity was measured after each addition. It can be seen from Fig. 5 that the presence of PVOH reduced the amount of dispersant required to reach a specific viscosity compared to the suspension without PVOH. 3.6. Example 6 - Calcium hydroxide

A pre-made, non-stabilised 30 wt.-% suspension of Pigment D was homogenised with Dispersant C and 0-0.6 wt.-% PVOH. The PVOH used was PVA 1. Dispersant C was also added incrementally with stirring, and the viscosity was measured after each addition. It can be seen from Fig. 6 that the presence of 0.6 wt.-% PVOH (grey line with diamonds) resulted in lower viscosities at lower dispersant concentrations compared to the suspension without PVOH.

3.7. Example 7 (Pilot plant trials)

An aqueous suspension of Pigment A with a solids content of 22.1 wt.-% was used as starting material. 0.4-0.8 wt.-% Dispersant A (and 0.5-0.8 wt.-% PVA 2 if used) were added (according to the amounts outlined in the Table 8 below). The obtained suspension was dewatered first with an Andritz centrifuge to a solids content of 55 wt.-% and then with an Epcon evaporator to a solids content of 72 wt.-% (final slurry).

The viscosity of the final suspension without PVOH (comparative sample) was 150 mPa s, while the viscosity of the inventive slurry 3613/2 with PVOH was 164 mPa s, although the concentration of dispersing agent was halved. However, increasing the concentration of PVOH did not further reduce the viscosity. The results of the viscosity measurements are shown in Table 8 and Fig. 7.

Table 8: Composition and viscosity of aqueous mineral suspensions prepared according to Example 7 (INV=inventive).

3.8. Example 8 (Pilot plant trials)

An aqueous suspension containing a mixture of Pigment E and Pigment F (solids content 76 wt.-%) and 0.29 wt.-% of Dispersant D was provided as feed material. Dispersant E, and PVOH if used, were added to said feed material, and subsequently the samples were ground in a 200 I horizontal mill (in the presence of 245 kg of grinding beads with a size of 1 .8 to 2.0 mm) until 95 wt.-% of the pigments had a particles size of less than 2 pm. The ground suspensions were treated with 250 ppm AMP and sieved at 63 pm.

The results confirm that PVOH can be added before grinding for samples with solids contents up to 72 wt.-%. The results of the viscosity measurements are shown in Table 9 and Fig. 8, wherein the columns in Fig. 8 refer to the viscosity and the grey line to the solids content.

Table 9: Composition and viscosity of aqueous mineral suspensions prepared according to Example 8 (INV=inventive).

3.9. Example 9 - Application trials on light weight coated (LWC) paper

Table 10: Composition of coating compositions prepared and used in Example 9 (pph: parts per hundred, dry weight; wt.-% are based on total weight of dry pigment (except for the solids content, wherein the wt.-% is based on the total weight of the suspension)). Paper coating trials 1 and 2 were run at a pilot coater at the following conditions:

■ Speed: 1400 m/min

■ Base paper: 46 g/m² wood-containing commercial LWC base

■ Stiff blade application, blade thickness: 0.5 mm

■ Coat weight: 10.5 g/m²1 side ■ Calendering speed: 300 m/min

■ Line load 300 kN/m

■ Temperature: 90°C

Table 11 : Results of paper coating trial 1 .

It can be seen that coating composition 10 including the same amount of dispersing agent as comparative coating composition 1 , has higher brightness as well as significantly improved dry pick and Deltack (influences the coating strength, resulting in less “picking” during printing) at good gloss, and good roughness,

The comparison of inventive coating compositions 6 and 10, wherein coating composition 6 has the same amount as coating composition 10, but only half the amount of dispersing agent , exhibits slightly higher paper gloss and brightness with similar PPS roughness.

The comparison of inventive coating compositions 5 and 6, wherein coating composition 5 contains the same amount of dispersing agent but only half the amount of PVOH, shows that both compositions exhibit very similar paper properties, both optically and in terms of surface strength.

Furthermore, the results compiled in Table 11 show that by introducing PVOH in the paper coating composition similar Brookfield viscosities and improved water retention without rheology modifier can be achieved (see coating compositions 5 and 6). The ACAV high shear viscosity increased as PVOH was introduced in the mineral suspension and was further increased when the PVOH amount was increased. Based on these results coating compositions 5 and 6 showed good results.

Table 12: Results of paper coating trial 2.

The results compiled in Table 12 show that by introducing a PVOH-containing mineral suspension in a paper coating composition, similar surface strength can be achieved despite reducing the amount of latex binder by 1 wt.-%, based on total weight of dry pigments (cf. coating compositions 1 and 8). Furthermore a significantly higher surface strength was obtained despite reducing the amount of starch binder by 1 .5 wt.-%, based on total weight of dry pigments (cf. coating compositions 1 and 9). Paper gloss and UV-brightness were improved when PVOH was introduced and the amount of total binders was decreased.

Furthermore the results in Table 12 show that similar Brookfield viscosity and improved water retention can be achieved without a rheology modifier (cf. coating compositions 1 and 9, and coating compositions 3 and 8) with only a minor impact in the high shear viscosity. 3.10. Example 10 - Application trials on board

Table 13: Composition of coating compositions prepared and used in Example 10 (pph: parts per hundred, dry weight; wt.-% are based on total weight of dry pigment (except for the solids content, wherein the wt.-% is based on the total weight of the suspension)). Board coating trials were run at a pilot coater at the following conditions:

■ Speed: 650 m/min

■ Base board: 230 g/m² commercial folding boxboard (FBB) base

■ Bent blade application, blade thickness: 0.5 mm

■ Coat weight: 11 g/m² Table 14: Results of comparison trials (comp.: comparative).

The results compiled in Table 14 show that by introducing PVOH in the paper coating composition and reducing the amount of binder by 1 wt.-%, based on the total weight of dry pigments, in said coating composition, similar paper gloss and brightness values could be achieved. The board coated with coating composition 15 even showed a significant improvement in paper gloss, and the board coated with coating compositions 13 and 14 exhibited an improved water retention. In addition, it was found that the surface strength was slightly increased. 3.11. Example 11

Aqueous mineral material suspensions were prepared by adding 0.5 wt.-% PVA to a 68 wt.-% Pigment B suspension with 0.7 wt.-% Dispersant A, wherein the wt.-% are based on the total weight of the mineral suspension. In addition, a comparative suspension without PVOH was prepared. The aqueous mineral suspensions listed in Table 15 below were used to prepare the coating compositions compiled in Table 16 below.

Table 15: Composition and viscosity of aqueous mineral suspensions prepared according to Example 11 (comp.: comparative).

Table 16: Composition of coating compositions prepared and used in Example 11 (pph: parts per hundred, dry weight; wt.-% are based on total weight of suspension). Paper coating trials were run at a pilot coater at the following conditions:

■ Speed: 20 m/min

■ Base paper: 58 g/m² Sappi Magno

■ Bent blade application, blade thickness: 0.3 mm ■ Line load: 700 N/m

■ Calandering temperature: 70°C

Table 17: Results of comparison trials (comp.: comparative).

The results compiled in Table 17 show that by introducing PVOH in the paper coating composition improvements in paper gloss, surface roughness, and dry pick were achieved. Coating compositions 17, 18, and 19 also showed improvements in water retention. Furthermore, coating compositions 17, 18, 19, and 21 exhibited a significantly improved deltack.

Claims

1 . Use of polyvinyl alcohol as co-dispersing agent for an aqueous suspension comprising a mineral material and a dispersing agent, wherein the mineral material is selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, and the dispersing agent is a polyacrylate-containing dispersant.

2. The use of claim 1 , wherein the polyvinyl alcohol has a degree of hydrolysis of at least

75 mol-%, preferably at least 85 mol-%, more preferably at least 90 mol-%, even more preferably at least 95 mol-%, and most preferably at least 98 mol-%, and/or the polyvinyl alcohol has an average molecular weight M_w from 10 000 to 150 000 g/mol, preferably from 12 000 to 100 000 g/mol, more preferably from 14 000 to 50 000 g/mol, and most preferably from 16 000 to 30 000 g/mol.

3. The use of any one of the preceding claims, wherein the aqueous suspension comprises the polyvinyl alcohol in an amount of 2 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 1 .8 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, even more preferably in an amount from 0.08 to 1 wt.-%, and most preferably in amount from 0.1 to 0.6 wt.-%.

4. The use of any one of the preceding claims, wherein the dispersing agent is a polyacrylate homopolymer, a polyacrylate copolymer, or a mixture thereof, and preferably the dispersing agent is a polyacrylate homopolymer, preferably at least partially neutralized, preferably with sodium ions, potassium ions, lithium ions, ammonium ions, calcium ions, and mixtures thereof, more preferably with sodium ions, calcium ions, or mixtures thereof, and most preferably with sodium ions.

5. The use of any one of the preceding claims, wherein the dispersing agent has a neutralization degree of at least 30 mol-%, preferably at least 40 mol-%, more preferably at least 60 mol-%, and most preferably at least 80 mol-%, and/or the dispersing agent has an average molecular weight M_w from 1000 to 15 000 g/mol, preferably from 2000 to 12 000 g/mol, more preferably from 3000 to

11 000 g/mol, and most preferably from 4000 to 10 000 g/mol.

6. The use of any one of the preceding claims, wherein the aqueous suspension comprises the dispersing agent in an amount of 3 wt.-% or less, based on the total weight of the aqueous suspension, preferably in an amount from 0.01 to 2 wt.-%, more preferably in an amount from 0.05 to 1 .5 wt.-%, and most preferably in an amount from 0.1 to 0.8 wt.-%.

7. The use of any one of the preceding claims, wherein the mineral material is in the form of particles having a weight median particle size dso from 0.05 to 100 pm, preferably from 0.1 to 50 pm, more preferably from 0.15 to 25 pm, even more preferably from 0.25 to 10 pm, and most preferably from 0.5 to 2 pm, and/or having a weight determined top cut particle size dgs from 0.1 to 200 pm, preferably from 0.2 to 100 pm, more preferably from 0.3 to 50 pm, even more preferably from 0.25 to 20 pm, and most preferably from 1 to 4 pm.

8. The use of any one of the preceding claims, wherein the aqueous suspension comprises the mineral material in an amount from 10 to 78 wt.-%, preferably from 20 to 75 wt.-%, and most preferably from 50 to 72 wt.-%, based on the total weight of the aqueous suspension.

9. A method for producing an aqueous suspension comprising a mineral material and a dispersing agent in combination with a co-dispersing agent, comprising the steps of: i) providing a mineral material selected from an alkaline earth metal mineral material, clay, calcined clay, titanium dioxide, and mixtures thereof, ii) providing a dispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, iii) providing a co-dispersing agent, wherein the co-dispersing agent is a polyvinyl alcohol, iv) providing water, and v) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii) and the water of step iv).

10. The method of claim 9, wherein step v) comprises the steps of: a1) contacting the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv), and a2) adding the mineral material of step i) to the mixture obtained in step a1), or b1) contacting the dispersing agent of step ii), and the co-dispersing agent of step iii), and b2) adding the mineral material of step i) and water of step iv) to the mixture obtained in step b1), wherein the mineral material and the water are added separately or in combination, preferably in combination, or c1) contacting the dispersing agent of step ii), the mineral material of step i), and the water of step iv), and c2) adding the co-dispersing agent of step iii) to the mixture obtained in step c1), or d) contacting the mineral material of step i), the dispersing agent of step ii), the co-dispersing agent of step iii), and the water of step iv) simultaneously.

11 . The method of claim 9 or 10, wherein the method further comprises the step of grinding and/or dewatering the aqueous suspension during and/or after step v).

12. A method for producing an aqueous suspension comprising a mineral material, and a dispersing agent in combination with a co-dispersing agent, wherein the mineral material is precipitated calcium carbonate, and the method comprises the following steps:

I) providing a calcium oxide-containing material,

13. A dispersing agent composition comprising a dispersing agent in combination with a codispersing agent, wherein the dispersing agent is a polyacrylate-containing dispersant, and the codispersing agent is polyvinyl alcohol.

14. An aqueous suspension obtainable by a method according to any one of claims 9 to 12.

15. Use of an aqueous suspension according to claim 14 in paper applications, packaging applications, polymer applications, or water treatment applications.