WO2008125873A1

WO2008125873A1 - Grinding method

Info

Publication number: WO2008125873A1
Application number: PCT/GB2008/001366
Authority: WO
Inventors: John Claude Husband; Mikko Henrik Likitalo
Original assignee: Imerys Minerals Limited
Priority date: 2007-04-17
Filing date: 2008-04-17
Publication date: 2008-10-23
Also published as: GB0707457D0

Abstract

The present invention provides a method for grinding an inorganic particulate material in an aqueous suspension, wherein the suspension comprises a latex binder and the grinding process is carried out at a temperature greater than about 35°C.

Description

GRINDING METHOD

Field of the Invention

This invention relates to a method of grinding an aqueous suspension of an inorganic particulate material, products obtained thereby and to methods of making coating compositions and uses of said compositions.

Background of the Invention

Aqueous suspensions containing inorganic particulate material, for example an alkaline earth metal carbonate (e.g. calcium carbonate) or kaolin, are used widely in a number of applications. These include the production of pigment or filler containing compositions which may be used in paper manufacture or paper coating, and the production of filled compositions for paints, plastics and the like.

One important part of the preparation of inorganic particulate pigments is the grinding process. Ground calcium carbonate (GCC) is typically obtained by grinding a mineral source such as chalk, calcite, marble or limestone, which may be followed by a particle size classification step, in order to obtain a product having the desired degree of fineness. Similar grinding techniques are used for other inorganic particulate pigment materials.

In a wet grinding process, an aqueous slurry of course inorganic particulate pigment material may be ground using a stirred mill in the presence of a suitable grinding medium such as a ceramic grinding medium. At high solids, in particular, the temperature can increase considerably during the grinding process. Grinding is often undertaken at lower or reduced temperatures. Typically, lower or reduced temperatures at which grinding takes place are about 25°C.

Paper coating compositions, which are suitable for use in coating paper products, normally comprise one or more finely ground inorganic particulate pigments (such as hydrous or calcined kaolin, ground or precipitated calcium carbonate or titanium dioxide), one or more binders (such as starch or latex) and optional further additives. The term paper products, as used in connection with the present invention, should be understood to mean all forms of paper, including board, card, paperboard, and the like. The present invention seeks to provide alternative and/or improved methods for preparing coating compositions and which are, preferably, high solids containing coating compositions. The present invention also seeks to provide a more economical process for grinding inorganic particulate materials and wherein the properties of the resulting ground material are not significantly compromised. As such, the present inventors have surprisingly found that latex, typically in the form of an aqueous suspension can be combined with an inorganic particulate material, itself typically in the form of an aqueous slurry, before or during the grinding process, and the grinding process can be carried out at a high temperature and the process does not result in any significant deterioration in the performance of the resulting coating composition.

Summary of the Invention

According to the present invention, in a first aspect, a method of grinding an inorganic particulate material in an aqueous suspension, wherein the suspension comprises a latex binder and the grinding process is carried out at a temperature greater than about 35°C is provided.

According to a further aspect of the present invention, the resultant ground inorganic particulate material according to the first aspect of the present invention may be formed into a paper coating composition and according to a yet further aspect of the present invention a method of coating a paper product with said paper coating composition is provided.

The co-grinding of the inorganic particulate material and the latex binder allows the solids content of the coating suspension to be raised above that normally obtainable by conventional mixing of ground mineral slurries and binder dispersions. The solids content of the coating suspension, often referred to in the art as the "coating colour" may be above 70wt% and as high as 74wt%.

The grinding process may typically involve a high solids weight content aqueous sample of inorganic particulate material, such as a course ground calcium carbonate or kaolin slurry, and optionally comprising a dispersant, being introduced into a grinding mill along with an aqueous binder suspension, as well as an appropriate grinding medium, such as ceramic beads. The contents are then ground together until a required particle size is reached. During the process, the temperature may reach in excess of 8O⁰C and possibly 95°C, which is well in excess of the glass transition temperature of the latex binder.

Detailed Description of the Invention

The Inorganic Particulate Material

The inorganic particulate material may, for example, be an alkaline earth metal carbonate or sulphate, such as calcium carbonate, magnesium carbonate, dolomite, gypsum, a hydrous kandite clay such as kaolin or ball clay, an anhydrous (calcined) kandite clay such as metakaolin or fully calcined kaolin, talc or mica.

The preferred inorganic particulate material for use in the method according to the first aspect of the present invention is calcium carbonate. The particulate material used is preferably coarse ground calcium carbonate slurry which has a high solids concentration (between about 60 and 78 wt%). Hereafter, the invention may tend to be discussed in terms of calcium carbonate, and in relation to aspects where the calcium carbonate is processed and/or treated. The invention should not be construed as being limited to such embodiments.

The particulate calcium carbonate used in the present invention may be obtained from a natural source by grinding or may be prepared synthetically by precipitation (PCC), or may be a combination of the two, i.e. a mixture of the naturally derived ground material and the synthetic precipitated material. The PCC may also be ground.

Ground calcium carbonate (GCC) is typically obtained by grinding a mineral source such as chalk, marble or limestone, which may be followed by a particle size classification step, in order to obtain a product having the desired degree of fineness. The particulate solid material may be ground autogenously, i.e. by attrition between the particles of the solid material themselves, or alternatively, in the presence of a particulate grinding medium comprising particles of a different material from the calcium carbonate to be ground.

Wet grinding of calcium carbonate involves the formation of an aqueous suspension of the calcium carbonate which may then be ground, optionally in the presence of a suitable dispersing agent. Reference may be made to, for example, EP-A-614948 (the contents of which are incorporated by reference in their entirety) for more information regarding the wet grinding of calcium carbonate.

When the inorganic particulate material of the present invention is obtained from naturally occurring sources, it may be that some mineral impurities will inevitably contaminate the ground material. For example, naturally occurring calcium carbonate occurs in association with other minerals. Also, in some circumstances, minor additions of other minerals may be included, for example, one or more of kaolin, calcined kaolin, wollastonite, bauxite, talc or mica, could also be present. In general, however, the inorganic particulate material used in the invention will contain less than

5% by weight, preferably less than 1% by weight of other mineral impurities.

PCC may be used as the source of particulate calcium carbonate in the present invention, and may be produced by any of the known methods available in the art. TAPPI Monograph Series No 30, "Paper Coating Pigments", pages 34-35 describes the three main commercial processes for preparing precipitated calcium carbonate which is suitable for use in preparing products for use in the paper industry, but may also be used in the practice of the present invention. In all three processes, limestone is first calcined to produce quicklime, and the quicklime is then slaked in water to yield calcium hydroxide or milk of lime. In the first process, the milk of lime is directly carbonated with carbon dioxide gas. This process has the advantage that no byproduct is formed, and it is relatively easy to control the properties and purity of the calcium carbonate product. In the second process, the milk of lime is contacted with soda ash to produce, by double decomposition, a precipitate of calcium carbonate and a solution of sodium hydroxide. The sodium hydroxide must be substantially completely separated from the calcium carbonate if this process is to be commercially attractive. In the third main commercial process, the milk of lime is first contacted with ammonium chloride to give a calcium chloride solution and ammonia gas. The calcium chloride solution is then contacted with soda ash to produce, by double decomposition, precipitated calcium carbonate and a solution of sodium chloride.

The process for making PCC results in very pure calcium carbonate crystals and water. The crystals can be produced in a variety of different shapes and sizes, depending on the specific reaction process that is used. The three main forms of PCC crystals are aragonite, rhombohedral and scalenohedral, all of which are suitable for use in the present invention, including mixtures thereof. Following the grinding process, the inorganic particulate material may have a d₅o in the range of about 0.4 μm to about 1.5 μm. Preferably, the inorganic particulate material, following grinding has a d₅₀ of less than or equal to about 0.75 μm. Suitable commercially available calcium carbonates include Carbital 90^® and Carbilux^®.

Unless otherwise stated, the mean (average) equivalent particle diameter (d₅₀ value) and other particle size properties referred to herein for the inorganic particulate materials are as measured in a well known manner by sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a Sedigraph 5100 machine as supplied by Micromeritics Instruments Corporation, Norcross, Georgia, USA (telephone: +1 770 662 3620; web-site: www.micromeritics.com). referred to herein as a "Micromeritics Sedigraph 5100 unit". Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having a size, referred to in the art as the 'equivalent spherical diameter' (esd), less than given esd values. The mean particle size d₅₀ is the value determined in this way of the particle esd at which there are 50% by weight of the particles which have an equivalent spherical diameter less than that d₅₀ value.

The Binder

The latex binder may comprise, for example, one or more of the following: a styrene- butadiene rubber latex; an acrylic polymer latex, a polyvinyl acetate latex or a styrene acrylic copolymer latex. The latex, may, optionally, be carboxylated. Preferably the latex binder possesses a glass transition temperature of between about -1O⁰C and 4O⁰C as measured by differential scanning calorimetry. Preferably, the latex binder possesses a particle size between 80 and 200nm as measured by a dynamic light scattering method. Both of these methods of measurement are described in the article Fundamentals of Latex Technology by RT. Klun, published in the 1988 TAPPI Coating Binders Seminar, TAPPI Press Atlanta. Commercially available latex products are typically sold as aqueous emulsions containing 50wt% latex. These products are suitable for use in the present invention.

The binder may form 0.99% to 18%, e.g. 1% to 15.2%, by weight of the solids content of the composition during the grinding process. The Aqueous Suspension

The method according to the present invention is preferably carried out on an aqueous suspension which comprises up to about 78 wt% of the inorganic particulate material, typically of the order of about 65 to 75 wt%, for example about 68 to 72 wt%.

The amount of binder used in the method is preferably less than about 14% by weight, more preferably less than 12% by weight, typically less than about 11% by weight, based on the weight of dry inorganic particulate material. However, the precise amounts may be varied easily by those skilled in the art, to achieve the effect provided by the present invention.

The aqueous suspension may suitably be prepared by conventional mixing techniques, and may suitably include optional additional components, as will be well appreciated and understood by those skilled in this art.

Paper coating suspensions (commonly referred to as coating colours) are generally high solids dispersed systems. The solids content and viscosity of the coating colour when applied to the paper are significant. For example, if the solids content of the coating colour is too high, then the viscosity will be too high for easy metering and runnability problems may result, giving rise to so-called scratching and splashing. However, if the solids content is too low, the coating will flow too easily in the basepaper pores and poor coverage will result. In addition, there will be more water to remove thermally and the basepaper fibres may swell and reduce the quality further. At the desired levels of solids and viscosity, the coating does not penetrate into the pores, so it is held out onto the surface of the paper and is able to give a smooth surface. The runnable solids content in the colour solids is typically about 65 to 70wt%.

When calcium carbonate is ground at high solids, such as 78wt% solids, this gives rise to a broad particle size distribution (psd) which results in a high degree of packing of the particles. However, a broad psd gives a low pore size and volume in the dried coating, which, in turn, lowers the light scattering and hence the opacity (or hiding power) of the final coating. It is possible to address this problem by grinding at lower solids in order to provide a steeper particle size distribution. This gives rise to larger pores and thus increased scatter and opacity. However, as mentioned previously, there are other problems associated with low solids grinding. By co-grinding the inorganic particulate material, such as calcium carbonate, with latex according to the present invention, the solids content may be kept low enough to generate the desirable psd, which in turn will give rise to acceptable hiding power. However, the solids content will not be lowered to an unacceptable level by the subsequent addition of the binder dispersion which would ordinarily contribute significantly to the amount of water present.

The upper limit of colour solids present in the coatings composition, containing 11 parts per hundred of latex, is about 74wt% based on the combination of a 50wt% aqueous emulsion of latex added to a slurry of 78wt% ground calcium carbonate. In order to provide a runnable colour solids composition, e.g. about 65 to 70wt%, this particular example would typically require further water to be added. It is therefore preferred that the amount of ground calcium carbonate solids is, for example, around 72wt% which when co-ground with 11 parts per hundred of latex added as a 50wt% aqueous emulsion, will provide a colour solids of around 69wt% which is generally considered suitable as a runnable solids content.

The Grinding Process

The grinding is suitably performed in a conventional manner. The grinding may be an attrition grinding process in the presence of a particulate grinding medium, or may be an autogenous grinding process, i.e. one in the absence of a grinding medium.

The particulate grinding medium, when present, may be of a natural or a synthetic material. The grinding medium may, for example, comprise balls, beads or pellets of any hard mineral, ceramic or metallic material. Such materials may include, for example, alumina, zirconia, zirconium, silicate, aluminium silicate or the mullite-rich material which is produced by calcining kaolinitic clay at a temperature in the range of from about 1300⁰C to about 1800⁰C. Alternatively, particles of natural sand of a suitable particle size may be used.

Generally, the type of and particle size of grinding medium to be selected for use in the invention may be dependent on the properties, such as, e.g. the particle size of, and the chemical composition of, the feed suspension of inorganic material to be ground. Preferably, the particulate grinding medium comprises particles having an average diameter in the range of from about 0.1mm to about 6.0mm and, more preferably in the range of from about 0.2mm to about 4.0mm. The grinding medium (or media) may be present in an amount of from about 40% to about 70% by volume of the charge; and, more preferably in an amount from about 50% to about 60% by volume of the charge.

The grinding may be carried out in one or more stages. For example, the feed suspension may be partially ground in a first attrition grinder, the suspension of partially ground inorganic particulate material then being fed to a second attrition grinder for further grinding, after which the suspension of ground material may be fed to one or more subsequent attrition grinders.

After the grinding has been carried out, the suspension may be dewatered to a high solids suspension, and any grinding medium removed.

A high solids suspension formed by said dewatering may suitably have a solids level of the order of 72% by weight, and may be formed using a dispersing agent. The dispersing agent used may or may not be the same as that optionally used in the grinding step. However, the dispersing agent used at the post-grinding stage will be required to restrict flocculation of the particulate inorganic material in the high solids suspension, and will therefore be present in a dispersant-effective amount, typically at least about 0.3% by weight of dry inorganic particulate, more preferably at least about

0.4% by weight, for example at least about 0.5% by weight.

According to the present invention, high solids grinding is preferably carried out at a calcium carbonate solids content of up to about 78wt%. When grinding at high solids it is customary to use one or more dispersants. For example, in "Processing of Calcium Carbonates" by S. M. Fortier et al in the 36^th Annual Meeting of the Canadian Mineral Processors, held at Ottawa, 20-22 January 2004, and published by The Canadian Institute of Mining, Metallurgy and Petroleum, Paper 11 , pages 167 to 175, the contents of which are hereby incorporated by reference in their entirety, methods are described for grinding up to calcium carbonate solids contents of 78wt%. Also, US 6003795, the contents of which are also hereby incorporated by reference in their entirety, describes methods for grinding up to calcium carbonate solids contents of 78wt% comprising the use of water soluble polycarboxylate dispersing agent, wherein substantially all of the carboxylic acid groups of the polycarboxylate dispersing agent may be in a neutralised state. In particular, the methods described therein and which are useful in the present invention include those where different dispersing agents are used. The different agents may comprise a fully neutralised polycarboxylate salt and an acid form of polycarboxylate which may be added separately, one after the other, or in either order. The basic polymer or copolymer of the different dispersing agents may be the same in each case. Alternatively, a partially neutralised polycarboxylate for use as a dispersing agent may be added at one or more stages. Partially neutralised polycarboxyates are also described in JP55-40715, the contents of which are also incorporated herein, by reference, in their entirety. The degree of neutralisation of the partially neutralised agent may typically be about 30% or less. The partially and fully neutralised polycarboxylates may comprise the same basic polymer or copolymer, e.g. a sodium polyacrylate. The molecular weight of the sodium polyacrylate may be less than about 20,000 and preferably in the range 700 to 10,000, as measured by the method of gel permeation chromatography using a low angle laser light scattering detector. Typically, the aqueous suspension, during at least one grinding stage, has a pH in the range from 8.5 to 9.8.

US 5317053 and US 5084254, the contents of which are also hereby incorporated by reference in their entirety, describe suitable methods and dispersants for grinding calcium carbonate for solids contents up to 75wt%. Suitable dispersants for use in the present invention include an alkali metal, e.g. sodium, or ammonium salt of a poly(acrylic acid) or a poly(methacrylic acid) having, for example, a number average molecular weight not greater than about 10,000. The dispersant is typically present up to about 1wt%, for example, 0.05 to 1wt%, based on the dry weight of the inorganic particulate material.

The grinding process is carried out in the absence of any significant cooling. As such the temperature of the material being ground is greater than about 35°C, for example greater than about 40⁰C, for example greater than about 50⁰C or 60°C or 70⁰C or 80⁰C or 95°C. The temperature of the material being ground may be as high as about 100⁰C. Preferably, the temperature of the suspension will be above the glass transition temperature (Tg) of the latex which will mean the latex will be in a rubbery condition during the grinding process. This rubbery condition, however, does not impart adverse effects to the invention. The Coating Composition

The aqueous suspension produced in the grinding process according to the present invention, can be used as a coating composition without the addition of further additives. However, optionally, a small amount of thickener such as carboxymethyl cellulose may be added.

The coating composition according to the present invention may contain one or more optional additional components, if desired. Such additional components, where present, are suitably selected from known additives for paper coating compositions. Some of these optional additives may provide more than one function in the coating composition. Examples of known classes of optional additives are as follows:

(a) one or more additional pigments: the inorganic particulate materials described earlier in the present invention may be used as sole pigments in the paper coating compositions, or may be used in conjunction with one another or with other known pigments, such as for example, calcium sulphate, satin white, and so called 'plastic pigment'. When a mixture of pigments is used the total pigment solids content is preferably present in the composition in an amount of at least about 75wt% of the total weight of the total coating composition;

(b) one or more binding or cobinding agents: for example, latex, which may, optionally, be carboxylated, including: a styrene-butadiene rubber latex; an acrylic polymer latex; a polyvinyl acetate latex; or a styrene acrylic copolymer latex, starch derivatives, sodium carboxymethyl cellulose, polyvinyl alcohol, proteins; (c) one or more cross linkers: e.g. in levels of up to about 5% by weight; for example glyoxals, melamine formaldehyde resins, ammonium zirconium carbonates; one or more dry or wet pick improvement additives: e.g. in levels up to about 2% by weight, for example melamine resin, polyethylene emulsions, urea formaldehyde, melamine formaldehyde, polyamide, calcium stearate, styrene maleic anhydride and others; one or more dry or wet rub improvement and/or abrasion resistance additives: e.g. in levels up to about 2% by weight, for example glyoxal based resins, oxidised polyethylenes, melamine resins, urea formaldehyde, melamine formaldehyde, polyethylene wax, calcium stearate and others; one or more water resistance additives: e.g. in levels up to about 2% by weight, e.g. oxidised polyethylenes, ketone resin, anionic latex, polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde, melamine formaldehyde, polyamide, glyoxals, stearates and other materials commercially available for this function;

(d) one or more water retention aids: e.g. in levels up to about 2% by weight, for example sodium carboxymethyl cellulose, hydroxyethyl cellulose, PVOH (polyvinyl alcohol), starches, proteins, polyacrylates, gums, alginates, polyacrylamide bentonite and other commercially available products sold for such applications;

(e) one or more viscosity modifiers and/or thickeners: e.g. in levels up to about 2% by weight; for example acrylic associative thickeners, polyacrylates, emulsion copolymers, dicyanamide, triols, polyoxyethylene ether, urea, sulphated castor oil, polyvinyl pyrrolidone, CMC (carboxymethyl celluloses, for example sodium carboxymethyl cellulose), sodium alginate, xanthan gum, sodium silicate, acrylic acid copolymers, HMC (hydroxymethyl celluloses), HEC (hydroxyethyl celluloses) and others;

(f) one or more lubricity/calendering aids: e.g. in levels up to about 2% by weight, for example calcium stearate, ammonium stearate, zinc stearate, wax emulsions, waxes, alkyl ketene dimer, glycols; one or more gloss-ink hold-out additives: e.g. in levels up to about 2% by weight, for example oxidised polyethylenes, polyethylene emulsions, waxes, casein, guar gum, CMC, HMC, calcium stearate, ammonium stearate, sodium alginate and others; (g) one or more dispersants: the dispersant is a chemical additive capable, when present in a sufficient amount, of acting on the particles of the particulate inorganic material to prevent or effectively restrict flocculation or agglomeration of the particles to a desired extent, according to the normal processing requirements. The dispersant may be present in levels up to about 1% by weight, and includes, for example, polyelectrolytes such as polyacrylates and copolymers containing polyacrylate species, especially polyacrylate salts (e.g. sodium and aluminium optionally with a group Il metal salt), sodium hexametaphosphates, non-ionic polyol, polyphosphoric acid, condensed sodium phosphate, non-ionic surfactants, alkanolamine and other reagents commonly used for this function. The dispersant may, for example, be selected from conventional dispersant materials commonly used in the processing and grinding of inorganic particulate materials. Such dispersants will be well recognised by those skilled in this art. They are generally water-soluble salts capable of supplying anionic species which in their effective amounts can adsorb on the surface of the inorganic particles and thereby inhibit aggregation of the particles. The unsolvated salts suitably include alkali metal cations such as sodium. Solvation may in some cases be assisted by making the aqueous suspension slightly alkaline. Examples of suitable dispersants include: water soluble condensed phosphates, e.g. polymetaphosphate salts [general form of the sodium salts: (NaPO₃)χ] such as tetrasodium metaphosphate or so-called "sodium hexametaphosphate" (Graham's salt); water-soluble salts of polysilicic acids; polyelectrolytes; salts of homopolymers or copolymers of acrylic acid or methacrylic acid, or salts of polymers of other derivatives of acrylic acid, suitably having a weight average molecular mass of less than about 20,000. Sodium hexametaphosphate and sodium polyacrylate, the latter suitably having a weight average molecular mass in the range of about 1,500 to about 10,000, are especially preferred; (h) one or more antifoamers/defoamers: e.g. in levels up to about 1% by weight, for example blends of surfactants, tributyl phosphate, fatty polyoxyethylene esters plus fatty alcohols, fatty acid soaps, silicone emulsions and other silicone containing compositions, waxes and inorganic particulates in mineral oil, blends of emulsified hydrocarbons and other compounds sold commercially to carry out this function; (i) one or more optical brightening agents (OBA) and/or fluorescent whitening agents (FWA): e.g. in levels up to about 1% by weight, for example stilbene derivatives;

(j) one or more dyes: e.g. in levels up to about 0.5% by weight; (k) one or more biocides/spoilage control agents: e.g. in levels up to about 1% by weight, for example metaborate, sodium dodecylbenene sulphonate, thiocyanate, organosulphur, sodium benzoate and other compounds sold commercially for this function e.g. the range of biocide polymers sold by Nalco;

(I) one or more levelling and evening aids: e.g. in levels up to about 2% by weight, for example non-ionic polyol, polyethylene emulsions, fatty acid, esters and alcohol derivatives, alcohol/ethylene oxide, calcium stearate and other compounds sold commercially for this function;

(m) one or more grease and oil resistance additives: e.g. in levels up to about 2% by weight, e.g. oxidised polyethylenes, latex, SMA (styrene maleic anhydride), polyamide, waxes, alginate, protein, CMC, HMC.

Any of the above additives and additive types may be used alone or in admixture with each other and/or with other additives, if desired.

For all of the above additives, the percentages by weight quoted are based on the dry weight of inorganic particulate material (100%) present in the composition. Where the additive is present in a minimum amount, the minimum amount may be about 0.01% by weight based on the dry weight of pigment. The Coating Process

The coating process is carried out using standard techniques which are well known to the skilled person. The coating process may also involve calendaring or supercalendering the coated product.

Methods of coating paper and other sheet materials, and apparatus for performing the methods, are widely published and well known. Such known methods and apparatus may conveniently be used for preparing coated paper. For example, there is a review of such methods published in Pulp and Paper International, May 1994, page 18 et seq. Sheets may be coated on the sheet forming machine, i.e., "on-machine," or "off- machine" on a coater or coating machine. Use of high solids compositions is desirable in the coating method because it leaves less water to evaporate subsequently. However, as is well known in the art, the solids level should not be so high that high viscosity and leveling problems are introduced. The methods of coating may be performed using apparatus comprising (i) an application for applying the coating composition to the material to be coated; and (ii) a metering device for ensuring that a correct level of coating composition is applied. When an excess of coating composition is applied to the applicator, the metering device is downstream of it. Alternatively, the correct amount of coating composition may be applied to the applicator by the metering device, e.g., as a film press. At the points of coating application and metering, the paper web support ranges from a backing roll, e.g., via one or two applicators, to nothing (i.e., just tension). The time the coating is in contact with the paper before the excess is finally removed is the dwell time - and this may be short, long or variable.

The coating is usually added by a coating head at a coating station. According to the quality desired, paper grades are uncoated, single coated, double coated and even triple coated. When providing more than one coat, the initial coat (precoat) may have a cheaper formulation and optionally coarser pigment in the coating composition. A coater that is applying coating on each side of the paper, will have two or four coating heads, depending on the number of coating layers applied on each side. Most coating heads coat only one side at a time, but some roll coaters (e.g., film press, gate roll, size press) coat both sides in one pass.

Examples of known coaters which may be employed include, without limitation, air knife coaters, blade coaters, rod coaters, bar coaters, multi-head coaters, roll coaters, roll/blade coaters, cast coaters, laboratory coaters, gravure coaters, kisscoaters, liquid application systems, reverse roll coaters, curtain coaters, spray coaters and extrusion coaters.

Water may be added to the solids comprising the coating composition to give a concentration of solids which is preferably such that, when the composition is coated onto a sheet to a desired target coating weight, the composition has a rheology which is suitable to enable the composition to be coated with a pressure (e.g. a blade pressure) of between 1 and 1.5 bar.

Calendering is a well known process in which paper smoothness and gloss is improved and bulk is reduced by passing a coated paper sheet between calender nips or rollers one or more times. Usually, elastomer coated rolls are employed to give pressing of high solids compositions. An elevated temperature may be applied. One or more (e.g. up to about 12, or sometimes higher) passes through the nips may be applied.

Examples

Embodiments of the present invention will now be described by way of example only, with reference to the following examples.

Test Methods

Brookfield Viscosity

The Brookfield viscosity values were measured at ambient temperature (22°C) using a Brookfield Viscometer set to operate at a spindle speed of 100rpm. The suspension was thoroughly mixed using a Heidolph ST-1 laboratory stirrer. Immediately after mixing, the suspension was transferred to the viscometer. The viscometer spindle was immersed in the suspension. The viscometer spindle was activated 30 seconds after cessation of homogenisation and the viscosity was recorded a further 15 seconds later.

High Shear Viscosity

The high shear viscosity was measured on a Bohlin CSR rheometer using a 0.5° cone at 12 ks^'1. Water Retention

Water retention was measured using a Gradek water retention meter using an applied pressure of 2 bar and a time of 60 seconds.

ISO Brightness

The ISO brightness of coated paper was measured by means of an Elrepho Datacolour 2000™ brightness meter fitted with a No. 8 filter (457nm wavelength), according to ISO 2470: 1999 E.

Gloss

The gloss of a coated paper surface may be measured by means of a test laid down in TAPPI Standard No 480 ts-65. The intensity of light reflected at an angle from the surface of the paper is measured and compared with a standard of known gloss value. The beams of incident and reflected light are both at an angle of 75° to the normal to the paper surface. The results are expressed in TAPPI gloss units.

Smoothness

The Parker Print Surf ("PPS") test provides a measure of the smoothness of a paper surface, and comprises measuring the rate at which air under pressure leaks from a sample of the coated paper which is clamped, under a known standard force (1000 kPa), between an upper plate which incorporates an outlet for the compressed air and a lower plate, the upper surface of which is covered with a sheet of either a soft or a hard reference supporting material according to the nature of the paper under test. From the rate of escape of the air, a root mean cube gap in μm between the paper surface and the reference material is calculated. A smaller value of this gap represents a higher degree of smoothness of the surface of the paper under test.

Opacity

Opacity, as used herein, is a measure of percent reflectance of incident light from a coated substrate. The standard test method is ISO 2471. The opacity of a sample of paper can be measured by means of an Elrepho Datacolor 3300 spectro-photometer using a wavelength appropriate to opacity measurement. First, a measurement of the percentage of the incident light reflected is made with a stack of at least ten sheets of paper over a black cavity (Rinfinity). The stack of sheets is then replaced with a single sheet of paper, and a second measurement of the percentage reflectance of the single sheet on the black cover is made (R). The percentage opacity is then calculated from the formula:

Percentage opacity = 100 x R/Rinfinity.

Offset Printing

Samples of the coated and calendered papers were printed with a sheetfed magenta ink on a IGT laboratory printing unit. 0.3 cm³ of ink was used and applied at a printing speed of 0.5 ms^~1 and a pressure of 500N. A portion of the paper was predamped with 1 gnrf² of water using a condensation method. Print gloss and print density were measured on the undamped (dry) and damped (litho) regions of the paper.

Print Gloss

The print gloss of a printed paper surface is measured with a Hunter glossmeter according to TAPPI Standard No 480 ts-65. The intensity of light reflected at an angle from the surface of the paper is measured and compared with a standard known gloss value. The beams of incident and reflected light are both at an angle of 20 degrees or 75 degrees to the normal to the paper surface. The results are expressed in TAPPI gloss units.

Print Density

Print density was measured on the same printed samples as were used for print gloss using a Gretag SpectroEye® densitometer.

Wet Pick

The wet pick strength was measured using a Prufbau inking unit with predamping using a 4 second delay before printing with a standard ink. The printing speed was accelerated from 0 to 3 ms^"1 and the results quoted as the speed at which failure of the coating or the basepaper is first noted.

Dry Pick

The dry pick strength of the coated strips was measured using an IGT inking unit and a standard oil of viscosity 17.5 Pa. s. The printing speed was accelerated from 0 to 6 m s^"1 and the point on the strip where the coating or basepaper began to fail was noted and reported as a pick velocity.

Specific Gravity

The specific gravity was measured by weighting a known volume (100cm³) of slurry.

Materials

In Examples 1 to 7, the source of calcium carbonate sample used was Carbital 60^® which is a ground calcium carbonate commercially available from Imerys. The particle size distribution of the sample was such that 60wt% of the dry material possesses an esd of < 2 μm. The average particle size (esd) (dso) was 1.5 μm.

Carbolite™ 16/20 is a ceramic grinding media commercially available from Carboceramics.

DL966™ is a carboxylated styrene-butadiene latex emulsion of Tg = 20⁰C and particle size between 130 and 160nm commercially available from Dow Chemical.

Polyacrylate dispersant (Jaypol 1183™), is commercially available from M and J Polymers.

Polyacrylate dispersant (PA30™) is commercially available from BASF.

Sodium carboxymethyl cellulose CMC (Finnfix 10™) is commercially available from CP Kelco. Antifoamer RD™ is commercially available from Dow Corning and is a silicone emulsion.

Example 1 is present for the purposes of comparison and assists in illustrating the effect of the presence of the latex binder during the grinding process.

Example 1

7.75 kg of a slurry of ground calcium carbonate (Carbital 60^®) at 78.3wt% solids, based on the total weight of the slurry, was prepared. The slurry was introduced into a grinding vessel and 13 kg of ceramic grinding media (Carbolite™ 16/20,

Carboceramics) was added. The mixture was stirred at 400 rpm until an energy input of

114 kWh t^"1 had been achieved. During the grind, 0.20 wt% of a polyacrylate dispersant (Jaypol 1183™) was added to maintain fluidity. The final temperature of the mixture immediately after grinding was 70⁰C. The grinding media was separated from the slurry, which had a particle size distribution of 92wt% < 2 μm, 23wt% < 0.25 μm and a solids content of 75.7 wt%.

Example 2

7.75 kg of the same Carbital 60^® slurry as described in Example 1 was weighed into the grinding vessel and 720 g of DL966™ styrene-butadiene latex emulsion was added to give a dose of 6 pph (expressed as dry latex on dry carbonate). 13.8 kg of ceramic grinding media (Carbolite™ 16/20) was added and the vessel was stirred until the energy input had reached 114 kWh t^"1. During the grind, 0.20 wt% of a polyacrylate dispersant (Jaypol 1183™) was added to maintain fluidity. The media was separated from the slurry and the solids content of the slurry measured as 78.5 wt%. This is 3 solids units higher than could have been prepared by mixing 78 wt% ground calcium carbonate slurry with 50 wt% latex emulsion. The slurry had a particle size distribution wherein 79 wt% < 2 μm, and 14wt% < 0.25 μm.

Example 3

7.75 kg of the same Carbital 60^® slurry as described in Example 1 was weighed into the grinding vessel and 720 g of DL966 latex emulsion (which is equivalent to 6 pph, expressed as dry latex on dry carbonate) was added. 13.7 kg of ceramic grinding media (Carbolite™ 16/20) was added and the vessel was stirred until a work input of 270 kWh f¹ had been reached. The temperature of the mixture rose to 73°C. Water and dispersant (Jaypol 1183™) were added during the grinding to maintain fluidity and, after separation from the media, the solids content of the slurry was 68 wt% and the particle size distribution was 89 wt% < 2 μm and 18wt% < 0.25 μm.

Example 4

7.75 kg of Carbital 60^® was weighed into the grinding vessel and 13 kg of ceramic grinding media (Carbolite™ 16/20) was added. The vessel was stirred until a work input of 82 kWh f¹ had been reached. At this stage the particle size distribution was 86 wt% < 2 μm. 720 g of DL966™ latex emulsion was then added (equivalent to 6 pph expressed as dry latex on dry carbonate) together with 700 g Carbolite™ grinding media and the grind restarted. A further energy input of 20 kWh t^"1 was added and the product separated from the grinding media. The final temperature of the mixture was 68°C. The solids content of the slurry was 74 wt% and the particle size distribution was 91 wt% < 2 μm and 22wt% < 0.25 μm.

Example 5

5.3 kg of the same Carbital 60^® slurry as described in Example 1 was weighed into the grinding vessel and 660 g of DL966™ (Dow Chemical) styrene-butadiene latex emulsion (glass transition temperature = 20⁰C, particle size 140 nm, gel content 78%) was added to give a dose of 8 pph (expressed as dry latex on dry carbonate). 8.8 kg of ceramic grinding media (Carbolite™ 16/20) was added and the vessel was stirred until the energy input had reached 150 kWh f¹. During the grind, 0.20 wt% of a polyacrylate dispersant (PA30™) was added to maintain fluidity. After grinding, the temperature of the grind was 48°C. The media was separated from the slurry and the solids content of the slurry measured as 72.3 wt%. The particle size was 88 wt% < 2μm and 12.5 wt% < 0.25 μm. The specific gravity of the suspension was measured as 1.026 g cm^"3. This is lower than the density calculated from the measured solids content, which is 1.733 g cm^"3. This density difference is believed to be due to entrained air.

Example 6

5.3 kg of the same Carbital 60^® slurry as described in Example 1 was weighed into a water cooled grinding vessel and 660 g of DL966™ (Dow Chemical) styrene-butadiene latex emulsion (glass transition temperature = 20⁰C, particle size 140 nm, gel content 78%) was added to give a dose of 8 pph (expressed as dry latex on dry carbonate). 8.8 kg of ceramic grinding media (Carbolite™ 16/20) was added and the vessel was stirred until the energy input had reached 182 kWh t^"1. During the grind, 0.20 wt% of a polyacrylate dispersant (PA30™) was added to maintain fluidity. After grinding, the temperature of the grind was 26°C. The particle size was 88.7 wt% < 2μm and 17.2 wt% < 0.25 μm. The media was separated from the slurry and the solids content of the slurry measured as 71.3 wt%. The specific gravity of the suspension was measured as 0.846 g cm^"3. This is lower than the density calculated from the measured solids content, which is 1.714 g cm^"3. This density difference indicates that this sample has a high level of entrained air.

Example 7

5.3 kg of the same Carbital 60^® slurry as described in Example 1 was weighed into a grinding vessel and 660 g of DL966™ (Dow Chemical) styrene-butadiene latex emulsion (glass transition temperature = 20⁰C, particle size 140 nm, gel content 78%) was added to give a dose of 8 pph (expressed as dry latex on dry carbonate). 8.3 g (0.2 pph) of Antifoamer RD™ (Dow Chemical) was also added, followed by 8.8 kg of ceramic grinding media (Carbolite™ 16/20) and the vessel was stirred until the energy input had reached 182 kWh t^"1. During the grind, 0.20 wt% of a polyacrylate dispersant (PA30™) was added to maintain fluidity. After grinding, the temperature of the grind was 49°C. The particle size was 90 wt% < 2μm and 17.7 wt% < 0.25 μm. The media was separated from the slurry and the solids content of the slurry measured as 74.5 wt%. The specific gravity of the suspension was measured as 1.312 g cm^"3. This is lower than the density calculated from the measured solids content, which is 1.77 g cm^" ³. This density difference shows that this sample has a lower level of entrained air than Examples 5 and 6.

Example 8 - Preparation of Coating Compositions

1.2 kg (dry weight) aliquots of the mineral slurry from Example 1 were mixed with 8 and 10 pph of DL 966™ latex. 1 pph of sodium carboxymethyl cellulose CMC (Finnfix 10™) was added. Aliquots of the products from Examples 3 and 4 containing 1.2 kg dry calcium carbonate were weighed and 2 and 4 pph DL966™ latex added, so that the total binder levels present were the same as with Sample 1. 1 pph of CMC (Finnfix 10™) was added to each of these.

Aliquots of the products from Examples 5 to 7 containing 1.2 kg dry calcium carbonate were weighed and no further latex was added so that the levels of total binder present were 8 pph. 1 pph of CMC (Finnfix 10™) was added to each of these.

The coating colours were applied to a woodfree basepaper (60 grrf²) using a cylindrical laboratory coater (Heli-Coater™) at a speed of 600 m min^"1 and the blade extension varied to obtain coat weights of between 8 and 12 grrf². The solids and the viscosity of the coating colours as applied are listed in Table 1.

Table 1 : Coating colour rheological properties

B'field = Brookfield viscosity at 100 rpm; h/s = high shear viscosity measured on Bohlin CSR rheometer at 12 ks^"1.

The coated sheets were calendered using a Perkins laboratory supercalendar. The pressure was 68 bar and the temperature was 65⁰C. The coated strips were passed through the roll 10 times. The optical properties (75° gloss, brightness and opacity) of the calendered coated papers were measured, together with the surface smoothness using a Parker Print-Surf instrument. The results are summarised in Table 2.

Table 2: Coated and calendered sheet properties

Example 9 - Printing Performance

The coated sheets were laboratory offset printed using an IGT inking unit and a standard quickset ink. The speed was 0.5 m s^"1 at a pressure of 500 N. A part of the strip was predamped using a condensation method. The print gloss and print density were measured on both the dry and predamped regions.

The dry pick strength of the coated strips was measured using an IGT inking unit and a standard oil of viscosity 17.5 Pa. s. The printing speed was accelerated from 0 to

6 m s^'1 and the point on the strip where the coating or basepaper began to fail was noted and reported as a pick velocity. The wet pick strength was measured using a

Prufbau inking unit with predamping using a 4 s delay before printing with a standard ink. The printing speed was accelerated from 0 to 3 ms^"1 and the results quoted as the speed at which failure of the coating or the basepaper is first noted. The print evaluation results are summarised in Table 3. Table 3: Offset printing evaluation

Discussion

The results indicate that the coating strength achieved using the methods according to the present invention, is comparable to that of the conventionally prepared control. The rubbery condition of the latex binder, due to the grinding process being carried out above the Tg of the binder, has not adversely affected the binding ability. Cooling the grind to reduce the temperature gives no discernible differences in behaviour.

Claims

1. A method for grinding an inorganic particulate material in an aqueous suspension, wherein the suspension comprises a latex binder and the grinding process is carried out at a temperature greater than about 35°C.

2. A method according to claim 1, wherein the temperature of the suspension during grinding is above the glass transition temperature (Tg) of the latex binder.

3. A method according to claim 1 , wherein the temperature is greater than about 40⁰C.

4. A method according to claim 1 , wherein the temperature is greater than about 60⁰C.

5. A method according to claim 1 , wherein the temperature is greater than about 80⁰C.

6. A method according to claim 1 , wherein the temperature is greater than about 95°C.

7. A method according to any one of the previous claims, wherein the upper limit of the temperature is about 100⁰C.

8. A method according to any one of the previous claims, wherein the inorganic particulate material is selected from one or more of an alkaline earth metal carbonate or sulphate such as calcium carbonate, magnesium carbonate, dolomite, gypsum, a hydrous kandite clay such as kaolin or ball clay, an anhydrous calcined kandite clay such as metakaolin or fully calcined kaolin, talc or mica.

9. A method according to the previous claim, wherein the inorganic particulate material is calcium carbonate.

10. A method according to the previous claim, wherein the calcium carbonate is natural calcium carbonate, or precipitated calcium carbonate (PCC).

11. A method according to any one of the previous claims wherein the d₅₀ is in the range of about 0.4 μm to about 1.5 μm.

12. A method according to any one of the previous claims, wherein the latex binder is selected from one or more of the following: a styrene-butadiene rubber latex; an acrylic polymer latex, a polyvinyl acetate latex or a styrene acrylic copolymer latex.

13. A method according to any one of the previous claims, wherein the binder is in the form of an aqueous emulsion comprising 50wt% latex.

14. A method according to any one of the previous claims, wherein the latex binder possesses a glass transition temperature of between about -1O⁰C and 4O⁰C.

15. A method according to any one of the previous claims, wherein the aqueous suspension comprises between about 60 to 78wt% of the inorganic particulate material and between about 0.99wt% to about 18wt% of latex binder.

16. A method according to any one of the previous claims, wherein the amount of binder present is less than about 14wt%.

17. A method according to any one of the previous claims, wherein during grinding the aqueous suspension further comprises at least one dispersant.

18. A method according to the previous claim, wherein the at least one dispersant is a polyacrylate or polymethacrylate.

19. A method according to the previous claim, wherein the dispersant is sodium polyacrylate.

20. A method according to any one of the previous claims, wherein the ground aqueous suspension is formed into a paper coating composition.

21. A method according to the previous claim, wherein the suspension and any grinding media present are separated and, optionally, the suspension is dewatered or ddiilluutteedd iinn oorrddeerr ttoo ffoorrmm ssaaiidd ppaappeerr ccooaattiinngg ccoommppoossiittiioonn..

22. A method according to claim 20 or 21, wherein the total solids content of the coating composition is about 65wt% to about 74wt%.

23. A method according to the previous claim wherein the total solids content of the coating composition is about 69wt%

24. A method according to any one of claims 20 to 23, wherein the coating composition further comprises a thickener.

25. A method according to the previous claim wherein the thickener comprises, consists essentially of, or consists of carboxymethyl cellulose.

26. A method according to any one of claims 20 to 25, wherein the coating composition is combined with one or more additives.

27. A method according to the previous claim, wherein the one or more additives are selected from one or more cross linkers; one or more water retention aids; one or more viscosity modifiers and/or thickeners; one or more lubricity/calendering aids; one or more dispersants; one or more antifoamers/defoamers; one or more dry or wet pick improvement additives; one or more dry or wet rub improvement and/or abrasion resistance additives; one or more gloss-ink hold-out additives; one or more optical brightening agents (OBA) and/or fluorescent whitening agents (FWA); one or more dyes; one or more biocides/spoilage control agents; one or more levelling and evening aids; one or more grease and oil resistance additives; one or more water resistance additives; one or more additional pigments; one or more additional binding or cobinding agents; or any combination thereof.

28. A method for coating a paper product comprising coating said paper product with a composition made according to any one of claims 20 to 27.

29. A method according to the previous claim, further comprising calendaring the coated paper product.

30. A method according to claim 28 or 29, wherein the paper product is selected from paper, board, card, paperboard.

31. A method for grinding particulate calcium carbonate in an aqueous suspension, wherein the suspension comprises a latex binder and the grinding process is carried out at a temperature greater than about 35⁰C.

32. A method according to claim 31 , wherein the temperature of the suspension during grinding is above the glass transition temperature (Tg) of the latex binder.

33. A method according to claim 31, wherein the temperature is greater than about 40⁰C.

34. A method according to claim 31 , wherein the temperature is greater than about 60°C.

35. A method according to claim 31 , wherein the temperature is greater than about 80⁰C.

36. A method according to claim 31 , wherein the temperature is greater than about 95°C.

37. A method according to any one of claims 31 to 36, wherein the upper limit of the temperature is about 100⁰C.

38. A method according to any one of claims 31 to 37, wherein the calcium carbonate is natural calcium carbonate, or precipitated calcium carbonate (PCC).

39. A method according to the previous claim wherein the d₅₀ is in the range of about 0.4 μm to about 1.5 μm.

40. A method according to any one of claims 31 to 39, wherein the latex binder is selected from one or more of the following: a styrene-butadiene rubber latex; an acrylic polymer latex, a polyvinyl acetate latex or a styrene acrylic copolymer latex.

41. A method according to any one of claims 31 to 40, wherein the binder is in the form of an aqueous emulsion comprising 50wt% latex.

42. A method according to any one of claims 31 to 41 , wherein the latex binder possesses a glass transition temperature of between about -1O⁰C and 4O⁰C.

43. A method according to any one of claims 31 to 42, wherein the aqueous suspension comprises between about 60 to 78wt% of the calcium carbonate and between about 0.99wt% to about 18wt% of latex binder.

44. A method according to any one of claims 31 to 43, wherein the amount of binder present is less than about 14wt%.

45. A method according to any one of claims 31 to 44, wherein during grinding the aqueous suspension further comprises at least one dispersant.

46. A method according to the previous claim, wherein the at least one dispersant is a polyacrylate or polymethacrylate.

47. A method according to the previous claim, wherein the dispersant is sodium polyacrylate.

48. A method according to any one of claims 31 to 47, wherein the ground aqueous suspension is formed into a paper coating composition.

49. A method according to the previous claim, wherein the suspension and any grinding media present are separated and, optionally, the suspension is dewatered or diluted in order to form said paper coating composition.

50. A method according to claim 48 or 49, wherein the total solids content of the coating composition is about 65wt% to about 74wt%.

51. A method according to the previous claim wherein the total solids content of the coating composition is about 69wt%

52. A method according to any one of claims 48 to 51 , wherein the coating composition further comprises a thickener.

53. A method according to the previous claim wherein the thickener comprises, consists essentially of, or consists of carboxymethyl cellulose.

54. A method according to any one of claims 48 to 53, wherein the coating composition is combined with one or more additives.

55. A method according to the previous claim, wherein the one or more additives are selected from one or more cross linkers; one or more water retention aids; one or more viscosity modifiers and/or thickeners; one or more lubricity/calendering aids; one or more dispersants; one or more antifoamers/defoamers; one or more dry or wet pick improvement additives; one or more dry or wet rub improvement and/or abrasion resistance additives; one or more gloss-ink hold-out additives; one or more optical brightening agents (OBA) and/or fluorescent whitening agents (FWA); one or more dyes; one or more biocides/spoilage control agents; one or more levelling and evening aids; one or more grease and oil resistance additives; one or more water resistance additives; one or more additional pigments; one or more additional binding or cobinding agents; or any combination thereof.

56. A method for coating a paper product comprising coating said paper product with a composition made according to any one of claims 48 to 55.

57. A method according to the previous claim, further comprising calendaring the coated paper product.

58. A method according to claim 56 or 57, wherein the paper product is selected from paper, board, card, paperboard.