NO180598B - Aqueous paper coating composition, preparation thereof and preparation of paper - Google Patents

Abstract

There is disclosed an aqueous paper coating composition which comprises at least 45 % by weight of a particulate pigment dispersed with a dispersing agent, and an adhesive; characterised in that said dispersing agent comprises an anionic polyelectrolyte and a cationic polyelectrolyte, the cationic polyelectrolyte being present in an amount sufficient to render the particles cationic, in that said adhesive is a cationic or non-ionic adhesive and in that said particulate pigment is one which is not capable of being dispersed in water at high solids, and following vigorous mixing, in the sole presence of said cationic polyelectrolyte. Also disclosed is a method for making the paper coating composition, a method for coating a paper with a paper coating composition and the resulting coated paper. The coated paper of the invention is particularly suited to recycling.

Description

The present invention relates to a paper coating mixture as specified in claim 1's preamble, a method as specified in claims 12 - 15 for producing a paper coating mixture, and a method for coating paper with a paper coating mixture as specified in claim 16. The coated paper can be used as " Waste paper" in the paper-making process. "Recycled paper" is the term used for paper, cardboard and the like that is recycled.

Calcium carbonate is known as a paper coating pigment and because it normally carries a positive charge it is conventionally dispersed with an anionic dispersant. Other paper coating pigments that carry a neutral or positive charge exist, such as gypsum, talc, calcined kaolin clay, these too must be dispersed using an anionic dispersant (these materials are also recognized to be deficient in negative positions).

A full discussion of the composition of paper coating compositions and methods of applying such compositions to paper is given in Chapter XIX, Volume III of the second edition of the book by James P. Casey entitled "Pulp and Paper: Chemistry and Technology". A further discussion is also found in "An Operator's Guide to Aquous Coating for Paper and Board", edited by T.W.R. Dean, The British Paper and Board Industry Federation, London, 1979.

DE-3707221 and EP-0307795 disclose a cationic pigment dispersion. The pigment is first given a protective colloidal coating using a cationic polymer and then, under certain conditions, dispersed with a cationic polymer.

TAPPI, Vol. 65, No. 4, April 1982, pages 123 - 125, Atlanta, Georgia, U.S.A.; A.J. Sharpe, Jr. et al.: "Impro-ved Cationic Conductive Polymer Displays Outstanding Filmability" describes a polysalt formed by the interaction of a strong cationic polymer, such as a polydiallyldimethylammonium chloride, and a weak anionic polymer, such as polyacrylic acid. The polysalt thus obtained is added in significant quantities (of the order of 50% by weight, calculated on the weight of the pigment) to a pre-dispersed pigment solution with a low content of solids, in order to provide a conductive coating mass which is used in the production of paper with a conductive surface .

According to a first feature of the present invention, an aqueous paper coating mixture is provided which is characterized by what is stated in the characterizing part of claim 1, namely that the dispersant comprises an anionic polyelectrolyte and a cationic polyelectrolyte, the cationic polyelectrolyte being present in an amount sufficient to render the pigment particles cationic, the binder is a cationic or nonionic binder, and the particulate pigment is one which, in the mere presence of the cationic polyelectrolyte, is not capable of being dispersed in high solids water with subsequent strong stirring, further features appear in claims 2 - 11.

According to one feature of the present invention, a method for coating paper is provided, comprising coating the paper sheet with a paper coating mixture according to the first feature of the invention.

The coated paper can advantageously be recycled in the papermaking process.

The particulate pigment used according to

the invention is one that is not capable of being dispersed in water to high solids contents (such as greater than 60% by weight) and after vigorous mixing (for example sufficient to release at least 10 KJ of energy per kg of pigment), in only

presence of the cationic polyelectrolyte. This means that the pigment surface should have a neutral or a total positive charge. This is, for example, the case for inorganic pigments such as calcium carbonate, calcium sulphate, talc and calcined kaolin clay. Most preferably, the pigment is calcium carbonate, any form, natural or synthetic, most preferably ground or crushed marble, but chalk or precipitated calcium carbonate (PCC) can also be used. In this regard, it should be noted that although raw chalk can be dispersed using a cationic polyelectrolyte in the absence of vigorous mixing, this is not the case if the chalk is subjected to vigorous mixing. It is believed that this can be attributed to the powerful mixture stripping off the anionic aluminosilicate-containing layer normally present on the raw chalk. In the absence of vigorous stirring, the aluminosilicate-containing layer will be able to give a negative charge on the surface of the lime particles, which makes it possible for a dispersion to be obtained by means of a cationic polyelectrolyte.

It is preferred that the ground pigment has a particle size distribution such that at least 50% by weight has an equivalent spherical diameter smaller than 2 fj. m, and preferably that at least 60% by weight has an equivalent spherical diameter less than 2 fim.

Ground marble for use in the present invention is preferably mixed by crushing batches of marble in an aqueous suspension in the absence of a chemical dispersant and using a particulate grinding medium. Any agglomerates formed can be broken up by dewatering the suspension of ground marble, for example by filtration in the absence of a flocculant with subsequent drying of the pigment and pulverization of the dried product in a conventional mill. The particulate pigment is dispersed with a combination of an anionic polyelectrolyte and a cationic polyelectrolyte. Preferably, the anionic polyelectrolyte is a water-soluble vinyl polymer, an alkali metal or ammonium salt thereof or an alkali metal or ammonium salt of polysilicic acid. Most preferably, the anionic polyelectrolyte is a polyacrylic acid, a polymethacrylic acid, a substituted polyacrylic acid or a substituted polymethacrylic acid or alkali metal or ammonium salt of one of these acids. The substituted polyacrylic acid may be a partially sulfonated polymer. A particularly effective anionic polyelectrolyte is an alkali metal or ammonium salt of a copolymer of acrylic acid and a sulfonic acid derivative of acrylic acid, in which the proportion of sulfonic acid monomers is preferably 5-20% of the total number of monomer units.

The number average molecular weight of the anionic polyelectrolyte is preferably at least 500, and preferably not greater than 100,000. The amount used is preferably in the range 0.01 - 0.5% by weight, calculated on the weight of the dry pigment, preferably in the range 0.1 - 0.2% by weight.

The cationic polyelectrolyte may be a water-soluble, substituted polyolefin containing quaternary ammonium groups. The quaternary ammonium groups can be in the linear polymer chain or can be branches of the polymer chain. A number average molecular weight for the substituted polyolefin is preferably at least 1,500 and preferably not greater than 1,000,000, and is preferably in the range of 50,000 to 500,000.

The required amount is generally in the range of 0.01 to 1.5% by weight, based on the weight of the dry pigment. Advantageous results obtained near the substituted polyolefin is a polydiallyl dihydrogen or lower alkylammonium salt. The lower alkyl groups, which can be the same or different, can for example have 2-4 carbon atoms and each is preferably methyl. The ammonium salt can, for example, be a chloride, bromide, iodide, HS04", CH3S04" or nitrite. Preferably, the salt is a chloride. More preferably, the cationic polyelectrolyte is a polydiallyldimethylammonium chloride. Alternatively, the water-soluble substituted polyolefin may be a product obtained by the copolymerization of epichlorohydrin and an aliphatic secondary amine, which product has the formula:

wherein R and R', which may be the same or different, are each hydrogen or a lower alkyl group of 1-4 carbon atoms, preferably methyl or ethyl and X is Cl"' BR"- I"' HS04"' CH3SO4" or nitrite. The preferred number average molecular weight for this epichlorohydride product is in the range of 50,000 - 300,000.

Alternatively, the cationic polyelectrolyte can be a water-soluble, organic compound with a number of basic groups and preferably with a number average molecular weight of at least 10,000, and preferably not greater than 1,000,000. Most preferred is the number average molecular weight of at least 50,000. These water-soluble organic compounds can be described as polyacid organic bases (multivalent organic bases) and were preferably compounds consisting only of carbon, hydrogen and nitrogen and which are free of other functional groups, such as hydroxy-carboxylic acid groups, which would increase their solubility in water and thus increase the probability that they are desorbed from the clay minerals in an aqueous suspension. Preferably, the organic compound is polyethyleneimine (PEI) with a number average molecular weight in the range of 50,000 to 1,000,000. A further example of a water-soluble organic compound that can be used is a polyethylenediamine which can be a copolymer of ethylenediamine with an ethylene dihalide or with formaldehyde.

The cationic polyelectrolyte is used in an amount sufficient to make the mineral particles cationic. Experiments have shown that the zeta potential of the particles will normally be at least 20 mV after treatment, typically in the range 30 - 40 mV and usually not greater than +50 mV to +60 mV. These potentials are determined using a dilute (0.02% by weight) solids suspension using a carrier electrolyte of calcium chloride (10"<4>M) with a "Pen Kem Laser Z" meter.

The weight ratio of cationic polyelectrolyte to the weight of the polyelectrolyte used is preferably in the range 2:1 to 20:1, more particularly when the calcium carbonate is ground marble.

According to a further feature of the present invention, there is provided a method of preparing a paper coating mixture, comprising the following steps: (i) dispersing an aqueous suspension of a particulate pigment, and (ii) combining the dispersed aqueous suspension with a binder, and if necessary, adjust the dilution so that the particulate matter is at least 45

% by weight of the mixture;

the method is characterized by the fact that the pigment is dispersed using a dispersant comprising a combination of an anionic electrolyte and a cationic electrolyte, the cationic polyelectrolyte being used in an amount sufficient to make the pigment particles cationic, and that the binder is a cationic or non-ionic binder ;

and that the particulate pigment is one which cannot be dispersed to a high solids content in water, even after vigorous stirring, in the presence of only the cationic polyelectrolyte.

In the present method, it is normal for the raw pigment to be received as a filter cake with a relatively high solids content. To this, the dispersant is added to provide a slurry with a high dry matter content (45 - 85% dry matter by weight) which can then be subjected to vigorous stirring. This slurry was then converted into a paper coating mixture by dilution and addition of the required amount of cationic or non-ionic binder, and other conventional auxiliaries for a paper coating mixture.

Preferably, the pigment is mixed with the anionic polyelectrolyte before mixing with the cationic polyelectrolyte. This seems to make it possible to achieve a more liquid suspension at a high solids concentration.

The aqueous dispersion of the pigment may also include other conventional auxiliaries for the paper coating composition, such as an agent to reduce solubility (for example, melamine formaldehyde resin), a lubricant such as calcium stearate and a catalyst to catalyze crosslinking of the cationic latex if present: a suitable such catalyst is sodium bicarbonate. The necessary quantities of these aids are known to the person skilled in the art.

The binder used according to the invention should be a non-ionic or cationic binder. These binders are used in contrast to anionic binders which are normally used in paper coating mixtures in which the pigment is anionic. Thus, cationic guar gum and cationic starch binder can be used, as well as cationic or non-cationic latexes. Such cationic and non-ionic binders are commercially readily available. The particular cationic or non-ionic binder used will, for example, depend on the printing process to be used, for example offset lithography will require that the adhesive is water-soluble. For paper to be used in an offset technique, the amount of the binder should preferably be of the order of 7 to 25% by weight, calculated on the weight of the pigment, but for gravure paper, the binder should be used in an amount of 4 - 15% by weight, calculated on the weight of the pigment . The exact amount of binder required will depend on the nature of the binder and the material to be coated, but this can easily be determined by a person skilled in the art.

The pigment suspension for incorporation into the paper coating mixture according to the invention should preferably be subjected to vigorous stirring before or after dispersion. Typically, the vigorous stirring will be sufficient to contribute at least 10 KJ of energy per Kg pigment and preferably no more than approx. 50 KJ per Kg. Normally, the introduced energy will be in the range 18 - 3 6 KJ per Kg of pigment.

The coating mixture can be coated on a paper sheet using a normal coating machine and under normal coating conditions. It has been found that paper coated with a cationic mixture according to the present invention provides largely equivalent results to those obtained with a conventional anionic system.

The coated paper according to the invention is advantageous when it is used as "scrap paper" or return paper in the paper-making process. Generally, large quantities of paper will be recycled at the point of manufacture for one or more reasons, and the recycling advantages of the paper according to the invention are of the utmost importance to the paper manufacturer.

A papermaking pulp formed from such recovered paper may include conventional papermaking pulp, such as a bleached sulfite pulp, and typically waste fibers, and the pulp will be used in a ratio of 10:90 to 60:40.

Also included in the papermaking pulp will be a filler, for example a calcium carbonate filler, as well as a retention aid. As the waste fibers will include a proportion of calcium carbonate from the coating, it is possible to reduce the amount of calcium carbonate filler used to give a total amount of filler in the range of 5-20% by weight of the total papermaking mass. The weight of dry waste paper that is added (fibre and filler) should preferably be in the range of 5-30% by weight of fibres.

It has been found that when waste fibers used are derived from a coated paper according to the present invention, this will enable the amount of retention aid used in the papermaking pulp to be reduced.

The invention shall be further illustrated by the following examples:

Example 1

Three batches of raw/crushed marble were ground in an aqueous suspension containing 30% dry matter by weight, and in the absence of a chemical dispersant, using a particulate grinding medium. The duration of the painting was different from each case so that three different food products were obtained with the particle size distribution so that particle size distributions containing respectively 50%, 68%, and 87% by weight with an equivalent spherical diameter less than fim were obtained. In each case, the suspension of ground marble was dewatered by filtration in a tube pressure filter, in the absence of flocculant, and the filter cake was dried and pulverized in a laboratory hammer mill.

Samples of each of the ground mar powders were mixed with water and with two different dispersants, according to each of the different methods described below. The dispersants were: (E) an anionic electrolyte which was sodium polyacrylate having a number average molecular weight of 4,000; and (F) a cationic polyelectrolyte which was a polydiallyl dimethylammonium chloride, having the number average molecular weight

of 50,000.

In each case, the weight ratio of (F) to the weight of large (E) was 4:1, with the optimum total amount of dispersants being determined for each of the ground marble powders and found to be different in each case. The two methods for preparing the aqueous suspension of the mother powders were: (i) the powder was mixed with water containing the required amount of (E) and after careful mixing, the required amount of (F) was added followed by further mixing, and (ii) ) the powder was mixed with water the necessary contents

amounts of both (E) and (F) together.

In each case, the viscosity of the suspension was determined by a "Brookfield Viscometer" with a spindle speed of 100 rpm, and the percentage by weight of solids was determined by completely drying a known weight of the suspension and the subsequent weighing of the dry residue. The suspension was then diluted with small amounts of water, and further determinations of viscosity and % solids by weight were determined. Viscosity versus wt% solids was plotted and the solids concentration for a suspension with a viscosity of 500 mPa.s was determined by interpolation. The results obtained are shown in the following table I.

These results show that suspensions can be obtained at higher solids concentrations by method (i) (mixing powders first with the anionic polyelectrolyte and then with the cationic polyelectrolyte) than by method double (ii) (mixing the powder with both dispersants together). This effect is more marked with finely ground marble powder than with a coarser product.

Example 2

A further batch of finely ground marble powder was prepared by the method described in Example 1, the particle size distribution of the ground product being such that 87% by weight consisted of particles with an equivalent spherical diameter of less than 2 µm.

Samples of this marble powder were incorporated into a paper coating mixture, prepared according to the following compositions:

The three different mixtures of type (b) were prepared containing the following three different cationic starches: (i) cationic starch with a low degree of substitution (ii) another cationic starch with a low degree of substitution

(iii) cationic starch with a high degree of substitution

The three mixtures are identified as mixtures (b) (i) , (b) (ii) and (b) (iii) respectively

For the cases of the cationic mixtures (a) and (b), an aqueous suspension of the marble powder was first prepared using as dispersants 0.16% by weight, based on the weight of dry marble, of an anionic dispersant

(E) and 0.65% by weight, calculated on the weight of the dry marble, of a cationic dispersant (F), in the process

described under (i) in Example 1 above.

In the case of the anionic mixtures (c) and (d), the marble powder was treated with 0.30% by weight, based on the weight of dry marble, of the dispersant (E) alone.

In addition, 0.8 parts by weight of a melamine formaldehyde resin as an insolubilizing agent, and 0.5 parts by weight of calcium stearate were added to each of the mixtures (a), (b), (c) and (d). To each of the cationic mixtures (a) and (b) 0.2 parts by weight of bicarbonate was also added to catalyze the crosslinking reaction of the cationic latex.

Each paper coating composition was diluted with water to give a shear viscosity, determined with a "Ferraniti-Shirley Viscometer" at a shear rate of 12,800 s"<1> in the range of 60-70 mPa.s, if possible. The high shear viscosities and wt% the proportion of solids in diluted mixtures is shown in the following table II.

Each composition was coated onto a lightweight offset base paper having a basis weight of 48 g/m"<2> using a paper laboratory coating machine of the type described in British Patent No. 1032536. The coated paper samples were then super-calendered at a pressure of 6.89 MPa, and at a temperature of 65°C with 10 passes through the nip of the calender rolls at a speed of 36 m/min"<1>.

Each sample of the calendered coated paper was examined for gloss, according to the TAPPI standard method, for smoothness using "Parker Print Surf", at 10 Kgf, for percentage reflection of light of wavelength 457 nm and for percentage opacity. In each case, the determinations of gloss, smoothness, percent reflectance, and percent opacity were obtained from coated samples of paper in a range of different coating weights, determining these properties for each coating weight and interpolating to find the value of the property at a coating weight of 8 g.m." <2> The results are given in the following table II.

When the results for the cationic mixture (a) are compared with those for the corresponding anionic mixture (c), and the results for the cationic mixture of type (b) are compared with the properties for the corresponding anionic mixture (d), it will it is noted that the properties of the coated paper for a cationic system according to the invention essentially correspond to those obtained with a conventional anionic system.

Samples of the paper coated with the above-mentioned coating mixtures were used as waste paper or scrap paper in a papermaking process. Bleached sulphite pulp was milled according to TAPPI Standard T200 to a grinding grade of 45 SR or 270 Canadian Standard Freeness and pulp was prepared, consisting of aqueous suspensions of the following ingredients:

The calcium carbonate filler had a particle size distribution such that 43% by weight consisted of particles with an equivalent spherical diameter less than 2 µm. When the waste paper contained about 20% by weight of inorganic filler material, the amount of added fresh calcium carbonate was reduced to give a total amount of filler of 50 parts by weight. Correspondingly, the weight of added scrap paper (fibre + filler) was such that 30 parts by weight of fiber were obtained.

The retention of calcium carbonate filler in the paper prepared from the mixtures containing waste paper coated with each of the coating mixtures (a), (b), (ii), (c) and (d) was measured using a retention vessel with the stirrer set at speed 5 (1050 OMD/min) and stirred for 30 s. As a comparison, the retention of the same calcium carbonate filler in the papermaking mixture containing no waste paper was also determined. The results are given in the following table III:

These results show that although the incorporation of waste paper in the pulp, which was coated with an anionic mixture (c) and (d) reduces the retention of calcium carbonate filler compared to a pulp containing no waste paper, the incorporation of waste paper coated with the cationic mixture in actually improve the retention of the filler.

Example 3

A batch of raw, crushed marble was ground according to the procedure described in Example 1 to give a ground product with a particle size distribution such that 60% by weight consisted of particles with an equivalent spherical diameter of less than 2 µm. The suspension of ground marble was dewatered by means of a centrifuge and the centrifuge cake containing 68% dry matter f was used in the following experiments.

Samples of the ground marble centrifuge cake were first mixed with an anionic dispersant and then, after careful mixing, with a cationic dispersant. In each case, a predetermined amount of anionic dispersant was used, but for the cationic dispersant, a small amount of dispersant was first added, after which the suspension was vigorously mixed for one minute and the viscosity determined using a "Brookfield Viscometer" with a spindle speed of 100 rpm /my. The amount of dispersant and viscosity was recorded and a further amount of dispersant was added and this procedure repeated. Further small increments of dispersants were added until the suspension viscosity reached a minimum, at which point the total amount of cationic dispersant added was considered optimal.

The dispersants were:

(G) an anionic electrolyte which was a sodium polyacrylate with a number average molecular weight of 70,000, (H) sodium silicate which was a sodium salt of polysilicic acid and acted as an anionic dispersant, (I) a cationic polyelectrolyte which was a polydiallyldimethylammonium chloride with a number average molecular weight of 50,000 ,

(J) a cationic polyelectrolyte which was polyethyleneimine.

When the dispersant (J) was used, it was also necessary to add sufficient sulfuric acid to adjust the pH to 7.8 because polyethyleneimines are sensitive to pH, and do not act effectively as dispersants at pH greater than 8.

As a comparison, a sample of the centrifuge cake was treated with dispersant (J) at pH 7.8 alone, without the use of any anionic dispersant. For each combination of the dispersants, the weight % of dry marble in the suspension, the minimum viscosity of the suspension and the amount of anionic and cationic dispersant were determined and the results are given in Table IV below.

These results show that generally lower viscosities will be obtained when polyethyleneimine is used as cationic dispersant instead of polydiallyldimethylammonium chloride, but use of the cationic dispersant must precede the addition of an anionic dispersant.

Example 4

A batch of marble flour with a particle size distribution such that substantially all of the particles passed through a 300 mesh British Standard sieve (nominal opening 53 /zm) was subjected to abrasion grinding in a concentrated, deflocculated aqueous suspension, the quantities of marble flour, water and abrasive sand our:

615g marble flour.

330g water + anionic and cationic dispersants.

1,500g of sand.

The grain size of the sand used was less than No. 18 mesh British Standard sieve (nominal aperture 0.850 mm) and larger than 30 mesh British Standard sieve (nominal 500 mm). The anionic dispersant used was (E) and the cationic dispersant used was (F), both as described as in Example 1 above. The portions of marble flour were milled using different total amounts of (E) and (F), but in each case the weight ratio of (F):(E) was 4:1. In each case, the paint was continued for a period of time sufficient to supply the suspension with 3 96 KJ of energy per KG of dry marble, and in each case the product had a particle size distribution such that about 50% by weight consisted of particles with an equivalent spherical particle diameter of less than 2 µm. When painting was finished, the suspension of ground marble was separated from the sand and the viscosity of the suspension was measured using a "Brookfield Viscometer" with a spindle speed of 100 rpm. The suspension was then diluted with a small amount of water, and the viscosity measured again. The weight % of dry marble in the suspension was also determined by drying a known small weight of the suspension and weighing the dried residue. The steps of diluting with water and measuring the viscosity and % by weight of dry marble were repeated several times. Viscosity versus wt% dry marble was plotted and the wt% of dry marble in the suspension having a viscosity of 500 mPa.s was determined by interpolation. The results are shown in the subsequent table V.

Example 5

Further batches of the same marble flour used in example 4 were ground using the method described in example 4, and as anionic dispersant 0.07% by weight, calculated on dry marble of (E) was used and as cationic dispersant was used 0.28% by weight, calculated on the weight of dry marble, one of a selection of polydiallyldimethylammonium chloride polyelectrolytes of various molecular weights. In each case, the weight % proportion of dry marble in the suspension was one which gave a viscosity of 500 mPa.s, determined as described in Example 4 and the results are shown in the following Table VI:

it should have a number average molecular weight of at least 50,000 if a marble suspension with acceptable fluidity is to be obtained.

Example 6

A batch of raw, crushed marble was ground by the method described in Example 1 to give a ground product with a particle size distribution such that 60% by weight consisted of particles with an equivalent spherical diameter of less than 2 µm. The suspension of ground marble was dewatered by means of a centrifuge cake containing 73% by weight dry matter was used in the following experiments: Samples of the ground marble centrifuge cake were first mixed with 0.1% by weight, calculated on the weight of dry marble, of the anionic dispersant (G) as described in example 3 (that is, a sodium polyacrylate with a number average molecular weight of 70,000). After thorough mixing, to each of the samples of the aqueous suspension of anionically dispersed ground marble was added an amount of one or more of the following cationic dispersants: (I) a cationic polyelectrolyte which was polydiallyldimethylammonium chloride having a number average molecular weight of 50,000 - 100,000; (J) a cationic polyelectrolyte which was polyethyleneimine; (K) a polyethyleneimine having a number average molecular weight lower than that of (J); (L) a polyethyleneimine having a numerically smaller molecular weight lower than that of (K).

The amount of each cationic dispersant was that found experimentally to give the lowest viscosity for a suspension with a given solids content. For the dispersant (I), this amount was found to hold. For the dispersant (I) this amount was found to be 0.45% by weight, calculated on the weight of dry marble and for the dispersants (J), (K) and (L) the amounts were found to be 0.40% by weight calculated on the weight of dry marble.

For the case of the polyethylene minidispersant (J), (K) and (L), sufficient sulfuric acid was also added to adjust the pH to 7.8. In each case, the viscosity of the suspension was determined by means of the "Brookfield Viscometer" with a spindle speed of 100 rpm and the percentage by weight of dry matter in the suspension was determined by completely drying a known weight of the suspension and weighing the dried residue. The suspension was then diluted with small amounts of water and further determinations of viscosity and % by weight of solids in the suspension were determined by completely drying a known weight of the suspension and weighing the dried residue. The suspension was then diluted with small amounts of water and further determinations of viscosity and % dry matter by weight were determined. The procedure of diluting the suspension and determining viscosity and % dry matter by weight was repeated two or three times and viscosity versus % dry matter by weight was graphically produced. The solids concentration for the suspension with a viscosity of 300 mPa.s was determined by interpolation and the results are shown in the table

VII:

Example 7

A batch of raw crushed marble was ground in an aqueous suspension containing 30% by weight of dry soffit and in the absence of chemical dispersants, using a particulate grinding medium to give a ground calcium carbonate product of paper coating quality having a particle size distribution such that 90 % by weight of the particles had an equivalent spherical diameter less than 2 µm. The ground marble suspension was dewatered by filtration in the absence of flocculant and the filter cake dried and pulverized in a laboratory hammer mill.

Samples of the finely ground marble powder were mixed with water to give a suspension containing 60% by weight solids and varying amounts of an anionic and cationic dispersant. The anionic dispersant was a sodium polyacrylate dispersant with a number average molecular weight of 4,000 and the cationic dispersant was polydiallyldimethylammonium chloride with a number average molecular weight of about 50,000.

In each experiment, the anionic dispersant was first added to the ground marble suspension, after which the mixture was stirred at 9,000 rpm using a stirrer rotating at 1,420 rpm. The cationic dispersant was then added and the mixing procedure repeated. Viscosity was determined immediately after the second mixing procedure was completed using the Brookfield Viscometer. The results are shown in the following table VIII. These results show that optimal dispersion was obtained where the weight ratio between cationic dispersant and anionic dispersant was approx. 4:1. When this ratio was increased to 6.9:1 it was still possible to obtain a very liquid dispersion, but this at the expense of a somewhat higher dose of mixed dispersants.

Example 8

A batch of raw crushed marble was ground in an aqueous suspension containing 30% by weight solids, in the absence of chemical dispersants, using a particulate grinding medium to give a ground product with a particle size distribution such that 78% by weight of the particles had an equivalent spherical diameter less than 2 µm. The suspension of ground marble was dewatered in the absence of a flocculant and a vacuum drum filter to a solids content of 64% by weight. Some of the filter cake thus formed was heat-dried and mixed back with the moist filter cake to give a mixture with a solids content of 70% by weight.

This mixture was divided into 3 portions for treatment with cationic polydiallyldimethylammonium chloride dispersants, having three different number average molecular weights. Each of these three parts was further divided into three smaller portions, each of which was treated with different doses of anionic dispersants (E), as described in Example 1. In each case, the anionic dispersant was added first to the cake of ground marble, and thoroughly mixed with this, after which the cationic dispersant was added and mixed. The dose of cationic dispersant used in each case was approx. 3.5 times the dose of anionic dispersant. In each case, the viscosity of the resulting suspension was measured "Brookfield Viscometer" with the spindle speed of 100 rpm and the weight % solids was determined by completely drying a known weight of a suspension and weighing the dried residue.

The suspension was then diluted with a small amount of water and further determinations of viscosity and % solids by weight were made. Viscosity was plotted against wt% solids and the solids content of a suspension with a viscosity of 300 mPa.s was determined by interpolation. The results obtained are shown in the subsequent table IX. These results show that slightly more liquid suspensions, for a given solids content, were obtained when the cationic dispersant used had a number average molecular weight of 500,000.

Claims (16)

1. Aqueous paper coating mixture comprising (i) at least 45% by weight of a particulate inorganic pigment dispersed in a dispersant and (ii) a binder, characterized in that the dispersant comprises an anionic polyelectrolyte and a cationic polyelectrolyte, the cationic polyelectrolyte being present in an amount sufficient to render the pigment particles cationic, the binder is a cationic or non-ionic binder, and the particulate pigment is one which does not disperse to a high solids content in water, even after vigorous agitation, in the presence of only the cationic polyelectrolyte. 2. Paper coating mixture according to claim 1, characterized in that the pigment is selected from calcium carbonate, calcium sulphate, talc or calcined kaolin clay. 3. Paper coating mixture according to claim 1, characterized in that the pigment is calcium carbonate pigment. 4. Paper coating composition according to any one of the preceding claims, characterized in that the number average molecular weight of the anionic polyelectrolyte is in the range 500 - 100,000. 5. Paper coating composition according to any one of the preceding claims, characterized in that the anionic polyelectrolyte is used in an amount in the range of 0.01 - 0.5% by weight, calculated on the weight of dry pigment. 6. Paper coating composition according to any one of the preceding claims, characterized in that the cationic polyelectrolyte is a water-soluble, substituted polyolefin containing quaternary ammonium groups. 7. Paper coating mixture according to claim 6, characterized in that the number average molecular weight for the substituted polyolefin is in the range 1,500 - 1,000,000. 8. Paper coating mixture according to claim 6 or 7, characterized in that the amount of cationic polyelectrolyte is used in an amount in the range of 0.01 - 1.5% by weight, calculated on the weight of dry pigment. 9. Paper coating composition according to any one of claims 1-5, characterized in that the cationic polyelectrolyte is a water-soluble organic compound with a number of basic groups and with a number average molecular weight in the range of 10,000 - 1,000,000. 10. Paper coating mixture according to claim 9, characterized in that the organic compound is polyethyleneimine with a number average molecular weight in the range 50,000 - 1,000,000. 11. Paper coating composition according to any one of the preceding claims, characterized in that the ratio between the weight of cationic polyelectrolyte to the weight of anionic polyelectrolyte is in the range 2:1 to 20:1. 12. Process for the preparation of a paper coating composition, comprising the following steps: (i) dispersing in an aqueous solution a particulate pigment, and (ii) combining the dispersed aqueous suspension with a binder, and if necessary adjusting the dilution so that the particulate material constitutes at least 45% by weight of the mixture, characterized in that the pigment is dispersed using a dispersant comprising a combination of an anionic polyelectrolyte and a cationic polyelectrolyte, the cationic polyelectrolyte being used in an amount sufficient to make the pigment particles cationic, the binder being a cationic or non-ionic binder, and the particulate pigment is one which does not disperse to a high solids content in water, even after vigorous agitation, in the presence of only the cationic polyelectrolyte. 13. Method according to claim 12, characterized in that the pigment is selected from calcium carbonate, calcium sulphate, talc or calcined kaolin clay. 14. Method according to claim 13, characterized in that the pigment is a calcium carbonate pigment. 15. Method according to claims 12, 13 or 14, characterized in that the pigment is first mixed with the anionic polyelectrolyte before mixing with the cationic polyelectrolyte. 16. Method for the production of coated paper, characterized by coating the paper sheet with a paper coating mixture according to any one of claims 1 - 11.