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INORGANIC MATERIAL SLURRY
This invention relates to a cationically dispersed, high solids, aqueous suspension of an inorganic pigment or filler, such as calcium carbonate. It is known to disperse inorganic pigments and fillers such that the particles have an overall positive charge. Such cationically dispersed suspensions are useful in paper making (EP-0278602A) and paper coating. It has now been found that the rheology of a cationically dispersed suspension of a mineral pigment or filler can be improved by using a mineral having a particular particle size distribution.
Thus, in accordance with the present invention, there is provided a high solids, aqueous suspension of a cationically dispersed particulate inorganic material, characterised in that the particulate inorganic material has a particle size distribution such that not more than 10% by weight of the particles have an equivalent spherical diameter (esd) smaller than 0.25 microns.
The high solids suspension should preferably be at least 60% by weight solids.
It is preferred that the inorganic material is a material which, when ground to a particulate mass, exists in the form of regular, approximate spherical particles having a low mean particle aspect ratio. Thus, the material may be a calcium carbonate, in any form, natural or synthetic. Particularly preferred is ground marble, although precipitated calcium carbonate (PCC) and chalk are operable, Other possible inorganic materials are gypsum, talc and calcined kaolin clay. However, it is to be appreciated that other minerals having a plately structure, e.g. layer lattice silicates such as kaolin clay, are within the scope of the present invention. Preferably , the inorganic material employed in the present invention should have a specific surface area, as measured by the BET N2 method, of less than about 7.5m2g_1, more preferably less than about 6.5m2g"1, and preferably at least 2m2g"1.
The particulate material should also preferably have a particle size distribution such that no greater than 1% of the particles have an esd larger than 10 microns and at least 65% have an esd smaller than 2 microns.
The inorganic material may be ground, before dispersion, to the desired particle size distribution. The grinding conditions can be adjusted in a manner known per se to produce materials having varying distributions.
It has been found that a cationic slurry prepared in accordance with the present invention can be formed into a slurry having a given viscosity at higher solids than a slurry in which the inorganic material has a broader particle size distribution.
Where the inorganic material is a pigment or filler which carries a neutral or positive charge, such as marble, talc, gypsum or calcined kaolin clay, the particles of the material may be dispersed using a dispersing agent comprising a combination of an anionic polyelectrolyte and a cationic polyelectrolyte, the cationic polyelectrolyte being used in an amount sufficient to render the particles cationic. Although chalk particles, when in a raw state, do not carry a positive charge because of natural anionic species absorbed to the particle surface, chalk can be subjected to vigorous agitation in order to strip off such anionic species and render the mineral capable of being effectively dispersed at high solids using a combination of an anionic polyelectrolyte and cationic polyelectrolyte. The high solids aqueous suspension of the present invention can be "made down" into a paper coating composition by dilution (if necessary) to a solids concentration of at least 45% by weight and by addition of an adhesive, which should be non-ionic or cationic in nature.
A full discussion of the constituents of paper coating compositions and of the 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 given in "An Operator's Guide to Aqueous Coating for Paper and Board", edited by T.W.R. Dean, The British Paper and Board Industry Federation, London, 1979.
The aqueous suspension of the present invention should preferably be subjected to vigorous mixing before or after dispersion. Typically, the vigorous mixing should be sufficient to impart at least lOkJ energy per kg of the inorganic material, and preferably no more than about 50kJ per kg. Normally, the amount of energy input will be in the range of from 18-36kJ per kg of the inorganic material.
The paper coating composition can be used in a method of coating a sheet member. The thus formed coated paper is particularly suitable for recycling.
Ground marble for use in the present invention is preferably formed by crushing batches of marble in aqueous suspension in the absence of a chemical dispersing agent using a particulate grinding medium. Further size reduction is achieved by dewatering the suspension of ground marble, for example by filtration in the absence of a flocculating agent and then drying the pigment, and pulverising the dried product in a conventional mill.
The particulate pigment is dispersed with the combination of an anionic polyelectrolyte and a cationic polyelectrolyte. Preferably, the anionic polyeletrolyte 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 polyeletrolyte is a poly(acrylic acid), a poly(methacrylic acid), a substituted poly(acrylic acid) or a substituted poly(methacrylic acid), or an alkali metal or ammonium salt of any of these acids. The substituted poly(acrylic acid) may be a partially sulphonated polymer. An especially effective anionic polyelectrolyte is an alkali metal or ammonium salt of a copolymer of acrylic acid and a sulphonic acid derivative of acrylic acid, in which the proportion of the sulphonic acid derivative monomer is preferably from 5% to 20% of the total number of monomer units.
The number average molecular weight of the anionic polyelectrolyte is preferably at least 500, and preferably no greater than 100,000. The amount used is generally in the range of from about 0.01% to about 0.5% by weight based on the weight of dry pigment, preferably in the range of from about 0.1 to 0.2% by weight. The cationic polyelectrolyte may be a water- soluble substituted polyolefine containing quaternary ammonium groups. The quaternary ammonium groups may be in the linear polymer chain or may be in branches of the polymer chain. The number average molecular weight of the substituted polyolefine is preferably at least 1500 and preferably no greater than 1,000,000, and is more preferably in the range of from 50,000 to 500,000. The quantity required is generally in the range of from about 0.01% to about 1.5% by weight based on the weight of dry pigment. Advantageous results have been obtained when the substituted polyolefine is a poly (diallyl di(hydrogen or lower alkyl)ammonium salt). The lower alkyl groups, which may be the same or different, may for example, have up to four carbon atoms and each is preferably methyl. The ammonium salt may be, for example, a chloride, bromide, iodide, HS04~, CH3S04" or nitrite. Preferably the salt is a chloride. Most preferably, the cationic polyelectrolyte is poly (diallyl dimethyl ammonium chloride). Alternatively, the water-soluble substituted polyolefin may be the product of co-polymerising epichlorohydrin and an aliphatic secondary amine, said product having the formula
in which R and R', which may be the same of different, are each hydrogen or a lower alkyl group having from one to four carbon atoms, preferably methyl or ethyl and X is Cl"* Br"* I-* HS04_* CH3S0«- or nitrite. The preferred number average molecular weight of this epichlorohydrin product is in the range of from 50,000 to 300,000.
Alternatively, the cationic polyelectrolyte may be a water-soluble organic compound having a plurality of basic groups and preferably having a number average molecular weight of at least 10,000 and preferably no greater than 1,000,000. Most preferably, the number average molecular weight is at least 50,000. These water-soluble organic compounds may be described as polyacidic organic bases, and are preferably compounds of carbon, hydrogen and nitrogen only and are free of other functional groups, such as hydroxy or carboxylic acid groups, which would increase their solubility in water and thus increase the likelihood of their being desorbed from the clay mineral in an aqueous suspension. Preferably, the organic compound is polyethyleneimine (PEI) having a number average molecular weight in the range 50,000 to 1,000,000. A further example of a water-soluble organic compound which may be employed is a polyethylene diamine which may be a copolymer of ethylene diamine with an ethylene dihalide or with formaldehyde.
The cationic polyelectrolyte is employed in an amount sufficient to render the mineral particles cationic. Experiments have shown that the zeta potential of the particles will normally be at least +20mV after treatment, typically in the range of from +30 to +40 mV and usually no greater than +50 to +60mV. These potentials have been measured using a dilute (0.02 weight %) solids suspension using a supporting electrolyte of potassium chloride (10"4M) with a "Pen Kern Laser Z" meter.
The ratio of the weight of cationic polyelectrolyte to the weight of anionic polyelectrolyte used is preferably in the range of from 2:1 to 20:1, more preferably in the range of from 2:1 to 10:1.
In the method of the making the slurry of the invention, it is normally the case that the raw pigment is received as a filter cake having a relatively high solids content. To this is added the dispersing agent in order to provide a dispersed high solids slurry (45- 80% by weight solids) which may then be subjected to vigorous mixing.
Where the pigment is to be dispersed using a combination of an anionic and cationic polyelectrolyte, the pigment is mixed with the anionic polyeletrolyte before mixing with the cationic polyelectrolyte. This appears to enable a more fluid suspension to be obtained at a higher solids concentration. When the aqueous suspension is to be used in paper coating, it, may also include other conventional paper coating composition adjuvants such as an insolubilising agent (e.g. a melamine formaldehyde resin), a lubricant such as calcium stearate and a catalyst to catalyze cross-linking of the cationic latex if present: a suitable such catalyst is sodium bicarbonate. The quantities of these adjuvants required are known to those skilled in the art.
The adhesive used in making the paper coating composition should be a non-ionic or a cationic adhesive. Such adhesives contrast with the anionic adhesives which are normally used in paper coating compositions in which the pigment is anionic. Thus, cationic caesin and cationic starch adhesives can be used as well as cationic or non-ionic latices. Such cationic and non-ionic adhesives are readily commercially available. The particular cationic or non-ionic adhesive used will depend, for example, on the printing process to be used, e.g. offset lythography requires the adhesive to be water- insoluble. For paper to be used in an offset printing technique, the amount of adhesive should preferably be of the order of from 7 to 25% by weight, based on the weight of pigment whilst, for gravure printing paper, the adhesive should be used in an amount of 4-15% by weight, based on the weight of pigment. The precise quantity of adhesive required will depend upon the nature of the adhesive and the material being coated, but this can readily be determined by the person skilled in the art. The coating composition may be coated on to a sheet member using normal paper coating machinery and under normal paper coating conditions. It has been found that the paper coated with a cationic composition in accordance with the present invention provides broadly similar results to that obtained with a conventional anionic system. The coated paper which may be made using the present invention is of advantage when it is employed as "broke" or recycled paper in a paper making process. Commonly, large quantities of paper are recycled at the point of manufacture for one reason or another, and the advantages of the paper of the present invention in recycling are most important to the paper manufacturer. Such a method for recycling paper includes the step of reducing the paper into a fibrous recyclable state and incorporating said fibre in a paper-making composition. Such a paper-making composition may include conventional paper-making pulp, such as a bleached sulphite pulp and, typically, the broke fibre and the pulp will be employed in a ratio of from 10:90 to 60:40.
Also included in the paper making composition will be a filler, for instance a calcium carbonate filler and also a retention aid. Since the broke fibre will include a proportion of calcium carbonate from the coating, it is possible to reduce the amount of calcium carbonate filler employed to give a total quantity of filler in the range of from 5 to 20 percent by weight of the total paper-making composition. The weight of dried broke added (fibre and filler) should preferably be in the range of from about 5 to 30 percent by weight of fibre.
It has been found that, when the broke fibre employed is derived from a coated paper in accordance with the present invention, this enables the amount of retention aid employed in the paper making composition to be reduced.
The aqueous slurry of the present invention is also particularly suited to paper filling and reference here is made to our EP-278602A. The present invention will now be illustrated by the following Example: EXAMPLE Two calcium carbonate pigments were prepared by low solids sand grinding of marble flour. Adjustment of grinding conditions allowed products of varying widths of distribution to be compared. Sedigraph data was obtained as shown in Table 1 below (percentages given are weight %):
Table 1
Surface Area (BET N2) δ.Om1 8.6m2g-1
Both samples were filtered to give a filter cake of between 70 - 75 % solids. This cake was then cationically dispersed using a pretreatment of sodium polyacrylate (Molecular weight 4000) followed by addition of a larger dose of polydadmac (i.e. a poly(dialyl dimethyl ammonium chloride)) of molecular weight - 500,000 followed by addition of a larger dose of the polydadmac. The ratio of cationic to anionic polymer was maintained at between 3.2 and 3.5:1 by weight. The suspension was diluted with water until a viscosity, measured at 100 rpm using a Brookfield
Viscometer, of approximately 600 mPa.s was reached and the solids content of the suspension determined.
Table II SAMPLE A dispersion on high speed mixer Dose of anionic Dose of poly- Solids wt% Brookfield polyacylate wt% dad ac wt% viscosity mPa.s
0.11 0.36 70.3 600 Table III SAMPLE B, Dispersion
Polyacylate dose Polydadmac dose S wt% wt%
Hence in this example, a ground marble having a broad size distribution gives approximately 4 units lower solids for a given rheology when cationically dispersed.