COMPOSITIONS FOR COATING SHEET MATERIALS

Abstract: A method of treatment and use of an aqueous coating composition which includes the steps of (a) concentrating the composition to increase the solids content thereof and (b) employing the concentrated composition to coat a sheet material, e.g. paper, wherein the concentration step (a) is carried out by use of thermal evaporation under reduced pressure. The composition concentrated may be a previously employed coating composition which has been recovered or it may be a fresh coating composition.

Description: COMPOSITIONS FOR COATING SHEET MATERIALS The present invention relates to compositions for coating sheet materials and to methods of treatment and use of such compositions. The invention relates particularly to the concentration and use of aqueous compositions intended for coating cellulosic fibrous sheet materials such as paper, board, paper coated board, card, cardboard and the like. Aqueous coating compositions for coating the aforementioned sheet materials are well known and are widely used and generally comprise a concentrated suspension in water of a pigment, generally a white pigment such as hydrous or calcined kaolin clay, calcium carbonate, calcium sulphate, titanium dioxide, satin white, barium sulphate, or the like, or a mixture of any two or more of these, an adhesive or binder to attach the pigment to the sheet material, usually a dispersing agent for the pigment, and possibly other additives. The coating composition may for example be applied to a web of the sheet material moving at a relatively high speed by means of various types of applicator device. Relatively large proportions of the quantity of coating composition prepared for a coating operation are lost from the operation in that they are not actually applied to the surface of the sheet material. Some of the coating composition is lost by spillage or splashing on or around the coating machine. Further losses can be incurred when the sheet material breaks on the coating machine, and surplus coating composition is inevitably left behind on the walls of tanks in which it is prepared and stored, and of pipes through which it is distributed to the coating machine. The surplus coating composition is removed from the floor or from parts of the coating equipment by washing with copious amounts of water. The result is a dilute aqueous suspension or solution of the ingredients of the coating composition which might be suitable for the relatively low value use of being incorporated as a filler material in the composition used for forming a cellulosic sheet material, or is otherwise discharged to a sewer. Materials from waste sheet material coating compositions which are discharged to a sewer present a difficult settling problem in a waste water treatment plant and add to the difficulty and cost of ensuring that the aqueous effluent discharged from the plant is environmentally acceptable. Douglas L. Woerner & John L. Short, in "Recovery of Coating Losses", TAPPI Proceedings of the 1991 Coating Conference, pages 251-256 describe a procedure for reconcentrating a dilute suspension of waste coating composition and reusing the reconcentrated composition in a paper coating operation. Waste coating composition is simulated by diluting a coating composition to a solids concentration of 10% by weight, and then reconcentrating the suspension to 60% by weight by ultrafiltration using membrane filtration modules. A reusable coating composition could be produced in this known way, but the process has the disadvantage that soluble components of the coating composition, such as the dispersing agent, certain types of starch adhesive, stabilising components of latex adhesives and optical brightening agents pass through the filter medium with the excess water, and are lost from the coating composition. An alternative known procedure for recovering material comprising a spilled coating composition comprises selective flocculation of the dilute suspension containing the material. This procedure also results in the loss of valuable ingredients of the composition. It also introduces chemical flocculant which may be undesirable in subsequent processing stages using the recovered composition. Another disadvantage is that the procedure is not widely applicable to different pigments in coating compositions. Mainly kaolin-containing suspensions may be treated by the procedure. However, the procedure is not sufficiently effective with mainly calcium carbonate-containing suspensions . The present invention is concerned therefore with concentration of a coating composition in a manner which can be employed to provide an improved method of recovery and re-use of useful material from a dilute waste coating composition. The present invention is also concerned with concentration of a coating composition in a manner which can be employed to improve the properties of a coating composition (whether or not recovered from waste coating material) and the properties of a sheet material coated with such a composition. According to the present invention there is provided a method of treatment and use of an aqueous coating composition which includes the steps of (a) concentrating the composition to increase the solids content thereof and (b) employing the concentrated composition to coat a sheet material, wherein the concentration step (a) is carried out by use of thermal evaporation under reduced pressure. The sheet material to which the coating composition is applied may be any of the materials known to be treated with a water based coating composition comprising coating pigment and adhesive and other optional ingredients. Such sheet material may comprise any of the sheet materials commercially coated with such a composition, eg. paper, paper board, card, cardboard and like products made from cellulosic and/or synthetic fibre compositions. The aqueous coating composition is desirably substantially free of volatile organic solvents. The aqueous coating composition may include as inorganic pigment material any one or more of the inorganic pigment materials known or used in the art, eg. selected from kaolin, calcined kaolin, calcium carbonate derived from natural or synthetic sources, dolomite, calcium sulphate, mica, talc and titanium dioxide or composite pigments, eg. as described in GB 2,273,701, containing these materials. The pigment material may form from 10% to 90% by weight of the solids content of the composition. The present invention is particularly suitable to concentrate compositions wherein the pigment material forms from 70% to 95%, especially 80% to 90%, by weight of the solids content of the composition. Coating compositions for use in coating sheet materials vary depending upon the materials to be coated which vary throughout the world depending upon the geography of the region in which the material is produced. However, the method according to the present invention generally is applicable to concentrate all such compositions. The composition to be concentrated may include as adhesive or binder, depending on the type of composition concerned, any one or more of the hydrophilic adhesives known or used in the art, eg. selected from starches and other polysaccharides, proteinaceous adhesives, and latices. The amount of adhesive or binder present in the composition to be treated in accordance with the present invention depends upon whether the composition is to be applied as a relatively dilute or concentrated pigment-containing suspension to the material to be coated. For example, a dilute pigment -containing composition (binder-rich composition) could be employed as a top-coat for underlying more pigment-rich compositions. The adhesive or binder present in the composition may range from 1% to 70% by weight relative to the dry weight of pigment (100% by weight) especially 4% to 50% by weight. Where coating composition is not to be employed as a binder rich composition the adhesive or binder may form from 4% to 30%, eg. 8% to 20%, especially 8% to 15% by weight of the solids content of the composition. The amount employed will depend upon the composition and the type of adhesive, which may itself incorporate one or more ingredients. For example, the following adhesive or binder ingredients may be used in the following stated amounts: (a) Latex: levels range from 4% by weight for self thickening gravure latices to 20% by weight for board coating latices. The latex may comprise for example a styrene butadiene, acrylic latex, vinyl acetate latex, or styrene acrylic copolymers . (b) Starch and other binders: levels range from 0 to 50% by weight, eg. 4% by weight to 20% by weight for pigment-rich compositions. The starch may comprise material derived from maize, corn and potato. Examples of other binders include casein and polyvinyl alcohol . Additives in various known classes may, depending upon the type of coating and material to be coated, be included in the coating composition to be concentrated by the method according to the present invention. Examples of such classes of optional additive are as follows : (a) Cross linkers: eg. in levels 0 to 5% by weight; for example glyoxals, melamine formaldehyde resins, ammonium zirconium carbonates. (b) Water retention aids: eg. in up to 2% by weight, for example sodium carboxymethyl cellulose, hydroxyethyl cellulose, PVA (polyvinyl acetate) , starches, proteins, polyacrylates, gums, alginates, polyacrylamide , bentonite and other commercially available products sold for such applications. (c) Viscosity modifiers or thickeners: eg. in levels up to 2% by weight; for example polyacrylates, emulsion copolymers, dicyanamide, triols, polyoxyethylene ether, urea, sulphated castor oil, polyvinyl pyrrolidone, montmorillonite, CMC (carboxymethyl celluloses) , sodium alginate, xanthan gum, sodium silicate, acrylic acid copolymers, HMC (hydroxymethyl celluloses), HEC (hydroxyethyl celluloses) and others. (d) Lubricity/Calendering aids: eg. in levels up to 2% by weight, for example calcium stearate, ammonium stearate, zinc stearate, wax emulsions, waxes, alkyl ketene di er, glycols. (e) Dispersants: eg. in levels up to 2 per cent by weight, for example polyelectrolytes such as polyacrylates (sodium and ammonium) , sodium hexametaphosphates, non- ionic polyol, polyphosphoric acid, condensed sodium phosphate, non-ionic surfactants, alkanolamine and other reagents commonly used for this function. (f) Antifoamers/defoamers : eg. in levels up to 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. (g) Dry or wet pick improvement additives: eg. in levels up to 2% by weight, for example melamine resin, polyethylene emulsions, urea formaldehyde, melamine formaldehyde, polyamide, calcium stearate, styrene maleic anhydride and others. (h) Dry or wet rub improvement and abrasion resistance additives: eg. in levels up to 2% by weight, for example glyoxal based resins, oxidised polyethylenes, melamine resins, urea formaldehyde, melamine formaldehyde, polyethylene wax, calcium stearate and others . (i) Gloss-ink hold-out additives: eg. in levels up to 2% by weight, for example oxidised polyethylenes, polyethylene emulsions, waxes, casein, guar gum, CMC, HMC, calcium stearate, ammonium stearate, sodium alginate and others. (j) Optical brightening agents (OBA) and fluorescent whitening agents (FWA) : eg. in levels up to 1% by weight, for example stilbene derivatives. (k) Dyes: eg. in levels up to 0.5% by weight. (1) Biocides/spoilage control agents: eg. in levels up to 1% by weight, for example metaborate, sodium dodecylbenene sulphonate, thiocyanate, organosulphur, sodium benzonate and other compounds sold commercially for this function eg. the range of biocide polymers sold by Calgon Corporation. (m) Levelling and evening aids: eg. in levels up to 2% by weight, for example non- ionic polyol, polyethylene emulsions, fatty acid, esters and alcohol derivatives, alcohol/ethylene oxide, sodium CMC, HEC, alginates, calcium stearate and other compounds sold commercially for this function. (n) Grease and oil resistance additives: eg. in levels up to 2% by weight, eg. oxidised polyethylenes, latex, SMA (styrene maleic anhydride) , polyamide, waxes, alginate, protein, CMC, HMC. (o) Water resistance additives: eg. in levels up to 2% by weight, eg. 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. (p) Insolubiliser: eg. in levels up to 2% by weight. For all of the above additives, the percentages by weight quoted are based on the dry weight of pigment (100%) present in the composition. Where the additive is present in a minimum amount the minimum amount may be 0.01% by weight based on the dry weight of pigment. Step (b) in the method according to the present invention may be carried out in a known way which will depend upon the material to be coated, the coating composition to be applied and other factors as determined by the operator, eg. speed and ease of runnability eg. using a conventional coating machine. Methods of coating paper and other sheet materials are widely published and well known. 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, ie . "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 levelling problems are introduced. All known methods of coating for use in step (b) of the method according to the present invention require (i) a means of applying the coating composition to the material to be coated, viz an applicator; and (ii) a means for ensuring that a correct level of coating composition is applied, viz a metering device. 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, eg. as a film press. At the points of coating application and metering, the paper web support ranges from a backing roll, eg. via one or two applicators, to nothing (ie: 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. A coater that is applying a double coating, ie . a coating on each side of the paper, will have two or four coating heads, depending on the number of sides coated by each head. Most coating heads coat only one side at a time, but some roll coaters (eg. film press, gate roll, size press) coat both sides in one pass . Examples of known coaters which may be employed in step (b) include air knife coaters, blade coaters, rod coaters, bar coaters, multi -head coaters, roll coaters, roll/blade coaters, cast coaters, laboratory coaters, gravure coaters, kiss coaters, liquid application systems, reverse roll coaters and extrusion coaters. We have found that compositions conventially employed in coating operations are not ideal. In some cases the maximum solids level runnable on the coating machine is not attained because of unnecessary dilution of one or more of the components, eg. the pigment slurry and/or the adhesive polymer, employed to produce the composition. Such dilution may have occurred because processing or handling of such materials at lower solids levels may have been necessary. We have found unexpectedly and beneficially that the concentration of the coating composition as in step (a) can be increased without substantial loss through concentration of valuable components of the composition. This provides the benefits of concentration without any unduly detrimental effect on the performance of the composition, eg. rheology, runnability and properties when used in coating, eg. brightness and abrasiveness . Better coverage of the sheet material may be achieved allowing coating to be carried out more cheaply. Also, less fibre swell from the presence of water occurs and this can allow the sheet coated with the concentrated composition to have a smoother, higher gloss surface finish with improved print properties. The present invention is particularly (although not exclusively) suited to the concentration of coating compositions wherein the particle size distribution of the pigment particles contained in the composition is of a steep form. The particle size distribution, "psd" which may be measured by the well known method of sedimentation, eg. using a SEDIGRAPH™ machine, is a graph of the percentage by weight of particles less than a given size (equivalent spherical diameter) versus particle size (equivalent spherical diameter) . A steep psd may be defined in various ways, for example as in GB2058734 wherein the particle size range factor d 5 o is less than 3 especially less than 2, or alternatively wherein the steepness factor [0 d 50 '20 15 is less than 2.2 especially less than 1.8, wherein d 90 , d 10 , d 50 and d 20 are respectively the particle size values at which there are less than 90, 10, 50 and 20 per cent by weight of the pigment particles present. Such a distribution may be obtained in a known way 0 by applying one or more particle size classifications. Pigments having a steep psd which are becoming increasingly used in coating compositions are well known to be more difficult to dewater than those having a non- steep or "broad" psd; the present invention 25 allows suspensions comprising steep psd pigments as well as broad psd pigments to be concentrated efficiently without substantial detrimental effect on the suspension caused by the concentration procedure. The present invention is especially suitable for use in the recovery and re-use of a waste aqueous coating composition. The components recovered may be components as in a prior art sheet coating composition as aforesaid. Substantially all of the involatile components except water may be recovered. The recovered coating composition may be re-used in the coating operation in which the composition was applied and from which it was recovered. If desired, fresh coating composition may be blended with recovered coating composition for use in the coating operation (step (b) ) . Beneficially, the present invention provides, in contrast to the prior art waste coating recovery processes, an economically attractive process by which substantially all of the useful ingredients can be recovered from a diluted aqueous suspension of sheet material waste coating composition, and by which the recovered ingredients can be re-used as a concentrated aqueous coating composition eg. for coating sheet material as aforesaid. The recovered composition in aqueous slurry or suspension form can be reconcentrated to substantially the same solids content of the original coating composition without significant deterioration in rheology or coated sheet properties. As noted above, some properties may be unexpectedly improved. In addition, the present invention beneficially prr .des a process which allows avoidance of discharge to sewers of liquid effluents containing dissolved contaminants or solid particles which settle only with difficulty. In addition, the present invention may be employed to concentrate various types of pigment, including those of various chemical compositions, or those of various psd forms, including steep psd. In the method according to the present invention where applied to recover coating composition from a dilute suspension thereof the said dilute suspension may contain not more than 20% by weight of solid material, for example, not more than 10% by weight of solid material. The extent of concentration achieved in step (a) will depend on the extent of dilution of the composition to be concentrated and the final desired solids content. Generally, the suspension after concentration has a rheology suitable for a coating operation as specified by the user of the coating composition. Generally, the solids content (on a dry weight basis) of the suspension after concentration is greater than 55 per cent, eg. greater than 60 per cent, especially in the range 60 per cent to 75 per cent. Where the suspension to be concentrated is a dilute recovered suspension the solids content may be raised by at least 40 per cent by weight, eg. from less than 20 per cent by weight to greater than 60 per cent by weight; for example, in this case the solids content may be raised by a differential percentage in the range of from 45 per cent to 70 per cent by weight. Where the suspension to be concentrated comprises a fresh composition the suspension may initially have a solids content of at least 55 per cent, eg. at least 60 per cent by weight. The concentration step (a) may in this case raise the solids content by a differential percentage of from 1 per cent to 20 per cent, eg. 2 per cent to 10 per cent, by weight. The evaporative concentration step (a) in the method according to the present invention may be carried out in an evaporator of a known type having no unduly narrow passages or channels through which the material to be treated must pass. Thus, the evaporator may for example be a plate evaporator, a rotary evaporator or an evaporator of the forced transfer or circulation type wherein transfer or circulation is forced by a pump. Where the evaporator is of the forced transfer type it may be connected to a heat exchanger whereby an aqueous suspension to be concentrated is delivered from the heat exchanger or heater in which heat is applied to the evaporator wherein the pressure is reduced to cause evaporation without substantial application of heating. In such an arrangement, heat may be applied by a heating fluid, eg. hot water, in a jacket formed around a channel through which the aqueous suspension has to pass. In the said arrangement, the aqueous suspension to be treated may be circulated around in a loop by passing, eg. pumping, the suspension between the heat exchanger and evaporator. The suspension may be introduced and removed from such a loop as a batch or series of batches or may be introduced and removed continuously. The aqueous suspension to be evaporated may depending on the extent of concentration required conveniently be passed in turn through each of a series of evaporators which may be of the same or different types . For example, the suspension may be passed in turn through each of a series of chains or loops each containing a heat exchanger or heater and an evaporator . The or each evaporator employed in the method of the present invention may include a vessel in which water vapour is collected and extracted at an upper part of the vessel and concentrated aqueous suspension is removed from a lower part of the vessel. The extracted water vapour may be passed to a condenser, eg. cooled by a coolant liquid such as water. The heat extracted in the coolant liquid may be re-used by recirculation of the liquid to a heat exchanger used in heating the suspension. Preferably, the suspension to be concentrated in the method according to the present invention, especially where dilute, eg. containing less than 20 per cent solids, enters the evaporator at a temperature in the range of from 65°C to 85°C, and the concentrated suspension leaves the evaporator at a temperature in the range of from 35°C to 55°C. Conveniently, the suspension to be concentrated is heated to the required temperature by waste heat which may be recovered from a neighbouring plant for paper coating or paper making or waste treatment and optionally some may be obtained from the condensed water vapour. Preferably the pressure acting upon the suspension in the evaporator is reduced to within the range of from -0.5 bar to -0.95 bar. This is done in order to reduce the temperature at which water contained in the treated aqueous suspension boils to within the range from about 38°C to 85°C. This minimises the thermal degradation of heat sensitive components of the coating composition being concentrated, and also permits the use of waste heat from another part of a paper coating or paper making plant, which will typically be in the form of steam or hot water at a temperature not greater than about 85°C It may be desirable in the method according to the present invention to add one or more increments of an antifoaming agent to the dilute aqueous suspension to be concentrated by evaporation. Such an agent will form typically less than 1% by weight of the solids content and helps to avoid excessive foaming in the evaporator. The product slurry or suspension formed following concentration by evaporation in the method according to the present invention may be supplied to the plant in which it is to be used or re-used as a coating composition in a known way, eg. via a slurry transport pipeline by the action of one or more pumps. The product slurry or suspension may or may not be further treated before it is re-used in a coating process. For example, new coating pigment material may be added to the suspension if recovered from a previous operation to improve the optical properties of the pigment content . As noted above, the product slurry comprising the product of concentrating a coating composition especially where recovered from a previous operation may be mixed together with fresh unused unconcentrated coating slurry, eg. in a percentage by weight of from 2% to 30% by weight of the product slurry mixture. According to the present invention in a second aspect there is provided an aqueous coating composition for use in coating a sheet material as aforesaid which composition has been concentrated by evaporative concentration under reduced pressure. According to the present invention in a third aspect there is provided a sheet material which has been coated with a coating composition which contains partly or wholly the coating composition according to the second aspect . Examples of use of evaporation to concentrate compositions to be used for painting applications are known, eg. from WO92/07900, but such compositions are essentially based upon an organic solvent rather than water and the art relating to such compositions is a quite different art from that of coating cellulosic sheet materials using aqueous compositions treated and used as in the method according to the first aspect of the present invention. Also, evaporative concentration has been disclosed for concentration of a mixture of liquids separated from a painting suspension, eg. as in EP488449. However, use of evaporation for concentration of an aqueous coating composition and the benefits which are obtained by such concentration have not been disclosed or suggested hitherto. Embodiments of the present invention will now be described by way of example only with reference to the following Examples. Example 1 A coating composition was prepared by suspending in water the following ingredients. Such a formulation is typical of the compositions used for preparing Northern European lightweight coated paper. Ingredient Parts by Weight English coating clay 100 Styrene-butadiene latex 11 adhesive Carboxymethyl cellulose 0.3 The English coating clay was kaolin clay having a particle size distribution such that 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2 μm. The styrene-butadiene latex adhesive was supplied as an aqueous emulsion containing 50% by weight of polymer solids. The parts by weight given above represent the dry weight of polymer solids. The carboxymethyl cellulose was added as a viscosity modifier or thickener. The quantity of coating composition prepared contained 3 kg of dry solids and the final solids concentration was found to be 63.5% by weight. About half of the original total coating composition was separated to represent waste coating material and was diluted with water to a solids concentration of 5% by weight to simulate a suspension of a waste coating composition following washing. The dilute suspension was then reconcentrated to 63.0% by weight in an evaporator of the rotary type to form a recovered coating composition. The rotary evaporator was placed in a water bath at a temperature of 70°C and the pressure in the evaporator was reduced to within the range from -0.84 to -0.86 bar, under which conditions water boils at a temperature in the range from 50°C to 58°C. The viscosities of the original and recovered coating compositions were measured at low shear rate by means of a Brookfield Viscometer at 100 rpm using Spindle No. 3, and at the higher shear rates of 1280s " and 12800s " by means of a Ferranti-Shirley Viscometer. The results are set forth in Table 1 below. Table 1 Coating Composition Original Recovered Viscosity (mPa.s) Brookfield 730 870 Ferranti - Shirley 85 78 1280 s "1 Ferranti-Shirley 69 63 12800 s "1 Samples of coated paper were prepared using each of the two coating compositions at a range of coat weights from about 7 g.m -2 to about 11 g.m-2 by a bar coating technique on a precoated base paper. The samples of coated paper were tested for opacity, brightness and light scattering coefficient (S) and each measured property was plotted graphically against coat weight. The value of the property corresponding -2 to a coat weight of 8 g.m was found in each case by interpolation . The opacity of each sample of coated paper was measured by means of a DATACOLOR 2000 brightness meter fitted with a No . 10 filter (a green filter embracing a broad spectrum of wavelengths) . A measurement of the percentage of the incident light reflected was made with a stack of ten sheets of paper over the black cavity (R ) . The ten sheets were then replaced with the single sheet from the top of the stack over the black cavity and a further measurement of the percentage reflectance was made (R) . The percentage opacity was calculated from the formula: Percentage opacity = 100.R/R ∞ The procedure was performed a total of ten times with each time a different sheet of paper on the top of the stack, and the average value of the percentage opacity was determined. The brightness, or percentage reflectance to violet light of the paper formed from each of the three portions of stock was measured by means of a DATACOLOR 2000 brightness meter fitted with a No . 8 filter (457 n wavelength) . In order to determine the Kubelka-Munk Scattering Coefficient (S) pieces of the uncoated base paper were first tested for percentage reflectance to light of wavelength 457 nm when placed over a black background by means of a DATACOLOR 2000 brightness meter fitted with a No . 8 filter to give the background reflectance Rk . Each piece of coated paper was then placed (a) on a black background (Rg) ; and (b) on a pile of uncoated pieces of the paper (R^) . Finally the reflectance to light of wavelength 457 nm was measured for the pile of uncoated pieces alone (r) . From these measurements the reflectance R c of the coating alone was calculated from the formula : - Rr Rl-R " Rθ- r (Rl - Rg) - R + R - r and the transmission T c of the coating from the formula : - T c 2 = (Rg - R c ) (1 - R c R b ) Rb From these two quantities it is possible to calculate a theoretical value for the reflectance R of a coating layer of infinite thickness of the same material from the formula :- 1 - T c 2 + R c 2 1 + R-, Rr R„ The Kubelka-Munk scattering coefficient S in m 2 .kg " 1 for a coating of weight X g.m may now be calculated from the formula : - SX 1 cotr 1 i-aR c bR r where R„ The scattering coefficient S was plotted against the coat weight X and the value of S in each case for a coat weight of 8 g.m " 2 wa s found by interpolation. The results are set forth in Table 2 below. Table 2 Coating Opacity Brightness Scattering composition coefficient (S) Original 89.8 88.7 120 Recovered 89.8 88.0 140 These results show that the quality of the coating achieved by using the recovered composition is very similar to that achieved with the original composition. The brightness of the coating is very slightly reduced when the original composition is replaced by the recovered composition, but the light scattering properties are improved. Example 2 A coating composition was prepared by suspending in water the following ingredients. Such a formulation is typical of the compositions used for preparing European wood-free coated paper. Ingredien Parts by weight Ground marble 100 Acrylic latex 10 Carboxymethyl 0.5 cellulose Optical brighteni .ng 0.5 agent The ground marble had a particle size distribution such that 90% by weight consisted of particles having an equivalent spherical diameter smaller than lμm. The acrylic latex was "ACRONAL S694" manufactured by BASF. This is a styrene/n- butylacrylate/acrylonitrile co-polymer suitable for offset paper. The final solids concentration of the coating composition was found to be 66.8% by weight. A portion of the original total coating composition was diluted with water to a solids concentration of 5% by weight to simulate waste coating composition, and was then reconcentrated to 67.0% by weight in an evaporator to form a recovered coating composition. The viscosities of the original and recovered coating compositions were measured at low shear rate by means of a Brookfield Viscometer at 20 rpm using Spindle No. 5, and at the higher shear rates -1 -1 of 1280s and 12800s by means of a Ferranti -Shirley Viscometer . The results are set forth in Table 3 below. Table 3 Coating composition Original Recovered Viscosity (mPa.s) Brookfield 8100 17400 Ferranti -Shirley 140 359 1280s "1 Ferranti -Shirley 56 113 12800s "1 A blended paper coating composition was formed by mixing 15 parts by weight, based on the dry weight of solids in the composition, of the recovered coating composition with 85 parts by weight, based on the weight of dry solids in the composition, of the original coating composition. Samples of coated paper were prepared using both the original and the blended compositions by applying the compositions to a _2 precoated wood-free base paper of weight 80 g.m , using a "HELI-COATER""" laboratory paper coating machine according to GB-A-2225261. In each run the rotational speed of the drum was such as to give a paper speed of 600 m.min and the blade angle was 45°, but the force applied to bias the blade against the drum was varied from run to run to give a range of different coat -2 weights in the range of from about 7 to about 13 g.m The samples of coated paper were dried using a combination of infra red lamps and hot air and were then conditioned overnight at 23 °C and 50% relative humidity. The samples were then calendered by being passed 10 times between the rolls of a laboratory supercalender at a temperature of 65°C and a pressure of 1000 psi (6.89 mPa) , and were then conditioned again in the same way as before. The samples of calendered coated paper were tested for opacity, brightness and paper sheet gloss and each measured property was plotted graphically against coat weight . The value of the property corresponding to a _2 coat weight of 8 g.m was found in each case by interpolation. The opacity and brightness values were obtained by the techniques described in Example 1 above . The paper sheet gloss was measured by the method prescribed in TAPPI Standard No. T480 ts-65 using a Hunterlab Glossmeter set at 75° to the normal to the paper. The results are set forth in Table 4 below. Table 4 Coating Opacity Brightness Paper sheet composition gloss Original 89.8 83.0 69.0 Blended 89.5 83.1 68.0 These results show that the opacity and brightness of the coatings obtained with the blended coating composition are virtually the same as those obtained with the original composition. The apparent difference between the gloss of the calendered, coated sheet obtained with the blended composition and that obtained with the original composition is considered to be within the limits of experimental error. Example 3 A coating composition was prepared by suspending in water the following ingredients. Such a formulation is typical of the compositions used for preparing North American light weight coated paper. Inσredient P rts by weight Delaminated clay 65 No. 2 coating clay 30 Calcined clay 5 Styrene butadiene latex 8 adhesive Starch adhesive 8 The delaminated clay was a kaolin clay from Georgia, USA having a particle size distribution such that 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm. ("ASTRAPLATE 100") . The No. 2 coating clay was a kaolin clay from Georgia, USA having a particle size distribution such that 83% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm. ("KCS") . The calcined clay was a metakaolin prepared by calcining a kaolin clay from Georgia, USA, and had a particle size distribution such that 90% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm. ( "ALPHATEX" ) . The starch adhesive ("PG 280") was a hydroxyethyl ether derivative of maize starch, having a degree of substitution of 0.05. The final solids concentration of the coating composition was found to be 62.3% by weight. A portion of the original total coating composition was diluted with water to a solids concentration of 5% by weight to simulate waste coating composition, and was then reconcentrated to 62.1% by weight in an evaporator to form a recovered coating composition. The viscosities of the original and recovered coating compositions were measured at low shear rate by means of a Brookfield Viscometer at 20 rpm using Spindle No. 4, and at the higher shear rates of 1280s " and 12800s " by means of a Ferranti -Shirley Viscometer . The results are set forth in Table 5 below. Table 5 Coating composition Original Recovered Viscosity (mPa.s) Brookfield 6000 7900 Ferranti -Shirley 212 300 1280s "1 Ferranti -Shirley 153 242 12800s "1 A blended paper coating composition was formed by mixing 30 parts by weight, based on the weight of dry solids in the composition, of the recovered coating composition with 70 parts by weight, based on the weight of dry solids in the composition, of the original coating composition. Samples of coated paper were prepared using both the original and the blended compositions by applying the compositions to a base _2 paper of weight 45.4 g.m , using the "HELI-COATER' M " laboratory paper coating machine. In each run the rotational speed of the drum was such as to give a paper speed of 600 m.min and the blade angle was 45°, but the force applied to bias the blade against the drum was varied from run to run to give a range of different coat weights in the range of from about 6 to _2 about 10 g.m . The samples of coated paper were dried using a combination of infra-red lamps and hot air and were then conditioned overnight at 23 °C and 50% relative humidity. The samples were then calendered by being passed 10 times between the rolls of a laboratory supercalender at a temperature of 65 °C and a pressure of 1000 psi (6.89 mPa) , and were then conditioned again in the same way as before. The samples of calendered coated paper were tested for opacity, brightness and paper sheet gloss by the techniques described in Examples 1 and 2 above, and each measured property was plotted graphically against coat weight. The value of the property corresponding -2 to a coat weight of 8 g.m was found in each case by interpolation . The results are set forth in Table 6 below. Table 6 Coating Opacity Brightness Paper sheet composition gloss Original 91.3 74.2 61.0 Blended 91.3 74.2 61.5 These results show that the opacity, brightness and gloss of the coatings obtained with the blended coating composition are virtually the same as those obtained with the original composition. Example 4 A coating composition was prepared by suspending in water the following ingredients. Such a formulation is typical of the compositions used for preparing a coated paper for rotogravure printing. Ingredient Parts by weight Coating clay 50 Coating talc 50 Acrylic latex 5 adhesive The coating clay had a particle size distribution such that 52% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm and was "platey" , in other words of a very high particle aspect ratio, eg. greater than 25:1. The coating talc had a particle size distribution such that 45% by weight consisted of particles having an equivalent spherical diameter smaller than 2μm. The acrylic latex was "ACRONAL 538V" manufactured by BASF (an alkali- swellable styrene/acrylic copolymer) . The final solids concentration of the coating composition was found to be 58.1% by weight. A portion of the original total coating composition was diluted with water to a solids concentration of 5% by weight to simulate waste coating composition, and was then reconcentrated to 60.1% by weight in an evaporator to form a recovered coating composition. The evaporator was placed on a water bath at 70 °C and was operated under a vacuum of from -0.84 to -0.88 bar. Under these conditions, water boiled at a temperature in the range of from 50°C to 58°C. The viscosities of the original and recovered coating compositions were measured at low shear rate by means of a Brookfield Viscometer at 100 rpm using Spindle No. 3, and at the higher shear rates of 1280s "1 and 12800s 1 by means of a Ferranti -Shirley Viscometer. In order to obtain a direct comparison, before the measurement of the viscosity at the low shear rate, both suspensions were diluted to a uniform solids concentration of 55% by weight. The results are set forth in Table 7 below. Table 7 Coating Original Recovered j composition Viscosity (mPa.s) Brookfield 500 260 Ferranti-Shirley 83 67 1280 s "1 Ferranti -Shirley 62 60 12800 s "1 Blended paper coating compositions were formed by mixing the recovered coating composition with the original coating composition in the proportions, respectively: 90 parts by weight original/10 parts by weight recovered 85 parts by weight original/15 parts by weight recovered 75 parts by weight original/25 parts by weight recovered. Samples of coated paper were prepared using both the original and the blended compositions by applying the compositions to a precoated wood- free base paper of weight 36 g.m "2 , using a "HELI-COATER"™ laboratory paper coating machine according to GB-A-2225261. In each run the rotational speed of the drum was such as to give a paper speed of 800 m.min "1 and the blade angle was 45°, but the force applied to bias the blade against the drum was varied from run-to-run to give a range of different coat weights in the range of from about 5 to about 9 g.m " . The samples of coated paper were dried using a combination of infra red lamps and hot air and were then conditioned overnight at 23 °C and 50% relative humidity. The samples were then calendered by being passed 10 times between the rolls of a laboratory supercalender at a temperature of 65°C and a pressure of 1000 psi (6.89 mPa) , and were then conditioned again in the same way as before . The samples of calendered coated paper were tested for opacity, brightness and paper sheet gloss and each measured property was plotted graphically against coat weight . The value of the property corresponding to a coat weight of 7 g.m was found in each case by interpolation . The opacity and brightness values were obtained by the techniques described in Example 1 above . The paper sheet gloss was measured by the method prescribed in TAPPI Standard No. T480 ts-65 using a Hunterlab " " Glossmeter set at 75° to the normal to the paper. The results are set forth in Table 8 below. Table 8 Coating Opacity Brightness Paper composition sheet gloss Original 86.4 70.0 49 Blended 86.3 70.1 52 90/10 Blended 86.3 70.4 51 85/15 Blended 86.8 70.1 52 75/25 Within the limits of experimental error, there is no detectable difference in the opacity, brightness or sheet gloss results for the four different coating compositions . Even up to 25% by weight of the recovered coating composition can be included without causing any deterioration in these properties. Further samples of the calendered coated paper were subjected to rotogravure printing tests to determine, respectively, the print gloss, print density and the percentage of missing dots by methods described in the article "Realistic paper tests for various printing processes" by A. Swan, published in "Printing Technology", Vol. 13, No. 1, April 1969, pages 9-22. A gravure printing cylinder was used with an area of deeply etched cells to give a solid black area, and an area of less deeply etched cells to give a half-tone area. The half-tone area was used to estimate the percentage of gravure dots which were missing from the test print. This percentage was expressed as "% missing dots" . The solid black area was used to measure the gravure print density using a Macbeth™ density meter and the print gloss using the Hunterlab™ Glossmeter set at 75° to the normal to the paper in accordance with TAPPI Standard No. T480ts-65. The results are set forth in Table 9 below. Table 9 Coating Print Print % missing composition gloss density dots Original 73 1.88 3.5 Blended 73 1.88 3.3 90/10 Blended 73 1.89 3.4 85/15 Blended 73 1.89 3.5 75/25 Again there was no detectable difference in any of the three measured properties for the four paper coating compositions, even with a mixture containing 25% by weight of the recovered composition. Example 5 A wood free paper topcoating composition was prepared incorporating the following solids components present in the stated parts by weight: Pigment 100 parts by weight Calcium carbonate slurry having a solids content of 78 per cent by weight and 90 per cent of the particles having a size less than 2μm Acrylic latex binder 12 parts by weight Acrylic thickener 0.15 parts by weight Polyvinyl alcohol 0.5 parts by weight Optical brightening agent 0.5 parts by weight The polyvinyl alcohol acts as an enhancer to the acrylic latex binder and to the optical brightening agent . A batch of this composition was prepared at 71% solids level, diluted to 15% solids and evaporatively concentrated back to 71% solids. A further batch was kept at 71% solids without dilution. Samples of the diluted and reconcentrated batch were blended with samples of the undiluted batch. The undiluted batch samples are referred to as samples A. The samples from the diluted and reconcentrated batch constituted respectively amounts of 10%, 20% and 30% by weight of the blend in each case and are referred to as samples B, C and D. The blended composition in each case and also the undiluted sample A itself were each separately applied to sheets of a woodfree base paper using a Helicoater™ machine in its bent blade (variable blade angle) mode. Coat weights of 9 g "1 , 11 g "1 and 13 g "1 were applied on different bases for each blend investigated using different coater blade angles. Runnability of the coating machine employing each composition was investigated in the usual way by plotting graphs of coating weight against blade angle, the concentration of each composition being varied in small steps. This allowed the lowest composition concentration at which scratching first occurred to be determined from the graphs. The results obtained are given in Table 10 as follows Table 10 Sample Percentage of Lowest concentration diluted and at which scratching reconcentrated occurred composition (% solids by weight) A 0 69.3 B 10 70.2 C 20 70.5 D 30 above 70.5 Table 10 shows that improvement in the amount of runnable solids of the fresh composition (Sample A) is obtained when the product of dilution and reconcentration (Samples B to D) is added to the fresh composition (Sample A) . The improvement is seen by observation of the lowest concentration at which postblade scratching occurs.

Claims: CLAIMS 1. A method of treatment and use of an aqueous coating composition which includes the steps of (a) concentrating the composition to increase the solids content thereof and (b) employing the concentrated composition to coat a sheet material, wherein the concentration step (a) is carried out by use of thermal evaporation under reduced pressure. 2. A method as claimed in claim 1 and which comprises a method for recovering and recycling suspended and dissolved components from a dilute aqueous suspension of a waste coating composition from a sheet material coating operation, the recovered components being treated in the form of a dilute suspension in the said step (a) . 3. A method as claimed in claim 2 and wherein the recovered components are re-used in the coating operation from which they were recovered. 4. A method as claimed in claim 2 or claim 3 and wherein the dilute suspension contains not more than 15% by weight of solid material. 5. A method as claimed in claim 1, 2, 3 or 4 and wherein the evaporation is applied in one or more steps until the solids content of the suspension has reached at least 55% by weight. 6. A method as claimed in any one of the preceding claims and wherein the evaporation in at least one step is applied in an evaporator which is evacuated but in which no heat is applied, the suspension being heated by a heat exchanger or heater prior to delivery to the evaporator. 7. A method as claimed in claim 6 and wherein the suspension to be concentrated is circulated in a loop or in each of a series of loops containing a heat exchanger or heater and an evaporator. 8. A method as claimed in claim 7 and wherein the suspension enters the evaporator at a temperature in the range of from 65°C to 85°C and leaves the evaporator at a temperature in the range of from 35°C to 55°C. 9. A method as claimed in any one of the preceding claims and wherein the composition concentrated in step (a) includes inorganic components comprising one or more of hydrous kaolin clay, calcined kaolin clay, calcium carbonate, dolomite, calcium sulphate, titanium dioxide, mica, satin white and barium sulphate and composite pigment materials containing any one or more of these materials. 10. A method as claimed in claim 1 and wherein the coating composition has not previously been used in coating. 11. A method as claimed in claim 10 and wherein the coating composition has a steep particle size distribution. 12. An aqueous composition for a sheet material coating operation which incorporates material concentrated in a method as claimed in any one of the preceding claims. 13. A sheet material which has been coated with a coating composition which contains partly or wholly the coating composition as claimed in claim 12.

Inventors: GOLLEY CHRISTOPHER ROBIN LANGD, TOIVONEN HANNU OLAVI ENSIO, GOLLEY, CHRISTOPHER, ROBIN, LANGDON, TOIVONEN, HANNU, OLAVI, ENSIO
Applicants: ECC INT LTD, GOLLEY CHRISTOPHER ROBIN LANGD, TOIVONEN HANNU OLAVI ENSIO, ECC INTERNATIONAL LTD., GOLLEY, CHRISTOPHER, ROBIN, LANGDON, TOIVONEN, HANNU, OLAVI, ENSIO
Application year: 1997
Main classification: D21H23
IPC codes: B01D1, B05D3, B05D7, C09D5, D21H23