MANUFACTURE OF PAPER AND PAPERBOARD

Abstract: A process of making paper or paper board comprising forming a cellulosic suspension, flocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, characterised in that the suspension is flocculated using a flocculation system comprising a siliceous material and an anionic branched water soluble polymer that has been formed from water soluble ethylenically unsaturated anionic monomer or monomer blend and branching agent and wherein the polymer has (a) intrinsic viscosity above 1.5 dl/g and/or saline Brookfield viscosity of above about 2.0 mPa.s and (b) rheological oscillation value of tan delta at 0.005 Hz of above 0.7 and/or (c) deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding unbranched polymer made in the absence of branching agent.

Description: Manufacture of Paper and Paperboard This invention relates to processes of making paper and paperboard from a cellulosic stock, employing a novel flocculating system. During the manufacture of paper and paper board a cellulosic thin stock is drained on a moving screen (often referred to as a machine wire) to form a sheet which is then dried. It is well known to apply water soluble polymers to the cellulosic suspension in order to effect flocculation of the cellulosic solids and enhance drainage on the moving screen. In order to increase output of paper many modern paper making machines operate at higher speeds. As a consequence of increased machine speeds a great deal of emphasis has been placed on drainage and retention systems that provide increased drainage. However, it is known that increasing the molecular weight of a polymeric retention aid which is added immediately prior to drainage will tend to increase the rate of drainage but damage formation. It is difficult to obtain the optimum balance of retention, drainage, drying and formation by adding a single polymeric retention aid and it is therefore common practice to add two separate materials in sequence. EP-A-235893 provides a process wherein a water soluble substantially linear cationic polymer is applied to the paper making stock prior to a shear stage and then reflocculating by introducing bentonite after that shear stage. This process provides enhanced drainage and also good formation and retention. This process which is commercialised by Ciba Specialty Chemicals under the Hydrocol ® trade mark has proved successful for more than a decade. More recently there have been various attempts to provide variations on this theme by making minor modifications to one or more of the components. US-A-5393381 describes a process in which a process of making paper or board by adding a water soluble branched cationic polyacrylamide and a bentonite to the fibrous suspension of pulp. The branched cationic polyacrylamide is prepared by polymerising a mixture of acrylamide, cationic monomer, branching agent and chain transfer agent by solution polymerisation. US-A-5882525 describes a process in which a cationic branched water soluble polymer with a solubility quotient greater than about 30% is applied to a dispersion of suspended solids, e.g. a paper making stock, in order to release water. The cationic branched water soluble polymer is prepared from similar ingredients to US-A-5393381 i.e. by polymerising a mixture of acrylamide, cationic monomer, branching agent and chain transfer agent. In WO-A-9829604 a process of making paper is described in which a cationic polymeric retention aid is added to a cellulosic suspension to form floes, mechanically degrading the floes and then reflocculating the suspension by adding a solution of a second anionic polymeric retention aid. The anionic polymeric retention aid is a branched polymer which is characterised by having a rheological oscillation value of tan delta at 0.005Hz of above 0.7 or by having a deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding polymer made in the absence of branching agent. The process provided significant improvements in the combination of retention and formation by comparison to the earlier prior art processes. EP-A-308752 describes a method of making paper in which a low molecular weight cationic organic polymer is added to the furnish and then a colloidal silica and a high molecular weight charged acrylamide copolymer of molecular weight at least 500,000. The description of the high molecular weight polymers indicates that they are linear polymers. However, there still exists a need to further enhance paper making processes by further improving drainage, retention and formation. Furthermore there also exists the need for providing a more effective flocculation system for making highly filled paper. According to the present invention a process is provided for making paper or paper board comprising forming a cellulosic suspension, flocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, characteπsed in that the suspension is flocculated using a flocculation system comprising a siliceous material and an anionic branched water soluble polymer that has been formed from water soluble ethylenically unsaturated anionic monomer or monomer blend and branching agent and wherein the polymer has (a) intrinsic viscosity above 1.5 dl/g and/or saline Brookfield viscosity of above about 2.0 mPa.s and (b) rheological oscillation value of tan delta at 0.005Hz of above 0.7 and/or (c) deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding unbranched polymer made in the absence of branching agent. It has surprisingly been found that flocculating the cellulosic suspension using a flocculation system that comprises a siliceous material and anionic branched water soluble polymer with the special rheological characteristics provides improvements in retention, drainage and formation by comparison to using the anionic branched polymer in the absence of the siliceous material or the siliceous material in the absence of the anionic branched polymer. The siliceous material may be any of the materials selected from the group consisting of silica based particles, silica microgels, colloidal silica, silica sols, silica gels, polysilicates, aluminosilicates, polyaluminosilicates, borosilicates, polyborosilicates and zeolites. This siliceous material may be in the form of an anionic microparticulate material. Alternatively the siliceous material may be a cationic silica. Desirably the siliceous material may be selected from silicas and polysilicates. The silica may be for example any colloidal silica, for instance as described in WO-A-8600100. The polysilicate may be a colloidal silicic acid as described in US- A-4,388,150. The polysilicates of the invention may be prepared by acidifying an aqueous solution of an alkali metal silicate. For instance polysilicic microgels otherwise known as active silica may be prepared by partial acidification of alkali metal silicate to about pH 8-9 by use of mineral acids or acid exchange resins, acid salts and acid gases. It may be desired to age the freshly formed polysilicic acid in order to allow sufficient three dimensional network structure to form. Generally the time of ageing is insufficient for the polysilicic acid to gel. Particularly preferred siliceous material include polyalumino-silicates. The polyaluminosilicates may be for instance aluminated polysilicic acid, made by first forming polysilicic acid microparticles and then post treating with aluminium salts, for instance as described in US-A-5, 176,891. Such polyaluminosilicates consist of silicic microparticles with the aluminium located preferentially at the surface. Alternatively the polyaluminosilicates may be polyparticulate poiysicilic microgels of surface area in excess of 1000m 2 /g formed by reacting an alkali metal silicate with acid and water soluble aluminium salts, for instance as described in US-A- 5,482,693. Typically the polyaluminosilicates may have a mole ratio of alumina:siiica of between 1 :10 and 1 :1500. Polyaluminosilicates may be formed by acidifying an aqueous solution of alkali metal silicate to pH 9 or 10 using concentrated sulphuric acid containing 1.5 to 2.0% by weight of a water soluble aluminium salt, for instance aluminium sulphate. The aqueous solution may be aged sufficiently for the three dimensional microgel to form. Typically the polyaluminosilicate is aged for up to about two and a half hours before diluting the aqueous polysilicate to 0.5 weight % of silica. The siliceous material may be a colloidal borosilicate, for instance as described in WO-A-9916708. The colloidal borosilicate may be prepared by contacting a dilute aqueous solution of an alkali metal silicate with a cation exchange resin to produce a silicic acid and then forming a heel by mixing together a dilute aqueous solution of an alkali metal borate with an alkali metal hydroxide to form an aqueous solution containing 0.01 to 30 % B 2 0 3 , having a pH of from 7 to 10.5. The anionic branched polymer is formed from a water soluble monomer blend comprising at least one anionic or potentially anionic ethylenically unsaturated monomer and a small amount of branching agent for instance as described in WO-A-9829604. Generally the polymer will be formed from a blend of 5 to 100% by weight anionic water soluble monomer and 0 to 95% by weight non-ionic water soluble monomer. Typically the water soluble monomers have a solubility in water of at least 5g/100cc. The anionic monomer is preferably selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, crotonic acid, itaconic acid, 2-acrylamido-2-methylpropane sulphonic acid, allyl sulphonic acid and vinyl sulphonic acid and alkali metal or ammonium salts thereof. The non-ionic monomer is preferably selected from the group consisting of acrylamide, methacrylamide, N-vinyl pyrrolidone and hydroxyethyl acrylate. A particularly preferred monomer blend comprises acrylamide and sodium acrylate. The branching agent can be any chemical material that causes branching by reaction through the carboxylic or other pendant groups (for instance an epoxide, silane, polyvalent metal or formaldehyde). Preferably the branching agent is a polyethylenically unsaturated monomer which is included in the monomer blend from which the polymer is formed. The amounts of branching agent required will vary according to the specific branching agent. Thus when using polyethylenically unsaturated acrylic branching agents suvh as methylene bis acrylamide the molar amount is usually below 30 molar ppm and preferably below 20 ppm. Generally it is below 10 ppm and most preferably below 5 ppm. The optimum amount of branching agent is preferably from around 0.5 to 3 or 3.5 molar ppm or even 3.8 ppm but in some instances it may be desired to use 7 or 10 ppm. Preferably the branching agent is water-soluble. Typically it can be a difunctional material such as methylene bis acrylamide or it can be a trifunctional, tetrafunctional or a higher functional cross-linking agent, for instance tetra allyl ammonium chloride. Generally since allylic monomer tend to have lower reactivity ratios, they polymerise less readily and thus it is standard practice when using polyethylenically unsaturated allylic branching agents, such as tetra allyl ammonium chloride to use higher levels, for instance 5 to 30 or even 35 molar ppm or even 38 ppm and even as much as 70 or 100 ppm. It may also be desirable to include a chain transfer agent into the monomer mix. Where chain transfer agent is included it may be used in an amount of at least 2 ppm by weight and may also be included in an amount of up to 200 ppm by weight. Typically the amounts of chain transfer agent may be in the range 10 to 50 ppm by weight. The chain transfer agent may be any suitable chemical substance, for instance sodium hypophosphite, 2-mercaptoethanol, malic acid or thioglycolic acid. Preferably, however, the anionic branched polymer is prepared in the absence of added chain transfer agent. The anionic branched polymer is generally in the form of a water-in-oil emulsion or dispersion. Typically the polymers are made by reverse phase emulsion polymerisation in order to form a reverse phase emulsion. This product usually has a particle size at least 95% by weight below 10μm and preferably at least 90% by weight below 2μm, for instance substantially above 100nm and especially substantially in the range 500nm to 1μm. The polymers may be prepared by conventional reverse phase emulsion or microemulsion polymerisation techniques. The tan delta at 0.005Hz value is obtained using a Controlled Stress Rheometer in Oscillation mode on a 1.5% by weight aqueous solution of polymer in deionised water after tumbling for two hours. In the course of this work a Cammed CSR 100 is used fitted with a 6cm acrylic cone, with a 1°58' cone angle and a 58μm truncation value (Item ref 5664). A sample volume of approximately 2-3cc is used. Temperature is controlled at 20.0°C ± 0.1 °C using the Peltier Plate. An angular displacement of 5 X 10 "4 radians is employed over a frequency sweep from 0.005Hz to 1 Hz in 12 stages on a logarithmic basis. G' and G" measurements are recorded and used to calculate tan delta (G'VG') values. The value of tan delta is the ratio of the loss (viscous) modulus G" to storage (elastic) modulus G' within the system. At low frequencies (0.005Hz) it is believed that the rate of deformation of the sample is sufficiently slow to enable linear or branched entangled chains to disentangle. Network or cross-linked systems have permanent entanglement of the chains and show low values of tan delta across a wide range of frequencies, Therefore low frequency (e.g. 0.005Hz) measurements are used to characterise the polymer properties in the aqueous environment. The anionic branched polymers should have a tan delta value at 0.005Hz of above 0.7. Preferred anionic branched polymers have a tan delta value of 0.8 at 0.005Hz. Preferably the intrinsic viscosity is at least 2 dl/g, for instance at least 4 dl/g, in particular at least 5 or 6 dl/g. It may be desirable to provide polymers of substantially higher molecular weight, which exhibit intrinsic viscosities as high as 16 or 18 dl/g. However most preferred polymers have intrinsic viscosities in the range 7 to 12 dl/g, especially 8 to 10 dl/g. The preferred branched anionic polymer can also be characterised by reference to the corresponding polymer made under the same polymerisation conditions but in the absence of branching agent (i.e., the "unbranched polymer"). The unbranched polymer generally has an intrinsic viscosity of at least 6 dl/g and preferably at least 8 dl/g. Often it is 16 to 30 dl/g. The amount of branching agent is usually such that the intrinsic viscosity is reduced by 10 to 70%, or sometimes up to 90%, of the original value (expressed in dl/g) for the unbranched polymer referred to above. The saline Brookfield viscosity of the polymer is measured by preparing a 0.1% by weight aqueous solution of active polymer in 1 M NaCI aqueous solution at 25°C using a Brookfield viscometer fitted with a UL adaptor at 6rpm. Thus, powdered polymer or a reverse phase polymer would be first dissolved in deionised water to form a concentrated solution and this concentrated solution is diluted with the 1M NaCI aqueous. The saline solution viscosity is generally above 2.0mPa.s and is usually at least 2.2 and preferably at least 2.5mPa.s. Generally it is not more than 5mPa.s and values of 3 to 4 are usually preferred. These are all measured at 60rpm. The SLV viscosity numbers used to characterise the anionic branched polymer are determined by use of a glass suspended level viscometer at 25°C, the viscometer being chosen to be appropriate according to the viscosity of the solution. The viscosity number is η-η 0 /η 0 where η and η 0 are the viscosity results for aqueous polymer solutions and solvent blank respectively. This can also be referred to as specific viscosity. The deionised SLV viscosity number is the number obtained for a 0.05% aqueous solution of the polymer prepared in deionised water. The salted SLV viscosity number is the number obtained for a 0.05% polymer aqueous solution prepared in 1 M sodium chloride. The deionised SLV viscosity number is preferably at least 3 and generally at least 4, for instance up to 7, 8 or higher. Best results are obtained when it is above 5. Preferably it is higher than the deionised SLV viscosity number for the unbranched polymer, that is to say the polymer made under the same polymerisation conditions but in the absence of the branching agent (and therefore having higher intrinsic viscosity). If the deionised SLV viscosity number is not higher than the deionised SLV viscosity number of the unbranched polymer, preferably it is at least 50% and usually at least 75% of the deionised SLV viscosity number of the unbranched polymer. The salted SLV viscosity number is usually below 1. The deionised SLV viscosity number is often at least five times, and preferably at least eight times, the salted SLV viscosity number. According to the invention the components of the flocculation system may be combined into a mixture and introduced into the cellulosic suspension as a single composition. Alternatively the anionic branched polymer and the siliceous material may be introduced separately but simultaneously. Preferably, however, the siliceous material and the anionic branched polymer are introduced sequentially more preferably when the siliceous material is introduced into the suspension and then the anionic branched polymer. In a preferred form of the invention the water soluble anionic branched polymer and siliceous material are added to the cellulosic suspension, which suspension has been pre-treated with a cationic material. The cationic pre-treatment may be by incorporating cationic materials into the suspension at any point prior to the addition of the anionic branched polymer and siliceous material. Thus the cationic treatment may be immediately before adding the anionic branched polymer and siliceous material although preferably the cationic material is introduced into the suspension sufficiently early in order for it to be distributed throughout the cellulosic suspension before either the anionic branched polymer or siliceous material are added. It may be desirable to add the cationic material before one of the mixing, screening or cleaning stages and in some instances before the stock suspension is diluted. It may even be beneficial to add the cationic material into the mixing chest or blend chest or even into one or more of the components of the cellulosic suspension, for instance, coated broke or filler suspensions for instance precipitated calcium carbonate slurries. The cationic material may be any number of cationic species such as water soluble cationic organic polymers, or inorganic materials such as alum, polyaluminium chloride, aluminium chloride trihydrate and aluminochloro hydrate. The water soluble cationic organic polymers may be natural polymers, such as cationic starch or synthetic cationic polymers. Particularly preferred are cationic materials that coagulate or flocculate the cellulosic fibres and other components of the cellulosic suspension. According to another preferred aspect of the invention the flocculation system comprises at least three flocculant components. Thus this preferred system employs a water soluble branched anionic polymer, siliceous material and at least one additional flocculant/coagulant. The additional flocculant/coagulant component is preferably added prior to either the siliceous material or anionic branched polymer. Typically the additional flocculant is a natural or synthetic polymer or other material capable of causing flocculation/coagulation of the fibres and other components of the cellulosic suspension. The additional flocculant/coagulant may be a cationic, non-ionic, anionic or amphoteric natural or synthetic polymer. It may be a natural polymer such as natural starch, cationic starch, anionic starch or amphoteric starch. Alternatively it may be any water soluble synthetic polymer which preferably exhibits ionic character. The preferred ionic water soluble polymers have cationic or potentially cationic functionality. For instance the cationic polymer may comprise free amine groups which become cationic once introduced into a cellulosic suspension with a sufficiently low pH so as to protonate free amine groups. Preferably however, the cationic polymers carry a permanent cationic charge, such as quaternary ammonium groups. The additional flocculant/coagulant may be used in addition to the cationic pre- treatment step described above. In a particularly preferred system the cationic pre-treatment is also the additional flocculant/coagulant. Thus this preferred process comprises adding a cationic flocculant/coagulant to the cellulosic suspension or to one or more of the suspension components thereof, in order to cationically pre-treat the cellulosic suspension. The suspension is susbsequently subjected to further flocculation stages comprising addition of the water soluble anionic branched polymer and the siliceous material. The cationic flocculant/coagulant is desirably a water soluble polymer which may for instance be a relatively low molecular weight polymer of relatively high cationicity. For instance the polymer may be a homopolymer of any suitable ethylenically unsaturated cationic monomer polymerised to provide a polymer with an intrinsic viscosity of up to 3 dl/g. Homopolymers of diallyl dimethyl ammonium chloride are preferred. The low molecular weight high cationicity polymer may be an addition polymer formed by condensation of amines with other suitable di- or tri- functional species. For instance the polymer may be formed by reacting one or more amines selected from dimethyl amine, trimethyl amine and ethylene diamine etc and epihalohydrin, epichlorohydrin being preferred. Preferably the cationic flocculant/coagulant is a polymer that has been formed from a water soluble ethylenically unsaturated cationic monomer or blend of monomers wherein at least one of the monomers in the blend is cationic or potentially cationic. By water soluble we mean that the monomer has a solubility in water of at least 5g/100cc. The cationic monomer is preferably selected from di allyl di alkyl ammonium chlorides, acid addition salts or quaternary ammonium salts of either dialkyl amino alkyl (meth) acrylate or dialkyl amino alkyl (meth) acrylamides. The cationic monomer may be polymerised alone or copolymerised with water soluble non-ionic, cationic or anionic monomers. More preferably such polymers have an intrinsic viscosity of at least 3 dl/g, for instance as high as 16 or 18 dl/g, but usually in the range 7 or 8 to 14 or 15 dl/g. Particularly preferred cationic polymers include copolymers of methyl chloride quaternary ammonium salts of dimethylaminoethyl acrylate or methacrylate. The water soluble cationic polymer may be a polymer with a rheological oscillation value of tan delta at 0.005Hz of above 1.1 (defined by the method given herein) for instance as provided for in copending patent application based on the priority US patent application number 60/164,231 (reference PP/W-21916/P1/AC 526) filed with equal date to the priority of the present application. The water soluble cationic polymer may also have a slightly branched structure for instance by incorporating small amounts of branching agent e.g. up to 20 ppm by weight. Typically the branching agent includes any of the branching agents defined herein suitable for preparing the branched anionic polymer. Such branched polymers may also be prepared by including a chain transfer agent into the monomer mix. The chain transfer agent may be included in an amount of at least 2 ppm by weight and may be included in an amount of up to 200 ppm by weight. Typically the amounts of chain transfer agent are in the range 10 to 50 ppm by weight. The chain transfer agent may be any suitable chemical substance, for instance sodium hypophosphite, 2-mercaptoethanol, malic acid or thioglycolic acid. Branched polymers comprising chain transfer agent may be prepared using higher levels of branching agent, for instance up to 100 or 200 ppm by weight, provided that the amounts of chain transfer agent used are sufficient to ensure that the polymer produced is water soluble. Typically the branched cationic water soluble polymer may be formed from a water soluble monomer blend comprising at least one cationic monomer, at least 10 molar ppm of a chain transfer agent and below 20 molar ppm of a branching agent. Preferably the branched water soluble cationic polymer has a rheological oscillation value of tan delta at 0.005Hz of above 0.7 (defined by the method given herein). Typically the branched cationic polymers have an instrinsic viscosity of at least 3 dl/g, Typically the polymers may have an intrinsic viscosity in the range 4 or 5 up to 18 or 19 dl/g. Preferred polymers have an intrinsic viscosity of from 7 or 8 to about 12 or 13 dl/g. The cationic water soluble polymers may also be prepared by any convenient process, for instance by solution polymerisation, water-in-oil suspension polymerisation or by water-in-oil emulsion polymerisation. Solution polymerisation results in aqueous polymer gels which can be cut dried and ground to provide a powdered product. The polymers may be produced as beads by suspension polymerisation or as a water-in-oil emulsion or dispersion by water-in-oil emulsion polymerisation, for example according to a process defined by EP-A-150933, EP- A-102760 or EP-A-126528. When the flocculation system comprises cationic polymer, it is generally added in an amount sufficient to effect flocculation. Usually the dose of cationic polymer would be above 20 ppm by weight of cationic polymer based on dry weight of suspension. Preferably the cationic polymer is added in an amount of at least 50 ppm by weight for instance 100 to 2000 ppm by weight. Typically the polymer dose may be 150 ppm to 600 ppm by weight, especially between 200 and 400 ppm. Typically the amount of anionic branched polymer may be at least 20 ppm by weight based on weight of dry suspension, although preferably is at least 50 ppm by weight, particularly between 100 and 2000 ppm by weight. Doses of between 150 and 600 ppm by weight are more preferred, especially between 200 and 400 ppm by weight. The siliceous material may be added at a dose of at least 100 ppm by weight based on dry weight of suspension. Desirably the dose of siliceous material may be in the range of 500 or 750 ppm to 10,000 ppm by weight. Doses of 1000 to 2000 ppm by weight siliceous material have been found to be most effective. In one preferred form of the invention the cellulosic suspension is subjected to mechanical shear following addition of at least one of the components of the flocculating system. Thus in this preferred form at least one component of the flocculating system is mixed into the cellulosic suspension causing flocculation and the flocculated suspension is then mechanically sheared. This shearing step may be achieved by passing the flocculated suspension through one or more shear stages, selected from pumping, cleaning or mixing stages. For instance such shearing stages include fan pumps and centri-screens, but could be any other stage in the process where shearing of the suspension occurs. The mechanical shearing step desirably acts upon the flocculated suspension in such a way as to degrade the floes. All of the components of the flocculating system may be added prior to a shear stage although preferably at least the last component of the flocculating system is added to the cellulosic suspension at a point in the process where there is no substantial shearing before draining to form the sheet. Thus it is preferred that at least one component of the flocculating system is added to the cellulosic suspension and the flocculated suspension is then subjected to mechanical shear wherein the floes are mechanically degraded and then at least one component of the flocculating system is added to reflocculate the suspension prior to draining. According to a more preferred form of the invention the water-soluble cationic polymer is added to the cellulosic suspension and then the suspension is then mechanically sheared. The siliceous material and the water-soluble branched anionic polymer are then added to the suspension. The anionic branched polymer and siliceous material may be added either as a premixed composition or separately but simultaneously but preferably they are added sequentially. Thus the suspension may be re-flocculated by addition of the branched anionic polymer followed by the siliceous material but preferably the suspension is reflocculated by adding siliceous material and then the anionic branched polymer. The first component of the flocculating system may be added to the cellulosic suspension and then the flocculated suspension may be passed through one or more shear stages. The second component of the flocculation system may be added to re-flocculate the suspension, which re-flocculated suspension may then be subjected to further mechanical shearing. The sheared reflocculated suspension may also be further flocculated by addition of a third component of the flocculation system. In the case where the addition of the components of the flocculation system is separated by shear stages it is preferred that the branched anionic polymer is the last component to be added. In another form of the invention the suspension may not be subjected to any substantial shearing after addition of any of the components of the flocculation system to the cellulosic suspension. The siliceous material, anionic branched polymer and where included the water soluble cationic polymer may all be introduced into the cellulosic suspension after the last shear stage prior to draining. In this form of the invention the water-soluble branched polymer may be the first component followed by either the cationic polymer (if included) and then the siliceous material. However, other orders of addition may also be used. In one preferred form of the invention we provide a process of preparing paper from a cellulosic stock suspension comprising filler. The filler may be any of the traditionally used filler materials. For instance the filler may be clay such as kaolin, or the filler may be a calcium carbonate which could be ground calcium carbonate or in particular precipitated calcium carbonate, or it may be preferred to use titanium dioxide as the filler material. Examples of other filler materials also include synthetic polymeric fillers. Generally a cellulosic stock comprising substantial quantities of filler are more difficult to flocculate. This is particularly true of fillers of very fine particle size, such as precipitated calcium carbonate. Thus according to a preferred aspect of the present invention we provide a process for making filled paper. The paper making stock may comprise any suitable amount of filler. Generally the cellulosic suspension comprises at least 5% by weight filler material. Typically the amount of filler will be up to 40%, preferably between 10% and 40% filler. Where filler is used it may be present in the final sheet of paper or paper board in an amount of up to 40%. Thus according to this preferred aspect of this invention we provide a process for making filled paper or paper board wherein we first provide a cellulosic suspension comprising filler and in which the suspension solids are flocculated by introducing into the suspension a flocculating system comprising a siliceous material and water- soluble anionic branched polymer as defined herein. In an alternative form of the invention we provide a process of preparing paper or paperboard from a cellulosic stock suspension which is substantially free of filler. The following examples illustrate the invention. Example 1 (comparative) The drainage properties are determined using a modified Schopper-Riegler apparatus, with the rear exit blocked so the drainage water exits through the front opening. The cellulosic stock used is a 50/50 bleached birch/bleached pine suspension containing 40% by weight (on total solids) precipitated calcium carbonate. The stock suspension is beaten to a freeness of 55° (Schopper Riegler method) before the addition of filler. 5kg per tonne (on total solids) cationic starch (0.045 DS) is added to the suspension. A copolymer of acrylamide with methyl chloride quaternary ammonium salt of dimethylaminoethyl acrylate (75/25 wt./wt.) of intrinsic viscosity above 11.0 dl/g (Product A) is mixed with the stock and then after shearing the stock using a mechanical stirrer a branched water soluble anionic copolymer of acrylamide with sodium acrylate (65/35) (wt./wt.) with 6 ppm by weight methylene bis acrylamide of intrinsic viscosity 9.5 dl/g and rheological oscillation value of tan delta at 0.005Hz of 0.9 (Product B) is mixed into the stock. The drainage time in seconds for 600ml of filtrate to drain is measured at different doses of Product A and Product B. The drainage times in seconds are shown in table 1. Table 1 Example 2 The drainage tests of Example 1 is repeated for a dose of 500g/t of Product A and 250g/t product B except that an aqueous colloidal silica is applied after the shearing but immediately prior to the addition of Product B. The drainage times are shown in table 2. Table 2 As can be seen even a dose of 125 g/t colloidal silica substantially improves drainage. Example 3 (comparative) Standard sheets of paper are produced using the cellulosic stock suspension of example 1 and by first mixing Product A into the stock at a given dose, then shearing the suspension for 60 seconds at 1500 rpm and then mixing in product B at a given dose. The flocculated stock is then poured onto a fine mesh to form a sheet which is then dried in a rotary drier at 80°C for 2 hours. The formation of the paper sheets is determined using the Scanner Measurement System developed by PIRA International. The standard deviation (SD) of grey values is calculated for each image. The formation values for each dose of product A and product B is shown in table 3. Lower values indicate better results. Table 3 Example 4 Example 3 is repeated except using doses of 500 g/t product A and a dose of 250 g/t product B and 125, 250, 500, 750 and 1000 g/t of aqueous colloidal silica applied after the shearing but immediately prior to the addition of Product B. The respective formation values for each dose of colloidal silica are shown in table 4. Table 4 A comparison of doses required to provide equivalent drainage results demonstrates that the flocculating system utilising cationic polymer, colloidal silica and branched anionic water soluble polymer provides improved formation. For instance from Example 2 a dose of 500g/t polymer A, 250g/t polymer B and 1000g/t silica provides a drainage time of 6 seconds. From Table 4 it can be seen the equivalent doses of product A, silica and product B gives a formation value of 18.05. From Example 1 a dose of 2000g/t product A and 1000 g/t product B in the absence of silica provides a drainage time of 6 seconds. From Table 3 the equivalent doses of product A and product B provides a formation value of 29.85. Thus for equivalent high drainage the invention improves formation by more than 39%. Even for equivalent higher drainage values, for instance 11 seconds, the improvements in formation can still be observed. Thus it can be seen from the examples that using a flocculating system involving cationic polymer, colloidal silica and branched anionic water soluble polymer provides faster drainage and better formation than cationic polymer and branched anionic water soluble polymer in the absence of colloidal silica. In Figure 1 Curve A is a plot of drainage versus formation values for the two component systems of Examples 1 and 3 employing 1000 g/t of branched anionic polymer (Product B) and 250, 500, 750, 1000, 2000 g/t cationic polymer (Product A). Curve B is a plot of drainage versus formation values for the three component systems of Examples 2 and 4 employing 250 g/t of branched anionic polymer (Product B), 500 g/t of the cationic polymer (Product A) and 125, 250, 500, 750, 1000 g/t of colloidal silica. The objective is to approach zero for both formation and drainage. It can be clearly seen that the process of the invention provides best overall drainage and formation. Example 5 (comparative) The retention properties are determined by the standard Dynamic Britt Jar methods on the stock suspension of example 1 when using a flocculating system comprising cationic polymer (Product A) and a branched anionic polymer (Product B) in the absence of colloidal silica. The flocculating system is applied in the same way as for Example 3. The total retention figures are shown as percentages in Table 5 Table 5 Example 6 Example 5 is repeated except using as the flocculation system 250g/t cationic polymer (Product A), 250 g/t branched anionic polymer (Product B) and 125 to 1000 g/t colloidal silica. The flocculating system is applied in the same way as for Example 4. The total retention figures are shown in Table 6. Table 6 From the results shown in Table 5, a dose of 250g/t cationic polymer (Product A), 250 g/t branched anionic polymer (Product B) gives retention at 81.20. By introducing 500g/t of colloidal silica the retention is increased to 94.13. In order to achieve equivalent retention in the absence of colloidal silica a dose of 500g/t Product A and 500g/t Product B is required.

Claims: Claims 1. A process of making paper or paper board comprising forming a cellulosic suspension, flocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, characterised in that the suspension is flocculated using a flocculation system comprising a siliceous material and an anionic branched water soluble polymer that has been formed from water soluble ethylenically unsaturated anionic monomer or monomer blend and branching agent and wherein the polymer has (a) intrinsic viscosity above 1.5 dl/g and/or saline Brookfield viscosity of above about 2.0 mPa.s and (b) rheological oscillation value of tan delta at 0.005Hz of above 0.7 and/or (c) deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding unbranched polymer made in the absence of branching agent. 2. A process according to claim 1 in which the material comprising the siliceous material is selected from the group consisting of silica based particles, silica microgels, colloidal silica, silica sols, silica gels, polysilicates, cationic silica, aluminosilicates, polyaluminosilicates, borosilicates, polyborosilicates and zeolites. 3. A process according to claim 1 or claim 2 in which the siliceous material is an anionic microparticulate material. 4. A process according to any one of claims 1 to 3 in which the anionic branched polymer has an intrinsic viscosity above 4dl/g and tan delta at 0.005Hz of above 0.7. 5. A process according to any one of claims 1 to 4 in which the components of the flocculation system are introduced into the cellulosic suspension sequentially. 6. A process according to any one of claims 1 to 5 in which the siliceous material is introduced into the suspension and then the anionic branched polymer is included in the suspension. 7. A process according to any one of claims 1 to 5 in which the anionic branched polymer is introduced into the suspension and then the siliceous material is included in the suspension. 8. A process according to any one of claims 1 to 4 in which the components of the flocculation system are introduced into the cellulosic suspension simultaneously. 9. A process according to any one of claims 1 to 8 in which the cellulosic suspension is pre-treated by inclusion of a cationic material into the suspension or component thereof prior to introducing the anionic branched polymer and siliceous material. 10. A process according to claim 9 in which the cationic material is selected from water soluble cationic organic polymers, or inorganic materials such as alum, polyaluminium chloride, aluminium chloride trihydrate and aluminochloro hydrate. 11. A process according to any one of claims 1 to 10 in which the flocculating system additionally comprises at least one additional flocculant/coagulant. 12. A process according to claim 11 in which the flocculant/coagulant is a water soluble polymer, preferably a water soluble cationic polymer. 13. A process according to claim 10 or claim 12 in which the cationic polymer is formed from a water soluble ethylenically unsaturated monomer or water soluble blend of ethylenically unsaturated monomers comprising at least one cationic monomer. 14. A process according to claim 10, claim 12 or claim 13 in which the cationic polymer is a branched cationic polymer which has an intrinsic viscosity above 3dl/g and exhibits a rheological oscillation value of tan delta at 0.005Hz of above 0.7. 15. A process according to claim 10 or any of claims 12 to 14 in which the cationic polymer has an intrinsic viscosity above 3 dl/g and exhibits a rheological oscillation value of tan delta at 0.005Hz of above 1.1. 16. A process according to any one of claims 1 to 15 in which the suspension is subjected to mechanical shear following the addition of at least one of the components of the flocculating system. 17. A process according to any one of claims 1 to 16 in which the suspension is first flocculated by introducing the cationic polymer, optionally subjecting the suspension to mechanical shear and then reflocculating the suspension by introducing the anionic branched polymer and siliceous material. 18. A process according to claim 17 in which the cellulosic suspension is reflocculated by introducing the siliceous material and then the anionic branched water soluble polymer. 19. A process according to claim 17 in which the cellulosic suspension is reflocculated by introducing the anionic branched polymer and then the siliceous material. 20. A process according to any one of claims 1 to 19 in which the cellulosic suspension comprises filler. 21. A process according to claim 20 in which the sheet of paper or paper board comprises filler in an amount up to 40% by weight. 21. A process according to claim 19 or claim 20 in which the filler material is selected from precipitated calcium carbonate, ground calcium carbonate, clay (especially kaolin) and titanium dioxide. 22. A process according to any one of claims 1 to 21 in which the cellulosic suspension is substantially free of filler.

Inventors: CHEN GORDON CHENG I, RICHARDSON GARY PETER, CHEN, GORDON, CHENG, I, RICHARDSON, GARY, PETER
Applicants: CIBA SPEC CHEM WATER TREAT LTD, CIBA SPECIALTY CHEMICALS WATER TREATMENTS LIMITED
Application year: 2000
Main classification: D21H23
IPC codes: D21H17, D21H21, D21H23