NO332614B1 - Procedure for producing paper - Google Patents

Abstract

The invention relates to a process for the production of paper from an aqueous suspension containing cellulose fibers and optional filler, which comprises separately adding to the suspension a cationic organic polymer having one or more aromatic groups and an anionic polymer having one or more aromatic groups, the anionic polymer of everything from step growth polymers, polysaccharides and naturally occurring aromatic polymers and modifications thereof, to forming and draining the suspension on a wire, with the proviso that if the anionic polymer is a step growth polymer, it is not an anionic melamine sulfosic acid condensation polymer. The invention further relates to a process for producing paper from an aqueous suspension containing cellulose fibers, and optional filler, which comprises separately adding to the suspension a cationic organic polymer having one or more aromatic groups and an anionic polymer having one or more aromatic groups, forming and draining the suspension on a wire, with the proviso that the anionic polymer is not an anionic polystyrene sulfonate or anionic melamine sulfoic acid condensation polymer.

Description

This invention relates to papermaking and more particularly methods for the production of paper from an aqueous suspension containing cellulose fibers, and optionally fillers, where cationic and anionic polymers with aromatic groups are added to a papermaking pulp. The procedure provides improved drainage and retention. Reference is made to the present requirements.

In the papermaking technique, an aqueous suspension containing cellulose fibers and optional fillers and additives, referred to as pulp, is fed into an inlet box which ejects the pulp onto a form wire. Water is drained from the pulp through the forming wire so that a wet paper web is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine. The resulting water, usually referred to as tailwater and containing fine particles such as fine fibers, fillers and additives, is usually recycled in the papermaking process. Drainage and retention aids are conventionally introduced into the pulp to facilitate drainage and to increase adsorption of fine particles on the cellulose fibers so that they are retained with the fibers. A wide variety of drainage and retention aids are known in the art, such as anionic, nonionic, cationic and amphoteric organic polymers, anionic and cationic inorganic materials, and many combinations thereof.

GB 1177512 describes a process for making paper from an aqueous cellulose suspension, where the suspension is treated with a cationic compound selected from cationic polymers, cationic starches or cationic gums, and an anionic polymer.

WO 9833979 describes an aqueous adhesive dispersion for use in papermaking, containing a dispersion system consisting of a cationic compound and an anionic stabilizer.

WO 99/55964 and WO 99/55965 describe the use of drainage and retention aids comprising cationic organic polymers with aromatic groups. The cationic organic polymers can be used alone or in combination with various anionic materials such as, for example, anionic organic and inorganic condensation polymers, for example sulphonated melamine formaldehyde and silica-based particles.

It would be advantageous to be able to provide a papermaking process with improved drainage and retention. It would also be advantageous to be able to provide drainage and retention aids comprising cationic organic polymers and anionic polymers with improved drainage and retention performance.

According to the present invention, it has been found that increased drainage and/or retention can be achieved by using drainage and retention aids comprising a cationic organic polymer with an aromatic group and an anionic polymer with an aromatic group. More specifically, the present invention relates to a method for the production of paper from an aqueous suspension containing cellulose fibers, and optionally fillers, which is characterized in that the method comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, the cationic polymer is a cationic polysaccharide; and an anionic polymer having one or more aromatic groups, the anionic polymer being selected from step growth polymers, polysaccharides and naturally occurring aromatic polymers and modifications thereof, which form and drain the suspension on a wire, with the proviso that if the anionic polymer is a step growth polymer, is it not an anionic melamine sulfonic acid condensation polymer. The invention further relates to a method for the production of paper from an aqueous suspension containing cellulose fibers, and optionally fillers, which is characterized in that the method comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, the cationic polymer being a cationic polysaccharide and an anionic polymer having one or more aromatic groups which forms and drains the suspension on a wire, with the proviso that the anionic polymer is not an anionic polystyrene sulfonate or anionic melamine sulfonic acid condensation polymer. The invention further relates to a method for the production of paper from an aqueous suspension containing cellulose fibers, and optionally fillers, which is characterized in that the method comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, and an anionic polymer with one or multiple aromatic groups, the anionic polymer is selected from anionic polyurethanes, anionic polysaccharides and naturally occurring aromatic polymers and modifications thereof, which form and drain the suspension on a wire, with the proviso that the anionic polymer is not an anionic polystyrene sulfonate or anionic melamine sulfonic acid condensation polymer. The invention thereby relates to a method as further defined in the claims.

The term "drainage and retention aids", as used herein, refers to two or more components which, when added to an aqueous cellulose suspension, provide better drainage and/or retention than is achieved when the two or more components are not added.

The present invention results in improved drainage and/or retention in the production of paper from all types of pulp, especially pulps with a high content of salts (high conductivity) and colloidal substances, and/or in papermaking processes with a high degree of tailwater closure, i.e. extensive tailwater recycling and limited fresh water supply. The present invention thereby makes it possible to increase the speed of the paper machine and to use a lower dosage of additives to provide a corresponding drainage and/or a retention effect, which thereby leads to an improved papermaking process and economic benefits. The present invention also provides paper with improved dry strength.

Cationic organic polymers with an aromatic group according to the present invention can be derived from natural or synthetic sources, and they can be linear, branched or cross-linked. Preferably, the cationic polymer is water soluble or water dispersible. Examples of suitable cationic polymers include cationic polysaccharides, for example starches, guar gums, celluloses, chitins, chitosans, glycans, galactans, glucans, xanthan gums, pectins, mannans, dextrins, preferably starches and guar gums, suitable starches including potato, corn, wheat, tapioca , rice, waxy corn, barley, etc; cationic synthetic organic polymers such as cationic chain growth polymers, for example cationic vinyl addition polymers such as acrylate, acrylamide, vinylamine and vinylamide based polymers, and cationic step growth polymers, for example cationic polyurethanes. Cationic starches and cationic acrylamide-based polymers with an aromatic group are particularly preferred cationic polymers.

The cationic organic polymer according to the invention has one or more aromatic groups, and the aromatic groups can be of the same or different types. The aromatic group of the cationic organic polymer may be present in the polymer backbone (main chain) or in a substituted group attached to the polymer backbone in a substituted group attached to the polymer backbone, preferably a substituent group. Examples of suitable aromatic groups include aryl, aralkyl and alkaryl groups, for example phenyl, phenylene, naphthyl, phenylene, xylylene, benzyl and phenylethyl; nitrogen-containing aromatic (aryl) groups, for example pyridinium and quinolinium and derivatives of these groups, preferably benzyl. Examples that cationically charged groups may be present in the cationic polymer as well as in monomers used to prepare the cationic polymer include quaternary ammonium groups, tertiary amino groups and acid addition salts thereof.

According to a preferred embodiment of this invention, the cationic organic polymer with an aromatic group is a polysaccharide represented by the general structural formula (I):

where P is a residue of a polysaccharide; Ai is a group which attaches N to the polysaccharide residue, suitably a chain of atoms comprising C and H atoms, and optionally O and/or N atoms, usually an alkylene group of from 2 to 18 and suitably 2 to 8 carbon atoms, optionally interrupted or substituted by one or more heteroatoms, for example 0 or N, for example an alkyleneoxy group or hydroxypropylene group (-CH2-CH(OH)-CH2-); R 1 and R 2 are each H or preferably a hydrocarbon group, suitable alkyl having from 1 to 3 carbon atoms, preferably 1 to 2 carbon atoms; Q is a substituent containing an aromatic group, suitably a phenyl or substituted phenyl group which may be attached to the nitrogen by means of an alkylene group usually of from 1 to 3 carbon atoms, suitably 1 to 2 carbon atoms and preferably Q is a benzyl group (-CH2-C6 -C5); n is an integer, usually

from about 2 to about 300,000, suitably from 5 to 200,000 and preferably from 6 to 125,000, or alternatively R1, R2 and Q together with N form an aromatic group containing from 5 to 12 carbon atoms; and X" is an anionic counterion, usually a halide-like chloride. Suitable polysaccharides represented by the general formula (I) include those mentioned above. Cationic polysaccharides according to the invention may also contain anionic groups, preferably in a smaller amount. Such anionic groups may be introduced in

the polysaccharide by means of chemical treatment or be present in the native polysaccharide.

According to another preferred embodiment of this invention, the cationic organic polymer with an aromatic group is a chain growth polymer. The term "chain growth polymer" as used herein refers to a polymer obtained by chain growth polymerization, which is also referred to as chain reaction polymer and chain reaction polymerization, respectively. Examples of suitable chain growth polymers include vinyl addition polymers prepared by polymerizing one or more monomers with a vinyl group or ethylenically unsaturated bond, for example a polymer obtained by polymerizing a cationic monomer or a monomer mixture comprising a cationic monomer represented by the general structural formula (II):

wherein R 3 is H or CH 3 ; R 1 and R 2 are each H or preferably a hydrocarbon group, suitable alkyl having from 1 to 3 carbon atoms, preferably 1 to 2 carbon atoms; A 2 is 0 or NH; B2 is an alkyl or alkylene group of from 2 to 8 carbon atoms, suitably from 2 to 4 carbon atoms, or a hydroxypropylene group; Q is a substituent containing an aromatic group, suitably a phenyl or substituted phenyl group which can be attached to the nitrogen by means of an alkylene group usually of from 1 to 3 carbon atoms, suitably 1 to 2 carbon atoms, and preferably Q is a benzyl group (-CH2-C6H5 ); and X" is an anionic counterion, usually a halide such as chloride.

Examples of suitable monomers represented by the general formula (II) include quaternary monomers obtained by treating dialkylaminoalkyl (meth)acrylates, for example dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate and dimethylaminohydroxypropyl (meth)acrylate and dialkyl- aminoalkyl (meth)acrylamides, for example dimethylaminoethyl (meth)acrylamide, diethylaminoethyl (meth)acrylamide, dimethylaminopropyl (meth)acrylamide and diethylaminopropyl (meth)acrylamide with benzyl chloride. Preferred cationic monomers of the general formula (I) include dimethylaminoethyl acrylate-benzyl chloride quaternary salt and dimethylaminoethyl methacrylate-benzyl chloride quaternary salt. The monomer of formula (II) can be copolymerized with one or more non-ionic, cationic and/or anionic monomers. Suitable copolymerizable nonionic monomers include (meth)acrylamide; acrylamide-based monomers such as N-alkyl(meth)acrylamides, N,N-dialkyl(meth)acrylamides and dialkylaminoalkyl(meth)acrylamides, acrylic-based monomers such as dialkylaminoalkyl(meth)acrylates and vinylamides. Suitable copolymerizable cationic monomers include the acid addition salt and quaternary salts of dimethylaminoethyl(meth)acrylate and diallyldimethylammonium chloride. The cationic organic polymer may also contain anionic groups, preferably in a smaller amount. Suitable copolymerizable anionic monomers include acrylic acid, methacrylic acid and various sulfonated vinyl monomers such as styrene sulfonate. Preferred copolymerizable monomers include acrylamide and methacrylamide, i.e. (meth)acrylamide, and the cationic or amphoteric organic polymer is preferably an acrylamide-based polymer.

Cationic vinyl addition polymers according to this invention can be prepared from a monomer mixture usually comprising from 1 to 99 mol%, suitably from 2 to 50 mol% and preferably from 5 to 20 mol% of cationic monomer with an aromatic group, and from 99 to 1 mol%, suitable from 98 to 50 mol%, and preferably from 95 to 80 mol% of other copolymerizable monomers which preferably comprise acrylamide or methacrylamide ((meth)acrylamide), the monomer mixture suitably comprises from 98 to 50 mol% and preferably from 95 to 80 mol% of (meth)acrylamide, the sum of the percentages is 100.

Examples of suitable cationic step growth polymers according to the invention include cationic polyurethanes which can be prepared from a monomer mixture comprising aromatic isocyanates and/or aromatic alcohols. Examples of suitable aromatic isocyanates include diisocyanates, for example toluene-2,4- and 2,6-diisocyanates and diphenylmethane-4,4'-diisocyanate. Examples of suitable aromatic alcohols include dihydric alcohols, i.e. diols, for example bisphenol A, phenyldiethanolamine, glycerol monoterephthalate and trimethylolpropane monoterephthalate. Monohydric aromatic alcohols such as phenol and derivatives thereof can also be used. The monomer mixture may also contain non-aromatic isocyanates and/or alcohols, usually diisocyanates and diols, for example any of those known to be useful in the manufacture of polyurethanes. Examples of suitable monomers containing cationic groups include cationic diols such as acid addition salts and quaternized products of N-alkanediol dialkylamines and N-alkyl dialkanolamines such as 1,2-propanediol-3-dimethylamine, N-methyldiethanolamine, N-ethyldiethanolamine, N- propyldiethanolamine, N-n-butyldiethanolamine and N-t-butyldiethanolamine, N-stearyldiethanolamine and N-methyldipropanolamine. The quaternized products can be derived from alkylating agents such as methyl chloride, dimethyl sulfate, benzyl chloride and epichlorohydrin.

The weight average molecular weight of the cationic polymer can vary within wide limits depending, inter alia, on the type of polymer used, and is usually at least about 5,000 and often at least 10,000. More often it is above 150,000, normally above 500,000, suitably above 700,000, preferably above about 1,000,000 and most advantageously above about 2,000,000. The upper limit is not critical; it may be approximately 200,000,000, typically 150,000,000 and suitably 100,000,000.

The cationic organic polymer may have a degree of cationic substitution (DSC) varying over a wide range depending, inter alia, on the type of polymer used; DSC may be from 0.005 to 1.0, usually from 0.01 to 0.5, suitably from 0.02 to 0.3, preferably from 0.025 to 0.2; and the degree of aromatic substitution (DSQ) may be from 0.001 to 0.5, usually from 0.01 to 0.5, suitably from 0.02 to 0.3 and preferably from 0.025 to 0.2. In case the cationic organic polymer contains anionic groups, the degree of anionic substitution (DSA) may be from 0 to 0.2, suitably from 0 to 0.1, preferably from 0 to 0.05, the cationic polymer having a total cationic charge . Usually the charge density of the cationic polymer is in the range from 0.1 to 6.0 meqv/g dry polymer, suitably from 0.2 to 5.0 and preferably from 0.5 to 4.0.

Examples of suitable cationic organic polymers with an aromatic group which can be used according to the present invention include those described in WO 99/55964, WO 99/55965 and WO 99/67310.

Anionic polymers with an aromatic group according to the invention can be selected from step growth polymers, chain growth polymers, polysaccharides, naturally occurring aromatic polymers and modifications thereof. The term "step growth polymer", as used here, refers to a polymer obtained by step growth polymerization, also referred to as step reaction polymer, respectively step reaction polymerization. Preferably, the anionic polymer is selected from step growth polymers, polysaccharides and naturally occurring aromatic polymers and modifications thereof, most preferably step growth polymers. The anionic polymers according to the invention can be linear, branched or cross-linked. Preferably, the anionic polymer is water soluble or water dispersible. The anionic polymer is preferably organic.

The anionic polymer according to the invention has one or more aromatic groups and the aromatic groups can be of the same or different types. The aromatic group of the anionic polymer may be present in the polymer backbone or in a substituent group attached to the polymer backbone (main chain). Examples of suitable aromatic groups include aryl, aralkyl and alkaryl groups and derivatives thereof, for example phenyl, tolyl, naphthyl, phenylene, xylylene, benzyl, phenylethyl and derivatives of these groups. Examples of anionic charges that may be present in the anionic polymer as well as in the monomers used to prepare the anionic polymer include groups that carry an anionic charge and acid groups that carry an anionic charge when dissolved or dispersed in water, the groups herein referred to collectively as anionic groups such as phosphate, phosphonate, sulfate, sulfonic acid, sulfonate, carboxylic acid, carboxylate, alkoxide and the phenolic groups, i.e. hydroxy-substituted phenyls and naphthyls. Groups that carry an anionic charge are usually salts of an alkali metal, alkaline earth, or ammonia.

Examples of suitable anionic step growth polymers according to the present invention include condensation polymers, i.e. polymers obtained by step growth condensation polymerization, for example condensates of an aldehyde such as formaldehyde with one or more aromatic compounds containing one or more anionic groups, and optionally other comonomers useful in the condensation polymerization such as such as urea and melamine. Examples of suitable aromatic compounds containing anionic groups include benzene- and naphthalene-based compounds containing anionic groups such as phenol and naphthol compounds, for example phenol, naphthol, resorcinol and derivatives thereof, aromatic acids and salts thereof, for example phenyl, phenol- . Examples of suitable anionic step-growth polymers according to the invention include anionic benzene-based and naphthalene-based condensation polymers, preferably naphthalenesulfonic acid-based and naphthalenesulfonate-based condensation polymers.

Examples of further suitable anionic step growth polymers according to the present invention include addition polymers, i.e. polymers obtained by step growth addition polymerization, for example anionic polyurethanes which can be produced from a monomer mixture comprising aromatic isocyanates and/or aromatic alcohols. Examples of suitable aromatic isocyanates include diisocyanates, for example toluene-2,4- and 2,6-diisocyanates and diphenylmethane-4,4'-diisocyanate. Examples of suitable aromatic alcohols include dihydric alcohols, i.e. diols, for example bisphenyl A, phenyldiethanolamine, glycerol monoterephthalate and trimethylolpropane monoterephthalate. Monohydric aromatic alcohols such as phenol and derivatives thereof can also be used. The monomer mixture may also contain non-aromatic isocyanates and/or alcohols, usually diisocyanates and diols, for example any of those known to be useful in the production of polyurethanes. Examples of suitable monomers containing anionic groups include monoester reaction products of triols, for example trimethylethane, trimethylolpropane and glycerol, with dicarboxylic acids or anhydrides thereof, for example succinic acid and anhydride, terephthalic acid and anhydride such as glycerol monosuccinate, glycerol monoterephthalate, trimethylolpropane monosuccinate, trimethyl -olpropane monoterephthalate, N,N-bis-(hydroxyethyl)-glycine, di-(hydroxymethyl)propionic acid, N,N-bis-(hydroxyethyl)-2-amino-ethanesulfonic acid and the like, optionally and usually in combination with reaction with a base, such as alkali metal and alkaline earth hydroxides, for example sodium hydroxide, ammonia or an amine, for example triethylamine, which thereby forms an alkali metal, alkaline earth or ammonium motion.

Examples of suitable anionic chain growth polymers according to the invention include anionic vinyl addition polymers obtained from a mixture of vinyl or ethylenically unsaturated monomers comprising at least one monomer with an aromatic group and at least one monomer with an anionic group, usually copolymerized with nonionic monomers such as acrylate and acrylamide-based monomers. Examples of suitable anionic monomers include (meth)acrylic acid and paravinylphenol (hydroxystyrene).

Examples of suitable anionic polysaccharides include starches, guar gums, celluloses, chitins, chitosans, glycans, galactans, glucans, xanthan gums, pectins, mannans, dextrins, preferably starches, guar gums and cellulose derivatives, suitable starches including potato, maize, wheat, maize, tapioca, rice, waxy corn, barley, preferably potato. The anionic groups in the polysaccharide can be native and/or introduced by chemical treatment. The aromatic groups in the polysaccharide can be introduced by chemical methods known in the art.

Naturally occurring aromatic anionic polymers and modifications thereof, i.e. modified naturally occurring aromatic anionic polymers according to the invention, include naturally occurring polyphenolic substances which are present in wood and organic extracts of bark of some types of wood and chemical modifications thereof, usually sulfonated modifications thereof. The modified polymers can be obtained by chemical treatment such as, for example, sulphite boiling and power boiling. Examples of suitable anionic polymers of this type include lignin-based polymers, preferably sulphonated lignins, for example lignosulphonates, kraft lignin, sulphonated kraft lignin and tannin extracts.

The average molecular weight of the anionic polymer may vary within wide limits depending, inter alia, on the type of polymer used and is usually at least about 500, suitably above about 2000 and preferably above about 5000. The upper limit is not critical; it may be about 200,000,000, usually 150,000,000, suitably 100,000,000 and preferably 10,000,000.

The anionic polymer may have a degree of anionic substitution (DAA) which varies over a wide range depending, inter alia, on the type of polymer used; DSAs usually from 0.01 to 2.0, suitably from 0.02 to 1.8 and preferably from 0.025 to 1.5; and the degree of aromatic substitution (DSQ) may be from 0.001 to 1.0, usually from 0.01 to 0.8, suitably from 0.02 to 0.7 and preferably from 0.025 to 0.5.

In case the anionic polymer contains cationic groups, the degree of cationic substitution (DSC) can be, for example, from 0 to 0.2, suitably from 0 to 0.1 and preferably from 0 to 0.05, the anionic polymer having a total anionic charge. Generally, the anionic charge density of the anionic polymer is in the range of from 0.1 to 6.0 meqv/g of dry polymer, suitably from 0.5 to 5.0 and preferably from 1.0 to 4.0.

Examples of suitable anionic aromatic polymers which can be used according to the present invention include those described in US 4,070,236 and US 5,755,930, and WO 95/21295, WO 95/21296, WO 99/67310 and WO 00/49227.

Examples of particularly preferred combinations of anionic and cationic polymers with aromatic groups, as defined above according to the present invention include (i) cationic polysaccharides, preferably cationic starch and anionic step growth polymers, suitable anionic benzene-based and naphthalene-based condensation polymers and anionic polyurethanes, preferably naphthalene-based condensation polymers ;

(ii) cationic polysaccharides, preferably cationic starch and naturally occurring aromatic anionic polymers and modifications thereof, suitable anionic lignin-based polymers, preferably sulfonated compounds; (iii) cationic chain growth polymers, suitable cationic vinyl addition polymers, preferably cationic acrylamide based polymers, and anionic step growth polymers, suitable anionic benzene based and naphthalene based condensation polymers; and (iv) cationic chain growth polymers, suitable cationic vinyl addition polymers, preferably cationic acrylic based polymers and naturally occurring aromatic anionic polymers and modifications thereof, suitable anionic lignin based polymers, preferably sulphonated lignins.

The cationic and anionic polymers according to the invention are preferably added separately to the aqueous suspension containing cellulose fibers or pulp, and not as a mixture containing said polymers. Preferably, the cationic and anionic polymers are added to the mass at different points. The polymers can be added in any order. Generally, the cationic polymer is added to the stock first and the anionic polymer is added subsequently, although the reverse order of addition can also be used. The polymers can be added to the mass to be dewatered in amounts that can vary within wide limits depending, inter alia, on the type of mass, salt content, type of salts, filler content, point of addition, etc. Generally, the polymers are added in an amount that provides better drainage and/ or retention than that obtained when they are not added, and usually the cationic polymer is added to the stock before the addition of the anionic polymer. The cationic polymer is usually added in an amount of at least 0.001%, often at least 0.005% by weight, based on dry pulp substance, while the upper limit is usually 3% and suitably 2.0% by weight. The anionic polymer is usually added in an amount of at least 0.001%, often at least 0.005% by weight, based on dry pulp substance, while the upper limit is usually 3 and suitably 1.5% by weight.

The polymers with aromatic groups according to the invention can be used in conjunction with additional additive(s) which are beneficial for the overall drainage and/or retention performance, thereby forming drainage and retention aids comprising three or more components. Examples of suitable pulp additives of this type include anionic microparticulate materials, for example silica-based particles and smectite-type clays, low molecular weight cationic organic polymers, aluminum compounds, anionic vinyl addition polymers and combinations thereof, including the components and use thereof described in WO 99/55964 and WO 99/55965 .

Low molecular weight (hereafter LMW) cationic organic polymers which can be used according to the invention include those usually referred to as anionic trash collectors (ATC). The LMW cationic organic polymer may be derived from natural or synthetic sources, and preferably is a LMW synthetic polymer. Suitable organic polymers of this type include LMW highly charged cationic organic polymers such as polyamines, polyamidoamines, polyethyleneimines, homo- and copolymers based on diallyldimethylammonium chloride, (meth)acrylamides and (meth)acrylates. Relative to the molecular weight of the cationic organic polymer with an aromatic group of this invention, the molecular weight of the LMW cationic organic polymer is preferably lower; it is suitable at least 2000 and preferably at least 10,000. The upper limit of the molecular weight is usually about 700,000, suitably about 500,000 and usually about 200,000.

Aluminum compounds which can be used according to the invention include alum, aluminates, aluminum chloride, aluminum nitrate and polyaluminium compounds, such as polyaluminium chlorides, polyaluminium sulphates, polyaluminium compounds containing both chloride and sulphate ions, polyaluminium silicate sulphates and mixtures thereof. The polyaluminium compounds can also contain anions other than chloride ions, for example anions from sulfuric acid, phosphoric acid, organic acids such as citric acid and oxalic acid. The process of this invention is applicable to all papermaking processes and cellulose suspensions, and it is particularly useful in the manufacture of paper from a pulp having a high conductivity. In such cases, the conductivity of the mass dewatered on the wire is usually at least 2.0 mS/cm, suitably at least 3.5 mS/cm and preferably at least 5.0 mS/cm. Conductivity can be measured with standard equipment such as, for example, a WTW LF 539 instrument supplied by Christian Berner. The values referred to above are suitably determined by measuring the conductivity of the cellulose suspension fed to or present in the headbox of the paper machine or, alternatively, by measuring the conductivity of the bottom water obtained by dewatering the suspension. High conductivity levels mean high contents of salts (hydrolysis) which can be derived from the materials used to form the pulp, from various additives introduced into the pulp, from the fresh water supplied to the process, etc. Furthermore, the content of salts is usually higher in the process where the bottom water is recycled in large extent, which can lead to significant accumulation of salts in the water circulating in the process.

The present invention further encompasses papermaking processes where waste water is recovered to a large extent, or recycled, i.e. with a high degree of waste water closure, for example where from 0 to 30 tonnes of fresh water is used per tonne of dry paper produced, usually less than 20, suitable less than 15 , preferably less than 10 and especially less than 5 tonnes of fresh water per tonne of paper. Recycling of tailwater obtained in the process includes suitably mixing the tailwater with cellulose fibers and/or optional filler to form a suspension to be dewatered; preferably it comprises a mixture of the bottom water with a suspension containing cellulose fibres, and optional filler, before the suspension enters the formwire for dewatering. The tailwater can be mixed with the suspension, between, simultaneously with or after the introduction of the drainage and retention aids of this invention. The fresh water can be introduced into the process at any stage; for example, it can be mixed with cellulose fibers to form a suspension, and it can be mixed with a thick suspension containing cellulose fibers to dilute it to form a thin suspension to be dewatered, before, simultaneously with, or after mixing the suspension with tailwater.

Further additives which are conventional in papermaking can of course be used in combination with the polymers according to the invention, such as for example dry strength agents, wet strength agents, optical brightness agents, dye, adhesives such as resin-based adhesives and cellulose-reactive adhesives, for example alkyl and alkenyl ketene dimers, alkyl- and alkenyl ketene multimers and succinic anhydrides, etc. The cellulose suspension, or pulp, can also contain mineral fillers of conventional types such as, for example, kaolin, kaolin, titanium dioxide, gypsum, talc and natural and synthetic calcium carbonates such as lime, ground marble and precipitated calcium carbonate.

The method of this invention is used for the production of paper. The term "paper" as used here of course includes not only paper and the production thereof, but also other cellulose fiber-containing sheets or net-like products, such as for example cardboard and cardboard and the production thereof. The method can be used in the production of paper from different types of suspensions of cellulose-containing fibres, and the suspensions should suitably contain at least 25% by weight and preferably at least 50% by weight of such fibres, based on dry substance. The suspension can be based on fibers from chemical pulp such as sulphate, sulphite and organosolv pulp, mechanical pulp such as thermomechanical pulp, chemothermomechanical pulp, refined pulp and ground pulp, from both hardwood and softwood, and can also be based on recycled fibres, optionally from de-inked pulps and mixtures thereof.

The invention is further illustrated in the following examples. Parts and percentages refer to parts by weight and percentage by weight respectively, unless otherwise stated.

Example 1

Cationic polymers used in the tests were purchased on the market or prepared by generally known methods. The cationic polysaccharides used in the tests were prepared by reacting native potato starch with a quaternizing agent according to the general method described in EP-A 0 189 935 and WO 99/55964. The cationic polymers used in the tests, hereafter also collectively referred to as cationic polymer, Cl to C3 according to the invention and Cl-ref to C3-ref meant for comparison purposes were the following: Cl: Cationic starch obtained by quaternization of native potato starch with 3-chloro -2-Hydroxypropyldimethylbenzylammonium chloride to 0.5% N.

C2: Cationic starch obtained by quaternization of native potato starch with 3-chloro-2-hydroxypropyldimethylbenzylammonium chloride to 0.7% N.

C3: Cationic vinyl addition polymer prepared by polymerization of acrylamide (90 mol%) and acryloxyethyldimethyl-benzylammonium chloride (10 mol%), molecular weight approximately 6,000,000.

Cl-ref: Cationic starch obtained by quaternization of native potato starch with 2,3-epoxypropyltrimethylammonium chloride to 0.8% N.

C2-ref: Cationic starch obtained by quaternization of native potato starch with 2,3-epoxypropyltrimethylammonium chloride to 0.5% N.

C3-ref: Cationic vinyl addition polymer prepared by polymerization of acrylamide (90 mol%) and acryloxyethyltrimethylammonium chloride (10 mol%), molecular weight approximately 6,000,000.

Anionic polymers used in the tests were purchased on the market or prepared by general known methods. The anionic polymers used in the tests, hereafter also collectively referred to as anionic polymer, Al to A8 according to the invention and Al-ref to A2-ref meant for comparison purposes were the following: Al: Anionic polycondensate of formaldehyde with naphthalene sulphonate, molecular weight approximately 20,000.

A2: Anionic polycondensate of formaldehyde with naphthalene sulphonate, molecular weight approximately 110,000.

A3: Anionic polycondensate of formaldehyde with naphthalene sulphonate, molecular weight approximately 40,000.

A4: Anionic polycondensate of formaldehyde with naphthalene sulphonate, molecular weight approximately 210,000.

A5: Anionic polyurethane obtained by reacting glycerol monostearate with toluene diisocyanate to form a prepolymer containing terminal isocyanate groups which is then reacted with dimethylol propionic acid.

A6: Anionic polyurethane obtained by reacting phenyldiethanolamine with toluene diisocyanate to form a prepolymer containing terminal isocyanate groups which is then reacted with dimethylolpropionic acid and N-methyldiethanolamine.

A7: Anionic sulfonated kraft lignin.

A8: Anionic lignosulfonate.

Al-ref: Anionic melamine formaldehyde sulfonate polycondensate. A2-ref: Anionic inorganic condensation polymer of silicic acid in the form of colloidal silica particles with a particle size of 5 nm.

A low molecular weight cationic organic polymer, also referred to as ATC, which was used in some of the tests, was commercially available and producible by generally known methods. The ATC was as follows: ATC: Cationic copolymer of dimethylamine, epichlorohydrin and ethylenediamine with a molecular weight of approximately 50,000.

All polymers were used in the form of dilute aqueous polymer solutions.

Example 2

Drainage performance was evaluated using a Dynamic Drainage Analyzer (DDA) available from Akribi, Sweden, which measures the time to drain a fixed volume of mass through a wire when a plug is removed and applies vacuum to the side of the wire opposite the side on which the mass is present.

A standard pulp was prepared from a pulp composition based on 56 wt% peroxide bleached TMP/SGW pulp (80/20), 14 wt% bleached birch/pine sulphate pulp (60/40) refined to 200° CSF and 30 wt% kaolin. 25 g/l of a colloidal fraction, bleach water from a paper mill, was added to the pulp. Mass volume was 800 ml and pH approximately 7. Calcium chloride was added to the mass to adjust the conductivity to 0.5 mS/cm. The obtained mass is referred to as standard mass. Additional amounts of calcium chloride were added to the standard stock to produce a medium conductivity stock (2.0 mS/cm) and a high conductivity stock (5.0 mS/cm).

The mass was stirred in a glass with baffles at a speed of 1500 rpm throughout the test and chemical additions were carried out as follows: i) addition of cationic polymer to the mass followed by stirring for 30 seconds, ii) addition of anionic polymer to the mass followed by stirring for 15 seconds, iii) to drain the mass while the draining time was automatically recorded. If used, ATC was added to the mass followed by stirring for 30 seconds before i) addition of cationic polymer and ii) addition of anionic polymer according to the procedure described above.

Table 1 shows the dewatering (drainage) effect at different dosages of the cationic polymer Cl, calculated as dry polymer on a dry pulp system, and different dosages of the anionic polymer Al-ref, Al and A2, calculated as dry polymer on a dry pulp system. The standard compound was used in test numbers 1-5 and the high conductivity compound was used in test numbers 6-9.

Example 3

First pass retention was evaluated using a nephelometer by measuring the turbidity of the filtrate from the Dynamic Drainage Analyzer (DDA), the tailwater obtained by draining the mass obtained in Example 2. The results are shown in Table 2.

Example 4

Drainage performance was evaluated using cationic and anionic polymers according to example 1 and the standard mass and method according to example 2. The results are shown in table 3.

Example 5

Drainage performance was evaluated using cationic and anionic polymers according to example 1 and medium conductivity mass and method according to example 2. The results are shown in table 4.

Example 6

Drainage performance was evaluated using the cationic and anionic polymers of Example 1 and the high conductivity mass and method of Example 2. The results are shown in Table 5.

Example 7

Drainage performance was evaluated using the cationic and anionic polymers according to example 1 and high conductivity mass and method according to example 2. The results are shown in table 6.

Example 8

Drainage performance was evaluated using the cationic ones

and anionic polymers according to example 1 and the standard conductivity mass and methods according to examples 2 and 3. The results are shown in table 7.

Example 9

Drainage performance was evaluated using the cationic and anionic polymers and ATC according to Example 1 and medium conductivity mass and method according to Example 2. The results are shown in Table 8.

Example 10

Drainage and retention performance was evaluated using the cationic and anionic polymers and ATC of Example 1 and medium conductivity mass and methods of Examples 2 and 3. The results are shown in Table 9.

Example 11

Drainage performance was evaluated using the cationic and anionic polymers of Example 1 and the standard conductivity mass and methods of Example 2. The results are shown in Table 10.

Claims (26)

1. Process for the production of paper from an aqueous suspension containing cellulose fibres, and optional fillers, characterized in that the process comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, the cationic polymer being a cationic polysaccharide; and an anionic polymer with one or more aromatic groups, the anionic polymer being selected from step growth polymers, polysaccharides and naturally occurring aromatic polymers and modifications thereof, which form and drain the suspension on a wire, with the proviso that if the anionic polymer is a step growth polymer, is it not an anionic melamine sulfonic acid condensation polymer. 2. Process for the production of paper from an aqueous suspension containing cellulose fibres, and optional fillers, characterized in that the process comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, the cationic polymer being a cationic polysaccharide and an anionic polymer with one or more aromatic groups which form and drain the suspension on a wire, with the proviso that the anionic polymer is not an anionic polystyrene sulfonate or anionic melamine sulfonic acid condensation polymer. 3. Method according to claim 1 or 2, characterized in that the anionic polymer is selected from condensates of an aldehyde, anionic polyurethanes, and naturally occurring aromatic anionic polymers and modifications thereof. 4. Method according to any of the preceding claims, characterized in that the anionic polymer is selected from condensates of an aldehyde and naphthalene-based compounds, anionic polyurethanes which are produced from a monomer mixture comprising aromatic isocyanates and/or aromatic alcohols, and lignin-based polymer. 5. Method according to any one of claims 1-3, characterized in that the anionic polymer is selected from step growth polymers which are anionic benzene-based or naphthalene-based condensation polymers. 6. Method according to claim 1 or 2, characterized in that the anionic polymer is prepared from one or more aromatic compounds selected from phenyl, phenol, naphthalene, naphthol and derivatives and mixtures thereof. 7. Method according to claim 1 or 2, characterized in that the anionic polymer is selected from tannin extracts, sulphonated lignins, benzenesulfonic acid, benzenesulfonate, xylenesulfonic acid, xylenesulfonate, naphthalenesulfonic acid, naphthalenesulfonate, phenolsulfonic acid, phenolsulfonate and mixtures thereof. 8. Method for the production of paper from an aqueous suspension containing cellulose fibers, and optional fillers, characterized in that the method comprises separately adding to the suspension a cationic organic polymer with one or more aromatic groups, and an anionic polymer with one or more aromatic groups, the anionic polymer is selected from anionic polyurethanes, anionic polysaccharides and naturally occurring aromatic polymers and modifications thereof, which form and drain the suspension on a wire, with the proviso that the anionic polymer is not an anionic polystyrene sulfonate or anionic melamine sulfonic acid condensation polymer. 9. Method according to claim 1 or 2, characterized in that the cationic polymer is cationic starch. 10. Method according to claim 8, characterized in that the cationic polymer is a vinyl addition polymer. 11. Method according to claim 10, characterized in that the cationic polymer is an acrylamine-based polymer. 12. Method according to any of the preceding claims, characterized in that the cationic polymer has an average molecular weight above 1,000,000. 13. Method according to any one of the preceding claims, characterized in that the cationic polymer has a benzyl group. 14. Method according to any one of claims 1 and 2 and 8-13, characterized in that the anionic polymer is selected from anionic polyurethanes, and naturally occurring aromatic anionic polymers and modifications thereof. 15. Method according to any one of claims 1 and 2 and 8-14, characterized in that the anionic polymer is selected from anionic polyurethanes which are produced from a monomer mixture comprising aromatic isocyanates and/or aromatic alcohols, and lignin-based polymers. 16. Method according to claim 8, characterized in that the anionic polymer is anionic polyurethane. 17. Method according to claim 16, characterized in that the anionic polymer is produced from a monomer mixture comprising aromatic isocyanates and/or aromatic alcohols. 18. Method according to any one of claims 1, 2 and 8, characterized in that the anionic polymer is a lignin-based polymer. 19. Method according to any one of claims 1-5, characterized in that the anionic polymer is a formaldehyde-naphthalene sulfonate condensation polymer. 20. Method according to any one of the preceding claims, characterized in that the anionic polymer has a weight average molecular weight within the range of from 500 to 1,000,000. 21. Method according to any one of the preceding claims, characterized in that the cationic polymer is added in an amount of from 0.005 to 2% by weight, based on dry suspension. 22. Method according to any one of the preceding claims, characterized in that the anionic polymer is added in an amount of from 0.005 to 1.5% by weight, based on dry suspension. 23. Method according to any one of the preceding claims, characterized in that it further comprises adding a low molecular weight cationic organic polymer to the suspension. 24. Method according to any one of the preceding claims, characterized in that it further comprises adding an anionic microparticulate material selected from the group consisting of silica-based particles and clays of the smectite type. 25. Method according to any one of the preceding claims, characterized in that the suspension has a conductivity of at least 2.0 mS/cm. 26. Method according to any one of the preceding claims, characterized in that it further comprises recycling backwater and introducing from 0 to 30 tonnes of fresh water per tonne of produced paper.