EP1918456A1

EP1918456A1 - Method of producing a fibrous web containing fillers

Info

Publication number: EP1918456A1
Application number: EP06396018A
Authority: EP
Inventors: Cherryleen Garcia-Lindgren; Sune WÄNNSTRÖM; Lars Wagberg; Linda Gärdlund
Original assignee: M Real Oyj
Current assignee: Metsa Board Oyj
Priority date: 2006-10-31
Filing date: 2006-10-31
Publication date: 2008-05-07
Also published as: RU2007139693A

Abstract

A method of producing a fibrous web on a paper or cardboard machine from an aqueous furnish containing fibres mainly derived from lignocellulosic material, fillers and any conventional additives. According to the method the fillers are admixed with a nanometer sized polyelectrolyte complex before they are contacted with the fibres and any additives. By means of the invention, surface strength, tensile strength, tensile stiffness and/or bulk can be improved at a given filler content. Also dusting of such webs can be reduced.

Description

The present invention relates to a method according to the preamble of claim 1 of producing a fibrous web on a paper or cardboard machine.
According to such a method, the web is produced from an aqueous furnish containing fibres mainly derived from lignocellulosic material, fillers and conventional additives.
Paper mills are in constant pursuit of lowering production costs. One common scheme to reach this goal is to increase the filler content and thereby reduce the use of fibres. As a consequence, there are less fibres present for forming fibre-fibre bonds and a weakening of the paper web (e.g. loss in tensile strength and bending stiffness) is observed. Limiting factors for increasing filler level are runnability on the paper machine and paper performance. Use of high filler loading (specifically for PCC) gives increased consumption of internal sizing agents (e.g. ASA, AKD) and increased risk for size reversion.
In highly-filled papers, treatment of the fillers with different types of polymers for improving dry strength has been tested. Natural polymers, such as cationic starch, guar and chitosan, provide good dry strength but at the same time affect drainage properties. Polyvinyl alcohol is a good strength-enhancing polymer but is not retained on the fibre to give any effects. Moreover, unreacted charged polymers in the circulating whitewater system is not desired because it eventually leads to collapse of the retention system or deposits in the machine. So far carboxymethylcellulose (CMC) has shown promise in this area, but when tested on filler-containing furnishes, strength increase diminishes.
It has also been suggested in the art to disperse mineral fillers, such as kaolin, calcium carbonate, and silicon dioxide, with polyelectrolyte complexes. Sunden, O. & Sunden, A. disclose a method of mixing low DS cationic starch and carboxymethylcellulose or alginate in order to obtain an amphoteric, highly viscous solution, which then envelopes filler particles into droplets dispersed in water (2 - 20 % to filler by weight). The filler particles are then further stabilised or cured by the addition of polyaluminium-oxy-citrate compounds. The result claimed by Sundén & Sundén was high strength and high filler retention in the papers thus produced.
When balancing strength increase against costs, the use of most of the mentioned polymers cannot be justified. For this reason there is still a need for a strength additive that is robust and efficient both performance-wise and cost-wise.
It is an object of the present invention to eliminate the problems related to the known solutions and to provide filler-containing papers of a novel kind.
It is another object of the invention to provide a method of making sized paper products having a high loading of fillers.
These and other objects, together with the advantages thereof over known paper products and methods, which shall become apparent from specification, which follows, are accomplished by the invention as hereinafter described and claimed.
The invention is based on the concept of replacing at least a part of the filler traditionally used in sized papers and cardboard products with a filler containing a nanometer-sized polyelectrolyte complex adsorbed attached to the filler. The present invention is based on the fact that the surface area of fillers is orders of magnitude greater than the surface area of fibres in the furnish, and that the surface of a mineral filler, e.g. the precipitated calcium carbonate (PCC) filler, is porous and is easily accessible by to adsorption of the nanometer-sized PEC.
Therefore, the invention provides for admixing the fillers with a polyelectrolyte complex before contacting the fillers with the fibres and any additives.
The complex can be employed in the form of a solution or a colloidal dispersion, which is admixed with the filler to provide a modified filler, and the modified filler is then admixed with a fibrous stock (furnish) together with the other components, including the sizing agents and retention agents and, optionally, conventional filler.
Surprisingly, the use of a modified filler of this kind will give rise to a further improvement of the strength properties and efficiency and stability of the internal sizing..
The new paper and cardboard products according to the invention comprise an uncoated product or optionally base paper for coating constituted by a fibrous web containing, in combination, 0.01 to 20 % by weight of a sizing agent, 1 to 50 % by weight of a filler, and 0.01 to 20 % by weight of a polyelectrolyte, at least 20 percent of which is adsorbed to the filler. The percentages are calculated from the total weight of the product. The paper can contain other components, including conventional fillers, retention agents and traditional auxiliary agents.
The paper and cardboard products can be produced by the steps of providing a fibre furnish, adding to the fibre furnish comprising 100 parts by weight of cellulosic or lignocellulosic fibres, in optional order, 1 to 50 parts by weight of a filler which is modified by 0.01 to 20 % by weight of a polyelectrolyte complex, and 0.01 to 20 % by weight of a sizing agent, the percentage being calculated from the dry weight of the filler, and forming from the stock comprising the fibres, the modified filler and the sizing agent a paper or cardboard web on a paper or cardboard machine.
More specifically, the present method is mainly characterized by what is stated in the characterizing part of claim 1.
The uses of the invention are characterized by what is stated in claims 25 to 27.
Considerable advantages are obtainable by the present invention. In addition to the cost savings from increased filler levels and reduced consumption/ increased stability of internal sizing agents, the method can be used for improving surface strength, e.g for reducing dusting of a filler-containing lignocellulosic web. With cationic PEC treated filler significant savings of retention aids are possible. By imparting a net cationic charge on the filler, retention is drastically improved such that retention aids can be significantly reduced or eliminated. Also a positive effect on bulk has been observed. Optionally cost savings can be obtained by reduced beating of fibres.
Compared to the natural and semisynthetic polymers mentioned above, with the present polyelectrolyte complexes there is the advantage that they will form a 3-dimensional structure capable of binding fibres and filler together. The complexes have a high water content, i.e. are highly swollen, and therefore the 3-D structure is created without excessive use of chemicals. This is important for a comparison with for example polymeric latexes used in paper coating formulations. Another important aspect is the competitive cost of the PEC, since the components of complex are bulk chemicals commonly used in papermaking.
The paper and cardboard products obtained by the present invention have been extensively tested and, e.g., formation has been studied. It appears that the same formation can be obtained with a polyelectrolyte complex treatment of the present kind as compared to the reference.
This invention solves two important problems with using higher filler levels in paper production. The treatment increases paper strength and thereby compensating for the use of less fibre in the paper. Moreover, the treatment decreases internal sizing demand brought about by the increased filler level.. Additionally, sizing stability and efficiency are improved with the invention. The potential for savings in paper production is therefore significant. In terms of strength, to give an example, the proportion of fibre can be decreased from 80 % to 72 %, saving 8 % fibre with the same (or even better) paper strength. Just this proportion difference corresponds to approximately 20 - 25 €/ton paper. In terms of internal sizing, an average of 1.25 kg AKD (5 €/kg) or roughly 6 E/t may be saved.
Next the invention will be examined more closely with the aid of a detailed description.

Figure 1 shows the effect of filler treatment on sizing demand and sizing stability in a plot of AKD dosage against Cobb 60 for test specimens from Example 1;
Figure 2 shows the geometric tensile index for a series of three sample series from Example 2;
Figure 3 shows the tensile stiffness index for the same samples; and
Figure 4 shows the bulk of the same samples.

According to the present invention it has been observed, unexpectedly, that polyelectrolyte complexes can be adsorbed effectively on fillers used for paper and board manufacture during simple processing. As was mentioned above, the present invention provides for filler treatment with a polymer and a polyelectrolyte complex that enables better sizing efficiency and stability during the production of highly-filled papers. It has been observed that this filler modification decreases the sizing demand significantly. Thus, papers comprising the modified filler in a concentration of more than 50 % of the total amount of fillers will need about 20 % less of the conventional sizing agent than a conventional paper containing unmodified filler at the same Cobb60 -value.
The modification process can be carried out separately, only for the filler. According to a first preferred embodiment, the present invention provides for modification of a filler, which is then - after the modification - added to the stock and admixed with fibres and sizing agents. However, the present treatment can be combined with a similar treatment also of the fibres, as discussed on our co-pending patent application. Thus, according to a second embodiment, the polyelectrolyte complex is added separately to both the fillers and the fibres before they are mixed together.
As was mentioned above, CMC as such can be used for filler treatment which also leads to improved sizing efficiency, sizing stability and decreased sizing demand. However, the present invention provides even better results. Without being bound to any particularly theory, it would seem that the action of the present treatment with, e.g., polyamideamine epichlorhydrine (PAE) and with a complex formed by PAE and caboxymethylcellulose (CMC) is quite different from treating a filler with CMC alone. The initial addition of PAE imparts positive charges to the surface of the particle. This layer anchors the polyelectrolyte complex to the PCC particle. The complex forms a 3-dimensional structure that binds PCC particles and fibres together, resulting in the strength increase observed in the trial described below. At the same time, the surface of the filler is modified resulting in better AKD sizing.
The current invention is basically therefore a simple 2-component (anionic polymer + cationic polymer) complex that has a net charge on the filler particle. It does not require a third component for curing, although such components are not excluded. The dosage of the complex is low (typically about 0.001 to 1 % by weight of the filler, in particular about 0.01 to 0.1 % by weight, e.g. about 0.08 % by weight), therefore no significant agglomeration of filler takes place. The interaction between the complex and the filler is rather the opposite of the above mentioned Sunden solution, agglomeration takes place as shown by the particle size distribution at the mm-range. But the association between the particles in the present invention is weak, hence after ultrasound treatment, the particle size distribution comes close to that of the untreated filler, In this case, formation and filler distribution in the paper are not affected.
According to the present invention, a filler, typically a mineral filler, is modified by contacting it with a polyelectrolyte complex. The modification step comprises typically admixing, continuously or intermittently, the filler with a polyelectrolyte complex before feeding it into the headbox where it is mixed with the fibres and any additives in aqueous suspension. The mixing and contacting time can amount to about 1 second to about 24 hours, typically some 10 seconds to 5 hours is sufficient and times in the range of about 1 minute to about 1 hour are preferred. The pH is in the range of 4 to 9, preferably 5 to 8,5 the temperature 10 to 65 °C and the pulp consistency 0.1 to 5 %, preferably 2 to 4 %.
The polyelectrolyte complex (in the following also abbreviated "PEC") is preformed and added as such (cf. below).
The filler can be any conventional filler which, in the present context, means a particulate filler which is pulverous and comprises loose particles. As specific examples of conventional fillers, the following can be mentioned: calcium carbonate (natural or in particular precipitated), kaolin, talc, hydrogenated aluminum oxides (aluminum trihydroxides), calcium sulfate, barium sulfate, calcium oxalate, silicates or titanium dioxide. Precipitated calcium carbonate is preferred.
As a pre-treatment step, the filler can be treated with a polymer having a net charge. The treatment can be carried out with a cationic or anionic polymer. In either case, a portion of one of the components of the complex can be used for improving attachment. The pre-treatment step is preferably carried out separately from the admixing with the PEC.
Typically, the same polymer that is used as a cationic component of the complex is also employed for the pre-treatment, although it is possible to pre-treat the filler with a first cationic component and to use a second for forming the complex. Of the total amount of cationic polymer employed during the treatment, about 10 to 95 % by weight of the polymer, preferably 20 to 90 % by weight and in particular about 20-60 % by weight is separately attached to the filler material in a pre-treatment step.
According to a first embodiment, the polyelectrolyte complex is a complex that has a negative net charge. The negative charge can be at least -0.1 meq/g, preferably about -0.4 to -4 meq/g.
According to a second embodiment, the polyelectrolyte complex has a positive net charge. The polyelectrolyte complex can have a positive net charge of at least 0.1 meq/g, preferably about 0.4 to 4.0 meq/g.
With the aid of a pre-treatment the affinity of the polyelectrolyte complexes for filler is increased, in particular when the polyelectrolyte complex has a negative net charge. By treating the filler with a cationic polymer, the attachment of negatively charged polyelectrolyte complexes to the filler surface is facilitated. With cationic polyelectrolyte complexes pre-treatment is generally not needed. It would appear that the cationic polymer, such as polyamideamine epichlorhydrine (PAE) or other polyamine derived polymers which have a tertiary or quaternary amine functionality, enable increased attachment of PEC to the filler.
The contacting times can be of equal length in both stages and as explained above.
In the invention, the aim is to attach a substantial proportion of the polyelectrolyte complex so that at least 10 % by weight, preferably at least 20 % by weight, in particular at least 30 % by weight and most suitably at least 40 % by weight, of the complex is sorbed from the solution attached to the fillers. The fillers then contain typically approx. 0.1 - 30 %, of the polyelectrolyte complex.
As mentioned above, the polyelectrolyte complex used in the present invention is a complex formed by a cationic polymer and an anionic polymer. The polyelectrolyte complex typically is charged, and in particular it has a negative net charge, but it can also have a positive net charge.
Generally, the cationic polymer can be selected from the group of:

cationic acrylic polymers;
cationic polyacrylamides;
polydiallyldialkyl-ammonium polymers;
cationic condensation amido-amine polymers;
condensation products formed between dicyandiamide, formaldehyde, and an ammonium salt;
reaction products formed between epichlorohydrin or polyepichlorohydrin and ammonia, a primary amine or a secondary amine;
polymers formed by reacting a di-tertiary amine or secondary amine and dihaloalkanes;
polyethylamine formed by polymerization of ethylimine; and
polymers formed by polymerization of a N-(dialkyl-aminoalkyl)-acrylamide monomer.

One type of cationic polymer is formed by polyamide derivatives with tertiary and quaternary amine functionality. Another type of cationic polymer is formed by carbohydrate polymers, such as cationized starch. The anionic polymer is, for example, derived from a carbohydrate polymer having a negative net charge. Examples of such polymers are anionic cellulose derivatives, hemicellulose derivatives, starches and mixtures thereof. Synthetic anionic polymers may include polyacrylic acid, polymethacrylic acid, polyvinylamine, and polymers containing carboxyl groups, primary and secondary amine functionalities as well as mixtures of two or more of the aforementioned polymers.
According to one working embodiment, the polyelectrolyte complex comprises polyamide amine epichlorhydrine (PAE) or cationized starch and carboxymethyl cellulose (CMC) or a similar cellulose derivative. Other polymer combinations have also been tested and proven to work. Thus, the PEC can be made by adding a polyamide amine epichlohydrine resin (PAE) to carboxymethyl cellulose (CMC) having a degree of substitution between 0.4 and 1.3 and preferably forming that PEC in water. (See Colloid and Surfaces A: Physicochem. Eng. Aspects 213 (2003) 15-25), the content of which is herewith incorporated by reference). Generally, molar ratios of PAE to CMC of about 1:1 to 1:5 can be used. But preferably, a proportion based on charge density is used. The charge ratio (meq/g of cationic polymer/meq/g of anionic polymer as absolute value) most often used is in the range of 0.3 to 1.5.
Similar proportions can be used with starch derivatives.
It is preferred to contact the components of the PEC at turbulent conditions, e.g., under intensive agitation by dynamic or static mixers. Alternatively, the turbulence can be generated by utilizing the velocity differences of two feeds flowing at different flow rates. The contacting can be carried out in a continuous operation or batch-wise or semibatchwise, although continuous or semibatch operation is preferred.
The preparation of a suitable PEC complex is described in our copending patent application title "Method of producing paper and board", the contents of which is herewith incorporated by reference.
The molecular weight of polymer components of the complexes may vary within large ranges. Typically, the degree of polymerization (DP) of the polymer, e.g. the PAE, is approx. 100 - 20,000, in particular approx. 200 - 5,000.
The aqueous suspension containing the modified filler can be used as such in paper or cardboard making. If separation of the filler is desired, it is usually not dried before papermaking. During papermaking, the filler is mixed with cellulosic or lignocellulosic fibers, sizing agents, retention chemicals, and other additives. Retention chemicals to be mentioned include polymeric products such as polyethylene imine, low molar mass polyacrylamide and polyamine, as well as cationic starch, guar or polyacrylamine combined with colloidal silica, alumina or montmorillonite. The amount of retention chemicals is in general at least 0.5 % of the dry matter of the fiber, typically approx. 0.6 - 1 % of the dry matter of the fiber.
The furnish may also contain other conventional additives, such as internal sizing agents and retention agents, including AKD or ASA resins, rosins and ionic and polymeric retention agents. Many of the additives in the paper furnish serve to ensure that the fibers have the required properties to attract and bond. Conventionally, the amount of such additives does not exceed 5 % by the dry weight of the paper/board, typically the amount are is in the range of about 0.1 to 4 % by weight.
The paper pulp is slushed in a manner known per se to a suitable consistency (typically a solids content of approx. 0.1-1 %). The above-mentioned filler is added to the fiber slush, before the headbox of the paper or board machine, usually in an amount of approx. 1 - 40 % by weight of the weight of the fibers in the fiber pulp. Usually the filler constitutes at least 5 % by weight, most suitably 10 - 100 % by weight, of the total filler in the base web, and respectively 10 - 50 % by weight of the fiber material in the base web.
The filler modified according to the present invention can be used as such for filling papers or it can be used in combination with conventional, untreated fillers, for example mineral fillers. The weight ratio between modified fillers and conventional fillers (if any), is typically about 1:100 to 100:1, preferably about 30:70 to 99:1, in particular about 50:50 to 90:10. Thus, a portion (at maximum 95 %, usually 90-10 % by weight, of the total amount) of the filler used in the slush can consist of conventional fillers, such as calcium carbonate (natural or precipitated), kaolin, talc, hydrogenated aluminum oxides (aluminum trihydroxides), calcium sulfate, barium sulfate, calcium oxalate, silicates or titanium dioxide and mixtures thereof.
In the paper or board machine the fiber pulp is formed into a paper or board web. The fiber web is dried and most suitably optionally coated, and optionally or after-treated by for example calendering.
With the help of the invention it is possible to produce uncoated or coated and optionally also calendered cellulose-containing material webs having excellent printing properties, high smoothness, as well as high opacity and whiteness. The web can be coated with a pigment, for example, calcium carbonate, gypsum, aluminum silicate, kaolin, aluminum hydroxide, magnesium silicate, talc, titanium dioxide, barium sulfate, zinc oxide, synthetic pigment, or mixtures thereof.
By "cellulose-containing material" is meant here generally paper or board or a corresponding cellulose-containing material derived from a lignocellulose-containing raw material, in particular wood or annual or perennial plants. The said material may be wood-containing or woodfree, and it may be prepared from mechanical, semimechanical (chemimechanical) or chemical pulp. The chemical pulp and the mechanical pulp may be bleached or unbleached. The material may also contain recycled fibers, in particular from recycled paper or recycled board. The grammage of the material web varies typically within the range 35 - 500 g/m², in particular it is approx. 50 - 450 g/m².
In general, the grammage of the base paper is 20 -300 g/m², preferably 30 - 80 g/m². The invention is particularly suitable for producing highly-filled uncoated papers. Such papers are exemplified by offset printing papers and office papers (copying papers).

Example 1

A trial with an experimental pilot paper machine, the XPM, was performed for testing the use of treated precipitated calcium carbonate (PCC) in highly-filled papers. The filler treatment was performed by first adding the polymer, in this case, polyamideamine epichlorhydrine to a 15 % PCC slurry. The final concentration of the slurry contained 2 % of the polymer based on the dry weight content of PCC. The mixture was allowed to stand with constant stirring for 15 minutes. A polyelectrolyte complex solution was prepared containing equal weight polyamideamine epichlorhydrine and carboxymethylcellulose (anionic treatment). The complex solution (2 % by weight) was then added to the slurry with constant stirring and allowed to react for 30 minutes.
A furnish consisting of 70 % mixed hardwood (22 SR) and 30 % softwood (24 SR) was pH adjusted to 8. The following additives were also used: 2,5 kg/t PAC 18 and 60 g/t Cartaretine 30AE for retention; Aquapel J320 for AKD stock internal sizing and 4 kg Hi Cat 5244 A stock starch. The paper produced on the XPM had a grammage of 80 g/m² with 28 % filler.
Figure 1 shows the effect of filler treatment on sizing demand and sizing stability. The figure shows a plot of the AKD dosage against Cobb 60. The solid lines for the untreated PCC for both 1 and 4 weeks are shown. It took 2 kg/t AKD for the untreated PCC to reach a reasonable Cobb value (approx. 20). Cobb values taken after 4 weeks show a slight increase, indicating a loss of sizing effect over a period of time. The dashed lines represent the treated PCC with polymer+polyelectrolyte complex for both 1 and 4 weeks time. One can observe that a reasonable Cobb value was attained with 1,25 kg/t AKD. This is a savings of 0,75 kg/t AKD to reach the same degree of sizing with the treatment.
Additionally, the Cobb values remain unchanged taken after 4 weeks time, This indicates an improved sizing stability with the filler treatment.

Another important result observed with the invention was the significant increase in strength in terms of tensile index, tear index and Scott Bond. All papers produced from treated filler exhibited higher strength values than untreated filler. The average increase in terms of tensile strength was 51 %, while the average increase in Scott Bond was 85 %.

Table 1

	AKD kg/t	Tensile index Nm/g	Tear index mN m² /g	Scott Bond J/m²
PCC	0	14.4	4.,7	81
	0.5	14.6	4.7	81
	1.25	13.7	4.6	84
	2	14.3	4.8	83
	3	14.2	4.7	76
PEC-treated PCC	0	22.6	6.0	166
	0.5	22.1	5.7	157
	1.25	21.9	6.0	148
	2	20.9	5.8	143
	3	19.9	5.5	136
Average (PCC)		14.24	4.69	81.00
Average (PEC)		21.49	5.82	150,00
Increase %		51	24	85

Example 2

A trial on the experimental paper machine (XPM) was performed to test the effects of complex-treated (both anionic and cationic) filler at high filler level in terms of strength, sizing, retention and optical properties.
A cationic complex was added to a PCC slurry resulting in a final concentration of 0.2 % based on the dry weight of the filler. A furnish consisting of 70 % mixed hardwood (22 SR) and 30 % softwood (24 SR) was pH adjusted to 8. The following additives were used: 2.5 kg/t PAC 18 and 60 g/t Cartaretine 30AE for retention; Aquapel J320 for AKD internal sizing and 4 kg Hi Cat 5244 A stock starch.
The paper produced on the XPM had a grammage of 80 g/m² with 28 % filler. A reference series with untreated PCC was performed with increasing AKD dosages (0 to 2 kg/t) and similar series with anionic and cationic polyelectrolyte complexes, respectively, were performed. Additionally, a trial point was performed using cationic polyelectrolyte complex-treated filler without any retention chemical system.
The papers were then tested for strength and optical properties. Figure 2 shows the geometric tensile index for all three sample series. Both anionic and cationic polyeletrolyte complex filler treatment showed increased tensile index as compared to the reference, with anionic treatment giving the highest increase. A similar trend was observed for tensile stiffness index. Figure 3. However, the cationic polyelectrolyte complex treatment exhibited a slight increase in bulk. The anionic complex did not affect this property. Another interesting observation was for the cationic treatment without retention chemicals. It showed even a higher increase in both tensile index, tensile stiffness index and bulk as compared to the cationic anionic-treated series. Figure 4. This illustrates that a 28 % filler level is possible without retention chemicals.

Example 3

PEC filler treatment on surface strength
PEC treatment on the filler has been shown to have positive effects on sizing efficiency, sizing stability and strength, with relatively low dosages. But results also show that the treatment has good effects on surface strength. Surface strength refers to the resistance of the surface layer of a sheet to the break-away of surface fragments when the sheet is separated from the inked blanket during printing. Loosely bonded fibers, filler particles and starch may be pulled away from the paper surface by moist conditions during printing. A considerable build-up of such material on printing blankets must be cleaned off resulting to unnecessary maintenance stops. This leads to a requirement from the customers for a paper with improved surface strength.

An XPM trial was performed to study the effect of PEC filler treatment on surface strength on stock-sized papers to varying levels. The fiber furnish used was 80 % unbeaten birch kraft pulp and 20 % beaten (27 SR) softwood kraft pulp. Except for the reference, the filler (PCC) was otherwise treated first with 0.3 % PAE (based on the dry weight of the filler) under vigorous stirring and allowed to react for 15 minutes. 0.18 % anionic PEC (PAE/CMC) (based on the dry weight of the filler) was thereafter added under similar conditions. The targeted filler level is 28 % and grammage of 80 g/m². Varying dosages of ASA (0 to 0.25 %), 0.2 % stock starch and retention aids were used. The results are as follows:

Table 2

	ASA %	Cobb 60s	Surface strength Picking (m/s)
Reference	0.16	23.2	0.47
	0.20	19.2	0.50
	0.25	17.9	0.51
PEC-treated filler	0.16	22.6	0.54
	0.2	18.4	0.52
	0.25	17.3	0.52
PEC-treated filler with PEC on fibre	0.20	22.3	0.57

Following the results of the reference, with increased ASA dosage, the degree of hydrophobicity increases (decreasing Cobb value) and surface strength increases. With PEC-treated filler however, both hydrophobicity and surface strength are higher than the reference with the same ASA dosage. The last point is also interesting since it combines PEC both as a filler and fiber treatment. Although the degree of hydrophobicity is more or less maintained as compared to the reference, the surface strength increased significantly.

Claims

A method of producing a fibrous web on a paper or cardboard machine from an aqueous furnish containing fibres mainly derived from lignocellulosic material, fillers and any conventional additives, characterized by admixing the fillers with a nanometer sized polyelectrolyte complex before contacting them with the fibres and any additives.
The method according to claim 1, wherein the polyelectrolyte complex is a complex formed by a cationic polymer and an anionic polymer.
The method according to claim 2, wherein the polyelectrolyte complex has a negative net charge.
The method according to claim 3, wherein polyelectrolyte complex has a negative net charge of at least -0.1 meq/g, preferably about -0.4 to -4 meq/g.
The method according to claim 2, wherein the polyelectrolyte complex has a positive net charge.
The method according to claim 5, wherein the polyelectrolyte complex has a positive net charge of at least 0.1 meq/g, preferably about 0.4 to 4.0 meq/g.
The method according to any of claims 2 to 6, wherein the cationic polymer is selected from the group of:
- cationic acrylic polymers;

- cationic polyacrylamides;

- polydiallyldialkyl-ammonium polymers;

- cationic condensation amido-amine polymers;

- condensation products formed between dicyandiamide, formaldehyde, and an ammonium salt;

- reaction products formed between epichlorohydrin or polyepichlorohydrin and ammonia, a primary amine or a secondary amine;

- polymers formed by reacting a di-tertiary amine or secondary amine and dihaloalkanes;

- polyethylamine formed by polymerization of ethylimine; and

- polymers formed by polymerization of a N-(dialkyl-aminoalkyl)-acrylamide monomer.
The method according to any of claims 2 to 7, wherein the cationic polymer is a synthetic cationic polymer selected from the group of polyamide derivatives having amine functionality, such as polyamide amine epichlorohydrine (PAE).
The method according to any of claims 2 to 8 wherein the cationic polymer is a polymer derived from a carbohydrate polymer.
The method according to any of the preceding claims, wherein the polyelectrolyte complex comprises a cationic polymer and an anionic polymer derived from a carbohydrate polymer.
The method according to claim 10, wherein the anionic carbohydrate based polymer is selected from the group of anionic derivatives of starch, hemicellulose, cellulose and mixtures thereof
The method according to claim 11, wherein the anionic polymer is carboxymethyl cellulose (CMC).
The method according to any of claims 1 to 9, wherein the polyelectrolyte complex comprises an anionic polymer selected from the group of synthetic anionic polymers, including polyacrylic acid, polymethacrylic acid, polyvinylamine, and polymers containing carboxyl groups, primary and secondary amine functionalities, as well as mixtures thereof.
The method according to any of claims 1 to 13, wherein the polyelectrolyte complex is preproduced under turbulent mixing, and the complex is fed, in an amount of about 0.01 to 10 wt-%, preferably 0.03 to 5 wt-%, to a filler slurry based on the dry weight of the filler.
The method according to any of claims 1 to 14, wherein the fillers are first treated with a polymer.
The method according to claim 15, wherein the fillers are treated with the polymer in order to cationize a significant part, preferably 40 % or more, of the available surface groups on the fillers.
The method according to claim 15 or 16, wherein the fillers are treated with 0.01 to 1 wt-%, preferably 0.05 to 1 wt-%, of a polyamide derivative having a tertiary or quaternary amine functionality, the percentages being calculated on the dry weight of the filler.
The method according to any of claims 15 to 17, wherein the fillers are pretreated with a portion of the same cationic polymer used for producing the polyelectrolyte complex.
The method according to any of clams 1 to 17, wherein after contacting with the polyelectrolyte complex after a retention time, of about 1 to 60 minutes, typically about 10 to 30 minutes, the treated fillers is added in an amount of 1 to 50 wt-%, preferably about 5 to 40 wt-%, in particular about 20 to 30 wt-% based on the dry weight of the paper or board, to a pulp slurry containing fibres and other paper chemicals are added to the pulp slurry, which is then fed to the paper or board machine.
The method according to any of the preceding claims, wherein the filler is selected from the group of synthetic and natural calcium carbonate, kaolin, titanium dioxide and silicates and mixtures thereof.
The method according to any of the preceding claims, wherein the dosage of the polyelectrolyte complex is less than 0.1 % of the dry weight of the filler, preferably about 0.01 to 0.09 %.
The method according to any of the preceding claims, wherein the polyelectrolyte complex consists essentially of a cationic polymer and an anionic polymer.
The method according to any of clams 1 to 21, wherein the polyelectrolyte complex comprises a cationic polymer and an anionic polymer along with at least one further component.
The method according to any of the preceding claims, wherein the polyelectrolyte complex is non-mucous.
The use of a method according to any of the preceding claims for improving surface strength, tensile strength, tensile stiffness and/or bulk at a given filler content.
The use of a method according to any of claims 1 to 24 for reducing dusting of a filler-containing lignocellulosic web.
The use of a method according to any of claims 1 to 24 for reducing the amount of size of a filler-containing lignocellulosic web.
The use according to claim 27, wherein the filler concentration is at least 25 % by weight of the dry fibres.