strategies for improving the formation/retention ... · 2) formation aids: class i, gums and...

Strategies for Improving the Formation/Retention Relationship during

Papermaking Part 1. A Heteroflocculation Approach

Tom Lindström and Gunborg Glad Nordmark

October 2005

According to Innventia Confidentiality Policy this report is public since 2010-10-13

a report from STFI-Packforsk

Strategies for Improving the Formation/Retention Relationship during

Papermaking Part 1. A Heteroflocculation Approach

Tom Lindström, Gunborg Glad-Nordmark

STFI-Packforsk Report No.: 96 | October 2005

Cluster: Paper Chemistry Restricted distribution to: Billerud, Eka Chemicals, Holmen, Kemira, Korsnäs,

Mondi Packaging Paper, M-real, Norske Skog, Stora Enso, Södra, Voith

According to STFI-Packforsk's Confidentiality Policy this report is assigned category 2

Strategies for Improving the Formation/Retention Relationship during Papermaking STFI-Packforsk Report 96

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Acknowledgements The authors acknowledge the financial contributions from Billerud, Eka Chemicals, Holmen, Kemira, Korsnäs, Mondi Packaging Paper, M-real, Norske Skog, Stora Enso, Södra and Voith.




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Table of contents Page

1 Summary.......................................................................................................4

2 Introduction ..................................................................................................5

2.1 Experimental .......................................................................................7

2.2 Materials..............................................................................................7

3 Methods ........................................................................................................9

3.1 Surface carboxymethylation (CMC-grafting).......................................9

3.2 Charge determination..........................................................................9

3.3 Retention tests ....................................................................................9

3.4 Sheet forming and sheet formation.....................................................9

4 Results and Discussion ............................................................................11

5 References..................................................................................................17




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1 Summary This paper deals with a retention strategy to improve the formation of paper, while maintaining the retention, i.e. improving the retention/formation relationship.

The technology is based on topochemical surface grafting of cellulosic fibres with carboxymethylcellulose (CMC). Surface grafting increases the number of surface charges on the cellulosic fibres, enabling the deposition of cationic filler particles onto the negatively charged cellulose surfaces in the heteroflocculation mode. This is gene-rally a very inefficient deposition mode unless the fibres have a high surface charge density.

When cellulosic surfaces are charged, the friction between the fibres are decreased, so the internal stresses in fibre flocs can be released. This results in a break up of flocs and better fibre dispersion. The combination of heteroflocculation and the decreased friction between fibres results in sheets with an improved retention/formation rela-tionship compared to a strategy using a cationic high molecular weight polyacryla-mide (C-PAM) as the retention aid.

Sammanfattning Denna rapport behandlar en retentionsstrategi för att förbättra sambandet mellan retentionen vid papperstillverkningen och arkets formation.

Teknologin baserar sig på topokemisk ytgrafting av fibrer med karboxymetyl-cellulosa (CMC). Behandlingen ökar antalet ytladdningar på cellulosafibrerna, vilket möjliggör deposition av katjoniska fyllmedelspartiklar till negativt laddade cellulosa-ytor i ett heteroflockuleringsförfarande. Detta är allmänt sett mycket svårt såvida inte fibrerna har en hög ytladdning.

När ytladdningstätheten på fibrerna ökar, minskar friktionen mellan fibrerna, så att inre spänningar i fiberflockar kan utlösas. Detta resulterar i att fiberflockar bryts upp och fibrerna dispergeras. Kombinationen av heteroflockuleringsförfarandet och den minskade friktionen mellan fibrerna resulterar i att retentions/-formationssambandet kan förbättras jämfört med en strategi där man använder en katjonisk högmolekylär polyakrylamid som retentionsmedel.




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2 Introduction The impact of paper formation on both strength properties and printability of paper is well recognized among papermakers, albeit the importance of specific structural levels of uniformity of paper is being the subject of continued scrutiny.

Basically, headbox and forming zone design together with running conditions on the machine will determine sheet formation for a given stock composition and chemical retention aid system. The subject of mechanical factors influencing sheet formation have also been reviewed recently (Norman and Söderberg 2001).

It is also well known that there is a correlation between fines retention and floccula-tion (e.g. Pierre and Carré 1993; Tichy and Karnis 1978) and laboratory scale floccu-lation measurements have also been connected with mill trials. (e.g. Rutland 1977). It is also recognized that high retention levels are associated with an impaired forma-tion, but very little is known about the impact of different retention aids on the retention/formation curve (Albinsson et al 1995; Swerin and Ödberg 1997).

The chemistry and mechanisms of action of different retention aids is known to a reasonable extent and has been the subject of many reviews to which the reader is referred (e.g. Eklund and Lindström 1992; Lindström 1989; Gess 1998; Norell et al 1999).

Retention aids are usually classified in groups such as coagulants, patch flocculants, bridging flocculants, microparticulate systems, network systems etc. but even a closer considerations will give little guidance as to what systems would be better than others or which dosage strategies should be applied. There are numerous of publications in the trade literature claiming superior retention/formation characteristics for certain retention strategies, but very little or no data have been supplied to support such claims.

In the only review available on formation/retention relationships, Swerin and Ödberg (1997) do indeed suggest that there are differences between different systems, but no hard data were given.

There are a few laboratory studies on the issue. The study of Krogerus (1994) suggested that low to medium Mw flocculants should be selected and the study by Huber et al (2004) suggested certain multi-component applications of flocculants were better than others, but none of these studies made any efforts to discuss possible mechanisms to gain some better understanding. Presumably, these authors realized the complexity of the systems under study.

Neglecting the chemistry for a moment, the most successful approach to gain some understanding of the tendency for fibre flocculation is the crowding factor concept, developed by Kerekes and Schell (1992). In this concept the crowding factor concept is related to the consistency, fibre length and coarseness and Kiviranta and Dodson




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(1995) have also shown that this concept can be well correlated to formation of paper, ignoring the influence of chemical additives.

Obviously, the crowding factor concept does not take chemistry into account, but it is fairly obvious that if the friction between fibres will be decreased, it will be more difficult to build up the stresses keeping the flocs together as internal stresses in the flocs are released when the friction between fibres decreases. Indeed, this stress release mechanism was demonstrated in a paper by Beghello and Lindström (1998). These authors carboxymethylated a pulp and studied the flocculation by optical means in a flow loop and found that the floc size was decreased, when the fibres became carboxymethylated. The strong repulsion between fibres will keep the fibres form approaching each other at distances smaller than the Debye-Huckel shielding length at the corresponding electrolyte concentration.

There are also other means of dispersing fibres and a survey reveals that dispersion aids may be grouped into the following classes:

1) Additives increasing the dispersion medium viscosity, Soszynski and Kerekes (1988), Zhao and Kerekes (1993).

2) Formation aids: class I, gums and mucilages, which decrease the coefficient of friction between fibres, De Roos (1958), Yan et al (2005)

3) Formation aids: class II, high molecular weight polymers affecting the rheological properties of the suspending media, Wasser (1978), Lee and Lindström (1989).

Gums and mucilages have been known since ancient times to disperse fibres and the mechanism is believed to be the decreased friction between fibres when these sub-stances are being adsorbed onto fibre surfaces.

High molecular weight polymers, such as anionic polyacrylamides have also been known to improve formation. In this specific case, adsorption is not required for dis-persion. The mechanism is most likely due to turbulence suppression as suggested by Lee and Lindström (1989). Such additions will, however, also hamper the drainage of the pulp and such systems must be carefully controlled, as they are dependent on the presence of the additive in the white-water loop. A dispersant system must be compatible with other wet-end adjuvants; particularly retention aid systems and specific strategies must therefore be developed in order for them to comply with practical papermaking conditions. This was also demonstrated for turbulence suppressants on the FEX-machine in our laboratory some decades ago (Lindström et al 1986). The technology has, indeed, also been used by a number of fast-draining new paper-machines, but the technology has not enjoyed wider practical application.

The previously discussed carboxymethylation technology (Beghello and Lindström 1998) is, of course, not a practically viable technology but this study partly initiated the search for viable surface charging technologies of papermaking fibres, which lead to the development of a topochemically selective grafting technology using carboxy-methylcellulose (CMC) in this laboratory (Laine et al 2000). With this technology,




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cellulosic fibres are contacted with CMC at a high temperature and ionic strength and under these conditions CMC will co-crystallise onto cellulosic surfaces in an irreversible way. If the molecular weight of the CMC is sufficiently high not to penetrate the cell wall of fibres, the fibres will be surface grafted with CMC in a very selective way. The surface charge of bleached softwood kraft pulps can easily be increased an order of magnitude. Hence, the friction between fibres is expected to be lowered, resulting in fibre dispersion, as was also later demonstrated by Yan et al (2005).

This paper will demonstrate a laboratory study aimed at evaluating a retention strategy, by which, an improved formation/retention relationship can be obtained, by fusing the fibre charging (CMC-grafting) technology with a heteroflocculation concept, where cationic filler particles are deposited onto negatively charged fibres. In this way, both the decreased surface friction between fibres can be utilized and the high surface charge of fibres can be used to deposit the filler onto the papermaking fibres.

The idea of using the heteroflocculation mode, i.e. depositing cationic fillers onto anionic fibres, is far from new and has been around in the industry for many years (see e.g. Gess 1998; Alince 1983). Unless high filler contents are targeted it is undesir-able to deposit filler particles as flocs in paper, because this will decrease the scattering coefficient of paper. Therefore heteroflocculation may offer some advantages. The idea has also been practised, but has never really taken off. One reason is most likely that bleached pulps contain very few carboxylic groups, resulting in less interaction between a cationic filler and the fibre surface. It will be demonstrated that the charging technology will alleviate this drawback and make the fused technologies work.

2.1 Experimental

2.2 Materials The pulp used in all experiments was a never-dried unbeaten ECF (Elementary Chlorine Free)-bleached softwood (spruce/pine) kraft pulp (M-real, Husum, Sweden). A Celleco-filter with 100 μm screening slots was used to remove the fines (20-25 %) prior to the experiments. Before use, the pulp was first transferred to its H-form using 0.01 M HCl, after which the pulp was transferred to its Na-form using 10-3 M NaHCO3 for 10 minutes, after which the pH was adjusted to pH 9 and kept there at 30 minutes before being transferred to the grafting vial.

The CaCO3 used was a commercially available ground calcium carbonate (HC-60 GG from OMYA AG). It was used without further purification in the experiments.




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A fractionated poly-DiallylDiMethylAmmoniumChloride (poly-DADMAC, Ciba, Yorkshire, UK) was used as the polyelectrolyte in the polyelectrolyte titrations. The same polyelectrolyte was used to recharge the anionic calcium carbonate. The charge density was determined by direct polyelectrolyte titration to be 5.9·10-3 eq/g (theo-retical value is 6.19·10-3 eq/g) using a commercially available potassiumpolyvinyl-sulphate (Waco Chemicals). The charge density of this polymer is 6.16·10-3 eq/g and the molecular mass was 3·105.

The Mw (Mw=9.2·105) and molecular mass distribution (Mw/MN = 6.3) was deter-mined on TSK-gel columns (Tosoh Corp. Japan) using appropriate data software (PL caliber GPC/SEC Software version 7.01, Polymer Laboratories). Molecular mass standards were polyethylene oxides (Tosoh Corp. Japan).

The cationic polyacrylamide (C-PAM) used was labelled PL1520 and was obtained from EKA Chemicals (Bohus, Sweden). This polymer has a molecular weight of 7 million and a 20 % molar charge density of cationic groups (according to the manu-facturer).




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3 Methods

3.1 Surface carboxymethylation (CMC-grafting) Surface carboxymethylation was carried out using carboxymethylcellulose (CMC) according to a method developed by Laine et al (2000). The pulp, in its sodium form, was placed in a solution of 5·10-2 M CaCl2. CMC (Finnfix WRH, Metsä-Serla, Finland, D.S. = 0.52, Mw = 1·106 Da) was added to the pulp, which was subsequently diluted to a pulp consistency of 2.5% and then heated at 120 ºC for two hours in a pressurized vessel. This was followed by washing with deionised water until the con-ductivity of the filtrate was below 5 μS/cm. Five different grafting levels was used, corresponding to the addition of 1,2,5,10 and 20 mg/g CMC. This resulted in seven different surface charge densities: 2.1, 2.4, 3.2, 5.0, 8.7, 14.5 and 27.8 μeq/g. The surface charge of the reference pulp was 1.8 μeq/g. They were all used in their Ca-form in the experiments to follow.

3.2 Charge determination The total charge of the pulps was determined by conductometric titration, according to Katz et al (1984). Polyelectrolyte adsorption followed a procedure by Winter et al (1986). The adsorption was carried out with pulp in its sodium form at a pulp con-centration of 5 g/l. The pH was 8.0 +/- 0.1 in all adsorption experiments. An appropriate excess of poly-DADMAC was added and the suspension was shaken until adsorption equilibrium was reached (30 minutes). The fibres were separated from the solution by filtration and dried in order to record the dry weight. The filtrate was saved and titrated using the polyelectrolyte titration procedure in order to determine the amount of polyelectrolyte adsorbed.

3.3 Retention tests The laboratory retention trials were conducted using a vaned “Britt Dynamic Drain-age Jar” (BDDJ) with a standard steel screen (holes: diameter = 76 μm) at 750 RPM.

3.4 Sheet forming and sheet formation Isotropic sheets were formed using a Finnish sheet former according to the SCAN-CM 64:00 standards. Two series of experiments were performed. One series with different additions of a cationic polyacrylamide (C-PAM). In this series of sheets, the calcium carbonate containing (15 %) stock was poured into the hand-sheet former after which the C-PAM was added. Contact time between C-PAM addition and sheet forming was 30 sec. In a second series of experiments, the pre-cationized calcium




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carbonate was added to the sheet-mould, contacted with the fibres for 30 sec, before forming a sheet.

In these series of experiments the filler retention was measured in the hand-sheet former and the formation of the sheets were measured using the STFI Formation method (β-radiography).




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4 Results and Discussion In a first series of experiments, the importance of fibre anionic charge density on deposition of a cationized ground calcium carbonate (GCC) filler was demonstrated.

In a heterocoagulation experiment of this type, the filler should be cationized, with a cationic polyelectrolyte, which is not active in the bridging mode. For this purpose poly-diallyldimethyl ammonium chloride (poly-DADMAC) was chosen. This is a highly charged polyelectrolyte with a comparatively moderate molecular weight (920 kDaltons). Such polyelectrolytes are conceived to act as “patch-flocculants” in reten-tion operations (Lindström 1989). As a reference retention system, a high molecular weight (7000 kDaltons, molar cationic charge density = 20 %) bridging flocculant, cationic polyacrylamide (C-PAM), was chosen.

A reference retention experiment was first carried out using a Britt Dynamic Drain-age Jar (BDDJ). A stock consisting of unbeaten, fines free, ECF-bleached reference softwood kraft pulp (85 %), which was mixed with 15 % of the GCC-filler and different amounts of the flocculants were added in what is here referred to as the homoflocculation mode. The results are displayed in figure 1.

As shown in figure 1 the bridging flocculant is C-PAM is very efficient in retaining the filler, whereas the patch-flocculant has very moderate effect on retention, as expected from theory. (The arrow in the figure indicates the total amount of poly-DADMAC that was added in a later experiment using the heteroflocculation mode).

Figure 1 The effects of cationic polyacrylamide (C-PAM) and poly-diallyldimethylammonium chloride (poly-DADMAC) on filler (anionic ground calcium carbonate (A-GCC) retention in Britt Dynamic Drainage Jar (BDDJ) retention experiments (750 RPM/vaned jar). Stock: Unbeaten, fines free, bleached softwood kraft pulp (85 %) and 15 % A-GCC. The arrow indicates the total amount of poly-DADMAC used in the heteroflocculation experiments displayed in figs 3, 5 and 6.




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Figure 2 Adsorption isotherm of poly-DADMAC onto anionic GCC. GCC-conc. = 10g/l. The arrow indicates addition (0.5 %) of poly-DADMAC, when cationizing the GCC.

Then, an experiment was designed where the negatively charged GCC-filler was separately recharged with poly-DADMAC. In this series of experiments, 0.5 % poly-DADMAC was added to a slurry of the GCC-filler. The adsorption isotherm of poly-DADMAC onto the filler under these conditions is shown in figure 2. With this addition of poly-DADMAC, the filler surface is effectively saturated, and recharged, with poly-DADMAC, and there is a slight excess of poly-DADMAC in solution.

A series of experiments were then carried out using fibres with different surface charge densities. Different surface charge densities of the fibre materials were obtained by grafting the cellulose with carboxymethylcellulose (CMC). This can be done by heating cellulose fibres to high temperatures at an elevated electrolyte con-centration in the presence of CMC (see exp. section). This method is an effective sur-face charging method for papermaking fibres. A series of 6 pulps including the reference pulp was produced for this purpose and subjected to a series of experiments where the cationized filler was added in the heteroflocculation mode. The results are displayed in figure 3. In this series, different amounts of the cationic GCC-filler were added to the pulp.

As seen in the figure, filler retention can only be maintained at very low filler additions for the ref. pulp (surface charge density = 1.8 μeq/g). The explanation is simply that the cationized GCC-filler will recharge the negatively charged fibres and in this way only a limited amount of cation filler can be deposited onto the fibre sur-faces. If, on the other hand the anionic charge density of the fibres is increased, more




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Figure 3 The effects of fibre surface charge density on filler retention (cationized GCC) in Britt Dynamic Drainage Jar (BDDJ) retention experiments (750 RPM/vaned jar). Stock: Unbeaten, fines free, bleached softwood kraft pulps (grafted with various amounts of CMC) and various amounts of cationized GCC. The GCC was cationized with an addition of 0.5 % poly-DADMAC (based on filler) before use. Heteroflocculation mode.

cationic filler can be retained. For the fibres with the highest charge density (28 μeq/g), the retention of the cationic filler is very high due to the strong interaction between the cationic filler and the anionic surface of the fibres. In essence, these experiments demonstrate the efficiency of the heteroflocculation mode, provided the fibre surface is sufficiently negatively charged.

It was also of general interest to investigate if the heteroflocculation mode is a robust retention process, with respect to its sensitivity to electrolytes. Modern, closed papermaking systems can contain rather high levels of electrolytes, interfering with retention processes. Hence, four series of experiments were run with two different electrolytes, NaCl and CaCl2. One series was a reference series with the C-PAM and the reference pulp (S.C. = 1.8 μeq/g) and another was the series with the cationic GCC-filler in the heteroflocculation mode using a highly grafted pulp (S.C. = 28 μeq/g). The results are displayed in figures 4 and 5. Figure 4 shows the effect of the electrolytes on the retention with an addition of 0.1 % C-PAM and Figure 5 shows the results with the cationic GCC-filler in the heterocoagulation mode. Both systems can withstand high levels of NaCl, whereas CaCl2 will inhibit the retention at higher addition levels. The bridging flocculant system was found to be more sensitive to high CaCl2 concentrations than the heteroflocculation system.




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Figure 4 The effect of electrolytes (NaCl, CaCl2) on filler (A-GCC) in Britt Dynamic Drainage Jar (BDDJ) retention experiments (750 RPM/vaned jar). Stock: Unbeaten, fines free, bleached softwood kraft pulp (85 %) and 15 % A-GCC, Bridging flocculation mode with an addition of 0.1 % C-PAM.

Figure 5 The effect of electrolytes (NaCl, CaCl2) on filler (A-GCC) in Britt Dynamic Drainage Jar (BDDJ) retention experiments (750 RPM/vaned jar). Stock: Unbeaten, fines free, CMC-grafted (28 μeq/g) bleached softwood kraft pulp (85 %) and 15 % cationized GCC. The GCC was cationized with an addition of 0.5 % poly-DADMAC (based on filler) before use. Heteroflocculation mode.




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Finally, the two systems were compared with respect to their retention/formation characteristics. In this series of experiments standard laboratory handsheets were manufactured, while the single pass filler retention was monitored and the final hand-sheets were then subjected to formation analysis.

Figure 6 shows the results of this experiment. Two different relationships were obtained, one for the homoflocculant system (Bl. Kr. + An. CaCO3 + C-PAM) and one for the hetoroflocculation mode addition (Graft Bl. Kr + Cat. CaCO3). For all stocks the filler content was 15 %. In the bridging flocculant system, the addition of C-PAM was increased and retention levels and formation numbers were determined.

In the heteroflocculation mode, the addition of poly-DADMAC was kept constant (0.1 % based on the total stock) and pulps with different charge densities were used. The charge densities (expressed as μeq/g) of the different pulps are indicated for each point in the figure.

The bridging flocculant system shows, as expected, a continuously deteriorated formation with an increased filler retention, whereas there is a minimum in the formation value vs. retention for the heteroflocculation system.

Figure 6 also shows that the heterocoagulation system gives better formation for filler retention levels above 20 %.

The minimum in the curve in the heterocoagulation can be explained by the combi-nation of two effects. One effect is the decreased friction between fibres, when they are increasingly charged. A handsheet made from the reference pulp had a formation number = 12.0, whereas a handsheet made from grafted pulp without any other additives, with the highest charge density had a formation number = 9.3. As explained in the introduction of this paper, stresses, keeping a floc together, are released, when the friction between fibres decreases. This has also has been confirmed using appropriate equipment for flocculation analysis (Yan et al 2005).

This means that an increasing charge density of the pulp, should improve the forma-tion value in the heterocoagulation curve in figure 6. When the cationic filler is added, it will act as a flocculant in itself and the combination of the two effects will yield a curve with a minimum in the formation value as displayed in figure 6.

The flocculant effect of the cationic filler is the reason the two formation/retention curves intersect in figure 6.

In conclusion, this paper demonstrates a retention strategy (based on the fusion of surface grafting of fibers with CMC and heteroflocculation) by which paper with improved formation can be produced at a certain retention level.




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Figure 6 Formation (CoV, %) for standard handsheets vs CaCO3 retention. = addition of C-PAM to a stock consisting of bleached softwood kraft pulp (85 %) and anionic GCC (15 %). = Heteroflocculation experiments with a stock consisting of unbeaten, fines free, CMC-grafted (various degrees of grafting indicated by surface charge density (μeq/g) of the pulp) bleached softwood kraft pulp and 15 % cationized GCC. The GCC was cationized with an addition of 0.5 % poly-DADMAC (based on filler) before use.




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5 References Albinsson C-J, Swerin A and Ödberg L (1995) ”Formation and retention during twin-wire blade forming of a fine paper stock” Tappi J., 78(4), 121

Alince B and Lepoutre P (1983) “Interaction of cationic clay particles with pulp fibres” Tappi J., 66(1), 92

Beghello L and Lindström T (1998) ”The influence of carboxymethylation on the fiber flocculation” Nordic Pulp Paper Res. J., 13(4), 269

De Roos A (1958) ”Stabilization of fibre suspensions” Tappi J., 41(7), 354-358

Eklund D and Lindström T (1992) ”Paper Chemistry - an Introduction" DT Paper Science Publications, Grankulla

Gess J M (1998) ”Retention of fines and fillers” Tappi Press, Atlanta, USA

Huber P, Pierre C, Bermond C and Carré B (2004) ”Comparing the fiber flocculation behavior of several wet-end retention systems” Tappi J., 3(4), 19

Katz S, Beatson R P and Scallan A M (1984) “The determination of strong and weak acidic groups in sulfite pulps” Sv. Papperstidn., 87(6), R48

Kerekes R J and Schell C J (1992) ”Characterization of fibre flocculation regimes by a crowding factor” J. Pulp Paper Sci., 18(1), J32

Kiviranta A and Dodson C T J (1995) ”Evaluating fourdrinier formation performance” J. Pulp Paper Sci., 21(11), J379

Krogerus B (1994) ”Impact of retention polymers on flocculation, retention, drainage and sheet formation. A laboratory study” 1994 Papermakers Conference Proceedings, Tappi Press, Atlanta, 445




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Laine J, Lindström T, Glad-Nordmark G and Risinger G (2000) “Studies on topochemical modification of cellulose fibres” Part 1” Nordic Pulp Paper Res. J., 15(5) 520

Lee P and Lindström T (1989) ”Effects of high molecular mass anionic polymers on paper sheet formation” Nordic Pulp Paper Research J., 4(2), 61

Lindström T, Wågberg L and Hallgren H (1986) “Chemical Additives Strategies for the Production of Highly Filled Fine Papers” EUCEPA/ATICELCA 22nd Conf., Florence. Devt. & Trends in Sci. & Technol. of Pulp & Papermaking, Vol. 1, Paper no 13: 17

Lindström T (1989) ”Some fundamental chemical aspects on paper forming" in Fundamentals of Papermaking, Ed by Baker C F & Punton V W. Mech. Eng. Pub. Ltd. (London), Vol I, 309

Norell M, Johansson K and Persson M (1999) ”Retention and Drainage” Chap 3 in Book 4 ”Paper Chemistry” in ”Papermaking Science and Technology, Fapet Oy, Helsinki

Norman B and Söderberg D (2001) ”Overview of forming literature, 1990-2000” in ”The Science of Papermaking”, Trans. of the 12th Fundamental Res. Symp. held in Oxford, Sept. 2001, The Pulp & Paper Research Society/PIRA International, London, Vol I, 431

Pierre C and Carré B (1993) ”Complementarity between two dewatering and retention microparticle systems. cationic starch/anionic colloidal silica” 1993 Tappi Papermakers Conf., Atlanta, USA, Tappi Press, Atlanta, 163

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Soszynski R and Kerekes R (1988) ”Elastic interlocking of nylon fibers suspended in liquid: Part 1. Nature of cohesion among fibers” Nordic Pulp Paper Research J., 3(4), 172

Swerin A and Ödberg L (1997) ”Some aspects of retention” in ”The Fundamentals of Papermaking Materials” Trans. of the 11th Fundamental Res. Symp. held at Cambridge, Sept. 1997, The Pulp & Paper Research Society/PIRA International, London, Vol. I, 265




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Wasser R (1978) ”Formation aids for paper- An evaluation of chemicals additives for dispersing long fibered suspensions” Tappi J., 61(11), 115

Tichy J and Karnis A (1978) ”Flocculation and retention of fibres and filler particles in flowing suspensions” Trans. of the Techn. Sect. of the Canadian Pulp and Paper Ass. 1, TR19

Winter L, Wågberg L, Ödberg L and Lindström T (1986) “Polyelectrolytes adsorbed on the surface of cellulosic materials” J. Colloid Interface Sci., 111, 537

Yan H, Lindström T and Christiernin M (2005) “Some ways to decrease fibre suspension flocculation and improve sheet formation” Accepted for publ. in Nordic Pulp Paper Res. J.

Zhao R and Kerekes R (1993) ”The effect of suspending liquid viscosity on fiber flocculation” Tappi J., 76(2), 183-188


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