This investigation is the third contribution in a series. Tom Lindström and Gunborg Glad-Nordmark, STFI-Packforsk AB, Stockholm, Sweden

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1 A study of AKD-size retention, reaction and sizing efficiency. Part 3: The effects of fibre charge density and electrolyte concentration on size retention Tom Lindström and Gunborg Glad-Nordmark, STFI-Packforsk AB, Stockholm, Sweden KEYWORDS: AKD-sizing, Alkyl ketene dimers, Radioactive labelling, AKD retention, Surface charge SUMMARY: This investigation deals with a laboratory study of the combined effects of fibre surface charge density and electrolyte concentration (NaCl, MgCl 2, CaCl 2 ) on the retention of a cationic AKD-dispersion. A bleached softwood kraft pulp was grafted with carboxymethylcellulose (CMC) to various degrees. Hence, a series of pulps with surface charge densities ranging from 1.7 µeq/g to 28 µeq/g were manufactured and used in the study. The AKD used was a radioactively (C-14) labelled, laboratory manufactured, cationic (Z-potential = mv) dispersion stabilized by means of ligno-sulfonate/cationic starch. This stabilizing system is commonly used in many commercial formulations. The laboratory study showed that AKDretention is sensitized by electrolytes and that there is an optimum electrolyte concentration for optimum AKD-retention. Moreover, there is an optimum surface charge density for optimum AKD-retention. The results could be explained in terms of the coupling (swelling) of the surface layer of the stabilizing system and the amphoteric nature of the stabilizer and electrosteric interactions during deposition of cationic particles onto anionic surfaces. ADDRESSES OF THE AUTHORS: Tom Lindström (tom.lindstrom@stfi.se) and Gunborg Glad-Nordmark (gunborg.glad-nordmark@stfi.se): STFI-Packforsk AB, Box 5604, SE Stockholm, Sweden. Corresponding author: Tom Lindström Alkyl ketene dimers (AKD) are commonly used for sizing of paper/board materials. It is well known that the sizing process occurs in three consecutive steps, namely retention, spreading and reaction. The sizing efficiency has been shown to be directly linked to the amount of reacted AKD (Lindström and Söderberg 1986) and size retention will obviously be of critical importance. Recirculated AKD-size will become the subject of alkaline hydrolysis and therefore single pass retention is an important parameter for the improvement of the sizing process. Combinations of lignosulfonates or naphthalenesulphonic acids and cationic starch usually stabilize AKD sizing formulations, although anionic dispersions also are available on the market. Surface-active agents should be avoided as they interfere with sizing (Lindström and Söderberg 1986c) and the mentioned stabilizers are fairly cheap and therefore cost-effective dispersions can be manufactured. Although, cationic AKD-dispersions are net cationic, most commercial formulations are often amphoteric in nature due to the presence of lignosulfonates/naphtalenesulphonic acids. This investigation is the third contribution in a series of papers aimed at examining the AKD-size retention process using C-14 labelled AKD. The methodology is not new and was used in this laboratory already many years ago (Lindström and Söderberg 1986a, c-d; Lindström and O Brian 1986b). The retention of AKD was also studied in these early investigations (Lindström and Söderberg 1986c), and it was found that electrolytes decrease retention and cationic polyelectrolytes increase the retention. During recent years the subject of AKD-size retention has been examined in much greater detail. In our first paper in the more recent investigations (Johansson and Lindström 2004a), the effects of pulp bleaching of various pulps (TCF/ECF/ SW/HW) on self-retention of cationic AKD were investigated. This paper basically indicated that, the higher the surface charge of the pulp, the better was the retention of cationic AKD. It has also been reported by Isogai et al (1997) that the retention is affected by the surface charge density. A higher surface charge density is expected to be beneficial for the retention of cationic species, but these authors also reported an optimum charge density for optimum size retention. In a second investigation (Johansson and Lindström 2004b), the effects of various retention aids and modes of addition were investigated on both anionically and cationically stabilized AKD-dispersions. This investigation showed that there are many powerful retention aids for both anionic and cationic AKD-formulations. Both investigations actually indicated that the self-retention of AKD was of minor importance, compared to the effects of the retention aids. It was also reported that anionic size formulations were in many cases easier to retain than cationic formulations. This is not an unexpected feature, as cationic retention aids are not expected to interact with cationic species. Moreover, this latter publication also indicated, unexpectedly, that there was an optimum electrolyte concentration for the self-retention of cationic AKD. Hence, these investigations raised the issue of the role of surface charge density and electrolyte concentration on the self-retention of cationic AKD. This will, therefore, be the focus in this investigation on AKD retention. Different fibres have different surface charge densities, but in order to make a systematic investigation it is necessary to have the same type of fibre material and only change the surface charge density. This has, hitherto, not been possible to do in a practical way, but now a new fibre grafting technology using carboxymethylcellulose (CMC) has been developed in this laboratory (Laine et al 2000). With this technology, cellulosic fibres are contacted with CMC at a high temperature and ionic strength Nordic Pulp and Paper Research Journal Vol 22 no. 2/

2 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 a high selectivity. The surface charge of bleached softwood kraft pulps can in this way easily be increased an order of magnitude. Therefore, the effects of surface charge density on AKD-retention can be investigated. When investigating AKD-retention, it is critical to have suitable procedures in terms of mixing conditions, contact times etc. This was already obvious in our first investigations many years ago (Lindström and Söderberg 1986c). When the kinetics of AKD-deposition was studied, it was found that at low fibre consistencies, size retention increases with time after addition. Deposition is limited by fibre-akd-particle collision frequency. At higher fibre concentrations, the initial deposition of AKD is very fast, but the deposited AKD is sheared off very fast. This behaviour has also been highlighted by recent investigations by Champ and Ettl (2004). Therefore, it is more practical to study AKD retention and deposition at lower fibre consistencies, where deposition experiments can be performed in a reproducible way, albeit not exactly carried out as in commercial applications. In a previous communication from this laboratory (Johansson and Lindström 2004b), it was noted that, in a graph where the retention of AKD was given vs the CaCl 2 concentration, the retention was decreasing with electrolyte conc. for an ECF-bleached softwood kraft pulp (surface charge = 2.3 µeq/g), whereas, the retention went through a maximum for a TCF-bleached hardwood kraft pulp (surface charge = 5.8 µeq/g). These observations were one of the basis points in this investigation. Materials: Manufacture and Treatments Manufacture 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 NaHCO 3 for 10 minutes. Then the ph was adjusted to ph 9 and kept there at 30 minutes before transfer to the grafting vial. The cationic polyacrylamide (C-PAM) used was labelled PL1520 and was obtained from EKA Chemicals (Bohus, Sweden). This polymer had a molecular weight of 7 million and a 20% molar charge density of cationic groups (according to the manufacturer). 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 M CaCl 2. CMC (Finnfix WRH, Metsä-Serla, Finland, D.S. = 0.52, M w = Da) was added to the pulp, which was subsequently diluted to a pulp consistency of 2.5% and heated at 120ºC for two hours in a pressurized vessel. This was followed by washing with deionised water until the conductivity of the filtrate was below 5 µs/cm. Six different grafting levels were used, corresponding to the addition of 1, 2, 3, 5, 10 and 20 mg/g CMC. This resulted in five different surface charge densities: 3.2, 5.0, 6.5, 8.7, 14.5 and 27.8 µeq/g. The surface charge of the reference pulp was 1.8 µeq/g. Pulp washing (Na- vs Ca-form of the pulps) Before sheet forming, the pulps were first transferred to their H-form using 0.01 M HCl then adjusted to ph = 2 and kept there for 30 minutes. The pulps were then washed with deionized water to a conductivity below 5 µs/cm. This step removes the adsorbed metal ions. The pulps were then transferred to their Na-form using 10-3 M NaHCO 3 for 10 minutes and the ph was adjusted to 9 and kept there for 30 minutes. Water-soluble substances were then removed by washing with deionized water to a conductivity below 5 µs/cm. This was the standard procedure for bringing the pulps to their Na-form. When the Ca-form of the pulp was used, the pulp was washed with 0.05 M CaCl 2 for 30 minutes, after which excess electrolyte was removed by washing the pulp with deionized water to a conductivity below 5 µs/cm. Preparation of C-14-labelled AKD-dispersion Standard C-14-labelled-alkyl ketene dimer (AKD) (mp: 58-62ºC) was made from a mixture of AKD wax (EKA Chemicals, Sweden) and C-14-labelled AKD wax (Amersham Pharmacia Biotech, Buckinghamshire, England). The dispersion was stabilized with a mixture of cationic starch (C-starch), HiCat (Roquette Freres, France), and lignosulfonic acid (LISA), Wanin S (Lignotech, Borregaard, Norway), according to a standard procedure given by EKA Chemicals. The fatty acid distribution was C18: 65%, C16: 30% and C14: 10%. LISA, g/g AKD, was dissolved in g deionized water after which 0.25 g/g C-starch/AKD was added to the solution. The ph was set to 3.2 with sulphuric acid. The dispersion was then refluxed for 1h at a temperature of C in order to gelatinize the C- starch. Molten AKD wax, 2.5 g of C-14-labelled AKD wax and 22.5 g unlabelled AKD wax, was poured into the LISA/C-starch solution and mixed using a Polytron PT 3000 (Kinematica AG, Switzerland) for 2 minutes at a speed of rpm. After this treatment, the emulsion was dispersed using a high-pressure homogenizer (Minilab type 8.30H, APV, Denmark) at a pressure of 600 bar. The AKD emulsion was then quickly cooled. The obtained AKD dispersion had a concentration of 8% and could be stored at 8 C, up to 6 months. Particle size distribution and stability features of such dispersions were discussed in our previous communication (Johansson and Lindström 2004a). 162 Nordic Pulp and Paper Research Journal Vol 22 no. 2/2007

3 Sheet forming Hand sheets were made according to the SCAN C 26:76 standard. The stock was mixed in a vaned Britt Dynamic Drainage Jar (BDDJ) at a speed of 750 rpm. The pulp consistency was 0.342%. The AKD was added to the stock and after 60s stirring, the stock was poured into a British sheet former maintaining a total volume of 8 l and a pulp consistency of 0.171%. This procedure is identical as the procedure labelled modified procedure in a previous publication (Johansson and Lindström 2004b). Sheets were formed from pulps either in their Naform, when retention experiments were carried out using NaCl as the electrolyte or in their Ca 2+ /Mg 2+ -form when carrying out experiments in these electrolytes. Note that all experiments were carried out in the - presence of 1mM NaHCO 3 (ph = 8). The HCO 3 was added as a catalyst for the AKD-reaction. The grammage of the hand sheets were 80 g/m 2. Sheets were then wet-pressed according to SCAN 26:76 and dried in a photo drier at 90 C for 15 minutes in order to cure the sheets. Analytical Methods Conductometric titration The total amount of charged groups, i.e. carboxylic groups, was determined by conductometric titration (Katz et al 1984). Surface charge determined by polyelectrolyte titration The surface charge density of the fibers was determined by a polyelectrolyte titration procedure described by Wågberg et al (1985). The adsorption probe used, a high molecular mass poly-diallyldimethyl-ammonium chloride (p-dadmac, Alcofix 109, Ciba Speciality Chemicals Inc. Basel, Switzerland), was fractionated by ultrafiltration (Pellicon XL Device Biomax 500, cut-off 5x105). The average molecular mass of the polycation was determined to be 6x10 5 using SEC (Size Exclusion Chromatography). The polydispersity (M w /M n ) was around 3. Pulp, in which acidic groups had been converted to their sodium form, was immersed in 10-5 M NaHCO 3. An appropriate excess of p-dadmac was added and the suspension was stirred until adsorption equilibrium was reached (30 min). The fibers were separated from the solution by filtration. The remaining amount of polycation in the solution was determined by titration with a standard anionic polyelectrolyte solution, potassium polyvinylsulphate (KPVS, Wako Pure Chemical Industries, Japan) and orthotoluidine blue (OTB) was used to detect the end point. The surface charge was determined by making a full adsorption isotherm (4-6 polymer additions) of p-dadmac and extrapolating back to zero p-dadmac concentration. Using this procedure, the surface swelling of p-dadmac can be taken into account and stoichiometry prevails (Wågberg et al 1985). Tetrahydrofuran extraction of hand-sheets The determination of the reacted amount of AKD was done in the following way. Every hand sheet was extracted with tetrahydrofuran (THF) with the purpose of removing unreacted AKD. The hand sheets were first soaked in deionized water for 1h, 100 ml/g sheet, and thereafter soaked in THF, 100 ml/g sheet, for 1 minute. The water treatment step is necessary in order to swell the fibers so THF has access to fiber-fiber bonds where residual AKD is present, as has been discussed earlier (Lindström and Söderberg 1986a). The sheets were then extracted in THF using a Soxhlet extractor for 6 cycles, air-dried and then reconsolidated in deionized water. AKD retention and reaction The extent of AKD retention and reaction of each hand sheet was determined by measuring the radioactivity of the sheets before and after THF extraction. The activity was determined by first oxidizing a piece of a handsheet in a Packard Sample Oxidizer Model 307 (Hewlett Packard, USA). The 14 CO 2 was absorbed in Carbosorb E (Packard BioScience Company, USA) and diluted with Permaflour E+ (Packard BioScience Company, USA) before the 14 C- activity was measured in a Tri-Carb Liquid Scintillation Analyzer Model 2100TR (Hewlett Packard, USA). Sizing test Water absorbency was determined by Cobb 60 (SCAN-P 12:64). Z-potential measurements of AKD-dispersion The Z-potential of the AKD dispersions was determined using a Z-potential apparatus (Rank Brothers, Bottisham, Cambridge, England). The Z-potential of the cationic dispersion was determined to mv. Results In order to obtain a series of pulps with different surface charged densities an ECF-bleached softwood kraft pulp Fig 1. AKD-retention vs. molar concentration of CaCl 2. Reference pulp: Unbeaten, fines free ECF-bleached softwood kraft pulp (surface charge: 1.8 µeq/g). The CMC-grated pulps were grafted to two different surface charge densities (8.7 and 12.2 µeq/g). Addition of AKD: 1 mg/g. Nordic Pulp and Paper Research Journal Vol 22 no. 2/

4 (surface charge = 1.8 µeq/g), was grafted with carboxymethylcellulose (CMC) to two different surface charge densities (surface charge = 8.7 and 12.2 µeq/g respectively). In a first series of sizing experiments using C-14 AKD (with the pulps in their Ca-form) the effects of different CaCl 2 concentrations on size retention was investigated. It should be noted that a sizing catalyst (10-3 M NaHCO 3 ) also was added to the stock). The results from these experiments are displayed in Fig 1. All three pulps have approximately the same AKDsize retention with no added CaCl 2. For the reference pulp, AKD-size retention decreases monotonously with an increasing electrolyte conc., whereas the AKD-retention goes through a sharp maximum, after which the AKD-retention decreases for the CMC-grafted pulps. This is exactly what was observed in the publication previously referred to (Johansson and Lindström 2004b), but with a TCF-bleached softwood kraft pulp. Obviously, the similar behaviour shown between the softwood and hardwood kraft pulps in these two investigations suggests that if pulps have a similar surface charge content, they behave in a similar fashion with respect to the effects of electrolytes on the AKD retention. It is also obvious, that the reference pulp is extremely sensitive towards electrolytes, so in many practical cases, the advantage of having a self-substantive size (cationic AKD) is non-existent. In order to further explore the nature of this sensitization process the terminology used for this phenomenon a series of experiments were carried out using different electrolytes. Thus, a surface grafted pulp with a surface charge density of 12.2 µeq/g was transferred to its Na, Ca and Mg-form and a series of retention experiments at different electrolyte concentrations (NaCl, CaCl 2, MgCl 2 ) were carried out. Again it should be noted that a sizing catalyst (10-3 M NaHCO 3 ) also was added to the stock. The results of these experiments are given in Fig 2. Again, the electrolyte sensitization effect is similar as in Fig 1 and it may be noted that this sensitization effect also occurs for NaCl, albeit the maximum is shifted to a higher electrolyte concentration. There is a slight, but significant difference, between MgCl 2 and CaCl 2. Ignoring this difference, it appears as if the critical parameter is the valency of the cation. A comparison between the monovalent and the divalent cation, immediately suggests that the cause of the sensitization effect should be sought mainly in terms of electrostatic charge interactions, to be discussed below. In order to illuminate the combination of the effects of surface charge and electrolyte concentration a series of experiments were carried out with surface grafted pulps over a wider region of surface charge densities and using different electrolyte concentrations of NaCl. The results of these experiments are displayed in Figs 3 and 4. For pedagogical reasons the data have been plotted in two ways. In Fig 3 the AKD-retention has been plotted vs the surface charge of the fibres, with the molar NaCl concentration as a parameter and in Fig 4, the AKD-retention has been plotted vs. the molar NaCl concentration. In order to get less cluttered graphs, some curves connecting the data points have been deleted for an easier visualisation of the results. Fig 2. Effect of different electrolytes on AKD-retention. Surface grafted unbeaten, fines free, ECF-bleached softwood kraft pulp (surface charge: 12.2 µeq/g). Addition of AKD: 1 mg/g. Fig 3. AKD-retention vs. surface charge of CMC grafted, unbeaten, fines free, ECF-bleached softwood kraft pulps at different molar concentrations of NaCl. Addition of AKD: 1 mg/g. Fig 4. Effects of molar concentrations of NaCl on AKD-retention for CMC grafted, unbeaten, fines free, ECF-bleached softwood kraft pulps. Addition of AKD: 1 mg/g. The following conclusions can be drawn from these graphs. Firstly, the AKD-retention increases sharply with surface charge and there is an optimum surface charge for maximum AKD-retention. Secondly, there is on optimum electrolyte concentration for optimum AKDretention. The optimum surface charge decreases from around 15 µeq/g at low electrolyte concentrations to about 5 µeq/g at higher electrolyte concentration. An overall conclusion is that AKD-retention is solely governed by electrostatic interactions as the AKD- 164 Nordic Pulp and Paper Research Journal Vol 22 no. 2/2007

5 As the range of surface charge densities explored in this latter experiment covers the whole range of surface charge densities in commercial pulps known to us, we can conclude that the surface charge has no impact on required amount of reacted AKD for sizing to a particular sizing degree. The reason why deposition mode of the size has no effect on sizing efficiency lies in the fact that the size will spread all over the open surfaces in the sheet structure as was discussed in detail in a previous communication (Johansson and Lindström 2004b). Therefore this specific aspect of these results will not be further discussed. Fig 5. Cobb 60 vs. reacted amount of AKD. : Reference: unbeaten, fines free, ECF-bleached softwood kraft pulp with addition of 0.1%C-PAM. : CMC-grafted unbeaten, fines free, ECF-bleached softwood kraft pulp with addition of 0.1%C-PAM. : CMC-grafted unbeaten, fines free, ECF-bleached softwood kraft pulp with addition of 0.01 M CaCl 2 : CMC-grafted unbeaten, fines free, ECF-bleached softwood kraft pulp with combined additions of 0.01 M CaCl 2 and 0.1% C-PAM. Addition of AKD between mg/g. Surface charge of CMC-grafted pulp = 14.5 µeq/g. retention approaches zero, when the electrolyte conc. is sufficiently high to have swamped all electrostatic interactions. In this lab, it has been found that sizing efficiency is only governed by the amount of reacted AKD, irrespective of curing conditions, amount of reaction catalyst added, choice of retention aid, mode of AKD deposition etc. It was therefore of interest to determine, whether the amount of surface carboxyl groups would influence the sizing curve. It is possible, for instance to hypothesize that the presence of carboxyl groups could be beneficial for AKD-size retention but detrimental to sizing as the surface energy of cellulose is expected to increase when the number of carboxyl groups is greatly increased. Hence, four series of sizing curves were run in order to determine whether surface charge or deposition mode would affect sizing development. A reference pulp (1.8 µeq) and a surface grafted pulp with a carboxyl group content of 14.5 µeq were chosen for this purpose. Four different series of experiments were run. All experiments were run in deionized water to which a sizing catalyst had been added (10-3 M NaHCO 3 ). In one reference series, the reference pulp (Na-form) was sized, using various amounts of AKD (0.1-2 mg/g). 0.1% cationic polyacrylamide (C-PAM) was added as a retention aid. In a second series of experiments, the grafted pulp was used and various amounts of AKD was added and 0.1% C-PAM was used as a retention aid. In a third series of experiments, the grafted pulp was used and the AKD was retained by an addition of 0.01 M CaCl 2. Finally, in a fourth series of experiments, the AKD was retained by a combination of 0.01 M CaCl 2 and 0.1% C-PAM. The results from these series of experiments are displayed in Fig 5. As in all our previous communications the Cobb 60 - values fall on one single line, with a threshold amount for sizing around 0.01 % reacted AKD in the sheet. Discussion There are two major findings in this communication, which need to be elaborated onto in some further detail. One finding was that there was an optimum charge density for maximum AKD-retention. A second major finding was that there is an optimum ionic concentration for optimum AKD-retention for a number of electrolytes (Na, Mg, and Ca-salts). For a surface with a low charge density of carboxylic groups, the deposition of a cationic particle is driven by the increased entropy, associated with the release of counter-ions from the surface, when a cationic particle is deposited onto an anionic surface. When the surface charge of the anionic surface is increased, the surface structure will swell and becomes more solubilized. This surface-swelling effect of grafted cellulosic surfaces has also been verified and discussed in previous communications from this laboratory (e.g. Laine et al 2002). Hence, when the surface becomes increasingly charged, the charged groups will protrude out from the surface into the solution, creating the swollen surface layer of the pulp. This is a general aspect, also expected to occur for commercial pulps with different charge densities, although such pulps have not been charged through a surface grafting procedure. As the results showed, a more highly charged surface will exhibit an electrosterically related restriction to the deposition of cationic particles. This may be explained by the following arguments. If a cationic particle is deposited onto such a particle surface, the deposition will be hampered by a loss of conformational entropy of the protruding chain ends during adsorption. Hence, the increased entropy loss associated with the release of counter-ions will be balanced by loss of conformational entropy of the protruding chain ends during deposition of the cationic particle. This balance between the two entropy terms will basically regulate the extent of particle deposition and the maximum in AKD-particle deposition may be explained in this way. The fact that there is an optimum electrolyte concentration for an optimum retention of cationic AKD-particles also needs an explanation. The AKD-dispersion is net cationic (Z-potential = 11.5 mv), but is dispersed using a mixture of lignosulphonic acid (LISA) and cationic starch (Cstarch). Hence, the surface is actually covered by a polyelectrolyte complex of an anionic and a cationic polyelectrolyte. Nordic Pulp and Paper Research Journal Vol 22 no. 2/

6 Fig 6. Schematic illustration of interactions on an AKD-particle surface stabilized by a polyelectrolyte complex (e.g. cationic starch and lignosulphonate) at different electrolyte concentrations. The charge interaction between the anionic and cationic sites on the surface of the AKD-particles is thus regulated by the electrolyte concentration and the type of electrolyte present in the system and is illustrated in Fig 6. Under electrolyte free conditions, the cationic sites on the surface of an AKD-particle are therefore blocked by the interaction with the interaction with the LISA. Hence, the AKD-particle will have a low interaction with anionic fibre surfaces as the cationic sites are basically blocked. If the electrolyte concentration is increased, the mutual attraction between LISA and C-starch will be decreased, enabling the surface layer of the AKD-particle to swell and interact stronger with an external anionic surface. The interaction is therefore said to be sensitized by the presence of an electrolyte. From these considerations it is also expected that a divalent ion (Mg, Ca) will have significantly higher interaction with the surface complex, enabling a lower electrolyte concentration for sensitization of the retention process, as demonstrated by the experiments displayed in Figs 1 and 2. If the electrolyte concentration is further increased, the electrostatic interactions are swamped out and the interaction between the cationic AKD-particles and the anionic fibre surface will be decreased, leading to a decreased AKD -particle retention. These combined effects may also explain why the optimum charge density for maximum AKD-retention decreases with an increasing electrolyte concentration in the system. At low electrolyte concentrations, there are strong interactions within the surface complex at the AKD-particle surface. The cationic sites can be said to be shielded from the anionic fibre surfaces through this strong interaction. It is only when the fibre surface has a sufficiently high surface charge that there will be a significant interaction between the AKD-particle and the anionic fibre surface. When the electrolyte concentration is increased, the polyelectrolyte surface complex on the cationic AKDparticles will swell, enabling a better interaction with external anionic groups (on the surface of fibres). Hence, there will be an increased interaction with anionic surfaces and less anionic groups on the surface of fibres is required for interaction, enabling better deposition at a lower surface charge density compared to conditions at a lower electrolyte concentrations. This explains the shift to lower optimum charge densities when the electrolyte concentration is increased. In conclusion, the presented results for the self-retention of cationic AKD-particles, when the combined effects of surface charge density and electrolyte concentration are considered, may be explained by quite simple electrostatic charge considerations. It is acknowledged that macroscopic data do not prove mechanisms on the molecular level and, hence, the mechanism suggested should be conceived as a hypothesis. Acknowledgments 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. Literature Champ, S. and Ettl, R. (2004): The dynamics of alkyl ketene dimer (AKD) retention, J. Pulp Paper Sci. 30(12), 322. Isogai, A., Kitaoka, C. and Onabe, F. (1997): Effects of carboxyl groups in pulp on retention of alkyl ketene dimer, J. Pulp Paper Sci. 23(5), 215. Johansson, J. and Lindström, T. (2004a): A study on AKD-size retention, reaction and sizing efficiency. Part 1: The effects of pulp bleaching on AKD-sizing, Nord. Pulp Paper Res. J. 19 (3), Johansson, J. and Lindström, T. (2004b): A study on AKD-size retention, reaction and sizing efficiency. Part 2: The effects of electrolytes, retention aids, shear forces and mode of addition on AKD-sizing using anionic and cationic AKD-dispersions, Nord. Pulp Paper Res. J. 19 (3), Katz, S., Beatson, R.P. and Scallan, A.M. (1984): The determination of strong and weak acidic groups in sulfite pulps, Svensk Papperstidning, 87, R48. Laine, J., Lindström, T., Glad-Nordmark, G. and Risinger, G. (2000): Studies on topochemical modification of cellulose fibres. Part 1 Chemical conditions for the attachment of carboxymethyl cellulose onto fibres, Nord. Pulp Paper Res. J. 15:5 (2000) 520. Laine, J., Lindström, T., Glad-Nordmark, G. and Risinger, G. (2002): Studies on topochemical modification of cellulosic fibres. Part 2 The effect of carboxymethyl cellulose attachment on fibre swelling and paper strength, Nord. Pulp Paper Res. J. 17:1, Lindström, T. and Söderberg, G. (1986a): On the mechanism of sizing with alkyl ketene dimers. Part 1. Study on the amount of alkyl ketene dimer required for sizing different pulps, Nord. Pulp Paper Res. J. 1(1), 26. Lindström, T. and O Brien, H. (1986b): On the mechanism of sizing with alkyl ketene dimers Part 2. The kinetics of reaction between alkyl ketene dimers and cellulose, Nord. Pulp Paper Res. J. 1(1), 34. Lindström, T. and Söderberg, G. (1986c): On the mechanism of sizing with alkyl ketene dimers. Part 3. The role of ph, electrolytes, retention aids, extractives, Calignosulfates and mode of addition on alkyl ketene dimer retention, Nord. Pulp Paper Res. J. 1(2), 31. Lindström, T. and Söderberg, G. (1986d): On the mechanism of sizing with alkyl ketene dimers Part IV. The effects of HCO 3 -ions and polymeric reaction accelerators on the rate of reaction between alkyl ketene dimers and cellulose, Nord. Pulp Paper Res. J. 1(2) (1986d), 39. Wågberg, L., Winter, L. and Lindström, T. (1985): Determination of ionexchange capacity of carboxymethylated cellulose fibers using colloid and conductometric titrations, In: Punton, V. (ed.), Papermaking Raw Materials, Mechanical Engineering Publ. Ltd., London, Vol. 2, p 917. Manuscript received October 2, 2006 Accepted January 22, Nordic Pulp and Paper Research Journal Vol 22 no. 2/2007