Flocculation of fillers with polyelectrolyte complexes

Transcription

1 Flocculation of fillers with polyelectrolyte complexes Markus H.J. Korhonen, Susanna Holappa, Per Stenius and Janne Laine KEYWORDS: Polyelectrolyte Complexes, Flocculation, Molar mass, Charge density, Filler SUMMARY: The objective of this study was a better understanding of the effect of charge density and of the molecular weight of polyelectrolyte complex components on the flocculation of kaolin and precipitated calcium carbonate (PCC) fillers. The results show that the components of the complexes had a major impact on flocculation. In order to achieve an effective flocculation one of the components of the polyelectrolyte complex (PEC) should be a high molecular weight polyelectrolyte so that the PEC could effectively form polymer bridges between filler particles. More specifically, the component in excess of the complex should have a higher molar mass than the other component. When the polyelectrolyte with lower molecular weight is in excess in the PEC, the flocculation efficiency of the PEC decreases significantly. The charge ratio of the PECs has a great effect on the flocculation. With charge ratios other than 1, maximum flocculation was achieved with low dosages, but with complexes near 1:1 ratio the dosing range is broader, and higher maximum flocculation can be achieved. A premixed complex where one component has a high charge density is not beneficial for flocculation. This study showed that a one step addition of PEC could be an effective and robust way for filler flocculation. ADDRESS OF THE AUTHORS: markus.korhonen@aalto.fi susanna.holappa@kemira.fi per.stenius@aalto.fi janne.laine@aalto.fi Department of Forest Products Technology, Aalto University, FI Aalto, Finland Corresponding author: Markus Korhonen The papermaking furnish includes several different raw materials such as fibers, fines, fillers and various additives. Recently, one of the greatest interests in terms of the reduction of raw material cost has been to increase the filler content of paper. Fillers, such as kaolin and PCC, are of colloidal size, meaning that they are typically retained only in low percentages in the wire section of paper machine if no chemicals are used to flocculate them. In order to control retention of fillers in the wet-end various different chemical systems, both single and multicomponent, have been developed and studied (Wågberg, Lindström 1987, Järnström et al. 1995, Roberts 1996, Petzold et al. 1996, 1998, 2003a, 2003b, Shin et al. 1997, Nyström et al. 2003a, 2003b, Nyström, Rosenholm 2004, Zhou 2006). It is well known that under hydrodynamic conditions (high-speed machines), a large expansion of the polymer chain is needed to achieve high filler retention. Therefore, many retention aid systems today are mainly based on the bridging flocculation mechanism. In the traditional twocomponent polymeric bridging flocculation system (e.g., PEI + A-PAM), the polyelectrolytes are added in sequence, i.e. polymers are added one after another to the system. This kind of polyelectrolyte bilayer structure has been reported to have advantages such as the improvement of paper strength compared to that of a single polyelectrolyte system (Gernandt et al. 2003, Gärdlund et al. 2003, 2007, Vainio et al. 2006, Cho et al. 2006). These type of systems have also been shown to improve dewatering and flocculation (Somasundaran, Yu 1993, Petzold, Lunkwitz 1995, Petzold et al. 1996, 2003b, Somasundaran, et al. 2000). Another dual component system that has gained great interest in recent years is polyelectrolyte complexes (PEC). They are formed by mixing two oppositely charged polyelectrolytes that associate due to the entropy gain resulting from the release of a large number of counter-ions into solution. However, only a few flocculation studies have been made with pre-mixed complexes (PECs), i.e. complexes formed by mixing the two polymers before adding them to the suspension (Petzold et al. 1996, 1998, Nyström et al. 2003a, 2003b, 2004, 2005). Flocculation studies of PEC have shown that the range of the optimum flocculation concentration is wider, and that flocculation can be enhanced compared to that of single polymer system. The properties of the PECs depend on the polyelectrolytes used and their mixing ratio. As a consequence, the efficiency of the PECs may alter significantly, especially near 1:1 stoichiometric ratio (Lofton et al. 2005; Hubbe, 2005). The main focus of this study was to understand the effect of the properties of the components of a PEC on flocculation. Particular attention was paid to the charge densities and to the molecular weights of the polyelectrolytes and their mixing ratios in the complexes. In order to obtain unequivocal knowledge of the mechanisms involved in flocculation it is necessary to use simplified systems. In this study kaolin and precipitated calcium carbonate (PCC), two very common fillers, were chosen to represent mineral particles. The experiments were made at ph 5 (with kaolin) and at ph 8.5 (with PCC) to model usage of polyelectrolyte complexes under both acidic and alkaline papermaking conditions. The experiments were made with the Turbiscan Online optical analyzer (Bordes et al. 2002), which enables monitoring of flocculation of filler suspensions under different shear conditions and polyelectrolyte systems. The results show that the components of the complexes had a major impact on the flocculation. It was found out that in order to achieve an effective flocculation one of the components of the polyelectrolyte complex (PEC) should be a high 239 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013

2 molecular weight polyelectrolyte, so that the PEC could effectively form polymer bridges between filler particles. Materials and methods Fillers Two different fillers, kaolin and PCC, were used in the experiments. The kaolin was Intrafill C obtained from Imerys International, France. Dried precipitated calcium carbonate (PCC) was provided by Specialty Minerals Inc., USA. Both fillers were dispersed to a dry solids content of w-% in distilled water without dispersing agents. Mean particle size was ca. 5 m and ca. 2 m for kaolin and, PCC respectively (given by the manufacturers). For the experiments, filler suspensions were diluted to a concentration of 0.1 w-% in a saline solution of 1 mmol/l NaHCO mmol/l NaCl. The ph of the suspensions was adjusted with 0.1 M NaOH to 8.5 for PCC and with 0.1 M HCl to 5.0 for kaolin. Under these conditions the surface charge of PCC was cationic (z-potential 15 mv) and the surface charge of kaolin was anionic (-43 mv). Polyelectrolytes Cationic and anionic polyacrylamides were obtained from Kemira Ltd, Finland. Anionic polyacrylamides (A-PAM) were synthesized by copolymerization of acrylamide and acrylic acid. Cationic polyacrylamides (C-PAM) were copolymers of acrylamide and dimethylaminoethyl acrylate quarternized with methyl chloride (Fig. 1). The Three A-PAMs and three C-PAMs with different molar masses and charge densities were used in the experiments. Molecular weights of polyelectrolytes were used as given by the producer and charge densities were determined by using polyelectrolyte titration (Koljonen et al. 2004). The properties of the polyacrylamides are shown in Table 1. Other reagents Electrolytes (NaCl and NaHCO 3 ) were analytical grade. Deionized and distilled water was used. Preparation of the polyelectrolyte complexes Six polyelectrolyte complex systems were examined (Table 2). The effect of the molecular weight, the charge, and the mixing ratio of the components on flocculation was measured. However, the main focus was to study PECs formed from low charge polyelectrolytes and only one of the studied polyelectrolytes had a high charge density (A-LWHC). Anionic and cationic polyacrylamides were mixed together at the following charge ratios: (A-PAM:C-PAM) 3:1, 2:1, 1:1, 1:2, 1:4, 1:7 and 1:10. Before complex formation the A-PAM and C-PAM stock solutions (1g/l, prepared one day before experiments) were diluted to a concentration of 0.25 g/l and the salt concentration was adjusted to 1 mmol/l NaHCO 3 and 9 mmol/l NaCl (ph with buffer was 8.5). PECs were then prepared by mixing these solutions together by using a magnetic stirrer (500 rpm, 1 hour). Hence, the final concentrations of PECs were 0.25 g/l. Particle size The particle size distribution of the complexes was determined by photon correlation spectroscopy using the Coulter N5 particle size analyzer (Coulter Electronics Inc, New England, USA). The N5 is equipped with a nm helium-neon laser and the system can analyze particles from six different angles. The measurement range is from m to 3 m. Electrophoretic mobility The charge characteristics of the complexes were measured as electrophoretic mobility with a Coulter Delsa 440 Doppler electrophoretic light scattering analyzer (Coulter Electronics Inc, New England, USA). The analyzer is equipped with a helium-neon laser and it measures the mobility distribution of particles whose size lies between 0.01 and 30 m. The system analyzes particles by making independent laser-doppler measurements at four different angles (7.5º, 15º, 22.5º and 30º) simultaneously. Fig. 1 Schematic picture of C-PAM molecular structure Table 1. The properties of the polyacrylamides used in the experiments. Molecular weights are given by the producer. Sample code Properties Mw, 10 6 g/mol Charge density, meq/g A-HW Anionic, high Mw, low charge density A-MW Anionic, medium Mw, low charge density A-LWHC Anionic, low Mw, high charge density C-HW Cationic, high Mw, low charge density C-MW Cationic, medium Mw, low charge density C-LW Cationic, low Mw, low charge density Nordic Pulp & Paper Research Journal Vol 28 no. 2/

3 Table 2. The compositions of the polyelectrolyte complex systems. Polyelectrolyte A-HW: Anionic, high Mw, A-MW: Anionic, medium A-LWHC: Anionic, low Mw, low charge density Mw, low charge density high charge density C-HW: Cationic, high Mw, low charge density x x x C-MW: Cationic, medium Mw, low charge density x x C-LW: Cationic, low Mw, low charge density x Fig 2. Schematic picture of the circulation system used in the experiments. Electrophoretic mobility The charge characteristics of the complexes were measured as electrophoretic mobility with a Coulter Delsa 440 Doppler electrophoretic light scattering analyzer (Coulter Electronics Inc, New England, USA). The analyzer is equipped with a helium-neon laser and it measures the mobility distribution of particles whose size lies between 0.01 and 30 m. The system analyzes particles by making independent laser-doppler measurements at four different angles (7.5º, 15º, 22.5º and 30º) simultaneously. Flocculation tests The flocculation of fillers was studied with a Turbiscan online optical analyzer (Formulaction, France). In this device a near-infrared focused LED (wavelength 850 nm) illuminates the scattering medium flowing through a cylindrical glass tube. Both the transmitted (detection angle 0º) and the backscattered (detection angle 135º) light flux were measured with the device. The volume fraction is obtained by using the light scattering theory. However, the calculated particle diameter values are not exact values and they should be considered as estimates. The details of the use and the measurement of this device are presented elsewhere (Bordes et al. 2002). In order to measure flocculation as a function of time the Turbiscan Online was built into a circulation system (Fig 2). Filler slurry (200 ml) was continuously pumped from the sample vessel equipped with the magnetic stirrer (mixing rate 500 rpm) through the measuring cell with the flow rate of ca. 10 ml/s. Diameter of tubes was 3 mm. The transmission and the backscattering values were measured every 0.5 s. After a polyelectrolyte or a PEC addition to the sample slurry, a mixing of the sample was continued with mixing rate of 500 rpm for a chosen period of time. The concentration of the slurry was 0.1% and the salt concentration was 1 mmol/l NaHCO 3 and 9 mmol/l NaCl in every test. For the particle size calculations the refractive index of 1.56 was used for both kaolin and PCC. Results and discussion Characterization of pre-mixed polyelectrolyte complexes Mixing of the polyelectrolytes resulted in clear or turbid solutions, or precipitation of the polymer complexes as is typical for PECs (25, 26). In non-stoichiometric ratios (charge ratios differing strongly from 1:1) clear solutions were formed. Near the isoelectric point (IEP), solutions became more turbid and at the stoichiometric ratio some precipitation of the complex was observed in some cases (for this reason the 1:1 ratio was not used in flocculation experiments). In addition to visual observation of the PEC mixtures, they were characterized with regard to size and electrophoretic mobility (z-potential). The charge of the colloidal particles formed upon mixing of the oppositely charged polyelectrolytes depended primarily on the mixing charge ratio. As shown by the results presented in Fig 3a and 3b, the net charge of all complexes changed from negative to positive when the polyelectrolyte in excess changed from A-PAM to C-PAM. For all PECs formed by low charged polyelectrolytes the IEP was close to the stoichiometric charge ratio. For the complex formed by the high charge density polyanion A-LWHC and the low charge density polycation C-HW (Fig 3b) the IEP clearly deviated from the stoichiometric ratio, which is typical for this kind of polyelectrolyte combination (Thünemann et al 2004, Koetz et al. 1996, Xiao et al. 2009). The biggest particles were formed when the charge ratio of the oppositely charged polyelectrolytes was close to the IEP (Fig 4a and b). At this point the electrostatic repulsion between PEC particles was weak but apparently the water-soluble co-monomers were able to prevent 241 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013

4 PECs from precipitating. The molecular weight appeared to be the dominant contributor to the size of the PECs formed by low charge density polyelectrolytes. The higher the Mw of the components, the larger were the complexes. The higher molecular weight polyelectrolytes have larger dimensions and they are also able to complex a higher number of oppositely charged polyelectrolyte (Xiao et al. 2009). The combination of the highly charged polyanion A- LWHC with the low charged high Mw polycation C-HW behaved differently (Fig 4b). The particles were very large at low A-PAM additions but were, on the contrary, very small at the stoichiometric mixing ratio or at anionic excess. Clearly, on association with a stoichiometric amount or an excess of small, highly charged polyanions, the properties of the polyanion dominate. There is probably some free polyanion in the solution when A- LWHC was added in excess. On the other hand, the formation of very large particles when low amounts of A- LWHC are added was probably due to the formation of bridges between the high molecular weight C-HW molecules (Mende et al. 2002). Flocculation of kaolin and PCC Flocculation experiments were started with single polyelectrolyte systems in order to find the relevant conditions for filler flocculation. In these experiments only high Mw bridging flocculants were used. The shear conditions were kept constant (500 rpm), and they were chosen in a way so that the breakage of the flocs was as low as possible. However, some particle surface erosion, polyelectrolyte transfer and cleavage and reflocculation of particles may occur. The extent of their effect on flocculation is difficult to estimate precisely in our set-up. The capability of PEC to withstand shear forces, and their reflocculation ability is under investigation in our laboratory and will be reported in forthcoming papers. Since PCC has a cationic surface charge, anionic polyelectrolytes adsorb on its surface and cause flocculation of PCC particles. Fig 5 shows how PCC particles flocculate when anionic high Mw A-PAM (A- HW) is added to the PCC suspension. The flocculation of PCC can be seen as an increase in the transmission level of the PCC sample shortly after an addition of the anionic A-HW (at 10 s). This increase is due to the flocculation of PCC particles into bigger aggregates which allows more light from Turbiscan s light source to pass through the sample to the detector. The net surface charge of kaolin is anionic, meaning that cationic polyelectrolytes can be used to flocculate kaolin particles (Fig 6). Shortly after the addition of cationic flocculant (C-HW), the transmission level of the kaolin sample increased rapidly. As can be seen from Fig 5 and 6 there were no signs of floc breakup since the floc size (transmission level) increased or levelled out as the experiment continued. Fig 3. The mobilities of the polyelectrolyte complexes as a function of the charge ratio (anionic/cationic) of the components added to the solution (logarithmic scale). In a) anionic component is the same (A-HW) and molecular weight of cationic component varies. In b) side are other combinations of PECs with different properties of components (molecular weight, charge density). Fig 4. The particle diameters of the polyelectrolyte complexes as a function of the charge ratio (anionic/cationic) of the components (logarithmic scale). In a) anionic component is the same (A-HW) and molecular weight of cationic component varies. In b) are other combinations of PECs with different properties of components (molecular weight, charge density). Nordic Pulp & Paper Research Journal Vol 28 no. 2/

5 Fig 5. Effect of contact time and A-HW polyelectrolyte dosage (% of dry weight of PCC) on flocculation of 0.1% PCC suspension. A-HW was added at 10s. The flocculation phenomenon was different for kaolin and PCC. With PCC the highest flocculation was already achieved at a dosing level of 0.025% of A-HW, whereas with kaolin, the highest flocculation was obtained with the addition of 0.5% of C-HW. This is probably a consequence of the differing surface charge density of kaolin and PCC (the properties of A-HW and C-HW were similar, Table 1). Kaolin has a higher surface charge than PCC and more polyelectrolytes can adsorb on its surface. Therefore, the maximum flocculation occurs at a higher polymer dosage for kaolin. For PCC surface saturation is achieved at a 0.025% addition before steric hindrance (steric stabilization) starts to interfere with flocculation. The decrease in flocculation efficiency at high polymer dosages, Fig 5, is a well-known effect for bridging flocculants (Swerin et al. 1996). Effect of polyelectrolyte complexes on flocculation Different PEC systems were investigated in order to determine the effect of molecular weight and charge density of polyelectrolytes on the properties of PECs and finally on flocculation. Due to the surface charge differences, the main focus of the experiments was on anionic complexes for PCC and on cationic complexes for kaolin. Six different addition levels were used for different PECs: 0.01%, 0.025%, 0.05%, 0.1%, 0.2% and 0.5%. The same addition levels were used with both kaolin and PCC. One component systems of A-HW and C-HW were used as a reference for PCC and kaolin, respectively. Measurements of floc sizes were taken after 60 s of PEC addition for both PCC and kaolin. Similarly to single polyelectrolyte systems (Fig 5 and 6), higher amounts of PECs were needed to reach optimum flocculation level with kaolin than with PCC, probably due to its high surface charge. Different combinations of PECs resulted in clear trends. Due to the extensive amount of data, examples of major observations are summarized in Fig. 7 to 11. Fig 7a and 7b show the flocculation (presented as particle size) of PCC and kaolin, respectively, as a Fig 6. Effect of contact time and C-HW polyelectrolyte dosage (% of dry weight of kaolin) on flocculation of 0.1% kaolin suspension. C-HW was added at 10s. function of the polyelectrolyte addition level at 60s for the following systems: the single polyelectrolytes A-HW and C-HW, the PEC (A-HW+ C-HW) at charge ratios of 3:1 (anionic), 2:1 (anionic), 1:2 (cationic) and 1:4 (cationic). Even though the cationic polyelectrolyte alone (C-HW) does not flocculate PCC particles, they can be flocculated using cationic complexes (charge ratio 1:2 and 1:4). Apparently, at low charge ratios the cationic complex contains free anionic domains (owing to the loose structure of the complex), which are able to adsorb on the PCC particles. Indeed, it has been found in adsorption studies of PEC that a complex with a given net charge adsorbs on a surface with same sign (Saarinen et al. 2008); however, a high dosage of this complex is required. With anionic A-HW + C-HW complexes (3:1 and 2:1) the flocculation of PCC was greatly enhanced in comparison to the one component system. The maximum floc size was about 18 m whereas with the single polyelectrolyte system (A-HW) the corresponding value was about 11 m. The maximum floc size was obtained when 0.2% PEC was added. The floc size was already significantly larger at the addition level of 0.05% over the single polyelectrolyte system. As has been reported earlier (Petzold et al. 1996, 1998, Nyström et al. 2003a, 2003b, 2004), the dosage window with polyelectrolyte complex systems is significantly wider compared to single polymer systems. This is explained by the zwitterionic character of PECs. Despite the surface saturation of PCC particles, PECs are able to bridge particles by interacting electrostatically with other complexes on adjacent particles (Nyström et al. 2005). Other reason for the efficient flocculation of PEC is probably the large size of the complex. Petzold, Lunkwitz et al. (1995) and Nyström et al. (2005) suggest that complexes form large polymer networks between particles. It has also been shown earlier by Salmi et al. (2007) that PECs form long-range interactions between surfaces, which supports this conclusion. Our results are consistent with this interpretation. Although oppositely 243 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013

6 charged polyelectrolytes neutralize each other in the premixing, the complex formed is significantly larger than a single component alone. Therefore PEC forms larger complexes, which increase the probability of interaction between opposite charges of complexes at adjacent particles, thus increasing flocculation. The charge ratio of PECs greatly influences flocculation. Nyström et al. (2003b) have suggested that, along with bridging, charge neutralization also plays a key role in flocculation. When the complex is more anionic (3:1), it gave rise to stronger flocculation than the PEC with the lower charge ratio (2:1) at lowest dosage (0.01%). However, when more than 0.05% of PEC was added, the flocculation of the high charge complex started to decrease slowly, probably due to charge reversal and stabilization of the PCC particles. With the lower charge ratio (2:1) optimum flocculation was shifted to a higher dosage level and the flocculation also remained higher in a wider concentration range. In addition, a higher maximum floc size could be achieved with the lower charge ratio. This is probably because it is possible to add higher amounts of PECs with lower charge ratio to the system before the restabilization of PCC particles begins to hinder flocculation. It seems that the theory for the optimum polymer dosage shown for bridging flocculants (Swerin et al. 1997) is also valid for PECs. With kaolin, probably due to its higher surface charge, higher amounts of polyelectrolytes were needed to achieve maximum flocculation (Fig 7b). In contrast to PCC one could flocculate kaolin with both anionic and cationic single component polyelectrolytes. This can be assumed to be due to the amphoteric nature of kaolin. In one component systems, the maximum flocculation was achieved with the addition level of 0.5% and 0.1% for C- HW and A-HW, respectively, and the maximum efficiency was higher for the cationic flocculant (C-HW). Surprisingly, the highest flocculation efficiency of kaolin was achieved with anionic PECs (ratio 2:1). With anionic PECs, flocculation was significantly higher compared to the single component anionic polyelectrolyte system (A-HW) and cationic PECs. The particle size increased as high as 50 m with an addition level of 0.5% (anionic PEC with charge ratio of 2:1 and 3:1) compared to the maximum particle size of 15 m with A-HW. An explanation for this can not be given at this point. With cationic PECs the difference was smaller when compare to the single polymer system. With 1:4 cationic PEC, flocculation was at the same level as single C-HW at low dosages but more efficient flocculation was obtained at an addition level of 0.5%. With 1:2 PEC, which is less cationic than 1:4 PEC, flocculation was clearly smaller than with C-HW at lower dosages. However, at 0.5% it was already at the same level, and the rising trend of flocculation indicates that one can achieve higher flocculation by adding more than 0.5% of this PEC. PAPER CHEMISTRY Effect of charge and molar mass of PEC components on flocculation A polycation-polyanion system where one component is a high charge density polyelectrolyte and the other component is a high molecular weight polyelectrolyte is a very typical example of a system giving rise to a combination of patching and bridging mechanisms. These types of PEC systems have been shown to improve flocculation compared to single polyelectrolytes (Somasundaran, Yu 1993, Petzold et al. 1995, 1996, 2003b, Somasundaran, et al. 2000, Nyström et al. 2005). However, in this study it was found that flocculation of kaolin with the pre-mixed PEC containing the high charge density anionic component and the high molecular weight cationic flocculant (A-LWHC + C-HW) was significantly lower than with the other PEC systems containing the same polycation. Fig 8 shows that the kaolin flocculation is clearly lower for the cationic PEC containing the high charge polyanion compared to the single C-HW polyelectrolyte or the other polyelectrolyte complex (A-MW+C-HW)with the same charge ratio (anionic:cationic 1:4). Although the size of the A-LWHC+C-HW complex was very large when the cationic charge was in excess in the complex (see Fig 4b.), poor flocculation was still observed. Hence, it is not only the absolute size of the complex that determines its flocculation efficiency. The structure of the PEC of this type is probably locally very compact and therefore there are only few available loops and tails for bridging flocculation (Chen et al. 2003, Nyström et al. 2003a, 2005). To achieve high flocculation efficiency with this PEC, higher concentrations than with PECs formed from lower charge density polyelectrolytes is needed (Nyström et al. 2005). There was also a strong influence of the Mw of the components on the flocculation. Fig 9 presents the flocculation of PCC with three different complexes. The anionic polyelectrolyte was same in all of the complexes (A-HW), while the Mw of the cationic component changed from low to high (C-LW, C-MW and C-HW). It was shown in Fig 7a that cationic C-PAM (C-HW) does not flocculate cationic PCC particles alone. By varying the properties of the polycation and using the same polyanion, the changes in PCC flocculation have to be related to the properties of the complexes rather that the characteristics of the individual components alone. Because the surface charge of the PCC is cationic, anionic PECs were chosen. The charge ratio of the complexes was constant (anionic:cationic 2:1). Flocculation was enhanced by increasing the molar mass of the cationic component, which concomitantly increases the size of the complex particles (Fig 4). It is likely that the improved flocculation was a consequence of the bigger size of the PEC; this would provide a larger network structure between PCC particles, which therefore are more likely to interact with each other (Salmi et al. 2007). Nordic Pulp & Paper Research Journal Vol 28 no. 2/

7 Fig 7. Flocculation of (a) PCC and (b) kaolin with the A-HW+C-HW complexes with different charge ratios (anionic/cationic) after 60 s as a function of the dosage (% of dry weight of PCC or kaolin). Note that the scale is different in Fig 7a and b. Fig 8. Flocculation of kaolin with two PECs with the charge ratio of 1:4 (anionic:cationic) after 60 s as a function of the dosage (% of dry weight of kaolin). Polycation is the same (C-HW) in all of the PECs and anionic component varies. Fig 10. Flocculation of kaolin with different complexes with the charge ratio of 1:7 after 60 s as a function of dosage (% of dry weight of kaolin). The effect of Mw of polyelectrolytes in PEC was also studied with kaolin (Fig 10). In the two upper curves the Mw of the cationic component is same (C-HW), while the Mws of the anionic components changes (A-HW and A-MW). As with PCC (Fig 9), the larger complex size provides somewhat improved flocculation for kaolin. However, flocculation was still quite strong with both of Fig 9. Flocculation of PCC with different complexes with charge ratio of 2:1 after 60 s as a function of the dosage (% of dry weight of PCC). The anionic component is same in all of the complexes (A-HW) while the molar mass of the cationic component varies. the complexes. On the other hand, when both components were changed to middle molar mass polyelectrolytes, A- MW and C-MW, the flocculation was substantially lower than with the other PECs. This indicates that for efficient flocculation at least one of the PEC components should be a high molecular weight polyelectrolyte. If not, there might be too few loops and tails in the PEC to achieve strong bridging between particles and therefore flocculation will be weak. In Fig 9 and 1 the effect of the Mw of the complex components is presented. In these cases the Mw of the polyelectrolyte that was in excess in PEC, was either higher or about the same as the Mw of the other polyelectrolyte. It seems to be that the component that is in excess in the PEC plays a significant role in flocculation. Hence, flocculation efficiency of the PECs is substantially affected by the Mw of the excess poly-electrolyte, see Fig 11a and b. Due to the amphoteric nature of kaolin it can be flocculated efficiently with both anionic and cationic polyelectrolytes. When the anionic charge was in excess in the PEC (Fig 11a), flocculation was substantially higher when the high Mw polyanion (A-HW) was the dominant component compared to the medium Mw polyanion (A-MW). 245 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013

8 Fig 11. Effect of the dominant component of complex on the flocculation of kaolin with (a) the charge ratio of 2:1 and (b) the charge ratio of 1:10 after 60 s a function of the dosage (% of dry weight of kaolin). Similarly, when the cationic charge was in excess, the flocculation was substantially higher when the high Mw polycation (C-HW) was the dominant component instead of the medium Mw polycation (C-MW) Fig 11b). This can not be explained by the size of the PECs, because they were quite similar with all charge ratios (except with 1:1). This indicates that the flocculation mechanism is dependent on the Mw of the polyelectrolyte in excess in PEC. A possible reason for the decreased flocculation for the PEC in which low Mw polyelectrolytes are used in excess, could be that, in these cases, the polyelectrolyte with the lower Mw formed complexes with the higher Mw polyelectrolyte with the charge ratio close to 1:1, and the excess of lower Mw polyelectrolyte remained free in the solution. As a result, the complex is not an effective flocculant. This is most clearly seen with the PEC that contains a low Mw and high charge density component (e.g. A-LWHC + C-HW, see Fig 4 and 8). As mentioned earlier, an excess of polyelectrolyte in the complex probably plays important part in the flocculation (Nyström et al. 2003b). Hence, it is beneficial that the excess polyelectrolyte has a high molecular weight which leads to an extensive bridging between particles. Conclusions The results show that flocculation of both PCC and kaolin can be significantly improved by adding polyelectrolyte complexes in comparison with the corresponding single polyelectrolytes employed in this work. The complexes can also be used in significantly wider concentration ranges. The components of the complexes had a major impact on the flocculation. The largest complexes were formed and the strongest flocculation was yielded when both of the components in PECs had high Mw. The molar mass of the components had a great impact on the flocculation. In order to achieve efficient flocculation one of the components should be a high molecular weight polyelectrolyte so that complex could effectively form polymer bridges between filler particles. More specifically, the component in excess of the complex should have higher or the same molecular weight than the other component because when the low molecular weight polyelectrolyte is in excess in the complex, the flocculation efficiency of complex decreases significantly. Pre-mixed complexes, in which one component has a high anionic charge density, are not beneficial for flocculation. In that case, excess cationic charge led to a very large complex, which probably had a low amount of free cationic sites. When the anionic charge was in excess the size of the complex was very small. In both cases, poor flocculation was observed. The charge ratio of the complexes has a great effect on the flocculation. With the higher charge ratio (e.g. 3:1) maximum flocculation was achieved with lower dosages, but a broader dosing range and a higher maximum flocculation could be achieved with the complex possessing lower charge ratio (e.g. 2:1). The PECs formed from low charged polyelectrolytes showed enhanced flocculation compared to a single polyelectrolyte but further experiments should be made in high shear conditions in order to study how they affect floc strength and re-flocculation. Acknowledgements Kemira LTD, Finland is acknowledged for financial support. Specialty minerals, USA and Imerys International, France are acknowledged for contributing the samples. Literature Bordes, C. Garcia, F. Snabre, P. Frances, C. (2002): On-line characterization of particle size during an ultrafine wet grinding process, Powder Technology 128, Chen, J. Heitmann, J.A. Hubbe, M.A. (2003): Dependency of polyelectrolyte complex stoichiometry on the order of addition. 1. Effect of salt concentration during streaming current titrations with strong poly-acid and poly-base, Colloids surf. Part A: Physicochem.Eng. Aspects 223, Nordic Pulp & Paper Research Journal Vol 28 no. 2/

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Langmuir 22, Manuscript received October 9, 2012 Accepted March 23, Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013