Langmuir 2003, 19, 3981-3986
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Flocculation of Semidilute Calcite Dispersions Induced by Anionic Sodium Polyacrylate-Cationic Starch Complexes Roger S. Nystro¨m,*,†,‡ Jarl B. Rosenholm,† and Kari Nurmi§ Department of Physical Chemistry, Åbo Akademi University, FIN-20500 Åbo, Finland; Graduate School of Materials Research, Åbo, Finland; and Raisio Chemicals, FIN-21201 Raisio, Finland Received January 9, 2003. In Final Form: February 12, 2003 Polyelectrolyte complexes (PECs) formed by mixing oppositely charged polyelectrolytes are widely used as flocculation agents of aqueous solid/liquid dispersions. The complexes have been found superior to single polyelectrolytes in showing a substantially wider optimum concentration range for flocculation. In this work the flocculation performance and mechanisms induced by premixed complexes of cationic starch and anionic sodium polyacrylate (NaPA) on semidilute calcite dispersions were investigated by measuring the particle size and the dynamic mobility. The flocculation performance was studied as a function of the ratio of NaPA to starch, and the total amount of polyelectrolyte added. The influence of pretreatment of the calcite dispersions on the complex-induced flocculation was also investigated. The investigation clearly shows that, by using appropriate amounts of premixed NaPA and starch, the flocculation of the calcite dispersions was strongly enhanced compared to the case of using single NaPA or starch, respectively. Several mechanisms are involved in the enhanced flocculation induced by the two oppositely charged polyelectrolytes. These mechanisms are strongly dependent on the ratio of NaPA to starch, and the total amount of polyelectrolytes added. However, interparticle bridging by the polyelectrolyte complexes, and charge neutralization, induced by the deposition of the complexes, were found as the main reasons for the enhanced flocculation. Pretreating the bare calcite with anionic sodium polyacrylate changed the charge characteristics of the calcite from cationic to anionic. Despite the change in charge characteristics, the pretreatment did not drastically change the flocculation behavior of the semidilute calcite dispersions induced by the subsequent addition of premixed polyelectrolyte complexes. However, the pretreatment resulted in a slight shift in the ratio of NaPA to starch required for optimum flocculation.
Introduction The interactions between the numerous charged materials of different concentrations in, for instance, the wet end of a paper machine are impossible to monitor exactly. The only way of predicting the behavior of such multicomponent dispersions is to limit the number of substances investigated. In this investigation, calcium carbonate, low molecular weight anionic sodium polyacrylate (NaPA), and high molecular weight cationic starch represent the mineral and two oppositely charged polyelectrolytes, respectively. The binary systems calcium carbonate-cationic starch1-3 and calcium carbonate-NaPA4-7 have been previously studied. Introducing a second polyelectrolyte in such a binary system increases the complexity substantially. Both similarly8,9 and oppositely charged polyelectrolyte pairs have been utilized for optimizing the flocculation in a wide range of applications. The focus of this work was placed on the flocculation of calcite dispersions induced by adding * Corresponding author. Address: Porthansgatan 3-5, FIN-20500 Åbo, Finland. Telephone: +358-2-215 4617. Fax: +358-2-215 4706. E-mail:
[email protected]. † Department of Physical Chemistry, Åbo Akademi University. ‡ Graduate School of Materials Research, Åbo. § Raisio Chemicals. (1) Nystro¨m, R.; Backfolk, K.; Rosenholm, J. B.; Nurmi, K. Colloids Surf., A, in press. (2) Hedborg, F.; Lindsto¨m, T. Nord. Pulp Pap. Res. J. 1993, 3, 319325. (3) Ja¨rnstro¨m, L. Nord. Pulp Pap. Res. J. 1993, 1, 27-33. (4) Rogan, K.; Bentham, A.; George, I.; Skuse, D. Colloid Polym. Sci. 1994, 272, 1175-1189. (5) Ma¨kinen, M.; Laakso, A.-P. Nord. Pulp Pap. Res. J. 1993, 1, 5961. (6) Joseph, R.; Vangani, V.; Devi, S.; Rakshit, A. Colloid Polym. Sci. 1994, 272, 130-140. (7) Sanders, N. J. Pulp Pap. Sci. 1992, 18 (5), 169-175.
varying amounts of premixed low molecular weight anionic NaPA and high molecular weight cationic starch. The presence of oppositely charged polyelectrolytes in the dispersions results in an entropically and electrostatically driven complex formation between the two polyelectrolytes. The properties of such polyelectrolyte complexes (PECs) in various aquatic conditions have been extensively studied by using a wide range of different techniques.10-16 The polyelectrolyte complexes, instead of a single polyelectrolyte, have been found to substantially improve colloidal surface modification17-24 and dispersion flocculation.25-31 The flocculation induced by polyelectro(8) Swerin, A.; Glad-Nordmark, G.; O ¨ dberg, L. J. Pulp Pap. Sci. 1997, 23 (8), 389-393. (9) Walldal, C. Colloids Surf., A 2001, 194, 111-121. (10) Mende, M.; Petzold, G.; Buchhammer, H.-M. Colloid Polym. Sci. 2002, 280, 342-351. (11) Koetz, J.; Koepke, H.; Schmidt-Naake, G.; Zarras, P.; Vogl, O. Polymer 1996, 37 (13), 2775-2781. (12) Webster, L.; Huglin, M.; Robb, I. Polymer 1997, 38 (6), 13731380. (13) Dautzenberg, H. Macromolecules 1997, 30, 7810-7815. (14) Karibyants, N.; Dautzenberg, H. Langmuir 1998, 14, 44274434. (15) Ko¨tz, J. Nord. Pulp Pap. Res. J. 1993, 1, 11-14. (16) Pergushov, D.; Buchhammer, H.-M.; Lunkwitz, K. Colloid Polym. Sci. 1999, 277, 101-107. (17) Petzold, G.; Schwarz, S.; Buchhammer, H.-M.; Lunkwitz, K. Angeew Makromol. Chem. 1997, 253, 1-15. (18) Buchhammer, H.-M.; Kramer, G.; Lunkwitz, K. Colloids Surf., A 1994, 95, 299-304. (19) Kramer, G.; Buchhammer, H.-M.; Lunkwitz, K. J. Appl. Polym. Sci. 1997, 65, 41-50. (20) Kramer, G.; Buchhammer, H.-M.; Lunkwitz, K. Colloids Surf., A 1997, 122, 1-12. (21) Oertel, U.; Schwartz, S.; Buchhammer, H.-M.; Petzold, G.; Jacobasch, H.-J.; Ra¨tzsch, M. Angew Makromol. Chem. 1993, 207, 203213. (22) Petzold, G.; Buchhammer, H.-M. J. Appl. Polym. Sci. 2000, 75, 16-25.
10.1021/la034037j CCC: $25.00 © 2003 American Chemical Society Published on Web 03/13/2003
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lyte complexes is mostly less dependent on the concentration of the flocculation agents compared to traditional bridging, where restabilization frequently occurs when the flocculation agent is added in excess. The mechanisms of the enhanced flocculation observed when utilizing polyelectrolyte complexes are strongly dependent on the properties and on the amounts of the polyelectrolytes present in the dispersion.31 The main focus of this investigation was on the mechanisms involved in the enhanced flocculation induced by the oppositely charged NaPA and starch. A deeper understanding of the mechanisms was reached when investigating the influence of the ratio of NaPA to starch, the total amount of added polyelectrolyte, and the pretreatment of the calcite on the particle size and mobility of the calcite dispersions. The final aim of this work was to increase the understanding of reaching optimum flocculation in whatever system where a solid substance was treated with oppositely charged polymers. When kept controlled, the ratio and the amount of polyelectrolytes present are parameters that are useful tools for tuning the flocculation behavior of such aqueous solid/liquid dispersions. Materials and Methods Materials. The ground calcium carbonate (GCC) used was supplied by CarboMinerals, Macon, USA, and it was used as received. The dry GCC was dispersant free grade and ground to a particle size of (90 wt %)
