3424
Langmuir 2007, 23, 3424-3431
Nonaqueous Suspensions of Surface-Modified Kaolin Yan Zhang,† David I. Gittins,‡ David Skuse,‡ Terence Cosgrove,† and Jeroen S. van Duijneveldt*,† School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K., and Imerys Minerals, Ltd., Par Moor Centre, St. Austell, PL24 2SQ, U.K. ReceiVed October 16, 2006. In Final Form: December 19, 2006
A range of different stabilizers have been used to render natural kaolin clay particles hydrophobic and dispersible in nonpolar solvents such as heptane. Both silanol and aluminol groups are known to be present at the kaolin surface. Use of a Hammett indicator showed that silanes would not neutralize the acidic aluminol sites, whereas amines would neutralize these sites. Both types of stabilizer adsorbed chemically onto the clay. In addition, a combined silane + amine treatment and a polyisobutylene-based stabilizer with a succinimide/amine head group (SAP230) were also considered. Both would neutralize the acid sites. The final sediment density after settling under gravity was used to gauge suspension stability, which varied with the kaolin surface treatment as silanes < amines < silane + amine , SAP230. This behavior was very similar for suspensions in heptane and in a higher molecular weight branched alkane, polydecene. This trend of increasing stability correlated very well with an increase in surface coverage of the stabilizing moieties, a decrease in particle size found using small-angle light scattering, and a decrease in Bingham yield stress obtained by fitting rheological data.
Introduction Clay-polyolefin composites have been studied widely because incorporation of the clay particles can lead to desirable changes in the material properties of the polymers, such as improving their mechanical response or lowering their gas permeability.1 Major challenges in preparing such composites involve the surface modification required to obtain a hydrophobic clay surface and, subsequently, to obtain a homogeneous particle distribution with minimal aggregation.2-6 There are several ways to carry out the surface modification, including cation exchange, chemical or physical adsorption of a stabilizer, and grafting a layer of polymer having the same composition as the bulk polymer.7,8 Rather than attempting to create clay-polymer composites directly, in this paper we set out to apply various surface modifications to a specific clay (kaolin) and to characterize suspensions of these particles in low molecular weight solvents, with a view to correlate the suspension properties with the nature of the surface treatment applied. It is hoped that such information will provide a good basis for choosing surface modifications suitable for use in polyolefin-based composites. Clays are layered silicates. Some common clays, such as smectites, have a relatively high cationic exchange capacity (CEC) because of their permanent negative charge due to isomorphous substitution, so that their surface can be modified by ion exchange. Furthermore, the layers can be delaminated/exfoliated because * Corresponding author. † University of Bristol. ‡ Imerys Minerals, Ltd. (1) Sinha, R. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539. (2) Buggy, M.; Bradley, G.; Sullivan, A. Composites, Part A 2005, 36, 437. (3) Buining, P. A.; Veldhuizen, Y. S. J.; Pathmamanoharan, C.; Lekkerkerker, H. N. W. Colloids Surf. 1992, 64, 47. (4) Herder, P. C.; Claesson, P. M.; Herder, C. E. J. Colloid Interface Sci. 1987, 119, 155. (5) Shah, D.; Fytas, G.; Vlassopoulos, D.; Di, J.; Sogah, D.; Giannelis, E. P. Langmuir 2005, 21, 19. (6) Wang, Z. M.; Nakajima, H.; Manias, E.; Chung, T. C. Macromolecules 2003, 36, 8919. (7) Rungruang, P.; Grady, B. P.; Supaphol, P. Colloids Surf., A 2006, 275, 114. (8) Su, S.; Jiang, D. D.; Wilkie, C. A. Polym. Degrad. Stab. 2004, 84, 279.
of the relatively weak interlayer interaction. The family of kaolinite clays, however, is different. Although it was believed that kaolinite carries a small amount of permanent negative charge (
