Surfactant intercalated koppies and boane bentonites for polymer nanotechnology

Abstract

This research aimed to develop technology and processes to further beneficiate two southern African bentonites for applications in polymer/clay nanotechnology. The bentonites were from the Koppies mine in South Africa, and the Boane mine in Mozambique. The work was divided into two parts: (i) preparation of organomodified nanoparticulate smectite clays, and (ii) preparation of their poly(ethylene-co-vinylacetate) nanocomposites. Nanoparticulate organobentonites were prepared using purified bentonites. The conventional organomodification process uses a very low concentration of bentonites at 80 oC. In this study, a novel method was developed: concentrated slurries of naturally occurring Ca-bentonite partially activated with soda ash in the presence of a proprietary dispersant were contacted at ambient temperature with quaternary ammonium surfactants. A known amount of bentonite dispersion was placed in a planetary mixer before the mixture. Likewise, a known amount of surfactant, up to 50% excess, based on the estimated cation exchange capacity (CEC) of the bentonites, was added while mixing the dispersion. The surfactants added were either in solution or in powdered form. The intercalated bentonite was recovered by centrifugation and washed repeatedly with water until halide ions could not be detected using a 1M silver nitrate solution. The solids were dried at ambient temperature and humidity, and then crushed and milled into a fine powder using a mortar and pestle. Several instrumental techniques were used to characterise and examine the properties of the bentonite samples before and after organic treatment.The X-ray diffraction (XRD) results were consistent with: (i) paraffin-type extended chain intercalation; and (ii) interdigitated monolayer intercalation of the C12 and C14 single-chain alkyl surfactants and bilayer intercalation of the single-chain C16 surfactant and the surfactants with double alkyl chains. Fourier transform infrared (FTIR) spectroscopy analysis of the organobentonite powders confirmed disordered chain conformations. XRD also detected significant amounts of cristobalite in the samples of Boane bentonite (from Mozambique). This impurity could not be removed cost-effectively. The onset decomposition temperature ofthe present organobentonites was around 200 ºC, which is within the typical range of polymer/organoclay processing temperatures. The thermal stability of the organobentonites was independent of both the number of alkyl chain substituents and their length, and also independent of the degree of clay intercalation. SURFACTANT INTERCALATED KOPPIES AND BOANE BENTONITES FOR POLYMER NANOTECHNOLOGY ii Poly(ethylene-co-vinylacetate) nanocomposites were prepared with South African Koppies bentonite, organomodified with single-chain C12 (and polar 2-hydroxyethyl side chain) and double-chains C18 alkyl ammonium cationic surfactants. The later surfactant was intercalated both below and above the clay CEC. Nanocomposites were prepared by twin-screw melt compounding. Transmission electron microscopy (TEM) indicated the presence of mixed nanoand micron-sized clay morphologies. XRD studies revealed that the crystallinity of the particles improved and that the d-spacing values increased on incorporation of the modified bentonites in the polymer matrix. It is postulated that, rather than indicating polymer co-intercalation, this was caused by further intercalation of either excess surfactants or surfactant residues that were released by shear delamination of the clays during compounding. Improved mechanical properties were realised, especially when using the bentonite containing the longer double-chains surfactant intercalated at levels in excess of the CEC of the clay. The nanocomposites showed improved tensile modulus and elongation at break values at the expense of a reduction in impact strength, while tensile strength was about the same as for the neat polymer.