(34a) Influence of Supercritical Carbon Dioxide on the Morphology of Modified Clays

Organic modified montmorillonites are an important class of materials with wide industrial applications including structure-forming and thickening agents, organic absorbents, and recently as novel additives to the growing field of polymer-layered silicate nanocomposites. In nanocomposites manufacture, special consideration must be given to the mechanisms of intercalation and delamination for an organoclay species within the polymer medium. These physical phenomena are generally rate-limiting relative to the residence time of typical processing machinery, and thus nanocomposites rarely offer the full benefits of the value added by inclusion of these modified clay minerals (i.e. achieving a fully exfoliated structure). Disruptive technologies have been proposed to advance the rate and extent of exfoliation for a nanocomposite which include high energy ultrasound, electrical fields, and the use of supercritical fluids. Supercritical carbon dioxide (scCO2) is perhaps the most attractive of these technologies as it is easily adapted to standard processing machinery, requires little capital investment, and does not present the same safety issues as the others. Several current patented or patent-pending practices involve treating the organoclay with scCO2 prior to its addition to an extrusion or injection molding process. The work to be discussed in this talk examined the effects of this pre-treatment practice on a selection of organoclay species. Five organoclays were prepared by Southern Clay Products for the study using four different alkyl-ammonium surfactants and one phenyl-ammonium surfactant. The alkyl surfactants varied in chain length (C12-C18) and number of long chains. The organoclay species were place in a batch vessel and subject to scCO2 at different pressures (7.6 and 9.7 MPa) and temperatures (50 and 200oC) and then subsequently analyzed after releasing the pressure at different rates (0.2 and 4.8 MPa/s). The prepared samples were compared against the original materials and ethanol/water washed samples. Extensive characterization was conducted to examine the morphology of the coating surfactant and the clay mineral, which included pH and contact angle measurement, XRD, TEM, SEM, and particle size analysis (PSA). The results to be discussed show that scCO2 does not necessarily act as an inert solvent with the organoclay species, and while its use can be regarded as beneficial to exfoliation, there is evidence that the improvement does not arise from the rapid expansion of the supercritical solution as has been suggested in the literature, at least not under the conditions examined.