The Use of Supercritical CO2 in the Processing of Thermoplastic Elastomer Nanocomposites

Abstract

This research studied the use of supercritical C02 (scC02) in the processing of layered silicate thermoplastic olefinic polymer (TPO) nanocomposites. Focus of the work was to understand how C02 interacts with the nanofiller in a manner that may lead to increased interlayer spacing of the clay platelets during processing, and if that interaction can be exploited as a mechanism for creating a fully exfoliated structure. After a series of studies, it is understood that scC02 indirectly improves the level of exfoliation and dispersion for a TPO/clay nanocomposite by plasticizing the maleated polypropylene (compatibilizer species used for preparing TPO nanocomposites) and the surfactant of an organoclay filler. The effect of plasticization aided intercalation of the compatibilizer into the interlayer areas of the clay. For intercalation, concentrating scC02 at the compatibilizer/clay interface was found to be more effective than when the clay was annealed alone under the gas before mixing or when all components of the nanocomposite formulation were heated together in the presence of the gas. In the latter case, the effectiveness of scC02 to plasticize the compatibilizer/clay association was diminished by inclusion of the TPO matrix which similarly sought to absorb the gas. In the case of the clay alone in the presence of scC02, surfactant rearrangement was noted to occur increasing its surface coverage over the oxygen plane. This in turn, was hypothesized to reduce the available sites for the compatibilizer to form associations within the galleries as opposed to the broken edges of the clay particles, thereby lowering the driving force for interlayer intercalation. The aiding effect of scC02 on the interlayer intercalation was found dependent on the organoclay species, i.e. chemical structure of its surfactant and the manner which that surfactant was arranged within the galleries of the silicate. For a surfactant with two alkyl chains and paraffinic arrangement, scC02 significantly aided the interlayer intercalation. However, this surfactant appeared fairly unique with other examined organoclays not displaying scC02 influence, namely an organoclay with surfactant having three alkyl chains and paraffinic arrangement, or one with surfactant possessing either monoalkyl or dialkyl chains and a lateral bilayer arrangement. Surfactant rearrangement occurring due to the presence of scCO2 was ascribed to the different response of the organnoclay to the intercalation. As observed by X-ray diffraction and differential thermogravimetric analysis, for the organoclay with two alkyl chains and paraffin-type arrangement the presence of scC02 resulted in increased interlayer distance and decreased surfactant packing at the edge area of silicate platelets, both of which were favourable factors to the interlayer intercalation. Finally, adding a cosolvent of either water or ethanol with scC02 was observed by infrared analysis to significantly promote chemical associations between the compatibilizer and clay during melt processing. Water enhanced the compatibilizer/clay carboxylation whereas ethanol reacted with the compatibilizer generating ester bonds. The increased chemical affinity was believed to be responsible for the significantly improved compatibilizer/clay intercalation, and resulting exfoliation and dispersion of the TPO nanocomposite revealed by XRD, TEM and rheology analysis.