Date of Award
Master of Applied Science (MASc)
Michael R. Thompson
Polymer-layered silicate (PLS) nanocomposites are of great interest presently due to their
significant enhancement of properties compared to conventional polymeric materials. However, achieving a high extent of exfoliation, which refers to the complete separation and uniform dispersion of clay layers, is a key challenge in the preparation of PLS nanocomposites, particular for non-polar polymers like polyolefins. Recently, several novel processing technologies utilizing scCO₂ as an exfoliation aid have been developed. But how to achieve an optimal processing method by which fully exfoliation occurs with the aid of scCO₂ has not been fully studied.
A novel clay-CO₂ injector apparatus was designed and constructed for this project in order to maintain the organoclay in suspension with scCO₂ during its melt compounding into a nanocomposite. Thermoplastic polyolefin elastomer (TPO) based nanocomposites were prepared by three methods: 1) directly injecting the CO₂ into a twin screw extruder (TSE); 2) injecting an organoclay-CO₂ suspension along with its compatibilizer into a twin screw extruder; and 3) injecting the organoclay-CO₂ suspension into a single screw extruder (SSE). Under Method 1 both as-supplied organoclay and a previously scCO₂- pretreated organoclay (from batch) were tested, while for the other two methods only the as-supplied material was used. The structure and properties of the resultant
nanocomposites were characterized by X-ray diffraction (XRD), transmission electron
microscopy (TEM), rheology and mechanical testing.
It was found scCO₂ has a great influence on the morphology of clay within TPO nanocomposites. When preconditioned in scCO₂, chain mobility of surfactant within clay interlayers increased and led to further cation exchange reactions, thus resulted in an expansion in interlayer spacing and greater chance for penetration of large polymer specimen. It was shown in XRD patterns and TEM graphs that nanocomposites prepared using these pretreated organoclays (s-C20A) improved the degree of exfoliation over those based on as-supplied organoclay, in the absence of CO₂. On the other hand, the clay-CO₂ injector could also be beneficial for achieving an exfoliation structure. With increasing CO₂ pressure, the mobility of surfactantlcompatibilizers chains increased, allowing greater intercalation of matrix chains and further delamination under shear flow. The greatest exfoliation structure so far was achieved by maintaining the C20A/CO₂ suspension above the critical pressure of the gas during first-pass melt compounding in a single screw extruder then following with a second-pass through a twin screw extruder which provided sufficient shear stress for further exfoliation without the plasticizing effect of CO₂. Both pretreating clays and clay-CO₂ injection approaches showed improvement in clay dispersion over conventional nanocomposites melt compounding method. However, this improved exfoliation structure did not directly bring on improvement of rheological and mechanical properties. A possible thermo oxidative degradation was considered to be responsible for the reduction in rheological and mechanical properties.
Zhuang, Zijin, "Use of Supercritical Carbon Dioxide in the Processing of Thermoplastic Elastomer Nanocomposites" (2009). Open Access Dissertations and Theses. Paper 4255.
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