The Manufacture of Polymer Nanocomposite Materials Using Supercritical Carbon Dioxide
The use of supercritical carbon dioxide (scCO₂) as a processing aid to help exfoliate nano-clays and improve their dispersion during melt blending in polymer matrices has been reported in the literature. One of the best processes in terms of improving the degree of nano-clay dispersion and composite mechanical properties was developed in our laboratory. This process allows the clay to be in direct contact with scCO₂ and expanding the clay-CO₂ mixture via rapid depressurization into a two-stage screw extruder to mix with the polymer pellets. However, composites with clay loading higher than 6.6 wt % were not reported. In addition, the scCO₂ aided processing method has not been applied to carbon nanotube (CNT) based composites.
This dissertation initially focused on applying the scCO₂ aided processing technique to the field of CNT expansion and CNT/polymer composite preparation. The relationship with the expanded CNT morphology and the experimental conditions of the expansion procedure (including pressures, temperatures, exposure time, and depressurization rates) was studied. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂ expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/ poly(phenylsulfone) (PPSF) composites prepared with scCO₂ aided method provided continuous improvements in Young's modulus up to the addition of 7 wt % CNTs. However, the Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt % CNT.
The second part of this work is concerned with the development of a semi-continuous process using scCO₂ to process polymer-clay composites with clay loading higher than 6.6 wt % (i.e. 10 wt %). Two major modifications are involved in the new procedure: exfoliating the nano-clay directly into the hopper filled with pellets followed by processing the composite immediately and sequentially mixing the clay into the melt. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) results show that this modified procedure help to reduce the clay collapse when processing the composites with high clay loadings. Surface modified montmorillonite (MMT) nano-clay/polypropylene (PP) composite at 10 wt % nano-clay with improved clay dispersion was obtained with increased modulus and tensile strength of 63 % and 16%, respectively, compared to the pure PP matrix.
Additional mechanical property improvements for nano-clay based composites are then obtained with the use of high crystallinity polypropylene (HCPP) and polypropylene grafted with maleic anhydride (PP-g-MA). HCPP has higher crystallinity and stiffness than conventional PP and, therefore, composites made from HCPP have better mechanical properties to start with. PP-g-MA has polar groups grafted on the PP chains that promote the intercalation of PP with clay. By using the newly developed procedure, the HCPP nanocomposite at 10 wt % of nano-clay has a Young's modulus as high as 3.236 GPa, and the modulus of the 10% MMT/PP-g-MA sample is found to be 2.595 GPa, both higher than that of the composite prepared by the direct blending method and that of a composite based on a conventional PP matrix.