Development of clay modifications for polymer/clay nanocomposites

Three commercial clays: Cloisite Na+ , Cloisite 30B and Cloisite 15A were used to melt compound with a number of polymer matrices, including polypropylene (PP), poly(butylene terephthalate) (PBT) and polyamides (PA). X-ray diffraction (XRD), contact angle measurement and thermogravimetric analysis (TGA) showed small interlayer space, poor compatibility and low thermal stability of these nanocomposites, resulting in poor mechanical properties in composites. A number of modifications of commercial clays were carried out, including silylation and introduction of thermally stable surfactants, e.g. alkyl quaternary phosphonium cations and polyhedral oligomeric silsesquioxane (POSS). It was found that poor compatibility between polymer matrices and organically modified clays, especially in those POSS modified clays, again restricted the formation of exfoliated structure in polymer matrices. A mode of dual-surfactant modification for clay was developed, and large interlayer spacing (>3.2 nm), good thermal stability (decomposed at ~300ºC) and improved surface properties for those dual-surfactant modified clays promoted a better dispersion of clays in polymer matrices and consequently better mechanical properties. For example, the tensile moduli of PP, PBT and PA 12 were improved from 515 MPa, 1065 MPa and 490 MPa to 1020 MPa, 1470 MPa and 800 MPa of their nanocomposites containing 3 wt% dual-surfactant modified clays, respectively. However, the dispersion of these organoclays varied in different polymer matrices, due to the different compatibility and the existence of polar type interactions between organoclays and polymer matrices. In order to further address the effect of polymer matrix on clay dispersion, three polyamide matrices (PA 6, PA 11 and PA 12) were employed, and PA 6 based nanocomposites showed the most effective exfoliation and mechanical enhancements due to the strong polar type interactions between the polymer and the organoclays.