Mechanism of Silicate Platelet Self-Organization during Clay-Initiated Epoxy Polymerization

Organoclays have been found to initiate epoxy self-polymerization. The requisite organoclays have an expanded interlayer gallery at 5.2 nm (X-ray basal spacing) as compared with original 1.2 nm for the clay intercalated with poly(oxypropylene)-amine salt (POP-amine salt) of M-w 2000 g/mol. Upon homogeneous mixing of 1-15 wt % organoclays in epoxy and heating, the clay initiation suddenly occurred in an explosive manner. We have elucidated the mechanism by analyzing the progressive reaction mixtures. After an inductive period, the epoxy monomers entered into the clay galleries, and the silicate platelets started to propagate the epoxy self-polymerization and expanded the d spacing beyond the detection limit of XRD analysis (i.e., >10 nm). The chain growth was eventually restricted because of consumption of the epoxy monomer and fixing of the polymer chains on the platelets. The resultant powder product was analyzed to contain some self-organized microstructures with silicate platelet piled skeletons (by SEM) and lamellar structure with neighboring platelet spacing over 36-96 nm (by TEM). The alignment of silicate platelets results from clay-tethered epoxy self-polymerization, silicate pushing, and end cross-linking in a synchronized mechanism.