Epoxy/epoxies cationic clays

Figure 7.7 XRD plots of organoclay (modified with octadecyl ammonium cation), epoxy/DDS/clay (intercalated) and epoxy/DETDA/clay (exfoliated) nanocomposites. Reprinted from D.Ratna, N.R. Manoj, R.K. Singh Raman, R. Varley and G.R Simon, Polymer International, 2003, 52, 9,1403 2003 John...

Figure 7.8 TEM micrographs of epoxy/DETDA/clay (modified with octadecyl ammonium cation) nanocomposite cured at a) 100 °C and b) 160 C. Reprinted with permission from O. Becker, Y.B. Cheng, RJ. Varley and G.P. Simon, Macromolecules, 2003, 36, 5, 1616 2003, American Chemical Society...

The model illustrated in Figure 2 summarizes the overall mechanism for formation of epoxy polymer - clay nanocomposites. Upon solvation of the organoclay by the epoxide monomers, the gallery cations reorient from their initial monolayer, lateral bilayer, or inclined paraffin structure to a perpendicular orientation with epoxy molecules inserted between the onium ions. A related reorientation of alkylammonium ions has been observed previously for e-caprolactam intercalated clay intermediates formed in the synthesis of Nylon-6 -exfoliated clay nanocomposites (9). Thus, the ability of the onium ion chains to reorient into a vertical position in order to optimize solvation interactions with the monomer may be a general prerequisite for pre-loading the clay galleries with sufficient monomer to achieve layer exfoliation upon intragallery polymerization. 

Thermal Decomposition of Polymeric Nanocomposites Based on Anionic Clays The thermal decomposition of DGEBA nanocomposites cured with polyoxypropylene diamine (Jeffamine D230) and containing 4-toluenesul-fonate/LDH was investigated by simultaneous thermal analysis (STA) in air. The LDH nanocomposite (TS/LDH) is compared to the neat epoxy and to a bis(2-hydroxyethyl)ammonium montmorillonite nanocomposite (30B). The clay content was 5 wt% for both nanocomposites. In Figure 9.24, differential thermal analyses obtained by STA are shown. A main exothermic peak is observed at about 550° C for neat epoxy. In the LDH nanocomposite this peak is split in two parts, so the heat release rate is decreased and the heat evolution delayed, where as no relevant difference is observed between neat epoxy and the cationic clay nanocomposite. 

Thermosetting nanocomposites exhibit a reduced rate of heat release compared to neat polymer. However, the approach to nanocomposites itself is not sufficient to comply with the actual fire test standards. For this reason, traditional flame retardants are currently used in combination with nanofillers, and researchers are focusing on the individuation of synergistic systems. As an alternative to the most common cationic clays, anionic clays show improved performance in terms of flame retardancy. Epoxy nanocomposites based on anionic clay exhibit unique self-extinguishing behavior in a UL-94 horizontal burning test never observed before in a pure nanocomposite. The formation of a continnous intu-mescent ceramic layer on the surface of a polymer during combustion reduces the heat release rate to a higher extent than do montmorillonite nanocomposites. 

Fig. 12 Changes in properties of isothermally cured epoxy monomer and montmorillonite surface-treated with methyl, tallow, bis-2-hydroxyethyl ammonium cations at 10wt% clay (a) d-spacing at a range of temperatures and (b) oscillatory rheological parameters and d-spacing at 70 °C [63]...

---