Free volume and storage stability of one-component epoxy nanocomposites
One-component epoxy formulations consist of epoxy resin, hardener, filler and the accelerator. They are preferred for industrial applications, but unlike two-component epoxy resin systems they have a limited shelf-life due to the presence of the accelerator. As in order to achieve a lower hardening temperature it is necessary to introduce an accelerator in the formulation, which in turn reduces the shelf-life. The accelerator is encapsulated in Zeolite and Nanosilica carriers. These encapsulations, named “NANOMODULES”, have a controlled release of accelerator in the Epoxy-Hardener matrix to increase the shelf life and a reduced hardening temperature. Positron annihilation lifetime spectroscopy (PALS) is used to study the in-situ curing behavior of epoxy resins of DGEBA with a hardener of dicyandiamide (DICY), from the initial viscous state of the formulation to the solid state of cured polymer. Several isothermal measurements were done in the temperature range from 50 °C to 80 °C for the DGEBA-DICY benchmark system. PALS parameters seem to correlate very well with the curing mechanism in the samples. Gelation was observed as an increase in the intensity of ortho-positronium (o-Ps) and a sharp decrease in the lifetime. Change of (o-Ps) lifetime is used as a measure for the degree of curing. Applying the Johnson-Mehl-Avrami-Kolmogorov approach under reasonable assumptions, these experimentally observed changes could be characterized by determining the growth exponent ‘n’ and reaction constant ‘k’. It is possible to extrapolate the shelf life of one-component epoxy formulation at room temperature. Furthermore, for a selected series of formulations the effect of free accelerator on storage stability is studied in comparison to the accelerator encapsulated in micro and nano-sized Zeolite carrier. A temperature dependence of sub-nanometer level free volume studies on their cured composites indicate no real changes in the glass-transition temperature Tg. Similarly, PALS and JMAK analysis were performed on various one-component formulations, with varying concentrations of accelerator in micro and nano-sized Zeolite and Nanosilica based encapsulations. These results proved helpful in further development and synthesis for “NANOMODULES”, resulting in several one-component formulations with desirable properties. This work was supported by the German Ministry of Education and Research (BMBF), under the Project No.: 03X0026F “NANOMODULE”.