Shelf Life and Controlled Cure of Epoxies by Loaded Zeolite

Epoxy formulations based on the multi-functional amine hardener, dicyandiamide (Dicy), regularly contain a free accelerator for reducing the curing temperature and the time needed to complete the network formation. Unfortunately, all accelerators reduce the shelf life of these adhesives at 25 degrees C. In order to solve this problem, accelerator-loaded zeolites fillers were developed, optimised with respect to host-guest interactions and characterised by Fraunhofer IFAM (Bremen, Germany) with regard to the release and curing behaviour in epoxy adhesive formulations. They are added to an epoxy adhesive (digly-cidylether of bisphenol A (EP) and dicyandiamide (Dicy), mass ratio 100: 6.7), stored at 25 degrees C in regular air or cured (heated with beta = 10 K/min to 170 degrees C subsequent isothermal curing for 45 min). That shelf life and curing behaviour are investigated by FT-IR spectroscopy and modulated DSC. Compared to the EP containing free accelerator, the zeolite-filled EP possesses a threefold increase in shelf life at 25 degrees C due to the immobilization of the accelerator in the pores of the zeolites. While the free accelerator acts steadily during heating, it is shown that the loaded zeolite releases the accelerator at about 76 degrees C. Surprisingly, the released accelerator is not only involved in the chemical formation of the epoxy network but it accelerates the dissolution of Dicy considerably. As the result, network formation at 170 degrees C finishes after not more than 19 min and the starting temperature for curing could be reduced to 140 degrees C. (C) Koninklijke Brill NV, Leiden, 2010