Thermo-mechanical properties and structural features of epoxy resins cured by electron beam radiation

Complete text of publication follows. Radiation induced polymerisation offers many advantages compared to the traditional thermal curing of composites essentially for the fast and out-of-autoclave curing operation as well as for the ability to manufacture complex and large structures at low development costs. However a better control of the elaboration of the network under radiation is still required to reach the specifications of high performance composites. Macroscopic properties such as the glass transition temperature (Tg) as well as the microstructure of the formed network are of central importance in the design of matrices with demanding mechanical properties. In this work we have examined in particular the effect of thermal post cured treatment and of the initiator nature, iodonium or sulfonium salts with various counter ions, on the thermo-mechanical properties and on the microstructure of the formed network. The samples were characterized to determine the the relation between the degree of cure, the post-cure treatment and Tg. Network microstructure was investigated by atomic force microscopy (AFM) and NMR relaxometry, bringing information at various dimension scales. The AFM phase images revealed the heterogeneous character of the network that can be interpreted in term of variations of crosslink densities. The decay of transverse magnetization measured by solid state proton T2 NMR relaxation experiments exhibits two kinetic components which can be assigned to the highly cross-linked and the loosely cross-linked phase. The effects of the thermal history on Tg and on monomer conversion in aromatic epoxy materials cured by EB irradiation have been examined in some detail.