## 1 Introduction
As a consequence of the chain-growth mechanisms of epoxies initiated by Lewis or Bronsted acids, it is possible to control the polymerization process at the different stages: from the initiation to the propagation either by the active chain end mechanism (ACE), where the neutral monomer adds to the cyclic onium chain end by a nucleophilic attack, or by the activated monomer (AM), where a stable hydroxylic terminal group adds to a protonated monomer, with the possibility of forcing transfer reactions which may favorably affect the architecture and the cross-link density of the formed network (Scheme 2). Most of the industrial solutions are currently based on robust systems that polymerize upon exposure to an energetic stimulus by activating latent onium salt initiators that are sensitive to high temperatures or to different types of radiation (UV-visible, accelerated electrons, X-rays and \(\gamma\)-rays) [15; 16; 17]. The cationic chain polymerization process which was selected for our current work will be preferably triggered by exposure to high-energy irradiation which efficiently induces the polymerization of epoxy resins. The choice for an appropriate acceleration voltage is primarily guided by the need for sufficient penetration of the electron beam into the uncured material shaped by the mold and associated tooling. The plots of Figure 1 represent the simulated deposition of energy in a material of unit density as a function of the EB energy. This shows that composite materials of thicknesses or cross-sections up to some centimeters can be treated with a rather uniform energy deposition, provided that the mold itself does not abate dramatically the incident beam. This is actually a strong advantage of this technology operated initially at room temperature (RT) and which does not to require strong and complex tools. Moreover, lower energy EB (few hundreds eV) can, however, be used for laboratory test experiments, particularly for filler-free resin samples or simple pre-impregnated fabrics which do not require metal casting tools. Other processing parameters include the power of the accelerator and the features of the beam (scan rate and dimensions, pulse frequency and duration), which all affect the instant and time-average dose rate [18]. Figure 1: Simulated depth–dose distribution (expressed as percent relative of the front dose absorbed at the entrance face of an object exposed of unit density exposed to EB radiation of energy 3, 6 and 10 MeV (adapted from Ref. [19]). The interaction between matter and the high-energy photons or particles is essentially a non-selective process producing randomly excited states, radical cations and thermalized electrons as the primary species. In organic materials, the primary species are essentially converted into free radicals. Long-lived cationic chain reactions due to carbenium or oxonium intermediates generated and evidenced at low temperature in ultra-pure epoxy monomers free from any initiator are unlikely. The high reactivity of cationic centers renders transfer and deactivation reactions very fast unless special conditions (purity of involved chemicals, low temperature) are maintained throughout the process. There are still debates on the precise mechanism of cationic initiation under EB irradiation; however, when irradiation is applied to epoxies formulated with onium salts, a reasonable description of the initiation is based on an electron transfer from the in situ generated free radicals (with appropriate reduction potential) or from thermalized electrons that reduce onium salts efficiently [18]. These mechanisms were confirmed by pulse radiolysis experiments on DGEBA in the presence of an iodonium salt [17; 20]. For this reason, onium salts, in particular diaryliodonium salts, which exhibit favorable redox properties in the presence of organic free radicals, are introduced in the epoxy formulations processed by high-energy radiation.