Radiation processing for synthesis of structural materials

Radiation processing has given rise to more and more interest in the production of structural materials because of the several advantages that it can offer. Besides the economic considerations, concerning energy saving due to the short processing times, radiation curing provides a non thermal process way, thus reducing mechanical stresses in the final product. On the other hand epoxy resins matrices for advanced composites, used in the aerospace and automotive applications, cured by ionizing radiation, generally suffers from poor fracture toughness. This mechanical property can be enhanced by the introduction of engineering thermoplastics, but is significantly affected by the morphology and by the distribution of residual stresses in the material. Considering that radiation curing can cause an increase of temperature, due to both the exothermic polymerization reactions and the absorption of radiating energy, depending upon process and system parameters, a right choice of operating conditions has to be done in order to obtain the thermal profile which could provide the desired final properties. In this work epoxy resins toughened blends, for use as matrices for advanced composites, have been cured by electron beam with a moderate temperature profile. The samples cured in different operating conditions, including a post irradiation thermal treatment out of the mould, have been characterized in terms of both thermal behaviour by DMTA analysis and mechanical properties by fracture toughness test. The results, discussed also in the light of the morphological analysis investigated by SEM, indicate that the required properties for such applications (in terms of Tg and KIC) can be achieved by a dual cure process consisting of irradiation at moderate temperature followed by a slight thermal treatment. The use of a second treatment is needed in order to complete the cure, overcoming vitrification effects due to the low temperature during irradiation. (author)