Effects of electron beam radiation and annealing on the crystallinity and tensile properties of a 3D-printed PLA-based material

This article reports on the influence of electron beam irradiation on the tensile properties and crystallinity of 3D-printed PLA-based materials. Monitoring the evolution of key tensile properties as a function of dose confirmed the detrimental effects of irradiation with a typical loss of about 50 % of tensile strength and ultimate displacement after a 75 kGy treatment. Particular attention was paid on the nature of crystalline domains and degree of crystallinity χ in samples analyzed by calorimetry and X-ray diffractometry, as obtained after 3D-printing, after irradiation and after annealing in optimized conditions. The χ value in irradiated samples was shown to initially increase from 19 to 24 % and return to 19 % at 100 kGy before a stronger decay at 300 kGy. The maximum crystallization capacity of the same samples was much higher (χ>60 %) within this dose range. Crystallinity can therefore be adjusted after irradiation with possible beneficial effects to mitigate the undesired impact of irradiation. For samples irradiated at 75 kGy with a loss of 50–60 % on tensile strength and ultimate displacement, part of the mechanical damage due to irradiation was recovered by soft annealing at 60 °C, increasing the tensile strength by 20 % and ultimate displacement by 50 %. This remarkable result can be interpreted by considering the particular sensitivity to the evolution of tie chains. While chain scission under radiation is effective and mechanically impactful in domains closed to crystallites, chemi-crystallization is believed to occur with partial healing of the damages due to chain scission.