Cationic photopolymerization and hot lithography technology: characterization and 3D printing study of epoxy-based photopolymers

Photopolymers are a class of thermosets that polymerised after light exposure: the photochemical reactions start and the formation of the polymer network leads to mechanical and chemical modification of the final material. Cationic photopolymerization is a powerful and efficient method for photocuring the liquid resin and to obtain the thermosetting polymer with excellent characteristics: an important class of cationic photocurable monomers is the epoxy-based one. In the polymeric additive manufacturing sector, the photopolymers still represent the most employed material: in particular the stereolithography (SLA) process, which ensures the highest spatial resolution and object definition, takes advantage of liquid photopolymers solidified through the interaction with laser light emission, that starts the photocuring reaction of the resin. Nowadays the research for new and highly performing photopolymers is incessant: the development of modified SLA-based process like hot lithography allows the processability in 3D printer of new type of photopolymers, which are normally too viscous or not enough reactive at room temperature. The aim of this project is the modification of a cationic photopolymerized system, based on an epoxy resin (BADGE resin), in order to obtain proper structural properties of the final thermoset polymer and to allow formulation printability through high temperature 3D printing technology (hot lithography process). The work is essentially divided in 3 parts: first of all the cationic system, BADGE and cationic photoinitiator, modified with the introduction of different concentrations of chain transfer agents has been characterized in terms of mechanical properties and its photocuring process is also studied thanks to photo-DSC and photo-rheology. After the selection of a proper cationic formulations, only the resin has been processed in the hot lithography printer: the printing job parameters and the influence of 3D printing technique on the photopolymer properties have to be studied. The third part of the project regards the identification of proper additive to modify the mechanical properties of the thermoset, limiting its own brittleness: silica nanoparticles, core-shell rubber particles and poly-siloxanes have been taken into account. In conclusion the presence of trimethylolpropane inside the basic cationic photocurable formulation (BADGE and cationic photoinitiator), in combination with hot lithography machine, allows optimal 3D printability of the liquid resin: the influence of the additive manufacturing process has been generally positive in terms of mechanical properties of the printed samples; the spatial resolution of the printed object has been considered, overall, good. Finally thanks to the addition of fumed silica, the photopolymer has shown a general improvement of the mechanical properties, in particular the elongation at break has almost doubled without decreasing of rigidity and mechanical strength. In addition thanks to the particles dimension in the nanoscale the formulation processability in the hot lithography printer has been unvaried. Issues and possible solutions for future development and application of this class of photopolymers has been discussed.