FDM 3D Printing of Main-Chain Polybenzoxazine with Diels-Alder Moieties

Fused Deposition Modeling (FDM) permits rapid, inexpensive printing of precise, complex 3D thermoplastic structures. Thermosets are generally printed with vat photopolymerization systems and are mostly limited to radical cure polymerization chemistry. Our approaches to 3D printing thermosets with FDM printers is to prepare <i>thermoplastic</i>, B-stage thermoset filaments that thermally cure during printing. The challenge is to design a lower temperature transition that will permit extrusion of filament without curing and a higher temperature cure chemistry that will ultimately crosslink the thermoset once its been printed. In this work, this was done by designing a polymer with benzoxazine groups in the polymer backbone that will cure and crosslink above 200 °C and Diels-Alder cycloadducts in the backbone that will fall apart below 150 °C to permit filament extrusion. The resin melts in the filament extruder through retro Diels-Alder fragmentation of the polymer allowing filament to be formed. On cooling the cycloadducts re-assemble yielding the filament with the desired thermoplastic properties. In the printer head at over 200 °C, the filament liquefies by cleavage of the cycloadducts, but now the benzoxazine groups also start to undergo ring opening polymerization. The molten polymer readily flows through the extruder to form the desired print composed of the cured thermoset.