Incorporating Crosslinks in Fused Filament Fabrication: Molecular Insight into Post Deposition Reactions

Abstract 3D printed objects suffer from intrinsic mechanical weakness due to low interlayer adhesion and defects that result in anisotropy in their mechanical properties. In this report, we examine the ability of a protocol to incorporate covalent bonds between layers to strengthen interlayer adhesion by depositing multi-amines between layers during the fused filament fabrication printing process. The multi-amines may then react with oxygenated functional groups on the deposited filament to form covalent crosslinks between layers. Determination of the interfacial fracture energy and infrared (IR) spectroscopy studies indicate the successful formation of covalent bonds and strengthening of the interlayer interface with a variety of multi-amines. More importantly, IR results also elucidate the relative rate of reaction of the amines with oxygenated functional groups found on the oxidized filament. These results show that the crosslinking reactions primarily occur shortly after deposition of the filament, i.e. at elevated temperatures. The data also show that the reactivity of the amines is not the prevailing factor in determining interfacial strength, as the aromaticity of the crosslinker plays a key role in creating a stronger interface. These results therefore provide important foundational understanding that can be used to apply this protocol to a variety of extrusion based additive processes and materials.