The influence of rheology on 3D printing with heterogeneous reactive polyurethane reagents

Abstract

The successful printing of multi-layer, multi-material polyurethane woodpile structures with good structural fidelity and rigidity was demonstrated via polymerisation-based reactive extrusion processing. Two heterogeneous, viscoelastic feed-streams, i.e. a diol and a polymeric isocyanate reagent (which was selected to improve flow-process safety), which contained fumed silica microparticle reagents were mixed and the printing performance was shown to be governed by relative rheological properties of the feeds and by the polymerisation exotherms generated upon mixing. The influence of the polymer rheological characteristics on the polymerisation, exotherms and extent of reaction achieved has not been extensively explored to date. To understand the impact of feedstock characteristics on the printing process, polydimethylsiloxane coated fumed silica particles were added to the two polymer feeds. These were designed to act as: (a) inorganic functional materials to modify the material properties of the printed devices, (b) additives that modify the feed materials’ rheology and (c) inorganic fillers that moderate the polymerization exotherm. The experimental results demonstrated that the viscosity and elasticity of the poly(isocyanate) feed, which is tuned through silica particle content, must be comparable to or lower than those of the diol feed. This requirement was attributed to the mixing characteristics that developed when the two feed streams were combined, which ultimately determined the interfacial reactive area created and dictated the rate/extent of the step-growth reaction.