Engineering Structure Dynamics and Rheological Properties of Functional Inks

Functional inks provide endless possibilities for 3D printing, as structures can not only be designed with certain morphology but also with specified macroscopic properties such as enhanced thermal or mechanical properties. Graphene and reduced graphene oxide (RGO) nanoplatelets are of great interest because of their favorable aspect ratio and conductive properties. They are currently used in a number of applications such as supercapacitors and thermal heat shield materials. Processing of the composite inks for these applications, which consist of highly loaded particles, first involves a detailed understanding of the underlying rheological properties. However, little is known about the effect of processing/print parameters, e.g., print speed has on the orientation of such 2D particles during the printing process and how this subsequently influences the macroscopic properties of the final cured composite. Understanding this relationship is the main focus of this work.

In this work, inks with solids loading from 3 – 18 wt% with both graphene and RGO were dispersed into epoxy resin EPON 862 to form a viscoelastic shear thinning fluid. The optimal loading for printing was determined from rheological measurements and the resultant thermal and mechanical properties measured as a function of print speed. The results show a two to three fold increase in thermal conductivity as a function of print speed for a fixed particle concentration and a shift in the glass transition temperature with increasing print speed to higher values. These results can be attributed to alignment of the particles during printing due to the imposed shear and this result was verified by Xray diffraction measurements. Thus, the present work helps develop design rules for processing of 3D structures with enhanced properties using additive manufacturing and we envision the use of such materials in a number of applications mentioned above.