Electrical Anisotropy and its Mitigation in Conductive Polymers Printed by Digital Light Processing

In most 3D printing technologies, objects are realized layer by layer. This layer-by-layer construction leads to inherent anisotropic physical properties. Controlling, understanding and sometimes mitigating such anisotropy is a critical issue in the development of 3D printing. We demonstrate and quantify in this work electrical anisotropy in conductive materials processed by the so-called Digital Light Processing (DLP) method. In this method, which enjoys high resolution and high speed, layers of polymers, are successively cross-linked by UV irradiation of 2D patterns. Here, we use acrylate based resins and carbon nanotube as conductive fillers for their low percolation threshold that allows realizing conductive and still sufficiently transparent materials for UV irradiation. Conductivity parallel to the layers of 3D printed objects is found to be much greater than conductivity perpendicular to the layers. This electrical anisotropy is explained by the high contact resistance between printed layers. High contact resistance results from the slow diffusion of carbon nanotubes from the uncured material towards the interface of the cured object. We found that implementing a delay time before curing successive layers, or decreasing the matrix viscosity with temperature, to promote diffusion of the conductive particles allow substantial reduction of the contact resistance between layers. As a result, conductivity anisotropy can be reduced by almost two orders of magnitude. This control and mitigation of conductivity anisotropy allows reconciliation of the high resolution of the DLP technology with the possibility to realize uniform 3D materials.