Direct Ink Writing of High-Strength Polymer Composites

Abstract

Polymeric composites with various functionalities are widely used in the aerospace, automotive, medical industries. Direct ink writing (DIW) is an extrusion-based 3D printing method that builds the object from 3D model data in a layer-by-layer fashion. In this dissertation, high-performance thermosetting polymers, thermosetting composites, and hydrogels were printed via DIW. Modifications on printing material formulations were made for either advancing the manufacturing process with improved production efficiency or enhancing the material performance.

Firstly, an energy-efficient and rapid frontal curing assisted in-situ printing-and-curing process was developed for epoxy resin-based thermosets and composites fabrication. The dual initiating systems consisting of cationic and thermal initiators were incorporated for epoxy resin’s frontal propagation. Reducing the cationic initiator concentration from 2mol% to 0.05 mol% can lower the front temperature from ~290°C to ~240°C. Nano/micro-filler was incorporated as catalysts to further reduce the front temperature by tuning the reaction profile. With the incorporation of 1wt% CNT, the front temperature can be reduced to ~227.14°C while the front velocity remains (~6 cm min⁻¹ ). The as-developed printing process was furthered extended to continuous carbon fiber reinforced thermosetting composites (c-CFRC) fabrication with an exceptional tensile strength of ~1.147 GPa with a fiber volume ratio of 48%. This new process could potentially lead to 3.6 billion kilowatt-hours (kWh)/year energy-saving in comparison to traditional oven curing processes.

Secondly, carbon fibers with modified surface chemistry were incorporated for frontal polymerization enabled printing of dicyclopentadiene (DCPD) composites. Norbornene functional groups were grafted onto the carbon fiber surface for an in-situ filler-polymer matrix interface enhancement via frontal polymerization. Incorporation of 3wt% norbornene-grafted discontinuous carbon fibers (CFs) led to 1.7- and 2.6-folds enhancement in tensile and interlayer bonding strength with reference to the neat resin, respectively.

Lastly, single network hydrogel with phenyl acrylate (PA), and soft acrylamide (AAm) components were printed and crosslinked under ultraviolet irradiation for load bearing tissue applications. Cellulose nanocrystal (CNC) was added for rheology modifications. Hydrogel with a PA/AAm ratio of 3:2 demonstrates a tensile strength of ~4.4 MP and toughness of ~6 MJ m⁻³.