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    Additive manufacturing of nanotube-loaded thermosets via direct ink writing and radio-frequency heating and curing
    (Pergamon-Elsevier Science Ltd, 2022) Sarmah, Anubhav; Desai, Suchi K.; Crowley, Ava G.; Zolton, Gabriel C.; Tezel, Güler Bengüsu; Harkin, Ethan M.
    Direct Ink Writing (DIW) is an extrusion-based additive manufacturing method where the print medium is a liquid-phase 'ink' dispensed out of nozzles and deposited along digitally defined paths. Conventional DIW of thermosetting resins relies on viscosity modifying agents, novel crosslinking chemistries, and/or long curing schedules in an oven. Here we demonstrate the use of a co-planar radio frequency applicator to generate an electric field, which can be used to rapidly heat and cure nano-filled composite resins as they are printed. This method avoids the need for an oven or post-curing step. This process consists of a sequential print-and-cure cycle which allows for printing of high-resolution, multi-layered structures. Every extruded layer is partially cured using RF before depositing the next layer; this allows the printed part to maintain structural integrity. The process enables both increased throughput and decreased touch time relative to traditional manufacturing. Commercial epoxy resin with varied nano-filler loadings were examined as DIW candidates. After printing, the thermo-mechanical properties, surface finish, and shape retention of RF-cured samples were comparable to conventionally cured samples. This method of manufacturing establishes RF heating as a suitable alternative to conventional methods, facilitating rapid, free-form processing of thermosetting resins without a mold.
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    Kinetics of carbon nanotube-loaded epoxy curing: rheometry, differential scanning calorimetry, and radio frequency heating
    (Elsevier, 2021) Tezel, Güler Bengüsu; Sarmah, Anubhav; Desai, Suchi; Vashisth, Aniruddh; Green, Micah J.
    The isothermal curing kinetics of carbon nanotube loaded epoxy was investigated using rheometry and differential scanning calorimetry (DSC) at a range of temperatures. Rheo-kinetics was used to observe time-dependent rheological changes in elastic (G?) and viscous (G?) moduli, and complex viscosities of epoxy-CNT samples during isothermal curing. DSC measurements were also performed to monitor the curing reaction, in order to compare against the rheo-kinetic parameters. The Kamal-Sourour kinetic model describes the curing of the epoxy-CNT system for rheo-kinetics and DSC well. The activation energies of the curing reaction were found to be ?36 kJ/mol and ?33 kJ/mol using rheo-kinetics analysis and DSC, respectively. In addition, radio-frequency (RF) electromagnetic fields were used to heat and cure the epoxy-CNT sample; such heating techniques are valuable in a number of epoxy processing technologies. G?, G?, and complex viscosities of RF heated samples were measured to monitor RF-aided curing. This allows us to monitor the curing kinetics inside samples being heated by RF fields; the data indicate that RF-aiding curing is faster than curing rates for samples undergoing curing inside a measurement device such as a rheometer or DSC, because the heat generated is immediate and volumetric.
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    Rapid manufacturing via selective radio-frequencyheating and curing of thermosetting resins
    (Wiley-V C H Verlag GMBH, 2022) Sarmah, Anubhav; Desai, Suchi K.; Tezel, Güler Bengüsu; Vashisth, Aniruddh; Mustafa, Mazin M.; Arole, Kailash
    A new method for additive manufacturing of thermosetting resins using selective, localized radio-frequency (RF) heating and curing in a thermoset reservoir is demonstrated. The use of a local RF applicator addresses the challenge of selective curing and printing of heat-curable thermosets from a reservoir of resin, without the addition of photocurable acrylates. The filler of interest is multi-walled carbon nanotubes, which heat up rapidly in response to an RF field. A target temperature can be maintained by modulating the RF power. Multilayered structures were 3D printed by moving the RF applicator relative to the resin reservoir, selectively curing the resin exposed to the field; this process was repeated for each layer. Thermal and mechanical properties of RF-printed samples were compared against conventional samples, with both methods showing similar glass transition temperatures and storage moduli; the RF-heated samples showed a more uniform morphology with lesser voids. The 3D printing process (temperature and conversion varying in space and time) is modeled to demonstrate the scope of this method in printing complex structures. This method of multilayered additive manufacturing of thermosetting resins allows for rapid, free-form processing.