New curing technique speeds up direct ink writing

Thermosetting polymers, such as epoxy resins, are ubiquitous in the modern world. Their high mechanical strength and thermal stability has seen them used in everything from batteries to construction materials. But manufacturing components from such polymers is a slow and intensive process, and the reliance on moulds limits available geometries.

Additive manufacturing solves many of these issues, which is why techniques like Direct Ink Writing (DIW) have been getting lots of attention in the cross-linkable resins community. DIW is an extrusion-based 3D printing method that involves dispensing a liquid (the ‘ink’) through a nozzle onto a surface along defined paths, to fabricate 3D structures layer-by-layer. The primary challenge with using DIW for manufacturing thermoset parts is retaining the shape of the structure during printing and curing. An international collaboration, led by scientists at Texas A&M University, may have found a solution.

Writing in Carbon [DOI: 10.1016/j.carbon.2022.08.063], they report on the use of radio frequency (RF) fields to rapidly cure commercial epoxy resins. This followed on from the group’s earlier work which demonstrated that carbon nanomaterials heat up rapidly in response to RF between 20 kHz and 300 GHz. They reasoned that, by using these materials as a filler in a liquid, printable epoxy, and exposing the resulting structure to RF, they could eliminate the need for large ovens to heat and cure printed specimens.

They started with an epoxide and curing agent, to which varying quantities (3-7 wt%) of multi-walled carbon nanotubes were added. The rheological properties of the resin samples were characterised before being extruded using a commercial direct ink writing printer. In order to be suitable for DIW, an uncured resin should have a yield stress of between 1 and 100 Pa, with a high post-shear viscosity and storage modulus. Samples with 4, 5 and 7 wt% CNT loading all fitted these conditions. The resin with 4 wt% also showed a rapid heating rate in response to RF (approaching > 100 °C in ~15 seconds at 1 W) and so the team chose to proceed with this sample for all experiments.

The fabrication process consists of sequential print-and-cure cycle, with each extruded layer partially cured using RF before depositing the next layer. While this may sound slow, conventional DIW printing requires oven-curing for 12-24 hours. The RF application takes a just 45- 60 seconds per layer, and so reduces the total processing time. And because each layer is partially cured, the structure can maintain its shape throughout the printing process.

The team printed a variety of complex, high-resolution shapes, including a gear and a single-trace hexagon. Bars printed using this technique had a higher tensile strength than conventionally-made samples (~23.1 MPa vs. ~18.6 MPa). This was attributed to the fact that RF heats the sample from inside-to-outside, which eliminates trapped air bubbles, producing a uniform morphology and smooth surface finish. In contrast, the conventionally-made samples contained multiple voids, as confirmed by SEM imaging.

Simulations carried out by the team confirmed that the same technique could be used to fabricate three-dimensional shapes, such as an open cylinder with a small thickness. In addition, they say that it could be applied to “most heat-curable resins”. They write that, “This work facilitates rapid, free-form processing of commercial thermosets which establishes RF heating combined with DIW printing as a promising method for additive manufacturing of thermosetting systems with reduced processing time and energy requirements.”

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Anubhav Sarmaha, Suchi K. Desaia, Ava G. Crowley, Gabriel C. Zolton, Guler Bengusu Tezel, Ethan M. Harkin, Thang Q. Tran, Kailash Arole, Micah J. Green. “Additive Manufacturing of Nanotube-loaded Thermosets via Direct Ink Writing and Radio-Frequency Heating and Curing,” Carbon, Vol 200 (2022), Pages 307-316. DOI: 10.1016/j.carbon.2022.08.063

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