Whether you’re constructing homes through your 3D-printing company, creating RPG tabletop terrains and cosplay weapons through your garage-based modeling operation, or providing disaster relief and building structures in distant locales or even on other planets, you don’t want to waste your medium. On Earth, that’d be financially and ecologically expensive, but in space, on the Moon, or on Mars, depleting your printing resource could be a disaster.
But no matter how much a modeler plans, human and mechanical errors during design or printing will always result in wasted PLA, resin, concrete, food, or other media. So, why not just recycle the incomplete or flawed printings?
Because sometimes you just can’t.
Until recently, the finest 3D printing available was stereolithography, and its method for printing high-resolution structures at the atomic scale required curing its resin with ultraviolet (UV) rays. However, exposing resin to UV radiation is like exposing a raw egg to a hot frying pan: you’re never going to make a solidified egg-white fluid and clear again.
But because of a breakthrough at Japan’s Yokohama National University – which was published in ACS Omega – doing a reverse Humpty-Dumpty with a 3D-printing medium is now possible, inaugurating a new era of sustainable, micro-detailed, recyclable resin printing.
“Conventional resins form irreversible cross-linked networks,” explains co-corresponding author Shoji Maruo, professor at Yokohama Faculty of Engineering. "Previously proposed ‘recyclable’ resins either require the addition of chemical additives for reuse or rapidly degrade after one or a few reuse cycles.”
The key to the Yokohama breakthrough is a component of coal tar, anthracene (C14H10), a solid polycyclic aromatic hydrocarbon with three fused benzene rings. Used to create plastics and dyes, anthracene and its derivatives can preserve wood, and because of how they react to light, form cross-linked three-dimensional chemical bonds in a process called photodimerization. But when heated, these substances can return to their original flattened forms.
“Reversible photodimerization reaction of anthracene could be a practical method for developing a truly reusable resin free from initiators [chemicals that induce reactions] that can maintain performance through multiple recycling cycles while supporting high-precision stereolithography,” says Maruo.
Employing a reversible adhesive, Maruo’s team synthesized anthracene resin and devised two means for curing it: single-photon, layer-by-layer microstereolithography, and dual-photon lithography, causing the resin to absorb two photons simultaneously. They assessed the accuracy of 3D-printing butterfly models at different speeds and found their method performed as well as traditional means.
According to co-first author Masaru Mukai, specially appointed assistant professor on the Faculty of Engineering at Yokohama and currently at the Tokyo University of Science, “Recyclable resin can be precisely patterned in arbitrary shapes using laser-scanning, confirming its suitability for two-photon lithography.”
In the Yokohama step-growth polymerization method, curing resin requires no initiators, meaning that, as Mukai explains, “Reversible photodimerization of anthracene provides a practical route toward recyclable materials for light‑based 3D printing.”
The Yokohama team's initiator‑free, step‑growth mechanism proves that “a photocurable resin can be recycled multiple times with comparatively small changes in material performance, while remaining compatible with both two‑photon and single‑photon stereolithography.” The result, Mukai says, is “reusability and high‑precision fabrication can be combined more effectively than in previously reported recyclable resins.”
According to Maruo, that recyclability is multi-multi-generational. “Previously reported recyclable resins depended on photo-initiators, but the resin in this study cures via stepwise polymerization without the need for initiators,” he says. “This unique feature simplifies resin formulation, eliminates contamination from additives, and enables near-complete recyclability.”
The proof was in the 3D pudding: 10 rounds of printing a letter, melting it, and printing another letter, and printing a cube, melting it, and then printing a disc. Material degradation was negligible even as compared with previous studies on reusable stereolithography resins. For the Yokohama team, the next step will be adapting resin for large-scale 3D printing and improving heat-response and longevity. If they’re successful, they’ve moved potential colonies on the Moon and Mars one significant step closer towards viability.
Source: Yokohama National University via EurekAlert
