Photopolymer resins, which harden (cure) on exposure to light, are commonly used in the manufacture of bespoke 3D printed parts. However, while technologies to improve the resolution of 3D printing and its manufacturing speed have advanced considerably over the past few decades, the resins themselves have not changed much since the 1980s.

The resin components—composed of reactive monomers and/or oligomers containing (meth)acrylates and epoxides—come mostly from petrochemical feedstocks. Although some progress has been made in the use of more sustainable resins derived from biomass, the derivatization of renewable biomass, and the introduction of hydrolytically degradable bonds, the recyclability of these is still limited. This is largely because they rely on irreversible bonds being created when the resin hardens. To break these bonds, additional chemicals have to be added at each stage, resulting in a “snowballing effect” in which the only way to recycle the material is to make more of it.

Currently, there is not a photopolymer resin that can be depolymerized and directly re-used in a circular, closed-loop pathway.

Now, a new type of recyclable resin, made from biosourced materials, has been designed for use in 3D printing applications. Researchers from the University of Birmingham in the U.K. showed that high-resolution, 3D printed structures can be manufactured from an entirely bio-sourced feedstock. Once they have reached the end of their useful life, the products can be recycled within an almost fully closed-loop system.

For the first time, a photopolymer resin has been produced that can be printed at high resolution but can then be broken back down to its constituent parts, recycled, and reprinted, with the addition of just a small amount of photoinitiator to maintain the material’s curable properties.

This work is published in Nature in the paper, “A renewably sourced, circular photopolymer resin for additive manufacturing.

Josh Worch, PhD, assistant professor at the University of Birmingham, tweeted that he was “over the moon” to see this work published. “We disclose a fundamental advance in photoresin technology for vat photopolymerization 3D printing that is congruent with principles of green chemistry and a circular economy.” He added that, “by swapping conventional (meth)acrylate or thiol-ene chemistry with cyclic disulfides, the closed-loop photoresin can be printed and then efficiently depolymerized to regenerate a printable resin. The resin is also 100% bio-derived and has the potential to be scaled.”

The feedstock for the process is made from lipoic acid, a naturally occurring fatty acid molecule that is commonly sold as a dietary supplement. The team made a combination of two monomers from the lipoic acid from which they were able to make a resin that could be recycled either back into the monomers, or right back to the original molecule for recycling.

In the study, the researchers completed two “recycles,” but anticipate that further recycles would be possible.

“Our approach is an important step away from relying on 3D-printable resins made from petrochemicals, which cannot be efficiently recycled,” noted Andrew Dove, PhD, professor School of Chemistry, University of Birmingham. “While we still have improvements to make to the properties of the new resin, this research opens up exciting new avenues for development.”

Uses for the material could include industries where rapid prototyping is used to test products before moving to mass production. Although currently the material is more flexible than might be commonly used in industry, future applications could include medical and dental components.

“Enabling recycling within the light-mediated 3D printing industry is essential since it is a rapidly expanding method for materials production,” noted Worch. “We now have the prospect, with our technology, to help ensure that recycling becomes a built-in feature of 3D printing.”

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