Bio-based resins could offer recyclable future for 3D printing

In a study, published today [15 May 2024] in Nature, researchers from the University of Birmingham 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.

Photopolymer resins, which harden -- or 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 speed of manufacture have advanced considerably, the resins themselves have changed very little since the process first emerged in the 1980s.

The basic materials -- usually epoxies or acrylics -- come mostly from petrochemical feedstocks. Although some progress has been made in the use of more sustainable resins derived from biomass, the recyclability of these is still limited, 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.

In contrast, the Birmingham-led team has, for the first time, succeeded in producing a photopolymer resin 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.

Lead researcher Professor Andrew Dove said: "Our approach is an important step away from relying on 3D-printable resins made from petrochemicals, which cannot be efficiently recycled. While we still have improvements to make to the properties of the new resin, this research opens up exciting new avenues for development."

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.

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 automotive parts, medical and dental components, and even jewellery design.

Co-lead researcher Assistant Professor Josh Worch said: "Enabling recycling within the light-mediated 3D printing industry is essential since it is a rapidly expanding method for materials production. We now have the prospect, with our technology, to help ensure that recycling becomes a built-in feature of 3D printing."

University of Birmingham Enterprise has filed a patent application covering the resin and its use in 3D printing.