A renewably sourced, circular photopolymer resin for additive manufacturing

Additive manufacturing using vat photopolymerization offers rapid fabrication of bespoke 3D-printed parts. However, the resins used, mostly composed of petroleum-derived (meth)acrylates and epoxides, are not recyclable. While some progress has been made using renewable biomass and degradable bonds, true circularity—depolymerization and reuse—remains elusive. This research aims to address this gap by developing a completely renewable and recyclable photopolymer resin. The challenge lies in creating a dynamic covalent bond system that is both easily photopolymerized for high-resolution printing and efficiently depolymerized for reuse. Previous attempts using dynamic covalent bonds have resulted in either inefficient recycling or a change in material composition with each cycle. This work proposes a closed-loop system where the dynamic bond (cyclic disulfide) is formed in situ during photopolymerization and depolymerizes back to the original monomers, enabling repeatable photocuring and printing.

Existing photocurable dynamic networks often rely on open-loop recycling, requiring the addition of extra photoactive resin components in each cycle. This leads to altered material composition and a "snowballing" effect, requiring increasing amounts of new material with each recycling step. Two-dimensional photoset materials based on reversible cycloaddition reactions and thiol-ene chemistry partially address this, but scaling to high-fidelity 3D printing presents significant challenges. The authors review the limitations of these approaches, highlighting issues like complex synthesis, high energy light requirements, incomplete depolymerization, and property changes after recycling. The use of strained cyclic disulfides, like lipoic acid, is proposed as a potential solution due to their ease of radical polymerization and established dynamic behavior, including depolymerization.

Lipoic acid was esterified using renewable isosorbide and menthol to create isosorbide lipoate (IsoLp₂) and menthyl lipoate (MenLp₁), acting as a multivalent crosslinker and reactive diluent, respectively. While the initial synthesis involved EDC coupling and chlorinated solvents, the authors demonstrate a greener alternative using acid-catalyzed Fisher esterification in bulk. The MenLp₁-IsoLp₂ (30:70 wt%) resin demonstrated good ambient stability. Printing was performed using a commercial digital light processing (DLP) printer with a 405 nm light source. Printing resolution and fidelity were assessed using test structures with intricate features. Chemical depolymerization was investigated using a base-catalyzed method involving phosphazene and thiophenol in 2-methyl-tetrahydrofuran (MeTHF). The depolymerization efficiency and recycled resin yield were analyzed using size-exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. To explore the structure-property relationships, the ratio of MenLp₁ to IsoLp₂ was varied, and other renewable alcohols were used to synthesize different lipoates, allowing for tuning of mechanical properties (UTS, Young's modulus, Tg). The depolymerization process was optimized and then applied in a closed-loop 3D printing cycle with catalyst-free thermal depolymerization in DMF. The resulting recycled resin was characterized, and its printability and properties were compared to the pristine resin across multiple cycles. Finally, a method for hydrolytic depolymerization was also developed to regenerate the original alcohols and lipoic acid, enabling a potentially infinite closed-loop system.

The MenLp₁-IsoLp₂ (30:70 wt%) resin demonstrated high-resolution 3D printing capabilities, achieving a minimum feature size of 100 µm. The x-y resolution was excellent, with high agreement between theoretical and printed surface areas. Complex 3D parts were successfully printed. Base-catalyzed depolymerization yielded up to 98% of recycled resin, with NMR analysis showing 85 mol% purity. The recycled resin's composition was comparable to the original, demonstrating efficient depolymerization. Varying the crosslinker-to-diluent ratio yielded a wide range of mechanical properties (UTS, Young's modulus, Tg), demonstrating the platform's modularity. Different renewable alcohols were used to create various lipoates resulting in a broad range of mechanical properties. A catalyst-free thermal depolymerization method in DMF proved efficient and consistent, achieving high yields of recycled resin (up to 97%). A closed-loop 3D printing cycle was successfully conducted with EtLp₁-GlyLp₃ (31:69 wt%) resin, completing two recycling sequences with high yield (91% and 94%) and consistent resin composition and printability. Finally, a hydrolytic depolymerization method allows for the regeneration of the original components, enabling a potentially infinite closed-loop recycling pathway.

This work successfully demonstrates a closed-loop 3D printing system using a completely renewably sourced photopolymer resin. The use of lipoates addresses the critical limitations of current photopolymer resins, offering both renewability and recyclability. The high resolution and fidelity of the 3D prints demonstrate the potential for practical applications. The modularity of the lipoate platform allows for tuning of mechanical properties, widening its applicability. The high efficiency and consistency of the catalyst-free thermal depolymerization method simplifies the recycling process. The development of a hydrolytic depolymerization method offers the potential for an indefinitely repeatable recycling process. The minor differences in composition observed between the original and recycled resins are attributed to a small amount of oligomeric impurities, and further research is focused on eliminating these to improve the process's long-term consistency.

This research establishes a proof-of-concept for circular DLP printing using a renewably sourced photopolymer resin based on lipoates. The platform's modularity, high printing resolution, efficient depolymerization, and potential for indefinite recycling highlight its significant advantages over current petroleum-based alternatives. Future work will focus on eliminating oligomeric impurities to achieve perfect orthogonality in the depolymerization process and further enhance the mechanical properties of the resin to compete with state-of-the-art commercial resins.

While the closed-loop recycling process demonstrated high efficiency over two cycles, further investigation is needed to assess the long-term stability and consistency of the process over many cycles. The presence of minor oligomeric impurities in the recycled resin suggests that complete depolymerization is not yet fully achieved. The current hydrolytic depolymerization method requires further optimization for scalability and practical integration into a closed-loop system. Addressing the practical challenges of waste collection and recycling infrastructure will also be essential for broader adoption.