Methacrylated Kraft lignin as a double-edged sword: a quantitative analysis of Kraft lignin-containing objects printed by stereolithography

Finding an alternative to fossil-based resins which is renewable, sustainable, and affordable is an important step to enable large-scale industrial manufacturing using stereolithography. Lignin has shown some potential to fulfil these requirements. However, examples that contain Kraft lignin, which is produced industrially at a rate of 70 million tons per year, are relatively scarce compared to examples using organosolv lignin or lignin model compounds. In this work, we fractionated and modified Kraft lignin and blended it with a commercially available stereolithography resin produce 3D printed objects. We thoroughly characterized the lignin, the resins, and the 3D printed objects, and conclude that while our modification scheme allows for smooth incorporation of large amounts Kraft lignin into resins (up to 10% (w/w), or 10 times the amount previously demonstrated as possible), and the lignin itself aids in the photopolymerization process (Ec = 662(2) mJ·cm-2 for the base case, Ec = 0.763(1) mJ·cm-2 for resins containing 10% (w/w) lignin), the printed objects are weaker (σ = 20(1) MPa) and more brittle (ε = 1.12(5)%) than the control that contains no Kraft lignin (σ = 46.3(5) MPa, ε = 3.95(9)%). Together, these results demonstrate the importance of lignin modifications to create miscible blends for stereolithography resins but highlight the need for further understanding how these modifications can simultaneously enhance photo- and mechanical properties of printed objects. These are an important first step towards creating 3D printable objects that contain industrially relevant lignin.