# Lignin to Materials: A Focused Review on Recent Novel Lignin Applications
## 5 3D Printing Lignin-Plastic Composites
However, when 10 wt % acrylonitrile butadiene rubber (NBR41) was added to the composite, the chemical and physical crosslinks between the NBR41 and lignin resulted in the improved mechanical properties of the composite relative to common petroleum-based thermoplastics. This composite containing 40 wt % lignin, 10 wt % NBR41, and 50 wt % ABS showed great 3D-printability. In an experiment where organosolv hardwood lignin was blended with nylon, the lignin was found to reinforce the thermoplastic matrix by increasing the stiffness of the structure, leading to increased 3D printability [50]. The melt viscosity was also reduced as a result of blending the lignin, which further increases 3D printability. The proposed mechanism for the bonding in the composite was lignin domains forming hydrogen bonds with the thermoplastic matrix. In addition, a study shows that 20-40% lignin can be used with PLA to form a matrix material for 3D printing. In the study, characterization techniques including thermogravimetric analysis, X-ray diffraction, and scanning electron microscope indicated that lignin is a nucleating agent that increases the crystallization of PLA [52]. This composite material resulted in great extrudability and flowability; it was also observed that lignin did not agglomerate. In another study, alkali lignin and organosolv lignin were acetylated to improve the compatibility with PLA to form a composite [53]. Even though the lignin decreased the crystallization behavior of PLA, it drastically improved the thermal stability of PLA and increased the elongation at break. The acetylated lignin was observed to prevent PLA from undergoing hydrolytic degradation. Besides combining lignin with PLA, other studies have shown the diverse types of plastics that lignin can be blended with to construct effective 3D printing materials. For instance, authors of a study constructed photoactive acrylate resins used for 3D printing from up to 15 wt % acrylate organosolv lignin, resin bases, a reactive diluent, and other compounds [54]. Figure 6 shows the reaction used for lignin acylation to construct resins. Even though the resulting lignin resin had decreased thermal stability compared to the commonly available resin, the resin exhibited characteristics that were favorable for 3D printing. The resin had increased ductility and resulted in 3D prints that were uniformly fused, high-resolution, and tough. Additionally, a study reports that 20 wt % biorefinery lignin from _P. radiata_ combined with polyhydroxybutyrate (PHB) improved the surface quality and reduced the shrinkage of the 3D printing material compared to the PHB alone [55]. Figure 7 below is a flow chart depicting how lignin can be extracted from softwood chips and combined with PHB to construct an improved 3D printing material. The presence of lignin was reported to reduce wrapping by values between 38% and 78% when compared to PHB, used as the sole printing material. This reduction of shrinkage was possibly due to the complex bulk lignin structure in addition to the lack of interfacial tension between the lignin and the polymer matrix. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Lignin Feedstock** & **Polymer Feedstock** &
\begin{tabular}{c} **Lignin Content in 3D** \\ **Printing Composite (wt \%)** \\ \end{tabular} & **Source** \\ \hline Kraft lignin & ABS & 40\% & [49] \\ Organosolv hardwood lignin & Nylon & 40-60\% & [50] \\ Softwood lignin from soda cooking process & PLA & 20\% and 40\% & [52] \\ Alkali lignin and organosolv lignin & PLA & 0.5–20\% & [53] \\ Organosolv lignin & Polymer resins & 5–15\% & [54] \\ Softwood lignin from _P. radiata_ & Polyhydroxybutyrate & 20\% & [55] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the studies on the 3D printing applications of lignin. Figure 6: Reaction to synthesize acylated lignin for the construction of resin. Reprinted with permission from [54]. Copyright (2018) American Chemical Society.