3D printable lignin-caprolactone material†

Abstract

The use of lignin in three-dimensional (3D) printing materials has been considered a viable strategy to generate sustainable 3D printing objects. However, complex molecular structures, high viscosity, and charring of lignin impair its 3D printability. This study investigated the synthesis of lignin-caprolactone polymer and its fused deposition modeling (FDM)-3D printing performance. Lignin-caprolactone polymerization was carried out with ethanol-soluble fractionated birch alkali lignin (L_E) and caprolactone (CL). Results showed that ethanol fractionation reduced lignin's molecular weight from 22 870 to 3827 g mol⁻¹ and increased its hydroxyl group concentration. The melt temperature, viscosity, and polymerization degree were considered in the Box-Behnken surface approach to obtain lignin-caprolactone with the best results. Compared to unfractionated lignin caprolactone (LPO), fractionated lignin-caprolactone polymer (L_EPO) had a 10.9% higher grafting ratio, 69.43% rise in melt temperature (T_m), and 85.71% increase in glass transition temperature. The melt rheological investigation showed that L_EPO's lower viscosity (160.9 Pa s) and shear-thinning behavior than those of LPO made it more suitable for the 3D printing application. The 15 °C delay in G′ and G′′ crossover points of L_EPO compared to LPO improved 3D printing adhesion layers. Furthermore, L_EPO exhibited superior mechanical characteristics and a greater water contact angle (92°) than LPO. The reduction in molecular weight distribution of lignin (due to ethanol fractionation) prior to copolymerization facilitated the production of a 3D-printable polymer containing 75% lignin. By tailoring the melt and viscosity parameters of the lignin-caprolactone copolymer, the lignin-copolymer exhibited improved 3D printing performance, which offers advantages over lignin composite 3D printing.