Role of in situ added cellulose nanocrystals as rheological modulator of novel waterborne polyurethane urea for 3D-printing technology

Abstract

Cellulose nanocrystals (CNC) have been studied as rheological modulator in order to improve the printing performance of a novel polycaprolactone-polyethylene glycol (PCL-PEG) waterborne polyurethane urea (WBPUU) based ink for direct ink writing 3D-printing technology. The cellulose nanocrystals are chemically bonded to the WBPU, since they were added during the synthesis of a WBPUU, in order to tune the rheological properties of the ink and thus, improve the printing performances. The WBPUU-CNC inks have been extensively characterized from the rheological viewpoint and have been subsequently used to print different pieces. To determine the capacity of the CNC to improve the printing performance of this type of inks, we establish the relationship between the rheological properties of the inks and their printing viability by an optimal window of compositions of the inks. Additionally, we have measured the mechanical and thermomechanical properties of printed WBPUU-CNC pieces to evaluate the effective reinforcement of in situ incorporated CNC in the WBPUU-CNC inks. The results showed that the addition of CNC accentuates the shear-thinning behaviour, presenting lower tan δ, especially at the lowest CNC contents compared with the matrix, which resulted in inks with excellent printable properties. Moreover, the shape fidelity is increased by the solid-like behavior due to the filler effect of CNC and confirmed by SEM images. Finally, a large increase in Young’s modulus and thermal and thermomechanical stability were observed on the printed pieces that contained only 0.5 wt% CNC compared to the matrix, suggesting the formation of effective interactions between CNC and WBPUU, as corroborated by FTIR, that lead to a greater reinforcement.