Design, synthesis and characterization of inks based in waterborne polyurethane urea dispersions suitable for direct ink writing 3D-printing.

Abstract

A series of waterborne polyurethane urea dispersions were prepared to study their viability as inks for direct ink writing 3D-printing. In this context, waterborne polyurethane urea dispersions were successfully synthesized using a hydrophilic polyethylene glycol (PEG) and a hydrophobic polycaprolactone (PCL) as soft segment to ease the physical gelation of the inks. The PCL/PEG ratio as well as the molar mass of the PEG has been modified and the resulting dispersions, as well as the films prepared from the dispersions, were characterized from the physicochemical, mechanical, thermomechanical and morphological viewpoints to select the better formulation that fulfils with the required end-use properties, particularly for 3D-printing process. Once the selection of the composition of the waterborne polyurethane urea was stablished, inks with different solid content were prepared, analysed by means of rheology and subsequently printed so as to establish relationships between the rheological behaviour and the 3D-printing performances.To pilot the rheological behaviour and the printing performances, cellulose nanocrystals were used as viscosity modulator. Different cellulose contents were added to the waterborne polyurethane urea ink using two different methods called the in situ and the ex situ methods which differ in the moment of the addition of the nanoentities. The composites prepared from both methods were analysed from the rheological viewpoint and printed in the aim to select the best compositions, as well as to compare both incorporation methods. Additionally, the physicochemical, mechanical and thermomechanical properties of the printed pieces were studied to ensure the successful reinforcement of the cellulose nanocrystals into the composites. It was seen that the different addition methods resulted in different disposition of the cellulose nanocrystals leading to the obtain of nanocomposites presenting different rheological, mechanical and thermomechanical properties.Two potential applications were studied to demonstrate their viability. In the first one, shape customized scaffolds were prepared via 3D-printing and freeze drying using a previously developed waterborne polyurethane urea ink with in situ addition of different contents of cellulose nanocrystals, as well as crosslinking by immersion in CaCl2 before freeze drying. The morphology, mechanical and physicochemical properties of the prepared scaffolds were evaluated and tested as an absorption material against cationic methylene blue dye presenting an interesting absorption capacity. For the second application, inks based on a waterborne polyurethane urea with Salvia extracts incorporated by in situ method and different contents of ex situ added cellulose nanocrystals were developed in order to prepare scaffolds with antimicrobial properties. The antimicrobial activity of the scaffolds was tested successfully against Gram positive Staphylococcus aureus and Gram negative Escherichia coli. Additionally, the influence of the inclusions of Salvia extracts on the rheological properties of the inks and the properties of the dispersion were studied.