3D printing of polymeric composites with cellulose micro-fibrils

The thesis is focused on the production of a 100% bio-based composite, obtained by reinforcing a matrix consisting of a thermosetting resin, poly-furfuryl alcohol (PFA), with hemp micro-fibrillated cellulose (MFC). The material is used for the production of a three-dimensional final object by 3D printing process. Cellulose micro-fibrils are crystalline filaments (diameter of 10-100 nm and length of the order of µm) contained in vegetable fibers with excellent mechanical properties; the common extraction methods (fibrillation) however require a significant amount e of energy and are followed by several steps, so that the production of MFC is not yet affirmed on the industrial scale. In this work the MFC is produced from hemp vegetable pulp from sustainable cultivation, by two pre-treatments (mechanical refining and enzymatic hydrolysis) and then fibrillation using an innovative process, a twin-screw extrusion: the product, with a concentration higher than 30% wt, has good quality and the process requires less energy consumption compared to common processes. The quality and properties of the MFC were evaluated through morphological analysis (MorFi®), the turbidity measurement of aqueous suspensions, optical microscopy and the production of films; these were then characterized by traction tests and transmittance measurement. The hemp crystallinity index was also assessed before and after the process steps and compared with that of the eucalyptus fibers. The MFC has been successfully incorporated into the resin from a minimum content of 5% wt to a maximum of 35% wt, obtaining a good dispersion. The rheological characterisation of the resin showed that the addition of MFC changes the Newtonian behavior into a pseudoplastic response (necessary for 3D printing). The pseudoplastic composite revealed to be printable and the degree of dispersion of the MFC achieved was sufficient to avoid the clogging of the 0.76 mm diameter nozzle of the printer. The printed object, a 2cm x 2cm cube, mintained its shape at the end of the printing process. Further printing tests are currently conducted to identify the most suitable operating parameters to improve the shape and the dimensional resolution of the printed objects.