Sustainable re-entrant hemp/polylactic acid nonwoven composites with in-plane and out-of-plane auxeticity

Abstract Auxetic composites are unique materials with a negative Poisson’s ratio (NPR), that has been of great interest for various applications. Regardless of their exceptional properties, they require special attention due to their non-biodegradable nature. This work focuses on the use of natural hemp fibers as a reinforcement and biodegradable polylactic acid (PLA) as a matrix for making biodegradable auxetic composites. Hemp nonwovens were prepared by the needle-punching method, followed by adding different content of PLA into the nonwoven to fabricate the composite. The out-of-plane auxetic response of needle-punched hemp/PLA composite was achieved by a heat-compression protocol whereas the in-plane auxetic effect was obtained by creating a re-entrant honeycomb structure. Heat-compressed composites exhibited a prominent auxetic effect upon uniaxial loading resulting in the achievement of an out-of-plane auxetic effect. Composite with 50% PLA showed the highest NPR of—6.5 with a tensile strength of 2.1 MPa. The out-of-plane auxetic effect in the heat-compressed composites is due to the tilting and buckling of vertical fiber columns that tend to reorient themselves back to their vertical position upon the application of uniaxial force. Further increase of PLA content not only decreases the NPR but affects the mechanical properties negatively as the excess amount of PLA creates defects inside the composite. Re-entrant honeycomb composite having a PLA content of 50% showed an in-plane NPR of −0.40. The study provides a new approach for the development of biodegradable composites with out-of-plane and in-plane NPR for various applications of auxetic materials.