Co-authors​ :

 Geoffrey GINOUX (FRANCE), Xikun WU , Chaimae LAQRAA , Damien SOULAT (FRANCE), Samir ALLAOUI (FRANCE) 

The industrial development and the need in performance for high-end applications require more and more petroleum-based materials, while environmental issues are rapidly growing. Therefore, the use of partially or fully bio-based composites with high mechanical performance is of high interest to replace existing petroleum-based materials. In addition, additive manufacturing (AM) has grown as a significant trend thanks to its advantages, namely flexible design, functional manufacturing, and waste minimization over conventional methods [1]. More recently, continuous AM of vegetal fibers-based composites is a promising solution for lightweight and multifunctional parts due to the freedom in the structure design and the use of high-performance materials [2]. However, this technology is limited by the range of commercially available materials and by the impregnation quality of the vegetal fibers that prevents the development of such materials for industrial applications [3].
This study aims to develop new additively manufactured bio-based materials with high performance thanks to an adapted preparation method of the input materials, namely by commingling of vegetal fibers with thermoplastic fibers, to improve the impregnation of the continuous reinforcement in the end materials. For this purpose, commingled hybrid hemp/PLA yarns were used in continuous yarn coating (CYC) AM to produce hemp yarn-reinforced bio-composites (HYRB) by comparison to the use of pure hemp yarns. Microstructure, composition formulation and mechanical behavior were then assessed by X-ray micro-computed tomography (X-μCT), solvent extraction, tensile test, and digital 3D microscopy to evaluate the materials-process-properties relationships.
X-µCT and solvent extraction showed lower hemp fiber fraction but greater impregnation with the HYRB prepared with commingled yarns, compared to the pure hemp yarns, which is attributed to the addition of PLA fibers in an intimate mixing during commingling. In addition, reduced void content, more aligned fibers, more homogeneous distribution, and better dimensional accuracy were observed with the specimens prepared with the commingled yarns. This led to a significant enhancement of the mechanical properties during tensile loading with materials going from pseudo-ductile to brittle behavior as impregnation increased due to greater stress transfer with a more developed hemp/PLA interface.
This original commingling/AM method can provide bio-based and continuous reinforced-composites with enhanced quality and performance, close to those obtained with conventional manufacturing processes. The findings of this work are thought to be transposable to other vegetal yarn-based composites to increase the range of available materials and to develop the technology towards high-demand applications that require functionality.