Micro-braiding and co-wrapping techniques have been developed over a few decades and have made important contributions to biocomposites development. In this present study, a set of flax/polypropylene (PP) micro-braided and co-wrapped yarns was developed by varying different PP parameters (PP braiding angles and PP wrapping turns, respectively) to get different flax/PP mass ratios. The effects on textile and mechanical characteristics were studied thoroughly at the yarn scale, both dry- and thermo-state tensile tests were carried out, and tensile properties were compared before and after the braiding process to study the braidabilities. It was observed that PP braiding angles of micro-braided yarn influenced the frictional damage on surface treatment agent of flax roving, the cohesive effect between PP filaments/flax roving, and the PP cover factor; PP wrapping turns of co-wrapped yarn had a strong impact on the flax roving damage and the PP coverage, which further influenced the characteristics. Micro-braided yarn and co-wrapped yarn with the same flax/PP mass ratio were compared to evaluate the two different hybrid yarn production techniques; it was proven that micro-braided yarn presented better performance.

The identification of thermomechanical in-plane shear behavior of preform is one of the most important factors to ensure the quality of the thermoplastic composites during the thermoforming process. In this present work, the non-symmetric in-plane shear behavior of flax/polypropylene 2D biaxial braided preform for thermoplastic biocomposites was characterized at elevated temperature chamber by using bias-extension test. Analytical models of a bias-extension test based on non-symmetric unit cell geometry for 2D biaxial braids were defined and applied; the thermo-condition-dependent experiments were conducted to study the temperature and displacement rate dependences. The influence of unit cell geometry parameters including braiding angle, tow waviness, and cover factor on the thermal in-plane shear behavior was deeply invested, experiments in both axial and transversal directions were performed for a complete study, and asymmetric scissor mechanisms for in-plane shear behavior were introduced and studied. Finally, a simulation of thermal impregnation distribution based on unit cell geometry was made to clarify the importance of the overall fiber volume fraction.

With the growing popularity of hybrid yarn techniques, the micro-braided yarn is becoming a good choice as one kind of intermediate materials for thermoplastic biocomposites, by presenting favorable morphology during the preform process and lowering the resin flow distance during the thermo-compression process. In this article, different flax/polypropylene (PP) based multi-core micro-braided hybrid yarns with the similar total number of flax core fibers were manufactured, by varying the parameters: multi-core configuration and braiding angle; both dry-and thermo- states tensile tests on yarns were carried out, since it is necessary to simulate the deforming behavior of a single hybrid yarn during the thermoforming process. The objective is to determine the cover factor especially for multi-core micro-braided yarn as a comprehensive textile indicator; and to study the influence of the cover factor parameters on mechanical tensile properties at the yarn scale. It has been observed that the cover factor parameters contributed the braider effect (friction and compression) on flax cores in the dry-state and lubricant effect (distribution and viscosity) in the thermo-state; further influenced the characterizations. Increasing multi-core configuration and braiding angle can both increase the tensile strength; larger cover factor results in greater tensile stiffness both in dry-and thermo-states.