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We performed an experimental study to manufacture flax fiber textile reinforced thermoplastic composites by direct impregnation method in a closed mold. To reduce the process time, we adopted compression resin transfer molding resin because of its short flow path during the highly viscous resin impregnation process. Fiber preheating process was introduced before the impregnation process in order to avoid the resin cooling down when contacting the flax fibers. Optimal process temperature conditions such as mold temperature, resin temperature and fiber temperature were experimentally determined while minimizing the effect of thermal degradation of flax fiber and obtaining good impregnation quality. In particular, a sequential pressure cycle was proposed to improve the impregnation quality and to obtain the uniform fiber volume fraction in the final part. The impregnation quality was assessed by the density measurement and by the micrographic image analysis. The mechanical properties of the final parts were comparable with the literature values obtained by the compression molding of matrix-fiber premixed materials such as commingle yarns or film stacking, whereas the process cycle time of was much shorter than the other conventional methods in the literature. [ABSTRACT FROM AUTHOR]

• Ten flax varieties cultivated in two Belgian regions in two years were investigated. • Accumulated temperature at germination and emergence affects the flax fiber yield. • Precipitation during early phase of growth influences tensile strength of fibers. • A couple of flax varieties with good tensile properties were identified. • Pulling date and retting degree are key factors affecting the tensile properties. Flax (Linum Usitatissimum L.) fiber composites serve as sustainable replacement for glass fiber composites. Given that these composites are based on natural sources, their performance is susceptible to variation induced by several factors. Hence, ten flax fiber varieties cultivated in two different regions of Belgium in 2017 and 2018 are compared in terms of their yield and their tensile strength. Statistical analyses are carried out to determine the varietal difference in terms of tensile strength of bundles and of composites. The mechanical tests of flax fibers are carried out in two ways, i.e. dry fiber bundle tests and impregnated fiber bundle tests (IFBT). The results from the two consecutive growth years show their significant influence on the yield of the flax fibers, which are highly dependent on the weather conditions, in particular the precipitation during the growing season. The influence of the date of pulling and the retting degree on the yield of flax fibers and the tensile strength of fiber bundles is also investigated. The early pulling and lower retting degree result in a detrimental effect on the fiber strength whereas the late pulling shows little to no effect on the fiber strength. The difference of flax fiber strength among ten flax varieties over two years can be easily identified using IFBT and a couple of flax varieties exhibit a good tensile strength with low variation. This methodology can be used to select good flax varieties which have high mechanical properties with a small variation, which are suitable for a fiber reinforcement of high performance natural fiber reinforced composites. [ABSTRACT FROM AUTHOR]