Optimal design of flax fiber reinforced polymer composite as a lightweight component for automobiles from a life cycle assessment perspective

The present study combines the generalized rule-of-mixture (ROM) model and the Ashby material selection method for the life cycle assessment (LCA) of flax fiber reinforced polymers (FRPs) and glass FRPs (GFRPs). The ROM model allows life cycle environmental impact predictions according to specific parameters of flax FRPs such as fiber format, volume fraction, manufacturing technique, and load-bearing capacity. The comparisons applied in this study are constructed on two common composite structures: mat panels and injection molded struts with equal stiffness and strength as the design criteria. On the one hand, the parametric LCA predicts that the equal strength design criterion for flax FRPs contributes to consistent mass increases, subsequently resulting in higher life cycle environmental impacts compared to the reference GFRPs; on the other hand, under the equal stiffness criterion the flax mat polypropylene (flax mat-PP) film helps with mass reduction in reference to the glass mat-PP composite, leading to the 20–50% life cycle environmental impact reductions for most impact categories. The subsequent evaluation of the influences of the fiber volume fraction on flax FRPs shows different patterns. For the short flax fiber-PP composite, a steady decrease of the life cycle CO2 emissions can be observed with the increasing fiber volume fraction. However, for the flax mat-PP composite, depending on the tensile modulus of the flax fiber, the optimal volume fractions of the fiber change from 28 to 32% v/v, whereby the lowest life cycle greenhouse gas (GHG) emissions can be achieved.