Mechanical Properties of Flax-Polypropylene Composites from Dry Flexible Towpregs: Influence of Radial Position of Flax Fibers in the Towpreg

Abstract

The radial position of the flax fibers as reinforcement and polypropylene (pp) fibers as matrix was altered using friction spinning in three distinctive sequences from core to sheath, namely, (a) pp-flax-flax-pp (PFFP), (b) flax-pp-flax-pp (FPFP), and (c) flax-flax-pp-pp (FFPP). Interestingly, at the towpregs stage, PFFP demonstrated higher tensile characteristics than FPFP and FFPP. The towpregs were consolidated to yield three unidirectional composites (UDCs), namely, UDC-PFFP, UDC-FPFP, and UDC-FFPP, using compression molding. The tensile strength of the composites ($${{\rm{\sigma}}_{{\rm{UDC}} - {\rm{FFPP}}}} \gt {\rm{}}{{\rm{\sigma}}_{{\rm{UDC}} - {\rm{FPFP}}}} \gt {\rm{}}{{\rm{\sigma}}_{{\rm{UDC}} - {\rm{PFFP}}}}$$) was contrary to the breaking load (f) of the corresponding towpregs ($${f_{{\rm{FFPP}}}} \lt {\rm{}}{f_{{\rm{FPFP}}}} \lt {\rm{}}{f_{{\rm{PFFP}}}}$$) which necessitated the current investigation. Remarkably, the UDC with the lowest tensile as well as interlaminar shear strength i.e. UDC-PFFP, yielded the maximum energy absorption in Charpy and Notch impact tests. In the towpreg, the flax fibers when placed at the core position, slid over each other resulting in poor strength in FFPP while post-curing the pp matrix impregnated the core flax fibers in UDC-FFPP, resulting in the highest strength amongst the three UDCs in tensile and flexural modes. Micro-computed tomography (µCT) was carried out to confirm the same. Thus, the user can select the requisite fiber-matrix radial positions within towpreg to suit the ultimate applications.

Keywords

• flexible towpreg
• natural flax fibers
• radial position
• damage mechanics
• mechanical testing
• micro-ct