Effect of Moisture Cycling Duration and Temperature on the Strengthening and Stiffening of Cycled Flax Fibres
Topic(s) : Material science
Co-authors :
Garing CLARE (BELGIUM), Yasmine MOSLEH (NETHERLANDS), Aart Willem VAN VUUREAbstract :
Natural fiber composites, based on renewable sources such as flax, present environmentally-friendly solutions with notable specific mechanical properties. However, despite their advantages in lightweight design and cost-effectiveness, their inherent hydrophilic nature poses a significant challenge, leading to moisture absorption and subsequent performance degradation. Several studies have investigated various methods to address this issue. Accordingly, this research contributes to this pursuit by examining the impact of moisture cycling on the durability of flax-epoxy composites. Specifically, this study aims to investigate the influence of cycling duration and temperature on the stiffening and strengthening of flax fibers after exposure to high-low humidity cycling, based on previous research.
Figure 1. Tensile strength of moisture-cycled flax fibres.
Figure 2. Tensile moduli of moisture-cycled flax fibres.
This research employs the Impregnated Fibre Bundle Test (IFBT) method to determine fiber stiffness and strength using a fiber volume fraction of approximately 40%. Four moisture cycling protocols are implemented, involving respectively three hours per cycle and four days per cycle, both at a lower temperature of 21ºC and a higher temperature of 60ºC. Flax fibers are subjected to 20 cycles of alternating high (85%) and low (23%) humidities. Analysis of the back-calculated properties from flax-epoxy composites reveals that the applied cycling protocols enhance the tensile strength and modulus of the fibres as shown. Figure 1 illustrates a notable increase in the tensile strength of the fibres after a longer cycle duration (4 days per cycle) while variation in temperature has no significant effect. Figure 2 shows enhancements in tensile modulus but the difference in temperature and cycling duration have no significant influence for this property. The fibres undergoing 4 days of cycling at 21ºC (4D21 fibres) show remarkable improvements in tensile strength (31%), as well as tensile moduli E1 (40%) and E2 (45%) after being subjected to 10 cycles. This fiber improvement is attributed to a phenomenon similar to a hornification effect in wood, which is to be explored further in this study. Various spectroscopic and microscopic analyses are currently being conducted to elucidate the mechanism behind this increase in modulus and strength after moisture cycling. This strengthening and stiffening effect could be due to changes in the distribution and chemistry of the primary constituents of flax, such as pectin, cellulose, lignin, and hemicellulose, accompanied by changes in the microstructure of the flax fibers. Analysis of water uptake also suggests that cycling duration affects the sorption capacity of flax fibers.
Figure 1. Tensile strength of moisture-cycled flax fibres.
Figure 2. Tensile moduli of moisture-cycled flax fibres.
This research employs the Impregnated Fibre Bundle Test (IFBT) method to determine fiber stiffness and strength using a fiber volume fraction of approximately 40%. Four moisture cycling protocols are implemented, involving respectively three hours per cycle and four days per cycle, both at a lower temperature of 21ºC and a higher temperature of 60ºC. Flax fibers are subjected to 20 cycles of alternating high (85%) and low (23%) humidities. Analysis of the back-calculated properties from flax-epoxy composites reveals that the applied cycling protocols enhance the tensile strength and modulus of the fibres as shown. Figure 1 illustrates a notable increase in the tensile strength of the fibres after a longer cycle duration (4 days per cycle) while variation in temperature has no significant effect. Figure 2 shows enhancements in tensile modulus but the difference in temperature and cycling duration have no significant influence for this property. The fibres undergoing 4 days of cycling at 21ºC (4D21 fibres) show remarkable improvements in tensile strength (31%), as well as tensile moduli E1 (40%) and E2 (45%) after being subjected to 10 cycles. This fiber improvement is attributed to a phenomenon similar to a hornification effect in wood, which is to be explored further in this study. Various spectroscopic and microscopic analyses are currently being conducted to elucidate the mechanism behind this increase in modulus and strength after moisture cycling. This strengthening and stiffening effect could be due to changes in the distribution and chemistry of the primary constituents of flax, such as pectin, cellulose, lignin, and hemicellulose, accompanied by changes in the microstructure of the flax fibers. Analysis of water uptake also suggests that cycling duration affects the sorption capacity of flax fibers.