Aart Willem Van Vuure

The escalating environmental concerns associated with the non-renewable nature of petrochemical-based composite constituents have accelerated the development of sustainable and renewable alternatives. This study evaluates the potential of woven hemp-reinforced polyfurfuryl alcohol (PFA, furan) composites as fully bio-based composite materials. The use of PFA, a fully bio-based resin renowned for its high rigidity and fire-retardant properties, has been hindered by challenges associated with water vapour evolution, acid-catalysed fibre degradation, and porosity formation. Novel woven long hemp fibres were combined with a polyfurfuryl alcohol resin formulated with a mild acid catalyst, while an early-stage venting procedure during compression moulding was investigated to mitigate porosity and fibre degradation. Thermogravimetric analysis was used to determine the venting moments during the moulding cycle. Despite the limited improvement in porosity reduction achieved through the investigated venting strategy, the hemp balanced satin 6/6 fabric–furan composites exhibited commendable stiffness with a maximum modulus of 17.0 ± 0.4 GPa. However, the inherent brittleness of the resin and possibly the presence of fire-retardant fillers limited the tensile strength (a maximum of 71.8 ± 4.2 MPa) and failure strain (a maximum of 0.72 ± 0.07%). The bending properties of neat furan resin produced using an improved curing protocol were comparable to those of conventional thermoset resins, with a modulus of 3.2 ± 0.2 GPa, strength of 110.5 ± 17.3 MPa, and failure strain of 4.1 ± 0.8%. Although several challenges remain, this study demonstrates the potential of natural fibre–furan composites for high-performance natural fibre composite applications and provides guidance for their further development. Future research should focus on optimising venting strategies, avoiding fire retardants to minimise resin brittleness, incorporating matrix tougheners, and enhancing the inherent toughness of the matrix.

Global warming necessitates the development of bio-based materials, with natural fibre composites offering a carbon-negative solution for large-volume applications such as civil engineering structures. However, their hygro-thermal durability, particularly moisture sensitivity, remains a challenge. This research investigated the moisture aging, fatigue, and creep behaviour of natural fibre composites, alongside strategies to enhance their long-term performance. Experimental studies involved cyclic moisture loading, immersion testing, and outdoor exposure, often combined with mechanical fatigue and creep tests conducted in controlled climate chambers. Results indicate that composite stiffness decreases with increasing humidity, with the extent of reduction (e.g., up to 40% in poorly bonded systems) highly dependent on the matrix and fibre-matrix adhesion. Fatigue behaviour of natural fibre composites was comparable to glass fibre systems, with stiffness tending to stabilize or even increase over fatigue cycles, a distinct characteristic. Creep studies revealed that the secondary creep rate is predictive of failure time, and time-temperature superposition enabled lifetime predictions, with estimates reaching 142 years under specific conditions. Good fibre-matrix adhesion consistently limited creep, with plasma treatments demonstrating reduced creep strain. Durability improvement strategies explored include using lignin-rich fibres like bamboo to reduce water uptake, applying enzymatic treatments to lower water absorption and diffusion, and utilising pre-swollen fibres during manufacturing to mitigate initial swelling-induced damage and improve mechanical property retention during cyclic moisture exposure. While immersion testing typically showed property degradation, cyclic moisture tests sometimes revealed an initial property decrease followed by an improvement, termed "hornification." Overall, enhancing fibre-matrix adhesion and employing pre-swollen fibres show significant promise for improving durability. Future work will investigate barrier coatings to further prevent component leaching and improve long-term performance.

This study investigates unidirectional flax fibre composites with epoxy (EP) and partially bio-based unsaturated polyester (UP) matrices conditioned to three humidity conditions (20%RH, 50%RH, and 85%RH) to assess how the resin system influences mechanical performance under hygroscopic exposure. Specimens were subjected to quasi-static tension, flexion, and tension-tension fatigue. Moisture uptake was observed to strongly influence both the quasi-static and fatigue behaviour of the flax fibre composites. Increasing the relative humidity from 20% to 85% led to a reduction in both tensile and flexural modulus, with decreases of approximately 12% for EP-Flax and about 24% for UP-Flax. The modulus measured during fatigue decreased by approximately 25% for both composites with increasing humidity. Despite this reduction in modulus, the fatigue life of EP-Flax increased, likely due to the approximate 20% rise in strength measured under higher humidity. This behaviour at higher levels of humidity is attributed to an increased plastic deformation, as indicated by the loss factor, hysteresis energy, and residual (permanent) strain. The resulting matrix plasticisation while reducing modulus can promote stress redistribution and compliance, thereby delaying failure. Furthermore, microscopic analyses revealed humidity-induced degradation of the fibre-matrix interface, with longer fibre pull-out lengths and more internal fibre cracking at high humidity.

This study investigates unidirectional flax fibre composites with epoxy (EP) and partially bio-based unsaturated polyester (UP) matrices conditioned to three humidity conditions (20%RH, 50%RH, and 85%RH) to assess how the resin system influences mechanical performance under hygroscopic exposure. Specimens were subjected to quasi-static tension, flexion, and tension-tension fatigue. Moisture uptake was observed to strongly influence both the quasi-static and fatigue behaviour of the flax fibre composites. Increasing the relative humidity from 20% to 85% led to a reduction in both tensile and flexural modulus, with decreases of approximately 12% for EP-Flax and about 24% for UP-Flax. The modulus measured during fatigue decreased by approximately 25% for both composites with increasing humidity. Despite this reduction in modulus, the fatigue life of EP-Flax increased, likely due to the approximate 20% rise in strength measured under higher humidity. This behaviour at higher levels of humidity is attributed to an increased plastic deformation, as indicated by the loss factor, hysteresis energy, and residual (permanent) strain. The resulting matrix plasticisation while reducing modulus can promote stress redistribution and compliance, thereby delaying failure. Furthermore, microscopic analyses revealed humidity-induced degradation of the fibre-matrix interface, with longer fibre pull-out lengths and more internal fibre cracking at high humidity.

Natural fibres like flax and hemp are gaining traction to provide a sustainable alternative for particularly glass fibres in structural applications, also outdoor. Three flax fibre bridges have already been constructed and wind turbine blades are targeted. To meet the requirements and provide sufficient longevity, research has focused on understanding and improving the durability of these materials in moist conditions. For well bonded matrices, static stiffness reduction at the winter outdoor condition of 85%RH is limited to about 20%. Static tensile strength improves at high humidity. The fatigue behaviour of well bonded flax fibre composites is competitive with glass fibre composite. Accelerated hygrothermal creep can be modelled by time-Temperature superposition. Creep can be accommodated by applying pre-creep after production. Strategies to enhance hygrothermal durability include not drying fibres before composite part production and improving fibre-matrix adhesion. Future research will include developing better barrier top coatings for natural fibre composites. When flax fibre composites are aged by moisture cycling only, their properties actually improve over time. For simulating outdoor ageing, water immersion seems quite predictive, because of the inclusion of the effects of leaching; a good coating would prevent the latter. This paper summarises the findings of eight PhD theses, conducted in the Sustainable and bio-based composites group at KU Leuven.