Extraction optimisation for and hygroscopic behaviour of flax fibres in composite applications

Flax fibres (Linum Usitatissimum) combine excellent mechanical properties with a very low density. In terms of specific stiffness they can compete with E-glass fibres. This is why they are currently used in lightweight applications such as composites. However, there are two main factors inhibiting their breakthrough in this market:

1. The cost of flax fibres compared to glass fibres remains high due to the energy and labour-intensive production process of the fibres and limited fibre yield.

2. Flax fibres are sensitive to moisture absorption. This makes composite producers are wary to use these fibres in their applications as they are worried about the implication of this absorption on the short- and long-term composite properties. This raises the question on the durability of these materials, especially in conditions where cyclic variations of relative humidity are expected.

This dissertation therefore aims to identify the critical parameters and fibre characteristic needed to obtain fibres which can be used in composites and reveal the effects of moisture during static and dynamic hygroscopic loading.

It was found that fibre fineness has a large influence on the transverse composite properties. In the current production process, this parameter is controlled in part by the degree of retting of the stems but largely by scutching and hackling. For coarse fibres, even a mild combing operation is sufficient to drastically increase transverse performance of the composites. The degree of retting impacts the longitudinal tensile response of the composites. A higher degree of retting lowers the modulus of the fibres, except in the initial elastic response region. The fibre variety nor the purity have a direct effect on the mechanical performance of the fibres.

In the second part of this thesis kinetic and equilibrium parameters of the moisture absorption and desorption process were collected and connected to the microstructure of the fibre and composites. It was established that the diffusion behaviour of flax fibres both in- and ex-composite was Fickian in absorption. During desorption, deviations from this behaviour were found when equilibrating to extremely low relative humidity. The diffusion coefficients were found to be dependent on the moisture concentration of the fibres. Furthermore, the longitudinal and transverse mechanical properties of flax fibre composites decrease as their moisture content increases. An exception is the longitudinal tensile strength which increases, due to the plasticizing effect of water. To link these observations to the microstructure, the fibres were chemically modified aiming at altering their moisture behaviour. From this a theory is proposed which describes the diffusion process in technical flax fibres.

Finally, the effect of hygroscopic fatigue of the composites was examined. After very few cycles, flax fibres composites show extensive debonding and technical fibre splitting. These damage modes severely impact transverse performance of the composites.