When natural fibres are applied as reinforcement in polymer composites the moisture sensitivity, causing fibre swell and ultimately rotting through fungi attack, can be a very serious problem. A number of methods have been developed dealing with this problem that change the chemical and / or physical composition of the fibre, resulting in reduced moisture sensitivity. To this category belong acetylation and hydrothermal treatment. For acetylation, acetic anhydride is used as a chemical that reacts with reactive OH-groups of the lignocellulose material, increasing hydrophobicity. In hydrothermal treatment no chemicals are used, only water and energy.
The main focus of this paper is on hydrothermal treatment, the Duralin® process in particular, of bast fibres such as flax, jute and hemp. Acetylation is reviewed briefly. A survey is given of the structure and composition of bast fibres and the moisture adsorption and desorption mechanisms in these fibres.
The Duralin® process involves three steps, hydrothermolysis, drying and curing. The raw material for the Duralin® process applied to flax is green rippled flax straw. This eliminates the need for the traditional dew-retting, a risky process where the freshly harvested flax stems lay on the field for about four weeks. The Duralin® process reduces moisture absorption and biological degradation, the fibre yield is higher than after dew-retting and the shives can be used as a filler material in polymers and for making a water proof particle board. The main causes for reduced water uptake after Duralin treatment will be reviewed.
Duralin® fibres have equal or higher tensile strength and higher flexural modulus than fibres extracted from dew-retted flax. Compounds reinforced with these fibres have apart from decreased moisture sensitivity a better mechanical performance. Both the amount and the release rate of decomposition products resulting from compounding with polypropylene are significantly less for Duralin® fibres than for dew-retted or green fibres. Duralin® fibres are on a weight basis competitive with glass fibres.