Natural plant fibres are hierarchically structured materials combining a number of interesting properties, such as exceptional mechanical and thermal properties and biodegradability. The aim of this work was to analyse the physicochemical properties of seagrass fibres (Posidonia oceania) to better understand their suitability as green biopolymers in applications like flame retardancy or fibre composites. The mechanically pretreated, dried seaweed fibers were sequentially washed, sonified, treated with NaOH and HCl, and the influence of the chemical treatment on composition, micromorphology, crystallinity, thermal and mechanical properties was analyzed.
The results of the physico-chemical investigations showed that the alkali treatment caused some of the hemicelluloses and lignin to be dissolved from the fibre, thereby increasing the crystallinity content. The mechanical properties such as tensile strength and elasticity were only slightly lower compared to the untreated fibers, while the modulus of elasticity and tangent modulus were slightly higher. The fibers treated with HCl showed significantly rougher micromorphology, largely degenerated crystalline cellulose, a lower ignition temperature and a shorter burning time during thermo-oxidative degradation. In addition, the content of the elements Mg2+ and Ca2+ decreased, and the values for tensile strength and elasticity were significantly reduced. These results correlated well with a burning test showing an increased flammability and a drastically reduced burning time upon chemical treatment of fibres.
In conclusion, treatment of seagrass fibers leads to significant changes in chemical composition, microstructure, thermal and mechanical properties. For insulation and flame retardancy applications no treatment while for composite materials treatment with NaOH seems to be advantageous to gain good fiber-matrix bonding and preserve good mechanical properties.