Microstructure and mechanical properties of bark cloth extracted from Milicia excelsa: A promising naturally occurring fabric for use in composite materials?

Abstract

This study evaluated the potential of iroko ( Milicia excelsa ) bark waste from Cameroon's tropical forests as a bio-based resource for engineering applications, particularly for use as fibrous reinforcement in composite materials. Microstructural characterisation of the iroko bark waste revealed that the inner part consisted of a layer, the secondary phloem, which was extremely rich in fibres and sap in the form of latex particles. An extraction process was then used to separate the phloem and remove the sap from the rest of the bark. The microstructure of the resulting bark cloth was characterised using synchrotron X-ray microtomography and scanning electron microscopy. Micrographs revealed its intricate, trellis-like fibrous architecture. Thermal, hygroscopic and mechanical properties were characterised by tensile, bending and fibre bundle tensile tests. These tests revealed significant anisotropy in the bark cloth, with high longitudinal stiffness and strength. The mechanical behaviour was interpreted using a simple wavy fibre bundle network model incorporating key microstructural descriptors and fibre-scale mechanical properties. The properties of the individual fibre bundles were comparable to those of other plant fibres, such as hemp, which are already used in the composites industry. This naturally occurring fabric, derived from forest residues, shows promise as a reinforcement material in composites.