Permeabilities along fiber direction of ramie bundles and through-thickness of ramie fabric stack for liquid composite molding

Natural fiber has been a focus for environmental and recyclable polymer composite. Liquid composite molding process is an attractive manufacturing technique for natural fiber-reinforced polymer composites with high quality and low cost. Understanding the permeability along different directions of fiber preform is important for liquid composite molding to design and optimize mold and processing parameters. This paper addresses issues of the permeabilities along longitudinal direction of ramie fiber bundles and through-thickness direction of ramie fabric stack. Two simple methods were designed to detect axial and transverse infiltration with assistance of external vacuum pressure in ramie bundles and ramie fabric stack, respectively. Different surface chemical treatments, including flame retardant, silane and alkali treatments, were done on ramie fabric. The effects of fiber content, liquid type and surface treating method on the permeability and capillary pressure were studied. The results show that surface treatment obviously changes the surface morphology and surface energy of ramie fiber. The relationships between defined relative velocities of penetration flow and applied pressure for ramie fiber bundles and fabrics perfectly follow linear relationships, indicating that Darcy's law is suitable for describing permeation behavior in ramie fibers. Moreover, fiber content and liquid type, including silicone oil and epoxy resin, significantly impact axial permeability and capillary pressure. Surface treatment significantly decreases the permeability along the thickness direction of ramie fabric stack followed by increasing capillary pressure, which are attributed to the changes of treated ramie fibers in surface energy and morphology. Finally, a unique difference in the permeabilities along axial and thickness directions was pointed out.