Down feathers are remarkable insulating materials that reputedly have the greatest warmth-to-weight ratio of all natural fibres, and they possess excellent compressibility and compression recovery. Despite their outstanding performance as thermal insulators, down feathers have been relatively overlooked by the academic community and their structure and properties remain quite poorly understood. To provide insight into the fundamental properties of these feathers and to inspire the design and development of future synthetic insulation materials, a study into the structural, mechanical, and thermal properties of down feathers and their assemblies has been conducted.
The appearance, mass, size, and geometric shape of goose, duck, and eider down plumes and their barbs were assessed. While goose and duck down plumes were very similar in both appearance and size, the eider down feathers were found to be larger and their barbules had a greater number of prongs and nodes. In each type of down, barbules were adapted to occupy maximum space and utilised a planar cross-section as they divided from the barb to optimise compression recovery. The microstructure and morphology of goose and duck down barbs and barbules were analysed using SEM, TEM, and AFM and were found to be analogous to wool fibres. Melanin granules were found in goose down but not in duck down, and the examined goose down barbs had irregular, hollow cross sections, whereas duck down’s were solid and more elliptical. With the help of novel down-based nonwovens developed to capture X-ray diffraction data from goose and duck down with excellent clarity, goose and duck down were found to share great similarities in their crystal structures.
The mechanical properties of goose, duck, and eider down feathers were studied, including a comparison of their barbs’ tensile properties and the compression resistance and recovery of individual down plumes. Eider down barbs were found to have greater Young’s moduli, ultimate strength, and strain at break than goose or duck down barbs, and individual eider down plumes were also more compression resistant than those from geese and ducks.
The compression resistance and recovery of goose and duck down assemblies were studied using a novel apparatus. Goose down proved to be more compression resistant than duck down, attributed to its cross sectional shape and hollow geometry. The thermal resistances of goose and duck down-filled test squares were extremely similar, but the densities of the down inside the face fabrics strongly influenced their thermal resistances.
Air-laid and thermal-bonded down-feather-based nonwovens were developed to alleviate the problem of constraining individual down feathers in insulated products. Their structural and thermal properties were evaluated and they possessed industry-leading warmth-to-weight ratios, which could be further improved by better engineering of these composite materials.
Great efforts have been made to provide a comprehensive investigation into the structure and properties of both individual and bulk down feathers in relation to their thermal insulation properties. It is hoped that this research will prove useful to the development of superior biomimetic synthetic insulations as well as the high performance products made from down, a world-leading natural material.
