Nanofiber Applications for Lightweight, Thin Thermal Insulation

Electrospinning and new melt-spun air jet technology were used to produce novel nanofiber-based inorganic insulation material. NSRDEC used a transdisciplinary approach to investigate factors affecting the fundamental mechanisms of heat transfer in these nanofibrous materials. High efficiency, light weight, and thin thermal insulation materials will decrease the weight and bulk of current military thermal insulation battings to provide lightweight and low bulk cold weather clothing systems, sleeping bags, and tent and shelter liners. This will significantly enhance the Warfighter comfort and mobility, thus materially improving the Army capability. New thermally efficient thermal insulation material will also contribute to the national welfare by increasing energy efficiency and reducing the weight and bulk of thermal insulation layers for a variety of applications, including apparel, vehicles, food service equipment, and shelter. Key Findings: Electrospun nanofibers in the as-spun condition are not useful for high-loft thermal insulation by themselves, but may be useful as components in hybrid battings with high bulk densities. Fibers less than 1 μm in diameter are not thermally efficient at low fiber volume fractions. Performance gains in existing vacuum insulated material are possible by incorporating a proportion of nanofibers into the structure, but large diameter fibers are necessary for durability and compression recovery. Modification of fiber infrared radiation properties may also improve the thermal insulating efficiency of nanofiber layers. Both techniques involved in this process (pyrolyzing polymeric nanofibers and use of carbon melt-blown fibers) produce nonflammable fibers. Accomplishments: This is the first scientific research on radiation and natural/forced convection in electrospun nanofibrous porous media; the key innovation of the NSRDEC nanofiber thermal research was focusing on the radiative properties of the fiber itself. A series of carbonized thermal insulation materials (an additional processing step), created from the original polyacrylonitrile electrospun mats, resulted in a large improvement in the thermal insulation properties. Another approach produced a composite structure that incorporated nanofibers, but relied on larger diameter polyester fibers to support the nanofibers and provide better compression recovery for the composite samples. A 10% by weight add-on level of carbon nanofibers resulted in a significant increase in thermal insulating efficiency of the continuous filament polyester insulation. NSRDEC research program has helped to pioneer a fundamental understanding of the dominant heat transfer mechanisms (and effectiveness) of electrospun nanofibers for thermal insulation applications at moderate temperatures. It is anticipated that nanofiber battings will decrease the weight and thickness of current military thermal insulation layers to provide lightweight and low bulk Army cold weather clothing systems, sleeping bags, and tent and shelter liners. Additionally, this high-efficiency lightweight thermal insulation may have fire-retardant or fireproof properties because the processes used produce nonflammable fibers.