Application of Nanofiber Technology to Nonwoven Thermal Insulation

Nanofiber technology (fiber diameter less than 1 micrometer) is under development for future Army lightweight protective clothing systems. Nanofiber applications for ballistic and chemical/biological protection are being actively investigated, but the thermal properties of nanofibers and their potential protection against cold environments are relatively unknown. Previous studies have shown that radiative heat transfer in fibrous battings is minimized at fiber diameters between 5 and 10 micrometers. However, the radiative heat transfer mechanism of extremely small diameter fibers of less than 1 micrometer diameter is not well known. Previous studies were limited to glass fibers, which have a unique set of thermal radiation properties governed by the thermal emissivity properties of glass. We are investigating the thermal transfer properties of high loft nanofiber battings composed of carbon fiber and various polymeric fibers such as polyacrylonitrile, nylon, and polyurethane. Thermal insulation battings incorporating nanofibers could decrease the weight and bulk of current thermal protective clothing, and increase mobility for soldiers in the battlefield. INTRODUCTION We are addressing the mechanisms of heat transfer through fibrous insulation where the fiber diameter is less than 1 micrometer (μm). The thermal insulating efficiency of fiber-based insulation is known to increase as the fiber size is reduced. Recent advances in the technology of producing nanofibers have revealed a gap in our knowledge about the heat transfer behavior of low-density nanofibrous layers. Radiative transfer through beds of fibers where the fiber diameter is much less than infrared wavelengths is not well understood, either theoretically or experimentally. Understanding heat transfer through nanofiber structures will allow us to exploit the unique properties of polymer nanofibers for applications such as improved military cold weather clothing and hand wear, sleeping bags, and tent liners, as well as applications for military food service refrigeration and storage equipment. Note: Nanofibers are generally defined in the U.S. textile industry and Japanese and Korean strategic research initiatives as fibers of less than 1 μm in size. This is in contrast to the National Science Foundation current definition of nanotechnology, where structures are less than 0.1 μm in some critical dimension. Convective heat transfer (heat carried by gas flow) through nanofiber beds is qualitatively different when fiber diameters begin to approach the mean free path of air molecules (≈100nm). We are investigating models and experimental verification of the influence of slip flow on convective heat transfer in nanofiber beds. However, radiation heat transfer is the most challenging aspect of this problem. There are deficiencies in the classical approaches to treating thermal radiative transfer through nanofibrous layers, especially when the fibers are composed of polymers, conductive materials (such as carbon), or contain nanoparticulate fillers that act as infrared absorbers or emitters. The greatest challenge in this work is to reconcile radiation models with experimental measurements of heat transfer through nanofiber insulation materials. APPROACH Heat transfer through porous media consists of conduction, convection, and radiation. Many practical applications focus on fibrous materials that have a low fiber volume fraction (less than 10% fiber for the most part). Lightweight and compressible insulation materials maximize insulating value at a minimum weight. For these types of materials, heat conduction through the solid portion of the matrix (the fibers) is negligible, so it is not necessary to focus on solid conduction heat transfer. However, Journal of Engineered Fibers and Fabrics http://www.jeffjournal.org Volume 2, Issue 2 2007 32 conduction through the still air trapped within the insulation is important, and the thermal conductivity of air, total gas volume fraction, and thickness of air within the material is required to properly analyze both radiation heat transfer and convection heat transfer mechanisms. Radiation Heat Transfer Research on glass fiber insulation circa 1940-1960 suggests that fiber diameters in the range of 5-10 μm possess the best thermal barrier properties (Figure 1). Infrared radiation (heat) wavelengths are in the range of 0.7 to 100 μm, suggesting those fiber diameters less than 0.5 μm would be too small to interact with thermal radiation. However, it is known from later studies that fibers smaller than 1 to 3 μm can increase the thermal resistance of polymer fiber insulation materials (as evidenced by commercial microfiber insulation materials such as Thinsulate® and Primaloft®). Experimental data for fibers less than 5 μm is sparse, incomplete, and sometimes contradictory. Figure 2 shows some contrasting data for glass fibers [1]. It is evident that there are still benefits to decreasing the fiber size down to 2.5 μm even for fiberglass. FIGURE 1. Optimum glass fiber size for minimum thermal radiation is about 5 μm [1]. The importance of the radiation component in heat transfer through fibrous insulation increases with temperature, and is important even at the relatively low temperatures experienced in clothing applications. Thermal radiation can account for 40 to 50% of the total heat transfer in low density fibrous FIGURE 2. Contrasting data shows thermal conductivity is still decreasing below 5 μm fiber diameter [2]. insulation at moderate temperatures. Modeling techniques account for radiative transfer by assuming that the fibers absorb, emit, and scatter thermal radiation [3-7]. Many of the simplest models in the past assumed that the fibers only absorb or emit infrared, but this can lead to significant errors [3]. The electromagnetic/optical properties of fibers have been found to be very important in radiative transfer (optical includes infrared wavelengths). The scattering and absorption parameters of fibrous insulation materials depend on the optical properties of the polymeric material, as well as the size, shape, and orientation of the fibers. For glass fibers, the standard assumption is that glass does not absorb the thermal radiation, and there is little interaction between radiation and other modes of heat transfer, unlike strong absorbers like most polymers. Predictions from a particular model [3], (Figure 3) show an example for polyester fibers where the optimum diameter is around 1 μm, but changes to the dielectric constant result in further increases in thermal efficiency for smaller fibers (conducting fibers reduce radiation heat transfer). A material such as meltblown pitch carbon fiber should also show this effect, because the fiber is conductive, colored black, and is a strong absorber and emitter of infrared. Another example is that the reflective coatings on silica fibers used for high-temperature applications (where thermal radiation is dominant) have been shown to reduce radiative heat transfer [8]. Convective Heat Transfer The other major mode of heat transfer influenced by the submicron fiber size parameter is convective heat transfer. Convection may be natural (driven by Journal of Engineered Fibers and Fabrics http://www.jeffjournal.org Volume 2, Issue 2 2007 33 heated air rising through the material), or it may be forced, driven by external pressure gradients, such as FIGURE 3. Predicted radiative conductivity as function of fiber diameter (figure redrawn from [3]). external wind, or motion of the body. Very fine fibers tend to damp out all convection because of their huge surface area, which impedes the free flow of air past the fibers. Theoretical relationships between gas permeability and fiber size for various volume fractions of fiber are useful in predicting the convective flow through fiber beds. Figure 4 shows this type of correlation for beds of fibers greater than 1 micron in size, where air flow at different pressure gradients, humidities, and compression levels is used to characterize the mean fiber diameter [9].