Phillip Gibson

Transport properties of porous membranes based on electrospun nanofibers

Abstract Electrospinning is a process by which high voltages are used to produce an interconnected membrane-like web of small fibers (10–500 nm in diameter). This novel fiber spinning technique provides the capacity to lace together a variety of types of polymers, fibers, and particles to produce ultrathin layers. Of particular interest are electrospun membranes composed of elastomeric fibers, which are under development for several protective clothing applications. The various factors influencing electrospun nonwoven fibrous membrane structure and transport properties are discussed. Performance measurements on experimental electrospun fiber mats compare favorably with transport properties of textiles and membranes currently used in protective clothing systems. Electrospun layers present minimal impedance to moisture vapor diffusion required for evaporative cooling. There may be special considerations in the application of elastomeric membranes for protective clothing. Effects of membrane distortion upon transport behavior of the structure might be significant. Preliminary measurements have found that changes in elastomeric membrane structure under different states of biaxial strain were reflected in measurements of air flow through the membrane. Changes in membrane structure are also evident in environmental scanning electron microscope (SEM) images of the pore/fiber rearrangement as the membrane is stretched. Experimental measurements and theoretical calculations show electrospun fiber mats to be extremely efficient at trapping airborne particles. The high filtration efficiency is a direct result of the submicron-size fibers generated by the electrospinning process. Electrospun nanofiber coatings were applied directly to an open cell polyurethane foam. The air flow resistance and aerosol filtration properties correlate with the electrospun coating add-on weight. Particle penetration through the foam layer, which is normally very high, was eliminated by extremely thin layers of electrospun nanofibers sprayed on to the surface of the foam. Electrospun fiber coatings produce an exceptionally lightweight multifunctional membrane for protective clothing applications, which exhibits high breathability, elasticity, and filtration efficiency.

Different electrostatic methods for making electret filters

Three charging techniques (viz., corona charging, tribocharging, and electrostatic fiber spinning) were used to charge fibers or fabrics of different polymer types. Corona charging is suitable for charging monopolymer fiber or fiber blend, or fabrics. Tribocharging is only appropriate for charging fibers with dissimilar electronegativity. Electrostatic fiber spinning combines the charging of polymer and the spinning of the fibers as a one-step process. It was observed that two dissimilar fibers following tribocharging had higher filtration efficiency than the corona-charged polypropylene fibers. An electrostatic spinning process produced nanofibers exhibiting extremely high efficiency by mechanical filtration mechanisms. Little charge was retained in electrospun polyethylene oxide fibers; however, polycarbonate and polyurethane retained a great amount of charge.

Factors Influencing Steady-State Heat and Water Vapor Transfer Measurements for Clothing Materials

Several techniques exist to evaluate the water vapor transport characteristics of clothing materials. The most common techniques include guarded hot plate sweating skin simulants and cup-type moisture vapor transmission rate tests. Theoretically, all such tests measure an identical property, water vapor resistance, but the results from different test methods rarely agree. The reasons for the discrepancies are the different conditions present in each test: in some cases the intrinsic properties of the materials are altered by the test conditions. The results of three studies illustrate important factors to be considered when evaluating the thermal and moisture vapor transport properties of textile materials. Each study concentrates on one particular aspect of the problems encountered in measuring relative performance characteristics of these ma terials. The first study involves an experimental correlation between two kinds of water vapor permeability tests. The second study looks at the influence of air per meability on heat and water vapor transport through woven and nonwoven fabrics. The final study determines the agreement between three different guarded hot plate (sweating skin simulant) test facilities that differ mainly in the air velocity over the test samples.

Effect of temperature on water vapor transport through polymer membrane laminates

This paper determines the extent to which the water vapor transport properties of nine different polymer membranes and membrane/textile laminates are affected by temperature. A particular test method, the Dynamic Moisture Permeation Cell (DMPC), is ideally suited for this type of study, owing to its complete control over the humidity and gas flow rate on the two sides of the test sample, and the ability to control the temperature of the test system. This allows temperature-dependent effects to be separated from concentration-dependent effects on mass transfer phenomena. The DMPC permits the experimenter to explore the temperature dependence of the diffusion behavior at different points on the vapor sorption isotherm of the hydrophilic polymer component of a polymer film or membrane laminate. Temperature effects are shown to be much less important than concentration-dependent effects in a hydrophilic polymer layer. Observed changes in water vapor flux at different temperatures are primarily due to the relationship between temperature and the saturation vapor pressure of water, and not to intrinsic changes in polymer permeability.

An Automated Water Vapor Diffusion Test Method for Fabrics, Laminates, and Films

An automated apparatus has been developed to measure the transport of water vapor through coated and uncoated fabrics, fabric laminates, thin foams, and solid films under a variety of conditions. The apparatus is more convenient to use than the traditional test methods for textiles and clothing materials; it allows one to use a wider variety of test conditions to investigate the concentration-dependent and nonlinear transport behavior of many of the semipermeable membrane laminates that are now available. The dynamic moisture permeation cell (DMPS) has been auto mated to permit multiple setpoint testing under computer control and to facilitate in vestigation of transient phenomena. Results generated with the DMPC are in agree ment with and of comparable accuracy to those from the ISO 11092 (sweating guarded hot plate) method of measuring water vapor permeability.

Modeling convection/diffusion processes in porous textiles with inclusion of humidity-dependent air permeability

A set of partial differential equations describing time-dependent heat and mass transfer through porous hygroscopic materials was developed. Water in a hygroscopic porous solid may exist in vapor or liquid form in the pore spaces or in bound form when it has been absorbed by the solid, which is typically some kind of hydrophilic polymer. Factors such as the swelling of the solid due to water imbibition, and the heat of sorption evolved when the water is absorbed by the polymeric matrix, were incorporated into the appropriate conservation and transport equations. A numerical code to solve the set of nonlinear coupled equations was developed, and applied to an experimental apparatus designed to simulate transient and steady-state convection/diffusion conditions for textile materials. Experimental measurements of air permeability and diffusion properties as a function of relative humidity provided fundamental data on the changes in transport properties as hygroscopic textiles fibers swell and decrease the free gas phase volume within the porous structure. When these relations were incorporated into the numerical model, it was possible to directly compare the predictions of the numerical code with results generated in the experimental apparatus. Results are shown for hygroscopic porous textiles under several conditions. Under pure diffusion, with no convective flows across the sample, the temperature changes of hygroscopic textiles subjected to step changes in environmental relative humidity are shown to agree with the numerical predictions. These temperature changes are due to sorption of water vapor from the flows on the two sides of the material, and relate to textile fiber equilibrium sorption isotherms and sorption kinetics, as well as the physical structure and thermal properties of the textile. Under conditions of both a concentration gradient and a pressure gradient across the fabric, which results in combined diffusion and convection, it is shown that the effect of fiber swelling, which results in significant changes in the resistance to convective flow, also has an effect on the resulting total mass flux across the textile layer.

Electrospinning Technology: Direct Application of Tailorable Ultrathin Membranes

that offer advantages for protective outerwear for military applications. A materials evaluation program was initiated by the Army Survivability Directorate in late 1985 to identify potential, alternative commercial waterproof and moisture vapor permeable materials capable of meeting military needs for the Extended Cold Weather Clothing System. After a market survey identified many new semipermeable laminated materials, screen tests for fabric weight, strength, and hydrostatic resistance were used

The use of volume‐averaging techniques to predict temperature transients due to water vapor sorption in hygroscopic porous polymer materials

Volume-averaging techniques developed for modeling drying processes in porous materials offer a convenient framework for analyzing vapor sorption in porous hygroscopic polymeric materials. Because of the large temperature changes associated with water vapor sorption in these materials (from 10° to 20°C), sorption/diffusion processes are best characterized through the coupled differential equations describing both the transport of energy and mass through the porous structure. Experimental and numerical results are compared for a variety of natural and man-made porous polymeric materials (textiles) using the volume-averaging technique. Boundary heat and mass transfer coefficients and assumptions about thermal radiative properties of the experimental apparatus are shown to influence results obtained with the numerical solution method.

Humidity-Dependent Air Permeability of Textile Materials1:

Changes in fabric structure as hygroscopic fibers swell at high humidities can have a large influence on the measured air permeability of fabrics such as cotton, wool, silk, and nylon. The variation of air permeability as a function of relative humidity is of practical importance in ranking and evaluating candidate textiles for protective clothing applications. This paper describes a test method used to determine the relative humidity dependence of the air permeability of hygroscopic woven textile fabrics. The instru mentation also permits dynamic measurements during a step change in relative humid ity. Typical results are shown for woven fabrics, nonwoven battings, and novel elec trospun fiber mats.

Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores

Small-diameter carbon nanotubes (CNTs) are shown to enable exceptionally fast transport of water vapor under a concentration gradient driving force. Thanks to this property, membranes having sub-5 nm CNTs as conductive pores feature outstanding breathability while maintaining a high degree of protection from biothreats by size exclusion.