Moisture vapor transport

Cunningham D, Lowery M. Moisture vapor transport channels for the improved attachment of medical devices to the human body. Biomedical Microdevices 2004, 6, 149-154. 

SPMs can now be found in commercial markets and specialty clothing due to their lightweight structure, liquid and aerosol repellent properties, and facilitation of moisture vapor transport. However, for military use, SPMs have limitations (Wilusz 2007). SPMs may act as liquid-repellents but may allow vapors to pass and therefore need an activated carbon layer to add extra protection capabilities. Moreover, military garments experience tremendous stress on a day-to-day basis. SPM-based ensembles are more susceptible to tearing as compared to activated carbon-based textile fabrics (Wilusz 2007). Optimizing the permselectivity of the membrane by surface modification or other such techniques is necessary to achieve a balance between comfort (e.g., moisture vapor transmission) and chemical vapor barrier properties. Furthermore, SPMs or membrane-carbon ensembles must possess acceptable mechanical strength to sustain daily military operations. 

Fig. 14.2 Comparison of SEM micrographs (a) mioroporous membrane, (b) layered fabric system with electrospun PU nanofiber web, and (c) spunbond nonwoven fabric, (d) SEM micrographs of electrospun PU nanofiber web. (e) Effects of area density of electrospun polyurethane web on air/moisture vapor transport properties of layered fabric systems, (f) Cross section of laminated nanofiber nonwoven web and (g) vapor permeability before/after wash (a-c Reprinted with permission from Lee and Obendorf [23]. Copyright 2007, The Korean Fiber Society, d, e Reprinted with permission from Lee and Obendorf [24]. Copyright 2007, SAGE Publications, f, g Reprinted with permission from Kimura et al. [25]. Copyright 2010, SAGE Publications)...

Diverse appHcations for the fabric sometimes demand specialized tests such as for moisture vapor, Hquid transport barrier to fluids, coefficient of friction, seam strength, resistance to sunlight, oxidation and burning, and/or comparative aesthetic properties. Most properties can be deterrnined using standardized test procedures which have been pubHshed as nonwoven standards by INDA (9). A comparison of typical physical properties for selected spunbonded products is shown in Table 2. 

While the decomposition of these compounds under the right conditions produces exquisitely tailored materials, it is important to recall that most of these compounds are highly toxic, pyrophoric, and air and moisture sensitive. Therefore, special equipment is necessary to produce thin films by chemical vapor transport. The method itself appears quite simple. The appropriate vapors are brought into contact with a substrate maintained at a particular temperature. The vapors decompose, leaving behind the desired material. In Figure 3.20, the three vapors introduced into the reaction chamber decompose and produce the desired... 

The availability of worldwide gridded reanalysis (NCEP, ECMWF, NASA-DAO) has allowed assessments of the water vapor transport into Amazonia, and in fact some recent works have shown contradictory results in terms of trends in input moisture into the Amazon basin. For instance, Costa and Foley (1999) identified a statistically significant decreasing trend in the atmospheric transport of water both into and out of the Amazon basin, based on 20 years (1976-96) of the NCEP reanalysis. On the other hand, Curtis and Hastenrath (1999) have identified statistically significant upward trends of lower tropospheric convergence, upward flow, convergence of atmospheric water vapor transport, and precipitable water over the Amazon basin, based on the analysis of 40 years (1958-97) of the NCEP reanalysis. 

Farnworth [14] reported a numerical model describing the combined heat and water-vapor transport through clothing. The assumptions in the model did not allow for the complexity of the moisture-sorption isotherm and the sorption kinetics of fibers. Wehner et al [30] presented two mechanical models to simulate the interaction between moisture sorption by fibers and moisture flux through the void spaces of a fabric. In the first model, diffusion within the fiber was considered to be so rapid that the fiber moisture content was always in equilibrium with the adjacent air. In the second model, the sorption kinetics of the fiber were assumed to follow Fickian diffusion. In these models, the effect of heat of sorption and the complicated sorption behavior of the fibers were neglected. 

The marc generally contains 30-32% hexane (known as solvent hold-up), which also must be recovered and recycled to the extractor. Heat must be used to evaporate the hexane from the meal. Live steam is injected as the primary heat source and to provide moisture vapor as a stripping gas to transport solvent vapors to condensers. The extracted flakes, known as spent flakes, must be drained of solvent as much as possible to reduce the amount of energy required to desolventize the meal. 

Hydrogen bonds were also responsible for blending of chitosan and cellulose [147]. The mechanical and dynamic thermomechanical properties of the material appear to be dominated by cellulose. The reduced rate of water vapor transport through the blend appears suitable for wound dressings, providing an optimal moisture environment of the wound by preventing excessive dehydration. The blend demonstrated effective antimicrobial activity against E. coli and S. aureus. 

Lee et al. explored the application of zinc oxide nanoparticles to polypropylene nonwoven fabrics via electrospinning as one approach to impart UV-protective properties. In addition, moisture and air vapor transport characteristics of layered clothing systems with various levels of electrospun mat area density are assessed to observe the consequence of electrospun web layers on thermal comfort level of the layered structures (Fig. 14.6c) [56]. Ultrafine cellulose fibers from natural cotton lines, whose degree of polymerization was above 10,000, were electrospun. The cellulose nanotibers treated with the hydrothermal incorporation of Ce02 nanoparticles into the nanofiber substrate surface showed good UV-shielding properties... 

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