Water moisture transfer

Water is introduced into closed pharmaceutical systems either accompanying the input materials or in the headspace as relative humidity [79]. Whatever water is contained within the dosage form and its container will ultimately equilibrate among the components according to its affinity for the solid ingredients and the number of association sites. The Sorption-Desorption Moisture Transfer model has been used to evaluate the thermodynamically favored state that will result after the equilibration process is complete [79]. 

A subtle aspect of stability analysis of freeze-dried products in vials with rubber stoppers is the tendency for water vapor to be transferred from the stopper to the solid during storage. Representative data for residual moisture as a function of time at different temperatures are shown in Figure 11. As expected, the residual moisture level increases more rapidly at higher temperature, but the plateau level is independent of temperature as equilibrium is established between the freeze-dried solid and the stopper. The extent to which this is observed depends on several factors. First, the nature of the rubber stopper formulation affects the diffusivity of water in the rubber. Second, the processing of the stopper can affect the level of residual moisture present. It is not uncommon for extended drying of the stopper to be necessary to minimize residual moisture. Finally, the mass of the freeze-dried solid determines the extent to which the percent residual moisture is affected by water vapor transfer from the stopper, where large cakes may be relatively unaffected by the small amount of water vapor that is... 

Figure 11 Changes in residual moisture during storage of freeze-dried solids due to water vapor transfer from the stopper at different storage temperatures 5°C (diamonds), 25°C (squares), and 40°C (triangles).

Moisture transfer through fibres, which increases with the moisture absorption. Of these mechanisms the water vapour permeation seems to be the most important... 

The measurements first reveal an increase in the water level, which can be attributed to moisture transfer from the stopper into the product. The decrease observed subsequently could be due to water expulsion as a result of some product transformation (crystallization of sugar within the substrate, for example). The water would then be adsorbed by the stopper. As a matter of fact, it is well known that the capacity of stoppers to adsorb water depends on their composition [13-15]. Finally, in the third stage, transfer by diffusion through the stopper could account for the slow moisture recovery in each sample. 

Nevertheless, in the field of physical chemistry of foamed polymers the transition from quantity to quality has not yet occurred practically no generalizations, even of semi-empirical nature, are available which relate the kinetics of moisture and water absorption to the main morphological parameters of polymeric foams (specific gravity, portion of open ceUs, etc.) Very little is known about molecular mechanisms of vapor and moisture transfer taking into account the chemical and physical structure of foams. 

Here both the moisture content XAl at the water surface and the mass flux mA of the water being transferred are still unknowns. At the surface of the water saturation prevails and so the moisture content XAi, as taught by thermodynamics [1.33], is given by... 

Temperature influences the rate of drying in a number of ways. The principal reason for kiln drying at high temperatures is to increase the rate of moisture transfer to the wood surface. Raising the temperature dramatically enhances the rate of diffusion of water molecules across cell walls. The rate of diffusion increases with temperature at approximately the same rate, as does the saturated vapour pressure (Table 8.1 Figure 8.9b). 

It is a well-documented fact that changes in reaction rate correlates well to changes in a, with references too numerous to include. Because reactions alter their behavior as a function of researchers often mistakenly redefine either as (1) a measure of the ability of water to participate in reactions, (2) a ratio of the amount of water that participates in chemical reactions and microbial growth to water that cannot participate, or (3) a ratio of "free" to "bound" water. The actual role of a in food reactions, microbial growth, and moisture transfer has been extensively documented and reviewed but not completely understood (Labuza, 1980 Rockland and Nishi, 1980 Chirife and Buera, 1994). 

Li and Holcombe [31] developed a two -stage model, which takes into account water-vapor-sorption kinetics of wool fibers and can be used to describe the coupled heat and moisture transfer in wool fabrics. The predictions from the model showed good agreement... 

Crystallization can be accounted for by the moisture dependence of Tg according to Roos and Karel (1991b). Crystallization releases water, which in closed containers is absorbed by the amorphous portion of the food. As a result when Tg drops, T-Tg increases and rapid crystallization follows. Products, which have high moisture transfer rates in the environment, will lose water, but the moisture content in the amorphous part remains fairly constant. Crystallization proceeds at a rate defined by a constant T-Tg. Crystallization leads to a complete change in physical structure. It may considerably decrease stability. Lactose crystallization in milk powders... 

Recently, Diaz et al. [101] have demonstrated that exposure of water-set hair tresses to a lower humidity can also produce a loss of set. In addition, Robbins and Reich [102] have demonstrated this same phenomenon with single hair hbers. Thus, we conclude that exposure of water-set hair to changes in humidity results in the flow of additional moisture either into or out of hair. This moisture transfer, therefore, breaks hydrogen bonds critical to water-set stability, resulting in a decrease in style retention. 

Hambleton, A., Perpinan-Saiz, N., Fabra, M. J., Voilley, A., Debeaufort, F 2012. The Schroeder paradox or how the state of water affects the moisture transfers through edible films. Food Chemistry, 132(4) 1629-2230. 

The main property of the outer shell is to provide a protection against the outside weather conditions, mainly rain, snow and wind. In order to avoid excessive sweat accumulation, the outer layer should allow moisture transfer from the body to the environment. Such waterproof, windproof and breathable fabrics (WBF—mainly membranes and coatings) have been on the market for more than 30 years. The waterproof properties and, at the same time, the water vapour transfer are either achieved with a micro-porous or a hydrophilic structure, or a combination of both technologies (bi-component WBF). Waterproofness and breathability are contradictory requirements and, therefore, a compromise has to be found between protection and comfort properties. This compromise is usually achieved by adapting the porosity and thickness of such WBF layers. 

Comparing the nanoCber mats with different porosities, shows that the nanoCber mat with higher porosity and air permeability has the higher moisture transfer. This result indicates that the high resistance to the air permeability of nanoCber mats dose not impeded the moisture transfer through the bulk nanoCber mats. This difference is related to the size of water vapor and air molecules. 

Electrospun nanoiiber mats by modified electospinning system have high porosity. Results show that the porosity of nanofiber mats increases by decreasing speed rotation of dmm. Air permeability and moisture transfer of nanofiber mats were reduced with reduction the porosity of nanofiber mats. Based on experimental results, moisture transfer behavior is influenced by nanofiber mats porosity. Bulk nanofiber mats with high rates of water vapor diffusion and low air permeability are promising candidates for protective clothing applications. 

---