Gradient moisture

MoistureResista.nce, Plastic foams are advantageous compared to other thermal insulations in several appHcations where they are exposed to moisture pickup, particularly when subjected to a combination of thermal and moisture gradients. In some cases the foams are exposed to freeze—thaw cycles as well. The behavior of plastic foams has been studied under laboratory conditions simulating these use conditions as well as under the actual use conditions. 

Moisture gradient is the moisture profile ia a material at a specific moment duriag dryiag, which usually reveals the mechanisms of moisture movement ia the material up to the moment of measurement. 

Moisture gradient refers to the distribution of water in a solid at a given moment in the diying process. 

The study of internal moisture gradients estabhshes the particiilar mechanism which controls during the drying of a solid. The experimental determination of rehable moisture gradients is extremely difficult. 

FIG. 12-40 Two typ es of internal moisture gradients obtained in drying solids,... 

Sometimes the stiffness matrix for a lamina, [Q j], Is not constant through the thickness of the lamina. For example, if a temperature gradient or moisture gradient exists in the lamina and the lamina material properties are temperature dependent and/or moisture dependent, then [Qij]i( is a function of z and must be left inside the integral. In such cases,... 

Moisture gradient, 9 97 Moisture levels, in sugar, 23 472 Moisture permeability, 10 2 Moisture permeability, of higher olefin polymers, 20 423-424 Moisture properties, of polyamide fibers, 19 744-745... 

The formation of moisture gradients within the material during extrusion, resulting in migration of liquid from within the interparticulate spaces to surface of the extrudate, has been reported (40,25). At low initial moisture content, the friction within the chamber can result in higher temperatures leading to loss of moisture of up to 12% from the extrudate surface. This effect is more pronounced with A-A extruders. 

Among formulation variables, the moisture content particularly on surface of the material needs to be optimized. While excess surface moisture could lead to uncontrolled agglomeration, a dry extrudate can break up into fine powder. Upon spheronization for prolonged periods at high speeds, moisture gradients develop within pellets by migration from core to the surface. This is also a function of the concentration of dissolved solids and spheronizing time. The surface moisture could be lost over time, due to... 

Plastic foams are advantageous compared lo other thermal insulations in several applications where lhey are exposed lo moisture pickup, particularly when subjected to a combination of thermal and moisture gradients. 

Pan Y. McGuire A.D. Melillo J.M. Kicklighter D.W. Sitch S. and Prentice I.C. (2002). A biogeochemistry-based dynamic vegetation model and its application along a moisture gradient in the continental United States. Journal of Vegetation Science, 13, 369-382. 

The internal moist layer has neither a temperature nor moisture gradient. 

Fig. 24. Determination of the diffusion coefficients from the observed diffusion fronts shown in Fig. 23 (a) Diffusion along moisture gradient (b) self-diffusion scenario for 30% w/b moisture content. (Reproduced from Ref. 90 with permission from Elsevier.)...

Hougen, McCauley, and Marshall [Tram. Am. Imt. Chem. Eng., 36, 183 (1940)] discussed the conditions under which capillary and diffusional flow may be expected in a drying solid and analyzed the published experimental moisture-gradient data for the two cases. Their curves indicate that capillary flow is typified by a moisture gradient involving a double curvature and point of inflection (Fig. 12-40a) while diffusional flow is a smooth curve, concave downward (Fig. 12-40b), as would be predicted from the diffusion equations. They also showed that the liquid-diffusion coefficient is usually a function... 

When drying thin material, there is little effect on the drying time by friction due to flow in the capillaries. In many of these cases the effect of gravity is negligible and there will be no moisture gradient within the solid. 

The ratio of the slopes of the regression lines mil m2 gives the transmittance r. If the sea surface temperature gradient is much larger than the moisture gradient, that is, AL AL, then by the radiative-transfer equation, r = AL/AL, and r = mi/m2 (11),... 

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