Equilibrium moisture level

If polyurethanes are used to entrap cells, the diffusion wiU depend on the polyol used to build the polyurethane since the polyol defines equilibrium moisture. Later in this chapter, we will discuss a number of entrapment systems, including acrylates and polysaccharides. Each has its own equilibrium moisture and therefore unique diffusion constant. Only polyurethanes, however, offer the opportunity to affect changes in the constants. Conventional hydrophilic polyurethanes have equilibrium moisture levels around 70%. It is possible, however, to increase the molecular weight of a polyol (an ethylene glycol of 1000 molecular weight) to 3000 or more. This increases the equilibrium moisture to greater than 90%. 

Our discussion of cells in an HPU12 may shed some light on this. Entrapping the cells in a hydrophilic polyurethane certainly affects the diffusion of substrate into the domains of the cells. An interesting study would involve hydrogels with high equilibrium moisture levels. 

On removal from the heated environment and return to ambient conditions, the film initially picks up to 50% of its equilibrium moisture level within a few minutes while cooling but the remaining moisture to reach equilibrium is picked up much more slowly. Films can typically take over 12 h to reach equilibrium. Figure 7.5 shows the effect of RH on this process and indicates that final (equilibrium) moisture levels will change significantly with different RH. 

The relative wettabilities of hydrophobic pharmaceutical powders have been determined using a vacuum balance technique to measure the rates of w ater vapor uptake and the equilibrium moisture levels of the powder samples (1()5. ... 

Numerous efforts have focused upon the nature of moisture transport of epoxy systems. Previous-sorption desorption work demonstrated that equilibrium moisture levels In an epoxy system can be related to thermodynamic states (1,2,3). Transient and equilibrium dynamic mechanical experiments are performed In this work with two epoxy systems TGEBA-TETA and N-5208. These experiments provide Insight Into the nature and extent that network changes have on the dynamic mechanical properties as a result of hygrothermal cycling. 

Changing the atmosphere surrounding the product by absorbing the moisture (e.g. a desiccant or substance which is hygroscopic in nature) or the emission of moisture whereby an equilibrium moisture level between the product and air space is set up. Both absorption and emission may be temperature-dependent. 

An example of typical equipment used to make PET fibers is shown in this figure. Prior to melting, the polymer chips must be thoroughly dried, typically under vacuum at elevated temperatures for several hours. The equilibrium moisture content of PET under ambient conditions is about 0.4 % [3] and this much moisture would cause unacceptable levels of degradation. The dried polymer chips are fed to an extruder which melts the chips and forwards them to a... 

Analyses reported on a dry basis are calculated on the basis that there is no moisture associated with the sample. The moisture value (ASTM D-3173 ISO 331 ISO 589 ISO 1015 ISO 1018 ISO 11722) is used for converting as determined data to the dry basis. Analytical data that are reported on a dry, ash-free basis are calculated on the assumption that there is no moisture or mineral matter associated with the sample. The values obtained for moisture determination (ASTM D-3173 ISO 589) and ash determination (ASTM D-3174) are used for the conversion. Finally, data calculated on an equilibrium moisture basis are calculated to the moisture level determined (ASTM D-1412) as the equilibrium (capacity) moisture. 

Equilibrium moisture basis data calculated to the moisture level established as the equilibrium moisture (ASTM D-1412 ASTM D-3180). 

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... 

All nylons absorb some moisture from environmental humidity. Moisture absorption characteristics must be considered in designing and joining these materials. They absorb from 0.5 to 2 percent by weight of moisture after 24-h water immersion. Freshly molded objects contain less than 0.3 percent moisture since only dry molding powder can be successfully molded. Once molded, these objects absorb moisture when they are exposed to humid air or water. The amount of absorbed moisture increases until an equilibrium condition is reached based on the relative humidity of the environment. Equilibrium moisture contents of two commercial nylon resins for two humidity levels are as follows ... 

Epoxy glasses aged at 140 °C were subjected to 40 °C/98 % relative humidity moisture penetration. Figure 31 shows the results of this transport experiment. We observed both a decrease of initial sorption kinetics as well as a decrease of equilibrium sorption level as a function of aging time. This supports the idea that during sub-Tg annealing, the resin contrasts and densities, resulting in decreased free volume. 

Equilibrium Dynamic Mechanical Data. Dynamic mechanical properties of both the DGEBA-TETA and the N-5208 epoxy systems exhibit characteristic transitions observed in many polymeric materials. Figures 2a and 2b Illustrate "equilibrium" dynamic mechanical tan 6 as a function of temperature for samples saturated at different moisture levels. 

Fig. 10 Examples of the kinetics of secondary drying. Triangles = mannitol (crystalline) squares = poly (vinylpyrrolidone) circles = moxalactam di-sodium (amorphous). All solids were prepared by freeze-drying a 5% aqueous solution from a 1-cm fill depth, followed by hydration to a uniform moisture level of 7%. The quantity, F, is the fractional attainment of equilibrium, which corresponds to near zero water content. The secondary drying conditions were product temperature = 18°C chamber pressure = 200mTorr. (From Ref °l)...

Environmental Factors. Moisture. Wood, which is a hygroscopic material, gains or loses moisture to equilibrate with its immediate environment. The equilibrium moisture content (EMC) is the steady-state level that wood achieves when subjected to a particular relative humidity and temperature. The eventual EMC of two similar specimens will differ if one approaches EMC under adsorbing... 

In spite of this situation, soybeans generally present fewer storage problems than corn since the soybean crop usually dries to a lower moisture level in the field prior to harvest. Fungi likely to be present on soybeans stored at 25°C at various relative humidities and equilibrium moisture contents are shown in Table 3.5. 

The second parameter is the time to reach an equilibrium uptake for a given moisture level in the environment. Whereas there is still much debate regarding the exact nature of water adsorption and, consequently, tte interpretation of the isotherm, there is even less known about the kinetics of water adsorption. Up to now the only existing models are purely based on experimental data and, as such, they are completely empirical [93-97]. 

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