Rate of water permeation through

The WVTR through the container is determined by the container wall thickness, the permeability of the material and the difference between the external and internal relative humidity environments. Waterman et al. [99] determined the theoretical rate of water permeation through a standard 60-cc bottle when stored at 40°C/75% RH. This equated to an uptake of 1 mg of water per day. They commented that even if the product had been packed under low water vapour conditions the relative humidity conditions within the container would be equate to 50% RH within 1 day. The WVTRs (see Table 2.8) for some common packaging materials were reported by Waterman et al. [99]. 

We tried to photocontrol water permeation through a porous poly(vinyl alcohol) membrane coated with the polyacrylamide gel containing triphenylmethane leuco-cyanide groups [59]. The photoresponsive behavior of the gel has already been described in Sect. 3.1. Figure 26 shows that the rate of water permeation through... 

The statements made above apply regardless of the crosslinking system used. When the crosslinker is changed, the skin formation time and the tack free time may change but the effect on the deep section cure rate is minimal, since the rate of water permeation through the cured skin is generally the rate-controlling factor in the cure of one-component RTV silicone sealants. 

The permeability of liposomes prepared from synthetic lecithin have provided useful data which help to explain functional changes in membranes in terms of an alteration in bilayer structure. For example, the rate of water permeation through liposomes decreases 10-fold, and the of permeation increases from about 9 to 26 kcal mol" below the transition temperature for the lipids (Block et al., 1975). Rates of permeation for electrolytes and nonelectrolytes were found to be maximum at, or near, the temperature of the transition where fluid- and solid-phase lipids coexist, and decreased in both the gel and fluid phases below and above this temperature (Block et al., 1976). Since the order-disorder transition of lipids is accompanied by a decrease in the area per molecule, it was postulated that pores developed in the bilayer at the transition temperature and that the number and lifetime of these pores was dependent upon the fatty acid chain length of the lipids (Block eta/., 1976). 

Fig. 8 (a) Rate of water pomeation through 1100 EW EP-Nafion at 30°C water vapor at a = 0-30 into a dry N2 stream. The dry membrane thicknesses are shown in the graph. The permeation rate increases invCTsely with decreasing mcanbrane thickness, (b) Rate of water permeation through 1100 EW tP-Nafion at 80°C from liquid water into a dry N2 stream. The dry membrane thicknesses are shown in the graph. The permeation rate changes by 20% when the membrane thickness changes by a factor of 5... 

One very common method of measuring the rate of water permeation through a film is cup method which is the gravimetric method described by Fu et al. [22]. The membrane to be tested is placed and fixed over a standard cup, at least half filled with water. The cup is placed in a chamber in which air at constant relative humidity is circulated at a constant temperature. This method measures water permeation rate at a high water activity on the feed side of the membrane since the atmosphere is saturated with water vapour. The VPS described above provides more flexibility in the feed water vapour composition and allows for the measurement of water transport in a mixture. 

Continuity of fhe wafer flux fhrough the membrane and across the external membrane interfaces determines gradients in water activity or concentration these depend on rates of water transport through the membrane by diffusion, hydraulic permeation, and electro-osmofic drag, as well as on the rates of interfacial kinetic processes (i.e., vaporization and condensafion). This applies to membrane operation in a working fuel cell as well as to ex situ membrane measuremenfs wifh controlled water fluxes fhat are conducted in order to study transport properties of membranes. 

Even in a dead-end mode, hydrogen has to be periodically purged, because of accumulation of inerts or water. The frequency and duration of purges depends on purity of hydrogen, rate of nitrogen permeation through mem-... 

A = 4.05 X lO " cm/(s-kPa)(4.1 X 10 cm/(s-atm)) and = 1.3 x 10 cm/s (4)//= 1 mPa-s(=cP), NaCl diffusivity in water = 1.6 x 10 cm /s, and solution density = 1 g/cm . Figure 4 shows typical results of this type of simulation of salt water permeation through an RO membrane. Increasing the Reynolds number in Figure 4a decreases the effect of concentration polarization. The effect of feed flow rate on NaCl rejection is shown in Figure 4b. Because the intrinsic rejection, R = 1 — Cp / defined in terms of the wall concentration, theoretically R should be independent of the Reynolds... 

This handbook makes no attempt to describe environmental transport properties because these phenomena are specific to environmental conditions such as wind speed and water current velocity. A notable exception is the diffusion or permeation rate of a substance through biological membranes, which is the key process controlling dermal absorption and therefore a key determinant of the dose actually available to exert a toxic effect. Chapter 11 treats this topic. The reader seeking estimation methods for molecular diffusiv-ities in air and water should consult the text by Reed et al. (1987). 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

It Is reasonable to assume that a mechanical stress would supply energy to the elastomer thereby Increasing the rates of any processes. For example. In aging, plastics or elastomers which are highly elongated will decompose more rapidly than unstressed controls. There Is, however, no published Information on the effects of ultrasonic radiation on water permeation through elastomers. 

This extends the previous work (I ) In which the Lennard-Jones type surface potential function and the frictional function representing the Interfaclal forces working on the solute molecule from the membrane pore wall were combined with solute and solvent transport through a pore to calculate data on membrane performance such as those on solute separation and the ratio of product rate to pure water permeation rate in reverse osmosis. In the previous work (1 ) parameters Involved in the Lennard-Jones type and frictional functions were determined by a trial and error method so that the solutions in terms of solute separation and (product rate/pure water permeation rate) ratio fit the experimental data. In this paper the potential function is generated by using the experimental high performance liquid chromatography (HPLC) data in which the retention time represents the adsorption and desorption equilibrium of the solute at the solvent-polymer interface. 

Table IV.07. Permeation Rate of Water Through Fluoroplastic Films...

The effects of water and temperature on the adhesive itself are also of utmost importance to the durability of bonded structures. In the presence of moisture, the adhesive can be affected in a number of ways, depending on its chemistry and how rapidly the water permeates through and causes significant property changes [51,86-88]. The potential efficacy of moisture penetration on the locus of failure of bonded joints has been discussed in the previous section. As expected, elevated temperature conditions tend to degrade joint strength at a faster rate. 

The cured adhesive acts as a barrier for the permeation of water to the uncured material. Any water that passes through this barrier quickly reacts with uncured material, and thus the barrier is thickened. The rate (dn/dr) at which moles of water permeate unit cross section of the cured layer is given by Eqn. 1. Here p is the vapour pressure of water in the surroundings and P is the water permeability coefficient of cured adhesive. 

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