Permeability, Permeation water vapor

A gravimetric method is often used for the measurement of water vapor permeability. The water vapor transmission rate (WVTR) according to this method is the amount of water vapor in g that permeates in 24 hours under standard test conditions (temperature, air relative humidity, water vapor sorbant) through aim2 sample surface area. The method measure a flux and not a permeability coefficient. 

Under natural exposure, an organic coating can be in contact with water in different forms, such as rain, dew, or humidity. Some coatings are more permeable to water vapor than to Kquid water, whereas with others liquid water permeates faster [37]. The permeability is also different depending on the film being applied on a substrate or as a freestanding film [38]. 

The apparatus used in our laboratory to measure water vapor permeability is schematically shown in Figure 10-4 (Tohge, 1996 Tadanaga, 1996) as an example. The sample to be evaluated is placed between two parts of the cell and clamped. The temperature of the sample can be controlled by the thermostat, or an electronic heater. Both sides of the sample in the permeation cell are evacuated to 10 Torr and the sample is degassed in the permeation apparatus for several hours. A saturated water vapor of at 0°C (4.58 mmHg) is then introduced to the upstream side and the pressure change on the downstream side due to the permeated water vapor is measured with a pressure meter. The water permeation coefficient (P) is evaluated from the increasing rate of the pressure (dp/dt) at a steady state and normalized with the thickness of the sample. When the vacuum line is not used, a test gas is supplied to the upstream side of the sample. The pressure difference between the upstream side and down stream side is determined and the flow rate is also measured, by, for example, soap-bubble displacement (Klein, 1990). 

The procedures for membrane casting and gas permeability measurement were the same as above. The permeability of water vapor was measured separately according to GB 1037-70 (China). The gas permeation properties of PEK(S)-L and DIDMPEK(S)-C are shown in Table IV. 

Foamed or Cellular CeUular plastics such as polyurethane and polystyrene do not hold up or perform well in the ciyogenic temperature range because of permeation of the cell strnc tnre by water vapor, which in turn increases the heat-transfer rate. CeUular glass holds up better and is less permeable. 

There are substantial differences in the rates at which water vapor and other gases can permeate different plastics. For instance, PE is a good barrier for moisture or water vapor, but other gases can permeate it rather readily. Nylon, on the other hand, is a poor barrier to water vapor but a good one to other vapors. The permeability of plastic films is reported in various units, often in grams or cubic centimeters of gas per 100 in.2 per mil of thickness (0.001 in.) of film per twenty-four hours. The transmission rates are influenced by such different factors, as pressure and temperature differentials on opposite sides of the film. 

It should be recognized that all plastic materials over a time period allow a certain amount of water vapor, organic gas, or liquid to permeate the thickness of the material. It is only a matter of degree of permeation between various materials used as barriers against vapors and gases. It has been found that the permeability coefficient is a function of the solubility coefficient and diffusion coefficient. The process of permeation is explained as the solution of the vapor into the incoming surface of the barrier, followed by diffusion through the barrier thickness, and evaporation on the exit side. 

FIG. 13 Reduction of the relative permeability coefficient is dependent on the clay platelet aspect ratio in the system of polyimide-clay hybrid with water vapor as the permeate. Each hybrid contains 2 wt% clay. The aspect ratios for hectorite, saponite, montmorillonite, and synthetic mica are 46, 165, 218, and 1230, respectively. (From Ref. 71.)... 

As stated, the capability of plasma deposits to reduce the access of water to corrosion-sensitive surfaces may be an important motivation for their application in corrosion protection. In order to study this property, Kapton polyimide film was selected as the substrate because of its high inherent permeability to water and its ability to resist elevated temperatures. The response of Kapton film overcoated by PPHMDSO to the permeation of water vapor is shown in Fig. 1. Clearly, the presence of the organo-silicone plasma film greatly reduces water permeation. The magnitude of the effect is much enhanced when plasma polymers are produced at high T and p. 

While reasonably accurate for the permeation of simple nonpolar gases, this equation is less suit-able for water vapor, so the permeability data quoted for this substance are of qualitative. significance only. 

Most of the current industrial development efforts are focused on processes that separate water from the overhead ethanol/water vapor of the distillation column, replacing the molecular sieve drier as shown in Figure 8.18(b). The overhead vapor mixture is sent to a water-permeable membrane, producing a dry ethanol residue and a low-pressure permeate enriched in water, which is recycled to the column. Another option, shown in Figure 8.18(c), is to use the membrane-separation step to replace... 

The mechanisms involved in the deterioration of insulation characteristics of LDPE are analogous to the principle of water vapor permeation through it. Two distinctively different paths can yield the water vapor permeability at the same value, i.e., many water molecules moving rather slowly or few molecules moving... 

Liquid water on one side of a silicone contact lens permeates through the lens by a solution-diffusion mechanism and evaporates on the other side quickly according to the permeability of water, whereas the solubility of water in silicone polymer is low [1]. The high water vapor permeability was speculated to be one of the reasons causing the suction cup effect that makes the lens stationary on one spot and tenaciously stick to the cornea this may damage the corneal epithelium and result in other complications. However, the high permeability per se cannot be the reason for the suction cup effect if the exterior surface is covered by the tear film, i.e., if there is no driving force for water permeation. 

---