Plasticizers partial pressure

Many of the important applications for the separation of COj and CH correspond to C02 partial pressures well above 20 aim (e.g., cases 1-3 in the previous discussion of enhanced oil recovery applications). In such cases, competiiiou for umelaxed volume should be of recond-order importance, since the condensable" nature of C02 causes it to dominate the microvoid environments in the polymer even in the face of substantial methane panial pressures. At high COj partial pressures, plasticizing behavior nich as that illustrated in Fig. 20.4-4 may he obeerved. On the other hand, at relatively low C02 pressures (20 atm) (cases 4 and 5 in the discussion of enhanced oil recovery applications), competition effects would be anticipated for most glassy polymers. These effects could be mesked in the case of cellulose acetate because or iis strong tendency to internet with C02 and exhibit plasticization at low penial pressures. 

The effect of plasticizers and temperature on the permeabiUty of small molecules in a typical vinyUdene chloride copolymer has been studied thoroughly. The oxygen permeabiUty doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeabiUty are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a Htde more comphcated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeabiUty and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeabiUty doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

Plasticization Gas solubility in the membrane is one of the factors governing its permeation, but the other factor, diffusivity, is not always independent of solubility. If the solubility of a gas in a polymer is too high, plasticization and swelhng result, and the critical structure that controls diffusion selectivity is disrupted. These effects are particularly troublesome with condensable gases, and are most often noticed when the partial pressure of CO9 or H9S is high. H9 and He do not show this effect This problem is well known, but its manifestation is not always immediate. 

The gas concentration (partial pressure) at temperatures above critical can act as a super critical solvent. Rubbers in this environment are subjected to high swells leading to subsequent extraction of plasticizers, low molecular weight polymers etc. 

When the gas or vapor feed stream contains a component that is highly soluble in the polymer membrane and causes plasticization, then the selectivity as defined by Equation 4.6 will depend on the partial pressure or the amount of the plasticizing component sorbed into the membrane. Furthermore, pure-gas permeation measurements are generally not a good indicator of the separation performance, and mixed-gas permeation measurements will be needed [21-23]. Often, the mixed-gas selectivity is less than predicted from pure-gas measurements [8] however, the opposite has been observed [24], Competitive sorption effects can also compromise the prediction of mixed-gas behavior from pure-gas measurements [25], For gas pairs where each component is less condensable than C02, like 02/N2, it is generally safe to conclude that the selectivity characteristics can be accurately judged from pure-gas permeabilities at all reasonable pressures. When the gas pair involves a component more condensable than C02, plasticization is likely to be a factor and pure-gas data may not adequately reflect mixed-gas selectivity. When C02 is a component, the situation depends on the partial pressures and the nature of the polymer. 

In addition the plasticizer, due to its low molecular weight, will tend to evaporate from the binder system at elevated temperatures like the solvent during dr3dng. The partial pressure of the plasticizer is a complicated function of temperature and composition. 

When the interaction between one penetrant and the polymer is not affected by the presence of another penetrant, the pure-component permeabilities of the two penetrants in the mixture can be used in Equation 7. For rubbery polymers at low penetrant partial pressures, this assumption of independent-permeation appears satisfactory (19-20). It does not, however, appear to hold in general for glassy polymer membranes (12,13,21-25). Moreover, it also has been shown that plasticization of both rubbery (26) and glassy (27) polymers can occur at higher penetrant activities. 

Semipermeable containers Containers that allow the passage of solvent, usually water, while preventing solute loss. The mechanism for solvent transport occurs by absorption into one container surface, diffusion through the bulk of the container material, and desorption from the other surface. Transport is driven by a partial pressure gradient. Examples of semipermeable containers include plastic bags and semirigid, low-density polyethylene (LDPE) pouches for large volume parenteral (LVPs), and LDPE ampoules, bottles, and vials. 

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