Polymers vapor permeability constants

Table 10.6 Vapor Permeability Constants (10 ° P) at 35°Cfor Polymers 10.69...

TABLE 10.7 Vapor Permeability Constants (1010P) at 35°C for Polymers All tabulated values are multiplied by 1010 and are in units of seconds-1 (centimeters of Hg)-1. 

The deviation from the near-constant rule also occurs if the gas interacts specifically with polymer molecules. This is seen in the case of water vapor permeability. The molecular size of H2O is approximately the same as that of O2 however, the solubility of H2O is much greater than that of O2 in orders of magnitudes and varies greatly depending on the nature of polymers. Consequently, there is no near-constant value of oc that is found for many gas pairs, and the permeability ratio of H2O/O2 for polymers spreads in orders of magnitude. 

D. G. Pye, H. H. Hoehn, and M. Panar, Measurement of Gas Permealnlity of Polymers. I. Permeabilities in Constant Volutne/Vatiable Pressure Apparatus, J. Appl Polym. Sci., 20, 1921 (1976) Measurement of Gas Permeability of Polymers, n. Apparatus fiwDetennination of Mixed Gases and Vapors, J. Appl. Polym. Sci., 20, 287 (1976). 

Nontoxic Citrates Nontoxic citrate plasticizers derived from natural citric acid, such as triethyl citrate (TC), tributyl citrate (TBC), acetyl triethyl citrate (ATC), acetyl tributyl citrate (ATBC), and triacetine, have been shown to be effective plasticizers for PLA [27-29]. Some gas permeability tests have been performed to assess the potential use of PLA and nontoxic citrate plasticizer blends in food packaging and other applications. The effect of ATBC on PLA barrier properties was studied by Coltelli et al. [30] using PLA mixed with ATBC (10-35 wt%), followed by compression molding. Yu et al. [31] blended PLA/ATBC mixmres with carbon black (CB) to form electrically conductive polymer composites. Fourier transform infrared (FTIR) experiments revealed that the interaction between the PLA/ATBC matrix and the CB filler was increased by the addition of ATBC. Water vapor permeability values decreased with an increase in ATBC content (at constant CB levels). For example, at 30wt% CB, the WVP of the PLA decreased from 0.66 x 10 kgm/(msPa) (at 0% ATBC) to 0.10 X 10 kgm/(msPa) with the addition of 30% ATBC. 

FIGURE 2.10 Theoretical predictions based on path tortuosity [eq. (2.9)], as a function of (a) filler aspect ratio a = 1 to 1000 (b) filler aspect ratio and alignment (5 = 1 perfect smectic alignment—dashed lines S = 0 random orientation —solid lines) (c) filler aspect ratio for a constant volume fraction (pv = 5%. (d) Comparison of the same theoretical predictions (parameters as indicated) with experimental values for water vapor permeabilities in various polymer-montmorillonite nanocomposites. (From Refs. 39-41.)... 

Vinylidene chloride polymers are more impermeable to a wider variety of gases and liquids than other polymers. For example, commercial copolymers are available with oxygen permeabilities of 0.03 nmol/m s-GPa. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase m either tends to reduce permeability. Permeability is affected by the kind and amounts of comonomer as well as crystallinity. A more polar comonomer, e.g., an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. All VDC copolymers, are very impel meable to... 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Figure 2.60 [38] presents the permeability of water vapor through several polymers as a function of temperature. It should be noted that permeability properties drastically change once the temperature exceeds the glass transition temperature. This is demonstrated in Table 2.16 [66], which presents Arrhenius constants for diffusion of selected polymers and CH3OH. 

The water ingress properties of various polymers can be assessed by values of their permeability coefficient and the diffusion constant of water (Table 15.13). The permeability coefficient is defined as the amount of vapor at standard conditions permeating a sample that is 1 cm2 and 1-cm thickness within 1 s with a pressure difference of 1 cmHg across the polymer. The diffusion coefficient is a measure of the ease with which a water molecule can travel within a polymer. There is a wide variation in the maximum amount of water absorbed by polymeric materials. Certain systems have very low absorption at lower temperatures, but the rate of absorption increases significantly at higher temperatures. 

Accurate description of barrier films and complex barrier structures, of course, requires information about the composition and partial pressure dependence of penetrant permeabilities in each of the constituent materials in the barrier structure. As illustrated in Fig. 2 (a-d), depending upon the penetrant and polymer considered, the permeability may be a function of the partial pressure of the penetrant in contact with the barrier layer (15). For gases at low and intermediate pressures, behaviors shown in Fig. 2a-c are most common. The constant permeability in Fig.2a is seen for many fixed gases in rubbery polymers, while the response in Fig. 2b is typical of a simple plasticizing response for a more soluble penetrant in a rubbery polymer. Polyethylene and polypropylene containers are expected to show upwardly inflecting permeability responses like that in Fig. 2b as the penetrant activity in a vapor or liquid phase increases for strongly interacting flavor or aroma components such as d-limonene which are present in fruit juices. 

A major breakthrough in the study of gas and vapor transport in polymer membranes was achieved by Daynes in 192016. He pointed out that steady-state permeability measurements could only lead to the determination of the product DkD and not their separate values. He showed that, under boundary conditions which were easy to achieve experimentally, D is related to the time required to achieve steady state permeation through an initially degassed membrane. The so-called diffusion time lag , 6, is obtained by back-extrapolation to the time axis of the pseudo-steady-state portion of the pressure buildup in a low pressure downstream receiving volume for a transient permeation experiment. As shown in Eq. (6), the time lag is quantitatively related to the diffusion coefficient and the membrane thickness, , for the simple case where both kD and D are constants. 

It appears that regardless of the film material involved, oxygen permeates about four times a fast as nitrogen, and carbon dioxide about 25 times as fast. The fact that the ratios of the permeabilities for all gases, apart from water vapor, are remarkably constant, provided there is no interaction between the film material and the diffusing gas, leads one to express the permeability as the product of three factors [49] one determined by the nature of the polymer film, one determined by the nature of the gas, and one accounting for the interaction between the gas and the film i.e.. 

Depending on the degree of affinity for moisture, plastic resins can be divided into two classes (1) hygroscopic and (2) nonhygroscopic. Moisture adsorption and/or absorption capability depends on the type of resins as well as the ambient temperature in which it is placed. In some instances, exposure of only few minutes can be detrimental. If the material is exposed to a certain temperature and relative humidity for a period of time, it will reach the equilibrium point, referred to as the equilibrium moisture content (EMC). Prior to drying it is important to know the permeability (product of the diffusion constant of water vapor-polymer system and the solubility coefficient) of polymer to water vapor since this dictates the condition for relative humidity for the safe storage of the polymer [16]. 

Permeability. Many polymers are used in packaging and, in particular, for food. In this latter case the permeability to gases and vapors is of prime importance. The permeation or transmission of a gas or vapor is a function of the solubility of a gas or vapor in the polymer and the rate of diffusion through the matrix. The permeability coefficient, diffusion constant, and solubility coefficients can all be measured and are influenced by the chemical structure and morphology. In order to achieve the required permeability characteristics it is common to co-extrude a series of polymers to form a laminated structure. Such materials allow selective permeation of a specific species and enhance the life of the product (190,191) (see Transport Properties). 

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