Permeability of Polymers to Gases and Vapors

Permeability of Polymers to Gases and Vapors Brochure P302-335-79, The Dow Chemical Company, Midland, Mich., 1979. 

This chapter is a revised and updated version of Chapter 50, Permeability of Polymers to Gases and Vapors by S. A. Stem, B. Krishnakumar, and S. M. Nadakatii, published in the first edition of Physical Properties of Polymers Handbook , J. E. Mark, Editor, AIP Press, Woodbury, NY, 1996, pp. 687-700. 

Polymers are permeable to liquids having similar solubility parameters. Several different celts have been designed to measure the permeability of membranes to gases and vapors. 

Permeability of Polymers to Gases, Vapors, and Liquids, Alexander Leborits, Modern... 

The permeability of cellular polymers to gases and vapors depends on the fraction of open cells as well as the polymer-phase composition and state. The... 

Methods u.sed to measure the permeability of polymers to vapors fall broadly into two categories, namely those for water vapor and those for volatile liquids. In both cases the most common measuring technique is the weight change or gravimetric method. Several test standards are based on this. Other methods include techniques similar to those used for gases. 

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). 

We test the permeability of polymer films or sheets to various vapors and gases by mounting the polymer between chambers that contain different concentrations of the migrant molecules. We can determine the permeability from pressure changes, volumetric changes, or by microanalytical techniques that measure the concentration of the migrant molecules in a stream of gas flowing across the low concentration side of the barrier. 

The rate of transmission of gases and vapors through polymeric films varies with the structure of both the diffusate molecule and the polymer. Polymers with polar groups, such as cellulose and cellulose acetate, are permeable to water vapor, but polymeric hydrocarbons, such as PIB, are essentially impervious to water vapor. 

VINYLIDENE CHLORIDE MONOMER AND POLYMERS. Vinyli dene chloride copolymers most valuable property is low permeability to a wide range of gases and vapors. From the beginning in 1939, the word Saran has been used for polymers with high vinylidene chloride content, and it is still a trademark of The Dow Chemical Company in some countries. Sometimes Saran and poly(vinylidene chloride) are used interchangeably in the literature. 

Nonlinear, pressure-dependent sorption and transport of gases and vapors in glassy polymers have been observed frequently. The effect of pressure on the observable variables, solubility coefficient, permeability coefficient and diffusion timelag, is well documented (1, 2). Previous attempts to explain the pressure-dependent sorption and transport properties in glassy polymers can be classified as concentration-dependent and "dual-mode models. While the former deal mainly with vapor-polymer systems (1) the latter are unique for gas-glassy polymer systems (2). 

Gases and vapors permeate FEP at a rate that is lower than for most plastics. It occurs only by molecular diffusion, because the polymer was melt-processed. Because of the low permeability and chemical inertness, FEP is widely used in the chemical industry. Its permeation characteristics are similar to those of PTFE with some advantage because of the absence of microporosity often present in PTFE. For the permeation through FEP films an inverse relationship between permeability and film thickness applies 49... 

The similar breakdown of the surface state was also observed with parylene C film. Parylene C is a semicrystalline polymer and one of the most effective barriers for gases and vapors according to permeability values. It was thought that such an excellent barrier film would provide superior corrosion protection of a metal when it was deposited on the metal surface. Contrary to the expectation, parylene C film didn t provide good corrosion protection due to the surface state breakdown described above. This conclusion was ascertained by studies using electrochemical impedance spectroscopy (EIS), which is described in Chapter 28. 

Hansen CM. The three-dimensional solubility parameter— key to paint component affinities. J. Paint Tech. 1967 39(505) 104—117. Rogers CE. Permeation of gases and vapors in Polymers. In Comyn J. ed.. Polymer Permeability. New York Elsevier Applied... 

The models most frequently used to describe the concentration dependence of diffusion and permeability coefficients of gases and vapors, including hydrocarbons, are transport model of dual-mode sorption (which is usually used to describe diffusion and permeation in polymer glasses) as well as its various modifications molecular models analyzing the relation of diffusion coefficients to the movement of penetrant molecules and the effect of intermolecular forces on these processes and free volume models describing the relation of diffusion coefficients and fractional free volume of the system. Molecular models and free volume models are commonly used to describe diffusion in rubbery polymers. However, some versions of these models that fall into both classification groups have been used for both mbbery and glassy polymers. These are the models by Pace-Datyner and Duda-Vrentas [7,29,30]. 

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