Transport Properties of Gases in Polymers: Bibliographic Review

After some general considerations and basic equations on transport phenomena in polymers, this review, exclusively bibliographical, presents different concepts and theoretical models that have been proposed and developed to describe the transport mechanism of molecular species in polymers by diffusion. Based on numerous previous studies, it will show how the permeability of gases (or organic vapours) depends strongly on the polymer structure (degree of crystallinity, thermal and mechanical histories), on the penetrant size and nature as well as on the conditions of temperature and pressure

Transport Properties of Gases in Polymers: Experimental Methods

The permeability of gases in polymers is a property inherent to their structure, which results, firstly, from the absorption of fluids by the material, then, from the diffusion of these products through the polymer matrix. The capacity of a gas to cross more or less fast a material can be used in numerous industrial domains. For example, polymers of low permeability are looked for in domains as different as the oil production, the food packaging or the automotive industry. For oil applications, the main function of polymers is to ensure the leakproof of pipes for example. In that case, materials are in contact of gas at high temperature and high pressure. As the information concerning the gases transport coefficients in these extreme conditions was not available in literature, devices able to give access to these properties were developed. In this paper, the methods for obtaining the transport coefficients and a brief bibliographical review of the various existing experimental techniques are presented. Then, the various experimental devices developed are described in detail

Characterization of Polymer Layered Silicate Nanocomposites by Rheology and Permeability Methods: Impact of the Interface Quality

Polymer clay nanocomposites are mostly described as materials with improved properties. However, many studies are not concluding about the possible benefits. This work reports permeability measurements on different polyolefins nanocomposites. Clay exfoliation has been proved by various techniques. Unfortunately, the nanocomposite does not exhibit good barrier properties. Hence, permeability coefficients were found to increase. A poor quality of the interface between filler and medium is likely to be responsible for this degradation. Insufficient interactions between silicate layers and the surrounding polymer lead to preferential pathways for diffusion. Organoclay exfoliation does not necessarily lead to better barrier properties. A good quality of the interface between polymer and filler is required to reach a high performance level. The standard techniques used to characterize exfoliation degree do not permit to highlight this kind of phenomena

Permeability, Diffusion and Solubility of Gases in Polyethylene, Polyamide 11 and Poly (Vinylidene Fluoride)

The gases transport coefficients, permeability, diffusion and solubility, are determined by the time lag method on a specific permeation cell. Three semicrystalline polymers, polyethylene (PE), polyamide 11 (PA11) and poly(vinylidene fluoride) (PVF2), are studied in the presence of helium (He), argon (Ar), nitrogen (N2), methane (CH4) and carbon dioxide (CO2) for temperatures ranging from 40 to 80°C in the case of PE, and from 70 to 130°C for both other materials. The applied pressures are, in the majority of tests, of 10 MPa for He, Ar, N2 and CH4, and of 4 MPa for CO2, except in some particular cases where the influence of pressure was studied. In the case of PE, the influence of the volume fraction of the amorphous phase, ranging from 0. 21 to 0. 70, the influence of temperature and the influence of the nature of the gas on the transport processes are investigated. Also, the independence of these phenomena related to pressure and sample thickness, between 0. 5 and 6 mm, is shown. For PA11, after determining the influence of temperature and of the nature of the gas used, the effect of the plasticizer incorporation in this polymer was studied. Regarding PVF2, apart the classic parameters that are temperature and the kind of gas used, we compare the coefficients of transport of CH4 and CO2 in PVF2 made up by extrusion or by compression moulding. For each polymer, it is shown that permeability, diffusion and solubility depend on temperature following Arrhenius' laws. It also seems that diffusion is directly related to the gases molecule size and that the solubility coefficient can be linked to the epsilon/K gases parameter. The comparison of the results obtained with the available data in the literature seems satisfactory

Phénomène de porosité et instabilités ductiles dans les matériaux hybrides polyéthylène/argile

De nombreuses études actuelles traitent des mélanges de polymère avec de l’argile. Elles sont fréquemment référencées sous le nom de nanocomposites car elles se focalisent sur la dispersion à l’échelle nanométrique des feuillets d’argile. Cette faible dimension fait que les taux de charge sont peu importants. Les nanocomposites présentent potentiellement beaucoup d’intérêt face aux composites conventionnels. Toutefois beaucoup d’études ne sont pas encore concluantes. Le présent travail s’attache à décrire l’existence de phénomènes indésirables dans les mélanges polymère/argile, et cherche à comprendre l’origine de tels mécanismes. Plusieurs mélanges à base de polyéthylène, d’argile organophile et d’agents compatibilisants ont été réalisés avec des conditions de mise en œuvre variées (mélangeur interne, extrusion bivis et monovis…). Dans quelques cas, l’allongement à la rupture chute de manière catastrophique. La diminution de la ductilité semble dépendre fortement de la mise en œuvre. Les faciès de rupture révèlent l’existence de porosité. La « pseudo-fragilité » de certains mélanges pourrait être reliée à la taille des pores. Dans certains cas, les pores atteindraient des tailles critiques, éventuellement par coalescence, qui empêchent l’éprouvette dans son ensemble de supporter l’étirement même si localement le polymère reste ductile, tandis que dans d’autres cas, les dimensions des cavités sont plus faibles, de sorte que localement les chaînes peuvent se réorganiser à tout instant, et donc la striction se propage sur toute la longueur de l’éprouvette. Cette porosité de la structure ainsi mise en évidence pourrait être l’explication des résultats de perméabilité médiocres de ces échantillons. Quelques résultats complémentaires viennent montrer comment les techniques communes qui permettent de caractériser le degré d’exfoliation de l’argile (DRX, MET, Rhéologie), sont insensibles à ce genre de phénomènes