The Effect of Polymer Structure on the Gas Permeability of Model Polyurethanes

Abstract

In the face of impending CFC elimination, a large variety of low-boiling alternate blowing agents have come under scrutiny for use in rigid foam insulation applications. The permeability of both the alternate blowing agent and atmospheric gases through the rigid foam thermoset polymer contributes to the rate at which the k -factor of the resulting foam deteriorates.In this study, a compression molding method for synthesizing polymer films from polyurethane formulation components (a variety of diols with pure and polymeric methylenediphenyl diisocyanate) was developed and utilized to produce films containing distinct structural features. The permeabilities of the films to oxygen and carbon dioxide were then determined using a diffusion cell combined with either an infrared or coulombic detector.The effects on permeability of structural features such as diol molecular weight, alkylene oxide repeat unit, and isocyanate functionality are presented. Structural features determined to promote improved barrier performance in polymer films have been incorporated into a fully formulated foam system. Foam permeability and k -factor aging comparisons were then made which demonstrate the improved foam barrier performance. The resulting structure/permeability correlations derived from these experiments will be useful in designing rigid foam polymers for use in alternate blowing agent applications.

Preparation of microcellular thermoplastic elastomer foams from polystyrene‐b‐ethylene‐butylene‐b‐polystyrene (SEBS) and their blends with polystyrene

“…The permeability of the gas molecules is determined by the T g of polymer. Low T g rubbery polymers usually, need lower energy to create microcavity for gas molecules diffusion as compared to high T g glassy polymer 27. By considering this fact, it could be speculated from our CO 2 diffusivity data that blending PS into SEBS could decrease the CO 2 permeability and thus, minimize the foam shrinkage.…”

Effects of Humidity and Elevated Temperature on the Density and Thermal Conductivity of a Rigid Polyisocyanurate Foam Co-Blown with CCl3F and CO 2

“…This enhanced the gas permeability of the polymer as well as its capability to sorb solvents. Kaplan and Tabor [22] have shown that humidity increased the oxygen permeability of thermoset polyurethane films having glass transitions near or below 25°C. The effects of water vapor on chain mobility in polymers with affinity for water have been documented [23].…”

“…In addition, environmental conditions such as moisture also affect the diffusion rate of R11. For example, using an FTIR techmque, Nguyen et al [7] determined values of Dff at 60°C to be 10.1 X 10-8, 22.2 X 10-a and 38.4 X 10-' cm2/s for Rll in rigid PIR foam exposed to relative humidities of < 10, 60, and 75%, respectively. Furthermore, the mass change of the foam is a function of the amount and density of the constituent gases within the cell cavities.…”

Gas Diffusion in Closed-Cell Foams

“…This is unfortunate since previous studies [25] showed that the gas diffusion coefficient through polyurethane membranes depends on the type of polyol used, whereas the influence of the isocyanate functionality has not been clearly observed. Since D c,i and S c,i may vary by several orders of magnitude from one polyurethane foam to another [25], any reliable assessment of gas diffusion models through polyurethane foams should be performed for polyurethane foams made out of the same polyol. For instance, Alsoy [15] used the experimental data for the diffusion coefficient of an unknown type of polyurethane membrane [16] to validate the Bart and Du Cauzé de Nazelle's model against the experimental data taken from other studies without checking if the polyurethane foams were generated using the same polyol.…”

“…Note that the specific type of polyols used for polyurethane foams is rarely mentioned and often unknown to the authors [17]. This is unfortunate since previous studies [25] showed that the gas diffusion coefficient through polyurethane membranes depends on the type of polyol used, whereas the influence of the isocyanate functionality has not been clearly observed. Since D c,i and S c,i may vary by several orders of magnitude from one polyurethane foam to another [25], any reliable assessment of gas diffusion models through polyurethane foams should be performed for polyurethane foams made out of the same polyol.…”