Material BehaviourEffect of temperature on water vapor transport through polymer membrane laminates
Introduction
Polymer membranes laminated to textiles are used extensively in waterproof breathable clothing items such as jackets, gloves, and boots. The polymer membrane acts as a barrier to liquid water entry from the environment, but is sufficiently permeable to water vapor to allow significant amounts of sweat to evaporate through the clothing system. Familiar examples are Gore-Tex® and Sympatex® clothing products.
Laboratory testing is usually a necessary first step to evaluate the comparative water vapor transport properties of candidate materials for new clothing system designs. However, comparison of material properties often becomes complex owing to changes in tested properties at different test conditions. One material may be rated better than another material at one particular set of test parameters, yet the ranking may reverse under a different set of conditions. The two effects which are usually responsible for changes in ranking of materials are concentration-dependent permeability, and temperature-dependent permeability.
Concentration-dependent permeability: Membranes which contain a continuous hydrophilic component, such as Gore-Tex® and Sympatex®, change their transport properties based on the amount of water contained in the hydrophilic polymer layer. The magnitude of the relative changes in water vapor transfer rate as a function of membrane water content is quite large for several common clothing materials and systems. The water content of these materials is a function of the water vapor content (humidity) of the environment on either side of the clothing layer. Test methods which evaluate concentration-dependent permeability need to be capable of independently varying the relative humidity of the environment on the two sides of the material. Concentration-dependent permeability has been studied and reported on extensively 1, 2, 3, 4, 5. Reported work on measured temperature-dependent water vapor transport properties of clothing membrane laminates is less common [6].
Temperature-dependent permeability: Some polymer membranes may exhibit lower intrinsic water vapor transfer properties at low temperatures. This effect is of practical importance for the ability of cold weather clothing to dissipate water vapor during active wear, or for boots, gloves, and sleeping bags to dry out under cold conditions. Knowledge of temperature-dependent permeability is also important when comparing test results between test methods or laboratories which may conduct standard testing at different temperatures. Analysis of temperature-dependent permeability must distinguish between changes in the intrinsic transport properties of the material, and the apparent decrease in water vapor transport rates due simply to the lower vapor pressure of water at lower temperatures.
Osczevski [6]has shown that water vapor diffusion in hydrophilic films and membranes can be affected by the ambient temperature. The intrinsic water vapor diffusion resistance increases at lower temperatures (Fig. 1), which lowers the rate at which water vapor is transported across the layer.
It is difficult to evaluate temperature-dependent permeability with common textile test methods. The method used by Osczevski was a modified cup method that could not independently control the humidity on the two sides of the sample. Thus the temperature-dependent results obtained by Osczevski were not obtained at similar polymer water content levels (or equivalent points on the water vapor sorption isotherm). This means that the water vapor transport results were also affected by the concentration-dependent transport properties of the materials which were tested.
The objective of this study is to determine the degree to which water vapor transport properties of several different polymer membranes and membrane/textile laminates are affected by temperature. A particular test method, the Dynamic Moisture Permeation Cell (DMPC), is ideally suited for this type of study, owing to its complete control over the humidity and flow rate on the two sides of the test sample, and the ability to control the temperature of the test system. This allows temperature-dependent effects to be separated from concentration-dependent effects on mass transfer phenomena. The DMPC permits the experimenter to explore the temperature dependence of the diffusion behavior at different points on the vapor sorption isotherm of the hydrophilic polymer component of a polymer film or membrane laminate.
Section snippets
Materials
Nine materials were selected for testing under various conditions. The materials are all polymer membranes or membrane laminates. These materials are of interest for various types of protective clothing systems. They include commercially available polymer membrane/fabric laminates used in items such as gloves, boots, and cold weather parkas. Some of the materials are under development for chemical protective clothing applications and the composition of the membrane component is proprietary.
Method
The Dynamic Moisture Permeation Cell (DMPC) 7, 8, 9is an automated device that can test the mass transport properties of very small pieces of woven and nonwoven fabrics, membranes, and foams. The apparatus is more convenient to use than the traditional test methods for textiles and clothing materials, and allows one to use a wider variety of test conditions to investigate concentration-dependent and nonlinear transport behavior of the many types of semipermeable membrane laminates which are now
Test conditions
The setpoints used for each material are summarized in Table 2. The first six setpoints are at a constant humidity gradient of 0.50 (50%). The final setpoint is at the maximum humidity gradient of 1.0 (100%). This final setpoint is the one used to show the temperature dependence of the water vapor transport properties. Some of the samples tested proved to have very low water vapor fluxes at the low temperatures, so it was necessary to maximize the possible water vapor flux by providing the
Conclusions
The Dynamic Moisture Permeation Cell (DMPC) was used to evaluate the temperature-dependent water vapor transport behavior of nine polymer membranes and membrane/textile laminates. The DMPC's control over vapor concentration, gas flow rate, and temperature allowed concentration-dependent effects to be distinguished from temperature-dependent effects.
Measured changes in water vapor flux over the temperature range of 3–40°C were primarily due to the fundamental physical relationship between
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