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This article surveys the physiological impact of waterproof textiles on the wearers of protective clothing. Wearer trials with test subjects in a climatic chamber involve ambient temperatures of +20, 0, and -20 deg C. The physiological function of breathable materials in comparison to a water vapor impermeable construction is quantified. Results show that water vapor permeable constructions offer a clear benefit to wearers at all tested temperatures: moisture accumulation in the breathable protective garments and in whole clothing systems are much smaller than in the nonbreathable one. Additionally, the ratio of evaporated sweat to produced sweat E/P is much higher for breathable constructions. Differences are statistically significant at levels of p > 0.995 or higher. There is no indication of a temperature dependency of the water vapor resistance of hydrophilic membrane laminates, but results show that, especially at ambient temperatures far below the freezing point, such breathable foul weather protective textiles still offer a great benefit to wearers.

Foul weather protective clothing for sports and occupational wear is extremely important to the textile industry throughout the world. However, feelings of uncertainty have been growing in the market about the function of so-called breathable (i.e., water impermeable but water vapor permeable) materials at different climatic scenarios. The literature describes some basic work. Osczevski and Dolhan [9] and Farnworth et al. [2] reported a strong dependency of the water vapor resistance of hydrophilic membranes or coatings: the higher the relative humidity at the membrane, the lower the water vapor resistance (i.e., the higher the water vapor permeability or breathability). In a temperature dependent experiment, Osczevski [10] placed a hydrophilic film on an ice block. Water vapor sublimating from the ice could diffuse only through the film and was collected by a dessicant. Osczevski measured mass transport through the film, and he found that water vapor resistance is an exponential function of temperature. In his experiment, water vapor permeability vanishes nearly completely with decreasing textile temperature: e.g., at -10 deg C he reported only 2% of the water vapor flux at room temperature. Because diffusion in hydrophilic materials is non-Fickian, he also derived from his results a theory of diffusion speed depending on activation energy epsilon, and he accounted for different relative humidities. He concluded that "the advantage of a `high-tech' vapor permeable waterproof shell over a standard [i.e., nonbreathable] waterproof coating is less pronounced in subfreezing weather." Additionally, Gretton et al. [7] reported an increase in the moisture vapor transmission rate (MYTh) of hydrophilic and microporous textiles when measuring with a heated dish instead of an unheated dish. They interpreted their results by the increased motion of water vapor and polymer molecules, which they claimed would also work for microporous constructions. For hydrophilic membranes, they also discussed the influence due to the change in relative humidity as reported in references 9 and 2. Contrary to their own results for single layer fabrics, Gretton et al. [7] found a decrease in the MVTR for some of the microporous constructions as a part of a clothing system measured with the heated dish method (see Table III of Gretton et al. [7]).

However, the results for temperature dependency published by Osczevski [10] and Gretton et al. [7] are based on experimental laboratory setups, and they have not been validated by wearer trials with test persons. Gretton et al. [7] announced such trials in their conclusions, and we may have overlooked them, but to our best knowledge, the works of Osczevski and Gretton et al. for practical use of foul weather protective clothing by man have yet to be tested on people.

Our approach is physiological and directly related to the wearer of protective clothing-breathability is a physiological parameter of clothing and textiles, hence this approach promises valuable results. To measure a wearer's related quantity free of doubt, the best procedure is human subject testing. Thus, in our work we compare hydrophilic breathable laminates with a water vapor impermeable coating in wearer trials. The main aim of this article is to determine whether or not a temperature dependency of the water vapor transport properties of hydrophilic membranes and coatings can be detected, and if their physiological function, especially at low temperatures, is still better than that of nonbreathable constructions.

For readers interested in details, a technical report (in German) [1] is available from us.

EXPERIMENT

WATER PROOF TEXTILES

The characteristics of textiles used for foul weather protective clothing worn in the wearer trials are given in Table I. The textiles were supplied by German manufacturers from their actual production. Physiological properties were tested by means of Skin Model measurements of the water vapor and thermal resistances R^sub et^ and R^sub ct^ according to ISO 11092 [8]. In comparison, 1 mm of still air has a water vapor resistance of R^sub et^ = 2.27 m^sup 2^ Pa/W. For thermal resistance (thermal insulation), the well known unit clo in official SI units is I clo = 155 X 10^sup -3^ m^sup 2^ K/W.

The aim of this work is to survey the temperature dependency of the physiological function, especially the water vapor transport properties, of hydrophilic foul weather protective textiles. Therefore, we tested two different hydrophilic constructions (samples 2 and 8). In comparison, a microfiber woven fabric (sample 7) and a water vapor impermeable coating (sample 5) were also tested as reference materials. Because constructions differ significantly, the water vapor resistances Ret also vary widely. On the other hand, their thermal resistances Rct (i.e., thermal insulations) are always very small. Thus, when the constructions are integrated in ready-made clothing systems, their contribution to the total thermal insulation is minor in comparison to, for example, air layers or underwear.

Textiles were tailored as two-piece suits of foul weather protective garments consisting of jacket and trousers. The cut of suits 5, 7, and 8 was identical, and suit 2's pattern was only slightly different. Thus, differences in the physiological function of the clothes could be directly attributed to the foul weather protective textiles.

Additional clothing components were worn, each depending on the test temperature. These additional clothes were identical for all foul weather protective suits tested. For each ambient temperature, the additional clothing components are described in Table II, and details can be found in reference 1.

CONTROLLED WEARER TRIALS WITH TEST PERSONS

Four young healthy men served as test subjects for each clothing system and each ambient temperature. Every suit was worn twice (i.e., one test repetition) at each temperature. Additionally, test persons were trained in climatic and activity conditions in pre-tests.

RESULTS & DISCUSSIONS

The hypothesis to be proved is that the physiological function of water vapor permeable foul weather protective garments is superior to nonpermeable clothing, not only above 0 deg C, but as well at temperatures far below the freezing point.

During the wearer trials, it turned out that the amount of moisture being accumulated in the clothing system led to very significant differences between the foul weather protective garments (see Figure 1), i.e., the amount of condensed water sometimes differed dramatically between the various ensembles. At T^sub a^ = -20 deg C the condensing water froze beneath the foul weather protective garments and ice (white frost) formed. Again the amount of ice strongly depended on the foul weather protective clothing used.

Moisture as well as frost accumulation is due to the condensation of evaporated sweat. Especially at cold temperatures and high temperature gradients, the dew point is reached within the clothing system. The barrier material leads to the steepest moisture gradient of all clothing worn, so condensation is usually comparatively high at that position.

Figure 1 shows the amount of moisture accumulated in the foul weather protective suits as well as in the whole garment system. Clearly, at all temperatures, especially at T^sub a^ = -20 deg C, moisture accumulation in the water vapor impermeable foul weather clothing system, sample 5, is much higher than in all "breathable" constructions. These differences between sample 5 and the other constructions are highly significant, mostly at a level of p > 0.999, but always at p > 0.995.

For physiological tests, our data show large differences and very high significance levels. Thus, results are free of doubt that in real wearing situations, moisture accumulation in water vapor permeable suits is much smaller than in unbreathable garments at all different temperature conditions. The high moisture accumulation in the water vapor impermeable suit not only impedes comfort, but it also lowers the thermal insulation of the clothing system by enhanced heat conductivity, which decreases the protection against cold. Especially at low temperatures (for example, the test temperature of T^sub a^ = -20 deg C), this may lead to a critical cooling of the body, e.g., during rest periods following strenuous work. Hence, high moisture accumulation can be harmful to the wearer's health (so-called post exercise chill).

We mentioned that suit 8 had been worn by different test persons. Therefore, results can only be compared to other garments on a limited scale. For example, it is not understandable that suit 8 shows a smaller moisture accumulation at T^sub a^ = - 20 deg C than suit 7, made of an identical textile material but without the membrane, thus having one water vapor barrier less. Therefore, we must conclude that this result is caused by the different test persons. On the other hand, differences in the water vapor impermeable suit 5 are significant at p > 0.999, so that even with other test persons, the differences in moisture accumulation are still trustworthy.

The data in Figure 1 show that moisture accumulation, especially in sample 2, increases with decreasing ambient temperature. However, this does not necessarily mean that water vapor resistance has increased as well: suit 7 has no membrane and so has no temperature dependency of the water vapor resistance at all. Nevertheless, sample 7 also shows an increase in moisture accumulation with decreasing temperature (although smaller than suit 2). Therefore, we must conclude that the increased moisture accumulation is not due to increased water vapor resistance. But it is likely that increased moisture accumulation at low temperatures is due to the changing temperature and water vapor gradients to be found in the thicker clothing systems. Especially at low temperatures, the dew point will be reached much sooner in the clothing system.

The ratio E/P of evaporated to produced sweat is a powerful relative assessment of the physiological function of different clothes. As Figure 2 shows again that the differences between the water vapor impermeable sample 5 and the breathable constructions are very large. Data demonstrate that the E/P ratio, and thus the physiological function, of the water vapor permeable constructions is much higher and better than for the nonbreathable one, especially at T^sub a^ = -20 deg C. These differences are again highly significant, usually at p > 0.999 and always at p > 0.998.

E/P decreases slightly with decreasing temperature. This has to be expected, because at lower temperatures more clothes are worn than at higher T^sub a^, thus leading to more water vapor barriers. On the other hand, the change in E/P is less likely to be due to increased water vapor resistance of the foul weather protective textiles at lower temperatures, because all clothing systems show this effect, especially suits 5 and 7, with temperature-independent water vapor resistances.

As we already reported for moisture accumulation, the other test persons wearing sample 8 yielded slightly different results. Particularly, the higher E/P ratio at T^sub a^ = -20 deg C in comparison to suit 7 is not logical. However, again the differences for suit 5 are large enough to be trustworthy.

A high E/P ratio is especially important for people engaged in heavy physical activity. Even at low temperatures, this is the case for winter sports like cross-country skiing. For such wear situations, water vapor permeable constructions obviously allow much better sweat transport, cause less moisture accumulation, and so offer better cold protection.

Hence, if Osczevski was right, at least at T^sub a^ = - 20 deg C, the ratio E/P and the moisture accumulation should be quite similar for all the different garment systems we have tested, except for the microfiber sample 7. However, our wearer trial results tell a different story. With highly significant differences, the water vapor permeable materials offer a superior physiological function compared to the impermeable one. Thus, Osczevski's results show no correlations to our wearer trials, and so from a physiological point of view, his technique and predictions cannot be regarded as quantitative, at least down to ambient temperatures of T^sub a^ = - 20 deg C.

We cannot figure out exactly why Osczevski's theory fails: It could be due to the measuring technique with an ice block, which, from a physiological point of view, is not an ideal simulation of the human skin or body.

Checking Osczevski's theory [10] at even deeper temperatures was not possible for us due to the technical limitations of our climatic chamber. However, most wear situations for foul weather protective textiles are within the range of temperatures we have covered with our experiments, especially for heavier sweating. All our results clearly show a great benefit to the wearer of hydrophilic, breathable laminates in comparison to a water vapor impermeable coating.

Particularly our results for sample 7, a microfiber woven, can be interpreted without any temperature dependency of the water vapor resistance, but can be explained by the thicker underwear, which we believe to be much more likely. On the other hand, Gretton et al. [7] claimed that a small temperature dependency leads to a slightly higher water vapor transmission rate for the heated dish method and single layers. They interpreted this as an increased motion of the water molecules and polymers caused by a higher temperature. They claimed that this would not work in clothing systems, because there the outer layer would be cooler. In this case, these authors even reported a decrease of the MVTR with higher temperature for some of their material combinations (see Table III of reference 7).

In our opinion it is more likely that there is no temperature dependency of the water vapor resistance of microfiber or microporous textiles at all. The water vapor resistance for such materials is mainly determined by the amount of pores in the textile [13], which is a matter of construction. This construction is macroscopic in comparison to the size of a water vapor molecule. The textile's construction, and thus the motion of water vapor molecules, is also not affected by the polymer motion, which is on an atomic scale [3, 4].

Higher water vapor molecule motion at an elevated temperature, which is also used as an explanation by Gretton et al. [7], in our opinion should not affect the overall steady-state diffusion rate in microporous or woven fabrics: In such textiles (different from hydrophilic constructions) Fickian diffusion under geometric constraints takes place. These geometric constraints result from the fact that free volume is occupied by the textile. The easiest way to treat these constraints mathematically is to change the diffusion constant D. The speed of the water vapor molecules and their typical path lengths may change, but diffusion is a statistical process. If steady state has been reached, a given number of water vapor molecules passes from inside to outside per area, water vapor partial pressure, and time units. But other molecules take the opposite way. Both processes may be speeded up if the temperature is increased, but they are faster by the same amount. Thus, the ratio of molecules going out to those coming into the clothing still remains constant. So the argument of Gretton et al. might explain a faster achievement of the steady state or modified unstationary properties, but not a change of the steadystate value itself.

In our opinion, the results of Gretton et al. [7] could be interpreted much more like this: The authors are measuring water vapor permeability in comparison to a standard material, and the result is given in percent. The differences they found for each microporous sample at different temperatures (i.e., heated or isothermal dish method) are quite small. Nevertheless, the authors interpret them to be significant, because they claim to have extremely good accuracy: The error they report for the relative water vapor permeability is only 1-2%, i.e., this method would be much more accurate than any other technique we are aware of. However, if the accuracy they report was only slightly poorer as given, around 5%, all of their results for the microporous constructions would be perfectly compatible with a water vapor transmission rate unchanged by temperature.

Our explanation is supported by another fact: If one calculates the mean value for the differences Gretton et al. report for the microporous textiles (single layers + clothing systems), a value of only 0.2% is obtained, which would be extremely close to our theoretical prediction of 0.

Actually, the results of Gretton et al. [7] for the hydrophilic textiles fit slightly better into our picture. Again, we would not agree to interpreting their results as increased molecular motion. But we do support their findings that applying a temperature gradient changes the relative humidity at the membrane significantly, which also changes the water vapor permeability of hydrophilic materials.

CONCLUSIONS

We have surveyed the water vapor transport properties of foul weather protective textiles and clothing as a function of temperature by means of wearer trials with human subjects. We have shown that the water vapor transport properties and the physiological function of water vapor permeable clothing, especially based on hydrophilic components, are much better than for impermeable clothing, also at temperatures down to -20 deg C. In particular, the ratio E/P of evaporated to produced sweat as well as the moisture accumulation in the clothing are much better in breathable than in nonbreathable garments. These differences are highly significant on a level of at least p > 0.995. Our results clearly indicate that the ability to transport water vapor and the physiological function of breathable foul weather protective clothing still exist at subzero ambient temperatures down to -20 deg C, and thus are relevant to most common wear scenarios.

ACKNOWLEDGMENTS

We are grateful to the Forschungskuratorium Textil for the financial support of the research project (AiF-No. 11674), which was funded by the German Ministry of Economy through a grant of the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen "Otto-von-Guericke." Samples of textiles and clothes were supplied by German textile manufacturers.

LITERATURE CITED

• Bartels, V. T., Survey on the Moisture Transport Properties of Foul Weather Protective Textiles at Temperatures Around and Below the Freezing Point (in German), technical report no. AiF 11674, Hohenstein Institute of Clothing Physiology, Boennigheim, Germany, 2001.
• Farnworth, B., Lotens, W. A., and Wittgen, P. P. M. M., Variation of Water Vapor Resistance of Microporous and Hydrophilic Films with Relative Humidity, Textile Res. J. 60(1), 50-53 (1990).
• Flory, P. J., "Statistical Mechanics of Chain Molecules," Interscience Pub., NY, 1969.
• de Gennes, P. G., "Scaling Concepts in Polymer Physics," 3rd ed., Cornell University Press, Ithaca, NY, 1988.
• Givoni, B., and Goldman, R. F., Predicting Metabolic Energy Cost, J. Appl. Physiol. 30(3), 429-433 (1971).
• Givoni, B., and Goldman, R. F., Predicting Rectal Temperature Response to Work, Environment, and Clothing, J. Appl. Physiol. 32(6), 812-822 (1972).
• Gretton, J. C., Brook, D. B., Dyson, H. M., and Harlock, S. C., Moisture Vapor Transport Through Waterproof Breathable Fabrics and Clothing Systems Under a Temperature Gradient, Textile Res. J. 68(12), 936-941 (1998).
• ISO 11092, Measurement of Thermal and Water-vapour Resistance under Steady-state Conditions (Sweating Guarded-hotplate Test), 1993.
• Osczevski, R. J., and Dolhan, P. A., Anomalous Diffusion in a Water Vapour Permeable, Waterproof Coating, J. Coated Fabrics 18, 255-258 (1989).
• Osczevski, R. J., Water Vapor Transfer Through a Hydrophilic Film at Subzero Temperatures, Textile Res. J. 66(1), 24-29 (1996).
• Umbach, K. H., Investigation of Constructional Principles for Clothing Textiles Made of Synthetic Fibers Worn Next to the Skin with Good Comfort Properties (in German), technical report no. AiF 3653, Hohenstein Institute of Clothing Physiology, Boennigheim, Germany, 1977.v
• Umbach, K. H., Methods of Measurement for Testing Physiological Requirements of Civilian, Work and Protective Clothing and Uniforms, Melliand Eng. 68, E379-- E383 (1987).
• Umbach, K. H., Moisture Transport and Wear Comfort in Microfibre Fabrics, Melliand Engl. 74, E78-E80 (1993).