http://jit.sagepub.com/Journal of Industrial Textiles

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 DOI: 10.1177/152808379602500408

1996 25: 311Journal of Industrial TextilesBarbara PauseLaminates

Measuring the Water Vapor Permeability of Coated Fabrics and  

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- Apr 1, 1996Version of Record >>

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311

Measuring the Water Vapor Permeabilityof Coated Fabrics and Laminates

BARBARA PAUSE

Gateway Technologies, Inc.6680 Gunpark Dr., Suite 200

Boulder, CO 80301

ABSTRACT: The water vapor permeability of a given material plays an importantpart in evaluating the physiological wearing comfort of clothing systems or deter-mining the performance characteristics of textile materials used in technical applica-tions. Many methods are used for measuring the parameters of water vapor transferthrough textiles; they vary in effectiveness and are difficult to correlate to one

another. This report discusses the mechanism of the water vapor transfer in textiles,and common methods used to determine parameters of the water vapor transfer.

Finally, a new measuring method will be introduced and compared to some existingmethods.

1. INTRODUCTION

THE THERMOPHYSIOLOGICAL WEARING comfort of a clothing system isdetermined mainly by the water vapor permeability of the textilematerials in the system. A high degree of water vapor permeability of theclothing system supports the moisture transfer emitted from the skin of thewearer through the textile layers into the environment. In this way a com-fortable microclimate can be established, even at different levels of physicalactivity on the part of the wearer. But the water vapor permeability is notthe only important property for materials that are applied to clothing; tech-nical applications of textiles also require knowledge of the water vaportransfer as a condition for suitable use.Because the knowledge of the water vapor permeability through a given

material is so important for many applications, the measurements of suchparameters should be very accurate as well as time- and cost-effective. Fur-

JOURNAL OF COATED FABRICS, Volume 25-April 1996

0093-4658/96/04 0311-10 $10.00/0@1996 Technomic Publishmg Co., Inc.

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thermore, the test equipment should be easy to handle. Using portable test-ing equipment would be most desirable in order to allow for measurementsnot only in laboratories but also in the production mills in which thematerials are produced.With these objectives in mind, I have developed a new testing method for

determining water vapor transfer through textile materials. In this report thenew testing method will be introduced and compared with the existingstandard testing methods. Furthermore, the testing apparatus will bedescribed and some results of test measurements will be presented and dis-cussed.

2. WATER VAPOR TRANSFER THROUGH TEXTILES

Water vapor flux through a textile barrier results from a water vapor con-centration gradient that exists in a clothing system, for example, between themicroclimate near the surface of the skin and the exterior environment. Thewater vapor flux through the textile material occurs in four different ways:1. Diffusion of the water vapor through the air spaces between the fibers2. Absorption, transmission, and desorption of the water vapor by the

fibers3. Adsorption and migration of the water vapor along the fiber surface4. Diffusion of the water vapor through the fiber or yarn capillariesWater vapor diffusion through the air spaces between the material fibers is

the main transport mechanism for water vapor in textiles. It is dependent onthe textile structure, i.e., the volume of air trapped in the fabric. An openfabric structure in a given material promotes this diffusion process. Butwater vapor transfer through these air spaces is also influenced by thematerial thickness. The thicker the material, the more limited the watervapor diffusion because the moisture will be stored in the air spaces or ab-sorbed by the fibers before it passes through the fabric. The water vapor ab-sorption of fibers depends especially on their chemical structure. In absorb-ing water vapor, the fibers swell reducing the size of the air spaces, whichleads to a delay in the diffusion process and hence to a decrease in watervapor transfer through the textile. Water vapor absorbed by the fibers canalso be transferred through the fibers and again emitted into the air spaces orreleased to the environment by desorption. However, compared to the watervapor diffusion through the air between the fibers, water vapor transferthrough the fibers themselves occurs at a much slower rate and involves amuch lower quantity of water vapor. Therefore, the fibers act more likemoisture sinks and sources rather than as a mean of water vapor transporta-tion.

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The effectiveness of the water vapor migration process along materialfibers is determined by the fibers’ wetting capacities and especially by thesize of their surface. For example, in textiles made of microfibers, the fibersurface is very large, allowing significant amounts of water vapor to migrate.Water vapor transfer through the fiber capillaries is based on the capillary ac-tivity and is determined by the fiber fineness. This process is especially com-mon in textile materials made of synthetic fibers [1],[2].Water vapor transfer through a textile material is not constant. Besides the

fabric structure and the moisture absorption capability of the material, thewater vapor transfer is dependent also on the difference in water vaporpressure between the two material surfaces, which is determined by the pre-vailing temperatures and humidities on both sides [3].The amount of water vapor transfer through a textile material is usually

expressed by the followmg values: water vapor permeability (Wd), watervapor transmission (WVT), and water vapor resistance (R,.,). Water vaporpermeability (Wd) and water vapor transmission (who) have a direct rela-tionship to the water vapor transfer across the fabric. Water vapor perme-ability (Wd) indicates the quantity of water vapor that has moved through aunit area of the sample material in a certain period of time as a result of thedifference in water vapor pressure between the two sample surfaces. Thewater vapor transmission value (WVT) also indicates the water vapor flowthrough the sample within a given time interval, but without reference tothe water vapor pressure difference exerted during the measurement. Finally,the water vapor resistance (Ret) describes a material’s resistance to moistureor evaporative transport through a material; therefore, it is inversely propor-tional to the other two values. The water vapor resistance (Ret) is determinedfrom the evaporative heat flux through the sample per unit area and the dif-ference in water vapor pressure.

3. COMMON MEASURING METHODS

In order to determine the values for water vapor transmission, a variety ofdifferent measuring methods are used. The most common methods are theASTM E 96-94 Standard Method and the ISO 11092 Standard Method. Im-

portant characteristics of these two methods are summarized in Table 1.

The ASTM E 96-94 Standard Method involves filling a dish with distilledwater and covering the opening with the test sample. The dish with the sam-ple is then placed in a climatic chamber over a given period of time. Duringthe measurement interval, water penetrates the sample fabric and enters thechamber atmosphere above, which results in a water loss inside the dish. Thewater dish and test sample are weighed during the measurements at specifictime intervals. When the water vapor transfer through the sample is stabi-

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Table 1. Common measuring methods.

lized, the water vapor transmission (WVT) is determined from the weightloss during the time interval referred to the sample size. The StandardMethod DIN 53 333 is based on the same measuring principle, but the wateris replaced with a desiccant. The water vapor transmission (WVT) is thendetermined from the weight increase of the desiccant due to the moisture ab-sorption at the specific time interval and the sample size [4],[5].The ISO 11092 Standard Method, also known as &dquo;Skin Model,&dquo; involves

a different measuring method to determine the water vapor transfer througha material. In this measuring method a &dquo;sweated skin&dquo; is simulated by aheated porous metal plate. The water vapor generated on the plate penetratesthe sample as a result of a difference in water vapor pressure between theplate surface and the outside environment. The water vapor resistance (R,,)is determined from the heating power necessary to maintain a constant watervapor flux through the sample and the water vapor pressure difference. Thewater vapor permeability (Wd) of the samples is calculated from the watervapor resistance and the latent heat of vaporization of water under the testconditions [6].Because the measurements of the above methods are carried out while the

water vapor flux through the sample remains constant, the measuringperiods last several hours. Due to these long measuring intervals neithermethod is suitable for routine tests. Furthermore, these methods requirelarge sample sizes. The maximum sample thickness for tests carried out us-ing the ASTM E 96-94 Standard Method is 32 mm. An essential disadvan-tage of these measuring methods is that different measurements are deter-mined under different test conditions; consequently, the measuring resultsobtained with the different methods are not comparable to one another. Ex-penditure on the test equipment used for the ISO 11092 Standard Method,as well as its acquisition cost, is extremely high.

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4. A NEW MEASURING METHOD

Based on the study of the existing standard measuring methods - as wellas other measuring techniques not discussed in this paper-and the compari-son of their advantages and disadvantages, I have developed a new mea-suring method. It is based on the determination of the humidity change inan air volume over a certain period of time. The use of a very sensitive mea-suring technique allows for a short measuring procedure that lasts only a fewminutes. Therefore compared to the common measuring methods, thesetests are carried out much faster (Table 2).

In the new measuring process, all of the common measuring values of thewater vapor transfer are determined simultaneously under precisely-definedtesting conditions. The tests can be carried out at humidities between 5%and 90% and in a temperature range between 20°C (68°F) and 70°C(158°F). Taking into consideration all testing conditions that the othermethods are based on, the values can be determined; therefore, a direct com-parison of results obtained with the other methods is possible. A sample sizeof about 76 mm X 76 mm is necessary for the tests. This sample size ismuch smaller than is necessary with the other methods. Materials with athickness up to 50 mm can be investigated. Therefore, in comparison withthe ASTM E 96-94 Standard Method, much thicker samples can be testedwith the new method.The measuring apparatus consists of two pieces: the basic device and the

measuring computer. The measuring computer is a laptop equipped with adocking station that includes the measuring system. The complete measur-ing system is portable; it fits into a medium-sized carrying case. The entiremeasuring procedure runs computer-controlled. The manual labor neces-sary is therefore minimal compared to other testing methods [7].The basic device consists of two chambers; one is arranged atop the other.

Table 2. Parameter of the New Measuring Method.

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The lower chamber is filled with distilled water. The upper chamber con-tains an air volume with a certain percentage of humidity. During the mea-surement, the test sample is fixed onto the sample holder and arranged be-tween the two chambers. The sample lies on a fine meshed lattice so that adefined amount of water vapor will transfer from the water bath to the sam-

ple. The testing device also contains a moistening/drying system in order toestablish the desired humidity in the upper chamber at the beginning of atest and to remove the moisture at the end of the test. A heating system alsomakes measurements at different temperatures possible. The apparatus is

equipped with humidity and temperature sensors.Before the tests are initiated, the desired humidity in the upper chamber is

established by moistening or drying the air inside. If tests are to be carriedout at temperatures above room temperature, the air and the water bath are

preheated. After the desired initial conditions are realized, the sample is fedinto the testing device by means of a slide-type mechanism. If the sample isinserted in the correct position, the slides separating the sample from thetwo chambers are removed. At the same time, the measurement processbegins. Due to a difference in water vapor pressure between the two cham-bers, water vapor generated in the water bath penetrates the sample, reachesthe upper chamber, and increases the humidity therein. The humidity in-crease within the measuring interval is then determined. The temperatures ofthe water bath in the lower chamber and the air in the upper chamber are

also measured. The measurement ends when a humidity increase of approx-imately 3% is reached.Based on the temperature and humidity measurements in both chambers,

the water vapor pressure difference and the latent heat of vaporization of thewater are determined. The increase in moisture content in the upper cham-

ber during the measuring interval is also calculated. Based on this data thecommon measurements for the water vapor transfer, water vapor permeabil-ity (Wd), water vapor transmission (WVT), and water vapor resistance (Ret)are determined. All of these values are calculated by means of a computerprogram and presented together with test conditions in the form of a testingrecord.

5. TEST AND COMPARISON MEASUREMENTS

In order to test the new measuring apparatus, comparison measurementswere carried out using the ASTM E 96-94 Standard Method and the ISO11092 Standard Method. Included in the comparison tests were the follow-ing samples:0 polyester (PES) knit

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. fabric [85% polyester (PES) /15% cotton (CO)]

. polyamid (PA) knit with polyurethane (PUR) coatingThe water vapor absorption capability of all samples was low due to the

low moisture absorption of the polyester and polyamid fibers, the low quan-tity of the cotton fiber with a high moisture absorption capability, and thesmall thickness of the samples. The water vapor transport within thesamples was therefore not strongly influenced by moisture absorption. As aresult of the moisture absorption, measuring errors which occur when usingsuch dynamic measuring methods did not need to be considered. The in-vestigations were carried out under the same testing conditions that thestandard methods are performed-ASTM E 96-94: 21°C (70°F), 50%relative humidity; ISO 11092: 35°C (95°F), 40% relative humidity. Thesamples were exposed to these climatic conditions for 12 hours before thetests were carried out. In this way a moisture content comparable to thesamples tested with the Standard Methods were reached.The test results are shown in Figure 1 and Figure 2. In the comparison test

using the ASTM E 96-94 Standard Method, the correlation of the measur-ing results was between 84% and 88%. A correlation between 90% and93% was obtained in the comparison using the ISO 11092 StandardMethod. The expectations of the comparison test were exceeded. The bestcorrelation was reached in both comparison of the polyester knit sample,probably due to the low moisture absorption of the polyester fiber and theregular structure of the sample.

FIGURE 1. Comparison of measured results, obtamed m tests usmg the ASTM E 96-94Standard Method and the New Measunng Method (NMM)

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The maximum deviations from the mean value amounted to 10% for themeasurements carried out with ASTM E 96-94 Standard Method. The vari-ation coefficients of the measurements made using the ISO 11092 StandardMethod and our new method were 6%. Therefore, the accuracy of ourmethod is comparable to that of the tests carried out using the ISO 11092Standard Method and much higher than that of the tests carried out with theASTM E 96-94 Standard Method.The measuring apparatus was further tested under variation of the test

conditions. Included in the tests were the following samples:. polyester (PES) knit· foam with polyurethane (PUR) coating. laminate of the coated foam with the polyester knit

The tests were carried out under various relative humidities between 45 %and 75% in the upper chamber of the device. The chamber temperature was

kept at a constant 21 °C (70°F) during the tests. The values obtained forwater vapor transmission (WVT) are shown in Figure 3.The water vapor transmission (WVT) of the samples changes as the hu-

midity changes. Thus, it is confirmed that tests carried out under differenttest conditions lead to different measuring results in all cases and a compari-son of such test results is therefore useless. For each of the samples, a

decrease in water vapor transmission (WVT) occurred while an increase inthe humidity was taken place, but the changes in the water vapor transferwere different. The water vapor transmission (WVT) in the studied humid-ity range was reduced by 80% for the polyester knit sample, by 70% for thefoam sample and by 50% for the laminate. Thus, samples like the laminate

FIGURE 3. Water Vapor Transmission (WVT) and relative humidity.

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with a low overall water vapor transfer are also influenced by the test condi-tions, which must be taken into account when comparing materials as wellas when considering potential material applications.

6. SUMMARY

The results obtained from the use of the new measuring method correlateto those obtained from the common Standard Testing Methods. Comparedto the ASTM E 96-94 Standard Method, our new method yields more ac-curate results. Furthermore, the previous tests showed a level of accuracycomparable to that obtained with the ISO 11092 Standard Method. Usingour method all common measurement parameters of water vapor transfer intextiles are determined simultaneously under defined testing conditions. Thetests can therefore be carried out under various conditions, which makescomparisons with other measuring methods possible, as well as measure-ments under specific desired application conditions. One great advantage ofour measuring process is the short measurement interval, which makes rou-tine tests especially cost- and time-effective. The portable design allows formeasurements to be taken from any location and the testing device cantherefore be implemented in the production process as well as at sales pre-sentations.

REFERENCES

1. Umbach, K.-H. 1993. "Feuchtetransport und Tragekomfort in Mikrofaser-Textilien," Melliand Textilberichte, 2:174-178.

2. Wehner, J. A., B. Miller and L. Rebenfeld. 1988. "Dynamics of Water VaporTransmission through Fabric Barriers," Textile Research Journal, 10:581-592.

3. Farnworth, B., W. A. Lotens and P. P. M. M. Wittgen. 1990. "Variation of WaterVapor Resistance of Microporous and Hydrophilic Films with Relative Humid-ity," Textile Research Journal, 60(1):50-53.

4. 1994. ASTM E 96-94: Standard Test Methods for Water Vapor Transmission ofMaterials, Annual Book of ASTM Standards, Philadelphia, pp. 1-8.

5. 1981. DIN 53 333: Pr&uuml;fung von Leder, Kunstleder, und Textilien; Bestimmungder Wasserdampfdurchlassigkeit. (8.1981), Beuth Verlag, Berlin.

6. 1993. ISO 11 092: Textiles-Physiological Effects-Measurement of Thermal andWater-Vapour Resistance under Steady-State Conditions (Sweating Guarded-Hotplate Test). (10.1993) ISO Switzerland.

7. Pause, B. 1996. "Portable Device to Measure Parameter of the Water Vapor Trans-fer through Textiles," Unpublished technical documentation of the developedmeasurement method.

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