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R E S E A R C H A N D D E V E L O P M E N T B R A N C H

DEPARTMENT OF NATIONAL DEFENCE

CANADA -~~

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DEFENCE RESEARCH ESTABLISHMENT OTTAWA—-

7 1 TECHNICAL NOTE NO. 79-1

~4N ~ PPARATUS FOR THE M~ASUREMENT OF THEWA’fER-VAPOUR PERMEA~TEITY OF TEXTILES #

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byB. Farnwor-th

Environmental Protection SectionProtective Sciences Division

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ABSTRACT

An apparatus has been constructed to measure the water-vapourresistance of textiles , in particular those such as battings and pilesthat are highly air permeable and have low vapour resistances . Thetechnique has been tested by measuring the water vapour resistance ofair gaps in the range 1 to 8 mm thickness . No systematic errors wereobserved with a random error in the resistance measurement equivalentto 0.5 nan of still air.

RE SUME

On a mis au point un appareil pour mesurer la résistancedes textiles ~ la vapeur d’eau, et par ticulièrement de ceux,coinme lea tissus peluchés et d’autres ~ fo rmation en nappes ,qul sont tr~s perméables ~ l’air et offrent *me faible résistance~ la vapeur. La méthode a été éprouvée par la mesure de larésistance ~ la vapeur d’eau , d~espaces d’air, d’une épaisseurde 1 ~ 8 mm. On n’a pas relevé d’erreurs systématiques, et cepour ime erreur aléatoire dans la mesure de la résistance,équivalente ~ 0.5 nan d’air immobile.

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INTRODUCTION

Since the accumulation of water In clothing as a resul t ofsweating seriously degrades its thermal insulation, a knowledge of thepropagation of water is essential to the design of good clothing. Inorder to predict the water vapour transport properties of the clothingsystem as a whole, a knowledge of the water vapour resistance of eachconstituent material is required. This paper describes an apparatus

• designed to measure the water vapour resistance of all types of materials,including insulating fabrics such as piles and battings, that are usedin arctic clothing systems.

The propagation of water vapour through a dry fabric is governedby an equation of the form :

(1) M = At~P/R

where M is the mass transmitted per unit timeA is the area of the fabrict~P is the difference In partial pressure of water vapour across

the fabricand R Is the vapour resistance of a unit area of the fabric.

Standard methods for the determination of R consist basicallyof placing the sample over a dish of pure water, at a fixed distancefrom the water surface, in a controlled temperature and humidityenvironment, and measuring the weight loss of the dish (l)(2). Sucha test measures not only the resistance to the passage of water vapourof the sample but also that of the air gap between the water and thesample and that from the upper surface of the sample to the surroundingatmosphere. These extraneous resistances can be eliminated orestimated by a variety of techniques. For example, the effect of theair gap can be estimated by varying its thickness and noting the changein overall resistance (3) or the resistance between the sample andthe atmosphere can be minimized by generating an air flow over the

• sample (1). However, neither of these methods is applicable tomaterials such as battings where the surf ace Is not sufficientlywell defined to permit accurate adjustment of an air gap and where thehigh air permeability of the material precludes generating an air flow.

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These di fficulties have been over come by the apparatusdescribed in this paper. In this apparatus , the sample Is san~ iichedbetween two layers of a niicroporous Teflon sheet which is highlypermeable to water vapour (4) . Thus , on one side , the liquid watercan be brought In to con tact with the Teflon , eliminating the air gap

• and , on the other , dry air can be blown over the Te flon , minimi zingthe sample — to—atmosphere resistance. The resistance of the two layersof Teflon is taken into account by measuring the overall resistance ofthe apparatus with and without the sample in place.

EQUIP~~NT AND PROCEDURES

Description of the Apparatus

The apparatus Is shown schematically in Figure 1 with thedetails of the sample holder in Figure 2. The upper pa~rUon of thesample holder con tains distilled water 8eparated from the sample bya sheet of microporous Teflon (4) supported by a stainless—steel mesh .The sample is sealed in to a ring of silicone rubber (5) to preventescape of water vapour at the edges . The sample is separated from thelower portion of the chamber by a second sheet of Teflon . Dry air iscirculated , at a cons tant rate of about 4.8 1/mm , through this chamberand drying tubes of CaSO~.. The sample chamber si ts on a scale , readableto 0.1 g and Its mass is noted every 5 or 10 minutes to obtain themass loss rate . The vapour pressure above the samp le is taken as thesaturation vapour pressure of water at the temperature of the waterbath , measured with a coppe r— cons tantan thermocouple to an accuracyof 0.2 °C. The vapour pressure of the returnIn g dry h r is assumed tobe that given by the manufacturer of the drying tubes (6) (dew point~—8O°C) which is negligible .

The resistance for the passage of water vapour from the bathto the drying tubes is then given by equation 1 wi th & equal to thesaturation vapour pressure at the water bath temperature . The massloss rate is measured over a period of several hours , af ter an initialstabilization period of sufficient duration to establish a constantloss rate (about one—half hour for a non—hygroscopic material).

The resistance of the sample is taken as the diffe rence inoverall resistance with and without the sample in pla ce , times asmall correction factor;

(2) R =(R — L )xCsample with sample without sample

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The correction factor C arises because the area of the sample is about15% larger than the area of the water surface. Hence the flow of watervapour is not strictly perpendicular to the water surface and the rateis slightly larger than that expected from equation 1. C has beencalculated from a numerical solution of Laplace’s equation, whichgoverns the vapour flow, in the actual experimental geometry (7).The results are shown in Figure 3, as a function of sample thickness,for a constant sample area equal to 1.15 times the water surface area.

Sample Preparation

The samples are sealed into a ring of sIlicone rubber by meansof a rotational mold shown in cross section in Figure 4. The sample isheld between two aluminum plates separated by the nominal thickness ofthe sample. The mold is spun at 1800 rpm and a mixture of roomtemperature vulcanizing rubber and hardener (5) Is fed onto the topplate. The rubber flows to the outside rim of the mold forming a ringwhich seals the edges of the sample. The height of the rIng is thethickness of the sample plus 12 mm above and below. The sample holderhas a 12—nun shoulder on the side of each of the upper and lower chambers(see Figure 2) so that when the sample is placed in the holder the twolayers of Teflon are held at a spacing equal to the nominal thicknessof the sample. In this way the compression of the sample Is maintainedat a known value during the measurement.

RESULTS AND DISCUSSION

The performance of the apparatus was tested by measuring thevapour diffusion resistance of air gaps of thickness in the range 1 to8 mm . The results are shown in Figure 5 where the overall resistance ofthe apparatus is plotted versus the gap thickness. To correct for thearea discrepancy mentioned above the thickness has been divided by thecorrection factor C. The resistance is expressed as the thickness ofa layer of still air expected to have that resistance. This latterquantity is calculated from handbook (8) values of the diffusion constantfor water vapour Into air .

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The graph is expected to be a straight line of unit slope,the zero thickness intercept giving the residual resistance of theapparatus without a sample . The line drawn in Figure 5 is the best— - •

fit straight line of unit slope and agrees with the data withtn therandom experimental error.

The error arises primarily from the inaccuracy in ~~e weightmeasurements which are sensitive to fluctuations in room temperature

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and to air currents. The measurements improve with the length of timeover which they are taken. These data are an average of three trialsat each point taken over about 3 h per trial. The error bars indicatestatistical standard error among the three trials.

The resistance values measured for a few fabrics of currentinterest to the clothing research program are listed in Table I.

CONCLUSION

It Is concluded from the results shown in Figure 5 that theapparatus is accurate to a resistance of about 0.5 nun equivalent airgap without systematic error.

5

TABLE I

Water—Vapour Resistance of Some Textile Materials

Sample Thickness R R(mm) (in2 s pa kg ’) (mm equivalent

still air)

Thinsulatea 7.7 (6.3 ± .3)x l0~ 11.5 ± .5M 400

Thinsulate1’ 6.4 (4.0 ± .6) xlO’ 7.3 ± 1.0M 530

ThinsulateC 3.2 (2.6 ± .2) -xiO 7 4.8 ± .4CS 150

coretex*d 0.36 (1.83 ± .04) X1O7 4.10 ± .07

Text 7~~~5e 0.38 (0.42 ± .02) xlO7 0.76 ± .04

* Laminated between Nylon—Tricot and Nylon—Poplin

a. 3M Company, microfibre non-woven polypropylene batting,400 g/m2.

b. As above, 530 g/m2.

c. 3M Company, microfibre non—woven polyester/polypropylenecomposite batting, 150 g/m2.

d. Gore Industries Ltd, microporous/Teflon Sheet.

e. 50/50 Nylon/cotton twist fabric, 170 g/m2, OGlO7 dye.

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REFERENCES

*

1. Canadian Government Specifications Board 4 —GP—2 , Method 49(Control Dish Method).

2. Whelan E .M. et al. Textile Research Journal 25, 197 (1955).

3. Spencer—Smith J.L. , Clothing Research Journal 5, 82 (1977).

4. Gore—Tex. Gohlke, D.J. and Tanner , J.C. J. Coated Fabrics 628 (1976). —

5. RTV—51l, General Electric Corp. (USA).

6. IndIcating Drierite, W.A. Hammond Drierite Co. (USA).

7. The solution technique is similar to that in:Pitts, D.R. and Sissom, L.E. “Theory and Problems in HeatTransfer”, McGraw—Hill (1977).

8. American Institute of Physics Handbook. McGraw—Bill (1957).

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Fi gure 1. Schews2tic diagram of the apparatus. The ecv’~ le i8 he ldbeiveen ~~o layers of nn oropor oua Teflon with water onone aide cmd dry air on the other. The n~zea of thesamp le holder is monitored to give the water vapourdiffusion ra te.

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Figure 2. De tails of the ~~~ holder. The samp le is embedded in aring of silicone rubber to prevent leakage of wa ter vapourat the edges. The upper layer of Te fl on is supported bya 8 tain less -s teel mesh.

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Fi gure 4. Rotational mold for imbedding samples into silicone- rubber. The room-temp era t~re vulcanizing rubber is• injected at the top and flows to the outside rim- where it sets into a ring.

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THICKNESS/C (mm)

Fi gure 5. Overall resistance of the apparatus with the camiile repby air layers of various thickness. The thicknecseo havebeen divided by the corre ction fac tor C to account for thearea discrepancie s. The resistance is expressed an thethickness of a per fe c t ly sti l l air layer having tha tresistance . The line is of unit s lope with interceptadjusted to f i t the da ta point s.

UNCLA$SryrEr~Sicenly Ciaft,f,c*i,on

DOCUMENT CO NTROL DATA — R & 0( Seccc. n y I as su I n c ct c i i i u t i l l , - , IxxIy ut abst rac t ac,,I oil,. n.j ali ,ncIalc uo 110.1 lye entered wu i ,-~ . I he- ,,- C l •lII (k)tcj i erec t 5 1 1 aSSI I CMI -

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Defence Research Establishment OttawaNational 1)efence Headqullrter8• Qtt ava, Onta rio, Cnn~ e4~ - LIA OZ~ - - •• ___ -

- - 4 DC (CUMI N I I I I I

An Apparatus for the Measurment of the Water Vapour Permeabilityof Textiles

4 DESCRIPT IVE NOTES (Type ccl report and InClusIve’ dates)

Technical Note.5 AUTH OR(S ) Last name , Inrst name midd le c n c t I a ) (

Farnworth , Brian

~~. DOCUM~~~T ~~~.j j la T OTAL I~O OF PAGES 1L NO )~ R E F SMarch 1979 ii. 8

8a. PROJECT OR GRANT NO. 9a ORIGINATOR S DOCUMENT NUM 1IER IS,

14B00 DREO Technical Note 79-1

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10. DISTR IB UTI ON ST A T E M ENT

Distribution unlimited.

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13 ABST CT• Unclassified

apparatus has been constructed to measure the vat~r vapourresistance of textiles, in particular those such as battings andpiles that are highly air permeable and have low vapour resistances.The technique has been tested by measuring the water vapour resistanceof air gaps in the range 1 to 8 mm thickness. No systematic errorswere observed with a random error in the resistance measurementequivalent to 0.5 of still air.

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UNClASSIFIEDSecu r ity Class ,f,ca i,on

KEY WORDS

Test equipmentWater vapo~~PermeabilityFabrics

INSTRUCTIONS

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