Indi an Journal of Fibre & Textile Research Vo l. 29, June 2004, pp. 122-1 28

Air and water permeability characteristics of nonwoven fabrics

D Bhattacharjee, A Ray & V K Kothari"

Department of Tex tile Technology, Indian Institute of Technology, New Delhi 110 016, India.

Received II November 2002; accepted 4 April 2003

Ex perimental investigations on ai r and water permeability characte ri stics o f spunlaid heat-bonded Typar fabrics show a very close linear correlation between both the permeability va lues. The ra ti o of water permeability to air permeability is a fU llction of the pressure head at whi ch these val ues are measured . The values of air and water permeabilities themselves are a po lyno mial function of the fabri c porosity . I-I ence, the know ledge of fa bric porosity and the ac tive pressure head would be enough to predict the air permeability and water permeability va lues of spun laid heat-bonded fabrics.

Keywords: Air permeability, Darcy ' s coefficient , Fabric porosity, Pressure head , Typar fabri c, Water permeabili lY IPC Code: lnl. C I. 7 D041-13/00; GO IN 15/00

1 Introduction Air and water permeabi liti es are two very important

properties of geo textiles and require criti cal control in many geotechnical app lications. Advent of new instruments has helped in faster and non-destructive measurement of air permeability. Water permeability tes ts, on the other hand, are time consuming, difficult to perform and prone to errors. The relati onship between ai r and water permeabiliti es for di fferent nonwoven constructions is important for process and quality control during the manufacture and fi e ld performance of these fabrics. Although both ai r and water are fluids, there are certain d ifferences in the properties of the two fluids. These d iffe rences can contribute to dev iations from the expected results and therefore should be taken into consideration. This IS

di scussed in the fo llowing secti ons:

1.1 Fluid Flow

Both air and water are Newtoni an fluids and follow Darcy's law l oftl uid fl ow, given by

aQ = K ~ ah at f.-l ax

. .. ( I )

where aQ is the rate of flow of fluid; all , the at ax

hydraulic gradient across the medium; A, the surface area of medium; J1 , the viscosity of the fluid; and K, t e Darcy's coefficient of permeability.

aTo whom all the correspondence should be addressed. Phone: 2659 140 1: Fax: +9 1- 11- 2658 11 03: E-IIl:l il: ko th ari @tex tile. iild .ernel.i n

1.1.1 Effect of Pressure

For an incompress ible fluid like water, the solution of Darcy's equation becomes

Q= KA b..p I-L L

.. . (2)

where b..p is the pressure di fference between the

opposite faces; and L , the thi ckness of the media. However, fo r a co mpress ible fluid like air, both the

vo lume flow and velocity vary with pressure from one face of the sample to another. Whereas Darcy's eq uati on considers these quantities as constant. In that case, the perfect gas law is cons idered and the eq uati on is solved to its integral form as shown below:

PV = nRT

Q= KA(PI - P2)

f.-l X 2P1L

KA P'II ---

where Pili = (PI -P2)12

... (3)

Thus, while water fl ow rate depends upon pressure difference only, airflow rate also depends upon the mean pressure.

1.1.2 Effect of Viscosity

Viscosity (;1) dec ides Darcy 's permeability constant and Reynolds' number (Re). The relationship of viscos ity with temperature is opposite in gases and liquids, i.e whil st the viscosity of gases increases wilh

BHATTACHARJEE el al. : AIR AND WATER PERMEABILITY CHARACTERISTICS OF NONWOVEN FABRICS 123

temperature, in liquids it decreases with temperature. So, any relati onship found will be valid for a small temperature range. For the present work, all the

experiments have been carried out at 30°C.

1.1.3 Drift Velocity

In case of gases, the velocity at the solid walls cannot be considered zero and a 's lip ' or 'drift' ve locity takes place whi ch must be taken into account. This effect increases when the mean free path of gases is much less than the pore size. As shown by Klinkenberg2, the permeability of fluid through porous medi a depends upon the pressure (P) as shown by the following equati on:

- b K=K(l+-)

p . . . (4)

where b is a parameter dependent upon the fluid; and

K , the modi fied coefficien t of permeability. Carro l1 3 recommended the use of ASTM tes t 0 737

to determine water permeability coefficients and showed a linear proportionality between results of air permeability and water permeability tests on fibres . Bottcher4 studied a w ide range of needle-punched fabrics and found close correlation fo r both horizontal (in plane) and vertical (normal ) permeabilities .

From the above discussion , a relationship can be expected between air and water permeabilities subject to certain conditions, e.g . (i ) the pressure difference should be low (100 Pa), (ii) the te mperature range shoul d be small, and (iii ) the relationship will depend upon the pressure diffe rence due to Klinkenberg effect.

In the present study, an attempt has been made to find out a correlation between air and water permeabiliti es for a range of spun laid heat-bondec! nonwovens used for geotextiles and to determine the parameters on whi ch these values are mostly dependent.

2 Matel"ials and Methods A range of Typar fabrics, having the speci fications

as shown in Table 1, was used for study. ASTM standards fo r geosynthetics and nonwovens were fo llowed, wherever possible, for tests.

2.1 Measurement of ominal Thickness The nomi nal thickness was measured accord ing to

the ASTM D 5199-9 1. The instrument used was Essdiel thickness gauge with a least count of 0 .01

Table I - Specifications of spunlaid heat-bonded polypropylenc Typar fabrics

Sample

Typa l' I Typal' 2 Typar 3 Typal' 4 Typar 5 Typar 6 Typar 7

MassI area g/m2

53 63 99 113 132 194 289

Thickness mm

0.28 0.32 0.33 0.39 0.41 0.55 0.68

Porosity Fibre Fi bre di am. denier mm

0.8 0.042 11.3 0.79 0.044 1.4 0.67 0.043 12.0 0.68 0.041 10.7 0.65 0.041 10.7 0.61 0.040 10.4 0.53 0.039 10.2

mm. The tes t specimens were removed at random di stributed pattern with no specimen taken nearer than 100 mm from selvedge or roll edge.

2.2 Measurement of Fabric Weight Electronic balance with a least count of O.OOlg was

used for the measurement of fabric weight per unit area.

2.3 Measurement of Fibre Diameter and Denier Fibre diameter was measured with a projection

microscope at x 400 magnification . The readings were taken directly from the fabrics.

2.4 Measurement of Air Permeability The samples were tested for air permeability

according to ASTM 0737-75 (Reapproved 1980). The instrument used was TEXTEST FX 3300 (Switzerland). This is a digita l portable instrument and the tests can be performed on the fabri c without cutting out the small test specimens from it. T he lest pressure in the instrument was maintained at 98 Pa and the air permeability values were obtained d irectly from the instrument.

2.5 Measurement of Water Permeability The tests were carried out according to ASTM D

4491 -92 in a water permeability tester. The main parts of the instrument (Fig. I) are a cyli ndrical base prov ided with an outlet nozzle and piezometer, perforated plate, pl unger, ai r bleeder, rubber ring, thin geogrid and dial gauge.

A circular fabric speci men of 95 mm diameter was prepared and the thickness was determined at 2 kPa pressure. The specimen was saturated in distilled water for 24 h and a thin layer of sil icon grease was applied on the sides of the perforated piston and the plunger. The perforated plate was then placed over the fla nge of the base and then a thin geogrid was put on the perforated plate. The saturated specimen was

124 INDIAN J. FIBRE TEXT. RES., JUNE 2004

placed on the geogrid and a rubber ring was put over the spec imen. The experiment was carried out without any normal load on the fabric. The system was backfi lled through the inlet; hydraulic head was adjusted to the requ ired val ue by adjusting inflow rate and the height the discharge pipe. However, the test was requ ired to be carried out in laminar flow . This test was repeated at different hydraulic heads.

Multiple layers of fabrics of the thickness 10mm were used5

. The instrument was assembled inside water to avoid leakage and air entrapment inside the fabric .

Accordi ng to ASTM D449 1-92, the dissolved oxygen content (DOC) of water was kept at 4.5 ppm (a llowed 6ppm) and pH at 6.8 .

3 Results and Discussion

3.1 Ai r Permeability Air permeability va lues were measured over a wide

range of pressure heads and are given in Table 2. The permeability is a strong function of the pressure head

RAM

Fi g. I-Instru ment for measuring water permeability

INLET

PERFORATED DISC

OUTLET

(LJp) and g ives a linear regression line with correlation coeffici ent of 0 .98 - 0.99. It shows the followin g relationship (Fig. 2):

.. . (5)

where PlI is the air permeability; and 0, and ((2 . the constants.

The val ues of constants C/, and C/2 depend upon the porosity (t ) for the range of the Typar fab ri cs studied as shown by the following relationship:

((, = -0.1723 + 0.2167t (r = 0.98) ... (6)

(/2 = -0.00 18+ 0 .001St (r = 0.97) ... (7 )

3.2 Water Permeability Table 3 shows the water permeability values o f

different samp les. The water discharge rates were found to be linear with respect to the pressure heads. Water permeability decreases slightly with the increase in pressure head (Fig. 3).

6

o 200 400 600 800

Pressure Head (Pa)

1000

+ Typ.u I

e Typar2

b.Typ&r )

X Typaf ..

X TYP:II S

e Typar 6

+ Typar l

1200

Fig.2-Effect of pressure head on air permeabil ity for different Typar fabrics

Table 2 - Air permeability va lues of Typar fabrics

Pressure Air Eermeabili tl', m3/m2/s head, Pa Typar I Typal' 2 Typar3 Typar4 Typar 5 Typar 6 Typar 7

100 2.306 2.0 17 0.673 0.718 0.800 0.281 0.104 200 3.599 3.082 1.052 1.224 1.36 1 0.499 0.205 300 4.521 3.917 1.385 1.483 1.788 0.700 0.285 400 5.531 4.62 1 1.686 1.952 1.783 0.863 0.379 500 6.535 5.23 1 2.006 2.08 2.09 1 0.978 0.455 600 6.238 2.215 2.494 2.546 1.049 0.518 700 2.388 2.745 3.109 1.331 0.603 800 2.832 3. 11 3.405 1.424 0.63 900 2.734 3.416 3.532 1.536 0.719 1000 3.036 3.559 3.824 1.645 0.767

BHATTACHARJEE el al .: AIR AN D WATER PERMEABILITY CHARACTERISTICS OF NONWOVEN FABRICS 125

3.3 Correlation between Air and Water Permeability A very good linear relationship (Fig. 4) of the

fo llowing form was observed between ai r permeability (Pa) and water permeability (PII .).

.. . (8)

As the lines pass through the ongll1, it can be deduced that at the limiting porosity value at which the ai rflow rate reduces to zero, the water fl ow through the fabrics is also totally stopped. This is expected as water, being a fluid with higher viscosity and density than air, would not be able to pass through a pore through which air cannot pass.

The constant 'b' was found to be dependent on pressure and fo llows a non-linear relationship with pressure (Fig.5) as given below:

b = 255.8 p .O.S IOS ( r = 0.99) . .. (9)

This gives a good prediction of the constant' b' and the values of water permeability can be found from the known values of air permeability and pressure head. However, both air and water permeabilities were found to fo llow a close correlation with fabric porosity as shown by the following relationship (Figs 6 and 7) :

. .. (10)

16 .-------------------------------~==~

"' 14 N~ M~ 12

1; 10 x

~ 8 :0

'" ~ 6 Q; Cl. 4 Q; iii s: 2

-G--_ --...0_ •

+ Typar I

OT)'Par2

b.Typar J

XTypar4

- X TyparS

e Typar6

~

0 ~~i=~~~=I~~c===~~~ o 100 200 300 400 500 600

Pressure Head (Pa)

Fig.3-Effec l of pressure head on water permcabilily for different Typar fabrics

PIV = alV + bilE + CIIE2

... (1\) The constants a, band c (the subscript a is fo r air

permeability and w is fo r water permeability) have a very good correlation with pressure heads and give a linear regression (r = 0 .97 - 0.99) which is as fo llows:

aa = 0.0256 iJp + 4.407 a,l' = -0.0852 iJp+6 1.90 \ ... (12)

ba = -0.0983 iJp - 17.212 blV = 0.2937 iJp-225.57 . .. ( 13)

Ca =0.0979 iJp + 16.836 elV =-02531 iJp+208.71 ... ( 14)

4S .-------------------------------.-----,

"' 40 N~ '5 M- J

E 1~ 30

o

.,:; 25

.~

{J 20

§ 15 '" Cl.

iii 10

~

o

. 100 I'a

. 200 Pa

1>. 300 P.

X400 Pa

+ 500P,

2

.. . _e-· -..... . /; ..

--------~-

4 3 2

A ir permeability (m 1m Is)

6

Fig.4--Linear relationship between air permeability and water permeabilily

6

x

~ 3

2

o L--L __ ~~ __ ~~ __ -L __ L-_L __ ~~ __ ~~

o 100 200 300 400 500 600

Pressure Head (Pa)

Fig.5-Non-linear relationship between pressure head and constant 'b'

Table 3 -- Water permeability values of Typar fabrics

Pressure Water permeability x JO-4, 1ll3/m2/s

head, Pa Typar I Typar2 Typar 3 Typar4 Typar 5 Typal' 6 Typar7

100 13.4 12.5 3.85 4.41 3.340 2.67 0.62 200 12.97 11.6 3.92 4.14 3.3 2.54 0.71 300 12.94 9.52 3.82 3.63 3.26 2.55 0.63 400 11.89 9.59 3.77 3.28 3.22 2.3 0.62 500 7.7 1 3.75 2.89 3. 18 2.22 0.55

126 INDIAN J. FIBRE TEXT. RES. , JUNE 2004

2.5 ,---------------------,

• •

< 0.5

0.5 0.55 0.6 0.65 0.7 0. 75 0.8 0.85

Porosity

fig.6- Re lat ionship between air permeability and fabric poros ity for Typar fabri cs

16 r----------------------~

14

0.5 0.55 0.6 0.65 0.7 0.75 O.S 0.85

Porosity

Fig.7-Relationship between water permeability and fabric poros ity for Typal' fabri cs

By eliminating /Jp between these sets of eq uati ons , following relationships were obtained:

a ll' = 76.56 -3 .380a ... ( 15)

bll· = -276.98 - 2.987 ba .. . ( 16)

ell = 252.23 - 2.S8S ca ... (17)

These sets of equations were used to predict air and water permeabilities from the porosity of fabrics and the pressure head in which they are to be measured

Using equations 8 and 9 with known values of pressure head (100 Pa - 500 Pa), the values of P", were obtained (Table 4). It can be seen that the error percentage vanes to a very high degree with in a pressure head from fabri c to fabric. Thi s was more prominent III samples Typar 6 and Typar 7 respectively. This can be attributed to the variati on in thi ckness and hence the porosity of the fabrics. Excluding these two fabrics, the average value of error between predicted and actual values is 5.96%. Using Eqs (10)-(14), similar trends were obtained III

VlO\OCNVl 11'-:1'-00 NVl

I'- r-i N ,..; N 0

~ ~ {::: ~ fl M ~ 0\ M ("'j ~ N d

I:C Table 5 - Actual vs theoretical values of ai r penneability [Eqs( I 0)-( 17») :r:

» Sample

Air penneability, mJ/m2/s ~ » 100" 200' 300' 400' 500' n

Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % :r: » ;>0 '-

Typar I 2.31 2.37 2.95 3.60 3.34 -7.32 4.52 4.30 -4.95 5.53 5.26 -4.92 6.54 6.22 -4.81 tTl tTl

Typar 2 2.02 2.22 10.12 3.08 3.13 1.41 3.92 4.03 2.87 4.62 4.93 6.77 5.23 5.84 11.61 ~

Typar 3 0.67 0.80 19.06 l.05 1.17 1l.21 1.39 1.54 11.09 1.69 1.91 13.12 2.01 2.28 13.45 ~

Typar4 0.72 0.89 24.00 1.22 1.29 5.62 1.48 1.70 14.32 1.95 2.10 7.47 2.08 2.50 20.21 » ;>0

Typar ~ 0.80 0.64 -20.10 1.36 0.95 -30.50 1.79 1.25 -29.94 1.78 1.56 -12.53 2.09 1.87 -10.75 » z Typar 6 0.28 0.38 34.85 0.50 0.59 17.33 0.70 0.79 13.15 0.86 1.00 15.71 0.98 1.21 23.23 0

Typar 7 0.10 0.11 13.06 0.21 0.21 13.06 0.29 0.31 10.05 0.38 0.41 9.32 0.46 0.51 13.06 ~ » ' Pressure head in Pa. -l

tTl ;>0

-0 tTl ;>0

3:: tTl » I:C r =i -< n :r: »

Table ~ - . A.ctual and theoretical values of water penneability [Eqs (10) - (14») ;>0 »

Water Penneability x 10-4, mJ/m2/s n -l

Sample tTl ;>0

100' 200' 300· 400' 500' (n

Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % Actual Theoretical Error % -l n (n

Typar I 13.4 13.8 2.96 12.97 12.57 -3.05 12.94 11.35 -12.27 11.89 10.13 -14.48 &91 0 ."

Typar2 12.5 12.84 2.74 11.6 11.73 1.11 9.52 10.62 11.51 9.59 9.5 ' -D.92 7.71 8.39 8.8 z 0

Typar 3 3.85 4.26 10.53 3.92 4.05 3.35 3.82 3.85 0.73 3.77 3.64 -3.34 3.75 3.44 -8.26 z Typar4 4.41 4.77 8.14 4.14 4.52 9.12 3.63 4.27 17.15 3.28 4.01 22.38 2.89 3.76 30.18

~ 0

Typar 5 3.34 3.34 -0.07 3.3 3.21 -2.59 3.26 3.09 -5.17 3.22 2.97 -7.81 3.18 2.85 -10.52 < tTl

Typar 6 2.67 Z

1.94 27.34 2.54 1.92 -27.41 2.55 1.9 -25.57 2.3 1.88 -18.45 2.22 1.85 -16.51 ."

Typar 7 0.62 0.91 0.71 »

47. 11 0.85 19.52 0.63 0.79 24.62 0.62 0.72 16.39 0.55 0.66 19.66 I:C ;>0

• Pressure head in Pa. n (n

tv -..J

128 INDIAN 1. FIBRE TEXT. RES. , JUNE 2004

case of Typar 6 and Typar 7 fabrics (Tables 5 and 6). Excluding the val ues of these two fabrics, the average error percentage was -2.51 % for water permeability and - J.6 % for air permeability.

The above results can thus be useful in predicting the water permeability from air permeability. While G a, b" and Ca can be used to find the air permeability for a gi ven pressure head , Eqs (12)-( 17) can be used to predict the water permeability of the fabrics.

4 Conclusions 4.1 The porosity of the fabrics plays a very

important role in deciding the air permeability of the fabrics.

4.2 Excellent linear correlati on was observed between air and water permeabilities. The relationship indicates that for the limiting case for which the air

permeability is zero, the water permeability also becomes zero. The ratio of air permeability to water permeability is a function of the pressure head.

4.3 The knowledge of fabric porosity and pressure head makes it possible to determine both the air and water permeabilities and their relationshi p for the range of fabrics studied.

References I Darcy H, Les FOlltaines Publiqlles de la ville de Dijoll (Vicwr

Dalmont Publica tion, Pari s), 1856. 2 Klinkenberg L J, Drilling alld Prodllctioll Practices (Ameri can

Petroleum Institute, Washington), 1941. 200-213. 3 Carroll R G (11') , Geotechnical Testillg. 4 (2) ( 1991) 83-85. 4 Bottcher P, 1111 Text BIIII. NOllwovellS, 4 (1993) 15-1 6. 5 Geolexliles alld Geoll1elllbral1.es ill Civil Ellgilleerillg. ed ited

by R V Van Zanten (A A Balakama Publications, Rotterdam). 1986.