Performance Characteristics of Waterproof Breathable Fabrics

Abstract

The effect of atmospheric conditions on the water vapour permeability characteristics of waterproof breathable fabrics has been studied.
Several types of waterproof breathable fabrics were tested for vapour permeability under a wide range of atmospheric temperatures and relative humidities. It was found that atmospheric conditions have a considerable effect on the vapour permeability characteristics and that there are differences in behaviour between the various types of fabric.
The two main variables influencing vapour permeability are identified. Regression equations for the relationship between vapour permeability and the main atmospheric parameter are presented. Conclusions are drawn about the capabilities of the fabrics under conditions of use.

Full text

Perfoirnance Characteristics of
Waterproof Breathable Fabrics
DAVID
A.
HOLMES
Facirlty
of
Technology
Boltori Iiistitiite
Dearie Road
Bolton,
BL3
JAB,
UK
ABSTRACT:
The effect
of
atmospheric conditions on the water vapour permeability
characteristics of waterproof breathable fabrics has been studied.
Several types
of
waterproof breathable fabrics were tested
for
vapour permeability
under a wide range of atmospheric temperatures and relative humidities. It was found
that atmospheric conditions have
a
considerable effect on the vapour permeability char-
acteristics and that there are differences in behaviour between the various types
of
fab-
ric.
The two main variables influencing vapour permeability are identified. Regression
equations for the relationship between vapour permeability and the main atmospheric
parameter are presented. Conclusions are drawn about the capabilities of the fabrics un-
der conditions of use.
INTRODUCTION
URING PHYSICAL ACTIVITY
the body provides cooling partly by producing
D
insensible perspiration. If the water vapour produced increases the relative
humidity of the atmosphere inside the clothing sufficiently to cause sensible
perspiration and increased thermal conductivity of the insulating air then the
clothing becomes uncomfortable. In extreme cases hypothermia can result
as
the body loses heat too rapidly causing
a
decrease in core temperature.
If
the
sensible perspiration cannot evaporate and the thermal insulation
of
the cloth-
ing remains high then the body is prevented from cooling and hyperthermia can
result.
Even
small deviations from normal skin and core temperature can cause
discomfort and reduction in performance. The heat energies produced during
306
JOURNAL
OF
INDUSTRIAL
TEXTILES,
Vol.
29,
No.
4-April
2000
1528-0837/00/010306-1
I
$10.00/0
0
2000
Technomic
Publishing
Co.,
lnc.

Perfortitatice Cliaracteristics of lvaterproof Breathable Fabrics
307
various activities and the corresponding perspiration rates have been published
[1,2].
To
take an extreme example, during very strenuous activity the body can
produce over 1 kW of heat. In order to remain at
a
physiologically acceptable
surface temperature of
33
to
35°C
and
a
core temperature of
37OC
under these
conditions, the skin has to produce perspiration at
a
rate
of
over
1000
cm3 per
hour. In order to be comfortable, therefore,
a
garment should be able to transmit
the water vapour produced by perspiration from inside the clothing to the out-
side atmosphere at
a
similar rate. Some fabric manufacturers claim water
vapour permeabilities of up
to
10,000
g.m-2 per 24 hours.
There has been increasing development of fabric that is water and wind resis-
tant yet permeable to water vapour. Such fabric provides weather protection
and comfort. The range of uses of
so
called “breathable” fabric is increasing.
Over twenty-five applications ranging from the well known foul weather leisure
clothing to specialised medical and military uses can be identified
[
1,3].
There are several ways of achieving fabrics with waterproof breathable prop-
erties [4]. The first to be scientifically engineered were the densely woven fab-
rics made from
100%
cotton. When the surface of the fabric becomes wet the
cotton fibres swell transversely reducing the size of the pores in the fabric thus
requiring very high pressure to cause water penetration. These fabrics, known
as
Ventile
[5],
were developed for the Ministry of Defence in the 1940s to pro-
vide protection to air crews from the severe conditions of the North Atlantic
Ocean. Ventile fabric is still in use for military applications and is being manu-
factured and marketed for
a
wider range of applications. The synthetic equiva-
lent uses densely woven micro-filaments but unlike Ventile these cannot make
use
of
the transverse swelling of the filaments to provide the waterproof proper-
ties.
Probably the most significant development was the micro-porous membrane
Goretex first reported in 1976 [6]. Goretex is
a
thin film of
FTFE
containing
a
network of very small pores (1.4 billion per cm2) much smaller than raindrops,
but large enough to allow the passage of water vapour molecules-hence the
name poromeric. Non-poromeric hydrophilic membranes which were devel-
oped later are made from chemically modified polyester containing
intermolecular “pores” that permit water vapour to diffuse through the mem-
brane at
a
relatively high rate. These membranes are only
a
few micrometers
thick and have to be laminated to one
or
between two conventional textile fab-
rics.
Following the commercial success of Goretex, several manufacturers devel-
oped breathable coatings based on polyurethane. The first coatings were mi-
cro-porous, the pores being incorporated using
a
variety of techniques
[4].
The
mechanism by which they operate is similar to that of the
porous
membranes,
but the coatings are much thicker and less delicate
so
that they do not have to
be protected by fabric on both sides. More recently developed coatings are

308
DAVID
A.
HOMES
non-poromeric hydrophilic polyurethane. The polyurethane is chemically modi-
fied
so
that water vapour can diffuse through the solid coating
[
11.
The densely woven fabrics have the aesthetic appeal of soft handle and flexi-
bility; however, few of the synthetic microfilament types are truly waterproof.
Some manufacturers claim that they can achieve waterproof properties by coat-
ing each filament with
a
proprietary fluorocarbon finish. The laminated and
coated fabrics can be stiff and make
a
rustling noise when flexed. New coating
techniques enable much thinner coatings to be applied and the use of
microfibres reduces this adverse influence on aesthetics. Fabrics possessing
a
high degree of stretch have
also
been developed thus extending the range of
ap-
plications
[4].
D
&
K
Consulting carried out
a
survey of the Western European market be-
tween November
1996
and July
1997 [7].
The survey predicts that the market
will grow to about
45
million linear metres per year by the year
2000.
Since the
beginning of the
1980s
when the market started to expand rapidly almost
40
million metres of waterproof breathable fabric have been used in Europe with
a
value of more than
f270
million. Initially laminated fabrics dominated the mar-
ket, but coated fabrics had increased to
55%
of the market by
1996.
The
UK
market accounts for more than
30%
of the total European market. Breathable
fabric is now being made into garments that can be used for everyday fashion
wear rather than high performance work and sportswear.
Various procedures for determining the water vapour permeability of textile
fabrics have been developed. There are two main approaches
to
the design of
suitable procedures. In the vapour cup (dish) methods the water vapour
pro-
duced above the surface of liquid water held in
a
container passes through the
fabric into the ambient atmosphere. The Water Vapour Permeability (WVP) is
the mass of water vapour passing through unit area of the fabric in unit time.
The mass of water vapour
is
taken
as
the
loss
in mass of the water in the con-
tainer. The other methods attempt to simulate sweating by using heated wet
plates
or
water soaked membranes covered by the test fabric. Various measures
of permeability and related properties are determined. Standard procedures are
carried out under standard atmospheric conditions of water temperature, atmo-
spheric temperature, and relative humidity. However, in use fabrics are exposed
to
a
variety of atmospheric conditions and the water from perspiration is
at
33
to
35°C.
The aim of this work was to compare the water vapour permeability
characteristics of different types of waterproof breathable fabrics under
a
range
of atmospheric conditions.
EXPERIMENTAL
Five waterproof breathable fabrics were selected representing different types
of construction
as
detailed in Table
1.

Pet-foniiaiice Characteristics of Waterproof Breathable Fabrics
309
fable
7.
Fabric samples.
Moss
Per
Unit
Area
Thickness
bm-21
(mml
Densely woven cotton
Woven polyamide hydrophilic
PU
coating
2
loyer: woven polyomide hydrophilic
3
loyer:
knitted polyamide microporous
Woven polyester microfiloment
polyester membrane
PTFE
membrane
220
165
185
175
70
0.33
0.43
0.33
0.44
0.1
The water vapour permeability of the fabrics was determined using
a
cup
method similar
to
BS7209
Appendix
B
[S].
Open plastic cups were filled with
water to within
10
mm
of
the top.
Fabric specimens were placed over the open top and sealed
to
the rim using
silicone sealant and
a
cover ring
so
that vapour could only pass through the
fab-
ric. A section
of
the assembly is shown in
Figure
1.
In order to simulate the ef-
fects
of
body temperature the cups were placed in
a
water bath maintained at
a
temperature of
37°C
using
a
temperature controller and
a
water cooling circuit.
The bath was surrounded
at
the bottom and sides by expanded polystyrene insu-
lation. The effects
of
external atmospheric conditions were simulated by plac-
ring
FIGURE
1.
Section
of
test
assembly.

310
DAVID
A.
HOMES
Thermocouple
~ontro~~er
1
FIGURE
2.
Diagram
of
apparatus.
ing the insulated bath inside an environmental chamber. The cup assemblies sat
in holes in the bath lid. The top of the assemblies was flush with the bath lid
so
that only the fabric was exposed to the chamber conditions. The chamber was
fitted with
a
fan which removed the microclimate caused by the permeating
vapour that developed above the specimens. This vapour would otherwise in-
fluence the vapour permeability of the fabrics due to its higher relative humid-
ity than the chamber atmosphere. A schematic diagram of the apparatus
is
shown in Figure
2.
The fabric specimens were exposed to atmospheric conditions ranging from
15
to
45°C
and
40
to
90%
relative humidity for five hours at each combination.
The water vapour permeability
was
determined by weighing the assembly be-
fore and after exposure and calculating the permeability in g.rn-’.day-*.
RESULTS
Table
2
shows the average WVP values of the fabrics at each temperature
and relative humidity. Vapour permeability is caused by a difference in water

Perforinnrice Characteristics of Wnterproof Brcntliable Fabrics
311
Table
2.
Values
of
WVP
under various atmospheric conditions (g-m-2day1).
RH
(%)
RH
(%)
40
65
90
40
65 90
Hydrophilic Coating Hydrophilic Mernbrone
Temp.
('C)
15 4463 4027 3737 4898 4822 4027
25 31 93 3090 2712 3809 3604 3071
35 1986 1378 1197 2361 1705 1342
45 1487 798 -290 1560 726 -290
Micro-porous Membrane Woven Cotton
15 6240 5422 4426 7039 6046 4825
25 3955 3374 2752 5515 4426 3005
35 3264 1923 1388 4465 2685 1524
45 2358 798 -468 2866 798 -762
Woven Micro-filornents
15 7401 6607 5297
25 5551 4559 3144
35 4417 2576 1596
45 301
1
762 -871
vapour pressure between the two faces of the fabric. Figure
3
shows graphs of
water vapour permeability
(WVP)
values against difference in vapour pressure
between the two faces of the fabrics. This difference is the difference in vapour
pressure between water at
37°C
and that corresponding
to
the various tempera-
tures and relative humidities (i.e., the vapour pressure at the corresponding dew
point temperatures).
DISCUSSION AND CONCLUSIONS
It can be assumed that
in
the experiments the space between the water sur-
face and the lower fabric surface is saturated with water vapour.
It is difficult
to
compare the results of this work with those presenkd by
manufacturers and to compare different manufacturers' claims because of the
different conditions used in the tests. Standard procedures use different condi-
tions.
For
example, British Standard
7209
Appendix
B
[S]
uses water at
20°C
and atmospheric conditions of
20°C
and
65%
relative humidity. ASTM
[9]
use
a
relative humidity of
50%
and
a
recommended water temperature of
32.2"C
or
a
desiccant.
No
atmospheric test conditions
are
quoted in manufacturers' tech-

312
DAVID
A.
HOMES
+
Coated
nFE
mmbrane Woven microfibre
x
Hydrophilic
membrane
x
Woven cotton
0
1
2
3
4
5
6
Vapour
pressure difference
(kPa)
FIGURE
3.
WVP
against vapour pressure difference.
nical literature. The results agree approximately with manufacturers’ claims in
the middle of the range of conditions used..
Some manufacturers claim much higher
WVP
values than have been found
in this work, presumably because they use test conditions showing the most fa-
vourable results
as
part of their marketing strategy. Some manufacturers quote
percentage breathability which does not mean that the fabric will transmit the
quoted percentage of perspiration produced by the body.
It
is the breathability
of the fabric compared with
a
reference fabric. Claimed
WVP
values can be
misleading
to
technically unqualified consumers who do not appreciate the ef-
fects of ambient atmospheric conditions. When it is raining the ambient humid-
ity is very high, reducing the
SVP
of the fabric
to
below the quoted value.
The fabrics can be ranked in decreasing order of permeability
as
follows:
Tightly woven synthetic micro-filament
]
Tightly woven cotton} Tightly woven
Micro-porous membrane)
Hydrophilic membrane} Membranes
Hydrophilic coating) Coatings
The results indicate that the main variables affecting
WVP
are the fabric con-
struction and the water vapour pressure gradient between the fabric faces. Pre-
sumably, the tightly woven fabrics have the highest
WVP
values due
to
their
high ratio of inter-fibre and inter-yarn spaces. The coated fabrics have the low-
est
WVP
values due
to
the high coating thickness. Membranes have higher
permeabilities than coatings because they are much thinner. The micro-porous

Perforiliarice Clraracteristics
of
Waterproof Breathable Fabrics
313
membranes have higher permeabilities than the hydrophilic versions
as
they
contain physical holes whereas the latter
are
solid.
The influence of vapour pressure can be seen from the graphs
in
Figure
3,
which show that WVP is
a
function
of
vapour pressure difference. All the fab-
rics tested showed negative WVP values at very high temperatures and relative
humidities. In other words vapour can pass from the outside to the inside of
clothing due to the vapour pressure on the outside being higher than that on the
inside. These conditions are unlikely to be met in practice. The relationship be-
tween WVP and vapour pressure difference is non-linear. The positive values
have been used
to
determine the nature of the relationship. If it is assumed that
the WVP is zero at zero vapour pressure difference, then the best
fit
is given by
a
third degree polynomial of the type
Y
=
CLX
+
b$
+
CA?
+
C
where
a,
b,
and
c
are constants,
Y
is the vapour permeability, and
x
is the vapour pressure differ-
ence. The best
fit
curves are presented in Figure
3.
The regression equations
and correlation coefficients are presented
in
Table
3.
The correlation for this re-
lationship
is
very good
as
indicated by the high correlation coefficients, the
lowest of which is over
0.93.
These regression equations can be used to predict the vapour permeabilities
of the fabrics under any atmospheric conditions. The average temperature of
rain in the British Isles is about
6°C
and when it is raining the relative humidity
is approaching
100%.
When it is freezing, the relative humidity is reduced to
virtually zero. Table
4
compares the vapour permeabilities of the various fab-
rics under these conditions. It is assumed that the atmospheric conditions inside
the clothing are
37°C
and
100%
relative humidity. If an estimate is made of the
area of fabric
in
a
suit consisting of
a
jacket and trousers at
2.4
m2,
then the ap-
proximate total vapour permeability of
a
suit can be calculated, which is also
shown
in
Table
4.
This performance can be compared with the perspiration
rates for various activities in Table
5.
It can be seen that the woven cotton fab-
Table
3.
Regression equations and correlafion coefficients.
Fabric
Type Correlation
Regression Equation Coefficient
Densely woven cotton
Woven polyamide
hydrophilic
PU
coating
2
layer: woven polyamide
hydrophilic polyester membrane
3
layer: knitted polyamide
micro-porous
PTFE
membrane
Woven polyesfer micro-filament
Y
=
1788~
-
4401~2
+
62.02
-
25
Y
=
929x
-
194~'
+
30.42
-k
58
Y
=
wax
-
1
4aX2
+
25.52
+
78
Y
=
1856~
-
580x2
+
81.92
-
38
Y
=
1835~-
473~'
+
68.82-
11
0.9876
0.9325
0.9434
0.9703
0.9877
Noie:
Y
=
water vapour permeability
(g-m-2.day');
x
=
vapour pressure difference
(kPa).

314
DAVID
A.
HOMES
Table
4.
WVP
under "realistic" conditions.
T
("C)
RH
(%)
VP
(kW
Inside 37
100
6.28
Outside
0
0
0.56
6 100 0.93
Vapour pressure difference (kPa)
5.84
5.33
WVP
(gm-2.day-1)
v.p.
diff
(kPo)
5.33 5.84
Woven micro-fibre 6700
8200
Woven cotton 6100
7300
Microporous
PTFE
membrone
5700
7300
Hydrophilic membrane
4900
5800
Hydrophilic coating
4200
5100
'
Calculated Total
WVP
(9-day-')
Woven micro-fibre 16000 19700
Woven cotton suit 15000
17000
Microporous
PTFE
membrane 13800 17400
Hydrophilic membrane 11700 13900
Cooted suit 10000 10900
ric can only cope with work levels up
to
active walking with
a
light
to
heavy
pack. The coated fabric can only cope with conditions between light and active
walking with no pack carried.
It
is
difficult
to
compare the results with those of other workers due
to
the
different conditions used. In general the conclusions are in very good agree-
ment with those of others. Other workers agree on the rank order
of
the differ-
ent types of fabric
[10,11].
Various
workers have
also
concluded that the
vapour transport characteristics
of
breathable fabrics are subject to change de-
pending on the weather.
Saltz
[lo]
used
a
heated cup method in combination
with an artificial rain installation
to
test
a
variety of fabrics similar
to
those
used
in the present work. The water in the cup was maintained
at
36°C
and the
water for the shower at
20°C.
It
is concluded that rain has
a
major influence on
vapour transport. In most
cases
wetting by rain reduces the vapour tra9smission
rate. The two-layer
PTFE
laminate was an exception
to
this general rule:
it
showed an increase in vapour transport when exposed
to
rain. This increase is
claimed
to
be caused by the condensation of escaping water vapour on the re-
verse side of the fabric due
to
the cooling effect of the rain. Although the best
fabrics studies allowed water vapour transport
of
up
to
200
g.m-2.h-1
in
dry
conditions, rain reduced the value of most fabrics to
a
maximum of
50
g.rn-*.h-'. The effect of rain has been found to considerably reduce vapour per-

Perlorinnrice Cliaracteristics
of
Waterproof Breothable Fabrics
315
Table
5.
Approxirnafe work and perspiration
rates associated with various acfivities.
~~
~~ ~
Work
Rate Perspiration Rate
Activity Watt) (g.da)rl) Limit
of
Use
Sleeping
60
2280
Sitting 100
3800
Genile wolking
200
7600
Active wolking
300
11500
.:.
with light pack
400
15200
.
9
with heavy pock
500
19000
.
*.
>
with heavy pock
in
mountains
600-800
22800-38400
*.
r-
Very heovy
work
1000-1
200
38000-45600
Note:
=
cooled;
*
=
hydrophilic membrone;
micro-fibre.
=
microporous membrane;
=
woven
cotton;
*
=
woven
meability due to blocking of the micro-pores
or
saturation of hydrophilic mem-
branes by water in some cases to
as
low
as
1200
g.m-*-day-*. They show that
the best fabrics can only cope with activity levels slightly beyond active walk-
ing
(324
Watt) in dry conditions and, for most of the fabrics, inactive condi-
tions
(84
Watt) during rain.
Ruckman draws general conclusions from her extensive studies
[
121:
water
vapour transfer of breathable fabrics depends very much
on
atmospheric condi-
tions. In general, wind increases and rain decreases the water vapour transfer
rate of fabric, giving
in
descending order of water vapour transfer performance:
windy, dry, wind driven rainy, rainy.
The present work confirms the conclusion drawn by several workers that
vapour transport through most breathable fabrics may prove insufficient, partic-
ularly for persons
in
strenuous working conditions.
The performance of waterproof fabrics should not be judged on weather pro-
tection alone.
Factors
such
as
physiology and durability should
also
be taken
into account. Weder carried out
a
very extensive study
of
all the above factors
[13].
One of the conclusions drawn is that the question “which product
performs the best” cannot be answered. The conditions of application and cor-
responding requirements imposed on
a
product are quite different.
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G
R,
Textiles,
1991,
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4,
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Keithley
J
H,
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S
J,
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W.
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and Schuierer
M,
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2,
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Mill,
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DAVID
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HOMES
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itttp://~~7viv.gorefabrics.com,
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B.
Detennination
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Method
E96-80, Standard Test
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Z,
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A
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J
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J
E,
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Weder M,
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