International Textile Conference, Terrassa 2001
FAST DETERMINATION OF SURFACE MOISTURE
ABSORPTIVITY OF SMART UNDERWEAR KNITS
Lubos Hes, Technical University of Liberec, Czech Republic
e-mail luboshes@hotmail.com
Abstract
In order to explain the thermal comfort of underwear or shirts touching the
sweating human skin, a new parameter called moisture absorptivity was
introduced in the paper and a simple equation of the moisture transfer between the
fabric and skin was derived. Since the direct measurement of the moisture
absorptivity is complicated, an indirect method for its experimental determination
was proposed and used for the evaluation of thermal comfort of fabrics when
coming into contact with wet human skin. The experimental results obtained by
means of this new method correlate with practical experience. The results also
confirmed, that the so called smart knits may provide better thermal contact
comfort in wet state than common cotton fabrics.
1. Introduction
Many people believe, that 100% cotton underwear (t-shirts) provide the best thermal
contact comfort, even in hot days, due to its easy and fast sweat sorption (wetting), and
their experience based on common life of a clerk or a bussinesman confirms this statement.
Nevertheless, when the wearer has to exhibit some physical effort, the excess of the sweat
keeps accumulated in the cotton fabric in the proximity of sweating glands for long time
and causes thermal discomfort. On the other hand, when we wear the PES/cotton fabrics in
hot day, the thermal discomfort appears as well (even without physical effort), but such
fabric gets dry soon. Some customers believe, that both fabrics differ in their water vapour
permeability mainly.
In order to explain the effect of this parameter, various underwear fabrics were measured
in this study. From the measurements made on the PERMETEST (Sensora) instrument (see
in Tab. 2) resulted, that water-vapour permeability of the measured underwear fabrics
depends more on their mass per area then on their composition, and that in all cases the
relative vapour permeability was very good, exceeding 15%.
The next parameter in question is the moisture sorption capacity (absorbency) of shirt
fabrics. There are many methods to determine this parameter 1. Nevertheless, the
moisture absorbency characterises just the specific moisture retention corresponding to the
state of full saturation of the fabric volume by water or sweat, and is directly proportional
to the fabric mass. No transient aspects are considered here, and no different boundary
conditions of moisture transmission between the skin and a fabric are respected. Therefore,
Scheurell and al. 3 reminded the importance of studying the dynamic surface wetness of
fabrics, and developed a new method of their determination, which is based on humidity
dependent colour changes of a special chemical agent deposited on the fabric surface. In
their study, a cotton fabric freely exposed to saturate water-vapour increased its surface
humidity 2-3 times faster then a PES fabric of similar parameters. The authors concluded,
that the dynamic surface wetness is a very important factor influencing the clothing
comfort of garments. A survey of other techniques to measure trans-planar liquid transport
into fabrics published Kissa 3. Nevertheless, all the found measuring methods are not
suitable for simple standard measurements of transient fabric wetting, due to quite
complicate preparation of the measurements and poor dynamic properties of some of these
methods. Moreover, the reduced comfort caused by wearing e.g. the PES/cotton shirts in
hot day is felt mainly in the moment, when the suddenly wetted fabric touches the skin.
Consequently, local cool feeling occurs, which is considered unpleasant. Within the contact time, heat is transferred by conduction through a thin intermediate layer, created by wet
outstanding fibres. Thus, the boundary condition approximates to the heat transfer of 1st
order, which should be respected within a measuring method in question.
Therefore, the first objective of the research work was to develop a method of an indirect
experimental determination of the so called surface moisture absorptivity B 4, whose
higher level apparently increases the contact comfort of wet fabrics and on the contrary.
Such parameter should present an integral factor, embracing the effect of moisture surface
adhesion (given by the contact angle) and the moisture conduction (depending on capillary
forces). The highest surface moisture absorptivity appears in the moment, when the
adhesion and conduction mechanisms, which in some cases act again each other, create a
specific synergic effect.
A new measuring method described in the paper is easy and reproducible, and reflects the
real moisture and heat transfer conditions between the fabric and the skin.
2. Theoretical
2.1. Introduction of Moisture Absorptivity
The amount of liquid inside any porous structure or textile fabric can be expressed
in terms of the fabric free volume saturation s [1]. Thus, for s = 0 the fabric is dry, and for s
= 1 all the pores are full of a liquid. In this case, the saturation propagation within a fabric,
either along its surface, but also perpendicularly to its surface, can be characterised by the
classical partial differential equation of diffusion processes:
(s / ) = A ( 2s / x2)
(1)
Here, A [m2/sec] is so called moisture diffusivity. This parameter is for textile fabrics
sometimes moisture dependent due to swelling. The solution of equation of this kind for A
= const is generally known. If we consider just short time moisture conduction, then we can
convert a textile fabric to a semi-infinite body, where the 1st order boundary condition is
applied. In this case, the moisture saturation propagation in the x direction is given by the
equation
s = erfc (x /2 A1/21/2)
(2)
The experimental determination of the moisture diffusivity from the moisture
propagation along the measured fabric is possible. Unfortunately, the moisture diffusivity
in this form does not characterise the volumetric capacity V of the fabric expressed in this
case in m3/m2s to conduct the moisture (sweat) from the contacted skin away towards a
fabric interior. To cope with this task, a Darcy law modified for the saturation gradient
should be introduced as follows:
V = - s (s/x)
(3)
Here s [m2/s] is the volumetric moisture flow conductivity, which is proportional to the
fabric permeability. In the next step, we should remind, that in the first Fick´s diffusion
law, which is used to express the mass flow in the form formally identical with Eq. (3), the
same diffusion coefficient D occurs, as in the second Fick´s law for transient mass transfer
by diffusion. By simplifying the problem solved to a simple diffusion, we can express the
moisture flow conductivity in Eq. (3) s by means of the moisture diffusivity A. From
applying this relation in equation (2) follows:
V = A1/2(s/1/21/2)
(4)
The first term in this equation fully characterises the fabric ability to absorb the moisture
from any moist surface, which contacts the fabric. Then this so called moisture
absorptivity B [m3 s1/2] is defined by the next relation:
B = A1/2
(5)
As shown in 3, many researchers have already measured the time-dependent longitudinal
wicking of fabrics. From these results, the moisture diffusivity A could be determined and
its square root used for the calculation of the spontaneous moisture uptake according to Eq.
4. Nevertheless, this approach may produce inaccurate results, since longitudinal wicking
rates not always correlate with the corresponding trans-planar ones, due to the complexity
of the wicking processes, which besides the diffusion processes include capillary
penetration of moisture inside fabrics, and also moisture absorption of the fibre surface.
Therefore, the goal of this paper is to develop a technique, which would determine not the
moisture absorptivity itself, but its real impact on the comfort properties of a surface wetted
fabric. To achieve this, an indirect way was chosen, as explained in the next chapter.
2.2. Indirect Method of the Moisture Absorptivity Measurement
The suggested method is based on the objective evaluation of warm-cool feeling perceived
by a wearer of a cloth, which suddenly comes into contact with a wetted skin. In this
moment, the cotton fabric absorbs the liquid sweat rapidly, and conducts it away from the
fabric surface towards to the fabric interior. Due to high adhesion forces, the sweat keeps
accumulated in the fabric close the places where the sweat was generated. If the amount of
sweat is not too high, within a short time the moisture concentration close to the fabric
contact surface reduces, and the wearer feels the pleasant contact of nearly dry fabric.
The other mechanism of achieving the pleasant dry feeling of underwear and shirts is based
on the use of PES micro-fibres, which, due to higher surface, absorb in some extend the
humidity also, but the liquid sweat is rapidly distributed by capillary forces in larger area
surrounding the perspiration zone, thus reducing the average relative humidity of fabric
under the limit, which would result in unpleasant wet feeling. Unfortunately, this
mechanism requires also some additional dynamic contact forces typical for sport
activities. In the case of blended fabrics containing too much poorly absorbing PES fibres
of common section and fineness, the sweat keeps adhered on the skin, and provokes an
unpleasant cool feeling due to sweat evaporation.
The suggested method is based on the objective evaluation of cool feeling effect within an
experimental procedure, which simulates the real fabric wearing conditions described
above. Before the method is explained, the instruments for the objective warm-cool feeling
determination are described.
2.2.1. Instruments For the Evaluation of Thermal Contact Feeling of Textile Fabrics
Warm-cool feeling means the feeling we get when the human skin touches shortly any
object, in our case textile or other fabric used in clothing, furniture or carpets. It was found,
that this parameter characterises well the transient thermal feeling which we get in the
moment, when we put on the undergarment, shirts, gloves or other textile products. Since
this feeling strongly affects the choice of people when buying the clothes or garments, the
objective assessment of this feeling became very important in the last decade.
The first instrument, which was able to evaluate the warm-cool feeling of fabrics objectively, was developed by YONEDA and KAWABATA in 1983 [5]. They have introduced
the maximum level of the contact heat flow qmax [W/m2K] as a measure of this transient
thermal characteristics, and KAWABATA has published the first objectively determined
values describing the thermal-contact properties of textile fabrics. Their instrument, called
THERMO-LABO, was commercialised. In 1986, another instrument for the objective evaluation of warm-cool feeling of fabrics, but of different concept, was completed at the
Technical University in Liberec. This computer controlled semi-automatic instrument
called ALAMBETA calculates all the statistic parameters of the measurement and exhibits
the instrument auto-diagnostics, which avoids faulty instrument operation. The whole
measurement procedure, including the measurement of thermal conductivity , thermal
resistance R, qmax, sample thickness and the results evaluation, lasts less than 3 -5 min. As
the objective measure of warm-cool feeling of fabrics, so called thermal absorptivity b
[Ws1/2/m2K] was introduced [6]. The meaning of this parameter (formerly used in the civil
engineering and ergonomics) is explained in next paragraph.
Provided that the time  of thermal contact between human skin and a fabric is short,
textile fabric was idealised to a semi-infinite body of thermal capacity c[J/m3] and initial
tem-perature t2. Transient temperature field between human skin (characterised by a
constant temperature t1) and a fabric is then given by the following partial differential
equation
(t / ) = a ( 2t / x2)
(6)
and can be used for the calculation of the initial level of heat flow q passing between the
skin (characterised by a constant temperature t1) and textile fabric according to the next
equation, whose derivation for the boundary condition of 1st order is similar to derivation
of the Eq. (4):
qdyn = b (t1 - t2)/( )1/2
(7)
Thus derived thermal absorptivity b [Ws1/2/m2K] is given by the following relation:
b = (c)1/2
(8)
As it can be see, the level of thermal absorptivity depends neither on the temperature
gradient between the fabric and skin, nor on the measurement time. This value just depends
on the contact pressure, which also correspond to the real situation. The pressure is
adjustable.
1
2
3
8
11
H
6
4
7
5
9
10
Fig. 1 Principle of the ALAMBETA instrument
The simplified scheme of the instrument is shown on Fig. 1. The principle of first version
of this instrument depends in the application of ultra thin heat flow sensor 4, which is
attached to a metal block 2 with constant temperature, which differs from the sample
temperature. When the measurement starts, the measuring head 1 containing the mentioned
heat flow sensor drops down and touches the planar measured sample 5, which is located
on the instrument base 6 under the measuring head. In this moment, the surface
temperature of the sample suddenly changes and the instrument computer registers the heat
flow course. Simultaneously, a photoelectric sensor measures the sample thickness. All the
data are then processed in the computer according to an original programme, which
involves the mathematical model characterising the transient temperature field in thin slab
subjected to different boundary conditions [7]. To simulate the real conditions of warmcool feeling evaluation, the instrument measuring head is heated to 32ºC (see the heater 3
and the thermometer 8), which correspond to the average human skin temperature, while
the fabric is kept at the room temperature 22ºC. Similarly, the time constant of the heat
flow sensor, which measures directly the heat flow between the automatically moved
measuring head and the fabrics, exhibit similar value (0,07 sec), as the human skin.. Thus,
the full signal response is achieved within 0,2 sec. The validity of thermal absorptivity as a
new warm-cool feeling parameter of fabrics was confirmed by several tests where the
results of relative subjective feeling of 100 persons were compared with the values of
thermal absorptivity found by means of the ALAMBETA instrument, see in 4. Within
various research projects the thermal-insulation and thermal-contact properties of all
common textile products were experimentally investigated. It was found, that practical
values of thermal absorptivity of dry fabrics range from 20 to 300, – see Tab. 1. The higher
is this value, the cooler feeling represents.
Tab. 1
ALAMBETA
EFFECT OF FABRIC STRUCTURE, COMPOSITION AND TREATMENT ON THE
LEVEL OF THERMAL ABSORPTIVITY b [Ws1/2/m2K], contact pressure 200 kPa
20 - 40
30 - 50
40 - 90
70 – 120
100 - 150
130 - 180
150 - 200
180 - 250
250 - 350
300 - 400
330 - 500
450 - 650
600 - 750
 750
1600
Micro-fibre or fine PES fibre non-woven insulation webs
Low density raised PES knits, needled and thermally bonded PES light webs
Light knits from synthetic fibres (PAN) or textured filaments, raised tufted carpets
Light or rib cotton RS knits, raised wool/PES fabrics, brushed micro-fibre weaves
Light cotton or VS knits, rib cotton woven fabrics
Light finished cotton knits, raised light wool woven fabrics
Plain wool or PES/wool fabrics with rough surface
Permanent press treated cotton/VS rough weaves, dense micro-fibre knits
Dry cotton shirt fabrics with resin treatment, heavy smooth wool woven fabrics
Dry VS, Lyocell, silk weaves, smooth dry resin-free heavy cotton weaves (denims)
Close to skin surface of wetted (0,5 ml of water) cotton/PP (or spec. PES) knits
Heavy cotton weaves (denims) or wetted knits from spec. PES fibres (COOLMAX)
Rib knits from cotton or PES/cotton, knits from micro-fibres, if superficially wetted
Other woven and knitted fabrics in wet state
Liquid water (evaporation effect not considered)
As results from the table, the thermal - contact feeling of the fabrics is strongly affected by
their structure and composition. It was found 8], that fibres and fibre polymers of higher
moisture regain, provide also cooler feeling. Therefore, the warmest feelings can be achieved at fabrics made from PVC, PP, PAN, whereas viscose, flax, cotton and PAD fibers
show the coolest feeling. Which feeling is better, depends on customer: for hot summer
garments cooler (cotton) feeling is demanded, whereas in the north of Europe warmer
clothing, based on the PES/wool is preferred.
An important aspect of the “warm-cool” feeling evaluation is the change of this feeling
when the textile product gets wet. Because the time of the warm-cool feeling evaluation of
samples in the ALAMBETA instrument is very short, less than 3 minutes, the evaluation of
humid samples is reliable (the sample does not turn dry during the measurement). Because
the thermal conductivity and thermal capacity of water is much higher than these of dry
textile structure, the negative feeling of coolness of garments moistened by sweat can
exceed 1000. Thermal contact properties of socks in wet state were studied in [9]. Since the
thermal absorptivity is mainly the superficial property, its level can be changed by any
superficial or finishing treatment, like raising, brushing and coating. The instrument was
commercialised [10.
2.2.2. Methodology of the Indirect Measurement of the Fabric Moisture Absorptivity
The intention of this research was to characterise the contact comfort felt by a wearer of a
shirt during a hot day. For this purpose, a special thin interface fabric was found, which
should simulate the effect of a sudden sweat discharge on the skin. This sweat simulator
should be as thin as possible, in order not to influence (in dry state) the thermal capacity of
the measured fabric, but this interface fabric should absorb a certain amount of liquid
injected in the centre of this interface fabric and mainly - it should distribute the liquid fast
and uniformly within a circle of approx. 50 mm diameter (in order to cover the area 25x25
mm of the heat flux sensors). After some trials, a thin (0,1 mm) nonwoven fabric
containing PP on one side and viscose fibres on the other side was found to fulfil all
demands. In order to reduce the amount of liquid, the interface fabric was perforated.
At the beginning of the measurement, the ALAMBETA instrument is switched on and the
measured underwear is placed on the measuring base of the instrument. Then, the volume
of 0,2 ml of water (containing detergent) was injected on the centre of the interface fabric
surface, covered by the viscose fibres. Within one minute, the liquid distributed uniformly
within a circle of 45-50 mm, and stopped. When this occurred, this interface fabric was
turned by the viscose side down and inserted into the space between the measured sample
and the centre of the measuring head of the instrument - see Fig. 2. At the same time, the
interface fabric and the measuring head of the instrument dropped down towards to the
measured underwear or shirt fabric.
Within a few seconds, the liquid from the interface fabric was more (in case of pure cotton
shirt) or less (in other cases) taken away by absorption into the lower fabric. If this fabric
exhibits low absorption, the thermal capacity of the interface fabric is maintained quite
large and the initial level of thermal absorptivity b is significantly higher.
In the case of measurement of “warm-cool” feeling on pure cotton fabrics, characterised by
higher moisture absorptivity, the moisture is rapidly distributed within the whole thickness
of the fabric, so that the interface fabric gets nearly dry, and the instrument shows a lower
level of the resulting thermal absorptivity.
Guiding shaft
Measuring
head
Upper heat
flow sensor
Interface
fabric
Bottom HF
sensor
Fabric
fated
Measuring plate
Fabric
Fig. 2 - Simulation of the underwear wetting and wicking by means of the ALAMBETA
instrument
2.2.3. Theoretical Analysis of the Underwear Wicking and Wetting Effect on Cool Feeling
In the new version of the ALAMBETA, not only one, but two heat flow sensors are
applied, as shown on Fig. 1 - see the second sensor 7 . This enables to simplify the heat
flow signals evaluation and moreover, the instrument can check the level of heat, which is
during the measurement conducted away (in the surrounding air) from the sample. This
increases the measurement precision.
During the measurement, the computer integrates the heat Q[W] passing through both heat
flow sensors, which is accumulated inside the measured sample, according to the Eq. 1:
Q = q() d =
T

0
( q1 –qo) d
(9)
The measurement finishes for the time level T, when q1 (T ) equals qo (T). Then, the heat
Q causes the increase of the average temperature inside the measured sample, as follows:
Q=
1
moco (t1 – to)
2
(10)
This equation then yields the surface related heat capacity moco [J/m2]. Simultaneously,
the sample thickness h[mm] is measured and used for the determination of the fabric heat
capacity oco [ J/m3] from the equation
oco = moco/h
(11)
The consequent steady state measurement of heat flow passing through the sample then
enables the easy determination of the coefficient of thermal conductivity [W/m.K] and
the fabric thermal resistance R[m2K/W]. The warm-cool feeling parameter - thermal
absorptivity, then follows from Eq. 3.
In the next step, the mentioned measuring principle will be applied in the simple analysis
of the wetting and wicking simulation. In this case, heat balance of the space between both
heat flow sensors should include the effect of heat capacity per area m1c1 of the interface
fabric and its initial moisture Mwcw, provided that the measuring head has just dropped
down and completed the thermal contact between the interface and underwear fabrics:
QM =
1
(moco + m1c1 + Mwcw )(t1 – to)
2
(12)
Within a few seconds, the moisture will be absorbed by the underwear and distributed in its
volume. In fabrics exhibiting good moisture conduction the sweat will be transported by
the capillary action outside the area of heat flow sensors. Hence, the effective moisture
content in central part of the underwear will be reduced to lower level mw, thus reducing
the volumetric thermal capacity of the system consisting of interface fabric and underwear.
The integral heat detected by both heat flow sensor will be as follows:
Qm =
1
(moco + m1c1 + mwcw)(t1 – to)
2
(13)
Because mw  Mw, and due to the fact, that the specific heat of water cw is very high –
approx. 3 times higher than that of fabrics, even small differences in the moisture amount
absorbed form the sweat simulating fabric and conducted away form the sensing area of
heat flow sensors will result in relative big changes in heat capacity of various tested fabric
system and hence (according to Eq. 8) in big changes in their thermal absorptivity levels. In
fact, the resulting sensitivity will be even bigger, due to varying evaporation effects, which
were not considered in this simple analysis (the more moisture keeps in the interface fabric
after contact, the cooler is ist surface, and the higher is the resulting thermal absorptivity).
3. Experimental Results
The composition of the investigated plane fabrics varied from 100% cotton to 100% PES
or PP fibres. Medium values of the results are shown in the following Table 2.
Tab. 2. Cool feeling (thermal absorptivity) of various fabrics measured by the ALAMBETA
instrument when simulating their sudden thermal contact with wetted skin, pressure 200 Pa
SAMPLE COMPOSITION
AND STRUCTURE
45% cotton, 45% PP+PAD
Italian 2 layers smart knit
50% cotton 50% PP spec. structture Czech smart knit
100% spec. Section PP + common
PP, Czech 1 layer smart knit
100% spec. Section PES, single
layer knit Dupont Coolmax
100 % cotton denim
100% cotton shirt, no resin
100 % cotton shirt, no resin
70 % cot. + PES woven shirt
35 % cot. + PES woven shirt
75 % cot. + PES woven shirt
35 % cot. + PES woven shirt
100% cotton shirt resin treated
Sample
thickness
H [mm]
CV up
3%
Thermal
conductivity 
[mW/mK]
CV up 3%
1.15
105
Relative
water vapour
permeability %
CV up 6%
41,4
Thermal
absorptivity b
[Ws1/2/m2K]
CV up 5%
0,66
100
43,0
421
1,21
112
40,2
430
0,54
97.2
32,1
443
0,71
0.43
0.38
0.21
0,28
0,23
0,26
0.22
86,2
83.1
90.1
78,7
120
88,9
123
149
13,5
22,5
24,7
25,4
24,8
19,4
22,4
22,8
452
508
565
731
751
875
935
1178
415
3.2.Results Evaluation
3.2.1.With an increasing portion of PES fibres in common woven shirt fabrics increases the
unpleasant cool feeling (i. e. increases thermal absorptivity) when worn in conditions of
surface wetting, which matches the practical experience of wearing the tested shirts.
3.2.2. Special smart fabrics with improved thermal comfort properties like double layered
knits or T shirts knitted from Coolmax or Coolplus (Taiwan) modified PES fibres reveal
more pleasant contact feeling in conditions of superficial wetting.
3.2 3. Exceptionally some cotton/PES blend fabrics made from common fibres may exhibit
relatively good thermal contact comfort in the wet state, even with quite high portion of
PES fibres, due to some unknown effect or due to a special fabric structure (confirmed by
wearers).
3.2.4. Cotton shirt weaves containing too much chemical agents deposited inside the fabric
may show worse contact comfort feeling in the wet state, in spite of the fact, that their
steady-state water vapour permeability keeps very high. Lukas proved in 11, that closing-
up the finest capillary channels (e.g. by resins) should reduce the vertical suction height of
water in these fabric (which should result in worse moisture uptake).
4. Conclusions
From the application of the indirect method of experimental determination of the moisture
absorptivity described in this paper may be concluded, that superficially wetted non finished 100% cotton fabrics show substantially warmer (more pleasant) feeling then those
of cotton/PES blends, which correlate with practical experience. Special smart products
like Coolmax knits made of modified PES fibres, 100% PP knits containing modified
filaments or double layered cotton/PP knits exhibit even warmer feeling after their thermal
contact with the wetted surface which indicates, that their resulting moisture absorptivity is
higher as that of the pure cotton woven fabric. The described method thus proved its
applicability for the purposes of designing or fast evaluation of fabrics with increased
moisture absorptivity.
Acknowledgements
The presented results of the project LN00B090 were supported by the Ministry of
Education of Czech Republic
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