O o d h e a d p u b L i s h I n g L i m I t e d

11.3
Textile properties relevant to sports apparel
11.3.1 Thermal resistance
The physical quantity associated with heat transfer through textiles is thermal
resistance, which is a measure of the ability of a material to resist heat flow.
Heat flow between any two points in a material occurs in response to a
gradient in temperature between those points. Thermal resistance is generally
measured in steady-state conditions, that is, once the heat flow resulting
from the applied temperature gradient has stabilised.
Fibres typically make up less than 20% of the total volume of textile
materials, even for dense structures such as woven fabrics. This falls to
about 5–10% for knitwear and can be as low as 0.1% in insulating webs. For
knitted fabric structures of the type used in active sportswear, air contributes
about half the total thermal resistance (Holcombe and Hoschke, 1983). The
remainder of the resistance can be attributed to a combination of fibre thermal
properties and fabric packing density. The thermal conductivities of
conventional apparel fibres are all of a similar order of magnitude and much
greater than that of air (see 
Table 11.1).
Figure 11.1
 shows fabric thermal resistance plotted against thickness for
a large number of fabrics of various structures. The thermal resistance of
conventional textile materials largely reflects their thickness – the thicker
the material, the greater its thermal resistance, which implies that there is no
intrinsic fibre ‘warmth’ property (Holcombe and Hoschke, 1983). Thermal
resistance is mainly determined by fibre characteristics that influence the
resulting fabric thickness and hence the amount of air trapped in the fabric.
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Wool’s traditional reputation as a warm fibre is firmly grounded in its
natural properties. The level of technology of earlier times was limited to
converting the natural crimp, elasticity and bulk of wool into fabrics that
were characteristically hairy, conferring a warm, soft touch and thickness for
good insulating properties. These fabrics were ideally suited for garments
needed to provide warmth in times when domestic heating was a luxury
rather than taken for granted as it is today.
Table 11.1 Thermal conductivities of textile fibres
Fibre
Conductivity mW/(m K)
Cotton
461
Polyacrylonitrile
200
Polyamide
243
Polyester
141
Polypropylene
117
Polyurethane
126
Polyvinylchloride
167
Viscose rayon
289
Wool
193
Air
26
Water
600
Thermal resistance (
K.
m
2
/W)
0.10
0.08
0.06
0.04
0.02
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thickness (mm)
11.1
 Relationship between fabric thickness and thermal resistance.
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11.3.2 Vapour resistance
The mechanism of moisture vapour transport through textile materials has
some parallels with heat flow. In steady-state conditions, in this case with
the rate of vapour flow constant, moisture movement is opposed by a resistance
that is largely a function of fabric thickness. The rate of diffusion of moisture
vapour through air is very much greater than that through fibres, so most of
the vapour flow takes place through the air spaces between fibres and yarns.
As with thermal behaviour, fibre properties have little direct influence on
steady-state resistance to moisture vapour flow.
Rain protective shell products are an integral part of the clothing system
of many outdoor sports participants. They incorporate coatings or waterproof
breathable membranes for which the vapour and wind resistance are
significantly higher than that of a conventional textile of equivalent thickness.
Measurements of steady-state thermal and vapour resistance are useful
for quantifying the performance of clothing worn for protection against the
climatic extremes encountered in outdoor activities such as skiing and climbing.
They are far less important in the lighter and thinner fabrics used for active
sportswear where the air gaps between clothing layers can have greater
resistance than the textiles themselves. In addition, bellows ventilation of air
inside the clothing induced by body activity and external air movement can
contribute significantly to heat and moisture loss from the body independently
of heat and moisture losses through the textiles themselves (Bouskill et al.,
2002).
11.3.3 Fibre moisture sorption
Water vapour moves through the air spaces between fibres and yarns as the
result of either forced convection caused by external wind and body movement,
or by diffusion from regions of high vapour concentration to regions of low
concentration. Similarly, water vapour moves in and out of fibres and through
their internal molecular structure under the influence of moisture concentration
gradients between regions. As mentioned earlier, this process is slower because
the diffusion coefficient of water vapour inside fibres is less than the diffusion
coefficient of water vapour in air.
The amount of water vapour inside the fibre is determined by the relative
humidity and, to a lesser extent, the temperature of the adjacent air (Watt and
D’Arcy, 1979). An equilibrium exists between fibre moisture content and
relative humidity. If the moisture content of the fibre is below its equilibrium
level for the relative humidity of the surrounding air, water molecules diffuse
from the air into the fibre. Conversely, if the moisture content of the fibre is
above equilibrium for the external conditions, water molecules are released
from within the fibre and diffuse back into the air. This behaviour, known as
hygroscopicity, is common to all textile fibres.
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An important difference between natural fibres such as wool and synthetic
fibres is that the chemical structure of natural fibres enables them to store
much more water vapour internally. 
Table 11.2
 shows that wool has a sorption
capacity of about 35% at saturation whereas for fibres such as polyester and
polypropylene, the corresponding figure is of the order of 1% (CSIRO,
undated).
Steady-state measurements take no account of the responses of fibres to
changing temperature and humidity conditions. In practice the vapour pressure
in and around clothing is never constant, particularly at the skin, and fibre
properties play a very important role in the management of moisture in the
microclimate adjacent to the skin. The extent of that role is dependent on
fibre dynamic moisture vapour sorption characteristics, that is, the speed and
extent of their response to changes in the moisture in the air around them.
11.3.4 Heat of sorption
An important feature of the behaviour of hygroscopic materials is the energy
associated with changes in moisture content. Water molecules absorbed by
such materials associate with the relatively polar polymer chains within the
structure, dropping to a lower energy state and thereby releasing heat. The
reverse of the absorption process requires energy (Hearle, 2002).
Thus moisture absorption by fibres as humidity rises causes the fibre
temperature to rise, and moisture release following a decrease in humidity
lowers the fibre temperature. For fibres such as wool that can absorb a large
amount of moisture, the amount of heat involved is quite significant. A
kilogram of dry wool placed in an atmosphere of air saturated with moisture
releases about the same amount of heat as that given off by an electric
blanket running for 8 hours! A good demonstration of this is to take a loose
handful of wool fibre that has been oven dried and lightly spray it with water
from an atomiser. The heat released is very obvious – sufficient to cause the
temperature of the wool to rise by as much as 10–12
°C.
Table 11.2 Saturation moisture regain of some common
sports apparel fibres
Fibre
Saturation moisture content
(% regain)
Wool
35
Cotton
24
Polyamide
7
Polyester
1
Polyolefin
0.05
Polyacrylonitrile
7
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11.3.5 Moisture vapour transport
In most everyday wear conditions moisture is present in clothing only as
vapour or as adsorbed water within the fibre, that is, in the form of individual
gaseous water molecules. However during active sport where the metabolic
heat loss from the athlete is typically quite high, liquid water and water
vapour often coexist. The physical processes that control the behaviour of
these two states of water in textile materials are largely independent.
The diffusion of moisture vapour into and out of fibres, known as dynamic
moisture sorption, is a time-dependent process. The main rate-determining
factors are fibre diameter, saturation moisture regain and the diffusion
coefficient of moisture vapour inside the fibre (Li, 2001). The finer the
diameter, the higher the sorption capacity or the higher the diffusion coefficient,
the more rapid the change in fibre moisture content.
Dynamic moisture vapour sorption behaviour is responsible for a property
known as buffering. This is only significant in fibres such as wool where the
moisture sorption capacity is high. As the rate of moisture evaporated from
the skin increases, the relative humidity of the microclimate adjacent to the
skin increases and the fibres respond by increasing their moisture vapour
content. This slows (or buffers) the rate of rise in humidity within the
microclimate. As moisture levels decrease, the fibres give up some of their
stored moisture, again slowing the rate of humidity change.
Wool has a favourable combination of the key rate-determining factors
and so its buffering effect is far more pronounced than that of other fibres.
By slowing the rate of change in humidity, wool worn next to the skin makes
the wearer less conscious of changes in humidity and reduces sensations of
discomfort that might otherwise occur. This ability to buffer humidity change
is responsible for wool’s widely recognised ‘breathable’ reputation.
Buffering is in effect a form of skin-clothing microclimate humidity control
that is most important at low to moderate activity levels or in the early stages
of vigorous activity. It sets wool apart from other materials used in sport and
is one of the cornerstones of the product known as ‘Sportwool™’. This is
described in 
Section 11.5.
11.3.6 Liquid moisture transport
The behaviour of textile materials in contact with liquid sweat is a critical
factor in their ability to influence wear comfort at high levels of physical
activity. Water is picked up and moved either laterally in the plane of the
fabric or transversely from one face of the fabric to the other by the process
of wicking.
Unlike moisture vapour, the driving force for liquid sweat movement is
the surface energy of the fibre, that is, the attraction between the liquid and
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the fibre surface, and the size and configuration of the spaces within the
fabric (Das et al., 2007). The mechanisms underlying wicking in textiles
closely parallel those responsible for the wicking of liquids in capillaries.
The two key factors that influence the pressure driving liquids along a capillary
are the radius of the capillary and the surface energy (wettability) of its solid
surfaces. The smaller the radius and/or the higher the surface energy, the
more readily wicking occurs. 
Table 11.3
 shows the surface energies of some
typical fibres used in active sportswear.
Although the spaces between fibres and yarns do not have the continuous
circular form on which wicking theory is based, close proximity between
fibres and their parallel alignment in yarns create discontinuous capillaries
of irregular shape through which liquids can migrate. Capillaries are effectively
formed both between fibres within yarns and between the outer fibres of
adjoining yarns. The apparent capillary radius of spun yarns depends on the
relative alignment and proximity of the fibres, which in turn depend on
properties such as:
• fibre cross-sectional shape and diameter;
• yarn structure (ring, open-end or vortex spun);
• yarn twist factor and count; and
• knit or weave structure and tightness factor.
The terms hydrophilic, meaning water loving or attracted to liquid water,
and hydrophobic, meaning water hating or repelling liquid water, are widely
used to describe the properties of the surface of fibres. In general, the more
hydrophilic the fibre, the higher its surface energy and the more readily a
fabric made from it will wick liquid water. These terms are often wrongly
used interchangeably with the term hygroscopicity. The former relate to the
interaction of liquid water with the surface of fibres, the latter to the exchange
of water vapour with the internal structure of the fibre. These two properties
are quite independent.
Table 11.3 Surface energy of some common sports
apparel fibres
Fibre
Critical surface energy
(mJ/m
2 
@ 20
°C)
Cotton
200
Polyamide (nylon 6)
46
Wool (natural)
29
Wool (chlorinated)
45
Polyester
43
Polyvinylchloride
37
Polypropylene
29
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Fibre surface chemistry has become quite an advanced area of textile
science and is today an important aspect of many textile products. It is
possible to chemically modify the surface energy of a fibre so that its surface
energy is anywhere between the extremes of strongly hydrophobic and strongly
hydrophilic without changing the underlaying hygroscopic behaviour
significantly.
A key point here is that although it is relatively easy to modify the surface
properties of fibres to change them from hydrophilic to hydrophobic and
vice versa, modification of their internal structure to increase or decrease
hygroscopic moisture vapour capacity is extremely difficult. Although the
synthetic fibre industry has tried very hard to increase the moisture vapour
capacity of its products to match that of wool or cotton by processes such as
hydrolysis in the case of polyester, synthetic fibres are still well short of the
performance of most natural fibres in this regard.
11.3.7 Drying of textiles
An important requirement of sports apparel is that it is able to dry as quickly
as possible after absorbing liquid sweat or after washing. Apart from the
discomfort aspect, the replacement of the air in a textile by water dramatically
increases its thermal conductivity. As 
Table 11.1
 shows, the thermal conductivity
of water is more than 20 times greater than that of the air that makes up the
bulk of a textile. In cold or windy conditions where the rate of evaporative
heat loss from the wet textile is high, post-exercise chill can result.
The rate of moisture evaporation from wet textiles is a function of the
surface area available for evaporation and the surrounding climate conditions.
Any two fabrics of the same area that contain the same amount of interstitial
water will take the same time to dry in a given environment, regardless of the
type of fibre involved. This is illustrated by 
Fig. 11.2.
 In each case the rate
of loss of moisture, represented by the slope of the line, is about the same.
The total time taken for the fabric to dry depends on the amount of water
present in the fabric at the start of the drying process, indicated by the
intersection of each line with the vertical axis.
Hydrophobic fabric manufactured from fibres such as polypropylene retain
very little water after complete immersion when compared with strongly
hydrophilic fibres such as cotton and appear to dry more quickly as a result.
Yet polypropylene films used for plastic wrapping are commonly treated to
make them hydrophilic so that they retain the printing inks used for labelling.
A strongly hydrophilic fibre such as cotton can be treated with a hydrophobic
agent so that it picks up little or no water after immersion.
The key factors that determine the amount of water retained in fabrics
after wetting are the thickness or physical volume of the fabric and as with
wicking, fibre surface energy and effective capillary size. Treating a textile
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with a hydrophobic agent will shorten its drying time but at the expense of
wicking behaviour and vice versa.
Speed of drying is considered a negative for wool in the active sports and
outdoor areas. Although the surface of dry wool fibres is relatively hydrophobic
due to fatty acids in the outer scale structure, in the presence of moisture the
surface inverts to a ‘protein out/fatty acid in’ arrangement (Huson et al.,
2008) that is more hydrophilic and leads to significant water retention after
immersion. Similarly, machine wash treatments for wool generally leave the
fibre surface quite hydrophilic and increase water retention.
The recently development of the QuickDry Merino concept (Denning,
2006) is a means of shortening drying time in applications where speed of
drying is an issue. In this process, hydrophobic, low-temperature activating
polymers are applied to the wool fibre surface, reducing drying time by as
much as 70% in the case of machine-wash treated wool. These treatments
have negligible influence on moisture vapour absorption by the fibre and
therefore are not detrimental to buffering behaviour. They have the added
advantage of improving the stain resistance properties of wool fabric.
11.3.8 Odour management
The high levels of moisture generally present in clothing worn during active
sports and the presence of bacteria originating from the skin provide the
ideal environment for bacterial and fungal growth. This is particularly the
case if the garments are worn and then left damp for a while before washing.
Polyester 1
Polyester 2
Polypropylene / cotton
Cotton
0:00:00
0:28:48
0:57:36
1:26:24
1:55:12
2:24:00
Elapsed time (h:m:s)
Moisture content (%)
1.6
1.2
0.8
0.4
0
11.2
 Drying behaviour of typical textiles.
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One side-effect of the growth of these micro-organisms is the development
of unpleasant odours.
The use of antimicrobial agents that inhibit the growth of micro-organisms
is becoming more widespread in an effort to add a ‘hygienic’ or ‘fresh’
image of active sports apparel. Wool has its own natural way of dealing with
this issue. Athletes such as long-distance runners and mountaineers who are
obliged to wear the same clothing for long periods have reported less odour
build-up in their clothing when wearing wool than with synthetic fibres.
In addition to the ability to buffer and thereby reduce moisture levels in
the clothing microclimate that facilitate the growth of micro-organisms, the
wool fibre has a very complex internal chemical structure that includes
many side-chains. These side-chains are able to bind the acidic, basic and
sulphurous compounds that are important components of body odour and
thus capture them inside the fibre. When wool garments are washed, the
odour compounds are readily removed.