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

11.4
Textiles for sports apparel
For a clothed athlete, body heat loss occurs both from exposed skin and from
the outer surfaces of clothing. Some convective and evaporative losses also
occur at the inner surface of clothing and from covered skin through the
process of forced or bellows ventilation via openings such as those at the
sleeves and neck. Heat lost from the outer clothing surfaces must first pass
through the clothing itself. Similarly, some of the moisture evaporated from
the skin must diffuse through the clothing and most of the liquid sweat
picked up by the clothing must wick to the outside for evaporation to occur.
Clothing interferes with metabolic heat loss. Reducing fabric thickness
increases the rate of heat and moisture flow through clothing but there is a
lower limit to thickness determined by the point at which the garment becomes
too transparent to be viable, or basic structural characteristics such as strength
and elasticity are below practical limits.
The ideal clothing for active sports where sweat is an issue should maximise
the rate of evaporation of sweat while minimising the discomfort associated
with wet clothing against the skin. It should pick up sweat from the skin,
transport it to the outer fabric face, expose the maximum wetted surface area
for evaporation and draw the latent heat required for evaporation from the
body. During periods when the rate of sweat output from the skin is greater
than the rate of evaporation from the clothing, it should store this excess
liquid in such a way that the clothing neither feels wet to the skin nor clings
to it.
Unfortunately these attributes cannot all be achieved simultaneously and
the most practical sports apparel will inevitably be a compromise. As an
example, a simple knit manufactured from fibres that have a very high
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Advances in wool technology
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surface energy will readily pick up sweat from the skin and give good
evaporation from its external surface. However, the sweat is held uniformly
through the fabric and so feels damp when in contact with the skin. If its
inner surface is dense enough and hydrophilic enough it may form liquid
‘bridges’ across to the skin that cause ‘wet cling’ or drag and interfere with
physical movement.
In order to avoid skin contact issues while maintaining good evaporation
performance, some active sportswear manufacturers use double-knit fabric
structures, also known as double layer knits or double-faced knits. These
consist essentially of two discrete fabrics joined during the knitting process,
either by interlacing the two faces, or by the addition of a third binder yarn
crossing from one to the other. An example of an interlaced double-knit is
shown in 
Fig. 11.3.
The almost complete separation of the two faces enables different physical
characteristics to be achieved at the internal and external surfaces of the
fabric. Manufacturers have sought to use the separation available with this
structure to build functional characteristics into textiles to improve sweat
transport and evaporation. If the two surfaces of the fabric have different
sweat attracting characteristics, then by putting the less wettable face in
contact with the skin, sweat picked up from the skin is transported to the
outside of the fabric where it is more readily exposed to air movement for
evaporation into the surrounding atmosphere.
Early versions of double-faced fabrics comprised fibres such as
polypropylene or polyester on the skin side, with cotton or viscose on the
outside. Current products tend be 100% synthetic, consisting of textured
continuous filament yarns or spun staple yarns (the latter made from filaments
cut up into individual fibres of specific length) on the inner face, and a
textured, low bulk continuous filament yarn on the outer face, both in the
same fibre type.
Another option is to use density gradient structures – fabrics in which the
diameter of the fibres on the inside face is greater than that on the outside.
In such structures the packing density of the fibres, that is, the effective
capillary size, increases from inside to outside, encouraging liquid migration
away from the skin.
Face
Back
11.3
 An example of a double knit structure.
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At the other extreme is the apparel worn in some of the more technically
aware sports such as climbing and skiing where sweat is not such an issue.
These are often treated with hydrophobic agents to prevent liquid pick-up
from snow or rain, avoiding the enormous loss in insulating properties that
results from water in the fabric structure.
Most synthetic fibres tend to be relatively hydrophobic (water repellent)
in the raw polymer state. Over the past decade or so a range of treatments has
been developed to make synthetic fibres more hydrophilic. Many treatments
and treated fabrics have proprietary brand names that are widely recognised
by active sportswear users, such as Akwatec™, Climalite™, HydroMove™,
H2-Out™, Dri-FIT™ and Coolmax™. Most consist of a hydrophilic polymer
coating applied to the fabric after manufacture so that sweat wicked up by
the fabric is uniformly distributed throughout the structure.
The incorporation of natural fibres such as cotton and wool to active
sportswear fabrics adds another dimension to moisture management behaviour
that cannot be matched by all-synthetic products – that of vapour buffering.
This is particularly the case if the natural fibre is directly adjacent to the
skin. Sportwool™, wool’s first serious attempt at a high-performance textile
suitable for active sports, emerged in 1994 from a need for a cycling product
with the buffering capabilities of wool with the ease of transfer printing of
polyester.