The inner moisture barrier layer
is a woven fabric, which protects the wearer from dampness and provides a soft,
draping surface for good thermal transfer between the wearer and the clothing.
The inner fabric is inherently resistant to water and wind. The outer layer is
a fabric which allows free passage of water vapour and limited passage of
water. Suitable thread count and thickness of the inner and outer layer fabrics
provide the required properties. No coatings or surface finish treatments are
used for desired results. Garment assembly needs to be dipped in water for 1-2
minutes prior to use, in order to reserve the cooling liquid in the absorbent
core of super absorbent polymer. Then the excess water needs to be rinsed and
the cooling vests are ready for use.
The great advantage of this type
of cooling garment is that it is light, flexible and therefore portable. But
this type of garment is not suitable for humid conditions as because the
effectiveness of evaporative cooling is inversely proportional to ambient
relative humidity. Another limitation of this technology is cooling takes place
on the outer surface of the garment, not inner surface which is nearer to skin.
If evaporation rate is low, it may act as a thermal insulator and increase the
Super absorbent fibres (SAFs)
Super absorbent polymers (SAP) are
hydrophilic networks that can absorb and retain a large amount of water in an
aqueous solution. Most commonly available SAP in the market is partially
neutralised lightly cross-linked polyacrylic acid, due to its best performance
verses cost ratio. Water molecules are held in the hydrogel by means of
hydrogen bond formation. Cross-linking between the polymer chains form a three
dimensional structure and prevent infinite swelling, i.e. dissolving. Most of
the SAPs are now made from acrylic acid (AA), its salt and acrylamide (AM) via
solution or inverse-suspension polymerisation techniques.
Super absorbent fibres (SAF) are
produced by Technical Absorbents Ltd. are basically sodium salt cross linked
polyacrylates. Raw materials used for manufacturing of SAF are acrylic acid
(AA), methyl acrylate (MA) and special Acrylate/Methacrylate monomer (SMMA) in
which the acrylic acid is partially neutralised to the sodium salt of acrylic
acid (AA Na). SAFs are normally not considered to be the main structural
component due to two reasons: (1) its strength in dry state is not as good as
other fibres of same denier and (2) the fibre changes its structure to a gel
form after absorption. SAFs can be blended with other fibres and nonwovens from
the can be processed by any dry-laid technology followed by bonding by needle
punching, or thermal bonding, or chemical bonding. SAFs should be processed at
45% RH to 75% Rh for best performance.
For designing the three layer
evaporative cooling fabric assembly, the inner layer (which is nearer to skin)
is made from some commercially available water repellent breathable fabric.
Evaporative resistance of the inner layer fabric was measured as per ASTM
F186812 standard on Sweating Guarded Hot Plate. The fabric has an evaporative
resistance value of 12.329 Pa.m2/W. The outer layer fabric is woven
from 150/96 denier multifilament yarns with varying warp thread density and
weft thread density to achieve different air permeability values. Prior to
weaving the 150/96 denier multifilament yarn was twisted at 316 twist per meter
to avoid the problem of filamentation during weaving. Air permeability of the
outer layer fabrics was measured as per ASTM D737 04(2012) standard on
Textest FX 3300 instrument.