Page 71 - Basic Principles of Textile Coloration

60 SYNTHETIC FIBRES

room temperature. Like other hydrophobic fibres, accumulation of static electric
charge poses some problems in handling and promotes accumulation of dirt. Anti-
static spin finishes are invariably applied by the fibre manufacturer. The three major
types of PET fibres are high tenacity (highly oriented by drawing), regular tenacity
and staple fibre. Large amounts of texturised staple polyester fibres are produced for
blending with cotton.

4.3.2 Properties of PET fibres

The standard regain of PET is only 0.4% and so it is a hydrophobic material. Even
above the Tg, water penetration is minimal. Because of this, PET is unaffected by
aqueous solutions of ionic dyes. The many ester linkages make it slightly polar and
the fibres can be dyed with non-ionic disperse dyes as for cellulose acetate and
nylon. Disperse dyes only have very limited water solubility. The dye in the bath is so
finely divided that the high specific surface area of the particles allows a rapid
equilibrium with the small amount of dye in true solution. This dyeing process
involves the fibre extracting the small amount of dye dissolved in the aqueous bath,
and more dye dissolving from the reservoir of dispersed particles. At 100 °C, the
temperature is not more than 10–15 °C above the value of the Tg, the rate of
diffusion of dyes into the polyester is very low and dyeing is slow. This is because
polymer chains in the amorphous regions of the polymer still do not have adequate
mobility to continually create large enough voids into which dye molecules can
diffuse. Increasing the dyeing temperature from 100 °C to 130 °C, however, has a
very favourable effect on the dyeing rate and dyeing of PET under pressure has
become the accepted practice. Even under these conditions, however, the dyes used
are invariably of small molecular size in comparison with other types of dyes to
ensure an acceptable rate of diffusion into the fibres.

Dyeing at the boil is possible if the Tg of the PET has been lowered by absorption
of a swelling agent or carrier. Dyeing carriers are liquids present in emulsified form in
the dyebath, which absorb into the PET causing swelling. They act as plasticisers
and assist chain movements and create free volume. The use of dyeing carriers is
now in decline because of their odour, the difficulties of removing them from the
fibres after dyeing, and their adverse effect on the light fastness of some dyes.

Small amounts of cyclic oligomers, such as the cyclic trimer with three glycol and
three diacid units, are always bi-products of the polycondensation reaction. These
oligomers cause complications in dyeing. Although they are not very soluble in
water, they leech out of the polyester during high temperature dyeing. On cooling