Page 72 - Basic Principles of Textile Coloration

POLYESTERS 61

the dyebath, there is a risk that the oligomers will deposit on the fibre and machine
surfaces. For this reason, the exhausted dyeing bath is often drained while still hot.

In PET, the molecular chains are fully extended and relatively rigid because of the
repeating benzene rings and the short chains separating them. PET filaments
therefore have a higher elastic modulus than the nylons and are less extensible and
flexible. They are therefore not suitable for the manufacture of ladies’ silk hose but
provide fabrics with good dimensional stability, crease resistance and shape
retention, even when wet. Such properties are also improved by heat setting.
Because PET is a thermoplastic material, permanent pleats can be introduced by hot
pressing. Polyester fibres are strong, easy to dry, and relatively stable to processing
conditions. Much of their value is attributable to the excellent textile properties of
fabrics of both 100% PET and those in which cotton and polyester are blended.

Even though PET is less extensible than nylon, unset filaments will shrink by
relaxation when exposed to dry or wet heat. This is a consequence of the relaxation
of tensions built up in the filaments during processing. It can cause consolidation
problems with packages of yarn and uneven dimensional changes in fabrics.
Materials are usually heat set before exposure to high temperatures. Steam setting
can be used for packages, but hot air is preferred for woven fabrics using a pin
stenter. In this, small pins mounted on two parallel continuous chains pierce and
hold the selvages of the fabric as it passes through a hot air oven. The setting
temperature may be as high as 220 °C. If pin marks on the selvages are undesirable,
setting on heated cylinders is possible but does not allow width control. Heat setting
after dyeing usually involves lower setting temperatures and care in dye selection to
ensure that the non-ionic disperse dyes do not vaporise from the fibre surface.

Being an ester, PET is susceptible to hydrolysis but under normal conditions this
does not limit its use as a textile. It is, for example, unaffected by treatment with
cold, aqueous 20% NaOH solution in the mercerisation of cotton in cotton/polyester
blends (Section 5.4.6). PET does hydrolyse and slowly dissolve in hot alkaline
solutions, however. Treatment with 3–5% NaOH for 30–60 min at 95 °C gives a
considerable weight loss and filaments of decreased denier and softer handle. This
type of process led to the deliberate manufacture of fine microfibres (Section 4.3.4).
The polymer is reasonably stable to solutions of dilute acids and is stable to pressure
dyeing at 130 °C at pH 4.5–6.0, apart from the leeching out of low molecular
weight oligomers. PET is more stable to sunlight than nylon and has good resistance
to the chemicals used in textile processing. This is partly a consequence of its
hydrophobic nature. It is not particularly soluble in common solvents although it
may swell somewhat. Methylene chloride (dichloromethane) or aqueous N,N-