Page 58 - Basic Principles of Textile Coloration

THERMAL PROPERTIES OF POLYMERS 47

Usually, Tg is not a sharp, well-defined temperature and the changes in
temperature dependent properties occur over a range of values around Tg.

As the temperature increases, the volume of a rigid polymer sample expands in
the usual manner. This involves proportionate increases in the actual volume
occupied by the molecules, as they vibrate, and that of the voids between
molecules in the amorphous zones and around the chain ends. These voids are
called the polymer free volume. Molecular packing in the crystalline state is very
compact and ordered so the free volume is a characteristic of the amorphous
regions of a polymer. At temperatures above the Tg, bond rotations allow sections
of the polymer chains to move into voids with increasing ease and thus create new
voids. Segmental motion increases rapidly with increasing temperature above Tg,
increasing the free volume and the coefficient of expansion. Eventually, on
continued heating, chain mobility becomes so pronounced that the true liquid
state forms at the melting point.

The Tg has considerable significance in textile processing. Two of the most
important processes that depend upon a synthetic filament having some plasticity
are drawing and texturising. In drawing, newly formed continuous filaments are
first wrapped several times around a rotating feed cylinder, and then around a
second one running at 4–5 times the linear speed of the first. Provided that the
polymer is not at a temperature too far below its Tg, the stress applied by the faster
running cylinder causes the polymer molecules to unfold and slide by each other in
the filament as it extends in length. This aligns the molecules in proximity,
increasing the degree of molecular interaction and therefore the strength of the
filament. It is in this way, in varying degrees, that the orientation of the molecules
of all artificially made fibres improves the filament tenacity and rigidity. If the
filament is at a temperature too far below Tg, it is too rigid to be drawn uniformly
and may even break on application of stress. At temperatures around Tg, the
filament is sufficiently plastic for drawing to be practicable.

In texturising, the filaments are deformed by application of a transverse stress.
The objective is to buckle the initially straight filament so that it develops a
permanent crimp. Several technologies for texturising depend on bending the
filaments while they are warm and plastic, and then stabilising the crimp by rapidly
cooling. The degree of crimping determines whether the fabric handle will be
smooth or rough. Texturising increases the bulk of an assembly of filaments
trapping more air and improving the insulating power.

When a fabric made from thermoplastic fibres such as nylon or polyester is
heated to a temperature around Tg, the mobility of the polymer chains allows the