Page 59 - Basic Principles of Textile Coloration

48 FIBROUS POLYMERS

relaxation of any tension. This causes deformation of the shape of the material,
usually with considerable shrinkage. It can occur in hot water. The tensions in the
polymer molecules can be present from the original drawing of the filaments
immediately after their production, from texturising, or from stresses introduced
during winding, weaving or knitting. The strained polymer molecules, held in
place in the rigid, glassy structure, become mobile once the temperature is above
Tg. Their movement in the plastic condition eases the stress on them.

Fabric deformation or shrinkage on heating is minimised by a process called
heat setting. The speed and separation of two moving parallel chains, which grip
the fabric edges as it passes through the setting oven, determine the final
dimensions of the relaxed fabric. The setting temperature used is above Tg. In heat
setting, inter-chain bonds, such as hydrogen and dipole bonds, break. This allows
the molecular chains to move and adopt new, stress-free positions. New
intermolecular bonds then form with the fabric in a relaxed condition at the
setting temperature. After cooling, the polymer molecules in the filaments become
frozen in place. The new bonds are stable up to the heat setting temperature. The
reorganised internal polymer structure, and the material’s dimensions, will be
stable, provided the material is not heated to temperatures above the heat setting
temperature.

Most synthetic fibres cannot be dyed at temperatures below the Tg existing
under the dyeing conditions. This is because the dye has no mechanism for
diffusion into the fibre. The dye can only penetrate into the polymer structure
when motion of the polymer chain segments creates sufficient free volume with
adequate mobility to allow dye molecules to penetrate into the polymer. The
temperature at which polymer chain segment mobility allows a significant increase
in the rate of dye diffusion is called the dyeing transition temperature, Td. Dyeing
of thermoplastic synthetic fibres always requires temperatures above Td. This poses
no problems with nylon and acrylic fibres that have quite low values of Tg and Td.
The Tg of polyester, however, is around 80–90 °C. The diffusion of dyes into
polyester filaments is therefore quite slow at 100 °C and dyeing must be carried
out at temperatures well above this, by dyeing in water above 100 °C under
pressure. Because of the higher Tg, polyester drawing requires warm filaments.
Cellulosic and protein fibres do not have a Td. Unlike most synthetics, natural
fibres are quite porous, being constituted of many long fibrils laid side by side along
the fibre axis. Dyes are able to penetrate into the bulk of these fibres along the
many narrow channels permeating them.