Page 67 - Basic Principles of Textile Coloration

56 SYNTHETIC FIBRES

from polymer production and textile manufacturing processes. We have already seen
that heating nylon to a temperature above its Tg causes rearrangement of the
molecular chains in the amorphous regions of the material, and a reorganisation of
the intermolecular attractive forces (Sections 3.4 and 4.2.1). Because the nylon
polymer molecules are still potentially reactive, strong heating may establish a new
polycondensation equilibrium with a new molecular weight distribution and more or
fewer amino end groups. Level dyeing with anionic dyes is difficult if thermally
induced changes in polymer structure have not been uniform. Filaments under
tension undergo a drawing effect that can change the orientation and crystallinity of
the polymer. During weaving and knitting, nylon filaments under prolonged mild
stress elongate but do not recover their original length on releasing the tension.
Variable mechanically and thermally induced stresses can build up in the filaments
and may not relax until the fabric is subsequently wetted or heated. This can result
in undesirable changes in shape from the shrinkage of filaments. Processes such as
drawing, bulking (texturising) and heat setting, in which it is difficult to ensure
uniform treatment, can result in physical and chemical variations along the length of
filaments that only become visible after dyeing with anionic acid dyes. These effects
are the origin of a phenomenon called barré. The extreme case of this is the
occurrence of individual filaments in a woven or knitted nylon material that have
absorbed less dye than neighbouring filaments and show up as a pale streak on the
material surface.

Heat setting of nylon materials may be carried out under dry conditions in hot air
at 190 °C for nylon 6, or 205 °C for nylon 6.6. Alternatively, setting is achieved in
steam at 120 or 135 °C, respectively. As explained in Section 3.4, this allows
relaxation of any strain in the filaments or fibres introduced during manufacture and
improves the wrinkle resistance and dimensional stability of a material, avoiding
shrinkage when it is heated in water, as in dyeing, or in hot air as in drying. It also
stabilises the yarn twist and filament crimp. Heat setting is carried out after dyeing
unless the fabric has a tendency to form crease marks.

It is very difficult to obtain accurate data for the Tg values of nylons. In particular,
absorption of small amounts of water significantly depresses the value of Tg, but it
also depends on the extent of drawing and the method of heat setting, and therefore
on the degree of orientation and crystallinity. Nylon 6 has a lower Tg than nylon 6.6
for comparable conditions. It also has a less compact internal structure. At a given
temperature, nylon 6 will usually have greater and more mobile free volume and
therefore it usually has a higher rate and degree of dye absorption. Although dyes
diffuse more rapidly into nylon 6 fibres during dyeing, they also diffuse out more