Page 73 - Basic Principles of Textile Coloration

62 SYNTHETIC FIBRES

dimethylformamide can be used to extract disperse dyes without influencing PET
filaments.

Pilling is the name given to the undesirable formation of small bundles of knotted
fibres on a fabric surface. These are formed by the matting together of fibre ends that
have been rubbed up from the fabric surface by abrasion. This is much easier for the
smooth surfaced synthetic fibres than for natural fibres. Because of the high tenacity
of synthetic fibres, these knots of matted fibres do not easily break off, as do those of
the weaker natural fibres. The balls of fibres therefore accumulate on the fabric
surface and produce an unattractive appearance. This can be quite pronounced in
fabrics made from 100% PET staple fibre. Decreased fibre tenacity reduces the
pilling, as does singeing after dyeing. Optimising the yarn and fabric construction,
heat setting, and chemical finishes all help to minimise pilling.

4.3.3 Modified polyesters

As for the nylons, PET type fibres are available in a considerable number of different
physical and chemical forms. Besides ethylene glycol and terephthalic acid, other
monomers may be added in the polymer-forming reaction. Copolymers produced
with ethylene glycol plus other diols (such as 1,4-butanediol), or diacids (such as
suberic acid, 1,8-octanedioic acid), have a lower melting point and Tg, and improved
dyeing properties. The introduction of longer alkyl groups into the polymer chain
gives it greater flexibility and the fibres absorb disperse dyes at the boil without use of
a carrier. They also give less fibre pilling on abrasion. Incorporation of halogenated
or phosphonated derivatives gives polyesters with improved flame resistance.

Polyester dyeable with cationic acid dyes has some terephthalic acid units
replaced by 5-sulphoisophthalic acid (2, Figure 4.3). This again decreases the
regularity of the PET chains and also the crystallinity and orientation so that dyeing
can be carried out at the boil by a cation exchange mechanism. Similarly, dyeing
with anionic dyes is possible if an acid such as 1,5-pyridinedicarboxylic acid (3, in
Figure 4.5) is used as a comonomer. Basic and acid-dyeable PET are used for special
colour effects.

All these chemically modified PET fibres have lower Tg values. They all absorb
disperse dyes at the boil without the use of a carrier.

Kodel fibres (Eastman Chemical, USA) are also polyesters produced by ester
exchange between dimethylterephthalate and a mixture of cis- and trans-1,4-
bis(hydroxymethyl)cyclohexane. The polymer is called poly-1,4-cyclohexylene-
dimethyleneterephthalate or PCDT (4). In many respects, this polymer is very