Page 79 - Basic Principles of Textile Coloration

68 SYNTHETIC FIBRES

4.4.2 Modacrylic fibres

Modacrylic fibres have between 35 and 85% acrylonitrile units in the polymer chain.
Fibres with such high relative proportions of comonomers such as vinyl chloride
(chloroethene), along with the acrylonitrile, provide specific properties such as
improved flame resistance or chemical resistance. Because of the wide variations in
comonomer type and proportions, fibres with a wide range of properties can be
produced. The dyeing characteristics of modacrylics are much like those of regular
PAN. They are usually more soluble in common solvents such as acetone, which
may be used for solution spinning. Some of the fibres of this type, with a high
proportion of a comonomer, may have lower softening temperatures and care is
required to avoid creasing in dyeing.

4.5 OTHER SYNTHETIC FIBRES

The three major synthetic fibre types are the nylons, polyesters and acrylics, with
polyesters predominating. In addition, there are a number of other kinds used to a
much lesser extent in textiles. These include a variety of addition polymers made
from vinyl compounds. Fibres from polypropylene are relatively cheap but quite
difficult to dye. Large quantities are coloured by pigmentation of the molten polymer
before spinning. This provides cheap coloured filaments of good fastness properties
but only in a restricted range of colours. Additives can be incorporated into the
polypropylene before spinning that are capable of increasing dye substantivity and
the rather poor heat and light resistance of this polymer.

Polyvinyl fibres have limited textile application. Polyvinyl and polyvinylidene
chlorides are valued for their high resistance to chemicals and their low-
flammability, and have specialised applications. They tend to have low softening
temperatures and shrink on heating. This polymer is soluble in water and filaments
of it are used as support yarns in the construction of delicate materials. Washing the
final article dissolves out the polyvinyl alcohol leaving the delicate structure of the
other yarns behind.

Polyurethane elastomeric fibres are being used more and more. The generic name
of such fibres is Spandex. They have the unusual characteristic of high elasticity and
elastic recovery. Such elastomeric fibres are essentially block copolymers consisting
of strong, rigid polymer segments among weaker more flexible segments. The flexible
segments are composed of low molecular weight aliphatic polyesters or polyethers.
These are reacted with a di-isocyanate and the resulting product is reacted with an
appropriate diamine (Figure 4.9). The urethane and urea type groups produced are