Page 76 - Basic Principles of Textile Coloration

ACRYLIC FIBRES 65

processes (such as differential shrinkage of warp and filling filaments, sanding or
sueding to give the fashionable ‘peach skin’ look, or napping and shearing) produce
special surface appearances. A variety of chemical finishes are used to impart water
or oil resistance, soil release, and resistance to pilling. The application of anti-static
lubricants is common.

Fabrics of microfibres are delicate and require careful treatment. They also pose a
number of problems in wet processing and dyeing not found with those produced
from higher denier filaments. The extra fine filaments have a much higher specific
surface area than regular filaments. A major consequence of this is the much higher
reflection of light from the filament surface. A higher dye concentration in the
filaments is therefore needed to produce a given depth of shade. The greater fibre
surface area also results in higher rates of dyeing at lower temperatures, and
decreased fastness to light, crocking (fastness to rubbing), water and ozone. These
problems will be more fully discussed in Section 15.7.5.

4.4 ACRYLIC FIBRES

4.4.1 Polyacrylonitrile

Radical polymerisation of acrylonitrile (cyanoethene) in aqueous solution, using
sodium persulphate (Na2S2O8) and bisulphite (NaHSO3) as initiators, gives poly-
acrylonitrile (PAN) with sulphate (SO4–) and sulphonate (SO3–) end groups (Figure
3.2). The homopolymer decomposes on heating before it melts. Melt spinning, as
carried out for polyamide and polyester fibres, is therefore impossible. In addition,
the homopolymer has limited solubility in common solvents so solution spinning of
filaments was not a simple option for commercial fibre production. Pure PAN also
proved to be very difficult to dye because of its compact, crystalline structure and a
Tg above 100 °C.

Commercial acrylic fibres are copolymers obtained by polymerisation of
acrylonitrile in the presence of other monomers, usually in aqueous solution or
emulsion. An acrylic fibre consists of polyacrylonitrile containing at least 85% by
weight of acrylonitrile units. The presence of other types of monomer unit along the
polymer chain destroys the regularity of the structure and decreases the crystallinity.
Non-ionic comonomers such as vinyl chloride or acetate, or methyl esters of acrylic
or methacrylic acids (propenoic and 2-methylpropenoic acids), lower Tg to around
75 °C and considerably improve the dyeability of the polymer. The disruption of the
regular repeating structure by a different comonomer not only increases dye diffusion