Page 62 - Basic Principles of Textile Coloration

NYLON FIBRES 51

Figure 3.4), produced from terephthalic acid (1,4-benzenedicarboxylic acid) and
ethylene glycol (1,2-ethanediol), came on the market in 1948, followed by
polyacrylonitrile copolymers in 1950.

H NH2(CH2)5CO2H + H
N O H2O N

O

NH2(CH2)5CONH(CH2)5CO2H NH2(CH2)5CO NH(CH2)5CO nNH(CH2)5CO2H

Scheme 4.1

All synthetic fibres are produced by extrusion of continuous filaments, either of
the molten polymer or of its solution in a solvent, through tiny holes in a metal
plate. This is called a spinneret. Extrusion of polymer filaments is called spinning,
but must be distinguished from the spinning of yarns of staple fibres by drawing and
twisting. In melt spinning, the solid polymer filaments are obtained by rapidly cooling
the extruded molten polymer, whereas in solution spinning they remain after
evaporation of the solvent, or coagulation in a bath of suitable chemicals.

4.2 NYLON FIBRES

4.2.1 Production of nylon 6.6

Heating the salt of adipic acid and hexamethylene diamine (1, Figure 4.1) in
concentrated aqueous solution in the presence of a little acetic acid gives nylon 6.6,
or polyhexamethylene adipamide. Once most of the volatile diamine has reacted,
water is gradually removed by distillation. Finally, the molten mixture is heated to
280 °C under nitrogen to avoid oxidation. Figure 3.3 illustrates the sequence of
condensation reactions. The final liquid polymer is extruded as a thick ribbon,
cooled to solidify it, and cut into chips. Using the exact 1:1 stoichiometric ratio of
the monomers (from the diacid-diamine salt), and forcing the polycondensation to a
high conversion, are important for obtaining the desired high molecular weight of
the product. Low values of molecular weight give weak filaments but too high a
value results in a more viscous molten polymer that is difficult to extrude. Addition
of a small amount of acetic acid in the polymerisation assists in controlling the final
molecular weight of the nylon. The added acetic acid functions as a chain stopper by