Page 413 - Handbook of Properties of Textile and Technical Fibres

386 Handbook of Properties of Textile and Technical Fibres

The melt spinning process of fiber formation begins with the melting of the PA
chips followed by extrusion of the melt through the fine orifices of spinnerets, solid-
ification of melt streams into viscoelastic filaments by quenching in air, and then
winding the filaments. The rate of cooling during solidification is characterized by
2
1
the heat transfer coefficient c [Wm K ], for fibers c ¼ 33e800. Thermal conductiv-
1
2
ity of polymers is low (order is 0.1e0.3 [Wm K ]) and therefore the difference be-

tween temperatures of the fiber surface and core can be up to 30 C. This is one reason
for the appearance of a surface “skin”. During cooling the polymer crosses the temper-
ature zone of maximum crystallization rate at T*, which depends on the melting tem-
perature T m given by the relation T* ¼ 0.8 T m .
The spun yarn is then drawn and heat-set in other machines. At present, it is standard
practice to aggregate the spinning-drawing-winding procedures. A modified version of
the melt spinning process, in which a liquid bath is used between spinneret and a take-up
roller is called liquid isothermal bath spinning (Najafi et al., 2017b). The liquid
allows other processing variables in the spinning line to be used in order to modify

the mechanical properties of the fibers. The liquid temperature is about 30e40 Chigher
than the polymer T g , to provide enough mobility of the polymeric chains. It was found
that for PA 6 fibers, the liquid increased the tenacity by approx. 44%, modulus by
approx. 69%, and reduced elongation till break by approx. 41%. Such modification

was obtained only through entering the yarn in a warm (88 C) liquid bath for a few mil-
liseconds (Najafi et al., 2017b).
Dry spinning, wet spinning, and gel spinning need a polymer solution. For dry spin-
ning, mixtures of volatile solvents of formic acid and chloroform or dichloromethane are
used. After the extrusion of the polymer solution, the hot air stream evaporates the sol-
vent and the fiber is solidified. The tenacity (tensile strength at break) and initial moduli
of PA 6 fibers, prepared by dry spinning from mixtures of formic acid and chloroform
solution are, 1 GPa and 16e19 GPa, respectively, compared to 0.5 and 5 GPa for PA 6
fibers prepared by melt spinning. It is also shown that mechanical properties of PA 6
fibers strongly depend on the draw ratio, molecular weight, concentration of polymer,
and solvent ratio (Gogolewski and Pennings, 1985).
In wet spinning, a viscous solution of polymer is used, which is extruded into a
continuous filament, and then a coagulation of fibers is usually carried out by using
a nonsolvent for the polymer. For the higher molecular weight PA 6, concentrated sul-
furic acid and formic acid/lithium chloride solvents are used for wet spinning. The
disadvantages of this process are that it is difficult to obtain fibers with uniform
diameters and it is not possible to use higher spinning speeds (Vasanthan, 2008).
For gel spinning of PA 6 fibers, benzyl alcohol is used as a solvent. Gel spun fibers
drawn to a draw ratio of 5.3 have an initial modulus of 6.2 GPa (Cho et al., 1996).
Solid-state coextrusion of PA 6, plasticized with ammonia, was investigated by
Zachariades and Porter (Zachariades and Porter, 1979). They obtained fibers with
moduli up to 13 GPa. The ammonia changes only the amorphous regions of PA 6,
whilst iodine can change both the crystalline and amorphous regions. It was found
that hydrogen bonds in polyamides can be replaced by GaCl 3 (see Fig. 12.20).
Draw ratios of 7e13 are achieved for the melt spun complex of PA 66/GaCl 3 fibers,
under low strain-rate stretching. PA 66 fibers made from the GaCl 3 complexed state,