Membrane and Desalination Technologies - TCE Moodle Website

88 J. Ren and R. Wang
In the spinning process of hollow finer membranes, the air gap can also be used for the
release of the tensile stress, especially for the instaneous demixing process. The influence of
air gap distance and DR on the tensile stress at the bottom of the air gap for the same spinning
speed is shown in Fig. 2.33. The tensile stress increases with the increase of the drawing ratio
and the decrease of the air gap. A critical tensile stress exists, which may cause the breakage
of the nascent hollow fiber membranes, the formation of some defects, the poor performance,
irregular shapes, et al. Since the tensile stress increases greatly with a very small increase of
DR in the wet spinning process (air gap is zero), it is difficult to control the tensile stress. In
order to release the tensile stress, a small air gap should exist between the spinneret exit and
the coagulation bath. Because of the air gap, the drawing ratio will be mainly attained at the
air gap and the tensile stress will be released greatly due to the relative low viscosity of the
polymer solution in the air gap. Thus, it is easy to control the tensile stress below the critical
tensile stress. And the hollow fiber breakage and some defects caused by a high tensile stress
can be overcome.
Free fall spinning
When the take-up speed is the same as that of the free-gravitational flow dictated at
the bottom of the air gap, the elongation stress at the bottom of the air gap is zero. In this
case, the gravitational force component at the distance z (from the exit of the spinneret) is
equal to the elongation force
Z L
F ¼ rg dz; ð46Þ
Tensile stress
τ critical
L=0
1
DR
z
Small air gap
Qp
vzðzÞ
large air gap
Fig. 2.33 Schematic diagram of the relationship between tensile stress and drawing ratio at different
air gaps.