Civil Engineering Reference
In-Depth Information
as well as solution concentration. As the needle diameter is decreased, the diameter
of the i bers decreases greatly. However, the dif erences in morphology of electrospun
mats were not signii cant with increasing rotation speed of the grounded collector. h e
cellulose solution in NMMO with 50% H
2
O could not be electrospun into i bers at
concentrations below 4 wt%.
Recently, electrospinning and melt-blown techniques were harnessed for the inves-
tigation into the preparation of textile materials of thin cellulose i bers. In the study,
Tomaszewski
et al.
[81] performed electrospinning by using spinning solutions with
cellulose concentration of 3 wt% and 6 wt% in NMMO. For preparing the cellulose
spinning solution, the aqueous NMMO solution was i rst concentrated to the 70
wt% content. Crumbled cellulose at er the hydrothermal processing pretreatment or
untreated was added to the NMMO solution at 40°C with stirring and the antioxidant
(propyl gallate) was added in the amount of 0.1 wt% on cellulose. Hydrothermal pro-
cessing pretreatment of cellulose was adopted for reducing the cellulose polymerization
degree from 1400 to 370. h e mixture was put into a vacuum autoclave, preheated to
40°C. h e autoclave was equipped with an illuminated sight glass enabling the inspec-
tion of the cellulose pulp surface in the course of dissolving. h e reactor content was
slowly heated up to 120°C with a step-wise pressure reduction from atmospheric to
about 8 mmHg, enabling the evacuation of a sui cient amount of water to reach its con-
tent in the mixture below 13.3 wt%, thus enabling the dissolution of the cellulose in the
NMMO. At that point, the hitherto white and opaque pulp began to darken and acquire
transparency. h e process was continued at a pressure of 3 to 5 mmHg and temperature
of 120°C at usually no longer than half an hour. Beyond 150°C, cellulose undergoes an
exothermic decomposition in the presence of NMMO. In adverse conditions like big
mass and excessive water evacuation, temperature may increase, causing an explosive
decomposition of the solvent. It, therefore, seems that the water content during the dis-
solutions must not be reduced to below 10%. Also important is the inspection of the
pulp surface in the autoclave through the sight glass to control and prevent ef ervescent
boiling which may occur at an excessive decrease in the pressure, high temperature
and presence of a large amount of water. Compared to hydrothermal processing of pre-
treated cellulose solution, the quality of the untreated cellulose solution was not good.
h erefore, only solutions of the hydrothermal processing pretreated cellulose were used
in the spinning.
For the electrospinning of cellulose i bers by the Lyocell process, the solutions solidi-
i ed at ambient temperature (some solutions solidify with delay) and must be heated up
to at least the melting point, i.e. above 80°C. h e thin cellulose solution streams formed
in the electrostatic i eld must be quickly coagulated to become i bers. h e process is
mostly accomplished in water where it proceeds rapidly in less than one second. h e
earthed spinning head has two temperature-controlled heating zones. h e spinning
solution is melted in the cylindrical chamber and pressed with nitrogen to a steel pipe,
where the temperature of the beginning of spinning is adjusted. h e spun i brous mate-
rial is taken up on the surface of the water in which a metallic mesh is immersed, con-
nected to the high potential of the high voltage power supply (11-30 kV). h e i brous
material is collected on the mesh and then transferred onto a plastic mesh on which it is
washed many times with distilled water and, then, slowly dried at ambient temperature.
From a variety of these spinning solutions with dif erent cellulose concentrations, 3 and
