Dry spinning
Dry spinning is similar to the melt spinning. The polymer concentration
in the dope ranges from 20 % to 30 % High polymer concentration results in
higher solution viscosity and difficulties in material handling.
However, physical
properties of the resulting fibre are better with higher polymer concentration.
The polymer solution is generally heated ‘to a higher temperature to reduce
dope viscosity, The polymer solution is
extruded into a dry spinning cell. It is a jacketed vertical box, where heated
air is circulated to evaporate the solvent. After extrusion, in the solidification stage, the solvent is removed by evaporation in presence of heat and suitable inert gas current.
air is circulated to evaporate the solvent. After extrusion, in the solidification stage, the solvent is removed by evaporation in presence of heat and suitable inert gas current.
Because of this, the spacing for holes
in spinnerette plate s wider. For such operation, the solvent should have
certain properties like low boiling point, low heat of evaporation, good heat
stability, inertness, ease of recovery and free from explosion hazards and
toxicity.
Industrial fibre production process consists of the following operations
(a) Dissolution of the polymer
in the solvent.
(b) Mixing and ripening of
the polymer solution.
(c) Metering of the polymer solution through the
extruder.
(d) Filtration of the
solution to remove mechanical impurities and undissolved polymer particles
(e) Extrusion of the polymer
solution through the spinnerette into a vertical drying chamber, at 100-150
Kg/cm2.The diameter of the hole is in between 0.2-0.4 mm.
(f) Solidification of the polymer solution by means of heated inert gas, mostly air.
(f) Solidification of the polymer solution by means of heated inert gas, mostly air.
(g) Application of
spin-finish for lubrication and anti-static effect.
(h) Collection of the
filaments in take-up bobbin.
Advantages of dry spinning
Dry spinning process has the following advantages:
(a) it is a recommended process
for heat-sensitive polymers
(b) High spinning speed can
be achieved.
(c) Lower solvent is required
because of high polymer concentration in the dope.
(d) The dry spinning method
is a relatively flexible one and spinning conditions can be modified.
(e) The process is more
suitable for filament yarns.
(f) The post spinning
operations are simple like that of melt spinning.
Disadvantages of dry spinning
The process has the following disadvantages
(a) The process requires
solvent and so solvent recovery process.
(b) Because of the presence
of solvents, there is a limit to maximum spinning speed.
(c) As the evaporation rate of the solvent is higher than the diffusion rate, a circular cross-section of the fibre is difficult to achieve.
(c) As the evaporation rate of the solvent is higher than the diffusion rate, a circular cross-section of the fibre is difficult to achieve.
(d) Additional post spinning
operation is required for complete solvent removal
(e) spinnerette hole configuration in the
spinnerette is very much critical for uniform outward mass transfer.
The fibres which are formed by this method and solvent with their boiling
points are given in Table
Polymers and Solvents in Dry
Spinning
Polymer solvent Boiling
point of the solvent
Cell-acetate → Acetone → 56°C
Cell-triacetate → Methylchloride → 41°C
PVC → Acetone → 56°C
PAN/PU → DMF → 153°C
PVA → Water → 100°C
