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. 

Dry spinning

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.
(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.
(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 

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