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Dry Spinning Process | Advantages and Disadvantages of Dry Spinning

Dry spinning technique is used for such kind of Polymers which won’t melt but degrade on heating. Dry spinning is required for polymers with a melttemperature equal to or close to their thermal degradation temperature, therefore they require dissolving in a solvent in order to be processed into fibres. In dry spinning the polymer is dissolved in its solvent and then extruded. As fibres emerge through the spinneret the solvent is evaporated off with hot air. This process may be used for the production of fibres such as Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandex and Vinyan

What is Dry Spinning?

Dry spinning technique is used for such kind of Polymers which won’t melt but degrade on heating. Dry spinning is required for polymers with a melttemperature equal to or close to their thermal degradation temperature, therefore they require dissolving in a solvent in order to be processed into fibres. In dry spinning the polymer is dissolved in its solvent and then extruded. As fibres emerge through the spinneret the solvent is evaporated off with hot air. This process may be used for the production of fibres such as Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandex and Vinyan. Dry spinning process is shown in fig.1.
Dry Spinning
Fig.1 Dry Spinning

Steps involved in dry spinning

  1. Solution preparation. (Polymer + Solvent): In dry spinning, a volatile solvent is used to dissolve the raw materials and form a spinning solution/dope solution.The polymer concentration in the dope ranges from 20 to 30 %. Higher 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 heated to a higher temperature to reduce dope viscosity.
  2. Then the solution is transferred to the spin block and purified by filter.
  3. The solution is extruded through a spinneret into a warm air chamber where the solvent evaporates, solidifying the fine filaments.
  4. Finally, the filament yarn either is immediately wound onto bobbins or is further treated for certain desired characteristics or end use.
  5. Application of spin finish for lubrication and antistatic effect.
  6. Collection of filaments in take-up bobbin.

Why dry spun fibres have serrated cross section?

The polymer solution is extruded into a dry spinning cell. It is a jacketed vertical box, where heated air (inert gas) is circulated to evaporate the solvent. The evaporation of the solvent vapor from the core to the surface is quicker than the transfer of the solvent from the core to the surface. So, the surface of the skin will be relatively dry. This results in the skin drying up quickly, and the core remaining relatively viscous. Because of this, the cross-section collapses, and non-circular cross-section of the fibre results from a circular spinnerette cross section.

Advantages

  • It is suitable for heat sensitive polymer 
  • Higher spinning speeds can be easily achieved
  • The post spinning operation is simple
  • Moderate concentration of polymer is required.
  • It is relatively flexible process and spinning conditions can be modified.

Disadvantages

  • Flammable solvent hazards.
  • Solvent recovery is required.
  • Additional post spinning operation is required for complete removal of solvent.
  • Difficult to achieve exact cross section of fibre.
  • Slow (200-400 yds/min).

Polymers and Solvents in Dry Spinning

Polymer
Solvent
Boiling Point Solvent (oC)
Cellulose Acetate
Acetone
56
Cellulose Triacetate
Methylchloride
41
PVC
Acetone
56
PAN
DMF
153
PU
DMF
153


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