Dry-jet-wet spinning Mukesh Kumar Singh

Introduction Polymer is dissolved in an appropriate solvent to make the fibre solution. This solution is then extruded under heat and pressure into an air gap before it enters a coagulation bath. The produced fibre is then washed and dried before it is heat treated and drawn. This is an alternative method to wet spinning and is required as spinning directing into the bath For some fibres, creates microvoids that negatively affect the fibre properties. this is due to the solvent being drawn out of the liquid too quickly. An inert atmosphere may be required to prevent oxidisation in some polymers, if so fibres are extruded into a nitrogen atmosphere.

This small difference in arrangement of spinneret (i. e This small difference in arrangement of spinneret (i.e. placing it outside the coagulation bath) brings about significant difference in spinning performance and ultimate structure of the fibre. Dry-jet wet spinning imbibes the benefits of both dry spinning and wet spinning. However unlike dry spinning, the air gap in dry-jet wet spinning is too small to allow any appreciable removal of solvent before the protofibres enter the coagulation bath. It is still sufficient to bring about changes on the surface of the extruded filament. It is hypothesized that in PAN spinning, a kind of dense cuticle (probably hydrophobic) is formed on the extruded fibres as they pass through the air gap. This in turn changes the diffusion dynamics of the solvent and nonsovent during the coagulation of the fibres

This method is often required for high performance fibres with a liquid crystal structure. Due to their structural properties their melt temperature is either the same as, or dangerously close to their decomposition temperature Therefore they must be dissolved in an appropriate solvent and extruded in this manner

Benefits of dry-jet wet spinning High speed of spinning High concentration of dope High degrees of jet-stretch ratios Control of coagulation kinetics by monitoring coagulation bath parameters. Among these benefits, (a) to (c) are derived because of the use of dry-jet and the air gap, while (d) is derived from the use of wet coagulation.

The diffusion coefficients of solvent and nonsolvent and their relative values are extremely important in deciding the protofibre structure. The effect of various parameters on these diffusion coefficients in dry jet wet spinning and immersion-jet wet spinning is explained below: Diffusion coefficients of solvent (DS) and nonsolvent (Dns) in both cases are known to decrease with increasing dope concentration and reducing coagulation bath temperature. However, diffusion coefficient of solvent is always larger than diffusion coefficient of nonsolvent. This may be due to the higher affinity of the solvent to the polymer matrix even though the size of the nonsolvent molecule (such as water) is much smaller than that of solvent molecules such as DMF, DMSO. Ratio of the two coefficients, DS/Dns increases with decreasing temperature and with increasing concentration of the dope. This means that as the conditions for diffusion are made more difficult, the differences between the solvent and non solvent diffusion are markedly enhanced.

There are distinct differences between the diffusion behaviour of solvent and nonsolvent in dry-jet and immersion jet wet spinning. Diffusion coefficients for both solvent and non solvent in dry-jet wet spinning are smaller than the immersion-jet wet spinning. This is possibly because of the formation of more dense structure of the entire protofibre or the formation of cuticle on the surface of the protofibre as it passes thorough the air gap in the dry-jet wet spinning. However, the ratio of DS/Dns is always greater for the dry-jet wet spinning than the immersion-jet.

Dry-jet-wet spinning of PAN fibre

Effect of jet-strech on diffusion coefficients of solvent (DS) and non-solvent (D ns) for immersion-jet wet spun (IJWS) and dry jet wet spun (DJWS) system

Mol orientation during dry-jet-wet spinning

Effect of jet stretch on diffusion coefficient ratio for dry jet wet spun and wet spun system

Jet-stretch ratio has a significant effect on diffusion coefficients Jet-stretch ratio has a significant effect on diffusion coefficients. The diffusion coefficient of solvent increases and of water (non solvent) decreases with increasing jet stretch ratio However, they both increase abruptly after a certain critical jet stretch ratio in the immersion-jet wet spinning possibly due to the formation of voids or crevice openings on the fibre surface On the other hand, in dry jet wet spinning, the diffusion coefficient of solvent keeps on increasing and that of water keeps on decreasing with increasing jet stretch ratio. This implies that on increasing jet-stretch ratio, the ratio of DS/Dns increases rapidly in dry-jet wet spinning resulting in a more compact and finer filament structure without large voids.

Therefore, dry-jet wet spinning is beneficial in making high strength fibres, such as PAN precursors for carbon fibres, where the formation of defect free structure of fine denier is an essential requirement. A further advantage claimed for the dry-jet wet spinning system is improved retention of cross-sectional shape so that it is possible to spin filaments of controlled non-circular cross-sections from orifices of appropriate shapes. The existence of even a small air gap has large influence on the diffusion coefficients of solvent and nonsolvent, however, the magnitude of the air gap has only a mild effect on the diffusion coefficients in dry-jet wet spinning..

Moderate increase in coagulation bath temperature allows application of larger jet-stretch ratio. However, it leads to the formation of voids after a certain critical jet-stretch ratio and the cross-section of the fibre tends to become round. On the other hand, in dry-jet wet spinning, the higher jet stretch ratio has no such detrimental effect on protofibres rather it enhances the outward movement of solvent compared to inward movement of nonsolvent.

Effect of coagulation parameters