Wet Spinning of Polyacrylonitrile Fibers

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Polyacrylonitrile (PAN) fiber wet spinning is a common process for producing PAN fibers, often as precursors for carbon fibers. This method is used for polymers that degrade before melting, as it relies on a solution rather than a melt. 

Process Steps

The general steps for wet spinning PAN fibers are as follows:

Dope Preparation: Fiber-grade PAN resin (or copolymer, typically with methyl acrylate and itaconic acid comonomers) is dissolved in a suitable solvent to form a viscous "spinning dope" or solution. Common solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and aqueous solutions of sodium thiocyanate (NaSCN) or zinc chloride.

Extrusion (Spinning): The prepared dope is accurately metered by a pump and extruded through a multi-hole spinneret (a plate with many fine capillaries) into a coagulation bath.

Coagulation: The nascent polymer filaments enter the coagulation bath, which is a non-solvent for the PAN but miscible with the solvent in the dope (e.g., a water/DMSO mixture when DMSO is the solvent). Solvent diffuses out of the filaments, and the non-solvent diffuses in, causing the polymer to precipitate and solidify into continuous fibers. This phase separation process is critical in determining the fiber's initial structure, such as porosity and cross-sectional shape.

Washing: The newly formed fibers are then thoroughly washed in a series of baths (typically using water) to remove residual solvent and any remaining non-solvent from the coagulation bath.

Drawing (Stretching): The fibers are stretched, either in a wet state (in hot water or steam baths) or after drying, in multiple stages. This drawing process aligns the polymer chains and microfibrils in the axial direction, which significantly enhances the fiber's mechanical properties, such as tensile strength and modulus. The total stretching ratio can be substantial, often 15-25 times the original length.

Oiling and Drying: A finish or oil may be applied, and the fibers are then dried on heated rolls to remove moisture. The drying conditions affect the final fiber structure and properties.

Heat Setting/Thermal Molding: The fibers may undergo further heat treatment or thermal finalization steps to stabilize their structure and properties before being wound onto bobbins. 

Key Parameters and Characteristics

Solvent Type: The choice of solvent (e.g., DMSO vs. DMF) affects the dope's viscosity and the fiber's formation and physical properties, such as cross-sectional shape and surface smoothness.

Coagulation Conditions: Coagulation bath parameters, including temperature and non-solvent concentration, influence the diffusion rates of solvent/non-solvent, which in turn determines the fiber's morphology (e.g., porosity, density, and cross-section).

Drawing Ratio and Conditions: The amount and conditions (wet/dry, temperature) of stretching are critical for molecular orientation and the development of the fiber's final mechanical strength.

Applications: PAN fibers produced by wet spinning are primarily used as precursor material for manufacturing high-performance carbon fibers. 



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