An experimental and theoretical study of the wet spinning process

Wet spinning apparatus was designed and built which is capable of producing commercial quality fibers. The materials studied were nylon-6, nylon-66, redissolved DuPont Nomex® and Kevlar® polyacrylonitrile/7.5% vinyl acetate copolymer, and Bisacid A-2. Several solvents and coagulants were examined and their effects on the fiber superstructures were studied.

The superstructure of wet spun fibers may be classified as either (1) hollow core, (2) fluted core, or (3) sponge-like. All wet spun fibers may be considered to represent one or more of these classifications. Examples of each type were spun and examined by scanning electron microscope. The fibers were also studied with small angle x-ray diffraction.

A microscopic static coagulation technique was used to observe coagulation and the void forming process. By this technique and a hydrodynamic stability analysis, the fluted fibers were associated with a dimensionless group, F = ^DQC_kT , i.e., the product of the diffusivity, heat of mixing, and the concentration divided by the product of the thermal conductivity and the temperature. The characteristic value for this group is ^DQ_k (^dC_dT)_equil. Assuming that _k (^dC_dT)_equil is 0.1. fluted fibers are formed for values of F greater than 0.018.

The draw down of the fiber during the coagulation process was mathematically modeled. Plots of dimensionless fiber diameter vs. dimensionless distance from the spinneret are presented and compared to experimental data. Hollow fibers were formed when a chemical reaction occurred at the fiber surface between the bath and solvent. This increased the mass transfer rate and it was shown that hollow fibers were prevalent at initial external Sherwood numbers greater than 20.