Rheology, Dynamics and Structure Development During Melt Spinning of Nylon-6

A comprehensive study of melt spinning dynamics and structure development of nylon-6 was performed. The major emphasis of the research consisted of on-line rheological and morphological measurements. These included diameter, temperature and tension measurements of the fiber as well as x-ray diffraction and birefringence measurements. The effects of annealing spun fibers in air, water and 20 percent formic acid solution were also studied. Tensile properties of spun fibers were measured and related to the spinning variables and structural characteristics.

In the rheological portion of the work, the shear viscosity and the normal stresses in nylon-6 melts were measured. Nylon-6 melt shows more Newtonian-like behavior in shear experiments than does poly-ethylene and shows very low normal stresses. Rheological response in melt spinning was interpreted in terms of an elongational viscosity. In the spinline, nylon-6 behaves more like a viscoelastic fluid with the ratio elongational viscosity to shear viscosity being about 15.

In studies of the dynamics of melt spinning various force components in the spinline were calculated. The gravity force was negligible at high take-up speed but was significant at low speed. The inertia and the aerodynamic drag shwoed an opposite behavior. The rheological force decreased along the spinline at low take-up speeds, remained constant at intermediate speeds and increased at higher speeds.

In the structure development studies, on-line x-ray patterns of nylon-6 exhibited an amorphous halo. This was true whether spinning was done was in 65 percent or 100 percent relative humidity. Nylon-6 crystallized in a y-pseudohexagonal form on the bobbin in the presence of moisture. The small angle x-ray scattering from conditioned fibers showed a diffuse ring at low take-up speeds but a two point diagram for fiber spun at high take-up speeds.

Nylon-6 changed slowly from a y-pseduohexagonal structure into an alpha-monoclinic structure when annealed at increasingly higher temperature. The rate of change was greatest when annealed in formic acid, but lowest in air, while intermediate in water.

The tensile strength and the modulus of spun fibers increased but the elongation to break decreased with increasing take-up speed or spin-line stress. Similar behavior was observed when the properties were correlated with the birefringence and the c-axis orientation factor.