Fiber spinning from the nematic melt. V. Flow instabilities in the 75:25 copolyester of p‐hydroxybenzoic acid and 2‐hydroxy‐6‐naphthoic acid

We have investigated the rheological properties of the Celanese copolyester with the composition 75 mol% p‐hydroxybenzoic acid and 25 mol% 2‐hydroxy‐6‐naphthoic acid (designated as 75HBA/25HNA). Three different samples having inherent viscosities 3.0, 6.0, and 9.2 dL/g were studied. A flow instability is observed at low shear stress which produces an irregularity in the fiber diameter. The surface irregularity becomes less pronounced above a minimum shear stress, indicating that the flow instability originates in the capillary. For these nematic melts, the minimum shear stress marking the onset of more regular flow is found to decrease with increasing temperature and with decreasing inherent viscosity of the copolyester. The die swell ratio of extrudates decreases with increasing shear stress. Fibers were spun from the samples having ηinh = 9.2 and 3.0 dL/g. The initial modulus and tenacity to break for 75HBA/25HNA fibers spun at sufficiently high shear stress to produce smooth filaments are significantly lower than the values we previously reported for fibers of the 58HBA/42HNA copolyester. Moreover, the optimum properties are obtained at relatively low spin‐draw ratios. The 75HBA/25HNA polyester also exhibits a yield stress which decreases with increasing temperature. This observation indicates the presence of crystallities at the test temperatures. We believe that the higher content of HBA in the present copolymer gives rise to crystallization of HBA blocks in the thread line and that defects are introduced at higher spin‐draw ratios which cause the mechanical properties to become worse.