Evaluation of the requirements for the rheological characteristics of the polymer solution for production of high-strength polyaramid fibres

Rheological studies of samples of spinning solutions on a Rheotest 2 viscometer showed that for the same viscosity values determined with the falling ball method, the rheological characteristics such as the flow index, initial Newtonian viscosity, and apparent viscosity, differ. The highest strength of aramid fibres is attained in spinning from a spinning solution with the following rheological characteristics: apparent viscosity of 59.3 Pa⋅sec, initial Newtonian viscosity of 73.2 Pa⋅sec, and flow index of 0.808. The general problems in the chemistry of polycondensation processes related to reactions of formation of polymers and the molecular-weight distribution of the products obtained are set out in great detail in [1-4], but the rheological characteristics of the polymer solutions in these processes have been insufficiently investigated [5]. Determination of the viscosity of the polymer solution by the falling ball method is the only method for plant supervision of the polycondensation process in production of spinning solutions for spinning polyaramid fibres. As practice in industrial use has shown, this method of plant supervision is ineffective for stabilizing the quality of the spinning solutions and operational correction of the polycondensation process parameters. An analysis of the operation of such plants showed that for the same viscosity values determined with this method, the strength characteristics of aramid fibres obtained from these polymer solutions differ. In spinning the fibres, both the geometric parameters of the spinneret and the drawing conditions, as well as the temperature conditions of drying the fibres and heat treatment conditions remain unchanged. From a practical point of view, it was also useful to determine the other characteristics of polymer spinning solutions which could provide additional information on the character of the polycondensation process, in particular, estimating the flow index and initial Newtonian viscosity, which is a physical constant of the polymer and correlated with such a characteristic as the molecular weight (MW). The optimum MW of the polymer that ensures a given set of physicomechanical properties of the fibres is usually determined from the spinning conditions and the requirements for the strength of the fibre obtained from the given solution. The problem of increasing the fibre strength will be practically solved due to the maximum possible orientation of chain macromolecules relative to the axis of the fibre. This is ensured by the corresponding flow rate of the polymer solution relative to the walls of the spinneret channel and is closely correlated with the information concerning the rheological properties of the polymer, in particular, with the reproducibility of the ratios between the MW, molecular weight distribution (MWD), and the non-Newtonian viscosity. Evaluating the quality of a polymer by the viscosity of the solution alone without considering the shear characteristics is insufficient for determining the effect of any parameters on the synthesis process. For this reason, we focused our primary attention on studies of the rheological characteristics of the polymer solutions that ensure the highest strength of the fibres. Rheological studies were conducted on spinning solutions with the same viscosity determined with the falling ball method.