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 synthes...
We have conducted rheological studies of polymer solutions synthesized using solvents having different dielectric constants. We have determined the effect of this characteristic on the rheological properties of the spin dopes. Estimation of the strength indices of aramid fibers obtained from solutions based on solvents with different dielectric constants made it possible to identify the benchmark value for the dielectric constant.As we know, in "wet" forming the polymer solution (in this case, polyamide benzimidazole) is extruded in the form of fine jets into the coagulation bath in which, as a result of mass exchange processes causing phase decomposition of the solution, the solution-to-freshly formed fiber transition occurs. The gelatinous filament emerging from the coagulation bath is subjected to orientational drawing in air between the take-up and draw godet rolls. Depending on the composition of the coagulation bath, the kinetics change for the process of separating the phase concentrated in polymer, and consequently the viscoelastic properties of the freshly formed fiber also change. We should note one more important point: since aromatic polyamides are rigid-chain polymers, they either form liquid-crystalline solutions or can form a mesophase in the freshly formed fiber as a result of concentration of the polymer. But depending on the mesophase content, the conditions vary for orientational drawing and subsequent treatments to achieve optimal mechanical characteristics of the finished fiber. This underlines the importance of the choice of composition of the coagulation bath (which affects the kinetics of the phase separation process) and the residence time of the forming fiber in the bath.In obtaining fibers from aromatic polyamides by a "wet" method, coagulation bath compositions are used that provide either slow phase decomposition kinetics ("soft" baths) or conversely fast fixation of the phase state ("hard" baths). In this case, we should not forget that the stability of the "wet" forming process (we mean the frequency of breaks in the filament between the take-up and draw godet rolls, leading to frequent shutdown of the spinning machine) and the strength characteristics of the filaments also depend both on the synthesis process parameters and on the purity of the monomers and solvents used: dimethylacetamide with lithium chloride (DMAA+LiCl). From a chemistry standpoint, the solvent used in the process (both regenerated DMAA and fresh DMAA) is subjected to chemical analysis for impurity content (the major impurities are assumed to be water, acetic acid, dimethylamine, and also slight amounts of monomethylamine), which is strictly regulated. However, note that a change in the ratio of fresh to regenerated DMMA affects the stability of the plasticizing stretch process and the strength characteristics of the fiber formed.Traditional rheological studies of spin dopes have shown that the forming process for aramid filaments is not always stable even with practically identical characteristics of the reacti...
Rheological studies of polyamidebenzimidazole (PABI) solutions with various calculated mean molecular weights (MWs) were carried out. A relationship was found between the ratio of the greatest Newtonian viscosity to the apparent viscosity and the calculated mean polymer MW. The effect of this characteristic on the rheological properties of spinning solutions and the strength of aramide fibers was determined. It was established that the strength characteristics of aramide fibers were higher if they were spun from PABI solutions with mean MW > 30,000.Many polymer properties are complicated functions of molecular weight (MW). If the rheological characteristics of polymer solutions are examined, then a so-called critical MW cr that corresponds to a sharp change in the rate of viscosity increase with increasing chain length (the exponent a in the viscosity as a function of MW, η = kMW a , increases from 1 to 3.4) is found for a homologous series [1,2]. For this, a viscoelasticity typical of polymers, a viscosity anomaly, appears. The activation energy of flow becomes constant. A transition through MW cr is associated with the formation in polymers of a network of intermolecular contacts (linkages) that increases considerably the resistance of the system to deformation. It is thought that a transition from oligomers to actual polymers occurs namely at MW cr .The viscosity as a function of the product c[η], where [η] is the intrinsic viscosity that depends on MW according to the Mark-Kuhn-Houwink equation ([η] = kMW a ) and reflects the volume occupied by a single macromolecule, is more convenient to use for polymer solutions. However, this does not mean that the polymer strength does not change starting at MW cr . Conversely, an increase of MW increases the strength characteristics. The viscosity defining the ability to process a given polymer or a given polymer solution into an item (in our instance, a fiber) is the limit for the lengthening.In practice, the problem of increasing fiber strength is solved by orienting the macromolecular chains as much as possible relative to the fiber axis using spinning and orientational drawing. It should be kept in mind that deformation of semi-rigid-chain polymers such as polyamidebenzimidaozle (PABI) is capable of transforming the formed system, e.g., a gel-fiber, into a liquid-crystalline state characterized by a stable uniaxial orientation along the drawing direction. In other words, deformation can be counted among thermodynamic factors such as pressure, volume, and temperature that affect the phase state of the system. This is a somewhat ideal case that can be realized during formation of PABI fibers only under special conditions. The optimal polymer MW that provides this array of fiber physicomechanical properties in usual processing practice is determined, as a rule, using the formation conditions and fiber strength requirements obtained from the given solution.Thus, the effects of changes of MW and viscosity properties during synthesis of an aromatic copolymer (PABI) on the...
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