Modeling of pneumatic melt spinning processes

Abstract

Computer simulation of the pneumatic processes of fiber formation from the polymer melts is discussed. The dynamics of air-drawing of thin polymer streams in supersonic air jets formed in the Laval nozzle is presented versus the melt blowing process. In the Laval nozzle process, the air flow takes place with high Reynolds number and the k-x model is used which considers kinetic energy of the air flow and the specific dissipation rate of the kinetic energy. For melt blowing, the air fields are simulated with the use of the k-e turbulent model. The air velocity, temperature, and pressure distributions along the centerline of the air jets are considered in the modeling of both pneumatic processes. The air fields are predetermined at the absence of the polymer streams for several air compression values in the Laval nozzle inlet and several initial air velocities in the melt blowing process. Each polymer stream in a usual configuration of a single row of the filaments in the process is considered as non-interacting aerodynamically with other streams, and the air jet is assumed to be undisturbed by the polymer streams. Air-drawing of the polymer filaments is simulated as controlled by the distribution of air velocity, temperature, and pressure on the air jet centerline with the use of a stationary model of melt spinning in a single-, thin-filament approximation. Effects of non-linear viscoelasticity of the polymer melt subjected to fast uniaxial elongation are accounted for in the modeling. Strong influence of the air jet velocity, the melt viscosity which controls response of the polymer melt on the air-drawing forces, and the dieto-collector distance has been predicted. Influence of initial air temperature, geometry of the air die, initial velocity and temperature of the melt, extrusion orifice diameter can be also predicted from the model. The example computations concern air-drawing of isotactic polypropylene with the use of the Laval nozzle are compared with the predictions for the melt blowing process. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: [4402][4403][4404][4405][4406][4407][4408][4409][4410][4411][4412][4413][4414][4415] 2012

Modeling of pneumatic melt drawing of polypropylene super-thin fibers in the Laval nozzle

“…An average diameter of the sub-filaments created during the ideal hypothetical splitting under the negative extrapressure p rh (z s ) at a point z s is estimated by the formula [23] …”

“…For uniaxial tension, the constitutive equation in thin-filament approximation leads to the set of two equations involving axial velocity gradient dV/dz, local tensile stress ∆p(z) and rheological extra-pressure p rh (z) [23] …”

“…where = 300, 000 [23,25]. Under the tensile stress, uniaxial molecular orientation of the polymer melt is formed in the filament at the air-drawing.…”

Kinetics of oriented crystallization of polymers in the linear stress-orientation range in the series expansion approach

“…The influence of crystallization on the rheological behavior has been investigated experimentally, as well as by using modeling concepts [24,[29][30][31][32][33][34][35][36][37][38][39][40]. Significant effects of oriented crystallization has been indicated by modeling of melt spinning processes that include effects of hardening of the polymer by online crystallization, such as high-speed melt spinning [4,5,24,28], melt blowing [41][42][43], pneumatic melt spinning in the Laval nozzle under supersonic air jet [43,44]. In the modeling of polymer processing under variable molecular orientation and temperature conditions, the crystallization rate has been expressed by a quasi-static formula [5,6] obtained by an extension of the Avrami-Evans formula basing on the nonisothermal Nakamura approach [45,46].…”

“…The formulas have been determined in the range of large undercoolings, and there is no reliable information, experimental or theoretical, from which the coefficient could be determined in the entire temperature range. Modeling of melt spinning processes with the oriented crystallization effects indicates that the oriented crystallization coefficient is responsible for important effects in the process dynamics and structure formation [4,5,28,[41][42][43][44]. None of the actually available models are satisfactory for calculating the kinetics of oriented crystallization under variable temperature in the entire range of undercooling with the use of the oriented crystallization coefficient proposed by Ziabicki [2,47].…”