Studies on melt spinning. V. Draw resonance as a limit cycle

The exact wave form of draw resonance in isothermal spinning of Newtonian liquids was sought by solving numerically the simultaneous partial differential equations¹ of melt spinning in their original nonlinear form without recourse to perturbation. When the draw-down ration of spinning exceeded 20, solution of the equations became a limit cycle, a sustained oscillation having amplitude and period independent of initial conditions. As the draw down ratio was further increased, the amplitude of the limit cycle grew very rapidly, and the wave form became close to a pulse train predicting an extreme thinning of the thread at regular intervals along the thread. The above solution for Newtonian liquids agreed well with experiment with respect to oscillation period. Agreement, however, was poor in amplitude, indicating that possibly the amplitude of draw resonance is affected by deviations of polymer viscosity from Newtonian.     

Nonisothermal orientation-induced crystallization in melt spinning of polypropylene

Two kinds of polypropylene with different molecular weight (MI = 15 and 30) were melt-spun at the spinning temperatures of 210–290°C and take-up velocities of 0.15–3 km/min. In the cases of the spinning temperatures of 270 and 290°C for MI15 and 250 and 290°C for MI30, the density showed a minimum with increasing take-up velocity at around 0.5–1 km/min. This result suggests that crystallization behavior is influenced by two competitive effects, i.e., cooling rate and crystallization rate both of which are enhanced by the increase in take-up velocity. Crystal structures of slightly oriented monoclinic, slightly oriented pseudohexagonal, highly oriented pseudohexagonal, and highly oriented monoclinic were successively observed with increasing take-up velocity. The change of crystallization temperature may result in the different kinds of crystal modifications. Numerical calculations on nonisothermal orientation-induced crystallization in the melt spinning process and experimental results showed qualitative agreement in the change of crystallinity with take-up velocity, spinning temperature, and molecular weight.     

Crystallization of running filament in melt spinning of polypropylene

The crystallization kinetics of the running filament in melt spinning have been studied for three cases: isothermal crystallization of an isotropic melt, non-isothermal crystallization of an isotropic melt, and non-isothermal crystallization of a non-isotropic melt. Both the temperature and the orientation dependences of nucleation rate and growth rate are estimated for polypropylene. Calculated curves for non-isothermal crystallization of a non-isotropic melt with partial high orientation closely approximate the experimental data. In particular, the experimental data are best explained by crystallization with two-dimensional growth. The crystallization processes in melt spinning may be governed by localized molecular orientation of the supercooled melt in the initial stage.     

Elongational viscosity in the isothermal melt spinning of polypropylene

The elongational viscosity of polypropylene has been investigated by isothermal melt spinning, carried out over a range of experimental conditions. The filament diameter and the elongational force were measured for running filaments and the relationship between elongational viscosity and elongational strain rate reported. The elongational viscosity was observed to decrease in the vicinity of the spinneret and then remained constant before increasing along the thread line. An increase in elongational viscosity did not occur within the isothermal zone until the elongational flow was fully developed. The onset of an increase in elongational viscosity was determined from the constant total elongational strain. The degree of molecular orientation was also studied by birefringence measurements and was investigated as a function of elongational stress. At a high elongational stress, the relation between birefringence and elongational stress departed from linearity and exhibited a rapid increase which can be related to the increase in elongational viscosity.     

Extrusion and melt spinning characteristics of thermally degraded polypropylene

A basic study of the influence of thermal degradation of polypropylene on its extrudate swell and melt spinning characteristics is reported. The degradation reduces swell, changes the shape of the spinline diameter/velocity profile (i.e., the spinline apparent elongational viscosity function), and stabilizes the melt spinning process. The importance of molecular weight distribution narrowing during the degradation process is noted in these studies.     

Sensitivity analysis of low-speed melt spinning of isotactic polypropylene

Linearized sensitivity analysis of fiber spinning has been studied using a two-phase constitutive model that includes the effects of crystallization. The analysis focuses on sensitivity predictions for the low-speed melt spinning of isotactic polypropylene and comparison to the experimental results of Young and Denn [1989, Disturbance propagation in melt spinning. Chemical Engineering Science 44, 1807-1818]. Modifications in an earlier-developed two-phase model enable comparisons of two different constitutive equations for the melt phase, namely the Giesekus and extended pom-pom models. Comparisons with sensitivity data for low-speed spinning conditions demonstrate that the incorporation of crystallization effects leads to improved predictions of the magnitude and trends of the perturbation frequency dependence for both constitutive equations. At low spin speeds where flow-enhanced crystallization effects are negligible, the sensitivity is predicted to decrease with increasing cooling and this trend is also shown to be consistent with increased crystallinity.     

Finite-amplitude stability and draw resonance in isothermal melt spinning

The spinning of fibers from polymer melts is sometimes limited at high takeup rates by the onset of a flow instability known as “draw resonance,” which is characterized by large amplitude fluctuations in the takeup area and the force. In this work, the onset and growth of the spinning instability is analyzed for an isothermal Newtonian liquid by examining the nonlinear dynamics of the most unstable spatial mode. At draw ratios below the critical value D_R = 20·21 computed from linear theory the system is stable to finite-amplitude perturbations. At higher draw ratios all disturbances approach a stable finite-amplitude oscillation which agrees in period and magnitude with experimental observations of draw resonance.     

Studies on melt spinning. III. Flow instabilities in melt spinning: Melt fracture and draw resonance

An experimental study has been carried out to investigate threadline instabilities in melt spinning. Two types of melt threadline instabilities, draw resonance and melt fracture, were observed under both isothermal and nonisothermal spinning conditions. Polymers investigated were polypropylene and polystyrene. Draw resonance was observed as an increase of the take-up speed above a critical value. It was also observed that an increase in take-up speed reduces the severity of melt fracture, whereas once draw resonance occurs the amplitude and frequency of the pulsing thread diameter increases with the take-up speed. The phenomenon of draw resonance was investigated by taking motion pictures of the pulsing molten threadline spun vertically downward. Furthermore, a stability analysis was carried out to explain the experimentally observed draw resonance.     

Studies on the mechanism of draw resonance in melt spinning

The physical mechanism of draw resonance of polymer fluids in melt spinning has been studied. It is proposed, according to the cross-sectional area dependence of the local draw ratio of the filament along the spinline, that filament drawing can be divided into three modes which are given in the main text. In particular, it is found that the nonuniformizing draw mode is the necessary condition of draw resonance. The mechanism is certified through the analytical solution of isothermal melt spinning under uniform spinning stress and the critical draw ratio of isothermal and uniform tension melt spinning. The mechanism was employed to analyze the promotive and suppressive factors of draw resonance in a real spinning system and the development of filament unevenness along the spinline.     

Computer simulation of melt spinning of polypropylene fibers using a steady-state model

A mathematical model that includes crystallization in the spinline and the effect of crystallization on the extensional viscosity and the various physical properties of polypropylene has been developed and used to help in identifying the various factors that can affect the spun yarn characteristics. The model is used to simulate effects of spinning parameters on fiber physical properties, temperature, and stresses. The experimental observation of a minimum in density of the spun yarn at high throughput rates, when density is plotted as a function of take up velocity, has been investigated in some detail. It has been found that all conditions which can substantially affect the rate of cooling and the orientation of the polymer in the spinline, viz, throughput rate, spinning temperature, and spinning speed have an important bearing on the temperature range in which crystallization can take place in the spinline and thus affect the density. It is suggested that in addition to these factors, the formation of different crystal modifications at different spinning speeds could also contribute to the reduction in density of these samples. The model cannot reflect the observation of density changes occurring due to the formation of different crystal modifications. Nevertheless, it can be of use in understanding the effects of various process conditions on the cooling rate and the orientation of the polymer in the spinline. © 1995 John Wiley & Sons, Inc.     

Numerical simulations of draw resonance in melt spinning of polymer fluids

This article deals with numerical simulations of draw resonance of polymer fluids by employing direct difference methods to solving the governing equations in melt spinning. The stability of each difference method was studied by a comparison of the results obtained from simulations with the theoretical solutions or values. The numerical simulation confirms that the critical draw ratio of draw resonance in an isothermal and uniform tension spinning of a Newtonian fluid is between 20 and 21. The cross-sectional area of a spinline in draw resonance was found to decrease monotonically from a spinneret toward a take-up bobbin, although the taken-up filament shows periodical variation. This study has also illustrated the mechanism of draw resonance previously proposed by the author. © 1993 John Wiley & Sons, Inc.     

聚丙烯/聚苯乙烯共混熔纺中分散相的形态演变

采用共混熔融纺丝法制备聚丙烯(PP)/聚苯乙烯(PS)共混纤维,在纺程上不同位置收集纤维,研究了纺程上分散相PS的形态演变以及共混组成和拉伸比对形态变化的影响。结果表明:纺程上分散相由挤出丝中的球形变为卷绕丝中的椭球形;随着分散相含量的增加,挤出丝中分散相数目增多,分散相之间发生聚并,直径增加,纺丝过程中在拉伸应力作用下,分散相发生形变;随着拉伸比的增加,卷绕丝中分散相长径比增加,分布变宽。     

Crystallization in polymer melt spinning

The crystallization behavior of poly(ethylene) terephthalate (PET) melt spun into fiber monofilaments was examined using a laboratory set-up. The wind-up speeds ranged from free fall under gravity to 1500 m/min. The major additional variables that were manipulated included the mass flow rate and the filament temperature profile. The structure of the as-spun fibers was probed using tensile tests, differential scanning calorimetry, optical birefringence, and x-ray diffraction. It was found that while the filaments that had been spun nonisothermally were essentially amorphous, those that had been made under isothermal conditions at temperatures ranging from 180°C to 240°C were oriented and crystalline. In addition, the rate of oriented crystallization was much greater than that under quiescent conditions at the same temperature. This is perhaps the first published study which shows that highly crystalline (up to 40% crystallinity) PET fibers can be obtained at low spinning speeds merely by altering the fiber temperature profile while the material is still above the polymer glass transition temperature.     

Production and characterization of polypropylene fiber upon addition of selective peroxide during melt spinning and comparison with conventional polypropylene fibers

In the present work, effect of selective peroxide on reactor grade polypropylene (PP) (known as V30S) during melt spinning process on the physical and thermal properties of as‐spun resultant multifilament yarn was studied. Attempts have also been made to compare this yarn sample with other fiber samples produced from reactor and controlled rheology grades polypropylene. The results show that the multifilament yarn spun from V30S/Peroxide sample shows higher birefringence and tensile strength and also lower modulus, elongation at break, and shrinkage compared with that of spun from pure V30S granule. Density and thermal behavior studies show low variations compared with original sample. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Dog-legging in the melt spinning process

Experiments were performed in a continuous capillary rheometry to investigate the dog-legging mechanism in a melt spinning process. The bending angle of the spinline was measured under various spinning conditions while spinning PP, PET, and PEN fibers. Experimental results indicate that obstacles inside the capillary of the spinnerette are the primary cause of bending of the spinline. The bending angle is greatly influenced by the obstacle's dimensions and position inside the capillary. An increase in the obstacle's radius or a decrease in the obstacle's length caused the bending angle to increase. The closer the obstacle was to the exit of the capillary implied the bending angle would be the wider except for the block immediately above the exit. Also at high shear rates, PEN was found to be relatively insensitive to spinline-bending than PET polymers. In accounting for the dog-legging behavior, our results suggest that the die swell model is more realistic than the conventional vortex model.     

Correlations between relaxation time spectrum and melt spinning behavior of polypropylene. II: Melt spinning simulation from relaxation time spectrum

Using an integral nonlinear constitutive equation including the Log-Normal relaxation spectrum, the fiber-spinning behavior of commercial polypropylenes is studied. The relaxation spectrum is strongly related to the molecular weight distribution. Moreover, by means of a finite element analysis, simulated stress and velocity profiles are obtained either in isothermal or nonisothermal cases. In the isothermal case, broadening the relaxation spectrum leads to more pronounced curvature of the velocity profile. In the nonisothermal case, the critical draw ratio is sensitive to the mean relaxation time which depends on both average molecular weight and distribution breadth. It is shown that increasing the average relaxation time leads to a less rapid growth of the velocity. Furthermore, this effect is less pronounced when the relaxation spectrum is wider.     

Nonuniform cooling in multifilament melt spinning of polypropylene fibers: Cooling air speed limits and fiber-to-fiber variations

The cooling of the spinning stage in a commercial compact-spinning line has been studied. A rectangular fiber bundle is extruded from the spinneret and cooled by air entering from one side. The speed of the cooling air is considerably reduced through the fiber bundle. There are practical lower and upper limits for the cooling air entrance speed, corresponding to filament breakage at the leeward and windward sides, respectively. These limits are functions of the material and processing parameters. Due to the nonuniform cooling, fibers sampled at the windward side generally have higher molecular orientation, lower amorphous fraction, higher density, and higher tensile modulus and strength. For most combinations of spinning and material parameters, the structure is either bimodally oriented α-crystalline or uniaxially oriented mesomorphic at all spinneret positions. Fibers with different structure types show opposite windward/leeward side trends with regard to local order and melting behavior. The structure may be mesomorphic at the leeward side and α-crystalline at the windward side, if the average spin-line stress is close to a critical value for orientation-induced crystallization, and the air speed difference across the spinneret is large. The cooling air speed affects the spin-line stress. Hence, the fiber-to-fiber variations due to nonuniform cooling are discussed in terms of the molecular orientation in the melt and the effective time available for arranging molecules into ordered structures. © 1995 John Wiley & Sons, Inc.     

Structure development during polymer processing: Studies of the melt spinning of polyethylene and polypropylene fibers

Experimental studies of structure development in melt spinning of polyethylene and polypropylene fibers are described. Emphasis is given to the influence of applied stresses on the rates of crystallization and on the development of crystalline morphology. The relationship of fiber morphology to mechanical properties, especially “hard elastic fibers” is considered. The relevance of such studies to other polymer processing operations such as film extrusion is discussed.