Dynamics and structure development during high speed melt spinning of nylon 6. I. On-line experimental measurements

On-line experimental measurements of filament diameter, temperature, and birefringence as a function of distance from the spinneret were carried out during melt spinning of two nylon 6 resins of differing molecular weight. Filament tension was also measured. A rapid diameter attenuation or “necking” was observed in the spinline for both resins at take-up velocities above 6000 m/min. Evidence of crystallization in the spinline was also observed at these high spinning speeds. An analytical technique was used to extract apparent elongational viscosity for each resin and heat transfer coefficient for the melt spinning process from the experimental results.     

On-line studies of structure development during melt spinning of poly(butylene terephthalate)

An on-line study of structure development during poly(butylene terephthalate) melt spinning was carried out. Two polymers with different molecular weights (intrinsic viscosities of 0.75 and 1.0 dL/g) were used. The range of take-up velocities studied was 1500 to 4500 m/min. On-line measurements included diameter, temperature, birefringence, and tension. The phenomenon of diameter thinning (necking) was observed for both polymers at take-up velocities of 3500 and 4500 m/min with a mass throughput of 4 g/min. At a constant mass throughput, the distance from the spinneret at which the necking occurred varied with take-up velocity and molecular weight of the polymer. Increasing the take-up velocity at constant mass throughput caused an increase in cooling rate and a slight increase in the rate at which the temperature decreased with distance from teh spinneret. A small but detectable change in the rate of temperature decrease was observed at a position near or just beyond the formation of the neck. It is suggested that this effect is due to the increased heat transfer caused by the rapid increase in filament velocity and increased surface to volume ration in the neck. Increased take-up velocity also caused necking to occur at higher temperature, as did an increase of polymer molecular weight. Birefringence increased with distance from the spinneret and indicated substantial molecular orientation was developed in the filament prior to the necking zone. A sharp increase of birefringence in the necking zone was observed for take-up velocities of 3500 and 4500 m/min. A discussion of the mechanism of neck formation is presented, and it was concluded that necking is intimately associated with stress-induced crystallization in PBT. An increase of spinline stress resulting from either an increase of take-up velocity or an increase of molecular weight can cause stress-induced crystallization and, hence, necking to occur nearer the spinneret and at higher temperature. For a given polymer this leads to filaments with higher levels of crystallinity, crystalline orientation, and crystalline perfection (greater crystal size). These changes in morphology result in changes in the filament mechanical properties. The effect of molecular weight change on the structure and properties is complicated by the fact that the development of crystallinity seems to be affected by the molecular weight independent of the spinline stress.     

涤纶HOY结构形成的计算机模拟

在基本纺丝动力学方程的基础上,引入显著的应力诱导结晶作用,运用改进的Euler方法和牛顿二分法,通过计算机求解动力学方程组,模拟涤纶HOY结构的形成过程,获得了纤维结构形成与卷绕速度、质量流量和侧吹风温度的关系。结果表明,增大卷绕速度或减小质量流量,有利于增大纤维的结晶度及双折射,侧吹风温度降低使结晶位置移向喷丝孔方向。计算机模拟的结果与相关文献给出的实验结果吻合良好。     

Effect of metallic wire inserted in nozzle in high-speed melt spinning of poly(ethylene terephthalate)

High-speed melt spinning of poly(ethylene terephthalate) was performed using a spinning nozzle with an inserted metallic wire of various lengths (0, 8, 30, and 45 mm). The molecular orientation of as-spun fibers increased with the increase in the wire length at all the take-up velocities examined. Along with the enhanced molecular orientation, the longer wire length led to the starting of orientation-induced crystallization at lower take-up velocities. The structure of crystallized fibers obtained at low speeds can be characterized by high crystallinity and relatively low molecular orientation. From the on-line measurement of the diameter and temperature profiles of the spin line with the 30-mm metallic wire, it was revealed that the spin-line had a maximum diameter of about 6 mm at the wire end. The spin-line temperature at this position was about 190°C. The solidification of the spin-line occurred at positions much closer to the spinneret in comparison with ordinary high-speed spinning. These results show that high-speed spinning with a wire inserted in the nozzle corresponds to a spinning process operated at extremely low extrusion temperature using a nozzle with an extremely large diameter. From the starting of orientation-induced crystallization at lower levels of birefringence in comparison with ordinary high-speed spinning, the alteration of the inherent fiber structure that cannot be represented by birefringence was also suggested. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 665–674, 1998     

PTT的纺丝稳定性和聚集态结构

利用毛细管流变仪研究PTT熔体挤出时的破裂现象,讨论PTT纺丝稳定性和初生纤维聚集态结构。结屎表明,PTT熔体是一种拉伸变稀型流体。自由挤出时,即使剪切速率达到1.5×105 s-1时,挤出熔体也没有出现明显的熔体破裂;而在纺丝过程中,在卷绕速度达到3.8 km／min(剪切速率2.1×103 s-1)时,就出现明显的熔体断裂现象。在高速纺丝中,控制PTT初生纤维取向结构的关键是喷丝头拉伸比,决定结晶结构的关键是卷绕速度。增加喷丝头拉伸比可以提高初生纤维的取向度;提高卷绕速度可以提高纤维结晶度。     

Structure development in the early stages of crystallization during melt spinning

The earliest stage of crystallization during melt spinning was examined for four polymers: HDPE, PVDF, nylon 6 and poly(oxymethylene). The four polymers have very similar melt viscosities. Of particular interest is the dependence of the time for the onset of detectable crystallization on the take-up speed. The results for all four polymers lie on the same onset time versus take-up speed curve, indicating that this condition depends chiefly upon chain orientation and not appreciably on chain chemistry or specific undercooling. The result is consistent with a condition of critical strain level. A similar, but less stringent, result is found for further crystallization in the spinline.     

PET高速纺丝中渐近流变力的模拟计算

本文设计了一个简单的数学模型,并假设和温度有关的牛顿粘度和对流控制热传递.运用该数学模型研究了通过每个纺丝孔的质量流量、横向侧吹风的风速及风温、卷绕速度、喷丝板处的挤出温度以及PET聚合物本身的流动性对渐近流变力的影响.结果表明:聚合物本身的流动性、卷绕速度、喷丝板处的挤出温度对渐近流变力的影响显著.随着卷绕速度的增加、PET聚合物本身流动性的减弱,喷丝板处挤出温度的降低,渐近流变力显著增加.     

Melt spinning of nylon 6: Structure development and mechanical properties of as-spun filaments

The structure of melt-spun nylon 6 filaments was studied using on-line x-ray diffraction and birefringence measurements. Measurements were also made on as-spun and treated filaments. On-line wide-angle x-ray scattering measurements indicated that crystallization did not occur on the nylon 6 spinline at spinning rates up to 1000 m/min when spinning was done into either ambient air of 60% relative humidity or into wet saturated air. The filaments did crystalline gradually on the bobbin to a paracrystalline pseudohexagonal (γ) form. The rate of crystallization was dependent on the molecular orientation developed in the spun filaments. Crystalline orientation factors based on hexagonal symmetry were computed as a function of take-up velocity for fibers which were conditioned 24 hr in air at 65% relative humidity. Annealing in air or treatment in water or 20% formic acid solution causes a transformation from the pseudohexagonal form to the α monoclinic form. The tangent modulus of elasticity and tensile strength of spun and conditioned filaments increase with increasing take-up velocity and spinline stress, while elongation to break decreases with these variables.     

Radial temperature differences during the melt spinning of fibers

In this investigation, a numerical model was developed to predict the temperature distribution in a fiber during melt spinning. This model uses the implicit Crank–Nicolson method to solve the governing differential equation for the problem. The model was applied to a series of numerical experiments on a liquid crystalline fiber which is melt-spun. These simulations used typical sets of operating conditions to determine the effect of various operating parameters on the predicted radius profile, spinline tension, and temperature distribution. The effects of spinneret capillary diameter, mass flow rate, ambient air temperature, spinning temperature, and elongational viscosity were investigated. The results of the various runs showed that ambient air temperature and mass flow rate had a significant effect on the predicted radius profile, spinline tension, and temperature distribution. The spinning temperature was an important parameter, but its only significant effect was on the spinline tension. Spinneret capillary diameter and elongational viscosity had little effect on the predicted results.     

Structure development during melt spinning of linear polyethylene fibers

Apparatus has been developed for studying the development of crystallinity and orientation during the melt spinning of synthetic fibers. Tension in the fiber and temperature, diameter, and x-ray diffraction patterns are measured as a function of distance from the spinneret for a running monofilament. Measurements are presented for linear polyethylene over a range of spinning variables together with other investigations carried out on the final as-spun fibers. These data indicate that the development of crystallinity in polyethylene is controlled by a balance between increased crystallization kinetics caused by the stress in the fiber and a tendency for increased supercooling with change in any spinning variable that increases cooling rates in the fiber. The type of crystalline orientation observed, its development during the spinning process, and the changes observed with changes in spinning conditions suggest a model for the as-spun fiber structure in which varying amounts of row nucleation and twisting of lamellar, folded-chain crystal overgrowths occur depending on the spinning conditions. As-spun fiber birefringence was shown to depend primarily on the crystalline orientation. Mechanical properties correlated well with c-axis crystalline orientation function and spinline stress.     

On the neck-like deformation in high-speed spun polyamides

The neck-like deformation process occurring in high-speed melt spinning of polyamide 66 and polyamide 6 filaments was investigated at take-up speeds of 4200 to 5500 m/min by on-line laser light scattering, thermographic contrast compensation, and wide-angle X-ray scattering (WAXS) measurements. New information about the onset of crystallization along the spinline was obtained by measuring simultaneously diameter and temperature profiles in the neighborhood of the neck. Crystallization rates, as a function of take-up speed, are estimated for both polyamides. Based on the present experimental results of diameter profiles, temperature profiles, and WAXS patterns, a picture of the physical mechanism responsible for the neck-like deformation of high-speed melt spun polyamides is proposed. © 1993 John Wiley & Sons, Inc.     

Fiber structure formation in ultra-high-speed melt spinning of poly(ethylene 2,6-naphthalene dicarboxylate)

The high-speed melt spinning of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) was performed up to the take-up velocity of the ultra-high-speed region, 9 km/min. From the investigations of the structure and physical properties of the as-spun fibers, the high-speed spinning of PEN was divided into three regions in terms of the mechanism of fiber structure formation. The first region is the take-up velocity of up to 2.5 km/min and the birefringence of up to 0.08 where only a slight increase in molecular orientation was attained. At the take-up velocity of 2.5–4.5 km/min and the birefringence of 0.08–0.25, although some experimental evidences indicated that the orientation-induced crystallization did not occur, there was an increase in the fiber density which suggested the formation of some ordered structure. At the take-up velocity > 4.5 km/min and birefringence > 0.25, the orientation-induced crystallization occurred. The fibers obtained in this region were characterized by the formation of the crystalline structure dominated by the β form. The presence of the necklike deformation in the spinning line was also confirmed. The solidification temperature of the spinning line analyzed from the diameter profile suggested that the formation of β modification crystals occurred at relatively low crystallization temperatures in comparison with that in an isotropic state. Therefore it was indicated that the presence of elongational stress in the spinning line promoted the formation of the β modification crystals. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1415–1427, 1997     

On-line studies and computer simulation of the melt spinning of nylon-66 filaments

A mathematical model was developed to describe the high-speed melt-spinning behavior crystallizable polymers. This model included the effects of acceleration, gravity, and air friction on the kinematics of the process; temperature and molecular orientation on the crystallization kinetics of the polymer; and temperature, molecular weight, and crystallinity on the elongational viscosity of the material. Experimental on-line diameter, birefringence, and temperature profiles were obtained for a 12,000 Mn nylon-66 at 2.5 g/min spun at take-up speeds ranging from 2800 to 6600 m/min. These profiles were qualitatively and reasonably quantitatively in agreement with the predicted profiles. They indicated that orientation induced crystallization occurs at spinning speeds greater than 4000 m/min. The experimental diameter and birefringence profiles were compared to those predicted by the model using Avrami indices of 3, 2, and 1. There was a small increase in the crystalline index at the lower speeds with decreasing index. The effect of the strain hardening was more significant at the higher speeds, this being shown by decreasing the exponent in the relationship for the crystallinity on the elongational viscosity. The model developed in this study indicates that high spinning speeds provide the high stress environment that increases the molecular orientation within the fiber. It is this higher molecular orientation that is the driving force for rapid crystallization on the spinline. This rapid crystallization causes a strain hardening, preventing any further drawdown in the fiber diameter and an abrupt rise in the birefringence. This behavior closely corresponds to the observed spinline profiles.     

Comparison of structure development in quiescent crystallization,die extrusion and melt spinning of isotactic polypropylene and its compounds containing fillers and nucleating agents

The influences of fillers and nucleating agents on crystalline structure and stress induced crystallization of isotactic polypropylene were studied under a range of cooling and processing conditions, including die extrusion and melt spinning. Continuous cooling transformation curves were determined for polypropylene and various polypropylene filler compounds. The influence of spinline stress on crystallization was studied. The experiments reveal that under quiescent conditions, the kinetics and crystalline forms produced by the crystallization of polypropylene are dominated by nucleating fillers and impurities. The crystalline orientation‐spinline stress relationship, on the other hand, was found to be the same for polypropylene and its compounds. At high uniaxial stresses, kinetics and orientation development are dominated by homogeneous orientation crystallization.     

A CORRELATION OF THE ONSET OF DRAW RESONANCE SPINLINE INSTABILITY

Draw-resonance instability observed in fiber spinning is experimentally examined with an isothermal monofilamcnt spinning setup. Two polypropylene resins are used in this study. An apparent spinline elongation rate is obtained and found to be a key parameter influencing the onset of draw resonance. From dimensional analysis, the critical draw-down ratio is seen to depend on several dimensipnless groups. An important one can be expressed by the ratio between the spinline elongation rate (?) and the spinneret extrusion shear rate (Y).

A dimensionless critical stability contour is constructed in a plot of the critical draw-down ratio versus the ratio of (T/y). This stability contour provides a guideline for stable spinning conditions based on rheologically significant parameters. The validity of this stability contour is confirmed by the critical spinline stability data of the two different polypropylene resins operated at conditions with the same Theological properties.     

Melt spinning of metallocene catalyzed polypropylenes. I. On‐line measurements and their interpretation

The melt spinning of metallocene catalyzed isotactic polypropylene resins was investigated. The details are presented for on‐line studies performed on six miPP resins with melt flow rates (MFRs) between 10 and 100 and a Ziegler–Natta catalyzed isotactic polypropylene resin with a MFR of 35 for comparison. The on‐line studies indicated that, as the molecular weight and polydispersity increased, crystallization occurred closer to the spinneret at higher crystallization temperatures and under lower spin line stresses. Further, as the spinning speed increased, crystallization occurred closer to the spinneret at higher crystallization temperatures because of increased stress in the spin line. These observations were interpreted in terms of an increased rate of crystallization caused by increased molecular orientation in the spin line with increasing molecular weight and increasing spinning speed. This “stress‐enhanced” crystallization was further interpreted in terms of an increased rate of crystal nucleation. It was further concluded that the narrower molecular weight distribution of metallocene resins was the primary factor that produced differences in the structure and properties of fibers spun from these resins compared to those of Ziegler–Natta catalyzed resins of similar weight‐average molecular weight or MFR. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3223–3236, 2001     

Raman spectroscopy for spinline crystallinity measurements. II. Validation of fundamental fiber‐spinning models

The original Doufas–McHugh two‐phase microstructural/constitutive model for stress‐induced crystallization is expanded to polyolefin systems and validated for its predictive capability of online Raman crystallinity and spinline tension data for two Dow homopolymer polypropylene resins. The material parameters—inputs to the model—are obtained from laboratory‐scale material characterization data, that is, oscillatory dynamic shear, rheotens (melt extensional rheology), and differential scanning calorimetry data. The same set of two stress‐induced crystallization material/molecular parameters are capable of predicting the crystallinity profiles along the spinline and fiber tension very well overall for a variety of industrial fabrication conditions. The model is capable of predicting the freeze point, which is shown, for the first time, to correlate very well with the measured stick point (i.e., the point in the spinline at which the fiber bundle converts from a solid‐like state to a liquid‐like state and sticks to a solid object such as a glass rod). The model quantitatively captures the effects of the take‐up speed, throughput, and melt flow rate on the crystallization rate of polypropylene due to stress‐induced crystallization effects. This validated modeling approach has been used to guide fiber spinning for rapid product development. The original Doufas–McHugh stress‐induced crystallization model is shown to be numerically robust for the simulation of steady polypropylene melt spinning over a wide range of processing conditions without issues of discontinuities due to the onset of the two‐phase constitutive formulation downstream of the die face, at which crystallization more realistically begins. Because of the capturing of the physics of polypropylene fiber spinning and the very good model predictive power, the approximations of the original Doufas–McHugh model are asserted to be reasonable. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Numerical simulation of the spin‐draw process of poly(ethylene terephthalate) fibers

Numerical simulation for the calculation of the profile developments of the spin‐draw process in the melt spinning of poly(ethylene terephthalate) was performed. Both spinning and drawing profiles were analyzed and included the structure development of birefringence and crystallinity in the draw line. By applying a simple model describing the continuous drawing process, we made it possible to simulate the spin‐draw process. The strain rate of the spinline had a broad distribution, and that of the draw line had a narrower peak. The calculated birefringence ranged from 0.176 to 0.192 and the crystallinity ranged from 0.37 to 0.44 with draw ratio. The birefringence profile had a similar pattern as the stress profile, and the crystallinity gently increased along the draw line more than the birefringence did. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2522–2527, 2007     

High-strength poly(L-lactide) fibers by a dry-spinning/hot-drawing process. II. Influence of the extrusion speed and winding speed on the dry-spinning process

This paper is concerned with the influences of the extrusion speed and the winding speed during dry spinning of 4 wt % solutions of poly(L -lactide) (PLLA) in mixtures of chloroform and toluene, on the ultimate fiber tenacities after hot drawing. It was found that high-strength PLLA fibers (1.5 GPa) can be produced at high spinning rates (> 180 m/min) if rupturing of the entanglement network and oriented crystallization during spinning is suppressed. This could be accomplished by avoiding spinline stretching and applying low elongational deformation rates in the spinneret during spinning.