Studies on melt spinning. VI. The effect of deformation history on elongational viscosity,spinnability, and thread instability

The effect of deformation history on the elongational behavior and spinnability of polypropylene melt was investigated by carrying out isothermal melt-spinning experiments. For the study, spinnerettes of different die geometries were used to investigate the effect, if any, of the entrance angle, the capillary length-to-diameter (L/D) ratio, and the reservoir-to-capillary diameter (D_R/D) ratio on the elongational behavior of molten threadlines. An experimental study was also carried out to investigate the phenomenon of draw resonance in the extrusion of polypropylene melts through spinnerettes of different die geometries. Draw resonance is the phenomenon which gives rise to pulsations in the threadline diameter when the stretch ratio is increased above a certain critical value. The results of our study show that the critical stretch ratio at which the onset of draw resonance starts to occur decreases as the L/D ratio is decreased, as the entrance angle is increased, as the D_R/D ratio is increased, as the melt temperature is decreased, and as the shear rate in the die is increased. Of particular interest is the observation that, at 180°C, the severity of fiber nonuniformity increases as the stretch ratio is increased, whereas at 200°C and 220°C, the severity of fiber nonuniformity first increases and then decreases as the stretch ratio is increased considerably above the critical value. A rheological interpretation of the observed onset of draw resonance is presented with the aid of the independently determined rheological data.     

Non-isothermal rheological response in melt spinning and idealized elongational flow

Almost all polymer processing operations involve moving and shaping the polymer as a melt and then cooling it, usually quite rapidly, to a solid state. In order to develop models for such processes we have begun systematic studies in non-isothermal rheology; here we interpret the results in the context of melt spinning. Theoretical predictions of stress vs distance from the spinneret were calculated from generalized (non-isothermal) viscoelastic theory and compared with Dees' melt spinning data on high density polyethylene. Despite certain experimental and theoretical difficulties, the agreement is good. Surprisingly, an additional theoretical curve, based on a simple Trouton viscosity, also gave a reasonable approximation over much of the distance, despite the transient nature of the flow. To understand this phenomenon further, a more tractable theoretical problem was studied in detail; the problem, of constant elongational flow (? = constant) in the presence of a constant rate of temperature change (dT/dt = constant). The results depend on two dimensionless groupings; the first is the usual product of a relaxation time and ?; the second involves the ratio of normalized dT/dt to ?. When the second group is large, a quasi-viscous state exists. The melt spinning data for the HDPE may be near this state.     

Fluid flow stability analysis of multilayer fiber drawing

We here investigate drawing of multi-layered Newtonian and non-Newtonian fluid fibers, drawn under isothermal and non-isothermal conditions. We first develop one-dimensional equations governing mass, momentum, and energy balances and solve them numerically to obtain steady state draw root shape, velocity, and temperature profiles. These solutions are then used to perform linear stability analysis. For the case of isothermal draw, the system displays an oscillatory instability when the draw ratio (ratio of cross-sectional areas of fiber at the entrance and exit of the drawing) is higher than a critical draw ratio (highest stable draw ratio) of about 20.21. Investigation of stability behavior under non-isothermal draw conditions is performed by considering radiative heating and convective cooling. Employing only radiative heating enhances the critical draw ratio, and simultaneous heating and convective cooling increase the critical draw ratio even further. For the case of simultaneous heating and cooling, with increasing convective cooling strength, the critical draw ratio first increases, reaches a maximum, and then gradually decreases. However, with only convective cooling, the critical draw ratio decreases with an increase in convective cooling strength. We also find that the stabilizing effect of a non-isothermal operation can be enhanced by considering fluids with higher viscosity sensitivity to temperature, increasing the maximum temperature, and for sharper attenuation of the fiber cross-sectional area with length. For the case of isothermal drawing of non-Newtonian fluid fibers, the system has a higher critical draw ratio for shear thickening fluids (power-law exponent, n>1). In contrast, the use of a shear thinning fluid (n<1) reduces the critical draw ratio. Consideration of a non-isothermal operation of non-Newtonian fluid fibers reveals that the critical draw ratio is primarily determined by the non-Newtonian behavior rather than the non-isothermal drawing.     

改善N-PP纤维加工性能的关键因素

探索了制备高等规度聚丙烯 (N-PP)高强纤维的加工条件 ,讨论了纺丝拉伸条件及原料的熔融指数 (MI )和等规度对 N -PP纤维的结构和性能的影响 ,发现采用降低卷绕丝取向度的方法可以改善 N-PP纤维的拉伸性。结果表明 :用 MI较低的 N -PP原料 ,在适宜的纺丝温度、缓冷的条件下 ,再配以一定的拉伸倍数及紧张热定型 ,可制得强度大于 7c N/ dtex的 N-PP高强纤维。     

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.     

Analysis of molecular orientation and internal stresses in extruded plastic sheets

The development of molecular orientation and internal stresses in extruded sheet made of polypropylene was analyzed, and their correlations to operating conditions such as draw ratio, cooling rate, die temperature, melt temperature, and die gap opening were studied. Measurements of attenuated-total-reflectance infrared dichroic ratio for the surface molecular orientation, birefringence for the orientation stress distribution in the thickness direction, and free shrinkage ratio for the overall frozen-in stresses were carried out to determine the amount of orientation stresses in the extruded samples. As expected, the overall orientation stress depends strongly on draw ratio, while higher melt temperature reduces the overall orientation. It was found that faster cooling rates and lower die temperatures cause surface orientation stresses to increase as the core orientation stresses remain almost unchanged.     

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.     

Preparation and properties of waterborne poly(urethane acrylate)/silica dispersions and hybrid composites

Abstract

Simulations of the isothermal and non-isothermal filling of a rectangular cavity were carried out for polystyrene using a Giesekus viscoelastic constitutive equation, whereby in the non-isothermal case the thermal resistance at the mould wall was modelled with different heat transfer coefficients to investigate the effect of the thermal resistance on the development of the molecular orientation. Results for stress development along the flow front and the cold wall were compared showing that the principal stress differences in the middle of the flow front are lower than those at the mould wall. In case of the non-isothermal filling, the latter one will increase further while the melt is gradually cooling down, which is especially true if the thermal resistance at the mould wall has been properly considered. Consequently, the high molecular orientation at the wall seems to be rather a result of the non-isothermal shear flow than of the extension at the advancing front as usually assumed.     

Kinetics of non-isothermal crystallization of polyethylene and polypropylene

Differential scanning calorimetry (d.s.c.) data on non-isothermal crystallization of isotactic polypropylene and high-density polyethylene from the melt at different cooling rates were treated in terms of the Ozawa equation. It was found that the crystallization of polypropylene follows the Ozawa equation but in the case of polyethylene the Ozawa theory is not valid.     

Studies on melt spinning. VIII. The effects of molecular structure and cooling conditions on the severity of draw resonance

The effects of molecular structure and cooling conditions on the severity of draw resonance was investigated by carrying out carefully controlled melt spinning experiments. For the study, two types of polymeric materials were used: one which exhibits viscoelastic behavior (high-density polyethylene, polypropylene, and polystyrene), and the other which exhibits almost Newtonian behavior [nylon-6 and poly(ethylene terephthalate)]. In order to investigate the effect of cooling on the severity of draw resonance, different methods of cooling the molten threadline were employed. In one set of experiments, isothermal chambers of various lengths (3, 6, and 12 in.) were attached to the spinnerette face, so that the molten threadline, upon exiting from the spinnerette, began to cool in the ambient air only after it had passed through the isothermal chamber. This method of cooling is called “delayed cooling,” providing both an isothermal region (inside the isothermal chamber) where only stretching occurs, and a nonisothermal region (outside the isothermal chamber) where both stretching and cooling occur simultaneously. In other experiments, the temperature profile of the molten threadline was controlled by adjusting the temperature of the heated chamber. This method of cooling provides a gradual drop of the threadline temperature, compared to the more sudden drop when spinning into a cold environment provided at the spinnerette exit. The severity of draw resonance was recorded on movie film, and the thread tension was measured with a low-force load cell transducer and recorded on a chart recorder. The temperature of the threadline along the spin direction was measured using a fiber optical probe attached to a Vanzetti Infrared Thermal Monitoring System (Model TM-1). It was found that the severity of draw resonance depended on the molecular structure and the way the molten threadline was cooled. Of particular interest is the observation that, for the viscoelastic materials investigated, cooling destabilized the molten threadline outside the isothermal chamber. This gave rise to more severe resonant behavior, at and above the critical draw-down ratio, in contradiction to the theoretical prediction by Fisher and Denn. It was observed, also, that the elasticity of the materials tended to destabilize the molten threadline (i.e., it increased the severity of draw resonance), again in contradiction to the theoretical prediction of Fisher and Denn. It is believed that morphological changes of polymers may play an important role in the occurrence of draw resonance when a melt threadline is stretched under cooling. Our study indicated that a good understanding of draw resonance of viscoelastic fluids requires more careful study than the classical hydrodynamic stability analysis reported by Fisher and Denn. They based their analysis on several convenient and yet unjustified assumptions, and solely on phenomenological considerations. We suggest that future theoretical analysis of draw resonance be carried out by considering a fluid model with a nonlinear memory function in order to properly account for the deformation history of the fluid, and the relaxation and cooling processes in the die swell region and the region below it.     

Improvement of melt spinning properties and conductivity of immiscible polypropylene/polystyrene blends containing carbon black by addition of styrene‐ethylene‐butene‐styrene block copolymer

Conducting polymeric materials prepared from immiscible blends, such as polypropylene (PP)/polystyrene (PS), together with carbon black (CB), are known to have a relatively high electrical conductivity, because of a selective distribution of CB (double percolation). Melt spinning of immiscible blends containing CB has, however, not been extensively reported on previously. An immiscible 1:1 blend of PP and PS to which 4 wt% CB was added exhibited a very low melt draw‐down ratio at rupture compared wit PP with the same content of CB. By adding 5 wt% SEBS (styrene‐ethylene‐butene‐styrene block copolymer), the ultimate melt draw‐down ratio increased about 10 times, which made the material more suitable for melt spinning. As‐extruded samples of the immiscible blends (with CB) did not have higher electrical conductivities than PP/CB. A heat treatment increased the conductivity of immiscible PP/PS/CB composites, and longer treatment times and higher temperatures promoted the conductivity. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

High density polyethylene/polystyrene blends: Phase distribution morphology,rheological measurements,extrusion, and melt spinning behavior

An experimental study of the development of phase morphology, rheological properties, and processing behavior of mechanical blends of a polystyrene (PS) and a high density polyethylene (PE) is presented. Phase morphologies were determined by scanning electron microscopy for (i) products prepared in a screw extruder/static mixer system, (ii) samples removed from a cone-plate viscometer, (iii) extrudates, and (iv) melt spun fibers. Disperse phase dimensions were measured. The values varied from 1–5 μm in the products from static mixers. The dimensions of the dispersed phase in the blend products from the cone plate and capillary die were of the same order. The melt-spun fibers exhibited disperse phase dimensions as low as 0.35 μm. Polystyrene was extracted from the blend fibers producing small diameter, PE fibrils, or minifibers. Both the initial melts and the blends were rheologically characterized. The shear viscosity and principal normal stress difference N₁ exhibit maxima and minima when plotted as a function of composition. The characteristics of extrudates and melt spinning behavior of the blends were investigated. The shrinkage of extrudates of PE is much greater than PS. Additional small amounts of PE to PS greatly increase its shrinkage. Addition of PE to PS initially increases extrudate swell, though the swell shows maxima and minima when considered as a function of composition. The positions of the maxima and minima correspond to those of N₁. The onset of draw resonance has been investigated in isothermal melt spinning. Wide angle X-ray diffraction studies have been carried out on blend fibers and the orientation of the crystalline polyethylene regions has been determined as a function of process conditions. This orientation decreases rapidly with the addition of polystyrene when the melt-spun filaments are compared at the same spinline stress or drawdown ratio.     

提高熔体直纺涤纶长丝强度的工艺探讨

通过对聚合及纺丝工艺条件的调整试验,研究原料及工艺条件变化对直纺涤纶全拉伸丝(FDY)断裂强度的影响,找出了影响直纺FDY断裂强度的因素有对苯二甲酸(PTA)质量、熔体黏度、冷却吹风速度、纺丝速度及拉伸比等。结果表明,选用优质的PTA原料,控制较高的聚合物熔体特性黏度,再通过调整纺丝及卷绕工艺条件,可改善纺丝性能,生产的涤纶FDY的强度可达4.8 cN/dtex左右,从而提高了长丝质量。     

Influence of melt deformation history on orientation in vitrified polymers

An experimental study of the development of orientation in polystyrene melts during flow and its retention in vitrified parts is described. It is shown on the basis of elongational and shear flow experiments that orientation in vitrified polystyrenes may be predicted from a knowledge of the stress field at the time of vitrification and application of stress-optical laws. More generally a relationship between birefringence and principal stress difference is found which correlates (1) on-line isothermal shear flow, (2) on-line non-isothermal elongational flow (melt spinning), and (3) vitrified samples formed in both shear and elongational flow. It is further proposed that orientation in polymer chains in deforming melts is uniquely dependent on stress—specifically, that the ratio of the stress-optical constant C to the intrinsic birefringence Δ° is approximately a constant.     

Formation of hollow fibers in the melt‐spinning process

This article reports an investigation of the formation of hollow fibers in a melt‐spinning process. Experimental results indicate that die swelling is largely responsible for a negative effect on hole formation. The factors that positively affect die swelling, including a decrease in temperature, a decrease in capillary length, and an increase in shear rate, are thus not recommended for the spinning of hollow fibers. For vinyl‐type polymers such as polypropylene, in which the apparent elasticity leads to serious die swelling, the formation of hollow fibers is more complex than that of a typical condensation polymer. Our results further demonstrate that when hollow fibers are being made in a variety of shapes (but of the same denier), spinning a polygonal hollow fiber is significantly more unstable than spinning a circular one. Moreover, an asymmetric bridge along the polygonal contour leads to a melt twist and interrupts the entire spinning process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2896–2902, 2001     

Melt processability of polytetrafluoroethylene: Effect of melt treatment on tensile deformation mechanism

It is widely accepted that the melt processibility of polytetrafluoroethylene (PTFE) is poor. In this article, a high‐molecular‐weight PTFE was extruded smoothly with a modified die; and critical shear rate could be raised to 4 s^?1, using a die with L/D (length to diameter) ratio of 200. Meanwhile, we compared the current PTFE fiber spinning method with melt spinning to investigate the effects of high‐temperature treatment on the drawability of PTFE and found that the processing sequence could play a key role. The deformation imposed before or after the high‐temperature treatment could determine whether the fibrillation can be achieved continuously and effectively. Based on the experiment phenomenon, together with the results of differential scanning calorimetry, X‐ray diffraction, and scanning electron microscopy characterization, we proposed a model to describe the submicron structural change of PTFE during extension. From this model, the fundamental mechanism for the poor melt processibility of PTFE was elucidated. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A study of bicomponent coextrusion of molten polymers

An experimental study has been carried out of coextruding polystyrene with lowdensity polyethylene and polystyrene with high-density polyethylene, using both slit and circular dies. Two melt streams were separately fed to the die entrance and forced to flow side by side through a die. When using the slit die, wall normal stresses were measured with three melt pressure transducers flush-mounted on each of the rectangle's long sides, directly opposite each other. When using the capillary die, three different capillary length-to-diameter (L/D) ratios were employed: 4, 11, and 18. Wall normal stresses were measured for dies having L/D ratioes of 11 and 18 only. The measurements of wall normal stresses permitted one to determine the pressure gradient, and hence the viscous property. For each set of extrusion conditions (L/D ratio, flow rate, and component ratio), extrudate samples were collected. These were later carefully cross sectioned and photographed in order to examine the shape of the interface between the two components. At the phase interface of the polystyrene/low-density polyethylene system, it has been observed that the polystyrene, which is more viscous and yet less elastic than the low-density polyethylene, has a convex surface. However, at the phase interface of the polystyrene/high-density polyethylene system, the high-density polyethylene, which is more viscous and also more elastic than polystyrene, is seen to be convex. This then appears to indicate that the viscosity ratio of two components is primarily responsible for the final shape of the interface.     

Structure–property relationship studies in amine functionalized multiwall carbon nanotubes filled polypropylene composite fiber

Amine functionalized multiwalled carbon nanotubes (a‐MWNT) based polypropylene (PP) composite fibers were prepared in the presence of polypropylene‐g‐maleic anhydride (PP‐g‐MA) by melt‐mixing followed by melt‐spinning with subsequent post‐drawing of the as‐spun fibers of varying draw ratio (DR). In order to enhance the interfacial interaction, a‐MWNT were utilized in combination with PP‐g‐MA during melt‐mixing. Fourier transform infrared spectroscopic analysis revealed the formation of imide bonds between MA functionality of PP‐g‐MA and amine functional group of a‐MWNT. Higher tensile properties of PP/a‐MWNT/PP‐g‐MA composite fibers were registered with varying DR of the as‐spun fiber. Orientation factors of a‐MWNT and PP chains along the fiber axis were correlated with the higher tensile modulus and tensile strength of PP/a‐MWNT/PP‐g‐MA composite fiber of varying DR. Crystallization studies indicated the role of hetero‐nucleating action of a‐MWNT in PP/a‐MWNT/PP‐g‐MA composite fiber. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

PBT/PET conjugated fibers: Melt spinning,fiber properties,and thermal bonding

This work examines the PBT/PET sheath/core conjugated fiber, with reference to melt spinning, fiber properties and thermal bonding. Regarding the rheological behaviors in the conjugated spinning, PET and PBT show the smallest difference between their melt‐viscosity at temperatures of 290°C and 260°C respectively, which has been thought to represent optimal spinning conditions. The effect of processing parameters on the crystallinity of core material‐PET was observed and listed. In order of importance, these factors are the draw ratio, the heat‐set temperature, and the drawing temperature. The crystallinity of sheath material‐PBT, however, can be considered to be constant, independent of any processing parameters. The bulk orientation, rather than the crystallinity of PET core, dominates the tenacity of PBT/PET sheath/core fiber. Moreover, heat‐set treatment after drawing is recommended to yield a highly oriented conjugated fiber. With respect to thermal bonding, PBT/PET conjugated fibers processed via high draw ratio but low‐temperature heat setting can form optimal thermal bonds at a constant bonding temperature of 10°C above the T_m of PBT.     

Analysis of melt spinning master-curves of low density polyethylene

The extensional rheological properties of a low density polyethylene (LDPE) melt were studied by using melt spinning technique. Based on the extension properties of the LDPE melt under experimental conditions, the melt spinning master-curves were plotted by introducing scaling factor b and the draw ratio λ. The scaling factor b shows the combination effects on the preorientation before extension, the unwrapping and orientation of molecular chains during extension. Several linear relationships between b and temperature, log b and the logarithm of extrusion flow rate v₀ (log v₀) have been investigated. By using the values of b and the reference curve, the extension viscosity curves with different temperature and extrusion flow rate could be calculated, thus the measurement range of melt spinning technique would be extended effectively. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012