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.     

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.     

The role of crystallization kinetics in the development of the structure and properties of polypropylene filaments

The processing, structure, and properties of filaments melt spun from three polypropylenes with similar rheology but substantially different crystallization kinetics were studied. The crystallization kinetics of the homopolymer was increased by the addition of a nucleating agent, whereas slower crystallization kinetics was obtained through a small amount of random copolymerization with ethylene. The relative crystallization kinetics of these three polymers was examined under quiescent conditions using differential scanning calorimetry. The technique of on-line diameter and birefringence measurement was used to show the characteristics of the on-line crystallization of the different resins. It was found that changing the quiescent crystallization kinetics by either the addition of a nucleating agent or through copolymerization with ethylene can produce profound effects on the structure and properties of polypropylene as-spun filaments when they are spun under relatively low stress and low takeup velocity conditions. Higher takeup velocities and spinline stresses reduce the effect of differences in quiescent crystallization due to the influence of on-line stress-induced (molecular orientation-enhanced) crystallization. © 1993 John Wiley & Sons, Inc.     

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.     

Effect of molecular weight on high-speed melt spinning of nylon 6

An extensive experimental study of the structure and properties developed in as-spun nylon 6 filaments is reported. Five polymers representing different molecular weights in the range 25,000–73,000 g/mol (viscosity average) were studied. These polymers were melt spun over a range of spinning speeds using an air drag type of drawdown device. Maximum take-up velocities achieved were in the neighborhood of 4000 m/min. The structure and properties of the as-spun filaments were characterized using density, DSC, WAXS, SAXS, birefringence, and tensile tests. The structural characteristics and properties of the filaments are strongly dependent on molecular weight. Generally, higher molecular weight leads to higher modulus and filament tenacity and lower elongation to break in the as-spun filaments. The structural changes with molecular weight are rather complicated; the complications are explained in terms of changes of crystallization rate and attainable crystallinity.     

Correlation and modeling of the occurrence of different crystalline forms of isotactic polypropylene as a function of cooling rate and uniaxial stress in thin and thick parts

A study of structure development in thin melt spun isotactic polypropylene filaments is described, which is then applied to the prediction of the behavior of thick parts. Conditions under which different crystalline forms of polypropylene are obtained as a function of cooling rate and spinline stress were investigated. Continuous cooling transformation (CCT) curves are developed. This also allows us to develop a map of crystalline form as a function of these variables. We have applied the CCT curves and this map to predict the development of cross‐sectional variation structure in thick filaments and rods. This is applied in particular to the quenching of a cylindrical rod and the structural characterizations observed through the cross section are compared with predictions from the CCT curves and solutions of Fourier's transient heat conduction equation.     

Structure development in melt spinning filaments from polybutylene terephthalate based thermoplastic elastomers

Structure development in polybutylene terephthalate (PBT) and thermoplastic elastomers (TPEs) based on PBT during the melt spinning process was investigated. Melt spun filaments were examined using wide angle X‐ray diffraction and birefringence. Crystalline orientation was characterized in terms of Hermans‐Stein orientation factors. Crystalline orientation and birefringence were found to correlated with spinline stress better than drawdown ratio. Crystalline orientation development was strongly dependent on spinline stress. The TPEs with low PBT content had low birefringence although the crystalline orientation was often high.     

Structure development in melt‐spinning syndiotactic polystyrene and comparison to atactic polystyrene

Syndiotactic polystyrene (s‐PS) and atactic polystyrene (a‐PS) were melt‐spun into filaments. The s‐PS filaments exhibited increasing amounts of crystallinity and orientation with increasing drawdown ratio and spinline stress. The a‐PS filaments were amorphous but exhibited birefringence. The birefringence and Hermans orientation factors for a‐PS were proportional to this spinline stress. In ice water and at low drawdown ratios, the s‐PS is glassy or mesomorphic. At higher drawdown ratios and spinline stresses, it crystallized. The crystalline form was the zigzag TTTT hexagonal α‐form. The birefringence and orientation factors of the s‐PS filaments were higher than those of the a‐PS filaments and the difference of the birefringence increased with increasing spinline stress. Mechanical testing results showed that the Young's modulus and tensile strength generally increased with increasing spinline drawdown ratio for both a‐PS and s‐PS filaments. The elongation to break was enhanced for both materials by increased chain orientation. Polym. Eng. Sci. 44:2141–2147, 2004. © 2004 Society of Plastics Engineers.     

Structure development in melt spinning of poly(ethylene‐co‐octene) filaments with various comonomer contents

An experimental study of the spinnability and the variation in crystallinity and orientation of melt spinning of poly(ethylene‐co‐octene) with different contents of comonomers was carried out. The spinning behavior of these polymers was investigated under different draw‐down ratios and temperatures and correlated to spinline stress. The melt‐spun filaments were characterized by wide‐angle X‐ray diffraction birefringence, and differential scanning calorimetry. S‐1 is a high‐density polyethylene and S‐2, S‐3, and S‐4 have 16, 22, and 38 wt % octene. An orthorhombic unit cell was found in all four polymers, but a dominant hexagonal structure (perhaps mesophase) was found for the highest octene level (S‐4). The orientation factors for the a‐, b‐, and c‐axis of the orthorhombic crystal structure and a‐axis of the hexagonal phase were then calculated. The crystalline orientation behavior of the lower octene copolymers (S‐1, S‐2, and S‐3) are similar and can be represented as a “row‐nucleated“ structure. However, the orientation behavior of S‐4 was different. The uniaxial mechanical properties were also measured. The Young's modulus and tensile strength generally increased with birefringence for all polymers. With increasing content of octene, the Young's modulus showed a decrease from semicrystalline thermoplastic toward an elastomer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 9–22, 2004     

Enhancement of fiber structure formation of a liquid crystalline copolyester via ultra-high speed bicomponent spinning with poly(ethylene terephthalate)

A thermotropic liquid crystalline copolyester, poly(hydroxybenzoic acid-co-ethylene terephthalate) (LCP), and poly(ethylene terephthalate) (PET) were coextruded using two different extrusion systems to form sheath-core type biocomponent fibers. The bicomponent fibers could be spun up to a take-up velocity of 8 Km/min. The structural characterization of the individual components in the as-spun fibers showed that the orientation development in the PET component was significantly suppressed compared with the corresponding single component fibers. A significant increase in the tensile modulus of the LCP core component, which was estimated by the simple rule of mixtures, was observed above a take-up velocity of 4 km/min. The increase in tensile modulus was attributed to the increase in the overall orientation of the LCP core resulting from the combination of the high levels of stress generated during spinning at very high speeds and the altered thermal and stress generated during spinning at very high speeds and the altered thermal and stress histories provided by the bicomponent spinning process. On-line study of the thinning behavior of single component and bicomponent spinning was carried out in order to gain an understanding of the spinline dynamics, which improved the processability and structure development of LCP.     

聚丙烯纺丝中纤维不均匀性的研究

本文研究了聚丙烯树脂、纺丝条件等对聚丙烯纺丝过程中纤维不均匀性的影响。试验证明仅仅从流变学的角度来研究聚丙烯纺丝过程中纤维直径的不均匀性是不够的,因为初生纤维的屈服应力和初始模量的不均匀性主要取决于其取向的不均匀性和结晶性的差别。因而为了改善初生纤维的牵伸性能、提高聚丙烯纤维的质量,应该以控制初生纤维的织构和织构的均匀性为原则来合理选择聚丙烯树脂原料和控制纺丝工艺条件。     

Influence of molecular weight distribution on the structure and properties of melt-spun polypropylene filaments

The role of molecular weight distribution on the spinnability, structure, and properties of melt-spun isotactic polypropylene filaments was studied with the aim of clearly distinguishing the effect of the breadth of the distribution from the effect of the average molecular weight and resin melt flow rate (MFR). Nine resins were chosen for this purpose, ranging in MFR from 16 to 78 and in polydispersity from 2.6 to 5.4. It was observed that the spinnability, structure, and properties of the spun filaments were all strong functions of the breadth of the distribution. Spinnability decreased with increasing breadth. At given spinning conditions and polydispersity, an increase in the weight-average molecular weight (decrease in MFR) produces an increase in crystallinity, birefringence, tensile strength, and tensile modulus. But at given spinning conditions and resin MFR, broadening the molecular weight distribution (increasing the polydispersity) produces an increase in crystallinity, tensile modulus, and elongation-to-break while birefringence and tensile strength decrease. The major influence of the polydispersity on the structure and properties developed was attributed to its effect on both the elongational viscosity of the resin and the ability of high molecular weight tails in the distribution to influence the stress-induced crystallization that occurs in the spinline. © 1995 John Wiley & Sons, Inc.     

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     

Crystalline structure and thermodynamic analysis of ultra‐low diameter VGCF‐polypropylene nanocomposite monofilaments

Nanocomposites of VGCF reinforced polypropylene monofilaments were created using melt compounding extrusion. The monofilament diameter and VGCF loading were varied to characterize the effects of spinline stress and imposed confinement on material properties, including crystalline structure and thermodynamic behavior. A correlation was found between spinline stress, degree of confinement, depression of the melting temperature, and changes in crystallite dimensions and orientation. POLYM. COMPOS., 37:1641–1649, 2016. © 2014 Society of Plastics Engineers     

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.     

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.     

The influence of resin characteristics on the high speed melt spinning of isotactic polypropylene. I. Effect of molecular weight and its distribution on structure and mechanical properties of as-spun filaments

The fine structures and tensile mechanical properties were characterized for high speed melt spun filaments prepared from three polypropylenes with melt flow indices in the range 12–300. It was found that spinnability and the resulting structure and properties are affected by both the weight average molecular weight and the polydispersity of the polymer. Higher tenacities, with consequent lower percent elongation to break, could be achieved by spinning the narrow molecular weight distribution polymers at high spinning speeds. This effect was associated with the development of higher birefringence in these samples. Modulus did not correlate with birefringence in the present study, but it was found to be controlled by the nature and level of crystalline order developed in the filaments.     

Spinning speed–throughput rate relationships for polyester,nylon, and polypropylene fibers

The relationship between spinning speed and throughput rate has been investigated for fibers having the same fiber denier in the drawn state when produced by melt spinning of poly(ethylene terephthalate), nylon 6, and polypropylene polymers over a range of take-up speed (750–3000 m min^-1) and throughput rate. To understand the structural origin of the relationship, a limited amount of characterization of structure and properties of the as-spun and drawn fibers was also done. A comparison of the results for the three polymers shows that while the increase in productivity with increase in spinning speed is relatively less for polyester and nylon 6, it is quite high for polypropylene. The birefringence data show that while molecular orientation increases rapidly with increasing wind-up speed in polyester and nylon 6, the rate of increase is relatively less in the case of polypropylene. The possible reasons for the observed differences in behavior are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1773–1788, 1997     

High-strength poly(L-lactide) fibers by a dry-spinning/hot-drawing process. I. Influence of the ambient temperature on the dry-spinning process

In this paper the influence of the ambient temperature of the spinline during dry spinning of PLLA solutions has been discussed. Variation of this ambient temperature induced variation of the extrudate swell of the spinline, affected by the rate of solidification of the PLLA. By way of phase separation, porous filaments were achieved in which the morphology depended on the applied ambient temperature. Hot drawing of these filaments led to tensile strengths varying between 1.1 and 2.2 GPa. An optimal tensile strength of 2.2 GPa was achieved by hot drawing of a filament which was spun into a surrounding of 25°C.     

Development of Structure and Properties during Spunbonding of Metallocene Catalyzed Polypropylene

Spunbonding is one of the most widely used processing techniques to convert polymers into nonwoven fabrics. metallocene-catalyzed polypropylene is becoming more and more important. A study was carried out to understand the development of structure and properties of a metallocene-catalyzed isotactic polypropylene during spunbonding. This research was conducted using the Reicofil spunbond line at the University of Tennessee, Knoxville. The filaments at three different throughput rates were studied for tensile properties and for structural features by birefringence, X-ray diffraction, and thermal measurements. The fabrics produced at different process conditions were tested for various mechanical properties. The failure mechanism of the fabrics at different bonding temperatures was studied using the scanning electron microscope. The results are compared with a conventional polypropylene processed under similar conditions. The results showed that the mPP produced fabrics with better strength and elongation at comparable processing conditions. Also, failure mechanisms were found to be different for the fabrics produced from the two polymers, which are due to differences in the produced structures.