Fundamental understanding of the effect of air‐gap distance on the fabrication of hollow fiber membranes

The objectives of this work are, fundamentally, to understand hollow fiber membrane formation from an engineering aspect, to develop the governing equations to describe the velocity profile of nascent hollow fiber during formation in the air gap region, and to predict fiber dimension as a function of air‐gap distance. We have derived the basic equations to relate the velocity profile of a nascent hollow fiber in the air‐gap region as a function of gravity, mass transfer, surface tension, drag forces, spinning stress, and rheological parameters of spinning solutions. Two simplified equations were also derived to predict the inner and outer diameters of hollow fibers. To prove our hypotheses, hollow fiber membranes were spun from 20 : 80 polybezimidazole/polyetherimide dopes with 25.6 wt % solid in N,N‐dimethylacetamide using water as the external and internal coagulants. We found that inner and outer diameters of as‐spun fibers are in agreement with our prediction. The effects of air‐gap distance or spin‐line stress on nascent fiber morphology, gas performance, and mechanical and thermal properties can be qualitatively explained by our mathematical equations. In short, the spin‐line stresses have positive or negative effects on membrane formation and separation performance. A high elongational stress may pull molecular chains or phase‐separated domains apart in the early stage of phase separation and create porosity, whereas a medium stress may induce molecular orientation and reduce membrane porosity or free volume. Scanning electron microscopic photographs, coefficient of thermal expansion, and gas selectivity data confirm these conclusions. T_g of dry‐jet wet‐spun fibers is lower than that of wet‐spun fibers, and T_g decreases with an increase in air‐gap distance possibly because of the reduction in free volume induced by gravity and elongational stress. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 379–395, 1999     

Studies on high‐speed melt spinning of noncircular cross‐section fibers. III. Modeling of melt spinning process incorporating change in cross‐sectional shape

A numerical analysis program for high‐speed melt spinning of flat and hollow fibers was developed. Change in cross‐sectional shape along the spin line was incorporated adopting a formulation in which energy reduction caused by the reduction of surface area was assumed to be equal to the energy dissipation by viscous flow in the plane perpendicular to the fiber axis. In the case of flat fiber spinning, the development of temperature distribution in the cross section was considered. It was found that the empirical equations for air friction and cooling of the spin line of circular fibers can be applied for the flat fiber spin line if the geometrical mean of long‐axis and short‐axis lengths was adopted, instead of fiber diameter, as the characteristic length for Reynolds number and Nusselt number. Three features expected through the high‐speed spinning of noncircular cross‐section fibers could be reproduced: (1) although cooling of the flat fiber spin line was enhanced, calculated tension at the position of solidification was not affected much by the difference in cross‐sectional shape; (2) change in cross‐sectional shape proceeded steeply near the spinneret; and (3) temperature at the edge became significantly lower than that at the center in the cross section of flat fibers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1589–1600, 2001     

Modeling of the tensile moduli of mechanical,thermomechanical, and chemi‐thermomechanical pulps from orange tree pruning

Stiffness is one of the most relevant properties of composite materials. Although fiberglass has been traditionally used as reinforcement, natural fibers are seen as possible replacements due to concerns for environmental protection. In this work fibers from orange tree prunings were prepared and converted into mechanical, thermomechanical and chemi‐thermomechanical pulps, to be used as reinforcement for polypropylene. Polypropylene composite materials with 20–50% of reinforcing fibers were prepared and mechanically characterized. The intrinsic Young's modulus of the fibers was back calculated by means of the Hirsch model. The moduli were also obtained by Halpin‐Tsai equations with Tsai‐Pagano methods and then compared to establish the influence of the aspect ratio. Finally, a fiber tensile modulus factor was defined in order to characterize the contribution of the fibers to the Young's moduli of the composites. POLYM. COMPOS., 34:1840–1846, 2013. © 2013 Society of Plastics Engineers     

Formation and properties of polyester/copolyester conjugate fibers prepared by ultra‐high‐speed melt spinning technique

In this study, three types of conjugate fibers, sea‐islands type, orange split type, and side by side type, were prepared by using an on‐line steaming process through a high‐speed spinning technique in order to improve the processibility, efficiency, and properties of the fibers. It was found that the weight reduction ratios of orange split and sea‐island polyester/copolyester fibers were higher than that of polyester fibers. The SEM results indicate that the split time was shortened by using the ultra‐high‐speed melt spinning process. The nozzle‐draft increased and elongation decreased for side by side conjugate fibers after the spin speed was increased. The elasticity and crimp ratio of side by side conjugate fibers were significantly affected by the drawing temperature. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Film insert molding as a novel weld‐line inhibition and strengthening technique

Specimens with weld lines were produced via conventional and film insert molding techniques using two types of materials as the substrate resin, i.e. a polycarbonate/acrylonitrile‐butadiene‐styrene (PC/ABS) blend and glass fiber‐filled polycarbonate (PC‐gf). The formation and morphology of the weld line region was assessed with and without the presence of 0.5‐mm‐thick PC film inserts. The weld line formation and characteristics were found to be dependent on the extent of interaction between the injected resin and the mold surface or the film insert. Better interfacial interaction between the substrate and film led to the distortion of the weld line orientation, which significantly enhanced the mechanical properties of the weld line. The incorporation of glass fibers into the substrate resin would usually reduce the resistance of the weld line towards tensile, flexural and impact loadings. However, with the attachment of film inserts, the mechanical properties of the weld line region have significantly improved, even with the presence of rigid fibers. Upon examination of tensile and impact fracture surfaces of film insert specimens, a unique orientation of fibers across the weld line (parallel to the flow direction and perpendicular to the weld line) could be observed at regions directly under the film. The combination of favorable properties from the unique fiber orientation and distortion of the weld line, as well as the ability of the film to effectively dissipate forces towards a larger area, have synergistically contributed towards the mechanical property enhancement of the weld line region in film insert moldings. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

The Online Measurement of Lyocell Fibers and Investigation of Elongational Viscosity of Cellulose N‐methylmorpholine‐N‐oxide Monohydrate Solutions

In this paper, the development of diameter and surface temperature of Lyocell fibers was measured online. The diameter and tensile force on the spin line in the coagulation bath were traced. The velocity, velocity gradient and the tensile stress profiles development of the fibers in the air gap were studied. The apparent elongational viscosity of cellulose N‐methylmorpholine‐N‐oxide monohydrate (NMMO‐MH) solutions was studied by steady‐state melt spinning theory. The decrease of the fiber diameter was mainly taking place near the spinneret, and the decrease of the diameter became more dramatic with increasing taking‐up speed. The surface temperature of the fibers was also dropping faster with increasing taking‐up speed for the heat transfer coefficient increased. The diameter of the Lyocell fibers almost did not change before and after it entered the coagulation bath. The tensile force on the spin line increases with increasing taking‐up speed and coagulation bath length. The velocity and the tensile stress increase slowly near the spinneret, and then accelerate. The apparent elongational viscosity of cellulose NMMO‐MH solutions decreases with increasing temperature at the same elongation rate and decreases with increasing elongation rate at the same temperature. The fiber of the Lyocell process was not really solidified in the air gap and a gel or rubbery state was formed.     

Stress development in drying fibers and spheres

Stress development during drying is a critical factor that affects the final structure and properties of a coated fiber or spherical product. Stress development during drying of the coating is due to nonuniform shrinkage and physical constraints. In this study, a large deformation elasto‐viscoplastic model is developed to predict stress development in drying fibers and spheres after the coatings solidify. From the model, stress evolution in the drying fibers/spheres can be predicted by a partial differential equation of diffusion in one dimension, a first‐order partial differential equation of pressure distribution, and two ordinary differential equations on local evolution of the stress‐free state. The system of equations is solved by the Galerkin/finite element method in the one dimensional axial/spherical symmetric coatings. Solutions show changes in solvent concentration and viscous stress as the coating dries. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3934–3944, 2003     

Studies on high‐speed melt spinning of noncircular cross‐section fibers. II. On‐line measurement of the spin line,including change in cross‐sectional shape

On‐line measurement was performed in the high‐speed spinning of flat, hollow, and circular fibers of poly(ethylene terephthalate), paying particular attention to the change in cross‐sectional shape along the spin line. The diameter profiles of hollow and circular fibers were essentially identical, whereas the deformation of flat fiber shifted to the region closer to the spinneret. The necklike deformation of hollow and circular fibers started at the takeup velocity of 5 km/min. In the case of flat fibers, presence of the necklike deformation was confirmed at 4 km/min, and extremely steep diameter attenuation was observed at 5 km/min. The spin‐line tension of the flat fiber was also larger than that of circular fibers. Combined measurements of fiber velocity and thickness enabled us to evaluate the aspect ratio of the flat fiber and hollow ratio of the hollow fiber in the spin line. These two factors were found to decrease steeply near the spinneret. Accordingly, the thinning of the spin line and the change in cross‐sectional shape appeared to proceed independently. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1582–1588, 2001     

Studies on high‐speed melt spinning of noncircular cross‐ section fibers. I. Structural analysis of as‐spun fibers

Flat fibers and hollow fibers were prepared through the high‐speed melt spinning of poly(ethylene terephthalate) (PET), and the structures of these fibers were compared with those of circular fibers. The cross‐sectional shape of each fiber changed to a dull shape in comparison with that of the respective spinning nozzle. The change in the cross‐sectional shape was slightly suppressed with an increase in the take‐up velocity. There was a significant development of structural variation in the cross section of flat fibers in that the molecular orientation and crystallization were enhanced at the edge. Despite the difference in the cross‐sectional shape, the structural development of flat, hollow, and circular fibers with increasing take‐up velocity showed almost similar behavior. Considering that the tensile stress at the solidification point of the spin line is known to govern the structure development of high‐speed spun PET fibers, it was speculated that the effects of the enhancement of cooling and air friction on the tensile stress at the solidification point cancel each other. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1575–1581, 2001     

Melt‐spinning process of a tetrafluoroethylene– hexafluoropropylene copolymer

An industrial melt‐spinning process of tetrafluoroethylene– hexafluoropropylene copolymer (FEP) using an extruder was studied. The novel “spinneret,” having both a large‐diameter spinning nozzle and a high‐temperature vessel, was used to solve the problem of filament breakage on the spinning line caused by high melting viscosity of FEP. The extruder, with its long feed zone, was newly designed to function with a geared pump. The strength of fibers increased with drawing of as‐spun fiber. FEP fibers up to six denier were continuously produced through long‐run production. According to this new process, FEP fibers can be supplied for textile or industrial application. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2366–2371, 2002     

Electrically conductive polymeric bi‐component fibers containing a high load of low‐structured carbon black

Melt spinning at semi‐industrial conditions of carbon black (CB) containing textiles fibers with enhanced electrical conductivity suitable for heating applications is described. A conductive compound of CB and high density polyethylene (HDPE) was incorporated into the core of bi‐component fibers which had a sheath of polyamide 6 (PA6). The rheological and fiber‐forming properties of a low‐structured and a high‐structured CB/HDPE composite were compared in terms of their conductivity. The low‐structured CB gave the best trade‐off between processability and final conductivity. This was discussed in terms of the strength of the resulting percolated network of carbon particles and its effect on the spin line stability during melt spinning. The conductivity was found to be further enhanced with maintained mechanical properties by an in line thermal annealing of the fibers at temperatures in the vicinity of the melting point of HDPE. By an adequate choice of CB and annealing conditions a conductivity of 1.5 S/cm of the core material was obtained. The usefulness of the fibers for heating applications was demonstrated by means of a woven fabric containing the conductive fibers in the warp direction. By applying a voltage of 48 V the surface temperature of the fabric rose from 20 to 30°C. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42255.     

Reactive electrospinning of poly(vinyl alcohol) nanofibers

We aim to couple the electrospinning in‐line with solution chemistry to fabricate novel crosslinked polymer nanofibers. Poly(vinyl alcohol) (PVA) was used as a model polymer due to its high amount of hydroxyl groups. To obtain ideal parameters for electrospinning, pure PVA was explored primarily. To gain crosslinked fibers, PVA was first crosslinked partially with glutaraldehyde (GA) followed by transferring this precursor to a long hot tube which was used as reactor and then electrospun right before gelation. The preheating time and tube‐passing time were determined with viscometer and rheometer. The reactive as‐spun fibers could maintain their original morphology after water immersion due to their high crosslinking degree. The thermal stability and mechanical property of reactive as‐spun fibers were improved drastically compared with pure and GA vapor crosslinked PVA fibers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

High Stiffness Cellulose Fibers from Low Molecular Weight Microcrystalline Cellulose Solutions Using DMSO as Co‐Solvent with Ionic Liquid

There is a need to develop high‐performance cellulose fibers as sustainable replacements for glass fibers, and as alternative precursors for carbon filaments. Traditional fiber spinning uses toxic solvents, but in this study, by using dimethyl sulfoxide (DMSO) as a co‐solvent with an ionic liquid, a novel high‐performance fiber with exceptional mechanical properties is produced. This involves a one‐step dissolution, and cost‐effective route to convert high concentrations of low molecular weight microcrystalline cellulose into high stiffness cellulose fibers. As the cellulose concentration increases from 20.8 to 23.6 wt%, strong optically anisotropic patterns appear for cellulose solutions, and the clearing temperature (T _c) increases from ≈100 °C to above 105 °C. Highly aligned, stiff cellulose fibers are dry‐jet wet spun from 20.8 and 23.6 wt% cellulose/1‐ethyl‐3‐methylimidazolium diethyl phosphate/DMSO solutions, with a Young's modulus of up to ≈41 GPa. The significant alignment of cellulose chains along the fiber axis is confirmed by scanning electron microscopy, wide‐angle X‐ray diffraction, and powder X‐ray diffraction. This process presents a new route to convert high concentrations of low molecular weight cellulose into high stiffness fibers, while significantly reducing the processing time and cost.     

High‐strength regenerated cellulose fibers spun from 1‐butyl‐3‐methylimidazolium chloride solutions

High‐performance regenerated cellulose fibers were prepared from cellulose/1‐butyl‐3‐methylimidazolium chloride (BMIMCl) solutions via dry‐jet wet spinning. The spinnability of the solution was initially evaluated using the maximum winding speed of the solution spinning line under various ambient temperatures and relative humidities in the air gap. The subsequent spinning trials were conducted under various air gap conditions in a water coagulation bath. It was found that low temperature and low relative humidity in the air gap were important to obtain fibers with high tensile strength at a high draw ratio. From a 10 wt % cellulose/BMIMCl solution, regenerated fibers with tensile strength up to 886 MPa were prepared below 22 °C and relative humidity of 50%. High strengthening was also strongly linked with the fixation effect on fibers during washing and drying processes. Furthermore, an effective attempt to prepare higher performance fibers was conducted from a higher polymer concentration solution using a high molecular weight dissolving pulp. Eventually, fibers with a tensile strength of ～1 GPa and Young's modulus over 35 GPa were prepared. These tensile properties were ranked at the highest level for regenerated cellulose fibers prepared by an ionic liquid–based process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45551.     

Effects of the spin line temperature profile and melt index of poly(propylene) on melt‐electrospinning

Effects of the spin line temperature and melt index of polymer on fiber formation by the melt‐electrospinning process have been studied. Employing four levels of primary heating temperatures and two levels of secondary heating temperatures provided the necessary temperature profiles. Cooling time was altered through variation in tip‐to‐collector distance. Effects of the polymer melt index were also investigated using two types of poly(propylene) with different melt index values. Changes in diameter and structure of the electrospun fibers were then observed using scanning electron microscopy and differential scanning calorimetry. It is worth to note that providing enough cooling time in the spin line is effective in producing finer fibers. However, introducing the higher heating temperature in the spin line adversely affected fibrous shape formation. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Evolution of tension during the thermal stabilization of polyacrylonitrile fibers under different parameters

Complicated physical and chemical reactions can occur during the thermal stabilization of polyacrylonitrile (PAN) fibers, and they can be macroscopically reflected by the evolution of tension in the fibers. In this work, PAN fibers were oxidized under different parameters in a continuous production line. The tension in the fibers was examined in detail and found to be influenced greatly by the stretching ratio, temperature, and time, as well as the porosity of the PAN precursors. As the thermal stabilization proceeded, tension with different characteristics could result from various reaction mechanisms. At the initial stage, a higher temperature was helpful for lowering the tension, but the tension increased with an increasing stretching ratio. In a later stage, the tension was dominantly dependent on the cyclization reaction and increased with increasing temperature or time. Under the same stabilization conditions, the tension in low‐porosity fibers was higher than that in high‐porosity fibers. The microstructures, characterized by high‐resolution transmission electron microscopy, provided some direct evidence for the partially stabilized fibers that the stabilization in the crystalline phase was slower than that in the amorphous phase. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5500–5506, 2006     

Shear flow of molten polymer in melt blowing

Besides the air flow field, the flow field of molten polymer plays a key role in fiber formation in the melt‐blowing (MB) process. In this article, the flow field of molten polymer was discussed, and its effects on the fiber microstructures were studied through theory and experiments. First, this field was supposed to be a kind of shear flow field. Two equations were introduced and solved. Then, analyzing the solutions combining with the actual melt‐blown practice, we concluded that the distribution profile of this flow field was a series of inverse parabolic in the course of the polymer stream attenuating. Further inferring from this flow pattern, we could also assume that there could have been a novel cross‐sectional microstructures in the melt‐blown fibers. Finally, the comparison experiments concerning the MB and its fibers were designed and carried out. The results indicated that the shear flow field could be qualitatively described by the equations, and the assumptions about the microstructures are basically in agreement with the experimental results. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Wettability investigations on the cellulosic surface of alfa fibers

A wettability study was performed on samples of alfa fibers with the Wilhelmy plate technique. The set of test liquids employed in the measurement of the contact angles was composed of water, heptane, diiodomethane, α‐bromonaphthalene, and formamide. During their first immersion in high‐surface‐energy test liquids, the alfa fibers showed anisotropic behavior: they had an advancing contact angle of 67 ± 6° in one orientation of immersion and an angle of 112 ± 9.5° in the opposite one. Optical microscopy revealed the existence of fibrils on the alfa‐fiber surface. They kept almost the same orientation and were responsible for the interesting hydrophobic/hydrophilic behavior of the fibers. Contact angle measurements and investigations of the hysteresis were also performed. The various results were examined according to the heterogeneities of the fibers. The surface energy of the alfa fibers was determined with three theoretical models: the geometric model, the Good–Van Oss–Chaudhury model, and the Chang model. A comparative study of these models was undertaken. The study of the wetting properties of alfa fibers will provide essential information for optimized composites and so will help us in choosing the right chemical treatment necessary to enhance adhesion in alfa‐fiber‐based composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Cellulose fibers modified by silicon dioxide nanoparticles

The sol–gel method is one of the most suitable ways for producing glasses, glass films, glass fibers, and glass nanoparticles. The relatively mild reaction conditions and simplicity of the sol–gel method make it an excellent tool for producing substances with precisely tailored properties. This technique opens the possibility for the synthesis of various new compounds, including pH sensors, ion sensors, bioactive nanoparticles, dyes carriers, and so forth. An attempt was made to combine the sol–gel technique with the advanced technology in the production of cellulose fibers in order to obtain fibers with new and unique properties. Cellulose fibers were prepared with N‐methylmorpholine‐N‐oxide as the direct solvent. The obtained fibers contained up to 30% (w/w) silicon dioxide nanoparticles. In order to observe the influence of the modifier on the fibers, their mechanical properties were examined. Modified fibers were also examined by means of thermogravimetry, wide‐angle X‐ray scattering, and ²⁹Si‐NMR solid‐state spectroscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1793–1798, 2005