Effect of dope flow rate on the morphology,separation performance,thermal and mechanical properties of ultrafiltration hollow fibre membranes

We have determined the effects of dope extrusion speed (or shear rate within a spinneret) during hollow fibre spinning on ultrafiltration membrane's morphology, permeability and separation performance, and thermal and mechanical properties. We purposely chose wet-spinning process to fabricate the hollow fibres without drawing and used water as the external coagulant in the belief that the effects of gravity and elongation stress on fibre formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibres. An 86/14 (weight ratio) NMP/H₂O mixture was employed as the bore fluid with a constant ratio of dope fluid to bore fluid flow rate while increasing the spinning speed from 2.0 to 17.2 m/min in order to minimise the complicated coupling effects of elongation stress, uneven external solvent exchange rates, and inner skin resistance on fibre formation and separation performance. Hollow fibre UF membranes were made from a dope solution containing polyethersulphone (PES)/N-methyl-2-pyrrolidone (NMP)/diethylene glycol (DG) with a weight ratio of 18/42/40. This dope formulation was very close to its cloud point (binodal line) in order to speed up the coagulation of nascent fibres as much as possible so that the relaxation effect on molecular orientation was reduced. Experimental results suggested that a higher dope flow rate (shear rate) in the spinneret resulted in a hollow fibre UF membrane with a smaller pore size and a denser skin due to a greater molecular orientation. As a result, when the dope extrusion speed increased, pore size, water permeability, CTE and elongation of the final membranes decreased, but the separation performance, storage modulus, tensile strength and Young's modulus increased. Most surprisingly, for the first time, we found that there was a certain critical value, when the dope extrusion rate was over this value, the final fibre performance could not be influenced significantly. The results suggested that it was possible to dramatically enhance the production efficiency of hollow fibre UF membranes with the same fibre dimension and similar separation performance by the method proposed in this paper.     

Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes

Polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by dry/wet and wet phase inversion methods. In spinning these PVDF hollow fibers, dimethylacetamide (DMAc) and polyvinyl pyrrolidone (PVP) were used as a solvent and an additive, respectively. Water was used as the external coagulant. Water or ethanol was used as the internal coagulants. The membranes were characterized in terms of water flux, molecular weight cut-off for the wet membranes. Gas permeation fluxes and effective surface porosity were determined by a gas permeation method for the dried membranes. The cross-sectional structures were examined by scanning electron microscopy. The effects of polymer concentration, air-gap, PVP molecular weight, PVP content in the polymer dope, and the internal coagulant on the permeation properties and membrane structures were examined. Highly permeable PVDF hollow fiber membranes could be prepared from a polymer dope containing low molecular weight PVP and using ethanol as the internal coagulant.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

Preparation and characterization of inorganic hollow fiber membranes

Inorganic hollow fiber membranes were prepared by spinning a polymer solution containing suspended aluminum oxide (Al₂O₃) powders to a hollow fiber precursor, which is then sintered at elevated temperatures. In spinning these hollow fiber precursors, polyethersulfone (PESf), N-methyl-2-pyrrolidone (NMP), and polyvinyl pyrrolidone (PVP) were used as a polymer binder, a solvent, and an additive, respectively. The inorganic hollow fiber membranes prepared were characterized using scanning electron microscope (SEM), gas permeation techniques Coulter porometer, and gravimetric analysis. Some primary factors affecting the structure and performance of the membranes such as the sintering temperature and the ratio of the aluminum oxide to the PESf polymer binder were studied extensively. The prepared inorganic membranes show an asymmetric structure, which is similar to the conventional polymeric membranes prepared from the same phase-inversion technique. The inorganic hollow fiber membrane with a higher porosity and better mechanical strength could be prepared by blending the spinning solution with a smaller amount of aluminum oxide powder.     

Fabrication of high performance Matrimid/polysulfone dual-layer hollow fiber membranes for O2/N2 separation

Matrimid/polysulfone (PSf) dual-layer hollow fiber membranes were fabricated by using co-extrusion and dry-jet wet-spinning phase-inversion techniques. The effects of the spinning dope composition, spinneret dimension, spinneret temperature and the air gap distance on the hollow fiber membranes separation performance were studied. Aging phenomenon was also studied. After coated by 3 wt% silicon solution, the hollow fiber membranes have an O₂/N₂ selectivity of 7.55 at 25 °C, 506.625 kPa which exceeds the intrinsic value of Matrimid. The membranes have an O₂ permeance of 9.36 GPU with an apparent dense-layer thickness of 1421 Å calculated from the O₂ permeability. SEM images show the high porosity underneath the dense skin. It indicates that non-solvent addition is not necessary in the inner spinning dope to induce the macroviod formation. The binodals of the Matrimid/solvent/H₂O and PSf/solvent/H₂O indicate that the composition of the spinning dope plays an important role in the structure and the gas separation performance of the dual-layer hollow fiber membranes. The delayed demixing of the inner spinning dope may fabricate low resistance support layers in the dual-layer hollow fiber membranes.     

Study of cellulosic fibres morphological features and their modifications using hemicelluloses

The microfibrilar structure and the morphology of the fibre are some of the most important characteristics that determine fibre performance during yarn making. This article is focused on understanding of morphological features of manmade cellulosic fibre and exploring an alternate way to alter fibre morphology. It is observed that though the chemical nature of different types of cellulose fibres viz. viscose, modal and lyocell is same; different processing routes lead to different cross-section and morphologies of fibres which leads to their characteristic properties and spinning behavior. A novel way is attempted to alter the fibre morphology of viscose fibre by changing the fibre regeneration kinetics and molecular weight distribution through addition of low molecular weight and branched structured hemicelluloses in spinning dope. Addition of hemicelluloses in the spinning dope prior to spinning and regeneration of viscose fibres is found to alter the morphology of the fibres without affecting tensile properties of the fibres.     

The effects of flow angle and shear rate within the spinneret on the separation performance of poly(ethersulfone) (PES) ultrafiltration hollow fiber membranes

The effects of dope flow rate and flow angle within a spinneret during spinning hollow fiber membranes on the morphology, water permeability and separation performance of poly(ethersulfone) ultrafiltration hollow fiber membranes were investigated. For this purpose, two spinnerets with different flow angles were designed and used. The dope solution, containing polyethersulphone (PES)/N-methyl-2-pyrrolidone (NMP)/diethylene glycol (DG) with a weight ratio of 23/41/36, which was very close to its cloud point (binodal line), was used in order to speed up the coagulation of nascent fibers so that the relaxation effect on molecular orientation was reduced. The wet-spinning process was purposely chosen to fabricate the hollow fibers without extra drawing. Therefore, the effects of gravity and elongation stress on fiber formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibers. Experimental results suggest that higher dope flow rates (shear rates) in the spinneret produce UF hollow fiber membranes with smaller pore sizes and denser skin layers due to the enhanced molecular orientation. Hence, the pore size and the water permeability decrease, but the solute separation increases. Hollow fibers spun from a conical spinneret have smaller mean pore sizes with larger geometric standard deviations, thus exhibiting lower water flux and greater solute separation than hollow fibers spun from a traditional straight spinneret. In addition, SEM studies indicate macrovoids response differently for the 90° straight and 60° conical spinnerets when increasing the dope flow rate. Macrovoids can be significantly suppressed and almost disappear in the 90° spinneret at high dope flow rates. This phenomenon cannot be observed for the 60° conic spinneret.     

The role of additives on dope rheology and membrane formation of defect-free Torlon hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon^® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon^® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon^® hollow fiber membranes with an O₂/N₂ selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O₂/N₂ selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O₂/N₂ selectivity for compressed air varies from 7.12 to 9.00.     

The preparation and characterization of silver‐loading cellulose acetate hollow fiber membrane for water treatment

Silver‐loading asymmetric cellulose acetate (CA) hollow fiber membrane was spun via the dry jet‐wet spinning technique. The spinning solution was prepared by dissolving AgNO₃ and CA in N,N‐dimethylformamide (DMF). The silver ions were reduced in the spinning dope into silver nano‐particles. The morphology of the resulting hollow fibers was examined using a scanning electron microscope and the silver content in the fiber was measured using an inductively coupled plasma atomic emission spectrometer. The antibacterial activities were evaluated. These hollow fibers had a sponge‐like structure and dense inner and outer surfaces. At a 50 k magnification, the pore on the skin layer was not observable, while the nodule size was smaller than 10 nm. The residual silver content of as‐spun hollow fiber was about 60% of the original silver added in the polymer solution. After immersing in water bath for 180 days, the silver content in the bulk of the hollow fibers decreased to 60% and the silver content on the surface reduced to 10%, yet still showed antibacterial activity against Escherichia coli and Staphylococcus aureus. After permeating with water for 5 days, the silver content in the hollow fibers decreased, and did not show antibacterial activity against E. coli and S. aureus. Thus, silver content must be periodically replenished after permeation. The proper range of AgNO₃ in the spinning solution for CA hollow fiber should be about 100–1000 ppm. Copyright © 2005 John Wiley & Sons, Ltd.     

Fabrication of magnesium bentonite hollow fibre ceramic membrane for oil-water separation

In this study, low-cost magnesium bentonite (MB) was used for the fabrication of bentonite hollow fibre (BHF) membrane with high pure water flux. MB powder was initially characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), particle size distribution (PSD) analyser, Brunnauer -Emmett- Teller (BET) method, and field emission scanning electron microscope (FESEM). The BHF membrane obtained was then fabricated through dope suspension mixing, phase inversion and sintering process. The dope suspension was prepared by mixing MB, dispersant, polymer binder, and solvent using a planetary ball mill. While the spinning process was carried out at the extrusion rate of 8 mL/min, a fluid bore rate of 10 mL/min and air gap of 5 cm, and this was followed by sintering operation at 950 °C, 1000 °C, 1050 °C, and 1100 °C. The resulting BHF membrane was characterized by scanning electron microscopy (SEM) and XRD; the porosity test, water flux and oil rejection were also examined. The SEM surface morphology of BHF at sintering temperature of 950 °C showed spongy-like and nested macrovoids structure; the porosity was 49.09% with a mean pore size of 3.9 µm. The performance test on the bentonite-based hollow fibre membrane showed that the membrane prepared at 20 wt% and sintering temperature of 1000 °C, which induced high and stable permeate water flux and oil rejection of BHFC membrane were 544 L/m² h and 97%, respectively. The results have shown that the presence of magnesium in bentonite can enhance and promote the needed support material for the fabrication of hollow fibre ceramic membrane.     

Preparation and characterisation of SrCe0.95Yb0.05O2.975 hollow fibre membranes

A mixed proton and electronic conducting hollow fibre membrane, SrCe_0.95Yb_0.05O_2.975 (SCYb), has been prepared by spinning a polymer solution containing suspended SCYb particles to a hollow fibre precursor, which is then sintered at elevated temperatures. The SCYb powders having a sub-micron size, i.e. an essential size for fabrication of the hollow fibre with good mechanical strength, were synthesised through a polymerised water-soluble complex method. By controlling weight ratio of the SCYb ceramic powder to the polymer binder and sintering temperatures, the SCYb ceramic hollow fibres with gas-tight properties can be prepared. Some primary factors affecting microstructures, gas tightness and mechanical strength of the mixed conducting hollow fibre membranes were studied in details.     

Direct measurement of rheologically induced molecular orientation in gas separation hollow fibre membranes and effects on selectivity

Asymmetric polysulfone hollow fibre membranes for gas separation were spun using a dry/wet spinning process. An optimised four component dope solution was used: 22% (w/w) polysulfone, 31.8% (w/w) N,N-dimethylacetamide, 31.8% (w/ w) tetrahydrofuran and 14.4% (w/w) ethanol. Fibres were spun at low- and high-dope extrusion rates and hence at different levels of shear. Molecular orientation in the active layer of the membranes was measured by plane-polarised infrared spectroscopy. Gas permeation properties (permeability and selectivity) were evaluated using pure carbon dioxide and methane. The spectroscopy indicated that increased molecular orientation occurs in the high-shear membranes. The selectivities of these membranes were heightened and even surpassed the recognised intrinsic selectivity of the membrane polymer. The results suggest that increased shear during spinning increases molecular orientation and, in turn, enhances selectivity.     

Effect of wet spinning parameters on the properties of novel cellulosic fibres

Novel cellulose fibres (Biocelsol) were spun by traditional wet spinning technique from the alkaline solution prepared by dissolving enzyme treated pulp directly into aqueous sodium zincate (ZnO/NaOH). The spinning dope contained 6 wt.% of cellulose, 7.8 wt.% of sodium hydroxide (NaOH) and 0.84 wt.% of zinc oxide (ZnO). The fibres were spun into 5% and 15% sulphuric acid (H₂SO₄) baths containing 10% sodium sulphate (Na₂SO₄). The highest fibre tenacity obtained was 1.8 cNdtex⁻¹ with elongation of 15% and titre of 1.4 dtex. Average molecular weights and shape of molecular weight distribution curves of the celluloses from the novel wet spun cellulosic fibre and from the commercial viscose fibre were close to each other.     

Effect of solution extrusion rate on morphology and performance of polyvinylidene fluoride hollow fiber membranes using polyvinyl pyrrolidone as an additive

<正>Polyvinylidene fluoride(PVDF) hollow fiber membranes prepared from spinning solutions with different polyvinyl pyrrolidone(PVP) contents(1%and 5%) at different extrusion rates were obtained by wet/dry phase process keeping all other spinning parameters constant.In spinning these PVDF hollow fibers,dimethylacetamide(DMAc) and PVP were used as a solvent and an additive,respectively.Water was used as the inner coagulant.Dimethylformamide(DMF) and water(30/70) were used as the external coagulant.The performances of membranes were characterized in terms of water flux,solute rejection for the wet membranes.The structure and morphology of PVDF hollow fiber were examined by BET adsorption,dry/wet weight method and scanning electron microscopy(SEM).It is found that the increase in PVP content and extrusion rate of spinning solution can result in the increase of water flux and decrease of solute rejection.The improvements of interconnected porous structure and pore size are induced by shear-thinning behavior of spinning solution at high extrusion rates,which could result in the increase of water flux of hollow fiber membranes.The increase of extrusion rate also leads to the increase of membrane thickness due to the recovery effect of elastic property of polymer chains.     

The ageing phenomenon of polyethersulphone hollow fibre membranes for gas separation and their characteristics

We have studied the ageing behaviour of PES/NMP (polyethersulphone/N-methyl pyrrolidone) hollow fibres for gas separation that were prepared from 35% and 37% dope. The effect of ageing on hollow fibres spun from low and high shear rate (103 vs. 862 s⁻¹) has also been investigated, in terms of their transport properties (permeation flux and separation performance), thermal, mechanical and tensile properties. Hollow fibres in this study were aged for around four months in ambient air at room temperature prior to testing.In general, the gas permeation flux drops steeply during the 40 days following fabrication and levels off thereafter. The O₂/N₂ selectivity decreases slightly over time. Hollow fibres spun with high shear rate seem to age faster than those spun with low shear rate. The gas fluxes of both membranes were found to follow a log–log relationship with ageing time. For almost all the gases used in this study, the gas flux decay rate, calculated from the slope of the log–log plot of gas flux vs. ageing, is higher for membranes spun with high shear rate. The effect of shear rate on ageing is less significant for smaller gas molecules that travel faster such as He and H₂. No significant effect of ageing on gas selectivity was observed. Experimental results also indicate that the storage modulus and loss modulus of the hollow fibres increase with ageing. Hollow fibres spun with high shear rates give a slightly higher increase in these moduli than those spun at low shear rates. Surprisingly, tangent δ (energy dissipation) and glass transitional temperature are not sensitive to ageing. We also found that the tensile yield strength and Young's modulus of the hollow fibres increase slightly with ageing. The hollow fibre membranes spun at high shear rates also show a higher increment in tensile yield stress. However, the change in Young's modulus due to ageing was similar for fibres spun with high and low shear rates.     

Comparison of adsorption performances of metal–chelated polyamide hollow fibre membranes in lysozyme separation

Commercially available microporous polyamide hollow fibres are modified by acid hydrolysis to activate the reactive groups and subsequently binding of the ligand, i.e. Cibacron Blue F3GA. Then the Cibacron Blue F3GA-derived hollow fibres were loaded with different metal ions (i.e. Zn(II), Cu(II), Ni(II)) to form the metal chelate. The internal polymer matrix was characterised by scanning electron microscopy. The effects of pH, initial concentration of lysozyme, metal type and temperature on the adsorption of lysozyme to the metal–chelated hollow fibres were examined in a batch reactor. The non-specific adsorption of lysozyme onto the polyamide hollow fibres was 1.8 mg/g. Cibacron Blue F3GA immobilisation increased the lysozyme adsorption up to 62.3 mg/g. Metal–chelated hollow fibres showed a significant increase of the adsorption efficiency. Lysozyme adsorption capacities of Zn(II), Cu(II) and Ni(II)-chelated hollow fibres were different. The maximum capacities of Zn(II), Cu(II) or Ni(II)-chelated hollow fibres were 144.2, 75.2 and 68.6 mg/g, respectively. Significant amount of the adsorbed lysozyme (up to 97%) was eluted in 1 h in the elution medium containing 1.0 M NaSCN at pH 8.0 and 25 mM EDTA at pH 4.9. Repeated adsorption–desorption process showed that this novel metal–chelated polyamide hollow fibres are suitable for lysozyme adsorption.     

Gel spinning of poly(vinyl alcohol) from dimethyl sulfoxide/water mixture

Gel spinning of poly(vinyl alcohol) (PVA) was attempted from the PVA dope prepared from the mixture of dimethyl sulfoxide (DMSO) and water. The DMSO/H₂O = 80/20 (w/w) mixture and methanol were found to be the best solvent for the spinning dope and the coagulant, respectively, to give PVA fiber with the highest drawability. PVA fiber with the highest strength and Young's modulus were obtained from the undrawn gel fibers when subjected to hot two-stage drawing under conditions such as to produce maximum drawability. Furthermore, higher draw ratios of PVA fiber were attained at 6 wt % dope by lowering the coagulating temperature of methanol. In the present work, the highest tensile strength (2.8 GPa) and the highest Young's modulus (64 GPa) were realized, when the spinning dope was prepared from PVA with DP of 5,000 and the DMSO/H₂O (80/20) mixed solvent to have the PVA concentration of 6 wt %, the coagulating temperature of methanol was ?20°C, and the two-stage drawing was carried out at 160 (first) and 200°C (second). The PVA fiber prepared under this gel spinning condition could be elongated to 45 times draw ratio. The very high drawability of PVA fibers obtained from the DMSO/H₂O (80/20) mixture dope was ascribed to the ability of the DMSO/H₂O mixture to promote gelation. © 1994 John Wiley & Sons, Inc.     

Y2O3/ZrO2中空纤维陶瓷膜的制备与表征

本文结合相转化技术和干湿法纺丝工艺制备了以YSZ(以摩尔分数为8%的Y2O3稳定的ZrO2)为原料的中空纤维陶瓷膜, 考察了YSZ粉体的粒度分布和形貌, 并研究了YSZ中空纤维陶瓷膜的气密性、 孔特性、 机械性能、 微观结构及晶型变化等.     

BPA transfer rate increase using molecular imprinted polyethersulfone hollow fiber membrane

Bisphenol A (BPA) imprinted polyethersulfone (PES) hollow fiber membrane was spun using a dry–wet spinning method, the membrane was then prepared as a filter with an effective area of 200 cm². The hollow fiber filter was employed to study the BPA transport behavior. The transport ability of the prepared hollow fiber membrane was measured using 100 μmol/l BPA aqueous solutions at a flow flux of 50 and 75 ml/min, respectively. The BPA transfer rate increased for the imprinted hollow fiber membranes due to the larger amount of binding sites, comparing with the non-imprinted one. In the present study, hollow fiber membrane and the molecular imprinting technique were combined for advanced separation and the data suggested that small molecules could transfer in the direction opposite to the concentration gradient due to different pH.     

Hyperlayer separation in hollow fiber flow field-flow fractionation: effect of membrane materials on resolution and selectivity

Hollow fiber flow FFF (HF FlFFF) has recently shown its capability to separate and characterize the size of submicrometer particles and has demonstrated the potential to be developed into a disposable flow FFF channel. In this work, HF FlFFF was used for the hyperlayer separation of micron-sized particles and the separation capability was examined by using various hollow fiber membrane materials (Polysulfones, cPVC, and PAN). From the experiments, PAN (polyacrylonitriles) showed an outstanding performance in particle separation compared to the other membranes. By orienting the fiber module in an upright direction, the upstream flow migration reduced band broadening of eluted peaks. When the efficiency of the PAN hollow fiber system was tested by varying the ratio of outflow-rate to radial flow-rate, it was found that optimum separation in hyperlayer HF FlFFF can be obtained at the ratio of about 6–7. From the examination of retention at or around steric inversion diameter, it was observed that experiments showed a good agreement with predictions by semi-empirical calculation. In hyperlayer HF FlFFF the diameter based selectivity values were shown to be 1.2–1.7 depending on the type of membranes and the field strength (the radial flow-rate) conditions.