Dope Extrusion Research Articles

Measurement of rheologically induced molecular orientation using attenuated total reflection infrared dichroism in reverse osmosis hollow fiber cellulose acetate membranes and influence on separation performance

The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s −1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

The deduction of fine structural details of reverse osmosis hollow fiber membranes using surface force–pore flow model

In order to elucidate the relationship between the dope extrusion shear rate and membrane performance, sodium chloride transfer through the asymmetric cellulose acetate reverse osmosis hollow fiber membranes is modeled, allowing fine details of the fiber structure to be deduced from the NaCl-H2O rejection characteristics. The structural information such as the pore size radius and skin thickness of the active layer deduced from the sodium chloride separation experimental data and surface force-pore flow model (SF-PF) is then used to interpret the relationship between the rheological conditions during spinning and membrane performance. The modeling results revealed that increased extrusion shear rate would decrease both pore size and thickness of the active layer, thus increasing the separation performance of the RO hollow fiber membranes.

Optimization of cellulose acetate hollow fiber reverse osmosis membrane production using Taguchi method

Cellulose acetate hollow fiber membranes for reverse osmosis (RO) were spun using a forced convection technique. In this study, a systematic experimental design based on Taguchi’s method (which is a fractional factorial method) has been employed for discussing the relationship between the rejection rate coefficient, permeation rate and the dry–wet spinning conditions for making cellulose acetate hollow fibers for RO. The factors considered in the experimental design included the polymer contents (PCs), the ratio of the solvent (acetone) to swelling agents (formamide) in the dope solution, the dope extrusion rate (DER), the type of bore fluid (BF), the residence time (RT) and the nitrogen gas flushing rate (GR). The results indicate that the BF and the DER are the two most important factors in determining the performance of the RO membranes.

Study of shear rate influence on the performance of cellulose acetate reverse osmosis hollow fiber membranes

The effect of shear rate on the separation performance of reverse osmosis hollow fiber membrane is discussed. Experiments involving six different dope extrusion rates (DERs) (ranging 2.5–5 ml/min) are performed with the other process factors set at the optimum conditions determined by the Taguchi analysis. This will enable an assessment to be made on the relationship between the DER and the rejection rate. The regression method is used to analyse the experimental results and an empirical model has been developed. Simultaneously, it is found that there is a fairly strong correlation between extrusion shear rate and the rejection rate of the membranes, whereby as the shear rate increases, the rejection rate increases until a critical level of shear is achieved, beyond which reverse osmosis membrane performance deteriorates, suggesting that there exists an optimum shear rate which yields optimal membrane morphology for reverse osmosis hollow fiber membranes.

Visualization of the effect of die shear rate on the outer surface morphology of ultrafiltration membranes by AFM

We have demonstrated the effect of shear rate on the outer surface morphology of polyethersulfone (PES) hollow fiber ultrafiltration (UF) membranes by an atomic force microscope (AFM). A digital instrument (DI) AFM was used to reveal the surface morphology of hollow fiber membranes prepared with varying shear rates from 1305 to 11,066 s −1. A tapping mode was operated for studying the polymeric membranes when AFM was applied to image the surface of a fiber in air. AFM images of the outer surface have revealed that the nodules in the outer skin appeared to be randomly arranged at low shear rates but formed bands that were aligned in the direction of dope extrusion when the shear rate increased. Both nodule sizes in the fiber spinning and transversal directions decreased with increasing shear rate possibly because of chain disentanglement and thermodynamically favored. This result has not been reported so far. The analysis of AFM images showed that the roughness of the outer surface of hollow fiber UF membranes in terms of R ms, R a and R z decreased with an increase in shear rate. The pure water flux of the membranes was nearly proportional to the mean roughness and higher mean roughness resulted in lower separation of membranes. AFM data also imply that there was a certain critical value of shear rate around 3585 s −1, the roughness decreased significantly with an increase in shear rate below 3585 s −1 and almost leveled off or in a much slower pace above this shear rate.

Production of super selective polysulfone hollow fiber membranes for gas separation

Polysulfone gas separation hollow fiber membranes were manufactured using a dry/wet spinning process with forced convection in the dry gap. Hollow fibers were produced using two different bore coagulants: one pure water and one with reduced water activity. An optimized multi-component dope was used which proved to be a shear-thinning power-law fluid. For each bore system, membranes were spun at low and high dope extrusion rates (DERs) corresponding to shear rates of around 4000 s −1 and 10 000 s −1, respectively, at the outer spinneret wall. Plane polarized infrared spectroscopy was used to probe the membrane active layer. Pressure-normalized fluxes and selectivities were evaluated using pure carbon dioxide and methane. For the spinning conditions used here, the combination of reduced water activity in the bore and high DER produced highly selective membranes. Some selectivities reached about three times the recognized intrinsic value for the polymer.

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/H 2O 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.

The deduction of fine structural details of gas separation hollow fibre membranes using resistance modelling of gas permeation

Gas transfer through asymmetric polysulfone hollow fibre membranes has been modelled, allowing fine details of fibre structure to be deduced from gas permeation characteristics. The structural information is used to interpret the relationship between spinning conditions and fibre properties. Dope concentration determines the general morphology of the fibre, such as the porosity (voidage fraction), thickness of the active layer and order of magnitude of surface porosity (fraction of surface area that is pores), and thus it sets the permeability and level of selectivity that are likely to be achieved on coating. The selectivity of the solid polymer (the maximum selectivity achievable by any membrane if coating is highly effective or if no surface pores are present) was found to increase with increasing dope extrusion rate. The elevated levels of shear in the spinneret may enhance the orientation of polymer molecules. Increasing the jet stretch ratio during spinning had a detrimental effect on solid polymer selectivity. Increased elongational strain possibly results in an unfavourable polymer structure.

The rheology of fibre spinning and the properties of hollow-fibre membranes for gas separation

The effect of spinning rheology on gas-separation hollow-fibre membranes has been investigated. For polysulfone fibres, permeability decreased with increasing dope concentration. The lowest fibre selectivities occurred with the medium spinning-dope concentration, suggesting that both surface pore area and active layer thickness were high. Both permeability and selectivity increased with increasing dope extrusion rate, possibly due to enhanced orientation of the membrane skin. Permeability increased and selectivity decreased with increasing jet stretch. Polyacrylonitrile membranes, because of the low polymer permeability, proved insensitive to spinning conditions, showing no separation capability due to the dominating effect of surface pores.

Biodegradable hollow fibres for the controlled release of hormones

Poly( l- lactide), (PLLA), hollow fibres were prepared using a dry-wet phase inversion spinning process. The effect of several spinning parameters (i.e. bore medium flow rate, spinning dope extrusion rate, fibre take-up rate, and spinning height) on the hollow fibre dimensions is reported. The use of several spinning systems (i.e. different solvent/non-solvent pairs with or without additive) resulted in PLLA hollow fibres with varying asymmetric membrane structures, i.e. a porous matrix covered by an internal and external skin varying from very thick and dense to very thin and porous. Some of the differences in membrane structure were qualitatively explained on the basis of a model developed by Reuvers [52] for the formation of flat-sheet membranes by immersion precipitation. Release experiments were carried out using PLLA hollow fibres filled with a 25 wt.% dispersion of micronized 3H-levonorgestrel in castor oil, and a receiving fluid consisting of 40 wt.% aqueous ethanol. The hollow fibre levonorgestrel release rates were found to be dependent on the membrane structure of the hollow fibre wall. For the different hollow fibre samples, zero-order levonorgestrel release rates were found, in the range of 0.1–10 μg/cm/day. Possible release mechanisms are discussed. Preliminary in vivo (rabbit) release experiments showed that constant levonorgestrel blood plasma levels could be obtained for a period up to 210 days. It is concluded that the new biodegradable hollow fibre reservoir device shows very promising properties for possible application as a long-acting contraceptive delivery system.

9 Adjuvant therapy with thymic factors in solid tumours

The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s−1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

“Bishydrazid A2”: A new semiaromatic film and fiber-forming polymer based on bisacid A2

We report the preparation of the dihydrazide of bisacid A2 (“bisdihidrazide A2”). Its composition and structure have been verified by elemental analysis and by means of infrared and NMR spectra. This material was then converted into a polyhydrazide (“bishydrazid A2”) in an HCl-accepting solvent. The polymer was converted to fiber by wet spinning the dope obtained during the polycondensation reaction without prior neutralization or dilution. Film was cast from a solution of the precipitated and washed polymer which had an inherent viscosity of about 0.8; mechanical tests on the film—both before and after treatment with formic acid—were carried out. Differential scanning calorimetry showed an α/β transition comparable to that shown by bisacid A2 polyaramides and described in preceding papers1,2. Wide-angle x-ray scattering showed that the polymer is amorphous. Scanning electron micrographs revealed the physical structure of wet-spun fiber and the probable mechanism of fiber formation by dope extrusion into a water bath.

Spinning of Peanut Protein Fibers1

Abstract Raw Altika peanuts were blanched, partially defatted, ground and the protein was removed using an aqueous alkaline extraction, precipitated at pH 4.0 and freeze dried to yield a protein concentrate with 85.5% protein, 3.1% fat and 2.0% ash on a dry weight basis. Peanut protein concentrate was used to produce dope solutions for the spinning of fibers. Viscosity of dope solutions increased rapidly with increases in protein concentration from 11 to 14%; the highest protein concentration gelled within a short time after mixing. Dope solution viscosity increased with increasing NaOH concentration from 0.85% to 0.90%. Higher NaOH concentrations, up to 1.05%, however, resulted in a continued decrease in dope viscosity. Dope viscosity increased as a function of maturation time, especially at NaOH concentrations that yielded the highest viscosities. The best conditions for spinning peanut protein fibers were: 1) dope pH 11.4, 2) maturation time of 12 hours for a 13.0% protein dope or 2 hours for 13.5% protein dope, 3) coagulating bath conditions of 2N acetic acid and 20% NaC1, and 4) dope extrusion pressure of 15 psi. Suitability of dope solutions for spinning depended on the interaction between protein concentration, pH and dope maturity.