Fiber Porosity Research Articles

Crystallinity, Crystalline Quality, and Microstructural Ordering in Boron Nitride Fibers

A qualitative and quantitative analysis of the microtexture/microstructure in several experimental boron nitride fibers was investigated by X‐ray diffraction and electron microscopy according to their mechanical properties. Young's modulus was controlled by grain orientation. The disordered microstructure of low modulus fibers consisted of amorphous regions and nanometer‐sized crystallites arranged in a wide range of orientation. In contrast, high modulus samples showed a coarse‐grained microtexture in which crystallites were ordered in a meso‐hexagonal arrangement along the fiber‐axis. Cracks were easily propagated as crystallinity increased, leading to decreased failure strain. The strength‐microstructure relationship was obscured by the fiber porosity and defects.

Cathodoluminescence of Late Triassic terebratulid brachiopods: implications for growth patterns

Upper Triassic terebratulid brachiopod Rhaetina shows differential preservation of shell structure under cathodoluminescent (CL) and scanning electron microscope (SEM), indicating intra- and interspecific differences in brachiopod growth patterns.In longitudinal sections, the CL lines appear firstly in inner, ontogenetically older parts of the shell and their outlines are parallel with the internal valve surface in posterior parts of the shell. In anterior parts of the shell, CL lines run diagonally towards the external valve surface and terminate in external growth lines. The shell structure of external growth lines indicates that major growth lines (associated with gradual reorientation of calcitic crystals due to reflection of the mantle edge) and disturbance lines (associated with abrupt mantle regression) are present at the terminations of CL lines. These data indicate that CL lines correspond to zones recording slowing/cessation of the growth rate. Under SEM, shell layers that are luminescent can be locally recognized as amalgamated or altered relicts of the secondary fibres. The origin of luminescence in these areas can be related to (1) the increased Mn-bearing diagenetic fluid flow favoured by higher porosity of altered secondary fibres, or can be caused by (2) temporal changes in the redox state controlling the solubility of Mn2+ and Fe2+ during the mantle anaerobiosis. Similarly, amalgamated/altered shell structure could either point to (1) the primary modification of shell secretory regime during the slowing/cessation in growth rate, or could be (2) the product of diagenetic processes responsible for post-depositional alteration of a shell structure, due to the primary shell heterogeneity/porosity in areas of growth discontinuities. Although presented data do not convincingly solve this uncertainty, the important point is that CL lines of studied brachiopods reflect changes in the growth rate and can be used in the investigation of growth dynamics in brachiopods, complementary to external growth lines and isotope or trace element analysis. In order to verify the modification of shell structure and understand the origin of associated luminescence during the change in growth rate unbiased by diagenetic overprint, it is necessary to examine modern brachiopods.The distribution pattern of CL lines that correlates with growth rate fluctuations potentially reflects a short-term environmental instability on the time scale of an individual life span. The data show correlation between CL line pattern and different bathymetric settings. Regular repetition of CL lines is preserved in Rhaetina gregaria, Rhaetina hybensis and Rhaetina pyriformis that occur in relatively uniform depositional settings below a normal storm wave base. In contrast, irregular pattern of CL lines is typical of R. gregaria from highly variable deposit associations derived from very shallow, subtidal settings above a normal storm wave base. In the case of R. gregaria, this difference reflects intraspecific variations in growth dynamics among bathymetric settings, indicating environmental control on their growth rates. Rhaetina gregaria is mostly the only one brachiopod species present in benthic assemblages from very shallow, marginal marine environments in the Western Tethys. The implication is that R. gregaria, in contrast to most other brachiopods, could live in physically stressed habitats characterized by higher environmental instability/unpredictability.

Surface hydrolysis of filaments based on poly(trimethylene terephthalate) spun at high spinning speeds

AbstractThe surface alkaline hydrolysis of fibers made from poly(trimethylene terephthalate) (PTT) was studied after extruding the polymer at high spinning speeds from 2000 to 6000 m/min and heat setting in the range of temperatures from 100 to 180°C. Fiber weight loss increased with an increasing heat‐setting temperature but it was also dependent on the spinning speed. Some of the partially hydrolyzed fibers had a well‐developed, hydrophilic surface, and pore size in the range of 0.69 to 1.20 μm. The optimum reaction and morphological conditions for increasing porosity in PTT fibers depends on spinning speed and heat‐setting temperature. A temperature of 180°C is the upper limit for heat‐setting PTT filaments but seems to be the most effective for making porous fibers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1724–1730, 2004

Effect of Hydration–Dehydration Processes on the Structure and Porosity of Detemplated Silica Fibers

A study was carried out on the conditions of template removal (calcination or extraction by solvents) and hydration–dehydration processes on the hexagonal structure and porosity of detemplated fibers. The hydration–dehydration cycle leads to a decrease in fiber porosity, especially in fiber detemplation by extraction with surfactant solvents such as ethanol and acetic acid.

Acrylonitrile–sodium methallylsulfonate copolymer. DSC approach to membrane porosity of foam and hollow fibers

The porosity of membranes formed from acrylonitrile–sodium methallylsulfonate copolymer was characterized from the analysis of the depression of the melting point of absorbed water. Membranes were obtained either as a foam or as a hollow fiber; the foam consisted of interconnected macrocavities (mean diameter about equal to 1 mm) while the hollow fiber was a symmetric membrane used for blood ultrafiltration. Differential scanning calorimetry (DSC) of water revealed both the Gaussian distribution of pore sizes and correspondingly, their mean size: 5.2 nm for the pores through the walls separating macrocavities in the foam and 5.6 and 10.6 nm associated with two distributions representing nearly equal amounts of absorbed water, for the hollow fiber. In addition to DSC, the water magnetic relaxation showed that the isothermal dehydration of the foam was due to the deswelling of macrocavities while the increasing amount of absorbed water in pores reflects its strong interaction with the polymer.

Study on the effect of a non-solvent additive on the morphology and performance of ultrafiltration hollow-fiber membranes

Hollow-fiber membranes were prepared by the dry/wet spinning technique from polyvinylidene fluoride (PVDF) dope solutions containing the solvent N, N-dimethylacetamide and the non-solvent additive 1,2-ethanediol. Ethanol aqueous solution, 50% by volume, was used as internal and external coagulants. The effect of the non-solvent additive concentration on the morphological properties of the hollow fibers was studied in terms of atomic force microscopy (AFM) and scanning electron microscopy (SEM). The pore size, the nodule size and the roughness parameters at the inner and outer surfaces of the hollow fibers were studied by AFM. The liquid entry pressure of water ( LEPw) and the porosity of the hollow fibers were evaluated. The pore size was also determined by the gas permeation test and by the solute transport using ultrafiltration of polyethylene glycol (PEG) and polyethylene oxides (PEO). A comparative study was made between the membrane characteristics parameters obtained from the different characterization techniques.

Preparation and characterization of gas separation hollow fiber membranes based on polyethersulfone-polyimide miscible blends

In this work the preparation and characterization of gas separation hollow fibers based on polyethersulfone Sumikaexcel (PES) and polyimide Matrimid 5218 (PI) blends are reported. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and structure of the asymmetric hollow fibers. Differential Scanning Calorimetry (DSC) was used to determine the glass transition temperatures of the blends. The permeation rates of CO 2 and N 2, for both uncoated and PDMS coated fibers, were measured by the variable pressure method. Experimental results were used in conjunction with the resistance model in order to determine the surface porosity of uncoated fibers. PES/PI fibers exhibit an asymmetric structure and one glass transition temperature. Increase of the air gap distance during spinning results in higher surface porosity and gas permeation rates. PDMS coated fibers exhibit a CO 2 permeance varying from 31 to 60 GPU and a CO 2/N 2 selectivity varying from 35 to 40, at room temperature. The thickness of the skin layer, which corresponds to these permeation rates, varies from 0.1 to 0.15 μm. After plasticization with CO 2, ultrathin hollow fibers especially rich in PES, exhibit reversible reduction of CO 2/N 2 selectivity. These properties establish PES/PI hollow fibers as excellent candidate membranes for the separation of gaseous mixtures in industrial level.

The Macroporosity of Polyacrylonitrile Fibre

The closed porosity of polyacrylonitrile fibres wet-spun from dimethyl acetamide solutions was investigated in raising the spinning bath temperature from 2 to 20°C. The dependence of the porosity of the fibre on the conditions of plasticization treatment is established. The fibre treated in saturated vapor medium has higher porosity values (narrow transmission factors in chlorobenzene) than the fibre treated with a hot aqueous solution of dimethyl acetamide. The dependence of the fibre porosity on the drying and repeated washing and subsequent drying temperatures is demonstrated. The mobility of the structural elements increases and the porosity of the fibre decreases with an increase in the drying temperature.

The determination of porosity and cellulose content of plant fibers by density methods

A combination of the Archimedes method and the technique of helium pycnometry was used to perform accurate measurements of porosity and cellulose content. For this purpose, the densities of hemp, sisal, jute and kapok fibers were measured. An equation was developed using the bulk and absolute density of the fiber and the density of pure cellulose to determine the amount of cellulose a particular fiber contains. As a result, true plant fiber porosity and cellulose content were accurately determined.

Structure of Coreless, Long Silicon Carbide Fibres and Aluminum—SiC Composites

Using electron microscopic, x-ray, and sorption methods, the internal structure, surface, and porosity of coreless silicon carbide fibres and composites were investigated. A model of the interaction at the aluminum melt —SiC fibre interface was proposed. The effect of low-temperature annealing on the structure and properties of domestic and Japanese coreless silicon carbide fibres was observed. The reaction rate in composites reinforced with domestic annealed fibres decreased.

Cellulose nitrate-based multilayer composite membranes for gas separation

Cellulose nitrate (CN)-based composite hollow fiber membranes are prepared for gas separation. The membrane performance is studied in terms of the concentration of CN coating solution, polysulfone (PS) substrate, pre-wetting treatment, solvent used in CN solution, and the various substrate materials. In order to fabricate a useful multilayer composite membrane, the surface porosity of composite hollow fiber has to be reduced to a certain degree, before silicone rubber coating, by adjusting the CN coated layer and the PS substrate structure. The effect of pre-wetting treatment on the membrane performance is found to be minor, which may be due to the coating technique and the relatively highly permeable substrate used in this study. The solvent used in dissolving CN also affects the gas permselective property of the multilayer composite membranes and this effect may be attributed to the formation of different membrane morphology of CN layer caused by the different physical properties of solvents. The selection of substrate materials is important and this work suggests that a permeable material is more suitable to serve as the substrate. Besides, the substructure resistance in the substrate should be kept as low as possible in order not to deteriorate membrane performance of the resultant multilayer composite hollow fiber membranes.

Construction of a thread coater and use of azocasein release To characterize the sealant coat porosity of fibers coated with latex biocatalytic coatings

A single-stage annular fiber coating method with co-current dry-air drying at 30 degrees C has been developed for multilayer coating of 128 &mgr;m diameter polyester thread (yarn) with latex films as a model for enzyme immobilization and development of a filament biocatalytic filter. Acrylic vinyl acetate polymer coatings were sequentially metered onto the fibers by the combination of a flexible squeegee and a red rubber annulus. The thread coater can operate over a range of 0.07-1.37 m/min thread velocities while delivering a nearly constant and reproducible polymer loading of 30. 8 +/- 1.3 mg/m. A 100% polyester, 278.9 denier thread was precoated with latex to generate an approximately 369 denier sealed filament. The filament was then coated with a latex + sulfanilamide-azocasein mixture and sealed with a polymer top coat. The permeability of the polymer sealant top coat was characterized using entrapped azocasein as a tracer molecule and monitoring the azocasein release upon rehydration of the coated threads. Azocasein release rate decreased with curing time at 30 degrees C until 2 days and was invariant after 2-3 days of curing. A 282 mOsm rehydrating solution was sufficient to suppress increased azocasein release due to top coat blistering. No enhancement in the permeability of the top coat was observed when high molecular weight water soluble polymers (WSPs) were used as fillers. This probably results from the low WSP to latex ratio used (0.05-0.1) and the slow rate of WSP leaching compared to the release of azocasein. A method using 60-120 mesh silica was also developed to study the effect of mechanical abrasion of the coated threads as measured by azocasein release kinetics.

New developments in separators for valve-regulated lead–acid batteries

In valve-regulated lead–acid (VRLA) batteries the electrolyte solution has to be immobilized to ensure tiny channels left open for the transfer of oxygen from the positive to the negative electrode. So far microfibre glassmats have predominantly been used, which based on their high porosity and good wettability of the glass fibres are able to retain durably large electrolyte volumes. The tensile strength of such microfibre glassmats remains unsatisfactory. Developments to produce absorbing mats from organic fibres have recently succeeded due to advanced developments in polymers and in fibre production processes as well as in achieving permanent hydrophilisation. Such polypropylene-microfibremats have excellent tensile and puncture strength and—as pockets—can be well integrated into highly automated assembly processes. Test data for polypropylene-microfibremats are presented and compared to microfibre glassmats. Another approach to hamper the electrolyte in its free mobility is to gel it: batteries with gelled electrolyte have been shown to require conventional microporous separators—both for secure fixing of plate spacing as well as for preventing electronic shorts. Despite their complex filling process gel batteries are well accepted for cycling applications, when simultaneously freedom from maintenance is required. Due to the high power requirements for EV batteries there is a trend towards thinner plates and thinner separation; also substantial pressure on the positive electrode and thus also on the separator is desirable to improve the cycling life decisively. A new separator development is presented, which in spite of high porosity (>80%), suffers only little deformation even under very high pressure. It effectively prevents acid stratification, forms no filling profile and permits oxygen transfer.

Investigation into Cotton Fibre Morphology Part I: Effect of Chemical and Physical Treatments on Fibre Porosity

The porosity of cotton fibre of various degrees of maturity has been assessed using differential dyeing technique and glycerol retention tests. It has been found that, irrespective of fibre maturity, additional pores are created when the fibre undergoes treatments such as scouring, sequential bleaching or, more unusually, final alcohol boil. The results indicate that, for each of the above treatments, porosity increases are due to the creation of voids rather than to any fine changes in fibre morphology. The increase in fibre porosity is due to the dissolution of water- and alcohol-soluble substances, as well as waxes within the various layers of the cotton wall structure.

Chemical Vapor Deposition of Heterocyclic Compounds over Active Carbon Fiber To Control Its Porosity and Surface Function

The chemical vapor deposition (CVD) of some heterocyclic compounds was examined to control the porosity and surface functionality of the active carbon fiber (ACF). The deposition took place only on the pore wall of the ACF, when the heterocyclic compound as the precursor and the deposition temperature were selected carefully to be thermally stable and around 700 °C, respectively. Moderately activated ACFs modified with pyridine, pyrrole, and thiophene demonstrated molecular sieving activity for selective adsorptions of CO2/CH4 and O2/N2 through selective CVD. In contrast, furan decomposed at this temperature, failing to provide molecular sieving activity. The thermal stability of the depositing molecules is a key factor to obtain the molecular sieving performance after CVD. Pyridine, pyrrole, and thiophene produced amorphous carbon within the pore which appears to implant the nitrogen and sulfur atoms over the surface of the ACF, respectively.

Effects of Enzymatic Removal of Xylan and Glucomannan on the Pore Size Distribution of Kraft Fibres

The effects of enzymatic removal of xylan and glucomannan from pine and birch kraft pulps on the pore size distribution and median pore width of the pulp fibres were determined by modified solute exclusion and 'H NMR techniques. With the NMR method, clearly higher median pore sizes of the kraft fibres were obtained compared with the solute exclusion method. However, the effects of enzymatic treatments on pore size distributions and median pore widths of fibres were detected by both techniques. An increase in the pore width of both pine and birch kraft fibres was observed even after a limited xylanase treatment. The NMR technique appeared to be more suitable than the solute exclusion method for determination of the further increase of the median pore width and the widening of the pore size distribution of fibres by a more extensive xylanase treatment. Both techniques revealed an increase of the median pore width of fibres caused by the extensive removal of pine kraft pulp glucomannan. Despite their basic differences and specific limitations, both the NMR and solute exclusion techniques showed the enzymatic effects on the porosity of pulp fibres.

Effect of steam explosion on the physicochemical properties and enzymatic saccharification of rice straw

The effects of steam explosion pretreatment on the physical and chemical properties of rice straw and on the enzymatic saccharification of the straw were investigated. A wide range of pretreatment conditions were tested. Pretreatment effects were assessed by morphological changes, pore size distribution, pH, soluble sugars concentration, amounts of extracted components, and by enzymatic saccharification of pretreated samples. Rice straw was very finely defiberized by steam explosion. A great increase in pore volume accessible to cellulolytic enzymes was observed during pretreatment at 3.53 MPa for 2 min. On the other hand, pretreatment under very high steam pressure of 4.02 MPa reduced fiber porosity. The steam explosion products were separated into hemicellulose, cellulose, methanol-soluble lignin, and klason lignin. Increasing the steaming time at high steam pressures (temperatures) increased the amount of klason lignin and reduced the amount of hemicellulose, thus emphasizing the need to identify optimal pretreatment conditions for efficient use of biomass. A maximum 83% glucose yield was obtained by enzymatic saccharification of the sample exploded at 3.53 MPa for 2 min. These explosion conditions produce a material that is highly susceptible to attack by cellulolytic enzymes.

Evolution of Surface Composition, Porosity, and Surface Area of Glass Fibers in a Moist Atmosphere

The aim of this work is to study the influence of moisture on the surface chemistry and texture of C and E glass fibers. Thermal analysis (in the conditions of controlled rate transformation analysis) coupled with mass spectroscopy, surface area determination (by krypton adsorption at 77 K), adsorption of water by gravimetry, and AFM were used to characterize both novel and aged fibers (a few days at 50 °C in 95% humidity). Whereas aging has little effect on E glass fibers which can be interpreted by the appearance of both a small roughness and hydrophilic superficial sites, the changes observed on the C glass fibers are drastic. The surface area is increased by at least 10 times, micropores appear (as shown by gas adsorption), and clusters (evidenced by AFM) are formed on the surface. The migration of sodium and calcium from the bulk toward the surface (where they are stabilized under the form of carbonates in agreement with thermal analysis results) could explain the formation of both micropores and clu...

Thin-Film-Composite Gas Separation Membranes: On the Dynamics of Thin Film Formation Mechanism On Porous Substrates

Abstract A wide range of gas separations of interest in energy applications are carried out using membranes. Growing attention has been paid to the technology of making thin-film-composites(TFCs) membranes. Understanding the polymer solution and substrate property is key to successfully preparing TFCs membranes. This paper reports on some fundamental issues of coating hollow fibers with polymer from solution by dip coating. Polymeric porous hollow fibers with varying porosities and permeances were coated with polymer solutions of different viscosities in a continuous process. In addition, the fibers were coated dry and by presoaking in the coating solvent. It was found that the thickness of the coating on the low permeance/porosity/wet and dry fibers could be approximated by the Deryaguin model (h/R = 1.33 (Ca) 0.67). For dry fibers, as the fiber porosity increased, the measured coating thickness was significantly underestimated by the Deryaguin equation. It is believed that the pores in the fiber allow r...

Analysis of the Non-Viscous Flow Effect in Liquid Molding Process

Non-viscous flow effect in liquid molding processes has been analyzed and evaluated. Ergun equation is used to describe the non-viscous flow through fibrous media. Overall flow resistance of the fibrous media is considered as the summation of the viscous inertial components. The generalized permeability of the fibrous media is found to be not only a function of the structural parameters such as fiber diameter and porosity, but also related to the fluid properties and flow velocity. Two-dimensional mold filling flow equation has been established. In order to evaluate the magnitude of the non-viscous flow effect in various process conditions, solutions of simplified one-dimensional filling flow cases are discussed. Parametric study with numerical simulations has been carried out to assess the non-viscous flow effect on the filling time and the inlet fluid pressure.