密封材料的透水性研究

本文探讨了橡胶密封材料的结构对透水性的影响,介绍了透水性的测试方法以及几种材料的水蒸汽透过系数。     

Measurement of water vapor transmission rate in highly permeable films

Measurements of water vapor transmission rate (WVTR) are often based on the wet cup method described by ASTM E 96‐95. In attempting to compare the performance of thin polymer films with moderate to high water vapor permeability, it was observed experimentally that the ASTM method did not give reliable results for highly permeable films. In particular, the WVTR depended on film thickness and the ratio of film area to water surface area. It was determined that the high water vapor flux through the more permeable films caused a reduction in the driving force for water vapor transmission, that is, the relative humidity at the inner surface of the film. Consequently, the WVTR was underestimated. Comparisons based on a small area ratio and a constant small‐flux condition were considered as alternative approaches for evaluating performance using the wet cup method. The constant flux approach produced the best correlation with WVTR that was measured with a commercial instrument. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1624–1633, 2001     

Effects of parameters on nanofiber diameter determined from electrospinning model

In this paper the effects of 13 material and operating parameters on electrospun fiber diameters are determined by varying the parameter values in an electrospinning theoretical model. The complexity of the electrospinning process makes empirical determination of the effects of parameters very difficult. The results show that the five parameters (volumetric charge density, distance from nozzle to collector, initial jet/orifice radius, relaxation time, and viscosity) have the most significant effect on the jet radius. The other parameters (initial polymer concentration, solution density, electric potential, perturbation frequency, and solvent vapor pressure) have moderate effects on the jet radius. Parameters relative humidity, surface tension, and vapor diffusivity have minor effects on the jet radius. Knowing the relative effects of parameters on jet radius should be useful for process control and prediction of electrospun fiber production.     

Permeation of oxygen and water vapor through EVOH films as influenced by relative humidity

The transport properties of oxygen and water vapor through EVOH films as functions of relative humidity (RH) and temperature were studied. The results of oxygen and water vapor permeation demonstrated that temperature and RH markedly affected barrier properties of these films. In general, the EVOH films had minimal oxygen and water vapor permeabilities at a low RH, attributed to the reduced mobility of the polymer resulting from strong interactions between small water molecules and the polymeric matrix at low RH. Beyond 75% RH, the permeabilities increased considerably. In addition, the barrier performance of the EVOH films was found to be dependent on their ethylene content and orientation. From the experimental data, semiempirical equations describing oxygen transmission rates (O₂TR) as functions of RH and temperature were developed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1866–1872, 2001     

Initiated chemical vapor deposition of polyvinylpyrrolidone-based thin films

Initiated chemical vapor deposition (iCVD) is used for the first time to deposit a non-acrylic carbon-based polymer, polyvinylpyrrolidone (PVP). PVP is known for its hydrophilicity and biocompatibility, and its thin films have found many applications in the biomedical community, one of which is as antibiofouling surfaces. From vapors of 1-vinyl-2-pyrrolidone (VP) and tert-butyl peroxide (TBPO), iCVD produces PVP thin films that are spectroscopically identical to bulk PVP without using any solvents. iCVD works by selectively fragmenting gaseous TBPO with heat to create radicals for initiation of polymerization. This selectivity ensures that the monomer VP does not disintegrate to form species that do not conform to the structure of PVP. Fourier-transform infrared (FTIR), nuclear magnetic resonance, and X-ray photoelectron spectroscopy (XPS) show full retention of the hydrophilic pyrrolidone functional group. Number-average molecular weights range between 6570 and 10,200 g/mol. The addition of ethylene glycol diacrylate (EGDA) vapor to the reaction mixture creates a cross-linked copolymer between VP and EGDA. Films with different degrees of cross-linking can be made depending on the partial pressures of the species. Methods for quantifying the relative incorporation of VP and EGDA using FTIR and XPS are introduced. The film with the lowest degree of cross-linking has a wetting angle of 11°, affirming its high hydrophilicity and iCVD's ability to retain functionality.     

Orientation analysis of individual electrospun PE nanofibers by transmission electron microscopy

A systematic orientation analysis of individual electrospun polyethylene (PE) nanofibers was performed by transmission electron microscopy (TEM). PE nanofibers with a wide diameter distribution, ranging from 150 nm to several micrometers, and a variety of morphologies, including cylindrical, beaded- and ribbon-like fibers, were produced by the high-temperature solution electrospinning. In each fiber, crystalline orientation and its morphology were investigated by TEM. In the cylindrical fibers, development of fiber structure was strongly related to the fiber diameter. Depending on the diameter, three different structural models based on 1) the random-oriented crystalline structure, 2) the shish-kebab structure, and 3) the fibrillar structure composed of extended-chain crystals were proposed. In addition, orientation analysis of beaded fibers and that of ribbon-like fibers was also performed.     

UV-cured clay/based nanocomposite topcoats for wood furniture: Part I: Morphological study,water vapor transmission rate and optical clarity

The combination of UV-curing technology and nanotechnology has been applied in this study to synthetize by in situ photopolymerization method, UV-cured topcoats based on acrylate matrix reinforced (1 and 3 wt%) individually with three different types of commercial organoclays, namely Cloisite 10A (C10A), Cloisite 15A (C15A) and Cloisite 30B (C30B). The morphological study was quantitatively and qualitatively performed by X-ray diffraction (XRD) and by transmission electron microscopy (TEM) respectively. Water vapor transmission rate (WVTR) and optical clarity of these nanocomposites were also assessed. TEM images obtained for those UV-cured coatings respectively reinforced with C10A and C30B showed that the absence of diffraction peaks in XRD patterns of these samples do not mean necessarily a possible exfoliation of their layered silicate nanoparticles by acrylate matrix (AM). Indeed, according to TEM images, we believe that C30B was not dispersible in the AM; while both UV-cured nanocomposites containing C10A and C15A respectively seemed to have an intercalated morphology regardless of the clay content (1 and 3 wt%). All the organoclays used in this study have had an effect on both WVTR and optical clarity. The tortuous path created by the organoclay dispersed into the AM, by retarding the progress of water vapor through a sample, best explain the decrease of WVTR whereas the decrease of optical clarity is due to the light scattering by organoclay particles. Based on the above mentioned results, among the three different reinforcing agents used in this study, C10A appears to be the ideal organoclay for practical application.     

In situ capacitance measurements for in-plane water vapor transport in paint films

A capacitance technique has been adapted to study in-plane water vapor transport in paint films. The technique requires an application of electrical contact materials on the paint film surface for capacitance measurements by electrochemical impedance spectroscopy (EIS). The capacitance obtained by EIS using Cu tape and Ag paste as the contact materials are presented. A direct comparison of capacitance and gravimetric measurements demonstrates that the change in the coating capacitance is quantitatively correlated with the total amount of in-plane water vapor transported in paint films. The water vapor diffusion coefficient derived from the capacitance technique agrees with one from the gravimetric method.     

Characterization of waterborne acrylic based paint films and measurement of their water vapor permeabilities

Recently, production of waterborne coatings has increased significantly as a consequence of strict pollution regulations. Waterborne coatings are sensitive to humidity, thus, their barrier properties with respect to permeation of moisture needs to be determined. Among various coating constituents, binder represents the matrix structure and its amount relative to the amounts of pigments and fillers can significantly affect the structure, hence the barrier property of the coating. In this study, waterborne acrylic based paints applied as protective coating on interior and exterior wall of the buildings are studied. The paint samples formulated with four different binder contents by a commercial paint company are used in the experiments. We first determine unknown ingredients of the paint samples using different characterization tools and then investigate the effect of the binder content on the structure of the paint films. In addition, water vapor permeability of the paint films is measured using a permeation cell. Results show that the barrier property of the waterborne acrylic based paint films against humidity decreases with decreased binder content due to uneven distribution of the pigments, consequently, porous structure formation in the films.     

Promoting effect of water vapor on catalytic oxidation of methane over cobalt/manganese mixed oxides

Methane oxidation was conducted in a fixed bed quartz tubular reactor on a series of mixed oxides of cobalt/manganese prepared by a sol–gel method. A unique promoting effect of water vapor on methane conversion was observed for the first time on these cobalt/manganese mixed oxides calcined at 450 or 600 °C. However, these mixed catalysts lost their catalytic activities after being calcined at 850 °C. The catalytic activity of methane oxidation was significantly improved by supporting the cobalt/manganese mixed species onto the high surface area SiO₂ or Al₂O₃–SiO₂ materials. It was noteworthy that the water enhancement effect was retained on these supported catalysts.     

The effect of relative humidity gradient on water vapor permeance of lipid and lipid-hydrocolloid bilayer films

The water vapor (WV) permeance of lipid and lipid-hydrocolloid films exposed to relative humidity (RH) gradients of 100–0%, 100–50%, 100–65% and 100–80% RH were determined. The lipids used were beeswax (BW) or a blend of BW and acetylated monoglycerides (AG). Hydrocolloids used were methylcellulose, carboxymethylcellulose or ethylcellulose (EC). All films, except those containing EC, exhibited increased water vapor permeance as the RH gradient was reduced by raising the low-end RH. This increase in permeance was apparently caused by hydration and swelling across the entire film thickness, thus facilitating water movement through the film. Because of its hydrophobicity, EC likely lessened this swelling. Knowledge of the WV properties of edible films at relatively small gradients in the upper half of the RH spectrum, such as those used in this study, is useful because these conditions are far more common to foods than are the 100–0% gradients that are often used when evaluating films. Even though the WV permeance of BW and BW/AG films increased greatly at the 100–80% RH gradient, as compared to gradients ranging from 100–65% to 100–0%, they still possess WV barrier properties sufficient to be useful for foods.     

Effects of water vapor on the crystallization and microstructure manipulation of MgO ceramic fibers

Magnesium oxide (MgO) ceramic fibers are a promising candidate material in high temperature insulating area, supporting area, adsorption area and catalytic area, and most of the properties are determined by the microstructure. In the present work, MgO precursor fibers were fabricated by the centrifugal-spinning combined with sol-gel method. The thermal decomposition and crystallization process of the ceramic fibers were fully characterized. Different atmosphere preheat treatment results suggested that water vapor promoted the thermal decomposition and crystallization of precursor fibers at a lower temperature. Three kinds of particles, including round particles, polyhedron particles to plate particles in the microstructure of the fibers could be manipulated by adjusting the water vapor preheat treatment. The change in textural properties (BET surface area, pore size and pore volume) of the MgO fibers, heated at different temperatures in air after pre-heated treatment in water vapor, was analyzed. Furthermore, the formation mechanisms of the microstructures of the fibers were also presented. The easy manipulation of the microstructures of MgO ceramic fibers may make it a promising material in various areas.     

Polymer-water interactions. Origin of perturbed infrared intensities of water in polymeric systems

We explore the possible reasons for the huge infrared intensity perturbations of water molecules sorbed onto a variety of polymer surfaces. We demonstrate that strong polymer-water interactions, not always obvious from qualitative approaches to examination of the water vibrational spectra, are probably responsible for such effects. The deviations from Beer’s law for water adsorption is semi-quantitatively correlated with the level of water uptake (i.e. water activity in the polymer). This clearly demonstrates the dependence on water-polymer proximity and intermolecular interaction strength, to the extent that donor-acceptor charge transfer interactions may well control this interesting, and potentially useful, phenomenon.     

Latent orientation in the skin layer of electrospun isotactic polystyrene ultrafine fibers

Isotactic polystyrene was electrospun into ultrafine fibers from the chloroform or tetrahydrofuran solutions. Tubular or ribbon-like fibers with many small pores were obtained from the chloroform solution. Densely stacked lamellae were formed by annealing them. On the other hand, fibers with many small dents on the skin layer were obtained from the tetrahydrofuran solution. The skin layer changed into the densely stacked lamellae by annealing, while the interior was not crystallized under the utilized annealing condition. These results suggested that the skin layer of the as-spun fiber should contain a finite amount of highly-oriented molecular chains, while the interior may be unoriented. Despite the annealing, the orientation of the chains in the skin layer has been preserved and worked as the nuclei for the densely stacked lamellae.     

The effect of water vapor and inert gases on the carbon-oxygen reaction

The interpretation of the effect of small quantities of water vapor on the kinetics of the gasification of carbon by oxygen is re-examined in the light of recent studies describing conditions under which an explosive reaction may occur in this mixture of reactant and products: oxygen, carbon monoxide, carbon dioxide and water vapor. The generation of water from residual hydrogen in the carbon is a key factor in the development of explosive conditions through a radical chain reaction in the gas phase. Some effects of inert gases on the rate of gasification are also explained by their influence on gas-phase reactions through stabilization of radicals and inhibition of diffusion of atoms to the walls. It is concluded that the possibility of gas-phase radical reactions in mixtures of carbon monoxide and oxygen, which depends critically on the presence of water vapor, should be considered in the interpretation of the kinetics of carbon gasification by oxygen.     

Role of water vapor in oxidative decomposition of calcium sulfide

CaS formed from the CaO sorbent during desulfurization in coal gasifiers has to be converted to CaSO₄ before disposal. CaS is mainly decomposed to CaO and SO₂ by O₂ and then CaO is converted to CaSO₄ by SO₂ and O₂. The role of H₂O in the oxidative decomposition of CaS with O₂ was studied using reagent grade CaS and H₂¹⁸O. The following results were obtained: (1) there is a synergistic effect of H₂O and O₂ on the oxidative decomposition of CaS to CaO and SO₂; (2) H₂O reacts with CaS to form CaO, SO₂ and H₂ in the absence of O₂; (3) the oxidative decomposition of CaS to CaO and SO₂ occurs stepwise; (4) H₂O directly reacts with CaS in the presence of O₂; (5) H₂O plays an important role in the oxidative decomposition of CaS even if the O₂ concentration is high.     

Investigation of post-spinning stretching process on morphological, structural, and mechanical properties of electrospun polyacrylonitrile copolymer nanofibers

Electrospun polyacrylonitrile (PAN) copolymer nanofibers with diameters of ∼0.3 μm were prepared as highly aligned bundles. The as-electrospun nanofiber bundles were then stretched in steam at ∼100 °C into 2, 3, and 4 times of the original lengths. Subsequently, characterizations and evaluations were carried out to understand morphological, structural, and mechanical properties using SEM, 2D WAXD, polarized FT−IR, DSC, and mechanical tester; and the results were compared to those of conventional PAN copolymer microfibers. The study revealed that: (1) the macromolecules in as-electrospun nanofibers were loosely oriented along fiber axes; although such an orientation was not high, a small extent of stretching could effectively improve the orientation and increase the crystallinity; (2) most of macromolecules in the crystalline phase of as-electrospun and stretched nanofibers possessed the zig-zag conformation instead of the helical conformation; and (3) the post-spinning stretching process could substantially improve mechanical properties of the nanofiber bundles. To the best of our knowledge, this study represented the first successful attempt to stretch electrospun nanofibers; and we envisioned that the highly aligned and stretched electrospun PAN copolymer nanofibers could be an innovative type of precursor for the development of continuous nano-scale carbon fibers with superior mechanical strength.     

Effect of adsorbed water vapor on Mg intercalation in electrochromic a-MoO3 films

The nature of interaction between the water vapor and MoO₃ thin films has been investigated using infrared spectroscopy technique. On prolonged exposure to high humidity conditions, presence of free water vapor and formation of Mo–OH bonds was detected in MoO₃ films. Films exposed to high humidity for various durations and freshly deposited were intercalated with Mg ions under identical experimental conditions. Free water vapor and Mo–OH bonds did not prevent/hinder the intercalation process. The diffusivity (D) of Mg ions in MoO₃ films has been estimated using Galvanostatic Intermittent Titration Technique (GITT) as a function Mg concentration. It is observed that D values are lesser in case of films exposed to humidity. Cyclic voltammetry studies carried out on films revealed reduced cyclability in exposed films compared to that of fresh films.     

Fabrication of microstructure controlled cathode catalyst layers and their effect on water management in polymer electrolyte fuel cells

Although cathode catalyst layers (CCLs) are at the center of water management in polymer electrolyte fuel cells (PEFCs), the understanding of water movement in CCLs and their roll on fuel cell performance is still limited. In this present study, several CCLs with controlled microstructure, including main pore size, pore volume and porosity ranging from 30 to 70 nm, 0.443 to 0.962 cm³/g_Pt/C, and 45.4 to 64.4%, respectively, were prepared by changing the hot-pressing pressure in a decal process, and their water management ability and cell performance were evaluated. The electrochemical analyses reveal that, as the pore size and pore volume of CCLs increase, the diffusion resistance mainly arising from water accumulation in the pores is evidently reduced by capillary water equilibrium, which leads to better cell performance. Water balancing between accumulation and discharging in the pores also depends on the CCL pore structure, and the CCLs with greater pore sizes and larger pore volumes reveal more stable cell performance by better water management in steady state operation, even under extremely humid conditions. Based on these MEA technologies such as fabrication of CCLs, further study will be performed to understand microscopic phenomena in nano pores of CCLs by combining the experimental approach with CCL numerical modeling.