Some aspects of wetting/dewetting of a porous medium

Various features of wetting/dewetting of porous media are examined. The phenomenon of capillary hysteresis is illustrated by a vertical capillary tube which consists of an alternating sequence of convergent—divergent conical sections. A study of the kinetics of wetting of this tube by a liquid shows that when the velocity of the liquid/vapour meniscus is plotted against the height of penetration, it oscillates about the Washburn velocity—distance curve and performs Haines jumps. A general macroscopic equation is derived for the rate of wetting/dewetting of a porous medium having randomly distributed, finely divided particles or pores. Use is made of the Forchheimer equation, which is an extension of Darcy's equation to higher Reynolds numbers. Dissipative energy terms due to internal fluid calculaton and to irreversible movements of the meniscus strongly affect the initial rate of imbibition, but as the wetting progresses the Reynolds number decreases and Washburn's equation prevails.The application of percolation theory to wetting/dewetting phenomena in porous media is studied. The use of percolation theory by Kirkpatrick and Stinchcombe to find the electrical conductivity of inhomogeneous solid mixtures is adapted to determining the permeability of a porous medium to fluid flow. It is also shown how the relation between the “precolation probability” and the concentration of “unblocked” channels or pores can be applied in calculating the capillary pressure—desaturation curve in drainage. In particular, percolation theory predicts that a threshold pressure or break-through pressure is required before a non-wetting fluid can displace a wetting fluid in a porous medium. It is often convenient to use tree-like or branching lattice networks as models of a porous medium, because these are amenable to exact solutions in regard to percolation probability and permeability. The percolation properties of porous medium models which consist of lattice networks of cylindrical channels with a distribution of cross-sections and also of randomly packed rotund particles are examined and their relevance to wetting/dewetting phenomena discussed.     

Copper-alumina nanocomposite coating on copper substrate through solution combustion

The main objective of the present research is to investigate the production of Cu-Al₂O₃ nanocomposite coating on a copper substrate using solution combustion synthesis. Solution combustion synthesis is mainly used to produce nanocomposite powders; however, in this study it is applied to produce nanocomposite coat. For this purpose, both copper and aluminum nitrates (Cu (NO₃)₂·3H₂O and Al (NO₃)₃·9H₂O) are used as oxidizers. Also, urea and graphite are respectively used as fuel to synthesize the Cu-Al₂O₃ nanocomposite and as inhibitor to prevent the oxidation of the synthesized copper. The microstructure and morphology of the nanocomposite coating, which includes 25 wt% alumina as the reinforcing phase, was studied using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy at different fuel/oxidizer ratios ranging from 0.9 to 2. The temperature variation during the process was measured as a function of time using a precise thermocouple. Finally, micro-hardness and wear tests were conducted on the nanocomposite coating. The results verified the formation of Cu-Al₂O₃ nanocomposite coating. Time-temperature curve illustrated that the highest temperature was achieved at the fuel/oxidizer ratio of 1.25. The results of the microhardness and wear resistance test showed that these properties depend heavily on the fuel/oxidizer ratio, with the best condition attained at the ratio of 1.25.     

Synthesis and slurry spray coating of barium strontium alumino silicate on SiC substrate

Barium strontium alumino silicate (BSAS); (Ba_0.6Sr_0.4Al₂Si₂O₈) was synthesized through solid state reaction between BaCO₃, SrCO₃, Al₂O₃ and SiO₂ subjected to wet milling in isopropanol for about 24 h. The sequence of the solid state reaction was studied by subjecting to DG/DTG from room temperature to 1550 °C. The crystallographic phase evolution was confirmed by X-ray diffraction of the powders calcined in the range 1000 to 1300 °C for 2 h. The monoclinic celsian phase obtained at 1300 °C, pelletized through uniaxial pressing was sinterable to 67 to 78% density in the temperature range of 1300 to 1500 °C. The density improved to 75 to 94% after ball milling for 76 h, while ZrO₂ addition further improved the density by 2%. The celcian phase of BSAS was dispersed in isopropyl alcohol, milled for about 24 h and spray coated on to plain SiC and mullite precoated SiC substrates. Sintering of coated samples and characterization for weight gain/loss, microstructure, scratch test prove that mullite + BSAS coating is more effective than single layer coating of BSAS on SiC substrates.     

An insight into pitch/substrate wetting behaviour. The effect of the substrate processing temperature on pitch wetting capacity

Pitch/substrate interactions at the mixing stage (<200 °C) were studied by means of a drop spreading wetting test. The substrates were obtained from a petroleum pitch by thermal treatment in the temperature range of 300–1900 °C. The results show that thermal treatment has a significant influence on the physical and chemical properties of the substrates, and consequently, on pitch/substrate wetting behaviour. Substrates with plastic properties (softening point below 350 °C) deform and/or agglomerate during the wetting experiment and thereby stop pitch penetrating. Moreover, the presence of aliphatic hydrogen in these substrates facilitates oxidative stabilization, which in turn facilitates pitch/substrate wetting behaviour. Substrates obtained above 400 °C are wetted by the pitch. However, the pre-graphitic order obtained on carbonization does not seem to have a significant effect on pitch wetting capability under the conditions used in this study. The oxidative stabilization of the substrates does not exert a significant influence on unfused substrates. However, in the case of plastic substrates, pitch wetting capacity is greatly affected by oxidation.     

Aluminum borophosphate glaze-coated aluminum alloy substrate: Coating properties and coating/substrate coupling

Thermal fatigue can cause irreversible damage in aluminum alloys restricting their use in the automotive industry, despite their excellent mechanical and technological properties. The application of ceramic coating is an alternative to obtain a protective barrier to improve the wear resistance at high temperatures. However, the low melting point and high coefficient of thermal expansion (CTE) of aluminum alloys limit the coating options. Thus, a suitable coupling feature can be obtained between aluminum alloys and a glazed coating. A glazed coating based on the aluminum borophosphate system was developed and applied onto an aluminum-silicon-copper commercial alloy. The coating was characterized by X-ray diffractometry, scanning electron microscopy, hardness tests, and thermal analysis. The coupling between the glazed coating and the aluminum alloy surface was studied employing optical dilatometry and optical fleximetry. A dense, good adhesion coating and presenting adequate dilatometric coupling (effective coupling temperature of 345 °C) related to the investigated aluminum alloy was obtained at 500 °C. The good compatibility of CTE between the layers (24.54 × 10^?6 °C^-1 for the substrate and 14.56 × 10^?6 °C^-1 for the coating) led to a crack-free material. For this reason, microhardness increased from 136 (aluminum alloy) to 325 HV (glazed aluminum alloy). The glazed coating can expand the use of this alloy, improving its performance and thermal efficiency. This result suggests an enormous potential of applications in the automotive industry, for instance.     

Controlled drug release through carboxymethyl‐chitosan/poly(vinyl alcohol) blend films

Carboxymethyl‐chitosan (CMCS)/poly(vinyl alcohol) (PVA) blend film was studied for an application as a coating material in the site‐specific drug delivery system. Films were prepared by blending varying amounts of 4 wt% CMCS with 4 wt% PVA, casting and drying at 50°C for 24 h. The cross‐sectional SEM micrograph of the film revealed that an increase in the amount of CMCS in the blend resulted in the film surface less smooth in the dry state and the network less uniform and more porous in the hydrated state, which became appreciable at 50%. The inclusion of CMCS in the blend also made the swelling of the films pH‐dependent, and lead to an increase in the degree of swelling with pH increase. When the permeation of three model drugs, salicylic acid, theophyline, and ornidazole, was studied using a static diffusion vessel, it followed a zero‐order kinetics and increased with an increase in the CMCS content in the blend, a decrease in the molecular weight of drug, an increase in the pH of medium, and a decrease in the film thickness. POLYM. ENG. SCI., 47:1373–1379, 2007. © 2007 Society of Plastics Engineers     

RAFT copolymerization of amphiphilic poly (ethyl acrylate-b-acrylic acid) as wetting and dispersing agents for water borne coating

Additives are very crucial in any paint formulation, as they play a significant role during various stages, when incorporated in a small amount. This additive exhibit unique molecular architecture which could be achieved by living radical polymerization, i.e., ATRP, RAFT, and NMP. An amphiphilic diblock copolymer of poly (ethyl acrylate)-b-poly (acrylic acid) (EA-b-AA) has been synthesized via reversible addition-fragmentation chain transfer (RAFT) technique at three different molecular weights, viz., 5000, 7500, and 10,000 having an acid value of 130 mg KOH/g of sample. Cumyl dithiobenzoate was synthesized and characterized as a chain transfer RAFT agent. Here, block length was kept identical and molecular weight of the copolymer was varied. The ability of additives to function as wetting and dispersing agent were evaluated by analyzing their performance properties such as mechanical, optical, chemical, and rheological at different PVC’s. Polydispersity index (PDI) less than 1.6 determined by gel permeation chromatography (GPC) indicates good control during polymerization.     

Composition fluctuations before dewetting in polystyrene/poly(vinyl methyl ether) blend thin films

We report structural development in blend thin films of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) below 200 nm in two phase region during the incubation period before dewetting using neutron reflectivity (NR) and atomic force microscopy (AFM). As was predicted by the former optical microscope (OM) and small-angle light scattering (LS) measurements on blend thin films of protonated PS and PVME [Ogawa H, Kanaya T, Nishida K, Matsuba G. Polymer 2008;40:254–62.], the NR results clearly showed that the tri-layer structure consisting of the surface PVME layer, the middle blend layer and the bottom PVME layer was formed in the one phase region. After the temperature jump into the two phase region, it was found that the phase separation of the middle blend layer proceeded in the depth direction during the incubation period before dewetting, suggesting that the dewetting was induced by the composition fluctuations during the incubation period.     

Electrical explosion spray of Ag/C composite coating and its deposition behavior

Ag–C composite coating exhibits excellent electrical and thermal conductivities, good arc mobility, and low contact resistance, making it has a good prospect in switch contact of high voltage isolators. At present, the electro-deposition method is mainly used to prepare Ag/C composite coatings. However, the production efficiency of the electro-deposition method is low and the obtained coatings are thin. The electrical explosion spraying, due to its simplicity and high efficiency, has attracted significant attention in coating preparation. In this study, a new method that confines Ag and graphite powders in a tube for electrical explosion spraying was proposed. Powder electrical explosion spraying was used for preparing an Ag/C composite coating by employing a self-designed device. The heating behavior of the powder during exploding, macroscopic morphology, micromorphology, deposition efficiency, and thickness of the coatings, as well as the deposition behavior of the sprayed particles, were investigated. After a single spraying, a dense and uniform Ag/C composite coating was obtained at the charging voltage of 13 kV and a spray distance of 18 mm. The results show that the coating area is approximately 39.25 mm², the coating thickness was 50 μm, and the deposition efficiency was 35%. the coatings have good adhesion with the substrate. the interface between the coating and the substrate appeared as an inter-diffusion of elements, which was typical of a metallurgical bonding interface. Graphite is evenly distributed in the coating. Furthermore, the underlying deposition behavior of the coating was carefully characterized.     

Scratch resistant low friction/low surface energy coating for silicon substrate

A smooth, ultrathin film of a polydimethylsiloxane (PDMS) on a silicon substrate has been prepared by spin-coating. This film gives a 0.06 dynamic coefficient of friction against paper, the lowest value ever reported for polymer–paper sliding pairs. The value is only about one-third of the coefficient of friction (0.21) between polytetrafluoroethylene and paper. The coating is not scratchable by sliding a stainless steel stylus over the surface with a pressure greater than 3.6 × 10¹⁰ dyn/cm². The film displays a surface tension of 20.5 dyn/cm. It is stable in water and propylene glycol. The film is an effective and durable solid lubricant. The surface characteristics of a spray-coated PDMS and a plasma-copolymerized thin film of perfluoropropane and 3,3,3-trifluoropropylmethyldimethoxysilane have also been investigated. Both films show much lower scratch resistance, weaker adhesion to the silicon substrate, and higher friction. The plasma film yields the same surface tension as the spin-coated PDMS. Its surface energy, however, increases after soaking in water or propylene glycol. The exceptionally low friction and the unusually high scratch resistance of the ultrathin film of PDMS are attributed to the absence of deformation and tearing components and a low adhesion components in the sliding friction mechanism.     

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.     

Multilayer GZ/YSZ thermal barrier coating from suspension and solution precursor thermal spray

Rare-earth (RE) zirconates, such as gadolinium zirconate (GZ), gained much attraction to be used for the next generation TBC. A double-layer and triple-layer TBC were deposited using the suspension and solution precursor high velocity oxy fuel (HVOF) thermal spray. A dense solution precursor GZ layer was intended to minimise the crack propagation from underneath, thereby inhibiting the CMAS infiltration. In the furnace cycling test, the double- and triple-layer coatings had a comparable cyclic lifetime. For the CMAS test, both the double- and triple-layer coatings were exposed to CMAS at 1250 °C for 30 mins. The CMAS deposits melted and infiltrated both coatings through the dense vertical cracks (DVCs). Interestingly, the GZ reacted with the molten CMAS to form a gadolinium apatite phase (Ca₂Gd₈(SiO₄)₆O₂) that was detected in the double- and triple-layer TBC. Both the double- and triple-layer TBCs succeeded in reacting with CMAS.     

Multi-walled carbon nanotube/Co composite field emitters fabricated by in situ spray coating

Multi-walled carbon nanotube(MWCNT)/Co composite powders, in which MWCNTs are homogeneously implanted in Co nanoparticles, were fabricated by polyol method. Homogeneous field emitters were fabricated by in situ spray coating of MWCNT/Co composite powders without using polymer binder and followed by sintering MWCNT/Co composite powders to form powder-powder bonding and powder-substrate bonding. Field emission properties of MWCNT/Co composite field emitters made by in situ spray coating were enhanced compared to those of MWCNT/Co composite field emitters made by screen printing due to self-activation of MWCNTs and minimization of contamination and structural defects of MWCNTs.     

Fabrication of functionally graded Fe-TiC wear resistant coating on CK45 steel substrate by plasma spray and evaluation of mechanical properties

The main challenge in production of metal matrix composite coatings is the existence of thermal residual stress in coating – substrate interface which results in delamination of the coating eventually. The aim of this paper is to enhance the tensile bond adhesion of the coating by fabricating functionally graded coating. In this regard, raw materials including titanium carbide and iron powders were milled with different compositions. From the substrate to the surface, the weight fraction of TiC particulates increased from 25% to 100%, while the weight fraction of Fe particulates decreased in mentioned direction. Moreover to make a comparison between mechanical properties of the graded coating with those of duplex and single layer coatings, a coating system comprising NiCrAlY bond coat and 100?wt% TiC top coat and a single layer titanium carbide coating were prepared as well. X-Ray diffraction method was used to identify obtained phases from each composition. In addition, microstructural properties of the coatings were investigated by scanning electron microscope. Mechanical properties such as adhesion, hardness and wear resistance were evaluated by tensile bond test, Vicker's method and pin- on- disc method, respectively. The results revealed that the FGC sample has higher coating adhesion in comparison with other coating. Moreover the wear test results showed that the FGC sample faced with less weight loss which means higher wear resistance.     

Controlled degradation of poly(ethyl cyanoacrylate-co-methyl methacrylate) (PECA-co-PMMA) copolymers

Copolymers of poly(ethyl cyanoacrylate-co-methyl methacrylate) (PECA-co-PMMA) with various compositions were synthesized by free radical bulk polymerization in an effort to control degradation and stability as well as glass transition temperature to overcome intrinsic poor processability of the poly(ethyl cyanoacrylate) (PECA) homopolymer. The copolymers were found to have an alternating random tendency, which was responsible for the efficient inhibition of the unzipping degradation from the polymer chain. Consequently, the stability of the copolymers at elevated temperature and in solution was significantly improved compared to the PECA homopolymer. The glass transition temperatures of the copolymers were lowered by the incorporation of the methyl methacrylate (MMA), thereby further widening the operating temperature range of the polymer. On the other hand, the copolymer films exhibited hydrolytic degradation in phosphate buffered saline (PBS) solution at 37 °C, which is promising for their use as novel biomaterials.     

HfC-based coating prepared by reactive melt infiltration on C/C composite substrate

Reactive melt infiltration based on alloy design is proposed in the present work for preparing HfC-based coating on C/C composite substrate. A 50Hf10Zr40Si alloy ingot was prepared and infiltrated into a C/C preform at temperatures much lower than the melting point of the alloy to obtain the HfC-based coating. An obvious layered microstructure of the coating was formed. The carbonization reactions occurring between Hf and carbon of the surface layer of the C/C composite is the primary reason for the reactive melt infiltration process to proceed at relative low temperatures. Acetylene flame test showed that the HfC-based coating protected the C/C composite from serious oxidation.     

PDMS/ZIF-8 coating polymeric hollow fiber substrate for alcohol permselective pervaporation membranes

In order to develop high performance composite membranes for alcohol permselective pervaporation (PV), poly (dimethylsiloxane)/ZIF-8 (PDMS/ZIF-8) coated polymeric hollow fiber membranes were studied in this research. First, PDMS was used for the active layer, and Torlon®, PVDF, Ultem®, and Matrimid® with different porosity were used as support layer for fabrication of hollow fiber composite membranes. The performance of the membranes varied with different hollow fiber substrates was investigated. Pure gas permeance of the hollow fiber was tested to investigate the pore size of all fibers. The effect of support layer on the mass transfer in hydrophobic PV composite membrane was investigated. The results show that proper porosity and pore diameter of the support are demanded to minimize the Knudsen effect. Based on the result, ZIF-8 was introduced to prepare more selective separation layer, in order to improve the PV performance. The PDMS/ZIF-8/Torlon® membrane had a separation factor of 8.9 and a total flux of 847 g·m^-2·h^-1. This hollow fiber PDMS/ZIF-8/Torlon® composite membrane has a great potential in the industrial application.     

Two-fluid spray atomisation and pneumatic nozzles for fluid bed coating/agglomeration purposes: A review

In fluid bed processing in the chemical, food or pharmaceutical industries, pneumatic nozzles are typically used to convert binder or coating liquids into droplets. Producing fine droplets from liquids in a gas phase is termed atomisation, and it involves complex phenomena which are not yet fully understood. This paper provides a systematic and up-to-date review of two-fluid nozzle designs and principles together with a presentation of nozzle fundamentals introducing basic nozzle theory and thermodynamics. Correlations for the prediction of mean droplet diameters are reviewed, compared and accompanied by a discussion of their use.     

Surface coating with poly(trifluoroethyl methacrylate) through rapid expansion of supercritical CO2 solutions

Rapid expansion of supercritical solutions (RESS) of poly(trifluoroethyl methacrylate), poly(TFEMA), was performed to produce ultrafine particles for spray coating application to improve the hydrophobicity of moisture-sensitive biodegradable materials. Carbon dioxide (CO₂) was used as the RESS solvent. Thermoplastic starch/poly(butylene adipate-co-terephthalate) (TPS/PBAT, 60:40 wt/wt) blend was used as the coating substrate. The objectives of this work were to determine the capacity of the RESS process for coating TPS-based material with poly(TFEMA), and to investigate the effect of RESS parameters – i.e. pre-expansion pressure and temperature (P_pre, T_pre) and poly(TFEMA) concentration – on the surface morphology and hydrophobicity of the coated materials. It was found that RESS produced poly(TFEMA) particles precipitated onto the surface of the TPS/PBAT substrate, with particle sizes ranging from 30 nm to several microns, depending on processing parameters. Rapid expansion of fluoropolymer solutions (0.3–1.0 wt%) with P_pre of 331 bar initiated from unsaturated conditions produced nanoparticles with a narrow size distribution of ∼30–70 nm; whereas larger particles with broader size distributions and a lower degree of agglomeration were obtained when supersaturated solutions were expanded with P_pre of 172 bar, especially at T_pre (80 °C) – higher than the glass transition temperature (73 °C) of poly(TFEMA). The surface coverage by the fluoropolymer increased with increasing P_pre and poly(TFEMA) concentration, but decreased with increasing T_pre. In addition, the hydrophobicity of the coated substrate, determined by water contact angle and water vapor transmission rate measurements, increased with increasing surface coverage.     

Residual thermal stresses prediction for CVD coating/substrate system based on a numerical model

For a chemical vapor deposition (CVD) coating/substrate system, an improved and optimized numerical model is established to predict the residual thermal stresses. This model takes into account both the normal and bending strains and is developed based on the concept of a misfit strain between coating and substrate. Comparisons are presented between predictions from this model and from finite element analysis. The effects of coating thickness, elastic modulus, temperature difference, and multiple deposition on the residual stresses in the coating/substrate system have also been analyzed in detail. Furthermore, some confirmatory CVD SiC experiments with different layers have also been conducted according to the analysis model. The predictions that the multiple deposition system can relieve the residual thermal stresses and reduce the microcracks in the outermost coating effectively, are consistent with the numerical model.