Inorganic/organic hybrid coatings for aircraft aluminum alloy substrates

A series of water-based stable sol–gel systems have been developed. Various functional groups including amino, epoxy, vinyl, and allyl groups can be incorporated into the sol–gel network to interact with organic polymer resins. The solid content of these sol–gel-based coating formulations varies from 2.5 to 45%. The sol–gel coating of alumina–silica networks derived from low solid content solutions (2.5%) has been developed and evaluated to replace the current conversion coating pretreatment process. Sol–gel coatings derived from the high solid content solutions (17–45%) have shown excellent mechanic strength, good adhesion, and provide corrosion protection of the aluminum substrate when cured at elevated temperatures. Sol–gel/epoxy resin hybrid coatings have been formulated and studied. The hybrid coating showed enhanced mechanical strength such as hardness and abrasion resistance. When cured at elevated temperatures (80°C), all of the hybrid coatings studied passed wet adhesion testing. Some of the hybrid coatings pass wet adhesion testing when cured at room temperature. However, water-sensitivity remains for most of the room temperature cured hybrid coatings.     

An organically modified zirconate film as a corrosion-resistant treatment for aluminum 2024-T3

Current coating systems for aircraft corrosion protection are based on a traditional chromate surface treatment, primer, and topcoat. The Air Force is seeking environmentally benign corrosion-resistant surface treatments for aluminum-skinned aircraft as a replacement for environmentally hazardous surface treatments involving chromates. Performance of replacement treatments must be able to satisfy the durability needed for dramatically extended lifetimes, be compatible with present and future environmental requirements, and be easily integrated into current primer/topcoat paint systems.

Organically modified zirconate sol–gel films were investigated as an environmentally compliant replacement for chromated surface treatments, which included functionalized components to tailor the chemistry at both the aluminum oxide substrate/surface treatment interface and the surface treatment/topcoat interface. Sol–gel films were applied to aluminum 2024-T3 alloy coupons, and the resulting film chemistry was investigated. FTIR was used to identify organic components in the film and X-ray photoelectron spectroscopy was used to investigate the interface chemistry. The result of the chemically modified sol–gel synthesis was a coating in which a concentration gradient was formed at the surface, which is discussed. The corrosion protection attributes of these films was also studied and the electrochemical evaluation of sol–gel films will be discussed, both as stand-alone coatings and as part of a full coating system.

Organically modified sol–gels exhibited significantly better protection in terms of barrier properties in comparison to a typical chromate-based processes. The resulting data from evaluations of sol–gel produced coatings show promise towards the goal of producing a robust chemical interaction/bonding of such corrosion-resistant coatings on the surface of aluminum-skinned aircraft without the use of environmentally hazardous chromate agents.     

Transparent UV curable antistatic hybrid coatings on polycarbonate prepared by the sol–gel method

UV curable, hard, and transparent organic–inorganic hybrid coatings with covalent links between the inorganic and the organic networks were prepared by the sol–gel method. These hybrid coating materials were synthesised using a commercially available, acrylate end-capped polyurethane oligomeric resin, hexanedioldiacrylate (HDDA) as a reactive solvent, 3-(trimethoxysilyl)propoxymethacrylate (MPTMS) as a coupling agent between the organic and inorganic phase, and a metal alkoxide, tetraethylorthosilicate (TEOS). The materials were applied onto polycarbonate sheets and UV cured, followed by a thermal treatment to give a transparent coating with a good adhesion and abrasion resistance. The high transmission and the thermogravimetric behaviour indicate the presence of a nanoscale hybrid composition. In a taber abrasion test, uncoated polycarbonate sheets exhibit a 48% decrease in light transmittance at 633 nm after 300 wear cycles, whereas the hybrid coating system containing 10 wt% silica shows only 10% decrease in light transmittance. For obtaining antistatic coatings, an intrinsically conductive polymer (ICP) was added to the optimised coating formulation. It is shown that the surface resistivity of the organic–inorganic hybrid coating can be reduced from 10¹⁶ to 10⁶ Ω for a high concentration of ICP in the coating formulation.     

Ternary evaluation of UV-curable seed oil inorganic/organic hybrid coatings using experimental design

Inorganic/organic hybrid coatings were prepared using epoxidized linseed oil with combinations of the two sol–gel precursors (titanium(IV) isopropoxide, tetraethyl orthosilicate), and a telechelic silicate based on a modified oligomeric caprolactone. The coatings were UV-cured with sulfonium initiators which concomitantly cured the epoxy functional organic phase and the sol–gel inorganic phase to form a co-continuous inorganic/organic system. A ternary experimental design was employed to elucidate the influence of inorganic modifier on the mechanical properties of the inorganic/organic hybrid coatings. Small angle X-ray scattering (SAXS) was used to evaluate radius of gyration of the metal-oxo-cluster. Various coating properties, such as hardness, impact resistance, adhesion, solvent resistance, and surface energy were investigated as a function of sol–gel precursors. Inorganic/organic hybrid coatings containing both tetraethyl orthosilicate and the modified caprolactone resulted in improved hardness and solvent resistance with no loss of impact strength. The inclusion of titanium(IV) isopropoxide in to the systems resulted in a systematic reduction in the coatings properties. This was attributed to inhibition of the organic crosslinking process as a consequence of absorption of ultraviolet light by the titanium-oxo-clusters.     

Nanostructured coatings approach for corrosion protection

Nanostructured surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach were investigated as potential replacement for chromate-based surface treatments on aircraft aluminum alloys. In the traditional sol–gel method, hydrolysis-condensation processes are followed by condensation polymerization upon film application. This process sequence provides a low temperature route to the preparation if thin coatings which are readily applied to most metallic substrates. The recent discovery of a method of forming functionalized silica nanoparticles in situ in an aqueous sol–gel process, and then cross-linking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This Self-assembled Nanophase Particle (SNAP) process can be used to form thin, dense protective organic surface treatment coatings on Al aerospace alloys. The ability to design coating components from the molecular level upward offers tremendous potential for creating multifunctional coatings.

The important components of Al alloy corrosion inhibition by chromate are storage and release of Cr^VI species, inhibition of cathodic reactions (primarily oxygen reduction), and inhibition of attack at active sites in the alloy. Unlike chromate-based treatments, current SNAP coatings provide barrier-type corrosion resistance but do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. In this study, organic inhibitors were tested for corrosion protection of aluminum alloys in combination with the (SNAP). Scanning Vibrating Electrode Technique, anodic polarization, electrochemical impedance spectroscopy, and salt spray test were used to study this new approach for chromate replacement.     

Coating films prepared by photoreactions of the surface‐modified diamond powder and PET film with a binder agent

The surface‐modified diamond and PET film underwent photopolymerization rapidly with a binder agent to afford coating films of interpenetrating network (IPN) structure. The coating films thus formed exhibit higher tensile strength, thermal stability, and adhesion strength to the PET film. The inert surfaces of pristine diamond (PD) and PET film were modified by different chemicals and procedures to introduce epoxide and methacryloyl groups, respectively, on their surfaces. A coating agent consisting of an epoxide group containing modified diamond (called ED), a binder agent, and photoinitiators was prepared. After applying the coating agent to the substrate (a glass plate or a methacryloyl group containing PET film, MMA‐PET) and degassing under reduced pressure, the thin film of the coating agent was exposed to UV light (λ_max; 365 nm) at room temperature to yield a coating film of IPN‐structure. The tensile strength and thermal properties of the ED‐containing free coating film (called free film) increased with the amount of ED embedded, whereas the strength of the PD‐containing free film decreased with the amount of PD embedded. The adhesion strength of the coating film on the MMA‐PET improved significantly by the free radical polymerization of the methacryloyl groups on the MMA‐PET and the acrylate resin in the binder agent. The surface photoreactions of ED and MMA‐PET with the binder agent were confirmed by modeling. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

Physicochemical properties and photocatalytic activities of TiO2-films prepared by sol–gel methods

The physicochemical properties, the photocatalytic activities in aqueous solution and the adhesion properties of supported TiO₂ films prepared by different sol–gel methods have been studied. The thickness, the TiO₂ loading and the photocatalytic activities are influenced by the nature of the stabilising agent. By contrast, the nature of the organic titanium precursor, as well as the solvent and the absence of stabilising agents are determining for the resulting photocatalytic activities. Titania-sol generated by non-controlled hydrolysis of titanium isopropoxide was used to determine the influence (i) of number of coating, (ii) of the calcination temperature and (iii) of the nature of support on the photocatalytic activity under direct and backside irradiation. The higher the coating number, the thicker the TiO₂-film and the higher the photocatalytic efficiency. the optimum calcination temperature was found to be 400 °C. Migration of cationic species into TiO₂-films and the decrease of thickness at higher temperatures lead to the decline of activity.     

On the modification of photocatalysts for improving visible light and UV degradation of gas-phase toluene over TiO2

The photocatalytic behavior of different TiO₂-based photocatalysts was reported for gas-phase toluene removal under both UV and visible light illumination, and compared to that of commercial P25 (Degussa) TiO₂. Promotion by sulfates and the use of nanosized anatase TiO₂ were reported to strongly increase the toluene removal efficiency under UV illumination. Nanosized-anatase was prepared by a protecting group sol–gel synthesis using hexamethyldisilazane as crystallite growth inhibitor. Sulfates played a double positive role, with photogenerated electrons transfer effects limiting charge recombination and as repulsive species for strongly adsorbed aromatic intermediates that act as poisons. The decrease in particle size obtained on nanosized anatase TiO₂ (5 nm) yielded a considerable enhancement in the toluene removal efficiency. Pure high surface area rutile has been synthesized at low temperature by a polyethylenglycol-containing sol–gel method for visible light activation purposes. A two-way semiconductor coupling phenomenon, consisting of a reciprocal electron/hole transfer between two visible light-activated oxides, rutile TiO₂ and WO₃, was proposed to explain the large gain in efficiency when adding low amounts of WO₃ to rutile TiO₂.     

TiO2 optical coating layers for self-cleaning applications

TiO₂ films deposited by various coating techniques were investigated for self-cleaning applications. The optical coating layers of TiO₂ films prepared from a sol–gel precursor were deposited on glass substrates using spin coating, dip coating and screen printing techniques. Effects of film deposition techniques on crystal structure, microstructure, thickness, photocatalytic activity, hydrophilicity and optical properties of the films were investigated using XRD, AFM, SEM, surface profilometer, UV–vis spectrophotometer and contact angle measurement. Dip coating the TiO₂ optical film two and three times resulted in superhydrophilic surfaces. Increasing number of dipping times was found to increase the photocatalytic activity.     

Surface hydrophilic–hydrophobic property on transparent mesoporous silica thin films containing chromium oxide single-site photocatalyst

Using a sol–gel/spin-coating method with organic template, the transparent mesoporous silica thin films containing chromium oxide moieties can be prepared on quartz plate. The spectroscopic characterization has revealed that these thin films contain isolated and tetrahedrally coordinated chromium oxide moieties (single-site photocatalyst) in their frameworks. Even before UV light irradiation these Cr-containing mesoporous silica (CrMS) thin films have demonstrated a hydrophilic surface property more strongly than Cr-containing non-porous silica films and mesoporous silica thin films without chromium oxide. Furthermore, the Cr-containing mesoporous silica thin films showed the super-hydrophilic property not only under UV light irradiation but also even under visible light irradiation from a fluorescent lamp. In the presence of ethylene gas, the polyethylene was successfully formed on the surface of the CrMS thin film under UV light irradiation. After the formation of polyethylene the surface property of the CrMS thin film was converted into hydrophobic while keeping its transparency. The isolated and tetrahedrally coordinated chromium oxide moieties are responsible for these photo-induced surface reactions.     

The self-assembled nanophase particle (SNAP) process: a nanoscience approach to coatings

In the corrosion protection of aluminum-skinned aircraft, surface pretreatment and cleaning are critical steps in protecting aerospace alloys from corrosion. Our recent discovery of a revolutionary new method of forming functionalized silica nanoparticles in situ in an aqueous-based sol–gel process, and then crosslinking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This coating method is called the self-assembled nanophase particle (SNAP) process.

The SNAP coating process consists of three stages: (1) sol–gel processing; (2) SNAP solution mixing; (3) SNAP coating application and cure. Here, we report on key parameters in the ‘sol–gel processing’ and the ‘coating application and cure’ stages in the GPTMS/TMOS system. The SNAP process is discussed from the formation of the nanosized macromolecules to the coating application and curing process.

The ‘sol–gel processing’ stage involves hydrolysis and condensation reactions and is controlled by the solution pH and water content. Here, the molar ratio of water to hydrolysable silane is a key factor. SNAP solutions have been investigated by NMR, IR, light scattering, and GPC to identify molecular condensation structures formed as a function of aging time in the solution. In moderate pH and high water content solutions, hydrolysis occurs rapidly and condensation kinetic conditions are optimized to generate nanophase siloxane macromolecules.

In the ‘SNAP solution mixing’ stage, crosslinking agents and additives are added to the solution, which is then applied to a substrate by dip-coating to form the SNAP coating. The chemical structure and morphology of the films have been characterized using X-ray diffraction (XRD), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and atomic force microscopy (AFM). SNAP films are amorphous but exhibit nanostructured assembly of siloxane oligomers at a separation of about 1.8 nm as well as molecular level ordering of O–Si–O species. The surface analytical data indicate that the films retain the basic chemical arrangement of the siloxane macromolecules/oligomers and crosslinking process creates a network of siloxane oligomers tethered together. Results of these analyses are then used to construct a model of the SNAP coating. Results of these analyses are discussed in detail.     

Visible light photodegradation of 4-chlorophenol with a coke-containing titanium dioxide photocatalyst

Photocatalysts based on titanium dioxide have been prepared by a modified sol–gel process using different alkoxide precursors. Depending on the precursor and the calcination temperature of the gels, carbon-containing catalysts with large surface areas, capable to photodegrade p-chlorophenol (4CP) with visible light (λ>400 nm), have been obtained. Photodegradation and mineralisation were confirmed by HPLC and TOC measurements. The catalysts were characterised by physisorption of argon, elemental analysis, EPR, UV/VIS, X-ray powder diffraction (XRD), FT-IR and high-resolution transmission electron microscopy (HRTEM). A highly condensed, carbonaceous species formed during calcination is responsible for the photosensitisation. When used as a photoelectrode, the appearance of a photocurrent indicated the semiconductor nature of these novel materials. The catalysts exhibit a surprisingly good long-time stability despite of the carbonaceous nature of the sensitising species. It is also shown, that commercially available TiO₂ can be photosensitised by impregnation with suitable alcohols followed by pyrolysis.     

Influence of the methods of TiO2 incorporation in monolithic catalysts for the photocatalytic destruction of chlorinated hydrocarbons in gas phase

Three different methods of fixing titanium dioxide on a monolithic, natural magnesium silicate matrix, onto-the-wall extrusion, wash-coating and sol–gel, are compared. Photo-assisted oxidation tests with chlorinated hydrocarbons, trichloroethylene (TCE) alone and in mixtures with perchloroethylene (PCE) were carried out with the monolithic photocatalysts.

Results show that the use of extruded titania monoliths provides significant advantages for best stability of anatase, porosity and resistance to loss of active phase due to erosion. Catalysts obtained by the sol–gel method maybe a good option for this application, but the coating method must still be improved.     

Observations on chromate conversion coatings from a sol–gel perspective

Chromate conversion coatings (CCCs) have been applied to aluminum substrates for many years to improve corrosion resistance and paint adhesion. In recent years the so-called sol–gel chemistry based coating systems have been examined as possible replacements for CCCs on aluminum. A mechanism is proposed for the formation of CCC on aluminum that is consistent with sol–gel chemistry principles. The conclusions illustrate important considerations for developing non-CCCs for aluminum.     

Nanocrystalline orthoferrite powders: Synthesis and magnetic properties

Nanocrystalline orthoferrite powders were synthesised at low temperatures by employing an aqueous sol–gel process. Colloidal sols and water re-dispersible gels of orthoferrite precursors were prepared by room-temperature processing of inexpensive metal salts. The average diameter (Z_av) of the precursor particles was in the size range from 4 to 7 nm; the diameters had a narrow size distribution. Water re-dispersible translucent gel monoliths were obtained by concentrating the aqueous sols followed by drying them under reduced pressure (10⁻² Torr) at room temperature. The sol–gel transition was found to be completely reversible. Nanocrystalline fine powders of orthoferrites of general formula, LnFeO₃ (Ln = La, Sm, Gd, Dy, Er, Yb and Y) having a crystallite size of about 25 nm were prepared by heating the gel precursors at 650–700 °C in air. Powder X-ray diffraction and thermogravimetry, respectively, were employed to identify perovskite phase formation and delineate thermal events that lead to gel to crystallite conversion. Magnetic measurements were carried out on the resultant powders at room temperature and down to 40 K. Nanocrystalline orthoferrite powders exhibited weak ferromagnetic behaviour, and reduced magnetic moments.     

Electromagnetic shielder compatible ZnO transparent conducting oxides hybridized with various sizes of Ag metal nanoparticles

In this paper, the effect of different sizes of Ag-nanoparticles dispersed in ZnO matrix using sol–gel method has been focused. Low-temperature crystallized ZnO thin films was achieved by sol–gel process, using zinc acetate dihydrate and 2-methoxyethanol as starting precursor and solvent, respectively. Various sizes of Ag-nanoparticles could be prepared by the spontaneous reduction method with changing the preparation temperatures and mole concentrations of Ag 2-ethylhexanoate in dimethyl sulfoxide solvent. The crystallographic structure of the Ag–ZnO hybrid film was analyzed by X-ray diffraction. Ag-nanoparticle size and optical property of Ag–ZnO hybrid films were measured by UV–vis spectrophotometer.     

Dye sensitized artificial photosynthesis in the gas phase over thin and thick TiO2 films under UV and visible light irradiation

Perylene diimide based organic sensitizers capable of electron generation under illumination were used to initiate gas phase photo reduction reactions on TiO₂ thin and thick film surfaces. For comparison [Ru(Bpy)₃]²⁺ dye sensitizers were also studied. The photo reduction of CO₂ was carried out under static conditions in the gas phase. TiO₂ films were coated on hollow glass beads via a sol–gel procedure. Pt was incorporated on the films either by adding the precursor salt in the sol, Pt(in), or by wet impregnation of calcined film with an aqueous solution of the precursor salt, Pt(on). Organic sensitizers were incorporated on the films by wet impregnation of the film from an aqueous solution. Under UV illumination, the methane yields of platinized TiO₂ thin films decreased in the following order: Pt(on)·TiO₂ > Pt(in)·TiO₂ > TiO₂. The presence of organic sensitizers inhibited the catalytic activity of pure and platinized TiO₂ thick films under UV illumination. The relative enhancement of the reaction yields in the presence of the organic sensitizer under visible light illumination depended on the presence of Pt as well as the incorporation method of Pt in the TiO₂ structure. The reaction yields were better when Pt was impregnated on the TiO₂ film than when Pt was incorporated in the Ti sol. On the other hand, pure or platinized TiO₂ under visible light illumination was totally inactive indicating the role of the organic dye in generating catalytically active electrons under visible light.     

Preparation of rough anatase films and the evaluation of their photocatalytic efficiencies

Sol–gel derived rough anatase films without controlled particle sizes were prepared by surfactant templating. The coating sol–gel was obtained by hydrolysis of Ti(OC₃H₇)₄ in ethanol/HNO₃ solution. The gel films, prepared by dipping glass substrates in surfactant solutions, were dried after immersion under an atmospheric pressure. The rough films of TiO₂ anatase were obtained after calcining at 500 °C. The resultant films were transparent, semitransparent or opaque and 136–402 nm thick. It was found that the TiO₂ films prepared from the sol–gel with surfactant showed a granular nanostructure, and they were composed of regular particles, for example; between 50 and 70 nm. The roughness of the films was found to depend on the surfactant concentration in the sol–gel solution and can show a roughness between 0.82 and near of 17 nm. The photocatalytic activity of the films for the degradation and mineralization of phenol, an industrial pollutant, in water and under 365 nm irradiation was improved by the surfactant modification. Kinetic analysis of degradation and mineralization of phenol in water were employed to evaluate the different TiO₂ films under the same experimental conditions. The global photonic efficiency for degradation and mineralization of phenol ξ_g, was calculated to facilitate comparison with a TiO₂ standard photocatalyst named Degussa P-25.     

Study of the effect of hyperbranched polyols on cationic UV curable coating properties

Cycloaliphatic epoxide‐based cationic UV curable coatings containing three different hyperbranched polyols (HBPs) were systematically formulated and characterized. Polyether polyols were found to deter epoxide conversion under low UV intensity. For high UV intensity, the cycloaliphatic epoxide conversion increased with polyol content. Coatings with HBPs had better flexibility and solvent resistance than those containing a reference triol. Compared to the triol and polyester HBPs, polyether HBPs imparted lower T_g and hardness but a better flexibilizing effect to the coatings. At higher R values, the coating solvent resistance decreased except for those with one polyether HBP. A two‐peak phenomenon was discovered in the residual thermal reaction heat curve after photoDSC experiments. These two peaks were found to be related to the thermal relaxation behavior of the photocured inhomogeneous film, and the reactivation and reaction of the ‘trapped living cationic species’. The capability of the polyol to lower the high‐temperature‐peak temperature corresponded well with the flexibilizing ability. Copyright © 2006 Society of Chemical Industry