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.     

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.     

Surface analytical study of self-assembled nanophase particle (SNAP) surface treatments

The recent discovery of a Self-assembled NAnophase Particle (SNAP) process of forming functionalized silica nanoparticles in situ from hydrolyzed tetramethoxysilane (TMOS) and glycidoxypropyltrimethoxysilane (GPTMS) in an aqueous sol–gel process, and then crosslinking the nanoparticles to form a thin, dense, protective film on Al aerospace alloys, is an excellent example of a nanoscience approach to coatings. To investigate the surface chemistry of the SNAP films, X-ray photoelectron spectroscopy (XPS) was utilized to obtain detailed chemical state information on the coating constituents. During the course of these surface analytical studies, a charge referencing method from which accurate and reliable photoelectron peak binding energies could be determined was developed.

Use of an internal standard allowed the spectra to be charge referenced, and the referenced data enabled accurate identification of chemical states in the SNAP coatings. Results indicate that the Si bonds present in the SNAP film are a combination of the bonds in the individual precursors TMOS and GPTMS. These data support the concept that the nanosized siloxane macromolecule structures of the precursors are retained through the coating application process and basically comprise the film. There is little evidence of other chemical reactions occurring.

This chemical state information was verified by a silicon chemical state plot and the calculated modified Auger parameter, which fell between the precursors’ Auger parameter values. The XPS peak data and Auger parameter data are both self-consistent and consistent with the presented model of the SNAP film. The XPS analysis provides an overview of the film chemistry and changes that could occur during the processing. Researchers involved in the XPS analysis of organic coatings could benefit greatly from the demonstration and application of good charge referencing techniques.     

Plasma treatment of automotive steel for corrosion protection – a dry energetic process for coatings

In the thrust of pursuing new environmentally friendly technology for automotive application, a new corrosion protection coating system for automotive steel has been developed through interface engineering affected by an energetic plasma process. The plasma treated coating system outperformed the current phosphated galvanized steel system in scab corrosion tests. In the plasma process, the steel substrate was subjected to plasma cleaning and in situ plasma polymerization deposition. Plasma of a mixture of argon and hydrogen was created to remove the surface contaminants and the inherent oxide layer. A very thin film (50–100 nm) was then deposited by a plasma generated from alkylsilanes (e.g. trimethylsilane (TMS)). The interface can be so designed that strong corrosion-resistant interfacial bonds such as Fe–Si, Fe–C, and Si–C can be obtained. The interfacial chemistry involved in the plasma process and corrosion reaction are characterized by reflection absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), and sputter neutral mass spectroscopy (SNMS).     

Structure, thermal-stability and reducibility of Si-doped Ce–Zr–O solid solution

The Si-doped Ce–Zr–O solid solutions have been prepared by the reverse microemulsion method. The effects of Si and its content on the structure characters, thermal-stability and reducibility of the Ce–Zr–O solid solution have been studied by N₂ adsorption, XRD, laser Raman (LR), TPR, FT-IR, NMR and XPS methods. The results indicate that, there are the bonds of Si–O–M (Ce or Zr) in the Ce–Zr–Si–O solid solutions, and the presence of Si can increase obviously the surface area, thermal-stability, crystal lattice distortion rate, and reducibility of the solid solution. The surface area of the sample with 20 wt.% Si reaches 153 m² g⁻¹ after being calcined at 900 °C for 6 h. The Ce–Zr–O solid solution with 5.2–10 wt.% Si shows excellent thermal-stability and reducibility.     

Nanostructured sol–gel derived conversion coatings based on epoxy- and amino-silanes

Inorganic/organic hybrid conversion surface coatings for long-term protection of aluminum alloys against atmospheric corrosion have been developed based on a unique self-assembled nanophase particle (SNAP) coating process. Nano-particles with peripheral epoxy functional groups are pre-formed in an aqueous sol–gel process and then assembled and crosslinked upon application on the substrate surface. Mono-, di-, and tri-functional amino-silanes have been used as crosslinking agents. Corrosion resistance properties of these hybrid nanocomposite coatings studied by a variety of electrochemical testing methods including electrochemical impedance spectroscopy, scanning vibrating electrode technique, and potentiodynamic scan method, indicate excellent barrier protection performance of the coatings. For comparison, coatings crosslinked with amino-silanes offer significant improvement in coating performance over the previously described SNAP formulations with a conventional amine crosslinker—diethylenetriamine.     

Three-dimensional compositional gradients in water-borne urethanes and latexes: spectroscopic approaches

Stratification of individual components in thermosetting coatings plays an important role during film formation and affects a number of macroscopic properties. This behavior is particularly pronounced for water-borne coatings, and therefore molecular level understanding of interactions in latexes and responsiveness of individual components during coalescence of water-borne polyurethanes are of particular interest. The presence of macromolecular arrangements and interactions among various components near the film–air (F–A) and the film–substrate (F–S) interfaces can be effectively monitored using attenuated total reflectance (ATR) and step-scan photoacoustic (SSPA) Fourier transform infrared (FT-IR) spectroscopy. Both approaches are capable of obtaining information from various surface depths and complement each other if one seeks molecular level information from 0 to 150 μm into the film. In this presentation combined ATR and PA information along with IR and/or Raman surface imaging will be presented to establish three-dimensional representation of polyurethane film formation and the effect of copolymer/polymer blend latexes on coalescence processes in Sty/n-BA.     

Combustion synthesis of Si3N4–TiN composite powders

Si₃N₄–TiN composite powders have been prepared by self-propagating high-temperature synthesis (SHS). Y₂O₃ plays a dominant role on the formation of rod-like Si₃N₄ whiskers. Full nitridation occurred of the Si–Ti mixtures. In the SHS process, TiN formation preceeds that of Si₃N₄. The formation of TiN inhibits the grain growth of rod-like Si₃N₄ crystals. Meanwhile some solid solutions between Si₃N₄ and TiN have formed.     

Multiscale modeling of the effects of unlimited four‐membered ring formation on sol‐gel silica film molecular structure

A modified multiscale model of a sol‐gel silica drying process is presented that treats first‐shell substitution effects and unlimited four‐membered ring cyclization. The inclusion of four‐membered (8‐atom) rings allows the model to simulate the formation of the tetrasiloxane rings and cubic silsesquioxane cages known to be prevalent components of sol‐gel silica systems, as well as a full range of other ring‐containing structures. The polymerization process is treated using a dynamic Monte Carlo method where a discrete population of one million monomers evolves according to kinetic rules, including unimolecular‐like closure of three‐bond blocks. Compared with prior simulations with extensive cyclization that allowed only three‐membered rings, the molecular structures formed with unlimited four‐membered ring are more complex, and the occurrence of “skinning” (the rapid formation of a gel only at the surface of the film) is more pronounced and leads to more severe structure gradients. © 2012 American Institute of Chemical Engineers AIChE J, 59: 707–718, 2013     

The superlattice structure and self-adaptive performance of C–Ti/MoS2 composite coatings

To improve the self-adaptability of MoS₂ coating in different environments, the coatings were doped with functional C and Ti by unbalanced magnetron sputtering system. The clear superlattice structure with minimal modulation period was investigated by High Resolution Transmission Electron Microscope (HRTEM). The co-doped coatings have better mechanical properties due to the special structure and the formation of C–Mo, Ti–S and Ti–O bonds, and better lubrication performance in both high humidity and vacuum than those of the single-doped ones. The doped Ti not only facilitates the formation of the MoS₂ (002) basal plane, but also improves the oxidation resistance of the composite film. The degree of friction-induced graphitization on the wear tracks and the quality of transfer films on the wear scars are key factors affecting the lubrication performance of the composite film. In the high-humidity environment, the reasonable doping elements can promote the formation the high-quality transfer film by interacting with H₂O water molecules, which will benefit the lubrication of the coating better. Our findings deepen the understanding of MoS₂ composite coating and provide a new solution for improving the self-adaptability of the coating.     

Preparation of microstructure compatible porous supports by sol–gel synthesis for catalyst coatings

To enhance the inner surface of microchannel reactors, porous metal oxide coatings based on alumina, silica, and titanium oxide have been developed as support for catalytically active components. The oxides were made by the sol–gel process. Thin film coating was accomplished by dip coating using an Fe-based alloy as support. The influence of the sol composition, sol viscosity, and thermal treatment procedure on the surface enhancement factor and the porosity of the coating was investigated. The resulting surface enhancement factor reached 800 m²/m².     

Curing of non-pigmented alkyd coatings detected by in-situ photoacoustic fourier transform infrared spectroscopy (PA FT-IR)

Photoacoustic Fourier transform infrared spectroscopy (PA FT-IR) has been utilized for in-situ measurements of the curing processes in alkyd coatings. The spectroscopic data have been correlated to the weight loss measurements and indicate that ca. 33% w/w (solvent-based) highly volatile solvent molecules remain in the coating and may be involved in the curing process. Analysis of infrared spectra indicates that both film formation and degradation of the coating occur almost simultaneously. Alternative mechanisms for network formation are proposed.     

On the plasma-chemical synthesis and/or regeneration of ultradispersed catalysts for ammonia production

The equilibrium compositions of multi-component heterogeneouse systems N–Al–O–Ca–Mg, Cu–Zn–Al–O and Fe–Al–K–Ca–Si–O in gaseous and condensed state are established. Further, we have developed a three-dimentional model dealing with the motion, heating, melting and evaporation (thermal destruction) of micron-size particles—(5–60 μm—Fe, Fe₂O₃, Fe₃O₄, FeO, Ni, NiO, Cu, Al, Al₂O₃, CaO, Mg, MgO) in an axial-symmetric plasma-chemical reactor (PCR). Based on the model calculations, we have designed and build plasma-chemical installations and used them to study the mechanisms of preparation of catalysts (and regeneration of spents of deactivated catalysts) for reforming of natural gas (steam conversion of CH₄), for low-temperature steam conversion of CO and for synthesis of NH₃.

A physical-chemical analysis of plasma-chemically synthesised (PCS) and/or regenerated ultra-dispersed catalysts in electric-arc low-temperature plasma conditions has been performed by X-ray structural and phase analysis, electron microscopy, Mössbauer spectroscopy, derivatographic, thermal-magnetic, chemical and analytical methods, by studing the dynamics and kinetics of formation of the active surface due to the reduction (the “plasma” catalysts undergo reduction 2–5 times faster than their respective commercial analoges and show activity by 15–20% higher than that of the conventional catalysts).     

Enhancing coating functionality using nanoscience and nanotechnology

Nanoscience is currently enabling evolutionary changes in several technology areas but new paradigms will eventually have a much wider and revolutionary impact. In the area of coatings, new approaches utilizing nanoscale effects can be used to create coatings with significantly optimized or enhanced properties. The ultimate impact of nanoscience and nanotechnology in the area of coatings, and other potential application areas, will depend on our ability to direct the assembly of hierarchical systems that include nanostructures. Two fundamentally different approaches to direct self-assembly are discussed. One approach involves ordering existing identifiable components into the desired coating or structures. The second approach involves the formation of new structures during the coating process. Potential impacts of nanostructure properties on film characteristics and applications are discussed with a focus on coating reactivity, corrosion resistance, strength and durability.     

Novel copper immersion coating on magnesium alloy AZ91D in an alkaline bath

Copper immersion coating of magnesium alloys has, to date, been conducted only in acidic baths. This article describes a novel alkaline bath for copper immersion coating on AZ91D magnesium alloy. Prior to the coating process, a chemical etching process of the magnesium substrate was optimized using orthogonal experimental methodology. The copper immersion coating was then investigated with regard to the effect of pH and fluoride content in the deposition bath. It was revealed during the coating process that an increase of pH and fluoride content led to a surface film formation on the magnesium substrate. The surface film formation occurred simultaneously with copper reduction, rendering a controlled magnesium dissolution, thereby a controlled copper deposition. With optimized conditions of chemical etching and immersion coating processes, uniform copper deposits were achieved.     

Experimental evidence of the interface/interphase formation between powder coating and composite material

The present study focuses on the physicochemical characterization of the interface between an epoxy/polyester-based powder coating and an epoxy/thermoplastic-based composite material. An in-mold process has been used for the powder coating deposition on carbon/glass fibers reinforced composite substrates, and different methods have been performed to characterize the interface. We evidence here the strong dependence of the structural, chemical and thermal behaviors of the interface on the cure time/temperature conditions at which the powder coating was crosslinked. At a low crosslinking rate of coating (∼48%), experimental results reveal the development of a large heterogeneous organic/organic interphase between coating and substrate. However, thin interphases have been detected when the crosslinking rate goes beyond 69%. Besides, a phase segregation of thermoplastic additive within the composite matrix was identified in the formation of this interphase. Energy-dispersive X-ray spectroscopy (EDS) as well as FTIR/Raman experiments enabled us to put in evidence the diffusion process of the thermoplastic additive toward the coating. From thermal analysis, glass transition temperature Tg for both components was observed, which confirm the proposed mechanism. This study highlights the importance of the thermal processes on the complex competition between the interdiffusion between two epoxy matrix and the existence of thermoplastic toughening agent at the interface of powder coating and composite material.     

氧化铝粒子化学气相包覆氧化钛的过程研究

探讨了回转炉化学气相淀积（CVD）包覆新工艺,利用Ti（OC_4H_9）_4水解,在Al_2O_3粒子表面实现了TiO_2包覆,研究了包覆过程特征。结果表明,TiO_2在Al_2O_3粒子表面的包覆是成功的,并具有一定的致密性,包覆量为0.1%～10%;化学气相包覆过程中既存在成膜包覆,又存在成核包覆,通过控制反应物浓度等方法可以强化成膜包覆,提高包覆效果。     

Film formation of coatings studied by diffusing-wave spectroscopy

The coating industry is facing nowadays a growth in environmental regulations, and imposed substitution of suspected toxic components or solvents and a constant need for improving performances. The new formulations undergo new film-formation processes, influencing the coating performance and appearance.

We have studied the film-formation process of several coating systems (water-based and solvent-based) using a new optical technology based on diffusing-wave spectroscopy. This unique and simple technique allows a non-intrusive monitoring of the drying process on the appropriate substrate (concrete, metal, plastic, wood, etc.). The kinetics of film formation, displayed in real-time, provide a new vision of the successive steps of the mechanisms taking place (evaporation, packing, etc.) as well as accurate information on drying times such as open-time, for an in-depth characterization of the film-formation process.     

乳化型水性固化剂的固化行为与成膜机理探讨

乳化型水性固化剂在成膜过程中兼具乳化与固化功能,直接影响了水性涂料的固化成膜和涂层性能。比较了水性环氧树脂涂料与溶剂型环氧树脂涂料的成膜过程,探讨了双组分水性环氧树脂涂料的成膜机理,分析了影响水性环氧树脂涂料成膜的因素;环氧乳液的粒径、固化剂与环氧树脂的相容性以及水从环氧乳液中的挥发速率等都会影响水性环氧树脂涂料的成膜过程,进而影响涂膜的物理化学性能。     

Synthesis and characterization of urea bridged hybrid periodic mesoporous organosilica materials

Urea bridged organic–inorganic hybrid mesoporous SiO₂ materials (U-BSQMs) were synthesized through a sol–gel procedure by co-condensation of bis(triethoxysilyl propyl) urea (BSPU) under basic conditions using cetyltrimethylammonium bromide (CTAB) as organic template. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the mesoporous structure of the sample. Fourier-transform infrared spectroscopy (FT-IR), solid state CP-MAS NMR spectroscopy of ²⁹Si (²⁹Si CP-MAS NMR) and ¹³C (¹³C CP NMR) indicated that most of the Si–C bonds are unbroken during the synthesis process. The N₂ adsorption–desorption results revealed that these hybrid mesoporous SiO₂ materials have bimodal distribution of pores with pore diameters of 2.4 and 3.8 nm, respectively. Thermogravimetric analysis (TG) demonstrated that about 16% Si–C bonds have been broke during the synthesis progress. This kind of material is expected to find possible application in ion supporting, drug delivery and catalysis.