ABSTRACTS
The following is a list of the abstracts for papers which will be presented in THE THIRD INTERNATIONAL SYMPOSIUM ON SILANES AND OTHER COUPLING AGENTS. The listing is alphabetical by presenting author. This list is updated continually to add abstracts as they become available and to make appropriate corrections. This list may be conveniently searched by using the editor provided with most popular browsers (e.g. Microsoft Explorer, Netscape, ... etc.)
1) ICSI-CNRS ,15, rue Starcky, BP 2488, 68057-Mulhouse (France)
2) IGC Lab Carreau Rodolphe, route de Guebwiller, 68840-Pulversheim (France)
3) Wacker Chemie GmbH, Werk Burghausen, D-84480 Burghausen (Germany)
Influence of the Controlled Silylation by Trimethylchlorosilane On the Surface Properties of Some Silica Sample Assessed by IGC Techniques
For PDMS rubber applications, one has to perform a partial desactivation of the surface of the silica filler by a controlled silylation. It is then of the most importance to understand how varies the surface properties of silica that was submitted to a well-controlled silylation process. For this purpose, silica samples having different origins, that says 2 fumed silica samples, 2 precipitated samples and a xerogel, were silylated using different amounts of trichlorosilane in order to obtained a large range of coverage ratio of the silica surface by the TMS groups. Then, the surface properties were examined using both IGG in infinite dilution conditions and IGC in finite concentration conditions. On silica having a flat surface at the molecular level , like a fumed silica having a specific surface area of 130 m2/g, the variation of the surface properties with the TMS coverage ratio could be described by calling on a simple surface model based on the random scattering of the TMS groups on an hexagonal array. That is no more the case on silica exhibiting a rough surface like the silica having 300 m2/g, or precipitated silica samples that present much more complex surface structure that look more or less like polysilicon network.
1) Dow Corning S.A.,7180 Seneffe, BELGIUM
2) Department of Chemistry, De Montfort University, The Gateway,
Leicester, LE1 9BH, UK, and Institute of Polymer Technology and Materials
Engineering, Loughborough University, Loughborough, Leicestershire, LE11
3TU, UK
3) Dow Corning Corporation, Midland, Michigan, USA
Kinetics of Cure, Cross-link Density and Adhesion of Water-reactive Alkoxysilicone Sealants
When a silicone sealant is exposed to wet air, a surface skin is formed, which becomes thicker with time. Studying a "neutral" cure system based on the hydrolysis and condensation reactions of alkoxysilanes, used as cross-linking and coupling agents, it has been observed that, at constant temperature and humidity, the thickness of the cured layer is initially proportional to the square root of time, but later the gradient of such a plot increases. There are thus two regions of cure, an outer region and an inner one. Swelling in toluene, which has been measured for samples taken at various depths, shows that the cross-link density is greater in the outer regions. Single lap joints in aluminium or glass have been cured in these two regions and joints from the outer region are always stronger than corresponding ones from the inner region. This behaviour has been ascribed to the mobility of low molar mass cross-linking and coupling agents. During the initial stages of cure they migrate into the outer region, but once cure has passed into the inner region there is now a paucity of these compounds.
Measure of the water vapour permeability coefficient (P) of the cured siloxane network has shown that P is independent of humidity and stretching of the material, but a plot of logP against reciprocal absolute temperature is non-linear, showing that the heat of permeation is negative and varies with temperature. This can be explained by the formation of water-clusters. When the network is formed water is absorbed and alcohols are released. Gravimetric monitoring of the cure process yields a value of D, which equals that of methanol in the network, showing that it is the removal of methanol that controls weight loss and the overall rate of the curing reaction.
Controlled Adhesion Through Mixed Functionality Silanes and Resins
The network properties of the interphase are influenced by the silane-matrix reaction chemistry, the thickness of the deposited silane layer on the glass surface, the number of reactive versus non-reactive sites of silane on the glass, and the miscibility between the silane and the matrix phase. [1] It has been determined that the molar fraction of reactive receptor groups present on the substrate surface (y) and in the matrix phase (x) profoundly alter the fracture energy and strength of adhesive bonds. [2,3] The ultimate strength of the fiber-matrix bond is a critical aspect to consider when designing fiber reinforced composite structures for integral armor applications, as high strength is not necessarily equated to high energy absorption during a ballistic event. Mixed silane compositions can be used to control the availability of reactive sites (y) on the treated glass fiber surfaces, therefore influencing the peak load measured during pull-out. Additional control of x can also be realized though the use of mixed functionality interpenetrating polymer network (IPN) adhesives. IPN adhesives hold much promise with respect to processing ease and mechanical performance, but relatively little research has been performed on these type of adhesive systems. [4,5] The goal of this research is to understand the relationships between y, x, and bond strength using silane coupling agents and IPN adhesives which have potential use in Army composite systems.
1) N. Suzuki and H. Ishida, Macromol. Symp., 19, 108 (1996).
2) L. Gong, A. D. Friend, R. P. Wool, Macromolecules, 31, 3706 (1998).
3) I. Lee and R. P. Wool, Macromolecules, 33, 2680 (2000).
4) L. Zhang, H. Liu, S. Yan, G. Yang, H. Feng, J. Polym. Sci.: Part B: Polym. Phys., 35, 2495 (1991).
5) P. Gong and L. Zhang, J. Appl. Polym. Sci., 68, 1313 (1998).
End-Capped Fluoroalkyl-Functional Silanes
Though there have been a lot of surface fluoroalkylating agents, no agents applicable to various surfaces have not been reported. In this work, we succeeded in the synthesis of novel oligomeric silanes having end-capped fluoroalkyl group. Glass, alumina, cellulose, polyethylene, and polyester surfaces were effectively modified by those oligomeric silanes. Especially, oligomeric type silanes were more reactive and effective than monomeric type silanes. From contact angle measurements, surface free energies were reduced to 15-20 mJ/m2 for dispersive component and 1-3 mJ/m2 for polar component, respectively, and the surfaces were shown to be both highly water and oil repellent. Modified surfaces were analyzed using XPS measurements. A linear correlation was observed between the area ratio of the F1s peak to the Si2p peak for glass, and the thickness of siloxane layer on the surface is shown to be less than 5 nm for polyester. The structure of the siloxane layer is discussed in terms of a network interphase model.
It was also quite easy to add more functional groups such as hydrophilicity (FLIP-FLOP character), antimicrobacterial property, and so on.
1) Division of Chemistry, California Institute of Technology, Mailcode 210-41, 1200E California Blvd, Pasadena, CA 91125
2) University of Nijmegen, Dept. of Organic Chemistry, Toernooiveld 1, 6525 ED, Nijmegen, THE NETHERLANDS
Hybrid Silane Materials for Biosensors
In this paper we present the preparation of hybrid silane materials for biosensor applications.
The first example describes the development of biocompatible coatings based on sol-gels, which can be used as a protective coating for implantable glucose sensors. Blending the sol-gels with different organic polymers varied the properties of the sol-gel materials. The biocompatibility and the applications on biosensors were investigated.
In the second example, polysilanes which posses' electronic properties that allow for a number of applications e.g. as conductive, electroluminescent, non-linear optical and micro-lithographic materials were investigated. Many of these properties can be expected to be more readily exploited through the incorporation of the polysilane moiety into a block copolymer system, which would allow for the manipulation of the macroscopic order of the material through supramolecular assembly.
Finally, some initial results will be shown on the development of silica networks based on manipulating giant vesicles using micropipettes. Complex lipid structures can be formed starting from giant vesicles with this technique that can act as a template for the deposition of silica leading to nano- and microsized structures with possible applications in the field of biosensors.
G. Maes; Chemistry Department, KULeuven, Celestijnenlaan 200 F, B-3001 Heverlee (BELGIUM)
On the Use of Silanes for Interfacial Biosensor Design
Affinity biosensors allow the detection of affinity based interactions between complementary molecules, which occur e.g. in antibody-antigen recognition or DNA hybridisation. The presence of antibodies/antigens in the analyte can be verified by detection of the binding of these molecules to their complements (selective probes), immobilised onto a biosensor surface. For the immobilisation of biomolecules on oxide materials, silanes (especially APTES) are commonly used.
In order to meet the requirements of miniaturized affinity based biosensors regarding sensitivity, selectivity, stability and reproducibility, tailored bio-interfaces have to be developed and evaluated. To that end, our approach to immobilize antibodies on SiO2 and other metal oxides shifted from the use of ethoxysilanes (with NH2, COOH, SH and glycidoxy end-groups) to the use of chlorosilanes with bromo end-groups. These chlorosilanes are deposited from both vapor and liquid (toluene) phase.
Contact angle goniometry, XPS and grazing angle FT-IR are used to evaluate the quality of the silane films. Parallel to the interface optimization, we are identifying and validating additional analytical techniques and methods to study the density (or concentration), distribution and activity of the coupled linkers (i.e. silanes) or bio-molecules, and the type and stability of binding between the coupled molecules and the substrate.
Monosilane obtaining from silica using lithium hydride
(Abstract not yet available)
Surface Aging of Sized Glass Fibers and its Influence on the Adhesion of Composites
In composites the interphase plays a key role in determining their performance. Considerable effort was applied to develop surface treatments for fibers to enhance adhesive interaction with the surrounding polymer matrix. An unbridged gap between the surface chemistry, thermodynamic work of adhesion and the composite properties still exists that does not enable to predict composite properties from usual surface characteristics. Information available on aging behavior of surface treatments before composite formation are very scarce. Therefore, the present work highlights the importance of both the initial and aged fiber surface properties, which accounts for the much different performance of interphase, particularly subjected to combined moisture and temperature attack for differently sized glass fibers (aminopropyltriethoxy silane, lubricant, polyurethane or epoxy film former). Our recent Atomic Force Microscopy studies have revealed an application utilizable for the characterisation of both surface geometry in detail and mechanical stiffness for virgin glass fibers, aged ones, and fibers pulled out from single-fiber model composites.
Thus, the effective surface area for interfacial interaction can be determined and provides necessary information to evaluate the failure behavior more exactly. In addition, due to water and humid air treatment of sized glass fibers the surface chemistry and the adhesion strength are affected.
Taking into account effects of roughness alteration due to different sizings/aging and resulting different failure behaviour, there is a strong correlation between the results of surface chemistry and those for microcomposites (adhesion strength, energy release rate, phase angle after repeated cyclic loading).
Effect of Surface Pre-treatments on the Corrosion Protection Afforded by Silane Coatings on Aluminum and its Alloys
Silane coupling agents provide potential alternatives to the use of chromate coatings for corrosion protection on various metals. Two research directions in this context are being pursued in our laboratory. One concerns probing the chemical bonding interactions between silanes and aluminum substrates, and the other extends to using silane treatments for the corrosion protection of Al alloys. These studies emphasize the use of X-ray photoelectron spectroscopy (XPS), secondary electron microscopy (SEM), atomic force microscopy (AFM) for surface characterization, as well as the various corrosion tests. Studies on high-purity Al (99.99%) show how the form of pre-treatment (mechanical polishing, alkaline etching and acid etching) can strongly influence the degree of silane adsorption. Etching pre-treatments on the Al-2024 samples may lead to the surface enrichment of Cu, which in turn can affect the corrosion protection afforded by subsequently-applied silane layers.
Silane and Mixed/Hybrid Coupling Agents for Natural Fiber composites: Recent Developments
This presentation highlights our developments in designing novel silane-based hybrid coupling agents for natural fiber composites. Recent developments of silane coupling agents in designing superior strength natural fiber composites will also be highlighted. Natural fiber composites are mainly price-driven commodity composites that have useable structural properties at relatively low cost. In recent years, the manufacture and use of traditional composite structures made of glass, carbon and aramid fibers are considered critical and composites made of natural fibers have received increasing attention in light of growing environmental awareness. Although thermoplastics have the added advantage, of recyclability, thermosets are desired because of their superior mechanical properties in the resulting composites. The natural fiber reinforced polypropylene composites already have attracted the attention of auto-makers in making interior parts of automobiles. In addition, natural fiber-reinforced thermoset composites are wining importance in construction and other infra-structure applications. Natural fiber composites are now emerging as a realistic alternative to -glass fiber reinforced composites. Advantages of natural fibers over man-made fibers like glass and carbon are low cost, low density, acceptable specific strength, ease of separation, carbon dioxide sequestration and biodegradability. The main drawback of natural fibers is their hydrophilic nature that lowers their compatibility with relatively hydrophobic polymer matrices.
Adhesion is critical to the performance of natural fiber composites. Since a significant feature of these composites is their low cost, surface treatments that rely on water-based systems, unlike organic-based ones, is our main target. The increase in strength of natural fiber composites is quite significant in case of silane treated natural fibers. For a specific matrix system, one has to take the care to use right silane coupling agent with the desired functionality. The silane coupling agent like 3-methacryloxypropyltrimethoxysilane ( y-MPS) with an optimum concentration is a suitable silane coupling agent for polyester resin based natural fiber composites. The epoxy resin system since has the chemical unit DGEBA (Diglycsdylether of bisphenol A), needs an epoxy compatible silane coupling agent. Dow corning 6040: 3-glycidopropyltrimethoxy silane (gamma-GPS) is successfully used in making superior strength composites from natural fibers and epoxy resin. Aminofunctional alkoxy silane cum maleated polypropylene hybrid water-based coupling agent is successfully used to enhance performance of natural fiber thermoplastic composites.
1) Department of Applied Chemistry, Osaka Institute of Technology, 5-16-1, Ohmiya, Asahi-ku, Osaka, 535-8585 JAPAN
2) Ceramics Division, Yokohama Plant Showa Denko KK., 8 Ebisucho Kanagawa-ku Yokohama, 221-8517 JAPAN
Interface Structure of Vulcanizates Filled with Al(OH)3 Modified by gamma-mercaptopropyltrimethoxysilane
The influence of the condition of hydrolysis of gamma-mercaptopropyltrimethoxysilane (MGPS) and addition of alkali on the interface structure between Al(OH)3 particles and EPDM vulcanizates was investigated. Al(OH)3 particles were modified with MGPS by following methods : (1) in MGPS methanol solution added with NaOH aqueous solution of ~ 0.1wt% for Al(OH)3 ; (2) in cyclohexane added with MGPS hydrolyzed in methanol / H2O mixture solvent for 24 hours. The amount of MGPS deposited on Al(OH)3 particles prepared by method(2) was more than the one by method(1). The amount of MGPS deposited on Al(OH)3 particles increased with the concentration of NaOH. The results of ESCA measurements suggested the presence of chemical bonding and/or dipole-dipole interaction between Al(OH)3 particle and hydrolyzed MGPS. The influence of the interface structure of MGPS deposited on Al(OH)3 particles on mechanical properties of EPDM vulcanizates filled with the modified Al(OH)3 was discussed.
1) Department of Applied Chemistry, Osaka Institute of Technology, 5-16-1, Ohmiya, Asahi-ku, Osaka, 535-8585 JAPAN
2) Composite Material Division, Industrial Technology Center of Okayama
Prefecture, 5301, Haga, Okayama, 701-1296 Japan
Topography and Chemistry of Silane-Treated Inorganic Particle Surface and Their Influences on The Composite Properties
The silane coupling agents are used to improve the interfacial adhesion between the inorganic filler and the matrix resin in the particulate-filled polymer composite. In this study, the influences of the silane structure, the substrate nature and the treatment condition on the topographical and chemical properties of silane-treated layer were investigated in detail. First, the influence of the substrate nature on the reactivity of silane was investigated. For this purpose, silica, alumina, calcium carbonate, magnesium hydroxide, aluminum hydroxide were used as substrate. The reactivity of silane was measured using ESCA and by the carbon analysis. It was found that calcium carbonate has the lowest reactivity and effect on the filled composite properties. Secondly, some different model silane structures were prepared on the glass bead surface as model particle by varying the amount, the kind of organic functional group, the alkoxy groupnumber of silane and the treatment conditions (the medium and heating temperature, etc.). The topography of the silane-treated layer on the particle surface was observed using an atomic force microscopy. As a result, it was clarified that both topographical and chemical difference of the silane-treated layer strongly affected on the mechanical properties of the filled composite.
Silane Couping Agents as Dispersion Promoters for Magnetic Particles. Effect of Silane Functionality on the Dispersion Rheology
Our objective is to replace the organic solvents used in magnetic tape manufacture with liquid monomers. These solventless formulations would eliminate air pollution in magnetic tape manufacture. This necessitates the use of high solids formulations and presents a challenging fluid rheology problem. Our approach has been to treat the magnetic particles with silane coupling agents to aid in dispersion. Four different silanes were used, methyltrimethoxysilane, phenyltrimethoxysilane, octyltrimethoxysilane, and a commercial coupling agent (Z-6030, from Dow Corning). The magnetic properties for the treated particles were not adversely affected by the surface treatment. The treated particles were used to prepare a series of solventless acrylate dispersions. Coatings were cured by uv irradiation to give sold films, whose magnetic properties depended on the choice of coupling agent. Coatings containing particles treated with phenyltrimethoxysilane and Z-6030 had the best magnetic properties. The viscosity for the solventless dispersions was orders of magnitude higher than that for the conventional, solvent-based dispersion. However, the viscosity decreased exponentially with increasing shear rate. The particles treated with the commercial coupling agent, Z-6030, had the lowest viscosity.
University of Oklahoma, Norman, OK 73019
Admicellar Polymerization--An Alternative Method of Surface Modification
Surface modification of filler materials like porous aluminum oxide powders or glass fibers can be achieved by admicellar polymerization(AP) as well as by silanation. AP uses common commercial materials to form polymeric "coatings" as thin as nanometers in scale. Measurements of advancing contact angle and composite void size/spatial distributions confirm the significant change in surface character on application of a polymer layer. Composites made with fibers coated by AP exhibit increased adhesion, comparable to conventional sizing in single fiber pull out tests and in three-point bend tests. Similarly, the standard tests of mechanical properties of elastomeric formulations important to the tire industry improve with modified filler.
The Effects of Silane Hydrolysis on Aluminum Oxide Dispersion Stability in Ceramics Processing
The controlled hydrolysis of organosilanes has been shown to greatly influence both the surface adsorption of silanes on alumina powder, and the resultant dispersion stability of alumina in organic solvent slurries. Plateau adsorption concentrations from settling experiments show that surface coverage for hydrolyzed forms of both n-octyltrimethoxysilane (NOS) and N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) is typically achieved at concentrations of approximately 6mmoles/m2. Moreover, the settling densities of dispersions prepared with hydrolyzed silanes are consistently higher than densities achieved with monomeric silanes alone (as seen in case studies involving NOS in toluene, and AAPS in isopropyl alcohol). Similarly, ceramic slips prepared with either polystyrene and alumina, or with polyisobutylmethacrylate and alumina, also lead to ceramic green bodies with increased densities when the slips are prepared with hydrolyzed silanes. In addition, solid state NMR and dynamic mechanical analyses of resultant green bodies reveal that the molecular motional behavior of these polymers is strongly influenced by the presence of hydrolyzed silanes. These results collectively add to a growing body of evidence which supports the idea that not only is hydrolysis required for silanes to produce controllable and predictable effects in many industrial processes, hydrolysis must be made to occur at the "right time" within any sequence of steps that define an industrial process. In the case of ceramic slurries, the "right time" is the period just prior to the mixing of the ceramic slip ingredients.
Sterically Hindered Silanes for Waterborne Systems
Alkoxysilanes are useful as crosslinkers and adhesion promoters in numerous coating, adhesive and sealant formulations. Previously, silanes with sterically hindered alkoxy groups have been used in one-component waterborne formulations with long-term shelf-stability. Model compounds are used to study the rate of hydrolysis of silanes in waterborne systems. The effects of the leaving group on the silane hydrolysis rates are investigated using vinyltrialkoxysilanes in an aqueous acetone solution. The acid-catalyzed rates of hydrolysis are measured under pseudo-first order conditions.
Functional Silanes as Silicone Elastomer and Coating Crosslinkers
Hydrolyzable functional silanes are used as silicone elastomer crosslinkers. The hydrolysis triggered by air moisture, leads to condensation of linear polymer chains to three-dimensional networks. The properties of these elastomers depend on the silane structure. The influence of the structure has been studied for RSi(OR')3 silanes with R = Me, Et, Pr, Bu, Pentyl, Vi, Phenyl for hydrolyzable acetoxy or oximino groups.
Reactivity differences have been observed: kinetic and thermodynamic Vi>Ph~Me>Et>Pr>Bu>Pe. They lead to a decrease in surface curing rate and a decrease in the crosslinking density. They are related to steric and inductive effects of the functional groups.
Oximinosilanes showed a lower reactivity compared to acetoxysilanes, coming from a Si-ON bond more stable than the Si-OAc bond. This also leads to slower surface cure and lower crosslinking densities.
Organometallic tin used for the catalysis of oximinosilane systems gave results similar to organometallic titanium. For acetoxysilane systems, the titanium system was found more efficient.
Knowledge of these data allows the design of coatings tailored to specific requirements, such as antifouling coatings. Cure rate, release and mechanical properties can be adjusted with the selection of well adapted silanes.
Film Growth Oscillations of Organofunctional Silanes on Metal Oxide Surfaces
The kinetics of adsorption of propyltrimethoxysilane (PTMS, a model representative of many commercial organofunctional silanes) and aminopropyltriethoxysilane (APS, a widely used industrial orgaonosilane) onto polycrystalline metal oxide surfaces have been investigated using X-ray Photoelectron Spectroscopy (XPS) and static Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS). In each case, the kinetics does not follow the behaviour predicted by classic adsorption models. Non-linear oscillations are observed in every case, indicating that complex mechanisms are involved. Based on the surface processes that can occur and dynamics theory, models of organosilane film growth mechanisms have been developed to describe the observed oscillatory behaviour and an insight into the surface adsorption processes has been obtained.
The results illustrate that the conditions for optimum adhesion are likely to occur at the first peak in adsorbate coverage as a function of exposure, which is significantly different from an intuitive understanding of film growth and quite important for industrial applications of these materials.
* Present Address: Physics and Astronomy and Laboratory for Surface
Modification, Rutgers University, Busch Campus, 136 Frelinghuysen Rd,
Piscataway, NJ, 08854-8019.
Reactive Mixing of Silica Rubber
For most applications, rubbers are reinforced with active fillers. Usually carbon blacks or silica particles are used to enhance properties and service life. An advantage of silica compared to carbon black is: silica gives lower hysteresis loss, which for tyre applications leads to a lower rolling resistance and consequently fuel savings.
The compatibility of hydrophilic silica with the hydrophobic polymer matrix is low. Filler-matrix compatibility can be enhanced by adding a bi-functional coupling agent. The filler surface will only be partly activated, and this can create serious problems for the properties of the final product. The objective of the project is to examine the influence of mixing processing parameters - fill factor, rotor speed, order of adding ingredients - on the filler-matrix-coupling. Irreproducible conditions during mixing and vulcanisation, combined with different possibilities of adding the coupling agents to the filler/rubber substrate are no doubt major factors in the reproducibility of silica-reinforced rubber compounds.
It turns out that the ultimate temperature obtained during the mixing process governs the reaction mechanism of the coupling agent: formation of a silica - rubber bond vs. the action as a curing agent.
ICOSAP Programme - Silane as a Primer and for Adhesive Incorporation
For aluminium structures, surface treatment of the alloy prior to bonding is generally regarded as vital for the long-term durability of adhesive joints. Until recently, the strongest and most durable adhesive bonds to metal substrates have been achieved by pre-treating the substrate with a range of aggressive and toxic chemicals to modify the surface topography and chemistry. This is particularly true for aluminium where there is extensive use of hexavalent chromium and strong acids during surface treatment for bonding. There has been growing pressure from government and environmental bodies to remove such substances from these processes. To this end considerable effort has been directed toward the application of organosilanes as a potential pre-treatment process. The most efficient methods of application, together with a knowledge of the mechanisms by which these materials enhance strength and durability, are not fully understood and it is this lack of understanding that the current programme has addressed.
During the last three years, the adhesives team at the Defence Evaluation and Research Agency in Farnborough (UK) has led an extensive research programme into the use of organosilanes as coupling agents for structural adhesive bonding. Termed ICOSAP, the International Collaboration on Organo Silane Adhesion Promoters, the project has involved sixteen research groups in six countries. The aim of this project has been to understand the relationships between silane application variables, solution chemistry, surface chemistry, joint durability and mechanisms of failure. This work has examined the effects of silane solution concentration, pH, buffering, hydrolysis time, drying time and temperature, substrate hydration, and processing time lags between application and bonding, on bond strength and durability. In addition to the use of silane as a primer recent work has examined the effect of incorporating the silane into the adhesive prior to bonding.
Results obtained from this work will be summarised, highlighting the key scientific acheivements and the potential of this environmentally friendly pre-treatment process compared with phosphoric and chromic acid pre-treatments.
Supramolecular Structures from an Amphiphilic Polysilane-based Block Copolymer
Polysilanes possess electronic properties that allow for a number of potential applications e.g. as conductive, electroluminescent, non-linear optical and micro-lithographic materials. Many of these properties can be expected to be more readily exploited through the incorporation of the polysilane moiety into a block copolymer system, which would allow for the manipulation of the macroscopic order of the material through supramolecular assembly (e.g. by microphase separation or aggregation in aqueous systems).
The multi-block poly(methylphenylsilane)-co-poly(ethylene oxide) depicted below allows the generation of defined macromolecular assemblies from aqueous dispersions. The generated morphologies include micellar rods, vesicles, helical aggregates and 2 dimensional solid films. This work shows that block copolymers containing semiconductor component parts may be used to form nanostructures with optoelectronic properies.
1) Material and Process Engineering, Cessna Aircraft Company, Wichita, KS 67215
2) Department of Materials Science and Engineering, University of Cincinnati, OH 45221-0012
Interface Studies of Bis-silanes on Metals
Recent research on silane technology for corrosion protection of metals has been focused on bis-silanes due to its superior performance over the conventional monosilanes. In this paper, two functional bis-silanes namely bis-amino and bis-polysulfur silane on different metals have been analyzed due to its "universal" nature towards enhanced corrosion protection and paint adhesion. Previous research on these silanes were primarily focused on the use of electrochemical impedance spectroscopy in order to reveal the mechanism behind the anticorrosion performance of these thin films of silanes on metals. However, little information regarding the chemistry of the interface could be known. Hence, this paper focuses on the use of ToF-SIMS and XPS for understanding the metal-silane interface. These studies reveal that the metal-silane interface could be much more complex due to formation of interfacial products and diffusion of metal into the film. The interfacial structures are associated with the anti-corrosion performance of these silane films. However, it is to be noted that a systematic structure-property relationship with respect to corrosion performance of these silanes is yet to be developed. NMP (N-methyl pyrollidone) swelling test results have also been given in order to establish the enhanced paint-adhesion of these silanes on various automotive metals over the conventional phosphate treatment.
Silane Films on Metals for Corrosion Control
Films of organofunctional and non-organofunctional bis-silanes were deposited on various metals including cold-rolled steel, galvanized steel and aluminum alloys. It will be shown that such films can provide outstanding corrosion protection properties and, therefore, silane-based processes are candidates for replacing chromates in several metal-finishing industries. The properties of the silane films were studied by EIS using a non-intrusive electrolyte. The mechanism of performance of silanes is based on a passivation of the metal-film interface which also becomes hydrophobic after complete cure of the silane. In some cases a new compound is formed between the silane and the metal oxide. In such systems the silane film can be seen as a conversion coating.
Comparison of Silane Adsorption on Silica from the Gas Phase, Liquid Phase and from Supercritical CO2: Infrared Studies and their Relationship to Xerographic Properties
Silica particles are dispersed on the surface of xerographic toner additives for control of toner flow and charge. Therefore, a chemical modification of the surface of the silica particles is tantamount to performing a chemical modification of the surface of the toner. The aim of our work is to develop an understanding of the relationships between the molecular surface chemistry and the macroscopic property of triboelectrification. Infrared spectroscopic techniques have been developed to monitor the adsorption of various functionalized chloro and alkoxysilanes on silica surfaces. Of particular use has been a thin film technique that provides access to the low frequency region where characteristic Si-O-Si, Si-O-C and Si-Cl modes appear. In this presentation, I will provide a brief overview of the methods we have developed to extend the thin film technique for adsorption studies conducted in non-aqueous, aqueous and supercritical fluids. The information derived from these studies has been critical in developing protocols for performing silanization reactions that lead to novel modified silica particles for use as additives in xerographic toners.
Factors Influencing the Effectiveness of Silanes for Structural Adhesive Bonding to Aluminum
The application of adhesive bonding to structural applications involving aluminum is limited by the resistance of the bonds to moisture. The use of silanes as environmentally benign primers can result in significant improvements in moisture resistance. However, when used with structural epoxy adhesives, such as FM73, these treatments generally fall short of the ideal performance, which is zero "adhesive" failure in the Wedge test. In an effort to gain an understanding into what factors are limiting the performance of the silane process, we have examined the effect of varying the composition of the silane layer and the pretreatment of the aluminum. The results show that the performance in the wedge test is largely insensitive to the composition of the silane layer over a fairly broad range of compositions and thicknesses. On the other hand, pretreatments of the aluminum can result in significant improvements. Phosphoric acid anodization, FPL etch and boiling for short periods of time can result in the elimination of all "adhesive " failure. On the other hand, anodization using solutions that produce barrier-type oxides result in no improvements. Hot water treatments that result in thick pseudo-Boehmite layers are also less than ideal. The results suggest that nanometer-scale interlocking contributes significantly to the performance of the bond. Interpenetration of the silane layer and the adhesive does not appear to happen.
Ureido Silanes on E-glass Fibres, Deposition and Surface Characteristics.
A review of eight articles which examine the behaviour of methoxy and ethoxy ureido silanes deposited onto E-glass fibres from both organic and aqueous solutions. Ureidopropyltrimethoxy silane is used as a coupling agent for paints to metal surfaces, it has not found much use as a coupling agent for E-glass fibres. On metal surfaces the silane is deposited with the ureido functional group outermost available for reaction with a paint/polymer matrix1,2 . On E-glass surfaces, however, unless specific conditions were employed, the ureido silanes tend to deposit with a highly crosslinked siloxane outer surface. silane oriented outermost.
The silanol groups condensed, not only with silanols for adjacent silanes and the E-glass surface but also with aluminium hydroxides leached from the E-glass surface. This formed a highly crosslinked alumino-siloxane outer layer. The alumino siloxane was formed when the silane is deposited onto E-glass surfaces at all pHs between 4 and 10 and also from a range of organic deposition solutions.
We present 29Si NMR results which illustrate the high level of condensation, and also demonstrate that silane to silicate binding of the form SiO- +NH2 was not present. A small proportion of Si-O-Si silane silicate bonds were detected, indicating that the silane was bound to the E-glass surface by a small number of sparsely distributed Si-O-Si links.
Electro kinetic analysis of the surfaces confirmed patchy deposition with a siloxane outer surface. Using AFM spectroscopy and quantum chemistry calculations it was estimated that the patchwise deposition of the silane w in the form of bilayers (or multiples of bilayers). The bilayer structure consisted of siloxane outer layers with the inner ureido layers forming hydrogen bonds.
A graphical fit using the mole and volume fractions confirmed the various types of siloxane surface as deposited from organic solvents. Chlorinated solvents tended to produce a ureido outer surface which was condensed, but solvent swollen, deposition from alcohols and acetone resulted in a siloxane surface. Toluene and THF both produced minimally condensed ureido surfaces, which were easily removed, by washing.
1) J. K. Premachandra, W. J. van Ooij, and J. E. Mark. J. Adhesion Sci. Technol. 12 (1998) 1361
2) P. Puomi, H. M. Fagerholm. J. Adhesion Sci. Technol. accepted (2000)