A Self-Assembled Monolayer (SAM) is a one molecule thick layer of material that bonds to a surface in an ordered way as a result of physical or chemical forces during a deposition process. Silanes can form SAMs by solution or vapor phase deposition processes. Most commonly, chlorosilanes or alkoxysilanes are used and once deposition occurs a chemical (oxane) bond forms with the surface rendering a permanent modification of the substrate. Applications for SAMs include micro-contact printing, soft lithography, dip-pen nanolithography, anti-stiction coatings and orientation layers involved in nanofabrication of MEMs, fluidic microassemblies, semiconductor sensors and memory devices.
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Common long chain alkyl silanes used in the formation of SAMs are simple hydrocarbon, fluoroalkyl and endgroup substituted silanes. Silanes with one hydrolyzable group maintain interphase structure after deposition by forming a single oxane bond with the substrate. Silanes with three hydrolyzable groups form siloxane (silsesquioxane) polymers after deposition, bonding both with each other as well as the substrate. For non-oxide metal substrates, silyl hydrides may be used, reacting with the substrate by a dehydrogenative coupling.
The perpendicular orientation of silanes with C10 or greater length can be utilized in micro-contact printing and other soft lithography methods. Here the silane may effect a simple differential adsorption, or if functionalized have a direct sensor effect.
Acrylates, methacrylates, and unsaturated polyesters easily undergo free-radical polymerization and can therefore be modified by copolymerization with silanes that have unsaturated organic substitution. The usual coupling agents for thermoset polyesters undergo radical copolymerization in such systems. These resins, usually of loosely defined structure, are often mixed with a second monomer like styrene to reduce viscosity. The silane monomer should match the reactivity of the styrene rather than the maleate portion of the polyester for better reinforcement. Methacrylyl- and styryl-functional silanes undergo addition much more readily than vinylsilanes. Vinyl-functional silanes mismatch the reactionary parameters of most unsaturated polyesters but can be used in direct high pressure polymerization with olefins such as ethylene, propylene, and dienes.
Thermoset urethanes can be effectively coupled with two types of silanes. The first type, including isocyanate-functional silanes, may be used to treat the filler directly or integrally blended with the diisocyanate (TDI, MDI, etc.) prior to cure. Amine- and alkanolamine-functional silanes, on the other hand, are blended with the polyol rather than the diisocyanate. Alkanolamine-functional silanes react with the isocyanate to form urethane linkages, while amine silanes react with the isocyanates to yield urea linkages. A typical application for coupled urethane systems is to improve bond strength with sand in abrasion-resistant, sand-filled flooring resins.
Phenolic resins are divided into base-catalyzed singlestep resins called resols or acid-catalyzed two-step systems called novolaks. Although foundry and molds are formulated with resols such as aminopropylmethyldialkoxysilanes, the commercial utilization of silanes in phenolic resins is largely limited to novolak/glass fabric laminates and molding compounds. The phenolic hydroxyl group of the resins readily reacts with the oxirane ring of epoxy silanes to form phenyl ether linkages. When phenolic resins are compounded with rubbers (as in the case with nitrile/phenolic or vinyl butyral/phenolic adhesives) or impact-resistant molding compounds, additional silanes'particularly mercapto-functional silanes'have been found to impart greater bond strength than silanes that couple to the phenolic portion.
Thermoplastics provide a greater challenge in promoting adhesion through silane coupling agents than thermosets. The silanes must react with the polymer and not the monomeric precursors, which not only limits avenues for coupling, but also presents additional problems in rheology and thermal properties during composite formulation. Moreover mechanical requirements here are stringently determined. Polymers that contain regular sites for covalent reactivity either in the backbone or in a pendant group include:
A surprising number of these are coupled by aminoalkylsilanes. Chlorinated polymers readily form quaternary compounds while the carboxylate and sulfonate groups form amides and sulfonamides under process conditions. At elevated temperatures, the amines add across many double bonds although mercaptoalkylsilanes are the preferred coupling agents. The most widely used coupling agents, aminoalkylsilanes, are the most economical but are not necessarily the best. Epoxysilanes, for example, are successfully used with acrylic acid and maleic acid copolymers.
The group of polymers that most closely approaches theoretical limits of composite strength does not appear to contain regular opportunities for covalent bond formation to substrate. Most of the condensation polymers including polyamides, polyesters, polycarbonates, and polysulfones are in this group. Adhesion is promoted by introducing high energy groups and hydrogen bond potential in the interphase area or by taking advantage of the relatively low molecular weights of these polymers, which results in a significant opportunity for end-group reactions. Aminoalkylsilanes, chloroalkylsilanes, and isocyanatosilanes are the usual candidates for coupling to these resins. These silanes improve the mechanical strength of the thermoplastics, allowing them to replace the cast metals in such typical uses as gears, connectors, and bobbins.
The polyolefins and polyethers present no direct opportunity for covalent coupling. Until recently, the principal approach for composite formulation was to match the surface energy of the filler surface'by treating it with an alkyl-substituted silane'with that of the polymer. For optimum reinforcement, preferred resins should be of high molecular weight, linear, and have low melt viscosity. Approaches to improved composite strength have been through compatibility with long-chain alkylsilanes or aminosilanes. It is more effective to couple with vinyl or methacryloxy groups, particularly if additional coupling sites are created in the resin by addition of peroxides. Dicumyl peroxide and bis(t-butylperoxy) compounds at levels of 0.15% to 0.25% have been introduced into polyethylene compounded with vinylsilane-treated glass fibers for structural composites or vinylsilane-treated clay for wire insulation. 50% increases in tensile and flexural properties have been observed in both cases when compared to the same silane systems without peroxides.
Another approach for coupling polypropylene and polyethylene is through silylsulfonylazides. Unlike azide bound to silicon, sulfonyl azides decompose above 150 °C to form a molecule of nitrogen and a reactive nitrene that is capable of insertion into carbon-hydrogen bonds to form sulfonamides, into carbon-carbon double bonds to form triazoles, and into aromatic bonds to form sulfonamides. Fillers are treated first with the silane and then are fluxed rapidly with polymer melt.
One of the more innovative ways of modifying the surfaces of polyolefins is to apply a multipodal oligomeric coupling agent, such as . Such oligomers provide better adhesion to polyolefins and still have the linking effect of silane dipodal chemistry in attaching to surfaces. The olefin-based backbones provide great compatibility with all hydrophobic olefins as well as with elastomers of various types. Finally, an oxygen plasma treatment prior to applying silane coupling agents produces hydroxyl radicals on a polyolefin surface. These hydroxyl radicals provide good linkage sites for any silane coupling agent to link onto the polyolefin surface, which opens a larger range of applicable silanes.
On the other hand, inorganic surfaces can impose enormous steric constraints on the accessibility of organic functional groups in close proximity. If the linker length is long, the functional group has greater mobility and can extend further from the inorganic substrate. This is important if the functional group is expected to react with a single component in a multicomponent organic or aqueous phase found in homogeneous and phase transfer catalysis, biological diagnostics, or liquid chromatography. Extended linker length is also important in oriented applications such as self-assembled monolayers (SAMs). The typical linker length is three carbon atoms, a consequence of the fact that the propyl group is synthetically accessible and has good thermal stability.
Mineral fillers are increasingly being used as important additives and modifiers for organic polymers. A mineral's surface can be made more compatible and dispersible in polymers or a reinforcing additive by the alkoxysilanes.
The use of silane-modified minerals in organic rubber has become increasingly important. In general, the minerals with aluminum and silicon hydroxyl groups on their surfaces are receptive to alkoxySilanes bonding. The treatment of a mineral surface by an organosilane is depicted in Figure 1.
Figure 1. Treatment of a mineral surface by an organosilane
Silane treatment can improve processing, performance, and durability of mineral-modified products by:
Figure 2. Example of transforming a mineral filler from hydrophilic to hydrophobic with a phenyl-functional silane
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Silane coupling agents have been effectively used in filled polymer systems with silica (both fumed and precipitated), glass beads, quartz, sand, talc, mica, clay, and wollastonite. Other metal hydroxyl groups, such as magnesium hydroxide, iron oxide, copper oxide, and tin oxide, may be reactive to a lesser extent, but often benefit from silane treatment. Traditionally, silane coupling agents give poor bonding to carbon black, graphite, and calcium carbonate.
Minerals are treated with either neat silane or a solution of silane in water and/or alcohol. With a neat silane, the adsorbed water on the filler surface is often sufficient to hydrolyze the alkoxysilane and simultaneously bond the silane to the filler surface. It is important that the filler be coated uniformly through the use of intensive mixing, such as with a Henschel mixer. Commercial processes are continuous, often in heated chambers, followed by further heat treatment to remove byproducts of alcohol and water and to complete the bonding of the silane to the surface.
The loading level of silane on the filler surface is a function of the surface area of the filler. While it was previously thought that one monolayer of silane should be sufficient, experimentation has shown that several layers of silane give optimal results. For example, typical fillers with average particle sizes of 1 to 5 microns often give best results when treated with about 1% silane (based on weight of the filler). The optimal level of silane treatment should be determined experimentally.
The choice of which silane to use in a particular application is determined by the nature of the desired benefit. All alkoxysilanes will bond to a receptive filler or mineral surface. If the silane treatment is designed to provide surface hydrophobicity, then a silane with a hydrophobic group, such as butyl, octyl, fluorocarbon, or phenyl, should be chosen. If the silane treatment is designed to provide compatibility of the mineral in a polymer matrix, then the nature of the organic group on the silane should be similar to the chemical structure of the polymer (i.e., an octyl or longer-chain alkyl group will help provide compatibility and dispersibility of the mineral in a polyolefin matrix). If the silane treatment is to bond a filler to a polymer matrix, then an organo-reactive silane should be chosen that would bond chemically to reactive sites present in the polymer.
In , Dow Corning was facing an increasing number of competitors around the world that were getting into the standard silicones business. At the same time, for a growing customer segment called 'price-seekers' who needed little or no service attached to the products, price had become the driving force.
Dow Corning needed to defend their position as a provider of innovative silicon-based materials and solutions (which, for the most part, came bundled and priced together). Dow Corning knew that they needed to find a better way to meet customers' needs exactly.
It was time for a game-changing strategy ' one that would simultaneously:
Dow Corning decided the best way to take advantage of the potential in the mature market segment, without detracting from the value of the Dow Corning offering, would be to create a separate brand. This brand would offer a clearly defined value proposition and set of products via a web-enabled platform to provide competitive pricing.
The product set would be a wide range of standard silicone products generally thought of as 'commodities' by customers. These standard products would be ' and still are ' manufactured by Dow Corning.
The tone would be 'no frills' ' just straightforward business terms and conditions, high-quality products, a reliable supply, and market-driven prices. Short. Sweet. To the point.
With the launch of the XIAMETER® brand in March , Dow Corning revolutionized the way they do business. Specialty silicone products, service and solutions continued as part of the business under the Dow Corning® brand. The XIAMETER brand met the needs of 'price-seekers' who purchased standard silicone products via www.xiameter.com.
The global drive to find greater efficiencies in business through a web-enabled business model worked in their favor. It helped Dow Corning maximize productivity and reduced human error, while keeping costs competitive for customers. Rather than cannibalizing the Dow Corning brand as some feared, the XIAMETER® brand made it stronger. The two brands worked in harmony, helping increase Dow Corning's financial results dramatically during this period.
Between and , the XIAMETER® brand was a web-enabled business with clear business rules, offering commonly used standard silicones at transparent, market-driven prices. However, those business rules only allowed for large-volume orders. There was no technical service. Lead times ran 7-20 days. And only 400 standard silicone products were available through www.xiameter.com.
In , Dow Corning expanded the XIAMETER® business model ' for the same reason we launched it in the first place ' to meet customer needs based on smart customer segmentation analysis. Xiameter saw a need for all customers ' not just large-volume buyers ' to be able to purchase standard silicone products at market-based prices. Based on the success of the original Xiameter model, company leaders were confident expanding the model would work ' both for customers and Dow Corning. And it did!
The XIAMETER® business model, which was expanded in June and expanded again in , now offers thousands of standard silicone products. Plus, the model is no longer for large-volume customers only. There are more volume-quantity options. With the Xiameter transparent price tiers, customers can choose the pricing most appropriate for them based on the volumes they need. They can purchase multiple items within a product family and receive greater discounts. They can lock in price and volume commitments through an online supply agreement. They also can choose credit terms that work best for them, which can be varied each time they order. Plus, customers who need smaller quantities, shorter lead times or more flexible ordering options can have them ' by purchasing XIAMETER® brand products through local distributors.
With today's XIAMETER® brand, customers can get all the standard silicones they need in the way that works best for them. Whatever they want. Wherever and however they want it. In a world of traffic jams, on-hold music and long supermarket lines, we think it's a great option.
Source: Xiameter.
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