Organofunctional Silanes
Properties and Applications
Organofunctional silanes are hybrid resins that contain at least one reactive organic functionality and one or more alkoxy groups. The most common alkoxy groups are methoxy and ethoxy which hydrolyze in the presence of moisture to form reactive silanols which then condense to Si-O-Si bonds by either alcohol or (more common) water elimination.1 The silanol groups (hydrolyzed alkoxy groups) can also react with hydroxyl groups present on almost all mineral and metallic surfaces and thus can bond covalently to these materials. The type of organic functionality will depend on the application and the nature of the base resin. The most common functionalities include amino, epoxy, (meth)acryloxy and vinyl groups. Thus, these resins can form durable bonds between organic and inorganic materials and often function as coupling agents.
Silane coupling agents are incorporated into many formulated products and plastic resins to improve their properties. For example, they can function as crosslinkers in moisture curing adhesives and sealants or as adhesion promotors for a variety of substrates. They also improve the physical and mechanical properties of filler-reinforced products. The choice of functionality depends on the application.
Amino and glycidoxy functional silanes are often blended with
thermosetting resins such as epoxies and phenolics. They enhance adhesion to many inorganic substrates and reduce moisture sensitivity. They also improve
the dispersibility of fillers in the resin and thereby increase the mechanical strength.
One of the most important applications of amino and glycidoxy
functional silanes is pretreatment of (short) glass fibers and glass fabrics used in
thermosetting laminates and prepregs which find many applications in aerospace, automotive and electrical
industries.
Another important application is the synthesis of silane terminated prepolymers that are used in moisture-curing products such as sealants and adhesives.
For example, moisture-cure silicone polyurethanes (SPUR) can be prepared by reacting amine functional silanes with diisocyanate-terminated
polyurethanes yielding urea linkages and alkoxy functional end groups.
Vinyl and (meth)acryloxy functional silanes are
frequently used in thermoplastic resins and sulfur crosslinked EPDM
to incorporate inorganic fillers and to improve adhesion to inorganic
substrates. The filler particles are typically pretreated with the vinyl silane to achieve maximum wetting and compatibility with the resin system. Two important examples are vinylsilane-treated glass
fibers for structural composites and vinylsilane-treated clay for wire insulation.
Vinyl alkoxy silanes are also used as crosslinking agents in polyolefins
such as polyethylene. The silane is grafted via its vinyl group on
the thermoplastic through a peroxide initiated free-radical reaction. The resulting copolymer is then cross-linked via exposure to hot water or steam. The silane grafting reaction is typically carried out in
the molten plastic by reactive extrusion.
Manufacturers & Distributors
1The reactivity of a coupling agent depends on the size of the alkoxy groups. With increasing size, steric crowding around the silicon atom increases which, in turn, reduces the reactivity of the alkoxysilane. Thus, methoxy groups are more reactive than ethoxy groups. The latter is often chosen when improved product stability or increased pot-life is required. The reactivity and properties of a coupling agent also depend on the linker between the organic functionality and the silicon atom. With increasing linker length, the mobility of the functional group increases which allow the functional group to extend further into the resin. The typical linker length is three carbon atoms.