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Why Silane Coupling Agents Are Used

ilane coupling agents are silicon-based chemicals that contain two types of reactivity – inorganic and organic – in the same molecule. A typical general structure is
where RO is a hydrolyzable group, such as methoxy, ethoxy,or acetoxy, and X is an organofunctional group, such as amino,methacryloxy, epoxy, etc.

A silane coupling agent will act at an interface between an inorganic substrate (such as glass, metal or mineral) and an organic material (such as an organic polymer, coating or adhesive) to bond, or couple, the two dissimilar materials.

When organic polymers are reinforced with glass fibers or minerals, the interface, or interphase region, between the polymer and the inorganic substrate is involved in a complex interplay of physical and chemical factors. These factors are related to adhesion, physical strength, coefficient of expansion,concentration gradients and retention of product properties. A very destructive force affecting adhesion is migration of water to the
hydrophilic surface of the inorganic reinforcement. Water attacks the interface, destroying the bond between the polymer and reinforcement, but a “true” coupling agent creates a water-resistant bond at the interface between the inorganic and organic materials. Silane coupling agents have the unique
chemical and physical properties not only to enhance bond strength but also, more importantly, to prevent de-bonding at the interface during composite aging and use. The coupling agent provides a stable bond between two otherwise poorly bonding surfaces.

With silane, the epoxy coating on the silica particles is apparent; without silane, clean silica particles can be seen in the epoxy matrix.