Organofunctional silanes are used for many purposes including adhesion promotion, coupling of a resin matrix to inorganic fillers, moisture scavenging, improvements in compounding, and crosslinking. When used for crosslinking silanes have been generally applied to formulations of organic polymers that are used as sealants. These are often called silylated polymers or "hybrid" polymers since the final polymer often has the characteristics of both a silicone sealant and a polyurethane sealant.

However, there are several other crosslinking mechanisms that may be of interest to the adhesion formulator. These have the potential for creating new products in the adhesives and sealants area including crosslinkable thermoplastic adhesives.

This article will offer a general review of organosilanes as crosslinkers for conventional polymers such as acrylates, polyesters, polyurethanes and polyesters. Innovative crosslinking methods that are outside the norm for these materials will also be discussed. Previous SpecialChem articles have described the specific use of silanes specifically in sealant formulations.¹

Introduction to Organosilanes

Silanes have multiple commercial uses which include coupling agents for reinforced plastics and adhesives, internal adhesion promoters, primers for adhesives and coatings, and dispersants for paints and printing inks. Silanes can also be used to promote crosslinking, and for this purpose they can be incorporated in the polymer chain by various reaction mechanisms.

The properties and potential uses of silanes are dictated by their molecular structure:

(RO)₃-Si-R-X.

These materials offer two types of functionality. Silicone (Si) is the center of the silane molecule, which contains an organofunctional group (RO) with a second functional group (X). RO is a hydrolyzable group, typically methoxy, ethoxy, or acetoxy, which reacts with water to form silanol (Si-OH) which is relatively reactive and will crosslink with other organic groups. X is an organofunctional group, such as amino, epoxy, or methacrylate, which is generally reactive to organic resins. This group must be chosen to ensure maximum compatibility with the resin. R is typically a small alkylene linkage.

The small amount of water required for hydrolysis is commonly supplied by trace moisture on the surface of the substrate or in the air, from a pre-hydrolysis step, or from moisture inside the polymer.

Silanes as Crosslinkers for Thermosetting Elastomers

Properly selected silanes can be used to endcap polymers, such as polyurethanes, polyethers, and silicones for improved sealant properties. These are often referred to as "silylated" material. The addition of the silane endcap generally improves modulus and tensile strength and decreases elongation. In addition to endcapping, silanes can be incorporated in the polymer chain by free radical polymerization or by reaction with active groups on the polymer chain.

In the presence of moisture and an appropriate catalyst, the reactive silane end groups undergo hydrolysis and condensation reactions to form a stable three dimensional polymer network. This approach has been primarily used with polyurethane and polyether polymer systems. The types of silanes that have been used for these applications are summarized in Table 1.

Crosslinking occurs due to the presence of ambient moisture. The crosslinking rate can be varied by suitable choice of catalysts or by adjusting the pH of the system. Silane crosslinking offers formulations that provide a fast, room temperature cure. The resulting crosslink is strong, tough, and resistant to moisture and other chemicals.

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