Organofunctional silanes as crosslinkers in adhesives and sealants

Functional silanes as crosslinkers are organofunctional compounds. They are designed to chemically bond polymer chains through moisture-activated reactions, forming durable three-dimensional networks. 

The properties and potential uses of silanes are dictated by their molecular structure. Its molecular formula is given below. 

(RO)₃-Si-R'-X

where,

• (RO)₃ = Hydrolyzable alkoxy groups (e.g., methoxy, ethoxy, or acetoxy)
• Si = Central silicone atom
• R' = Short alkylene linkage
• X = Reactive organic functional group (e.g., amino, epoxy, or methacrylate) 

The hydrolyzable RO group reacts with water to form silanol (Si-OH). It is relatively reactive and crosslinks with other organic groups. The organofunctional X group is generally reactive to organic resins. This group must be chosen to ensure maximum compatibility with the resin. 

Mechanism of action

Silane crosslinking undergoes a moisture-triggered mechanism that involves hydrolysis and condensation reactions. The small amount of water required for hydrolysis is commonly supplied by the trace moisture on the surface of the substrate, in the air, during pre-hydrolysis, or within the polymer. 

• Step 1: Upon exposure to atmospheric moisture, alkoxy groups in the silane molecule hydrolyze to form silanol groups.
• Step 2: The silanol groups subsequently condense to create siloxane bonds. This process results in a tightly crosslinked polymer network. 

Hydrolysis and condensation reactions in silane crosslinking

The ability of organofunctional silanes to react under ambient moisture conditions makes them particularly suitable for one-component, moisture-curing systems widely used in construction, automotive, and industrial bonding applications. 

This crosslinking mechanism improves mechanical strength, chemical resistance, and long-term durability. Thus, enabling excellent adhesion to inorganic substrates such as glass, metals, and mineral surfaces. The ambient curing capability also eliminates the need for high-temperature processing, supporting energy-efficient formulation design.

For thermosetting elastomers

For improved sealant properties, properly selected silanes can be used to endcap polymers, such as:

• polyurethanes,
• polyethers, and
• silicones

These are often referred to as "silylated" material. The addition of the silane endcap generally improves modulus and tensile strength, while decreasing elongation. In addition to endcapping, silanes can be incorporated into the polymer chain through free radical polymerization or by reacting 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 is primarily used with polyurethane and polyether polymer systems. 

The types of silanes that are 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 a 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.

Polyurethanes

Silylated urethane polymers (SPUR) allow formulators to produce room temperature, moisture-curing construction sealants. These are free of unreacted isocyanate (NCO) monomer and exhibit superior adhesion and physical properties.

Key properties of these sealants include:

• Enhanced adhesion to a wide range of plastic substrates, in addition to the more conventional architectural substrates,
• Improved UV stability and weatherability,
• Immediate paintability,
• Improved resistance to automotive fluids

The NCO-terminated urethane prepolymers are made in the usual way. The organofunctional silane is added to the prepolymer for end-capping under similar reaction conditions. Silane amount is based on the percentage of NCO in the urethane prepolymer. An excess of 5-10% silane ensures that all of the NCO is reacted.

The NCO on an isocyanate-terminated prepolymer reacts readily with an amino silane:

RNCO + NH₂C₃H₆Si(OC₂H₅)₃ → RNH(CO)HNC₃H₆Si(OC₂H₅)₃ → R' Si(OEt)₃

where,

• RNCO = Isocyanate
• NH₂C₃H₆Si(OC₂H₅)₃ = Amino functional silane
• RNH(CO)HNC₃H₆Si(OC2H₅)₃ = Urea-functional organosilane
• R′–Si(OEt)₃ = Organofunctional trialkoxysilane

A typical SPUR sealant formulation is presented in Table 2.

Table 2: An example of a one-part SPUR sealant formulation³

Polyethers

Sealants based on silyl-terminated polyethers (MS sealants) are developed in Japan. They have gradually seen application in Europe and the US. Similar to the SPUR sealants, MS sealants offer a wide range of properties. They can be formulated into a variety of moisture-curable sealants. 

They are particularly useful for applications where their non-staining characteristics and excellent adhesion provide long-term use without significant changes in properties. They are also noted for low temperature gunnability. 

KANEKA MS POLYMER™, offered by Kaneka Corporation, Japan, is silyl terminated polyether prepared from high molecular weight polypropylene oxide. It is end-capped with allyl groups, followed by hydrosilylation to produce a polyether end-capped with methyldimethoxysilane groups. 

In the presence of an appropriate catalyst, the methoxysilane group can be cured by moisture. The water reacts with the methoxysilane group to liberate methanol and produce silanol. Further reaction of the silanol with either another silanol or methoxysilane produces siloxane linkages. 

A typical MS sealant formulation is presented in Table 3.

Table 3: Typical one-component MS Sealant formulation ⁴

Silicones

Hydrolyzable functional silanes are also used as silicone elastomer crosslinkers. The hydrolysis, triggered by air moisture, leads to crosslinking of linear polymer chains to three-dimensional networks. The properties of these elastomers depend on the silane structure. The influence of the structure on properties and reactivity has been extensively studied.⁵

For other crosslinking polymers

Organofunctional silanes can be used to crosslink several other polymers apart from thermosetting elastomers. The properties that are generally improved include tear resistance, elongation at break, tear propagation resistance, and abrasion resistance. 

Organofunctional silanes can even be used with thermoplastic polymers such as acrylics, polyolefins, polyvinyl chloride, etc. In these cases, the silane is incorporated into the polymer either by grafting onto the polymer backbone or by copolymerization during the polymerization reaction. Crosslinking in the presence of water is generally catalyzed by tin compounds or other suitable catalysts. 

Silanes have also been used to crosslink acrylic latex polymers. Silanes containing a reactive free radical group, such as vinyl or methacryl, can be copolymerized with the other acrylic monomers during the synthesis of the latex. Many of these silane-modified acrylic latex coatings are claimed to have excellent adhesion and chemical resistance.⁶ 

Organosilanes have also been used as additives and crosslinkers in hot-melt adhesive systems. In these systems, it is very important that the organosilane is compatible with the thermoplastic resin and the other additives that are commonly employed in hot melt adhesives. 

Table 4 provides examples of organofunctional silanes with various functionalities that are useful in certain types of hot melt adhesive applications.

Table 4: Examples of organofunctional silanes used in hot melt adhesive formulations

Now that we are aware of the material-level benefits, let's see how this translates directly into a range of practical applications and emerging developments across industries.

The development of organofunctional silanes has increasingly moved towards application-specific and performance-driven design. Branched and structurally modified organosilanes are being optimized to deliver enhanced performance in sealant systems.

Let's take a look at the emerging trends and applications of organofunctional sealants.

Adhesion to difficult substrates

Chemically blocked organofunctional silanes are developed to improve adhesion to difficult substrates. They are particularly used in automotive sealant applications. They are claimed to bond effectively to glass (windshields) as well as to lead-free ceramic frit. The blocking mechanism allows adjustment of the curing properties of these systems. Thus, making it possible to align with production line requirements while maintaining strong adhesion performance.

Adoption in hot-melt adhesives

Organofunctional silanes have also been applied to non-reactive hot melt adhesives. These adhesives are gaining rapid adoption in the product assembly market. This is due to their speed, simplicity (one-component), and environmental friendliness. In these systems, organosilanes enhance heat resistance through crosslinking while improving overall adhesion.

Durability in construction sealants

New organofunctional silanes are developed for specific end-use industries. Much of this development has been focused on organosilane additives in the building and construction industries. For example, branched organofunctional silanes are developed to improve the properties of sealants. These organosilanes provide increased flexibility through the nature of their branched molecular structures. Thus, enabling better elasticity and movement accommodation in demanding applications.

Environmental and low-VOC technologies

There is a strong shift towards low-volatile organic compound (VOC) and environmentally compliant formulations. Traditional solvent-based systems are replaced by solvent-free or moisture-curing technologies. Here, organofunctional silanes play a critical role as they help develop one-component moisture-curing systems. Thus, reducing the need for organic solvents. This reduces the emission of VOC and complies with EU REACH standards. The silane-modified systems are positioned as isocyanate-free alternatives to conventional PU chemistries. Thus, addressing environmental and occupational safety concerns.