Silanes for Adhesives
Silane adhesion promoters are bifunctional organosilicone compounds which act as molecular bridges between the polymer matrix of an adhesive or sealant and the substrate, either inorganic or organic.

The silane end contains hydrolysable alkoxy groups that are activated by reaction with ambient moisture. The hydrolysable alkoxy groups attached to the silicon end of the silane are typically either methoxy or ethoxy. Once activated (hydrolyzed), the resultant silanol groups will condense with o ther silanols or with reactive groups on the surface of a substrate such as SiOH, AIOH, or other metal oxides or hydroxides.The silane’s ability to bond to a surface will generally be determined by the concentration of such sites on the surface. Selecting the optimal silane for an application requires matching the reactivity of the silane‘s organofunctional group to that of the polymer.

The silanes can be blended into an adhesive formulation or used as primers on substrates. The structure and re activity of the silane will affect the ability of the silane to migrate. The most effective way to promote adhesion is to apply the silane as a primer to the surface, followed by application of the adhesive/sealant.

In this way, the silane will be on the s urface and therefore at the interface where it can enhance adhesion between the polymer and the substrate. Silane primers are usually dilute solutions of 0.5 to 5 percent silane in alcohol or water/alcohol solvent. They are wiped or sprayed on the substrate, after which the solvent is allowed to evaporate. While the concentration needed for a specific application may vary, one percent (1%) based on resin content is recommended as a good starting point.

The organofunctional group of the silane can react, and bond to, the polymer backbone. Residual moisture activates the silane’s alkoxy groups to the active silanol form which react with each other, liberating
moisture, and forming siloxane bonds between the polymers. The resulting Si-O-Si crosslink is extremely durable, offering excellent weather, UV, temperature, chemical and moisture resistance.

The filler may either be treated with silane before it is added to the sealant formulation (pretreatment method), or it can bind with the filler during compounding (additive method).

Alkoxysilanes react very rapidly with water;they are usually used to capture excess moisture in sealants and adhesives.

Vinyltrimethoxysilane is the most common moisture scavenger , due to the electron interactions of the vinyl group it reacts with moisture faster than other alkoxy silanes, enabling it to function as a moisture scavenger in the presence of other silanes incorporated as adhesion promoters, crosslinkers or coupling agents. The amount of silane added will depend on the water content of the formulation constituents.

Methanol is formed as a byproduct, and the vinyl silane crosslinks into an inactive species in the formulation. Other alkoxy silanes, such as methyltrimethoxysilane, are also used as waters.
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