1. Inorganic Reactive Silanes

In the presence of water, coupling agents produce highly reactive silanols. Subsequently, these silanols begin to condense, forming oligomeric structures while also forming weak hydrogen bonds to the surface of inorganic materials. Finally, drying the inorganic materials leads to further condensation and dehydration between the coupling agent and the surface. This process yields multiple strong, stable, covalent bonds to the surface.

2. Organic Reactive Silanes

The improved adhesion between the surfaces of inorganic materials treated with silane coupling agents and organic resins is caused by:
(1) Improved wetting of the treated inorganic surface by the resin.
(2) Improved compatibility between the treated inorganic surface and the resin.
(3) Hydrogen bonding between the treated inorganic surface and the resin.
(4) Multiple covalent bonds between the treated inorganic surface and the resin.

Many different factors affect the above four items, such as the type of thermoplastic or thermosetting resins, whether or not functional groups remain, the abundance and reactivity of the remaining functional groups, and the overall polarity or non-polarity of the resins.
 

Thermoplastic resins
For thermoplastics, the chemical bonds introduced by reactive silanes are often relatively weak. A limited number of highly polar thermoplastics will develop weak interactions with some silane coupling agents. In these specific situations, both the thermoplastic resin and the silane are capable of forming hydrogen bonds. Therefore, the effectiveness of resin modification is highly dependent on the compatibility of each organic resin and its ability to form hydrogen bonds.

Thermosetting resins
Unlike thermoplastics where considerations such as the critical surface tension, the dissolution parameters, and other similar factors may be used to evaluate resin compatibility, these factors are not meaningful for predicting the strength of composite materials prepared from thermosetting resins. To maximize the strength and other physical properties of a thermosetting composite, it is generally recommended to first react the organic functional group of the silane with the thermosetting resin before curing the composite. It is important to use a reactive silane coupling agent bearing an appropriate functional group that matches the functional reactivity of the thermosetting resins.