Silane adhesion promoters hydrolysis

The excellent adhesion of MS Polymer sealants to conventional construction substrates such as glass, metal, and plastics (e.g., PVC) is derived finm the widely used silane adhesion promoter V-(P-aminoethyl)-3-aminopropyltrimethoxysilane, DYNASYLAN DAMO-T. Another aspect of using a diaminosilane is the intrinsic change of pH that results in fast hydrolysis and condensation reactions of the MS Polymer . To guarantee a certain shelf life, MS Polymer sealants are generally formulated with vinyltrimethoxysilane, DYNASYLAN VTMO as a water scavenger. 

Thus dispersion force adsorption may be sufficient, in principle, to provide good fundamental adhesion. In practice, it is often necessary to produce stronger bonds that are less susceptible to attack by medium m, e.g., less susceptible to hydrolysis. Silane adhesion promoters, discussed above, increase the hydrolytic stability of interfaces between adhesives and glass or metals through covalent bond formation. 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

The kinetics of the hydrolysis and condensation of organic functional trialkoxy silanes has been reported by Pohl and Osterholtz [17-19]. The silane coupling agents used as adhesion promoters [1-3] usually have a trialkoxy silane as one of the functional groups, i.e. (MeO)3Si—(CH,),—0,CC(Me)=CH2. If this attaches to a glass substrate, it will form Si—O—Si bonds or if it attaches to metal substrates, it can form M—O—Si bonds. Thus, the work described here can be applicable to providing additional understanding for those processes. 

Under optimum conditions, hydrolysis is rapid, and precedes slower condensation reactions. However, the silanol groups formed in hydrolysis are very reactive, and condensation can be a significant factor even in dilute aqueous solutions. Understanding the kinetics of the hydrolysis and condensation reactions for a specific silane in solution is important because its effectiveness as an adhesion promoter is influenced by the extent of condensation, which, in turn, affects the structure of the interphase in composites. 

Keywords silane, crosslinking, hydrolysis, coating latex, adhesion promoter... 

The aminosilane coupling agent 3-aminopropyltriethoxysilane or y-amino-propyltriethoxy silane—also abbreviated as 3-APS, y-APS, APS or A1100 (Union Carbide)—is widely used to promote adhesion between polyimide thin films and mineral surfaces such as native-oxide silica, alumina and various glass ceramics [1, 2]. The structure of APS and the hydrolysis reaction sire shown in Fig. 1. Typically, dilute aqueous solutions of 0.1 vol% or approximately 0.080 wt % are employed to prime the mineral surface. The mechanism for the interaction of the bifunctional aminosilane with the mineral surface is the subject of much speculation, although it is conjectured by Linde and Gleason [3] that the amine end initially forms an electrostatic bond with surface hydroxyls. Subsequently, possibly as the result of elevated temperatures, the silanol end of the molecule proceeds to form a siloxane-like bond with the surface and the amine... 

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