Silane coupling effect

The molecular weight of the condensed silane is also influenced by the same factors. Another factor that is important for the silane coupling effect is the molecular architecture of the siloxane networks. Caged structures and ladder structures are the extreme cases of the different siloxane structures (11,12). The solution pH, topochemical effects of the solid surface, the structure of the organofunctional group, and the concentration of the silane are all believed to influence siloxane structure, although no detailed study has yet been reported. 

Silane coupling agents may contribute hydrophilic properties to the interface, especially when amino functional silanes, such as epoxies and urethane silanes, are used as primers for reactive polymers. The primer may supply much more amine functionality than can possibly react with the resin at the interphase. Those amines that could not react are hydrophilic and, therefore, responsible for the poor water resistance of bonds. An effective way to use hydrophilic silanes is to blend them with hydrophobic silanes such as phenyltrimethoxysilane. Mixed siloxane primers also have an improved thermal stability, which is typical for aromatic silicones [42]. 

Ethylene-vinyl acetate Fetterman [37] reinforced compounded ethylene-vinyl acetate (EVA) copolymer by using short hbers and found that silane coupling agents were effective at establishing improved hber-matrix adhesion. Das et al. [38] prepared carbon fiber-filled conductive composites based on EVA and studied the electromagnetic interference shielding effectiveness of the composites. 

Another highly effect chain extender is trimellitic anhydride (TMA) which gives rise to branching of the PET structure. Note that the multifunctional epoxies (see Table 14.2) react quickly with the terminal carboxylic acid groups of PET but can also react with the film former and the silane coupling agent on glass fibre reinforcements. 

Ishida, H. and Koenig, J.L. (1980). Effect of hydrolysis and drying on the siloxane bonds of a silane coupling agent deposited on E-glass fibers. J. Polym. Sci. Polym. Phys. Edition 18, 233-237. 

Fig. 14. Effect of the chemical structure of silane coupling agents on their adsorption on the surface of a CaC03 filler, r=1.25pm (O) aminosilane (AMPTES), (A) aliphatic silane (SPIES)... 

Han [17] has shown that the effect of silane coupling agents on the viscosity of filled thermoplastics is not consistent. Melt viscosity may be decreased or increased depending on the chemical structure of the treatment and the nature of the polymer/filler combination under consideration. These observations probably reflect the effectiveness of the coupling agent in promoting bonding between filler and polymer, and hence the extent of polymer immobilization. 

J. Kim, K. Lee, K. Lee, J. Bae, J. Yang, and S. Hong, Studies on the thermal stabilization enhancement of ABS synergistic effect of triphenyl phosphate nanocomposite, epoxy resin, and silane coupling agent mixtures, Polym. Degrad. Stab., 79(2) 201-207, 2003. 

Silane coupling agents are very effective adhesion promoters between glass and certain thermoplastic polymers where no chemical reaction between coupling agent and polymer is observed. 

Although acid-base effects can be important in adhesion our studies indicate that bonding to metals through silane coupling agents is not by an acid-base mechanism, but probably through Si-O-M oxane bonds. As with glass, the hydrolysis and formation of oxane bonds are true equilibria, but the individual equilibrium constants are not known. 

The equilibrium constant for condensation of two silanetriols to the simple dimer siloxane—forming one siloxane link—is in the range of 400-500. If the equilibrium constant for the second and third condensation is of comparable size, siloxane bond formation is highly favored over reversal. This observation is probably the basis for the extreme effectiveness of silane coupling agents in protecting composites against wet environments. 

Our interest in silane coupling agents and in the preparation of silicates has led us to study the mechanisms involved in the various reactions. In a previous paper, we investigated the kinetics and mechanism of the alcoholysis of TMOS [13]. This present work studies the effects of changing the substituents on a para-substituted phenyl attached directly to the alkoxysilane. The alkoxysilane silanes used have only one alkoxy group present to eliminate complications from a competing second or third alcoholysis reaction. The resulting Hammett plot yields additional information on the mechanism of this reaction. 

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