Silane Coupling Agents

Abstract

Much progress has been made in the last 8 years in understanding the theory and practice of silane coupling agents. A major advance in this direction was the measurement of true equilibrium constants for the hydrolysis and formation of siloxane bonds. Equilibrium constants for bond retention are so favorable that a silane coupling agent on silica has a thousandfold advantage for bond retention in the presence of water over an alkoxysilane bond formed from hydroxy-functional polymers and silica. In practice, the bonds of certain epoxies to silane-primed glass resist debonding by water about a thousand times as long as the epoxy bond to unprimed glass.Oxane bonds of silane coupling agents to metal oxides seem to follow the same mechanism of equilibrium hydrolysis and rebonding, although equilibrium constants have not been measured for individual metal-oxygensilicon bonds. This suggests, however, that methods of improving bond retention to glass will also improve the water resistance of bonds to metals. Modification of standard coupling agents with a hydrophobic silane or one with extra siloxane cross-linking have improved the water resistance of bonds to glass and metals another hundredfold over that obtained with single coupling agents.Bonds of polymers to silane-treated surfaces may involve simple reaction of the polymer with functional groups of the silane, but most often involve an interdiffusion of siloxane oligomers into the polymer with subsequent cross-linking to form interpenetrating polymer networks in the interphase region. This mechanism correlates with the observation that a silane primer that bonds two different polymers to glass can also be used to bond the two polymers to each other.A simple screening test has been used for evaluating silane primers in bonding many thermosetting and thermoplastic polymers to glass and v vi Preface metals. Ratings from these tests provide a guide for formulators in developing optimum bonding primers.Bonding through silane coupling has been disappointing in processes, such as extrusion or injection molding, that involve high shear in the melt. Bonds between coupling agent and polymer are made and broken during the time of high-temperature shear, so that the full value of coupling agents are not retained. A new method has been proposed that bonds silane-treated particulate fillers to polymers through ionomer linkages. Under conditions of high-temperature shear, the ionomers are fluid, but they set to bonded structures when cooled to room temperature.

Optimisation of wet chemical silane deposition to improve the interfacial strength of stainless steel/epoxy

“…1a (right)) and with the substrate surface (Fig. 1c), forming, respectively, a siloxane network (Si O Si) and metal-siloxane (Si O M) bonds [11,16,18]. It is understood that the self-condensation of the reactive silanol groups in the solution prior to application on the metal substrate should be avoided as much as possible as this consumes hydroxyl groups, necessary for coupling to the steel surface.…”

“…Moreover this can lead to the adsorption of oligomeric siloxane species on the steel surface, leading to weak points in the resulting polymer-metal hybrids. [16,18,20].…”

“…Among the different adhesion promoters, especially silane-based molecules are frequently used [17,18]. In addition, silanes may increase corrosion protection and improve the composite thermal and mechanical properties as well as its chemical or moisture resistance [16,19]. Functional organosilane coupling agents have X n -Si-(R-Y) 4−n as general structure, in which X represents a hydrolysable group, mostly alkoxy or chlorine, R a spacer or linker group and Y the organofunctional group.…”

“…Another approach involves chemical modification of the surface [7,[10][11][12] by e.g. the introduction of a coupling agent [7,11,[14][15][16][17][18][19] which facilitates chemical bonding. The coupling agent acts as a molecular bridge between substrate and matrix, being able to establish covalent chemical bonds or physical interactions with both sides, in this way increasing adhesion [16].…”

“…the introduction of a coupling agent [7,11,[14][15][16][17][18][19] which facilitates chemical bonding. The coupling agent acts as a molecular bridge between substrate and matrix, being able to establish covalent chemical bonds or physical interactions with both sides, in this way increasing adhesion [16]. Among the different adhesion promoters, especially silane-based molecules are frequently used [17,18].…”