Silane coupling agents, adhesion bonding

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

Abstract—When K. L. Mittal asked me to provide a historical account of the applications of silane coupling agents in adhesion. I decided to write in the form of a personal account of my last 45 years in this line of study. No attempt is made to make the history comprehensive, or to recognize the host of other researchers who have contributed to our understanding of adhesion across an interface of dissimilar materials. It has been an immensely interesting area of study with many practical applications in composites and bonded structures. 

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 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. 

The problems in bonding polymers to hydrophilic mineral surfaces were discussed and a theory of adhesion in which silane coupling agents provide a bond at the interphase was proposed by Plueddemann in 1974 [3]. However, silane coupling agents specifically designed to impart hydrophilicity to mineral surfaces have been proposed only recently [34], A trimethoxysilyl-terminated poly( /V-... 

However, the chemical bonding theory cannot account for the increase in adhesion experienced between non-reactive matrices such as polyolefins and inorganic reinforcements in which chemical bonds will not be formed [4], This observation, among others, leads to an alternative proposal that an interphase composed of various constituents forms surrounding the reinforcement. This third phase in the composite is possibly formed through interdiffusion of physisorbed silane and matrix molecules in the interphase and perhaps via preferential adsorption of both matrix components as well as silane coupling agents on the reinforcement surface [5],... 

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