science when progress in the art has outstripped that in basic understanding. The evergrowing number of new high poly- mers that are useful for adhesi...
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approximately the middle ' ~ O ' S , the art of bonding solid adherends together by means of synthetic high polymeric adhesives had emerged as a technology which has since developed extensively. About this time many workers in the field began to realize that basic understanding of the nature of adhesion was required for continued rapid development of the technology of adhesive bonding. This need gave rise to a widespread search for adequate chemical, physicochemical, and mechanical theories to explain a large mass of accumulated empirical information, as has happened so often in the history of science when progress in the art has outstripped that in basic understanding. The evergrowing number of new high polymers that are useful for adhesives have made easier the tasks of those searching for sound theories of how they work by providing material for intercomparisons among types of high polymers. At the same time, the many and varied new formulations of the same high polymers into marketable products have injected new behaviors requiring explanation. One need only to relate to time the number of conferences, symposia, and committees devoted solely to various aspects of adhesives and adhesion to appreciate the great increase of interest in and importance of this field of research in the last 10 to 15 years. We come to the time of this symposium with many theoretical questions unanswered. There is far too little systematic information on the stability of adhesive bonds, as a function of time, in various environments-hot, cold, wet, and radioactive. There is doubt as to what is the best method for measuring the strengths of adhesive bonds. The nature of the bonds between metal surfaces and adhesives is imperfectly. understood. Here we seem to be in an era of transition away from the point of view that van der Waals forces can account adequately for almost all of the bonds encountered and toward the idea that covalent, ion-dipole, and electrostatic charge transfer types of bonds are operative in adhesive-

adherend systems. The structures and properties of oxide films on metals and their influence on bond strength and stability are under rather extensive investigation. Specific effects of metal (metal oxide) surfaces on the stability of adhesive bonds, especially at elevated temperature, are beginning to be found and studied. Considering adhesion as an adsorption phenomenon represents a significant, more or less recent, advance in the field along theoretical lines. As the chemistry of epoxy resins becomes better understood, these versatile components of adhesives find their way into an ever-increasing number of formulations having explicitly tailored properties. Much basic information still is needed, particularly concerning bonding mechanisms, in these systems. Interest is still high in the problems of adhesion encountered in polyester-glass laminates and the effects of finishes on the strength of adhesion, with a number of basic problems still requiring solution. Theoretical calculations of the strengths of adhesive bonds, considering the interface between adherend and adhesive, have frequently been made on the basis that all sites on the surface of a metal participate in the bonding. Because strong bonds would be required to provide observed adhesional strengths if very few surface sites interacted with the adhesive, and weak bonds would more than account for the experimental values of the strengths if many interactions were possible, atfention is being focused on arriving at some sort of realistic evaluation of the number of adhesive-adherend interactions per unit area of surface. Only a few of the current problems in the field of adhesives and adhesion are mentioned here. The articles comprising the present symposium contain material that bears on one or another aspect of most of these problems, and hence they contribute measurably to the progress of understanding in this complicated field. JOHN E. RUTZLER, JR. Case Institute of Technology VOL. 5 0 , NO. 6

JUNE 1958