Adhesive bonding aspects

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Even though the substrates are quite dissimilar, adhesive bonding of composite material is comparable to that of aluminum. Only aspects unique to composites will be discussed here. There are many details of composite part design, manufacture and performance that are not necessarily related to adhesive bonding that also will not be discussed. 

There are, of course, many more aspects of composite hardware design that differ from metallic bonded structure but do not necessarily involve adhesive bonding. For instance there are many types of reinforcement tape and fabric to choose from, the orientation of the plies must be chosen, the ply stackups must be balanced to avoid part warping after cure, a minimum number of plies must be used to prevent non-visible impact damage that significantly affects the load carrying capability of the part, etc. 

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. 

In deciding which surface chemistry tools to use for the broad area of adhesion and for adhesive bonding in particular, a number of aspects must be considered. More often than not, a combination of instruments must be used to take advantage of the unique information provided by each method. Table 1 shows some of the important aspects of adhesive bonding and some of the characterization methods... 

Aspects of adhesive bonding and applicable surface characterization methods1... 

Aspects of Adhesive Bonding and Applicable Surface Characterization Methods... 

Kinloch, A. J., Interfacial Fracture Mechanical Aspects of Adhesion Bonded Joints, Review Article, Journal of Adhesion, vol. 10, 1979, p. 193. 

Sharpe, L. H., Aspects of the Permanence of Adhesive Joints, in Structural Adhesive Bonding, M. J. Bodnar, ed., Interscience, New York, 1966. 

Morris, C. E. M., Strong, Durable Adhesion Bonding Some Aspects of Surface Preparation, Joint Design, and Adhesive Selection, Materials Forum, vol. 17, 1993, pp. 211-218. 

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of low-surface free energy is suggested. 

We develop certain aspects of the adsorption theory of adhesion more fully than has been done previously, based solely on (free) surface energetics, and show how they can be applied to real or practical systems. As a consequence, it is shown that the deBruyne adhesion rule [7] is incorrect in part. Our concern is mainly with what we believe to be the most important problem in the making of adhesive bonds—that is, the achievement of extensive and proper (no intermediate phase) interfacial contact. We discuss also the breaking strength of certain adhesive joints where this is necessary to the development of our thesis. However, the processes of making and breaking adhesive joints bear no... 

Several terms are used to describe important aspects of the formulation or functioning of adhesives, and of equipment for doing adhesive bonding. Some of these terms, and their definitions, are the following ... 

In general, the applications of spectroscopic techniques in the study of various aspects of adhesive bonding that have been reported are the following ... 

The techniques highlighted here are XPS, AES, SIMS, various forms of FTIR, Raman spectroscopies, and HREELS. This selection is based on their relative ease of application and interpretation, their commercial availability, and the unique capabilities that each technique possesses for the study of an aspect of adhesive bonding. These capabilities are also highly complementary. The applications discussed are chosen to illustrate the applications in three major areas described earlier surface characterization, modification of metal or polymer surfaces, and analysis of interfaces. 

This chapter summarizes the principles of some of the many spectroscopic techniques that are available for the analysis or study of aspects of adhesive bonding science and technology. As indicated in Table 1, there are dozens of techniques and new acronyms appear almost on a daily basis. The number of instrumental spectroscopies available today to the scientist is bewildering, especially the many techniques for surface characterization. Therefore, it is likely that some techniques have been missed, although it was attempted to cover them all, at least in Table 1. The choice of techniques from that listing that were actually discussed in this chapter had to be limited and was in some cases somewhat arbitrary and subjective. However, some emphasis was put on techniques that can be used in the study of the science of adhesive bonding technology. Techniques for routine analysis, e.g., NMR or the various mass spectrometries, were not discussed in depth. 

It is clear that tremendous developments have been made in recent years with certain techniques. A large number of spectroscopic techniques are now available that can be adapted, with little or no adaptation, to the study of various aspects of adhesive bonding. 

The formation and operation of adhesive-bonded joints in liquids is characterized by a niunber of featirres, some physical-chemical aspects of which will be considered in the present chapter. 

One of the most frequent procedures in the formulation of adhesives is the addition of adhesion promoters. Of these, silanes are by far the most frequently used. The silanes form a multi-membered family of chemicals. Numerous publications have described aspects of silane chemistry and the applications of silanes [15-18]. Although many silanes are readily available, in many industrial formulations of adhesives the most frequently used of these are the primary amine and diamine versions because of their relatively low cost. However, these do not necessarily produce optimum benefits in adhesion for all applications. Guidelines for the selection of silanes best suited for specific applications would therefore be useful. These should be based on a fundamental understanding of the manner in which silane compounds affect the interface and interphase in adhesively bonded systems. The... 

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