Adhesive—adherend interactions

Further molecular mechanics investigations in the same direction but for UF resins also followed, with equally interesting results. In these the efficiency of resin adhesion to both amorphous and crystalline celluloses was computed by following the synthesis of the resin. This was achieved by calculating by molecular mechanics the adhesive/adherend interactions with the two types of cellulose for each isomeride produced through the reaction of urea with formaldehyde. This was done up to the level of trimer. The adhesive/adherend interactions were calculated for urea, monomethylene diureas. 

An important facet of adhesion bonds is the locus of the proposed action or the scale to which the adhesive and adherend interact. Table 1.1 shows a scale of action for each mechanism, which is intended to aid in the understanding of these mechanisms. Of course, adhesive-adherend interactions always take place at the molecular level, which is discussed later in this chapter. 

A brief review of each technique will be followed by a discussion of results illustrating the application of the particular technique to adhesion. Examples are given of surface characterization of pretreated adherends, adhesive/adherend interactions, failure surface analysis, and correlation of these results to bond performance. Good summaries of the four major surface analytical techniques, namely XPS, AES, ISS, and SIMS, are given by Hercules(5) and Hofmann.( ) (See also Chapter 6 by Davis.)... 

Adhesives must function solely through surface attachment. Therefore, the nature of the condition of the adherend surface is crucial to the formation of strong and durable bonds. By surface we usually mean that region of a material which interacts with its surroundings. There is some region of a bonded assembly where the adhesive and the adherend interact, but only rarely is this a sharp boundary. Usually it is a very diffuse, somewhat ill-defined region of interaction that has become... 

B. Beck, An Investigation of Adhesive/Adherend and FiberjMatrix Interactions, Part B, SEMjESCA Analysis of Fracture Surfaces, NASA Report NAGl-127, January 1983. 

For plastics, we do have the D-3929, Practice for Evaluating the Stress Cracking of Plastics by Adhesives Using the Bent-Beam Method. It recognizes that some adhesives may interact with plastic adherends in such a way as to induce areas of weakness leading to stress cracking. 

The two issues that are dominant in determining the interfacial strength in the case of contact adhesion are (1) the completeness and intimacy of contact between the adhesive and adherend at the interface and (2) the strength of the intermolecular interactions across the interface. Methods for predicting both of these factors are discussed below. 

There is no unify ing theory of adhesion describing the relationship between practical adhesion and the basic intermolecular and interatomic interactions which take place between the adhesive and die adherend either at die interface or within the interphase. The existing adhesion theories are, for the most part, rationalizations of observed phenomena, although in some cases, predictions regarding the relative ranking of practical adhesion can actually be made. 

In all adhesive joints, the interfacial region between the adhesive and the substrate plays an important role in the transfer of stress from one adherend to another [8]. The initial strength and stability of the joint depend on the molecular structure of the interphase after processing and environmental exposure, respectively. Characterization of the molecular structure near the interface is essential to model and, subsequently, to maximize the performance of an adhesive system in a given environment. When deposited on a substrate, the silane primers have a finite thickness and constitute separate phases. If there is interaction between the primer and the adherend surface or adhesive, a new interphase region is formed. This interphase has a molecular structure different from the molecular structure of either of the two primary phases from which it is formed. Thus, it is essential to characterize these interphases thoroughly. 

Comyn [1] has pointed out that maximum bond strength and consequently greater adhesion between the substrate and polymer could be achieved with a monolayer of silane bound to both the adherend and adhesive. The current investigation was undertaken to evaluate the possibility of monolayer level depositions on silicon substrates by employing a few w -functionalized alkanoyl-substituted derivatives of APTES which will provide polar moieties as well. The interactions of these functionalized silanes covalently immobilized on silicon with octadecylamine and octadecanoic acid, used as models for basic and acidic polymeric adhesives, were also examined in this study. Characterization of the silanized surfaces as well as studies on their interactions with the above two chemical probes were carried out through ellipsometric and XPS measurements. 

Abstract—The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive. 

Strong chemical bonds between the adhesive and adherend help stabilize the interface and increase joint durability. Aluminum joints formed with phenolic adhesives generally exhibit better durability than those with epoxy adhesives. This is partially attributable to strongly interacting phenolic and aliphatic hydroxyl groups that form stable primary chemical bonds across the interface. 

By far the dominant adhesion mechanism, particularly in the absence of covalent linkages, is the electrostatic attraction of the polar groups of the adhesive to polar groups of the adherends. These are mainly forces arising from the interaction of permanent dipoles, including the special cases of hydrogen bonding (10-25 kJ/mol) and Lewis acid-base interactions (<80 kJ/mole).25 26 These forces provide much of the attraction between the... 

Diffusion Theory. The diffusion theory of adhesion is mostly applied to polymers. It assumes mutual solubility of the adherend and adhesive to form a true interpliase. The solubility parameter, the square root of the cohesive energy density of a material, provides a measure of the intemiolecular interactions occurring witliin the material. Thermodynamically, solutions of two materials are most likely to occur when the solubility parameter of one material is equal to that of the other. Thus, the observation that "like dissolves like." In other words, the adhesion between two polymeric materials, one an adherend, the other an adhesive, is maximized when the solubility parameters of the two are matched ie, the best practical adhesion is obtained when there is mutual solubility between adhesive and adherend. The diffusion theory is not applicable to substantially dissimilar materials, such as polymers on metals, and is normally not applicable to adhesion between substantially dissimilar polymers. 

Molecular Forces Between Adherend and Adhesive. The various theories of adhesion invoke the occurrence and interplay of physical and chemical interactions across the adherend-adhesive interface, as well as the deformation behavior of the adhesive (6, 7). Therefore, bond formation depends upon the development of intermolecular attraction, both within the bulk of the polymer and between adhesive and adherend. 

Adhesive force Act between the adherend surfaces and the adhesive layer molecules and are mainly based on electrical interactions (dipoles). 

---