Interfaces primary bonding

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

Examples of the diversity of possible interface structures are as follows. Topotactic interfaces. Primary valence forces may link closely juxtaposed, or perhaps coherent, reactant and product phases so that the crystalline product retains the orientation and some structural features of the reactant [29,30]. The interface, with thickness of molecular dimensions, is defined by the discontinuity of structure and bonding within which reactivity is locally enhanced by the strain field. Product catalysis. If the residual solid is a catalyst for breakdown of a reactant constituent, decomposition may occur within chemisorbed material at the product... 

It is easily understandable that chemical bonds formed across the adhesive substrate interface can greatly participate to the level of adhesion between both materials. These bonds are generally considered as primary bonds in comparison with physical interactions, such as van der Waals, which are called secondary force interactions. The terms primary and secondary stem from the relative strength or bond energy of each type of interaction. The typical strength of a covalent bond, for example, is on the order of 100 to 1000 kJ/mol, whereas those of van der Waals interactions and hydrogen bonds do not exceed 50 kJ/mol. It is clear that the formation of chemical bonds depends on the reactivity of both adhesive... 

For FRCs, the stress transfer at the interface between the matrix and fibre phases is determined by the degree of adhesion. Effective transfer of stress and load distribution throughout the interface is possible when strong adhesion exists at the interfaces. The adhesion phenomenon is described in several theories including mechanical interlocking, absorption, primary bonding, interdiffusion, electronic theories, etc. 

There are relatively few well-documented examples of interfacial primary bonding in the literature but it is possible to find examples in the areas of organic coatings on steel, metallized plastics and adhesion promoters. As the following examples will show, the exact definition of the chemistry resulting from primary bond formation at the interface has only become possible with the advent of surface analysis techniques. Such investigations rely heavily on XPS and ToF-SIMS for interfacial analysis. 

Thus, when investigating the nature and mechanism of adhesion between an adhesive, coating or polymer matrix and the substrate, it is important to consider the possibility of primary bond formation in addition to the interactions that may occur as a result of Dispersion forces and Poiar forces. In addition to the Adsorption theory of adhesion, adhesion interactions can sometimes be described by the Diffusion theory of adhesion, Electrostatic theory of adhesion, or Mechanical theory of adhesion. Recent work has addressed the formation of primary bonding at the interface as a feature that is desirable from a durability point of view and a phenomenon that one should aim to design into an interface. The concept of engineering the interface in such a way is relatively new, but as adhesives become more widely used in evermore demanding applications, and the performance XPS and ToF-SIMS systems continues to increase, it is anticipated that such investigations can only become more popular. 

However, the formation of such bonds is very difficult to demonstrate uniquely and most of the evidence presented is circumstantial or indirect (see Primary bonding at the interface). Other theories have also been put forward, but the covalent bond theory remains the most widely accepted one. The remaining silane groups on the silicon atom can then condense with themselves so that a three-dimensional network is believed to be formed that is of the type shown in Fig. 1. 

An unprimed silicone adhesive implies that it is free of any adhesion promoter. The substrate on the other hand, may still need to be activated or primed. Adhesion relies mainly on chemical and/or mechanical mechanisms (see Mechanical theory of adhesion and Primary bonding at the interface). The chemical adhesion depends on both the reactivity of the selected silicone cure system and on the natural presence of reactive groups on the surface of the substrates. 

Primary bonding at the interface J F WATTS Examples in organic coatings, metallized plastics and adhesion promoters... 

The extent of adhesion at the polymer/flller interface may be related to various parameters of adsorption and vetting. Factors related to the adsorption of the polymer onto the filler are types of interfacial forces (secondary, primary bonds), molecular orientation/conformation at the interface, and polymer mobility. Contact angle, surface tension, and substrate critical surface tension are among factors related to wetting. 

Figure 6.9 Specific adhesion mechanism. In this example we see the structure and the interfacial bonding of y-amino-propyltriethoxysilane on a silica surface. Chemical bonds (ionic, covalent, metallic) are formed across the interface via the adhesion promoters that bind to surfaces with these primary bonds. Reprinted from Chiang et al. (1980), with permission from Elsevier...

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