Primary bonding at the interface

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... 

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

For example, in an investigation of silanes as adhesion promoters for ethylene/ vinyl acetate co-polymer encapsulants reported by Koenig et aL, the organo-silanes (referred to as primers ) were shown to generate primary chemical bonds at the polymer/substrate interface [27]. 

In a good adhesive bond, the joint is able to transmit both tensional and shear forces across the interface. Generally, an adhesive bond will be weaker than either bulk material joined by it since the predominant forces at the interface are Van der Waals while those in a bulk material are usually covalent. If one phase is a microorganism, this may not be true since a proportion of the "bonds" holding the organism together are not covalent (e.g. hydrophobic association in the membrane glycolipid-protein complexes). In macroscopic adhesive bonds, flaws are the primary source of weakness since the area for bacterial adhesion... 

Quinones are essential components of the electron transport chain of both bacteria and higher plants.127 Since quinones can act as primary acceptors, investigations were made of the charge transfer processes in models in which porphyrin and quinone are bound by covalent bonds at different distances from one another.128 An investigation of the mechanism of formation and disappearance of radicals during the interaction of chlorophyll and quinone under laser illumination showed that the most optimal conditions for the charge separation process exist at the interface, where the recombination of charges129 in different phases is difficult. 

The model suggests that charge transfer is a primary mechanism of Schottky barrier formation, and a good agreement with the experimental results is found for polycrystalline inorganic semiconductors. It should be emphasized that the model is based on the thermod)mamic equilibrium of electrons across the interface between the metal and the semiconductor, and is facilitated by the interface bonds. Therefore, it does not depend on the details of the interface reactions, so long as the physical properties of the semiconductor, such as IP and Eg, remain intact at the interface. The model does not apply to interfaces where strong chemical reactions result in the domination of the interface by new reacted species. 

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