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7.1.1 Mechanical interlocking Many users of adhesives will intuitively understand the importance of mechanical interlocking. Often referred to as “keying” a surface prior to bonding, painters and bonders have improved adhesion by using mechanical abrasion of some form for many years.The benefits of this surface preparation include removal of surface contaminants and increasing the effective surface area for bonding due to surface topography and roughness. Many studies have been performed to correlate the relationship between surface roughness and adhesion performance and, although there is obviously a link, it is not possible to explain all adhesion phenomena by this mechanism alone. 7.1.2 Diffusion theory Proposed in the early 1960’s by Voyutskii, and based on a series of experiments which related adhesion between two polymers to time and temperature, modelled on the basis of a diffusion process using Ficks law.The inter-diffusion of two polymers requires a degree of chain mobility and mutual solubility. The diffusion theory can be used to explain the auto-adhesion of polymers, and solvent welding of amorphous polymers, but has no place in the consideration of adhesion between organic and inorganic material phases. 7.1.3 Electronic theory The basis of the Electronic theory is that if two materials with different electron band structures are brought into intimate contact, there will some transfer of electrons, lead- ing to electronic interaction of the molecules. There is much controversy surrounding this theory and it is not generally accepted as the prime reason for adhesion. 7.1.4 Adsorption theory The adsorption theory states that provided intimate contact is obtained between substrate and adhesive, adhesion will occur because of intermolecular forces that occur between atoms, molecules and ions of the two phases. There are a variety of bond types that can occur but he most common is thought to be Van der Waals forces. 7.1.5 Weak Boundary Layers The concept of a “proper” joint is based on cohesive failure of the adhesive or substrate. Any joint that fails interfacially can be considered to be improper. Such failures are said to be due to a “weak boundary layer”. The simplest WBL is the presence of a layer of contamination (eg grease) on the surface of a substrate. However WBLs can be due to many sources such as incorrect pre-treatment processes, segregation of components within the adhesive or substrate, or the development of a weakened layer due to exposure to environmental attack. 58 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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