Reaction with Unreactive Surfaces

To explain the adhesion of isocyanate-based adhesives to impervious, apparently unreactive surfaces such as glass and metals, it is considered that isocyanate groups react with the molecular film of tightly absorbed water always present on such surfaces, the reaction of the isocyanate groups with the oxyhydrate layer on metal surfaces and isocyanate polymerization on (alkaline) glass. All of these processes provide intimate contact if not actual chemical bonds between adhesive and adherend. 

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The situation does not improve with the use of trifunctional silanes. It appears that a tridentate reaction with the surface is impossible. Omsequently, a large numbm of new silanols is formed on hydrolysis of the unreacted functional groups. Some of these groups, however, condense with other newly formed silanols to form siloxane bridges. The overall result of both effects is that the total content of silanols in a bonded phase based on a trifunctional silane is at least as high as the silanol content of bonded phases based on monofunctional or difunctional silanes. 

Mechanical adhesion. Cracks and pits are produced on the treated rubber surface which favor the mechanical interlocking with the adhesive. Eurthermore, unreacted solid prismatic TCI crystals on the treated rubber surface can be dissolved by the organic solvent into the adhesive, favoring the reaction with the adhesive. 

These results take into account three possible processes in series mass transfer of fluid reactant A from bulk fluid to particle surface, diffusion of A through a reacted product layer to the unreacted (outer) core surface, and reaction with B at the core surface any one or two of these three processes may be rate-controlling. The SPM applies to particles of diminishing size, and is summarized similarly in equation 9.1-40 for a spherical particle. Because of the disappearance of the product into the fluid phase, the diffusion process present in the SCM does not occur in the SPM. 

There are two mechanisms which allow a radical to react in situations where the parent molecule is inert. First, the reaction between a surface and parent molecule may be exothermic but require a large activation energy to proceed. (This is probably the case for methane which is very unreactive toward a large variety of clean surfaces .) When the activation energy is provided by dissociating the molecule while it is in the gas phase, then the resulting fragments spontaneously react with the surface. 

Primary interest goes to the surface hydroxyls found after modification. Unreacted Q3 and Q4 sites are detected. Concurrent with the silanol number before reaction, also the number of surface silanols after reaction decreases with increasing pretreatment temperature. From the general reaction scheme, these bands may be attributed either to surface silanols in hydrogen bonding interaction with amine groups, or to unreacted surface groups. 

The overall process of coal combustion involves the following three steps in sequence transport oxygen from the bulk stream of gas to the outer surface of the particles, diffusion of oxygen through the ash layer to the reaction interface, and finally reaction with the combustible matter in the unreacted core. The overall flux of oxygen per unit surface area can be expressed as follows ... 

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