Bonding - Interphase or Interface Considerations

The interphase is a thin region which exists between the bulk adherent and the bulk adhesive [7]. A surface oxide, either native or one produced by pre-treatment, is present on most metal adherends. These oxide layers will frequently be contaminated, even after cleaning. The net effect of absorbed layers is  

A primer is often applied in a production process after pre-treatment and before the application of the adhesive. Typical thickness for the oxide are 0.003 - 0.4 pm, for the primer 5-10 pm and for the adhesive 10 - 15 pm. The interphase region is expected to have mechanical properties different from either the adherend or the adhesive. Filbey and Wightman [9] comment that measurement of these properties is important in understanding adhesion, for example, poorly durable bonds are often a consequence of 

Minford [12] discusses at some length cohesive versus adhesive failure. On the topic of failure due to water desorption of the adhesive Laird [13] has shown that water can progress by diffusion along the interface as much as 450 times faster than by permeation. Adhesive systems can also be sensitive to certain of the strong polar solvents as they contain polar elements themselves. Dilution of an adhesive can be achieved by the use of this principle but after the adhesive has been cured in the bondline the same solvent can attack the adhesive and destroy the adhesion at the interface. 

A further interfacial factor can be the presence of non adsorbable or non desorbable contaminating films (as previously mentioned above) at the interface. Such materials can be oils, fatty acids, plasticisers from the rubber and metal processing oils from inadequately cleaned metal components. Some of these lubricants can be absorbed by the adhesive if it is solvent-based but in the case of the new waterborne rubber to metal systems this absorption cannot take place, for the systems are neither miscible or compatible with oils. These new waterborne systems have a critical tolerance level for surface contamination of the metal and if this is exceeded then wetting out of the metal by the adhesive will not, at the worst be possible, or at the best complete. 

When examining the surface of a failed bonding it is extremely difficult to determine whether the failure has taken place at the original interface or whether a new interface has been opened, either in the adhesive or in the rubber, for the distance from the old to the new interface can be extremely small. Thus it may not be possible to determine whether contamination of the interface was the cause of the failure or not. Examination of the surface of the failed bond using very specialised equipment such as Secondary Ion Mass Spectroscopy (SIMS), Ion Scattering Spectroscopy (ISS) and Auger Electron Spectroscopy (AES) can greatly assist with determination of some of the causes of this type of failure. 

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