The service life of adhesive joints

Adhesive joints are frequently expected to perform satisfactorily under service conditions which include dynamically and statically applied loads and exposure to hostile environments such as water, organic solvents, etc and, in many instances, combinations of these conditions may be experienced. It is therefore of prime importance for the adhesives technologist to be able to develop and recommend adhesive systems which will possess an adequate service life under the operating conditions which are to be experienced by the bonded structure. This, in turn, leads to the need to understand the mechanisms of failure and to develop test methods for (i) developing and selecting adhesive systems and (ii) predicting quantitatively the expected service life. 

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Life prediction of adhesive joints in service is difficult because several degradation mechanisms may be operating simultaneously. However, a process sequence to use in estimating the service life of adhesive joints is as follows ... 

The present chapter has emphasised the types of loading conditions and environments which may be experienced by a bonded component and which may greatly reduce the service-life of adhesive Joints, unless consideration is given to all aspects of the material selection, choice of surface pretreatment and the design of the bonded component i.e. unless an adhesive system approach is adopted. 

Two other points of interest are that it appears that such hydration inhibitors are only really effective on an aluminium alloy substrate which has already been pretreated by the CAE process. It would obviously be useful for the technologist if such simple primers were effective on a degreased or abraded surface. Secondly, the above work [82] also suggested that some silane-based primers could function as hydration inhibitors, so adding another possible mechanism whereby such organometallic primers might increase the service life of adhesive joints. 

To predict the service life of adhesive joints exposed to a hostile environment requires a knowledge of the mechanisms and kinetics of attack. In the relatively simple epoxy/abraded steel joints exposed to water the work described in previous sections has identified the mechanism as being the rupture of interfacial secondary forces, as predicted from a consideration of the thermodynamics of the system, and the kinetics as being governed by the diffusion of water through the adhesive to the interface. Thus, in this system one might hope to describe a model to predict quantitatively the service life, and Kinloch and co-workers [47,123] have proposed the following model. 

Moisture has a profound effect on the service life of adhesive joints made with some adhesives between some adherends. This point has been made in the section on Water Sensitivity in Chapter 5. There are four aspects of the problem which need to be drawn together when looking at joints which have been exposed to moisture or are to be designed to withstand such exposure. These comprise the ability of moisture to diffuse through the adhesive, its susceptibility to displacement from the substrate, the corrosion susceptibihty of the substrate and, lastly, the role of any primer or sealing agent in modifying the first three considerations. 

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