Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 29Since nearly every adhesive application is unique, the adhesivemanufacturers often do not have data concerning the aging characteristicsof their adhesives in specific environments. Thus, before anyadhesive is established in production, a thorough evaluation shouldbe made in either a real or a simulated operating environment.With most structural adhesives, strength is more directional thanwith mechanical fasteners. Generally, adhesives perform better whenstressed in shear or tension than when exposed to cleavage or peelforces. Residual stresses inside the joint can also present serious problems.Such stresses arise from shrinkage due to cure or aging, fromdifferent coefficients of thermal expansion between substrates, andfrom other circumstances.The adhesive material itself should never be used as a structuralsubstrate. Very heavy bondlines with uneven joint thickness result inundesirable concentrations of stresses. Many adhesives cure by an exothermicchemical reaction whose intensity is dependent on the massof material. Adhesives are generally formulated to cure in thin sections.Therefore, certain epoxy adhesives, when applied in significantbulk, could over-heat due to their own crosslinking reaction and, infact, burn or degrade when cured in thick sections.1.4.7 Design limitationsThe adhesive joint must be carefully designed for optimum performance.Design factors include the type of stress, environmental influences,and production methods that will be used. Many rigid adhesivesdo not work well when external stresses act to peel or cleave the substratesfrom one another. These stresses can often be reduced or eliminatedby careful joint design. Seldom can a joint, which is designedfor mechanical fastening, be used successfully for adhesive bondingwithout revision. Sometimes such revisions result in added expenseor manufacturing steps.There are no standards to guide the user with regard to design limitsor to provide a safe design margin. These will depend on the adhesiveand substrate, on the production methods, on the specificend-use environment, and on many other factors that are often notforeseen at the time of design development. Therefore, it is very difficultto predict the useful life of a bonded joint. Simple life estimationprocesses used in other industries (such as Arrhenius plots to predictthe aging of electrical wire insulation) are not effective with adhesivesbecause of the numerous and sometimes competing reactions that cantake place within a bonded joint. The only effective method of estimatingthe useful life of an adhesive bond is to test prototypes underenvironmental conditions that will accelerate the stress on the bond.