Handbook of Adhesives and Sealants

Effect of the Environment 713to their base metals. Resistance of adhesive joints to moisture can beimproved either by preventing water from reaching the interface or byimproving the durability of the interface itself. The first method ofpreventing degradation can be achieved through formulation of theadhesive system. These possibilities include:1. Proper selection of a base polymer having low water permeability2. Incorporating inert fillers into the adhesive to lower the volumethat can be affected by moisture3. Coating the exposed edges of the joint with sealants having verylow permeabilities to inhibit wicking of moisture along the interface4. Chemically modifying the adhesive to reduce water permeationMethods that are available to improve the durability of the interfacecenter on surface preparation and use of primers or coupling agents.These are most beneficial on metallic surfaces that are prone to degradationby corrosion. Primers and surface treatments tend to hinderadhesive strength degradation in moist environments by providingcorrosion protection to the adherend surface. A fluid primer that easilywets the interface presumably also tends to fill-in minor discontinuitieson the surface. Organosilane, organotitanate, and phenolic primershave been found to improve the bond strength and permanence ofmany adhesive systems.Approaches for the development of water resistant surface treatmentsinclude application of inhibitors to retard the hydration of oxidesor the development of highly crystalline oxides as opposed to moreamorphous oxides. Standard chemical etching procedures, which removesurface flaws, also result in improved resistance to high humidity.17.4.3 Combined effects of stress,moisture, and temperatureMechanical stress accelerates the effect of the environment on the adhesivejoint. A great amount of data is not available on this phenomenonfor specific adhesive systems because of the time and expenseassociated with stress-aging tests. However, it is known that moisture,as an environmental burden, markedly decreases the ability of certainadhesive to bear prolonged stress especially at slightly elevated temperatures.The interaction of temperature and moisture causes greaterdegradation than can be attributed to either environment by itself.