Page 111 - Handbook of Adhesives and Sealants
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Theories of Adhesion  79


            was exceeded, it is likely that the effects would be different. That is,
            the material would expand because of increased thermal expansion
            coefficient, and the modulus would decrease because of increased free-
            dom of rotation in the molecule. Many of these effects will be described
            in detail in later chapters. However, it is important to realize that
            these changes could either increase or reduce the stress on the joint.
            This is why the ‘‘environmental stress’’ effect that was shown in Fig.
            1.11 can either positively or negatively affect the natural bond
            strength of a joint.
              Aging can also increase degradation of the adhesive, adherend or
            interphase regions. These degradation mechanisms generally lead to
            a lowering of the cohesive strength of the adhesive. For example, ther-
            mal heating may first cause softening of the adhesive and, then, ad-
            ditional heating will cause oxidation and finally pyrolysis of the mo-
            lecular chains. Exposure to moisture can cause cohesive weakening
            through a hydrolysis reaction mechanism.
              Environmental factors may also cause new interphase areas that
            could provide a weak boundary layer in the joint. Environmental mois-
            ture can diffuse through the adhesive and/or adherend, accumulate
            at the interface and cause corrosion of metallic adherends. Plasticizer
            or moisture migration could also accumulate at the interface and form
            a weak boundary layer.


            2.5  Special Mechanisms Related to
            Sealants
            All of the modes of adhesion failure described in the preceding sections
            also apply to sealants. Adhesion failure, a loss of bond between the
            sealant material and its substrate, is a common type of sealant failure.
            Sealant failures most commonly occur under the following conditions.

            1. The sealant is improperly formulated and, therefore, does not ad-
               here to the substrate (i.e., the surface energetics do not allow good
               adhesion to occur).
            2. There is a weak boundary layer either initially or formed during
               service, and the weak boundary layer fails in service.
            3. The sealant becomes brittle and more limited in its movement ca-
               pability because of aging or environmental conditions. Thus, it can
               no longer compensate for the required substrate movement, and
               bond failure results because the cohesive strength is greater than
               the adhesive strength.

              Cohesive failure is a failure within the body of the sealant material.
            This failure frequently begins with a small nick or puncture of the
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