An adhesive, by traditional definition such as DIN-EN 16920, "is a non-metallic cement which bonds materials by surface adhesion in such a way that the bond exhibits a sufficient inner strength (cohesion)." In modern engineering, this definition is not sufficient in that the bond line is recognized to be an integral part of the body it helps to assemble. Apart from the obvious properties - adhesion and cohesion - the dynamic mechanical parameters such as modulus of elasticity and other viscoelastic properties play an important role in determining the overall mechanical behavior of the body. Such functional bond lines, when properly engineered, can be instrumental in absorbing vibrational or static stress. The latter is typical in situations where two materials of different thermal expansion coefficients are joined.
It is well known that there is a clear correlation between the chemical nature of the backbone polymer of the adhesive and the properties of the bond. Polymers with a rigid chemical structure and a high degree of crosslinking yield high bond strengths and high moduli. In other words, they are tough but brittle. Epoxies are typical representatives of such adhesive materials. At the other extreme are polymers, which form flexible networks. They are highly elastic, i.e., they exhibit low elastic moduli and limited bond strengths. Various siloxane compounds fall into this class.