A wide array of chemically setting adhesives is available. Initially, there are low — molecular (and consequently flowable or low-viscosity) substances in which the reactive groups may react with each other under specific conditions (see Sections
5.4 to 5.8); this transforms the low-molecular systems into polymer substances with a high relative molecular mass and high mechanical resistance [3].
In the chemical setting process, the build-up of strength by means of chemical reactions must take place only after application of the adhesive and after joining of the adherents; otherwise, the adhesive would lose its wetting capacity. The chemical reaction can be initiated by mixing two or more reactive components shortly before applying the adhesive. After application, the materials transform into a macromolecular substance within the adhesive layer at room temperature. The adherents must be fixed until this substance has at least been partly formed because, in the low — molecular state, the adhesive is not able to transmit forces. In most cases, the time needed for the setting process can be shortened by the application of heat.
It is possible to force the reaction to start under specific conditions in the bond-line, for example when heat is applied, by using reactive components with specific chemical structures. These adhesives are known as ‘one-part adhesives’, although from a chemical standpoint they mostly consist of two or more reactive components.
4.3.1