Collanti, nastri adesivi

According to DIN 16920, adhesives are “non-metal materials which bond assembly parts by means of surface adhesion and internal strength”. Before adhesives were standardized, nature herself made use of the technique: the carnivorous sundew produces an adhesive bond with its victim, wasps and bees glue their constructions; and anyone familiar with the sea has seen muscles and barnacles which are almost impossible to remove from a ship’s bow or a rock. We the people started using glue early on – the use of adhesives is older than welding, screwing or riveting. Around 3500 BC, the Sumerians boiled animal skins in order to make a glue; furniture with glued veneers has been attributed to the Egyptians in the 5th century BC.

Adhesion and Cohesion

Back to the DIN norm: the assembly parts are on the one hand bonded by means of the surface adhesion between the glue and the assembly part, i.e. the adhesion, and on the other hand bonded by means of the internal strength, i.e. the cohesion, of the adhesive material.

The bond between the glued piece and the glue itself takes place for different reasons. On the one hand, physically or chemically binding forces on the molecular level are responsible. This is, for example, the case with the bonding of metals and some plastics. On the other hand, very porous materials like, for example, wood mostly attach to the glue mechanically: the glue migrates (diffuses) into the microscopically fine surface pours and then hardens. The configuration of the adhesive and the combined attributes of the adhesive and the adherend determine the strength of the adhesion.

If the adhesive has bonded to the surface of the adherend, the tiny space between two assembly parts can be bridged. After hardening, the adhesive must have attained a sufficient amount of internal strength – the cohesion – for the bond to hold.

An exception to the adhesion-cohesion mechanism is found in the case of gluing thermoplastic plastics with pure solvents like, for example, dichloromethane or acetone. In this case, the solvent solvates (etches) the surface to such an extent that the long molecular chains that make up the solid plastic are loosened and then join with the chains from the assembly part to form a bond. If the solvent evaporates, none of the “glue” will remain behind and the interior strength of the joint will be solely determined by the plastic.

When working with adhesives it is advisable to take account of the temporal progress of the hardening process. Based on experience, the strength of an adhesive bond is very weak at the time of the joining, increases rapidly from there but then slows down. Even in the case of quick setting adhesives, the adhesive strength continues to increase for many hours and even days and only attains its maximum strength with the passage of time. This maximum strength, however, will, depending on environmental influences, then tend to drop off as weeks and months go by until a constant, though somewhat lesser strength is finally attained. (...)