1.1.13.2.2 Thermosetting Adhesives

1.1.13.2.2 Thermosetting Adhesives

Epoxy, urethane and acrylate adhesives are common examples of thermosetting adhesives. Thermosetting adhesives cure by an irreversible chemical reaction, referred to as crosslinking, which is brought about by the linking of two linear polymers, resulting in a rigid three-dimensional chemical structure. Some crosslinked reactions require heat for initiation and completion, while others can be completed at room temperature. With certain ‘room temperature’ curing adhesives, it is the internal heat of the reaction generated by the curing mechanism, called the exotherm, which provides the energy required to completely cure the polymeric material.

Thermosetting adhesives (like thermosetting plastics), once cured and crosslinked, cannot be reheated, melted or reused. This thermosetting property gives them good heat resistance compared to thermoplastic varieties, which may be remelted. High enough heat may soften and degrade thermosetting adhesives through oxidation or molecular chain growth polymerisation.

Thermosetting adhesives may be one-component (1C) or two-component (2C) systems. A 2C system, such as the resin and hardener of epoxy, is stored separately and mixed at a specified ratio (weight or volume) just before use to initiate the chemical reaction and crosslinking. Mixing may be done by hand, in dual cartridges with static mixer tips or in large industrial equipment designed for the purpose. Care should be taken to observe the manufacturer’s stated ‘open time’ as the best bond to the substrate will occur before crosslinking thickens and begins to harden the adhesive. A 2C adhesive system generally offers a longer shelf life because the reactive components are separated from each other until mixed for use.