Reactive hot melt adhesives are based on polyurethane (PUR).

Sequence of the adhesive reaction
The moisture-curing hotmelt adhesives bond via a two-stage reaction:

In the first stage, the adhesive passes through a physical setting mechanism, which gives the adhesion a certain degree of strength. This strength is sufficient to allow further processing or transport.

In the second stage, which can last from hours to days depending on the type of adhesive, a chemical post-crosslinking takes place. This chemical reaction depends on the moisture content and accessibility of the adhesive joint to air humidity. After the hotmelt adhesive has reacted, it loses its thermoplastic properties, i.e. it does not become soft again, even if the object to be bonded heats up. The resulting thermoset has exceptionally good polyurethane adhesion values, combined with high strength, high heat resistance, good low-temperature flexibility and good solvent resistance.