Most polyurethane adhesives do not immediately have a high bonding strength when bonding and still need to be cured. Curing refers to the process by which liquid adhesives turn into solids. The curing process also includes post-curing, that is, the reactive groups in the initially cured adhesives further react or crystallize to achieve the final curing strength. For polyurethane adhesives, the curing process is one that enables the -NCO groups in the adhesive to react completely, or allows the solvent to evaporate completely and the polyurethane molecular chains to crystallize, thereby generating a sufficiently high adhesive force between the adhesive and the substrate.
Polyurethane adhesives can cure at room temperature. For reactive polyurethane adhesives, when room temperature curing takes a relatively long time to complete, an appropriate amount of catalyst can be added to promote the curing of the adhesive. In addition, to shorten the curing time, heating can also be adopted to promote curing. Heating not only benefits the curing of the adhesive itself, but also accelerates the reaction between the -NCO groups in the adhesive and the active hydrogen groups on the substrate surface. Meanwhile, heating can also soften the adhesive layer, increasing its wetting of the substrate surface and facilitating molecular movement to find a "partner" that generates molecular forces at the bonding interface. Heating is also very beneficial for enhancing the adhesive force of polyurethane adhesives. For instance, when a two-component
Polyurethane Adhesive is used to bond steel plates, its bonding strength varies at different curing temperatures and curing times.
The heating methods for curing include oven or drying tunnel heating, drying room heating, and fixture heating, etc. For metal substrates with fast heat transfer, fixtures can be used for heating, and the adhesive layer heats up faster than when heated in an oven. The heating process should be carried out gradually to increase the temperature. For solvent-based polyurethane adhesives, attention should be paid to the evaporation rate of the solvent. Most of the solvent has evaporated during the drying process, and the remaining solvent will gradually diffuse outward through the adhesive layer. If the heating is too fast, the solvent will vaporize and bubble in the softened adhesive layer, forming bubbles in the joint. In severe cases, most of the uncured, fluid adhesive polyurethane adhesive can be squeezed out of the joint, thus creating voids and affecting the bonding strength of the adhesive.
For two-component solvent-free polyurethane adhesives and one-component wet-curing polyurethane adhesives, heating should not be too fast either. Otherwise, the -NCO groups will accelerate the reaction with the adhesive, the surface of the substrate and the moisture in the air, causing the generated CO2 gas to have no time to diffuse. Meanwhile, the viscosity of the adhesive layer will increase rapidly, and eventually the bubbles will remain in the adhesive layer. One-component wet-curing polyurethane adhesives mainly cure by reacting with moisture in the air. Therefore, it is advisable to maintain the air humidity within a certain range to allow them to cure slowly at room temperature. If the air humidity is insufficient, a small amount of water can be added to the adhesive surface to promote the curing of the adhesive. If the adhesive is sandwished between dry and hard adherents and the adhesive layer is relatively thick, moisture from the interface and the outside is not easy to penetrate into the adhesive, which may lead to insufficient and incomplete curing of the adhesive. In such cases, a very small amount of moisture can be injected into the adhesive.
