Curing of polyurethane adhesives

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 in which the NCO groups in the adhesive react completely or the solvent evaporates completely, and the polyurethane molecular chains 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, if curing at room temperature takes a relatively long time, a catalyst can be added to promote curing. To shorten the curing time, heating can be adopted. 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 surface of the substrate. Heating can also soften the adhesive layer, thereby increasing its penetration into the surface of the substrate and facilitating molecular activity to find "companions" that generate molecular forces at the bonding interface. Heating is beneficial for further bonding. The solidified heating bodies include oven or drying tunnel heating, drying room heating, fixture heating, etc. For metal substrates with fast heat transfer, fixtures can be used for heating. The adhesive layer heats up faster than in an oven.

 

The heating process should be gradually increased in temperature. The evaporation rate of the solvent in solvent-based polyurethane adhesives should be carefully controlled. During the drying process, most of the solvent has evaporated, and the remaining solvent gradually diffuses 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. This can squeeze out most of the uncured adhesive in a fluid adhesive state from the joint, creating vacancies that will affect the bonding strength. For two-component solvent-free adhesives and one-component wet-curing adhesives, heating should not be too fast either, otherwise the NCO groups will react with the adhesive or the surface of the substrate. The moisture in the air accelerates the reaction, and the produced CO2 gas has no time to diffuse. As a result, the viscosity of the adhesive layer increases rapidly, and the bubbles remain in the adhesive layer.

 

One-component moisture-curing polyurethane adhesives mainly cure by moisture in the air, so a certain air humidity should be maintained. It is advisable to cure slowly at room temperature. If the air is dry, a small amount of water can be applied to the glue-coated surface to promote curing. 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, resulting in incomplete curing. In such cases, a very small amount of moisture can be injected into the adhesive.