Polyurethane adhesives contain urethane (-NHCOO-) or isocyanate (-NCO) groups in their molecular chains. These groups make them highly reactive and polar, resulting in excellent chemical bonding to substrates containing active hydrogen, such as porous materials like foam, plastic, wood, leather, fabric, paper, and ceramics, as well as smooth surfaces like metal, glass, rubber, and plastic. So, what are the specific bonding processes involved when using polyurethane adhesives for bonding?


Gluing

Application methods include spraying, brushing, dipping, and roller coating. These methods are generally determined by the adhesive type, viscosity, and production requirements. The key is to ensure an even, bubble-free, and gap-free adhesive layer.

The amount of adhesive applied (which is actually related to the thickness of the adhesive layer) is also a key factor affecting shear strength. Generally, shear strength is high within a certain range. If the adhesive layer is too thin, the adhesive will not fill the gaps in the uneven surface of the substrate, leaving gaps and low bond strength. As the thickness of the adhesive layer increases, the bond strength decreases. It is generally believed that when a lap shear specimen is subjected to a load, the adherend and the adhesive layer deform, breaking the adhesive layer into a peeling state. The peeling force reduces the apparent shear strength.

Airing

For solvent-based polyurethane adhesives, after applying the adhesive, it should be allowed to air-dry for several minutes to several dozen minutes to allow most of the solvent in the adhesive to evaporate, which helps improve initial tack. If necessary, appropriate heating and air drying (such as in a composite film lamination process) should be performed. Otherwise, due to a large amount of solvent remaining in the adhesive, bubbles may form in the adhesive layer during the curing process, affecting the bond quality. For solvent-free polyurethane adhesives, the adherends can be bonded immediately after application.

Bonding

This step involves bonding the previously applied adherends. A clamp can also be used to secure the adherends to ensure perfect alignment. If necessary, pressure can be applied to promote plastic flow of the adhesive, allowing it to penetrate the adherend surface and achieve maximum contact with the substrate.