In recent years, with the introduction of environmental protection policies, waterborne polyurethane adhesives have gradually replaced solvent-based adhesives. Due to their high viscosity, water resistance, cold resistance and heat resistance, waterborne polyurethane adhesives are widely used in various fields. The editor of Beijing Kaimat has sorted out the following points to note when using waterborne polyurethane adhesives for everyone.
1. Appearance treatment
One of the conditions for forming an excellent bond is to carry out the necessary treatment on the surface of the substrate.
The surface of the adherend often has weak interface layers such as grease and dust. Affected by these, the bonding strength obtained by bonding based on the weak interface layer is not easy to improve. For those substrate appearances that do not match the surface tension of the adhesive, chemical treatment must also be carried out.
2. Cleaning and degreasing
The appearance of some metal and plastic substrates is often easily contaminated by sweat, oil, dust, etc. In addition, there are mold release agents on the plastic surface, which reduce the contact surface area between the adhesive and the substrate and lower the cohesion within the adhesive layer. Therefore, surface cleaning and drying treatment must be carried out before bonding.
3. Rough areas
For smooth surfaces, roughening treatment is generally required to increase the contact area between the adhesive and the substrate. The adhesive penetrates into the concave or porous areas on the surface of the substrate. After curing, it acts like a nail, hook or rod, firmly bonding the substrates together.
4. Bonding construction
(1) Apply glue
The methods of coating (gluing) include spraying, brushing, dipping, roller coating, etc. The decision should be made based on the type, viscosity of the adhesive and production requirements to ensure that the adhesive layer is uniform, free of bubbles and intact.
(2) Let it dry
For solvent-based polyurethane adhesives, after application, they need to be left to dry for several minutes to tens of minutes to allow most of the solvents in the adhesive to evaporate, which is conducive to improving the initial adhesion. When necessary, appropriate heating and forced air drying should also be carried out (such as in the composite film lamination process). Otherwise, due to a large amount of solvent remaining in the adhesive, bubbles are likely to form in the adhesive layer during the curing process, which will affect the bonding quality. For solvent-free polyurethane adhesives, the objects to be bonded can be adhered immediately after application.
(3) Bonding
This step involves joining the bonded surfaces of the objects that have been coated with adhesive together. A fixture can also be used to fix the bonded parts to ensure that the bonding surfaces are fully adhered and positioned. When necessary, a certain amount of pressure should be applied to allow the adhesive to better produce plastic flow, so as to wet the surface of the bonded objects and achieve maximum contact between the adhesive and the surface of the substrate.
5. Curing of 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.
