Polyurethane-based adhesives, whether one-part or two-part, are high performance and very versatile with a wide range of properties including adhering to a wide range of substrates. Their viscosity, pot life, and the flexibility achieved in the product after curing is critical to their bonding success.
Both the 1K and 2K adhesives are non-flammable, can be solvented or solvent-free, and have cure times and pot life which can be varied by changing formulations. They have good resistance to solvents when cured. These adhesives do not require heat for curing but it can be used to speed up the cure between two substrates when necessary, and the adhesive will stay bonded through a wide range of operating temperatures. Another advantage is that many polyurethanes (dependent on their formulation) can be applied using sprays, brushes or rollers either automatically or manually.
While most polyurethane-based adhesives are the result of the same chemical reaction between an isocyanate and a polyol, they are separated into two classifications, 1K for one part polyurethane adhesives, and 2K for two part systems. They are then split further according to their differing strengths and the few limitations brought about by the way that chemical reaction is utilized.
For 2K systems the isocyanate and the polyol are manufactured and supplied separately, and are mixed just prior to application according to a prescribed ratio of components and a recommended amount of mixing to get the required reaction and totally cross-link the system. Following these guidelines, and getting the timing is vital to the effectiveness of the cure.
1K systems have the advantages of requiring no mixing, and are therefore easier to apply. They also have excellent chemical resistance. This premixed adhesive’s reactive component is a polyurethane prepolymer created during manufacture by the reaction between polyol and excess isocyanate, and cross-linking is achieved through exposure to ambient moisture, by using a misting spray before the second substrate is bonded, or by dissolving the prepolymer in a solvent carrier which will cause it to react to the moisture provided as the solvent evaporates.
However, these high-performance adhesives do have limitations caused by their total reliance on moisture and the right ambient conditions to ensure cure time and pot life. Once the container is opened, the same ambient moisture can also shorten both the adhesive’s shelf life and its viscosity.
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