One-component polyurethane adhesives do not need to be prepared under normal circumstances and can be used directly according to operational requirements. This is also the convenience of using one-component adhesives.
For two-component or multi-component polyurethane adhesives, they should be prepared in accordance with the instructions. If the hydroxyl content and isocyanate group content of each component are known, the distribution ratio of each group can be determined through chemical calculation.
Generally speaking, when preparing two-component solvent-based polyurethane adhesives, the tolerance of the two-component ratio is larger than that of non-solvent-based ones. However, if there are too many NCO groups in the prepared adhesive, the curing will be incomplete, and the cured adhesive layer will be relatively hard, or even brittle. If there is an excessive amount of hydroxyl components, the adhesive layer will be soft and sticky, have low cohesion and poor bonding strength. The tolerance of the ratio of solvent-free two-component adhesives is slightly smaller than that of solvent-based ones. This is because the initial molecular weight of each component is relatively small. If one component is in excess, it will result in slow curing and difficulty in complete curing, with a sticky appearance and low strength of the adhesive layer.
The prepared glue should be used up on the same day as it has a limited shelf life. The pot life refers to the period during which the prepared adhesive can maintain its operability for application. The viscosity increases with the storage time, making the operation difficult until the glue solution loses its fluidity, gels and becomes ineffective. The pot life of different types and grades of polyurethane adhesives varies, ranging from a few minutes to several days. When used in large quantities in industrial production, a shelf life test should be conducted in advance.
If the adhesive components contain a catalyst or a catalyst is added during the compounding process to accelerate the curing speed, the pot life will be shorter. In addition, the ambient temperature has a significant impact on the pot life. The pot life is shorter in summer and longer in winter. Two-component polyurethane adhesives diluted with ammonia-grade organic solvents have an extended shelf life. The pot life of common solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, should be more than 8 hours (i.e., one working day).
If the prepared glue is not used up on the same day, it can be appropriately diluted, covered and sealed, and stored in a cool place. Check for any signs of turbidity or gelation when you go to work the next day. If there is no obvious change on the surface of the glue and it has good fluidity, it can still be used. Generally, it can be mixed in small amounts in batches into newly prepared glue. If it has gone bad, it should be discarded.
To reduce viscosity, facilitate operation, ensure uniform application of the adhesive, and facilitate control of the application thickness, organic solvents can be added for dilution. The diluents available for polyurethane adhesives include acetone, methyl ethyl ketone, toluene, ethyl acetate, etc.
Adding a catalyst can accelerate the curing speed of the glue. Curing catalysts are usually organotin compounds.
