27 августа, 2015 
Malyar Most urethane adhesives are based on urethane prepolymers. A prepolymer is made by reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminated urethane as shown in Fig. 11. Prepolymers may have excess isocyanate present (“quasiprepolymers”) or they may be made in a 2:1 stoichiometric ratio to minimize the amount of free isocyanate monomer present. Most moisture-cured prepolymers are based on 2:1 stoichiometric ratios. Two-component adhesives generally are based on quasi-prepolymers, which use the excess isocyanate to react with either chain extenders present in the other component or with the substrate surface.
Prepolymers are isocyanates and react like isocyanates, with several important differences. Prepolymers typically are much higher in molecular weight, are higher in viscosity, are lower in isocyanate content by weight percent, and have lower vapor pressures. Prepolymers are important to adhesives for a number of reasons. The desired polymeric structure of the adhesive can be built into the prepolymer, giving a more consistent
Urethane Prepolymer
structure with more reproducible physicals. In addition, since part of the reaction has been completed, reduced exotherms and reduced shrinkage are normally present. For two — component systems, better mixing of components usually occurs, since the viscosity of the two components more closely match. In addition, the ratios of the two components match more closely. Side reactions such as allophanate, biuret, and trimer are lessened. Finally, prepolymers typically react more slowly than does the original diisocyanate, allowing longer pot lives.
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