26 августа, 2015 
Malyar Isocyanates react with active hydrogens as depicted in Fig. 3. This addition reaction occurs with the active hydrogen adding to the nitrogen atom and the electron-rich nucleophile (Lewis base) reacting with the carbonyl group. Generally, the stronger the base, the more readily it reacts with the isocyanate. Table 2 shows typical reaction rates of some active hydrogen-containing compounds.
As expected, the aliphatic amines and aromatic amines (the strongest bases in the table) react the fastest. The urethanes industry has taken advantage of this reactivity in two-component commercial processes, demanding fast cure by using specially designed metering equipment and spray heads.
Alcohols and water react readily at room temperature. Most urethane adhesives depend on the — NCO group reacting with either water or alcohols. Primary OH groups are two to three times as fast as sterically hindered secondary OH groups under equivalent conditions. The reaction rates shown in Table 2 reflect uncatalyzed reaction rates and should be used as an indication of relative reaction rates. Actual rates are dependent on
R-N=00 + HA -► R-tjI^=0
H A
Isocyanate Active hydrogen Adduct
Figure 3 Reaction of isocyanate with active hydrogen.
solvent, temperature, and the presence of catalysts. Catalysts can significantly accelerate these reactions and can in some cases alter the order of reactivity [20].
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