Possible excess -NCO groups consumption reactions performed under water assistance

6 Possible excess -NCO groups consumption reactions performed under water assistance 

As shown [17, 331], the postcure process is very intense as the index number I is higher and is strongly influenced by the presence of the atmospheric humidity. 

As in other similar cases [337], the main postcure process should be, the appearance of the urea groups according to equations 4.22(a)-(c), together with the slow reaction leading to the appearance of the allophanate group equation 4.22(d). 

Even though such reactions usually need high temperatures, it should be verified whether or not they would, however, take place to some extent during the long period of the postcure process. The relative higher concentration of urethane and ester groups from the macromolecular chain in the solid polymer could encourage such a reaction. 

In order to verify the possibility of the contribution of these reactions to the change in the mechanical properties, some quantities of 4,4 -diamino diphenyl methane (MDA) were incorporated into the polymeric material, so to obtain an equivalent quantity of -NH2 groups equal to 0.02 g / 100 g of a linear PU (equal to the excess initial concentration of -NCO groups corresponding to I = 110). Thin films of PUs with MDA about 0.5 mm thick were obtained by evaporation from solution at 60°C as described elsewhere [17]. 

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