Isocyanates with sodium bisulfite

Bisulfite addition products are formed from aldehydes, methyl ketones, cyclic ketones (generally seven-membered and smaller rings), a-keto esters, and isocyanates, upon treatment with sodium bisulfite. Most other ketones do not undergo the reaction, probably for steric reasons. The reaction is reversible (by treatment of the addition product with either acid or base ) and is useful for the purification of the starting compounds, since the addition products are soluble in water and many of the impurities are not. ... 

The use of blocked isocyanates for aqueous systems is of special interest. The sodium bisulfite blocking of hexamethy1ene diisocyanate was already described in Petersen s classic paper on blocked isocyanates ( ). Its application in cross-linking paper (75) and acrylamide copolymers (76) has been described. The stability of bisulfite-blocked aromatic isocyanates in water can present a difficulty, and various stabilizer systems have been proposed. Peters and Reddie (77) have made an extensive study of the various factors affecting stability and have found that optimum stability was obtained at pH of 2-3 in aqueous ethanol with excess hydrogen peroxide. 

Sodium bisulfite-blocked isocyanate-terminated prepolymers are extensively used as shrink-resistant finishes for wool. Polycarbamoylsulfonates (PCS) are produced from NCO-terminated prepolymers with bisulfite salts in aqueous alcohol as the solvent, shown schematically as follows (78-80) ... 

Burning in a chemical incinerator equipped with an afterburner and scrubber is a suitable method of destruction. In a laboratory small amounts can be destroyed by slowly adding an aqueous solution of sodium bisulfite to the isocyanate. The stable water-soluble derivative may be drained down with large volumes of water. 

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