Hydrolytic stability of toluene diisocyanate and polymeric methylenediphenyl diisocyanate based polyureas under environmental conditions

Polyureas were prepared by reacting toluene diisocyanate (TDI) or polymeric methylenediphenyl diisocyanate (PMDI) with water under prolonged vigorous mixing at room temperature. Hydrolytic degradation of these (powdered) TDI- and PMDI-based polyureas was studied by measuring the rates of formation of free toluenediamine (TDA) or methylenedianiline (MDA) in water as a function of time by utilizing HPLC. The heterogeneous hydrolysis reactions were carried out in glass reaction tubes under nitrogen with initial polyurea loadings of 2 g/L in deionized water or buffer solution. In the case of TDI-polyurea, concentrations of free 2,4- and 2,6-toluenediamine in water were measured after hydrolysis at 120, 140, and 160 degreesC. The hydrolysis of PMDI-polyurea was carried out at 150, 160, and 170 degreesC, and concentrations of 2,4'-methylenedianiline (2,4'-MDA), 4,4'-methylenedianiline (4,4'-MDA), and 2,4-bis(paminobenzyl)aniline were measured. In both cases the rate of formation of diamine was well represented by both a pseudo-first-order reaction and a zero-order reaction. The temperature dependence of rate constants fit Arrhenius behavior. The half-time for hydrolysis of TDI-polyurea at 25 degreesC was calculated to range from about 18 000 to 300 000 years and that of PMDI-polyurea was estimated to range from about 110 000 to 12 million years, depending on the kinetic assumptions made. The half-times for hydrolysis at buffered pH levels of 4, 7, and 9 were within a factor of 2 of those in deionized water. These results are of importance in understanding the fate of polyureas formed in the event of a release of TDI or PMDI into the environment.