Study On Reaction Course And Kinetics Of Water-isocyanate Reaction

The reaction of isocyanate with water is one of the most important reactions in the production of polyurethane foam,during which the polyurea obtained would help stabilize the bubbles by enhancing the strength of cell membrance,and the carbon dioxide released become an important source of gas for foaming.Moreover,water-isocyanate reaction is vital to isocyanate adhesion and moisture curing polyurethane coating.In the present study,the reactions between water and p-,m-,and o-tolyl isocyanate and 2,4-,and 2,6-tolyl diisocyanate were investigated by high performance liquid chromatography(HPLC),combined with 1H-NMR and electrospray ionization mass spectrum,followed by discussion of mechanism and kinetics of water-isocyanate reaction.According to HPLC analysis,in water-isocyanate reaction systems there is a high concentration of intermediate amine which experiences an apparent increase followed by decrease as the reaction proceeds.Hence,it is necessary to take amine concentration into consideration when kinetic equation of water-isocyanate reaction is established.Furthermore,existence of amine will interfere the accurcy of traditional dibutylamine back-titration method,resulting in a lower isocyanate concentration,which determines that dibutylamine back-titration method is not applicable in the study of water-isocyanate reaction.Material balance calculations show that isocyanate is merely converted to amine and substituted urea during the reaction process.There exist no carbamic acid,carbamic acid anhydride or ammonium salt in the reaction systems.No biuret was found under any reaction conditions either.All above indicate that reaction between isocyanate and urea can hardly proceed at normal temperature or without catalyst.Compared with ethers,amides accelerate water-isocyanate reaction more obviously.It is speculated that the promoting mechanism of amides may be similar to that of amine catalysts.It can be concluded from the study of diisocyanates that reaction between diisocyanates and water is a process of stepwise oligomerization.Two-NCO moieties of diisocyanate participate in the reaction simultaneously,instead of one by one.However,there is a concentration distribution of their corresponding products in the reaction mixture due to different reactivity between the two-NCO moieties.In addition,the effects of temperature,water concentration and isocyanate concentration on reaction rate and amine content were studied.It was found that temperature and water concentration have a relatively significant effect on reaction rate,but the effect of isocyanate concentration on reaction conversion is small.Water concentration and isocyanate concentration affect amine content apparently,but temperature acts reversely.Based on above experimental findings,the concentration of intermediate amine was taken into consideration and a consecutive-parallel reaction scheme was proposed to describe the reaction of isocyanate with water.A kinetic model that contains three ordinary differential equations was deduced from the two-step reaction mechanism.The new kinetic model fitted the reaction of p-,o-and m-tolyl isocyanate with water well with an average relative deviation of 4.13%,5.20%and 4.36%respectively,which indicates that consecutive-parallel reaction scheme and derived kinetic model are valid for water-isocyanate reaction.