Study On Blocking Isocyanate With Substituted Phenols

Polyurethane resin synthesized by isocyanate is widely applied in industries of wood, shoemaking, textile and clothing, packaging, plastics processing, automobile, cryogenic, paint, magnetic tape and so on. Due to the effect of cumulated double bond and atoms of N,O on either hand of carbon atom, isocyanate group has high reaction activities and is easy to react with compounds containing active hydrogen. That limits the use of isocyanate to some extent. So the study on the blocking of isocyanate is significant. Remarkable achievements have been obtained through a number of studies so far. However, many achievements can not be applied to practice. The study on the blocking of isocyanate is going on. Especially in recent years, we advocate to use environment-friendly adhensives and paints all long, which makes this study receive more and more attention. In this study, substituted phenols were used to block isocyanate and the results were satisfactory.P-nitrophenol, p-chlorophenol, phenol and p-methylphenol were applied to block isocyanate as blocking reagengts, respectively. The effect of influencing factors including single-component catalysts, composite catalyst, temperature, feeding mode and feed ratio on the blocking reaction were investigated and optimized.When p-nitrophenol was used as the blocking reagent, the optimal blocking reaction conditions were as follows. The 1,4-dioxane solution containing PAPI was added to the 1,4-dioxane solution containing p-nitrophenol at the preheated temperature of 60℃.The scheduled addition time was fixed at about 90 min. The amounts of the composite catalysts, dibutyl tin dilaurate and triethylamine, added to the reaction solution, were 2.0% and 4.0% of the p-nitrophenol amount. After addition, the reaction temperature was controlled at 100℃and maintained 10 h. The blocking efficiency could reach up to 100%. The unblocking reaction of blocked isocyanate began to occur at 106.1℃and ended at 121.94℃detected by DSC.P-chlorophenol, phenol, p-methylphenol were used as the blocking reagents, respectively. The influencing factors were all investigated and optimized. It was found that the blocking efficiency was best when using p-chlorophenol as the blocking reagent compared with phenol and p-methylphenol.The trichloromethane solution of PAPI was added into the trichloromethane solution of p-chlorophenol at the preheated temperature of 40℃.The scheduled addition time was fixed at about 60 min. The blocking reaction was catalyzed by the composite catalysts similar to the above mentioned. And the amounts of dibutyl tin dilaurate and triethylamine added to the reaction solution were 2.0% and 2.0% of the p-chlorophenol dosage, respectively. The blocked product was white powder after purification. The unblocking temperatures of blocked isocyanate were 87℃,112℃and 170℃,respectively, when using p- chlorophenol, phenol and p-methylphenol as blocking reagents.Compared with other blocking reagents, p-chlorophenol is a kind of blocking reagent for its blocked products can be unblocked at lower temperature.