Synthesis And Curing Mechanism Of Curing Agent Of Blocked Isocyanate

In order to achieve novelty one-component aqueous polymer-isocyanate adhesive, functionality curing agents, stabilization isocyanats, were blended or copolymerized with aqueous adhesive. In this paper, low temperature deblockable blocked-isocyanate, i.e., functionality curing agent, was prepared by2,4,6-trichlorophenol (2,4,6-TCP) blocked polymethylene polyphenlene isocyanate (PAPI).Firstly, on the basis of the previously study, we studied systematicly the various influencing factors when P-chlorophenol and O-chlorophenol blocked toluene diisocyanate (TDI), and discussed the effects of molar ratio of blocking agents and isocyanate, the structure of blocking agents, blocking reaction temperature, blocking reaction time, catalyst, reaction medium on the blocking reaction. The blocked adducts were characterized by the methods of di-n-butylamine back titration and Fourier transform infrared (FTIR) spectroscopy. And stability of blocked adducts was studied.Next, PAPI was checked as reagent because of its better performance and lower toxicity than TDI, which was blocked by blocking agents such as2-chlorophenol,4-chlorophenol,2,4-dichlorophenol and2,6-dichlorophenol. The experimental results showed that the reaction velocity of dichlorophenol as blocking agent was faster than that of monochlorophenol, since one more attracting electron substituent group, Cl atom, was at adjacent and contraposition of phenol hydroxyl.2,4,6-trichlorophenol was selected as a final blocking agent of polymethylene polyphenlene isocyanate to the purpose of low temperature deblocking. The effects of molar ratio of—OH/—NCO, reaction temperature, solvent, reaction time and catalyst on this blocking reaction were discussed. Fourier transform infrared was used to characterize these blocking adducts. Experimental results showed that it was propitious to the blocking reaction when the optimal molar ratio of-OH/-NCO was (1.1-1.2):1and the optimal reaction temperature at80℃. Meanwhile, experimental results also exhibited that the polar solvents could accelerate the blocking reaction and the reaction velocity was faster when the solvent polarity was stronger, and that it made for increasing blocking rate when the reaction time was prolonged, but to some extent, the reaction reached the equilibrium. In addition, experimental results also showed that the catalysts of organic tin could quicken significantly the blocking reaction velocity and reduced the blocking reaction time. TCP-blocked PAPI adduct was characterized by1H NMR and13C NMR. The results showed that there was-NH/-COO—formed in adducts.The method that determine blocking ratio using Fourier transform infrared was founded in this paper. The quantitative analysis peak was checked the characteristic absorption peak of the two isolated H on the four-substituent benzene which was at848cm-1. Then the rudimental content of—NCO was calculated. This method was of easy, little quantity, precision and rapidness.In different conditions, o-toluidine method was used to test the deblocking temperature of blocked isocyanate. It was discussed that the deblocking temperature of blocked isocyanate and the key influencing factors of deblocking, such as different solvents, catalyst and heating velocity, etc. Deblocking temperature was analyzed by DSC instrument, Fourier Transform Infrared Spectroscope and Thermal Gravimetric Analysis. The FTIR results showed that the characteristic absorption peak of—NCO which was presented at2250cm-1was increased gradually along with the deblocking reaction. The thermal analysis indicated that the initial deblocking temperature of2,4,6-TCP blocked PAPI was75℃. And the experimental results showed that the deblocking temperature would be declined when the blocked adduct in polar solvents.The emulsifiable2,4,6-TCP blocked PAPI was synthesized by the method of self-emulsification. The shape and size of the latex particles waterborne blocked isocyanates were characterized by laser particle size analysis instrument and scanning electron microscope (SEM). The results showed that, when TEA was neutralizing agent, the latex particle size was larger and distribution was narrower than that of KOH was neutralizing agent. And differential scanning calorimetry was showed that the blocking adduct was deblocked at70℃and deblocked perfectly at120℃.Finally, the curing mechanism of aqueous polymer-blocked isocyanate was studied by differential scanning calorimetry. The experimental results demonstrated that the dynamic parameters were deduced that ΔE=4.190kJ/mol, n=0.7for the system of poly vinyl acetate emulsion adhesive-blocked PAPI and ΔE=14.483kJ/mol, n=0.87for polyvinyl alcohol-blocked PAPI.