Cure‐reaction kinetics of amine‐blocked polyisocyanates with alcohol using hot‐stage Fourier transform infrared spectroscopy

Abstract

Cure reaction between a series of N-methylaniline-blocked polyisocyanates, based on 4,4 0 -methylenebis(phenyl isocyanate), poly(tetrahydrofuran) and several substituted N-methylanilines, and n-decanol has been studied. The solid-state isothermal cure reaction was carried out using hot-stage FTIR spectroscopy, in the temperature range of 125-1458C. The urea carbonyl absorption band of blocked polyisocyanate moiety was used to monitor the conversion of blocked polyisocyanate into polyurethane. Kinetic and thermodynamic parameters were calculated using normalized conversion curves. The overall order of cure reaction, for each of the blocked polyisocyanates was found to be first order. Based on the results of kinetics and reaction conditions used in this study, the elimination-addition (S N 1 ) mechanism was suggested for the cure reaction between N-methylaniline-blocked polyisocyanates and n-decanol. The effect of substituents present in the blocking agents on the cure reaction of N-methylaniline-blocked polyisocyanates was investigated and found that the cure reaction of N-methylaniline-blocked polyisocyanates was retarded by electron-donating substituents and facilitated by electron withdrawing substituents. The observed high negative entropy of activation value supports the formation of a four-centered, intramolecularly hydrogen-bonded ring structure during transition state of the cure reaction.

Polyurethane Powder Coatings: A Review of Composition and Characterization

“…On the other hand, elevated curing temperature and/or long curing time leads to poor properties and yellowing of the coating. 43,44 The advantages and disadvantages of this system are summarized in Table 2.…”

Synthesis and Deblocking Studies of Low Temperature Heat-Curable Blocked Polymeric Methylene Diphenyl Diisocyanates

“…It has been shown that structures of blocking agents and isocyanates equally have a strong influence on deblocking temperature (1,(15)(16)(17). Several compounds, having active hydrogen atom, have been reported as blocking agents, including phenols, oximes, amides, imides, imidazoles, amidines, pyrazoles, N-methyl anilines, and active methylene compounds (1,2,(15)(16)(17)(18)(19)(20)(21). It has been reported that substituent present in the blocking agent has a very strong influence on the deblocking temperature.…”

Improved “cure on demand” of aromatic bismaleimide with thiol triggered by retro-Diels-Alder reaction

“…The cleavage and release of blocking group, at high temperature, yields again an isocyanate function which can react and lead to crosslinking of polymer. Various blocking groups are used industrially or studied by research groups [18][19][20][21][22][23][24][25]. However, the blocking of isocyanate suffers from many limitations: the moisture sensitivity of isocyanate function, the high toxicity of free isocyanates and the general drawback common at all system, the undesirable release of volatile blocking agents.…”