S. Kalaimani

Bio: S. Kalaimani is an academic researcher from University of Madras. The author has contributed to research in topics: Arrhenius equation & Oceanography. The author has an hindex of 3, co-authored 5 publications receiving 44 citations.

Synthesis and studies on forward and reverse reactions of phenol-blocked polyisocyanates: an insight into blocked isocyanates

Abstract: Blocked isocyanates are an important class of raw materials used in the polyurethane industry. In this study, the synthesis and kinetics of blocking and deblocking reactions of a series of phenol-blocked polyisocyanates have been studied in detail using a hot-stage FT-IR spectrophotometer adapting to neat conditions. The results were compared with an aim to resolve complex questions on the relationship between the forward and reverse reaction parameters. As a result, double Arrhenius plots for thermally reversible reactions were proposed for the first time. Using these plots, the most probable equilibrium temperatures for the forward and reverse reactions and the equilibrium rate constants of these reactions were assessed. It was found that the trend present in the rate constants of the forward reaction, reverse reactions and equilibrium were uniform i.e., the rate of these reactions decreased or increased with respect to the acidity of the blocking agent. A phenol with more acidity and less nucleophilicity, e.g., 2-chlorophenol, was found to be a better blocking agent; it blocks the isocyanate quickly, and at the same time, it cleaved off easily. The most probable temperatures assessed using the double Arrhenius plots were found in accordance with deblocking temperatures. The data, such as time required for conversion into product, the equilibrium temperature range for forward and reverse reactions and most probable equilibrium temperature in combination with deblocking temperature, reported in this work are very attractive from the manufacturing and application points of view.

Successful synthesis of blocked polyisocyanates, using easily cleavable phenols as blocking agents, and their deblocking and cure studies

Abstract: Phenols with electron withdrawing substituents at the 2,4-positions are important for use as blocking agents for isocyanates. Blocked polyisocyanates derived using such blocking agents are attractive for producing heat-cured polyurethane products at relatively low temperatures, i.e., below 160 °C. In this study, a series of blocked polyisocyanates were prepared using phenol, 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol; their blocking and deblocking kinetics, deblocking temperatures, equilibrium temperatures, equilibrium rate constants and cure-times were studied using a hot-stage FT-IR spectrophotometer, adapting neat conditions. Double Arrhenius plots for these thermally reversible systems were reported with an aim to understanding the relationship between forward and reverse reactions. It was found that the rates of forward and reverse reactions and equilibrium rates increased with increasing the acidity of phenol, except in the case of 2-chloro-4-nitrophenol; correspondingly, the deblocking temperature and cure-time of blocked polyisocyanates decreased. Blocked polyisocyanates obtained using unsubstituted phenol showed equilibrium temperature as a range in the double Arrhenius plot, whereas, in the case of 2,4-dichlorophenol and 2-chloro-4-nitrophenol, the Arrhenius plots showed distinct equilibrium temperatures. The equilibrium temperature range or equilibrium temperature of 2-chloro-4-esterphenol-blocked polyisocyanate was not determined, as extrapolation of its plot was found to extend out of the temperature range studied. Importantly, as expected with strong background, all three di-substituted phenols were found to deblock remarkably below 55–70 °C, compared to unsubstituted phenol, which deblocks at 135 °C. More importantly, 2-chloro-4-nitrophenol deblocks at 65 °C and its blocked polyisocyanate cures with polyol within 25 minutes at 110 °C.

Crystal structure of phenyl N-(4-nitro-phen-yl)carbamate.

Abstract: The asymmetric unit of the title compound, C13H10N2O4, contains two independent mol­ecules (A and B). The dihedral angle between the aromatic rings is 48.18 (14)° in mol­ecule A and 45.81 (14)° in mol­ecule B. The mean plane of the carbamate N—C(=O)—O group is twisted slightly from the attached benzene and phenyl rings, making respective dihedral angles of 12.97 (13) and 60.93 (14)° in A, and 23.11 (14) and 59.10 (14)° in B. In the crystal, A and B mol­ecules are arranged alternately through N—H⋯O hydrogen bonds and C—H⋯π inter­actions, forming chains along the a axis. The chains are further linked by C—H⋯O hydrogen bonds into a double-chain structure.

Crystal structure of phenyl N-(3,5-di-methyl-phenyl)carbamate.

Abstract: The asymmetric unit of the title compound, C15H15NO2, contains two independent mol­ecules (A and B). The di­methyl­phenyl ring, the phenyl ring and the central carbamate N—C(=O)—O group are not coplanar. In mol­ecule A, the di­methyl­phenyl and phenyl rings are inclined to the carbamate group mean plane by 27.71 (13) and 71.70 (4)°, respectively, and to one another by 84.53 (13)°. The corresponding dihedral angles in mol­ecule B are 34.33 (11), 66.32 (13) and 85.48 (12)°, respectively. In the crystal, the A and B mol­ecules are arranged alternately linked through N—H⋯O(carbon­yl) hydrogen bonds, forming –A–B–A–B– chains, which extend along [100]. Within the chains and linking neighbouring chains there are C—H⋯π inter­actions present, forming columns along the a-axis direction. The columns are linked by offset π–π stacking inter­actions, forming a three-dimensional network [shortest centroid–centroid distance = 3.606 (1) A].

Successful synthesis of blocked polyisocyanates, using easily cleavable phenols as blocking agents, and their deblocking and cure studies

Abstract: Phenols with electron withdrawing substituents at the 2,4-positions are important for use as blocking agents for isocyanates. Blocked polyisocyanates derived using such blocking agents are attractive for producing heat-cured polyurethane products at relatively low temperatures, i.e., below 160 °C. In this study, a series of blocked polyisocyanates were prepared using phenol, 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol; their blocking and deblocking kinetics, deblocking temperatures, equilibrium temperatures, equilibrium rate constants and cure-times were studied using a hot-stage FT-IR spectrophotometer, adapting neat conditions. Double Arrhenius plots for these thermally reversible systems were reported with an aim to understanding the relationship between forward and reverse reactions. It was found that the rates of forward and reverse reactions and equilibrium rates increased with increasing the acidity of phenol, except in the case of 2-chloro-4-nitrophenol; correspondingly, the deblocking temperature and cure-time of blocked polyisocyanates decreased. Blocked polyisocyanates obtained using unsubstituted phenol showed equilibrium temperature as a range in the double Arrhenius plot, whereas, in the case of 2,4-dichlorophenol and 2-chloro-4-nitrophenol, the Arrhenius plots showed distinct equilibrium temperatures. The equilibrium temperature range or equilibrium temperature of 2-chloro-4-esterphenol-blocked polyisocyanate was not determined, as extrapolation of its plot was found to extend out of the temperature range studied. Importantly, as expected with strong background, all three di-substituted phenols were found to deblock remarkably below 55–70 °C, compared to unsubstituted phenol, which deblocks at 135 °C. More importantly, 2-chloro-4-nitrophenol deblocks at 65 °C and its blocked polyisocyanate cures with polyol within 25 minutes at 110 °C.