Effect of isocyanate structure on deblocking and cure reaction of N-methylaniline-blocked diisocyanates and polyisocyanates

Two series of N-methylaniline-blocked isocyanates based on monomeric diisocyanates such as 4,4′-methylene bis(phenyl isocyanate), toluene-2,4-diisocyanate, isophorone diisocyanate and 1,6-diisocyanato hexane and their NCO terminated polyurethane prepolymer (polyisocyanates) were prepared and characterized thoroughly by FTIR, ¹H NMR, ¹³C NMR and EI-Mass spectroscopic methods. The blocking reaction of N-methylaniline with aromatic isocyanates and aromatic polyisocyanates occur faster when compared to the aliphatic isocyanates. The deblocking reactions of blocked isocyanates were carried out under dynamic and isothermal conditions using hot-stage FTIR spectrophotometer. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate kinetics and thermodynamics parameters. Cure reactions of blocked isocyanates with hydroxyl-terminated polybutadiene were also followed to establish the structure-property relationship of the N-methylaniline-blocked isocyanates. The deblocking studies of blocked isocyanates reveal that the aromatic isocyanates undergo deblocking easily compared to aliphatic isocyanates. The rate of deblocking reaction of N-methylaniline-blocked aromatic polyisocyanates was found to be higher compared to N-methylaniline-blocked aromatic monomeric diisocyanate adducts. On the other hand, this trend was just reverse in the cure-reaction studies. The dissolution behavior of N-methylaniline-blocked isocyanates in Terathane-2000, polypropylene glycol-2000, polycaprolactone diol-2000 and hydroxyl-terminated polybutadiene-2500 was also studied and found that all adducts are soluble in these polyols.     

Kinetics of urethane formation from isophorone diisocyanate: The catalyst and solvent effects

The dependence of the kinetic parameters of urethane formation in the reaction between isophorone diisocyanate and alcohols of different structure (n-propanol, isopropanol, propargyl alcohol, 1,3-diazidopropan-2-ol, and phenol) in diluted solutions on the natures of solvent (toluene, carbon tetrachloride) and catalyst (dibutyltin dilaurate, diazobicyclooctane) was found using an original IR spectroscopic procedure. The ratio of the apparent rate constants for the reactions involving the aliphatic and cycloaliphatic NCO groups of isophorone diisocyanate was determined, and the efficiency of catalysis in these reactions was estimated. The reaction conditions under which the difference between the reactivities of isocyanate groups can reach 40 were determined.     

Kinetics of urethane formation from isophorone diisocyanate: The alcohol nature effect

The kinetics of the noncatalytic reactions of isophorone diisocyanate with n-propanol, isopropanol, 1,3-diazidopropan-2-ol, propargyl alcohol, and phenol in toluene in the temperature range from 20 to 90°C at the stoichiometric ratio of reactive groups has been investigated by IR spectroscopy. The apparent rate constants for the reactions of the aliphatic and cycloaliphatic isocyanate groups of isophorone diisocyanate with all of the alcohols have been measured. The activation parameters of the reactions of isophorone diisocyanate with n-propanol, isopropanol, and 1,3-diazidopropan-2-ol have been determined. The data obtained are considered in terms of the alcohol structure and molecular organization of solutions.     

Ring opening polymerization of epoxides with urea‐derivatives of 4‐aminopyridine as thermally latent anionic initiator

In this study, a series of urea‐derivatives of 4‐aminopyridine (4AP) were evaluated as thermally latent initiators for the anionic ring‐opening polymerization of diglycidyl ether of bisphenol A (DGEBA). The urea‐derivatives were synthesized by the reactions of 4AP with the corresponding iso(thio)cyanates (phenyl isocyanate, tert‐butyl isocyanate, methylene diphenyl diisocyanate, and phenyl isothiocyanate). The ability of the urea‐derivatives as latent initiators was investigated with differential scanning calorimetry (DSC): Upon heating formulations comprised of DGEBA and the urea‐derivatives in a heating rate at 10 °C/min, the resulting DSC profiles indicated exothermic peaks to confirm that DGEBA underwent the polymerization efficiently. The corresponding DSC‐peak top temperatures (T_{peak top}) was higher than that observed for the formulation comprised of DGEBA and pristine 4AP, to clarify that the urea are useful initiators with thermal latency. A possible mechanism for the initiation step involves the thermal dissociation of the urea into 4AP and the corresponding isocyanates. 4AP thus generated readily initiated the ring‐opening polymerization of epoxide. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2518–2522     

Interpenetrating polymer networks from the novel bismaleimide and cyanate containing naphthalene: Cure and thermal characteristics

The novel interpenetrating network bismaleimide-triazine polymers were derived from A bismaleimide, viz. 2,7-bis(4-maleimidophenoxy)naphthalene (BMPN), and a dicyanate ester, viz. 2,7-dihydroxynaphthalene dicyanate (DNCY), possessing similar backbone structures. The cure process of the blends including of 0.11 mmol/mol Fe(AcAc)₃ and 2% nonyl phenol is characterized by DSC and in situ FTIR. The DSC curves show two-stages curing process for the mixture systems including of 0.11 mmol/mol Fe(AcAc)₃ and 2% nonyl phenol. In the in situ FTIR spectra of cured resin, the absorption peak of the CCH bending vibrations decreases continuously until the end of cure reaction after that of the OCN group disappears mostly. These illuminate that the structure of IPNs can be formed due to the respective polymerization of two monomers for mixtures. Interlaced patterns are seen obviously in the region by SEM for IPNs BT resins. Thermal stabilities of cured resins are characterized by TGA. The pure polyDNCY, polyBMPN and typical IPNs BT resins show good thermal stability.     

Investigation of the competitive rate of derivatization of several secondary amines with phenylisocyanate (PHI), hexamethylene-1,6-diisocyanate (HDI), 4,4'-methylenebis(phenyl isocyanate) (MDI) and toluene diisocyanate (TDI) in liquid medium

The stabilization of the isocyanate (NCO) groups during workplace sampling is necessary for their subsequent laboratory analysis. Most derivatization reagents are secondary amines. By carrying out a test in which two secondary amines are added to an isocyanate, the relative rates of these reactions can be evaluated. This evaluation is known for a monoisocyanate, phenylisocyanate (PHI), but is being developed for diisocyanates. This study deals with the relative reactivity (RR) of four diisocyanates: hexamethylene 1,6-diisocyanate (HDI), 4,4'-methylenebis(phenyl isocyanate) (MDI), and the ortho and para isomers of toluene diisocyanate (TDI) in addition to PHI, with four secondary amines: 1-(2-methoxyphenyl)piperazine (MOPIP), 9-(N-methylaminomethyl)anthracene (MAMA), 1-(9-anthracenylmethyl)piperazine (MAP), and dibutylamine (DBA). These competitive derivatization reactions are studied in three reaction solvents, namely acetonitrile, toluene, and acetonitrile doped with water (1% v/v). The results show that the order of reactivity, which doesn't change with the isocyanate as well as with the solvent used, is the following: DBA > MAP > MOPIP > MAMA. The relative difference in reactivity is a function of both the isocyanate and the solvent used. Hindered aromatic diisocyanates (TDI and MDI) show a greater difference in reactivity with the derivatization agents. These differences in reactivity are also modified by the solvent used. For example, larger differences are observed in acetonitrile than in toluene, but the introduction of water to acetonitrile, which does not affect the reaction yield, makes these differences smaller.     

First Principle Simulations of Ethylene Glycol Addition to Diisocyanates

The reaction of addition of ethylene glycol to diisocyanates was studied with the B3LYP method to gain an insight into the mechanism of polyurethane synthesis. It was found that the functional basis set should contain at least one diffusion function for the simulation in order to properly describe thermo‐chemical features of the model reaction. Using the B3LYP/6‐31+G** level the values of Gibbs free energy were estimated for the addition reaction of ethylene glycol to toluene‐2,4‐diisocyanate, toluene‐2,6‐diisocyanate, diisocyanate‐[5‐isocyanato‐1‐(isocyanatomethyl)‐1,3,3‐trimethylcyclohexane], 1,6‐diisocyanatohexane, 4,4′‐methylenebis(cyclohexyl isocyanate) and methylenebis(phenyl isocyanate). Both the gaseous phase and the benzene environment were taken into consideration. Spontaneity of the reaction proved to be dependent on both substrate type and product isomeric configuration. The trans‐urethane isomer has been found to be a more stable product. Considering the values of activation energy the minor dependence on the structure of diisocyanate was observed. This confirmed Flory's postulate to be valid for the polyurethane synthesis. The highest value of activation energy was found for the first stage, which consists of ethylene glycol approach and simultaneous proton transfer to the isocyanate group. For that reason the first stage has been estimated as that limiting the general rate of the urethanisation reaction.

     

A DSC Study on Cure Kinetics of HTPB-IPDI Urethane Reaction

The kinetics of theurethane-forming cure reaction of hydroxyl terminated polybutadiene (HTPB) with isophorone diisocyanate (IPDI) in presence of ferric tris (acetyl acetonate) (FeAA) catalyst was investigated using differential scanning calorimetry (DSC). The Arrhenius activation parameters, viz., activation energy E and pre-exponential factor A were evaluated using the non-isothermal integral Coats-Redfern equation. The cure reaction was catalysed by ferric acetyl acetonate (FeAA), as revealed from the decrease in reaction temperatures and the increase in rate constants; however, the computed activation energy did not show any correlation to the catalyst concentration. The values of E and A for the uncatalysed reaction at different heating rates showed interdependence through kinetic compensation (KC) effect. Using KC correction, E values were normalised for the value of A for the uncatalysed reaction under identical conditions. The normalised E values decreased exponentially with increase in concentration of FeAA, showing high propensity of the HTPB-IPDI system for catalysis.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal analysis of polymer layered silicate nanocomposites

It has been shown, for three different polymer layered silicate (PLS) nanocomposite systems, how differential scanning calorimetry (DSC) can identify the different reactions of homopolymerisation and of crosslinking that occur in the intra- and extra-gallery regions of these nanocomposites, respectively, and hence how DSC can be used to assess the cure conditions for optimising their nanostructure. The PLS nanocomposites are based upon: (i) diglycidyl ether of bisphenol-A (DGEBA) cured with a polyoxypropylene diamine; (ii) DGEBA cured with an –NH₂ terminated hyperbranched polymer (HBP); and (iii) tri-glycidyl p-amino phenol (TGAP) cured with a diamine. In each case, the existence of both intra- and extra-gallery reactions in the DSC cure curves, and whether they occur simultaneously or sequentially, and in what order, are identified and correlated with the nanostructure as observed by small angle X-ray scattering and transmission electron microscopy. In particular, it is shown that the intra-gallery reaction must precede the extra-gallery for significant exfoliation to occur. In accordance with this scenario, the TGAP/diamine system displays the greatest degree of exfoliation, the DGEBA/diamine system the least, with the DGEBA/HBP system intermediate. For those systems in which significant exfoliation occurs, the DSC cure curves also allow the optimum cure conditions, such as the isothermal cure temperature, to be determined.     

Substituent effect of N‐aryl‐N′‐pyridyl ureas as thermal latent initiators on ring‐opening polymerization of epoxide

A series of N‐aryl‐N′‐pyridyl ureas were synthesized by the reactions of 4‐aminopyridine (4AP) with the corresponding isocyanates such as phenyl isocyanate, 4‐methylphenyl isocyanate, 4‐methoxyphenyl isocyanate, 4‐chlorophenyl isocyanate, 4‐(trifluoromethyl)phenyl isocyanate, and 4‐nitrophenyl isocyanate. Bulk polymerization of diglycidyl ether of bisphenol A (DGEBA) in the presence of the ureas as initiators was evaluated by differential scanning calorimetry (DSC) at a heating rate of 10 °C/min. The resulting DSC profiles indicated exothermic peaks above 140 °C, while the DSC profile measured for a formulation composed of DGEBA and pristine 4AP indicated an exothermic peak at around 120 °C, implying that the derivation of 4AP into the corresponding ureas is a useful strategy to achieve thermal latency. The peak top temperatures were correlated with the electron density of the aromatic ring of the ureas, that is, as the electron‐withdrawing nature of the substituent on the aromatic ring became larger, the peak increases. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2569–2574     

Pyrolysis Kinetics and Flammability Evaluation of Rigid Polyurethane with Different Isocyanate Content

Polyurethane (PU) is a typical product of the reaction between isocyanate and polyol, whose ratio would greatly influence material properties. In this paper, to investigate the influence of isocyanate on PU thermal stability and flammability, three kinds of rigid polyurethanes (RPUs) with different isocyanate ratio (1.05, 1.1, and 2.0) were manufactured in a laboratory and employed to have a series of TG (thermogravimetry), DSC (differential scanning calorimetry), and cone calorimetry tests. Kissinger’s method was used to calculate the activation energy and judge their stabilities. However, for such a complex degradation which consists of five reactions, it does not make sense by Kissinger method to obtain only two peak active energies. Considering complexity of PU degradation in air, genetic algorithm (GA) was employed to calculate kinetic triplets of five sub-reactions. The effects of isocyanate contents on each sub-reaction stability were obtained and then analyzed. By cone calorimeter testing, we found that great differences in heat release rate data. However, DSC analysis showed a complete opposite changed trend. Such difference is caused by DSC and calorimeter’s sample morphology, the former using grinded polyurethane powders but the latter polyurethane foam block.     

Kinetics of Uncatalyzed Reactions of 2,4′‐ and 4,4′‐Diphenylmethane‐Diisocyanate with Primary and Secondary Alcohols

The kinetics of the uncatalyzed reaction of an industrially important 50/50 blend of isomers of 4,4′‐diphenylmethane‐diisocyanate (4,4‐MDI) and 2,4′‐diphenylmethane‐diisocyanate (2,4′‐MDI) with primary and secondary alcohols was studied using high‐performance liquid chromatography coupled with photodiode array detector. The alcohols such as 1‐propanol, 2‐propanol, 1‐hexanol, 2‐hexanol, 3‐hexanol, 1‐methoxy‐2‐propanol, and 3‐methoxy‐1‐propanol were used in high molar excess to diisocyanate in toluene at 80°C, and pseudo–first‐order dependences on the concentrations of 4,4′‐MDI and 2,4′‐MDI were found. Appropriate treatments of the kinetic data allowed us to determine the corresponding pseudo–first‐order rate constants. According to the kinetic results, the reactivity of the isocyanate group in the para‐position is about four to six times higher than that of the ortho‐positioned isocyanate group, depending on the reacting alcohol. Furthermore, the substitution effect, i.e., change in the reactivity of the free isocyanate group after the other has been reacted, was found for both 4,4′‐MDI and 2,4′‐MDI isomers. The differences in the reactivities of the isocyanate groups of 2,4′‐MDI and 4,4′‐MDI isomers before and after one of two isocyanate groups has been reacted are explained in terms of partial positive charges on the corresponding carbonyl carbon atom calculated by high‐level quantum chemical calculations. In addition, the UV‐spectral properties of the products obtained by quenching the reaction mixture with methanol are also discussed in light of practical implications.     

Polyurethane Ionomers from Cycloaliphatic Diisocyanate and Polytetramethylene Glycol

Abstract

Segmented polyurethane (PU) ionomers were prepared from cycloaliphatic diisocyanate [methylene bis(4-cyclohexyl isocyanate) (H₁₂MDI) and isophoron diisocyanate (IPDI)] and polytetramethylene glycol (PTMG) by using an anionic-type chain extender, viz., dimethylol propionic acid (DMPA). The effect of ionic content and butanediol (BD) on the state of dispersion and physical properties of emulsion-cast film was determined using Autosizer, transmission electron microscopy (TEM), Instron, and Rheovibron. With increased incorporations of DMPA in PU, particle size of emulsion decreased asymptotically, tensile modulus and strength increased, and the glass transition temperature (T _g) moved toward the higher temperature. On the other hand, with increased incorporation of BD in PU, particle size of emulsion, tensile modulus, and strength of the emulsion cast film increased, and the major transition of soft segment moved toward higher temperature. With regard to the structural effect of the isocyanate, H₁₂MDI gave finer dispersion and better mechanical properties over IPDI.     

Kinetics study of imidazole-cured epoxy-phenol resins

The reaction kinetics of diglycidyl ether of bisphenol A (DGEBA) cured with different concentrations of imidazole and bisphenol A (BPA) were investigated by using differential scanning calorimetry. Both dynamic and isothermal DSC were studied. Two initiation mechanisms were found to play roles in the curing reactions. One was based on adduct formation of epoxy groups with pyridine-type nitrogen and the other was based on ionic complexes of imidazole and BPA. The subsequent propagation was composed of three main reactions, viz. the epoxide/phenol reaction, the acid/base reaction, and the epoxide/R-O⁻ reaction. A generalized kinetics model was developed and used to predict the conversion of epoxide groups using a wide range of imidazole and BPA concentrations, and cure temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3233–3242, 1999     

On the mechanism of C ⇄ N migration of alkoxycarbonyl groups in reactions of pyridinium ylides with isocyanates

New examples of reversible C N migrations of alkoxycarbonyl groups, which occur in the reactions of pyridinium and 3-(diethylcarbamoyl)pyridinium ylides, viz., derivatives of dimethyl and diethyl malonates, with aryl isocyanates were studied. The mechanism of migration of the methoxycarbonyl group from the carbon atom to the nitrogen atom was considered on the basis of quantum-chemical calculations. The product of the primary attack of the isocyanate group by pyridinium ylide was established to be rearranged with low potential barriers to form carbamate without formation of cyclic intermediate compounds.     

Preparation and reactions of hydrophilic isocyanate micelles dispersed in water

We investigated the abnormal reaction behavior of NCO units in micellar hydrophilic isocyanate in water through titration, particle size, FT-IR spectroscopy and GPC and mechanical properties. Hexamethylene diisocyanate (HDI) allophanate was modified with methoxy polyethyleneglycol (MPEG), yielding novel hydrophilic isocyanate, which formed stable and homogeneous micelles in water. The results we obtained showed that the rapid reduction of the NCO group took place at a specific time (with an induction period) after dispersing hydrophilic isocyanate into water. The induction period, which is equal to the lifetime of the NCO group, was largely dependent on the diameter of micelles determined by the content of MPEG introduced. The result of GPC measurement indicated that there are three types of reactions between NCO groups and water. The rapid reduction of NCO in water was explained by the stepwise progress of the reactions, according to the location of NCO units in the core-shell structure of hydrophilic isocyanate micelles.     

Quantum-chemical study on the reaction of phenyl isocyanate with linear methanol associates. Addition at the C=N bond

Quantum-chemical calculations at the B3LYP/6-311++G(df,p) level of theory showed that reactions of phenyl isocyanate with methanol associates involve formation of pre-and post-reaction complexes. The reactions proceed through late asymmetric cyclic transition states. The height of the energy barrier decreases as the degree of association of the alcohol increases. The relative change in the Gibbs energy in the reaction of phenyl isocyanate with methanol also becomes smaller as the degree of alcohol association increases.     

Observations of Gelation and Vitrification of a Thermosetting Resin during the Evolution of Polymerization Shrinkage

Summary: We report a new methodology to determine the gelation and vitrification of a thermosetting material during the polymerization process by detecting the evolution of cure shrinkage through a thermomechanical analyzer (TMA) and a differential scanning calorimeter (DSC). The gelation and vitrification determined by the evolution of cure shrinkage correspond favorably with that measured by conventional rheological techniques. The isoconversional phenomenon at gelation point was further confirmed by monitoring cure shrinkage at temperatures ranging from 90 to 110 °C. Whereas, vitrification was observed to occur at higher degrees of cure with increasing cure temperatures. Inhibited cure shrinkage was also observed in the vitrification region where the reaction transitioned from chemical to diffusion controlled.

Combination of dimension change detected by DMA and heat flow detected by DSC for determining the relationship of cure shrinkage and degree of cure.     

Diol-diisocyanate polymerization by gamma irradiation

Gamma irradiation has been used as an alternative way of curing diol-diisocyanate by means of specific catalysts and/or elevated temperatures. A mixture of toluene diisocyanate (TDI) and hydroxy terminated polybutadiene (HTPB) oligomer were irradiated over a dose range of 0–720 kGy using a 5 kCi⁶⁰Co -source. Various compositions were formulated and also suitable additives were incorporated. Disappearance of the isocyanate peak in the IR spectra was considered as a measure of its consumption. The conversion characteristics showed a profound influence of irradiation. A suitable mechanism is envisaged.