Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.     

New telechelic polymers and sequential copolymers by polyfunctional initiator-transfer agents (inifers)

Summary Novel oxycarbonyl isocyanate telechelic polyisobutylenes have been prepared by conversion of hydroxylterminated polyisobutylenes with N-chlorocarbonyl isocyanate. This reaction is free from side-reactions and provides highly reactive isocyanate-terminated polyisobutylene prepolymers. Conversion of these prepolymers with glycol, urea, pentaerythritol, and polyethylene glycol led to linear chain extension, crosslinking, and multiblock copolymer formation, respectively.     

Mechanistic investigation of the simultaneous addition and free-radical polymerization in batch miniemulsion droplets: Monte Carlo simulation versus experimental data in polyurethane/acrylic systems

A detailed kinetic Monte Carlo simulation was used to predict the characteristics of the batch miniemulsion polymerization of an isocyanate and an acrylic monomer mixture that contains a hydroxyl functional monomer (HEMA). The simulation takes into account the simultaneous polyaddition of the polyurethane prepolymer with the hydroxyl group of HEMA and the free radical polymerization of the acrylic monomers and all reactions in aqueous and polymer particle phases. The model has been assessed by batch miniemulsion polymerizations carried out using an aliphatic isocyanate prepolymer, n-butyl acrylate, 2-hydroxyethyl methacrylate monomers and potassium persulfate as an initiator. It was found that partitioning of water had a significant effect on both kinetics and microstructure of the resulting polymer. Evolution of different species of PU prepolymer produced in the reaction and the sol and gel fractions revealed that the terminal pendent double bond of the HEMA in polymer chains has significantly lower reactivity than that of the HEMA free monomer. Detailed information on gel microstructure has been derived in the model by both distribution of molecular weight between crosslinking points in acrylic chains and distribution of chain extension of PU prepolymers. These crosslinking density distributions can be related to mechanical and adhesive properties of the polymer.     

Synthesis and properties of ultraviolet/moisture dual‐curable polysiloxane acrylates

Ultraviolet (UV)/moisture dual‐curable polysiloxane acrylates (PSAs) were prepared from N,N‐bis[3‐(triethoxysilyl)propyl]amine (G402) and ethoxylated trimethylolpropane triacrylate (EB160) through Michael addition. The obtained prepolymers were characterized by ¹H‐NMR and FTIR. The rheological behavior of the prepolymers exhibited the properties of a Bingham fluid and the apparent viscosity was directly correlated with molecular weight. The photocuring kinetics of PSA were studied using photo‐DSC and all the polymerization conversions were high. With increasing content of tertiary amine in the prepolymer, the photocuring rate in air increased as well. The moisture‐curing kinetics of the prepolymers was studied using FTIR. It was found that the curing mechanism may be described as the transforming of Si O C into Si O Si structure, which was consistent with the theoretical expectation. DSC and TGA were used to characterize the glass‐transition temperatures and the thermomechanical stability of the prepolymers. Measurements of physical properties showed excellent gloss, impact strength, and high electric resistance for both UV‐ and moisture‐cured films, but poor adhesion for UV‐cured films and lower hardness for moisture‐cured films. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 846–853, 2005     

Curing reaction of saturated aliphatic polyester modified unsaturated polyester resins

Two series of unsaturated polyesters (UPE from isophthalic acid, fumaric acid, and propylene glycol) were prepared. In series-A resins, UPEs wee thickened with isocyanate-terminated saturated aliphatic polyestes, i.e., an isocyanate-terminated polycaprolactone diol (PE-di-OL), through reaction of the isocyanate group with the hydroxyl group of the UPE. In series-B resins, the UPEs were mixed with saturated aliphatic polyesters i.e., PE-di-OL. The curing reaction of these two series of UPEs with styrene was studied by using differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The DSC data show that for a fixed PE-di-OL molecular weight, the curing reaction rate of series-A UPE is faster than that of series-B UPE. The variation of microgel size during curing ws studied by GPC. These results revealed that microgel formation has a great effect on the kinetics of cure for the unsaturated polyester-styrene system. The curing of these two series of UPEs is found to strongly depend on the compatibility of the components in the curing system.     

Moisture curing kinetics of isocyanate ended urethane quasi‐prepolymers monitored by IR spectroscopy and DSC

The study of the kinetics of the curing of isocyanate quasi‐prepolymers with water was performed by infrared spectroscopy and differential scanning calorimetry. The influence of the free isocyanate content, polyol functionality, and of the addition of an amine catalyst (2,2′‐dimorpholinediethylether) in the reaction kinetics and morphology of the final poly(urethane urea) was analyzed. A second‐order autocatalyzed model was successfully applied to reproduce the curing process under isothermal curing conditions, until gelation occurred. A kinetic model‐free approach was used to find the dependence of the effective activation energy (E_a) with the extent of cure, when the reaction was performed under nonisothermal conditions. The dependence of E_a with the reaction progress was different depending on the initial composition of the quasi‐prepolymer, which reveals the complexity of the curing process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Blocked isocyanate-terminated prepolymers in a delayed-cure durable-press finish for wool. Influence of isocyanate and blocking group structure on reactivity

A series of prepolymers containing terminal blocked isocyanate groups was prepared by reaction of isocyanate-terminated prepolymers [derived from a poly(propylene oxide) diol (MW = 2000) or a triol (MW = 3000) and different diisocyanates] with various blocking agents. Their curing rates with a polyepoxide crosslinking agent were measured to assess their suitability for a delayed-cure durable-press treatment for wool. The curing rates varied over a wide range, but commonly used blocking groups such as phenols, caprolactam, β-diketones, and alcohols were not sufficiently reactive for the intended application, although others, e.g., oximes and certain blocking agents containing a basic center such as N,N-diethylhydroxylamine, hydroxypyridines, and hydroxyquinolines, had the required reactivity. Blocking by 1,3-dipolar cycloaddition of nitrones was examined, but the reactivities were too low. Prepolymers containing blocked aromatic isocyanates were more reactive than those containing blocked aliphatic isocyanates. Difunctional butanone oxime-blocked pre-polymers cured slightly more slowly than analogous trifunctional prepolymers.     

In situ sensor monitoring and intelligent control of the resin transfer molding process

An intelligent closed-loop expert control system has been developed for automated control of the resin transfer molding process of a graphite fiber preform using an epoxy resin, E905L. The sensor model system has been developed to make intelligent decisions based on the achievement of landmarks in the cure process, such as full preform impregnation, the viscosity, and the degree of cure of the resin rather than time or temperature. In-situ frequency dependent electromagnetic sensor (FDEMS) and the Loos resin transfer model are used to monitor and control the processing properties of the epoxy resin during RTM impregnation and cure of an advanced fiber architecture stitched preform. Once correlated with viscosity (η) and degree of cure (α), the FDEMS sensor monitors and the RTM processing model predicts the reaction advancement of the resin, viscosity and the impregnation of the fabric. This provides a direct means for monitoring, evaluating, and controlling intelligently the progress of the RTM process in situ in the mold throughout the fabrication process and for verification of the quality of the composites.     

Block polymers from isocyanate-terminated intermediates. III. Preparation and properties of cured diene–urea block polymers

Isoprene–urea and butadiene–urea block polymers have been prepared by reaction of isocyanate-terminated prepolymers with diamines and diisocyanates. It was found that the per cent of blocked urea in these copolymers is dependent on the isocyanate–diamine stoichiometry. Stress–strain data were obtained on sulfur-cured copolymers. Stress levels at any given extension were directly dependent on the urea content, however, ultimate strength was maximized at about 35 wt-% urea. Slight variations in urea structure produced rather dramatic changes in the modulus of these cured elastomers. Diamine–diisocyanate stoichiometry did not affect the physical properties of these cured copolymers at equal or excessive diamine concentrations. However, at excess diisocyanate levels, elastomeric properties deteriorated rapidly. Finally, stress–strain properties were affected by polymerization solvent. Copolymers prepared in hexane gave higher values of stress at low strains than corresponding elastomers prepared in toluene. These latter results are explained in terms of the nonaqueous emulsion in which the urea blocks form.     

Synthesis and Characterization of Polyurea Resin for Dielectric Coating Applications

In this article, polyurea coatings were synthesized by reaction between toluene diisocyanate, and polyether difunctional and trifunctional amines PO-1 and PO-2 by incorporating urea linkage. The isocyanate-terminated prepolymers formed were further reacted with short-chain amines as chain extender as well as curing agents. These resins were synthesized by varying the molar ratio of amines and isocyanate and keeping the molar ratio of other monomers constant. The presence of functional groups were determined using Fourier transform infrared spectroscopy. The thermal stability of polymers was determined using the thermogravimetric analysis and differential scanning calorimetry. The polymers were further tested for mechanical and electrical properties. It was observed that on increasing the hard segments in the polymer the electrical properties increase whereas elongation decrease. As isocyanate content increases, crosslinking increases resulting in improvement in dielectric properties. The maximum dielectric strength observed was 45 kV mm^?1 and dissipation factor was 0.068.     

聚丙烯塑料胶粘剂的研制

以甲苯为溶剂,过氧化二苯甲酰(BPO)为引发剂,通过自由基聚合,采用马来酸酐(MAH)接枝改性氯化聚丙烯(CPP)；研究了处理条件、固化条件、反应温度、反应时间、MAH的用量、BPO的用量对聚丙烯(PP)塑料胶粘剂粘接性能的影响,得到一种对PP塑料粘接性能较佳的胶粘剂。采用FTIR表征了产品的结构。     

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

Cure reaction between a series of N-methylaniline-blocked polyisocyanates, based on 4,4′-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 125–145°C. 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_N¹) 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. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Optimizing the properties of isocyanate-terminated polyurethane prepolymers by statistically designed experiments

The properties of isocyanate-terminated polyurethane prepolymers were optimized through the use of statistically designed experiments. A simple 2 × 2 factorial with centerpoint statistical design was used to determine the effect the diol-to-triol ratio and per cent free isocyanate had on the viscosity of the prepolymer. Combinations of a 1000 or 2000 molecular weight diol with either a 1500 or 4100 molecular weight triol showed that a reasonable range of viscosity could be maintained only with the 4100 molecular weight triol. A nine-point design was then utilized to predict the best levels of free isocyanate and polyol ratio that would yield cured urethane polymers with the highest tensile strengths, moduli, and elongations. With this design, second-order equations were computed that predict the mechanical responses of urethane polymers based on the free isocyanate and polyol ratio used in its formulation.     

Emulsion terpolymerization of dimethyl meta-isopropenyl benzyl isocyanate (TMI®) with acrylic monomers: Process development and kinetics

Dimethyl meta-isopropenyl benzyl isocyanate (TMI®) is a bifunctional monomer possessing a double bond and an isocyanate group. Emulsion terpolymerization of TMI with the acrylic monomers, methyl methacrylate and n-butyl acrylate, was studied. Polymerizations were carried out at 40°C using a redox initiator system in order to minimize the extent of hydrolysis of the isocyanate in the aqueous emulsion environment. The kinetics of the polymerization reaction were investigated. The polymerization rate was found to decrease with increasing TMI concentration. The effects of several preparative variables such as the monomer, surfactant, and initiator concentration on the polymerization kinetics was studied. Several semicontinuous polymerization processes were developed in order to enhance the incorporation of TMI at appreciable rates. These processes also allow us to control the polymer composition, latex particle size, and the locus of isocyanate groups in the latex particle. Process variables such as the mode of initiator addition (batch versus semicontinuous) were found to greatly influence the polymerization behavior. © 1996 John Wiley & Sons, Inc.     

The phase mixing of moisture cured polyurethane-urea during cure

Moisture cured polyurethane-ureas (MCPUs) is one of the industrially important polymer, which shows good thermal–mechanical and weathering properties and widely used in the reactive hot melt adhesives and coatings. In this study, chemically crosslinked MCPUs were prepared by reacting isophorone diisocyanate (IPDI) with polyethers like polytetramethyleneglycol (PTMG)-1000 and polyethyleneglycol (PEG)-1000, with NCO/OH ratio 1.6:1. Trimethylol propane (TMP) was used as a crosslinking agent during the prepolymer synthesis. The excess isocyanate of the prepolymers was cured with moisture at 25 °C and humidity of 40%. Fourier transform infrared spectroscopy (FTIR) and dynamic mechanical thermal analyzer (DMTA) measurements were used to monitor curing process of polyurethane-urea systems. Higher correlation coefficient (R²) values were obtained for the second-order cure model compared to the first- and third-order for both the synthesized prepolymers.

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The change in short range ordering associated with hydrogen bonding as well as decrease in crystallinity of soft segment during the phase mixing was observed from differential scanning calorimetry (DSC) measurements. The change in thermal stability was assessed by thermogravimetric (TG) analysis. Characterizations of the curing process provide an essential base to obtain best polymer.The phase mixing phenomenon was confirmed from the angle resolved X-ray photoelectron spectroscopy (AR-XPS).     

The kinetics of the reaction of isophorone di-isocyanate with mono-alcohols

Isophorone di-isocyanate (IPDI) is used to react with hydroxyl containing prepolymers in the preparation of polyurethane elastomers, particularly with hydroxy-terminated polybutadiene (HTPB) in composite propellant systems. In the present work, the kinetics of the reaction of IPDI with monoalcohols are studied, as a model for the polyurethane systems. A gas chromatography method is used to follow the reaction, which allows direct measurement of the reaction of individual isocyanate groups. The rates of individual isocyanate groups are described by a second order equation modified to include catalysis by the urethane products. The rates of primary and secondary isocyanate groups in the two isomers of IPDI are compared. Differences between the various isocyanate groups are relatively small, and are offset by the urethane catalysis effect, so that the overall disappearance of isocyanate is roughly second order. The reaction rate of IPDI with HTPB is faster than with the model compounds, and it is suggested that this arises because there is tendency for the isocyanate to complex with the hydroxyl end groups of the polymer.     

Prevention of Hydrosilation Cure Inhibition in a Polysiloxane Adhesive by Surface Oxidation

For a variety of reasons epichlorohydrin-ethylene oxide (ECO)-based compounds, in the form of rain erosion boots or sleeves, are bonded to aircraft radomes using a hydrosilation-cured RTV polysiloxane adhesive. Unfortunately, cure of the adhesive can be completely inhibited by unreacted vulcanizing agent and/or ECO cure by-products. We have earlier shown that this inhibition can be prevented by treating the ECO surface with hydrogen peroxide to oxidize the hydrosilation catalyst poisons to a harmless state. In this study we have used spectroscopic techniques to monitor the hydrosilation reaction kinetics and in turn to identify the poison, define the poisoning level and monitor the poison removal by hydrogen peroxide treatment. The degree of poison removal is also correlated with adhesive bond strength using a 180°; peel test. The critical poison in the system was excess ECO vulcanizing agent which can be completely removed from the surface using a 30 vol. % H₂O₂ treatment for 30 minutes as long as the initial vulcanizing agent concentration is 1 p.p.h. or less.     

Graphene oxide-anchored reactive sulfonated copolymer via simple one pot condensation polymerization: proton-conducting solid electrolytes

A rapid and efficient post-polymerization functionalization of poly(urea-co-urethane) (PUU) onto the graphene oxide (GO) nanosheets has been developed to produce super-acidic polymer/GO hybrid nanosheets. Thus, the surface of GO nanosheets were functionalized with 3-(triethoxysilyl)propyl isocyanate (TESPIC) from hydroxyl groups to yield isocyanate functionalized graphene oxide nanosheets. Then, sulfonated polymer/GO hybrid nanosheets were prepared by condensation polymerization of isocyanate-terminated pre-polyurea onto isocyanate functionalized graphene oxide nanosheets through the formation of carbamate bonds. FTIR and TGA results indicated that TESPIC modifier agent and poly(urea-co-urethane) were successfully grafted onto the GO nanosheets. The grafting efficiency of poly(urea-co-urethane) polymer onto the GO nanosheets was estimated from TGA thermograms to be 205.9%. Also, sulfonated polymer/GO hybrid nanosheets showed a proton conductivity as high as 3.7 mS cm^?1. Modification and morphology of GO nanosheets before and after modification processes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD).     

异氰酸酯/环氧树脂的固化机理

详细研究了异氰酸酯/环氧树脂体系的固化反应和固化机理。采用差示扫描量热法（DSC）和红外光谱法（FTIR）跟踪了异氰酸酯/环氧树脂固化反应过程,定量分析了异氰酸酯、环氧基团和新生成的异氰脲酸酯和口恶唑烷酮的变化。DSC分析结果表明,DSC曲线上出现3个放热峰,说明固化过程中存在至少3种反应;FTIR分析结果表明,在140℃以下固化体系主要发生异氰酸酯的三聚反应生成三嗪环（异氰脲酸酯）;在200℃下,异氰酸酯-NCO基团与环氧基团开环反应生成口恶唑烷酮;在230℃ 下,三嗪环（异氰脲酸酯）进一步与环氧基团开环反应生成口恶唑烷酮。研究了不同温度下环氧基团、异氰酸酯基团、异氰脲酸酯基团、口恶唑烷酮基团随反应时间的变化规律。     

Prediction of the mechanical properties of a crosslinking polymer from a combination of the measured kinetics and a model for network growth

The mechanical properties of a crosslinking isocyanate–hydroxy system were predicted by a combination of the measured curing kinetics and a model for polymer network growth. The kinetic parameters were determined from FTIR using the linear rising temperature method (the activation energy = 52 kJ/mol, reaction order = 2.9). This data, combined with the network model, was used to predict the rise in elastic modulus as measured by dynamic mechanical analysis (DMA). The agreement between the predicted and experimental results was excellent over the early part of the curing, but the model failed at higher isocyanate conversions. In addition a novel method for obtaining the activation energy for the reaction directly from DMA results is presented; the value obtained from this method was in excellent agreement with that obtained by FTIR.