NCO Concentration Research Articles

Self-healing epoxy coatings via microencapsulation of aromatic diisocyanate in lignin stabilized Pickering emulsions

Isocyanate-filled microcapsules are gaining increased attention for their role in developing self-healing materials, thereby reducing maintenance costs and increasing polymer durability. Nevertheless, microencapsulating highly reactive -NCO groups remains a challenging task in the literature. Likewise, considerable efforts have been directed towards developing polymers from renewable and biobased sources, which could also be applied to microcapsule synthesis. In this work, lignin, an abundant biopolymer, was used as a solid stabilizer for oil-in-water (O/W) interfaces, enabling the encapsulation of highly reactive isocyanate. Hybrid polyurethane/polyurea microcapsules containing reactive methylenediphenyl diisocyanate (MDI) were obtained via optimized O/W Pickering emulsions using lignin for system stabilization. This optimized process facilitated shell formation via the reaction of diisocyanate with lignin and water, eliminating the need for additional chain extenders. Scanning electron microscopy revealed the formation of spherical and rough microcapsules, while infrared spectroscopy confirmed the presence of residual free -NCO groups, indicating effective encapsulation of MDI. Additionally, a core -NCO concentration of 11 wt% was confirmed by titration. The microcapsules were further assessed as components in self-healing epoxy coatings. Their incorporation resulted in the retardation of corrosion on a low carbon steel panel after 72 h of submersion in a saline solution. Electrochemical impedance spectroscopy (EIS) confirmed a significant increase of approximated 620% in impedance modulus after 83 days of immersion in a 3.5 wt% NaCl solution, compared to the neat epoxy coating. These findings suggest a promising technological application of this material for the advancement of self-healing epoxy-based coatings and composites.

Combustion of biomass pyrolysis gas: Roles of radiation reabsorption and water content

Biomass energy has drawn increased attention owing to its zero carbon emissions and large reserves. Pyrolysis instead of direct combustion is an efficient and clean way for biomass energy conversion. As a strong radiative species, H2O greatly affects the combustion processes. Despite the large concentration and fluctuation of H2O content in biomass pyrolysis gases (py-gas), radiation reabsorption effect has not been systematically investigated for py-gas combustion. In this study, one-dimensional simulations of premixed py-gas/air flames were conducted, using PREMIX code with both adiabatic and radiative models. The effects of H2O content and radiation reabsorption were examined. As H2O content in the py-gas mixture increased from 40% to 50%, the flame speeds decreased from 42.86 to 28.08 cm/s, while the relative differences caused by radiation reabsorption increased from 9.92% to 17.42%. Radiation reabsorption affected laminar flame speed primarily through the preheat-induced chemical effect, which was mainly controlled by HCO radical. The outlet mole fraction of NO was reduced by up to 13.56% when radiation reabsorption was considered. Reaction pathway analyses revealed that the NO emission was closely related to the outlet temperature and the peak NCO concentration, which were the limiting factors for the thermal-NO route and the fuel-NO route, respectively.

Nontarget Analysis Reveals a Bacterial Metabolite of Pyrene Implicated in the Genotoxicity of Contaminated Soil after Bioremediation.

Bioremediation is an accepted technology for cleanup of soil contaminated with polycyclic aromatic hydrocarbons (PAHs), but it can increase the genotoxicity of the soil despite removal of the regulated PAHs. Although polar biotransformation products have been implicated as causative genotoxic agents, no specific product has been identified. We pursued a nontarget analytical approach combining effect-directed analysis (EDA) and metabolite profiling to compare extracts of PAH-contaminated soil from a former manufactured-gas plant site before and after treatment in a laboratory-scale aerobic bioreactor. A compound with the composition C15H8O2 and four methylated homologues were shown to accumulate as a result of bioreactor treatment, and the C15H8O2 compound purified from soil extracts was determined to be genotoxic. Its structure was established by nuclear magnetic resonance and mass spectroscopy as a heretofore unidentified α,β-unsaturated lactone derived from dioxygenation of pyrene at an apical ring, 2H-naphtho[2,1,8-def]chromen-2-one (NCO), which was confirmed by synthesis. The concentration of NCO in the bioreactor was 11 μg g-1 dry soil, corresponding to 13% of the pyrene removed. It also accumulated in aerobically incubated soil from two additional PAH-contaminated sites and was formed from pyrene by two pyrene-degrading bacterial cultures known to be geographically widespread, underscoring its potential environmental significance.

Exposure to Diisocyanates and Their Corresponding Diamines in Seven Different Workplaces.

Biological monitoring to assess exposure to diisocyanates in the workplace is becoming increasingly widespread due to its relative ease of use and ability to look at all exposure routes. Currently, biological monitoring measures the corresponding isocyanate-derived diamine in urine, after hydrolysis. Because of this, any exposure to the diamines themselves released during the industrial process could confound the assessment of diisocyanate exposure. This paper reports an initial assessment of the extent of diamine formation and exposure during different processes involving diisocyanates including casting, grouting, core making, spray painting, foam blowing, and floor screeding. Air monitoring and glove analysis were conducted for both the relevant diisocyanate (measured as total NCO) and its corresponding diamine; urine samples were analysed (after hydrolysis) for the isocyanate-derived diamine. Processes that generated aerosols (as demonstrated by impinger analysis) such as spray painting and foam blowing were associated with the detection of diamines. Those processes that did not generate aerosols (casting, grouting, core making, and screeding) had no diamines detected, either in air or on gloves. In spray-painting tasks, diamines were a minor component (<15%) of the ambient concentration whereas in the foam blowing processes, where water is added to the process, diamine generation is more marked (up to eight times the airborne NCO concentration). Some non-aerosol processes gave rise to substantial diamine levels in urine (in exceedance of international guidance values, >5 µmol mol-1 creatinine) despite airborne levels being well within occupational exposure limits (20 µg m-3 total NCO in Great Britain); measurement data and statistical modelling indicated that skin absorption was the most likely exposure route. Foam blowing exposures were more complex, but urinary levels were greater than those expected from diisocyanate inhalation alone (measured as total NCO). This study provides evidence that biological monitoring for diisocyanates based on measuring the corresponding diamine in urine is valid, although any co-exposure to diamines themselves should be considered when interpreting results. It also demonstrates the potential for substantial skin absorption of diisocyanates in certain processes such as floor screeding and foam production.

Determination of Polyurethane Foam Growth Kinetics by a Simple Column Height Test

In the present work, the height of foam column has been used to monitor the reaction of the isocyanate group. Kinetic modeling describes the conversion versus time curves. The global reaction can be divided into two different paths: the blowing reaction and the polymerization. The kinetics of the reaction of toluene diisocyanate with a polypropylene glycol has been studied with auxiliary blowing agents; the concentration of NCO and OH groups could be considered equivalent during all the experiments as a model for similar polyurethane systems. Despite the complexity of the system in which different kinetic orders are possible, as well a physically controlled diffusion process, data were fitted to a simple kinetic model of an apparent order. The results obtained have enabled us to propose a reaction path and catalyzed second-order kinetic model.

Study on Mechanical Properties of Polyurethane Elastomers for Drum Scraper

Polyurethane elastomers (PUE) were prepared by casting method using the prepolymer and the chain extender. In here, the prepolymer synthesized by using poly(tetramethylene glycol ether) (PTMG) and toluene diisocyanate (TDI), the chain extender was a mixture of 3,5-dimetylthio toluene diamine (E-300) and triethanolamine. The effects of the NCO concentration in prepolymer, the molar ratio of E-300/triethanolamine, and the chain extension coefficient of NCO/NH2 on the mechanical properties of the prepared PUE were studied. The results showed that the prepared PUE possesses excellent mechanical properties which can meet the drum scraper’s application requirements when the NCO concentration in prepolymer was 5.06% and the molar ratio of composite chain extender was 0.92/0.08.

THE ROTATIONAL SPECTRUM OF THE NCO–ANION

The rotational spectrum of the negative molecular ion NCO– has been observed both in a supersonic molecular beam and in a low-pressure glow discharge. The identification is ironclad because of the previous infrared detection of NCO–, the presence of well-resolved nitrogen quadrupole hyperfine structure, and the observation of nine harmonically related transitions in the millimeter band. The spectroscopic constants B and D are three orders of magnitude more accurate than those derived from the earlier IR measurements, and the theoretical eQq is in good agreement with that measured. The entire rotational spectrum can now be calculated well into the THz region to 1 km s–1 in equivalent radial velocity or better. NCO– is an excellent candidate for radio astronomical detection because of its high stability, polarity, and favorable partition function. The fairly high concentration of NCO– in our laboratory source implies that other molecular anions containing the NCO group may be detectable in the radio band.

Determination of the Rate Constant for the Radical−Radical Reaction NCO(X2Π) + CH3(X2A2‘ ‘) at 293 K and an Estimate of Possible Product Channels

The rate constant for the reaction of the cyanato radical, NCO(X2Pi), with the methyl radical, CH3(X2A2' '), has been measured to be (2.1 +/- 1.3(-0.80)) x 10(-10) cm3 molecule(-1) s(-1), where the uncertainty includes both random and systematic errors at the 68% confidence level. The measurements were conducted over a pressure range of 2.8-4.3 Torr of CH4 and at a temperature of 293 +/- 2 K. The radicals were generated by the 248-nm photolysis of ClNCO in a large excess of CH4. The subsequent rapid reaction, Cl + CH4, generated the CH3 radical. The rate constant for the Cl + CH4 reaction was measured to be (9.2 +/- 0.2) x 10(-14) cm3 molecule(-1) s(-1), where the uncertainty is the scatter of one standard deviation in the data. The progress of the reaction was followed by time-resolved infrared absorption spectroscopy on single rovibrational transitions from the ground vibrational level. Multiple species were detected in these experiments, including NCO, CH3, HCl, C2H6, HCN, HNC, NH, and HNCO. Temporal concentration profiles of the observed species were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of the residuals between experimental observations and model calculations. Both HCN and HNC seem to be minor products (<0.3% each) of the NCO + CH3 reaction. The peak concentrations of NH and HNCO were small, accounting for <1% of the initial NCO concentration; however, their temporal profiles could not be fit by the model kinetics. The observed C2H6 temporal profile always peaked at significantly higher concentrations than the model predictions, and several reaction models were constructed to help explain these observations. The most likely product channel seems to be the recombination channels, producing CH3NCO and CH3OCN.

Determination of the Rate Constants for the NCO(X2Π) + Cl(2P) and Cl(2P) + ClNCO(XA‘) Reactions at 293 and 345 K

The rate constant for the reaction of the isocyanato radical, NCO(X2Pi) with chlorine atoms, Cl(2P), has been measured at 293 +/- 2 and 345 +/- 3 K to be (6.9 +/- 3.8) x 10(-11) and (4.0 +/- 2.2) x 10(-11) cm3 molecules(-1) s,(-1) respectively, where the uncertainties include both random and systematic errors. The measurements were carried out at pressures of 1.3-6.2 Torr with either Ar or CF4 as the bath gas and were independent of both pressure and nature of the third body. Equal concentrations of NCO and Cl atoms were created by 248 nm photolysis of ClNCO. The reaction was monitored by following the temporal dependence of NCO(X2Pi) using time-resolved infrared absorption spectroscopy on rotational transitions of the NCO(10(1)1) <-- (00(1)0) combination band. The reaction rate constant was determined by using a simple chemical model and minimizing the sum of the residuals between the experimental and computer generated temporal NCO concentration profiles. The reaction Cl + ClNCO --> Cl2 + NCO was found to contribute to the observed NCO. The rate constant for this reaction was found to be (2.4 +/- 1.6) x 10(-13) and (1.9 +/- 1.2) x 10(-13) cm3 molecules(-1) s,(-1) at 293 and 345 K, respectively, where the uncertainties include both random and systematic error.

DSC monitoring of the cure kinetics of a castor oil-based polyurethane

Differential scanning calorimetry (DSC) has been used to monitor the reaction between castor oil and isophorone diisocyanate (IPDI), using a non-isothermal method, at different heating rates. The NCO/OH ratio in these systems was 1, so that the concentration of NCO and OH groups could be considered equivalent during all the experiments. Despite the complexity of the system, in which different order kinetics are possible, as well as physically controlled diffusion processes, data were fitted to a simple kinetic model, of apparent order n, apparent activation energy E A, and apparent frequency factor A 0. The dependency of these parameters on heating rate was used to analyze kinetics of polymerization of these systems.

Mechanism of the selective catalytic reduction of NOx by C2H5OH over Ag/Al2O3

The mechanism of the selective catalytic reduction (SCR) of NOx by C2H5OH over silver catalyst (Ag/Al2O3) was investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Attention was focused on the formation and reactivity of novel enolic species on the Ag/Al2O3 surface. DRIFTS spectra show that the surface enolic species, which was derived from partial oxidation of C2H5OH over Ag/Al2O3 in the presence of excess oxygen, play a crucial role in the formation of isocynate species (NCO) by reaction with NO+O2 or NO3− absorbed on the surface of Ag/Al2O3. 2,3-dihydrofuran was used to an enolic model compound in our DRITFS study, and the results support our assignment. A novel mechanism of the SCR of NOx by C2H5OH is proposed based on the DRIFTS studies. The mechanistic differences between the SCR of NOx by C2H5OH and that by C3H6 are discussed. The high reactivity of the enolic species results in high surface concentration of NCO and high efficiency of NOx reduction using C2H5OH as a reductant. The results of density functional theory (DFT) calculations are in good agreement with the DRIFTS spectra, and support our suggestions about the surface enolic species.

Surface/interfacial changes during polyurethane crosslinking: A spectroscopic study. V *

AbstractCrosslinking reactions and stratification processes in polyurethane films were investigated using attenuated total reflectance (ATR) FTIR spectroscopy. The HDI trimer levels near the film–substrate (F–S) interface appear to increase at the early stages of reactions, and after reaching a maximum, decrease. This is attributed to solvent depletion and subsequent film shrinkage, thus causing local NCO concentration changes. At the later stages of crosslinking reactions, stratification of the 1,6‐hexamethylene diisocyanate (HDI) trimer occurs, with higher concentration levels at shallower penetration depths from the F–S interface. At the same time, NCO‐containing groups assume preferentially parallel orientation to the F–S interface. On the other hand, H‐bonded carbonyl groups tend to orient themselves in a perpendicular direction. Quantitative analysis indicates that at extended reaction times the amount of H‐bonded CO groups at shallower penetration depths increases and their orientation tends to be more random, which is correlated with migration toward the F–S interface. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 81: 2045–2054, 2001

Interfacial studies of crosslinked urethanes: Part IV. Substrate effect on film formation in polyester waterborne polyurethanes

These studies examine the effect of substrate surface tension on crosslinking reactions in water-borne polyurethanes (PUR) applied to tin-plated steel, steel, polypropylene (PP), thermoplastic olefin (TPO), and glass. The results show that, relative to tin-plated steel, removal of tin-plating from a steel substrate increases isocyanate (NCO) consumption by 42% near the film-substrate (F-S) interface. When PUR is allowed to crosslink on steel, PP, TPO, and glass, the NCO concentration is greater at the film-air (F-A) interface. Furthermore, crosslinking reactions result in a greater amount of urea-hydrogen bonded species near the F-S interface for all substrates. While an increase of substrate surface tension decreases the amount of urea-hydrogen bonded carbonyl groups near the F-S interface, TPO was found to exhibit different behavior due to talc stratification near the surface. In this case, the presence of talc in WB-PUR coatings or thermoplastic substrates decreases the amount of hydrogen-bonded species and increases NCO consumption.

Defect free coatings from two-pack isocyanate curable acrylic dispersions

The combination of isocyanate crosslinking groups and water as continuous phase in coatings provides a real challenge to the coatings industry. Especially the ability of the crosslinker to react with water and liberate carbon dioxide is a problem. CO 2 will be entrapped in the film, resulting in poor aesthetic properties and porous films with reduced resistance properties. In this paper it will be shown that the choice of acid monomer and the acid value have a significant effect on the physical drying rate of the films’ on the level of water remaining in the film after cure, and on the rate of decrease of the NCO concentration and CO 2 blister formation. Increasing the acid value will result in faster NCO loss and more CO 2. The same effects are reached when acrylic acid is chosen over methacrylic acid, which in turn yields a faster NCO concentration decrease and more CO 2 blisters than βCEA. A faster decrease of the NCO group concentration does not necessarily lead to a faster tack free time. By careful selection of the type of acid defect free films without CO 2 blisters can be formed with a thickness of at least 300 μm (dry). CO 2 blister formation is caused by the presence of water soluble poly-acid which remains in the film enabling the reaction of the isocyanate group with water.

Interfacial studies of crosslinked polyurethanes; Part I. Quantitative and structural aspects of crosslinking near film-air and film-substrate interfaces in solvent-borne polyurethanes

One of the reactions leading to the formation of polyurethane (PU) crosslinked networks is the reaction of NCO and OH functionalities. In this study, we examined how crosslinking reactions of hexamethylene diisocyanate isocyanurate and polyacrylate near the film-air (F-A) and film-substrate (F-S) interfaces in urethane coatings may affect crosslink density as well as other network properties. While at the initial stages of the crosslinking reactions, solvent evaporation competes with the urethane network formation and isocyanate consumption changes at various depths from the F-A and F-S interfaces. Quantitative analysis of the NCO consumption as a function of depth showed that the NCO concentrations change from 2.35×10−5 to 2.09×10−5 M, while going from 0.27 to 1.14 µm. During reaction times not exceeding two to three hours, the NCO consumption at the F-A and F-S interfaces is consumed more rapidly. At low relative humidity conditions, excessive amounts of unreacted NCO exists at both the F-A and F-S interfaces. However, at the extended reaction times, NCO concentration levels at the F-S are greater than at the F-A interface, and the NCO concentration differences can be as high as 3×10−5 M. In this study we also examined how crosslinking reactions of hexamethylene diisocyanate (HDI) isocyanurate and polyacrylate near the F-S interfaces in urethane films may affect orientation and distribution of urethane functionalities.

Probing the Cure and Postcure Reactions in Polyurethanes by FTIR and GPC

Fourier transform infrared (FTIR) and gel permeation chromatography (GPC) wre respectively utilized in determining the changes in isocyanate (NCO) concentration and molecular weight (MW) and its distribution (MWD) in polyurethanes during their cure and postcure stages. Samples, originally cured at 45°C, were taken through accelerate aging tests at various temperatures. The FTIR and GPC analytical conditions were optimized, for both molecular weight and molecular weight distribution and NCO group determination. In FTIR spectroscopy Beer's law was obeyed over the ranges: 2.33-22.34 and 0.03-1.61 mmol lit-1. RSD, detection limit and molar absortivity were found to be 1.46%, 4´10-6 mol lit-1 and 1095 lit mol-1 cm-1 respectively. It was found the NCO concentration decreases with the progress of polymerization and at the same time MW of the sol increases, going through a maximum and then decreases to a constant level. The time to reach this level and termination of NCO concentration, i.e., the end of polymerization decreases with the increase in temperature. The results obtained from GPC and FTIR methods in following the different steps of polymerization as well as those from the linear plots of the end time of reaction vs. the 1/T, were in good agreement.

Kinetics of the reactions of NCO with NO and NO2

AbstractThe rate coefficients of the reactions of NCO radicals with NO and NO2: (1) NCO + NO → products (293–836 K) and : (2) NCO + NO2 → products (294–774 K) were measured by means of laser photolysis and laser induced fluorescence technique in the indicated temperature ranges. NCO radicals were produced from the reaction of CN, from photodissociation of ICN or BrCN, with O2. The concentration of NCO was monitored with a dye laser set at 414.95 nm. We determined k1 = 1.73 × 10−5 T−2.01 exp(−470/T) cm3 molecule−1 s−1 that agrees with published results at room temperature and confirms the temperature dependence of an early report. A non‐Arrhenius negative temperature dependence of k2 was observed in this work that agrees satisfactorily with results for a shock tube18 near 1250 K. We obtained k2 = 6.4 × 10−10 T−0.646 exp(164/T) cm3 molecule−1 s−1 for 1250 K ≥ T ≥ 294 K by combining data of these two measurements. Our result at 294 K and the temperature dependence disagree with results of two previous investigations. © 1995 John Wiley &amp; Sons, Inc.

Bulk anionic copolymerization of ε‐caprolactam in the presence of macroactivators derived from polypropylene glycol

AbstractBulk anionic copolymerization of ε‐caprolactam (CPL) was conducted, under four different conditions by changing temperature (110 or 125°C) and [NCO]/[NaH] ratio (1, 2, or 3), in the presence of NCO‐terminated polypropylene glycol (P1) and its CPL‐blocked prepolymer (P2). Under the same conditions and reaction time, the conversion of CPL and reduced viscosity of the P2 system were higher than those of the P1 system. However, at the same conversion the P1 system showed higher viscosity for reactions at 125°C with [NCO]/[NaH] = 3 and at 110°C with [NCO]/[NaH] = 2. These results were attributed to cyclotrimerization of NCO groups of P1 (formation of isocyanurate) at the initial stage, which not only consumed the effective concentration of NCO but also increased the viscosity of the P1 system. Comparing IR spectra of the reaction products of model compounds, phenyl isocyanate and CPL‐blocked phenyl isocyanate, with NaH/CPL also supported this conclusion. The crystalline melting temperature (Tm = 198–208°C) and melting enthalpy of the final products depended on the conversion of CPL and the types of macroactivators. © 1993 John Wiley &amp; Sons, Inc.

Product branching in the CO+NF (a 1Δ) reaction

The product channels for the NF(a 1Δ)+CO quenching reaction, either chemical reaction with formation of NCO+F or physical quenching with formation of NF(X 3Σ −)+CO, have been studied in a flow reactor at 300 K. Laser-induced fluorescence measurements of the NCO concentration show that the spin-conserving chemical reaction is the main pathway. The yield of NCO was calibrated from the stoichiometric F+HNCO reaction giving HF+NCO.

Quantitative high temperature absorption spectroscopy of NCO at 305 and 440 nm

A spectral survey of NCO absorption near the P 2 + p Q 12 head of the [ A 2 Σ + (00 00)← X 2 Π i (00 10)] band was obtained at 1450°K, 0.6 atm using a remotely located cw ring dye laser source and a shock tube. Mixtures of hydrogen cyanide, oxygen and nitrous oxide diluted in argon were shock heated to provide a reproducible steady-state concentration of NCO, and narrow-line absorption was measured in repeated experiments with the laser set at different wavelengths. The peak absorption was found at 440.479 nm (vac). The experimental spectrum was compared with a theoretical model to yield an average Voigt parameter a ≅ 0.1. Additional experiments, in mixtures of cyanogen, oxygen and nitrous oxide diluted in argon, provided a known plateau level of NCO, which was used to infer an absolute absorption coefficient β(1450°K, 0.60 atm) = 110(-50, +130) cm -1 atm -1 at 440.479 nm. This value of β corresponds to an oscillator strength of 0.0026 for the (00 00←00 10) band. Similar experiments were conducted to monitor the absorption around the R 1 head of the [ B 2 Π i (10 10)← X 2 Π i (00 10)] band of NCO, using a frequency doubled cw ring dye laser. The observed spectrum displayed strong broadening, indicating predissociation of the upper state. At the peak absorption wavelength (304.681 nm, vac), we inferred β(1470°K, 0.63 atm) = 40(-19, +48) cm -1 atm -1 and a ≅ 9. This value of β corresponds to an oscillator strength of 0.0031 for the (10 10←00 10) band.