Isocyanate calculation

The tensile properties at room temperature and at intervals up to 240°C of the two series of polyurethane elastomers studied are given in Table 3.27 (block ratio 1 2 1 and BDO chain-extended) and in Table 3.28 (block ratio 1 3 2 and BDO/CHDM chain-extended). In both these series the percentage of free isocyanate calculated to be present in the elastomer on first casting is varied from 0 to 50% in 5% steps. 

More recent matrix isolation studies on benzoyl nitrene, however, did show conversion to the isocyanate. Calculations show that the acyl nitrene has oxazirene (108) character and that this 0-N interaction stabilizes the singlet state of the nitrene. Interestingly, a reexamination of benzoyl nitrene, trapped in an argon matrix and irradiated at 308 nm, revealed that in addition to phenyl isocyanate, a small amoimt of phenyl cyanate (110) was formed. Direct irradiation of benzoyl azide (254 nm) also generated the... 

Hydroxyl Number. The molecular weight of polyether polyols for urethanes is usually expressed as its hydroxyl number or percent hydroxyl. When KOH (56,100 meg/mol) is the base, the hydroxyl number is defined as 56,100/equivalent weight (eq wt). Writing the equation as eq wt = 56,100/OH No. allows one to calculate the equivalents of polyol used in a urethane formulation, and then the amount of isocyanate required. The molecular weight can be calculated from these equations if the fiinctionahty, is known mol wt = / eq wt. 

The acetyl content of cellulose acetate may be calculated by difference from the hydroxyl content, which is usually determined by carbanilation of the ester hydroxy groups in pyridine solvent with phenyl isocyanate [103-71-9J, followed by measurement of uv absorption of the combined carbanilate. Methods for determining cellulose ester hydroxyl content by near-infrared spectroscopy (111) and acid content by nmr spectroscopy (112) and pyrolysis gas chromatography (113) have been reported. 

Formulations should be based on stoichiometric considerations. Based on a knowledge of the hydroxyl value of the polyol the amount of isocyanate necessary to cause chain growth should be calculated. The gas evolved will depend on the water content and additional isocyanate must be incorporated corresponding to the water present. When the isocyanate used equals the theoretical amount the system is said to have a TDI index of 100. In practice a slight excess of isocyanate is used (TDI index 105-110) to ensure complete... 

Isobutylene, polymerization of, 1207 Isocyanate. Hofmann rearrangement and,933-934 Isoelectric point (pJ), 1024 calculation of, 1024 table of, 1018-1019 lsoleucine, metabolism of, 911 molecular model of, 304 structure and properties of, 1018 Isomer, conformational, 93 Isomerase, 1041-1042 Isomers, 81... 

For polyol and isocyanate blends, the total equivalents of isocyanate divided by file total equivalents of OH equals the reaction index. For more on urethane calculations, see refs. Id and lk. 

The curves describing the isocyanate decrease, calculated by the mathematical model, have been fitted with measured curves to estimate the reaction rate constants. 

The analytical determination of the Isocyanate decrease during curing of the paint has been used to estimate the reaction rate constants. A reasonable curve fitting between the calculated and the measured curves has been obtained for a reaction rate constant (ki and kz in Scheme II) of approx. 0.01 cm . mmol". s =-. 

Using known and estimated parameters, calculations have been performed with regard to the decrease of isocyanate and the carbon dioxide concentration in the paint film. 

Figure 8 gives the calculated isocyanate decrease during curing at different reaction rate constants and at a water concentration of 1.4Z (m/m). 

Figure 8. Influence of the reaction rate constants on the isocyanate and hydroxyl decrease during curing model calculations.

The concentration of each chemical species, as a function of time, during cure can be calculated numerically from Equations 3-6 using the Euler-Romberg Integration method if the initial concentrations of blocked isocyanate and hydroxyl functionality are known. It is a self-starting technique and is generally well behaved under a wide variety of conditions. Details of this numerical procedure are given by McCalla (12). 

The effectiveness of the above described computational procedure was tested by generating an analytical ("ideal") data curve by calculating the isocyanate concentration as a function of time assuming rate constants of k = 1.0/min and k2 = 1.0 A/mol/min and initial concentrations of blocked isocyanate and hydroxyl of 1.0 M. The objective function, for various values of k. and k, for this "ideal" data was calculated and a contour plot for constant values of F was generated and is shown in Figure 2. 

FIGURE 4 EXPERIMENTAL DATA AND CALCULATED ISOTHERMAL CURE CURVES (A PREDICTEDD [BLOCKED ISOCYANATE], 0 EXPERIMENTAL [NCO], PREDICTED [NCO], + PREDICTED [CROSSLINKS])... 

Once k. and k2 have been determined it is then possible, if the initial concentrations are known from knowledge of the resin system, to calculate curves for the concentration of each species during cure. In this way the absorbance of isocyanate at its maximum can be calculated. 

Comparison of results using concentration and normalized absorbance for the experimental data demonstrates the effectiveness of this approach. The concentration procedure results in k1 = 1.85/min and k = 0.35 A/mol/min. The normalized absorbance procedure results in k. = 1.94/min and k of 0.33 A/mol/min. This demonstrates the ability of the normalization procedure to accurately determine the rate constants from the shape of the curve. The maximum isocyanate absorbance calculated by the normalization procedure,. 712 M, agrees very well with the experimentally determined value of. 692 M. 

Thermally induced intra-intermolecular criss-cross cycloaddition of nonsymmetrical azines 363 in the presence of phenyl isocyanate provides the corresponding products of the mixed criss-cross cycloaddition 364 (Scheme 55) <2002TL6431>. Two different reaction mechanisms, intra-intermolecular and inter-intramolecular, of the mixed criss-cross cycloaddition with opposite sequence of reaction steps are possible. Quantum chemistry calculations suggest the intra-intermolecular mechanism as the most probable mechanism of this reaction <2004CCC231>. 

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

The data on isocyanate crosslinked polybutadiene (B2) is given in Table II, and for this the molecular weight (Mc) was calculated using eqn. (2) (Mc=l0,1OOF3). The expected value of Mc as obtained from GPC on uncrosslinked polymer should be somewhere in the range of Mn (5100) to Mw (7400). Comparison of GPC and swelling data yields that F3 is in the range of 0.51-0.73, which in turn determines the extent up to which the chains deform non-affinely. 

In remembrance of the isolation of carbene 78 (Section m.C) it seems acceptable that 141, formed from 140 by a [l,2]-migration of the substituent, functions as a second intermediate. Carbene 141 can subsequently fragment into a nitrene 143 and carbon monoxide or open to an isocyanate 144, which alternatively may also be generated from an acid azide via acylnitrene 142. In comparison to calculated reaction pathways 75 78 and 82 81 (Schemes... 

Itoh and co-workers reported the ruthenium(n)-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes with isocyanates to afford the corresponding bicyclic pyridones 163 (Scheme 72).356 357 For previously reported ruthenium-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes see Refs 358 and 358a, and for theoretical calculations of the cyclocotrimer-ization of alkynes with isocyanates, isothiocyanates, and carbon disulfide see Refs 359 and 359a. 

Calculations were performed with few of these systems, either for their formation or properties. Thus, PM3 calculations of the cycloaddition of pyridinc-A -oxide with an isocyanate were performed, with R = H for simplification. Two consecutive transition states were postulated, pointing out a nonsynchronous process for this reaction, via a... 

The rearrangements 67 —> 70, 71 —> 72 and 74 —> 75 include the transformation of conjugated dienes to cumulenes. Nevertheless, these reactions take place with very high yields in some cases, because either an irreversible step of hydrolysis such as 69 —> 70 is involved or the very exothermic transformation from cyanates to isocyanates is used. Comparison of the energies, calculated by ab initio methods [121], shows that, for example, the energy of methyl isocyanate is lower than that of methyl cyanate by 26.8 kcal mol-1 and that of vinyl isocyanate is lower than that of vinyl cyanate by 28.1 kcal mol-1. 

The first literature report of a reaction of an isocyanate with wood is that due to Clermont and Bender (1957). In this study, DMF impregnated wood samples 1/8 in thick were suspended above phenylisocyanate liquid in a vessel heated at temperatures from 100 °C to 125 °C for various time intervals. Treated samples were washed with DMF, then water, then acetone, and dried in an oven at 105 °C. ASEs in the range of 60-80 % were reported for these samples. In view of the reactivity of DMF with isocyanates, the lack of an efficient clean-up procedure and the fact that ASE values were calculated from the first water-soak cycle only, this study is of limited value. 

Ab initio calculations on aza-Diels-Alder reactions of electron-deficient imines with buta-l,3-diene show that these reactions are HOMO (diene)-LUMO(dienophile)-controlled and that electron-deficient imines should be more reactive than alkyl-or aryl-imines. The Diels-Alder reaction of r-butyl 2//-azirine-3-carboxylate (80) proceeds with high diastereoselectivity with electron-rich dienes (81) (Scheme 28). The hetero-Diels-Alder additions of imines with sterically demanding dienes yield perhydroquinolines bearing an angular methyl group. The asymmetric hetero-Diels-Alder reaction between alkenyloxazolines and isocyanates produces diastereometri-cally pure oxazolo[3,2-c]pyrimidines. °... 

Analysis of the Isocyanate was accomplished by performing an amine equivalent determination (per ASTM D1638). The polyol component was analyzed for hydroxol equivalent by an acetylation procedure developed at Sandla National Laboratories. An 0.8 gram sample Is acetylated with a 1/9 acetic anhydrlde-pyrldlne mixture for 2 hours at reflux temperature and then the free acetic acid Is back titrated with base and compared to a blank. From this Information a hydroxol equivalent can be calculated. The polyol acid number was determined by ASTH D2849. 

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