Isocyanates unreacted

Titrations with dibutylamine [111-92-2] can also be used to determine the NCO content of isocyanates and prepolymers. Generally, an excess of amine in a suitable solvent such as chlorobenzene [108-90-7] is added to the sample. The resulting solution is allowed to react and the unreacted amine is back- titrated with dilute hydrochloric acid. For low NCO content levels, a colorimetric method is often used. The isocyanate-containing species is titrated with amine and the unreacted amine is deterrnined using malachite green [569-64-2]. 

A specific example of the process represented by Figure 4b occurs when a multihmctional isocyanate is dissolved in a Hquid, water-immiscible core material and the mixture produced is dispersed in an aqueous phase that contains a dispersing agent. The aqueous phase reacts with some of the isocyanate groups to produce primary amine functionaHties. These amino groups react with unreacted isocyanate groups to produce a polyurea capsule shell (13). 

Sodium methanenitronate reacts with phenyl isocyanate in benzene to give the readily separable sodium salts of nitroacetaniUde and nitromalondiarkhde. Except as the salt, nitromethane is unreactive with phenyl isocyanate at temperatures up to 100°C the higher homologues do not give condensation products that can be isolated. 

In these systems the prepolymer is prepared with an excess of isocyanate to give an isocyanate-terminated molecule. This is then reacted with unreacted glycol and other ingredients. The disadvantage of this system is that component streams are of similar volume and viscosity, this facilitating both metering and mixing. The system is used mainly for the manufacture of microcellular products. 

Polyurethanes (PURs) are usually described as being prepared by the reaction of diols with diisocyanates. However, this is an oversimplification because often water is deliberately added in the production of flexible polyurethane foams. Unreacted isocyanate groups react with water to form carbon dioxide and urea groups in the polymer chain. The carbon dioxide acts as a blowing agent in the production of PUR foams. Also, polyurethanes can be formed by the reaction of bischloroformates with diamines. 

ABA type poly(hydroxyethyl methacrylate) (HEMA) and PDMS copolymers were synthesized by the coupling reactions of preformed a,co-isocyanate terminated PDMS oligomers and amine-terminated HEMA macromonomers312). Polymerization reactions were conducted in DMF solution at 0 °C. Products were purified by precipitation in diethyl ether to remove unreacted PDMS oligomers. After dissolving in DMF/toluene mixture, copolymers were reprecipitated in methanol/water mixture to remove unreacted HEMA oligomers. Microphase separated structures were observed under transmission electron microscope, using osmium tetroxide stained thin copolymer films. 

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. 

It is of interest to note that the concentration of active chains is highly sensitive to the extent of reaction. Such results because, as a first approximation, three inactive chains result from each isocyanate group that remains unreacted in the gel. Two molecules of monohydroxy PPO are required, again as a first approximation, to produce three inactive chains. 

Crosslinking and branching can be promoted either by the use of a triol as a chain extender, or by using less chain extender than is theoretically required the unreacted isocyanate end groups then react with urethane groups in the main chain to form allophonate or biuret crosslinks. 

Reductive amination of an aldehyde with excess primary amine, using a support-bound borohy-dride, provides the desired secondary amine contaminated with the primary amine precursor. Covalent capture of the primary amine with a support-bound aldehyde provides the pure secondary amine. Treatment with excess isocyanate yields the final urea product, which is purified by reaction with a support-bound amine to remove unreacted isocyanate. For the full potential of this method to be realized, further development of support-bound reagents and scavengers for most of the important chemical transformations will be necessary. Al-... 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

If isocyanate remains unextracted, the extent of reaction would apparently decrease with the extraction time. However, the amount of naphthylamine is obviously constant in the range of extraction duration studied, supporting complete removal of the unreacted isocyanate from the film. 

Method A TBA-F in THF (1M, 0.84 ml, 0.84 mmol) is added to the aryl isocyanate (4.2 mmol) and the mixture is stirred at 70°C. After ca. 1 min, the mixture solidifies. After subjecting it to low pressure to evaporate unreacted isocyanate, the solid is taken up in CH2C12 (25 ml). The solution is washed well with brine, dried (MgS04), and evaporated to yield the aryl isocyanurate (e.g. 85% from PhNCO 65% from 4-MeOCOC6H NCO 91% from 4-Me,SiOCOCAH4NCO). 

The rate limiting step In this side reaction Is the reaction of Isocyanate and water. The Importance of this reaction depends on the polarity of resin, the level and type of catalyst, and the humidity In the bake oven. The effect of this reaction Is to reduce the number of urethane crosslinks but Increase the number of urea crosslinks. The total crosslink density will likely decrease since two Isocyanate groups are necessary to form one urea link while only one isocyanate group is required to form a urethane link. If excess isocyanate groups are present, this reaction will consume the excess Isocyanate and Increase the crosslink density. This reaction can occur not only in the bake oven but also on aging of the coating if unreacted isocyanate groups are present after cure. 

A small amount of unreacted isocyanate groups in the hardened foam can be detected by IR spectroscopy. 

Bhopal, the site of the largest chemical accident in the past 50 years, wasn t an explosion but rather an uncontrolled reaction. The plant in Bhopal, India, owned jointly by Union Carbide and the Indian government, was producing methyl isocyanate as an intermediate for a pesticide. Water was admitted into a storage tank containing this chemical, which caused it to hydrolyze and produce CO, HCN, and unreacted MIC, which spread downwind into a crowded slum nearby and caused many deaths and casualties. The desired reaction to synthesize MIC was... 

Potlife. Batch qualification and casting usually take several hours during which time the propellant must stay fluid. This time is called potlife and obviously depends on such factors as catalyst level and temperature of the propellant batch. Reduction of the catalyst concentration to obtain an acceptable potlife is not always advisable because it may prolong unduly the time for complete cure. Further, unreacted hydroxyl and/or isocyanate groups tend to undergo undesirable side reactions. 

Isocyanates react readily with water to form amines and polyureas (Chadwick and Cleveland 1981 Hulse 1984 Kennedy and Brown 1992) and hydrolysis of HDI is expected to occur much more rapidly than biodegradation (HSDB 1995). Consequently, reaction with water is expected to be the only significant fate proeess of HDI in moist soil or sediment. The HDA resulting from hydrolysis, however, should be subject to various types of biodegradation (HSDB 1995). Gilbert (1988) has summarized the results of laboratory experiments on TDI in undistiubed moist sand, which indicate that TDI is converted to polyureas at a rapidly decreasing rate, with 5.5 and 3.5% of unreacted TDI remaining after 24 hours and 8 days, respeetively. The toluene diamine hydrolysis produet was not found above the detection limit of 0.01 ppm. These results were... 

In contrast to isocyanates, isothiocyanates are relatively unreactive toward hydroxyl-containing compounds thus they are not even affected by aqueous media [31]. Their reaction with hydroxyalkyl thioureas has been reported [32]. With free 2-mercaptoethylamine, 1 mole of phenyl isothiocyanate gave the N-substituted product, l-(2-mercaptoethyl)-3-phenyl-2-thiourea, and 2 moles gave the N,S-disubstituted product. Isocyanates, regardless of the relative amounts, reacted with 2-mercaptoethylamine to give the N,S-disubsti-tuted products. Cleavage with silver nitrate affords the l-(2-mercaptoethyl)-3-alkylureas [33]. 

Isocyanates readily react with primary alcohols at 25°-50°C, whereas secondary alcohols react 0.3 as fast, and tertiary alcohols approximately 0.005 as fast as the primary [2]. Thus, the effect of steric hindrance shows a pronounced effect on these reactions. For example, triphenylcarbinol has been reported to be completely unreactive [3a, b]. Other tertiary alcohols such as f-butanol may react under uncatalyzed conditions with isocyanates to give olefin formation, as shown in Eq. (4), using phenyl isocyanate [4, 5]. 

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