Water reaction with isocyanates

Blowing Agent - COj produced by isocyanate reaction with water... 

From this emphasis on hydroxyl groups, it might be thought that the hardening process consists solely of urethane formation between resin and polyisocyanate. This is obviously not so, since water vapour is usually present and the isocyanate reactions with water must proceed in competition with the reaction with hydroxyl. This will lead to consumption of isocyanate groups and linking of isocyanate molecules by urea linkages. With a difunctional isocyanate this tends to space the resin chains farther apart,... 

The steric effects in isocyanates are best demonstrated by the formation of flexible foams from TDI. In the 2,4-isomer (4), the initial reaction occurs at the nonhindered isocyanate group in the 4-position. The unsymmetrically substituted ureas formed in the subsequent reaction with water are more soluble in the developing polymer matrix. Low density flexible foams are not readily produced from MDI or PMDI enrichment of PMDI with the 2,4 -isomer of MDI (5) affords a steric environment similar to the one in TDI, which allows the production of low density flexible foams that have good physical properties. The use of high performance polyols based on a copolymer polyol allows production of high resiHency (HR) slabstock foam from either TDI or MDI (2). 

An isocyanate 2 formed by a Curtius rearrangement can undergo various subsequent reactions, depending on the reaction conditions. In aqueous solution the isocyanate reacts with water to give a carbaminic acid 6, which immediately decarboxylates to yield an amine 3. When alcohol is used as solvent, the isocyanate reacts to a carbamate 7 ... 

The ketocarbene 4 that is generated by loss of Na from the a-diazo ketone, and that has an electron-sextet, rearranges to the more stable ketene 2 by a nucleophilic 1,2-shift of substituent R. The ketene thus formed corresponds to the isocyanate product of the related Curtius reaction. The ketene can further react with nucleophilic agents, that add to the C=0-double bond. For example by reaction with water a carboxylic acid 3 is formed, while from reaction with an alcohol R -OH an ester 5 is obtained directly. The reaction with ammonia or an amine R -NHa leads to formation of a carboxylic amide 6 or 7 ... 

Barbitone (barbital, 282) was irradiated in aqueous solution at pH 10 with light of 254 nm wavelength to give the ureide (289) in 68% yield. In ethanol, the derivative (290) was formed (62%). With A-methylbarbitone (283) in buffer at pH 10, a mixture of the ureide (291) and the imidazole (292) was obtained. It was suggested that the ring-opened compounds were formed via isocyanates (288). Reaction with water would then give an acid which would spontaneously decarboxylate, but in ethanol a stable urethane (290) would form [173]. 

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

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... 

The reactions with l-Iithio-2-butyI- and l-Iithio-2-phenyI-l,2-dihydro-pyridines have. been described with several other reagents including isocyanates, esters, and diethyl chlorophosphate.147 A less familiar adduct from 10-methyl-10//-pyrido[3,2-ft][l,4]benzothiazine and butyllithium has also been tested for reaction with water, deuterium oxide, and diethyl chlorophosphate.148... 

Chlorosulfonyl isocyanate is a colorless, fluid liquid which fumes slightly in moist air. The vapors have a tussive effect. The compound shows an extraordinarily violent, almost explosive-like reaction with water. The contact of a small amount of the compound with the skin has no deleterious effect if it is rapidly removed by rinsing with plenty of water. Contacts which last longer than a few seconds may result in severe burns. 

Several reaction intermediates can be observed by varying the reaction conditions. If equimolar amounts ofbromine and hydroxide are added to acetamide (26, R = Me, Scheme 2.11), the product is N -bromoacetamide (27, R = Me). Further reaction with base gives unstable salts (28), which rearrange to isocyanates (29) reaction with water and an excess of hydroxide finally leads to the amine product (30), but in alcoholic solutions urethanes (31) are formed. 

For those products laminated using reactive polyurethane based adhesives, suppliers have been aware for many years of the potential migration of amines formed by the reaction of unreacted isocyanate monomer with water and recommend that laminated products are given time to fully cure before they are used in contact with food. Adhesives have been developed that contain low levels of monomeric aromatic isocyanates, in order to reduce any potential migration of aromatic amines into food in critical applications. Such adhesives are not considered to present any notable amine migration hazard unless used in high-temperature applications. 

Foams may be prepared by either one of two fundamental methods. In one method, a gas such as air or nitrogen is dispersed in a continuous liquid phase (e.g. an aqueous latex) to yield a colloidal system with the gas as the dispersed phase. In the second method, the gas is generated within the liquid phase and appears as separate bubbles dispersed in the liquid phase. The gas can be the result of a specific gasgenerating reaction such as the formation of carbon dioxide when isocyanate reacts with water in the formation of water-blown flexible or rigid urethane foams. Gas can also be generated by volatilization of a low-boiling solvent (e.g. trichlorofluoromethane, F-11, or methylene chloride) in the dispersed phase when an exothermic reaction takes places, (e.g. the formation of F-11 or methylene chloride-blown foams). 

TDI s production and uses may result in its release to the environment through various waste streams. If released to air, TDI will exist solely as a vapor in the ambient atmosphere and will be degraded in the atmosphere by reaction with photochemically produced hydroxyl radicals the half-life for this reaction in air is estimated to be 2.7 days. Atmospheric degradation may also occur through contact with clouds, fog, or rain. If released to water or moist soil, toluenedi-isocyanate is not expected to leach or adsorb to solids due to its rapid degradation reaction with water. It is not expected to bioconcentrate in aquatic organisms. 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

The above described adducts of the Isocyanates are cleaved on reaction with water to give ultimately an amine, CO2, and the active hydrogen containing progenitor. For example, the overall hydrolysis reactions of a urethane can be shown as ... 

TABLE II. SYNERGISTIC EFFECT OF DABCO CATALYST AND DIBUTYLTIN DILAURATE FOR THE CATALYSIS OF PHENYL ISOCYANATE (0.07M) REACTION WITH WATER (0.035M)... 

Emulsification occurs when the partially extended urethane is added with high-speed mixing to 3% aqueous acetic acid. Curing of the latex takes place either by reaction of water (150. 151) with the terminal isocyanate groups or by reaction with water-soluble diamines ... 

An amino group can also be acylated <84JHC697, 86JHC935) it reacts with sulfonyl chlorides and aryl isocyanates <86JHC935>, and also with nitrous acid to give, via the diazonium salt (281), either the triazinone by reaction with water or the chloro-triazine by reaction with chloride ions (Scheme 50). Nitrosation of 3-amino-1,2,4-triazine 2-oxides (282) and subsequent thermolysis of the diazonium tetrafluoroborate salts (283) afforded 3-fluoro-1,2,4-triazine 2-oxides (284) (Scheme 51). In one instance the diazonium tetrafluoroborate was isolated <85H(23)1969>. 

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