Diisocyanate unreacted

Methylene dianiline is normally a very reactive diamine in the presence of diisocyanates. However, a sodium chloride complex that is relatively unreactive at room temperature is commercially available. When the complex is heated to 21°C, it activates to quickly cure the urethane [76]. 

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. 

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. 

Hexamethylene diisocyanate is a highly reactive synthetic chemical that is widely used in the production of polyurethane materials. There is no natural somee of HDI. All of the potential exposures to this compound are associated with the production, handling, use, and disposal of HDI and HDI-containing products or materials. Exposures to HDI are often associated with exposures to its prepolymers, especially to a trimeric biinetic prepolymer of HDI (HDI-BT) (see Figure 5-Ia), whieh is widely used as a hardener in automobile and airplane paints, and whieh typieally contains 0.5-1% unreacted HDI (Alexandersson et al. 1987 Hulse 1984 Karol and Hauth 1982). There is evidence that diisocyanate prepolymers may induce asthma at the same or greater frequency as the monomers (Seguin et al. 1987) therefore, there is a need to assess the potential for human exposme to prepolymeric HDI as well as monomeric HDI. Except for limited data on occupational exposures, no information was foimd in the available literature related to the potential for human exposure to prepolymers of HDI. 

On completion of the reaction, the material may be carefully degassed to remove any entrapped gasses and unreacted diisocyanate. Care must be taken to prevent foaming. Applying vacuum to the glassware does create the potential for it to implode. Suitable safety precautions must be taken. Table 3.2 indicates some typical time/temperature reaction conditions for some simple prepolymers. 

Free isocyanate specimen. A measure of the free, unreacted diisocyanate monomer present in an isocyanate prepolymer. Low free isocyanate prepolymers are desirable, as they contain very low levels of volatile isocyanate, making them inherently safer for the end user. 

The time required for complete reaction is dependent on the reaction temperature, on the design of the phosgene inlet tube, on the efficiency of agitation, and on the rate of phosgene addition. It is important that the reaction be continued until practically all the hexamethylenediammonium chloride has disappeared. If unreacted amine salt is present, it has a tendency to sublime with the diisocyanate during distillation. 

Isocyanate-Terminated Adducts and Polymers. Monomeric diisocyanates such as TDI, MDI, or Hi2MDI, because of their irritant characteristics, are seldom used in an unreacted form for the compounding of urethane coatings. They are normally converted into isocyanate-terminated polymers or adducts of polyols such as hydroxyl-terminated polyesters and polyethers, castor oil, etc. 

The diisocyanate and polyols can be reacted in various combinations by using diols, triols, and tetrols, or combinations of these in which the NC0 0H ratio is lower than 2. Typical idealized structures are shown in Figure 2. The adducts and polymers can be produced with a relatively low content of free unreacted disocyanate. Because of the volatility and irritant characteristics of diisocyanates such as TDI, it is Important to keep the unreacted TDI to a minimum, particularly for spray applications. A vacuum stripping step is often recommended to eliminate this hazard. 

Gagne, S., Lesage, J., Ostiguy, C., and Tra, H. V., Determination of unreacted 2,4-toluene diisocyanate (2,4TDI) and 2,6-toluene diisocyanate (2,6TDI) in foams at ultratrace level by using HPLC-CIS-MS-MS, Analyst, 128, 1447-1451, 2003. 

Enol ethers from (2-oxoalkyl)phosphonic diesters are themselves highly reactive in hydrolysis and addition reactions. (2-Butoxyethenyl)phosphonic dichloride in CCI4 solution readily adds bromine in the cold, but attempts to distil the resultant (1,2-dibro-mo-2-butoxyethyl)phosphonic dichloride result in dehydrobromination the product 334 is unreactive to further attempted bromination, but suffers ready hydrolysis to (1-hydroxy-2-oxoethyl)phosphonic acid, and similarly, hydrolysis of the precursor dichloride yields (2-oxoethyl)phosphonic acid. Reactions between enol ethers and amides, carbamates or phosphoramidates, under acidic conditions, yield the enamides 335 [R = CO-alkyl, CO-aryl, COO-alkyl or P(0)(0Pr%f (2-Alkoxyethenyl)phosphonic diisocyanates act as precursors to phosphapyrimidines 336 and analogous phosphapurines ... 

An excess of isocyanate groups is not permitted, so that the final product, a urethane oil, contains no unreacted -NCO. It is therefore stable to moisture, non-toxic and differs from bodied oil in that it contains urethane linkages and can be much higher in molecular weight. The amount of diisocyanate used and the molecular weight of... 

Volatile isocyanates, such as TDI and hexamethylene diisocyanate, are irritants to the eyes, nose and throat. Higher molecular weight polyisocyanates for paints therefore should contain very low levels of these isocyanates, so that the isocyanate content of the air around them remains below the threshold limit value (see p. 124). Even if they contain only low levels of volatile isocyanates, precautions must be taken to prevent inhalation of spray droplets containing unreacted isocyanates (either excellent extraction of overspray in the spray booth, or the wearing of fresh air hoods by those in the vicinity). The uncured liquid finishes can also be skin irritants. 

Because the diisocyanate is used in excess, there is usually free monomer present. Isocyanates are hazardous materials particularly upon inhalation and skin contact. Chronic exposure can lead to sensitization. The adhesives must therefore be used with proper ventilation and should not come in contact with the skin in the unreacted state. Vapor monitoring badges for employees and periodic real time vapor monitoring around process equipment is recommended. 

Taymaz [44] developed a direct method for TDI and diphenylmethane diisocyanate in polyurethanes by direct injection of the sample solution on to three-column system (500 +100 +100A) using methylene chloride (lmLmin ) with refractive index detection. A sample of crude MDI was shown to contain about 43% of unreacted MDI when examined by this method. 

Phenyl diisocyanate (VIII) was used as a grafting molecule following the same strategy discussed above in the case of the double anhydride in [14—17]. As expected, the FTIR spectrum of the modified wood surface, after extraction of the unreacted VIH, displayed a strong peak at 2 250cm, characteristic of the presence of isocyanate functions (see Fig. 20.5). These modified wood samples were treated with the biocides IV and V, which provided an excellent resistance to fungi [18]. 

The molar fraction (-FN) of unreacted diisocyanate was determined as a function of the reduced time t (Fig. 1.13) [44]. Due to the changes in the rate constants during reaction, both the initial and average rate-constant values were calculated. The rate constants of the initial and average stages of the process and the values of the rate-constant ratios were determined. These values corresponded to the reactions of the first and second NCO groups of the symmetrical DBDI isocyanates (2,2 and 4,4 ) and to those of the 4,4 -MDI reaction. Note that the reactivity values of 4,4 -DBDI... 

In all cases the prepolymer obtained from the reaction between macrodiol and diisocyanate, was reacted with EG in a quantity calculated so that there should remain theoretically no excess of unreacted NCO groups. This corresponds to an isocyanic index I = 100 where I = (lOOx [NCO])/([OH]m + [OHJce). The description of the poliadditon procedures are detailed elsewehere [127,207],... 

Thermoplastic linear polyurethanes which are usually chain-terminated so that no unreacted free NCO groups remain available. Environmental considerations direct growing attention to these newer non-polluting urethane adhesive forms, e.g. powders, films, aqueous dispersions and 100% solids reactive systems. Some systems do possess blocked diisocyanates which are activated on heating to produce chemically reactive solid systems. 

Skin irritation can also be caused by amine accelerators, e.g., MDA (van Joost et al. 1987), triethylene-diamine (Adams 1983) and triethylamine (Malten 1964). Concentrated liquids may even be corrosive. The irritant effect of diisocyanates in the hardeners, glues and paints may also be attributed to the solvents used in these products. Completely hardened PU products usually do not cause skin problems. However, some unreacted isocyanate monomer may remain in surplus inside the PU foam, even after curing. During machining and cutting, PU dust containing isocyanate is produced. When heated to above 250 °C, PU polymers decompose into isocyanates and nitrogen oxides, and may then cause dermatitis (Bjorkner 1992). 

Phosgenation is usually carried out continuously as follows. The diamine feed is injected into a solution of phosgene in an inert solvent (e.g., o-dichloro-benzene) at 25—100°C (the cold stage) and then the product is passed through a heater at 150-160°C (the hot stage). Hydrogen chloride and unreacted phosgene are taken off and the diisocyanate is separated from the solvent by distillation. 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

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