Diisocyanates Methylene diphenyl diisocyanate

Similarly, nitroben2ene, carbon monoxide, and methanol can react sequentially in the presence of noble metal catalysts, to produce methyl A/-phenylcarbamate [2603-10-3] (4). The phenylcarbamate is subsequently coupled with formaldehyde [50-00-0] to yield the methylenebis(carbamate) (5) which is pyroly2ed to yield methylene diphenyl diisocyanate (MDI) (23). 

For methylene diphenyl diisocyanate (MDI), the initial reaction involves the condensation of aniline [62-53-3] (21) with formaldehyde [50-00-0] to yield a mixture of oligomeric amines (22, where n = 1, 2, 3...). For toluene diisocyanate, amine monomers are prepared by the nitration (qv) of toluene [108-88-3] and subsequent hydrogenation (see Amines byreduction). These materials are converted to the isocyanate, in the majority of the commercial aromatic isocyanate phosgenation processes, using a two-step approach. 

Maleamic acid, cyclization of, 293 Maleic anhydride, 59 Maleimido azine, 307 Manganese diacetate catalysts, 71 Mark-Houwink-Sakurada equation, 57 Material safety data sheets (MSDSs), 246 Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), 385, 388 McGrath, J. E., 327 MDI isomers, 210 MDIs. See Methylene diphenyl diisocyanates (MDIs)... 

Methylene diphenyl diisocyanates (MDIs), 201, 219. See also MDI isomers liquefied, 211, 226-227 Methyl placement, precise versus random, 447... 

Figure 2. Pressure profile of isothermal decomposition of 4,4 -methylene diphenyl diisocyanate.

Phenolic-isocyanates (phenolic-urethanes). The binder is supplied in three parts a phenolic resin in an organic solvent (0.8%), methylene diphenyl diisocyanate (MDI) (0.5%), and a liquid amine catalyst. When mixed with sand, the amine causes a reaction between the resin and the MDI, forming urethane bonds, which rapidly set the mixture. The speed of setting is controlled by the type of catalyst. The optimum cure temperature is 25 to 30°C. Compression strength is typically over 4000kPa (600psi). 

Di-hydroxyl end-capped PPG (18) is an intermediate in the formation of a common polyurethane prepolymer (20). End group functionality of this intermediate is important, as this hydroxyl functionality is modified to form the prepolymer. Any different end group structures could lead to the presence of prepolymer that will not form polyurethane of the desired structure. The desired reaction of the intermediate (18) to form the prepolymer (20) is described in Figure 21. Reaction of one unit of the intermediate (18) and two units of methylene diphenyl 4,4 -diisocyanate (MDI) results in the formation (nominally) of the prepolymer (20). 

Commonly used isocyanates are toluene diisocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [polyethylene phthalate) or polyethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene diisocyanate (which refers to the ratio of 2,4-toluene diisocyanate to 2,6-toluene diisocyanate). High-resilience foam contains about 80% 80/20 toluene diisocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Petsonk, E.L. et al., Asthma-like symptoms in wood product plant workers exposed to methylene diphenyl diisocyanate, Chest, 118, 1183, 2000. 

Reuzel PGJ, Arts JHE, Lomax LG, et al Chronic inhalation toxicity and carcinogenicity studies of respirable polymeric methylene diphenyl diisocyanate (polymeric MDI) aerosol in rats. Fundam Appl Toxicol 22 195-210, 1994... 

Uses. Production of methylene diphenyl diisocyanate (MDI), which is used to produce polyurethanes hardening agent for epoxy resins, anticorrosive materials, printed circuit parts, dyestuff intermediates, filament-wound pipe, and wire coatings... 

The uses of aniline obtained from nitrobenzene are given in Table 11.5. Aniline s use in the rubber industry is in the manufacture of various vulcanization accelerators and age resistors. By far the most important and growing use for aniline is in the manufacture of jP,jP-methylene diphenyl diisocyanate (MDI), which is polymerized with a diol to give a polyurethane. 

The average prices of commercial quantities in cents per pound are given in Table 13.1. Six chemicals are over 1.00/lb hexamethylenediamine (59), methylene diphenyl diisocyanate (68), toluene diisocyanate (78), phosphorus (86), 1,4-butanediol (89), and potassium hydroxide (90). In 1992 only three chemicals, and in 1988 only two chemicals, were over 1.00/lb. The cheapest of the second 50 chemicals is tall oil, with sodium sulfate close. The average price for all the second 50 chemicals is 0.55/lb. 

Uses of phosgene include the manufacture of toluene diisocyanate (45%), methylene diphenyl diisocyanate and polymer diisocyanates (38%), and polycarbonate resins (12%). 

Major uses of aniline include -methylene diphenyl diisocyanate (MDI) (75%) and rubber chemicals (15%) production. It is also used to a smaller extent in herbicides (4%), dyes and pigments (3%), and specialty fibers (2%). 

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

A synthetic approach to poly(ester-urethanes) was recently published by Xue et al. [48]. Glycerol was employed as a trifunctional initiator in the enzymatic ROP of CL. The three-arm PCL triol was then reacted with methylene-diphenyl diisocyanate (MDI) and hexanediol to yield a three-arm PCL-based poly(ester-urethane) with shape-memory properties. 

Important commercial isocyanates include the diisocyanate monomers toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and MDI-, TDI-, and HDI-based isocyanates (e.g., prepolymers and polyisocyanates). World-wide production volume is estimated at over 12 billion lb. Isocyanates (diisocyanates, polyisocyanates, and prepolymers) all cause similar health effects, most commonly asthma [32]. Isocyanates are reported to be the leading attributable cause of work-related asthma [16]. Isocyanates are potent sensitizers that can trigger a severe and potentially fatal asthma attack in sensitized persons at very low isocyanate exposure levels [16]. Toluene diisocyanate is reasonably anticipated to be a human carcinogen by National Toxicology Program. 

Reuzel, P.G.J., Arts, J.H.E., Lomax, L.G, Kuijpers, M.H.K., Kuper. C.F., Gembardt, C., Feron, V.J. Loser, E. (1994) Chronic inhalation toxicity and carcinogenicity study of respirable polymeric methylene diphenyl diisocyanate (polymeric MDI) aerosol in rats. Fundam. appl. Toxicol., 22, 195-210... 

Sepai, 0., Schtitze, D., Heinrich, U., Hoymann, H.G, Henschler, D. Sabbioni, G. (1995b) Hemoglobin adducts and urine metabolites of 4,4 -methylenedianiline after 4,4 -methylene-diphenyl diisocyanate exposure of rats. Chem.-biol. Interact, 97, 185-198... 

Processes are under development to manufacture methylene diphenyl diisocyanate (MDI) without using toxic and corrosive phosgene. The proposed process schemes usually consist of three steps alkoxycarbonylation of nitrobenzene or aniline with CO and an alcohol to alkyl plienylcarbamate, condensation of the carbamate, and then thermal decomposition of the resulting urethane to MDI. For example, the condensation of methyl N—phcnylcarbamate (MPC), and IICHO into methylene diphenyl diurethane (MDU) is carried out in the presence of an acid catalyst. 

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