Isocyanates recovery

There are numerous applications in solvent recovery processes where evaporation equipment are employed. Figure 14 provides an example of a process scheme for toluene-di-isocyanate recovery. This is an example of continuous vacuum evaporation of distillation residues. 

The process can also be run to produce urethanes (2.15).30 The urethane could be pyrolyzed to the isocyanate. Recovery of the base would require treatment with, perhaps, sodium hydroxide, which would give sodium chloride as waste. Thus, the cost of the NaOH and Cl is wasted. This group has also used guanidines (2.18) to prepare mixed dialkyl carbonates. 

Semiflexible molded polyurethane foams are used in other automotive appHcations, such as instmment panels, dashboards, arm rests, head rests, door liners, and vibrational control devices. An important property of semiflexible foam is low resiHency and low elasticity, which results in a slow rate of recovery after deflection. The isocyanate used in the manufacture of semiflexible foams is PMDI, sometimes used in combination with TDI or TDI prepolymers. Both polyester as well as polyether polyols are used in the production of these water-blown foams. Sometimes integral skin molded foams are produced. 

According to the submitters l-cyano-S-a-naphthylurea is obtained similarly from a-naphthyl isocyanate in 85-90% yields. Crystallization from acetone-petroleum ether (12 and 6 ml., respectively, per gram of crude product recovery approximately 60% per crystallization) yields lustrous prisms, m.p. 148-149 with decomposition. 

Respiratory sensitizers, e.g. isocyanates or formaldehyde, result, in mild cases, in a sense of tightness of the chest and occasionally a troublesome cough. Severe cases involve bronchial asthma. Refer to Table 4.6. With such sensitizers, complete cessation of contact is often followed by rapid recovery but no further exposure is generally permitted. 

The mouse sensory irritation potentials of HDI, toluene-2,4-diisocyanate (TDI), isocyanatoethyl methacrylate (lEM), and isocyanatoethyl propionate (lEP) were determined in another study. Male Swiss albino CD-I mice were exposed for a 2-minute control period with room air, 3 minutes of exposure to one of 4 isocyanate vapors, then 2 minutes of recovery with room air. The range of concentrations tested were 0-0.82 ppm for HDI, 0-3.44 ppm for TDI, 0-2.51 ppm for lEM, and 0-1.95 ppm for lEP. The concentration that produced RDjo was determined. HDI was determined to be approximately 3 times... 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

Boorman, G.A., Uraih, L.C., Gupta, B.N., Bucher, J.R. (1987). Two-hour methyl isocyanate inhalation and 90-day recovery study in B6C3F1 mice. Environ. Health Perspect. 72 63-9. 

New processes include synthesis of /V-alkylated anilines from olefins and aniline in an inert solvent with at least one catalyst from a range that includes alkali metal alcoholates, alkaline earth metal alcoholates, alkali metal amides and alkaline earth amides36. The uses for /V,/V-dimethylaniline (11) include in the manufacture of polyester resins, sulfur recovery (in copper refining), insecticides and fungicides, dyes, pharmaceuticals, explosives, rubber products, specialty isocyanates and petroleum additives. The /V-ethylaniline (26) is a dye intermediate and rubber additive, and is used for bum control in explosives, while /V,/V-diclhylaniline is used in production of polyester resins, pharmaceuticals, diazo prints (lithographic), and dyes, and as a petroleum additive37. 

The fact that these concentrations have been measured does not mean that occupational exposures occur persons may use respiratory protection equipped to prevent exposure. All persons who have excessive occupational exposure to isocyanates may experience primary irritant effects in the respiratory tract depending on the extent of excessive exposure. Brief accidental exposures to concentrations of isocyanates tenfold or more above the TLV may cause short term respiratory irritation with recovery 24-48 hours following cessation of exposure. Continuing repeated workday exposures several-fold higher than the TLV can cause chronic respiratory irritation. All individuals will not suffer the same degree of respiratory irritation from excessive exposures due to individual biochemical and physiological differences. Studies have shown that on the order of five percent of persons who have had an occupational exposure to TDI develop a bronchial asthmatic type of response to subsequent exposures that are below concentrations causing any detectable primary irritation. 

Carbon dioxide analysers, based on heat capacity, electrical conductivity or partial pressure measurements, but especially on IR light absorption, are therefore used at various points in industrial processes. Carbon monoxide and dioxide are the species most commonly analysed for by the last of the above-mentioned detection techniques, which is also employed in a variety of processes [6], namely (a) control of the hydrogenation of plant oils in order to avoid production of unwanted trans isomers (b) measurement of sugars and CO2 In soft drinks (c) measurement of moisture (d) determination of CO2 In Industrial environments (e) determination of isocyanates In the production of polyurethane (f) determination of methane In argon from nuclear plants (g) control of the efficiency of solvent clean-up and recovery systems. 

Bucher, J. R., Gupta, B, N., Adkins, B Thompson, M., Jameson, C. W Thigpen, J, E., and Schweiz, B. A. (1987). Toxicity of inhaled methyl isocyanate in F344/N rats and B6C3F1 mice. 1. Acute exposure and recovery studies. Environ. Health Perspect. 72, 53-61,... 

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