Aniline polyurethane manufacture

The major uses of aniline are in the manufacture of polymers, mbber, agricultural chemicals, dyes and pigments, pharmaceuticals, and photographic chemicals. Approximately 67% of the wodd production of aniline is used in the manufacture of rigid polyurethanes and reaction-injection-molded (RIM) parts for the constmction, automotive, and durable goods industries. 

Aniline, which is used not only to synthesise drugs, pesticides and explosives but also as a building block for materials such as polyurethane foams, rubber, azo dyes, photographic chemicals and varnishes, is manufactured at a quantity of approximately three million tons each year [61]. The toxic effects of aniline include increased nitration of proteins in the spleen [62]. 

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. 

Ethylbenzene is made from ethylene and benzene and then dehydrogenated to styrene, which is polymerized for various plastics applications. Cumene is manufactured from propylene and benzene and then made into phenol and acetone. Cyclohexane, a starting material for some nylon, is made by hydrogenation of benzene. Nitration of benzene followed by reduction gives aniline, important in the manufacture of polyurethanes. 

This process was elaborated as a heterogeneously catalyzed variation by Asahi Chemicals (Japan) in order to open a new route to diisocyanates, not depending on the use of phosgene [120, 134]. Ethyl phenylcarbamate, which in a first step is obtained by catalytic oxidative carbonylation of aniline, CO, oxygen, and ethanol (eq. (17)), is condensed with aqueous formaldehyde to yield methylene diphenyl diurethane. Thermal decomposition leads to methylene diphenyl diisocyanate (MDI), which is one of the most important intermediates for the industrial manufacture of polyurethanes (eq. (18)). The yields and selectivities of the last reaction step seem to be the main reasons why this process is still inferior to the existing ones. 

Drawing on the Allied technical reports, ICI in England began the manufacture of diisocyanates. Production of polyurethanes in the US started in 1954, and by the following year they were available on a commercial scale as adhesives, coatings, foams and elastomers. The main American manufacturers of the intermediate isocyanates were DuPont, Mobay (a joint venture of Mobil and Bayer) and National Aniline. 

Spandex stretch fiber, based on polyurethanes, was developed by DuPont and appeared in 1962. From this time, polyurethanes would account for the greater part of demand for anilines. Aniline production alone had more than doubled, to over 100 million lbs. per year, between 1939 and 1957, in part to satisfy demand in products other than dyes. Half the US output was consumed in the production of rubber additives, mainly diphenylamine and cyclohexylamine, the latter used as a chain stopper in manufacture of polyurethanes (also as a boiler water additive and, in the US until banned in 1970, in the manufacture of cyclamate sweeteners). Other polymers, such as epoxy resins, relied on the bulk availability of various aromatic amines (Chapter 14). 

Nowadays, aniline remains one of the basic starting materials, not only in the dye industry but also in the chemical industry (for polyaniline, polyurethane) and pharmaceutical industry (for the manufacture of drugs such as antipyrine, antifebrin etc.)33. World demand for aniline is expected to grow at an impressive rate of several percents per year... 

Aniline has been one of the most important compounds in the first years of the chemical industry and its importance is still high. Nowadays, the major use of aniline is in the manufacture of polyurethanes. Aniline itself is produced mainly from nitrobenzene by reducing it with iron and dilute hydrochloric acid. Aniline is released from the formed anilinium hydrochloride by sodium carbonate treatment. 

Process improvements in the manufacture of aniline have been driven by tremendous demand, particularly from the rubber industry, and for use in the manufacture of isocyanates for polyurethanes, dyestuffs, sulfa drugs, agrochemicals, and detonators and stabilizers for explosives. Two widely used catalytic processes have been developed, one vapor phase, the other liquid phase. Both processes are highly exothermic, and the exchange and use of heat is important4. 

The production of aniline is a major international business, carried on in the US, Europe and Asia, mainly for the conversion, by reaction with formaldehyde under acid-catalyzed conditions, into diaminodiphenylmethanes 9a, 9b and 9c, and then into isocyanates, mainly 4,4/-methylenebis(phenylisocyanate) (MDI, also known as 4,4 -methylene-di-paraphenylene isocyanate, 4,4 -diphenylmethane diisocyanate, methylene diphenylene diisocyanate and diisocyanato diphenyl methane) (9d), from which polyurethanes are produced. This accounts for well over 60% of total demand (Figure 1). Aniline is also used in bulk for the production of antioxidants and vulcanization accelerators for rubber. Some 15.5 million lbs. of cyclohexylamine are made each year mainly by catalytic hydrogenation of aniline. Half the demand is for use as a boiler water additive. Other major uses include in the manufacture of herbicides, plasticizers, emulsifying agents, dyes, dry-cleaning soaps, acid gas absorbents and, in Asia, cyclamate sweeteners. Apart from India, the use of aniline for dyestuff manufacture represents about 10% of demand. 

In Japan, Tosoh Corporation commenced construction in 2003 of a 160,000 metric tons per year plant at Nanyo, to supply Nippon Polyurethane Industry Co. Ltd, manufacturer of 170,000 metric tons per year of MDI (9d)23. Nippon also manufactures 25,000 metric tons per year of a blend of 2,4- and 2,6-toluene diisocyanates, TDI (10a and 10b), by a process that involves toluidines as intermediates24. Tosoh has since embarked on construction of a new 159 million aniline plant. Japan produced 196,000 metric tons of aniline in 2005, based on 1.1% annual growth. 

Distribution of aniline and aromatic amines depends on the level of waste treatment, which in turn determines the amount released to the environment. Aniline, unlike secondary and tertiary amines, is moderately soluble, and is released to the environment primarily in wastewater from its manufacture, and at sites where polyurethanes, rubber,... 

An example of a sequential-reaction extractive reaction is the manufacture of 2,4-dinitrotoluene, an important precursor to 2,4-diaminotoluene and toluene diisocyanate (TDl) polyurethanes. The reaction involves nitration of toluene by using concentrated nitric and sulfuric acids which form a separate phase. Toluene transfers into the acid phase where it reacts with nitronium ion, and the reaction product transfers back into the organic phase. Careful control of liquid-liquid contacting conditions is required to obtain high yield of the desired product and minimize formation of unwanted reaction products. A similar reaction involves nitration of benzene to mononitrobenzene, a precursor to aniline used in the manufacture of many products including methylenediphenylisocyanate (MDI) for polyurethanes [Quadros, Reis, and Baptista, Ind. Eng. Chem. Res., 44(26), pp. 9414-9421 (2005)]. 

Phosgene is sometimes a byproduct of manufacturing aniline dyes, polycarbonate resins, coal tar, pesticides, isocyanates, polyurethane, and pharmaceuticals. Phosgene also occurs in uranium enrichment processes and bleaching sand for glass production. 

The primary use of nitrobenzene is in the captive production of aniline, with about 97.5% of nitrobenzene production consumed in this process. The major use of aniline is in the manufacture of polyurethanes. Nitrobenzene is also used as a solvent in petroleum refining, in the manufacture of cellulose ethers and acetate, and in Friedel-Crafts reactions to hold the catalyst in solution. It is also used in the synthesis of other organic compounds including acetaminophen, which is an over-the-counter analgesic commonly known as Tylenol . 

The industrial manufacture of MDI and polymeric MDI, particularly by ARCO (Atlantic Richfield Co.), BASF, Bayer, ICI (Imperial Chemical Industries), Mitsui, Mobay, Montedison, Nippon Polyurethane, Olin, Takeda and Upjohn, starts with the condensation of formaldehyde and aniline, with the formation of diaminodiphenyl-methane or methylene diphenyldiamine ... 

Aniline is the second largest petrochemical use of gaseous hydrogen. About half as much hydrogen is used for aniline as for cyclohexane production. Approximately 80% of the aniline produced worldwide is used in the manufacture of MDI for polyurethanes. The balance is used for rubber processing chemicals, agricultural chemicals, dyes, pigments, pharmaceuticals, and specialty polymers. DuPont s aromatic polyamide fiber Kevlar is also derived from aniline [1). 

Isocyanates are formed by reacting phosgene with an appropriate hydrocarbon substrate. Many isocyanates are possible depending upon the hydrocarbon starting material. The commercially important polyurethanes are manufactured from toluene diisocyanate, based on toluene, and methylene diphenyl isocyanate, based on aniline. Both toluene diisocyanate (TDI) and methylene diphenylene isocyanate (MDI) can be used to manufacture foamed products, but only MDI is used as the primary feedstock for elastomeric polyurethanes. 

The basic building blocks for polyurethanes are polyisocyanates and macroglycols, also called polyols. The commonly used polyisocyanates are tolylene-diisocyanate (TDI), diphenyimethane diisocyanate or methylenediphenyl isocyanate (MDl), and polymeric methylenediphenyl isocyanate (PMDI) mixtures manufactured by phosgenating aromatic polyamines derived fi-om the acid-catalyzed condensation of aniline and formaldehyde. MDl and PMDI are produced by the same reaction, and separation of MDl is achieved by distillation. The synthetic routes in the manufacture of commercial polyisocyanates are summarized in Figme 4.28. 

The first step in refining benzole is steam distillation that is employed to remove compounds boiling below benzene. To obtain pure products, the benzole can be distilled to yield a fraction containing benzene, toluene, and xylene(s) (BTX). Benzene is used in the manufacture of numerous products including nylon, gammexane, polystyrene, phenol, nitrobenzene, and aniline. On the other hand, toluene is a starting material in the preparation of saccharin, trinitrotoluene, and polyurethane foams. The xylenes present in the light oil are not always separated into the individual pure isomers since xylene mixtures can be marketed as specialty solvents. 

Aniline is a starting product for the manufacture of drugs such as acetamino-phen/paracetamol (Tylenol), which is )V-acetyl-4-hydroxyaniline (100). Another commercial product is 4,4 -MDI (methylene diphenyl diisocyanate, 101), which reacts with a polyol (102) to give polyurethanes such as 103. Polyurethanes are used in applications ranging from the foam in upholstery to thermal insulation material in refrigerators. 

Aniline is used as a chemical intermediate to prepare isocyanates for making polyurethanes, antioxidants, and vulcanization accelerators, as well as in the manufacture of agricultural fungicides, herbicides and insecticides and in the preparation of certain dyes. Similarly, pyrrole is used as a chemical intermediate in the preparation of electrically conducting polypyrrole by means of an electrochemical polymerization process. Pyrrole has few other industrial uses. 

The chief outlets are for polyurethane (di-isocyanates) 40%, rubber chemicals, herbicides minor users include dye makers (approx. 5%) and pharmaceutical manufacturers. Benzene is the feedstock and the traditional route is to nitrate this and then to reduce the nitrobenzene to aniline. Catalytic hydrogenation has displaced iron/ferrous chloride reduction in this and analogous reductions e.g. in the manufacture of toluidines. Amination of phenol manufactured from cumene (Vol. I, p. 366) has been patented (Figure 2.8). The yield claimed is 99% though the economic viability is uncertain. 

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