In 1978, demand for phosgene was an estimated 1,630 million pounds. The growth in U.S. demand for phosgene from 1970 to 1979 was 9.2%/yr; future growth in demand is forecast at 7.0%/yr through 1984 (Anonymous, 1980). The capacity for the production of phosgene in the United States (12 producers at 15 sites) in 1983 was estimated to be < 2,101 million pounds (SRI International, 1983).
In 1973, the pattern of phosgene use was as follows: production of toluene diisocyanate (TDI), 61.7%; production of polymethylene polyphenylisocyanate (PMPPI), 23.6%; production of polycarbonate resins, 3.9%; and other uses (including production of acyl chlorides, chloroformate esters, diethylcarbonate, dimethylcarbamylchloride, isocyanates other than TDI and PMPPI, dyes, biocides and pharmaceuticals and use as a chlorinating agent), 10.7% (SRI International, not dated).
In 1977, the polycarbonate industry consumed approximately 6% of the phosgene produced and herbicide manufacture and processing used 9% (Hardy, 1982).
World production of isocyanates in 1978 has been estimated as 635,000 metric tons of TDI and 454,000 metric tons of diphenyl methane-4,4’-diisocyanate (MDI) products. The estimated 1981 capacities of U.S. manufacturers of isocyanates amounted to 318,000 and 444,000 metric tons for TDI and polymeric isocyanates, respectively (Chadwick and Cleveland, 1981).
TDI is a precursor of polyurethane resins, which are widely used to make foams, elastomers, and coatings. Polycarbonate resins based on phosgene find use in appliance and electric-tool housings, electronic parts, and break-resistant glazing. A rapidly growing use of phosgene is in the preparation of PMPPI for the production of rigid polyurethane foams (SRI International, not dated). The reaction of phosgene with primary alkyl and aryl amines, referred to as phosgenation, yields carbamoyl chlorides, which can be dehydrohalogenated readily to isocyanates:
RNH2+COCl2 → RNHCOCl → RN=C=O+HCl.
This procedure is used almost exclusively for the production of isocyanates (Chadwick and Cleveland, 1981). All commercial manufacturing processes for aromatic isocyanates currently in use appear to have the following steps:
A solution of an amine in an aromatic solvent—such as xylene, monochlorobenzene, or o-dichlorobenzene—is mixed with a solution of phosgene in the same solvent at a temperature below 60°C.
The resulting mixture slurry is digested in one to three stages for several hours at progressively increasing temperatures up to 200°C; digestion is accompanied by the injection of additional phosgene.
The final solution of reaction products is fractionated to recover hydrogen chloride, unreacted phosgene, and solvent for recycling, isocyanate product, and distillation residue.