Environ. Sci. Technol. 1999, 33, 2579-2583
Fate of Methylenediphenyl Diisocyanate and Toluene Diisocyanate in the Aquatic Environment
They are among the major isocyanates used in the manufacture of polyurethane resins and products. TDI is usually manufactured and used as a mixture of the 2,4-isomer, 80%, and the 2,6-isomer, 20%. MDI is most often used as an oligomeric mixture, commonly known as “polymeric” MDI (pMDI), containing about 50% of the “monomer”, 4,4′-MDI.
YOSHIKUNI YAKABE Kurume Research Laboratories, Chemicals Inspection and Testing Institute, 19-14 Chuo-Machi, Kurume-Shi Fukuoka 830, Japan KAREN M. HENDERSON AND WILLIAM C. THOMPSON Environmental Testing Services, Bayer Corporation, New Martinsville, West Virginia 26155-0500 DENIS PEMBERTON AND BERNARD TURY Gilbert International Ltd., Bridgewater House, Whitworth Street, Manchester M1 6LT, United Kingdom ROBERT E. BAILEY* Bailey Associates, 4115 Elm Court, Midland, Michigan 48642
Toluene diisocyanate (TDI) and methylenediphenyl diisocyanate (MDI) are highly reactive materials used on a large scale in the production of polyurethanes. Their behavior in the aquatic environment has only been reported in general terms. This work provides a clearer understanding of their heterogeneous interaction with water to enable a better prediction of the environmental impact of a spill. The kinetics and product distribution have been studied for both stirred and unstirred reactions and have been correlated with the degree of agitation, surface area, and stoichiometry. TDI reacts rapidly, in less than 5 min, when well stirred, and MDI reacts more slowly because of its greater viscosity. However, under poorly mixed conditions, typical of an environmental spill, the reaction of both materials may take several weeks for completion because of the formation of insoluble solid polyurea crusts. This polyurea is the predominant product under all conditions except at very low loadings of diisocyanate. Under all conditions studied, only low concentrations of watersoluble products, including diamines, were formed. These results are indicative of a reaction occurring predominantly in the organic phase where the initial hydrolytic product, amine, can react further with isocyanate to produce ureas. These studies are consistent with the minimal effects noted from the accidental spillage of TDI and MDI in the environment.
Introduction Toluene diisocyanate (TDI) and methylenediphenyl diisocyanate (MDI) are each manufactured and used on a large scale, greater than one billion (1 × 109) kg/year worldwide. * Corresponding author telephone: (517)631-5064; fax: (517)8353410; e-mail:
[email protected]. 10.1021/es981350c CCC: $18.00 Published on Web 06/15/1999
1999 American Chemical Society
Isocyanates are chemically reactive species and are wellknown to react with water. However, the rates and the course of reaction of TDI and MDI with water under conditions relevant to the environment have only been reported in general terms (1, 2). This paper describes and discusses a series of experimental studies designed to enable better prediction of the aquatic fate, and hence the potential environmental impact, of these commercial chemicals. The chemistry of the reaction of an aryl isocyanate with water is as follows. The isocyanate adds water forming a carbamic acid, which rapidly splits off carbon dioxide leaving a primary amine:
ArsNdCdO + H2O f ArsNHsC(dO)OH f ArsNH2 + CO2 (1) This primary amine can also react with the isocyanate forming a urea:
ArsNH2 + ArsNdCdO f ArsNHsC(dO)sNHsAr (2) The literature on aryl isocyanate reactions with water has focused on simple monofunctional compounds such as phenyl isocyanate. The pseudo-first-order rate constant for the reaction of phenyl isocyanate in water at 25 °C was reported to be 3.39 × 10-2 s-1 by Castro et al. (3). The reaction was base catalyzed but not acid catalyzed. Thus in water solution at pH 7 or less, the half-life of phenyl isocyanate was calculated to be 20 s and less at higher pH values. Results of a study of this reaction in dioxane/water mixtures (4) concur with this. In dioxane containing 90% water, at 25 °C, the rate constant was found to be 3.9 × 10-2 s-1. Isocyanates are even more reactive with amine than with water. In Ekberg and Nilsson’s study (4), the reaction of phenyl isocyanate with aniline, leading to the formation of diphenylurea, was observed to be concurrent with hydrolysis. In dioxane containing 90% v/v water, the reaction with aniline was found to be approximately 4 × 104 faster than hydrolysis. A closely similar reactivity ratio can be calculated from the results of Hegarty et al. (5), who related the reactivity of nucleophiles with phenyl isocyanate in water to their basicity. Thus, the products of the reaction of an aromatic isocyanate with water are expected to be not only an aromatic amine but also a urea. With diisocyanates such as TDI and MDI, the ureas will be polymeric, and polyureas have been reported to be the main products arising from environmental contact of TDI and MDI with water (1, 2). It is to be expected therefore that TDI and MDI will react very rapidly in aqueous solution, in which they can have only transient existence, and will therefore be virtually unavailable. However both materials are hydrophobic and relatively dense, and pMDI is viscous. As a result when poured VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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into water they sink, and reaction occurs heterogeneously at or below the diisocyanate/water interface. The overall reaction is much slower than the homogeneous reaction described above. Carbon dioxide bubbles are produced, which can cause the reacting mass to float and move about, and solid crusts of polymer form. The studies described below were designed to explore this heterogeneous reaction in more detail for use in assessing the potential impact of accidental spillage of TDI or MDI.
Materials and Methods Test Compounds. Samples of commercial TDI consisting of a mixture of 80% 2,4-TDI and 20% 2,6-TDI were provided by Nippon Polyurethane Industry Co. Ltd. and Bayer Corp. Pure 2,4-TDI and 2,6-TDI were also obtained from Nippon Polyurethane Industry Co. Ltd. The reference compounds 2,4-toluenediamine (2,4-TDA) and 2,6-toluenediamine (2,6TDA) were purchased from Tokyo Chemical Industry Co. Ltd. and Tokyo Kasei Kogyo Ltd., respectively. Polymeric MDI, Millionate MR-200, viscosity 172 mPa s, was supplied by Nippon Polyurethane Industry Co. Ltd. and contained 54.5% of 2-ring MDI (monomer) and 32.4% 3-ring MDI with the remainder higher oligomers. The reference compound 4,4′methylenedianiline (4,4′-MDA) 99.8%, was supplied by Nippon Polyurethane Industry Co. Ltd. Reference urea compounds N,N′-bis(aminomethylphenyl)urea (mixed isomers) and the corresponding diurea were synthesized from 2,4-TDA at Sumitomo Bayer Urethane Co. Ltd. for this project. Reagents, including the waters used, were from commercial sources and were guaranteed reagent grade. TDI Reaction with Water. In a large-scale study, commercial TDI was mixed in 3.5 L of purified water at ambient temperature at a loading of ca. 28 mg/L using a 134 mm paddle stirrer (Ace Glass 8085-19) in a 5-L reaction flask with indents (Ace Glass 6481-10) at about 200 rpm. The effects of less efficient dispersion of the TDI was also studied using a slower stirrer speed and a simple round-bottom flask. Aliquots of the mixture were periodically withdrawn and analyzed for TDI by reverse-phase HPLC after derivatization with 1-(2pyridyl)piperazine in toluene. A series of smaller scale studies were conducted mainly with 2,4-TDI; 2,6-TDI and the commercial 80/20 mixture were used in less extensive comparative experiments. Various amounts of TDI were stirred into water (250 mL) with a 40-mm magnetic stir bar in an Erlenmeyer flask. A stirring rate of 1300 rpm was found to give a satisfactory dispersion of TDI in water at 27 °C. At intervals, aliquots of the reaction mixture were taken and analyzed for TDI (as described above or by GC after toluene extraction), dissolved organic carbon (DOC; after filtration, by Shimadzu total organic carbon analyzer), TDA (by HPLC after filtration), and solid reaction product (by filtration). In one experiment, at 1000 mg/L 2,4-TDI, the solid reaction product was extracted with a 10 mM solution of LiCl in dimethylformamide (DMF). The molecular weight of the extracted material was determined by gel permeation chromatography coupled with low angle laser light scattering. To study the reaction under static conditions, 5-g samples of 2,4-TDI were placed in open cylindrical vials of varying sizes, providing different surface areas (2.5, 5.7, 13.8, and 19.6 cm2) and depths (1.6, 0.72, 0.30, and 0.21 cm, respectively) of TDI. The vials were each carefully placed in a beaker containing 300 mL of water, which was allowed to stand in a thermostat at 25 °C. Periodically the vials were removed, and solids filtered from the 300 mL of water were combined with the organic material in the vial for determination of TDI remaining. The water solution was analyzed for DOC and TDA as described above. MDI Reaction with Water. A variety of configurations were tried for the reaction of pMDI with water. Since pMDI is a viscous liquid at ambient temperature, it tended to 2580
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FIGURE 1. Disappearance of 28 mg/L 80/20 TDI mixed isomers in water with vigorous agitation. accumulate on the magnetic stirring bar at 1500 rpm. The addition of glass beads provided more surface area for reaction. The most consistent results were obtained by coating the interior wall of a 100-mL Erlenmeyer flask with 20, 50, or 500 mg of pMDI and stirring in 50 mL of water with a 40-mm magnetic stirring bar or shaking on a rotary shaker at 200 rpm. However, even these methods did not satisfactorily disperse the pMDI into water. At appropriate intervals, 1-mL aliquots of the reaction mixture were taken for MDA and DOC determination. For determination of MDI, the entire reaction mixture was quenched by derivatization with an excess of dibutylamine in toluene. At the end of reaction, the residue insoluble in toluene and water was extracted with DMF containing 10 mM LiCl. A rough estimate of the amount of polymeric material in the extract was obtained by HPLC. The reaction rate under static conditions was studied by the same procedure as described above for TDI.
Results Reactions of TDI with Water. In the larger-scale experiment, with very good dispersion of the TDI into water at ambient temperature, approximately half the nominal concentration of 28 mg/L 80/20 TDI was lost in 30 s as shown in Figure 1. With less efficient stirring the half-life was in the region of 3-5 min. Preliminary smaller-scale experiments also showed the reaction to be heterogeneous with the reaction rate to be dependent on stirring energy, apparently related to the TDI-water interfacial area. Fast stirring with a magnetic bar was found to give the most consistent results in these smallscale studies and was used thereafter. The rates of TDI disappearance could be approximately fitted to first-order kinetics. The rates of reaction of TDI at a loading of 1000 mg/L in experiments at 27 °C are shown in Figure 2. The half-lives of 2,4- and 2,6-TDI can be seen to be about 0.7 and 1.7 h, respectively. Results of studies with various loadings of 2,4-TDI are summarized in Table 1. As can be seen, reaction rate was a function of the loading of TDI. The extent of reaction after 30 min stirring varied from 85% at 10 mg/L to 20% at 10 000 mg/L. At 12 °C, the dispersion of TDI into water was poor and led to difficulty in sampling and in analysis of the reaction mixture and to considerable scatter of the results. Consequently, kinetic analysis could not be performed, but overall the reaction was seen to be slower than at 27 °C. After 8 h reaction, 24-45% of the TDI remained unreacted at 12 °C whereas none could be detected at 27 °C. Using synthetic seawater, the reaction was slighter slower than in purified water. Under unstirred conditions, the reaction was much slower. A constant (zero-order) reaction rate showing a dependence
FIGURE 2. Disappearance of TDI, at 1000 mg/L, in small scale stirred experiments. [, 2,4-TDI; 9, 2,6-TDI; 0, 80/20 mixed isomers. FIGURE 4. Concentrations of toluenediamine (TDA) produced from reaction of 80/20-TDI and 2,4-TDI with water: [, with rapid stirring and 0, unstirred.
FIGURE 3. Disappearance of 2,4-TDI under static conditions showing the effect of TDI depth: b, 4.3 mm; 0, 7.2 mm; and [,10 mm (at 5.7 cm2 surface area).
TABLE 1. Kinetics of 2,4-TDI Reaction with Stirred Water at 27 °C and the 2,4-TDA Produced at the Completion of Reaction initial TDI
TDA produced
loading (mg/L)
extent reaction after 0.5 h (%)
half-life (h)
concn (mg/L)
yield from TDI (%)
10 100 1 000 10 000
85 75 32 20
