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Environmental fate & pathways

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Information from a variety of sources has been collected and summarized to facilitate an overview of the atmospheric fate and potential environmental effects of emissions of methylenediphenyl diisocyanate (MDI) or the read accros approach to toluene diisocyanate (TDI) to the atmosphere (Tury et.al., 2004). Atmospheric emissions of both MDI and TDI are low, both in terms of concentration and mass, because of their low volatility and the need for careful control over all aspects of their lifecycle from manufacture through disposal. Typical emission losses for TDI are 25 g/t of TDI used in slabstock foam production. MDI emission losses are lower, often less than 1 g/t of MDI used. Dispersion modeling predicts that concentrations at the fenceline or beyond are very low for typical releases. Laboratory studies show that TDI (and by analogy MDI) does not react with water in the gas phase at a significant rate. The primary degradation reaction of these aromatic diisocyanates in the atmosphere is expected to be oxidation by OH radicals with an estimated half-life of one day. Laboratory studies also show that this reaction is not expected to result in increased ground-level ozone accumulation.

Experiments were carried out with a variety of TDI concentrations at 27 °C and relative humidity 7–70% in a 17-m³, Teflon-lined environmental chamber. Atmospheres were monitored for TDI (by several methods), TDA, TDA-urea, organic carbon, and total bound nitrogen, and for presence of aerosols. A significant TDI loss rate was observed, which was matched by an equal loss of organic carbon and nitrogen, indicative of physical removal from the gas phase by deposition onto the chamber’s walls rather than chemical conversion. No evidence for a gas-phase reaction with water vapor was found, and no TDA or ureas were detected.

Overall, the study clearly indicated degradation by photolytically generated radicals, rather than by direct photolysis, and the absence of any hydrolysis in the vapor phase.

A substantial loss rate of TDI caused by adsorption onto the walls was again observed.

Irradiation substantially increased the loss rate. The incremental loss caused by irradiation was not affected by the presence of common urban pollutants: hydrocarbon mixtures, NH3, and ammonium sulfate aerosol. It was enhanced by the presence of relatively high concentrations of diazabicyclooctane (DABCO, a PU catalyst) but was suppressed by nitric oxide, a free radical scavenger. No TDA was detected in any experiment, corresponding to less than 0.05% conversion of TDI. Overall, the study clearly indicated degradation by photolytically generated radicals, rather than by direct photolysis, and the absence of any hydrolysis in the vapor phase.

The degradation of most trace organic gases in the atmosphere is initiated by reaction with OH radicals, themselves generated photochemically. This was confirmed as the dominant process for TDI by a study of the degradation rate of 80/20 TDI in a large photoreactor, at ambient temperature and pressure, in the presence of photolytically generated OH radicals. The reaction rate of TDI with OH radicals, measured relative to that of toluene, was estimated as=7.4 x 10E12 cm³/molecule/ sec.

Both isomers of TDI were found to inhibit ozone formation and radical levels in all experiments. An 18-hr irradiation showed that the ozone inhibition effects extended over at least two days. The prediction is made, that TDI is unlikely to have a positive effect on ozone formation under any atmospheric conditions in the United States, and it should not be considered an ozone precursor.

This atmospheric degradation of MDI and TDI by OH radicals raises concerns that the process might lead to ozone or smog formation. Smog chamber studies have shown that this is not the case. TDI, in particular, should not be considered an ozone precursor.

Overall, it can be concluded that no significant long-term or wide-ranging environmental effects would be expected from current emissions of MDI or TDI to air.