Sampling of diisocyanates (HDI, TDI) in air by derivatisation with secondary amines as reagentsPart 1. Partial rate factors (PRF) of reagents

Abstract

Relative differences in reactivity between reagents (all of which are secondary amines) particularly affect polyfunctional isocyanates, such as hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI), compared with monoisocyanates. Thus, in 1994–1995, we carried out a programme of work to study the relative reactivity of the standard reagents, and of two alternative derivatising agents, by performing comparison tests on reaction rates. In the meantime, the reactivity of standard reagents used internationally for the determination of isocyanates in workplace atmospheres, as compared with some proposed “more reactive” alternative competitors, has been addressed by various authors. We measured the relative rates of reactions (partial rate factors) using the monomeric diisocyanates HDI and TDI and the monoisocyanate phenylisocyanate (PHI) separately in absorber solutions containing twin mixtures of different reagents. The partial rate factors of the reactions were found to vary by orders of magnitude using the diisocyanates HDI and TDI. They show a dependence on the chemical structure. Using the monoisocyanate PHI, our experimental results are consistent with literature data. Furthermore, our results clearly demonstrate that the proposed “most reactive” reagent dibutylamine (DBA) is less reactive towards HDI and TDI than is the reagent 1-(2-methoxyphenyl)piperazine (2-MP). In spite of the different reaction rates found, in our experience, up to now, there is “no-effect” on the analytical results when comparison tests are performed by monitoring isocyanates in air using DBA and the well known standard reagents. Work continues and results will be given in a following paper.