Emission of Isocyanates from Pur-adhesives. Time Course of Emission from Curing Pur-adhesive

The time course of isocyanate emission from curing PUR-adhesives was studied in an emission test chamber. The adhesives under investigation were commercial products for fixing of textile floor coverings and based on methylene bisphenyl diisocyanate (MDI). Influence of the curing mechanism was studied by comparing the emission curves of one-component adhesive (OCA) and two-component adhesive (TCA). For TCA the decrease in isocyanate emission was found to follow a two-step process during curing. In the first step the emission is dominated by surface evaporation and the decay of emission is mainly caused by the decrease in monomer content due to reaction. In the second step the release is limited by internal diffusion. The influence of monomer reactivity on the emission profile could be demonstrated for 2,4 ́and 4,4 ́-MDI. The less reactive 2,4 ́-MDI caused prolonged emission. INDEX TERMS VOCs and SVOCs, Indoor air chemistry, Measurement methods, Air and pollutant transport modeling and measurement, Monomers INTRODUCTION Polyurethane adhesives are widely used because their chemical and rheometric properties can be adapted to various materials. The high reactivity of diisocyanates leads to very durable bonding, but also gives rise to health risks for persons under exposure (Wolff and Stirn, 2000). Therefore there is a demand for information on isocyanate emission from fresh PUR adhesive systems. Such adhesives systems are based on “polymeric MDI” (PMDI). In the synthesis of PMDI not only the symmetric 4,4 ́-MDI is obtained but also the asymmetric 2,4 ́MDI in smaller amounts and, to an even lesser extent, 2,2 ́-MDI (Ulrich, 1999). Additionally different amounts of MDI homologues with three and more benzyl rings depending on the reaction conditions will be obtained. Previous work has shown that the emission of monomers from fresh PUR-adhesives can be described in a similar way as for isocyanate resins (Wirts and Salthammer, 2002). The emission process was found to be limited by gas-phase diffusion, i.e. fresh polyurethane (PUR) adhesives are best described as "wet sources". In contrast to curing processes of wet paints, which are dominated by physical "drying" caused by solvent evaporation (Clausen, 1993; Sparks et al., 1999; Zeng et al., 1999), PUR adhesives harden by a reaction which leads to a depletion of the emitting monomer-substance. Furthermore, the reaction leads to an alteration of the matrix which may influence the diffusion of monomers through the bulk. For separation of these two effects we conducted two experiments. The depletion of monomer in the adhe∗ Contact author email: [email protected] Proceedings: Indoor Air 2002