Environ. Sci. Technol. 2002, 36, 1827-1832
Isocyanate Emission from PUR Adhesives: Influence of Temperature, Monomer Content, and Curing Mechanism
TABLE 1. Important Diisocyanates for PUR Adhesives
M A R T I N W I R T S * ,† A N D TUNGA SALTHAMMER‡ Fraunhofer Institut fu ¨ r Fertigungstechnik und Angewandte Materialforschung (IFAM), Wiener Strasse 12, 28359 Bremen, Germany, and Fraunhofer Institut fu ¨ r Holzforschung, Wilhelm-Klauditz-Institut (WKI), Bienroder Weg 54 E, 38108 Braunschweig, Germany
The isocyanate emission potential of polyurethane (PUR) adhesives was evaluated by measuring the area-specific emission rate (SERa; µg m-2 h-1) in a laboratory emission test chamber. The chamber was constructed for the analysis of reactive substances allowing quantitative trapping of all isocyanate monomers emitted from the substrate. Isocyanates were analyzed as their 1-(2-pyridyl)piperazine derivatives by HPLC with fluorescence and UV detection. The influence of temperature, the content of the diisocyanate in the resin, and the system pressure were investigated. These parameter studies gave evidence that the diisocyanate emission from the resins is a gas-phase-limited mass transfer. The overall mass-transfer coefficient (β ) Dδ-1) was calculated for HDI and MDI. From the temperature dependence of emission rates, the enthalpies of evaporation for 2,4′-MDI, 4,4′-MDI, and HDI from their resins were determined. The influence of monomer content on SERa followed Henry’s and Raoult’s laws in the cases of HDI and MDI, respectively, allowing the prediction of emission rates proportional to diisocyanate content. Therefore, from simple parameters such as application temperature and monomer content, isocyanate emission rates can be predicted. Additionally, MDI emission from curing PUR adhesives was studied. These results elucidate the influence of monomer reactivity and curing mechanism on the decrease of MDI emission with time.
Introduction The growing variety of different materials with low weight and high stability requires a bonding technology to give highly durable material composites. Adhesive bonding technology with polyurethane (PUR) adhesives fulfills these demands due to their high reactivity, which gives strong bonding to many substrate surfaces (1). On the other hand, the reactivity gives rise to health risks on exposure. Usually exposure levels in the indoor environment are close to or below detection limit, but isocyanate emission during industrial application of PUR adhesives can lead to exposure of workers (2). Despite * Corresponding author present address: Labor Dr. Wirts und Partner,Rutenbergstr.59,30559Hannover,Germany;e-mail:
[email protected]; telephone: +49 40 391628; fax: +49 511 95079829. † Fraunhofer Institut fu ¨ r Fertigungstechnik und Angewandte Materialforschung. ‡ Wilhelm-Klauditz-Institut. 10.1021/es015538z CCC: $22.00 Published on Web 03/08/2002
2002 American Chemical Society
1) TLV is the German occupational exposure limits for 8 h (Maximale Arbeitsplatzkonzentration, MAK; 3).
the wide use of PUR adhesives, the knowledge about the risk for workers applying these adhesives is limited, because most of these adhesives are used in small and medium-sized enterprises, e.g., car repair shops, which do not monitor workplace air quality. For that reason, there is a demand for predicting and assessing isocyanate emissions from fresh PUR adhesives. The structures of the two most important diisocyanates used in PUR adhesives are shown in Table 1. In comparison with p-xylene, a typical solvent and volatile organic compound (VOC), both diisocyanates possess a lower volatility and a higher potential for causing adverse health effects as indicated by their low threshold limit values (TLV). As a consequence and for hygienic reasons, the contents of the diisocyanate monomers with higher evaporability like HDI in adhesives is generally
