Characterization of Aerosol Release during Spraying of Isocyanate Products

The aerosol release during the professional application of two different isocyanate based two component spray systems was identified and the physicochemical properties of the released airborne aerosols were characterized. For this purpose, aerosol release fractions were measured using a mass balance method described by Schwarz and Koch. Besides the release of total aerosol mass special emphasis was directed to the content of free monomeric MDI (4,4'- and 2,4'-diphenylmethane diisocyanate) in three particle size fractions relevant for inhalation uptake: inhalable, thoracic, and respirable size fraction. Two products were investigated: a two component PUR (polyurethane) spray foam (Elastopor) and a polyurea spray coating (Elastocoat). The mass fraction of the applied products released with the overspray as inhalable aerosol is 6.3 × 10-4 (Elastopor) and 4.0 × 10-4 (Elastocoat). Of the released total overspray aerosol 75 or 80% were in the thoracic size range, and 26 or 47% in the respirable regime for the PUR spray foam or the polyurea spray coating, respectively. At the time point of release the content of monomeric MDI in the aerosol corresponds to the composition of the bulk product. However, analysis of air samples indicates that <1% of the spray foam aerosol mass release fraction is attributed to free monomeric 4,4'- and 2,4'-MDI. For the Spray Coating the monomeric MDI fraction is <0.1%. Higher oligomers of MDI and prereacted oligomeric reaction products make up a few percent of the aerosol. This results in a total fraction of 0.0023% (spray foam) and 0.00015% (spray coating), respectively, of the sprayed monomeric MDI that is transferred into an inhalable aged aerosol. This data demonstrates, that during professional spraying only a small fraction of the total applied mass is released as airborne aerosol. The potential distribution of the theoretically inhalable aerosol in the respiratory tract and a low residual monomer content is described, significantly contributing to a refined safety assessment of the spray applications at the workplaces.