The industrial applications of polyurethane coatings have been steadily growing. The reaction of polyisocyanates to polyols is the basis for all polyurethane coatings. The Oregon State OSHA PEL for HDI-polyisocyanates is 0.5 mg/m3 8-hour TWA and 1.0 mg/m3 ceiling. The recommended impinger sampling method for HDI-polyisocyanates is cumbersome and potentially hazardous.
Previous comparisons of impinger versus filter sampling in paint spray environments have shown that the filters can underestimate the polyisocyanate concentration. However, a recent NIOSH study concluded that an upper limit for the isocyanate concentrations in a paint-spray environment could be measured/calculated by a gravimetric method.
In this study, a PVC-filter gravimetric method was compared to an impinger-sampling method for measuring isocyanate concentrations during spray painting of automobiles. Seven side-by-side impinger and gravimetric sample sets were collected inside a custom-made chamber from a 2-gal plastic bottle. Each set consisting of four impingers and four filter cassettes with the same inlet orientation was mounted symmetrically inside the chamber. Tygon® tubing passing through the base connected the samplers to battery-operated pumps outside. Additionally, a vacuum pump connected to five holes at the base permitted the paint spray to be drawn into the chamber at isokinetic velocity.
Gravimetric samples were weighed with a precision analytical balance. Impinger samples were analyzed for isocyanates by a standard HPLC/UV method. The gravimetric weights were converted to isocyanate weights using a factor based on the paint formulation. The average isocyanate concentrations in mg/m3 by the two methods were statistically compared. The average ratio of gravimetric measurements to impinger measurements was 1.06 ± 0.15 % for n = 6 (total 48 samples). The gravimetric isocyanate concentrations are the theoretical maximums for the paint atmosphere sampled, whereas, the impinger isocyanate concentrations are the true in-situ values. The predictable correlation between the two methods suggests that the gravimetric method could serve as a screening tool for monitoring isocyanates in validated paint atmospheres.