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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.
isocyanates, diisocyanates, polyisocyanates, industrial hygiene, polyurethane paint spray environment, air sampling, gravimetry, method evaluation, field validation, field comparison, impinger sampling
Senior Industrial Hygienist, Worldwide Facilities Group-Chemical Risk Management, General Motors Corporation, Wentzville, MO
HES Principal, Bayer Corporation, Pittsburgh, PA