to reaction with polyols or water. Second, it improves detection of the isocyanate by increasing sensitivity and selectivity. The two most important factors in achieving efficient derivatization are the inherent reactivity of the reagent and the ability of the collection medium to dissolve or disperse collected particles or droplets and/or make derivatizing reagent accessible to the isocyanate groups. Derivatizing reagents in use today are most commonly primary or secondary aliphatic amines and their inherent reactivities with isocyanates typically differ by less than a factor of five.61,62 This difference is probably not very important. Probably of greater importance is the efficiency of mixing the collected particles and derivatizing reagent. Aerosols generated from spray applications of isocyanate products typically contain a mixture of isocyanate and polyol. If the polyol and isocyanate are not separated and the derivatizing reagent is not accessible to the isocyanate group at the time of collection, the isocyanate will be lost to reaction with the polyols within the droplet.2,63 This would appear to be a significant problem for collection of droplets on reagent-coated GFFs. Micrographs of spray paint droplets on GFFs show that droplets typically make only minimal contact with the fibers.51 This is not conducive to dispersal of the droplet or mixing of the reagent coated on the fiber with the isocyanate species. The larger the droplet, the greater the problem is likely to be because of the greater potential deficiency of reagent at or near the sites of contact with the fibers. Derivatization of isocyanate species present in particles or droplets is expected to be more efficient using impinger collection. The solvent not only serves to disperse or dissolve the droplet or particle, thereby interfering with the curing reaction but also provides a means for bringing the reagent and the isocyanate species together. Two practices have been investigated that appear to improve the performance of reagent-coated GFFs when sampling aerosols. One is the presence of a small amount of nonvolatile solvent, such as diethyl phthalate or diphenylmethane, on the filter.64,65,66,67 Another is the desorption of the filter in a solution of derivatizing reagent immediately after sampling.66 c. Sampling: Collection plus Derivatization Reagent-coated GFFs and sorbents, impingers, and bubblers have all been investigated as samplers for isocyanates. Some sampler comparisons have been conducted under laboratory conditions and others in the field. The majority of comparisons have found that solventless samplers give higher results than impingers or bubblers in laboratory evaluations and impingers and bubblers give higher results in the field.2 A likely explanation is that collection efficiency is most important in laboratory evaluations, under which conditions the polyols that compete with derivatizing reagent are generally not present. Under conditions where derivatization kinetics are relatively unimportant, reagent-coated GFFs may give higher results than impingers because the GFFs collect small particles more efficiently. In the field, where derivatization rate is clearly important, impingers and bubblers tend to give higher results than solventless samplers. Evidence for this is especially strong in environments where rapidly curing MDI-based products are sprayed.67,68,69,70 The literature is more ambiguous concerning environments where less reactive HDI-based products are used.71,72,73,74 Table II gives recommendations for sampler use based on the size and cure rate of the isocyanate aerosol. The cure rate of isocyanate systems depends on the nature of the isocyanate, the nature of the co-reactant (typically polyols), the type of catalysis (if any), and the temperature. The half-lives of isocyanate species in reactive systems (i.e., the time it takes for half of the isocyanate groups to undergo a curing reaction) can vary from a few seconds to many hours. For the purposes of this document, isocyanate systems having half lives of a few minutes or less are considered fast cure. The faster the product cures, the more strongly an impinger is 1/15/98
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