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Can wipe sampling be used for quantifying the amount of isocyanate on a surface?

The wipe sampling methodology was developed as a qualitative tool for education and housekeeping improvement. Wipes are useful in detecting isocyanate contamination on surfaces and/or objects that workers might expect to be clean (e.g., doorknobs, handrails, tools, safety glasses, pencils) or for determining if surfaces are clean after decontamination activities. In this mode they can be used to indicate areas which need to be cleaned or, when this is impractical, where PPE is required. The presence of dirt, grime or other components, however, can affect the shade of the indicator color so that interpretation beyond a semi-quantitative determination of high, low, or not contaminated may be difficult.

The manufacturer can provide a color wheel which may be useful in evaluating isocyanate surface contamination (ideally from surfaces that are clean with respect to dirt and grime). However, difficulties can exist when interpreting the wipe sampling results due to the variability from the reader’s color perception and if the color is not consistent across the wipe pad. For example, if particles with reactive isocyanate groups are picked up by the pad, a non-homogeneous discoloration may result. For quantification, a defined area needs to be “wiped” and this is not always possible (e.g., doorknobs). Furthermore, such data is most beneficial if it can be interpreted in terms of potential for exposure (i.e., transferability from the contaminated surface to unprotected skin) and to aid in decision making with regard to increased risk reduction measures. Even then, care must be taken as it should be understood that it is not only the monomers that will give a color reaction, but also the oligomeric NCO-species or partly reacted NCO-species which could be partially bound to the surface and not readily transferable to the skin.

To conclude: Wipes are useful qualitative tools and not designed to provide reliable quantification of isocyanates on surfaces.

Can wipes be used to identify available isocyanates on a polyurethane surface that might not be completely cured?

Potentially, but care needs to be exercised. For example, the wipe sampling methodology can provide a qualitative assessment of the presence of an unreacted isocyanate species but cannot determine if the contact will result in transfer to (unprotected) skin. Further, wipes should be used as recommended by the manufacturer. They are sold with mineral oil as a developing solution. Use of more aggressive or polar solvents may (1) enhance the extraction of free diisocyanate from within the polyurethane material in a manner that is not representative of skin contact, or (2) solubilize the reaction chemistry of the wipe allowing it to cause discoloration of the polyurethane product due to reaction with a bound, partially reacted diisocyanate. This surface discoloration would not indicate a transferable exposure to skin and could be misleading if the process is not understood.

To conclude: Wipes are useful qualitative tools, but care should be exercised in using this tool in accordance with the manufacturer’s recommendations.