Objective Color Scale for the SWYPE Surface Sampling Technique Using Computerized Image Analysis Tools

Abstract

Colorimetric SWYPE pads are useful tools for identifying unpolymerized aliphatic isocyanates on a variety of surfaces. This technique has been used in autobody shops to determine the presence of hexamethylene diisocyanate and other aliphatic isocyanates that are important constituents of many automotive coatings. SWYPEs have the advantage of being relatively inexpensive, rapid, and portable. The color change elicited by aliphatic isocyanates (from yellow to red) provides a visual indication of the extent of surface contamination. To quantify isocyanate contamination based on the colorimetric response, an objective color scale was developed for isocyanate loading. Sampled pads were digitized on a calibrated, portable flatbed scanner, and red-green-blue (RGB) histograms of SWYPE images were created. A calibration curve was created from a series of reference images derived from SWYPEs loaded with an isocyanate-containing product. The SWYPE RGB analysis allowed for quantification over a range of isocyanate loadings: from ∼0.01 to 24.0 μg/cm², with reproducibility of >90%, accuracy >90%, and a surface extraction efficiency of >90%. RGB analysis exhibited a lower detection limit than visual scoring (∼3 μg versus ∼10 μg). The colorimetric response was cross validated with a high-performance liquid chromatography quantitative assay. When combined with RGB analysis, SWYPE colorimetric wipes represent a rapid and inexpensive method to assess objectively surface contamination with aliphatic isocyanates.

Keywords:

• autobody
• collision repair industry
• diisocyanates
• hexamethylene diisocyanate
• surface contamination

ACKNOWLEDGMENTS

The authors would like to acknowledge and thank Neal McConnell for his constant support and technical assistance; Jacqui Ahmad and Jianbo Yu for chemical analysis at the Environmental Health Laboratory, University of Washington; chemist Tom Klingner at CLI, Colorimetric Laboratories, Inc., for his guidance; Lisa Weiland at WISHA Laboratory for supplying hardener samples, and the spray painting training facility for providing a sampling location.

This project was funded by the National Occupational Research Agenda Project.