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Progress made on test standard for identifying hexavalent chromium

Hexavalent chromium, Cr(VI), has traditionally been used as a pigment for paints and plastics, with both Chromium Yellow (PbCrO4) and Chromium Red (PbCrO4•Pb(OH)2) extensively applied in the U.S. and Europe. However, due to the adverse health effects associated with Cr(VI) and lead, these pigments and other hexavalent chromium compounds were largely banned for use in electronic products under the EU RoHS Directive (2006). But how can processors and their customers be assured these pigments no longer are in their products?

PlasticsToday Staff

July 19, 2011

3 Min Read
Progress made on test standard for identifying hexavalent chromium

Hexavalent chromium, Cr(VI), has traditionally been used as a pigment for paints and plastics, with both Chromium Yellow (PbCrO4) and Chromium Red (PbCrO4•Pb(OH)2) extensively applied in the U.S. and Europe. However, due to the adverse health effects associated with Cr(VI) and lead, these pigments and other hexavalent chromium compounds were largely banned for use in electronic products under the EU RoHS Directive (2006). But how can processors and their customers be assured these pigments no longer are in their products?

NF_0720_SbCrInterfere.jpg

Color formation of extracts: alkaline digestion solution (left); Cr(VI) standard (center); Cr(VI) standard plus antimony trioxide (right).

That is the question tackled by Joe Kuczynski and Sophia Lau, both of whom work for IBM at its facility in Rochester, MN. Kuczynski works at IBM's Material and Process Engineering Lab, while Lau is employed in the IBM Center of Excellence for WW Product Environmental Compliance. The two authored this article for readers of PlasticsToday and also offer insight into ongoing work that may help answer the question and work to keep illegal pigments out of plastic products.

With product manufacturing continuing to move out of developed countries and into regions with less stringent environmental regulatory requirements, the possibility of finding these pigments resurfacing in plastics destined for use in electronic assemblies is of concern to electronic product manufacturers worldwide. In some cases, random batch-to-batch testing may be necessary to ensure product compliance.

X-ray Fluorescence (XRF) spectrometry, in general, is an effective screening tool to detect if chromium species are present as pigments in plastics; however, it lacks the capability to distinguish between the carcinogenic and regulated Cr(VI) species versus the non-regulated and more stable species of Cr(III). International test standards organizations, such as the International Electrotechnical Commission (IEC), are working to develop test standards that can discern and differentiate between these two forms of chromium; however, fully validated methods for testing of hexavalent chromium in plastics are not currently available. 

One of the challenges in developing such a test method is the low recovery of the hexavalent chromium compounds in certain plastic matrices. While the base resins can certainly influence the extraction efficiency, in a recent study, IBM scientists discovered another key factor that directly affects hexavalent chromium recovery in plastics.

In this joint IEC/IBM study, hexavalent chromium recovery in various plastic matrixes was evaluated via colorimetric reaction of Cr(VI) and diphenylcarbazide (DCP)1-3.  IBM scientists noted a low Cr(VI) recovery even in cases where the majority of the plastic matrices was dissolved. This pointed to potential matrix interference rather than an extraction efficiency issue. 

Plastics with specific flame retardancy requirements are often used in electronic products to prevent the spread of fire. The most common chemical flame retardants used in electronic applications are the halogenated flame retardants. In a controlled experiment, IBM scientists were able to further demonstrate that antimony trioxide, a synergist to enhance the activity of halogenated flame retardants, reacts with hexavalent chromium by converting Cr(VI) to Cr(III)4, leading to an artificially low detected hexavalent chromium levels.

Shown in the photo are the final DPC-containing solutions for the blank (alkaline digestion solution), a Cr(VI) standard, and the Cr(VI) standard with antimony trioxide. Cr(VI) forms a brightly colored pink-purple solution upon complexation with DPC, as evidenced by the center image in the photo. However, in the presence of trivalent antimony, Sb(III), color formation is completely absent. Therefore, simultaneous extraction of both the trivalent antimony flame retardant and the hexavalent chromium pigment in plastics results in facile reduction of Cr(VI) leading to erroneous quantification of hexavalent chromium.

The discovery of antimony(III) synergist matrix interference is key to unlocking the Cr(VI) recovery puzzle; there are still challenges ahead to research and develop methods to counteract the effect of the antimony synergist. IBM and IEC experts will continue to collaborate on developing viable solutions to this industry challenge, but with this discovery, are one step closer to a reliable test standard for Cr(VI) in plastics to confirm regulatory compliance.

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