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Development and use of screening methods to determine chromium (VI) and brominated flame retardants in electrical and electronic equipment 1 Introduction
1.1 Executive Order on RoHSIn Denmark the RoHS Directive has been implemented by the Statutory Order on the restriction of import and sale of electrical and electronic equipment containing certain hazardous substances, Executive Order No. 873 of 11/08/2006. Pursuant to the Executive Order, import and sale of electrical and electronic equipment containing more than 0.1% lead, mercury, hexavalent chromium, polybrominated biphenyls (PBBs) or polybrominated diphenyl ethers (PBDEs) by weight in homogeneous materials and more than 0.01% cadmium by weight in homogeneous materials has been prohibited as from 1 July 2006. For a period of time the brominated flame retardant deca-BDE was subject to an exemption, but the exemption expired on 30 June 2008¹. 1.2 Homogeneous materials and passivation layersNeither the RoHS Directive nor the Danish Executive Order defines what is meant by "homogeneous materials". According to the EU Commission's interpretation as seen on its website, "homogeneous material" means a material which cannot be mechanically disjointed into different materials. Examples of homogeneous materials are individual types of plastic, ceramics, glass, alloys, paper, board, resins and coatings. Mechanical actions are stated to include unscrewing, cutting, crushing and abrasive processes. Examples mentioned on the Commission's website:
The uncertainty as regards the definition of homogeneous materials applies particularly to thin coatings, e.g. passivation layers on zinc, aluminium, copper and stainless steel. For example, a thin passivation layer on a thin coating of zinc may contain hexavalent chromium. In principle, it is possible to remove the hexavalent chromium surface by abrading it with fine-grade sandpaper, but in practice it will be difficult to obtain sufficient material using this method to analyse the concentration of hexavalent chromium in the homogeneous surface layer. Normally, the passivation layer can be dissolved with a suitable chemical solution, and the weight loss of a test piece can then be determined. Analysis standards for determination describe how to determine the weight of the passivation layer for chromated surfaces on zinc and cadmium coatings and on aluminium. These methods determine the volume per surface unit, and they cannot be directly used to determine the concentration of chromium(VI) in the passivation layer. It seems to be generally accepted that passivation layers with hexavalent chromium contain more than 0.1% in the thin surface layer that is generated either on top of an electrolytically precipitated zinc layer or generated through a chemical reaction with pieces of aluminium, copper and stainless steel. Such passivation layers have widely different thicknesses and thus widely different chromium(VI) release potential per unit area. Passivation layers on copper and stainless steel are so thin that they cannot be seen with the naked eye (0.01-0.1 µm), while passivation layers on zinc and aluminium are typically somewhat thicker (0.2-2.0 µm). As a consequence, it may be difficult to accurately determine the concentration of hexavalent chromium in the surface. The volume is often stated as a total volume per cm² and not as a concentration. The concentration can then be calculated if the surface thickness is known, but this may also be difficult to determine in practice. In many cases it is necessary to base the determination on the general knowledge about the thickness of the layers and the concentration of the substances in the layer, and then, based on an analysis showing the presence of the substances in the surfaces, to infer that the product is not compliant with the requirements of the RoHS Directive. Thus, the analysis of chromium(VI) in the surface does not establish directly that the concentration exceeds 0.1% in the passivation layer itself, or that the passivation layer can be regarded as a homogeneous material. At the time of writing (March 2009), no court of law has delivered a ruling as to the documentation needed to determine the presence of excessive concentrations of the substances in thin surface coatings. 1.3 Probability considerations in relation to the selection of samplesElectrical and electronic products are composed of a large number of components, each consisting of different parts made of different materials: plastic, metal, ceramic materials, glass, etc. A complete analysis for RoHS substances in all electrical or electronic product parts would easily comprise hundreds of analyses and be very costly. In this connection it is important to distinguish between the procedures in relation to documentation and control. Documentation of RoHS compliance should include documentation of all individual parts and materials. On the other hand, for control purposes it may be relevant to be able to select the parts in which the RoHS substances are most likely to be found. This guidance includes instructions as to where looking for individual substances is most relevant, and points out the exceptions to the RoHS Directive that should be taken into account. 1.4 Screening using X-ray fluorescence spectrometry (XRF)A widespread screening method used by the regulatory authorities in a number of countries is screening using X-ray fluorescence spectrometry (XRF). Handheld instruments are available on the market for elemental analysis of a sample in less than a minute. The handheld instruments typically have a 1 x 1 cm measuring window, so only larger parts can be analysed. Laboratories have more sensitive instruments with measuring windows of less than 1 x 1 mm, which can be used to scan more complex parts such as printed circuit boards. The handheld instruments are useful for analyses for lead, cadmium and mercury in large plastic parts, ceramic materials, glass, metal alloys, etc. However, it should be noted that the measurements may be highly uncertain because they often have to be taken under less than ideal measuring conditions. The instrument works by exposing the material to an X-ray beam. This causes the individual elements of the material to emit light with characteristic wavelengths, thereby making it possible to determine the elemental composition based on the intensity of the individual wavelengths of the emitted light. The "depth" of the measurement depends on the light dispersal in the material. In plastic materials, measurements taken using a handheld instrument typically provide a picture of the composition in the top millimetres, while measurements in metals reveal the composition in the top approx. 0.4 mm. Ideally, measurements can only be taken of homogeneous materials. In relation to inhomogeneous materials, e.g. with several coatings on top of each other, the measurements should be interpreted very cautiously. In relation to the PBDE and PBB substance groups, which both contain bromine, the instrument can be used to measure the presence of bromine, but this method cannot be used to determine the type of brominated compound. Consequently, an XRF screening can be used as the first screening step, but a positive XRF screening should be supplemented with a laboratory test as described in chapter 5 to get an indication of whether the bromine is present in the form of PBDEs or PBBs. In relation to hexavalent chromium, an XRF screening can only be used to analyse for chromium in the material, but it is not possible to determine whether the chromium is in the hexavalent form (see the detailed explanation in Chapter 3). The use of handheld XRF instruments is also limited in connection with chromium occurring in very thin coatings, because the instrument measures the average content of chromium in a layer that is considerably thicker than the coating of hexavalent chromium. Furthermore, ideal measuring conditions are difficult to obtain with a handheld instrument with a relatively large measuring window of about 1 x 1 cm if, for example, the measurement concerns a screw. Here, the readout may well indicate a chromium content in the material of less than 0.1%, although the actual content is much higher in the thin surface coating. This guidance assumes that the users of the guidance have access to screening for the elemental composition using an XRF instrument. [1] Executive Order No. 449 of 03/06/2008: "Executive Order amending the Executive Order on restriction of import and sale of electrical and electronic equipment containing certain hazardous substances".
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