Homogenous Material

Both the ELV and RoHS directives specify maximum concentration values for certain “homogeneous materials”.  A great deal rides on what is meant by a homogeneous material, especially for coatings and surface treatments.  As defined by the European Commission, a homogeneous material is one that cannot be mechanically disjointed into different materials.

But what does that mean?  According to the UK Department of Trade and Industry, “The term ‘mechanically disjointed’ means that the materials can be, in principle, separated by mechanical actions such as unscrewing, cutting, crushing, grinding and abrasive processes.”

On a gross scale, this says that you cannot sell a car and claim it is compliant because it only has Cr6+ on the wheels, and so the weight of Cr6+ is only 0.000001% of the weight of the car.  However, the meaning is much more complicated for coatings.

For example, take a typical coating stack for a painted steel.  Cr6+ is often used in a chromate conversion layer on the surface of the zinc, and some high-corrosion applications have used a chromated primer (although that is now only done for aircraft).  If the primer is chromate-free, then the only likely problem with restricted materials is the thinnest layer of all: the chromate conversion layer.  As a percentage of the total the chromate layer is miniscule, and in fact zinc platers often regard chromated zinc as a single material.

So is this stack RoHS-compliant?  The easiest way to answer the question is to think of sanding the stack.  Even thought you might not be able to do so in practice, you could, in principle, sand off the layers one at a time, and the surface chemistry would be different as you remove each layer.  So each layer can be “mechanically disjointed” and therefore each coating in the stack is considered a homogeneous material.

Thus, what matters in the case of the chromate conversion layer is the percentage of Cr6+ in this layer alone.  And any hexavalent chromate layer, no matter how thin, presumably contains more than 0.1 wt.% Cr6+ – after all, that is why it works.

Life is, of course, more complicated than this.  It could be argued that although the layers in the coating stack could be separated in principle, this is not possible in practice.  It could also be argued that the layers are not themselves homogeneous since their chemical composition varies through their thickness.  This definition of a homogeneous material has been a source of argument and confusion since its inception; however this has not prevented the same wording being adopted in other regulations.