What is a Homogeneous 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 in some high-corrosion applications, a chromated
primer is also used. 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. We suspect that in the end this whole
question will have to be fought out in court in the various
EU jurisdictions, with results that may or may not make any
engineering sense.
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