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SCIENCE:
DEGRADABILITY
The topic of degradability is discussed
here as an example of a key issue for the chemical industry.
Chemicals should, ideally:
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be sufficiently stable to fulfil the
purpose for which they are being used – such
stability may be intrinsic or conferred by formulation |
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be sufficiently degradable in the environment that
they are not considered as persistent according to
regulatory criteria. Degradability by higher organisms
such as mammals may be advantageous if the products
are not toxic. |
We have heard it stated frequently that
the above criteria are mutually exclusive. It is our experience
that this is not true – in other words, functional
stability does not necessarily mean persistence. The molecular
features that make for environmental degradability (partial
or complete) do not imply, for example, thermal instability.
Environmental degradability usually
implies biodegradation, perhaps assisted by hydrolysis
or oxidation. Photodegradation is an additional route,
but is not always reliable or possible!
What kind of features help biodegradability,
and which hinder?
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presence of labile functions such as
esters, alcohols or activated double-bonds is beneficial |
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use of linear alkyl groups and avoidance of branching/fused
rings can help primary degradation start |
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limiting use of halogens, especially multiple substitution
(some halogenation is not necessarily a problem) is
usually a good strategy |
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limiting molecular weight and size helps bioavailability |
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avoidance of some types of cyclic systems, and of
certain functional groups (e.g. nitro) on aromatic
rings can be necessary |
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use of functions founds in natural compounds e.g.
acids, amines can help degradation, although there
can be toxicity risks in mimicking biotic substances
too closely. |
Polymers represent a different set
of issues, since their high molecular weight limits bioavailability.
Nevertheless, degradation of polymers for certain applications
is desirable and achievable.
Complete and rapid degradation is not
always required. It may be sufficient for there to be
rapid partial degradation followed by slower mineralisation.
Photography courtesy of Emanuel
Lobeck
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