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Global Journal of Inorganic Chemistry. Volume 1, Issue 2 (2010) pp. 119-131
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Review Article
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The Mechanism of Chrome Tanning
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Anthony D. Covington
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The University of Northampton, Boughton Green Road,
Northampton, NN2 7AL, UK
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Abstract |
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For over half a century, the mechanism by which chromium(III) salts can stabilise
collagen, converting it into leather, has been accepted as based on the notion
of crosslinking. This classical view
involves chemical bonding directly between carboxyl groups on adjacent triple
helices, whereby the highly structured protein is supported in the way stitching
strengthens a tear in material, so that it can resist wet heat up to 120oC
or even higher, when the transition from helix to random coil occurs, observed
as macroscopic shrinking. However, that
view is showed here to be erroneous; the currently assumed mechanism is fatally
flawed.
In
its stead, it is postulated that the reaction is better explained by a simpler
mechanism, named ‘Link-Lock’. All
tanning processes are either single- or multi-component. All single component reactions confer the
same degree of moderate hydrothermal stability increase, regardless of the chemistry,
because the bound species merely interfere with the shrinking transition: this
is termed ‘linking’, because the reagent only links to the collagen structure, but
even if the reagent is capable of creating crosslinks, the outcome is always
the same. Multi-component processes may additionally
involve reaction between a primary tanning agent and a secondary reagent, to
create a stable matrix within the collagen structure: by crosslinking the
primary reagent, ‘locking’ the molecules together, analogous to scaffolding around
a building, a new chemical matrix is formed.
This synergistic interaction means higher temperature is required to
cause the breakdown of the supramolecular matrix and the collapse of the
helical structure, so this is the origin of high hydrothermal stability.
Chromium(III)
molecular ions are merely linking agents, so to achieve high shrinkage
temperature, a locking agent must be present: several chemical species can
perform this function, but the commonest is sulfate ion. Crosslinking in the classical sense may
occur, but the evidence is that its occurrence is rare enough to be unimportant,
so the tanning mechanism does not depend on it.
The
role of masking, changing the ligand field of the chromium(III) complex, is to
affect the way further complexation can occur with collagen. Here, it is shown that the traditional view
is incorrect: however, the reality is that the tanner can benefit, but not in
the way currently assumed.
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Keywords |
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Chrome tanning; tanning chemistry; tanning mechanism
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