98 Chapter 5
2.3 Tanning Materials
The chemical structure of collagen, a long-chain protein with significant quan-
tities of free acidic, basic and hydroxy-, as well as other reactive, groups on the
outer surface of the molecule, allows it to react readily with a diverse range
of agents. A number of these are multifunctional, with the ability to form stable
links between adjacent protein chains which have the properties required to
produce a true leather. Some of the more commercially significant groups will
be considered.
Oil and Aldehyde Tannages. The technique of impregnating skins with oils
and fats to make a variety of leather-like materials has been outlined. Among
other factors, the properties of the final product are critically dependent on the
type of oil used. Brains, marrows and, in particular, marine oils have beenfound
to give a particularly soft, spongy, stable product. It is now known that these
contain fatty acids with a medium degree of polyunsaturation. Unsaturatedoils
and fats such as tallow only lubricate. Highly saturated materials such as lin-
seed oil polymerise around and within the fibre structure, eventually giving a
hard, cracky product. Another factor found to be essential for the production
of a truly leather-like material is that after the skin is impregnated with the
oil, it has to be left in the open air for an extended period of time. It is now
known that during this period oxidation takes place at the unsaturated bonds
and that this reaction can be enhanced and accelerated by hanging the skins
in heated stoves or otherwise treating them with warm air.
The exact nature of the tanning reaction is not known and a number of mech-
anisms have been proposed. It has been shown, however, that as the process
progresses, the degree of unsaturation of the oil reduces, peroxy-derivatives
are formed, hydroxyl functions appear and, more specifically, acrolein,
CH 2 CH · CHO, is produced. It is thought that this and other aldehyde com-
pounds are responsible for the chemical cross-linking and that coating the fibres
with polymerised oils imparts the special physical characteristics to the leather.
More is known of the mechanism of interaction between the simpler alde-
hydes, particularly formaldehyde, and collagen where a reaction first takes place
at the free amino groups:
(1)Crosslinking then occurs between the N-hydroxymethyl group and a free
amino group from an adjacent polypeptide:
(2)Collagen NH CH OH + H N Collagen
Collagen NH CH HN C22
2
→ oollagenCollagen NH 22 HCHO Collagen NH CH OH →