Chemistry of the crosslinking of collagen during tanning

The materials commonly used for crosslinking collagen as part of the process of converting animal hides into leather fall into three main groups: mineral tannages, aldehyde tannages, and "vegetable" tannages. The most important mineral crosslinking agents are hydrated basic chromium III sulfate complexes. These compounds form extended polynuclear coordination complexes containing hydroxol, oxo, and sulfato bridges into which ionized carboxyl groups on collagen enter readily as coordinating ligands accomplishing crosslinking. On pH adjustment and partial drying, highly stable complexes are formed with oxo bridges predominating and protein amide groups entering the coordination complex. The aldehyde tannages proceed through aldehyde condensation reactions with collagen amino groups to give alpha-hydroxyamines which can condense with other collagen amine groups to effect crosslinking. The vegetable type tanning agents, whether natural plant extracts or synthesized, are complex, high molecular weight polyhydroxy compounds that do not rely on crosslinking as such to be effective. Their effectiveness appears to depend on other properties. This and additional information concerning these commerical tannages are reviewed.