Phenolic reactions for leather tanning and dyeing
Kraft lignin degradation by a biomimetic system was investigated, using haemin and hydrogen peroxide, which mimics the catalytic mechanism of lignin peroxidase to produce phenolic compounds. The degradation products were identified using spectroscopic methods and gas chromatography-mass spectrometry. 2-methoxyphenol, 4-hydroxybenzaldehyde, vanillin and vanillic acid were produced and their formaldehyde polymerisation products were used for tanning collagen. The denaturation (shrinkage) temperature of collagen was raised to 80°C through hydrogen bonding interactions between the polymers and the protein. For dyeing of hide powder, the lignin degradation products were reacted with laccase (a polyphenol oxidase); 2-methoxyphenol gave the darkest colour. These products have potential to be used as raw materials for tanning and dyeing of animal skins. Therefore, this can add value to the industrial byproduct (Kraft lignin) and reduce its environmental impact. Polyphenols are the basic building blocks of natural skin and hair pigments: they can be polymerised to create versions of natural black, brown and orange melanins. These reactions can be catalysed by laccase; not only do they create colour, but also if they are conducted in the presence of collagen, the resulting pigment is bound to the leather in a covalent tanning manner. Therefore, the leather becomes coloured by a lightfast pigment, which is firmly fixed. It appears to be a feature of this type of stabilising mechanism for collagen, the creation of a polyphenol matrix around the triple helix, that it has the added benefit of actually strengthening the leather. Since every traditional process step effectively weakens collagen, especially tanning, this is the first chemical reaction that has been shown to reverse that effect. Leather tanning reactions between collagen and dihydroxynaphthalenes (DHNs) and oxazolidine have been investigated, using hide powder and sheepskin pickled pelt. This investigation showed that some DHNs have a tanning effect on collagen. The measurement of combined and cross-linked DHNs on collagen showed that 30-40% of 1,6- and 2,6-DHNs were fixed through covalent bonding. Shrinkage temperature of the leather changed little after the non combined DHNs had been removed from the leather, indicating that the high stability of the combination tanned leather comes from the covalent bonding formed between DHNs and collagen through oxazolidine.