An innovative new application for waste phenolic compounds: Use of Kraft lignin and naphthols in leather tanning

Abstract

Degradation of Kraft lignin by using hemin and hydrogen peroxide was investigated to produce phenolic compounds. The degradation products were identified using spectroscopic methods and gas chromatography–mass spectrometry (GC–MS). 2-Methoxyphenol, 4-hydroxybenzaldehyde, vanillin, and vanillic acid were produced. Their polymerisation products were used as synthetic tanning agents for tanning collagen. Polymerised 2-methoxyphenol can increase the hydrothermal stability of collagen, like a conventional syntan, through hydrogen bonding interactions between the polymers and the protein. These products have potential to be used as raw materials for tanning of animal skins and other industries, such as chemical, pharmaceutical, food, and perfumery industries. Therefore, this can add value to the industrial byproduct (Kraft lignin), often considered a waste, and reduce its environmental impact. Leather tanning reactions between collagen and dihydroxynaphthalenes (DHNs) and oxazolidine have also 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. This work indicates that Kraft lignin degradation products could be used as new and innovative agents for leather production in a way that opens up new markets and reduces environmental pollution. Keywords: Waste phenolic compounds; Kraft lignin; Syntans; Dihydroxynaphthalenes; Cross-links; Tanning; Covalent bond