Sulfide unhairing: rethinking the received wisdom

Content: The removal of hair from a hide or skin by dissolving it with a mixture of lime and sulfide is a fundamentally understood feature of leather technology. Or is it? For a long time, it has been accepted within the leather literature that, in water, sulfide may be present as either hydrogen sulfide (H2S), hydrosulfide (HS-) or sulfide (S2-), depending on the pH. pH 12 H2S(aq) ⇌ HS-(aq) ⇌ S2-(aq) The generally accepted mechanism of hair burning is sulfide attack at the cystine disulfide linkages in keratin. Also, it is believed that the unhairing reaction only proceeds at an appreciable rate in the presence of the dianionic S2- species, because that fits with the technological observation that unhairing reactions only proceed at pH greater than 12. However, recent publications have provided substantive proof that the S2- species does not exist in aqueous media at any pH: researchers were unable to observe any evidence of the S2- species in a solution of Na2S dissolved in hyper-concentrated NaOH and CsOH using Raman spectroscopy. The assigned second pKa for removal of the second proton has now been estimated to be 19, making the concentration of S2-(see below) vanishingly small. HS- ⇋ S2- + H+ There is a clear contradiction between the currently accepted mechanism for sulfide unhairing with the evidenced speciation of sulfide species in aqueous environment. Here the implications for this important process are discussed and possible alternative mechanisms postulated that fit with the new knowledge. Take-Away: It is a truism that we must understand the mechanistic principles of a process in order to control it. Here, we have a big change in thinking for ‘sulfide unhairing’, so it is vital that we understand the implications for leather science and leather technology of that change

Towards zero solid waste: utilising tannery waste as a protein source for poultry feed

Zero waste is now a strongly emerging issue for sustainable industrial development where minimisation and utilisation of waste are a priority in the leather industry. In a tannery hides and skins converted in to leather through various processes. Approximately 20% (w/w) of the chrome containing tannery solid waste (TSW) is generated from one tonne of raw hides and skins. However, tannery solid waste may also be a resource if it is managed expertly as we move towards zero waste. This research illustrates the potential of tannery solid waste as a poultry feed additive. An oxidation method was used to achieve 95% of dechroming rate of chrome tanned waste followed with thermal and enzymatic treatment to produce gelatin solution and collagen concentrates. The thermal stability and fibre structure of samples were analysed by Differential Scanning Calorimeter (DSC) and Scanning Electron Microscope (SEM). Protein content and fourteen amino acid concentrations were determined using amino acid analysis. High Performance Liquid Chromatography (HPLC) was used to compare the amino acid composition with wheat and soya bean meal that is conventionally used in poultry feed. The nutrient requirements for poultry feed vary according to the purpose for which they have been developed. The high content of arginine, leucine, threonine, serine and methionine in the extract were of a sufficient level for poultry feed. Hexavalent chromium test was performed and showed that levels of the metal were low enough to be used in feed additives. In addition, the extracted product showed 75% digestibility (in vitro) and appears that treated TSW may be utilised in poultry feed, this demonstrates a clear example of waste utilisation. In Bangladesh plans are being formed to use the extract in poultry feed production

Methods of isolation and identification of pathogenic and potential pathogenic bacteria from skins and tannery effluents

Currently there is no standard protocol available within the leather industry to isolate and identify pathogenic bacteria from hides, skins or tannery effluent. This study was therefore carried out to identify simple but effective methods for isolation and identification of bacterial pathogens from the effluent and skins during leather processing. Identification methods based on both phenotypic and genotypic characteristics were investigated. Bacillus cereus and Pseudomonas aeruginosa were used as indicator bacteria to evaluate the isolation and identification methods. Decontaminated calfskins were inoculated with a pure culture of the above mentioned bacterial species followed by a pre-tanning and chromium tanning processes. Effluent samples were collected and skins were swabbed at the end of each processing stage. Bacterial identification was carried out based on the phenotypic characteristics; such as colony appearance on selective solid media, cell morphology following a standard Gram-staining and spore staining techniques, and biochemical reactions, e.g., the ability of a bacterial species to ferment particular sugars and ability to produce certain enzymes. Additionally, an identification system based on bacterial phenotypic characteristics, known as Biolog® system was applied. A pulsed-filed gel electrophoresis (PFGE) method for bacterial DNA fingerprinting was also evaluated and used for the identification of the inoculated bacteria. The methods described in the study were found to be effective for the identification of pathogenic bacteria from skins and effluent

Use of Viscoelastic Changes to Demonstrate the Relationship Between Drying Parameters – A Preliminary Study

Dynamic mechanical thermal analysis (DMTA) is routinely used for mechanical analysis by the polymer industry to provide information on the viscoelastic properties of a material. This report reveals how DMTA has been used to further previous studies by providing insight into the differences between post-tanned leathers (chromium and chromium-free). It demonstrates the potential to correlate the results with an industrial application such as optimization of the drying conditions during cell rotary conditioning (CRC). DMTA can indicate leather fiber response to changes in atmospheric humidity and temperature, potentially facilitating real-time adaptation of conditions during leather drying. Initial DMTA results show that post-tanning, particularly fatliquoring, changes the rate of drying and allows scientists to advise on optimal leather drying conditions based on viscoelastic changes

Investigating the Cell Rotary Conditioning Mechanism Using Dynamic Mechanical Thermal Analysis

Jeyapalina et al. established that dynamic mechanical thermal analysis (DMTA) can be used to gauge the progression of leather drying. This work has now been advanced in order to understand the mechanism surrounding sorption/desorption of moisture during cell conditioning, e.g., cell rotary conditioning (CRC). This paper will demonstrate how the use of DMTA could be used to monitor changes in leather stiffness. A gravimetric moisture analysis was performed on identical leather samples to gauge the progression of desorption. The change in storage modulus (E’) was coupled to the moisture content (leather moisture and atmospheric relative humidity) to obtain a better understanding of the physical properties (specifically stiffness) of leather during a drying process. The research presented illustrates how DMTA can indicate leather fiber response to changes in atmospheric humidity and facilitate real-time adaptation of drying conditions during leather dehydration. The use of a cell conditioning system allows a tanner to control the flexibility of the material through the tension applied and the drying conditions. DMTA shows that the favorable conditions inside a CRC unit result in detectable changes to the leather fiber, similar to findings by Abrahamson and Williams-Wynn. Using this technique a researcher can dry chromium-containing and chromium-free leathers in a manner that is highly customizable to produce desired physical properties. Difficulties experienced in chromium-free leathers can also be investigated in detail using this technique

Parameters for composting tannery hair waste

Solid hair waste is generated by the leather industry as a by-product of the leather manufacturing process. Keratin, the main structural constituent of hair proteins, is highly resistant to degradation and their disposal is of environmental concern. The aim of this study was to develop conditions favorable for the degradation of bovine hair in a composting environment as an environmentally friendly option for the management of solid tannery hair waste. The thermophilic optimum temperature, 40 – 50C, moisture content 55%, pH 7.0 and a carbon to nitrogen ratio of 35:1 were found to be favorable to sustain metabolic functions of thermophilic microbial flora, responsible for degrading keratins. The biodegradation and structural transformation of the substrate was assessed using scanning electron microscopy. The results show that under these conditions the bovine hair lost most of its integral structural stability and that the cuticular components were more resistant to degradation. The compost stability as evaluated by monitoring the degree of humification and carbon to nitrogen ratio indicated that the final product achieved reasonable stability by attaining 73% degree of humification, 26% humification rate and carbon to nitrogen ratio of 29:1. Hence the composting technology used in this study has potential application in the leather industry for the production of an economically viable produc

NEW ENVIRONMENTALLY BENIGN LEATHER TECHNOLOGY: COMBINATION TANNING USING VEGETABLE TANNIN, NAPHTHOL AND OXAZOLIDINE

ABSTRACTLeather tanning is a process of converting of skin or hide protein into leather with adequate strength properties, resistance to various biological and physical agents, and capable of being used for a wide range of purposes.  Leather tanning reactions between collagen-vegetable tannin-oxazolidine and collagen-dihydroxynaphthalenes (DHNs)-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 vegetable tannin and DHNs on collagen showed that 20-50% vegetable tannin, 1,6- and 2,6-DHNs were fixed through covalent bonding.  Shrinkage temperature of the leather changed little after the non combined vegetable tannin and DHNs had been removed from the leather, indicating that the high stability of the combination tanned leather comes from the covalent bonding formed between vegetable tannage or DHNs and collagen through oxazolidine. Covalently bound tannin on collagen was more stable and could not be extracted by lyotropic agents. Keywords: tanning, leather, hydrothermal stability, vegetable tannage, dihydroxynaphthalenes, oxazolidine

NEW ENVIRONMENTALLY BENIGN LEATHER TECHNOLOGY: COMBINATION TANNING USING VEGETABLE TANNIN, NAPHTHOL AND OXAZOLIDINE

ABSTRACTLeather tanning is a process of converting of skin or hide protein into leather with adequate strength properties, resistance to various biological and physical agents, and capable of being used for a wide range of purposes.  Leather tanning reactions between collagen-vegetable tannin-oxazolidine and collagen-dihydroxynaphthalenes (DHNs)-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 vegetable tannin and DHNs on collagen showed that 20-50% vegetable tannin, 1,6- and 2,6-DHNs were fixed through covalent bonding.  Shrinkage temperature of the leather changed little after the non combined vegetable tannin and DHNs had been removed from the leather, indicating that the high stability of the combination tanned leather comes from the covalent bonding formed between vegetable tannage or DHNs and collagen through oxazolidine. Covalently bound tannin on collagen was more stable and could not be extracted by lyotropic agents. Keywords: tanning, leather, hydrothermal stability, vegetable tannage, dihydroxynaphthalenes, oxazolidine

Microscopic examination of skin structure

Microscopic examination of the skin structure may help in identifying possible lesions as well as studying the integrity of the tissue; two dimensional examination of the structure may not be able to easily identify the complexity of the collagen bundle network in skin sections. In this area our research involves examination of skin sections in three dimensions. Thin dermal sections were stained with Haematoxylin and Eosin (H & E) and mounted on glass slides with a cover slip. The sections were imaged using a Leica TCS SP2 confocal microscope (Leica Microsystems GmbH, Germany). We have developed software techniques for 3D visualisation of the confocal data allowing display on a computer monitor or an observation cube. The image shows an example of visualisation from confocal microscope data of collagen bundles in a skin section (microscope image width 750 microns)